Photochemistry (Volume 22)

Photochemistry Volume 22

A Specialist Periodical Report

Photochemistry Volume 22 A Review of the Literature published between July 1989 and June 1990 Senior Reporters D. Bryce-Smith, Department of Chemistry, University of Reading A. Gilbert, Department of Chemistry, University of Reading Reporters N. S. Allen, Manchester Polytechnic A. Cox, University of Warwick R. B. Cundall, MRC Radiobiology Unit, Didcot M. Edge, Manchester Polytechnic W. M. Horspool, University of Dundee S. T. Reid, 7he University of Kent A. C. Weedon, The University of Western Ontario, Canada

ISBN 0-85 186-205-5 ISSN 0556-3860 Copyright 0 1991 The Royal Society of Chemistry All Rights Reserved No part of this book may be reproduced or transmitted in any form or by any means - graphic, electronic, including photocopying, recording, taping, or information storage and retrieval systems - without written permission >om the Royal Society of Chemistry

Published by The Royal Society of Chemistry, Thomas Graham House, The Science Park, Cambridge CB4 4WF Printed in Great Britain by Billings & Sons Ltd., Worcester

Introduction and Review of the Year BY D. BRYCE-SMITH AND A. GILBERT

There have been several changes this year. Firstly, we have been able to reintroduce a Chapter on Solar Energy Conversion. The present Chapter is rather short, partly in reflection of the reduced interest in this topic among many researchers. However, further references relevant to this topic will be found in some of the other Chapters, in particular that dealing with photo-oxidation and -reduction. Largely in the interests of keeping the length of the Volume and its price within manageable limits, we have decided to discontinue the coverage of inorganic and organometallic aspects of photochemistry in specific Chapters, but numerous aspects of these fields receive attention in other Chapters. Certain problems of presentation have arisen in one or two recent Volumes of these Reports. We believe that readers will now find no significant problems of this type, although the ideal of arranging for a completely uniform typeface throughout the Volume still eludes us. We turn firstly to some of the more interesting developments in the photophysical field. Overlap and interaction between the organic and physical aspects of photochemistry has unfortunately continued to decline this year. Such overlap as remains is largely restricted to electron transfer processes. Apart from this area, studies of photophysical processes have mostly involved the virtually pure physics of excited states. In short, the physical and organic photochemists have increasingly been going their separate ways. The Reporters regret this development. Szabo has presented a unified and comprehensive analysis of diffusion controlled by molecular fluorescence quenching. Kopelman’s review of fractals includes much of photochemical interest. Attention is drawn to Nickel’s useful work on the theoretical and practical aspects of the measurement of luminescence properties, etc., under various experimental conditions. Scaiano has produced a valuable two-volume handbook covering all aspects of photochemical methodology. Peffluoroalkanes are increasingly proving valuable as solvents because of their negligible interaction with excited states (Ikematu et al.; Maciejewski). The -maximum entropy method is being increasingly used in fluorescence studies, for example by Ware and his group. McGown is among those who have put timeresolved spectroscopic techniques to good use in chemical analysis. Frequency

vi

Iritroductioti arid Review of the Year

domain fluorimetry is finding increased applications, for example, in chromatography (Cobb and McGown) and the study of photon migration in human living tissues (Lakowicz and Berndt). Pouget et al. have suggested an ingenious solution to the problem of correcting for systematic phase errors in frequency domain spectroscopy, and Barbieri et al. have used a differential method to overcome the problem of phase noise. Kelly et al. have developed a videofluorimeter to produce images of tissue metabolism. Two groups have presented pioneering studies of oscillatory reactions, particularly the Belousov-Zhabotinsky reaction. We expect to see much more work in this field in the future. Heldt et al. have demonstrated competitive triple fluorescence of aminosalicylates and benzanilides. There has been markedly increased interest in the photochemistry and photophysics of organosilicon compounds during the past year. Fruitful future extensions to other non-metallic elements can be confidently expected. Ultrafast techniques are being widely used to probe the photophysics of charge transfer processes: (see e.g. Bagchi). Marcus and his collaborators have continued their pioneering theoretical work in this field, although a number of other workers are now making important theoretical contributions. Several groups have added significantly to the knowledge of twisted internal charge transfer states [TICT]: to form a bridging TICT Lounay et al. have used N,N'-bis[4-~yanophenyl]piperazine molecule with properties that might prove useful in optical switching devices. Nag and Bhattacharyya have developed biochemical applications of TICT. Cyclodextrin complexes continue to attract both photophysical and photochemical enthusiasts. Special attention is drawn to Andrews' unified theory of radiative and radiationless molecular energy transfer processes. Hochstrasser and co-workers have achieved the remarkable feat of using fs fluorescence time-dependent anisotropy for the direct measurement of energy transfer between identical chromophores. Strouse et al. have most ingeniously used intervening anthryl groups along a polymer chain as energy relays for a long-distance energy cascade. Zelent et al. have described the properties of photoconducting polymers having carbazolyl and fluorenyl pendant chromophores to constitute a donor-acceptor system. Blok and Dekkers have described a valuable new method for discriminating between 37m* and 3nn* states in organic molecules based on application of the circular polarisation of phosphorescence. Locke and Lin have defended their previously challenged detection of triplet naphthalene excimers. A number of papers have been concerned

Iiirroducrioii arid Review qf the Y2ar

vii

with proton and electron transfer processes involving triplet states. Ford et al. have reported triplet states of alkoxy-substituted phthalocyanines: these might have applications in photodynamic therapy. Attention is drawn to a useful review of the dynamics of flexible biradicals by Doubleday, Turro, and Wang. Very highly excited states can be produced by cavitational collapse caused by ultrasonic irradiation: alkanes under argon produce emission from C2H, and CH, whereas under nitrogen, emission from CN predominates. Creed et al. have demonstrated photochemically induced chromophore aggregation in a liquid crystalline poly(ary1cinnamate). We turn to some of the more strictly photochemical developments. Irradiation of the diketones (1) in the crystal phase leads mainly to the cyclobutanes (2) rather than the expected fission products (Lewis et d.).Yoshioka et al. have described an interesting example of trapping by 0 2 of photochemically generated biradicals. Irradiation of the acetophenones (3) in the crystal phase gives the indanols (4)almost quantitatively: a 1,6-hydrogen shift is involved (Wagner and Zhou). A somewhat related process has been employed as a step in the synthesis of aflatoxins by Krauss et al. Those tempted to believe that all possible photoreactions of acetone have already been described will be interested in the isolation of oxetan ( 5 ) by Kagan et aZ.: evidently addition of the keto to the enol tautomer is involved. Conversion of the cubanecarboxylic acid (6) into the phenyl derivative (7) by irradiation in benzene in the presence of lead tetra-acetate may have applications in less esoteric systems (Moriarty et al.). Hochstrasser and Wirz have made the interesting observation that prolonged irradiation of ketene in a matrix gives the tautomer hydroxyethyne. Although buta- 1,3-dienes normally quench the enone triplet state, Demuth et al. have reported that 2-(trimethylsily1oxy)buta-1,3-diene gives cyclobutane adducts with cycloalkenones in good yields. Ue et al. have made the remarkable observation that ethene undergoes photoaddition to 2-naphthol in the presence of aluminium chloride to give the cyclobutane (8): the keto form (9) of the naphthol has been reasonably proposed as an intermediate. This appears to provide a rare example of catalysis of a photoreaction by a Lewis acid. Photoisomerisation of the enone (10) to the tetracyclic product (1 1) as a single diastereoisomer appears to provide the first example of the use of a furan moiety in an intramolecular cycloaddition process (Crimmins and Thomas). The use of photochemical cyclobutane formation as a step in the synthesis of complex natural products is now fairly commonplace; but a particularly interesting example involving the production of the tetracyclic species (12) has been described this year by Winkler and Hershberger. Photoisomerisation of the enone (13) has provided a route to the previously unknown noradamantanone

...

Ititroductioti arid Review of the Year

Vlll

(3) R1 and R2 = H,CH3, or Ph

(2)

H3C&H3 H3C

OH

(5)

(4)

(6) R=C02H (7) R = Ph

m0+c5H11

& 0 2 H 3 H

0

Introduction and Review of the Year

ix

(14): Sindler-Kulyk et af. Formation of the triplet state of azathymine (15) in acetone leads to trapping of (probably ground state) acetone to give the novel product (16) in good yield: Guo et af. Quinkert et al. have described an elegant synthesis of (+)-aspilkin (17) in which photochemical ring-opening of the cyclohexadienone (18)to a ketene followed by intramolecular lactonization is a key step. Pavlik and Farahat have described a rare example of oxetan formation by addition of keto-compounds to 1-methylpyrrole. However, the oxetans are unstable and undergo photoelimination to give 3-acetyl- 1-methylpyrrole. The orange acenaphthylene (19) gives a colourless dimer on irradiation at 390 nm in the solid phase: the reaction is reversible on irradiation at 300 nm (Rieke et af.). In the field of quinone photochemistry, the photocyclization of compound (20) to the phenol (21)in yields of up to 90% is a notable example (Iwamoto). Kira et al. have described a photochemical 1,3-~igmatropicshift which appears to proceed in an antarafacial manner, contrary to the Woodward-Hoffman postulates. The presence of a silicon atom adjacent to one of the reaction centres may be a significant factor. An increasing number of single electron transfer reactions are now being reported: Arnold and Mines have described a particularly interesting example involving deconjugation of phenylalkenes in the presence of 1,4dicyanobenzene, biphenyl, and 2,4,6-trimethylpyridine. Further interest has been shown in the asymetric induction of the di-z-methane rearrangement but only in the solid phase reaction is any significant control evident (Chen et al.). Armesto, Horspool, et al. have provided full details of their synthesis of dihydroisoquinolinones, e.g. (22) (23). An interesting feature is the use of perchloric acid to inhibit an undesired electron transfer process. Tetrahedranes are less novel than they used to be but the formation of (24)by irradiation of the cyclobutadiene (25)deserves mention (Maier and Born).

-

Kitamura et af. have described a remarkable high-yield photochemical synthesis of isoquinolinones (26)from the vinyl bromides (27) in the presence of water, dichloromethane, potassium isocyanate and a phase transfer agent. Binkley and Hehemann have described a novel high-yield procedure for the removal of protecting benzyl groups in carbohydrate derivatives. Photoaddition reactions of azo compounds and photochemical ~ a r a additions to benzenes are both rare; so formation of the adduct (28) from N-methyltriazolinedione and benzene is an unusual process on both counts (Hamrock and Sheridan). In the metpcycloaddition of alkenes to benzenes, there are both theoretical and experimental grounds for believing that the S1 benzene undergoes

Ititroductiorr orrd Review of the Ymr

X

Ph

n

OH

Introduction and Review of the Year

xi

slight polarisation at an early stage along the reaction coordinate. So it is particularly interesting that Gilbert and Rodwell have now found that the direction of this polarisation is reversed in the corresponding meta-photoaddition of 1,3dienes. Wender et al. have continued their pioneering work on applications of intramolecular photochemical meta cycloadditions to the benzene ring. A new synthesis of retigeranic acid (29) by application of this approach has been described this year. Irradiation of the styrene derivative (30) gives the intramolecular ortho adduct as a single stereoisomer (31): this type of process is less common than the corresponding meta process, possibly because the styrene rather than the benzene moiety is excited, in accordance with orbital symmetry predictions. It is also unusual to have a process of this type in which the reacting units are separated by four atoms (Aoyama et aZ.). Okada et al. have described a new photochemical procedure for decarboxylation of carboxylic acids. Photoadditions to the pyridine ring are much less common than those to benzenes, so it is interesting that the production of (32) from 1,3,5-tricyanobenzene and tetramethylethylene (Ohashi et al.) can be extended to give the corresponding derivative (33) from 2,4-dicyanopyridine (Bernardi et al.). Strong interest continues in the photochemical generation of vinyl carbocations from vinyl halides (Kitamura et al., inter alia). Arene radical cations can be produced by irradiation of slurries of semiconductors in polar solvents containing arenes as a result of electron transfer to photogenerated holes in the semiconductor. Related processes in aqueous systems can produce hydroxyl radicals which then react further. This procedure has attracted interest as a method for the destruction of pollutants such as lignin by-products of paper manufacture (Kobayakawa et al.). The photoisomerisation of the anthracene (34) to (35) provides another rather rare example of m-cycloaddition to the benzene ring (Usui et al.). Benzylic positions and other centres from which carbocations can readily be generated can be fluorinated by irradiation in the presence of titanium dioxide and fluoride (Wang and Mallouk). Nakabayashi et al. have described an interesting retro Paterno-Bbchi photoreaction in the presence of triethylamine as an electron donor. Pac et al. have reported that aromatic ketones can be photoreduced to alcohols or pinacols with cadmium sulphide powders and with triethylamine as sacrificial donor in acetonitrile. Rather surprisingly, alkenes can similarly be reduced. Wu and Morrison report that self-sensitized photoreduction of the 17-keto-group in some steroidal ketones can be achieved by energy transfer from a dimethylphenyl-

xii

Imroductioii aiid Review of the Year

Ph

NC

R

CN

Introduction and Review of the Year

...

Xlll

siloxy group at the 3a-position: this serves as an antenna chromophore. In a further example of the use of Ti02-suspensions, CO has been photoreduced in aqueous solution to HCHO (Getoff et d.). Similarly, using hydrated cuprous oxide, C02 has been photoreduced to CH3OH (Tennakone et d.). Tungstosilicate-loaded Ti02 is active in the photoinduced oxidation of methane (Graetzel et d.). Hill et al. have achieved anaerobic functionalisation of remote unactivated C-Hbonds using tungstates excited by blue light. These workers have also reported selective production of isobutane hydroperoxide under similar conditions. In a reaction that may be relevant to the improved dispersal of oil slicks at sea, the photo-oxidation of butylcyclohexane under the influence of sunlight has been shown to be promoted by traces of anthraquinone (Rontani and Giusti). Nakata and Frei have shown that franpbut-2-ene undergoes stereoselective photo-oxidation to its epoxide in a cryogenic matrix using NO2 and excitation with red light. The low photon energy appears to be important: see also Adam et aZ. Ethylene glycol has been selectively produced by irradiation of nitrogen-saturated methanol containing hydrogen peroxide (Shimizu et d.), The long-sought direct oxidation of benzene to phenol has been achieved photochemically using Ti02 suspensions in water (Kato et d.). Irradiation of 2-phenylbenztriazole in aerated solvents leads remarkably to cleavage of the benzene ring and formation of 2-phenyl-l,2,3-triazole-4,5dicarboxylic acid (Amiet et d.). Ylidions, a new class of reactive intermediates, have been generated by photo-oxidation of phenacylsulphonium ylides using 9,lOdicyanoanthracene as sensitizer ( n a n g and Schuster). The blue isomer obtained by irradiation of the well-known photochromic compound 2-(2,4-dinitrobenzyl)pyridine has been identified as the enamine (36) (More O’Ferrall and Quirke). There has been a resurgence of interest in photochromic fulgides: see e.g. Suzuki et al. Wilcox et al. have de: :ribed a new photolabile protecting group for amino-acids. An interesting example of intramolecular oxetane formation has been provided by the photoisomerization of indole (37)to (38) (Takechi et ul.). The semilocalized biradicals (39) and (40) have been produced by irradiation of the azo-compound (41) and directly identified by e.s.r. spectroscopy (Ham et d). Photoelimination of N2 from l-aryl-l,2,3-triazoles has provided a useful route to indole derivatives (Nagawa et d). Tsuda and Oikawa have proposed a new mechanism for the photo-Wolff rearrangement. Photoelimination of N2 from compound (42) gives the carbapen-2-ems (43): this ring system is of pharmacological interest and has previously only been accessible by

xiv

Itttroductiott uad Review of the Year

AC

(37) II = 2 - 5

& 0

/

N I

(39)

AC

(40)

(38) II = 2 - 5

RFgkN2 0

&y

CH3

C02CH202C(CH3)3

P h - E a Ph

C02CH202C(CH3)3

Iiitroductiori arid Review of the Yeor

XV

multistep syntheses (Rosati et al. ). Irradiation of N-acyloxyphthalimides (44)in the presence of 1,4-diazabicyclo[2.2.2]octane,carbon tetrachloride, and t-butanol gives the corresponding alkyl chloride, carbon dioxide, and phthalimide (Okada et al.). Hinrichs and Margaretha have described the novel conversion of the thiophenone (45) into the thiet (46). Shi et al. have described a most unusual a,a-elimination of two phenyl groups on irradiation of the l,l,l-triphenylalkane (47)to give biphenyl and the carbene (48). We turn now to developments in polymer photochemistry. This Part of the series usually has by far the greatest number of references, and this year’s contribution contains the record number of 639 references. Much, though not all,of the high activity in this area of photochemistry stems from its industrial importance, though there continues to be a substantial amount of basic physical and organic photochemistry having importance and interest beyond the field of polymers. Numerous new photoinitiation systems have been described this year including polymeric types and two- and three-component systems. Charge-transfer initiators are attracting much interest, e.g. aniline-TCNE (Dongyi and Feng). There is growing interest in liquid crystal systems. Excimer and exciplex studies of aromatic groups in polymer systems sometimes provide useful information on conformation. Three-component systems involving N-phenylglycine, dyes, and diphenyliodonium chloride have been found to be highly synergistic for sensitization of polymerization (Kawabata et ul.; cf. Harada et ul.). Novel highly conducting addition polymers have been prepared by irradiation of 2,s-thiophenedithiol with 2,5-diethynylthiophene (Kobayashi et al.). There is evidence that the photopolymerisation of maleimide occurs via a free radical mechanism whereas that of N-ethylmaleimide occurs anionically (Aida et al.). Some confirmation of this unexpected finding would appear to be desirable. Although cyclohexene has long been known to form a cyclobutane photoadduct with maleic anhydride by a chargetransfer mechanism, Li et al. now report that hydrogen atom abstraction and polymerisation also occur. Interest continues to be shown in photopolymerisation within micelles and bilayers. For example, Higashi et al. have produced extremely high molecular weight polymers under mild conditions in a bilayer system by locating the initiator at the most effective site. It used to be thought that photoinitiation of polymerisation involved purely radical species; but photoinduced cationic and anionic polymerisations now attract increasing interest. Likewise, photoinitiation procedures involving metal ion species, e.g. FeC13, are now known to be important. Many examples are described this year. Irradiation of propargyl halides at 77K

xvi

Irirroductiori arid Review of the Year

gives semi-conducting polymers by a radical mechanism (Zakirov et ul.). Jacobine et ul. have prepared various prepolymers by cycloaddition of cyclopentadiene with various acrylic monomers: the photochemical interest in these lies in the fact that they undergo photochemical crosslinking very rapidly to give very tough plastic materials. Photocrosslinking of polyethylene has long been known to result in interesting changes in properties, including thermal stability. A number of interesting developments in this area have appeared this year: see Chen and Ranby, inter alia. Among a considerable amount of work on liquid crystal polymers, that of Ikeda et ul. on photoreversible nematic-isotropic phase changes appears particularly notable. Reversible photoinduced swelling of polymer gels has been observed (Irie et ul.). This phenomenon might lead to useful applications. Although calculations have shown that it is possible to measure the end-to-end distances of polymer chains by tagging with donor-acceptor probes (Liu and Guillet), the introduction of such probes must almost inevitably alter the conformation of a polymer. Allen et al. have confirmed the identity of the luminescent species in nylon 6,6 as a triscycloalkenobenzene moiety. Several areas of conflict and uncertainty remain in the field of polyolefin photo-oxidation. This year has seen a massive increase in studies of the photoablation of polymers for etching purposes. This doubtless reflects the possibilities of industrial applications. Solar energy photochemical research has become something of a poor relation recently, partly owing to the disappointment over efforts to simulate systems that mimic natural photosynthesis, and partly because of the development of increasingly efficient photophysical devices such as amorphous silicon multilayer cells. Nevertheless, Maruthamuthu and Ashokkwnar have reported success this year in the long-sought photosplitting of water into hydrogen and oxygen with high efficiency using visible radiation. Their system employs a suspension of noble metals on WO3 particles in aqueous methylviologen. In view of past disappointments, the results of confirmatory studies will be eagerly awaited. Fox et al. have described a multielectrode photoelectrochemical cell reportedly successful for the unassisted photolysis of water. As a final general comment, we note that the centre of gravity of international photochemistry has now clearly moved to Japan where a great deal of fine work is being done. However, we cannot avoid a sense of regret in witnessing a marked decline in research activity in the U.K. This appears to result partly from policy changes among grant-giving bodies, and partly from the failure to perceive the potential for important practical applications.

Contents

PART I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY Photophysical Processes in Condensed Phases Cundall

3

1

General

3

2

Singlet Processes

By R.B.

2.1

2.2 2.3 2.4 2.5 2.6 3

Electron and Proton Transfer Reactions and Exciplexes Dyes Photoisomerization and Related Processes Electronic Excitation Energy Transfer Polymeric Systems Colloidal and Heterogeneous Systems

Triplet State Processes

4 Other Processes

4.1 4.2 4.3

Chemiluminescence Photochromism Photochemical Reactions

References

15 18 21 23 26 27 30 37 37 38 38 41

ORGANIC ASPECTS OF PHOTOCHEMISTRY

PART I1 Chapter

10

Photolysis of Carbonyl Compounds Horspool

59

1

Norrish Type I Reactions

59

2

Norrish Type I1 Reactions

68

3

Oxetan Formation

73

1

By W.M.

4 Miscellaneous Reactions

References

80

91

xviii Chapter

2

1

96

Cycloaddition Reaction

96

Intermolecular Intramolecular

96 108

Rearrangement Reactions

115

a, p-Unsatumated Systems p, -Unsaturated Systems

115 124

3

Photoreactions of Thymines, etc.

133

4

Photochemistry of Dienones

135

Cross-conjugated Dienones Linearly Conjugated Dienones

135 137

5

1,2-, 1,3-, and 1,4-Diketones

139

6

Quinones

153

References

161

Photochemistry of Alkenes, Alkynes, and Related Compounds By W.M. Horspool

169

Reactions of Alkenes

169

cis-trans Isomerization Hydrogen Abstraction Reactions Addition Reactions Rearrangement Reactions Halogenated Alkenes

169 172 172 174 176

2

Reactions involving Cyclopropane Rings

178

3

Reactions of Dienes, Trienes, and Higher Po1yene s

190

[2+2] Intramolecular Additions

200

Dimerization and Intermolecular Additions

202

Miscellaneous Reactions

204

References

212

2

Chapter

Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By W. M. Horspool

3

1

contem Chapter

Chapter

Chapter

xix Photochemistry of Aromatic Compounds By A. C. Weedon

221

Introduction

221

1

Isomerization Reactions

222

2

Addition Reactions

226

3

Substitution Reactions

239

4

Intramolecular Cyclization Reactions

250

5

Dimerization Reactions

260

6

Lateral Nuclear Rearrangements

266

7

Peripheral Photochemistry

272

References

281

5

Photo-reduction and -oxidation By A. Cox

295

1

Introduction

29 5

2

Reduction of the Carbonyl Groups

295

3

Reduction of Nitrogen-containing Compounds

300

4

Miscellaneous Reductions

302

5

Singlet Oxygen

303

6

Oxidation of Aliphatic Compounds

304

7

Oxidation of Aromatic Compounds

309

8

Oxidation of Nitrogen-containing Compounds

313

9

Miscellaneous Oxidations

315

References

316

Photoreactions of Compounds Containing Heteroatoms Other than Oxygen By S.T. R e i d

329

Nitrogen-containing Compounds

329

Rearrangements Addition Reactions Miscellaneous Reactions

329 347 355

2

Sulphur-containing Compounds

355

3

Compounds Containing Other Heteroatoms

360

References

368

4

6

1

Cot1tents

xx

Chapter

7

Photoelimination By S.T. R e i d

375

1

Elimination of Nitrogen from Azo-compounds

375

2

Elimination of Nitrogen from Diazo-compounds

380

3

Elimination of Nitrogen from Azides

388

4

Photoelimination of Carbon Dioxide

394

5

Fragmentation of Organosulphur Compounds

395

6

Miscellaneous Decomposition and Elimination Reactions

398

References

402

POLYMER PHOTOCHEMISTRY By N.S. A l l e n and M . Edge

4 11

1

Introduction

411

2

Photopolymerization

411

2.1

413 432 433

PART 111

2.2 2.3

Photoinitiated Addition Polymerization Photografting Photocrosslinking

3

Optical and Luminescence Properties of Polymers

443

4

Photodegradation and Photooxidation of Polymers

460

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9

461 462 464 467 468 469 469 472 474

Polyolefins Polyvinylhalides Polystyrenes Polyacrylates Polyesters Polyamides/Polyimides Natural Polymers Polymer Ablation Miscellaneous Polymers

5

Photostabilization of Polymers

47 6

6

Dyes and Pigments

479

References

482

xxi

Contents PART I V

PHOTOCHEMICAL ASPECTS OF SOLAR

ENERGY CONVERSION B y A. COX

505

1

Introduction

505

2

Homogeneous Photosystems

505

3

Heterogeneous Photosystems

507

4

Photoelectrochemical Cells

509

5

Luminescent Solar Concentrators

510

References

5 10

AUTHOR INDEX

514

Part I PHYSICAL ASPECTS OF PHOTOCHEMISTRY By R. B. CUNDALL

Photophysical Processes in Condensed Phases BY R.B. CUNDALL The divergence between the organic and physical aspects of photochemistry has increased during the past decade. Research in photophysics is now largely occupied with the detailed examination of chemical physics of excited states using the techniques of ultrafast spectroscopy rather than elucidation of reaction mechanisms. As a consequence, most papers are concerned with minutae of specific systems and the general principles and theoretical basis of the subject have not changed greatly over the last few years. For this reason the reviewer has chosen, as in past years, to survey the topic generally rather than make arbitrary choice of some subjects for detailed examination. The application of techniques in the subpicosecond region i s probably the most exciting area during the period of review. The practical application of photophysics to electronics on any great scale although it would appear to have considerable promise for microelectronics is still awaited. The use of fluorescence in imaging seems to be one of the most promising photophysical techniques being exploited. A number of references to this procedure are included in the text.

Researches on theoretical topics have not been reported very extensively. A few papers are mentioned here and some others at appropriate points later in the article. Weberl has re-examined the famous Perrin equation for quantifying the rotational depolarization of fluorescence. The arguments presented in the paper are applied to the temperature dependence of the local motions of tyrosine and tryptophan residues observed in proteins. A unified and comprehensive analysis of diffusion influenced bimolecular quenching of fluorescence by Szabo’ i s also noteworthy for its in-depth treatment of this important topic. Monomer-excimer interaction kinetics have been studied further and a theoretical analysis of energy transfer from both species to an acceptor p r e ~ e n t e d . ~Fractals are now very much in fashion in all aspects of science and a review by Kopelman4 includes topics which are

4

Phorochernistty

specifically of photochemical interest. Systems selected for discussion include naphthalene dimerization, excitation migration, and the role of excitons in doped naphthalene crystals. The consequences of lowered dimensionality on migration phenomena needs to be considered by all photochemists. The most significant effect is a very considerable increase in the efficiency of electronic energy transfer. Complex kinetic systems with time-dependent rate coefficients, including fluorescence quenching, have been analysed and show considerable variation with system type. The theory of excited state relaxation still offers considerable challenge. In particular this is exemplified by a paper in which the contribution of vibronic coupling to intramolecular radiative and non-radiative singlet state processes of large conjugated organic molecules has been assessed. Nickel,7 in a very useful paper, has discussed the elimination of polarization bias effects from the measurement of luminescence properties and transient absorption in isotropic solutions. The theoretical treatment is fully developed and recommendations are given f o r making reliable observations under a variety of experimental conditions a.re detailed. Determination of quantitative data from steady state luminescence spectroscopy is by no means as straightforward as many workers assume; this work very convincingly demonstrates otherwise. A number of books and reviews which describe some of the new experimental techniques have appeared. Topics covered include hole burning spectroscopy, various forms of picosecond and femtosecond spectroscopy, femtosecond coherent spectroscopy, ultrafast timeresolved fluorescence spectroscopy,ll ultrafast spectroscopy and its application to analytical chemistry, applications of ultrafast laser spectroscopy for the study of biological systems,1 3 All aspects of photochemical and luminescence in general. methodology are covered comprehensively in the two-volume handbook edited by Scaiano. Observations of evanescent luminescence is increasingly recognized as a useful procedure for the study of interfaces. Reichertl has assessed optical considerations to be taken into account for evanescent detection of absorbance and fluorescence at crystal/solution and polymer/solution interfaces. The technique is becoming referred to as total internal reflection fluorescence spectroscopy (TIRF). A combination of fluorescence detection with two photon excitation gives a very high level of sensitivity.l 7 The technique is especially well suited to

*

*

I: Photophysical hocesses it1 Condensed Phases

5

measurement on small volume samples and provides a very useful probe of microenvironments, surfaces, thin films, micelles, and membranes. A ps absorption-emission spectrometer design which uses both pump-probe and streak camera measurement with a single-mode locked Nd-YAG laser has been described in detail.18 The theory of nonstationary time-dependent emission measurement and its application to ultrafast processes has been exemplified by analysis of data on the fs time-resolved emission from dye molecules in water.19 The power of this experimental technique is exemplified by the determination of the evolution of the vibronic absorption spectrum in molecules following excitation with a 6 fs optical pulse.2o Nile blue A and malachite green are two specific systems which have been followed after a delay of up to 120 fs over 1.5 ps. It has been demonstrated by this type of experiment that conformational changes of excited states can be followed with a time resolution of 10 Solvent cage effects and intra- molecular twisting are ps.” specific processes that can be observed. This is very well illustrated by published measurements of the ultrafast vibrational changes which occur in four co-ordinated nickel (11) porphyrins after excitation using fs time-resolved spectroscopy.22 The steady state condition of the initial excited state is reached in about 20 ps. The authoritative feature article by Bagchi and FlemingZ3 reviews both theoretical and practical aspects of ps and sub-ps laser spectroscopy in considerable detail. The dependence excited state relaxation on viscosity, polarity, temperature, and wavelength are experimental variables examined by these authors. Environmental influences are basic to the whole field of photochemistry and the depth of understanding of these effects is a good measure of psogress in photophysical research. Perfluoroalkanes provide unique solvents of special interest because of their slight influence on excited states. Spectral, photophysical, and photochemical studies on these interesting systems have been very comprehensively reviewed by Macie jewski 24 A general relationship between the time dependence of the energy gap between electronic states and fluorescence Stokes shift has been interpreted using models of solvation dynamics by Chandra and Bagchi.25 An improved method of measuring the magnitude of dipole moments of molecules in excited states and also their direction from solvatochromic shifts is likely to be a very useful extension of presently available methods. 26 Other related papers published during the year have dealt with influence of electric fields on the fluorescence decay

.

6

Photoclwinistn,

of polar molecules27 and solvent effects on the magnetic field perturbation of photogenerated radical ion pairs.2 8 Photoacoustic calorimetry has been used as method for the measurement of reaction volumes of excited states.2 9 Partial volumes of excited states obtained thereby can be used for estimating structural changes which occur on excitation. A dual laser beam flash photolysis technique in which fluorescence signals from both the sample and reference, separated in time, can be recorded has been employed in magnetic field effect studies.30 Time-resolved luminescence measurement techniques for the measurement of fluorescence lifetimes continue to be re-appraised and improved. For example, measurement of submicrosecond time scales using a high-speed photomultiplier gating circuit has been described,31 the turn-off and turn-on times are 60 and 4 0 ns. Optimization of signal to noise ratios in time-filtered fluorescence detection can allow satisfactory subnanosecond lifetime measurement to be achieved.32 Data treatment of luminescence decay using pulse excitation is still undergoing active development. Probably the most significant papers consider the maximum entropy method, a technique similar to that used in resolving of astronomical images, which has been fully described and tested by Ware and his group.33 The procedure is compared with results achieved with the well established exponential series method f o r recovering a distribution of lifetimes from fluorescence lifetime data. It is clearly demonstrated that the maximum entropy method is well adapted to dealing with situations in which a distribution of decay times is expected and must be determined. Libertini and Small34 have applied the older method of moments to determine the decay lifetime distribution. A related paper by B e e ~ h e mdescribes ~~ a second generation global analysis program for the recovery of complex inhomogeneous fluorescence decay kinetics, A contribution from the Belgian group also describes a systematic study of global analyses of multiexponential fluorescence decay surfaces using reference convolution.36 A comparatively simple deconvolution procedure has been designed for application to the analysis of stretched exponential decay functions.37 It is pointed out that single photon counting lifetime measurements on long-lived samples (hundreds of ns up to microseconds) are complicated by the long tails in the emissions found with both A2 and D2 lamps.38 On the practical side, equipment for measuring longer luminescence lifetimes from 100 ps

I: Photophysical fiocrsses in Cotidetised Phases

7

up to 1s using low-cost analog input interfaces has been described which can be employed as an accessory for use with standard spectrofluorimeters.39 Powerful data handling capacity is involved in a general global analysis program which allows simultaneous analysis of single photon timing data for complete determination of activation energies, frequency factors, and quenching rate constants.40 This has been applied to analysis of data from tryptophan photophysics as a foundation for the general treatment of protein luminescence. The "long time" approximation for the intensity decay equation in diffusion limited fluorescence quenching has been critically reexamined and the importance of the usually neglected terms in the theory applied to the analysis has been indicated.41 Wang and Harris42 describe a method for the quantitative estimation of component amplitudes in multiexponential data obtained from timeresolved fluorescence spectroscopy. A design of apparatus which uses time correlated and single photon counting with alternate recording of excitation and emission minimises troublesome lamp instability effects and consequently simplifies and improves the analysis of data.43 Time-resolved spectroscopy is finding increasing use in chemical analysis to judge from some papers in analyticallyoriented journals. For example, M ~ G o w ndescribes ~~ how fluorescent components can be separated by differences in lifetimes, a technique which allows the elimination of interfering signals from unwanted luminescence signals. Multicomponent fluorescence analyses can be carried out by use of a fibre optic probe.45 Processing of data is aided by a canonical correlation technique for rank estimation of excitation-emission matrices which allows determination of the number of components.46 Phase shift fluorimetry, the other important method for measuring fluorescent lifetimes, also continues to be developed and improved. The effects of inaccurate reference lifetimes on the interpretation of frequency domain fluorescence data can be removed or minimized by a least squares analysis method.47 The direct collection of multi-frequency data for obtaining fluorescence lifetimes can be achieved by the use of digital parallel acquisition in frequency domain fluorimetry.48 Frequency domain lifetime measurement has been used for on-line fluorescence lifetime detection of eluents in chromatography.49 An unusual use of frequency domain measurement which has been reported is for the examination of photon migration in living tissue.50 Photons in the

8

Phoroclwmistty

region between 620 and 7 4 0 nm were detected in both the human arm and finger. The correction of systematic phase errors in frequency domain spectroscopy can be achieved by use of a fluorophore of known lifetime as standard. It has been pointed out that a simple scattering solution can be used as standard. This ingenious suggestion very conveniently dispenses with the need for a fluorescent standard with a previously-determined lifetime value. 5 1 Phase noise, another troublesome factor encountered in frequency domain fluorimetry, can be eliminated by use of a differential method. 52 Diode lasers are Finding use as light sources for the generation of molecular fluorescence and subsequent time a n a l y ~ i s . ’ ~The high sensitivity obtainable by use of such sources is exemplified by detection of concentrations as low as 5 x M dye. A visible semiconductor laser fluorimeter system has reportedly been used for the detection of as low as 4 x M rhodamine 800.s4 This is a very useful development for the measurement on dyes which emit in the deep red and require excitation at comparatively long wavelengths. Modern imaging techniques are beginning to be increasingly exploited in applications of photophysics. For example, a videofluorimeter has been used to image tissue metabolism.55 Reduced nicotinamide adenine dinucleotide (NADH) fluorescence was used to acquire digital metabolic images using emission collected during mitochondria1 oxidative metabolism by this non-invasive technique. An image analyzer has similarly been used for data acquisition from biological systems in studies of phototropism.56 Optical fibres are useful accessories in a variety of photophysical measurements. The current status and prospects for their further use in chemical analysis are assessed in an article by Norris.57 Luminescence spectroscopy and evanescent wave devices are topics highlighted in this review. By way of example of this developing technology a device described for luminescence examination of inorganic semiconductors may be cited.5 8 Spectroscopic techniques based on the optical microscope are being used with increasing success in photophysics. Microscopic fluorescence decay measurements have been made on both thin liquid films and droplets of concentrated dye solutions.59 Illustrative data are given for rhodamine B in 2 0 pm films. A luminescence lifetime microscope spectrometer based on time-correlated single photon counting with an avalanche diode detector has measured

9 luminescence decay times of 10 f 2 ps upwards on GaAs surfaces.60 A versatile set up for observing the effect of pressure and temperature (up to 1 1 GPa and 77-400 K) using a fluorescence microscope has also been described.6 1 A new cell design and improved analysis algorithm for photoacoustic calorimetry has improved the sensitivity which can be achieved by this method by a factor of about 10; also the use of thicker samples allows use of bigger exciting laser pulses with an increase in detectable signal.6 2 A time-resolved photoacoustic method described by Terazima and A ~ u m i can ~ ~be used for direct measurement of inter-system crossing yields. A combination of laser flash and optoacoustic calorimeter has been used to study relaxation in two laser and two photon excited species.64 Diphenylmethyl and l-naphthylmethyl radicals are two systems which have been studied by this interesting technique for study of reactive intermediates. More esoteric experimental procedures which can be applied to complex molecules include, for example, picosecond time-resolved circular dichroism. A spectrometer for this purpose has been described and used to observe carbon monoxide elimination from excited carbon monoxymyoglobulin,65 Fluorescence detected CD has also been used to observe riboflavin in capillary electrophoresis.66 Liquid rare gases have been used as unfamiliar solvents which are well suited for the application of time-resolved infra-red spectroscopy to the direct measurement of an intermolecular C-H oxidative additive reaction of alkenes . 67 The methylene-blue sensitized photo-oxidation of mesodiphenylhelianthrene has been proposed as a convenient new chemical actinometer for the extended wavelength range 610-670 nm.68 Chelation enhanced fluorescence can be used to detect non-metal ions such as carboxylate, sulphate, and phosphate groups .69 Anthrylpolyarnines can be employed as effective probes for this purpose. Oscillatory reactions provide one of the most active areas of research in contemporary chemical kinetics and two published studies on the photochemistry of Belousov-Zhabotinsky reaction are very significant in this respect. One deals with Ru(bpy)j3+ photocatalysed formation of spatial patterns70 and the other is an analysis of a modified complete Oregonator (model scheme) system which accounts for the 0 2 sensitivity and photosensitivity These papers probably represent significant pioneering studies of

.

10

Photochemistry

nonlinear effects, arising from feedback reactions, in photochemistry. This seems likely to be a significant area for both intellectual and practical development. In spite of the use of oxygen as a fluorescence quencher for many decades, the fact that many hours may be required for equilibration, especially in biological systems, has recently been pointed out and effectively demonstrated by careful experimentation.72 Andre and his co-workers continue to bring out their unique sequence of papers on the technical aspects of industrial photochemistry. In the past year the series has involved detailed discussions on the optimization of conditions,73 light distribution around an illuminated e1nitter7~and the design of industrial photoreactors.75 Photochemistry induced by circularly polarized light has been shown to make possible both partial chiral resolution and racemization of rhodamine 69.76 It has been found that the rate of racemization can be measured for rapidly inverting molecules of these types in achiral solvents. The availability of software for the on-line analysis and documentation of data for transients from kinetic flash photolysis will be useful for examining the now very extensive compilation of data.77 A series of vignettes of new developments in the areas of photophysics by Phillips,78 chemiluminescence and bioluminescence by McCapra,7g and physical organic chemistry by TurroBO give interesting, if personal, overviews of the named topics. The proceedings of a conference on dynamical processes of excited states in solids is largely concerned with inorganic solids but some papers of organic interest are included.8’ Since a fluorescence based method is now used in automated DNA sequencing procedure photophysics is set to play a significant part in the highly publicized human genome project. This technique is fully described in the paper by Trainor.82

The temperature dependence of the fluorescence yields and lifetimes of excited produced states in liquid butane, pentane and hexane have been examined.83 It is concluded that decomposition of initially formed upper excited states into free radicals competes

I: Phorophysical Pt.ocessrs in Cotidc.ri.scw' Phases

11

with internal conversion to the fluorescing state. Absorption and emission spectra of benzene, naphthalene, anthracene, and tetracene in a variety of n-perfluoroalkenes have been interpreted and the extent of solvent-solute interaction in these four systems assessed .B4 Solvatochromic shifts in trifluoromethyl derivatives of benzene and naphthalene show that the change in polarizability of the solute consequent upon elektronic excitation is surprisingly sma11.85 It is found to be only 10% greater than for the ground state and the induced dipole of the excited state is close to the ground state value. A theoretical paper considers both the radiative and nonradiative singlet state deactivation processes of biphenylene. 86 A ps time resolved absorption measurement on the same molecule shows that configurational change after electronic excitation occurs within 15-30 ps and the lifetime of emission for the relaxed state ~ photophysical properties of 1,3,5-tri-(pis 245 k 15 ~ s . 8 The oligophenylene)-benzenes have been reported88 while the much investigated properties of singlet states of 2,lO-diphenylpolyenes have been reviewed by Allen and Whitten.89 The temperature dependence and decay kinetics of the high energy band in DPH fluorescence in a nonpolar solvent show two exponential lifetimes of 62 ps and 13.4 ns at 380 nm, consistent with the previously proposed establishment of equilibrium between the IBu* and 'Ag* excited state .go Itoh9l has deduced the coupling constants between the lAg*(S1) and lBu*(Sz) states for DPH and DPO to be 140 and 90 cm-I respectively. A fluorescence depolarization study of the dynamics of lI8-diphenyloctatetrene in a nematic liquid crystal is related studyg2 on this type of molecule. The co-existence of two stereoisomers of tetrabenzonaphthalene has been shown by spectroscopy in Shpol'ski matrices at 4 . 2 K.93 A comparison of iodide quenching of naphthalene and pyrene in cyclodextrin complexes in the presence of alcohols shows quenching only with pyrene.g4 Theoretical studies on the rotamerism of the S1 to s6 excited states of 1,2'-binaphthyl define the various possible rotamer conformations.95 Fluorescence studies have been made on conformers of trans-2-naphthylethylene and its azo analogues at 4 Kg6 and on the 2'-naphthylbutadiene chromophore of a fluorescent cholesterol analogue probe.97 Hydrophobic effects influence the photophysical properties of molecular aggregates of Il4-di(2-naphthyl) cyclohexane and enhance the formation of fluorescent dimers within the aggregate.98 The transient spectral hole burning technique has been used to

12

Pliotoch umistn

-

study details of the structure of the S o S1 absorption band for anthracene in PMMA.99 Two stereoisomers of 9,lO-dimethoxyanthracenes are identified by observation of two separate fluorescence decay rates in rare gas complexes O0 and internal heavy atom effects on the photophysics and photo-chemistry of 2 styrylanthracene derivatives has also been examined. O 1 Fluorescence studies of interest have been reported on a range of anthryl compounds by Singh and Roy102 and also on the reversible photochemistry of 9-anthrylazomethines and related compounds. O3 Pyrene photophysics has produced the usual, and now to be expected, crop of papers on diverse topics. Aggregation in concentrated solutions has been evidenced by the Shpol'ski effectlo4 and two photon excitation spectra have yielded new electronic state assignments.l o 5 The maximum entropy method, mentioned earlier, for the determination of fluorescence lifetimes shows that in dipyrenylpropane the data for luminescence decay do not fit a simple 3-state model.lo6 This study adds more information upon a system hitherto open to much dispute. It is unlikely that the problem will be considered as solved. The presence of a ground state dimer of pyrene moieties has been shown by NMR in the bichromophoric molecules of (pyrenylcarboxyl) alkaneslo7 and also with racemic and meso dipyrenyl alkanes.l08 Strong circular polarization in excimer emission has been detected from pairs of pyrene groups linked to a polypeptide chain.1°9 Other studies on pyrene involve heavy atom quenching, association in y-cyclodextrins'l 1 l 2 and the use of the excimer formation from pyrene as a probe to investigate recombination of germinate ion pairs in squalane. The recovery time following upon bleaching induced by S1-Sn absorption of p-terphenyl in cyclohexane at room temperature is The temperature dependence of found to be about 380 ps. photophysical processes in perylene, tetracene, and some derivatives is shown to arise from thermal activation of the S1-T2 intersystem crossing process and also for the S1-So internal conversion, which is particularly important in rubrene. 1 A strong Tin* fluorescence from bi~-4,4'-diethylaminobenzene has been characterized in 2 - m e t h y l . t e t r a h y d r o f u r a n at low temperature' and transition moment directions have been determined in some Qcyano-N,N- dimethylaniline derivatives. A ps spectroscopic study shows the existence of competitive triDle fluorescence for aminosalicylates and benzanilides.1 18 Resonance Raman studies have been reported on the S1 state of

'

I: Photophysical

it] Cotidcltiscld Phases

13

~QWSSC~S

’

P-carotene’’” 2 o and S1 and Ti states of all-trans-P-apo-8carotenal.1 2 ’ Detailed excited state studies have also been completed with 9-phenylacridine, 2 2 pyrazinyl Schiff base derivatives, 23 and acetylnaphthalenes.1 2 4 Resolution of La and Lb transitions of indole derivatives which occur in propylene glass at low temperatures are compared with theory12’ and steady state and time-resolved fluorescence of tyrosine and related compounds’2 6 have some relevance to biophysical applications of fluorescence. A g ~ n a n ’has ~ ~ examined the dynamic Stokes shift seen in coumarin fluorescence in considerable detail and resolution of the rotational effects of methyl group substituents as well as the anisotropic rotation of flavin has been revealed by ps resolved fluorescence depolarization.128 Among detailed photophysical investigations reported are studies on the tautomer of 3-hydroxyflavone,12’ s-indacene, an example of an anti-aromatic molecule,I3O and three isomeric 2 An interesting report on solvent (aminophenyl) benzoxazoles. 1 3 ’ effects gives results of the high resolution spectroscopy of some substituted N-phenylpyrrolidines in a supersonic jet.1 3 2 Even for nonpolar solvents, where the intermolecular forces are weak, local site selectivity for solvent interaction appears to be the rule. An interesting didactic article by Henderson’3 3 reanalyses the conventional assignment electronic transitions in carbonyl compounds. A study of the molecular dynamics of excited formaldehyde molecules dissolved in water by time-resolved fluorescence shows that the major proportion of the vibrational and environmental relaxation occurs within 100 fs. 34 Electronic spectra of benzil in solution and entrapped in thin films show evidence for the formation of complexes with oxygen and also have been used to determine the energetics of conformers.1 3 5 Anthrene singlet states in solution are found to undergo reactive selfquenching. 36 The photophysics of the doubly charged form of quinine’37 and some phenoanthrenic alkaloids’38 are amongst studies published on natural products. The anomalous photophysical properties of sulphur compounds continue to attract interest. Quenching studies of the luminescent S 2 state of a number of aromatic thioketones show a range of lifetimes between 1 0-9 and 10-1 s . 39 A variety of solvents shows the role of transient interactive effects and environmental viscosity. In the case of the S 2 state of thiophosgene in perfluoro-solvents, molecular dissociation is found to be the most

’

’

’

’

’

important deactivation process. 40 Excitation of adamantanethione, thiocamphor and thiofenchone to S 2 states shows the importance of subpicosecond processes for all these rnolecules.l4l Fluorescence is weak and significant cross-over to the T2 state seems involved in all cases. Silicon provides an example of another element that produces molecules with interesting photophysics. The fluorescence of cyclotetrasilanes in rigid glass at 7 7 K shows a remarkably large Stokes shift.142 This effect seems to arise from the considerable variation of the silicon-silicon bond lengths which occurs between the S1 and S2 states. The dual fluorescence of aryldisilane and related compounds is taken as evidence for formation of (an*) orthogonal intramolecular CT states. 4 3 It would appear that whole areas of photophysics await investigation amongst the non-metallic elements. Radicals and radical ions provide fruitful subjects of research. Room temperature fluorescence from the arylmethyl radicals Ph3C., Ph2CH. and PhCH2- 44 and theoretical studies of rotational barriers in the benzyl cation, radical and anion as well as the singlet and triplet states of d i p h e n y l ~ a r b e n e lare ~ ~ typical examples of such contemporary studies. A very detailed paper considers the problems of the state assignment and reactivity of excited states of p-substituted benzyl radicals. 46 Ketyl radicals containing the anthrone and the 4-(methyl sulphonate) benzophenone ketyl radical anion148 are related studies in this field. Radical ions which have been studied include the phenylalkene cation in perfluorocarbon polymer films at 7 7 KI49 and both the 9fluorenol radical cation and 9-fluorenyl cation. Carbocations are conveniently generated for such investigations from photolysis of phosphonium chlorides?52 Fluorescence of the phenyldimethylsilyl radical in 3-methylpentane at 7 7 KIs3 and the solvation of photolytically generated p-aminophenylthiyl radical by sub-ps transient absorption1s4 are of particular interest in view of the unusual behaviour of silicon and sulphur in other non-radical systems. A remarkably detailed study in which four pathways for excited radical decay are identified involves diphenylketyl radicals in acetonitrile.1 5 5 Electron ejection and dissociative OH cleavage reactions are amongst the alternative primary processes involved.

15

1: Photophysical Procrss~.~. it1 Cotidrtisrd Phasrs

The elucidation of the details of charge transfer processes appears to be the most active area of photophysics at the present time and the subject, due largely to the application of ultrafast techniques, is being studied in considerable depth and detail. Bagchil S6 has reviewed in admirable fashion the detailed dynamics of solvation and charge transfer reactions in dipolar liquids. New aspects of the theory of the dynamics of electron transfer and the theory of charge transfer (CT) spectra have been extensively developed during the year by Marcus, who so largely pioneered the field, and his collaborators. 57-159 Other theoretical papers of note deal with solvent polarization in electron transfer,160 the role of translational modes in electron transfer,161 the effect of dielectric saturation, 62 the importance of fast solvent relaxation,1 6 3 the influence of t1.e enexgy balance on the photoinduced electron transfer,1 6 * and the form of the relationship between the electron transfer rate ys energy gap relationships in polar solvents. 65 An interesting correlation which has been established is one between rates of photoinduced long-range intramolecular transfer in rigidly linked donor-acceptor systems and the splitting of nn* energies in structurally related dienes.166 The same group have also examined through bond modulation of intramolecular electron transfer in rigidly linked donor-acceptor systems.167 Another very interesting model study relates to the influence of the parameters of transfer distance, ionic strength and temperature on electron transfer in acetonitrile-water mixtures.168 The time-dependent absorption dynamics of photogenerated electrons in water is a basic study for the understanding of the whole range of charge transfer effects. Missmer and Simon169 have compared various models for this system using published data for this fs time range process. CT complex formation from excited states is a widespread phenomenon and consequently has been very extensively studied. 70 A structure-energy correlation of intramolecular CT state formation in a series of (N,N-diethylanilino) amines indicates that high polar excited states are involved (pe values range between 1 3 . 4 20.9 D) Fs-ps laser studies have provided much detailed information on the nature of the relaxation of CT states.1 7 2 The reverse charge recombination processes have also been examined in comparable detail. In the case of CT complexes, the processes involved in recombination are usually quite different from those

occurring in germinate ion pairs. 173 Direct observation of reactions in electron-cation germinate pairs has been achieved in several non-polar solvents.174 The thermalization length for an electron produced by photoionization in one system at 295 nm is about 6 nm; a value which changes with excitation energy. An interesting example is the measurement of the (H3O+ + e) process in 11 M ~~1.175 The rate constant measured is 1.17 x 10l2 s-l. There have been many experimental studies on charge transfer in specific systems and only a selection can be cited here. 185 nm photo-ionization of 1,4-dioxan may either occur directly or indirectly through second photon absorption by the T1 state.176 Synchrotron radiation of wavelength which is continuously variable in the range 10-40 eV has been used to examine details of photoionization by means of quenchers in solutions in the alkane squalane.177 Ion pair formation competes with internal conversion from the S2 state in the chloranil-diphenylamine complex178 and photoionization of 2,2’-bipyridine in aqueous solution involves multiphoton excitation of T1 and S1 states. 79 Monophotonic ionization from the vibrationally unrelaxed excited singlet state of N,N,N’,N’-tetramethyl-p-phenylenylenediamine in acetonitrile shows ionization increases with energy in the S1 state and slightly decreases for excitation around the S2 excitation energy level.180 A method for determining quantum yields of photoionization in solution using conductivity published during the year extends the range of presently available techniques. 8 1 Photoionization of anion radicals, studied by pulse radiolysis and flash photolysis apparently shows promise for research with materials that can be used in processes which can be deployed solar energy conversion.182 Other studies involve p-cyano-p‘-methyl-thiodiphenylacetylene, 83 rhenium complexes, 84 and ethyl-4 ( N , N dimethylamino)- and 4 (N,N diethylamino)benzoates.185 Electron transfer studies have also been made on alkane nitrile solutions186 and also on wavelength dependent effects on behaviour with dual-function fluorescein-anthracene methyl ester.187 The polarity of the microenvironment is found to play a dominant role in solvent affected relaxation of 2-methoxy-6-chloro9-aminoacrldine.18U Other excited CT complexes reported upon include anthracene-tetracyanoethylene, 89 some polyenes and carotenoids, viologen-quinone linked molecules,l g l phenoltriethylamine, 9 2 naphthol-triethylamine, 93 viologen-linked porphyrins, 94 toluquinone, 95 linked porphyrin-phenolphthalein

’

’

I: Photophysical

~oct.sst.sit1 Cotidetised

P/ia.se.s

17

molecules, 96 peptide chains, 97 chloranil sensitization, 98 and ionization of benzidine derivatives.lg9 CT physical quenching is also involved in the interaction of 02 with various hydrazines.200 The twisted internal charge transfer (TICT) state also provides a topic of special fascination for photochemists. Zimmt201 has studied the twisted state of tetraphenylethylene by a picosecond light pulse technique using an optically detected calorimeter. The method provides an interesting new experimental approach to this subject. Other systems examined by conventional techniques include p-cyano-N,N-dialkylaniline,202 dual emission from 7-amino-4-methyl coumarin,203 bis (Q-N,N-dimethylamino) benzylidene acetone,204 dimethylaminobenzonitrile,2 0 5 ‘ 206 p-nitroN,N-dimethylaniline (which was found not to give a TICT state),207 9-morpholino-acridine,208 ethyl 4- (N,N-dimethylamino)benzoate,209 aminosalicylates,2 1 and N,N‘-bis (4-cyanophenyl)piperazine.2” The latter forms a bridging TICT molecule with properties that show promise for the exploitation of this compound in optical switching. Solvatochromic effects relate closely in their nature to those found in photo-induced charge transfer. The influence of solvent medium on the energy of electronic states in general has been reviewed by Suppan.212 Experimental studies in this area are exemplified by studies of dipole moment changes on excitation which occur in 7-hydroxyflavone.213 Exciplex systems on which work has been reported during the period of review include picosecond studies of benzene interaction with indole,2 1 styrene derivatives of 5,6-benz-2,2‘-diquinoyl and the role of exciplexes in photoisomerization,2 1 aminophthalimides and aliphatic halocompounds,2 1 6 1 -naphthyl derivatives, triethylamine systems,2 1 7 magnetic field effects on luminescence from pyrene-dimethylaniline complexes,2 1 tryptophan-N,Ndimethylformamide,2 1 dicyano-anthracene-stilbene, 220 and trans-9styrylphenanthrene and diethylamine.22 1 Exciplex emission observed from folded and extended donor and acceptor molecules with limited flexibility has been examined in 222 detail by use of time-resolved microwave conductivity. Fluorescence quenching of mesosubstituted anthracenes is brought about by electron acceptors223 and in particular by nitroxyl radicals which also induce intersystem crossing.224 The solvation dynamics of N-methylamide, the fluorescent polarity probe, PRODAN, and coumarin 1 0 2 are other systems examined in detail. 225 A complete issue of the Journal& Chemical Phvsics226 deals

18

Ph otock em istry

exclusively with the spectroscopy and dynamics of proton transfer in polyatomic systems. The papers published here are not reviewed. Solvent control of proton transfer occurs with l-naphthol.227 In the case of water, the transfer has been shown to take place initially from small clusters of water and eventually from bulk solvent. Intramolecular proton transfer takes place from a variety of electronic states in q-benzaldehyde.228 Excited state proton transfer which occurs in 3-hydroxy-picolinamide results in a negligible energy barrier for tautomerization in this molecule.229 Solvent and acidity effects on excited states have been investigated in other systems.230’ 2 3 1 Femtosecond spectroscopy has been used for direct observation of excited state proton transfer in benzotriazole p h o t o ~ t a b i l i s e r sand ~ ~ ~an estimation has been made of the intrinsic energy barrier in the same class of molecule.233 A time-resolved study of 2-aminopyridine in a 1 : 1 complex with carboxylic acid shows that the double proton transfer process involved is very fast.234 Other detailed studies have included the deprotonation of aromatic amines , 235 acid-base equilibrium in the flash photolysis of mesophenyl-3,4,5,3’,4’,5’hexamethyl dipyrinyl methane236 and reverse proton transfer in 2-(2-hydroxyphenyl) benzothiazole which has several different phototautomerically active states.2 3 7 Cyclodextrin complexes provide fruitful media for photophysical studies. Multifrequency phase and modulation fluorescence spectroscopy have shown that there are distributions of environments in 1 : 1 P-cyclodextrin-ANS probes.238 Other investigated systems include hydroxyphenylazo-derivatives of sulphanilic acid with a- and P-cyclodextrin,239‘ 240 dual luminescence of dimethylaminobenzonitrile in y - c y ~ l o d e x t r i n ~ ~ ~ where enhancement of nonpolar emission occurs, proton transfer in carbazole-y-cyclodextrin,242 and the effect of y-cyclodextrin on intramolecular CT processes in aminocoumarin laser dyes. 243 DveS - Dyes in general have convenient properties for the investigation of primary processes by the whole range of fast reaction techniques. An example of a very much state of the art investigation is the quantum mechanical analysis made of the observed 6 fs dynamic absorption spectrum of Nile blue.244 Another is the ingenious study of the spontaneous emission and dephasing in front of a mirror measured by the ultrafast dynamics of the same dye in a polymeric matrix.245 Applications of dyes in optical devices are considered in a

2.2

I: Photophysical Processes itr Cotidetised Phnses

19

number of reports. This is exemplified by the photophysical and structural study made on dye molecules with potentially useful nonlinear optical properties.246 The effect of donor/acceptor groups and heteroatoms on the 2nd order hyperpolarizability coefficient, which controls the manifestation of optical nonlinearity, is determined by observation of solvatochromism. The addition of detergents influences the luminescence polarization of dyes247 and barriers to rotation have been assessed in a series of fluorescent perylene dyes.248 Bagchi and his collaborators249 have analysed experimental results for barrierless processes of crystal violet in n-alcohols. The dynamics of such processes give rise to nonexponential fluorescence decay and a fractional dependence of the rate of nonradiative relaxation on viscosity.250 The photophysics of surface active styrylcyanine dyes shows that the presence of the hydrocarbon group significantly influences both the molecular organization and solvent interaction.2 5 1 Optical spectra and excited state dynamics of cis-thioindigo show that the first excited nn* singlet state decays rapidly to be the 3nn* state, which is close in energy, effectively ruling out a singlet mechanism for the cis-trang-isomerization. 252 Fluorescence and photoisomerization studies of amphiphilic aminostilbazolium dyes show that environmental effects compete with decay and isomerization through their influence on the radiationless So deactivation process.253 The effect of solvent on the S1 process, which also influences CT and cA-trans isomerization of DCM styrene dye, has been studied by a combined application of both ps and ns spectroscopy, as well as HPLC and NMR. 254 Photon echo and optical hole burning experiments on ionic dyes in ethanol glass show a solvation shell effect and spectral diffusion.255 Excited state relaxation times of cationic-anionic polymethine dyes have been examined by using IR absorption for the cationic moiety and the visible region for the anionic unit. 256 Not surprisingly interactions between these two entities have been detected . A test of different theories of dielectric effects has been made by measuring the spectroscopy and rotational dynamics of oxazine 725 in alcohol.257 Time-resolved pump probe spectroscopy has detected methanol-dye complexes with oxazine 4 in MeOH/CH3CN solvent mixtures.258 The UV and visible CT absorption and emission spectra of some polar dyes and 4-nitroaniline have been reported as a function of pressure up 6 kbar.259 Other studies of considerable

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20

Photochtvnismy

photophysical interest deal with the viscosity dependent isomerization and fluorescence yields of merocyanine 540,260 the photophysics and photochemistry of phenosafranin in aqueous and acetonitrile solutions,2 6 1 an analysis of the fluorescence spectra of N and P isomers of DODCI,262 and the effect of viscosity on the photo-ionization of Victoria blue B leucocyanide which involves intra-molecular electron transfer.263 Intramolecular fluorescence self quenching of fluoresceinamine (fluorescein with an NH2 substituent) involves electron transfer through covalent or electrostatic binding.264 The fluorescence efficiency is increased considerably when the lone pair of electrons on the nitrogen atom is made unavailable. The photophysics and dynamics of rigidized coumarin laser dyes,265 static and dynamic electrolyte effects on the excited state behaviour of coumarin 153 in ethyl acetate,266 and the photoquenching of rhodamine B and coumarin 495 laser grade dyes, which show a variety of effects267 have also been reported. Solvent relaxation effects on the transient hole burning spectra of rhodamine 6G, rhodamine 575, and erythrosin illustrate the power of this technique in studying such processes.268 Fluorescence self quenching in rhodamine B solutions occurs through aggregates which are more significant in ethanol solutions than in water. 269 The role of TICT in diethylaminocoumarin laser dyes, which are useful biological probes, is demonstrated by the large fluorescence enhancement found on binding to bovine serum albumin.270 The effect is due to reduced TICT formation in the hydrophobic interior of the protein. A photophysical study, including use of opto-acoustic techniques, on 3,3'-diethylthiacarbo-~yanine iodide (DTDCI) shows that isomerization occurs in the S1 state.271 The photochromic dye merocyanine 540 is another dye system for which many details of the photophysics have been worked out.272 Laser induced anharmonic thermal grating effects have also been used in studies of excited state absorption and photoisomerization of cyanine dyes. 273 Spectroscopic and electrochemical techniques have both been employed to study the oxidised and reduced forms of bis(2,5-ditert-butylphenyl)imide dimers . 274 A comprehensive investigation of the photochemical properties of the benzophenoxazine and thiazine derivatives is of considerable interest in view of the medical applications of these compounds.275 The search for luminescent dyes suitable for use as solar energy collectors has lead to a detailed examination of the effects of solvent and pH on the spectral characteristics of Schiff's base

I: Photophysical Processes it1 Condwsc.d Phows

21

derivatives derived from Q-substituted benzaldehyde and 2aminopyridine.276 Large Stokes shifts due to proton shifts occur and pKa* values for the excited states involved have been estimated by Forster's method. Of particularly strong physical interest is a study which deploys the dephasing induced damping of super-radiant emission arising from superexciton involvement in pseudo-cyanine bromide.277 Fluorescence quenching has been used to study the nature of the precursor of the charge carrier in the organic photoconductor rhodamine GG-2,5-bis-(p-diethyl-aminopheny1)-1,3,4oxadiazole.278 Spectral differences caused by infra-red irradiation in the hole burning spectra have been observed for metal free phthalocyanine in PMMA. 279 The use of cyclodextrins as either an inducer or quencher of fluorescence of dyes has been studied with rhodamine 6G, rhodamine B and fluorescein.2 8 0 Other striking effects reported in these media include the fluorescent enhancing effect of small amounts of organic solvents in cyclodextrins with coumarin laser dyes.281 ycyclodextrins also have been shown to affect the intramolecular CT process in amino-coumarin dyes.282 Processes - The photoisomerization of stilbene and the related photophysics of this molecule must surely be the most thoroughly examined of all single photochemical systems. Nevertheless further investigations continue to reveal new aspects which require investigation. Saltiel & d.283 have observed and measured the elusive cis-stilbene fluorescence in solution at 3OoC and were able to make estimations on the extent of the alternative fates of excited S1 state of the is exceedcis-isomer. The emission observed from the --isomer ingly weak and this indicates that isomerization occurs by movement over a single potential energy surface. The determination of zero point vibration levels of trans-stilbene collision in So and S1 electronic states in a collision free environment are useful data for the eventual ( ? ) completion of an energy base for the stilbene isomerization mechanism.284 The potential energy barrier for twisting the trans-stilbene S1 state in hydrocarbon solvents has been re-examined by Saltiel and Sun.28s The observed rate constants for It* lp* twisting gives measured AH values in disagreement with earlier work. Another analysis of the data in alcohols shows that isomerization in homologous alcohols above ethanol are consistent with a rearrangement of a state which has incomplete solvation.286 The stabilization of the polar transition

2.3 Photoisomerization -Related

-

22

Pliotockrtnistry

state is rapid compared with the dielectric response occurring in all alcohols. A different type of environmental effect is manifested in the photoisomerization of trans-stilbene complexed to cyclodextrin. In picosecond laser studies a single decay in acyclodextrin shows binding occurs at one site, whilst in the P-form two decays give evidence for the existence of two sites which are in equilibrium.287 It has been deduced that the dimensionality of stilbene isomerization may not be one-dimensional, as commonly assumed, from an examination of the kinetics of isomerization of the meta-methyl ; substituted derivative of stilbene in decane as a function of temperature.288 This conclusion differs from the findings of Hochstrasser and his c o - w o r k e r ~ ~on ~ ’ both stilbene and diphenylbutadiene. Their interpretation is based on measurements carried on the “stiff“ diphenylbutadienes, where the phenyls are held rigid by saturated chains of 2-4 carbon atoms linked through the 2 positions of aromatic ring to the double band. The apparent energy barriers and the dependence on viscosity deviate from the Kramer‘s model but are consistent with the modified Kramers-Hubbard model and are effectively 1-dimensional in character. Park and Waldeck,290 on the other hand, conclude from their studies on 4,4’-dimethyl-stilbene in n-alkane solvents that the I-dimensional hydrodynamic Kramers equation is not appropriate. In their view a 2-dimensional co-ordinate system with a reaction trajectory related to solvent viscosity is more appropriate. Further contributions to this subject are certain. Related studies have been carried out on isomeric stilbene radical anions by a quantitative ESR method. 291 Direct measurements of the and trans-isomers concentrations are shown to be possible. It has been established by steady state and time-resolved methods that the --trans photoisomerization of p-styrylstilbene involves both a one- and two-fold adiabatic process. 292 A quantum mechanical explanation for these results has been proposed in this paper. The trans to m- photoisomerization of trans-4-nitro-4 (dialkylamino) stilbene and analogues with a nonrotable amino group has been reported.293 Solvents, &. were varied in a very detailed experimental investigation. The photoisomerization of diphenylbutadiene in low viscosity nonpolar solvents investigated by ps absorption spectra indicates that reactant-solvent clusters are involved in this

a-

1: Phorophysical Prot.rsses it! Condetised Phases

23

rearrangement.294 Accordingly in this case a multidimensional Kramers behaviour is manifest. The absorption and fluorescence spectra of a - 2 - s t y r y l anthracene derivatives have been correlated with their photochemical behaviour by Caste1 and Fischer.295 The complex fluorescence emission seen with 9-amino-6-chloro2-methoxyacridine is due to a --trans interconversion of the methoxy group.296 This compound is of considerable interest in view of its relationship to the quinacrine drug family. A molecular orbital calculation to provide a theoretical explanation of the results of this work is also published.297 The photochemical -/trans isomerization of an amphiphilic merocyanine dye (4hydroxy-l-cetyl-stibazolium betaine) has been compared in homogeneous solution and reversed micelles.2 9 8 Other detailed studies have been reported on charge transfer and cLs-trans photoisomerization of aand ~ - 1 , 2 - b i s ( I - m e t h y l - 4 - p y r i d i n e ) ethylene dication in acetonitrile,299 and substituent effects on the photoisomerization of 5(E)-styryl-1,3-dimethyluracil derivative where charge transfer in both singlet and triplet excited states is involved.300 A comprehensive review of the photoisomerization of azobenzene containing polymers draws attention to the many useful physical properties which these compounds possess.3 0 1 The photoisomerization of carvone is solvent dependent, a property which offers possibilities for use of this system in the photochemical storage of light energy.302 Shatalin303 has examined the property of photoanisotropy in systems which undergo photochemical transa isomerization. 2.4 Electronic Excitation m a y Transfer - Andrews304 has put forward a unified theory of radiative and radiationless molecular energy transfer. This paper is of considerable interest in view of the frequently under-rated difficulty in separating the two effects in experimental systems. A very timely review is that written by Evesque305 on energy migration in fractal spaces such as those found in polymers and similarly organized systems. A number of papers deal with other more specific aspects of the theory of energy transfer. These include the long-time depolarization decay due to excitation transport,306 enhancement of energy transfer between donor and acceptor molecules near a long metallic wire or dielectric fibre in which guided modes are excited,307 and energy transfer by exchange interaction in non-

24

Photochrtnist~

uniform doped solid state crystals.308 In the latter system the kinetics are analysed where donor and acceptor are positionally correlated with one another. A particularly useful paper by Wagner and Ware3'' applies the maximum entropy method for the recovery of fluorescence lifetime distributions to Forster transfer in rigid and viscous media. As shown by treatment of specific examples it is evident that this constitutes a powerful treatment for determining the distribution of rate constants in this type of system. A generalized model has been given for radiationless energy transfer in solutions of dyes (an improved version of an earlier model) . 3 1 0 The theory is compared with data in some experimental systerns. It is necessary to take full account of the kinetic complications which arise when using comparative quenching to measure electronic energy transfer in polymers. 3 1 1 The competitive kinetics generally used for homogeneous solution of small molecules are not adequate since account needs to be taken of the macromolecular nature which effectively change the dimensionality. The strength of quencher binding also has effect on the concentration efficiency relationship for energy transfer. Valeur et al.3 1 have calculated the distribution of donoracceptor distances in flexible bichromophonic molecules. The rotational isomer theory for a 3 or 4 bond sequence is found to be in good agreement with experimental results. Topical examples of intramolecular energy transfer involve studies in which chromophore/quencher complex can transfer excitation from a metal ion to a ligand CT state,313 and singlet energy transfer in conformer restricted Zn free base diporphyrine is mainly due to a dipole-dipole Forster process, although at small separations the Dexter exchange mechanism may also be effective.3 1 4 The number of solid state energy transfer studies is smaller than in previous years. Energy diffusion in binary solid solutions of 1,4-dibromonaphthalene and l-bromo-4-chloronaphthalene is one example which may be cited.315 A thermal activation energy has been determined for diffusion of singlet excitons in biphenyltetra-cyanobenzene crystals doped with TCNQ.3 1 6 A crosscorrelation phase fluorometer was used in this work. The effectiveness of excitation energy migration and transfer has been measured in polystyrene solutions in dioxane by using anthracene as the acceptor.3 1 7

I: Photophysical Processes itt Cortderised Phases

25

A single photon technique has been used to study nonradiative transfer between 7-aminocoumarin dyes and thiazine dyes in methanol solution.3 1 8 A measured critical transfer distance range of 30-70 A is consistent with the Forster mechanism. Direct measurements of energy transfer between identical chromophores has been determined by fs fluorescence time-dependent anisotropy in 9,9-difluorenone.319 This is a remarkable achievement and represents a very much state of the art experiment. Time-resolved anisotropy measurements of excitation hopping between two anthryl moieties attached to both ends of alkane molecules have been interpreted by a model based on conformational analysis.320 This type of system has the experimental advantage of being a completely isolated bichromophoric system. Wu and Morrison321 have used model systems specially synthesised to test the efficiency of different aryl antenna groups for photon harvesting and examined effectiveness of singlet-singlet energy transfer to target ketone acceptors. Another important discovery reported in the past year is the remarkable dependence of the rate of singlet-singlet energy transfer on the configuration of the hydrocarbon bridge in bichromophoric systems.322 Transfer can occur through up to 8 0 trans-oriented bonds in confirmation of previous conclusions. The interpretation offered is supported by observation of solvent effects. Energy transfer in monolayers of cyanine dyes in Sheibe aggregates shows the operation of strong coherent energy transfer between domains which increase with temperature.323 Transfer between layers is found to be much less efficient. Long-range excitation energy transfer measurements in LB multilayer systems yields transfer distances of up to 1 0 0 nm for dyes.324 These exceptionally long ranges have been attributed to the highly favourable mutual orientation of dye molecules. Forster energy transfer in 2-dimensional amphiphile arrays at air/water interfaces has also been observed for several donor/ acceptor pairs.325 Electronic energy transfer in an anisotropic system is very well exemplified by studies of 2,5,8,11-tetra-tert-butylperylene in vesicles.326 Rotational motion is suppressed in this model system. A detailed study of energy transfer from phenyl groups of micelle forming Triton-405 to solubilized pyrene molecules is interpreted in terms of a postulated structure of the colloidal particle structure.327

26

Ph otochrinistry

Time gated fluorescence spectroscopy of the tumour localizing fraction of haematoporphyrin derivative, used as a photochemically destructive sensitizer, in the presence of cationic surfactant provides evidence for energy transfer in the premicellar region which is absent in buffer solution or detergent micelles.328 2.5 Polvwric Svst- A two-volume compilation edited by Fouassier and Rabek329 contains many papers of photophysical interest which, although largely concerned with polymer systems, constitute a useful review of photophysical topics. Fractal concepts have been applied to luminescence decay in polymers,330 an approach which, although not yet of particularly widespread application, needs to be considered seriously for the treatment of macromolecular systems. The distribution of electronic relaxation functions and non-exponential fluorescence decay kinetics in polymers has also been considered in terms of the possible reversibility occurring in donor-excimer pairs using a perturbation type theory.3 3 1 Molecular mobility in both polymer films and in bulk polymers can be examined by fluorescence methods. As an example, the quenching of pyrene fluorescence by phthalic anhydride has been used in studying the structure of bulk poly(dimethylsiloxanes). 3 3 2 The photophysics and photochemistry of bis-2-(9-anthryl)-ethyl glutamate and pivalate have been employed as a pair of models of different polymeric systems with the same luminescent An ingenious method which has been used chromophores.3 3 3 successfully to enhance excitation transfer in solid polymer is the introduction of intervening anthryl groups along a polymer chain which are capable of acting as energy relays for a long distance energy cascade.3 3 4 The use of polymer molecules as light collecting antenna has been used in inducing a photosensitized oxidation of perylene in aqueous solutions of poly(sodiumstyrenesulphonate-co-2-vinyl-naphthalene). 3 3 Time-resolved fluorescence spectra of T I C T compounds bonded to polymers are exemplified by studies on dimethylaminobenzene derivatives.336 Some polymeric systems which open up other areas of general photophysical interest include an examination of the properties of photoconducting polymers with the two pendant chromophores, carbazole and fluorene, which together constitute an electron donor acceptor system. 337 Time-dependent fluorescence spectral shifts and the unusual slow decay of exciplexes in the presence of

I: Photophysical fiocesses in Conderised P/inse.s

27

dimethylterephthalate and a-dicyanobenzene has been related to the photochemical formation of ions by charge transfer.338 o w S v s t w - The high level of 2-6 C activity in research in this particular area continues unabated for several reasons. Perhaps the two most significant causes are the use of luminescence in structural studies and possibilities for application of heterogeneous systems in some form of technology. In the first of these classes is a new approach to the study of fluorescence anisotropy in LB films and membranes using the fluorescence rhodamine labelled probes.339 A combination of both aspects is featured in the survey article on electronic excitation transfer in organized molecular assemblies including vesicles, LB films, and photosynthetic light harvesting systems.340 Analysis of fluorescence decays reveals fractal behaviour arising from a spatial distribution in sites with slightly different energy levels. Fractal behaviour applies also to luminescence quenching processes brought about by electrolyte percolation which can occur in water in oil microemulsions.341 Limiting fluorescence anisotropies of perylene and xanthene derivatives have been compared in solvents, micelles and lipid vesicles.342 The structural causes of the observed differences are discussed. Pyrene fluorescence has been used to examine conditions in premicellar s o l ~ t i o n s and ~ ~ ~the effects of solvents and also temperature have been studied in this situation. ESR and fluorescence techniques for measuring micelle sizes have both been 344 applied to potassium octanoate and dodecanoate in water. Photorheological effects in cationic and nonionic micellar solutions are brought about by changes in bulk viscosities such as those which can be induced by the photoinduced isomerization of Fluorescent additives such as 94-hydroxy-u-stilbene .345 substituted anthracenes become less active during illumination due to photodimerization. In the case of sodium dodecyl sulphate (SDS) the formation of clusters between an ionic species and SDS has been shown to occur below the critical micelle concentration. The interaction of SDS with ethidium and the divalent metal ions, Zn2+, Cu2+, w. have been examined by time-resolved emission.346 Time-resolved rotational diffusion of acridine orange attached to SDS micelles shows there is only one type of A 0 above the cmc whereas below the cmc partition occurs between aggregates and solution.347 Application of the maximum entropy method, discussed

28

Photochunistr?,

elsewhere in this article, to time-dependent fluorescence of pyrene in SDS micelles in the presence of Cu2+ ions shows a Poisson distribution of fluorescence decay in such systems.348 The distribution broadening arises from varying extents of interaction of water with pyrene. Time-resolved fluorescence quenching has been applied to measurement of degrees of aggregation of SDS and 1-pentanol in mixed micelles.349 Micelles also are suitable and interesting media in which to conduct energy transfer experiments. This is clearly illustrated by experiments with polyaromatic hydrocarbons in sodium taurocholate and SDS micelles.350 Fluorescence quenching of pyrene by both Cu2+ and Co2+ in SDS micelles and the change measurable in relative intensities of the usual vibronic bands due to polarity effects shows that the presence of salts changes both the tightness of packing in the micelles and the influence arising from the higher concentration of salt near the micelle.3 5 1 The association of anionic surfactants, such as alkane sulphonates, with P-cyclodextrin has been shown to involve 1 : l and 1:2 complexation by the use of fluorescence probes. 352 The study of eosin Y and rose bengal triplet states in SDS by flash photolysis and ESR shows that triplet eosin does not associate with surfactant whereas rose bengal triplets show a considerable hydrophobic interaction.353 This shows that probe studies are not restricted exclusively to fluorescent singlet states. Anthracene in frozen aqueous micellar solutions of cetyltrimethylammonium halides forms CT complexes.354 Fluorescence quenching, together with elastic and quasielastic light scattering techniques has been used in the characterization of elongated micelles which are formed in solutions of cetyltrimethylammonium chloride (CTAC) in the presence of sodium salicylate.355 Quenching of the triplet states of 9-methylanthracene is particularly sensitive to small differences in environment and allows movement of hydrophobic molecules in the long rod-like micelles to be detected.356 The kinetics of the fluorescence quenching of n(9-anthroyloxy) stearic acids by tertiary amines which occurs in nonionic micelles of Triton X-100 have been examined in a detailed study by Melo and costa.357 Inverted or reverse micelles are topical subjects in contemporary colloid chemistry and photophysics has its own quota of papers on effects in these media. Characterization and

I: Photophysical Bocesses in Condmsrd Phases

29

stabilization conditions have been described for didodecyldimethylammonium chloride and derivatives in apolar solvents.358 Magnetic field effects on the decay rates of photogenerated germinate radical pairs in reversed m i ~ e l l e sand ~ ~photoionization ~ of alkylphenothiazine s u l p h o n a t e ~are ~ ~ other ~ systems researched in considerable detail. Sodium (2-ethylhexyl)sulphosuccinate (AOT) is the most familiar inverted micelle forming system. Photophysical techniques have been deployed to measure interfacial micropolarity ,36 colloidal semiconductor growth in reverse micelles,362 fluorescence quenching of indole compounds,363 and deactivation of naphthalene and pyrene by fumaronitrile and a ~ e t o n i t r i l ein ~ ~this ~ particular system. The sol-gel conversion is a colloidal process which can be followed by the changes in the vibronic bands in the fluorescence Changes in the spectra of pyrene and pyrene-3-carboxyaldehyde.365 fluorescence spectra of 1 -,naphthol have also been used for this purpose.366 The photophysical properties of the well known natural photosensitizerhypericinhas been compared in solution and micellar dispersion367 and excited state redox potentials measured for benzothiadiazole-4,7-dicarbonitrile in the presence of cationic micelles.368 Significant studies on phospholipid bilayers and vesicles which have been reported include an investigation of physical properties by quinoline based probes,369 treatment of phospholipid vesicles as fractal objects by use of pyrene probes,370 and photoactivated proton transfer from hydroxylated aromatic compounds indicates two regions for localization in these systems.371 H ~ r t u b i s ehas ~ ~ prepared ~ an extensive review of all aspects of solid surface luminescence on which there have been a variety of experimental investigations during the year. Spectroscopic studies of pyrene on silica interfaces shows the distribution of aromatic ring orientations and dimer/ excimer involvement.373 An interesting technique has been used to measure the effect of pressure on the decay of erythrosin bonded to aminopropyl silica gel particles in which measurements on samples mounted in a capillary tube are made with a microscope.374 The detailed kinetics of quenching of fluorescence of pyrene and 9,lOdiphenylanthracene by oxygen at silica gel gas/solid interfaces have been treated by a Langmuir-Hinshelwood type scheme.3 7 5 The kinetics of desorption and also those for excimer formation occurring with pyrene on A1203 illustrate a photochemical approach

Photocheinisry

30

to the study of adsorption.376 Total internal reflection fluorescence has been used as a technique for investigation of the microenvironmental polarity of chromatographic surfaces.377 Pyrene absorbed on chemically modified quartz plates has been examined by making use of the environmentally-induced changes in the ratio of III/I fluorescence vibronic band intensity. The lifetimes of isolated rhodamine B molecules adsorbed on semiconductor crystals378 and the very short lifetimes of J aggregates of dyes on AgBr and ~ i 0 are 2 ~other ~ ~ systems which have been investigated recently. The effects of co-adsorbed water on the photophysical properties of pyrene on zeolites380 and electron transfer from cyanine dyes to AgBr381 are systems relevant to the important practical applications of the respective substrates. Time-resolved 2nd harmonic generation from fluorescence on surface provides a method which can be applied to the study of ps time scale isomerization processes occurring with adsorbates.382 This specialized technique has been used to investigate three different dye species at submonolayer coverage. An extensive survey of photochemistry on solid surfaces which covers all aspects of the subject has been published in 1989.383 Specific papers on this topic appearing elsewhere include electron transfer reactions and H2 evolution from rose-bengal complexes on SiO2,384 anthracene sulphonates adsorbed on colloidal alumina coated silica particles,385 rose bengal on Si02, A1203 and Ti02,386 and an internal reflection flash photolysis study of the photochemistry of eosin on Ti02 semiconductor electrodes.387

3

TriDlet State Processes

The fundamental nature of triplet state processes is mainly influenced by the spin forbidden nature of the transitions which means that experimental techniques suitable for their elucidation tend to be quite different from those used for the study of singlet states. A new method for discriminating between 3nn* and 3nn* states in organic molecules by examination of the circular polarization of phosphorescence has been described and its use demonstrated by several examples.388 The detection of S-T transitions by circular dichroism has also been studied and applied to the case of b ~ t y l a m i n e . ~ ~Heavy ’ atom and magnetic field effects can be ,xamined through observations of phosphorescence hole burning spectra3” and the effect of magnetic fields in

I: Photophysical Processes it1 Cotdmscd Pllmcs

31

photolysis by polarized light provides evidence of homolytic decomposition of the triplet states into radicals.391 A direct measurement of the quantum efficiency of energy transfer from higher excited triplet states can be achieved using the thermal lens method.392 This is illustrated by the measurements reported on 2-acetylphenanthrene and biphenyl in acetonitrile. Picosecond multiphoton 266 nm laser photolysis of liquid alkane solutions of aromatic hydrocarbons shows that there is ultrafast solute triplet formation on a ps to ns time scale.393 The mechanisms of dissociation and activation of the solvent under these conditions is closely related to those involved in pulse radiolysis experiments. The photochemistry of 2-butyne in solid xenon at 10 K involves an efficient external heavy atom effect producing a triplet state whose reactions have been identified.394 Studies on aliphatic triplet states include a use of microwave optical double and multiple resonance on triplet excitation of cyclopentanone, a system which does not show strong intermolecular excitation exchange.395 Time-resolved ESR studies on the lowest excited triplet states have been made on both acyclic and cyclic ketones and aldehydes as well as methyl derivatives of cyclohexanone.396 Time-resolved triplet state absorption spectra from five isomers of p-carotene produced by anthracene sensitization allow the direct observation of isomerization between some of these isomers in the triplet manifold.397 Another study of related compounds is an examination of the triplet state properties of retinal homologues as a function of chain length.398 The magnetic and optical properties of triplet excitons and traps in lI4-dibromobenzeneshow that triplet excitons, at least in this system, have a one-dimensional character.399 Time-resolved resonance Raman spectra of Ti states derived from trans- and cis-stilbene show that both the generated spectra are identical and also that the conformation of the common state is 400 planar or near planar with a high degree of txans geometry. Another structural study has been reported of the first excited triplet state of trane-stilbene in diphenyl- acetylene TCND single crystals by electron nuclear double resonance.* 0 1 Interconversion of rotamers of tranq-1,2-diarylethylenes in the triplet state has been investigated by Momicchioli et al. 402 Temperature, pressure and deuterium effects on the phosphorescence decay-rate constant of naphthalene in single crystals of durene show that photo-induced hydrogen abstraction

32

Photochetnistry

from the solvent by the triplet state from durene may be occurring.403 This is an example of a complication which may be inherent in many phosphorescent systems of this type. Quantum mechanical tunnelling plays an essential role in the process. The effect of substitution on the triplet state energy and decay of some disubstituted 2,6- and 2,7-naphthalenes in ethanol glass at 77 K has been studied in some The fierce dispute over the suggested detection of triplet excimers of naphthalene derivatives continues. Locke and Lin4O5 report that in solutions of naphthalene and 1,n-di-a-naphthylalkane the measured emission is definitely & due to biacetyl-like impurities as suggested by other workers in a reinterpretation and re-examination of earlier data. The final decision on the matter is still awaited. The triplet state photophysics of naphthalene and a,w-diphenylpolyenes photogenerated in trapped heavy cation exchanged zeolites have been characterized.lo6 The application of the modified zeolite environment provides a powerful new method for induction and observation of external heavy atom effects. Triplet excitation energy transport kinetics in vapour deposited naphthalene has been followed by an analysis of the kinetics of phosphorescence and delayed fluorescence.lo7 The assorted decays observed are related to the domain type in the solid and the annihilation kinetics are of the fractal type. A laser flash photolysis study of the sensitized and heavy atom induced production of the acenaphthylene triplet in solution emphasizes the importance of heavy atom effects on intersystem crossing.l o 8 The role of the T2 state in 9,lO-dibromoanthracene excited singlet-triplet crossover has been assessed by the use of 1,3-octadiene which for this system is a selective triplet quencher for triplet states higher than the lowest, T1 .log The T2 state mediates intersystem crossing but does not participate in any T-T annihilation process. Triplet state properties of 9-acetoxy-10phenylanthracene derivatives have also been reported in detail. Static and dynamic quenching of the phenanthrene triplet by So transitions in Cu(I1) ions411 and nonradiative T diazaphenanthrenel' are other examples of photophysical interest. Time-resolved ESR studies of mobile excited triplet states in anthracene-lI2,4,5-tetracyano-anthracene shows the nature of exciton and lattice motion, the latter involves the interaction which occurs between adjacent molecular stacks.4 1 3 Triplet energy transfer in which ketones were used as donors was found to be

-

I: Photophysical Processes

iri

Coridertsed Phases

33

involved in observation of sensitized delayed fluorescence solutions of 3,9-dibromo-perylene.414 The T-T annihilation process in pyrene and several of its derivatives has been characterized under the conditions of laser excitation.4 1 5 Pyrene derivatives also show the expected P-type delayed fluorescence and also high excimer contributions to the induced emission. The usefulness of triplet states of pyrene derivatives generated by laser light excitation as a probe of mobility in biological systems is apparent. The intensity dependence of UV laser flash excitation of diphenylamine in methanol shows the occurrence of two quantum A l s o the binding photoionization involving the triplet state. energies of the triplet excimers observed in poly(N-vinylcarbazole) films between 15 and 55 K have been estimated.417 Triplet states, biradicals, radical ions and heavy atom effects can all be involved in the photodimerization of aceanthrylene.418 It is established that the triplet state reaction mechanisms give rise to four stereoisomeric dimers. The basicity of the T1 state of phenazine is found to have a pKa of 1.9, which is appreciably different from an earlier value, by flash p h o t ~ l y s i s . ~ ~The g discordant results obtained in earlier work are satisfactorily explained. Carbonyl triplet states continue to be significant subjects of photochemical research. Time-resolved ESR studies on excited triplet 2-methylacetylacetophenone and photoenolization are particularly significant since the enol triplets are not phosphorescent.420 Benzophenone triplet states have been characterized by photothermal beam diffraction spectroscopy. This is a calorimetric method which depends upon sensing the temperature dependent refractive index gradient at the edge of the excitation beam and production of a measurable change in the optical behaviour of the probe laser beam.421 The time resolution of the technique is from nano- to milliseconds and it has been used to follow the well known reaction of the benzophenone triplet with ethanol. A dual phosphorescence from benzophenone at 77 K in a mixed solvent system of 2,2,2-trifluoroethanol and water arises from emission containing both a short-lived free 'nn* state and a long-lived complex of the 422 triplet with the solvent which has mixed nn* and nn* character. Multifunction quenchers of the benzophenone triplet state where exciplex formation can be involved are more effective than when monofunction species are involved.423 Silane acts as a quencher of benzophenone and benzol triplets as shown by both T-T absorption

34

Photocheinistry

and phosphorescence techniques.424 Transient resonance Raman studies of the lowest triplet states of 4-phenylbenzophenone and related compounds have been analysed for evidence of both nn* and nn* character.425 Triplet states, due to their longer lifetimes, are more sensitive to environmental changes than the corresponding fluorescent systems. For this reason the phosphorescence of 4methylbenzophenone has been used to probe successfully the structure and environment of cholic acid crystals.426 The laser specific luminescence observed on pulsed laser excitation of benzophenones in Freons is found to be due to the formation of free radicals with an excited diphenylmethylene moiety generated as a consequence of a two photon process.427 The luminescence arises from sensitization of generated radicals by the presence of the benzophenone triplets. This is an interesting example of the complexity which can arise in techniques using laser light and caution is always required in interpreting data obtained by such methods. Twisted non-charge transfer excited triplet states of aromatic aldehydes and ketones are revealed by resonance Raman scattering and transient dielectric loss interconversion between two populated triplet states in the case of 2-naphthaldehyde and 2-acetophenone in cyclohexane.428 Solvent polarity effects on the 3nn* and 3nn* states of 1,2-naphthoquinone and 9,IO-phenanthrenequinone examined by time-resolved ESR and CIDEP indicates that there is only a small energy of separation between these states in the two g-quinones.429 An interesting example of an excitation transfer mechanism has been found when acenaphthenquinone is used as an acceptor.430 Timeresolved ESR shows that a variety of donors in the TI state selectively populate the T2 state of this particular acceptor. It is a matter for speculation as to whether this is a general result. Triplet excited states and semiquinone radicals of 1 , 4-disubstituted a n t h r a q ~ i n o n e s ~and ~ ’ neighbouring group participation in the very rapid photoreduction of 1,1,4,4tetramethyl-1, 4 - d i h y d r 0 - 2 , 3 - n a p t h a l e n e d i o n e ~ ~are ~ papers with a more organic photochemical content. In the latter system the remarkable reactivity is interpreted as arising from intramolecular hydrogen bonding in the transition state. A laser spectroscopic investigation of excited state processes in a-sulphonyl ketones shows that P-cleavage of the ketone takes place through the triplet state.433 Several investigations have been reported during the year on proton transfer processes involving triplet states. In four

I: Photophysical ?+ocesses irt Condemed Phases

35

substituted amino-salicylates molecular phosphorescence is enhanced through the effect of proton transfer,434 reverse proton transfer of 3-hydroxyflavone occurs in the triplet state,435'436 intramolecular proton transfer and tunnelling in the triplet state of 2 - ( 2 '-hydrophenyl) benzoxazole,437 and proton hyperfine structure ~~~ ESR of triplet states of 1 -naphthol and 1 - n a p h t h ~ l a t eare examples of such systems. Charge transfer processes in which triplet states are involved include electron transfer reactions between bis(4-methoxyphenyl) methyl-cations and I-methoxynaphthalene,439 geminate recombination kinetics of triplet radical ion pairs on porous glass and the effect of magnetic fields,440 and magnetic field effects on the decay kinetics of CT exciplexes of triphenylamine with quinones and ketones.44 Photophysical studies of various triaryl-2-pyrazolines in solution and in microcrystalline forms show that in solution triplet states are formed only by excitation transfer from a donor whilst by contrast they are formed directly in the microcrystalline environment.442 Extinction coefficients of the triplet states of some laser dyes443 and the influence of quenchers on laser dyes444 are papers of obvious technical interest. The spectra of triplet state emission from solid dye samples examined by ODMR show that the phosphorescence observed arises entirely from aggregate forms.445 Photo-induced electron transfer between phenosafranine and triethylamine molecules in perfluorosulphonate membranes is a phenomenon which has been examined in some Triplet states of metalloporphycenes ZnPC1, PdPC2, PtPC2 and NiPC2 have been characterized in low temperature liquid crystals and at room temperature in isotropic solution.447 The alkoxysubstituted phthalocyanines have deep-red optical absorbance and reports of triplet states of these molecules are of considerable topical interest in view of possible application of these compounds in photodynamic therapy.448 Measurements on solvent effects on the energies of T-T transitions of p h e n y l h e t e r o b o r i n e ~is ~ ~an ~ example of work carried out on a group of less familiar compounds in photochemistry. Room temperature phosphorescence which can occur in mixed organic media is observed in synthetic enzyme model surfactant systems.450 The effect of pH on the 3)-aminobenzoic acid photosensitized dimerization of free thymine base shows that the triplet state is involved in this reaction.451 The nature of the phosphorescence of nucleic acids and DNA components at 77 K is discussed in the paper by

Triplet states of biradical systems continue to be subjects of strong photophysical interest. The dynamics of flexible biradicals has been reviewed by Doubleday, Turro, and Wang453 in a useful general article. The effects of solvent polarity and structure on inter-system crossing in diphenylcarbenes is exemplified by a ps laser study on dimesityl carbene which shows a strong dependence of this process on the magnitude of the S-T energy gap.454 A detailed resonance Raman investigation of triplet state and radical cation resonance of biphenyl derivatives,455 the triplet sensitization of doublet-doublet fluorescence of the e-methoxybenzyl radical,456 effects of 2,2-dimethyl substitution on the lifetimes of cyclic hydrocarbon triplet II3-hiradicals, 457 laser-induced fluorescence spectra of aromatic carbenes at 77 K,458 and fluorescence Shpols 'ki spectra of diphenyl carbene4S9 are studies on biradicals of particular spectroscopic interest. Spin polarization conservation effects during triplet-triplet energy transfer in fluid solution has been observed by timeresolved ESR spectroscopy using the pyridinyl radical dimer and a sensitizer.460 The relaxation parameters for spin orbit coupling and paramagnetic relaxation in micellized triplet radical pairs have been measured by the magnetic field dependence of the decay kinetics.46 Singlet oxygen remains a significant subject for photochemical research. The rise and decay kinetics of ' 0 2 produced by porphyrin sensitization have been followed by ns time-correlated single photon counting.462 Schmidt and A f ~ h a n iprovide ~ ~ ~ data on the measurement of the phosphorescence decay constant of '0902 as a function of solvent which relate to disagreements in the earlier literature. The latter have been critically reviewed during the year by Ogilby.464 Heavy atom quenching effects on the decay of singlet oxygen have also been described.465' 466 Effects on the lifetime of ' 0 2 in four solvents under hydrostatic pressures up to 400 MPa at 25OC show that the observed lifetimes decrease with pressure to an extent which is dependent upon the particular solvent used.467 The mechanism of the effect is analysed on the basis of collisional deactivation using a hard sphere model of the liquid. Studies of the quenching of in the heterogeneous silica gel/cyclohexane system by time-resolved infra-red phosphorescence shows that quenching is influenced by both adsorbed water and silanol groups.468 Quenching of ' 0 2 by anthracyclines in aqueous solution

I: Phorophysicol Processes in Cojidemed Phows

37

involves a CT quenching mechanism as shown by the extensive formation of .02-.469 Rate constants for reaction of with a number of aromatic hydrocarbons in the weakly deactivating solvent, carbon disulphide have been determined.470 The mechanism of the interaction of simple conjugated olefin triplet states with oxygen takes place through a singlet encounter complex which gives rise either to or to a 1,4-biradical of the olefin.471 The efficiency of the former process decreases as deviation from planarity of the olefin triplet structure increases. Singlet oxygen quenching by saturated and unsaturated fatty acids and also by cholesterol all have rate constants of the order of l o 4 M-l s-l indicating that in biological systems damage to red blood cells from singlet oxygen arises mainly from attack on amino acid residues in proteins, largely histidine which has the highest value rate constant of lo7 M-l s-l .472 Quenching of by human blood plasma has in fact been directly studied by the 1270 nm emission. 473 Quenching by proteins and water is dominant, uric acid, tocopherols, carotenoids, ascorbic acid, and bilirubin make only minor contributions. The yield of '02 in aqueous dispersions of small unilamellar vesicles and time-resolved IR phosphorescence show that I02 molecules diffuse through vesicle bilayers sufficiently quickly to establish equilibrium between hydrophilic and hydrophobic phases.474

4.1 - C a m ~ b e 1 1 ~has ~ 5 produced a very C h e comprehensive book on chemiluminescence which deals particularly with bioluminescence. T0wnshend~~6reviews aspects of solution chemiluminescence particularly related to analytical chemistry. Induction periods in which free radical oxidation is inhibited by the presence of phenolic inhibitor can be measured by observation of the 1270 nm chemiluminescence of which arises from reaction of hydroperoxide with di-tert-butyl hyponitrate. 477 Sonoluminescence created by ultrasonication of nonaqueous liquids is due to light emission from small excited molecules.478 Alkanes under argon give rise to emission from C2, C2H1 and CHI whilst under nitrogen emission is detected largely from CN. These luminescent species are produced from the chemical reactions of the very high energy species produced during the rapid heating which

38

Photochernistw

arises from cavitational collapse of bubbles formed by ultrasonication. Clearly much higher levels of excitation are involved than in the case of conventional photochemistry. Triboluminescence, which arises from the effects of fracture of crystals, occurs with 1,2,5-triphenylphosphole and related compounds and is reported in a very usefully referenced paper.479 4.2 Photoc- Much interesting work continues on this topic with an aim of improving materials for various technological applications. A very comprehensive (and expensive) text has been edited by Durr and B ~ u a s - L a u r e n t ~and ~ O must be an indispensable source of information to anyone with a serious interest in this field. A few papers which have strong photophysical aspects will be mentioned. These include a spectroscopic study of N-(2hydroxybenzylidene) aniline and its photoproducts,481 photophysics and kinetics of two photochromic indolinospirooxazines and one indolinospironaphthopyran,482 photochromism of spiropyrans in aluminosilicate gels,483 and a qualitative study of the photostability of spiropyrans.484 Laser flash photolysis studies of nitrospiropyran units which are linked intra-molecularly with triplet quenching or triplet sensitizing side groups show that in these compounds the triplet state does not contribute to the generation of the photochromically active photomerocyanine.485 4 . 3 Photochemical React.ionS - A two-laser technique for measuring quantum yields of fluorescence for excited reaction intermediates has been applied to the study of diphenylketyl radicals.486 The use of a photoacoustic method for the determination of the photostability of 3-phenyl-1,2-dihydronaphthalene shows what should prove to be a useful and widely applicable experimental method.487 A theoretical paper dealing with the fundamental aspects of photochemical mechanisms is a quantum mechanical examination of the effects of the nodal plane of wave functions upon the nature of photochemical reactions of organic molecules.488 This gives a qualitative explanation of the nature of various photochemical reactions. Some examples of the formation and reactivity of solvated electrons have been cited earlier. Another paper is concerned with an investigation of dynamics of electron and cation generated in neat water over the femto-second timescale reported for both H20 and ~20.489 A quite different reaction studied on the sub-ps

I: Photophysical Processes irt Cortdertsed P1iase.s

39

timescale is the ultrafast photodissociation of six co-ordinate Ni(I1) porphyrins by a delegation process which occurs as the 3nn* state relaxes after intersystem crossing has taken place.4 9 0 Another ps kinetic study considers the mechanism of the photocyClOreVerSiOn of aromatic endoperoxides.491 Two steps which are not influenced by solvent involve formation of a biradical intermediate and a upper nn* singlet state. Both ps and ns measurements have been made on the photoreduction of benzophenone by 1,4-diazobicyclo[2,2,2]octane and the transient intermediate characterized.492 Two other laser flash photolysis studies which are of considerable photophysical interest deal with dipyridyl ketones493 and aroyl xanthates and carboxylic dithiocarbonic anhydrides.4 9 4 Photodehydroxylation of 9-phenylxanthen-9-01and examination of the behaviour of the related intermediates by laser flash photolysis reveals both homolytic and heterolytic processes.495 Hydrogen abstraction by the nn* state is involved in the influence of molecular geometry on the spectroscopy and photochemical properties of a-oxo[l,n]paracyclophanes in molecules when n varies between 8 and 12.496 Skuratova4g7 has put forward an alternative mechanism for the primary step in the extensively studied photochemical reduction and hydroxylation of anthraquinone sulphonates. In distinction to the conventionally accepted mechanism it is proposed that hydroxyl radicals are involved. Hydrogen abstraction from lipids by triplet states of benzophenone derivatives followed directly by the use of laser flash techniques allows the separation of physical quenching processes from chemical reactions of the excited state.498 The production of 02 by the photochemical decomposition of aromatic endoperoxides has also been studied by ps kinetic procedures.499 A mechanistic study has also been reported on the phototransformation of 3-nitrophenol in aqueous solution.500 There is a strong wavelength dependence of the low quantum yield for the phototransformation in this system. An interesting method has been devised for estimating the approximate quantum yield for the photodimerization of solid Srans-cinnamic acid in crystalline films.501 It appears that ciscinnamic acid formation occurs at defect sites. Photocylization of [ S ] helicene to dihydrobenzoperylene in Shpols'ki matrices at 4.2 K has been investigated by fluorescence, phosphorescence, and the influence of hydrogen acceptors.502 Chromophore aggregation and

40

Photochrmisty

the concomitant wavelength dependent photochemistry of the main chain of liquid crystalline poly (aryl cinnamate) is an unusual reaction reported this year. 503 Laser flash photolysis studies of dibenzosuberenyl cations and radical cationsS0* are further studies in a presently fashionable field of ion and radical intermediate photochemistry. Mechanistic studies on systems related to commercial photoinitiators for polymerization include work on arene diazonium salts,505 sensitized photolysis of iodonium salts,506 and the use of excited states of ketones as electron donors in iodonium salt systems employed as photoinitiators.507

I: Photophysical Processes iii Condensed

41

Phii.w

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141. 1989, 5Q,

D. Creed, A. C. Gr if f in, C. E. Hoyle and K. Veukataraman,

u, 4049.

Ip,

.-

J.Phvs.Cham., 1989,

, 1990,

504

L.J.Johnston, J.Lobaugh and V.Wintgens,

505

H.B.Ambroz, G.K.Przybytmak, C.Stradowski and M.Wolszezak, J.Photochem.Photobio1.A., 1990, 369.

506

J.P.Pouassier, D.Burr and J.V.Crivello, J.Photochem.Photobio1.A., 1989, 317.

507

p3, 7370.

u,

H.-J.Timpe, K.P.Rronfeld, V.Lamme1, J.P.Pouaseier and D.J.Lougnot, 1990, 111.

,.-

a,

fi,

Part I1 ORGANIC ASPECTS OF PHOTOCHEMISTRY

1 Photo1ys is of Carbonyl Cornpounds BY W.M. HORSPOOL

Roth has written a review dealing with the historical beginning

A review article has described the

of organic photochemistry.'

of

analysis 2 photochemistry of ions of ketones. oximes and olefins. use

n.m.r.

techniques

applied

to

the

of

Norrish Type I Reactions

1

A detailed study of the irradiation of 0-acyl-a-oxinino ketones has been carried out. Both N-0 bond fission and Norrish Type I reactivity

was

observed.

The

outcome

dependent upon the substituents.3

of

the

reaction

was

A study of the triplet decay

of the acetophenones (1. 2 ) has been reported.4 The influence of ring

substituents

on

the

photochemical

acetophenone derivatives (3. X

=

behaviour

of

the

8 , 3-Me, 4-0Ye. 3-OH, 4-08. 3-

C1. 4-C1. 4-CN and 4-CO Me) has been evaluated. The reactions 2 involved afford the products shown below (Table 1 ) . The 8cleavage reaction. fission of a C-0 bond, is influenced by the type of

substituent

and

the best

yields

are

electron releasing substituents such as 4-MeO.

obtained

with

5

5 Table 1. Products formed from the Photoreactions of (3).

Products ( X ) Compound (3)

(3) x

=H

Acetophenone

Phenol

Deoxybenzoin

25.8

38.0

26.9

( T ' 2

9.3

(3) X

=

4-CN

46.8

53.2

-

-

(3) x

=

4-Meo

33.2

32.3

16.7

17.6

60

Photochemistty

R'

(1) R ' = CN, H, Me, OMe; R 2 = OCH,CH,CH=CMe, R ' = OCH,CH,CH=CH, or OCH,CH,CH=CMc2

Aldehyde

J $ p ;::;;: I;:',

Ph

P

h

b

; R2=H

Cycl i z a t i o n

60 '1' 5 'I'

Benzene Zeol i t e

32'1. 6 5 '1'

60 Oie 35 'I'

Benzene Zeolite

2'1. 5O'f0

9 8 '1' 5 0" l o

Scheme 1

(7) n = 8-12

"/I: Phorolysis qf Crrrboti-vlCompoutids

61

The photochemistry of the ketones (4-6) in faugasite zeolites has shown

that

enhanced

the

over

Norrish the

I aldehyde-yielding

Type

solution

phase

examples are shown in the scheme 1. of

( 7 ) have

the cyclophanes

normal benzophenone systems.

