A ~~
Volume 20
A Specialist Periodical Report
Photochemistry Volume 20 A Review of the Literature published between...
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A ~~
Volume 20
A Specialist Periodical Report
Photochemistry Volume 20 A Review of the Literature published between July 1987 and June 1988 Senior Reporters D. Bryce-Smith, Department of Chemistry University of Reading A. Gilbert, Department of Chemistry University of Reading Reporters N. S. Allen, Manchester Polytechnic A. Cox, University of Warwick R. 8. Cundall, MRC Radiobiology Unit, Didcot W. M. Horspool, University of Oundee S. T. Rdd, 7he University of Kent A. C. Weedon, The University of Western Ontario, Canada
SOCIETYOF
ISBN 0-85186-185-7 ISSN 0556-3860 Copyright @ 1989 The Royal Society of Chemistry
All Rights Reserved N o part of this book may be reproduced or transmitted in any form or by any means-graphic, electronic, including photocopying, recording, taping, or information storuge and retrieval systems-without written permission from The Royal Society of Chemistry Published by The Royal Society of Chemistry Thomas Graham House, Cambridge, CB4 4WF
Printed in Great Britain by Whitstable Litho Printers Ltd., Whitstable, Kent
Introduction and Review of the Year D. BRYCE-SMITH A. GILBERT W e may p e r h a p s be p a r d o n e d f o r f e e l i n g some s e n s e o f p e r s o n a l s a t i s f a c t i o n and a c h i e v e m e n t i n r e a c h i n g t h e t w e n t i e t h o f t h e s e a n n u a l r e v i e w s of p h o t o c h e m i s t r y ; t h o u g h i t must a l s o be s a i d t h a t w e have been u n s u c c e s s f u l i n p e r s u a d i n g t h e Royal S o c i e t y of C h e m i s t r y t h a t a c e l e b r a t o r y b a n q u e t f o r t h e hard-working Reporters would be i n o r d e r ,
The c o v e r a g e t h i s y e a r i s much t h e same as i n Volume 1 9 , w i t h o n e major e x c e p t i o n . C i r c u m s t a n c e s beyond o u r c o n t r o l h a v e n e c e s s i t a t e d omission o f P a r t V d e a l i n g with photochemical a s p e c t s o f solar energy conversion. W e hope t h a t i t w i l l p r o v e p o s s i b l e t o r e i n s t a t e t h i s s e c t i o n i n f u t u r e Volumes. Some of t h e R e p o r t e r s have been f e e l i n g t h a t c o n s t r a i n t s on l e n g t h h a v e been p r e v e n t i n g them from d o i n g f u l l j u s t i c e t o t h e work r e v i e w e d . Changes d e s i g n e d t o ameliorate t h i s problem are now u n d e r d i s c u s s i o n , and we hope t o be able t o g i v e d e t a i l s i n Volume 21. W e s t a r t t h i s Review by r e f e r r i n g t o some of t h e more i n t e r e s t i n g developments on t h e t h e o r e t i c a l s i d e o f photochemistry.
Kusba h a s s o l v e d t h e d i f f u s i o n e q u a t i o n f o r l o n g r a n g e e n e r g y t r a n s f e r by d i p o l e - d i p o l e i n t e r a c t i o n accompanied by m a t e r i a l d i f f u s i o n . F r a c t a l models are i n c r e a s i n g l y b e i n g u s e d t o e x p l a i n some e n e r g y t r a n s f e r p r o c e s s e s (von Borczyskowski and K i r s k i ; Tamai e t al.; P i n e s and Huppert; Newhouse and Kopelman). A t t e n t i o n is drawn t o Najbar's i m p o r t a n t t r e a t m e n t of heavy atom effects. V e r y s h o r t t i m e - r e s o l u t i o n t e c h n i q u e s are b e i n g i n c r e a s i n g l y a p p l i e d ' t o t h e s t u d y of v e r y fast processes, f o r example s o l v e n t r e l a x a t i o n and e l e c t r o n - t r a n s f e r t o and from e l e c t r o n i c a l l y e x c i t e d species (Peters i n t e r a l i a ) , Real t i m e femtosecond t e c h n i q u e s now p r o v i d e a probe f o r t h e s t u d y of t r a n s i t i o n s t a t e s ( D a n t o s e t a l . ) . T h i s is a v e r y p r o m i s i n g development. The F i r s t I n t e r n a t i o n a l L a s e r C o n f e r e n c e was h e l d i n 1987 - i n p a r t a V
M
r e f l e c t i o n o f t h e growing u s e of l a s e r t e c h n i q u e s i n p h o t o c h e m i s t r y A v a l u a b l e s u r v e y g i v i n g access t o R u s s i a n and p h o t o p h y s i c s . l a s e r work on p i c o s e c o n d s p e c t r o s c o p y and b i o l o g i c a l p h o t o c h e m i s t r y h a s been p r o v i d e d by Letokhov. Amongst numerous a p p l i c a t i o n s o f f i b r e o p t i c s i n l u m i n e s c e n c e s t u d i e s , UR would p i c k o u t t h e i n g e n i o u s oxygen s e n s o r which u s e s f l u o r e s c e n c e d e c a y t i m e as t h e i n f o r m a t i o n c a r r i e r ( L i p p i t s c h e t a l . ) , and t h e u s e of l i q u i d The informc o r e f i b r e s i n laser f l u o r i m e t r y (Fujiwara e t a l . ) . a t i o n a v a i l a b l e from s t e a d y s t a t e l u m i n e s c e n c e s t u d i e s can be i n c r e a s e d by a computer-based m u l t i d i m e n s i o n a l f l u o r e s c e n c e t e c h Cundall i n n i q u e which h a s been r e v i e w e d by P a t o n a y e t a l . P a r t I of t h i s Volume h a s i d e n t i f i e d c h i r a l i t y c h a n g e s on e l e c t r o n i c e x c i t a t i o n as a f r u i t f u l area for f u r t h e r r e s e a r c h . B r a u c h l e ' s n o v e l phase-modulated h o l o g r a p h i c g r a t i n g t e c h n i q u e h a s been s u c c e s s f u l l y a p p l i e d t o e x c i t e d m o l e c u l e s i n polymer matrices. A p i o n e e r i n g s t u d y by Chatzidimitriou-Dreisman and Brandas h a s been c o n c e r n e d w i t h p h o t o c h e m i c a l e f f e c t s i n m o l e c u l e s s i t u a t e d i n amorphous e n v i r o n m e n t s , i n c l u d i n g l i q u i d s : t h e r m a l motion c a n g e n e r a t e l o c a l c o h e r e n t o r d i s s i p a t i v e s t r u c t u r e s which a f f e c t t h e b e h a v i o u r o f n e i g h b o u r i n g m o l e c u l e s . Bulska h a s proposed 2,2' - b i p y r i d y l - 3 , 3 ' - d i o l a s an a d v a n t a g e o u s new f l u o r e s c e n c e s t a n d a r d , and Wintgens e t a l . have described a f u l g i d e a c t i n o m e f e r p a r t i c u l a r l y u s e f u l f o r imrk on laser e x c i t a t i o n .
Locke and Lim have r e p o r t e d t h e f i r s t example o f a s p e c i e s formed by t h e c o m b i n a t i o n o f tux, e l e c t r o n i c a l l y e x c i t e d m o l e c u l e s : t h e y t e r m t h i s a 'bicemer'. Alkane p h o t o c h e m i s t r y , n o r m a l l y a r a t h e r i n a c c e s s i b l e f i e l d , has been s t u d i e d u s i n g a n i o n i z i n g e x c i t a t i o n s o u r c e (Yoshida and L i p s k y ) . Much s t i l l r e m a i n s t o be u n d e r s t o o d a b o u t t h e p h o t o c h e m i s t r y and p h o t o p h y s i c s o f b e n z e n e , and t h i s hydrocarbon and its d e r i v a t i v e s , including diphenylpolyenes, continue t o provide p e r e n n i a l c h a l l e n g e s . D o u b l e t - d o u b l e t f l u o r e s c e n c e of f r e e b e n z y l r a d i c a l s i n s o l u t i o n h a s been o b s e r v e d by Tokumura e t a l . : nonr a d i a t i v e d e c a y a l s o o c c u r s . Azulene c o n t i n u e s t o a t t r a c t p h o t o c h e m i c a l a d m i r e r s , i n c l u d i n g Hopkins and R e n t z e p i s , i n t e r a l i a . A t t e n t i o n i s drawn t o a two-photon s t u d y o f p y r i d i n e u s i n g a t h e r m a l l e n s i n g t e c h n i q u e ( S a l v i e t a l . ) . The f l u o r e s c e n c e d e c a y k i n e t i c s o f 1 , 2 - d i (1-pyreny1)propane are t h e s u b j e c t o f s t r o n g d i s a g r e e m e n t between t w o g r o u p s ( S i e m i a r e z u k and Ware; Zachariasse and S t r i k e r ) .
Introduction and Review of the Year
vii
M i a l o c q h a s s t u d i e d t h e f o r m a t i o n o f s o l v a t e d e l e c t r o n s by p h o t o e x c i t a t i o n of i n o r g a n i c a n i o n s and t h e b i p h o t o n i c e x c i t a t i o n of water.
G a r t s t e i n and Zakhidov have proposed t h a t charge-
t r a n s f e r s t a t e s i n mixed s t a c k d o n o r - a c c e p t o r compounds c o u p l e w i t h c r y s t a l l a t t i c e phonons t o form c o m p a r a t i v e l y s t a b l e s o l i t o n s . Examples o f c o m p e t i n g s i n g l e t a n d t r i p l e t p a t h w a y s f o r photochemi c a l t r a n s + cis i s o m e r i z a t i o n o f c e r t a i n s t i l b e n e s h a v i n g i n t r a m o l e c u l a r d o n o r - a c c e p t o r c h a r a c t e r h a v e b e e n d e s c r i b e d by G o r n e r . S e v e r a l o t h e r i n t e r e s t i n g s t u d i e s i n t h e f i e l d have a p p e a r e d t h i s year. V a r i o u s p h o t o p h y s i c a l t e c h n i q u e s c o n t i n u e t o be u s e d i n t h e s t u d y o f p o l y m e r s : some p a r t i c u l a r l y i n t e r e s t i n g work o n e l e c t r i c a l l y - c o n d u c t i n g p o l y m e r s h a s been d e s c r i b e d b y R o t h a n d B l e i e r , i n t e r a l i a . The p h o t o p h y s i c s o f t h i n f i l m s a n d c o l l o i d a l s y s t e m s , i n c l u d i n g micelles, c o n t i n u e s t o be a n i m p o r t a n t a n d a c t i v e f i e l d see e.g. K a l y a n s u n d a r a m ; Debe. C y c l o d e x t r i n s have of research: been found t o i n c r e a s e t h e chemiluminescence y i e l d s from aqueous p e r o x y o x a l a t e s by up t o 3 0 0 - f o l d (Woolf a n d G r a y e s k i ) . Enzymeg e n e r a t e d e x c i t e d s t a t e s o f a c e t o n e h a v e been f o u n d t o i n d u c e q u a s i - p h o t o c h e m i c a l b e h a v i o u r o f r i b o f l a v i n i n t h e d a r k (Rojas a n d S i l v a ) . From s t u d i e s o f t h e l u m i n e s c e n c e o f lo2, Schmidt a n d B r a u e r have c o n c l u d e d t h a t most o f t h e p r e v i o u s l y r e p o r t e d r e d e m i s s i o n d o e s n o t come from t h i s m o l e c u l e . D e l a y e d f l u o r e s c e n c e h a s been o b s e r v e d i n t h e d e a c t i v a t i o n o f h i g h l y e x c i t e d t r i p l e t s t a t e s ( S k v o r t s o v a n d A l f i m o v ; cf. McGimpsey and S c a i a n o ) . The p h o t o l y s i s of a r y l a z i d e s , long used as a source o f a r y l n i t r e n e s , h a s been shown a l s o t o p r o d u c e a t r a n s i e n t d e h y d r o a z e p i n e ( S h i e l d s e t a l . ; Liang and S c h u s t e r ) . I n a r e m a r k a b l e d e v e l o p m e n t , Nguyen e t a l . claim t h a t t h e y have been a b l e t o d e t e c t s i n g l e m o l e c u l e s by a l a s e r - i n d u c e d fluorescence procedure. B i o c h e m i s t s c o n t i n u e t o b e t h e main e x p l o i t e r s o f p h o t o p h y s i c a l t e c h n i q u e s , t h o u g h i t s h o u l d be p o i n t e d o u t t h a t t h e c o v e r age o f t h e s e p a r t i c u l a r a s p e c t s i s s e l e c t i v e r a t h e r t h a n comprehens i v e . Even p r o c e s s e s i n l i v i n g c e l l s are now b e i n g s t u d i e d (Vigo et a l . , i n t e r a l i a ) . Some a s p e c t s o f b i o l o g i c a l p h o t o c h e m i s t r y are u n d o u b t e d l y a n a c q u i r e d t a s t e . F o r e x a m p l e , a t h r e e - d i m e n s i o n a l p r e s e n t a t i o n of t h e t o t a l f l u o r e s c e n c e from u r i n e h a s e n a b l e d L e i n e r et a l . t o i d e n t i f y s e v e r a l f l u o r e s c e n t m e t abo 1i t e s
.
...
Introduction and Review of the Year
Vlll
W e now come t o d e v e l o p m e n t s i n i n o r g a n i c p h o t o c h e m i s t r y . G r g t z e l and co-workers have d e s c r i b e d a p h o t o e l e c t r i c d e v i c e
(1) R
= Me or Et
(3)
(4)R = H or Bun
(5) R’s
Ar
Q
RO
(9) R=Me, Et, or Pr’
(10)
H or Me
Introduction and Review of the Year
ix
r e s p o n s i v e t o t h e n e a r - i n f r a r e d , a n d w i t h an e f f i c i e n c y o f 37%. Carbon monoxide and hydrogen are produced by c o n c u r r e n t p h o t o r e d u c t i o n o f c a r b o n d i o x i d e and water u s i n g v i s i b l e l i g h t i r r a d i a t i o n o f s o l u t i o n s c o n t a i n i n g [ R ~ ( b p y ) ~ and I ~ +v a r i o u s C o ( I 1 ) species (Ziessel e t a l . ) . P h o t o e x c i t a t i o n o f a s y s t e m c o n t a i n i n g t h e same r u t h e n i u m complex, m e t h y l v i o l o g e n , and q u a d r i c y c l a n e c a n i n i t i a t e a c a t a l y t i c c y c l e f o r t h e valence isomerization of Ishida et a l . quadricyclane t o norbornadiene (Kutal et a l . ) . r e p o r t t h e p h o t o r e d u c t i o n o f C02 c a t a l y z e d by CRu(bpy)2(CO)Z!2+. B i a n c h i n i and Meli have described a n i n t e r e s t i n g new method f o r t h e i n t r o d u c t i o n o f s u l p h i d o , d i s u l p h u r , and d i s e l e n i u m l i g a n d s B e l m o r e et a l . have described t h e f i r s t i n t o complex frameworks. example of t h e p h o t o a c t i v a t i o n o f a m e t a l - c a r b o n d i o x i d e complex i n s o l u t i o n . Dobson e t a l . have d e v i s e d a p r o c e d u r e f o r t h e i d e n t i f i c a t i o n of co-ordinatively unsaturated s u b s t i t u t e d metal c a r b o n y l t r a n s i e n t s which e n a b l e v a l u e s of a l l r a t e c o n s t a n t s f o r l i g a n d d i s p l a c e m e n t r e a c t i o n s t o be o b t a i n e d . P h o t o i n s e r t i o n o f i s o n i t r i l e s i n t o t h e C-H bond of a r e n e s p r o d u c e s a l d i m i n e s , and is c a t a l y z e d by i r o n (Jones e t a l . ) . M e s s e l h h s e r e t a l . have d e s c r i b e d t h e p h o t o c h e m i c a l i n s e r t i o n o f an a l k e n e i n t o an S-S bond i n a c o - o r d i n a t i o n complex. S a k a k u r a and co-workers have d e s c r i b e d t h e r e g i o s e l e c t i v e c a r b o n y l a t i o n of a l i p h a t i c a n d a r o m a t i c h y d r o c a r b o n s i n t h e p r e s e n c e o f a rhodium complex. I n r e l a t e d work, these a u t h o r s have u s e d t h e same complex i n an i n t e r e s t i n g c h a i n - s h o r t e n i n g s y n t h e s i s o f t e r m i n a l a l k e n e s from a l k a n e s , e.g. p e n t a n e + b u t -1-ene. I r r a d i a t i o n o f t h e 0 3 / B r 2 complex l e a d s t o a n e w species w h i c h is b e l i e v e d t o be B r 2 0 ( T u r n e r e t a l . ).
I n t h e f i e l d of o r g a n i c p h o t o c h e m i s t r y , g r e a t l y i n c r e a s e d u s e is b e i n g made o f p h o t o c h e m i c a l p r o c e d u r e s i n t h e s y n t h e s i s o f n a t u r a l p r o d u c t s and other complex m o l e c u l e s . T h e r e i s growing
i n t e r e s t i n t h e e f f e c t s o f n o v e l e n v i r o n m e n t s o n t h e c o u r s e of p h o t o r e a c t i o n s , e.g. z e o l i t e s , membranes, and c y c l o d e x t r i n complexation. The p h o t o i s o m e r i z a t i o n of c y c l o b u t a n o n e s t o t r a n s i e n t c a r b e n e s h a s been u s e d a s p a r t o f an i n t e r e s t i n g s y n t h e s i s o f Intramolecular oxetan formation m u s c a r i n e ( P i r r u n g and D e A m i c i s ) . h a s been u s e d as p a r t o f n o v e l s y n t h e s e s o f medium-ring e t h e r s (1) and t h e t r i c y c l o - o c t a n e ( 2 ) (Carless e t a l . ; G l e i t e r and Kissler). Cossy e t a l . have employed t h e p h o t o r e d u c t i v e c y c l i z a -
X
Introduction and Review of the Year
t i o n of a m i d e s o r u n s a t u r a t e d k e t o n e s i n a new s y n t h e s i s o f hirsutene. P a t t e n d e n and T e a g u e h a v e a c h i e v e d t h e t o t a l s y n t h e s i s of t h e a n g u l a r t r i q u i n a n e ( 3 ) by p r o c e d u r e s which i n c l u d e i n t r a m o l e c u l a r ~ I T + ~ pI hTo t o c y c l o a d d i t i o n o f a n e n o n e . The u s e of c i r c u l a r l y p o l a r i z e d l i g h t f o r i r r a d i a t i o n h a s been l i t t l e e x p l o i t e d i n t h e p a s t , so t h e s y n t h e s i s o f t h e o p t i c a l l y a c t i v e e n o n e s ( 4 ) i n t h i s manner by C a v a z z a a n d Zandomeneghi i s of p a r t i c u l a r i n t e r e s t . Wang a n d P a q u e t t e h a v e d e s c r i b e d t h e The photochemical r o u t e to 1,3-bridged c y c l o - o c t a t e t r a e n e s . a z o m e t h i n e g r o u p i s n o r m a l l y of l o w p h o t o c h e m i c a l r e a c t i v i t y , b u t Kaneko e t a l . h a v e d e s c r i b e d a s y s t e m i n which a CF3 g r o u p a p p e a r s t o h a v e an a c t i v a t i n g e f f e c t , The o x a - d i - r - m e t h a n e p h o t o c h e m i c a l r e a r r a n g e m e n t o f B , y u n s a t u r a t e d systems normally o c c u r s v i a a t r i p l e t pathway; b u t F u c h s and c o - w o r k e r s h a v e d e s c r i b e d a n example i n which d i r e c t i r r a d i a t i o n (presumably s i n g l e t ) g i v e s t h e oxa-di-r-methane product whereas t r i p l e t s e n s i t i z a t i o n f o l l o w s a d i f f e r e n t path. The oxa-di-n-methane r e a r r a n g e m e n t o f b i c y c l o C 2 . 2 . 2 3 o c t e n o n e s h a s been e x p l o i t e d by S c h a f f n e r and Demuth t o p r o d u c e t r i c y c l o C 3 . 3 . 0 . 0 2 ~ 8 1 0 c t - 3 - o n e s , and R a j u and Deota h a v e e x t e n d e d t h i s procedure t o a s y n t h e s i s of l i n e a r t r i q u i n a n e s . Nagamatsu e t a l . h a v e shown t h a t l i q u i d c r y s t a l a n d v a r i o u s o t h e r media c a n g r e a t l y i n f l u e n c e t h e s t e r e o c h e m i s t r y o f u r a c i l p h o t o d i m e r i z a t i o n . P i r r u n g a n d Nunn h a v e shown t h a t i r r a d i a t i o n of a s e r i e s of q u i n o n e m o n o a c e t a l s ( 5 ) i n a c e t i c a c i d p r o v i d e s a f l e x i b l e high-yield r o u t e t o s u b s t i t u t e d cyclopentenones. Tu a n d M a r i a n o h a v e d e s c r i b e d a n i n t e r e s t i n g l - s t e p s y n t h e s i s o f t h e l-azabicycloC3.3.0loctane ( 6 ) by x a n t h o n e - s e n s i t i z e d i r r a d i a t i o n of t h e p y r r o l i n i u m p e r c h l o r a t e (7). Mukai e t a l . have d e s c r i b e d t h e u n e x p e c t e d c o n r o t a t o r y r i n g - o p e n i n g o f a c y c l o b u t e n e t o t h e c o r r e s p o n d i n g d i e n e which o c c u r s v i a c h a r g e - t r a n s f e r T s u j i and e x c i t a t i o n t o g i v e t h e r a d i c a l - c a t i o n of c y c l o b u t e n e . N i s h i d a h a v e r e p o r t e d t h a t i r r a d i a t i o n of t h e s t r a i n e d d i e n e ( 8 ) i n a l c o h o l i c s o l u t i o n g i v e s t h e p r o d u c t ( 9 ) which i s s u g g e s t e d t o r e s u l t from a d d i t i o n o f t h e s o l v e n t t o t h e photoisomer ( l o ) , A t t e n t i o n i s drawn s p e c t r o s c o p i c e v i d e n c e f o r which i s p r o v i d e d . t o t h e e x t r a o r d i n a r y p h o t o i s o m e r i z a t i o n (11) + (12) d e s c r i b e d b y Prinzbach et al.
Introduction and Review of the Year
xi
Cohen a n d co-workers have p r o v i d e d a v a l u a b l e r e v i e w o f c o n f o r m a t i o n e f f e c t s i n s o l i d - s t a t e p h o t o r e a c t i o n s . Mukai e t a l . have r e p o r t e d t h e r e m a r k a b l e d e s e n s i t i z e d p h o t o e l e c t r o n - t r a n s f e r i s o m e r i z a t i o n r e p r e s e n t e d by (13) + (14). T u r r o e t a l . have r e p o r t e d t h a t i r r a d i a t i o n of b e n z o c y c l o b u t e n e g i v e s t h e d i h y d r o p e n t a l e n e s ( 15 ) and (16 ) They p r o p o s e t h e intermediacy of t h e prefulvene-type biradical (17): t h i s t y p e o f i n t e r m e d i a t e was p r o p o s e d e a r l i e r by t h e S e n i o r R e p o r t e r s t o a c c o u n t f o r t h e p h o t o c h e m i c a l f o r m a t i o n o f f u l v e n e and benzv a l e n e from benzene. I t was o n c e t h o u g h t t o be i n v o l v e d i n t h e m e t a - c y c l o a d d i t i o n o f a l k e n e s t o b e n z e n e , b u t a d i f f e r e n t mechani s m is now g e n e r a l l y a c c e p t e d . L a l l e y a n d S p i l l a n e have d e s c r i b e d a n o v e l p h o t o c h e m i c a l r e a r r a n g e m e n t o f t h e sodium s a l t o f m-aminobenzenesulphonic a c i d . Wagner and Nahm h a v e described a n i n t r a m o l e c u l a r p h o t o c h e m i c a l c y c l o a d d i t i o n t o t h e benzene r i n g t h a t i s u n u s u a l i n o c c u r r i n g by a t r i p l e t pathway.
.
C y c l i c osmic esters have l o n g been known t o be i n v o l v e d i n t h e osmium t e t r o x i d e - c a t a l y z e d c i s - d i h y d r o x y l a t i o n o f a l k e n e s , b u t n o t a r e n e s . The i s o l a t i o n o f compound (18) by Wallis a n d Kochi f o l l o w i n g i r r a d i a t i o n o f t h e c h a r g e - t r a n s f e r complex between o s m i u m t e t r o x i d e and benzene i s t h e r e f o r e of p a r t i c u l a r i n t e r e s t , T h i s s u g g e s t s t h a t t h e c o r r e s p o n d i n g u s e of c a t a l y t i c q u a n t i t i e s of osmium t e t r o x i d e i n c o n j u n c t i o n w i t h hydrogen p e r o x i d e c o u l d l e a d t o t h e f o r m a t i o n of p o l y h y d r o x y l a t e d c y c l o h e x e n e s a n d - a n e s . A rare example o f p h o t o c h e m i c a l c y c l o a d d i t i o n t o a CZN g r o u p is p r o v i d e d by t h e f o r m a t i o n o f b e n z o x a z o l e ( 1 9 ) from p e n t a c h l o r o p h e n o l i n a c e t o n i t r i l e . The p h o t o c o n v e r s i o n of 2-methylbenzophenone i n t o t h e a n t h r o n e ( 2 0 ) i n v o l v e s t w o s e q u e n t i a l p h o t o processes *the E-enol ( 2 1 ) which u n d e r t h e i n f l u e n c e o f h i g h i n t e n s i t y laser l i g h t a b s o r b s a s e c o n d p h o t o n t o g i v e t h e c y c l i z e d i n t e r m e d i a t e ( 2 2 ) which u n d e r g o e s a i r - o x i d a t i o n t o ( 2 0 ) (Wilson e t a l . ). N i s h i o e t a l . h a v e described a n i n t e r e s t i n g p r o c e d u r e f o r w i t h pteridin-2,4,7-trione, a s o l i d which c a n be s t o r e d i n d e f i n i t e l y a t room t e m p e r a t u r e b u t which on warming r e v e r t s t o t h e p a r e n t t r i o n e w i t h l i b e r a t i o n o f s i n g l e t oxygen, A d a m and co-workers have d e s c r i b e d a c o n v e n i e n t one-pot s y n t h e s i s o f e p o x y - a l c o h o l s v i a p h o t o - o x y g e n a t i o n o f a l k e n e s i n t h e p r e s e n c e of a T i ( 1 V ) c a t a l y s t .
' s t o r i n g ' s i n g l e t oxygen as a n a d d u c t
Introduction and Review of the Year
xii
CL (19)
Ph
Me H
(22)
N
(24)
(23)
“6
Me3Si -N=Si
Mee i 0H 2 P h
NHBz
=N-SSiMe3
...
Introduction and Review of the Year
XI11
D k r e t a l . h a v e d e s c r i b e d some n o v e l complex c i n n o l i n e d e r i v a t i v e s which show p h o t o c h r o m i c p r o p e r t i e s .
A l b i n i e t a l . have
p r o v i d e d some new e v i d e n c e o n t h e mechanism f o r t h e p h o t o i s o m e r i z a t i o n o f h e t e r o c y c l i c !-oxides. exceptional.
Simple p y r i d i n e N-oxides
Thus i r r a d i a t i o n o f p y r i d i n e x-oxide
a f f o r d s t h e ring-opened product (23).
are
i n aqueous base
Aoyama e t a l . h a v e
d e s c r i b e d t h e u n p r e c e d e n t e d p h o t o c y c l i z a t i o n o f t h e amide ( 2 4 ) t o t h e lactam ( 2 5 ) .
The f i r s t e x a m p l e s o f C2a+21~1p h o t o r e a c t i o n s o f
a three-membered r i n g and an azo-compound h a v e b e e n d e s c r i b e d by Hunig a n d S c h m i t t . N i c o l a o u et a l . h a v e p r e p a r e d t h e f i r s t s t a b l e example o f a 1 , 2 - d i t h i e t h a n e :
t h e procedure involves (21~+2~r)
p h o t o d i m e r i z a t i o n o f C=S g r o u p s . T r a n s i e n t t r i p l e t b i r a d i c a l s h a v e been t r a p p e d by r e a c t i o n w i t h oxygen:
t h e procedure is claimed t o provide an e f f e c t i v e
method f o r e s t i m a t i n g t h e l i f e t i m e s o f t h e s e s p e c i e s (Adam e t a l . ). S t r e i t h e t a l . h a v e d e s c r i b e d t h e f i r s t example o f a p y r i d i n o cyclopropene (26 )
.
The r e m a r k a b l e s i l a n e d i i m i n e (273 h a s been p r e p a r e d by i r r a d i a t i o n o f t h e a z i d e ( 2 8 ) ( K l i n g l e r and P r i n z b a c h ) .
The
u n u s u a l l y s t a b l e t r i a z i r i d i n e ( 2 9 ) h a s b e e n p r e p a r e d by p h o t o l y s i s of t h e corresponding azide. Delduc e t a l . have r e p o r t e d t h a t t h e p h o t o l y s i s of 2 - a l k y l and S - a c y l x a n t h a t e s p r o v i d e s a u s e f u l s o u r c e o f f r e e a l k y l a n d acyl radicals.
P h o t o l y s i s of t h e imide (30) g i v e s tetramethyl-
c y c l o b u t a d i e n e (Kashima e t a l . ).
W e conclude w i t h r e f e r e n c e s t o developments i n polymer p ho t o c h e m i s t r y
.
Polymer f i l m s h a v e been o b t a i n e d by p l a s m a p o l y m e r i z a t i o n o f hexafluorobenzene, N-vinylpyrrolidine,
and c h l o r a c r y l o n i t r i l e
Higuchi e t a l . h a v e shown t h a t i r r a d i a t i o n o f an a z o b e n z e n e -mod i f i e d po 1y ( Y -met h y 1-L -g 1u t a m a t e -CO-L -g 1u t am i c ac i d ) (Munro).
i n b i l a y e r membrane v e s i c l e s o f distearyldimethylammonium c h l o r i d e l e a d s t o t r a n s - c i s i s o m e r i z a t i o n o f t h e polymer:
t h i s leads t o
t r a n s f e r o f t h e p o l y p e p t i d e f r o m t h e h y d r o p h o b i c b i l a y e r membrane inter.ior to the hydrophilic surface. As a r e s u l t , ' t h e r e w a s a d e c r e a s e i n t h e i o n p e r m e a b i l i t y t h r o u g h t h e b i l a y e r membrane a n d t h e f o r m a t i o n of i n t e r v e s i c u l a r a d h e s i o n .
E l s n e r a n d Ritter h a v e
p r e p a r e d p h o t o s e n s i t i v e membranes from a n a r o m a t i c p o l y a m i d e a n d
a c i n n a m a t e t h a t i n c o r p o r a t e s a l i q u i d c r y s t a l l i n e component.
Contents PART I
PHYSICAL ASPECTS OF PHOTOCHEMISTRY Photophysical Processes in Condensed Phases By R.B. Cundall
3
1
General
3
2
Singlet State Processes
9
2.1 2.2 2.3 2.4
2.5 2.6
Electron Transfer Reactions and Exciplexes Dyes and Related Systems Photoisomerization and Related Processes Electronic Excitation Energy Transfer Polymeric Systems Colloidal and Heterogeneous Systems
14 17 19 22
24 25
3
Triplet State Processes
29
4
Other Chemical Systems
34
5
Biological Systems
36
References
40
PART I1
PHOTOCHEMISTRY OF INORGANIC AND ORGANOMETALLIC COMPOUNDS
Chapter 1
The Photochemistry of Transition-metal Complexes By A . Cox
65
1
Introduction
65
2
Titanium
65
3
Vanadium, Niobium, and Tantalum
66
4
Chromium, Molybdenum, and Tungsten
67
5
Manganese and Rhenium
69
6
Iron
70
7 Ruthenium
71
8
Osmium
77
9
Cobalt
77
xv
Contents
xvi 10 Rhodium and Iridium
79
11 Nickel
81
12 Platinum
82
13 Copper and Gold
83
14 Lanthanides
84
15 Uranium
86
16 Actinides
88
17 Miscellaneous
88
References
88
The Photochemistry of Transition-metal Organometallic Compounds By A. Cox
103
1
Introduction
103
2
Titanium
103
3
Niobium
105
Chapter 2
4 Chromium, Molybdenum, and Tungsten
105
5
Manganese and Rhenium
108
6
Iron
111
7
Ruthenium
116
8
Osmium
117
9
Cobalt
117
10 Cobalt and Iridium
118
11 Rhodium and Iridium
118
12 Nickel
122
13 Palladium and Platinum
123
14 Copper and Silver
124
15 Miscellaneous
124
References
124
The Photochemistry of Compounds of the Main Group Elements By A. Cox
135
Introduction
135
Chapter 3
1
Contents
xvii 2
Boron and Indium
135
3
Silicon, Germanium, and Tin
137
4
Nitrogen and Phosphorus
141
5
Oxygen, Sulphur, and Selenium
142
6 Halogens
142
Miscellaneous
143
References
143
PART I11
ORGANIC ASPECTS OF PHOTOCHEMISTRY
151
Chapter 1
Photolysis of Carbonyl Compounds By W. M . Horspool
151
1
Norrish Type I Reactions
151
2
Norrish Type I1 Reactions
156
3
Oxetane Formation
162
4
Miscellaneous Reactions
164
References
170
Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By W.M. Horspool
173
Cycloaddition Reactions
173
7
Chapter 2
1
Intramolecular Intermolecular Dimerization 2
Rearrangement Reactions ai,
p-Unsaturated Systems
p, y,-UnsaturatedSystems
173 185 195 197 197 208
3
Photoreactions of Thymines and Related Compounds
211
4
Photochemistry of Dienones
214
I
Cross-Conjugated Dienones Linearly Conjugated Dienones
214 222
5
1,2-,1,3-, an 1,4-Diketones
222
6
Quinones
232
Reference
237
Contents
xviii Chapter 3
Photochemistry of Alkenes, Alkynes, and Related Compounds
245
By W.M. Horspool 1
Reactions of Alkenes Addition Reactions cis-trans Isomerization Hydrogen Abstraction Miscellaneous Reactions
245 245 245 249 251
2
Reactions involving Cyclopropane Rings
253
3
Reactions of Dienes, Trienes, and Higher Polyenes
261
4
[ 2 + 2 ] Intramolecular Additions
267
5
Dimerization and Intermolecular Additions
273
6
Miscellaneous Reactions
273
Reference
280
Photochemistry of Aromatic Compounds By A . C . Weedon
285
Introduction
285
1
Isomerization Reactions
287
2
Addition Reactions
299
3
Substitution Reactions
309
4
Intramolecular Cyclization Reactions
317
5
Dimerization Reactions
327
6
Lateral Nuclear Rearrangements
328
7
Peripheral Photochemistry
333
Reference
338
Photoreduction and -oxidation By A. Cox
344
1
Introduction
344
2
Reduction of the Carbonyl Group
344
3
Reduction of Nitrogen-containing Compounds
347
4
Miscellaneous Reductions
348
5
Singlet Oxygen
348
6
Oxidation of Aliphatic Compounds
3 50
Chapter 4
Chapter 5
Contents
xix
7 Oxidation of Aromatic Compounds
352
8
Oxidation of Nitrogen-containing Compounds
355
9
Miscellaneous Oxidations
356
References
357
Photoreactions of Compounds containing Heteroatoms other than Oxygen By S.T. R e i d
366
Chapter 6
1 Nitrogen-containing Compounds Rearrangements Addition reactions Miscellaneous Reactions
366 366 383 392
2 Sulphur-containing Compounds
392
3
Compounds containing other Heteroatoms
40 1
References
407
Photoelimination By S.T. R e i d
413
1
Elimination of Nitrogen from Azo-compounds
413
2
Elimination of Nitrogen from Diazo-compounds
423
3
Elimination of Nitrogen from Azides
429
4
Photoelimination of Carbon Dioxide
434
5
Fragmentation of Organosulphur Compounds
436
6
Miscellaneous Decomposition and Elimination Reactions
439
References
447
POLYMER PHOTOCHEMISTRY By N . S . Allen
455
1
Introduction
455
2
Photopolymerization
455
2.1 2.2 2.3
456 462 466
Chapter 7
PART IV
Photoinitiated Addition Polymerisation Photocrosslinking Photografting
3
Optical and Luminescence Properties
466
4
Photodegradation and Photooxidation of Polymers
476
Contents
xx
4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9
Polyolefins Poly(vinylha1ides) Polystyrenes Polyacrylics Polyamides Poly(Organosi1anes) Rubbers and Polywethanes Natural Polymers Miscellaneous Polymers
476 477 478 478 479 479 479 480 481
5
Photostabilisation Process
483
6
Dyes and Pigments
487
References
490
Part I PHYSICAL ASPECTS OF PHOTOCHEMISTRY By R. B. CUNDALL
Photophysical Processes in Condensed Phases BY R. 6. CUNDALL
It h a s b e c o m e a p p a r e n t o v e r t h e l a s t f e w y e a r s t h a t photophysical research has been largely concerned with the very detailed analysis o f
particular systems.
f w d i n g are reflected
in
The economics of research
t h e s t y l e and e x t e n t t o w h i c h g r o u p s i n
ditferent parts o f t h e world are able to investigate specific problems.
