Organic Photochromic and Thermochromic Compounds: Volume 2: Physicochemical Studies, Biological Applications, and Thermochromism (Topics in Applied Chemistry)

Organic Photochromic and Thermochromic Compounds Volume 2: Physicochemical Studies, Biological Applications, and Thermochromism

TOPICS IN APPLIED CHEMISTRY Series Editors: Alan R. Katritzky, FRS University of Florida Gainesville, Florida Gebran J. Sabongi 3M Company St. Paul, Minnesota

Current volumes in the series: ANALYSIS AND DEFORMULATION OF POLYMERIC MATERIALS Paints, Plastics, Adhesives, and Inks Jan W. Gooch CHEMISTRY AND APPLICATIONS OF LEUCO DYES Edited by Ramaiah Muthyala FROM CHEMICAL TOPOLOGY TO THREE-DIMENSIONAL GEOMETRY Edited by Alexandru T. Balaban LEAD-BASED PAINT HANDBOOK Jan W. Gooch ORGANIC PHOTOCHROMIC AND THERMOCHROMIC COMPOUNDS Volume 1: Main Photochromic Families Volume 2: Physicochemical Studies, Biological Applications, and Thermochromism Edited by John C. Crano and Robert J. Guglielmetti ORGANOFLUORINE CHEMISTRY Principles and Commercial Applications Edited by R. E. Banks, B. E. Smart, and J. C. Tatlow PHOSPHATE FIBERS Edward J. Griffith POLY(ETHYLENE GLYCOL) CHEMISTRY Biotechnical and Biomedical Applications Edited by J. Milton Harris RESORCINOL Its Uses and Derivatives Hans Dressler A Continuation Order Plan is available for this series. A continuation order will bring delivery of each new volume immediately upon publication. Volumes are billed only upon actual shipment. For further information please contact the publisher.

Organic Photochromic and Thermochromic Compounds Volume 2: Physicochemical Studies, Biological Applications, and Thermochromism

Edited by

John C. Crano Late of PPG Industries, Inc. Monroeville, Pennsylvania

and

Robert J. Guglielmetti University of Aix-Marseille II Marseille, France

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

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In Memoriam DR. JOHN C. CRANO On January 10, 1998, Dr. John C. Crano, 62, one of the co-editors of this book, and his wife Dorothy were killed instantly in an automobile accident in Florida. In this tragic death the world of photochromism has lost the person chiefly responsible for the success of the largest commercial application of organic photochromics. Dr. Crano received a B.S. degree in Chemistry in 1957 from Notre Dame and M.S. and Ph.D degrees from Case Western Reserve University in 1959 and 1962, respectively. He joined PPG Industries in 1961, and spent his entire career with

that firm, at first in various roles in the Chemical Research and Development area.

In 1974 PPG began research, upon means to impart photochromic properties to ophthalmic lenses made from plastics, in particular from poly(allyl diglycol carbonate), CR-39®. Plastic lenses command over 85% of the total ophthalmic market in the US, principally because a lightweight plastic product is more comfortable to wear and permits more attractive fashion designs. Comparative testing of all classes of photochromics, including inorganic compounds, indolinospirobenzopyrans and naphthoxazines, occupied the first few years. In the early 1980s expanding research and development efforts focused upon the indolinospirobenzoxazines and quinolinooxazines and their close structural relatives. In 1986 Dr. Crano became leader of a constantly enlarging team of organic, physical, polymer, and photo-chemists, and engineers. The first generation plastic photochromic ophthalmic lenses, the Transitions® Comfort Lens, were manufactured in Ohio and test-marketed in June of 1989. Transitions Optical, Inc., a joint venture between PPG and the French lens manufacturer Essilor, was formed in 1990 and began manufacturing lenses early in 1991 in a new plant in Florida. A second generation lens, the Transitions® Plus lens, was introduced in November 1992. In the years since, Transitions Optical has marketed a succession of new lenses. In September 1994 the EuroBrown™ lens appeared, formulated to give when activated a brown color, especially favored in the European market. The mid-index Transitions® III lenses were launched in the United States and in Europe during 1996, and the Transitions XTRActive™ lenses were introduced in v

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In Memoriam

the US in January 1997. The most recently introduced lenses are Transitions® III lenses in a standard index matrix. These use the latest technology in organic photochromic dyestuffs and polymer science and engineering, and maintain the company’s position of technical leadership in the field of plastic ophthalmic photochromic lenses. Transitions Optical now sells its various lenses in over 50 countries. In his position as Associate Director of Research and Development, Optical Products, Dr. Crano managed the entire photochromics research program. He led the team of scientists that synthesized and evaluated hundreds of candidate photochromic dyestuffs, and directed all of the product and process development involved in the various Transitions® lenses. In addition, his responsibilities included R&D on non-ophthalmic photochromic applications, and on other optical coatings and resins. He has a number of US patents in various areas of technology, including three

during 1990–1992 that cover the basic compositions and methods for producing

the first generation of Transitions® lenses. These were important in establishing a strong proprietary position in photochromic plastic ophthalmic lenses. In addition, he had published several reviews and invited lectures on photochromism and photochromic polymers. In 1985 he was General Chairman of the Central Regional Meeting of the American Chemical Society, and in September 1996 he was Co-Chairman of the 2nd International Symposium on Photochromism. This Symposium, which included participants from 16 countries, owed much of its success to Dr. Crano’s excellent organization.

Recent Publications and Presentations of J. C. Crano Photochromic Systems for Plastic Ophthalmic Lenses, J. C. Crano, 1997 Gordon Conference on Organic Photochemistry. Photochromic Compounds: Chemistry and Application in Ophthalmic Lenses, J. C. Crano, T. Flood, D. Knowles, A. Kumar, and B. Van Gemert, Pure Appl. Chem, 68(7), 1395–1398 (1996). Photochromic Materials, John C. Crano, in “Kirk-Othmer Encyclopedia of Chemical Technology,” 4th Edn., 1993, J. Wiley & Sons, New York. Photchromic Organic Compounds in Polymer Matrices, J. C. Crano, C. N. Welch, B. Van Gemert, D. Knowles and B. Anderson, in “Photochemistry and Poymeric Systems” J. M. Kelly, C. B. McArdle and M. J. de F. Maunder, Eds., 1993, Royal Society of Chemistry, Cambridge. Spirooxazines and Their Use in Photochromic Lenses, J. C. Crano, W. S. Kwak and C. N. Welch, in “Applied Photochromic Polymer Systems” C. B. McAedle, Ed., 1992, Blackie ans Sons. R. Guglielmetti and R. Bertelson

viii

Contributors Angelo Alberti, I.Co.C.E.A.—CNR, 40129 Bologna, Italy Serguei Aldoshin, Institute of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia

Jean Aubard, Institut de Topologie et de Dynamique des Systèmes, Université Denis Diderot, Paris, France M. H. Deniel, Lab. I.M.C.R.P., UMR 5623 au CNRS, Paul Sabatier University, F-31062 Toulouse, France Kunihiro Ichimura, Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Yokohama 226-8503, Japan

Masahiko Inouye, Department of Applied Materials Science, Osaka Prefecture University, Sakai, Osaka 593, Japan

D. Lavabre, Lab. I.M.C.R.P., UMR 5623 au CNRS, Paul Sabatier University, F-31062 Toulouse, France Vladimir Lokshin, Université de la Méditerranée, Faculté des Sciences Luminy, ESA 6114 CNRS, 13288 Marseille Cedex 9, France Klaus Lorenz, Department of Organic Chemistry, University of Regensburg, D-93040 Regensburg, Germany

V. Malatesta, Great Lakes Chemical Italia, Via Maritano 26, 20097 S. Donato (MI), Italy Albrecht Mannschreck, Department of Organic Chemistry, University of Regensburg, D-93040 Regensburg, Germany

J. C. Micheau, Lab. I.M.C.R.P., UMR 5623 au CNRS, Paul Sabatier University, F-31062 Toulouse, France ix

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Contributors

Shinichiro Nakamura, Yokohama Research Center, Mitsubishi Chemical Corporation, Yokohama 227, Japan André Samat, Université de Méditerranée, Faculté des Sciences Luminy, ESA 6114 CNRS, 13288 Marseille Cedex 9, France Michael Schinabeck, Department of Organic Chemistry, University of Regensburg, D-93040 Regensburg, Germany

Foreword Experiments showing a rapid and reversible change of color seem like magic and are always fascinating. The process involved, photochromism, has a few real and many potential applications. Photochromic glasses that darken in the sunlight (protecting eyes from excessive light intensity) and bleach in dim light are today a part of everyday life. Organic photochromic compounds in plastic ophthalmic lenses, more comfortable to wear, are now competing with silver salts in glasses, despite the longer lifetime of the inorganic system. This successful commercial application has given a new impetus to research in the general field of photochromism, which had its most recent revival in the early eighties.

The story of organic photochromism with its ups and downs, from the breakthroughs of the pioneering period in the fifties, through the hard times due to the drawbacks of photodegradation, to the recent successes is in many ways a saga. The upsurges in this domain were marked by an increasing flow of articles in scientific journals and the publication of several books (in 1971, 1990, and 1992) that have collected the important accumulated knowledge. Over this period, a considerable number of patents have been issued. International meetings have accompanied this activity, the most recent being held in 1993 (ISOP-93 at Les Embiez Island, France) and in 1996 (ISOP-96 in Clearwater, Florida). Remarkably, these meetings had good representation from both academia and industry. The next ISOP is planned for 1999 in Fukuoka, Japan. Since publication of the most recent books, new areas have been explored and a large number of new results have been obtained, and it seemed, therefore, timely to publish them. This was the endeavor of the chairmen of ISOP-93 and ISOP-96, Robert Guglielmetti and John Crano, respectively, who have acted as coeditors. R. Guglielmetti, a professor at the University of Marseilles, is a recognized leader in the field, and the late Dr. J. Crano Associate Director of Research and Development for optical products at PPG Industries since 1986, led research in the development of plastic ophthalmic eyewear. The efficient cooperation of a scholar and an industrial scientist has led to the publication of Organic Photochromic and Thermochromic Compounds in two volumes of about 400 pages each. Volume 2 includes ten chapters, of which six are devoted to physicochemical methods applied to the investigation of photochromism and four address some specific topics selected for their wide interest. Nine out of the ten main authors have not written chapters for previous books, and two are from companies. The literature has been reviewed to 1995–1997. xi

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Foreword

Physicochemical methods have been referred to in preceding books, but, in this volume, systematic studies have been carried out with the same method on different classes of photochromic compounds: kinetics under continuous irradiation, electronic paramagnetic resonance (EPR), molecular modeling, X-ray diffraction, Raman studies [particularly surface-enhanced resonance Raman spectroscopy (SERRS)], and chirality and photochromism (contributing to a better understanding of the ring opening of spiro compounds). The authors are among the leading specialists in their respective fields. The other specific topics are: (i) the influence of various types of matrices

(polymer, liquid crystal, Langmuir–Blodgett (LB) films, sol-gel) on the behavior of photochromic systems (spiropyrans, azobenzenes, viologens, and fulgides); (ii) the study of photodegradation mechanisms, an important topic—given that the so-called “fatigue” is the genetic disease of all organic materials—that has only recently been methodically tackled; (iii) the application of photochromic molecules to the reversible control of peptide and protein conformation, antigen– antibody reactions, and the cation-binding affinity of complexing agents that differ from the photochromic biological systems (e.g., rhodopsins, phytochromes) which are not dealt with in this book; and (iv) last but not least, thermochromism of organic compounds, stressing its structural and mechanistic aspects. It is clear that this volume is truly different from the preceding accounts. Photochemists will appreciate Volume 2 as a nice complement to Volume 1, although it can be read independently. Organic photochromic systems are known for their applications in variable-transmission optical materials, ophthalmic lenses, authentification devices (photochromic inks), and novelty items, but they also have great potential in any domain where reversible physical properties are desired (optical memories, gradation masking, optoelectronic systems, nonlinear optical devices, etc.). This book is thus strongly recommended to anyone interested in materials science.

Henri Bouas-Laurent University Bordeaux 1

Preface Organic Photochromic and Thermochromic Compounds, of which this book is the second volume, is the fourth major treatise on photochromism involving organic molecules and derived systems. The first such book, edited by G. H. Brown, was published in 1971 as a volume in the Weissberger series, and the second, edited by H. Dürr and H. Bouas-Laurent, was published in 1990 in the Elsevier series.2 To this list should be added a third book, edited by C. B. McArdle,3 which focuses on the very important topic of the behavior of photochromic systems in polymer matrices. The two volumes of this new treatise are an outgrowth of the large increase in the number of publications and patents concerning photochromic compounds and their use in various applications (e.g., ophthalmic lenses, security printing, etc.) during the past 10 years. As a result of this increased interest, two successful International Symposia on Photochromism (ISOP) have been held: ISOP-93, held in France on Les Embiez Island near Bandol (September 12–16, 1993), and ISOP96, held in the United States in Clearwater, Florida (September 8–12, 1996). The number of countries represented at each of these symposia (17 and 16, respectively) attests to the international scope of the photochromic research community. This global interest is also exemplified by the authors of the chapters within this book. The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. Our thanks go to all the colleagues who volunteered to collaborate on this book and who had the patience and perseverance to undertake all the steps xiii

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Preface

required to prepare the final manuscripts. In addition to the chapter authors, several other people have contributed their time and talent. Dr. A. Samat and Dr. V. Lokshin (Laboratory of Organic Chemistry and Materials, Marseille) must be acknowledged for many fruitful discussions throughout the process of putting together the book. We are deeply indebted to Diana Gronholm, PPG Industries, Inc., whose help in the revision and copying of manuscripts, communication with contributors and the publisher, and a variety of other tasks during the preparation of the book was invaluable. Dr. Anil Kumar, also with PPG, helped with the conversion of files into usable program formats and provided technical assistance in the preparation of files for the final manuscripts. Finally, we express our appreciation to Audrey Anderson, Denise Callihan, and Beverly Weston at PPG Industries for obtaining missing titles of several references and providing general assistance when it was needed to verify details of references. 1. G. H. Brown (ed.), Photochromism (Techniques of Chemistry. Vol. III), Wiley-Interscience, New York (1971).

2. H. Dürr and H. Bouas-Laurent (eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam (1990). 3. C. B. McArdle (ed.) Applied Photochromic Polymer Systems, Blackie, New York (1992).

J. C. Crano and R. Guglielmetti

Contents Introduction

1. Aim and Organization of the Book. . . . . . . . . . . . . . . . . . . . . . . . 2. Brief Historical Survey of Photochromism. . . . . . . . . . . . . . . . . . . 3. Definitions of Photochromism . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Brief Overview of the Chapters in the Book. . . . . . . . . . . . . . . . . . 5. Conclusion and Future Developments . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 2 4 7 8

1. Photochromic Polymers Kunihuro Ichimura 1.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Photochromism in Polymer Matrices . . . . . . . . . . . . . . . . . . . . . 1.2.1. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2. Matrix Effect on Photochromism . . . . . . . . . . . . . . . . . . . 1.2.2.1. Spiropyrans and Related Compounds. . . . . . . . . . 1.2.2.2. Azobenzenes. . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2.3. Viologens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2.4. Fulgides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2.5. Photochromism in Sol-Gel Inorganic Matrices . . . . 1.2.3. Polarization Photochromism in Polymer Solids. . . . . . . . . . 1.2.3.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3.2. Polymers Doped with Photochromic Compounds. . . 1.2.3.3. Polymers with Covalently Bound Photochromic Units . . . . . . . . . . . . . . . . . . . . . . 1.3 Photochromic Liquid-Crystalline Polymers . . . . . . . . . . . . . . . . . 1.3.1. Photochromism in Polymeric Mesophases . . . . . . . . . . . . . 1.3.1.1. Nematic and Smectic Phases . . . . . . . . . . . . . . . . 1.3.1.2. Cholesteric Phases . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Polarization Photochromism of Liquid-Crystalline Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 11 11 15 15 16 18 20 21 24 24 24 26 31 31 31 34 35 xv

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1.4. Surface Photochromism for Regulation of Liquid-Crystal Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1. Surface-Assisted Liquid-Crystal Alignment Control . . . . . . . 1.4.2. Out-of-Plane Alignment Photocontrol . . . . . . . . . . . . . . . . 1.4.2.1. Spin-Coated Films.. . . . . . . . . . . . . . . . . . . . . . . 1.4.2.2. Polymeric Langmuir–Blodgett Films.. . . . . . . . . . . 1.4.3. In-Plane Alignment Photocontrol.. . . . . . . . . . . . . . . . . . . 1.5. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix. Preparation of Photochromic Polymers . . . . . . . . . . . . . . . . A.1. Polymethacrylates Substituted with Spiroindolinebenzothiopyrans (5) . . . . . . . . . . . . . . . . . . . . A.2. Polymethacrylates with Spirofluorenylindolizines ... . . . . . . . . . A.3. Poly[4´-[[2-acryloyloxyethyl]ethylamino]-4-nitroazobenzene] (12) A.4. An Ionene Polymer Incorporating Viologen Units (17) . . . . . . References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39 39 40 40 44 47 50 52 52 53 53 53 54

2. Photodegradation of Organic Photochromes V. Malatesta 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Properties and Reaction Mechanism(s) of Spiropyrans and Spirooxazines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Multiplicity of the Photoreactive State . . . . . . . . . . . . . . . . 2.2.2. The cis-cisoïd X Isomer . . . . . . . . . . . . . . . . . . . . . . . . . ................. 2.2.3. Nitro-Substituted Spiropyrans, Unsubstituted Spiropyrans and Spirooxazines ........... 2.2.4. 2.3. Colorability and Photostability (Photocoloration Quantum Yields) . . 2.4. Photo- and Thermal Degradation of Photochromes . . . . . . . . . . . . 2.4.1. Degradation of Spiropyrans: General Approach . . . . . . . . . . 2.4.2. Photodegradation: Dependence on the Nature and Position of the Substituent . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3. Substituents on the Indoline Moiety of Spiropyrans: Inductive and Mesomeric Effects . . . . . . . . . . . . . . . . . . . 2.4.4. Substituents on the Indoline Moiety. . . . . . . . . . . . . . . . . . 2.4.5. Substituents on the Chromene Moiety of Spirobenzopyrans. . 2.4.5.1. Monosubstituted Compounds . . . . . . . . . . . . . . . . 2.4.5.2. Disubstituted Compounds. . . . . . . . . . . . . . . . . . . 2.4.5.3. C3-Substituted Spiropyrans. . . . . . . . . . . . . . . . . . 2.4.5.4. Substituents on N and C3´ . . . . . . . . . . . . . . . . . . 2.5. Photodegradation of Benzothiazolinospiropyrans . . . . . . . . . . . . . . 2.6. Saturated Five-Membered Ring Azaheterocyclic Spiropyrans. . . . . 2.6.1. Pyrrolidine Series ...................

65 66 67

68 68 70 74 75 75 77

81 82 84 84 84 84 89 93 96 97

Contents

2.6.2. Thiazolidine Series (X=S). . . . . . . . . . . . . . . . . . . . . . . . 2.6.3. Oxazolidine Series (X=O). . . . . . . . . . . . . . . . . . . . . . . . 2.7. Saturated Six-Membered Ring Azaheterocyclic Spiropyrans . . . . . . 2.8. Spiropyran Degradation: A Quantitative Approach . . . . . . . . . . . . 2.9. Non-Azaheterocyclic Spiropyrans. . . . . . . . . . . . . . . . . . . . . . . . 2.9.1. 6- and 8-Substituted Spirobenzodithiolane and Spirobenzoxathiolane Benzopyran Series. . . . . . . . . . . . . . 2.9.2. Spiro[bibenzopyrans] . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.3 Dithiole Series. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10. Photodegradation of Spirooxazines. . . . . . . . . . . . . . . . . . . . . . 2.10.1. Solvent Polarity and Photodegradation. . . . . . . . . . . . . . 2.10.2. Structure and Photodegradation . . . . . . . . . . . . . . . . . . 2.10.3. Mechanism of Photooxidation of Spirooxazines: Singlet-Oxygen Reactivity. . . . . . . . . . . . . . . . . . . . . . 2.10.4. Oxidation in Dark Reactions of Spirooxazines . . . . . . . . 2.10.5. Reaction of Merocyanines with Free Radicals . . . . . . . . . 2.11. Photodegradation Measurements: Experimental Apparatus . . . . . . 2.11.1. Irradiation Source.. . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11.2. Analysis Setup.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11.3. Signal-Conditioning Circuits (SCC).. . . . . . . . . . . . . . . 2.11.4. Operating Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12. Photostability of Fulgides . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13. Commercial Spirooxazines and Chromenes: Relevant Properties and Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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99 99 99 99 111 111 115 115 115 119 119 124 136 141 143 143 147 147 148 151 153 164

3. Photokinetics under Continuous Irradiation M. H. Deniel, D. Lavabre, and J. C. Micheau

3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Dynamic Analysis of the Isolated Photochemical Step:

167

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 170 3.2.1. Establishment of the Photochemical Rate Equation . . . . . . . 170 3.2.2. Photochemical Reactor Uniformly Stirred and Equipped with a System for UV/Visible Spectroscopic Analysis . . . . . 172 3.2.3. Experimental Verification of the Validity of the Photochemical Kinetic Equations under Continuous Irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 3.3. AB Systems Involving Two Species . . . . . . . . . . . . . . . . . . . . . . 174 3.3.1. Establishment of the General Kinetic Equation under Continuous Irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

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1k 3.3.2. Properties of the Photostationary State of the AB, System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1k Systems . . . . . . . . . . . . . . . . . . . . . . 3.3.2.1. AB, Systems . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2.2. AB, 3.3.3. Photostationary Methods . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4. Dynamical Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. ABC Photochromic Systems Involving Three Species . . . . . . . . . . 3.4.1. General Comments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2.1. Systems with Two Photoisomers . . . . . . . . . . . . . . 3.4.2.2. Photodegradation of a Spiro[indoline-benzopyran] . . 3.5. ABC Systems Exhibiting Nonlinear Behavior. . . . . . . . . . . . . . . . 3.5.1. Ideal ABC System Giving Rise to Bistability . . . . . . . . . . . 3.5.2. Photochromism and Photodegradation of TPID . . . . . . . . . . 3.6. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 1. Flash Photolysis versus Continuous Irradiation: Influence on the Accumulation of Reaction Intermediates. . . . . . . . . Appendix 2. Photostationary Methods . . . . . . . . . . . . . . . . . . . . . . . . A2.1. AB, , 1k Systems . . . . . . . . . . . . . . . . . . . . . . A2.2. AB, 1k Systems . . . . . . . . . . . . . . . . . . . . . . A2.3. AB, Systems . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 3. Thermal Relaxation of an ABC System . . . . . . . . . . . . . . Appendix 4. Localization of an Elementary Relaxation Process in ABC Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 5. Localization of a Photodegradation Process . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

175 176 177 177 177 182 182 183 183 186 190 190 191 194 195 195 195 197 198 199 202 203

205

4. EPR and Radical Processes Angelo Alberti 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Viologens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Radical Cations from the Reduction of Viologens . . . . . . . . 4.2.2. Some Particular Viologens. . . . . . . . . . . . . . . . . . . . . . . . 4.2.3. EPR of Viologens in Micellar Systems. . . . . . . . . . . . . . . . 4.3. "!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 4.4. Spiroindolinic Photochromes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1. Paramagnetic Species in Photochromism and Photodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2. Radical Ions from Spiro Compounds . . . . . . . . . . . . . . . . . 4.4.3. Spin Probes and Spin Traps with a Spiroindolinic Structure. . References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

211 212 212 219 220 224 226 226 229 234 236

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5. Molecular Modeling Calculations Shinichiro Nakamura 5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Absorption Wavelength and Intensity . . . . . . . . . . . . . . . . . . . . . 5.3. Thermal Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1. Photon–Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2. Photon–Photon Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Reaction Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1. Spiropyran Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2. Fulgide Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3. Other Photochromic Systems. . . . . . . . . . . . . . . . . . . . . . 5.5. Quantum Yield. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

241 241 247 247 249 251 251 253 253 255 256

6. The Enantiomers of 2-Donor-Substituted Benzopyrans and Benzo-1,4-oxazines and Their Thermal Racemization Albrecht Mannschreck, Klaus Lorenz, and Michael Schinabeck 6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Methods of Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1. Remarks Pertaining to All Methods . . . . . . . . . . . . . . . . . 6.2.2. Preparative Enrichment of Enantiomers by Enantioselective Liquid Chromatography . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3. Determination of Enantiomeric Purities . . . . . . . . . . . . . . . 6.2.4. Assignment of Relative Configurations . . . . . . . . . . . . . . . 6.2.5. Measurement of Barriers by Thermal Racemization.. . . . . . 6.3. Results of the Thermal Racemizations . . . . . . . . . . . . . . . . . . . . 6.3.1. Transition State of C(sp3)–O Bond Cleavage. . . . . . . . . . . 6.3.2. Remarks Pertaining to All Types of Benzopyrans and Benzo-l,4-Oxazines.. . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.3. 2-Aryl-2-methylbenzopyrans . . . . . . . . . . . . . . . . . . . . . . 6.3.4. 2-Alkoxybenzopyrans.. . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.5. Spirobibenzopyrans . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.6. Spiro[oxaindane-benzopyrans] . . . . . . . . . . . . . . . . . . . . . 6.3.7. Spiro[indoline-benzoxazines]. . . . . . . . . . . . . . . . . . . . . . 6.3.8. Spiro[indoline-benzopyrans] . . . . . . . . . . . . . . . . . . . . . . 6.4. Conclusion.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

261 263 263 263 265

267 269 271 271 273 274 274 280 283 284 288 290 292

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7. Structural Studies by X-Ray Diffraction Serguei Aldoshin 7.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Spiropyrans.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1. Indolinospiropyrans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1.1. Structural Features of SP Molecules. . . . . . . . . . . . 7.2.1.2. The Nature of Structural Changes in SP Molecules upon Photoexcitation. . . . . . . . . . . . . . . . . . . . . . 7.2.1.3. Effect of the Nature and Position of Substituents in SP Molecules on the Length of the Cspiro–O Bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2. The Structure of Permanent Merocyanines—Models of the

297 298 298 298 303 304

Open Forms of Indolinospiropyrans. . . . . . . . . . . . . . . . . . 306 7.2.2.1. Molecular Structure of Merocyanines . . . . . . . . . . . 307 7.2.2.2. Geometric Isomers of Merocyanine Forms . . . . . . . 312 7.2.2.3. Nature of the Molecular Packing and Energy of Intermolecular Interactions in Merocyanine Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 7.2.3. Effect of the Nature of Heteroatoms in the Spiro Center on the Structure and Photochemical Properties of Spiropyrans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 7.2.3.1. Structural Features of Indoline Spirothiopyrans Containing S and N Heteroatoms in the Spiro Center. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 7.2.3.2. Structure of “Symmetrical” Spiropyrans Consisting of Two Benzopyran Fragments . . . . . . . . . . . . . . . 317 7.2.3.3. Structure of the First Representative of Spiro-2HNaphtho[l,8-bc]oxepins (SPNO) . . . . . . . . . . . . . . 318 7.2.4. 2-Oxaindan Spiropyrans with Polycondensed Chromene Fragments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 7.2.4.1. Structure of 2-Oxaindan Spiropyrans . . . . . . . . . . . 320 7.2.4.2. Thermal Valence Isomerization of SPs . . . . . . . . . . 323 7.2.4.3. Photochromism of 2-Oxaindan Spiropyrans . . . . . . . 324 7.3. Spirooxazines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 7.4. Photochromic [2H]-Chromenes.. . . . . . . . . . . . . . . . . . . . . . . . . 330 7.5. Spirans of the Perimidine Series . . . . . . . . . . . . . . . . . . . . . . . . . 333 7.6. Fulgides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 7.6.1. Arylidene Derivatives of Fulgides (ArF). . . . . . . . . . . . . . . 339 7.6.2. Fulgides of the Indole Series . . . . . . . . . . . . . . . . . . . . . . 343 7.7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350 References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350

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8. New Trends in Raman Studies of Organic Photochromes Jean Aubard

8.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. Survey of Vibrational Studies on Spiropyran and Spirooxazine Photochromes.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3. Dynamics and Structure of Transient Species Involved in the Photochromic Process of Spiro Compounds as Probed by Time-Resolved Raman Techniques.. . . . . . . . . . . . . . . . . . . . . . 8.3.1. Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2. Survey of Recent Time-Resolved Raman Studies of Spiro Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3. Time-Resolved Resonance Raman Spectroscopy of Indolinospironaphthoxazines: Experimental Evidence for a TTC Resonant transoid Open Form in Different Solvents. . . 8.4. Surface-Enhanced Raman Spectroscopy of Spiro Compounds: A Route for Direct Detection and Identification of Their Photodegradation Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1. SERS Detection of Spirooxazines and Spiropyrans at Trace Levels in Silver Colloidal Solutions . . . . . . . . . . . . . . . . . 8.4.1.1. Brief Description of the SERS Effect . . . . . . . . . . 8.4.1.2. Analysis of SERS Spectra of Some Spiro Compounds.. . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.2. Detection by SERS of Degradation Products from UV-Irradiated Solutions of 8-Methoxy-6-Nitro-BIPS . . . . . . 8.4.2.1. Assignments of SERS Spectra of 8-Methoxy-6Nitro-BIPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.2.2. SERS Analysis of UV-Irradiated Solutions of Photochromes . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

357 359 366 367 369

372

378 378 378 380 382 382 386 389

9. Biological Applications—Supramolecular Chemistry Masahiko Inouye 9.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Photocontrol of Peptide and Protein Characteristics by Spiropyrans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1. Photomodulation of Polypeptide Conformation. . . . . . . . . . 9.2.2. Photoregulation of Protein Characteristics . . . . . . . . . . . . . 9.2.2.1. Photoregulated Binding of Spiropyran-Modified Receptor Proteins. . . . . . . . . . . . . . . . . . . . . . . .

393 394 394 397 397

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9.2.2.2. Photoregulation of Enzyme Activity by Use of Spiropyrans . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2.3. Photocontrol of Antigen–Antibody Reactions.. . . . . 9.3. Spiropyran Derivatives Possessing a Molecular Recognition Site . . . . 9.3.1. Spiropyran Derivatives as Artificial Signaling Receptors . . . . 9.3.1.1. Crowned Spiropyrans as Alkali (Alkaline-Earth) Metal Cation Receptors . . . . . . . . . . . . . . . . . . . . 9.3.1.2. Spiropyridopyrans as Nucleobase Receptors. . . . . . . 9.3.2. Photoregulation of Polymer Characteristics by Crowned Spiropyrans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.3. Other Applications Utilizing Spiropyran Derivatives. . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

398 401 402 402 402 406 408 410 412

10. Thermochromism of Organic Compounds André Samat and Vladimir Lokshin

10.1. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2. Spiroheterocyclic and Related Compounds. . . . . . . . . . . . . . . . . 10.2.1. Compound Families. . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1.1. Spiropyrans and Spirooxazines. . . . . . . . . . . . . 10.2.1.2. [2H]-pyrans.. . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1.3. Other Spiroheterocyclic Compounds . . . . . . . . . 10.2.2. Mechanism of Thermal Isomerization . . . . . . . . . . . . . . . 10.2.2.1. Theoretical Studies. . . . . . . . . . . . . . . . . . . . . 10.2.2.2. Activation Parameters of the Isomerization Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3. Evaluation of Equilibrium Constants. . . . . . . . . . . . . . . . 10.3. Schiff Bases and Related Nitrogen-Containing Compounds . . . . . . 10.3.1. Thermochromism of Salicyl-Schiff Bases.. . . . . . . . . . . . 10.3.2. Structural, Spectroscopic, and Theoretical Studies of Intramolecular Hydrogen Bonding . . . . . . . . . . . . . . . . . 10.4. Biathrones and Other Overcrowded Ethenes . . . . . . . . . . . . . . . . 10.5. Miscellaneous Compounds.. . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

415 416 416 416 421 421 422 422

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

467

425 431 436 436 445 451

457 459 460

Introduction 1. AIM AND ORGANIZATION OF THE BOOK The principal aims of this volume and the first volume of Organic Photochromic Thermochromic Compounds are to complement earlier books covering the phenomenon of photochromism1–3 and to survey the main representative series of photochromic compounds having actual or potential industrial applications. In contrast to the book edited by H. Dürr and H. Bouas-Laurent,2 which was organized around the type of reactions involved in the photochromic phenomenon (cis–trans isomerization, electrocyclic reactions, cycloaddition reactions, tautmerization, dissociation processes etc.), this collection of reviews is organized around

the main categories of organic photochromic compounds that have real or potential uses in various applications. The many important developments of photochromic pigments and materials in recent years have necessitated that this collection of reviews be issued in two volumes. This volume 2 contains ten chapters complementing the nine chapters of Volume 1 devoted to specific families of photochromic compounds. The first eight chapters of this volume concern physicochemical studies and describe different methods for studying photochromic behavior through a variety of physicochemical approaches. The following topics are addressed: 1. Interactions between photochromic compounds and polymer matrices: a crucial discussion because polymer systems are generally used as supports for photochromic substances in most applications 2. Photodegradation mechanisms: an important topic for the practical use of photochromic compounds in variable-optical-transmission materials 3. Spectrokinetic studies, expecially of compounds in a photostationary state (conditions similar to those encountered in most applications) 4. Radical processes in the photocoloration mechanism and their investigation by electron paramagnetic resonance 5. Chemical computations and molecular modeling 6. Chirality in spiroheterocyclic and chromene systems 7. Structural studies by X-ray diffraction 8. Structural studies by Raman spectroscopy

1

2

Introduction

The technical coverage in the first eight chapters is nicely supplemented by the last two chapters, which address potential biological applications of photochromic systems and supramolecular chemistry (Chapter 9) and organic thermochromic compounds (Chapter 10).

2.

BRIEF HISTORICAL SURVEY OF PHOTOCHROMISM

The first examples of the phenomenon of photochromism in both inorganic or organometallic complexes and organic substances were discovered near the end of the 19th century.4,5 The terms phototropy6 and phototropism7 were employed to describe this phenomenon but are now used largely for biological systems. The term “photochromism” was suggested by Hirshberg in 1950.8 Interest in the photochromism of organic materials began to increase substantially around 1940. The principal studies of photochromic compounds involved acquiring an insight into mechanisms of the photoprocesses, determining the structures of the uncolored and colored forms, and developing synthetic methods. The pioneering work by the team of Hirshberg and Fischer in Israel1,8,9 is worthy of a special mention. The development of time-resolved or flash spectroscopy by Porter10–12 and, more recently, the use of laser photophysical techniques opened new approaches to the study of the excited states and transient species involved in the photoreactivity of photochromic molecules.

3. DEFINITIONS OF PHOTOCHROMISM We can consider a photochromic organic compound to be a reversible dye under photochemical control. Photochromism can be defined as a reversible transformation of a chemical species, induced in one or both directions by electromagnetic radiation, between two states having observable light absorptions in different regions of the spectrum:

Ordinarily, B has at least one absorption band appearing at longer wavelength than the absorption bands of A. The activating radiation generally is in the ultraviolet (UV) region (300–400 nm) but may be in the visible (400–700 nm). The most prevalent photochromic systems are based on unimolecular reactions and those described in this book correspond to this type. Reversibility is the main criterion for photochromism. The back reaction may occur predominantly by a thermal mechanism, as is the case with

Introduction

3

spiropyrans, spirooxazines, and chromenes. For these systems, the thermally driven back reaction may be accompanied by one that is photochemically driven, but the thermal reaction normally predominates. In other systems (e.g., fulgides or arylethenes), the photochemically induced forms (B) are thermally stable. For such systems, the back reactions are predominantly photochemical. The photochromic transformation and the observed spectral changes or changes in physical or chemical behaviour are related to the modifications of the geometry of the system and its electronic distribution. This will be seen in the examples given below. A recurring theme throughout this book is the importance of the medium in which the photochromic compound is incorporated. The medium can strongly influence or control the kinetics of the thermal back reaction, if one occurs, the color of the species formed in the forward photochemical reaction, and other properties of the photochromic process. Photochromism and its various characteristics can also be defined according to the type of application at which they are targeted. Two general types of applications can be defined: 1. Applications directly dependent upon the color change caused by the differences between the molecular and electronic structures of A and B and their corresponding absorption or emission spectra. Examples would be:

• • • • • •

Variable-Transmission optical materials such as photochromic ophthalmic lenses or camera filters Fluid-flow visualization Optical information storage Novelty items (toys, T-shirts, etc.) Authentication systems (security printing inks) Cosmetics

2. Applications dependent upon changes in physical or chemical properties that accompany the more easily observed color change during the photochromic reaction. Examples of such properties are conductivity, refractive index, electrical moment, dielectric constant, chelate formation, ion dissociation, phase transitions, solubility, and viscosity. Certain physical changes that occur when the photochromic entity is chemically attached to the macromolecular backbone of polymers are of special interest (see Chapter 1). Some examples of potential applications utilizing the physical or chemical changes which accompany the observed shift in the absorption maxima are:

4

Introduction

• • • • • •

Optoelectronic systems (semiconductors modulated by photochromic pigments) Reversible holographic systems Optical switches Optical information storage Photochemically switchable enzymatic systems nonlinear optical devices

Of all these potential applications, a few have been commercially successful (polymer-based photochromic eyewear, novelty items, and security printing inks) or demonstrated to be useful (fluid-flow visualization). Several others have shown considerable promise and may very well be utilized in commercial products in the future.

4. BRIEF OVERVIEW OF THE CHAPTERS IN THE BOOK Chapter 1 (Photochromic Polymers). Like most chemical reactions, photochromic reactions are influenced significantly by the media in which they occur. This chapter surveys the interactions between photochromic entities and polymer matrices in which they are incorporated either by binding them covalently to polymer backbones or by dissolving or suspending them in polymer solids. Polymeric materials play a very crucial role in studies on photochromism, particularly from a practical viewpoint since the various applications of photochromic materials invariably require that they possess definite forms like films, sheets, plates, fibers, beads, and so on, depending on the application. The photochemical as well as thermal behavior of organic photochromic molecules is influenced in various ways by the characteristics of the polymer media in which they are incorporated. More importantly, the interaction between the photochromic and polymeric entities can lead to novel photoinduced properties beyond the obvious color changes. Because of this, photochromic polymers have been of extensive interest in interdisciplinary research. Most studies have been carried out with organic polymers, although there have been quite a few studies involving the incorporation of organic photochromic compounds into inorganic matrices through the sol-gel process. Of special interest are photochromic liquid-crystalline polymers, which can be prepared using nematic, smectic; or cholesteric phases. In these systems, the photochromic reaction can be used to influence the characteristics of liquid crystals (e.g., phase changes) and, in turn, lead to novel applications (e.g., polarization holography). Chapter 2 (Photodegradation of Organic Photochromes). This chapter deals with the crucial problem of photodegradation of photochromic materials, largely

Introduction

5

in the presence of oxygen. In any application of photochromic materials, there is no more important consideration than that of photostability, often called fatigue resistance. The main focus of this chapter is on two well-studied series of photochromic compounds, spiropyrans and spirooxazines, and the comparison of their photoreactive excited states and molecular structures. The discussion of their photodegradation takes into account the results obtained from spectrophotometric measurements together with the identification of the fragments resulting from photodegradation. Different mechanisms are proposed for the photooxidation reaction. The experimental apparatus and procedures for photodegradation studies are also described. This chapter is a complete survey of the contributions of the leading research teams involved in the pursuit of an understanding of the photodegradation phenomenon. Chapter 3 (Photokinetics under Continuous Irradiation). A methodology for analyzing the reactions of typical photochromic systems under continuous irradiation is described in detail in this chapter. For dynamic photochromic

systems, the basic photochemical rate equation is established, and a suitable apparatus and the numerical techniques for studying the systems are presented. An examination of simple systems involving the interconversion of only two isomers, designated as “AB” systems, taking into account thermal and/or photochemical bleaching, is presented. Also examined are photochromic systems involving several photoisomers or photodegradation processes, designated as “ABC” systems. This work has allowed an evaluation of the molar absorptivity of the colored species and the quantum yields of photocoloration and photodecoloration. The examples given include a dihydroindolizine compound, a spiro[benzothiazolinebenzopyran], a 2H-chromene, a spiro[indoline-benzopyran], and the dimer of the triphenylimidazolyl radical. Chapter 4 (EPR and Radical Processes). The radical species sometimes resulting from photochromic systems can be investigated by electron paramagnetic resonance (EPR) spectroscopy. This technique can also be employed to detect radical species formed during photodegradation. There are many literature references on EPR studies of radical cations obtained through the reduction of viologens. The results of these studies and parallel electrochemical and spectroscopic studies of different structural types of viologens (thiophenic, micellar, or porphyrinic viologens) are discussed. The photocoloration of ketonaphthalene is also discussed in terms of homolytic or heterolytic processes. The indolinospirobenzo-pyrans and -naphthopyrans and the indolinospironaphthoxazines yield paramagnetic species during cleavage under UV irradiation which can be detected with the aid of spin trapping

6

Introduction

using nitric oxide. Nitro-substituted spiropyrans or spirooxazines give stable radial anions that are easily studied by combined electrochemical and EPR techniques. Chapter 5 (Molecular Modeling Calculations). A theoretical approach to photochromic behavior of different systems is important for the understanding of mechanisms and also for the ability to design functional materials having, for example, a desired absorption wavelength or thermal stability after irradiation. The purpose of this chapter is to review the more recent computational studies, involving various semiempirical and quantum-mechanical methods, on the principal series of photochromic compounds: spiropyrans, spirooxazines, fulgides, and diarylethene derivatives. The relationships between the computational results and and nuclear magnetic resonance (NMR) data and X-ray diffraction structures are also discussed. Chapter 6 (The Enantiomers of 2-Donor-Substituted Benzopyrans and Benzo-1,4-oxazines and Their Thermal Racemization). This chapter is interesting

among other reasons because it contributes to the linkage between photochromism and thermochromism. Thermal racemization of the separated enantiomers of chiral electron-donating substituted benzopyrans, and similarly substituted spirobibenzopyrans, spirobibenzoxazines, or spiro[indoline-benzopyrans or benzoxazines], and spiro[oxaindane-benzopyrans] is presented as a novel and convenient method for determining the barrier energies to thermal ring opening. A quantitative rationalization in terms of steric and, particularly, electronic effects is attempted. Various techniques used in the investigations include enantioselective liquid chromatography, enantioselective spectroscopy, polarimetry, and circular dichroism spectroscopy. This work offers a fundamental new outlook on transition states and on the relationship between the structures of photochromic compounds and the corresponding energy barriers of their ring-opening reactions. Chapter 7 (Structural Studies by X-ray Diffraction). The geometric parameters of photochromic compounds have been determined by X-ray diffraction on single crystals and yield interesting correlations with photochromic behavior (colorability, thermal bleaching rates, or absorption spectrum of the colored species). More intensive studies have been carried out on members of the spiropyran series (indolinospiropyrans), examining both closed spiranic forms and permanent merocyanines. The latter served as models of open photomerocyanines, the transient species produced by UV irradiation of spiropyrans. The investigations by X-ray diffraction have been extended to other series such as fulgides and more recently developed series such as spriooxazines, 2Hchromenes and perimidine derivatives. Chapter 8 (New Trends in Raman Studies of Organic Photochromes). This chapter presents an overview of past and recent vibrational data on spiropyrans and spirooxazines, obtained by various Raman approaches.

Introduction

7

Fundamental questions related to the electronic configuration of the open or

colored forms and the number and structures of the photomerocyanine isomers are considered on the basis of the results of continuous-wave (stationary) and timeresolved (picosecond, nanosecond, and millisecond) Raman experiments. For spironaphthoxazine photochromic compounds, the Raman spectra may be attributed to the TTC (trans–trans–cis) isomer having a dominant quinoidal electronic

configuration. Surface-enhanced resonance Raman spectroscopy (SERRS) is demonstrated as a new analytical method for the study of the photodegradation process in solution for nitro-BIPS derivatives. The development of this method could lead to the identification of the photoproducts in thin polymer films or sol-

gel matrices and ultimately to control of degradation. Chapter 9 (Biological Applications—Supramolecular Chemistry). Indepen-

dent of their easily observed photochromic behavior, characterized by their absorption spectra, photochromic compounds are interesting models for supramolecular chemistry involving selective host–guest recognition. This is especially

true for biological applications. Photochromic molecules have been used to develop photoresponsive systems suitable for mimicking and controlling biological processes. In this chapter, spiropyran derivatives are chosen to illustrate the photocontrol of enzyme activity. In another type of system, spiropyran compounds possessing molecular recognition sites such as crown ethers were prepared as alkali-(alkaline-earth) metal ion receptors and shown to have their binding affinities controlled by irradiation. This is an excellent example of the exploration of the changes in properties other than color that are brought about through the photochromic reaction. Chapter 10 (Thermochromism of Organic Compounds). This final chapter,

emphasizing the literature of the last 12 years, is devoted to organic compounds whose reversible color changes (coloring and fading reactions) result from

temperature changes (thermochromism). Examples of such thermochromic compounds are spiroheterocyclic compounds, Schiff bases and related nitrogencontaining molecules, bianthrones and other overcrowded ethenes, and miscellaneous compounds. Recent work in this field has mainly been directed toward understanding the thermodynamic, kinetic, structural, and mechanistic aspects of thermochromism rather than merely toward the discovery of new thermochromic

compounds.

5. CONCLUSION AND FUTURE DEVELOPMENTS Volume 2 of ‘Organic Photochromic and Thermochromic Compounds’ is focused on the most interesting and useful physicochemical methods employed in the

investigation of the photochromic behavior and structural properties of organic compounds. A short survey of thermochromic compounds (Chapter 10) is also

8

Introduction

included to demonstrate the similarities (and differences) between photochromism and thermochromism. This comparison is complemented by the discussion of chiral therrno- and photochromic materials (Chapter 6). The combination of photochromic spiropyrans with enzymes could lead to enzymatic systems with photoswitchable catalytic behavior (Chapter 9). The technical improvements in physicochemical investigations of photochromic behavior combined with the synthesis of new interesting models of photochromic substances are anticipated to lead to new advances in the knowledge of the very complex phenomenon that is photochromism, regardless or independently of the various suggested photophysical mechanisms for photocoloration.

REFERENCES 1. G. H. Brown (ed.), Photochromism Techniques of Chemistry, Vol. III), Wiley-Interscience, New York (1971).

2. H. Dürr and H. Bouas-Laurent (eds), Photochromism: Molecules and Systems, Elsevier, Amsterdam (1990). 3. C. B. McArdle (ed.) Applied Photochromic Polymer Systems, Blackie, New York (1992). 4. M. Fritsche, C. R. Acad. Sci., 64, 1035 (1867). 5. E. ter Meer, Ann. Chem. 181, 1 (1876). 6. W. Marckwald, Z. Phys. Chem. 30, 140 (1899). 7. O. A. Neumüller, Basis-Römmp, Franckhsche Verlagshandlung, Stuttgart (1977). 8. Y. Hirshberg, C. R. Acad. Sci, 231, 903 (1950). 9. R. Dessauer and J. P. Paris, in: Advances in Photochemistry (W. A. Noyes, G. S. Hammond, and J.

N. Pitts, eds.), Vol. 1, p. 275, Interscience, New York (1963). 10. G. Porter, Angew. Chem. 80, 882 (1968).

11. P. G. W. Norrish and G. Porter, Nature 174, 658 (1949). 12. G. Porter, Proc. Roy. Soc., Ser. A 200, 284 (1950).

1 Photochromic Polymers Kunihuro Ichimura 1.1. INTRODUCTION Photochromism proceeds, in general, in condensed phases and is affected markedly by the nature of the matrix. Polymeric materials play a very crucial role in studies on photochromism, in particular from a practical viewpoint since various applications require photochromic materials in the form of films, sheets, plates, fibers, beads, and so on. Photochromic molecules have usually been incorporated into polymer matrices by binding them covalently to polymer backbones or by dissolving or suspending them in polymer solids. The photochemical as well as thermal behavior of organic photochromic molecules is influenced in various ways by the characteristics of the polymeric media so that photochromic polymers have attracted extensive interest, leading to interdisciplinary research.1,2 Effects of polymer matrices are closely coupled with the regulation of photochromic reactions and with the emergence of novel photoinduced properties other than color changes, giving rise to versatile photofunctionality.2,3 In this respect, the behavior and properties of photochromic materials can only be reasonably envisaged by taking the structural features of matrices into account. One of the ways to understand the matrix effect is based on the stratum of orderedness of molecular structures of matrices, as summarized in Table 1.4 For the sake of convenience, a dimensional classification of various matrices is employed here. Matrices of each dimension possess their own optical properties. The differentiation between one-dimensional matrices like solution phases or amorphous polymer solids and ones of higher dimensions is based on whether the matrices display distinct phase changes. Molecular structural alteration of photochromic substances embedded in a zero-dimensional matrix brings about ordinary optical property changes including changes in light absorption, emission, and refraction. On the other hand, matrices exhibiting phase separation belonging to the Kunihuro Ichimura • Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Yokohama 226-8503, Japan. Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert

Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999.

9

10

Chapter 1

one-dimensional category can exhibit a reversible modification of light scattering whereas photochromic reactions carried out in two-dimensional matrices like liquid crystals and stretched polymer films can lead to changes in birefringence. Changes in optical rotary power can be induced in a cholesteric mesophase, a three-dimensional matrix. The major aim of this chapter is to review the matrix effect of polymers on photochromism and the versatile photofunctionality induced by photochromic reactions in polymer matrices. It should be noticed here that the matrix effect has a dual meaning: the effect of the matrix on photochromism and the effect of photochromism on the matrix. In the former case, the rate of photochromism and photoinduced optical property alterations are markedly affected by polymer matrices, in particular when photochromic molecules are embedded in polymer solids. This kind of matrix effect was reviewed by Krongauz,2 who discussed the photochromism of spiropyrans and azobenzenes in macromolecules. The latter case involves the reversible changes in the properties of polymer materials triggered by photochromic reactions, as listed in Table 1. As indicated previously,3 photochromism in polymer materials induces a great number of property changes involving not only optical characteristics, but also chemical, electrical, and bulk properties. In order to avoid too much diversification, the discussion in this chapter is focused primarily on optical properties of polymeric materials. Photochromism in amorphous solids as a zero-dimensional phase is treated in Section 1.2 while photoinduced birefringence as well as dichroism is discussed in Section 1.3, focusing on the photochromic reactions in liquid-crystalline polymers as twodimensional phases but also briefly considering photochromism in cholesteric phases as three-dimensional phases and the photoinduced modification of circular

Photochromic Polymers

11

dichroism or optical rotary power. The specific topic of photocontrol of the surface-assisted alignment of liquid crystals is presented in Section 1.4 to depict the molecular amplification of structural transformations of photochromic molecules localized at uppermost surfaces in liquid-crystal cells.

1.2. PHOTOCHROMISM IN POLYMER MATRICES 1.2.1. Synthesis

This section deals predominantly with polymers substituted with photochromic molecules displaying the 4n + 2 type of electrocyclic reactions since the photochromism of spiropyrans and related compounds continues to attract extensive interest. A large number of papers have been published on the synthesis of polymers bearing 6-nitrospirobenzopyrans (SP) (1 Figure 1; arrows indicate the positions of vinyl groups) in their side chains because of their distinct color change upon photoirradiation. One of the conventional ways to obtain polymers bearing spiropyrans is via the radical vinyl polymerization of the corresponding monomers.2 An acryloyl or methacryloyl residue is introduced usually at the 1´- or 8-position through an alkylene spacer to be subjected to homopolymerization or copolymerization with suitable vinyl monomers.5–7 In connection with the applicability of photochromic polymers in the fabrication of erasable optical recording materials written by a diode laser, the introduction of spirothiopyran (SSP) units (2) has been attracting interest as a means of producing polymer thin films which absorb longer-wavelength light after ultraviolet (UV) irradiation owing to the formation of the merocyanine structure (3) (Figure 1). A variety of spirothiopyrans were synthesized and dissolved in a mixed solvent of cyclohexane and tetrahydrofuran (THF) together with a vinyl chloride/vinylidene chloride copolymer to prepare a photosensitive layer.8,9 UV light irradiation brought about dark-green coloration, which was stabilized by the introduction of a nitro group at the 6-position. The same unit was introduced onto poly(methacrylate) chains at the 8position. Photochromic monomers (4) prepared according to the scheme shown in Figure 110 (see Appendix for details of the synthetic procedure) were copolymerized with methyl methacrylate through radical as well as ionic polymerization.11 While the polymers (5) showed normal photochromism in the solution phase, UVirradiated films retained a deep green color in the dark after partial decoloration at an initial stage. In particular, when the nitrogen atom of the indoline moiety was substituted with an octadecyl group the photocolored state (6) exhibited remarkable thermal stability upon prolonged storage in the dark. In contrast, the colored form of the corresponding monomer dissolved in a poly (methyl methacrylate) (PMMA) film faded gradually even at room temperature. Studies of the kinetics of the thermal reverse process revealed that the Arrhenius plots for a copolymer film

12

Chapter 1

showed a distinct break point at 54°C. This temperature may correspond to the transition temperature attributable to the local mode relaxation of polymer main chains. A long alkyl substituent at the nitrogen atom of the indoline moiety enhances the effect of local mobility of polymer chains on the thermal reversion. Negative photochromic polymers were obtained by introducing a spirobenzoselenazolinobenzopyran (SSeP) moiety onto PMMA backbones. Monomers (7)

Photochromic Polymers

13

prepared12 as shown in Figure 2 were subjected to radical polymerization, yielding atactic polymers, and to ionic polymerization using phenyl magnesium bromide, yielding an isotactic copolymer (8).13 Both the monomers and the copolymers displayed negative photochromism. It is noteworthy that of the thermally stable colored form was dependent on the tacticity: for an atactic polymer having 0.23–0.24 mol % of SSeP, was at 579 nm in chloroform, whereas the absorption maximum of isotactic PMMA having 0.10 mol% of SSeP was at 597 nm. Although visible-light irradiation of the SSeP polymers bleached the blue-purple color to yield the colorless spiro form, a partial thermal reversion (8– 17%) occurred in the dark. After the partial coloration in the dark, no essential change in the absorption spectrum occurred upon prolonged storage at room temperature. The Arrhenius plots for the thermal coloration indicated that the rate

14

Chapter 1

changes discontinuously at the in a similar manner as in the case of SSP copolymers. The extent of the thermal isomerization was again influenced markedly by the tacticity of the PMMA. Although 85% of the colorless form was fixed in the dark in a film of the atactic PMMA bearing SSeP after irradiation with visible light, only 24% of the spiro form was restored in the case of the isotactic PMMA. Further investigation of the effect of the tacticity on the photochromic processes is required. An acryloyloxy residue was introduced at the 3-position of a benzothiazolinospiropyran (BSP) through a spacer to prepare photochromic vinyl polymers (9) (Figure 2).10 Interestingly, polymers having BSP units attached through a phenylene spacer (9a) display no photochromism even in solution, probably because of the severe suppression of the mobility of the photochromic units due to the rigid p-phenylene group. UV irradiation of a film of copolymers incorporating BSP units attached through a flexible spacer (9b) resulted in deep-green coloration due to the formation of an open form which has an absorption maximum at ca.

690 nm.14 The thermal decoloration of these polymers in solution was markedly slower than that of a low-molecular-weight model compound, indicating that steric effects play an essential role in this type of attachment mode. This was in contrast to the photochromisc solution of polymers bearing 6-nitrospiropyrans (1) linked to the polymer backbones at their 1´, 5´-, or 8-positions; the decoloration of photoirradiated solutions of these polymers took place at a rate quite similar to that for the corresponding model compounds. The thermal reverse isomerization of the photomerocyanine of BSP in a film deviated from first-order kinetics in a manner similar to the behavior of films of polymers having spiropyran pendant groups. In ternary copolymers consisting of BSP monomer, methyl methacrylate,

and methacrylic acid, the decoloration rate decreased with an increase in the content of methacrylic acid. Spirofluorenylindolizines (10)15 displaying the 4n + 2 type of electrocyclic reactions, similarly to spiropyrans, were introduced into PMMA chains for kinetic analysis (Figure 3).16 The monomer (11) was prepared by the condensation of 2methacryloyloxymethylpyridine and the corresponding spirocyclopropenefluorene

Photochromic Polymers

15

compound according to Dürr’s procedure.15 The momoner was copolymerized with ethyl methacrylate to give a polymer containing 1.1 mol% of the photochromic unit with The rates of decoloration of the polymer in solution obeyed strictly first-order kinetics and agreed well with those of a lowmolecular-weight model compound. The thermal decoloration in a polymer solid was much slower than in solution; in films, the copolymer covalently linked with the photochromic unit faded about 10 times more slowly than the model compound dissolved in poly(ethyl methacrylate). Below the of the copolymer, plots of the logarithm of the decrease of optical density against reaction time did not exhibit a linear relationship and could be expressed as a sum of three firstorder processes. All these findings led to the conclusion that the kinetics of spirofluorenylindolizines were quite similar to those of spirobenzopyrans. Because photochromic reactions in polymer matrices deviate in general from first-order kinetics of both photochemical and thermal isomerization below the (vide infra), polymers with lower than room temperature are of interest in order to perform photochromic reactions smoothly even in the solid state. In this respect, polysiloxanes17 and polyphosphazenes18 with flexible main chains are attractive.

In the application of photochromic substances to optical memory media, the photosensitive layer should be as thin as possible while maintaining a sufficient optical density so that the optical readout can be made with high accuracy. This means that the most efficient recording thin film can be produced solely with photochromic compounds without the use of polymeric matrices, which act like diluents. Some attempts have been made to fabricate photochromic thin layers by vacuum deposition of photochromic compounds on a substrate plate.19–21 The attachment of long alkyl residues to a spiropyran framework brought about a marked suppression of the crystallization of vacuum-deposited thin films, thereby allowing the photochromic reaction to proceed effectively.20 It was pointed out that photochromic vacuum-deposited films seem to be problematic for practical applications owing to poor mechanical strength and the degradation of photochromic properties due to gradual crystallization. A novel technique to overcome these drawbacks of spiropyran molecular films was offered by the plasma polymerization of a conventional spiropyran, 6-nitrospiro[indoline-benzopyran].22 When compared with a vacuum-deposited thin film having a multigranular surface due to crystallization, a plasma-polymerized thin film showed a smooth surface and did not exhibit deterioration of its photochromic properties. 1.2.2. Matrix Effect on Photochromism 1.2.2.1. Spiropyrans and Related Compounds In his excellent review,2 Krongauz has summarized the effects of polymer matrices on spiropyran photochromism as reported in papers appearing before

16

Chapter 1

about 1987. The matrix effects investigated were predominantly those on the thermal cyclization of photomerocyanines which were dissolved in or attached covalently to polymer chains. In solution, the thermal reversion of methacrylate polymers substituted with spirobenzopyrans was often accompanied by a spectral shift of the photomerocyanines and a deviation from first-order kinetics. These observations were ascribable to intermolecular interaction between the merocyanine form and the

ester residue of methyl methacrylate. This was supported by the emergence of photoviscosity effects in solutions of these photochromic polymers. The conclusions drawn from studies of the thermal reversion of low-molarmass photomerocyanines derived from 6-nitrospiropyrans in polymer solids can be summarized as follows according to Krongauz’s review12: 1.

The decoloration deviates from first-order kinetics below the glass transi-

tion temperature

although the reaction obeys first-order kinetics above

2. The complex decoloration kinetics were attributed to nonhomogeneous distribution of free volume and not to the isomerism of the merocyanine form. 3. The decoloration can be fitted with the Williams-Lamdel-Ferry (WLF) equation, which is consistent with the fact that the lower the temperature, the longer was the time for chain rearrangement. 4. The spectral shift observed during the decoloration was explainable in terms of the aggregation of spiropyrans in polymer solids. 5. The kinetic behavior of polymers containing dissolved spiropyrans was not far different from that of bulk polymers to which the spiropyran moiety was covalently linked. 1.2.2.2. Azobenzenes Although the photochromism of azobenzenes is not accompanied by a distinct

color change owing to the overlapping of the absorption bands of the E- and Zisomers, the reversible geometrical photoisomerization gives rise to the versatile photofunctionality of polymeric materials incorporating azobenzenes. This is mainly due to the alteration of chemical as well as physical properties induced by changes in the molecular structure of the azobenzenes.23 The ready availability and relatively good photofatigue resistance of this kind of simple chromophore may attract extensive interest from a practical standpoint, which may in turn stimulate fundamental studies on the effect of polymer matrices on the photochromic behavior. Since the early study by Paik and Morawetz24 on the E–Z thermal isomerization of azobenzenes attached as side groups in polymers, a number of papers

Photochromic Polymers

17

have been devoted to elucidation of the kinetic behavior of azobenzene chromophores dissolved in or attached to various polymers in the solid states.25,26 In general, the E–Z photoisomerization of azobenzenes in solid polymer matrices consists of two processes and has been modeled by the sum of two exponentials.27,28 One process, occurring at an early stage of the photoreaction, was found to be as fast as in a dilute solution and was followed by a much slower process. The contribution of the fast process decreases with physical aging29–32 and increases with plasticization and with an increase in temperature. The thermal reversion of azobenzenes in the side chains of polymers obeys first-order kinetics in solution and above . In contrast, below the thermal relaxation deviates from first-order behavior and comprises an anomalously fast component at the beginning, due to the strained conformation of the Z-isomer trapped in a rigid matrix, followed by a slower process.33–40 The enhancement of the thermal rate at the beginning is rather a general phenomenon for azobenzenes regardless of whether the chromophores are of the ‘azobenzene type’ or the “pseudo-stilbene type” having a couple of push–pull substituents.41 Usually, the isomerization kinetics have been studied using polymeric systems containing a

small amount of azobenzenes in order to extract information on the monomolecular behavior in solid matrices. Barrett et al.42 analyzed the thermal reversion of a blend of a homopolymer having Disperse Red residues (12) with PMMA and a copolymer with MMA (13) (Figure 4). The results were that, after the fast relaxation, the thermal isomerization proceeds according to a sum of three exponential processes corresponding to three kinds of triads, DDD, DDM and

18

Chapter 1

MDM, where D and M denote the Disperse Red monomer and MMA, respectively.

In contrast to the first-order kinetics of the conventional azobenzene isomerization in solution, a slight deviation from first-order kinetics was observed even in aqueous solutions for polyelectrolytic azobenzene polymers substituted with hydrophobic alkyl residues (14, Fig. 4).43 In an aqueous solution of the amphiphilic polysulfonates containing a small amount of azobenzene moiety, the extent of E-to-Z photoisomerization under UV irradiation was reduced as a result of the compartmentalization of the azo chromophores in a hydrophobic core, which resulted in motion of their restriction.

1.2.2.3. Viologens Viologens are among the chromophores exhibiting photochromism based on reduction–oxidation cycles. Light irradiation of viologens generates, under suitable conditions, colored cation radical species, which readily revert to the colorless viologens upon exposure to oxygen. Some efforts have been made to synthesize polymers incorporating covalently linked viologens in order to obtain improved photochromic properties, including the elongation of the lifetime of the colored cation radicals. It was observed that the coloration rate of viologens upon exposure to UV light was enhanced markedly by physically embedding the dicationic molecules in aprotic polymer solids like poly(N-vinyl-2-pyrrolidone).44 Polymers substituted with viologens units were prepared by copolymerization of styryl derivatives (15) or methacrylates (16) bearing a viologen residue with polar monomers (Figure 5).45 The rate of color development upon photoirradiation under aerobic conditions was influenced by the nature of the comonomers and decreased in the order N-vinylpyrrolidone N,N-dimethylacrylamide > acrylamide > 2-hydroxyethyl acrylate. The effect of N-vinylpyrrolidone was remarkable. This can be interpreted in terms of the destabilizing effect of protic units on the viologen ion pair and the stabilization of the photogenerated cation radicals by aprotic polar units. This means that both effects facilitate the forward electron-transfer reaction producing the coloration. Polymers incorportating viologen units in their main chains were also prepared (Figure 6; see Appendix for details of the procedure).46,47 The effects of counterions and additives on the photochromism of viologen ionene polymers

Photochromic Polymers

19

were investigated. Although bromide anion did not induce any coloration upon UV irradiation, the addition of poly(N-vinylpyrrolidone) generated a green coloration.46 The photochromic behavior of poly(tetrahydrofuran) incorporating viologen units in the main chain was studied in detail.47 A polycationic polymer (17) having the tetrakis [3,5-bis(trifluoromethyl)phenyl]borate anion (TFPB–) as counterion displayed broad absorption bands due to an ion-pair charge-transfer complex between the bipyridinum moiety and the anion. Photoirradiation in vacuo with light exciting the charge-transfer band (>365 nm) resulted in the formation of colored species ascribable to cation radicals with blue coloration. A schematic representation of photoinduced electron-transfer processes is presented in Figure 7.

20

Chapter 1

The photogenerated colored state was rather stable below 0°C and disappeared upon heating owing to the back electron transfer. The coloration took place very rapidly and was complete within 20 ps.48 1.2.2.4. Fulgides Fulgides and related diarylethenes have been investigated extensively because of the long-term thermal stability of their photocyclized colored forms, which could lead to this application in erasable optical recording materials and photoswitchable optical elements. The entrapment of these photochromic molecules in polymer films is necessary for these practical applications. Picosecond laser photolysis was employed to study the electrocyclic reaction of a furylfulgide (18, Figure 8) in polymer solids and revealed that the colored structure was formed with a time constant of ca. l0ps irrespective of the nature of the polymer matrix.49 The photofatigue resistance of a series of fulgides (19–24, Figure 8) has been studied not only in relation to the molecular structure, but also by focussing on the effect of the polymer matrix.50 The reversibility upon alternate irradiation with UV and visible light in toluene solutions was in the order 20 = 23 > 22 > 24 > 19. The photochromism of these fulgides was examined in various polymers including PMMA, poly(styrene) (PSt), polycarbonate (PC), and a copolymer of styrene with maleic anhydride (StMA). The concentration in the polymers was 20wt%. As well as having good photofatigue resistance, 21 and 22 exhibited good reversibility in PSt and PC films whereas the reversibility was reduced in PMMA and StMA. The effect of the polymer matrix on the reversibility has not been elucidated. The thermally induced ring-opening reaction of the colored forms was also investigated in polymer matrices. The results showed that the thermal stability of the closed form cannot be estimated from the solution behavior and was dependent on the nature of the polymers. The difference in the thermal stability of the colored forms of fulgides between solutions and polymer solids

Photochromic Polymers

may arise mainly from the polymer structure and the concentration factor. The higher concentration of fulgides in polymer solids may cause or enhance irreversible side reactions probably based on bimolecular processes. 1.2.2.5. Photochromism in Sol-Gel Inorganic Matrices The sol-gel process is a novel technique for the preparation of various inorganic oxide glasses at low temperatures without melting.51 They are produced by condensation-polymerization of a suitable metal alkoxide. For instance, the solgel process of tetraethyl orthosilicate (TEOS) consists of the hydrolysis reaction

and the condensation reaction

21

22

Chapter 1

The sol-gel process involves two main steps. In the first step, metal alkoxides are hydrolyzed at a low temperature to form a microporous glass (xerogel) with a large surface area. In the second step, the porous glass is annealed at an elevated temperature in order to give a shrunken, nonporous glass. Because of the mild preparative conditions, organic molecules can be embedded in inorganic networked matrices, including silica glasses, by the sol-gel process without degradation of their structures or deterioration of their functionality.52-55 Some photochromic compounds, including a fulgide56 and a spiropyran,57 have been incorporated in sol-gel matrices. Two methods have been employed to obtain metal oxide glasses doped with organic molecules. The first consists of putting a thin layer of sol-gel solution on a substrate to form a thin film, followed by drying for a relatively short time. On the other hand, thick bulk glasses are formed by the second method, in which a sol-gel solution is very slowly and carefully evaporated at ambient temperature to give a monolithic and crack-free glass.

Detailed studies of the matrix effect of sol-gel silica glasses on the photochromism of azobenzenes were conducted to elucidate the microenvironmental characteristics of the inorganic substances.58–60 In sol-gel films, hydrogen bonds were formed between the azo groups of azobenzene probe molecules and silanol groups of the inorganic glass, resulting in a marked blue shift of the transition absorption band. In particular, the intensity of the absorption band due to the forbidden transition of the E-isomer was enhanced markedly in the silica matrix, when compared with its intensity in the spectra of azobenzenes in solution as well as in organic polymer solids such as a PMMA film (Figure 9). The increase in the absorption coefficient can be reasonably explained in terms of the distortion of the azobenzene moiety due to its strong adsorption on the walls of the porous silica, which leads to the mixing of forbidden n-orbitals with allowed p-orbitals.59 The photochemical and the thermal isomerization of azobenzenes in sol-gel films were influenced by the water content of the sol-gel solution.58 On the basis of microenvironmental conditions, the photochromic molecules were classified into three types with different isomerization behavior. Some of the organic molecules were embedded in the sol-gel matrix and exhibited quenching of the photoisomerization while others that underwent photoisomerization exhibited enhancement of the Z-to-E thermal reversion process owing to a strained conformation, just as in the case of the organic polymer solids described above. The third group of azobenzene molecules exhibited retardation of the thermal isomerization because of hydrogen bonding between the guest molecules and silanol groups. The photochromism of azobenzene probe molecules tells us furthermore that bulk materials differ distinctively from silica films.59 The pores in the sol-gel bulk materials were larger than those in the corresponding sol-gel films. The first-order kinetics of the thermal isomerization of the photogenerated Z-isomer revealed that the steric effects restricting the molecular motion increased in the order; sol-gel

Photochromic Polymers

23

bulk < PMMA < sol-gel film. It should be stressed that the thermal back isomerization of an azobenzene entrapped physically in a silica bulk material completely obeyed first-order kinetics quite similar to solution kinetics, in sharp contrast with the fact that the thermal reactions of photochromic guests usually deviated from first-order plots in polymer solids at temperatures below as mentioned above. This suggests that the photochromism of molecules entrapped physically in bulk silica materials is not subject to the suppressive effect caused by silica matrices. The difference in the matrix effect of silica between sol-gel films and monoliths has also been observed for the photodimerization behavior of anthracenes entrapped in silica solids.61 Since the photochromism in a sol-gel silica matrix is affected at least partially by the nature of the walls of the pores, the photochromism of molecules absorbed on a silica surface seems to be of interest. In order to investigate this, colloidal silica with a diameter smaller than light wavelengths (150nm) was employed because a suspension of tiny inorganic particles in an organic solvent with an appropriate refractive index affords an apparently transparent solution that allows one to monitor the photochromic reactions of azobenzenes60 and spiropyrans simply by UV-visible spectroscopic measurements.62,63 In the case of nitrospiropyran, the normal photochromism was observed when a colloidal silica surface to which the photochromic residues were covalently linked through an alkylene spacer was dispersed in organic solvents. A peculiar aspect of the photochromic silica particles was the reversible photocontrol of their dispersibility in organic sovlents.62 UV irradiation of the photochromic particles dispersed in carbon tetrachloride induced floculation, leading to sedimentation of the colloidal silica. Subsequent visible-light irradiation redispersed the particles.

24

Chapter 1

1.2.3. Polarization Photochromism in Polymer Solids 1.2.3.1. Background

The interaction of chromophores with linearly polarized light causes dichroism because of the axis-selective photochemical conversion, bringing about the

so-called Weigert effect.64 Michl65 discussed the potential application of photochromism based on the molecular orientation induced by linearly polarized light as photomemory principle and pointed out the following requirements for photochromism based on molecular orientation. The chromophoric units must have anisotropic absorption, be alignable with linearly polarized light, and be prevented from spontaneously losing their alignment by random thermal motion on the observation time scale. Michl presented some examples involving destructive photoorientation, in which photoactive molecules having a transition moment parallel to the electric vector of polarized actinic light were consumed selectively, resulting in dichroism. In another system polarized-light-induced dichroism was caused by nondestructive orientation by photoinduced molecular rotation. Although Michl claimed in his review article65 that samples exhibiting photoinduced reorientation of this type had not yet been found, there have been reports that the photoirradiation of photochromic molecules in polymer films with linearly polarized light brings about molecular reorientation. In particular, intensive work has been carried out on the linearly-polarized-light-induced molecular reorientation of photochromic molecules embedded in polymer matrices. This type of photochemical process is referred to here as polarization photochromism. This section deals predominantly with photoinduced molecular reorientation in amorphous photochromic polymer solids; other systems such as liquid-crystalline polymers are considered in Section 1.3.2 because each system has its own characteristics. 1.2.3.2. Polymers Doped with Photochromic Compounds When a PMMA film doped with the cyclized form of a furylfulgide was illuminated with linearly polarized Ar laser light at 514nm, photoselection took place, resulting in an axially selective ring-opening reaction leading to photoinduced, dichroism. This axially selective photoreaction, leading to transient optical dichroism, probably involved destructive orientation which was randomized by heating the film at a temperature above the of PMMA.66 If photochromic molecules in a polymer solid are exposed to polarized light which is absorbed by both chemical species participating in the photochromic reaction, thus bringing about the forward and the backward reaction simultaneously, the photodichroism arises from the nondestructive mode of molecular reorientation. Irradiation of a PMMA film containing molecularly dispersed 6-nitrospirobenzopyran with linearly polarized UV light, which triggers both the

Photochromic Polymers

25

forward and backward reactions, caused the dichroism of a photomerocyanine.67–69 The extent of photoinduced dichroism depended on the nature of the photochromic compound and on the composition and viscosity of the

polymeric binder. p-Dimethylaminoazobenzene molecularly dispersed in a PMMA film similarly displayed dichroism after irradiation with linearly polarized light. When the actinic light was turned off, the photodichroism disappeared due to the rotational randomization of the azo molecules. Since the thermal randomization reflects the molecular mobility in polymer solids, the photoselection induced by irradiation of a spirobenzopyran with linearly polarized light was

used to monitor free volumes of polymer matrices.70,71

One of the applications of polarized photochromism in polymer solids is polarization holography. Todorov and co-workers72,73 employed a film of poly(vinyl alcohol) (PVA) doped with water-soluble azo dyes like methyl orange and methyl red for the formation of a holographic grating. They employed a PVA film incorporating the azobenzene dyes as a representative photoactive layer which

they exposed to an Ar laser beam. It was observed that the film demonstrated dichroism during the laser irradiation as a result of the molecular reorientation of azo-dye molecules. The photodichroism was accompanied by photoinduced birefringence which resulted in the formation of diffraction grafting upon holographic exposure with an Ar laser light. The photodichroism disappeared relatively rapidly in the dark (Figure 10). In contrast to this, the disappearance of the photoinduced birefringence and hence the diffraction efficiency took place only partially: in the dark at room temperature, approximately half of the birefringence was retained for a long time. This probably suggests that the PVA

26

Chapter 1

chains were deformed in accordance with the photoinduced ordering of dye molecules, resulting in the relatively stable birefringence.73 1.2.3.3. Polymers with Covalently bound Photochromic Units In contrast to polymer solids physically containing photochromic molecules, the covalent binding of photochromic units to polymer backbones results in the marked stabilization of the molecular orientation induced by irradiation with linearly polarized light with implications for the fabrication of optical recording media and various optical elements. As discussed in Section 1.3, liquid-crystalline polymers substituted with azobenzene moieties show marked photobirefringence

owing to the reorientational ordering of mesogens. Amorphous polymers bearing azobenzenes also display photodichroism and photobirefringence and have been attracting increasing interest. Tredgold et al.74 prepared photosensitive polymers bearing azobenzene units

(25, Figure 11) with varying degrees of substitution (50%, 70%, and 90%) by the reaction of a styrene–maleic anhydride copolymer of MW=1600 with the corresponding azobenzenes substituted with a hydroxyl group in the presence of sodium hydride, followed by quenching with methanol. The polarized-light irradiation of films was carried out at 70°C, followed by cooling down under

continuous polarized illumination. There was a distinct difference in the emergence of photobirefringence between the 50% and 70% loaded polymers, on the one hand, and the 90% loaded polymer, on the other. The former polymers acquired lower birefringence while the latter showed relatively large birefringence. Thermal treatment of these polymer films after polarized illumination disclosed a very interesting behavior. Whereas the birefringence of both 50% and 70% loaded polymers was lost upon annealing at 70°C for a while, the birefringence of the 90% loaded polymer film was instead enhanced and reached a large birefringence This anomalous behavior of the 90% loaded polymer film may arise from a cooperative effect between the azobenzene-containing side groups of the polymer, which demonstrates a polycrystalline texture. Owing to the crystalline nature, side groups having photoinduced E-configuration align themselves parallel to existing E-forms to enhance the birefringence. A closely related phenomenon was observed for a p-phenylenediacrylate polyester incorporating a Disperse Red (p-nitro-p´-aminoazobenzene moiety) side group (26, Figure 11) as a nonlinear optical unit.75 The polyester was first investigated by Shi et al.,76 who reported that large birefringence was induced by irradiation with linearly polarized light. Prolonged irradiation with 488-nm light with large exposure energies of more than several hundred joules per square centimeter resulted in a birefringence of which was quite thermally stable. According to Shi et al., the thermal stability of the molecular ordering of the films was ascribed to the photoinduced cross-linking of the p-phenylenediacrylate units via a [2 + 2] cycloaddition. On the other hand, Natansohn et al.