6

reactions.

process

Some

is

selected

The photophysical properties

been measured

and

compared

with

7

Irradiation of benzoin in its first excited singlet state has been shown to involve the formation of radicals by a-fission. The detection technique.

of

the

radicals

was carried

out

the CIDEP

by

The behaviour of radicals produced by irradiation of

( 8 ) in faugasites has been studied. The control exerted by these

substrates on the reactions was assessed.

Dibenzyl ketone has

also been irradiated in faugasite zeolites. The products formed

from the reaction are dependent upon the cation density and the cation

type.

studied.

The

influence

of

magnetic

field

effects

was

affords

the

10

Direct

irradiation

of

the silyl

(9)

ketone

products shown in scheme 2. The same products can be obtained on sensitised irradiation. An analysis of the reaction suggests that two

reaction

paths

photochemically

are

operative."

The

reactive in methanol. The

esters

(10)

ere

preference for the

reaction

paths observed is dependent upon the substituent in the

naphthyl

ring. Thus

substituent

the

with

reaction

unsubstitution mode

is

or

with

dominated

by

a

4-methyl

C-0

bond

heterolysis yielding intermedintes from which the ether ( 1 1 ) and the acid ( 1 2 ) are formed. With a 4-cyano substituent the reaction is dominated by C-0 homolysis yielding the alkane ( 1 3 ) a s the major

product.

The

introduction

of

a

3-methoxy

substituent

produces a system which is intermediate between the two extremes.

62 Ph

XR

Ph

0

(8) R = Me, Et, Bu, P e n t y l , or Octyl

PhCH,COSiMe,Ph L

hv

PhCH2SiMe2Ph

+

+

PhCH,CH,Ph

2 - 9 "1.

29-84 'lo

Ph,SiMe2 12 ./a

(9)

Scheme 2

'

& O/r 'p h

X

t 10)

hv

MeOH

+ X

X = H =4-CN X = 3-Me0 X = 4-Me

x

PhCH,COOH

X

84 12

16 80

60 12

31 93

52 7

38 72

cat ioni c /anionic

radical

cationic /anionic

Scheme 3

II/I: Photolysis of Cnrbotryl Cotnimurrds

63

These results are shown in the scheme 3.12

Ring expansion to a carbene is one of the many reactions of interest which small ring cyclic ketones undergo. The kinetics of the photo-ring

expansion of ketones to carbenes has been

studied and the formation of the carbene in alcohol has been shown to be irreversible. Thus the irradiation of the ketone (14) and (15b) by trapping of 13 trans- to the ester group.

affords the two isomeric ethers (1%) the carbene by attack

cis- or

Another example of this ring expansion process has shown that the irradiation

of

the

cyclobutanone

(16) in

the

presence

of

succinimide affords the ring expanded sdduct (17). The carbene in this example is subsequently trapped by insertion into the N-H bond. Other nitrogen acids such as phthalimide. imidazole. 3 . 5 dimethyl

pyrazole,

adducts. l4

labile

expansion process. l5 Ring of

some

the

and

shown in scheme

irradiation

and

Interestingly

photochemically products

purine

on

benzimidazole also afford azetidinones

(18)

irradiation

fragment

4 rather

than undergoing

are to

ring

compounds

such

also the

the ring

expansion also takes place

larger

1:l

on the

as

the

cyclopentanone derivative (19). On irradiation in methanol this compound undergoes ring expansion by a Norrish Type I process to

afford the carbene (20) which is trapped ns the corresponding ether

(21) in 41% yield. The tricyclic ketone

(22)

is also

reactive in the Norrish Type I mode but in this instance fission

of the C2-carbony1 carbon bond does not result in a oarbene. Hydrogen abstraction affords a ketene which is trapped a8 the ester (23). This is a potential precursor for prostaglandin-H analogues. 16

Pitorocit misrr?,

64

\COZMe

(15)a; b;

(14)

M:RQ

Me+e

Me

R’ = MeO, R 2 = H R’ = H, R 2 =Me0 0

Me Me Me

R

0 (18) R = OMe or NMe2 Scheme L HO

”cy”” ’Bu

0 (19)

(20)

C?,Me

?iY’

0

(22) R = (CH,),CO,Me

ia‘

0--

’R’

(23)R’ = CH=CHCHISiMe2But)C5H,,

U , I :Photolysis of Carbor 1-vl Compo ut ids

65

A study of the photochemical behaviour of cyclohexanone in the gas phase has examined 13C enrichment factors.” his

coworkers’*

have

reported

further

a

Turro and

study

on

the

photochemical behaviour of the 2-phenylalkanones ( 2 4 ) . These were reported to undergo para-cyclophane formation on irradiation in solution. The present report examines the photochemical activity in the cavity of cyclodextrin. Irradiation under these conditions apparently suppresses the cyclophane formation and mixtures of cis-

and

hydrogen

trans-enals

are

formed

abstraction

route.

by

The

a C-C

change

bond

in

fission and

was 18 attributed to the confined environment within the cyclodextrin.

The

triplet

states

of

the

silylated

behaviour

ketones

(25)

are

photochemically reactive and undergo Norrish Type I fission to afford the aldehydes (26) and the esters (27) when the ketones are irradiated in methanol. The yields of product are high as is the regioselectivity of the process. The high regioselectivity is thought to be due

to the influence

of

the Si o n the

B-

radical. In the bicyclic ketone (28) a similar influence is seen and rearrangement, a process typical of a 8 . y-unsaturated enone. affords the enone ( 2 9 ) in 51% yield.’’

The steroidal amides ( 3 0 )

also undergo Norrish Type I cleavage this time of a C-N bond. The resultant biradical rebonds at the o-position of the N-arsl group 20

affording the rin6 expanded ketones (31).

Irradiation

of

the

ketone

(32) in methanol

with

added

titanium tetrachloride affords the cyclized adduct (33). This is formed by the addition of solvent to the carbonyl group followed by lactonisation. In the absence of TiClq irradiation yields only

products of Norrish Type I reactivity.

21

Photochenisty

66

bn=5

n = 10 enal

n= 12 enal

enal

MeOH a-cyclodextrin

0.1 2 3.4

0.1 1 5.6

0.07 9.0

0 - cyclodextrin

1.3

3.6

9.0

d -cyclodextr in

0.5

2.0

4.0

0

LyR2 R’

Me02C

w

R1

(27)

(25)

a; R’ = SiMe3, R 2 = H, n = 1 or 2 b; R1 = SiMe,, R 2 = Me, n = 2

Me34=-0

67

W l : Photolysis of Carbotyl Cotiipoutids

b"' (311

(30)

R =

C8H,7, R ' = H, Me, MeO; R = O A c , R1= H

ieea Cope

CH,OH M

:

m H

(32)

(33)

0

f r ag me n t u t ion/cy c lizut ion rut io Zeolite

' 6 6'

(34) a; n = 1 b; n = 2 c; n = 6 d; n = 1 0 e; n=12 f ; n :16

T /N

R

73 >>loo >>loo

6.2

3.0 1.9

> >loo >>loo

1.2 2.7 2.7

>>loo

%

Ph

zcoz

(35)

OH /NCH2"ZMe R'

(36) R = d -camphor

- 10 - sulphonyl

0

68

PJiotoc*Jieitiistn

Norrish Type I 1 Reactions

2

The influence of the confined environments of zeolites o n the photochemical outcome of the irradiation of the ketones ( 3 4 ) has been evaluated.

Irradiation of the ketones in benzene solution

gives mixtures of cyclization and fragmentation the ratios of which are

shown beside

the appropriate

structures.

When

the

irradiations are carried out in pentasil zeolites the cyclization reaction is totally suppressed. This is presumed to be due to the constricted nature of the environment which prevents rotational motion of the central bond in the biradical so that only fission occurs. 22 The affords

the

photochemical cyclobutanol

cyclization (36).23

of

the

Earlier

ketone

studies

on

(35)

the

photochemical reactions of pyridones has shown that Norrish Type

I 1 reactivity

A

is observed.24

further

study

at

313 nr

examined the behaviour of (37) and deuteriated derivatives.

An

examination

of

the

photochemical

behaviour

of

has 25

the

substituted adamantane (38) has been carried out in conjunction with a crystal-structure analysis. adamantane has a y-hydrogen

This

has

shown

that

the

favourably placed for abstraction

by a Norrish Type I 1 process and thus irradiation of adarantane

(38) in the

crystalline

phase

yields

the

26

derivative (39).

Interest in the crystalline phase photochemistry of the diketones

(40) has been reported. An X-ray study of these compounds ha% established the spatial relationship between the carbonyl groups and the hydrogens undergoing abstraction. T h e products obtained from these ketones o n irradiation in the crystalline phase are shown in

scheme 5. It is interesting to note in the solid state

that cyclization

to the cyclobutane products

is preferred

to

fission. 27 The influence of different corplexing agents ( 4 1 - 4 3 )

69

I I A : Photolysis of Carboriyl Cornpourids

R'

Ph

R3

R3

R'

R2

H

H

H

H Me

H H

Me H

P- MeOC6H,

H p-MeOC6H,

(38)

(39)

(40)a; n = 8 b;n = 7

3OIO

04 " 0

8 9 "10

1o o / o

13 O I O 1 1 O/O

c;n =lo

4'lo

9 1 "lo

5'lo

Scheme 5

on the outcome of the cyclization of ( 4 4 ) into the j3-lactams ( 4 5 ) and ( 4 6 ) has been studied.

28

Previous work by Yoshioka et a l . 1.3-diketones

( 4 7 ) could

undergo

29.30

demonstrated that the

Norrish

Type

I1

hydrogen

abstraction from the triplet state. The resultant biradical ring closed to yield 3-hydroxycyclobutanones. In another investigation of the photochemical shown that

behaviour of the ketones

irradiation

in pentane

(47)

in the presence

they have of

oxygen

affords the products shown in scheme 6. The formation of these products is due to the trapping by oxygen of

the biradicals ( 4 8 ) 31 formed by the conventional 1.5-hydrogen transfer.

The hydrogen abstraction path within the Norrish Type I 1 process and related reactions can be controlled by the presence of hetero atoms. Typical of this is the work on the synthesis of prolines ( 4 9 ) by the irradiation of the

glycines ( 5 0 , R

=

Me,

2.4.6-trimethylphenyl, 2,4,63‘ triisopropylphenyl). In this reaction hydrogen abstraction by the i-Pr ,

excited

Me2CHCH 2 .

benzoyl

Me

group

diastereoselectivity

takes

for

place

from

the

the formation of

6-position.

The

( 5 0 ) is apparently

dependent upon the barrier of rotation of the amide function. Wagner and his

32

have reported on the photochemical

behaviour of the acetophenone derivatives (51). These have been shown to undergo cyclization. following 1,6-hydrogen transfer, on irradiation in the crystalline phase.

The chemical yield of

the products (52) is almost quantitative although there is some variation in the quantum yield as shown. The authors” that the molecules

must

exist

in the crystalline

believe

phase

in a

geometry which allows for ready abstraction of the &hydrogen

11/1: Photolysis of Ccrrbotyl Coniporrr1d.s

71

/- R 1 R &“iR1

0

144) R 1 = H, Me, Et, Pr, P r i , P h ; R 2 = a l k y l or c y c l o a l k y l R’-R1 = CH2XCH2, X = CH,, 0, CH,CH2; R 2 = a l k y l or c y c l o a l k y l

“.EJvR, .E,,I~ H0

R,

0

0

Ar

+

+

ArC02H

A r = Ph, p-MeC6H4, m-MeC6H, Scheme 6

Ar

j;Q. (48)

(49)

HO

7'

OCYC

(52)

(51)a; R1=H, R 2 = M e , R 3 = H b; R'=Me, R2=Me,R 3 = H c; R'=R2=R3=Me d ; R 1 = P r n JR 2 = R 3 = Me e ; R' = P h J R 2 = R 3 =Me

67 "10 loo"/. 94"Ie 10O " / O

& OR

R'

0.05 0.24 0.12

0.03

CH30CH20

0

Me

/

1.o

100°/o

M e 0&OM.

OR

(53)R' OMe, or H; R = CH20CH, or Me R 1 = H; R = a l k y l

(54)

Ac 0

AcO

Ph Ph

(56)

(55)

Ph

(57)

OH

yielding biradicals which can adopt the appropriate alignment for efficient

cyclization.33

photochemical

reactivity

Krauss

et

the

of

a1 . 3 4

ketones

have

reported

(53). All

the

these

of

compounds are reactive yielding the benzofuran derivatives ( 5 4 ) in reasonable yields. The reaction is typical of photochemical 1.6-hydrogen

transfer

to

provide

biradicals

which

undergo

cyclization. The products ( 5 4 ) are proposed as intermediates in the synthesis of aflatoxins.

34

The gibberellin derivative (55) is photochemically reactive and

on

irradiation

affords

the

(56).

lactone

The

reaction

presumably involves a photoreaction akin to the Norrish Type I1 hydrogen

abstraction

process,

in

thk:

case

a

1.8-hydrogen

transfer, from the allylic site to yield the biradical (57). Bond formation and rearrangement by a trans-esterification yields the final product.

35

The phosphate ester (58) is photochemically reactive

=

at 254 nm

0.025)

( adis

in methanol

under nitrogen. T h e product

produced from this is the isomer

( 5 9 ) . The formation of this

product

is proposed

to arise

via

a Norrish

Type

I1 process

involving the P=O system. This hydrogen abstraction yields the biradical

(60) which subsequently

attacks

the

o-site

on the

pendant phenyl group yielding ( 6 1 ) . Ultimately the final product 36 (59) is obtained in quantitative yield.

3

Oxetan Formation

The

photochemical addition of benzaldehyde

to cyclic alkenea

(62.

63) has been studied. T h e products produced

from these

reactions are the oxetans shown in scheme 7. In addition to the

74

0

II

0y-J

PhO

PhO-

;/I 'O

Qn

'

r s

OH

OH

PhO-

I

\

o

+ H

(62)n = 1 or 2

(64)n = 1

n = 2

endo 61

74

Ph

exo 3 9 y i e l d 55°/r

2 6 y i e l d 34"/*

(ap H

Q"+ ( 6 3 ) n = 1 or 2

H

(65)n= 1 n= 2

en do 82

90

Scheme 7

em 18 y i e l d 98"/. 10 y i e l d

oxetans

(64. 6 5 ) . products

of

reduction

are

also

obtained.

However, the important result from this study relates to the stereochemistry of the oxetans. Thus, there is a preference in all of the cases studied for the formation of the endo-isomer and the ratios obtained are shown under the appropriate structures. The authors reason that the preference shown is attributable to secondary

effects. 37

orbital

photochemically

to

furan

to

The yield

aldehydes the

(66)

exo-adducts

add (67)

exclusively in high yield. The study w a s extended to the addition of the same aldehydes to dihydrofuren ( 6 8 ) . With this alkene the addition of

(66a.b) afforded mixtures

of

the

endo-and

exo-

adducts ( 6 9 ) and ( 7 0 ) respectively in the ratios shown beside the appropriate structures. When the bulkier aldehydes (66c.d) were 38

used only the endo-adducts ( 6 9 c , d ) were obtained.

Kagan e t

reported that the irradiation of neat acetone

afforded a low yield of the oxetan (71). The mechanism for the formation of

this was not

certain although

logically

it was

suggested that addition of acetone to its enol produced the final product.

A re-examination

of

this reaction

has verified

the

mechanism. Thus irradiation of acetone in protic solvents such as secondary alcohoIs at 0-5OC under Argon at 254 nm afforded the same product ( 7 1 ) although other compounds ( 7 2 1 , (73). and ( 7 4 ) In a n extension of this work it has been

were also obtained.

demonstrated that acetone will add successfully to other enols again affording 3-hydroxyoxetans. The enols used for the addition were obtained from acetaldehyde, cyclopentanone. pentan-3-one. 2,4-dimethyl-

pentan-3-one,

2-methylpentan-3-one.

Turro,

Le

butan-2-one,

and

40

Noble

and

coworkersa1

have

studied

the

76 CHO

Ratio endo : exo

a; 7 : 1,

b; 1 5 : 1,

c; > 2 0 : 1, d ; > 2 0 : 1

HO MeCOCH2CMe20H OH

I

Me2C-

CMe2

I

Me C 0I C H C ), 0 Me

OH

NC

&

B.

(77)a n t i

(75)

R = F, CI, Br, OH, Ph, But

77

[I/]: Photolysis of Carbonyl Compounds photochemical behaviour of 5-substituted-adamantane-2-ones

R

= F.

(75,

C 1 , Br. HO, P h , and t-Bu) o n irradiation in acetonitrile

and aqueous solutions of P-cyclodextrin

with fumaronitrile. The

influence o f environment on the formation of the syn-

and the

anti-oxetans ( 7 6 ) and ( 7 7 ) respectively w a s studied.

Acetone

and

acetophenone-sensitized

irradiation

of

benzvalene (78) affords benzene. However, when the energy of the sensitizer is less than 65 kcalmol

-1

, photo cycloaddition. at

-30°C,results in the formation of ~ x e t a n s . ~ * T h utho s addition of the ketones ( 7 9 ) or p-benzoquinone or tetrachloro-p-benzoquinone affords the oxetans ( 8 0 ) . (81). and ( 8 2 ) respectively. The yields of products

are

shown below

the appropriate

structures.

The

addition of biacetyl also yields the oxasemibullvalene (83). This product is formed from the common intermediate biradical which either ring closes to the oxetan (83). Another

product

( 8 5 ) is

(84)

( 8 0 ) or rearranges

produced

in

the

addition

to of

p-chloranil. This, however, is presumed to arise by an electron transfer path.

A continuation of a study of chiral induction

in the Paterno-Buchi reaction has examined the influence of aryl 43 substituents on the addition of glyoxalates ( 8 6 ) to furan.

Intramolecular cycloaddition is used to synthesis0 strained polycyclic oxetans. Thus the photochemical

cyclization of the

ketone ( 8 7 ) yields the oxetan (88). This compound has been used as

the

key

intermediate

in

a

synthetic

route

to

tricycle[ 3,3,0.03’7]octane derivatives.44 The acetone-sensitized irradiation isomerization

of

the

enone

affording

the

(89)

brings

enone

about

(SO).

trans-cis-

Intramolecular

cycloaddition also occurs yielding the two oxetans ( 9 1 ) and ( 9 2 ) in a total yield of 42%. A further cyclization mode, a photoene

78

R' h R 2

0 (79)a; R ' = R 2 = M e b; R ' = R 2 = Ph c ; R ' :Ph, R 2 = OMe

(78)

YMe

Me

R

0

&

!R

I

I

R

0 (80)(19'Iol

(81)b; 22'1, C; 38'10

+(84)

(83)

0

0

(86) R' = (+)

- Menthyl

R * = aryl

A Me

(87)

(82)

R H or CI '=

Cl (85)

47 (88)

II.1: Photolysis of Corbotiyl Cornpoutidh

79

(95)

m Me3Si I

0

0

(971

(100)

(98)

(96)

reaction, yields the compound (93) whose identity was verified by x-ray studies.

45

Oxetans can also undergo cleavage reactions under conditions of photoelectron transfer. Thus the irradiation of the oxetans ( 9 4 ) with

triethylamine

conversion

to the alkenes

obtained

at

254

nm

brings

( 9 5 ) . The best

in THF as solvent . 4 6 The

about

yields

electron

efficient

(of 73%) are

transfer

mechanism

involves the formation of the biradical (96) followed by transfer

of hydrogen and bond cleavage.

4

Miscellaneous Reactions

The irradiation of the ketoaldehyde (97) affords the deconjugated enone ( 9 8 ) in 98% yield. The corresponding also

photochemically

reactive

and

( 9 8 , 8 9 % ) . The efficient decarbonylation to the presence of prevents

the

trimethylsilyl

the oxa-di-c-methane

trans-isomer (99) is

yields the same product

process

is presumed to be due group which apparently

(see chapter I I I / P

for

examples of this reaction) from taking place. The authors suggest that the intermediate alkyl radical formed on decarbonylation is stabilised by the silyl group. 4 7 Decarbonylation is also found in photochemical

reactions of some cyclic ketones. A detailed

laser flash study of the decarbonylation of the ketone (100) has shown

that

tetremethyl-o-xylylene

converted exclusively

(101) is

into the cumene

study of the photodecarbonylation

(102).

48

formed

and

is

A laser flash

of the ketone (103) and the

81

II/I: Photolysis of Carborryl Coinpouirds

@o Ph Ph

(103)

R (110) R = OMe, Me, F, or H

R'

R'

I

I

N HCHC02H

NHCHD

I

i >

0

H

H

(ll?) R' = HI Me, P r i , CH,OH, CH,CO,H,

M eO,C

CH,Ph or 3 - benz imidazolylmethyl

BfCozH EfPh Me02C

82 formation

of

the

quinodimethide

derivative

(104) has

been

reported. An examination of the decay of the product (104) was carried out. 4 9 The enone ( 1 0 5 ) undergoes photoconversion at 300 nm in Argon degassed solutions to yield the keto thiol (106). This is formed by decarbonylation of the starting material and quantitative

formation

of

the

thiete

( 1 0 7 ). 5 0

Hinrichs

and

Margaretha

report that the thiete (107) is readily hydrolysed 51 into the thiol (106).

The photodecarboxylation of a series of carboxylic acids to afford the corresponding alkanes in high yields

(z.84%)

has

been reported. The reaction involves SET from an azaaromatic such 52 as acridine in the presence of t-butylthiol as a proton donor.

A quantitative

study of

the photochemical

decarboxylation

of

pyruvic acid ( 1 0 8 ) has shown that the formation of products, e . g . (1091, is more efficient at lower pH. This effect is interpreted as evidence for the photodecarboxylation of one of the hydrated forms of (108).5 3 A photooxypenative decarboxylation of the aryl acetic acid derivatives

(110) has been described. The reaction

affords the corresponding aldehyde and involves a S E T process from pyrimido[ 5,14-g]pteridine-lO-oxide.

''

Photodecarboxylation

of ( 1 1 1 ) in MeOD or D 0 affords the deuteriated products ( 1 1 2 ) .

55

2

A method for the conversion of the cubane carboxylic acid (113) into the phenyl derivative ( 1 1 4 ) uses the irradiation of the acid in benzene in the presence of lead tetraacetate. The product is obtained in 66% yield.

An

electron

photochemical

56

transfer

mechanism

is

involved

in

conversion of the phthalimide derivatives

the (115)

144: Photolysis of Carboiiyl Coinpourids

83

go N-

R= p

CO,R

RC I

- B u ' C6H,(CH2),

(PhCH,),CH,

I

Z - C,H,CH

=CH (CH,),,

c y c l o h e x y l or 1-adamantyl

g: N-C0,R

0 (119) n = 1 or 2

(118)

0 R A O - D

R*

I

S

R-CI

R =

Cf3.

Q

H

Scheme 8

(121) X

NAc,O, or S

+

+

PI1ot ochLW istty

84 into phthalimide

( 1 1 6 ) and

the corresponding chloro

compound

(117). The electron transfer takes place to DABCO affording the radical anion

( 1 1 8 ) which

on protonation

eliminates the acyl

radical. Decarboxylation of this radical affords an alkyl radical 57 which abstracts chlorine from the solvent, tetrachloromethane.

A study of the biradicals formed

by photodecarboxylation of the

carbonates ( 1 1 9 ) has been carried out.

58

Dauben and his coworkers have described a light initiated halogenative

decarboxylation

of

the

esters

(120).

The

path

followed is outlined in scheme 8 and involves the production of a radical by decarboxylation. 59 This reaction is a development of a previous procedure described by Barton and his coworkers. An example of this is the irradiation of the esters ( 1 2 1 ) at O°C in methylene dichloride which gives good yields of the cyclized

(122) (Scheme 9 ) . The reaction involves the formation

products o f the

radical

( 1 2 3 ) be photodecarboxylation. Cyclization of

( 1 2 3 ) affords the primary radical ( 1 2 4 ) which combines with the thiopyridyl radical to yield (122). The derivative ( 1 2 5 ) is also Photochemically reactive and yields the heterocycle ( 1 2 6 ) in 68% yield. No evidence was obtained for the formation of bicyclic products

in

this

reaction."

The

use

of

the

photochemical

decomposition of these Barton e s t e r s , e.g.(127) has been made i n the alkylation of the enones (128). Irradiation of the mixture of ( 1 2 7 ) and ( 1 2 8 ) affords the products (129).61 Barton and Sarma have

also

reported

alkylation of 62 (130).

further examples of

methyl

vinyl

ketone

this reaction

to afford

in the

the derivatives

Irradiation of the epoxide ( 1 3 1 ) in cyclohexane or acetone affords the chromanone

( 1 3 2 ) in 65% yield. The reactivity of

II/I: Phorolysis of Carlwiyl ('ompounds

85

Scheme 9

R

cyclohexyl

R5NJ R3NA Mi' (128) R = COOMe

M:

(129)

Phorochtvnistry

86

Me

“

‘

O

a

M

e

(132)

(1311

“

(135)

b

‘I

R2 a R’

R

‘

2

“Q“

H

R2 CHO

(136)

R1

R2

hexane acetone

a; M e 0 b; Me c; CL

H H

58 56

48

H

50

d; H e; Me0

H Me

48 36 52

60

LO 45

S c h e m e 10 CFCL,CHFCO,R

C F X Y C C IZC0,R

(137A) X = Y = 2 = C I or

F

hv

\

0.

(1378)

(138) R1 = H, R 2 = M e 0 or R ’ = R 2 = M e 0

(134)

I I h : Phorolysis of Carboriyl Cornpoutids

87

these epoxides appears to be dependent on the substitution in the aryl ring. Thus the irradiation of the derivatives hexane

affords

the dihydrofurans

(134) while

(133) in

irradiation

in

acetone yields the chromanones (135). The yields are shown below the appropriate structures. The formation of the two product types presumably arise from a common intermediate (136) (scheme 10)

which

by

decsrbonylation

path to

"a"

yield

undergoes

ring

(134) while

by

contraction path

"b"

and

hydrogen

migration yields the chromanones (135). No clear statement on why this

reaction

preference

is

observed

was

made.

It

is

not

impossible that differences in reaction multiplicity or electron 63 transfer processes could influence the outcome.

The

influence of

reductive

cleavage

of

the ester C-Halosen

group on bonds

of

the photochemical (137a) has

been

evaluated. The reaction occurs at the a-site unless the halogen is fluorine. In this instance. e . 6 . with (137b). the reduction can take place at the 0-position if there is a chlorine at that carbon.64 Irradiation of (138) in acetonitrile/water using

1 >

250 na brings about the formation of (139). This compound is presumed to be formed via a n intermediate to which water adds to yield (140). Rearrangement of this affords the fins1 product in high yield.

65

A study of the influence of the direthylphenylsiloxy group attached at C-4 of the steroid (141) has shown that excitation of the siloxy group leads to exclusive reduction of the 17-keto

group when the reaction is carried out

in triethylamine and

acetonitrile. Direct irradiation of the keto group in (141) leads to epimerization and reduction.

66

'

Pliotochet nistn,

88

& R2

R*

0N

\

R1%

\

(141 R = SiMe2Ph

&

O

"0

H

HO O x f ? H

@oH 0

HO o@ H

(147)

t H 02N

89

Il/l: Photolysis of Carboriyl Compounds

0 '

0 '

\

I

o' ONy-J} HO

HON

H ~

H (150)

(149)

ON

\

H

d qN*b' 0

0

(152)

-7 '02N& 0 Et

(154)

(1551

}

Extended irradiation (308 n m ) of ketene isolated in a matrix has demonstrated

that

ethynyl

alcohol

( 1 4 2 ) is obtained. The

process can be reversed by irradiation at shorter wavelengths.

The

novel

photochemical

(143) have been

reactions

investigated.

The

of

67

the dinitroketones

irradiation

of

(143) as

a

mixture of keto and enol isomers under nitrogen through a Pyrex a 1:l mixture of the two ketones ( 1 4 4 ) and (145)

filter affords

in a total yield of 55%. The isomeric dinitro ketone (146) gives a 48% yield of the ketone (147). Two main mechanistic paths were considered

but

the failure of

the mononitro

ketone

(148) to

undergo photochemical reaction ruled out one of the routes. Thus the authors

suggest

that

the

products

are

formed

by

a

path

involving CN bond fission with the expulsion of a nitro group to yield a radical ( 1 4 9 ) which is transformed via ( 1 5 0 ) and (151) into the observed products.68 (153) were also investigated

The mononitro ketones ( 1 5 2 ) and and i t was shown that

(153) was

inert. However, the isomeric ketone (152) is transformed into the ester

(154)

on

irradiation

in

ethanol.

The

reaction

is

interpreted as involving C-C bond fission to the biradical (155). This is transformed into a ketene which is trapped by solvent to yield

(154).69 A further detailed report of the photochemical

reactivity of nitroketones has been published supplementing the material published earlier.

70

W I : Photolysis of Curboriyl Cornpourids 5

91

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2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones BY W.M. HORSPOOL

1

Cycloaddition Reactions

Intermolecular.- An earlier study had shorn that enolic diketones such as ( 1 ) undergo photoaddition to dienes.

The thermolysis of

the photochemical product yielded a variety of products end the present publication reports on the structure identification of these. Thus

the photoaddition

( 1 ) to isoprene followed by

of

thermolysis yields the four products ( 2 ) . (3). ( 4 ) ' and ( 5 ) . The addition to other dienes w a s studied and the euthors2 claim that this synthetic systems

(e.g.

path

provides

a good

2). In another

route to cyclopentanone

example

they have

studied

the

photoaddition of ( 1 ) to indene the products of which are shown in scheme 1.2 The photochemical dimerization of the enone ( 1 ) by direct irradiation in benzene/cyclohexane affording (6) in 17% yield

has

also

been

reported.'

Dimerization

to

afford

analogous product occurs with the corresponding ethyl ester.

Photochemical results

in

addition

the formation of

of

acetylene

indanone

to

the

an 3

enone

(7s)

(8) in 7% yield.

This

product is formed by a double addition of acetylene followed by elimination

of HCN

to

yield

the

final

product.

Addition

of

acetylene to ( 7 b ) also gives ( 8 ) but in a better yield (23%). Several other products were obtained among which ( 9 . 4 . 5 % ) , ( 1 0 , 13x1, and (11. 3.1%) were identified. Compound ( 9 ) is formed by

W 2 : Eiione Cycloaddirioris arid Rrcrrrcrrigrtiierits

97

18 %

7 oo/

+

+

Q+Me02C

Scheme 1

98

Photochemistry

,"$

Qf,

tE2oc$-$M ;

0

AcO H

0 (14)

Me

(foR 0

0 1R, 4S, 5s

(23)

1 S, 4S, 5 R

(24)

11,2: Etiorie Cydondditiotis atid Rwrratig.rnetit.s

99

a straightforward (2+2)-cycloaddition and (11) by a second (2+2)addition

to

(9) while

rearrangement of (9) photochemical

(10)

arises

by

The cyclopentenone

cycloaddition

and

with

an

oxa-di-e-methane

(12) also undergoes

2-(trimethylsilyloxy)-

buta-1.3-diene as the substrate affords the (2+2)-adduct (13) in

65% yield as a mixture of epimers. The report also gives details of

the photoaddition

of

this diene

to a variety

of

cyclo-

alkenones. The adducts can be used as precursors in syntheses of natural products as exemplified by the conversion of (13) into (14). The use of dienes in cycloaddition reactions of this sort is unusual since normally the diene acts as a quencher of the enone triplet state.

Photochemical addition of the enone acetal (15) to the ( S ) butenolide (16) affords the (2+2)-adduct (17). This adduct was readily transformed into the enone (18) and subsequently into optically pure

(-)-O-bourbonene.'

The lactone

(19) affords a

mixture of head-to-head and head-to-tail isomers (20) and ( 2 1 ) respectively on irradiation in the presence of the alkyne. 2prop-2-yn-1-01.

Addition

also

occurs

chloroprop-2-ene and hept-1-ene, the

photochemical

(2+2)-addition

to

prop-2-en-1-01,

1-

Hof fmann e t a1 . 8 have studied

of

ethylene

the chiral

to

lactone ( 2 2 ) . The acetone-sensitized addition affords the two adducts

(23) and

(24). The

reaction

exhibits

a

temperature

dependence and the diastereoisomerio excess shorn by the products obtained

from

the

reaction

ranged

from

47%

to

29%

in

the

temperature range of -85 to +14OC.

The photochemical reactivity of the enones (25) has been studied. On irradiation in acetonitrile or benzene the diaers ( 2 6 ) and (27) are formed. When 2.3-direthylbut-2-ene is added to

the eolution in benzene and irradiation with

1 < 350

nm is used

H H

(26)

Me

Ph H Ph

;

J Me @:-2c00Et Me

Me

Me

J

J

e

M

Me

Me&R1 Me

(33)

Me R

Me

R = H, R’ = Me, Et; R = Me, R’ = Et

&

C02Et

Me

Me

Me COOEt

(35)

(34)

Me

(37)a; R = H b;R=Me

W 2 : Etiotie CyclotrddiIiotu arid Rearmtigetnetits the adducts (28) are formed behaviour

is different

101

in reasonable yields. The photo-

in alcohols when hydrogen

abstraction 9

reactions are dominant and the reduced products ( 2 9 ) are formed.

The photochemical reactivity of some (2+2)-adducts are also of interest and the photochemical epimerization of the imidate (30) 10 by photolysis has been described.

The photoaddition of allenic esters (31) to the enone (32) has been studied. The major products from these reactions are the dienes (33) and the cyclobutanes (34) and (35). It seems likely that the addition proceeds by the traditional biradical path to an intermediate such as (36). Dis-proportionation affords (33) while C-C

bond

formation yields

the

two cyclobutanes.

The

cycloaddition of isobutylene to the enone (37a) in a variety of solvents

affords

the

adduct

(39, 18-24%).

derivative

(38. 20-40%)

and

the

elkylated

These products again arise from the

formation of a biradical common to both products and i t either undergoes ring closure or hydrogen abstraction to produce the observed products. The authors12 report that the outcome of this reaction

is

sensitive

to

change

in

medium

and

when

the

cycloaddition is carried out in silica gel the reaction affords the adduct products with

(40, R

= H)

in 32 X

yield

as well a s

the other

(38) and (39) in smaller amounts. This cycloaddition

isobutylene

has

been

used

synthetic path to oxosilphenene

with

the

enone

(37b) a s

(41). Thus the adduct (40. R

a

=

Me), obtained in 31 X from the photocycloaddition of the alkene 12 to the enone (37b), is elaborated into the final product (41).

Three

products

(42),

(43), and

(44)

are

obtained

on

the

irradiation of nootkatone (45) in the crystalline phase. Product

(42) is formed by a normal intermolecular while

compounds

( 4 3 ) and

( 4 4 ) are

(2+2)-cycloaddition

produced

by

free

radical 13 processes involving hydrogen migration and cycloaddition.

Photochetnistn

102

po bo

H

(39)

(40) R = H, 32 %; R = Me, 41 %

Me I Me

Q

0

0 (43)

?JMe (44)

0

W2: Etionr Cycloadditiotis atid Rearrartgetnetits The

photochemical

103

(2+2)-cycloaddition of

both

electron

deficient and electron-rich alkenes to the dihydropyridone (46) has been reported. The addition affords the products shown in scheme 2. The reaction proceeds in high yield and with reasonable selectivity as shown by the exo/endo ratios quoted. The process also takes place with 2.3-dimethylbut-2-ene

and yields the two

adducts (47) and (48) in a ratio of 6:4 and a total yield of 85%. The trans-isomer

(48) is unstable and readily converts to the

(47).l4

cis-isomer

The

pgridones

(49) undergo

photochemical

(4+4)-addition when irradiated in the presence of an excess of cyclic conjugated dienes. Thus with cyclopentadiene the adduct

(50) is formed. l5 The pyrid-2-ones (51) undergo (4+4)-addition with cyclopentadiene to yield the isomeric adducts ( 5 2 ) and ( 5 3 ) . Open chain dienes did not react in the same manner and. for 16 example, 2.5-dimethylhexa-2,Q-diene gave the (2+2)-adduct (54).

The photochemical additions of the naphthalene derivatives (55)

have

been

in

studied

detail.

The

intramolecular

cycloaddition encountered gives the adducts ( 5 6 ) . Kinetic and photophysical

details

reaction arises

from

are reported the

triplet

as is the state. l7

fact

that

the

The photochemical

addition of ethylene to a series of P-naphthols (57) has been reported. The reaction takes place in the presence of aluminium chloride and the intermediate (58) is proposed as the species to which the photoaddition takes place. The yields o f the adducts

(59) are high ( R 68%; R

=

The

C02Me,

enones

= H,

68%).

70%; R 18

(60a-d)

P

HO, 49%. R

undergo

=

MeO, 66%. R

photochemical

=

Br,

(2+2)-cyc10-

addition with electron deficient alkenes. Typical of the products obtained are the adducts (6la) and (61b) obtained in 45 and 55% yield respectively from the addition of methyl methacrylate to

Ph otochemistty

104

I

C02Me

C02Me

(46)

R2

R’ H

H

(CH2)3 C02Me C02Me H C02Me H O(CH2)3Me H CH2Ph

Yield (%) 93 73 85 96 80 73

exo lendo 6:4

7:3 7:3 8:2

Scheme 2

I3

R’

\ R I ,

(52)

(51) R

k (53) R = H, R’ = H or Me R = Me, R’ = CMe3

(54)

IU2: Emnr Cydoudditions mid Rrorrmtigrnirrits

R

105

qo /

(59)

R’ (61) a; R3 = Me or COOMe b; R4 = COOMe or Me

(62) a; H b; H c; Me d; Me

R2

Bu

(63) R = Me, Bu, iso-pentyl or Ph

iso-pentyl Bu iso-pentyl

H H

R2

(68) R’ = H, R2 = Me, R3 = H or Et R‘ = CH2C02Etl R2 = Me, R3 = Et

(69) R’ = R2 = H or OBu, R3 = Et

R’ = H, R2 = CN, R3 = Et R’ = H, R2 = CN, R3 = H

Photochetnistty

106

the enone (60a). T h e enones are also reactive on irradiation in propan-2-01 with triethylamine

from which good yields of

dihydro dimers

( 6 2 ) are obtained. The photoreactivitg 19 enones ( 6 3 ) was also studied.

of

the the

Photodimerization of coumarin ( 6 4 ) in the crystalline phase in the presence

of a crystal

controlling substance

has been

examined. Thus when dimerization is brought about in antipyrine only the syn-dimer ( 6 5 ) is formed. When 5-iodo-1,3-dimethyluracil is employed as the crystal controlling substance the anti-isomer ( 6 6 ) is obtained while with a mixture of 5-Br- and 5-iodo-1.3dimethyluracil a 1:l mixture of the syn- and anti-isomers is obtained.20 The singlet excited state of

the enones

( 6 7 ) add

styrene to afford

the (2+2)-adducts ( 6 8 ) in which the phenyl

group

endo-position.21

is

in

the

Photochemical

addition

of

=

Me,

n-butyl vinyl ether or acrylonitrile to the enone ( 6 7 , R2

R3

= H)

in its singlet state has also been studied and'affords

the adducts (67, R2

=

(69). 2 2 The photochemical

Y e , R3

= H)

with

reactivity of the enone

dimethyl acetylenedicarboxylate and

other electron deficient compounds such as N-phenylmaleimide and diethyl maleate has also been studied. Typically products of the type represented by (70) were obtained. A mechanism involving electron transfer cannot be excluded.

A

review

photochemistry

has of

highlighted psoralen

some

23

specific

derivatives.24

The

aspects ester

of

the

(71) is

reported to react inefficiently in (2+2)-cycloaddition reactions in the presence of DNA. 2 5 8 26 However, different reactivity is found when the ester ( 7 1 ) is irradiated in benzene solution in the presence of alkenes (72). This process affords the (2t2)adducts

(73) efficiently.

exclusively

at

the

The

addition

site illustrated

apparently

and no evidence

occurs

for the

II,2: Enone Cycloadditions and Rearrangements

107

a ’

C02Et

R’

Rg

R2

(73)

R4

(72)

M

e

w

R’ Me Me Et EtO Ph p-tolyl

R2 Me Me Et H H H

o

R3 Me H H H H H

rn

R4 Me H H H H H

CH300C(CH2)7 (CH&CH3

Me

(75)

(74)

(76) cis H-H

(77) cis H-T

(78)

R‘ = (CH2)$02Me R2 = (CH2)$Ha

Me

Me

%

55 73 70 a5 71 62

formation

of

adducts

by

addition

to

the

furan

moiety

was

observed." Four adducts have been obtained on irradiation of the psoralen ( 7 4 ) and methyl olsate ( 7 5 ) in ethanol using

1 = 360 nm.

The four adducts are formed with a quantum yield of 0.003 and have been identified as (76-79). A fifth product was identified as the methyl ester of elaidic acid (80) arising by trans-cisisomerization.28 The dimerization of the psoralen (74) yields the two products ( 8 1 ) and (82). Interestingly the quantum yield of the

dimerization

trifluoride.

was

diminished

by

the

addition

of

boron

29

Intramolecular.-The (2+2)-cycloaddition reactions of the enamine (83. R

( 8 3 ) have been studied. Irradiation of mixture of the two products

70%.

When

the

bicyclic

= H)

affords a

( 8 4 ) and ( 8 5 ) in a total yield of

product

(84)

is

treated

with

wet

acetonitrile an almost quantitative yield of ( 8 5 ) is obtained. The reaction is temperature dependent and at room temperature the ratio of products

(84):(85)

is 1:3.5

while at O°C

obtained is 6:l. The photoconversion of (83, R bicyclic compound ( 8 4 , R

=

=

Me) is efficient (82%).

the ratio

Me) into the 30. 31

The influence of solvent on the direct photoisomerization of

(-)-carvone

( 8 6 ) has

been

studied

at

293K.

The

(2+2)-

cycloaddition product (87) and the ester (88) formed by Norrish type I reactivity of ( 8 6 ) were observed in ethanol a s solvent. The best yields of product were obtained on irradiation at 335 nm for 24 h. When acetonitrile and cyclohexane were used only the adduct ( 8 7 ) was produced. 32 Cyclization in a constrained system

is also

found

in

the

irradiation

acetone- or acetophenone-sensitized

of

the

lactone

(89). The

irradiation of this lactone

results in intramolecular addition. Two products (90) and (91) are obtained arising from a common biradical intermediate ( 9 2 ) .

W 2 : Etione Cycloadditioris nird Rearratigernrrirs

109

Me

Me” QR Me”L) (84)

P

C02Et

(87)

Me

Me (89) a; R’ = C02Et, R2 = Me

b; R’ = Me, R2 = H

Me

PhorochP I N istty

110 Two reaction paths are available

to this intermediate and

it

either undergoes ring closure to yield ( 9 0 ) or else reacts by a hydrogen abstraction pathway to yield the ene product (91). The ratio of the products obtained varies due to stabilising effects 33 which the substituents exert on the biradical.

Several examples of intramolecular cycloaddition in enones have been reported. Becker and his coworkers have examined the influence

that

point

of

attachment

plays

in determining

the

outcome of the reaction. Thus the irradiation of (93) affords both the isomeric compounds ( 9 4 ) and ( 9 5 ) in the ratios shown. The reaction clearly proves that there is a preference for the retention of the stereochemistry at the terminal position of the double bond. This specificity is the result of the formation of the biradical ( 9 6 ) formed during the initial addition step. 34 The photoreactions

of the enone

( 9 7 ) are related

to this and the

structures of the products obtained w e r e established by n.m.r. s ~ e c t r o s c o p y .The ~ ~ photochemical cyclization of the enone (98) at 3 5 0 nm in hexane affords a 74% yield of the adduct ( 9 9 ) as a single diastereoisomer. The chemical conversion of this compound ( 9 9 ) into Ginkolides A and B was postulated. The authors suggest that this is the first time that a furan has been used in such an intramolecular cycloaddition.

36

Intramolecular photoaddition in the enones (100. 101) has been studied. The photochemistry observed this

system

and

yields

the

is conventional

(2+2)-adducts

(102)

and

for

(103)

respectively. It is of interest to note that the reactions of the enone

(104) yield both

the adducts

(105) and

(106). This

is

evidence that the bond forming process involves the fi-carbon of the enone

producing the intermediate biradical (107). In this

particular

case ring opening of the cyclopropane

to give the

111

IU2: Etiotie Cycloadditiotis atid Remratigtwwtits

ratio of products

35 : 1 5.8 : 1 3.5 : 1

*q&

C02Me

OCH20CH3

(97)

(99)

(98)

C5H1 1

5(J

R (104) R = H or M e

R

(105)

112 biradical ( 1 0 8 ) occurs in competition with normal ring closure. The

biradical

f~rmation.~'

( 1 0 8 ) affords

the

(106) by

adduct

C-C

bond

The influence o f polarization of the enones (109-

111) in the determination of the type of addition which occurs has been assessed. The rules obtained from the study show that

= 3 ( i .e.

when n

a chain of 3 atoms separating the enone from the

vinyl ether double bond) only

(2+2)-heed-to-head cycloadducts

(112) are formed. In this instance no effect of polarization of

= 4

the vinyl ether is apparent. When n

the same rule applies

although the quantum yields f o r the formation of (113) are lower. Only when n

=

2 is polarization important and this affords the

adduct

(1141, by 38 addition.

crossed addition, and

(115) by head-to-head

A total synthesis of (-)-perhydrohistrionicotoxin (116) has been

reported.

This

synthesis

utilises

the

intramolecular

cycloaddition of the enone ( 1 1 7 ) to sfford the product ( 1 1 8 ) by a head-to-head addition. This is achieved by irradiation of (117) in acetonitrile with a P y r e x filter. The yield of product is 95% and only about 5% of an epimeric product was detected. The adduct (118) is

then

chemically

elaborated

to afford

( 1 1 6 The ~ ~ ~

photocycloaddition reactions of other nitrogen substituted enones have

been

studied. Thus

the

enones

(2+2)-cycloaddition on irrediation

(119) and

in methylene

( 1 2 0 ) undergo dichloride

or

acetonitrile. The cyclobutane ring formed in these reactions is unstable and undergoes ring opening to afford the final products (121) and (122) respectively in the yields shown. When a shorter chain length separates the enone double bond from the alkene component

an

alternative

reaction

path

is

followed.

Thus

irradiation of ( 1 2 3 a ) under the same conditions affords the enone (124a). This is produced v i a the biradical (125) which fails to form a cyclobutane and instead fragments by the rupture of the

W2: Etiorie Cycloadditioris atid Rrarruugrnwtits

113

0

0

R'

Q? R2

(112)X=CH2,R'=OMe,R2=H X = CH2, R' = H, R2 = OMe X=O,R' =R2=H

(1 13) R' = OMe, X = CH2 R'=H,X=O

C5H11

&

c5H1 1

0

C+l

0

0

H o

Photocketnistry

I14

H

H

(120) a; R = H b;R=Me

(1 19) a; R = H

b;R=Me

(121) a; 70 O h

b; 60 %

&

Me

Me

(1 22) a; 75 O h b; 80 %

(123) a; n = 1, R = H bin = 2 , R = H c ; n =2,R=Me

MMe eQ

M Me e f l

NHR

N

R (1 24) 57 Yo

ao&

Me2

Me

/

Me

CMe2

H H

I

Me (127)

Tbc

HO

C-N

bond

labelled "a".

In the

case

of the irradiation of

(123b, c) the reaction also yields the enones ( 1 2 4 ) but these are accompanied

by

the

cyclized

products

(126)

and

(127)

r e s p e ~ t i v e l y .A~ ~study of the photoreactivity of a series of enones related to (128) has shown in all the cases examined that the

(2+2)-adduct

( 1 2 9 ) was

formed.

However,

the

reaction in most cases was simple Z-E-isomerization.

A

review

has

highlighted

the

principal 41

topochemistrg

(2+2)-cyclo additions in the pdi-(cinnamoy1)benzene

of

the

(130).42 The

(2+2)-dimerization of the arylpropenoic acids ( 1 3 1 ) and (132) to yield truxinic and truxillic derivatives has been reported. The irradiation of afforded

a mixture

of

( 1 3 1 ) and

( 1 3 2 ) as a thin film

three cyclodimers in a total yield of 16%. Under the

same conditions a thin film of ferulic acid (132) afforded a low yield of a dimer (2%) while irradiation of ferulic acid in the crystalline state failed to yield products.

The interest in

dimers from these acids arises from the fact that such dimers are present

in

graminaceous

cell

walls. 43

Int ram0 1ecul ar

(2+2)-c~cloaddition occurs on irradiation of

the dicinnamoyl

compound (133) using light of

1 > 300 nm. Under these direct

irradiation conditions the yield of the corresponding cyclobutane derivative is 31.6 %.

T h e cyclobutane ring can be cleaved to

afford the starting material by irradiation at 220 nm.

2

44

Bearrangement Reactions

a.8-Unsaturated

Systems.-

A review has discussed

the photo-

chemical rearrangements of a.9-unsaturated enones .45 A study of the photoactivation and deactivation of thrombin by the ester (134) has been carried

out. The model

system

(135) was also

examined with a view to obtaining more mechanistic details of the

Photoch rtnistty

C02Et Et2N (1 35) a; R = Me b;R=H

“‘“rnO Me

\

Me

W2: Etiorie Cycloadditiotisarid Rearmri~c.raotrs

I17

interaction. The irradiation of the ester (135a) at 366 nm brings about trans-cis-isomerization and the resultant photostationary state contains 60% of the cis-isomer. Irradiation of (135b) also effects the isomerization and in addition the lactone (136) is formed.46

A flash photochemical study of the behaviour of the

ynone (137) has been reported. Apparently the triplet state of (137) is long lived. Decomposition of the ynone takes place on irradiation in cyclohexane but in benzene the compound (137) is unreactive.

47

The irradiation of the trans-enone (138) in B-cyclodextrin brings about the formation of the cis- and trans-cyclopropane derivatives

(139). The

mechanism

for

the

formation

of

the

compounds was discussed. Interestingly the reaction only occurs in the

cyclodextrin medium

and

in its absence

the starting

material merely undergoes trans-cis-isornerization.The influence of the cyclodextrin substrate on the outcome of the reaction must be due to restricted movement within the cavity which prevents the trans-cis-isomerization. 48 Irradiation of (140) in benzene or

toluene

photon-one

under bond

sensitized

isomerization

typical example of

the

conditions as

the

brings

principal

results obtained

are

about

one

reaction.

those with

A

the

sensitizer zinc octaethylporphyrin which affords the following: 13-cis- f 19.3%) , 49 7 - c i s - ( 0.2%).

9- c i s - ( 5.7%)

,

9.13-di-cis- ( 1.5%)

and

A n account of the facile photocyclization of the ionones ( 1 4 1 ) to afford high yields of the dihydropyrans (142) has been

reported.

Different reactivity is recorded on irradiation of

the dienone (143). Usins

1 > 347 nm EIZ-isomerizstion is the

principal reaction. The isoaerized product is transformed into the enol ether ( 1 4 4 ) . Using shorter wavelength light (

>

280

Photocherni s t y

1 I8

MeMe

Me Me

Me

WMe

Me

Me

(146)

(144)

(148) R = COMe or CN

cR

Me Me

Me

Me

(151)

yield (%)

(153) a; 25 b; 10 c; 5 d; 26

0 (154) R = Et, Me, H, Pi, PhCH2,But, C6Hl,CH2, PhCH2CH2,or PhCH2CH2CH2

W2: Etiorie Cycloadditiotisorid Rearrarigeinrrlts nm or

1 I9

1 = 254 n m ) three new photoproducts (145). ( 1 4 6 ) and ( 1 4 7 )

are formed by cyclization or hydrogen migration steps. 51 Direct irradiation of the enone ( 1 4 8 , R ring expanded compound (149, R

=

=

COMe) at 254 nm yields the

Y e ) as the main product in 40%

yield. Similar behaviour is observed for the nitrile ( 1 4 8 , R (149, R

CN) when

= CN)

=

is obtained. The process presumably

involves the photochemical generation of the slid ( 1 5 0 ) as the key intermediate. Irradiation in alcohols e.g. methanol provides

a route f o r the synthesis of the ketals ( 1 5 1 ) .

Continued

interest

has

been

shown

52

in

de-conjugation

reactions and the possibility of bringing about enantioselective processes. In this regard Pete and his the

photochemical

have studied

of

deconjugation

the

(152)

lactones

by

irradiation at 254 nm. Deconjugation results in the formation o f the 0,y-unsaturated

lactones

(153) in the yields

shown. The

esters (154) were also studied and deconjusation was found to be more efficient with yields around 70% being achieved. Reactions in the presence of ( t ) - or

(-)-ephedrine were also studied and

an enantiomeric excess of 30% was achieved in the process. Irradiation of conditions

the

ionone

affords

a

derivative

mixture 54 deconjugated enone (156).

of

( 1 5 5 ) under

products

53

sensitized

containing

the

The photochemical rearrangement of (157a) into the bicyclic compound

(158) in

acetone

of methanol

is reported

to

be

a

di-%-methane process involving interaction with the benzyl group on the O-carbon atom. The compound (157b) does not follow this reaction path but instead hydrogen abstraction reactions such as the addition of solvent take place? thiophenes

(159) and

The annulated tetrahydro-

( 1 6 0 ) respectively

are

obtained

irradiation of the enone (161) in methanol containing

on the

Piiotociwrnistrv

120

OR

OR

(155) R = H, TBDMS or BOM

rn

MeO,C Ph

Me

H

H

iV2: Etiorie Cycloadditiotis atid Rearrarigt.r?ierrts penta-l14-diene

in

or

121

alcohols. 5 6

allylic

Details

of

photochemical ring opening reactions of the thiophenones

the (161)

yielding the esters (162) have been described. Further details of

ring closure

irradiation

of

reactions the

to

(159. 160) were discussed.

thiophenones

(161, R

=

Me,

The

alkyl,

or

propargyl) in the presence of 2,3-dimethylbut-2-ene affords the 57

cycloadducts (163).

The photochemical reactions of phenylethynyl phensl ketone in cyclohexane have been studied. The product from the reaction was 3-phenylindanone ( 1 6 4 ) obtained in 14% yield. Other products such as chalcone (1.5%), phenylacetylene (10%) and benzaldehyde ( 9 % ) were also formed.58 The enone (165) is converted into the

3,4-benzotropone (166) o n irradiation in an EPA glass at 77K 59 using Pyrex filtered light.

The photochemical deconjugation of the enone (167) into (168) has been studied. Apparently the reaction is affected by the presence of acid. Thus at low concentrations of acetic acid ( 4 x 10% converted

the excited into the

singlet state of the enone

trans-isomer

(167) is

(169) and i t is this species

which is chemically converted by acid into (168). The presence of the isomer (169) was demonstrated by irradiation of (167) in benzene

methanol

when

the

ether

(170) was

isolated.

The

conversion of (167) into (168) is concentration dependent and at a concentration of 0.111 9 = 0.044 while at 1.0111 = 0.035. 60

Scheffer

and

his

coworkers

have

continued

their

investigations of the influence which phase exerts on the outcome

of photochemical hydrogen abstraction reactions of enones. This work is exemplified br the difference in behaviour of the enone

(171). In solution irradiation yields the hemiacetal ( 1 7 2 ) while in the crystalline

phase photoreaction

brings about hydrogen

abstraction by the $-carbon of the enone f r o m one of the benzylic sites.

Bonding

within

the resultant 61 polycyclic compound (173).

biradical

affords

the

Two reports have dealt with the photochemical behaviour of the dienones ( 1 7 4 ) and (175). Zimmerman and Larers6* have shown that on afford

irradiation a

mixture

( 1 7 4 ) rearranges

of

isomers

of

by

phenyl migration structure

gross

to

(176). The

secondary photochemistry of these compounds was also studied. A detailed photochemical study of the rearrangement of (175) has 63 shown that direct irradiation yields products (177) and (178). The

reaction

is

reminiscent

of

the

behaviour

reported

by

Zimmerman e t a 1 . 6 2 for the parent system.

The

dimethylaminocoumarin

photochemical

reactions

with

a

(179) series

was of

subjected

alkyl

iodides

to and

bromides. The products formed from this treatment were the 3alkyl

derivatives 64 reactions.

(189)

formed

by

free

radical

addition

The photochemical reactivity of testosterone on irradiation in

the

solid

state

has

been

studied.65

The

testosterone

derivative (181) is also photochemically reactive in the solid state producing

the dimer

( 1 8 2 ) in 10.4% yield. The reaction

presumably takes place by hydrogen abstraction by the @-carbon of the enone from the allylic site in another molecule.

The 66 combination of the resultant radical pair affords the product.

The transformation of ( 1 8 3 ) into ( 1 8 4 ) by photohydration has been reported.

67

II.2: Etione Cycloadditiotts atid Rearrarigernents

123

Me Me

#OH

d}

0

OH

124

Pliotoclitmistty The ketone

(185) o n irradiation

through Pyrex

in hexane

saturated with oxygen affords the diketone ( 1 8 6 ) in 40% yield.

A study of the photoreactivity of the optically active compound ( 1 8 7 ) in degassed another

reaction

hexane path

gave

yielding

evidence the

for

the operation

trans-isomer

(188) in

of a

photostationary mixture (ratio of 4:l) with the starting material

(187).The composition of the mixture from the irradiation could be altered by the inclusion of a triplet quencher whereby after 3h irradiation the mixture of trens:cis was 8:l and the mixture had

a

composition

of

50:l

trans:cis

after

6h

irradiation.

Prolonged irradiation of either the trans-(187) or cis-(185) gave two new products identified as the ketone (189) and the aldehyde ( 1 9 0 ) in 30% and 6% yield respectively. The route to the products was thought to involve initial fission of the C-l,C-12 bond to afford a biradical. This biradical can be trapped by oxygen and reduction then results in the formation of the diketone (186). In the absence of oxygen rebonding will afford the mixture of cis-(185) and trans-(187) or the ketone (188). This compound on secondary photolysis will yield both ( 1 8 9 ) and (190) by Norrish Type

I processes. Analogous cis-trans-isomerization occurs with

the ketone

(191) affording

( 1 9 2 ) in 37% yield. The secondary

photolysis affording (193) and (194) is inefficient in this case giving yields of 5% and 2% respectively.

68

The irradiation of Bentazone (195), a contact herbicide, in hexane yields the rearrangement product (196). The mechanism by which this conversion occurs is unknown.

69

f5,y-Unsaturated Systems.- Irradiation of the methyl enone (197) affords

the

fragmentation 1.3-acyl

1,3-acyl

migrated

products were

migrated compound

compound

obtained.

The

(198).

In

addition

irradiation of

the

( 1 9 8 ) does not reform the starting

W 2 : Eiiom Cycloadditioru arid Rearmrigernerits

125

Me0

Me0

Me

Me

Me

Me0

Me0

CHO

Me (1 89)

0

t;' CHO (194)

0 (195)

Me

Ph otocliemist ty

I26

material and only products of fragmentation were detected. T h e alternative mode for rearrangement of such enones. the oxa-di-xmethane

process,

is

observed

on

irradiation

of

the

phenyl

analogue ( 1 9 9 ) which gives a low yield of the cyclopropane ( 2 0 0 ) . The difference

in reaction mode

shown by

( 1 9 7 ) and

( 1 9 9 ) is

presumably due to a difference in multiplicity with the triplet state required for the oxa-di-x-methane process populated only on irradiation of the benzoyl

keto group in

( 1 9 9 ). 7 0 Similar

changes in reactivity dependent o n multiplicity are also shown by the enone (201). Thus direct irradiation of this enone (201) in t-butanol brings about conversion product

to the 1,3-acyl migrated

( 2 0 2 ) from the singlet state and

the oxa-di-n-methane

product (203) from the triplet state. Acetone-sensitization also affords the latter compound (203) as well as the acetone adduct ( 2 0 4 ) . The involvement of the singlet state in the formation of

the migration product ( 2 0 2 ) was verified by irradiation of (201) in the formed.

presence Compound

of

a

triplet

(202)

quencher

is also

when

(202)

photochemically

was

again

reactive

and

acetone-sensitization brings about the oxa-di-n-methane process to

yield

(205).

Enone

irradiation yielding

(206) is

also

the migration product

reactive

on

direct

( 2 0 7 ) . The authors

have analyzed these reactions in considerable detail and have proposed that the stereochemical

outcome of the 1,3-migration

reactions are dependent upon the ground state stereochemistry of 71 the original enone.

Gandhi et al.72 report that the irradiation of the lactams ( 2 0 8 ) brings about a 1,3-migration of benzyl or ally1 groups from

nitrogen to carbon to afford the derivatives (209). Irradiation o f the N-methyl derivative follows a different path end yields the aziridine derivative

(210). The formation of this compound

could be considered to arise by a n ora-di-c-methane

process or

W 2 : Etrotre Cydoadditions arid Rearratrgetnetits

(214)

127

I fission and rebonding.

by Norrish type

The photochemical rearrangement of the enone (211) provides a route to the previously unknown noradarantanone (212) vie the oxa-di-=-methane

process. 73 The oxa-di-u-methane

mode is also

observed on irradiation of the enone (213). This reaction path provides a reasonable route to the triquinane (214). The enones such as (213) are readily prepared by the addition of dienophiles to

the

(215).7 4

cyclohexadienone

The

triplet

sensitized

irradiation of the enone (216) affords the tetracyclic ketone (217) via

an

efficient

starting 75 synthesis of the triquinane (218).

compound

was

used

oxa-di-=-methane

as

the

rearrangement. This

material

for

the

first

The tricyclic enone (219) has been synthesised successfully. The direct irradiation of this compound results in the formation of the two products (220) and (221). These are formed from the two intermediates (222) and

(223) produced as a result of a-

fission in the starting material followed by rebonding. When the reaction is carried out in methanol the formation of (220. 221) is suppressed and the labile methanol adduct (224) is produced by trapping of the ketene (222).76 A 1.3-ally1 migration occurs on the irradiation of the thio ketones (225) in benzene solution.

This process yields the isomeric allylthioindoles (226) in yields which can be as high as 65%.

77

The irradiation of the ionones (227) at 1

>

254 no at room

temperature affords the three ethers ( 2 2 8 ) , (229), and (230). The products (228) and (229) are the result of oxetan formation by way of the two possible addition modes of the carbonyl group to the ring double bond. The other product (230) arises from the biradical

(2311, again an oxetan

forming mode. but

hydrogen

129

W2: Eitoiie Cycloadditiorts arid Rearrartgernertrs

4

0

0

(225)

(226)

R1 = Ph, Me, Bu; R2 = Me, Ph; R3 = H, Me

PM OR

(227) R = H, Ar or Pt

(228)

SR Photochfinistry

130 PMe Me M

e

R2 “-Me

Me

OR

OR

(232) R’ = Me, R2 = OH

(2311

(230)

(235)

(233)

R’ = H, D, or Me

TMS

TMS

I

I

EbN/MeCN

(239)

“U-Scheme 3

W2: Etione Cycloadditionsarid Rwrrarigrmcwts abstraction

competes

with

ring

131 closure

and

yields

(230).

Irradiation at lower temperatures yields the ethers ( 2 3 2 ). 7 8 The irradiation of the lactone (233) in alcoholic solution using 254 nm light has been studied and products such as ( 2 3 4 ) and ( 2 3 5 ) are obtained. These compounds arise by two paths following the initial C-0 bond fission in the lactone to yield the biradical (236). This intermediate can decarbonylete to yield (237) from which the ethers (235) are formed. An alternative process is the fragmentation of (236) by loss of ketene. This yields the enone (238) which reacts with solvent

.”

(234)

process.

Earlier 8 0 , 81

Unsaturated

work

had

aldehydes

also

to give the dihydrofuranones reported

such a s

on

(239) have

this

reaction

been shown to

undergo a photo-electron transfer induced cyclization to afford a mixture of alcohols ( 2 4 0 ) (scheme 3). This use of aldehydes has

been extended in the development of a route to spiro compounds. Thus

the

irradiation

transfer conditions

of

the

aldehydes

affords a mixture

( 2 4 1 ) under of products

electron ( 2 4 2 ) and

(243). The details of this a r e recorded in table ( 1 ). 8 2 Pete and his coworkers have made use of their reductive cyclization of unsaturated

ketones

in

a

synthesis of hirsutene

(244). The

reaction involves the irradiation of the ketone ( 2 4 5 ) at 254 nm in the presence of triethylamine whereby the tricyclic compound (246) is formed in 58% yield. This is subsequently transformed 83 into hirsutene.

Table ( 1 ) Products from the Irradiation of enals (241). Compound ( 2 4 1 )

Products (242) and (242)

n 2 1 2

trsns:cis ratio

m 1 2 2

90: 10

low selectivity 58 : 4 2

yield ( X I 65 40 24

82

Photochp i n istw

I32

w

H

H

H

H

H

(245)

(244)

3-indolyl

Me

Me

Me

(249)

(248)

(247)

H H

R

C02Me

Meo2c$JJ

O

b

H (250) R = 1, 3-dimethyluracil

(255) n = 2-6 or 12

C02Me

Meo2cT I

I

Me

Me

W2: Etiotie Cycloadditiotis mid Rearratigetntwrs 3

133

Photoreactions of Thymines etc.

Irradiation of the uracil derivative

(247) in solution in the

presence o f indole yields only uracil

(248) by photochemical

decarbonylation. In the solid phase, however. irradiation yields the derivative (249). This is thought to be formed by nay of the (250) as

oxetan

the

reactivity of uracil

intermediate. 84

key

The

photochemical

(248) in alcohols is well known and the

products from this have been identified p r e v i o u ~ l y .A~ ~recent report has duplicated these results but has also isolated two new products from the reaction in ethanol. These have been identified as the dihydropyridine derivatives ( 2 5 1 ) and (252). The mechanism

by which these products is formed is not known.86 The azathymine (253)

is photochemically

reactive in its triplet state. When

reaction is carried out in acetone the new heterocyclic compound (254) is formed in 66% yield. The route to the formation of this product is thought to involve trapping of ground state acetone by the triplet state of the thymine followed by reaction of the 87 resultant biradical with another molecule of acetone.

Intramolecular

cycloaddition

derivatives (255) have (255) as adducts

lo-* (256)

to

been

reactions

studied.

in

the

thymine

Irradiation at 365nm of

M solutions leads exclusively to the

regardless

of

chain

lenpth.88

The

isomeric

compounds (257) were also studied. The photoreactivity of these did not yield products of photoaddition but instead the compounds ( 258) were obtained by opening of the pyrone ring,89 2-Thiouracil

( 2 5 9 ) is photo- chemically reactive in the presence of alkenes. The reaction appears to be dependent on substitution and some alkenes yield products from reaction at the thiocarbonyl group

134

0

(257) n = 3-6or 12

(258)

0

0

N‘LR1 R2

HNKNR1 2R2 ; +J

R2

H (260)

(261) R’ = R2 = Ph or Me R’ = R2 = p -tolyl or Me R’ = R2 = Ph or Pr

(262) R3 = H (263) R3 = CHNMe2 I

Me

(265) R’ = R2 = H or Me

0

0

(266) R’ = H,R2 = CH20H, OEt

or C02Me

R3 = R4 = Me

II/2: Enotie Cycloadditioris orid Rearratigetnrrits

I35

as well as forming (2+2)-cycloadducts by addition to the C - 5 . C - 6

double bond. The thioquinazolinedione ( 2 6 0 ) only yields products by attack at C - 2 .

90

The pyrimidones (261) undergo reduction o n irradiation in the

presence

of

amines.

Thus,

for

example.

irradiation

in

triethylamine affords the reduction product (262) and the amine addition product ( 2 6 3 )l'.

Further studies on the conversion of

pyrimidines ( 2 6 4 ) into the corresponding Dewar systems has been reported. 92 Photochemical addition of alkenes to the pyridazinone derivatives

(265) yields the adducts (266). These are photo-

chemically reactive and are converted readily into the tricyclic compounds (267). The rearrangement encountered in the reaction involves a 1.2-bond migration, resulting in expansion of the four membered ring, followed by bond formation within the resultant 93 biradical.

4

Photochemistry of Dienones

Cross-conjugated Dienones.-The extended dienones (268) and ( 2 6 9 ) have been s t u d i e d e g 4 The irradiation of (268) yields only the phenyl migrated compound

(270). When the triphenyl derivative

( 2 6 9 ) was irradiated the usual cyclization path occurs analogous to

that

reported

many

ago

years

diphenylcyclohexa-2,4-dienone

by

for

the

Zimmerman

parent

et

a1

.''

4.4-

The

zwitterion developed during the cyclization is trapped by the solvent methanol to yield the ether (271). This compound is Prone to attack by oxygen. T h e irradiation of the cyclohexadienone (272) at 366 nm at -78OC in pentane yields only the cycloadduct

(273) with

no

evidence

for

the

formation

of

the

other

diastereoisomer. The introduction of a methoxy substituent at C-3

Pliotochernistry

I36

8

OH

0

OH

QPh h Ph

Ph Ph

Ph (271) R = Me0

(270)

Me0 Me Me

A Me Me

(274)

(273)

0

0

Me0 Me @O2Me Me Me

Me (277)

(276)

0

BukBut pJ 0

BuQBut

Me@2R1 /

(278) R = H, Me or P t

(279)

(280) R' = OCHzPh, R2 = CH20H R' = OMe, OCH2Ph, R2 = Me

apparently

influences

the

regioselectivity

exhibited

in

the

cycloaddition. Thus the irradiation of a mixture of (274) and ( 2 7 5 ) affords the two adducts (276) and ( 2 7 7 ) with a preference for the formation of (276).96 The photochemical or ground state ring opening of the spiro compounds ( 2 7 8 ) affording ( 2 7 9 ) has 97 been studied.

Linearly

Conjugated

hexadienones

Dienones.-

Irradiation

of

(280) in the presence of morpholine

the

cyclo-

affords the

adducts (2813 by the usual ring opening. ketene formation, and trapping route. The author suggests that these products may be 98 of value in the synthesis of phomenoic acid and phomenolactone.

A detailed description of the synthesis of (+)-Aspilicin

(282)

by photochemical intramolecular lactonization as a key step has been published.

99

Irradiation ( A

>

350 n m ) of the tropone derivative ( 2 8 3 ) in

chloroform at -6OOC affords the adduct ( 2 8 4 ) in 41 X yield. This compound was then chemically

converted

into dactylol

marine sesquiterpene obtained f r o m Aplysia dactylomela.

(285) a 100

A report o f enantiocontrol of photochemical reactions of tropolone published.'"

ethers

in

prochiral

guest

complexes

has

been

A free radical path is reported to be involved in

the conversion of the borate complexes of tropolone (286) into the products shown in scheme 4. These compounds are isolated after oxidation and methylation

of the reaction mixture. The

irradiation at 366 nm of benzene solutions under an atmosphere of Argon is presumed to produce the radical pair (287) from which the products are formed.

102

The enone (288) is photochemically reactive and undergoes

138

Photochetnistry

0. ..OH

A 0 N-COCHMeCH=CR%H=CMeCH2R1 W

"'OH

(2811

Me

R = n-Butyl R=c-CBH~~ R = ally1 R = benzyl

22 24 20 18

6 4 5 7

6 8 11 9

1 1 2 : 1 1 6 1 18 : 1 9 :

Scheme 4

0

0

0

6 4

-

IU2: Ettorie Cycloaddirioris arid Rearrangernem

139

the addition of maleates. acrylates and crotonates to afford the 103 (2+2)-cycloadd~ct~ (289).

5

The

dione

1,2-,

1,3-. and 1.4-Diketones

(2901, on

dihydrofurans (291, R

irrediation

= H

in

the

presence

2,3-

of

or M e ) , yields two products identified

as (292) and (293).lo4 The reaction producing ( 2 9 2 ) could involve decarbonylation followed by trapping of the resultant biradical by the dihydrofuran. The route to ( 2 9 3 ) could be more complicated and involve either a concerted process or else a diverted oxetan forming

reaction. Oxetans are often

Pavlik and Farahat"'

formed by diketones

and

have reported the formation of an oxetan

from the photochemical addition of pentane-2,4-dione end methyl acetoacetate

to

1-methylpyrrole.

The

oxetan

adducts

formed

initially are unstable and undergo photoelimination to yield 3acetyl-1-methylpyrrole. Other additions were also described. A study of

the photochemical

addition of alkenes

(294) to the

trione ( 2 9 5 ) has been carried out. Thus with stilbene (294n) in benzene solution the excitation involves the stilbene and its interaction with the trione to yield the oxetan (296a), The other alkenes used

in the study apparently

involve a ground state

complex of the alkene and the trione. The excitation of this again affords

the oxetans

(296b-e). In the case of the 2.3-

dimethylbut-2-ene hydrogen abstraction is important and gives the isomeric products (297) and (298). In alcoholic solvents such as methanol different reactivity is observed. Typical of this is the behaviour of 1,l-diphenylethylene which yields the products (299) and ( 3 0 0 ) shown in scheme 5. The alcohol incorporation products are thought to arise from an electron transfer process a s shown in scheme 6.

106

Photochemistry

140

0

Ph

H

(290) E = C02Et

(291) R = H or Me

(293)

(292)

Me

Me I

Rg4 R2

R3

(294)

(295)

a; 95 Yo b; 81 Yo c; 21 Yo d; 78 Yo e; 50 Yo

Me

(298) 13 Yo

(297) 11 Yo

Me

OH (299) 68 Yo

Scheme 5

Me

Me (300) 20 Yo

141

lU2: Eriorir Cydoaddiiions arid Rrarrar1grmrtit.s

The irradiation of the dione ( 3 0 1 ) in methanol yields the furan derivative (302) presumably by way of the carbene (303) and trapping

by

solvent.

photochemistry examples) .lo'

(305) and benzene

of

Such

ring

expansion

cyclobutanones

(see

is

common

Chapter

in

the

l/III

for

The diketone (304) is converted into the products

(306) on irradiation using light of

solution. T h e products

are

thought

1 > 340 nm in

to be

formed

by

Norrish Type I fission and mono-decarbonylation. This affords a biradical which either closes to the ketone ( 3 0 5 ) or undergoes further decarbonylation to yield the dihydronaphthalene This

compound

is

not

isolated

and

either

oxidises

(307). to

the

108

naphthalene (306) or ring opens to the divinyl benzene (308).

(309)

The thiodiones

are photochemically

reactive in the

presence of alkenes and yield oxetans and thietanes. lo'

Yoshioka

e t al.l10 have studied the outcome of the irradiation of the 1.3-

diketones yields

(310). In hexane solution using a Pyrex filter good

(44-66%) of

formation reaction ketone

of

the cyclobutanols

these

compounds

(311) are obtained. The

proceeds

a Norrish

by

Type

to yield an enol. The reversion of the enol

is proposed

remaining

carbonyl

formation

of

the

I1

to the

to be retarded by ,hydrogen bonding to the group.

Cyclization

observed

products.

then The

results

results

thermolysis of the compounds (311) were reported. 'lo

in

the

from

the

The keto-

esters (312) on irradiation undergo a SET from nitrogen to the keto group. This is followed by hydrogen abstraction to afford the products (313). When the chain separating the ester 0 and the N-group

is Cg or C 4

compounds

(314)

in

a n alternative low

yield. '11

reaction The

path gives

irradiation

of

the the

monothioimides (315) affords the products shown in scheme 7. The principal product in each case is the bicyclic compound ( 3 1 6 ) and this is formed via the biradical (317) produced by interaction

Photochemistry

142 Me

+

Ph

*

Ph Scheme 6

&

&o /

wo

0

/

/

/

/

/

PP

PP

PP

Prn

PP

PP

(306)5 %

(305)23 Oo/

(304)

Y

Pr"

Prn

(309)X = S,Y = 0 or S

(308)70 Yo

(307)

X=O,Y=S

R'

R2

R3

H H H Me

H Me H

H H Me H

H

W2: Eiione Cycloadditioris and Rearrangements

n=2;

n=3; n=4;

I43

R1

R2

R3

H H Me H H H

H Ph Ph H H H

H Ph Ph Ph H Ph

n

R

1 1 2 2 3 3

H Me H Me H Me

% yield

24 61 71 56 38 46

31 34 trace trace trace trace

Scheme 7

RO *N-

Me

/

0 (320) a; R = H b; R - R = (CH=CH)2

144 of

Ph otochet r ist y the

thio group with

the alkene moiety. A

subsequent

hydrogen shift affords the main product of the reaction.

1,4-

112

The photochemical reactivity of phthalimide and N-methylphthalimide with silyl compounds ( M e SiCH R , R = OEt, SPr, NEt2) 3 2 in acetonitrile or methanol has been studied. The reaction encountered

is

a

hydrogen

abstraction

process

to afford

the

derivatives (318) which on dehydration yields the isoindolones ( 3 1 9 ) . '13

Details

phthalimides

of

the

photochemical

behaviour

of

the

( 3 2 0 ) in the presence of phenylcyclopropane

have

been published and shown to involve electron transfer processes. Irradiation of (320a) in methanol affords the adduct (321) while irradiation in acetonitrile yields the isomeric cyclized products ( 3 2 2 a ) and

(323a). Analogous

formed

the

on

products

irradiation

of

the

(322b) and

( 3 2 3 b ) are 114 (320b).

phthalimide

Irradiation of N-methylphthalimide in the presence of the indoles ( 3 2 4 ) affords the (2tZ)-cycloaddition products ( 3 2 5 ).l15 Intra-

molecular

addition

irradiation

of

of

the

this

The

reactive

in

isomeric this

mode

is

phthalimide

(2+2)-cycloaddition gives acetone.

type

also

reported

derivatives

from

(326).

the The

the oxetans ( 3 2 7 ) o n irradiation in

phthalimide and

instead

deacetylation to yield (328b).

derivative irradiation

(328a) brings

is

not

about

116

Yazzocchi and his coworkers have studied the cyclization of the phthalimide derivative (329) yielding the two compounds (330) and (331) in a ratio of 52:48. These products are a n essential step in a route to pyrrolo[l,4]benzodiazepine antibiotics. 11' photochemical

The

cyclization of

thiophthalimides with a n N-side 118 chain affords the cyclic derivatives (332) and (333).

The regioselective reduction of the phthalimide derivative

W2: Enorie Cycloadditioris arid Rearrangernetits

@

M

~N-Me

$&,R

/

145

N-Me /

R

0

N-Me

R$

R

/

0

0 (323)

0 R3

\

R' \

R'

(324) R' = C02Me, R2 = H, R3 = Me or R2 - R3 = (CH2)4or (CH& R' = COMe, R2 = H, R3 = Me or F? - R3 = (CH2)4 or (CH2)5

(325)

0

AC (326) n = 2-5

(327)

0

mv2)"-N33 \

R

0

n =2or3 (328) a; R = Ac b;R=H

R'y$&

R' R2 O *N\

R*

0

-

(329) R' = MeO, R2 = PhS03

0 (330) R' = PhS03, R2 = Me0 (331) R' = MeO, R2 = PhS03

146

Photochemistry

dn

Ph

Ph

( + /J $ %

/

0

0

(332)I/ = 1-4

(333)

@

:Me

M e& ; \

0

(335)

(334)

/

gNMe (336)

(337)

\

@!Me /

/

0

flp (339)

*

Me Me

hv

-

+

Me

0

/

16 Yo

35 %

(336)

Me Me

Scheme 8

+ @ M\e

/

dimer

35%

147

II/2: Eriorir Cycloadditioris arid Rearratigrtntwts (334) has been reported to give the derivative (335).ll'

A dimer

is formed on the irradiation of the phthalimide derivative (336) in benzene using b addition

to

the

>

360 nm. The process involves (2+2)-cyclo-

9.10-bond

of

the

phenanthrene

moiety.

Both

alkenes and dienes are reactive and undergo addition to this phthalimide (336). Thus the photoaddition of 1,l-diphenylethylene yields the two products (337) and (338). These products arise by different

reaction

paths. The

former

is produced

by

a

path

analogous to the reaction of other phthalimides with alkenes similar to that described by Mazzocchi et el .ll' is

obtained

by

(2+2)-addition

to

phenanthrene. The use of heavy atom

the

while the latter

9.10-bond

of

the

solvents such as methyl

iodide has shown that this latter process arises from the triplet state while

the former

is a singlet state reaction. The two

adducts (337) and (338) are themselves photochemically reactive and irradiation of (337) yields the isoquinolinone (339) while (338) affords

starting

material

and

the

alkene. 120

Further

examples of the addition of alkenes to (336) have been reported and the results from the addition of 2-methyl propene are shown 121 in scheme 8.

Irradiation at 240 nm of the anhydride (340) in an Argon matrix

at

11K

cyclopropenone irradiation

at

affords (341). 240

nm

an This but

intermediate

identified

as

compound

stable

further

on

is

irradiation

at

to

1

>186

the

nm

decomposition results in the formation of CO, difluoroethyne, and the ketene (342).122 Irradistion of the anhydride (343) in the presence of 1.1-diphenylethylene oxetans. Oxetans anhydride

results in the formation of

(344) and (345) are formed when the isomeric

(346) is

irradiated under

the same

conditions

in

benzene under an atmosphere of nitrogen. However. in this case

Photochrmistiy

148

0 F F>C=-C=0

(343)

(342)

Ph Ph Ph

(344)15 %

(346)

(345)7 %

(347)50 %

(348)14 %

OH (352)

W2: Enone Cycloaddirions arid Rearrarigrmenrs

I49

the yields are low. 15 and 7% respectively, and the reaction is dominated by an alternative reaction path yielding the insertion products ( 3 4 7 ) and ( 3 4 8 ) in 5 0 and 14% yields respectively. No clear evidence for the presence of a charge transfer complex wes found. This reaction is reminiscent of the type of reactivity found

with

derivatives. 123

phthalimide

Irradiation

of

the

anhydride ( 3 4 9 ) in solution fails to yield a product but using 390 nm light in the solid converts the orange anhydride ( 3 4 9 )

into a white

crystalline

compound on irradiation

compound at

identified

as

(350).

This

300 nm. again in the crystalline.

phase. is converted back to starting material ( 3 4 9 ) . The process 124 can be repeated several times.

A study of the matrix-isolated photochemical behaviour of o-phthalaldehyde has identified t w o isomeric forms only one of which.

the

(351).

E-2-isomer

is

photochemically

reactive.

Irradiation of this at 313nm yields the E- and 2-isomers ( 3 5 2 ) and

(353). The

irradiation

at

&-isomer 357x110

can be into

the

photochemically phthalide

converted

(354).

accompanied by benzaldehyde and carbon monoxide, 125

This

by is

A micro-lab

experiment has been described for the trans-cis-isomerization of 1.2-dibenzoylethylene using sunlight. 126 Further studies o n the photochemical behaviour of the enediones ( 3 5 5 ) has been carried out. This report has dealt with the influence of bridgehead a w l substitution.

'*'

and undergoes

The trione ( 3 5 6 8 ) is photochemically reactive

a phenyl miqration on

yielding the new product

irradiation

in methanol

(357). T h e actual details of how the

1.5-migration occurs is not known but it could arise by either

a concerted process or by a bond fission recombination path. The diester (356b) is a l s o reactive and undergoes a n oxa-di-x-methane reaction to yield the product ( 3 5 8 ) .

128

Photochemisry

150

(354)

(353)

‘‘03 RCO

PhO

(356) a;R = Ph b; R = M e 0

R’

0 (359)

Ph

(357)

R’

o

a

(358) E = MeOCO

0 (360)

R‘=Ph,R2=Me;R1=Me,R2=Ph

e

0 (3611

151

W2: Errorre Cycloadditiorrs arid Rearrarrgrrnertts Griesbeck12’

has demonstrated

that the acetone-sensitized

irradiation of the Diels-Alder adducts ( 3 5 9 ) converts into the pentacyclic diketones (360). These compounds can be ring opened on thermolysis to give the tetraquinane derivatives ( 3 6 1 ) in high yield. 12’

(PtZ)-Cycloaddition

Diels-Alder adduct

(362. R

is

= H

also

reported

for

the

or CH OMe). The irradiation of

2

this converts i t into the cage compound (363). This new crown ether is photoresponsive and selectively complexes Rb’ when R Pandey diketones

et

al. 13’

(364) at

report

that

the

irradiation

1 > 300nm in alcoholic solution

efficient way for the formation of the monoacetals

of is

=

the an

(365). In

addition to this reaction they also observed that irradiation of the

diketones

carbonyl

brought

groups

to

about

yield

fission

the

of

the

rearomatized

bonds

6 to the

diketone

Isomerization of the ketone (366) to the exo-isomer 131 also observed.

(366).

(367) was

The quinone adduct ( 3 6 8 ) is photochemically reactive in the presence of tolan. The product isolated from this was identified as (369) presumably

v i a the intermediacy of e n oxete. Similar

behaviour was also demonstrated

for thioxanthone and anthrone

when the adducts ( 3 7 0 ) were isolated. Anthraquinone also affords a similar product which is itself photochemically reactive and is converted into the cyclized product (371). Spirooxetans (372) are obtained o n irradiation of thioxanthone and anthrone in the presence

of

reactivity

cyclohexene. 13* An of

the

quinone

account

derivative

of

the photochemical

(373) in benzene

with

alkenes and vinyl derivatives of pyridine afforded endo-adducts b s addition to the 1.3-dipole formed by ring opening of the C-C bond of the oxirane ring. These adducts were also photochemically 133 reactive and rearranged into the products (374) and ( 3 7 5 ) .

152

Phoroclianisrr?,

RR’

R

0

R’

(364) R-R = (CH=CH)2 R’-R’ = 0 (365) R’ = MeO, EtO, Prb, or CHzCH2

(366)

(367)

Yh

0

CPhCOPh

0

0

(370) X = S or CH;!

(369)

Me

0

0 (372) X = S or CH2

(371)

(373)

&‘ / /

0 (374)

(376)

(375)

OH

Ph (377)

W 2 : Etiotre Cydoadditiotis arid Rearratigetnetits 6

153

Quinones

A study of the reaction of benzoquinone with alkenes has sought to identify the nature of the transients involved. In the case of

reaction

with

tetraphenylallene

irradiation

in

carbon

tetrachloride at 355 or 532 nm yields the indene (376) as the sole product. When methanol is added to the reaction mixture the same indene ( 3 7 6 ) is accompanied by the methoxy derivative (377). The suggested mechanism for the formation of the indene (376) involves electron transfer to yield the radical cation / radical anion pair (378). Protonation followed by combination yields the cation

(379)

product. 134 lactones

which

is

Benzoquinone

(380) and

the

key

adds

intermediate

to

photochemically

(381) to give the oxetans

the

to

final

the

( 3 8 2 ) and

enol (383)

respectively when irradiation is carried out under argon. In the presence of oxygen the biradicals produced during the addition of the quinone are trapped adducts

( 3 8 4 ) and

as

( 3 8 5 ) are

1.2.4-trioxanes, obtained

from

Thus the two

either

of

the

lactones.135 Following irradiation at 330 nm and above of the barbaralone adducts

( 3 8 6 ) and p-chloranil

( 3 8 7 ) and

produced

(388) are

in benzene solution the two

formed.

The

first

of

these

is

v i a an exciplex while the latter product arises from

interaction of the radical cation of (386) and the chloranil. Benzophenone

also adds affording

the adduct

(387, R

=

Ph) . A

detailed study of the influence of solvent upon the reaction was r e ~ 0 r t e d . lAnother ~~ report by Kim and his coworkers has detailed the results of the photochemical addition of 1,4-diphenylbuta1,3-diyne

to

p-quinones

(389).

This

reaction

yields

the

quinomethanes ( 3 9 0 ) by (2+2)-cycloaddition and ring opening of Irradiation of the sulphonimide ( 3 9 1 ) in 138 ethanol results in its conversion into the products (392). the resultant oxete. 13'

154

Photoclionistiy

?

Ph

Q p)tph7:n0h Ph

Ph

0-

(378)

Ph

(379)

m

0

0

Q

Q

0

0 (383)

0

00

bQ

0

(384)

(385)

0

(387)

CI

CI

CI

CI

(388)

R-R=

fkR1 “x“ PhC3 C C O P h

R’

R2

R2

0

R’

0

(389)

R’= R~ = H or CI

(390)

0

155

W 2 : Enone Cycloaddirioris arid Rearrangmerits

V' o-" NS02Ph

NHSOZPh

NS02Ph

NHS02Ph

pfkp 0

(392)R = OH or CI

(391)

(393) OH

(395)

(394)

SCHRR'

0 (399)

An

investigation of

the photochemical

quinone ( 3 9 3 ) in crown ethers ( 3 9 4 ) ( n

=

reactivity of

the

1 , 2 , or 3 ) has been

carried out. The products from the reaction have been identified as the adducts (395). T h e formation o f these is thought to arise by hydrogen Coupling

abstraction

between

products . 13’

by

the quinone from the crown ether.

the resultant

The

radicals affords

photochemical

reactions

the observed

of

the

quinone

derivatives ( 3 9 6 ) has shown that the reactions are dominated by a 1,5-hydrogen transfer as the photochemical step. This affords a biradical ( 3 9 7 ) which cyclizes in the main to yield the product ( 3 9 8 ) in yields up to 90%. In some case (396d and e ) and all of the sulphur analogues cyclization also affords another product 140 identified as (399). T h e quinones (400. R Et) also undergo

photochemical

cyclization o n

benzene. The reactions presumably formed

by

hydrogen

abstraction

= H.

by

Me, R 1

=

irradiation in

involv? the biradical a

quinone

Me or

carbonyl

(401) group.

141

Cyclization within this biradical affords the product (402).

A

mechanistic

quinones,

study

(403).

e.g.

of

with

the

photochemical

1,l-diary1 alkenes

reaction such

as

of

1,l-

diphenylethene has been reported. T h e products from the reaction are

shown

in

scheme

9. The

detailed

study

investigated

the

dependence of quantum yield o n concentration and evidence for the involvement of electron transfer processes was sought. 14* The photochemical reaction between 1,l-diphenylcyclopropane and the cluinone ( 4 0 3 ) has also been studied. Irradiation through Pyrex in acetonitrile solution affords the adduct (404). The mechanism by which this is formed involves a n electron transfer process resulting

in

naphthoquinones

ring

opening

( 4 0 5 ) react

of

the

cyclopropane. 143

photochemically with

The

the allenes

( 4 0 6 ) to afford the products shown in scheme 10. The reaction appears

to be

dependent upon the substitution pattern o n the

157

IV2: Eiiorir Cydoadditiorts arid R~.nrrnngrmrtits

Ph

(403)

15 %

4%

+ @kPh

CI Ph

0 14 %

Scheme 9

Pltorociwnisrry

158

mx 0

Y

“’2-

R1

+

\

0

R4

(405)

hv

A

X=Y=CI

X = Y = Br

a;R’=R2=R3=R4=H b; R‘ = R2 = R3 = H, R4 = Me c; R’ = R 2 = R3 = H, R4 = Me0 d; R’ = R2 = H, R3 = p -CIC& R4 = CI e; R‘ = R2 = H, R3 = p -FC6H4,R4 = F f; R’ = R2 = H, R3 = Ph, R4 = H g; R’ = R2 = H, R3 = p-MeC&, = Me h; R1 = R2 = H, R3 = Ph, R4 = H

Scheme 10

8

14 61

25 89 85 52

B 74 66 0 0

0 0 0 0

II.2: Etiotie Cycloadditioris atid Rearratgemem

*o

I59

*O

NHR

/

0

NHR

0 (407)

(409)

0 - L O

CHO 0

Me0

Me

Ph CCOPh

0 (413)

Me0

Me

1 60 allene

portion

and

only

allenes

( 4 0 6 8 ) and

( 4 0 6 b ) react

by

(2+2)-addition to the C=C bond of the quinone. A l l the other allenes react primarily by (4+2)-addition. The reaction involving the formation of a pyran proceeds by an electron transfer process 144 with decay to a biradical and cyclization.

The irradiation of the quinones ( 4 0 7 , R

=

H or A c , Z

=

Nbz.

C H Z . or 0) in benzene affords the enaminoquinones (408) a s the primary photoproducts. These compounds then undergo thermal or photochemical cyclization to afford the heterocyclic quinones ( 4 0 9 ) and

( 4 1 0 ).145 The quinone

(411)

undergoes photochemical

oxidation and lactonization to afford (412) when irradiated under normal room lighting. The reaction is efficient affording (412). 146 a biologically inactive lactone, in 9 8 X yield.

Anthraquinone photochemically reacts with diphenyl acetylene to yield the anthrone ( 4 1 3 ) and the cyclized compound (414). The latter product is formed by photochemical cyclization of ( 4 1 3 ) in

a

cis-stilbene

abstraction

type

reactions

cyclization. 14' by

the

A

quinone,

study

of

hydrogen

9.10-dihydro-9.10-

benzenoanthracene-l,4-dione ( 4 1 5 1 , has been reported.

148

IV2: Eiiorir Cycloadditioris arid Reurrartgemwts

I61

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Chem..

Sect.

B.

1989. 28B. 539 ( C h e r n . A b s t r . ,

1990. 112, 98972). 73.

M.

Sindler-Kulyk,

Z.

Majerski,

D.

Mlinaric-Majerski, T e t r a h e d r o n L e t t . ,

Pavlovic.

and

K.

1989, 30. 3577.

74.

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76.

M.

(Chem. A b s t r . . (Chem. A b s t r . ,

Ohkita.

1989, 110, 172744). 1989, 111, 97536).

T. Tsuji.

M.

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M.

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and

S.

W2: Etrottr Cydouddiitiorisarid Reurrattgemwrs

I65

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3

Photochemistry of Alkenes, Alkynes, and Related Compounds BY W.M. HORSPOOL

1

The

Beactions of Alkenes

results

of a

study

of

the

gas-phase

ethylene at 183 nm have been reported.

ais-trans 1sorerization.-

A

photochemistry

of

1

detailed

study of

the

cis-trans

isomerization of the propenylcycloelkenes ( 1 ) has been carried out. The work examined the influence of sensitizers on the photostationary state composition. Typical of this are the results using benzophenone when the following values for quantum yields were obtained: alkene (la) quantum yield for the formation of the cis isomer conversion

=

0.53, the quantum yield for the trans-ois interis 0 . 4 8 and the corresponding values for (lb) are

0.45 and 0.57. The photobehsviour of the alkene (lo) is different and

in

addition

to

the

isomerization

process

*

cycloaddition affords the dimers (2) and (3). with

the study

of

triplet

states of

aryl

an

efficient

Ueviews dealins

alkenes

by pulse

r a d i o l ~ s i s ~and with non-reversible cis-trans-isorerization of aryl substituted alkenes4 have been published.

Examinations of the photochemical iaomerization of trsnsatilbene in alcohols'

and in alkanes'

have been reported. A study

of the photoaherical behaviour of trans-stilbene in faujasites

Photochemistry

170

' O 03 B"'

e, I

(4)

(8) R = a l k y l , p h e n y l o r 2 - n a p h t h y l

But (5)

(10) a; n = 2 b;n = 1

II/3: Photochemistry of Alkenes, Alkyries, arid Related Cornyourids has appeared reported. and

The photoisomerization of trans-stilbene

4,4'-direthoxystilbene

of

examined.

171

in

polar

trans-cis-Isomerization

of

solvents the

has

been

photoresponsive

complexing stilbene derivative(4)can bebrought about by direct

A description of the photochemical 2-E-isomer-

irradiation. ization of physical

the styrylstilbene

aspects

of

the

( 5 ) has been published."

photochemical

reactions

of

The the

dinaphthylethylene (6) have been evaluated. l1 The irradiation of the stilbene analogue (7) has shown that Z-E-isomerization occurs

from both the singlet and the triplet state. In polar solvents the intersystem crossing is decreased and under these conditions the singlet state isomerization predominates.

12

A review lecture has dealt with the isomerization of anthryl ethylenes. l 3 A study of the anthracenyl substituted ethylenes ( 8 ) has

shown

that

the

photoisomerization

one-way isomerization from cis-trans. reported

the

trans-cis-isomerization

undergone

involves

a

Laarhoven et a1 .I5 have of

pmethyl-2-styr~l

anthracene and its 9-bromo derivative. The p r e t h y l derivative shows

little

Spectroscopic

isomerization details

of

the

on

irradiation

molecules

are

at

366

nm.

reported

and

discussed. Apparently fob both of these compounds the decay of the excited states is faster than isomerization.

15

The triplet-sensitized isomerization of cis-cyclooctene ( 9 ) to the trans-isomer (10a) is temperature dependent and the quantum yield for the process doubles on reduction of temperature from

25 to -78OC when the reaction is carried out in benzene. A similar effect is also observed in toluene and p x y l e n e . In t h e a e

I72 cases the quantum yield is lower but this could be associated with

the

magnetic

lower

triplet

resonance

cycloheptene

energy

study

(lob) has

of

been

of the

these arenes.16 A pseudorotation

reported.

The

nuclear

in

trans-

trans-isomer

was

prepared by the irradiation of cycloheptene in the presence of 17 Cu(I) triflate. Photochemical isomerization of one, two or three double bonds in [26]-paracyclophanes has been reported.

Hydrogen

Abstraction

photochemicaily

Reactions.-

reactive

on

The

irradiation

imine

18

(11)

is

under

an

in benzene

atmosphere of nitrogen. In the absence of a trapping agent the adduct (12) is formed. The route to this material is proposed to involve a Norrish type I 1 like hydrogen abstraction to afford the quinodimethane (13). This is trapped by unreacted imine and after elimination

of

tri-methylsilyl

amine

the

final

product

is

obtained in 67% yield. When dimethyl maleate or dimethyl fumarate are added to the reaction mixture irradiation affords, after work up, the adduct ( ' 1 4 ) in 45 % yield.''

The styryl amines ( 1 5 ) are

photochemically reactive v i a the singlet excited state of the styryl moiety. Following excitation an intramolecular exciplex affords the biradicals

(16) and

(17) by way of an electron

transfer followed by a hydrogen transfer. The reactions of the biradicals formed are dependent upon the length of the chain separating the centres and yield the products (18) and ( 1 9 ) in the relative ratios shown in scheme 1.

Addition

Reactions.-

electron-acceptor phenanthrene

The

20

irradiation

alkenes

and hexamethyl

through

Pyrex

of

the

(20) in acetonitrile solution with disilane

brirys about a regio-

173

W 3 : Photochemistry qf :I Ikc)ies, Alkynes, and Related Compounds '

C0,Mc p

H2N r 0

H 2

M

e

NHSiMe3

(15) n = 1 - 5

(17)

/-r'JzMe

Ph

Me

Ph

Ratio n = 1

n = 2 n = 3

4 n=5

n=

(18)

1

(19)

>20: 1 1 4 : 1 2.4: 1 0.15 : 1 >8: 1

Scheme 1

x%H NC

CN

NC (211

a; X = H

b; X = M e c; x = C I d; X = C N e; X = O M e

NCACN (22)

#I.

91.

75 80

10 8

54 84

32

10

trace

6

Pimtochanistry

174

selective addition to afford the adduct ( 2 1 ) and the alkane ( 2 2 ) . The reaction proceeds v i a an electron transfer from phenanthrene to the alkene to yield the radical anion (23). This intermediate is the precursor to both products. The yields of products are shown under the appropriate structure and only when the aryl moiety

is

electron

donating. are

poor

reaction was extended to the esters ( 2 4 ) .

Rearrangement Reactions.- Kira et al. chemical

isomerization

of

'*

ally1

yields 21

obtained.

The

have described the photosilyl

derivatives.

The

irradiation at 254 nm in hexane brings about the conversion of (25.

R = Ye, Ar

product

mixture

behaviour was

=

Ph) into (26, R

of

the

two

shown for

naphthsl) and ( 2 5 . R

=

in

=

the

Ye. Ar

=

Ph) affording a 29:54.

ratio of

the derivatives

€IA,r

=

(25,

R

=

Similar Ye, Ar

=

phenyl). The reaction was shown

to be intramolecular. When the irradiation of a derivative with an

optically

proceeded

active

with

centre. This

silyl

inversion

centre

of

was

studied

reaction

of

the migrating

with

the Woodward-

configuration

is in apparent disagreement

the

Hoffmann postulates. 22

A quantitative yield of the trans-alkene

(27)

photochemical

was

obtained

on

ring

cleavage

of

the

vinylcyclopropane ( 2 8 ) . The biscyclopropanes (29) were found to be photochemically inert under the same reaction conditions. The authors suggest that a free radical mechanism is involved whereby homolysis of the C-Br bond is the first step.23 dealt

with

photochemical

isomerization

electron transfer processes. 24 Lewis"

of

A review has

alkenes

through

has published an artiale

dealing with the formation of radical ions of orsanic molecules containing C-C multiple bonds. A study of the photoiaomerization

175

W3: Photochemistry of Alkenes, Alkynes, and Related Compounds

xy

X

Y

CO-Me L

(24)

X H

X

Cl CN

Y CN C02Me

CO,M C0,Me

Me R &SiMe2A

S i Me, Ar

r

R

(25)

R = Me, Ar = Ph R = Me, Ar = N a p h t h y l R : H , Ar = Ph

(26)

Br

[XI [iFI' .+

176

Pltotochmisty

of the radical cation

that

ring

butadiene

opening

(30) of hexamethylcyclobutene has shown

affords

the

radical

(31).26 SET photochemistry

cation of

of

the

the alkene8

trans-

(32-34)

results in deconjugation when the irradiation is carried out in acetonitrile solution and in the presence of 1.4-dicyanobenzene. biphenyl, and 2.4.6-trimethylpyridine. The reaction involves the formation of the corresponding radical cation, deprotonation by the base, with the production of a free radical. The formation of 3-phenylpropene from (32a) and 3-phenyl-2-methyl- propane from

(32b) is typical of the behaviour reported. The reection with (32c) is more complicated and gives a mixture of products as shown in scheme 2.27 The SET photochemistry of (-)-pinene (35) in the presence of l,4-dicyano-benzene as sensitizer in methanol has been studied. The reaction encountered involves the formation of the radical cation of the pinene followed by ring opening of the four membered ring. The resultant radical cation (36) is trapped by methanol and the radical so produced reacts with the 28 dicyanobenzene to afford the final products (37) and (38).

Halogenated A1ksnes.-

A

previous

study of

the photochemical

behaviour of the insecticide Bromodan (39) shored that there was a

solvent

dependency

on

the

outcome

of

the reaction.

Thus

irradiation in hexane using a quartz filter gave products ( 4 0 ) and

(41) resulting

from the reduotive

dehalogenation

of

tho

double bond. In methanol dehalosenation of the double bond a l s o took place affording products

(41) and (42). In a mixture of

acetone and hexane as solvent a more complex mixture of products (43-44) was obtained

while in triethylamine only bridgehead

dehalogenation took place affording (45). 29 A reinvestigation of

11.3: Photochemistry of A1kew.s. Alkynes, and Related Compounds

177

(JPh (32)a; R ' = H , R 2 = Me b ; R 1 = Me, R 2 = Me C ; R 1 = M e , R 2 = Et

(34)

(33)

Me

Me

3 .I.

9 */.

40 ' l a

Scheme 2

(36) Ar = 4

C

CI

(39)

b Br

CI

C ' $ s -

CI

- cyanophenyl

Br

C1 bC

Br

CI

the

reactivity

(39)

of

casts

considerable

doubt

on

the

selectivity claimed. Furthermore a repeat of the reaction in triethylamine

has

shown

that

the

product

(45)

is

also

photoreactive and is converted into the debroninated product 30 (44).

2

The

Reactions involvinl: Cyclopropane B i 4 s

irradiation

with

visible

light

of

a

mixture

of

the

cyclopropane (46) and tetracyanoethylene affords the sdduct (47). A study of the analogous thermal process was also reported.31 Shi et

.

al 32 have

reported

the

photochemical

conversion

of

the

alkynes ( 4 8 ) into biphenyl and its derivatives. The reaction is brought about by irradiation under argon in methanol solution. The formation of the products

is thought

to involve a di-x-

methane interaction between the two aryl groups followed by the extrusion of a carbene.

9.6.

( 4 9 1 , which is trapped subsequently

by solvent. A typical example of this is shown in scheme 3 for the reaction of (48a). The zwitterionic nature of the proposed bridged

intermediate

(50)

is

substantiated

by

subatituent

effects. Thus the irradiation of (48d) affords biphenyl (7.1%) and p-methoxybiphenyl (30%) which is interpreted as showing a marked preference f o r bonding between a methoxy substituted a w l 33 group and a phenyl group rather than between two phenyl groups.

An

earlier

publication

gave

a

preliminary

account

of

the

photobehaviour of the alkenee (51). 3 3 The work has been extended and i t has been shown that a mc

8

singlet state is involved.

Irradiation produces the biradical ( 5 2 ) within which 1.2-hydrogen transfer affords the transient (53) which is proposed to exhibit

W3: Photochemistry of A1kerie.x Alkynes, and Related Compounds

NC

( 4 6 ) R ' = R 2 = Me

(47)

R' = Me, R 2 = c y c l o p r o p y l

R2

Q R3 (48) a ; R' = R 2 = R 3 = H b; R 1 = R 2 = H , R 3 = M e c ; R ' = R2 = R 3 = M e d; R1 = R 2 = H , R 3 =OMe e ; R' = R 2 = R3 = O M t

hv

MeOH

Argon atmos.

OMe

Scheme 3

179

Photochemistry

180

0 0

Ph-

C-C

( CH2) 3CH

Ar (50)

(49)

CN

Me

(51) X = p - M e 0 X = m-Me0 X = p-Me x = p-CI X = p-CN

Scheme 4

Ph

Ph

5 p:: NC

Ph NC

Ph

Ph

'Ph

NC&Ph

NC

CN

(551

(57)

Ph& Ph F

Ph

e

(59)

h

Ph H

(60)

Ph

181

W3: Photochemistry of Alkenes, Alkynes. and Related Compounds

ionic behaviour. This subsequently ring closes to the cyclopropane

(54)

(scheme 4). Quantum

yield

determinations

carried out and a slight solvent dependency was detected.

were 34

Last year the photochemistry of some systems in a confined environment was reported. 3 5 Thus the solid state photoreactivity of the dienes ( 5 5 ) and ( 5 6 ) have been shown to be different from

the photochemical reactivity in solution. The quantum efficiency in the crystalline phase is, however, much lower than the quantum efficiency in solution phase. The dicyanodiene ( 5 5 ) yields only the cyclopentene ( 5 7 ) v i a the biradical ( 5 8 ) in the crystalline phase. The tetra-phenyl diene (56) also shows different behaviour under the same reaction conditions as above and reacts only by vinyl- vinyl bridging to afford the biradical ( 5 9 ) and ultimately product

(60).