T h e a p p l i c a t i o n o f p h o t o p h y s i c a l p r o c e s s e s in a r e a s s u c h
as molecular electronics
i s s t i l l i n a n e m b r y o n i c s t a t e and m u c h
m o r e w o r k n e e d s t o be d o n e b e f o r e s u b s t a n t i a l p r o g r e s s t o w a r d s u s e f u l d e v i c e s w i l l be a c h i e v e d . T h i s r e v i e w f o l l o w s t h e p a t t e r n o f p r e v i o u s y e a r s and deliberately
s e t s o u t t o p r o v i d e a broad
over a wide area.
survey o f published w o r k
T h i s is c o n s i d e r e d t o be m o r e g e n e r a l l y u s e f u l
than a discussion of a limited number o f special topics. Biochemists remain t h e leading exploiters o f
photophysical
t e c h n i q u e s and it is n e c e s s a r y t o p o i n t o u t t h a t c o v e r a g e o f t h e b i o l o g i c a l and m e d i c a l a r e a s i s not c o m p r e h e n s i v e . Experimentally
t h e past year has seen t h e ingenious
a p p l i c a t i o n o f t h e very
short time resolution spectroscopic
t e c h n i q u e s o f very r a p i d p r o c e s s e s s u c h a s s o l v e n t r e l a x a t i o n and electron transfer following electronic excitation. Subpicosecond o f detecting
t i m e s c a l e r e s o l u t i o n has e v e n r e a c h e d t h e s t a g e
w h a t c a n l o o s e l y be d e s c r i b e d a s t h e “ t r a n s i t i o n
states“ of some o f the most elementary reactions such as unimolecular dissociation.
1
A
General
very l a r g e number o f papers presented
at t h e F i r s t
I n t e r n a t i o n a l L a s e r C o n f e r e n c e i n 1987 g i v e a c o m p l e t e representation lasers’.
o f photochemical research involving t h e use o f
Extensive applications t o l u m i n e s c e n c e and photochemistry
are reported.
A
b o o k e d i t e d by L e t o k h o v 2 o n l a s e r p i c o s e c o n d
s p e c t r o s c o p y and p h o t o c h e m i s t r y
of
3
biomolecules
is largely
Photochemistry
4 concerned w i t h
techniques
and c a t e g o r i e s
o f
compound.
The s u r v e y
i s e s p e c i a l l y v a l u a b l e i n i t s emphasis on Russian work.
The
b i e n n i a l r e v i e w o f luminescence published i n A n a l y t i c a l Chemistrv, i s very
compsehensive and p a r t i c u l a r l y
analysis3
.
special issue of
A
proceedings
o f
i n biomedical. scierice:;.
u s i n g f l u o r e s c e n c e decay
t i m e as
e x c i t e d s t a t e behawiour
wide variety
o f
photoactions.
fl.uosescence
spectrometry
p h o t o c h e m i s t r y and examines t o .solvent
effects
A
technique
t h e power
review o f
available adjunct t o
the multidimensional fluorescence
and u t i l i t y o f
maximising information obtainable from steady
t h i s method f o r s t a t e luminescence7.
T i m e r e s o l v e d l u m i n e s c e n c e i s now t o b e r e g a r d e d a s e x t e n s i v e reviews d e a l w i t h phase r e s o l v e d a n a l y s i s and w i t h l a n t h a n i d e probes. value of of
identification’.
biomolecule tagging strongly fieldg.
The
by
recommmends
former
lanthanide chelates
t h e use o f
t h r e e year
emphasises t h e
selectively as
a means
i n immunoassay e t c .
time resolved
Transmission spectroscopy o f
organized
Two
The l a t t e r d i s c u s : j e s a p p l i c a t i o n o f
systems
review o f the esoteric
luminescence d e a l s w i t h equipment
fluorescence
and
in t h i s
o r i e n t e d molecules by
is a l s o s i g n i f i c a n t i n view o f
p o l a r i z a t i o n modulation spectroscopy the importance o f
routine.
fluorescence i n chemical
incorporating fluorescence l i f e t i m e
i:omponent
i n a
C u m p u t e r s make r n u l t i d i r n e n s i u n a l
a readily
e s t a b l i s h e d methods. illustrates
oxygen
the information carrier5.
has reviewed picosecond o r g a n i c
the relation o f
inany
typic^. 1. o f
i s o n e w h i c h d e s ~ : r j . b e s a f J . b s ~ ?0 p t J . c
~.ntesr?:;tiny p a p e r s
Peters‘
chemical
2nd I n t e r n a t i o n a l Symposium on q u a n t i t a t i v e
luminescence spectrometry
sensor
relevant t o
A n a l y t i c a l C h i m i c a A c t a p ~ ’ u v i . d t , st h e
i n technology
subject o f
and biology.’’.
A
circu1arl.y polarized
and r e s u l t s
largely for
chiral
m e t a l ~ o m p l e x e s ’ ~ .C h a n g e s i n c h i r a l i t y o n e l e c t r o n i c e x c i t a t i o n should prove a A
f r u i t f u l f i e l d
for
further
research.
NATO A S 1 w h o s e p r o c e e d i n g s h a v e b e e n p u b l i s h e d 1 2
i t s e l f with
supramolecular photochemistry.
Amongst
c:oncerned
t h e 30
papers
are contributions which deal w i t h photoinduced charge separation and energy m i g r a t i o n .
excited
The
Journal o f
Luminescence published
a c o l l e c t i o n o f papers on d y n a m i c a l processes i n
during the year
s t a t e s of
solids’3.
c o v e r e d i n some o r g a n i c h o l e b u r n i n g was
one o f
A l l
i n t e r d i s c i p l i n a r y aspects
b u t m a i n l y i n o r g a n i c systems. the techniques described.
u s u a l l y o n l y occurs on t h e picosecond o r because o f t h e circumstances
s h o r t h o l e decay
are
Optical
This
phenomenon
femtosecond t i m e s c a l e
t i m e a l t h o u g h it c a n u n d e r
be found on t h e nanosecond t i m e
scale.
some
R e s u l t s hawe
5
I: Photophysical Processes in Condensed Phases been g i v e n for p - t e r p h e n y l ” .
Brauchlel’
p r e s e n t s a phase
m o d u l a t e d h o l o g r a p h i c g r a t i n g t e c h n i q u e f o r use in o p t i c a l solid State spectroscopy.
It has been applied t o p h o t o a c t i v e m o l e c u l e s
in p o l y m e r i c e n v i r o n m e n t s .
N e w r e a c t i o n paths c a n be i n d u c e d by
two photon laser excitation16.
T h e s e may o c c u r in e x p e r i m e n t a l
systems by e i t h e r a c c i d e n t o r d e s i g n and a r e s h o w n t o o p e r a t e in a w i d e variety o f systems. T h e N o t r e D a m e g r o u p d i s c u s s t h e p r o c e d u r e s used i n t h e a s s e m b l y o f n u m e r i c d a t a b a s e s on t h e k i n e t i c s o f t r a n s i e n t s p e c i e s in SOlUtiOn”.
Recent c o m p i l a t i o n s are d e s c r i b e d and m e t h o d s of
d a t a c o l l e c t i o n and use e x p l a i n e d . An e x t r e m e l y e x c i t i n g a c h i e v e m e n t i s t h e u s e o f r e a l t i m e f e m t o s e c o n d p r o b i n g o f t r a n s i t i o n s t a t e s in c h e m i c a l reactions’?. The m e t h o d , w h i c h shows p r o m i s e f o r t h e s t u d y o f b i m o l e c u l a r and u n i m o l e c u l a r r e a c t i o n s , has a l r e a d y been applied t o t h e d e c o m p o s i t i o n o f ICN. picosecond
R ~ l l i e r e ’ has ~ discussed the application of
s p e c t r o s c o p y t o study a variety o f e l e m e n t a r y
processes. T h e r e a r e f e w t h e o r e t i c a l p a p e r s on p h o t o p h y s i c s i n t h e period u n d e r r e v i e w and no p a r t i c u l a r t h e m e s a r e e v i d e n t . i n t e r e s t i n g t o p i c s h o w e v e r h a v e been c o n s i d e r e d . fractals in luminescence
Some
T h e use o f
s h o w s h o w t h i s s i g n i f i c a n t concept can
be applied t o t h e b e h a v i o u r o f n o n h o m o g e n o u s s y s t e m s z 0 .
A few
i n v e s t i g a t i o n s h a v e a l r e a d y used t h i s a p p r o a c h in t h e study of energy transport.
T h e t h e o r y of b a r r i e r l e s s e l e c t r o n i c r e l a t i o n
in s o l u t i o n t a k i n g i n t o a c c o u n t solvent v i s c o s i t y , c h a n g e o f polarity w i t h t e m p e r a t u r e has been t r e a t e d by Bachi2’.
A combined
e x p e r i m e n t a l and t h e o r e t i c a l study o f h o r i z o n t a l t r a n s i t i o n s in i n t e r n a l m o l e c u l a r t w i s t i n g h a s been m a d e f o r a r o m a t i c m o l e c u l e s including ~ - d i m e t h y l a m i n o b e n z o n i t r i l e 2 2 . Luminescence studies o f i n e r t i a l m o t i o n in l i q u i d s c o n s t i t u t e s an i m p o r t a n t part o f present d a y p h o t o p h y s i c s 2 3 .
Subpicosecond molecular dynamics can
be s t u d i e s by d e g e n e r a t e f o u r - w a v e m i x i n g a s e x e m p l i f i e d by w o r k w i t h CS2 and n i t r o b e n z e n e Z 4 .
A r i g i d polar p r o b e has a l s o b e e n
used t o e x a m i n e t h e r o l e o f s o l v e n t m i c r o s c o p i c r e l a x a t i o n i n highly p o l a r a p r o t i c s o l v e n t ~ 2 ~ .T h e r e i s a good c o r r e l a t i o n b e t w e e n s o l v a t i o n t i m e and t h e m i c r o s c o p i c r e l a x a t i o n t i m e o f individual solvent molecules.
The effect of small clusters in t h e
m o l e c u l a r e n v i r o n m e n t on o s c i l l a t o r s t r e n g t h and s p e c t r a h a v e been examined f o r t e t r a c e n e + Ar and c y a n o a n t h r a c e n e dynamics o f polar solvation has been measured
+
Ar or X e Z 6 .
The
for several probe
6
Photochemistry
molecules by fluorescence upconversion with subpicosecond r e s o l u t i ~ n ~ ~SimonZa . has reviewed the subject for polar media including solvation of e l e c t r o n s , rotational diffusion as evidence of dielectric f r i c t i o n , and time dependent Stokes shifts a s probes o f solvation dynamics.
The influence o f non-Debye relaxation and
molecular s h a p e on the time dependence of the Stokes shift i s examined also by Castner et a 1 J 9 . in polar solvents.
Electron transfer i s possible
A comprehensive analysis of solvent
reorganization in optical and thermal electron transfer p r o c e s s e s , including solvatochromism and intramolecular electron-transfer in spheroidal molecules gives an expression which can be correlated
.
with earlier work3'
Excited state d i p o l e moments and polarization
in centrosymmetric and dimeric mo1ei:ules
have been theoretically
c:alculated for bichromophoric m o l e c u l e s , p o l y e n e s , p o l y y n e s , and cumulenes3'
I
32
.
A q u i t e detailed theory of the effect o f heavy atoms on singlet and triplet states of aromatic molecules has been presented by Najbar33. F r e d r i c k s 0 n 3 ~ has formulated expressions f o r the concentration depolarization o f fluorescence in the presence of molecular rotation.
A
theoretical examination o f diffusion
influenced fluorescence quenching by nearest possible quenching neighbours in liquids has been m a d e 3 = . Smoluchowski
A modified version of
- Collins - Kimball formulation
of
- Volmer
the Stern
equations has been matched w i t h experimental data f o r quenching of anthraquinone derivatives by
N,N-dimethyl-g-toluidine.
Another
paper d i s c u s s e s this work on t h e basis o f t h e kinetics o f partly diffusion controlled reactions36. An interesting state model has been applied t o e x c i t e d - s t a t e proton transfer reactions and used t o analyse data for a number of ~ y s t e m s 3 ~ .End t o end distance distribution of flexible m o l e c u l e s have been estimated from steady state fluorescence energy transfer and the e f f e c t o f quenching induced changes o n the Fbrster exchange distance3'.
For naphthalene ( d o n o r ) and aminonaphthalene
(acceptor) separated by a methylene chain results agree w i t h frequency domain
measurement^^^.
A pioneering study o f some note has been reported which d e a l s w i t h the r o l e o f coherence in disordered matter40.
This paper
examines t h e existence o f coherent ( o r d i s s i p a t i v e ) structures i n amorphous matter l i k e liquids w h i c h a r i s e from thermal motion. Electronic transltiOnS affect the behaviour o f many molecules
I: Photophysical Processes in Condensed Phases
7
around t h e e x c i t e d s p e c i e s and t h i s l e a d s t o c o o p e r a t i v e e f f e c t s w h i c h may be d e t e c t e d by e x p e r i m e n t . As
p h o t o c h e m i s t r y b e c o m e s used i n o t h e r f i e l d s s u b j e c t s w h i c h
m i g h t be c o n s i d e r e d " c l a s s i c a l " c o n t i n u e t o f i g u r e in t h e literature.
F o r e x a m p l e a paper c o n s i d e r s t h e s t a b i l i t y o f l i g h t
s o u r c e s and i m p l i c a t i o n s f o r p h o t o b i o l o g i c a l s t u d i e s h 1 .
A
new
f l u o r e s c e n c e s t a n d a r d , [ 2 , 2 / - b i p y r i d y 1 1 - 3 , 3 - d i o l . has been suggestedh2.
It has t h e a d v a n t a g e o f having a broad g r e e n
f l u o r e s c e n c e band w e l l s e p a r a t e d f r o m t h e a b s o r p t i o n and a q u a n t u m f i e l d o f 0.3 2 0.03 at r o o m t e m p e r a t u r e .
The photoreversible
f u l g i d e , A b e r c h r o m e 5 4 0 . i s a u s e f u l a c t i n o m e t e r f o r s i n g l e or double laser excitation experiments43.
Measurement o f primary
p r o c e s s e s by l a s e r i n d u c e d o p t o c o u s t i c s p e c t r o s c o p y is a p o w e r f u l technique44.
A method
has been r e p o r t e d f o r m e a s u r i n g a b s o l u t e
f l u o r e s c e n c e y i e l d s by m e a s u r e m e n t o f r e l a t i v e f l u o r e s c e n c e e m i s s i o n and n o n r a d i a t i v e
(photoacoustic) datah5.
Low
c o n c e n t r a t i o n s o f a heavy a t o m q u e n c h e r ( C 2 H g I ) very c o n v e n i e n t l y e l i m i n a t e s t h e need f o r a b s o l u t e c a l i b r a t i o n p r o c e d u r e s . Three papers deal with the effect of variation o f temperature on t h e second d e r i v a t i v e o f f l u o r e s c e n c e
Derivative
s p e c t r a g i v e b e t t e r r e s o l u t i o n o f o v e r l a p p i n g bands and d i s c r i m i n a t e very w e l l t h o s e w i t h n a r r o w b a n d w i d t h s . F l u o r e s c e n c e p o l a r i z a t i o n has proved t o g i v e high s e n s i t i v i t y in d e t e r m i n a t i o n o f a v i d i n and b i o t i n by m e a n s o f t h e f l u o r e s c e i n conjugateh7.
S y n c h r o n o u s d e r i v a t i v e s p e c t r o s c o p y c a n a l s o be
u s e f u l as e x e m p l i f i e d by m e a s u r e m e n t o f p e s t i c i d e
composition^^^.
T h e r e has been a c o n s i d e r a b l e d e c l i n e in t h e n u m b e r o f p a p e r s w h i c h d e a l w i t h t h e d e t a i l s of t e c h n i q u e s o f m e a s u r e m e n t o f fluorescence decay.
This is no doubt d u e to the fact that t h e
alternative methods are now essentially w e l l established. Nevertheless a microcomputerized
u l t r a h i g h speed t r a n s i e n t
d i g i t i z e r and l u m i n e s c e n c e l i f e l i n e i n s t r u m e n t has been describedh9.
A
very u s e f u l m u l t i p l e x e d a r r a y f l u o r o m e t e r a l l o w s
s i m u l t a n e o u s f l u o r e s c e n c e d e c a y at d i f f e r e n t e m i s s i o n w a v e l e n g t h u s i n g s i n g l e p h o t o n t i m i n g array d e t e c t i o n s 0 .
Data c o l l e c t i o n
r a t e s could a p p r o a c h t h a t f o r a r e p e t i t i v e l a s e r p u l s e s y s t e m and t h e t e c h n i q u e could be u s e f u l l y applied
t o H P L C or m i c r o s c o p y . T h e
p o w e r of t h i s e q u i p m e n t has been e x e m p l i f i e d by s t u d i e s o n
e-
a m i n o t e t r a p h e n y l p o r p n y r i n s at e m i s s i o n w a v e l e n g t h s u p t o 6 8 0 nm. T h e u s e and p e r f o r m a n c e o f t h e d e l t a f u n c t i o n c o n v o l u t i o n method for t h e e s t i m a t i o n o f f l u o r e s c e n c e d e c a y p a r a m e t e r s has been
Photochemistry
8 c r i t i c a l l y appraised. combining t h e with
from a
that
There
simultaneous
reference samples3.
d i s t r i b u t i o n o f decay r a t e s of
are simplifying
analysis of
for
a f a s t d e c o n v o l u t i o n method
analysis o f
i s a l s o l i k e l y t o be v e r y u s e f u l s 4 .
time
t y p e PM t u b e s
several side-on basis
for
the proper
selection o f
the analysis of
Details
f l u o r e s c e n c e decay t h a n t h e decay
Time r e s p o n s e d a t a
either
of
these tubes
Fluorescence
Time-resolved
i n o r g a n i c powders;
apart
peak h a l f w i d t h ,
and G r a t t o n ”
discuss various
a
i n single
l i f e t i m e s measured by
luminescence
peak maxima,
for
provides
have been used t o m o n i t o r e l u e n t s
l i q u i d chromatographys6.
measurements,
t o a Poisson
(Hamamatsu R 9 2 8 and R 9 5 5 )
photon counting experimentss5. use o f l a s e r e x c i t a t i o n
i n
is considered.
pulse r e p e t i t i o n p e r i o d is l e s s
when t h e e x c i t a t i o n
for
Application
a sample for
advantages
s i n g l e photon t i m i n g data
during
has been used
from l i f e t i m e and i n t e n s i t y d a t a a r e
employeds7. Thompson
t h e phase s h i f t
fluorometric
Rayleigh scattering
shows
w a v e l e n g t h dependence The m e a s u r e m e n t fluorometry
apparentcolour
associated
other
forms
of
anisotropy
10 p s
of
e f f e c t s due t o t h e times by
i n the
frequency
PM t u b e .
- domain
i n details9. A
f l u o r o m e t e r w h i c h uses t h e o p t i c a l K e r r e f f e c t over
for nm d o w n
t h e range 390-900
is u s e f u l f o r t h e s t u d y o f u l t r a f a s t dynamics i n
solution60.
i n g e n i o u s waveguide detecting very
for
0 2 , pH,
capillary
flow c e l l for
passing along the fibre63.
c o o l i n g b e l t designed
keeps t h e sample
at
15K for
fluorometry
for
samples t o e v a p o r a t e q u i c k l y a f t e r
A
l o w temperature
number o f
systems
fluorescence
spectra
An
stage6‘.
intensity i n of
noise i n
has been reduced by u s i n g an OMA66.
f o r making measurements on l i v i n g c e l l s OMA
has been used t o o b t a i n subnanosecond
and a n i ~ o t r o p y ~ ~ T h. i s
study t h e s t i m u l u s induced p l a t e l e t granules.
luminescence
t h e 5 0 pdm3
leaving the cold
f l a s h p h o t o l y ~ e sa~n d~ t h e e f f e c t
have been d e s c r i b e d .
fibres
self-cleaning
Dye f l u o r e s c e n c e h a s b e e n u s e d t o m o n i t o r nitrogen laser
capable o f
l i q u i d core
measurement b u t a l l o w s
s t u d y i n g p h o t o l a b i l e samples
for
a n d e n ~ y m e s ~ ’ , ~ *A. v e r y
l o w l e v e l s i n v o l v e s t h e use o f
with laser l i g h t continuous
f i n d many u s e s i n l u m i n e s c e n c e ,
Optical fibres
example w i t h sensors
A
aspects
equipment have been r e p o r t e d .
o b t a i n i n g t i m e resolved emissions to
technical
f o r determining lifetimes.
electron transit
has been d e s c r i b e d
Various picosecond
of
of
method
secretion o f
A r a p i d scanning
has been used t o
acriflavine-loaded
fluorescence
blood
spectrometer
has
I: Photophysical Processes in Condensed Phases
9
been used w i t h an acoustooptical tunable filter t o monitor pH and Ca2+ concentrations i n living cells68.
A
set up for measuring
fluorescence d e c a y s by means of microscope optics has been used w i t h living cells6'.
An improved apparatus for measurement o f
correlation functions by fluorescence in observed volumes o f about 1 0 pm3
c a n give diffusion data70.
T w o other items are o f particular g e n e r a l interest.
The
observation that picosecond optical phase conjugation can be induced in conjugated organic molecules is of considerable significance for optoelectronics7'. Locke and L i m t 2
have reported the first example o f bicemer,
a d i m e r i c species formed by association o f two electronically excited molecules.
Charge-transfer stabilization occurs by
association of two identical triplet states or intramolecular association in 1 , l - d i - a - n a p h t h y l e t h a n e and I , l - d i - ( 9 - a n t h r y l ) ethane in polar solvents73
2
Sinqlet State Processes
The details o f t h e mechanism of decay o f S 1 retain
their interest.
states in alkanes
The effect of deuterium on fluorescence
lifetimes has been discussed in terms of the theory o f radiationless tran'sition~~'.
Analysis of fluorescence l i n e shapes
and Raman excitation profiles o f t e t r a d e s m e t h y l - @ - c a r o t e n e i n isopentane has been carried out at 1 9 0 and 23QK7'.
Solvation
occurs over a time scale o f about 100 f s whilst vibrational relaxation has a time scale o f about 2 5 0 fs.
The kinetics o f the
interaction o f alcohols with the excited state o f triethylamine shows involvement o f a charge transfer e x c i p l e ~ ~ ~Ionizing . radiation i s a means o f exciting saturated hydrocarbons and t h e complexity of three component systems containing saturated hydrocarbons, aromatic s o l v e n t , and fluorescent solute has been examinedT7. Benzene and its simple derivatives still provides intriguing problems for photophysics.
Tests for scrambling of the ' B z u
state
of benzene have been suggested7B and a theoretical study o f the mechanism of i n t e r n a l conversion of S ,
state m a d e by K a t ~ ~ ~ .
Measurements of two photon cross-sections for liquid benzene and methylbenzene have used two photon induced fluorescence observations for t h e first t i m e a o .
Picosecond transient absorption
measurement o f geminate electron-cation recombination have been
Photochemistry
10
studied by 2 photon ionization of benzene o r azulene in liquid hexane”.
T h e generate pairs have a radius o f about
absorption at 266 nm.
50A
and s h o w
Radiationless decays have been investigated
i n a range o f substituted alkylbenzenes o f the type Ar(CH2 1 . ~ 8 2 . The effects of values.
X
on k f and k i , d o not extend to n = 2 and higher
Two papers describe the photophysical properties o f the
anaesthetic 2-aminobenzoates.
One o f t h e s e deals w i t h solvent
relaxation processesB3 and the other with the photophysical behaviour in defined solvents and phospholipid v e s i c l e s B 4 .
Time
resolved analysis o f data in t h e latter shows t h e multiplicity o f environments w h i c h exist in biological assemblies. doublet
The doublet-
fluorescence o f benzyl radicals in solution shows
nonradiative relaxation occurs in these species8’.
An excimer o f
9-methyladenine which is not formed by diffusion in aqueous solution is observable by ps time resolved spectroscopye6. The excimer i s formed v i a a weakly coupled stacked d i m e r which exists between t w o ground state monomers. The hindrance o f rotational relaxation i n excited states
of
biphenyl and e - t e r p h e n y l by methyl
substituents affects relaxation processes: experimental data are consistent w i t h theoretical calculations”.
T h e photophysical
properties have been measured and photoisomerization mechanisms proposed in case of the 1 , 3 - d i p h e n y l a l l y l c a r b a n i o n 8 8 , 8 9 . Hydrostatic pressure i s useful for obtaining details of photochemical mechanisms.
It has been used t o s h o w that in the
excited s t a t e o f tetramethyl-g-terphenyl hindrance t o conformational relaxation of a twisted ground state is incomplete even in g l y c e r o l s o l v e n t g 0 .
The emission spectrum o f triphenylene
has been reported in detailg’.
Diphenylpolyenes continue t o
provide f r u i t f u l subjects o f research.
Evidence for S2 emission
and for solvent effects o n this has continued t o be a c c u m u l a t e d g 2 . Fluorinated diphenylpolyenes have given information on ground and excited s t a t e geometryg3.
Nonplanarity o f So
indicated and excitation t o the S ,
ground state i s
state changes t h e molecular
geometry significantly. T h e search for useful phototropic molecules continues. For 2-hydroxyazobenzenes light induces changes in pLa and transfer between phases acts as a conveneient detector for changes of ionization statesg4. T h e cis-trans isomerization o f some hydroxyazobenzenes can be used to construct a light powered hydrogen pump by exploiting the difference i n lipophilicity o f t h e geometric i s o m e r s g S .
Other systems involving proton transfer
11
I : Photophysical Processes in Condensed Phases effects o n their photochemistry reported include , N - a l k y l p h t h a l i m i d e s g 7 , and 5 - a m i n o i n d o l e 9 * phenylenediamine~~'
.
A three component self modelling technique has been applied t o a n a l y s i s o f t h e l u m i n e s c e n c e s p e c t r a o f t h e t w o r:onformers
trans-1
, 2 - d i ( 2-naphthyllethene which has a background
d o m i n a t e d by R a m a n e m i s s i o n g 9 .
of
spectrum
Exciplex formation in 1 , 8 -
n a p h t h a l i m i d e s has bee,n e x a m i n e d i n p r o t i c and a p r o t i c solutlon~'~~. Aggregation OCCUI-s in naphthalene carbohydroxamic acids even a t concentrations as l o w a s
dm-3 l o o .
O n e arid t w o
photon processes in the photochemistry o f 1,3-bis(l-naphttiyl)-2propanone s h o w an example o f a " r e l u c t a n t ' Norrish type I reaction o c c u r r i n g f r o m both s i n g l e t and t r i p l e t e x c i t e d states1'*. state properties o f
Excited
n a p h t h y l , phenanthry1,and biphenyl substituted
arylmethyl radicals have been measuredlo3.
T h e corresponding
h a l i d e w a s p h o t o l y s e d a t 3 0 8 n m and t h e r e s u l t i n g r a d i c a l e x c i t e d by 3 3 7 n m .
T h e insecticide carbaryl i s a naphthalene derivative
w h i c h f o r m s 1 - n a p h t h o l a s i t s m e t a b o l i t e . A spectrofluorornetric study has been reportedlo4. Azulene is one o f t h e most interesting molecules from the photophysical point o f view.
A
picosecond measurement o f t h e
vibrational energy decay i n matrix isolated palyatomic molecules shows a t 4 K that molecular modes in a polyatomic matrix d o e s not a f f e c t d e c a y i n t h e S2
v i b r a t i o n a l manifoldlo'.
T h e S2
+
So
fluorescence of pseudoazulenes has been studies in Shpolski matriceslo0.
T h e ionization threshold o f azulene in hydrocarbon
l i q u i d s h a s b e e n d e t e r m i n e d by m u l t i p h o t o n i o n i z a t i o n 1 0 7 .
The
t e m p e r a t u r e d e p e n d e n c e of t h e f l u o r e s c e n c e l i f e t i m e o f 4 , 6 , 8 -
t r i m e t h y l a z u l e n e - 1 - a l d e h y d e s h o w s a s t r o n g t e m p e r a t u r e b e t w e e n 77 and 1 8 0 K d u e t o v i b r a t i o n a l l y a s s i s t e d i n t e r n a l conversionlo'. T i m e resolved synchrotron spectroscopy o f excited fluorescence o f anthracene single crystals has been publishedlo9 a n d t h e t i m e r e s o l v e d and t e m p e r a t u r e d e p e n d e n t f l u o r e s c e n c e s p e c t r a o f a n t h r a c e n e and p y r e n e i n both t h e l i q u i d and c r y s t a l l i n e reported states1I0
Excimer emission i s observed.
T h e spectroscopy o f 9,lO-dihydroanthracenes i n Shpolskii matrices s h o w s that t h e benzene ring fluorescence i s red shifted from t h e excitation origin'11. s p l i t t i n g of t h e 'B2"
This is interpreted as exciton
state w i t h t h e forbidden state having lowest
e n e r g y ; a n a s s i g n m e n t c o n f i r m e d by o n e a n d t w o p h o t o n spectroscopy.
D e t a i l e d a n a l y s i s of t h e p h o t o p h y s i c a l p r o c e s s o f
d i n a p h t h o l C 1 , Z - a : l f , Z f - h 1 a n t h r a c e n e , a s t r o n g l y nonplalnar
Photochemistry
12
overcrowded aromatic molecule has been compared with benzoCc] phenanthrene112. s t ud ied 1n
Intramolecular mixed excimer formation h a s been
a n t h r a :I e n e - p h e n a n t h sene a nd a n t h 1’a ce n e - p y 1-e n e 1 i n k e d
s y s t e m s 1 1 3 . It is noteworthy that the fluorescence decay of 9 , l O dicyanoanthracene in a jet 1:ooled excess e n e r g y 1 1 4 .
alter the decay features. excited
experiment i s insensitive to
Complexation with Ar. X e . and ethanol d o e s not Other relevant papers involve the
state d i p o l e moments of 9 , 9 - b i a n t h r y 1 1
the fluorescence
of 1 , s - d i h y d r o x y - a n t h r o q ~ i n o n r - d ~in n - h e x a n e at 1 0 K 1 l 6 , and
dihydroxyanthraquinone-do
d2
in n-octane at 1 O K 1
and 1 , 8 -
.
The absorption spectrum of the pyrene ~irystal ex~iimrr is similar t o that in s o l u t i o n 1 1 8 , and its formation studied a s an example o f a n oriented bimolecular r e a c t i o n l l g . time i s about 140fs.
The formation
Further studies indicate that relaxation o f
an initially excited delocalized exciton o c c u r s into a selftrapped distorted excimer s t a t e 1 Z 0 .
A
high pressure study on
intramolecular excimer formation of 1 , 3 - d i - l - p y r e n y l p r o p a n e in various solvents shows a strong dependence upon solvent viscosity but not o n polarity12‘.
hindered rotation m o d e l for excimer
A
formation i s based on K r a m e r ’ s theory. Siemiarezuk and W a r e l Z 1 have reinvestigated t h e fluorescence decay kinetics of 1 , 2 - d i ( l - p y r e n y l ) p r o p a n e and concluded in contradiction t o previous r e p o r t s , that there is a distribution o f short lifetimes in addition t o two longer lived fluorescence components.
This proposal has produced a strong dissent by
Zachariasse and Striker
123
w h o on t h e basis o f a global a n a l y s i s ,
maintain that only three d e c a y s are o b s e r v e d , namely two excimer emissions and o n e from the m o n o m e r . Detailed analyses o f
intramolecular structures are possible.
Comparison of NMR and fluorescence data shows meso- and racemic diastereoisomers are found from 2 , 4 - d i ( 2 - p y r e n y l ) p e n t a n e 1 2 4 .
The
polarization o f monomer and excimer o f 4 , 9 , disubstituted pyrenes have been measured i n nematic liquid crystals12’.
Quenching o f
pyrene fluorescence by alcohols in cyclodextrin inclusion complexes has a l s o been studied i n d e t a i l 1 Z 6 .
Solvent effects on the
photophysical properties o f pyrene-3-carboxylic acid has been used t o measure t h e pEa i n different s 0 l v e n t s l 2 ~ . Geminate recombination i n excited state proton transfer reactions has been studied w i t h B - h y d r o x y p y r e n e - l , 3 , 6 - t r i s ~ l p h o n a t e ~ 2 ~A. numerical solution Of
the Debye-Smoluchowski equation w i t h back reaction is formulated for interpretati‘on o f t h e data.
I: Photophysical Processes in Condensed Phases
13
S o l v e n t s h i f t s in van d e r W a a l s c o m p l e x e s o f p e r y l e n e h a v e been studied i n s u p e r s o n i c jet f r e e e x p a n s i o n w i t h r a r e g a s and organic solvent m o l e c u l e s 1 2 9 .
The 1:l
complexes give results
c o n s i s t e n t w i t h solvent s h i f t t h e o r y .
Orientation measurements
have b e e n m a d e w i t h p e r y l e n y l and p e r y l e n o y l p r o b e m o l e c u l e s i n a variety o f s o l v e n t s and l i q u i d c r y ~ t a l s l 3 ~ . Solvent perturbation
o f t h e e x c i t e d s t a t e symmetry o f
r a n d o m l y o r i e n t a t e d m o l e c u l e s of f l u o r e n e has been studied by f l u o r e s c e n c e e x c i t e d by t w o - p h o t o n a b s o r p t i o n 1 3 1 . T h e t e m p e r a t u r e dependence of the rates o f photophysical processes o f fluorenone is a t t r i b u t e d t o energy s h i f t s o f t h e v a r i o u s e x c i t e d states’32. P h o t o p h y s i c s o f a n u m b e r o f o t h e r f l u o r e n e d e r i v a t i v e s have been r e p o r t e d by Dogra and h i s c 0 w o r k e r s ~ 3 3 , ~ ~ E~l,e c~t ~ r o~n i. c s t a t e s o f a c e p h e n a n t h y l e n e have been analysed by a b s o r p t i o n and e m i s s i o n in cyclohexane136.
T i m e r e s o l v e d f l u o r e s c e n c e has been used t o
i n v e s t i g a t e c o n f o r m a t i o n a l c h a n g e p a t h w a y s for 1 , 3 - d i ( U carbazoly1)propane in toluene, another interesting example of i n t r a m o l e c u l a r e x c i m e r f o r m a t i 0 n l 3 ~ . P h o t o c y c l i z a t i o n and f l u o r e s c e n c e s t u d i e s on h y d r o x y s t i l b a z o l e s hydroxyazaphenanthrenes
and
r e p o r t e d in an e x t e n s i v e paper is very
r e l e v a n t t o t h e a p p l i c a t i o n o f t h e s e c o m p o u n d s as f l u o r e s c e n t probe~l3~. T h e f l u o r e s c e n c e s p e c t r o s c o p y o f t h e S2 been examined
state o f coronene has
in t h e solid s t a t e , as i s o l a t e d m o l e c u l e s , and
a l s o in van d e r W a a l s c l u s t e r s 1 3 9 .
The fluorescence o f
f l u o r a n t h r e n e has a l s o been r e p o r t e d at ~ E I K ’ ~ ~ . The dependence o f intramolecular
proton t r a n s f e r on s o l v e n t
f r i c t i o n has been e s t a b l i s h e d f o r 2 - ( 2 ‘ - h y d r o x y - 5 ~ r n e t h y l p h e n y l )
solvent^'^'.
b e n z o t r i a z o l e in a l c o h o l and o t h e r
Excited-state
proton transfer i n 2 - ( 2 / - h y d r o x y p h e n y l ) b e n z o t h i a z o l e has a l s o been s t u d i e d ’ 4 2 .
P h o t o p h y s i c a l p r o p e r t i e s and l a s e r p e r f o r m a n c e o f
w , w ~ - b i s ( o x a z o l - 2 - y l ) - ~ - o l i g o p h e n y l e nin e s d i o x a n e h a v e been m e a s u r e d at r o o m t e m p e r a t u r e 1 4 3 .
9,
increases with the number o f
p h e n y l r i n g s between t e r m i n a l l y p o s i t i o n e d o x a z o y l g r o u p s . Pyridine is another molecule o f enduring interest.
The two
p h o t o n s p e c t r u m o f liquid p y r i d i n e has been o b t a i n e d by t h e r m a l lensing techniques144.
The two lowest
TIIT*
states are of benzene
t y p e and t h e next h i g h e r e x c i t e d s t a t e is n n * .
T h e f l u o r e s c e n c d of
t h e t r a n s - 2 - s t y r y l q u i n o x a l i n e c o n f o r m e r has been found t o c h a n g e markedly with s o l v e n t 1 4 5 .
The photophysics of 6-(2/-hydroxy-4/-
m e t h o x y p h e n y l l - s - t r i a z i n e photostabilizer
has been e x a m i n e d i n
Photochemistry
14
d e t a i l and tautomerization through proton transfer can be important146.