Photochromic Polymers

27

claimed on the basis of infrared (IR) spectral measurements that there was no loss of the double bond even after prolonged irradiation and concluded that the large birefringence of the polyester film was enhanced by the physical reorientation of p-phenylenediacrylate units, which were rather photochemically inactive, induced by the molecular orientation of the Disperse Red units resulting from irradiation with linearly polarized light. The photoselection in azobenzene polymers (Figure 11) is rather a general phenomenon.77–84 Systematic studies have been conducted by Natansohn and coworkers on the photoinduced molecular reorientation of azobenzene units to induce birefringence and dichroism of thin films of acrylate amorphous polymers with Disperse Red (DR) pendant residues, which have been attracting extensive

28

Chapter 1

interest because of their applicability to second-order optical devices.77,78 The birefringence was generated by irradiation with a linearly polarized Ar laser beam and erased by irradiation with a circularly polarized laser beam. The authors proposed that this could serve as a principle of rewritable optical memory. The investigation of this system was concerned not only with its practical applicability, but also with the dynamics of photochromic residues in cooperation with amorphous polymer backbones. A schematic representation of the birefringence changes is presented in Figure 12. A homogeneous sample film was placed between a couple of polarizers so as to be illuminated with linearly polarized light. The actinic light caused an increase in the birefringence. When the light was turned off, a rapid decrease in the birefringence to about 60% of the saturation value occurred. This indicates that some of the photoaligned molecules did not retain their orientation in the long term, but a significant number of the chromophore residues remained unchanged such that the birefringence persisted for several months. A comparison has been made between the reorientational behavior of a series of copolymers of DR with MMA (13, Figure 4) and that of series of blends of a DR homopolymer (12) with PMMA to elucidate the sequential effect of the DR monomer units on the orientability caused by intramolecular interactions between dye chromophore groups in polymer solids.79 The maximum birefringence induced by linearly polarized light for the blends80 increased linearly with an increase in the DR fraction. In contrast, the copolymers showed a nonlinear

Photochromic Polymers

29

relationship between the maximum birefringence and the increase in the azo fraction. This difference in the effect of azo content on the birefringence has been discussed in terms of the structure of triads of the polymers.81 The linearity of the birefringence as a function of the azo content of the polymer was attributable to the same triad (DDD). Copolymers were composed of three kinds of triads. DDD, MDD, and MDM, where D and M represent the azo unit and the MMA monomer unit, respectively, so that the microenvironment of D was affected by the nature of the triads. Natansohn et al. suggested that the order of relative optical orientability was MDM > DDD > MDD. This means that the isolated azo group of the MDM triad has the highest weighted contribution to the maximum birefringence. These authors also analyzed both the writing curve, reflecting the emergence of photobirefringence, and the thermal relaxation curve after switching off of the actinic light as sums of fast and slow processes, using biexponential fitting equations. The reorientation of azobenzene molecules induced by linearly polarized light is the result of a reversible geometrical photoisomerization, as mentioned in several papers. Azobenzene molecules in the E-configuration with a transition moment parallel to the electric vector of the actinic polarized light absorb photons preferentially and undergo photoisomerization to give the Z-counterpart. Because the Z-isomer can be converted by actinic light back to the E-isomer, some of the Eisomers have a chance to suffer from the orientational rearrangement of molecules with a molecular axis perpendicular to the polarization axis of the actinic light (Figure 13), leading to the molecular reorientation. Since the photoisomerizability in polymer solids was influenced by the free-volume distribution, the efficiency of photodichroism as well as photobirefringence should be affected by the size of the azo units. The photoisomerization of azobenzenes occurs through two mechanisms: the inversion mechanism and the rotation mechanism. The estimated sweep volumes for the isomerization of azobenzene and 1-azonaphthalene are presented in Table 2.85 In this context, the photochemical behavior of a polymer with pendant DR groups was compared with that of a polymer (27, Figure 11) with the pendant azo-groups, which possess a naphthyl instead of a phenyl residue.82 Both the photoinduced birefringence and the thermal relaxation were again treated as sums of fast and slow processes, and biexponential functions gave a good fit. The NDR-loaded polymer with the bulkier substituent (27) exhibited a slower rate of photobirefingence. On the other hand, there was no significant difference in the maximum birefringence and in the thermal relaxation rate between the two polymers. Since the photoinduced molecular reorientation of azobenzenes was stabilized by the rigid polymer network, this in turn meant that in order for linearly polarized light to give rise to efficient photoselection, a large exposure energy was required. As mentioned above, the polyester bearing DR units (26) required an exposure energy of several hundred millijoules per square centimeter while the

30

Chapter 1

photobirefringence leveled off after 2s of irradiation with 514 nm Ar laser light at an intensity of 0.7 mW/cm2.76 Similarly, the photodichroism of a thin film of poly(4-methacryloyloxyazobenzene) (28, Figure 11) was caused by illumination with polarized blue light at an exposure energy of a few joules per square centimeter.83 However, the exposure energy was reduced by one order magnitude for 29, in which the azobenzene moiety is attached at the ortho-position.84 The marked enhancement of the photosensitivity with this type of attachment arises probably from the decrease in the sweep volume for the photoisomerization, by comparison with that in the case of the conventional head-on type attachment. The photoselection process resulting in the preferential distribution of azochromophores in their perpendicular orientation has been mainly performed by irradiation with visible (blue) light. It was confirmed by the attenuated total reflection method that polarized pumping UV light also caused the formation of the Z-isomer, again with perpendicular orientation, in a thin film of poly(L-glutamate) with azobenzene residues in the side chains.86

Photochromic Polymers

31

1.3. PHOTOCHROMIC LIQUID-CRYSTALLINE POLYMERS 1.3.1. Photochromism in Polymeric Mesophases

1.3.1.1. Nematic and Smectic Phases

Liquid-crystalline phases with two-dimensional order (Table 1) influence photochromic processes because their molecular orientation is reversibly altered by photochromic reactions owing to the marked structural changes of the photoactive molecules.3 Intensive interest has been focused on the photoinduced alteration of mesophases admixed with photochromic compounds, since photochromism can be transformed to the reversible modification of the optical anisotropy of liquid-crystalline states. This phenomenon is applicable to optical storage of information as nondestructive readout can be performed with light of wavelengths far from those causing the photochromic reactions. One of the typical phenomena induced by photochromism in mesophases is the photoinduced phase change of nematic liquid crystals doped with photochromic compounds such as azobenzenes. Mesophase changes are triggered by photoinduced transformations of the rodlike E-isomer of azo molecules into the Vshaped Z counterpart. The latter acts as an impurity to destroy the mesophase because of the drastic alteration in the molecular structure. For example, when a low-mass cholesteric liquid crystal was admixed with azobenzene, reversible modification of the cholesteric pitch was brought about upon alternate illumination with UV and visible light owing to the photoisomerization of azobenzene.87 Adiabatic phase changes between nematic and isotropic phases are other examples of photoinduced birefringent changes.88–96 Polymeric films demonstrating reversible birefringence alteration have been readily prepared by dispersing droplets of nematic liquid crystals with dissolved azobenzenes in an aqueous solution of poly(vinyl alcohol) to give cast films.97 Extensive studies on photochromic liquid-crystalline polymers have been made by Krongauz et al.2 Liquid-crystalline phases caused marked colour changes of poly(acrylates)98 and poly(siloxanes) substituted with spiropyran side chains upon UV irradiation owing to the aggregation of the photomerocyanines.99 In contrast, spirooxazines attached to liquid-crystalline polymer backbones displayed no aggregation and hence exhibited normal photochromism similar to that in solution.100 Fulgimides bound covalently to the side chains of nematic liquidcrystalline polymers also showed normal photochromism. There have been many papers concerning liquid-crystalline polymers having azobenzenes in their side chains (Figure 14) because of the mesogenic nature of the azo chromophore. Liquid-crystalline polymers incorporating azobenzene moieties in their main chains (30) have also been prepared.l02 A family of poly(acrylates) substituted with azobenzenes having chiral alkyl groups (31) showed smectic A as well as cholesteric phases.103 The photochromic behavior

32

Chapter 1

Photochromic Polymers

33

of azo chromophores in the side chains of polymers has been investigated using Langmuir–Blodgett (LB) films. These films exhibited phase changes as a result of the photoisomenzation.104 Systematic studies have been made by Tazuke and Ikeda and their co-workers on the photoinduced mesophase alteration of nematic liquid-crystalline polymers involving azobenzene copolymers (32),105,106 liquid-crystalline polymers doped with low-mass azobenzenes,107–109 and ternary mixtures of liquid-crystalline polymers, low-mass liquid crystals, and azobenzenes.110 UV irradiation of their

films induced the transformation of the nematic phase into an isotropic phase because of the formation of the nonmesogenic Z-isomer, just as in the case of lowmass liquid-crystalline systems. The photoinduced mesophase change is illustrated in Figure 15. The essential difference between low-mass and polymeric systems was in the efficiency of the photoinduced phase change. Although UV exposure triggers the phase alteration of low-mass liquid crystals even at room temperature,88-97 heating of the polymeric systems at a temperature close to the nematic–isotropic transition temperature was necessary to enhance the phase transformation. The photoinduced mesophase transformation of polymeric systems from a nematic into an isotropic phase was influenced by various factors including the molecular structure of the azo chromophores as well as that of the nonphotoactive mesogenic units, the molecular weight of the polymers, the spacer length between the mesogenic units and the polymer backbones, the content of azobenzene units, and, in particular, the temperature during photoirradiation.

34

Chapter 1

The order parameter of the polymeric mesophase plays an important role. A decrease in the order parameter resulted in an increase in the photoresponsiveness. It takes approximately 200ms at a temperature close to to complete the relaxation of the reorientation of the nematic phase of a liquid-crystalline polymer doped with a low-mass azobenzene into an isotropic state after the rapid photoisomerization (<10ns).110 Interestingly, the relaxation proceeded somewhat faster for the liquid-crystalline copolymers covalently linked with azobenzene residues in their side chains than for the low-mass liquid-crystalline systems containing azobenzenes.111 This was due to the presence of azobenzene units close to mesophasic residues so that the photoinduced structural conversion of azo chromophores was more effective in causing the reorientational motion of nonphotoactive mesogenic units. Thermal reversion was suppressed in a glassy

state, enabling photoimages written in as a photoinduced mesophase alteration to be retained for a long time at temperatures below The reverse isomerization was also affected by the nature of the mesophase; it took place more rapidly in a nematic phase than in a smectic one. This can be interpreted in terms of the difference in free volume, as discussed frequently for conventional polymeric matrices.112 Liquid-crystalline polymers with azobenzene side chains other than poly(acrylates) have also been used to make birefringent photoimages.113 The photoinduced flop of the polarization of ferroelectric liquid crystals showing a quick response was achieved by dissolving azobenzene moieties in the liquid crystals.114,115 Photocontrol of the switching behavior of ferroelectric liquid-crystalline polymers (33, Figure 14) was induced by admixing azobenzene derivatives.116 1.3.1.2. Cholesteric Phases In contrast to the observation that the cholesteric pitch of low-mass cholesteric liquid crystals was modified by the E–Z photoisomerization of doped azobenzene,

essentially no change in the pitch has been induced photochemically for polymeric cholesteric liquid-crystalline systems. However, an attractive optical phenomenon was caused by photochromism when cholesteric polymer liquid crystals were doped with photochromic guests. In general, dye molecules dissolved in cholesteric phases display liquid-crystal-induced circular dichroism (LCICD) owing to the three-dimensional phase. Since photochromism corresponds to the reversible interchange between two kinds of chromophores, reversible LCICD can be observed for cholesteric polymers containing photochromic molecules. When a nitrospiropyran was dispersed monomolecularly in a thin film of poly(L-glutamate) which had a cholesteric phase, UV irradiation to form a merocyanine structure resulted in the emergence of a large optical rotary power as a result of LCICD.117 Reversible transformation of the optical rotary power arose from alternate exposure of the film to UV and visible light to induce the photochromic reaction of the spiropyran. This kind of event has also been observed for

Photochromic Polymers

35

cholesteric liquid-crystalline polymers containing photochromic guests other than spiropyrans like azobenzenes and fulgides. Photopolymerization of acrylated cholesterol monomers in the presence of photochromic molecules afforded other systems exhibiting reversible LCICD.118 These systems have been proposed as optical recording materials affording nondestructive readout of optical information since the readout can be performed with the use of light of wavelengths far from those causing photochromic reactions.117 LCICD induced by photochromism has been also observed for a solution of polypeptides.119 Furthermore, even photoinactive chromophores like a diaminoanthraquinone moiety demonstrated reversible LCICD in a film of poly(Lglutamate) (35, Figure 14) when a dye molecule was covalently bound to an azobenzene chromophore to yield 34 (Figure 14), which was then dissolved in the cholesteric polymer matrix. The photoisomerization of the azobenzene moiety caused a reversible change in the LCICD in the spectral region corresponding to the diaminoanthraquinone residue.120,121 This behavior was interpreted in terms of the modification of the orientation of the anthraquinone dye molecule as a result of the structural change of the azobenzene moiety due to the photoisomerization. The

structure–ICD relationship was studied systematically using guest molecules

having an azobenzene moiety and the dye unit attached via different spacers.122 1.3.2. Polarization Photochromism of Liquid-Crystalline Polymers

As discussed in the previous section, polarized actinic light is selectively absorbed by photochromic molecules with a transition moment parallel to the electric vector of the light, resulting in the preferential formation of a perpendicular orientation which was retained in polymer solids at temperatures below Liquid-crystalline polymers exhibiting polarized photochromism are listed in Figure 16. The first report of polarization photochromism in liquid-crystalline polymers was made by Eich and Wendorff and their (co-workers,123,124 who achieved the generation of a holographic grating by exposure of a layer of a liquidcrystalline polymer having azobenzene side chains (36 and 37) to Ar laser beams at 488 run. Polarization holography based on mesophasic polymer systems has been reviewed.125 A photosensitive mesophase layer with a monodomain texture of good optical quality was obtained by the action of an aligning layer of a rubbed polyimide covering a substrate surface to give a homogeneous (uniaxially parallel) alignment, followed by holographic exposure. Liquid-crystalline polyesters bearing azobenzene side chains (38, Figure 16) were also applied to the formation of holograms with high resolution and excellent storage (stability.126 Holograms thus formed were thermally stable upon prolonged storage at an ambient temperature below and could be erased by heating to temperatures above or by light irradiation. Excitation of the transition of the azobenzene moiety with

36

Chapter 1

polarized visible light brought about the reorientation of the azo-chromophores to give photodichroism and photobirefringence.127 Consequently, azobenzene chromophores in a layer of a photoirradiated liquid-crystalline polymer (37) showed, for instance, an order parameter one order of magnitude greater (S = 0.56) than those of an amorphous polymer (S = 0.03). Polarized-light irradiation of a layer of another azobenzene liquid-crystalline polymer (39) with a homeotropic (perpendicular) alignment gave rise to the

Photochromic Polymers

37

emergence of birefringence due to a homogeneous orientation even at temperatures below In connection with investigation of the photoinduced reorientation mechanism, factors affecting the photobirefringence were examined.129 A remarkable temperature effect was found. Below photoinduced birefringence was only slightly influenced by temperature whereas an increase of the photobirefringence by an order of magnitude occurred above . Maximum birefringence was attained at temperatures about 30–35°C above . Heating to much higher temperatures resulted in a reduction of the birefringence, and no photoinduced birefringence was observed at the clearing temperature, with the formation of an isotropic melt. It should be noticed that the induction of optical anisotropy proceeded very slowly whereas a photostationary state was generated easily, implying that the molecular reorientation arose from repeated E–Z photoisomerization, causing the rotational diffusion of azobenzene chains. It was first recognized that exposure of liquid–crystalline polymers having azobenzene side chains (40, Figure 16) to linearly polarized light led to the selective reorientation of the azo-chromophores without any transformation of the orientational direction of nonphotochromic residues in the glassy state.130 On the other hand, there was a cooperative effect in the nematic phase of the polymer which resulted in the reorientation of both the azo units and the photoinactive mesogenic units as a result of photoselected rotational diffusion of the former residues in the liquid-crystalline side-chain polymer.130 Subsequent studies revealed that the reorientation of molecules in liquid-crystalline polymer layers induced by linearly polarized light could be classified into three types, as sketched in Figure 17.131 In case A, the reorientation is almost completely suppressed because of the very stable orientational order of the mesophasic matrix. This was the case for polymer 41. In case B, reorientation of the azobenzene residues occurs but does not influence the orientational order of nonphotochromic side groups. The polymer 42 mentioned above belongs to this case. In case C, exemplified by polymer 40, the reorientation of the photochromic moieties of the polymers causes a continuous reorientation of nonphotochromic mesogenic groups even below such that a large optical anisotropy is exhibited. Note that the structural difference between 40 and 42 is solely the spacer length in the mesogenic unit. The extent of molecular reorientation of nonphotochromic residues has been assessed by monitoring the stretching vibration of the CN group attached to the mesogenic units by polarized IR spectroscopy.132–134 Although a large optical anisotropy was generated by polarized-light irradiation of thin films of liquid-crystalline polymers bearing azobenzene side chains, the preparation of uniformly aligned films required a rather cumbersome procedure, whereas spin-coating of amorphous azo polymers provided a convenient and easy way to prepare thin films of excellent optical quality. The values of the photobirefringence were small for these polymers, as mentioned above (Section

38

Chapter 1

<

1.2.3.3). This situation led Stumpe and coworkers to achieve photoinduced reorientation of azobenzene chromophores attached to liquid-crystalline polymer backbones (43, Figure 16) using convenient film preparation techniques including the spin-coating procedure to fabricate amorphous films.134–137 Polarized-light irradiation of these amorphous films below caused a markedly large birefringence of indicating the physical reorientation of nonphotochromic mesogenic residues triggered by the photoinduced reorientation of azobenzene groups. Indeed, polarized IR spectroscopic measurements revealed that the anisotropic direction of the probe CN group attached to nonphotochromic mesogenic residues was approximately perpendicular to the electric vector of the polarized actinic light. This indicated clearly that the molecular reorientation of nonphotoactive units was induced by the polarization photochromism of azobenzene residues as a result of cooperative physical reorientation.

Photochromic Polymers

1.4.

39

SURFACE PHOTOCHROMISM FOR REGULATION OF LIQUID-CRYSTAL ALIGNMENT

1.4.1. Surface-Assisted Liquid-Crystal Alignment Control The orientation director (direction) of the molecular axis of liquid crystals relative to a surface of a substrate is dependent on the nature of the surface. The

most common way to achieve a homeotropic alignment involves the treatment of substrate surfaces with amphiphilic compounds or long-chain alkyl silylating reagents. A typical method yielding a homogeneous (unidirectionally parallel) alignment is based on the rubbing treatment of polymer thin films covering substrate surfaces. When a substrate surface is modified with photochromic molecules to alter the structure of the surface at the molecular level, the alignment of nematic liquid crystals can be performed by alternate irradiation leading to surface photochromism. This type of photoresponsive liquid-crystal cell was first reported in 1988.138 Typically, the photoinduced alignment change has been achieved on substrate surfaces with a surface density of one photochromic unit per square nanometer or less.139,140 Such photoactive surfaces have been called “command surfaces”139 since each photochromic unit brings about the reversible alteration of the alignment of about 104 liquid-crystalline molecules on it.141,142 Three kinds of photoregulation of liquid crystals using photochromic surfaces have been described, as illustrated in Figure 18. The first one, called “out-of-plane alignment” photoregulation, involves the reversible change between homeotropic and planar alignment triggered by the geometrical photoisomerization of azobenzenes,138,141,142 stilbenes,143 and esters.144 When UV actinic light was linearly polarized to lead to the polarization of photochromism of azobenzenes on surfaces, a planar alignment became uniaxial to give a homogeneous alignment, which reverted to the homeotropic alignment due to the back,isomerization.145,146 Slantwise exposure with nonpolarized UV light gave rise to a homogeneous alignment. 147 This is the second kind of photoregulation. The third kind, referred to as “in-plane alignment” photocontrol, is based on the surface polarization photochromism and has been so far achieved by irradiation with linearly polarized light to induce photochromic reactions of azobenzenes,148–51 spiropyrans,152,153 stilbenes,154,156 and cinnamates.156 Thin films of photochromic polymers plays a crucial role in commanding the alignment of liquid crystals. They have been fabricated by spin-coating, adsorption, or Langmuir-Blodgett deposition of the polymers on substrate surfaces. From a practical viewpoint, the methods for preparing command surfaces based on covering substrate surfaces with photochromic polymers are of great significance owing to the easy handling and the versatility of photoactive polymeric materials. Furthermore, the elucidation of the working principle of the alignment photoregulation assisted by photochromic polymer surfaces has provided a deeper understanding of interfacial phenomena, including friction, lubrication, wettabil-

40

Chapter 1

ity, adhesion, epitaxial growth, and bioadaptability, at the molecular level. In this respect, photochromic polymers have opened a novel route to photosensitive molecular systems exhibiting reversible alteration of birefringence and dichroism which can be visualized with the aid of a polarizer or a pair of crossed polarizers.

1.4.2. Out-of-Plane Alignment Photocontrol

1.4.2.1. Spin-Coated Films

By analogy with the surface modification of silica substrates, a thin film of poly(viny alcohol) (PVA) was subjected to surface-selective esterification with phexylazobenzene acid chloride or to acetal formation with p-hexylazobenzene aldehyde in benzene.157 The surface-modified PVA thin films were subsequently rubbed to fabricate liquid-crystal cells. Upon UV irradiation of a cell made by placing a nematic liquid crystal between a couple of plates covered with the modified PVA so that the rubbing direction of both films was parallel, the homeotropic alignment was converted into a homogeneous one (Figure 19a). A photoresponsive twisted nematic (TN) cell could be assembled by setting the rubbing directions of the two plates at 90° to each other (Figure 19b). These observations mean that two factors can contribute to the uniaxially oriented, homogeneous liquid-crystal alignment: the parallel reorientation due to the

Photochromic Polymers

41

formation of Z-isomers and the uniaxial alignment owing to the rubbing effect. Guest–host cells were assembled by dissolving dichroic dyes in liquid crystals to visualize photoimages with the use of a single polarizer. Alternate irradiation with UV and blue light leading to the photoisomerization of azobenzene side chains attached to vinyl polymers induced the reversible switching of the alignment of nematic liquid crystals between homeotropic and

42

Chapter 1

planar modes (Figure 18a) when liquid-crystal cells were made from a couple of transparent plates that were spin-coated with the azobenzene polymers.158,159 Two kinds of polymers have been prepared (Figure 20). Polymers of type (44) were synthesized by the esterification of PVA with an azobenzene acid chloride.158 The second type was prepared by the radical polymerization of acrylated or methacrylated azobenzenes (45).159 Acrylamide-type polymers were also prepared. It is noteworthy that azobenzene chromophores attached to side chains of vinyl polymers sometimes displayed a blue shift of the UV absorption band due to the transition because of the formation of chromophoric aggregates. For example, heat treatment of a thin film of a PVA substituted with an azobenzene (44; m = 5, n = 1) resulted in a spectral change due to the aggregation of the azo chromophore, which reverted back to the monomeric form after one cycle of alternate UV and visible-light irradiation. Similar spectral characteristics have been observed for some poly(acrylates) as well as poly(methacrylates), as shown in Table 3.

Photochromic Polymers

43

Whereas the preparative method for command surfaces was easy and convenient using these azobenzene polymers, the functionality of photochromic thin films in regulating liquid-crystal alignment was complicated and dependent on the structure of the polymers, and sometimes no photoresponse was induced. The results are summarized in Table 3. Although distinct photocontrol of alignment was achieved using a thin film of an azobenzene-modified PVA (44),158 regulation of the liquid-crystal alignment was not performed until cells were heated to produce an isotropic melt of nematic liquid crystals in the case of substrates covered with poly(acrylates) and poly(methacrylates) bearing azobenzene side chains.159 These facts indicated that microscopic mixing at an interface region between a liquid-crystal layer and the surface of an azobenzene polymer

44

Chapter 1

film must be a key step in establishing the photoresponsiveness. Since the alignment photocontrol was triggered by the structural alteration of monolayered photochromic units at a topmost surface, the coating of thick polymeric films was not a necessary condition, and the film thickness could be considerably reduced without deterioration of the photoregulativity. For this purpose, the adsorption of azobenzene polymers on substrate surfaces was a useful procedure to give photoalignment control. Photoresponsive cells were simply assembled by sandwiching a nematic liquid crystal in which a polymer with azobenzene pendant groups was dissolved between two substrate plates.159 The polymer was adsorbed spontaneously to make the silica surface photoactive. The photoinduced alignment alteration described so far took place reversibly between bistable states: the homeotropic and planar states (Figure 10.18a). An extraordinary change in the alignment of a nematic liquid crystal was observed when a thin film of a polymethacrylate substituted with p-hexylazobenzene in the

side chain through a long alkylene spacer (45i, Figure 20) was used (Figure 21).160,161 Before irradiation, the nematic liquid crystal showed a marbled texture. UV irradiation resulted in a change in the alignment to form a transient homeotropic mode at an early stage of the irradiation, followed by the formation of a schlieren texture. This transformation between the marbled and schlieren textures through a homeotropic state occurred reversibly; the transient homeotropic state was stable in the dark. This unique phenomenon was interpreted in terms of the microscopic modification of the polymer thin film. The azobenzene homopolymer having a long spacer exhibited a blue-shifted band owing to the aggregation of the E-isomer even at room temperature. It follows that the

aggregated state of the E-form was destroyed at an early stage of UV irradiation by the partial formation of the bent Z-isomer, bringing about a homeotropic alignment, followed by the formation of a photostationary state containing the Z-isomer as a major component, giving a schlieren texture. This process proceeded reversibly upon illumination of a UV-exposed film with blue light, which resulted in the transient recovery of the monomeric E-form, which was converted into the aggregated state upon further illumination to regenerate a marbled texture. 1.4.2.2. Polymeric Langmuir–Blodgett Films Since the Langmuir–Blodgett (LB) technique affords significant information concerning the nature of the photochromic molecular films, photochromic LB films acting as command surfaces may provide a clue to the working mechanism of this novel surface-assisted phenomenon. Because of photochromic reactions like the geometrical photoisomerization of azobenzenes and thioindigos are accompanied by a marked change in their molecular structures, a sufficient free volume is required in LB films embedding these kinds of photochromic moieties. In fact, photoisomerizations accompanied by the expansion of the areas occupied

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by photochromic amphiphiles were severely suppressed in LB films.162,163

Polymeric amphiphiles incorporating photochromic units were good candidates to give photoactive LB films applicable to command surfaces because the free volume was determined not by the photochromic units, but by the polymeric backbone chains, which occupy larger areas. Indeed, reversibility of the photochromism of azobenzene chromophores was attained when they were bound to a polyelectrolyte through a coulombic interaction.164 A family of PVA polymers substituted with azobenzenes in their side chains (44, Figure 20) were prepared in order to obtain photochromic LB films which were active for alignment photoregulation. Owing to the partial esterification of hydrophilic PVA, the azo-modified polymers exhibited an amphiphilic character due to the hydrophilic vinyl alcohol units and the hydrophobic azobenzene units substituted with a long-chain alkyl tail. Application of an appropriate surface pressure during the deposition of a polymeric monolayer on a plate enabled the control of the average density of azo chromophores on the substrate surface. The minimum density of the photochromic units for photoregulation of the liquidcrystal alignment was around one unit per square nanometer,165 quite in line with the value (1.2 nm2) obtained by the covalent adsorption method through silylation.139 Similarly to other deposited LB films, multilayered azo polymers demonstrated molecular orientation during the dipping procedure, leading to a homogeneous liquid-crystal alignment parallel to the direction of dipping.165 Using these novel LB films, systematic studies have been carried out to elucidate which features of the molecular structures of azo chromophores govern the photoinduced switching of the liquid-crystal alignment.165–169 The effects of spacer length (n) and tail length (m + 1) on the photoresponse of liquid-crystal

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cells are summarized in Table 4. It should be stressed here that the number of molecular layers plays an important role. Though non photochemical alteration of liquid-crystal alignment was achieved with a monolayered film of an azo-modified PVA (44, Figure 20; m = 5, n = 1), a cumulative layer gave a positive response. The photoregulativity was influenced not only by the spacer length, but also by the tail length. Furthermore, whereas the rates of the photochemical as well as of the thermal isomerization were not affected by the tail length, longer tail residues brought about a reduction in the rate of the photoswitching from the homeotropic to the planar mode. The longer the tail was, the slower the photoresponse became. This slower response suggested that the reorientation of photoisomerized azobenzene chromophores embedded in LB films resulted from the subsequent interaction of the chromophore with liquid-crystalline molecules after a sufficient relaxation period. These facts implied that photochromic units with suitable lengths of both spacer and tail interacted intimately with liquid-crystalline

molecules at the surface to bring about a perpendicular conformation of the command molecules, resulting in a homeotropic alignment. The photoisomeriza-

tion of command molecules takes place rapidly and induces the reorientation of liquid crystals surrounding them. The rearrangement of mesophasic molecules at the boundary region is transferred from molecule to molecule, resulting in the change of alignment in the whole cell. It has been observed that the electronic absorption spectra of LB monolayers were markedly dependent on the average density of the chromophore. The spectral shape and absorption maxima of monolayered azo-modified PVA (44, Figure 20; m = 5, n = 1) were roughly in line with those observed in solution when the monolayer was deposited at an average density of 1.02nm2 per unit, showing strongly that the azo chromophores were oriented in a random fashion without any specific interaction among them. In contrast, a remarkable hypsochromic shift of

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the absorption band due to the transition was observed when the film deposition was made at an occupied area of per unit, indicating the aggregation of the azobenzene moieties through side-by-side interaction due to dense packing. Further studies on the molecular orientation of LB films of the amphiphilic azo-modified PVA have been carried out with the aid of Fourier transform infrared (FTIR) measurements to obtain more information concerning the surface-induced change in the alignment of liquid-crystalline molecules.170 1.4.3. In-Plane Alignment Photocontrol The regulation of azimuthal alignment by means of surface polarization photochromism (Figure 18c) has practical applicability for active optical devices for erasable optical information storage, laser-addressed displays, spatial modulation, light waveguides, and so on, being driven by rotating the electric vector of polarized actinic light from a single light source. Another interesting application is the production of aligning layers for fabricating display devices. Though uniaxial orientation of liquid crystals in conventional display device technology has been achieved by buffing a thin polyimide film to get homogeneous alignment, this

technique has an essential drawback in that the rubbing treatment brings about the buildup of static electricity on the polymer film; this attracts dust particles, which causes the performance of precise devices to deteriorate. In this respect, the azimuthal photocontrol of liquid crystals by means of thin films of photochromic polymers is of great significance from a practical viewpoint. Linearly polarized UV irradiation of liquid-crystal cells assembled from silica plates covered with LB films of amphiphilic azobenzene PVAs (44, Figure 20) yielded the in-plane reorientation of liquid crystals from a homeotropic mode thus displaying the second type of surface-assisted alignment photoregulation shown in Figure 18b. The direction of the photoinduced homogeneous alignment was perpendicular to the electric vector of the polarized UV light as in the case of cells fabricated with a silica plate whose surface was covalently modified with azobenzene silylating reagents.165–169 Table 5 shows the effect of structural features of the LB films on their ability to control the azimuthal reorientation of liquid crystals. The table indicates that the photoresponsive behavior was dependent on the number of accumulated layers. In particular, azimuthal reorientation could be achieved specifically with monolayered films in general, although there were some exceptions. On the other hand, multilayered films caused homogeneous alignment parallel to the dipping directions, and no azimuthal reorientation was induced by irradiation with linearly polarized UV light. This showed that the conformational orientation of the polymer chains predominates over the photoinduced orientation of azo chromophores. The third type of alignment photoregulation (Figure 18c) using a single polarized light source has been performed by coating substrate surfaces with

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polymers doped with dichroic dyes171–173 and polymers bearing azobenzene side chains.83, 84 When a nematic liquid–crystal cell fabricated with the use of silica plates coated with a polyimide thin film containing a molecularly dispersed azo dye was exposed to a polarized Ar gas laser beam, a homogeneous alignment was generated in the direction perpendicular to the plane of polarization of the actinic light.171 Although experimental details were not available, it has been assumed that the photoalignment takes place as a result of the repetition of polarized E–Z photoisomerization of the azo dye.174 In the case of PVA doped with water-soluble azo dyes, the polarization photochromism gives rise to photodichroism, as previously reported by Todorov et al.72 A homogeneous alignment of liquid crystals was induced in the cell assembled using the dye-doped PVA thin film. The photoinduced liquid-crystal orientation was retained without any deterioration although the photoinduced dichroism of the azobenzene chromophere disappeared rapidly. It was observed that the photoinduced birefringence of the PVA film decreased slightly even after the disappearance of the azobenzene dichroism. According to Todorov et al., the stability of the photobirefrigence probably arose from the reorientation of PVA chains as a result of the photoinduced reorientation of the azo moiety. Thus, the efficient emergence of the uniaxial orientation of liquid crystals may be assisted by the anisotropic, physical reorientation of polymer chains at the topmost polymer surface.

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Spin-coated films of polymers with pendant azobenzenes were other candidates for photocontrol of the azimuthal alignment of liquid crystals. When a homopolymer of 4-methacryloyloxyazobenzene (28, Figure 11) was coated on a silica plate and exposed to linearly polarized light, prior to assembly of the cell, a homogeneous alignment was obtained.83 The uniaxial orientation of the mesophase faded away gradually even in the dark, however. This arose probably from the permeation of liquid-crystalline molecules into the polymer film, resulting in relaxation of the oriented state of the azobenzene moiety. This suggests that the combination of liquid crystals with azobenzene polymers plays an essential role in the persistence of the photoinduced in-plane alignment of liquid crystals. In fact, the storage stability of the photoinduced homogeneous alignment was considerably enhanced by using thin films of poly(methacrylates) having laterally attached azobenzene side chains.84 In marked contrast to the case of the head-on type of azobenzene polymer (28), the homogeneous alignment induced by polarized-light irradiation of thin films of azo polymers of the side-on type (29) displayed excellent storage stability in the dark. Azo polymers of this type showed the following peculiarities. First, the reorientation of the azo chromophore by polarized light required an exposure energy of approximately 100 mJ/cm2 whereas the exposure energy required to effect the complete photoisomerization of azo groups of the head-on type of polymer (28) amounted to a few joules per square centimeter. Second, although the photodichroism of the chromophore in a polymer film was rather thermally labile so that about 25% was lost in 30min, the homogeneous alignment was not altered essentially even after the film was left standing in the dark for more than one month. It is very likely that the stabilization of liquid-crystal photoalignment arose from the molecular interaction between the azo molecules and liquid-crystal molecules to form supramolecular aggregates at the interface. The stabilized photoalignment generated by the polymer thin films enables one to achieve reversible reorientation of liquid crystals by rotating the electric vector of polarized actinic light. It is noteworthy that with these azo polymers of the side-on type the reorientation of liquid crystals required an order of magnitude greater exposure energy than in the case of the head-on types. Thus, the molecular aggregates constituted by surface azo residues and liquid-crystalline molecules had a suppressive effect on the photoinduced reorientation of the command molecules. As revealed distinctively by experiments on the photoinduced regulation of in-plane alignment by silica plates whose surfaces were modified with azobenzenes,148–151 the azimuthal reorientation of liquid crystals was achieved by the uniaxial conformational change of command molecules localized at the topmost substrate surface. This situation was confirmed for polymer films by the results obtained with the following systems. The first consisted of the chemical modification of PVA thin films. Careful treatment of PVA films with azobenzene acid

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chlorides in hexane yielded the surface-selective introduction of the chromophores with a reasonable surface coverage of 2–3 units per square nanometer.175 Polarized-light irradiation of a liquid-crystal cell assembled with plates coated with the azo-modified PVA film resulted in a homogeneous alignment. The second system was based on the deposition of a single LB monolayer of a polyimide bearing p-cyanoazobenzene side chains (46, Figure 20) on a silica plate. A homogeneous alignment was obtained in a cell fabricated using these plates after polarized-light irradiation of the LB monolayer.176 When the cell was assembled after keeping the photoirradiated monolayer for 2 days in the dark, no homogeneous alignment was observed, indicating that the surface photodichroism of the azo chromophore disappeared upon prolonged storage. These facts support the supposition that the photoinduced uniaxial orientation of the photochromic residues at the topmost surface was dramatically stabilized by the presence of liquid-crystalline molecules. The surface-assisted photoalignment of liquid crystals has been extended to certain organic dye molecules to give a novel way to produce optical polarizers.177 The procedure was quite simple. A thin film of an azobenzene polymer coated on a substrate plate was exposed to linearly polarized blue light, followed by spincoating of an aqueous solution of a water-soluble azo dye. Dye molecules aligned parallel to the plane of polarization of the actinic light. Since the orientational direction of the azo chromophore was regulated at will by changing the plane of polarization, polarized photoimages could be made in an azobenzene polymer film using a photolithographic technique. Polarized patterns of anisotropically aligned dye molecular films were thus obtained and visualized with a polarizer. In summary, the polarization photochromism of thin films of azobenzene polymers has been shown to induce the homogeneous alignment of nematic liquid crystals. Although the extent of the photodichroism as well as the photobirefringence of these thin films was quite minute, the optical information written-in in the films was molecularly amplified by the self-organization of liquid crystals and visualized with the aid of polarizer(s) and readily overwritten by rotating the electric vector of the polarized writing light.

1.5. CONCLUDING REMARKS The investigation of the photochromic materials that exhibit distinct modifications of their absorption spectra has been motivated in a practical sense predominantly by the possibilities of developing erasable optical recording media with a high memory density179 and reversibly color-changeable dyestuffs with excellent photofatigue resistance. Tremendous advances have been made in these areas over the past few years owing to the exploration of novel photochromic molecules suitable for specific purposes. Progress has been achieved by combining conventional photochromic molecules with polymeric materials. As discussed

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previously3 and mentioned briefly in Section 1, organic photochromism can provide a versatile means of altering the optical, electrical, chemical, and even biological properties of polymeric materials because the molecular structural changes of photochromic moieties can induce changes in the structural, conformational, orientational, and aggregational states of macromolecular matrices entrapping photochromic molecules in various manners. The following are some examples of systems displaying photo-optical effects. Polymers exhibit a reversible refractive index change as a result of the molecular photochemistry of photochromic molecules dissolved in or attached to macromolecular skeletons. This provides a novel route to photo-optical switching of thin-film waveguides composed of polymers doped with photochromic compounds like fulgides,179 azobenzenes,180 diarylethenes,181 and so on. Marked birefringence modification has been achieved by dissolving photochromic molecules in thermotropic liquid crystals or by attaching photochromic molecules to surfaces of substrates in nematic liquid-crystal cells, as described in Sections 1.3 and 1.4. A novel photoresponsive electro-optical effect has been realized by assembling a cell filled with a ternary composite of liquid-crystalline polymers, low-molecular-weight liquid crystals, and azobenzene guests.182 The ionic conductivity of polymeric materials has been controlled photochemically by employing a thin film of a polymer composite containing a liquid-crystalline azobenzene and LiClO4183 or another system consisting of polymers, spiropyran, and LiClO4.184 These systems were applicable to electrostatic photoimage formation.185 As reviewed in this chapter, among the photochromic moieties that have been combined with polymeric materials, one of the most popular has been the azobenzene moiety, although this chromophore generates such a poor color change that it has traditionally played only a minor role in research to develop photochromic materials for the purposes mentioned just above. The extensive interest attracted by this kind of simple molecule arises not only from its ready availability, but also from the fact that its isomerization is accompanied by the drastic alteration of its molecular structure, and these structural changes have been readily coupled with changes in the properties of materials. It should be stressed that this situation offers novel routes possibly leading to new types of photofunctionality of organic photochromic compounds. For example, as shown in Sections 1.3.2 and 1.4.3, the molecular reorientation of chromophores has been achieved by polarized-light irradiation of photochromic molecules. Note that the efficient reorientation of photochromic molecules results from their repeated photoisomerizations in polymeric matrices which stabilize the reoriented states of these chromophores. In other words, polarization photochromism is a prerequisite condition for photoinduced molecular reorientation. This opens a novel route to a photochemical method for the control of the alignment of organic molecules, which is a fundamental technique in the fabrication of various optical

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elements including light waveguides, light modulators, optical polarizers, and so on. In the systems described for the surface-assisted alignment photocontrol of nematic liquid crystals, it becomes rather inappropriate to call photoactive molecules like azobenzene photochromic since a single photochromic molecule triggers a drastic alteration in the alignment of a number of liquid-crystalline molecules. In these systems, the functionality is no longer based on the color change of photochromic molecules but on the inititation of the molecular reorientation of a large number of guest molecules. The state of a molecular assembly has been modified and controlled by specific photoactive molecules. In this respect, it has been suggested that the photoactive molecules triggering the orientational, conformational, and even aggregational changes of a number of photochemically inactive molecules as well as macromolecules be termed “commander molecules” while guest molecules like liquid crystals have been

referred to as “soldier molecules” because of the manner in which they act.186 Host–guest chemistry related to molecular recognition phenomena has been widely accepted and deals with rather static phenomena in terms of the structures of host–guest components whereas the commander–soldier concept involves essentially dynamic aspects. This concept might be extended to a wide variety of other photoresponsive systems including, for example, visual sensory systems and solar energy converters, both initiated by retinal proteins. The photoisomerization of retinylidene as a commander embedded in a biopolymer as a soldier macromolecule causes the structural deformation of the ordered opsin chain, leading to the triggering of subsequent processes. The commander–soldier concept may open a novel route to versatile photofunctional polymeric materials incorporating photochromic molecules.

APPENDIX. PREPARATION OF PHOTOCHROMIC POLYMERS A.1. Polymethacrylates Substituted with Spiroindolinebenzothiopyrans (5)

The reaction scheme employed in the preparation of 5 10 , 11 is shown in Figure 1. 5-Nitrosalicylaldehyde was reacted with chloromethyl methyl ether in the presence of aluminum trichloride to yield 3-chloromethyl-5-nitrosalicylaldehyde. 3-Methacryloxymethyl-5-nitrosalicylaldehyde, which was obtained by the reaction of the chloromethylaldehyde with silver methacrylate in refluxing toluene, was converted to 3-methacryloxymethyl-O-(N,N-dimethylthiocarbamoyl)-5-nitrosalicylaldehyde in dimethylformamide (DMF) at room temperature. The rearrangement of the thiocarbamoyl group was performed in refluxing toluene to form 3methacryloxymethyl-S-(N, N-dimethylcarbamoyl)-5-nitrosalicylaldehyde, which was purified by column chromatography on silica gel. Then, the aldehyde was hydrolyzed with aqueous sodium hydroxide at 20°C to yield 3-methacryloxy-

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methyl-5-nitrothiosalicylaldehyde. 8´-Methacryloxymethyl-3,3-dimethyl-6´-nitrospiro[(2´H)-l´-benzopyran-2,2´-indoline] (4) was prepared by condensation of the methacryloxythiosalicylaldehyde with 3,3-dimethyl-2-methyleneindoline in refluxing 2-butanone in 36–49% yield.10 The copolymers with methyl methacrylate (MMA) were prepared by radical polymerization using azobis (isobutyronitrile) (AIBN) as an initiator or by ionic polymerization using phenylmagnesium bromide) as a catalyst.11 Polymethacrylates having a spirobenzoselenazolinobenzopyran moiety (8, Figure 2) were prepared similarly by the polymerization of the corresponding monomer (7) with MMA.13 (see Figure 2). A.2. Polymethacrylates with Spirofluorenylindolizines

Polymethacrylates with spirofluorenylindolizines were prepared by the following procedure.16 2-Methacryloyloxymethylpyridine was prepared by the reaction of 2-hydroxymethylpyridine with methacryloyl chloride in benzene in the presence of triethylamine and condensed with the corresponding spirocyclopropenefluorene compound to give a photochromic monomer (11, Figure 3). Fifty milligrams of 11, 950 mg of ethyl methacrylate, and 5 mg of AIBN were dissolved in 5 ml of butanone and polymerized under a nitrogen atmosphere at 65°C. The polymer was precipitated in methanol. A.3. Poly[4'-[[2-acryloyloxyethyl]ethylamino]-4-nitroazobenzene] (12)

For the preparation of 12 (Figure 4),78 an acryloyl chloride solution in tetrahydrofuran (THF) was added dropwise to a cooled mixture of Disperse Red 1 and triethylamine in THF at 0°C, followed by stirring for 6h or more. The resulting salt was filtered, and the solution was concentrated and poured into water. The precipitated monomer (60%) was then filtered, dried, and recrystallized from ethanol to give the monomer (m.p. 82–84°C). Polymerization was performed in dry toluene with 10% AIBN as an initiator at 60°C for 4 days. The polymer was precipitated in methanol and reprecipitated from a THF/methanol system. Gel permeation chromatography indicated an equivalent molecular weight of 4000.

A.4. An Ionene Polymer Incorporating Viologen Units (17) The reaction scheme for the preparation of 1746 is shown in Figure 6. Dried THF was stirred in Ar for 15 min at room temperature in the presence of trifluoromethanesulfonic anhydride as a catalyst. The solution was cooled to –70°C, and a THF solution of 4,4´-bipyridine was added. The mixture was stirred for 7 h at – 70°C and for 1 h at 2°C to cause the reaction of 4,4´-bipyridine with living dicationic poly(tetrahydrofuran). The highly viscous products were

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purified by repeated precipitation and washing with water. The content of 4,4´bipyridinium ions was determined from the absorbance at 265 nm. The counterions of the white polymer were exchanged in chloroform with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion (TFPB) to yield a pale yellow elastic polymer.

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156. H. Tomita, K. Kudo, and K. Ichimura, Command surfaces 15. Photoregulation of liquid crystal alignment by cinnamoyl residues on a silicia surface, Liq. Cryst. 20, 171–176 (1996). 157. K. Aoki, T. Seki, M. Sakuragi, and K. Ichimura, Reversible alignment change of liquid crystals induced by photochromic molecular films, Part 15. Convenient methods to prepare “command surfaces” by surface-selective modifications of poly(vinyl alcohol) thin films with azobenzene units, Makromol. Chem. 193, 2163–2174 (1992). 158. K. Ichimura, Y. Suzuki, T. Seki, Y. Kawanishi, T. Tamaki, and K. Aoki, Reversible alignment change of liquid crystals induced by photochromic molecular films, 2. Reversible alignment change of a nematic liquid crystal induced by pendent azobenzene groups-containing polymer thin films, Makromol. Chem., Rapid Commun. 10, 5–8 (1989). 159. Y. Kawanishi, T. Seki, T. Tamaki, K. Ichimura, and M. Ikeda, Reversible alignment change of nematic liquid crystals by photochromic polymer films, Polym. Adv. Technol. 1, 311 –318 (1991). 160. Y. Kawanishi, T. Tamaki, T. Seki, M. Sakuragi, Y. Suzuki, and K. Ichimura, Multifarious liquid crystalline textures formed on a photochromic azobenzene polymer film, Langmuir 7, 1314–1315 (1991). 161. Y. Kawanishi, T. Tamaki, T. Seki, M. Sakuragi, and K. Ichimura, Alignment of nematic liquid crystals controlled by the photochromic reaction of aggregated surface azobenzenes, J. Photpolym. Sci. Technol. 4, 271–278 (1991). 162. H. Nakahara, K. Fukuda, M. Shimomura, and T. Kunitake, Molecular arrangements and photoisomerization of amphiphilic azobenzene derivatives in mono- and multilayers, Nippon Kagaku Kaishi 1988, 1001. 163. L. Collins-Gold, D. Möbius, and D. G. Whitten, Interfacial effects on excited-state potential energy surface. Interrelationship between photoreactiviry and surface properties, Langmuir 2, 191–194 (1986). 164. K. Nishiyama and M. Fujihira, Cis-trans reversible photoisomerization of an amphiphilic azobenzene derivative in its pure LB film prepared as polyion complexes with polyallylamine, Chem. Lett. 1988, 1257–1260. 165. T. Seki, M. Sakuragi, Y. Kawanishi, Y. Suzuki, T. Tamaki, R. Fukuda, and K. Ichimura, “Command surface” of Langmuir–Blodgett films. Photoregulation of liquid crystal alignment by molecularly tailored surface azobenzene layers, Langmuir 9, 211–218 (1993). 166. M. Sakuragi, T. Seki, Y. Kawanishi, T. Tamaki, K. Ichimura, R. Fukuda, H. Hiramatsu, and K. Fujiwara, Photoregulation of liquid crystal alignment by Langmuir–Blodgett layers of azobenzene polymers, J. Photopolym. Sci. Technol. 4, 279–282 (1991). 167. T. Seki, M. Sakuragi, Y. Kawanishi, R Fukuda, H. Hiramatsu, K. Fujiwara, and K. Ichimura, Structural effects of LB molecules on the command-surface-type photocontrols of liquid crystal alignment, J. Photopolym. Sci. Technol. 5, 569–572 (1992). 168. T. Seki, M. Sakuragi, Y. Kawanishi, Y. Suzuki, T. Tamaki, K. Ichimura, R. Fukuda, and H. Hiramatsu, Modulated photoregulation of liquid crystal alignment by azobenzene Langmuir– Blodgett layers: Reversible alignment changes of liquid crystals induced by photochromic molecular films, Part 11, Thin Solid Films 210/211, 836–838 (1992). 169. T. Seki, K. Ichimura, F. Fukuda, and T. Tamaki, Liquid crystal-induced orientational changes of azobenzene LB monolayers: Effect of the tail structure, Kobunshi Ronbunsu 52, 599–605 (1995). 170. N. Katayama, Y. Ozaki, T. Seki, T. Tamaki, and K. Iriyama, FT-IR study of molecular orientation and anisotropy in “command surfaces” formed by Langmuir–Blodgett films of azobenzene amphiphilic polymers, Langmuir 10, 1898–1902 (1994). 171. W. M. Gibbons, P. J. Shannon, S.-T. Sun, and B. J. Sewtlin, Surface-mediated alignment of nematic liquid crystals with polarized laser light, Nature 351, 49–50 (1991). 172. W. M. Gibbons, P. J. Shannon, and S.-T. Sun, Optically controlled alignment of liquid crystals: Devices and applications, Mol. Cryst. Liq. Cryst. 251, 191–208 (1994).

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173. Y. Iimura, J. Kusano, S. Kobayashi, T. Aoyagi, and T. Sugano, Alignment control of a liquid crystal on a photosensitive polyvinylalcohol film, Jpn. J. Appl. Phys., Part 2 32, L93– L96 (1993). 174. C. Jones and S. Day, Shedding light on alignment, Nature 351, 15 (1991). 175. H. Akiyama, M. Momose, K. Ichimura, and S. Yamamura, Surface–selective modification of poly(vinyl alcohol) films with azobenzenes for in-plane alignment photocontrol of nematic liquid crystals, Macromolecules 28, 288–293 (1995). 176. H. Akiyama, K. Kudo, K. Ichimura, S. Yokoyama, M. Kakimoto, and Y. Imai, Azimuthal photoregulation of a liquid crystal with an azobenzene-modified polyimide Langmuir–Blodgett monolayer, Langmuir 1 1 , 1033–1037 (1995). 177. K. Ichimura, M. Momose, K. Kudo, H. Akiyama, and N. Ishizuki, Surface-assisted photolithography to form anisotropic dye layers as a new horizon of command surfaces, Langmuir 11, 2341–2343 (1995). 178. M. Irie (ed.), Photo-reactive Materials for Ultrahigh Density Optical Memory, Elsevier Science, Amsterdam (1994). 179. T. Kardinahl and H. Franke, Grating in nonlinear optical polymers for integrated optical device applications, Proc. SPIE 2025, 574–578 (1993). 180. W. Knoll, Integrated optics for the characterization of photoreactive organic thin films, Pure Appl. Chem. 67, 87–94 (1995). 181. N. Tanio and M. Irie, Photooptical switching of polymer film waveguide containing photochromic diarylethenes, Jpn. J. Appl. Phys. 33, 1550–1553 (1994). 182.

T. Kajiyama, H. Kikuchi, and K. Nakamura, Photoresponsive electrooptical effect of liquid

crystalline polymer/low molecular weight liquid crystal composite system, Proc. SPIE 1911, 111–121 (1993). 183. K. Kimura, T. Suzuki, and M. Yokoyama, Photoresistive ionic conductivity of polymer composite films containing azobenzene liquid crystal, J. Phys. Chem. 94, 6090–6093 (1990). 184. N. Kubo, N. Kobayashi, K. Ikeda, and R. Hirohashi, Control of ion conductivity in solid polymer electrolyte containing spiropyran derivative by photoirradiation, Nippon Kagakuu Kaishi 1992, 1148–1153. 185. K. Kimura, T. Suzuki, and M. Yokoyama, Image storage based on the photo-induced ionic conductivity jump of polymer films containing azobenzene liquid crystal, J. Chem. Soc. Chem. Commun. 1989, 1570–1571.

186. K. Ichimura, A new trend in photopolymers based on the commander/soldier concept, J. Photopolym. Sci. Technol. 8, 343–350 (1995). 187. K. Ichimura, Surface-assisted photoregulation of molecular assemblages as commander/soldier molecular systems, Supramol. Sci. 3, 67–82 (1996). 188. T. Ikeda, T. Miyamoto, T. Sasaki, S. Kurihara, and S. Tazuke, Effect of structure of photoresponsive molecules on photochemical phase transition of liquid crystals IV Photochemical presentation behavior of photochromic azobenzene guest/polymer liquid crystal host mixtures, Mol. Cryst. Liq. Cryst. 188, 235–250 (1990).

2 Photodegradation of Organic Photochromes V. Malatesta 2.1. INTRODUCTION Photochromism1,2 is a phenomenon based on the absorption of light of wavelength by a generally uncolored compound A, which gives after isomerization a colored form B, absorbing at The metastable B form reverts back to A either thermally1–3 or photochemically.4 The compound A can be used in liquid solutions or in viscous solutions such as a polymer matrix.

Several classes of photochromes A have been described.1,2 These classes may be differentiated according to the chemical transformation upon which the phenomenon is based. In general, the molecular rearrangement responsible for the reversible color change may consist in: 1. E–Z isomerization of double bonds (e.g., stilbenes, azo compounds, polymethines, indigoids, etc.) 2. Electron transfer (e.g., cell chlorophyll, Fe(II) thiazines, etc.) 3. Hydrogen transfer (tautomerism) (e.g., metal dithizonates, N-salicylideneanilines, etc.) 4. (Photo)dissociation (e.g., triarylmethanes)

V. Malatesta • Great Lakes Chemical Italia, Via Maritano 26, 20097 S. Donate (NI), Italy. Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert

Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999.

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5. Pericyclic reaction: intramolecular electrocyclic reaction (e.g., spiropyrans, spirooxazines, fulgides, etc.), cycloaddition (2 + 2) reaction. Practical applications such as in recording, displaying, and copying materials, optical filters, sunlenses, and special gadgets have been envisaged.1,2 Most of these applications, however, require photochromes endowed with good colorability and acceptably long durability. The most important and promising classes of photochromic materials so far reported are spiropyrans (SP) and spirooxazines (SO) in which a heterocyclic and a chromene moiety are orthogonally linked through a carbon atom. Since they were first reported, SPs5 and SOs6 have been studied extensively, with the nitro derivatives of spirobenzopyrans, , receiving the most attention. These nitro derivatives are endowed with high colorability but lack sufficient durability for long-term applications. In 1980 Hovey et al.7 were the first to recognize the high stability of SOs with respect to photoexcitation. This observation has spurred intense research efforts, both in academia and in industry, aimed at designing SOs endowed with good colorability as well as longer durability. Durability with respect to photoexcitation or resistance to photochemical fatigue is, in principle, determined by the reactivity of the excited states of the spiro and/or merocyanine form(s), which in turn depends on their lifetime and is directly linked to the photophysical properties (e.g., colorability) of the given photochrome.

2.2. PROPERTIES AND REACTION MECHANISM(S) OF SPIROPYRANS AND SPIROOXAZINES

Several models have been proposed to explain the properties and photochromic reaction mechanisms of spiropyrans and spirooxazines based on both frequency2,5 and time8–10 field spectroscopic studies. The photochromic reaction features the dissociation of the bond, producing a distribution of isomers3–15 (Scheme 2).

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67

The experiments carried out by Ernsting and Arthen-Engeland12,13 were the first aimed at elucidating the ultrafast photochromic reaction dynamics. They studied the unsubstituted indolinospiropyran 3 and its naphthopyran analog 4 in n-pentane.

They found that the first merocyanine isomer is formed in its ground state in 0.9– 1.4ps. The internally hot (molecular temperature of ca. 900 K) primary merocya-

nine (MC) product isomerizes on the picosecond time scale to give a distribution of MC isomers. The open merocyanine form of both SPs and SOs may, in

principle, originate from either a singlet state of the closed spiro structure.

or triplet

excited

2.2.1. Multiplicity of the Photoreactive State

Laser flash-photolysis8–13 and time-resolved resonance Raman scattering, studies14,15 have revealed that the reaction dynamics for spiropyrans carrying a nitro group are significantly different from those for spiropyrans without a nitro group (SP). Indeed, a triplet state, has been found8,11 to play an important role in the photocoloration process of the former, as indicated by the sensitivity of the dynamics to the presence of (Scheme 3).

For molecules containing no nitro groups, the photoreaction is dominated by excited singlet states, (Scheme 4).

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2.2.2. The cis-cisoid X Isomer

Before proceeding to the analysis of the photodegradation mechanisms of SPs and SOs, it is useful to briefly discuss the nature of the primary steps of the bond cleavage process. The conclusion that emerges from the study thus far published is that different intermediate species may be responsible for the two concurrent processes, namely, photocoloration and photodegradation, initiated by UV irradiation of SPs and SOs both in a solvent and in a polymer matrix. Furthermore, besides photodegradation, a second degradative process, namely, photooxidation, needs to be considered; the two degradative processes seem to involve intermediate states of different multiplicity and/or geometry. In 1962, Fischer and co-workers reported that “ultraviolet irradiation of spiropyrans results in formation of an unstable isomer, X, of the colored modification” and proposed the existence of an open cis-cisoid isomer.16 The geometry of this isomer resembles very closely that of the parent spiropyran, i.e., the central C3=C4 bond has a cis–cisoid conformation:

2.2.3. Nitro-Subsriruted Spiropyrans,

The role of the X isomer in the photocoloration of 6-nitro-1´,3´,3´-trimethylspiro[2H-1-benzopyran-2,2´-indoline] (6-nitro-BIPS, 6) has been discussed by several authors.

In a picosecond transient absorption spectroscopy study of 6-nitro-BIPS derivatives, Krysanov and Alfimov8 identified a transient absorbing at 440 nm and proposed its formation within 8 ps from the excited singlet state of the

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69

spiropyran form. This cis-cisoid X isomer isomerized to the transoid form in less than 300 ps (see below). Lenoble and Becker11 reported that in nonpolar solvents 6-nitro-BIPS forms the merocyanine from the excited triplet state of the spiro form. via the cis-cisoid X isomer triplet, in 10ns (Scheme 5), the transformation being faster (<1 ns) in polar solvents.

In 1990 Zhang et al.10 identified, by means of picosecond and femtosecond transient electronic absorption spectroscopy, a metastable species X that formed in less than 100 fs after excitation, at 295 nm, of 6-hydroxy-1´,3´,3´-trimethylspiro[2H-l-benzopyran-2,2´-indoline] (6-OH-BIPS, 7).

A small fraction of this metastable species was reported to re-form the bond in about 200 fs. The major fraction relaxed to give by isomerization, in ca. lOOps, a merocyanine product (B; Figure 1). In a quantitative study of the photostability of spiropyrans, Malkin et al.17 concluded that the photodegradation quantum yield is independent of the heterocyclic part and is determined instead by the structure of the chromene part of SPs. According to these authors, photodegradation of nitro-substituted SPs originates from the reaction of the cis-cisoid X isomer with solvent or impurities (Scheme 6):

Conversely, photooxidation is the result of the reaction of the triplet state of the closed SP form with oxygen. The two processes are initiated by the same step, i.e., cleavage of the bond (Scheme 7):

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In that study, it was empathized that the lifetime of the X isomer determined the

extent of the SP degradation. 2.2.4. Unsubstituted Spiropyrans and Spirooxazines

Photochromism of spiropyrans and spirooxazines has been examined by picosecond transient absorption spectroscopy9 and nanosecond time-resolved spontaneous coherent anti-Stokes Raman scattering (CARS).18 The results obtained by transient absorption spectroscopy have been used, in the case of spirooxazines, to suggest that the photochromic reaction, transforming the spiro form via ring cleavage to merocyanine isomers, occurs within 10ps after the light flash and proceeds through the excited state of a cisoid intermediate absorbing at 616nm and designated X*.9 The time-resolved CARS measurements18 suggest that

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71

merocyanine isomers are formed within 20 ns, with the initial isomer distribution depending on the solvent used. In laser photolysis studies of photoinduced aggregation in polymers containing nitro-substituted spiropyran units, Kalisky et al.19 have proposed that the photoinduced ring opening to he fully isomerized merocyanine form proceeds via a short-lived (<10 –9 s in toluene) nonplanar ringopened intermediate X. In a more recent time-resolved investigation, Ernsting and Arthen-Engeland13 have searched for evidence of the existence of however brief, of the X isomer in a subpicosecond study of the primary photochemical reaction step of the unsubstituted benzospiropyran 3 and naphthospiropyran 4 but found no evidence for its transient formation. The same conclusion was reached through a picosecond absorption and picosecond time-resolved resonance Raman investigation conducted by Aramaki and Atkinson14 on alkyl-substituted spiro[indolinenaphthopyrans]. The main argument used by the authors was that the merocyanine could not be induced to undergo photochemical ring closure back to the spiro form and therefore the observed 400–460-nm transient absorption was due to a excitation of the merocyanine form rather than to the Xisomer absorption. The nonplanar intermediate “X” generated by ultraviolet (UV) irradiation of indolinospirooxazines has a buildup time of less than 2ps and rearranges to the planar merocyanine forms in 2–12 ps, depending on the nature of the solvent as well as the kind of substituent present on the molecule. However, Tamai and Masuhara20 conducted a 200-fs time-resolved transient absorption spectroscopic investigation on the parent spiro[indoline-naphthoxazine], 1,3,3,trimethylspiro[indoline-2,3´-[3H]-naphtho-[2,l-b]-[l,4]-oxazine] (8) and reported that the rate of the bond cleavage is approximately 700 fs – 1 , which is the rate of population of a transient state A that relaxes in about 470 fs to give a metastable merocyanine B. The metastable transition state was identified as the cis-cisoid X isomer (Figure 2).

As for the multiplicity of the excited state from which the X isomer is generated, we have already mentioned that Krysanov and Alfimov8 proposed a mechanism in which the vibrationally excited triplet state of acts as the precursor to the cis-cisoid ring-opened photoproduct X. On the other hand, Kalisky et al.19 claimed that X is formed from in less than l0ps. An excited singlet precursor is also postulated for the formation of X, with the

72

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intermediacy of the excited singlet state derived from (10ps) and yielding X in about 300 ps. Of the main transient absorption bands at 440 and 580 nm that are observed upon laser irradiation of nitrospiropyrans, the first one has been assigned in part or exclusively to either X, its excited triplet state and/or the spiropyran triplet . The 580-nm absorption has been assigned to the merocyanine form and to From what has been so far reported, it would seem that the intermediacy of the cis-cisoid ring-opened nonplanar intermediate X is much better documented for nitrospiropyrans than for spiroazines, for which the matter is still controversial. No studies have so far been reported on the importance of the X isomer in the photodegradation mechanistic schemes. The detailed Schemes 8 to 11 in which the X isomer has been invoked by various authors are summarized below: •

Krysanov and Alfimov (1982)8

Photodegradation of Organic Photochromes

•

Lenoble and Becker (1986)11

•

Tamaki et al. (1989)21

•

Malkin et al. (1990)17

73

Only in the scheme of Malkin et al. does the X isomer seem to be involved in the formation of photodegradation products. Finally, contrary to what was accepted until 1990, Sakuragi et al.22 have claimed that in the case of 6-nitro-BIPS (6) the photocoloration is mostly (>90%) from the spiropyran excited singlet state whereas the photodegradation originates predominantly from the interaction of the spiropyran triplet state with the solvent, oxygen, impurities, etc. In this study, no distinction was made between photodegradation and photooxidation, two processes that seem to have common reactive intermediates (see Scheme 12):

74

Chapter 2

•

Sakuragi et al. (1990)22

2.3. COLORABILITY AND PHOTOSTABILITY (PHOTOCOLORATION QUANTUM YIELDS) Determining the quantum yield of photocoloration for a reversible system yielding a metastable colored form B, has proven to be not a trivial problem.23,24 Difficulties stem from the uncertainty in the calculated value of the molar absorption coefficient of merocyanine B due to the metastable nature of this form. Colorability is a term frequently encountered in the literature1,2 of spiropyrans and spirooxazines and refers to the propensity of the colorless species A to be transformed, by irradiation, into the colored form B. It has been expressed in terms of the apparent absorption coefficient where is the optical density of the colored species B, [A] is the initial concentration of A, and l is the cell pathlength. Colorability of different photochromes has been compared by keeping constant the experimental conditions (excitation source, filters, distance, irradiation geometry, absorbance of A).25 A different and more quanti-

tative measure of Colorability, , has been proposed26 and defined as the product of the photoreaction quantum yield and the molar extinction coefficient of the merocyanine. This definition, although independent of the irradiation conditions, does not take into account the role of the thermal backreaction rate that limits the maximum intensity of color obtained at a given temperature. Recently, we have described27 a treatment of the spectroscopic data for thermoreversible systems that allows the determination of the molar absorption coefficient of the metastable merocyanine form, the quantum yield of the color-forming reaction, and the kinetic parameters of the thermal bleaching. Under steady-state stationary conditions, Colorability depends on all these parameters and can be calculated for any experimental conditions from

Photodegradation of Organic Photochromes

75

where is the optical density of species B, is the initial concentration of A, I0 is the initial irradiation light intensity, is the molar absorption coefficient of B, is the molar absorption coefficient of A, k is the thermal bleaching rate constant, is the quantum yield of the reaction, and is a photokinetic factor defined by the experimental conditions. Depending on the molecule and the solvent, quantum yields ranging from 0.18 to 0.65 have been calculated for some representative spirooxazines.27 For other systems, had already been reported.23,24 It has been observed that, in general, photochromes having high are characterized by a lower photostability.

2.4. PHOTO- AND THERMAL DEGRADATION OF PHOTOCHROMES 2.4.1. Degradation of Spiropyrans: General Approach Spiropyrans, especially the nitro-substituted ones, are in general endowed with

good colorability. However, one of the major deficiencies of spiropyrans for many potential applications is their short life cycle because of the extensive thermal and photochemical decomposition that they undergo by repeated and/or continuous light exposure. In 1968 Gautron28 was the first to investigate the photodegradation of 6-nitro-BIPS (6) and four other derivatives.

76

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Irradiation of aerated, thermostated anhydrous toluene solutions of 6 and 9–12 (0.2%) was carried out with a 500-W high-pressure mercury lamp for an average time period of 24 h. The analysis of the gas-phase products revealed formation of CO, and formaldehyde, whereas by distillation of the reaction liquid phase, benzaldehyde and phenol were collected. The latter compounds derived from oxidation of the solvent and were already present in it initially, although in trace amounts. In the solid phase, three main compounds were identified, namely, 3,3-dimethyloxindole (13, Y = H), 1,3,3-trimethyloxindole (14, Y = H), and derivatives of o-salicylaldehyde 15, together with dichlorooxindoles derived, probably, from further reaction of the degradation products with HC1, and used in the workup operations. It is worth noting that about 50% of the starting spiropyran gave tarry compounds that, when analyzed by nuclear magnetic resonance (NMR) spectroscopy, showed, among others, resonances typical of groups on quaternary carbons and aromatic and methoxyl protons. In the remaining 50%, salicylaldehyde accounted for only 33% (mol/mol). A typical mass balance gave the following in the case of 6 (in terms of mole percent): 6, 18%; 5-nitro-o-vanillin (15, ), 30%; 1,3,3trimethyloxindole (14, Y = H), 10%; 3,3-dimethyloxindole (13, Y = H), 8.7%. The latter two compounds were always found in quantities much lower than the

salicylaldehyde derivative although the stoichiometry would have required a 1:1 ratio. With a very detailed product analysis, Gautron showed by labeling of the benzopyran oxygen atom that the label was transferred, upon oxidation, to salicylaldehyde(s) and not to the oxindoles. This indicates that in the photochemically driven coloration process, it is the cleavage of the bond, not that of the O–aryl bond of the benzopyran moiety, that gives rise to the formation of the merocyanine. It was also concluded that the oxygen of the C=O group of oxindoles was incorporated from atmospheric oxygen. By labeling of the group, it was shown that labeled formed and therefore that the 3,3-dimethyloxindole (13) was derived from oxidative degradation of the group of the spiropyran. In control experiments, 1,3,3-trimethyloxindole did not give 3,3-dimethyloxindole, and therefore demethylation of the spiropyran had to

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77

take place before the formation of the final products. It was suggested that the lower than stoichiometric amount of salicylaldehyde(s) that was generally isolated could be due to its side reaction with formaldehyde to give polycondensates of the formaldehyde/phenol type. 2.4.2. Photodegradation: Dependence on the Nature and Position of the Substituent In a pioneering study, Gautron28 examined the influence of substituents on the photodegradation process of SP substituted on both the indoline and chromene moieties. The main conclusion that could be drawn from this study was that the introduction of electron-donor groups (EDG) at any position of the molecule except the C3´ position of the bridging C3´=C4´ bond, decreased photodegradation whereas electron-withdrawing groups (EWG) enhanced degradation. To monitor and quantify photodegradation, Gautron had to propose a reaction model (supposedly more complicated than the simple reversible reaction responsible for photochromism) and chose a physical parameter (e.g., the optical density variation as a function of the light exposure time) with which to gauge the degradation progress. The photodegradation was induced by flash-photolyzing airequilibrated toluene solutions (10 –4 M) of variously substituted 6-nitrospiropyrans. For a simple fully reversible, photochromic system,

when photodegradation is not taking place or disregarded, under pulsed photolysis conditions, the measured optical density of the photochrome solution at the end of the nth flash, is related to its maximum optical density at the end of the flash by

where is the rate constant for the thermal bleaching process Equation (2.4.2.1) can be rewritten as

where is the molar extinction coefficient of merocyanine B at l is the cell pathlength, is the concentration of A at time zero for the nth flash, and

78

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is a reaction constant of the photochrome that takes into account the global quantum efficiency of the photocoloration process. E is the energy of the flash:

where i is the light intensity, and T is the time at the end of the flash, whose duration is usually much shorter than the photocoloration time. If one uses flashes of increasing energy E, the maximum optical density of the solution at the end of one flash varies as indicated in Figure 3. This behavior obtains when photodegradation is not occurring. However, it is experimentally observed that the maximum optical density at the end of the flash depends strongly on the number of flashes; i.e., Clearly, the simplistic scheme does not correctly describe the behavior of the system as photodegradation processes slowly deplete the concentration of the A and B species. A more complete scheme was then adopted in which degradation products D were introduced:

Photodegradation of Organic Photochromes

79

Reactions 1 and 2 describe reversible photochromism, and 3 and 5 irreversible photochemical degradation processes having rate constants of and The thermal degradation processes are reactions 4 and 6, with the latter having a negligibly small rate constant as SPs do not degrade appreciably in the absence of UV light. For photoreactions of 1, 3, and 5, one can write

where is the concentration of A after the nth flash. Similarly, for the thermal reactions 2 and 4 the time dependence of the concentrations of B and D is described by

and mass balance considerations require that

where is the concentration of D after the nth flash. In Eq. (2.4.2.8) v is a factor describing the photodegradation process and having the general expression

where

and

take into account the kinetic constants of reactions 1–5 (Scheme

14).

One can now write an equation relating the optical density value number of flashes:

to the

where g(E) is a factor that takes into account the reaction rate constants, the energy of the photolysis light flash, the cell pathlength, and the extinction coefficient of the merocyanine at the absorption maximum. For the first flash,

80

and

Chapter 2

is related to the optical density after the nth flash by

The last equation can be rewritten as

By plotting versus the number of n flashes, one should get a straight line having a slope of log(l – v). For most of the compounds studied by Gautron,28 Eq. (2.4.2.14) was not obeyed. Instead of a straight line, curves were obtained whose deviation from linearity was more pronounced for aerated solutions (Figure 4).