Change

in

reactivity

is also

evident

in

the

reactions of cyclohexenone (61) where the endo-ketone (62) and the bicrclic isomer

(63) are formed in the solid state. The

authors suggest that "conf inement control" is responsible for the selectivity

observed.

A

full

report

published. 36 Acetophenone-sensitized

of

this

has

now

been

irradiation of the diene

(64) affords the cyclopropane ( 6 5 ) in 35 X yield. The reaction is reasonably efficient with a quantum yield of 0.041.37 Direct irradiation in acetonitrile is more complex and in addition to the di-rc-methane rearrangement product (65) six other products are formed. These are the result of rearrangement. ayclization 38 or fragmentation and are shown in scheme 5 .

Further work has been carried out on the aza-di-r-methane rearrangement

of acyclic b,y-unsaturated

iminea. The present

Photochemistry

182

of!:wPh

Ph

Ph

aCN

(63)

(62)

(611

Pr' Pr'

Ph&CN

Ph

Ph

CN

CN

Scheme 5

Me

Ph

Me

Ph

A'

r

II.3: Photochemistry of Alkenes, Alkynes. and Related Compounds report

deals

with

the

photochemical

183

reactivity

the

of

N-

benzylimines ( 6 6 ) . Acetophenone-sensitized irradiation of all of the

examples

brims

about

conversion

to

the

corresponding

oyclopropane derivatives (67). 38 Quantum yield studies have shorn that

the efficiency of

the conversion is better

measured for the N-arylinines another

report

Arnesto,

(68)

Horspool

than

that

reported previously. 39 In and

coworkers41

have

demonstrated that the acetone-sensitized irradiation of the aza1.4-diene (69) affords a mixture of the isomeric cyclopropanes (70). Interestingly the reaction does depend for its success on the presence of the two substituents on C-5. When only one substituent is present as with azadiene fails and only 2-&-isomerization

results.

(71) the cyolization 40

Both direct and acetone-sensitized irradiation of the diene (72) leads to the formation of the two products (73) and (74). These are formed by way of a di-n-methane process involving the biradicals (75). ( 7 6 ) . and (77). The inter-relationship of these biradicals and those derived by the irradiation (or thermolysis)

of the diazocorpounds ( e . ~ ,78) w a s a~sessed.~' The tricyclic enone (79) is photochemically reactive in ohloroforr solution. On direct irradiation three products (80-82) are formed in the yields shorn. The biradical intermediate (83). produced by a dir-methane

bridging process,

is the precursor

oommon

to the

formation of all the products. Thus fission of bond "a" yialdm (80) and fission of bond "b" yields (81). Rupture of bond "a" affords

another

intermediate

(84)

rhioh

is

subsequently

transformed into (82).42A further study of thie system has been reported with the bromo-mubstituted tetraene ( 8 5 ) . Irradiation

Photochemimy

184

Me Me

OAc

Me

D

D

R

D

(72)

(791

D

D (73)

(80) 19 '/o yield

OA c

(74)

(81) 18% yield

(82)15'/. yield

II/3: Photochemimy of Alkenes, Alkynes, and Related Cornpourids

185

Br (85) C02Me

& I

Me02C

E = C02Me

(89)

d!$->.

Pr

"

Pr "

N-

R '

R (95) R = H or CN

(96) 53 '10

(97) R = H; 2 5 "10 R = C N ; 57'/0

Pr"

CN CN

R (98) R = H ; 2 2 '/o R=CN; 43O/o

(99)

(100a)76 "lo

Pliotochernistry

186

of this affords the two products ( 8 6 ) and ( 8 7 ) . Again, these are formed by way of di-u-methane bridging to yield the biradical ( 8 8 ) . Bond

"a"

fission affords

product

(86) while

bond

"b"

fission yields (87). The two products are thought to be formed v i a the triplet state and the authors suggest that intersystem

crossing

is aided

substituent.

by

the heavy

Interestingly

atom

no

of

effect

the bromine

for

evidence

a

product

corresponding to (82) was obtained in these experiments.

43

Direct irradiation of (89) in benzene or acetonitrile gives a quantitative yield of the cyclooctahetraene (90) in accord with ear 1 ier work. 4 4 * 4 5 Previously Grovenstein et

had failed

to identify three products from the sensitized irradiation of ( 8 9 ) . However. benzophenone sensitization has been shorn to yield

the semibullvalene (91) and direct irradiation of it, ( 9 1 ) . gives the isomer (92). Acetone-sensitized

irradiation of the cyclo-

octatetraene ( 9 0 ) affords the (2+2)-product (93) which undergoes a di-x-methane rearrangement to afford the third semibullvalene

A study of the photochemical reactivity of the pyrazine derivatives of barrelene (95) and (96) hes shown that the di--kmethane process

is dominated by pyrazino-vinyl

Irradiation of the derivative (95. R

= H)

yields the preducts

( 9 6 ) by a vinyl-vinyl bridging path and (97,

=

bonding made.

R = H) and (98, €2

H ) from the pyrazino-vinyl bridging. This, ( 9 5 , R

f

H) , is the

only compound in the group which shows this reactivity. Compound

(95, R

= CN)

yields only products (97, B

=

CN) and ( 9 8 . B

= CN)

from the pyrazino-vinyl path. When the more sterically crowded

system ( 9 9 ) was used irradiation gave the products (100a)and

(lOOb).'he former is produced in 76% yield and is the result of

187

II.3: Photochemistry of Alkenes. Alkynes, and Related Compounds

But Me

0

% qhQgM Me

/

RN

0

/

Me

I

RN

NR

0

0

**

Me

(106)a; E = C02Me, R ' = R 2 = M e b; E = COzMe, R1 = Me, R 2 = Ph c; E = COZMe, R1 = Ph, R2 = H

E

Me

-

E

Me

/

Me

Me'*

C02Me

Me

2.7-bridging while

? the latter arise from 3 , 8 - b r i d g i n ~ . ~ The

direct irradiation of the barrelene (101) yields the cyclooctatetraene (102).4a Trifonov e t ~ 1 . ~have ' slso studied the irradiation of

the phenyl

photoreactivity wavelenath

of

this

independent.

substituted compound

The

derivative

is

product

solvent.

from

the

(103). The oxygen

and

reaction

was

identified as (104). This conceivably could arise by a di-rmethane process by benzo-vinyl bridging to yield the biradical

(105) which, rather than forming a cyclopropane, yields the final 49

product (104).

Direct irradiation of dibenzobarrelene (106s) in solution yields the di-%-methane product (107) and the cyclooctatetraene (108). Acetone-sensitized

irradiation

affords

compound

(107)

only. In the crystalline phase, however, a new product (109) is obtained as well as (107) and (108). A reinvestigation of this has

suggested

that

formation of both

the biradical the ester

(110) is

involved

in

the

(109) and the cyclooctatetraene

(108). The formation of the biradical could involve a tri-rmethane intermediate such as (111). Other examples (106b. c) have been studied and while the solid state irradiation does not yield the

appropriate

diester

aorresponding

to

(109) the

cyclo-

octatetrsene obtained does have the correct substitution pattern in confirmation of the involvement of a biradical analogous to (110) rather than the (2+2)-cycloaddition reaction path which has been favoured in the past.50 A further study of the asymmetric induction in the di-x-methane

rearrangement has examined the

IV3: Photochemistry of

AIX.ciios. Alkynes. and

Related Compounds

189

CO, ment hyl

CO, mcnt hyl

(V)

(113)

CO, menth y l I

kco2

C02Mc

menthy'

(114)

(116)

E E

Ph P h C O O N y h R

(117) E = C0,Pr'

(119)a; R= Ph b;R=p -MeOC6H4 c;R=p -C\C,H,

4R = p - Me C,H4

PhCOOJ 0 Ph

(120) a ; R=H b; R=OMe C; R= CI d; R=Me

NJph

Ph

%

Ph

(121)

190

Photochetnistty

influence of chiral esters. Thus the irradiation of (112) yields the cyclooctatetraene (113) and a diastereoisomeric mixture of the semibullvalenes

(114) and

(115) in a ratio of

60:40

in

solution and 2 0 : 8 0 in the solid phase. The position of attachment is vital and the diastereoselectivity shown by the triene (116) is very

poor

and

yields

a

1:l mixture

of products

in both

solution and solid phases. 51 The photochemical cyclization of the dibenzobarrelene

(117) to

the

semibullvalene

(118)

in

the

crystalline phase can occur by four recornisable paths. These paths are ( i ) initial bonding between 9a and 11. (ii) initial bonding betreen 10a and 12. (iii) initial bonding between 4a and

12 or ( i v f initial bonding between 8a and 11. It is possible to differentiate betreen paths ti) / (ii) and paths (iii) / (iv) by determining the absolute configuration in the reactant crystal. The crystal of the barrelene (117) is homochiral and irradiation in the solid gives laevorotatory (118). This means that there is a preference for path ( i ) 1 (ii) and it is argued that within this pair of routes path (ii) is favoured.

3

52

Reactions of Dienes. Trienes, and Hichrr Polyenas

A study of the photophysical properties of p,p'-disubstituted 1,6-diphenylhexa-1,3,5-trienes has shorn the presence of dual

fluorescence.53 Liu and his coworkers report that the photoisomerization of 7-cis-retinal on direct irradiation at 365 nm occurs

with

high

efficiency.

A

quantum

considered as an explanation of this effect.

chain

process

wms

54

The synthesis and the photochemical reactions of a Serie8

IU3: Photochemistry of Alkenes, Alkynes. and Related Cornpourids

191

of N-benzoyloxy-4-aryl-2.3-diphenyl-l-azabuta-l,3-dienes

(119)

have been described. The direct irradiation of these compounds brings about an efficient six-electron type cyclization to yield the 2,3-diphenylquinoline derivatives (120).55 Armesto, Horspool, and their coworkersb6 have also published full details of their synthetic

approach

to

dihydroisoquinolinone

derivatives.

A

typical example of the process is the cyclization of the enol benzoate (121) into the final product (122) on irradiation in the presence of perchloric acid. The involvement of the acid inhibits an electron transfer process and a photo-Mannich reaction yields the observed products. This paper supplements and expands the material already published in note form. 57 A MIND0/3 analysis of the potential

energy

surfaces

involved

in

the photochemical

conversion of the diene (123) into the naphthalene derivative ( 1 2 4 ) has been published. 58 Semi-empirical

MIND0 3 calculations

have been carried out on the cyclohex-1,2-diene / hexa-1.3.5triene system.

59

A photo-1,5-sigmatropic migration of hydrogen is reported on irradiation of the ionol (125) in ether or aqueous sodium dodecyl sulphate. The product from the reaction was identified as (126). When the reaction is carried out in a liquid crystal medium of butyl stearate no migration occurs. If isotropic butyl stearate

is

used

both

cis-trans-isorerization

and

the

1.5-migration take place , 6 0 The triene (127) is photo-chemically labile and on direct irradiation is oonverted into the three primary photoproducts

(128),

(1291,

and

(130) in a ratio of

9:2:1. The formation of the polycyolia compounds (129) and (130) occurs from the excited singlet state."

The vinylstyrl furan

192

Photochetnist~

(

a

Me Me ‘

W Me O

H

@OH

HO

HO (133)

(134

R = (CH,),CHMeZ or CH-CHCHMeCHMe, HO

W 3 : Photochemistry of Alkenes, Alkynes. and Related Compounds

193

(131) undergoes conversion v i a a crossed Diels-Alder reaction into

the bicyclo(3,2,l)octadiene 62

(132)

on

irradiation

in

benzene.

A design for a flow-through photochemical reactor for the conversion of ergosterol into previtamin D Irradiation of

2

has been described.

the provitamin D analogues

(133) affords

63

the

trienes (134). Thermal isomerization of these analogues (134) affords the previtamin compounds (135).64 Patents dealing with the

laser

irradiation

of

vitamin

K i 5

the

photochemical

conversion of 3,24B,25-trihydroxycholesta-5,7-diene into 2 4 B . 2 5 dihydroxycholecalciferol have been reported.

66

A study of the photochemical reactivity of the fulgide (136) has been reported." have

demonstrated

Other studies on the related fulgide (137) that

photocyclization

(138)

and

(139).68

yields

the

isomeric

photochemical 69 cyclization of the fulgide (140) has also been reported.

naphthofurandiones

The

Irradiation of the spiro compounds (141 , R1-R2

= (CHSCH)2 ,

R3 = H: R1 = R2 = C 1 , R3 = H) results in their trans-formation into the three isomeric compounds (142), (143), and (144). The compounds (143) and (144) are also photochemically reaotive and can

be

converted

into

the

tricyclic

isomer

(14s)

by

a

conventional (2+2)-cycloaddition. 7 0 A study of the irradiation at 2 5 4 nm of indenes in the gas phase has been carried out. Various transpositions occur such as illustrated ?or the indene (146) in scheme

Swenton and his ~onorkera'~have reported

the facile irradiative conversion of (147) into the indanones

0

\ Me

R2w

R*+@3

\

RZ

R’

R

R3

W-QQ hv

Me

+

Me

( 146)

Scheme 6

R3

@Me \

W3: Photochemistry of Alketiex Alkynes, and Related Compounds

@ ?lR2 R

195

?&lR2 R

0

R3

0

R3

H (147)

R’ H H H H H 0-CH2-0

R2 H H H H H

R3

Yield

Ofo

02 96 87 98 96 72

H Me p-MeC,H, p M e O C,H, Ph

-

H

Meo2

Me

NMe2

Me

I l 4 9 ) R’ = Br, OEt, or CN; (150) R = Pri or N M e z R 2 = Me, Ph, or OMe; R 3 = CO,Me, CO,Et, or COMe

(151)

Ph

I

C

Ph (152)R-R R =

= (CH,), Me

or (CH2)&

NC

CONH2

Ph

I

CH ( 154)

(148) in high yield using light of 300 nm and piperylene as a quencher. T h e reaction presumably involves the population of the excited

singlet

state

and

fission

of

bond

recyclization within the resultant biradical.

"a"

followed

by

72

The photochemical reactivity of the pyran derivatives ( 1 4 9 ) has been studied and a reversible C-0 bond fission is reported. et a 1 . 7 4 have reported

Krasnaya pyrans

(150) results

dienones

(151).

reported

a

in

ring

Armesto,

new

that

photochemical

the irradiation of the

opening

Horspool

73

and

and

reaction

the

others" for

the

formation

of

previously substituted

4H-pyrans (152). The irradiation of these compounds ( 1 5 2 ) through Pyrex in methylene dichloride solution gives the cyclobutenes ( 1 5 3 ) in about 30% yield. These products are accompanied by the

fragmentation products

(154) and

(155) which are produced by

secondary irradiation of the cyclobutene. The reaction affording the cyclobutenes

is presumed

illustrated

(156).

in

to involve bridging of the type

Collapse

of

this

intermediate

as

illustrated yields the observed products. A full report of this work

has

been

published

with

the

results

of

experiments

substantiate the proposed electron transfer mechanism.

to

75

The photochemical conversion of the triene ( 1 5 7 ) into the tricyclic isomer active

ester

(158) has been carried out. Using optically derivatives

such

as

diacetone-o-D-glucose

irradiation brought about conversion into the mixture of produots (158) and ( 1 5 9 ) in a total yield of 45%. The diastereoisoreric excess was 83%. A n X-ray crystal structure confirmed the nature of the

product.

77

Two reports

''' ''

have discussed

the low

197

W 3 : Photochemistry of Alkenes, Alkyties, and Related Compounds

E (157)

(159)

(158)

E = COOR, R =

diacetone- a -0-glucose

[a]*+ [0]*+ (161)

hv pentane h >220 nm

oMe +

H

+

&Me

H

H 26 ' l o

22

Me

Me

+

4 *I*

3 'I.

Scheme

7

10

+o 2 1 '1.

Pliotochernismy

198

temperature (Freon matrix at 77 K ) photochemical behaviour of the radical cation (160) generated by the radiolytic oxidation rf cyclo-octatetraene.

Apparently

irradiation with visible

light

yields the radical cation ( 1 6 1 ) by a symmetry allowed photocyclization.

Dai

et

al. 78

have

demonstrated

that

the

photoproduced radical cation (161) is identical to that formed br the radiolytic oxidation of semibullvalene (162).

irradiation of the methyl

The

substituted cyclic allene

(163) has been studied to evaluate the influence of substitution o n the reactions encountered. This work follows on a n earlier study of the photoreactivity of the unsubstituted allene. Direct irradiation using light of wavelength

>

220nm in pentane brought

about the formation of the products shorn in scheme 7 in the yields

shown

obtained

below

from

thermally

this

generated

the

appropriate

reaction

were

carbene

structure.

compared with

intermediate

and

The

products

those from a

the

conclusion

reached is that carbene intermediates are involved in the photochemical reaction. This is different from the unsubstituted case and the authors for

the

80

change

suggest that the methyl group is responsible in

behaviour

in

the

singlet

state

photo-

chemistry.80 An approach to the cembranoids (164), with either a

cis-

or

trans-arrangement

photochemical

isomerization. (165)

*'

properties

of

carried out

.*' The photochromic

and

at A

bond study

its conversion

"a", of

has the

into

involved

a

photochromic

(166) has been

behaviour of the closely redated 83

tetraenes (167) has also been s t u d i o 4

W3: Photochemistry of Alkerws. Alkynes, and Related Compounds

199

qCN CN

(166)

-

(167)R = 9-anthryl, p N02C6H,

p-Me,NC6H,,

-

o N02C6H,

&.' R2 (169) R' = 1-or 2 - n a p h t h y l R2 = H, CO, 1-naphthyl or 2-naphthyl

(168)

-

bPh CH=X

QPh CH=X (1711

(170)

X = 0, NPh, or NC&OMe-p Me Me

I

"YNYO N=N

OYNYO N- N

b/e0 NyN,Me

0 (172)

200 4

1 2 + 2 ] Intramolecular Additions

A description of the electron-transfer

norbornadienes acridine

to

orange

quadricyclanes as

sensitizers

induced ring-closure of acridine

using has

been

yellow

reported.

and

Photo-

isomerization is accompanied by quenching of the fluorescence of the dye.84 A study of the conversion of norbornadiene ( 1 6 8 ) into quadricyclane has been made using electron donating sensitizers. The mechanism by which the cyclization takes place is thought to involve an exciplex of the radical-ion pair. The excited triplet state of the diene is produced by back electron transfer between the radical-ion pair in a solvent cage.85 The influence of CsI and KBr on the

quadricyclane to norbornadiene photochemistry has

been studied .86 The photochemical reactions of the norbornadienes (169) with biacetyl

has been reported. The mechanism

for the

isomerization of ( 1 6 9 ) into the corresponding quadricyclanes by biacetyl

sensitization

quadricyclans (171,

is discussed. 87 The

formation

of

the

(170) occurs on irradiation of the norbornadiene

X = 0 ) . Copper

(11)

porphyrins

rapidly

brought

reformation of the norbornadiene. The imines (171, X

NC H OMe-p) were also photoreactive.

=

about NPh or

88

6 4

The adduct (1721, formed by the photochemical addition of the dione (173) to benzene, is also photochemically reactive and on irradiation at -6OOC

with the dione

(174) is formed in 50% yield.*' or

ether

solution

cyclobutadiene

using

(175)

The irradiation in cyclohexane

light

into

(173) the (2+2)-adduct

the

of

I

>

300nm

tetrahedrane

converts (176).

the This

M3: Photochemistry of Alkenes, Alkynes, and Related Compounds

F

Si(Me),

<:

(177) a; R , R = H

b;

R, R =

n = 1 para-attachment n = 1 meta-attachment n = 2 para-attachment n = 2 meta-attachment

85 "/o 4 0 "/a

6 3 "lo mixture o f cis 26"/" and trans 6"L

20 1

202

Photochrrnistr?,

tetrahedrane has a m.p. of 162OC and is reasonably stable. The stability is thought to be due to an electronic effect of the silsl group.

90

(2+2)-Cycloadditions are frequently used in the synthesis of cage compounds. Thus the acetone-sensitized irradiation of the

trienes (177) to afford the cage compounds (178) has been used in a synthetic approach to the [I]-peristylanes.

91

Irradiation of the styrene derivatives (179) at 280 nm in benzene solution brings about a (2+2)-cycloaddition affording the oxacyclophanes (180) in the yields shorn. 92 Other examples of this type of addition have been reported such as the photochemical (2+2)-c~cloadditionof

(181) using

1 >

280 nm under

nitrogen to afford product (182) up to 90% yield. In some cases another product identified as (183) is formed in low yield. This is

thought

to be

produced

by

oxidative

trapping of

a

free

radical. 93 A paper describing the synthesis of the dialkenes (184-186) and the photochemical conversion into the corresponding (2+2)-c~cloadductshas been published.

5

94

Direrization and Intermolecular Additions

The dimerization of acenaphthylene (187) in faugasites has been studied. The

irradiation gives a mixture of

cis- and

trans-

dimers. The work has shown that the dimers are formed from the singlet state when the faugasite has low atomic weight cations

W 3 : Photochemistry of Alktwcx Alkynes, and Related Compounds

(184) a;n = 1 , R = H

203

(185)

b;n = 1 , 2, or 3 R-R = (CH-CH),

Me

(189)

(190)

/-

7iMe

MeMe

Me

- eM

R2

/ \ -

x

(193)R' = R 2 = Me, X = H, Me, o r M e 0 R' = Me, R 2 = Et, X = M e 0 R' = R 2 = Et, X = M e 0

Ph otock rni istr?,

204

and from the triplet state with high atomic weight cations. Irradiation of the alkene (188) affords the dimers

*

(190).

95

(189) and

Photochemical dieerization of 3-methylbuta-1 ,Z-diene is

reported to yield the cyclobutane derivatives (191) and (192) in 31%

and

products

16%

yields

were

also

A

respectively. observed."

A

number

laser

of

flash

unidentified study

of

addition of fumaronitrile to stilbene has been reported.

the 97

A

detailed study of the pressure induced diastereoselectivity of electron

transfer

photo-Diels 98 reactivity of cyclohexa-1.3-diene has been published.

6

and

triplet

sensitized

Alder

Miscellaneous Beactions

The SET induced fission of the C-C bond in the bicumenes (193) has been reported."

The photochemical fission of the triaryl

alkane (194) in methanol using quartz apparatus has been studied. The reactions observed are shorn in scheme 8. Several processes take place but

the one of

interest

is the formation of the

carbene (195) by the extrusion of biphenyl. The carbene is the precursor to the ether (196) and the alkene (197). This study is similar to that reported by the same authors for the photoreactivity of triarylmethyl substituted alkgnes and is in essence 32 a di-x-methane process. The photoreactivity of several other triarylmethyl substituted alkyl systems was examined, for example the alkane (198) and the ayclopropyl derivatives (199). Again the predominant

reaction

path

was

the

formation

of

a

carbene

intermediate. loo A study of the matrix isolated cyclic allrlic and 1-adamantyl radicals activated by ultra violet irradiation

W 3 : Photochemistry of Alkcwi~.s.Alkynes, and Related Compounds

(196)16’1.

205

2 7 OIO

(197)1 O o I o

Scheme 8

Ph I

ph?-

Ph

Ph

(199)n = 1 , 3, or 5

(198)

R‘&. R~CO

N , 1

C02E t

C 0 2 Et (200)R’ = R 2 = R3 = H or Me; X = O (202)R’ = R 2 = R3 = H or Me; X = NC0,Et

EtO 2CN

- R2

R’&6t1

R’

R*R’ R’ (204)

R2

(205)

R’

Photochemistry

206

has shown that hydrogen abstraction from the matrix material 101 (adarnantane) takes place.

Irradiation of the tricyclic diazaoctenes (200) results in their transformation into the bicyclic isomers (201). The aza analogues (202) behave differently and react by NN-bond cleavage to

yield

the

bicyclopentanes

(203).lo2

reactions of the bicyclohexanes

The

photochemical

(204) under electron transfer

conditions have been studied. The principal products from the ring opening are the hexadienes

(2O5).lo3 Photolyses

(254 nm

under nitrogen) of a series of aryl cyclobutanes (206-211) in acetonitrile have been carried out.

104

The irradiation of the chlorinated bornenes (212) as a thin film on silica gel has been carried out. Several products are formed by

reduction, dechlorination,

dehydrochlorination

and

oxidation.lo5 Irradiation of the exo-isomer (213) in hexane using

254 nm

light yields norbornan-2-one

(5.8%). exe6-chloronor-

bornan-2-0119 ( 9 . 8 % ) , and a trace of 2-trimethylsilorynorbornene (214). When sec-butylamine is added to the mixture irradiation affords only the alkene (214). The endo-isomer (215) is much less

* *

reactive due to the absence of u /r

iAiteractions.lo6 A study of

the photochemical behaviour of the syn- and anti-isomers of the norbornenone (216) has sought to examine the extent of r * and u* 107 LWO mixing. The photochemical C-Br fission in the broroalkenes

(217)

irradiation of

continues

to

be

(217) in a mixed

of

synthetic

solvent, rater

value.

Thus

/ rethylene

dichloride, with a phase transfer agent and potassium isocyanate affords

the

isoquinolinones

(218). The

reactions

are

quite

W 3 : Photochemistry of Alkenes. Alkvnes, and Related Compounds

207

w H H

(206)

Me

Me

Ph' h\ Ph,

PhAPh

(210)

cia+

R

CI H (212)R = CH,CI

Me3Si 0

Me3Si 0

CI CHCI:, H or Me

(213)

cL& Me3Si0

CI

AxAr

Ar

OSi Me3

Br

R

JrJ$;r 0

(217)

208

Photochemistry

efficient and, for example, the all-phenyl compound (217, A r

=

Ar

=

Ph) C

yields

the corresponding

isoquinolinone

(218,

H OMe-p) in 46% while the all-p-methoxyphenyl compound (217,

6 4

Ar

=

C

(218,

H 0th-p) yields 93% of the corresponding isoquinolinone

6 4

R = OYe, Ar = C H OYe-p). 108 The vinyl bromide (219) is 6 4

photochemically

reactive

(Pyrex

dichloromethane

/

isopropoxide

sodium

/

filter) in a propan-2-01 mixture.

The

product

obtained from this treatment is the spiroallene (220). This is formed via a cationic intermediate produced by ionization of the

C-Br bond. This ionization also occurs when

irradiation

is

carried out in alcohols with pyridine as the base. These reaction conditions

yield

the

ethers

Te t rac hlorome t hane

(221). O9

undergoes photochemical rearrangement in an Argon matrix at 12R to afford the species identified as (222). The formation of this

is presumed to involve C-C1 homolysis.

The

benzyl

alcohols ROH (R

ethers

=

110

(223) photochemically

cleave

to form

alkyl, cycloalkyl, or 2-pyrrolidinoethyl) in

acetonitrile/nater.

The

reaction

involves

single

electron

transfer to 1.4-dicyanonaphthalene as the sensitizer. '11 and

Lament''' have described

Arnold

the SET photochemistry of a variety

of ethers. A typical example

is the irradiation of

(224) in

methanol / acetonitrile mixtures with a sensitizer. This yields diphenylmethane and the ether (225). Several systems of this type were studied.'12

A detailed account of the spplication of C-C

bond fission in radical cations has been published. study

of

the

photochemical

transformations

of

113

a

series

A of

chromenes and related compounds has been carried out. h variety

of

reactions

was uncovered

some

of

which

are shorn in

209

IV3: Photochemistry of Alkuies, Alkynes, and Related Compounds

Ph

$7

0I-H

Ph

Ph

(221)R= Me, Et, or Pr'

(219) CI

>

ROCH,

CI--- CL

Cl

(223)R = al k y 1, c ycloal k y I,gr 2 - pyr r 01 idinoet hy l

(222) P hC , H C H,O Me

CH,OCH,

0 CH

(225)

(224)

TosH*

0 Me

OMe

(226)

"'"w Me

a; R =

H, v e r y unstable

b; R = OAC

"'

0

0

Scheme 9

Me

Photochemistry

210

R' 0

(229)

(228)

R' = Me3CSiMe2 or CH,CO,

R 2 = PhCH,

f OH -0Ac

&OH

OH

HO

HO OH

OAc (232)

HoQoH

&OAc

HCOO OH (233)

(234)

OH

@ OH

R (237)

(238)R =

H, Ph

or

Me

21 1

W3: Phorochemistry of Alkenes, Alkynes, and Related Compounds scheme 9.114 Binkley and Hehemann"'

have

reported a facile

photochemical debenzylation reaction. A typical example of this is

the

irradiation

of

the

carbohydrate

(226)

in

tetra-

chloromethane solution with calcium carbonate and NBS yielding the product

(227) in 86% yield. The method

application to sensitive molecules. '15

is suitable

for

SET induced deprotection

(with dicyanoanthracene or anthraquinone as sensitizer) of the carbohydrate

ethers

(228) gives 116 corresponding alcohols (229).

reasonable

yields

of

the

An earlier report on the photobehaviour of D-glucose (230) and D-mannose (231) in the presence of Fe(II1) chloride followed by acetylation described the sywthesis of the arabinopgranose (232).11'

Similar

reaction

conditions

have

been

used

with

D-galactose (233) yielding the lyxopyranose (234). The arabinopyranose (232) has now been used as a starting material in the 118 synthesis of xylopyranose sugars.

Wan and his c o w ~ r k e r s " ~have reported the photochemical conversion of the alcohol ( 2 3 5 ) into dibenzosuberone (236) with a quantum yield of m. 0.003. The reaction involves the S1 state populated

by

irradiation

conversion. which was

at

254

nm.

The

mechanism

of

the

substantiated by deuteriation studies,

involves the conversion of the alcohol to the anion (237) by ionization of the C-H bond at C-5.119 A study of the photosolvolysis of the fluorenol derivatives (238) has been made in methanol/water.

120

Photochetnistw

212 7

1.

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Maier and D. Born. Angew.

Chem. I n t . E d . Enrl., 1 9 8 9 ,

28, 1050. 91.

L. A. Paquette, C.-C. Shen, and P. Engel, J . O r g . Chem., 1989. 5 4 , 3329.

92.

J.

Nishimura,

Y. Horikoshi, Y. Wada. H. Takahashi. S.

Yechino, and A. Oku, T e t r a h e d r o n L e t t . , 1 9 8 9 , 3 0 , 5 4 3 9 .

IU3: Photochemistry of Alkenes, Alkynes, arid Related Compounds 93.

S. Inokuma, T. Yamamoto.

and

219

J . Nishimura.

Tetrahedron

L e t t . , 1990. 3 1 . 9 7 . 94.

J.

Nishimura,

U.

Takeuchi.

H. Takahashi.

E.

Ueda.

Y.

Matsuda. and A . Oku, B u l l . Chem. SOC. J p n . , 1 9 8 9 . 6 2 , 3 1 6 1 . 95.

V . Ramamurthy.

D. R . Corbin, C. V. Kumar, and N. J . Turro,

Tetrahedron L e t t . , 1990. 3 1 , 4 7 . 96.

Ya. U . Slobodin.

Zh.

Org. Khim..

1988,

24.

1556

(Chem.

A b s t r . , 1989. 110, 172721). 97.

K.

S.

Peters, S. A . Angel, and E. O'Driscoll.

Pure A p p l .

Chem., 1 9 8 9 , 6 1 , 6 2 9 . 98.

I.-S. Chung, N. J . Turro. J . Mertes. and J. Mattay, J . Org. Chem., 1 9 8 9 . 5 4 . 4 8 8 1 .

99.

P. hlaslak and H. Chapman, Jr., J. Chem.

SOC.,

Chem.

Commun., 1 9 8 9 , 1 8 0 9 . 100.

M. Shi , Y . Okamoto, and S. Takamuku, T e t r a h e d r o n L e t t . , 1989. 3 0 , 6709.

101.

H.-G. Korth. F . Lange. F. Lubbe. W . Muller. M. Seidel, and

R. Sustmann. J. Chem. S O C . , P e r k i n T r a n s . 2 , . 1 9 8 9 , 1 2 9 3 . 102.

J. Kurita, H. Sakai. and T. Tsuchiya, Chem. Pharm. B u l l . , 1 9 8 8 . 3 6 , 2887 (Chem. A b s t r . , 1 9 8 9 . 1 1 1 , 5 6 7 4 7 ) .

103.

T. Tsuji, T. Yiura. K. Sugiura, and S. Nishida. J .

Am.

Chem. S O C . , 1 9 9 0 , 1 1 2 , 1 9 9 8 . 1 0 4 . C . Pac,

K. Go-an, and S. Yanagida, B u l l . Chem. SOC. J p n . .

1989, 6 2 , 1951. 105.

H. Parlar, Chemosphere.

1988. 1 7 , 2141 ( C h e m . A b s t r . . 1989,

111. 7 7 3 0 8 ) . 106.

B. D. Yarwell, J . J . Nash. H. Morrison, M. L. Faloetta. and

K. D. Jordan, J . Am. Cheat. 107. J .

S O C . , 1 9 8 9 , 111, 7 9 1 4 .

J . Nash and H. Morrison, J . Or# . Chem., 1 9 9 0 , 5 5 , 1 1 4 1 .

Photocheinistw

220

108. T. Kitamura. S. Kobayashi. and H. Taniguchi, J . 01-6.Chem., 1990, 55. 1801. 109. T. Kitamura, I . Nakamura, S. Kobasashi, and H. Taniguchi, J . Chem. S o c . , Chem. Commun., 1989, 1154.

110. G. Yaier, H. P. Reisenauer. J . Hu, B. A. Hess, and L. J . Schaad, T e t r a h e d r o n L e t t . , 1989, 3 0 , 4105.

1 1 1 . G. Pandey and A. Krishna. S y n t h .

Commun..

1988. 18, 2309

(Chem. A b s t r . , 1989, 111, 38584). 112. D. R. Arnold and L. J. Lamont, Can. J . Chem.. 1989, 6 7 , 2119. 113. R. Popielarz and D. R. Arnold, J . A m .

Chem. S O C . , 1990,

112, 3068. 114. Y . A . tdiranda, J . Primo, and R. Tormos, T e t r a h e d r o n . 1989, 4 5 . 7593.

115. R. W . Binkley and D. G. Hehemann. J . Org. Chem.. 1990. 5 5 . 378. 116. A. Nishida. S. Oishi, and 0. Yonemitsu. Chem. Pharm. B u l l . , 1989. 3 7 . 2 2 6 6 ( C h e m . A b s t r . . 1990. 112, 179617). 117. S. Ichikawa, 1. Toeita. A . Hosaka, and T. Sato. B u l l . Chem. SOC. J p n . . 1988, 61. 513.

118. A . Hosaka. S. Ichikara. H. Shindo. and T. Sato, B u l l . Chem. SOC. J p n . . 1989. 62, 797.

119. P. Wan, D.

Budac,

and

E. Krogh, J .

Chem.

S O C . , Chem.

Commun., 1990, 2 5 5 . 120. P. Wan and E. Krogh, J . A m . Chem. S o c . , 1989, 111, 4887.

4

Photochemistry of Aromatic Compounds BY A.C. WEEDON

Introduction This year's report on the photochemistry of aromatic compounds follows the format of the previous two years with the material grouped in sections devoted to photochemical reactions which involve isomerisation of the aromatic ring, addition to the ring, substitution on the ring, intramolecular cyclisation of substituents onto the ring, dimerisation, and lateral nuclear rearrangements in which part of a substituent cleaves and rearranges to a new ring position. A final section deals with reactions of substituents on the periphery of the'aromatic ring whose reactivity depends upon the presence of the aromatic system. In order to avoid unnecessary overlap with the contents of other chapters it has been this reporter's policy this year, as in the previous two years, not to include in this chapter any discussion of literature reports dealing with the following three topics: 1.

Di-x-methane rearrangements of homo-conjugated arenes.

2.

Photo-oxidation reactions of the alkyl side chains of arenes;this includes light-induced autooxidation reactions as well as singlet oxygen oxidations. In general, singlet oxygen addition to arene rings is also omitted.

3.

The reactions of nitrenes produced in the photoelimination reactions of aryl azides.

Several reviews whose topics include photochemical reactions of aromatic compounds have appeared duringthe year. The preparation of indole alkaloids by enamide photocyclisation reactions has been surveyed.' A very useful four volume work dealing with the theory and experimental practicalities of light-induced electron transfer reactions has been published;2 the third volume of this set is

2.22

Photochemistry

devoted to the reactions of organic substrates and contains chapters contributed by various authors reviewing the light-induced electron transfer reactions of aromatic systems. These include accounts of the fates of the radical ions produced from arenes by light-induced of the photochemical 1,S reactions of electron transfer, heteroarenes, of the intramolecular photocyclisation reactions of

'

aryl substituted iminium salts, and the cleavage of benzylic substituents following electron donation to the excited states of arenes. The photochemical enolisation of ortho-alkyl aryl ketones (u.formation of (1) from (2) by intramolecular hydrogen abstraction) has been reviewed7 as has the photochemical arylation of furans and thiophenes.8 The influence of a variety of medium effects upon the outcome of some photochemical reactions of aromatic compounds has been r e ~ i e w e d ;this ~ includes the effect of electric fields upon the photochemistry of acetoxyfluorenes, the effect of liquid crystals upon the photodimerisation of acenaphthylene and the light induced electron transfer mediated substitution of cyanoarenes by amines. In the most recent volume of Padwa's Organic Photochemistry series are chapters comprehensively reviewing recent work on the photochemical addition of alkenes to arenes" and the intramolecular photochemistry of conjugated aryl alkenes. 1.

Isomerisation Reactions

The nitrogen-oxygen bond of 1,2,4-oxadiazoles is dissociative in the excited state and this can initiate rearrangements to isomeric oxadiazoles. New examples of such rearrangements have now been p~blished.'~-'~ Buscemi and Vivona and coworkers have found that the primary products of irradiation of the 5-aryl-3-acetylamino-l,2,4oxadiazoles (3) are the isomeric oxadiazoles (4) which are presumed to be formed from the intermediates (5).I2 The primary products (4) This secondary are further photolysed to the quinazolones (6) rearrangement may proceed by cleavage of the nitrogen-oxygen bond of (4) to give (7) followed by cyclisation of the nitrogen onto the aryl ring, or it could be the result of an electrocyclic ring closure in (4) of the type commonly observed for N-aryl benzamides (for an example of this see the rearrangement of (182) to (181) in Section 4 below). The latter process would involve the intermediate (8). A similar choice of mechanisms is possible14 in the photochemical

223

W 4 : Photochemistty of Aromatic Compounds

d..a H

0'

R (4) R = H, Me, or OMe

(3) R = H, Me, or OMe

H

JsNr..f

R

R

(6) R = H, Me, or OMe

(5)R = H, Me, or OMe

(7)

NHCOCH,

(8)

(9) R = Me or Ph

224

Photochemisrty

rearrangement observed for the styryl oxadiazoles (9). Irradiation of E-(9), R=Ph, results in rapid E-2 isomerisation of the side chain double bond and slower formation of the quinoline (lo), which could result from either N-0 bond cleavage to (11), followed by cyclisation of the nitrogen onto the benzene ring, or from electrocyclic closure of (9) to (12), followed by thermal rearrangement to (10). The isomerisation to the quinoline (10) is much less efficient for E(9), R=Me, although E-2 isomerisation of the styryl double bond is still fast. The authors argue that the phenyl group of (9), R=Ph, causes the excitation to be localised in the oxadiazole ring whereas in (9), R=Me, excitation is localised in the styryl chromophore; this implies that isomerisation to the quinoline therefore proceeds by NO cleavage via (11) rather than electrocyclic closure to (12).14 Photolysis of pyridine N-oxides leads to a variety of products depending upon the reaction conditions or the structure of the pyridine; included among the products can be species resulting from deoxygenation of the pyridine nitrogen, inter- or intramolecular aromatic substitution by the oxygen, and rearrangement to 2-acylpyrroles. It has now been reported15 that pyridine N-oxides substituted with bulky groups in the 2- and 6-positions, e.g. (13), will, if irradiated in methylene chloride at -7O"C, yield 2-acylpyrroles such as (14) as the major isolable products. The photochemical isomerisation of 5-membered heterocycles can lead to scrambling of the ring carbons, either by 1,3-sigmatropic rearrangements in Dewar type intermediates as in the interconversion of (15) and (16) in Scheme 1, or by ring expansion of hetero-vinyl cyclopropene intermediates as in the interconversion of (17) and (18) Evidence exists for the operation of both shown in Scheme 2. mechanisms and the pathway followed appears to depend upon the structure of the heterocycle, with the second route favoured for furans. The results of semi-empirical molecular orbital calculations have now been published16 which support nicely the route shown in Scheme 1 for the rearrangement of 2-cyanopyrrole to 3-cyanopyrrole. The same calculations performed for furans suggest that this mechanism is unfavourable for these compounds because the barrier for the 1,3-sigmatropic shift in the Dewar intermediates is much larger than that for rearomatisation. In a report" on the photochemical cyclisation of the terthiophene (19) to give (20) it is shown that (19) also produces (21). This suggests that in competition with cyclisation to (20), (19) also rearranges to (22) which is then

225

W 4 : Photochemistry of Aromuiic Compounds

& H

0-

O

(1 3)

Me

Me I Me-Si-SiMe3 &Si

I

Me-Si-SiMe3

Me2CH2CHMe, SiMe3

I

Q

SiMe2CH2CHMe2

SiMe,

(23)

(24)

Photochemistry

226

photocyclised to (21). If (22) is indeed an intermediate then this is a rare example of a 3-substituted thiophene rearranging into its 2-substituted isomer. The isomerisation more commonly observed involves formation of the 3-substituted isomer from the 2-substituted thiophene. Photoisomerisation of a silyl substituted benzene has been observed in the case of (23) which rearranges to give a mixture of (24) and (25).la The rearrangement of azulene to naphthalene induced by excitation into the S2 state has been examined under collision free conditions in a molecular beam and an approximate estimate of the rate constant for the rearrangement under these conditions has thereby been determined. l9 2.

Addition Reactions

The photochemical addition of alkenes to the singlet excited state of simple benzenes most commonly results in the formation of adducts in which bonding of the alkene terminii has occurred either to adjacent aromatic carbons (to yield ttorthott adducts) , or to aromatic carbons which are situated 1,3 to each other (to yield ttmetatt adducts) Less commonly the alkene becomes bonded to the para positions of the benzene ring. A new example of an adduct resulting from this rarer mode of addition is the para adduct (26) which is obtained from irradiation of benzene with N-methyltriazolinedione. 2o

.

The extent to which ortho rather than meta adducts are produced correlates with the magnitude of charge transfer interactions between the alkene and the excited arene; a greater degree of charge transfer favours the production of ortho adducts over meta adducts, although if electron transfer between the alkene and the excited arene becomes exergonic, substitution rather than addition can become the dominant reaction. An example of this trend is apparent in the photo-addition of acrylonitrile to the substituted phenols (27)(30) 21 Para-cyanophenol (27) gives a product identified as (31)

.

which is presumably formed by ring opening of the ortho adduct (32), while the major product from para-carboethoxyphenol (28) is the ortho-adduct (33). However, with hydroquinone (29) or its monomethyl ether (30) the greater electron donating ability of the arenes apparently leads to the production of substitution products; thus (29) leads to (34) and (30) leads to (35) and (36).

W4: Photochemistry of Aromatic Compounds

227

Me I

Me@SiMe2CH2CHMe2 -Si- SiMe3

OH

x

SiMe3

(27) X = CN (28) X = C02Et (29) X = OH (30) X = OMe

(25)

ON

NC

OH

&N

0

NC

OH

yHCH3

OH

CHCH3

CN

(34)

HO (35)

d

& @ J H 2

(37)

I

CN

228

Photochemistry

In the meta addition of alkenes to the singlet excited state of substituted benzenes the orientation of addition is understood in terms of the existence along the reaction pathway of a polar species such as (37) which closes to the meta adduct (38) by bonding between the 1-position and either the 3- or the 5-positions in (37). Species (37) may be an intermediate, or may merely correspond to a polarisation developed during the interaction of the excited arene and alkene along a concerted pathway to meta products. It is formed by preferential addition of the alkene to those meta carbons which place the arene substituent at positions it can most stabilise. Thus a benzene ring with an electron withdrawing substituent will produce meta adducts derived from closure of (37) in which the substituent resides at the 3- or 5-positionsf while a benzene ring with an electron donating substituent will be attacked by an alkene to give products derived from (37) in which the donating substituent resides at the 1-position. An example of this is seen in the photo-addition of trans-dichloroethylene to benzonitrile and other substituted benzenes, which has previously been described by Gilbert and coworkers in preliminary form,22-24and has now been published in a full paper.25 Here the polar species corresponding to (37) is (39). This reaction also shows remarkable stereoselectivity since the major product is derived from the polar species in which the chlorine closest to the arene substituent is 8x0 to the allylic system, as shown in (39), and closure to the product occurs by bonding between the 1- and 5-positions to give (40) rather than between the 1- and 3-positionsf which would yield (41). This selectivity is attributed to steric and electronic interactions, with the endo chlorine in (39) forcing the polar species into a conformation in which bonding between the 1- and 5-positions is favoured.25 Only small amounts of adduct are isolated which result from a polar species formed by meta bonding of the arene to the alkene with the chlorine nearest to the cyano substituent in an endo orientation; presumably this diastereoisomer of (39) is not formed as readily because of steric interactions between the approaching alkene and the substituent on the excited arene. Ortho addition is the favoured mode of photochemical addition of alkenes to hexafluorobenzene26 and this has now been extended to the addition of the dienes 1,4-octadiene and lf3-hexadiene.*’ Ortho adducts are also formed in the irradiation of pentafluoroalkoxybenzenes with cyclic alkenes; the reaction is regioselective and

IV4: Photochemisty of Aromatic Compounds

229

yields primary products such as (42) which undergo electrocyclic closure to the isolated products (43).28 The photochemical addition of maleic anhydride to benzene was one of the first benzene cycloaddition reactions studied and also yields products derived from o r t h o addition. A paper has now appeared in which the effects of variation of factors such as temperature, irradiation wavelength and time have been investigated in order to optimise the preparative yield of the reaction.2g Gilbert and Rodwell have examined the photo-addition of furan to various substituted benzenes. 30 With benzonitrile the meta adduct ( 4 4 ) , which results from bonding of the arene to the terminii of the 1,3-diene system of furan, is the sole product. This is an interesting result because it implies that the charge distribution in the polar species which is assumed to develop from interaction of the diene and the excited arene is the reverse of that in alkene addition. Thus the polar species would have structure (45) where addition has occurred in such manner that the cyano substituent stabilises the negative charge at the 1-position, rather than at the 3- and 5-positions as in the alkene-arene analogue (39). The endo orientation of the furan oxygen relative to the allylic system of (45) is also consistent with this charge distribution. Similar regioselective meta addition of furan is also seen with phenyl alkynes and styrene, although in the latter case (2+2) addition of the side chain x-bond to the furan competes and this becomes the dominant reaction of furan with a-methyl styrene, benzaldehyde and methyl ben~oate.~' With acyclic 1,3-dienes such as 2,3-dimethoxy- or

2,3-dimethyl-1,3-butadiene, para addition to benzonitrile is observed, although with 1,2-dimethylenecyclohexane, in which the diene is held in a cisoid orientation as in furan, meta addition is still seen. 30 Several other examples of photochemical addition of alkenes to furans have been published during the period of coverage of this ~ h a p t e r . ~ ~ ' ~U.V. ~ light irradiation of khellin, (46), in the presence of dimethyl fumarate is reported to yield a (2+2) adduct from interaction of the fumarate with the furan ring,31 while the structurally related psoralen (47) undergoes a photodimerisation reaction to a product thought to result from cyclisation of the furan ring of one molecule onto the coumarin ring of a second.32 Intramolecular addition of the enone double bond to the furan in (48)

Photochemistry

230

[1

,]F

’*

F

H o

F F H

(45)

(44)

(46)

0

/o

(47)

(48)

0

$yJ) 0*-*

& /

/

R

A

(00 (49)

(50) R = H or Me

(51) R = H or Me

R4

R’ R

(52)R = Me, R’ = H (57) R = Me, R’ = Me

(53)R = Me, R’ = H

W 4 : Photochemistry of Aromatic Compounds

23 1

has been found to yield the adduct (49), which is used as a precursor in the synthesis of a natural product A different mode of intramolecular photo-addition of an alkene to a furan is seen in (50), which gives (51) upon i r r a d i a t i ~ n . ~ ~ Wender's group has made extensive use of the meta photocycloaddition reaction of alkenes with benzenes for the synthesis of natural products, and much of this work has been reviewed. One common application of this reaction has been intramolecular photochemical cycloaddition of 5-phenyl-1-pentenes such as (52) for the synthesis of triquinanes; in this application the products are derived from addition of the side chain alkene across the 1- and 3positions of the benzene ring. The side chain alkene can also add across the 2- and 6-positions of the ring but 1,3-addition is preferred if an ortho substituent is present which is capable of stabilising the polarisation that develops as shown in (53). A problem for the synthetic utilisation of this reaction is the ability of the polarised species (53) to close to adducts in either of two ways. That is, by bonding between the 2- and 6-positions or between the 2- and 4-positions of (53). In the latter mode of closure the product is (54), an angular triquinane precursor, while in the former (55) is obtained, which is a linear triquinane precursor. This lack of selectivity can sometimes be surmounted since the two products (54) and (55) can be in thermal or photochemical equilibrium via a vinyl cyclopropane rearrangement. This is the case in a new synthesis of the sesterterpene retigeranic acid (56) in which the precursor of the angular triquinane ring system present in this molecule is constructed by intramolecular meta photocycloaddition of the phenylpentene (57).36 Here a mixture of the desired adduct (58) and the product of the alternative mode of ring closure, (59), is obtained in an unfavourable ratio of 1:2. Fortunately, the unwanted isomer (59) can be equilibrated with (58) by further irradiation. A possible means of overcoming the non-selectivity of the closure reaction in the intramolecular addition of phenylpentenes has been suggested by Gilbert.37 A s noted above for the addition of trans-dichloroethylene to substituted benzenes, the chlorine substituents can direct the orientation of closure of the polarised species such as (39) by a repulsive interaction of the chlorine with one end of the allylic system. Gilbert has now shod' that the adducts formed from intramolecular addition of the alkene side chains

Photochemistry

232

(54) R = Me, R' = H (58) R = Me, R' = Me

(55) R = Me, R' = H (59) R = Me, R = Me

OMe I

Ph

W4: Photochemistry of Aromatic Compounds

233

of trans-l-chloro-5-phenyl-l-pentene (60) and 2-chloro-5-phenyl-lpentene (61) across the 1- and 3-positions of the benzene ring are also derived from a preferential direction of closure which appears to be governed by the chlorine substituent. Thus with (60) the major photoproduct isolated is derived from closure across the 2- and 4positions of the polarised species (62), while the major product isolated from (61) is derived from the adduct produced by closure across the 2- and 6-positions of (63). A further interesting feature of this reaction is that whereas for 5-phenyl-l-pentene addition of the side chain alkene across the 2- and 6-positions of the benzene ring is preferred, with (60) and (61) addition of the side chain alkene occurs across the 1- and 3-positions of the ring, even though no ortho substituent is present to stabilise the developing positive charge in (62) and (63). In contrast to the preferred meta mode of intramolecular photoaddition of 5-phenyl-l-pentenes, where the alkene and benzene groups are separated by three atoms, irradiation of the styrene (64) yields a single stereoisomer of the ortho adduct (65). 3 8 In (64), not only are the reacting units separated by 4 atoms, but also it is the styrene rather than the benzene which is excited. Comparable photoreactivity is seen for phenanthrene-styrene systems such as (66) which yield 2+2 adducts (67) along with products derived from competing Paterno-BUchi reaction of the ester carbonyl with the alkene side chain. The photochemical cycloaddition also proceeds in an intermolecular fashion between the ester of 9-phenanthrene carboxylic acid and para-methoxy-B-methylstyrene. 39 The mechanism of this reaction is shown to involve addition of the styrene to the singlet excited state of the phenanthrene derivative. 40 Several years ago Wagner4', 42 reported that alkenyloxyacetophenones such as (68) also undergo ortho intramolecular photoaddition of the side chain alkene to the benzene ring to give adducts which are derived from further rearrangement of (69). The reaction proceeds from the triplet excited state of the arene. This is in contrast to the above reactions of (64) and (66), and to most other benzene cycloadditions, which generally proceed from the singlet excited state. The efficiency of this reaction has now been probed as a function of the structure and substitution pattern of the Basc 3 upon the effects observed it is alkenyloxyacetophenone 43 concluded that the reaction involves an initial charge transfer

.

234

Photochemistry

4 “Me0

\

0

,OMe

W4: Photochemistry of Aromatic Compounds

235

interaction of the X-X* triplet excited state of the acetoghenone with the side chain alkene which collapses to a biradical en route to the primary product. As with the 5-phenyl-1-pentenes the cycloaddition is most efficient when the benzene and the alkene are separated by three atoms. This work has been extended to alkenyloxyacetonaphthones such as (70) and (71) which yield (72) and (73), respectively,44 also by a triplet excited state mechanism. In the absence of acyl or other substituents which accelerate intersystem crossing naphthalenes normally react with alkenes from their singlet excited state to give ortho products resulting from addition to the a and 8 positions of the naphthalene ring. In a new example of this type of reactivity the regiochemistry of addition of the polar alkenes acrylonitrile and dihydropyran to methoxynaphthoThe photochemical addition of nitriles has been investigated. 45 ethylene to 2-naphthol in the presence of Lewis acids has also been i n ~ e s t i g a t e d . ~It ~ is found that the product is (74) and the authors suggest that this results from addition of ethylene to the excited keto tautomer of the phenol. The light induced reaction of alkenes with phthalimides and These include other arene imides can occur in a number of ways. addition of the alkene to the imide C-N bond, addition to one of the imide carbonyls, and ortho or para addition to the arene ring itself .47 A new example of the latter mode of reaction has been reported for N-methylphthalimide; 48 irradiation of this compound in the presence of allyltrimethylsilane gave compounds identified as both stereoisomers of the para adduct (75) and the ortho adduct (76). The ortho adduct is photolabile and is efficiently photolysed back to the starting materials unless long wavelength light is used for the irradiation. From the results of sensitisation experiments the authors conclude that (75) is formed from the triplet excited state of the phthalimide and that (76) is a singlet excited state product. Irradiation of the phenanthrene imide (77) with alkenes or dienes in benzene solution also gives products of 2+2 addition, (78), along with adducts (79) which result from insertion into the imide C-N bond.49 On the basis of external heavy atom effects upon the product distribution it is concluded that (79) is singlet excited state derived while (78) is produced from the triplet excited state. When a polar solvent such as methanol-acetonitrile is used, (78) and (79) are still formed5' but products of light induced electron transfer

Photochemistry

236

are also observed if the alkene is sufficiently electron rich. Thus with a-methylstyrene the major compound isolated is identified as

-

(80)

The same group has investigated the photo-addition of alkenes to 1 ,2- and 2 ,3-naphthalenedicarboxylic acid anhydride. 51 No products of addition of the alkene to the naphthalene ring were observed; instead the alkene inserts into the C(=O)-0 bond of the anhydride or adds to one of the carbonyl groups to produce an oxetane. By analogy with bides, the insertion reaction can be viewed as an addition to the partial C - 0 double bond illustrated by the contributing structure (81) to give adduct (82) which collapses to the isolated insertion product (83). Phthalimides in which the nitrogen is connected by an alkyl chain to the 3-position of an N-acyl indole will form intramolecular Paterno-Biichi adducts such as (84) 52 If, however , the phthalimide is attached to the 2-position of the N-acyl indole then no oxetane formation occurs; instead photo-Fries rearrangement of the N-acyl indole takes place to give products such as (85). Both of these reactions also proceed when the indole and phthalimide are irradiated separately; the photo-Fries reaction of N-acylindoles is well known53 while oxetane formation occurs when phthalimides are irradiated in the presence of N-acylindole. 54

.

The exciplexes formed between arenes and good acceptors such as dicyanobenzene can dissociate to radical ion pairs if the solvent used is sufficiently polar. The radical cation of the arene is then susceptible to attack by nucleophiles and this can lead to products of addition or substitution of the arene. A preliminary account of how salts such as tetrabutylammonium tetrafluoroborate can serve to mediate the charge separation in less polar solvents was reported A second paper from the same group has now been last year.55 published which describes the formation of the amino substituted dihydrophenanthrene (86) during the irradiation of a solution of phenanthrene, dicyanobenzene and propylamine in relatively non-polar solvents such as THF in the presence of tetrabutylammonium tetraflu~roborate.~~In the absence of the salt no product is formed The photo-addition of saturated heterocyclic amines containing two heteroatoms (e.s, morpholine and piperazine) to the singlet excited state of benzene has been e~amined;~' the outcome of the

.

237

W4: Photochemistry of Aromtrric Compounds

0-

0A (84)

0 (85)

238

Photochemistry

reaction is similar to that with the analogous amines containing a single heteroatom. Light induced electron transfer from an amine to an excited arene leading to a contact radical ion pair is proposed to account for the products observed when 9,lO-dicyanoanthracene is irradiated in wet benzene in the presence of an a-amin~ketone.~~ The non-polar solvent maintains the proximity of the radical ion pair which would normally then undergo non-productive back electron transfer; however, in this case the ion pair reacts, ultimately to furnish 9,lOdihydro-9,lO-dicyanoanthracene along with products of fragmentation of the a-aminoketone. Alkyl borate and borohydride salts can also serve as electron donors in the photoreduction of aryl cyanides and aryl halides and an abstract of a report on this topic has appeared.5 9 Reductive alkylation of N-methylacridinium (87) occurs when it is irradiated with carboxylic acid salts.60 The reaction is thought to proceed by electron transfer from the carboxylate to the excited acridinium ring followed by decarboxylation of RCOO'; coupling of the alkyl radical produced with the acridinyl radical then gives (88). A very similar sequence probably occurs in a reaction proposed as a synthetic procedure for decarboxylation of carboxylic acids. 61 In this case an aza-aromatic comF,clnd such as acridine is irradiated with a carboxylic acid in benzene in the presence of tert-butyl thiol. The authors propose that a hydrogen bonded acridine-acid complex is excited and that adiabatic proton transfer is followed by electron transfer. This produces RCOO' which decarboxylates, and reduction of the alkyl radical then ensues. The major fate of the acridine is coupling to (89) if the reaction is performed in the absence of oxygen. Irradiation of aqueouE methanolic solutions of the pyridinium salt (90) yields a thermally and photochemically labile species. 62 Its structure is uncertain and the only information reported is the absorption spectrum; this exhibits long wavelength.bands which may indicate a product of addition to the pyridinium ring. The photohydration of pyridine to 2-pyridone by irradiation in alkaline aqueous solution proceeds by hydrolysis of a Dewar pyridine intermediate. Some years ago this was proposed as an undergraduate experiment,63 and improvements to the procedure have now been pub1 ished 64

.

Ii/4: Phorochemisiry of Aromatic Coinpounds 3.

239

Substitution Reactions

Photochemical substitution of groups on nitroanisoles can occur by direct attack of a nucleophile upon the excited state of the arene (the SN2Ar* mechanism) ; this mechanism frequently involves i p s 0 attack of the nucleophile at a substituted ring carbon followed by departure of the substituent as an anion. An alternative mechanism for substitution in nitroanisoles involves electron transfer from the nucleophile to the excited arene followed by a coupling-elimination sequence. The mechanistic alternatives lead to a different regiochemistry of substitution. This is exemplified in a full paper,65 which expands on an earlier preliminary report, 66 describing the outcome of the photosubstitution reaction of 4-nitroanisole with 1hexylamine and ethyl glycinate. The authors argue that ethyl glycinate, which displaces the nitro group to yield ethyl N-(paramethoxyphenyl) glycinate, reacts by an SN2Ar* mechanism, while 1hexylamine, which displaces the methoxy group to give N-hexyl-4nitroaniline, reacts by electron transfer from the amine to the triplet excited state of the nitroanisole followed by coupling of the amine nitrogen to the methoxy-substituted carbon of the arene. The authors report the results of semi-empirical calculations of the energies and charge distributions nf the excited state and intermediates and attempt to correlate these with the observed regiochemistry. Similar reactivity is seen for 2-fluoro-4-nitroanisole upon irradiatlon with a number of aminesi6’ the photoproducts are a mixture of the appropriate l-alkylamino-22-alkylamino-4-nitroanisole. These fluoro-4-nitrobenzene and correspond to displacement of the methoxy or fluorine substituent, respectively. The reaction is complicated by competing displacement of the fluorine and methoxy substituents by solvent (aqueous methanol). On the basis of quenching and kinetic experiments the authors suggest68 that fluoride substitution occurs by SN2Ar* attack of the amine on the x-x* triplet excited state’of the anisole derivative while displacement of the methoxy group takes place via electron transfer from the amine to the n-n* triplet excited state. Much of the interest in photosubstitution reactions originates An in their potential application to photoaffinity labelling. example has been published6’ of the use of 2-f luoro-4-nitroanisole for this purpose in which the methoxy group is displaced by the

240

Photochemistry

nitrogen atom of amino acid derivatives. In a similar application the photosubstitution reactions with amines and hydroxide ion of the steroids (91) and (92) have been studied.70 Photolysis of (91) in an aqueous solution of hexylamine resulted in selective displacement of the methoxy group by hydroxide, while with (92) the products isolated following irradiation in aqueous methylamine were tentatively assigned structures corresponding to displacement of one or other of the two methoxy groups by the amine. Quantum yields have been determined for displacement of alkoxy groups in 3-alkoxynitrobenzenes by hydroxide ion in aqueous and micelle solutions to give 3-nitrophenol. 71 The quantum yields are higher in the micellar medium. The photolysis of 3-nitrophenol itself has been studied in aqueous solution.72 The quantum yields for disappearance are very low (in the order of lo-') and numerous products are formed which include resorcinol, nitroresorcinol and nitrocatechols. A chain (Sml) mechanism is proposed for the light initiated substitution of halogen by sulphite in haxo-substituted naphthols. 73 The reaction is initiated'by electron transfer from sulphite to an excited sensitiser dye. The sulphite anion radical produced then attacks the halonaphthol; loss of halide from this species to give the product is coupled with electron transfer from a second sulphite, so propagating the chain. Several papers have appeared reporting further examples of the light induced, electron transfer mediated substitution reactions of cyano arenes in polar ~ o 1 ~ e n t s . In ~ ~ these - ~ reactions ~ ~ ~ ~ the first step is electron transfer from a potential nucleophile to the singlet excited state of the arene. Depending upon the conditions and the solvent, the radical cation-radical anion in-cage pair can then conibine to produce a substitution product by an addition-elimination sequence. Alternatively, if the radical cation is sufficiently acidic then it can lose a proton (possibly to the radical anion) and the resultant radical either couples with the radical anion or with the radical produced from it by protonation. If nucleophiles such as water or alcohol are present then these can intercept cationic sites also. The arene can yield products of substitution resulting from attack at positions adjacent to substituents already present on the ring, or substitution products arising from i p s 0 attack. Thus irradiation of 1,3,5-tricyanobenzene with an electron rich alkene

II.4: Photochemistry of Arotmtic Compounds

241

such as 2,3-dimethyl-2-butene yields the indanone derived from hydrolysis of (93);74 this may result from coupling of the initially produced radical ion pair to give (94) followed by cyclisation. If toluene is used as the electron donor instead of alkenes, the toluene radical cation produced ionises to give a benzyl radical which attacks the tricyanobenzene radical anion to give, ultimately, a mixture of (95) and (96).75 With alkenes in polar solvents 2,4-dicyanopyridine reacts similarly to give products of the type (97);76 however if the reaction is performed in less polar solvents then proton transfer in the initially produced radical 'ion pair apparently competes to produce an ally1 radical and pyridyl radical. These couple and eventually give (98). With amine donors coupling of the radical ion pair results in the isolation of (99) and (100).77 Arnold has previously reported the photosubstitution reactions of alkenes with 1,4-dicyanobenzenes in the presence of nucleophilic solvents such as alcohols.78 Here the light induced electron transfer is followed by attack of the solvent on the alkene radical cation prior to coupling with the arene radical anion. The reaction has now been examined using the alkenes a- and B-pinene in methanol s~lution.~' The structures of the products indicate that the pinene radical cation undergoes a ring opening reaction before attack by methanol and coupling with the arene. Thus with 6-pinene the radical cation produced by transfer of an electron to the singlet excited state of 1,4-dicyanobenzene is (101) which opens to (102) and is attacked by methanol to give (103) prior to reaction with the dicyanobenzene to furnish the substitution product. mechanism of photosubstitution light induced In the 1,S electron transfer from a good nucleophile to an acceptor such as an aryl halide yields the arene radical anion which loses halide to produce an aryl radical. This couples with the nucleophile to give an intermediate which proceeds to product by transferring an electron to a second aryl halide, thus propagating a chain mechanism. This has now been successfully applied to the coupling of N-methyl-2pyrrolidone enolate (104) with various aryl halides by photolysis in liquid ammonia. The products (105) are obtained in high yield." The efficiency of the analogous reaction of haloaryl ketones with benzene thiolate has been compared with that of the same reaction induced electrochemically.

242

Photochemistry

NC

NC (93)

(94)

Ncq Ph

CN (96)

CN

(97)

6

CN

(99)

Q Q

Q-

Me/

0

OMe

QAr Me/ 0 (106) R = NO2, R' = Me, CH20H, or CH2CI (107) R = CN, R' = Me, CH20H, or CH2CI

W4: Photochemistry of Aromatic. Compounds

243

Photochemical cyanation of nitroimidazoles (106) to give cyanoimidazoles (107) has been reported as a synthetically useful procedure. 82 Irradiation of aryl vinyl halides such as (108) in the presence of nucleophiles leads to substitution of the halide to give (109). If a para-alkoxy substituent is present on the aryl group geminal with the halide, then ips0 substitution can also occur if good nucleophiles are used. This leads to the formation of products (110) in addition to (109) These reactions are understood in terms of the intermediacy of a vinyl carbocation represented by (111) and (112).83184 Evidence has now been obtained for the intermediacy of such c a r b o ~ a t i o n s . ~ ~ ~Thus ' ~ irradiation of (113) in the presence of

..

bases strong enough to deprotonate the alcohol appreciably results in the intramolecular trapping of the carbocation by ips0 attack to give (114),85 while irradiation of (115) in ethanolic sodium ethoxide gives (116) and (117).86 The formation of (116) and (117) requires a 1,2-aryl shift in the initially produced vinyl carbocation (118) to give more stable (119) which is then'trapped by the ethoxide. These photochemically produced vinyl carbocations have also been trapped by cyanate and thiocyanate nucleophiles. 87 The efficiency at various values of pH of the photo-Smiles rearrangement of (120) to give (121) has been examined in the presence and absence of a-cyclodextrin. 88 The reaction is thought to proceed by intramolecular attack of the side chain amine upon the photo-excited benzene ring at the position meta to the nitro group. Thus the meta compounds (120) rearrange to (121) via a spiro intermediate resulting from ips0 attack of the amine on the ring at the alkoxy-substituted position. The ortho and para analogues (122) and (123) react differently; (122) yields (124) while (123) produces These products are formed by a mixture of (125) and (126).89 rearrangement of dihydro intermediates resulting from attack of the side chain amine upon the position meta to the nitro group; these dihydro intermediates have now been observed by 'H-nmr spectroscopy and structures assigned. As indicated in the introduction, the photolysis of halothiophenes and halofurans in the presence of arenes to yield arylheterocycles has been reviewed.' In this reaction the aryl group couples at the position on the heterocycle formerly occupied by the halogen. The reaction is particularly efficient if a ketone or

Photochemistry

244 Ar\

c=c(

Ar'

Ar Nu

(109)

Ph

Me0

Me0

0/(CH2)nNH2

(120)n = 2 o r 3

245

IV4: Photochemistry of Aromatic Compounds NO2

g

I

ACH2)”OH

H

(121)n = 2 o r 3

Q

246

Photochemistry

aldehyde carbonyl is present in the 2-position of the heterocycle. The effect upon the reaction of the presence of a cyano or carbomethoxy group in the 2-position of the heterocycle has also been reported. 91 The carbomethoxy group is equally effective for the promotion of the photoarylation reaction. However, the cyano group is deactivating so that 2-bromo-5-cyanothiophene is photochemically inert in the presence of arenes while the 2-iodo analogue undergoes photoreduction to 2-cyanothiophene only. The coupling of a thiazoline ring to the 2-position of 2-iodothiophenes and 2iodofurans possessing acetyl or carboxaldehyde groups in the 5position has also been e~emplified.’~The reaction has been extended to the coupling of 2-halofurans and thiophenes containing a carbonyl substituent in the 5-position with aryl acetylenes and styrenes.9395 For example, irradiation of 2-iodothiophene-5-carboxaldehyde with phenyl acetylene and styrene yields (127) and (128), re~pectively.’~ In what may be a related reaction irradiation of pyrrole with acetone is reported to yield a tetrapyrr~le.’~ In other new reports of photoarylation reactions, photolysis of polyhalobenzenes in benzene has been shown to yield terphenyls” while 4-iodoterphenyl couples with both benzene and pyridine on Photolysis of 2-iodopyridine in irradiation with U.V. light. benzene gives 2-phenylpyridineI and 2-f luoropyridine irradiated in

’’ ’’

the presence of indole gives 2- (2-indolyl)pyridine. loo Sulphur extrusion and biaryl formation is reported for (129) which gives (130) on irradiation in the presence of trimethyl phosphite; however, the reaction fails for (131), which is recovered unchanged.”’ Biphenyls are also the major products formed on irradiation of triphenylmethanelo2 and various triphenylphoslo* phonates. The conversion of naphthalene to 2-naphthoic acids by irradiation with carbon dioxide and electron donors (e.g. amines or dimethoxybenzene) has been further investigatedlo5 and the quantum yields of the reaction measured for different solvents and donors. Electron transfer also occurs in the photochemical phosphonation of naphthalene and phenanthrene achieved by irradiation with trialkylphosphites and electron acceptors such as l13-dicyanobenzene.lo6 The photonitration of phenol in aqueous solutions of nitrate ion has been reported”’ and phenols have been prepared by irradiation of substituted benzenes with the aromatic N-oxide (132) lo’

.

II/4: Photochemistry of Aromatic Compounds

247

U.V. light irradiation of catharanthine (133) with vindoline (134) in aqueous solution is reported to give vinblastine (135) in 15-20% yield. log The major product of excitation of benzaldehyde is benzoin which is produced by coupling of the ketyl and benzoyl radicals formed by hydrogen abstraction from a ground state benzaldehyde molecule. When the reaction is performed using solid complexes of benzaldehyde with B- or 7-cyclodextrin some asymmetric induction is observed in the benzoin; in addition, the two radicals can combine in a competitive mode such that the substitution product 4-benzoylbenzaldehyde is also obtained. 'lo Two Japanese patents have been filed which deal with the photochemical chlorination of pyridine. '11# Friedel-Crafts acylation of furan gives 2-acylfurans. A complementary photochemical procedure has been published for the preparation of 3-acylfurans which involves irradiation of a furan in the presence of a thioamide.'l3 The reaction is proposed to occur by regioselective Paterno-BUchi addition of the thioamide to the furan; the thietane produced then eliminates and hydrolyses to the observed product. Irradiation of chlorophyll a with bis-2-(5-nitropyridyl)disulphide results in the substitution of the 5-nitropyridylthiyl group into a variety of positions in the chlorophyll m01ecule.l~~ Some of

'*

the isomeric products were partially purified and characterised by 'H-nmr spectroscopy, and it was concluded that in these substitution had occurred at the meso positions of the porphyrin ring. Photolysis of alkyl carboxylic acids with lead (IV) acetates can generate alkyl radicals by decarboxylation and this has been used to place an aryl group on a cubane. Thus photolysis of a benzene solution of the cubane dicarboxylic acid (136) containing lead tetraacetate yields the phenyl cubane (137) '15 A mechanistically related

.

procedure uses iodosobenzene diacetate for the photochemical arylation of cyclohexane carboxylic acid with heteroaromatics such as pyridine or quinoline. '16 Irradiation of slurries of semiconductors in polar solvents containing arenes can result in electron transfer from the arene to photogenerated holes in the semiconductor; the fates of the radical cation of the arene can include the formation of products of substitution. Alternatively, in aqueous systems interaction of the

Photochemistry

248

(134)

a

CN

gRJtL Br

H02C

R

I OH

(139) R = CI (140) R = H

(138)

(136) R = C02H (137) R = Ph

H (141)

F!

R C02Me H (143) R = CI (145) R = H

(144) R = F (146) R = OMe

R/4: Photochemistry of Aromatic. Compounds

249

photogenerated holes with water can produce hydroxyl radicals which attack arenes present to yield products of substitution. This has attracted interest as a method for the destruction of pollutants and has been applied to the degradation of aqueous solutions of lignin by-products of the Kraft process used in the wood pulping and paper The lignins are polyhydroxy acylbenzenes manufacturing industry. '17 which can be effectively hydroxylated and ultimately oxidised to carbon dioxide under these conditions. Several groups have also published details of the phenols and polyhydroxybenzenes formed from benzene, chlorobenzenes and chlorophenols upon U.V. light irradiation of titanium or zinc oxidelalp122 suspended in aqueous solutions of the arenes. Under these conditions the arene rings are hydroxylated and the position of substitution is directed by any hydroxyl groups already present in the ring. A number of new reports on the removal of chlorine or other halogen from aryl halides by direct photolysis have appeared. Thus the degradation of chlorophenols in the atmosphere has been studied123 as has the wavelength dependency of the solution phase dechlorination The herbicide bromoxynil (138) of tetrachlorodioxins 124

.

photodebrominates in waterla5 and the antidepressant clomipramine (139) photodechlorinates to imipramine (140) when photolysed in aqueous solutions containing 8-cyclodextrin. Photodechlorination of polychlorinated biphenyls in aqueous solutions of cyclodextrins has also been in~estigated.'~~ In ethanol solution U.V. light irradiation of the heterocycle (141) results in dechlorination although for (142) loss of the sulphonamide group occurs instead.128 In order to probe the reason for the low fluorescence efficiency of 4-haloindoles relative to the 5- and 6-halo substituted isomers Yang and coworkers have examined the photochemistry of the 4-chloro- and 4-f luoroindoles (143) and (144) in methanol solution. 12' Loss of halogen occurs in both cases but apparently by different mechanisms since (143) gives the product of photoreduction (145) while (144) is converted to the 4-methoxy derivative (146). Irradiation of haloarenes in the presence of amines in polar solvents promotes electron transfer from the amine to the excited arene; this can be followed by loss of halide from the resulting arene radical anion. Ultimately dehalogenated arenes are obtained. Two new reports of this process have been published, one for 9,lOd i c h l o r o a n t h r a ~ e n eand ~ ~ ~the other for hexabromobenzene lJ1

.

250

Photochemistry

Several Physical studies, not involving product isolation, of the photo-dissociation of aryl halides have been published. 132-135 4.

Jntramolecular Cvclisation

Reactions

The cyclisation of the singlet excited state of stilbene systems is a concerted electrocyclic reaction which proceeds in a conrotatory fashion to yield trans-dihydrophenanthrenes such as (147). If the reaction is performed in the presence of an oxidant then aromatisation occurs and phenanthrenes are isolated. A new review of the reaction has appeared." L a a r h ~ v e n ' ~has ~ examined the fate of the dihydrophenanthrene intermediate (148) formed by photocyclisation of the styryl benzophenanthrenes (149). When the reaction is performed in the presence of iodine as oxidant the expected arene (150) is obtained but in the absence of the oxidant compound (148) rearranges to the more stable, isolable dihydroarenes (151) and (152). This rearrangement is base catalysed and the proportions of (151) and (152) isolated depend upon the nature of the base and the solvent. Benzo-fused pyridinium systems also undergo the reaction13" 13' and a new example, the photocyclisation of (153) to (154) in the The quantum efficiency of presence of iodine, has been published. 13' photocyclisation of 1,2-bis-(a-naphthyl)ethyleneto dihydropicene has been examined in m i ~ e l l e s ' ~and ~ the cyclisation of [ 51-helicene (155) to dihydrobenzperylene (156) has been carried out at 4K; (156) was characterised by its fluorescence and phosphorescence emission. 14' The effect of different substituents on the photocyclisation of the symmetrically tetra-substituted stilbene (157) has been probed. 14* A s might be expected, cyclisation is inefficient when the substituents are bulky tert.-butyl groups. More interestingly, evidence is seen for two equilibrating dihydrophenanthrene intermediates (158). Presumably these are conformers which are slow to interconvert due to a barrier to passage of the closest R groups past one another. If an ortho substituent is present in a stilbene it can direct the regiochemistry of the photocyclisation towards or away from the position of the substituent. In addition, it can act as a potential leaving group, obviating a need for an oxidant to generate a phenanthrene from the dihydrophenanthrene intermediate. Thus, for

II,4: Photochemistry of Aromatic Compounds

25 1

R

(1 57) R = But, F,or OMe

R

$+R

/

Me0

A (158)R = But, F, or OMe

Me0

Photochemistry

252

example, compound (159) can photocyclise to either (160) or (161), both of which are in thermal and photochemical equilibrium with (159) under the reaction conditions. Elimination of methanol from (160) results in irreversible conversion of the stilbene to the phenanthrene in competition with oxidation of (161) to the alternative phenanthrene product (162). Mallory has now reported that performance of the reaction in the absence of an oxidant in alcohol solvent containing sulphuric acid favours elimination of methanol from (160) so that the regioselectivity of the cyclisation is improved. 143 Since dihydrophenanthrene formation is a singlet excited state reaction, stilbenes which are substituted by groups that accelerate intersystem crossing photocyclise less readily; however, irradiation of anthraquinone with diphenyl acetylene yields (163) as the primary photoproduct which apparently photocyclises to (164) under the irradiation conditions. '44 Aryl groups attached to adjacent positions on heterocyclic rings frequently photocyclise to dihydrophenanthrenes readily. Thus the reaction has been reported to proceed smoothly for (165),14' (166),146 (167),14' and (168),14* although it fails for the sulphur Heterocyclic arenes linked by analogue of (168), compound (169).I4' an ethylene also photocyclise and new examples of structures which do this are (170),150 (171),15' (172),l3 (173),lS2 (174),lS2 (175)lS3 and (176).lS3 An example has also been published of a 1-aryl-2heteroarylethylene possessing structure (177) which cyclises to (178) in the presence of oxygen as the oxidant.lS4 In a reaction which is mechanistically related to the stilbenedihydrophenanthrene photocyclisation, the anilide derivative of (178), compound (179), photocyclises to give (180) '54 A s mentioned in the Introduction to this chapter, such enamide cyclisations have now been reviewed.' Other new examples of this reaction are the formation of (181) from (182)15' and the formation of the regioisomers (183) and (184) from (185)156 by irradiation of the enamides in the

.

presence of sodium borohydride. These enamide photocyclisation reactions can be viewed as an electrocyclic closure of the amide contributing structure (186) to give (187); in the presence of borohydride the iminium function in (187) is reduced to the observed products.

8

& 253

IV4: Photochemistry of Aromaric Compounds

&o

\

’

OMe

0

\

\

0

/

\

\

/

0

0

(167)

I

CH3

(168) X = 0 (169) X = S

I

CH3

(174) X = S (176) X = NCH3

(177) x = 0 or s

(178) X = 0 or S

254

Photochemistry

W 4 : Photochemistry of Aromatic Compounds

255

The enamide photocyclisation can also proceed oxidatively or, if a potential leaving group is present in an ortho position, rearomatisation by elimination of the ortho substituent in the dihydroaromatic intermediate can occur. This is the case with (188) which cyclises to (189).157 The formation of vinyl carbocations in the photochemistry of triaryl vinyl halides such as (108) was discussed in the previous section dealing with photosubstitution reactions. These compounds also undergo light induced homolytic cleavage of the halogen and yield triaryl ethylenes as products of photoreduction. The triaryl ethylenes are subject to secondary photochemistry characterised by stilbene-dihydrophenanthrene cyclisation.87 The vinyl carbocations produced by light induced heterolytic cleavage of the halide can be intercepted by nucleophiles such as cyanate ion; the vinyl isocyanates (190) which are produced are also photochemically labile and react by a 6 electron cyclisation pathway to give, after proton transfer, isoquinolines (191).87 A very similar photocyclisation is seen for the oxime ether (192) which yields quinolines (193) after elimination of benzyl alcohol from the primary photochemical product.lS8 The related oxime ether (194) photocyclises in an analogous manner to yield quinoxaline (195), although if the reaction is performed in the presence of boron trifluoride etherate then the system undergoes oxidative cyclisation to the phenanthridine (196) instead. The formation of a diphosphadihydrophenanthrene is not observed when diphosphene (197) is irradiated with U.V. light. Instead the phospha-indane (198) is isolated. This is presumably derived from cleavage of the P-P bond of (197) to generate an unstable phosphinidine (199) which inserts into an adjacent C-H bond. The photochemical stilbene-dihydrophenanthrene cyclisation is essentially a hexatriene-cyclohexadiene interconversion. This interconversion is also the basis of the photochromic properties of fulgides. The fulgides are formally arylidene-alkylidene derivatives of succinic anhydride, (200), which are in photochemical equilibrium with the corresponding cyclohexadiene, e.g. (201). The potential applications of these compounds for photo-erasable and photo-writable data storage have resulted in a great deal of effort devoted to the synthesis of new analogues and the characterisation of their photochromic properties. During the period of coverage of

Photochemistry

256

U

(190)

OCHPPh

(191)

(192) R = MeO, Me, or CI (193) R = MeO, Me, or CI

W4: Photochemistry of Aromatic Compounds

257

this chapter a number of patents and other reports have appeared in which new fulgides have been described or in which the properties of known fulgides in different media have been examined. Photocyclisation of the bis arylidene succinic anhydride (202) in the presence of iodine is followed by oxidative aromatisation to give mainly (203),174 which is derived from preferential cyclisation onto the unsubstituted ortho position of (202). This contrasts with the corresponding cyclisation reaction of stilbenes described above, in which an ortho substituent that is a potential leaving group directs the regiochemistry of cyclisation and obviates the need for an oxidant for rearomatisation. The authors do not indicate whether the reaction regiochemistry is reversed if iodine or air are excluded during the irradiation. 174 The thyroid hormone thyroxine, (204), is reported to yield a light. complex mixture of products when irradiated with U.V. However, the synthetic hormone analogue (205) photocyclises in fair Intramolecular displacement of yield to the dibenzofuran (206) 17' ortho halide is also reported to occur in the photocyclisation of (207) to (208),176and in (209) to (210),177 and for the enolate of (211) to give indole (212),17* Benzannelated indoles such as (213) have been found as products of photoextrusion of nitrogen from the triazole substituted naphthalenes such as (214) 17' Reductive closure to indanone (215) has been reported to occur during photolysis of phenyl ynone (216) 180 In addition, products of photoreduction and cleavage of the ynone were observed. The photochemical cyclisation of phenylamino-enones (217) is a route f o r the generation of tetrahydrocarbazoles such as (218).la' The reaction has now been applied to the synthesis of the Aspidosperma alkaloid aspidofractinine (219) la2

.

.

.

.

The photocyclisation of the pyridyl amine (220) to yield carboline (221) was described some years ago.la3 It has now been found that the regiochemistry of this reaction can be changed so that C-N bond formation occurs instead of C-C bond formation if a chlorine atom is present to direct the orientation of the c y c l i s a t i ~ n . ~ ~ ~ Thus irradiation of (222) yields the heterocycle (223). The photolysis of diphenylamines has been performed in the present of bromoform, chloroform and tribromomethylbenzene and is found to yield acridines (224).Ia5

258

Photochemistry

\

\

0

CI

CI

CI

Ar=

@ 0 L O

0

6 0

H

OMe

II.4: Photochemistry of Aromatic Compounds

259

H

(224) R = H, Me, or Br R'= H or Ph

@ MeoYR Me0

Me0

Me0

//

OMe

(225) R = Me (226) R = COZEt

Me0

OMe

(227)

Photochemistry

260

The alkaloid glaucine ( 2 2 5 ) has been prepared by reduction of the photocyclisation product ( 2 2 6 ) obtained from irradiation of ( 2 2 7 ) .186 The light induced coupling proceeds in very low yield. However, the chemical efficiency of the reaction was improved by utilising ( 2 2 8 ) in which the bromine substituent directs the regiochemistry of a stilbene-dihydrophenanthrene cyclisation v i a the Elimination of HBr from this species presumed intermediate ( 2 2 9 ) . yields the isolated product which on reduction also yields ( 2 2 5 ) . Photocyclisation of analogues of ( 2 2 7 ) in which not all of the phenol groups are methylated has also been described. 18’ Mariano has published a full account describing the results of a mechanistic study of the photocyclisation reactions of benzyliminium salts ( 2 3 0 ) .lS8 Irradiation of the iminium salts initiates light induced electron transfer from the excited arene to For systems the iminium group to give the biradical cation ( 2 3 1 ) . without o r t h o substituents this is followed by intramolecular coupling to give, ultimately, ( 2 3 2 ) . If an ortho-silylmethyl group is present, as in ( 2 3 3 ) , then the biradical cation ( 2 3 4 ) produced upon U.V. irradiation loses a silyl cation competitively so that ( 2 3 5 ) is also formed by an alternative mode of biradical coupling.

5.

Dinerisation Reactions

The photodimerisation of anthracenes normally occurs by a 4x+4x cycloaddition in which bonds are formed between the 9- and 10positions of an excited anthracene molecule and the 9- and 10positions of a ground state partner. The photodimerisation reaction has been investigated for two new anthracenes possessing structures ( 2 3 6 ) and (237).189#190 The formate ester ( 2 3 6 ) yields the usual 4x+4x dimer ( 2 3 8 ) , which possesses head-to-tail regiochemistry, in moderate yield. No products from photo-Fries cleavage of the formyl group were reported.lag A head-to-tail dimer was also obtained from the difluorophosphine substituted anthracene ( 2 3 7 ) ; the structure ( 2 3 9 ) was assigned and it was noted that rotation around the C-P bonds in the photoadduct is apparently prohibited, presumably because of interaction between the fluorines and the adjacent bridgehead methine.

261

II,4: Photochemistry of Aromatic Compounds

Me0

Me0

Me0

Me0

OMe

a I

Q R

R (230) (233) R = 2-CH2SiMe3

(232)

R (2311 (234) R = 2-CH2SiMe3

(235)

R’

I

f

(236) R’ = 0-C-H, R2 = Me (237) R’ = PF2, R2 = H

f

(238) R’ = 0-C-H, R2= Me (239) R’ = PF2, R2 = H

262

Photochemistv

The course of the anthracene dimerisation reaction can be changed by joining the two anthracene rings with a linkage connecting the 9-positions. In recent years it has been shown that not only does tethering of the anthracenes direct the regiochemistry of the dimerisation by forcing a head-to-head orientation of addition, but also, depending upon the length and nature of the linkage, it can result in 2x+4x addition instead of 4x+4x addition and the addition may involve bonding to the outer rings of the anthracene nucleus instead of across the central ring. An example of this is provided by the photochemistry of dianthrylketone (240) and dianthrylmethanol (241) lgl,lg2 The dianthrylmethanol photodimerises efficiently from

.

its singlet excited state to give the %ormaln 4x+4x head-to-head adduct (242), while the dianthrylketone photodimerises to a 2x+4x adduct (243) in which an outer ring of one of the anthracenes has added in a Diels-Alder manner across the 9,lO-positions of the second anthracene. This latter reaction proceeds much less efficiently and from the triplet excited state. The x-ray crystal structures of (240) and (241) have now been determined with a view to probing whether their different photoreactivity has its origins in structural effects in the linked anthracenes. The crystal structures show no differences between the two molecules which would explain their contrasting photoreactivity and the authors conclude that the difference arises from the change in the reactive excited state.lg3 Normal head-to-head 4x+4x photodimerisation has been found for the series of anthracenes (244)-(251) which are all linked by a three carbon chain. The dimers have structures (252)-(259), respectively, and their quantum yields of formation have been determined and are found to vary ~ide1y.l'~ The authors find that the reaction proceeds by either the singlet excited state or the triplet excited state, depending upon the nature of the linkage, and they attempt to correlate the variation in quantum yield with the ground state conformations of the molecules. Desvergne and coworkers recently reported that photodimerisation of the siloxane linked anthracenes (260) yields an unusual 4x+4x adduct (261) in which the 9,lOpositions of one anthracene had added across the 1,4-positions of the second ring.lg5 The same group has now examined the silane linked analogue (262) and find that it also photodimerises, although the product was not isolated and its structure fully characterised. While not a dimerisation, the recently published photochemistry of the novel cyclophanes (263) will be reported here because of its

263

W 4 : Photochemistry of Aromatic Compounds

\

\

&

(240) X = CO (241) X = CHOH (244) X = CH2CHOHCH2 (245) X = C H d O - C H 2 (246) X = CH&(OH)MdH2 (247) X = CHOHCH2CHMe (248) X = CHMe-CO-CH2 (249) X = CH2CH2CO (250) X = CHMeCHMeCO (251) X = CH2CHMeC0 (260) X = SiMezOSiMep (262) X = SiMe2CH2SiMe2

(242) X = CHOH (252) X = CH2CHOHCH2 (253) X = CH2COCH2 (254) X = CH2C(OH)MeCH2 (255) X = CHOHCH2CHMe (256) X = CH2COCHMe (257) X = CH2CH2CO (258) X = CHMeCHMeCO (259) X = CH2CHMeCO

(243)

R

CN'

+ j J (263) R

&g

RN R = CHO, H, S02Ph, COCH3, COCF3, Me, or COCMe3

(264) \

264 similarity to the photochemistry of the linked product of U.V. light irradiation of (263) is the in which para positions of the benzene ring have positions of the anthracene.”’ The reaction

Photochemistry anthracenes.

The

4x+4x adduct (264)

added to the 9,lOis thermally and

photochemically reversible and the efficiencies and rates of the forward and reverse processes depend upon the nature of the nitrogen substituents. A cage compound is formed from photodimerisation of the tethered naphthalene rings of (265); the product has structure (266) which is derived from 2x+2x cycloaddition of the side chain propenyl groups as well as double 2x+4x cyclodimerisation between the naphthalene rings. lg8 When the length of the polymethylene chain linking the naphthalenes was extended, or when the substituents on the side chain ethylenes were changed, the cage compound was no longer formed and the only products isolated were from 2n+2x cycloaddition of the side chain ethylenes. The photodimerisation of acenaphthylene (267) yields s y n and a n t i adducts (268) and (269). In solution the singlet excited state forms the s y n dimer only, and the triplet excited state forms both the s y n and the a n t i dimers.lg9 The photodimerisation has now been examined for acenaphthylene included within zeolites. The variation of the s y n to anti product ratio has been determined as a function of the nature of metal cations present as counterions in the zeolite structure and as a function of the concentration of acenaphthylene in the zeolite.’” The triplet excited state dimerisation is promoted in zeolites containing heavier atoms as the cations. The benzanellated analogue of acenaphthylene, aceanthrylene, (270), also photodimerises and yields four isomers resulting from 2x+2x cycloaddition of the double bond in the five-membered ring. The mechanism of this reaction in solution has now been investigated using sensitising and quenching experiments and by observation of the effect of external heavy atoms and the effect of variation of concentration of (270).201 The results indicate that the dimers are formed non-concertedly via a 1,4-triplet biradical intermediate produced by interaction of triplet excited aceanthrylene with a ground state partner. The anhydride of acenaphthylenedicarboxylic acid, (271), has been reported to photodimerise in similar manner to acenaphthylene itself, but only in the solid state and not in solution.’” The dimer

W 4 : Photochemistry of Aromatic Compounds

Y

265

Photochemistry

266

has the a n t i configuration and is converted back to (271) by irradiation at the appropriate wavelength. The phenanthrene imide (77) is in photochemical equilibrium with a similar adduct possessing syn stereochemistry, as shown in structure (272).49 Photolysis of the perfluoro 4,6-diisopropyltriazine (273) extrudes nitrogen and yields the azete (274) which dimerises thermally to (275).203,204 It has been reported205that photolysis of deacetylaspidospermine (276) yields a dimer assigned the structure (277). The reaction only proceeds when dichloromethane is used as solvent and the product is reported to be formed in 60% chemical yield and with a quantum yield of 0.92! 6.

Lateral Nuclear Rearransements

This section is concerned with the photochemical rearrangements of substituted arenes in which a substituent cleaves to give a fragment which recombines with the arene, usually at the ortho or para position of the ring. In the case of the rearrangement of phenyl esters to give acyl phenols, and the rearrangement of anilides to ring acylated anilines the reaction is termed the photo-Fries rearrangement. The photo-Fries rearrangement of phenyl acetate in aqueous solution in the presence and absence of 6-cyclodextrin has been reexamined.206 In aqueous solution a mixture of phenol and the ortho and para isomers of hydroxyacetophenone is produced. When 6-cyclodextrin is present the quantum efficiency of formation of phenol and of the ortho product is increased. It is suggested206 that the enhanced photochemical yield of the o r t h o product reflects the less polar environment of the cyclodextrin cavity while the increased quantum yield of phenol formation reflects the availability of abstractable hydrogen within the cyclodextrin cavity. The suitability of the photo-Fries reaction for the conversion of phenyl succinates (278)-(283) to the phenols (284)-(289), respectively, has been examined and compared with the corresponding thermal reaction catalysed by aluminium trichloride. 207 Photochemists will be pleased to read that the authors conclude that the photochemical route is more general than the ground state procedure

II.4: Photochemistty of Aromutic Cornpounds

267

(290) R‘ = H,R2 = Ac, R3 = OH (291) R’ = OH,R2 = Ac, R3 = OMe (292) R’ = OMe, R2 = Ac, R3 = OH

(293) R‘ = HI R2 = AC (294) R1 = OAC, R2 = OMe (295) R’ = OMe, R2 = OAc

PODq 0

(297)

& S

0

Photochemistry

268

and gives superior chemical yields if alkoxyl or hydroxyl groups are already present on the aromatic nucleus. Photo-Fries rearrangement has been suggested as a procedure for the preparation of acetyl hydroxychromans (290)-(292) from (293)The photo-Fries rearrangement of aryl (295), respectively. 208 thiosalicylates (296) has also been examined for the preparation of benzophenones such as (297) which can be converted thermally to the thio-xanthones (298) 209 The photo-Fries rearrangement of the series

.

of acetoxyindoles (299)- ( 302) has been found to be regioselective;2'0 Thus the 5-acetoxyindole (299) yields mainly (303), while the 6acetoxy isomer (300) gives mainly ( 3 0 4 ) , and 5,6-diacetoxyindole yields predominantly (305). The doubly substituted anilide esters (306) could, in principle, give products from photo-Fries cleavage of either the ester or the anilide groups; the major products are reported to be the hydroxyacylanilides (307) which derive from selective cleavage of the ester. 211 Products resulting from intramolecular photo-Fries reaction of an anilide have been observed during the U.V. light irradiation of the azacholestenones (308); the rearranged compounds have structure (309).212 Photo-Fries reactivity has also been found for N-aroyldiphenylamines 213 Thus photolysis of N-benzoyldiphenylamine yields a mixture of carbazole and ortho and para benzoyldiphenylamine. When the reaction is carried out in the presence of iodine the ortho isomer cyclises to (310) in a secondary photochemical step. A full paper has appeared214 expanding on the previous report2" concerning the photo-Fries rearrangements of N-aroylcarbazoles. In polar solvents mixtures of carbazole and both (311) and (312) are obtained, while in non-polar solvents carbazole and low yields of (311) only are isolated instead. An intramolecular cyclisation reaction competed successfully with rearrangement in the case of N-(ortho-chlorobenzoy1)carbazole to yield (313) as the major product. In a report from a different laboratory216 the photo-Fries

.

rearrangement of N-sulphonylcarbazole is described and the products identified as (314) and (315); in contrast to the results described above, these authors report that N-benzoylcarbazole is photochemically inert. Photo-Fries migration has been reported for the sulphonate A homolytic cleavage-recombination ester (316) to give (317).217 mechanism akin to the photo-Fries rearrangement of sulphonamides may

iU4: Photochemistry of Aromatic Compounds

269

1

R2

0 R3)$-R4

NHCOCH3

(299) (300) (301) (302) (303) (304) (305)

R' R2 H H H H H H CH3 H H Ac H H H H

R3 R4 OAc H H OAc OAc OAc OAc OAc OH H H OH OAc OH

R5 H H H H H

NHCOCH3

(306)

(307)

AC

Ac

(308) R = H, Me,or OMe

(309)R = H, Me, or OMe

R' R2 H (311) COPh H COPh (312) H (314) S02Ph H S02Ph (315)

w (313)

Photochemistry

270

aSiMe2SiMe3 SiMe2SiMe3 Si Me3 (322)

(323)

(325)

(324)

(326)n = 10,12,or 15

(327)

II.4: Photochemistry of Aromatic Cornpounds

27 1

be involved in the photochemical rearrangement of the herbicide bentazone (318) to give (319).218 Two groups have published results of a study of the photochemistry of diphenyliodonium salts. 219e220 They conclude that the reaction can proceed from both the singlet and triplet excited states by both heterolytic and homolytic cleavage of the phenyliodonium bond. Among the products isolated are iodobiphenyls arising from recombination of the fragments. Similar photochemistry is seen for sulphonium salts.221 In these reactions fragmentation of the arylonium salt followed by coupling of the fragment to the aryl ring leads to a neutral product and the release of a proton. This has been discussed as a method for photoactivation of acid catalysed polymerisation processes. 221 A fragmentation-recombination pathway accounts for the formation of ortho- and para-aminophenol by photolysis of N-phenylhydroxylamine222 and for the formation of (320) from the ally1 phosphate (321) 223 The arylsilane (322) undergoes light induced silicon-silicon bond cleavage and recombination to (323) which is trappable by isobutene in an ene reaction to give (23) whose photochemistry was discussed above. Similar cleavage of a siliconsilicon bond is presumably occurring during the photochemical conversion of the thienylsilane (324) to give (325).224 Norrish Type I cleavage of benzylketones occurs very readily to yield benzyl and acyl radicals. In competition with other processes, these can couple by attack of the acyl radical upon the aromatic nucleus of the benzyl radical to give as a product an acyl toluene derivative. Turro has examined the photochemical properties of the 2-phenylalkanones (326) in solution and complexed with cyclode~trins.~ In~ ~solution the major fate of the Type I biradicals formed is intramolecular recombination by attack of the acyl radical at the para position of the benzyl radical to generate (327). However, complexation of (326) within the cyclodextrin cavity inhibits formation of (327) and promotes disproportionation of the biradicals to give (328). Turro has also examined the photochemistry of dibenzylketone included in zeolites.226 Recombination of the Type I radicals in this case can produce a mixture of (2-methylpheny1)benzylketoneand (4-methylpheny1)benzylketone;decarbonylation of the acyl radical competes and leads to the isolation of 1,2diphenylethane. It is found that the relative yields of the products

.

Photochemistry

212

can be varied by altering the nature of the metal cation present in the zeolite structure, or by applying a magnetic field across the sample during the irradiation. These effects reflect competition between intersystem crossing in the radical pairs and their lateral motion within the zeolite. The photochemistry of a-phenoxy acetophenones has been studied. 227 Cleavage occurs to generate phenoxy and phenacyl radical pairs which suffer several fates, including formation of compounds such as (329) which can arise from attack of the phenacyl group upon the aryl ring of the phenoxy radical. 7.

: e ' t

The photochemistry of ortho-alkyl aromatic carbonyl compounds has been studied for many years' and is characterised by a photoenolisation reaction in which light induced migration of hydrogen from the ortho-substituent to the carbonyl oxygen occurs to generate reactive ortho-quinodimethanes. The hydrogen migration can occur from the triplet excited state of the carbonyl compound, in which case the initial photochemical step is similar to that of the Norrish Type I1 reaction of the triplet excited state of saturated ketones and generates the triplet 1,4-biradical (330). Spin inversion allows the biradical to relax to the h' and 2 enols (331) and (332). The hydrogen migration can also occur from the singlet excited state of the aryl carbonyl compound in which case it can be regarded as a concerted sigmatropic hydrogen shift by which the 2 enol (332) is produced stereospecifically. The formation of the 2 enol is one of the few singlet excited state processes of aryl carbonyl compounds which can compete with intersystem crossing. The triplet 1,a-biradical (330) can be regarded as the triplet excited state of the E and 2 enols (331) and (332). In support of this view, Scaiano has shown that the transient observed by flash photolysis of ortho-methylacetophenone and assigned to triplet biradical (330) can be quenched by ground state oxygen. This presumably occurs by a triplet-triplet annihilation mechanism to give singlet oxygen which was detected by its emission.228 The enol formed by irradiation of ortho-phthalaldehyde has been observed spectroscopically in a nitrogen matrix at 15K and assigned structure (333).229 Photolysis of the enol gave the phthalide (334) as well as producing cleavage to benzaldehyde and carbon monoxide.

W 4 : Photochemistry of Arotncrtic Compounds

273 OH

(330)

OH

(335)

(334)

(333)

WN

C02Me

I

hO2 C02Me

Meo2c$J H

(337)

(336)

w

N

"

0

2

I

wn02 Me02C OH NO2

hO2 C02Me

(339)

(338)

akSiMe3 q eph WPih Ph

Ph

Ph

NHSiMe3

(343)

274

Photochemistry

Many unsaturated aryl substituents will abstract hydrogen from ortho groups in an excited state process closely analogous to the photoenolisation reaction of ortho-alkyl aryl ketones. F o r example, ortho-alkyl nitrobenzenes can be converted photochemically to nitroso arenes in which the ortho substituent has been oxidised. This is believed to proceed by way of intermediates with structure (335) which rearomatises by hydroxyl transfer from nitrogen to the adjacent benzylic position. This reaction has been reviewed in the Russian literature230 and is implicated in the photochemical degradation of the vasodilator nifedipine (336) to produce nitrosopyridine The nitroaryl quinoline (337) is in tautomeric derivatives. 231 equilibrium with the benzylidene form (338) and irradiation of this This reaction would appear to be an mixture leads to (339).232 example of nitro group abstraction of hydrogen from the benzylic position in (337) to yield an intermediate analogous to (335) which rearranges to (339); however, the authors discount this mechanism since they perform the reaction using light of wavelength such that only (338) is excited. The imine derivative of ortho-tolualdehyde, compound (340), also rearranges by a photochemical abstraction of hydrogen from an ortho position to give an intermediate which, based upon the nature of the adducts formed with dienophiles, is assigned structure (341) 233 The chemistry of the ortho-quinodimethane (342) generated by extrusion of carbon monoxide from 1,3,3-triphenyl-2This species cyclises to indanone has been reported. 234 benzocyclobutene (343) and also to an intermediate which is oxidised to 9,lO-diphenylanthracene. It is shownthat closure of the unstable intermediate (342) leading to the anthracene product can be induced photochemically. Agosta has been investigating the ability of ring nitrogen in aromatic nitrogen heterocycles to abstract hydrogen from alkyl side chains in a reaction which he has shown resembles the Norrish Type I1 6-cleavage reaction of the triplet excited states of ketones. 235, 236 Thus the pyrimidine (344) and the quinoline (345) yield the cleavage products (346) and (347), respectively. Aryl substituted alkenes and alkynes become more basic in the excited state so that electrophilic hydration of the double bond occurs under very mildly acidic conditions. Kresge has used this effect for the generation of an enol from mesitylacetylene in water,237 and McClelland has used the reaction for the generation of

.

275

II84: Photochemistry of Aromatic Compounds

OMe YOMe

Ph Ph OH

6;q & R

R

(344) R = C2H5 (346) R = H

(345) R = C2H5 (347) R = H

R'

CH$Ph3 CII

CH2CI

CH2PPh3 CI-

CHZCI

(350) R, R' = H or OMe

OH

I

(352)

Br

I

Ar3C-X CH2 (353)

(354)

(355)

(356) X = alkyl or CN

276

Photochemistry

a vinyl carbocation from para-methoxyphenylacetylene in trifluoroethanol. A number of papers concerning the formation of carbocations by photolysis of mono-, di-, and triarylmethane derivatives have been published. 239-244 Wan and Krogh have examined the photosolvolysis of

’”

9-fluorenol in methanol and propose that the primary step is heterolysis to form the fluorenyl carb~cation’~’ and a transient assigned to this ion has been observed by flash photolysis of 9-fluoren01.~~’ Flash photolysis has also been used to monitor the decay of the carbocations produced by excited state heterolysis of the carbinol (348)241 and the amide acetal (349).242 Photochemical cleavage of the phosphonium salt (350) has been reported to yield the corresponding diarylmethyl carbocation, which was also observed by flash p h o t ~ l y s i s , ~and ~ ~ a similar process may be involved in the photosubstitution of (351) to give (352) 244 The photochemistry of ketyl radicals is currently of interest and has been reviewed. 245 Photolysis of benzophenone ketyl radical

.

in acetonitrile solution has been shown to yield a mixture of 0-protonated benzophenone, formed by electron ejection, and benzophenone, formed by homolytic cleavage of a hydrogen atom.246 In non-polar solvents the cyclic analogues of benzophenone ketyl radical such as (353) are also reported to undergo hydrogen atom loss and regeneration of the parent ketone upon excitation. 247 Photolysis of triarylmethyl radicals has been described and leads to the production of triarylmethyl cations. 248 The above studies involved pulse excitation of a precursor to yield the short-lived radical species whose photochemistry was then probed by application of a second light pulse. Sequential two photo absorption is also apparently involved in the photochemistry of the 1,8-bis-(bromomethyl)naphthalene (354);249 photolysis of (354) with a high intensity laser source yielded acenaphthene (355), whereas no acenaphthene was formed using an arc lamp source for excitation. Carbenes and biaryls are generated during the photolysis of triarylmethane derivatives (356),250-252 and carbene intermediates have been trapped during photolysis of stilbene and 1,l-diphenylethylene. 253 The efficiency of the photochemical decarboxylation of arylacetic acids to give arylmethyl carbanions has been shown254 to be greatly enhanced if a hydroxyl substituent is present on the

II.4: Photochemistry of Aromatic. Compounds

277

methyl carbon; thus in aqueous acetonitrile at pH 11.5, under which conditions phenylacetic acid is photochemically inert, mandelic acid was photolysed to benzyl alcohol with a quantum yield of 0.4. A transient observed by flash photolysis of para-nitrobenzyl phosphonic acid has been assigned to the para-nitrobenzyl anion resulting from light induced heterolysis of the carbon-phosphorous bond. 255 Irradiation of dibenzosuberenol (357) in aqueous base leads to dibenzosuberone (358); this transformation is thought to proceed by ionisation of the benzylic proton thus generating a delocalised benzylic anion which can reprotonate at the asterisked positions shown in structure (359) to generate an enol which reketonises to (358) 256 The mechanism of the photochemical retro-aldol reaction of 2- (meta-nitrophenyl)-1-phenylethanol (360) has been studied;257 the nitrobenzyl anion (361) is the primary intermediate when the irradiation is performed in neutral water, but under acidic conditions the non-Kekul6 structure (362) is implicated. Evidence for this is the isolation of (363) as the final product, which may be formed by attack of water on (362) to give nitrosobenzyl alcohol (364) followed by condensation of the nitroso group with (362). Photolysis of the naphthylmethyl esters (365) gives naphthylmethyl radicals and phenylacetoxyl radical. The radical pair may transfer an electron to yield phenyl acetate and naphthylmetkyl cation which is quenched by nucleophilic solvent. Alternatively, the radical pair may escape their solvent cage and so yield free radical derived products. Pincock has estimated the rate constant for the electron transfer in the radical pair for different X-substituents on the naphthalene by monitoring the ratio of free radical products to ionic products produced in the photolysis reaction, and has correlated this rate constant with the free energy of the electron transfer reaction in the radical pair.258 The results are discussed in terms ofthe Marcus theoretical relationship between reaction rate constant and equilibrium constant. If electron transfer is thermodynamically feasible then an arene may act as a donor to an excited state species, or an excited arene can donate an electron to a ground state acceptor. In either case an arene radical cation is produced. If the radical cation possesses a benzylic substituent capable of ionising as a cation then this can be rapid and can lead to a benzylic radical. The final product of such a sequence results from benzylic substitution of the arene. A number of new examples of this type of reactivity have

.