Fast isomerization of the excited en01 form i s
important in the excited state relaxation o f dibenzoylmethane in nonpolar s o l v e n t s 1 4 7 .
Fluorescence spectra and lifetimes o f
chalcone (benzalacetophenone) k e t y l radical anions have also been i n ~ e s t i g a t e d ’ ~ ’ . Various aspects of the photophysics o f 3 hydroxyflavone involving phenyl t o r s i o n ‘ 4 g , intramolecular proton transfer solvent effects and changes in dipole m o m e n t l s 0 , intramolecular proton transfer in isolated molecules i n solid argonlS1 , l 5 2 have reported also during the year. Coumarins are another much investigated g r o u p .
This period
of review has included excited state interactions of coumarin with nucleotide b a s e s 1 5 3 and quenching by chloride i o n s 1 5 4 . Fluorescence lifetimes of angular furocoumarins in several solvents have been measured and correlated with triplet y i e l d s 1 5 5 . Structural and solvent effects on the fluorescence properties o f benzodihydropyranones have been examined a l s o 1 s s .
N-
naphthylsubstituted pyridine cations s h o w unusual S t o k e s ‘ shifts d u e to torsional relaxation of aromatic groups a s shown by the dependence o n solvent
A
two dimensional
fluorescence sensor has been used t o examine solvent effects on 1 ,
5-diphenyl-3-vinyl-A2-pyrazoline1 5 8
.
The laser behaviour o f
variously substituted pyrylium and thiopyrylium salts gives a range
much better than many d y e s 1 5 3 . T h e curious behaviour o f sulphur compounds i s o f great
interest. of the S 2
This i s seen in the effects of deuteration on the decay state o f
anth hi one'^^.
i n the 1 and 8 positions. to S ,
S2
and S 2
Large deuterium effects a r e seen
T h e results are ascribed to competing
t o S o processes; t h e latter results from motions
of the C ( l ) - H and C ( B ) - H atoms towards the thionyl g r o u p .
2.1
Electron Transfer Reactions and Exciolexes
- Photoinduced
electron transfer i s o n e of the most important areas o f research. A
r e v i e w of photoinduced
electron transfer and electron acceptor
complexes usefully surveys the s u b j e ~ t ’ ~ ’ Details . o f the mechanisms can be obtained by very short t i m e resolution spectroscopy.
Dynamic solvent effects on intramolecular electron-
transfer involve solvent fluctuations. Time resolved ps emission spectroscopy has been used t o examine t h e kinetics o f intramolecular c h a r g e transfer in b i s ( 4 - a m i n o p h e n y 1 ) s u l p h o n e i n ethanol a s a function o f temperature i n this respect162.
It has
I: Photophysical Processes in Condensed Phases
15
been shown that electron transfer times are not e q u a l to longitudinal relaxation times in aprotic solvents163.
This shows
that electron transfer and longitudinal relaxation times are not generally t h e same as recent published work i m p l i e d , but micrOSCOplC solvation times are found t o be similar t o electron transfer times i n a broad range o f polar aprotic solvents.
The
influence of dielectric relaxation o n intramolecular electron transfer shows an unusually strong temperature dependence f o r bridged donor acceptor compounds in propylene g l y c 0 1 ” ~ . An extremely interesting study i s that o n charge transfer state formation from ( N , N - d i m e t h y l a m i n o ) b e n z o n i t r i l e in C F 3 H i n the supercritical s t a t e 1 6 5 .
Solvent molecules aggregate around
the Solute and i t 1 s possible t o c o n t r o l the solvation numbers. 1:l
A
solvent exciplex i s not formed and this cannot b e t h e source
of anomalous C T emission.
Dipole moments and the direction o f t h e
transition d i p o l e moment of some intramolecular exciplexes have been determined by the effect of electric field on fluorescence166 . M i a l ~ c q ’has ~ ~ examined t h e formation of the solvated electron by
UV
photolysis of inorganic anions and neutral
molecules l i k e tryptophan in polar solvents and by the biphotonic photolysis o f w a t e r .
Problems o f electron localization and
solvation a r e analysed w i t h reference t o theoretical studies. Specific examples o f electron transfer studies made include a time resolved spectroscopic investigation o f C T complexes o f 2naphthol w i t h triethylamine in polar and non-polar s o l v e n t s 1 6 8 , fluorescence quenching o f carbazole and indole by ethylene thiodicarbonate w h i c h forms ground state c o m p l e x e s ’ 6 9 , and the luminescent charge transfer complex of 4 , b l - b i p y r i d i n i u m ion w i t h tetrakis [ 3 , 5 - b i s ( t r i f l u o r o m e t h y l ~ p h e n y l l b o r a t eanion’’’. Picosecond l a s e r spectroscopy o f 4 - ( 9 - a n t h y r y l ) - N , N dimethylaniline and related compounds in various solvents indicates multiple excited states w i t h different d e g r e e s o f c h a r g e transfer17’. Similar studies o n W - ( l - p y r e n y l ) - P - ~ , ~ dimethylamino-alkalenes in acetonitrile have allowed the intramolecular electron transfer and subsequent recombination deactivation o f t h e charged state t o be s t u d i e d 1 7 2 .
Various forms
of spectroscopy have been a p p l i e d , f o r example t h e mechanism o f donor acceptor electron transfer has been examined by photon gated persistent spectral hole burning in metal-
t e t r a b e n z o p o r p h y r i n / h a l o m e t h a n e systems i n thin films at liquid H e
Photochemistry ternperaturel73.
Study of charge-transfer states o f 4 -
dimethylamino-3,5-dimethylbenzonitrile are two types o f
in a free jet shows there
These arise from a CT state and
also from a twisted higher excited S2 state. T h e soliton concept i s likely t o prove f r u i t f u l in many aspects of photochemistry especially w h e r e organized systems are involved,
It has been proposed that charge transfer excitons in
mixed-stack donor-acceptor compounds exist in a variety of solitortic states'75.
In this effect charge transfer states couple
to lattice phonons in the crystal lattice to f o r m the comparatively stable solitons. The combination o f electrooptical absorption and emission techniques i s valuable.
In the case o f 9 - ( 4 - d i m e t h y l a m i n o p h e n y l ) -
10-cyanoanthracene such experiments s h o w that this molecule fluoresces f r o m two excited states in low and medium polarity solvents'76. solvent.
O n e highly polar excited state only emits i n a polar
In t h e fluorescence quenching
Q-
N-phenyl-l-
naphthylamine by n i t r i l e s , e s t e r s , and amines t h e CT interaction arises through hydrogen bonding for which equilibrium constants have been d e t e r m i n e d 1 7 7 .
Other related systems reported are
fluorescence quenching o f 9-methylanthracene by pipera~ine'~', magnetic field effects of lanthanide ions on pyrene-dimethylaniline exciplex l u m i n e ~ c e n c e ' ~ and ~ , electron transfer from excited states o f anthracene derivatives to methylviologen which results in generation of a cation radical and h y d r o g e n l E O . Fluorescence yields can be chemically mediated by electron transfer in nitroxide/naphthalene adducts and this effect can be used as optical sensors o f radical/redox reactions'''. increases t h e fluorescence yields. 1'4-disubstituted
Chemical reduction
Persistence hole burning of
anthraquinone molecules with electron donating
groups and stearylamino groups show intermolecular hydrogen bonding w i t h t h e matrix i s importantl'2.
Intramolecular excited s t a t e
charge transfer and fluorescence decay o f p-cyano-i,bJdimethylaniline in mixed hydrocarbon/halocarbon solvents shows a viscosity effect d u e t o slowing o f the rate o f TICT stabilizationl'3.
In cerium(1V)-porphyrin sandwich complexes
excited s t a t e deactivation involves neutral exciton or charge transfer excited s t a t e s l e 4 .
T h e neutral exciton state i s
~
l
and ~ l a
substantial CT character is involved in the rapid radiationless decay.
Electron recomination kinetics following laser
photoionization o f t j , N , N ' , ~ ' - t e t r a m e t h y l - ~ - p h e n y l e n e d i a m i n ein
I: Photophysical Processes in Condensed Phases
17
nonpolar solution has been compared w i t h t h e o r y l n 5 . The r o l e of f r e e volume on TICT emission of d i m e t h y l a m i n o b e n z o n i t r i l e and related compounds has been examined i n polymeric m e d i a l a 6 .
The increase in emission w i t h i n c r e a s e in
free volume rules out t h e possibility of specifl~: s o l u t e - s o l v e n t interactions being responsible for TICT emission i n PVA polymer matrices.
Fluorescence quenching o f phenanthrene and chrysene by
K I in m e t h a n o l - e t h a n o l ,solutions shows both electron transfer processes and exciplex formation between aromatic hydrocarbon and perturber are important' e 7 . The influence o f solvent polarity and viscosity on the deactivation of the S l state o f donor-acceptor in substituted t r a n s - s t i l be n e s ha s been d e t e rmi n ed by mea s u r i ng t h e f 1.LA o r e s c. e n c e and adiabatic twisting rate
constant^'^^.
Exciplexes are formed
between amines and t r a n s - 9 - s t y r y l p h e n a n t h r e n e 1 a g . protonation
Effects o f
on t h e photophysical properties of pyrydyl- and
d i m e t h y l a m i n o - d i p h e n y l h e x a t r l e n e d e r i v a t i v e s , wh1.ch can be i~sed a s biochemical p r o b e s , have been reported a l s o l g n .
A
dual
fluorescence observed by picosecond time resolved sprectroscopy in trans-4-dimethylamino-l'-cyanostilbene
is d u e to a TICT state 191.
Another example o f intramolecular CT complex formation is provided by t r a n s - 4 - d i m e t h y l a m i n o - 4 ' - ( l - o x ~ b ~ t y l . ~ ~ t i l b e n e ~ ~ ~ . Solvent effects on t h e spectrum give a value of 220 for the excited state dipole moment. The effect o f electric field on the fluorescence o f 4 - ( 9 - a n t h r y l ) - ~ , N . - 2 , 3 , 5 , 1 i - h e x a m e t h y l - a n i l i n eshows this compound forms an excited state w h o s e dipole moment d o e s not change with solvent'93. between
Chiral discrimination i n exciplex formation
1-dipyrenylarnine and chiral amines is very weak'94.
In the
probe molecule PRODAN ( 6 - p r o p i o n y l ) - 2 - ( d i m e t h y l a m i n o ) - n a p h t h a l e n e the initially formed excited state converts t o a lower CT state as directly evidenced by time-resolved spectra in n - b u t a n o l l g 5 .
Rate
constants f o r intramolecular electron transfer have been measured in both singlet and triplet states o f covalently porphyrin-amideq u i n o n e moleculesl96. Intramolecular excimer formation occurs during the lifetime o f t h e excited state o f bis(naphthalenelhydrazides which a r e used as photochemical deactivators o f metals in polyethylenelg7.
2.2.
Dves and Related Svstems
- T h e s m a l l number
o f citations i n
this section certainly does not represent t h e extent o f interest i n d y e photophysics.
Much work o f technological nature is not
18
Photochemistry
published i n o p e n l i t e r a t u r e .
An i n d i c a t i o n o f a r e s e a r c h o f
c o n s i d e r a b l e t e c h n o l o g i c a l i n t e r e s t i s p r o v i d e d by a p a p e r d e a l i n g with S o - S ,
t w o photon absorption dynamics in d y e
solution^'^^.
A n o t h e r e x a m p l e i s a s t u d y o f l a s i n g potential. o f a d y e , t h e d i h y d r o x y - d e r i v a t i v e o f 2 , 2 ’ - b i p y r i d y 1 1 9 9 .I n t r a m o l e c u l a r d o u b l e p r o t o n t r a n s f e r i s i n v o l v e d and p r o d u c e d by p u l s e i n t h e 4 9 2 - 5 7 2 n m range.
A
r a n g e o f v e r y s o l u b l e and p h o t o s t a b l e p e r y l e n e d y e s h a v e
been r e v i e w e d 2 0 0 .
T h e relation between t h e dipnle moments o f
m e r o c y a n i n e s i n t h e g r o u n d and e x c i t e d s t a t e s h a s b e e n i n v e s t i g a t e d by R u s s i a n w o r k e r s 2 0 1 . P o l y m e t h i n e d y e s h a v e a l s o b e e n i n v e s t i g a t e d by p i c o s e c o n d s p e c t r o s c o p y 2 0 2 .
Picosecond spectroscopy
m e a s u r e m e n t s h a v e b e e n m a d e on t h e s o l v e n t d e p e n d e n c e o f r o t a t i o n a l diffusion b e h a ~ i o u i - ~ F o~r~ c .r e s y l v i o l e t t h e r o t a t i o n a l b e h a v i o u r i s a f f e c t e d by c h a n g e s i n m o l e c u l a r e n v i r o n m e n t a r i s i n g f r o m
It i s an oblate rotator i n
differences in local solvation.
e t h y l e n e g l y c o l and I - d o d e c a n o l a t 3 7 O C b u t a p r o l a t e r o t a t o r i n 1-dodecanol at 26OC. The interpretation requires an assignment of the transition d i p o l e direction. Observation o f photophysical hole b u r n i n g h a s b e e n c a r r i e d o u t w i t h c r y s t a l v i o l e t and e t h y l v i o l e t in v a r i e t y o f s o l v e n t s and t h e d a t a u s e d t o d e t e r m i n e r e l a x a t i o n r a t e c o n s t a n t ~ 2 ~ ~ , T2h~e ~u .l t r a f a s t c o n f o r m a t i o n e q u i l i b r a t i o n i n t r i p h e n y l m e t h a n e d y e s h a s b e e n o b s e r v e d by t i m e r e s o l v e d i n d u c e d p h o t o a b s ~ r p t i o n ~M~a ~ l a.c h i t e g r e e n and c r y s t a l v i o l e t w h e n e x c i t e d by f s p u l s e s s h o w s r e l a x a t i o n c o m p o n e n t s i n t h e s u b p i c o s e c o n d t i m e s c a l e d u e t o t o r s i o n a l m o d e s and s o l v e n t l i m i t e d v i b r a t i o n a l relaxation206. These involve t h e nonrigid nature o f the triphenylmethane moiety. Subpicosecond time-resolved intramolecular electronic energy transfer has been measured i n bichromophoric rhodamine d y e s i n solution207.
A
n e w subpicosecond fluorescence
s p e c t r o s c o p i c s y s t e m h a s b e e n used t o s t u d y t h e t r a n s i e n t s o l v a t i o n o f t h e p o l a r d y e m o l e c u l e s , c o u m a r i n s 102 and 3 1 1 2 0 8 .
R e t t i g and
h i s g r o u p h a v e a l s o m a d e a s e r i e s o f very e l e g a n t
investigation^^^^-^".
For rhodamine dyes with different
l-
s u b s t i t u t i o n p a t t e r n a n e f f i c i e n t d e a c t i v a t i o n p a t h w a y is l i n k e d with rotation o f amino groups towards a state w i t h increased charge localization, a non-emissive TICT state209. Riradicaloid C T states a r e f o r m e d i n x a n t h e n e and r e l a t e d d y e s s h o w i n g t h a t f o r t h e nonradiative process flexible amino groups interact with the x a n t h e n e s k e l e t o n s t o a n e x t e n t w h i c h d e p e n d s on t h e e l e c t r o n a c c e p t o r strengthz”. strongly reduced.
F o r br.idged a m i n o g r o u p s k,,
is zero or
The n o n - e x p o n e n t i a l f l u o r e s c e n c e d e c a y o f
19
I: Photophysical Processes in Condensed Phases crystalline phase222.
T h e s y n t h e s i s and e f f e c t s o f p H o n t h e
spectral characteristics of substituted 2-phenylquionoxalines
have
been reported as part o f a search for luminescent d y e s c a p a b l e o f acting as solar energy collectors223.
T h e bleaching of r o s e
b e n g a l o n i u m s a l t s h a s b e e n i n v e s t i g a t e d and t h e s t a t e s i n v o l v e d identified224.
S i n g l e t and t r i p l e t s t a t e p r o p e r t i e s o f b i s - ( 2 , 5 -
d i - t e r t - b u t y l p h e n y l l i m i d e d e r i v a t i v e h a v e been i n v e s t i g a t e d a s part of an extended
investigation of perylenetetracarbonylic
d i a n h y d r i d e dyes22'.
2.3
Processe s
Photoisomerization a n d Related
- Bagchi226 has
r e v i e w e d p h o t o c h e m i c a l i s o m e r i z a t i o n d y n a m i c s i n s o l u t i o n and considers the diverse behaviour which arises from the nature o f t h e s o l v e n t , v i s c o s i t y , and s h a r p n e s s o f t h e e n e r g y b a r r i e r . a r t i c l e by B a r b a r a and J o r z e b a 2 Z 7
An
considers dynamic solvent
e f f e c t s on p o l a r and n o n p o l a r i s o m e r i z a t i o n s i n c l u d i n g p r o t o n and e l e c t r o n t r a n s f e r , e x c i t e d s t a t e s , and c i s - t r a n s i s o m e r i z a t i o n s particular.
in
An e x t e n d e d f o r m o f t h e K r a m e r s e q u a t i o n , m u c h
employed in interpreting data for photoisomerization proposed22e.
has been
Experimental data for seven different cis-trans
i s o m e r i z a t i o n in t h e l i q u i d s t a t e o v e r r e a s o n a b l y h i g h b a r r i e r s have been fitted t o a simple t h r e e parameter form o f Kramer's equation.
B o t h h i g h and l o w v i s c o s i t y
l i m i t s h a v e been f i t t e d t o
i s o m e r i z a t i o n r a t e c o n s t a n t s f o r t h e s t i l b e n e and D O D C I s y s t e m s
A m e m o r y k e r n e l c a n be f o r m e d i n l i q u i d p h a s e
amongst others.
photochemical cis-trans isomerizations d u e to a time dependent friction effectZz9.
Computer modelling
o f
cis-trans isomerization
reactions where there is transport over l o w barriers indicates t h a t c o r r e l a t i o n o f r a t e s w i t h s o l v e n t v i s c o s i t y i s u n l i k e l y t o be s a t i s fa c t o r y300
.
A q u a n t u m c h e m i c a l study o f t h e mechanism o f t h e cis-trans photochemical isomerization in r e t i n a l l i k e protonated bases uses t h e m o d e l compound l o w e s t e x c i t e d s t a t e is ' 6 :
Schiff
l - i m i n o - 2 , 3 - ~ e n t a d i e n e ~ ~Tlh.e
like state is l i k e but t h e s e c o n d l A 9 The photoisomerization of
particularly labile photochemically.
all trans-retinal has been studied in a variety o f o r g a n i c s o l v e n t s and o r g a n i z e d m e d i a 2 3 2 . involved
in t h e photoisomerization
The structure of the isomers o f r e t i n o i c a c i d and
photoprotective effects i n model physiological determined233.
&-trans
A picosecond
time-resolved
solutions have been
absorption study o f
isomerization of retinylidene acetaldehyde
shows S,
Photochemistry
20
t r i p h e n y l m e t h a n e d y e s i s d u e t o b a r r i e r l e s s r e l a x a t i o n by a n intramolecular rotational relaxation which allows the viscosity e f f e c t s o f s o l v e n t s t o b e examined2l'.
Ground state recovery f r o m
the electronically excited malachite green molecule has been studied v ! transient vibration heating212.
S1
states are produced
by a n u l t r a s h o r t l i g h t p u l s e a t 600 n m and r e l a x a t i o n f o l l o w e d by s p e c t r o s c o p y prohirig b e t w e e n 6 2 5 and 7 2 5 n m . conversion
( T , , ~
heating t o 6 0 0 K . constant o f
-
Rapid i n t e r n a l
3 p s ) t o t h e vibrdtional manifold gives transient T h e second observed relaxation with a time
llps represents the subsequent vibrational energy
dissipation to the solvent.
T h e two-photon l a s e r photochemistry o f
the coumarin l a s e s d y e , 7 - d i m e t h y l a m i n o - 4 - m e t h y l c o u m a r i n , has also been investigated213. The influence o f the counter anion on t h e excited state relaxation time o f cationic polymethine d y e s has a l s o been repoi-ted214
T h e f l u o r e s c e n c e l i f e t i m e is d e p e n d e n t o n t h e a n i o n
in weakly polar media but independent i n polar media.
The
fluorescence behaviour o f highly concentrated rhodamine 6G s o l u t i o n s i n m e t h a n o l and w a t e r c a n b e s e p a r a t e d i n t o m o n o m e r and dimer contributions215.
A b s o r p t i o n e m i s s i o n and e x c i t a t i o n
spectral data support the view that t h e d y e r o s e bengal forms H - t y p e a g g r e g a t e s i n w a t e r and p o l a r p r o t i c ~ o l v e n t s z ~ ~ T h.e s p e c t r o s c o p i c b e h a v i o u r o f r h o d a m i n e 6 G i n p o l a r and n o n p o l a r solvents as w e l l a s i n thin glass and PMMA films shows dimer f o r m a t i o n o c c u r s and t h e i r s t a b i l i t i e s h a v e b e e n c o m p a r e d u n d e r different conditions217.
T h e equilibrium between t h e neutral
z w i t t e r i o n i c and l a c t o n i c ( c o l o u r l e s s ) f o r m s o f r h o d a m i n e 1 0 1 i n p o l a r s o l v e n t s is s t r o n g l y a f f e c t e d by t e m p e r a t u r e 2 l B .
An
excitonic treatment o f the bonding o f aggregates i n rhodamine 6 G i n e t h a n o l w h i c h h a s b e e n used t o r a t i o n a l i z e t h e i n t e r p r e t a t i o n o f d i m e r i z a t i o n and t r i m e r i z a t i o n c o n s t a n t s d e t e r m i n e d a t different temperatures suggests linear structures for aggregates219
T h e polarized spectra o f a serles o f stilbazollum
merocyanines i n polyvinyl alcohol films shows that only exclplexes fluoresce220. Aggregation o f r o s e bengal units i n synthesised dimeric systems has been used as a m o d e l for the medically interesting haematoporphyrin derivative d i m e r 2 2 1 The deactivation o f t h e UV stabilizers o f t h e 2 (hydroxyphenyl1-benzotriazole
class which involve intramolecular
hydrogen bonds has been researched i n m u c h d e t a i l in t h e
I: Photophysical Processes in Condensed Phases
21
lifetimes of about 500ps are longer than for r e t i n a l i ~ o m e r s 2 3 ~ . There is no configurational relaxation of the S ,
states w h i l s t
spectra of the triplet changed w i t h time showing that molecular changes only occur i n t h e triplet manifold.
Solvent effects on
the photoisomerization of bilirubin arise from solvent interference w i t h intramolecular hydrogen bonding235. Cis-trans -
photoisomerization o f diarylethylene i n e t h a n o l
glasses i n the temperature range o f 4 . 2 t o l O O K shows that structural nonequilibrium occurs in such m a t r i c e ~ 2 3 ~ . Theoretical calculations have been made on stilbene which are relevant t o photoisomerization d y n a m i c s .
M N D O calculations o f
stilbene potential energy properties shows no evidence o f a doubly excited "phantom" state but a singly excited ' 8 ,
state w i t h
adiabatic rotation around the c e n t r a l ethylene bond has only a small barrier on this p a t h 2 3 7 .
Calculations o f d i p o l e m o m e n t s ,
optical s p e c t r a , and second order hyperpolarizability coefficients of
some m o n o - and disubstituted stilbene molecules allows t h e
design of useful nonlinear optical molecules238. Solvent reorientation and isomerization of trans-stilbene in alkane solutions has been studied by ps time scale anisotropic absorption and p o l a r i ~ a t i o n ~ ~Coupling ~. of solute and solvent decreases as the size of the solvent molecules increases.
The
applicability of currently favoured models for the activated barrier crossing in the photoisomerization o f stilbene i s discussed.
A
method for measuring quantum yields in the
photoisomerization o f trans-stilbene gives high accuracy w i t h o u t use of a c h e m i c a l a ~ t i n o m e t e r 2 ~ ' .
Evidence has been found for
dynamic solvent effects on the photoisomerization of 4 , 4 ' dimethoxystilbene in w h i c h the effects o f temperature and hydrostatic pressure w e r e made in n - a l k a n e and n-alkyl alcohol24'. A
ps laser time-resolved study fits frequency dependent solvent
shifts but gives results inconsistent w i t h the free volume model. Photophysical and theoretical studies o f trans - 1 - . 2 - ,
and 9 -
styryl anthracenes have studied in different ~ o l v e n t s ~ ~ 2Similar . experimental and theoretical investigations have been m a d e o f the photoisomerism and rotamerism of t r a n s - s t y r y l p h e n a n t h r e n e ~ ~ ~ ~ . This provides a particularly isomers.
comprehensive study of 5 positional
The solvent dependence o f the excited state reactivity
of 1-styrylisoquinoline has been examirted by the direct and photosensitized forms24)4.
isomeritation o f both t h e neutral and protonated
Other related work i s on t h e kinetics o f
Cis-
Photochemistry
22
trans photoisomerization o f 1 , 2 - d i - ( l - n a p h t h y l l e t h y l e n e i n single crystals2"
and regioselective photoisomerization of m - s t y r y l
~ t i l b e n e s ~ ' ~ . T w o groups have studied =-transphotoisomerization
of
1 - ( 9 - a n t h r y l ) - 2 - p h e n y l e t h y l e n e ~ ~ ~ ~A - ~ ~ ~ .
number of 4 - n i t r o - 4 ' - d i a l k y l a m i n o s t i l b e n e s in nonpolar solvents have a mixed singlet and triplet mechanism for the trans
-+
cis
p h o t o i s o m e r i z a t i ~ n ~A~ ~related . study has been made w i t h l-(l-naphthyl)-2-(4-nitro-phenyl)-ethyl)-ethylene
examined for triplet i n v o l v e m e n t 2 5 1 .
which has a l s o been
In 5 . 6 b e n z - Z . Z ' - d i q u i n o l y l
photoisomerization about the carbon-carbon d o u b l e bond in l o w yield
to
i s dependent on the solvent.
T h e electronic
excitation i s localized on the azaphenanthrene m o i e t y z s 2 . Two studies have been reported o n a z o b e n z e n e ~ ~ ' ~ , ~ ' ~ .In the case of the sterically hindered p , Q , g ' , g ' - s u b s t i t u t e d azobenzines the effect of exciting lower '(n,rr*) and higher ' ( r , r r * ) states has been ~ t u d i e d 2 ~ ~In. cyclodextrin inclusion complexes of azobenzene there i s partial blockage of rotational motion about the N = N bondZ5'. A
holographic method has been used t o investigate t h e
cis-
trans isomerization w h i c h occurs in the d y e methyl red dissolved in poly(methy1 m e t h a c r y l a t e ) and polystyrene2s0.
Back isomerization
from the excited state of the laser d y e 3 , 3 ' diethyloxadicarbocyanine
fluorimetric methods2s7
2.4
iodide ( D O D C I ) has been examined by
.
Electronic Excitation Enerav Transfer
-
Studies dealing
exclusively with energy t r a n s f e r , theoretical o r e x p e r i m e n t a l , in homogeneous systems are very f e w in the year under review. Nevertheless some very seminal investigations have been m a d e . The diffusion equation for l o n g - r a n g e energy transfer by the d i p o l e - d i p o l e interaction mechanism which i s accompanied material diffusion has been solved numerically258.
by
T h e theory o f
enhanced energy transfer between molecules embedded in spherical dielectric particles has been developed for application t o dipoledipole energy transfer taking place between molecules embedded in aerosol d r o p l e t ~ 2 5 ~ .T h e experimental systems studied involved the use of the d y e s coumarin as donor and rhodamine 6 G as acceptor. T h e nature o f energy transport and percolation has been examlned in mixed molecular crystals w h i c h are regarded a s fractal structures*60.
Strong guest host interaction produces induced
energy funnels which a r e found to mask the fractal nature o f t h e
I: Photophysical Processes in Condensed Phases
23
percolating triplet guest clusters. The r e l a t i on s h i p between ex c i t a t 11)n t r a n sport a nd f 1 u o r e s c e n I: e depolarization i n two and three d i m e n s i u n d l disordered systems has been discussed by Anfinrud and S t r u v e Z G 1 .
I n t h e usual discuqsion
of excitation transport by dipole-interaction it i s conventional to assume that excitation i s completely depolarized after a single This supposition has been critically examined and a theory
hop.
formulated suitable for application to Langmuir Blodgett films and absorbed species. A
comparison of D O D C I fluorescence depolarization in g l y c e r o l
and ethylene glycol shows the effect of a orientational
orr relation
on excitation t r a n ~ f e r 2 ~ ~T h . e effect of rotational diffusion o n fluorescence depolarization i s separated
from the influence of
excitation transport at higher concentrations.
T h e decay function
analysis o f energy transfer from acriflavin to erythrosin B in ethanol and glycerollwater solution shows hopping at l o w acceptor concentration and a Forster mei:hanism
at higher levels263 .
Energy
transfer from coumarin 4 6 0 to rhodamine 6t is complex and reflects the time dependence of suitable d o n o r - a c c e p t o r pairs264. A
fractal model has been used in interpretation o f singlet
excitation energy migration among different sites which energetically and spatially disordered in Langmuir Blodgett monolayer f i l m s 2 6 5 .
Long-range exchange contributes t o singlet-
singlet energy transfer i n a series o f rigid bichromophoric molecules w h e r e a cyclic ketone acts as acceptor for energy from an excited
1 , 4 - d i m e t h o x y n a p h t h a l e n e moiety266.
Exchange interaction
is found t o be more efficient than dipole-dipole coupling. Energy transfer and chiral asymmetry effects have been examined i n bichromophoric molecules with camphor structures267
.
Bimanes
are other molecules i n which fluorescence and phosphorescence have been used t o observe both singlet-singlet and triplet-triplet
', energy t r a n ~ f e x - ,2z ~6 ~ S e v e r a l other papers introduce concepts derived f r o m solid state physics. For example picosecond spectroscopy is used t o study polaritons in anthracene c r y ~ t a l s 2 ~ ' . Initially formed polaritons are relaxed by phonon interactions w i t h the lattice and these relaxed polaritons act as efficient means o f transporting excitonic energy. T h e behaviour i s complicated by restricting influences referred t o a s "bottlenecks".
Polariton effects i n
anthracene single crystals can be observed in transient grating experiments a t low t e m p e r a t u r e ~ z ~ l .A t high temperature excited
Photochemistry
24
state transfer i s incoherent and excitation i s localised on a given site in the lattice and then randomly hops.
At l o w temperatures
phonon interactions are reduced and coherent states become important. These states are no longer localised and energy transfer occurs by propagation o f excitation wavepackets.
At very l o w
temperatures the polariton structure involves an interaction of the whole m a t e r i a l with the illuminating electromagnetic field.
In
s o l i d phases o f octasubstituted phthalocyanine derivatives e x c i t o n
diffusion lengths o f 1 0 0 - 2 0 0 A occur in molecular superlattices formed by columnar mesophases arising from segregation aromatic cores and flexible side c h a i n s 2 7 2
of
rigid
. The nonlinear spectra
of J aggregates o f pseudoisocyanine in solution shows evidence o f singlet-singlet annihilation of exciton d o m a i n s * 7 3 . i n
Harri~nanZ~~
reviewing energy transfer in synthetic porphyrin arrays show:;
that exciton coupling i s important i n some
o f
these systems.
Exciton migration processes occur in phycocyanin 6 6 5 and allophycocyanin27 2.5
.
Polymeric Systems
-
M 0 r a w e t z 2 ~ ~has recently reviewed the
study o f synthetic polymers by nonradiative energy transfer. Excimer dynamics in poly-(N-vinyl carbazole) films have been revealed by t i m e correlated single photon counting in the picosecond region277.
An emission f r o m an overlap excimer i s
observed immediately whereas the alternative sandwich codformation emission shows delayed formation. T h e s a m e g r o u p have investigated a l s o the fluorescence of the monomer and t h e role o f tacticity o n
the behaviour o f the solvent in tetrahydrofuran278.
Tacticity
influences t h e t i m e resolved spectra and affects t h e rise and decay of t h e fluorescence spectra. Non-exponential picosecond trapping of singlet excitation in poly-(&-vinyl
carbazole) has been modelled by
making allowance for excimer dissociation and the t i m e dependence for trapping o n monomer and excimer fluorescence decays2”.
Another
example of picosecond timescale dynamics is a study made o f ionized and excited states in polystyrene films following
2
photon laser
p h o t o l y s i ~ 2 ~ ~The . dynamics and mechanism o f generation of excited states by recombinatiori fol lowing ionization and excimer formulation have been elucidated.
T h e photophysics o f alternating
copolymers o f both acenaphthylene and methacrylic acid w i t h maleic acid has also been i n v e ~ t i g a t e d ~ ~ ’ . T h e u s e o f fluorescence probes t o determine polymeric structures is of w i d e application.
Excimer formation i n pyrene
I: Photophysical Processes in Condensed Phases
25
labelled hydroxypropyl Cellulose i n water has been investigated by picosecond fluorescenceza2.
Two different structureless bands d u e
t o different excimers are found.
The quenching o f fluorescence of
probes covalently bound t o polyelectrolytes by m e t h y l v i o l o g e n , sulphonated propylmethyl v i o l o g e n , and a neutral zwitterionic viologen has also be studied by time resolved t e c h n i q u e ~ 2 ~ 3 . Aggregation numbers in polymeric micelles can be determined by luminescence quenching.
Intramolecular micelles formed by a
copolymer of maleic anhydride and hexyl vinyl either have been studied by the quenching of tris ( 2 , 2 ’ - b i p y r i d i y l ) r u t h e n i u m ( 1 1 ) by 9 - m e t h y l a n t h r a c e n e 2 B 4 . T h e influence o f alkoxysilanes on pyrene excimer production can be used to indicate the speed of g e l formation2e5. T i m e dependent absorption and emission of the excimer can both be used and i t w a s shown that in methoxysilane g e l production w a s faster than w i t h t h e ethoxy homologues. T h e developing interest in the possible use o f organic materials i n electronics i s opening up n e w areas o f photophysics. Polyacetylene i s the prototype conducting polymer and its properties a r e being examined by photophysical techniquesza6. Nonlinear optical properties o f polymeric media a r e important since l a r g e second order nonlinear optical enhancement can occur d u e t o cooperative interaction between dipoles in polymer c h a i n s Z e 7 . Polydiacetylene is another interesting system w h o s e absorption and fluorescence spectroscopy has been researched in Picosecond time resolved and frequency domain coherent Raman scattering has been related t o nonlinear optical processes i n a soluble p ~ l y d i a c e t y l e n e ~ Relaxation ~~. measurements i n polydiacetylene polymers shows singlet exciton lifetimes o f about psZgo.
1
Deactivation occurs to a conformatianally relaxed ( k i n k e d )
singlet state.
Rice and P h i l p 0 t 2 ~ ’ have examined t h e r o l e o f
polarons and bipolarons in m o d e l tetrahedrally bonded homopolymers such as p o l y - ( o s g a n o ~ i l y l e n e ) 2 ~ ’ .T h e s e states are available in the polymer for either the addition or the excitation of electrons Or holes. T h e width of these polarons are a f e w bond lengths and interchain photoexcitation is possible. Picosecond orientations of transition dipoles i n polysilanes has been studied by fluorescence a n i s o t r 0 p y 2 ~ 2 . Excitation energy transfer in these polymers i s ultrafast in accord w i t h the Rice and Philpot polaron m o d e l . 2.6
Colloidal and Heteroqeneous Systems
-
Photophysics in
colloidal systems remains a very significant area o f research in
Photochemistry
26
spite o f the sparse practical returns from work related to solar energy. The recently published book by K a l y a n s ~ n d a r a m provides ~~~ an extensive survey o f the whole subject.
A review paper considers
fluorescence a s a means of studying microemulsions and reversed micelles and applications in analysis294.
Colloidal media offer
interesting possibilities f o r selective photochemical reactions. L a t t e ~shows ~ ~ ~that i s o m e r i s a t i o n , d i m e r i z a t i o n , and photoaddition can be achieved and he correlates structure-chemical and photochemical reactivity relationships in microemulsions.
Another
review considers the significance of static fluorescence quenching in the study o f micellar systems296.
Static quenching i s shown to
be the dominant effect in luminescence quenching in interpolyelectrolyte c o m p l e x e s 2 9 7 . Fluorescence probes can be used to investigate anionic polymer interactions2”.
- cationic surfactant
Perylene has been employed as a probe o f the
rigidity of sodium taurocholate r n i c e l l e ~ and ~ ~ ~its effects o f metal cations on the fluorescence intensity of polycyclic aromatic hydrocarbons g i v e further information on the same systems300. Analysis of fluorescence decay curves can be used t o determine the mean aggregation number o f aqueous micelles.
T h e use o f 1 -
methylpyrene quenching by the immobile quencher ltetradecylpyridinium chloride in S D S micelles gives an aggregation number of 6 9 in excellent agreement w i t h literature values3”. Excimer formation o f a water soluble fluorescence probe in anionic micelles and nonionic polymer a g g r e g a t e s , such as a pyrene d e r i v a t i v e , is an indicator of cluster s t r u c t u r e 3 0 2 .