Equation (2.4.2.14), which considers only photodegradation, describes then only approximately the behavior of irradiated spiropyran solutions. The observed deviation is, in the author’s opinion, derived from an oxidation process not included in Scheme 14 as the data reported in Figure 4 for 8-methoxy-6-nitroBIPS (11) seem to indicate. The reactions responsible for the deviation are, in Gautron’s opinion, of the autooxidation type, i.e., oxidative chain reactions of free-radical intermediates with the initiators being, most likely, peroxides from the solvent or solute. This

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hypothesis was supported by the results obtained when an antioxidant such as 2,6di-tert-butylcresol (BHT) was used. BHT at concentrations below behaved as an oxidation inhibitor, although degradation as not completely suppressed. The exact nature of the oxidation steps was not known, and it as not possible to include them in a more elaborate model reaction scheme. It was suggested that a biradical intermediate derived from the homolytic cleavage of bond was most likely involved in the oxidative degradation process. Equation (2.4.2.14) was, however, considered adequate for a cursory description of the spiropyran degradation, and log(1 – v) was taken as a parameter describing the degradation process. For dilute solutions the initial velocity v´, i.e., the initial slope of the curve, was considered to be more appropriate. However, for concentrated solutions the parameter i.e., the number n of flashes necessary to decrease the optical density of the solution to half of the original value was instead used to monitor degradation. The experimental conditions adopted by Gautron were as follows: solutions of spiropyran in benzene, toluene, or dioxane; high-pressure 500-W mercury lamp; light filtered through borosilicate glass (2mm thick; cutoff, or Wood glass (2mm thick; passing bandwidth, flash energy, 300 J (450 J when Wood glass was used). The author rightly noted that deaerated solutions could not be considered completely oxygen-free. 2.4.3. Substituents on the Indoline Moiety of Spiropyrans: Inductive and Mesomeric Effects The reactivity (photodegradation) of spiropyrans substituted on the aromatic ring of the indoline moiety has been rationalized in terms of inductive and mesomeric effects and quantified on the basis of the Hammett equation:

where k and are physicochemical constants for the substituted and unsubstituted spiropyrans, respectively. The Hammett constant is a factor that depends on the electronic nature and position (ortho, meta, or para) of the substituent, and the reaction constant, measures the response of that particular reaction to the electronic perturbations introduced by the substituents. When Eq. (2.4.3.1) is used to describe the degradation processes undergone by photochromic systems, k and are the parameters defining the degradation reaction(s). In general, is positive when degradation is favored by electron-withdrawing groups Br, etc.). On the other hand, when degradation is favored by an increase of the electronic density induced by electron-donating groups is negative. In Gautron’s study, the k values were either the v´ (initial velocity) or

82

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values. For substituents on the benzopyran ring, the values were those used by the National Cash Register Company (NCR). Taft or McDaniel and Brown values28 were used for substituents on the indoline moiety; for 5-substituted pyrans, values were found to be adequate. The reactivities, in terms of photodegradation, of indoline-substituted nitrospiropyrans are presented in Table 1. By plotting log v' for the 5-substituted compounds versus a straight line is obtained (Figure 5). The electron donors and have a stabilizing effect (19 and 20 in Table 1) whereas the electron acceptors Cl, F, and Br enhance degradation (9, 17 and 18 in Table 1). Table 1 also includes the values of the durability parameter, and the colorability, defined as the apparent molar extinction coefficient along with the thermal bleaching rate constants k and after the first and nth flash, respectively. It may be noted that k generally increases with the number of flashes. 2.4.4. Substituents on the Indoline Moiety

Methyl substitution at C4´, C5´, C6´, and C7´ stabilizes the photochrome in the order C7´ > C6´ > C4´ > C5´ (Table 1; cf. v´ for 24, 23, 22, and 19). A phenyl substituent on N destabilizes the photochrome whereas and stabilize it (cf. 12, 25, and 26). Ethyl groups at C3 also have a stabilizing effect (see 27). These results are rationalized in terms of substituent-induced changes of the bond polarizability. Electron-donor groups on N and C3 decrease the

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83

84

Chapter 2

polar character of the bond whereas the opposite is true for electronwithdrawing substituents. Gautron suggested that, in general, the more polarized the bond, the lower the degradation. 2.4.5. Substituents on the Chromene Moiety of Spirobenzopyrans

The values obtained for dioxane solutions of spirobenzopyrans substituted on the chromene moiety are presented in Table 2. In the nitrosubstituted spiropyran series (6, 28, and 29), the 6-nitro derivative is usually more stable. and CHO substituents are also more stabilizing when on C6 (cf. 6 and 32; 30 and 32). Indeed, the electron-donor efficiency of the methoxy group is higher when at C6 as indicated by the Hammett constant values, which are –0.76 for the C6 (para) position and –0.39 for the C8 (ortho) position. For the nitro-substituted spiropyrans, the electron-withdrawing character of depends on the substitution position, with Hammett constants of 0.66, 0.78, and 0.80, for at C7, C6, and C8, respectively. Having observed that C6 substitution had a major effect on photodegradation, Gautron prepared a series of C6-substituted derivatives and studied their behavior with respect to photoexcitation. The results presented in Table 3 show that indeed electron-donor groups at C6 efficiently stabilize the photochrome against photodegradation (cf. 33, 36, and 3; 3 and 33 vs. 34 and 6). 2.4.5.1. Monosubstituted Compounds The results presented in Table 3 show that among the spiropyrans with a nitro substituent at C6, C7, and C8, the C6-substituted compound is the most stable. and CHO have the same effect. Here again, electron-donor substituents, e.g., and stabilize the photochrome against photodegradation. 2.4.5.2. Disubstituted Compounds

The analysis of the results presented in Table 4 indicates that EDGs on the chromene moiety decreases photodegradation (37 vs. 40), whereas EWGs on C8 have the opposite effect (see 41). 2.4.5.3. C3-Substituted Spiropyrans A CH3 group on C3 greatly decreases stability whereas

substitution yields stabilization (Table 5, 47 vs. 48). It was noted that, in general, the rate constant k of the thermal bleaching reaction was not affected by the Hammett constants of substituents on the indoline moiety whereas for substituents on the chromene moiety EDGs produced an increase of k.

Photodegradation of Organic Photochromes

85

86

Chapter 2

Photodegradation of Organic Photochromes

87

88

Chapter 2

From the above results, the following conclusions are drawn: degradation is partially an oxidation process; autoxidation plays a role in photodegradation;

EDGs on all positions (except for C3) decrease degradation; degradation of spiropyrans occurs at the C3=C4 level; and ring opening by heterolytic cleavage of the bond results in a better stability of the spiropyran toward photodegradation29 (Scheme 15).

On the other hand, as already noted, homolytic cleavage yields biradicals 51 and 53, and their reaction with molecular oxygen results in formation of peroxy radicals 52 and 54, whose structure depends on the resonance form of the biradical blocked by O2. The most important contributors are believed to be 51 and 53 (Scheme 16).

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89

The biradicals may also abstract H atoms from the solvent or solute and/or react with other radicals to give high-molecular-weight products. This is a rather important point as it suggests that degradation without oxidation is also possible.

2.4.5.4. Substituents on N and C3 Table 6 contains results showing propensity for degradation of spiropyrans variously substituted on the indoline nitrogen atom and/or the bridging C3. The influence of solvent polarity on the photodegradation of 11 has been studied by Gautron.29 It was found that photodegradation was much lower (by up

to one order of magnitude) in protic and polar aprotic solvents

450, 800, and 1450 for 11 in toluene, dimethoxyethane, acetonitrile, and dimethyl sulfoxide, respectively). The dielectric constants for these solvents increase in that order and are and 49, respectively. Similar results were obtained for derivatives of 11 substituted on the indoline moiety.

90

Chapter 2

Photodegradation of Organic Photochromes

91

After the 1968 papers of Gautron, despite the large number of publications on the photophysical properties of spiropyrans and spirooxazines, no further studies on photodegradation of organic photochromes appeared until 1979. Renewed interests and a new impetus to better understand the degradation process arose when it was realized that a major application of photochromes could be in the field of sunlenses and prescription glasses. The possibility of grafting spiropyrans to living polymers30 to obtain photochromic transparent polymers was studied. Living (co)polymers with both flexible and rigid sequences were designed as it had been realized that a photochrome needed a “free volume” sufficient for the molecular reorganization on which photochromism is based in order to fully display its photochromic properties. The spiropyrans were grafted on the flexible region, with the rigid part acting to protect the photochrome. The polymers studied were homopolymers such as poly(styrene) (PS), poly(methyl methacrylate) (PMMA), poly(hexyl methacrylate) (PHMA), and poly(isoprene) (PI) and block copolymers. The rigid phases were those of PS and PMMA whereas PI and PS constituted the soft ones. The photochromic polymers were synthesized by grafting the photochrome, conveniently substituted in order to generate a carbocation intermediate, onto the

living polymer, i.e., a polymer obtained by anionic polymerization.30 Polymerization was carried out at –70 °C under nitrogen, and grafting was done in the dark using an excess of spiropyrans. The photodegradation studies were performed

either in solution (Table 6) or in polymer matrices (Table 7). Pulsed irradiation was used for the solution degradation studies whereas for the photochromic matrices, for which the merocyanine thermal bleaching rate constant depends strongly on the viscosity of the medium, a continuous irradiation method was used. For continuous irradiation it was necessary to take into account, in the expression for the maximum absorption Am, the degradation and photocoloration process undergone by the photomerocyanine:

where I is the initial UV light intensity; is the photocoloration reaction rate constant; is the thermal decoration rate constant, is the rate constant for the UV-induced photodecoloration of the merocyanine; is the rate constant for the visible-induced photodecoloration of the merocyanine; I´ is the visible light intensity; and and are the rate constants for the photochemical UV-induced degradation of the spiropyran and the merocyanine, respectively. Equation (2.4.5.4.1) applies to Scheme 17. For flash-photolysis experiments, a different equation was used to describe the absorption after the first flash:

92

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where and are the absorption coefficients of SP and MC, respectively; l is the cell pathlength; is the total coloration quantum yield; k, a constant, is a function of the flash energy E and the experimental conditions of the reaction. Spiropyrans SP1, SP2 and SP3 (56, 60, and 61 in Table 6) were grafted on each of the four polymers PS, PMMA, PHMA, and PI and some of their copolymers, and the time for total degradation was measured. The results reported in Table 7 are in line with the previous findings of Gautron28 on the effect of substituents on photodegradation. For the homopolymers, degradation is related to the glass transition temperature (see 69–73) The reactivity of to the double bonds present in the polymer chain could contribute to the degradation by peroxidation of the allyl C–H bonds and explain the lower photostability of photochromes in PI. For toluene solutions of photochromic polymers, it was noted that photodegradation of the grafted spiropyran

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93

was not substantially different from that of the same photochrome in solution. However, the matrix affects the colorability and decoloration rate, mostly because of the higher viscosity of the polymeric medium.

2.5. PHOTODEGRADATION OF BENZOTHIAZOLINOSPIROPYRANS Spiro compounds other than spiro[indoline-benzopyrans] have been synthesized with the aim of finding photochromic systems endowed with both good colorability and long durability. Benzothiazolinospiropyrans 77 have been extensively studied by Guglielmetti’s group.31 Studies were carried out on aerated toluene solutions, and photodegradation was quantified in terms of The effect of substituents on both the benzothiazoline and chromene moieties was assessed. Photocolorability of this class of photochromes is, in general, lower than that of indolinospiropyrans (Table 8).

77

94

Chapter 2

In quantitative degradation studies, in order to separate electronic from steric effects, the Taft equation, relating the degradation parameter V1/2 to steric r°v, inductive and resonance parameters, was used:

where and h are constants for the indicated parameters and r °v is the steric hindrance radius of the R3 substituent. The following values were found to be adequate for a good fit of the experimental data: and h = 1.2. Based on this data treatment, the steric parameter emerged as the dominant parameter defining photostability. A more systematic analysis of the effects of substituents on photostability was carried out on polysubstituted benzothiazolinospiropyrans (Table 9). With an increase in the electron-donating power of the R3' substituent on the thiazolino N atom, photodegradation decreases

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95

96

Chapter 2

(78 vs. 93), paralleling the results obtained for the indolino series.28 With substituents at C8 having close to zero, varies between 3 and ca. 10 (101, 102, and 104). Only groups affect photodegradation in a significant way. and groups at C6' gave the best results in terms of improved stability (98, and 99). It was noted that photodegradation was more pronounced in more dilute solutions, whereas polar protic or aprotic solvents increased photostability. However, the behavior of benzothiazolinospiropyrans was found to be very similar to that of their indolino counterparts.28 The degradation studies were extended to aza-, oxaza-, and thiazacyclopentane and cyclohexane spiropyrans32:

where The substituents R were either electron-donor or electron-acceptor groups, and H was either a benzo-condensed or a simple saturated heterocyclic ring. The effects of substituents on the photodegradation may be appreciated by examining the values presented in Table 10. In the indoline series the introduction of a methyl group at C3 increases photodegradation (11, and 47). The benzothiazolinospiropyrans (X = S) are much less stable than their indoline counterparts, probably because the bond cleavage has a more homolytic character. For the benzoxazoline series (X = 0), no

major effects were observed.

2.6. SATURATED FIVE-MEMBERED RING AZAHETEROCYCLIC SPIROPYRANS For 105 ongoing from benzo-condensed (see Table 10) to a simple saturated heterocycle ring 111, it may be seen that the absence of the benzo group on the heterocyclic moiety leads to a hypsochromic shift of the absorption maximum of the merocyanine form, although the colorability is not greatly affected (Table 11).

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2.6.1. Pyrrolidine Series

The introduction of a group at C3 does not affect degradation whereas a phenyl group at this position improves photostability (112, 113, and 114). Compared to the indoline series, the pyrrolidine spiropyrans offer no special advantages.

98

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Photodegradation of Organic Photochromes

99

2.6.2. Thiazolidine Series Derivatives with an ethyl or phenyl group at C3 have better photostability (117, 121, and 122). No major differences by comparison with the indoline series were observed. 2.6.3. Oxazolidine Series

This series has a photostability comparable to that of the indolinospirobenzopyrans and better than that of the thiazolidino- and pyrrolidinospiropyrans (127 vs. 112). The substitution of the benzene ring in the oxazolidine and thiazolidine series decreases light stability, whereas electron-donor substituents on the heterocyclic moiety do not yield any substantial improvement.

2.7.

SATURATED SIX-MEMBERED RING AZAHETEROCYCLIC SPIROPYRANS

No major improvement of photodurability was observed on going from the five- to the six-membered ring azaheterocyclic spiropyrans.32 In the piperidine series, a methyl group at C3 diminishes photostability by a factor of, 2 (Table 12, 142 vs. 144). The introduction of electron-acceptor atoms such as sulfur and oxygen at the position to the nitrogen atom of the azaheterocycle results in a decrease of the photostability of the modified spiropyran. On the other hand, the indoline series

compounds are more photostable than the pyrrolidinospiropyrans. As noted previously, electron-donor groups (e.g., on the azaheterocyclic moiety increase photostability.

2.8.

SPIROPYRAN DEGRADATION: A QUANTITATIVE APPROACH

The substituent effects reported so far have been interpreted in terms of modification of the bond polarity, with those increasing it being beneficial for light stability of the substituted spiropyran (Figure 6). In 1990 Malkin et al.17 in a study of a large number of variously substituted spiropyrans, critically analyzed the structure–photostability correlation proposed by Gautron28 in 1968. In particular, the Russian authors argued that the unified empirical parameter used by Gautron to analyze and compare the photostability of the many spiropyrans studied could not be used in structure–property correlations for different series of spiropyrans as was both concentration and light-pulse energy dependent. Furthermore, from previous studies it was not known which

100

Chapter 2

Photodegradation of Organic Photochromes

101

form of SP (spirocyclic or open merocyanine or both) underwent irreversible photodegradation. Malkin et al.17 were the first to propose a quantitative method of determining the quantum yield of photodegradation, for photochromic spiropyrans. During continuous photoirradiation of a spiropyran solution in parallel with the photocoloration process photo-, and thermal degradation may take place to give degradation products D (see Scheme 17). The

irreversible photodegradation of SP leads to a concentration change that can be described by

where is the intensity of the incident light at the exciting wavelength; and are the extinction coefficients of the spiropyran and the ith photodegradation

product, respectively, at wavelength solution at

and

is the optical density of the

By integration, Eq, (2.8.1) gives

where C° Sp the initial concentration of SP, l is the optical pathlength, t is the irradiation time, and V is the volume of the solution. Under conditions of pulsed photolysis, the merocyanine optical density variation at wavelength is

since

substituting in Eq. (2.8.2), one obtains

where

and

are the MC optical density variations at wavelength

after time 0 and t of photoirradiation with continuous light; optical densities of the solution at wavelength

and

are the

is the intensity of the light

102

Chapter 2

Photodegradation of Organic Photochromes

103

absorbed by SP at is the Avogadro number; V is the volume of the solution (litres); t is the irradiation time(s); and l is the optical pathlength. Irradiation of solutions of SPs was carried out with a 1000-W high-pressure mercury lamp for the continuous-irradiation mode and with a pulsed-photolysis setup, having a resolution, for the pulsed UV irradiation studies. Filters were used to isolate the 320–380-nm lamp wavelength range. The spiropyrans illustrated in Figure 6 could be divided into five main classes differing in the dependence of the photodegradation quantum yield on the oxygen concentration and the degree of conversion of SP: 1.

2. 3.

does not depend on the degree of conversion or the presence of oxygen

(Figure 7 and Table 13). increases in the presence of oxygen as the degree of conversion of SP increases (nitro-substituted SP) (see Figure 8 and Table 14). increases as the degree of conversion of SP increases (no oxygen effect) (Figure 9 and Table 15).

104

3. 4. 5.

Chapter 2

increases as the degree of conversion of SP increases (no oxygen effect) (Figure 9 and Table 15). decreases in the absence of as the degree of conversion of SP increases (see Figure 10 and Table 10). decreases in the presence or the absence of as the conversion of SP increases (Figure 10 and Table 16).

For most of the SPs studied, Eq. (2.8.5) is obeyed and the dependence is linear up to a 90% conversion as evidenced in Figure 7.

Photodegradation of Organic Photochromes

105

SPs belonging to the dithiolane series (150–154) are thermally and photochemically unstable and decompose in the presence of acids, and it is thus not possible to compare their values to those of other spiropyrans. It is worth noting that for all nitrospiropyrans and a few other derivatives, changes during photolysis, for the nitrospiropyrans, it already changes beyond 5% conversion. For a few spiropyrans (Table 15), changes during photolysis whether or not is present. This phenomenon is thought to be due to side reactions of the SP or MC with degradation products D which further consume the starting spiropyran (Scheme 18). For all nitro-substituted SPs, photooxidation processes are responsible for photodegradation. These compounds are characterized by a relatively long-lived

106

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Photodegradation of Organic Photochromes

107

triplet state of the spirocyclic form SP, and degradation results from its interaction with molecular ground-state oxygen (Scheme 19).

On the other hand, for the triplet-state lifetimes shorter than microseconds photodegradation is not affected by the presence of oxygen. At variance with Gautron’s conclusions, Malkin et al.17 claimed that it is not possible to assume that the irreversible degradation processes are entirely determined by the reaction of MC with oxygen as for many SPs the degradation quantum yield is not affected

108

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by Furthermore, even when a dependence on is observed, degradation takes place also in the absence of oxygen (Figure 10). Central to the reasoning of Malkin et al. is the observation that photooxidation plays an important role only in the case of SPs with a relatively long-lived triplet state 3SP and that the dependence of on the degree of conversion is due to further thermal side reactions of both SP and MC forms with photoproducts (see Scheme 18). Gautron, from his structure–reactivity correlation for the photodegradation process of SPs, had concluded that electron density factors that induced a decreased polarization of the bond would increase the efficiency of its homolytic cleavage, and, because of the high reactivity of the resulting biradical with photodegradation would become more efficient and pronounced.28 Malkin et al.17 disputed these conclusions mostly because the photodegradation efficiency is highest for nitro-substituted SPs, for which the quantum yield of photocoloration is the highest, and for some of the compounds

studied by Gautron28,29 in which the polarity of the bond changed drastically, photodegradation was not appreciably affected. According to Malkin et al.17 photodegradation is due to the reaction of the cis-cisoid X isomer with the solvent S and/or impurities (Scheme 20):

It is not necessary then to invoke the existence of a biradical formed by homolytic bond cleavage in order to explain photodegradation. Indeed, calculations on model pyrans show that because of the convergence of the and terms of the excited spiropyran during bond cleavage, the structure formed by cleavage ofthe C–O bond in the state is similar to the biradical triplet structure of the merocyanine form (Figure 11). The authors found no dependence of on the exciting wavelength, and this was taken to mean that only one photoprocess leads to photodegradation and that the bond cleavage takes place from the lower excited states of SP to give the cis-cisoid X isomer. The efficiency of photodegradation is then determined by the quantum yield of formation of the X isomer and the rate of its reaction with the solvent and/or impurities. It was also

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noted that photodegradation and photocoloration are initiated by the same process,

i.e.,

bond cleavage and formation of the cis-cisoid X isomer, and in

general, at variance with Gautron’s conclusions, the highest values were obtained for those compounds having the highest yields of photocoloration. Factors that increase the lifetime of the X isomer lead to a more efficient photodegradation. In summary, Malkin et al. have shown that is almost independent of the heterocyclic part of the SP structure and is determined mostly by the structure of the chromene moiety. Photodegradation and photooxidation are two parallel and concurrent processes, the first one being determined by the reaction of the ciscisoid X isomer of MC with the solvent and impurities. Photooxidation results from the reaction of the triplet state of the colored form of SP with This is

generally observed for NO2-substituted SPs. Sakuragi et al.22 have shown instead that photodegradation (fatigue) of 6-nitro-BIPS originates mainly from the reaction of 3SP to give unknown by-products Z, whereas photocoloration occurs, contrary to what is commonly accepted, 90% through the excited singlet state and 10% through the triplet state when directly excited (see Scheme 12). Irradiation of a toluene solution of 6-nitro-BIPS with 313-nm light afforded several degradation products of different polarities, and one of them was identified as 2-hydroxy-5-nitrobenzaldehyde. Acidic products were also found. The factors influencing fatigue were analyzed. The solvent affected photodegradation, with the half-life increasing with increasing solvent polarity. 6-Nitro-BIPS had a half-life of ca. 1 min in hexane and longer than l00 min in acetonitrile (Figure 12). It was suggested that in nonpolar solvents strong intermolecular interactions would accelerate photodegradation processes. Increasing the concentration was found to

110

Chapter 2

affect photodegradation more severely in the case of spiropyrans. Samples of 6-nitro-BIPS in acetonitrile, upon irradiation alternately at 365 nm and 580 nm, underwent a fatigue process much slower (6:1) than that observed in toluene (Figure 13). Excited-state quenchers or sensitizers free in solution or linked as substituents R on 211 were found to affect the fatigue resistance of the spiropyran toward photoexcitation. Photochemical fatigue of 6-nitro-BIPS (6) depended on the presence in solution of triplet sensitizers or quenchers. The triplet energy ET for 6 is 64.3 kcal When benzophenone, a triplet-state sensitizer having a triplet energy was added to solutions of 6, a considerable acceleration of the fatigue was observed (Figure 14). On the other hand, l,1'-binaphthyl acted as a triplet-state quencher, and its presence in solution decreased the colorability (Figure 15). Fatigue was alleviated by the presence of amines that behaved also as quenchers. l,4-Diazabicyclo[2.2.2]octane (DABCO) was the most effective. DABCO and other amines acted mostly as neutralizing agents of acidic fragments which accumulated as result of photodegradation (Figure 16). Quenchers (naphthyl moiety; see 212, and 213) and sensitizers (benzophenone moiety, see 214) of the SP triplet state exerted similar effects when linked to the SP molecule. The half-life cycles of compounds 211–214 were more than doubled (Figure 17).

Photodegradation of Organic Photochromes

111

2.9. NON-AZA HETEROCYCLIC SPIROPYRANS 2.9.1. 6- and 8-Substituted Spirobenzodithiolane and Spirobenzoxathiolane Benzopyran Series

Compounds of general formula 215 have been synthesized, and their colorability and photostability have been studied and compared to those of azaheterocyclic spiropyrans.33 In this latter series, it has been shown32 that electron-donor

112

Chapter 2

Photodegradation of Organic Photochromes

113

114

Chapter 2

substituents on the heterocyclic portion (H in 215) and at C3 improve fatigue resistance, whereas a on the benzene ring of the benzopyran part diminishes the negative effect of a group on the fatigue phenomenon. The subsequent effects found in this series are very similar to those previously reported.28 In particular, electron-donor groups (e.g., at C8 increase the photostability of C6 nitro-substituted derivatives (216 vs. 217, Table 17). For C8 nitro derivatives (223–230), and at C5 decrease photodegradation (224 vs. 223 and 226 vs.227). In general, dilute solutions of these spiropyrans have a better colorability and a poorer photostability (Figure 18). C3 phenyl-substituted benzothiazole spiropyrans are endowed with a better colorability, as already observed for the benzoxathiolane series. The presence at C6 of mesomeric electron-acceptor group improves photostability much more than that of a mesomeric donor group (Table 18).

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115

2.9.2. Spiro[bibenzopyrans]

As observed for the previous series,33 derivatives are more photostable than the ones (235 vs. 237, Table 19). No major differences are observed with respect to the behavior of the series discussed above. 2.9.3. Dithiole Series

The colorability of this series is rather poor although electron-withdrawing groups (e.g., at C4' and C5' ) improve it (see 245, Table 20).

2.10. PHOTODEGRADATION OF SPIROOXAZINES From the results reported in the preceding tables, one can state that photochromes having the heterocyclic indoline moiety perform better in terms of both colorability and fatigue resistance. It is interesting to compare the behavior of

116

Chapter 2

spiro[indoline-naphthopyrans] (4), spiro[indoline-naphthoxazines] (8) and spiro[indoline-benzopyrans] (11).34 We have seen that photocolorability has been defined as the apparent extinction coefficient where is the maximum absorbance after the first flash, [SO] is the concentration of spirooxazine, and l is the pathlength. The parent spirooxazine 8 exhibits absorption at 294 nm, 319 nm, and 350 run with a shoulder

Photodegradation of Organic Photochromes

117

118

Chapter 2

at 367 nm in toluene solution. The value depends on the wavelength composition of the irradiation light. By using filters having different cutoff values, it was possible to show that the 294-nm and 319-nm absorption bands were the ones determining the degree of colorability. When Wood glass (passing bandwidth and plain glass (cutoff nm) filters were used, the colorability dropped by a factor of 2 (Table 21). Colorability is a function of the light flash energy. It was shown that photodecoloration induced by visible light was weak and did not affect colorability appreciably. For most of the spirooxazines studied, colorability was higher for more dilute solutions for and for indicating the deactivation, at higher concentration, of SO excited states yielding MC. Colorability increases for a given SO on passing from toluene to methanol (see 247 and 256, Table 22). A sharp increase was also observed for nitro- (251) and piperidinyl- (256), substituted naphthoxazines and for the phenanthrene analog 262 (Table 22). Photodegradation is a function of the light flash energy and irradiation wavelength. By increasing the flash energy, photocoloration is improved whereas photostability decreases. For unfiltered light, for most of the spirooxazines studied. Colorability dropped by a factor of ca. 2 when a Wood

Photodegradation of Organic Photochromes

119

glass filter was used, but photostability increased by a factor of 40. Spirooxazines are even more stable if irradiated with light filtered through plain glass without losing too much colorability (Table 23). From the above, it is clear mat absorption at 294 run and 320 nm is the most effective in promoting photodegradation. 2.10.1. Solvent Polarity and Photodegradation As result of photodegradation, the thermal bleaching rate constant increases by a factor ranging from 2 to 4 as reported in the case of 247 and 256, probably because of an increasing polarity of the medium. On the other hand, polarity does not appreciably affect photodegradation of spirooxazines. It is believed that the oxazine ring opening proceeds through a heterolytic cleavage of the bond, and a further increase of bond polarity has no major effects. 2.10.2. Structure and Photodegradation Of the three compounds studied, 4, 8, and 11, 8-methoxy-6-nitro-BIPS (11) had the worst performance although it exhibited a very good colorability. The colorability of spironaphthopyrans was low, and their fatigue resistance slightly better. Spirooxazine 8 has the best photostability and an acceptable colorability (Table 24). Photodegradation and colorability change in opposite ways. Indeed, as mentioned before, more dilute solutions have a better colorability and poorer

120

Chapter 2

fatigue resistance. This dichotomy seems to be the real bottleneck for achieving an acceptable performance with organic photochromes. A new photochromic spirooxazine, 263, was reported by Me and coworkers35 in 1990. 1',3',3'-Trimethylspiro[3H-naphth[2,1 -b][1 ,4]-oxazine-3,2'-piperidine] (263) excited at 308 nm with a pulsed excimer laser (XeCl, 60 mJ per pulse) and bleached at nm showed an excellent fatigue resistance that was the same

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121

as that exhibited by the parent spiro[indoline-naphthoxazine] 8 up to ca. 1000 flashes. Yoshida and Morinaka36 have studied the photodegradation of vacuumdeposited films of spiro[1´,3´,3´-trimethylindoline-6-hydroxybenzopyran] (7).

122

Chapter 2

The amorphous films were irradiated with a 100-W super-high-pressure mercury lamp and exhibited a pale to dark blue color. With long exposure times (>2 h), peaks at 400 am and 600 nm decreased in intensity whereas the 490 nm absorption band steadily increased and the film turned reddish. The oxygen content of the irradiated film increased by a factor of ca. 2.4. From the NMR spectra modifications during UV irradiation, the authors inferred that the bond was cleaved and a new N–H bond formed. Furthermore, a new signal due to a group developed. The cleavage positions were those indicated below:

Up to 1993, the major products from photodegradation of spiro[indolinenaphthoxazines] had not been reported. In 1993, a comparative study of the products from the photodegradation of (11), spiro[indolinenaphthopyran] 4, and spiro[indoline-naphthoxazine] 8 was reported by Guglielmetti and co-workers.37

Photodegradatlon of Organic Photochromes

123

This study showed37 that irradiation of aerated toluene solutions of these three photochromes yielded common products from the heterocyclic moiety, namely, 3,3-dimethyl- and 1,3,3-trimethyloxindole (13 and 14) and 1,2,3,4tetrahydro-2,3-dioxo-4,4-dimethylquinoline (265). From degradation of the chro-

mene moiety, three products were formed: napthol (267), and naphth[l,2-d]oxazole (268).

(266),

The solutions were photolyzed for variable periods of time ranging from 2 hours for 11 to 2 days for 4 and 5 days for 8. The products were isolated by preparative high-pressure liquid chromatography (HPLC) and identified through a combination of mass, NMR, and Fourier transform infrared (FT-IR) spectroscopic data. The results obtained by Guglielmetti and co-workers37 confirmed those of Gautron28 on photooxidation of 4 although, at variance with Gautron’s report, only

124

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small amounts of high-molecular-weight products were isolated. The formation of l-nitroso-2-naphthol and naphthofuran was not observed. No hypotheses concerning the reaction mechanism(s) were put forward. The authors wondered whether photooxidation resulted from the interaction of the biradicaloid form of MC with as previously suggested by Gautron29 or from the reaction of singlet oxygen, derived from the sensitizing action of an unspecified sensitizer, with the

merocyanine. 2.10.3. Mechanism of Photooxidation of Spirooxazines: Singlet-Oxygen Reactivity

It was then important to find out whether spiropyrans and spirooxazines could act as sensitizers, and therefore promote formation of singlet oxygen, and to ascertain

the reactivity of this activated oxygen species with the spirocyclic or the merocyanine form or both. In order to investigate this, methylene chloride solutions of 4, 8, 247, 256, 269, and 270 were excited with a frequency-tripled Nd-YAG laser (355 nm), and emission detected by germanium diode that is generally used to monitor the formation of through its luminescence emission at 1269nm.38

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No emission at 1269 nm was detected, which was taken to mean that no was formed or that its formation quantum yield, was lower than the sensitivity limit of the detector In two-laser, two-color experiments, after excitation at 355 nm the merocyanine formed was excited (2-ns delay) with a green layer (532 nm), and again no was detected, a clear indication that the (photo)merocyanine did not participate in the sensitized formation of singlet oxygen, the species that could induce oxidative degradation of the photochromes. The only noticeable exception was found for 11 for which was measured to be 0.15.

A typical singlet-oxygen sensitizer such as methylene blue (MB) has Having assessed that spirooxazines and their merocyanine forms were not able to sensitize formation of it was necessary to ascertain whether they could behave as quenchers. When MB-containing methylene chloride solutions of the compounds studied were induced to produce by directly exciting the sensitizer with 532-nm laser light, SOs and/or their MC forms behaved as singletoxygen quenchers. The measured quenching rate constants are total quenching constants. The technique that we have used does not allow one to distinguish between the contributions of purely physical and chemical processes to the rate constants The values are reported in Table 25.

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Only 8-methoxy-6-nitro-BIPS (11) behaved as both a good sensitizer and a quencher of The quenching ability of merocyanines was found not to be very

different from that of the corresponding closed spirooxazine form. was also generated thermally in the dark by decomposing 1,4-dimethylnaphthalene endoperoxide (271) at 50 °C (or for a longer time at 40 °C), yielding 1,4-dimethylnaphthalene (272) and singlet oxygen40 (Scheme 21):

When spirooxazines were present in toluene (or acetonitrile) solutions of the endoperoxide, after periods of time longer than ca. 20 half-lives, no appreciable degradation of spirooxazines was observed. This would seem to indicate that even when is independently generated, SOs quench it through a mostly physical deactivation process, and the chemical reaction component is small or exceedingly small. In that study, a solvent dependence of the photooxidative degradation process was noted. Indeed, degradation was much faster in the more polar solvent

acetonitrile than in a nonpolar solvent (e.g., toluene). This behavior was suggested to be indicative of a merocyanine–oxygen interaction that produced a polar intermediate which would be stabilized by solvents of larger dielectric constant for acetonitrile and 2.2. for toluene). A charge-transfer (CT) complex of the type was suggested to be on the reaction coordinate (Scheme 22):

Under controlled photolytic conditions, the rearranged oxidized product 273 was isolated from acetonitrile solution, and its structure was confirmed by X-ray

analysis (Figure 19). Extensive photolysis led eventually to formation of di- and trimethyloxindoles, 13 and 14, a quinolone derivative, 265, and, in the case of

spirooxazines, naphthoxazole 268. In the case of the two spiropyrans included in the study, 2-hydroxyl-l-naphthaldehyde and 5-nitrovanillin were also formed. It is

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worthnoting that, as already observed by Guglielmetti and co-workers,37 naphthofuran (an analog of 268 in which N is substituted with CH) was not formed from oxidative degradation of spiro[indoline-naphthopyran]. The authors suggested that a possible reason could be the different attack site of the nucleophilic superoxide anion on the pseudo carbocation of the partially oxidized (one electron)

merocyanine (Scheme 23).

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The two (crypto)carbocations 275 and 276 are stabilized in the case of the naphthopyran by the naphthyl in the and in the case of the naphthoxazine by the neighboring indoline N atom (Scheme 23).

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It is interesting to note that spirooxazine yields more 1,3,3-trimethyloxindole (14) whereas spironaphthopyran yields more 3,3-dimethyloxindole (13). It was suggested that could, in the case of spirooxazine, attack the indoleninium quaternary nitrogen atom to give an adduct (277) that by rearrangement would give 3,3-dimethyloxindole via N-formyl-3,3-dimethyloxindole (278) (Scheme 24). Other products isolated were 279 and 280.

In their work on quenching of singlet oxygen by spirooxazines and their merocyanine forms, Firth et al.41 reached the same conclusions as Malatesta et al.38 The results reported in the previous two papers indicate the spirooxazines and spiropyrans are not able to induce sensitized formation of singlet oxygen. This

species had, however, been invoked by Guglielmetti and co-workers42 to explain the different product distributions that were observed when toluene solutions of SOs and SPs were photolyzed with a 250-W xenon arc lamp in the presence or absence of a typical singlet-oxygen quencher such as l,4-diazabicyclo[2.2.2]octane (DABCO). The authors reasoned that of the four products formed from the photodegradation of 4 and 8 two were derived from the reaction channel and the other two were derived through the reaction channel of molecular oxygen with the biradical form of the merocyanine (Scheme 25).

The ratio of 3,3-dimethyloxindole to 1,3,3-trimethyloxindole found when photolysis was conducted in the presence of DABCO (20 for SP and 4.6 for SO) and in its absence (6.5 for SP and 0.5 for SO) was taken as an indicator of the incidence of the concurrent reaction channels in the overall photodegradation process of the two classes of photochromes. 267 and 268 are thought to derive from the peroxy derivatives 284 and 285 of merocyanines (Scheme 26).

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The fatigue resistance was measured with an apparatus simulating irradiation conditions49 close to those of daylight exposure; this apparatus is further described in Section 2.11. The above dark–light sequence was repeated, and by interpolation the exposure time

required to decrease the optical density of the solution to

half of its original value was calculated. This value was taken as an indicator of the fatigue resistance of the particular photochromic system. Guglielmetti and

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co-workers42 noted that in the case of 11 a much larger amount of DABCO (100fold excess) was necessary to improve the fatigue resistance of this compound (Figure 20), whereas for spirooxazine the required DABCO/photochrome ratio was considerably smaller, ranging from 1.28 to 1.69 (Figure 21).

We have shown38 that 8-methoxy-6-nitro-BIPS (11) is the only photochrome able to sensitize the formation of 1O2 with a quantum yield that is about one-fourth that of a commonly used singlet-oxygen sensitizer, i.e. methylene blue, for which This benzopyran behaves as a good quencher of as well. Unfortunately, in the paper by Guglielmetti and co-workers an analysis of the

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reaction products formed in the presence or absence of DABCO is not reported for this compound, and it is not possible to check the validity of the concurrent reaction pathways model. In control experiments, it was shown that 1,3,3-trimethyloxindole, under experimental conditions similar to those of photodegradation, did not undergo demethylation to 3,3-dimethyloxindole. Demethylation must then occur on reaction intermediate having the SO and SP skeleton still intact Guglielmetti and coworkers43 have suggested that peroxyl, intermediates 286 formed by reaction of the biradical form of MC with molecular oxygen undergo rearrangement that results in the elimination of the indolyl methyl group as formaldehyde (Scheme 27). A possible alternative mechanism is that proposed by Malatesta et al.38 in which the excited zwitterionic form of MC yields, by charge transfer to molecular oxygen, the superoxide radical anion and the MC radical cation, which by further reactions give the same biradical adduct put forward by Guglielmetti and co-workers (Scheme 28). When the polarity of the medium increases, due to the enhanced polarization of the bond, its cleavage is thought to be almost completely heterolytic. According to Guglielmetti and co-workers43 the only viable reaction pathway is

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that involving the merocyanine (quinoidal or zwitterionic) and singlet oxygen. Indeed, these authors claimed44 that, in methanol only, singlet-oxygen-induced degradation takes place with intermediates such as 289 and 290, to give, in the case of spironaphthopyran, 14, 291–293 (Scheme 29 and Figure 22). Although the proposed mechanism seems attractive and all the fragmentation products, except for the o-naphthoquinone 293, have been identified, it still remains to be demonstrated that the merocyanine forms of spiropyrans and spirooxazines have the ability to induce formation of singlet oxygen, a species on which the entire mechanistic scheme has been based. (This question is being addressed by Guglielmetti’s group). l-Formylimino-2(lH )-naphthalenone (294), 1,3,3-trimethyloxindole (14), 3,3-dimethyloxindole(13), and naphthoxazole (268) have been reported to form in photoaged polyurethane (PU) films containing the parent spiro[indolinenaphthoxazine] (Figure 23).

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These compounds were extracted by refluxing the aged photochromic PU films. A kinetic analysis of the thermal bleaching of merocyanine in PU showed that, at variance with that was observed in toluene solution, no acceleration of the ring closure rate took place. The decoloration was described by a biexponential function:

The kinetic parameters are reported in Table 26.

It has been observed that in toluene solutions the thermal bleaching kinetics of a large number of spirooxazines (Figure 24) changed as photodegradation proceeded.46 For compounds having fast bleaching kinetics, the fading rate increased during photodegradation whereas those characterized by slower kinetics

Photodegradation of Organic Photochromes

exhibited a reduction of the fading rates. It was demonstrated that photodegradation products previously reported did not affect the rate constants. Instead, benzaldehyde, a toluene degradation product, was effective in accelerating or slowing down the ring-closure kinetics of the merocyanine, depending on the structure of the SO. The percent variation of the half-life was calculated as

where is the initial half-life of the merocyanine form, and is its half-life when, because of photodegradation, the absorbance of MC reaches a value i.e., one-half of the initial value. It has been observed that the

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slower the initial bleaching rate of the compound, the more it tended to decrease during irradiation (Figure 25).

2.10.4. Oxidation in Dark Reactions of Spirooxazines

Malatesta et al.47 have shown that thermal dark reactions yield the same intermediate oxidation product(s) as those formed during photodegradation as a result of electron-transfer processes from SO or its MC form to a suitable electron acceptor EA, such as 7,7,8,8-tetracyanoquinodimethane (TCNQ, 316). A complex ion-radical salt (317) was easily formed by simply refluxing a solution of TCNQ and SO in the dark, under an argon atmosphere, where, is the radical anion of TCNQ and X is the neutral TCNQ. A likely reaction mechanism is reported in Scheme 30, where the thermal (b) and

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photochemical (a) routes that lead to the same oxidized and rearranged product 317 are indicated. The X-ray structure of 317 shows that the is arranged in a helical fashion, with the cationic oxidized merocyanine forming a fish-bone-like network (Figures 26 and 27). The complexes are paramagnetic and have conductivity values typical of semiconductors. Powder and single-crystal electron spin resonance (ESR) investigations have indicated the extensive delocalization of the ion-radical electron over tetrameric units (tetrads) formed by the species, with the cation being arranged in segregated layers nearly orthogonal to the TCNQ stacks. The powder ESR spectra of the complex salt have a temperature dependence typical of spin-coupled radical-pair systems; i.e., the electrons residing on adjacent couples interact strongly and their reciprocal orientation may be parallel (triplet biradical) or antiparallel (singlet biradical) (Figure 28). The separation energy (J) between the singlet ground state and the triplet excited state has been calculated to be ca. 1 kcal The helical arrangement of the couples along the crystal b axis was also

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deduced from the dependence of the ESR signal linewidth on single-crystal orientation within the spectrometer applied magnetic field. When was used as oxidizing (EA) agent, a complex salt of similar structure was isolated in which the anion was This salt showed no conductivity. The existence of a thermal degradation route of the type reported above renders even more complex the task of designing a spirooxazine endowed with high stability with respect to photoexcitation. From what has been reported

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so far, it is evident that degradation may occur by interaction of photochromes in

their spiro or open merocyanine form with molecular oxygen; activated oxygen species solvent and/or solvent degradation products (e.g., benzaldehyde or benzoic acid in toluene); impurities, or electron acceptors. 2.10.5. Reaction of Merocyanines with Free Radicals

To the list of interactions that may lead to degradation must be added the easy reaction of free radicals with the merocyanine forms. We have recently reported48 that photochromic PMMA and polycarbonate cast matrices develop a violet– reddish color on aging, due to the formation of free-radical adducts (FRA),

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320–325, of merocyanines. Compounds 321–324 were prepared by photolysis

with a UV lamp (emission maximum at 310 nm) of deaerated acetonitrile solutions of the spirooxazines in the presence of the free-radical generator 2,2´-azobis(isobutyronitrile) (AIBN); the benzyl adduct 325 was prepared by photolyzing the spirooxazine in toluene containing di-tert-butyl peroxide (BOOB) (Scheme 31). The bond adjacent to the carbonyl group of the merocyanine 326 reacts with radicals to give free-radicals adducts (327) and, by further reaction, diamagnetic products (Scheme 32).

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FRAs are compounds characterized by large molar absorptivities (monoadduct: biadduct: The isolated yields are not high (5–20%), but because of the deep color, formation of FRAs gives a permanently colored background to the photochromic system and limits its practical applications.

2.11. PHOTODEGRADATION MEASUREMENTS: EXPERIMENTAL APPARATUS We will now describe a computer-controlled apparatus designed to monitor and analyze the photodegradation process of photochromes induced by either cyclic xenon lamp irradiation or the flash photolysis.49 The apparatus is illustrated in Figures 29 and 30. Two compounds, 11 and 8 will be considered, and relevant data obtained using this setup reported. 2.11.1. Irradiation Source

The continuous irradiation source, IL in Figure 30, is a 250-W short-arc xenon lamp fed by a power supply, ILS. The lamp has a daylight spectral type output, a high radiance, and an almost constant color temperature. The pulsed irradiation

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source, IF, is a 150-J linear flash (5 lamp with an ORTHOTRON flash power supply, FS. Three flash triggering modes are available:

• • •

Manual (useful for test purposes) Computer-controlled Free-running

The free-running mode has an internal clock generator that periodically triggers the flash lamp. For long-duration degradation experiments, it is important that the irradiation lamp intensity reaching the measurement cell, MC, remains constant.

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Indeed, a possible problem may derive from the fact that even lamps of the same type set at the same operating point may not deliver an identical light intensity. However, by using a photodiode light sensor, LS, and an OPA amplifier, it is possible to convert the current into a stabilized output voltage proportional to the lamp intensity. 2.11.2. Analysis Setup

The analysis lamp, AL, is a 50-W quartz tungsten halogen lamp placed in a lamp housing including a condenser and a rear reflector and is powered by a DCstabilized supply, ALS. The measuring cell is a 5-cm-optical-path quartz cylinder with a water jacket connected to a thermostat (±0.1 °C). The analysis shutter control circuit, SCC, is used for the measurement of the analysis light intensity. The grating monochromator, M, uses a Czerny–Turner optical arrangement. The wavelength can be varied from 200 to 1000nm with a fixed spectral slit width set to ca. 2nm. The photomultiplier, PMT (Hamamatsu 931, a nine-stage, sidewindow tube), has a spectral bandwidth matching the spectral absorption range of the photochromes, and its rectangular-shaped photocathode suited to catch the image of the output slit of the monochromator, M. The PMT has a low sensitivity to stray light because of the side-window geometry. The PMT sensitivity may change, especially during photodegradation studies lasting for several hours. However, when the light is turned off, sensitivity is recovered quickly at a rate that is faster than the rates of the fatigue processes when the anode current is low enough, i.e., 10mA. In general, the sensitivity loss of a given photomultiplier is rather fast during initial operation, and in order to get reproducible results, the measurements are usually carried out after a one-minute stabilization period. Reduction in loss of sensitivity is also achieved by using a frosted glass in front of the PMT in order to make the monochromator exit slit image cover the entire cathode area. When all these precautions are taken, the sensitivity fluctuation is rather limited (less than a few percent), even in long-lasting (>10h) degradation experiments. 2.11.3. Signal-Conditioning Circuits (SCC)

Computer control of the apparatus is obtained by a PC LAB 812 PG interface card that provides the input and output control signals, performs analog-to-digital (A/D) conversion, and clocks the data acquisition rate. The three main parts of the SCC are:

• • •

Shutter command Line circuits I–V converter

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2.11.4. Operating Modes The photochemical fatigue (PF) of a photochromic system can be measured with the experimental setup described above by choosing one of the following modes or a combination thereof: A. B. C. D.

Cyclic degradation Continuous degradation Flash degradation Kinetic measurement

A schematic representation of the first three modes is presented in Figure 31. The cyclic degradation mode A is particularly usefiil to simulate the degradation induced by daylight exposure and is characterized by a sequence of successive periods of brightness and darkness The brightness duration and the darkness duration (hours, minutes, seconds) and the total number of cycles (brightness + darkness) are entered via a user-friendly graphic software interface.

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For kinetic data acquisition, the desired number of points, the sampling step and the number of degradation cycles before measurements are entered. At the end of the experiment, the fatigue resistance time (time at which the initial absorbance value is reduced by half) is calculated by interpolation. In the continuous degradation mode B, the input time sequences and parameters are the same as in the cyclic mode except for the darkness duration, which is reduced to one minute, i.e., the time required to stabilize the PMT. The fatigue resistance is calculated as for mode A. The flash degradation mode C requires a preset number n of flashes followed by one measuring flash, with the input being the number of degradation flashes and sequences. The fatigue resistance is now defined as the number of flashes needed to reach Finally, the thermal bleaching rate constant of the merocyanine back to the colorless form can be measured. In the kinetic mode, once the initial transmittance data has been acquired, the flash is fired and the kinetic decay is sampled with preset rate. For biphasic or multiphasic kinetics, the decay can be adequately described by a sum of exponentially decaying functions f(t):

where the are the amplitudes and the the rate constants for individual steps (Figure 32). The computation of spectrokinetic parameters is carried out by a combination of the Marquardt nonlinear least-squares (NLLSQ) and the Padè– Laplace (PL) method. When comparative fatigue resistance studies are carried out, it is essential to set up standard measurement conditions. In general, anhydrous toluene (on 4-Å sieves) is used as solvent. In order to be able to compare lowcolorability photochromes, e.g., spironaphthopyrans (4) or spirooxazines (8) with the high-colorability ones, e.g., nitrospiropyrans (11), it is important to operate under conditions in which absorbance is a linear function of concentration.

For 11, linearity was observed over the 0.2–2.0 absorbance range. The standard concentration was set at Temperature control is also essential. Indeed, with an increase in the operating temperature, the merocyanine lifetime

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becomes shorter (faster thermal bleaching rate) and the fatigue resistance tR increases (Figure 33). By filtering the xenon lamp emission with a long-pass WG 295 Schott filter, it is possible to remove the high-energy UV-C and UV-B wavelength range. F3 (Figure 30) attenuates the intense IR peaks in the 800 to 1000-nm region. The xenon spectrum would then match the daylight spectrum. Fatigue resistance was 450 and 90 s, respectively, in the presence and the absence of the WG 295 filter in the case of the nitrospiropyran 11. For 8, decreased from 4890 to 3300 s when the filter was not used. The reproducibility of the measurements is, in general, not better than ca. 5%. By using this measurement apparatus, it is possible to screen molecules in terms of fatigue resistance within the same series or between different series of photochromic compounds. While recent, although not numerous, studies4,33,34,36–38,42–48 of spiropyrans

and spirooxazines have addressed the mechanistic aspects of the degradation of these photochromes and elucidated the structure of the intermediates and photooxidation products, only qualitative investigations have been reported regarding the stability of other classes of organic photochromes.

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2.12.

151

PHOTOSTABILITY OF FULGIDES

We present here a summary of what has so far been published on the thermal stability and photostability of fulgides.

Photochromism of fulgides is a molecular phenomenon based on the photocyclization of substituted bis-methylenesuccinic anhydrides or fulgides (328) to l,8a-dihydronaphthalene (l,8a-DHN, 329)2 (Scheme 33).

Many fulgides undergo a range of irreversible reactions, including [l,5]-hydrogen shifts (330), photochemical [l,3]-hydrogen shifts, and oxidation to naphthalene.

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These reactions ate all due to the 8a-hydrogen in l,8a-DHN (329), and replacement of this hydrogen by a methyl group gives a thermally stable 1,8aDHN which does not undergo methyl shifts or ethane elimination reactions50 (Scheme 34).

Fulgide 331 has shown a good performance in PMMA films containing 15– 29% (w/w) of fulgide. However, its photochromic activity was reduced to less than 10% of its original value on heating the film at 80 °C for 10 days.50 When heated at 80 °C, 332 lost 90% of the color in a few hours. The system underwent photochemical fatigue, but the degradation products were not described51. In 20 coloration–decolorization cycles, it was reduced to half its initial concentration. The thienyl analog 33351 (Scheme 35)

exhibits a similar photochemical fatigue but was found to be much more thermally stable and underwent only 3.5% degradation after 249 hours at 80 °C. The most promising fulgides for technological applications are the oxazolyl fulgides51 (335 and 336, Scheme 36).

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They are characterized by a good photochromic response thermal stability (10% degradation after 10 days at 80 °C), and resistance to photochemical fatigue Addition of nickel quenchers was found to improve the photochemical fatigue resistance by a factor of 2–3. On going from 331 to the oxazolyl fulgide 335, the C=C bond is replaced by a C=N bond. This modification had already been shown to considerably improve the photochemical fatigue resistance on going from spiropyrans to spirooxazines.1,2 The same improvement is observed in the fulgide series. Under the above conditions, the photochemical fatigue is 0.63, 0.42, and 2.8% for 335, 336, and 331, respectively. Finally, the anhydride functionality in fulgides undergoes slow and irreversible hydrolysis to esters, especially in alcohols, and this reactivity further contributes to the degradation of this class of photochromes.

2.13. COMMERCIAL SPIROOXAZINES AND CHROMENES: RELEVANT PROPERTIES AND BEHAVIOR The structures of three spirooxazines and one chromene commercialized by Great Lakes Chemical under the trade name Variacrol52 are shown in Figure 34. The fatigue resistance of these dyes may be significantly increased by:

• • •

increasing the dye concentration (Figure 35)52 adding a light stabilizer such as a hindered amine (HALS) or an excitedstate quencher (e.g., DABCO or its derivatives ) (Figure 36)53 excluding oxygen from the system (microencapsulation, hard coating, etc.)54

The relevant data concerning spectral features, colorability, expressed as luminous transmittance Y as perceived by the human eye, and accelerated weatherability for

UNACTIVATED

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the four Variacrols in RAV7, a polyallyl diglycol carbonate, are presented in Figures 37–40. Acknowledgments Dr. L. Crisci helped with the drawings and Dr. M. L. Wis read the manuscript. Their help is gratefully acknowledged. I am thankful to Dr. C. Neri for his continuous interest in the photochrome work. We would also like to thank the European Commision (Bruxelles, B) for partial financial support through the BRITE/Eurarn Project No. BE 3380 BRPR CT 96 0228. REFERENCES 1. G. H. Brown, in: Photochromism (G. H. Brown, ed.), pp. 1–11, Wiley-Interscience, New York

(1971). 2. H. Dürr, in: Photochromism: Molecules and Systems (H. Dürr and H. Bouas-Laurent, eds.), pp. 1– 14, Elsevier, Amsterdam (1990). 3. G. Favaro, F. Masetti, U- Mazzucato, G. Ottavi, P. Allegrini, and V. Malatesta, Photochromism, thermochromism and solvatochromism of some spiro[indolinoxazine]–photomerocyanine systems: Effects of structure and solvent, J. Chem. Soc., Faraday Trans. 2 90, 333–338 (1994). 4. C. Bohne, M. G. Fan, Z.-J. Li, J. Lusztyk, and J. C. Scaiano, Photochromic processes in spiro(l,3,3trimethylindolo-2,2´-naphth[l,2-b]-l,4-oxazine) studied using two-laser two-colour techniques, J. Chem. Soc., Chem. Commun. 1990, 571–572. 5. E. Fischer and Y. Hirshberg, Formation of coloured forms of spiropyrans by low-temperature irradiation, J. Chem. Soc. 1952, 4522–4524. 6. R. E. Fox, Research reports and test items pertaining to eye protection of air crew personnel, final report on contract AF41(657)-215 (April 1961), AD 440226. 7. R. J. Hovey, N. Y. C. Chu, D. G. Piusz, and C. H. Fuchsman, Photochromic compounds, U.S. Patent 4215010(1980). 8. S. A. Krysanov and M. V Alfimov, Ultrafast formation of transients in spiropyran photochromism, Chem. Phys. Lett. 91 77–80 (1982). 9. S. Schneider, A. Mindl, G. Elfinger, and M. Melzig, Photochromism of spirooxazines: Investigation of the primary process in the ring-opening reaction by picosecond time-resolved absorption and emission spectroscopy, Ber. Bunsenges. Phys. Chem 91, 1222–1224 (1987). 10. J. Z. Zhang, B. J. Schwartz, J. C. King and C. B. Harris, Ultrafast studies of photochromic spiropyrans in solution, J. Am. Chem. Soc. 114, 10921–10927 (1992). 11. C. Lenoble and R. S. Becker, Photophysics, photochemistry, kinetics, and mechanism of the photochromism of 6´-nitroindolinospiropyran, J. Phys. Chem. 90, 62–65 (1986). 12. N. P. Ernsting, B. Dick, and T. Arthen-Engeland, The primary photochemical reaction step of unsubstituted indolino-spiropyrans, Pure Appl. Chem. 62, 1483–1488 (1990). 13. N.P. Ernsting and T. Arthen-Engeland, Photochemical ring-opening reaction of indolinospiropyrans studied by subpicosecond transient absorption, J. Phys. Chem. 95, 5502–5509 (1991). 14. S. Aramaki and G. H. Atkinson, Spiroxazine photochromism: Picosecond time-resolved Raman and absorption spectroscopy, Chem. Phys. Lett 170 181–186 (1990). 15. S. Aramaki and G. H. Atkinson, Spironaphthopyran photochromism: Picosecond time-resolved spectroscopy, J. Am. Chem. Soc. 114, 438–444 (1992). 16. R. Heiligman-Rim, Y. Hirshberg, and E. Fischer, Photochromism in spiropyrans. Part IV Evidence for the existence of several forms of the colored modification, J. Chem. Soc. 1962, 2465–2470.

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17. Ya. N. Malkin, T. B. Krasieva, and V A. Kuzmin, Quantitative study of the photostability of spiropyrans, Izv. Akad. Nauk SSSR, Ser. Khim. (Engl. Ed.) 1990, 236–243. 18. S. Schneider, F. Baumann, U. Kluter, and M. Melzig, Photochromism of spirooxazines. CARS investigation of solvent effects on the isomeric distribution, Ber. Bunsenges. Phys. Chem. 91, 1225– 1228 (1987). 19. Y. Kalisky, T. E. Orlowski, and D. J. Williams, Dynamics ofthe spiropyran–merocyanine conversion in solution, J. Phys. Chem. 87, 5333–5338 (1983). 20. N. Tamai and H. Masuhara, Femtosecond transient absorption spectroscopy of a spirooxazine photochromic reaction, Chem. Phys. Lett. 191, 189–194 (1992). 21. T. Tamaki, M. Sakuragi, K. Ichimura, and K. Aoki, Laser photolysis studies of nitrospiropyrans intramolecularly linked with a triplet quenching or sensitizing side group, Chem. Phys. Lett. 161, 23–26 (1989). 22. M. Sakuragi, K, Aoki, T. Tamaki, and K. Ichimura, The role of triplet state of nitrospiropyrans in their photochromic reaction, Bull. Soc. Chem. Jpn. 63, 74–79 (1990). 23. F. Wilkinson, J. Hobley, and M. Naftaly, Photochromism of spiro-naphthoxazines: Molar absorption coefficients and quantum efficiencies, J. Chem. Soc., Faraday Trans. 88 1511–1517 (1992). 24. N. Y. C. Chu, Photochromism of spiroindolinaphthoxazine. I. Photophysical properties, Can . J. Chem. 61, 300–305 (1983). 25. A. Samat, D. De Keukeleire, and R. Guglielmetti, Synthesis and spectrokinetic properties of photochromic spiropyrans, Bull. Soc. Chim. Belg. 100, 679–700 (1991). 26. P. Appriou, R. Guglielmetti, and F. Garnier, Study o f the photochemical processes in the benzopyranic ring opening reaction of photochromic spiropyrans, J. Photochem. 8, 145–165 (1978). 27. G. Favaro, V. Malatesta, U. Mazzucato, G. Ottavi, and A. Romani, Thermally reversible photoconversion of spiroindoline-naphtho-oxazines to merocyanines: A photochemical and kinetic study, J. Photochem. Photobiol, A 87 235–241 (1995). 28. R. Gautron, Photochromisme des indolinospiropyranes. IV—Étude de la dégradation par voie physique. Relation avec la structure, Bull. Soc. Chim. Fr. I968, 3190–3200. 29. R. Gautron, Photochromisme des indolinospiropyranes V—Mecanisme de la photodegradation, Bull. Soc. Chim. Fr. 1968, 3200–3204. 30. D. Gaude, C. Le Bacmon, R. Guglielmetti, and R. Gautron, Photochromisme des spiropyrannes. Étude de la photodégradation de dérivés indoliniques susbititués et polymériques, Bull. Soc. Chim. Fr. 1979, 9–10, 489–498. 31. D. Gaude, R. Gautron, R. Guglielmetti, and J. C. Duffy, Photochromisme et photodégradation de dérivés benzothiazoliniques, Bull. Soc. Chim. Fr. 1981, II, 14–18. 32. D. Gaude, R. Gautron, M. Maguet, and R. Guglielmetti, Photochromisme des spiropyrannes. Photodégradation d’aza, oxaza- et thiaza-cyclopentane- et cyclohexanespiropyrannes, Helv. Chim. Acta 66, 342–349 (1983). 33. D. Gaude, R. Gautron, and R. Guglielmetti, Photochromisme des spiropyranes. Étude de la colorabilité et de la photodégradation de dérivés non-azaheterocycliques, Bull. Soc. Chim. Belg. 100, 299–313(1991). 34. R. Gautron, D. Eloy, P. Escaffre, R. Guglielmetti, E. Pottier, and P. Tardieu, Etude par la photolyse à éclairs répétés de la photodégradation et de la colorabilité de quelques spiro[indoline-oxazines], Bull. Soc. Chim. Belg. 100, 315–328 (1991). 35. S. Kawauchi, H. Yoshida, N. Yamashina, M. Ohira, S. Saeda, and M. Irie, A new photochromic spiro[3H-l,4-oxazine], Bull. Chem. Soc. Jpn. 63, 267–268 (1990). 36. T. Yoshida and A. Morinaka, Decomposition process of the photochromic compound spiro[l,3,3trimethylindoline-6´-hydroxy benzopyran] in the solid state under UV irradiation, J. Photochem. Photobiol. A 63, 227–234 (1993). 37. G. Baillet, G. Giusti, and R. Guglielmetti, Comparative photodegradation study between spiro[indoline-oxazine] and spiro[indoline-pyran] derivatives in solution, J. Photochem. Photobiol. A 70 157– 161 (1993).

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38. V Malatesta, M. Milosa, R. Millini, L. Lanzini, P. Bortolus, and S. Monti, Oxidative degradation of organic photochromes, Mol. Cryst. Liq. Cryst. 246, 303–310 (1994). 39. F. Wilkinson, W. P. Helman, and A. B. Ross, Quantum yields for the photosensitized formation of the lowest electronically excited singlet state of molecular oxygen in solution, J. Phys. Chem. Ref. Data 22, 113–262(1993). 40. N. J. Turro, M.-F. Chow, and J. Rigaudy, Mechanism of thermolysis of endoperoxides of aromatic compounds. Activation parameters, magnetic field, and magnetic isotope effects, J. Am. Chem. Soc. 103, 7218–7224 (1981). 41. A. A. Firth, D. J. McGarvey, and T. G. Truscott, Photochemical properties of spirooxazines, Mol. Cryst. Liq. Cryst. 246, 295–298 (1994). 42. C. Salemi, G. Giusti, and R. Guglielmetti, DABCO effect on the photodegradation of photochromic compounds in spiro[indoline-pyran] and spiro[indoline-oxazine], J. Photochem. Photobiol. A 86, 247–252 (1995). 43. G. Baillet, M. Campredon, R. Guglielmetti, G. Giusti, and C. Aubert, Dealkylation of N-substituted indolinospironaphthoxazine photochromic compounds under UV irradiation, J. Photochem. Photobiol. A 83, 147–151 (1994). 44. G. Baillet, V. Lokshin, R. Guglielmetti, and G. Giusti, Photooxydation du photochrome 1,3,3trimethylspiro[indoline-naphthopyrane] dans le methanol, C. R. Acad. Sci. Paris 319, Série II, 41– 46 (1994). 45. G. Baillet, G. Giusti, and R. Guglielmetti, Study of the fatigue process and the yellowing of polymeric films containing spirooxazine photochromic compounds, Bull. Chem. Soc. Jpn. 68, 1220–1225 (1995). 46. G. Baillet, R. Guglielmetti, and G. Giusti, Variation of the bleaching rate of some photochromic compounds under irradiation in toluene, Mol. Cryst. Liq. Cryst. 246, 287–290 (1994). 47. V. Malatesta, R. Millini, and L. Montanari, Key intermediate product of oxidative degradation of photochromic spiro-oxazines. X-ray crystal structure and electron spin resonance analysis of its 7,7,8,8-tetracyanoquinodimethane ion-radical salt, J. Am. Chem. Soc. 117, 6258–6264 (1995). 48. V. Malatesta, F. Renzi, M. L. Wis, L. Montanari, M. Milosa, and D. Scotti, Reductive degradation of photochromic spiro-oxazines. Reaction of the merocyanine forms with free radicals, J. Org. Chem. 60, 5446–5448 (1995). 49. R. Dubest, P. Levoir, J. J. Meyer, J. Aubard, G. Baillet, G. Giusti, and R. Guglielmetti, Computercontrolled system designed to measure photodegradation of photochromic compounds, Rev. Sci, Instrum. 64, 1803–1808 (1993). 50. H. Suzuki, A. Tomoda, M. Ishizuka, A. Kaneko, M. Furui, and R. Matsushima, Photochromism of 4-oxazolyl fulgides, Bull. Chem. Soc. Jpn. 62, 3968–3971 (1989). 51. A. Kaneko, A. Tomoda, M. Ishizuka, H. Suzuki, and R. Matsushima, Photochemical fatigue resistance and thermal stability of heterocyclic fulgides in PMMA film, Bull. Chem. Soc. Jpn. 61, 5368–5373 (1988). 52. Products catalogue, Great Lakes Chemical s.r.l, Milan, Italy (1994). 53. V. Malatesta, P. Allegrini, and L. Crisci, Stabilizing mixture having photochromatic activity, U.S. Patent 5242640 (1991). 54. V. Malatesta, L. Crisci, and P. Allegrini, Procedimento per aumentare la resistenza all’invecchiamento di articoli fotocromatici, Italian Patent Application MI/93A01216 (1993).

3 Photokinetics under Continuous Irradiation M. H. Deniel, D. Lavabre, and J. C. Micheau 3.1. INTRODUCTION In photochromic systems, the number, nature, and kinetic and spectral properties of the transient species formed under irradiation cannot be established without a good knowledge of the reaction mechanism. Flash photolysis is often used for analysis of this type of problem. 1–9 However, fast methods are not well suited to the study of slow photochromic systems such as those used in variable-transmission glasses. In practice, the fast time scales of observation in

flash photolysis are completely different from those of the slower reactions characterizing the accumulation of a slow photoisomer under a low-powered photon flux.10 Among the different photoisomers formed, only those with the longest lifetimes (> 10 s) will be observed by techniques using continuous irradiation (see Appendix 1 of this chapter). Although technically simple, continuous irradiation has the following advantages:

• Methods employing continuous irradiation consider the photochromic system as a dynamic system that is open to light flux energy. The kinetic equation of the photochemical reaction can thus be established. This equation is not taken into consideration in the pulsed methods as they study only the thermal relaxation process after the flash. • The experimental conditions for continuous irradiation that can be set up in research laboratories are quite comparable to those encountered in natural, environmental, or industrial photochemical systems.

M. H. Deniel, D. Lavabre, and J. C. Micheau • Lab. I.M.R.C.P., UMR 5623 au CNRS, Paul Sabatier University, F-31062 Toulouse, France. Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999.

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In this chapter, we describe a methodology for analyzing the reaction of typical photochromic systems under continuous irradiation. In Section 3.2, we establish the basic photochemical rate equation and describe a suitable apparatus and the numerical techniques required to analyze the results (see Table 1 for a list of abbreviations and symbols). The photoreduction of ferrioxalate will be used to illustrate the first part.11 In Section 3.3, we present a study of the complete dynamics of simple photochromic systems, i.e., those involving the interconversion of only two isomers. These systems are designated to “AB systems” (see Table 2). The influence of thermal and/or photochemical bleaching as opposed to direct photocoloration is then examined using appropriate experimental examples. In Section 3.4, we will examine photochromic systems involving several photoisomers or photodegradation processes. These systems are designated as “ABC systems” (see Table 3). The thermal bleaching of chromenes12 and the photodegradation of spiropyrans13

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will be used to illustrate our approach. Dynamic analysis of these systems under continuous irradiation provides information on the reaction mechanism and enables extraction of parameters that are often difficult to obtain directly, namely, the quantum yields of the photochemical steps and the ultraviolet (UV) visible spectra of the nonisolable photoisomers. The photochromism of the dimer of the triphenylimidazolyl radical (TPID)14 illustrates the nonlinear dynamics and the complexity of the behavior of ABC systems under continuous irradiation (Section 3.5). At the end of this chapter, supplementary material is presented in five appendixes.

3.2.

DYNAMIC ANALYSIS OF THE ISOLATED

PHOTOCHEMICAL STEP: 3.2.1. Establishment of the Photochemical Rate Equation

The expression signifies that overall a photosensitive reagent A is transformed under irradiation into a product B. is the incident monochromatic photon flux (in moles of photons per liter per second) at the irradiation wavelength In the following discussion, the prime (´) always indicates the irradiation wavelength. Abs' is the total absorbance of the solution at this wavelenth15:

where and are the decadic molar extinction coefficients of the photosensitive reagent A and the product B, respectively, and 1´ is the optical pathlength of irradiation (in centimeters). The detailed mechanism of this reaction involves at least one excited state of A, namely, A*. We assume that product B also absorbs light, but that it does not give rise to any visible chemical transformation. Under these conditions, the overall reaction comprises at least three elementary reactions:

The first process is an activation step; all the photons, absorbed by A give rise to

the formation of an excited state A*. The rate of this step is a direct function of the photonic flux absorbed by A, which has the dimensions of a chemical rate

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(moles per liter per second)† and is proportional to the total amount of light absorbed by the reaction medium,

The excited state undergoes two competitive processes of rapid decay: deactivation (radiative or not) and a reaction leading to the formation of the final product B. Under low-power continuous irradiation, the concentration of the excited state [A*], which is a species of short lifetime remains low and changes little. This is thus compatible with Bodenstein’s assumption (quasi-stationary state):

Hence, the equation for the conservation of matter

can be simplified as

and by combining with Eq. (3.2.3):

letting the quantum yield of the overall process The rate equation of an isolated photochemical step is thus

This equation contains three terms: (i) which is constant under given conditions of irradiation, (ii) the concentration of the photosensitive reagent, [A], and (iii) the photokinetic factor16–19

which is a function of [A] and [B]. F´ is a decaying function of Abs´ when and when During the reaction, F´ falls if †

It should be noted that our photonic fluxes are volumic: hence:

and

do not have the same

dimensions as the classical photon flux (moles per square meter per second), and so the volume of the irradiated solution must be taken into account.

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or rises of If the irradiation wavelength corresponds to an isosbestic point20,21 remains constant and the kinetics are first-order. Otherwise, Eq. (3.2.7) cannot be integrated analytically and must be solved by numerical methods.22 In all cases, the changes in the photokinetic factor under continuous irradiation can be obtained by monitoring Abs´. Equation (3.2.7) is rigorous and is applicable to both high and low absorbances.

3.2.2. Photochemical Reactor Uniformly Stirred and Equipped with a System for UV/Visible Spectroscopic Analysis To investigate the photochemical kinetics under continuous irradiation, the

absorbances at all wavelengths including the irradiation wavelength need to be measured experimentally. Figure 1 illustrates a typical experimental setup.23 The type of reactor shown is suitable for studying reactions in the liquid phase (or even in a polymer film; the cell is then replaced by a 45° holder) where -the progress of the photochemical reaction is reflected by changes in absorbance. The experimental kinetics are thus represented by plots of Abs versus t.

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3.2.3. Experimental Verification of the Validity of the Photochemical Kinetic Equations under Continuous Irradiation

The photoreduction of potassium ferrioxalate is a classical example whose kinetic behavior can be summarized by a single photochemical step of the It is a photochemical transformation of ferric into ferrous ions:

The validity of the photochemical kinetic equation (3.2.7) for this simple reaction was demonstrated by simulation and numerical fitting. Equation (3.2.7) is first integrated numerically (semi-implicit Runge–Kutta method) using physically realistic values for all the unknown parameters. From Eq. (3.2.1) (Beer–Lambert’s law), the change in absorbance can be calculated and compared with the measured change. In this case, it is simply expressed by the change in Abs´; the two parameters and are fitted automatically using an iterative algorithm of the Powell type, designed to minimize the residual quadratic error between the experimental and the calculated curves (n is the number of experi-

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mental data points). Each loop of the minimization routine involves a numerical integration of the differential equation (3.2.7). Figure 2 shows the quality of the fit and demonstrate that the kinetics of the photochemical reaction are well described by Eq. (3.2.7). Although it requires equipment for numerical calculations, a simulation and fitting method is of particular interest for more complex mechanisms. It can be employed to analyze thermal or photochemical reisomerizations and reactions involving several photoisomers as well as photodegradation processes. This approach is also referred to as “inverse treatment” as it proceeds back from the experimental data to the parameters of a model. A block diagram of the method of simulation and fitting is presented in Figure 3.

3.3. AB SYSTEMS INVOLVING TWO SPECIES The previous example consisted of single photochemical step In photochromic systems, such a photochemical step is often associated with either a

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reverse photochemical step or/and a thermal step photochromic systems (Table 2) may include a maximum of three interconversion processes between the two isomers A and B (one thermal and two photochemical steps).34–37

3.3.1. Estabishment of the General Kinetic Equation under Continuous Irradiation In AB systems, the two species A and B obey the law of conversation of matter given by

The kinetic equation of the overall systems thus only includes a single variable, for example, [A], giving

or by combining with Eq. (3.3.1.1):

Canceling the rate constant or/and the quantum yield leaves all the sets of subsystems that can be constructed from the AB, 1k system, namely, the AB the AB, 1k, and the AB systems. 3.3.2. Properties of the Photostationary State of the AB,

1k System

For long durations of irradiation, the system reaches a photostationary state (denoted here by the subscript PS) where giving

* Thermochromic systems will not be considered here, and we will assume that the product A is stable.

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This relationship shows the relative importance of the various experimental constraints on the rate of photostationary conversion.38 However, it is more convenient to express Eq. (3.3.2.1) in terms of three nondimensional parameters: (i) (ii)

the rate of conversion into photoisomer B which assesses the relative importance of the photocoloration reaction with respect to the thermal bleaching

(iii)

the ratio of the rates of the reverse and direct photochemical reactions.

This gives

The steady-state behavior of two typical and relatively well-documented AB systems can be described using Eq. (3.3.2.2), namely, AB, 1k and AB, systems. 3.3.2.1. AB, For AB,

1k Systems

1k systems,39–41 the reactions are

In such systems, for which

or

Eq. (3.3.2.2) can be simplified to

Equation (3.3.2.1.1) shows that in this particular case to obtain, for example, a rate of photostationary conversion of 50% must be 1. Assuming that and then or As the order of magnitude of is liter mol –1 and conventional sources of continuous irradiation produce value of of then must be , with order of magnitude for the lifetime of B of s. For shorter lifetimes, increases and decreases.

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3.3.2.2. AB, For AB,

177

Systems the reactions are

Here, the relationship given by Eq. (3.3.2.2) always applies with or The rate of conversion in the photostationary state is thus given by

which involves the parameter and depends only on the irradiation wavelength via the activation ratio For AB, systems, the photostationary state, once attained, remains stable indefinitely even in the dark. 3.3.3. Photostationary Methods In Appendix 2 of this chapter, we review all the available photostationary methods that can be employed to extract the characteristic parameters of simple photochromic systems such as the quantum yields of the photochemical species or the UV/visible spectra of the transient species. Nevertheless, these steady-state methods have limited application. They can only deal conveniently with the simplest photochromic systems, such as the thermally reversible AB, 1k or photochemically reversible AB, For type AB, 1k systems (both thermally and photochemically reversible), the steady-state method is theoretically valid, although numerous practical difficulties restrict its use to a few particular cases.51 For more complicated reaction mechanisms such as those involving photodegradation, photostationary methods are not applicable as the photostationary state is not defined. Another limitation of the photostationary methods is that they only exploit the positions of the steady states and not the underlying kinetic curves. They do not generate enough data to validate the relevant mechanistic hypotheses. We shall show that analysis of kinetic curves under continuous irradiation

represents a much more general method of investigation.

3.3.4. Dynamical Method Using the example of a simple AB, 1k photochromic system, we can define the experiments required to obtain the quantum yield and the complete UV/ visible spectrum of the unstable photoisomer B The equation for the change in absorbance of such a system can be obtained by making in the kinetic

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and steady-state equations, Eqs. (3.3.1.3) and (3.3.2.1), and by application of the Beer–Lambert law, Eq. (3.2.1):

or

where Abs is the absorbance of the system at any wavelength and at time t and AbSPS, is that at the photostationary state Expressing Eq. (3.3.4.1) in the form (3.3.4.2) shows that the estimation of the rate of change in absorbance (–dAbs/dt) and the steady-state absorbance AbsPS will give a value for the coefficient of the apparent rate Kinetic analysis of the decay in the dark will give a value for and so only a knowledge of F´ is required to obtain a value for the product A value for is then obtained after an independent determination of Once is known, is calculated from relationship (3.3.2.1) (with and then the molar extinction coefficient is calculated from the Beer–Lambert law (Eq. 3.2.1). The following measurements are therefore required to obtain and (i) the changes in Abs under continuous irradiation, along with Abs´ (irradiation should be long enough to facilitate estimation of AbsPS) (ii) the rate of change of Abs in the dark after irradiation, from which a value for will be obtained.