Photochemistry

278

o^""

(369)

0

Ph

S02NHR

(373) w c H P h A O3 o*oq

'

Me0

f

ocH3

Ph

(372)

OMe

(374)

CH,

(-$ OMe

phisiMe3

NC

(376)

(375) Ph-

(CH2),-NHMe

CN

(377)

(378)n = 1-5

W 4 : Photochemistry of Aromatic Compounds

279

appeared during the period of coverage of this report. Albini has published the results of a systematic and comprehensive study of a series of benzyl compounds with the general formula PhCHRX where X is hydrogen or C, 0, S, Si, or Sn bonded sub~tituents.~~'Irradiation of 1,4-dicyanonaphthalene in the presence of these compounds induces electron transfer to give the radical cation (PhCHRX)" and the radical anion of the sensitiser. The radical cation can ionise to a benzyl radical by loss of a proton, or of the species X+; from product studies the competition between these two pathways is A new method for fluorination of determined for different X.259 benzylic positions (and of any readily oxidised centre capable of producing a stable carbocation) has been proposed which involves U.V. light illumination of titanium dioxide in the presence of the benzyl substrate and fluoride ion.260 It is suggested that electron transfer from the benzyl compound to a photogenerated hole in the semiconductor generates a radical cation of the benzyl derivative ; loss of a proton and a second electron to the semiconductor then yields the benzyl cation which is quenched by fluoride. Arnold's group has published further details of their work on the electron transfer sensitised fragmentation of phenethyl ether They report that irradiation of 1,4-dicyanoderivatives .261n262 benzene in solutions of phenethyl ethers (366) and the acetals (367) in polar solvents leads to electron transfer from the benzylic derivative to the excited sensitiser, followed by C-C bond cleavage in the radical cation produced to yield benzyl radicals and an oxygen stabilised carbocation. Separation of these leads to products of radical abstraction and coupling, and nucleophilic attack on the cation by solvent. The ability of the reaction to proceed is correlated with the oxidation potential of (366) and (367) and upon the stability of the carbocations produced on cleavage.261 The fate of the radical cation obtained by irradiation of l,4-dicyanobenzene in the presence of the cyclic ether (368) is C-C bond cleavage to generate species (369) which contains a radical and a carbocation centre.262 In the presence of aqueous perchlorate the carbocation of (368) is attacked by water and the benzyl radical is oxidised by perchlorate. cyclisation then yields the isolated product (370) Cleavage of (367) to a benzylic radical and a carbocation has also been achieved by direct two photon excitation of (367) which results in electron ejection. 263

280

Photochemistry

Two papers have been published which examine the electron transfer sensitised photochemistry of 1,2-polyaryl substituted Irradiation of solutions of 1 4-dicyanobenzene in ethanes.264t265 polar solvents containing the ethanes yields the radical cations of the ethane by electron transfer to the sensitiser. Carbon-carbon bond cleavage in the radical cation produces benzyl radical and benzyl carbocation. By examination of the reaction products it is then possible to determine which of the benzyl fragments take the charge and hence obtain estimates of the relative oxidation potentials of a series of benzyl radicals.265 Carbonyl ylids have been proposed as intermediates trappable by alkenes in the energy transfer and electron transfer sensitised opening of styrene oxides. 2668267 Benzyl ethers in which the arene is electron rich can be used as protecting groups and removed by electron transfer sensitisation. Two new examples of this have been reported for (371)268and (372).269 Using a photoexcited cyanoarene as electron acceptor leads to electron transfer from the benzyl ether and loss of a benzylic proton from the radical cation. Loss of a second electron is presumed to yield a benzyloxy cation; this is attacked by water to produce a hemiketal which collapses to free the alcohol. Solvent and structural effects on the electron transfer sensitised anti-Markovnikov addition of methanol to styrenes has been studied.270 Transformations of arene substituents can also be achieved by transfer of an electron from a ground state donor to the excited state of the arene ring, or from an excited state donor to the ground state arene. Thus the retro Paterno-BUchi reaction of the benzophenone-tetramethylethylene oxetane (373) can be induced by irradiation in the presence of triethylamine as the electron donor. 271 This yields l,l-diphenyl-2-methylpropene; the regiochemistry of this fragmentation is the opposite to that obtained by irradiation of electron acceptor arenes in the presence of (373) in which case benzophenone is the product. 272 Two new conditions for the cleavage of tosyl groups from tosyl alcohols and amines by light induced electron transfer to the tosyl group have been reported.273t274 Thus irradiation of (374) in the presence of triethylamine results in selective detosylation while the benzoyl and benzyl groups protecting groups are untouched. 273

W4: Photochemistry of Aromatic Cornpounds

28 1

Similarly, excitation of the naphthyl group of (375) induces intramolecular electron transfer to the aryl sulfonamide which cleaves and allows isolation of good yields of the free a m i ~ ~ e . ~ ~ ~ Irradiation of phenanthrene in the presence of the electron deficient styrene (376) and hexamethyldisilane is reported to yield (377) by a mechanism involving electron transfer to the styrene.275 The regiochgmistry of the light induced intramolecular addition of amines to excited styrenes in (378) has been examined.276 This reaction also proceeds by electron transfer from the amine to the excited styrene chromophore.

References 1.

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H.-D. Becker, L. Hansen and K. Andersson, J. Ora. Chem. ?

193.

H.-D. Becker, V. Langer, B.W. Skelton and A.H. White, Jiust. J. Chem., 1989, a, 603. H.-D. Becker and K.A. Amin, J. Ora. C h e L, 1989, a,3182.

1986,

194.

x,2956.

W 4 : Photochemistry of Aromatic Cornpounds 195. 196.

29 1

J.-P. Desvergne, N. Biti, A. Castellan, M. Webb and H. Bouas-Laurent, J. Chem. SOC.. Perkin 2 , 1988, 1885. J.-P. Desvergne, N. Biti, J.P. Pillot and H. Bouas-Laurent,

J. Chem. Res. SvnOD., 1989, 146. 197.

M. Usui, T. Nishiwaki, K. Anda and M. Hida, FiDDon Kaaaku

Kaishi, 1989, 237; Chem. Abstr., U,152977~. 198.

. and M. Sato, J. Nishimura, M. Takeuchi, €ITakahashi

u,

199. 200. 201. 202. 203. 204. 205. 206. 207. 208. 209. 210. 211.

Tetrahedron T,etters, 1990, 2910. P. de Mayo, K. Okada, M. Rafalska, A.C. Weedon and G.S.K. Wong, J. Chem. SOC.. Chem. Commun., 1981, 820. V. Ramamurthy, D.R. Corbin, C.V. Cumar and N.J. Turro, T e t r w o n Letters, 1990, U , 47. B.F. Plummer and S.F. Singleton, J. Phvs. Chem., 1989, 5515. R.D. Rieke, G.O. Page, P.M. Hudnall, R.W. Arhart and T.W. Bouldin, J. Chem. SOC.. Chem. Commun. , 1990, 38. R.D. Chambers, T. Shepherd and M. Tamura, J. Chem. SOC., P e r k i n , 1990, 975. R.D. Chambers, T. Shepherd, M. Tamura and P. Hoare, J. Chem. SOC., Perkin 1, 1990, 983. R. Erra-Balsells, Phvtochemistry, 1988, 2 7 , 3945.

s,

A.V. Veglia, A.M. Sanchez and R.H. De Rossi, & Ora. Chem . I 1990, 4083. L. Fillol, R. Martinez-Utrilla, M.A. Miranda and I.M. Morera, Monatsh. Chem., 1989, m, 863. C.S. Vijayalakshmi, P. Shanmugam and K.J.R. Prasad, Ind. J, Chem. , 1989, m, 510. Ser. C. Belled, M.A. Miranda and A. Simon-Fuentes, m. Quim 5;, 1989, a,39; Chem. Abstr., 112, 216634~. A.C. Chan and P.R. Hilliard, Tetrahedron Letters, 1989, 6483. V . G . Syromyatnikov, L.P. Passkal and A.Y. Kolendo, Ukr. KhigL 1989, a,855; Chem. Abstr., 112, 197775s. R.P. Ghandi, M. Singh, Y.P. Sachdeva and S . M . Mukherji, End, J. Chem., 1989, 288, 535. I. Datta, T.K. Das and S. Ghosh, Tetrahedron Letters , 1989, X I 4009. S. Ghosh, D.B. Datta, I. Datta and T.K. Das, , 1989, 3775.

..

u,

a,

212. 213. 214.

292 215. 216. 217. 218. 219. 220. 221. 222.

223. 224. 225. 226. 227. 228. 229. 230. 231. 232. 233. 234. 235.

Photochemistry S.N. Ghosh, T.K. Das, D.B. Datta and S. Mehta, T e t r w d r o n Mtters, 1987, 28, 4611. A. Chakrabarti, G.K. Biswas and D.P. Chakaraborty, Tetrahedron, 1989, u, 5059. P.M. Miranda and A. Factor, J. Polvm. Sci.: Polvm. Chem., 1989, 21, 4427. K. Hustert and M. Mansour, Tetrahedron Letters, 1989, 6159. H.J. Timpe and V. Schikowsky, J. Prakt. Chem., 1989, m, 447. 639. J.L. Dektar and N.P. Hacker, J. Oru. Chem. , 1990, F.D. Saeva, polvm. Mater. Sci. Ena,, 1989, u, 72. A.M. Fahmy, M.M. Aly, M.Z.A. Badr and A.M. Abd El-Gawad, Bull. P01. Acad. Sci., 1988, X I 97; Chem. Abstr.1 & A 2 1 76487r. D.R. Anderson and C.N. Eley, Tetrahedron Letters, 1989, a, 4059. S.S. Hu and W.P. Weber, J. Oraanometal Chem., 1989, m, 155. V.P. Rao, N. Han and N.J. Turro, Tetrahedron Letter@ , 1990, u, 835. N.J. Turro and 2. Zhang, Tetrahedron Letters, 1989, 30, 3761. J.C. Netto-Ferreira, I.G.J. Avellar and J.C. Scaiano, J. O m . Chem,, 1990, B,89.

x,

x,

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R.W. Redmond and J.C. Scaiano, J. Phvs. C h m I 19891 % I , 5347. J. Gebicki, S. Kuberski and R. Kaminski, J. Chem. SOC., in 2, 1990, 765. R.N. Nurmukhametov and A.M. Sergeev, Bh. Fiz. Khim., 1990, a,308. Y. Matsuda, R. Teraoka and I. Sugimoto, m t . J. Pharm. , 1989, 54, 211. O.P. Petrenko and V.P. Mamaev, Jzv. Akad. Nauk S.S.S.R., Ser. Khim,, 1989, 2323. S.V. Kessar, T. Singh and A.K.S. Mankotia, J. Chem. Soc., a e m . Co-, 1989, 1692. J.C. Netto-Ferreira, V. Wintgens and J.C. Scaiano, Tetrahedron Tletter@, 1989, a,6851. S. Prathapan, S. Loft and W.C. Agosta, o n Letters, 1988, 2,6853.

IV4: Photochemistry of Aromatic Cornpounds 236. 237. 238.

293

..J

S. Prathapan, S. Loft and W.C. Agosta, Chem. Soc. I 1990, u, 3940. A . J . Kresge and N.P. Schepp, J. Chem. SOC.. Chem. Comun., 1989, 1548. R . A . McClelland, F. Cozens and S. Steenken, Tetrahedroq 2821. &ette=, 1990, P. Wan and E. Krogh, J. Am. Chem. SOC., 1989, U,4887. S . L . Mecklenburg and E.F. Hilinski, ,1989, u,5471. R . E . Minto and P.K. Dae, J. Am. Chem. SOC,, 1989, 8858. R . A . McClelland, V.M. Kanagasabapathy and S. Steenken, J. Chem., 1990, 68, 375. E . O . Alonso, L.J. Johnston, J.C. Scaiano and V.G. Toscano, J. Jim. Chem. SOC., 1990, U , 1270. A.S. Cherkasov, N . S . Shelekhov, T.V. Veselova, L.N. Denisova and V.M. Shershukov, m t . Spektrosk., 1989, H I 1286; Che148787~. mstr., U , J.C. Netto-Ferreira and J.C. Scaiano, pes. Chem. Intermed., 1989, ;L2, 187. R.W. Redmond, J.C. Scaiano and L.J. Johnston, 6 . Am. Chem, soc. , 1990, 398. J.C. Netto-Ferreira, W.F. Murphy, R.W. Redmond and J.C. Scaiano, J. Am. Chem. Socc, 1990, J&&, 4472. J.L. Faria and S. Steenken, J. Am Chem. Soc., 1990,

u,

239. 240. 241. 242. 243. 244.

245. 246.

u,

u,

247. 248. 249. 250. 251. 252. 253. 254. 255. 256.

.

u,

1277. A. Ouchi and A. Yabe, Tetrahedron Lettera, 1990, ;L;L, 1727.

M. Shi, Y. Okamoto and S. Takamuku, J. Chem. Res. S v a , 1990, 131. M. Shi, Y. Okamoto and S. Takamuku, Tetrahedron Letters, 1989, uI 6709. M. Shi, Y. Okamoto and S. Takamuku, J. Ora. Chem,, 1990, 55, 3821. J. Woning, A. Oudenampsen and W.H. Laarhoven, J. Chem. SOC.. perkin 2, 1989, 2147. 2809. P. Wan and X. Xu, Tetrahedron LetterS, 1990, Y. Okamoto, N. Iwamoto, S. Toba and S. Takamuku, ,1989, 1325; Chem. Abstr., J&&, 118117~. P. Wan, D. Budac and E. Krogh, J. Chem. SOC.. Chem. C o n u ! ~ ~ , 1990, 255.

Photochemistty

294 257.

S. Muralidharan, K.A. Beveridge and P. Wan, - J , 1989, 1426.

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D.P. De Costa and J.A. Pincock, J. Am. Chem. Sect, 1989, 8948. A. Sulpizio, A. Albini, N. D'Alessandro, E. Fasani and S. Pitera, J. Am. Chem. SO C . , 1989, u, 5773.

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C.M. Wang and T . E . Mallouk, J. Am. Chem. SOC, I 1990, LU, 2016. 1989, pZ, 2119. D.R. Arnold and L.J. Lamont, can. J. Chem D.R. Arnold and L.J. Lamont, D n . J. Ch-, 1990, 68, 390. S. Steenken and R.A. McClelland, J. Am. Chem SOC., 1989, 4967. P. Maslak and W.H. Chapman, Tetrahedron, 1990, u,2715. R. Popielarz and D.R. Arnold, J. Am. Chem. SOC., 1990, 3068. P. Clawson, P.M. Lunn, D.A. Whiting J. Chem. SOC.. Perkin 1, 1990, 153. P. Clawson, P.M. Lunn, D.A. Whiting J. Chem. SOC.. Perkin &, 1990, 159. G. Pandey and A. Krishna, Svnth. Commun., 1988, &@, 2309. A. Nishida, S. Oishi and 0. Yonemitsu, chem. Pharm. B\aLL, 1989, 37, 2266. K. Mizuno, I. Nakanishi, N. Ichinose and Y. Otsuji, Chem. Lett., 1989, 1095. K. Nakabayashi, S. Fujimura, M. Yasuda and K. Shima, Bull. Chem. SOC. JaDan, 1989, 62, 2733. K. Nakabayashi, J. Kojima, K. Tanabe, M. Yasuda and K. Shima, pull. Chem, SOC. JaDan., 1989, a,96. R.W. Binkley and D.J. Koholic, J. Ors. Chem., 1989, 54, 3577.

.,

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w, 264. 265. 266. 267. 268. 269. 270. 271. 272. 273. 274. 275.

u,

T. Hamada, A. Nishida and 0. Yonemitsu, Tetrahedron Lette1s, 1989, 30, 4241. K. Mizuno, K. Nakanishi, J. Chosa, T. Nguyen and Y. Otsuji, 3689. Tetrahedron Letters, 1989, F.D. Lewis, G.D. Reddy, S. Schneider and M. Gahr, J. Am. Chem. SOC., 1989, U, 6456.

x,

276.

u,

5 Photo-reduction and =oxidation BY A. COX 1. Introduction Reviews have appeared of advances in alkane functionalisation,l non-sensitized

photooxygenation

of

organic

compounds,

sensitlzed

photooxygenation,

current aspects of photooxidation involving singlet

oxygen,

oxygen

singlet

pathways,

and

eiectron-transfer

electron-transfer

induced

in organic synthesis,

oxygenation

electron-transfer

reactions of sulphoxides and s~lphones,~ flash photolytic generation of some react i ve species , the photocher ica 1 reduct i on of C02 in homogeneous and microheterogeneous systems, photoinduced catalytic hydroformylatlon and carbonylation,1° 1 Ight-induced electronic processes at interfaces,l1 photocatal ys i s

on

t i tanlum

dioxide,l2 l3 @

photocata 1 ysed

ox idat ions

involving copper c ~ m p i e x e s ,and ~ ~ photoreduction of aromatic species.15 2. Reductfon of the Carbonyl Group The

photoreduction

9,10-anthraquinone,

efficiency

of

1,4-anthraquinone,

2-phenyl-9,10-anthraquinone,

carbonyl

compounds

such

6,13-pentacenequinone,

as and

in the presence of anthracene, pyrene,

naphthalene, biphenyl, l,4-benzoquinone, and 1,4-naphthoquinone has been shown to depend on the relative positions and nature of the electronic levels of the substrate and acceptor.16 Photoreduct ion of arorat ic ketones to

alcohols

or

pinacols

can

be

catalysed

by

CdS

powders

using

triethylamine as sacrificial donor in HeCN,17 and morphology is reported by the same authors to affect the tuo-electron transfer photoreductions of aromatlc ketones on CdS conditions olefins behave

induced by

visible light; under analogous Aldehydes of the type RCHO (R

-

L-toiyl, p-anisyl, hexyl 1 have been photoreduced to the corresponding

296

Photochemistry

alcohol by HC02Na in aqueous methanol in the presence of a catalyst photogenerated from C r ( t 0 ) ~ . ~Self-sensitized ~ photoreduction of the 17-keto-group

in some steroidal ketones has been achieved by energy

transfer from a dimethylphenylsiloxy group appended at the 3a-position, which serves as an antenna chromophore.20 It was also found that in the presence of an ll-keto group, this functions as a singlet-triplet switch during transnission of excitation from the aryl antenna to the ultimate 17-keto target. Oxygen

quenching

of

the

1,a-biradical

formed

during

the

photoenolisation of 2-methylacetophenone, occurs by energy transfer and leads

to

production

phenylpenta-1,4-diyl,

of the

singlet triplet

oxygen. 21

diradical

l-Hydroxy-4-methyl-l-

formed

by

laser

flash

photolysis of PhCO(CH2)2CHHe2 in HeOH, has been found by 02-trapping to have a triplet llfetime of 131

f

5 ns.22 This result correlates well with

values obtained from other sources and In this way establishes the reliability

of

the

technique.

l-aryl-2,4-dimethylpentan-l,3-diones

Type

I1

photoreaction

of

in the presence of oxygen gives

furan-3(2H)-ones and tetrahydrofuran-2-ones via a biradical which Is subsequently trapped by oxygen.23 Various B-(p-tolyl )proplophenones are reported to give 1,2,3,4-tetrahydro-2-naphthols in high chemical but low quantum yields via their triplet state, from which E-H abstraction is in competi tion wl th rapid Internal charge transfer quenching.24 These studies provide new Information about how rate constants for reaote hydrogen abstractions depend on molecular conformation.

Decay kinetics have been

obtained for the long-lived intermediate formed in the photoreduction of benzophenone by isopropyl alcohol in MeCN.25a26 Laser flash photolysis experiments on Ph2CO/i-PrOH/HeCN and Ph2CO/Ph2CHOH/HeCN show that the intermediate arises from coupling of a diphenylketyl with a dinethyiketyl radical and that dimerisation of diphenylketyl radicals is unimportant. A

II/S: Photo-reduction and -oxidation

297

time-resolved ESR study of the photoreaction between benzophenone and some amines in polar and non-polar solvents suggests that direct H atom abstraction occurs in most system regardless of solvent p~larity.~’ However,

an

ionic

process

may

benzophenone/N,N-diethyianiline/acetonitrile. PhCOC6H4R-4 (R

-

H, Br) by 4-R1CgH4R2 (R1, R2

occur

-

The

in

the

system

photoreduction

of

H, OHI Me, OHI Br, OHI H,

NH21 HI NHe2tBr, NHe2) has been investigated in solvents of different viscosity as well as in micelles.28 Introduction of a heavy aton and reduction of molecular mobility amplification coefficient.

A

leads to a decrease

in the CIDNP

CIDEP study of the photoreduction of

benzophenone and xanthone with alkoxides in several alcohols shows that, even in the early stages of the photoreaction, there exists a hyperfine interaction between the ketone anion radical and the alkali metal cation.29 In solution some poiyf luorophenyl ketones possess enhanced reactivity in hydrogen abstractlon and charge transfer processes compared with bentophenone,30 and an

investigation of the photochemistry of

4,4-dimethyi-l-phenylpent-2-yn-l-one

has shown its triplet to be long

i ived in inert solvents.31 Under appropriate conditions, the transient

will undergo hydrogen abstraction reactions, charge transfer quenching by aromatics, and electron-transfer processes.

In the presence of tertiary

amines, p-amino alcohols or alcohols, the primary photochemical reaction of 8-lapachone (1) is a direct photoretiox process.32 This involves either electron transfer or a H atom abstraction which quenches the quinone excited state, and generates a relatively stable semireduced quinone radical. The effect of methylene chain length on the photophysicai and photochemical processes of n-aikyl anthraquinone-2-carboxylates has been studied by a variety of

technique^.^^

Hydrogen atom abstraction from the

rethylene side chain by the triplet state of the anthraquinone occurs with

Photochemistry

298

q Me

ROCH

R' i

t; 0

Ph*R2 R

(2)

Me

(1)

C H2CH2C0,Na

@

CH2CH2C02Na

(3)

R3

299

ZZ/5: Photo-reduction and -oxidation

a rate which increases with increasing chain length. A kinetic study of the photoreduction of p-benzoquinone

in normal and reversed micellar

systems up to pressures of 150 HPa has appeared, and

bromide micelles, but not in SDS miceiles,

hexadecyltrimethylamaonium pressure

promotes

the

in cationic

In

photoreaction.34

the

photoreduction

of

fluorescein, eosin and erythrosln, the yield of radicals from the cage uas found to be increased by application of a 0.18 T magnetic field.35 The effect was smaller with eosin and erythrosin because of the presence of heavy atoms.

A correlation has been established between the rates of the

acid-cataiysed photoinduced eiectron-transfer reactions from the excited state of [Ru(bpy)3I2+ to ketones and the rates of their acid-catalysed reduction

by

unprotonated

transfer

reactions

between

10-methylacridan.36 aromatic

benzophenone and anthraquinone, and

carbonyl

Photoinduced compounds

electron such

as

1,4-diazabicyclo[2.2.2 ]octane and

PhNHe2 as electron donors, have been compared with those of cyano compounds such as 9-cyanoanthracene and 9,lO-dicyanoanthracene.37 The resulting free ion yield appears to depend upon the spin state of the geminate

ion

pair

and

is

efficient

for

triplets

(carbonyl

compounds/donors) and inefficient for singlets (cyano coapounds/donors). The competition between photoinduced hydrogen abstraction and electron transfer in a mixture comprising 9,10-phenanthraquinone/glycidyl ether (2;

R

-

Ph, E- and p-tolyl, p-ClCsH4, (Et0)3SiCH$H2, Et, H~~CHCH~)/P~~I+PFG'

has been investigated by quantum yield

measurement^^^

and a kinetic study

of the photochemical deconjugation of methyl geranate has shown that the quantum yields of the deconjugated esters and of their ratios are dependent upon the concentration of added base. 39 &-Unsaturated

and

a-alkylidene lactones undergo photochemical deconjugation, and in the presence of (+I-

or (-)-ephedrine C-protonation of the photodienol ic

intermediate can occur enantioselecti~ely.~~ A TSLIF investigation has

Photochemistry

300

suggested that the long-lived species previously assigned as a tautomer singlet state of 3-hydroxyflavone Is in fact a triplet state.41 An

investigation

of

the

excited

state

tautonerisation

of

7-hydroxyquinoline using ps time-resolved fluorescence has revealed the presence of two different excited state tautomer isat ion processes. 42 The faster of the two occurs within electronically excited l:2 solute-alcohol complexes and the slower is associated with tautonerisation in excited solute-alcohol corplexes contalning more than two solvent molecules. 3 . Reduction of Nitroaen-contalnina Conwounds A study of the photoreactivity of Fe-doped Ti02 powders for the

reduction of N2 to NH3 has shown that Fe(II1) ions are better traps than Ti(IV) ions.43 This is important since the charge separation helps adsorption of the nitrogen by its reaction with surface species activated by holes.

CdS/VC13 and CdS/V02+ are photocatalytical ly active for the

reduction of nitrobenzene to aniline using ~ 1 ~ ~ 3 2Evidence 0 . ~ ~ is presented to suggest that these are truly hybrid photocatalytic systems. An

intercalation

nethylviologen

has

compound

been

composed

of

layered

HTiNbO5

s y n t h e ~ i s e d . ~ ~In n ~ ~oxygen-free

and

conditions

irradiation induces fornation of a blue colour and this has been interpreted in terms of an electron-transfer process which gives .'VH

The

sane workers have also investigated the montmorillonite-methylviologen

con pound^.^^

The effect of pH and the counterion of oxalate on the

photoreduction

of

methylviologen

durlng

continuous

photolysls

of

HV2+/oxalate mixtures has been studied48 and an increased efficiency of photosensitized

reduction

tr i plet-tr lplet

energy

of

methylviologen

transfer

using

has

been

observed

by

9-anthracenecarboxy late. 49

Homogeneous photoreduction of paraquat salts in isopropanol occurs by electron

transfer

and

has

been

studied

by

the

spin-trapping-ESR

te~hnique.~' The rate of photoreduction of MY2+ to HV+* in the system

ZZ/": Photo-reduction and -oxidation

30 1

HV2+/EDTA/Acridine Yellow, is retarded by the presence of small amounts of nitroprusside as a consequence of its effect on the excited state of Acridlne

Use can be made of this observation as a method for the

microdetermination of nitroprusside.

Hethanolic methylviologen can be

photoreduced under catalysis by phthalocyanine complexes of yttriur(II1) and lanthanoid(II1) metals in the presence of triethan~lamine.~~ Ground state adduct formation between methylviologen and the complex is reported to suppress generation of HV'.

Temperature effects on the photosensitized

reduction of methylviologen have been studied, as well as hydrogen formation

in

aqueous

cetyl

trimethylammonium

bromide

containing

methylvioiogen, EDTA, zinc phthalocyanine and/or [ R ~ ( b p y ) ~ J A~ *kinetic .~~ investigation has been reported of photosensitized electron transfer from non

uater-soluble

(ZnTPPC%Na l4

to

ZnTPP,

water-soluble

duroqulnone,

ZnTHePyP4+,

methylviologen

and

ZnTPPS4'

and

anthraquinone-2-

in reversed micelles of benzyldimethyl-n-hexadecylammonium

sulphonate

chloride.54 Photoinduced evolution of hydrogen has been achieved using viologen-linked water-soluble zinc porphyrins (Zn-PC3(CnV)) with different methylene chain lengths (n

-

in a suitable system 55 and the

2-61

chemistry of HV+* produced photochemically and of its dimer have been examined under a variety of conditions.56 Photooxygenation of methanolic pyrromethenones has been achieved in the

presence

of

propentdyopent-alcohol tetraphenylporphyrin

oxygen

and

adducts.57

porphyrin In

a

sensitizer,

quenching

and

study

(ZnTPP) by p-cyano-N-methylpyridine

gives

of

zinc

in DHSO, the

redox-ion product (ZnTPP+) has been produced by flash photolysis, and the quenching

rate constant

solution. 58

measured

9-Alkyl-lO-methylacridan

in

Triton is

formed

10-methylacridinium ion with fatty acids in basic presence of cysteine, peptide-like

micelle

X-100

derivatives

by

aqueous

reduction

of

and in the such as Ac-Cys-One,

302

Photochemistry

irradiation of 5-bromouracil at 308 nm gives uracil and the corresponding cystine derivative in a process which occurs by initial electron transfer from the disulphide cysteine derivative to the triplet state of the 5-bromouraci 1

Prolonged

irradiation produces a S-uraci lylcystelnyl

adduct. Benzophenone-sensitized photoreduction of N-phenyl-p-benzoquinone iiine

and

phenothiazin-3-one

proceeds

quantitatively

to

give

the

corresponding phenols.61 The reduction occurs by H atom transfer to the carbonyl oxygen rather than to the azomethine nitrogen. 4. Miscellaneous Reductions

CO has been transformed into HCHO by UV irradiation in the presence and

of a TIO2-suspension as well as a Ti02 colloid in aqueous

selective photoreduction of carbon dioxide to methanol has been achieved using an aqueous suspension of hydrous cuprous oxide ( C U ~ O ' X H ~ O ) .In~ ~ perhalogenated chlorofluoropropanoates of the type CFXYCClZC02R (X, Y, Z

-

C1, F), the ester strongly directs the photochemical reduction of a C-Hal bond

to

the

a-position.64

High

regioselectivity

occurs

even

in

CFC12CHClC02R, and in those esters possessing an a-CC12 group, the first and

second

stages

can

be

separated.

Iron

=-tetra-

(2,6-dichlorophenyi)porphyrin has been shown to be a very efficient

catalyst for the photoreduction of CC14 by alcohols.65 The photoreduction of phenacyl halides by the NADH model compounds, i0-methylacridan (AcrH2) l-bentyl-1,4-dihydronicotlnaaide

and

(BNAH)

proceeds

by

dissimilar

mechanistic pathways.66 This has been ascribed to the difference in pK between AcrH2'.

and BNAH'.,

and the difference in the one-electron

oxidation potentials between AcrH' and BNA-. The same authors also report that [Ru(bpy)3I2+ will efficiently catalyse the photoreduction of phenacyl ha1 ides by 9,10-dihydr0-1O-methylacridine.~~ Arylcyclopropanes (38 R R1

-

-

H, C02He, R2

Ph, R3

-

-

C02He, COzEt, CN, COMe, R3

P(0)(OHe)2; R2

-

H, R3

-

-

H, C02Et; R

- R1

-

Ph,

H, R2

C02Me) are reported to act as electron

IV5: Photo-reduction and -oxidation

303

acceptors on irradiation in MeCN containing (R4CH2)3N (R4

-

H, Melt

regioselective add1tion products (R4CH2)2NCHR4CPhRCHR1CHR2R3 as we1 1 as reduct ion products PhCHRCHR1CHR2R3 are

formed.68 The reduct Ion and

photoreduction of some carbonic acid derivatives in HMPT have been compared

and

the

highest

N, N-dimethy 1 thiocarbamates .69

yield Sol vent

of

alkane

vi scosi ty

was has

obtained been

from

found

to

influence the effect exerted by a magnetic field on radical yields during the photoreduction of xanthene dyes .70 5. Sinalet Oxyqen

Measurements of the luminescence of singlet oxygen in various media have shown that the small but significant wavelength shifts observed in aromatic solvents are not-attributable to exciplexes or to charge-transfer The 1 ifetime of singlet oxygen has been determined directly by monitoring the phosphorescence at 1270 nm and is dependent mainly on the property of the solvents used,72 and the time-correlated single photon counting technique has been applied to the measurement of photosensitized slnglet oxygen luminescence induced by flash photolysis e ~ c i t a t l o n .It ~~ has also been reported that the lifetime of singlet oxygen in solution decreases monotonicai ly with increasing pressure.74 This is interpreted In terms of the pressure dependence on the collision frequency as estimated by

the

hard

sphere

model.

Quantum

yields

of

singlet

oxygen

phosphorescence have been measured in five different solvents75 and the dependence of the quantum yield and decay time of singlet oxygen lminescence studied in CCl4 and CS2 as a function of photoexcitation intensity and the concentration of Pd-mesoporphyrin as s e n s l t i ~ e r . ~ ~ Bimolecular rate constants have been measured for the physical quenching of singlet oxygen by canthaxanthin and all-trans-&carotene in polar and non-polar solvents, in order to evaluate the effect of medium on the efficiency of the quenching process.77

304

Photochemistry Singlet oxygen has been generated by thermal decomposition of the

water-soluble

endoperoxide

(41,

and

cherilurinescent transition of 02('Ag)

light

emission due

to

the

to the ground state has been

measured directly using a cherical trap.78 Some natural bithiophenes have been shown to be sensitizers of singlet oxygen,79 and metailotexaphyrins are reported to be a new family of photosensitizers for the efficient generation of singlet oxygen.80

A

comparison of the photosensitized

oxygenation efficiency of several kinds of sensitizers measured using

cis-cycioocta-l,5-diene

a,

as substrate, has appeared81 and the methylene

blue sensitized photooxygenation of resodiphenylhelianthrene can be used as a highly sensitive and accurate chemical actinometer in the range 610

-

670 nm.82 Triplet energy surfaces of some oiefins have been investigated

.

by the sing 1et oxygen 1urn i nescence techn i que 83 6. Oxidation of Aliphatic Compounds EthaneE4 and c y ~ l o h e x a n e have ~ ~ been partial iy oxidised at moderate temperatures using singlet oxygen produced from N20 with an ArF excimer laser. The room temperature photo-activation of methane on Ti02 supported on molybdena has been investigated.86 The same authors also demonstrate that

tungstosi 1 icate

(SiW12040)~--loaded Ti02

is

active

in

the

photoinduced oxidation of CH4, and the study further reveals that the polytungstate greatly modifies the photocataiytic effect of the Ti02 surface.87 Lower coordinate surface sites play an important role in the oxidative coupling of methane over ultrafine crystalline magnesium oxide doped with lithium,88 and a study of the photocataiytic activation of the lower alkanes in aqueous solutions of polyoxocomplexes of tungsten suggest that charge transfer participation in the terminal W I O group gives an electron-deficient site which is capable of activating the alkane.89 The same workers have also examined the photooxidation of lower alkanes in aqueous solutions of polytungstates.

Cycloalkanes such as adarantane and

W 5 : Photo-reduction and -oxidation

305

cyciohexane undergo an electron transfer photooxidation in the presence of [ (Bu4Nl4 (W10032) I,

and another report describes the successful use of t

the system 111 PhCH2NEt3Ci’-oxometallate, H3PY12040

Na~jH7HOgV503g

> NagV10028

with yields such that NaV03 ::

-

H3PHo12040

>

H4SiH0120~0.~~

Anaerobic functionalisatlon of remote unactivated C-H bonds has been achieved using such polyoxometaliates as a-PU120403- and U10032~- excited by blue 1 1ghtrg3 selective production of the tertiary hydroperoxide of isobutane under these conditions is also de~cribed.’~ A

If0 study of the photodimerlsation of cyclohexene by

excited

[Yl~032J4-has concluded that a radical monomer coupling occurs which possibly operates for polyoxometailates in general and Is probably the same as on metal oxide s~rfaces.’~Photooxidation of heptane in synthetic air containing HeONO and NO gives alkyl nitrates, and these have been shown to arise from a prlmary reaction between alkyiperoxy radicals and NO.96 trans-Rh(PHe3)2(CO)Ci is reported to be an efficient and high y selective catalyst for the photochemical dehydrogenation of cycioalkanes 97 the room temperature oxidation of liquid cyclohexane by oxygen over Illuminated Ti02 is photocataiysed and shows a high selectivity to cyci hexanone, and

photooxidation

of

some

hydrocarbons

sensitized

by

the

2,7-dlatapyrenlum dication in the presence of an iron catalyst involves formation of

free hydroxyl

radical

from

oxygen.99

Butyicyclohexane

undergoes photooxidatlon by solar rays in synthetic seawater containing traces of anthraquinonet reaction mainly occurs on the ring carbons.loO Discharge flow cheailuminescence has been used to determine the rate constant of the reaction of atomic oxygen wlth ethene.”’ Semi-empirical and ab initiQ calculations have been used to study the to amlnoethylene and ethylene. Electron-rich 1,2-cycloaddition of 02(1~g) oleflns react via a twitterionic intermediate and the total reaction paths were determined using Fakui’s IRC theory.lo2 The same group has examined

306

Photochemistry

the mechanism of the [2+21 cycioaddition of singlet oxygen to propyiene using tlIND0/3 with the energy gradient technique.lo3 Stereoseiective photooxidation of trans-but-2-ene to epoxide has been achieved in a cryogenic

matrix

using

nitrogen

dioxide

and

exciting

with

red

iight.104~105 The low photon energy is thought to be an important factor contributing to product control and the dynamics of the intermediate nitrite radicals have

been

studled.lo6 Photocatalytic oxidation

of

isobutylene on grafted and supported metal-complex catalysts occurs by adsorption on photogenerated 0-,lo'

and a mixture of acetaldehyde,

propionaldehyde, and acroiein is formed by photooxidation of propene using a vanadium oxide cata1yst.lo8 This latter reaction occurs by interaction of the substrates with the charge-transfer excited triplet of (V4+-O-)" supported on Vycor glass, to form 02- and the propene cation radical. Photooxygenation of olefins in the presence of transition-metal catalysts derived from Ti, V, or no constitutes a one-pot synthesis of epoxy a l c o h o i ~ .The ~ ~ ailylic ~ hydroperoxide is formed in an ene reaction and is subsequently transformed into the epoxy alcohol by the transition metal catalyst.

At

254

nm,

irradiation

of

cyciohexene

in

oxygenated

cyciopentane leads to a variety of products.l1° These arise from peroxyi radicals which are formed following a solute to 02 charge transfer absorption.

Studies have also been made at 185 nm in both the presence

and absence of oxygen.

Irradiation of ceric ammonium nitrate in MeCN

containlng cyclohexene, oct-l-ene or a series of styrene derivatives eads to formation of the 1,2-dinitrate adducts via the nitrate radica .lI1 Photooxygenation of methanol ic solutions of the cyciopropyl idenes (5) ( 6 ) and ( 7 )

in the presence of DCA gives methoxy hydroperoxides,l12 and

photoinduced

liquid-phase

oxidation of

studied using PhCOCPhROMe (R

-

tetrachioroethylene has

been

H, One) as photoir~itiator.~~~ Various

olefins have been subjected to dye-sensitized photooxidation including

307

ZI.5: Photo-reduction and -oxidation cholesterol, whose product distribution is dependent on both sensitizer and solventI1l4 9,ll-didehydroestrone derivatives,'15

drlmenyl acetate,

which leads to allylic alcohols potentially useful as chiral synthons for natural sesquiterpenes, of

the product

distribution

(Z)-7-methyl-B-ionone

A study

and (t)-11,12-diacetoxydrlrn-8-ene.

has

of

the photooxygenation

revealed

its

dependence

of

on

(El- and

the

skewed

ground-state conformation of the dienone chromophore.'18 Singlet oxygenation of some 6-substituted cycloocta-lI4-dienes has been shoun to be a convenient selective route to ~-5,8-disubstituted and trans-5,C-dlsubstituted cycloocta-1, 3-dienes.l19

The stable conformation

of the starting material in the ground state appears to be responsible for the

observed

photooxidlsed

selectivity. to

u-5,8-

6-Substituted and

cycloocta-l,4-dienes

trans-5,6-difunctionalised

cycloocta-

1,3-dienes stereoselectlvely.120 The same authors also report that

cis-cycloocta-1,5-dlene -1, 5-diene.lZ1

in

cis,

gives ~-5,8-dihydroperoxylcyc1oocta

HeOH

Rose-bengal

are

sensitized

a-phellandrene produces a mixture of &s-

of

oxidation

methanol ic

and trans-endoperoxides and all

the hydroperoxides derivable by ene reaction of the substrate uith 102.122 A common first step for endoperoxlde and hydroperoxide fxmatlon has been

developed.

An effective

enecyclohexy1idene)ethanol

stereoselective synthesis of 2-2-(2-methylby

the

photosensitized

oxygenation

of

l-vinylcyclohexene has been described.123 Singlet oxygen oxidation of

cis-cyclohexa-3,5-diene-l,2-diol

gives a mixture of endoperoxides which

can be readily converted to conduritols A chemiluminescent uranlum(IV),

autoxidatlon

of

various

the emitting species has been

Bis(adamanty1idene)methane

and

photosensitized oxygenation at -78'

and

D,124

and

in the

cyclopentadlenides

of

identified as U022t.125

vinylldeneadarantane

undergo

to afford adamantanone.126 On being

warmed in the presence of a fluorescer, the former compound emits light,

308

Photochemistry

and a 3-methylene-1,2-dioxetane has been suggested as the chemiluminescent intermediate. The second-order rate constant for the reaction between atomic oxygen Ratios of products arising from

and cyclopentanone has been

the photooxygenation of 2-methoxy-3-methylcyclopent-2-en-l-one

in CDjOD

-

CD6COCD6 or -CD2C12 are highly sensitive to the concentration of CDjOD and this observation has been

interpreted in terms of an

intermediate

e x ~ i p l e x . ~ Ethylene * ~ ~ ~ ~ glycol ~ has been selectively synthesised by UV irradiation of N2-saturated methanol containing H2O2. 130 Alcohol-der ived radicals are produced in the photoreactlon of [ptC16I2- with alcohols, and in pure alcohols this radical is usually RCH2O-r in the presence of water, ~ ratios of dimethoxymethane and only RiHOH Is p r 0 d ~ c e d . l ~Stoichiometric dihydrogen are generated in the photocatalytic dehydrogenation of MeOH using Sn(I1)-coordinated

iridium(II1) catalysts,132 and the effect of

substrate structure and photon energy on the photochemical dehydrogenation

of

alcohols

under

porphinato)rhodium cyclohexanol

to

homogeneous

has

been

cyclohexanone

catalysis

discussed.133 and

hydrogen

by

chloro(tetrapheny1-

Photodehydrogenation of can

be

induced

by

chloro(tetraphenylporphinato)rhodium(III) excited in its B or 0 bands, and may occur by the same mechanism as has previously been suggested for propan-2-01.~~~In the presence of some metal catalysts (Pt, Pd, Rh2O3) loaded uith CdS, photoinduced dehydrogenation of aqueous ethanol gives h ~ d r 0 g e n . l ~Optimum ~ amounts of hydrogen are generated using a 5% Pt-5% Cu/CdS catalyst system. The effect of surface composition and structure of CdS and Pt/CdS in the photocatalytic dehydrogenation of aqueous EtOH has also been examined and it is concluded that

HO formed from Cd(OH12

reacts with EtOH.Ij6 In the photooxidation of EtOH and 2-methylpropan-2-01 using Ti02 or TI(ORl4 (R

-

Et, iso-Pr), identical radical intermediates

are involved in both cases.137 The same workers have also studied the

309

II/5: Photo-reduction and -oxidation photocatalytic oxidation of several alcohols on Ti02 by spin trapplng, photocurrent measurement, and product analysls,138 and the photocatalytic reaction of EtOCH2CH20H on T102 to glve ethyl f 0 r ~ a t e . l In ~ ~ thls latter case the substrate reacts with 02- adsorbed on the catalyst surface to produce a surface formate-like species. Conditions for the photoevolution of hydrogen from H20-HeOH mixtures uslng Ru/Ti02 powders have been optlmlsed,140 studies of the photooxidatlon of methyl dehydroabietate,141 b-cadlnene,la2 and of free radical photooxidative transformatlons of androst-4-ene-38,9a, 178-trio1 3,17-dlacetate uslng Pb(0Ac)l or HgO/I, have been de~cr1bed.l~~ Reports have appeared of various other photooxidations including those of THF and THP,144 adamantane enol ethers to glve 1,2-dloxetanes via a chain electron transfer m e c h a n l s ~ ithe ~ ~ ~ benzopyran acetaldehyde in the gas phase,147 and of 2,2-dihalo-l,3-diketones to give 1,2,3-triketones as their ~ o n o h y d r a t e s .The ~ ~ ~ quantum yield of formatlon of HeCOCH(0H)He from pyruvic acid has been found to rise with decreasing

pH.14'

This

has

been

interpreted

In

terms

of

photodecarboxylation of one of the hydrated forms of pyruvic acid. Second-order rate constants have been determined for the reactions of 3A2uPt2 (Pt2 (RRlCHOH) by

[Pt2(P205H2)414-) wlth organic halides (RX) and alcohols a Stern-Volrer

study.15*

Flash photolysis

experiments

establish that the primary step is X and H transfer wlth RX and alcohols respectively.

Poly(viny1idene

fluoride)

has

been

shown

to

be

photocherlcal ly Inert at long and short ~ave1engths.l~~ Introduction of photoactive plgments such as ZnO or Ti02 do not Induce any photocatalysed oxidation even on prolonged exposure. 7. Oxldation of Aromatic Comwunds Alkylbenzenes have been photooxldlsed In air uslng Q-phenanthroline and 2,2'-bipyrldine as sensitizers,152 and also under both aerobic and anaerobic condltlons in the presence of H3PW12040 or ( B U ~ N ) ~ P W ~ ~ O ~ ~ . ~

3 10

Photochemistry

Electron-transfer photooxygenations of and l,&dimethylnaphthalene

Q-, 1-,p-xylene,

p-methoxytoluene,

can be sensitized by dicyanoanthracene and

chlorani 1, and give the corresponding aldehyde and carboxyl ic acid.154 The presence

of

added

photooxygenation.

biphenyl

significantly

enhances

the

rate

of

Photocatalytic formation of phenol by oxidation of

benzene using Ti02 powders in water has been reported and occurs at the benzenehater 1nterfa~e.l~~ The influence of the preparation methods of Ti02 on the photocatalytic degradation of phenol in aqueous dispersion has been described, and the rutile phase is active or inactive according to the

conditions

employed.156

m-Phenoxytoluene

has

been

selectively

photooxidised to A-phenoxybenzaldehyde using Ti02 particles incorporated in

Nafion

membranesi5’

and

at

colloidal

Ti02/non-aqueous

solution

interfaces.158 Catalytic oxidation of l,I-diphenyIbuta-l,3-diene on CdS powder in HeCN is promoted by addition of H20, Ti02 and Na2S03,15’ and the same group has also reported the photooxidation of this diene and of various cyclohexa-l,3-dienes on silica gel .160 Benzyl alcohol undergoes photooxidation at Xexit

>

300 nr to give a

mixture of products,161 and protonated pterldine and f lavin analogues are reported to be efficient and substrate-selective photocatalysts in the oxidation of its derivatives by oxygen.162 Related alcohols such as benzhydrol, and benzoin have been photooxidised using CdS powder as photolysis catalyst! the use of sulphite as hole scavenger will prevent the photocorrosion of CdS and promote the react10n.l~~ A lH CIDNP investigation of the photooxidation of benzylic acid and benzhydrol by UO*(NO3)2 has shown that benzophenone is formed in both cases by reactions which proceed hydroxylated

via triplet radical using

Fenton’s

pairs.164 Benzoic acid has been

reagent

that

has

been

generated

photochemical ly.165 Thls photoinitiated reduction of Fe(II1) to Fe(I1) has the advantage that Is uses a substantially lower iron concentration.

II.5: Photo-reduction and -oxidation

31 1

Photocataiytic decomposition of chiorobenzene in Ti02 suspensions is affected both by pH and oxygen, and is thought to involve electron attachment to the substrate, followed by formation of phenol, and finally decomposition of the phenol to C02 and oxygen under catalysis by Ti02.16' The use of biphenyl as a cosensitizer has been reported to lncrease the DCA-sensitized

l,l-diphenyl-2-vinyicyciopropane to

photooxygenation of

3,3-dlphenyl-5-vinyi-l,2-d ioxoiane.167

Neither

processes

electron-transfer

as

the

eiectron-transfer

nor

A report has appeared of some

charge transfer emission were observed. cosensitized

em I ssion

excipiex

photooxygenations

stereochemical

outcome

of

photooxygenation

of

m-

and

includes

the

DCA-sensitized

and

such

trans-2,3-

bis(4-methoxyphenyl)oxirane, in which a chain reaction leads exclusively to the &-ozonide.16*

Benzylic ethers such as ROCH2C6H40Me-p (R

-

aikyl,

cycioalkyl, 2-pyrrolidinoethyi) can be converted to the corresponding alcohols

by

photolysis

in

tleCN-H20

in

the

presence

of

lt4-dlcyanonaphthaiene as electron acceptor.169 This is a m i id and efficient procedure for cleaving the benzylic ether protecting group. a-Diaikoxy carbocations have been produced from benryl acetals or ketais via radical cations generated by electron transfer, and photoionisation in aqueous solution.170 Using hypocreilin A as sensitizer, various xylenols will undergo photooxygenation

by

1,4-cycloaddi tion,

and

both

nono-

and

diendoperoxides can be produced from the photooxygenation of substituted sti l b e n e ~ .This ~ ~ ~ latter study suggests that endopergxide formation occurs via a polar intermediate such as a perepoxide or zwitterion. Photooxygenation of l-isopropylideneindan-2-one uslng singlet oxygen is reported to occur through an

initial 4+2 cycioaddition to give an

endoperoxide,173

dye-sensi t ized

and

the

photooxygenation

9,ll-didehydroestrone also leads to endoperoxide formation.lc14

of

312

Photochemistry Irradiation

9-(formyloxy)-lO-nethyianthracene

of

promotes

head-to-tai 1 4n+4n photodimerisatlon.175 The photooxidation of anthracene derivatives has been studied in AOT/heptane reversed rice1

and

photosensitized oxidation of perylene soiubilised in an aqueous solution of

styrenesuiphonate-co-2-vinylnaphthalene)

poly(sodium

gives

3,lO-perylenequinone and 1,12-perylenequinone in a process which

is

thought to involve the participation of singlet oxygen.177 Photoexcltation of

9.10-dicyanoanthracene

biphenyl

In

HeCN

and

under

9-cyanoanthracene oxygen

leads

to

in the presence of anthraquinone

and

9(1OH)-anthracenone respectively via the reaction of the radical anions of cyanoanthracenes wi th molecular oxygen.178 The photodegradation of methyl violet has been studied in an 02-purged solution containing TiO2, and rate constants for mineralisation determined as a function of pH, temperature, and concentration of the ~ a t a 1 y s t . l ~Some ~ rethoxyf lavones have been dehydrogenated

using

2,4,6-triphenylpyryiiur

photosensitizer to give the corresponding cond i t ions 4-n i trof 1 avone rema i ns unchanged.

tetrafluoroborate

fiavone: under

as

the same

The react I on appears to

occur by a singie-electron transfer process from the flavonone B ring to the excited pyryiium salt.

A study of the products of photooxidation of

psoralen (9) by spin trapping suggests that one contains a hydroperoxy group.181

~2

-

Dye-sensitized photooxygenation of 2-rethoxy-5-phenylfurans (101 R1

C O ~ H ~~1 :

endo-peroxides

-

C O ~ H ~AC, , ~2

- HI

-

in non-participating solvents gives

which on thermal rearrangement yield 3H-1,2-dio~oies.~*~

Nunerous other singlet photooxygenations have been described including those of furfuryl benzoate to give new Baeyer-Villiger rearrangement products,

4,7-dinethoxy-2,9-di - m - b u t y ioxepino [2,3-bIbenzofuran,la4

and polyhydroxyalkyi and g i y c o ~ y l f u r a n s . In ~ ~ ~the autoxidation of the narine sesquiterpene tetronic acid variabiiin, the tetronlc acid moiety

W 5 : Photo-reduction and -oxidation

313

acts as a sensitizer for the production of singlet oxygen: this then leads to oxidation of the furan moiety.186 Cheri luminescence has been observed during the liquid-phase autoxidation of PhjCNat the emitter has been identlfied as the excited radical Ph$.18’ 8. Oxidation of Nitroaen-containinq Compounds

Photoredox reactions of amino alcohols having several bonds between the substituents can be Induced in the presence of acceptors such as thioindigo and 8-lapachone, and

in some cases these transformations

proceed by selective C-C photofragmentation.188 Phenyldimorphol inoethane undergoes a clean oxidative photofragmentation to the corresponding aldehydes (PhCHO and CH,0).189

The mechanism is presumably similar to that

involving amino alcohols and occurs by hydrolysis of the irinium ions generated in the presence of light absorbing electron acceptors. Singlet oxygen-mediated

photofragmentation of amino alcohols has

also been

described.lgO Iminiur cations have been generated by exclplex dissociation of excited l K N * ,

.. amines,

(DcN

-

1,4-dicyanonaphthalene). This single

electron transfer oxidative reactlon has been used to convert the amino alcohol (11) to the oxaatabicyclodecane (12)191 and, by hydrolysis of the iminium cation, as a mild method of N-debenzylatior~.~~~ Photooxidation of aqueous (*)-iupinane leads to the formation of a variety of compounds including alkaloids, amino acids and

sugars.lg3 Neutral nitroaminyl

radicals capable of undergoing intramolecular hydrogen abstraction to give N-nitroimine compounds are formed on photolysis of nitroamino steroids in the presence of various oxidlsing agents, and use has been made of this transformation

to

prepare

6&nitroamino-5a-cholestane

the

N-nitroepimino

compound

(13)

from

.Ig4

A study of the dye-sensltlzed photooxygenation of the carbon-nitrogen

double bond has revealed a reactivlty pattern of C-N compounds towards ‘02 which suggests that reaction is facilitated by electron donors attached to

Photochemistry

3 14

nitrogen.lg5 The ultimate products are highly particular to the substrate structure.

At

366

nm,

irradiation

of

nifedipine

2,6-dimethyl-3,5-dicarbomethoxy-4- (2' -n i tropheny i Ipyrid ine

gives

in

an

Intramolecular hydrogen abstraction process which occurs from the triplet state.lg6 This nitroso compound is also produced at 254 nm, but on further irradiation in the presence of oxygen the corresponding nitro compound is formed.

The known photochromic change from coiourless to violet when

2,4,4,6-tetraphenyl-l,4-dihydropyridine

is

irradiated

in

deaerated

soiutlon has been accounted for in terms of an irreversible change to give 2,3,4,6-tetraphenylpyr1dine.197 Other examples are reported, and ai 1 seem to involve a di-n-methane rearrangement. excited state of the [Ru(bpy)3i-'21

Electron transfer from the

ike centre in N-ethylated copolymers of

4-vi ny lpyr i di ne and [Ru(bpy12 (MeVbpy1 12+

(MeVbpy

-

4-methy 1-4 ' -vinyl-2 ,2 ' -

bipyridyl) to methylviologen has been studied as a function of ionic strength.lg8

Irradiation

9,9-dihydro-lO-methylacridine,

of

the

in

NADH

model

oxygen-saturated

MeCN

compound, gives

the

10-methyiacridinium ion in the presence of HC103, and 10-methyi-9-acridone in its absence.lg9 Kinetic studies have revealed the presence of radical chain reactions initiated by eiectron-transfer from the singlet state of the model compound to oxygen. Endoperoxide corresponding

(14) has been obtained quinol inoquinoline.200

by

photooxidation of

the

Lumiflavin-sensitized

photooxygenation o f aqueous solutions o f indole gives the corresponding semiquinone.201 Oxygen reacts with the semiquinone radical anion of iuniflavin to form superoxide radical and also with the triplet state'of iumiflavin to form singlet oxygen. uslng HCrO4- as sensitizer at Xirr

Tryptophan has been photooxidised

365 or 436 nmt N-formyikynurenine the

only photoproduct, arises in a 2-electron process.202 Similarly, it has been found that irradiation of a mixture of tryptophan and aqueous uranyl

II.5: Photo-reduction and -oxidation

315

sulphate leads to ring cleavage with formation of kynurenine and its N-forryl derlvative.203 Photooxygenation of pyrazin-2-ones with singlet oxygen gives stable 3,6-epidioxypyrazin-2-ones and a-0x0 hides, 204 and in aerated solvents, irradiation of 2-pheny1benzotriazole leads to cleavage of

the

benzene

dicarboxylic

ring

acid.205

4,5-diarylimidazoles N,N'-diaroylureas, formation

and

of

formation

Using

singlet

RCONHCONHCOR

(R

diaroylureas

iridazolin-2-0ne.~'~

The

2-phenyl-1,2,3-triazole-4,5-

rethylene

undergo

the

of

blue

oxygenation

-

Ph,

may

as and

441CgH4,

involve

cleavage 2-ClCgHq);

the

chemiluminescent

sensitizer, to the

corresponding

oxidation

of

4-(diethylamino)phthalhydrazide with H202/02 is catalysed by metal ions, leading to an enhancement of the emission,207 and the effects of axial ligands on electron transfer from the excited triplet state of zinc

.

tetrapheny 1porphyr i n (ZnTPP1 to benzoqu i none have been stud ied *08 In solvents having a dielectric constant 9

<

Q

<

18, axial ligation of

pyridine promotes a large enhancement of 4ion. 9. Miscellaneous Oxidations Iodide

photooxidation

has

been

studied

on

the

heterogeneous

photocatalyst W03/a-Fe203/W in aqueous solution,209 and an investigation of the photooxidation of CF31 in low temperature matrices has produced spectroscopic evidence for the participation of CF3Ob and CF36 in the oxidation of iF3.210 Photooxidation of RCF2S02Na (R

-

C1 (CF2)80CF2,

F (CF2)80CFZ, F (CF217, F2C-CFOCF2CF (CF31OCF2 1 is an effective method for the synthesis of perhalofluorocarboxylic acids and their esters from the corresponding organic

sulphonyl

substrates

fluorides R C F Z S O ~ F . ~Some ~ ~ easily

such

as

di-

and

triphenyimethane.

oxidised di-

and

triphenylacetic acid, 2-nitropropane, and olefins can be selectively fluorinated in MeCN/AgF solutions using a Ti02 photocatalyst.212 rechanism has been suggested which involves attack of 'F

A

on either a

3 16

Photochernisty

radical cation or carbocatlon at the serlconductor surface. In contrast to

ZnTtlPyP(2)

and

ZnTHPyP(4),

20-tetraki s (l-methyl-2-, 3-,

ZnTtlPyP(3)

(THPyP(n1,

5,10,15,

and 4-pyr id in lo )porphyrin 1, has proved to be

an effective sensitizer for the photoreduction of m e t h y l ~ l o l o g e n . ~ ~ ~ Haeaatoporphyrln, tetraphenylporphyrln and metal 1 ic complexes with Cu(II), Co(II), Ni(I1) and Ag(I1) are also reported to be sensitive to visible light and oxygen, and are easily ~ x i d l s e d .Photosensitized ~~~ one-electron oxidation of phenacyl sulphonlur ylldes using 9,lO-dicyanoanthracene glves ylidlons, and the ground state properties of this new class of reactive lnterrediates have been investigated.215 Studies have also appeared of reactions of singlet oxygen with a series of allyllc sulphoxldes, sulphones and temperature,

sulphides as a function of the extent of reaction, and

solvent,216

the

electron

transfer

oxidation

of

2-substituted 1,3-dlthi0lane~~~ and diphenyl diselenide,218 the near UV enhancement of Si o ~ l d a t i o n ,and ~ ~ ~of the effect of oxygen on the cherllurlnescent oxidation of ethylaluminlum diethoxide with XeF2 In solution.220 REFERENCES 1.

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200. K. Kitahara and H. Nishi, J. Heterocycl. Cher., 1988, 2 5 , - 1 0 6 3 .

328

Photochemisny

201. A. Yoshirura and T. Ohno, Photocher. Photobiol., 1988, 48, 561. 202, H. Bolte and J. Leraire, J. Photocher. Photobiol., A , 1989, 46, 285. 203. R. Bhatt, P. S. Shrirali, S. C. Areta, and H. C. Chowdhry, Zh. Orq.

m.,1989, 25, 431. 204. T. Nishio, N. Tokunaga, H. Kondo, and Y.

Perkin Trans. 1,

Omote, J. Chem. SOC.,

1988, 2921.

205. R. G. Ariet, N. A . Evans, and P. H. Wynne, Aust. J. Chem., 1989, 42, 441. 206. E. Purushothaman and V. N. R. Pillai, Indian J. Chen., 1989, 288, 290. 207. I. V.

Pyatnitskii, 0. A .

Zaporozhets, and L. I. Dubovenko, &

Obstich. Khim., 1988, 58, 1121. 208. H. Seki, H. Hoshlno, and H. Shituka, J. Phys. Chem., 1989, 93, 3630. 209. D. Chen, X. Wen, and W. Jin, Huaxue Tonabao, 1989, 32. 210. K. C. Clemitshaw and J. R. Sodeau, J. Phvs. Chem., 1989, 93, 3552. 211. C. Hu, 2. Xu, and W. Huang, J. Fluorine Chem., 1989, 42, 145. 212. C. H. Wang and T. E. Hallouk, J. Am. Chem. SOC., 1990,

u,2016.

213. K. Takahashi, Y. Kamatasuchi, T. Komura, and H. Imanaga, NipDon Kaaaku Kalshi, 1988, 1756. 214. F. Gao and B. Pu, Huaxue Shiii, 1988,

lo, 328.

111, 7149. 111,8212.

215. J. J. Zhang and G. B. Schuster, J. Am. Chem. SOC., 1989, 216. E. L. Clennan and X. Chen, J. Am. Chem. Soc., 1989,

217. B. Pandey, S. Y. Bal, and U. R. Khire, Tetrahedron Lett., 1989,

30,

4007 218. G.

Pandey, V.

J. Rao, and U. T. Bhalerao, J. Chem. Soc., CheQ.

Coaaun., 1989, 416. 219. V. Nayar and I. W. Boyd, Chemtronics, 1989, 4, 101. 220. R. G. Bulgakov, V. N. Yakovlev, G. Ya. Maistrenko, G. A . Tolstikov, and V. P. Kazakov, Izv. Akad., Nauk SSSR, Ser. Khim., 1988, 2732.

Photoreactions of Compounds containing Heteroatoms other than Oxygen BY S.T. REID

1.

Nitrogen-containing Compounds

Few novel photoreactions have been described in nitrogencontaining compounds during the period covered by this report; emphasis has again been placed on the further investigation and application of known nrocesses. Aspects of the Dhotochemistry of heteroaromatic compounds have been reviewed. Rearrangements.- Z,,-Photoisomerization has been observed in imines and related systems containing carbon-nitrogen double bonds, although in the case of imines, only one stereoisomer is normally stable at room temperature. The 9,lO-dicyanoanthraceneand E-oxime ethers of 2-acetylsensitized isomerization of naphthalene has been described. g-+E Photoisomerization has also been reported in a-keto oximes. X-Ray crystal structures of the photochromic formazans, 3-methyl-1,5-diphenylformazan and 3-t-butyl-l,5-diphenylformazan , have been determined, and solvent polarity has been shown to have an effect on the photoThe photoreactions isomerizations of 1 ,3 ,5-triarylformazans. of azines have been reviewed.6 -Z ,E-Isomerization competes with elimination of nitrogen on irradiation of azoalkanes. Laser flash photolysis studies have shown that isomerization of E-azocumene to 2-azocumene is In almost all examples the dominant process at room temperature.' of photoisomerization of 2-azoalkanes, reaction appears to take place 2 a non-radical inversion pathway, although a diazenyl radical may be involved in the rearrangement of the hindered The solution photoazoalkane, g-E-t-butyl-N' -norbornyldiazene. chemistry of g - [ (trifluoromethyl)azolcyclopropane (1) has been reinkestigated.and the 2-isomer ( 2 ) and the pyrazoline ( 3 ) are formed in addition to other products; the product distribution is different from that previously d.escribed and is similar to

z-

*

Photochemistry

330

(6) n = 6,8, or 12

( 7)

R' = R2 = Me, R3 = H R' = H, R2 = Me, R3 = H or Me R', R2 = (CH2)5, R3 = H

I M : Photoreactions of Compounds containing Heteroatoms other than Oxygen

33 1

that observed for azocyclopropane. New photoresponsive polymers and peptides have been developed by incorporation of an azobenzene chromophore. The photochemistry of azobenzene-containing polymers has been reviewed,” and the ion-conducting polymer composite film of poly (vinyl chloride)/the azobenzene liquid crystal (4)/lithium perchlorate is a promising material for use in photoinduced Photoisomerization has also been demonstrated image storage. in the azo-quinone ( 5 ) which has potential as a new type of chemical transducer. l2 E+Z Photoisomerization of the azobenzene moiety is similarly responsible for the photoresponsive properties of L-lysinel and L-orn ithine14 containing peptides and‘polypeptides. -E-Dithiadiaza[~.2lmetacyclophenes (6) undergo quantitative isomerization to the ;-isomers on irradiation (X = 369nm).I5 The E,g-bis-azobenzenophane (7) is similarly converted on irradiation into the Z,Z-isomer in 88% yield;16 an extremely rapid thermal isomerization of the E,Z-isomer to the -Z,Z-isomer is involved. A kinetic study of photochemically induced Z,E-isomerization in other bis-azo compounds in chloroform solution has been described. 17 Photochromism arising by reversible intramolecular proton transfer has been reported in 4-hydroxy- and 4-methoxybenzylidenesalicyloylhydrazines,I8 and the blue isomer obtained by irradiation of the well-known photochromic compound, 2-(2,4dinitrobenzy1)pyridine ( 8 1 , has been identified as the enamine (9);19 a short-lived species detected in this conversion is thought to be the aci-nitro tautomer. Many examples of photorearrangement arising by 4 ~ -and 6~-electrocyclicpathways have been reported. Full experimental details for the photohydration o f the pyrimidin-4-ones (10) to the enamides (11) via the Dewar pyrimidinones (12) have been published. 2o 6~-Electrocyclizationfollowed by elimination of benzoic acid is observed in the conversion of the 4-aryl-Ebenzoyloxy-2,3-diphenyl-l-azabuta-1,3-dienes (13) into the quinoline derivatives (14);21 analogous 2-azabuta-l13-dienes undergo photochemically induced cyclization in the nresence of acid to give novel isoquinolines.22 The triphenyl oxime ester (15: R * Ph) is similarly converted into the phenanthridine (16)

332

Photochemistry

mp

R

I

I

0

0

Ph

OAPh

OAPh

0-C-Ph

N

d

R

Ph

hv

Ph

t

/

Me

phHPh Nl " MexMe

hv

0

Me

Me Me

Me Me

Me

Me

dI

Me

(211

IIB: Photoreactions of Compounds containing Heteroatoms other than Oxygen

333

by o x i d a t i v e p h o t o c y c l i z a t i o n , whereas t h e i s o p r o p y l analogue (15; R = CHMe2) undergoes an a l t e r n a t i v e c y c l i z a t i o n l e a d i n g t o t h e 2g-imidazole ( 1 7 ) by a pathway t h a t is t h o u g h t t o i n v o l v e t h e i n i t i a l f o r m a t i o n of t h e r a d i c a l (18) followed by c y c l i z a t i o n and loss o f benzoyloxyl r a d i c a l . 23 6n-Electrocyclizations a r e a l s o responsible f o r the r e v e r s i b l e photochromism e x h i b i t e d by an i n c r e a s i n q l y l a r g e number of f u l g i d e s . Photochromic f u l g i d e s i n c o r p o r a t i n g t h e 24-26 i n d o l e n u c l e u s have a t t r a c t e d p a r t i c u l a r a t t e n t i o n t h i s y e a r , and t h e 4-oxazolyl f u l g i d e (191, which is c o n v e r t e d on i r r a d i a t i o n i n t o t h e c o l o u r e d isomer ( 2 0 ) , e x h i h i t s qood t h e r m a l s t a b i l i t y 27 and l o w photochemical f a t i g u e . R e l a t e d e l e c t r o c y c l i c r i n g opening r e a c t i o n s p l a y an i m p o r t a n t r o l e i n photochromic systems. I n t h i s c o n n e c t i o n , f u r t h e r s t u d i e s o f t h e i n d o l i n o s p i r o p y r a n s have been d e s c r i b e d and t h e s u b j e c t b r i e f l y reviewed.28 I t h a s been shown, f o r example, t h a t i n t h e c a s e of 1',3',3'-trimethyl-6-nitrospiro[2g-l-benzopyran-2,2'-indolinel more t h a n 90% of t h e t o t a l amount of c o l o u r e d photomerocyanine isomer is g e n e r a t e d from t h e e x c i t e d s i n g l e t s t a t e whereas photochemical f a t i g u e arises mainly via t h e e x c i t e d t r i p l e t s t a t e . 2 9 30 I n d o l i n o s p i r o n y r a n s have been i n c o r p o r a t e d i n t o d i o c t a d e cyldime t h y l a m o n i u m b i l a y e r membranes ,31 and l a s e r f l a s h p h o t o l y s i s of t h e m e t h o x y - s u b s t i t u t e d s p i r o p y r a n ( 2 1 ) i n t h e p r e s e n c e o f t h e metal i o n s , P b ( I I ) , C o ( I I ) , Mn(I1) and Y ( I I 1 ) , a f f o r d s complexes o f t h e merocyanine isomer. 32 I n c r e a s i n g a t t e n t i o n h a s been devoted t o t h e p r e p a r a t i o n and development o f analogous photochromic i n d o l i n o s p i r o - o x a z i n e s . I n p a r t i c u l a r , spiro(l,3,3-trimethylindolo-2,2'-naphth~l12-~l1,4-oxazine ( 2 2 ) 3 3 and i t s n i t r o are p o t e n t i a l l y v a l u a b l e photochromic systems. I n t h e p r e s e n c e o f s u i t a b l e

derivative^^^

s u b s t i t u e n t s , t h e c h e l a t i o n of a photomerocyanine of t h i s t y p e t o d i v a l e n t metals can b e a c h i e v e d , a complexation which r e t a r d s d e c o l o u r i z a t i o n i n t h e dark. 35 O t h e r new photochromic s p i r o p y r a n ~and ~ ~s p i r o - o ~ a z i n e s have ~ ~ been d e s c r i b e d . N e w examples o f t h e stilbene-to-dihydrophenanthrene c o n v e r s i o n i n n i t r o g e n - c o n t a i n i n g systems have been r e p o r t e d . O x i d a t i v e p h o t o c y c l i z a t i o n o f t h e E-phenylsulphonylindoles (23) t o t h e 6-carbomethoxy-7~-pyrido[4,3-~lcarbazoles (24) is accompa n i e d by l o s s o f t h e N-phenylsulphonyl group ,38 whereas a similar

Photochemistty

334

SO, C02Me I

Ph

(23)

R = H or Me0

hv 12, MeOH

@ /

\

\ y /

CO l,

ClO,

(26)

(25)

0 \

Y o

CH2Ph

CH2Ph

0

(29)

R = H, Me, or Me0

(30)

IV6: Photoreactions of Compourlds containing Heteroatoms other than Oxygen

335

photocyclization in 4 ,5-diaryl-A4-imidazolin-2-ones is facilitated by the presence of acid.39 The azonia analogue ( 2 5 ) of hexahelicene has been prepared in low yield by oxidative photoperchlorate cyclization of 2 - s t y r y l n a p h t h o [ l , 2 - a ] q u i n o l i z i n i u m (26).40 Photocyclization reactions of enamides are also common and are of particular value in synthesis. 5-Benzylbenzofuro[2,3clquinolin-6 (5H) -one (27), for example, has been prepared in 82% yield by irradiation of ~-benzyl-~-phenyl-2-benzofurancarboxamide ( 2 8 ) in a ~ e t o n i t r i l e ,and ~ ~ an analogous process is presumably involved in the photochemically induced conversion of the 3-aroylamino-5-methyl-1,2,4-oxadiazoles (29) into the 2-acetylaminoquinazolin-4-ones (30) in methanol. 4 2 Photoreductive cyclization of enamides is usually carried out in the presence of sodium borohydride. The use of this approach in the synthesis of indole alkaloids has been reviewed,43 and the conversion of enamide (31) into lactam (32) is a key step in a new general strategy for the synthesis of ipecac and heteroyohimbine alkaloids.44 Acyl group migration can be effected photochemically in nitrogen-containing systems. Regioselective photo-Fries rearrangements in 5- and 6-acetoxyindoles have been observed in aqueous solution, the major products being 4-acetyl-5-hydroxyindole and 7-acetyl-6-hydroxyindole respectively.45 1-Aroyland 3-aroyl-carbazoles have similarly been prepared by irradiation of 9-aroylcarbazoles in p o l a r solvents.46 g-Benzoyldiphenylamine (33) is converted on irradiation in the presence of iodine as oxidant into the benzoyl derivative (34) and 9-phenylacridine (35) the latter arising as the result of a novel photocyclization of the 2-substituted benzoyl derivative (36) 47 A 1,3-acyl migration is also involved in the photorearrangement of certain 4-aza steroids (37) to the corresponding fused azocines (38).48 Competing [ 3 , 3 1 - and [1,31- sigmatropic amino-Claisen rearrangements have been observed on irradiation of 1,3,1',3'-tetraallyl2,2'-biimidazolidinylidenes. 49 The acetophenone-sensitized aza-di-IT-methane rearrangement of a series of l-aryl-4,4-dimethyl-6,6-diphenyl-2-azahexa-2,5dienes (39) to cyclopropylimines ( 4 0 ) has been described;'' the efficiency of the conversion is dependent on the nature of the N-benzyl group, with higher quantum yields being observed in the

-

.

-

Photochemistry

336

Me0

Me0 Me0

Me0

hv D

NaBH4

\--I (32)

0 hv b

12, MeOH

(34)

+ Ph

Ph hv 4

H

(36)

(35)

&

0

6"

hv

R = H, Me, or Me0

(37)

Me

Me hv

Ph

CH2R

(39)

N-CH~R

II.6: Photoreactions of Cornpoutids containing Heteroatoms other than Oxygen

337

p r e s e n c e of e l e c t r o n - w i t h d r a w i n g s u b s t i t u e n t s . The analogous d i n-methane rearrangement o f t h e oxime a c e t a t e of e t h y l 2 , 4 , 4 trimethyl-5-oxopent-2-enoate i s of p o t e n t i a l v a l u e i n t h e s y n t h e s i s of p y r e t h r o i d components. 51 The formation o f t r i p h e n y l 1 , 3 , 5 - t r i a z i n e from 2,2,4,6-tetraphenyldihydr0-1,3,5-triazine on i r r a d i a t i o n i n t h e s o l i d s t a t e may i n v o l v e an i n i t i a l aza-din-methane rearrangement. 5 2 Competing di-n-methane rearrangements have been r e p o r t e d i n 2-acetyl-2-azatricyclo[6.2.2 .O1 p 5 ldodeca4 , 6 , 9 ,ll-tetraen-3-one ( 4 1 ) l e a d i n g t o t h e formation of photoisomers ( 4 2 ) and ( 4 3 ) t h e b i r a d i c a l ( 4 4 ) ~t h~e ~e f f e c t of a 9-bromo s u b s t i t u e n t on t h i s photorearrangement h a s been s e p a r a t e l y i n v e s t i g a t e d . 5 4 Various p y r a z i n o b a r r e l e n e s a l s o undergo competing di-n-methane photorearrangements. 55 L i t t l e i n t e r e s t h a s been shown t h i s y e a r i n t h e photoc h e m i s t r y o f five-membered h e t e r o a r o m a t i c compounds. A t h e o r e t i c a l s t u d y of t h e p h o t o i s o m e r i z a t i o n of 2-cyanopyrrole t o 3-cyanopyrrole h a s been d e s c r i b e d . 5 6 The f o r m a t i o n o f 2 - a c e t y l aminoquinazolin-4-ones on i r r a d i a t i o n o f 3-acetylamino-5-aryl1,2,4-oxadiazoles (45) i n methanol i s t h o u g h t t o i n v o l v e i n i t i a l rearrangement t o t h e c o r r e s p o n d i n g 3-aroylamino-5-methyl-l,2,4o x a d i a z o l e s ( 4 6 ) . ” Photochemically induced e p i m e r i z a t i o n a t C-7 i n t h e i m i d a t e (47) i s t h e r e s u l t of r i n g c l e a v a g e followed by Ring recycli~ation.~ ~ c l e a v a g e h a s a l s o been o b s e r v e d i n t h e p y r a z o l i n e s (48) and gave t h e 2-amino-3-cyano-5-azabicyclo[2.1.0]pentanes (49) 2 i n i t i a l cleavage of t h e nitrogen-nitrogen bond. 59 F u r t h e r examples o f photorearrangement of i s o x a z o l i n e s t o enamino a l d e h y d e s have been r e p o r t e d . 6 0

*

Photorearrangements r e p o r t e d i n six-membered c y c l i c systems i n c l u d e t h e d i s s o c i a t i o n of t h e cyclomers (50) i n t o 1 , l ’ -( 1 , 2 - e t h a n e d i y l ) b i s ( p y r i d i n y l ) b i r a d i c a l s , 61 r i n g c o n t r a c t i o n o f t h e 4 - s u b s t i t u t e d 2-amino-3,5-dicyano-6-phenyl-4H-pyran (51) t o t h e cyclobutene ( 5 2 ) f o l l o w e d by photochemically induced f r a g m e n t a t i o n t o t h e a l k y n e ( 5 3 ) and t h e amide (54),62 and t h e unusual c o n v e r s i o n of 3-methoxy-!-substituted pyrid-4-ones i n t o t h e c o r r e s p o n d i n g 3-hydroxymethylpyridones on i r r a d i a t i o n i n benzene 63 The p h o t o c h e m i s t r y o f n i t r o n e s and h e t e r o a r o m a t i c N-oxides h a s a t t r a c t e d much a t t e n t i o n i n p r e v i o u s y e a r s . L i t t l e

.

-

of s i g n i f i c a n c e h a s been r e p o r t e d i n t h e p e r i o d covered by t h i s

338

Photochemistry

'Ac

R2 R1

R2 R1

Et02C-N R3

A3

C02Et R' = R2 = R3 = H or Me R

R

(50) R = H, Me, or CMe3

(49)

'NHC02Et

339

11.6: Photoreactions of Compoutids containing Heteroatoms other than Oxygen

#" K hv

Me

NOH I1

-

::

Me

C-NHMe

hv

hv

Ph

___c

0

Me3C

0

0

340

Photochemistry

report. F u r t h e r examples of t h e c o n v e r s i o n o f n i t r o n e s i n t o o x a z i r i d i n e s have been d e s c r i b e d , 64 and a combined s F e c t r o p h o t o m e t r i c - p o l a r o g r a p h i c t e c h n i q u e h a s been developed t o d e t e r m i n e 65 t h e quantum y i e l d s of a r y l p r o p e n a l n i t r o n e p h o t o t r a n s f o r m a t i o n s . C r y s t a l s t r u c t u r e a n a l y s i s of t h e 1:l i n c l u s i o n complex of (~,~)-(-)-1,6-bis(~-chlorophenyl)-l,6-diphenylhexa-2,4-diyne-l,6d i o l and ( - 1 -benzylidene-2- (3-methylbutyl) azane o x i d e h a s been used t o e s t a b l i s h t h e c o n f i g u r a t i o n of t h e n i t r o n e and t h e n a t u r e of t h e p r e v i o u s l y r e p o r t e d p h o t o c y c l i z a t i o n t o an o p t i c a l l y a c t i v e o x a z i r i d i n e . 66 A l t e r n a t i v e pathways have been r e p o r t e d f o r c e r t a i n h e t e r o c y c l i c N-oxides and i n v o l v e photo-

(z)

c h e m i c a l l y induced oxygen t r a n s f e r from a c r i d i n e !-oxides to c y ~ l o h e x e n eand ~ ~ from pyrimido[ 5 , 4 - q ] p t e r i d i n e 5-oxide t o benzene and i t s s i m p l e d e r i v a t i v e s . 68 O x a z i r i d i n e s undergo rearrangement and f r a g m e n t a t i o n on exposure t o l i g h t and some of t h e s e t r a n s f o r m a t i o n s have been employed i n s y n t h e s i s . 69 O x a z i r i d i n e s have a l s o been proposed as i n t e r m e d i a t e s i n t h e photorearrangement o f oximes. Oximes ( 5 5 ) a r e c o n v e r t e d i n t h i s way i n t o t h e b u t e n o l i d e s (56) i n h i g h I n c o n t r a s t , however, a d i f f e r e n t r e a c t i o n h a s been yie1d.l' o b s e r v e d on i r r a d i a t i o n o f By- u n s a t u r a t e d oxime-boron t r i f l u o r i d e complexes l e a d i n g t o t h e formation o f d i h y d r o i s o x a z o l e s ;71 t h e 2,2-dimethyl-4,4-diphenylbut-3-enal oxime (571, f o r example, a f f o r d s t h e d i h y d r o i s o x a z o l e ( 5 8 ) by a pathway which i s b e l i e v e d t o involve intramolecular electron transfer. Photochemically induced r i n g opening a l s o o c c u r s i n t h e f u s e d a z i r i d i n e ( 5 9 ) l e a d i n g t o t h e ' D e w a r ' pyridone (60) by way of t h e i n t e r m e d i a t e (61),72 whereas o t h e r a z i r i d i n e s have been shown t o b e photochromic.73 A n i t r i l e y l i d e h a s been d e t e c t e d on l a s e r f l a s h p h o t o l y s i s of 3- ( 4 - b i p h e n y l y l ) - 2 g - a z i r i n e i n cyclohexane; 74 analogous n i t r i l e y l i d e s have p r e v i o u s l y been d e t e c t e d i n r i g i d m a t r i c e s a t 77K. Aspects of t h e photochemistry 75 of three-membered h e t e r o c y c l e s have been reviewed. Photochromism i n 2 - n i t r o b e n z y l compounds h a s been reviewed. 76 5-Nitrobenzyl-containing compounds undergo photorearrangement t o t h e c o r r e s p o n d i n g n i t r o s o isomers by a p r o c e s s i n v o l v i n g i n i t i a l i n t r a m o l e c u l a r hydrogen a b s t r a c t i o n . The c o n v e r s i o n of 2-nitrobenzaldehyde t o 2 - n i t s o s o b e n z o i c a c i d h a s been shown t o proceed i n t h i s way & y direct excitation t o the

II.6: Photoreactions of Compoutids containing Heteroatoms other than Oxygen

341

s i n g l e t s t a t e and t h e subsequent formation o f a h i g h l y r e a c t i v e t r a n s i e n t k e t e n e . 77 An analogous t r a n s f o r m a t i o n h a s been r e p o r t e d i n a methyl o - n i t r ~ p h e n y l a c e t a t e and ~ ~ [ 1 - ( 4 @, 5 @ dimethoxy-2'-nitrophenyl~ethyl]glycine ( 6 2 ) h a s been d e s i g n e d a s a p h o t o l a b i l e p r e c u r s o r o f g l y c i n e (63) :79 i r r a d i a t i o n r e s u l t s i n t h e l i b e r a t i o n of t h e amino a c i d presumably via t h e u n s t a b l e n i t r o s o hemiketal (64) P h o t o l a b i l e 2 - n i t r o b e n z y l anchoring groups have been used i n t h e s o l i d - p h a s e s y n t h e s i s o f peptides.8082

.

N i t r o a l k a n e s and aB-unsaturated n i t r o a l k e n e s a r e known t o undergo photorearrangements which y i e l d t h e corresponding n i t r i t e s and p r o d u c t s d e r i v e d therefrom. By-Unsaturated n i t r o a l k e n e s have now been examined: 8 3 3-€luoro-4-methyl-, 3-chloro4-methyl- and 3,4-dimethyl-4-nitro-2,5-cyclohexadienyl a c e t a t e s a r e c o n v e r t e d on i r r a d i a t i o n i n methanol i n t o t h e r e l a t e d 4-hydroxy d e r i v a t i v e s w i t h r e t e n t i o n o f c o n f i g u r a t i o n , s u g g e s t i n g t h a t t h e recombination of t h e carbon r a d i c a l s g e n e r a t e d by carbon-nitrogen bond homolysis and n i t r o g e n d i o x i d e must be extremely rapid. The photochemistry o f a - n i t r o k e t o n e s i s complicated by t h e e x i s t e n c e of a t a u t o m e r i c e q u i l i b r i u m between t h e a - n i t r o k e t o n e and t h e corresponding e n o l . 16-Nitroandrostan17-one (651, which l a r g e l y e x i s t s a s t h e e n o l tautomer i n e t h a n o l , undergoes rearrangement t o t h e c y c l i c g-hydroxyimide (661, a r e a c t i o n which i s w e l l e s t a b l i s h e d i n five-membered c y c l i c an i t r o k e t o n e s and is viewed a s proceeding an i n t e r m e d i a t e n i t r i t e . 84 15a-Nitr0-5a-androstan-16-one ( 6 7) , however, e x i s t s e x c l u s i v e l y a s t h e k e t o tautomer i n e t h a n o l and i s converted on i r r a d i a t i o n i n t o t h e c y c l i c imide (68); t h e same p r o d u c t i s o b t a i n e d from t h e a-oximino ketone ( 6 9 ) which i s t h e r e f o r e presumed t o be an i n t e r m e d i a t e i n t h i s conversion. An unexpected a-cleavage r e a c t i o n h a s been observed on i r r a d i a t i o n o f 2anitrocholest-4-en-3-one (70) i n methanol o r e t h a n o l and a f f o r d s t h e corresponding 3-alkoxy-2-nitro-2,3-secocholest-4-en-3-ones (71). 85 N i t r i t e s a r e a l s o probably i n v o l v e d i n t h e photochemically induced l o s s of b o t h n i t r o groups from 2,4-dinitro-5aand 2,4-dinitro-5B-cholestan-3-ones. 86 F u r t h e r examples o f f a c t o r s such a s c h a i n l e n g t h , 8 7 magnetic f i e l d , 8 7 and a-cyclod e x t r i n cornplexation88 which i n f l u e n c e t h e e f f i c i e n c y o f photoSmiles rearrangements have been d e s c r i b e d .

*

Photochemistry

342

/o @ 0 L N H 2

-0””””. N/

hv

-

Me 0

Me0 Me0

Me0

Me0 (62)

(64)

II/t5: Photoreactions of Cornpourids containing Heteroatoms other than Oxygen

(72)

(73)

343

(75)

Ihv 0

o

a

O

(74)

0

0

0

0

o&Me Et Et

Et

Scheme 1

Et

H

344

Phorochemistry

N i t r o x i d e r a d i c a l s g e n e r a t e d by t h e a c t i o n o f r e d l i g h t on b o t h t h e needle8' and t h e p l a t e l e t " form o f c r y s t a l l i n e humulene n i t r o s i t e are t h o u g h t t o a r i s e by an i n i t i a l i n t r a m o l e c u l a r hydrogen a b s t r a c t i o n by t h e n i t r o s o group. P h o t o r e a c t i o n s i n n i t r o g e n - c o n t a i n i n g c a r b o n y l compounds m e r i t b r i e f d i s c u s s i o n i n t h i s s e c t i o n as w e l l a s i n P a r t 111, C h a p t e r 1. Type I1 r e a c t i o n s are e s p e c i a l l y common. 5,5-Diethylb a r b i t u r i c a c i d ( 7 2 1 , f o r example, undergoes Type I1 photoc y c l i z a t i o n on i r r a d i a t i o n i n a c e t o n i t r i l e t o g i v e t h e c y c l o b u t a n o l (73) igl t h e a d d i t i o n a l formation of cyclobutanol (74) i s presumably t h e r e s u l t o f Type I1 c l e a v a g e t o t h e monoethyl d e r i v a t i v e (751, f o l l o w e d by p h o t o c y c l i z a t i o n . Intramolecular hydrogen a b s t r a c t i o n from t h e p r o p y l group i s p r e f e r r e d , however, i n g - p r o p y l b a r b i t a l ( 7 6 ) and l e a d s t o p h o t o p r o d u c t s ( 7 7 ) t o ( 7 9 ) by t h e pathways shown i n Scheme 1. Greater s e l e c t i v i t y h a s b e e n a c h i e v e d i n t h e known c o n v e r s i o n of oxamides ( 8 0 ) i n t o t h e B-lactams ( 8 1 ) by s o l i d s t a t e i r r a d i a t i o n o f 1 : l - i n c l u s i o n compounds o f t h e oxamides w i t h N , N , N ' ,~'-tetracyclohexylbiphenyl-2,2 I-dicarboxamide. 9 2 I n t r a m o l e c u l a r hydrogen a b s t r a c t i o n i s presumably i n v o l v e d i n t h e f o r m a t i o n o f t h e d i h y d r o n a p h t h i m i d a z o l e s ( 8 2 ) and t h e naphthi m i d a z o l e s (83) from t h e 3-amino s u b s t i t u t e d 2 - ( c y c l o a l k y l a m i n o ) 1 , 4 - n a p h t h a q u i n o n e s ( 84) t h e enaminoquinones ( 85) Examples of p h o t o c y c l i z a t i o n a r i s i n g by way o f l16-hydrogen t r a n s f e r g 4 and 1,7-hydrogen t r a n s f e r g 5 t o oxygen have been r e p o r t e d i n n i t r o g e n c o n t a i n i n g s y s t e m s , and remote hydrogen t r a n s f e r i s r e s p o n s i b l e f o r t h e c o n v e r s i o n o f t h e ( E - d i a l k y l a m i n o ) a l k y l 8 - o x o e s t e r s (86) i n t o t h e medium s i z e d a z l a c t o n e s ( 8 7 ) a n d / o r t h e a m i n o - s u b s t i t u t e d l a c t o n e s ( 8 8 ) . 9 6 Nitrogen-oxygen bond homolysis competes w i t h i n t r a m o l e c u l a r hydrogen a b s t r a c t i o n on i r r a d i a t i o n o f 1-benzyloxypyrid-2-one i n p o l a r s o l v e n t s , 9 7 whereas f r a g m e n t a t i o n t o t h e b e n z o t r i a z i n e s (89) i s t h e p r i n c i p a l p r o c e s s o b s e r v e d on i r r a d i a t i o n o f azapropazone ( 9 0 ) . 98 I n t r a m o l e c u l a r hydrogen a b s t r a c t i o n h a s now been u n e q u i v o c a l l y e s t a b l i s h e d i n 2- (N-trimethylsilyliminomethyl) t o l u e n e ( 9 1 ) and a f f o r d s t h e 2-quinonedimethane ( 9 2 ) which c a n b e t r a p p e d as t h e r T 4 + T 2 1 a d d u c t w i t h d i m e t h y l f u m a r a t e o r d i m e t h y l maleate.

-

.

-

IIh: Photoreactions of Compouirds containing Heteroatoms other than Oxygen

R’

‘c=o

hv

I

345

c

O= C-NR22 R1% 0

*“rj

NHR

hv

w

NHR

0

0 (84)

*o

R=HorAc

+ 0 (82) R2H2C,

,CH2R3 N

Photochem isti-y

346

(89) R = NMe2 or NHMe hv b

K N - S i M e 3

NHSiMe3

(911

(92) 0

0 hv MeCN

R1

I

C02Et

R2 R' = R2 = Me R' = OMe, R2 = Me R' = Me, R2 = OMe

(93)

C02Et (94)

H

CO, HO

HO

HO

(96)

(95)

1.9

-

O

hv

Aorhv

R

(97)

R = CH2Ph or CHMe2

(98)

W6: Photoreactions of Cornpourids containing Heteroatoms other than Oxygen

347

A d d i t i o n r e a c t i o n s . - A wide v a r i e t y o f i n t e r m o l e c u l a r and i n t r a m o l e c u l a r [,2 + ,23 p h o t o c y c l o a d d i t i o n s have been d e s c r i b e d i n n i t r o g e n - c o n t a i n i n g systems. [ T 2 + ,23 P h o t o d i m e r i z a t i o n h a s been r e p o r t e d i n a c l a y - i n t e r c a l a t e d s t i l b a z o l i u m c a t i o n l o o and i n t h e d i s o r d e r e d p h o t o r e a c t i v e c r y s t a l s t r u c t u r e o f B - n i t r o s t y r e n e . 101 have been r e s o l v e d Photodimers of 5~-indolo[1,7-&][1]benzazepine by l i q u i d chromatography on m i c r o c r y s t a l l i n e t r i a c e t y l c e l l u l o s e , and a k i n e t i c s t u d y o f i r r a d i a t i o n o f thymine i n aqueous s o l u t i o n s u g g e s t s t h a t d i m e r i z a t i o n o c c u r s from b o t h s i n g l e t - and t r i p l e t - s t a t e a g g r e g a t e s . 103 E t h y l 2,3-dihydro-2,2-dimethyl-3-oxo-l~-pyrrole-lc a r b o x y l a t e ( 9 3 ) is claimed t o be t h e f i r s t 3-oxopyrrole t o undergo [,2 + ,21 p h o t o a d d i t i o n t o a l k e n e s ; t h e fused cyclobutanes (94) are o b t a i n e d i n good y i e l d on i r r a d i a t i o n i n a c e t o n i t r i l e . N-Methoxycarbonyl-5,6-dihydropyrid-4-one undergoes [,2 + ,21 photoa d d i t i o n t o a series of a l k e n e s w i t h h i g h r e g i o s e l e c t i v i t y t o g i v e c y c l o a d d u c t s w i t h improved s t a b i l i t y . l o 5 Two c y c l o b u t a n e d e r i v a t i v e s ( 9 5 ) and ( 9 6 ) were o b t a i n e d by p h o t o a d d i t i o n o f u r o c a n i c a c i d t o thymidine ,Io6 and t h e p h o t o r e a c t i o n o f g i l v o c a r c i n w i t h DNA has been shown t o i n v o l v e [,2 + ,.,2] a d d i t i o n of g i l v o c a r c i n V t o a thymine r e s i d u e . l o 7 Photoresponsive cyclobutane-1,2-dicarbonyl-capped d i a z a crown e t h e r s have been p r e p a r e d by i n t r a m o l e c u l a r [,2 + ,21 photoc y c l o a d d i t i o n i n p,E'-trimethylenedicinnamoyl-capped d i a z a crown ethers log Photochemically induced c o n v e r s i o n o f t h e N-benzyls t y r y l a c e t a m i d e s (97) i n t o t h e a d d u c t s ( 9 8) i s q u a n t i t a t i v e l y T r a n s f o r m a t i o n s analogous t o t h e r e v e r s e d by h e a t o r light.'" de Mayo r e a c t i o n have been reported. i n v i n y l o g o u s amides. The i n t r a m o l e c u l a r a d d u c t s ( 9 9 ) , formed on i r r a d i a t i o n of t h e c y c l i c enaminones (100;n = 2 ) , underqo retro-Mannich c l e a v a g e t o g i v e t h e A p r o c e s s which resembles an aza-Claisen t r i c y c l e s (101) rearrangement h a s , however, been r e p o r t e d i n t h e enaminones (100; n = l ) and y i e l d s t h e i s o m e r i c a l k e n e s (102). I n t r a m o l e c u l a r p h o t o c y c l o a d d i t i o n s f o l l o w e d by retro-Mannich f r a g m e n t a t i o n s have a l s o been o b s e r v e d i n a c y c l i c t e r t i a r y vinylogous amides. The

.

p r e p a r a t i o n o f 3 - s u b s t i t u t e d 8i-3-azaheptalen-8-ones (103) by i r r a d i a t i o n o f t h e t r i c y c l i c arnines ( 1 0 4 ) may i n v o l v e i n i t i a l f o r m a t i o n o f t h e u n s t a b l e i n t r a m o l e c u l a r a d d u c t s ( 1 0 5 ) . 112 [,2 + ,21 P h o t o a d d i t i o n s t o carbon-nitrogen and n i t r o g e n -

Photochemistry

348

R = H or Me

(99)

fl

R R

hv

=

O R = C02Et, COPh, or S02Ph

H hv

CN

a (1 05)

W6: Photoreactions of Compounds containing Heteroatoms other than Oxygen

349

n i t r o g e n double bonds a r e a g a i n r e l a t i v e l y r a r e . Ouinoxalin-2ones undergo a r e g i o s e l e c t i v e b u t n o n - s t e r e o s e l e c t i v e photocycloa d d i t i o n t o e l e c t r o n - p o o r a l k e n e s ; ’13 t h e quinoxalin-2-one (1061 , f o r example, is c o n v e r t e d i n t h i s way i n t o t h e a z e t i d i n e (107) with m e t h a c r y l o n i t r i l e , whereas i r r a d i a t i o n i n t h e p r e s e n c e o f amines l e a d s t o t h e formation o f r e d u c t i v e dimers. O p t i c a l l y a c t i v e 8-lactams have been p r e p a r e d by p h o t o a d d i t i o n o f chromium carbene complexes o f ( 2 ) - v a l i n e and (R)-phenylglycine t o v a r i o u s imines 1 1 4

.

Evidence f o r t h e formation o f a t r a n s i e n t t e t r a z e t i d i n e 115 on i r r a d i a t i o n of t h e azobenzenophane (108) h a s been o b t a i n e d , whereas i n c o n t r a s t ~-methyl-1,2,4-triazoline-3,5-dione h a s been found t o undergo a I n 4 + ,21 p h o t o a d d i t i o n t o benzene.l16 An analogous benzophenone-sensitized a d d i t i o n o f t h e same t r i a z o l i n e dione (109) t o dibenzocyclooctatetraene (110) gave t h e adduct (111) which was f u r t h e r c o n v e r t e d photochemically i n t o t h e isomer ( 1 1 2 ) by way o f a di-r-methane rearrangement. 117 P h o t o a d d i t i o n s t o p h t h a l i m i d e s and r e l a t e d imides have a g a i n a t t r a c t e d much i n t e r e s t and it h a s been c l e a r l y shown t h a t v a r i a t i o n i n t h e a r e n e s t r u c t u r e c a u s e s a change i n t h e s i t e of a d d i t i o n . D e t a i l e d s t u d i e s of t h e photochemistry of ;-methyl9,lO-phenanthrenedicarboximide (113) have been r e p o r t e d . S t e r e o i s o m e r i c c y c l o b u t a n e s (115) a r e formed a s w e l l a s imide i n s e r t i o n In p r o d u c t s ( 1 1 4 ) on i r r a d i a t i o n i n t h e presence of a l k e n e s . a similar f a s h i o n , methanol-incorporated a d d i t i o n o f a l k e n e s t o t h e same dicarboximide (1131, which t a k e s p l a c e y & an e l e c t r o n t r a n s f e r pathway, a f f o r d s p r o d u c t s ( 1 1 6 ) of a d d i t i o n t o t h e a r e n e system a s w e l l a s t h e e x p e c t e d p r o d u c t s o f a d d i t i o n t o t h e carbonyl group (117).119 I n contrast, regioselective lr2 + n21 and I T 4 + n 2 1 a d d i t i o n s t o t h e benzene r i n g a r e p r e f e r r e d on ir r a d i a t ion o f !-me t h y 1ph t h a 1im i de and a 11y 1t r im e t hy 1si 1ane ; 3-methoxy-g-methylphthalimide and n-hex-l-ene a r e s i m i l a r l y converted i n t o t h e a d d u c t s (118) and ( 1 1 9 ) . 1 2 1 Y e t a n o t h e r photoa d d i t i o n pathway, t h a t of i n t r a m o l e c u l a r oxetane f o r m a t i o n , is observed i n t h e g- (w-indol-3-ylalkyl) p h t h a l i m i d e s ( 1 2 0 ) i n a c e t o n e s o l u t i o n and r e s u l t s i n t h e formation of o x e t o [ 2 , 3 - ~ l i n d o l e s Examples of p h o t o r e d u c t i o n i n p h t h a l i m i d e ~ land ~~ in ( 1 2 1 ) . 122 N-methyl- 1,2-naphthalenedicarboximide12 have a l s o been r e p o r t e d ,

-

amd d e t a i l s and t h e mechanism o f t h e p h o t o i n i t i a t e d e l e c t r o n -

Photochemistry

350

4

-

+ kN-"" hv

Ph,C=O

0

+

MeCN

hv

benzene

(114)

+

II’6: Photoreactions of Compounds containing Heteroatoms other than Oxygen ?Me

R3 -C-R4

(113)

hv

+

MeOH

-Me

+

P -Me Me0 @N Me0

35 1

352

Photochemistty ( 1 2 2 ) t o phenylcyclo-

t r a n s f e r a d d i t i o n o f !-methylphthalimide

propane (123) t o g i v e t h e s t e r e o i s o m e r i c s p i r o t e t r a h y d r o f u r a n y l l a c t a m s ( 1 2 4 ) have been d e s c r i b e d . 12’ I n t r a m o l e c u l a r p h o t o c y c l o a d d i t i o n i n t h e N-pentenylp h t h a l i m i d e (125) t o g i v e t h e b e n z a z e p i n e d i o n e ( 1 2 6 ) h a s been u s e d a s a key s t e p i n t h e s y n t h e s i s of pyrrolo[lf41benzodiazepine a n t i b i o t i c s such a s p r o t h r a c a r c i n . Photochemically induced I T 4

+ T41

known i n n i t r o g e n - c o n t a i n i n g h e t e r o c y c l e s

of pyrid-2-ones Pyrid-2-one Ia4

+ ,.,41

c y c l o a d d i t i o n s are w e l l

t h e photodimerization

b e i n g one of t h e f i r s t a u t h e n t i c a t e d examples.

d e r i v a t i v e s (127) have now been shown t o undergo

p h o t o a d d i t i o n t o t h e c o n j u g a t e d c y c l i c d i e n e s (128) t o

g i v e t h e a d d u c t s ( 1 2 9 ) and ( 1 3 0 ) . 1 2 7 r 1 2 8

+

I n t r a m o l e c u l a r rn4 I (9 10)

-

p h o t o c y c l o a d d i t i o n s have been r e p o r t e d i n d i a z a [ 3.3 a n t h r a c e n o p h a n e s . 129,130

T43

O t h e r p h o t o c y c l o a d d i t i o n s of n o t e i n c l u d e t h e f o r m a t i o n o f o x e t a n e s from :,;I-dimethylimidazolidenetrione

and a l k e n e s 1 3 1

and a s i n t e r m e d i a t e s i n t h e a d d i t i o n o f 1 , 3 - d i c a r b o n y l compounds t o n i t r o g e n h e t e r o c y c l e s , 132 selective In4

t h e e x c e p t i o n a l s t e r e o - and r e g i o -

+ 21

a d d i t i o n o f l - n a p h t h a l d e h y d e t o a-morpholinoa c r y l o n i t r i l e 13’ and t h e r e a c t i o n of l13-dimethyl-6-azathymine with acetone t o give t h e heterocycle

(131).134 The o x e t a n e (132) h a s been p r o p o s e d a s an u n s t a b l e i n t e r m e d i a t e i n t h e s o l i d - s t a t e p h o t o a d d i t i o n of 5-formyl-1,3-dimethyluracil

(133) t o i n d o l e (134)

.

t o g i v e 5- ( b i s - l - i n d o l y l ) methyl-lf3-dimethyluracil (135) 135 Photoinduced e l e c t r o n - t r a n s f e r i n i t i a t e d . a d d i t i o n of 2,3-dimethylbut-2-ene (136)

t o 1 , 3 , 5 - t r i c y a n o b e n z e n e a f f o r d s t h e iminoindane

whereas s e n s i t i z e d i r r a d i a t i o n o f 4-methyl-7- ( d i e t h y l -

amino)coumarin (137) i n t h e p r e s e n c e o f d i e t h y l m a l e a t e and oxygen l e a d s u n e x p e c t e d l y t o t h e p i p e r i d i n e d e r i v a t i v e ( 1 3 8 ) . 13’ S i n g l e e l e c t r o n t r a n s f e r from s u i t a b l e d o n o r s i s r e s p o n s i b l e f o r t h e p h o t o a d d i t i o n r e a c t i o n s o f iminium s a l t s . A f u r t h e r i n t r a m o l e c u l a r example o f t h i s approach i n v o l v i n g photo-

s p i r o c y c l i z a t i o n of ! - s i l y l a l l y l

iminium s a l t s h a s been employed

i n t h e c o n s t r u c t i o n of t h e p e n t a c y c l i c s k e l e t o n o f t h e Harringtonine alkaloids;

i r r a d i a t i o n o f t h e iminium s a l t (139) i n

a c e t o n i t r i l e a f f o r d s t h e s p i r o c y c l i c s y s t e m ( 1 4 0 ) i n o v e r 70% yield. l3

A s e q u e n t i a l e l e c t r o n - p r o t o n - t r a n s f e r mechanism is i m p l i c a t e d i n t h e p h o t o r e a c t i o n o f l - a l k y l - 4 6 - d i a r y l - 2 (lg)

-

W6: Photoreactions of Cornpourids containing Heteroatoms other than Oxygen

___)

d

N 0 y

OCH,O(CH,),OMe 4

(127) R’ = H, Me, or PhCH2 R2 = H, Me, or CMe3

MeCN hv

6

(128)n = 1 or 2

M e ,0 ? T p e

0

353

OCH20(CH2)20Me

354

Photochemistry

HN

NC

do +

Me i

Et02CmC02Et

hv

Et2N C02Et (138)

Ph

Ph

1

I

CH2 i, h v , MeCN

Me3C-C-0

ii, NaHC03

Me3C-C-0 C104-

(140)

(139)

OCH2Ph

R=

<" 0

0

0

+ R2CH2NR3R4

hv

b

Ph

Ph

R' = Me or C3H7 (141 1

R2 = Me, R3 = Et, R4 = Et or Ph R2 = H, R3 = Me, R4 = Ph

R2/C'NR3R4 (142)

IIh: Photoreactions of Cornpourids containing Heteroatoms other than Oxygen pyrimidones ( 1 4 1 ) w i t h analogous a d d i t i o n s o f pyrid-2-ones have been molecular react i on h a s aminoalkanes (143) and (145) .141

355

amines t o g i v e a d d u c t s ( 1 4 2 ) , 1 3 ’ and a l i p h a t i c t e r t i a r y amines t o g - a l k y l An e q u i v a l e n t i n t r a reported. been d e s c r i b e d i n N-methyl-w-(6-styry1)l e a d s t o t h e c y c l i c amines ( 1 4 4 ) and

Photoamidations of f l u o r o a l k e n e s w i t h formamide have been c a r r i e d o u t i n t - b ~ t a n o l land ~ ~ i n microscopically heterogeneous media. 143 1 , 3 - D i m e t h y l u r a c i l (146) h a s been c o n v e r t e d i n t o t h e e s t e r s (147) and (148) on i r r a d i a t i o n i n methanol;144 t h e mechanism o f t h i s t r a n s f o r m a t i o n is u n c e r t a i n b u t t h e ester (1491, which is a l s o a p r o d u c t of t h i s r e a c t i o n , does n o t a p p e a r

t o be an i n t e r m e d i a t e i n t h e formation o f t h e d i h y d r o p y r i d i n e derivatives.