Fractal
modelling has been used for luminescence quenching in m i c r o emulsions w h e n quencher exchange between different droplets i s allowed303 Ru(bpy)32+ fluorescence in t h e presence o f Fe(CNIG3in water and o i l emulsions has been analysed in this w a y . Malliaris304
has used fluorescence t o follow changes induced
i n S D S micelles w h e n a l k a n e s , a l c o h o l s , and ketones are solubilised.
By means of steady s t a t e methods information on
micellar s i z e , m i c r o p o l a r i t y , interfacial charge d e n s i t y , etc. can be obtained.
P y r e n e and two oppositely charged derivatives have
been investigated in NaF ion30S.
Anthracene and 9-methylanthracene
have also be’en entensively studied in detergent m i ~ e l l e s ~ and ~ 6 Iions are shown by anthroyloxy probes t o interact w i t h cetyltrimethylammonium bromide m i c e l l e ~ ~ ’ ~ Internal . mobility o f Triton X - 1 0 0
micelles has been investigated by fluorescence
anisotropy and excimer p r 0 b i n g 3 ~ ~ .
I: Photophysical Processes in Condensed Phases
27
A number o f colloidal systems containing d y e s have been investigated.
T h e fluorescence lifetime of acridine o r a n g e has in the S D S premicellar region3”.
been measured
A short l i f e t i m e
of less than 3 n s is found for the monomer but the emission lifetime increases w i t h dimer formation and S D S concentration. Quenching o f 7-ethoxycoumarins by inorganic ions in S D S 3 l 0 ,
d y e solubilizates in
micelle l i k e complexes o f surfactants w i t h p o l y e l e ~ t r o l y t e s ~ ‘ ~ , time resolved fluorescence depolarisation o f rhodamine d y e s in Triton X - 1 0 0 micelles and Aerpsol OT reversed m i c e l l e s 3 l 2 , and Static and time resolved fluorescence in an amphiphilic flavin in Aersol OT reversed m i c e l l e ~are ~ ~ ~ other photophysical systems reported.
A picosecond timescale resonant energy transfer from
rhodamine 6G t o malachite green has been used t o study micellar size and s h a p e 3 1 4 .
In this very detailed work it i s found that
d y e s a r e solubilised at the surface o f the S D S micelles. CT
complexes are formed and electron transfer occurs from
excited molecules of anthracene derivatives t o methylviologen in aqueous micellar m e d i a 3 1 5 .
Methylene blue quenches pyrene
fluorescence by electron transfer in S D S micelles316
.
Electron
transfer between anthraquinone sulphonate radicals and duroquinone i n S D S micellar solution occurs in t h e aqueous phase; there i s n o evidence of intramicellar transfer3I7.
Photoionisation o f
).I,N,N’, N ’ - t e t r a m e t h y l b e n z i d i n e in anionic-cationic mixed micelles has been studied in d e t a i l by ESR318.
The photochemistry
o f the
semi-oxidised forms o f eosin Y and rose bengal have been investigated
in colloidal solutions319.
Relevant to t h e
fluorescence o f proteins i s a study of fluorescence quenching o f
indolic compounds by amino-acids in S D S . C T A B , and CTAC m i ~ e l l e s ~ * ~ Rate constants for proton transfer o f several hydroxyaromatic compounds have been measured solutions321. of
in a variety of surfactant
Photoprotolytic dissociation does not require exit
the reactant molecules from the micelles.
Micellar solutions
can b e used to improve t h e fluorescence determination of 2 - n a p h t h o l by inhibiting proton transfer or proton inducing reactions322.
The
decay of t h e radical pair composed of diphenylphosphonyl and 2 , 4 , 6 trimethyl benzoyl radicals in S D S is affected by magnetic f ields32 3
.
A useful r e v i e w o f optical probes in the study of thin organic films has been written by Debe3z4.
Apart f r o m conventional
luminesence the discussion of internal reflective fluorescence excited by t h e evanescent w a v e i s especially interesting.
Pressure
28
Photochemistry
area i s o t h e r m s and f l u o r e s c e n c e b e h a v i o u r o f 1 2 - ( l - p y r e n y i ) d o d e c a n o i c acid at a n air-aqueoius i n t e r f a c e s h o w s e x c i m e r f o r m a t i o n d o m i n a t e s t h r o u g h a g g r e g a t i o n d r i v e n by h y d r o g e n bonding o f c a r b o x y l i c acid g r o u p s 3 2 5 .
D a n s y l f l u o r e s c e n c e has a l s o been used
a s a p r o b e f o r polarity a t t h e a i r - w a t e r i n t e r f a c e 3 2 6 .
The
interaction of carboxylic porphyrins with dioleoylp h o s p h a t i d y l c h o l i n e i n spread m o n o l a y e r s and t h e e f f e c t s o f l o d i d e ion quenching i s another system studied327.
T h e f1uoresc:ence
lifetime of 5 - ( 4 - c a r b o x y p h e n y l ) - 1 0 , 1 5 , 2 0 - t r 1 t 0 1 y 1 p o r p h y r i t i
ln mixed
L B f i l m s w i t h d i o l e o y l - p h o s p h a t i d y l c h o l i n e has b e e n proposed
standard328.
as a
Fluorescence drainage profiles of thin liquid films
has been o b s e r v e d w i t h r h o d a m i n e c o n t a i n i n g
surf act ant^^^'.
F l u o r e s c e n c e and e n e r g y t r a n s f e r o f n e g a t i v e l y charged c y a n r n e d y e s bound t o m o n o l a y e r s i s a n o t h e r study r e p o r t e d 3 3 0 .
A
t l m e resolved
s t u d y o f t h e e f f e c t s o f m o l e c u l a r o r g a n i s a t i o n i n pyrene l a b e l l e d p h o s p h a t i d y l c h o l i n e is compared w i t h d a t a f r o m ~ y c l o h e x a n e .~ ~ Order p a r a m e t e r s a r e used t o i n t e r p r e t d a t a on ~ r d e rand f l u i d i t y o f a n u m b e r o f probes i n l i p i d m e m b r a n e s o b t a i n e d by m e a s u r e m e n t s of fluorescence anisotropy decay332.
Ambiqultles in the
interpretation of time resolved fluorescence anisotropy m e a s u r e m e n t s i n l i p i d v e s i c l e s y s t e m s w i t h D P H o r T M A - O P H probes a r e attributed to t h e u n s a t i s f a c t o r y m o d e l s being used t o i n t e r p r e t the data333.
T h e s o l u b i l i s a t i o n o f d i p h e n y l p o l y e n e s i n lipid
bilayers has been c r i t i c a l l y e x a m i n e d 3 3 4 .
I t i s c o n c l u d e d that
such p r o b e s a r e s a t i s f a c t o r y i f used a t low c o n c e n t r a t i o n s . F l u o r e s c e n c e t e c h n i q u e s c a n a l s o b e used t o study s o l i d surfaces. 6G
A f r a c t a l a p p r o a c h has been used t o i n t e r p r e t r h o d a m i n e
probed m o r p h o l o g y o f p o r o u s s i l i c a s u r f a c e ~ 3 ~ ~ M i.g r a t i o n o f
excitation agrees with a one step energy transfer mechanism. Excimer f l u o r e s c e n c e d e t e r m i n a t i o n o f s o l i d - l i q u i d i n t e r f a c i a l pyrene l a b e l l e d p o l y - ( a c r y l i c acid 1 i n d i c a t e s strand coiling336 . P i c o s e c o n d pump p r o b e s p e c t r o s c o p y has been a p p l i e d t o e l e c t r o n i c relaxation in aggregates o f pseudoisocyanine molecules on c o l l o i d a l s i l i c 0 n 3 3 ~ . Energy t r a n s f e r a t i n t e r f a c e s c a n a l s o b e s t u d i e d by d i f f u s e r e f l e c t a n c e l a s e r p h o t o l y s i ~ ~ E ~ l~e . ctron t r a n s f e r t o m e t h y l v i o l o g e n f r o m c o l l o i d a l C d S and T i 0 2 has been s t u d i e d by e x a m i n a t i o n o f t h e k i n e t i c s f o l l o w i n g l a s e r p u l s e ~ 3 3 ~ , 3 F~l~u o . r e s c e n c e , photochemistry. and s i z e q u a n t i s a t i o n e f f e c t s o f c o l l o i d a l s e m i c o n d u c t o r ( c d J s , Z n S , and T i O Z ) p a r t i c l e s h a v e b e e n surveyed by Henglein3('.
Very s m a l l p a r t i c l e s u n d e r g o a
transition from semiconductor t o molecular properties.
Emission
29
I: Photophysical Processes in Condensed Phases spectroscopic evidence i s obtained for Rr$nsted calcined Vycor g l a s s 3 4 2 .
acid sites i n
Other solid surface studies include
pyrene on T i 0 2 3 4 3 , evaporated merocyanine d y e l a ~ e r s . 3 ~ ~ ~ fluorescent probes o n clay p a r t i c l e s 3 4 5 , and diphenyl polymers on alurnina346.
3
Triolet State Processes
The longer lifetime and enhanced biradical character of the triplet State produce a difference in the style of research and the influence of time resolved spectroscopic techniques on the subject i s less marked than for excited singlet states. Selective enhancement of room temperatures phosphorescence i s achieved by cyclodextrin treated cellulose s u b s t r a t e s 3 4 7 . surface active a g e n t ~ 3 ~and ~ , for heterocyclic compounds by absorption on silica gel coated plates submerged in (~hlorofoi-m-containing solvents349. Delayed fluorescence can occur during deactivation o f highly excited triplet states3”.
Energy is transferred to the m e d i u m ,
which may be an electron acceptor andlor d o n o r .
Extended and
localised triplet states in disordered media can be examined by optical and ESR t e c h n i q u e ~ 3 ~. ’ Fashioning of electron spin echo spectra has been used in studying the triplet states of azaromatic m o l e c u l e ~ 3 ~ ~Diffusion . limited phosphorescence quenching in polymer solutions involving small molecule interactions has been interpreted by f r e e volume theory3’3.
Disordering enhances diffusivity o f triplet excit,ons
in condensed aromatic s y s t e m s 3 5 4 .
Fractal like triplet-triplet
annihilation kinetics operate in naphthalene doped poly-(methyl methacrylate) as shown by a delayed fluorescence decay t i m e w h i c h i s less than
that for phosphorescence, suggesting a geometric
fractal dimension o f 1 . 0 t o 1 . 6 3 5 5 .
Long distance intra-molecular
triplet energy transfer rates have been compared w i t h those for electron t r a n ~ f e r 3 ~ ~T.h e former process requires l i t t l e solvent reorganisation but the l a t t e r , because o f a need for t h i s , i s strongly temperature dependent.
Polystyrene i s a good m e d i u m for
the study of long distance electron transfer reactions involving triplet states and i n o n e c a s e t h e r a t e is temperature invariant between 7 7 and 1 C 3 K 3 5 7 .
T h e nature o f the energy g a p dependence
for electron transfer rates in triplet exciplexes has been examined358 and a l s o t h e geminate r a d i c a l pairs formed b y quenching o f phosphorescent states i n polar s o l v e n t ~ 3 ~ ~T h . e r o l e o f spin
30
Photochemistry
orbit
coupling effects
on t h e
geminate recombination o f Magnetic yields
field
for
kinetics
t r i p l e t
has been examined
radical pairs
show a dependence o f
effects
photogenerated b i r a d i c a l s o f
t h e modulated t r i p l e t
the type
upon t h e m e t h y l e n e c h a i n l e n g t h 3 6 1 . T h e r e i s the y i e l d micellized The e f f e c t
of
oxygen on c y c l i c
1,3-diradicals
the t r i p l e t
state
r e s o l v e d r e s o n a n c e Raman s p e c t r o s c o p y triplet
states
theoretical triplet
been used t o The d e c a y
symmetric
triplet
state
relaxation
of
explain experimental data triplet-triplet
biradical
(two sub-levels)
and 7 7 K i n S h p o l s k i m a t r i c e s 3 6 7 . carbene
isomers
and n o n - s y m m e t r i c
kinetics of
mesitylene
shows
lifetime363.
A
fluorescence
for
phosphorescence
of
the excited
has
substituted b e n ~ e n e s 3 ~ ~ .
fluorescence
i n the
h a v e b e e n m e a s u r e d b e t w e e n 10 state o f dimesityl
of
absorption spectra
spectra
of
4,5of
the
from
dimethylphenols
states determined37'.
a t
77K
A
a n d C N D O s t u d y h a s b e e n made o n t h e p y r i d i n e - B C L ,
complex372 and l a s e r p h o t o l y s i s bipyridine
the
k e t o n e ~ 3 ~ A ~ .
i s o m e r s h a v e been m e a s u r e d 3 7 0 and
and p h o s p h o r e s c e n c e
of
aromatic molecules
The t r i p l e t
b e n z o c y c l ~ h e p t a t r i e n y l i d e n e ~ ~T~- . T
the p s values
that
of
h a s a l s o b e e n c h a s a c t e r i ~ e d ~ a~n'd a l s o t h a t
three trichlorobenzene
on
Time
has a l s o been r e p o r t e d on t h e a - c l e a v a g e
states o f
mechanism f o r
1 n-* D-
has been used t o examine
produced from d i f f e r e n t
study
A + - ( CH2
a l s o an e f f e c t
radical p a i ~ s 3 ~ . 2
triplet
c o n f o r m a t i o n can a f f e c t
i n the
i n m i c e l l e ~ ~ ~ ~ .
triplet
has
characterised
i o n i n ~ y c l o h e x a n e 3 ~ 3 .Other
made o n t h e t r i p l e t
states
of
the 2 , 2 ' studies
have been
1,2,6-trimethyl-3,5-diphenyl-l-
p ~ r i d o n e 3 ? ~p ,h t h a l a ~ i n e 3 ~a n ~ d. t e t r a m e t h y l a n d t e t r a e t h y l - p phenylenediamine3' ODMR
stilbene;
.
has been a p p l i e d t o t h e i t
i s found
rigorously planar
that
i n single
cyanobenzene complexes on t h e
of
crystals378.
the t r i p l e t
spectra
of
trans-
i s not
i s almost
spectrum of
stilbene completely
1,4-
are most
Spectroscopic
significant studies
i n triplet
state
i n c l u d e t i m e r e s o l v e d ESR
enols of o-hydroxybenzaldehyde3a2, energy t r a n s f e r
states
of
state
I n crystals o f
excitation
acetophenones t o 9,10-dibromoanthracene
substituted
sub-levels
has also been p u b l i s h e d 3 " .
Carbonyl groups photochemistry.
triplet
of
~ t i l b e n e ~ ' ~T .h e p h o s p h o r e s c e n c e
diazatriphenylene
studies
study
t h e phosphorescent
of
(S,)
which involves
from
higher
t h e donor383, two photon e x c i t a t i o n o f
triplets
of
a ~ e t o p h e n o n e 3 ~t ~ r a, n s i e n t
r e s o n a n c e Raman
d e u t e r a t e d b e n z o p h e n ~ n e s ~a n ~ d~ ,d i f f u s e r e f l e c t a n c e o f
I: Photophysical Processes in Condensed Phases
31
t r i p l e t s i n b e n z o p h e n o n e r n i c r o c r y s t a l ~ 3 ~ ~T.w o p h o t o n e x c i t a t i o n of benzophenone triplets results in rapid energy transfer t o benzene solvent3a7 and quenching o f benzophenone triplets by electron donors does not give satisfactory correlation with ionization potentials388.
Details o f t h e mechanism o f
d e h y d r o g e n a t i o n o f e t h a n o l by a s e r i e s o f w a t e r s o l u b l e b e n z o p h e n o n e s h a v e a l s o b e e n r e p o r t e d 3'9
Rotational relaxation
s t u d i e s o f t r i p l e t s t a t e s o f t r i i s o p r o p y l b e n z o p h e n o n e ~ ~n~ ~T*, a n d TIT*
S t a t e s o f 4 - h y d r o x y b e n z o p h e n o n e i n ethanol3',
ketone in
, and Michler's
are other systems which have been studied
spectroscopically.
Conformers have been established i n triplet
b e n ~ i l s ~p o~l y~s ,t y r e n e q u e n c h e s t h e p h o s p h o r e s c e n c e o f b e n ~ i 1 3 ~ ~ , a n d t h e t r i p l e t a n d q u i n t e t s t a t e s f o r m e d by t r i p l e t - t r i p l e t r a d i c a l pairs o f benzoylphenylmethylene are produced by photolysis o f c r y s t a l l i n e a z i b e n z i l p o w d e r at 7 7 K 3 9 5 . The dynamics of excitons in isotopically mixed naphthalene c r y s t a l s h a s b e e n r e ~ i e w e d 3a~n d~ t h e e f f e c t s o f o r i e n t a t i o n o f m e t a l ion perturbers i n naphthalene-crown ether metal ion complexes on the external heavy atom examined397. The triplet excimer phosphorescence from liquid solutions o f naphthalene and di-2-naphthylalkenes in isooctane previously reported has not been confirmed and t h e emission probably arose from impurities like biacet~l3~'.
Exciton trapping has been
studied in doped 1,4-dibromonaphthalene3gg, ENDOR applied t o t h e s t u d y o f t r i p l e t t r a p s o f p h e n a n t h r e n e - T C N B i n naphthalene-TCNB'OO, and triplet 1 -
and 2 - n a p h t h y l p h e n y l c a r b e n e s produced by photolysis
of diazo compounds401.
Conformers o f triplet states o f 2-
n a p h t h a l d e h y d e at 300Kk02
,
T,
a n d T2
states from dual
p h o s p h o r e s c e n c e o f 1 , 4 - n a p h t h o q ~ i n o n e ~a n~ d~ ,T - T a b s o r p t i o n o f 5 , 8 - d i h y d r o x y - l , 4 - n a p h t h o q u i n o n e by f l a s h p h o t o l y s i s a r e a m o n g s t other interesting papers404.
Q u i n o l i n e , isoquinoline, and t h e i r
protonated cations in phosphorescent triplet states have been measured in C H 3 0 H / H 2 0 and stretched PVA fibres at 7 7 U 4 0 5 .
Heavy
atom effects on the room temperature phosphorescence of dibenzo [ f , h ] quinoxaline absorbed on filter paper have been examined in some detai1406.
Characterisation o f sub-levels o f the non-
p h o s p h o r e s c e n t t r i p l e t s t a t e o f c i n n o l i n e h a s b e e n m a d e by t i m e resolved ESRko7. Assignments of the I R spectrum o f matrix isolated anthracene t r i p l e t s 4 0 0 , i n v e r s e i n t e r s y s t e m c r o s s i n g , T~
-+
S n , studies o f
stepwise t w o photon excitation o f anthracenes409 triplet exciton
Photochemistry
32
f u s i o n at e d g e d i s l o c a t i o n s i n a n t h r a c e n e c r y s t a l s 4 1 0 , and t i m e r e s o l v e d ESR o f a n t h r a c e n e t r i p l e t i n a s e r i e s o f a l k y l t r i m e t h y l a m m o n i u m h a l i d e m i c e l l e s a f f e c t e d by h e a v y a t o m s a r e a n o t h e r group o f related studies.
T r i p l e t - t r i p l e t a b s o r p t i o n and
cis-
trans-isomerization have been correlated 2 - e t h e n y l - a r 1 t h r a c e n e ~ ~ 2 and 2 - s t y r y l - a n t h r a c e n e and r e l a t e d
compound^"^.
T h e photophysics
and p h o t o c h e m i s t r y o f s u l p h o n a t e d d e r i v a t i v e s o f 9 , l O - a n t h r a q u i n o n e shows that lowest
TTI*
t r i p l e t s a r e w e a k photosens.it.izers w h i l s t , i n
c o n t r a s t , nT* t r i p l e t s a r e s t r o n g s e n s i t i ~ e r s ~ ’ ~T.h e e f f e c t o f substitution o n triplet yields o f aminoanthraquinones shows internal hydrogen bonding i s effective i n causing i n t e r n a l c o n v e r s i o n 4 ’5
.
T r i p l e t s t a t e s o f x a n t h o n e d e p e n d u p o n t h e c o m p o s i t i o n of t h e solvent; t h e presence o f w a t e r enhances t h e yield o f 3
~
s~t a t*e 4 1 6
D u a l phosphorescence f r o m 2 - ( 2 ’ -hydroxyphenyl) benzoxazole
is d u e
t o k e t o - e n 0 1 t a u t o m e r i ~ m ~and ’ ~ the kinetics multi-exponential decay i s d u e t o differences o f environment418.
Triplet state
p r o p e r t i e s and t r i p l e t s t a t e - o x y g e n i n t e r a c t i o n s o f t h e b i o l o g i c a l l y i n t e r e s t i n g l i n e a r and a n g u l a r f u r o c o u m a r i n s a r e useful in view o f possible clinical application4Ig. Oxygen quenching o f phenanthrene phosphorescence involves a n exchange mechanism420 which also operates for triplet state energy transfer involving phenanthrene in biphenyl421.
Radiationless
p r o c e s s e s o f e x c i t e d s t a t e s o f 5 , E i - d i a z a p h e n a n t h r e n e i n h e x a n e and h e x a f l u o r o i s o p r o p a n o l h a v e b e e n s t u d i e d by both t r i p l e t and s i n g l e t luminescenceC22.
An e x t r e m e l y f a s t r a d i a t i o n l e s s t r a n s i t i o n i n t h e
t r i p l e t s t a t e o f 9 , l Q - d i a z a p h e n a n t h i - e n ei n b i p h e n y l and f l u o r e n e hosts at 3 K i s d u e t o softness o f t h e twisting m o d e o f t h e nitrogen-nitrogen d o u b l e bond423.
P h o s p h o r e s c e n c e e m i s s i o n and
p o l a r i s a t i o n o f h a r m a n e i n m e t h y l c y c l o h e x a n e and EPA g l a s s e s a t 77K
is from a
3 7 1 ~ *
s t a t e 4 Z 4 . V i b r o n i c c o u p l i n g and p r o x i m i t y e f f e c t s
are analysed. Phosphorescent states o f aromatic thioketones with l a r g e z e r o f i e l d s p l i t t i n g h a v e b e e n i n v e s t i g a t e d by ODMR42’. Excitation wavelength dependence o f phosphoresence o f thioxanthone and 2 , 4 - d i - i s o - p e n t y l - t h i o x a n t h o n e shows that red shifting o f the luminescence is d u e t o hydrogen bonding particularly with w a t e r C 2 6 . T I - T 2 inversion in aromatic thiones produces dramatic changes in t h e p h o s p h o r e s c e n c e s p e c t r a by i n t e r c h a n g i n g 3 n a * a n d 3 n ~ * s t a t e s 4 Z 7 . T h e d e c a y is n o t b i e x p o n e n t i a l .
T h e concentration
dependence o f t h e photochemistry o f 4 H - 1 - b e n z o p y r a n - 4 - t h i o n e i n v o l v e s g r o u n d s t a t e q u e n c h i n g of t h e t r i p l e t s t a t e , p o s s i b l y by
I: Photophysical Processes in Condensed Phases triplet excimer formation420.
33
Interesting photochemical studies
have been made o n the triplet states of monothioanthraquinone and pivalothiophen~ne~~'.
T h e acidity of the triplet state o f 2 -
nltrothiophen has been measured:
p%*
= -0.95
0 . 2 0 ~ 3 ' .
Triplet states are very important photosensitizers
and this
fact has prompted many investigations, for e x a m p l e , the triplet yields of a series of cyanines and photosensitization through singlet oxygen have been related t o substituent effects and quenching rate d a t a 4 3 2 .
The photophysical properties of the
porphyrin, c h l o r i n , a potent sensitizer for photochemotherapy
shows
that the high absorption of both the singlet and triplet states make this compound suitable for sequential biphotonic excitation4j3.
Intersystem crossing in p o r p h y c e n e ~ ~ 3 ~ .
phosphorescence quenching of z i n c tetraphenyl porphin on solid supports by molecular oxygen shows marked differences on silica g e l and sodium chloride c r y s t a l ~ ~ 3t i ~ m,e resolved spontaneous and coherent Raman scattering of nickel octaethylporphyrin in the triplet state436 a r e other related researches.
Triplet-triplet
energy transfer i n the Cu( I 1 1 poi-phyrin free base porphyrin ,.
and z i n c tetraphenylporphyrin and 2-piperidoanthraquinone in aligned nematic and isotopic phases o f liquid crystals exhibiting no enhancement of intersystem crossing in the nematic phase are other interesting porphyrin studies438.
A method
o f oxygen
detection using phosphorescent, Langmuir-Blodgett films of the m e t a l l o p o r p h y r i n , tetraphenylporphine palladium
( 1 1 ) has been
proposed439. Photochemical l i k e behaviour o f riboflavin i n the dark is promoted by energy transfer from e n z y m e generated excited triplet states of acetone440.
Energy conversion by energy transfer from
triplet lumiflavin t o ferricyanide ions occurs without electron transfer whereas w i t h ferrocyanide electron transfer gives semireduced lumiflaven and f e r r i ~ y a n i d e ~ ~ ' .Phosphorescence f r o m 2 - ( ~ - t o l u i d i n y l ) n a p h t h a l e n e - 6 - s u l p h o n a t e and l-anilinonaphthalene-
6 - s u l p h o n a t e bovine serum albumin has l i f e times o f milliseconds442 It is interesting to note that in these experiments oxygen w a s removed f r o m t h e system by glucose together with the enzymes g l u c o s e oxidase and catalase.
Electron transfer from triplet
states o f xanthene d y e s gives r i s e t o t h e methylviologen r a d i c a l 4 4 3 . Unreactive ion pair complexes form between the d y e eosin and M V 2 + and addition o f alcohol increases the efficiency o f energy transfer.
Photochemistry
34
Triplet states o f t h e a m i o d a r o n e , a n antiarrhythmic d r u g w h i c h shows cutaneous t o x i c i t y , are involved in its p h o t 0 1 y s i . s ~ ~ ~ The . lowest triplet states of the drugs codeine and morphine and their molecular s u b units veratrole and guaiacol are
Other Chemical Systems
4.
Ferradini and Bensa~son''~
have reviewed flash photolysis and
pulse radiolysis as tools for studying the kinetic and thermodynamic properties o f
+
02
,
. OH,
lo2.
and
The problems involved in detecting luminescence from singlet oxygen in biological systems have been assessed447 and examination of the red luminescence from ram seminal vesicle microsomes shows pitfalls in the use of spectrally resolved luminescence440.
Direct
measurement o f 1268nm emission shows most of t h e previously reported red emission is not from
lo2.
spectrometer has been used t o determine decay
( l A g , u = O -+
A
very sensitive
lo2
3 1 - g , u = O ) at 127Onm449.
lifetime by direct Using weaker excitation
intensities much longer lifetimes are found i n perhalogenated solvents, C S 2
and C D C 1 3 .
In this c a s e deactivation involves
coupling of highest fundamental vibrational mode o f the acceptor with a ( ' A g , u = O
---*
3 E = 9 , u = m ) transition.
Perturbations of this
transition on the luminescence yield o f ' 0 2
in organic solvents and
water has been examined experimentally4s0.
The yield in benzene i s
higher than expected when compared with results for a number of different
sensitizer^^^'.
Interaction o f O2
('Ag) with
disubstituted olefins gives evidence for physical quenching induced by the hydrocarbon chain452.
T h e interaction i s not w i t h d o u b l e
bonds but may be with CH3 and C H 2 groups o f the side chain substituents.
A
photochemical amplifier for liquid chromatography
based o n singlet oxygen sensitization involves '02 reacting w i t h a substituted f u r a n 4 5 3 . Singlet oxygen generating efficiency of aterthienyl and some o f its synthetic analogues s h o w these m a t e r i a l s , used as mosquito l a r v i c i d e s , f u n g i c i d e s , and n e m a t o d i c i d e s , are excellent ' 0 2 deactivation o f
s e n s i t i z e r s h s 4 . Generation and
l o 2 by helianthrene and its
derivative^^^^,^^^
hypercin4S7 have also been reported. T h e photosensitized
and
production
o f ' 0 2 P M M A glasses w i t h acridine sensitizers has been measured by time resolved spectroscopy4s8.
In these conditions sensitizers are
found t o quench the generated l o 2 . Deactivation of singlet oxygen by thiols and related compounds have been examined a s possible protectors against skin p h o t o s e n ~ i t i v i t y ~ ~Variation ~. of acidity
35
I: Photophysical Processes in Condensed Phases shows t h a t
'
O2
theoretical
reacts exclusively
study of
with thiolate
reactions o f
examined p o s s i b l e i n t e r m e d i a t e s . effects
Singlet
oxygen y i e l d s
properties
and
lo2
yields4G1.
energy
:I
pairs.
I
Types
photooxidation o f
crf
of
Typical rose
and
I 1 processes
tryptophan
and
and d r a w s
attention to
l i f e t i m e i n t h e membrane.
The
components.
concepts
emphasis
the
fact
sensitizing
h a e m a t o p o r p h y r i n d e r i v a t i v e components depends cellular
are involved
t y r o s i n e by
current
b i o l o g i c a l membranes w i t h
o x y g e n mechanisms
with different
dyes
r e q u i r e s t h e r e t o be a l o o s e
p h t h a l ~ c y a n i n e s ~ ~ V' . a l e n ~ e n oh~ a s~ d ~i s c u s s e d the photomodification
longer
st a te
to
from t r i p l e t dye t o t h e
i n detail463.
production.
b i n d i n g between i o n
singlet
t r ip l et
with
azodyes d u e
by u s i n g m e t a l complexed
transfer
have been i n v e s t i g a t e d
bengal behaviour,
sensitized
o r I' e l a t e d
fading of
I and Type I 1 s e n s i t i z e r s based o n r o s e b e n g a l
m e t a l i o n s 4 6 2 . Type onium s a l t s
The
c a n be i n h i b i t e d
photogenerated lo2
w h i c h quench by undergbing
has a
and r a d i c a l .
i n the photosensitized photooxidation o f ester o f
po l y u ns at u s a t e d f a t t y a c i d s h a v e been
i n the
a11i.ons~~~ A .
O2 w i t h o r g a n i c s u l p h i d e s has
on
on
that
lo2
efficiency
on i n t e r a c t i o n
Hydrophobic e f f e c t s
p o r p h y r i n a g g r e g a t i o n and f l u o r e s c e n c e
influence
Polyethylene
g l y c o l m o d i f i e d h a e m a t o p o r p h y r i n has t h e p o s s i b l e advantage as a photosensitizer i n that organic
Cherniluminescence of
i t i s
soluble
i n b o t h aqueous
e x c i t e d ketones
has
formed
and
been used t o measure t h e r e l a t i v e y i e l d s from s e l f
alkylperoxyl radical pairs46E. azides a dehydroazepine spectroscopy and t r i p l e t
nitrenes of
and nanosecond l a s e r
reaction of
i s detected
a t
and
media produces a t r i p l e t
state
have a l s o been d e t e c t e d . i n a ~ i d e s ~ ~ ' . Picosecond
t h e nn*
reaction
o f d i m e t h y l s i l y l e n e produced by
triplet471.
o f dodecamethylcycloherasilane review o f
techniques which
acetate
t h e p--nitrobenzylanion
a f t e r cleavage o f kinetics
Absorption,
i s another
i n aqueous and C 0 2
emission,
and
f l a s h photolyses
interesting
f l a s h p h o t o l y s i s s p i n resonance describes CIDEP can be used t o i d e n t i f y
free radicals473.
r e s o l v e d E S R a n d ENOOR h a s b e e n u s e d d u r i n g t h e p h o t o l y s e s
dimethoxy-W-phenylacetophenone, initiator474
red Singlet
o f f?-nitrophenyl of
aryl
room
4-nitrophenyl
photolysis
and
by t i m e r e s o l v e d i n f r a
and dehydroazepenes
3-
alkoxyl
I n the photochemistry o f
and f l a s h p h o t o l y s i s
t h e photochemistry
A
solution
solvents467.
an e f f i c i e n t
Time o f W,W-
UV c u r i n g
.
Some s t u d i e s o f
chemiluminescence can be c i t e d as b e i n g o f
36
Photochemistry
physicochemical interest.
It i s c o n s i s t e n t w i t h p r e s e n t d a y t r e n d s
t o w a r d s n o n - h o m o g e n e o u s k i n e t i c s t o find r e p o r t s o f o s c i l l a t i n g chemiluminescence w i t h luminol in a continuous f l o w stirred tank reactorh7'.
T h e first study o f peroxyoxalate chemiluminescence i n
microemulsions has a l s o appeared476.
Cyclodextrins increase the
y i e l d s o f c h e m i l u m i n e s c e n c e f r o m a q u e o u s p e r o x y o x a l a t e s by f a c t o r s u p to 3 Q 0 4 7 7 .
Many f a c t o r s a r e i n v o l v e d b u t irrcLusion o f b o t h t h e
r e a c t i o n i n t e r m e d i a t e and f l u o r o p h o r e i n t h e c y c l o d e x t r i n c a v i t i e s i s proposed. The study o f photochromic materials i s a n active area o f research.
P i c o s e c o n d t i m e r e s o l v e d s p e c t r o s c o p y h a s b e e n used t o
investigate primary processes i n ring opening i n the photochromism o f s p i r o x a ~ i n e s ~ ~C A~R.S h a s a l s o b e e n used t o i n v e s t i g a t e s o l v e n t e f f e c t s on i s o m e r i c d i s t r i b u t i o n s i n t h e s a m e s y s t e m s 4 7 9 . Resonance Raman studies o f the photochromism o f 1 ' , 3 ' , 3 ' - t r i m e t h y l spiro
- [ 2 H - I - b e n z o p y r a n - 2 , 2 ' - i n d o l i n e 1 show at least four
t r a n s i e n t s h a v i n g m e r o c y a n i n e s t r u c t u r e s a r e involved4".
A
two
step two colour l a s e r study o f piperdinospiropyran and a nitrochromene shows that photolysis a t 355nm followed excitation of t h e t r a n s i e n t s by a s e c o n d 5 3 0 n m p u l s e p r o b a b l y p r o c e e d s by a s i n g l e t mechanism"'. Photochromic behaviour o f salicylidene anilines incorporated in a L a n g m u i r - 8 l o d g e t t m u l t i l a y e r shows that thermal decoloration is s u p p r e s s e d by t h e h i g h l y o r d e r e d d e n s e l y packed e n v i r o n m e n t 4 8 2 .
.
B i s t a b i l i t y h a s b e e n o b s e r v e d w ~ t ht h e t r i p h e n y l i m i d a z o l y l r a d i c a l dimer when irradiated at
A
transition between t w o states
i s i n d u c e d by c h a n g i n g e i t h e r t h e f l o w r a t e or i n c o m i n g l i g h t f l u x . This is believed t o b e t h e first example o f chemical instability induced in a n i s o t h e r m a l p h o t o c h e m i c a l s y s t e m .
5 .
Biolocrical S v s t e m s
A p p l i c a t i o n s o f p h o t o p h y s i c s i n b i o l o g y and m e d i c i n e a r e very e x t e n s i v e and o n l y a f e w t o p i c s c a n b e m e n t i o n e d i n t h i s r e v i e w .
A
survey o f t h e u s e o f lanthanide ions a s luminescent probes o f b i o m o l e c u l a r s t r u c t u r e 4 1 4 and a g e n e r a l a c c o u n t o f l o n g d i s t a n c e e l e c t r o n t r a n s f e r i n p r o t e i n s and m o d e l s y s t e m s 4 8 5 a r e very helpful.
T h e m e t h o d s a p p l i c a b l e t o t h e s y n t h e s i s and a c t i v a t i o n o f
a n u m b e r o f p h o t o a c t i v a b l e f l u o r o p r o b e s h a v e b e e n d e s c r i b e d and photoactivation yields measured486. T h e e f f i c i e n c y and m e c h a n i s m o f f l u o r e s c e n c e q u e n c h i n g by a c r y l a m i d e and s u c c i n i m i d e f o r s i m p l e a r o m a t i c f l u o r o p h o r e s h a s
I: Photophysical Processes in Condensed Phases
37
been e x a m i n e d w i t h regard t o t h e i r e x t e n s i v e use as b i o c h e m i c a l probes in a q u e o u s s o l u t i o n 4 8 7 .
UV
resonance Raman excitation
p r o f i l e s o f t y r o s i n e s h o w that t h e t e c h n i q u e is a p p l i c a b l e for e x a m i n i n g e x c i t e d s t a t e interrnediates4O8.
Picosecond resolution of
i n d o l e a n i s o t r o p y d e c a y s and r e l a x a t i o n can be followed by 2 G H z f r e q u e n c y d o m a i n s p e c t r o ~ c o p y ~ ~ ’ .R o t a t i o n c o r r e l a t i o n t i m e s as s h o r t as 8 p s can be m e a s u r e d .