The desired parameters ca1n also be obtained by simultaneous simulation and fitting of the absorbance plots at and An illustration of the method of simulation and fitting for determining the kinetic and spectral parameters of dihydroindolizine 1, a thermally reversible photocrome of the AB, 1k

is shown in Figure 4.

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On the other hand, in the presence of a photochromic system of the AB, 1k type,55–57 such as 3-i-propyloxy-8-methoxy-3´-methyl-6-nitro[benzothiazolino-

the coefficient of the apparent rate now becomes

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and two additional parameters, and are unknown. Apart from the apparent quantum yield the method of simulation and fitting as described above cannot produce values for all these parameters. Nevertheless, by irradiation at two different wavelengths, the relative importance of the direct and reverse photochemical steps can be altered by varying the activation ratio generating enough kinetic information to produce values for the two extra parameters. An illustration of this method for the determination of the kinetic and spectral parameters of benzothiazoline 2, a photochrome that is both photochemically and thermally reversible, is illustrated in Figure 5. Figure 6 shows the UV/ visible spectra of the closed and the open form of benzothiazoline 2. If the process of photochemical bleaching is neglected (taking the four curves cannot be fitted satisfactorily. On the other hand, photochemical bleaching can also be demonstrated during the phase of thermal decay by irradiating B with 425-nm light (where However, the value of this acceleration cannot be exploited to obtain a value for the quantum yield as the calculation requires prior knowledge of Analyzing this system with a single irradiation wavelength (as in Figure 7) will produce a correct fitting of the corresponding kinetic curves, but only the “apparent quantum yield” will be obtained, not the true quantum yields.

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3.4. ABC PHOTOCHROMIC SYSTEMS INVOLVING THREE SPECIES The method of simulation and fitting enables extraction of significant kinetic information even with more complicated photochromic systems, such as those which give several photoisomers or those in which there is a photodegradation reaction. Such systems generally involve three overall species A, B, and C and will thus be designated as ABC systems (Table 3).

3.4.1. General Comments There are two main differences between AB systems with two species and ABC systems with three species:

(i) The number of variables required to describe the kinetics of the system: Since in both cases a single conservation-of-matter equation is obeyed, the

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number of variables and differential equations for an ABC system is 2 (for an AB system, it is 1). (ii) The number of processes involved: For the combined direct and reverse isomerization processes of thermal and photochemical nature, the maximum number of possible processes is 4 in an AB system (2 thermal and 2 photochemical) and 12 in an ABC system. This inherent complexity of ABC systems is reflected by a more complex dynamic behavior in the following respects: (i) In the case of the thermal relaxation, it is reflected by the presence of biexponential kinetics leading to uncertainty in the exact localization and also the number (two to six) of elementary processes involved (see Appendix 3 of this chapter). (ii) In the case of photochemistry under continuous irradiation, it is reflected by the presence of several competitive absorbing species which may give rise to nonlinear phenomena such as multiple steady states (see Section 3.5). In view of these considerations, it may not be possible to determine accurately the detailed reaction mechanism in such photochromic ABC systems. However, under some conditions, the experiments can be arranged in such a way as to discriminate between similar reaction mechanisms and extract the relevant parameters. This will include varying the incident photon flux the irradiation wavelength the duration of irradiation the temperature, and the initial concentrations. Appendixes 4 and 5 show how the particular thermal or photochemical processes in ideal photochromic systems of the ABC type can be identified. The dynamic behavior of numerous photochromic systems can be interpreted in terms of an ABC system, these include

(i) Systems that possess two unstable photoisomers whose lifetimes are long enough for both to be observed under continuous irradiation, (ii) systems that are photodegradable, for example, an AB system accompanied by a set of its photodegradation products designated by “C.”

3.4.2. Examples 3.4.2.1. Systems with Two Photoisomers An experimental example of a system with two slow photoisomers is represented by 7,7-diphenyl[7H]pyrano[3,2-e]indole or chromene 3.

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Figure 8a shows the changes in the UV/visible spectrum of the reaction mixture during the thermal bleaching after irradiation. Figure 8b illustrates the biexponential nature of the relaxation process. Although chromene 3 can give rise to four possible open forms (two cisoid and two transoid), the biexponential nature of the relaxation shows that only two different forms accumulate, namely, B and C with the chromene 3 playing the role of the stable isomer A. Several kinetic schemes relating these three species, involving up to six thermal processes (reduced to four in this case, because A is thermally stable), may be envisaged (see Appendix 3), but they cannot be discriminated by analyzing a single observation of a biexponential relaxation. On the other hand, if we assume a simpler mechanism such as one involving only two thermal processes, we show in Appendix 4 that both processes can be localized: by altering the initial concentrations of the photoisomers at the onset of the relaxation, we can find out whether the two thermal processes are convergent or successive (see Table 3). Different initial conditions can be set up by altering the intensity and duration of irradiation. Weak irradiation of long duration favors accumulation of the photoisomer with a long lifetime, whereas strong irradiation of short duration favors the photoisomer with a shorter lifetime (see Appendix 3). However, for chromene 3, the situation is more complicated as there is a photointerconversion reaction between the two photoisomers. If during the slow phase of the thermal relaxation, the solution is irradiated with a pulse of blue light, which is absorbed by photoisomers B and C, but not by chromeme 3, there is a transient acceleration in the decay kinetics (Figure 9). A possible intepretation of this phenomenon is a photochemical transformation of some “slow photoisomer” into a “fast photoisomer,” with a transient fast relaxation followed by resumption of the slow relaxation. The minimal kinetic scheme representing this phenomenon is shown in Figure 10. It appears that the complete establishment of the detailed reaction mechanism requires additional experiments. However, simplifying assumptions based on structural considerations and comparisons of several analogous molecules may be considered. A simplified model including in ABC, 2kc system has been tested successfully but not entirely proved.

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3.4.2.2.

Chapter3

Photodegradation of a Spiro[indoline-benzopyran]

Spiro[indoline-benzopyran] 458 is a highly photodegradable photochromic molecule.59,60

Overall, it can be seen to be an ABC system involving three species: the closed form A, the open form B, and the set of degradation products from the degradation of the closed form and from that of the open form). The consequences of the photodegradation will be seen as a continuous decay in the photostationary state, with an increase in the rate constant with increase in the duration of irradiation. This change in rate indicates that the photolysis products of act as catalysts of the thermal reverse reaction.

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The most parsimonious mechanism to account for those observations can be summarized by the following kinetic scheme:

The main component of the mechanism is a thermally reversible photochromic process of the type, supplemented by a couple of photodegradation reactions, one from and the other from Without going into the details of the mechanisms, we can assume that both the closed and open forms (A and B) undergo photodegradation. No assumptions are made about the structures of the photodegradation products. We only assume that they partly catalyze the thermal decay and are related to the catalysis by the photodegradation products of A and B, respectively).61–63 This mechanism is described by the following set of differential equations:

The dynamics of this mechanism were analyzed from the results of three experiments designed to examine the respective influences of the duration of irradiation, the initial concentration and the incident photon flux (Figure 11). The model reproduces the main characteristics of the experimentally observed kinetics, especially the influence of initial concentration and of the intensity and duration of irradiation. However, there is still uncertainty about whether the photodegradation processes originate either from the closed form (via route or from the open form (via route or both. One or other of the two processes may be neglected without significant deterioration in the quality of the fit. However, they cannot both be neglected. Whatever the assumption about the mechanism, the overall quantum yield of the photodegradation is close to or around 0.7% of the quantum yield of photocoloration Analysis of the model shows the true catalytic influence of the photodegradation products and gives a value for the catalytic rate constant of around

Fitting the other

parameters gives: and 10°C.

at

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Chapter 3

In Appendix 5, we show, however, that in an ideal photochromic system of the ABC type, photodegradation from either the stable form A or the unstable form B can be theoretically distinguished by performing two experiments and altering the intensity of irradiation

3.5. ABC SYSTEMS EXHIBITING NONLINEAR BEHAVIOR 3.5.1. Ideal ABC System Giving Rise to Bistability Some ABC systems can give rise to unusual and complex dynamic behavior such as photochemical bistability, corresponding to the possible presence of two different photostationary states for the same irradiation and initial concentration conditions. The system ABC, (see Table 3) is bistable under conditions in which and We assume that the molar extinction coefficients of A and B are equivalent and that C does not absorb Dynamic analysis of this ideal ABC, system gives an estimate of its behavior as a function of If is low, the photochemical reaction is balanced by the fast thermal back-relation B does not accumulate, the photochemical reaction does not take place, and the system stabilizes into a steady state enriched in On the other hand, if is high, B forms and C is then produced; C is relatively stable as the thermal reaction is slow, and the system stabilizes into a steady state enriched in C. Between these two extreme values of incident photon flux, the behavior of the system will be more complicated. If A is concentrated, it absorbs the largest fraction of and B is little exposed to irradiation. The photochemical reaction does not take place, and the system stabilizes at A. Thus, if A is concentrated at the start, it will remain concentrated in the steady state. On the other hand, if A is low, B is well exposed to irradiation, and the photochemical reaction occurs, C then accumulates instead of A. Thus, if A is low

at the start, A will remain low in the steady state. This will be reflected by the existence of two different photostationary states (A low or A concentrated) for the same value of incident photon flux and the same initial global concentration. This represents a bistable photochemical system (Figure 12). A feature of this ABC, kinetic scheme is its cyclic nature: photoproduct C regenerates the starting compound A. This enables it to operate in a closed system, since it does not photodegrade. Experimental photochromic, photodegradable systems involving similar processes are thus likely to exhibit similar behavior, but they need to be studied in an open system. An illustative case is the photochromism of the triphenylimidazolyl dimer (TPID).

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3.5.2. Photochromism and Photodegradation of TPID

The photochemistry of the dimer of the triphenylimidazolyl radical is an example of photochromism whose dynamic behavior resembles that of a bistable ABC system. It is characterized by the formation of colored, relatively persistent radicals (TPI, 5).

from homolytic cleavage of the dimer64–67:

A fraction of the radicals dimerizes:

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while another fraction are transformed into triphenylimidazole (TPIH) after capture of a hydrogen atom from the solvent:

This last process results in degradation of TPID. Furthermore, three additional

processes participate in the photodegradation: (i) Photosensitization of the solvent by TPIH results in decomposition of the solvent liberating ions:

(ii) The ions protonate the to TPIH:

radicals, which are subsequently reduced

(iii) Photooxidation of TPIH produces phenylphenanthroimidazole (PPI)68:

All these steps are required to accurately account for the progress of the photochromism and the photodegradation.69 The complete mechanism (photochromism + photodegradation) thus resembles the ABC, system. It involves two successive photochemical reactions: and the first being balanced by the thermal back-reaction which may be compared with and the second may be compared with The experimental system is, however, more complex, requiring four differential equations for a complete description:

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The change in absorbance is calculated from

assumming that the main absorbing species at the irradiation (366 nm) and observation (554 nm) wavelengths are TPID, TPI°, and TPIH. Figure 13 shows the fit of the mechanism represented by Eqs. (3.5.2.1) – (3.5.2.5) to the experimental data for the absorbance at 554 nm (TPI° radicals) during photodegradation of TPID under continuous irradiation at 366 nm. The curve in Figure 13 exhibits characteristic parts: (i) Initially, TPI° radicals are formed by homolytic cleavage of TPID. (ii) A “plateau” is reached where the dimerization reaction keeps pace with the formation of radicals. (iii) The photostationary state is not stable due to the photodegradation process, and as [TPIH] increases, more and more acid is formed, giving rise to

194

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a faster hydrogenation of radicals (autocatalytic loop); the photochemical formation of acid is a component of this autocatalytic loop as the proportion of incident light available to irradiate TPIH is inversely related to the concentraiton of radicals. (iv) The concentration of radicals goes to zero when all the initial TPID has been consumed. We have shown that, when carried out in a stirred, continuous-flow photochemical reactor, this reaction exhibits two different stable steady states (bistability). In this particular case, an open system (in energy and mass) is required to circumvent photodegradation. The model can account for this bistable behavior. The main concepts emerging from analysis of an ideal ABC, system can thus be applied to interpret the photochromism and photodegradation of TPID.

3.6.

CONCLUSION

Dynamic analysis of photochromic systems under continuous irradiation represents a powerful method of investigation of the reaction mechanisms. The characteristic kinetic and spectral parameters such as the quantum yields of the photochemical steps and the molar extinction coefficients of the transient species can be derived using this method. The essence of the method is the inverse treatment based on numerical simulation and fitting of the plots (Abs versus t)

obtained under continuous irradiation. This also exploits the information contained in the irradiation kinetics. In order to extract one or more of the relevant parameters of a given process, specially designed experiments need to be carried out in which the effect of the process under consideration is conspicuous. Another advantage of the method is that it can test the validity of a mechanistic hypothesis and discriminate between similar mechanisms, as well as help to define the types of experiments required to test the initial hypothesis. Reliable kinetic data can be obtained from an experimental setup that can both irradiate and record absorbances at seveal wavelengths at once, including the wavelength of irradiation. The set of matrices obtained (Abs versus form the data base for construction and testing of the model. Dynamic analysis is also a powerful method for the study of complex behavior. The description of an ideal ABC system with the possibility of multiple photostationary states illustrates the dynamic bistable behavior of TPID. In this chapter, we have also described a methodology for rationalizing the investigation of the reaction mechanism of photochromic systems. It is now clear that rapid techniques and/or structural determination are not sufficient to identify all the details of a photochromic reaction mechanism and to account quantitatively for all the available experimental observations under continuous irradiation.70–75

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APPENDIX 1. FLASH PHOTOLYSIS VERSUS CONTINUOUS IRRADIATION: INFLUENCE ON THE ACCUMULATION OF REACTION INTERMEDIATES Let us consider an ideal ABC, 3k system (see Table 3) involving two photoisomers C and B, one fast (B) and the other slow (C). Numerical simulation of the ABC, 3k system after flash and continuous irradiation is shown in Figure A1. These results show that after flash irradiation, the fast isomer B accumulates. The system thus behaves as:

and the unstable intermediate which is observed is B. On the other hand, after continuous irradiation, the slow isomer C accumulates and the system thus behaves as:

and the unstable intermediate which is observed is C. Simulations of different types of ABC systems also show that in all cases flash photolysis favor observation of species with a short lifetime, while continuous irradiation favors accumulation of longer lifetime species.

APPENDIX 2. PHOTOSTATIONARY METHODS The photostationary methods are only valid for AB systems without photode-

gradation. The only assumptions are that product A is thermally stable, that its spectrum is known, and that the Beer–Lambert law applies. A.2.1. AB,

1 k Systems

For AB, 1k systems,76–79 when one combines the photostationary equation (3.3.2.1.1) with the Beer–Lambert law (Eq. 3.2.l), one obtains a linear relationship involving the relevant kinetic and spectral parameters of photochromism such as the quantum yields and the UV/visible spectrum of the unstable photoisomer B

196

where

Chapter 3

This is thus the equation of a straight line, obtained by

plotting the measurable value (an observation wavelength is selected at which the phenomenon is conspicuous) against an experimental constraint that alters the term Application of this relationship requires observation of several steady states as a function of the variable experimental constraint.

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Either (constant temperature and variable incident photon flux) or (variable temperature and constant incident photon flux) can be used as the variable constraint. The photokinetic factor must be measured for each of the photostationary states. It should be noted, however, that the relationship in Eq. (A2.1) comprises four unknown quantities and and cannot be analyzed directly as the numerical values of the slope p and the intercept i with the y axis produce values for only two unknown parameters (Figure A2). In Section A2.2, we show how this can be resolved.

A2.2. AB,

In contrast, for formed into

1k Systems

systems,80–85

which only contains two unknowns:

and

and Eq. (A2.1.1) is trans-

198

A2.3. AB,

Chapter 3

Systems

In the case of a photochemically reversible, photochromic sytem of the AB, type, the following data are required for application of the photostationary method:

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(a) the spectra of A (b) the spectra of the two photostationary states that are obtained by long irradiation of A at two different wavelengths, and The principle of the method (Fischer’s method)86–89 is illustrated in Figure A3. The rate of conversion in the photostationary state (corresponding to irrradiation at is given by

and are the differential spectra between the two photostationary states and A (see Figure A3) and where and are the actual values of the differential spectra and at any given observation wavelength on Figure A3). The spectrum of photoisomer B is given by

and the ratio of quantum yields is

In this case,

APPENDIX 3. THERMAL RELAXATION OF AN ABC SYSTEM The thermal relaxation of an ABC system may involve up to six elementary firstorder processes relating the three components A, B, and C. A typical example would be the thermal ABC, 6k system (see Table 3). From the mass balance equation:

two equations are required to describe the kinetics of the system, which in this case can be represented in its integrated form (the demonstration of this is outside the scope of this chapter). An interesting feature of this system is that whatever the true number of processes (two to six), A, B, and C all change according to a biexponential law of the following form:

200

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where X represents one of the three species A, B, or C, is its initial concentration, and is its final equilibrium concentration; and are amplitude terms, expressed as concentrations, which depend on the initial conditions of the relaxation; and and are negative and correspond to apparent first-order rate constants whose values are a function of the set of all the elementary rate constants; they do not depend on the initial conditions:

where the respective

terms are

The shape of the relaxation kinetic curve depends on the elementary rate constants and on the three initial nonequilibrium concentrations and The first amplitude for species A is given by

where

with

is the equilibrium Concentration of [A]:

and the second amplitude is:

For species B, similar relationships are obtained:

with

where The amplitude for species C is given by

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If the three species A, B, and C absorb, the change in absorbance will be given by

where is the initial absorbance, and is the final absorbance. Figure A4 shows a few examples of biexponential relaxation kinetics as a function of the initial concentrations and relative rate constants. It emerges, however, that kinetic anaysis of a single biexponential relaxation does not provide any information on either the number or the localization of the elementary processes underlying the relaxation. Only the rate constants and the overall amplitudes of the exponentials can be obtained directly. Supplementary experiments need to be carried out in which the initial conditions are varied in order to make possible the elucidation of the detailed reaction mechanism.

202

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APPENDIX 4. LOCALIZATION OF AN ELEMENTARY RELAXATION PROCESS IN ABC SYSTEMS The kinetic equations describing the evolution of an ABC, 6k system (see

Appendix 3) show that simple extraction of the rate constants of the biexponential relaxation is not sufficient to determine the rate constants of the true elementary processes since up to six of them may be involved. However, the expressions

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(A3.8) and (A3.10) show that one can alter the amplitudes by changing the initial concentrations In the case in which there are only two possible processes, this property can be used to localize them and determine their elementary rate constants. As an example, we show in Figure A5 how one can discriminate between ABC, (convergent) and ABC, (successive) mechanisms by altering the ratio The results show that it is possible to distinguish a scheme of type ABC, from a scheme of type ABC, b y simultaneously fitting the relaxation kinetics carried out with two different ratios (see Table 3). In practice, it may be difficult or impossible to alter the initial concentrations at will (in this case the ratio since they are nonisolable species). However, Appendix 1 shows that in photochemical ABC system, the ratio obtained at the end of the irradiation depends on the photon flux and the duration of irradiation. In certain cases, this property may be of use in the analysis of the relaxation processes.

APPENDIX 5. LOCALIZATION OF A PHOTODEGRADATION PROCESS In the study of photodegradation by conventional methods, the different reaction steps are very difficult or impossible to localize. For example, in the case of spiro[indoline-benzopyran] 4, one cannot be sure whether the stable closed form A or the open unstable form B is the photodegradable species. However, from the dynamic viewpoint, it can be shown that this problem can be resolved by discriminating two ideal kinetic schemes of the ABC type characterized, either

by two parallel or two successive photochemical processes (ABC, 1k, or ABC, 1k; see Table 3). In the scheme ABC, 1k, the photocoloration stems from A, and the photodegradation from B, whose concentration is a function of the incident photon flux. The relative importance of these two processes (photocoloration versus photodegradation) will thus depend on the irradiation intensity On the other hand, in the scheme ABC, 1k, the two photochemical processes stem from the closed form A, and the intensity of the incident photon flux will have little influence. In order to discriminate between the two schemes, experiments must be conducted with different values of the incident photon flux (Figure A6). The results of the simulation and fitting demonstrate that it is theoretically possible to localize photodegradation process by conducting experiments at different levels of incident photon flux.

204

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79. G. Gauglitz, P. Stöbel, H. Meier, and H. Rau, Photokinetic examination of (Z,E,E)-4,4'-distyrylazobenzene, J. Photochem. Photobiol. A 85, 207–211 (1995). 80. F. Grégoire, D. Lavabre, J. C. Micheau, M. Gimenez, and J. P. Laplante, Kinetics in a continuously stirred photochemical tank reactor, J. Photochem. 28, 261–271 (1985). 81. J. C. Micheau and D. Lavabre, Stirred flow reactor. A new approach to photochemical kinetics, EPA Newsletter, No. 3, pp. 26–33 (1986).

82. H. Rau and G. Greiner, Determination of quantum yields of the X/Y isomerization system from spectroscopic data, EPA Newsletter, No. 41, pp. 40–55 (1991). 83. G. Gauglitz and E. Scheerer, Method for the determination of absorption coefficients, reaction rate constants and thermodynamic data in the system A–B, J. Photochem. Photobiol. A 71, 205–212 (1993). 84. V. Pimienta, G. Levy, D. Lavabre, A. Samat, R. Guglielmetti, and J. C. Micheau, Computer analysis of the thermoreversible photochromism of spiropyrans compounds: Evaluation of absorption spectrum and quantum yields, Mol. Liq. Mol Cryst. 246, 283–286 (1994). 85. G. Favaro, V. Malatesta, U. Mazzucato, G. Ottavi, and A. Romani, Thermally reversible photoconversion of spiroindoline-naphthoxazines to photomerocyanines: A photochemical and kinetic study, J. Photochem. Photobiol. A 87, 235–241 (1995). 86. E. Fischer, The calculation of photostationnary states in systems A–B when only A is known, J. Phys. Chem. 71, 3704–3706 (1967). 87. J. Blanc and D. L. Ross, A procedure for determining the absorption spectra of mixed photochromic isomers not requiring their separation, J. Phys. Chem. 72, 2817–2824 (1968).

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88. G. M. Wyman and B. M. Zarnegar, Excited state chemistry of indigoid dyes I, J. Phys. Chem. 77,

831–837(1973). 89. G. M. Wyman, A new method for calculating the absorption spectra of the components of photochromic systems A–B, Mol. Photochem. 6, 81–90 (1974).

4 EPR and Radical Processes Angelo Alberti 4.1. INTRODUCTION Free radicals and radical ions are paramagnetic species whose magnetic properties originate from the presence of an odd number of electrons in their molecules. The former species are normally obtained by homolytic cleavage of a chemical bond, induced either photolytically [e.g., the generation of tert-butoxy

radicals via ultraviolet (UV) irradiation of di-tert-butyl peroxide or the formation of alkyl radicals via hydrogen abstraction by alkoxy radicals or by excited carbonyl compounds, via halogen abstraction by trialkylstannyl radicals, or by direct photolysis of weak halogen–carbon bonds] or thermally [e.g., the decom-

position of azo compounds such as azoisobutyronitrile (AIBN) to give alkyl radicals or that of alkylhyponitrites to give alkoxy radicals]. On the other hand, radical ions are obtained through single-electron-transfer processes resulting in the uptake of an electron by a given substrate to form its radical anion (reduction) or in the loss of an electron to give the corresponding radical cation (oxidation). Both the reduction and oxidation processes are spontaneous when reactants with appropriate redox potentials are involved; in some less favorable cases, the electron transfer may still be achieved thermally or by photostimulation, sometimes with the aid of sensitizers. Radical anions and cations of particular substrates can also be obtained through electrochemical reduction or oxidation. Examples and discussion of all these processes can be found in any good textbook of physical organic chemistry, and their detailed description is outside the scope of this introduction. Electron paramagnetic resonance (EPR) spectroscopy is the technique of choice for the study of paramagnetic species. It can, in many cases, provide a detailed picture of the electronic distribution within the molecule of the examined free radical or radical ion through the knowledge of the spin density distribution, which in a broad sense reflects the distribution of the unpaired electron within the Angelo Alberti • I.Co.C.E.A.—CNR, I-40129 Bologna, Italy.

Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999.

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framework of the investigated species. The spin density distribution can be principally derived from two experimental observables: the hyperfine coupling [or hyperfine splitting (hfs)] constant at the individual magnetically active nuclei in the radical and its g-factor. The hfs constants characterize the interaction of the nuclei with the unpaired electron, while the value of the g-factor can be seen as an indication of how much the behavior of the unpaired electron in the examined radical differs from that of a free electron its knowledge being therefore useful in the elucidation of radical structure. Further information on the radical structure may be obtained from the spectral linewidth and its variation with temperature and from the observation of selective line-broadening effects, when present. EPR spectroscopy, besides providing information on species having only one unpaired electron, that is, on species in a doublet state can also provide information on species in higher spin states, in particular those having two

unpaired electrons; thus, the spectral parameters of triplet biradicals which, however, can only be observed in frozen matrices because in solution the lines are too broad to allow their detection, provide information on their geometry. For a detailed description of EPR spectroscopy, the un-acquainted reader is referred to specific treatises elucidating the theory underlying it.1–4 This chapter, far from being exhaustive, is aimed at outlining the properties of the radical ions and of the free radicals derived from photochromic compounds independently of whether or not these paramagnetic species are involved in the photochromic process itself. In particular, it will deal with the EPR of dihydroand dialkyldipyridyl radical cations (viologens) and of the radical anions of a variety of nitro-substituted spirofindoline-benzopyrans], spiro[indoline-naphthopyrans], and spiro[indoline-naphthoxazines] as well as their triphenylgermyloxy nitroxides and with the triplet spectra observed upon irradiation of 2,2,4,4tetrachloro-1 -keto-1,4-dihydronaphthalene.

4.2.

VIOLOGENS

There is in the literature a tremendous number of reports of EPR investigations on the radicals derived from viologens, often carried out in conjunction with electrochemical and spectroscopic studies of different types. The following sections are intended to give a general overview, based on some selected examples, of the information gained on these radical species through EPR studies. 4.2.1. Radical Cations from the Reduction of Viologens

Bipyridinium salts are nearly colorless or pale yellow compounds that in solution can be very easily reduced to deeply colored radical cations. Although the

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term viologens properly applies to the

213

-dialkyldipyridiniuni salts,

can be

considered as the radical cation from the simplest viologen, known as Tony blue.

The first reports of the detection of the EPR spectrum of date back to the early sixties.5,6 When a thoroughly degassed 95% ethanol solution of dipyridinium dichloride was photolyzed in the cavity of an EPR spectrometer with visible light, strong signals due to the -dihydro -dipyridyl radical cation were observed. As it happens, it would appear that in one of the reports5 the authors misinterpreted the observed spectra. The correct values were later confirmed by other authors7 in an investigation on the photochemical or electrochemical reduction of -dihydro -dipyridinium dichloride. The values of the hyperfine coupling constants are collected in Table 1. Because the hfs constants for aromatic protons are proportional to the spin densities at the adjacent carbon atoms through the McConnell relation,8 these values provide a clear-cut picture of the spin density distribution within the molecular framework of radical cation The hfs constants for the nitrogen atoms should reflect, besides the spin density at the nitrogen atom itself, the spin density at the adjacent atoms as well as the nature of their bonds to the nitrogen.7 The assignment of the hfs constants to the two nitrogen atoms was straightforward owing to the value of their nuclear spin quantum number while that of the hydrogen atoms could be unambiguously established by deuteration. The assignment of the splittings to the two sets of four equivalent protons in positions 2, and 3, was based on the results of simple McLachlan molecular orbital (MO) calculations,7,9 but because of the similarity of the values ambiguity could not be totally avoided. Methyl viologen ethyl viologen and benzyl viologen radical cations were obtained7 similarly to by photo-

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lyzing ethanol solutions of the parent salts with visible light; the radical was also obtained by electrochemical reduction of either methyl viologen or Nmethylpyridinium chloride. Figure 1 shows the EPR spectrum of and the values of the hfs constants for and are presented in Table 1. The hfs constants for the nuclei in the different positions, assigned again on the basis of MO calculations, indicate that alkyl substitution of the atoms has, despite the inversion of magnitude for positions 2 and 3, little influence on the spin density distribution in the aromatic rings, the only notable variations being experienced by the splittings of the nitrogen nuclei. These assignments were criticized in a later study reporting the results of INDO calculations10 pointing to an opposite trend of the ring splittings, but the reproduction of previous results11 and deuterium substitution of the hydrogens at position 3 of methyl viologen provided unambiguous support for the data reported in Table 1.12 The radical cations from several substituted -diphenyl viologens (PV) have also been investigated by means of EPR spectroscopy as well as electron nuclear double resonance (ENDOR) and triple resonance spectroscopies.

The hfs constants for -diphenyl- -dipyridynium radical cation reported in Table 2 indicate that the spin density distribution in the central heterocyclic rings is analogous to that of the unsubstituted radical cation and

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opposite to that of the methyl-substituted of geometrical differences between radicals

215

This is believed to be an indication and

In particular the

angle is thought to be larger in the latter species, owing to steric repulsion between the hydrogen atom bound to and the ortho hydrogens and the N-aromatic ring.12 Methyl substitution of the outer aromatic rings does not significantly affect the spin density distribution (Table 2), unless the substituents are in the ortho positions, where they maximize the steric repulsion with the proton bound to the carbon adjacent to the nitrogen atom.14 The decrease of the para-hydrogen hfs constant upon ortho and ortho disubstitution is to be attributed to out-of-plane rotation of the substituted aryl rings caused by steric hindrance. The introduction of a fluorine atom in the meta or para position of the N-aryl groups15 appeared to have little effect on the spin density distribution as compared with that observed in the unsubstituted cation The value of the hfs constant at the fluorine atom in the para derivative was about twice that of the corresponding hydrogen, a feature in line with expectations; on the other hand, substitution of fluorine for a meta proton led, unexpectedly, to an coupling constant smaller than that of the corresponding hydrogen The similar decrease observed when a fluorine atom was introduced in an ortho position was attributed to rotation of the aromatic ring out of the dipyridyl plane induced by the fluorine substituent: consistently, the small values of the orthofluorine splittings were accompanied by values for the para hydrogen much smaller than in the unsubstituted radical cation INDO calculations yielded results in reasonable,agreement with experiment.15 From the reversible variations of the EPR spectral intensities of radical cations with temperature, the existence of a monomer–dimer equilibrium

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has been inferred.14 Through studies over a wide temperature range, the kinetic parameters of the dimerization, believed to occur with the formation of face-toface dimers with linking, were determined; a subsequent reinvestigation by other authors13 proved the spectral intensity variations to be due to temperaturedependent EPR saturation rather than to the alleged monomer–dimer equilibrium. When radicals were generated in aqueous solution either electrochemically at a gold electrode or photochemically at colloidal particles, better resolved spectra were obtained in the latter medium. This was attributed to the fact that the formation of face-to-face dimers is favored at the surface of the gold electrode, where high local concentrations of can be reached, whereas the radicals produced from adsorbed on particles were spread over a much wider surface and therefore their dimerization was more difficult.16 This was also proved by the fact that the spectra at colloids could be observed unchanged for several days. The interaction between the diamagnetic dimer and the radical cation is held responsible for the broadening observed at the gold electrode. When one of the methyl groups in was replaced with a longer hydrocarbon chain the electrochemical reduction resulted in the massive deposition of a diamagnetic violet dimer at the electrode whose interaction with the paramagnetic monomer caused the spectrum of the latter species to collapse to a single broad line depending on the applied potential). On the other hand, the spectra observed by photoreduction at colloids were much more resolved. The formation of dimers, also called pimers because they originate from weak interactions of their systems, has also been studied in bridged viologens containing four pyridinium units,16,17 where the formation of intramolecular pimers was possible. In particular, the persistence of the radicals obtained from tetracations 15–18 by photoreduction at the surface of sols varied along the series

Under illumination conditions, the spectral intensity of and decreased while that of and remained constant after reaching a plateau, the former two species having a more pronounced tendency to pimerization.

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Electrodes covalently derivatized with dipyridinium salts gave upon reduction featureless EPR spectra characteristic of an immobilized species while electrodes covered with Nafion ion-exchange resin with incorporated gave EPR spectra similar to that of in solution, indicating the nearly free tumbling of the radicals.18 In the former case, electron self-exchange was higher than in the latter Successful attempts were also made to observe EPR spectra of radical incorporated into a colloidal dispersion, although the spectra were much less resolved than in clay-free solutions. Using the EPR spectral intensities as concentration standards, the equilibrium constant for the dimerization of the cation within the clay film was estimated as This value, much higher than that found11 in aqueous solution was explained in terms of the ability of ethyl viologen to segregate from other cations on the montmorillonite surface, thus producing locally high concentrations. The formation of dimers can also occur through a disproportionation between a dication and its final reduction product. In particular, simultaneous EPR and electrochemical experiments20 carried out in poly(acrylamido-3-methylpropanesulfonic acid) (polyAMPS) at a potential sufficient to completely reduce the dication to provided evidence of the formation of radical cation through the following reaction:

The two species approach in solution to form a sandwichlike and electron transfer takes place with formation of the dimer, whose dissociation leads to radical Electrochemical results as well as the EPR spectrum observed by reduction of the multicharged viologen N-ethyl- -trimethylaminopentyl

bipyridine

indicated that dimerization proceeds through a radical–

molecule coupling22 leading to From cyclic voltammetry measurements, indications were obtained that in the dimer, the systems of the two units, although not completely separated, do not conjugate and, as result, the dimer is more difficult to reduce than the monomer. Complexes of radical cations included in cyclodextrin were observed both in methanol/aqueous solutions and as microcrystalline powder.21 In the former situation, the spectral linewidth did not decrease monotonically with temperature, as is the case in the absence of The linewidth decrease

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observed with an increase in the temperature from 10 to 20 °C is attributed to motional narrowing, the increase between 20 and 50 °C is probably due to the increasing rate of the forward and reverse processes in the inclusion equilibrium, and the linewidth decrease observed above 50 °C is again to be attributed to motional narrowing. The signal from sealed, degassed, and UV-irradiated samples of polycrystalline powder consisted of a single line that persisted unchanged indefinitely, indicating that inclusion produces a remarkable stabilization of the radical also in the polycrystalline state21. 2,2'-Dipyridinium salts are not proper members of the family of viologens, and although these species have been often studied along with 4,4'-dipyridinium salts, comparatively less attention has been given to the radical cations originating from their reduction. A typical example is provided by the radical cation from diquat (Figure 2) that was obtained by zinc-dust reduction of the dication in alcoholic solution.13,23 The spin density distribution in

was

determined from the line broadening observed in the NMR spectra of by addition of a small amount of It was consistent with the results of INDO calculations,16 but contradicted MO calculations according to which position 3 should be characterized by a relatively large spin density23 (Figure 2). Changes of the spectral pattern observed with temperature were due to inversion of the central ring, and although the energy barrier could not be determined, a value of ca. was estimated.23 radicals incorporated in the cavities of zeolites-Y exhibit EPR spectra analogous to those of the radicals in solution, consisting of a broad single line with g-factor values of 2.0030 and 2.0029 Although incorporated in the zeolite cavity, the radicals react with dioxygen at – 78 °C to form the superoxide radical anion through an electron-transfer reaction, and the spectra are replaced by the anisotropic spectra of the radical anion interacting with either (g1 = 2.084 and g2,3 = 2.005)-or

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219

Electron transfer was immediate and irreversible for and slower and reversible for reflecting the different oxidation potentials of the two cations, i.e., –0.45 and –0.49V vs. SCE, respectively. On this basis, it was estimated that the reduction potential of dioxygen in zeolites lies near –0.4V, i.e., the value in the aqueous media, and far from that in aprotic polar media like acetonitrile (E° = –1.04V). 24 4.2.2.

Some Particular Viologens

One of the most important properties of viologens is their ability to reversibly accept one electron, giving stable and colored radical cations. New viologens have been created, where the two pyridine moieties are separated by an unsaturated chain or by an aromatic or heteroaromatic ring, such as compounds 21–24.25

While dications 21 and 22 only underwent a one-stage two-electron reduction without formation of the radical cations, the reduction of the thiophene and furan derivatives proceeded in two steps, in the first of which the radical cations and were formed. The stability of these radicals was proved by the fact that the EPR spectrum of consisting of a strong line (g = 2.0038) with rather complicated hyperfine structure, persisted with undiminished intensity for weeks at room temperature. Several viologen-functionalized pentaatomic heterocycles have been employed as conducting polymers26 or to modify electrodes.27 The doped polythiophenic conducting polymer gave an intense EPR signal that did not vary upon electrolytic reduction to its normal undoped state (PTV2+).

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Upon reduction in the region where the cation radical was formed, a new signal with hyperfine structure, due to the reduced viologen moiety, could be seen superimposed on the previous one.26 The presence of the hyperfine structure indicated the high flexibility of the alkyl-chain-bound viologen units.

Viologen-substituted pyrroles 26–28 were electrochemically polymerized on the surface of electrodes in order to modify their properties.27 While the reduced monomers give intense EPR signals with resolved hyperfine structure similar to that of in the poly(pyrrole-viologen) signals there was no sign of hyperfine structure. This was attributed to a high local concentration of radical cations and to low mobility of the dipyridinium units due to grafting to the polymer backbone. Liquid-crystalline viologens were obtained by substitution of the methyl groups of with oxa-alkyl chains (29) and (30) and were found to be resistive materials below their mesophase transition temperatures but semiconducting above these temperatures.28 When a 1-30-V DC potential was applied to either 29 or 30 in the mesotropic mesophase, a sharp conductivity increase was observed, while no such increase was observed in the isotropic phase or below the solid to mesophase transition temperature. It was concluded that the increase in conductivity was due to accumulation of and in the region between the electrodes due to slow back electron transfer in the mesotropic mesophase. 4.2.3. EPR of Viologens in Miceilar Systems

The EPR spectrum of radical cation in 70mA/ sodium dodecyl sulfate (SDS) solution was similar to that observed in water, indicating that the SDS micelles did not affect the tumbling of The addition of caused a broadening of the spectrum without completely washing out the hyperfine structure, indicating that the electron exchange between and responsible for the broadening was slower than in water. At variance with the anionic SDS micelles, cationic cetyltrimethylammonium bromide (CTAB) and nonionic Triton X-100 micelles at the same concentrations showed an exchange

EPR and Radical Processes

221

rate between the radical cation and the dication analogous to that observed in water in the absence of surfactants.29 A careful analysis of the spectra of in 0.1 M SDS, however, indicated a strong broadening of the lines corresponding to the nitrogen total quantum numbers and By assuming for the radical cation an anisotropy parameter of 1.73, analogous to that typical of nitroxides, the correlation time for in SDS micelles was evaluated as ca. The EPR spectra of several N-alkyl- -methyl viologens, with alkyl chains of different lengths and in frozen SDS micelles were all very similar (a broad line with Also very similar were the spectra in dihexadecyl phosphate (DHP) vesicles. On the other hand, electron spin echo modulation (ESEM) spectra for the various viologens in were different and mostly reflected different interactions of the cations with water.31 The decrease of these interactions with increasing length of the N-alkyl group was attributed to the fact that cations with longer chains were more deeply embedded into the vesicle interface. The interactions of with water are much lower in SDS micelles than in DHP vesicles, in agreement with a disordered open model for the former medium; the fact that is already inserted in the SDS interface region explains why there was no difference in the water–cation interaction with changes in the length of the alkyl chain, i.e., when going from to and In this system the radicals were obtained by photolysis of the different viologens and a ruthenium complex in the frozen micelles, and the different embedment of the viologen molecule in the SDS micellar interface was reflected in the different intensities of the EPR signals, which in fact varied with the length of the N-alkyl chain, being maximum for and Radicals were also photogenerated in frozen dioctadecydimethylammonium chloride (DODAc) and in dipalmitoylphosphatidylcholine (DPPC) frozen vesicles, and the intensity of their spectra increased with increasing time of irradiation.33 However, if irradiation was prolonged over 200 minutes, the single broad-line spectrum typical of alkylmethylviologen radical cations was converted into an octet attributed to alkyl radicals from either DODAC (DAC) or DPPC (PC), having a similar structure.

The proposed mechanism for the formation of these carbon-centered radicals produced in the hydrocarbon interior of an organic aggregate by photoinitiated radical conversion from involves hydrogen abstraction from a location of the

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surfactant followed by migration of the radical site along the surfactant chain to the next-to-last carbon atom. EPR, ESEM, and ENDOR studies of the photoreduction of these alkyl viologens in DHP vesicles proved that also in these micelles the longer viologens were more deeply embedded into the vesicles. The yield of photoreduction as measured from the EPR spectral intensity increased with the chain length.34 Addition of cholesterol to the system resulted in an increased water penetration and in a reduction of the effective proton density in the interface and caused an increase of the photoreduction yield (see Table 3). Similar studies carried out in dodecyltrimethylammonium chloride (DTAC) and SDS micelles showed a strong dependence of the photoreduction yield on the chain length in the former medium.35 Thus, the radical yield increased from to due to deeper embedding into the vesicle, but as the length of the viologen chain approached that of the DTAC, the cation was pushed back towards the interface. The deeper the viologen molecule was inside the vesicle, the larger was the distance from water and hence from the electron donor (the chloride ion). The increased radical yield is attributed to back electron transfer becoming more difficult.34 The situation in SDS micelles was different, and the embedment in the anionic aggregate did not depend on the viologen alkyl chain length.35 It is possible that in this case the electron transfer leading to took place between the viologen and the anionic head of the surfactants rather than the chloride ion, the photoreduction yield being strongly dependent on the distance separating the viologen and the sulfate anion. A hydrocarbon radical similar to DAC was formed upon prolonged irradiation in DTAC, and, according to ESEM and ENDOR results, this species was located within the interior of the micelles.35

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223

When the dication was photoreduced in DODAC micelles in the presence of variable amounts of -tocopherol (vE), the nature of the observed radicals depended on the concentration of the latter compound.36 was observed in the absence of vE, and its signal increased slightly with the amount of tocopherol. When the vE concentration exceeded ca. 9%, a second radical was also observed, i.e., the neutral radical originating from proton abstraction from the tocopherol cation formed in the electron-transfer process, this latter species becoming the only observed radical for tocopherol concentrations in the range of 17 to 23%. The disappearance of the radical is attributed to its reduction to AV by tocopherol with formation of more Laser-photoinduced electron transfer in the three tetraphenylporphyrin-bound viologens 34–36 in reversed micelles led to radical pairs whose chemically induced dynamic electron polarization (CIDEP) spectra at room temperature proved dependent on the length of the “spacer.”37

A normal emission/absorption/emission/absorption pattern that did not vary in the time interval was observed for the radical pairs from 35 and 36, while the spectrum from 34 was initially totally absorptive then rapidly changed to emissive/absorptive and eventually became totally emissive It is suggested that for ZP4V, due to the smaller spacer chain, the time elapsed between laser excitation and radical-pair formation was shorter than for ZP6V and ZP8V and the spin polarization in the porphyrin triplet was retained to a larger extent before electron transfer took place.37 The amphipathic 1 -ethyl- 1'-cetyl-4,4'-bipyridinium dibromide in aqueous solution formed micelles, with the bipyridinium units distributed around the surface. The EPR spectra of the radical cations generated by irradiating aqueous samples of 37 containing variable amounts of cetyltrimethylammonium chloride (CTAC) in the presence of a photosensitizer showed progressive broadening with increasing molar fraction of the viologen.38 The effect was

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attributed to electron exchange between the adjacent bipyridinium units rather than to increased viscosity of the system, the estimated electron exchange rate being very fast, i.e.,

4.3. β-TETRACHLORO-α-KETONAPHTHALENE Colorless crystals of 2,3,4,4-tetrachloro-l-oxodihydronaphthalene 38) assume an amethyst color upon UV irradiation. This photochromic behavior of was initially explained39 in terms of the reversible photochemical dissociation of the ketone into a trichloronaphthoxyl radical and a chlorine atom:

Because both 39 and the chlorine atom have an unpaired electron, the crystal, when brought to the excited state, should be paramagnetic. Actually, irradiating a powder of inside the cavity of an EPR spectrometer at 77 K led to the observation of a triplet spectrum characterized by two weak extra lines in the middle of the main spectrum were attributed to a second triplet spectrum characterized by and The main spectrum was attributed to magnetic coupling between 39 and a chorine atom, and the weaker one to the interaction between two naphthoxyl radicals. The triplet spectrum, only observed when irradiating the sample at was later attributed to an excited state of the undissociated 38 rather than to the radical pair 39.41 It was also found that irradiation with the unfiltered light from a Hg lamp resulted in a strong EPR single line that persisted when the photochromic state disappeared, while irradiation of 38 at 333 K in benzene solution led to the observation of an EPR signal attributed to the 2,3,4-trichoro-l-naphthoxyl radical in equilibrium with a dimeric quinolether 40.41 In a later EPR investigation of single crystals of 38 irradiated at 223 K at 366 nm, the dependence of the spectral intensity on temperature was studied, and the values of D and E were redetermined together with their signs as and On the basis of these results and considering the two small zero-field parameters, the possibility that the triplet spectrum could be attributed to the molecular triplet state of was excluded. Also apparently to be excluded

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225

was the hypothesis of the triplet arising from the interaction between a chlorine atom and a trichloronaphthoxyl radical: in fact, fast recombination of the two moieties should result in a very short lifetime for the triplet. A more likely possibility is that the chlorine atoms abstracted another chlorine from position 4 of a neighboring molecule to form and a new radical 39 and that the two interacting electrons were thus located on two adjacent radicals.42 On the other hand, when the formation of the triplet signal of the radical pair

and the growth of the absorption of the colored species were simultaneously monitored, they were found to occur at different rates, as did their decay, thus providing an indication that the radical pair was not responsible for the photochromic effect.43

Studies of radical 39 in solution further proved that the naphthoxyl radical is not involved in the photochromic process. The radical, whose EPR spectrum consisted of a 1:2:1 triplet due to interaction of the unpaired electron with two protons, identified as those in positions 5 and 7 of naphthalene on the basis of semiempirical calculations, was found to exist in equilibrium with its dimer 40.43 Radical 39 or its dimer 40, was also found to absorb at 305 nm with whereas the product responsible for the photochromic effect absorbed at 530 nm and was characterized by a much higher extinction coefficient. Strong indications have instead been gathered indicating that the dissociation of 38 is a heterolytic process and that the carbenium ion 41 is most likely responsible for the photochromism of

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4.4 SPIROINDOLINIC PHOTOCHROMES 4.4.1. Paramagnetic Species in Photochromism and Photodegradation The photochromic behavior of spiro[indoline-benzopyrans], spiro[indolinenaphthopyrans], and spiro[indoline-naphthoxazines] has been extensively studied and has been attributed to a light-induced heterolytic reversible cleavage of the carbon–oxygen bond of the pyranic (oxazinic) ring44 as shown in the equilibrium for the simple spirofindoline-benzopyran] 42 and its corresponding merocyanine 43: }

Because the moieties present in the spiro compound are orthogonal, as are their systems, the possibility that the biradical 44 is intermediate in the conversion has been considered.45

Indeed, a markedly polarized triplet spectrum, attributed either to a biradical similar to 44 or to a doubly charged ion, was observed when crystals of l',3',3'trimethyl-8-nitrospiro[indoline-benzopyran] (45) were irradiated at 77 K.46 The

observed absorption and emission signals were attributed to forbidden transitions from the singlet state of the biradical to the and components of the triplet biradical. By observing the variations of line position, line intensity, and in linephase variations with orientation in the triplet spectrum of a single crystal of 46 irradiated at 105K inside the EPR cavity, it was concluded47 that they were consistent with formation of a triplet biradical exhibiting optical spin polarization; theoretical studies indicate that the observed anisotropy of the EPR intensity was not due to anisotropy of the magnetic transition moment, but is linked to the photophysics of triplet production and quenching.48 In particular, spin polarization would be a consequence of the formational intersystem crossing (ISC) process, chemical quenching of the triplet taking place without selection rules.

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227

Laser flash-photolysis (LFP) studies in the nanosecond region carried out on 47 in hexane solution also suggested the presence of a triplet biradical, which, however, would not lie on the main reaction coordinate of the photochromic process, being instead involved in the dimerization of As already mentioned, it is not possible to directly detect triplet biradicals in solutions by means of EPR spectroscopy, yet indirect indications of their presence in fluid benzene solutions of some spiro compounds50 could be obtained using the so-called double spin trapping technique.51 Spin trapping, normally used in studies of transient radical species whose lifetime is too short for a steady-state concentration to be reached that allows direct detection, consists in introducing into the system a diamagnetic species (the spin trap) amenable to undergoing fast reaction with the transient radical to give a spin adduct, i.e., a different radical, characterized by a longer lifetime and therefore readily detectable. In the case of biradicals, the trap must be a radical species, in order to give a paramagnetic adduct detectable by EPR spectroscopy. Nitric oxide is particularly useful for this purpose because its reaction with biradicals has been found to lead to rather persistent and easy detectable cyclic nitroxide radicals.51

The bubbling of NO through deoxygenated benzene solutions of spiro compounds 48–53 resulted in the detection of simple EPR spectra (see Figure 3),50,52 consisting of a 1:1:1 triplet due to coupling of the unpaired electron with

the nitrogen of the aminoxyl function These spectral parameters are consistent with the adducts being cyclic oxynitroxides of general structure 54 (see Figure 3), resulting from initial coupling of the NO radical with one site of the biradical to form a nitroso derivative which, in a subsequent step, intramolecularly trapped the other radical site. The detection of these adducts was taken as evidence of the presence of open biradicals similar to 44 in solution: however, the fact that in all cases the spectral intensity did not seem to increase when UV light was shined on the EPR cavity during the NO bubbling

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suggested, in agreement with the aforementioned LFP studies,49 that the biradicals are not directly involved in the photochromic process.50,52 Although the photochemical processes involved in photochromism are relatively well understood, limited data are available on the degradation of photochromic compounds. It has been recently found that the open forms of the photochromic compounds can readily react with free radicals, either from polymerization initiators or from other adventitious sources, to yield colored compounds that cannot revert to the spiro structure.53 Compound 55 was identified among the photoproducts of spirooxazine, and, although it has been proposed that electron transfer from an excited state of the open form (merocyanine) of the spirooxazine to oxygen to form the superoxide anion is an important step in the process, 55 may also form in the dark and in the absence of oxygen by treatment of the spirooxazine with a suitable electron acceptor,54 such as Cu(II) or tetracyanoquinodimethane (TCNQ). With the latter acceptor, a paramagnetic complex was isolated, for which EPR powder spectra and singlecrystal spectra were recorded. The integrated signal intensity at different temperatures suggested that the paramagnetic species is a mobile excited state (triplet exciton),54 while low-temperature studies of single crystals of the complex in different orientations allowed the determination of the parameters D and E for the triplet spectrum, which describe the spatial distribution of the correlated spins.

EPR and Radical Processes

In the presence of metal ions, such as Cu(II), and by inserting an appropriate substituent in the position adjacent to the heterocyclic oxygen, it might be possible to stabilize the colored form by chelation of the metal ion. The rate of formation of a Cu(II) chelate complex with the open form of spiropyran 56 was measured as ca. and was found to be independent of the copper salt concentration, indicating that the rate-determining step was the opening of the pyran ring.55 On the contrary, the rate of decoloration of the open merocyanine and of its complex after UV irradiation was found to decrease with increasing concentrations of

Decay of the complex may involve the reverse reaction of its formation to

give the free merocyanine, which can then revert to the initial closed form, or proceed through direct conversion of the copper chelate complex to the starting spiro compound. In the presence of small amounts of the former route is favored, and thus the observed decoloration rate is the rate of ring closure of the

merocyanine, while with higher concentrations the second process becomes more important: typical decay rates of the complex are for and for 4.4.2. Radical Ions from Spiro Compounds Spiro[indoline-benzopyrans], spiro[indoline-naphthopyrans], and spiro[indoline-naphthoxazines] can rather readily undergo electron-transfer processes in either direction; i.e., they can be readily oxidized and reduced. Because the presence of nitro groups in their molecular framework normally enhances the photochromaticity of these compounds, several combined electrochemical and

EPR studies have been carried out on a variety of mono- and dinitro-substituted spiroindolinic photochromes.52,56–60

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The peak reduction potential for nitrospiro[indoline-benzopyrans] 57–62 proved independent of the nature of the N-alkyl group (– 1.32 to – 1.40V vs. SCE in acetonitrile),57 as were the hfs constants measured in the EPR spectra of the radical anions.

In all cases the unpaired electron appeared to be exclusively localized on the ring bearing the nitro group,52,56–58 the presence of the quaternary carbon forbidding conjugation between the two molecular moieties. The more intriguing feature to explain was the variation of the spin density distribution with solvent in the radical

anions that were generated by the “dark” reaction of the spiro compounds with potassium tert-butoxide in either dimethyl sulfoxide (DMSO) or dimethylformamide (DMF) or by photoinduced reduction with tetrabutylammonium borohydride in tetrahydrofuran (THF). In the absence of direct proofs on the actual structures of the radical anions, and considering that the reduction in THF was photoinduced, the effect was tentatively justified by postulating that the radical anion was observed in the closed form in DMSO and in the open form in DMF or THF (Figure 4).57 The lower splitting of the nitrogen of the nitro group in the latter two media would be justified by the presence of the electron-withdrawing carbonyl group in the para position. Accordingly, a similar effect was not observed in the anions from the analogous naphthopyranic (63) and naphthoxazinic (64) derivatives, for which such interaction between the carbonyl group and the nitro group is expected to be weaker.59,60 The radical cations of the various spiroindolinic compounds could not be studied by EPR spectroscopy, being too short-lived to be observed. The oxidation and reduction potentials of nitro derivatives vary with the position and number of

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the nitro groups in the molecule. Oxidation is easier when the nitro group is on the benzopyranic, naphthopyranic, or naphthoxazinic units and more difficult when it occupies position 5 of the indolinic moiety, a position conjugated to the heterocyclic nitrogen atom; a similar trend was observed for the reduction potentials.58–60

As mentioned before, the radical cations were very short-lived (totally irreversible cyclic voltammograms), while all the radical anions were rather persistent,57–59 with the notable exception of that from the dinitrospironaphthoxazinic derivative 69.60 Actually, while the dinitro derivatives 65 and 67 exhibited a reduction voltammogram with two well-defined reduction peaks at potentials close to the reduction potentials of the corresponding mononitro derivatives, the voltammogram of 69 indicated an irreversible first reduction wave and a small, broad second wave attributed to reduction of degradation products of the radical anion formed in the first reduction step.

The EPR spectra of the radical anions of the mononitro derivatives and of the dinitro derivative were in line with expectations based on previous studies,52 while those of the dinitrospiro compounds and proved intriguing to rationalize. In particular, in the radical anions from the dinitro derivatives it is expected that the unpaired electron is confined on the molecular unit which is more prone to reduction, i.e., either the nitrobenzopyranic or nitronaphthopyranic moiety (see Table 4). This is actually what was observed when 65 and 67 were reduced with tetrabutylammonium borohydride in THF (see Table 5), while when the reduction was carried out in DMSO (Figure 5) with potassium tert-butoxide the spin density distributions observed in the spectra were unexpectedly very similar to those observed for the radical anions of the nitroindolinic derivatives 57 and 66.

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The spectra were initially attributed to “radical anions with an unusual localisation of the unpaired electron,”58,59 but when it was found that reduction of the dinitrospiro[indoline-naphthoxazine] 69 afforded the expected radical anion regardless of the reducing medium,60 the reduction of the other two dinitro derivatives was reinvestigated to ascertain the actual nature of the radicals responsible for the observed spectra. In particular, the reversibility of the first reduction step of 57 and 66 and the irreversibility of that of 69 suggested that for the former two compounds the reduction might have proceeded beyond the formation of the radical anion and that the spectra observed in the reduction in fact were due to radical trianions rather than monoanions. Ab initio calculations at the ROHF/3-21G level were carried out for the radical anions from compounds 57, 58 and 65 as well as for the dianions and trianions from 65 in order to check whether the formation of the trianions is energetically feasible. The results indicated that the unpaired electron in the “naked” radical anion is preferentially located on the benzopyranic moiety rather than on the indolinic unit and that the radical trianion would be characterized by a forbiddingly high energy On the other hand, these values are dramatically modified if the presence of alkali-metal (sodium) counterions is taken into account, the ion-paired trianions becoming by far the most stable species On this basis, it has been postulated that in the butoxide reduction of 65 and 67 formation of the potassium ion-paired trianions would be favored over that of the ion-paired monoanions, while reduction with tetrabutylammonium borohydride in THF or the electro-

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chemical reduction would stop at the “naked” radical anions. The reduction of 69, on the other hand, would always stop at the monoanions, whose short lifetime would not allow the process to proceed any further. This hypothesis received further support from the fact that addition of dibenzo-18-crown-6 to DMSO solutions of the trianions and loosened the ion pairs, leading to the observation of the “correct” anions and while addition of l-crown-4 ether, a crown ether too small to complex potassium cations, had no effect whatsoever.60 As to the photochromic activity, it should be pointed out that the radical ions originating in the reduction processes do not exhibit photochromism: in all cases, reduction leads to colored solutions which are not bleached by UV irradiation.57–60 A number of oxynitroxide radicals that, although not ionic, are structurally similar to the nitroanions were obtained by adding in situ-generated triphenylgermyl radicals to several nitro derivatives: attention has been addressed to nitrospiro[indoline-naphthopyrans] 63, 66, and 67, and the resulting adducts have been attributed structures

The spin density distribution was found to be analogous to that observed for the radical anions, but, as is typical of nitroxides, with larger hfs constants for the nitrogen atom. The attacking triphenylgermyl radicals were generated via hydrogen abstraction from the corresponding hydride by tert-butoxy radicals produced either through a “dark” thermal reaction, i.e., the thermal decomposition of tertbutyl hyponitrite, or through the photodecomposition of di-tert-butyl peroxide, and with each substrate only one nitroxide was observed, independent of the route used to produce it. It proved impossible to tell whether the nitroxide should be assigned a closed spiro structure or an open one, but it should be pointed out that the solutions of the germyloxynitroxide were colorless, or nearly so, and totally insensitive to light and UV irradiation.59 4.4.3. Spin Probes and Spin Traps with a Spiroindolinic Structure

Spin probes are persistent paramagnetic species which, once put in a given medium, can provide information on the properties of the medium itself through

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variations of their EPR spectral parameters. The class of compounds most exploited for this purpose is certainly the nitroxides, and 3-oxoindoline-2-spiro4'-[2',6'-diphenyl-4'H-pyran]-1-oxyl (73) represents perhaps the earliest example of a nitroxide with a spiroindolinic structure (Figure 6).61 The spiro structure of 73 is not associated with photochromic properties and does not appear to influence the spin density distribution, limited to the indolinic moiety, which is very much like that observed for a wide variety of nonspiro 2,2-disubstituted indolinonic aminoxyl compounds.62–66 It may, however, enhance the persistence of this nitroxide: indeed, it was reported that, the EPR spectrum of a deoxygenated benzene solution of 73 could be observed unchanged for over a year.61 More recently, the new photochromic spin probe 74 was synthesized.67 As the paramagnetic unit is provided by a 4-(2,2,6,6,-tetramethylpiperidinyl-l-oxyl) (TEMPO) moiety, it is not unexpected that the spectral parameters of this radical, i.e., and did not differ from those of TEMPO itself.

This compound is expected to provide an insight into the degradation processes responsible for the photoaging of these photochromes through the characterization of EPR-active fragments. For a similar purpose, the two photochromic spin traps 75 and 76 were also synthesized.67 If, as proved long ago for radiolytic degradation,68 the photodegradation of the spiroindolinic photochromes involves homolytic processes leading to the formation of transient radical species, these are likely to be trapped by other still intact molecules of 75 or 76, thus resulting in the formation of spin adducts amenable to identification by means of EPR spectroscopy.

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Acknowledgment The author wishes to thank Anna Rosa Ferri and Silvia Roda in the Documentation Service of the Area della Ricerca—CNR, Bologna (Italy) for

their kind cooperation.

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5

Molecular Modeling Calculations Shinichiro Nakamura 5.1. INTRODUCTION Photochromic molecules provide a unique opportunity for the practical application of theoretical chemistry, molecular modeling, and molecular simulation, since their molecular properties directly determine the function of devices.1 Recent remarkable developments of computer hardware and software have opened a new horizon in material design, namely, the combination of computational studies and experimental approaches to offer an understanding of function on a molecular level.2 This does well for the future design of functional materials. The requirements for the design of photochromic compounds are, among others, the adjustment of absorption wavelength and intensity, the attainment of thermal stability, clarification of reaction mechanism, and enhancement of quantum yield. The purpose of this chapter is to review recent computational studies in consideration of these requirements. 5.2. ABSORPTION WAVELENGTH AND INTENSITY The first event of the photochromic reaction is light absorption. Therefore, how efficiently the molecules of a photochromic compound absorb a given wavelength of light is the first concern. Practical applications often require the use of a limited range of wavelengths of the light source, and, as a result, molecules must be adjusted to the light source. We will include some work on organic functional dyes in this section, since most important studies are on organic dyes. The pioneering work reported in this area has used the Pariser–Parr–Pople Shinichiro Nakamura • Yokohama Research Center, Mitsubishi Chemical Corporation, Yokohama227, Japan. Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999.

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(PPP) method, and there are a large number of reviews based on this method.3–5 The indispensable role of the PPP method for practical molecular modeling is not likely to decrease in photochemistry, not only because it is a very simple method, but also because it takes electron correlation into account by taking advantage of the subtle balance between the approximation and the approximation.† Nonplanar molecules often show interesting properties. Their nonplanarity precludes the use of the approximation. The approximation is not adequate for the transition and intramolecular hydrogen bonding. The photochromic properties of many systems are, however, related to their nonplanarity as well as the electronic structure, including valence electrons. Recently, the ZINDO method has shown its efficiency for these problems, to which the PPP method is not applicable. The capability of this method to reproduce experimental wavelength and intensity data of various organic dyes was tested extensively.8 Examples of nonplanar molecules include the near-IR-absorbing dye naphthoquinone methide. Kubo et al. synthesized and determined the X-ray structure of the dye and found it to be nonplanar.9 Given its near-IR-absorbing property, this was a puzzle, since planarity would provide the extended conjugation that would account for the long-wavelength absorption. What then was the origin of the near-IR absorption? It was explained as the result of a configuration interaction (CI) between and The analysis led to a nonconventional guiding principle: the less planar the molecule, the more redshifted the absorption. This was confirmed by the syntheses of other related materials.10 Surprisingly, this is not unique to naphthoquinone methide, but rather is a general phenomenon exhibited by molecules having the possibility of CI between and such as indoaniline dyes.11,12 A schematic diagram is shown in Figure 1. Although in many previous studies orbital interaction, such as that the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), has provided useful pictures for experimental chemists, it failed to predict the twist-induced red shift in these systems. Instead, the configuration interaction has explained the generation of the low-energy state. There have been other studies concerning the absorption properties of organic functional molecules. The relation between the absorption and conformational change of porphyrin dimers was reported.13 An extraordinary spectral similarity between a new Ni(II) complex and Ni(II) phthalocynaine, as shown in Figure 2, was interpreted14 within the four-orbital model.15 Although further analysis at the ab initio level is required, this analysis has provided a clue for the design of absorption properties, which is far more important and difficult than † γ kk, Ip, and are the two-electron integral, ionization potential, and electron affinity, respectively. For a detailed discussion see Refs. 6 and 7.

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simple wavelength prediction from the industrial point of view. Again, the description based on CI was indispensable. Other studies focusing on the spectral patterns of important dyes have been reported. The extent of and its influence on the absorption of perylene derivatives was investigated.16 The factors determining the spectral shape were not only the structure of the molecule itself but also the intermolecular interaction. In this regard, the difference between the absorption of a pyrrolopyrrole pigment in the solid state and its absorption in solution was investigated.17 The calculated and observed values of the wavelengths and intensities in the absorption spectra of photochromic diarylethene derivatives are shown in Table 1. The standard way to obtain molecules absorbing at long wavelength was to replace the hydrogens in the positions X, Y, Z, and W by electron-withdrawing or electron-donating substituents. Calculations have indicated that the position Z is the right position for such substitutions, and this was confirmed by the observed

values. This is a typical example of a study in which the results of synthesis and calculation were in concert.18 In fact, given the diarylethene structure, as shown in

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Table 1, it is almost impossible, without the aid of calculations, to predict which is the best position at which to introduce substituents. Recently, a novel correlation was reported.19 As shown in Figure 3 there is a linear relationship between the of the colored open form of spironaphthoxazine derivatives and the chemical shift of the 2'-carbon atom in the 13C nuclear magnetic resonance (NMR) spectrum of the closed form. For the substituents at the and positions, the correlation coefficient was excellent (0.97), although it was 0.86 when substituents in the 5'-position were taken into account, where not only the electronic effect but also nonbonded interactions between the 5'substituent and the oxygen atom in the ortho position in the open form were involved. Although the reason for the occurrence of a relationship between a characteristic of the ground state of the closed form (A in Figure 3) and a characteristic of the excited state of the open form (B) is not explained, as the chemical shift is partly dependent on the electronic charge of the carbon atom, a linear correlation between the of the colored open form and the electronic charge of the C2' calculated by the parametric model number 3,20 was obtained as shown in Figure 4. This is an interesting result allowing the prediction, a priori, of the effect of a given substituent.

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Most interesting photochromic molecules are large molecules. This often prevents calculations using the ab initio method from being extensively applied. However, recent theoretical progress is providing a fundamental understanding of the absorption properties of these molecules (e.g., see Ref. 21).

5.3. THERMAL STABILITY For any application of photochromic molecules, a discussion of the thermal stability of the colored form is in order. We will discuss two types of photochromic molecules: (1) those that operate by a photon-heat mode (photochemical forward reaction and a thermal reverse reaction), such as spiropyran derivatives, and (2) those that exhibit a photon-photon mode, such as diarylethene derivatives. 5.3.1.

Photon–Heat Mode

Compounds which are to be applied in photochromic laminated glass

products are required to become colored under irradiation such as sunlight and to exhibit a thermally driven return to the uncolored form in the absence of the

irradiation. The coloration is caused by the formation of the open form of molecules such as spiropyrans and spironaphthoxazines. Figure 5 shows the schematic potential surface. The requirement on these molecules is that they are as thermally independent as possible; that is, the coloration must occur independently of the environmental temperature. Therefore, the energy barrier, Ea in Figure 5, should be as small as possible, so that the Arrhenius plot does not have a large slope. On the other hand, the barrier must be sufficiently high to give a sufficiently long lifetime of the colored form (B in Figure 5). Achieving such a subtle balance is the problem encountered in the design of the molecule. For this reason, there have been many attempts to calculate the most stable structure among the possible isomers of the open form.22 Special attention has been given to derivatives of naphthoxazine (see also Section 5.4), owing to their good fatigue resistance or photostability. Although

most calculations have been carried out by semiempirical molecular orbital (MO) methods because of the size of the molecules, Figure 6 shows the most stable open form of spironaphthoxazine as determined by an ab initio MO method. This result was confirmed by an NMR nuclear Overhauser effect (NOE) experiment.23 The description of the wave function at the Hartree-Fock (HF) level was sufficient for the keto type of the open form, which reconfirms the validity of the calculated results for the keto type shown in Figure 6. The zwitterionic electronic state is, however, beyond the capacity of calculations at the HF level and requires a description beyond this level. Unfortunately, there are reports that ignore this

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critical point. One exception is a study concerning the open-form structure of spirobenzopyran,24 in which the MNDOC/MRSDCI method25 was used.

5.3.2. Photon–Photon Mode The second category of compounds—those operating by a photon-photon mode—comprises photochromic systems functioning photochemically in both the forward and backward reactions. Fulgide and diarylethene derivatives are representative examples (Scheme 1).

In both systems, the photochemical forward reaction entails the cyclization of 1,3,5-hexatriene to 1,3-cyclohexadiene:

Therefore, the application of the Woodward–Hoffmann rules26 indicates the possibility of thermal reaction in a disrotatory mode as well as photochemical reaction in a conrotatory mode. Experimentally, no thermal ring-closing reaction was observed for these derivatives. Irie et al. have found photochemical products of ring closure that were thermally very stable.27 Semiempirical MNDO calculations have provided the

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explanation for this stability.28 Figure 7 shows the state correlation diagrams for the disrotatory mode reported for 1,2-di(3-furyl)ethene (a) and 1,2-diphenylethene (b). The relatively high energy of the product, 27 and 42 kcal/mol higher than the energy of the reactant in Figure 7a and 7b, respectively, prevents the thermal reaction. On the other hand, in a conrotatory mode the energy of the ring-closed product from l,2-di(3-furyl)ethene is only 9.2 kcal/mol higher than that of the

reactant (Figure 8a), while the ring-closed product from 1,2-diphenylethene is destabilized by 27.3 kcal/mol relative to the reactant (Figure 8b). As a result, even

for the symmetry-forbidden reaction in the case of 1,2-diphenylethene (Figure 8b), the backward reaction may occur, reflecting this destabilization. This is consistent with the experimental observation of a thermally unstable photochemical product. By contrast, in the case of 1,2-di(3-furyl)ethene, the barrier of the symmetryforbidden reaction mode would be high enough to prevent the forward as well as the backward thermal reaction. This explains the experimentally observed thermal stability of photochemical products of this type. In fact, it has been shown that the relative stability of the ground states of the open-ring form and the closed-ring form is an important factor for the formation of thermally stable photochemical products. This was confirmed with various examples,29 including dinaphthylethene derivatives.30 Moreover, the source of the energy difference was traced to the change of aromaticity before and after the ring-closing reaction.28

5.4.

REACTION MECHANISM

The contribution of computational chemistry to the study of mechanistic problems in photochromism is increasing since information on intermediates, transition states, and conical intersections is often very difficult to obtain experimentally. As a list of all such studies is beyond the scope of the present

chapter, our discussion will focus on representative molecules such as spiropyran derivatives and fulgide derivatives, but will also include examples of other recent work. 5.4.1. Spiropyran Derivatives

The investigation of the colored form of spiropyrans by theoretical calculations started at the dawn of studies on photochromic compounds. The pioneering studies of a benzothiazolinic spiropyran were published by Guglielmetti and coworkers, who investigated the different planar forms of the open-form stereoisomers by calculations of the atomic interaction energies,31 the extended Hückel method,31–33 the PPP method,32 and the CNDO/2 method.34 The results of molecular mechanical (MM) calculations on spiropyrans and spirooxazines were compared with the X-ray structures and used to explain 1H NMR data.35

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Semiempirical MO studies were also reported by other research groups.23,36–41 Recently, a systematic comparison of the various computational techniques for predicting the geometry and electronic spectra of spiropyrans and spirooxazines has been published.42 The validity of various semiempirical methods has been checked. This may provide a useful aid for practical uses of semiempirical methods.42 Molecular dynamics (MD) has also found use for these series of derivatives.43–45 It is relevant to cite here two more examples of the application of molecular calculations to spiropyran derivatives. First, in a study of the relationship between photochromism and second-order nonlinear optical (NLO) properties in spiropyran- and spirooxazine-doped polymer films, dipole moments, polarizabilities, and hyperpolarizabilities were estimated by the MNDO method.46 Second, the relative stabilities of the radical anions as well as their spin density distributions for a spiroindolinic series were calculated by an ab initio method in a combined electron paramagnetic resonance (EPR)/electrochemistry study.47 5.4.2. Fulgide Derivatives

Fulgide and related compounds are also typical photochromic molecules. Ilge and Colditz48 reported a PPP MO study on the mechanism of photoisomerization, substituent effects, and the quantum yield of the ring-closed reaction. Yokoyama et al.49 have studied the mechanism of the enantiotopomerization process in an isopropyl-substituted furylfulgide. The MNDO-AM1 calculated activation enthalpy was in good agreement with experiments in which the X-ray structure was obtained and the thermodynamic parameters were determined by variabletemperature measurements. The reported scheme is shown in Figure 9.49 These authors also reported the first study on diastereoselective photochromism of an (R)-binaphthol-condensed indolyfulgide.50 The heat of formation was calculated by MNDO-AMI, and the results were used to elucidate the stabilities of the conformers.50 5.4.3. Other Photochromic Systems

Although there are many other interesting photochromic systems which are less common than the systems described up to this point, computational studies of such systems have been limited. Some exceptions will be briefly cited here. Nespurek et al.51–53 reported the mechanism of the photochromism of sydnone, a mesoionic compound. Since this system exhibits photochromism in the solid state, the origin of the colored form was assumed to be the formation of a colored center or intermolecular charge-transfer state. In contrast to this view, the combined evidence from experiments and MO calculations led Nespurek et al. to conclude that the nature of the photochromism was intramolecular.

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In a review of the photochromic transformation mechanism for quinone derivatives,54 the MO studies by Gritsan and co-workers55,56 using MNDO-AM1, CNDO/S, and PPP are mentioned as pertinent aids for the understanding of the

mechanism in singlet as well as triplet potential surfaces. Photochromic Schiff bases were studied experimentally with the help of the MO calculations by Kownacki and co-workers.57–59 The proton-transfer mechanism along the internal hydrogen bonds was investigated, and it was shown that in molecules with two equivalent internal hydrogen bonds, only one proton translocated.59 Other examples in which theoretical calculations were applied in the study of photochromism include an MNDO/3 study on dihydroindolizines,60,61 the use of PPP electron densities as an aid for estimating the linear dichromism (LD) and magnetic circular dichromism (MCD) of symmetric dialkoxyanthracenes,62 and studies of the structures of unusual twisted cis zwitterionic azomethines,63 a

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synthetic 4',7-dihydroxyflavylium chloride,64 nitro derivatives of salicylideneaniline and 2-(2'-hydroxyphenyl)benzothiazole,65 4-bromo-N-salicylideneaniline,66 and pentafulvadiene derivatives.67 There was also an interesting study of proton transfer in the excited state.68

5.5.