-

Miscellaneous r e a c t i ons. Aminocyanopyridines have been p r e p a r e d by p h o t o i n i t i a t e d s u b s t i t u t i o n i n pyridine-2,4-dicarbonitrile w i t h v a r i o u s amines. 145 N i t r o i m i d a z o l e s can s i m i l a r l y be conv e r t e d i n t o cyanoimidazoles by i r r a d i a t i o n i n aqueous p o t a s s i u m cyanide s o l u t i o n . 14‘ An i n i t i a l photochemically induced carboncarbon bond homolysis h a s been proposed t o a c c o u n t f o r t h e a c i d c a t a l y s e d c o n v e r s i o n o f t h e ;-nitrobenzyl d e r i v a t i v e (150) i n t o t h e hydroxylamine (151) ;14’ photof ragmentation h a s been o b s e r v e d i n 2-phenylhydroxylamines Dihydro-3g-2-cyclopental c l a z e p i n e s have been s y n t h e s i s e d by p h o t o l y s i s of t h e i r c y c l o p e n t a d i e n y l i r o n d e r i v a t i v e s 1 4 ’ and t e r t i a r y amides have been c o n v e r t e d i n t o aamino esters by i r r a d i a t i o n of chromium aminocarbene complexes i n methanol o r t - b u t a n o l

.

2.

S u l p h u r - c o n t a i n i n g Compounds

New examples i n s u l p h u r - c o n t a i n i n g compounds o f p h o t o r e a c t i o n s a r i s i n g by e l e c t r o c y c l i c pathways have been r e p o r t e d . O x i d a t i v e

p h o t o c y c l i z a t i o n of t h e t h i o p h e n e (1521, f o r example, gave t h e c o r r e s p o n d i n g t h i e n o n a p h t h a l e n e (1531 ,15’ and analogous r e a c t i o n s have been r e p o r t e d i n c e r t a i n t e r t h i o p h e n e s 152 Ring opening, however, is r e s p o n s i b l e f o r t h e photochromism o b s e r v e d i n v a r i o u s s u l p h u r - c o n t a i n i n g s p i r o p y r a n s . lS3 lS4 A d i f f e r e n t pathway i n v o l v i n g i n i t i a l hydrogen a b s t r a c t i o n t o g i v e t h e b i r a d i c a l s

.

Photochemistry

356

C02Me

MeOH

Me

M e 0 2 C vC02Me

Meo2cyJ +

I

I

Me

Me

MeO-C,

+

Me-NH-C

,c=c,

,H NHMe

II.6: Photoreactions of Compounds corituining Heteroatoms other than Oxygen

357

(154) i s i m p l i c a t e d i n t h e p h o t o c y c l i z a t i o n of t h e quinones (155) t o t h e 5E-pyr ano [ 4,3,2-*l[ 3 I 1] ben z o x a t h i i n s ( 156 1 l5 Photo-Fries r e a r r a n g e m e n t s have been r e p o r t e d i n t h i o s a l i c y c l i c a c i d d e r i v a t i v e s 1 5 6 and i n 2- ( 2 ~ - b e n z o t r i a z o l - 2 - y 1 ) 4- t - o c t y l p h e n y l benzenesulphonate I 157 and N_-sulphonylcarbazoles are s i m i l a r l y c o n v e r t e d i n t o 1-sulphonyl- and 3-sulphonylc a r b a z o l e s . 1 5 8 S u r p r i s i n g l y I _N b e n z o y l c a r b a z o l e does n o t undergo an analogous photorearrangement. I n i t i a l n i t r o g e n - s u l p h u r bond homolysis i s a l s o i n v o l v e d i n t h e unusual rearrangement o f t h e c o n t a c t h e r b i c i d e bentazone (157) t o t h e l 1 2 , 4 - b e n z o t h i a d i a z i n 3 (4H)-one 1 , l - d i o x i d e (158) b u t d e t a i l s of t h e mechanism o f t h i s p h o t o i s o m e r i z a t i o n a r e n o t c l e a r . 159 Carbon-sulphur bond homolysis w i t h t h e formation of b i r a d i c a l s (159) h a s been proposed t o a c c o u n t f o r t h e photorearrangement of t h e a l l e n e t h i i r a n e s ( 1 6 0 ) t o t h e t h i i r a n e s (161) and t h e t h i e t a n e - 3 - t h i o n e s ( 1 6 2 ) 160 LOW t e m p e r a t u r e m a t r i x s t u d i e s have e s t a b l i s h e d t h a t photodecomposition of t h e 1 , 2 , 5 t h i a d i s e l e n o l e (163) o c c u r s via an i n i t i a l photorearrangement t o t h e n o v e l s p i r o t h i a s e l e n i r a n e (164) I 16' whereas i n i t i a l s u l p h u r s u l p h u r bond homolysis i s t h o u g h t t o a c c o u n t f o r t h e photochemi c a l l y induced i s o m e r i z a t i o n of a n o v e l f u s e d 1 , 2 - d i t h i o l e i n t o t h e c o r r e s p o n d i n g 1I 3 - d i t h i o l e . [,2 + ,21 P h o t o a d d i t i o n of t h i o n e s t o a l k e n e s y i e l d s t h i e t a n e s o r p r o d u c t s d e r i v e d t h e r e f r o m . Cycloadducts were o b t a i n e d i n t h i s way from v a r i o u s t h i ~ b a r b i t u r a t e s ,and ~ ~ t~h e photorearrangement of t h e 3-allylindoline-2-thiones (1651 t o t h e 2 - a l l y l t h i o i n d o l e s (166) i n benzene i s b e l i e v e d t o proceed via t h e u n s t a b l e t h i e t a n e s ( 1 6 7 ) . 164 Recjioselective a c y l a t i o n o f f u r a n d e r i v a t i v e s (168) t o g i v e 3 - a r o y l f u r a n s ( 1 6 9 ) can b e a c h i e v e d i n a s i m i l a r f a s h i o n by i r r a d i a t i o n i n t h e p r e s e n c e o f arenec a r b o t h i o a m i d e s (170) as shown i n Scheme 2. Products derived by [,2 + ,2] a d d i t i o n t o t h e carbon-carbon double bond and t o t h e t h i o n e have been o b t a i n e d on i r r a d i a t i o n o f 2 - t h i o u r a c i l i n t h e p r e s e n c e of a l k e n e s . 166 A n a l t e r n a t i v e pathway is p r e f e r r e d i n the E- (By-unsaturated c a r b o n y l ) t h i o a m i d e s (171) which are c o n v e r t e d v i a i n i t i a l carbon-sulphur bond f o r m a t i o n followed by 1,4-hydrogen t r a n s f e r i n t o t h e b i c y c l i c lactams (172) Intramolecular a d d i t i o n h a s a l s o been o b s e r v e d i n t h e d i t h i o n o l a c t o n e (173) and y i e l d s d i t h i a t o p a z i n e I a s t a b l e 1I 2 - d i t h i e t a n e (174) Contrary

.

.

-

.

Photochem istty

358

(155) R’ = H, R2 = H, Me, Et, or Ph (154) R’ = Me, R2 = Me or Et

hv

n-hexane

“CHMe,

0

CMe3 (160)

0 2

CMe3

R = CMe3 (159) R, R = Me2C(CH2)&Me2

CMe,

W6: Photoreactions of Compounds containing Heteroatoms other than Oxygen

(165) R' = Ph, Me, or C4H9 (167) R2 = Me or Ph R3=HorMe

Scheme 2

R = H or Me, n = 1-3

(171)

hv

0

H

S

0

H

t

(172)

359

360

Photochemistry

t o p r e v i o u s r e p o r t s , S2 s t a t e s a r e n o t now r e g a r d e d a s immediate precursors responsible f o r the intermolecular photoreactions of adamantanethione, thiocamphor and t h i o f e n c h o n e . The r e g i o s e l e c t i v e r a d i c a l a d d i t i o n of p-toluenes u l p h o n y l c y a n i d e t o a l k e n e s h a s been a c h i e v e d photochemically .I7* R e g i o s e l e c t i v e a d d u c t s have a l s o been o b t a i n e d on i r r a d i a t i o n of Se-phenyl ( s e l e n o t h i o p e r o x y )benzoate w i t h t e r m i n a l o r c y c l i c 171 alkenes. Ring c l e a v a g e i n i t i a t e d by carbon-sulphur bond homolysis

-

i s r e s p o n s i b l e f o r t h e c o n v e r s i o n of t h e 2 (5I3-thiophenone (175) i n t o t h e t e t r a h y d r o t h i o p h e n e (176) i n a l l y 1 a l c o h o l ;172 the l i k e l y pathway i s o u t l i n e d i n Scheme 3 . Type I1 c l e a v a g e h a s been employed i n t h e g e n e r a t i o n of a s y n t h e t i c a l l y u s e f u l t h i o n e from a phenacyl s u l p h i d e . 173 Type I1 p h o t o c y c l i z a t i o n , however, i s p r e f e r r e d i n t h e E-w-phenylalkyl s u b s t i t u t e d t h i o p h t h a l i m i d e s (177) and y i e l d s t h e t r i c y c l i c p r o d u c t s (178) and (179).174 The p o t e n t i a l use o f 4 - t h i o u r a c i l a s a photochemical probe i n n u c l e i c a c i d s t u d i e s h a s been i n v e s t i g a t e d i n 5 ' - 0 - ( 2 ' d e o x y - 4 - t h i o u r i d y l y l ) thymidine and i n 2'-deoxy-5'-g-thymidylyl-4thiouridine and t h e b i b e n z o t h i a z o l e (180) undergoes an unexpected photorearrangement t o t h e benzothiazino-benzothiazine (181) 176

.

3.

Compounds c o n t a i n i n q o t h e r Heteroatoms

As b e f o r e , t h e m a j o r i t y of t h e r e p o r t s i n c l u d e d i n t h i s s e c t i o n

are concerned w i t h t h e p h o t o r e a c t i o n s of o r g a n o s i l i c o n compounds. T r i p h e n y l - s u b s t i t u t e d s i l y l and germyl r a d i c a l s have been g e n e r a t e d from t h e c o r r e s p o n d i n g h y d r i d e s by m a t r i x i r r a d i a t i o n a t 77 P h o t o s i l y l a t i o n o f l-aryl-2,2-dicyanoethenes (182) can be achieved r e g i o s e l e c t i v e l y by p h e n a n t h r e n e - s e n s i t i z e d p h o t o a d d i t i o n o f , f o r example,tetramethyldisilane (183) t h e p r o d u c t s (184) are b e l i e v e d t o a r i s e by way of an e l e c t r o n t r a n s f e r mechanism. A d e t a i l e d s t u d y of photochemically induced 1 , 3 - s i l y l m i g r a t i o n i n a l l y l s i l a n e s h a s r e v e a l e d t h a t rearrangement o c c u r s w i t h i n v e r s i o n of s i l y l c o n f i g u r a t i o n I n a p p a r e n t d i s a g r e e m e n t w i t h p r e d i c t i o n s b a s e d on Woodward-Hof fmann r u l e s ,I7' and t h e formation of dimethylb i s ( 2 - t h i e n y l ) s i l a n e , 2 - m e t h o x y d i m e t h y l s i l y l t h i o p h e n e , and 2d i m e t h y l s i l y l t h i o p h e n e from 1,1,2,2-tetramethy1-1,2-bis(2-thienyl)-

36 1

IK/6: Photoreactions of Compounds containing Heteroatoms other than Oxygen

(175)

Scheme 3

Ph

S

0

Ph

Photochemistry d i s i l a n e i n methanol a p p e a r s t o i n v o l v e d i r e c t r e a c t i o n o f methanol w i t h t h e p h o t o e x c i t e d s t a t e . The g e n e r a t i o n and r e a c t i o n s of s i l y l e n e s ( s i l a n e d i y l s ) have a g a i n a t t r a c t e d a t t e n t i o n . The decay o f d i m e s i t y l s i l y l e n e , g e n e r a t e d by p h o t o d e c o m p o s i t i o n of t h e c o r r e s p o n d i n g t r i s i l a n e i n c y c l o h e x a n e , h a s been m o n i t o r e d by laser f l a s h p h o t o l y s i s The r e a c t i o n o f d i - t - b u t y l s i l y l e n e ( 1 8 5 ) , g e n e r a t e d i n t h e same way from hexa-t-butylcyclotrisilane (186) , w i t h p h e n y l i s o c y a n i d e a f f o r d s t h e d i s i l a c y c l o b u t a n e (187) ;182 t h e r e i s c o n s i d e r a b l e s u p p o r t f o r t h e i n t e r m e d i a c y o f a n o v e l s i l a k e t e n i m i n e (188) i n t h i s conversion. Further i r r a d i a t i o n with v i s i b l e l i g h t of t h e c y c l o p r o p e n y l s i l y l e n e (1891, o b t a i n e d by i r r a d i a t i o n of t h e t r i s i l a n e (190) i n a 3-methylpentane g l a s s a t 7 7 K , gave t h e s i l a c y c l o b u t a d i e n e (191) which can b e t r a p p e d a s t h e d i a s t e r e o isomeric s i l a c y c l o b u t e n e s (192) w i t h e t h a n o l . l E 3 Benzenoid d i s i l a n e s a r e known t o undergo p h o t o r e a r r a n g e ment via a s i l e n e i n t e r m e d i a t e . A c o n v e r s i o n of t h i s t y p e (Scheme 4 ) h a s been p r o p o s e d t o a c c o u n t f o r t h e p h o t o r e a c t i o n o f 1,2b i s ( p e n t a m e t h y l d i s i l a n y l ) benzene ( 1 9 3) w i t h i s o b u t e n e i ener e a c t i o n o f t h e p h o t o c h e m i c a l l y g e n e r a t e d s i l e n e (194) w i t h i s o b u t e n e a f f o r d s 2- ( i s o b u t y l d i m e t h y l s i l y l )-1- ( p e n t a m e t h y l d i s i l a n y l ) -3- ( t r i m e t h y l s i l y l ) benzene (195) 1,3-Bis (pentamethyld i s i l a n y 1 ) b e n z e n e undergoes an a n a l o g o u s p h o t o r e a c t i o n w i t h i s o b u t e n e t o g i v e 3- ( i s o b u t y l d i m e t h y l s i l y l ) -1- (Dentamethyldisilanyl)-4-(trimethylsilyl)benzene; t h e s i l e p i n ( 1 9 6 ) i s a minor p r o d u c t o f t h i s r e a c t i o n a n d a r i s e s from t h e s i l e n e (197) as shown i n Scheme 5. The key t o t h e s y n t h e s i s of s t a b l e d i s i l e n e s , which are r e a d i l y o b t a i n e d by d i m e r i z a t i o n o f s i l y l e n e s , i s p r o t e c t i o n of t h e s i l i c o n - s i l i c o n d o u b l e bond by s u b s t i t u e n t s . 1,2-Di-(1adamantyl) d i m e s i t y l d i s i l e n e (198) , a d i s i l e n e which i s r e s i s t a n t t o a i r o x i d a t i o n , h a s been p r e p a r e d i n t h i s way from t h e t r i silane (199) ; o n f u r t h e r i r r a d i a t i o n i t undergoes E+B isomerization. A s tudy of 1,2-diaryl rearrangement i n o t h e r t e t r a - a r y l d i s i l e n e s , p r e p a r e d i n the same way from t r i s i l a n e p r e c u r s o r s , h a s been described.lE6 P h o t o r e a c t i o n s i n which the s i l i c o n atom p l a y s a less i m p o r t a n t role have a l s o been r e p o r t e d . Type I c l e a v a g e w i t h loss of carbon monoxide i s o b s e r v e d on i r r a d i a t i o n o f b e n z y l dimethyl-

363

II.4: Photoreactions of Cornpourids containing Heteroatoms other than Oxygen

mN 'si'

hv Me&

CMe3

-c

Me3C-fi-Si-CMe3 I \ I

,Si: Me3C

-

PhNC Me3C,

Ph

Si=C=N'

Me3C'

-

Me&,

)( ,CMe3

Si Me&/

Ysi\CMe3

CMe3 CMe3

Nml

(186)

R

(185)

(188)

qR

(1 87)

Ry-JR Si :

,SiMe3

hv

&Me3 SiMe3

I k M e 3

77 K

-

Me3C

Me3C

CMe3

CMe3

R = CHMe2

i

h v, h> 400 nm

R EtOH

Me3C

SiMe2SiMe3

SiMe2SiMe3

CMe3

SiMe,S iMe3

MeyMe t

&SiMe2CH2CHMe2 SiMe3

(1 94)

Scheme 4

364

Photochemistry

SiMe2SiMe3

SiMe2SiMe3

SiMe2SiMe3

‘2

&

CMe2CH2SiMe3 @CMe2CH2SiMe3I ‘Me I ‘Me Me

SiMe2 @SiMe2 H SiMe3 (197)

K k

MeMS,‘ b C M e 2CMe2CH2SiMe3 CH2SiMe3 MeMS,‘ Me

SiMe2SiMe3 oCMe2CH2SiMe3

/

M d ‘Me

(196) Scheme 5 R1

‘si

,SiMe3

R’

R’\ Si :

hv

R2’ ‘SiMe3

D

‘si =si:

R2’

R2’

R2

R’

R’ = mesityi R2 = I-adamantyl

(199)

0

hv, MeOH

SiMe3

Me3Si

&

0

NaHC03

SiMe,

SiMe,

SiMe3

x 0 b

”$GePh3

hv_

R’YGe R2

/

R’ = Ph, p -MeC6H4,or p -CNGH4, R2 = H R’ = Ph, R2 = Me or PhCH20

1.h: Photoreactions of Compourids containing Heteroatoms other than Oxygen

365

phenylsilyl ketone. la7 The silicon-directed type I cleavage of B-(trimethylsily1)cyclohexanone (200) in methanol proceeds in a highly regioselective manner with an improved quantum yield and rate enhancement and affords the aldehyde (201) and the methyl ester (202). Silicon-directed decarbonylation has similarly been observed in the cis-trimethylsilyl aldehyde (203) which is converted on irradiation into the trimethylsilyl derivative (204) in 98% ~ie1d.l~’ a- (Trimethylsily1)vinyl alcohol has been generated from a-trimethylsilylethanol by acetoneThe initiated photochemically induced a-hydrogen abstraction. reactions of triplet carbonyl compounds with silanes have also 191 been investigated. Other reports of note in silicon-containing systems include the photochemical generation and reactions of trialkoxysilyl radicals,Ig2 the photoreactions of ethynylsilanes with 2-mercaptoethanol and thioglycolic acid,lg3 and the photochemically induced fluoroalkylation of the trimethylsilyl enol ether of pinacolone. 194 The photochemistry of organogermanium compounds is, not surprisingly, similar in many respects to that of organosilicon compounds. The photolysis of phenyl-substituted trigermanes affords digermanes and germylenes,l g 5 and photochemically generated dimethylgermylene undergoes efficient insertion into the carbon-chlorine and the carbon-bromine bonds of alkynyl and allenyl halides. Absorptions in the ultraviolet spectra obtained by photodecomposition of dialkylbis(phenylse1eno)germanes in 3-methylpentane at 77 K have been tentatively assigned to dialkylgermylenes 197 The photodecarbonylation of (a-arylacyl)triphenylgermanes (205) to (a-arylalkyl)triphenylgermanes (206) has been reported to take place in virtually quantitative yield: no addition products were obtained even in the presence of alkenes. In contrast, however, the alkylacylgermanes (207) undergo regioselective radical addition to styrene (208) to give 1:l-adducts (209) and 199 2 :1-adducts (210) A wide variety of photochemical transformations have been reported in organo-phosphorus compounds. Racemization of 1-phosphaallene enantiomers has been observed on exposure to light.200 Ally1 diphenyl phosphate (211) undergoes a type I1

’’’

.

.

Photochemistry

366

0 R'koPh,

+

PhCH=CHz

hv

GePh, + R

bR& Ph

(207)

GePh, Ph

(208) R = Me or PhCH2CH2 (209)

(210)

Scheme 6

NC,

,CN

NciJcN hv hv

Me

Me Me

Me

L

II,6: Photoreactions of Compourids containing Heteroatoms other than Oxygen

367

photorearrangement t o 2 - a l l y l p h e n y l p h e n y l DhosDhate ( 2 1 2 ) ; in c o n t r a s t t o c a r b o n y l t y p e I1 r e a r r a n g e m e n t s , t h i s t r a n s f o r m a t i o n proceeds a 1 , 3 - b i r a d i c a l (213) by t h e Dathway shown i n Scheme 6. 201 [,4 + ,41 P h o t o c y c l o d i m e r i z a t i o n h a s been o b s e r v e d i n 9- ( f l u o r o p h o s p h i n o )a n t h r a c e n e ,202 and an i n i t i a l l y formed photodimer, a s u b s t i t u t e d c y c l o t e t r a n h o s p h a n e , h a s been proposed t o a c c o u n t f o r t h e p h o t o r e a c t i o n s of (E)-l-mesityl-2(2,4,6-tri-

t-butylpheny1)diphosphene. 203

S i l y l a l k y l ( e t h e r phosphines) have

been p r e p a r e d by t h e photoinduced a d d i t i o n o f secondary phosphines t o w-alkenylchloro- and w-alkenylmethoxy-silanes. 204 T r i a r y l m e t h y l p h o s p h o n i c a c i d s undergo carbon-phosphorus bond c l e a v a g e t o g i v e t r i a r y l m e t h a n e s on i r r a d i a t i o n i n a l k a l i n e s o l u t i o n . 205 P h o t o f r a g m e n t a t i o n h a s a l s o been o b s e r v e d i n 2,3oxaphosphabicyclo[2.2.2loct-5-enes. 206 The f i r s t t i m e - r e s o l v e d s t u d i e s o f n a p h t h a l e n e - c o n t a i n i n g c a r b o c a t i o n s , o b t a i n e d by laser f l a s h p h o t o l y s i s o f (naphthylmethy1)phosphonium c h l o r i d e s , have been d e s c r i b e d , 2 0 7 and t h e r e a c t i v i t y of phosphinoyl and t h i o phosphinoyl r a d i c a l s , g e n e r a t e d by photochemically induced ac l e a v a g e o f acylphosphine o x i d e s and s u l p h i d e s , h a s been f u r t h e r s t u d i e d . *08 Details of t h e photodecomposition of t h e phosphorusc o n t a i n i n g p e s t i c i d e , g,g-diethyl-~-(4-methyl-2-0~0-2~-1-benzopyran-7-yl) p h o s p h o r o t h i o a t e , have been p u b l i s h e d . 209 Other r e p o r t s of n o t e i n t h i s s e c t i o n i n c l u d e t h e t h e r m a l l y r e v e r s i b l e nhotochromic c o n v e r s i o n of 1 , 2 - d i s e l e n y l e t h e n e ( 2 1 4 ) t o t h e t r i c y c l e (215)210 and photochemically induced a l k y l m i g r a t i o n i n d i a l k y l b o r y l t r o p o n a t e complexes. l1

Photochem istty

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372

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1 3 1 H. Aoyama, H. H a t o r i , and Y. Omote, J. Org. Chem., 1989, 54, 2359. 132 J. W. P a v l i k and M. S. F a r a h a t , B u l l . Chem. SOC. E t h i o p . , 1988, 2, 61 (Chem. A b s t r . , 1989, 111, 153554). 133 D. Dspp and H. R. Memarian, Chem. Ber., 1990, 123, 315. Y. Guo, P. Fan, and M. Fan, W&i Huaxue Xueb-ac, 1989, 2, 395 ( x m . 134 A b s t r . , 1990, E,157446). 135 J. Meng, D. Fu, Z. Gao, R. Wang, H. Wang, I. S a i t o , R. K a s a t a n i , and T. Matsuura, T e t r a h e d r o n , 1990, 46, 2367. 136 M. Ohashi, N. Aoyagi, T. Miyama, S. Kyushin, and S. Yamada, J. Chem. S O C . , P e r k i n Trans. 1, 1990, 189. 137 M. A. Kirpichenok, L. M. Mel'nikova, L. K. Denisov, I. I. Grandberg, and N. P. Akimova, Khim. G e t e r o t s i k l . Soedin, 1989, 30 (Chem. A b s t r . , 1989, 111, 194631). 138 R. W. Kavash and P. S. Mariano, T e t r a h e d r o n L e t t . , 1989, 30, 4185. 139 T. N i s h i o , M. Kato, and C. Kashima, L i e b i g s Ann. Chem., 1989, 611. 140 S. Ohmiya, C. Y . Chen, I. Murakoshi, and K. Otomasu, Chem. Pharm. B u l l . , 1989, 37, 2516. 141 F. D. Lewis, G. D. Reddy, S. S c h n e i d e r , and M. Gahr, J. Am. Chem. SOC., 1989, 111, 6465. 419. 142 M. G a u t i e r , I. Rico, and A . L a t t e s , J . F l u o r i n e Chem., 1989, 143 M. G a u t i e r , I. Rico, and A. L a t t e s , J. Org. Chem., 1990, 2 5 , 1500. 144 M. K r i s t i a n s e n , J. E r i k s e n , and K. A. Jorgensen, J. Chem. SOC., P e r k i n Trans, 1, 1990, 101. 145 R. B e r n a r d i , T. Caronna, S. Morrocchi, M. U r s i n i , and B. M. V i t t i m b e r g a , J. Chem. S O C . , P e r k i n Trans. 1, 1990, 97. 146 M. P.Crozet, P. V a n e l l e , 0. J e n t z e r , and J . Maldonado, Org. Prep. Proced. I n t . , 1989, Zl, 105. 14 7 S. M u r a l i d h a r a n , K. A. Beveridge, and P. Wan, J. Chem. S O C . , Chem. Cormnun., 1989, 1426. 148 A. M. Fahmy, M. M. Aly, M. Z. A. Badr, and A. M. Abd El-Gawad, B u l l . P o l . Acad. Sci.. Chem., 1988, 3,97. 149 I. U. Khand, T. Lanez, and P. L. Pauson, J. Chem. SOC., P e r k i n Trans,-l, 1989, 2075. 150 L. S . Hegedus, M. A. Schwindt, S. De Lombaert, and R. Imwinkelried, J. Am. Chem. S O C . , 1990, 112, 2264. 151 G. Karminski-Zamola and M. B a j i c , Synth. Comun., 1989, 2 , 1325. N. J a y a s u r i y a , J. Kagan, J. E. Owens, E. P. Kornak, and D. M. P e r r i n e , 152 J. Org. Chem., 1989, 54, 4203. 153 S o Watanabe, H. Nakazumi, K. Maeda, and T. K i t a o , J. Chem. S O C . . Chem. Commun., 1990, 421. 154 T. B. K r a s i e v a , Ya. N . Malkin, V. A. Lokshin, S. V. Mel'nichuk, S. A. Tikhomirov, and V. A. Kuz'min, Izv. Akad. Nauk SSSR. S e r . Khim., 1989, 2504 (Chem. A b s t r . , 1990, 112, 215912). 155 H. Iwamoto, B u l l , Chem. SOC. Spn., 1989, 62, 3479. 156 C. B e l l e d , M. A. Miranda, and A. Simon-Fuentes, An. Quim.. Ser. C, 1,989, 85, 39 (Chem. A b s t r . , 1990, 112. 216634). 157 P. M. Miranda and A. F a c t o r , J. Polym. S c i . , P a r t A: Polym. Chem., 1989, 27, 4427. 1 5 8 A. C h a k r a b a r t i , G. K. Biswas, and D. P. Chakraborty, T e t r a h e d r o n , 1989, 45, 5059. 159 K. H u s t e r t and M. Mansour, T e t r a h e d r o n L e t t . , 1989, g , 6159. 160 N. T o k i t o h , T. Suzuki, and W. Ando, T e t r a h e d r o n L e t t . , 1989, 3_0, 4271. 161 N. T o k i t o h , H. I s h i z u k a , A. Yabe, and W. Ando, T e t r a h e d r o n L e t t . , 1989, 30, 2955. 162 N. T o k i t o h , S. Imai, M. Goto, and W. Ando, T e t r a h e d r o n L e t t . , 1989, 10, 4825.

44,

I46: Photoreactions of Cornpourids containing Heteroatoms other than Oxygen

x.,

373

163 H. Takechi, M. Machida, and Y. Kanaoka, Chem. Pharm. 1989, 3-7, 1431. ~. T . N i s h i o , J. Chem. Res., Synop., 1989, 204. 164 165 K. Oda, M. Machida, and Y. Kanaoka, Tetrahedron L e t t . , 1989, 32, 4421. 166 E. S a t o , Y. Ikeda, M. Hasebe, T. N i s h i o , T. Miura, and Y. Kanaoka, Chem. Pharm. B k l . , 1988, 36, 4749. 167 M. Sakamoto, S. Watanabe, T. F u j i t a , and T. Yanase, J. Org. Chem., 1990, 5-5, 2986. Hwang, N. S t y l i a n i d e s , P. J. 168 K. C. Nicolaou, S. A. DeFrees, C.-K. C a r r o l l , and J. P. Snyder, J. Am. Chem. h., 1990, 1 2 , 3029. 169 K.J.Falk and R. P. S t e e r , 53 Am. Chem. SOC., 1989, 111, 6518. Fang, M.-Y. Chen, M.-C. Cheng, G.-H. Lee, Y. Wang, and S.-M. 170 J.-M. Peng, J. Chem. Res., Synop., 1989, 272. 1 7 1 T. Toru, T. Seko, E. Maekawa, and Y. Ueno, J.J_$em. SOC., - P e r k i n Trans., 1, 1989, 1927. 172 R. Kiesewetter and P. Margaretha, Phosphorus Sulphur, 1988, 39, 263. 173 E. Vedejs, R. A. Buchanan, and Y. Watanabe, J. Am. Chem. S O C . ~ , 1989, 111, 8430. 174 c o d a , E. Yoshida, K. Ohno, M. Machida, and Y. Kanaoka, Chem. Pharm. B u l l . , 1989, 37, 642. 175 J.-L. Fourrey, J. Gasche, C. F o n t a i n e , E. G u i t t e t , and A. Favre, J. Chem. SOC., Chem. Cornmug., 1989, 1334. 176 E. Tauer and K. H. Grellrnann, Chem. Ber., 1990, 123, 1149. 177 M. Wakasa, K. H o r i u c h i , and K. Mochida, Nippon Kagaku K a i s h i , 1989, 1469 (Chem. Abstr., 1990, 112. 77402). 178 K. Mizuno, K. Nakanishi, J. Chosa, T. Nguyen, and Y. O t s u j i , Tetrahedron L e t t . , 1989, 30, 3689. 179 M. Kira, T. Taki, and H. S a k u r a i , J. Ore. Chem., 1989, 54, 5647. 180 S. S. Hu and W. P. Weber, J. Organomet. Chem., 1989, 369, 155. 181 R. T. Conlin, J. C. N e t t o - F e r r e i r a , S. Zhang, and J. C. Scaiano, Organometallics, 1 9 9 0 , 4 , 1332. 182 M. Weidenbruch, B. Brand-Roth, S. Pohl, and W. Saak, Angew. Chem. I n t . Ed. Engl., 1990, 2, 90. 183 D. B. Puranik and M. J. Fink, J. Am. Chem. S O C . , 1989, 111, 5951. 184 M. Ishikawa, H. Sakamoto, F. Kanetani, and A. Minato, Organometallics, 1989, 8, 2767. 185 B. D.SKepherd, D. R. Powell, and R. West, O r g a n o m e t a l l i c s , 1989, 3, 2664, 186 IS. B. Yokelson, D. A. S i e g e l , A. J. M i l l e v o l t e , J. Maxka, and R. West, O r g a n o m e t a l l i c s , 1990, 2, 1005. 187 E. N. S t e p , V. F. Tarasov, and A. L. Buchachenko, Izv, Akad. Nauk SSSR, S e r . Khim., 1988, 2250 (Chem. Abstr., 1989, 111, 97336). 188 J. R. Hwu, B. A. G i l b e r t , L. C. L i n , and B. R. Liaw, J. Chem. SOC., Chem. Commun., 1990, 161. Am. Chem. SOC., 1989, 1 2 , 8834. 189 J. R. Hwu and P. S. F u r t h , 190 A. J. Kresge and J. B. Tobin, J. Am. Chem. S O C . , 1990, 1 1 2 , 2805. 1 9 1 A. A l b e r t i , S. D e l l o n t e , C. P a r a d i s i , S. R o f f i a , and G. F. P e d u l l i , J. Am. Chem. SOC., 1990, 112, 1123. 192 K. M. Johnson and B. P. Roberts, J. Chem. SOC., P e r k i n TransL.-&, 1989, 1111. 1 9 3 M. G. Voronkov, G. Yu. Z h i l a , N. N. Vlasova, 0. G. Yarosh, and A. I. Albanov, Zh. Obshch. Khim., 1988, 5 8 , 1827 (&em. Abstr., 1989, 111. 194835). 194 L. Zhang and W. Huang, Youji Huaxue, 1 9 8 9 , 2 , 38 (Chem. Abstr., 1989, 111, 77476). 195 M. Wakasa, I. Yoneda, and K. Mochida, J. Organornet. Chem., 1989, 3 3 , C1.

J.

Photochemistry

374 196 19 7 198 199 200 201 20 2 20 3 204 205 206 20 7 20 8 209 210

211

G. B i l l i b , H. Brauer, S. Maslov, and W.

P. Neumann, J. Organomet. C h h . , 1989, 373, 11. S. Tomoda, M. Shimoda, and Y. Takeuchi, Nippon Kagaku K a i s h i , 1989, 1466 (Chem. A b s t r . , 1990, 112, 158413). S. Kiyooka, T. Shibuya, F. S h i o t a , and R. Fujiyama, B u l l . Chem. SOC. Jpn., 1989, 62, 1361. S.-I. Kiyooka, M. Hamada, H. Matsue, and R. Fujiyama, Chem.Lett., 1989, 1385. M. Yoshifugi. K. Toyota, Y. Okamoto, and T. Asakura, T e t r a h e d r o n L e t t . , 1990, 3l, 2311. D. R. Anderson and C. N. Eley, T e t r a h e d r o n L e t t . , 1989, 32, 4059. L. Heuer, D. Schomburg, R. Schmutzler, Chem. Ber., 1989, 122, 1473. M. Y o s h i f u j i , T. S a t o , and N. Inamoto, B u l l . Chem. S O C . Jpn., 1989, 62, 2394. Lindner, A. Bader, E. G l a s e r , B. P f l e i d e r e r , W. Schumann, and E. Bayer, J. Organometal. Chem., 1988, 355, 45. M. S h i , Y. Okamoto, and S. Takamuku, B u l l . Chem. SOC. J p n . , 1990, 63, 453. L. D. Quin, F. H. Osman, N . D. Sadanani, A. N. Hughes, and R. 0. Day, Phosphorus, Sulphur S i l i c o n R e l a t . Elem., 1989, 41, 297. E. 0. Alonso, L. J. J o h n s t o n , J. C. Scaiano, and V. G. Toscano, J. Am. Chem. SOC., 1990, 112, 1270. T. Majima and W. Schnabel, J. Photochem. P h o t o b i o l . , A, 1989, 53, 31. W. M. Abdou. M. R. Mahran. M. M. Sidkv. and H. Wamhoff. PhosDhorus Sulphur, 1988, 39, 199. Y. Nakayama, K. Hayashi, and M. I r i e , J. Org. Chem., 1990, 55, 2592. K. Okada, H. Inokawa, and M. Oda, T e t r a h e d r o n L e t t . , 1989, 30, 4395. d

V

7

Photoelimination BY S.T. REID

T h i s Chapter i s p r i n c i p a l l y concerned w i t h t h e photoinduced f r a g m e n t a t i o n o f o r g a n i c compounds, accompanied by t h e formation of s m a l l molecules such a s n i t r o g e n , carbon d i o x i d e and s u l p h u r d i o x i d e . Photodecompositions r e s u l t i n g i n t h e formation of two or more s i z a b l e fragments a r e reviewed i n t h e f i n a l s e c t i o n . Fragmentations a r i s i n g by N o r r i s h Type I and I1 r e a c t i o n s of c a r b o n y l - c o n t a i n i n g compounds a r e c o n s i d e r e d i n P a r t 11, Chapter 1. 1.

E l i m i n a t i o n o f Nitroqen from Azo-compounds

.

Alkyl r a d i c a l s can r e a d i l y be g e n e r a t e d by photodecomposition of a c y c l i c azoalkanes. L i t t l e new photochemistry h a s been r e p o r t e d i n t h e s e systems. R a d i c a l s g e n e r a t e d by p h o t o e l i m i n a t i o n of n i t r o g e n from b i s (1-methylethyl) d i a z e n e s have been s t u d i e d by s p i n - t r a p p i n g and r a d i c a l scavenger techniques’ and amino group s t a b i l i z a t i o n o f one of the carbon r a d i c a l s produced by photodecomposition o f 2- ( t - b u t y l a z o ) -2- (dimethylamino)propane h a s been Evidence f o r s e q u e n t i a l homolysis of two carbonobserved. n i t r o g e n bonds h a s been o b t a i n e d from a s t u d y o f t h e vapour-phase p h o t o l y s i s o f azomethane; t h e i n t e r m e d i a t e methyldiazenyl r a d i c a l h a s a l i f e t i m e of 5 . 3 k 1 n s , 3 More a t t e n t i o n h a s been devoted t o t h e p h o t o r e a c t i o n s o f c y c l i c azoalkanes. D i a z i r i n e s a r e o f p a r t i c u l a r importance a s e l i m i n a t i o n o f n i t r o g e n l e a d s t o t h e formation of c a r b e n e s , a r e a c t i o n t h a t i s i n c r e a s i n g l y used f o r t h e g e n e r a t i o n of such s p e c i e s under mild c o n d i t i o n s . Rate c o n s t a n t s f o r 1,2-hydrogen m i g r a t i o n i n a l k y l c h l o r o c a r b e n e s , i n benzylchlorocarbene, and i n benzylbromocarbene,6 g e n e r a t e d i n t h i s way by l a s e r f l a s h p h o t o l y s i s o f t h e corresponding d i a z i r i n e s , have been measured. The r a t e o f 1,2-hydrogen m i g r a t i o n i n methylchlorocarbene i s 7 s u r p r i s i n g l y low i n comparison t o t h a t found f o r d i a l k y l c a r b e n e s ;

376

Photochemistry

i n t e r m o l e c u l a r a d d i t i o n t o a l k e n e s competes w i t h 1,2-hydrogen m i g r a t i o n i n methylchlorocarbene b u t n o t i n t h e less s t a b l e d i a l k y l c a r b e n e s . Time-resolved p h o t o a c o u s t i c c a l o r i m e t r y h a s been used t o o b t a i n k i n e t i c and thermodynamic d a t a on t h e rearrangement o f a l k y l c h l o r o c a r b e n e s t o t h e c o r r e s p o n d i n g v i n y l 8 chlorides. Photochemically g e n e r a t e d cyclopropylchlorocarbene e x h i b i t s c o m p e t i t i v e i n t r a m o l e c u l a r and i n t e r m o l e c u l a r c h e m i s t r y . The a b s o l u t e r a t e c o n s t a n t f o r t h e rearrangement of t h i s c a r b e n e t o l - c h l o r o c y c l o b u t e n e h a s been determined, and competing 1 , 2 carbon and 1,2-hydrogen m i g r a t i o n s a r e r e s p o n s i b l e f o r t h e f o r m a t i o n of c h l o r o a l k e n e s (1) and (2) from c y c l o b u t y l c h l o r o d i a z i r i n e (3).lo Absolute r a t e c o n s t a n t s f o r t h e i n s e r t i o n of photochemically gene r a t e d dime thoxy ca rbene and f 1uorome thoxycarbene i n t o t h e oxygen-hydrogen bond o f v a r i o u s a l c o h o l s have been measured,” and t h e r e a c t i o n s o f t h e c a r b o n y l y l i d e s (4), g e n e r a t e d from t h e d i a z i r i n e s (5) as shown i n Scheme 1, have been P y r i d i n i u m y l i d e s (6), examined u s i n g l a s e r f l a s h p h o t o l y s i s . 1 2 o b t a i n e d by photodecomposition of t h e d i a z i r i n e s ( 7 ) i n t h e p r e s e n c e of p y r i d i n e , undergo a d d i t i o n t o a - c h l o r o a c r y l o n i t r i l e t o g i v e t h e i n d o l i z i n e s (8).13 F u r t h e r examples o f t h e use of 14-17 d i a z i r i n e s i n p h o t o a f f i n i t y l a b e l l i n g have been r e p o r t e d . Cyclopropane d e r i v a t i v e s are r o u t i n e l y o b t a i n e d by p h o t o e l i m i n a t i o n o f n i t r o g e n from t h e c o r r e s p o n d i n g p y r a z o l i n e s ; 1 , 3 - b i r a d i c a l s have been shown t o b e i n t e r m e d i a t e s i n t h e s e transformations. Diphenylbicyclol2.1.Olpentane h a s been p r e p a r e d i n t h i s way ,I8 and t h e s i n g l e t b i r a d i c a l i n t e r m e d i a t e , g e n e r a t e d by d i r e c t i r r a d i a t i o n of t h i s p y r a z o l i n e , can be traDped w i t h oxygen. The p y r a z o l i n e p r e p a r e d hy t r e a t m e n t of g-benzyloxy~arbonyl-2~3-dehydroprolinet - b u t y l e s t e r w i t h diazomethane h a s been c o n v e r t e d i n t o t h e c o r r e s p o n d i n g f u s e d cyclopropane by p h o t o l y s i s i n d i e t h y l e t h e r . 2o P a r t i c i p a t i o n of a double bond p o s i t i o n e d i n c l o s e p r o x i m i t y t o t h e d e n i t r o g e n a t i o n s i t e h a s been observed. The formation o f t h e t r i c y c l i c d i e n e (9) i n a d d i t i o n t o t h e e x p e c t e d cyclopropane (10) by i r r a d i a t i o n o f 4,5-diazatetracyclo[6.2.11.3t602f7]dodeca-4,9-diene (11) can b e r a t i o n a l i z e d i n t h i s way as a formal homo-Cope rearrangement i n t h e b i r a d i c a l ( 1 2 ) . 2 1 Direct p h o t o l y s i s o f 5,6-dirnethylene-2,3-diazabicyclo[2.1.11-

377

W7: Photoelimination

(4)

Scheme 1

q

R

CI

(7)

i

N

R

-

h v , N2

pyridine

-

qQ

R H,C=C,

CI

(6)

R = H, Me, MeO, or NO2

CN

378

Photochemistry

hex-2-ene (13) i n s o l u t i o n gave 2,4-dimethylenebicyclo[l.l.OIbutane ( 1 4 ) , a t h e r m a l l y u n s t a b l e compound which d i m e r i z e s w i t h 2,4-Dimethylenesecond o r d e r k i n e t i c s between -8OOC and -25°C.22 1 , 3 - ~ y c l o b u t a n e d i y l , a non-Kekule isomer of benzene, was o b t a i n e d and c h a r a c t e r i z e d by e . p . r . s p e c t r o s c o p y on i r r a d i a t i o n of t h e same diazabicyclohexene under cryogenic/matrix i s o l a t i o n c o n d i t i o n s . 2 3 The s e m i l o c a l i z e d homotrimethylenemethane 1,3b i r a d i c a l (15) is formed a l o n g w i t h t h e r e a r r a n g e d t r i m e t h y l e n e methane b i r a d i c a l ( 1 6 ) on i r r a d i a t i o n of t h e diazabicyclo[2.2.11h e p t a n e (17) a t 1 2 K ; 2 4 i d e n t i f i c a t i o n by e.s.r. s p e c t r o s c o p y p r o v i d e s t h e f i r s t d i r e c t o b s e r v a t i o n of such a s p e c i e s . T r i p l e t d i y l s c a p a b l e o f b e i n g t r a p p e d i n t r a m o l e c u l a r l y have a l s o been d e t e c t e d i n t h i s way a t low t e m p e r a t u r e on p h o t o l y s i s o f t h e azoalkanes I n a r e c e n t s t u d y of t h e di-IT-methane rearrangement, c a u t i o n i s a d v i s e d i n u s i n g b i r a d i c a l s g e n e r a t e d by p h o t o e l i m i n a t i o n o f n i t r o g e n from azoalkanes a s m e c h a n i s t i c probes.26 E l i m i n a t i o n of n i t r o g e n i s p r e f e r r e d t o LT2 + ,.,21 p h o t o a d d i t i o n i n compounds c o n t a i n i n g m - p e r i p l a n a r b i s - a z o

.

chromophores F a i l u r e of 3,6-diphenyl-3,4,5,6-tetrahydro~yrazine ( 1 9 ) t o undergo e l i m i n a t i o n o f n i t r o g e n on i r r a d i a t i o n h a s been a t t r i b u t e d t o f a c i l e i s o m e r i z a t i o n t o t h e c o r r e s p o n d i n g hydrazone. Re-examination of t h e p h o t o l y s i s under b a s i c c o n d i t i o n s l e d t o t h e i s o l a t i o n of t h e s t e r e o i s o m e r i c cyclobutanes (20) and ( 2 1 ) and s t y r e n e ( 2 2 ) , formed y & 1,4-diphenylbutane-l,4-diyl (23) which can be t r a p p e d by oxygen i n t h e absense o f s o l v e n t . 2 8 R e s u l t s o b t a i n e d from t h e photochemically induced e l i m i n a t i o n of n i t r o g e n from d e u t e r a t e d 7,8-diazabicyclo[4.2.2ldec-7-ene can be r a t i o n a l i z e d i n terms o f p a r t i a l conformational e q u i l i b r i u m i n t h e b i r a d i c a l , 29 and t h e predominant i n v e r s i o n of s t e r e o c h e m i s t r y observed i n t h e bicyclohexane o b t a i n e d by p h o t o l y s i s of 2,3-diazabicyclo12.2.21oct-2-ene a p p e a r s t o r e q u i r e t h e intermediacy o f a l o n g - l i v e d d i a z e n y l b i r a d i ~ a l . ~ ' The b i r a d i c a l ( 2 4 ) formed i n a s i m i l a r (25) undergoes f a s h i o n from 1 , 4 - d i h y d r o f u r a n o [ 3 , 4 - d ] p y r i d a z i n e d i m e r i z a t i o n t o g i v e t h e cyclo-octane d e r i v a t i v e s (26) and (27) and y i e l d s a d d u c t s w i t h e l e c t r o n - d e f i c i e n t a l k e n e s . 31 311-Pyrazoles a r e known t o undergo photochemically induced e l i m i n a t i o n of n i t r o g e n t o g i v e t h e corresponding cyclopropenes by way of d e t e c t a b l e v i n y l d i a z o i n t e r m e d i a t e s . The

W 7 : Photoelimination

379

(18) R = CH20H or C02Me

502r2

R’

- -

hv

- N2

(29)

R1 = Me, R2 = Me or Ph R1 = Ph, R2 = Me or Ph

380

Photochemistry

5-sulphonyl substituted 3g-pyrazoles ( 2 8 ) are converted in this way in benzene into the cyclopropenes (29) in good yield.32 Retro-cleavage to the diazoalkane (30) and the ester (31) is also observed on benzophenone-sensitized irradiation of 2-methoxy2 ,5 I 5-trimethyl-A3-1I 3 I 4-oxadiazoline (32) and arises via the triplet diazenyl biradical (33 33 Photoelimination of nitrogen from l-aryl-1,2,3-triazoles provides a useful route to indole derivatives. Novel heteroaromatic indolo[6,7-glindoles (34) were obtained in this way from 1,l'- (lf8-naphthylene)-bis(1H-1,2I 3-triazoles) (35),34 and details of the conversion of (1,2,3-triazol-l-yl)naphthalenes into l~-benz[qlindoleshave been published in the patent literature.35 Short-lived nitrile imines are obtained on photolysis of 2g-tetrazoles Nitrile imines generated from bis2E-tetrazoles have been detected spectroscopically36 and the absolute rate constants for the reaction of diarylnitrile imines with various dipolarophiles have been determined.37 Irradiation of the 2,5-diaryl-2H-tetrazoles 136) in the presence of methyl 2-methylacrylate gave the nitrile imine adducts (37) 38 Photodecomposition of the first reported germapyrazoline, 4-dimesitylgerma-1-pyrazoline (381, has been examined and results in the formation of dimesitylgermylene and 9-methylenefluorene 2 the initially formed germirane (39). 39 The successful generation of azetes from perfluorinated derivatives of 1 I 2,3-triazines has recently been described. Irradiation of the perfluorodi-isoprooyl-lf2,3-triazine (40) gave a dimer (41) of the intermediate azete ( 4 2 ) which was also intercepted with f~ran.~' Polymer was the only product of irradiation of perfluoro-1 ,2 I 3-triazine itself ,40 although spectroscopic evidence for azete formation has been obtained in lowtemperature studies.41

.

.

.

2.

Elimination of Ni-trogen from-Diazoygom~oun_d~.

Carbenes can conveniently be prepared by photoelimination of nitrogen from a wide range of diazo-compounds. Investigations have been reported on the effect of solvent on the selectivity of singlet methylene, generated by direct photolysis of diazomethane ,42 and on the photolysis of aliphatic diazo compounds in the presence of oxygen.43 An attempt to obtain a second stable

II/7: Photoelimination

381

wc02r

R02C

-

hv -N2

R02C

R = Et or Me2CH

C02R

\

/

(34)

h v , - N2

CH2=C:

Me

Me

C02Me

(36)

C02Me

(37) R' = H or Me R2= CI, OH, OMe, or NH2

hv

- N2

-

R = mesityl

(42)

R = CF(CF&

382

Photochemistry

tetrahedrane by photoelimination of nitrogen from the diazocyclopropene (43) was unsuccessful, the isolated products being di-t-butylacetylene (44) and the t-butyltrimethylsilylacetylene (45);44 the tetrahedrane (46) was obtained, however, by thermal decomposition in the presence of copper (I) chloride followed by photoisomerization as shown in Scheme 2 . Reactions of diphenylmethylene, generated by photodecomposition of diphenyldiazomethane, with dimethyl thiophos~ h i t e and ~ ~ with silicon, germanium and tin h y d r i d e ~have ~ ~ been reported, and matrix isolation of diphenyloxiranone has been achieved in carbon-dioxide-doped arqon or xenon matrices at 35 K by irradiation of diphenyldiazomethane 47 Trinlet carbenes add non-stereospecifically to alkenes via a biradical intermediate; an analogous reaction of triplet dinhenylmethylene with [1.1.11propellane (47) has been described and leads to the cyclobutane (48) by the pathway shown in Scheme 3 . 48 Spin polarization of photochemically generated triplet fluorenylidene has been observed indirectly at room temperature by chemically induced dynamic electron polarization spectroscopy.4 9 Direct observation of the carbonyl ylide (49) at 10 K in an argon matrix has also been reported:50t51 the ylide is obtained by direct photolysis of (2-carbomethoxyphenyl)diazomethane (50) and undergoes reversible photoisomerization to the oxirane (51) as shown in Scheme 4. Competing intramolecular and intermolecular 0-H bond insertion pathways have been observed on analogous photochemically induced generation of [2-(hydroxymethyl)phenyllcarbene, [2-(hydroxymethyl)phenyllphenylcarbene, and 52 [ 2 (2-hydroxyethyl)phenyllcarbene in alcohols. a-Diazo carbonyl-containing compounds also undergo facile loss of nitrogen on irradiation. Photo-Wolff rearrangement is the most commonly observed and synthetically useful reaction in a-diazoketones and is thought to involve the initial generation of acyl carbenes, A new mechanism for the photo-Wolff rearrangement involving one-step ketene formation has been proposed and is based on semiempirical MIND0/3 MO calculations.53 Interception of the ketene (52) produced by Dhotodecomposition of diazo ketone (53) with the siloxyalkyne (54) leads to the formation of the phenol ( 5 5 1 , an important intermediate in a new synthesis of the 'host defense stimulant' maesanin. 54 Photolysis of the (l-diazo-

.

W 7 : Photoelimination

383

CMe3

hv

Me3Cx C MC I Ie -Si3Me3

c

-N2

N2

(43)

& Ill

5:CMe,

CMe3

+

Ill

5:5ime3

(44)

(45)

1

A, CuCl

Me3cxCMe3 Me3cx -

Me3C

SiMe3

hv

-

A

Me3C

SiMe,

(46)

Scheme 2

(48)

(47)

Scheme 3

aEH aCHN2 hv,-N2

b350 nm

C02Me

C02Me

c

b350 b254 nrn nm

/

q? \

OMe

(49)

Scheme 4

Photochernistry

384

OMe

OSi(CHMe2)3

(55)

R' = Me, R2 = SiEt3 or'Si(CHMe2)3 R' = CMe3, R2 = SiMe2CMe3,SiMe(CMe3)2,SiPh2CMe3,or Si(CHMe2)3 R' = Ph, R2 = SiEt3 or Si(CHMe&

M I eN Il2 0 II Me3Si-Si-C-C-R I

Me

(60)

hv

- N2

Ye.. E

Me3Si-Si-C-C-R I

-

Me (59)

R = Me, Me2CH, Ph, 4-MeCsH4,or adamantyl

Me2Si=C,

,SiMe3

rR

0 (611

W 7 : Photoelimination

385

2 - o x o a l k y l ) s i l a n e s (563 t a k e s a s i m i l a r c o u r s e a f f o r d i n g t h e s i l y l k e t e n e s (571, a l t h o u g h i n some c a s e s (R1 = CMe3) 2 - s i l y l c y c l o butanones (58) are a l s o formed by competing i n t r a m o l e c u l a r C-H bond i n s e r t i o n i n t h e a c y l carbene i n t e r m e d i a t e . 55 An a l t e r n a t i v e pathway i n v o l v i n g l 1 2 - t r i m e t h y l s i l y l m i g r a t i o n i s p r e f e r r e d i n t h e a c y l d i s i l a n y l c a r b e n e s ( 5 9 ) d e r i v e d from t h e d i a z o k e t o n e s Reaction ( 6 0 ) and y i e l d s t h e h i g h l y r e a c t i v e a c y l s i l e n e s ( 6 1 ) . 5 6 o f t h e c a r b e n e s and k e t e n e s g e n e r a t e d by p h o t o e l i m i n a t i o n o f n i t r o g e n from t h e 2-diazo-1,3-diketones (62) w i t h thioamides h a s been u s e d as shown i n Scheme 5 i n t h e s y n t h e s i s of 1,3-oxazines ( 6 3 ) and t h i a z o l e s (64) ; 5 7 5 - a c y l t h i a z o l e f o r m a t i o n is f a v o u r e d by an i n c r e a s e i n t h e e l e c t r o p h i l i c i t y o f t h e d i a z o compound o r by an i n c r e a s e i n t h e n u c l e o p h i l i c i t y o f t h e thioamide. I n t r a m o l e c u l a r c a r b e n e i n s e r t i o n i s p r e f e r r e d t o Wolff rearrangement on photodecomposition o f c e r t a i n 2-diazoacetoacetanilides. 58 Photo-Wolff rearrangement i n c y c l i c a-diazo k e t o n e s is accompanied by r i n g c o n t r a c t i o n . Photochemically induced conv e r s i o n o f t h e a-diazocyclopentanone ( 6 5 ) i n t o t h e c y c l o b u t a n e c a r b o x y l a t e (66) i n methanol, f o r example, h a s been used i n t h e s y n t h e s i s of junionone. 59 A Wolf f rearrangement pathway i s a l s o i m p l i c a t e d i n t h e p h o t o - o x i d a t i v e r i n g c o n t r a c t i o n of a-diazoc y c l o a l k a n o n e s . 6o A r e a c t i o n of p a r t i c u l a r i n t e r e s t i s t h e analogous d i r e c t c o n v e r s i o n of t h e d i a z o s u l p h o x i d e s (67) i n t o t h e carbapen-2-ems (681, a p r o c e s s which permits e n t r y i n t o a The r i n g system p r e v i o u s l y a v a i l a b l e o n l y by t o t a l s y n t h e s i s . 6 1 photodecomposition i n methanol of 1I 3-bis ( d i a z o ) -2-indanone h a s a l s o been examined. 62 S i m i l a r r i n g c o n t r a c t i o n s have been o b s e r v e d i n 1 , 2 quinone d i a z i d e s which have commercial importance a s p h o t o r e s i s t s . The i n d e n e c a r b o x y l i c a c i d s (69), f o r example, a r e t h e major p r o d u c t s of i r r a d i a t i o n of t h e l-oxo-2-diazo-1,2-dihydronaphthalR e~l a t e d s u l p h o n a t e s have been e n e s (70) i n aqueous d i ~ x a n e . ~ 64,65 shown t o undergo t h e same r i n g c o n t r a c t i o n s . The u s e o f d i a z o - c o n t a i n i n g compounds as p h o t o a f f i n i t y p r o b e s h a s a g a i n been w i d e l y r e p o r t e d . 66-71 Various 4-ethoxycarbonyl-5-hydroxy~yrazole d e r i v a t i v e s have been s u c c e s s f u l l y p r e p a r e d by p h o t o e l i m i n a t i o n of n i t r o g e n from t h e c o r r e s p o n d i n g 5-diazonium s a l t s . 72

Photochemistry

386

R1vC , Go hv

R1$N2 R’

c

0

15

1 s-

R2- C-NH2

R2- C NH2

Scheme 5

Me

0 h v , - N2

Me Me&N2

MeOH

0

Rl--BF;

0

R2 H I

0 C02CH202CCMe3

(67)

hv

- N2

R l -R -& 2H F D

0

I

’

Me

C02CH202CCMe3

.’--fie 0

C02CH202CCMe3

387

W7: Photoelim inarion

R

R

(70)

(72)

N3yC5cH2

R

R = H or p -MeC6H4S&

(69)

R1 = Me, R2 = Me or Ph

h v , - N2

(71)

HCN

*

R N-‘

R

R I: Me, CH20Me or CH2CI

388

Photochemistry 3.

E l i m i n a t i o n o€ N i t r o q e n from A z i d e s .

The p h o t o r e a c t i o n s o f a z i d e s c a n i n most c a s e s b e r a t i o n a l i z e d i n terms o f i n i t i a l l o s s of n i t r o g e n w i t h t h e f o r m a t i o n o f n i t r e n e s which u n d e r g o r e a r r a n g e m e n t

,

i n s e r t i o n or a d d i t i o n

F u r t h e r e x a m p l e s of r e a r r a n g e m e n t of t h e n i t r e n e s

reactions.

d e r i v e d from v i n y l a z i d e s t o 2 H - a z i r i n e s h a v e b e e n r e p o r t e d ; 2-azidobut-l-en-3-yne

and 2 - l - a z i d o b u t - l - e n - 3 - y n e

h a v e been

,73

c o n v e r t e d i n t h i s way i n t o t h e c o r r e s p o n d i n g 2;-azirines

and t h e 2-azirinylmethylphosphonates ( 7 1 ) h a v e b e e n o b t a i n e d by i r r a d i a t i o n o f t h e 2-azidoallylphosphonates ( 7 2 ) i n a c e t o n i t r i l e . The e x i s t e n c e o f v a r i o u s 2 - m e t h y l e n e - 2 g - a z i r i n e

74

derivatives

( 7 3 ) , g e n e r a t e d by p h o t o d e c o m p o s i t i o n o f t h e c o r r e s p o n d i n g 1azidoallenes

( 7 4 ) , h a s been e s t a b l i s h e d by t r a p p i n g e x p e r i m e n t s

w i t h hydrogen c y a n i d e which a f f o r d s t h e a d d u c t s ( 7 5 ) . 7 5 c o n t r a s t , i r r a d i a t i o n o f t h e 3-azido-2-cyanoacrylates

In

(76)

a f f o r d s n o t o n l y t h e 2 p a z i r i n e s ( 7 7 ) b u t a l s o t h e o x a z o l e s (781, p r e s u m a b l y formed

via

the p h o t o c h e m i c a l l y d e r i v e d n i t r i l e y l i d e s

( 7 9 ) .76 The p h o t o d e c o m p o s i t i o n o f a r y l a z i d e s c o n t i n u e s t o a t t r a c t most a t t e n t i o n i n t h i s a r e a a l t h o u g h few a d d i t i o n a l d e t a i l s c o n c e r n i n g t h e mechanism h a v e b e e n r e p o r t e d , p a r t i c u l a r l y with r e f e r e n c e t o r i n g expansion

via

l12-didehydroazepines.

Ring

e x p a n s i o n p h o t o p r o d u c t s h a v e I however, b e e n i s o l a t e d from v a r i o u s d i a z i d o n a p h t h a l e n e s ; 77 o b t a i n e d i n this way

& y

2 ,7 - d i a z i d o n a p h t h a l e n e

t h e n o v e l a z e p i n o a z e p i n e (80) h a s been t h e b e n z o a z e p i n e ( 8 1 ) on i r r a d i a t i o n o f ( 8 2 ) i n t h e n r e s e n c e o f sodium m e t h o x i d e .

P a r t i c u l a r a t t e n t i o n h a s been devoted t h i s y e a r t o t h e T h i s i n t e r e s t stems from t h e i r

study of f l u o r i n a t e d a r y l azides.

p o t e n t i a l u s e as p h o t o a f f i n i t y p r o b e s d u e t o t h e ease w i t h which p e r f l u o r o p h e n y l n i t r e n e u n d e r g o e s C-H bond i n s e r t i o n .

Perfluorin-

a t e d a z i d e d e r i v a t i v e s o f methyl b e n z o a t e a f f o r d photochemically g e n e r a t e d i n t e r m e d i a t e s which f o r m a d d u c t s w i t h a w i d e v a r i e t y o f f u n c t i o n a l g r o u p s . 78

I n t e r m e d i a t e s h a v e been d e t e c t e d i n t h e

p h o t o d e c o m p o s i t i o n o f o t h e r f l u o r i n a t e d a r y l a z i d e s . 79

Triplet

n i t r e n e s which r e a d i l y undergo h y d r o g e n a b s t r a c t i o n a r e o b t a i n e d i n p r e f e r e n c e from i o d o a r y l a z i d e s , t h u s l i m i t i n g t h e i r u s e as 1251l a b e l l e d p h o t o a f f i n i t y p r o b e s . 24% Nitrene i n s e r t i o n i n t o t h e N-H bond o f d i e t h y l a m i n e h a s , however, b e e n o b s e r v e d on

389

IU7: Photoelimination

CH2- CI H-C02H

H0

YH c=o

I

0 (84)

HO-@CH2-CH-C02H I hv, - N2

c

Et2NH, MeOH

R' = H or I

Y3

H' R' = H, Me, or OMe R2 = OMe or NEt2

I

YH

c=o

(83) R2 = NEt2or OMe

390

Photochemistry

irradiation of methyl 4-azido-2-iodo-3,5,6-trifluorobenzoate in cyclohexane. Singlet photochemistry resulting in rinq expansion to the azepines (83) is also preferred in the iodinated -N-(4-azidobenzoyl)tyrosines (84) in which the iodo group and the azido group are in different aryl nuclei.82 Other examples of the use of aryl azide derivatives in Fhotoaffinity labelling have been de~cribed'~-~' and 31 2-amino (4-azido-2-nitrophenyl)ethylaminocarbonyl]-g4-deacetyl-3-de (methoxycarbonyl)vincaleukoblastine has been prenared from vinblastine as a new photoreactive cytostatic substance with the ability to overcome multidrug resistance of tumor cells. Irradiation of sterically crowded 4-nitrophenyl azides affords the corresponding azo-compounds and nitroanilines.91 Intramolecular nitrene insertion is responsible for the conversion of the azide (85) into the pyrido[lt,2':1,21imidazo[4,5-~lindole (86)9 2 and the analogous Dyrene-sensitized photoreaction of 2-azidobiphenyl is much enhanced in a hydrophobic cage. 93 A new photosensitive orqanosilicon crosslinking agent, bis~4-~3-(4-azido~henyl)-l-oxo-2-propenyll~henyl~dimethylsilane, has been described.94 Heteroaryl azides undergo analogous photochemically induced ring expansion reactions. The 3-methoxy-4-azidopyridthe unstable 1g-1,2,5azines (87) were converted in this way &y triazepines ( 8 8 ) into the 4H-1,2,5-triazepines (89) on irradiation in the presence of a base such as sodium methoxide or diethylamine .95 The first recorded examples of fully unsaturated monocyclic 1,3,5-triazepines have been prepared in a similar fashion from a-azidoazines.96 Further investigations of the photochemically induced ring expansion reactions of 6-azidoquinolines and of 6-azidodiazines have been described and the anomalous behaviour of certain derivatives discussed. " The behaviour of 3-azido-5,7-di-t-butyl-2,1-benzisothiazole (90) has been clarified by the detection of the highly reactive 2,4-di-t-butyl6-cyanothionitrosobenzene (91) on irradiation at 12 K,98 and competing 1,2-aryl migrations in the nitrenes derived from the 9-aryl-9-xanthenyl azides (92) are clearly responsible for the observed formation of the imines (93) and the oxazepines (94). 99 6-Azido-4,5,7-trif l u o r o i n d ~ l eand ~ ~ adenylyl~ ( 2 I +5 ) adenylyl- ( 2 '+5 -8-azidoadenosinelo1 have shown promise in photolabelling experiments.

391

W7: Photoelimination

(93) R = Ph, p -MeOGH4, p -F&H4, or p -CI&H4

EtO OSiMe3

X

+

f EtO-C-N,

EtO OSiMe3

- & hv

- N2

NHC02Et

(97)

RO-C:

(98)

N-CiN N3 (1 00)

hv

- N2

N-CEN RO-C:

..

N: (99)

Photochemistry

392 Ethyl azidoformate i s a u s e f u l nrecursor of

(ethoxy-

c a r b o n y l ) n i t r e n e , a s p e c i e s which r e a d i l y u n d e r g o e s a d d i t i o n and insertion reactions.

The r e g i o c h e m i s t r y of

(ethoxycarbony1)nitrene

a d d i t i o n t o s i l o x y d i e n e s h a s been e x a m i n e d , l o 2 and a n a s y m m e t r i c s y n t h e s i s o f t h e !-substituted a-amino e s t e r ( 9 5 ) h a s b e e n a c h i e v e d by i r r a d i a t i o n of e t h y l a z i d o f o r m a t e i n t h e p r e s e n c e of t h e c h i r a l R e a c t i o n of n h o t o c h e m i c a l l y

s i l y l ketene a c e t a l ( 9 6 ) .

g e n e r a t e d ( e t h o x y c a r b o n y l )n i t r e n e w i t h 1-ethoxy-1- ( t r i m e t h y l s i l o x y ) c y c l o n r o p a n e ( 9 7 ) q a v e t h e i n s e r t i o n p r o d u c t ( 9 8 ) ,Io4 and o n l y s i n g l e t d e r i v e d n h o t o m o d u c t s , t h e i s o c y a n a t e formed b y C u r t i u s r e a r r a n q e m e n t and compounds a r i s i n g by C-H bond i n s e r t i o n ,

were o b t a i n e d on d i r e c t and s e n s i t i z e d i r r a d i a t i o n o f E - t r i m e t h y l s i l y l b e n z o y l a z i d e i n cyclohexane. 105

-N-Cyanoalkoxycarbimidoyl s i t u by -

(loo),

nitrenes

(991, g e n e r a t e d

in

i r r a d i a t i o n ( A = 300 nm) of t h e c o r r e s p o n d i n g a z i d e s a d d c l e a n l y and s t e r e o s p e c i f i c a l l y t o a l k e n e s t o g i v e 106

aziridines.

A m i n o s i l y l e n e ( a m i n o s i l a n e d i y l ) (101) h a s b e e n p r e p a r e d by i r r a d i a t i o n ( A = 2 5 4 nm) of s i l y l a z i d e (102) i n an a r g o n m a t r i x a t 1 2 K as shown i n Scheme 6 . lo7 D i m e t h y l s i l y l e n e ( d i m e t h y l s i l a n e d i y l ) , g e n e r a t e d b y p h o t o e l i m i n a t i o n o f n i t r o g e n from diazidodimethylsilane,

r e a c t s w i t h oxygen a t 35-42 K t o g i v e

d i m e t h y l d i o x a s i l i r a n e r a t h e r t h a n t h e s i l a n o n e 2-oxide e x p e c t e d

by a n a l o g y w i t h c a r b e n e c h e m i s t r y .

Dimethylgermylene can

s i m i l a r l y b e p r e p a r e d by i r r a d i a t i o n of d i a z i d o d i m e t h y l q e r m a n e i n

a c r y o s t a t i c matrix.”’

I r r a d i a t i o n of trinesitylazidogermane

(103)

i n a 3 - m e t h y l p e n t a n e m a t r i x a t 7 7 K a f f o r d s t h e germanimine ( 1 0 4 ) which c a n b e i n t e r c e p t e d b y e t h a n o l , w h e r e a s t h e same a z i d o germane (103) i s c o n v e r t e d on i r r a d i a t i o n i n c y c l o h e x a n e a t room t e m p e r a t u r e i n t o t h e a z a q e r m a c y c l o n e n t a n e (105) by a pathway i n v o l v i n g i n t r a m o l e c u l a r n i t r e n e i n s e r t i o n : ’lo i n d e p e n d e n t r o u t e s t o t h e n i t r e n e a n d t o t h e i m i n e have b e e n p r o p o s e d .

A

simple t e s t f o r d i f f u s i o n - l i m i t e d r e a c t i o n s which d o e s n o t r e q u i r e measurement of a b s o l u t e r a t e s h a s b e e n d e v i s e d and i s b a s e d on t h e i n s e r t i o n r e a c t i o n s o f n h o t o c h e m i c a l l y g e n e r a t e d (diethoxyphosphoryl) n i t r e n e

.

393

I . 7 : Photoelimination

H3Si-N3

hv

- N2

..

hv

H3Si-N:

*

H\,Si=NH H

(1 02)

Scheme 6

- -

hv. - Nr, cyclohexans

R3Ge-N3 (103) R = mesityl

I

hv -N2

R2Ge=N-R (104)

R’

0 (106) R’ = H or Me, R2 = Me

(1 07) R = H, Me, CHMe2,CH20H, or CH2Ph

0 R - C O 2 - N B

0 CMesOH, hv CC14

R-CI

0

+ C02 + HN

0

Scheme 7

Photochemistry

394 4.

P h o t o e l i m i n a t i o n o f Carbon Dioxide.

D e c a r b o x y l a t i o n s o f t h e chromone-3-acetic a c i d s (106) have been achieved i n a fas hi on s i m i l a r t o t h a t o f n h e n y la c e t i c a c i d . 1 1 2 P h o t o d e c a r b o x y l a t i o n of a - h y d r o x y a r y l a c e t i c a c i d s such a s mandelic a c i d o r b e n z i l i c a c i d p r o c e e d s via a-hydroxyarylmethyl c a t i o n s w i t h much h i g h e r quantum y i e l d s , however, t o q i v e t h e c o r r e s p o n d i n g arylmethanols. The e f f i c i e n c y of p h o t o d e c a r b o x y l a t i o n of p y r u v i c a c i d i n aqueous s o l u t i o n h a s been shown t o h e pH dependent ’14 and d e u t e r i u m - l a b e l l e d 4-N-substituted c y t o s i n e s have been p r e p a r e d by i r r a d i a t i o n o f t h e c o r r e s p o n d i n g c a r b o x y l i c a c i d s (107) i n D20 o r MeOD. ’15 R a d i c a l pathways have been proposed i n t h e p h o t o i n i t i a t e d d e c a r b o x y l a t i o n o f unmodified c a r b o x y l i c a c i d s u s i n g aza-aromatic compounds a s l i g h t a b s o r b e r s and t - b u t y l t h i o l as the hydrogen donor116 and i n a new method o f h o m o l y t i c a l k y l a t i o n of p r o t o n a t e d h e t e r o a r o m a t i c b a s e s which is based on photo117 d e c a r h o x y l a t i o n of c a r b o x y l i c a c i d s u s i n g iodosobenzene d i a c e t a t e . Decarboxylation-arylation p r o d u c t s have been o b t a i n e d on i r r a d i a t i o n o f a c u b a n e c a r b o x y l i c a c i d and l e a d t e t r a - a c e t a t e i n benzene o r i n s u b s t i t u t e d benzenes. ’18 The c o n v e r s i o n of 10-methyla c r i d i n i u m i o n (108) i n t o 9-alkyl-10-methylacridans (109) by i r r a d i a t i o n w i t h v i s i b l e l i g h t i n t h e p r e s e n c e of c a r b o x y l i c a n i o n s (110) i n v o l v e s i n i t i a l e l e c t r o n t r a n s f e r from t h e anion t o t h e s i n g l e t e x c i t e d s t a t e o f t h e a c r i d i n i u m i o n . ’19 Photochemically induced l o s s of carbon d i o x i d e from p y r e t h r o i d model esters120 and from naphthylmethyl esters”’ h a s a l s o been r e p o r t e d , and carbon d i o x i d e i s formed a l o n g w i t h o t h e r

p r o d u c t s of f r a g m e n t a t i o n on mercury-Dhotosensitized decomposition of t3-butyrolactone Monochromic p h o t o l y s i s (X = 2 4 0 nm) of d i f l u o r o m a l e i c a n h y d r i d e i n an argon m a t r i x gave a new s n e c i e s t o which t h e d i f l u o r o c y c l o p r o p e n o n e s t r u c t u r e h a s been a s s i g n e d , 1 2 3 and a new method of c h l o r o d e c a r b o x y l a t i o n of c a r b o x y l i c a c i d s h a s been d e s c r i b e d based on i r r a d i a t i o n of N-acyloxyphthalimides (111) i n t h e p r e s e n c e of 1,4-diazabicyclo[2.2.2loctane as shown i n Scheme 7; 124 an e l e c t r o n t r a n s f e r mechanism h a s been proposed.

.

The g e n e r a t i o n and s y n t h e t i c use of carbon r a d i c a l s o b t a i n e d by l i g h t - i n i t i a t e d e l i m i n a t i o n of carbon d i o x i d e from thiohydroxamic esters and i n p a r t i c u l a r from esters of 2-hydroxy2-thiopyridone have a g a i n been w i d e l y r e p o r t e d . The mechanism o f t h i s p r o c e s s h a s been t h e s u b j e c t of f u r t h e r investigation^,^^^'

126 and numerous examples o f i n t r a m o l e c u l a r 127-129

and i n t e r -

II/7: Photoelimination

395

molecular130-134 addition of such radicals to alkenes has been described. A transannular hydrogen migration has been observed 135 in the radical derived in this way from isolongifolic acid. The radical (112) obtained by irradiation of (+) -*-pinonic ester (113) can be trapped and converted into the synthetically useful acetate (114)i 136 in the absence of an efficient trapping agent, ring opening occurs in the radical (112) leading to the formation of the alkene (115). A useful new source of alkyl radicals, namely 5-alkoxycarbonyl dithiocarbonates , has also been described’ 37; the dithiocarbonate (116), for example, is converted in 87% yield into the 2-ethyl xanthate (117) by decarboxylation of the corresponding photochemically generated 2-alkoxy carhonyl radical. 5.

Fraqmentation of Organosulphur Compounds.

1-Substituted l12-dithiolanyl radicals are formed by intramolecular cyclization of thiyl radicals generated by photochemically induced sulphur-hydrogen bond homolysis in 1-substituted thio-3propanethiols.138 12.21 (2,6)- and [2.21(2,5)-Pyrazinophanes have been prepared from the correspondinq 2,11-dithia[3.3]pyrazinophanes by photoextrusion of sulphur. Carbon-sulphur bond homolysis is also involved in the novel conversion of the 2(3H)-thiophenone (118) into the thiet (119) as shown in Scheme 8;140- open chain photoproducts are obtained from the corresnondinq 2(3H)-furanones. Analogous photodecarboxylation in the dithiocarbonate (120) leads to the formation of the first cyclic dithio-oxalate, l,4-dioxane2,3-dithione (1211, a thermally unstable compound which can be 141 isolated as various [,4 + ,21 adducts. The additions of photochemically qenerated arylthiyl radicals to aryla~etylenes’~~ and to 11.1. l l p r ~ p e l l a n ehave ~~~ been described. Sulphur-sulphur bond homolysis i s also involved in the photoextrusion of ethylene from 1,2,6,9-tetrathiacyclodecane (122) as shown in Scheme 9.144 A new type of sulphur extrusion has been reported in the methanodithiomethano-bridged dibenzotrithionin (123) which on irradiation in the presence of trimethyl phosphite affords the tris-sulphide (124) 14’ Photolysis of cyclic polysulphides can lead to the formation of l12-dithiones; the cyclic polysulphide (125), for example, is converted in this

.

Photochemistry

396

/ Me% Me

OAc

Me

(1 19)

Scheme 8

ci

+ CH2=CH2

Scheme 9

/

397

W 7 : Photoelimination

Me Me

Me Me

&* S

*-

Me Me

+

Ph-CH

H

MeowMe "e: hv

-

\

398

Photochemistry

way i n t o t h e u n s t a b l e 1 , 2 - d i t h i o n e ( 1 2 6 ) which can b e t r a p p e d a s t h e a d d u c t (127) w i t h n o r h o r n e n e . l 4 5 - N i t r o p y r i d i n e t h i y l r a d i c a l s a r e i n c o r p o r a t e d i n t o c h l o r o y h y l l on i r r a d i a t i o n i n t h e p r e s e n c e o f 2,2'-dithiobis(5-nitropyridine). 1 4 7

Benzophenone-photosensitized c l e a v a g e o f B-phenylthioa l c o h o l s 1 4 a n d p h o t o i n d u c e d bond homolysis i n sulphenamides 149 has been r e p o r t e d , a n d p h o t o d e c o m p o s i t i o n o f N-benzenesulphonyloxyphthalimide i n acetone/dioxane s o l u t i o n a f f o r d s phthalimide, d i p h e n y l s u l p h o n e , and b e n z e n e s u l p h o n i c a c i d . If' Primary and s e c o n d a r y x a n t h i c a n h y d r i d e s undergo a v i s i b l e l i g h t - i n d u c e d r a d i c a l chain reaction t o give t h e corresponding xanthates with l o s s of c a r b o n o x y s u l p h i d e , 15' and S - a c y l d i t h i o c a r b o n a t e s (5a c y l x a n t h a t e s ) have been found t o b e a c o n v e n i e n t s o u r c e o f a c y l 152 radicals. P h o t o s o l v o l y s i s of m e t h a n e s u l p h o n a t e s i n aqueous a l c o h o l s h a s been reported,153 and e l e c t r o n t r a n s f e r mechanisms a r e i n v o l v e d i n t h e p h o t o c h e m i c a l l y i n d u c e d removal o f t h e E - t o l u e n e s u l p h o n y l g r o u p i n t h e a-D-arabinohexopyranoside (128) i n t h e p r e s e n c e of t r i e t h ~ l a m i n e land ~ ~ i n t h e c l e a v a g e o f t h e p h o t o s e n s i t i v e 4- ( 4 , 8 dimethoxynaphthylme t h y 1 1 benzenes u l p h o n y l n r o t e c t i n g group. 55

F r a g m e n t a t i o n and e l i m i n a t i o n r e a c t i o n s t h a t c a n n o t b e i n c l u d e d i n any o f t h e above c a t e g o r i e s are b r i e f l y reviewed i n t h i s s e c t i o n . I t h a s n o t proved p o s s i b l e t o c l a s s i f y t h e s e p r o c e sse s, b u t l i k e r e a c t i o n s a r e grouped t o g e t h e r . A n u n u s u a l a , a - e l i m i n a t i o n o f two phenyl g r o u p s i s proposed o n i r r a d i a t i o n of t h e l , l , l - t r i p h e n y l a l k a n e ( 1 2 9 ) i n methanol and r e s u l t s i n the f o r m a t i o n of b i p h e n y l (130) and t h e 156 p h o t o p r o d u c t s (131) a n d (132) d e r i v e d from t h e c a r b e n e ( 1 3 3 ) . The a n a l o g o u s f o r m a t i o n o f phenylcyanocarbene h a s been o b s e r v e d i n p h e n y l a c e t o n i t r i l e s . 157 V a r i o u s p h o t o r e a c t i o n s a r i s i n g by carbon-oxygen bond c l e a v a g e have been r e p o r t e d . Photoinduced s i n g l e e l e c t r o n t r a n s f e r - i n i t i a t e d c l e a v a g e of t h e b e n z y l i c e t h e r p r o t e c t i n g group h a s been a c h i e v e d i n t h e p r e s e n c e of 1 , 4 - d i c y a n o n a p h t h a l e n e as e l e c t r o n a c c e p t o r . 15* The mechanism of t h e a h o t o r e a r r a n g e m e n t o f 3,4-epoxyprecocene-I (134) t o t h e 3-chromanone (135) is

W 7 : Photoelim ination 159 uncertain although a triplet excited state appears to be involved. Other oxiranes undergo photodecomposition carbonyl ylidesI 160-164 and heterolytic carbon-oxygen bond cleavage is the primary photochemical step in 9-fluorenol.165r166 5-Cholest-3enoxyl radicals have been obtained on irradiation of 36-acetoxy4,5-epo~ycholestanes~~~ and both aryloxyoxalyl chlorides168 and aryloxyoxalyl t-butyl peroxides169 have proved to be convenient sources of aryloxyl radicals. The use of phthalimide hydroperoxides as efficient photochemical hydroxyl radical generators has also been reported.170 Single electron transfer-initiated fl-demethylation of N-methylated alkaloids171 and N , ; ’ - d i m e t h y l a n i l i n e ~ ~ has ~ ~ been described, and a photoremovable a-methylphenacylamido protecting group has been developed for the solid-phase synthesis of terminal peptide amides.173 A useful review of the applications of photosensitive protecting groups in carbohydrate chemistry has been p~b1ished. l ~ ~ The cis,transoid,cis-cyclobutadi-indene (136) undergoes photochemically induced [2 + 21 cycloreversion to indene.175 The mechanism of the light-induced enzymatic monomerization of pyrimidine photodimers has also been examined. 1 I 1-Diarylethylenes are obtained on photolysis of 2,2-diaryloxetanes in the presence of electron donorsI1” and the carbene (137) can be 178 generated by photodecomposition of the fused cyclopropane (138). Irradiation of certain fused 1,l-dihalocyclopropanes similarly affords dihalocarbenes,179 I 180

-

c-

Numerous examples of photoinitiated SRNl reactions have been described in the period covered by this r e p ~ r t . ~ ~ ~ - ~ ” Photocyclization reactions accompanied by elimination of HC1 and HBr have proved to be synthetically useful although in some cases details of the reaction mechanisms are not clear. The aporphine derivative (139) has been obtained in this way from the benzylisoquinoline (140) lgl Other aporphines can be obtained by the same procedure I lg2 and analoqous photocyclizations have been employed in the syntheses of d e h y d r o f u r a n~~”~ fused 2triazoles1194 isoquinolinesI lg5 naphthothiazolesI lg6 and acridThe unusual conversion of the chloroacetamides (141) ines. into the dibenz[~,f]azonine-5,7-diones (142) is thought to proceed via the alcohols (143). 19 8

.

’’’

-

Photochemisty

400

Me0 Me0

g

Me0 hv

q

c

- HBr

Me0

Me0

OMe

OMe (1 40)

(1 39)

R’

?!?2R R’

\

II.7: Photoelimination E q u i v a l e n t i n t e r m o l e c u l a r p h o t o r e a c t i o n s have been r e p o r t e d . The approach i s p a r t i c u l a r l y u s e f u l i n p h o t o a r y l a t i o n 199-202 and i n p h o t o h e t e r o a r y l a t i o n ,203-206 and 2- ( m e t h y l t h i o )adenosine h a s been p r e p a r e d by i r r a d i a t i o n of 2-iodoadenosine i n t h e presence o f dimethyl d i s u l p h i d e . 207 The p h o t o r e a c t i o n s o f amines w i t h c h l o r o a l k a n e s have a l s o been s t u d i e d . 208-2 11 Many other nhotodecompositions a r i s e by carbon-halogen bond cleavage. T h e m a j o r i t y of t h e s e a r e r a d i c a l p r o c e s s e s w i t h no s p e c i a l photochemical s i g n i f i c a n c e and d e t a i l s a r e n o t t h e r e f o r e i n c l u d e d i n t h i s r e p o r t . I n some i n s t a n c e s , however, t h e r e i s evidence f o r i o n i c i n t e r m e d i a t e s i n t h e s e t r a n s f o r m a t i o n s . T h i s i s p a r t i c u l a r l y so i n v i n y l h a l i d e s . 212-214 Evidence a g a i n s t cationic intermediates i n the photoreactions of cyclic iodides h a s now been o b t a i n e d from m a t r i x - i s o l a t i o n s t u d i e s . 215 S t u d i e s on t h e photodecomposition o f d i a r y l i o d o n i u m s a l t s have a l s o been de s c r i b e d 216 ,217 Photochemically induced nitrogen-halogen bond homolysis h a s been examined i n N-bromo imides. 218 I n t r a m o l e c u l a r hydrogen a b s t r a c t i o n i n t h e i m i d y l r a d i c a l d e r i v e d i n t h i s way from N-bromo-g-acetylhexanamide i s r e s p o n s i b l e for t h e formation of -N-acetyl-4-bromohexanamide. 219 D i a z i r i d i n y l r a d i c a l s can be g e n e r a t e d by i r r a d i a t i o n o f t h e corresponding N-chloro compounds. 220 The use o f a l k o x y l r a d i c a l s , q e n e r a t e d photochemically from hypobromites and h y p o i o d i t e s , c o n t i n u e s t o a t t r a c t much a t t e n t i o n , 221'222 p a r t i c u l a r l y i n t h e s t e r o i d f i e l d . 223-226 R e g i o s e l e c t i v e B-scission of t h e a-oxoalkyloxyl r a d i c a l d e r i v e d from t h e h y p o i o d i t e (144) i s i n v o l v e d , f o r example, i n t h e (145) and (146) on f o r m a t i o n of t h e novel a-hydroxy-e-lactones i r r a d i a t i o n of t h e s t e r o i d a l a l c o h o l i n t h e p r e s e n c e of m e r c u r y ( I I 1 o x i d e and i o d i n e . 227

.

-

Photochemistry

402 References

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II.7: Photoelimination 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

403

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e., Yc

Photochemistry

404 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109

110

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a,

s,

-

&.

a,

405

II/7: Photoelimination

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s,

z,

Photochemistry

406

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<.

W 7 : Photoelimination 184 185 186 187 188 189 190 191 192 19 3 194 195 196 197 198 199 200

20 1 20 2 20 3 204 20 5 206 20 7 20 8 209 210 211 21 2 21 3 214 215 216 217

407

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119,

s,

408 21 8 Y. L. T.-I. 219 Y. L. 220 G. V.

Photochemistry

Chow, D.-C. Zhao, M. K i t a d a n i , K. S. P i l l a y , Y. M. A. Naguib, and Ho, J. Chem. Soc., P e r k i n Trans. 2, 1990, 361. Chow and D.-C. Zhao, J. Chem. SOC., P e r k i n Trans. 2, 1989, 2023. Shustov, A. I. Prokbfyev, S. N. Denisenko, A. Yu. Shibayev, Y. V. Puzanov, and R. G. Kostyanovsky, J. Chem. SOC., P e r k i n Trans. 2, 1990, 141. 221 C. E. McDonald, T. R. Beebe, M. Beard, D. McMillen, and D. S e l s k i , Tetrahedron L e t t . , 1989, 30, 4791. 222 Z. L i u , Q. Chen, and Y . L i u , Huaxue Xueba?, 1989, 47, 609 (Chem. Abstr., 1990, u.2, 77563). 223 K. O r i t o , S. Satoh, and H. Suginome, 4. Chem. Soc., Chem. Cormnun., 1989, 1829. 224 H. Suginome, Y. Yamamoto, and K. O r i t o , J. Chem. SOC., P e r k i n Trans. 1, 1990, 1033. 225 H. Suginome, G, Satoh, J. B. Wane, S. Yamada, and K. Kobayashi, J. Chem. SOC., P e r k i n - T r - a n s d , 1990, 1239. 226 H. Suginome, S. Yamada, and J. B. Wane, J. Ore. Chem., 1990, 5 5 , 2170. 227 H. Suginome, J. B. Wane, and G. Satoh, J. Chem. Soc., P e r k i n Trans. 1, 1989, 1553.

Part III POLYMER PHOTOCHEM ISTRY By N. S. ALLEN and M. EDGE

Polymer Photochemistry

-

BY N.S. ALLEN AND M. EDGE

The format of this report is the same as that adopted in the previous one. In the field of photopolymerisation studies in micellar media appear to attract more interest whilst on the photoinitiator front new types of chromophores continue to be sought for more efficient cure as well as for specialised applications. In particular, non-extractable initiators due to their co-reaction with the monomer or prepolymer during cure are of major interest from a commercial point-of-view. The luminescence from polymer systems also continues to attract widespread interest with the emphasis on excited state properties, energy migration and molecular mobility. In the photodegradation of polymers the use of high energy laser ablation is growing in interest together with that of photodegradable polymers while in photostabilisation there is more emphasis on stabiliser performance, processing effects and interactions. On the dye and pigment side interests centre on improvements in stability and minimisation of photocatalytic activity.

2 PHO~pOJ~YNEUSUUN Once again numerous review articles in the field of radiation curing have appeared which reflects the strong academic and industrial interest in the subject. An ACS symposium book has appeared on the subject of radiation curing and encompasses many articles of interest which will be referred to throughout the

412

Photochemistry

following sectionsl. Two general topics include photocurable coatings and electron beam curing2r3 while more specifically depth of cure profiling4 and modification of the Kubelka-Munk expression5 have been carried out.

A

more comprehensive text has also appeared6

containing chapters on photoinitiators for free radical polymerisation 7 , cationic photoinitiators8, chemistry of photoresists9, photograftinglO, photochromic imagingll, four-centre type photopolymerisation12, use of lasers13 and the post-cured stability of acrylatesl4. Present trends in photopolymerisation have been reviewed15 with emphasis on the use of lasers while other articles by the same author concentrate on the use of lasers in studying water soluble photoinitiators16 and hologramsl7. The properties and preparation of photocurable monomers and prepolymers has always attracted interest and in this respect several reviews have appeared on various aspects of the subject. These include preparation and usesl8, oxygen inhibition19, general applications20, machine motionsa1, generation of acids22, metal protection23, resin types24, inks and composites25, f ~ r n i t u r e ~ ~ 1 2 ~ and leather2* coatings, structural designing29, paints30, pigmented coatings for wood31, dual ~ u r i n g 3 ~heat , resistant polyimides33 and marketing applications34. Other more specialised subjects include the crosslinking of adhesives35, limitations of curing36, antithrombic polymers37, photografting onto cellulose38, solid-state polyaddition39, topochemical polymerisation40,41, photodynamics of photocrosslinking42, charge-transfer

polymer is at ion^^^, polyfunctional polymers44, photoreactive p0lymers~~-4~, photocrosslinkable silicones49, photocationic initiators50, the influence of initiator absorbance on activity51 and microwave dielectrometry52.

HI: Polymer Photochemistry

. . .

413

. .

a

.

3.1 P h o t o l u L m t e d Addition Polvmerlsatlon Novel photoinitiators continue to attract much interest particularly with regard to faster cure. Diphenoxybenzophenone is one such initiator which has recently been found to effect the deep curing of multiacrylic systems53 while a novel polymeric hydroxyphenylalkylketone has been found to be superior to

the corresponding monomeric form in the curing of acrylates due to improved compatibility and lower volatility54. The excited state properties of the ketone chromophore were found to be unaffected by the polymer chain. Photochemical studies on a series of novel water soluble benzophenone photoinitiators indicates that their lowest excited triplet state is abstracting an electron from the tertiary amine co-synergist via a triplet exciplex followed by hydrogen atom abstraction55. In this work one of the photoinitiators, 4-methylsulphoniumbenzophenone sodium salt, exhibited a photoreduction quantum yield of 1.5 which was associated with secondary processes due to the reaction of the alkylamino radicals with a ground-state ketone molecule. The photoinitiator, 2-benzyl-2-dimethylamino-l-(4-morpholinophenyl)-butanone-l, has

been synthesised and shown to be highly effective for the photocuring of acrylates particularly pigmented systems56. As expected photochemical studies suggest that initiation occurs mainly via an a-cleavage reaction. Co-reactive photoinitiators have been prepared by the introduction of triethoxysilyl, epoxy and azo groups to the (2-hydroxy-2-propy1)phenone initiator units7. For example, the triethoxysilyl substituted initiators undergo a covalent coupling reaction on silica where upon a surface polymerisation becomes possible. The efficiency of the reaction

414

Photochemistry

with monomers has been found to be dependent on the intermolecular distance of the reactants and the process may be adapted for producing particulate materials for specialised applications. Laser flash photolysis studies on a,a-dimethylsulphonylacetophenones has identified benzoyl-2-propyl and benzene sulphonyl radicals formed through a &cleavage

process58 while the laser induced

polymerisation of multiacrylic monomers using 2,2-dimethoxy-2-phenylacetophenone

as a photoinitiator continued in

the dark for 90% of the total conversion process59.

Thioxanthone structures continue to attract interest and in this respect detailed photochemical studies on water soluble cationic structures has shown that their photochemical behaviour is similar to that of the unsubstituted parent compound60. Quenching effects by a tertiary amine co-synergist indicate that photoreduction occurs only in the triplet state with maximum efficiency being achieved by monosubstitution in position 3 of the thioxanthone chromophore, regardless of the nature of the water solubilising group. The introduction of methyl groups into other positions on the ring structure is shown to have a favourable effect on initiation efficiency. Photochemical studies on 2-acetoxythioxanthones have shown that the lower the radiationless conversion rate constant, the longer the triplet lifetime and the higher will be the photoreduction quantum yield and consequent photoinitiation activity of the initiators61. As found for the water soluble thioxanthones the introduction of electron donating groups enhances photoinitiation activity apart from the introduction of methyl groups in the a-position where the possibility of intramolecular hydrogen bonding reduces the rate.

III: Polymer Photochemistry

415

Polymer effects on the photoactivity of the thioxanthone chromophore have also been found to be negligible62 whereas in the anthraquinone induced photopolymerisation of methyl methacrylate the polarity of the solvent was found to markedly influence the rate of p ~ l y m e r i s a t i o n ~In ~ . this case the rate was directly proportional to the solvent polarity whereas the molecular weights were found to be inversely related.

The photophysical and photochemical properties of novel t-butylperester derivatives of fluorenone have been studied and found to be related to their behaviour on photoinduced polymeri~ation6~. The monoperesters were found to induce greater rates of photopolymerisation of methyl methacrylate than the bisperesters due to the much higher probability of self-termination in the latter case. In this work second derivative absorption spectroscopy and fluorescence analysis identified the presence of terminal fluorenone groups in the polymethylmethacrylate samples indicating the importance of benzoyloxy radicals in inducing polymerisation. It was interesting to note that the presence of a tertiary amine co-synergist prevented termination by the initiator fragments as shown in scheme 1. This confirmed the involvement of alkylamino radicals in inducing the polymerisation of the monomer. On laser flash photolysis of the same initiators triplet yields in acetonitrile were found to correlate with photopolymerisation activities65. However, in oxygenated solution a new long-lived transient absorption was observed which was tentatively assigned to decarboxylation followed by reaction of the aryl radicals with oxygen to give arylperoxy radicals (see scheme 2). In the laser induced polymerisation of methyl methacrylate using

Photochemistry

416

*A*

+

NR3

(A*) Bu'O

NR2R* Buo*

+

NR3

__c

I1-XMi*

+

*AH ( AH

NR,R*

-

Termination

1

ButOH

+

NR2R*

Termination

*AH

Where A XMi* NR,

= Benzophenone M

= =

9 - Fluorenone Any growing polymer radical Tertiary Amine

Scheme 1

derivatives

Ill: Polymer Photochemistry

417

0

COOBU'

II 0

0

0 Scheme 2

Photochemistv

418

2,2-dimethoxy-2-phenylacetophenone

as photoinitiator, an increase

in the laser repetition rate resulted in a decrease in both the polymer yield and molecular weight66. This was associated with the formation of small molecular radicals quenching living polymer radicals which in turn were directly related to the initiator concentration.

Mixtures of photoinitiators studied. Michler's

have

been

actively

Ketone and benzoyl peroxide have been shown to

effectively induce the photopolymerisation of methyl methacrylate through the formation of an initial complex shown in scheme 367. Although the exact initiating radical does not appear to be ascertained it is almost certainly the arylalkylamino radical from the Michler's

Ketone. In the interaction of benzil and thioxanthone

with triethylamine in the photoinduced polymerisation of acrylic monomers their is a competition between reverse electron transfer and ketyl radical formation68. As the carbonyl concentration increases the bimolecular termination rates due to radical recombination increases. The same workers also studied the same system but replaced the ketone initiators with pyrene69. Their inability to identify pyrene end groups indicated that the active initiating species arise from a complex between the pyrene and the triethylamine.

Copolymers of styrene with acetophenone oxime acrylate were produced using benzophenone as a sensitiser70. The total number of amino groups was found to be unaffected by the nature of the atmosphere. Organic dyes have been found to sensitise the photopolymerisation of a polyimide using the tetra-t-butylperester

III: Polymer Photochemistry

4 19

C6H5-C,

0 i

Michler's Ketone (MK)

c,H5-C\\

P 0

Kll

complexat ion

Initiating complex

(1)

Scheme 3

Photochemistry

420

derivative of benzophenone as the initiator71 while derivatives of fluorescein have been found to be effective photoinitiators for acrylic monomers72. Redox systems of phenazine and acridine with dimethylaniline were found to significantly enhance the rate of photopolymerisation of butyl methacrylate through the formation of a triplet e ~ c i p l e x ~In ~ .acetonitrile radical ions produced from the dissociation of the complex were believed to be responsible for inducing polymerisation whereas in benzene only free radical initiation was involved. Maleic anhydride has been found to copolymerise with phenyl glycidyl ether in the presence of potassium bromide due to the formation of a charge-transfer complex74 while the p-t-butylbenzoyl and benzoyl substituted monooxime ester derivatives have been found to be more effective initiators than the corresponding acetyl, methoxybenzoyl and dimethoxybenzoyl ester

derivative^^^.

N-phenylglycine with dyes and

diphenyliodonium chloride has been found to be highly synergistic for initiating uv curing76 as has a three component initiator system consisting of a bromine compound, benzil and N-methyldiethanolamine77.

In the photopolymerisation of methyl methacrylate induced by mixtures of chloroacetic acid derivatives and dimethylaniline the kinetics were found to be influenced by the degree of substitution of the chlorine atoms while the initiation occurred through a complex78. Ethyl aluminium sesquichloride has been found to induce the photocopolymerisation of 9-vinylanthracene with methyl methacrylate at -200C79 while photochromic polymers have been made by reacting u-bromoazobenzene with ethylene gas8O. Surface active N-cetylpyridinium bromide and chloride have been found to

111: Polymer Photochemistry

42 1

induce the emulsion photopolymerisation of methyl methacrylate in carbon tetrachloride81 while the photoinduced polymerisation of N-vinylpyrrolidone has been found to be inhibited by oxygen82 and benzil and phenylpropanedione monoxime esters photoinduce the polymerisation of monomers via the lowest excited singlet state83. The kinetics of the photopolymerisation of quaternary diallyl ammonium salts have been found to be first order in initiator concentration after compensating for viscosity effects84 while using dilatometry the rate of photopolymerisation of acrylamide induced by a dye:triethylamine complex is also first order with initiator concentration with termination occurring by degradative chain transferB5. In the latter case however, the molecular weight of the polymer was found to be independent of the rate of polymerisation and the initiation process whereas ellipsometric measurements have shown that collapse areas of monolayers prepared negatively charged surfactants such as b~s[2-(palmitoyloxy)ethyl]methyl(p-vinylbenzyl)a~on~umchloride

decreased during polymerisation86 while the photoinduced polymerisation of copolyimides by Michler's

ketone has been found to

exhibit both an autocatalytic and catalytic competitive reaction steps87. Radiotracing has been used to measure copolymer ratios for the benzil induced photopolymerisation of styrene with diethyl fumarateB8.

Polymers with functionalised end-groups have been obtained in the photopolymerisation of styrene induced by N,N-diethyldithioarninoformate as an iniferter89 while the photopolymerisation of methyl methacrylate induced by mercaptobenzothiazole in benzene exhibited kinetics which were

422

Photochemistry

consistent with those of an equilibrium complex with very high efficiencygo. The reaction of sodium 3-vinylphenylazosulphonate with acrylamide and methyl methacrylate gave highly water soluble photoreactive copolymers91 induced by an isomerisation process. Novel addition polymers have also been synthesised by irradiating 2,5-thiophenedithiol to 2,5-diethynylthiophene92. These polymers are insoluble in all conventional organic solvents and found to be 85% cis isomer as well as highly conductive. An applied magnetic field has been found to increase the rate of photopolymerisation of acrylamide induced by methylene blueg3. The magnetic field appears to increase the yield of semiquinone radicals which act as the initiator of free radical polymerisation. Some interesting work on the laser induced polymerisation of acrylic monomers is being carried out recentlyg*,g5. In single pulsed mode using 2,2-dimethoxy-2-phenylacetophenone

as photoinitiator polymers of

methyl methacrylate are produced with an extremely broad molecular weight distribution. An increase in the concentration of photoinitiator reduces the concentration of high molecular weight species while using a double pulsed mode produced polymers with a complex molecular weight distribution. Copolymers of p-(chloromethyl)styrene-4-vinylbenzophenone have been prepared and

quaternised with tertiary phosphines and found to be good photosensitisersg6 as have polymers containing pendant propargylg7 and 4-benzoyl-3-hydroxyphenoxy 98 moieties. Irradiation of polyamides containing thymine units gave photodimers with cis-anti and U - s y n arrangementsg9 while ortho-nitrobenzylester groups on flash photolysis give nitronic acid as an intermediate via a fast singlet route and slow triplet routeloo.

423

Ill: Polymer Photochemistry

In the photoinitiated polymerisation of _N-vinylpyrrolidinone and N-vinylcaprolactam in dioxane and ethanol,

the

rate was

higher in the latter solvent and monomerlol. This was attributed to the influence of the two additional methylene groups in the caprolactam ring which increases monomer reactivity. Other interesting effects have included the radiation dose on the photopolymerisation of diallyl oxydiethylene dicarbonatel02. Here long lived radicals were produced which continue to react in

the dark. The rate

appears

to

fit a

relaxation model that considers double bonds as traps with increasing lifetimes that are able to transfer to radical sites. The photoinduced polymerisation of methacrylic acid by sodium peroxide has been determined

to have an activation energy of 17.7

kJ/molel03. From this the macroradical lifetimes were determined to be 3.9 secs. In the intermittent photoinitiation of methyl methacrylate-styrene the dependence of the rate of propagation on monomer feed composition was understood in terms of a penultimate model and not a terminal model as previously assurnedlO4 with some evidence for cross-termination. An eosin-periodate combination has been found to induce the photopolymerisation of acrylonitrile where the dye was found to act as both the sensitiser and reducing agentlo5. Hydroxyl radicals are assumed to initiate polymerisation while the N-bromosuccinimide induced photopolymerisation of N,"-methylenebisacrylamide

is faster in 2-propanol solutionl06.

For another initiator combination involving Auramine 0 and benzoyl peroxide the rate of photopolymerisation of methyl methacrylate was found to be first-order in monomer concentration and 0.25 order in initiator concentrationlo7. The latter was associated with degradative chain transfer reactions. Photoreactive thin films of

Phorochemistry

424

amphiphilic diolefins have been prepared by the Langmuir-Blodgett techniquelo8. Analysis of the monolayers indicated that the molecules are arranged side by side with the stacking of aromatic rings and that on irradiation cyclobutane rings are formed initially followed eventually by polymer formation.

Thin films of

formaldehyde are apparently photopolymerised at 77 K using 2-nitrophenol as an initiator although the molecular weight is considered to be very 10w109. The typical benzophenone-amine induced photopolymerisation of butyl methacrylate has been found to decrease with increasing solvent polarity due to deactivation of the lowest excited triplet-state of the benzophenonellO. In the photoinduced polymerisation of methyl methacrylate by A-methylanthraquinone the rate was found to be dependent on the ratio of 2-propanol to hydrocarbon solvent system usedlll. Thus, lower concentrations of 2-propanol were used in the order dodecane > decane > octane > heptane > hexane. For solvents containing

unsaturation the effect of 2-propanol was absent. The effect was accounted for the basis of the ease of photoreduction of the A-methylanthraquinone in the mixed solvent systems.