P r o t o n induced q u e n c h i n g of
t r y p t a m i n e and r e l a t e d indoles4”
as w e l l a s tryptophan4”
have
been e x a m i n e d i n s o m e d e t a i l and e n v i r o n m e n t a l i n f l u e n c e s s t u d i e d , for example 18-crown-6 ether structure effects. lifetime distributions
Fluorescence
in h o m o t r y p t o p h a n d e r i v a t i v e h a v e been
m e a s u r e d a s a f u n c t i o n of t e m p e r a t u r e and v i s c o s i t y c 9 2 .
Decay
processes a r e affected by m o t i o n s of t h e t e t h e r e d s i d e c h a i n q u e n c h i n g g r o u p . P i c o s e c o n d f l u o r e s c e n c e has been used t o study s i n g l e t r y p t o p h a n r e s i d u e s i n a s e r i e s of p o l y p e p t i d e h o r m o n e ~ ~ ~ 3 . T h e effect o f u n f o l d i n g o f t h e p r o t e i n on t h e t r y p t o p h a n y l f l u o r e s c e n c e l i f e t i m e in a p ~ m y o g l o b i n ~ ’and ~ molecular dynamics s i m u l a t i o n s o f f l u o r e s c e n c e p o l a r i z a t i o n s of t r y p t o p h a n s i n myogl~bin~a ’ r~e o t h e r i n t e r e s t i n g p h o t o p h y s i c a l i n v e s t i g a t i o n s . high r e s o l u t i o n f l u o r e s c e n c e d e c a y and d e p o l a r i z a t i o n
A
study o f
a p o l i p o p r o t e i n s s h o w s t h a t t r y p t o p h y l f l u o r e s c e n c e s h o w s d o u b l e and tri-exponential decays496.
T h e s e a r e assigned t o i n t e r n a l m o t i o n s
of t h e p r o t e i n . A g e n e r a l a r t i c l e d e t a i l s t h e m e t h o d s o f f l u o r e s c e n c e q u e n c h i n g r e s o l v e d s p e c t r o s c o p y of p r o t e i n s 4 9 7 . T r y p t o p h a n r e s i d u e p h o s p h o r e s c e n c e in r e v e r s e d m i c e l l e s has been used in protein d y n a m i c s of l i v e r a l c o h o l d e h y d r o g e n a ~ e ~ ’and ~ a l k a l i n e p h o ~ p h a t a s e ~ ~ ’ .R o t a t i o n a l m o t i o n s of m y o s i n heads i n m y o f i b r i l s has been studied by using p h o s p h o r e s c e n c e a n i s o t r o p y d e c a y at 2 0 0 ~ 5 0 0 . 5 - E O S i n y l m a l e i m i d e i s a u s e f u l r o o m t e m p e r a t u r e t r i p l e t probe.
T h e d i s t a n c e b e t w e e n f l u o r e s c e n t probes a t t a c h e d t o
f o u r e s s e n t i a l l y s y l r e s i d u e s in p h o s p h o e n o l p y r u v a t e c a r b o x y l a s e has been m e a s u r e d by r e s o n a n c e e n e r g y t r a n s f e r s a ’ .
The topography
o f a c e t y l c h o l i n e r e c e p t o r has a l s o been r e v e a l e d by f l u o r e s c e n c e energy t r a n s f e r s o 2 .
T h e e f f i c i e n c y o f long r a n g e n o n r a d i a t i v e
e n e r g y t r a n s f e r among t r y p t o p h a n r e s i d u e s in b a c t e r i o p h a g e T4 l y s o z y m e has been studied using singlet e n e r g y t r a n s f e r , l o w t e m p e r a t u r e p h o s p h o r e s c e n c e and ODMR s p e c t r o s c o p y S o 3 . f a c t o r s a r e assigned
from crystallographic data.
Orientation
F l u o r e s c e n c e has
b e e n used t o m e a s u r e s u r f a c e d y n a m i c s o f l y s o z y m e s absorbed on hydrophobic silicaso4.
This is the first direct kinetic method
m e a s u r i n g c h a n g e s o f s t a t e o f a protein on a b s o r p t i o n .
Other
Of
38
Photochemistry
detailed studies have been made on b a c t e r i o r h o d o p ~ i n ~ ~ ~The - ~ ~ ~ . non-exponential
fluorescence decay functions of a single tryptophan
residue in erabutoxin b shows considerable freedom o f internal rotation around a covalent band in the s u b ns t o ns t i m e s c a l e s o 9 . Picosecond and nanosecond germinate recombination o f myoglobin w i t h C 0 , 0 2 , N 0 ,and isocyanides have been examined in details1’. The method o f triplet state detection i n d y e labelled proteins by fluorescence recovery spectroscopy has been described in a paper presenting this new m e t h 0 d 5 l 1 . A very
spectacular achievement i s the claim to b e a b l e t o detect
single molecules by fluorescence microscopy.
Single molecules of
phycoerythrin labelled w i t h 2 5 rhodamine 6 G chromophores are evidently detected in hydrodynamically focussed flows by laser induced f l u o r e ~ c e n c e ~. ’ ~ Acrylamide and O2
fluorescence quenching has been used as a
probe of solvent accessibility of aromatic fluorophores complexed with D N A 5 l 3 .
Binding o f c o r o n e n e , proflavin. and Hoechst 3 3 2 5 8
have been studied and a l l found insensitive t o quenching by acrylamide. Effects of excimer laser light in inducing strand breaks i n single strand514 and plasmid
DNA5”
have been
investigated in s o m e d e t a i l and two photon excitation demonstrated laser flash photolysis of DNA intercalated ethidium bromide i n the presence of methylviologen shows quenching by electron transfer516.
The hydrophobicity
o f DNA interior has been estimated
using 5 - and 8-methoxypsoralen as probesS17.
The environment
appears t o be very similar in polarity to methanol.
Fluorescence
energy transfer from tyrosine has been used as a probe for interactions of proteins w i t h D N A 5 1 e . Photochemical hole burning has been used for daunomycin i n solution and intercalated w i t h DNA519.
Experiments with d(ATIS and d ( C G I S
from environment.
s h o w differences arise
CIDNF has been used to study the reactivity o f
the furocoumarin photosensitizing
drugss2’.
Fluorescence lifetime distribution o f D P H r e v e a l the effects o f
cholesterol o n the microheterogeneity o f erythrocyte
membraness2 . All aspects of photodynamic t h e r a p y , including relevant photophysics, Angeles522.
are covered in a report o f a conference held in Los T h e r e is much information provided o n the nature of
haernatoporphyrin derivative structure.
A ,representative t y p e o f
study is the determination o f pK values f o r haematoporphyrin -a I X by absorption and fluorescence spectroscopy523. Fluorescence.
I: Photophysical Processes in Condensed Phases
39
quenching of a cationic porphyrin by cationic and anionic aromatics involves formation of ground state c o m p l e x e s 5 2 4 .
Axial-ligand
control o f the photophysical behaviour of Ru(I1) tetraphenyl and octaethylporphyrin produces contrasting properties of TI*)
excited states52’.
and ( r
(IT,IT*
The fluorescence and triplet properties of
water soluble tetra s o d i u m - m e s o - t e t r a ( 4 - s u l p h o n a t o p h e n y l ) porphine ( 1 2 - h y d r a t e ) in erythrocyte c e l l ghosts have been measured with the efficiency of
lo2
efficiency s 2 6 .
together
Multifrequency phase
and modulation fluorometry has been used for resolution o f porphyrin photoproduct mixtures527
.
Photophysics o f cof acial
porphyrin quinone cage molecules and electron transfer studies have been reportedsz8. Transient Raman spectroscopy has been used to observe the photodynamics of metal p o r p h y r i n ~ ~ 2 ~ . Picosecond fluorescence spectroscopy shows that incorporation of
haeinatoporphyrin derivative into malignant tumour cells occurs
iri vitro530.
The aggregate component i n H P D increases in cells
with incubation t i m e .
A
fluorescence imaging device for endoscopic
detection o f early stage cancer uses a fluorescent porphyrin mixture o f dihaematoporphyrin ether and e s t e r S 3 l .
Autofluorescence
is eliminated and the image intensified by a d i g i t a l image processing
system.
MerocyanineS4’
is
p r o b e w h o s e photophysical properties
another useful fluorescent have been investigated532.
It
is useful for identifying leukaemic c e l l s , analysis of plasma m e m b r a n e s , inactivation of v i r u s e s , etc. Theoretical calcul’ations have been made for photosynthetic pigrnents’33.
An extensive review of models o f energy and electron
transfer events of synthetic molecules for photosynthesis has been prepared by W a s i e l e w ~ k i ~ 3 ~Other . studies have made on tetraphenylporphyrin-polypeptide
pigmentss3’, photosensitization of
triplet ~ a r o t e n o i d s ’ ~ ~ fluorescence , yields and lifetimes for bacteriochlorophyll c 5 3 ? , triplet yields and ESR of chlorophyll a 5 3 8 , and quenching processes o f pheophytin
s39.
Other biologically relevant studies are on t h e fluorescence and phosphorescence of harmal and harmald at 7 7 K S c 0
and on also the
photophysics o f the antiflammatory drugs n a p r o x e n , benoxaprofen, and indornethacins4’.
It is perhaps an extreme case o f biological
relevance t o report a paper on t h e total fluorescence of human urine542.
A three dimensional presentation o f luminescence data
allows several fluorescent metabolites t o be detected and identified.
Photochemistry
40 References
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Part II PHOTOCHEMISTRY OF INORGANIC AND ORGANOMETALLIC COMPOUNDS
1 The Photochemistry of Transition-metal Comdexes ~ ~ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ _ _ _ _ _
BY A. COX
1, 1ntt d u c t ion
Reviews have appeared of the structure and reactivity of metal-centred
Ru( I I )
transitlon metal
complexes
photocatalysis campounds,
having
induced
excited
aromatic
states,
Jumineacent
heterocyclic
J igands,
light-sensitive
by
coordination
photochemical reactions between t x a n a it ion metal
complexes and gases at high pressuree ,4 inorganic photochemistry at
high
pressure,5
cobalt( I I [)-mine
and
the and
complexes6
p- peroxocobalt ( I I I )
photochemistry
complexes.
of
8-8
some
w-superoxo-
and
The
appl ication
of
metal-catalysed photor eact ions to sugar molecules has al.so bean reviewed - 8 2. A
Titanium
charge-transfer type complex formed by
[Pe(CN),J4photoresponse
by
Ti02
onset
is
particles over
700
nm
reported with
a
daorption o f to
have
its
photon-to-current
converaAon efficiency a8 high as 37%.9 Ti02/NI0 la an actlve catalyst. fur the photodecompoeition of water, lo a correlatlnn w j t.h pH-dependent surface structures has been astab.1 I shed P or the
photacat.alytlc activity of T i 0 2 suspended In aqueous AgN03,’’
and
the photocatalytic activity and melect-fvity of V/TIOz fn the
oxidation of lsobutene I R dependent an the c r p t a l modlf ication
of the Tio2.I2 Titania loaded with tungetosilicate ISiW,,0,,J4-
Is active in the photoproduction of hydrogen under band gap
65
Photochemistry
66
i l l ~ m i n a t i o n .Platiniaed ~~ titania has been used to catal.yse the photareduction of sodium carbonate to C and HCHO,"
and a new suspended in
method for the N-alkylation of ammonia by T i O Z / P t
alcohols has been published.l5 Semicanductor-semi ti zed radi cal Pormation is the initial step in the photacat.a.lyt.icreaction of dienea on irradiated Ti02 p0wders.l' sensitization of
Ti02
Reports have appeared of the
in the visible
light region usi.ng a
water-soluble anionic zinc porphyrin17 and of khe use o f t.ik3ny.l moncwners and dimera as
and vanadyl meso-tetraphenyJporphyrin
catalysts for the photooxidation of alkenerr.'O Photareduction of Ti( JV) in alcohol8 gives the act.ahec3ra.l complexes
[TiClz(ROH)qJ+, [TiC13(ROH)3J , and
the tetrahedral
complexes [TiQHC13J and [TiORC13J .l9 The same authors a l s o report the Pormation of hydrogen, and correlate its yie1.d to the a-CH bond energies in the alcohol. 3. Vanadium, Niobium, and Tant-a.Zum Hydrogen
chemiaorption
causes
large
increases
in
the
photoionisation threshold of isolated txana it inn metal. cl.nat-srs such as Vx, Nbx, and Fe,
( x =
induced shifts for
H
both
3-25).21
and
NH3
as
The dJrection of the
absorbatea haa
been
ratianalieed using a frontier molecular orbital model, Redox and photochemical propertiea of V(I1) polypyridine complexes have been
combined
to
achieve
the
first
example
of
photoredox
initiated 2-electron oxidation at a single transition metal centre.22 Studies have appeared of t h e photooxidation of V( 1 1 ) in acidic solution23 and
the
redox
chemistry of
water-soluble
vanadyl porphyr ins,24 and the mechanism of methane photooxidat ion
on
a
vanadium/silica
catalyat
has
been
shown
to
invol.ve
interaction of CHI with surface oxygen anions bonded to vanadium
1111: The Photochemistry of Transition-metal Complexes
67
ions25 The
photoredox
reaction8
of
the
cluster
compounds
'' have been deaclr
2+r [ NbgBr 123 2 t , and [ TafiC11~ ) [NbfiC11Z]
ihctd. 26
4. Chromium, Molvbdenum. and Tunaaten A
Cr(I),
study of the photoredox reactions of [M(C!N)5NOIn- (M Mn(I),
Mn(I1))
=
has shown that in the case of C r ( J ) and
Mn(1) the process is intermolecuJar h u t that. for Mn(J1) it. is intramolecular 27 Purther
temperature dependence
studies
of
the
doubl.et
lifetimes of coordination complexes of Cr ( I l I ) have tlhown that the RISC (reverse interaystem crosaing) is the lifetime Limiting process in compounds with a sma.ll energy separation.28 A small reduction
of
the
lifetimes
of
excited
[Cr(NH3)6I3+
and
[Cr(cyclam)(NH3)2J+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) with increased solvent mobility has been observed but which is absent in the deuterated analogues29. This has been interpret,ed to indicate that enhances
increased anharmonicity jn accepting modes
nonradiative
rates.
The
in
acidic
&-[ CK ( CYClam) (NH3)21 3+
--
ICr (cyclam)( H2O) (NH3 ) ) 3 t .
New
photoaquation
evidence
media shows
of g ivea
that
the
reaction proceeds yia a chemically reactive intermediate that is formed in concert with the decay of the loweat electronical.Ly excited doublet state. 30 An examination of the C.F photocherniatry
-
of [ ~ Y ~ - C Y ( N H ~ ) ~ ( C( X) X ] H20, ~ ~ NCS-, soiution suggests a c o m o n precuraor (%Z/4B2)
and )'P
in acidic
for all oP their
phuLorusctions implying that doublet deact lvatf on occurm mainly
rFrr
back lntersyetem crossing to the lowest excit-ed quartet
atate. 31
Mechanistic
implications
of
pKeSFJUKe-dependctnt
photochemical and photophya ical parameters of Cr ( I I I ) compl.exes
Photochemistry
68
have been considered.32 In particular, i.t i s suggested that a stxongly associative pathway for (4T2g) [Cr(NH3)613+ aguatjon i s followed-
study of the photoinduced water exchange of the
A
eight. possible amineaguachromIum( I I I ) ions uefng J80J abel l ing a
has allowed aemimpirical Cr(II1)
rationalisation of
current
empirical
and
tho photostereochemistry of
complexes.33 Ligand field irradiation oP [d~-Cr(en),F2]+
acid
in
critical evaluation of
solution
leads
successively
to
cia-[Cr (en)(en-H)F2 (H20)1 2+,
-.-cis- [ Cr (en)2F (H2D) ]
'+
[Cr(er~)(en-H)P(H,o)~]~+ and [Cr(en-H)2F2(Hp3)2J2t- 34 Lifetimes of the doublet excited states of some tr is( polypyr i w l ) complexes of Cr(II1) measured as a function of pH and [ C l - 1 ,
reflect. the
environment in the vicinity oP the met.a.1 centre and suggest. the existence of HZO, HO- or C1- within the intraligand pockets o f miss ion
2T,/2E; 35
properties
1,4,7-triazacyclononane, X
=
Cr (tacn) X 3
of
P, Rr,
CN,
(tam
=
NCS) have also bean
measured. 36 Electrogenerated chemiluminescence has been reported
for [Cr(m)6]3- and [Cr(bpy)3J3+ in pKOtiC
S O l V d S . 3 7
Various
aliphatic and aromatic hydrocarbons have been photooxidiaed using Cr03/AcOH
or Cr2072-/H$30q/H20/Bu4NBr38
and ZnO ha8 been s h m to
be an efficient catalyst for the photoreduction of Cr(V1) even in 8unlight.39 Studies of the kinetics of the photoaquatfon of [Mo(CN)8J4i n alkaline media,40 the 1 Ight-Induced redox reaction8 of [MS4 1"(
M
=.
Mo(VI), W(VI) , V ( V ) , Re(V1 I)) ,41 and of molecular hydrogen
formation on
photoreduction
of
Mo(V1)
in
aquenus-ethanolic
have been published. A kinetic invest tgation of the mechanism of photodissociation of WCl6 and W C l 4 in the ga8 phaae containhg H2 has been
A
study of the effect of
IIII: The Photochemistry of Transition-metal Complexes
69
counter-ions on the photooxidat ion of secondary alcohols Y3[PW12040] ( Y
-
H,
and y~'[W10032] (Y'
TBA)
tetrabutylanmnoniwa) has appeared; 44
UCIing
TBA) (TBA
K,
-
strong precomplexation is
required and the reaction occurs by a rapid two-electron transfer process.45
Hydrogen
eolutions
under
can
be
generated
photocatalysis
by
from
aqueous organic
polytungstatea 46
The
involvement of hydrogen-bonding protone in delocalisation of the paramagnetic
in
electron
a
aingle
crystal
photoreduced
of
decatungstate ha8 been reported47 and the same authors have also discusaed the photodjmerisation of o l d h a via abstraction of an a l l y l hydrogen using decatungstate.48
5. Manaanese and Rh enium Mn (TPP)CIOQ (TPP
-
5,10,15,20-tetta~enyl~orphyr inato) can
be photoconverted to Mn(TPP)Cl with concomitant oxidation of organic substrates, including alkane hydroxylation and alkene expoxidation.49 A n lnftial photoinduced oxygen a t o m
tlcansf el:
to
generate an active metal-oxo intermediate appears to be Involved. Fhotoreduct ion
of
tetrakis(4-methylpyridyl)porphyr inato-
manganese(II1) in the presence of a proton donor occurs by a reversible 1-electron process to give the corresponding I&I( I I) porphyrtn.
Photoirradiation
meso-tetraphenylporphinato)
In
of
Mn( I I I ) (TPP)N3 a
-
range
of
temperatures reveale a competition between oxidation of
the
2-MeTHF
over
(TPC
central manganese atom of the complex to yield Mn(V)(TPP)N, and
.
reduction to give Mn( I I ) (TPP) 51 Photodecompositlon products of aqueous Mn04- are reported to include a long-lived peroxo m(V) complex intermediate. 52 Discussions have appeared of! the photochemistry of some rhenium phosphite complexes such as t r w , m-ReC13py21P(OEt3) 1,
Photochemistry
70
cis,mer-ReC13(PEt3)[P(OEt)g]~, -
ReCI3 C P ( OEt)3 1 3,
and
ReC14[P(OEt)3]2, 53 and of the emission from [Re2Clg12-, which Beems to originate from the
l88*
excited state of the eclipsed
ion. 54 Iron
6.
Light-induced excited spin state trapping (LIESST) has been isothlocyanato)bis(l, 10-phan) iron( 1 I) J and
observed in --[bis(
[hexakis(1-propyl-1H-tetrazole)iron( I I ) 1 Solvent
of
tuning
the
ketrafJ u o r ~ b o r a t e , ~ ~
excited-state
propertjes
of
(BuqN)2[Fe(bpy) (CN)4] has been reported .56 Iaaer photolysis of [ FeL2 J 2+
( H3L
[Fe(H3L2)J3+
=
1,4,7-tr iazacyclononane) at pH 8-10 produces
and
[ Pe(H3L) (H2L)12+
hydrated
a
electron:
at
>
pH
7.0,
and a hydrated electron are formed at the same
rate. 57 The quantum yields of photodissociative axial ligand substitution reactions o f
low-spin FeN4XY
compl.exes,
(N4
=
bis(dimethylg1yoximate) or bis(naphthoquin0ne dioximate); X or Y =
CO,
PBug, P(OBu)3,
PhCHzNC, or methylimidazole) have been
observed to decrease with decreases in the u-acceptor strength of the axial 1 i ~ ~ d . 5Ratios 8 of quantum yields f o r loss of Y
x
yersua
in both ground and excited states were found to be similar The
kinetics
[Pe(CN)s(H20) J2' reported. 59
of
the
photochemical
.
decomposition
of
have been measured and activation parameters
Individual quantum yields of Pe( I I I) ( H C O Z - ) ~( n
1-4)60 and of [F~(III)(OX)~~-H,,,+] (Ox2'
= C2042-
n
=
1-3, m
=
0,1)61 have been determined in aqueous acidic solutions, and
photocatalytic effects of the pentan-2,4-dionates of Pe( I I C ) and Cr ( I I I )
on the decompos it ion of
1,2,3,4-tetrahydro-l-naphthyl
hydroperoxide have been measured,62 I n the presence oP excesa phenanthroline at pH
N
1.5, [Fe(phen)3l3+ can be photoreduced.
llll: The Photochemistry of Transition-metal Complexes
71
The process is wavelength dependent and the reaction is inhibited by some inorganic l i g a n d ~ s . ~ ~
7. Ruthenium Report8 have appeared of a timedependent shift of the emission spectrum of the M&CT excited state of [R~(bpy)3]~+at low temperature64 and of the properties of the lowest excited states
of
containing
8 ingle-crystal
[ Ru( bpy) ] ( PP6)
[O~(bpy)~]~+
redistribution
in
the
A
excited
host
mater ial
atudy
of
Ru(1L)
polypy~idyl
stater, o f
energy
complexes concludes that the excited atate energy is localised on the ligands in the lowest MLCT statea and is funnelled into the energetically favoured ligand after InitiaJ excitation.66 The recent assertion67 ascribing the polar ization anomaly observed in the
photoselection
heterogeneity,
has
of
spectrum now
been
[Ru (bpy)
12+
challenged.68
solvent
to The
MCPL
of
[Ru(bpy)3I2+ in PVA and 4:1 EtOH/MeOH 801fd solutions is reported
to
be
wavelength dependent
preferential distortion
excitation in
the
of
this
ions
with
ground
state
-
I n teract 4 ons 69
s o l vent/coun t.erIon measurements
of
and
is
interpreted
various d u e . to
~1r cuI. ar
electrochemilumineacence
the
degrees
species7'
and
[ (NH3)s R U ( I 1) ( m N )12+, [ (bpy)2Ru( II)Cl(DMABN)
1'
the
polar i sat J on
Prom
observed
of the
photoluminescence
properties
r (w3)
B I 3+, N I
(DMABN
-
II
r
~
~
of
asymaetric
(+)-I Ru( phen)3 J2+ show that the ECL arises from emission Ru( I t )
as
of
and
(dimethylamino)benzonitri~e),
complexes which exhiblt twisted internal charge transfer states,
of d - R u ( b ~ ) ~ ( N ~ ) ~at - H245 ~o have been d l s c ~ a s e d .Excitation ~~
nm and in an alcohol glass at 77 K leads to a st-rong luminescence; in MeCN-H2O photosubetltution occur8 to g i v e
Photochemistry
72
.
[ Ru (bpy) (N3)( CHsCN)3 + 7 2 The same author has also investigated
[ R U ( ~ W ) ~ ( C H ~ C N(PP6)2.73 )~]
Biphotonic
luminescence of
3 2+
[ Ru (bpy)2 ( AA)
effects
in
the
2,2 ' -biquinoline , or
( AA
6,7-d ihydto-5,8 -d imethyId ibenzo [6.1 ] [ 1 , l O ] phenanthro 1ine) associated with polytungstate anions such as [ C O ( H ~ O ) S ~ W ~ ~ O ~ ~ ] and
[Mn(OH)PWl103,]6-
proper t ies of
the new
(L
[ Ru (bpy)2L]2+
=
have been measured ability
on
have
complexes
CT
[Ru(bpy)~(bpz)12+
The
emission
[W u (bpy)Lz ] 2+
[ RuL3 ] 2t,
and
pyr id inopyrazoles and pyr idinopyrazolines) and the effect
properties
luminescence
reported .74
been
of
the
transfer
of
the
complex
i n ~ e s t i g a t e d ; ~the ~
of
spectra
ligand n-acceptor
[Ru(bpz) ] 2+
and
have also been measured .76 The photophysical
properties of Ru( biq)2 (CN)2, Ru( DMCH) ( C N )2 and Ru( i-biq) (CN)2 (biq
2,2'-biquinoline, DMCH
=
6,7-dihydro-5,8-dimethyldibenzo
[ 3,2-b.2 ' , 3 ' -/] [ 1,lO]phenanthroline,
i-biq
-
2,2 ' -bi-
=
isoquinoline) have been studied with particular reference to acidity
and
temperature
tuning observed
of in
the the
luminescence 77
Solvatochromism has
been
excited
state of
cis-[Ru(phen)2(CN)2]
and attributed to electron donor-acceptor
interactions involving the cyanide ligands.78 The visible spectra of C R ~ ( ~ P Y ) ~ ( P P ~ )and ~ I + C ( b p y ) ~ R u ( p p ~ ) R u ( b p y ) ~ I 4(ppz + planar
ligand,
localised on the different ligands, and the assigned to a ppz(r*) calculated
to
be
+
lr*
the
are
4',7'-phenanthrolino-5',6':5,6-pyrazihe)
composed of MLCT transitions which terminate in the
=
orbitals
luminescence is
Ru(II)t2 t ~ a n s i t i o n .These ~~ states are
weak
reductants
but
powerful
oxidants.
Picosecond absorption spectroscopy has been used to measure the spectra of
the primary MLCT e x c l t e d
pentaamineruthenium(I I ]
complexes and
states of
a series of
they are
Pound
to be
73
IIII: The Photochemistry of Transition-metal Complexes consistent with those expected from a model of these states as
Ru( I [I)L--
Excited state lifetimes are interpreted in
terms of the lowest excited state tuning model.
The effect8 of
the a-donating and n-accepting abilities of the axial ligands on the photophysical behaviour of Ru(I1) metalloporphyrinB have been In particular, two classes of comp.1 exes, RuP( cfQ)I. and
examined. RuPL2
(P =
tetraphenylporphyrin
or
octaethylporphyrin;
J.
=
piperidine, pyridine, W O , EtOH) have been investigated and the results compared with those for complexes in which t h e porphyrin macrocycle P is TPP or O E P . ~ ~ Measurements of quenching rate conatants of excited RuJ.s2+
(L = 4,4'-dialkyl-2,2'-bipyridine)
by aromatic amines and of the
activation parameters suggeat that in highly ewergic regions the process
is
adiabatic.82
liganda
cause
a
However, bulky
decrease
in
substituents on
transmission
coePficient
the with
increasing free energy of activation. The kinetics and mechanism of the [ Ru( bpy) 33 2+ sen8it ized chain react ion between formate and peroxydfaulphate have been reported,83
and comparison of t-he
rates of both reductive and oxidative quenching of [ R ~ ( b p y ) ~ ] ~ +
and cia-[Ru(phen)2(CN)2]
by aromatic amines and nltroaromatfcs
has shown that electrostatic interaction within product ion pairs determined the quenching mechanism, and hence the temperature dependence of the quenching.84 The
formation of
radicals by the photochemical ox idat ion of
semiqu inone
hydroquinones or
reduction of quinones using [Ru(bpy)3]2+ as sensitizer ha8 been examined in the range pH ~
8 Pair.
agreement ~ ~ was achieved
between the observed rate constants and those calculated using
Marcus theory. pyrldinium
--a I somerisatfon of N-methyl-4-pstyryl-
eensitized
by
[Ru(bpy)3I2+
proceeds
by
a
Photochemistry
74
one-electron transfer reaction, but it is reported that in the presence of colloidal s ilica an efficient electron-relay chai.n reaction on the colloidal surface supervenes .86 Photocatalyaia o f the Pachorr reaction by [ R ~ ( b p y J2+ ) ~ in t.he fluorenone, fluorene and dibenzofuran series has been reported and
is thought t.o
involve an electron tranafer mechanism.87 Measurements of the luminescence decaya of [Ru (bpy)3 3 2+ and [Ru(bpy)3 ] 2+ absorbed on metal oxide powders, 9 , S i 0 2 , SrTiOg, Ti02, have enabled t.he electron transfer rates from the excited Ru(1I) complexes to the semiconductors to be evaluated.88 The
dif€usion-controlled
constant f o r
rate
the
dynamic
~ +[Ru(bpy)z(4,4'-Cl2bpy)I2+ quenching of excited [ R ~ ( b p y ) ~ ]or the
heteropolytungstate
[ Co(H20)SiWll039 J6- shows
be Pitted by
phenanthrollne) by Ag'
=
in this
may
be
[ RhC1( dpm) J 3- (dpm
and
an ionic strength dependence which can 1,umineacence
bpy or 4,7-dirnethyl-l,10-
is no longer thought to occur by oxidative
electronlc transfer , but
exciplex
[Mn(OH)PWl,039]6-
the nebye-Smoluchowki equation.89
quenching of [RuL3J2+ (1,
exc iplexes.
anions
by
rather by
process
involved.
-
format ion of
luminescent
a termolecular metal
compI.ex
The
complex
Wilkineon
type
3-Ph2PCgH4S03-) ha8 been used successfully
as a homogeneous catalyst in conjunct-ion with [ R ~ ( b p y ) ~ ] ~and + ascorbic acid for the reduction of
Ascorbic acid has
also been photooxidieed to dehydroaecorbic acid with hydrogen evolution
using
Pt
loaded Ti02
in
an
aqueous solution of
[Ru(bpy)3 J2+.92 Carbon monoxide and hydrogen can be produced by the simultaneous photochemical reduction of C02 and H 2 0 by the visible
light
irradiation
of
organic
[Ru(bpy)3J2+ and a variety of Co(1i)
solutions
containing
species aa homogeneous
I I I l : The Photochemistry of Transition-metal Complexes
75
catalysts. 93
* [Ru (bpy)31 2+
Rate constants for the oxidative quenching of
by W2+ have been studied as a function of the concentration of
added electrolyte.94 The results have been used to evaluate critically the various literature expressiona for the dependence
of diffusional parameters on ionic strength. Rate constants for the quenching of [ R u ( b ~ ) ~ ]by ~ +various viologens with charges ranging from 0 to +4 have also been measured and variations interpreted in t e r m of Coulombic fnteractions of the reagents,95 and the effect of lipid and viologen structure on crosm membrane electron transfer [Ru(bpy),J2+
as
in donor
a
vesicular photosystem and
viologen
as
consisting of
oxidant
has
been
investigated. 96 Photoactivation of the three-component system [Ru(bpy)g J2+/KV2+/quadricyclene is
reported
to
initiate
a
catalytic cycle for the valence isomerisation of quadricyclene to norbornadiene.97 The primary photochemical step is oxidative quenching of the emissive ldLCT state of [ R ~ ( b p y ) ~ ] by ~+ W2+, followed by oxidation of quadricyclene by [Ru(bpy)3I3+ to give
the utructurally labile Q?.
A
photoinduced tranaacetalization
between 2-phenowytetrahydropyzans and octan-1-01, and involving the
Ru(bpy) 3 J 2+-methylviologen consens it izing system has been
described.98 It appears to proceed by initial electron transfer
from the excited ruthenium complex to the viologen, followed by electron
abstraction
from
the
acetal
by
[Ru(bpy)3I3+
Photohydrogenation of phenylacetylene and nethylphenylacetylene has been observed in a H2O-cyclohexane system using [ R u ( b ~ y ) ~ J ~ + as
photosensitizer, N,N'-dialkyl-4,4'-bipyridinium
as
charge
relay, FZDTA a8 sacrificial electron donor, and a Pt or Pd colloid stabilised in the organic phase as a hydrogenation
Photochemistry
76
Photoreduction o f CO;! t o HCO;!H
has been st.udi.t?d i.n t h e syst-em
TROW I Rii (bpy) 3 1'+/bi.s (viol.oqsn)/formake
dehydroqenase,
as
well. as i . t s photoreduct ion t o methane a.nd hi.gher hyrlroCAtrhonR us inq
co 1.1.0i d s - '-'I.
1: uthen ium
r Ru ( bpy ')3. ( CO)
The comp l.ex
3
'+ i.s
also an rtffic.ient- catalyst for khe photoradnct.ion of
The
e f f i c i e n c y of apat.ia.l charge separat.ion jn khe vets.icn.lar s y s k e r n
r Rl1( bpy)
12+
( internal.
(menJnrane)-oxidant.
(external
Sirnu1t-aneoua genaraki on of achieved
been
s o l l i t ion) ao.lut.inn)
heen
has
measursd
hydrogen and oxygen from water haft v.iaih.le
under
/octadecyl.v i.ol.oqen
1 ight
cho.1.ine
dipa.lmi toy.1 -D,T.-a-phoaphatidy.l
irradiat-ion
of
uo i nq
veo ic.1cta
rRn(bpy)3j2+ iinrler various condi.t*i.ons,1.04and t.he same qroup has
a
invest iqated [ R u (hpy) 3. ( 4 , 4
system
' -d ihept.adecy.l-7.,2' -hi pyr i d i n e ) 1
bil.ayer l i p i d mmbrane, K$3208 solution,
c0ntai.ni.nq
and
a
Co(II[)
surface.
The
trimetallic
compJex
12' ,3'-h]quinoxa.l h e ,
a
the
membrane
the new non7.i.nsar
1.i.feti.me o f
hexacheJat.ing
i nto
i.n t h e ext-arnal.
on
fixed
dipyrazino[2,3-f 1-
[[R~(bpy)~]~I.1~ (I.+ = a
htr .i 1 t.
as el.ect,ron donor
catalyst
luminescence
'+
.l .iganr11.06 and
of
the
photophysical pr0pert.j es of the din11c.lear and tetranuclear Ru( I I )
clusters
I (drnb);!Ru*JJt~(dmb)~]~+and
4 , 4 ' -dimethyl-:!,
[r ( d ~ n b ) ~ R ~ ~ r . J ~(dmb R u ) ~=+
2 ' -bipyr i d i n e ; L = I., Q - b i s / 2- ( 4 'methyl-
2,2 '-bipyridyl-4-y3 ) e t h y l I h e n ~ e n e ' lhave - ~ ~ appeared.
Measurement8
of
t-he
fluorescence
and
s p e c t r a of [ R ~ ( b p y J)3+ ~ i n aqueous s&i.um that
the
m i c e l l e . Io8
ruthenium
complex
Chemiluminescent
is
bonded
reactions
chemf J uminemence
dodecyl.sul.phate show
to of
the
surfactant
[Ru(bpy)3]+
and
[Ru(hpy)g13+ with solvent* a c e t - o n l t r i l e and also of [Mo6CIl41- and
under t-he same c o n d i t i o n s have heen report-ed.
'''
77
I I I l : The Photochemistry of Transition-metal Complexes 8.
Osmium
The metal-ox0
photoxidanta
trans-[Os(tmc) (0)212+
(bc
1r4,8,11-tetranrethyl-1,4,8,il-tetraazacyclotetradecane)
=
and
.
trans- ( 0 s ( C N ) (0) 2 J 2- have been reported 'lo
9 - Cobalt Photochemical
react ion
of
Co ( I I ) ( d q B P 2 ) 2
in
methanol
containing an excess of PBu3 gives H+[Co(I)(amsBpz)2PBu3]-, probably
involves
MeOH
as
reducing
agent..
'"
and
Cobalt ( I I )
tetraaulphophthalocyanine chemically bound to the surface of Ti02 is an efficient electron relay for the photocatalytic formation and depletion of hydrogen peroxide in aqueous suspensions.'I3. Free radicals do not appear to be involved. The same author8 have also propoaed a kinetic model for the oxidation of S(1V) to S ( V 1 ) using
In
this
a
mixture
compriaing
proflavine
monochlorohydrate a8 aeneitizer, [Co( 1 1 ) (bpy)3]Cl.z as t-relay, (HOCH2CH2)3N as donor and KqPtClg as CataJyst, the photoinduced electron transfer from sensitizer to the Co(l1) complex is more efficient in SDS anionic micellar solution than In water.l14 Oxidation of (Co(edta)'21
by S2Oa2-
is greatly accelerated by
irradiation in aqueous solutions containing [Ru(bpy)3 ]2+.115 A chain reaction is involved which is initiated by quenching of photoexcited [Ru(bpy)3 12+ by S2Oa2-.