QUANTUM YIELD

Heller reported the wavelength dependency of the ring-opening reaction of fulgides.69 Yokoyama and Kurita reported that the dependency was observed not only in the ring-opening reaction, but also in the ring-closing and E–Z isomerization of fulgide derivatives.70 Irie and Mohri reported the wavelength dependency in diarylethenes.27 These results provide an opportunity for theoretical analysis to suggest the mechanism. Although there exists Kasha’s rule and Vavilov’s rule,71 Turro et al.72 have shown that the wavelength effect is observed in a wide range of photochemistry. Several hypotheses have been proposed to explain the wavelengthdependent quantum yield of the photochemical reaction; these include (i)

isomerization of multiple geometries in the ground state, (ii) the existence of more than two absorption bands, with, for example, one being reactive and the other not, (iii) multiple basic reactions, and (iv) vibrational interaction or a “hot” ground state. The ultimate determination of the mechanism must be based on accurate experimental evidence and will be the principal target for both theory and experiment in the future. A related system, in that it contains the same 1,3,5-hexatriene skeleton, provitamin also showed the wavelength effect.73 As shown in Figure 10, Dauben et al.73 reported that the ring-closing reaction of

was caused by

wavelengths longer than 305 nm, producing as the main product and provitamin as the by-product. At wavelengths shorter than 302 nm, on the other hand, the main product was resulting from a cis–trans isomerization. Multi-reference double excitation (MRD) CI calculations by Share et al.74 together with the state correlation diagram of 1,3,5-hexatriene to 1,3-cyclohexadiene suggested the intervention of a doubly excited state. This was suggested as one of the sources of the wavelength effect.73 Robb et al.75,76 have put forward different arguments. Two conical intersections were proposed to be the source of the wavelength effect based on multi-reference calculations.75,76 Previous work had indicated that conical intersections were very rare.77 However, on the basis of accurate MC-SCF calculations, Robb et al. considered that they were more general than had been assumed, especially in order to understand photochemical reactions occurring within one vibrational motion78 or within 1 ps.79

In an attempt to understand the mechanism of the wavelength effect in diarylethenes, MRSD-CI calculations were reported, again on the reaction from

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1,3,5-hexatriene to 1,3-cyclohexadiene.80 The possibility of a “hot” ground state is also to be investigated in the future. Finally, a combined study of diarylethene derivatives by femtosecond spectroscopy and Franck–Condon wave packet dynamics was presented.81,82

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55. N. P. Gritsan, I. V. Khmelinski, and O. M. Usov, Experimental and theoretical study of photoenolization mechanism for 1-methylanthraquinone, J. Am. Chem. Soc. 113, 9615–9620 (1991). 56. N. R Gritsan, Quantum-chemical and experimental investigations of photochromic transformations in quinone compounds, J. Mol. Struct. 181, 285–296 (1988).

57. K. Kownacki, L. Kaczmarek, and A. Grabowska, Single versus double proton transfer in the photochromic Schiff bases. Electronic spectroscopy and synthesis of model compounds, Chem. Phys. Lett. 10, 373–379 (1993). 58. K. Kownacki, A. Mordzinski, R. Wilbrandt, and A. Grabowska, Laser-induced absorption and fluorescence studies of photochromic Schiff bases, Chem. Phys. Lett. 227, 270–276 (1993). 59. A. Grabowska, K. Kownacki, and L. Kaczmarek, Proton transfer along the internal hydrogen bonds in excited Schiff bases. Photochromism in symmetric systems with two equivalent reaction sites, J. Luminescence 60/61, 886–890 (1994). 60. N. A. Garcia, G. Rossbroich, S. E. Braslavsky, H. Dürr, and C. Dorweiler, Photoacoustic measurements and MINDO/3 calculations of energy storage by short-lived species: the spiro[l,8-a]dihydroindolizine-betaine system, J. Photochem. 31, 297–305 (1985). 61. C. Dorweiler, P. Spang, H. Dürr, K. Peters.and H. G. v. Schnering, Photochromic systems. Paper 9. Structural parameters of photochromic dihydroindolizines. X-ray analysis and MINDO/3-calculations, Israel. J. Chem. 25, 246–251 (1985). 62. T. Brotin, J. Walik, J.-P. Desvergne, and H. Bouas-Laurent, Electronic absorption properties of symmetrical dialkoxyanthracenes. Linear dichroism and magnetic circular dichroism, Photochem. Photobiol. 55, 335–347 (1992).

63. M. I. Knyazhansky and A. V Metelista, On photocolored product structure of photochromic azomethines in solutions and crystals, Mol. Cryst. Liq. Cryst. 246, 315–318 (1994).

64. P. Figueiredo, J. C. Lima, H. Santos, M.-C. Wigand, R. Brouillard, A. L. Macanita, and F. Pina, Photochromism of the synthetic 4 ,7-dihydroxyflavylium chloride, J. Am. Chem. Soc. 116, 1249– 1254(1994).

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65. R. S. Becker, C. Lenoble, and A. Zein, Photophysics and photochemistry of the nitro derivatives of salicylideneaniline and 2-( -hydroxyphenyl)benzothiazole and solvent effects, J. Phys. Chem. 91, 3517–3524(1987). 66. W.-H. Fang, X.-Z. You, and Z. Yin, Theoretical studies on the photochromic processes of 4-bromoN-salicylideneaniline, Theor. Chim. Acta. 92, 297–303 (1995).

67. M. Sakaguchi, Y. Takuma, K. Mitsuhashi, and S. Nakamura, A molecular orbital study on the photochromic property of pentafulvadiene, Nippon Kagaku Kaishi 10, 1109–1116 (1992). 68. S. Nagaoka, and U. Nagashima, Nodal-plane model in excited-state intramolecular proton transfer, Trends. Phys. Chem. 6: 55–87 (1998). 69. H. G. Heller, Fine Chemicals for the Electronics Industry, (P. Bamfield, ed.), p. 120, Royal Society of

Chemistry, London (1986). 70. Y. Yokoyama and Y. Kurita, Photochromic fulgides applicable to optical information storage.

Discovery of new nondestructive readout method, Nippon Kagaku Kaishi 10, 998–1005 (1992) and references therein. 71. N. Turro, Modern Molecular Photochemistry, Benjamin/Cummings, Menlo Park, California

(1978). 72. N. Turro, V. Ramamurthy, W. Cherry, and W. Farneth, The effect of wavelength on organic photoreactions in solution. Reactions from upper excited states, Chem. Rev. 78, 125–145 (1978). 73. W. G. Dauben, B. Disanayaka, D. J. H. Funhoff, B. E. Kohler, D. E. Schilke, and B. Zhou, Polyene and states and the photochemistry of provitamin D3, J. Am. Chem. Soc. 113, 8367–8374 (1991). 74. P. E. Share, K. L. Kompa, S. D. Peyerimhoff, and M. C. VanHermert, An MRD CI investigation of the photochemical isomerization of cyclohexadiene to hexatriene, Chem. Phys. 120, 411–419 (1988).

75. F. Bernardi, M. Olivucci, I. N. Ragazos, and M. Robb, A. new mechanistic scenario for the photochemical transformation of ergosterol: An MC-SCF and MM-VB study, J. Am. Chem. Soc. 114, 8211–8220 (1992). 76. F. Bernardi, M. Olivucci, and M. Robb, Modeling photochemical reactivity of organic systems a

new challenge to quantum computational chemistry, Israel J. Chem. 33 265–276 (1993) and references therein. 77. L. Salem, Electrons in Chemical Reactions, John Wiley & Sons, New York (1982).

78. U. Manthe and H. Koeppel, Dynamics on potential energy surfaces with a conical intersection: Adiabatic, intermediate, and diabatic behavior, J. Chem. Phys. 93, 1658–1669 (1990). 79. C. V. Shank, The first step in vision: Femtosecond isomerization of rhodopsin, Science 254, 412– 415 (1991) and references therein. 80. S. Nakamura, A. Murakami, M. Adachi, and M. Irie, Ab-initio and semiempirical MO studies on photochromic molecules, Mol. Cryst. Liq. Cryst. 246, 231–234 (1994).

81. H. Petek, K. Yoshihara, Y. Fujiwara, Z. Lin, J. H. Penn, and J. Frederick, Is the nonradiative decay of cis-stilbene due to the dihydrophenanthrene isomerization channel? Suggestive evidence from photophysical measurements on 1,2-diphenylcycloalkenes, J. Phys. Chem. 94, 7539–7543 (1990).

82. J. Frederick, Y. Fujiwara, J. H. Penn, Y. Yoshihara, and H. Petek, Models for stilbene photoisomerization: Experimental and theoretical studies of the excited-state dynamics of 1,2-diphenycycloalkenes, J. Phys. Chem. 95, 2845–2858 (1991).

6 The Enantiomers of 2-Donor-Substituted Benzopyrans and Benzo-1,4-oxazines and Their Thermal Racemization Albrecht Mannschreck, Klaus Lorenz, and Michael Schinabeck 6.1. INTRODUCTION The electrocyclic reactions of certain pyrans and 1,4-oxazines have received much attention during the last few years, mainly because they are connected with photochromism1–6 and, to a lesser extent, with thermochromism.7 Among the thermal electrocyclic rearrangements in such molecules, the well-known ringclosure reaction has been investigated in many cases because of its importance in relation to the persistence of colored merocyanines formed by irradiation of spiro or non-spiro benzopyrans or benzo-1,4-oxazines. On the other hand, the thermal ring-opening reaction has not been an important subject of research, probably because it does not seem to be relevant for practical applications, but also because the methods available for this purpose have been very much limited in their scope. One method works for molecules exhibiting a thermochromic equilibrium.8–13 For these, the barrier to thermal ring opening has frequently been calculated from the experimental barrier to ring closure plus the energy difference between the opened and the closed molecules. This indirect approach suffers from the addition of two experimental errors and from the fact that the ring-closure reaction Albrecht Mannschreck, Klaus Lorenz, and Michael Schinabeck • Department of Organic Chemistry, University of Regensburg, D-93040 Regensburg, Germany. Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999.

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may start from any isomer14 of the opened molecule, depending upon the conditions. In some cases9,10,12 direct monitoring of the ring-opening reaction has been possible inasmuch as a suitable thermochromic equilibrium existed. No such condition is to be met if the enantiomerization of benzopyrans or benzo-l,4-oxazines is used for the direct determination of the barrier to thermal ring opening (Figure 1). The easiest way to measure the rate constant of enantiomerization, is by dynamic 1H nuclear magnetic resonance (NMR) measurements of diastereotopic groups, which are a consequence of molecular chirality. Although this method has been used in the past,15–20 it has been limited for two reasons: (i) diastereotopic groups are not always present in the molecules of interest; and (ii) the availability of 1H-NMR spectrometers with low transmitter frequencies (approximately 100 MHz), as required for the relatively high barriers to ring opening, is decreasing. Separation of the enantiomers (Figure 1) and their thermal racemization20–24 represent a more difficult but less limited approach to the direct determination of the rate constants Such enantiomers had never been described, probably because they do not appear to be useful for the investigation of photochromism, but also because the classical methods of separation would be difficult to apply in the absence of suitable functional groups. Therefore, the present chapter will describe the physical methods available for the investigation of the enantiomers of 2-donor-substituted benzopyrans and benzo-l,4-oxazines. Some examples of their stereoanalytical characterization will be given. Subsequently, it will be shown that thermal racemization is a novel and

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fairly general approach to the measurement of the ring-opening barriers which had previously been difficult to determine. Kinetic results for some relevant classes of compounds in this area (Figure 1) will be presented, and their qualitative rationalization in terms of steric and, particularly, electronic effects will be attempted.

6.2. METHODS OF INVESTIGATION 6.2.1. Remarks Pertaining to All Methods

The use of physical methods in organic stereochemistry has made great

progress during the last 25 years,25 beginning essentially with the advances in enantioselective NMR and enantioselective chromatography. Such methods are required to attain the aims indicated above, i.e., the separation, analysis, and

kinetic investigation of the enantiomers of pyrans and 1,4-oxazines. The theory and the experimental details of the necessary methods are published. Therefore, we will restrict ourselves to brief descriptions and to citations of some literature. However, we will give a few representative unpublished examples of separations and analyses obtained for some pyrans and oxazines.

6.2.2. Preparative Enrichment of Enantiomers by Enantioselective Liquid Chromatography The application of liquid chromatography on nonracemic (“optically active”) sorbents has become a well-developed and successful method for the analytical and preparative separation of enantiomers.25,26 It is particularly useful for racemates that are difficult to resolve via diastereomeric derivatives, i.e., for molecules lacking functional groups for reaction or interaction. This is the case for many pyrans and oxazines, the first separations of which were published in 21,22 1990. Although a wealth of nonracemic sorbents for analytical use are known, their application to preparative enrichment, i.e., to the isolation of small, even milligram, amounts of the enantiomers, is often difficult because the larger quantity of sorbent required entails a lot of synthetic work or is expensive to purchase. In our experience, microcrystalline triacetylcellulose (TAC)25–27 and tribenzoylcellulose28 meet both requirements: they can be used analytically at high pressures, and the larger amounts required for high- or low-pressure preparative work are readily accessible. Preparative29 separations of racemates must be preceded by analytical separations in order to explore the experimental conditions such as sorbent, eluent, and wavelengths of detection during chromatography, all of which are difficult to predict. Detection can be performed by a photometer at a chosen

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wavelength and, simultaneously, by a polarimeter (or circular dichrograph) at a chosen wavelength.30,31 From a chromatogram such as that shown in Figure 2,32 the retention factors (capacity factors) can be easily determined.26 These are useful for comparisons and for optimizations because low values mean low substrate/sorbent interactions (and mostly low enantioselectivity) whereas high values mean that long times are required for separations. In cases of baseline chromatograms like that in Figure 2, semipreparative complete separations were accomplished by injection of about 15mg of substrate on a column 30 cm in length and 2.5 cm in diameter by cutting two fractions. The other extreme is an analytical chromatogram with no sign of separation at all, as far as photometric detection is concerned. Nevertheless, the corresponding chiroptical detection often indicates some degree of enantioselectivity (cf. the polarimetric curve for the first run in Figure 3). In such cases, recycling33 the racemate several times through the same column increased the separation, as in

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Figure 3.34 Careful cutting of fractions resulted in a preparative enrichment of the enantiomers that was sufficient for many purposes.

6.2.3. Determination of Enantiomeric Purities Preparative enrichment of enantiomers should be followed by determination

of their chemical and enantiomeric purities, e.g., by enantioselective liquid chromatography.25,26 In addition to the sorbents mentioned above, others may be used which are available in only smaller amounts. In our laboratory, this is true

for (+)-poly(trityl methacrylate) on silica,35 which can be used for analytical purposes. Preparative separations on a column 0.5 cm in diameter, however, would require many injections of small amounts of racemate. In the case of a baseline chromatogram like that in Figure 2, the determination of enantiomeric purity by measurement of the two peak areas of the enantiomers is straightforward. An analytical chromatogram showing some separation by photometric detection in spite of peak overlap (Figure 4) can still be used for optical purity determination.26,30,31 The simultaneous use of both photometric and chiroptical detection as

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in Figure 436 resulted in an experimental /A diagram and in computer deconvolutions of the experimental /V and A/ V chromatograms ( Rotation angle. A: Absorbance. V: Elution volume). Enantioselective 1H-NMR spectroscopy25,37 is another possibility for the measurement of enantiomeric purity when chromatography fails. The spectra have to be acquired in the presence of a nonracemic auxiliary, e.g., (S)-l-(9-anthryl)-

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2,2,2-trifluoroethanol in Figure 5.38 Sufficient intermolecular diastereomeric interactions, e.g., via hydrogen bonds, are among the conditions for the observation of additional splittings of the signals of the substrate. These splittings have to be sufficiently large for electronic integration (Figure 5). We have determined the enantiomeric compositions of several indolinospirooxazine and spirobipyran samples in this way.

6.2.4. Assignment of Relative Configurations The specific rotation can be measured at only a few wavelengths in most laboratories. This is one of the reasons why the significance of the full circular

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dichroism (CD) spectra has increased, compared to measurements of values.39 The latter were used for fast and independent characterization of all enantiomers that were preparatively separated. CD spectra39,40 have been obtained for many pyrans and oxazines. The enantiomeric purities of the samples used have to be taken into account when differential absorption coefficients are compared (e.g., in Figure 6).41 For molecules with similar chromophores and ultraviolet (UV) spectra, a common relative configuration may be derived from similar CD spectra. The absolute helicities42 (P) in Figure 6 cannot yet be determined by CD but were obtained by

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application of a successful rule35 concerning the elution order of antipodes on poly(trityl methacrylate) on silica. There are several reasons why some CD spectra are measured on-line with liquid chromatography, i.e., without preparative enrichment of the enantiomers.26,40,43,44 The main reason is fast interconversion as will be seen for many pyrans and oxazines in

Section 6.2.5. Their CD spectra were usually obtained during stops of the chromatographic flow at the two peaks of the enantiomers, as shown in Figure 7.41 The differential absorbance measured in this way can easily be calibrated43,44 to yield the usual differential absorption coefficient Such spectra are required, for instance, for the determination of relative configurations. 6.2.5. Measurement of Barriers by Thermal Racemization

The interconversion of enantiomers

can be treated as a reversible first-order reaction,45 starting from enriched (R) or (S):

where and are the rotation angles at times 0 and t, respectively. The rate constant of thermal enantiomerization is thus determined from the slope of a plot of versus t. The half-life25 is derived from the above equation as

The corresponding barrier, i.e., the free enthalpy of activation from

is calculated

and the temperature according to the Eyring equation.

In cases of higher temperatures of racemization, the thermal stability was occasionally checked by an independent experiment in which the racemate was

heated in the same solvent, or the deuterated solvent, at the same temperature and for the same period of time as in the kinetic experiments. spectra did not show any evidence of decomposition. Similar to circular dichroism, racemizations can be measured after preparative enrichment of the enantiomers (“offline”) or without enrichment (“on line”). The latter technique26,30 is mainly applied because of fast enantiomerization During a stop of the chromatographic flow at the peak of the antipodes, the decrease of the rotation angle (or the differential absorbance; Figure 7, top) at a fixed wavelength is monitored as a function of time. The results

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for the enantiomerization of 8 (Figure 7) will be transformed into results for the cleavage of the bond, and therefore discussion of these results will be postponed to Section 6.3.8. All racemizations were monitored in the absence of external light. A test experiment46 showed that the irradiation by the polarimeter lamp is too weak to cause significant photoracemization. Therefore, the decrease of the rotation angle is effected thermally.

6.3. RESULTS OF THE THERMAL RACEMIZATIONS 6.3.1. Transition State of

Bond Cleavage

Besides information about the ground states (Figure 1), knowledge of the structure of the transition states of enantiomerization would be a prerequisite for an interpretation of the corresponding barriers. In this respect, the available theoretical calculations represent the only information which can be taken as a basis for discussion. 2H-Pyran, 2-amino-2H-pyran, and benzo-2H-pyran have been treated19,47 by the MINDO/3 and the Pariser–Parr–Pople methods. The latter procedure has also been applied19 to several oxaindano- and indolinospiropyrans. The transition state of 2H-pyran48 has been calculated by several ab initio approaches, and that of an indolinospironaphthoxazine49 by means of the semiempirical AMPAC program with PM3 parametrization. All authors concluded that the bond is broken in the transition state. At least for the molecules studied, no additional barrier of similar or greater height has to be surmounted on the way from the ground state to any intermediate.47–49 The two substituents at the C(2) atom of the former 2H-pyran and -oxazine systems are situated47,49 out of the C(2)–C(3)– C(4) plane (cf. Figure 8) but apparently deviate47,49 from the orthogonal arrangement by an estimated angle of 20°; i.e., conjugation between the former C(2) and C(3) centers is weak in the transition state. At least in the case of the naphthoxazine, the three bonds of the C(2) atom of the former oxazine ring appear to be situated approximately in a single plane.49 In this molecule, there is a negative partial charge at the O atom, and positive partial charges reside at the C(2) and N atoms of the indoline systems.49 On the basis of these results, we have postulated the transition states depicted

in Figure 8. In the center part of the reaction profile, the molecule has to adopt some conformation(s) suitable for enantiomerization. The details of this region of the profile are not relevant to our kinetics, insofar as no further barrier of similar or greater height has to be surmounted. No such barrier had been found by the calculations.47–49 (However, it may exist in a few cases to be mentioned later, particularly for -disubstituted spirobipyrans, where additional steric hindrance occurs.) At the moment, we know that the reaction path involves intermediates but

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we do not yet know whether or not the ring-opened species identified in studies of thermochromic or photochromic behavior, e.g., the merocyanines, are intermediates of enantiomerization reactions. If we assume that the enantiomerization (Figure 8) proceeds via any intermediate, it can be shown statistically50 and derived formally51 that where k is the rate constant for cleavage of the bond and is the experimental rate constant of enantiomerization (see Section 6.2.5). This statistical factor of 2 has been included in all k, and values given in the following sections (except the values for symmetrical spirobipyrans, where two corrections cancel). Instead of bipolar transition states (e.g., as in Figure 8), biradical ones have been considered for the following cases: the ring closure of a nitro-substituted indolinospiropyran12 and the ring opening of three thermochromic carboxylic and sulfonic acids of an indolinospiropyran.9 No explicit information concerning this problem is provided by the computational results available. Some authors19,47,48 do not mention biradicals, and others49 exclude them a priori from the calculations. Occasional examinations of thermal stability in the presence of air have not shown any evidence of decomposition products (see Section 6.2.5). Therefore, we

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suggest that biradicals are not involved in our racemization experiments. It will be seen below that solvent effects on the barriers to bond cleavage provide some support for polar transition states.

6.3.2. Remarks Pertaining to All Types of Benzopyrans and

Benzo-1,4-oxazines

In some of the the tables presented below, it is mentioned that liquid chromatography was performed on-line with a polarimeter to monitor racemization. This statement means that preparative enrichment of enantiomers was not easy in these cases, mostly because of low barriers to enantiomerization. In such cases, circular dichroism spectra also were obtained by an on-line procedure described above in Section 6.2.4. For all other compounds in the tables, a preparatively enriched enantiomer was used for the racemizations. Such samples were characterized by physical methods, although in the present chapter the corresponding data are given for some representative examples only (see Section

6.2). In this context, “to characterize” means to determine the enantiomeric purity, P, to measure the specific rotation at up to five wavelengths, to monitor the circular dichrogram, and to calculate all chiroptical data for P = 100% by using the above experimental P value. In cases where both enantiomers were preparatively enriched, these chiroptical data (except their signs) agreed with each other within reasonable error limits. In addition, some of our samples prepared by enantioselective chromatography were checked by normal 1H-NMR spectroscopy; no impurities were found. This was done because impurities might have been contained in the eluent or in the chromatography equipment; they might also have been formed by some decomposition reaction in solution. With the present equipment in our laboratory, the minimum amount of material required for all the above investigations turned out to be ~ 4mg per enantiomer. With respect to the racemates used for chromatography, their method of synthesis is briefly indicated in each of the following sections insofar as the compounds were prepared in our laboratory. The discussions in the next sections will refer to enthalpy, i.e., to whereas the experimental results refer to free enthalpy, i.e., because, owing to the effort required to determine (and these parameters were determined in only a few cases mentioned in later sections. The absolute values obtained are below and, therefore, the differences between and are not large. The steric structures given in the following tables and formulas always represent one of the enantiomers, which is not necessarily the one used for racemizations.

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6.3.3. 2-Aryl-2-methylbenzopyrans The photochromism of benzo- and naphthopyrans at room temperature and below is well known. Substituents in the 2-position may act as electron donors and stabilize the system of the ring-opened isomer via conjugation.52 In this respect, 2,2-diarylnaphthopyrans have received considerable attention because of their photochromic properties at room temperature.6,53,54 Our approach to the investigation of thermal ring opening requires chiral substrates, e.g., 2-aryl-2-methylsubstituted compounds.32,55 The barrier heights (Table 1) of 2-methyl-2-phenylbenzopyrans, determined by our methods, are almost identical for 9–12, i.e., for different positions of a benzo or pyrido ring. However, the additional benzo ring in the phenanthropyran 13 decreases the value by (cf. 9 and 13 and 11 and 13 in Table 1 as well as 14 and 16 in Table 2). Apparently, the transition state for thermal ring opening of 13 is stabilized by additional

conjugation, relative to the transition states of 9 and 11 (cf. Section 6.3.1). In the case of 13, racemizations at temperatures between 29.5 and 70.0 °C in diglyme yielded the activation parameters and These results may be of interest for comparison with

the results of future theoretical calculations of and for future discussions of values of 2,2-disubstituted pyrans. Our barrier for the dibenzofuran derivative 1 refers to a very high temperature and therefore cannot be compared quantitatively with the other values in Table 1. Apparently, benzofurano annelation in 1 provides less additional conjugation than benzo or pyrido annelations in 9–12. In addition, an oxygen atom in the position para to the pyran oxygen atom may cause some increase of the barrier, as observed for spiro[indoline-benzopyrans].56 Finally, the values decrease in a reasonable way when the electron-donating substituents = methoxy or morpholino (Table 2) are introduced into the phenyl ring bound to the 2-position of the pyran system. Comparisons between 11 and 14 and between 13 and 16 in Table 2 result in methoxy contributions of whereas comparison between 11 and 15 shows a morpholino contribution of Apparently, the transition state for thermal ring opening of 14, 15, and 16 is stabilized by additional conjugation, relative to the transition state for ring opening of 11 and 13. This additional conjugation can be symbolized (Figure 9) by resonance formulas. 6.3.4. 2-Alkoxybenzopyrans

2-Alkoxybenzopyrans have played no role in the investigation of ringopening reactions, probably because no photochromism at room temperature has been expected. We have shown in several experiments57 that this negative expectation is fulfilled. Nevertheless, 2-alkoxybenzopyrans are of interest in the context of thermal behavior because they represent non-spiro model molecules

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that may be compared with spirobibenzopyrans. Mainly two types of 2-alkoxy compounds have been investigated; their structures and their known barriers to thermal enantiomerization are given in Tables 3 and 4. The substituted 2-ethoxy-3-methylbenzopyrans (Table 3) were prepared57 from 2-hydroxycinnamic aldehydes and ethanol using a general procedure.58 Comparison between 18 and 20 (15 kJ mol – 1 difference) and between 19 and 20 (6 kJ mol – 1 difference) both show a decrease of the barrier when is replaced by because the latter accommodates the negative charge in the transition state of ring opening. The extent of this decrease, i.e., the difference of the values compared, is different for a group in the 6- and in the 8position. The reason for this observation will be investigated in future work. In the case of the 2-benzyloxybenzopyrans24,59 21 and 22 (Table 4), the additional benzo ring in the naphtho compound 21 is again seen to decrease the value because the transition state for ring opening is stabilized by additional conjugation. The solvent effect of EtOH, relative to hexane, exerts its influence in the same direction and argues in favor of polar instead of biradical transition states. Finally, a comparison of the barriers for the non-spiro 2-benzyloxypyran 22 and the spirobipyran 23 in EtOH (Table 4) indicates that the pyran ring (Figure 10, bottom) apparently provides more stabilization of the transition state than the benzyloxy group (Figure 10, top). This suggestion is in agreement with better delocalization of the positive charge by the nonopened pyran ring, compared to its

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delocalization by the benzyloxy substituent. Therefore, at least in the case of 23 and 22, the spiro structure favors ring opening by comparison with the non-spiro structure. For comparison, a single 2-methoxybenzoxazine, namely, prepared57 60 by analogy to a procedure in the literature, was also studied.

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There was no decrease of the rotation angle of one of the enantiomers in hexachloro-1,3-butadiene at 81 °C, which means that It will be shown later for spirobenzoxazines that the introduction of the aza nitrogen atom renders ring opening more difficult. Two 1-cyanobenzo-1H-pyrans61 also show thermal racemizations62:

For 25,

at 81.0°C, and for 26, at 80.7°C. In 1,4-dioxane as solvent, no thermal decomposition occurred. However, it is not certain that these enantiomerizations are caused by cleavage of the C(1)–O bond because a negative charge on the former ring oxygen atom would not be stabilized by the MeO groups and a positive charge on C(1) would not be stabilized by the cyano substiruent.

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6.3.5. Spirobibenzopyrans

Although photochromism of spirobibenzopyrans is displayed mostly at lower temperatures, many molecules of this type were investigated in the past for several purposes.2 In our laboratory, the known and the unknown racemates have been prepared36,41 according to general procedures.2,63–66 For spirobipyrans consisting of two identical pyran moieties, we assume that the experimental rate constant for the enantiomerization is equal to k, the rate constant for the transformation of the spiro ground state to an intermediate, e.g., a merocyanine. The reason for this assumption is the fact that the usual correction50,51 for an intermediate and an additional correction for the two equal possibilities of bond cleavage in a spirobibenzopyran cancel. The compounds in Table 5 are listed in order of decreasing values although the fact that these values were not all obtained at the same temperature

may render a few of the rankings inaccurate. Because of the substituents in the 3and 3'-positions, steric factors add to the electronic ones and prevent a simple explanation of the order of barriers found. This is seen by looking at the low and

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the high values in Table 5. The short bridge in spirobipyran 31 will reduce the distance, i.e., the angle between the planes of the two pyran rings, relative to that in 5, the unsubstituted analog. Ring strain in 31 may increase the energy of the ground state and bring it closer to that of the transition state and thereby reduce the barrier. In addition to this comparison of the ground states of 31 and 5, some aspects of their transition states may have to be taken into account. On the other hand, the long bridges in 28 and 7 as well as the two methyl groups in 27 will exert steric effects on the ground state and particularly on the transition state of these compounds relative to 5, the unsubstituted molecule. In these cases, we cannot exclude the possibility that the relatively high values for the transformation of the spiro ground state to an intermediate contains a contribution from steric hindrance which adds to the usual electronic contribution of the bond cleavage. The decrease of the barriers ongoing from 32 to 33 (diglyme) and from 34 to 35 (hexane/2-propanol, 9:1) in Table 6 is in agreement with the behavior of 27 through 6 (Table 5), and the explanation of these decreases may require inclusion of steric effects in a similar way. X-ray analyses are planned in order to determine the angles between the planes of the pyran rings and to see whether their trend helps to explain the barriers.

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Racemizations in MeOH between 10 and 45°C resulted in the activation parameters presented in Table 7. For their detailed understanding, theoretical calculations will be required. The solvent effects in MeOH, relative to diglyme (5 and 6 in Table 5) and to hexane/2-propanol, 9:1 (34 in Table 6), again argue in favor of polar instead of biradical transition states. Apparently, the polar transition states are more stabilized by solvation than the less polar spiro ground states. As far as the spirobipyrans 27 (Table 5) and 32 (Table 6) are concerned, the additional benzo rings in the dinaphtho compound 32 decrease the value because the transition state (cf. Bottom of Figure 10) is stabilized by additional conjugation. Other approximate comparisons of the values in Tables 5 and 6 show the same trend. The ease of transformation of the dinaphtho compound 32 Table 6) relative to the dibenzopyran 27 suggests that for the benzonaphtho compounds in Table 8 opening of the pyran ring condensed to naphthalene, rather than the one condensed to benzene, will be preferred, as indicated by the arrow in the structural formula of Table 8. Indeed, the value for the dimethyl derivative 39 Table 8) is close to the value obtained for 32 Table 6). Removal of the methyl groups in 39 results in the spirobipyran 42 and reduces the barrier by approximately (Table 8). The same reduction was observed for 27 and 5, containing benzene rings (Table 5). The spirobipyrans in Table 8 consist of two unequal parts, of which only the pyran ring condensed to naphthalene will be opened. Therefore, the rate constant k for the transformation of the spiro ground state to an intermediate has been obtained from the experimental value by the relation i.e., the usual correction presented50,51 for an intermediate has been applied. Using k, we calculated the and values presented in Table 8. In this section, a few spirobibenzoxazines like 43 should be mentioned. Their enantiomers were separated but no thermal racemizations were observed,67 probably because their barriers were too high. The novel oxazine 44 was prepared

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via a new procedure36 and yielded at 50 °C in EtOH for the thermal ring-opening reaction.36 Unfortunately, we have not yet been able to separate the enantiomers of the known spirobibenzopyran 45 and determine its barrier for comparison.

6.3.6. Spiro[oxaindane-benzopyrans] Spiro[oxaindane-benzopyrans] are known18 to have interesting photo- and

thermochromic properties. In addition, activation parameters18,19 for thermal ring opening of several oxaindanospipnaphthopyrans have been determined by dynamic spectroscopy using the signal coalescence of diastereotopic methyl groups. The ∆G‡ values found cannot be compared with the results21,68 (108–112 kJ mol)–1 in Table 9 for three oxaindanospirobenzopyr-

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ans with no additional benzo rings. For the 3-methyl compound 49, at 89.7°C in diglyme was obtained.21,68

This value is considerably higher than that for 47 ( Table 9), which lacks the substituent in the 3-position. As discussed in Section 6.3.5, steric effects in the ground state and particularly in the transition state are responsible for this increase of the barrier. 6.3.7. Spiro[indoline-benzoxazines] Spiro[indoline-benzoxazines] are of great interest because of their photochromism and photostability3,5 as well as their thermochromism.7 Our racemic indolinospironaphthoxazines were prepared46 according to the published procedure3,69 starting from l-nitroso-2-naphthol. A few 5,7-dimethoxybenzoxazines70–72 were provided by Van Gemert and Crano. Some additional 7-methoxy- and nonmethoxybenzoxazines, which were not described but briefly mentioned in the literature,70,72–74 were synthesized38,46 according to these general procedures,

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starting from substituted 2-nitrosophenols. In addition, we developed a novel technique38 which uses the indoleninium iodides and the copper(II) complexes of 2-nitrosophenols. The complex is decomposed by dilute acid in an apparatus for liquid–liquid extraction, while the free 2-nitrosophenol is extracted and continuously led into the reaction flask. A methoxy group in the 7-position decreases the barrier to ring opening (cf. 50 and 51 in Table 10). A second methoxy group in the 5-position does the same (cf. 51 and 2), although to a much lesser extent. Apparently, the transition state is more stabilized by the methoxy groups than the spiro ground state. Schematic resonance formulas (Figure 11) cannot show these effects of substituents. However, calculations47,49 suggest a deviation from the ideal 90° dihedral angle along the C(2)–C(3) axis (see Section 6.3.1), which will generate some additional stabilization of the transition state; this is not possible in the ground state. The barrier of benzoxazine 2 is increased by an additional methyl group in the 8-

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position (cf. 2 and 52 in Table 10). It appears that the methyl group in 52 twists the

neighboring 7-methoxy substituent out of a position71 of effective conjugation, i.e., of effective electron donation. Much smaller changes in (Table 11) are produced by the chloro and the methyl substituents, which operate mainly by inductive, not conjugative, effects. The influence of EtOH as a solvent on the barrier of 54, compared to that of toluene, again is consistent with a stabilizing solvation of a polar transition state for bond cleavage. The benzoxazine 50 Table 11) can be compared with the corresponding naphthoxazine 57 (structure shown in Table 12) in the following way: racemizations20 of 57 in 96:4, between 11 and 55 °C yielded and The latter value was used for the correction of for 57 to a temperature of 64.8 °C. The adjusted value shows the expected effect of additional conjugation in the transition state of a naphtho (57) as compared to a benzo compound (50). If a chain between the indoline and the oxazine rings is introduced into a spiro[indoline-naphthoxazine], bridged compounds (e.g., 58 and 59) are obtained. For 58, the barrier to cleavage ( 33.5°C,MeOH) is considerably lower38 than that for nonbridged 57 (structure given in Table 12; 37.7 °C, MeOH). Similar behavior was found for bridged 5938 as compared to nonbridged 5146 (structure given in Table 10). Apparently,

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ring strain in 58 and 59 increases the energy of the ground state and thus reduces the barriers.

6.3.8. Spiro[indoline-benzopyrans]

The photochromism and thermochromism of spiro[indoline-benzopyrans] at room temperature are well described.1,2,4,7 Dynamic 1H-NMR spectroscopy has been employed for the determination of barriers to thermal ring opening, using the signal coalescence of diastereotopic methyl groups. The values15–17 found for several indolinospirobenzopyrans cannot be compared with the results56 in Table 13 because the substituents, temperatures, and solvents differ. The barriers of three indolinospironaphthopyrans have also been reported.15,19. The value of 56 ( 104°C, 20 biphenyl as solvent) is close to our finding 108°C, for 56 (Table 12). Our racemic indolinospiropyrans were prepared41,56 according to published procedures.1,2 A methoxy group in the 7-position decreases the barrier to ring opening relative to its value when the methoxy group is in the 6- or 8-position (cf. 61 and 60 and 61 and 62 in Table 13). The enantiomers of the corresponding nonmethoxy-substituted indolinospirobenzopyran could not be separated and hence were not racemized. However, the information in Table 13 is sufficient to draw conclusions similar to those in Section 6.3.7 concerning the results presented in Table 10. A methoxy group in the 7-position, but not one in the 6- or the 8position, may provide some additional stabilization of the transition state of 61 (cf. Figure 11). The results for the 5´-chloro compounds 63 and 64 in Table 13 are consistent with this statement. The racemization of the corresponding nonmethoxy-substituted analog 8 had to be conducted on-line with liquid chromatography in another solvent ( 96:4) which means that the results20 –1 ‡ (∆ G = 99.2 ± 0.2 kJ mol , 41.7°C) for the cleavage of the bond cannot be compared quantitatively with those in Table 13.

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When benzoxazines are compared with benzopyrans, i.e., 51 with 61 and 65 with 64 in Table 14, it turns out that the replacement of the methine group by an

aza nitrogen atom increases by as much as An increase of approximately is found20 for the corresponding naphtho compounds 57 and 56 when their barriers at 108 °C are compared (Table 12). One contribution to these differences in values may originate from the higher electronegativity of the oxazine nitrogen compared to the methine group in the pyrans. This effect reduces the electron density at C(2) of the oxazines and, therefore, makes ionic dissociation more enthalpy-consuming than in the case of the pyrans. The finding of a value of for 56 (Table 12) means that its half-life,

before thermal ring opening at room temperature is of the order of

only 10 s. Similarly low barriers were seen for several spirobipyrans in Tables 6 and 8. Therefore, the thermal ring-opening reactions of spiropyrans represent further examples of fast isomerizations.

6.4. CONCLUSION Despite extensive research activity concerning the photochromism of pyrans and oxazines, the corresponding enantiomers had never been described. In the present chapter, the physical methods available for preparative enrichment and for stereoanalytical characterization were described, together with a few examples of enantiomers representing the above classes of compounds. Microcrystalline

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triacetyl- and tribenzoylcellulose are particularly successful sorbents for the liquid chromatography of these lipophilic substrates.

Thermal racemization of enantiomers was presented as a novel approach to the measurement of the barriers to bond cleavage that avoids some of the disadvantages of earlier methods. It turned out that this type of enantiomerization is probably a general phenomenon of benzopyrans and benzo-1,4oxazines, at least if they possess a donor group in their 2-position, which accelerates bond cleavage. Tables 1–14 gave the kinetic results for some classes of spiro and non-spiro compounds. They extended from the 2-methyl-2-phenylbenzopyran 1 ( 135°C; Table 1), which exhibits no ring opening at room temperature, to the indolinospironaphthopyran 56 108°C; Table 12), the half-life of which at room temperature amounts to approximately 10s only. At least a few structural features of the transition state of the bond cleavage had to be assumed (Figure 8) for our rationalization of the kinetic results. In-this respect, several theoretical calculations of model compounds, performed in other laboratories, represented a basis of discussion. The extent of conjugation in the transition state, compared with that in the ground state, provided satisfactory qualitative explanations in many cases. The question is still open as to whether or not the ring-opened species identified in studies of thermochromism or photochromism are intermediates of the enantiomerization reactions in Figure 8. This question has now been answered for a single case: The known ring-opened species formed from 1',3',3'-trimethyl-6,8-dinitrospiro[2H-l-benzopyran-2,2´-indoline] cannot be identified with the intermediate of the enantiomerization reaction of this benzopyran75 (note added in proof, November 3, 1998). Since enantiomers of photochromic pyrans and oxazines are now available, further interesting experiments such as photoinduced transformations21 and racemizations41,57,67 as well as circularly-polarized-light-induced coloration and bleaching1 can be performed.

Acknowledgments We are grateful to Professor V. I. Minkin, Rostov/Don, who drew our attention to spiropyrans. Helpful discussions with Professor Minkin, Dr. R. Dubest (Paris), and Dr. V A. Lokshin and Dr. A. Samat (Marseille), are acknowledged. Valuable contributions to the present chapter originate from work done at Regensburg by Dr. G. Harié, S. Kraml, Dr. A. Leiminer, Dr. B. Stephan, and Dr. T. Trötsch. Chromatographic and spectroscopic help and advice were provided by Dr. T. Burgemeister, F. Kastner, N. Pustet, and D. Schuster. Typing was accomplished by C. Hübner. Some of the racemates were kindly provided by Dr. J. C. Crano (Monroeville, Pa.), Professor R. Guglielmetti

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(Marseille), Professor D. De Keukeleire (Gent), Professor V. I. Minkin, and Professor M. Mintas (Zagreb), as indicated in the corresponding references. Our project was supported by the French–German PROCOPE and the European COST program. A.M. is grateful to Professor A. Rassat for his hospitality during a sabbatical stay at the Ecole Normale Supérieure in Paris, during which work on the present chapter started. The revised manuscript was received January 22, 1997. REFERENCES 1. R. C. Bertelson, in: Photochromism (G. H. Brown, ed.), pp. 45–431, Wiley-Interscience, New York (1971). 2. R. Guglielmetti, 4n + 2 Systems: Spiropyrans, in: Photochromism, Molecules and Systems (H. Dürr and H. Bouas-Laurent, eds.), pp. 314–466, Elsevier, Amsterdam (1990). : 3. N. Y. C. Chu, in: Photochromism, Molecules and Systems (H. Diirr and H. Bouas-Laurent, eds.), pp. 493–509, Elsevier, Amsterdam (1990). 4. R. C. Bertelson, Volume 1 of this monograph, Chapter 1. 5. S. Maeda, Volume 1 of this monograph, Chapter 2. 6. V. Van Gemert, Volume 1 of this monograph, Chapter 3. 7. A. Samat and V. A. Lokshin, this volume, Chapter 10. 8. O. Chaudé, Etude spectrophotometrique de l’isomérisation de divers spiranes thermochromes, Cah. Phys. 50, 17–53 (1954). 9. Y. Sueishi and T. Nishimura, Effects of β- and γ-cyclodextrins on the thermal isomerization of 6SO3-spiropyran, J. Phys. Org. Chem. 8, 335–340 (1995) and references therein. 10. J. B. Flannery, Jr., The photo- and thermochromic transients from substituted 1´,3´,3´-trimethylindolinobenzospiropyrans, J. Am. Chem. Soc. 90, 5660–5671 (1968). 11. N. Y. C. Chu, Photochromism of spiroindolinonaphthoxazine, Can. J. Chem. 61, 300–305 (1983). 12. Y. Sueishi, M. Ohcho, and N. Nishimura, Kinetic studies of solvent and pressure effects on thermochromic behavior of 6-nitrospiropyran, Bull. Chem. Soc. Jpn. 58, 2608–2613 (1985). 13. G. Favaro, F. Masetti, U. Mazzucato, G. Ottavi, P. Allegrini, and V. Malatesta, Photochromism, thermochromism and solvatochromism of some spiro[indolinoxazine]-photomerocyanine systems, J. Chem. Soc., Faraday Trans. 90, 333–338 (1994). 14. J.-J. Meyer, P. Levoir, and R. Dubest, Upgrading a rapid-scanning spectrometer with microcomputerized data acquisition and treatment to measure spectrokinetic parameters of photochromic compounds, Analyst 120, 447–452 (1995) and references therein. 15. W. D. Ollis, K. L. Ormand, and I. O. Sutherland, Nuclear magnetic resonance studies of the electrocyclic reactions of thermochromic spiropyrans, J. Chem. Soc., Chem. Commun. 1968, 1697–

1699. 16. V S. Marevtsev, L. L. Zaichenko, V D. Ermakova, S. I. Beshenko, V. A. Linskii, A. T. Gradyushko, and M. I. Cherkashin, Effect of electron-donating and electron-accepting substituents on photo- and thermochromic properties of indolino-spiropyrans, Bull. Acad. Sci. USSR, Chem. Sci. 29, 1591– 1596 (1980); Chem. Abstr. 94, 29901 (1980). 17. N. L. Zaichenko, V. S. Marevtsev, V. D. Arsenov, and M. I. Cherkashin, Thermally induced inversion of configuration of the closed-form of indolinospiropyran molecules, Sov. J. Chem. Phys. 6, 22222233(1990). 18. S. M. Aldoshin, V. A. Lokshin, A. N. Rezonov, N. V. Volbushko, N. Shelepin, M. I. Knyazhanskii, L. O. Atovmyan, and V. I. Minkin, Structures and photo- and thermochromic properties of spiropyrans of the 2-oxaindan series with polycondensed chromene fragments, Chem. Heterocycl. Compd. 1987, 614–624; Chem. Abstr. 108, 131516 (1987).

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19. S. P. Makarov, B. Ya. Simkin, and V. I. Minkin, Theoretical prediction of barriers of thermal reactions involving valence isomerization of spiropyrans, Chem. Heterocycl. Compd. 1988, 140– 145; Chem. Abstr. 110, 7353 (1988). 20. A. Leiminer, B. Stephan, and A. Mannschreck, The enantiomers of indolino spiro compounds. Barriers to thermal cleavage of their C(sp 3 )–O bond, Mol. Cryst. Liq. Cryst. 246, 215–221 (1994). 21. B. Stephan, A. Mannschreck, N. A. Voloshin, N. V. Volbushko, and V. I. Minkin, Separation and photoinduced transformations of the enantiomers of 3´,3´-dimethylspiro[2H-l-benzopyran2,l´(2)oxaindans], Tetrahedron Lett. 31, 6335–6338 (1990). 22. B. Stephan, H. Zinner, F. Kastner, and A. Mannschreck, Enantiomers of 2,2´-spirobichromenes: Energy barrier for thermal racemization during HPLC on tribenzoylcellulose, Chimia 44, 336–338 (1990). 23. A. Leiminer, L. Loncar, K. Otocan, B. Stephan, T. Trötsch, F. Kastner, M. Mintas, and A. Mannschreck, The enantiomers of chromenes and their barriers to reversible ring opening, Chirality 5, 402 (1993). 24. L. Loncar, K. Otocan, M. Mintas, T. Trotsch, and A. Mannschreck, Chiral chromenes: Synthesis, separation of enantiomers and barriers to racemization, Croat. Chem. Acta 66, 209–216 (1993). 25. E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds, pp. 162–464, John Wiley & Sons, New York (1994). 26. S. Allenmark, Chromatographic Enantioseparation, Methods and Applications, 2nd ed., Ellis Horwood, New York (1991). 27. A. Mannschreck, H. Koller, and R. Wernicke, Microcrystalline cellulose triacetate, a versatile stationary phase for the separation of enantiomers, Kontakte (Darmstadt) 1985 (1), 40–48; Chem. Abstr. 103, 110494 (1985). 28. A. Mannschreck and R. Wernicke, Mikrokristalline Tribenzoylcellulose, ein vielseitiges Sorbens für die Enantiomerentrennung, Labor-Praxis 14, 730–738 (1990); Chem. Abstr. 113, 218852 (1990). 29. E. Francotte and A. Junker-Buchheit, Preparative Chromatographic separation of enantiomers, J. Chromatogr. 576, 1–45 (1992). 30. A. Mannschreck, Chiroptical detection during liquid chromatography; applications to stereoanalysis and stereodynamics, Chirality 4, 163–169 (1992). 31. W. Zelenka, A. Leiminer, and A. Mannschreck, A new type of polarimetric HPLC detector, GIT Fachz. Lab. 37, 97–103 (1993); Chem. Abstr. 118, 182432 (1993). 32. G. Harié, A. Samat, R. Guglielmetti, I. Van Parys, W. Saeyens, D. De Keukeleire, K. Lorenz, and A. Mannschreck, Chiral 2-aryl-2-methyl-[2H]-chromenes: Synthesis, characterization of enantiomers and barriers to thermal racemization, Helv. Chim. Acta 80, 1122–1132 (1997). 33. J. Dingenen and J. N. Kinkel, Preparative Chromatographic resolution of racemates on chiral stationary phases on laboratory and production scales by closed-loop-recycling chromatography, J. Chromatogr. A 666, 627–650 (1994). 34. B. Van Gemert, J. C. Crano, A. Leiminer, and A. Mannschreck, unpublished results. 35. Y. Okamoto and K. Hatada, in: Chromatographic Chiral Separation (M. Zief and L. J. Crane, eds.), pp. 199–218, Marcel Dekker, New York (1988). 36. K. Lorenz and A. Mannschreck, unpublished results. 37. D. Parker, NMR determination of enantiomeric purity, Chem. Rev. 91, 1441–1457 (1991). 38. M. Schinabeck and A. Mannschreck, unpublished results. 39. E. L. Eliel, and S. H. Wilen, Stereochemistry of Organic Compounds, pp. 991–1118, John Wiley & Sons, New York (1994). 40. K. Nakanishi, N. Berova, and R. W. Woody (eds.), Circular Dichroism; Principles and Applications, VCH Publishers, New York (1994). 41. B. Stephan and A. Mannschreck, unpublished results. 42. E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds, pp. 1120–1121, John Wiley & Sons, New York (1994).

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43. G. Brandl, F. Kastner, R. Fritsch, H. Zinner, and A. Mannschreck, On-line measurement of circular dichroism spectra during stops of chromatographic flow, Monatsch. Chem. 123, 1059–1069 (1992). 44. A. Mannschreck, On-line measurement of circular dichroism spectra during enantioselective liquid chromatography, Trends Anal. Chem. 12, 220–225 (1993); Chem. Abstr. 119, 194725 (1993). 45. E. L. Eliel, Stereochemistry of Carbon Compounds, p. 33, McGraw-Hill, New York (1962). 46. A. Leiminer and A. Mannschreck, unpublished results. 47. B. Y. Simkin, S. P. Makarov, and V I. Minkin, Calculation of the pathways of the thermal electrocyclic reactions of chromenes and their structural analogs by the MINDO/3 method, Chem. Heterocycl. Compd. 1983, 779–787; Chem. Abstr. 97, 162044 (1982). 48. P. N. Day, Z. Wang, and R. Pachter, Ab initio study of the ring-opening reactions of pyran, nitrochromene, and spiropyran, J. Phys. Chem. 99, 9730–9738 (1995) and references therein. 49. G. Garros, I. Chuev, M. Rajzmann, A. Samat, and R. Guglielmetti, unpublished results. 50. F. A. Bovey, L. Jelinski, and P. A. Miran, Nuclear Magnetic Resonance Spectroscopy, 2nd ed., p. 304, Academic Press, San Diego (1988). 51. A. Leiminer, University of Regensberg, unpublished results; M. Schmabeck, Diploma thesis, University of Regensberg (1995), p. 51. 52. W. H. Laarhoven, in: Photochromism, Molecules and Systems (H. Dürr and H. Bouas-Laurcnt, eds.), pp. 270–313, Elsevier, Amsterdam (1990). 53. J. L. Pozzo, V A. Lokshin, and R. Guglielmetti, New photochromic 2,2-diphenyl-[2H]chromenes annellated with nitrogenated six-membered ring, Mol. Cryst. Liq. Cryst. 246, 75–78 (1994). 54. V. Van Gemert, M. Bergomi, and D. Knowles, Photochromism of diarylnaphtho-pyrans, Mol. Cryst. Liq. Cryst. 246, 67-73 (1994). 55. W. Saeyens, D. De Keukeleire, N. Pustet, and A. Mannschreck, unpublished results. 56. S. Kraml and A. Mannschreck, unpublished results. 57. T. Trötsch and A. Mannschreck, unpublished results. 58. Yu, M. Chunaev, N. M. Przhiyalgovskaya, and L. N. Kurkovskaya, Intermolecular cyclization of 2hydroxycinnamaldehydes, Chem. Heterocyl. Compd. 1988, 376–381; Chem. Abstr. 109, 128781 (1988). 59. L. Loncar-Tomaskovic, M. Mintas, T. Trötsch, and A. Mannschreck, 2H-Chromenes: Synthesis, separation of enantiomers, circular dichroism and thermal racemization, Enantiomer 2, 459–472 (1997). 60. F. Chioccara, F. Della Gala, E. Novellino, and G. Prota, Synthesis of 3-phenyl-2H-l,4-benzoxazin2-one. Revision of some structural assignments, J. Heterocycl. Chem. 14, 773-775 (1977). 61. I. Shcherbakova, E. V. Kuznetsov, I. A. Yudilevich, O. E. Konpan, A. T. Balaban, A. H. Abolin, A. V. Polyakov, and Y. T. Struchkov, Oxygen analogs of Reissert compounds; molecular structure and reactions with sodium hydroxide, Tetrahedron 44, 6217–6224 (1988). 62. I. Shherbakova, T. Trötsch, and A. Mannschreck, unpublished results. 63. W. Dilthey, C. Berres, E. Hölterhoff, and H. Wübken, Zur Kenntnis der Spirodibenzopyrane, J. Prakt. Chem 114, 179-198 (1926). 64. R. Dickinson and J. M. Heilbron, Colour phenomena associated with benzonaphtha- and dinaphthaspiropyrans, J. Chem. Soc. 1927, 1699-1705. 65. W. Dilthey and H. Wübken, Zur Kenntnis der Spiropyrane, Ber. Dtsch. Chem. Ges. 61, 963-968 (1928). 66. P. Appriou, C. Trebaul, J. Brelivet, F. Garnier, and R. Guglielmetti, Synthèse de composés du type benzopyrane et spirobibenzopyrane. Etude de l’équilibre photochromique, Bull. Soc. Chim. Fr. 1976, 2039-2046. 67. Y. Okamoto, S. Honda, H. Yuki, H. Nakamura, Y. Iitaka, and T. Nozoe, Resolution, absolute configuration, and photoracemization of chiral troponoid acetals, Chem. Lett. 1984, 1149–1152. 68. N. A. Voloshin, V. I. Minkin, B. Stephan, and A. Mannschreck, unpublished results.

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69. E. Pottier, M. Sergent, R. Phan Tan Luu, and R. Guglielmetti, Synthèse de quelques spiro[indolinenaphthoxazines] et spiro[indolinepyridobenzoxazines]-photochromiques, Bull. Soc. Chim. Belg. 101, 719–739 (1992). 70. W. S. Kwak and C.-W. Chen (PPG Industries, Inc., Pittsburgh, Pa), U.S. Patent 4,816,584 (1989). 71. J. C. Crano, D. Knowles, P. Kwiatkowski, T. Flood, R. Ross, L. Chiang, J. Lasch, R. Chadka, and G. Siuzdak, Structure of three novel photochromic compounds; X-ray crystallographic and theoretical studies, Acta Crystallogr. Sect. B 50, 772–779 (1994).

72. J. C. Crano, W. S. Kwak, and C. N. Welch, in: Applied Photochromic Polymer Systems (C. B. McArdle, ed.), pp. 31–79, Blackie, Glasgow (1992). 73. M. Reichenbaecher and P. Gerney (Friedrich-Schiller-Universität, Jena, Germany), DD 238 611 (1986); Chem. Abstr. 106, P 157970 (1987). 74. S. Yamamoto and T. Taniguchi (Toray Industries, Inc.), JP 01 52 783 (1987); Chem. Abstr. III, P 222188 (1990).

75. R. Kiesswetter, T. Burgemeister, and A. Mannschreck, 1', 3', 3'-Trimethyl-6,8-dinitrospiro[2H-1benzopyran-2,2'-indoline], Enantiomer, submitted for publication.

7 Structural Studies by X-Ray Diffraction Serguei Aldoshin 7.1. INTRODUCTION During the past several years, the Institute of Chemical Physics in Chernogolvoka of the Russian Academy of Science has been involved in intense research in the field of crystal chemical “engineering” of photochromic systems, studying the possibility and the ways of controlling the photochemical transformations in such systems via their crystalline and molecular structure. These investigations include a thorough study of the structures of the initial compound and the resulting photoproducts, and, if possible, of intermediates, using different physicochemical and computational methods [X-ray analysis, infrared (IR) and ultraviolet (UV) spectroscopy, and quantum-chemical methods] and elucidation of the nature of structural conversion and of the correlations between structure and photochemical properties. When photochemically active compounds are studied, the above problem is particularly complicated because the structure is being solved for the ground electronic state, while chemical transformations occur in the excited state, which may be characterized by an essentially different structure of the molecule. The nature of structural changes in the electronically excited state can be studied by quantum-chemical methods which are based on the molecular structure in the ground state. Though the problem is difficult, the recent results of crystal chemical “engineering” of photochromic systems testify to the vast potentialities of this approach. In this chapter, we will consider the structures of photochromic compounds of various classes, and we will discuss the structural factors that have been determined to affect the molecules' reactivity and ways of modifying the structure

Serguei Aldoshin • Institute of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia. Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999.

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of photochromic molecules that have been proposed with the goal of improving their photochemical properties.

7.2. SPIROPYRANS Photochromic materials based on different classes of spiropyrans (SPs) are widely used in various fields of science and technology, such as in the production of light filters regulating luminous fluxes, as photochromic organic media for processing optical information, for photochromic optics, and in the production of nonlinear optical materials. In recent years, the study of new SPs has been conducted mainly in two directions, namely, the search for new classes of SPs and structural modification of the known systems to improve their basic characteristics (quantum yield of photoconversion, the stability of the photoisomer produced, the number of cycles of operations). Only a profound comprehension of mechanisms of photoinitiated rupture of the bond, structural isomerization, ways of stabilizing the photoisomer, routes of its breakdown, and influence of the structure of SPs on their properties can provide the basis for purposeful research in this area. Despite the vast number of investigations in this area, the mechanisms of the photochromic conversion of SPs and the influence of structural features on their

photochemical properties are not well understood. This complicates the search for and synthesis of new SP classes. 7.2.1.

Indolinospiropyrans

7.2.1.1. Structural Features of SP Molecules The main structural parameters of the SPs investigated are given in Table 1, and the general SP structure is shown in Figure 1. Figure 2 shows the structure of the closed form of SP1. The indoline and benzopyran moieties of the molecules

are approximately at right angles to each other but are nonplanar. The indoline moiety has the conformation of a flattened envelope with bending along the

line (dihedral angle a is in the range of 23–33°; Table 1). This type of structure of the indoline heterocycle is due to an essentially pyramidal configuration of the N(l') nitrogen atom, which reduces the conjugation of the lone electron pair (LEP) of the N( 1') atom and the system of the benzene ring. In fact, the N(l')–C(7a') bond length in spiropyrans SP1–SP12, ranging from 1.380(5) to 1.416(5) Å, is greater than that in the corresponding systems having a planar trigonal structure of the N atom (1.36–1.37 Å) but coincides with the length of the N–Ph bond (1.39–1.42 Å) in molecules with a pyramidal structure of the N atom.12–17 The deviation of the N(l') atom from the plane passing through the C(7a') and C(22') atoms and the carbon atom of the R' substituent linked to the N(1´) atom is 0.12–0.34 Å. This is an indication of the substantial character of

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the N-atom LEP situated approximately perpendicular to the plane of these three atoms. The LEPs of the O atom are not equivalent18; they are of and character and have different configurations: the density of one LEP is situated in the C(22')OC(8a) plane while the density of the other is perpendicular to this plane. With such a structure of the SP, the bond of the pyran heterocycle has a trans position with respect to the LEP of the N(l') atom, and the bond of the indoline heterocycle is trans with respect to the of the O atom. This type of structure can induce specific orbital interactions between the electronegative hereoatoms and the LEPs of the N and O atoms bonded to a

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tetrahedral carbon atom (the anomeric effect).19,20 This interaction can be described in terms of interaction between the LEP (n) of the heteroatom and the level of the adjacent polar bond located mainly on the positive carbon atom of the spiro center (Figure 3). Since an oxygen atom is more electronegative than a nitrogen atom, and the energy of the O-atom LEP is lower than that of the N-atom LEP, the antibonding orbital of the bond is energetically lower than the antibonding orbital of the bond, while the nonbonding orbital of the N(l') atom is energetically higher than the nonbonding orbital of the O atom. Consequently, interactions between the nelectrons of the N(l') atom and the orbital of the bond are quite important for determining the ease of opening of the spiro center.21 Recently, the concept of hyperconjugation has been widely used22 to account for structural effects in different molecules. Due to the population of the antibondong orbital of the this type of interaction results in a strengthening of the bond and a weakening of the bond, which is manifested as shortening and elongation, respectively, of these bonds. Actually, the bond lengths in the SPs studied are in the range of 1.432(4)–1.453(4) Å and are thus considerably shorter than normal lengths of bonds (1.47–1.48 Å)22,23 in five-membered heterocycles, while the are in the range of 1.452(4)–1.50(1) Å, which is considerably longer than normal C—O bonds (1.41.–1.43 Å)23–25 in six-membered heterocycles.

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The efficiency of this hyperconjugation must depend essentially on the geometric configuration of the LEPs of the N(1') and O atoms and the and bonds, on the one hand, and on the electronic state of the N and O atoms, on the other hand. In all of the SPs studied, the spiro center has a sterically strained structure. Because of the rigidity of the tetrahedral structure of the spiro center and the similar conformation of the indoline ring, the bond makes an angle of 165–171° with the LEP of the N(1') atom (Table 1).

The conformation of the benzopyran moiety is more flexible in these molecules.26 The pyran ring in SPs is nonplanar and is folded along the line by an angle of 0–17°. The bending along the line (angle is less (0–10°). The conformation of the six-membered heterocycle in SPs is evidently determined mainly by steric factors in the molecule and by molecular packing effects in the crystal rather than by the electronic effects of substituents. This is confirmed by the structure of SP4, which is characterized by two independent molecules—SP4a, with a virtually planar benzopyran ring, and SP4b, with bending along the and lines by 15.5° and 5.3°, respectively. In spiropyran SP1, the benzopyran fragment is bent toward the R' substituent while in spiropyrans SP2, SP3, SP4b, SP6, and SP7 it is bent toward the geminal methyl groups on the C(3') atom. In SP4a, SP5, SP9–SP11, the benzopyran moiety is approximately planar. These structural features lead to different orientations of the density of the O-atom LEP and the bond (angle Table 1). In SP2, SP3, SP4b, SP6, and SP7, the corresponding angle is only 116–130°; in SP4a and SP9–SP11, it is 141–145°; and in SPS and in SP1, it is 160.2° Thus, the interaction, which has an opposite effect to the interaction, will increase with an increase of the value, which means that the benzopyran moiety is bent more toward The electronic state of the N and O atoms depends on the nature and position of the substituents in the indoline and benzopyran moieties. Introduction of an electron-acceptor (EA) substituent into the benzopyran moiety in the positions para and ortho to the O atom should lead to increased conjugation of the electron density of the O-atom LEP with the benzene ring due to further transfer of electron density from the O atom into the system of the phenyl group and to the EA substituent. Actually, in spiropyrans SP1–SP4 and SP8–SP12, which have EA substituents in these positions, the O–C(Ph) bond lengths are shortened [1.32(1)–1.357(5) Å] and are similar to the lengths of the corresponding bonds in nitrophenols.27,28 A decrease of electron density on the O atom induces a further polarization of the bond, which leads to a decrease in the energy of the of the bond and its enhanced localization on the atom. This results in a strenghening of the interaction and further weakening and elongation of the bond compared to that in unsubstituted SP, in which the bond must be stronger and shorter. Thus, in photochromic

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indoline SPs the bond, which is cleaved on photoexcitation, is the weakest and most elongated bond even in the ground state as a result of specific orbital interactions. 7.2.1.2.

The Nature of Structural Changes in SP Molecules upon Photoexcitation

The specific structural features of SPs affect the photochromic properties of these compounds. Since photophysical and photochemical properties of organic molecules depend essentially on their structure in the excited state, we shall examine the nature of the structural changes in these molecules upon photoexcitation. The nature of such changes, in turn, depends on the type of electronic transition that converts the molecule into a photochemically active state (PCAS). The converstion of SP1–SP4 molecules to a PCAS upon UV irradiation with is known to be due to electronic transitions of the type in molecules without EA substituents and of the type in molecules with EA substituents in the benzopyran moiety.29 These transitions are characterized by intramolecular charge transfer (ICT), mainly from the l orbital of the oxygen atom and the

orbital of the

bond to the phenyl group and the EA

substitutents. This ICT must lead to a decrease of electron density on the O(1) atom. For 2H-chromenes, charge transfer from the O atom during the transition was found to be 0.060 e [CNDO(S) method].30 A decrease of the charge on the O atom is accomplished at the expense of electrons of the bond, which provides the driving force for further polarization. As was shown above, this type of redistribution of electron density leads to an increase of the interaction in SP molecules and a further contraction of the bond and weakening and elongation of the bond in the excited state. In spiropyran SP5, the lowest electronically excited PCAS is the state with ICT from the N(1') atom to the phenyl ring and group.31 Therefore, the efficiency of the interaction in the ground and excited PCAS of SP5 must be lower than in SP4. Actually, elongation of the bond in SP5 is less than in SP4 (Table 1), and the photochemical activity is lower than that of SP4. It has been shown32 that a significant change in the length of any bond in the excited state of he molecule induces the nonequilibrium transfer of the electronic excitation energy of the molecule to the vibrational energy of this bond. Therefore, in SPs the vibrations of the weakened and elongated bond can be excited, inducing its heterolytic cleavage. Thus, cleavage of the bond in the excited state can be due to the further weakening and elongation of this bond upon photoexcitation induced by the influence of the same factors that were responsible for its weakening and

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elongation in the ground state. Elongation of the bond can be one of the criteria for predicting the photochemical activity of these compounds.

7.2.1.3.

Effect of the Nature and Position of Substituents in SP Molecules on the Length of the Bond The results of X-ray diffraction analysis show that transferring the group from the para to the ortho position, as well as introduction of electronegative halogen atoms in these positions, does not influence the length of the

bond (Table 1). Introduction of EA substituents in the indoline moiety of SPs must induce an increase of the delocalization of the N(1') LEP along the system of the indoline phenyl ring and a reduced efficiency of interaction of this LEP with the orbital of the bond. The results of an X-ray diffraction analysis of

[indoline-2,2'-[2H-l]benzopyran] (SP5) with the

group at the

of

the indoline moiety support this assumption5 (Table 1). Introduction of the group at the position para to the N(1') atom of the indoline moiety leads to shortening of the N(1')—C(Ph) bond [1.380(2) Å] compared to this bond length [1.393(4)–1.413(5)Å] in SP1–SP4, SP8, and SP12. The lengths of the N(1')–C(Ph) and (Ph)C–N(NO2) bonds [1.380(2) and 1.461(2) Å, respectively] in SP5 are similar to the corresponding bond lengths in p-nitroaniline [1.371(4)– 1.380(5) Å and 1.450(5)–1.460(5)Å]33,34 which testifies to an increase of conjugation of the N(1')-atom LEP with the system of the phenyl ring in SP5. This results in weakening of the interaction, and the bond shortens only to 1.453(2) Å and the C–O bond is only

extended to 1.452(2) Å. Results of X-ray diffraction analysis of SP2335 confirm the effect of the electronic state of the N atom on the length of the bond. In SP23, due to the amide conjugation of the N-atom LEP with the system of the carbonyl group, the activity of the LEP and the efficiency of the interaction

must be lower than in SP5. Therefore, the while the

bond (1.47 Å) is not shortened

bond (1.44 Å) is much less elongated than in SP5.

Crystals of unsubstituted SP suitable for X-ray analysis are not available. The structure of the SP6 molecule appears to be the closest to that of unsubstituted SP.6 The group has very weak EA properties regardless of its position in the

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benzene ring while it has strong electron-donating (ED) properties when it is in the ortho and para positions of a benzene moiety. Thus, the substituent at the 7-position of the benzopyran moiety should have a weak effect on its electronic configuration, and the molecular structure of SP6 should be similar to the structure of unsubstituted SP. The main structural parameters of SP6 are given in Table 1. Its structure is similar to the structures of SP2, SP3, and SP4b. The methoxy group is turned with respect to the phenyl fragment plane about the C(7)–O(2) bond by 15.6°. The length of the bond [1.471(3) Å] is intermediate between the lengths of this bond in SP1–SP4 [1.485(4)–1.496(4)Å and in SP5 [1.452(2) Å]. Thus, even given that the group has a weak ED effect on the benzopyran moiety of the SP, the bond in unsubstituted SP should not be weakened and elongated less than in SP6. SP7 contains an additional benzene ring annelated at the 5- and 6-positions. The main structural parameters of SP7 are given in Table 1. The conformation of SP7 is similar to that of SP2, but annellation of the additional benzene ring and transfer of the substituent to the additional ring lead to shortening of the bond to [ 1.460(2) Å] compared to the length of this bond [ 1.492(3) Å] in

SP2. Although the bond is longer than a normal C–O its shorter length in SP7 compared to SP2 indicates that the interaction in this molecule is weakened. As the LEPs of the and O atoms and the and bonds in SP2 and SP7 have similar geometric configurations (Table 1), a decrease of the interaction in SP7 may be due to a change of the electronic state of the O(1) atom in the spiro center. Indeed, the O(1)–C(8a) bond in SP7 is longer [1.372(3) Å] than that in SP2 [1.356(3) Å]. This indicates a decrease in the conjugation of the system of the naphthalene moiety with the of the O(1) atom, which results in a reduction of the polarity of the bond and a weakening of the interaction. This is also confirmed by contraction of the bond to only 1.450(3) Å, whereas in SP2 the length of this bond decreases to 1.432(3) Å. Thus, an additional benzene nucleus at the 5- and 6-positions of the benzopyran moiety and the group on the added ring cause some contraction and strengthening of the bond in SP7 compared to SP1–SP6. This fact is consistent with the decreased photochemical activity of naphthospiropyrans (NSPs).

Particular attention should be given to the fact that the atom in the SPs with has the same pyramidal configuration and lies out of the plane of the adjacent atoms by 0.24–0.28 Å. Replacement of at the atom by the more bulky benzyl group, i.e., going from SP4 to SP8, leads to an increase of

the pyramid height to 0.34 Å but in other respects has no effect on the structure of the SP8 molecule7 (Table 1). Replacement of a group by a group makes possible the conjugation of the LEP with the system of the benzene

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ring. The efficiency of this conjugation is determined by the degree of coplanarity of the phenyl ring with the coordination plane of the atom. In SP2 and SP7, with values of the dihedral angle between these planes equal to 43.9 and 34.8°, respectively, the atom is displaced from the plane of the three adjacent carbon atoms by only 0.13 and 0.12 Å, respectively. This confirms that the LEP of the atom conjugates with the system of the phenyl substituent. The dihedral angle between the plane of the phenyl ring and the coordination plane of the atom in SP3 is 80°. Such a structure is not favorable for conjugation of the LEP with the system of the phenyl substituent, and atom lies out of the coordination plane by 0.21 Å conjugation, also must influence the orbital interactions in the center. However, the length of the bond in SP2 and SP3 is similar to that in SP1. (The reason for this phenomenon is discussed in Section 7.5.) The synthesis of new photochromic SPs based on 4-azaindole has been reported. The SPs obtained readily dissolve in polar and nonpolar solvents, and some of them even dissolve in water. The small bathochromic shift of the longwavelength absorption band of these compounds is due to the stronger EA properties of the pyridine ring as compared to those of a benzene ring. The photosensitivity of azaindoline SPs toward UV radiation proved to be no lower, and in some cases even higher, than the photosensitivity of their indoline analogs. In Ref. 8, the X-ray analysis of SP9, based on 4-azaindole, has been carried out. The main structural parameters of SP9 are given in Table 1. Since the pyridine ring has stronger EA properties than the benzene ring, the bond [1.386(6) Å] would be expected to be shortened; although the length of this bond [1.386(6) Å] is the same, within the limits of experimental error, as SP1 [1.398(5) Å] and in SP5 [1.380(2) Å], it is shorter than in SP1–SP4 and SP6– SP8 [1.394(3)–1.416(4) Å]. These results indicate that conjugation of the LEP of the atom with the system of the pyridine ring is slightly increased. However, the bond [1.486(6) Å] in SP9, with a bond length similar to that in SP1 [1.496(4) Å], is still elongated, which testifies to strong interactions in the SP9 molecule. Moreover, in SP9, in contrast to SP1, the benzopyran ring is virtually planar and the of the O(1) atom makes an angle of only 139.6° with the bond as compared to 160.2° in SP1. Therefore, the interaction in SP9 is weaker than in SP1. This is confirmed by the greater contraction of the bond in SP9, with a bond length of 1.434(7)Å compared to 1.453(4)Å in SP1.

7.2.2. The Structure of Permanent Merocyanines—Models of the Open Forms of Indolinospiropyrans Since the lifetimes of the open merocyanine (MC) forms are one of the important characteristics of photochromic systems based on SPs, it is necessary to

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carry out a detailed study of the structures of MCs and to identify the structural factors that stabilize SPs in the open state. The structures of different MCs, the energy of their intermolecular interactions in crystals, and the thermodynamic and kinetic stability of the isomers identified will then be examined. 7.2.2.1.

Molecular Structure of Merocyanines

Crystals of all MCs investigated (MC1–MC6) have been isolated from ethanol by recrystallization under UV radiation by Gal’bershtam,1 Kholmanskii, and the author.

MC1 molecules (Figure 4) in the MC1 crystals isolated from ethanol are solvated with water molecules.1 Transition to the open form is accompanied by the entire redistribution of the bond lengths, which is typical for a merocyanine chromophore.36 The distribution of the bond lengths is indicative of the bipolar structure of the MC1 molecule (Figure 5, form A) with a significant contribution from the resonance quinoid form B. The existence of a hydrogen bond between the O(1) and H(3) atoms has not been proved unambiguously. Although the distance [2.28(5) Å] is short (Figure 4), and the H(3) atom must be rather acidic, as follows from quantum-chemical calculations of charges on the atoms of MC1, a small bending about the C(4)—C(4a) bond and an increase of the bond angles C(3)C(4)C(4a) and C(4a)C(8a)O(1) to 127.4(5)° and 125.0(6)°, respectively, suggest repulsion between the O(1) and H(3) atoms. A similar repulsion apparently exists between the H(4) and atoms, which results in an increase of the bong angles and C(22')C(3)C(4) to 129.5° and 123.9(5)°, respectively, and in bending by 11.7° about the bond.

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Therefore, the open form of MC1 has a zwitterionic trans–cis structure (A) in Figure 5), which is sterically the most favorable one, with some contribution from the resonance forms B and C. The crystalline structure of solvated MC1 provides a basis for the assumption that replacement of a methyl group by a more bulky group at the atom in the indoline ring can hinder the solvation of the phenolate oxygen atom. To confirm this assumption, an X-ray diffraction analysis of the pentyl-substituted MC2 (Figure 6) was carried out.37,38 The pentyl substituent at the atom is similar to the methyl group in terms of its electronic properties. The MC2 molecules in the crystals are not solvated. MC2 molecules (Figure 6) have an almost planar trans–cis structure, similar to that of MC1. The distribution of the bond lengths in the MC2 molecule, is, in general, similar to that in the MC1 molecule. However, the absence of solvating water molecules in MC2 crystals leads to some redistribution of electron density in the MC2 molecule. The structure of the p-nitrophenolate moiety is indicative of the greater contribution of the resonance quinoid form B (see Figure 5) to the zwitterioinic structure (A) of unsolvated MC2 molecules compared to solvated MC1 molecules. Similarly to MC1, the contribution of the canonical form C is appreciable in MC2 molecules. The bond in MC2 [1.315(5) Å] is even shorter than in MC1, which suggests its complete double-bond nature and

localization of its positive charge mainly on the

atom.

Thus, the study of the MC crystals shows that the structure of open forms of SPs can be described by resonance forms A (which makes the greatest contribu-

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tion), B, and C, with the contribution of each form varying in accordance with the nature of the substituents in the molecule. Stabilization of open MC molecules must be related to stabilization of one of these resonance structures. The structure of MC1 crystals suggests that solvation of the O(1) phenolate atom stabilizes the zwitterionic structure A. Stabilization of the canonical structure C can be achieved, for example, by annellating additional benzene rings to the phenolate ring. Annellation of additional benzene rings at the 5- and 6- or 7- and 8-positions further stabilizes form (C) whereas annellation at the 6- and 7-position leads to its destabilization.