The photoinduced polymerisation of vinyl acetate by laser irradiation is associated with the decomposition of oligoperoxides present in the monomer112 while highly photosensitive copolymers have been prepared through the copolymerisation of methyl methacrylate with 1,2,3,4-tetrahydro-l-naphthylideneimino-~-sty~enesulphonate and

become alkali soluble after irradiationll3. The copolymerisation of N-vinylcarbazole with N,N-diethylaminoethyl methacrylate is retarded by oxygen but accelerated when the second coreactant was

111: Polymer Phorochemisrry

425

in excess114 while new non-yellowing photoinitiators based on a-hydroxy-[4-(methylvinyl)]isobutyrophenones have been

developedll5. Polymer molecular weights have been controlled in the photoinduced polymerisation of styrene and methyl methacrylate using dithiocarbamates as inifertersll6. Polymer yields and molecular weights increased significantly with reaction time via a living radical mechanism. The photoinduced polymerisation of acrylonitrile with styrene using azobisisobutyronitrile as initiator and imidazolium-p-chlorophenacylide as inhibitor was found to be directly proportional to the concentration of the acrylonitrile and the initiator117. The inhibitor decreased the rate of initiation but not that of the propagation or termination. ESR has shown that the photoinduced polymerisation of maleimide

occurs via a free radical mechanism while that of N-ethylmaleimide occurs via an anionic mechanismll8. Photopolymerisation rate was also found to be enhanced in hydrogen atom donating solvents. Copolymers of styrene and methyl methacrylate with acetophenone oxime acrylate were prepared by photoinduced polymerisation using benzophenone as photoinitiatorllg. $mino group formation in the copolymers was found to be higher in the former case and significantly enhanced the acid dye affinity of the polymer. Silicone acrylates have been found to photopolymerise more rapidly with increasing concentration of acryloxy groups120 while vinyl decanoate has been found to photopolymerise without a photoinitiatorl21.

Photoinduced electron transfer between anthracene and benzyltriphenylphosphonium tetrafluoroborate has been found to induce the photopolymerisation of methyl methacrylate122 while in

426

Photochemistty

the gas phase isoprene has been found to photopolymerise through its decomposition productsl23. Polyvinyl pivalate has been found to undergo an emulsion polymerisation124 whereas ethylene and acrylonitrile at 275oC will copolymerise using an excimer laser at 248 nm125 with the latter monomer reducing the quantum at higher concentrations. Carbon tetrachloride has been found to accelerate the rate of photopolymerisation of methyl methacrylate with l-benzyl-1,4-nicotinamide126.

Here hydrogen atom transfer between

the latter excited nicotinamide and the carbon tetrachloride is believed to occur where the nicotinamide radical then reacts with the solvent to produce a chloride salt and a corresponding trichloromethylene radical which acts as the initiator. The rate constants for photoinduced polymerisation of methyl methacrylate have been determined using 365 and 313 nm wa~elengthsl2~. In the former case the rate constants were significantly greater than those obtained thermally but not at the latter wavelength. Although no explanation was given partial reabsorption of the light at 313 nm by the monomer itself may be responsible for the lower efficiency. Microwave dielectrometry has been used to monitor the kinetics of photopolymerisation of butyl acrylatel28 as has the use of an intermittent light s0urce12~. Cyclohexene and maleic anhydride form a charge-transfer complex which on irradiation undergoes hydrogen atom abstraction and polymeri~ationl~~ while aniline and pyrrole will initiate the photopolymerisation of acrylonitrile through a charge-transfer complex with tetracyanoethylenel31. In this case the initiation rate depends upon the twist angle between the n orbital of the nitrogen atom and the p orbital of the benzene or pyrrole rings and how thisinfluences their ability to form a charge-transfer complex. Sulphur containing

III: Polymer Photochemistry

427

azo initiators have been shown to give free radicals on irradiation which are equally capable of inducing the polymerisation of acrylic monomers UI azobi~isobutyronitrile~~2.

Some interest continues to be shown in the use of micelles and bilayers for photopolymerisation. In two studies it was demonstrated that extremely high molecular weight polymers can be produced under relatively mild conditions in a bilayer state by fixating the initiator (1) at the most effective site for initiation (see Figure 1)133 I 134. A similar situation was observed for a chain transfer reaction at a bilayer surface. The chain transfer agent (3) was found to accurately control the molecular weight of the polymers produced from (2) whereas using the chain transfer agent (4) without a long alkyl group gave no control. It was also found that phase separation of binary layers composed of a nonpolymerisable fluorocarbon component (5) and a polymerisable hydrocarbon component is useful in controlling molecular weight. The molecular weights of polyacrylamide produced in reversed micelles have been found to be independent of the rates of polymerisation and initiation due to monoradical termination involving degradative chain transfe.135.

On the otherhand the

photopolymerisation of

N,N-d~dodecyl-N-methyl-N-[2-(methacryloyloxy)ethyl]a~on~um chloride in reverse micelles gave polymers whose molecular weights decrease with increasing initiator concentrationl36. The photopolymerisation of

b~s[2-(n-hexadecanoyloxy)ethyl]methyl(4-v~n~lbenzyl)a~on~um chloride and dioctadecyldimethylamonium bromide occurs in conditions of non-ideal miscibility and shows a surface

Photochemistry

t I

Photoini tiator transter agent Photoini t iator

non - polyrnerirable fluorocarbon domain

x

CH3+CH2,0!

NHC-CH2-SC-OEt

II S

CH3fCH2kOC I1

0

(1)

Polymer izable Amphiphile

8

0

II + NHC-CH,NMe,

CH3+CH2k OC CH3fCH&-Oi&

0,s

0 (2) Molecular

- weight

CH,+CH,kOC

Controller

8 J”C4;Br f.l Yr l2

CH,-fCH,)i;;OC

Br I

)- 0: -7-Bf 0 Br

II

0 (3)

(4)

F i g u r e 1. Schematic illustration ot the novel photopolymerization bilayer system and structures of the photoinitiator, polymerizable amphiphile .and molecular weight controllers

III: Polymer Photochemistry

429

a ~ e o t r o p e lwhereas ~~ the photopolymerisation of methyl methacrylate initiated by (4-benzoylbenzyl)dinethyldodecylammonium bromide and triethylamine was found to be dependent upon the concentration of the triethylamine and the light intensity138. The rate of photopolymerisation of acrylamide induced by 2,3-butanedione increased markedly in the presence of a surfactant compared with normal aqueous medial39. The nature of the hydrophilic group on the surfactant played an important role in controlling the rate which was in turn determined by the nature of the electrostatic charges on the micellar surface.

Photoinduced cationic and anionic polymerisations continue to attract some interest. In this respect simple alkyl iodides have been shown to be effective photoinitiators of isobutyl vinyl ether in the presence of tetrabutylammonium salts of hexafluoroarsenatel40. Sulphonium hexafluorophosphate salts having phenyl or benzyl substituents initiate the photopolymerisation of tetrahydrofuran141 while various diphenylhalonium salts gave 2-, 3-, and 4-halobiphenyls by an in-cage fragmentation recombination reaction on irradiationl42. Apparently, the iodonium salts gave more in-cage products and favoured homolytic cleavage more than the other halonium salts. N-alkylphenyl and N-alkoxyphenyl maleimides give polymers on irradiation by an anionic mechanisml43. On free radical polymerisation they give only low molecular weight oligomers. Triphenyl sulphonium hexafluorophosphate initiates the photopolymerisation of tetrahydrofuran in two stages involving firstly photolysis followed by catalytic ring 0peningl4~. The latter stage is a dark reaction and markedly controls the kinetics of the polymerisation. In contrast, p-acetylbenzylidene

430

Photochemistry

triphenylarsoniumylide is claimed to induce the photopolymerisation of styrene by a free radical mechanism with bimolecular terminationl45. Analysis of the photodecomposition products revealed that the phenyl radical generated from the bond scission between the arsenic atom and the phenyl group participated in the initiation of the polymerisation.

Photoinitiated polymerisation involving metal ion species also continues to attract interest. Ferric chloride acted as a catalyst in the photoinduced polymerisation of methyl methacrylate by sinigrinl46 while iron (11) porphyrin compounds have been found to photopolymerise in aqueous ethanol by second order kinetics which involves a five membered protoporphyrin pyrazine type intermediate147. Hydrogen chromate ions and polyacrylamide undergo a photoredox process to give polymer and chromium (111) ions148 whereas vanadium (V) oxotris(N,N-diethyldithiocarbamate) induces the thermal and photopolymerisation of methyl methacrylate through scission of the dithio bond and a consequent reduction of the vanadium (V) to vanadium (1~)149.Subsequent analysis of the polymer showed the presence of sulphur in the polymer due to chain transfer to the initiator. On the otherhand hexacarbonylmolybdenum and tungsten compounds have been found to induce the photopolymerisation of aromatic alkynes through metal carbene speciesl50. Ammonium dichromate has been found to induce the photopolymerisation of acrylonitrile but only in the pH range 5.5-1.5151 while tungsten hexacarbonyl induces the photopolymerisation of phenylacetylene the kinetics of which indicate that the propagating centres are conserved to high molecular weight con~ersions~~2. Analysis of the polymer indicated

III: Polymer Photochemistry

43 1

a coil like structure with a limited steric impurity. The UV induced polymerisation of cyclohexene oxide in the presence of titanium alkoxides gave only polymer while the thermal polymerisation gave polymer plus ~ - 1 , 2 - ~ y c l o h e x a n e d i o 1 ~In 5~. the presence of titanium dioxide however, W induced polymerisation gave only the diol while thermal polymerisation gave both polymer and the diol. The results were accounted for on the basis that while the alkoxides behaved as anionic initiators the oxide behaved as a radical initiator. Other studies of interest include the photopolymerisation of pyrrole by tris(2,2-bipyridine)ruthenium (II)154 and acrylamide by a methylene blue-trisacetylacetonate manganese (1111 systeml55.

The photopolymerisation of crystals also continues to attract interest. Polyacene was prepared by the Langmuir-Blodgett method using 22,24-pentacosadiynoic acid as the monomer for single layersl56. The solid state photoinduced polymerisation of 1,3,5-trithiane has a higher activation energy than that of the chemically initiated process157 while the laser induced polymerisation of functional derivatives of acetylene occurs via a cationic charged triple bond in the presence of an electron accept0rl5~.The liquid crystalline system 4-biphenylyl 4-(6-acryloyloxy-hexyloxy)benzoate will photopolymerise to give a

liquid crystalline orientated side chain polymer159,160. The phase of the monomer markedly influences the molecular ordering of the polymer. On the otherhand a cholesterol containing monotropic liquid crystalline acrylate monomer has been found to exist in two distinct crystalline states which undergo different processes during polymerisationl61. The photoinduced polymerisation of

Photochemistry

432

1,4-benzenedithiol occurs via a stepwise process to give a mainly cis product162 while the solid state photoinduced polymerisation of N-vinylcarbazole has been found to occur through a radical mechanism163. Finally, the anionic photoinduced polymerisation of acrylonitrile in the presence of N,N-dimethyl-p-toluidine

has been

found to occur at the surface of loosely associated monomer clusters close to the initiator molecules with glass formation influencing the rate of chain formation164. Propargyl halides at 77K have been found to undergo radical induced photopolymerisation

to give semi-conductive acetylenic polymersld5 and 4-[2-(2-pyrazyl)ethenyl]styrene crystals undergo topochemical

photoreactions to produce high molecular weight linear polymers containing a series of cyclobutane rings166.

2.2 Phatoaraftinq The process of photografting continues to attract some interest with a view to the improvement of properties. Multifunctional acrylates such as trimethylolpropane triaczylate have been found to accelerate the photografting of styrene onto the surface of polyethylene and are superior to the effect of organic a~idsl67,16~. Polypropylene, polyethylene and polystyrene have been photografted with glycidyl methacrylate followed by reaction with various light stabiliser molecules and their efficiency s t ~ d i e d ~ ~ It~was t ~ interesting ~ ~ . to note that whilst a grafted hindered piperidine molecule stabilised the polypropylene the other two polymers were unaffected. On the otherhand a 2-hydroxybenzophenone absorber stabilised polystyrene but not the two polyolefins. Polyethylene has also been thermally stabilised by photografting with N-(4-arninophenyl)metha~rylamide~~1.

III: Polymer Photochemistry

433

Dimethylformamide has been found to accelerate the photografting of methyl methacrylate onto chitin172 while the grafting of acrylamide onto cellulose acetate gave membranes with a high salt rejection173. Polyether-polyurethanes have been successfully photografted with N-vinyl-2-pyrrolidone in basic mediaI74 and styrene has been grafted onto rubber using benzophenone as ph~toinitiatorl~~ Methyl . methacrylate copolymers have been prepared with oxime groups capable of being photografted with acrylonitrilel76 and co-reactive photoinitiators have been immobilised onto c e l l ~ l o s e l ~ ~ .

2.3 Photocrossl inkinq The proceedings of the American Chemical Society congress in Dallas in April 1989 presents some new information on photocrosslinkable polymers. Polyolefins and ethylene-propylene-diene terpolymers may be photocrosslinked with conventional photoinitiators to give high gel ~ontents178,1~9,180. The use of polymer bound amine synergists for the UV curing of non-acrylate unsaturated resins has been utilised to effect the crosslinking of organic coatings through a hydrogen abstracting photoinitiator such as benzophenone181,182. This could be useful in terms of the toxicity problems associated with acrylates. A series of norbornene functional prepolymers have been prepared via a cycloaddition reaction of cyclopentadiene monomer with various acrylics183r184. These systems photocrosslink rapidly to give very tough plastics although their properties may be changed through the incorporation of polydimethylsiloxane precursors. A series of acrylated melamines have been prepared which offer the potential of being able to be cured through both free radical and condensation polymerisation p r o c e s ~ e s 1 ~ 5 twhile ~8~

434

Phorochemisrry

a series of linear polymers of propargyl ethers of bisphenols have been developed for corrosion protection187. Coatings containing chlorendic anhydride will undergo photocrosslinking without a photoinitiator presumably due to the formation of active chlorine radicals188 whereas the photocyclisation of Q-benzoyloxyphenyl ketone has been found to provide an efficient route to the synthesis of 2,3-diphenylbenzofurans which undergo photocrosslinking to give highly conjugated p-phenylene bisbenzofuransl89.

In order to improve performance and processing efficiency of radiation curable systems it has been proposed that curing should be monitored using a technique of mechanical s p e c t r o s ~ o p y ~ ~ 0 ~ ~ ~ 1 and in relation to this other workers have found that the molecular weight and structure of the diol component of multifunctional acrylates influenced the mechanical properties of the W cured films p r o d u ~ e d ~ ~The ~ tpresence ~ ~ ~ . of a urea group in the resin tended to enhance the modulus of the cured film whereas a vinyl caprolactam diluent reduced resin viscosity for ease of coating. The properties of UV curable urethane acrylates have been discussedlg4 while acrylated epoxy systems have been found to undergo significant changes in thermomechanical properties due to structural relaxation processes in the cured films195t196. Apparently there is a degree of incompatibility between the acrylic and epoxy components of the cured systems which can influence subsequent film performance on aging. Microencapsulated imaging (Cylithography) is an important growth field and here photocuring of such systems has been monitored using phase transformation diagramslg7. A novel technique has been developed involving the use

Ill: Polymer Photochemistty

435

of highly constrained microporous supports which simulate the

imaging mechanism in cylithography while at the same time avoiding the time consuming encapsulation step1g8- Vinyl ether functionalised urethane oligomers have been synthesised and cured using a cationic triaryl sulphonium salt initiatorlgg. W curable siloxanes are currently of major importance and in this respect linear siloxane monomers terminated with cyclohexene oxide groups cure much faster in the presence of a cationic photoinitiator than organic diepoxides under the same conditions200,201. Epoxy functional silicone monomers with both star and branches structures have been prepared and W cured using a cationic initiat0r~~2n203 while phenyl containing siloxane acrylates have been developed with good hydrolytic stability although they cured more slowly than their dimethoxy analogues204. High performance epoxy resins have been developed for the electronics industry which cure to depths of millimeters using a cationic phot0initiator2~5.

Benzoin tosylate significantly improved the latency and crosslinking efficiency of acrylic melamine formulations compared with that of p-toluenesulphonic acid salts206 and thiosulphate functionalised zwitterionic polymers have been prepared that photocrosslink to give a strongly hydrophobic network207. The photocuring of 1,6-hexanediol dimethacrylate has been found to have much higher photosensitivity than the corresponding diacrylate due to its insensitivity to oxygen208 and similarly acrylic polyurethanes containing polyether linkages cure faster than polyester based systems209. Excimer lasers have been found to be effective for the curing of coatings in electronics210. Polyimide precursors have been synthesised and their properties discussed21l

436

Photochemistry

while a laser flash photolysis study has shown that in the photocuring of polyimides hydrogen atom abstraction by the photoexcited triplet state of benzophenone is the key step in the mechanism212.

Peroxypolytungstic acid produces peroxo groups on irradiation and oxygen to give an insoluble film that is useful for

photo resist^^^^

while a magnetic field has been found to enhance the photocrosslinking of an azidomethylated polystyrene using thioxanthones as photoinitiators.214. The adhesion of radiation cured acrylates to polymer substrates has been investigated in depth and found to be dependent not only upon the surface tension properties of the two substrates but also the moisture regain and structure of the polymer substrate2I5. Nylon polymer for example, gives poor adhesion due to its high moisture regain whereas polymer substrates with aromatic groups extends the curing time due to some degree of self absorption. However, slower curing is preferred in order to allow the polymer coating to undergo relaxation processes and take up a suitable orientation on the surface of the substrate. It has been found that when the glass transition of a completely polymerised resin is higher than that of the polymerisation temperature the reaction usually stops before the polymerisation is comp1ete2l6. Thus, 1,6-hexanediol diacrylate continues to polymerise in the glassy state at a much slower rate due to the freezing of secondary relaxations in the polymer network.

The photocrosslinking of polyethylene has attracted some considerable interest. The properties of high density polyethylene crosslinked with triallyl cyanurate have been examined217~218while

Ill: Polymer Photochemistry

437

the photocrosslinking rate of low density polyethylene has been observed to increase at temperatures close to its melting point2lg. The photochemical crosslinking of polyethylene has been found to influence the subsequent thermal stability of the polymer which in turn is dependent upon the structure of the photoinitiator220. In this regard 2-chloroanthraquinone and benzophenone enhanced the thermal stability of the polymer due to the antioxidant behaviour of their photoconversion products. Both the structure of the monomer and the multifunctional acrylate has a marked influence on the photocrosslinking of polyethylene with xanthene and triallyl cyanurate being the most efficientz2l. Ethylene-vinyl acetate copolymer has been found to photocrosslink rapidly in the presence of 2-ethylanthraq~inone~2~ while UV curable epoxy acrylate oligomers have been used as effective primers for the application of polyethylene to pipes223-225.

The photocrosslinking of polymers containing cinnamoyl groups continues to attract much interest. Polycinnamides have been prepared through a palladium catalysed reaction of bisacrylamides with aromatic diiodides226 while polyvinyl cinnamoyl acetate has been prepared and a relation established between the number of cinnamoylacetoxy groups and the photosensitivity of the p 0 l y m e r ~ ~ ~ t 2 Comlike 2~. acrylic polymers containing side chajn cinnamoyl groups have been synthesised and their thermal properties ~ t u d i e d 2 ~while 9 liquid crystalline polymers containing main chain cinnamoyl groups underwent competitive crosslinking and a photo-Fries rearrangement230. Copolymers containing cinnamoyl groups however, have been found to undergo competitive intra- and intermolecular reactions depending upon the concentration of

438

Photochemi s t y

cinnamoyl groups in the polymer chain231n232. As expected dilute solutions of the copolymers undergo primarily intramolecular reactions whereas an increase in the concentration of cinnamoyl groups enhances intermolecular reactions. Blends of acceptor and donor substituted polyvinyl alcohol cinnamates have been observed to undergo rapid photocrosslinking when compared to the individual polymers233. Here intermolecular interactions dominated the initiation process. Cinnamate epoxy phenolic resins have also been investigated by thermal methods and found to undergo rapid crosslinking234.

Liquid crystalline polysiloxane elastomers have been prepared in a two step process by reacting polymethylsiloxane with phenyl cinnamate containing mesogens having terminal vinyl gr0ups2~~. These polymers undergo rapid crosslinking and exhibited only a smectic phase irrespective of the spacer and the terminal tail. Highly crosslinkable linear polysiloxanes have been prepared by the thermal depolymerisation of linear methyl and hydrogen siloxanes in the presence of acidic zeolite catalysts at 500-600oC236. Methacrylate siloxane polymers have been found to be more highly photoresponsive than conventional room temperature curing sil0xanes2~~ as have acrylic-epoxy s i l ~ x a n e s ~Using ~ ~ . dielectric loss and glass transition temperatures branched structures have been found to be present in photocrosslinked copolymers of adipic acid-diethylene glycol-ethylene glycol-maleic anhydride-styrene239 and polysulphones undergo marked photocrosslinking on irradiati0n2~0.

Copolymers of methyl methacrylate with an anthracene derivative

111: Polymer Photochemistry

439

have been found to photocrosslink r e a d i l ~ ~ ~Here 1 . the extent and nature of the crosslinking reaction is dependent upon the 9 and 10 substituents in the anthraquinone chroinophore which will influence the formation of endoperoxide formation. The vulcanisation efficiency of styrene-butadiene copolymer has been found to depend upon the initiator structure and follows the order deoxybenzoin > benzophenone > dibenzyl ketone > p-methylben~ophenone~~2. An applied magnetic field also accelerates the rate. Alicyclic interpenetrating polymer networks have been prepared by the copolymerisation of 3,4-epoxycyclohexy-3,4-epoxycyclohexanecarboxylate

with acrylic

monomers using cationic photoinitiators243. Copolymerisation yielded opaque polymers whereas grafting yielded transparent materials. In the former case incompatibility appears to be a major problem. Excimer laser studies on the rate of photopolymerisation of pigmented epoxy acrylates has been found to depend upon the square root of the energy of the p ~ l s e s 2 ~ 4On . the otherhand the photocationic polymerisation of iso-butyl vinyl ether in the presence of a butyl iodide catalyst has been found to be greatly affected by the addition of small amounts of quaternary ammonium salts245. In this respect the polymerisation rate increased with increasing size of the anion although for some unknown reason in the presence of two ammonium salts the rate was determined by the smaller anion. Crosslinked and non-crosslinked latex polymers have been obtained by the photopolymerisation of microlatexes of acrylic monomers in micellar systems246 while the laser thermolysis of 4-silaspiro[3,4]octane gives an organosilicon polymer produced from the formation of l-methylene-l-silacopentane by the elimination of ethylene from the parent m0lecule24~. A new type of photopolymer

Photochemistry

440

has been prepared from the linear addition of piperazine with diepoxy compounds248. Once again the absorption maxima of a series of commercial photoinitiators have been correlated with their photocuring efficiency in a urethane acrylate oligomer249. In the same study the diisocyanate backbone structure was an important factor in cure rate with high efficiencies being observed in the case of toluene diisocyanate, isophorone diisocyanate and hydrogenated methylene-4,4'-bisphenylene diisocyanate. In the case of cationic curing technology hydroxy containing vinyl resins have been found to constitute an attractive class of modifiers for improving film properties in terms of flexibility and hardnessz50. Remaining with the same technology cationic initiators with electron donor substituents have been found to be highly effective especially in the presence of a~etonitrile25~. A photopatternable epoxy phenyl methyl silicone has been described for use in integrated c i r ~ u i t s ~while 5~ for the W curing of inks the efficiency appeared to be a function of both the ink formulation and thickness of the coating253.

UV

curable urethane methacrylate oligomers have been prepared and

found to be highly effective254 while adipic acid modified epoxy acrylates have been developed for electrically insulating paint films255 as have water soluble coatings based on acrylic acid and a-methyl styrene for walls256 and polymers with pendent benzalacetophenone groups257. The photochemically catalysed cationic bulk polymerisation of aliphatic and cycloaliphatic epoxy resins has been found to depend upon the amount and type of epoxy group as well as the density of the polymer network258. In the case

of silicone polymers photodimerisable carboxylic acids have been

111: Polymer Photochemisty

441

bonded on to them through an amine catalysed esterification of pendent epoxypropyl groups259. Several poly a,w-alkadiyne monomers have been prepared and characterised which on irradiation undergo partial crosslinking to give a broad distribution of conjugated lengths which were restricted by the crystallite size and defects in the polymers260. Crosslinked films for dialysis membranes have been prepared by reacting 2-hydroxyethyl methacrylate with polyvinylpyrrolidone in the presence of benzoyl peroxide261 while pyridinium nitrite crosslinks have been identified in the photoquaternisation of a p-(nitrophenoxy)methylstyrene-styrene-4-v~nylpyrid~ne copolymer262. A novel two dimensional photopolymerisation at a highly orientated

bilayer surface has been studied263. The photopolymerisable amphiphile contained styrene sulphonate as a counterion and photoinitiators consisting of two long alkyl chains and a xanthate group which were coupled through spacers with different chain lengths. The spacer length controlled the photoreaction in a bilayer state. A benzoin terminated polypropylene glycol-2,4-toluene diisocyante oligomer polymerised vinyl acetate to give a linear block copolymer in the early stages of the reaction264. Electron spin resonance has been used to probe trapped radicals in photopolymerised dimethacrylates265. In the case of polyacrylates the propagation radicals resulted from a C-H cleavage with the highest conformational strain energy whereas for dimethacrylates the radicals produced were those of least conformational strain energy. An acryloyl chloropropanol has been used as diluent for epoxy acrylate resins and found to enhance the tensile properties of the cured coating266.

442

Photochemistry

An addition reaction of resorcin with conjugated double bonds of

tung oil gave a photocrosslinkable product in which two resorcin molecules are introduced into each eleostearic acid unit267 while diaminodiphenylmethane has been found to accelerate the W curing of an epoxy resin268. The diacrylate 1,4-phenylene bis[4-[6-(acryloyloxy)hexyloxy]benzene] exhibits mesomorphism with

a nematic phase between 108 and 155OC and a smectic phase below 88oC269. In the first phase the monomer is uniaxially orientated and photocrosslinking proceeds rapidly while in the second phase at lower temperatures the crosslinking reaction is isotropic. Transient structures in the phase separation of polystyrene/poly(2-chlorostyrene)

have been immobilised through a

photocrosslinking reaction involving the photodimerisation of anthracene labels270. The morphology of the blends showed immobilised spinodal like structures. Metal dimethyldithiocarbamates have been found to catalyse the

photocrosslinking of styrene-butadiene rubber containing anthracene as initiat0r~~1. The effect varied with the type of central metal atom present. The photocrosslinking of polyvinyl alcohol containing pendant 2,2'-iminostilbene groups has been found to achieve a maximum which was controlled by the micromotion of the polymer272 while the photocrosslinking of 1,3-bis(octadeca-trans-2-~-4-dienoyl)-~-glycero-2-phosphoric

acid is dependent upon pH273. Thermal methods have been used to optimise the photocuring of methacrylate monomers274 while polybutadienes have been esterified with 2,3-epoxypropyl methacrylate to give highly photocrosslinkable systems275. New visible light curing agents have been developed for filament winding276 as have autocurable benzophenonetetracarboxylic

Ill: Polymer Photochemistry

443

dianhydride based polyurethane met ha cry late^^^^. Other studies of interest include the photocuring of coloured glass l a m i n a t e ~ ~ ~ 8 , and the cationic curing of epoxy novolaks containing a caprolactone based p o l y 0 1 ~ ~ ~ .

IES OF POTBY-

Several specialist reviews of interest have appeared within the last twelve month period. These include the

&-trans

isomerisation

of azobenzene containing polymers280, polymer dynamics281, photochromism specific to spiropyran polymers282, photosensitive polymer gels.283, excimer formation284, general applications285, thermal and 0xyluminescence~~6,287, polymers of

(+)

and

(-)

catechin288 and the luminescence of p o l y ~ l e f i n s ~ ~ ~ .

Photochromism and &-trans

isomerisation processes have been

widely investigated recently. A fully reversible conformational change of a polypeptide chain has been observed following the

&-trans

isomerisation of attached azobenzene rnoietie~29~t2~~. The

circular dichroism of these grafted copolymers was found to be different in different solvent mixtures and to be influenced by the addition of trimethyl phosphate. Similar copolymers based on p-(pheriy1azo)benzyl-L-aspartate and n-octadecyl-L-aspartate undergo

conformational changes from right to left handed helices with increasing azobenzene groups2g2. At high temperatures the process was photoreversible which has interesting implications for solar energy harnessing. For copolymers of methyl methacrylate with trans-stilbene methacrylate photocontrolled isomerisation is limited due to an irreversible side reaction from the cis-stilbenyl pendant groups to the phenanthrenyl group293. Photoresponsive

444

Photochemistry

polypeptide membranes of poly(L-glutamic acid) containing pararosaniline leucocyanide groups in the side chains give cationic side groups on irradiation destabilising the helical structure of the polymer due to electrostatic repulsive forces294. Similar effects have been observed for the same polymers but with azobenzene sulphonate moeities295. On the otherhand the ch-trans photoisomerisation of 0-aminoazobenzene modified poly(L-glutamic acid) although accompanied by aggregation-disaggregation processes and precipitation they were nevertheless reversible and controllable through selective excitation296,297. The latter processes were simply hydrophobic interactions between the azobenzene groups. The &-trans

photoisomerisation of

poly[N-[p-(phenylazo)benzoyl]]allylamine

also undergoes similar

soluble-insolubilisation reactions on exposure to W and visible light respectively298. A photoreversible nematic-isotropic phase transition has been observed for polymer liquid crystals with mesogenic phenyl benzoate side groups299,300. The phase transition behaviour was found to be dependent upon the spacer length between the groups and the molecular weight of the polymer. Two classes of thermotropic polymers have also been developed with azobenzene groups with both mesogenic and photochromic properties3O1. In the former case the azobenzene group is incorporated into the main chain of polymalonates while in the latter case it is incorporated as a side chain group to a polyacrylate. Whilst the first type of polymer had smectic phases the second had both nematic and smectic phases. It was interesting to note that from studies on the rate constants for the --trans

isomerisation processes in both polymer

types and models that these processes are unaffected by the polymer chain length. Reversible parallel homeotropic alignment mode

Ill: Polymer Photochemistry

445

changes have been achieved in a liquid crystal through the

&-trans

isomerisation of pendant azobenzene ester chrornophores

attached to polyvinyl alcoho1302,303. Molecular control of the polymer layers was obtained by the Langmuir-Blodgett method since it gives significant information for designing the photochromic layer controlling the alignment mode of the liquid crystals. The photochromic activity of thionine has been found to be influenced by the presence of a polyelectrolyte such as polyacrylic acid304 while B and

T

cyclodextrins enhance the dimerisation of cyanine

dyes in solution and consequently the back reaction of the photoisomer to their original conformer305. Light scattering has been used to study the self-diffusion

coefficient of polystyrene

in three dimensional networks306. Here photochromic polystyrenes were prepared by reaction with spiropyran and from swelling experiments two different diffusion processes were revealed namely, a fast one due to chains free to move inside the network defects and a slow one corresponding to chains entangled in the gel. Acrylic copolymers with mesogenic and spiropyran-mesocyanine side chains produce a visibly absorbing photochromic mesophase which bleaches irreversibly on irradiation307 while photoinduced gel formation has been observed for copolymers of bis[4-(dimethylamino)phenyl(4-vinylphenyl)methyl]leucocyanide-N-iso propylacrylamide-N,N~-methylenebisacrylam~de~~~. In the latter

case the gels swelled on W irradiation and then shrank when the light was removed. Chromic transitions have also been observed in poly(a,w-alkadiyne) macromonomers309 and polymethyl methacrylate containing spirofluorenylindolizines~~~. Poly(trimethylsilylcyc1ooctatetraene)has been observed to undergo

reversible cA--

isomerisation with the latter exhibiting

446

Photochemistry

electrical conductivity311 while polythiophene has been observed to catalyse the cis-trans photoisomerisation of 2-octene312. A survey of optically active polymers describes how useful they are in terms of potential applications in photodevices and as models for photomodulated biological processes313.

Fluorescence emission studies on polymers continue to attract widespread interest with some specific areas on membranes, gel formation, curing and hydrophobic domains. The release kinetics of diacetylfluorescein in membranes of polystyrene and polysulphone have been observed to be first-order314 while temperature ramped fluorescence studies have been used to examine thermoreversible gels labelled with fluorescein i~othiocyanate~~5. The cure rates of epoxy resins have been monitored using two types of fluorescent probes316. The first type was an extrinsic probe based on perylene

as a dopant whereas the second was a fluorescent product of the curing process based on a reaction of the epoxy resin with trimethylamine valeramide. Whilst the fluorescence emissions increased monotonically during the initial stages of the curing process they remained constant at later stages indicating that the system was rigid enough to prevent rotational diffusion of the probe molecules. Reflection spectra have been used to monitor reversible colour changes associated with the photopolymerisation of a poly(heptacosa-10,12-diynoic acid) monolayer at a nitrogen gas-water interface317 and europium copolymer complexes of crown ethers give a blue fluorescence emission318. The light induced change in charge acceptance in molecularly doped polymers has been found to depend upon the type of polymer binder319. Marked changes occur in acidic or basic polymer binders exhibiting broad

III: Polymer Photochemistry

447

absorption spectra.

The micellar block copolymerisation of acrylamide and styrene has been monitored through the quenching of the fluorescence of a pyrene probe320. The fluorescence results indicated that the copolymers isolated and screened hydrophobic molecules from the aqueous phase. A coiled configuration of the polyacrylamide backbone about the styrene blocks was a contributor to the screening effect of the fluorescence. The effect of pH on the fluorescence of pyrene labelled polymethacrylic acid showed that at pH 5-6 the polymer has at least two hydrophobic guest sites and that at higher pH the carboxylic acid group was ionised resulting in the formation of a hydrophobic core surrounded by a hydrophilic mantle321. Both sites acted as a host for the pyrene and the ionised site also bound cationic quenchers which increased the rate of decay of the excited pyrene at this location. At increasing pH conditions the hydrophobic core was gradually reduced and the ionised mantle increased more uncoiled and eventually did not act as a host for the pyrene. Micelle formation in ethylene oxide-styrene copolymers has also been studied using pyrene as a fluorescent probe322. From shifts in the fluorescence spectra and intensity changes the location of the pyrene could be determined. Regions containing up to ten hydrophobic units have been observed using pyrene as a fluorescent probe in aqueous microemulsions of acrylamide-dodecyl methacrylate copolymers323 while cyclisation in doubly end tagged ethylene-propylene-styrene block copolymer micelles has been observed only in the disperse phase324. Amphiphilic copolymers of N-isopropylacrylamide and N-hexadecylacrylamide have been observed to undergo perturbations

448

Photochemistry

in aqueous solutions using pyrene derivatives as fluorescent probes when compared with the separate horn~polymers~~~. The binding of methyl orange to copolymers of 2-dimethylaminoethyl methacrylate and N-vinyl pyrrolidone has been found to be very pH d e ~ e n d e n t ~ ~ 6 . The polymer changed from an extended coil at low pH to a compact coil at higher pH with electrostatic attraction to the dye being greatest at lower pH values. At high pH the hydrophobic interaction between the hydrophobic moieties on the polymer and the dye are enhanced. Polarised luminescence measurements have been used to determine the surfactant effect on the intramolecular mobility of polyelectrolytes based on ~inylpyrrolidone32~ while methylviologen has been found to quench the fluorescence of copolymers of methacrylic acid with vinylphenylanthracene and related anthracene containing met ha cry late^^^^. At high pH the methylviologen was found to form a charge-transfer complex with the anthracene groups. Similar quenching studies have been undertaken on monomeric and polymeric complexes of tris(bipyridine)ruthenium and found to be different in each case329. The same type of complexes have been used to probe the structure of starburst dendrimers based on the successive reactions of methyl acrylate with ammonia and amidation with ethylene diamir~e~~*. Methyl viologen was again used as a quencher with electron transfer being primarily involved at the dendrimer surface.

The study of polymer blend miscibility continues to attract some interest although the level of activity appears to have declined. In this respect the non-radiative energy transfer method has been used to study the miscibility of blends of polyvinyl chloride (PVC) with chlorinated polyvinyl chloride (CPVC) and polycaprolactone

III: Polymer Photochemistry

449

(PCL)331. In this work the polymers were labelled with donor and acceptor chromophores and the efficiency of energy transfer determined in the solid state rather than in solution as has previously been done. Using methylnaphthalene and methylanthracene as donor/acceptor molecules blends of PVC with CPVC were immiscible whereas the incorporation of PCL to the immiscible blend of PVC and CPVC resulted in a ternary system with two co-existing phases. The same workers have also determined the blend miscibility of polymethylmethacrylate with PVC332. In this case as the isotactic content and molecular weight of the polymethyl methacrylate increased there was reduced miscibility. However, the ability and reliability of this method for studying blend miscibility appears to have received some detailed criticism333. It has been found that the sensitivity of this fluorescence technique depends on the range between the lower (total miscibility) and the upper (complete immiscibility) of the ratio of the intensity of fluorescence emitted by the donor/acceptor chromophores bound to the polymers. It is claimed that although measurements of the lower limit are reproducible determination of the upper limit is quite a problem with errors of 2-3 nm in the calculations. Apparently above this limit solvent cast films appear to be immiscible whatever the polymer tacticity. It is suggested that changes in the donor/acceptor emissions with tacticity are simply due to an effect on the chain conformation of the probability of intermolecular interactions. As a final criticism it is shown that heating a monophase blend above the lower critical solution temperature does not actually result in a significant enough change in the donor/acceptor emission ratio to be able to detect phase separation. However, it should be pointed out that the studies of

450

Photochemistry

the workers on the solid state are at least the beginnings of a new methodolgy to study phase miscibility in the real polymer system where blend miscibility matters at the end of the day. Whilst the second group of workers are perhaps somewhat justified in their criticism they should at least make some attempt to improve this methodology. Previous esoteric investigations into solution media whilst having a good scientific basis bears little or no relationship to the technological world. Intramolecular excimer formation of meso-bis[l-(2-pyrenyl)]ether

has been studied in

elastomer matrices334. Intramolecular excimer formation was found to be controlled by the segmental motions of the matrix polymer. Improvements in polymer miscibility have been made through hydrogen bonding by hydroxyl substitution335. This technique proved to be more effective in monitoring the degree of mixing of the component polymer chains as functions of both the intensity of hydrogen bonding and the concentration of the guest polymer. Time resolved fluorescence depolarisation has been used to examine electronic excitation transport in blends of methyl methacrylate-2-vinyl naphthalene copolymer and polymethyl methacrylate and found that dispersive transport occurs at room temperature336. The fluorescence anisotropy of anthracene labelled polybutadiene has been found to respond to the shear stress and orientation of a host polymer in solution (polyisobutylene)337.

Fluorescence probes to study micro- and macromolecular motions, interactions and reactions continue to attract much interest. The moisture content in silk has been found to shift its intrinsic fluorescence spectrum338 while the physical aging of epoxy resins has been monitored using a free volume dependent fluorescent probe

111: Polymer Photochemistry

45 1

namely, [p-(N,Nf-dialkylamino)benzylidenemalonitrile]3~~. In a similar way the processing and crosslinking of epoxy resins has been monitored using 1-(4-dimethylaminophenyl)-6-phenyl-1,3,5-hexatriene~~~and pyrene

has been used to investigate the polymerisation kinetics of methyl m e t h a ~ r y l a t e ~ ~ Excimer l. emission from a pyrene labelled hydrophobic monomer copolymerised with acrylamide has been found to be significant in a micelle when compared with simple solution media due to the closer proximity of the chromophores342. Real time changes in membrane permeability have been measured using a fluorescent p r 0 b e ~ 4while ~ calculations have shown that it is possible to measure the end-to-end distances of polymer chains by tagging with donor-acceptor pr0bes34~. Ion transport has been studied in polypyrrole membranes345 while exciplex formation of diethyl-2-(9-anthrylmethyl)-2-(p-NfN-dimethylam~nobenzyl)malonate

has been found to be very dependent on concentration in a polymer host matrix due to intermolecular interactions but not in solution346. Fluorescence analysis of sodium styrenesulphonate-2-vinylnaphthalene copolymer indicated that the

polymer adopts a pseudomicellar conformation in solution where the hydrophobic-hydrophilic interaction between water and pendant naphthalene groups induced the formation of a polymer core with naphthalene groups oriented toward the interior347. Here an increase in the ionic strength of the solution caused a decrease in the dimension of the polymer core and enhanced the efficiency of excimer formation. Apparently energy transfer from the electronically excited naphthalene groups to the solubilised probe induced its photochemical reaction. In this respect polynuclear aromatic compounds in the same copolymer were found to undergo

452

Photochemistry

oxidation via singlet oxygen formation348. Terminal 4-(N,N-dimethylamino)benzoate groups on poly(methy1 methacrylate)

form a twisted intramolecular charge-transfer state which is better developedthan when it is present as a side chain349. New multiarmed monomers containing five oligo(oxyethy1ene) chains on a phosphazine ring have been prepared and tagged with a fluorescent probe350. The latter indicates that the polymer has high hydrophilic characteristics.

Hydrophobic fluorescent probes have been bound onto copolymers of 2-dialkylaminoethyl methacrylate-N-vinyl-2-pyrrolidone and compared with effects of dye binding351 and in an amphiphilic copolymer of p-styrenesulphonate with butyl methacrylate containing vinyl carbazole as a fluorescent probe only monomer fluorescence was observed especially in water where the polymer formed latex particles352. Two molecular populations have been observed for an oxadiazole dye in poly(l-trimethylsilyl-l-propyne)353 while a system containing two oppositely charged polyelectrolytes has been studied in the presence of a photosensitiser, zinc tetrakis(sulphonatophenyl)porphyrin35*. In the latter case

charge-transfer complexation occurred between the photosensitiser and the hydrophobic parts of the polymer. Both the polyelectrolytes were found to preserve there own electric fields even when the oppositely charged polymer was in excess. Two polymeric derivatives of thiophene and pyrrole containing an anthracene model core have

been prepared355. The fluorescence from the latter was strongly quenched by the polymer backbone. A simple model has been developed to describe the rearrangement of pyrene terminated polyoxyethylene tracers absorbed from solution onto a planar surface containing

ZZI: Polymer Photochemistry

453

unlabelled p ~ l y o x y e t h y l e n e ~A~ ~high . molecular weight displacer was found to change the mean square end-to-end distance of the tracer chains more effectively. The fluorescence of dansylated cationically charged polyacrylamide containing both primary and quaternary amines has been found to be dependent upon pH357 while the fluorescence of dansyl doped styrene copolymers was found to be dependent upon their crosslink density358. A similar crosslinking dependence was found with dansyl labelled polymethyl methacrylate359 although it has been found that the fluorescence of this probe is strongly quenched by electrophiles360. Time resolved fluorescence studies on potassium salts of maleic anhydride-l-octadecene copolymer and polymethacrylic acid containing rhodamine dyes and pyrene probes shows the formation of a ground-state complex formed via an intramolecular energy transfer process361. Copolymers of N-(acry1oxy)succinimide-N-isopropylacrylamide with 4-(l-pyreneyl)butylamine have been found to give fluorescence

spectra typical of isolated random coil polymers362. Single polymer micelles were observed at low polymer concentrations in aqueous media whereas at higher concentrations both inter and intrapolymeric aggregation took place. Above the critical solution temperature excimer formation decreased due to disruption of the pyrene aggregates.

A

simple theoretical approach

has been developed to simulate the

decay of a donor excited state in the presence of electronic energy transfer in polymer coils with arbitrary degrees of non-nearest neighbour, cross-chain transfer and transfer distance dependencies363. Excimer fluorescence from pyrene end groups

454

Photochemistry

labelled in polyethylene glycol has been used to study the macromolecular motion of the probe chain during the formation of complexes with polymethacrylic

Hydrophobic attractive

interactions were observed between the pyrene groups for both free probe chains in water and when complexed with the acid. The same polymers have been examined on colloidal silica powder and the emission of both monomer and excimer species was significantly red shifted from those in solution368. It was proposed that preformed excimer sites already exist on the colloidal silica except when the glycol repeat unit is five units long. In the latter case polymer configuration prevented overlap of the end groups.

Fluorescence polarisation studies on acenaphthylene labelled polyacrylic acid has demonstrated complexation occurs with polyethylene oxide to give a tightly packed and sterically restricted conformation369. Neutralisation of the acidic groups restored the polarisation of the fluorescence and together with time resolved fluorescence analysis indicated that the motion of the probe is anisotropic. Solvent partitioning has been observed between glycol methacrylate copolymers from the fluorescence anisotropy changes of l-(4-tr~methylammon~umphenyl)-6-phenyl-l,3,5-hexatriene as a

probe370 and the macrocyclisation equilibrium theory has been adapted to the formation of intramolecular excimers in end-labelled chains371. On the otherhand dynamic rotational isomeric state formalism is used to calculate the rate of first passage from a non-excimer forming conformation to an excimer forming conformation in seven aromatic polyesters with different spacers between the aromatic rings372. Intramolecular excimer formation has been

III: Polymer Photochemistry

455

studied in phthalate containing polyesters with different numbers of methylene and oxyethylene spacers373. Here polymers containing phthalate ester groups did not exhibit any excimer emission whereas those containing iso- and tere-phthalate groups did exhibit excimer formation. Repulsive steric interactions in the former case is responsible for the absence of any emission. A similar study was carried out by the same workers on polyglycols containing 2,6-naphthalene dicarboxylic acid374. In this case the nature of the glycol controlled the excimer formation and showed an odd-even number effect for polymers with methylene spacers but not those containing oxyethylene units. Fluorescence studies on alkyl methacrylate-p-(dimethy1amino)benzoyloxyalkyl methacrylate copolymers have shown that the molecular twisting motion has no barrier and that the retarding effect of the polymer chain is entr0pic3~5. Fluorescence depolarisation has been used to examine temperature effects on phase transitions of a fluorescein isothiocyanate-derivatised polymer376 while time dependent

fluorescence analysis has shown that the delayed excimer fluorescence of poly(2-vinylnaphthalene) involves a triplet partner which is trapped prior to the annihilative event377. Exciplex formation in carbazole-terephthalate containing copolymers have shown the existence of two kinds of exterplexes378. The first is a partially overlapped exterplex where two chromophores of the carbazole unit have the same conformation as the second excimer and the other is a sandwich type one where two carbazole chromophores have the same conformation as the sandwich excimer. Both have different emission spectra.

Excimer forming sites in poly(methylphenylsi1oxane) have been found

456

Photochemistry

to be a function of temperature and molecular weight of the polymer379. Here rotational sampling of excimer forming sites was exothermic for meso diads whereas for racemic diads rotation from the predominant conformer to the corresponding excimer forming sites was practically athermic. Copolymers of siloxanes have also been studied by the same workers380 where it was concluded that the high flexibility of the polymer backbone is responsible for the temperature dependence of the fluorescence. One very interesting article by a leading authority in this area has studied in detail the fluorescence of carbazole and anthracene labelled methyl methacrylate-methacrylic acid copolymers and found that radiative

rather than non-radiative energy transfer was dominant and concluded that such studies on labelled polymers are unreliable381.

The study and identification of excimer forming sites in polymers is widespread. Polyethersulphone has been found not to exhibit intra- or intermolecular excimer formation in solution due to its rigid structure382. Apparently only at high concentrations of greater than 4.4 x lO'4M

was intermolecular excimer formation

evident. Excimer emission has been observed however, from finely dispersed perylene in polystyrene with new bands being observed due to free and self trapped e x ~ i t o n s ~Intramolecular ~~. excimer formation has been studied in diastereoisomeric model compounds of polystyrene in solution indicating that they are formed mainly in meso diads384. Singlet energy migration was also observed in styrene trimer and tetramer systems. Conformational motions were determined to be crankshaft in type rather than rotation about a single-single carbon bond. Pressure effects on excimer formation in meso-bis[l-(2-pyrenyl)ethyl]ether dissolved in polyisoprene

Ill: Polymer Photochemistry

457

indicates that chain mobility is independent of pressure385 but dependent upon microstructure386 while the same polymer in isooctane shows the existence of two conformers387. In poly(3,6-di-tert-butyl-9-v~nylcarbazole) the excimer formation is

complete within 100 ps after excitation388 while the fluorescence of copolymers based on 2-isopropenylnaphthalene show only excimer emission in the pH range 5-3 due to the presence of a dynamic component under these acidic conditions389. Polymers containing 1 , 2 - ~ - b i s ( 9 H - c a r b a z o l - 9 - y l ) c y c l o b u t a n e pendant groups exhibit

much faster drift mobility than that of poly(9-vinylcarbazole) due to a reduction in trapping sites390 whereas the monomer fluorescence emissions from poly(vinylnaphtha1enes) are completely depolarised before excimer formation391. Excimer formation and conformational mobility has also been found to be dependent upon pH for methacrylic acid-2-(l-naphthylacetyl)ethyl acrylate copolymer392 while in maleic anhydride-2-vinylnaphthalene copolymer excimer forming sites are due to vinylnaphthalene-vinylnaphthalene diad s e q u e n ~ e s ~ ~For 3 . poly(a-methylstyrene) excimer emission increases with increasing degree of polymerisation up to a value of 50 and is related to local motion about the polymer chain394. Intramolecular excimer formation on the otherhand in 1-pyrene and p-di-N,N-methylaniline capped polystyrene has been found to be inconsistent with the predictions of the two state Birks model but consistent with diffusion controlled exciplex formation395. A theoretical model for incoherent electronic energy migration and trapping by excimers for polymer chains has been p r e ~ e n t e d ~and ~6 fluorescence analysis has been used to determine the mobility and solvation of a fluorophore in a polymer solution3g7. An ensemble of distributed donor molecules which undergo rotational transitions

458

Photochemistry

into a discrete excimer state has been analysed in an attempt to model the distribution of electronic relaxation and non-exponential fluorescence of aromatic polymers in the presence of rotational sampling devices398. In the case of irreversible trapping the donor survival function was formulated in terms of a one-sided Laplace transformation and specified for a modified Gaussian distribution to yield a closed-form expression for the donor decay. This formalism permitted a time-dependent rate function to be derived which makes possible the construction of an excimer excitation function by means of the convolution theorem. A suitable model has also been developed to account for the increase in the number of excimer forming sites through rotational sampling399.

Energy transfer has been found to be very effective between acetophenone and naphthalene end groups in p o l y ~ t y r e n e ~while ~~t~~~ in copolymers from 1:l-styrene-Ig,p-(chloromethy1)styrene containing an anthryl derivative and osmium and ruthenium bipyridyl chromophores energy transfer has been observed from the ruthenium to the osmium complex with high efficiency by shuttling through the

intervening anthryl groups402. The limitations for comparative fluorescence quenching analysis in determining the magnitude of excited state migration rates has been determined403.

Fluorescence depolarisation studies on mixtures of isotactic and syndiotactic polymethylmethacrylate have shown the formation of a 1:2 complex in toluene4O4 while fluorescence anisotropy studies on polybutadienes have shown viscous rotational diffusion with slipping boundary conditions405. Fluorescence studies on quasi-rigid-rod like probes in anisotropic polyethylene have shown

III: Polymer Photochemistry

459

that there is a linear dependence between the factorisable orientation parameter and the length of rigid rods at a given orientation of the polymer406. Polarised fluorescence studies have been carried out on ethylene glycol-p-hydroxybenzoic acid-terephthalic acid copolymers and shows the molecular association of terephthalate units407. The photocyclisomerisation of bis(9-anthrylmethy1)ether in solid polymers strongly follows a temperature dependence relationship and is strongly determined by the free volume of polymer segments408. The local motion of polystyrene chains in solution has been studied using fluorescent probes409. Here the relaxation time in the interior of the chain was found to be twice that as those groups at the ends. A dimethylaminobenzoyloxy chromophore in polymethylmethacrylate has been found to form at twisted intramolecular charge-transfer complex410.

The photoluminescence of poly(3-alkylthiophenes) are enhanced by elongation due to the torsion of bonds between neighbouring groups411 and is also affected by a magnetic field412. The decay rates of singlet oxygen phosphorescence are different in polymer media413 and the fluorescence of wool is associated with tryptophan residues414. Luminescence fatigue has been observed in poly(p-pheny1ene~inylene)~l~ while the photoluminescence of

poly(methylphenylsily1ene) has been assigned to

R-T*

transitions416. Crystallised polyethylene may be used as a sensor in dosimetry417 while poly(phenylviny1ene) exhibit absorption bands due to the interchain separation of charges418~419.In poly-yne chains the excited states have relatively compact lattice defects4zo polyaniline exhibits luminescence properties similar to

Photochemistv

460

those of p ~ l y t h i o p h e n e ~Polydiacetylene ~~. crystals photogenerate charge carriers by direct i ~ n i s a t i o n ~ ~ ~ .

Other types of luminescence are also widespread with particular emphasis on chemiluminescence. This includes the efficiency of copper deactivators in p ~ l y p r o p y l e n e ~the ~ ~ ,role of hydroxy radicals in the oxidation of pulp424, l~cigenin4~5, various polymers426, polyphenylene ~ u l p h i d eand ~ ~epoxy ~ re~ins4~8. Thermoluminescence from polyethylene is considerably influenced by its draw ratio429 while crystals of polyethylene showed some evidence of trap d i ~ i n t e g r a t i o n ~The ~ ~ . thermal aging of epoxy resins quenches the thermoluminescence of dopants431 while that from polyurethanes increases during fatig~e4~2. The cathodoluminescence of polyorganosilsesquioxane films is influenced by proton implantation433 and the ultrasonic luminescence of polymer melts is based on a new electrical theory434. Finally, researchers appear to be closing in on the identity of the luminescent species in commercial thermoplastics with recent results confirming the presence of triscycloalkenobenzene structures in nylon 6,6 polymer435.

4.

PHOTODEGBADATION AND PHOTOOXIDATION OF POLYMERS

Several reviews of interest have appeared in this field within the last twelve months. Aspects of the photodegradation, photooxidation and ghotostabilisation of polymers have been well covered agah1436-4~8as have photodegradable polymer~~~9-443 and ~ e a t h e r i n g ~ 4 ~ -Photochemical 4~~. reaction kinetics have been considered in stationary and mixed layers447 as has energy migration in p0lyrners~~8.Photoablation has been covered in some

III: Polymer Photochemistry

461

depth449 as has the photolysis of polyesters450 and fibrous proteins451. Polyethylene photooxidation has been critically appraised452 as has its photocrosslinking453 as well as a more specific review on the photoetching of oxydianiline-pyromellitic dianhydride polyimide and polymethyl methacrylate454. The cationic degradation of polysulphides and polyethers has also been covered in some detai1455.

Many aspects of the photooxidation mechanisms of polyolefins remain to be evaluated. Form the kinetics of photolysis of hydroperoxide groups in thermally oxidised polyethylene both aldehydes and ketones were found to be derived from primary and secondary hydroperoxides and trans-vinylene groups from tertiary hydroperoxides456. A butadiene-styrene block copolymer has been found to oxidise much faster than an ethylene-propylene copolymer but the induction period in the former case was twice as long457. Differences in additive concentrations especially after processing however, may account for this discrepancy rather than any unusual mechanistic features. On the otherhand, hydroperoxides generated during pre-thermal oxidation of isotactic polypropylene have been found to be responsible for controlling the photooxidation r a t e ~ 4 5 ~Morphological . changes in the polymer were found to be unimportant under these conditions. On one rather controversial subject it was found that aromatic ketones and quinones do not photosensitise the decomposition of alkyl hydroperoxides in polyethylene film and is in complete conflict with much earlier published model system work in solution459. Surprisingly, naphthalene was found to act as a photosensitiser but only very

462

Photochemistry

weakly through a singlet-singlet energy transfer process from the lowest excited singlet state of the naphthalene to a repulsive state of the hydroperoxide. Changes in the amorphous-crystalline structure of blends of low and linear low density polyethylene during photooxidation have been determined by differential scanning calorimetry460.

4.2 PolV(vinv1halides~ Unlike the polyolefins efforts in the elucidation and understanding of the complexity of the photodegradation and photooxidation of polyvinylhalides continue to attract much interest. The activation spectrum of processed and pigmented polyvinyl chloride with titanium dioxide has been found to be in the region 300-320 nm461-463 while orientation has been found to markedly enhance the photooxidation rate of the polymer464. The latter was associated with the build-up, during orientation, of conformations which are especially reactive towards hydrogen atom abstraction by free radicals. However, the same group of workers have found that whilst the rates of dehydrochlorination of different polyvinyl chloride samples increased with increasing isotactic content the rates of carbonyl formation de~reased465t46~. In this case the latter result was associated with low rates of hydrogen atom abstraction at the isotactic triads due to a higher shielding of the hydrogen atoms at the CHCl groups. The former results however, are associated with both the occurrence of some chlorine atoms of low dissociation energy located at a definite type of isotactic triad and a favoured hydrogen atom abstraction reaction at the methylene group in syndio and heteroaromatic triads. Excimer laser fluorescence has been used to probe the concentration changes in plasticiser and carbonyl

ZZZ: Polymer Photochemistry

463

groups in photooxidised polyvinyl chloride with the former being reduced and the latter increased467. Mechanistic schemes have been derived for the infra-red photoproducts of polyvinyl chloride468 while polyvinylidene fluoride has been found by the same workers to be extremely stable to both short and long W irradiation conditi0ns~6~.The photodehydrochlorination of syndiotactic sequences in polyvinyl chloride is considered to be important at temperatures below its Tg470. Thus, above the glass transition temperature the polyene sequence length is short

< 6 units due

to oxidation of the amorphous phase where below the Tg longer polyene sequences are observed

h.>

21 units due to

dehydrochlorination in the highly ordered lamellar. Polyene sequences in the photooxidation of polyvinyl chloride have been modelled471 while metal powders have been found to influence the photooxidation rate of polyvinyl chloride with stabilisation increasing from iron, aluminium, copper and lead to tin and zinc having a virtual complete overall ~ t a b i l i s a t i o n ~The ~ ~ .thermal dehydrochlorination of polyvinyl chloride has been related to the presence of allylic chlorine atoms while photodehydrochlorination is related to carbonyl allylic groups and terminal double bonds473. In particular the molecular weight has been found to markedly control the concentration of these species in the polymer474. Electron beam irradiation has been found to significantly enhance the double bond formation and photocrosslinking of polyvinyl chloride475 while corona discharge has also been found to subsequently enhance physical property changes in the p0lymer~~6. Microhardness has been used to evaluate the weathering of polyvinyl chloride window frame materials477 while synchrotron irradiation of PTFE gave rise to surface crosslinking and etching for improved

Photochemistty

464

adhesi0n~~8.Plasticised polyvinyl chloride has been found to photoprotect cotton fibres479 while the photooxidation of polyvinyl chloride has been compared with numerous other thermoplastic materials480.

4.3 Folvstvrenes The long-wavelength W degradation of polystyrene is thought to involve in-chain peroxides and not hydroperoxides in the initiation step481. For poly(p-propionyl styrene) the initial photoreactions in photodegradation are Norrish type I a-cleavages with some &cleavage

to give methyl radicals as shown in scheme 4482. In this

scheme the carbonyl triplet is an important initiator and the polymer is considered to be more stable than poly(-p-acetylstyrene). Oxygenated functional groups have been observed to be produced in far

W

irradiated polystyrene but at a decreasing rate at high

dosages due to autoinhibition483. In the photooxidation of styrene -butadiene radical attack occurs initially on the hydrogen atom in the allylic position of the butadiene sequence (scheme 5)484. The primary photoproducts such as associated unsaturated hydroperoxides are decomposed into secondary allylic alcohols and a,B-unsaturated ketones which absorb long wavelength W light and produce unsaturated acids as shown. The decrease in double bonds as well as photoproducts results in a steady-state reaction and an eventual cessation of the photooxidation reaction. Modified polystyrene with acrylonitrile and butadiene greatly enhances its photostability with more free radical production and ~rosslinking48~ while the tert-butoxycarbonyl group in poly(t-butoxycarbonyl styrene) is deprotected on prior pyrolysis of the polymer486. In polystyrene sulphonic acid excimer to monomer formation is enhanced on

HI: Polymer Photochemistty

465 v,

I

+

+ I

5

I” u

I” u

t

I”

u

z n

k-

3

Y

466

Photochemistry

+

-CH2-CH=CH-CH2-

re

I I

I

I

I

saturation reactions

I I

I

h3

0 I

H

t

I

I

saturated ketones

I

I

I

saturation reactions

c i s , trans

saturated acids

isomerit at ion unsaturated acids

Scheme 5

III: Polymer Photochemistry

467

irradiation despite an overall decrease in emission intensity487 while tetramethylbenzidine has been found to photosensitise the degradation of a styrene-sulphur dioxide alternating copolymer488. Photooxidised surfaces of polystyrenes have been found to poison Pd catalysts due to carbonyl group formation489 and copolymers bearing -(formylamino)

styrene units have been found to undergo rapid

crosslinking on photolysis with reduced chain scission490. On the otherhand copolymers of styrene with acryloxyimino groups have been observed to undergo rapid main chain scission at the latter sites491. Photolysis of the acryloxyimino groups introduce double bonds into the polymer thus lowering its glass transition temperature and enhancing main chain scission.

4.4 Polvacrvlates Acrylonitrile-butadiene copolymers have been found to undergo photolysis reactions similar to those of styrene-butadiene copolymers described in scheme 5 above492. However, in this case the presence of the acrylonitrile favoured the formation of hydrogen bonds between hydroperoxides and alcohols and also lowered the permeability of the copolymer to oxygen. The addition of propyl alcohol to poly(methylmethacry1ate) enhances its rate of photolysis due to hydrogen atom transfer from the solvent493 while photoproducts in vinyl and methacrylate polymers with aromatic side groups act as energy traps4g4. Paper impregnated with polyacrylates shows enhanced thermal and p h o t ~ s t a b i l i t y ~The ~ ~ .photooxidative stability and photoyellowing of a series of novel W and electron beam cured dialkylamine terminated diacrylate monomers have been studied in some depth496-498. Here the structure of the amine group was found to be important in controlling both the photooxidative

468

Photochemistry

stability and photoyellowing of the cured resin film. The alkyl group adjacent to the nitrogen atom was concluded to be the prime site for oxidative attack particularly at an alkylmethylene adjacent to an oxygen atom to give unstable hydroperoxides which dissociate to give unsaturated carbonyl products. In general, amine structures with bulky substituents and hydroxyl groups exhibited the lowest degree of photoyellowing due to their ability to impair hydrogen atom abstraction. Amine structures with long aliphatic alkyl groups enhanced photoyellowing due to the increased probability of extended conjugation. The W cured coatings gave rise to more rapid photoyellowing than electron beam cured films due to facile hydrogen atom abstraction and subsequent

-

hydroperoxidation of the alkylamino groups by the residual photoinitiators.

The effect of several metal substrates have been examined on the photodegradation rate of poly(ethyleneterephtha1ate). Relative stabilities were in the order Cu > A1 > control polymer > Au > Ag under non-oxidative conditions. Under oxidative conditions only the Cu proved to be a photosensiti~er~~~. No mechanistic explanations were apparent. The effect of new catalyst systems have been examined on the rate of photodegradation of polyester fibres5O0. Apparently, whilst the use of Ti, Ge and A1 glycolate catalysts gave lower molecular weight products they proved to be more light stable than those prepared with antimony. Dye stability was also markedly improved.

III: Polymer Photochemistry 4.6 P o - / P o l w

469 .

*

The photoinduced oxidation of aliphatic polyamides has been investigated using light of 313 nm501 while under a tensile load the photooxidation of nylon 6,6 has been found to be sensitive to its morph0logy~~2.Samples of polymer moulded with a high mould temperature were found to be subject to the greatest degree of surface cracking and erosion whilst those which were quenched cooled were found to be more resistant to oxidative degradation on W exposure. At high temperatures nitrogen dioxide has been found

to influence the light stability and whiteness of nylon 6,6 fibres with yellowing being a particular problem above 8O0C503. Nitrated polyimides have also been found to degrade more rapidly on irradiation due to the formation of a 4-nitro-1,3-xylylene structure504 while linear dichroism has been induced on the photolysis of polyamides containing cyclobutane ring structures505.

w In order to understand the light induced yellowing of lignin the kinetics of the photoreactions of a simple model system based on the use of a carbonyl sensitiser and a phenolic substrate have been studied in some detai1506,5O7. A computer modelling system was set-up in order to predict the reaction schemes involved which consisted of 91 elementary reactions and 58 species. These reaction schemes are shown in scheme 6 for the carbonyl sensitised photoreactions, scheme 7 for the involvement of hydroxyl free radicals, scheme 8 for the role of hydroperoxy radicals and scheme 9 for the importance of singlet oxygen in the system. The main reaction pathways were hydrogen atom abstraction by the triplet sensitiser and singlet oxygen formation. In the latter case the amount of singlet oxygen formation was directly related to the

Photochemistry

470

LIGNIN MODEL COMPOUND AH

3

'02 REACTION c '02

+

(A*

A*

HOi)A02H

[>C=d*

1

PRODUCTS

PRODUCTS Different reaction pathways of the carbonyl - sensitized photo-oxidation of a lignin model compound (AH) in air - saturated dimethoxyethane

Scheme 6 PEROX I DES HzO

+ HO2*

I

A*

+

H2O

Different modes ot reaction for the hydroxyl radical in the system

Scheme 7

model

47 1

III: Polymer Photochemistry

t

H202

+

HSENSH*

HSENSH2

-

AOOH

Difterent reaction pathways tor the hydroperoxyl radical in the model system Scheme 8

HOf

H202

+

HSENS

AO2'

+

H3C0

OH Different reaction channels for ground state oxygen in the model system

Scheme 9

Phoiochemisity

472

substrate consumption. Spectral changes during the irradiation of wood pulp have also been undertaken by the same w o r k e r s 5 0 8 ~ ~The ~~. ionic state of the sulphonic acid groups appears to be an important factor with regard to the behaviour of the wood pulp. Ferric and ferrous ions are also important and may be deactivated through the use of EDTA or polyethylene glycol. Other work on lignin model compounds has resulted in the formation of oligomers through intramolecular 6-cleavage of the C-0 bond with the formation of phenacyl and guaiacoxy radicals510. Intramolecular photoreactions have also been observed for model quinones511 while irradiation of cellulosic polymers has been found to influence their subsequent moisture uptake512. Natural and artificial exposure of leather coated with nitrocellulose laquers has been found to give erroneous and inconsistent results in terms of different parameters513 while wood from Malaysia gave rise to greater yellowing due to a higher content of lignin over that of temperate zone wood514. Fabric finishing operations have been found to influence the subsequent photostability of the wool fibres515.

4.8 Polvmer Ablation

In previous years studies on the photoablation of polymers for etching purposes have been few. Over the last twelve months however, this area of work has expanded enormously and may be an indication of its commercial fruition. A dynamic model has been proposed for the laser ablation of polymer systems which implicates the successive absorption of two or more photons causing population to higher electronic excited states516. In this work the excitation time of laser pulses was stepped and indicated that short-lived radicals play an important role in ablation. The wavelength of the

III: Polymer Photochemisty

473

radiation however, is critical since in some cases such as the polyimides the products from the first laser pulse will reabsorb the energy from the second pulse thus slowing down the reaction. The same workers have also developed a pyroelectric detector in order to measure the ablation velocity of various polymers as a function of laser fluence517. In poly(ether ether ketone) high fluence laser ablation in the W region was associated with thermal effects whereas the reverse is observed at low fluence518. Three types of carbon clusters were identified on laser ablation of polyimides519 while the threshold for laser ablation of polymers is achieved when the density of the absorbed photons is the same as that of the density of the chromophores in the material520. Saturation effects on the laser ablation of polyimides have also been observed by other workers even at 355 nm521. In this case however, the possibility of blocking by the excited polymer followed by non-statistical disposal of energy in the ejection process was considered. Other workers have established some quantitative expression for the ablation depth ~ ~ T S Uthe G fluences curves for polymers522 while elsewhere mass spectrometric studies have been undertaken on the products of a b l a t i ~ n ~ ~In~ the r ~ ~ ~ . latter study524 a large distribution of non-pure carbon clusters was observed due to product interactions immediately following the laser ablation. Along the same lines polymers have been categorised into those that do not absorb strongly and ablate through thermal processes and those that absorb strongly and ablate through photochemical reaction525. For polystyrene smaller fragments are observed with increasing fluence of ablation526 while for the same polymer both the thermal and photochemical hole burning effects are equivalent at 308 nm527. Temperature changes during the

Photochemistry

474

photoablation of polyester films indicated that the process is primarily phototherma1528 and the use of 193 nm radiation has been more useful in this regard529. Surface analysis techniques have indicated peculiar surface morphologies in the ablation of polystyrene and poly~arbonates53~ and in poly(ethylene-2,6-naphthalate) microstructures have been observed

on the surface531. Kinetic models have been devised for polyphenylquinoxalines532 and dephasing and structural relaxation

have been undertaken in phenoxy resins doped with tetraphenylp~rphin~~~. Low energy excitation modes in polymers have also been probed by this t e ~ h n i q u e 5 ~ ~ .

4.9

Miscellaneous Polvmers

Laser flash photolysis studies on polyurethanes have been undertaken and provided evidence for C-N bond cleavage as well as indirect photolysis via t e r t - b u t y l p e r o ~ i d e s ~ ~Benzophenone ~,~~~. has been found to induce hydrogen atom abstraction in poly(viny1acetophenone) via its lowest excited triplet while in polycarbonates and polyesters the in chain carbonyl triplets were established as the primary species in photodegradation and the sulphone chromophore in polysulphones538. Dihydrobenzofuranols are produced in the photocyclisation of 1,4-bis(2-benzyloxybenzoyl)benzene539

and diffuse reflectance FTIR

has been used to monitor surface changes in the excimer laser irradiation of a polyimide540. Temperature effects are considered to be very important in the weathering of fibres and this has been taken into account using a modified expression541. The hardness of W cured bisphenol-A resins has been found to decrease on prolonged

irradiation and model systems were investigated to understand the

III: Polymer Photochemistry

475.

mechanisms involved542. Both a photo-Fries rearrangement and scission at the isopropylidene group were proposed although no account was taken for the residual photoinitiators. Poly[bis(4-isopropylphenoxy)phosphazene] undergoes photosensitised

degradation in solution in the presence of hexakis(4-benzoylphenoxy)cyclophosphazene but not in the solid

state due to phase separation of the ~ y s t e r n s ~ 4 Several ~. ketonic, ester, phenolic and ether impurities have been implicated in the photoinduced oxidation of EPDM r ~ b b e r 5 4while ~ _trans-polyphenylacetylene undergoes significant degradation in

chloroform and the solid state with a consequent reduction in conductivity545. Single crystals of poly(diacety1ene) have also been observed to undergo oxidation through a singlet oxygen mechanism involving the ene groups546. The photolysis of phenolic resins undergoes two mechanisms involving the formation of quinone methide structures and hydroperoxide groups at the ether linkages547 while the latter have been implicated in the photoinduced oxidation of melamine-urethane-acrylic coatings548. The photooxidation of paint films have been evaluated under controlled humidity c0nditions5~9while an imposed mechanical stress on melamine-acrylic coatings significantly decreased their light stability due either to increased oxygen access or more likely accentuated cohesive failure induced by the stress550. The synthesis and mechanisms of the photodegradation of novel poly(dimethylsilyl)thiophene, naphthalene and furan have been

investigated551-553 while photoactive carbenes have been generated in the photosensitised oxidation of polybutadiene by dimethyl diazomalonate554. Polymers with photocleavable monoclonal antifluorescyl antibodies have been prepared based on

Photochemistry

476

N-(2-hydroxypropyl)methacrylamide555

whereas novel dimeric telomers

have been observed in the photolysis of methyl-2-bromo-4,4,4-trichloro-2-methylb~tyrate~~~. The photolysis

of new ferromagnetic bisdiazines have been investigated557 while bisphenol-A epoxy polymers with aminoethyl thiosulphate groups undergo

S-S

bond cleavage on photolysis with a quantum yield of

0.2558. Carbonyl groups in orientated polyvinyl alcohol have been monitored during p h o t o o ~ i d a t i o nand ~ ~ ~photooxidised resins have been evaluated by thermal methods560. Carbazolyl polymers undergo photooxidation during the early stages of irradiation with gradual crosslinking in the inner layers due to screening and inhibition of oxygen diffusion561. The photooxidation of several flame retardant p-iodophenylcarbamoylarylhydrazidic bromides has been undertaken562

and photosensitisers have been immobilised onto isocyanate supports563.

5. PHOTOSTABILISATION OF POLY-, A

number of reviews have appeared to date on the subject of

photostabilisation. Photoantioxidant mechanisms have been concluded to be very dependent upon the thermal processing history of the polymer564 while various mechanisms have been considered g e n e r a l l ~ ~ ~ ~and - 5 ESR ~ 8 processes are discussed specifically569. The mechanisms of hydroxyphenylbenzotriazole stabilisers have been discussed in depth with emphasis on intramolecular hydrogen bonding and light stability570 while more specific reviews have appeared on poly(viny1 chloride)571 and the role of nitroxyl r a d i ~ a l s ~ ~ 2 .

The efficiency and mode of action of orthohydroxybenzophenone stabilisers continues to attract much interest. A yellow complex

111: Polymer Photochemistry

477

has been observed between 2-hydroxy-4-n-octoxybenzophenone and calcium stearate in p0lypropylene5~3while metal salts of 2-[(l-hydroxy-4-methyl-2-benzoyl)carbonyl]benzo~c acid have been

found to be effective stabilisers in poly(butadiene) through scavenging free radicals and quenching singlet oxygen574. Polymer bound stabilisers are also of some importance and in this respect the excited state properties of orthohydroxyphenylbenzotriazoles have been found to be important575. The longer lived the excited state of the stabiliser through grafting or co-reaction the less efficient will be the stabilisation. Light stabiliser molecules possessing the 4-hydroxybenzoate have been found to be extremely effective576 while mixtures of benzotriazole and tetrahydrobenzotriazole stabilisers are effective in stabilisation through the formation of adducts577. Phenylbenzoate esters are also claimed to be very effective light stabilisers worthy of ~ommercialisation5~8 while nickel complexes have been found to be effective in nylon twine579 and benzophenone stabilisers have been successfully grafted into butadiene block copolymers through hexamethylcyclotrisiloxane groups580. Surprisingly polyamides have been found to be effectively stabilised through the use of azobis-is~butyronitrile~~~ whereas other workers have found hydroxyarylketones to be ef fective582 as are zinc complexes of bis(naphthoylenebenzimida~oles)~83.Other articles of interest

include the stabilisation of polymeric insulators584, organotin stabilisers for poly(viny1 chloride)585, metal dialkyl dithiophosphates for poly(2,6-dimethyl-l,4-phenylene oxide)586, the behaviour of carbon black pigments587 and the effectiveness of absorber blends588.

478

Photochemistry

Of all the light stabilisers available the hindered piperidine structures are the most interesting in terms of understanding their mode of action and efficiency. The physical properties of stabilisers and their effect on light stabilisation have been discussed in detai1589 while other workers continue to observe antagonistic effects with thermal phenolic antioxidants590,591. Glycidyl derivatives of hindered piperidine compounds have been prepared592 while stabilisation has been found to be effective using mixtures of orthohydroxyaromatic stabilisers with hindered piperidine compounds in coatings for wood593, polyethylene594, and rubber595. Ethoxysilyl derivatives of hindered piperidines have also been prepared596 while initiators containing hindered piperidine structures have been found to be effective for leaving residual bound stabiliser molecules in coatings597. Diphenylamine type antioxidants have been found to synergise with hindered phenolic compounds on irradiation598. In a model system study hindered piperidine compounds were found to inhibit the photoreactions of heptane-2-butylanthraq~inone~~~ and other types of interactions have been discussed from a theoretical point-of-view600.

New aspects of the photostabilising mechanisms of hindered piperidine compounds continue to be discovered. In this regard some workers using isotopic labelling have found that substituted hydroxylamines will react with hydroperoxy radicals to give ketones and carboxylic acids and not dialkylperoxides as previously thought601. Reactions with acylperoxy radicals were found to be very fast in this process602. other workers however, implicate the quenching of singlet oxygen in rubber systems603 and nitroalkanes

III: Polymer Photochemistry

479

are believed to operate in a similar manner to the hindered amines604. The rates of photooxidation of nitroxy radicals have been considered to play an important role in determining the photostabilising efficiency of hindered piperidine compounds605 as have the nature of the substituent groups6O6. In the latter case 4-benzoyloxy and 4-stearoyloxy piperidine compounds were prepared with various substituents on the nitrogen atom. Nitroxyl and hydroxylamine derivatives were found to be the most effective and in turn dependent upon the processing history of the polymer. On the otherhand methyl and hydrogen derivatives of hindered piperidines were found to be the most effective stabilisers in acrylic melamine coatings607. In a continuation of this work it was found that stabilisation efficiency depended upon the rate of initial formation of nitroxyl radicals608. The decay kinetics of nitroxyl radicals in acrylic melamine coatings have also been used to monitor their weatherability609. Processing conditions and exposure to hydrochloric acid have both been found to impair the photostabilising efficiency of hindered piperidine compounds in

-

polypropylene film610.

This section refers to the effect of dyes and pigments on polymer systems and their colour fastness. Reviews have appeared on azo dye fading processes611, disperse dye fading612 and new trends in dye usage613.

The liquid phase coating of rutile pigments with silica markedly suppressed the activity of the pigments614 and the modelling of pigment discolouration has been undertaken615. In polyethylene

Photochemistry whilst carbon black was found to be an effective stabiliser, ultramarine blue had little or no effect whereas iron and chromium oxides were effective stabilisers and cadmium sulphide a sensitiser616.

The cis-trans isomerisation process in the photofading of azo dyes has been found to be a first-order process in polypropylene and independent of polymer crystallinity617. Dye lightfastness on leather has however been found to depend upon the colour of the dye618 while alkyl and peroxy radicals have been observed in the photofading of Rhodamine and azo dyes in solution61g. More specifically basic triphenylmethane dyes have been found to give benzophenone products on photofading620. The structure of a series of heterocyclic azo dyes has been related to their lightfastness621 while long-lived transient species have been observed on laser flash photolysis of a bis-styryl biphenyl dye which is associated with a radical cation622. Indole and tryptophan quench this species effectively and is related to the photofading of the dyes in wool fibre. Reactive dyes have been found to impair the orientation of stressed poly(vinylalcoho1) during irradiation623 and metal ions have been found to influence the daylight fluorescence of pigments in polyethylene624. Flash photolysis of an aluminium phthalocyanine dye in cotton has shown that the excited states are similar to those produced in solution but longer l i ~ e d 6 ~In ~ .the presence of oxygen their was no quenching of the excited singlet state in wet fibre and on prolonged laser irradiation at 354 nm oxidation of the phthalocyanine occurred to give a red product. Fluorescent brighteners for polyesters have been developed based on 2,4,5-trisubstituted oxazoles626 and

Ill: Polymer Photochemisry

481

pyridino[l,Z-b]pyrazol-2-0nes~~~. New halogenated

succinylfluorescein dyes have been synthesised and bound to polymer supports as singlet oxygen sensitisers628. In the latter form the dyes were much more light stable and had high quantum yields of singlet oxygen generation which also increased with increasing iodo substitution.

The stabilisation of dyes is also an area of much interest. Crystal violet lactone in pressure sensitive recording systems has been stabilised by varying the nature of the c o ~ n t e r i o n 6 ~Nickel ~. salts of orthohydroxybenzophenone have been found to be particularly

effective in this respect630 as have nickel salts of g-toluenesulphonic acid and orthohydroxybenzophenone sulphonates for other dye classifications631-633. Their ability to quench singlet oxygen is believed to be an important process. Nickel compounds have also been found to be effective in inhibiting dye fading in leather by complexing with the dye and impairing singlet oxygen f0rmation6~~. Nickel dithiolato complexes have been found to improve the lightfastness of cyanine dyes whereas azo dyes containing reactive epoxy groups for grafting into nylon have also been found to have improved lightfa ~ t n e s s ~Modified ~~. cyclodextrin containing phthalic acid groups gives rise to improved lightfastness for dyes637 whereas other methods have involved the use of dye mixtures638 and disperse dyes have been stabilised in cellulose diacetate639.

Photochernistly

482

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m.,

a, a, a,

m,

a,

x,

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.

-

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I

s,

m,

m,

572. 573. 574. 575 * 576. 577. 578. 579.

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u.,

500

Photochemistty

580.

I. Jansen, H. Schilling and K. Ruehlmann, Acta Polvm..

581.

I.S. Lukomskaya, G.G. Makarov, L.M. Postnikov and V.Ya. Shlyapintokh, Vvsokomol. Soedin.. Ser A, ( 1 9 8 8 ) , 30, 2064. M.D. Bhavsar and B.M. Shah, Man-Made Text. India ( 1 9 8 8 1 , X,

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585. 586.

? iytm R. Chandra, B.P.

583.

(1990),

(1989)p

a,271.

Mas=! ( 1 9 8 9 ) , 4, 90. Thapliyal and R.P. Singh, Br. Polvm. J.,

22, 1 9 .

589.

L.C. Whitney, t. Ena. [Brookfield. Conn.), ( 1 9 8 8 ) , 44, 2 9 . F.M. Egidis, V.P. Kolomytsyn, O.A. Zhivilkova, N.N. Shuvaeva and A.N. Dolgachev, Plast. M a w t ( 1 9 8 9 ) t 85V.B. Ivanov and V.Ya. Shlyapintokh, golvm. Deu. & S t u - I

590.

C. Andrei and P. Ioana, Mater. Plast. [Bucharest).

587. 588.

( 1 9 9 0 ) , 251, 249.

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(1989)

26, 39. 592.

M. Barabas, R. Iring, 2. Fodor, P. Balint and F. Tudos, Maav. Kem. Folqr., ( 1 9 8 9 ) , 25, 248. J. Luston and F. Vass, Makromol. Chem.. Macromol. Svm~.,

593. 594.

H. Kastien, Farbe Lack , ( 1 9 8 9 ) , 95, 1 6 . J. Malik, A. Hrivik and E. Tomova, Plastv Kauc.,

591.

(1989),

595. 596. 597. 598.

22, 231.

(1989),

26,

204. G. Ivan

and M. Giurginca, Rev. Poum. Chim., ( 1 9 8 9 ) , 2 , 1351. I. Jansen, D. Helbig, A.Al.S. Hammoud and K. Ruehlmann, Acta. Polvm., ( 1 9 8 9 ) , 421, 1 2 1 . P.A. Callais, V.R. Kamath and J.D. Sargent, Hod. Paint Coat. , ( 1 9 8 8 ) , 28, 41. V.B. Yakovlev, N.I. Litsov, V.I. Nikolaevskaya, V.L. Andreev and A.A. Kachan, Wsokomol. Soedin.. Ser. B . I ( 1 9 8 8 1 , m, 812.

600.

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S . Feng, Z. Gao and S . Wu, Gaofenzi XuebaQ, ( 1 9 8 9 ) , 1t-37. 6 0 4 . S . Al-Malaika, T. Czeckaj, G. Scott and L.M.K. Tillekeratne, Polvm. Dea. 61 Stabia., ( 1 9 8 9 ) , 26, 375. 605. K. Ohashi, K. Nakai and T. Kawaguchi, polvm. Dea. & S t a m . I ( 1 9 8 9 1 , 26, 7 9 . 606. S. Chmela and P. Hrdlovic, golvm. Dea. & Stabil., ( 1 9 9 0 ) , 22, 159. 607. D.R. Bauer, J.L. Gerlock, D.F. Mielewski, M.C. Paputa Peck and R.O. Carter, Polvm. Dea. & Stab ( 1 9 9 0 ) ! 28, 39. 608. D.R. Bauer, J.L. Gerlock and D.F. Mielewski, polvm. Deu. C Stabu., ( 1 9 9 0 1 , 215. 6 0 9 . J.L. Gerlock, D.F. Mielewski and D.R. Bauer, golvm. Dea. & Stabu., ( 1 9 8 9 ) , 26, 241. 6 1 0 . S. Chmela, D.J. Carlsson and D.M. Wiles, polvm. Dea. & 603.

Y. Yang, G. Li,

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a,

III: Polymer Photochemistry

50 1

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a.,

-

u,

a

.

maws

Part IV PHOTOCHEMICAL ASPECTS OF SOLAR ENERGY CONVERSION By A. COX

Photochemical Aspects of Solar Energy Conversion BY A. COX

1. Introduction Topics which have formed the subjects of reviews this year are solar photochemistry

and

heterogeneous

photocatal ysis,l

catalysis

in

the

conversion and storage of solar energy, photocatalytic reactlons involving titania,3 e 4

photoinduced catalytic reactions,

organised molecular systems, from inorganic materials,' energy

conversion

by

photoisomerisations.10

strategies for the production of methane

methods for solar energy

photosynthesis, An

photocieavage of water in

improved

and

laser

solar

solar energy

storage

by

scanning technique for the

evaluation of solar cells has been described." 2. Homoaeneous Photosystems Ionic

strength-dependent

scavenging of [Ru(bpyI3I3'

kinetics

have

been

observed

for

the

by EDTA, and the relationship of the rate

constant to the quantum yield of formation of .'YM

in the [Ru(bpy)312'

/HV2+/EDTA model photochemical system discussed.12 Photocatalytic cleavage

of water using a mixture comprising [Ru(bpy)3I2' as

photosensitizers,

MV2',

EDTA

and

and zinc phthalocyanine

colloidal

Pt

in

poly(viny1

alcohol 1-cetyltrimethylammonium bromide has been investigated.13 The same group has also shown that the efficiency of hydrogen formation is not affected by temperature.14 Hydrogen has been produced by irradiation of aqueous solutions of anthryl-substituted Co(II1) cage complexes in the presence of EDTA and Pt catalysts, and arises by energy trapping and intramolecular electron transfer.l5 PhotodGhydrogenation of propan-2-01 yielding

acetone

RhX(CO)(PR312

(X

and

-

dihydrogen halo)

with

is high

reported

to

turnover

be

catalysed

frequencies.16

by The

Photochemistry

506

three-coordinate species RhX(PR3I2 is responsible for the catalytic cycle and the same catalyst has also been successful in the dehydrogenation of a1kanes to a 1 kene and d i hydrogen.l7 Acetaldehyde and hydrogen are formed during prolonged photolysis of aqueous ethanol solutions of some cobalt cyano complexes.18 The amount of hydrogen corresponds to 3-4 full cycles of oxidation and reduction of the initial complex.

Measurement of the

proton kinetic isotope effect has enabled a mechanism to be proposeU.l9 Dihydrogen and dimethoxymethane are produced in a stoichlometric ratio by the

photodehydrogenation

of

methanol

using

iridium(II1) complexes,20 and

catalytic

hydrogen

reduction

of

C02

to

CO

has

been

observed

tin(I1)-coordinated evolution on

without

irradiation

of

[fac-Re(bpy) (C0)3Br 1 in THF/Et3N.21 Redox catalysts for the photochemical splitting of water have been prepared by dispersing cyclodextrins in aqueous solutions of MS04(M

=

Mn,

Fe, Co, Ni, Cu) by an ultrasonic

treatment.22 Second-order rate constants have been determined for the reactions of 3A2u Pt2 (Pt2 Stern-Volmer quenching

=

[Pt2(P205H2l4

experiment^.^^

14- )with secondary alcohols by

Broad band irradiation leads to the

formation of the corresponding carbonyl compound and hydrogen, and flash photolysis experiments point to H-atom transfer being the first step. Formation of hydrogen from water-alcohol solutions of polytungstates can be photocatalysed by colloidal platinum.24 At 1.5

<

pH

<

5,

the

O ~ ~and the efficiency of the alcohol predominant transient is [ W ~ O14-, with respect to hydrogen formation has been evaluated. The preparation of 4-hydroxyol igopyridines has been described25 and their metal complexes are useful as photocatalysts in solar energy conversion. In suitable systems, photoinduced evolution of hydrogen has been achieved using viologen-linked water-soluble zinc porphyrins (Zn-PC3(CnV)) with

different

methylene

chain

lengths

(n

-

2-6).26

N,N-Dimethylformamide-meso-tetra(sodium-~-sulphonatophenyl)porphyrinato-

I V: Photochemical Aspects of Solar Energy Conversion

507

rhodiua(I1) has been prepared and used for the photoreduction of water in a homogeneous 3-component system along with TiCI3 and K2PtC16.27 Various derivatives of TSPP

(tetrakis(4-sulphonatopheny1)porphyrin)

have been

examined as photosensitizers for the oxidation of H20 to oxygen on colloidal iridium oxides.28 Illumination of the porphyrin in the presence of sodium persulphate and colloidal iron will, in certain cases, result in the generation of 02. In a quenching study of zinc tetraphenylporphyrin (ZnTPP) by p-cyano-N-methylpyridine (ZnTPP')

in

DMSO,

the

redox-ion

product

has been produced by flash photolysis, and a quenching rate

constant obtained in Triton X-100

mice1 ie aqueous solution.29 Kinetic

studies of the photosensitized electron transfer by non water-soluble ZnTPP,

water-sol uble

ZnTMePyP4+,

ZnAnTPPS4-,

and

(ZnTPPC02Nal4

to

duroquinone, MV2+ and AQS- have been carried out in reversed m i ~ e l l e s . ~ ~ 3. Heteroqeneous Photosystems

Quantum yields of hydrogen photoevolution from suspensions of Pt/TiOZ in various electron donor solutions have been measured. 31 The mechanism probably involves a direct irreversible reaction of holes with adsorbed alcohol, which confers a potential on the Pt/Ti02 particles sufficiently negative that the electrons are necessarily used for hydrogen evolution. In situ photodeposition of Ru on TiOZ gives catalysts which are highly active for the photoevolution of hydrogen from H20/MeOH.32 Til-xSnx02 (0 < x

<

Platinised

1) shows photocatalytic activity in visible light for

hydrogen production from aqueous solutions containing sacrificial donors such as hypophosphite,33 and an evaluation of the photocatalytic activity of TiO2, activated by copper or copper oxides, has revealed that those prepared

by

a

chemical

deposition

technique

possess

the

highest

activity.34 The photocatalytic properties of some tertiary titanates have been assessed according to various criteria. These include their ability to

photocatalyse hydrogen and

oxygen evolution

in the presence of

Photochemistry

508

specified electron donors and electron acceptors, and the magnitude of their band gap.35 Polymerised n-butyl orthotitanate and methanol form a polymeric complex [pol-Ti(OBu14 t MeOHl in which Me0 replaces a BuO llgand to

introduce a

state which

may

be

important for

the catalytic

decomposition of water.36 Aqueous sod i um carbonate has been reduced to CH30H, HCHO, and HC02- using photoproduced electrons in the conduction band of platinised titar~ia.~~ Doping with Cr or Mn ions lowers the yield of products. An intercalation compound composed of layered HTiNb05 and methylviologen has been synthesised.38 In oxygen-free conditions UV irradiation turns the compound blue and this has been interpreted in terms of an electron-transfer process which gives .'VM

.

The simultaneous photosplitting of water into hydrogen and oxygen has been achieved with high efficiency by visible irradiation of a suspension of metal (Pd, Pt, Rhl-loaded WO3 particles in aqueous methylvi~logen.~~ The same authors also report hydrogen evolution from water using visible radiation in the presence of Cu(II)/W03 and an electron relay,40 and discuss

factors

partic1es.l'

influencing

Some

the

radiolytical ly

photocatalytic prepared

efficiency

Ni-Pd

sols

of

have

WOj

been

investigated as catalysts for the photoreduction of water and found to show an enhanced catalytic activity compared to pure Ni

Bimetallic

Pt-Au colloids also catalyse the formation of hydrogen from sacrificial

photo system^.^^

For systems not under kinetic control these colloids can

give improved rates of hydrogen evolution. The effective quantum yield of hydrogen generated from aqueous ethanol containing triethanolamine, a poiy(vinylpyrrolidone1-stabilised Pt catalyst, and octabromofluorescein as

sensitizer, has been studied as a function of photon energy over the range 1.8 - 3.12 eV.44 Photocatalytic

evolution

of

hydrogen

has

been

observed

on

zeolite-supported CdS particles whlch have been modified by an appropriate

509

IV Photochemical Aspects of Solar Energv Conversion catalyst (Pt or ZnS).45 Sustained evolution of hydrogen using thls system requires an added sacrificial donor.

In the presence of some metal

catalysts (Pt, Pd, Rh2O3) loaded on to CdS, photoinduced

dehydrogenation

of aqueous ethanol glves hydrogen.46 The optimum amount of hydrogen is generated using a 54 Pt-5% Cu/CdS catalyst system.

A study of the effect

of Pt loading on the efficiency of CdS-photocatalysed H evolution from aqueous Na2SO3 and on the intensity of the 510 nm emission of CdS suspended in the same solution has appeared.47 Luminescence measurements are shown to be useful in clarifying the mechanism of the photocatalysis. The same group also reports that the evolution of hydrogen from aqueous solutions of Na2S03 using Pt-loaded CdSl-xSe,

mixed crystal powder has

been achieved under illumination at > 520 nm.48 An investigation has appeared of the surface composition and structure of CdS and Pt/CdS used in

the

solar-induced

catalytic

dehydrogenation of

aqueous

EtOH.49

Decreases in photocorrosion of CdS and an increase in the wavelength of visible light absorption of CdS beyond 517 nm has been achieved by photodepositing Pd on the catalyst surface,50 and the CdS-catalysed photochemical conversion of H2S to H has been optimlsed with respect to Ru02 loading, CdS concentration, and S2-

concentration^.^^

and S032-

Electron transfer reactions on transparent colloidal Sn02 have been examined by the use of radiolytically generated reducing radicals and other methods. 52 4.

A

Photoelectrochemical Cells

multielectrode

photoelectrochemical

cell

for

Unassisted

photocatalysis and photosynthesis, and incorporating a bipolar Tl02/Pt multieiectrode cell with five panels has been d e ~ c r i b e d . It ~ ~ has been used for unassisted photoiysis of H20 and produces an H 2.4

:

1.

I

0 molar ratio of

In a new induced junction solar cell a strong inversion layer is

induced by an external bias voltage across an insulating layer.54

Its

510

Photochemistry

high efficiency is attributed to its having a very low dark saturation current compared with that of conventional nt-p junction cells.

The

performance of n-ITO/n-InGaAsP/p-GaAs solar cells has been evaluated as a function of chemical treatment of the ITO/InGaAsP heterointerface, using HN03 and HC1.55 Compared with non-treated cells, those treated with HNO3 have good solar cell performance. A comparison of transparent conducting electrodes on mercuric iodide photocel Is has appeared.56 These consist of ionic contacts of NaCl and LiCl, poly(viny1 alcohol )/phosphonic acid, and indium tin oxide. 5. Luminescent Solar Concentrators Tandem CdS/CdTe and Si solar cells have been described which are equipped with a LSC consisting of a transparent PMMA matrix containing fluorescent dye particles: this forms a flat plate on top of the multi-tandem structure.57

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32

A. Sobcrynski, T. Jakubowska, and S. Zieluski, Monatsh. Chem., 1989,

120, 101. 33

M. Avudaithai and T. R. N. Kutty, Mater. Res. Bull., 1989, 24, 1163.

34

H. Harada, M. Imamura, T. Kobayashi, K. Shimizu, Y. Nishimura, and S. Mine, Chem. Express, 1989, 3 , 287.

35

Y. Oosawa, R. Takahashi, M. Yonemura, T. Sekine, and Y . Goto, New J.

m.,1989, l3, 435. 36

T. Shinoda, J. Mol. Catal., 1989, 5l, 139.

37

M. U. Raphael and M. A. Malati, J. Photochem. Photobiol., A, 1989, 46, 367.

38

T. Nakato, H. Miyata, K. Kuroda, and C. Kato, React. Solids, 1988,

-6, 231. 39

P. Maruthamuthu and M. Ashokkumar, Int. J. Hydroqen Enerqy, 1989, 14, 275.

40

P. Maruthamuthu, M. Ashokkumar, K. Gurunathan, E. Subramanian, and M.

V. C. Sastri, Int. J. Hydroqen Enerqy, 1989, 14, 525. 41

M. Ashokkumar and P. Maruthamuthu, J. Photochem. Photobiol., A, 1989, 49, 249.

42

E. Amouyal, M. Ceorgopoulos, and M. 0. Delcourt, New. J. Chem., 1989,

13, 501. 43

A. Harriman, J. Chem. SOC., Chem. Commun., 1990, 24.

44

V.

S. Zakharenko, A. I. Yartsev, N.

P. Smirnova, and A.

El'tsov, Khim. Vys. Enerq., 1989, 23, 519.

V.

IV: Photochemical Aspects of Solar Energy Cot]version 45

M. A. Fox and T. L. Pettit, Lanqmuir, 1989, 2, 1056.

46

0. Zhuang, J. Chen, and Z. Wang, Xiamen Daxue Xuebao. Ziran Kexueban, 1988, 2 l , 537.

47

T. Uchihara, M. Matsumura, A. Yamamoto, and H. Tsubomura, J. Phvs.

w.,1989, 93, 5870. 48

T. Uchihara, H. Abe, M. Matsumura, and H. Tsubomura, Bull. Chem. SOC. Jpn., 1989, 62, 1042.

49

Z. Jin, 0. Li, L. Feng, Z. Chen, X . Zheng, and C. XI, J. Mol. Catal., 1989, 50, 315.

50

I. B. Rufus, V. Ramakrishnan, B. Viswanathan, and J. C. Kuriacose, Indian J. Technol., 1989, 27, 171.

51

L. R. Gazyli, J. J. Thomas, and R. G. Barile, Int. J. Hvdroqen Enerqy, 1989,

52

14,647.

P. Muivaney, F. Grieser, and D. Meisel, Lanqmuir, 1990, 5, 567.

u, US 4,793,910.

53

E. Smotkin, A. J. Bard, and M. A. Fox,

54

T. Fuyuki, S. Moriuchi, S. Miyagaki, and H., Matsunami, Mem. Fac. Enq. Kyoto Univ., 1988, 5 0 , 249.

55

S. Matsubara, H. Narui, N. Tsuchiya, N. S. Takahashi, and S. Kurita,

J. ApD1. Phys., 1989, 66, 3337. 56

J. M. Markakis and A. Y. Cheng, Nucl. Instrum. Methods Phvs. Res.. Sect. A, 1989, A283, 236.

57

M. Maejima, T. Hane, H. Makiguchi, and T. Horigome, Bull. Natl. Sci. Mus., Ser. E (Tokyo), 1988, U ,.

17.

513

Author Index

In t h i s Chapter numbers Chapter

i n d e x t h e numbers i n p a r e n t h e s e s a r e t h e Part and, where a p p l i c a b l e , numbers o f t h e c i t a t i o n and a r e f o l l o w e d b y t h e r e f e r e n c e number o r o f t h e r e l e v a n t c i t a t i o n s w i t h i n t h a t C h a p t e r , e . g . , (2.2) 1 4 Part 11 2, r e f . 14

Aaron, J.J. ( 1 ) 3 0 2 ; ( 2 . 2 ) 32

Abd El-Gawad, A.M. ( 2 . 4 ) 222; ( 2 . 6 ) 148

Abdel-Halim, S.T. ( 1 ) 298 Abdel-Kader, M.H. ( 1 ) 298 Abdel-Mottaleb, M.S.A. ( 1 ) 1 7 0 , 251, 265 Abdou, W.M. ( 2 . 5 ) 1 4 8 ; ( 2 . 6 ) 209 Abdulaziz, M. ( 3 ) 1 8 9 , 539 Abdul-Ghani, A. J. ( 2 . 5 ) 53; ( 4 ) 13, 14 Abdul-Kareem, S . ( 2 . 5 ) 5 3 ; ( 4 ) 1 3 , 14 Abdullaeva, Z.K. ( 3 ) 495 Abdullah, I. ( 3 ) 175 Abdunazarova, T.F. ( 3 ) 639 Abe, H. ( 4 ) 48 Abe, K. ( 2 . 2 ) 10; ( 2 . 6 ) 58 Abe, M. ( 2 . 5 ) 34 Aberg, V. ( 1 ) 249 Abou-Gamra, Z.M.A. ( 2 . 5 ) 5 8 ; ( 4 ) 29 Abraham, W. ( 2 . 7 ) 105 Abramovitch, R.A. ( 2 . 7 ) 74 Abuin, E.B. ( 1 ) 415 Achani, B. ( 1 ) 138 Ackermann, E. ( 2 . 7 ) 7 6 Adachi, G. ( 3 ) 318 Adachi, K. ( 3 ) 292 Adachi, T. ( 3 ) 1 3 3 , 134 Adam, C. ( 3 ) 4 8 4 , 492 Adam, G. ( 2 . 1 ) 35 Adam, W. (1) 457; ( 2 . 2 ) 135; ( 2 . 3 ) 4 1 ; ( 2 . 5 ) 2 2 , 109; ( 2 . 7 ) 1 9 , 26,

33

Adich, H.-J. ( 1 ) 6 8 ; ( 2 . 5 ) 82

Aebischer, J.N. ( 2 . 3 ) 26 Afans'ev, A. ( 3 ) 7 4 , 268 Afshari, E. ( 1 ) 463 Aglietto, M. ( 3 ) 29 Agman, N . ( 1 ) 127 Agosta, W.C. ( 2 . 4 ) 235, 236

Agostini, G. ( 1 ) 316 Ahmad, S.R. ( 3 ) 467 Ahmad, Z. ( 2 . 2 ) 33 Ahmad-Junan, S.A. ( 2 . 7 ) 126

Ahmed, S.A. ( 1 ) 310 Ahn, K.D. ( 3 ) 83 Aida, H. ( 3 ) 118 Ajayaghosh, A. ( 1 ) 494; ( 2 . 6 ) 80-82;

( 2 . 7 ) 173

Akabori, S . ( 2 . 2 ) 44; ( 2 . 6 ) 108

Akasaka, T. ( 2 . 5 ) 126 Akashi, R . ( 2 . 4 ) 167 Akerman, M.A. ( 3 ) 210, 244

Akimova, N.P. ( 2 . 2 ) 2 3 ; ( 2 . 6 ) 137

Akiyama, K. ( 1 ) 420, 429, 460

Akkaya, E.V. ( 1 ) 69 Alarid-Sargent, M. ( 2 . 6 ) 114

Albanov, A.I. ( 2 . 6 ) 193 Alberti, A. ( 2 . 6 ) 1 9 1 ; ( 2 . 7 ) 46

Alberti, G. (3) 621 Albini, A. ( 2 . 4 ) 3 , 259 Aleksandrova, L.N. ( 3 ) 164

Alfimov, M.V. ( 2 . 4 ) 185; ( 2 . 7 ) 197

Al-Hassan, K.A. ( 1 ) 206

Ali, M.A. ( 1 ) 310 Ali, M.B. ( 2 . 7 ) 153 Alif, A. ( 1 ) 500; ( 2 . 4 ) 72

Aliwi, S.M. ( 3 ) 149 Al-Jalal, N.A. ( 2 . 4 ) 21, 45

Al-Jarrah, M.M.F. ( 3 ) 63 Allaev, Zh. ( 3 ) 101 Allan, G. ( 3 ) 527 Allen, G.M. ( 3 ) 1 9 0 , 191 Allen, M.T. ( 1 ) 89 Allen, N.S. ( 3 ) 1 4 , 5 5 , 6 1 , 6 2 , 6 4 , 6 5 , 435, 496-498 Allmer, K. ( 3 ) 1 6 9 , 170 Al-Malaika, S. ( 3 ) 604 Almeida, R. ( 1 ) 1 5 7 , 158 Almgren, M. (1) 356 Alo-Aly, M.M. ( 1 ) 265 Aloisi, G.G. ( 1 ) 221; ( 2 . 3 ) 87 Alonso, E.O. ( 1 ) 152; ( 2 . 4 ) 243; ( 2 . 6 ) 207 Alonso, R.A. ( 2 . 4 ) 8 0 ; ( 2 . 7 ) 183 Al-Rawi, J.M.A. ( 3 ) 63 Al-Sayed, G. ( 2 . 4 ) 120 Alsins, J. ( 1 ) 356 Alt, R. ( 1 ) 496; ( 2 . 1 ) 7 Altmann, E. ( 2 . 7 ) 176 Altomare, A. ( 3 ) 297, 301, 313 Aly, M.M. ( 2 . 4 ) 131, 222; ( 2 . 6 ) 148; ( 2 . 7 ) 150 Amadelli, R. ( 3 ) 147 Amat-Guerri, F. ( 3 ) 628 Ambroz, H.B. ( 1 ) 505 Ameloot, M. ( 1 ) 36 Ameta, S.C. ( 2 . 5 ) 203 Amiet, R.G. ( 2 . 5 ) 205 Amin, K.A. ( 2 . 4 ) 194

Author Index Amine, C. (1) 104 Ammon, H.L. (2.2) 114; (2.6)

125

Amouyal, E. (1) 211; (4) 42

An, X. (4) 27 Anac, 0. (3) 554 Anda, K. (2.4) 197; (2.6) 129, 130

Anderson, A.B. (2.5) 95 Anderson, D.R. (2.1) 36; (2.4)

223; (2.6) 201 Andersson, K. (2.4) 192 Ando, H. (3) 513 Ando, W. (2.5) 126; (2.6) 160-162; (2.7) 110, 146 Andrade, J.D. (3) 555 Andrady, A.L. (3) 446, 461-463 And&, J.C. (1) 5, 73-75; (2.41.63 Andreev, V.L. (3) 222, 598 Andrei, C. (3) 590 Andresen, B. (1) 117 Andrews, D.L. (1) 304 Andrie, C. (3) 572 Andriessen, R. (3) 387 Andrzejewska, E. (3) 157 Angel, S.A. (2.3) 97 Angeloni, A.S. (3) 301 Angiolini, D. (3) 29 Anisimov, V.M. (3) 619 Anisimova, O.M. (3) 619 Anjaneyulu, A.S.R. (2.4) 174 Anklam, E. (2.7) 138, 144 Annable, T. (3) 391 Anpo, M. (2.5) 13, 88, 108; (4) 3 Anshyang, A.A. (3) 233 Antonettic, A. (1) 175 Antonies, M.S. (1) 265 Anufrieva, E.V. (3) 327 Aoki, K. (1) 485; (2.6) 29, 30; (3) 303 Aoyagi, N. (2.4) 74, 75; (2.6) 136 Aoyama, H. (2.2) 106; (2.4) 38; (2.6) 91, 109, 131 Aoyama, M. (3) 166, 290 Aparicio, F.J.L. (2.5) 185 Apikian, R.L. (3) 63 Apoita, M. (2.4) 159; (2.6) 23 Arai, H. (2.1) 30 Arai, M. (2.2) 110 Arai, S. (1) 189; (2.4) 139; (2.6) 40 Arai, T. (2.3) 13, 14,

515 24; (2.5) 83; (3) 223-225 Arai, Y. (1) 142 Arakawa, H. (2.5) 144 Araki, T. (2.2) 119-121, 123; (2.4) 48-51; (2.6) 118-120, 124 Aramaki, S. (1) 119 Aramendia, P.F. (1) 272 Arata, Y. (2.4) 38; (2.6) 109 Araux-Lara, J.L. (3) 353 Arbeloa, F.L. (1) 269 Arbeloa, I.L. (1) 269 Arbour, C. (1) 119 Arbuzov, B.A. (2.7) 45 Archer, S. (2.6) 38 Arct, J. (3) 574 Arevalo, M.A. (2.7) 87 Arhart, R.W. (2.2) 124; (2.4) 202 Ariamala, G. (2.1) 63; (2.7) 159 Aricioni, A. (1) 92 Ariel, S. (2.2) 6 1 Arlaud, P. (3) 450 Armesto, D. (2.3) 38-40, 55-57, 75, 76; (2.4) 158, 159; (2.6) 21-23, 50, 51, 62, 7 1 Arnold, D.R. (2.3) 27, 28, 112, 113; (2.4) 78, 79, 261, 262, 265 Aryan, R.C. (2.2) 50 Asahi, K. (3) 224 Asahi, T. (1) 173 Asai, R. (2.2) 52; (2.7) 163 Asaki, T. (1) 186 Asakura, T. (2.6) 200 Asami, Y. (2.4) 119; (2.5) 166 Ashihaga, K. (3) 272, 408 Ashok, K. (2.2) 127 Ashokkumar, M. (4) 39-41 Asimov, M.M. (1) 444 Askari, S. (2.2) 6 1 Askarov, M.A. (3) 101 Atabekyan, L.S. (2.6) 32 Atherton, S.J. (1) 346 Atkinson, G.H. (1) 119 Attanasio, D. (2.5) 153 Auband, J. (1) 482 Augugliaro, V. (2.5) 43 Aurich, J. (2.4) 73 Aussenegg, F.R. (1) 324 Avellar, I.G.J. (2.1) 5; (2.4) 227 Averbeck, D. (2.2) 25 Avetisyan, A.A. (2.2) 7; (2.6) 70 Avetisyan, T.V. (2.2) 7 '

Avgonov, A. (3) 623 Avudaithai, M. (4) 33 Anad, M.K. (2.5) 95 Ayachit, N.H. (1) 26 Ayotte, C. (2.7) 148 Ayyangar, N.R. (2.2) 131; (3) 611

Azadi-Ardakami , M. (2.1) 16

Azarov, V.A. (3) 479 Azumi, T. (1) 63, 206, 392; (3) 415

Baba, R. (2.5) 139 Baciocchi, E. (2.5) 111 Back, M.H. (2.3) 1 Back, R.A. (2.3) 1 Bader, A. (2.6) 204 Bader, M. (1) 246 Badr, M.Z.A. (2.4) 131, 222; (2.6) 150

148; (2.7)

Baeseler, M. (3) 576 Bagchi, B. (1) 23, 25, 156, 161, 249, 250

Bahar, I. (3) 372, 399 Bahr, N. (2.7) 27 Bai, Z. (3) 33 Bailey, W.P. (2.2) 122 Baillot, M.L. (1) 211 Baimuratov, E. (3) 559 Baird, W.M. (2.6) 106 Bajic, M. (2.4) 154; (2.6)

151

Baker, G.J. (1) 177 Bakhshinyan, R.V. (2.6) 70

Baklanov, M.V. (2.4) 184 Bakulev, V.A. (2.6) 75 Bal, S.Y. (2.5) 217 Balakrishnan, P. (2.5) 173

Balasubramanian, K.K. (2.1)

63; (2.7)

159

Balavoine, G. (2.7) 125 Balint, P. (3) 591 Ballesta, J.P.G. (2.7) 87 Bally, T. (2.3) 26 Balter, A. (1) 27 Bamji, S. (3) 584 Bandyopadhyay, A. (1) 58 Bandyopadhyay, S. (1) 138 Banerjee, U.K. (2.7) 59 Banert, K. (2.7) 73, 75 Banno, S. (3) 276 Bansal, W.R. (2.5) 6 1 Barabas, M. (3) 591 Baraldi, I. (1) 95, 402 Barbara, C. (2.5) 123 Barbara, P.F. (1) 163 Barbieri, B. (1) 52

Author Itidex

516

Barbosa, M.C.N. (2.4) 116; (2.7) 117 Bard, A . J . (4) 53 Barile, R.G. (4) 51 Barisas, B.G. (1) 3 1 Barkhash, V.A. (2.2) 48 Barlow, C.H. (1) 55 Barnes, J.C. (2.6) 7 1 Barni, E. (3) 621 Baros, F. (1) 5 Barrau, J. (2.7) 109 Barrow, C . J . (2.5) 186 Bartels, G. (3) 98 Bartnik, R. (2.6) 3 Bartocci, C. (2.5) 65; (3) 147 Barton, D.H.R. (2.1) 60, 62; (2.7) 125, 127, 130, 131, 134, 136 Bartus, J. (3) 573 Basak, S. (3) 345, 355 Basil, A . (1) 389 Bastiaens, P. (1) 128 Basu, S. (1) 218 Batchelder, D.N. (3) 546 Batrak, G.V. (3) 577 Bats, J.W. (2.2) 99 Battioni, P. (2.5) 65 Bauer, D.R. (3) 548, 607-609 Bauer, W. (2.3) 83 Baumann, H. (1) 278; (3) 25, 30 Baumler, W. (1) 262 Bayer, E. (2.6) 204 Bayer, H. (3) 205 Bayer, R. (2.2) 40; (2.6) 110 Bazhin, N.M. (1) 411 Bazhutin, Yu.V. (4) 24 Bean, D.L. (2.7) 109 Beard, M. (2.7) 221 Becker, D. (2.2) 34, 35, 37 Becker, H.-D. (2.4) 189, 192-194; (2.5) 175 Becker, P.C. (1) 20 Becker, R . S . (1) 275 Beckmann, E. (2.7) 27 Becknell, A.F. (3) 212 Beebe, T.R. (2.7) 221 Beechem, J.M. (1) 35 Begley, T.P. (2.7) 176 Behrens, S. (2.4) 16; (2.6) 56 Behruisch, J. (3) 470 Beileryan, N.M. (3) 112 Beinhert, G. (3) 395 Bell, T. (2.1) 2 Belled, C. (2.4) 209; (2.6) 156 Belletete, M. (1) 361

Bellinger, V. (3) 464, 465

Bellobono, I . G .