An
external mechanical load
applied to crystals of Co(NH3)5NO2C12 is reported to decelerate the rate of nitro-nitr ito photoisomer isation. The energy level transitions and the bond energiea in the ground and excited state of CoX5Ln ( X C l ' ,
NOZ-,
=
NH3, C";L
-
H20, NH3,
CN-) have been calculated by the INDO-CI mathod.'17
The results may have
implications €or
the photosubst itut ion
chemistry of Co(I11) complexes. A model has been advanced in an
Photochemistry
78
I
2+
1
2*
n
$)N '
2CIOi
i < )
2 c 1Oi
Ph
Ph
(4)a R = H ; R ' = N ~ b R =CMt3 ; R' = NEtz c
R=R'=H;
R =R'=OMc
IIII: The Photochemistry of Transition-metal Complexes
79
a t t e m p t t o u n i f y a p p a r e n t l y d i f f e r e n t patterns o f photmchemi.str:y
of some d6 system. of
pattern
tt iplettl,
T h i s is based upon a wavel.engt~h-i.ndspendent
ar k i n g
reacti.on8 and
from vi.htat.i.onal.1.y eqiii.l.i.hr:akad
a wavelength-dependent
reactimn
s i n g l e t st-ate. '.I8 . P h o t o s u b a t i t t i t ion react,i.ons o f anl
- O , N ) b i a ( et-hylenediamine) cobal.t( I I t )
wi.t,h
ar i.8 i.ng i.n t h e (bi.dent-at.o-O,O at*hyl.enttiii.ami.ne
have been atndi.ed i n a weakly al.kal.ine aqueons sol.uti.on i n the [ Ru (bpy) J
presence of
'+
118 i.rq
c ixcnl.ar l y polar: i.sed l.i.ght,.
E x c i t a t i o n o f t-he a x i a l . l.igand CT t r a n s f e r : [C0TXz]Is1.04 and CoIJ,Cl.Y](Cl.Oq),
(I,
R b m r p t i o n hand
in
Y = NCO-,
NCS-,
t h o presence of oxygen,
of
5,5,7,1.2, 1.2,'I.l-hexsmet.hyl.-
3 , 4 , 8 , J ~ - t e t r a a z a c y ~ ~ o t ~ ~ ~ r a d e c a - 4 , 1 1 - dX~ e = n e :Cl,
N O - , NCS-;
'.'"
Rr,
N3-,
(n-2)) i n methanol. and
N3 ( n = l . ) , H20
.leads to f o r m a t i o n of C ! O ( J ~ ) . ' ~ ~ " The
same aut.hors have also jnvestigated t.he photochami ca.1 propart-ies of
the
Co(1JJ)
related
hcsxamet.hyI-.l.4,8,
wit.h
complex
maao-S,'7,7,13.,1J,.14-
~ J - t e t . r a a ~ a c y c ~ o t ~ t . r a d s c a n ~ :Reports l2~
have
appeared of tho phot~odecarhoxy.lat.ion of ( I ) and f . C o ( e n ) T , ) C . l ~ ( H T .=
y-aminobatyr.ic acid)
to g i v e
(2)
and
[~~(csn)~(~XCH~CH 1 (C.104)2 3 . N H ~ ) rePlpect~vely,1.22,1.23 and cryst21 structure
of
of t h e
IC o ( bm)2( CH2NH) 1 ( Cl.04) 2 , the phot.ol.ye i.e
of product of [Co(bpy)2gl-y1( C 1 . 0 4 ) 3 . - H ~ 0 , ' . L 4 @-a phot.oi.somc3t:i.aat.i.on
cohaloximes .in the 80.1 Jd atat.e,'25 [Ru(CN),I4-
electron
to a I.iganrl t i . e l d exci.t-ed skate
g l y c i n e ) have been r e p o r t e d ;
is
and
r (CN) 5Ru ( I J ) (p-C!N)Ru ( J I J ) (CN) 1.0.
mod i.um and
transfer
by
o f C l ~ ( g l . y )( ~ ~ 9 l . y=
the product. i.n this l.att.er c a n e
16-
-
tr i.di.nm
Ext-erna.1 magneki c f iel d ef f ects on t h e e.wc.it.ed s t a t e pr0pert.i ec1
of R h ( I ) and J r ( J ) complexes have hean d i a c t ~ s s e c l : ~ ~ - ~ A
3cwr-temperat.urs phosphorescence
and WMR study o f
t.he
80
Photochemistry [ Rh(bpy)3 J
excited state of properties
have
been
Cm(trpy) ( ~ P Y(PY) ) i3+,
has appeared.128 Photophys ical
reported
and
2,2':6',2"-terpyridine)
'
for
[Rh(tr~y)~]~+,
C R ~ ( ~ ~(P~YP)Y ) c ~ I ~(trpy +
=
and photolysis of [Fth(trpy)(bpy)C1I2+ in
aqueous solution induces photolabilisation of the chlorine as seen with other Rh( I I I ) chloro m i n e complexes.I2'
A
discussion
of the photoinduced cis-trans isomerisation of cis-[Rh(bp~)~XZ)"+ (X,Z
=
C1, OH, n-1; X
respectively; X
=
= Z =
H20, Z
=
H20, n-3; X = C1, 2
-
OH, H20, n=l,2,
OH, n-2) showa that some sterically
congested isomers not available by other routes, can be prepared photochemically.130
Long
wavelength
excitation
solutions of the ion pairs [Rh(bpy)3]3t[M(CrJ),]4-
of
aqueous
(M = Pe(JI),
Ru(II), Os(I1)) promotes charge transfer from the metal of the cyan0 complex to the bipyridine ligand of the rhodium complex and leads
subsequently
to
photoaquation
and
formation
of
[ Rh( bpy)2 (H2O)2 J 3 + . 131 Photogeneration of hydrogen from water has
been reported using a platinum catalyst fixed on a bilayer lipid membrane in the system [Rh(bipy)3J3'-[Ru(bipy)3J2+-EDTA,
and rate
constants for the various steps determined. Differences have been discussed of the effect of deuteration of m i n e
and aqua llgands in
a-and
trans-[Rh(NH,)4(H2O)*I3+
and in cia- and trdnS-[Rh(NH3)4(H20)Cl]2+,
on the cis to trans,
and trans to cfs photoisomerisation processes - 133 Ligand P ield
(X excitation of cia- and tran~-[Rh(en)~X~]+
=
C1, Br) in acidic
aqueous solution gives [Rh(en)2(H20)XI2+ and excited-state halide dissociation and nonradiative deactivation rate constants have been described.134 The same group has also determined the triplet excited
state
NH3
and
C1
dissociative
rate
nonrad iat ive deactivation rate constants for
constants
and
IIIl: The Photochemistry of Transition-metal Complexes Direct I R evidence has appeared for the
[Rh(en)z(NH3)C1]2t photoejection
of
81
vinyl
chloride
from
Rh(acac)L2
(L
=
CH2:CHC1) .136 Irradiation of [ (trlphos)Rh(S2CO)IBPh4 (triphos
-
MeC( CH2PPh2)3 ) in CH2C12 promotes chelotropic elimination of both CO and COS to give [ (tripho~)Rh(~-S)~Rh(triphoa) 1 (BPh4)2.CH2C12
This ~~ and [(triph~s)Rh(p-S~)~Rh(triphos)J (BPh4)2, ( C H ~ C 1 2 ) 1 . 7 5 . ~ process, which can be extended to include diselenocarbonates, provides a new method of diselenium Uganda
introducing sulphido, disulphur and
-
into complex frameworks
Laser photolys is
studies of C1Rh( I I1)TPP (TPP = tetraphenylporphyrin) in ethanol have revealed both electron-transfer and axial ligand ejection processes,138 and a highly regtospecif ic 1,4-photoreduction of NAD(P)+ models using a R h ( I I 1 )
parphyrin axially substituted by
acetyl has appeared.13'
11. Nicke l The 1: 1 adduct of bis( 1,2-diphenyl-l,2-ethylenedithio1ato)nickel(0) (3) and quadr icyclane undergoes photodissociation to (3)
and
norbornadiene.140
[NiS4C4RR' It(
is
formed
on
photolysis of the dithiine complex (4) in poly(N-vinylcarbazole) and poly(acrylonitrile-styrene) matrices containing CHI3
The
process is believed to involve charge tranafer complex formation between the matrix and photooxidation of
(4).
CHI3 which then participates in the Resonance Raman spectra obtained from istence of
transient nickel porphyrin species demonstrate the
c
photoinduced
proto-
ligation
changes
in
nickel
3
and
~~ octaethylporphyrin species on a subnanosecond t i ~ n e s c a l e land carbon dioxide has been photoreduced to carbon monoxide in water
using
1,4,8,11-tetraazacyclotetradecanenickel(II)
catalyst .I43
chloride
as
Photochemistry
82
12. An
&-bis[
Plat inom
investigation of the photochemistry of orthometalated 2-(2-thienyl)pyridfneJplatinum(I I )
has
concluded
its reactions in CH2C12 involve Pt(thyp~)~Cland
CH2Cl
that
radicals
generated via a CTTS excited state, which itself is populated from the intraligand and MLCT states.144 The photophyaics and photochemistry
of
[ Pt2 (P205H2)4 ] 4-
Photoreduction
of
nitric
acid
have to
been
revi w e d . 14’
nitrite
inn
by
the
electronically excited platinum complex [Pt2( P ~ O S H ~ ) ~occurs ]~-
v i a the intermediates [Pt211,1J (P205H2)4]3- and HN03-.146
The
da*po triplet excited state o f this same complex is reported to
abstract a H atom from R3EH (E g ive [ Pt2 ( P2O5H2)4H2 J
Photochemical
-
Sn, Ge, Si; R
alkyl, Ph) to
=
147 ‘L48and PtzH2 as ultimate product. 14’
[Pt2(p-P205H2)4C12 J 4-
of
conversions
[Pt2(fi-P20sH2)4I2I4-
=
with
added
halide
ions
have
and been
describedlSO and the same authors have also shown that alkenes and alkynes will quench the triplet state of this complex by energy
transfer,
mechanisms.15‘ [Pt(CN),J2-
hydrogen
T12Pt( CN)4, containing
prepared. 152 The
abstraction,
and
a non-columnar luminescent form of platinum-thallium
bonds,
luminescent character istice of
dihalotetraamine
and
diradical
dihalod iethylenedlamine
has
been
a series of complexes
of
Pt(1V) have been reported, and constitute suitable models for studying the structure and properties o€ the unstable Pt(I1I) complexes having the same ligand environment.153 [Pt(CN)4X2J2- (X process, with complexes
=
C1, Rr) promotes photoaquation as the sole
formation of
display
Irradiakion of
different
[Pt(CN)4X0Hl2-; kinetics .154
however,
the two
Photosubstitution
processes a€ [PtC16J2- in MeCN involve homolysis of a Pt-C1 bond,
83
I I I l : The Photochemistry of Transition-metal Complexes and solvent effects appear to be determined by the efficiency of the photoinduced electron transfer from Ligand to metal, or medium to metal to give [PtC15]2--155,156 The same workers have also
investigated the mechanism of
[PtBKgJ2-
the photosubstitution of
in CH3CN. 157 Photochemically induced oxidation of
hexane with [ptC16I2- leads to the formation of B u C H - C H ~ P ~ , a compound also formed Prom hex-.2-ene and NaZPtClg in acetone-158 13.
C o m e r and Go1d
Picosecond flash photolysis studies of [Cu(dmp),]'
2,9-dirnethyl-l,lO-phenanthroline)
[ Cu (bcp)2 1'
and
2,9-dimethyl-4,7-diphenyl-l,lO-phenanthroline) excited
state absorption
spectra
respective ligand radical anions
to
(drnp
=
shown
the
those of
the
have
resemble
(bcp
=
Quenching kinetics have been
measured and interpreted in terms of a mectiwiisrn irivolving an intermediate *Cu....Q.
The
same
authors
also
report
that
photobleaching of [Cu(dmp);! ]+ in CHZC12 is a biphotonic procesa in which the second photon is absorbed by a relatively long-lived CTML
excited state. 160 Outer-sphere electron transfer between
CH2C12 and a high-lying excited state of the complex gives The temperature profile of t h e
[Cu(dmp)2J2' and C1- as products.
emission intensity is shown to depend strongly on the nature of the
phenanthroline
ePf iciency of
[Cu(dmp)P2]+
phenanthroline; P MV2'
ligand
-
as
well
(dmp
=
as
the
solvent.161
The
2,9-dimethyl-l,10-
tertiary phosphine) in the photoreduction of
has been improved by substituting PPh3 with PCyPh2 or
P ( E - C ~ H ~ O M ~ )and ~ ' ~advantage ~ has been taken of the special topography of [C~(dap)~]+[dap
=
2,9-bis(p-anieyl)-l,lO-
phenanthroline] in order to use it as an efficient photocatalyst for the reductive coupling of E - O ~ N C ~ H ~ C-163 H~B~
Photochemistry
84
The role played by the ground- and excited-state potential surfaces in the temperature dependence of the electronic spectrum of the planar [CuC14J2- ion has been investigated.164 A
study of the photoredox reactions of some gold pOKphyKins
has shown that chlorogold meso-tetrakis(4-N-methylpyridyl)porphine
tetrachloride
Is
durable
a
catalyst
for
eJectron
transfer processes involving water -1'5 14. Lan thana d -
The luminescence of divalent europium crown complexes in var ious solvents has been studied at various temperatures and the results
correlated
with
structural
differences
of
the
complexes. 166 A
number
of
papers describing
luminescence studies of
lanthanide complexes in the solid state have appeared. include emission from Ln(biq02)(N03)3'nH20 Sm; biqO2
=
(Ln -- I d , Ce, Pr, Nd,
2,2'-biquinoline N,N'-dioxide) which arises from the
transition *biqO2 + biq02,167 from Ce(II1) in LnOX (Ln
Y;
X
=
These
C1,
BK,
I),168
or
and
from the
Eu { p-XC6H4COCHCOBu t,
europi um ( I I I )
pipH[EU{E-XC6H,COCHCoBUt)ql
piper idine)
The
(X *
=
La, Cd or
p-diketonates of
and
CH3, CH30, H, F, C1, N02; pip
fluorescent
properties
of
=
PVC-Ce( [ I I),
P M - C e ( I I I ) and polystyrene-Ce( I I I) f ilma have been reported as well as those of analogous films containing 1 0 - c r 0 w n - 6 . ~ ~The ~ examination of
the excited state properties of
the lamellar
solids Ln0.33U02P04 (Ln = La, Ce, Pr, Nd, Tb, D y , and Yb) derived from
uranyl
phosphate
and
lanthanide
ions
shows
photoluminescence decay curves are exponential.
that
the
Lifetimes,
radiative quantum yields, unlmolecular radiative and nonradiative rate
constants
are
all
reported.
Luminescence
spectra
of
IIII: The Photochemistry of Transition-metal Complexes Tb(RLSA)3 and
in
(TMSA =
various
85
bi8(tKimethylsilyl)aIIIido) in the solid state
solvents,
have
been
used
to
analyse
the
coordinative behaviour OF the s01vent.s'~~ and the hydration of [Tb(PPP) 2 J
(PPP = bistr iphosphate) has been determined from
measurements of the luminescence lifetimes in H20 and D ~ 0 . l ~ ~ Studiea of
the luminescent properties of single crystals of
EU(NO~)~-~ down H ~ Oto liquid He temperatures suggest that water molecules are responsible of the emission.174
between Eu ( No3)
€OK
a considerable amount of quenching
The metal
ion sites of the 3:2
and the A- isomer
excitation and emission spectra.17' a88 ignment
dicyclohexyl-18-cro-6
of
(DC18C6), [Eu(N03)2(DC18C6)]2[Eu(N03)5]
complex
have been probed using
This ha8 made possible the
of different conformations of the complex cation.
Fluorescence measurements of Ln( I [I)
( ~ n EU, Gd, Tb, ~ y )
ions complexed with aminopolyacet ic ac ids have been obtained. These have enabled a simple theoretical model to be proposed from which certain generalisations concerning the fluorescence of the lanthanides have
der ived. 176 ,
been
luminescence of Eu(tI1)
and T b ( I I 1 )
Sene it izat ion
of
the
ions by orotic acids is
reported to involve excitation energy transfer Prom the acid. 178 The luminescence of Eu(N03)3*6H2O in n-propanol solution show8 a double exponential decay at temperatures below 240 K; the long wavelength
component
corresponds
to
emission
from
Eu( I I I)
containing one additional NOg- ligand in its first coordination sphere
compared
to
the
short
wavelength
component.17'
The
seneitized photoluminescence of Eu(II1) in acidified aolutions of sodium molybdate and anvnonium paramolybdate has been ascribed to energy
transfer
to
heteropolycomplex.180
Eu( I I I )
inside
Crystalline
the
europium-molybdate
europium
polymolybdate
Photochemistry
86
from
obtained
thia
has
~iol.at-i.on
( N H ~ ) ~ ~ R U ~ M O ~ ~ )and ~ Q shows O - ~ H a~ ~br iqht
Fluorescence
character i s t i c s
and
energy
the
cornpa i t. i.on
red
l.iimi.ne8cence.
t-canafer
wi.t-hin
a
Ce ( J J I ) -2,2 ’-bi pyr id i ne compl e x have heen descr i bed Tmninescence s t u d i e s of 7.,7.*-h.ipyridine-haaed crypt.at.es of Eli( J I J )
and Th( I I 3 ) show t.hat compared w.i th t.he corresponding
aqua comp.1a x , t h e w a v e . 1 angkh of khe l timinascence i~l una.ffect.ed by
tho cryptsate or watmr
.l.igands, b a t
the
that
. l . i f e t . i m e of
t.he
exc.i t . 4 skate and the .laminascence q n a n t u m yield a.re strong.ly aItered.l*’
The same workers have measured the ra.t.ea o f el.sct.ron
and energy t r a n s f e r r aact.i.ona bat-ween some cyani.de comp1.exe.s and
*
E u C 2.2.1 J
’+,
*Ruaq3+,
* 1TbC 2.3.- 1.3 3 t
Rnerqy
*Thaq3+.
and
t r a n s f e r quenching occurs much f a s t e r for t h e free i.ons khan for the cryptates,
while t h e r a t e const-ants for sl.ect.r-on t-canafar
qiienching are practi.cal.ly t h e
8-8
for c r y p t a t e and aqiio iona.
Some a,a’ -bipyridine and l., 1.0-phenanthrol.ina-cont.aining cryptate8 and Tb( I J I ) are repo.rtx4 t.o display energy transfer
of Eli( J J J )
luminescence of examinat.ion
of
viaihle t.he
.1 ight. iipon
IN
photophysical
. i r r a d . i a t . . i ~ n .and ’ ~ ~ an of
prope.rt..iea
the
hexaaz(UMcrcmyc.1ic complex [ R U ( C ~ ~ H (CH3COO) ~ ~ N ~ )1 (CH3croO)C.l ‘2H20
shown t.hat. , l u m i n e s c e n c e .in ef.Picient .for f-f
excitation and th at
energy transf t w from l .igand to mata.1 atom i a nsg.1i g.i bl e
Ce(1V)
and
Tb(JV)
p,recipit.at.ea
are
formed
-
’*’
in
th e
photaassist.ed oxidation of tho systems C a ( C . l 0 4 ) 3 - N a 4 P 2 0 7 -
(NH41~ S Z O ~ - H C3 04 !
and
Th(C.104) 3-NaqP3.07- (NH4) 2S208-HC.104
resprtcti.vs~y.18~’.’87 These procefjaes may fi.nd some a.ppl.i.cati.on i n t h e separation o f the c a r e earth el.ement.8. 1.5. A
Uranium
system c o n s i a t i n g of
aqueous sol.utiontl o f
riranirim and
I I I l : The Photochemistry of Transition-metal Complexes
87
copper salts with an organic electron donor such as propan-2-01
or cyclohexanol has been shown to be capable of the photolytic product ion of hydrogen. 188 Luminescence spectra and lifetimes have been descr ibed for U02(N03)2-XH20 (x
=
6,3,2)lg8 and earlier work of Marcantonatos
reporting emission from excited states of U022+ above that giving the
usual
green
investigation of
emission,
has
been
challenged.lgl An
the photophysical behaviour
of
some mixed
transition metal-uranyl complexes with polyketonate ligands has shown the uranyl chromophore to be vibronically isolated. lg2 Tertiary aromatic phosphines and arsines will photoreduce U022+ in a process whose primary step is electron trans€erlq3 and the the U(V)-U(V1)
kinetics of
self-exchange reactjons have been
investigated.lg4 U(IV) is formed on irradiation of a mixture of
and N2H4 at ~
UOz2+, HNO3,
o and ~ the mechanism c ~ of the ~ rapid ~
photostimulated exchange of 0 between U022+ and H20 has been
- ~ aH ~poly(viny1 o Irradiation of H ~ u o ~ ( I o ~ ) ~ in
discusaed.l96
alcohol) matrix leads to interactions of the excftd 2)022tand H+ ions
with
of
formation
l2
and
the
13-,lg7
steady-state
luminescence spectrum of solutions of U022+ containing F- has and U02F+,
been separated into contribution8 from [U022+],q
and IR multiphoton decomposition of solid etate U02(HCOO)2-H20 gives u308 v i a U02(0H) (HCOO) 'H2O as intermediate.199 The colour of the emission of U ( V 1 ) luminescence in oxides is
reported
to
shift
gradually
from
green
to
red
if
the
coordination of the uranium changes from 6 to 4,200 a Liebig ti tration involving the interlamellar ?ig+ ions of A ~ U O ~ P O Q - ~ H ~ O and
based
on
HU02PI4-4H20
photoluminescence has and
HUOflsO4-4H20
been form
descr ibed,201 solid
and
solutions
Photochemistry
88
representing
HU02( PO4) 1 - x ( A ~ 0 4 ) x ,
Emily
a
of
luminescent solids that can serve a8 hosts for chemistry.202
The
spectral distribution of
lamellar
intercalation
photodissociation
yield has been measured following [IF6 excitation in the B band and the dissociation found not to be uniform.203
16.
Actinides
2-Thenoyltr ifluoroacetone Pu(II1)
on
will
exposure to normal
Pu ( IV)
photoreduce
light204 and
jn nitric
to acid
solution, irradiation of compounds of Pu(III), Fe(III), and N2H5+ leads to their complete and rapid oxidation.2Q5 17. Miscellaneous An
ESR and ENDOR study of the photoinduced electron transfer
ins from magnesium and zinc tetrakis (4-sulphonatopheny1)porphy~ to K3Fe(CN)6 molecular
in
a
H20-Me2S0
glass
ha8
electron
transport
chain
composed
tetrakis(N-methyl-4-pyridyl)porphyr in
appeared.206
and
of
EDTA,
new
A
z Fnc
methylviologen
spatially organised by 1F-diameter zeolite 1, particles207 has been descr ibed and the mixed semiconductor catalyst (ZnS-CdS) supported on alumina has been used to photogenerate H from H20-208 Hg ( ) I I ) ions have been photocatalyt ically eliminated from aqueous solutions with ZnO a8 catalyst209 and the same authors have also photoreduced Hg( I I ) ions using aqueous suapensions of
Ti02 and WO3 .210
1.
J. F. Endicott, T. Ramasami, R. Tamilarasan, R. B. Lessard, C. K. Ryu, and G. R. Brubaker, Coord, Chern. R e v . , 1987, 1.
2,
R. A. Krause, Struct. Bonding (Berlin).
1987, 67, 1.
77,
Illl: The Photochemistry of Transition-metal Complexes 3.
H.
89
Hennig, R. Billing, and D- Rehorek, J. Inf.
1987,
Rec.
Meter.,
15, 423.
m, 139.
4.
M. J. M i r b a c h , ACS Symp. S e r . , 1987,
5,
R. V a n E l d i c k , NATO AS1
6.
P. N a t a r a j a n and P. Ramamurthy, Proc. lndlan Narl. Scl. Acad., Part A. 1986,
7.
N.
197, 357.
52, 865.
Shinohara,
Keiretsu. 1987,
8.
S e r . , Ser. C , 1987,
YOkOh8m8-shkltSU
Diageku
Ronso,
Shloen
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IIII: The Photochemistry of Transition-metal Complexes
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LIZ,
2
The Photochemistry of Transition-metal Oreanometallic Compounds
-
BY A. COX
Reviews have appeared of the photophysics of molybdenum complexes,
primary and secondary processes in organometallic
f laah phOtOly8i8 of Pe(C0)S and Cr(CO)gr3 dinuclear che~nistty,~ manganese
carbonyl
complexes
isolated
low
in
diene
complexes,6 p coordinatively
i c idium,
compounds,
redox
-
compounds lo Synthetic
photochemistry of
temperature
complexes,
unaaturated
and
the
species
metal
cluster
phatoproduction containjng
chemiluminescence and
matrices,
metal
organic
of
rhodium
of or
organometaL1i.c
photochemistry
in
industry has also been reviewed.l1 2. Ti t a n i urn Carbon dioxide has been photochemically inserted into the Ti-Me bond of (q5-C5H5)TiMeZ under photocleavage conditions to give (q5-CSH5)2Ti(OAc)Me,l”
and use has been made of the new
titanium complex [(C5Me4H)2TiMe~] €or the initiation of some ole€ in isomer isations. l3 Irradiation of the tltanacyclobutane (1)
brings
about
a
clean reductive elimination process to form
cyclopropanes and t itanocene.
Stereochemical studies suggest
that the pr imary photochemical step involves metal-carbon bond
homolya is
to
produce
a
metal-centred
1,4-biradical
intermediate.14 The primary process in the photolysis of the titanocene dichloride conrplexes (RCsH4)2TiC12 ( R pentyl)
iS
-
Me,
Et, Pr,
h o ~ l y a i sto give RC6H4’ and (Rc6Hg)TiCl~;~’ atlalC#gOUS
103
104
Photochemistry
4i
I ; R' = Me$ R = R'= Me
(1) R = H
Me\
Me2CH
(21
lIl2: The Photochemistry of Transition-metal Organometallic Compounds
105
reactions occur with zirconocene and haf nocene. l6 3.
Niobium
Nb clusters will photoreact with benzene, and at low laser intensi t b 8
the
probability
dehydrogenation
Of
NbXC6H6
Of
clusters has maxima for x = 5, 6, and ll.17 4. Chromium, Molybdenum. and Tunasten An ESR study of the radicals produced in the photoinduced
reactions of M(CO)6 (M of
-
CK, Mo, W) in CCl4 has shown the presence
together
eCl3
with
dlalkyl-
diphenyl-
and
acrylphenyl-nitroxides. l8 The average number of CO molecules produced
from
M(CO)6
using
infrared
SP~-sensltlzation is reported to be Cr(C0)6,
MO(Co)gr
and
3.0,
radiation
and
and
€or
3.0,
w(c0)6
1.5
PhotolySis
Of
Cr(C0)6 in ethene-saturated pentane solutions leads to sequential formation of (q2-ethene)Cr(CO)s at -2OOC and trang(q2-ethene)2Cr (CO)4
at -5OOC. 2o Analogous processes occur with
Mo(CO)6 and w(c0)6.
Irradiation of a solution of [b¶(CO)6] (M
-
CK, Mo, or W) in the presence of a pyridtne 2-carbaldehyde imine ligand 2-CsHqNCHNR
(L; R
-
transient complex [M(C0)5L]
Thermal ring
Ph, tert-Bu, n-Bu,
Pr) gives the
in which L is monocoordinated.21
closure to give
[M(CO)qL]
has been
monitored
kinetically. The same group has also reported the photoproperties and redox behaviour of a series of homonuclear and heteronuclear ligand-bridged (OC)~M-L-M'(CO)S complexea, (M and M' = Cr, Mo, or W;
L
=
pyrazine,
4,4'-bipyridlne,
I,2-bis(4-pyridyl)ethylene,
and
w-
1,2-bia(4-pyridyl)ethanne) .22
TKeahent of (2) with L i A l H 4 and UV light la reported to give (3),23
[ 4+6J -photocycloaddition
RCH:CR'CR':CHR2
(R, R2 = H, Me; R'
of conjugated dienes such as
-
H) to hexacatbonyl-
Photochemistry
106
6-1,1' -bi(cyclohepta-2,4,6-tr ien-1-yl)dichromium(O)
p-q6*
descr ibed ,24
been
photocycloadd it ion
and
1,1,2,2-tetrafluoro-t,2-disllacyclobutene R'
H,
=
Me; R
transition
=
metal
H, R'
=
pathway
migrations.25
M(CO)5L
and
involving and
between
and CH2:CRCR':CHz
(R,-
Me) can be mediated by Group VIR
carbonyls
I,l-addition
react ions
has
a
proceed
hydride
both
M(C0),L2
(M
Cr,
=
an
unusual
and
fluoride
CIO,
W;
L
=
l-phenyl-3,4-dimethylphosphole, 1-phenyl-3-methylphosphole and
1-phenylphosphole) have been synthesised photochemically from
M( CO)
and the
appropr iate phoaphole, and the photochemical
react.ions of these compounds wit.h alkenea have appeared.26 Ab i n i t i o
SCP
calculation8 have been used to generate state
correlation diagrams f o r
the photosubstitution reactions of
M(C0)sL (M = Mo, L = NH3, PMe3, CHOMe, C p 4 ; M
=
Tc, L.
=
Cl),27 and
mechanisms have been reported for the formation of ionic products on photolyaia OF MI(CO)~X (X = C1, Br, I), C1,
I),
and
(Q~-C~H~)F'~(CO) in~ I the
($-C~HS)MO(CO)~X ( X presence
of
=
various
liganda.28 Photolysie of (q5-C5Hs)2MoH2 in the presence of C02 produce8 (qs-C5H5)plo( q2-C02) which on continued irradiation
it3
converted to ($-CsH5)$40(qz-C03) .29 Thjs is the f i r s t solution phase example of the photochemical activation of a metal-carbon
dioxide complex.
Propene is produced by reaction of C2H4 w i t h
Mo(IV) in photoreduced Mo(VI)/Si02 catalysts, and a mechanism for Mo
carbene
formation
by
isomerisation
of
olef in-Mo( IV)
n
complexes proposed. 30 ESR data have appeared for the photolysie products o€ (q5-C5H5)Mo(CO)3Me,31 and it is also reported that (14-C5H5)(q5-C5Hg)m(OAc) (CO) and (n4-C5H5) (q5- C 5H 5 )MoMe(C0) are
formed when the substrate is
irradiated in the presence of
c y ~ l o p e n t a d l e n e . ~Photolysis ~ of
[ (q5-CsH5)Mo(dmp)H31
(dmp
=
107
III2: The Photochemistry of Transition-metal Organometallic Compounds Me2PC%%PMe2)
i n C&
containing M e 2 0 ,
AcOMe, PhMe and other
compounds causes H-D exchange between C6D6 and t h e H atoms of t h e starting
material.33
[ (q5-C5H5)Mo(CO)3COJ2CP2
undergoes
photoinduced l o s s of CO t o g i v e [ (q5-C5H5)Mo(CO)3COCP2Mo(CO)3(q5C S H ~ ) ]and ~ ~ i n r i g i d methylcyclohexane a t 93 K, i r r a d i a t i o n of give8 a mixture of t h e mote s t a b l e isomer
(q5-C5H,)Mo(CO)3Mn(CO)5
( Q ~ - C ~ H ~ ) M ~ ( C O ) ~ ( ~ - C O ) M ~ and. ( C O ) ~t h e
stable
less
isomer
possessing ~ - q ',q2-C0, wl t h l o s s of CO as t h e
(q5-C5H5)NoMn( C O ) only pho t o r eac t ion. A
of
k i n e t i c s method has been described for t h e i d e n t i f i c a t i o n
coordinat i v e l y
unsaturated
substituted
metal
carbonyl
t r a n s i e n t s and s u c c e s s f u l l y applied t o cie-[Ph3PW(C0)4] .36 The procedure a l s o enables v a l u e s of a l l r a t e constants for ligand displacement
reactions
t o be obtained.
Rates and a c t i v a t i o n
parameters have been measured for t h e ring-closure, coordination,
in
Cis-[(q'-pd~p)W(CO)~J
(pdpp
-
y&
coordinatively
olefin
unsaturated
CH2:CH(CH2)PPhZ),
produced
pulsed laser f l a s h photolysis of --(pip)(q'-pdpp)W(co)q p i p e r i d i n e ) . 3 7 A study of t h e carbonyl-loss
via
(pip
=
r e a c t i o n s of t h e
tungsten t h i o l a t e ( q5-C5Hs)W(CO) 3SR (R = Ph, _p-tolyl) show8 t h a t
i n t h e absence of o t h e r l i g a n d s ,
t h e y i e l d s of t h e products
[ (qS-C5Hs)W(CO)2SR]2 and [ (q5-C5H5)W(CO)SR]2 a r e dependent upon R
and t h e conditions. 38 D i m e r isat ion is suppressed i n t h e presence of a ligand such as Ph3P. the
photolysis
of
The same authors have a l s o discussed (q5-C5H~)W(CO)3S02R-3g
Photoinduced
carbonylation of t h e carbyne ligand in (q5-C5H5) (CO)2W!(JTol (To1 =
-p - t o l y l )
gives
(4);
recorded. 40 Bidentate
some
related
transformations
are
also
(alkenylcarbene) tungsten complexes r e a c t
photochemically with olef ins to g i v e o l e f in-carbene complexes i n
Photochemistry
108
or
which the g2-olefin and carbene liganda are trans.41
example
irradiation of (5) in the presence of C2H4 gives (6). A wide range
of
tungsten
and
molybdenum
2-oxaalkyl
(q'-enolate)
complexes are reported to undergo CO loss on irradiation leading to q3-enolates. 42
5. Mana anese and Rhenium A CAS-SCF
contracted-Ci investigation of the lowest excited
states of HMJI(CO)~and Pe(C05) has appeared.43 Laser photolysis and
electron
impact
ionisation W3
studies have
shown that
excitation of the a* state of Mn2(CO)1~ induces both metal-metal bond homolysis and ligand l o e 1 3 . ~However, ~ excitation of the n* state leads to ligand loss only.
Ionic cluster fragments derived
from Mn2(C0)1~ and carbonyls of Fe and Co have been studied by laser ion beam photodissociation, and relative photodissociation cross-sections for each ion and upper limits to bond dissociation energies for M-M and M-CO
Thiolate metal carhonyl
complexes have been prepared by irradiating M2(C0)10 (M = Mn, Re)
or R ~ ~ ( c o ) ~ ( P with R ~ ) ~RSSR (R
=
Me, Ph) or MeSSiMe3.46 Laser
flash photolysis kinetics studies reveal the electronic and steric effects of L ligands in the Re(CO)&
radical and the
steric effect of the R group in RSSR on the group transfer rate constantgives
a
Photoreaction of Mn2(C0)10 mixture
of
three
with 1,l-dimethylallene
mononuclear
and
two
dinuclear
complexesQ7 and the same authors have shown that low-temperature photol~ais of Rez(CO)lo
in the presence of C2H4, styrene or
isoprene give8 Re2(CO)8H(p-q2:'-R)
(R = CZHQ, C4H7, E- and
Z-CHICHPh, CsH7) and Re3(C0)13(p-q2:l-R) .48 eq-Re2(CO)g(N2) ha8 been obtained from photolysis of Re2(CO)10 in liquid Xe doped with N2 and also from narrow band photolyeis at 313 of Re2(CO)10
IIi2: The Photochemistry of Transition-metal Organometallic Compounds
(71
I
Mn(C013
(91
(10)
109
110
Photochemistry
isolated in a N2 matrix at 20 K.49 9,lO-phenanthraquinone to
Photochemical addition of
Re2(CO)Z0
gives
Re2(C0)7L2
(HL
=
9,lO-phenanthrasemiquinone) in which Re(C0)4L and Re(C0)3L units are linked by a bridge formed by one semiquinone 0 of the chelated ligand
of
the Re(C0)4L
coordination
site
of
unit; this 0 bridges the
Re(C0)3L
unit.50
to the vacant Photolysis
of
cymantrene, Ph2PC5H4Mn(C0)3, in the presence or absence of Ph3P gives Ph2PC5H$dn(CO)2(PPh3) and a mixture of (7) and (8),51 and in the presence of n-donor ligands, photolysia of the fulvalene bridged metal complex (9) gives (10) (L
ethylene, cyclooctene,
=
.
but-2-yne) 52 HOWBVBK, u-donor ligands yield the disubstituted complexes (11) (L = THF, PPh3,. Ph2PCH2PPh2, py). ( q5 -CsHs)H
e ( CO)3
Irradiation of
and Me2C=CCLS iMeg g ives the v inylidene complex
(q5-C5Hs)MnMe(CO)2( :C:CMe2)
as the major product together with
the butat 1: iene complex ( q5 -C5H5) W
e ( CO ) ( q2-Me2C :C :C :CMe2 ) .