In MC1 and MC2, substituents in the phenolate ring displace the equilibrium toward the quinoid form B and hence stabilize the unstable zwitterionic form. It was suggested31 that introduction of an additional group into the position ortho to the O(1) atom of this moiety should increase the stability of the open MC form. X-ray diffraction analysis of such a merocyanine, MC3, showed that this compound crystallizes in two polymorphic forms—a monoclinic form and rhombic form. In the monoclinic form, MC3 molecules have a nearly planar trans structure (Figure 7) similar to that of MC1 and MC2. However, the structure differs from

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that of MC1 and MC2 in that the phenolate oxygen atom in MC3 is disordered, occupying the positions s-cis- and s-trans- to the C(4)–C(4a) bond with almost equal probability. Thus, the trans–cisoid and the trans–transoid isomers, which were first found experimentally, exist in the crystals in equal ratios. The trans–transoid isomer, which constitutes of the crystal, is solvated by a water molecule which also occupies of independent positions. The hydrogen bond formed in the

solvate is a short one. Crystals of the rhombic form of MC4 also contain equal amounts of two isomers—the trans–cisoid and the trans–transoid isomer. The pattern of distribu-

tion of the bond lengths in MC4 is in line with that of MC3 within the limit of experimental error. Crystals of MC4, similarly to those of MC3, contain solvating water molecules which occupy 50% of independent positions. However, in contrast to MC3, it is the sterically less accessible O(1) atom of the trans–cisoid isomer that is solvated distance is 2.97(3) Å] rather than the O(1) atom of the trans–transoid isomer. This makes the phenyl substituent at the atom turn from the plane of the molecule by 87.9°. In crystals of MC4, there are also other

water molecules that do not solvate the MC molecules, and they occupy about 25% of independent positions and are situated at an distance of 2.30 Å from the solvating water molecules. Thus, solvation of the phenolate O atom is evidently typical for MC molecules in polar solvents if steric factors are favorable.

Introduction of an additional

group into the phenolate fragment

stabilizes another isomer—the trans–transoid one—which evidently exists in

solution in equilibrium with the trans–cisoid isomer, resulting in their cocrystallization. This conclusion made it necessary to perform a more detailed study of the structure of the trans–transoid isomer. An X-ray diffraction analysis of the o, p-

dinitro-substituted MC5, with the hydrogen atom of the vinyl group replaced by a group, was carried out. Such a replacement should hinder the formation of the trans–cisoid isomer. Indeed, in crystals of MC5 the molecules have a trans– transoid structure (Figure 8) and are joined together to form a chain via a strong hydrogen bond (1.70(4) Å]. The steric interactions between the atoms of the methyl group of C(10) and the syn H(5) atoms “eject” the phenolate moiety from the plane of the molecule due to a turn of 20.3° about the C(4)–C(4a) bond. However, this turn is evidently not sufficient to reduce the steric strain in the molecule, and the tendency to retain a planar structure with

delocalized electron density prevents a further loss of planarity by the molecule. This results are in distortion of the C(4)C(3)C(10) bond angle to 124.6(4)° and of the C(3)C(4)C(4a) and C(4)C(4a)C(5) angles to 133.3(4)° and 127.0(4)°. Despite the statistical disordering of the o, p-dinitro-substituted phenolate moiety in MC3 and MC4, the distribution of the bond lengths in these compounds

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is similar to that in MC5. This suggests that MCs with o- and groups in the phenolate fragment have mainly the bipolar structure E with appreciable distortion of aromaticity in the phenolate ring.

MC6 molecules (Figure 9)26 in the crystals are linked together by strong chains of O(1)···H0(4') hydrogen bonds [1.78(3) Å; O(1)···O(4') 2.64(1) Å], which results in elongation of the O(1)—C(8a) bond to 1.253(4) Å. This value is typical for zwitterionic molecules (1.26 Å). The distribution of the bond lengths in MC6 is indicative of an increased contribution of the quinoid form B (see Figure 5) to the structure of MC6. A decrease in the zwitterionic nature of the MC6 molecule and the greater contribution of the less reactive quinoid form to its structure may be responsible for the increased stability of this system. 7.2.2.2. Geometric Isomers of Merocyanine Forms To understand the mechanism of photochromism in SPs and to account for the spectroscopic behavior of MCs, in particular, the positions and nature of the

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long-wavelength absorption, which, together with the photoactivity of SPs and the stability of MC, determines the practical application of SPs, it is necessary to study the geometric isomerism and the relative stability of the isomers of the open merocyanine forms. Many theoretical studies have dealt with the problem of the relative stability of isomers of small model SPs and MCs and the effects of different substituents on their spectroscopic properties.39–43 X-ray diffraction analysis of MC1–MC6 showed that only planar trans–cisoid and trans–transoid isomers exist in the crystalline state. The former occurs in crystals of MC1–MC4, MC6, and the open form of a benzothiazoline MC studied by Miler-Strenger and Gugliemetti.44 The latter occurs together with the trans– cisoid isomer in crystals of MC3, MC4, and MC5, which have two nitro substituents in the phenolate fragment. Based on the results of quantum-chemical calculations,38 it was concluded that the trans–cisoid and trans–transoid isomers of MCs are energetically equal. In MCs with a single nitro group in the phenolate fragment, there is a substantial transition barrier between them, but introduction of two nitro substituents at the ortho and para positions of the phenolate fragment lowers this barrier. Cocrystallization of both isomers of the o, p-dinitro-substituted MC3 and MC4 may also indicate that these isomers have equal packing energy. The lowering of the rotation barrier about the C(4)–C(4a) bond when two electron-acceptor nitro groups are introduced into the phenolate moiety is evidently due to the increased contribution of form E to the structure of this

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type of molecule, which reduces the energy of the interaction of this moiety with the rest of the molecule. The results obtained cast doubt on the theory that the complex nature of the absorption spectrum of open merocyanine forms of SPs is due to the presence of four different stereoisomers in the solid state and in solution.45,46 Furthermore, calculations on the absorption spectra of model isomers of 2H-chromenes show that these isomers are indistinguishable in the region of long-wavelength absorption.41 In Refs. 47–51, to account for the spectroscopic properties of MCs, an

assumption is made about association of the colored and colorless forms of SPs. The similarity of the absorption spectra of MCs in nonpolar solvents (at low temperatures), in an amorphous state, and in thin homogeneous crystalline films suggest that the colored forms of SPs have the same structure in the liquid and solid states. Hence, the packing of MC molecules and the energy of intermolecular interactions in crystals are phenomena of definite interest.

7.2.2.3. Nature of the Molecular Packing and Energy of Intermolecular Interactions in Merocyanine Crystals The crystalline structures of permanent merocyanine forms of SPs are an indication of the well-defined tendency of MC molecules to form various types of aggregates. This is due to the planar structure and high polarity of MC molecules, and consequently, in addition to van der Waals forces, electrostatic and dipoledipole interactions play a significant role in the formation of aggregates. Calculations of the energy of crystalline MC structures show that the electrostatic interactions in the aggregates can amount to 50% of the van der Waals interaction. The strongest aggregates are formed in the MC2 crystals, which have molecules arranged in stacks on top of each other in a “head-to-tail” arrangement. The pairing energy due to the van der Waals and electrostatic interactions, excluding the dipole–dipole interactions, in these stacks is approximately –26 kcal mol–1 Solvated MC molecules form different aggregates via strong hydrogen bonds in “head-to-head” and “head-to-tail” arrangements. Bulky alkyl substituents at the N(1') atom can hinder the solvation of the trans–cisoid isomer of MCs, but they do not prevent the formation of “head-totail” aggregates. A methyl substituent at the N(1') atom does not hinder the solvation of trans–cisoid isomers of MCs. Solvation of the N(1')-phenyl-substituted MC results in the phenyl substituent rotating about 90° from the plane of the molecule and disrupting the conjugation of the N(1') LEP with the system of the phenyl moiety. This is evidently balanced by the gain in energy from solvation of the molecule. Solvated molecules of trans–cisoid MC isomers form aggregates through hydrogen bonds with solvating molecules. In the trans–transoid isomers, the substituents on the atom do not hinder the solvation. A solvated trans–transoid MC isomer can form both “headto-tail” aggregates and aggregates with solvating molecules. In MC1 crystals, the

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aggregates comprise the solvated MC molecules linked by hydrogen bonds. In these aggregates, the molecules are arranged in a “head-to-head” mode at an angle of 110° to each other. The electrostatic attraction energy between them is reduced, but the total energy of the van der Waals and electrostatic interactions is approximately In crystals of MC5, the strongest aggregates are formed by linking of the molecules in chains via hydrogen bonds as well as via intermolecular interactions between the molecules arranged in a parallel manner in layers in the “head-to-head” mode. The crystalline structure of MC6 is characterized by repeating chains, with the pairing energy of intermolecular interactions equal to – 10 kacl mol–1 The formation of strong aggregates in crystals of MC1–MC6 with different

types of packing allows the assumption to be made that these aggregates may also be preserved in solution. In crystals, the forms of molecular association that occur are usually those that are the most energetically stable and are the most favorable for packing. This means that in MC solutions, these aggregates can be even more diverse. In nonpolar aprotic solvents, MC molecules that are not linked by a specific intermolecular interaction with solvent molecules will preferentially form “head-to-tail” aggregates. In protic polar solvents, formation of solvates combined into different assemblies via hydrogen bonding is the most probable. The formation of solvates and aggregates is an additional factor stabilizing highly polar merocyanine forms, and the complicated absorption spectra of MCs can be accounted for by this rather than by assuming the existence of various isomers. 7.2.3. Effect of the Nature of Heteroatoms in the Spiro Center on the Structure and Photochemical Properties of Spiropyrans

The considerable influence of specific orbital interactions of heteroatoms in the spiro center of indoline SPs on the structure and photochemical properties of these compounds suggests that new classes of photochromic SPs can be obtained by changing heteroatoms and their relative orientation in the spiro center. In this section, we will examine the structure of SPs containing different heteroatoms in the spiro center. 7.2.3.1.

Structural Features of Indoline Spirothiopyrans Containing S and N Heteroatoms in the Spiro Center X-ray diffraction analysis of 1'-phenyl-3',3'-dimethyl-6-nitrospiro[indoline2,2'-[2H-1]benzothiopryan] (STP)52 (Figure 10) showed that, similarly to the C(22')–O bond in photochemically active SPs, the C(22')–S bond [1.866(2) Å], which is cleaved on photoexcitation, is considerably longer than the corresponding bonds in six-membered heterocycles (1.80–1.83 Å).24,25 The C(22')–N(1') bond [1.484(3) Å] in STP, in contrast to that in SP4, is not shortened, and its length agrees with that of normal bonds (1.47–1.48 Å) in five-membered

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heterocycles.23 Therefore, elongation of the bond in STP cannot be explained by using the concept of interactions in the spiro center as was proposed for SPs. Replacement of the oxygen atom at the spiro center by a sulfur atom, with an ionization potential similar to that of the nitrogen atom, evidently balances the electron-donating capacities of the LEPs of the S and N atoms. Furthermore, the angle in STP proved to be 101.9°, whereas in SP1–SP12 the angles are in the range of 120–124°. These data could suggest that hybridization of the S atom in STP is close to whereas that of the O atom in SP1–SP12 is close to When a heteroatom has hybridization, conjugation of its LEP with the system of the benzene moiety decreases; hence, the polarity of the bond in STP is much less than that of the bond in SP1–SP12,53 is and the interaction in STP is weaker than the interaction SP1–SP12. Elongation of the bond compared to the bond in SP1-SP12, and the resulting withdrawal of the LEP of the S atom from the orbital of the bond, also reduces the interaction. In STP, these interactions are evidently small, and they balance each other. Elongation of the bond to 1.866(2) Å in STP has been explained in terms of steric interactions between the gem-dimethyl groups and the heteroatom of the thiopyran ring. These interactions result in distortion of the indoline moiety and its bending along the line in the closed forms SP1–SP12. 7.2.3.2. Structure of “Symmetrical” Spiropyrans Consisting of Two Benzopyran Fragments It was demonstrated in Section 7.2.1.3 that the efficiency of orbital hyperconjugation in the spiro center depends both on the geometric configuration of the LEPs of the heteroatoms and adjacent bonds and on the electronic properties of the heteroatoms. Since “symmetrical” spiropyrans contain the same heteroatoms in the spiro center, efficiency of interactions depends mainly on the electronic state of the heteroatoms as determined by the nature and positions of substituents in the benzopyran moiety. In Bispiropyran (BSP)54 (Figure 11), the orientation of the of the O(1)and atoms with respect to the adjacent and bonds is the same. Nevertheless, the [1.459(1) Å] and [1.434(2) Å] bond lengths in the spiro center of BSP proved to be different. Introduction of the electron-acceptor group at the 8-position of BSP leads to contraction of the O(1)–C(8a) bond to 1.358(2) Å, which is indicative of an increase of conjugation of the O(1)-atom with the system of the benzene ring. Such contraction is similar to SP2 and SP3, where the lengths of these bonds are 1.356(3) Å. The length of the similar bond is 1.377 Å, which suggests that conjugation of the with the system of the

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naphthalene ring is weakened; a similar effect was seen with SP7, in which this bond length is equal to 1.372(3) Å. With this type of structure of the spiro center of BSP, the bond is more polar and the of the atom is more active so that in the spiro center the interactions, which lead to elongation of the bond, will predominate. In this case, the bond length proved to be only slightly longer than the normal value of 1.41–1.43 Å.

7.2.3.3.

Structure of the First Representative of Spiro-2H-Naphtho[l,8-bc]- oxepins (SPNO)

Spiropyrans containing seven-membered rings have not been available until recently,55,56 A spiropyran of this type, was first obtained and its structure studied in 198657 (Figure 12). Due to the rigid configuration of the spiro

atom, the seven-membered oxepin ring is strained and distorted. This conformation leads to a change in the mutual arrangemnt of the O(1)-atom and adjacent bond in the spiro center. If the of the O(2) atom and the adjacent bond are oriented at an angle of 165.0°, this proves to be favorable for an orbital interaction. Orientation of the O(1)-atom and the adjacent bond at an angle of 110.4° actually suggests that there is no reverse interaction. Therefore, in SPNO, the efficiency of the interaction between the of the O(2)

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atom and the orbital of the bond increases, which results in contraction of the bond to 1.409 Å and elongation of the C(22)-O(l) bond to 1.455(3) Å. In SPNO, the O(1)-atom is directed inside the heterocycle toward the C(4) atom, although it deviates from the line by approximately 38°. Taking into account the short interatomic distance (2.88 Å) and the distortion of the C(3)C(4)C(4a) bond angle to 130.3°, an attractive secondary interaction between the O(1) and C(4) atoms can be assumed. Apparently, these specific structural features of SPNO are responsible for its rapid conversion to compound P rather than to the open quinoid form upon its irradiation in solution with a mercury lamp.57

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7.2.4.

Chapter 7

2-Oxaindan Spiropyrans with Polycondensed Chromene Fragments

7.2.4.1. Structure of 2-Oxaindan Spiropyrans The conformations of the 2-oxaindan spiropyrans of SP14-SP17 have been investigated.58

All four have similar conformations. The naphtho(anthraceno)pyran and oxaindan moieties of the molecule are almost orthogonal to each other. In all the SPs studied, the pyran and oxaindan moieties are nonplanar. The former has a folded conformation with bending angles along the O(1)...C(3) and O(1)...C(4) lines in the range of 21.4–28.6° and 10.2–13.3°, respectively. The oxaindan ring has the conformation of an envelope with bending along the C(2)...C(13) line by 15.5–17.3°. The structure of one of the spiropyrans studied is shown in Figure 13, and their main structural parameters are given in Table 2. The Cspiro–O(1) bond is elongated in SP14–SP17, as in the indoline SPs studied before, While in SP1–SP11 the lengths of these bonds are in the range of 1.460(2)–1.496(4)Å, in SP14–SP17 these bond lengths are in the range of 1.444(1)–1.465(2)Å. In the unsubstituted SP14, this bond [1.444(1)Å] is the least elongated by comparison with the normal C–O bond lengths (1.41–1.43Å) in six-membered heterocycles. Annellation of an additional benzene ring has virtually no effect on the C(22')–O(1) bond length [1.447(1)Å] in SPI5]. The introduction of EA substituents at the 6-position of the naphthopyran moiety, for example, Br in SP16 and the NO2 group in SP17, results in elongation of this bond to 1.455(3)Å and 1.465(2)Å, respectively (Table 2). In the oxaindan moiety of SP14–SP17, the C(22')–O(2) bond appeared to be shortened [1.417(1)–

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1.407(3) Å] compared to the normal length of a C–O bond in five-membered heterocycles (1.426 Å).59 As the bond length increases, the bond has a tendency to be shortened whereas the bond in these SPs remains elongated (see Table 2). In the spiro centers of SP14–SP17, both heteroatoms are oxygen atoms. In those systems which have two oxygen atoms linked to a spiranic carbon atom, the efficiency of the interaction mainly depends on the geometric configuration of the LEPs of the oxygen atoms and the electronic state of these atoms. The compounds studied have the same conformation, with similar values

of the angles and

(see Table 2). However, the electronic states of the O(l) and

O(2) atoms in these compounds are different. The O(l) atom is linked to the

aromatic system of the naphthalene or anthracene moieties, and the O( 1 )–C(8a) bond lengths [ 1.354(2)–1 .377( 1) Å] are close to the lengths of the similar bonds in α-naphthol (1.37 Å).60 This is indicative of significant conjugation of the O(l) atom with the neighboring

system. This conjugation increases the polarity of the

bond compared to that of the

bond and decreases the

activity of the O(l)-atom LEP compared to that of the O(2)-atom LEP. Therefore,

in these spiropyrans the than the elongation of the

interactions must be stronger interactions, and they may be responsible for bonds and contraction of the

bonds.

These interactions are weaker in SP14–SP17 than in SP1–SP11, which contain different heteroatoms (O and N). This can also account for the lesser elongation of the

bond in SP14–SP17.

In SP18 and SP19,61,62 the of the O(2) atom is conjugated with the bond of the neighbouring carbonyl group, as confirmed by the O(2)–C(13) bond length (1.35–1.37 Å). Therefore, the interactions of the O(l) and O(2) atoms in these molecules are insignificant, and the bond lengths (1.42 and 1.399 Å) are normal.

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Introduction of the electron-withdrawing substituents Br and at the 6position of the naphthalene moiety results in elongation of the bond, this elongation being proportional to the electron-withdrawing properties of the substituents [1.455(3)Å in SP16 and 1.465(2)Å in SP17]. Simultaneous shortening of the O(1)–C(8a) bond occurs [1.366(3) Å in SP16 and 1.354(2) Å in SP17 as compared to 1.372(1) Å in SP14 and 1.377(1) Å in SP15]. This is indicative of increased conjugation of the LEP of the O(1) atom with the system of the naphthalene moiety on introduction of electron-withdrawing substituents. Elongation of the bond in SP16 and SP17 is related to the strengthening of the interaction of these compounds, which also results in shortening of the bond in SP16 and SP17 [1.413(3) and 1.407(3) Å, respectively], in comparison with these bond lengths in SP14 and SP15 [1.417(1)Å]. It should be noted that in the similar benzopyran SP2063 with an group in the position para to the O(1) atom, the bond is less elongated (1.452Å) and the O(1)–C(8a) bond (1.366Å) is less shortened than in SP17. These data suggest that the electron-withdrawing influence of the group is stronger in the naphthalene moiety than in the benzene ring.

7.2.4.2. Thermal Valence Isomerization of SPs Spiropyrans SP14–SP17 have distinct thermochromic properties in polar and nonpolar solvents. The values of the free energies of activation for the thermally induced valence isomerization of SP14–SP17 calculated from dynamic NMR data,58 together with the bond lengths in these compounds, allow the following conclusions to be made: 1. Variation of the structure of the heterocyclic nucleus and benzopyran moieties of the SP molecule has no substantial effect on the free energy of activation of the SP. 2. No correlation exists between the bond length in SP14–SP17 and the free energy of activation. The lack of this correlation is apparently due to the fact that the bond length in the ground transition state (the region of thermochromic cleavage)40

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is almost constant and differs considerably from this bond length in the initial state. Therefore, changes of this bond length due to the influence of structural factors do not have any substantial effect on the energy of activation of this bond cleavage. 7.2.4.3. Photochromism of 2-Oxaindan Spiropyrans SP14–SP17 in liquid and solid solutions are characterized by photochromic properties over a wide temperature range (from 77 to 273 K). The technique of flash photolysis allows reversible photocoloration to be observed even at 20 °C. To determine the relative photochemical activity of SP14–SP17,58 the ease of photocoloration (“colorability”) of these compounds has been studied. The results from a study in which “colorability” of SP14–SP17 was compared to the

bond length are given in Table 3. It is clear that these values increase simultaneously, the highest photochemical activity being exhibited by SP17, which has the most elongated bond. This is apparently due to the fact that the bond length in the excited transition state is much shorter than that in the ground transition state. Consequently, the reaction barriers of SPs in the excited states are markedly reduced. Therefore, the photochemical activity of coloration of SP proves to be sensitive to even small changes of the bond length in the ground state, which depends on different structural features of

SP molecules. 3.

SPIROOXAZINES

An increased interest in the synthesis and study of new photochromic spirooxazines (SOs) is due to their improved properties compared to SPs, i.e,. their stability to photodegradation.64 This has stimulated research on the influence of specific structural features of SOs on their properties. The SOs that have been studied by X-ray diffraction analysis to date can be divided into two groups: derivatives of benzoxazines and derivatives of annellated benzoxazines. The latter

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appear to comprise more members than the former, which differ in the position of the annellated benzene of pyridine rings. Table 4 shows the main structural parameters of the studied SOs. SO1–SO1665-71 belong to the class of naphthoand quinolinoxazines, whereas SO17-SO1972 are benzoxazines.

The configurations of all SOs investigated are almost the same and are similar to those of the SPs investigated before. The spiro center is tetrahedral. The dihedral angle between the planes is equal to 90–94°, similar to that in the indoline SPs. The indoline ring in each SO is nonplanar. Its nonplanarity is due to the bending along the line by an angle of 25–33°. Such distortion of the five-membered ring is due to a steric

repulsion between the gem-dimethyl groups and the O(1) and C(2) atoms. If the indoline ring is planar, these atoms are in the eclipsed configuration along the bond, while the distortion of this ring results in an increase of the torsion angle from to 33–40°. When the indoline ring is nonplanar, the N(2) has a pyramidal configuration and lies out of the plane of the adjacent atoms by 0.10–0.30 Å. The sum of the bond angles at the nitrogen atom N(2) is 340–356.1°. The oxazine moiety of the SO is more mobile, similarly to the benzopyran fragment in SP. This conformation can be characterized by the bending angles along the and lines. The MNDO method and the MOPAC program were used to reproduce various specific features of the structure of the naphthoxazine moiety68 that were revealed by X-ray diffraction analysis on crystals. This led to the conclusion that the influence of packing modes on the conformation of this fragment is secondary. However, it is suggested that its

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conformation is very lable and, owing to insignificant energetical differences, it can be determined mainly by the packing effects in a crystal. This conclusion is supported by the existence of the SP4 molecule with two different conformations of the benzopyran moiety in the crystal26. As in the indoline SPs,26,74 the structure of the spiro center in SOs is favorable for an interaction of the N(2)-atom LEP and the orbital of the

bond. The LEP of N(2) is directed approximately from the top of a pyramid orthogonal to its base, while the orbital of the bond is localized mainly along the continuation of this bond at the side of the electropositive atom. With such a conformation, the n(LEP) orbital of N(2) and the orbital of appear to be parallel to each other. Due to the overlap of these orbitals, the bond is shortened and the bond is elongated, compared to the C–N bond lengths (1.47–1.48 Å) and C–O bond lengths (1.41–1.43 Å) in five- and six-membered rings. The efficiency of this interaction depends considerably on the electronic state of the N(2) and O(1)

atoms. EA substituents in the benzopyran moiety of SP have been shown to strengthen this integration,26,74 whereas ED substituents weaken it. This result is connected with an increase or decrease of conjugation between the O(1)-atom

LEP and the system of the phenyl moiety. In the investigated SOs, the electronic properties of the substituents are not essentially different, so the differences in the bond lengths are often within the limits of experimental error. This should be taken into account when one compares SO structures, and it is more reasonable to consider certain trends in the SO series. Spirobenzopyran SP7 and spirobenzoxazines SO 17 and SO 18, as well as SO2 and its spiropyran analog, have the most similar compositions. The structures of these compounds (Table 4) are the same within the limits of experimental error, which suggests that the specific structural features of SPs and SOs are similar.

Comparison of the structures of SO1 and SO4 or of SO2 and SO3 indicates that annellation of benzene or pyridine moieties at positions 7 and 8 results in a

slight shortening of the bond and elongation of the bond compared to these bond lengths in SP17. However, the structures of all annellated SOs are the same. The presence of

and

substituents does not

influence the structure of SO5-SO8. In SO 10, the presence of the substituent at the positon para to the N(1) atom does not affect its structure, by comparison with that of the unsubstituted SO4. Within the limits of experimental error, the N(1)–C(3) [1.410(3)Å] and O(1)–C(4) [1.371(3)Å] bond lengths in

SO1 agree with the analogous bond lengths in SO4 [1.416(3) Å and 1.362(3) Å]. This confirms that conjugation of the azomethine bond and the LEP with the system of the naphthalene moiety, and hence, the interactions, has not changed in SO10. Therefore, the [1.456(3) Å] and [1.430(3) Å] bond lengths in SO 10 coincide with the analogous bond lengths in SO4 which are equal to 1.454(3) and 1.436(3) Å, respectively.

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The structure of SOI 1 was not affected by annellation of another potentially photochromic chromene ring at the 9- and 10-positions of the naphthalene moiety. Within the limits of experimental error, the [1.448(8) Å] and [1.424(8) Å] bond lengths coincide with the lengths of the analogous bonds in SO 10 and SO4. Introduction of an HO group at the position ortho to the O(l) atom of the naphthyl moiety in SO 12 (Figure 14) induces a slight elongation of the bond to [1.471(4) Å], compared to SO 10 and SO4, while the O(1)–C(4) [1.373(5)Å] and [1.434(4)Å] bond lengths are unchanged. The authors suggest that such elongation of the bond in SO 12 is not due to altered interaction, but rather is due to the participation of the O(l) atom in the intramolecular hydrogen bond (IHB) which forms a five-membered ring The deviations of the atoms from this plane are: O(l), –0.003 Å; C(4), 0.005 Å; C(5), The torsion angles O(1)C(4)C(5)O(2) and are equal to 1.2° and The IHB length and the angles i and are equal to 2.35(1) Å, 102.7°, 81.44°, and 149.8°, respectively. The atom participates in the bifurcate hydrogen bond, forming the second hydrogen bond with the oxygen atom of the water molecule: the (distance is 1.91 (5) Å, the distance is 2.81 Å, and the angle is 165°. Comparison of the geometrical parameters of these hydrogen bonds shows that the IHB is much weaker than the one, so its influence on the bond is weak.

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In SO 13, introduction of a group at the C( 10) position of the naphthyl ring does not affect the conjugation of the O(1)-atom LEP with the system of this moiety. The O(1)–C(4) bond length, 1.369(2) Å, agrees with this bond length in SO10–SO12 [1.371(3), 1.374(8), and 1.373(5) Å respectively]. The bond length, 1.446(2) Å, also agrees with the bond lengths in SP10 and SP11, 1.456(3) and 1.448(8) Å, while the bond in SO 13 [1.456(2) Å] is longer than in SO10–SO12 [1.430(3), 1.424(8), and 1.434(4)Å, respectively]. Elongation of this bond is not due to altered interactions of the spiro center, but rather is due to steric repulsions between the terminal vinyl group and the and C(2) atoms. Intramolecular distances between these atoms are shortened even though the bond is elongated: 3.064 Å; 2.940 Å. Elongation of the bond in SO 13 and of the bond in SO 12, induced by the secondary interactions, confirms once again the lability of these bonds in SO and SP molecules. For SO14 the value of which characterizes the “colorability,” is 6.00. This value is much higher than the values for SO 10–SO12: 1.03, 1.75 and 1.09, respectively.70 This indicates that SO 14 has increased photochemical activity, even if we take into account the difference in the extinction values of the SO open forms. The [1.45(5) Å] and [1.437(5) Å] bond lengths are close to these bond lengths in the spiro center of SO10–SO13 and SO4. However, introduction of an electron-withdrawing substituent at the position para to the N(2) atom of the indoline moiety results in increased conjugation between the N(2)-atom LEP and the system of the phenyl ring. This is confirmed by the shortening of the bond to 1.378(5) Å, compared to the analogous bond lengths in SO 10–SO 13 and SO4 1.409(3), 1.395(9), 1.413(5), 1.408(2), and 1.414(3) Å, respectively]. This increased conjugation results in the flattening of the pyramidal structure of the N(2) atom, which is manifested as smaller deviations of the N(2) atom from the plane of the adjacent atoms (0.16 Å compared to 0.27–0.32 Å in SO10–SO13) and an increase of the sum of the bond angles at the N(2) atom to 356.1°, compared to 345.8–349.3° in SO10– SO13 and SO4. However, flattening of the N(2)-atom pyramid in SO 14 is mainly due not to the decrease in the bending angle of the indoline ring to 26.8° as compared to 30.6, 26.7, 32.5 and 27.7° in SO10–SO13, respectively, but to the deviation of the methyl substituent at N(2) from the phenyl ring plane in the direction opposite to the atom. This results in a change in the orientation of the N(2)-atom LEP relative to the bond and in an increase of its pcharacter. This is confirmed by the change of the torsion angle in S014 [71.7° compared to 58.4(1), 57.2(2), 55.7(3), and 50.7(4)° in SO10–SO13]. As can be seen from the structure of SO14, the change in the configuration of the N(2) atom does not affect the efficiency of the interactions in the ground state, but it can influence the efficiency of this interaction in the excited state, and thus the photochemical activity of SO14.

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The bond lengths in SO15 and SO16 coincide with those in SO12. The bond lengths in SO5–SO16 indicate that substituents do not influence significantly the conjugation of the LEP with the system of the naphthyl moiety. Therefore, some elongation of the C(1)–O(1) bond in SO5– SO15, as well as in SO12, compared to the lengths of analogous bonds in other SOs, cannot be related to a change in interactions. This is also confirmed by the identical C(1)–N(2) bond lengths in SO10–SO12, SO14, and SO15. Similarly to SO12, elongation of the C(1)–O(1) bond length in SO15 and

SO16 may be due to participation of the O(1) atom in secondary interactions. The structures of SO15 and SO16 are favorable for attractive interactions with the hydrogen H(9) atom located in the peri position of the naphthyl moiety. In spite of the lower acidity of the naphthyl hydrogen atom in SO15 and SO16, compared to the hydroxyl hydrogen atom in SO12, the structures of SO15 and SO16 allow for existence of a weak intramolecular

bond, which further weakens and elongates the C(1)–O(1) bond.

4. PHOTOCHROMIC [2H]-CHROMENES As distinct from photochromic spiropyrans and spirooxazines, [2H]-chromenes do not contain a heterocyclic fragment such as an indoline nucleus. Consequently, in such compounds there are no specific orbital interactions that elongate and weaken the bond, the ruptures of this bond in the excited and ground states being responsible for the photo- or thermochromic properties of spiropyrans and spirooxazines as well as [2H]-chromenes. Several [2H]-chromene derivatives exhibit photo-, thermo- or solvatochromic behavior in solution. Specific structural features of the substituted di-phenylchromenes CH1–CH4, the dimethyl chromenes CH5, and the spiro[fluorene-benzopyrans] SFB1 and SFB2 have been studied.75–77 The annellation of a benzene ring to the 5 and 6 positions of the benzopyran moiety in CH2, instead of the pyridine ring in CH1, does not essentially affect the structure of the naphthopyran fragment. The pyran rings in CH1 and CH2 are nonplanar with folding along and in a similar manner as in SPs and SOs. The bond in CH1 and CH2 is elongated, and the bond is shortened (Table 5). The conformations of the phenyl groups are different. The mutual arrangement of the phenyl rings in CH2 and CH1 is similar. The dihedral angle between these fragments is 96.6(2)° and 91.0(2)°, respectively. However, the arrangement of the phenyl rings with respect to the pyran ring differs. In a Newman projection along the O(1) atom of the pyran ring in CH2 is almost eclipsed with one of the phenyl rings [the torsion angle O(1)C(1)C(14)C(15) is 14.2°]. In CH1, the O(1) atom is in a skewed conformation with both phenyl rings. The corresponding torsion angles along the bonds are 32.6 and 38.5°.

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Chapter 7

Hence, elongation of the bonds in the C(1) center may be related to the steric repulsion between the O atom and the nearest atoms of the phenyl rings, which are an eclipsed or slightly skewed conformations along the bonds. The fact that the values for the C(1)–O(1) bond lengths in CH1 and CH2 are the same suggests that this bond is not affected by the orientation of the phenyl rings. Therefore, the elongation of the C(1)–O bond is induced mainly by steric repulsions between the neighboring atoms in the tetrahedral C(1) center. Distances

between these atoms depend, in their turn, on the values of the bond angles at the C(l) atom. Indeed in CH1 and CH2, one of the bond angles is considerably reduced from the tetrahedral angle, 109.2°. Introduction of the bulky phenyl substituent to yield CH3 considerably distorts the conjugation of the naphthyl moiety with the ethylene bond of the pyran ring but does not essentially affect the structure of the molecule. The conformation and the distribution of the bond lengths in CH3 and CH2 are similar (Table 5).

The conformation of CH4 is similar to that of CH2. However, in CH4 the C(1)–O(1) bond is shortened [1.441(4) Å], and the bond is elongated [1.389(4) Å], compared to the analogous bond lengths in CH2, 1.459(4) and 1.372(4) Å. Elongation of the bond in CH4 may be due to the presence of the electron-donating O(2) atom of the furan ring in the position para to the O(1) atom. This results in strengthening of the C(1)–O(1) bond, which appears to be less elongated than in CH1–CH3. Therefore, steric interactions between the O(1) atom and the phenyl ring in the eclipsed conformation induce elongation not of the C(1)–O(1) bond but of one of the bonds: the bond length is 1.540(4) Å compared to 1.527(4) Å in CH2 and 1.510(8) Å in CH4. The length of another bond, 1.523(4) Å, coincides with the corresponding bond lengths in CH2 [1.524(4)Å] and in CH3 [1.528(8)Å]. Although the C(1)–O bond in CH4 is shorter than that in CH1–CH3, the photochemical activity of the former is higher.75 The photocolorability of these compounds increases in the order As calculated by the MMPD method, the energy of steric strain of the pyran rings, which is due to distortion of these rings at the expense of bending along and also increases in this order. The energy of steric strain appeared to correlate with photochemical activity for the chromenes investigated. The lack of photochromic shifts in CH3, although this chromene has the highest of the pyran ring, may be explained by steric hindrance arising upon opening of the ring containing the phenyl substituent. Quantum-chemical calculations of the electron structure of the CH4 molecule in the ground and excited states (PM3, MOPAC 6.0) indicate that, upon photoexcitation, density transfers from the LEP of the O(1) atom into the conjugated system of the annelated substituent and the orbital of the double bond of the pyran ring. Such transfer of

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density in CH4 is favored by the specific distribution of the bond lengths in conjugated heterocycles. The C(8)–C(8a), C(5)–C(6), and C(3)–C(4) bonds [1.363(3), 1.360(3), and 1.444(3) Å, respectively] are shortened. The O(2)–C(6) [1.390(3) Å] and O(2)–C(9) [1.380(3) Å] bonds are nonequivalent. Such a distribution of bond lengths indicates that the resonant structure which is due to donation of electron density from the O(2)-atom LEP into the of the

bond makes an essential contribution to the structure of CH4.

A considerable contribution of this resonance structure could explain the above-mentioned decrease of conjugation of the O(1)-atom LEP with the heterocyclic moiety and the increase of the O(1)C(8a)C(8) bond angle to 117.6(3)° compared to the analogous values in CH1–CH3 [115.6(3)°, 115.5(3)°, and 114.8(3)°, respectively]. Such a structure is favorable for the transfer of the electron density in the excited state, opening of the ring, and isomerization of the

open form, despite the fact that, among CH1–CH4, CH4 exhibits least elongation of the C(1)–O(1) bond. The large strain energy of the pyran ring is also favorable for its efficient opening. The structure of dimethylchromenes (CH5) is similar to that of diphenylchromenes CH1 and CH2 (Table 5). The dimethylchromene moiety in SO2 has almost the same structure: the C(1)O and bond lengths are 1.467(8) and 1.375(8) Å.75 As in CH1–CH4, elongation of the C(1)–O bond in CH5 and SO2 is induced by steric repulsion between the O atom and the methyl substituent in the tetrahedral center. One of the bond angles, is reduced to 104.1° in CH5 and 105.3° in SO4.

7.5. SPIRANS OF THE PERIMIDINE SERIES The synthesis of the new spirocyclic cyclohexadienoneperimidines SPR1a– SPR1c and 4H-naphthalenoneperimidines SPR2, which exhibit photo- and thermochromic properties, has been reported.78 X-ray diffraction analysis of SPR1b and SPR2 has been performed.79 The general view of the SPR1b and SPR2 molecules is shown in Figures 15 and 16. Both molecules have a spirocyclic structure, with dihedral angles between the N(1)C(1)N(2) and C(2)C(1)C(6) planes of 87.8 and 87.3° in SPR1b and SPR2, respectively. The perimidine moiety in the system is nonplanar, with bending

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along the lines at angles of 41.6 and 40.4°. The quinone part of the molecule is also nonplanar. The cyclohexadienone fragment of SPR1b is in a “boat” shape with bending along and by 10.8 and 21.3°, respectively. The naphthalene moiety of SPR2 has the same shape, with bending by 9.7 and 15.7°, respectively. This considerable distortion of the rings is apparently due to the rigid tetrahedral arrangement around the C(1) spiro atom, which determines a value of the intracyclic C(2)C(1)C(6) angle of 112.0° in SPR1b and 112.7° in SPR2, and to a significant steric and inductive influence of the tert-butyl groups, which reduce the values of the corresponding intracyclic angles C(2)C(3)C(4) and C(4)C(5)C(6) in SPR1b to 116.3° and 116.7°, respectively, and similarly reduce the value of the C(2)C(3)C(4) angle in SPR2 to 116.6°. In addition, the nonplanar structure of the perimidine ring is due to the rigid tetrahedral arrangement around the C(1) spiro atom, which determines a value of the endocyclic N(1)C(1)N(2) angle of 106.8 and 106.0° in SPR1b and SPR2, respectively. This structure results in considerable distortion of the planar trigonal conformation of N(1) and N(2) toward a pyramidal conformation. The deviations of the N(1) and N(2) atoms from the and

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coordination planes in SPR2 are equal to 0.22 and 0.14 Å, respectively, the sum of the valence angles at the N(1) and N(2) atoms is 350.0 and 355.6°, respectively, rather than the value of 360° that is typical for a planar trigonal arrangement. In SPR1b, deviations of the N(1) and N(2) atoms from the C(I)H(N)C(16) and C(1)C(15)C(25) planes are 0.25 and 0.28 Å, respectively, and the sum of the valence angles at the N(l) and N(2) atoms is 346.2 and 348.7°, respectively. The C(16)–N(2) bond [1.480(2) Å] in SPRlb is longer than the C(1)–N(1) bond [1.458(2) Å]. This may be due to the steric and electronic influence of the methyl group, which causes reduction of the endocyclic C(1)N(2)C(25) angle [116.2(2)°] and elongation of both the C(1)–N(2) bond and the N(2)–C(25) bond to 1.394(2)Å compared to the N(1)–C(16) bond length of 1.382(2)Å. In SPR2 the C(1)–N(1) bond length [1.465(2) Å] is equal, within the limits of experimental error, to the length of the analogous bond in SPRlb [1.458(2)Å], and the C(l)—N(2) bond [1.507(3) Å] appears to be longer than the analogous bond in SPRlb [1.480(2) Å]. The N(1)–C(15) [1.398(3) Å] and N(2)–C(24) [1.387(3) Å] bond lengths are also nonequivalent in SPR2, though to a lesser extent, and the reason for this phenomenon is not clear. Transformation of the open OSPR form into the spiran SPR may be connected with intramolecular nucleophilic attack of the quinonimine moiety by the LEP of the amine nitrogen, which is accompanied by proton transfer. We suggest that introduction of strong EA groups at one of the nitrogen atoms would reduce its basicity and allow examples of open forms of thermochromic spirans to be obtained. Bulky substituents at the nitrogen atom, which may stencally hinder the closure of the spiran ring, should produce a similar result. An effort to stabilize the quinonimine structure by introducing EA substituents,80 in order to study specific features of such model compounds, has been made. The bulky tosyl group and the less bulky acetyl group have been used as strong electron acceptors. Compounds OSPR1d–f have been obtained, with spectral parameters corresponding to the open quinonimine structures. An X-ray diffraction analysis of OSPR 1d showed that it actually has a quinonimine structure. Figure 17 presents a general view of the molecule. It is

essentially nonplanar due to turning around C(22)–N(2), C(14)–N(l), N(2)–C(24), and N(1)–S by 52.3, 52.3, 6.1, and 63.6°, respectively. The N(l) atom has a pyramidal arrangement. Its deviation from the C(14)SH(1) coordination plane is 0.24 Å, and the sum of the valence angles at the N(l) atom is equal to 346.9°. The pyramidal nature of the N(l) atom is likely due to the deviaiton of the H(l) atom from the C(14)N(1)S plane toward the N(2) atom with formation of the intramolecular hydrogen bond [1.96(3) Å]; the distance is 2.671(3) Å. Although the and distances are rather short, this hydrogen bond is not strong because of the unfavorable orientation of the N(l)atom LEP relative to the directions is 50.8°, whereas

bond direction. The angle between these two would be the most favorable for the intra-

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molecular hydrogen-bond formation. In addition, the six-membered N(2)C(22)C(23)C(14)N(1)H(1) ring is essentially nonplanar. The deviation of the H(l) atom from the C(22)N(2)C(24) plane is 1.1 Å. The intramolecular hydrogen bond is not linear; the N(1)H(1)N(2), H(1)N(2)C(22), and H(1)N(2)C(24) angles are 133(3), 101.1(9), and 126.9(9)°, respectively. The N(1)–C(14) bond [1.415(4) Å] is elongated compared to the analogous bonds in spirans of the cyclohexadienoneperimidine and 4H-naphthalenoneperimidine series (1.382–1.398 Å).79 Such elongation is apparently due to a weakening of the conjugation of the N(l)-atom LEP with the system of the naphthyl moiety. This is induced by, first, an increase of the rotation angle along the C(14)–N(1) bond (52.3°) and, second, the strong acceptor influence of the tosyl substituent. The shortening of the N(2)–C(22) bond to 1.405(4) Å may be due to conjugation of the N(2)-atom LEP with the system of the naphthalene ring. The angle between the LEP and the N-system is equal to 37.6°. Similarly to the spirocyclic structures of the cyclohexadienoneperimidine and 4H-naphthalenoneperimidine series,79 the cyclohexadienone moiety is nonplanar and has a “boat” conformation. The deviation of the C(24) and C(27) atoms from the C(25)C(26)C(28)C(29) plane is 0.13 and 0.23 Å, respectively. The angle of bending along and is 9.9 and 18.2°. In methanol, the absorption spectrum of OSPR1d is essentially similar to that in octane, while in alcohol solutions of SPR1a– SPRlc, the equilibrium is completely shifted toward the spiran structure. Variation of the temperature and photolysis of solutions of OSPRld do not affect the absorption spectra. The absorption spectra of OSPR1e and OSPR1f in hexane also confirm the presence of only the open quinonimine structure. Thus, the spectral data allow us to conclude80 that the presence of strong electron-withdrawing groups on the nitrogen atom stabilizes the quinonimine structures OSPRld–OSPRlf and hinders their tautomeric transition into spirocyclic analogs.

7.6.

FULGIDES

Many derivatives of dimethylenesuccinic anhydride, also called “fulgides,” exhibit photochromic properties, which are due to isomerization and cyclization reactions.

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The structures of arylidene derivatives of fulgides (ArF: or Ar) and the indole series (InF: or have been studied by an X-ray diffraction method. In their stereochemical description, fulgides are treated as derivatives of succinic acid. The trans arrangement of a substituent and the carbonyl group with respect to the C=C double bond is referred to as the E configuration, and the cis arrangement as the Z configuration. 7.6.1. Arylidene Derivatives of Fulgides (ArF) Three isomeric forms of ArF are available: EE, EZ, and ZZ. These three isomers are capable of photochemical intertransformations and cyclization. Cyclization only occurs from a sterically strained E form, and its efficiency depends on the structure of the E-isomer. To date, the structures of four arylidene derivatives of fulgides have been studied: and 85 The structure of the ZEisomer could not be investigated by X-ray diffraction analysis. For Arf1, only the E-isomer structure has been studied. A cyclic product of dehydrogenation has been studied only for ArF3 and ArF4. ZZ-isomers of ArF2–ArF4 have a sterically unstrained, almost planar structure. The dihedral angles between the cyclic moieties of the molecules do not exceed 10° (Figure 18) E-isomers which undergo cyclization have a strained structure. A general view of the E-isomer of ArF1 is shown in Figure 19. Below, the specific structural features of E-ArFl and EE-ArF2 are examined. The structures of the EE-isomers of ArF3 and Arf4 are similar to that of EE-ArF2. According to the selective rule of orbital symmetry preservation, cyclization of an E-isomer involves the methylene carbon atom of one benzylidene group and

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a phenyl substituent of the other to yield the cyclic intermediate and the product (Scheme 1). Photochemical cyclization occurs by a conrotatory mechanism, while thermal cyclization occurs by a disrotatory one. In the symmetrical fulgide EE-ArF2, the phenyl substituents of both benzylidene groups participate in the reaction with equal probability. In unsymmetrical benzylidene(diphenylmethylene)-succinic anhydride, E-ArFl, only one phenyl substituent of the diphenylmethylene moiety participates both in thermo- and in photochemical reactions. The selectivity of such a transformation is not determined by a rule of orbital symmetry preservation, so it was suggested that it may be related to the specific spatial structure of E-ArFl.81

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The steric repulsion that arises in the E-isomer leads to substantial distortion of the molecule. The five-membered heterocycle (A) is planar (Figure 19). In spite of the hybridization of the C(2) and C(3) atoms, the methylene C(5) and C(12) atoms in E-ArFl deviate from the ring plane in opposite directions by 0.26 and –0.33 Å, and in EE-ArF2 by 0.26 and –0.30 Å. The benzylidene and diphenylmethylene moieties are twisted about the C(2)=C(5) and C(3)=C(12) double bonds in the same direction by 15.0 and 24.4°, respectively. The trans phenyl substituents and (C) are shifted in opposite directions from the plane of the five-membered heterocycle. The phenyl substituents B and C are themselves rotated in the same direction with respect to the C(5)–C(6) and C(12)–C(13) bonds by 35.4 and 39.7°, which decreases steric repulsion between the substituents. In EE-ArF2, the benzene rings are twisted by about 30° in the same direction. Benzene rings B and C in E-ArFl are not parallel, the dihedral angle between them being equal to 22.9°; in EE-ArF2, this angle is 18.8°. The dihedral angles between the B and C planes and the heterocyclic plane A in E-ArFl are 39.5 and 46.8°, respectively. As in EE-ArF2, steric strains in E-ArFl are responsible for substantial deformation of the valence angles. However, whereas in EE-ArF2 the valence angles in both benzylidene moieties are distorted equally, in E-ArFl the distortions of the valence angles in the benzylidene and diphenylmethylene fragments are not the same. The valence angles C(3)C(2)C(5) and C(2)C(5)C(6) in the benzylidene fragment are increased to 136.6 and 131.1° and are close to the values of similar angles in EE-ArF2, 137.8° (average) and 129.5° (average). The deformation of the valence angles of the diphenylmethylene moiety, which is more sterically crowded, is determined by the nonvalence repulsion of the atoms of the benzine rings B and C, on the one hand, and of the atoms of the benzene ring D with the carbonyl group C(4)–O(2), on the other hand. The valence angles C(2)C(3)C(12) and C(3)C(12)C(13) are only increased to 132.0 and 122.8°,

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respectively. With such a structure of E-ArFl, benzene ring C is twisted inside the molecule to a greater extent than the B ring (see Figure 19), so that nonvalence distances between the potentially reactive centers C(5) and C(14) [3.303(3) Å] appear to be shorter than those between C(ll) and C(12) [3.460(3) Å]. In the symmetrical EE-ArF2, the distances between the reactive centers C(5)–C(14) and C(11)–C(12) are similar and equal to 3.392 A. Steric strains in the E-ArF1 molecule are the reason for the considerable difference in the valence bond lengths within the benzylidene and diphenylmethylene moieties. The C(2)=C(5) double bond [1.349(2) Å] in the benzylidene fragment is elongated due to its twisting. The sterically crowded diphenylmethylene moiety is twisted along the double bond to a greater extent than the benzylidene moiety, and the C(3)=C(12) bond is elongated to 1.368(2) Å. The barrier to rotation about these bonds is substantially reduced. Therefore, in EArFl, the barrier to rotation about the C(3)=C(12) bond must be lower than the barrier to rotation about the C(2)=C(5) bond. In addition, in the diphenylmethylene moiety, the bonds beween the C(12) atom and the phenyl substituents C and D are also elongated to 1.479(2) and 1.481(2) Å, respectively, because of steric repulsion between the two bulky substituents at the methylene C atom. Such elongation of the C(12)–C(13) and C(12)–C(19) bonds compared to the similar C(5)–C(6) bond [1.457(2) Å] in the benzylidene moiety suggests decreased conjugation between the electrons of the benzene rings C and D and the system of the C(3)=C(12) bond and a decrease of the barrier rotation about the C(12)–C(13) and C(12)–C(19) bonds. It should be noted that the lengths of the similar C=C bonds [1.337(8)–1.372(8) Å] and C-Ph bonds [1.440(8)–1.472(3) Å] in the benzylidene fragments of EE-ArF282,83 are essentially different though the distance between the reactive centers and the twisting angles about the double bonds are similar. This difference of bond lengths in EE-ArF2 is apparently due to the low precision of the experiment.82 Thus, substitution of a hydrogen atom by a bulky phenyl substituent in the benzylidene fragment of ArF2 results in an essential change in the structure of the E-ArFl isomer as compared to the EE-ArF2 isomer. The stereochemistry of phenyl substituents C and B in E-ArFl is different. The trans phenyl substituent C in the more sterically crowded diphenylmethylene moiety is located quite close to the corresponding reactive methylene center C(5) and, owing to the lower barriers to rotation about the elongated C(3)=C(12) and C(12)–C(13) bonds, must be more mobile than the trans phenyl substituent B in the benzylidene fragment. The results of the structural study are indicative of a stereospecific control of the thermo- and photochromic cyclization of E-ArFl. For ArF3 and ArF4, the structure of the cyclic product C obtained by dehydrogenation of (Scheme 2) has been studied. The naphthalene and anhydride rings are coplanar. The aryl ring is rotated with respect to the plane of the three rings by about 58°.

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7.6.2. Fulgides of the Indole Series Heterocyclic fulgides exhibit photochromic properties that are useful for practical applications.86,87 The structures and photochromic transformations of

fulgides of the indole series E-InFl and E-InF2, in which the indole substituent is joined to the methylene group at position 3, have been studied.88,89

The assignment of the configurations of the E- and Z-InFl geometrical isomers based on the spectral analysis data has been supported by X-ray diffraction analysis. The InFl molecule (Figure 20) is characterized by a sterically crowded conformation with a 1-methylindole substituent in the position trans to the carbonyl C(1)=O(1) group with respect to the C(2)=C(5) bond. The methyl group C(15) and the benzene ring of the indole moiety have an anti position with

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respect to the C(5)–C(6) bond. This induces a steric repulsion between the atoms of the indole and isopropylidene substituents, which is responsible for substantial distortion of the molecular structure. The methylene carbon atoms C(5) and C(16) deviate in opposite directions from the average plane of the five-membered oxygen-containing heterocycle by –0.47 and 0.53 Å, respectively. This distorts the planar arrangement of the bonds of the hybridized C(2) and C(3) atoms. The sum of the valence angles at these atoms is 359.1 and 358.1°, respectively. The indolylethylidene and isopropylidene moieties are twisted in the same direction with respect to the C(2)=C(5) and C(3)=C(16) double bonds by 24.3 and 24.5°. This twisting induces elongation of these bonds to 1.367(2) and 1.350(2) Å. The indole moiety is rotated about the C(5)–C(6) bond in the same direction by 33.4°. This structure results in an increase of the intramolecular nonvalence distances between the atoms of the methylene substituents. However, intramolecular contacts 3.27; 3.09; 3.35; 3.17; 3.30; 2.76; 2.72; and 2.79 Å] are essentially shortened compared to the sum of the van der Waals radii of the carbon atoms or the carbon and hydrogen atoms, this being indicative of steric crowding in the molecule.

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The five-membered oxygen-containing heterocycle is also nonplanar, being bent about the and lines by 9.2 and 8.3° in the same direction. This is due to steric repulsion between the O(l) and O(2) atoms and the methyl groups C(15) and C(18) with the distances between them being shortened 2.86;

2.91;

2.28;

2.23 Å]. Attention should be paid to the unusual distribution of the bond lengths in the five-membered heterocycle of the indole moiety with its C(6)=C(7) double bond [1.392(2) Å] being essentially elongated compared to the normal length of a C=C bond, 1.32 Å. It appears to be longer than the (C7)–N single bond [1.349(2) Å]. Such a redistribution of the bond lengths is indicative of strong conjugation of the nitrogen-atom LEP with the C(6) = C(7) bond. The structure of the InF2 molecule is similar to that of InFl. The isopropylidene substituent is twisted about the C=C bond by 23.5°. The indolylethylidene substituent is twisted by 19.1° about the C=C double bond and by 26.7° about the C–C single bond. The C=C double bonds are elongated to 1.361(2) Å. The structures of the E-isomers of InF3 and InF4 in which the indole substituent is attached to the methylene group at position 2, are similar to those of E-InFl and E-InF2.90

The molecules are sterically crowded (Figure 21). In E-InF3 the substituent and group of the indole moiety have a syn orientation which induces steric repulsion between the isopropylidene substituent and the group of the indole moiety. The intramolecular distances between these groups are shortened to 3.26–3.37 Å. In the E-InF4 molecule, the indole moiety is disordered in such a manner that 65% of molecules have the group arranged in an anti position with respect to the substituent, and 35% in a syn position. Intramolecular distances between these substituents are shortened to 3.23–3.40 Å. Owing to steric

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repulsion, the isopropylidene group is twisted about the C=C bond by 20.9° in EInF3 and 20.7° in E-InF4. The indolylethylidene substituent in E-InF3 and E-InF4 is twisted about the C=C bond by 20.6 and 21.8°, respectively, and about the single C–C (indolyl) bond by 41.4 and 48.4°, respectively. The structure of the Z-isomer of InF4 was successfully studied. As distinct from the approximately planar Z-isomers of InF2-InF4, the Z-InF4 molecule is nonplanar (Figure 22). Its nonplanarity is even slightly greater than that of the Eisomer. The isopropylidene group is twisted by 28.1° about the C=C double bond

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and by 50.8° about the –C(indolyl) single bond. Twisting about the double bonds is due to the mutual steric repulsion of the syn-methyl groups and between the latter and the methylene carbon atoms. The distances between these atoms are shortened to 3.13–3.35 A. Twisting of the indole moiety is due to the steric repulsion between the carbonyl hydrogen atom and the group of this moiety. Even with the twisting angle of 50.8°, the and distances are 3.00 and 3.10 Å, respectively. The structure of another fulgide of the indole series which contains the bulky adamantylidene group instead of the isopropylidene group, adamant-2-ylidene(1'-methyl-3'-indolylethylidene)succinic anhydride (AdF) (Figure 23), has been investigated.91 As in InFl, the indoline moiety is in a trans position with respect to the

carbonyl C(13)–O(3) group. Because of steric interactions between the synarranged bulky adamantylidene and indoline substituents, the latter is twisted

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about the C(2)-C(10) bond by 34.7°. Steric repulsion between these substituents also induces rotation of the entire indolylethylidene moiety about the C(10)=C(12) double bond by 17.5° and of the adamantylidene substituent about the C(15)=C(16) double bond by 13.3° in the same direction. The C(10)=C(12) and C(15)=C(16) double bonds are elongated to 1.374(3) and 1.341(3) A, respectively, compared to the normal value of 1.32 Å. In addition, the valence angles for the -hybridized atoms are as follows: C(10)C(12)C(15), 132.1; C(2)C(10)C(12), 123.4; C(3)C(2)C(10), 128.9; C(12)C(15)C(16) 130.7. C(15)C(16)C(21) 123.4°. However, the nonvalence distances in the molecule are shortened: 3.35; 3.49; 3.33; 3.24 Å. The five-membered oxygen-containing heterocycle is nonplanar, being bent about and by 7.7 and 6.8°, respectively. Such distortion of coplanarity is due to the steric repulsion between the carbonyl oxygen atoms O(2) and O(3) and the syn-arranged C(17) and C(ll) atoms 2.90; 3.01 Å]. As a result of this conformation of the heterocycle, the coordination angles at the C(12) and C(15) atoms are nonplanar. The sum of angles at these atoms is 359.2 and 357.9°. Calculation of the conformational energy of the molecule the AdF molecule supports the conclusion that the Eisomer containing a bulky adamantylidine substituent is characterized by strong steric interactions. Optimization of the molecular conformation by minimization of the energy of bond tension, valence-angle deformation, and torsion rotation about the bonds yields a conformational energy of the AdF molecule of This value is higher than that for fulgides with isopropylidene substituents In the five-membered heterocycle of the indoline moiety, similarly to InFl and InF2, considerable redistribution of the bonds is observed; the C(1)=C(2) double bond [1.373(3) Å] is elongated, and the single N–C(l) bond [1.350(4) Å] is shortened. This redistribution of the bond lengths is due to conjugation of the nitrogen-atom LEP with the orbital of the C(1)=C(2) bond. It induces polarization of this bond by increasing the nucleophilic properties of the C(2) atom. The C(10)=C(12) and C(15)=C(16) double bonds are also weakened due to torsion rotation about these bonds. This structure is favorable for the approaching of the reactive C(2) and C(16) centers and cyclization of AdF in the photoexcited state to form the C(l)–C(16) bond. Steric repulsion between bulky substituents of the E-isomer will favor isomerization of the excited molecule. Quantum-chemical calculations show that the order of the C(10)=C(12) and C(15)=C(16) double bonds is decreased in the excited electronic state, and steric repulsion induces an increase of the rotation angles about these bonds. This structure, on the one hand, favors the E–Z isomerization, and, on the other hand, it enables twisting about the C(2)–C(10) bond, the approaching of the reactive centers C(l) and C(16), and formation of the cyclic isomer.

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7.7. CONCLUSION Investigation of the structures of photochromic compounds has allowed structural factors that affect their photochemical properties to be determined. In the molecules of indolinospiropyrans and spirooxazines and of spiropyrans of the 2-oxaindane series, weakening and elongation of the bond, which is broken upon photoexcitation, have been found in the ground electronic state, mainly as a result of interactions. The orbital interactions increase upon molecular transitions into the electronically excited, photochemi-

cally active state. This results in the further weakening and elongation of the bond. The possibility of controlling both the effectiveness of these interactions and the bond length by changing heteroatoms in the spiro center and by varying the position and the nature of substituents in the benzopyran and indoline fragments has been indicated. Based on the structures of the resulting

photoproducts, the means by which association and solvation stabilize the open forms of spiropyrans has been studied. In the molecules of photochromic 2H-chromenes, weakening and elongation of the bond is only due to steric interactions in the The possibility of controlling this bond length in 2H-chromenes by varying the steric effect of substituents has been shown. The derivatives of photochromic fulgides that have been studied have sterically strained structures with substantially elongated and twisted C=C bonds and distorted valence angles. The electronic and steric influence of substituents in the fulgides on the distance between the relaxation centers, the distortion of the valence angles, and the polarization and weakening of the C=C bond and on the photochemical properties of these molecules has been described.

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dimethyl-11-nitrospiro(indoline-2,2'-[2H-l]naphthopyran). Structure of photochromic l',3,3'-

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trimethyl-7-methoxyspiro(indoline 2,2'-[2H]-1 benzopytan), Izv. Akad. Nauk SSSR, Ser. Khim. 1987, 190–192. 7. K. Sh. Karaev and N. G. Furmanova, On crystal and molecular structure of photochromic spiropyran

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and (E)-

Acta Crystallogr., Sect. C 39, 403–407

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48. V. Krongauz, J. Kiwi, and M. Aratzel, Laser photolysis studies of the light-induced formation of

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52. S. M. Aldoshin, A. S. Kholmanskii, and L. O. Atovmyan, Effect of heteroatomic nature in spirocenter of indoline spiropyrans on their structure and photochemical properties, Izv. Akad. Nauk SSSR, Ser. Khim. 1986, 576–582.

53. V. I. Vedeneev, L. V Gurvich, V. N. Kondrat’ev, V. A. Medredev, and E. L. Frankevich, Energiya Razryva Khimicheskikh Svyazei. Potensialy Ionzatsii i Srodstvo k Elektronu [Energy of chemical

bond cleavage. Ionization potentials and electron affinity], Handbook Tzdalel’stva Akad. Nauk SSSR, 1962, 163–168. 54. S. M. Aldoshin, Structural aspects of photochromic transformations of organic compounds,

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first representative of spiro-2H-naphtho(l,8-bc)oxepines, Dokl. Akad. Nauk SSSR 291, 639–641 (1986). 58. S. M. Aldoshin, V. A. Lokshin, A. N. Rezonov, N. V. Volbushko, N. R. Shelepin, M. T. Knyazhanskii, L. O. Atovmyan, and V. T. Minkin, Structure of photo- and thermochromic properties of spiropyrans of 2-oxoindane series with polycondensed chromene fragments, Khim. Geterotsikl. Soedin. 1987, 744–755. 59. L. E. Sutton, Tables of Interatomic Distances: Supplement 1956–1959, Chemical Society, London, (1965). Ada Crystallogr., Sect. A 17, 60. B. Robinson and A. Hargreaves, The crystal structure of 944–946 (1964).

61. K. Sh. Karaev, N. G. Furmanova, and N. V. Belov. The crystal and molecular structure of photochromic spiropyrane Kristallografiya 26, 199–202 (1981). 62. K. Sh. Karaev, N. G. Furmanova, and N. V. Belov. On the crystal and molecular structure of photochromic spiropyran Dokl. Akad. Nauk SSSR 262, 609–611 (1982). 63. K. Sh. Karaev, N. G. Furmanova, and N. V. Belov. On the crystal and molecular structure of spiropyran Dokl. Akad. Nauk SSSR 260, 1121–1124 (1981).

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64. J.-L. Pozzo, A. Samat, R. Guglielmetti, and D. de Keukeleire, Sovatochromic and photochromic

characteristics of new l,3,-dihyddrospiro[2H-indole-2,2'-[2H]-bipyrido[3,2,-f][2,3-h][benzoxazines]. J. Chem. Soc., Perkin Trans. 2 1993, 1327–1332. 65. W. Clegg, N. C. Norman, J. G. Lasch, and W. S. Kwak, Structure of a photochromic benzoxazine derivative, Acta Cryslallogr. Sect. C 43, 804–806 (1987). 66. Y. T. Osano, K. Mitsuhashi, S. Maeda, and T. Matsuzaki. Structures of photochromic spiroindolinobenzoxazines and a spiroindolinobenzopyran, Acta Crystallogr. Sect. C 47, 2137–2141 (1991). 67. R. Millini, G. Del Piero, P. Allegrini, L. Crisci, and V. Malatesta. Structure of photochromic

spiroxazines. I.l,33-Trimethylspiro[indoline-2,3'-[3H]-naphtho[2,l-b] [l,4]oxazine], Acta Crystallogr. Sect. C 47, 2567–2569 (1991). 68. W. Clegg, N. C. Norman, T. Flood, L. Sallans, W. S. Kwak, P. L. Kwiatkowski, and J. G. Lasch, Structures of three photochromic compounds and three non-photochromic derivatives; the effect of methyl substituents, Acta Crystallogr. Sect. C 47, 817–824 (1991). 69. R. Millini, G. D. Piero, P. Allegrini, V. Malatesta, and G. Castaldi, Structure of photochromic

spiroxazines. II. 1',3',3'-Trimethylspiro{anthra[2,1 -f][1,4]-benzoxazine-2-2'-indoline)-7,12-dione, Acta Crystallogr. Sect. C 49, 1205–1210 (1993). 70. P. Lareginie, E. Zaballos-Garcia, V. A. Lokshin, A. Samat, R. Guglielmetti, S. Aldoshin, and O. S. Filipenko, Synthesis of photochromic 6'-sulfanyl substituted spiro[indoline-naphthoxazines], Mendeleev Commun. 1997, 14–16. 71. S. Aldoshin, I. Chuev, A. Utenyshev, V. Lokshin, J. L. Pozzo, G. Pepe, and R. Guglielmetti, A

photochromic pyrido-annulated 2,2-diphenylchromene, Acta Crystallogr. Sect. C 51, 141–143 (1995). 72. J. Crano, D. Knowles, P. Kwiatkowski, T. Flood, R. Ross, L. Chiang, J. Lasch, R. Chadha, and G. Siuzdak, Structure of three novel photochromic compounds; X-ray crystallographic and theoretical studies, Acta Crystallogr. Sect. B 50, 772 (1994). 73. J. P. Reboul, A. Samat, P. Lareginie, V. Lokshin, R. Guglielmetti, and G. Pepe, Structure of a photochromic 8-ethoxy-2-methyl-spiro[syn-5,6-benzo-2-azabicyclo[2,2,2]octane-3,3'-[3H]napht[[2,l-b][l,4]-oxazine] Acta Crysallogr. Sect. C. 51, 1617–1619 (1995).

74. S. M. Aldoshin, Spiropyrans: Structural features and photochemical properties, Mol. Cryst. Liq. Cryst. 236, 207–214(1994). 75. S. M. Aldoshin, I. Chuev, A. Utenyshev, O. Filipenko, J. L. Pozzo, V. Lokshin, and R. Guglielmetti,

Specific structural features and photochemical properties of annulated 2,2-diphenyl[2H]chromenes, Acta Crystallogr. Sect. C 52, 1834–1838 (1996). 76. S. Aldoshin, I. Chuev, O. Filipenko, J. L. Pozzo, V. Lokshin, G. Pepe, and A. Samat, Photochromic

pyridoannulated 2,2,-dimethylchromene, Acta Crystallogr. Sect. C 52, 1537–1539 (1996). 77. S. Aldoshin, I. Chuev, O. Filipenko, J. L. Pozzo, V. Lokshin, and R. Guglielmetti, Structure and

photochromic properties of spiro[fluorene-chromenes], Acta Crystallogr. Sect. C (in press). 78. V. N. Komissarov, V. A. Harlanov, and L. Yu. Ukhin, Thermo- and photoinduced rearrangements of

spirocyclic compounds of 4H-naphthaleneperimidine series, Zh. Org. Khim. 26, 1106–1110 (1990). 79. S. M. Aldoshin, M. A. Novozhilova, L. O. Atovmyan, V. A. Komissarov, V. A. Harlanov, L. Yu. Ukhin, and V. I. Minkin, Synthesis, structure and properties of spirans of permidine series, Izv. Akad. Nauk SSSR, Ser. Khim 1991, 702–708. 80. S. M. Aldoshin, M. A. Novozhilova, L. O. Atovmyan, V. N. Komissarov, L. Yu. Ukhin, and V. A. Harlanov, Synthesis and structure of 2,6-di-tert-butyl-1,4-benzoquinone-4-N-(8-tosylaminonaphthyl-1)imine. Izv. Akad. Nauk. Ser. Khim. 1991, 1802–1808. 81. S. M. Aldoshin, L. O. Atovmyan, N. G. Furmanova, and O. A. Enikeeva. Stereospecific control of

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83. J. C. A. Boeyens, L. Denner, and G. W. Perold, Rotamers and isomers in the fulgide series. Part 2. Stereochemistry and conformational analysis of bis-(p-methoxybenzylidene)succinic anhydrides by X-ray crystallography and molecular mechanics, J. Chem. Soc., Perkin Trans. 2 1988, 1999–2007. 84. J. C. A. Boeyens, L. Denner, and G. W. Perold, Rotamers and isomers in the fulgide series. Part 1.

Stereochemistry and conformational analysis of bis-(3,4-dimethoxybenzylidene)succinic anhydrides by X-ray crystallography and molecular mechanics, J. Chem. Soc., Perkin Trans. 2 1988, 1749–1758. 85. J. C. A. Boeyens, C. C. Alien, and G. W. Perold. Rotamers and isomers in the fulgide series. Part. 3.

Structures of the bis(4-methoxy-3-methylbenzylidene)succinic anhydrides, J. Chem. Soc., Perkin Trans. 2 1993, 1161–1165. 86. H. G. Heller and S. N. Oliver, Photochromic heterocyclic fulgides. Part 1. Rearrangement reactions of (E)- -3-furylethylidene(isopropylidene)succinic anhydride, J. Chem. Soc., Perkin Trans. 1, 1981, 197–202. 87. A. P. Glazo, S. A. Harris, H. G. Heller, W. Johncock, S. N. Oliver, P. J. Strydom, and J. Whittall,

Photochromic heterocyclic fulgides. Part 4. The thermal and photochemical reactions of (E)isopropylidene-[ -(2- and -(3-thienyl)ethylidene]succinic anhydrides and related compounds, J. Chem. Soc., Perkin Trans. 1 1985, 957–961. 88. A. V. Metelitsa, O. T. Lyashik, S. M. Aldoshin, O. A. Kozina, N. V Volbushko, E. A. Medyantseva,

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90. S. M. Aldoshin and colleagues, unpublished results. 91. S. M. Aldoshin, A. N. Utenyshev, A. V. Metelitza, M. I. Knyazhanskii, O. T. Lyatik, Ye. A. Medyantzeva, and V. I. Minkin, Synthesis, structure, spectral and photochemical properties of fulgides of the indoline series with adamantylidene fragment, Russ. Akad. Nauk, Ser. Khim. 1996,

2301–2305.

8

New Trends in Raman Studies of Organic Photochromes Jean Aubard 8.1. INTRODUCTION During the past decade, considerable attention has been paid to the design and synthesis of novel types of organic photochromes which fulfill the requirements of various potential applications1 (optical filters, light energy conversion, data imaging and storage, optical switches, etc.). This recent work has also shown that new or modified photochromic properties are obtained when the compounds are trapped in polymers or gels. Fundamental research on the photochromic process has progressed as well and has been focused on the elucidation of photochemical and photophysical mechanisms and on the identification of transient species involved in the ring-opening/closing reaction. These studies have been mainly performed on the well-known classical series of organic photochromes, i.e., spiropyrans2,3 and, to a lesser extent, spirooxazines.4 To date, only scattered data are available for the other important series5–7 (diarylethenes, fulgides). Thus, for spiro compounds in various solvents, time-resolved techniques using laser flash photolysis associated with absorption and emission measurements have been extensively used to (i) identify the nature of the excited states (singlet and/or triplet), (ii) investigate the mechanism of the photochromic reaction, and (iii) measure the kinetics of the main transients involved in the reaction steps, from the primary ring-opening process (the C–O bond cleavage) to the final colored states (namely, the photomerocyanines). Less is known, however, concerning the structures of these intermediates and those of the different photomerocyanine isomers. Thus, to date, the structures of the isomeric open forms have not been firmly established experimentally. Indeed, Jean Aubard • Institut de Topologie et de Dynamique des Systèmes, Université Denis Diderot, Paris, France. Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999.

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direct spectroscopic detection of the four most probable trans isomers is difficult owing to the close similarity of their absorption spectra. Therefore, hypotheses regarding the geometry of these photomerocyanine isomers and the nature of their electronic distribution have been proposed on the basis of theoretical studies. In this context, vibrational spectroscopies would seem to be valuable since they provide subtle details about molecular structure in the liquid, gas, or solid state. However, owing to the transient character and the small amount of the colored species generated by ultraviolet (UV) light irradiation of spiro compounds, the “basic” vibrational spectroscopies are of very little use. Indeed, infrared (IR) spectroscopy has poor time resolution and low selectivity and is not sensitive enough to detect small amounts of photochromes and their photoproducts. Therefore, IR spectroscopy has barely been used to study photochromic

phenomena. Only a few results, mainly in the solid state, have been reported8. On the other hand, classical (spontaneous) Raman spectroscopy is suited to the study

of fast phenomena but is not a very sensitive technique. This prevents the study of transients in dilute media. Moreover, organic photochromes are sometimes (weakly) fluorescent when excited by visible laser lines, and this definitely precludes obtaining any good spontaneous Raman spectra. Last but not least, unambiguous assignments of the vibrational spectra of photomerocyanines (when available) have been difficult to make in the absence of realistic model compounds. Thus, up to the end of the 1970s, few research groups had tackled photochromism studies of organic compounds using classical vibrational spectroscopies, especially since these techniques brought little information! Owing to technological improvements and recent discoveries, new Raman methods have emerged in the last 15 years. Thus, Raman spectroscopy is today a manifold technique and covers numerous aspects of the study of very fast reactions in condensed phases. These include: Identification and structural characterization of short-lived species, with lifetimes in the picosecond range, or even shorter, in dilute media (ca. by means of time-resolved resonance Raman spectroscopy. Selective laser excitation within electronic transitions of photochromic systems showing fluorescence emission and low photostability, using resonance coherent anti-Stokes Raman scattering (CARS). Trace detection and analysis of species at pico- to femtomolar concentrations using surface-enhanced resonance Raman spectroscopy (SERRS). Therefore, today Raman spectroscopy is able to tackle the majority of the questions that arise in the study of the behavior of spiro compounds under UV irradiation, i.e.:

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What is the electronic configuration of the metastable colored photomerocyanine species? How many isomers are involved in the photochromic process and what are their structure? By which mechanisms are these colored species generated and what is the nature of the photophysical intermediates responsible for the ring-opening process? What is the mechanism of photodegradation and what are the main photoproducts involved in this process? As briefly outlined above, recent improvements in Raman techniques have opened up new possibilities for the study of the structure and dynamics of species involved in the photochromic reaction. Therefore, the purpose of this chapter is not to cover all aspects of vibrational spectroscopy, particularly Raman spectroscopy, but merely to focus on the new research trends in work on spiropyran and spirooxazine compounds. In the first part of this chapter, a brief survey of the most important IR and Raman studies performed before the early 1980s will be given with the aim of

establishing a concise data base for the main representative spiropyrans and spirooxazines. In the second part, studies on various series of photochromes will be reviewed. In particular, results obtained from our laboratory will be discussed in depth. The basic principles of this technique and a description of the instrumentation used by the author will be given first, so that the reader can be aware of “how it works.” The last part of the chapter will be devoted to recent studies in which surface-enhanced Raman spectroscopy (SERS) has been used with the aim of detecting and characterizing small amounts of photodegradation products in solutions and in polymer matrices. Here as well, a summary of the main parameters involved in this technique will be given prior to the discussion of the SERS results on spiropyran and spirooxazine derivatives obtained recently in our laboratory.

8.2. SURVEY OF VIBRATIONAL STUDIES ON SPIROPYRAN AND SPIROOXAZINE PHOTOCHROMES In the early seventies, Arnold and Paal published a detailed IR study on a series of 16 photochromic compounds.8 In this paper the authors reported a comparative assignment of the main vibrations of indolino- and benzothiazolinospiropyrans along with the corresponding spirooxazines. IR data from benzoxazolinospiropyrans were also included for comparison. Thus, this study provided an interesting data base which includes the vibrational characteristics of the two popular parent spiro compounds (since they have been used so often in various

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applications!), the indolinospironaphthopyrans (SPP) an the corresponding indo-

linospironaphthoxazines (SPOX) (Scheme 1).