Binana-Limbele, W. (1) 355

(3) 10,

102

Belotti, D. (2.2) 83 Belousor, V.M. (2.5) 107 Beltrame, P.L. (3) 617 Bender, C.O. (2.3) 45 Bengough, W.I. (3) 88 Beniska, J. (3) 573 Bensasson, R.V. (1) 472; (2.2) 49 Bera, S.C. (1) 236 Berces, T. (1) 217; (2.5) 25, 26 Berezhnaya, V.N. (2.2) 145; (2.6) 93 Bergel'son, L.D. (2.7) 66 Bergman, A. (2.3) 17 Bergman, R.G. (1) 67 Bergmark, W.R. (1) 281; (2.4) 58, 191 Bernardi, R. ( 2 . 4 ) 76, 77; (2.6) 145 Bernardinelli, G. (2.5) 146 Berndt, K. (1) 50 Berson, J . A . (2.7) 31 Bertinelli, F. (1) 92 Bertolotti, S . G . (1) 363 Bertram, J. (2.4) 65 Best, M.E. (3) 265 Bestova, J. (2.3) 63 Bett, S.J. (3) 168 Beugelmans, R. (2.7) 186 Beveridge, K. A. (2.4) 257; (2.6) 147 Bhakuni, D.S. (2.4) 186, 187; (2.7) 191, 192 Bhalerao, U.T. (2.5) 218; (2.7) 172 Bhatt, R. (2.5) 203 Bhattacharyya, K. (1) 179, 205, 241, 243, 270, 282 Bhavsar, M.D. (3) 582 Bhushan, K. (1) 276 Bianchini, G. (3) 29 Bickelhaupt, F. (2.7) 179 Biczok, L. (1) 217 Bigat, J.-Y. (1) 244 Bigger, S.W. (3) 570, 575, 616 Bigger, W. (1) 148 Bigogono, G. (3) 29 Bigot, J.-Y. (1) 20 Billhardt, U.T. (2.2) 99 Billib, G. (2.6) 196 Billiet, E.M. (3) 467 Billingham, N.C. (3) 428 Billington, A.P. (2.6) 79 Bilmes, G.M. (1) 271

Binegar, G.A. (2.6) 8 Binkert, T. (3) 376 Binkley, R.W. (2.3) 115; (2.4)

273; (2.7)

154

Birkhart, R.D. (3) 400 Bisagni, E. (2.2) 25 Bischofberger , N (2.2)

.

78

Bishop, T.E. (3) 192, 193 Bisht, P.B. (1) 192, 193 Biswas, G.K. (2.4) 216; (2.6)

158

Biswas, S. (3) 67, 107 Biti, N. (2.4) 195, 196 Biverstedt, A . (3) 215 Bizzaro, G. (1) 413 Bjelakovic, M. (2.5) 143 Blackstock, S.C. (2.7) 3 1 Blair, J.T. (1) 134 Blanchard, G . J . (1) 258 Blanchard-Desie, M. (1) 190

Blatt, E. (2.5) 157, 158 Blazej, A. (2.5) 12; (4) 4

Blazkova, A. (3) 271 Blok, P.M.L. (1) 388 Bloor, D. (3) 546 Blumen, A . K . (1) 306, 330 Blunt, J.W. (2.5) 186 Boate, D.R. (2.5) 30; (2.6)

7

Boboev, T.B. (3) 639 Boens, N. (1) 36, 40; (3) 387

Boese, W.T. (2.5) 97 Bogdanov, R . I . (3) 615 Bogdanov, V.S. (2.3) 73, 74

Bohne, C. (1) 409, 415; (2.6)

33

Bois-Choussy, M. (2.7) 186

Boivin, J. (2.7) 131, 135 Bokobza, L. (3) 334, 385, 386

Bolotin, A. (3) 600 Bolte, M. (2.5) 202; (3) 148, 151

Bolz, G. (2.7) 57 Bonet, J.J. (2.5) 115, 116, 174

Bonilha, J.B.S. (2.4) 7 1 Bonneau, I?. (2.7) 4-7, 12, 13

Bonnett, R. (2.5) 57 Borisenko, E.M. (3) 512 Born, D. (2.3) 90; (2.7) 44

Bornancini, E.R.N.

(2.7)

517

Author Index 185 Bortolus, P. (3) 543 Bostan, C. (3) 163 Boszczyk, W. (2.7) 208 Botija, J.M. (3) 628 Bouas-Laurent, H. (2.4) 195, 196 Bouchet, M.-J. (2.7) 69 Bouchy, A . (1) 73 Bouldin, T.W. (2.2) 124; (2.4) 202 Boule, P. (1) 500; (2.4) 72, 107, 121, 122 Bourdelande, J.L. (3) 329 Bourson, J. (1) 312 Boutevin, B. (3) 275 Bova, L.M. (3) 613 Boven, J. (3) 269 Bowerle, R.J. (1) 130 Bowman, W.R. (2.7) 167 Boyd, I.W. (2.5) 219 Boyde, S. (1) 313 Boyer, J.H. (1) 280 Bozhkova, N. (2.2) 41 Brackemann, H. (3) 124 Bradely, D.D.C. (3) 417-419 Brady Clark, K . (1) 427 Brahms, J.C. (2.2) 122; (2.7) 123 Brand-Roth, B. (2.6) 182 Braren, B. (3) 454, 516 Braslavsky, S.E. (1) 271, 474 Brauer, H.D. (1) 68, 470, 491; (2.5) 75, 82; (2.6) 196 Brault, D. (1) 472 Brauman, S.K. (3) 427 Braun, A.M. (2.5) 77 Braun, M. (2.1) 42; (2.5) 109 Bremner, J.B. (2.1) 65; (2.7) 198 Bren, V . A . (2.3) 88 Brenna, J.T. (3) 519 Brewer, W.E. (1) 237, 435 Brezova, V. (2.5) 12; (4)

4 Bridon, D.P. (2.1) 59 Bright, F.V. (1) 45, 47, 238 Britt, P.F. (1) 333 Brocklehurst, B. (1) 177 Brodilova, J. (2.5) 165 Broer, D.J. (3) 159, 160, 269 Broggi, F. (3) 115 Bromberg, A. (2.5) 71 Brooks, D.W. (2.3) 45 Brown, R.F.C. (2.2) 107 Bruckmann, R. (2.7) 55

Bruhnke, J . D . (2.2) 46 Brun, A.M. (1) 191 Buback, M. (3) 124 Buchachenko, A.L. (2.1) 11; (2.5) 164; (2.6) 187 Buchanan, R.A. (2.6) 173 Buchardt, 0 . (2.7) 82 Buchheim, T. (2.7) 64 Buchholtz, F. (3) 307 Buchholz, N. (3) 229 Budac, D. (2.3) 119; (2.4) 256 Budyka, M.F. (2.4) 185; (2.7) 197 Buenger, G.S. (2.7) 207 Bulgakov, R.G. (2.5) 125, 187, 220 Bulinski, A. (3) 584 Bunce, N.J. (2.4) 123 Bunce, R . A . (2.2) 63 Buntinx, G. (1) 455 Bur, A.J. (3) 337, 340 Burdik, D.S. (1) 46 Burdon, J.W. (3) 428 Burgemeister, T. (2.3) 83 Burger, R. (2.7) 57 Burger, U. (2.6) 117 Buriakov, V.M. (3) 511 Burkhardt, V . (2.7) 57 Burkhart, R.D. (3) 377, 401, 417 Burmenko, A.S. (3) 266 Burns, G.R. (2.6) 4 Burr, D. (1) 506 Burrows, J.A.J. (3) 400, 401 Burrows, J . P . (2.5) 147 Burshtein, A . I . (1) 195 Burton, K . A . (2.7) 3 Buscemi, S. (2.4) 12-14; (2.6) 42, 57 Buschmann, H . (2.1) 43 Busmann, H.-G. (1) 28 Butera, R . J . (3) 260, 309 Butsugan, Y. (2.4) 118; (2.5) 155 Byers, J . (3) 363 Byrne, L.T. (2.5) 184 Byteva, I.M. (2.5) 76 Cabana, L.A. (1) 197 Cabello, A . (3) 487 Cabrera, I. (3) 307 Cacuphat, C. (3) 510 Cadet, J . (2.2) 26 Cadona, L. ( 3 ) 115 Caffieri, S. (2.2) 24 Cagan, M. (3) 252 Cage, M.R. (3) 198 Cai, S.X. (2.7) 81

Caldwell, R.A. (2.2) 134; (2.4) 39, 40 Callais, P.A. (3) 597 Callinan, A . (2.3) 72 Callis, P.R. (1) 125 Calvo-Flores, F.G. (2.5) 185 Camilleri, P. (1) 368 Campa, C. (3) 329 Campbell, A.K. (1) 475 Campbell, T.C. (2.3) 44 Candau, F. (3) 85, 135 Cantos, A . (2.4) 66, 67, 70 Cao, W. (3) 43 cao, x. (3) 335 Cao, Y. (2.3) 84; (2.5) 159, 160, 163; (3) 155 Capuano, L. (2.7) 57 Carassiti, V. (2.5) 65 Caretti, D. (3) 301 Carless, H.A.J. (2.5) 124 Carlin, C.M. (3) 421 Carlini, C. (3) 29, 52, 128, 301, 313 Carlsson, D.J. (3) 610 Caronna, T. (2.4) 76, 77; (2.6) 145 Carpenter, B.K. (2.6) 79 Carpignano, R. (3) 621 Carre, C. (3) 16 Carroll, P . J . (2.6) 168 Carter, R.O. (3) 607 Carver, M.T. (3) 85, 135 Casal, H.L. (2.1) 48 Cascallo, M. (2.5) 174 Caspar, J.V. (1) 406; (2.3) 7 Cassel, J.M. (2.3) 37 Castagnino, E. (2.1) 60; (2.7) 127, 129 Castel, N. (1) 101, 103, 295; (2.3) 15; (2.4) 142 Castellan, A . (2.4) 195; (3) 510 Castillo, F. (3) 465, 466 Castle, R.N. (2.4) 157 Castro, C. (2.5) 195 Catalina, F. (3) 55, 61, 62, 64, 438, 566 Catena, G.C. (1) 238 Caudano, R. (3) 525 Cavatorta, P. (1) 90 Cavazza, M. (2.2) 4 Celewicz, L. (2.1) 55; (2.7) 115 Ceppan, M . (2.5) 12; (4) 4 Cerfontain, H . (2.2) 70; (2.3) 61 Cermak, V . (3) 74, 268

Author index

518 Cha, D.D. ( 2 . 7 ) 5 4 Chae, K.H. ( 3 ) 7 5 , 83 Chaidez, M.A. ( 2 . 3 ) 2 Chaiko, A.K. ( 3 ) 258 Chakrabarti, A. ( 2 . 4 ) 216; ( 2 . 6 ) 158

Chakrabarty, T. (1) 282 Chakraborty, D.K. ( 1 ) 417; ( 3 ) 377

Chakraborty, D.P. ( 2 . 4 ) 216; ( 2 . 6 ) 158

Chakraborty, S . ( 1 ) 138 Chakraborty, T. ( 1 ) 242, 243

Chakravorti, S . ( 1 ) 275 Challa, G. ( 3 ) 159, 1 6 0 , 269

Chaltykyan, R.O. ( 3 ) 112 Chalupa, V. ( 3 ) 27 Chambers, R.C. ( 2 . 5 ) 94 Chambers, R.D. ( 2 . 4 ) 203, 204; ( 2 . 7 ) 4 0 , 41

Chamizo, J.A. ( 2 . 6 ) 49 Champagne, B.B. ( 1 ) 284 Chan, A.C. ( 2 . 4 ) 210; ( 2 . 6 ) 45

Chan, B.G. ( 2 . 2 ) 146 Chan, C.L.K. ( 2 . 7 ) 149 Chan, S.C. ( 3 ) 254, 277 Chandler, C.J. ( 2 . 4 ) 175; ( 2 . 7 ) 193

Chandra, A. ( 1 ) 2 5 , 161 Chandra, R. ( 3 ) 586 Chang, C.H. ( 3 ) 9 4 , 9 5 , 368

Chang, V.C. ( 2 . 1 ) 13 Chang, W. ( 2 . 7 ) 24 Chanon, M. ( 2 . 4 ) 8 1 ; (2.5) 7

Chapat, J.-P. ( 2 . 7 ) 9 2 , 182

Chapman, C.F. ( 1 ) 225 Chapman, H., jun. ( 2 . 3 ) 99

Chapman, W.H. ( 2 . 4 ) 264 Char, K. ( 3 ) 356, 364, 366

Charubala, R. ( 2 . 7 ) 101 Chateauneuf, J.E. ( 2 . 7 ) 180

Chattopadhyay, N. ( 1 ) 235, 242

Chauveau, F. ( 2 . 5 ) 91 Chavan, A.J. ( 2 . 7 ) 1 4 Che, C.M. ( 4 ) 23 Chedekel, M.R. ( 2 . 2 ) 28 Chemerskaya, Z.F. ( 3 ) 110 Chen, C . ( 2 . 5 ) 101, 127 Chen, C.Y. ( 2 . 6 ) 140 Chen, D. ( 2 . 5 ) 5 0 , 119-121, 209; ( 2 . 6 ) 67

Chen, H. ( 3 ) 238

Chen, J. ( 2 . 3 ) 5 1 ; ( 2 . 5 ) 135, 160; ( 4 ) 46 Chen, L. ( 2 . 7 ) 189; ( 3 ) 285 Chen, M.-Y. ( 2 . 6 ) 170 Chen, N. ( 2 . 7 ) 178 Chen, Q. ( 2 . 6 ) 6 4 ; ( 2 . 7 ) 222 Chen, W. ( 3 ) 5 5 , 335 Chen, X. ( 2 . 5 ) 216 Chen, Y. ( 3 ) 219, 505 Chen, Z. ( 2 . 5 ) 136; ( 4 ) 49 Cheng, A.Y. ( 4 ) 56 Cheng, M.-C. ( 2 . 6 ) 170 Cherkashin, M.I. ( 2 . 6 ) 34 Cherkasov, A.S. ( 2 . 4 ) 244 Chernoivanov, V. A. ( 2 . 3 ) 88 Chi, C.M. ( 2 . 5 ) 150 Chiang, W.Y. ( 3 ) 254, 277 Chiba, K. ( 2 . 4 ) 1 0 5 ; ( 2 . 5 ) 48 Chiba, M. ( 2 . 5 ) 36 Chibisov, A.K. ( 2 . 6 ) 32 Chiellini, E. ( 3 ) 301 Chin, J.-J. ( 2 . 4 ) 1 0 1 ; ( 2 . 7 ) 145 Chmela, S : ( 3 ) 6 0 6 , 610 Cho, 1.4. ( 2 . 4 ) 188 Cho, I.H. ( 2 . 2 ) 1 3 2 , 1 3 7 ; ( 2 . 4 ) 144 Cho, I.N. ( 2 . 2 ) 147 Cho, K.C. ( 2 . 5 ) 1 5 0 ; ( 4 ) 23 Choi, W.C. ( 3 ) 75 Chokhavatia, A.S. ( 1 ) 310 Chosa, J. ( 2 . 3 ) 2 1 ; ( 2 . 4 ) 275; ( 2 . 6 ) 178 Chou, P.-T. ( 1 ) 229, 237, 4 3 5 , 436; ( 2 . 5 ) 41 Chou, S.H. ( 1 ) 347 Choudhry, G.G. ( 2 . 4 ) 125 Chow, Y.L. ( 2 . 7 ) 218, 219 Chowdhry, H.C. ( 2 . 5 ) 203 Chowdhury, M. ( 1 ) 218 Chowdhury, N. ( 1 ) 235, 242 Christensen, P.L. ( 1 ) 66 Christl, M. ( 2 . 1 ) 42 Chtaib, M. ( 3 ) 525 Chu, C.-L. ( 2 . 4 ) 188 Chu, D.Y. ( 1 ) 332; ( 3 ) 361 Chuang, M.C. ( 3 ) 521 Chughtai, A.R. ( 3 ) 499 Chunaev, Yu.M. ( 2 . 3 ) 6 7 , 6 9 ; ( 2 . 4 ) 170, 1 7 2 ; ( 2 . 6 ) 2 5 , 26 Chung, W.-S. ( 2 . 1 ) 4 1 ; ( 2 . 3 ) 98 Chupka, E . I . ( 3 ) 511

Chuvzshev, D.D. ( 3 ) 511 Ci, X. ( 2 . 4 ) 6 ; ( 2 . 5 ) 32, 188

Ciardelli, F. ( 3 ) 2 9 , 5 2 , 1 2 8 , 297, 313

Cimminiello, G. ( 2 . 5 ) 182 Cioni, L. ( 3 ) 29 Clancy, R. ( 3 ) 1 4 , 496, 497

Clark, M.D. ( 3 ) 392 Clark, V.M. ( 2 . 4 ) 183 Clawson, P. ( 2 . 4 ) 266, 267; ( 2 . 7 ) 160-162

Clemance, M. ( 2 . 7 ) 8 4 Clemitshaw, K.C. ( 2 . 5 ) 210

Clennan, E.L. ( 1 ) 200; ( 2 . 5 ) 216

Climent, M.J. ( 2 . 5 ) 180 Clough, R.L. ( 3 ) 413 Cobb, W.T. (1) 49 Coggiola, D. ( 2 . 4 ) 76 Colaneri, N.F. ( 3 ) 419 Colditz, R. ( 2 . 4 ) 161 Cole, E.R. ( 2 . 7 ) 149 Cole-Hamilton, D.J. (1) 368

Collart, P. ( 3 ) 334, 385 Collin, J.P. ( 2 . 5 ) 9 Collins, S . ( 1 ) 394; ( 3 ) 414

Colombo, N. ( 3 ) 510 Colston, J.E. ( 2 . 7 ) 89 Combs, L. ( 2 . 5 ) 93 Comeford, L. (1) 259 Comes, F.J. ( 1 ) 491 Corns, F.D. ( 2 . 7 ) 18 Condeau, S.J. ( 1 ) 355 Conesa, J.C. ( 2 . 5 ) 43 Conlin, R.T. ( 2 . 6 ) 181 Cook, J. ( 3 ) 624 Coombes, P. ( 2 . 7 ) 99 Cooper, G.F. ( 2 . 2 ) 67 Coqueret, X. ( 3 ) 259 Corbin, D.R. (1) 406; ( 2 . 1 ) 6 , 9 , 22; ( 2 . 3 ) 7 , 9 5 ; ( 2 . 4 ) 200 Cordola, E. ( 3 ) 601 Corey, E.J. ( 1 ) 465 Cornelisse, J. ( 1 ) 351; ( 2 . 4 ) 24 Corsano, S . ( 2 . 1 ) 6 0 ; ( 2 . 7 ) 1 2 7 , 129 Cortes, M. ( 2 . 5 ) 116 Corvaja, C. (1) 401, 413 Corval, A. ( 1 ) 252 Cosa, J.J. ( 1 ) 263, 363, 364 Cosstick, K. ( 2 . 4 ) 37 Cossy, J. ( 2 . 2 ) 8 2 , 8 3 Costa, S.M.B. ( 1 ) 357; ( 2 . 5 ) 5 4 ; ( 4 ) 30

5 19

Author index Cotter, W.D. ( 2 . 4 ) 8 8 ; ( 2 . 6 ) 88

Courtney, S.H. ( 1 ) 286; (2.3) 5

Couture, A. ( 2 . 4 ) 146 Cox, A. ( 2 . 4 ) 183 Cox, R.A. ( 2 . 5 ) 147 Coyle, J.D. ( 2 . 4 ) 47 Cozens, F. (2.4) 238 Crabbendam, A.M. ( 2 . 2 ) 70 Crabtree, R.H. ( 2 . 5 ) 1 Crago, K.T. ( 1 ) 280 Craig, B.B. ( 1 ) 144 Craig, D.C. ( 1 ) 167 Craig, R.A. ( 1 ) 148 Craik, D.J. ( 2 . 4 ) 175; ( 2 . 7 ) 193

Cranston, D.H. ( 3 ) 391 Creary, X. ( 2 . 7 ) 184 Creaser, 1.1. ( 4 ) 15 Creasy, W.R. ( 3 ) 519 Creed, D. ( 1 ) 503; ( 2 . 2 ) 134; ( 3 ) 230 Cremer, D. ( 2 . 7 ) 108 Crimins, M.T. ( 2 . 2 ) 36; ( 2 . 4 ) 33 Cristol, S.J. ( 2 . 7 ) 153 Crivello, J.V. (1) 506; ( 3 ) 22, 50, 202, 203, 236 Crocker, P.J. ( 2 . 7 ) 14 Crossen, C. ( 1 ) 245 Crossman, D. ( 3 ) 237 Crouch, S.R. ( 1 ) 450 Croucher, M.D. ( 3 ) 322 Crozet, M.P. ( 2 . 4 ) 8 2 ; ( 2 . 6 ) 146; ( 2 . 7 ) 182, 187 Csongar, C. ( 2 . 7 ) 36, 37, 38 Cubeddu, R. ( 1 ) 328 Cumar, C.V. ( 2 . 4 ) 200 Cunningham, C.W. ( 2 . 6 ) 4 Cuppen, T.J.H.M. ( 1 ) 101; ( 2 . 3 ) 15 Curran, D.P. ( 2 . 7 ) 133 Cusmano, G. ( 2 . 4 ) 14 Czeckaj, T. ( 3 ) 604 Czornik, A.W. ( 1 ) 69

Dabovic, M. (2.5) 143 Dad&, V. ( 2 . 1 ) 6 4 ; ( 2 . 5 ) 64

Dadomatov, Kh.D. (3) 639 Dagata, J.A. ( 2 . 4 ) 135 Dahlmann, J. ( 3 ) 576 Dais J. ( 2 . 5 ) 101 Dais S. (2.3) 78 Dailey, W.P. ( 2 . 7 ) 123 Daino, Y. (2.3) 16 Dainty, C. ( 1 ) 368

D'Alessandro, N. ( 2 . 4 ) 259

Daley, R.C. ( 3 ) 9 Dall'Acqua, F. ( 2 . 2 ) 24 Dalton, L.R. ( 1 ) 246 Daltrozzo, E. ( 1 ) 445 Damaskinos, S. ( 4 ) 11 Danheiser, R.L. ( 2 . 7 ) 54 Danilova, L.M. ( 3 ) 479 Dannals, R.F. ( 2 . 7 ) 86 Das, P.K. ( 1 ) 495; ( 2 . 2 ) 127; ( 2 . 4 ) 241

Das, R. ( 1 ) 41 Das, S. ( 1 ) 275 Das, T.K. ( 2 . 4 ) 213-215; ( 2 . 6 ) 46, 47

da Silva, E. ( 2 . 7 ) 131, 135

Date, T. ( 2 . 2 ) 9 2 ; (2.6) 20

Datta, D.B. ( 2 . 4 ) 214, 215; ( 2 . 6 ) 46

Datta, I. ( 2 . 4 ) 213, 214; ( 2 . 6 ) 46, 47

Datta Roy, R. ( 1 ) 315 Daub, J. ( 2 . 3 ) 8 2 , 83 Dauben, W.G. ( 2 . 1 ) 59 Daucher, B. (2.7) 56 Dauphin, G. ( 2 . 7 ) 92 D'Auria, M. ( 2 . 4 ) 8 , 91-95; ( 2 . 5 ) 7 9 ; (2.7) 199, 202, 203, 205, 206 Dave, B. ( 2 . 7 ) 71 David, C. ( 3 ) 460 Davidson, S. ( 3 ) 414 Davis, A. ( 1 ) 281 Davis, F.A. ( 2 . 6 ) 69 Davro, A. ( 3 ) 460 Davydov, B.E. ( 3 ) 158 Day, R.J. ( 3 ) 546 Day, R.O. (2.6) 206 DeAmicis, C.V. ( 2 . 1 ) 13 Dean, K. ( 3 ) 363 De Arms, P. ( 2 . 5 ) 194 Deblauwe, V. ( 3 ) 310 De Boer, S. ( 1 ) 277 DeFber, T.J. ( 2 . 5 ) 112 DeBruyn, V.H. ( 2 . 4 ) 89 DeCamp, A. ( 2 . 2 ) 117; ( 2 . 6 ) 126 Declemy, A. ( 1 ) 21 Declercq, J.P. ( 2 . 1 ) 4 5 ; (2.3) 9 De Clercq, R. ( 3 ) 455 DeCosta, D.P. ( 2 . 1 ) 1 2 ; ( 2 . 4 ) 258; ( 2 . 7 ) 121 Decout, J.L. (2.2) 8 8 , 89 Dedek, V. ( 2 . 1 ) 6 4 ; ( 2 . 5 ) 64 De Felippis, J. ( 2 . 3 ) 17 DeFrees, S.A. ( 2 . 6 ) 168 Deglise, X. ( 2 . 4 ) 63

De Graff, B.A. ( 1 ) 38 Degtyareva, A.A. ( 3 ) 144 de Haas, M.P. ( 1 ) 222 Deibel, R.M. ( 2 . 6 ) 106 Dekker, C. ( 3 ) 53, 59 Dekkers, H.P.J.M. ( 1 ) 388 Dektar, J.L. (2.4) 220; ( 2 . 7 ) 217; (3) 142 Delarue, M. (1) 188 Delatycki, 0. ( 3 ) 616 del Barrio, J.I. ( 1 ) 419 Delcourt, M.O. ( 4 ) 42 Del Giacco, T. ( 2 . 5 ) 111 Della Negra, S. ( 2 . 2 ) 26 Dell'Erba, C. ( 2 . 7 ) 188 Dellinger, M. ( 1 ) 472 Dellonte, S. ( 2 . 6 ) 191 De Lombaert, S. ( 2 . 6 ) 150 Deluca, J.P. ( 2 . 7 ) 42 de Lucas, C.M. ( 1 ) 39 De Lucchi, 0. ( 2 . 3 ) 41; (2.7) 26

de Man, X. ( 2 . 3 ) 9 De March, P. ( 3 ) 329 Demas, J.N. ( 1 ) 38 de Mayo, P. ( 2 . 4 ) 199 Dembele, A. (2.5) 69 Demchuk, M.I. ( 1 ) 256 Demeter, A. ( 2 . 5 ) 25, 26 De Meulemeester, R. ( 3 ) 525

DeMico, A. ( 2 . 4 ) 91-94; ( 2 . 7 ) 202, 203, 205, 206 Demmig, S. ( 1 ) 248 Demuth, M. ( 2 . 2 ) 5 , 75

Demyashkevich, A.B. (1) 371

Denisenko, S.N. ( 2 . 7 ) 220 Denisov, L.K. ( 2 . 2 ) 21-23, 6 4 ; ( 2 . 6 ) 137

Denisova, L.N. ( 2 . 4 ) 244 Densley, J. ( 3 ) 584 De Paoli, M.-A. ( 3 ) 544 De Piccoli, F. (1) 52 De Rossi, R.H. ( 2 . 4 ) 206 DeSaja, J.A. ( 3 ) 477 De Schryver, F.C. ( 1 ) 36, 40, 251, 358; ( 3 ) 136, 333, 334, 385-387 Deshayes, H. (2.5) 69 Deshmukh, B.T. ( 3 ) 416, 430 Desiraju, G.R. (2.6) 101 Desobry, V. ( 3 ) 56 Despres, A. ( 1 ) 458, 459; ( 2 . 7 ) 80 Desvergne, J.-P. ( 2 . 4 ) 186, 195 Devadoss. C. ( 1 ) 87. 492 Devadze, L.V. ( 2 . 6 ) 36 Deval, P. ( 2 . 3 ) 60 ~

520

Author Index

Devi, M.R. (2.4) (2.7)

176;

194

Devine, R.L.S. (1) 246 De Voe, R.J. (1) 204 de Wolf, W.H. (2.7) 179 Dey, J.K. (1) 131 Dhanya, S. (1) 179 Dhingra, R.C. (3) 515 Diamantino, M.T.R.S. (2.4)

71

Diener, G. (1) 278 Dietliker, K. (3) 56 Dietz, T.M. (2.5) 60 Dillon, M.P. (3) 413 Dilung, 1.1. (3) 73, 110 Di Mascio, P. (2.5) 78 Dimitrov, V. (2.3) 49 Ding, Y. (3) 150 Dirlikov, S.K. (3) 187 Dissinger, S. (2.7) 89 Dixit, R.R. (2.7) 67 Dixon, M. (2.5) 195 Dmitrieva, A.F. (3) 434 Dobkowski, J. (1) 207, 208

Docchio, F. (1) 328 Doeller, U. (2.2) 99 Dopp, D. (2.6) 133 Dorr, M. (2.7) 26 Dogra, S.K. (1) 131, 230 Dohi, M. (1) 456 Doi, M. (2.4) 155, 156; (2.6) 44

Doi, T. (2.5) 88 Doi, Y. (3) 79 Dolgachev, A.N. (3) 588 D'Oliveira, J.C. (2.4)

D'Souza, F. (1) 196 Du, X. (2.3) 28; (2.4) 79 Du, X.-M. (2.7) 11 Dubest, R. (1) 482 Dubonosov, A.D. (2.3) 88 Dubovenko, L.I. (2.5) 207 Duchowicz, R. (1) 272 Duek, E.R. (3) 544 Duerner, G. (2.2) 99 Duffield, A.M. (2.7) 149 Dufour, M. (2.4) 182 Duguid, R.J. (2.3) 7 1 Duley, W. (3) 527, 540 Dunbar, R.C. (2.4) 134 Dunn, B. (1) 483 Duportail, G. (1) 370 Durand, T. (1) 498 Dureja, P. (2.3) 29 Durocher, G. (1) 337, 361 Durr, €I. (2.3) 70 Durr, M. (2.3) 4 1 Dutt, M. (2.4) 178; (2.7) 195

Dutts, P.K. (1) 481 Duveneck, G.L. (1) 287 Dvorak, D. (2.6) 114 Dvornikov, A.S. (2.3) 73, 74

Dworjanyn, P.A. (3) 167, 168

Dyer, P.E. (3) 517 Dykstra, R.E. (1) 220 Dymond, C.M.G. (1) 346 Dzhalilov, A.T. (3) 165 Dzhaparidze, K.G. (2.6) 36

120

Do Minh, T. (2.6) 73 Dong, F. (3) 89 Dongyi, C.W. (3) 131 D'Onofrio, F. (2.4) 91-94; (2.5) 79; (2.7) 202, 203, 205, 206 Dorgham, K. (3) 563 Dorigo, A.E. (1) 145 Doubleday, C. (1) 453 Dougherty, D.A. (2.7) 18, 22, 23; (3) 557 Douglas, K. (2.7) 84 Dowd, P. (2.7) 24 Dowling, K.C. (3) 320 Drabkina, A.M. (3) 84 Drain, K.F. (3) 190, 191 Drake, K.E. (1) 245 Draxler, S. (1) 324 Dreeskamp, H. (1) 43 Drew, J. (1) 97 Drew, M.G.B. (2.4) 22, 23 Drew, P. (2.1) 58 Dreyer, U. (3) 85, 135 Drozdenko, I.V. (3) 73

Eastwood, F.W. (2.2) 107 Eaton, D.F. (2.1) 9, 22 Eaton, R.F. (3) 279 Ebbesen, T.W. (1) 299; (2.7)

63

Ebeid, E.M. (1) 123, 223 Eckberg, R.P. (3) 200, 20 1

Edmunds, A.J.F. (2.7) 30 Edwards, M. (2.2) 114 (2.6)

125

Efimov, A.A. (3) 577, 578 Eftink, M.R. (1) 72, 25 Eggert, L. (2.4) 73 Egidis, F.M. (3) 578, 588 Egorov, M.P. (2.7) 39 Egorov, S.Yu. (1) 462; (2.5)

73

Egusa, C. (2.2) (2.6)

119;

124

Ehret-Sabatier, L. (2.7) 68, 69

Ehrlich, P. (3) 152 Eichen, Y. (1) 384

Eichorn, M. (2.2) 99 Eiermann, U. (2.7) 139 Eisenthal, K.B. (1) 287, 454, 489, 499

Eitan, A. (3) 274 Ekelmans, G.B. (1) 428; (2.5)

42

Ekiz-Guecer, N. (2.2) 13, 65, 66; (2.6)

98

El-Azim, S.A. (1) 223 El-Daly, S.A. (1) 123 Eldin, N.K. (2.2) 138 Eley, C.N. (2.1) 36; (2.4)

223; (2.6)

201

Elisei, F. (1) 221, 493 El Ouriaghli, T. (3) 306 Elsaesser, T. (1) 130, 232

El-Sayed, B.A. El-Shall, M.S. El'tsov, A.V. Encinas, M.V.

(1) 265 (3) 123 (4) 44 (1) 363, 364; (3) 68, 69 Endo, Z. (2.3) 33 Engel, P.S. (2.3) 91; (2.6) 9; (2.7) 2 Engelhardt, L.M. (2.1) 65; (2.5) 184; (2.7) 198 Engstrom, S. (1) 326 Ennis, P.M. (2.5) 198 Enomoto, Y. (1) 165 Ensley, H.E. (2.5) 173 Ephardt, H. (1) 253 Erden, I. (2.5) 195 Ergonenc, P. (2.5) 195 Eriksen, J. (2.2) 86; (2.6) 144 Ermolenko, I.N. (3) 512 Ernsting, N.P. (1) 154 Erra-Balsells, R. (2.4) 205; (2.6) 95; (2.7) 209, 210 Essig, H. (3) 503 Eswaran, S.V. (2.7) 9 1 Eto, S. (3) 211 Evans, C. (1) 409 Evans, N.A. (2.5) 205 Evans, S.V. (2.1) 26 Evesque, P. (1) 305 Ezzell, S.K. (3) 342, 535, 536

Fabbri, D. (3) 297 Fabian, J. (1) 116 Factor, A. (2.4) 217; (2.6)

157

Fadnis, A.G. (2.5) 131 Fageol, P. (3) 148, 151 Fahmy, A.M. (2.4) 131, 222; (2.6)

148; (2.7)

Author Index 150 Fain, B. (1) 11, 19 Faizi, N.Kh. (3) 493 Falcetta, M.L. (2.3) 106 Falk, K.J. (1) 140, 141; (2.6) 169 Fal'kengof, A.T. (4) 18 Fan, H. (2.7) 11 Fan, M. (2.2) 87; (2.3) 85; (2.6) 134 Fan, M.G. (2.6) 33 Fan, P. (1) 296; (2.2) 87; (2.6) 134 Fanconi, B.M. (3) 340 Fang, J.-M. (2.6) 170 Fang, P. (3) 138 Farahat, M.S. (2.2) 105; (2.6) 132 Faria, J.L. (2.4) 248 Farmer, L. (2.7) 148 Farrow, S.J. (2.6) 106 Fasani, E. (2.4) 259 Fatunmbi, H.O. (1) 17 Favaro, G. (1) 493; (2.3) 87 Favre, A. (2.6) 175 Fayer, M.D. (1) 255; (3) 336 Feddersen, B.A. (1) 48 Fedevich, E.V. (2.5) 113 Fee, R.S. (1) 225 Feigelman, V.M. (2.4) 138 Feldman, K.S. (2.2) 100 Feldman, L. (3) 197, 198 Feldmann, D. (3) 526 Fendler, J.H. (3) 86, 137 Feng, L. (2.5) 136; (4) 49 Feng, S. (3) 488, 603 Feng, X. (3) 43, 131, 138, 174 Feng, P. (3) 187 Ferri, A. (3) 147 Ferri, T. (2.4) 95 Ferrigno, K. (3) 237 Fessenden, R.W. (1) 87, 182, 408, 447, 492 Ficini, J. (2.5) 123 Field, G. (3) 439 Fields, B.A. (3) 168 Figueredo, M. (2.4) 67 Fillol, L. (2.4) 207; (2.7) 112 Fink, M.J. (2.6) 183 Finzel, R. (2.7) 33 Fischer, A. (2.6) 83 Fischer, C. (2.3) 82 Fischer, E. (1) 101, 103, 295, 402; (2.2) 38; (2.3) 15; (2.4) 142 Fischer, K. (2.3) 70 Fischer, M. (2.7) 76

52 1

Fisera, L. (2.6) 60 Fissi, A. (3) 296, 297, 313 Fitch, R.M. (3) 135 Fite, C. (2.7) 25 Fitzgerald, E.C. (1) 387 Fitzmaurice, D.J. (1) 76 Flamuersheim, H.J. (3) 129 Fleming, G.R. (1) 23, 161, 286; (2.3) 5 Fleming, M.D.C.M. (2.7) 142 Fleury, E. (3) 275 Flint, E.B. (1) 478 Flippen-Andersen, J . (2.4) 115; (2.7) 118 Flynn, C.E. (3) 323 Fodor, 2. (3) 591 Fofana, M. (3) 405 Fogel, V.R. (3) 338 Fokin, E.P. (2.2) 145; (2.6) 93 Follows, G.W. (3) 435 Font, J. (3) 329 Fontaine, C. (2.6) 175 Fontana, F. (2.4) 116; (2.7) 117 Foote, C.S. (2.5) 71, 128, 129, 172, 184 Forbes, J.E. (2.7) 137, 151, 152 Ford, C.W. (2.2) 43 Ford, W.E. (1) 274, 385, 448 Fordeeva, N.A. (2.7) 211 Fornier de Violet, P. (3) 510 Forskkahl, I. (3) 506-509 Foster, S.J. (2.6) 103 Fouassier, J.P. (1) 433, 506, 507; (3) 13, 15, 16, 54, 58, 60 Fourrey, J.-L. (2.6) 175 Fox, M.A. (1) 333; (4) 45, 53 Fragnito, H.L. (1) 20, 244 Franiseo, C.G. (2.5) 194 Frank, C.W. (3) 324, 356, 364-367 Frankevich, E.L. (3) 422 Franzke, M. (2.5) 38 Frasca, A.R. (2.7) 209, 210 Freedman, P.K. (2.5) 39 Freeman, H.S. (3) 620 Frei, B. (2.2) 51, 78; (2.5) 118 Frei, H. (2.5) 104-106 Frey, W. (1) 232 Friar, J.J. (2.4) 145

Friedrich, J . (1) 445 Friend, R.H. (3) 417-419 Friere, J.J. (3) 379 Friesner, R.A. (3) 363 Frochtenicht, R. (2.4) 19 Frolov, A.N. (2.4) 184 Fromherz, P. (1) 253 Fronda, A. (2.7) 56 Fu, D. (2.2) 20, 84; (2.4) 96, 98; (2.6) 135 Fu, X. (3) 267 Fueki, K. (3) 461, 463 Fugiwara, Y. (2.2) 103 Fuhrmann, J. (3) 397, 406, 407 Fujihara, H. (2.4) 101; (2.7) 145 Fujii, T. (1) 366 Fujimoto, M. (1) 239, 240 Fujimura, S. (2.1) 46; (2.4) 271; (2.7) 177 Fujino, M. (3) 415 Fujishima, A. (2.4) 117, 118; (2.5) 137-139; (3) 153 Fujita, T. (2.2) 112; (2.6) 167 Fujiyama, R. (2.6) 198, 199 Fuka, J. (2.3) 63 Fukae, R. (3) 121, 125 Fukaya, H. (2.6) 100 Fukshansky, L. (1) 56 Fukumura, H. (3) 284 Fukunishi, K. (2.4) 127 Fukuoka, K. (2.5) 126 Fukuzumi, S. (2.4) 60; (2.5) 36, 59, 66, 67, 162, 199; (2.7) 119 Funayama, K. (2.1) 31; (2.5) 23 Funfschilling, F. (1) 390 Furabashi, M. (1) 178 Furlong, D.N. (2.5) 157, 158 Furth, P.S. (2.1) 47; (2.6) 189 Furui, M. (2.4) 173; (2.6) 27 Furukawa, J. (3) 92, 162 Furukawa, N. (2.4) 101; (2.7) 145 Furumatsu, Y. (2.7) 1 Furusawa, A. (3) 533 Furuuchi, H. (2.3) 13 Furuja, Y. (2.6) 17 Furuzawa, A. (3) 534 Futami, €I(2.1) . 8 Fuyuki, T. (4) 54 Gab, S. (2.3) 30

522 Gabelica, V. ( 2 . 7 ) 215 Gaber, J. ( 3 ) 542 Gaber, M. ( 1 ) 223 Gaborian, F. (2.2) 26 Gafurov, B.L. ( 3 ) 101 Gahr, M. ( 2 . 3 ) 20; ( 2 . 4 ) 276; ( 2 . 6 ) 141

Galdi, A. ( 2 . 4 ) 81 Gallego, M.G. ( 2 . 3 ) 40, 5 5 , 5 6 ; ( 2 . 4 ) 158, 159; ( 2 . 6 ) 21-23, 51 Galli, G. ( 3 ) 301 Ganapathy, S. ( 2 . 3 ) 54 Gandhi, R.P. ( 2 . 1 ) 20; ( 2 . 2 ) 50, 7 2 ; (2.6) 48 Gao, F. ( 2 . 5 ) 214 Gao, Y. ( 2 . 5 ) 127 Gao, Z. ( 2 . 2 ) 8 4 ; ( 2 . 4 ) 9 6 , 9 8 ; ( 2 . 6 ) 135; ( 3 ) 488, 603 Garaham, J.C. ( 3 ) 542 Garbarczyk, J. ( 3 ) 157 Garbarino, G. ( 2 . 7 ) 188 Garbassi, F. ( 3 ) 518, 530 Garcia, H. ( 2 . 5 ) 180 Garcia-Garibay, M. ( 2 . 3 ) 51, 52 Gardette, D. ( 2 . 4 ) 181 Gardette, J.L. ( 3 ) 468, 497 Garnett, J.L. ( 3 ) 167, 168 Garrido, J. ( 3 ) 6 8 , 69 Gasche, J. ( 2 . 6 ) 175 Gast, A.P. ( 3 ) 356, 364, 366 Gatechair, L.R. ( 3 ) 4 , 206 Gatilov, Yu.V. (2.5) 142 Gaudel, Y. ( 1 ) 175 Gaumet, S. ( 3 ) 468 Gaur, A. ( 1 ) 267 Gauss, J. ( 2 . 7 ) 108 Gauthier, S . ( 1 ) 337 Gautier, M. ( 2 . 6 ) 142, 143 Gautron, R. ( 1 ) 367 Gavrikova, E.V. ( 3 ) 23 Gavrilenko, V.N. ( 1 ) 444 Gavrilova, 1.1. ( 3 ) 327 Gawinecki, R. ( 3 ) 562 Gazyll, L.R. ( 4 ) 51 Gebel, R.-C. (2.2) 9 Gebicki, J. ( 2 . 2 ) 125; ( 2 . 4 ) 229 Gedanken, A. ( 1 ) 389 Geenevasen, J.A.J. ( 2 . 3 ) 61 Gehrke, G. ( 1 ) 294 Geiss, W. (2.2) 96 George, E. ( 1 ) 73 George, G.A. ( 3 ) 428, 502

Author Index (2.7) 68,

George, G.L. ( 3 ) 210 George, M.V. ( 1 ) 494;

Goeldner, M.P.

( 2 . 2 ) 127 George, S.M.

Goethals, E.J. ( 3 ) 455 Gohda, R. (2.7) 122 Golander, C.G. ( 3 ) 215 Golankiewicz, K. (2.1)

( 1 ) 376

Georgopoulos, M. ( 4 ) 42 Gerasimov, G.N. ( 3 ) 164 Gerber, S. ( 1 ) 77 Gerber, V.D. ( 3 ) 23 Gerlock, J.L. ( 3 ) 548, 607-609

Gero, S.D. ( 2 . 7 ) 130 Gerson, F. ( 2 . 3 ) 26 Gersten, J.I. ( 1 ) 307 Gerzon, K. ( 2 . 7 ) 90 Getoff, N. ( 2 . 5 ) 62 Geuskens, G. ( 3 ) 457 Ghande, M.E. ( 3 ) 601, 602 Ghandi, R.P. ( 2 . 2 ) 1 1 ; (2.4) 212

Ghiggino, K.P. ( 1 ) 37; ( 3 ) 369, 570, 575, 622 Ghiron, C.A. ( 1 ) 72 Ghoneim, N. ( 1 ) 8 5 , 216 Ghosh, A. ( 2 . 7 ) 59 Ghosh, S. ( 2 . 4 ) 213, 214, 215; ( 2 . 6 ) 46, 47 Giacometti, G. ( 1 ) 316 Giacomini, J.C. ( 2 . 7 ) 71 Giannotti, C. ( 2 . 5 ) 91 Gierer, J. ( 3 ) 424 Gierisch, S. ( 2 . 3 ) 83 Giese, B. ( 2 . 7 ) 133 Gigante, B. ( 2 . 5 ) 141 Gilabert, E. ( 1 ) 21 Gilardi, R. ( 2 . 4 ) 115; ( 2 . 7 ) 118 Gilbert, A . ( 2 . 4 ) 22-25, 30, 3 7 , 57 Gilbert, B.A. ( 2 . 1 ) 1 9 ; ( 2 . 6 ) 188 Gilges, S. ( 2 . 7 ) 133 Ginsburg, E.J. ( 3 ) 311 Giren, B.G. ( 1 ) 317 Girges, M.M. ( 3 ) 562 Giroux, L. (2.3) 1 Giurginca, M. ( 3 ) 595 Giusti, G. ( 2 . 5 ) 100 Glaser, D.M. ( 3 ) 6 4 , 65, 183, 184 Glaser, E. ( 2 . 6 ) 204 Glatthaar, J. ( 2 . 7 ) 107 Glaudemans, C.P.J. ( 2 . 7 ) 16 Gleiter, R. ( 2 . 1 ) 44; ( 2 . 2 ) 38 Gleixner, G. ( 3 ) 541 Glenneberg, J. ( 2 . 2 ) 99 Gleria, M. ( 3 ) 543 Gliesing, S. ( 2 . 3 ) 64, 66 Gloskey, D.J. ( 3 ) 188 Go-an, K. ( 2 . 3 ) 104; ( 2 . 7 ) 175

69

5 5 ; ( 2 . 7 ) 115

Goldberg, I. (2.6) 92 Gol'dfarb, E.I. ( 2 . 7 ) 45 Goldman, A.S. ( 2 . 5 ) 97 Golich, D.J. ( 1 ) 443 Gollnick, K. ( 2 . 5 ) 5 Golubkova, N.A. ( 3 ) 8 4 , 93

Golubski, Z. ( 3 ) 574 Gong, D. ( 2 . 2 ) 52; ( 2 . 7 ) 163

Gonzalez, A. ( 3 ) 477 Gonzalez-Lafont, A . ( 2 . 4 ) 65

Goodman, J.L.

( 1 ) 29;

( 2 . 7 ) 8 , 11

Goodwin, J.W. ( 3 ) 323 Goosen, A. ( 2 . 7 ) 99 Gopidas, K.R. ( 1 ) 261, 386, 446; ( 2 . 2 ) 127

Gopinathan, G. ( 1 ) 219 Gordienko, V.P. ( 3 ) 217 Gorman, A.A. ( 1 ) 471; (2.5) 4

Gorman, C.B. ( 3 ) 311 Gormin, D. ( 1 ) 118, 210, 434

Gorner, H. ( 1 ) 103, 293, 452

Gosh, P. ( 3 ) 6 7 , 78, 107 Gothe, S. ( 3 ) 215 Goto, M. ( 2 . 6 ) 162; ( 2 . 7 ) 34

Goto, Y. ( 4 ) 35 Gould, I . R . ( 1 ) 496; (2.1) 7

Gourdon, A. ( 1 ) 211 Grabowska, A. ( 1 ) 233 Grabowski, S. ( 2 . 5 ) 22 Grabowski, Z.R. ( 1 ) 224 Gradl, G. ( 1 ) 445 Graetzel, M. ( 2 . 5 ) 86 Graetzel, W. ( 2 . 5 ) 87 Graham, J.C. ( 3 ) 188 Graham, M.S. ( 3 ) 499 Gramain, J.-C. ( 2 . 4 ) 181, 182

Grammbitter, K. (2.7) 90 Granchak, V.M. ( 3 ) 110 Grandberg, A.A. ( 2 . 2 ) 64 Grandberg, 1.1. (2.2) 21-23; ( 2 . 6 ) 5 , 137; ( 2 . 7 ) 211 Granier, V. ( 3 ) 449, 522, 529, 532 Grassy, G. (2.7) 9 2

523

Author Index Gratton, E. (1) 48, 52 Gravel, D. (1) 337; (2.7) 148 Gray, H.B. (2.5) 150; (4) 23 Graziano, M.L. (2.5) 182 Greaves, P.H. (3) 414 Green, P.N. (3) 55 Green, W.A. (3) 55 Greenberg, D.B. (3) 314 Greenberg, M.M. (2.7) 31 Greiner, G. (1) 168 Grellmann, K.H. (1) 416, 437; (2.6) 176 Griesbeck, A.G. (2.1) 37, 38; (2.2) 129; (2.5) 109 Grieser, F. (1) 148; (4) 52 Griffin, A.C. (1) 503; (3) 161, 230 Grigoriadis, D.E. (2.7) 86 Griller, D. (1) 454 Grishin, I.Yu. (2.3) 67, 69; (2.4) 170, 172; (2.6) 25, 26 Gritson, N.P. (1) 411 Gritsyk, Ya.S. (2.5) 113 Grodzinsky, A.J. (3) 343 Grovenstein, E., jun. (2.3) 44 Grow, R.H. (2.6) 8 Grubbs, R.H. (3) 311 Gruen, H. (1) 293 Grutzmacher, H.-F. (2.6) 15 Grummt, U.W. (2.7) 36 Grunewald, J.O. (2.2) 95 Grunwald, E. (1) 259 Gruttadauria, M. (2.4) 14 Gsponer, H.E. (1) 263 Gu, J. (3) 248, 267 Gu, K. (2.5) 159, 163 Guan, H.W. (2.3) 53 Guardia, L.A. (2.5) 185 Gudkova, L.A. (3) 84 Gueiffier, A. (2.7) 92, 182 Guella, G. (2.2) 4 Guerin, B. (2.2) 47; (2.5) 31 Guerry, P. (2.2) 14; (2.6) 105 Gugliemetti, R. (1) 482 Gugumus, F. (3) 452, 456 Guido, C. (3) 621 Guillet, J.E. (1) 335; (2.5) 177; (3) 344, 347, 348 Guindy, N.M. (2.5) 58; (4) 29

Guittet, E. (2.6) 175 Gummeson, J.J. (3) 185, 186 Gunaherath, G.M.K.B. (2.6) 98 Gunderson, H.J. (2.6) 8 Guo, H. (2.7) 190 Guo, R.K. (1) 209, 336; (3) 346, 349, 375, 410 Guo, Y. (2.2) 87; (2.6) 134 Gupta, B.D. (3) 465 Gupta, S. (2.4) 186, 187; (2.7) 191, 192 Gurinovich, G.P. (2.5) 76 Gurova, L.P. (3) 26 Gurunathan, K. (4) 40 Guseinova, A.D. (4) 24 Gustav, K. (1) 6, 86 Gusten, H. (1) 88 Gutmann, R. (3) 500 Guzikowski, A.P. (2.6) 79 Guzinski, R. (3) 239 GUZZO, M. (3) 544 Ha, D.-C. (1) 465 Ha, H. (3) 174 Haajaiej, A. (3) 259 Haarer, D. (1) 279 Habata, Y. (2.2) 44; (2.6) 108 Habik, A.M. (1) 223 Hacker, N.P. (2.4) 133, 220; (2.7) 217; (3) 142 Haddad, H.Y. (2.2) 34; (3) 88 Haddad, N. (2.2) 30, 35, 37; (2.6) 111 Haddleton, D.M. (3) 230 Hafelinger, G. (2.6) 115 Hagedorn, M. (2.7) 75 Hageman, H.J. (3) 7, 19 Haggquist, G.W. (3) 400, 401 Haider, K.W. (1) 458, 459; (2.7) 80 Hakushi, T. (2.3) 16 Haley, B.E. (2.7) 14 Hall, R.A. (1) 325 Hallberg, A. (2.6) 102 Hallensleben, M.L. (3) 80, 98 Halley, F. (2.7) 125 Halpern, A.M. (2.4) 191 Halverson, A.M. (2.4) 89 Ham, S.-W. (2.7) 24 Hamada, E. (3) 636 Hamada, M. (2.6) 199 Hamada, T. (2.4) 274; (2.7) 155 Hamaguchi, H. (1) 425

Hamanoue, K. (1) 422 Hamashima, M. (3) 556 Hambright, P. (4) 28 h e r , N. (1) 19 Hamilton, C.A. (1) 30 Hammershoei, A. (4) 15 Hamoud, A.A.S. (3) 596 Hamoi, S. (1) 112 Hampton, T. (1) 280 Hamrock, S.J. (2.3) 89; (2.4) 20; (2.6) 116 Hamzah, R.Y. (1) 298 Han, H. (2.1) 18 Han, N. (2.4) 225 Hanaoka, T. (2.5) 144; (3) 79 Hanaya, K. (2.6) 61 Hane, T. (4) 57 Haney, C.A. (3) 620 Hanna, M.A. (3) 562 Hanna, M.M. (2.7) 89 Hanrahan, B.D. (3) 279 Hansen, H.J. (2.5) 77 Hansen, L. (2.4) 192 Hansen, S.H. (2.5) 161 Hanson, D.M. (1) 426 Hara, N. (2.2) 123; (2.4) 51 Harada, H. (4) 34 Harada, M. (3) 76, 77 Hard, T. (1) 296 Hardy, S.J. (3) 64, 65 Hargreaves, J.S. (3) 241 Harman, L.A. (1) 407 Harriman, A. (2.5) 80; (4) 28, 43 Harris, J.M. (1) 42, 273, 421 Harris, S.J. (2.5) 96 Hartke, K. (2.7) 141 Hartmann, F.X. (1) 308 Harvey, P.D. (2.5) 150; (4) 23 Hasabe, M. (2.2) 90, 93; (2.6) 166 Hasegawa, E. (3) 273 Hasegawa, K. (3) 81 Hasegawa, M. (2.2) 42; (3) 12, 41, 166 Hasegawa, T. (2.1) 30, 31; (2.2) 110, 111;. (2.5) 23; (2.6) 96 Hasegawa, Y. (3) 121 Haselbach, E. (2.3) 26; (2.4) 132; (2.5) 37 Haseltine, J.N. (1) 289 Hashimoto, H. (1) 120, 121, 397, 398 Hashimoto, T. (3) 286 Hatanaka, Y. (2.4) 69; (3) 326 Hatano, H. (1) 291

Author Index

524

Hatori, H. (2.2)

106; (2.6) 91, 131 Hatsui, T. (2.2) 1, 2 Hatton, W.G. (2.4) 133 Hattori, H. (3) 612 Hattori, Y. (1) 122 Haugen, C.M. (2.5) 190 Hawkins, W.L. (3) 565 Hayakawa, K. (2.2) 130 Hayashi, E. (2.2) 121; (2.4) 50; (2.6) 119 Hayashi, H . (1) 119 Hayashi, K. (2.4) 153; (2.6) 210; (3) 140, 141, 245 Hayata, H. (2.4) 162 Hayes, I.E.E. (2.1) 14 Hayes, M. (1) 177 Hayes, R . (2.7) 97 Haynes, D.R. (1) 376 He, H. (2.5) 81, 121 Healy, M. (3) 414 Heath, P. (2.4) 22, 25 Heeg, M.J. (2.7) 164 Hegedus, L.S. (2.6) 114, 150 Hehemann, D.G. (2.3) 115 Heim, N. (2.2) 99 Heinrich, A. (1) 437 Heinz, R.E. (3) 209 Heisel, F. (1) 211 Helbig, D. (3) 596 Held, S. (2.5) 5 Heldt, J. (1) 118, 410, 434 Heldt, J . R . (1) 410 Helene, C. (1) 188 Hellman, M.Y. (3) 486 Helmstreit, W. (1) 278, 424 Hemker, D . J . (3) 365-367 Hemmi, G. (2.5) 80 Henderson, G. (1) 133 Henin, F. (2.5) 40 Henin, H. (2.2) 53 Henkel, G. (2.2) 66 Hennecke, M. (3) 397, 406, 407 Henning, H.G. (2.1) 23, 32; (2.6) 94 Henriksen, U. (2.7) 82

Henrioulle-Granville, M. (3) 333

Herbert, E.J. (2.4) 183 Herbich, J. (1) 117, 207, 208

Herlinger, H. (3) 503 Herman, M.S. (1) 29 Hermont, R.M. (1) 222 Hernandez, R . (2.5) 194 Hernandez-Fuentes, I. (3) 380

Herrmann, J.M. (2.5) 98 Hershberger, P.W. (2.2) 39

Hess, B.A. (2.3) 110 Hess, B.C. (3) 412 Hess, G.P. (2.6) 79 Heuer, L. (2.4) 190; (2.6)

202 Hewgill, F . R . (2.5) 184 Heyward, J.J. (3) 369 Hida, M. (2.4) 139, 197; (2.6) 40, 129, 130; (2.7) 94 Higashi, F. (2.4) 97; (2.7) 201 Higashi, M. (2.3) 42; (2.6) 53 Higashi, N. (3) 133, 134, 263 Higashiura, M. (2.2) 55 Higashiyama, N. (3) 318 Higuchi, J. (1) 438 Higuchi, Y. (3) 383 Hikida, T. (1) 149 Hikmet, A.M.R. (3) 160 Hilborn, J. (3) 179, 180 Hilinski, E.F. (1) 18, 151; (2.4) 240; (2.7) 166 Hill, C.L. (2.5) 93, 94 Hilliard, P.R. (2.4) 210; (2.6) 45 Himbert, G. (2.7) 58 Hinrichs, H. (2.1) 50, 51; (2.7) 140 Hinsken, W. (2.2) 75 Hinterwaldner, R. (3) 35 Hiraga, T. (1) 61 Hirano, K. (2.4) 119; (2.5) 166 Hirano, Y. (3) 172 Hirata, Y. (1) 174, 180, 199; (2.4) 156 Kiratsuka, H. (1) 142, 153, 274 Hirayama, F. (2.4) 126 Hirayama, S. (1) 37, 59, 100, 374 Hiresaki, T. (2.3) 13 Hirokami, S. (2.2) 92; (2.6) 20 Hirose, M. (2.2) 104 Hirota, K. (2.1) 54; (2.4) 108; (2.6) 68 Hirota, M. (2.4) 97; (2.7) 201; (3) 127 Hirota, N. (1) 396, 403 Hirsch, E. (1) 355 Hirth, C.G. (2.7) 68, 69 Hitomi, I. (2.4) 163 Ho, D.M. (2.2) 134 Ho, G.-J. (2.7) 9, 10

Ho, H.Y. (3) 345 Ho, R. (2.7) 14 Ho, T.-I. (1) 291; (2.7) 218

Hoare, P. (2.4) 204; (2.7) 41

Hobel, K. (2.2) 79-81 Hoburg, E.A. (2.3) 8 Hochstrasser, R.M. (1) 289, 319; (2.1)

67

Hocking, M.B. (1) 479 Hoebeke, M. (1) 260 Hoffman, M.Z. (4) 12 Hoffmann, N. (2.2) 8 Hofland, A. (2.5) 112 Hogan, C. (2.5) 173 Holick, M.F. (2.7) 83 Holland, D.M.P. (1) 177 Holman, B. (3) 293 Holroyd, R.M. (1) 83 Holt, E.M. (2.2) 63; (2.7)

20

Holten, D. (1) 22, 490 Holzwarth, A.R. (1) 13 Honda, K. (2.4) 179; 11; (2.6) 12; 34, 35; (3) 154 Honda, T. (2.7) 63 Hong, H.H. (3) 553 Hong, L. (2.7) 189, 190 Hong, S. (3) 176 Hopf, G. (2.3) 64, 66 Hopkirk, A. (1) 177 Horie, K. (3) 42, 231, 232, 240, 384, 533, 534 Horigome, T. (4) 57 Horiguchi, Y. (2.2) 10, 104; (2.6) 58 Horikoshi, Y. (2.3) 92 Horiuchi, K. (2.6) 177 Horiuchi, S. (3) 299, 300 Horsky, J. (3) 381 Horspool, W.M. (2.3) 38-40, 55-57, 75, 76; (2.4) 158, 159; (2.6) 21-23, 50, 51, 62, 71 Horta, A. (3) 379, 380 Hosaka, A. (2.3) 117, 118 Hoshi, N. (1) 403 Hoshino, M. (1) 189; (2.5) 208 Hoshino, T. (2.4) 126 Hosokawa, T. (3) 326, 351 Hosono, H. (1) 194 Houk, K.N. (1) 145 Houlikan, F.M. (3) 486 Howard, D.D. (3) 209 Howells, B.D. (1) 132 Hoyle, C.E. (1) 503; (3) 2, 66, 94, 95, 161, 207, 230, 535, 536, 558 Hozumi, S. (3) 99 (2.5) (2.7)

Author Index Hradil, J. (3) 359 Hrdina, R. (1) 116; (2.3) 63 Hrdlovic, P. (3) 606 Hrivik, A. (3) 594 Hrivikova, J. (3) 271 Hrovat, D.A. (1) 499 Hsieh, H.-P. (2.3) 47; (2.6) 55 HSU, C . 4 . (2.4) 39, 40 HSU, W.N. (3) 620 Hu, C. (2.5) 211 Hu, J. (2.3) 110 Xu, M. (1) 360 Hu, S.S. (2.4) 224; (2.6) 180; (3) 551 Hua, X.M. (1) 307 Huang, A. (2.4) 129 Huang, J. (1) 47, 238; (3) 234 Huang, S.J. (3) 143 Huang, W. (2.5) 211; (2.6) 194 Huart, G. (2.2) 88 Huber, H. (1) 248 Hubig, S.M. (1) 191 Huch, V. (2.7) 57 Hudnall, P.M. (2.2) 124; (2.4) 202 Hueso-Rodriguez, B. (2.5) 117 Hughes, A.N. (2.6) 206 Hughes, K.D. (1) 32 Huhtikangas, A. (2.4) 109 Huizer, A.H. (1) 185, 234, 428; (2.5) 42 Hult, A. (3) 169, 170 Humer, W. (1) 184 Humel, K. (3) 242 Huppert, D. (1) 266 Hurtubise, R.J. (1) 372 Husler, R. (3) 56 Husson, H.-P. (2.4) 182 Hustert, K. (2.2) 69; (2.4) 218; (2.6) 159 Huston, M.E. (1) 69 Hutchens, D.E. (3) 207, 558 Hwang, C.-K. (2.6) 168 Hwu, J.R. (2.1) 19, 47; (2.6) 188, 189 Hynes, J.T. (1) 160 Iborra, S. (2.5) 180 Ibrahim, P.N. (2.6) 83 Ichikawa, K. (2.1) 30 Ichikawa, M. (1) 180 Ichikawa, S. (2.3) 117, 118 Ichimura, K. (1) 397, 485; (2.6) 29-31, 35;

525

(3) 47, 302, 303 Ichino, T. (2.3) 65 Ichinose, N. (2.4) 270; (2.5) 178 Iesce, M.R. (2.5) 182 Igarishi, K. (3) 34 Ihaya, Y.J. (2.5) 27 Iijima, T. (3) 243 Iizawa, T. (3) 504 Ikeda, N. (3) 387 Ikeda, T. (1) 61, 320, 336; (3) 127, 299, 300, 316, 349, 390 Ikeda, Y. (2.2) 15, 16, 90; (2.6) 127, 128, 166; (3) 156 Ikegami, K. (1) 429 Ikegami, Y. (1) 420, 460; (2.6) 61 Ikematu, S. (1) 149 Ikeo, S. (3) 612 Ikoma, T. (1) 420 Ilarinov, V.V. (3) 258 Ilge, H.D. (2.4) 161 Il'ichev, Y.V. (1) 371 Il'in, S.N. (3) 479 Imahori, H. (2.2) 142-144 Imai, A. (2.3) 65 Imai, N. (2.7) 14 Imai, S. (2.6) 162 Imai, Y. (3) 226 Imamura, M. (4) 34 Imamura, S. (3) 319 Imanaga, H. (2.5) 213 Imanishi, D. (3) 319 Imanishi, I. (2.2) 55 Imaoka, T. (2.2) 119; (2.6) 124 Imasaka, T. (1) 53, 54 Imazeki, S. (3) 317 Imonishi, Y. (1) 109 Imura, S. (2.4) 168 Imura, T. (2.4) 165, 169 Imwinkelried, R. (2.6) 114, 150 In, O.A. (3) 639 Inada, H. (3) 579 Inagaki, T. (2.1) 24 Inai, Y. (1) 109 Inaki, Y. (3) 99 Inamoto, N. (2.4) 160; (2.6) 203; (2.7) 98 Inman, W.D. (2.3) 2 Inokawa, H. (2.2) 102; (2.6) 211 Inokuma, S. (2.3) 93 Inomata, K. (3) 96, 97 Inoue, H. (2.1) 24, 25; (2.6) 97 Inoue, K. (3) 243, 350 Inoue, M. (2.4) 155, 156; (2.6) 44

Inoue, S. (3) 290, 291 Inoue, Y. (1) 194; (2.3) 16 Inubushi, S. (3) 316 Ioana, P. (3) 590 Ioannou, S. (2.5) 57 Ippen, E.P. (1) 10 Irie, M. (2.4) 152, 153; (2.6) 37, 210; (3) 44, 283, 308 hie, T. (2.4) 126 Iring, R. (3) 591 Irinyi, G. (2.5) 70 Isaksson, R. (2.6) 102 Ishar, M.P.S. (2.2) 11, 50 Ishchenko, A.A. (1) 256 Ishibashi, N. (1) 53, 54 Ishida, A. (2.6) 74 Ishiguro, T. (3) 636 Ishihara, M. (1) 70; (2.6) 112 Ishii, H. (3) 146 Ishii, K. (2.2) 52 163; (4) 21 Ishii, T. (2.4) 97 201 Ishikawa, A. (3) 2 3 Ishikawa, K. (3) 1 3 Ishikawa, M. (2.4) 18; (2.5) 199; (2.6) 184 Ishimoto, H. (3) 637 Ishizuka, H. (2.6) 161; (2.7) 146 Ishizuka, M. (2.4) 150, 151, 164, 166, 173; (2.6) 27 Ishizuka, Y. (3) 92 Ismail, L.F.M. (1) 251, 265 Isomura, K. (2.6) 74 Issa, R.M. (1) 223 Issa, Y.M. (1) 123 Itagaki, H. (3) 384, 394, 448 Itaya, A. (1) 338; (3) 284 Ito, D. (3) 249 Ito, H. (2.4) 162; (3) 249 Ito, 0. (2.7) 142; (3) 415 Ito, S. (3) 272, 388 Ito, Y. (1) 501; (3) 403 Itoh, H. (1) 268; (2.4) 39, 40 Itoh, M. (1) 146, 456; (2.5) 33 Itoh, T. (1) 91; (2.5) 85 Itoh, Y. (1) 311 Itoho, K. (2.6) 17 Ittah, V. (1) 266

526 Iu, K.-K. ( 1 ) 380, 468; ( 3 ) 413

Ivakhenko, E.P. ( 2 . 4 ) 138 Ivan, B. ( 3 ) 571 Ivan, G. ( 3 ) 595 Ivanov, P. ( 2 . 2 ) 41 Ivanov, V.B. ( 3 ) 471, 589 Ivanov, V.L. ( 2 . 4 ) 7 3 , 130

Iwabuchi, S. ( 3 ) 494 Iwai, K. ( 3 ) 139 Iwai, T. ( 2 . 4 ) 163 Iwamoto, A. ( 3 ) 7 0 , 119 Iwamoto, H. ( 2 . 2 ) 140, 141; ( 2 . 6 ) 155

Iwamoto, N. ( 2 . 4 ) 255 Iwamura, H. ( 2 . 2 ) 136; ( 2 . 7 ) 98

Iyer, R.M. ( 1 ) 219 Iyoda, T. ( 2 . 6 ) 12 Izumi, Y. ( 3 ) 318 Jabalameli, M. (2.4) 142 Jackers, C. ( 1 ) 358; ( 3 ) 136

Jackson, J.E. ( 2 . 7 ) 5 Jacobine, A.F. ( 3 ) 6 4 , 65, 183, 184 Jacobs, R. ( 3 ) 460 Jacobsen, R.E. ( 3 ) 11 Jacopcic, K. ( 2 . 6 ) 63 Jaganathen, A. ( 2 . 7 ) 69 Jaime, C. ( 2 . 2 ) 71 Jakubowska, T. ( 2 . 5 ) 140; ( 4 ) 32 Jandrisits, L. ( 2 . 1 ) 14 Jang, S.H. ( 3 ) 176 Jankiewicz, S.V. ( 3 ) 168 Jansen, I. ( 3 ) 580, 596 Janson, J. ( 3 ) 508, 509 Janssens, L.D. ( 1 ) 36, 40 Jardon, P. ( 1 ) 367 Jarzeba, W. ( 1 ) 163 Jaturonrusmee, W. ( 2 . 1 ) 65; ( 2 . 7 ) 198 Jayasuriya, N. ( 2 . 4 ) 1 7 ; ( 2 . 6 ) 152 Jayatissa, A.H. (2.5) 63 Jefford, C.W. ( 2 . 5 ) 146 Jeger, 0. ( 2 . 2 ) 51, 7 8 ; ( 2 . 5 ) 118 Jenks, W.S. ( 1 ) 496; ( 2 . 1 ) 7 ; ( 2 . 7 ) 49 Jennings, P. ( 3 ) 498 Jensen, N.-H. ( 2 . 2 ) 49 Jentzer, 0. ( 2 . 4 ) 8 2 ; ( 2 . 6 ) 146 Jerome, R. ( 3 ) 333 Jesse, K. (1) 491 Jia, D. ( 4 ) 27 Jia, X. (2.6) 64

Author Index Jiang, G.J. ( 3 ) 114 Jiang, Z. ( 2 . 5 ) 114 Jin, D. ( 2 . 5 ) 133 Jin, W. ( 2 . 5 ) 209 Jin, Y. ( 3 ) 248 Jin, Z. ( 2 . 5 ) 136; ( 4 ) 49 Jing, D.P. ( 3 ) 334, 385 Jinguji, M. ( 1 ) 70 Jitsumatsu, T. ( 3 ) 34 Job, K. (2.5) 118 Johansson, L.B.-A. (1) 326, 339, 342, 344 ( 2 . 6 ) 192 ( 2 . 6 ) 79 ( 2 . 5 ) 31 (2.3) 80; (2.7) 180 Johnston, L.J. ( 1 ) 152, 155, 439, 504; ( 2 . 2 ) 47; ( 2 . 4 ) 243, 246; ( 2 . 5 ) 3 0 ; ( 2 . 6 ) 207; ( 2 . 7 ) 48, 143 Jones, C.R. ( 2 . 6 ) 106 Jones, D.A. ( 3 ) 537 Jones, G. ( 1 ) 281; ( 2 . 4 ) 191 Jones, M. ( 2 . 7 ) 178, 179 Jones, W.E. ( 1 ) 313 Jongenelic, A.P.J.M. ( 1 ) 399 Jonsson, S. ( 3 ) 215 Jordan, K.D. ( 2 . 3 ) 106 Jorgensen, K.A. ( 2 . 2 ) 86; ( 2 . 6 ) 144 Joselvich, E. ( 1 ) 384 Joseph, J. ( 1 ) 280 Joshi, G.C. ( 1 ) 41, 137 Joshi, U. ( 2 . 7 ) 21, 28 Joussot-Doubien, J. (2.4) 63 Jovin, T.M. ( 1 ) 31 Jufishma, A. ( 2 . 5 ) 155 Jug, K. ( 2 . 4 ) 1 6 ; ( 2 . 6 ) 56 Juliano, V.F. ( 3 ) 544 Julliard, M. ( 2 . 4 ) 81 Jun, Q.B. ( 3 ) 218 Jung, C. ( 1 ) 6 Juranic, I. ( 2 . 5 ) 2

Johnson, K.M. Johnson, K.W. Johnson, L.J. Johnson, R.P.

Kabashima, T. ( 2 . 4 ) 83-86;

( 2 . 7 ) 212, 214

Kachan, A.A. ( 3 ) 171, 598 Kaczmarek, H. ( 3 ) 485 Kagan, J. ( 2 . 1 ) 39; ( 2 . 4 ) 1 7 ; ( 2 . 6 ) 152

Kagawa, T. ( 2 . 4 ) 111 Kakimoto, M. ( 3 ) 226 Kakitani, T. ( 1 ) 165 Kakiuchi, H. ( 3 ) 243 Kakiuchi, K. ( 2 . 2 ) 1 2 ,

1 8 ; ( 2 . 4 ) 46

Kalhousova, M. ( 2 . 3 ) 63 Kalinina, V.M. (3) 425 Kalman, B. ( 1 ) 326 Kalontarov, I.Ya. ( 3 ) 559, 639

Kalontarov, Yu.V. ( 3 ) 583 Kamachi, M. ( 3 ) 121 Kamalov, V.F. ( 1 ) 462; (2.5) 73

Kamaritskii, B.A. ( 3 ) 433 K a m a t , P.V. ( 1 ) 261, 274, 385, 386, 446 Kamatasuchi, Y. ( 2 . 5 ) 213 Kamath, V.R. (3) 597 Kamei, N. ( 2 . 4 ) 112 Kamel, E.M. (2.2) 138 Kametani, F. ( 2 . 4 ) 18 Kamiichi, K. (2.4) 156 Kaminski, A. ( 3 ) 485 Kaminski, R. ( 2 . 2 ) 125; ( 2 . 4 ) 229 Kamioka, K. ( 1 ) 311; ( 3 ) 403 Kamogawa, H. ( 2 . 6 ) 28

Kanabus-Karninska, J.M. ( 1 ) 64

Kanagasabapathy, V .M. (2.4) 242

Kanai, K. ( 2 . 2 ) 55 Kanamathareddy, S. (2.7) 74

Kanaoka, Y. ( 2 . 2 ) 15, 16, 9 0 , 93, 109, 115, 116, 118; ( 2 . 4 ) 5 2 , 54, 69, 99, 100, 113; ( 2 . 6 ) 122, 127, 128, 163, 165, 166, 174; (2.7) 200, 204 Kanazawa, J. ( 2 . 4 ) 124 Kanehiro, H. ( 3 ) 579 Kaneko, A. ( 1 ) 460; ( 2 . 2 ) 136; ( 2 . 4 ) 150, 151, 164, 166, 173; ( 2 . 6 ) 27 Kanematsu, K. (2.2) 130 Kanemitsu, Y. ( 3 ) 319 Kanetani, F. ( 2 . 6 ) 184 Kang, E.T. ( 3 ) 545 Kang, H.K. (2.4) 31 Kanigan, T. ( 1 ) 439 Kanner, R.C. (2.5) 128 Kanofsky, J.R. ( 1 ) 473 Kapili, L.V. ( 2 . 7 ) 61 Kapinius, E.I. ( 1 ) 195 Kapoor, S. ( 1 ) 219 Kar, R.K. ( 1 ) 236 Karakizawa, A. (2.2) 111; (2.6) 96 Karanjit, D.B. ( 3 ) 299, 300 Karatsu, T. ( 2 . 3 ) 1 3 ; ( 2 . 5 ) 83

527

Author Index Karau, M. (2.4) 105 Karminski-Zamola, G. (2.4) 154; (2.6) 151 Karoteev, N . I . (2.5) 73 Karpacheva, G P’ (3) 158 Karpuick, J. (1) 224 Kartsev, V.G. (2.6) 75 Karyukhina, M.O. (2.7) 66 Kasai, M. (3) 253 Kasai, P.H. (2.4) 133; (3) 265 Kasatani, K. (3) 305 Kasatani, R. (2.2) 84; (2.6) 135 Kascheres, C. (3) 544 Kasha, M. (1) 118, 434 Kashima, C. (2.2) 91; (2.6) 139 Kashuba, E.V. (2.5) 107 Kaska, W.C. (1) 483 Kastein, H. (3) 593 Kastha, G.S. (1) 404 Kasu, J.E. (3) 579 Kasuga, K. (2.5) 52 Kata, M.M. (2.5) 152 Katagi, T. (2.7) 120 Katalinikov, I.V. (1) 440 Katayama, A. (2.3) 65 Kato, C. (2.5) 45-47; (4) 38 Kato, M. (2.2) 91; (2.6) 139; (3) 45 Kato, N. (2.2) 1 Kato, T. (2.4) 118; (2.5) 155 Kats, M.M. (2.5) 92 Katsitadze, N.V. (3) 618 Kauffmann, H.F. (1) 331; (3) 398 Kavash, R.W. (2.6) 138 Kawabata, M. (3) 76, 77 Kawaguchi, T. (3) 569, 605 Kawahara, H. (3) 614 Kawahara, W. (2.4) 127 Kawahigashi, M. (1) 59 Kawai, A. (1) 430 Kawamura, T. (3) 298 Kawanishi, S. (2.7) 170 Kawanishi, Y. (3) 303 Kawase, Y. (2.6) 41 Kawatsura, K. (1) 422 Kawatuji, T. (2.7) 28 Kawauchi, S. (2.6) 37 Kazakov, V.P. (2.5) 125, 187, 220; (3) 425 Keana, J.F.W. (2.7) 81 Kearns, D.R. (1) 296, 297 Keeffe, J.R. (2.5) 195 Keijzer, F. (1) 487 Kelen, T. (3) 571 Keller, P. (3) 235

.

Kellett, M.A. (2.5) 189 Kellman, A. (1) 482 Kelly, G.P. (1) 442 Kelly, J.J. (1) 55 Kelly, J.M. (2.5) 198 Kemnitz, K. (1) 378, 379 Kemp, T.J. (2.5) 131 Kempka, U. (2.3) 66 Kennedy, S.R. (1) 409 Kenney, M.E. (1) 448 Kera, N. (2.4) 163 Kercha, S.F. (3) 264 Kernchen, F. (2.1) 23, 32; (2.6) 94 Kerr, A.J. (2.5) 96 Kerysig, D. (2.7) 105 Kessar, S.V. (2.3) 19; (2.4) 178, 233; (2.6) 99; (2.7) 195 Kesselmayer, M.A. (2.7) 11 Kessler, P. (2.7) 68 Kevan, L. (1) 360 Khalil, Z. (3) 450 Khan, A.U. (1) 465 Khan, S.I. (2.5) 172 Khan, V.A. (2.5) 142 Khan, Z.F. (2.6) 89, 90 Khand, I.U. (2.6) 149 Kharchenko, V.I. (3) 258 Kharitonova, G.A. (3) 26 Kharlanov, V.A. (2.2) 97 Khire, U.R. (2.5) 217 Khmelinski, I . V . (1) 411 Khoromskaya, V.A. (3) 493 Khoroshilova, V.V. (3) 158 Khosrowshahi, J.S. (2.1) 56; (2.4) 115; (2.7) 118 Khudyakov, I.V. (2.5) 28, 35, 70, 164; (3) 93 Khundkar, L.R. (1) 183 Kida, Y. (2.4) 165, 168, 169 Kidawara, M. (2.2) 133; (3) 81 Kido, H. (3) 24 Kiefer, R.L. (3) 538 Kiesewetter, R. (2.2) 56, 57; (2.6) 172 Kiguchi, T. (2.4) 1; (2.6) 43 Kikai, N. (2.3) 31 Kikuchi, J. (2.7) 93 Kikuchi, K. (1) 122; (2.4) 39, 40 Kim, A.E. (2.2) 147 Kim, A.R. (2.4) 144 Kim, D.-Y. (1) 426 Kim, D.J. (3) 176 Kim, H. (1) 450

Kim, H.B. (1) 61 Kim, H.J. (1) 160; (2.2) 113; (2.6) 123 Kim, H.K. (2.7) 62 Kim, J.M. (3) 75 Kim, M.S. (2.3) 12 Kim, S.K. (1) 286; (2.3) 5 Kim, S.S. (2.2) 132, 137, 147; (2.4) 144 Kimura, K. (2.6) 11 Kimura, T. (3) 556 Kimura, Y. (3) 494 King, A.D. (2.5) 19 King, R.B. (2.5) 19 Kinoshita, A. (3) 257 Kinoshita, K. (3) 350 Kinoshita, S. (1) 268 Kinoshita, T. (3) 294, 295 Kinstle, J.F. (3) 3 Kinugawa, M. (2.2))18; (2.4) 46 Kinumi, Y. (2.5) 55; (4) 26 Kira, M. (1) 143; (2.3) 22; (2.6) 179 Kirchhoff, M.M. (2.7) 180 Kirkar, E.S. (2.3) 86 Kirmse, W. (2.7) 52 Kirpach, A.B. (3) 327 Kirpichenok, M.A. (2.2) 21-23, 64; (2.6) 137; (2.7) 211 Kisa, F. (1) 455 Kiser, T.K. (3) 198 Kissler, B. (2.1) 44; (2.2) 107 Kitadani, M. (2.7) 218 Kitade, Y. (2.1) 54 Kitaguchi, N. (2.2) 148 Kitahara, K. (2.5) 200 Kitamura, N. (1) 61, 209 Kitamura, T. (2.3) 108, 109; (2.4) 83-87; (2.7) 212-214 Kitano, T. (2.4) 60; (2.5) 59; (2.7) 119 Kitao, T. (2.6) 153; (3) 629, 631, 632, 634, 636 Kitchen, D.B. (1) 134 Kiwi, J. (2.5) 87 Kiyooka, S. (2.6) 198, 199 Klann, R. (1) 130 Klein, U.K.A. (1) 214 Klemchuk, P.P. (3) 443, 601, 602 Klemm, D.V. (2.2) 126 Klick, D.I. (3) 210, 244 Kliem, U. (2.2) 135 Kligshtein, M.S. (3) 144

Author Index

528

Klimchuk, E.S.

( 2 . 5 ) 35,

7 0 , 164

Kline. T.B. ( 2 . 7 ) 70 Klinkenberg, 'W. ( 1 ) 428 Kloes, P. ( 3 ) 585 Kloosterboer, J.G. ( 3 ) 208, 216

Knepper, T. ( 3 ) 132 Knochel, T. ( 2 . 3 ) 82 Knochenmuss, R. ( 1 ) 227 Knoechel, F. ( 3 ) 229 Knoesel, R. ( 3 ) 8 5 , 135 Knyanskii, M.I. ( 2 . 4 ) 171 Knyazev, A . P . (2.3) 6 8 ; ( 2 . 4 ) 171; ( 2 . 6 ) 24 Knyazhanskii, M. I. ( 2 . 3 ) 6 8 ; ( 2 . 4 ) 138; ( 2 . 6 ) 24 Kobabushi, H. ( 1 ) 178 Koban, M.A. ( 3 ) 618 Kobata, T. ( 2 . 3 ) 16 Kobayakawa, K. ( 2 . 4 ) 117 Kobayashi, E. ( 3 ) 3 9 , 92, 162 Kobayashi, K. ( 2 . 7 ) 225 Kobayashi, S. ( 2 . 3 ) 108, 109; ( 2 . 7 ) 212-214 Kobayashi, T. ( 1 ) 397; ( 4 ) 34 Kobayashi, Y. ( 3 ) 116 Kobiro, K. ( 2 . 2 ) 12 Koboyashi, S. (2.4) 83-87 Koch, T.H. ( 2 . 5 ) 6 0 ; ( 3 ) 2 29 Kochany, J. ( 2 . 4 ) 125 Kocher, M. (1) 447 Kochetkova, I.S. ( 3 ) 327 Kochi, J.K. ( 1 ) 198 Koda, S. ( 2 . 5 ) 8 4 , 85 Kodera, Y. ( 1 ) 149 Koehler, M. ( 3 ) 5 7 , 177 Koerner, K. ( 2 . 5 ) 38 Koga, K. ( 2 . 2 ) 6 Koga, N. ( 2 . 2 ) 136 Kohguchi, Y. ( 2 . 2 ) 3 Kohmoto, S. ( 2 . 7 ) 21, 28 Koholic, D.J. ( 2 . 4 ) 273; ( 2 . 7 ) 154 Koike, K. ( 1 ) 375 Kojima, J. ( 2 . 4 ) 272 Kojima, K. ( 3 ) 494 Kojima, N. ( 2 . 4 ) 155; (2.6) 44 Kokubun, H. ( 1 ) 122 Kolendo, A.Y. ( 2 . 4 ) 211 Kolesnikov, S.P. (2.7) 39 Koloezkowski, S. ( 1 ) 119 Kolomytsyn, V . P . ( 3 ) 588 Kolontarov, I.Ya. ( 3 ) 495 Kolos, R. ( 1 ) 129 Kol'tsov, Yu.1. ( 2 . 7 ) 65 Komfort, K. ( 1 ) 115 Komissarov, V.N. ( 2 . 2 ) 97

Komiyama, M. (2.2) 111; ( 2 . 6 ) 96

Komura, T. ( 2 . 5 ) 213 Konak, -C. (3) 359 Kondo, M. ( 2 . 5 ) 204 Kondo, T. ( 3 ) 96 Konieczny, M. ( 2 . 7 ) 74 Konijnenberg, J . ( 1 ) 234; ( 2 . 5 ) 42

Konno, A. ( 2 . 2 ) 136 Konuk, R. ( 1 ) 351 Kook, S.-K. ( 1 ) 426 Kopecek, J. ( 3 ) 555 Kopelman, R. ( 1 ) 4 , 407 Koppes, M.J.C. ( 2 . 2 ) 70 Koput, J. ( 1 ) 207 Korchagina, D.V. ( 2 . 2 ) 48 Kornak, E . P . ( 2 . 4 ) 1 7 ; ( 2 . 6 ) 152

Korodenko, E.G. ( 3 ) 623 Korodenko, G.D. ( 3 ) 623 Korolev, V.V. ( 1 ) 411 Koroteev, V.F. ( 1 ) 462 Korth, H.-G. ( 2 . 3 ) 101 Koseki, K. ( 2 . 6 ) 1 6 ; ( 3 ) 71

Koshiishi, K. ( 3 ) 475 Koshioka, M. ( 2 . 4 ) 124 Kosower, E.M. ( 1 ) 266 Koster, T.P.M. ( 3 ) 549 Kostyanovsky, R.G. ( 2 . 7 ) 220

Kotelnikov, A . I . ( 3 ) 338 Kouchaku, Y. ( 3 ) 92 Kouda, Y. ( 3 ) 494 Kovalenko, M.F. ( 2 . 6 ) 5 Kovalesa, I . V . ( 1 ) 247 Kowalczyk, A. ( 1 ) 27 Kowalczyk, B . A . ( 2 . 1 ) 59 Koyama, K. ( 2 . 7 ) 104 Koyama, M. ( 3 ) 472 Koyama, N. ( 2 . 2 ) 133 Koyama, Y. ( 1 ) 120, 121, 397, 398

Koyanagi, M. (2.1) 8 Kozhushkov, S.I. ( 2 . 3 ) 23 Kozielski, K.A. ( 3 ) 428 Kozmenko, M.V. ( 1 ) 215 Krajewski, J.J. ( 3 ) 192, 193

Kralovaky, J. ( 2 . 3 ) 63 Kramer, W. ( 2 . 7 ) 17 Kramitz, G. ( 3 ) 397 Krasieva, T.B. ( 1 ) 484; ( 2 . 6 ) 154

Krasnansky, R. ( 1 ) 375 Krasnaya, 2h.A. ( 2 . 3 ) 7 3 , 74

Krasnovskii, A.A. ( 1 ) 462; ( 2 . 5 ) 73

Krasnoya, Z.A. ( 1 ) 256 Klaus, W. ( 2 . 6 ) 110

Krauss, G.A. (2.1) 34 Krauss, W. ( 2 . 2 ) 40 Krebs, A. (2.7) 39 Kremer, W. ( 3 ) 31 Kremminger, P. ( 3 ) 104 Kresge, A . J . ( 2 . 4 ) 237; ( 2 . 5 ) 8 ; ( 2 . 6 ) 190

Krestonosich, S. ( 2 . 4 ) 57 Krichevskii, G.E. ( 3 ) 619 Krieger, C. ( 2 . 7 ) 139 Krier, G. ( 3 ) 529 Krishan, V. ( 1 ) 196 Krishna, A. ( 2 . 3 ) 111; ( 2 . 4 ) 268; ( 2 . 5 ) 169; ( 2 . 7 ) 158 Krist, K. ( 4 ) 7 Kristiansen, M. (2.2) 8 6 ; ( 2 . 6 ) 144 Krogh, E. ( 2 . 3 ) 119, 120; ( 2 . 4 ) 239, 256; ( 2 . 7 ) 165

Krogh-Jespersen, K. (1) 134; ( 2 . 7 ) 9 , 11

Kron, A.A. ( 2 . 2 ) 48 Kronfeld, K.P. ( 1 ) 507 Krongauz, V. ( 3 ) 307 Kroon, J . ( 1 ) 167, 322 Kropp, M. ( 2 . 4 ) 59 Krug, H.-J. ( 1 ) 71 Krull, U.J. ( 1 ) 369 Kryszewski, M. ( 3 ) 239 Kryukov, A . I . ( 2 . 5 ) 44 Kubawaba, M. ( 2 . 4 ) 127 Kuberski, S. ( 2 . 2 ) 125; ( 2 . 4 ) 229

Kubo, Y. ( 2 . 2 ) 119-121, 123; ( 2 . 4 ) 48-51; ( 2 . 6 ) 118-120, 124 Kubota, M. ( 3 ) 514 Kubota, S . ( 3 ) 211 Kuboto, H. ( 3 ) 111 Kucher, R.V. ( 2 . 5 ) 113 Kucherova, I.Y. ( 1 ) 195 Kuchmii, S.Ya. ( 2 . 5 ) 44 Kudo, T. ( 3 ) 213 Kudryasheva, N. S ( 2 . 5 ) 16 Kuester, B . ( 3 ) 503 Kuhn, C.-S. ( 2 . 7 ) 15 Kuhn, H. ( 1 ) 323 Kuhnert, L. ( 1 ) 71 Kuhnle, W. ( 1 ) 107, 108, 207 Kukkala, P.K. (3) 542 Kuleshov, S.P. (2.5) 125 Kulik, S . V . (2.5) 44 Kulnig, R.K. (2.2) 96 Kumagai, T. ( 2 . 2 ) 4 4 ; (2.3) 3 3 , 3 4 ; (2.6) 108 Kumamoto, Y. ( 2 . 7 ) 146 Kumar, C.V. ( 2 . 3 ) 95 Kumar, G . S . ( 2 . 6 ) 1 0 ; ( 3 )

.