Room temperature ligand field luminescence has been observed from IReCl(C0)4L]
(L
pip, PPh3), I(q5-C5H5)Re(CO)2(NH3) J and
=
[ (~~-C,H,)Re(Co)~(pip)1 , 54
studies of ReBr (C0)3L [L
=
and
Ph,
H,
Me)]}
[ fE-Re( bpy) (CO)3Br 3
and
electrochemical
2-acetylpyridine, 2-pyr idylcarbonyl-
methylides [C~HQNC(O)CHZ, 2 CN,
emission
-
PPh3, AaPhg, SMe2, NC5H4R-4 (R =
have and
appeared.55
tr iethylamine
in
Irradiation
DMP
leade
of to
photobleaching of the complex to give the 5-ethyl-2,2'-bipyridine complex;56
the
same
photosens it ize the
group
has
also
isomer ieat ion of
used the
this
complex
to
tetrahydrodimera of
1-benzylnicotinamide. 57 ereparat ion of ( q5-C5Me5)ReO12 has been in the presence of achieved by photolysis of (Q'-C~M~~)R~(CO)~ iodosobenzene,58 photolysis of the rhenium( I ) 'enolate complex ~ A G - ( C O ) ~ ( P M ~ ~ ) ~ R ~ ( O C in M ~CHZC12 C ~ H ~ )solution leads to cleavage
IIl2: The Photochemistry of Transition-metal Organometallic Compounds
111 and in
of the Re-0 bond and formation of fac-(C0)3(PMe3)2ReC1,59 the
presence
of
propene,
(PPh3)2(q-CH2:CHMeCH:CHNe)ReH3
of
photolysis give8
the
or
(PPh3)2ReH7 bisallyl
hydride
(PPh3)2(q-C3H5)2ReH-60 (q4-C5H5)Re(PPhg)2H3 has been prepared by photolysis of R ~ ( P P ~ s ) in ~ Hthe ~ presence of cyclopentadiene.61 At
long
irradiation
times
(q5-C5H5)Re(PPh3)H4
is
formed.
Photolysia of Re02(CH2SiMe3)3 in hydrocarbon solvents gives the d'-d'
dimer
Re204(CH2SiMe3)462 forms
Re2(p-u:q3-CHCHCMe2) (CO)9
and
the
of
irradiation
p-butadienyl
complex
J. 63
[ Re2 (1-H) ( p-u: q2-CH-CHC( Me)=CH2 (CO)
a An ASED-MO study has shown that the oxidative addition of CH
bonds of methane and ethylene to ground state 3d64s2 F e atoms is hindered by a closed shell repulsion between the CH u bond pair and the 48 electrons of the Fe atoms.64 However, excitation of the Pe atom to the 3d64s14p1 configuration greatly reduces this barrier.
Laser-generated
La'
reacts
with
Pe(CO)5
to
LaFe(CO)3+ which after colliaional activation gives We',
give and
which itself reacts with a wide variety of linear, branched and cyclic alkanes in proceasea which differ from those of either La' or Pe+.65 An iron catalysed insertion of isonitrile into the C-H bond of arenes to produce aldimines has been described.66 Thus irradiation of Fe(PMe3)2(CNR)3
(R = Me, me3, CH2CMe3, Ph, or
2,6-xylyl) in benzene solution gives the corresponding aldimlne PhCH:NR.
~ gradual The related complex P R ( C N M ~ ) ~ ( C N )undergoes
ligand-solvent photosubstitution on irradiation in MeCN to give f iret
Fe(Me)3(NCMe) (CN)2
Pe(CNMe)2(NCMe)2(CN)2.67 matrix
isolated Pe(CO)4
and
subsequently
The IR laser-induced photochemistry of
ha8
been
discussed
in
terms
of
a
Photochemistry
112
distorted octahedron and an analogy drawn between the reactions of Pe(C0)4 and Hb,'* species
Pe(CO),
and Mkissbauer parameters of the unstable
-
(n
2-4),
generated
by
photolysie
of
matrix-isolated Fe(C0) 5, have been measured.69 Investigations of the photochemistry of Pe(C0)s deposited on to single crystals of A1203, Si(lOO), and Ag(ll0) s ~ r f a c e s , ~ and ~ ,a~Si(ll1)-(7 ~ x 7) surface72
have
been
descr ibed,
and
irradiation of
Pe(CO)
physisorbed on to porous Vycor glass 3s reported to lead to efficient formation of Fe(C0)4, a species which rapidly reacts with
the
surface
to
form
H-Fe(C0)4-OSi
and
H-Pe(M)g-OH.73
Continued photolysis promotes further decarbonylation and may lead to atomic iron or elemental iron aggregates. [M(CO),_l(~-C3H5)1+BPqformed when M(CO),
is
(M
=
Pe, n = 5 ; M = MO, W, n
irradiated with ally1 alcohol
= 6)
is
in the
presence of HBF4-OEt2; other similar reactions are reported.74 Irradiation of a mixture of ethylene and hydrogen containing a catalytic amount of Pe(C0)S is an efficient system for ethylene hyd~ogenation.~' A reservoir of Pe(CO),(C2H4)2
appears to be
formed which thermally dissociates to yield the active catalyst Fe(C0)3(C2H4).
Photolysis of trans, trans, cia-cycldodeca-1,5,9
-triene with Fe(CO)S
in benzene gives complex (12)76 and the
cycloaddition reactions of 1,1,2,2-tetrafluoro-l,P-disilacyclobutene with buta-1,3-diene have been studied in the presence of Fe(CO)5, and the effects of substituents i n ~ e s t i g a t e d . ~The ~,~~ same authors have also examined the metal carbonyl mediated isomerisation of 1,4-diailacyclohexa-2,5-dienes.79 Pe(CO)2(q2-S2CNMe2)2
and Fe(q2-S2CNMe2)2 have been prepared by
visible light photolysis of solutions containing Pe2 (CO)9 or Fe3(C0)12
and [ (v5-CgH5)(CO)3W(q1-SCSNMe2) J .80 Photolysis of a
113
lIi2: The Photochemistry of Transition-metal Organometallic Compounds
auapenston of PeZ.(CO)g and ( ~ J ~ - C ~ M ~ ~ ) ~ MinO ~ toluene ( C O ) gives ~ (q5-C5Mes)2Mo2Fe2(CO)9 (p2-C0)(qz, p 4 - C 0 ) ,
a
new
62-electron
butterfly Mo-Fe cluster .81
Low
temperature photolysis
(+-Ole€ in)Pe(C0)4
var ious
of
complexes of @-unsaturated
that loss of CO ia the predominant
matr ix
isolated
esters ha8 shown Three products are
observed, none of which is the expected (q2-olePin)Pe(CO)3, and haptotropic rearrangements involving these three occur under Dissociative loas of CO is the primary
selective irradiation. photochemical
event
following
species.83
(q4-cyclopentadiene)Fe(C0)3
transfer
of
5-&o
the
q5-cyclopentadienyl the
complexes of phosphite; L
=
group
complexes. type
This
or
(H
CO or MeCN)84
of
is
(P
various
followed
Me)
Photochemical
[(q5-CgHs)PeP2L]+
=
to
by form
studies
of
phosphine
or
and of the acyl-iron complexes
(R = H,
CF3) to
[ q5-C5H5 (CO)Fe[p-C( CF3) :C(R) (SMe))2Fe( CO)-$-C5Hs]
have
(cyclic P e - S
[q5-C5H5(CO)PeC(0)C(CP3)C(R)SMe]
give
excitation
appeared.8 5 The same authors also descr ibe the photochemistry of (qS-indenyl)2Pe2(CO)4 in the presence of 2e-donor ligands such as CO, PPh3, and PPh2H and report the reversible formation of the Photolyats of PpSiMepiMeR2 [ P p =
radical8 ( indenyl)Fe(C0)2L.86 ( q5-C5H5)Pe ( C O ) ;
R
=
Me, Et 3 g ives the monos ilyl-Pp der ivat ives ,
F pS iMe2Et ,
FpS iMeEt2,
P pS iMe3,
(~~-CgH5)2Pe,
and
( ~ J ~ - C ~ H ~ ) ~ P B ~The ( C products O ) ~ . ~ ~are explained by postulating the formation of silyl iron complexes as transient intermediates. Irradiation of a mixture of [(C0)3FeS]2 and ethylene leads to insertion of the alkene into the S-S bond; [(CO)3Pe]p2CO analogously reactions
produced of
if
ketenimines
CO
is
with
used -88
iron
Some
is
photochemical
carbonyls
have
been
Photochemistry
114
co
(13) R = C H M q
H2C=
CH/J
(15) R =TS-C5H5
@
Me I-
Fe
I181
(14) R =y5-C5H5
1112: The Photochemistry of Transition-metal Organometallic Compounds
qPh Me
C-t
Me
cKTcH2R
115
116
Photochemistry
descr ibed8’ and a mechanism
€OK
the photosubstitution reactions is assumed
of (13) has been proposed in which PhCH:CHCH:NRFe(C0)3 to be the initial primary photoproduct in The
alkylcarbene
photolysis of (15)-
complex
(14)
has
been
produced
by
The reactive intermediate Pe(DMPE)2 (DMPE
=
Me2PCH2CH2PMe2) generated by photolysis of FeH2(DMPE)2 has been shown to be sufficiently reactive to add intermolecularly to the
H bonds in pentane at -9OOC to form a-(l-pentyl)FeH(DHPE)2g2 A t -3OOC
the
analogous
reaction
Z-&-(pent-l-enyl)FeH(DMPE)2.
to
leads
E-
and
The ring-expanded complexes (16)
are formed on photolysis of the (aminocyclobutyl)iron complexes (17) (R
=
Me, CHMe2).93
The quantum yield of formation of the T1 state of ferrocene has been determined by the sensitized isomeriaation of D-glucose phenylosazone,94
and
2,2,2-tr ichloroethanol,
the
same
ferrocene
authors is
report
photoxidised
that
to
in the
€err icenium cation. 95 The heterocyclic derivatives of f errocene (18) and (19) undergo photolysia to (20) and (21).96
7. Ruthenium
Complexes of the type M(CO)4(~2-CF3CsCCp,)
(M
-
Ru, 0s)
have been prepared by photosubst itut ion of M( CO) 5 with the al-e and react with M(C0)s to give dimetallacyclobutenes, which in solution have P luxional atructurss .” M3(C0)12 (M
The photocherniatry of
Fe, Ru) in low temperature fluid solution and in
rigid organic solvents has been studied, revealing a wavelength, medium,
and
temperature
dependent
compet it Ion
between
The thermal and dissociative loss of CO and Pragrnentati~n.~~ photochemical behaviour of [Ru,(CO),z(PhC2R)] [ R U ~ ( C O ) ~ ~ ( P ~ CJ ~ Mhas ~ ) Zbeen describedg9 and
(R = Me, H) and irradiation of
IIf2: The Photochemistry of Transition-metal Organometallic Compounds ( q5 - C 5 m 5 )
H/D
P ( C H b 2 ) 3,PPhq ) in CgDg found to induce
RUH3 ( PR3 ) (R
exchange among solvent, hydr ide ligands and
phoraphine.lOO Re(C0)S; S i n
The =
117
complexes
m i n
[U
-
coordinated
(q5-C5HS)Ru(C0),,
MegSi, MegSi2, Me7Si3, CH2SiMe3, CH2SizMe~l have
been reported, and in contraet to the related (q5-C,H,)Fe(CO),, are
photolabile with
not
respect
to
deoligomerisation to
monosilane metal d e r i ~ a t i v e 8 . l ~ ~
LAalim Mult isubst itut ion of 0s (CO)5 by ethylene has been achieved
and has allowed isolation and full characterisation of moat members of the aeries O S ( C ~ ) ~ - ~ ( C (~xH =~ )1-4) ~ .lo2 Photolysis
of the carbene-containing cluster complex O S ~ ( C O ) ~ [ C H N M ~ ~ ] (p-me) (p-H)
give8
083(CO)~(p-Ct~Ne2) (p-SMe)(p-H)2,
a
compound
possessing a bridging (dimethy1amino)carbyne ligand-lo3
Halobenzoic acids and halobenzenes are photocarbonylated to benzenedicarboxylic acids and benzoic acids in the presence of some cobalt salts.lo4 This new procedure avoids the use of cobalt
carbonyl
and
complexes
Ph2C=C=CRC02Re ( R
-
reducing
Me, R'
=
agenta,
Irradiation
of
Et) in the presence of C02(CO)g is
reported to give PhaC=CH-CH-CPhZ together with
(22).lo5 !Fwo
ieomers of the coordinatively unaaturated species CH3Co(C0)3 have been produced photochemically and their reactions with the matrix and dihydrogen studied.lo6 Some related work on HCo(C0)q has also been
carried
out.
Photolysis
of
the
hydrido
complex
CoH[PPh(OEt)2]4 in the presence of allyl benzoate or allyl phenyl ether induces allyl-0 bond cleavage, and for s o m e allylic amides double
bond
migration
stereoisomers of
OCCUKS
to
give
kinetically controlled
an
E,Z
mixture
of
composition.lo7 The
Photochemistry
118
of
mechanism
the
photoisomer isat ion
1-phenylpropene in the presence of
of
3-phenylpropene
CoH[ PPh(OEt) J
to
ha8 been
studied and shown to be a photoinduced catalytic reaction rather than a photoassisted reaction.lo8 Photolysis of the aralkylcobaloxime (23) (X
=
CH2, (CH2)2,
(CH2)3, OCHZ) in CHCIQ has been discussed109 and the same authors have investigated the photolysis of 2- [ (alkylthio)carbonyl]-2[ (arylethy1)propylJ cobaloxime.'lo
Organocobalt
complexes
[(H20)L.CoRJ2+ ( R = Me, E t , Pr, CH2C1, CH2Br, CH20Me, CHzPh, L 1,4,8,1l-tetraazacyclotetradecane) photolysis
of
the
corresponding
have
been
prepared
alkylcobaloxlmes.'11
=
by
These
complexes are themselves photosensitive, readily decomposing to Co(I1) and carbon-centred radicals in visible light.
10. Cobalt and Iridium
Photoinduced homolysis of ( 2 4 ) (R
=
Co(sa1ophen)py) in the
presence of radical trapping agents such as 0, NO, S02, Ph$32, PhzSe2, MeSO2C1, BrCC13 and iodine gives adducts (24) (R
=
OH,
NHz, SPh, SePh, C1, Br, I , SOZH) after reductive workup.'12 11. Rhodium and Iridium
In the presence of PPh3, photolys is of [ Eth(NO) (CO)(PPh3)2 J leads to 108s of NO and formation of trana-[Rh(CO)Cl(PPh,)2]. similarly
[Ir(CO)Cl(PPh3)3]BP4
[ Lr(N0) (CO)C1(PPh3)2]BF4.'13
terminal
methyl
group
is
f ormed
and
from
Regioselective carbonylation of the of
n-pentane
has
been
achieved
by
irradiation of n-pentane under an atmosphere of CO and in the presence of EthCl( CO) ( PMe3)2 ; be R h C 1 ( W e 3 ) 2.
the active species is thought to
Both benzene and cyclohexane have been s imi,larly
carbonylated.115,116 The 8ame complex ha8 also been used f o r the regioselective synthesis of terminal olef ins from alkanes. lL7 For
lil2: The Photochemistry of Transition-metal Organometallic Compounds
119
example, pentane and decane have been converted into but-l-ene and
non-l-ene
respectively
acetaldehyde-
In
propan-2-01
is
the
with
liquid
simultaneous phase,
photocatalysed
by
format ion
of
dehydrogenation
of
the
related
complex
trans-[RhCl(CO) (PPh3)2J, and [RhC1(PPh3J2 has been suggested as the
active
species.'18
Evidence has
photolysis of (q5-C5H5)Rh(C2H,)2
appeared
to
show
that
in N2-doped liquid Xe at 173 K
leads to the formation of (q5-C5Hs)Rh(C2H4)N2; in the presence of CO,
brief
photolysis
(q5-C5H5)Rh(C2H4)C0.119
of
the
dini trogen
complex
gives
The same authors have also studied the
C-H and Si-H bond activation reactions of (q5-C5Hs)Fth(C2H4)C0 in low temperature matrices.l20,121 relay
A
system,
incorporating
[(s5-C5Me5)Rh(ppy)Ll2+ (ppy
the
novel
oligopyridine ligand, n
-
complex 1,2; L
=
H20, OH, C1, I), and capable of using weakly reducing electrons of a Ti02 colloid for hydrogen evolution has been described,122
The
photocatalytic
cis-[Rh2C12 (C0)2L2 J
system
(L
=
composed
of
acetone
and is
bis(dipheny1phosphino)methane)
effective in producing hydrogen from Me2CHOH123 and an ESR study of the mechanism of formation of hydrogen during photolysis of
alkyl(disalicylideneethylenediaminato)rhodium(lII)
shows
that
.
alkyl, hydroxyalkyl radicals, and atomic hydrogen are formed 124 Sunlight
irradiation
of
solutions
of
alkenes
and
(q5-C5H5)2Rh2(CO) (CP3C2CP3) induces transfer of H from the alkene to one of the "C(CP3) carbons to give the bis-alkenyl complexes ( q5-C5H5)2Rh2(alkene-H)[C ( CP3 )-C
(CF3)H] - 125
Photolysis of [ Ira ( CO)2212- in CH2Cl2, a complex consisting
of two t r 4 tetrahedral clusters linked by a single Lr-tr bond and unsupported by bridging ligands, gives [ 1r4(Cl) (CO)ll]-.126 This
120
Photochemistry
R
R'
127) R = 1 R'= I Me
, , CH2I
(28) R = Ph,
R'=
CF3
Bu
,
SiMcg
, OMc
Ill2: The Photochemistry of Transition-metal Organometallic Compounds
121
is thought to arise via hornolysis of the inter-cluster bond. Selective catalytic dehydrogenation of alkanes to alkenes has been achieved using the neutral catalyst [ IrH2(q2-O2CCP3) (PR3)2] (R
=
4-PCgH4, cyclohexyl)
Carbon-hydrogen bonds in THP, Me2C0
(I. = P(CH2CH2PPh2)3) a
and benzene can be .activated by [LIr]'
species which can,be generated by photolytic dehydrogenation of cis-[LIrH2)(S03CP3) .128
Similarly,
photolysis
of
(25)
and
(a5-C5Me5)Ir(C0) (q2-NCC6H4C1) in hydrocarbon solvents results in activation of
the solvent C-H
bonds,129
and a study of
the
photoactivation of methane matrices by complexes of the type [ M ( Q ~ - C ~ R ~ ) ( C O(M ) ~=~ Ir or Rh, R = H of Me) has been reported.130 Photoinduced charge separation and recombination kinetics have been measured for the dimeric [ I~(p-pz')(CO)(PPh2{0(CH2)2R))) -e
l 2 [pz'
pyridine (py), 4-phenylpyridine]
these
complexes
are
well
=
Ir( I)
complexes
3,5-dimethylpyrazolyl; R The results suggest that
adapted
to
detailed
studies
of
intramolecular electron-transfer reactions. Photolyeie of matrices
of
C2H4
(v5-C5H5)Ir (q1-CH2CH2)2 or
CO
(q5-C5H5)Ir(L) (&CHCH2)
gives
the
in low temperature
vinyl
(L = H2CCH2, CO)
hydride
complexes
Photooxidative
addition of (26) with 12, MeI, and CH212 produces the dinuclear dffridium(I1) complexes (27) (R [ Ir2AuCl2(CO) (p-dpma)2 1' [ IK2AUCl,(CO)2(r-dpma)2]+
=
has (d-
iodo, R' been
-
9
fodo, Me, C H Z I ) , ~ ~ ~ photoox id ised
ABPh(CH2PPh2)2)
to and
luminescence has been observed from [Ir~T1(Co)~Cl~(p-dpa)2JN03 and [IrzPb(Co)2~12(p-dpma)Z] (N03)2.135 Photolysis of LIrMe (L = octaethylporphyr in) in C& d imer .136
gives an Ir ( I I ) octaethylporphyrin
Photochemistry
122
12. Nickel Methane is reported to react with photoexcited nickel a t o m in AK matrices to give MeNiH. 13' the reactions of
A
PTIR matrix isolation study of
atomic and diatomic nickel
in solid argon
indicates that at very low concentrations of the metal, the nickel atom forms a n-complex with the triple bond
the
of
acetylene.13* Photorearrangement of this complex gives nickel vinylidene, NiCCH2. A
theoretical study of the photolytic reaction of Ni(CO)4
using LCGTO-X,
has appeared, and the observed luminescence is
assigned to emission from the charge-transfer excited fragment Ni( C0)3.
Multiple
luminescence
has
been
observed
from
continuous wave laser irradiation of gas-phase Ni(CO)4 at room temperature.140 oscillating
Two
of
the
reaction and
emissions
all three
molecular photofragments of Ni(CO)4
are
probably
coupled
in
an
originate from
rather than from Ni(CO)4
itself Mono- or zerovalent nickel can be produced by single or successive one-electron transfer steps during the photoreduction of silica-supported nickel catalysts in a hydrogen atmosphere. 14' The fundamental step is the formation of (Ni+-O-)* excitons from
ions and oxygen bound to
pairs of tricoordinated isolated N i 2 +
the surface of the Si. trans-[Ni[CCl:CCl(CgH~Y-4)}Cl(Pne3)21 trans-[Ni[C(C6H4Y-4) :CC12]C1(PMe3)2]
(Y
=
Me,
C1)
arise
and on
~ ) ( P M ~by ~)~] photolysis of ~ ~ ~ ~ s - [ N ~ ( C C ~ : C C L ~ ) ( C ~ H ~ Y -possibly
reductive
elimination
followed
by
oxidative
addition.142
Quenching of singlet oxygen by nickelocene is reported to be an efficient
process
and
to
involve both
reversible charge transfer interactions
energy
transfer and
lIl2: The Photochemistry of Transition-metal Organometallic Compounds 13.
123
Palladium and Platinum
Evidence derived from measurements of the lifetimes and electronic spectra of the tetrahedral complexes M(dppp)z (M = Pd, Pt; dppp in
-
bis(dipheny1phosphino)propane) in the solid state and
fluid
( It2(n*pu*)
solution
indicate
that
1 [t2(do*)ll). 144
the
emissive
Quantum
state
yields
of
(4-PAP-N02
photoisomerisation of trans-[Pd(PPrn3)(4-PAP-W2)C12]
pyridine-4-aldehyde-4-nitrophenylhydrazone)
-a
is
following
intraligand excitation have been determined
of
Photoisomerisation
[Pt(PPh3)2(RCICX)]
acetylido complex [Pt(PPh3)2R(CiCX)] (R = X = Ph, CH20H
or CMe20H and R = H, and
the
Me02C), but if R
= X
CH20H, C02H or CH2NH2, no
Details have appeared of the
analogous transformation occurs mechanism of
X =
=
gives
hydrogen production from photolyais of
aqueous
solutions of [PtH(PEt3)3]+ 147 and of a catalyst suitable for the hydros ilylat ion
of
ole€ ins. 148
[ PtClzYL]
(Y
=
subst ituted
pyridine, L = O-donor ligand) has been prepared by irradiation of [PtC12Y(CH2:CH2) 1 in solution.14g Thermally Induced loss of the O-donor ligand gives [ (PtC12Y)2J as a pair of cis/trana isomers whose
proportions
can
be
varied
photochemically.
a-bLs(2-phenylpyr ldlne)platinum( I I) and s;ia-bis[2'-thienyl)pyridinelplatinum(I1) undergo a oxidative
addition
with
stereoselective photochemical
various
organic
halides
to
yield
complexes of Pt(IV) with the halide and a u-bonded C-atom in the m - p o s it ion. 150
The
wavelength-dependence
of
the
photoelimination reactions of the diphenylplatinum(I1) complex (28)151 and of gj&-[Pt(PBu3)2(C6H4R-4)2J
(R
=
CP3, Me, CMe3, OMe)
have been atudied,lS2 In the latter case, the substituent is stable a t 2 5 3 . 5 run whereas at longer wavelengths it undergoes a
124
Photochemistry
regiospecif ic per icyclic reaction at platinum. 34.
CODDeK a d Silver
examination
An
of
the
wavelength
dependence
of
the
photorearrangements of CuCl complexes of cyclododeca-1,5,9trienes has revealed no reason to assume that the t w o types of reaction, cis-trans-photoisomer iaat ion and rearrangement, occur on
separate
(Ph3P)3CunX,
energy
surfaces .153 154
(n -- 1,2; X
C1, Br , I ; biL
=
The
photolysis
of
C1, Br, I, CN) and (PhgP)(biL)CuX (X
=
2,2 '-bipyr idyl, 1,lO-phenanthroline) has been
=
examined by ESR,15'
and Ph. and Ph2i) identified as spin adducts
with phenyl-tert-butylnitrone. photocatalyt ic
A
activity
[Cu212(PPh3)2(dpk) 1 ,
and
study has been made of the Of
[CuqIq(dpk)3]
[Cu(PPh3)2(dPk)lN03, (dpk
=
di-2-pyridyl
ketone) in the solar energy storage reaction, norbornadiene/ quadr icyclane. lS6 The effect of Ag( I)
on alkene E/Z phOtOiSOmt3Kif~atiOn has
been described together with further observations on the Ag( I) photoinduced 1,3-hydrogen shift .157 15. Miscellaneous
Luminescence spectra of a series of Ce(1") halide,
alkyl,
hydrocarbon
aryl,
ligands
ether,
have
been
conditions158 and a matrix synthesis
of
nitrile,
and
recorded
isolation W
( c ~ H 4 ) ~ E uusing
compounds containing cyclic
under
a
aromatic range
of
study of the direct
europium
atoms
ha8
been
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The Photochemistry of Compounds of the Main GrouP Elements BY A. COX 1.
Introduction
Reviews
have
appeared
of
laser
induced
photochemical
reactions of diborane and its mixtures with other compounds,'
the
~ of interaction of O('D) atoms with O3 during W p h ~ t o l y s i s ,and polysilane high polymers .3 2. Boron and Indium
Excitation of pentaborane(9) at 193 nm in the gas phase causes primary dissociation to BH3 and B4H6, followed by the ground state transformations shown below,
1 Reduction of the esters RC02R' (R and 4-MeC6H4, photoexc ited
2- and
mono-
=
4-C1C6H4, and
cyclohexyl ; R'
Me) has been
Their
Ph, 2-, 3-,
achieved
di-p-naphthoxyborane
tr i-B-naphthoxyborohydride.
=
react ivit Fee
and
lithium
have
rationalised in terms of the increased acidity of the excited state.
using
been (T,T*)
Tr imes itylbor irene ( 1) has been prepared us ing a
d i-n-methane-liFe photorearrangement of dimes ityl(mes itylethyny1)borane and its properties that
irradiation
of
NaBPh4
in
An earlier claim7 MeCN
or
THF
gives
the
diphenylborene anion has been challenged .8 4,4'-Bipyridinium ions
form ion-pair CT complexes with tetrakis[3,5-bis(trifluoro-
135
.
Photochemistry
136
M Ph ,e, Ph
Ph
(1) R = mesityl
1IN: The Photochemistry of Compounds of the Main Group Elements
methy1)phenyllborate
anion
and
show
for
the
a
broad
137 structureless
lumineacence at 553 nm. 9 The
quantum
EtIn(II1)TPP
(TPP
yield =
photodecomposition
tetraphenylporphyrin)
of
is increased in the
presence of pyridine and this has been interpreted in terms of the facile dissociation of the C-In bond in 3EtIn( I I I )TPP(Py) * assisted by the axial pyridine.l0
The same workers have alao
investigated the electron transfer reactions of EtIn(I1I)TPP
to
tetracyanoquinodimethane.I1 3.
Silicon, Germanium. and Tin
Si('D2) has been detected following photoexcitation of S i H Z into high bending vibrational levels of the A1B1 electron-transfer
photochemistry
state."
Some
a-silylamine-cyclohexenone
of
s y s t e m together with medium effects on the reaction pathways followed
have
been
describedl3
and
ethers
using
of
a
induced desilylation of trimethylsilyl enol
an
1-cyanonaphthalene polyailylated
isolation
two groups of ~ o r k e r s - ~ ~ t ~
silanimine has been reported by Electron transfer
matrix
excited haa
also
diazomethanea
(Me3Si)3SiC(N2)SiMe3
state been such
photosensitizer reported. l6
such
Photolysis
as Me3SiSiMe2C(N2)SiMe3
as of and
leads to the formation of sbonded silenea
by migration of a trimethylsilyl group to a carbane centre.17 That it is this group excluaively which migrates, may reflect vertical charge stabiliaation by the donating effect of the Si-Si u-electrons to the vacant p-orbitals of the silylcarbenes. A
ailylated
cyclobutene
has
been
prepared
by
the
photocyc loadd it ion of male ic anhydr ide with RC !CS i M e 3 ( R-H, Me, MegSi)18 and photolysis of H2C:CHSibfe2SiMe2CH:CH2
of butadiene gives a mixture of
a and
in the presence
trans vinylailacyclo-
138
Photochemistry
butanes
and
s i 1.acyc1.ohexans
Me2Si :CHCH2SiMe2CH:CH2.'I
v ia
the
The ene reacti.on o f al.l.yl.ai.l.anea wi.t.h
s i.ngl.et oxygen, 2o photoadd i t ion o f R9eH and
R!3eCH2CH2Si( Q M e ) 323
a i.l a n e
i.nt,ermedi.atx
t.0
s o h i t ion
hydros1.l y J a t i o n of ai J y.l acety.l enea"
H3.C :C H S i.( OMs)3 t-o 9 i.ve
phase
photochemi.ca1.
have been d e s c r ibed.
AbwoJute r a t e c o n a t a n t s have been measured for si.ly.lene r e a c t i o n s with hydrocarbons at. 3.98 K and t h e s e ahow that- si.ly.lsne
reacts r a p j d l y but unsel ect,ive.ly with iinsatiixat,ed hydrocarbons; hwever ,
a j .lyJ ene
j
a
unreacki ve
kowards
a.1kanea z 3
DimethyIailyIene has been produced hy y-irrad.iation saJutjons
of
dodecamsthy.lcyc.lohaxaaj .lane. 24
of
benzene
Rs.lat.jve
rake
c o n s t a n t s f o r the r e a c t i ons of methyl phenyl s i .1 y.l ene qenexated by p h o t o l y s i a of t h e Si-bridged 2,3-dimethylbu ta-I, 3-di ene,
arene have
( 3 . ) w.it,h EkOH,
been
Et3S.iH, and
pob.1ished. 3.5
R e 1ated
r e a c t i o n s have been urred t o prepare 2,2,6,6,8,8-hexamethy.l-~'r7-
4-thia-8-sj~ahicyc.loCS.3. O J o c t e n e ,
s t r u c t u r e shows ( 3 ) ,3.7 and
descr i b d .
j
compound
a
whose
X-ray
t to be pseudoaromat.ic,26 t h e a.i.lacyc.1opentenes
react ions o f di.methy1.ai 1.yl.ene w i.t-h p i.nenes have been
'*
The
photochemical
g e n e r a t ion
a
of
cyclopropeny-lsily-lene haa heen reported2'
and on photo.1ya.i a of
this
major
compound
in
a
Et2CHMe
glass,
the
product
a
ja
silacyclobutadi.ene. 3Q I r r a d i a t i o n o f 3.-cycl.op,:opyl.-j!-phenyl.hexamethyltr is ilane
i.n
2,3-dimethylbuta-I., 3-di.ene
preasnca
the
gi.ves a mi.xt.nre
of
of
product-a
whi.ch
arises from t h e intermediacy of cyc.loprapy.lpheny.ls.i.ly.lene-3'Follmwing n , n * e x c i t a t i o n of: arp-iinRaturat-ed si.1.yl. ketonea such
as
followed
(41,
the
by
y-H
main
phot.opxocea8ee
ahat-raction;
are
E/X-iaomexlmtlon
photo.iaomerjastlon
by
a b s t r a c t ion i.s also observed. 33. Photolys i.s of (Me3Si.)$3 i.COR
8-H
[IN: The Photochemistry of Compounds of the Main Group Elements
Me
Me
(6) R = mesityl
(7) R = mesityl
139
Photochemistry
140 (R
-
Et,
CHMe2,
CH2Ph)
gives
a
mixture and
Me3SiOCHR(Si(SiMe3)212C(OSiMe3):CHPh)
of
(w.
linear
cyclic
head-to-head
dimers ( 5) of the intermediate silenes, the proportion of cyclic dimers increasing with the bulk of R.33 The same authors have also examined the photoreactions of stable ailenes with dienes and alkenes;
[2+4],
[2+2],
and
ene
reactions
have a.11 been
observed. 34 A
study
of
the
phenylethynyldisilanea s i x-member ed
r inga
photophysical has
have
appeared. 35 been
formed
of
behavionr
some
Various by
novel
photolya i a
of
hexa-tert-butylcyclotrisilane in the presence of ni tr iles such as
MeCN and PhCN.36 Persilylcyclotrisilane,
[ (Et3Si)2SiJ3 has been
photolysed to produce (Et3Si)2Si:Si(SiEt3)2,37 and the same group reports that irradiation of hexaneopentyltrisilaoxetane leads to extrusion
of
dineopentylsilanediyl
and
of
formation
tetraneopentyldis ilaoxirane. 38 Some cyclotetras ilanes have been trradiation of the cyclopolysilanes (RR’Si), or 4; R = R ’ = Me3CCH2, MezCH, MeCHEt; R
alcoholic
medium
using
= Me3C,
9,lO-dicyanoanthracene
R’ as
(n
=
3
Me) in an
=
sensitizer
induces ring opening and formation of EtO(SiRR’)nH.40 An electron transfer reaction appears to be involved in which the polysilane functions as donor.
Photoreaction of tetrakis[bis(trimethyl-
silyl)methyl]disilene in methanol generates [(Me3Sl)2CH]2Si(OMe)H41
and photolysis of oxaailirane (6) in an inert matrix at
77 K gives the silacarbonyl ylid ( 7 ) . 4 2 Aryltriethylgermanes, diaryldiethylgermanes, benzyltriethylgermanee,
and
dlbenzyldiethylgermanes,
are
readily
photolysed to germyl radicals, which In the case of the radicals arlalng Prom the latter two compounds, add to C-C bonds.43r44
I I N : The Photochemistry of Compounds of the Main Group Elements
141
PhOtOlySiS of benzoyltriethylgerme, EtgCeCOPh has been shown to lead to the initial triplet radical pair Et36e and Phh0.45 The photochemistry OF dialkyl and diarylgermanium(1V) porphyrins ia consistent with
a mechanism
involving
bond. 46 Some organogermylenes R2Ge: (R been prepared by photolysis
=
cleavage
-
the Ge-C
Me, Et, Ph, mesityl) have
in a matrix and their electronic
spectra reported.47 The germylenes R2ge, R * & R , C1, OMe; R m
of
and Rm2ge ( R = F,
Et, Ph, mesityl) generated photolytically, undergo
a regioselective cycloaddition with 3,5-di-tert-butyl-~-quinone The first synthesis of a stable t o give 2-germa-1,3-dioxolane~-~* germathiirane has appeared4’ and a CIDNP study of the photolyais 0
(n = 1-3) to give Ph#e3-nGe
of Ph#eg,,GeLi
a contribution from
d
triplet mechanism.
’*
at 77 K has revealed Homolytic substitution
in trialkylstannyl iodides by photochemically generated iodine atoms has been descr ibed. 51
(Trialkylstannyl)acetylenes of the
form MegSnCiCPh photoreact with RI
(R
=
CF3, n-C3F7) to give
RCICPh.52 A proton CIDNP study of some benzyltin compounds,53
E-2
photoisomer isation
of
2-stannyl-l-borylalk-l-enes, 54
and
selective photoalkylation of the 10-methylacridinium ion using tetraalkylstannanes or d iethylmercury under vi s i ble i rrad i at ion5’ have all been reported.
Photoinduced allylation of aromatic
carbonyl compounds56 and of furyl- and th i e n y l d i c y a n o e t h e n e ~ ~ ~ have also been described. and
probably 4. An
involves
The latter process is regioselective an
electron
transfer
process.
Nitrocren and Phosohoxus
aqueous suspension of coprecipitated hydrous oxides OF
Fe(II1) and Ti(1V) is reported to photocatalyse the reduction of N2
to NH3
using
visible
light.58
Observations
of
the dual
Photochemistry
142 1.a.ser e x c i.tati.on
wavelength
diir i.ng
NO
of
phot-oexci.tat-i.on o f
S P ~ - N O mixtures59 and of t h e azi.da radi.ca1. u s i n g laser-i.ndnced f l u o r e s c e n c e 6o have been made. The f 1-uorescence of
form
Ph2P(0)R
=
PhCH2CHPh)
PhCH2CHMe,
1-,
(R
2-naphthyl.,
Me, i.8
dfphosphene
d e s c r ihed6'
Et,
p-RrC6H4,
PhCH:CMe,
PhCH:CPh,
weak wher ?as t h a t from Ph2P(O) R ' (R' =
9-phenant-hrenyl
r e p o r t e d t o be s t r o n g e r . the
t - e r t i a r y aryl.phosphi.ne oxidea o f t h e
'.'
9-MeOC6H4,
=
bi.phenyl.yl.)
i.8
phot-oi.somer i.nati.on o f
The c i . s - t r a n s
(R
RP=CR
,
has
2,4,6-(MegC)2C6H2)
and al.kenes have been phot.oepoxi.d i.sed
11s i.ng
been
1.i.ght- i.n
a r edox sya tern i nvo.1v.i ng t-etrapheny.1 porphyr i nant .imony(V) .6 3
5. Oxycren, Su'l p h w and Se.1end um comp1.s~ l.eadn t-o a n e w s p e c t e a
Photo1.ysis of t h e O g / R r 2
.
w h i ch may he RrORr 64
In
the
qaa
phase,
(R
H2C:CH(CH2),S.iMeR2_,(OAc)m
.