The IR frequencies and their assignments, proposed by Arnold and Paal, for these two particular photochromes, which have a key importance in the present review, are reported in Table 1. Referring to this table, the following comments can be made. (i) The assignments of the IR bonds between 2900 and to v(C–H) and of those between ca. 1480 and to v(ring) of either the indoline or the naphthalene moiety are unambiguous. (ii) In the spectral region below ca. particularly between 1200 and (which is a very important region since it concerns vibrations related to the C–O pyranic bond), some of the assignments proposed by Arnold and Paal seem to be questionable.9 (iii) The assignment of bands at 934 and to is unlikely since other authors have shown that these bands arise from in-plane deformation of the indolinic ring.10 It should be noted that the assignment of the IR band(s) at ca. to vibration(s) involving the bond was first proposed by Arnold in an extensive IR study of a series of spirobibenzopyran derivatives.11Arnold used this band, found in the region (depending on the compound), as a criterion for the pyranic structure of the derivatives studied. However, DavinPretelli et al. in a study of the IR and Raman spectra of unsubstituted bibenzopyrans, gave unambiguous assignments of the bands in this region and attributed them to vibrations involving mainly ring deformations and In view of the difficulties encountered in direct interpretation of the vibrational spectra of spiropyrans, Davin-Pretelli et al.10 proposed a more rational approach. They investigated first the IR and Raman spectra of the two constituent moieties of the photochrome under study. Thus, from a preliminary IR and Raman study of benzothiazolines and 2H-chromenes, which served as models, they were able to give a satisfactory assignment of the spectra of a series of benzothiazolinic spiropyrans.9 Moreover, on the basis of the vibrational data for 2H-chromenes, the set of bands at ca. was unambiguously assigned to vibrations involving

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the stretching of the bond, in agreement with Arnold and Paal.8 Since this bond has a central importance in the ring-opening process, one would expect that the associated vibrations would be very sensitive to the electronic perturbations induced in going from spiropyrans (closed forms) to photomerocyanines (open forms). Attempts to detect the main structural changes occurring in the ring-opening/ closing process in spiropyrans, using IR vibrational spectroscopy, were reported by Schiele and Arnold for a series of indolinonitrospiropyrans12 and later by

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Guiliano et al. for benzothiazolinonitrospiropyrans9 (Spiro., Scheme 2). In the latter study, the photochromic conversion was performed by UV-irradiating the spiropyrans, dispersed in a solid matrix or dissolved in carbon disulfide leading to thermodynamically stable colored species. The IR spectra of the colored states thus obtained were compared to those of a stable benzothiazoline photomerocyanine in the solid state and a permanent merocyanine (R=H), (Mero., Scheme 2). From a detailed analysis of the IR spectra, it appeared that two

spectral regions, namely, 1700–1500 and are of particular interest for the opening of spiropyrans. Briefly, on going from the colorless

spiropyrans to the colored photomerocyanines, the following observations were made.9 (i) A substantial shift towards lower wavenumbers was observed for the set of bands assigned in spirocompounds to v(C=C) modes of heterocyclic and benzopyranic rings. (ii) The location of the strong IR band at ca. assigned to was not affected by UV irradiation. (iii) Conversely, the band of medium intensity at ca. in the spectra of spiropyrans, assigned to the symmetric stretching vibration of the group, was shifted to lower wavenumbers in the spectra of the open form and is situated at ca. (iv) A shift toward higher wavenumbers of the bands assigned to v(CO) was observed; this probably indicates that vibrational modes involving are replaced in the open form by vibrational modes associated with a delocalized bond. All these observations and assignments, proposed by Guiliano et al.9 are presented in Table 2. It is clear from these results that the opening of nitrospiro-pyrans leads to extensive delocalization of the ethylene double bond along with an evident induced polarity of the C–O bond.

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Quite recently, on studying the Raman spectra of indolinonitrospiropyrans in powders, we obtained evidence for the opening of certain of these compounds under laser irradiation at probably through a thermochromic reaction induced by absorption of the laser light.13 Indeed, as shown in Figure la, a detailed analysis of the Raman spectrum of compound 1 (Scheme 3), indicated that some lines arise from the presence of a (small) amount of the open form. Thus, in addition to the main Raman lines observed at ca. 1640, 1580, 1480, 1360, 1330, and which are typical of nitrospiropyrans (see Table 2), we found additional weak lines at ca. 1610, 1390, 1285, and and a medium shoulder at ca. which may be assigned to the open form9,12 (see discussion above and Table 2). In order to confirm this hypothesis, we have recorded the Raman spectrum of the parent compound 2 (Scheme 3), which showed no coloration under laser excitation at 488 nm (Figure 1b). In this

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spectrum, only the main lines typical of the closed form for compounds in this series, are observed. All the proposed vibrational assignments of the Raman lines, for both the closed and open forms, are indicated on Figure 1. It should be noted that these assignments not only corroborate the previous ones9,12 but also clarify certain points. Thus, for the nitro derivatives in the open form, there is no doubt that the Raman line at ca. can be readily assigned to while those at ca. 1285 and are probably associated with

In the first part of this chapter, we have summarized the main vibrational characteristics of various spiropyrans and some spirooxazines. These data were based on previous work performed by using mainly IR absorption spectroscopy; some of the data were based on the very few results obtained by using Raman spectroscopy. Emphasis was placed on the vibrational data concerning the main spectral changes observed on going from the closed to the open forms of nitrospiropyran derivatives. Thus, we have been able to define some of the assignments in the literature8–12 by studying the Raman spectra of neat indolinonitrospiropyran derivatives.13 Indeed, for certain of these compounds, ring opening occurred when the samples were excited by the 488-nm laser line. In this way, it was possible to record and analyse the Raman spectra of both the closed and open forms in the solid state. All these spectral data are consistent with a delocalized -electron system for the merocyanine-like open form. In particular, for the thoroughly studied nitrosubstituted derivatives, it appears that the closed forms convert, under UV light irradiation, to highly conjugated open forms. In these structures, the electrons are principally delocalized along the dimethinic chain, with probably an involvement of the nitro substituent, in accordance with a zwitterionic form like that shown in Scheme 2 for the benzothiazoline photomerocyanine (Mero.). In the next part of this chapter, we will consider vibrational studies of organic photochromes in dilute solutions using time-resolved Raman techniques.

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8.3. DYNAMICS AND STRUCTURE OF TRANSIENT SPECIES INVOLVED IN THE PHOTOCHROMIC PROCESS OF SPIRO COMPOUNDS AS PROBED BY TIME-RESOLVED RAMAN TECHNIQUES The first report of time-resolved resonance Raman spectra, excited by a 532-nm, 30-ns laser pulse, of dilute solutions of nitrobenzoxazolinic spiropyrans

in toluene or benzene (ca. irradiated by a UV flash, was due to Aubard et al.14,15 In this study, because of the poor resolution of the UV flash (duration of ca. 1 ms), only the vibrational characteristics of the metastable photomerocyanine (lifetime of ca. 800ms) were obtained. Surprisingly, the main structural features observed for nitrospiropyran derivatives in the solid state9,13 were also observed in these experiments performed in nonpolar solvents. Indeed, the Raman spectra of the open forms in these solutions were consistent with a planar delocalized zwitterionic structure (see above and discussion in

Section 8.3.2). This electronic configuration, with the presence of partially charge atoms and in the photomerocyanines, was also confirmed at the same time by X-ray photoelectron spectroscopy.16 Following this study, the transient Raman spectra of 6-nitro-BIPS in various solvents, obtained with nanosecond UV irradiation and Raman excitation, were published by Takeda et al.17 These Raman spectra were interpreted as arising from a mixture of different photomerocyanine

isomers. Later, Schneider and co-workers18 reported nanosecond resonance coherent anti-Stokes Raman scattering (CARS) experiments on various spirooxazines. On the basis of the results of this study, the authors suggested the existence of several different isomers in thermal equilibrium that are formed in different solvents. Extensive investigations on spiropyrans (pyrrolidinic, indolinic), particularly those having a nitro group at the 6-position, were also reported by Aubard and co-workers. 19–21 Thus for 8-methoxy-6-nitro-BIPS (1, Scheme 3), from the detailed analysis of Raman spectra, obtained using nanosecond lasers for both UV irradiation (351 nm) and Raman excitation (532 nm), it was concluded20 that (i) the photomerocyanine (M) was already present at the end of the UV laser pulse (i.e., at (ii) this species (M) presented an almost planar structure with extensive -electron delocalization involving the group and a predominantly zwitterionic character, and (iii) the ring-opening process proceeded partly via a triplet intermediate the Raman characteristics of which revealed a nonplanar cisoid structure. Owing to the poor spectral resolution, no conclusion concerning the distribution of isomers was drawn. Following these first investigations, important and more detailed work, using time-resolved Raman techniques (mainly resonance Raman), has been reported since the end of the 1980s. However, before discussing these more recent results, we will review the main features of time-resolved resonance Raman spectroscopy by describing the experimental setup and the operating conditions we used

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recently in order to obtain good-quality Raman spectra of short-lived species in dilute media.

8.3.1. Instrumentation The time-resolved Raman detection of phototransients is performed by a pump–probe technique using two pulsed sources of light. The pump source, producing the phototransients (excited states, merocyanine isomers), is generally a UV pulsed laser. The probe source is a second laser excitation, tuned within an electronic absorption band of the transient species. In this case, the resonance Raman effect leads to a substantial enhancement of the scattered intensities. The probe pulse must be delayed in time with respect to the pump pulse, the delay corresponding to the kinetics of formation (or disappearance) of transients. The scattered light is dispersed in a spectrograph and detected by a gated intensified diode array, synchronized with the probe pulse. The experimental setup that we have used in recent studies of photochromic spiro compounds was developed in the Laboratoire de Spectrochimie Infrarouge et Raman22 (LASIR, Lille University, France) and is shown in Figure 2. It includes an excimer laser (Questek 2040) as the pump source (excitation conditions: 351nm, l–2mJ, 20ns) and a 10-Hz Q-switched Nd:YAG laser (Quantel 581C) coupled to a dye laser (Quantel TDL 50) as the probe source (excitation conditions: 340–640 nm, l–2mJ, 7–9 ns). The multichannel Raman spectrometer consists of a single monochromator equipped with a gated 20 ns full width at half-maximum (FWHM) intensified photodiode array detector. The spectral resolution at 625 nm, for example, is about and the corresponding width of the analyzed spectral field is The key feature of any time-resolved optical system is the synchronization module which monitors the delays between the pump pulse, the probe pulse, and the detector gate. In the apparatus described here, this has been achieved on the basis of a 100-MHz clock generator, which allows the time delay between the excimer and Nd:YAG pulses to be adjusted between 0 and 10 ms with a 10-ns step.23 The pump and probe beams are focused onto 1-cm quartz cells using the same lens. The Raman-scattered light is collected at 90° and imaged onto the entrance slit of the spectrometer. The spectra presented later in this chapter (see Section 8.3.3) have been obtained by substracting a spectrum recorded with the probe beam only (thus giving the solvent lines) from a second spectrum recorded with both the pump and probe beams (Figure 3). Solvent lines are thus eliminated. Spectra taken at different pump–probe delays are normalized with respect to the solvent bands before substraction.

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8.3.2. Survey of Recent Time-Resolved Raman Studies of Spiro Compounds Following their first experiments devoted to 6-nitro-BIPS,17 Takahashi and co-workers conducted a series of nanosecond studies24–28 on a limited number of photochromic compounds, namely l´,3´,3´-trimethylspiro[2H-l-benzopyran-2,2´-indoline] (BIPS), 6-nitro-BIPS, and spirobibenzopyran (SBP) (Scheme 4). This work was undertaken to elucidate the electronic distribution of the metastable photomerocyanines (ortho-quinoidal or zwitterionic) and the number of stereoisomers involved in the photochromic process. Therefore, the effects of temperature, solvent, laser excitation wavelengths, and isotopic substitutions and on the resonance Raman spectra were intensively studied to clarify the photochromic mechanism in the compounds investigated. The main conclusions drawn by the authors can be summarized as follows:

(a) For BIPS, the Raman spectra in various solvent arose form the mixture of four photomerocyanine trans isomers having TTC (trans–trans–cis), TTT, CTT, and CTC configurations with respect to the three C–C partial

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double bonds of the merocyanine-like skeleton. The relative abundance of these isomers was dependent on the polarity of the solvent. The electronic distribution of the photomerocyanines may be viewed as a resonance hybrid between the quinoidal and zwitterionic forms. The former structure is favored in all solvents except those which are hydrogen-bond donors (e.g., methanol), in which the zwitterionic structure is dominant.24 (b) For 6-nitro-BIPS, the ring-opening process involves a triplet state (identified as the state of the photomerocyanine), in agreement with both earlier and recent studies on 6-nitro-substituted spiro compounds.3 With regard to the isomeric configuration and electronic distribution in the open forms, the conclusions drawn were the same as those for BIPS (vide supra). However, contrary to previous assumptions13–15,21 it was suggested that the group would not be involved in the -electron delocalization.27 Lastly, detection of a long-lived colored transient generated from intense UV laser irradiation of 6-nitro-BIPS was interpreted as arising from the formation of merocyanine dimers or aggregates.27,29 (c) For SBP, studies revealed that at least two isomers exist in solution, their relative abundances depending on the nature of the solvent. Molecular orbital calculations indicate in this case that the TTT isomer would be the most stable and that an ortho-quinoidal structure should be favored.28 As already noted, Schneider and co-workers were the first to report nanosecond time-resolved Raman spectra of spirooxazine derivatives (including the spironaphthoxazine), using resonance CARS.18 Two years later, the same group

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published the first of a series of papers devoted to the investigation of the photochromic properties of compounds in this class, using numerous timeresolved spectroscopic techniques (including resonance CARS and Raman).30–32 On the basis of the results of picosecond transient absorption spectroscopic experiments, these authors postulated that in the photochromic process of spirooxazines the primary ring-opening step occurred within less than 2 ps and proceeded through the excited state of a nonplanar twisted cisoid intermediate. This short-lived species rapidly decayed, and rearrangements led to a mixture of photomerocyanine isomers; these rearrangements were complete on the nanosecond time scale. Time-resolved CARS and Raman spectroscopy were used in attempts to detect how many isomers were involved and which electronic structure they adopted. Although CARS measurements are directly related to the structure of the isomers in the mixture, difficulties arise in the assignment of CARS lines (due to their complex line shapes) to specific vibrational modes. However, this technique has the advantage that it is insensitive to any fluorescence emission, whether from the solvent or from the irradiated photochrome. From an expert analysis of CARS spectra, Schneider30 suggested that at least two different isomers (formed within 20 ns) were present in solution; their relative abundances

depending on the polarity of the solvent. Aramaki and Atkinson recently reported results from picosecond timeresolved experiments (Raman and absorption) for spironaphthoxazines33 and for a spironaphthopyran.34 In the study concerning spirooxazines, in contrast to the results of Schneider30 neither transient absorption nor spectra, recorded as a function of time (with a 50-ps interval), gave evidence for the formation of a cisoid intermediate during the opening of the spiroxazines.33 In agreement with previous nanosecond resonance CARS experiments,18,30 picosecond spectra showed a strong solvent dependence, which could be explained by the existence of different merocyanine isomers. Therefore, these results suggest that, in the photochromism of spironaphthoxazines and spironaphthopyrans, the photomerocyanine isomers (or distribution of isomers) are formed in less than 50 ps from the first excited singlet state of the spiro compound. In fact, photochromic reactions in the singlet manifold have been shown recently to occur on the subpicosecond time scale.35 It should be noted that in these studies, despite the high quality of the reported Raman spectra (particularly for spirooxazines), no assumption was made concerning the number of isomers and their structure. Indeed, as stated by the authors, none of the transient absorption or Raman data reported until now (including their own) provide clear evidence as to whether one or more isomers are present or allow their structure to be established. This could be the (pessimistic) conclusion of this section. Indeed, despite the increasing number of vibrational studies devoted to the most important series of organic photochromes, specific vibrational characteristics of the four different trans isomers expected remain to be established experimentally.

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In the last part of this section, we will report the results of recent experiments performed on spironaphthoxazines in various solvents. Comparison of the nanosecond Raman spectra with the resonance Raman spectra of a permanent open form in the spirooxazine series, the structure of which has been firmly established, have led for the first time to spectroscopic evidence for the identification of the isomeric structure of transient photomerocyanines in this series. 8.3.3. Time-Resolved Resonance Raman Spectroscopy of Indolinospironaphthoxazines: Experimental Evidence for a TTC Resonant transoid; Open Form in Different Solvents Following investigations of indolino spiropyrans and spirooxazines in various solvents,36 using nanosecond time-resolved absorption spectroscopy, the

spectra of these compounds in dilute solutions have been recorded at different time delays and at various laser excitation wavelengths.37 Since substantial fluorescence backgrounds are observed for the spiropyran derivatives and preclude the acquisition of any good resonance Raman spectra (especially in polar solvents), this study was focused on the investigation of the structural characteristics of open forms of spirooxazines. In this series, transient Raman spectra with high signal-to-noise ratios and good spectral resolution were recorded, thanks to an improved spectrometer with very sensitive detection in the red part of the spectrum (see Section 8.3.1). However, despite these improvements and the various experimental conditions used solvents, time delay, etc.), it was difficult to assign these Raman spectra to specific photomerocyanine isomers, and only hypotheses were proposed.37 These hypotheses were based on

recent theoretical studies, which gave TTC and CTC as the preferred stereoisomers38 (see Scheme 5). Recently, Guglielmetti and co-workers reported the synthesis and structural elucidation of the first permanent open forms described in the spiro[indolineoxazine] series.39 From nuclear magnetic resonance (NMR), UV spectroscopy and X-ray crystallography, it was unambiguously proven that both in the solid state and in various solvents these open forms have a quinoidal electronic configuration and a TTC (trans–trans–cis) isomeric structure with respect to the central (C–C–N–C) link39 (Scheme 6). The synthesis of these permanent open forms provided a unique opportunity to compare their Raman spectra to those of metastable photomerocyanines generated by UV irradiation of indolinospironaphthoxazine photochromes, which are expected to possess similar structural features. However, owing to their huge visible absorption in the 500–600-nm range (in the solid state and in solution) and the intense fluorescence emission when the usual excitation laser lines were used (i.e., 488 nm or 514.5 nm), it was hardly possible under these conditions to record the resonance Raman spectra of the permanent open forms. Aubard et al.40 used instead a newly developed

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-Raman spectrometer (DILOR, Labram system) with low-power

laser excitation in the red (He–Ne, 632.8 nm, or 647.2 nm). These authors reported, for the first time, the vibrational characteristics of a TTC isomer and a comparison with the transient Raman spectra of the indolinospironaphthoxazine compound obtained in the nanosecond, microsecond, and millisecond time ranges in the same solvents.40 The main results and conclusions of this study are summarized below. Figure 4 shows the spectra of the spirooxazine compound (SPOX) in various solvents (ca. excited at 600 nm and taken 10 ns after the 351-nm irradiation. It should be noted that these spectra resemble those recorded on the picosecond-nanosecond time scale by Aramaki and Atkinson33 but are of higher

quality and show more detailed spectral features. On going from dioxane to ethanol, large relative intensity changes were observed, whereas the line positions

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were only weakly (or not) affected. Thus, it appears that the lines at ca. 1555, 1450, 1425, and the triplet between 1150 and and the lines at ca. 1025 and were very sensitive to the nature of the solvent. The spectra were also recorded as a function of the pump–probe time delay37 (see Section 8.3.1). The spectra of SPOX in toluene presented in Figure 5 were obtained at 10 ns, and 1 ms. It can be clearly seen that the Raman spectra displayed no significant change over the whole time range. Moreover, these spectra were unaffected by the presence of oxygen in the solutions.37 Aubard et al.40 also reported the intensity variations of the spectra with the probe laser wavelength, the so-called excitation profiles. Figure 6 shows the intensity variations with the excitation wavelength (between 560 and 620 nm) for the line at ca. measured in two solvents of very different polarity (acetonitrile and dioxane). It should be noted that the excitation profiles displayed a clear red shift on going from dioxane to acetonitrile (shift of from 580 nm to 615 nm). This is in agreement with the positive solvatochromism of SPOX, which has been established previously from the analysis of transient absorption spectra, and corresponds to a quinoidal electronic distribution.36

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All these results showed that, in the photochromism of SPOX, (i) the ringopening process does not involve a triplet state, (ii) the observed spectra arise

from the open form(s), i.e., the metastable photomerocyanine species, and (iii) this species is already present at the end of the UV (pump) laser pulse (i.e., after 10 ns). It should be noted that the Raman spectra recorded over the 10 ns–10 ms interval by Aubard et al.40 resemble those obtained within a shorter time range (50 ps–1.5 ns) by Aramaki and Atkinson.33 This indicates that the photomerocyanine(s) is formed from the first excited singlet state of the spiro compound and probably in less than 1 ps.4,35 The strong solvent dependence of these Raman spectra (Figure 4) and their excitation profile characteristics (Figure 6) were previously interpreted in terms of the formation of two different photomerocyanine isomers having a dominant quinoidal electronic distribution.37 Even though recently theoretical studies have shown that the TTC and CTC isomers are probably the most favored,38 a direct vibrational analysis of the spectra in terms of these two isomers endowed with a quinoidal character is very tricky.

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In order to overcome these difficulties, Aubard et al.40 compared the spectra of SPOX to the resonance Raman (RR) spectra of a permanent open form in the spiro[indoline-oxazine] series, recently synthesized by Guglielmetti and coworkers39 (Scheme 6). The RR spectrum of this permanent open form in dioxane together with the nanosecond spectrum, in the same solvent, of a 351-nm UV-irradiated SPOX solution are shown in Figure 7. It should be noted that the Raman spectrum of the permanent merocyanine (Figure 7a) gives for the first time the vibrational characteristics of an open form in the spirooxazine series with a quinoidal electronic distribution and a TTC isomeric structure. As mentioned above, this spectrum was obtained by using a -Raman spectrometer and very low laser power excitation at 632.8 nm; in this way, the strong fluorescence background of the permanent open form was minimized.40 From a careful examination of spectra a and b in Figure 7, it is clear that they show almost a 1:1 correspondence. Moreover, this comparison reveals not only similarities in line positions but, interestingly, strong similarities in relative intensity distribution (Figure 7). This suggests that the spectrum of SPOX (Figure 7b) arises from a metastable photomerocyanine species with structural features and an electronic distribution very similar to those of the permanent open form. Moreover, in these RR experiments,40 the laser excitation

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wavelengths (ca. 550–650 nm) are located within the broad visible absorption band typical of the delocalized -electron system in photomerocyanines, that is, the central azomethinic chain. Under these conditions, the strongest observed RR lines draw their intensity from in-plane vibrations (v and ) involving C=C, C–N, and C=N groups of the central link. On the other hand, the ring vibrations of both the indoline and naphthalene moieties, which appear in the region (see Table 1), are probably less enhanced. Therefore, the strong similarities between the and RR spectra in Figure 7 clearly indicate that in both open merocyanine forms, the RR-active species have the same isomeric structure with respect to the central chain. AH these observations strongly indicate that in spironaphthoxazine photochromes the open forms have a quinoidal electronic distribution and a dominant TTC isomeric structure. Thus, from these experiments, it seems that photo-

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chromism of SPOX involves essentially the TTC photomerocyanine isomer as the predominant colored open form. However, further experiments are necessary to assess whether this conclusion might be considered as a general rule for the whole family of spiro compound photochromes.

8.4.

SURFACE-ENHANCED RAMAN SPECTROSCOPY OF SPIRO COMPOUNDS: A ROUTE FOR DIRECT DETECTION AND IDENTIFICATION OF THEIR PHOTODEGRADATION PRODUCTS

One of the major factors limiting the industrial applications of organic photochromes is their progressive degradation, with loss of their photochromism, after prolonged exposure to repeated light–dark cycles. Thus, the study of the mechanisms of photoproducts involved in the photodegradation is very important,

within the framework of a strategy aimed at obtaining more resistant photochromic compounds. Few studies addressing these questions have been performed and it was only very recently that qualitative aspects of the degradation of spirooxazines and spiropyrans in solution41–43 and in polymer matrices44 were reported. Recent experiments using specific spectroscopic detection of photoproducts to follow the kinetics of the spiro compound degradation have also been reported45–47 and have shown the possibilities of this approach. In particular, a new method, using SERS in silver colloids, has been developed to detect and identify spiropyran photochromes and their main photodegradation products.46–47 Since most of the available data concerning SERS of photochromes and photoproducts were obtained in our laboratory, the last section of this chapter is devoted to a brief description of this work. These data illustrate the ability of Ag-colloid SERS to detect small amounts of organic photochromes and photoproducts and to provide structural information on these species.

8.4.1. SERS Detction of Spirooxazines and Spiropyrans at Trace Levels in Silver Colloidal Solutions The first goal of these studies was to evaluate the feasibility of employing SERS for the detection of organic photochromes at very low concentration48 (ca. However, before discussing these results, we will summarize the main characteristics of the SERS method in order to clarify its potentialities and limitations. 8.4.1.1. Brief Description of the SERS Effect When organic molecules are close to (within a few Å) or adsorbed on rough surfaces of certain metals, such as silver, copper, or gold, a very large enhancement of their Raman spectra (by a factor of ca. may be observed.

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Although a great variety of surfaces have been shown to manifest a SERS effect, the most commonly used are working electrodes and metal colloids. Two mechanisms are considered to account for SERS enhancement. One is called an electromagnetic (EM) mechanism because it arises from plasmon excitation at the roughened metal surface. The amplitude of this EM effect depends largely on the shape and the size of the metal microstructures, which must be less than optical wavelengths. Aggregated silver or gold colloids satisfy this requirement, and EM enhancement factors of ca. have been reported.49 The other SERS enhancement mechanism, often referred to as the chemical (or molecular) effect, is related to the increase of the polarizability of molecules when they interact with the metal surface. The molecule–metal interaction, via the formation of a charge-transfer complex, often leads to the observation of striking differences between SERS and spontaneous (ordinary) Raman spectra. Nevertheless, this mechanism is believed to make a smaller contribution than the EM mechanism, and enhancements by factors in the range of 10–100, depending on the chemical nature of the adsorbates, have been estimated.49 However, the overall enhancement factor generally results from a contribution of both mechanisms, EM and charge transfer, leading to 6–8 orders of magnitude enhancement in favorable cases. In view of its relation to the resonance frequency of surface plasmons, on the one hand, and to the nature of the adsorbates, on the other hand, it is clear that the intensity of the SERS effect depends on various parameters. Thus, the state of the metal surface, i.e., the surface topography or the particle morphology in the case of metal colloids, and the laser excitation wavelength50 are of prime importance in the enhancement of the local electric field. The electronic configuration of the molecule under study and the nature and the concentration of the surrounding electrolytic salts also play a decisive role in the SERS detectivity and specificity.51 Finally, the combination of surface and resonance enhancement (SERRS) can occur when adsorbates have intense electronic absorption bands in the same spectral region as the metal surface plasmon resonances, yielding an overall enhancement factor as large as This situation is met in the case of the open forms of the spiro compounds studied in our work. Indeed, the absorption bands or open-form species are located in the 550–650-nm range, which corresponds to the wavelengths of the longitudinal surface plasmon resonance excitations. In this case, the SERRS effects leads to a high detectivity of photomerocyanines when Raman spectra are excited with either the green line (514.5 nm) of an laser or the red line (632.8 nm) of a He–Ne laser. Thus, the sensitivity of SERS far exceeds that of any other technique which is capable of providing structural information. A further advantage is that the intense luminescence background, that sometimes results from laser excitation under resonance conditions (and obscures Raman scattering) is quenched in SER(R)S. This is because the interaction of the molecule with the metal surface provides an efficient pathway for rapid nonradiative decay.

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The main features of the SER(R)S effect can thus be summarized as follows:

(i) SER(R)S is a highly sensitive method. (ii) It is rather selective since it allows the detection of small organic molecules even in the presence of complex macromolecules (polymers, biological molecules, etc.). (iii) Specific detection of different species in a mixture may be achieved when particular salts are added to the silver or gold colloids. (iv) The spectral features are related not only to the structure of the adsorbed species but also to their orientation with respect to the metal surface. As a consequence, SERS spectra are generally different from ordinary Raman spectra. (v) All luminescence emission is efficiently quenched, and this permits the study of highly fluorescent molecules such as functional dyes. (vi) Last but not least, SERS experiments need metal substrates that have been “activated,” such as roughened electrodes or aggregated colloids. In some cases, it has been shown that the presence of such metal surfaces may perturb the process under investigation.52 The possibility of specific detection of traces of organic molecules (ca. even in complex media, has opened up new perspectives in analytical chemistry and biochemistry in the last few years. We have recently developed a strategy, using Ag-colloid SERS in biological media, for ex situ and in situ detection and characterization of drug metabolites at trace levels.52 Since this method is general, it could be applicable to the detection and identification of photochromes and their photoproducts involved in the ring-opening process and in photodegradation. We report hereafter the main results and conclusions of some recent studies.46–48 8.4.1.2. Analysis of SERS Spectra of Some Spiro Compounds In order to demonstrate the usefulness of the method, SERS spectra of indolino-spironaphthopyran (SPP, 4) and indolino-spironaphthoxazine (SPOX, 3) (see Scheme 7) were recorded at low concentration and their spectral characteristics analysed.48 Figure 8 shows the SERS spectrum of 4 in Ag colloids (spectrum a) along with the corresponding near-infrared Fourier transform (NIR-FT) Raman spectrum of the neat compound (spectrum b). Strong differences between the SERS and spontaneous Raman spectra are apparent. As stated above, these differences arise mainly from the particular orientation of the adsorbed spiropyran with to the metal surface in SERS experiments. Therefore, in order to analyze the SERS spectra of these spiro compounds, model molecules of, respectively, the indolinic (5) and chromene (6) moieties have been synthesized (see Scheme 7) and their Raman spectra (spontaneous and

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SERS) recorded. From the comparison of the SERS spectra of the photochromes with those of the indoline and chromene model molecules (Figure 9), it was established that the SERS spectra of SPP (4) and SPOX (3) are mainly dominated by the vibrational characteristics of the indolinic moiety. Moreover, the detailed assignment of the SERS data, performed on the basis of the vibrational analysis of the NIR-FT Raman spectra of the pure substances, has enabled deduction of the geometry of the adsorbed species,48 shown in Figure 10.

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8.4.2. Detection by SERS of Degradation Products from UV-Irradiated Solutions of 8-Methoxy-6-Nitro-BIPS We noticed, at the beginning of this section that qualitative data on the degradation of spirooxazines and spiropyrans, in solutions and in polymer matrices have only recently been reported.41–44 In particular, the pioneering work of Baillet and Guglielmetti has established that degradation of spiro compounds is mainly a photooxidation process involving radical species or singlet oxygen. These conclusions were supported by analytical identification of the

different photoproducts generated after continuous UV irradiation of photochromic molecules (spiropyrans and spirooxazines) in polar and apolar solutions41,43 and in polyurethane films.44 On the basis of these newly available data, it seemed interesting to test the capability of SERS to detect and identify the main photoproducts involved in spiropyran photoaging. For this purpose 8-methoxy6-nitro-BIPS (7 in Scheme 8) was chosen because it is a highly colorable photochrome that degrades quite rapidly.53

8.4.2.1. Assignments of SERS Spectra of 8-Methoxy-6-Nitro-BIPS SERS analyses and assignments were performed on the basis of Raman data (spontaneous and SERS) for various model molecules which mimicked, respectively, the indolinic and benzopyran moieties of the photochrome47 (8–10) and fragments expected in the degradation process41 (11–13) (Scheme 8). The SERS spectrum of photochrome 7 in acetonitrile, in the absence of any UV light, is shown in Figure 11. After a few seconds of UV irradiation, leading to a deep-bluecolored solution, the recorded SERS spectra were identical to that presented in Figure 11 but were much more intense. This indicated that the SERS spectrum of 7 arises, at least in part, from open merocyanine species. Numerous other arguments were proposed in favor of this conclusion.47 Indeed, from an extensive

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comparison between the SERS spectrum of 7 and the spectra of the parent compound, BIPS, and, of the models 8–10, it was possible to identify the species giving rise to the observed photochrome SERS spectrum and to characterize its structure.47 In particular, as shown in Figure 12, comparison of the SERS spectrum of the photochrome (spectrum a) with that of the model 8 (tetramethylindoleninium iodide) (spectrum b) reveals strong similarities in the line positions and in the intensity distribution between the two spectra. This suggested that the SERS

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spectrum of 7 arises mainly from a species whose structural features and electronic distribution are very similar to those of the indoleninium salt 8. These observations and others (see Ref. 47) have led to the conclusion that the SERS spectra of 8-methoxy-6-nitro-BIPS are dominated by the vibrational characteristics of the open form(s) [photomerocyanine(s)], for which the electronic distribution is zwitterionic, in agreement with the proposed configuration for nitro derivatives (vide supra). These results may be compared with those of recent SERS experiments performed by Schneider et al.,54 who postulated that SERS spectra of nitro-BIPS derivatives in methanolic solutions originated from neutral open merocyanine species. The presence of open forms in solutions of 7, in the absence of any UV light, probably arises from solvatochromism, which is an efficient process in the opening of nitro-substituted spiro compounds in polar solvents.55 Moreover, as discussed above, the detection of photomerocyanines, even at trace levels, is favored with respect to that of the nonresonant closed form of the photochromes by a SERRS effect excited at 5l4.5nm. It should be noted that in the case of cyanine dyes, SERRS spectra, in Ag colloids, from M solutions have been recently reported.56

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8.4.2.2. SERS Analysis of UV-Irradiated Solutions of Photochromes In these experiments, SERS spectra were obtained by taking small aliquots at different time intervals from a continuously UV-irradiated solution. The aliquots were immediately deposited in silver colloids.47 The SERS spectrum of the nonirradiated photochrome 7 along with the spectra recorded after l0 min and 4h of intense UV photolysis in acetonitrile, using a focused 150-W high-pressure mercury lamp, are shown in Figure 13. Comparison between spectra 1,2, and 3 in Figure 13 shows that UV irradiation led to substantial spectral changes (changes in intensities and in line positions). Indeed, under these experimental conditions, after 4 h of extensive UV degradation in acetonitrile, it was observed that the photochrome had totally lost its photochromic properties. The yellow-colored solution thus obtained did not develop further. The SERS spectrum of this totally degraded solution (spectrum 3) was then expected to give structural information on the main final products involved in the degradation process. Because direct vibrational analysis of these SERS spectra was very tricky, their assignments were obtained by comparison with the spectra of the synthetic reference compounds (11–13). Indeed, it has been shown that degradation of photochrome 7 in solution is mainly a photooxidation

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process, which would lead to a mixture of tri- and dimethyloxindoles (11 and 12) and 5-nitrovanillin (13). As can be seen in Figure 13, the SERS spectrum of the totally degraded acetonitrile solution of 7, obtained in the presence of (spectrum 3), is very similar to the SERS spectrum of the model compound 13 (spectrum 4). On the other hand, the SERS spectrum of the same degraded solution but obtained upon addition of chlorides (spectrum 5) reveals the presence of only the closed form of the photochrome.47 Thus, depending on the nature of the anions added to the Ag colloids, specific detection of either the remaining photochrome (addition of or the photoproducts (addition of is achieved. However, quantitative measurements in the absence of an internal Raman standard are difficult. Only rough estimates of the amounts of 5-nitrovanillin present and photochrome remaining in the various degraded solutions can be obtained. By

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using specific Raman lines of these for the photochrome 7 and for the model compound 13—it has been possible to obtain estimates of their distribution in various solvents47 (Figure 14). The development of this method is based on the specificity of the SERS detection which is one of the key features of the SERS technique.51 We are currently using the method to study degradation and identify the photoproducts for other photochromic families (e.g., spirooxazines, 2H-chromenes) in solution and in other media such as thin polymeric films or sol gel matrices.

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Acknowledgments I wish to thank here all those who have contributed to the different Raman studies reported in this chapter: J. L. Albert, G. Buntinx, R. Dubest, R. Guglielmetti, K. Karlsson, G. Lévi, P. Levoir, J. J. Meyer, and J. Pantigny. I am very grateful for their help and useful discussions throughout this work. REFERENCES 1. G. J. Ashwell, in: Spectroscopy of New Materials (R. J. H. Clark and R. E. Hester, eds), pp. 61–85,

John Wiley & Sons, New York (1993). 2. S. A. Krysanov and M. Alfimov, Ultrafast formation of transients spiropyran photochromism, Chem.

Phys. Lett. 91, 77–81 (1982). 3. N. P. Ernsting and T. Arthen-Engeland, Photochemical ring-opening reaction of indolinospiropyrans

studied by subpicosecond transient absorption, J. Phys. Chem. 95,5502–5508 (1991) and references therein. 4. N. Tamai and H. Masuhara, Femtosecond transient absorption spectroscopy of a spirooxazine photochromic reaction, Chem. Phys. Lett. 191, 189–192 (1992). 5. H. Miyasaka, S. Araki, A. Tabata, T. Nobuto, N. Mataga, and M. Irie, Picosecond laser photolysis studies on photochromic reactions of 1,2-bis(2,4,5-trimethyl-3 thienyl) maleic anhydride in

solutions, Chem. Phys. Lett 230, 249–254 (1994). 6. N. Tamai and T. Saika, in: Fast Elementary Process in Chemical and Biological Systems (A. Tramer, ed.), pp. 99–105, AIP Press, Woodbury, New York (1996). 7. Y. Yokoyama and Y. Kurita, Photochromism of fulgides and related compounds, Mol. Cryst. Liq. Cryst. 246, 87–94 (1994). 8. G. Arnold and G. Paal, Spectroskopische strukturuntersuchungen an heterozyklischen spiroverbindugen, Tetrahedron 27, 1699-1713 (1971). 9. M. Guiliano, E. Davin-Pretelli, G. Mille, J. Chouteau, and R. Guglielmetti, Infrared and Raman

spectroscopic studies of benzothiazolinic spiropyrans and merocyanines, Helv. Chim. Acta 61, 1072-1085 (1978). 10. E. Davin-Pretelli, M. Guiliano, G. Mille, J. Chouteau, and R. Guglielmetti, Study of infrared and

Raman spectra of benzothiazolines and 2-H-chromenes, constitutive parts of benzothiazolinic spiropyrans. Synthetic aspect, Helv. Chim. Acta 60, 215–230 (1977). 11. G. Arnold, Cspiro–O valence vibrations in spiropyrans, Z. Naturforsch., B 21, 291–292 (1966).

12. C. Schiele and G. Arnold, Zur struktur des photochromen form des 6-nitro-trimethyindolinobenzopyranen, Tetrahedron Lett. 13, 1191–1195 (1967). 13. J. Aubard, J. 1. Albert, R. Dubest, and J. E. Dubois, Evidence for the opening of 6-nitroindolinospiropyran derivatives in the solid state upon laser irradiation: A comparative Raman study. Paper presented at the Societé Française de Chimie Meeting “Dynamique des Systémes Chimiques,” Paris, May 1989. 14. J. Aubard, J. L. Albert, and J. R. Dubois, in: Raman Spectroscopy, Liner and Nonlinear (J. Lascombe and P. V. Huong, eds), pp. 211–212, John Wiley & Sons, Chichester (1982). 15. J. Aubard, J. L. Albert, and J. R. Dubois, Time-resolved resonance Raman study of transient benzoxazolinic merocyanines, J. Raman Spectrosc. 14, 83–86 (1983). 16. M. Delamar, J. Aubard, J. L. Albert, and J. E. Dubois, XPS study of a benzoxazolinic spiropyran and

a related permanent merocyanine, J. Electron. Spectrosc. Relat. Phenom. 28, 289–293 (1983). 17. S. Takeda, K. K. Kuroyanagi, S. Matsubara, and H. Takahashi, in: Proceedings of the 9th

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of Japan, Tokyo (1984).

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18. U. Klüter, W. Hub, and S. Schneider, Resonance CARS spectra of metastable merocyanines produced by UV-photolysis of spirooxazines, Springer Proc. Phys. 4l, 152–155 (1985). 19. J. J. Meyer, J. Aubard, and J. C. Fontaine, A specific time-resolved resonance Raman spectrometer, J. Phys. E.: Sci. Instrum. 18, 430–433 (1985). 20. J. L. Albert, J. P. Bertigny, J. Aubard, R. Dubest, and J. E. Dubois, Etude cinétique et Raman du processus d’ouverture d’un spiropyranne indolinique, J. Chim. Phys. 82, 521–525 (1985). 21. J. Aubard, J. J. Meyer, J. C. Fontaine, P. Levoir, and J. E. Dubois, Sensitive optical multichanel spectrometer for time-resolved resonance Raman studies of transient photomerocyanines, Spectrosc. Lett. 19, 725–739(1986). 22. O. Poizat, N. Locoge-Karbowski, and G. Buntinx, Time-resolved resonance Raman spectroscopy of photochemical intermediates, Trends in Phys. Chem. 2, 199–219 (1991). 23. C. Allet, G. Buntinx, N. Locoge-Karbowski, and O. Poizat, Synchronization of a Q-switched Nd:YAG pumped dye laser and an excimer laser for time-resolved Raman experiments, J. Phys. VI C-7, 463–466(1991). 24. H. Takahashi, K. Yoda, H. Isaka, T. Ohzeki, and Y. Sakaino, Resonance Raman studies of transient species in the photochromism of l´,3´,3´-trimethylspiro-[2H-l-benzopyran-2,2´-indoline] (BIPS),

Chem. Phys. Lett. 140, 90–94 (1987). 25. T. Yuzawa, K. Ebihara, H. Hiura, T. Ohzeki, and H. Takahashi, Structures of transient species in the photochromic reaction of BIPS: Time-resolved resonance Raman study of isotopically substituted analogues, Spectrochim. Acta 50A, 1487-1498 (1994).

26. H. Takahashi, H. Murakawa, Y. Sakaino, T. Ohzeki, J. Abe, and O. Yamada, Time-resolved resonance Raman studies of the photochromic reaction of 6-nitro-l´,3´,3´-trimethylspiro[2H-1-

benzopyran-2,2´-indoline] (6-nitro BIPS), J. Photochem. Photobiol. 45, 233-241 (1988). 27. T. Yuzawa, S. Shimojima, and H. Takahashi, Photochromic reaction of 6-nitro BIPS: Time-resolved

resonance Raman and absorption study, J. Mol. Struct. 352/353, 497-507 (1995). 28. S. Hashimoto, A. Shimojima, T. Yuzawa, H. Hivra, J. Abe, and H. Takahashi, Time-resolved resonance Raman and molecular orbital studies of the structures of the transient species involved in

the photochromic reaction of 2,2'-spirobi[2H-1-benzopyran] (SBP), J. Mol. Struct. 242, 1–14 (1991). 29. H. Sato, A. Matsuzaki, S. Nishio, M. Yoshimi, K. Yamamoto, and H. Tomioka, A Raman Spectroscopic study of merocyanine aggregates of a N-l5 isomer of 6-nitro BIPS. A direct evidence for the association through the bridges, Chem. Lett. 8, 715–716 (1996). 30. S. Schneider, Investigation of the photochromic effect of spiro[indolino-naphthoxazine] derivatives by time-resolved spectroscopy, Z. Phys. Chem. N. F. 154, 91–119 (1987). 31. S. Schneider, A. Mindl, G. Elfingerand, and M. Melzig, Photochromism of spirooxazines. I. Investigation of the primary process in the ring-opening reaction by picosecond time-resolved absorption and emission spectroscopy, Ber. Bunsenges. Phys. Chem. 91, 1222–1224 (1987). 32. S. Schneider, F. Baumann, U. Klüter, and M. Melzig, Photochromism of spirooxazines. II. CARS investigation of the isomeric distribution, Ber. Bunsenges. Phys. Chem. 91, 1225–1228 (1987). 33. S. Aramaki and G. H. Atkinson, Spirooxazine photochromism: Picosecond time-resolved Raman and absorption spectroscopy, Chem. Phys. Lett 170, 181–186 (1990). 34. S. Aramaki and G. H. Atkinson, Spironaphthopyran photochromism: Picosecond time-resolved spectroscopy, J. Am. Chem. Soc. 114, 438–444 (1992). 35. N. Tamai, T. Saika, T. Shimidzu, and M. Irie, Femtosecond dynamics of a thiophene oligomer with a photoswitch by transient absorption spectroscopy, J. Phys. Chem. 100, 4689–4692 (1996). 36. A. Kellmann, F. Tfibel, R. Dubest, P. Levoir, J. Aubard, E. Pettier, and R. Guglielmetti, Photophysics and kinetics of two photochromic indolinospirooxazines and one indolinospironaphthopyran, J. Photochem. Photobiol. A49, 63–73 (1989).

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37. J. Aubard, G. Buntinx, R. Dubest, P. Levoir, J. P. Marsault, G. Lévi, A. Samat, A. Kellmann, F. Tfibel, and R. Guglielmetti, Time-resolved resonance Raman spectroscopy of transient indolinospironaphthoxazine photomerocyanines, in: Proceedings of the XIII ICORS (W. Kiefer et al. eds.), pp. 158-159, John Wiley & Sons, Chichester (1992). 38. S. Maeda, K. Mitsuhashi, Y. T. Osano, S. Nakamura, and M. Ito, The molecular design and applications of spirooxazines Mol. Cryst. Liq. Cryst. 246, 223–230 (1994).

39. P. Lareginie, V. Lokshin, A. Samat, R. Guglielmetti, and G. Pèpe, First permanent opened forms in spirofindoline-oxazine] series: Synthesis and structural elucidation, J. Chem. Soc., Perkin Treats. 2 (1996), 107–111. 40. J. Aubard, P. Lareginie, G. Buntinx, V. Lokshin, and R. Guglielmetti, Time-resolved resonance Raman spectroscopy of spirooxazine photochromes: Experimental evidences for a TTC isomer open form in various solvents, J. Raman Spectrosc. (to be submitted). 41. G. Baillet, G. Giusti, and R. Guglielmetti, Comparative photodegradation study between spiro[indoline-oxazine] and spiro[indoline-pyran] derivatives in solution, J. Photochem. Photobiol. A 70, 157–

161 (1993). 42. V. Malatesta, M. Milosa, R. Millini, L. Lanzini, P. Bortolus, and S, Monti, Oxidative degradation of organic photochromes, Mol. Cryst. Liq. Cryst. 246, 303–310 (1994). 43. G. Baillet, V Lokshin, R. Guglielmetti, and G. Giusti, Photooxidation of the photochromic compound l,3,3-trimethylspiro[indoline-naphthopyran] in the methanol, C. R. Acad. Sci., Paris 319, 41–46 (1994). 44. G. Baillet, G. Giusti, and R. Guglielmetti, Study of the fatigue process and the yellowing of polymeric films containing spirooxazine photochromic compounds, Bull. Chem. Soc. Jpn 68, 1220-

1225 (1995).

45. J. Biteau, F. Chaput, and J. P. Boilot, Photochromism of spirooxazine-doped gels, J. Phys. Chem. 100, 9024–9031 (1996). 46. J. Aubard, K. Karlsson, B. Luccioni-Houzé G.Lévi‚ and R. Guglielmetti, Analytical SERS: Detection and identification of photodegradation products in organic photochromes, in: Proceedings of the XIV ICORS (S. Asherand P. B. Stein, eds.), pp. 690–691, John Wiley & Sons, Chichester (1996). 47. J. Aubard, K. Karlsson, R. Dubest, G. Lévi, B. Luccioni-Houzé C. Salémi -Delvaux, and R. Guglielmetti, Identification of surface enhanced Raman spectroscopy of photodegradation products in organic photochromes, Mol. Cryst. Liq. Cryst. 298, 37–44 (19997). 48. J. Aubard, C. M’Bossa, J. P. Bertigny, R. Dubest, G. Lévi, E. Boschet, and R. Guglielmetti, Surface enhanced Raman spectroscopy of photochromic spirooxazines and related-spiropyrans, Mol. Cryst. Liq. Cryst. 246, 275–278 (1994). 49. R. K. Chang and T. E. Furtak, Surface Enhanced Raman Scattering, Plenum Press, New York (1982).

50. N. Felidj, J. Aubard, and G. Levi, Effects of various halide ions and probe molecules on inelastic Mie scattering from SERS active surfaces: Determination of particle size distributions from band shapes simulation, J. Chem. Phys. 24, 9735–9746 (1996). 51. J. Aubard, E. Bagnasco, J. Pantigny, M. F. Ruasse, G. Lévi and E. Wentrup-Byrne, An ion-exchange reaction as measured by surface enhanced Raman spectroscopy on silver colloids J. Phys. Chem. 99, 7075–7081 (1995). 52. S. Bernard, M. A. Schwaller, J. Moiroux, E. A. Bazzaoui, G. Lévi, and J. Aubard, SERS identification of quinone-imine species as oxidation products of antitumour ellipticines, J. Raman Spectrosc. 27, 539–547 (1996).

53. R. Dubest, P. Levoir, J. J. Meyer, J. Aubard, G. Baillet, G. Giusti, and R. Guglielmetti, Computer controlled system designed to measure photodegradation of photochromic compounds, Rev. Sci., Instrum. 64, 1803–1808 (1993).

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54. S. Schneider, H. Grau, and J. Ringer, Surface enhanced resonance Raman studies of spiropyrans (BIPS and derivatives), Mol. Cryst. Liq. Cryst. 246, 267–274 (1994).

55. R. Guglielmetti, in: Photochromism, Molecules and Systems (H. Dürr and H. Bouas-Laurent, eds.), pp. 314–466, Elsevier, Amsterdam (1990).

56. J. W. Macklin, New method for obtaining Raman spectra of extremely small quantities of lightabsorbing substances adsorbed on silver from very diluted solutions, J. Raman Spectrosc. 26, 1077– 1081 (1995).

9 Biological Applications– Supramolecular Chemistry Masahiko Inouye 9.1. INTRODUCTION One aspect of supramolecular chemistry is the development and characterization of simple and artificial models for biologically important functions as well as the exploration of their biological applications. (For a recent review, see Ref. 1). The two definitive and essential vital functions are heredity and metabolism. The former results from duplication of DNA, and the latter from catalytic action of

enzymes. Both phenomena involve molecular recognition at the initial stage of the

process. During the last two decades, artificial models developed in supramolecular chemistry have demonstrated the importance of strict complementarity in size, shape, and functional groups at the molecular level for selective host-guest recognition. (For a recent review, see Refs. 2–4). Recent investigations in the field reveal a shift of attention from static phenomena to dynamic ones, i.e., from simple synthetic hosts to real artificial receptors. (For a recent review, see Refs. 5– 13). A naturally occurring receptor is generally defined as a complex molecule or molecular assembly that, upon recognition of a specific substrate, undergoes a structural change that usually induces a series of functions (allosteric effect, signal transduction), which eventually results in a physiological response. (For a recent review, see Refs. 14–17). Thus, the construction of artificial receptors, in which the molecular recognition process is synchronized with the signal transduction, is of critical importance from the viewpoint not only of creating the next field of supramolecular chemistry, but also of producing devices of practical value such as molecular sensors. Photochromic molecules are suitable for mimicking such signal transduction processes, and, therefore, by combining molecular recognition

Masahiko Inouye • Department of Applied Materials Science, Osaka Prefecture University, Sakai, Osaka 593, Japan. Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert

Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999.

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with photochromic chemistry, conceptually new photoresponsive systems will be created. This chapter is devoted to biological applications of photochromic compounds. The relevant works, however, are too numerous to be surveyed within the space available. Thus, our discussion will be focused on the background and principles of an approach to the construction of new photoresponsible systems utilizing spiropyran derivatives (for general reviews on the chemistry of spiropyrans, see Refs. 18–20). The use of other photochromic compounds, such as azoarenes and diarylethenes, is not treated here and has been reviewed elsewhere.21–23 It is hoped that this chapter includes sufficient key and up-to-date references on biological applications of spiropyran derivatives. The chapter is divided into two main parts. The first part deals with photoregulation of peptide and protein characteristics by the use of spiropyran derivatives. Potential applications of such photoregulated proteins include their use as light-signal amplifiers, information storage devices, and sensoring assemblies. The second part is concerned with spiropyran derivatives possessing a molecular recognition site. These spiropyran receptors have been designed to have their binding affinities controlled by irradiation. From the opposite point of view, the receptors are also intended to enable molecular recognition information to be signaled as changes in the optical properties of the spiropyran unit. This type of new photoresponsive receptor is conceptually different from the signaling receptors synthesized thus far, such as crown ether dyes, and is functionally referred to as a self-indicating receptor.

9.2. PHOTOCONTROL OF PEPTIDE AND PROTEIN CHARACTERISTICS BY SPIROPYRANS 9.2.1. Photomodulation of Polypeptide Conformation

Synthetic polymers containing photochromic units can undergo reversible changes of their physical and chemical characteristics. (For recent reviews, see Refs. 24–26). Recent examples involving photochromic compounds other than spiropyrans are reported in Refs. 27–31. Spiropyran-containing polymers such as polyacrylates have also been prepared and were found to show photoinduced variations of their viscosity.18–20 The change in the viscosity of the polymers partly reflects the polymer conformation. Thus, spiropyran-attached poly(L-tyrosine) and poly(L-lysine) were synthesized by Vandewyer and Smets at the University of Louvain in 1970.32,33 No photoresponsiveness was observed however, for these modified peptides. On the other hand, in 1989 Ciardelli et al. at the Universtiá di Pisa reported the first photomodulation of the conformation of spiropyran-containing poly-

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(L-glutamic acid) 1.34 was linked to the carboxyl-side-chain groups of poly(L-glutamic acid). The resulting spirobenzopyran-modified peptide (1) was soluble only in hexafluoro-2-propanol (HFP). In HFP, 1 exhibited intense negative photochromism (Figure 1). Thus, at room temperature in the dark, 1 gave orange-colored solutions (merocyanine form 1: 385-365, and 312nm), which were bleached (spiropyran form 1´: by irradiation with visible light. The negative photochromic behavior of the peptide was attributed to the highly polar HFP solvent, which stabilized the zwitterionic merocyanine form. The circular dichromism (CD) spectra for the peptide showed that the polypeptide chain adopted a randomcoil conformation in the dark. The bleached solution after exposure to visible light displayed the typical CD pattern of an indicating that the isomerization of the merocyanine side chains caused the spiralization of the polypeptide chain. This random conversion was reversible upon alternate exposure to visible light and dark conditions. In contrast to the spiropyran-modified polypeptides derived from poly(Lglutamic acid), the poly(L-lysine) analogs have been reported to show no photomodulation of the side-chain conformation in pure HFP, although they exhibited similar photochromic behavior (Figure 2).32,33 However, when appropriate amounts of triethylamine were added to the HFP solution, exposure to visible light resulted in the reversible formation of a helix by the poly(L-lysine) chains.35 Thus, addition of triethylamine to the HFP solution induced the transition, but the amount of base necessary was different for the dark-adapted sample as compared with the irradiated one. The authors claimed

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that these results could be interpreted by assuming that in pure HFP the

conformation was determined by the protonated side chains of the unmodified lysine residues and as a consequence, the polypeptide adopted a coil conformation irrespective of the isomerization of the spiropyran units. Addition of triethylamine removed protons from the lysine side chains, which

resulted in helix formation by the polypeptide (2 and 2´). In the poly(L-lysine), the combination of base and light allowed modulation of the extent of the photoresponse, which provided a well-defined example of the combined action of physical and chemical stimuli on the secondary structure of polypeptides.

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In both cases (1 and 2) although the reason for the observed transitions on the basis of the photoisomerization of the spiropyran unit remains to be clarified, the spiropyran-modified polypeptides can be considered as macromolecular models resembling the behavior of naturally occurring photoreceptors.

9.2.2. Photoregulation of Protein Characteristics Light-controlled “on–off” switchable protein systems provide a fundamental route to bioelectronic materials.21–23 Naturally occurring receptors and enzymes whose binding and catalytic activities are photoregulated are of increasing interest from the viewpoint of various applications, such as their use as chemotherapeutic agents and biosensors. In this connection, various photoisomerizable proteins have been synthesized, with the aim of achieving photoregulation of protein characteristics.36,37

9.2.2.1.

Photoregulated Binding of Spiropyran-Modified Receptor Proteins

Willner and co-workers at The Hebrew University of Jerusalem have been investigating the photoregulation of the binding properties of various proteins by their chemical modification with photoisomerizable components.38–41 Willner et al.42 reported the photoregulated binding of a spiropyran-modified sugar-binding protein, concanavalin A (Con A). Con A is well known to associate strongly with and in vivo. These authors successfully followed the photocontrolled kinetics of association of a spiropyran-modified Con A to monosaccharide monolayers organized on Au electrodes. Con A was modified by condensation of its residues with a spiropyran-carboxylic acid derivative. The spiropyran-modified Con A (3) showed the usual reversible photochromic properties. Three sets of monosaccharide and functionalized monolayers—were associated with Au electrode (Figure 3). The kinetics of the monosaccharide binding to 3 and its merocyanine form were followed by cyclic voltammetry with as a solubilized redox probe. Upon addition of 3, a time-dependent decrease in the amperometric response of the cyclic voltammogram for the and monolayers was observed. No such decrease was observed in the case of the functionalized glucose, which is known to associate less strongly with Con A. Willner et al. claimed that this amperometric decrease was due mainly to binding of 3 to the monosaccharide monolayer associated with the Au electrode, so that insulation of the electrode toward the solubilized redox probe occurred. In these experiments, the rate of decrease in the amperometric response of the electrode reflected the kinetics of the 3-binding of 3. The kinetics of association of 3 to and ffun monolayers was photoregulated. For example, the time

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constant for binding of spiropyran-modified Con A to the lized monolayer was ca. 2.5-fold longer when the spiropyran is in the merocyanine

form (3´) than when it was in the spiropyran form (3). 9.2.2.2. Photoregulation of Enzyme Activity by Use of Spiropyrans Photoisomerizable polymers have been well investigated and applied in various systems to allow physical properties to be reversibly controlled by irradiation.24–31 Accordingly, the encapsulation of proteins in photoisomerizable polymers could provide a general strategy for photoregulating enzymes by means

of light-induced permeability of the substrate across the polymer membrane. Willner et al.43 reported the photoregulation of the activity of encapsulated in a photoisomerizable polymer. They chose azobenzene-, leucohydroxide-, and spiropyran-attached acrylamides 4–6 as photoisomerizable copo-

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lymers (Figure 4). The three photoisomerizable acrylamide copolymers containing exhibited reversible and cyclic photochromic properties. The enzyme’s activity toward the hydrolysis of N-(3-carboxylpropionyl)-L-phenylalanine p-nitroanilide (7) was assayed. On–off photostimulation of was observed upon immobilization of the enzyme in 6: when 6 was in the spiro configuration, the enzyme exhibited no hydrolytic activity, but when it was photoisomerized to the merocyanine form, the enzyme was activated toward hydrolysis of 7. This photochemical stimulation of the enzyme could be further cycled by irradiation of the copolymer between the two photoconvertible states. Similar “on–off” photostimulation of was also observed in the cases of 4 and 5. Willner et al. proposed that the enhanced activities of the enzyme in the merocyanine states could be attributed to the enhanced permeabilities of the anilide across the merocyanine copolymers because the increased polarity of the polymer matrices favored the transport of the polar substrate. Several spiropyran-assembled Au electrodes were prepared and used to control the activities of redox enzymes by photostimulation. Willner et al.44 organized a mixed self-assembled monolayer consisting of native cofactor pyrroloquinolinoquinone (PQQ) and spiropyran units in Au electrodes. PQQ monolayers have been reported to act as an electrocatalytic interface for the oxidation of NAD(P)H in the presence of With the mixed monolayer, in the

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presence of effective electrocatalyzed oxidation of NAD(P)H proceeded when the spiropyran units in the monolayer were in the spiropyran state. On the other hand, in the photoisomerized merocyanine state, the oxidation was inhibited (Figure 5). This finding was attributed to different interactions of with the photoisomerizable monolayer assembly: in the spiropyran state, is associated with PQQ, so that electron transfer from NAD(P)H to the Au electrode occurred, while in the positively charged protonated

merocyanine state, was repelled by the monolayer. Another example of amperometric transduction of photostimulated activation/deactivation of biomaterials was developed using spiropyran–GOD (glucose oxidase) Au electrodes. In this system, similar, but less effective, photoregulated oxidation of glucose was observed. Willner and co-workers also reported photoregulation of the reduction of by the cytochrome c (Cyt c)/cytochrome c oxidase (COX) system on spiropyran-modified Au electrodes.45,46 In addition, the reverse system, i.e., photochemical control of a redox-enzyme-assembled monolayer by spiropyranmodified soluble electron mediators, was investigated by the same research group.47 The photoregulated electrical interactions between the various redox proteins

and the electrode interfaces provide a means for the amperometric transduction and amplification of recorded optical signals. These integrated assemblies reveal the fundamental potential for future bioelectronic devices.

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Photocontrol of Antigen–Antibody Reactions

As a further extension of their approach, Willner et al.48 synthesized the anti(2,4-dinitrophenol) monoclonal antibody, Ab-DNP, by linking DNP with bovine serum albumin (BSA) with acid as spacer and then following a standard technique for the production of monoclonal antibodies. The Ab-DNP efficiently catalyzed the ring opening of 6,8-dinitrospiropyran 8 to its merocyanine isomer 8´. This result suggests that the Ab-DNP recognized the dinitrobenzopyran unit of 8 and that the binding site of An-DNP included a catalytic site for the isomerization of the spiropyran to its merocyanine form (Figure 6a). The reaction exhibited enzyme-like kinetics (the Michaelis–Menten kinetic model), from which the catalysis rate was obtained. The Ab-DNP acted as a cyclic catalyst for isomerization of 8, and ca. 200 cycles were repeated without loss of the Ab-DNP activity. Although the catalytic process was not so fast, the

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catalytic properties of Ab-DNP might have potential to be applied to a reversible optical recording system. The combination of a photoisomerizable spiropyran and Ab-DNP was also applied in the development of reversible amperometric immunosensors.49 The amperometric immunosensor was based on a 6,8-dinitrospiropyran-assembled monolayer on an Au electrode (Figure 6b). When Ab-DNP associated with the spiropyran monolayer as the electrode, the electrode surface was insulated toward the solubilized redox probe, and its amperometric response decreased. After irradiation of the electrode, photoisomerization of the spiropyran unit to the merocyanine occurred, which led to a distortion of the antigen monolayer that resulted in a dissociation of Ab-DNP from the monolayer. The rinsed electrode revealed a high amperometric response, indicating that the Ab-DNP was washed off. Thus, the two-step illumination of the monolayer enables its reversible cyclic performance, which would demonstrate a general means for cyclic operation of an amperometric immunosensor.

9.3. SPIROPYRAN DERIVATIVES POSSESSING A MOLECULAR RECOGNITION SITE 9.3.1. Spiropyran Derivatives as Artificial-Signaling Receptors

Spiropyran derivatives are a well-known and important class of photo- and thermochromic compounds which can be converted to the corresponding open colored zwitterionic merocyanine isomers. (For reviews, see Refs. 18–20). The isomerization is unique in terms of the accompanying large changes in the structural and electrical characteristics of the molecules. When the factors affecting this equilibrium are heat and light, the phenomena produced by the isomerization are called thermochromism and photochromism, respectively. When the factor is a chemical species, especially a specific substrate, another phenomenon can be produced, i.e., structural change and signaling based on molecular recognition. Figure 7 shows the equilibrium between a spiropyran (SP) possessing a molecular recognition site (H) and the corresponding merocyanine (MC). We expected that the equilibrium would be perturbed by recognition of the guest molecule G. Thus, when the equilibrium constant Keq2 is greater than Keq1, color will be developed, and when Keq2 is smaller than Keq1, the color will disappear.

9.3.1.1. Crowned Spiropyrans as Alkali- (Alkaline-Earth) Metal Cation Receptors In 1990, we introduced conceptually new artificial receptors, crowned spirobenzopyrans, in which recognition of alkali-metal cations induced a configurational change in the receptor framework that was accompanied by signaling

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(coloration).50 The design of the crowned spirobenzopyrans 9 (Figure 8) was based on the fact that the strong interaction between the complexed cations and the p-nitrophenolate oxyanion of the merocyanine from 9´ could favor the isomerization from the spiropyran to the merocyanine, i.e., in Figure 7. Thus, we synthesized spirobenzopyrans 9 possessing a monoaza-crown as a recognition site at their 1´-position. The crowned spirobenzopyrans 9 showed no absorption bands above 450 nm in aprotic solvents ( acetone, etc.), indicating that they were in the closed spiropyran forms. However, when a five-fold molar excess of Lil was added to an acetonitrile solution (0.2 mM) of 9a a new absorption band appeared On the other hand, only negligible changes were observed upon addition of NaI, KI, RbI, and CsI. The emerging absorption band was shown unambiguously to be due to the merocyanine structure 9a´ by nuclear magnetic resonance (NMR) investigations. Nuclear Overhauser effect (NOE) experiments suggested that the conformation of 9a´ is that depicted in Figure 8 and that neither the 3H-indolium nor the p-nitrophenolate unit rotates about the olefinic double bond on the NMR time scale. While 9b revealed a small but significant selective coloration with NaI, cation-induced hypsochromic band shifts were observed with and decreasing in that order; these were explained satisfactorily in terms of the electrostatic interactions between the complexed cations and the pnitrophenolate dipole of the merocyanine. Conversely, the hypsochromic band

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shifts demonstrated that the open, colored merocyanine exists mainly in a chargeseparated zwitterionic form and not a neutral keto form. Spirobenzopyran 11 (Figure 9) was designed to have a recognition site in which the complexed cations would interact with the ether oxygen of the closed spiropyran 11 and not the phenolate oxygen of the open merocyanine 11´; i.e., in Figure 7.51,52 Indeed, isomerization of 11 to the open, colored merocyanine 11´ was most strongly suppressed by the presence of a potassium cation, which was most strongly recognized by the crown ring. In addition to crowned spirobenzopyran, we also synthesized crowned spironaphthoxazines and examined their cation-induced isomerizations. These and other experiments showed that the selectivity of the isomerization was governed by several factors: (1) the size of the crown ring, (2) the position of recognition, (3) electrical properties of both the complexed cations and the merocyanine dipoles, and (4) the length of alkyl chains connecting the spirobenzopyran units and the crown units. We demonstrated that the spirobenzopyrans 9 bearing a short linkage showed color selectivities for and Selective coloration, for larger alkali-metal cations could not be obtained, however, even in the cases of the crowned spirobenzopyrans. While the spirobenzopyrans 10, in which the spirobenzopyran moieties were much further separated from the crown ether units by alkyl chains than in 9, showed a small selective coloration for larger alkali-metal cations, such as and the molar absorptivities in the presence of alkalimetal iodides were considerably smaller than compared to those for 9. We thought that the low coloring efficiency might result from the entropic and enthalpic disadvantages associated with the reduced probability of the existence of the complexed cations in the neighborhood of the phenolate oxygen of the merocya-

nines and the weak electrostatic interaction between the complexed large univalent cations and the p-nitrophenolate dipole of the merocyanines. Taking into account these points, we developed cryptand-type crowned spirobenzopyrans 12 (Figure 10).53

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The cryptand spirobenzopyrans 12 showed no absorption bands above 400 nm in nonhydroxylic solvents, indicating that they were in the closed

spiropyran forms. The absorption spectra were scarcely affected upon addition of any of the alkali-metal iodides in In the 1H-NMR spectra of 12b in however, downfield shifts (for aromatic and crown ring protons), splitting (crown ring and alkyl spacer protons), and sharpening (aromatic protons) of the signals of the spiropyran form were observed after the addition of KI. This result indicated that the alkali-metal cations were bound to the macrocycle of 12b; the lack of the coloration was attributed to the weak electrostatic interaction between the complexed univalent cations and the p-nitrophenolate dipole of the merocyanines. On the other hand, addition of alkaline-earth metal iodides to these solutions gave rise to changes in their spectra. Thus, 12a and 12b gave the most intense coloration with and respectively. Titration experiments demonstrated that almost 1 equiv of is enough to obtain the maximum coloration of 12b. Soon after our publication, Kimura et al. (Osaka University) reported other versions of crowned spirobenzopyrans (13, Figure 11).54,55 They connected

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spirobenzopyrans to a crown ether at the 8-position. Measuring the extraction ability toward alkali-metal cations, they found some enhancement in the affinity of 13 for as compared with that of the corresponding ordinary azacrown ethers. Indeed, the degree of isomerization of the crowned spirobenzopyrans 13 in the organic phase almost paralleled its selectivity in the extraction of cations. Thus, emergence of the absorption peak at 530–560 nm, due to the corresponding merocyanine forms, was observed. The enhanced selectivity was explained by the strong interaction between the complexed cation and the p-nitrophenolate oxyanion of the merocyanine form, and some evidence for such an interaction was obtained by 7Li-NMR spectroscopy. These authors also extended their approach to crowned spironaphthoxazines.56

9.3.1.2. Spiropyridopyrans as Nucleobase Receptors

The recognition and the selective binding of the nucleotide and oligonucleotide bases play critical roles in all living systems. Complementary hydrogen bonds arise in a very specific fashion between the purine and pyrimidine bases of the two strands of double-helix DNA that define its duplex structure and regulate its information-transfer functions. The specific binding of nucleotide bases, other than the bases pairing of oligonucleotides, is also seen in many natural enzymes, such as nucleases.1 In this connection, artificial receptors that recognize and bind to specific nucleoside bases are attracting much attention. In order to develop further examples of spiropyran receptors for biologically important species, we designed and synthesized spiropyridopyrans for recognition of guanine nucleoside and oligonucleotide derivatives.57 The design of the spiropyridopyran 14a (Figure 12) was based on the triple hydrogen-bond complementarity between the acetamidopyridopyran unit of 14a or the acetamidopyridone anion unit of the open merocyanine form 14a´ and guanine. We expected that the equilibrium between the colorless spiropyridopyran and the colored merocyanine would be shifted toward the latter by recognition of

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guanine (i.e., in Figure 7) because of highly polar hydrogen binds (Figure 12). The spiropyridopyran 14a showed only weak absorption bands above 350 nm in nonpolar solvents, indicating that 14a exists mainly as the closed spiropyran form. In however, addition of 2´,3´5´-tris-O-(tert-butyldimethylsilyl)guanosine (15G, 10 equiv) to 14a produced changes in the absorption spectra, and a strong absorption band appeared On the other hand, only negligible changes were observed upon addition of other nucleoside derivatives (15A, 15T, 15C, 15U) (Figure 12). The increase in the absorption band was attributed to the increase in the proportion of the merocyanine form relative to that of the spiropyran form due to recognition of 15G. Indeed, ca.10, 43, and 56% of the spiropyran (12.5 mM in existed as the merocyanine form in the presence of 15G at concentrations of 0, 6.3 and 12.5 mM, respectively,

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as compared to 10% in the absence of 15G as judged by integration of the 1HNMR spectra. As expected, the presence of the cytidine derivative interfered with the selective coloration of 14a produced by the guanosine derivative because of the competitive formation of the Watson–Crick G–C base pairs. Thus, addition of 15C (3.0 equiv of 15G) to the colored solution caused a dramatic fading of the color, but other nucleoside derivatives (15A, 15T, and 15U) had little influence on it. As compared to 14a, spiropyridopyran 14b, which was expected to bind guanine via two hydrogen bonds, exhibited less coloration in the presence of 15G, indicating that the triple hydrogen-bond complementarity between 14a and 15G was important for the coloration. This interpretation was corroborated by the results obtained with several N-methylated guanosine derivatives. Thus, the coloration observed for the monomethylated guanosine derivative 16G, which still had the possibility of triple-hydrogen-bond formation with 14a, was similar to that produced by 15G. On the other hand, much less coloration was observed for 17G, which was a regio-isomer of 16G but was not expected to form the triply hydrogen-bonded complex with 14a. Furthermore, little change in the spectra occurred upon addition of 18G, 19G, and 20G. Recent investigations have revealed that small nuclear RNAs (snRNA), which had N-methylated guanine bases, at the 5´-end, played a very crucial role in the combination of eukaryotic pre-mRNAs. Our spiropyridopyran could distinguish guanosine derivatives from N-methylated ones on the basis of its coloration. Subsequently, the color selectivities of 14a for dideoxynucleotide derivatives (21GA, 21AG, and 21AA) were examined. As expected, while 14a showed the almost similar absorption bands for 21GA and 21AG, indicating that the position of guanine bases in the oligonucleotides had little influence on it, no change in the spectra of 14a occurred in the presence of 21AA. This result suggested that such artificial receptors might operate well for larger oligodeoxynucleotides. In section 9.3.1, various spiropyran derivatives possessing a molecular recognition site were described. The coloration results from the structural change in the spiropyrans. Hence these artificial receptors have two conjugated functions, structural change and signaling based on molecular recognition. This is the reason why the spiropyrans are called multifunctional artificial receptors.58 9.3.2. Photoregulation of Polymer Characteristics by Crowned Spiropyrans In 1986, Ueno (Tokyo Institute of Technology) and Osa (Tohoku University) et al. reported the first example of crowned spiropyran 22 (Figure 13).59 Although 22 could not be isomerized to the merocyanine in the presence of any alkali-metal cations, photoregulation for cation-binding of the spirobenzopyran was observed.

Thus, in the dark, 22 showed different binding abilities for alkali-metal cations, the magnitude of binding lying in the order of The binding

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ability of the photoisomerized merocyanine structure was in the same order, but slightly diminished for all alkali-metal cations. They claimed that the depressed cation-binding ability of the opened merocyanine might be explained in terms of the electrorepulsive interaction between a positive charge of the nitrogen atom of the merocyanine unit and the crown-complexed cation. Poly(vinyl chloride) (PVC) membranes containing dibutyl phthalate (DBP) and spiropyran 22 showed a photoinduced potential change, a negative shift of the membrane potential being induced by ultraviolet (UV) irradiation, i.e., isomerization to the merocyanine. The differences for the membrane potential between crowned spiropyran state PVC and the photoisomerized merocyanine one, however, were smaller than those for a PVC membrane containing uncrowned spirobenzopyran, and remarkably depended upon both the kind and concentration of the alkali metal cations. The tendency for a shift toward more negative potential decreased in the order of at high concentration. The positive shifts observed for all alkali-metal cations seem reasonable since the cation-binding ability of 22 decreased with the isomerization. Kimura et al.60 reported photochemical control of ionic conduction in polymer films incorporating their crowned spirobenzopyran 13a (Figure 11). They prepared composite films containing PVC, bis(2-ethylhexyl) sebacate (DOS), 13a, and The photoinduced change of ionic conductivity for a 13a-containing film was quite reversible and was synchronized with the photoisomerization of the spirobenzopyran. The absorption peak of the merocyanine isomer in the spectrum of the composite film shifted to a shorter wavelength as compared to that in the spectrum of a film without The blue shift indicated that electron localization in the merocyanine form of the crowned spirobenzopyran was enhanced in the presence of the cation, suggesting interaction between the and the p-nitrophenolate oxyanion of the merocyanine form.