Author Index 280 Kumar, S. (2.2) 50 Kumaraswamy, G. (2.5) 191 Kun, Y.R. (1) 319 Kund, K. (2.7) 52 Kundu, T. (1) 205, 235 Kungwatchakun, D. (3) 308 Kunihiko, K. (3) 225 Kunkely, H. (2.3) 82 Kunze, R. (3) 287 Kuo, Y.H. (2.5) 3, 183 Kupka, H. (1) 412 Kurapov, P.B. (2.6) 5 Kuriacose, J.C. (4) 50 Kurihara, S. (3) 299, 300 Kurita, J. (2.3) 102; (2.6) 59 Kurita, S. (4) 55 Kurita, Y. (2.4) 162, 163 Kuriyama, A. ( 3 ) 116 Kuriyama, Y. (2.3) 13, 24 Kurkovskaya, L.N. (2.3) 67, 69; (2.4) 170, 172; (2.6) 25, 26 Kuroda, H. (3) 289 Kuroda, K. (2.5) 45-47; ( 4 ) 38 Kuroda, S. ( 3 ) 240 Kurokawa, K. (3) 383 Kurokawa, Y. (2.1) 68-70; (2.6) 84-86 Kuroki, K. ( 3 ) 533 Kurtsinovskaya, R.I. (1) 247 Kurumada, T. (3) 567 Kurz, G. (2.7) 88 Kurz, K. ( 3 ) 407 Kusaba, Y. (2.3) 65 Kutok, P. (3) 537 Kutschabsky, L. (2.1) 35 Kutty, T.R.N. (4) 33 Kutzhanov, R.T. (2.4) 29 Kutzner, J. (3) 526 Kuwata, K. (2.5) 29 Kuzmin, M.G. (1) 371; (2.4) 73, 130 Kuz'min, V.A. (1) 440, 441, 461, 484; (2.5) 35, 70; (2.6) 6 , 154; (3) 93 Kuznetsova, A.M. (3) 495 Kuznetsova, T.S. (2.3) 23 Kwiatkowski, S. (2.7) 14 Kwon, B.M. (2.5) 128, 129, 172 Kyuda, K. (3) 333 Kyushin, S. (2.6) 136

Laarhoven, W.H. (1) 101, 487; (2.3) 15; (2.4) 11, 136, 253

529 Lablache-Combier, A. (2.4) 4, 146; (2.6) 1 ; (3) 259 Lacaita, A. (1) 60 Lacoste, J. (3) 484, 492 Laermer, F. ( 1 ) 130, 232 Lahiri, S. (2.2) 128 Lahti, P.M. (2.7) 168, 169 Lakowicz, J.R. (1) 50 Lala, A.K. (2.7) 67 Lalitha, S. (2.3) 60 Lallemand, J.Y. (2.2) 54 Lammel, U. (1) 507; (2.6) 60 Lamont, L.J. (2.3) 112; (2.4) 261, 262 Lamotte, M. (1) 96, 104 Landl, G. ( 1 ) 331; ( 3 ) 398 Lando, J.B. ( 3 ) 260, 309 Lanez, T. (2.6) 149 Lang, J. (1) 349 Langa, F. (2.3) 38, 39, 57; (2.6) 50, 71 Langan, J.G. ( 1 ) 454, 499 Lange, F. (2.3) 101 Langer, V. (2.4) 193 Langhals, H. ( 1 ) 248 Langner, A. ( 3 ) 152 Lapcik, L. (2.5) 12; ( 3 ) 271, 476; ( 4 ) 4 Lapin, S.C. (3) 199 Lapouyade, R; ( 1 ) 170; (2.7) 13 Lappert, M.F. (2.6) 49 Laszlo, B. (2.5) 26 Latham, D.D. ( 3 ) 94 Latowski, T. (2.7) 208 Lattes, A. (2.6) 142, 143 Laukhina, O.D. (2.4) 185; (2.7) 197 Laumikonis, A. (4) 15 Laus, M. (3) 301 Lauter, A.G.Z. (1) 43 LaVilla, J . A . (2.7) 8, 11 Lawers, P.H. (2.2) 62, 94 Lawson, J.M. (1) 167 Lazare, S. (3) 449, 522, 529, 532 Lazareva, T.G. (3) 512 Le, N.A. (2.7) 179 Leclaire, M. (2.2) 54 Lederer , P. (2.5) 165 Ledward, M. ( 3 ) 435 Ledwig, T. (1) 412 Lee, A. ( 3 ) 337 Lee, B. ( 1 ) 320 Lee, C.S. ( 1 ) 300 Lee, D.U. (3) 87 Lee, G.-H. (2.6) 170 Lee, H.J. (2.5) 167

Lee, H.K. (2.7) 62 Lee, J.L. (3) 202, 203, 236 Lee, M. (1) 289 Lee, R.H. (3) 87 Lee, S.H. ( 1 ) 477 Lee, S.J. (3) 552 Lee, S.M. (2.5) 183 Lee, S.S. (2.2) 29; (2.4) 32 Lee, W.M. (2.5) 150; ( 4 ) 23 Lee, Y.J. (2.5) 196 Lee, Y.L. ( 3 ) 514 Leenders, R. (1) 128 Lee-Ruff, E. (2.1) 14 Lefedova, O.V. ( 3 ) 577 Leger, L. ( 3 ) 306 Lehmann, J. (2.7) 15, 16 Lehn, J.M. ( 1 ) 190 Lehner, B. (3) 205 Leihkauf, P. (2.7) 37 Leismann, H. (1) 202 Lejeune, V. (1) 459 Lemaire, J. (1) 500; (2.4) 72, 121, 122; (2.5) 151, 202; ( 3 ) 450, 468, 469, 484, 492, 547 Le Noble, W. (2.1) 41 Lenoir, D. (2.4) 107 Lenzi, M. ( 3 ) 563 Lepage, M.-E. (2.4) 181 Leroi, G.E. (1) 105 Leutwyler, S. (1) 227 Levanon, H. (1) 447 Level, M. (2.7) 84 Levesque, J. (3) 331 Levin, P.P. (1) 440, 441, 461; (2.5) 28 Levin, Ya.A. (2.7) 45 Levoir, P. ( 1 ) 482 Levy, R.L. (3) 339 Levy, R.M. ( 1 ) 134 Lewis, D.M. (3) 414, 451 Lewis, F.D. (1) 220; (2.3) 20, 25; (2.4) 276; (2.6) 141; ( 3 ) 293 Lewis, T.J. (2.1) 27 Leydier, L. (1) 211 Leyva, E. (2.7) 79 Lhome, J. (2.2) 88, 89 Li, G. (2.5) 62; (3) 603 Li, L. (2.6) 67 Li, Q. (2.5) 81, 121, 136; (4) 49 Li, S.W. (2.7) 101 Li, T. (3) 43, 51, 130 Li, X. (2.5) 133, 134 Li, Y. (1) 145; (3) 304 Li, Z. (3) 33, 89 Li, Z.-J. (2.6) 33

Author Index

530 Liang, R.C. (3) 198 Liang, 2. (3) 304 Lianos, P. (1) 341, 362, 370

Liao, C.-C. (2.2) (2.3)

47; (2.6)

108; 55

Liao, T.C. (3) 18 Liaw, B.R. (2.1) 19; (2.6)

188

Li Bassi, G. (3) 54, 115 Libertini, L.J. (1) 34 Lie, C.Y. (3) 218 Lijten, G.F.C.M. (3) 208, 216

Lim, H.S. (3) 393 Lin, C.T. (2.3) 53 Lin, E.C. (1) 405 Lin, G. (3) 282 Lin, L.C. (2.1) 19; (2.6) 188

Lin, M.C. (2.4) 135 Lin, S. (1) 390 Lin, S.-Y. (2.3) 47; (2.6) 55

Lin, S.H. (1) 11, 19 Lin, S.Q. (2.6) 67 Lin, T. (3) 304 Lin, Y. (3) 143, 256 Lindenblatt, T. (2.7) 141 Lindner, E. (2.6) 204 Lindrum, M. (1) 99 Ling, Y. (3) 256 Linn, D.E. (2.5) 19 Lipkowski, J. (1) 231 Lippitsch, M.E. (1) 324 Lipskaya, V.A. (3) 266 Lissi, E. (1) 363, 364; (2.5)

176; (3) 68, 69 Litsov, N.I. (3) 221, 598 Little, R.D. (2.7) 25 Litwiler, K.S. (1) 45, 47 Liu, A. (1) 181 Liu, D. (3) 150 Liu, G. (3) 344 Liu, H. (3) 155 Liu, J. (1) 496; (2.1) 7; (2.5) 101 Liu, L. (2.5) 171 Liu, L.B. (3) 429, 432 Liu, M.T.H. (2.7) 4-7, 12, 13 Liu, R. (2.5) 102, 103 Liu, R.S.H. (2.3) 54 Liu, W.-Q. (1) 328 Liu, X. (2.5) 102, 103; (3) 174 Liu, Y. (2.5) 114; (2.7) 222; (3) 542 Liu, Z. (2.7) 222 Liverton, N.J. (2.2) 68 Lluch, J.M. (2.4) 65 Lo, D. (3) 498

Lobaugh, J. (1) 504 Lobo, A.M. (2.5) 141 Lochmuller, C.H. (1) 373 Locke, R.J. (1) 405 Loft, S. (2.4) 235, 236 Loftus, G.C. (2.1) 16 Lohmannsroben, H.G. (1) 115, 410, 414

Lohse, V. (2.7) 37 Loi, A. (3) 621 Lokshin, V.A. (2.6) 154 Long, F.H. (1) 489 Long, S.A.T. (3) 538 Longkilde, F.W. (1) 400 Loo, B.H. (2.5) 137 Lopez, J. (2.5) 116 Lopez, L. (2.5) 145 Lorenc, L. (2.1) 45; (2.5)

143

Loreto, M.A. (2.7) 102, 103

Losev, A.P. (2.5) 76 Lotnik, S.V. (3) 425 Lottaz, P.-A. (2.6) 117 Loucheux, C. (3) 259 Lougnot, D.J. (1) 433, 507; (3) 15, 16, 54, 58, 60 Louis, T.A. (1) 60 Lounasmaa, M. (2.4) 109 Lounay, J.-P. (1) 211 Loutfy, R.O. (1) 170 Loutz, J. (3) 32 Lowry, R.E. (3) 337, 340 Lu, H. (1) 489 Lu, Q. (1) 62 Lu, R. (3) 256 Lu, Y. (1) 181 Lubbe, F. (2.3) 101 Lucas, P.C. (3) 483 Lucchini, V. (2.5) 109 Luddecke, E. (2.6) 115 Luettke, W. (1) 130; (2.4) 142 Lui, D. (3) 515 Lukashov, A.V. (3) 623 Lukomskaya, I.S. (3) 501, 581 Lunak, S. (1) 116; (2.3) 63; (2.5) 165 Lund, E.A. (2.5) 157 Luneva, N.P. (4) 22 Lunn, P.M. (2.4) 266, 267; (2.7) 160, 161 Lunsche, R. (1) 128 Luo, J.K. (2.4) 157 Luo, 2. (2.5) 101 Lupon, P. (2.5) 174 Lushchik, V.B. (3) 327 Luston, J. (3) 592 Lusztyk, J. (2.6) 33 Lutgen, P. (3) 529

Lwowski, W. (2.7) 106 Lyashenko, L.V. (2.5) 107 Lyashik, O.T. (2.3) 68; (2.4)

171; (2.6)

24

Lymarev, A.A. (3) 422 Lysenko, E.P. (2.5) 181 Lytle, F.E. (1) 32 Lyubarskaya, A.E. (2.3) 88

Lyubimov, A.V. (2.6) 34 Ma, X.H. (3) 141 Maas, G. (2.7) 55, 56 Mabuchi, T. (1) 366 Macaluso, G. (2.4) 12; (2.6)

57

McCapra, F. (1) 79 McClelland, R.A. (1) 150; (2.4) (2.5)

238, 242, 263; 170

McConnell, J.A. (3) 194 McCormick, C.L. (3) 342, 392

McCoy, R.K. (2.3) 53 McCray, J.A. (2.6) 79 McDonald, C.E. (2.7) 221 McGarry, P.F. (2.7) 48, 143

McGee, L.R. (2.6) 107 McGimpsey, W.G. (1) 409 McGlade, M. (3) 370 McGlynn, S.P. (1) 351 McGown, L.B. (1) 14, 44, 46, 49, 350

Machida, M. (2.2) 109, 115, 116, 118; (2.4) 52, 54, 113; (2.6) 122, 163, 165, 174 Machino, S. (2.3) 92 Macho, V. (3) 573 Maciejewski, A. (1) 24, 84, 139 Macintosh, A. (1) 281 McKee, V. (2.6) 4 Mackor, A. (3) 549 McLauchlan, K.A. (1) 30 McLaughlin, K.W. (3) 94 McLean, M.R. (1) 246 McMaris, G.E. (1) 163 McMillen, D. (2.7) 221 McMorrow, D. (1) 229 McPhail, A.T. (2.1) 61; (2.7) 132 Madadi, N. (2.7) 182 Maeda, K. (2.5) 197; (2.6) 52, 153 Maejima, M. (4) 57 Maekawa, E. (2.4) 118; (2.5) 155; (2.6j 171

Maekawa, Y. (3) 166 Maev, R.G. (3) 433

Author Index Magdinets, V.V. (3) 144, 258 Maguire, J.A. (2.5) 97 Mahgoub, S.A. (2.4) 131; (2.7) 150 Mahol, H.S. (1) 219 Mahoney, N.E. (2.2) 146 Mahran, M.R. (2.5) 148; (2.6) 209 Maier, G. (2.3) 90, 110; (2.7) 44, 107 Maistrenko, G.Ya. (2.5) 187, 220 Maisuradze, D.P. (2.6) 36 Maiti, A.K. (1) 404 Maiti, M. (1) 138 Maiya, B.G. (2.5) 80 Majerski, Z. (2.2) 73; (2.7) 60 Majima, T. (2.6) 208 Mak, T.C.W. (2.6) 66 Makarov, G.G. (3) 581 Makhashvili, N.I. (2.6) 36 Makhloufi, R. (1) 355 Makhonina, E.V. (4) 18 Maki, Y. (2.1) 54; (2.4) 108; (2.6) 68 Makiguchi, H. (4) 57 Makr, A.H. ( 1 ) 406 Maksimova, L.B. ( 3 ) 511 Mal, S. (2.6) 18 Malac, J. (3) 447, 480 Malatesta, V. ( 1 ) 469; (3) 518, 530 Malati, M.A. (4) 37 Maldonado, J. (2.6) 146; (2.4) 82; (2.7) 182 Maldotti, A. (2.5) 65; (3) 147 Malhotra, K. (2.5) 61 Malik, J. (3) 594 Malkin, Ya.N. (1) 484; (2.6) 6 , 154 Mallory, F.B. (2.4) 143 Mallouk, T.E. (2.4) 260; (2.5) 80, 212 Malone, K. (3) 536 Maloney, V.M. (2.3) 86 Mamada, A. (3) 308 Mamaev, V.P. (2.4) 232; (2.6) 78 Mamedov, Sh.V. (3) 338 Manda, Y. (3) 411 Mandzhikov , V .F. (2.3) 67, 69; (2.4) 170; (2.6) 25, 26 Manitto, P. (1) 469 Mankotia, A.K.S. (2.4) 233; (2.6) 99 Mansour, M. (2.2) 69; (2.4) 218; (2.6) 159

53 1 Mansueto, S.E. (3) 109 Mansuy, D. (2.5) 65 Manu, A.M. (3) 489 Manus, P. (3) 250, 279 Manzhikov, V.F. (2.4) 172 Marawetz, H. (3) 281 Marcandalli, B. ( 3 ) 617 Marcel-Curto, M.J. (2.5) 141 Marcus, R.A. (1) 157-159 Marder, S.R. (3) 311 Mareda, J. (2.6) 117 Marevtsev, V.S. (2.6) 34 Margaretha, P. (2.1) 50, 51; (2.2) 9 , 56, 57, 79-81; (2.6) 104, 172; (2.7) 140, 144 Margaryan, A.Kh. (2.2) 7; (2.6) 70 Margulis, L.A. (2.5) 35, 70, 164 Margulis, M.A. ( 3 ) 434 Mariano, P.S. (2.4) 5, 188; (2.6) 138 Mariero, A.L. (1) 401 Marinic, 2. (2.3) 62; (2.4) 34 Markakis, J.M. ( 4 ) 56 Markert, H. (3) 204 Markov, P. (2.4) 62 Markovic, D.Z. (1) 498 Marmonier, M.F. (3) 306 Maroncelli, M. (1) 225 Marples, B.A. (2.7) 167 Marquet, J. (2.4) 65-68, 70 Marta, F. (1) 217 Marta, P.J. (3) 247 Martin, A.R. (2.6) 102 Martin, M.E. (3) 628 Martin, N. (2.3) 75, 76; (2.6) 62 Martinez, C. (2.4) 57 Martinez, G . (3) 464-466 Martinez, M.L. (1) 237, 436; (2.5) 41 Martinez, M.T. (1) 132 Martinez-Lozano, C. (2.5) 51 Martinez-Utrilla, R. (2.4) 207; (2.7) 112; (3) 628 Martinho, J.M.G. ( 1 ) 5, 110; (3) 395 Martl, M.G. (3) 242 Maruthamuthu, P. (4) 39-41 Maruyama, K. (1) 314; (2.2) 133, 142-144 Marynick, D.S. (3) 355 Masatomi, T. ( 1 ) 153 Masetti, F. (1) 221

Masilamani, V. (1) 203 Masjedizadeh, M.R. (2.7) 25 Maslak, P. (2.3) 99; (2.4) 264; (2.7) 111 Maslov, S. (2.6) 196 Maslyuk, A.F. (3) 264 Mastalia, A. (2.7) 129 Masuda, T. (2.2) 1 Masuhara, H. (1) 393; ( 3 ) 284, 387 Masuhara, K. ( 3 ) 475 Masuhara, M. ( 1 ) 338 Masumi, S. (3) 114 Mataga, N. (1) 165, 172-174, 178, 180, 186, 199, 268, 393 Matasuzaki, T. (2.5) 144 Mateo, J.L. (3) 55, 61, 62, 438, 566 Mathies, P. (2.2) 51; (2.5) 118 Mathies, R.A. (1) 244 Mathivanon, N. (1) 150 Matkovskii, A.O. ( 3 ) 561 Matoike, A. (3) 139 Matsubara, M. (2.3) 31 Matsubara, S. (4) 55 Matsuda, K. (2.4) 100; (2.7) 204 Matsuda, M. (3) 352 Matsuda, T. (2.7) 120 Matsuda, Y. (2.3) 94; (2.4) 231 Matsue, H. (2.6) 199 Matsugo, S. (2.7) 170 Matsui, K. (1) 365 Matsumoto, H. (1) 142 Matsumoto, N. (3) 415 Matsumura, M. (4) 47, 48 Matsunami, H. (1) 378; ( 4 ) 54 Matsuo, T. (1) 359 Matsura, I. (2.5) 88 Matsusaka, M. ( 3 ) 472 Matsushima, R. (2.4) 173; (2.6) 27 Matsuura, T. ( 1 ) 501; (2.2) 2 0 ,'8 4 ; (2.5) 99; (2.6) 135; (2.7) 170 Matsuzaki, T. (3) 79 Matsuzaki, Y. (2.4) 55, 56 Mattay, J. (2.3) 98; (2.5) 6 Mattice, W.L. ( 1 ) 3; (3) 371-374, 396, 399 Matusch, R. (2.5) 122 Matuszewska-Czerwik, J. (3) 103

Ma;,-A.W.H. (2.5) 157, 158; ( 4 ) 15

Author Index

532 Maxka, J. (2.6) 186 Maxwell, B . D . (2.3) 106 Maxwell, V. ( 1 ) 368 Mazlumoglu, T. ( 3 ) 554 Mazzocchi, P.H. (2.2) 114, 117; (2.6) 125, 126 Meador, M.A. (3) 189, 539 Meador, M.A.B. ( 3 ) 189 Mecklenburg, S . L . ( 1 ) 151; (2.4) 240; (2.7) 166 Medyantseva, E.A. (2.3) 68; (2.4) 171; (2.6) 24 Meech, S . R . ( 1 ) 382 Meerts, W.L. ( 1 ) 284 Mehrotra, K.N. (2.1) 15 Mehta, S. (2.4) 215 Meic, 2. (2.6) 63 Meijer, D . K . F . (2.7) 88 Meille, S . V . (3) 543 Meirvenne, D. ( 3 ) 455 Meisel, D. ( 4 ) 52 Meister, E.C. (1) 93, 502; (2.4) 141 Meller, M. (2.2) 40; (2.6) 110 Mel'nichuk, S . V . (2.6) 154 Mel'nikova, L.M. (2.2) 21-23, 64; (2.6) 137 Melo, E.C.C. (1) 357 Melton, L.A. ( 1 ) 62 Memarian, H.R. (2.6) 133 Mendenhall, G.D. ( 1 ) 477 Mendicuti, F. (3) 373, 374 Mendola, D. (2.4) 91; (2.7) 202 Meng, J. (2.2) 20, 84; (2.4) 96, 98; (2.6) 135 Meng, X. (3) 488 Mentha, Y.G. (2.6) 117 Menzel, R. ( 1 ) 114 Mertes, J. (2.3) 98 Messier, P. ( 1 ) 337 Metelitsa, A.V. (2.3) 68; (2.4) 171; (2.6) 24, 65 Meyer, M. (1) 254 Meyer, T.J. (1) 313, 334; (3) 402 Mialocq, J.-C. (1) 254 Michael, C. (1) 442 Michaille, S. ( 3 ) 450 Michels, G. (2.6) 72 Michitani, H. (2.6) 41 Michl, J. (2.3) 86; (2.7) 109 Midden, W.R. (2.2) 28 Midoux, N. ( 1 ) 75 Mieche, J . A . (1) 211 Mielewski, D . F . ( 3 ) 548,

607-609

Migirdicyan, E. (1) 458, 459; (2.7) 80

Migus, A. ( 1 ) 175 Mihailov, S. ( 3 ) 540 Mihailovic, M.L. (2.1) 45; (2.5) 143

Mikami, N. (2.7) 120 Mikh, T.F. ( 3 ) 266 Mikhailov, Y.P. ( 1 ) 256 Mikhalyuk, S . A . ( 3 ) 251 Miki, T. ( 3 ) 531 Mikiharu, M. (3) 125 ' Millan, J. ( 3 ) 464-466 Millasson, P. (2.6) 117 Miller, R.S. (2.1) 56; (2.4) 115; (2.7) 118 Millevolte, A . J . (2.6) 186 Millican, D.W. ( 1 ) 46 Minaev, B . F . ( 1 ) 391 Minato, A. (2.4) 18; (2.6) 184 Mine, S. ( 4 ) 34 Minemura, N. ( 3 ) 514 Mines, S . A . (2.3) 27 Ming, Y. (2.3) 84 Minisci, F. (2.4) 116; (2.7) 117 Minkin, V . I . (2.2) 97; (2.3) 58, 59, 68, 88; (2.4) 171; (2.6) 24 Minto, F. (3) 543 Minto, R . E . ( 1 ) 495; (2.4) 241 Miranda, M.A. (2.3) 114; (2.4) 207, 209; (2.5) 180; (2.6) 156; (2.7) 112 Miranda, P.M. (2.4) 217; (2.6) 157 Misawa, H. (2.3) 13; (2.6) 2 Misev, L. ( 3 ) 56 Mishima, M. (2.1) 40 Mishra, P.C. ( 1 ) 135 Misra, R. (2.6) 107 Missmer, M.C. (1) 169 Mita, I. ( 3 ) 42, 231, 232, 240, 384, 448, 533, 534 Mitani, M. (2.7) 104 Mitchell, A . J . (2.4) 22 Mitsuhata, T. ( 3 ) 523 Mitsui, I. ( 1 ) 366 Mittel, J.P. (1) 126, 431 Miura, C. (2.4) 97; (2.7) 201 Miura, T. (2.2) 90; (2.3) 103; (2.6) 166 Miyagaki, S. (4) 54 Miyagawa, H. (2.5) 68

Miyagawa, K. (2.5) 27 Miyahara, T. (2.1) 40 Miyama, H. ( 3 ) 37 Miyama, T. (2.4) 74; (2.6) 136

Miyamoto, J. (2.7) 120 Miyasaka, H. (1) 172, 393 Miyasaka, K . ( 3 ) 429, 432 Miyashi, T. (2.2) 136; (2.3) 34

Miyashita, K. ( 3 ) 560 Miyashita, T. (3) 352 Miyata, H. (2:5) 45, 47; ( 4 ) 38

Miyata, K. (2.2) 111; (2.6) 96; (3) 614

Miyata, 0. (2.4) 1, 155, 156; (2.6) 43, 44

Mizuno, K. (2.3) 21; (2.4) 270, 275; (2.5) 178; (2.6) 178 Mizuno, T. (2.3) 31 Mjalli, A.M.M. (2.1) 16 Mlinaric-Majerski, K. (2.2) 73 Mobius, D. ( 1 ) 323 Mochida, K. (2.6) 177, 195 Mochizuki, S. (2.5) 66, 67 Modak, M. (3) 90 Modarelli, D.A. (2.7) 168, 169 Modes, S. (1) 341, 362 Modi, S.P. (2.6) 38 Moedzinski, A. (1) 437 Moghaddam, M.J. (3) 99 Mohan, D. ( 1 ) 267 Mohanty, A.K. (3) 38 Moharil, S.V. (3) 416, 430 Moiseev, V.V. ( 3 ) 171 Mokrushin, V.S. (2.6) 75 Mol, G.N. ( 3 ) 159, 269 Mol, W.E.M. (2.7) 88 Molloy, B. (1) 331; ( 3 ) 398 Molotkovskii, Yu.G. (2.7) 66 Momicchioli, F. ( 1 ) 402 Momose, T. (2.2) 55 Monnerie, L . ( 3 ) 334, 385, 386, 405

Montenay-Garestier, T. (1) 188 Moore, C.B. ( 1 ) 67 Moorgat, G.K. (2.5) 147 Moorman, R. (2.1) 58 Moorthy, P.N. (1) 318 Morand, P. (1) 97 Morawetz, H. (3) 381 Mordzinski, A. ( 1 ) 231,

533

Author Index 233 Moreno-Bondi, M.C. (3) 330 Moreno-Mazas, M. (2.4) 65-67, 70 More O'Ferrall, R.A. (2.6) 19 Morera, I.M. (2.4) 207; (2.7) 112 Morgan, F. (1) 96, 104 Morgan, T. (2.2) 114; (2.6) 125 Mori, A. (2.2) 3; (2.3) 42, 43; (2.6) 53, 54 Mori, K. (3) 475 Mori, T. (2.4) 97; (2.7) 201 Mori, Y. (1) 199; (2.6) 52 Moriarty, R.M. (2.1) 56; (2.4) 115; (2.7) 118 Morine, A. (2.4) 65 Morishima, Y. (3) 393 Morita, H. (3) 46, 214 Moriuchi, S. (4) 54 Moriwaki, T. (3) 211 Morkert, R. (1) 491 Moroi, S. (2.4) 119; (2.5) 166 Morooka, T. (2.4) 111 Morozov, V.A. (1) 195 Morrison, H. (1) 321; (2.1) 66; (2.3) 71, 106, 107; (2.5) 20; (2.6) 106 Morrissey, P. (2.7) 61 Morrocchi, S. (2.4) 76, 77; (2.6) 145 Mortezaei, R. (2.2) 53; (2.5) 40 MOSS, R.A. (2.7) 9-11 Moussa, K. (3) 53, 59 Moustacchi, E. (2.2) 25 Mu, W. (2.5) 98 Much, H. (2.7) 64 Mucha, M. (3) 458 Mueller, G . (3) 122 Mueller, U. (3) 424; (3) 49, 120 Mugmer, J. (1) 312 Mugnier, J. (1) 51 Mukai, M. (2.1) 8 Mukai, T. (2.2) 136; (2.3) 33, 34 Mukai, Y. (1) 398 IMukerjee, S.K. (2.3) 29 Mukherjee, G.S. (3) 78 Mukherjee, T. (1) 126, 431 Mukherji, S.M. (2.1) 20; (2.2) 72; (2.4) 212; (2.6) 48

Mukhin, R.R. (1) 391 Muller, C.L. (2.2) 31 Muller, J.F. (3) 529 Muller, M. (2.7) 88 Muller, W. (2.3) 101 Mulvaney, P. (4) 52 Muneer, M. (1) 494 Munkholm, C. (1) 264 Munoz, D. (2.7) 79 Munro, I.H. (1) 177 Munro, M.H.G. (2.5) 186 Muradov, N.Z. (2.5) 89, 90; (4) 24 Murai, H. (2.5) 27, 29 Murai, T. (2.4) 124; (2.5) 80 Murakami, H. (1) 268 Murakami, K. (3) 249 Murakami, M. (2.2) 76; (2.3) 31 Murakami, N. (3) 429 Murakata, Y. (2.1) 25; (2.6) 97 Murakoshi, I . (2.6) 140 Muralidharan, S (2.4) 257; (2.6) 147 Muramatsu, H. (2.7) 77 Muramatsu, T. (2.3) 65; (2.6) 61 Muramoto, A. (1) 438 Murase, T. (2.1) 54 Murasecco-Suardi, P. (2.5) 77 Murashko, V.L. (2.1) 17 Murata, I. (2.6) 112 Murata, K. (1) 142 Murata, S. (2.7) 50, 51 Murgia, S.M. (2.5) 111 Murphy, E.J. (3) 181, 182 Murphy, W.F. (1) 147; (2.4) 247 Murumami, K. (2.3) 65 Mustafa, M.B. (3) 315 Mustafin, A.Kh. (2.7) 45 Muszkat, K.A. (1) 96 Mutai, K. (2.6) 87 Muthal, P.L. (3) 416, 430 Muthusamy, S. (2.4) 177; (2.7) 196 Muzart, J. (2.2) 53; (2.5) 40 Muzykantov, V.N. (3) 23 Myakon'kii, A.G. (4) 18, 19

.

Nabeshims, M. (2.5) 197 Nag, A. (1) 205, 241-243, 270, 282 Nagagaki, R. (2.6) 87 Nagai, A. (2.6) 14 Nagai, Y. (1) 142

Nagaki, T. (3) 270 Nagakura, S. (2.6) 87 Nagamatsu, G. (3) 48 Nagano, H. (2.5) 197 Nagano, S. (3) 531 Nagaoka, S. (1) 488 Nagashima, V. (1) 228, 488 Nagata, M. (2.2) 92; (2.6) 20 Nagawa, Y. (2.7) 34, 35 Nagooka, S. (1) 228 Nagoya, I. (3) 156 Naguib, Y.M.A. (2.7) 218 Nagura, A. (3) 240 Nahm, K. (2.4) 41, 42 Nahor, G.S. (3) 354 Naik, D.B. (1) 318 Nair, V. (2.7) 207 Naito, A. (1) 291 Naito, I. (2.6) 74 Naito, R. (2.2) 130 Naito, T. (2.4) 1, 155, 156; (2.6) 43, 44 Nakabayashi, K. (2.4) 271, 272; (2.7) 177 Nakabayashi, N. (2.1) 46 Nakabayashi, S. (2.5) 138 Nakada, M. (2.4) 97; (2.7) 201 Nakagaki, R. (1) 113; (2.5) 15 Nakagawa, K. (2.2) 133; (3) 81, 126 Nakagawa, T. (3) 270 Nakahara, H. (3) 350 Nakahira, T. (3) 494 Nakai, J.S. (2.4) 123 Nakai, K. (3) 569, 605 Nakamura, H. (1) 359 Nakamura, I. (2.3) 109; (2.4) 85, 86; (2.7) 212, 214 Nakamura, J. (3) 292 Nakamura, K. (3) 45, 257, 383 Nakamura, S. (2.4) 117; (3) 34 Nakamura, Y. (3) 214 Nakanishi, F. (3) 108 Nakanishi, H. (2.4) 179; (2.7) 34, 35; (3) 108 Nakanishi, I . (2.4) 270; (2.5) 178 Nakanishi, K. (2.3) 21; (2.4) 275; (2.6) 178 Nakano, K. (2.3) 65 Nakashima, N. (1) 378 Nakata, M. (2.5) 104-106 Nakato, T. (2.5) 45, 46; (4) 38 Nakatsuka, K. (3) 612

Author Index

534 Nakawa, Y. (2.4) 179 Nakayama, H. (2.4) 69 Nakayama, T. (1) 422 Nakayama, Y. (2.4) 152, 153; (2.6)

210

Nakazawa, T. (1) 70, 365; (2.6)

112

Nakazumi, H. (2.6)

153;

(3) 634, 636

Nakhimovsky, L. (1) 104 Nakor, G . S . (4) 28 Nakos, S.T. (3) 183, 184, 237

Nalbandyan, G. (2.1) (2.2)

21;

7

Namariyama, Y. (3) 257 Namboodiri, K. (2.7) 70 Nan, C. (2.5) 127 Nanba, M. (2.2) 121, 123; (2.6)

119

Nandi, R. (1) 138 Naomura, K. (4) 17 Narabe, Y. (2.3) 65 Narasimhan, L.R. (1) 255 Narayan, D. (3) 117 Narui, H. (4) 55 Nash, J.J. (2.3) 106, 107 Nasioulas, G . (2.7) 90 Natarajan, P. (1) 182 Nath, D.N. (1) 218 Natsume, M. (2.4) 53 Nattrass, S.R. (1) 30 Nauba, M. (2.4) 50, 51 Navaratnam, S. (3) 65 Nayak, K. (3) 355 Nayar, V. (2.5) 219 Ndiaye, S . A . (1) 302; (2.2)

32

Ndou, T.T. (1) 327, 343 Neckers, D . C . (1) 301; (2.6) 341

10; (3) 72, 280,

Nedoshivin, V.Yu. (2.6) 34

Nefedov, O.M. (2.7) 39 Negievich, L.A. (3) 171 Negri, F. (1) 400 Neier, R. (2.2) 14; (2.6) 105

Neikov, G. (2.2) 41 Nelson, D.L. (2.7) 70 Nelson, G. (1) 94 Nelson, K.A. (1) 10 Nelson, W.L. (2.7) 71 Nemoto, Y. (3) 45 Neoh, K.G. (3) 545 Nepras, M. (1) 116; (2.3) 63

Neroznik, V.G. (3) 266 Nersesyan, K.A. (3) 112 Neta, P. (4) 28 Netto-Ferreira, J.C. (1)

147, 432; (2.1) 5, 48, 49; (2.4) 227, 234, 245, 247; (2.6) 181; (3) 212 Netzel, T.L. (1) 184 Neuenfeld, J . (3) 120 Neugebauer, F.A. (2.7) 139 Neugebauer, S.M. (2.7) 42 Neuman, R.C. (2.6) 8 Neumann, W.P. (2.6) 196 Newton, R.F. (2.1) 16 Nguyen, T. (2.3) 21; (2.4) 275; (2.6) 178 Nguyen, T.L.H. (3) 550 Ni, T. (1) 62 Ni, Y. (1) 297 Nickel, B. (1) 7, 99 Niclause, M. (2.4) 63 Nicodem, D . (2.5) 32 Nicolaou, K.C. (2.6) 168 Nicora, C . (3) 54 Niemeyer, N. (1) 130 Niessen, R. (2.4) 107 Nigam, S.K. (3) 117 Niino, H. (3) 531 Nikitina, S.V. (3) 251 Nikolaevskaya, V.I. (3) 598 Ninomiya, I. (2.4) 1, 155, 156; (2.6) 43, 44 Nishi, H. (2.5) 200 Nishida, A. (2.3) 116; (2.4) 269, 274; (2.6) 13, 14; (2.7) 155; (3) 298 Nishida, S. (2.2) 59, 76; (2.3) 31, 103 Nishihara, J. (3) 81 Nishijima, K . (3) 113 Nishikawa, S. (1) 186 Nishikubo, T. (3) 34, 45, 96, 97, 504 Nishimura, A.M. (1) 376 Nishimura, J. (2.3) 92-94; (2.4) 198 Nishimura, N. (2.7) 1, 122 Nishimura, Y. (2.3) 24; (4) 34 Nishimuta, K. (3) 139 Nishio, T. (2.2) 19, 51, 77, 90, 91; (2.5) 118, 204; (2.6) 113, 139, 164, 166 Nishiwaki, T. (2.4) 197; (2.6) 129, 130; (2.7) 94 Nishiyama, H. (2.4) 97; (2.7) 201 Nishiyama, N. (2.2) 3 Nishizawa, A. (3) 494

Nishizawa, K. (2.2) 111; (2.6)

110,

96

Nithipatikam, K. (1) 350 Niu, Q. (1) 477 Niwa, M. (3) 133, 134, 263

Niyazi, F.F. (3) 583 Niyogi, U. (3) 67, 107 Nizova, G.V. (2.5) 152 No, Y.G. (3) 536 Nobbs, J.H. (3) 5 Nocera, D.G. (1) 105 Noda, S. (1) 194 Noe, L . J . (1) 200 Noguchi, T. (1) 119 Nogueira, L . C . (2.4) 71 Nojima, C. (2.2) 2 Noll, K. (3) 28 Nomura, K. (2.5) 132; (4) 16, 20

Nomura, M. (2.4) 127 Nonell, S. (1) 474 Noren, G.K. (3) 181, 182, 192, 193

Norinder, U. (1) 292; (2.3)

10, 18

Norris, J.O.W. (1) 57 Novi, M. (2.7) 188 Novinson, T. (1) 483 Novis, Y. (3) 525 Novkirishka, M. (2.4) 62 Novoradovskii, A.G. (3) 638

Nowacki, J. (1) 208 Nowakowska, M. (1) 335; (2.5)

177; (3) 347, 348

Nozaki, A. (2.2) 103 Nozaki, K. (1) 291 Nozakura, S. (3) 393 Nozi, T. (3) 118 Numone, K. (1) 113 Nuriev, E.Sh. (3) 338 Nurkhodzhaev, Z.A. (2.4) 29

Nurmukhametov, R.N. (2.4) 230; (2.6)

76

NUBS, J.M. (2.4) 10 Nuyken, 0. (3) 91, 132 Oak, 0.2. (2.5) 124 Oandey, G. (2.5) 191 Obana, T. (2.1) 24 Obi, K. (1) 430 Occhiello, E. (3) 518, 530

Oda, H. (2.3) 31; (3) 629-633

Oda, K. (2.2) 109, 118; (2.4) 174

113; (2.6)

165,

Oda, M. (2.1) 52, 57;

Author Index (2.2) 102; (2.4) 62; (2.6) 211; (2.7) 116, 124 Odaira, Y. (2.2) 12, 18; (2.4) 46 O'Driscoll, E. (2.3) 97 Oedberg, L. (3) 357 Oelkrug, D. (1) 442 Oertel, U. (3) 122 Ofenberg, H. (2.4) 146 Ofir, H. (1) 447 Ogali, R.E. (2.4) 137 Ogawa, T. (2.2) 111; (2.6) 96 Ogilby, P.R. (1) 464; (3) 413 Ogilvie, R.J. (2.2) 30; (2.6) 111 Ogiwara, Y. (3) 111 Ogorodnikova, M.M. (2.1) 17 Ogura, H. (2.2) 104 Oh, S.C. (3) 114 Oh, S . M . (2.4) 143 Ohara, K. (2.5) 29 Ohara, S. (2.4) 108; (2.6) 68 Ohashi, K. (3) 569, 605 Ohashi, M. (2.4) 9, 74, 75; (2.6) 136; (3) 305 Ohashi, T. (3) 162 Ohashi, Y. (2.6) 52; (3) 166 Ohba, T. (3) 34 Oh-hira, Y. (2.6) 41 Ohira, M. (2.6) 37 Ohkatsu, Y. (3) 568 Ohkita, M. (2.2) 59, 76; (3) 223-225 Ohkura, K. (2.4) 99, 100; (2.7) 200, 204 Ohmiya, S. (2.6) 140 Ohngemach, J. (3) 57 Ohnishi, Y. (2.2) 12 Ohno, K. (2.2) 118; (2.5) 52; (2.6) 174 Ohno, T. (2.5) 201 Ohtawa, Y. (2.7) 50, 51 Ohtsuka, T. (2.2) 71; (3) 140 Ohtuka, T. (3) 245 Ohzeki, K. (1) 381 oi, Y. (3) 34 Oikawa, S. (2.7) 53 Oishi, S. (2.3) 116; (2.4) 269 Ojeda, P.J. (1) 269 Ojima, S. (1) 172 Oka, M. (2.1) 29 Okada, K. (2.1) 52, 57; (2.2) 102; (2.4) 61, 199; (2.6) 211; (2.7)

535

116, 124 Okada, S. (3) 108 Okada, T. (1) 178, 186, 268 Okada, Y. (3) 146 Okamoto, H. (2.3) 13; (3) 213 Okamoto, K. (2.1) 57; (2.7) 124 Okamoto, M. (1) 374, 467; (2.2) 103; (2.5) 74 Okamoto, Y. (2.3) 32, 100; (2.4) 102-104, 250-252, 255; (2.6) 200, 205; (2.7) 156, 157; (3) 237 Okawa, A. (4) 25 Okazaki, M. (1) 113 Okazaki, 0. (1) 291 Okazaki, R. (2.7) 98 Oku, A. (2.3) 92, 94 Okubo, K. (2.1) 52; (2.4) 61; (2.7) 116 Okubo, M. (2.4) 139; (2.6) 40 Okura, I. (1) 194; (2.5) 55; (4) 26 Okutsu, T. (1) 430 Olaj, O.F. (3) 104 Olaya, F.S. (2.6) 121 Oldham, P.D. (1) 377 Oldring, P.K.T. (3) 5 Olea, A . F . (3) 321 Oliver, A.M. (1) 322 Oliveros, E. (2.5) 77 Olkkonen, C. (3) 506, 507 Olson, D.R. (3) 50 Omate, T. (3) 71 Omote, Y. (2.2) 106; (2.4) 38; (2.5) 204; (2.6) 109, 131 Onal, A.M. (3) 163 O'Neill, J.A. (3) 210 Ongemach, J. (3) 177 Onoda, M. (3) 411 Onuki, M. (2.4) 105 Oosawa, Y. (4) 35 Opriel, V. (1) 470 Orakhovats, A. (2.2) 41; (2.3) 48, 49 Orellana, G. (3) 330 Oremus, V. (2.6) 60 Orito, K. (2.2) 71; (2.7) 223, 224 Orlandi, G. (1) 400 Orlowska, B. (2.6) 3 Orthwein, H. (1) 168 Ortiz, M.J. (2.3) 56, 57; (2.6) 22 Orwoll, R.A. (3) 538 Orzonowska, G. (1) 231 Osa, T. (1) 111; (3) 292

Osada, T. (3) 295 Osawa, E. (2.2) 71 Osawa, Z. (3) 289, 423, 426 O'Shea, M.S. (3) 502 Oshima, Y. (2.5) 84, 85 Osipov, A.N. (2.5) 181 Osman, F.H. (2.6) 206 Ostakhov, S.S. (2.5) 187 Osuka, A. (1) 314; (2.2) 133 Otis, C.E. (3) 524 Otomasu, K. (2.6) 140 Otsu, T. (3) 116 Otsuji, Y. (2.3) 21; (2.4) 270, 275; (2.5) 178; (2.6) 178 Otsuki, T. (2.2) 27 Ouchabane, R. (2.7) 171 Ouchi, A. (2.4) 249 Oudenampsen, A. (2.4) 253 Ouerfelli, 0. (2.5) 123 Ourisson, G. (2.7) 135 Ovchinnikov, A.A. (3) 433 Ovsepyan, V.V. (2.6) 70 Owens, J.E. (2.4) 17; (2.6) 152 Oxman, J.D. (2.4) 89 Oyabu, I. (2.1) 54 Oyama, H.T. (3) 366, 367 Ozaki, M. (3) 156 Ozaki, T. (1) 146 Ozawa, Y. (2.2) 143 Ozbalik, N. (2.7) 125, 136 Pac, C. (2.3) 104; (2.5) 17, 18; (2.7) 175; (4) 21 Pack, D.W. (1) 255 Pack, T.W. (3) 620 Paddon-Row, M . N . (1) 166, 167, 322 Padwa, A. (2.7) 32 Pae, A.N. (2.5) 196 Page, G.O. (2.2) 124; (2.4) 202 Paglia, E.D. (3) 617 Pal, H. (1) 126, 431 Paleta, 0. (2.1) 64; (2.5) 64 Palewska, K. (1) 93, 502; (2.4) 141 Palinisamy, P.K. (1) 203 Palit, D.K. (1) 126, 431 Palmer, C. (3) 86 Palmer, T.F. (1) 132 Palmisano, L. (2.5) 43, 156 Palomino, E. (2.7) 164 Pan, B. (3) 234

536 Pegram, J.E. (3) 446 Pan, J. (3) 130 Peinado, C. (3) 61, 62 Panarin, E.F. (3) 327 Pellacani, L. (2.7) 102, Pandey, B. (2.2) 5, 131; 103 (2.5) 217 Pelletier, M. (3) 529 Pandey, G. (2.3) 111; Pelzer, R. (2.1) 43 (2.4) 268; (2.5) 169, Peng, S.-M. (2.6) 170 192, 218; (2.7) 158, Pennanen, S. (2.4) 109 172 Pennington, W. (2.7) 74 Pannikottu, A. (3) 371 Pentegova, V.A. (2.5) 142 Pant, D. (1) 137 Penzkofer, A. (1) 262 Pant, D.D. (1) 137, 192, Pereira, M. (1) 319 193 Perez-Ossorio, R. (2.3) Pant, G. (2.6) 18 39, 57 Pappas, S.P. (3) 8, 445 Paputa Peck, M.C. (3) 607 Perez-Ruiz, T. (2.5) 51 Perisamy, N. (1) 41 Paquette, L.A. (2.3) 91 Perkins, T.A. (1) 184 Paradisi, C. (2.6) 191 Perly, B. (1) 254; (2.7) Paradiso, R. (3) 86 92 Paramasivarn, R. (2.4) Perrier, S. (1) 198 177; (2.7) 196 Perrine, D.M. (2.4) 17; Parisi, J.P. (3) 275 (2.6) 152 Park, H.R. (2.5) 62 Perry, J.W. (1) 183 Park, J.S. (3) 152 Pervov, V.S. (4) 18 Park, J.W. (1) 352 Pete, J.P. (2.2) 53, 82, Park, K.M. (3) 75, 83 83; (2.5) 40, 69 Park, N.S. (1) 288, 290 Peters, E.-M. (2.2) 135; Parkanyi, C. (3) 346 (2.3) 77 Parkinson, D.-R. (1) 264 Peters, K. (2.2) 135; Parlar, H. (2.3) 105 Parmon, V.N. (4) 2 (2.3) 77, 97 Peterson, K.A. (3) 336 Paro, K.A. (1) 124 Petrenko, Q.P. (2.4) 232; Parsons, B.J. (3) 65 (2.6) 78 Parsons, S. (3) 363 Pasch, H . (2.7) 64 Petrillo, G. (2.7) 188 Petrova, I.N. (3) 618 Pascual, A. (2.2) 78 Pettit, G.H. (3) 520 Pasimeni, L. (1) 413 Pasimi, P. (1) 316 Pettit, T.L. (4) 45 Pasquini, P. (1) 168 Pfanstiel, J.F. (1) 284 Passkal, L.P. (2.4) 211 Pfleiderer, B. (2.6) 204 Pastor, J.M. (3) 477 Pfleiderer, W. (2.7) 101 Patalakha, N.S. (2.7) 211 Philipart, J.L. (2.5) Patel, B. (3) 373, 374 151; (3) 469 Pathak, C.P. (3) 341 Philips, M. (3) 32 Patjens, J. (2.6) 104 Phillips, D. (1) 78 Patterson, H.H. (3) 421 Phillips, G. (2.7) 184 Piancatelli, G. (2.4) 91, Patterson, L.K. (1) 498 92, 94, 95; (2.7) 202, Patyk, A. (2.7) 108 Paul, A. (1) 230 205, 206 Paulauskas, A. (3) 635 Pichat, P. (2.4) 120; (2.5) 98 Paulmann, U. (2.5) 5 Paulson, D.R. (2.3) 2 Pichko, V.A. (2.3) 58, 59 Pauly, K.H. (2.3) 70 Picton, D.W. (1) 48 Pauson, P.L. (2.6) 149 Pierola, I . F . (3) 379, 380, 487 Pautov, V.D. (3) 327 Pavlik, J.W. (2.2) 105; Pieroni, 0. (3) 296, 297, 313 (2.6) 132 Pavlopoulos, T.G. (1) 443 Pietra, F. (2.2) 4 Pavlovic, D. (2.2) 73 Pietrasanta, Y. (3) 275 Pavlovic, V. (2.1) 45 Piette, J. (1) 260 Pedireddi, V.R. (2.6) 101 Pillai, V.N.R. (2.4) Pedulli, G.F. (2.6) 191 147-149; (2.5) 206; Peeters, S. (3) 32 (2.6) 39, 80-82; (2.7)

Author Index 173 Pillay, K.S. (2.7) 218 Pillot, J.P. (2.4) 196 Pimental, G.C. (1) 67 Pincock, J.A. (2.1) 12; (2.4) 258; (2.7) 121 Pipiraite, P. (3) 600 Pireaux, J.J. (3) 525 Pirrung, M.C. (2.1) 13 Pitera, S. (2.4) 259 Piva, 0. (2.2) 53; (2.5) 40 Planas, A. (2.5) 115, 116, 174 Plantenga, T.M. (3) 36 Platsch, H. (1) 457; (2.7) 19 Platz, M.S. (1) 458, 459; (2.7) 5, 78-80, 100 Pleixats, R . (2.4) 67, 68 Plieva, L.Kh. (3) 639 Plotnikov, V.G. (3) 493 Plummer, B.F. (1) 418; (2.4) 201 Plusquelles, D.F. (1) 284 Pohl, S. (2.6) 182 Pohlmann, L. (1) 71 Poisat, 0. (1) 455 Pokkuluri, P.R. (2.3) 50 Polacki, Z. (1) 317 Polavka, J. (3) 476 Polezhaeva, N.A. (2.7) 45 Politzer, I.R. (1) 280 Pollard, W.T. (1) 244 Pollet, A. (2.4) 146 Polo, F. (2.7) 87 Polowinski, S. (3) 103 Polozov, A.M. (2.7) 45 Pommeret, S. (1) 175 Ponstingl, H. (2.7) 90 Poole, N.J. (3) 546 Poornimadevi, C.S. (1) 250 Popescu, T. (1) 56 Popielarz, R. (2.3) 113; (2.4) 265 Popli, R. (3) 370 Popp, B. (2.3) 77 Porkhun, V.I. (2.2) 139 Port, H. (1) 232 Porte, A.L. (2.6) 89, 90 Portella, C. (2.2) 82 Porter, N.A. (2.1) 61; (2.2) 46; (2.7) 132 Porter, R.S. (3) 483 Porzel, A. (2.1) 35 Porzio, W. (3) 543 Pospisil, J. (3) 599 Postigo, J.A. (2.6) 95 Postle, R. (3) 515 Postnikov, L.M. (3) 501, 581

537

Author Index

Poston, P.E. ( 1 ) 421 Potapendo, A. Ya (2.5) 181

.

Pottier, E. (1) 482 Pouget, J. (1) 51, 312 Pouxziel, J.-C. (1) 483 Povazankova, M. (3) 573 Powell, D.R. (2.6) 185 Prabhakar, S. (2.5) 141 Pramod, K. (1) 447 Prasad, K.J.R. (2.4) 208 Prathapan, S. (2.2) 127; (2.4) 235, 236 Pratt, D.W. (1) 284 Preses, J.M. ( 1 ) 83 Preston, D.M. (1) 479, 483 Previtali, C.M. (1) 363, 364 Preyadorsini, K.I. (1) 318 Prica, M. ( 1 ) 148 Primo, J. (2.3) 114; (2.5) 180 Prinsen, W.J. (2.4) 136 Prinzbach, H. (2.7) 27 Prokofyev, A.I. (2.7) 220 Prudhomme, R.E. (3) 331, 332 Przhiyalgovskaya, N;M. (2.3) 67, 69; (2.4) 170, 172; (2.6) 25, 26 Przybytek, J.T. (2.1) 39 Przybytmak, G.K. (1) 505 Pu, B. (2.5) 214 Punchihewa, S. (2.5) 63, 179 Puranik, D.B. (2.6) 183 Purushothaman, E. (2.4) 147-149; (2.5) 206; (2.6) 39 Puzanov, Y.V. (2.7) 220 Pyatnitskii, I . V . (2.5) 207 Pyshchev, A . I . (2.4) 138

Qi, X. (2.3) 85 Qin, X.4. (2.3) 26 Qu, B. (3) 220 Qu, X. ( 3 ) 238 Quiclet-Sire, B. (2.7) 130

Quin, L.D. (2.6) 206 Quinkert, G. (2.2) 99 Quirke, A.P. (2.6) 19 Rabani, J. (3) 354 Rabeony, M.H. (3) 123 Rachinskii, A.G. (2.3)

11; (2.4) 140

Rachovskii, V.A. (3) 110 Rafalska, M. (2.4) 199 Rafellini, P. ( 3 ) 102 Raghavan, P.K.N. (1) 441 Raghuveer, K.S. ( 3 ) 72 Rahn, R. (1) 416 Rajendran, A.G. (2.5) 38 Rajeshwar, K. (3) 345, 355

Raju, B.N.S. (2.2) 74 Raldugin, V.A. (2.3) 81 Ramakrishnan, V.T. (2.4) 177; (2.7) 196; (4) 50

Ramalingarn, A. (1) 203 Ramamurthy, V. (1) 406; (2.1) 6 , 9, 22; (2.3) 7, 95; (2.4) 200 Rambke, J. (3) 80 Ramos, A. (2.3) 75, 76; (2.4) 159; (2.6) 23, 62 Ramponi, R. ( 1 ) 328 Ranby, B. (3) 169, 170, 178-180, 218-220, 437 Rangnekar, D.W. (3) 626, 627 Rangnekar, V.M. (1) 377 Rani, K.S. (2.5) 192; (2.7) 172 Rao, J.M. (2.4) 176; (2.7) 194 Rao, K.N. (3) 105, 106 Rao, O.S. (2.7) 106 Rao, T.N. (3) 105, 106 Rao, V.J. (2.5) 218 Rao, V.P. (2.1) 18; (2.4) 110, 225 Raphael, M.W. (4) 37 Rapp, K.M. (2.3) 82 Rashid, S.M.K. (2.5) 145 Raston, I. (2.3) 18, Rath, N.P. (2.2) 127 Ratiner, B.D. (2.2) 27 Ratovskii, G.V. (3) 511 Rau, H. ( 1 ) 168; (2.6) 115 Rauch, K. (2.4) 142 Ravert, H.T. (2.7) 86

Ravindranathan, K.J. (2.5) 86

Rawat, M.S.M. (2.6) 18 Ray, R. (2.7) 83 Ray, U.K. (2.2) 33 Rayez, M.T. (2.7) 4 Razumov, V.F. (2.3) 11; (2.4) 140

Rebato, J.R. (1) 419 Reddy, G.D. (2.3) 20; (2.4) 276; (2.6) 141 Redmond, R.W. ( 1 ) 64, 136, 147, 155, 486; (2.4) 228, 246, 247;

(2.5) 21

Regitz, M. (2.6) 72 Rehms, A.A. (1) 125 Reichenbaecher, M. (2.3) 64, 66 Reichert, W.M. ( 1 ) 16 Reichmanis, E. ( 3 ) 486 Reinert, G. ( 3 ) 436 Reinhard, G. ( 1 ) 457 Reinhardt, T.E. (2.4) 191 Reisch, J. (2.2) 13, 65, 66; (2.6) 98

Reisenauer, H.P. (2.3) 110; (2.7) 107

Reiser, A. (3) 233 Reiss, H. ( 3 ) 123 Reitberger, T. ( 3 ) 424 Rektonikova, L. (3) 480 Rembold, M. (3) 56 Renneke, R.F. (2.5) 9 3 Resemann, W. (2.4) 145 Retsema, J.A. (2.7) 61 Rettig, S.J. (2.1) 27 Rettig, W. (1) 202 Reynders, P. (1) 107, 108 Rhan, A.E. (2.2) 137 Rhodes, C.J. (2.3) 79 Richard, B. (3) 563 Richard, M. (3) 563 Richmond, K.N. (1) 55 Richter, W. ( 1 ) 279 Rickes, J. (3) 376 Rico, I. (2.6) 142, 143 Riding, K.D. (3) 200, 201 Ridley, D. (3) 252 Rieke, R.D. (2.2) 124; (2.4) 202

Riess, G. ( 3 ) 246, 322 Rigaudy, J. (2.7) 171 Righetto, L. (3) 102 Rihter, B.D. (1) 448 Rinke, M. ( 1 ) 88 Ripamonti, G. (1) 60 Rist, G. (3) 56 1 Ritter, G. (2.6) 115: Ritter, H. (3) 229 Rivas, C.'(2.4) 57 Rivaton, A. (3) 547 Roberge, P.C. (3) 331 Roberts, B.P. (2.6) 192 Roberts, S.M. (2.1) 16 Robey, M.J. (3) 439 Robinson, J.N. (1) 368 Robinson, P.J. (3) 14, 496, 497

Robles-Diaz, R. (2.5) 185 Rodgers, M.A.J. (1) 191, 448, 471; (3) 328

Rodionov, A.N. ( 1 ) 449 Rodriguez, B. (2.5) 117 Rodriguez, C.H. (2.4) 80; (2.7) 183

538 Rutherford, C.E. (1) 451 Rodriguez, F. (1) 39 Rodriguez, J. (1) 22, 490 Rutsch, W. (3) 56 Ruziene, A. (3) 600 Rodwell, P.W. (2.4) Ryan, M.A. (1) 387 22-25, 30 Roffia, S. (2.6) 191 Rye, R.R. (3) 478 Rygalov, L.N. (2.2) 139 Rogan, V. (2.7) 215 Rogers, C.E. (3) 550 Rykov, S.V. (2.1) 17 Rogler, W. (3) 204 Ryzhikov, M.B. (1) 449 'Rzepa,H.S. (2.5) 141 Rogozhin, N.A. (3) 578 Rohn, T. (2.4) 15 Rohner, Y. (2.4) 132 Saak, W. (2.6) 182 Roizard, C. (1) 75 Saalfrank, R.W. (2.7) 76 Rolandi, R. (3) 86 Sachdeva, Y.P. (2.1) 20; Rolinski, 0. (1) 27 (2.2) 72; (2.4) 212; Rolla, P.A. (3) 52, 128 Romani, A. (1) 493 (2.6) 48 Romano, S. (2.3) 56, 57; Sadanani, N.D. (2.6) 206 Sadimenko, L.P. (2.6) 65 (2.6) 22 Rongjiu, R. (3) 382 Sadlo, L. (2.3) 63 Rontani, J.F. (2.5) 100 Sadrai, M. (1) 204 Saeda, S. (2.6) 37 Rorvik, D.A. (1) 55 Saeva, F.D. (2.4) 221 Rosati, R.L. (2.7) 61 Rossi, R.A. (2.4) 80; Safarzadeh-Amiri, A. (1) (2.7) 181, 183, 185 369 Rossitto, F.C. (2.7) 168, Sahali, Y. (2.2) 34, 37 Sahyun, M.R.V. (1) 204 169 Rotella, D.P. (2.2) lo0 Said, H. (2.2) 5 Saidov, D. (3) 623 Roth, H.D. (2.1) 1, 2 Saigo, K. (3) 40, 505 Roth, K . (2.3) 26 Roth, S.C. (3) 337 Saika, T. (2.6) 12 Rotkiewicz, K. (1) 117, Saito, H. (2.5) 48 Saito, I. (2.2) 84; (2.5) 202 Rotman, S.R. (1) 308 99; (2.6) 135; (2.7) Rougee, M. (1) 188, 472 170 Roy, A.K. (1) 330 Saito, M. (2.5) 84, 85 Roy, M. (1) 102 Saito, N. (3) 514 Rozenkevich, M.B. (4) 18, Saito, S. (2.7) 95 Saito, T. (3) 490 19 Rubahn, H.G. (2.4) 19 Saito, Y. (2.5) 132, 134; (4) 16, 17, 20 Rubalcava, H. (1) 76 Rubinov, A. (1) 444 Saitoh, M. (2.1) 30 Rubio, A. (3) 379 Saitoh, Y. (3) 614 Rubio, M.A. (2.5) 176 Sajadian, S.K. (2.7) 91 Rudakoff, G. (3) 129 Sakada, T. (4) 9 Rud'ko, A.R. (3) 258 Sakai, H. (1) 338; (2.3) 102; (2.6) 59 Rudolph, A. (2.2) 60 Sakai, Y. (1) 37 Rudolph, M.J. (2.4) 143 Ruehlmann, K . (3) 580, Sakamoto, H. (2.4) 18; (2.6) 184 596 Rufus, I.B. (4) 50 Sakamoto, M. (2.1) 4; (2.2) 17, 52, 112; Ruggeri, G. (3) 29 Rulliere, C. (1) 21, 208 (2.4) 43, 44; (2.6) Runsink, J. (2.1) 43; 167; (2.7) 163 Sakata, T. (3) 383 (2.2) 8 Ruppmich, M. (2.7) 58 Sakharovskii, Yu.A. (4) Rusanov, A.L. (3) 583 19 Russell, G.A. (3) 195, Sako, M. (2.1) 54; (2.4) 108 196 RUSSO, P.S. (3) 315 Sakuragi, H. (2.3) 13; Rustamov, M.I. (2.5) 89, (2.4) 39, 40; (2.5) 83; (2.6) 2 90; (4) 24 Rusznak, I. (3) 635 Sakuragi, M. ( 1 ) 485;

Author Index (2.6) 29, 30 Sakurai, H. (1) 143; (2.3) 22; (2.6) 179 Sakurai, K. (1) 422 Sakurai, T. (2.1) 24, 25; (2.6) 97 Sala, N. (2.5) 115 Salakhutdinov, N.F. (2.2) 48 Salazar, J.A. (2.5) 194 Salbeck, J. (2.3) 82 Salim, M.S. (3) 498 Salom, C. (3) 380 Salter, L.F. (1) 451; (2.6) 103 Salthammer, T. (1) 43, 115, 414 Saltiel, J. (1) 283, 285; (2.3) 6 Saltvelt, M.E. (2.7) 20 Samadi, M. (2.7) 130 Samanta, A. (1) 87, 235, 408 Samarskaya, T.G. (3) 251 Samat, S. (2.5) 7 Samazhanova, K.B. (2.4) 29 Samuel, C.J. (2.7) 29, 30 Sanchez, A.M. (2.4) 206 Sanchez, K.A.J. (2.3) 2 Sancier, K.M. (2.5) 138 Sander, W. (2.7) 47, 108 Sandros, K. (1) 292; (2.3) 10 Sandstrom, J. (2.6) 102 Sangen, 0. (3) 121, 125 Sangster, D.F. (3) 168 Sankar, I.V. (2.7) 153 Sankararaman, S. (1) 198 Sano, T. (2.2) 10, 104; (2.6) 58 Santamaria, J. (2.7) 171 Santhosh, C. (1) 135 Santi, F. (1) 312 Santiago, A.N. (2.7) 181 Sarathy, T.P. (3) 105, 106 Sargent, J.D. (3) 597 Sargeson, A.M. (4) 15 Sarisky, M. (1) 319 Sarkan, M. (1) 213 Sarma, J.C. (2.1) 62; (2.7) 131 Sarpal, R.S. (1) 230 Sas, W. (2.7) 134 Sasakawa, T. (3) 390 Sasaki, D.Y. (3) 358, 360 Sasaki, T. (3) 404 Sasse, W.H.F. (2.5) 157, 158; (4) 15 Sastre, R. (3) 55, 61, 62, 438, 465, 466, 566,

Author Index 628 Sastri, M.V.C. (4) 40 Sastry, C.V.M. (2.4) 174 Sato, C. (1) 122 Sato, E. (2.2) 15, 16, 90, 93; (2.6) 127, 128, 166 Sato, H. (3) 232, 305 Sato, M. (2.4) 198; (2.6) 68; (3) 294, 295 Sato, S. (2.2) 44; (2.6) 108; (4) 31 Sato, T. (2.3) 117, 118; (2.4) 160; (2.6) 203 Sato, Y. (2.1) 6; (2.4) 117 Satoh, G. (2.7) 225, 227 Satoh, S. (2.7) 223 Satoh, Y. (2.6) 114 Satoru, H. (3) 79 Satyanarayana, P. (2.4) 174 Sauer, M.C. (1) 181 Sauerbrey, R . (3) 520 Sauvage, J.P. (2.5) 9 Savarino, P. (3) 621 Sawada, S. (3) 227, 228 Sawaki, Y. (2.6) 100 Sawanishi, H. (2.7) 77, 95, 96 Sazhnikov, V . A . (2.4) 185; (2.7) 197 Scaffardi, L. (1) 272 Scaiano, J.C. (1) 64, 136, 147, 152, 155, 409, 415, 427, 432, 433, 486; (2.1) 5, 48, 49; (2.4) 227, 228, 234, 243, 245, 246, 247; (2.5) 21, 30; (2.6) 7, 33, 181, 207; (2.7) 48, 143; (3) 17, 58, 202 Scarpati, R . (2.5) 182 Schaad, L.J. (2.3) 110 Schaap, A . P . (2.5) 145; (2.7) 164 Schaffner, K. (1) 474 Schaller, R . (3) 242 Schanze, K . S . (1) 184, 197 Schapp, A . P . (2.5) 168 Scharf, H.-D. (2.1) 43; (2.2) 8 Schauber, C. (3) 246 Scheffer, J.R. (2.1) 27; (2.2) 61; (2.3) 46, 50-52 Scheidhauer, P. (2.3) 70 Schell, F.M. (2.2) 40; (2.6) 110 Schepp, N . P . (2.4) 237

539 Scheuring, M. (2.7) 16 Schiavello, M. (2.5) 156 Schikowsky, V. (2.4) 219; (2.7) 216 Schild, H.G. (3) 325 Schilling, H. (3) 580 Schirrmeister, H. (3) 287 Schissel, P. (3) 499 Schlosser, U. (2.3) 77 Schlyer, B.D. (1) 406 Schmidt, B. (1) 88 Schmidt, E. (1) 204 Schmidt, F. (1) 345 Schmidt, G. (2.5) 122 Schmidt, J. (1) 18, 399 Schmidt, K.M. (1) 181 Schmidt, R. (1) 68, 463, 466, 470, 491; (2.5) 75, 82 Schmiegel, J. (2.6) 15 Schmitt, M. (2.7) 125, 136 Schmutzler, R. (2.4) 190; (2.6) 202 Schnabel, W. (2.6) 208; (3) 100 Schnapp, K.A. (2.2) 134 Schnatterer, A. (2.5) 5 Schneider, H. (3) 541 Schneider, K. (2.7) 55, 56 Schneider, S. (2.3) 20; (2.4) 276; (2.6) 141; (2.7) 17 Schnoll-Bitai, I. (3) 104 Schnorer, H. (1) 306 Schofield, J.M. (2.7) 97 Schomburg, D. (2.4) 190; (2.6) 202 Schott, T . A . (3) 392 Schroeder, J. (1) 294, 416 Schubert, J.E. (2.6) 90 Schuchmann, H.-P. (1) 176; (2.5) 110 Schultz, A.G. (2.2) 96 Schumann, W. (2.6) 204 Schupp, H. (3) 100 Schuster, D.I. (2.2) 45 Schuster, G.B. (2.4) 59; (2.5) 147, 215 Schwab, S.D. (3) 339 Schwarz, C. (2.2) 99 Schwarzer, D. (1) 294 Schwinden, M.D. (2.1) 34 Schwindt, M.A. (2.6) 150 Scopes, D.I.C. (2.7) 97 Scott, D.M. (2.1) 61; (2.7) 132 Scott, G. (3) 442, 564, 604 Scott, R.D. (2.2) 31

Scully, A.D. (3) 570, 575 Searle, N.D. (3) 462 Sears, D.F. (1) 283 ,Sebastiani, G.V. (2.5) 111 Sedlackova, M. (3) 247 Sedlak, P. (2.5) 165 Sedlar, J. (3) 599 Seely, G.R. (2.4) 114; (2.7) 147 Segawa, H. (3) 154 Segawa, T. (2.3) 33, 34 Seguchi, K. (3) 637 Segura, R.G. (2.6) 121 Sehili, T. (2.4) 121, 122 Seidel, M. (2.3) 101 Seiffarth, K. (3) 576 Seikel, K. (1) 470; (2.5) 75 Seitzinger, N.K. (1) 32 Seiyama, A. (1) 239, 240 Seki, H. (1) 189; (2.5) 208 Seki, K. (2.4) 99, 100; (2.7) 200, 204 Seki, T. (2.6) 31; (3) 302, 303 Sekine, T. (4) 35 Seko, T. (2.6) 171 Selafani, A . (2.5) 156 Selli, E. (3) 102 Sellini, E. (3) 10 Sellmer, H. (3) 20 Selski, D. (2.7) 221 Selvidge, L.A. (1) 125 Sengupta, P . K . (1) 213; (3) 90 Seoane, C. (2.3) 75, 76; (2.6) 62 Sepashvili, N.O. (2.6) 36 Sepiol, J. (1) 129 Serebrennikov, Yu.A. (1) 391; (2.5) 28 Serebryakov, E.P. (2.1) 21 Seret, A. (1) 353 Sergeev, A.M. (2.4) 230; (2.6) 76 Sergot, P. (3) 385 Serpone, N . (1) 204; (4) 1 Serwon, S. (3) 152 Sesselmann, T. (1) 279 Sessler, J.L. (2.5) 80 Sethuram, B. (3) 105, 106 Severance, D.L. (2.6) 106 Seves, A. (3) 617 Shaffer, M.W. (2.7) 100 Shafirovich, V.Ya. (4) 6, 22 Shah, B.M. (3) 582 Shah, M. (1) 280

Author Index

540 Shain, A.L. (1) 395 Shakespeare, W.C. (2.3) 80

Shank, C . V . (1) 20, 244 Shanlong, S. (3) 256 Shanmugam, P. (2.4) 208 Share, P. (1) 319 Sharma, A.K. (1) 267 Sharma, D.K. (1) 204; (2.6)

77

Sharma, K . S . (1) 276 Sharnoff, M. (1) 395 Shatalin, I.D. (1) 303 Shaw, D.A. (1) 177 Shea, K . J . (3) 358, 360 Sheketa, M.L. (3) 261 Shelekhov, N . S . (2.4) 244 Shen, C . 4 . (2.3) 91 Shen, G. (3) 238 Shen, J. (3) 89 Shen, S. (1) 187; (2.7) 9, 11

Shen, S.Y. (2.6) 67 Shen, T. (1) 187 Shen, Y. (3) 155 Sheng, X.C. (1) 477 Shenoy, G.R. (3) 626, 627 Shepherd, B . D . (2.6) 185 Shepherd, T. (2.4) 203, 204; (2.7)

40, 41

Sheppard, A.C. (2.6) 69 Shercef, A.M.K. (1) 251 Sheridan, R.S. (2.3) 89; (2.4) (2.7)

20; (2.6) 9, 11

116;

Sherief, A.M.K. (1) 265 Shershukov, V.M. (2.4) 244

Shetlar, M.D. (2.2) 85 Shevchuk, I.P. (2.5) 113 Shevstov, S . A . (2.3) 81 Shi, D . J . (3) 198 Shi, M. (2.3) 32, 100; (2.4) (2.6) 157

102-104, 250-252; 205; (2.7) 156,

Shi, W. (3) 218, 220 Shibata, J. (3) 34 Shibata, T. (2.3) 44 Shibayev, A.Yu. (2.7) 220 Shibuya, J. (2.5) 197 Shibuya, T. (2.6) 198 Shigorin, D.N. (2.5) 16 Shih, K . S . (2.5) 183 Shilov, A.E. (4) 6, 22 Shim, S . C . (1) 300; (2.2) 29, 137, 147; (2.3) 12; (2.4) 31, 32, 144; (2.5) 167, 196 Shima, K. (2.1) 46; (2.4) 55, 56, 106, 271, 272; (2.7) 177

Shimada, K. (2.4) 108; (2.6)

68 Shimada, S. (3) 126 Shimidzu, T. (2.6) 12; (3) 154 Shimizu, K. (4) 34 Shimizu, N. (2.1) 40 Shimizu, Y. (2.5) 130 Shimoda, M. (2.6) 197 Shimoiski, H. (1) 429 Shimokawa, T. (3) 97 Shimozawa, T. (2.6) 13 Shin, E.J. (2.4) 31 Shinar, J . (3) 412 Shindo, H. (2.3) 118 Shindo, Y. (2.6) 129; ( 3 ) 231, 232 Shinka, H. (2.2) 3 Shinn, N . D . (3) 478 Shinoda, S. (2.5) 132, 134; (4) 16, 20 Shinoda, T. (2.7) 94; (4) 36 Shiota, F. (2.6) 198 Shiozaki, H. (3) 634, 636 Shiragami, T. (2.5) 17, 18 Shirai, M. (3) 113, 490 Shirota, Y. (3) 114 Shishkin, L.P. (2.6) 36 Shishkina, R.P. (2.2) 145; (2.6) 93 Shizuka, H. (1) 142, 153, 178; (2.5) 208 Shlyapintokh, V.Ya. (3) 501, 581, 589 Shobatake, K. (1) 100 Shouji, E. (3) 273 Shpilevskaya, L. E. (3) 512 Shrimali, P.S. (2.5) 203 Shtern, A.M. (3) 638 Shul'pin, G . B . (2.5) 92, 152 Shustov, G.V. (2.7) 220 Shuvaeva, N.N. (3) 588 Siampiringue, N. (2.5) 151; (3) 469 Siccardi, G. (3) 29 Sidhu, K . S . (2.5) 61 Sidky, M.M. (2.5) 148; (2.6) 209 Siegel, D.A. (2.6) 186 Siegert, S. (2.7) 105 Siemiarczuk, A. (1) 33, 106, 348 Sienicki, K. (1) 3; (3) 396 Sies, H. (2.5) 78 Siggel, L. (2.4) 10; (2.5) 39 Sigwart, C . (2.1) 44

Silveira da Silva, R. (2.5)

32

Silver, J. (2.1) 41 Simic-Glavaski, B. (3) 260, 309

Simkin, B.Ya. (2.3) 58, 59

Simon, J . D . (1) 65, 169, 257

Simon-Fuentes, A. (2.4) 209; (2.6)

156

Simons, J.P. (1) 132 Simova, G. (2.2) 41 Sindler-Kulyk, M. (2.2) 73; (2.3) 62; (2.4) 34; (2.6) 63 Singer, L . A . (1) 171, 423 Singh, A.K. (1) 102; (2.3) 60 Singh, B.C. (3) 38 Singh, G.P. (1) 276 Singh, J. (1) 306 Singh, M. (2.1) 20; (2.2) 72; (2.4) 212; (2.6) 48 Singh, P. (2.4) 178; (2.7) 195 Singh, R. (2.2) 128 Singh, R.D. (1) 267 Singh, R.P. (3) 586 Singh, S.K. (2.4) 36 Singh, S.P. (2.1) 39 Singh, T. (2.3) 19; (2.4) 233; (2.6) 99 Singh, V. (2.2) 74

Singh Mankotia, A.K. (2.3)

19

Singleton, S.F. (1) 418; (2.4)

201

Sinibaldi, M.-E. (2.4) 182

Sirbiladze, K.Ya. (3) 619 Sisido, M. (1) 109 Sitzmann, E . V . (1) 287, 454, 499

Sivakumar, N . (2.3) 8 Skakovskii, E . D . (2.1) 17 Skapars, J.A. (3) 538 Skelton, B.W. (2.4) 189, 193; (2.5)

175, 184

Sket, B. (2.4) 26, 27, 28 Skiens, W.E. (3) 195, 196 Skubin, V.K. (3) 221 Skuratova, S.I. (1) 497 Sky, A.F. (2.7) 184 Slawin, A . J . (2.5) 141 Slezak, H. (2.7) 38 Slobodin, Ya.M. (2.3) 96 Sloma-Schwok, A. (1) 190 Small, E.W. (1) 34 Small, R.D. (3) 212 Smalley, R.K. (2.7) 97 Smets, G. (3) 310

Author Index Smirnova, N.P. (4) 44 Smit, K.J. (3) 622 Smith, A.B. (1) 289; (2.2) 68 Smith, B.J. (3) 546 Smith, D.M. (3) 499 Smith, P.A.S. (2.4) 145 Smith, T.A. (1) 37 Smotkin, E. (4) 53 Snegov, M.I. (1) 247 Snow, M.S. (2.4) 78 Snyder, D.M. (3) 262 Snyder, G.J. (2.7) 22, 23 Snyder, J.P. (2.6) 168 Snyder, S.W. (1) 38 So, H.Y. (2.4) 134 Sobczynski, A. (2.5) 140; (4) 32 Sobol, R.W. (2.7) 101 Sobry, M.M.F. (1) 123 Sodeau, J.R. (2.5) 210 Soen, T. (3) 270 Sokolik, I.A. (3) 422 Sokolov, O.V. (1) 215 Solaro, R. (3) 301 Solomonenko, G.V. (2.7) 65 Solovskii, M.V. (3) 327 Solvoveichik, O.M. (2.4) 130 Somei, M. (2.4) 53 Somich, C. (2.2) 114; (2.6) 125 Son, Y. (3) 421 Song, H.J. (1) 352 Song, K.H. (3) 70, 491 Sonnenschein, M.F. (1) 144 Soria, J. (2.5) 43 Sotnikova, N.P. (3) 495 Soumillion, J.Ph. (2.3) 9 Soundararajan, N. (2.7) 78, 80 Soutar, I. (3) 391 Southwell-Keely, P.T. (2.7) 149 Sowinska, M. (1) 211 Spangler, C.W. (2.3):53 Spasova, M. (2.7) 72 Specht, K.G. (2.2) 28 Spektor, V.N. (3) 433 Speranza, G. (1) 469 Spitler, M.T. (1) 387 Spoljaric, L. (2.3) 62 Spychala, J. (2.1) 55; (2.7) 115 Squillacote, M. (2.3) 17 Srinivas, P. (3) 105 Srinivasan, K.V. (3) 611 Srinivasan, R. (2.5) 110; (3) 454, 516, 517 Srinivasan, V.R. (2.4)

54 1

176; (2.7) 194 Srivastava, A.K. (1) 58; (3) 117, 145 Staab, E. (2.5) 109 Staab, H. (1) 496; (2.1) 7; (2.7) 139 Stadtmuller, S. (2.1) 37, 38 Staerk, H. (1) 28 Stammer, C.H. (2.7) 20 Standiford, M. (1) 435 Stapp, B. (3) 204 Steck, J. (2.7) 15 Steenken, S. (1) 150; (2.4) 238, 242, 248, 263; (2.5) 170 Steer, R.P. (1) 140, 141; (2.6) 169 Steier, W.H. (1) 246 Stein, A.D. (3) 336 Stein, Z. (2.6) 92 Steinmetz, M. (3) 528 Stel'tsova, Z.O. (3) 222 Stenius, P. (3) 215 Step, E.N. (2.1) 11; (2.6) 187 Stepuro, 1.1. (2.1) 53; (2.5) 149; (2.7) 114 Stern, S.A. (3) 353 Stiegman, A.E. (1) 183 Stierman, T.J. (2.3) 80 Stiplosek, 2. (2.6) 63 Stoddard, G.J. (3) 358, 360 Stoesser, R. (2.7) 43 Stoev, G. (2.2) 41 Stolte, S. (1) 487 Stone, K.J. (2.7) 31 Storch, M. (1) 6, 86 Stowe, R.W. (3) 21 Stradowski, C. (1) 505; (2.5) 56 Stramel, R.D. (3) 328 Strassel, A. (2.5) 151; (3) 469 Strehmel, B. (3) 120 Strel'tsova, 2.0. (3) 217 Strouse, G.F. (1) 313, 334; (3) 402 Struchkov, Yu.T. (2.2) 21 Strumban, E.E. (3) 433 Struban, O.E. (3) 433 Studer, S.L. (1) 229, 435, 436; (2.5) 41 Stunzhas, M.G. (2.4) 185; (2.7) 197 Sturm, B. (1) 56 Sturtevant, J.L. (3) 389, 393 Stylianides, N. (2.6) 168 Stytsenko, T.S. (2.3) 73, 74

Styzinski, M. (3) 439 Suarez, E. (2.5) 194 Suarez, M.L. (2.3) 71 Suau, R. (2.6) 121 Subbaraj, A. (2.7) 106 Suber, L. (2.5) 153 Suberlyak, O.V. (3) 261 Subocz, J. (3) 431 Subramanian, E. (4) 40 Subramanian, R. (2.7) 9 Suda, S. (2.5) 48 Sudesh Kumar, G. (1) 301 Sudnik, A.N. (3) 561 Sueishi, Y. (2.7) 122 Sugahara, Y. (3) 172 Sugawara, T. (2.7) 98 Sugawara, Y. (2.5) 47 Sugi, Y. (2.5) 144; (3) 79 Sugihara, Y. (2.6) 112 Sugimoto, I. (2.4) 231 Sugimoto, R. (3) 411 Sugimoto, S. (2.5) 130 Sugimura, T. (3) 231, 232 Suginome, H. (2.1) 68-70; (2.2) 71; (2.6) 84-86; (2.7) 223-227 Sugita, K. (3) 523 Sugita, M. (3) 513 Sugiura, K. (2.3) 103 Sugiyama, H. (1) 143 Suhadolnik, R.J. (2.7) 101 Sukhovtsky, A. (2.5) 195 Sulpizio, A. (2.4) 3, 259 Sumbetova, J. (3) 480 Sumino, M. (2.2) 103 Sumita, M. (3) 429, 432 Sumitani, M. (1) 299 Sun, J. (1) 188 Sun, M. (2.1) 3 Sun, Y.-P. (1) 283, 285; (2.3) 6 Sunamoto, M. (2.5) 88 Sundahl, M. (1) 292; (2.3) 10 Sundstrom, V. (1) 249, 339 Sunko, D.E. (2.7) 215 Suppan, P. (1) 85, 164, 212, 216; (2.4) 132; (2.5) 37 Supurovic, D. (3) 210, 244 Suresh, R.V. (2.7) 6 Sushkov, D.G. (1) 411 Suslick, K.S. (1) 478 Sustmann, R. (2.3) 101 Suvorov, N.N. (2.3) 67; (2.4) 170; (2.6) 26 Suwaiyan, A. (1) 214 Suzuki, H. (2.4) 150,

542 151, 164, 173; (2.6) 27 Suzuki, I. (1) 111 Suzuki, M. (2.2) 76; (3) 490, 579 Suzuki, N. (2.5) 130 Suzuki, T. (2.1) 29; (2.2) 136; (2.5) 108; (2.6) 11, 160 Suzuki, Y. (3) 303 Suzumoto, T. (1) 381 Svec, F. (3) 359 Sveinson, S.E. (1) 140 Sviridov, B.D. (2.2) 139 Swanson, F.J. (2.5) 57 Swaram, B.M. (1) 203 Swenton, J.S. (2.3) 72 Switzer, F.L. (2.7) 20 Sydnes, L.K. (2.5) 161 Sykora, J. (2.5) 14 Syromyatnikov, V.G. (2.4) 211 Szabo, A.G. (1) 2, 90, 97 Szklarczyk, M. (4) 8 Szneler, E. (1) 200 Tabata, Y. (3) 34, 384 Taborov, S.Kh. (3) 495 Tabuchi, K. (2.2) 133; (3) 81, 126 Tachiya, M. (1) 162 Tachizawa, 0. (2.7) 104 Tada, F. (2.6) 66 Tada, H. (3) 614 Tagaki, S. (3) 79 Tagawa, S. (3) 384 Tagaya, H. (2.4) 105; (2.5) 48 Tagoshi, H. (2.2) 1 Tahara, T. (1) 425 Tai, A. (2.3) 16 Tai, Y. (1) 113 Tailhan, C. (2.7) 152 Takacs, M. (2.2) 13, 65, 66; (2.6) 98 Takagi, K. (2.6) 100 Takagi, R. (2.4) 119; (2.5) 166 Takagishi, T. (3) 326, 351 Takahashi, A. (3) 172 Takahashi, H. (2.3) 92, 94; (2.4) 198 Takahashi, K. (2.5) 213 Takahashi, M. (2.7) 98 Takahashi, N.S. (4) 55 Takahashi, R. (4) 35 Takahashi, S. (2.5) 52 Takahashi, T. (2.2) 92; (2.6) 20 Takahashi, Y. (2.2) 136 Takamatsu, H. (3) 494

Author index Takami, K. (3) 388 Takamichi, N . (1) 456 Takamuku, S. (2.3) 3, 32, 100; (2.4) 102-104, 250-252, 255; (2.6) 74, 205; (2.7) 156, 157 Takao, H. (3) 257 Takase, I. (3) 118 Takashima, M. (2.5) 33 Takayama, M. (2.5) 99; (2.7) 170 Takayanagi, H. (2.2) 104 Takechi, H. (2.2) 115, 116; (2.4) 52, 54; (2.6) 122, 163 Takeda, K. (2.7) 1; (3) 415 Takeda, Y. (2.6) 2 Takeishi, M. (3) 82 Takematsu, A. (1) 189 Takematsu, M. (3) 263 Takemura, F. (3) 139 Takenaka, A. (1) 320 Takeshita, H. (2.2) 1-3; (2.3) 42, 43; (2.6) 53, 54 Takeuchi, H. (2.7) 104 Takeuchi, K. (2.5) 144; (3) 79 Takeuchi, M. (2.3) 94; (2.4) 198 Takeuchi, S. (3) 352 Takeuchi, Y. (2.6) 197 Taki, M. (2.4) 69 Taki, T. (2.3) 22; (2.6) 179 Takimoto, T. (2.6) 14 Takimoto, Y. (3) 34, 45, 76, 77 Takizawa, A. (3) 294, 295 Takuwa, A. (2.2) 120; (2.4) 49; (2.6) 118 Talapatra, G.B. (1) 315 Taljaard, B. (2.7) 99 Tam, A.C. (3) 521 Tamai, N. (1) 233, 314, 340 Tamai, T. (2.5) 178 Tamaki, T. (1) 485; (2.6) 29, 30, 35; (3) 303 Tamaoki, N. (2.6) 16 Tamir, I. (3) 274 Tammilehto, S. (2.4) 128 Tamura, K. (2.5) 34 Tamura, M. (2.4) 203, 204; (2.7) 40, 41 Tan, K.L. (3) 545 Tanabe, G. (2.2) 55 Tanabe, K. (2.4) 272 Tanaka, F. (1) 100, 467; (2.5) 74 Tanaka, H. (3) 357

Tanaka, K. (2.2) 10; (2.6) 58, 66, 92; (3) 523 Tanaka, M. (2.2) 6; (2.5) 10; (3) 70, 113, 119, 490, 491; (4) 5 Tanaka, N. (3) 317 Tanaka, T. (2.4) 60, 165, 168, 169; (2.5) 36, 59, 66, 67, 162, 199; (2.7) 119; (3) 308 Tanaka, Y. (2.2) 123; (2.4) 51 Tang, Q. (2.7) 186 Tang, Y. (2.5) 72 Tani, T. (1) 379, 381 Tanigaki, K. (2.7) 63 Tanigaki, T. (3) 350 Taniguchi, E. (2.4) 48; (2.6) 120 Taniguchi, H. (2.3) 108, 109; (2.4) 83-87; (2.6) 74; (2.7) 212-214 Taniguchi, T. (2.4) 167 Tanii, K. (2.5) 162 Tanimoto, Y. (2.5) 33 Tantrigoda, R.U. (2.5) 179 Tao, G. (3) 82 Taplick, T. (2.7) 43 Tarascon, R.G. (3) 486 Tarasov, V.F. (2.1) 11; (2.6) 187 Tarasov, Yu.1. (3) 615 Tardella, P.A. (2.7) 102, 103 Tarroni, R. (1) 92 Tasumi, M. (1) 119, 425 Tatarova, L.E. (2.2) 48 Tauer, E. (1) 231; (2.6) 176 Taylor, G.N. (3) 486 Taylor, V.L. (2.2) 63 Tazuke, S. (1) 61, 209, 320, 336; (3) 299, 300, 316, 346, 349, 375, 390, 410 Tedesco, A.C. (2.4) 71 Teixido, M. (2.4) 70 Tejedor, F. (2.7) 87 Tennakone, K. (2.5) 63, 179 Tenu, J.P. (2.7) 84 Teragaki, J. (3) 121 Terai, M. (2.5) 34 Terakawa, K. (2.4) 39, 40 Tera-Kubota, S. (1) 429 Teranishi, H. (1) 467; (2.5) 74 Teraoka, R. (2.4) 231 Terashima, M. (2.4) 99, 100; (2.7) 200, 204

543

Author Index Terazima, M. (1) 63, 392; (3) 415 Tercero, J.M. (3) 61 Tero-Kubota, S. (1) 420, 460 Teulade, J.4. (2.7) 92, 182 Teyssie, P. (3) 333 Tfibel, F. (1) 482 Thames, S.F. (3) 207, 536, 558 Thampi, K.R. (2.5) 86, 87 Thapliyal, B.P. (3) 586 Theodore, L.J. (2.7) 71 Thilegan, A. (1) 306 Thistlethwaite, P.J. (1) 325 Thomas, J.B. (2.2) 36; (2.4) 33 Thomas, J.J. (4) 51 Thomas, J.K. (1) 332, 375, 380, 468; (3) 320, 321, 361 Thomas, P.J. (2.1) 34 Thomas, R.C. (1) 451 Thommen, F. (3) 436 Thompson, A.N. (4) 28 Thompson, L.F. (3) 486 Thompson, M. (1) 369 Thompson, P.A. (1) 257 Thorpe, M.X. (3) 278 Thulstrup, E.W. (1) 117 Thurner, J.U. (2.7) 43 Tian, G.R. (2.3) 42, 43; (2.6) 53, 54 Tiefenthaler, A.M. (3) 4 Tikhomirov, S.A. (2.6) 154 Tillekeratne, L.M.K. (3) 604 Tillyaev, A.D. (3) 165 Timpe, H.J. (1) 424, 507; (2.1) 64; (2.4) 219; (2.5) 38, 64; (2.6) 60; (2.7) 216; (3) 25, 30, 49, 122 Tinant, B. (2.1) 45; (2.3) 9 Tipton, D. (3) 315 Tirrell, D.A. (3) 325 Tishin, B.A. (2.5) 187 Titov, V.I. (3) 615 Titz, M. (2.3) 63 Toba, S. (2.4) 255 Tobe, Y. (2.2) 12, 18; (2.4) 46 Tobin, J.B. (2.6) 190 Toblas, F.M.G. (1) 419 Tocho, J.O. (1) 271, 272 Tochtermann, W. (2.3) 77 Toda, F. (2.1) 28; (2.2) 101; (2.6) 92

Toennies, J.P. (2.4) 19 Toffoletti, A. (1) 316, 413 Togawa, S. (2.2) 120; (2.4) 49; (2.6) 118 Tokitoh, N. (2.6) 160-162; (2.7) 146 Tokmadzhyan, G.G. (2.6) 70 Tokumaru, K. (1) 146, 299, 456; (2.3) 4, 13, 14, 24; (2.4) 39, 40; (2.5) 83; (2.6) 2 Tokunaga, N. (2.5) 204 Tolentaev, B.N. (1) 462; (2.5) 73 Tolstikov, G.A. (2.5) 125, 187, 220 Tomalia, D.A. (3) 330 Tomas, V. (2.5) 51 Tomaschewski, G. (2.7) 37, 38, 43 Tombari, E. (3) 128 Tominaga, H. (2.5) 84, 85 Tominaga, K. (1) 396 Tomioka, H. (2.5) 68; (2.7) 50, 51, 62 Tomioka, K. (2.2) 6 Tomioka, Y. (3) 317 Tomita, I. (2.3) 117 Tomoda, A. (2.4) 150, 151, 164, 166, 173; (2.6) 27 Tomoda, S. (2.6) 197 Tomova, E. (3) 594 Tomura, Y. (3) 276 Tong, W. (2.1) 3 Tong, Z. (3) 488 Tonogai, K.H. (3) 119 Tonogai, S. (3) 70 Tonokura, K. (1) 153 Tonokura, T. (1) 142 Topchiev, D.A. (3) 84 Topgi, R.S. (2.2) 98 Toporowicz, M. (1) 447 Toriaki, A. (3) 461, 463 Toriyama, K. (1) 113 Torkelson, J.M. (3) 293 Tormos, R. (2.3) 114 Torski, Z. (2.5) 193 Toru, T. (2.6) 171 Toscano, V.G. (1) 152; (2.4) 243; (2.6) 207 Touillaux, R. (2.3) 9 Townshend, A. (1) 476 Toyota, K. (2.6) 200 Trainor, G.L. (1) 82 Tran, C.Q. (3) 270 Tran-Cong, Q. (3) 408 Trapp, M.A. (3) 66, 94, 95 Trautsch, W. (3) 129

Trehar, A. (2.3) 54 Trevisan, L. (3) 102 Trifonov, L. (2.3) 48, 49 Tripathi, H.P. (1) 137, 192, 193 Tripathi, V.C. (1) 137 Tro, N.J. (1) 376 Troe, J. (1) 294, 416 Troin, Y. (2.4) 181, 182 Troisi, L. (2.5) 145 Trojan, M. (3) 460 Tromsdorf, H.P. (1) 252 Trotter, J. (2.1) 26, 27; (2.2) 61; (2.3) 50, 52 Tsai, E.W. (3) 345 Tsai, F.J. (3) 293 Tseng, J.C.-C. (1) 171 Tsubomura, H. (4) 47, 48 Tsuchida, A. (3) 378 Tsuchida, E. (3) 273 Tsuchiya, M. (2.5) 83 Tsuchiya, N. (4) 55 Tsuchiya, T. (2.3) 102; (2.6) 59; (2.7) 77, 95, 96 Tsuda, H. (3) 210, 244 Tsuda, M. (2.7) 53; (3) 45 Tsuda, Y. (2.2) 10, 104; (2.6) 58 Tsuji, T. (2.2) 59, 76; (2.3) 31, 103 Tsuji, Y. (3) 378 Tsujimori, H. (2.2) 55 Tsujimoto, K. (2.4) 9 Tsujita, Y. (3) 294, 295 Tsukahara, K. (2.5) 52 Tsukamoto, A. (1) 54 Tsukamoto, R.M. (3) 560 Tsumo, Y. (2.1) 40 Tsumuraya, T. (2.7) 110 Tsunooka, M. (3) 70, 113, 119, 441, 490, 491 Tsurumi, K. (3) 423 Tsuzuki, K. (2.4) 111 Tu, X.M. (1) 46 Tudos, F. (3) 571, 591 Tuncay, A. (2.2) 126 Tung, C. (3) 488 Tung, C.-H. (1) 98 Turbeville, W. (1) 481 Turck, C. (3) 60 Turner, A.B. (2.4) 189; (2.5) 175 Turner, M.V. (2.3) 30 Turner, S.R. (3) 9 Turro, N.J. (1) 80, 287, 453, 496; (2.1) 6, 7, 9, 10, 18, 41; (2.3) 95, 98; (2.4) 110, 200, 225, 226; (2.7) 49; (3) 330

Author index

544

Tychinskaya, L.Yu. (2.1) 17 Tylli, H. (3) 506, 507 Tyndall, G.S. (2.5) 147 Uchida, K. (2.4) 152 Uchida, M. (3) 409 Uchida, T. (1) 61; (3) 227, 228 Uchihara, T. (4) 47, 48 Uchino, N. (3) 214 Udagawa, T. (2.3) 65 Ue, M. (2.2) 12, 18; (2.4) 46 Ueda, E. (2.3) 94 Uehata, A. (1) 359 Uekama, K. (2.4) 126 Ueng, Y.4. (2.2) 108 Ueno, A. (1) 111; (3) 292 Ueno, N. (3) 523 Ueno, Y. (2.6) 171 Ukegawa, K. (2.2) 133 Ukhin, L.Yu. (2.2) 97 Ulvi, V. (2.4) 128 Umasundari, P. (2.4) 174 Umehara, A. (3) 45 Upadhyaya, A.K. (2.1) 15 Urban, B. (2.1) 23 Urquhart, R.S. (1) 325 Ursini, M. (2.4) 77; (2.6) 145 Usacheva, M.N. (3) 73 Us&, H. (2.6) 100 Usanmaz, A. (3) 163 Ushakova, V.N. (3) 327 Ushida, K. (1) 422 Ushida, S. (3) 637 Usui, M. (2.4) 197; (2.6) 129, 130 usui, s. (1) 359 Usui, Y. (2.5) 49 Utschick, G. (2.5) 5 Vachl, V. (3) 27 Valdes-Aguilera, 0. (3) 72, 341 Valeur, B. (1) 51, 312 Vancik, H. (2.7) 215 Vandendriessche, J. (3) 386 Van der Auweraer, M.A. (1) 251 van de Voorst, A. (1) 260, 353 van Eijk, A.M.J. (1) 428 van Elburg, P.A. (2.7) 133 Vanelle, P. (2.4) 82; (2.6) 146; (2.7) 182, 187

Van Halbeek, H. (2.7) 20 van Herpen, W.M. (1) 284 Van Hoek, A. (1) 128 Van Meerssche, M. (2.1) 45 Van Pham, C.C. (3) 386 Varani, G. (2.5) 65; (3) 147 Varma, C.A.G.O. (1) 185, 234, 428; (2.5) 42 Vartanyan, S.A. (2.1) 21 Vasishtha, R. (3) 145 Vass, F. (3) 592 Vauthey, E. (1) 164; (2.5) 37 Vedachalam, M. (2.7) 74 Vedejs, E. (2.6) 173 Vedeneev, A.A. (3) 93 Veglia, A.V. (2.4) 206 Vehara, M. (2.5) 33 Veissier, V. (3) 405 Vencl, J. (2.3) 63 Venkataraman, K. (1) 503; (3) 230 Venkateswaran, R.V. (2.2) 33; (2.7) 59 Verbeeck, A. (1) 358 Verdu, J. (3) 464, 465 Verhoeven, J.W. (1) 167, 222, 322 Veselova, T.V. (2.4) 244 Vesely, M. (2.5) 12; (4) 4 Vever-Bizet, C. (1) 472 Viader, J. (2.2) 5 Vicente, M. (4) 10 Vichutinskaya, E.V. (3) 501 Vig, A. (3) 635 Vigny, P. (2.2) 26 Vijayalakshmi, C.S. (2.4) 208 Vikic-Topic, D. (2.6) 63 Viktorova, E.N. (1) 247 Villa, E. (2.4) 135 ,Vinkovic,V. (2.7) 60 Viola, R.W. (2.6) 79 Viols, H. (2.7) 92 Viovy, J.L. (3) 405 Viriot, M.L. (1) 73, 74 Vismara, E. (2.4) 116; (2.7) 117 Visnick, M. (2.2) 68 Visser, A.J.W.G. (1) 128 Viswanathan, B. (4) 50 Vittimberga, B.M. (2.4) 77; (2.6) 145 Vivona, N. (2.4) 12, 13; (2.6) 42, 57 Vlasova, N.N. (2.6) 193 Vogel, E. (1) 447 Vogl, 0. (3) 575

Vogler, B. (2.2) 40; (2.6) 110 Voisturiez, L. (2.2) 26 Voit, B. (3) 91, 132 Volbushko, N.V. (2.3) 68; (2.4) 171; (2.6) 24, 65 Volker, S. (1) 8 Volksen, W. (3) 519 Volnina, E.A. (3) 247 Voloshin, N.A. (2.6) 65 von Bunau, G. (1) 345, 354 von Geldern, T.W. (2.4) 35 von Schnering, H.G. (2.2) 135; (2.3) 77 von Sonntag, C. (1) 176; (2.5) 110 von Wandruzka, R. (1) 327, 343 Voortmans, G. (1) 358; (3) 136 Vorob'ev, A.V. (2.3) 81 Voronkov, M.G. (2.6) 193 Vos, F. (1) 294 Voyksner, R.D. (3) 620 Vurm, K. (3) 440 Vyprachticky, D. (3) 599 Vyskocil, P. (3) 376 Wada, T. (2.7) 85 Wada, Y. (2.3) 92 Wade, W.H. (1) 191 Wadle, A. (3) 397 Waeche, M. (3) 287 Wagner, B.D. (1) 33, 309 Wagner, H.N. (2.7) 86 Wagner, P.J. (2.1) 4, 33; (2.2) 17; (2.4) 41-44; (2.5) 24 Wakasa, M. (2.6) 177, 195 Waldeck, D.H. (1) 288, 290; (2.3) 8; (3) 373 Waldo, R.A. (3) 489 Walia, S. (2.3) 29 Walker, J. (2.4) 137 Walkington, A.J. (2.7) 126, 128 Waller, A. (1) 283 Walls, D. (2.5) 158 Walter, A. (1) 132 Waluk, J. (1) 117, 207 Wamhoff, H. (2.5) 148; (2.6) 209 Wan, P. (2.3) 119, 120; (2.4) 239, 254, 256, 257; (2.6) 147; (2.7) 113, 165 Wand, J.T. (2.3) 78 Wang, A.L. (1) 42 Wang, C.M. (2.4) 260;

Author Index (2.5) 212 Wang, D. (2.5) 81, 119, 120, 121, 171 Wang, E. (3) 138 Wang, F.W. (3) 337, 340 Wang, G. (3) 256, 285 Wang, H. (2.2) 84; (2.4) 96; (2.6) 135 Wang, J.B. (2.7) 225, 226, 227 Wang, R. (2.2) 20, 84; (2.6) 135 Wang, S. (2.3) 72; (3) 255, 314 Wang, W. (2.5) 72 Wang, X. (2.6) 64 Wang, Y. (2.5) 146; (2.6) 170; (3) 173 Wang, Z. (2.5) 135; (4) 46 Wannamaker, M.W. (2.7) 32 Ward, T.C. (3) 197 Ware, B.R. (3) 353 Ware, W.R. (1) 33, 106, 309, 348 Warman, J.M. (1) 222 Warncke, W. (2.7) 43 Warner, I.M. (1) 14, 94 Washio, M. (3) 384 Watanabe, J. (3) 290 Watanabe, N. (2.2) 133 Watanabe, S. (2.2) 112; (2.6) 153, 167; (3) 504 Watanabe, Y. (2.6) 173 Waterman, K.J. (2.4) 175; (2.7) 193 Watson, A.C. (2.4) 145 Watt, D.R. (1) 264 Watt, D.S. (2.7) 14 Wayne, W.L. (3) 288 Wayner, D.D.M. (1) 64 Weaver, M.J. (1) 163 Webb, K.K. (3) 50 Webb, M. (2.4) 195 Webber, S.E. (1) 311; (3) 328, 363, 368, 389, 403 Webber, W.P. (3) 551-553 Weber, G. (1) 1 Weber, H. (2.4) 15 Weber, S. (1) 354 Weber, W.P. (2.4) 224; (2.6) 180 Webster, G.R.B. (2.4) 125 Weedon, A.C. (2.2) 60; (2.4) 7, 199 Wegewijs, B. (1) 222 Wei, Y. (3) 248 Weidenbruch, M. (2.6) 182 Weidner-Wells, M.A. (2.2) 117; (2.6) 126 Weiland, R. (1) 168 Weiler, J. (2.3) 9

545

Weiller, B.H. (1) 67 Weinberg, P. (2.7) 36, 38 Weir, D. (1) 494 Weir, N.A. (3) 481, 482, 537, 574 Weisenborn, P.C.M. (1) 185 Weisman, R.B. (2.7) 3 Weiss, A.M. (3) 343 Weitemeyer, C. (3) 204 Welch, W.F. (3) 499 Welge, K.H. (3) 526 Weller, A. (1) 28 Welsh, K.M. (2.7) 109 Wen, T. (1) 200 Wen, X. (2.5) 209 Wender, P.A. (2.4) 10, 35, 36 Wennerstroem, 0. (1) 292; (2.3) 10, 18 Wenzel, T.J. (1) 373 Werner, T.C. (1) 124 West, R. (2.6) 185, 186; (2.7) 109 Wewers, D. (3) 204 Whitaker, C. (2.4) 90 White, A.H. (2.1) 65; (2.4) 189, 193; (2.5) 175, 184; (2.7) 198 White, B. (1) 335; (2.5) 177; (3) 347, 348 White, N.J. (3) 14, 496, 497 White, R.C. (2.1) 58 White, W. (2.7) 5 Whiting, D.A. (2.4) 266, 267; (2.7) 126, 128, 160-162 Whiting, K. (3) 481, 482 Whitnep, L.C. (3) 587 Whitten, D.G. (1) 89; (2.4) 6, 58; (2.5) 32, 188-190 Wibowo, T.S. (3) 98 Wickremanayaka, S. (2.5) 179 Widmaeir, J.M. (3) 306 Wiechmann, M. (1) 232 Wiersma, D.A. (1) 245, 277 Wietfeld, B. (2.2) 5 Wight, C.A. (3) 109 Wikander, G. (1) 344 Wilbrandt, R. (1) 400; (2.2) 49 Wilcox, M. (2.6) 79 Wild, U.P. (1) 93, 502; (2.4) 141 Wiles, D.M. (3) 610 Wilkinson, F. (1) 442 Will, B. (2.5) 109 Willard, F.K. (3) 194

Williams, B.D. (2.7) 89 Williams, D.J. (2.5) 141 Williams, F. (2.3) 78 Williams, G.R.J. (3) 420 Willner, I. (1) 384 Willsher, C. (3) 625 Wilson, A.A. (2.7) 86 Wilson, G.J. (3) 210 Wilson, R.M. (2.2) 134; (2.5) 22 Winitz, S. (2.4) 90 Winkler, J.D. (2.2) 30, 31, 39; (2.6) 111 Winnik, F.M. (3) 362 Winnik, M.A. (3) 322, 395 Wintgens, V. (1) 432, 504; (2.1) 48, 49; (2.4) 234 Wireko, F. (2.1) 27; (2.3) 52 Wirth, M.J. (1) 12, 17, 347 Wirth, U. (2.7) 76 Wirz, J. (1) 77, 457; (2.1) 67; (2.7) 19 Wismontski-Knittel, T. (1) 96 Witmer, M.R. (2.7) 176 Witte, P. (1) 114 Wojciechowski, P. (3) 239 Wolff, T. (1) 345, 354 Wolszczak, M. (1) 505; (2.5) 56 Wong, G.S.K. (2.4) 199 Wong, J.-F. (1) 453 Wong, S.S. (1) 166 Wong, W.K. (3) 100 Woning, J. (1) 487; (2.4) 253 Wood, J.L. (2.2) 68 Worl, A.L. (1) 334; (3) 402 Wu, G. (3) 282 Wu, J. (3) 174 WU, M.-J. (2.2) 100 Wu, Q. (2.3) 84 Wu, S. (3) 603 Wu, W.X. (2.6) 9; (2.7) 2 Wu, X. (3) 187 Wu, Y. (2.5) 154 Wu, 2. (3) 382 WU, Z.4. (1) 321; (2.1) 66; (2.5) 20 Wubbels, G.G. (2.4) 88-90; (2.6) 88 Wudl, F. (3) 412 Wurm, H. (3) 98 Wynne, P.M. (2.5) 205 Xenakis, A. (1) 341 Xi, C. (2.5) 136; (4) 49

Author Index

546

Xiao, J. (3) 33 Xiao, X. (2.5) 81, 119-121, 171 Xie, M. (2.6) 64 Xie, X. (1) 65 XU, C.-B. (1) 98 Xu, G. (2.5) 50 Xu, G.Z. (2.6) 67 Xu, H. (3) 395 Xu, H.J. (2.6) 67 Xu, J. (2.5) 154; (3) 267 Xu, P. (3) 314, 473, 474 Xu, S.Q. (3) 86, 137 Xu, X. (2.4) 254; (11) 113 Xu, Z. (2.5) 211 Yabe, A. (2.4) 249; (2.6) 161; (2.7) 146; (3) 531 Yafune, T. (2.4) 139; (2.6) 40 Yagci, Y. (3) 554 Yagi, M. (1) 438; (2.6) 92 Yague, E. (2.5) 51 Yakovlev, V.B. (3) 221, 598 Yakovlev, V.N. (2.5) 220 Yamada, K. (2.7) 21, 28, 28 Yamada, M. (2.6) 41 Yamada, N. (1) 175 Yaamda, S. (2.4) 9, 74, 75; (2.6) 136; (2.7) 225, 226 Yamada, T. (1) 423; (2.3) 16; (2.4) 124 Yamada, Y. (2.5) 108 Yamagata, S. (2.5) 137-139; (3) 153 Yamaguchi, H. (2.3) 42; (2.6) 53 Yamaguchi, K. (3) 114, 568 Yamaguchi, M. (2.6) 112 Yamaguchi, S. (2.6) 41 Yamamoto, A. (4) 47 Yamamoto, H. (2.6) 13, 14; (3) 298 Yamamoto, K. (2.2) 58; (2.4) 180; (2.6) 74 Yamamoto, M. (2.7) 21, 28; (3) 378, 388, 404, 408, 409 Yamamoto, S. (2.4) 167; (2.7) 122 Yamamoto, T. (2.3) 93; (3) 121, 125, 127 Yamamoto, Y. (1) 194; (2.2) 71; (2.7) 224; (3) 140, 141, 245, 272

Yamanaka, H. (2.4) 127 Yamane, K. (2.2) 120; (2.4) 49; (2.6) 118 Yamaoka, A. (3) 513 Yamaoka, T. (2.6) 16; (3) 45, 48, 71, 214 Yamasaki, M. (2.4) 126 Yamasaki, N. (2.3) 16 Yamashina, N. (2.6) 37 Yamashita, H. (3) 249 Yamashita, S. (1) 90 Yamashita, T. (2.4) 55, 56, 106 Yamauchi, S. (1) 396, 403; (2.1) 8 Yamawaki, Y. (3) 211 Yamazaki, I. (1) 314, 340 Yamazaki, T. (1) 340; (2.2) 92; (2.6) 20 Yamosaki, H. (1) 374 Yan, W. (3) 173 Yanagida, S. (2.3) 104; (2.5) 17, 18; (2.7) 175; (3) 312; (4) 21 Yanase, T. (2.2) 112; (2.6) 167 Yandell, M.D. (3) 575 Yang, D. (2.5) 102 Yang, D.D.H. (2.4) 129 Yang, F. (3) 174 Yang, H. (2.3) 85 Yang, K. (2.7) 189 Yang, N.C. (2.4) 129 Yang, P. (3) 173 Yang, Y. (2.1) 3; (2.5) 50; (3) 488, 603 Yankelevich, A.Z. (2.5) 28, 164 Yano, 0. (3) 270 Yao, X. (2.2) 20; (2.4) 96 Yap, M. (2.3) 46 Yapo, A. (2.7) 84 Yarosh, O.G. (2.6) 193 Yartsev, A.I. (4) 44 Yastrebinskii, A.A. (3) 495 Yasuda, M. (2.1) 46; (2.4) 55, 56, 106, 271, 272; (2.7) 177 Yeh, C.Y. (1) 11, 19 Yen, H.R. (3) 555 Yeung, A.S. (3) 324 Yeung, E.S. (1) 66 Yip, R.W. (2.6) 77 Yitzchaik, S. (3) 307 Ylitao, D.A. (3) 324 Yoda, S. (3) 125 Yokelson, H.B. (2.6) 186 Yokoyama, M. (2.6) 11 Yokoyama, Y. (2.4) 162, 163

Yoneda, I. (2.6) 195 Yonemitsu, 0. (2.3) 116; (2.4) 269, 274; (2.7) 155 Yonemura, M. (4) 35 Yoneyama, M. (3) 226 Yonezawa, H. (2.7) 85 Yonezawa, N. (3) 505 Yoo, D.Y. (2.2) 132, 137 Yoo, U.C. (2.2) 113; (2.6) 123 York, C. (1) 281 Yoshida, E. (2.2) 118; (2.4) 69; (2.6) 174 Yoshida, H. (2.6) 37 Yoshida, N. (1) 239, 240 Yoshida, T.M. (1) 31 Yoshifuji, M. (2.4) 160; (2.6) 200, 203 Yoshihara, K. (1) 233, 299, 378, 379, 382 Yoshimori, A. (1) 165 Yoshimura, A. (2.5) 201 Yoshino, K. (3) 411 Yoshioka, M. (2.1) 29-31; (2.2) 110, 111; (2.5) 23; (2.6) 96 Yoshizawa, H. (3) 408 Youda, A. (3) 290 Younathan, J.N. (1) 334; (3) 402 Young, E. (2.7) 125, 176 Young, G. (3) 535 Yu, C. (3) 155 Yu, J. (2.5) 102 Yu, J.-A. (1) 105 Yu, S. (2.5) 127; (3) 248, 267 Yu, T. (2.1) 3; (3) 335 Yu, W. (3) 238 Yu, X. (3) 238 Yu, Y. (2.5) 72 Yuan, H.L. (3) 346 Yuan, Z. (1) 187 Yudina, V.I. (2.7) 65 Yufit, D.A. (2.2) 21 Yymyanski, Y.R. (2.4) 138 Zabik, M.J. (1) 450 Zachariasse, K.A. (1) 107, 108 Zadok, E. (3) 274 Zaichenko, N.L. (2.6) 34 Zaidi, N.A. (2.7) 167 Zakharchuk, A.V. (3) 495, 623 Zakharenko, V.S. (4) 44 Zakharieva, R. (2.7) 72 Zakirov, K.K. (3) 165 Zakrzewski, J. (2.5) 91 Zamaraev, K.I. (4) 2

Author Index Zamotaev, A.A. (3) 217 Zamotaev, P.V. (3) 217, 222, 453, 459 h a , R. (1) 355 Zang, B. (2.5) 160 Zangler, F. (1) 56 Zannoni, C. (1) 92 Zapf, F. (3) 204 Zaporozhets, O.A. (2.5) 207 Zard, S.Z. (2.7) 131, 135, 137, 151, 152 Zavodnik, I.B. (2.1) 53; (2.5) 149; (2.7) 114 Zayed, A.H. (2.6) 38 Zayed, M.F. (2.2) 138 Zefirov, N.S. (2.3) 23 Zegers, C.P.G. (3) 208 Zehavi, U. (2.7) 174 Zelenay, P. (4) 8 Zelent, B. (1) 337 Zeng, D. (2.5) 114 Zeng, H. (2.3) 13 Zerbetto, F. (1) 97 Zhang, B. (2.5) 154, 159, 163 Zhang, J.J. (2.5) 215

547

Zhang, L. (2.5) 81; (2.6) 194; (3) 382 Zhang, S. (2.6) 181 Zhang, T. (2.5) 133 Zhang, Z. (2.1) 10; (2.4) 226 Zhao, C.L. (3) 322 Zhao, D. (3) 473, 474 Zhao, D . 4 . (2.7) 218, 219 Zhao, Y. (3) 331, 332 Zhao, Z. (3) 234 Zhen, J. (3) 346 Zheng, X. (2.5) 136; (4) 49 Zheng, Y. (3) 89 Zhila, G.Yu. (2.6) 193 Zhivilkova, O.A. (3) 588 Zhonggan, Z. (3) 130 Zhou, B. (2.1) 33; (2.5) 24 Zhou, D. (3) 473, 474 Zhou, J. (2.5) 50 Zlnou, Q. (1) 187 Zhou, S. (3) 353 Zhou, Y. (1) 187 Zhu, D. (3) 255

Zhu, X.R. (1) 273 Zhuang, Q. (2.5) 135; (4) 46 Zhubanov, B.A. (2.4) 29 Zhuravlev, M.A. (3) 471 Zielinski, S. (2.5) 140 Zieluski, S. (4) 32 Zimerman, H.E. (2.2) 62, 94, 95; (2.3) 35-37; (3) 470 Zimerman, J.B. (3) 192, 193 Zimnermann, G. (2.2) 99 Zimermann, H. (2.7) 76 Zimnt, M.B. (1) 201 Zink, J.I. (1) 479, 483 Zinukov, S.V. (1) 462; (2.5) 73 Zope, U.R. (2.2) 131 Zschokke-Granacher, I. (1) 390 Zubarev, V.E. (2.5) 181 Zuecker, I. (3) 122 Zuk, A. (3) 157 Zupan, M. (2.4) 26-28 Zupancic, N. (2.4) 26, 27 Zuraw, M.J. (2.3) 35, 36