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The transference number for in the 13a-containing film was decreased significantly when the spirobenzopyran unit isomerized to its merocyanine form. The results was explained by the enhanced binding in the merocyanine form. It is of much interest to apply a crowned spirobenzopyran to a single-ionic conducting system,61 where only a cation participates in ion conduction, because a cation transference number of nearly 1 is attainable in the system, which would result in drastic photoinduced changes in the ionic conductivity. Thus, composite films of lithium poly(perfluorosulfonate) (PPFS-Li) and oligooxyethylene diacetate (OOEAc) were selected. Under dark conditions, 13a was almost completely isomerized to its corresponding merocyanine form 13a´ in the composite film; i.e., negative photochromism was observed. Visible-light irradiation caused a smooth isomerization to the spiropyran form 13a. The isomerization back to the merocyanine form proceeded not with UV irradiation but with heating. The

magnitude of the photoinduced ionic-conductivity switching for this film was 20fold greater than that for the above-mentioned bi-ionic conducting film (PVC/ Effective transference of occurred more in the single-ionic conducting system than in the bi-ionic system, where both and its counteranion could migrate. Kimura et al.62 also applied this ion-conducting control to crowned spironaphthoxazines, aiming at the development of highly light-fatigueresistant switching systems.

9.3.3. Other Applications Utilizing Spiropyran Derivatives Sunamoto et al.63 (Kyoto University) published the first example of the photocontrolled transport of an amino acid across lipid membranes using spirobenzopyran embedded in liposomal bilayers of egg phosphatidylcholine (egg PC). An aqueous suspension of single-walled liposomes containing amino acids in the interior and the spirobenzopyran 23 (Figure 14) in the bilayer was prepared. The spirobenzopyran exhibited its normal photochromism in the bilayers. UV irradiation followed by visible-light irradiation of the liposome suspension brought about significant transfer of phenylalanine from the interior of the liposomes to the exterior, whereas no transport of the amino acid occurred in the dark. Figure 14 shows a schematic representation of the amino acid transport. The system is expected to function repeatedly until an equilibrium concentration of phenylalanine between the interior and the exterior of the liposomes is attained. Thus, Sunamoto et al. succeeded in presenting the first example of a system mimicking the transport of free amino acids across cell membranes. Sunamoto et

al.64 also synthesized a water-soluble spiropyran and investigated its thermocoloration in anionic AOT [sodium l,2-bis(2-ethylhexycarbonyl)-l-ethanesulfonate] reversed micelles in order to evaluate the effect of the reversed micelles in controlling the reaction rates or the reaction pathway.

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Acknowledgments Thanks are expressed to all my co-workers, especially Kaori Akamatsu, Kazuhisa Fujimoto, Seji Akiyama, and Gisen S. Sato, for conducting a literature search and for correcting errors in this manuscript. This chapter is dedicated to my mother, Yoshiko Inouye, deceased February 8, 1996.

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(1983). 38. I. Willner, S. Rubin, and A. Riklin, Photoregulation of papain activity through anchoring photochromic azo groups to the enzyme backbone, J. Am. Chem. Soc. 113, 3321–3325 (1991). 39. I. Willner, S. Rubin, and T. Zor, Photoregulation of α-chymotrypsin by its immobilization in a photochromic azobenzene copolymer, J. Am. Chem. Soc. 113, 4013–4014 (1991). 40. I. Willner, S. Rubin, J. Wonner, F. Effenberger, and P. Bäuerle, Photoswitchable binding of substrates to proteins: Photoregulated binding of α -D-mannopyranose to concanavalin A modified by a thiophenefiilgide dye, J. Am. Chem. Soc. 114, 3150–3151 (1992). 41. I. Willner, M. Lion-Dagan, S. Rubin, J. Wonner, F. Effenberger, and P. Bäuerle, Photoregulation of α -chymotrypsin activity in organic media: Effects of bioimprinting, Photochem. Photobiol. 59, 491–496 (1994). 42. I. Willner, S. Rubin and Y. Cohen, Photoregulated binding of spiropyran-modified concanavalin A to monosaccharide-functionalized self-assembled monolayers on gold electrodes, J. Am. Chem. Soc. 115, 4937–4938 (1993).

43. I. Willner, S. Rubin, R. Shatzmiller, and T. Zor, Reversible light-stimulated activation and deactivation of α -chymotrypsin by its immobilization in photoisomerizable copolymers, J. Am. Chem. Soc. 115, 8690–8694 (1993). 44. M. Lion-Dagan, E. Katz, and I. Willner, Amperometric transduction of optical signals recorded by organized monolayers of photoisomerizable biomaterials on Au electrodes, J. Am. Chem. Soc. 116, 7913–7914(1994). 45. M. Lion-Dagan, E. Katz, and I. Willner, A bifunctional monolayer electrode consisting of 4-pyridyl sulfide and photoisomerizable spiropyran: Photoswitchable electrical communication between the electrode and cytochrome c, J. Chem. Soc., Chem. Commun. 1994, 2741–2742. 46. I. Willner, M. Lion-Dagan, S. Marx-Tibbon, and E. Katz, Bioelectrocatalyzed amperometric transduction of recorded optical signals using monolayer-modified Au-electrodes, J. Am. Chem. Soc. 117, 6581–6592 (1995).

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47. M. Lion-Dagan, S. Marx-Tibbon, E. Katz, and I. Willner, Photoswitchable electrical communication of glucose oxidase and glutathione reductase with electrode surfaces through photoisomerizable redox mediators, Angew. Chem. Int. Ed. Engl. 34, 1604–1606 (1995). 48. I. Willner, R. Blonder, and A. Dagan, Reversible optical recording by a dinitrophenol antibodycatalyzed ring opening of 6,8-dinitrospiropyran, J. Am. Chem. Soc. 116, 3121–3122 (1994). 49. I. Willner, R. Blonder, and A. Dagan, Application of photoisomerizable antigenic monolayer electrodes as reversible amperometric immunosensors, J. Am. Chem. Soc. 116, 9365–9366 (1994). 50. M. Inouye, M. Ueno. T. Kitao, and K. Tsuchiya, Alkali metal recognition induced isomerization of spiropyrans, J. Am. Chem. Soc. 112, 8977–8979 (1990). 51. M. Inouye, M. Ueno, K. Tuschiya, N. Nakayama, T. Konishi, and T. Kitao, Alkali-metal cation recognition induced isomerization of spirobenzopyrans and spironaphthoxazines possessing a crown ring as a recognition site: Multifunctional artificial receptors, J. Org. Chem. 57, 5377– 5383 (1992). 52. M. Inouye, M. Ueno, and T. Kitao, Transmission of recognition information to other sites in a molecule: Proximity of two remote sites in the spirobenzopyran by recognition of alkali-metal cations, J. Org. Chem. 57, 1639–1641 (1992).

53. M. Inouye, Y. Noguchi, and K. Isagawa, Sensitive and selective coloration of cryptand-type crown spirobenzopyrans for alkaline earth metal cations, Angew. Chem. Int. Ed. Engl. 33, 1163–1166 (1994). 54. K. Kimura, T. Yamashita, and M. Yokoyama, Cation-specific isomerization of crowned spirobenzopyrans, J. Chem. Soc., Chem. Commun. 1991, 147–148. 55. K. Kimura, T. Yamashita, and M. Yokoyama, Synthesis, cation complexation, isomerization and photochemical cation-binding control of spirobenzopyrans carrying a monoazacrown moiety at the 8-position, J. Chem. Soc., Perkin Trans. 2 1992, 613–619. 56. K. Kimura, T. Yamashita, M. Kaneshige, and M. Yokoyama, Crowned spironaphthoxazine: Lithium ion-selective colouration and ion-regulated thermal stability of its coloured form, J. Chem. Soc., Chem. Commun. 1992, 969–970. 57. M. Inouye, K. Kim, and T. Kitao, Selective coloration of spiro pyridopyrans for guanosine derivatives, J. Am. Chem. Soc. 114, 778–780 (1992). 58. M. Inouye, Artificial signaling receptors for biologically important chemical species, Coord. Chem. Rev. 148, 265–283 (1996). 59. H. Sasaki, A. Ueno, J. Azai, and T. Osa, Benzo-l 5-crown-5 linked spirobenzopyran. I. Photocontrol of cation-binding ability and photoinduced membrane potential changes, Bull. Chem. Soc. Jpn. 59, 1953–1956(1986). 60. K. Kimura, T. Yamashita, and M. Yokoyama, Photochemical control of ionic conduction by crowned spirobenzopyran, Chem. Lett. 1991, 965–968. 61. K. Kimura, T. Yamashita, and M. Yokoyama, Photochemical switching of ionic conductivity in composite films containing a crowned spirobenzopyran, J. Phys. Chem. 96, 5614–5617 (1992). 62. K. Kimura, M. Kaneshige, T. Yamashita, and M. Yokoyama, Cation complexation, photochromism, and reversible ion-conducting control of crowned spironaphthoxazine, J. Org. Chem. 59, 1251– 1256 (1994). 63. J. Sunamoto, K. Iwamoto, Y, Mohri, and T. Kominato, Liposomal membranes. 13. Transport of an amino acid across liposomal bilayers as mediated by a photoresponsive carrier, J. Am. Chem. Soc. 104, 5502–5504(1982). 64. J. Sunamoto, K. Iwamoto, M. Akutagawa, M. Nagase, and H. Kondo, Rate control by restricting mobility of substrate in specific reaction field. Negative photochromism of water-soluble spiropyran in AOT reversed micelles, J. Am. Chem. Soc. 104, 4904–4907 (1982).

10

Thermochromism of Organic Compounds André Samat and Vladimir Lokshin 10.1. INTRODUCTION The reversible color change observed for a variety of compounds with temperature variation is known as thermochromism. A more precise definition has been given by Day1: “Thermochromism is defined operationally as an easily noticeable reversible color change in the temperature range limited by the boiling point of each liquid, the boiling point of the solvent in the case of solution or the melting point for solids.”

This chapter is mainly devoted to organic compounds for which the observed reversible color changes (coloring and fading reactions) are due to the effect of temperature, exclusively. The thermochromic behavior of inorganics, organometallics, macromolecular systems (such as polythiophenes), or supramolecular systems (such as liquid crystals) and of molecular systems for which the observed thermochromism is due to external intervention (association with another species like a metal ion or a proton) or to modification of the medium by a thermal effect (thermosolvatochromism, for instance) are not reviewed in this chapter. For many organic photochromic compounds (e.g., spiropyrans, anils, and hydrazones) for which the color change is photochemically induced, the bleaching reaction is at least partly thermally controlled. Most of these systems are discussed in other chapters of this monograph or have been discussed elsewhere2; among these, we will only review those systems for which the thermal equilibrium between the colorless and colored forms has been studied. The main review on thermochromism, by Day, appeared in 1963.1 Other important contributions were published successively by Vitry (1969),3 Day (1979),4 and Sidky (1983).5 In this chapter, we will review mainly the data described in the literature during the last 12 years. André Samat and Vladimir Lokshin • Université de la Méditerranée, Faculté des Sciences Luminy, ESA 6114 CNRS, 13288 Marseille Cedex 9, France. Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by Crano and Guglielmetti. Plenum Press, New York, 1998.

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This chapter is divided into four sections devoted to, respectively, spiroheterocyclic compounds (e.g., spiropyrans and spirooxazines), Schiff bases and related nitrogen-containing molecules, bianthrones and other overcrowded ethenes, and, finally, miscellaneous compounds.

10.2.

SPIROHETEROCYCLIC AND RELATED COMPOUNDS

10.2.1. Compound Families 10.2.1.1. Spiropyrans and Spirooxazines The thermochromism of spiropyrans, discovered in 1926, has been extensively studied. Nearly every known compound of this class leads to deep color on melting (generally red, purple, or blue). However, heating solutions of spiropyrans also causes coloration. Day1 in his review gave a table reporting essentially thermochromic spiropyrans of the indoline and spirobipyran series. Bertelson6 summarized the main spectroscopic and physicochemical data obtained up to 1971. Thermochromic properties of spirooxazines have been more recently reported.7 The thermochromic mechanism in these classes has been assumed to involve a thermally sensitive equilibrium between the colorless spiroheterocyclic form (SP) and the quasi-planar open merocyanine-like structure (MC) obtained after the breaking of the C–O bond (Scheme 1). For both these classes of spiroheterocyclic compounds, it seems certain that the thermally most stable photoinduced colored form and the species formed thermally are spectroscopically and kinetically indistinguishable. Depending on the structure, different isomers of the colored form can be involved, but the most stable corresponds to an E configuration.6,8–12 The electronic distribution of the open form is situated between two resonance forms, its proximity to one form or the other depending on the structure and the medium (Scheme 2). For example, spiro[indoline-pyrans] bearing a group would be zwitterionic,8 whereas spiro[indoline-naphthopyrans] or spiro[indolinenaphthoxazines] would be quinoidal.12–18

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During the past decade, new thermochromic spiropyrans have been described, particularly by Russian teams. For example, spiropyrans of the 2oxaindane or azaindanone series with polycondensed chromene fragments (1–8), exhibiting photo- and thermochromic properties, have been synthesized by Minkin and co-workers.19,20

Krasieva and co-workers21 mentioned the thermochromic behavior of spiropyrans of the dithiolane series (9, 10). These data confirm that the annellation of the benzopyran moiety favors the thermochromic properties of this class of

compounds.

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An important contribution from Hellrung and Balli22 concerned the thermochromism of a series of spiropyrans (11–20) in various solvents such as benzyl alcohol, dimethyl phthalate, decalin, toluene and xylene.

Bridged spiropyrans (or chromenoacridines) like 21 and 22 (Scheme 3) are colorless in the SP form. 23,24 Heating solutions of these compounds from 40 to 80°C results in reversible coloration (blue). The supposed mechanism involves the opening of the pyran ring (Scheme 3). Depending on the polarity of the solvent, these compounds show a negative solvatochromism (for 21, is 565 nm in toluene and 585 nm in ethanol). On the other hand, in the presence of bases like triethylamine the equilibrium is completely shifted toward the colored MC form. Although the thermochromic properties of monospiropyrans (with one pyran ring) have been described extensively, bis-spiropyrans with various structures have also been studied. For bis-spiropyrans in which the merocyanine fragments are

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separated by a group of atoms with saturated bonds, after ring opening (thermally or photochemically), the absorption spectrum of the bis-merocyanine with two identical noninteracting chromophores is similar to that of the monoderivative.25–27 The result can be different for bis-spiropyrans like 23–28 in which a system of

conjugated chromophores is formed when the pyran rings are thermally opened.

Compounds 23–28 are colorless at 20°C in toluene. Heating to 60°C apparently induces the opening of only one pyran ring according to the equilibrium:

where SP–SP represents the bis-spiropyran with two closed pyran rings and SP– MC represents the bis-spiropyran with one open and one closed pyran ring. In the case of 24 and 26 in n-propanol, an equilibrium is already established at about 20°C. When the temperature is gradually increased from 20 to 60°C, the equilibrium is significantly displaced to the right-hand side, in favor of the

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SP–MC form. At temperatures up to 70°C, the color changes from red to blue; apparently, the second chromophore in the bis-spiropyran opens during this process, and the equilibrium represented by Scheme 5 is established, where MC–MC represents a molecule with two open pyran rings.

The MC–MC form of 24 leads to only one absorption band while the MC–MC form of 26 leads to two absorption bands and respectively). According to the theory of interactions between chromophores,29,30 the presence of two conjugated chromophores in molecules that have similar absorption frequencies leads to a splitting of the singlet excited state of the “bis-dye” into two levels. In the absorption spectra, such splitting is characterized by the appearance of long-wavelength and short-wavelength absorption bands instead of the absorption band of a single chromophore. The ratio of band intensities depends on the angle between the chromophores and is determined by the relationship.30

For 24, and for 26, and Most of the known spiro[indoline-benzoxazines] and spiro[indolinenaphthoxazines] are certainly thermochromic, but the examples of thermochromic behavior described in the literature for spiro[indoline-naphthoxazines] (29– spiro[indoline-phenanthroxazines] (36 and 37),14–17 spiro[indoline-phenanthrolinooxazines] (38),13 and spiro[oxazepine-naphthoxazines] (39)33 are few in number.

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A 13C nuclear magnetic resonance (NMR) study in solution and in the solid state has been reported for three thermochromic spirooxazines, 29–31.32 From a quantitative analysis of I3C cross-polarization/magic-angle-spinning (CP-MAS) NMR spectra, shielding anisotropies were estimated and a correlation of these characteristics with thermochromic activity was suggested.

10.2.1.2.

[2H]-pyrans

2-Phenylamino-2H-pyran 4034 and benzo- or heteroannellated 2H-pyrans 41–4722 have been reported to exhibit thermochromic behavior in solution. For the latter compounds, of the colored form is in the range 443–600 nm in benzyl alcohol.

10.2.1.3.

Other Spiroheterocyclic Compounds

Minkin and co-workers35 recently developed a series of novel photo- and thermochromic perimidine spirocyclohexadienones for which the mechanism involved a ring–chain tautomerism. For instance, for 48 (Scheme 6) the absorption maxima for the closed and open forms are, respectively, 390 and 584 nm (log in octane as solvent. Interesting zwitterionic spirocyclic compounds like 4936 (Scheme 7) show a negative thermochromism. Indeed, in this case the colored spirocyclic form [absorption at about 400 nm and 530 nm in diglyme for R = alkyl] is more stable than the open colorless form even in an apolar solvent. At –30°C the

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ratio of the closed form to the open form is 1.0, whereas this ratio decreases to 0.35 at 70°C. 10.2.2. Mechanism of Thermal Isomerization 10.2.2.1.

Theoretical Studies In the past few years, quantum-chemical calculations have been used to study the thermal conversion of spiropyrans to merocyanines. Calculations of the potential energy surface of 2H-pyrans by the MINDO/3 method20 showed that the reaction coordinate for opening of the pyran ring is determined almost completely by stretching the C–O bond length by 1.95 , corresponding to the transition state of the thermal reaction The minimum-energy reaction path for the ring-opening reaction of pyran has been also recently investigated by ab initio calculations,37 using the 3-21G and 6-31G(d) basis sets, as well as by semiempirical calculations. It has been shown that the reaction path from the pyran to the most stable open form involved a two-step mechanism through a cis open form (Figure 1). The first step of this mechanism (transition state TS1) is the rate-limiting step, with a barrier of at the Hartree–Fock 6-31G(d) level, corrected to with Moller-Plesset second-order perturbation theory. This value is in agreement with experimental measurements of the activation energy of the ring

Thermochromism of Organic Compounds

423

opening of pyrans, spiropyrans (see Section 10.2.2.2), spirobipyrans, or chromenes.38 The second step has a negligible barrier at this level of theory and represents the isomerization of the cis open form to the trans open form. This two-step mechanism is also obtained using the semiempirical MNDO level of theory, but in this case the barrier is higher for TS1). Day et al.37 have shown that the discrepancy between their results and those of a previous study39 employing this method is due to the fact that in the latter work the authors wiped out the first step of the mechanism (a very low barrier, was found). Recent calculations performed on the spiro[indoline-naphthoxazine] 29 at the semiempirical PM3 level40 (TS option for determining the transition state and IRC option to connect the transition state to the spiro form and the most stable open form, respectively) have confirmed that the rate-determining step in the reaction path is the C–O bond breaking. According to calculations at this level, there is no stable cisoid open form and the transition state is directly connected to the stable trans open form (Figure 2).

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It is important to note that, in the models chosen for theoretical calculations, the reaction path from cis to the trans form is easy, particularly due to the lack of steric hindrance (R = H) (Scheme 8). If, for example, bulky substituents were linked to the C2 atom, the rotation around the C2–X3 bond would be certainly hindered, increasing the activation barrier of the cis–trans isomerization. In this case, it would be hazardous to take the C–O bond breaking to be the ratedetermining step in the thermal coloration reaction (Figure 3). It would be possible to observe a ring-opening followed by a ring-closure reaction without any formation of the stable trans colored isomer.

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10.2.2.2. Activation Parameters of the Isomerization Reactions Assuming that the rate-determining step of the reaction path from the uncolored spiro form to the colored merocyanine form is the ring opening of the pyran or the oxazine moiety (see above), the activation energy of the thermochromic process could be estimated as the racemization barrier measured from optically active compounds. Mannschreck et al.38 have largely developed this technique, which consists in measuring the rate of racemization: the energy of the C–O bond breaking is evaluated in the absence of any observable color change (Scheme 9).

The energy barrier to ring opening of the pyran cycle was also estimated by NMR spectroscopy for spiro[indoline-pyrans]41,42 and for spiro[oxaindanepyrans]19,20 (Scheme 10). At normal probe temperatures, the 1H-NMR spectra show geminal nonequivalence of the C-methyl substituents. At high temperatures, the spectra show a single, exchange-averaged C-methyl signal.42 This exchange is a consequence of fast electrocyclic ring-opening/ring closure. It has been shown41

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that the free energies of activation for inversion of spiropyrans depend on the structure, varying, for instance, from (at 104°C) for the spiro[indoline-naphthopyran] 50 to (at 205°C) for the 8-nitrospiro[indolinebenzopyran] 51.

In their NMR study, Menger and Perinis44 pointed out the catalytic effect of acid on the thermal C–O bond breaking of spiropyrans. In the case of spiro[oxaindane-pyrans] 1–7, the free energies of the ring opening vary from to depending on the structure.19 On the other hand, no correlation between the free energy of ring opening and the C–O bond length was found. There are only a few papers reporting the activation parameters of both the coloring and the fading reactions. In a series of papers published between 1985 and 1995, Japanese authors have studied the isomerizations involved in the thermochromism of spiro[indoline-benzopyrans]44–47 (Scheme 11). The thermal conversion of 6-nitrospiropyran 52 from a merocyanine form to a spiro form the thermal conversion of 8-carboxyspiropyran 53 from its spiro form to its merocyanine form and the rates of thermal isomerizations of 6-carboxy-8-nitrospiropyran 54 have been studied in solvents of different polarities and at high pressures Useful information on the action mechanism was expected, particularly concerning the breaking of the C–O bond. As shown in Scheme 12, two different routes are possible for ring

Thermochromism of Organic Compounds

427

opening via the C–O bond breaking, producing either a biradical or an ionic transition state. Activation parameters of the reactions studied, in chloroform and dimethyl sulfoxide as solvents, are presented in Table 1. An example of a volume profile for the isomerization reactions of 54 (in CHCl3) is given in Figure 4. For all the compounds studied, the volume profile

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exhibits a maximum. The authors concluded that these compounds probably isomerize via the same reaction pathway, i.e., via a transition state involving homolytic bond cleavage or bond formation, rather than via a transition state corresponding to heterolytic processes. They also concluded that the position of the transition state along the reaction path should be close to the SP form.

Thermochromism of Organic Compounds

429

The same authors have recently48 extended their investigations to the thermal equilibrium between the SP and MC forms of the 6-sulfonatospiropyran 55 (Scheme 11) in aqueous solution in the presence of varying concentrations of cyclodextrins (CD) at various pressures. With increasing CD concentration, the equilibrium between MC and SP favors the SP form, suggesting that the inclusion complex between the SP form and CD is more stable than that between the MC form and CD. Taking the information obtained thus far into account, a reaction scheme for isomerization in the presence of CD has been proposed (Scheme 13). Inclusion of the indoline moiety in the CD cavity would not occur due to the size of this entity (8.6 Å diameter). In contrast, the pyran moiety can be included though not completely. The sulfonato group may be exposed to an aqueous environment, owing to the hydrophobic nature of the CD cavity. The rate constants and activation volumes for the thermal isomerization of 6-sulfonato-spiropyran 55 at 313 K in water were studied for both coloration and fading reactions at different concentrations of CD. As previously described for 54, the volume profile has a maximum (the volume of the reaction can be estimated to be suggesting a nonpolar biradical transition state.

This assumption is consistent with experimental data obtained recently by Campredon et al.49 These authors trapped a biradical intermediate during electron spin resonance (ESR) experiments. Signals attributed to the radicals 61–64 were monitored when NO was added to solutions of spiropyrans 56–59. Similar results were obtained with spirooxazines 29 and 60 leading to radicals 65 and 66. Nevertheless there is no direct evidence indicating that the trapped biradicals are on the reaction path leading to the merocyanine. Activation energies for the ring opening and for the ring closure of spiro[indoline-benzopyrans] 67–76, spiro[indoline-naphthopyrans] 77–79, and spiro[oxaindane-pyrans] 4, 80 and 81 have been studied in amorphous vitreous state.50 The values obtained under these experimental conditions are somewhat smaller than the values obtained in solution. For instance, with the spiro[indoline-benzopyran] 73, substituted at the 5 -position by a nitro group, the activation energies are and for the ring opening and the ring closure, respectively. For comparison, the activation energy of the fading reaction

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of 73 in ethyl acetate is It is suggested that, under the experimental conditions used, the merocyanine forms would not be transoid. Favaro et al. studied the kinetic and thermodynamic behavior of seven spiro[indoline-oxazines], (29 and 32–37).15–17 The investigated molecules exhibited thermochromism in a polar solvent (ethanol) but not in a nonpolar solvent (methylcyclohexane). From thermodynamic data and the kinetic parameters of the bleaching reaction, the rate constants and the activation energies for the thermal

Thermochromism of Organic Compounds

431

breaking of the bond were obtained. The activation energies of the thermal coloration reaction were found to be in the range and the reaction rates varied over a very large interval 10–5–10–2s–1).17 10.2.3. Evaluation of Equilibrium Constants

The equilibrium constant for the thermochromic equilibrium between the spiro form SP and the merocyanine-like form MC

is given by

In general, for most of the compounds of the spiropyran and spirooxazine series, the SP form is more stable than the MC form UV-visible spectroscopy is the best method to use to study the equilibrium in this kind of situation. According to the laws of thermodynamics, the standard enthalpy of the equilibrium reaction can be calculated from where

where

is the standard enthalpy of the reaction. We can write

is the initial concentration of the thermochromic compounds, and

where AMC is the absorbance of the solution at of the MC form at equilibrium, and is the maximum possible absorbance that can be reached at the Thus, For

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A straight line should be obtained when the absorbance of the colored form is plotted against 1/T, giving the value. On the other hand, for compounds also exhibiting photochromism, the metastable form reverts to the stable form upon removal of the activating light. Monitoring the rate constant at different temperatures yields the activation energy of the fading reaction which in turn yields the activation enthalpy The complete energy diagram for the ground states of the colored and colorless forms can be derived. Examples are presented in Figure 5 for the spirooxazine 297 (positive thermochromism) and the spiropyran 5447 (negative thermochromism). Values of and for various spiroheterocyclic thermochromic compounds are given in Table 2. The determination of at a given temperature requires that a value be assigned to the molar absorption coefficient of the colored form, For this purpose, various assumptions or various methods have been used depending on the systems studied. For the determination of of different spiroheterocycles (11–20, 42–47), Hellrung and Balli22 chose for the molar absorptivity of the corresponding protonated form, thus causing a considerable error in the values obtained. In the case of chromenoacridines 21 and 22 (Scheme 3), the equilibrium is totally shifted toward the colored form in the presence of a base, making it possible to calculate the molar absorptivity and at 565 nm and 570 nm for 21 and 22, respectively) and then the concentration of the colored form under equilibrium conditions.23,24 At 40°C in ethyl acetate as solvent, the equilibrium constants are estimated to be and for 21 and 22, respectively. For the zwitterionic spirocyclic compound 49, the equilibrium is totally shifted toward the colored form at – 30°C in diglyme as solvent,36 allowing the measurement of the molar absorptivity and then the equilibrium constants at higher temperature by UV-visible spectroscopy at 20°C in diglyme). For photochromic compounds, the values can be estimated at low temperature after flash photolysis of the SP form. Indeed, the rate of thermal decay of the colored form decreases rapidly as the temperature is lowered and can be completely stopped. For example, at a temperature of – 70°C, Chu7 estimated for the spiro[indoline-naphthoxazine] 29. Favaro et al.16,17 estimated the molar absorptivity of the colored forms of spiro[indoline-naphthoxazines] 29 and 31–35 and spiro[indoline-phenanthroxazines] 36 and 37 from photostationary states, using iterative methods. In ethanol, was estimated to range from 31,000 to With these values, constants of were obtained for the thermochromic equilibrium of these compounds. Moreover, the values of the equilibrium constant were shown to be strongly structure-dependent, the percentage of the colored form in thermal equilibrium depending markedly on the contribution of zwitterionic

Thermochromism of Organic Compounds

433

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Thermochromism of Organic Compounds

435

structures. This contribution is postulated to be favored by electron-withdrawing substituents in the oxazine moiety and electron-donating substituents in the indoline moiety. This assumption has been confirmed for dipyridooxazines, as the highest value of the equilibrium constant is exhibited by the compound with a methoxy group on the indoline ring.13 As can be seen in Table 2, the standard enthalpies indicate an endothermic reaction of On the other hand, was very small (positive or negative), indicating that the reaction is almost isoentropic. From the results of their work, Favaro et al.17 defined the thermocolorability at each temperature as the absorbance at the maximum of the photomerocyanine band (AMC) relative to the total concentration The following expression was proposed:

This expression allows the thermocolorability of different molecules to be compared, if the values of are known. In the spirooxazine series, in general and the thermocolorability is equal to the product. In Flannery’s work on spiropyrans,51 arbitrarily estimated values were used, and usually up to 30% relative deviations of values were unavoidable. Recently, Zhou et al.52 proposed a new method of evaluating the equilibrium constants of spiropyrans. In this method, and (see Scheme 14) are measured independently. An example is given here for the spiropyran 82.

First, the first-order fading kinetics are monitored after the colored form has been photochemically induced, leading to Second, the rate constant is measured by taking advantage of the ability of the merocyanine form to complex Cu(II), leading to a new absorption band. Treating the process under the steady-state assumption, the authors monitored the disappearance of the merocyanine and showed that, in this case:

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due to the fact that the chelation process is the faster step and the ring opening is the rate-determining step in the complex-formation process. The value obtained for is significantly different from that evaluated by Flannery51 With compounds exhibiting negative thermochromism (i.e., the colored form is more stable than the colorless form), the concentration of the merocyanine form is high enough to allow the relative concentration of each species to be measured by 1 H - NMR spectroscopy. Using this technique, Nishimura et al.46 have estimated the equilibrium constant for the spiropyran 54 to be 2.83 at 25°C in DMSO-d6 as solvent.

10.3. SCHIFF BASES AND RELATED NITROGENCONTAINING COMPOUNDS

10.3.1. Thermochromism of Salicyl-Schiff Bases The Schiff bases of salicylaldehydes with arylamines, aminopyridines, and aryl- or thienylalkylamines show thermochromism and photochromism in the solid state due to hydrogen transfer.54 At the beginning of the century, Senier and Shepheard55 observed that many of the colored crystalline salicylideneanilines were thermochromic. In most cases, the color changed, upon heating, from yellow to orange or red. With this in mind, extensive investigations were carried out by Cohen and Schmidt,56,57 who found that photochromism and thermochromism were mutually exclusive properties of this series of compounds and suggested that this phenomenon is related to the crystal structure of the compounds and not to the chemical nature of the ring substituents. For instance, salicylidene-2-chloroaniline is photochromic whereas salicylidene-4-chloroaniline is thermochromic.

The presence of the ortho-hydroxy group is essential for the observation of both types of chromisms, and the mechanism involves intramolecular proton transfer via a six-membered-ring transition state, producing enol–keto tautomeric species, with the spectra of the keto forms showing a bathochromic shift (Scheme 16).

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437

Thermochromism is restricted to planar molecules and is attributed to a shift of the tautomeric equilibrium toward the “NH” form absorbing at longer wavelengths. For a nonplanar molecule, much energy is required for hydrogen transfer in the ground state, and the transfer occurs only in photochemically excited states. A subsequent Z-to-E isomerization leads to the “photo trans-keto 56,58,59 configuration.” These primary assumptions are summarized in Figure 60,61 6.

The molecular conformation and the possibility or impossibility of stack packing in the crystal have been shown to be responsible for photo- and thermochromism.60 Molecules for which the less favorable planar structure is stabilized by stack packing possess thermochromic properties. The strong inter-

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atomic interactions in the stacks prevent molecular acoplanarization, and thus the absorption wavelength of the keto isomer (or quinoid) is no higher than 470 nm. On the other hand, in photochromic crystals, with no stack-packing, a nonadiabatic acoplanarization occurs following the phototransfer, leading to an absorption at wavelengths up to 540 nm. A comparison of the photo- and thermochromic conversions for salicylideneanilines is given in Figure 7. From semiempirical calculations by the CNDO/2 method, Aldoshin and Atovmyan63 calculated the relative energy of photo- and thermochromic salicylidenearylamines. In all cases, a twisted conformer is the most stable, but the energy gap between the conformer and the planar form is narrow

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439

On the other hand, for thermochromic compounds the energy gain associated with packing can reach The major thermochromic Schiff bases prepared by condensation of salicylaldehydes with anilines are reported in Table 3. Replacement of the phenyl in the aniline part by a 2-pyridyl group64 leads only to thermochromic molecules (Table 4). From X-ray crystallographic studies of compounds in this series, it has been suggested that planarity is achieved because of the position of the N atom of the pyridine ring, which is always cis with respect to the (N=C)–H hydrogen atom.

The lone pair of the imino nitrogen atom is not conjugated with the pyridine ring, and, as a consequence, the hydrogen bond between the hydroxyl group and the imino nitrogen is strengthened [the mean N–H(–O distance is 1.8Å)]. Therefore, the thermochromic behavior of this class of compounds can be

interpreted as due to a shift in the tautomeric equilibrium (Scheme 16). Salicylidene-meta-aminopyridines and one salicylidene-para-aminopyridine were also studied,65,66 and, as expected, thermochromic (Table 4) and photochromic compounds were found. When the aminophenyl or aminopyridyl or aminothienyl group is separated from the aromatic ring by one or more methylene groups, the molecules are not planar. Compounds exhibiting both thermochromic and photochromic phenomena have been found,67–69 especially when two methylene groups are introduced in place of one70,71 (Table 5).

In addition, these data suggest that stack packing in the crystal is not absolutely necessary in order for thermochromism to be observed. When one or more methylene groups separate the imino nitrogen atom from the aromatic ring, the molecule is not planar but the nitrogen is more basic and the hydrogen bond with the hydroxyl group is strengthened, explaining both thermo- and photo-

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Thermochromism of Organic Compounds

chromic behaviors. In this case, substitution of the salicylidene ring that stabilizes the conjugated system is essential for thermochromism; for instance, N-(5methoxysalicylidene)benzylamine is only photochromic, whereas N-(4-methoxysalicylidene)benzylamine (176, Table 5) is thermochromic and photochromic. N-Salicylidenealkylamines also exhibit thermo- and photochromism.72 While Schiff bases are generally photochromic in rigid glasses irrespective of how they behave in the solid state, N-salicylidenebenzylamines, N-salicylidene-2phenylethylamines, N-salicylidene-2-aminomethylpyridines, and N-salicylidene2-aminoethylpyridines investigated in methylcyclohexane: isopentane at 77 K or

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in EPA (ether: isopentate: ethanol, 1:5:5) rigid glasses70 show strong negative thermochromism. The absorption of the colored form, depending markedly on the concentration, has been attributed to aggregates of the cis-quinoid form. Formation of aggregates has been observed for the Schiff bases when the imino nitrogen is sufficiently basic and the molecule is thermochromic. Examples of electron absorption spectra of positive and negative thermochromic compounds are shown in Figure 8. Pistolis et al.73 have shown that the complexation of thermochromic N-5chlorosalicylideneaniline and N-salicylidene-2-aminopyridine with cyclodextrins (CD) results in the disappearance of thermochromic properties and the appearance of photochromism. From 1H-NMR, UV-visible, and fluorescence studies in dimethylformamide (DMF) solutions, it has been found that and bind the thermodynamically more stable enol form of anils. Moreover, NMR data indicated that the binding site of the anil in the CD is the imino bridge, which dictates the position of the enol–keto equilibrium. Structural and optical studies74 of the thermochromisni of N-tetrachlorosalicylideneaniline (140) Table 3 and N-tetrachlorosalicylidene-1-pyrenylamine (171) Table 4 showed, in the latter case, an intermolecular charge-transfer interaction [the interplanar distances between the mean phases being short (3.38 Å)]. The consequence is a decrease of the thermochromic properties due to a modification of the proton transfer by this charge transfer.

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The salicylidene-6-aminomethyldihydroabietates 173 and 174 (Table 4) are other examples of molecules exhibiting photo- and thermochromic behavior in spite of the direct conjugation of the imino nitrogen atom with a phenyl ring.75,76 A typical thermochromic behavior for N,N´-bis(salicylidene)diamines77–79 (Table 6) and bis-N,N´-salicylideneanilines79 (Table 7) in the solid state has been also mentioned. Particularly, N, N´-bis(salicylidene)diamines bearing electronwithdrawing groups showed intense deep red coloration at 100–120°C.79 An Xray crystallographic analysis78 has shown that the molecules of N, N-bis(salicylidene)-para-phenylenediamine (189) are planar and are stacked in a parallel fashion to form one-dimensional columns. The relatively short interplanar spacing suggested the existence of an intermolecular charge-transfer interaction. 10.3.2. Structural, Spectroscopic, and Theoretical Studies of Intramolecular Hydrogen Bonding Intensive investigations have been conducted to elucidate the nature of the mechanism of thermochromism of salicylidene Schiff bases. Different techniques or methods suitable for the study of the tautomeric equilibrium between the enol form and the (Z)-keto form have been used, including X-ray diffraction, NMR, infrared (IR) and Raman spectroscopy, and theoretical calculations. In the anil-

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type systems considered, the prototropic equilibrium forms one part of a quasiconjugated network in which electron and proton motion are associated, and thus the shift of the proton may induce a remarkable change in the electronic states

related to the OH and NH forms. Wozniak et al.89 studied the crystal structures of four Schiff bases, 210–213, determined by X-ray diffraction and solid-state NMR. The data for 15 structures contained in the Cambridge Structural Database90 were used as reference values.

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When the O · · · N distance is relatively long, the proton is localized near the oxygen atom. The shorter the O · · · N distance, the greater is the shift of the proton toward the nitrogen atom. For intermediate distances, the proton may be delocalized between two different positions: one near the oxygen atom and the other near the nitrogen. On the other hand, it seems that the nature of the substitution of the salicylaldehyde part has more influence on changes of the hydrogen-bonding properties than the modification of substituents attached to the aromatic part of the amino fragment. The authors concluded that the mesomeric effect is dominant for the changes in geometry due to intramolecular hydrogen bonding. The C–O

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distance depends strongly on the position of hydrogen in the hydrogen bond. From IR81,91 and Raman81 studies on a series of salicylideneamines, the structure of the minor colored species has been identified as the (Z)-quinoid form. It has been shown91 that, in the crystal, thermochromism (and photochromism) are governed mainly by the packing and the availability of the nitrogen lone electron pair. The out-of-plane vibration is also controlled by these factors and consequently acts as a conformational probe. With the use of nuclear quadrupole resonance (NQR) and nuclear quadrupole double resonance techniques,83,92–94 the temperature dependence of and frequencies was measured at 77–289 K for a series of salicylideneanilines. All the experimental results agree with a model in which the exchange between the low-energy enol form and the high-energy tautomeric keto form is fast and reflects a dynamic equilibrium between the two forms.

In the case of N-5-chlorosalicylideneaniline,94 the proton–oxygen distance has been determined from the dipolar structure of the NQR lines. Moreover, the energy difference between the two proton sites has been estimated to be 74meV. The temperature dependence of the NQR frequencies has been measured83 for a series of seven thermochromic salicylideneanilines or salicylidenebenzylamines and one photochromic compound. The energy difference between the two forms varies from 56 meV to 104 meV depending on the structure and the substitution (Table 8). is larger for nonplanar photochromic compounds than for planar thermochromic ones. In the latter case, there is an equilibrium between the enol and (Z)-keto isomers, while for photochromic compounds, at 300 K, only the enol form exists. On the other hand, among the planar thermochromic compounds, variations of depend on the polarity of the molecules and also on the salicylaldehyde ring substitution. When the deviation from planarity increases, the resonance in the molecule decreases and the hydrogen transfer is more difficult. Substitution leading to a decrease of the electron density of the nitrogen atom of the bridge results in the same effect. A spectroscopic study in solution of the tautomers of several anils derived from salicylaldehyde and from 2-hydroxynaphthalene-l-carbaldehyde having the same amino part (141–145, Scheme 17) has been reported.82 Variable-temperature NMR spectroscopy and comparison with appropriate models show that while salicylideneanilines exist as phenolic tautomers, naphthylideneanilines exist as equilibrium mixtures containing appreciable amounts of both enol and keto tautomers, the latter being slightly predominant (Table 9). In fact, there is a negligible shift in the equilibrium on changing the electronic effect of the R substituent. A careful investigation of the spin-lattice relation rate of the protons of N,N´bis(salicylidene)-p-phenylenediamine (189) and of N,N´-di(2-hydroxy-l-naphthylidene)-p-phenylenediamine (193) crystals provided detailed information on the

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449

proton dynamics in the hydrogen bond.87,88 These NMR results are consistent with the optical measurements reported and with the crystal structure.78,86 In this kind of compound, the combination of the two possible forms (OH and NH) with

two hydrogen bonds may lead to four tautomeric forms, designated as [OH, OH], [NH, OH], [NH, NH], and [OH, NH] (Scheme 18). Among these, the [OH, NH] and [NH, OH] tautomers are thought to be the same or very close in energy, even in the crystalline state. The OH forms are more stable than the NH forms, populated as the temperature is decreased from room temperature to 108 K.78,86,87 The proton dynamics in the hydrogen bond is closely related to the thermochromism of the crystal, which is orange-colored at room temperature and turns pale yellow upon cooling in liquid nitrogen. NMR experiments clearly showed two processes exhibiting a large difference in their activation energies.87 These data suggested the presence of an interaction between two intramolecular hydrogen bonds through rearrangement of the π-electron distribution. This type of proton– electron interaction may represent the origin of proton–electron cooperation and can be controlled by chemical modifications. For instance, in the case of N,N´di(2-hydroxy-l-naphthylidene)-p-phenylenediamine (193), the measured activa-

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tion energies for the transitions between tautomers are smaller than those of N,N´bis(salicylidene)-p-phenylenediamine (189). This result suggests that the protons of 193 move much more easily, by a tunneling effect, and that the proton–electron coupling is stronger than in 189. Proton-transfer reactions of N-tetrachlorosalicylideneaniline (140) and Ntetrachlorosalicylideneaniline-1-pyrenylamine (171) have been investigated by using a semiempirical self-consistent field molecular orbital (SCF MO) method with an energy gradient technique.94 From the calculated potential barriers (81.27 and for 140 and 171, respectively, it can be seen that the

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thermochromic reactions occur easily at room temperature. The analysis of

molecular orbitals and electronic distributions show that thermochromism in

this case is intensified by a change in the π-electron configuration. Previous investigations of the structures and optical properties of these Schiff bases74 demonstrated a coupling between proton and electron motions. The strong hydrogen bonds in these molecules have mainly been attributed to a steric effect of the chlorine substituents at the 3- and 6-positions of the benzylidene moiety. In addition to this effect, the intermolecular interaction is sufficiently strong in the case of 171 to make the hydrogen-bond structure nearly intermediate between the NH and OH forms.

10.4. BIANTHRONES AND OTHER OVERCROWDED ETHENES

Bianthrones and related bianthrylidene systems undergo a reversible color change induced by light (photochromism), temperature (thermochromism), or pressure (piezochromism).

Since the original description of thermochromism of bianthrones at the beginning of the century, this phenomenon has evoked considerable interest and has been the subject of numerous reviews, the most recent being that published by Muszkat in 1988.96

The thermochromism of bianthrone in solution has been shown to result from a thermal equilibrium between two distinct and interconvertible isomeric species, Species A exists at room temperature as a yellow form upon heating of solutions of A, a significant fraction is converted to the green B form whose enthalpy is greater than that of the A form.97 In the A form, the severe repulsion between the two anthrone moieties is avoided by the adoption of a folded conformation (Figure 9). This feature has been confirmed by X-ray crystallographic investigation and dynamic NMR studies.97,98

Among the various proposed structures for the thermoinduced B species (supposedly identical to the photochromic ones96), the twisted conformation has gathered

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a variety of experimental and theoretical support99 (Figure 9). According to theoretical studies anthrone moiety would be planar, and the angle between them would be very large (57°99 or 64°101). The experimental value of the activation energy for the transformation is close to 84kJ mol –1.96,97 The thermochromism is prevented by bulky substiruents at the 1,1´- and 8,8´positions. In these cases, potential barriers that must be surmounted on the thermal path leading to the B form are too high, and the B form is therefore unobservable. For appropriately substituted bianthrones, two geometrical isomers are expected: the E form and the Z form (Scheme 19). Dynamic NMR studies97

provided information on the pathways for E–Z isomerization and conformational inversion in 2,2´-disubstituted bianthrones. These studies indicated that the highest-energy transition state for these processes is the folded-twisted structure and not the twisted ethylene (Figure 10). From a systematic study of the effects of steric interactions Tapuhi et al.97 searched for a possible relationship between thermochromism and the thermal

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E–Z isomerization in these systems (Scheme 20). At a given temperature, T, the equilibrium constant for the transformation is

where is the fraction of the compound in the B form, is the molar absorptivity of the B form at the thermochromic band maximum (in the region 650–730 nm) at temperature T, and is the molar absorptivity at the same band maximum of the pure B species (assuming that the values do not vary with temperature). Tapuhi et al.97 used values based on direct spectral measurements of pure B solutions obtained in studies of the photochromism of bianthrone and its derivatives, values were found to be equal to values at the lowest-energy band (maximum at of the A species and were practically independent of the substitution pattern. A value of

was used for

values and the free energy of activation at the coalescence temperature for the process are given in Table 10. The main conclusions from this study are

that (i) there is no direct correlation between values and the bulkiness of the substituents at the 2- and 2´-positions of bianthrone and (ii) the thermochromic phenomenon and the E–Z isomerization have a common transition state, the twisted-folded species. These results are consistent with early99 and recent101 calculations. Evans and Fitch102 developed an electrochemical method to determine the thermodynamic equilibrium constant K. The value obtained by this technique at 298 K) for bianthrone is in excellent agreement with spectrophotometric measurements.97 This method allows the estimation of the equilibrium constant of l,l´-disubstituted bianthrones, even if the substitution prevents any observable thermochromic behavior.

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Besides bianthrone derivatives, other sterically overcrowded alkenes103 have generated considerable interest owing to their intriguing thermochromic (and photochromic) properties.96,104 Other symmetrical bianthrylidenes in which the groups at the 10- and 10´-positions are identical [dixanthenylidene (230, X = O), bithioxanthylidene (230, X = S)], as well as several mixed thermochromic bianthrylidenes [9,9´-fluorenylideneanthrone (231), 9-diphenylmethyleneanthrone (232), xanthylideneanthrone (233), diphenylmethylenexanthene (234)] and a single overcrowded aromatic ene105 [2-(thioxanthen-9-ylidene)indane-l,3-dione (235)] have been described as exhibiting thermochromic behavior.

Solutions of difluorenylidene (236) and of its benzoannellated derivatives are deeply colored at room temperature; here the two fluorene rings are already twisted about the 9,9´-double bond, resembling the B conformers of bianthrylidenes.106 Molecular mechanics calculations107 and a molecular orbital study108 of the conformational behavior of 9,9´-bifluorenylidene confirmed that the twisted conformation (twist angle is more stable in this case. In general, bis(tricyclic) aromatic enes with central six-membered rings are mostly folded, whereas similar molecules with central five-membered rings are mostly twisted.103 Feringa et al.109 succeeded in resolving thioxanthene-type alkenes. A high racemization barrier was found for 2,2´-dimethylbithioxanthylidene (237), compared to values reported for other bis(tricyclic) alkenes

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The thermal racemization and the resolution of cis–trans isomers of benzoannellated bithioxanthylidene derivatives which show a stereospecific thermal isomerization process have been achieved.113 The study of the thermal isomerization of the unsubstituted bithioxanthene 238 by polarimetry at 75–

85°C in p-xylene, led to a racemization barrier of The mixture of trans-239 and cis-239 was separated into four stereoisomers: P-trans, M-trans, M-cis, and P-cis.

Stereospecific thermal isomerizations of P-trans to M-cis and of P-cis to Mtrans were observed. This behavior can be rationalized using the mechanism proposed by Tapuhi et al.97 (Figure 10). The cis–trans isomerization can only occur via the perpendicular twisted form, which is considered to have a lower free energy. Two pseudopolymorphic crystalline forms of 235 (235y and 235o) were isolated and characterized by X-ray diffraction.105 The first (yellow crystals)

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corresponds to a folded (average folding angle of 53.5°), nontwisted conformation, and the second (orange crystals) to a folded (folding angle of 44.1°), twisted conformation (twisting angle 11.1°). Thermochromism was observed at elevated temperature for crystals of 235o, giving rise to deep-red crystals (235r). Reversible conversion of crystals of 235o to 235r can be carried out repeatedly without any visible degradation. The thermochromic red crystals (235r) have been tentatively assigned to a twisted molecular conformation on the basis of semiempirical calculations (twisting angle of 45.5°).

10.5. MISCELLANEOUS COMPOUNDS Some indano[l,2-b]aziridines were reported to exhibit thermo- (and photo-) chromism.114,115 1-Cyclohexyl-6-(cyclohexylimino)-1 a-phenylindano[1,2-b]aziridine (240) leads to intensely red-colored solutions upon rapid heating in toluene or xylene to 135°C. The red color fades upon cooling. The proposed mechanism involves a thermal conversion of the aziridine to the tautomeric isoquinolinium

imine 241 (Scheme 21), despite the geometrical restrictions imposed by the molecule. The driving force in this system was attributed to the relief of ring strain in 240 and to the gain in resonance energy in 241. The X-ray structure of 240116 showed that the aziridine C–C bond (1.515 Å), which is cleaved on valence tautomerism, has a normal bond length. A reversible color change of 4,6,7-tri(alkoxy-substituted phenyl)-l,2,5-thiadiazolo[3,4-c]pyridines (242) was observed in the solid state.117 On grinding,

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yellow crystals of 242 became an orange amorphous solid which, on heating (or washing with an appropriate solvent), gave the original yellow crystals. The semibullvalenetetracarboxylates (243) and dicyanosemibullvalene (244) are yellow, 2,4,6,8-tetraphenylbarbaralane (245) and dicyanodiphenylbarbaralane (246) are orange-red, and semibullvalene-dicarbonitrile (247) is deep red.118–120

All exhibit thermochromism in solution and the solid state. For instance, crystals and solutions of 247 reversibly turn orange on cooling.120 On the other hand, while the UV-visible spectra of 243–245 exhibited long-wavelength shoulders, the spectra of 246 and 247 are characterized by a separated maximum at 436 nm and 444 nm respectively (acetonitrile). The long-wavelength absorption and thermochromism of these compounds have been interpreted in terms of a model based on different shapes for the energy surfaces of the ground state (shallow minimum) and the excited state (steep minimum). Probably, the transition

observed at long wavelength occurs from an upper vibrational level of the ground

state, the population of this level depending markedly on the temperature.118 Thermochromism of hindered amino-substituted cyclohexadiones due to C – N bond cleavage has been described by Russian authors.121–124 For instance, 2,6-di-tert-butyl-4-morpholino-4-(2-hydroxyphenyl)cyclohexadien-2,5-one 248 and 4-dimethylamino derivative 249 exhibit reversible thermochromic properties,121,122 (Scheme 22). X-ray diffraction studies showed that the occurrence of intramolecular bonding between the hydroxy group and the amino groups leads to a decrease of the C—N bond strength. Heating of 248 in heptane from 299 K to 368 K leads to the appearance of two absorption bands at 393 nm and 500 nm. There is no observable shift of the absorption wavelengths with a change in the polarity of the solvent, but polar solvents induce a hyperchromic effect. In the case of Mannich bases (250) showing thermo- (and photo-)chromism, the proposed mechanism is quite similar123,124 (Scheme 23). The color change is observed only in protic solvents, demonstrating the participation of the solvent during the proton transfer occurring before the morpholine elimination. Solutions of 250 are uncolored when dissolved and become yellow when thermal equilibrium is established. Depending on the

Thermochromism of Organic Compounds

substituents on the phenolic ring, the ratio of the colored form to the uncolored form varied from 0.81 to 1.1 in ethanol at 298 K.124

10.6.

CONCLUSION

During the past 12 years (the period covered by this review), very few new thermochromic molecular systems have been reported. The main families of compounds have continued to be the major focus of interest, with only a shift in the balance between them; for example, spiropyrans have been less studied (in terms of development of new compounds) while interest in anils has increased. There has also been a shift away from work essentially devoted to the description of new compounds, as, in recent years, the attention of authors has mainly been directed toward thermodynamic, kinetic, structural, and mechanistic aspects. For this purpose, a large variety of techniques, including not only spectroscopic techniques (UV-visible, NMR) but also X-ray diffraction and theoretical calculations, have been employed, leading to a better knowledge of the fundamental aspect of the thermochromism of organic compounds.

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Acknowledgments The authors are grateful to Professor A. Mannschreck (Regensburg, Germany) and Dr. J.-C. Micheau (Toulouse, France) for helpful discussions and

suggestions.

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Index Ab initio calculations, 233

Benzothiazolino spiropyrans, 13,93–95, 359,

Ab initio MO method, 247, 251 Absolute helicities 268 Activation energies, 423–426,452, 454–456

362, 363 Benzoxazolino spiropyrans, 359 Benzyl viologen, 213 Bianthrones, 451,453, 455 folded form, 452

Acryloyl residue, 11 Ag colloids, 387

Aggregates, 314 head-to-tail mode, 314 head-to-head mode, 315 Aggregates of the azo chromophore, 42 Alkali-metal cation, 402

Alkaline-earth metal cation, 404, 405

2-Alkoxy benzopyrans, 262, 274, 276, 278 Amino acid, 410 Amorphous polymer, 10 Amphiphilic azobenzene PVAS, 47

twisted form, 452 Bibenzopyrans, 360 Bilayer membrane, 10 Bipyridinium salts, 213 Birefringence, 37 Bis-merocyanines, 419 Bispiropyran, 317 Bis-spiropyrans, 418–420 Bistable photochemical system, 189 Bithioxanthylidene, 455,456

Anthracenopyran moiety, 320

Bovine serum albumine (BSA), 401

Antibody, 401 Antigen, 401 Apparatus for fatigue resistance measurement, 130, 145–149 Apparent absorption coefficient, 74 Artificial receptor, 393 2-Aryl-2-methyl benzopyrans, 262, 274, 291 Atomic interaction energies, 251 Au electrode, 397, 399 Aza-crown ether, 403, 406 Azobenzene(s), 16,39,398 Azobenzene-attached acrylamide, 398 Azo-modified polymers, 45 1-Azonaphthalene, 29

4-Bromo N-salicylidene aniline, 255

Barriers energy to thermal racemization, 269, 271 Barriers to thermal enantiomerization, 273, 280, 281 Benzo-l,4-oxazines, 262, 273, 277 Benzopyrans, 262, 273 Benzopyran moiety, 301, 305 Benzothiazoline moiety, 360 Benzothiazolinic spiropyrans, 251

Canonical quinoid structure of merocyanine, 309 6-Carboxy-8-nitro spiropyran, 426 8-Carboxy spiropyran, 426 Charge transfer interaction, 445 Chemically Induced Dynamic Electron Polarization, (CICEP), 223 Cholesteric phases, 34 Cholesterol liquid crystal, 10, 31 Chromatography enantioselective liquid, 263, 265 Chromenes, 153, 184, 185, 187

2H-Chromene, 303, 314, 330, 331 diphenyl and dimethyl chromenes, 330, 331 2H-Chromene moiety, 84, 360, 381, 388 Chromophores, 419, 420 α-Chymotrypsin, 399 Coherent anti-Stokes Raman scattering (CARS), 358, 366, 370, 371 Cinnamates, 39 Circular dichroism, 267, 268

467

468

Index

Cis-cisoid X isomer, 68, 69, 71 Cisoid intermediate, 371 Cis-trans-cis (CTC) isomer, 372, 375

Classical (spontaneous) Raman spectroscopy, 358–360, 363–366 CNDO/S calculations, method, 253,303 CNDO/2 method, 251 Colorability, 74 “Commander molecules,” 52

“Command surfaces,” 39 Commercial spirooxazines and chromenes, 153, 154 Complementary hydrogen bond, 406 Concanavalin,397, 398 Configuration interaction (CI), 242

Conical intersection, 256 Conrotatory mode, 249 Continuous irradiation, 170, 175, 183, 194, 195 Crowned spirobenzopyrans, 404–406,410 Crowned spironaphthoxazine, 404 Crowned spiropyran, 402, 403 Cryptand spirobenzopyrans, 405 Cyclic voltammetry, 217

2,2´-Dipyridinium salts, 218 4,4´-Dipyridinium salts, 217

Disrotatory mode, 249 Dixanthenylidene, 455 Dodecyltrimethylammonium chloride, 221 Doublet state, 212 Durability, 66 Dynamical method, 177

Electron Nuclear Double Resonance (ENDOR),214,222 Electron Paramagnetic Resonance (E.P.R)., 211–217, 219–228, 230, 233, 235, 253 Electron Spin Echo Modulation (ESEM), 221, 222 Electron spin resonance spectrometry, 139

Electron transfer, 211, 217, 219, 222, 223, 229 Egg phosphatidylcholine (egg PC), 410

1.3-Cyclohexadiene, 255,256

Enantiomeric purities, 265 Enantiomerization,271–273, 280 Enantiomers, separation, 262,265 Energy of intermolecular interactions, 314 Enol-keto tautomeric, 436 Equilibrium constants, 431, 432, 434, 435, 450

Cytochrome c, 400

1 -Ethyl-1´-cety1-4,4´-bipyridinium dibromide,

Cytochrome c oxidase, 400

223 Ethyl viologen, 213, 217 Extended Hückel method, 251 E-Z photoisomerization of azobenzenes, 17 E-Z photoisomerization of doped azobenzene, 34 E-Z thermal isomerization of azobenzenes, 16

Cyclodextrin, 217

Degradation products, 382, 386, 388 Dialkoxyanthracenes, 254 Diamino anthraquinone moiety (residue), 35 Diarylethene(s), derivatives, 244, 247, 249, 255, 256 1.4-Diazabicyclo [2,2,2] octane (DABCO), 110, 129, 131

Fatigue resistance, 115

Dichroic dyes, 41, 48

Ferrioxalate, 168, 173

Dicyano diphenyl barbaralane, 458 Dicyano semibullvalene, 458 1,2-Di (3-furyl) ethene, 250, 251 Dihydroindolizines, 178, 254 Dimethylene succinic anhydride, 339

Fischer's method, 198, 199 Flash photolysis, 167, 195, 432 9,9'-Fluorenylidene anthrone, 455 1-Formyl imino-2(lH)-naphthalenone, 133 1-Formyl-β naphthol, 123 Free radicals, 212 Free volume, 44, 45 Fulgides, 21, 338 adamant-2-ylidene-(1´-methyl-3´-indolylethylidene) succinic anhydride, 348,349 arylidene derivatives, 339–343 furyl fulgides, 20 indole series, 343–347 isomers, 341

3´,3´-Dimethyl-6-nitro spiro[indoline-2,2´-[2H1 ]benzothiopyran], 315 3,3-Dimethyl oxindole, 76, 123, 129, 387, 388 6,8-Dinitro spiropyran (merocyanine), 401 Dioctadecydimethyl ammonium chloride, 221 Dipalmitoyl-phosphatidylcholine, 221 1,2-Diphenyl ethene, 250, 251 I -Diphenylmethylene anthrone, 455 Diphenyl methylene xanthene, 455 7,7-Dipheny l[7H]pyrano[3,2-e]indole, 183 Diphenyl viologen, 214

Fulgide derivatives, 253, 255

469

Index Glucose oxidase (GOD), 400 Grafted spiropyrans on PS, PMMA, PHMA, PI polymers, 92

Marbled texture, 44 McConnell relation, 213 Mechanism of photooxidation of

Guanine, 407, 408

spirooxazines, 124 Merocyanine, 226, 228, 229 Merocyanines, 129, 133, 141 Mesoionic coumpound, 253 Mesomeric effects, 81 3-Methacryloxymethyl-5-nitro salicylaldehyde, 52, 53 Methacryloxy thiosalicylaldehyde, 53 Methacryloyl residue, 11 2-Methacryloyloxymethyl pyridine, 53 8-Methoxy-6-nitro BIPS, 126, 131, 366, 382, 385 Methylmethacrylate (MMA), 25,28, 53 Methyl viologen, 213, 214

Guanosine, 407, 408 Guest-host cells, 41 Hammett equation, 81 1,3,5-Hexatriene, 255,256 p-Hexyl azobenzene, 44 Homeotropic alignment, 39 Host-guest chemistry, 52 α-Hydrazones-β keto esters, 39 Hydrogen transfer, 436

2-Hydroxymethyl pyridine, 53 2-(2´-Hydroxy phenyl) benzothiazole, 255 6-Hydroxy-1´,3´,3´-trimethyl spiro[2H-1 -

benzopyran-2,2´ indoline], 69

Micelle, 10 Micelles, 220–222

INDO calculations, 214, 215, 218 Indoline moiety, 81, 301, 381

Micro Raman spectrometer, 373

Indoline spirothiopyrans, 315 Indolino-nitrospiropyrans, 363–365

MNDO calculations (semi empirical), 249, 253

Indolino-spirobenzoxazines, 265 Indolino-spironaphthopyrans, 288, 360,371, 380 Indolino-spironaphthoxazines, 284, 360, 371, 372, 373, 380 Indolino spiropyrans, 298, 359, 372

MNDO method (MOPAC), 325 MNDO/3 method, 254 MNDOC/MRSDCI method, 249, 255 MO calculations, 213, 218 Molar absorptivity, 432,454

Inductive effects, 81 Infrared (IR) spectroscopy, 358–364

Molar extinction coefficient, 75, 79

IR, Raman spectroscopy, 445 In-plane alignment photocontrol, 39,47

Intramolecular hydrogen bonding, 447, 449 Intramolecular proton transfer, 436 Isotactic copolymer, 13

MNDO-AM1 calculations, 253

Molecular packing, 314 Molecular recognition, 393 Monoclonal antibody, 401 Multifunctional artificial receptor, 408 Multiplicity of the photoreactive state, 67 NAD(P)H, 399

Laser flash photolysis (LFP), 227, 228 Length of the C spiro-O bond, 304 Leucohydroxyde, 398 Leucohydroxyde-attached acrylamide, 398 L-glutamic acid, 396 Linearly-polarized light, 24,38, 39 Liposomal bilayers, 410 Liquid crystal, 10

Nafion,217 4H-Naphthalenoneperimidines, 333, 338 Naphthopyran moiety, 320 o-Naphthoquinone, 133 Naphthoquinonemethido dyes, 243 Naphth[l,2-d]oxazole, 123 Nematic liquid crystals, 39, 44 Nematic-isotropic transition temperature

Liquid-crystal-induced circular dichroism (LCICD), 34 Liquid-crystalline polyesters, 35

Lone electron pair (LEP), 298, 299,301,317, 318, 322, 327, 330 Long-chain alkyl silylating reagents, 39 Low-mass liquid crystals, 33 L-lysine, 394

Nematic phases, 31 N-ethyl-N'-trimethylaminopentyl-4,4´bipyridine, 217

N-formyl-3´,3´-dimethyl oxindole, 129 Ni(II) phthalocyanine complex, 242, 243 6-Nitro BIPS, 366, 369 6-Nitrospiro benzopyran, 11

Index

470 6-Nitrospiro [indoline-benzopyran], 15 8-Nitro spiro[indoline-benzopyran], 426

Photochromism of sydnone, 253 Photocoloration, 68, 73, 78

6-Nitrospiropyrans, 39 Nitro-substituted spiropyrans, 68, 72 Nitroxides,212, 227, 228, 234 5-Nitro-o-vanillin, 76, 387, 388 N-methylated guanine, 408 N-methyl pyridinium chloride, 214 1H, NMR,421, 425, 436, 443, 445, 448–452 NMR Nuclear Overhauser Effect (NOE), 247 NMR, solid state, 421, 446 N,N´-bis (salicylidene) diamines, 446 N,N´-dihydro-4,4´-dipyridiniutn dichloride, 213 N,N'-dihydro-4,4'-dipyridyl, 213

Photocoloration quantum yields, 74 Photocontrol of antigen-antibody reaction,

proton transfer, 316 Non-azaheterocyclic spiropyrans, 111

Non linear optics, 253 1H Nuclear magnetic resonance (NMR) spectroscopy enantioselective, 266,267 Nucleobase, 406 Nucleobase receptor, 406 Nucleoside, 407 Oligooxyethylene diacetate (OOEAc), 410 Optical, kinetic and fatigue resistance, properties

of commercial pigments, 154–164 Orbital interactions, 300, 303, 318

Out-of plane alignment photocontrol, 331 Oxaindan moiety, 320 2-Oxaindan spiropyrans, 320 photochromism, 324 Oxaindano-spirobenzopyrans, 283 Oxaindano-spironaphthopyrans, 283 Oxazine moiety, 325 Oxidation in dark solutions, 136

Pariser–Parr–Pople method, (PPP), 241, 242, 253 Permanent merocyanines, molecular structure, 306–311

401 Photocontrol of ionic conduction, 408 Photocontrolled transport, 410 Photodegradation, 68, 73, 75, 78, 101–110, 168,182,183,191-193,195,203 Photodichroism, 25, 36, 48, 49 Photoexcitation, 66, 84, 110 Photofunctionality, 9 Photoinduced birefringence, 25, 37 Photokinetic factor, 168, 197 Photoinduced ionic conductivity change, 408 Photomerocyanines (open forms), 361, 364,

366, 372, 375 Photomodulation of peptide conformation, 394 Photon-heat mode, 247 Photon-photon mode, 249 Photooxidation, 68, 73, 382, 386, 388 Photoreceptor, 397 Photoregulation for cation-binding of spiropyran, 408 Photoregulation of enzyme activity, 398 Photoregulation of protein characteristics, 397 Photoresponsive liquid-crystal, 39 Photostability of fulgides, 151–153

Photostability, 74 Photostationary method, 177, 195, 198 Photostationary state, 175, 177, 189, 197 Pimers, 216 Polarization holography, 25 Polarization photochromism in polymer solids, 24 Polarization photochromism of liquidcrystalline polymers, 35

Polarized light induced dichroism, 24

hydrogen bond, 311 solvation, 310

Poly[4´-(2-acryloyloxy ethyl) ethylamino]-4nitro azobenzene, 53 Poly carbonate, 20 Poly (L-glutamate) film, 34, 35

trans-cisoid isomer, 311, 313, 314

Polymeric Langmuir–Blodgett films, 10, 33,

crystals, 314

trans-transoid isomer, 311, 313, 314 Permanent (open form) merocyanine, 372, 373

39, 44 Polymeric materials, 9

Photobirefringence, 36, 37

Polymeric matrix, 9

Photochemical fatigue, 110, 112–114. 145 Photochemical properties of spiropyrans, 315 Photochemical rate equation, 170 Photochromic liquid crystalline polymer, 31

Polymeric mesophases, 31 Polymers doped with photochromic compounds, 24 Polymethyl (ethyl) methacrylate (PM(E)MA), 25, 28 Poly (N-vinyl-2 pyrrolidone), 18, 19

Photochromic polymers, 9 Photochromic Schiff bases, 254

Index

471

Poly (perfluorosulfonate (PPFS-Li), 410

Polyphosphazenes, 15 Polysiloxanes, 15 Polystyrene (PS), 20 Poly (tetrahydrofuran), 19, 53

“Soldier molecules,” 52 Sol-gel inorganic matrices, 21 Solvatochromism, 374,385,418

Specific rotation

267

Quantum yield, 170, 177, 181, 195, 255 Quinoidal structure, 369, 370, 372, 374 Quinoid structure of merocyanine, 308, 309

Spin-coated films, 40 Spin -coating of amorphous azopolymers, 37, 39 Spin density and spin density distribution, 212-215, 230, 231, 234 Spin-probes, 234, 235 Spin trapping, 227, 234 Spiro [benzopyran-naphthopyrans], 283 Spirobenzopyrans, 262, 278, 280, 282 Spiro benzoselenazolino benzopyran, 12, 53 Spiro[bibenzopyrans], 115, 117,360 Spirobinaphthopyrans, 281 Spiro center, 301, 306, 315, 317, 322, 325, 329 Spiro compounds, 226, 227 Spirocyclic cyclohexadienone perimidine, 333, 338

Quinone derivatives, 254

Spiro cyclopropene fluorene, 53

Poly (trityl methacrylate) on silica, 265, 269 Polyurethane films, 382 Polyvinyl alcohol (PVA) 25, 40 Poly (vinyl chloride) (PVC), 408 Positive and negative thermochromism, 432,443 Potassium tert-butoxide, 230, 231 Previtamine 255, 256 3-i-Propyloxy-8-methoxy-3´methyl6-nitro-[benzothiazolino-[2,2´][2H-l]-benzopyran], 180– 182 Pyrroles, 220 Pyrroloquinolinoquinone (PPQ), 399

Spiro [dithiolene-benzopyrans], 118

Radical anion, 211,218, 230, 231, 233, 234 Radical cation, 211, 212, 214, 217-219, 223

Radical ions, 212 Random coil α-helix conversion, 396 Regulation of liquid crystal alignment, 39 Relative configurations, 267

Reorientation of azobenzenes, 29 Ring-opening process (step), 361, 371 Rubling treatment, 47 Salicylaldehydes, 76 Salicylidene-meta aminopyridines, 439 Salicylidene-para aminopyridines, 439 Salicylidene Schiff bases, 445,446 Salicyl-Schiff bases, 436 Saturated five-membered ring

azaheterocyclospiropyrans, 96–99 Saturated six-membered ring azaheterocyclospiropyrans, 99, 100 Schlieren texture, 44 Self-organization of liquid crystals, 50 Semi bullvalenetetracarboxylates, 458 Side-by-side interaction, 47 Single-ionic conducting system, 410 Singlet oxygen reactivity, 124 Smectic phases, 31 Sodium 1,2-bis (2 ethylhexyl carbonyl)-1-

ethanesulfonate (AOT)reversed micelles, 411

Spiro [fluorene-benzopyrans], 330, 331 Spiro fluorenyl indolizines, 14, 53 Spiro[indoline-benzoxazines], 262, 284, 285, 287, 290, 420 Spiro [indoline-benzopyrans], 186, 190, 203, 212, 226, 229, 262, 288–290,426 Spiro [indoline-naphthopyrans], 127, 135, 149, 212, 234,416 Spiro [indoline-naphthoxazines], 132, 149, 212, 233, 234, 286, 287,416, 420,432 Spiro[indoline-oxazines], 430

Spiro[indoline-phenanthrolinooxazines], 420 Spiro[indoline-phenanthroxazines], 420, 432

Spiro[indoline-pyrans], 416,425 Spiro indolino benzopyrans, 52 Spiro-2H-naphtho[ 1,8-bc]-oxepins, 318 Spironaphthoxazines, 245 I3 C NMR shift, 245 relative energy of open forms calculation, 248 visible absorption of the colored forms, 245 Spiro[oxaindane-pyrans], 425, 426 Spiro[oxazepine-naphthoxazines], 420 Spirooxazines, 359, 364,372 Spirooxazine-doped polymer films, 253 Spirooxazines, 324, 326 benzoxazines, 325 naphthoxazines, 325 quinolinoxazines, 325

Index

472 Spirooxazines photodegradation, 115, 120 effect of solvent polarity on photodegradation

and colorability, 119 effect of structure on photodegradation and colorability, 119 effect of wavelength irradiation on photodegradation and colorability, 119 Spiropyran(s), 298, 302, 323, 357, 364 colorability, 324, 329 Spiropyran-assembled Au electrode, 399 Spiropyran-attached acrylamide, 398 Spiropyran-containing polypeptide, 394, 395 Spiropyran(s) (derivatives), 247,251

l,2,3,4-Tetrahydro-2,3-dioxo-4,4-dimethyl

quinoline, 123 Tetrakis [3,5-bis (trifluoromethyl) phenyl] borate anion (TFPB–), 19, 54 Tetramethyl indoleninium iodide, 383,386 4-(2,2,6,6-Tetramethyl piperidinyl-1 -oxyl) (TEMPO), 235 2,4,6,8-Tetraphenyl barbaralane, 458 Theoretical calculations, 422,423,438,450, 452, 453 Theoretical MO calculations (transition state), 271 2,4,6,8-Tetraphenyl barbaralane, 458

Spiropyran-modified concanavalin, 397, 398

Thermal degradation of photochromes, 75

Spiropyran-modified poly (L-lysine), 394 Spiropyran-modified poly(L-glutamic acid), 396

Thermal isomerization, 422

Spiropyran-modifiedpolypeptides, 395–397

Thermal relaxation, 199

Spiropyridopyrans, 406, 408 Spiro [oxaindane-benzopyrans], 262, 283, 284 Spirothiopyran, 11 Stack packing, 439 Stilbenes, 39 Structural studies, 297

Thermal ring-opening reaction, 286 Thermal stability, 247 Thermal valence isomerization, 323 free energies of activation, 323

6-and-8-Substituted spirobenzodithiolane, 111,

116 6-and-8-Substituted spirobenzoxathiolane, 111, 117 Substituted spirobenzopyrans, 84–93

Substituents effects, 82 6-Sulfonato spiropyran, 429 Supramolecular aggregates, 49 Supramolecular chemistry, 393 Surfaces-assisted liquid crystal alignment control,

39 Surface-assisted phenomenon, 44 Surface enhanced (resonance) Raman spectroscopy (SE(R)RS), 358, 378-383, 385388 Surface photochromism, 39 Sweep volumes for isomerization, 30

Taft equation, 94 Tautomeric prototropic equilibrium, 439, 446, 448 Transition state of Csp3-O bond cleavage, 271 Triacetyl cellulose (TAC), 263 Tert-butoxy radicals, 211, 234

Tetrabutylammonium borohydride, 231–233 2,3,4,4-Tetrachloro-1 -oxo-dihydronaphthalene, 224 7,7,8,8-Tetracyano quinodimethane (TCNO), 136,228

Thermal racemization, 261, 269

Thermochromic perimidine

spirocyclohexadienones, 421, 458 Time-resolved resonance Raman troscopy, 358, 366–370, 372–377 216 2-(Thioxanthene-9-ylidene) indane-1,3-dione, 455 224, 225 Trans isomers, 372, 375 Trans-trans-cis isorner (TTC), 372, 373, 375378 Triads DDD, MDD, MDM, 29 Trianions, 233, 234 Tribenzoyl cellulose, 263

1´,3´,3´-Trimethyl-5´-nitrospiro[indoline2,2'[2H-1]benzopyran], 304 1´,3´,3´-Trimethyl-8-nitrospiro[indolinobenzopyran], 226 1,3,3-Trimethyl oxindole, 76, 123, 129,387, 388 TR3 instrumentation, 367, 368

Triphenylgermyl radicals, 234 Triphenyl imidazolyl radical, 191-194 Triplet biradicals, 212, 227 Triplet diradical, 227 Triplet spectra, 212 Triplet spectrum, 226, 228 223 Unsubstituted spiropyrans and spirooxazines, 70

473

Index Viologens, 212–214, 216–223 Viologen polyionenes, 19, 53

X-Ray diffraction, 439, 445, 446, 451, 456– 459

Water-soluble azo dyes, 48, 50 Watson–Crick base pair, 408 Woodward–Hoffmann rules, 249

Zeolites, 218 Zindo method, 242

Xanthylidene anthrone, 455 X-Ray diffraction, 297

Zwitterionic,416,49 Zwitterionic structure (form), 364, 366, 369, 370 Zwitterionic structure of merocyanine, 308, 309