HS ( CH2)n+7.Si.MeR(OAc) 6 5
photoreact-i.on =
Me, Et,; n
=
H2S
and
= 1,2)
gives
of
0,l; M
e m i . s a i.on
Fluorescence
fol.l.owi.nrJ
excjtatjon of SO3 at. 3 4 7 and 357 nm oriqinakea f r o m S 0 2 6 6
and
infrared f luoreacence has been reprt.Ftd Prom khe laser-induced
senai t f z e d reactfon in t,he SF6-IIF6-H3.
syskem. 67 SP5C!l
has been
photoadded to carbon-carbon doiihJe honda .in a procesa in which &P5
i n i t i a t e s t h e a t t a c k . 68 Analngous addi.ti.ons o f TeF5C1., but.
not SeP5Cl, have a1.m been observed. ehotoly8i.s of t h e ster ical.1.y protected I., 2,3-sel.enadi.axol.e
(8)
in
the
preaence
of
a
siiitab7.e
o1.efi.n
r e g i o s e l e c t i v e cycloadducts (9) ( R = C02Me,
l.eads
CN) v i a an
tm
the
n it. i.al.1.y
formed z w i t ter f on f c jnt.ermed i a ts .69 6.
Haloaea
Absolute r a t e constantxi have been o b t a i n e d for t h e Cl.z/ H R r
1113: The Photochemistry of Compounds of the Main Group Elements
143
chain r e a c t ion using t h e l a s e r - i n i t i a t e d chemical. chai.n xeacti.on technique. 70 The photochemistry
of
methyl bromide absorbed
on
plat inum( 111.) ,71- t h e photofragmentat ion dynamica of i.odoethane, l , l , l - t r i€luor0-2-iodoethane,~~
1-iodopropane
2 - i o d o p ~ o p a n e have ~~ been i n v e s t i g a t e d ,
and
and emission f r o m tF(R)
has been observed following t h e p h o t o l y s i s o f a mixture o f C P 3 t ,
P2 and Ha using a pulsed 248 nm KrP.excimar .laaer.74 The origin of fragment r o t a t i o n in cyanogen i o d i d e phot-odiaaociation has been discussed75 and t r a n s i e n t i n t e r m e d i a t e s have been r e p o r t e d i n t h e
photolysis of iodonium cations.76
Mi scel l a n e o u
7.
Aqueous persul-phate using
p l a t inised
p-CuCNS
i.8
photodecomposed
coated
with
i n v i s i.bl.e l i g h t
Rhorlamine
R
to
give
oxygen - 77
3. 2.
A. M.
M. R a r r i d a , A c t a Cisnt. Venez, 1986, 37, 607. P. Popovich and Yu. V. Pil.ippov, Vestm.
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H e , H. HOU, Q . Qin, Zhonuuuo, Jiuuanq, 1987, l4, 279. Grelbig,
Kruegerke,
T.
2. Anora. A . l J u . C h e m . , 3987,
69
-
A.
Wataru,
544,
K.
and
Sappelt.,
74.
Y. Kurnamoto, and N. T o k i t o , T e t r a h e d r o n I , e t t . ,
3986, 2 7 , 6107.
70
-
D. A .
R.
Dolson and S.
Leone,
J.
Phva.
Chem.,
1.987,
a.,
3543. 72.
S. A .
Coste11.0,
J. Phvs. Chem., 73..
C. P a t e r s o n ,
B.
Roop, Z .
M.
r,iu,
J.
M.
Whi.te,
Corry, M o l .
Phvs.,
and
1988, 92, 1.02.9.
F. G.
Codwin,
and P. A .
1987, 60, 729. 73.
Y. Huang, J. R. Cao, Y e Wen, X . Zhong, J . Zhang, W . Fang, X.
74.
13.
Wu, and Q. Zhu, Wu1.i Huaxue Xuebao, 1987, Raybone,
T.
Chem. Phvs. I.ett..,
M.
Watkinflon,
and
J.
3, 337. Whitehead,
C.
3 9 8 7 , L33, 442.
75.
C. H. Dugan and D. Anthony, J. Phvs. Chem., 1987,
76.
R.
J. nevoe,
M. R. V. Sahyuin, N. S e r p n e ,
Sharma, Can. J . Chem.,
77.
K.
Tennakone,
1987,
fi,
and
n.
K.
3.343..
S. Wicktamanayake,
J. Chem. SOC., Caradav Trans.
a,3929.
and
I , 1987,
M.
U.
83, 2 5 5 3 .
Gunasekara,
Part 111 ORGANIC ASPECTS OF PHOTOCHEMISTRY
1 Photolysis of Carbonyl Compounds
I
BY W. M. HORSPOOL Pfoertner has scale
up
reviewed
of
problems
photochemical
discussed
chirality
processes.e
Gilberta
which
can arise during
A
experiments.*
transfer
in
the
review
some
has
photochemical
has published a review dealing with the
use of the carbonyl group in the synthesis of steroid systems. The second edition of a textbook o n organic photochemistry by Coxon and Halton.
Norrish Type I Reactions
1 High
has been published.
intensity
irradiation
hexane/isopropanol
mixtures
carried
of
out
dibenzyl and
dibenzyl
ketone
1,2-diphenyl
in
ethane
and
A study of the photochemical
toluene as the sole products.' behaviour
of
yields
ketone
adsorbed
the results found
on
to be
zeolites has dependent
been
upon the
Si/A1 composition of the z e o l i t e e m A leser flash examination of
the behaviour
demonstrated
of dibenzyl
that
a
ketone in Nafion membranes has
modest
yield
of
benzyl
cations
is
f ~ r m e d .A ~ study of the physical photochemistry of the ketone ( 1 ) has shown that
the excited state is a carbonyl localized
singlet state. This ultimately decays to afford localized from
the singlet
radicals study of
state
to afford
radical
has
photochemical
been
behaviour
of
the photochemical ( 4)
ketones
reported.s of
derived
from
report also that
the
the 1-
from the triplet state. A
is formed
the modification
a-alkyldibenzyl
cyclodextrin
products
(3). The authors.
(2) and
naphthyl methyl the
a naphthalene
triplet state. Norrish Type I cleavage does occur
the
A
by
behaviour
complexation
similar
study
of
of in the
(5)
in
conversion
to
alkyldeoxybenzoins
cyclodextrin has also been made.lo Irradiation
of
( 6 ) at
300nm
brings
about
dehydrohumulinic acid (7) by epimerization at C-4 and (8)
151
152
Ph otochernistry
qR
s Ph+Ph
Ph
0
A
A
Ph
x
0
0 /
(81
/
(9) a; R’ = H, R2 = CN dr C02Mc b;R’=F, R 2 = C N orC02Me
(11) n = 2; 6 5 2 n = 3 ;47% n = 4 ; 7%
(12) n = 2 ; trace n = 3 ; 4% n = 4 ;10%
I I I I l : Photolysis of Carbonyl Compounds
153
Norrish Type I fission of the side chain carbonyl group again Laser flash irradiation has been used as a method
at C - 4 . I 1 for
the production of n-butylketene
chemistry
of
this
cyclohexanones processes
ketene
(9a)
on
was
undergo
irradiation.
from cyclohexanone. The
studied
both The
in
Norrish
detail.%=
I
Type
fluorinated
The
I1
and
compounds
(Sb)
showed a preference for Norrish Type I 1 behaviour. Within the Norrish Type
I 1 biradical
fluorine substitution leads
to a
preference for cyclization rather than cleavage. The Norrish Type I biradical afforded a ketene rather than a n alkenal.lm A study of the photochemical reactivity of the diones (10) has
shown that both Norrish Type I and Type I1 reactivity can take place. The Type 1:Type I1 product ratio is dependent upon ring size. Thus dione (lOa) affords the Type I 1 products ( 1 1 ) and (12) while
dione
( 1 0 ~ )yields
the Norrish
I products
type
(13c-15c) and low yields of the Norrish Type I1 products ( 1 1 ) and (12). Compound (lob) is intermediate between these results affording a Type I: Type I1 ratio of 0.3. A mechanistic study of the reactions was carried 0ut.l.
Irradiation (254 n m ) of the keto lactono brings about
a Norrish Type
(17). Elimination of ketene product
(18)
in
good
(16a) in t-butanol
I fission to yield a biradical from
yield.
this affords the isolated
Similar
irradiation
lactones (16b) yields only polymeric products.*'
the
of
Irradiation
of the keto epoxides (19) in methanol results in Norrish Type
I fission to afford a mixture of products the major components of which are the esters (20) and the aldehydo alkenes (21).
Minor products were detected in the ceses of (1%) and (19b) but these were not identified. A minor product from the irradiation of (19c) was identified as (22). formed by rinq opening of the epoxide following the Norrish Type J fission. Irradiation of the epoxide the ring
opening
of
identified.
Epoxyketone
Scheme
on
1
(19a) in ether gave evidence for
the epoxide although no products
(19b)
irradhtion
gave under
the the
products same
were
shorn
in
conditions.
Epoxyketones (19c) and (19d) were also photoreactive in ether but
affarded
complex mixtures
from
which
no
pure
material
could be isolated. The ketone (23) pave the alkenes (24) and
(25) on irradiation in ether and ( 2 6 ) and ( 2 7 ) in
Photochemistry
154
(13) n = 3 ; c + f = 8% n = 4 ; c + t = 24%
(16) a; R = But b; R = Et or Bui
(14) n = 3 ; 4%
n = 4 ; 19%
(15) n = 3 ; 3% n = 4 ; 8%
(17)
(181
,
(19) a; R1 = Me, R2 = C8H17, Sa 6a b; R1 = Me, R2 = C8H17, Sp 6p c;R1 = H , R 2 = O H , 5a,6a
,
d; R’ = H,
Scheme 1
R2=OH,
SB 9 6g
IIIII: Photolysis of Carbonyl Compounds
123)
155
(24)
c@
OHC H
' H
(26)
(27)
(28) a; R = H b;R = D
(29) a; R = H b;R = D
1301 a; R = H b; R = D
(31) a; R H b;R=D
(32) R ' = R 2 = C N R' = CN, R2=C%Et
( 33)
(34)
Photochemistry
156 Stiver of
and
Yatesi7
have
some hydroxy-keto
isomeric
compounds
obtained,
(30)
(28a.
and
configuration of
of
( 2 8 , 29).
29a)
(31)
the
attached. The use
studied the photochemical
steroids
showed
that
respectively,
carbon
to
which
reactions
Irradiation
had
the
the
retained
hydroxy
deuteriated derivatives
of
the
products
group
the is
(28b. 29b) has
identified the hydrogen abstraction processes involved in the conversion of these ketones into the lactones ( 3 0 b ) and (31b) respectively. The authorsi7
propose that there are two major
factors which control the stereospecificity of the reactions. These
are
the
shape
of
the
hydroxy-bearing
hydrogen transfer within the biradical
C-atom
and
the
formed o n Norrish Type
I fission. The stability of the biredical intermediate clearly plays
an
important
part
in
determining
the
outcome
of
the
reactions.
The
photophysics
of
the
a-cleavage
reactions
of
cyclobutanethiones such as ( 3 3 ) have been studied.ig ( 3 3 ) undergoes conversion at 20K
Matrix-isolated
heptan-3-one
to the carbene
( 3 4 ) . The identification of
achieved by
this
species was
trapping experiments and by i.r. spectroscopy.=)
Irradiation of
the ketone (35) in methanol
expansion to the acetal
brings about ring
(36) by way of the standard carbene
path. Chemical conversion of this afforded a synthetic path to moscarine ( 37 )
Norrish Type I 1 Reactions
2 The
.* *
amide
( 3 8 a ) is
photochemically
inert
on
irradiation
in
ether. The related compound (38b) is, however, photochemically reactive and undergoes fission by a Norrish Type I 1 process to yield a mixture of products.g1
The results of a study of the
enantioselective photodeconjugation reactions of the lactones ( 3 9 ) have been published.PP The behaviour of the ketones (40) and
(41)
in
the
isotropic
and
the
two
solid
phases , o f
heneicosane ( C P I H ~) ~has been evaluated. The influence of the various
phases
on
the
ratio
of
elimination
products of the ketones was discussed.es the photoreactivity of ketones
(42)
to
cyclization
The modification of
in cyclodextrin has been
11111: Photolysis of Carbonyl Compounds
(38) a; X = NMe
Me NyPh
PhyxK
0
b; X =CHR, R = alkyl
0
0
157
(39)
M (401 n
*
LnH
= 13,15,19,21
(41) n = 7 . 8,10,11 or 12
or 23
~1
( 4 2 ) R1=H,Me,Bu,pentyl ,decyl, R2=R3=H,X = CH2 R’ = R2= H ;R1= H R2= =R2 = Me ;X = 0,R3 = H
,
R3=p-Mc, R’=H,R2=butyl or pentyl, X = CH2 R3=m-Me,R1=H,R2=butyl or nonyl, X=CH2 R3=o -Me, R’ =H, R2=butyl or nonyl, X = CH2
R3
(441
&Jy HO ( 46)
-
k3
(44) (47) a; R1 =H b; R’ =Me
(45)
Photochemistry
158
Bu‘
(50)
Ph
/
Scheme 2
C02 H
Me
(531
OH
159
11111: Photolysis of Carbonyl Compounds studied
in
detail.
This
work
shows
the
consequences
of
restricted rotation on the Norrish Type I1 biradicals.e4 Two studies have
dealt
with
corresponding these
the
(43)
methylbenzophenone
photoenol
publications
laser-jet
where was
the
photochemistry
the
formation
observed.e6
In
the
the
second
of
was
successfully trapped by an electron transfer technique.='
The
(44) are photochemically reactive and on
quinone derivati'ves in
of
the
ketone
irradiation
photoenol
2-
of
of
/
benzene
t-butanol
undergo
conventional
Norrish Type I 1 hydrogen abstraction to afford good yields of the
cyclobutane
accompanied
by
(45).
derivatives the
rearrangement
These
compounds
products
(46)
and
were (47).
Subsequent experimentation shows that these are formed in good yield
from
secondary
irradiation
of
the
cyclobutane
derivatives (45).=? Irradiation of the ketone (48) affords the indanol (49) as the sole
product.
This
compound
is
produced
by
1.6-hydrogen
a process akin to a Norrish Type I 1 reaction by
transfer
the carbonyl oxygen from the proximate methyl group of the t butyl group
followed by
1,5-biradical. The
bond
formation within the
(50) behaves
ketone
somewhat
resultant
differently
( 5 2 ) as the major reaction produot
yielding the redox product
and a small amount of the indanol (51). The triplet lifetimes of the two ketones (48) and
(50) in toluene were found to be
1.7 ns and 25 ns respectively. The authorsPo suggest that the difference
in
difference
in the angle of
and
the
the
behaviour
di-t-butyl
formation
of
the
of
substituted
redox
the
twist
ketones
benzene
product
is
due
to
a
between the carbonyl group ring. The
method
( 5 2 ) is still unsure
but
of a
possible path could involve the route shown in Scheme 2. Some
benzophenone
quaternized behaviour
carboxylic
with has
benzophenone converted mice1 les . a o
to
been
amino
acids
alcohols
studied.='
carboxylic tertiary
acid
as
and
their
Amphiphilic and
alcohols
myristic by
(53) have
such
their
been
aggregation
derivatives acid
have
of
been
irradiation
in
Photochemistry
160
( 57)
(56)
R
(61)
(60) R = Me or Et
a;ratio 1:l
( 6 5 ) a ; R =Me, n 5 2 b ; R =Et, n = 2 c ; R =Me, n = 4
b; ratio 1:1.2
w
55YO
15%
Scheme 3
IIIII: Photolysis of Carbonyl Compounds
161
Q cb Me
' 0
0
(661 n = 4 or 5
Me
(671
(68) a ; n = 1 b;n=2
(70) (71)
he
( 7 2 ) a;R2=CN b;R2 =COzMe
(73) a;R' = H b;R' = Me
d;R= 0
f Si
e ; ~ =
*x' Y
R' (761
(97)R'=Me, R2=CN R' =Me, R2=CO~Me R' =H, ~ 2 CN =
(78) R1=H or Me
162
Photochemistry Oxetane Formation
3
The photoaddition of benzophenone to the vinyl sulphides ( 5 4 ) affords a mixture of the oxetanes ( 5 5 ) in yields ranging from 12 -
79%
dependent
on
the
alkene
used.as
Photoaddition of
aldehydes to 2,3-dihydrofurans has been studied. The addition affords oxetanes of the type shown by benzaldehyde derivative addition
affords ( 5 7 ) was
of
two
isomers.
also
aldehydes
studied.
afforded
(56) where addition of
Addition In
to
this
two
the
furan
instance
types
of
the
oxetane
illustrated by (58) and ( 5 9 ) . = = Intramolecular
cycloaddition
within
the
ketoalkenes
(60)
affords the oxetanes ( 6 1 ) as the principal products ( > 85%). These oxetanes can be ring-opened in methanol a
trace of
trifluoroacetic
acid) to afford
(acidified with the medium
ring
( 6 3 , 6%) was also obtained from the
ethers ( 6 2 ) . The product
irradiation of ( 6 0 , R = M e ) as a component of a mixture of minor products. The oxetane (64) was a minor product obtained from the irradiation of ( 6 0 , R=Et). The shorter chain keto-alkenes (65) are also photoreactive and afford the products shown in Scheme
3.
These
additions are
less
regioselective than the
previous examples.3a The intramolecular
photoaddition of the
keto-alkenes ( 6 6 ) to afford the oxetanes ( 6 7 ) has been used as a step in the synthesis of the tricyclo-octane tricyclononane ketones oxetanes
(68b).=*
( 6 9 a-d) ( 7 0 a-d)
Irradiation of
using
>
wavelengths
in good
yield. The
(68a) and the
the
endo-norbornenyl
300
nm
affords
thienyl ketone
the
(69e),
however, only affords the corresponding oxetanes (708) under sensitized
conditions.
irradiation
with
the
The
ketones
use
of
( 6 9 ) was
shorter also
wavelength
studied. Ketone
(69b) was shown to afford a complex mixture of products under such treatment while the ketone (69c) was converted into the oxetane (70c) and the aziridine derivative (71).==
Photochemical
addition of electron deficient
the pyridine
thione
(74). This reaction arises by spiro-thietane products.
In
( 7 5 ) which the
alkenes (72) to
(73a) affords the substituted
case of
the formation of an
pyridines unstable
fragments to produce
the isolated
the N-methylpyridine
thione
(73b)
111l1: Photolysis of Carbonyl Compounds
163
(81)
(83)
(82)
Photochemistry
164 addition
of
This
also
is
( 7 6 ) as
methylacrylonitrile yields formed
from
an
unstable
the product.
thietane
but
by
an
alternative fragmentation path resulting in the elimination of thioformaldehyde. The isolation of thietanes (77) was achieved by the irradiation of the thione ( 7 8 ) in the presence of the same
electron
Ramamurthyg7 deficient
deficient
report
alkenes
that
add
alkenes.s' both
to
Devanathan
electron
the
rich
and
(79) to
thione
and
electron
afford
the
(80) Scheme 4. The addition occurs from the lowest
oxetanes
triplet state of the enone and a biradical and an exciplex are presumed
to
be
involved
in
the
study of the photochemical
formation of
reaction of
products.07
A
(81) with
the thione
electron rich alkenes has reported that the formation of the (82)
adduct
and
the
(83) arise
oxetane
from
the
triplet
excited state. The formation of the products occurs from both S z
and S x
excitation.sg The photoaddition of the same enone
to electron deficient alkenes has also been investigated. The irradiation
in
this
system
affords
oxetanes
from
the
S n
singlet state.**
4 The
Miscellaneous Reactions
4.5-corane
(84)
decarbonylation
is
obtained
the
of
in
60%
yield
pentacsclic
on
photo-
(85).*O
ketone
Photochemical decomposition of the carbonate ( 8 6 ) , by the loss of carbon dioxide, affords a mixture oxirane. Triplet benzyl
styrene sensitized
radicals.4a
254 n m ) of phenyl
oxide,
the
bibenzyl
irradiation An
earlier
carbonate
carbene.
and
products
of
and
yields
products
study of
the
( 8 7 ) reported
carbon
containing
phenylacetaldehyde.
dioxide
solely
from
irradiation (at
that
were
benzaldehyde, produced.ee
A
reinvestigation of the irradiation of this compound (at 254 nm in
acetonitrile)
trans-stilbene
deoxgbenzoin and bibenzyl.
has
oxides
When
provided
evidence
(88) and
(89) are
that
the
formed
cis-
as
and
well
as
smaller amounts of diphenylacetaldehyde and methanol
is
used
as
the
solvent
the
same
products are produced accompanied by benzylmethyl ether. 1 . 2 diphenylethanol.
and
2.2-diphenylethanol.
These
authors*=
suggest that the oxiranes ( 8 8 ) and (89) are formed by ray of
I I I l l : Photolysis of Carbonyl Compounds
(91)
165
( 92)
CHflCOR
(9114%) a; R = H b; R =OAc
qH20Me
I
RCOOH
(96) R = Me,p-MeCgH4 benzyl or 1 naphthylmet hyl
-
( 97)
(99) n = 1 or 2
(98)
166
Photochemistry
the
1,3-diradical
( 9 0 ) produced
by
decarboxylation
of
the
carbonate. A study has examined
the influence of a g-nitro group on the acetate .*.
photochemical hydrolysis of phenyl
The enol ester
(91a) undergoes homolytic bond fission to afford the radical pair (92) as the key intermediate to the products formed. The authors."
propose
benzylic (93a)
radical
or
combines
Irradiation products product
that
of
a
which
1,2-hydrogen
either
with
the
enol
ester
the
migration
abstracts acyl
yields
hydrogen
radical
yielding
(91b) affords
a
affording
the
(94a).
analogous
(93b) and (94b) and in addition produces the cyclic (95). This is presumed to be formed by cyclization of
the radical (92a) with the 9-acetyl group followed by a series of
steps before
the
final product
is formed.."
Irradiation
(96) affords the acid ( 9 7 )
(340 n m ) of methanol solutions of and the ether (98).'= Irradiation
of
the
iodoketones
(99)
has
shown
that
the
products obtained are formed by competing radical and cationic A
processes . 4 7
previous
account
of
the
photochemical
solvolysis of the chloroketone (100) reported the formation of two photoproducts reinvestigation photoproduct
from
second product
(101) and the solvolysis product
of
this the
reaction
reaction
has
is
shown
the
(102).*.
that
alcohol
the
A
only
(101). The
(102) is formed by an acid catalysed thermal
process.
A
study
of
the
photoreduction
has
acetonitrile/triethylamine (103 a , b ) afford
the alcohols
the amides (103 c , d ) and
of
the
shown
oxoamides that
the
(103)
in
oxoamides
(104) in high yield. However,
(105) are also reactive and afford
the cyclized compounds (106
-
108) respectively. The failure
of the oxoamides (103 a,b) to undergo cyclization is presumed to be due to intramolecular hydrogen bonding."O which
this
reductive
cyclization
occurs
The ease with
with
amides
or
unsaturated ketones has been exploited in a new synthesis of hirsutene (109). Thus irradiation (254 n m ) of the ketone (110) in acetonitrile/triethylamine affords the alcohol (111, 20%) and
the
material
desired
is
cyclic
readily
compound
converted
to
(112,
58%).
hirsutene .=*
This
letter
The
ketone
IIIi1: Photolysis of Carbonyl Compounds
167
Rqy N‘R2
M Me e)(%
0
,
(103) a; R’ = allyl, $= H R3=Me b; R1 =CH$’=CH,R2=H,R3=Me c; R’ =R2=allyl,R3= H or Me d ;R’ = propargyl R2=R3 =Me
NH
0
1104) R =ally1 or propargyl
,
eN, Q
X$$J 0
0
0
A (109)
(107)
(1081
I4 (110)
(111)
A
A (112)
(113) Me
CHzOH
Me Me
Me Me
(114)
(115)
1J
Photochemistry
168
M . q c o M e
(1 18)
(117)
(116)
+ 0
R' &R2
R2 = Ph (120) R' = H R' =CN, R2=Me or H R' = C l , R 2 = P h R' = Me, R2 = p - M e C & ,
(119)
L & OH
Me3Sn
R'
(121) R = H or
Me
(122)
ph> R
(123) a; R =Me
b j R =Ph
M Ph>Aco2H t
trace
22Oh
11111: Photolysis of Carbonyl Compounds
169
(113) reacts photochemically with methanol in the presence of titanium
tetrachloride to yield
the stereoisomeric products
(114). In the absence of the metal chloride only the methanol adduct (115) is formed by a hydrogen abstraction path.== Wavelength
dependence
bicyclobutane
of
(116) has
the
been
photolytic studied
in
behaviour inert
of
solvents.
the
At
254 nm and 300 nm the irradiation affords the three products (117), (118). and (119) but the ratio of products is different at the different wavelengths. The authorsmm believe that the state. Irradiation at 300 nm in
reactions arise from the z-x* diisopropyl
ether
or
in
toluene
yields
only
the
methylene
cyclobutane (117).'= Takuwa
and
his
irradiation of presence
of
coworkersse
have
demonstrated
the aromatic carbonyl compounds
the
stannanes
(121)
affords
that
the
(120) in the
the
unsaturated
alcohols (122) as the principal products. An electron transfer mechanism the
is proposed. Electron transfer is also involved in
reaction
of
amines
with
alkenes
such
as
the
phenylethylenes (123). The electron transfer in this instance affords an alkenyl been
demonstrated
radical by
reaction
has
been
example,
the
alkene
a
anion the presence
variety
uncovered
in
(123a) with
of the
of which has
techniques.
A
photoreaction
N,N-diethylaniline
further of,
for
in
the
presence of carbon dioxide. This treatment affords the three carboxylated derivatives (124). (125). and
(126) by trapping
of the radical anion by carbon dioxide. Similar carboxylation
was demonstrated for (123b) and biphenylene. The influence of the amine on the yield of product was studied.='
170
Photochemistry 5
References
1.
K. H. Pfoertner. S p e c . P u b l . - R . SOC. C h e m . , 1986, 57
2.
3.
A . G . Griesbeck, EPA Nersl.. 1986. 28, 13. A. Gilbert, S p e c . P u b l . - R . S O C . C h e m . , 1986, 57
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J. M . Coxon and B. Halton, O r g e n i c P h o t o c h e m i s t r y , 2 n d
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L. V. Romashov, Yu. I. Kiryukhin. and Kh. S. Bagdasar'yan, Dokl. A k e d . Nauk S S S R , 1987, 295, 427 ( C h e m . A b s t r . , 1988,
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L. J . Johnston and J. C. Scaiano, J .
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B. N. Rao, M. S. Syamala,
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I I I I I : Photolysis of Carbonyl Compounds
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2 1 . H. G. Henning and C. Hentschel. 2. C h e m . .
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( C h e m . A b s t r . , 1 9 8 8 , 108, 1 3 0 8 0 1 ) . 22.
J . P . Pete, F. Henin, R. Mortezaei, J . Muzart, and 0. Piva, Pure A p p l . C h e m . . 1 9 8 6 , 5 8 , 1 2 5 7 .
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R. G. Weiss, J . A m . C h e m .
S O C . , 1987,
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6215. 24. G.
D. Reddy, B. Jayasree, and V . Ramamurthy, J . Org.
C h e m . , 1987, 5 2 , 3107.
25. R. Y. Wilson, K. Hannemann, K. Peters, and E.-M. Peters, J . Am. C h e m . S O C . , 1987. 109, 4741. 26.
R. M. Wilson, K. Hannerann, W. R. Heineman, and J . R. Kirchhoff, J . A m . C h e m . S O C . , 1 9 8 7 , 1 0 9 , 4 7 4 3 .
2 7 . P.
J . Wagner, B. P. Giri, R. Pabon. and S. B. Singh, J .
Am. C h e m . S O C . , 1987, 1 0 9 , 8104. 2 8 . A . Osuka,
H. Shimizu, H. Suzuki, and K. Maruyama, C h e m .
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H. J . Schaefer. L i e b i g s Ann. C h e m . ,
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30. A . Gogol and H. J. Schaefer, L i e b i g s Ann. C h e m . , 1 9 8 7 , 597. 31.
T. H. Morris, E. H. Smith, and R . Welsh. J . C h e m .
SOC.,
C h e m . C o m m u n . , 1987, 964.
32. H. Itokawa, H. Matsumoto, T. Oshima. and S. Mihashi, Y e k u g a k u Z a s s h i , 1987. 107, 767 ( C h e m . A b s t r . , 1988, 108, 186609). 33.
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34. R. Gleiter and B. Kissler, T e t r a h e d r o n L e t t . , 1 9 8 7 , 2 8 , 6151.
35. R. R. Sauers. A . A . Hagedorn, tert., S. D. Van Arnum, R. P. Gomez, and R. V . Moquin, J. Org. C h e m . , 1 9 8 7 , 5 2 , 5 5 0 1 . 3 6 . T. Nishio. J . C h e m . S O C . , P e r k i n T r a n s . I , 1 9 8 7 , 1 2 2 5 . 3 7 . S . Devanathan and V . Ramamurthy, J . O r g . C h e m . , 1988, 53, 741.
40.
P. Rao and V . Ramamurthy, J. O r g . C h e m . , 1 9 8 8 , 5 3 , 3 2 7 . V. Rasarurths, J. O r g . C h e m . . 1 9 8 8 , 53, 3 3 2 . L. Fitjer and U. Quabeck, Anpen. C h e m . Int. E d . E n g l . ,
41.
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38. V.
3 9 . V . P. Rao and
1987, 26,
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C. White and
S. Ma. J . Heterocycl. C h e m . . , 1 9 8 7 . 2 4 ,
1203 (Chem. A b s t r . .
1988,
108, 1 3 1 6 3 7 ) .
Photochemistry
172 4 2 . G.
W. Griffin, R. L. Smith, and A. Manmade, J . Or#. Chem..
1976, 4 1 , 338.
T. R i x , J. Org. Chem., 1 9 8 7 , 5 2 , 2 3 0 9 . P. Kuzmic, L. Pavlickova, and Y . Soucek, Collect. Czech.
4 3 . R . C. White and 44.
Chem. Commun., 1 9 8 6 . 5 1 . 1 2 9 3 ( C h e m . Abstr., 1 9 8 7 , 106, 195627). 4 5 . M. Alvaro, V. Baldovi.
H. Garcia, M . A. Yiranda, and J .
Primo, Tetrahedron Lett., 1 9 8 7 , 2 8 , 3 6 1 3 . 4 6 . M.
Inamura. K. Tokuda, N. Koga, and H. Iwamura, Chem.
Lett., 1 9 8 7 . 1 7 2 9 . 47
f
B. Sket and M . Zupan, Bull. Chem. SOC. Jpn.,
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50.
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5 4 . A. Takuna.
H. Tagana, H. Iwamoto, 0. Soga, and It.
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2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones BY W. M. HORSPOOL Intramolecular.- The apide ( 1 ) is converted on irradiation in benzene/ethanol
into a complex mixture of products
The
of
first
two
these
established
were
as
(2)-(5).
the
primary
photoproducts. The cyclopropyl aldehydes arise from secondary photolysis
of
photoproduct
(2)
while
the
ester
(4)
is
a
secondary
of irradiation of the aldehydes (3). Oxidation
also arises during the secondary photolysis of ( 2 ) and affords the aromatised product ( 5 ) . % The intramolecular photocycloaddition of the vinyl ketenes (6) has
been
reported.
(2+2)-adducts
This
shown
in
yields the syn-adduct one
case
only.
encountered
also
1.*
in
migration
the
the
cycloaddition
( 8 ) is formed in
reactions
formation of
photocyclizes
yields of
Primarily
(7). The anti-adduct
Hydrogen
resulting
(9)
Ketene
system affords good Scheme
are
isomers of
affording
the
also (7).
tricyclic
compounds (10) and (11) Scheme 2. A
previous
report
gave
an
account
of
the
intramolecular
photocyclization of the enone (12) to afford t h e head-to-head adduct
(13).* The adduct
(13) obtained in this ray has been
used as a starting material for ester (14).* While the above
pentalenolactone G methyl reaction only yields a
head-to-head adduct this is not the case with the enone (15a) which
undergoes
photocyclization
1 , 5 - ~ y c l o a d d i t i o nproduct biradical product.
(17)
followed
by
yield
the
bonding
yield
observed the the
( 1 5 ~ )which
(18) is
the
not
(15b) and
biradical
to
formed in this instance and the cyclization affording five-membered ring is predominant. This is not so with enones
alternative
to
This is formed by way of the presumably
arising
The
(16).
yield products
(19) and
(20)
from both modes of cyclization. The 1.6-cyclization
Photochemistry
174
Q R
N
M
NMc
e
0
+?
45)
C0,Et
R'
d2
0 NMe
0
2
1
( 3 ) R = CHO, R = H R 2 = CHO
R'= ti,
e$
w
0
R2
R' 2
( 6 ) a : R'= Me. R = ti
( 7 1 a: 43%
b: R ' = R 2 = ti c : R ' = H . R 2 = Me
H
(yo+ ti
( 8 ) a:
-
b: 10% C:
R a t i o 1: 1 . 5 Scheme 1
R'
b: 58% c : t o t a l yield 39%
+
11112 Enone Cycloadditions and Rearrangements
175
e05&
0
+
(9)
Scheme 2
boyc= 0 \I\
0
‘c*
-C* (12 1
O
H
(141
(151 a: R’
= Me, R 2 = H
b: R’ = R 2 = H c: R’
=
H , R 2 = Me
R’
Photochemistry
176
&
q:
R2
R2
(19 1
(18)
R'
= = R =
(21)a: R b: R c:
=1 H;n= 2
H: n
Me: n = l
(221 a : ratio 50 b : ratio 50 c : ratio 1
1 (231
1 50
Scheme 3
H
(24)
(251
177
11112 Enone Cycloadditions and Rearrangements
&
b
OR
=
(26) R
0
CH2CH=CH,, CH,C H W e 2 . CH,CMc =CMcH (cis and t r a n s )
=
(27)
Ph
0
-
( 2 8 ) A r = Ph, 1 naphthyl, 2 - naphthyl or 9 phcnanthrenyl
-
0 (29)
& 0
0
(30)
( 3 11
y C 0 2 M c
(32)
R2
( 3 3 ) R ' = H, R 2 = CO,& R'
=
C02Mc. R,=
H
Photochemistry
178
H
Me
Me
.'e 0 x 0
R2 (37) n
R2
= 2 or 3; R'= R ~ H = 1
2
n = 3 o r L ; R = R = C02Me n = f,; R'= H , R 2 = OSiMe2CMe3
Me
(39)
(38)
Me
(LO) X
=
H , Br o r OMc
11112 Enone Cycloadditions and Rearrangements
179
path does not go to completion and instead the biradical ring-opens
affording the enones
(20).5 Matlin
(18)
aL.=
9t
have
examined the influence of the position of the attachment
of
the side chain as well as enone ring size o n the cyclization of the enones (21). Closure to the head-to-tail products (221, following the r u l e of f i v e . is the preferred route as shown in Scheme 3. The notable
exception to this is seen with
enone
is preferred affording (23) as the
(21c) where the 1.6-closure principal product.
A
70%
yield
of
the
adduct
(24)
is
obtained
on
brief
irradiation through Pyrex of a hexane solution of the enone (25).7
Intramolecular
( 2 6 ) has been
cyclization. of
A
reported.
the
enone
typical product
derivatives
formed from
this
reaction is shown in (27) and involves a head-to-head addition of the alkenyl side chain to the enone double bond.'
A study
of
carried
the
out.s
photoreactions The
benzyl
intramolecular
of
the
( 2 8 ) has
(28, Ar=Ph) was
derivative
cycloaddition.
(28) did undergo
enones The
1-naphthyl
cycloaddition to yield
been
unreactive
to
derivative
of
the unstable adduct
( 2 9 ) which thermally or photochemically reverted
to starting
material. Both of the other enones also undergo cycloaddition affording
the
adducts
adducts photochemically
(30)
(31)
and
revert
respectively.
to starting material
These
and
the
authors'
propose that this is brought about by the excitation
of
dihydronaphthalene
the
moiety
either
or
by
(32)
in
directly
intramolecular energy transfer from the carbonyl group. The
intramolecular
photoaddition
chlorobenzene using wavelengths ( 3 3 ) in a ratio of mixture
of
achieve
a
products total
>
of
the
enone
350 nm affords the adducts
1 . 5 : l and in a total yield of 87%. can be
synthesis
carried of
the
forward
This
successfully
to
laurenene.*O
A
diterpene
total synthesis of (A)-pentalenene ( 3 4 ) has been carried out using the intramolecular photocycloaddition of the enone ( 3 5 ) affording the tricyclic ketone ( 3 6 ) as a key step.** An efficient route to the synthesis of the cscloalkanones (37) involving the intramolecular photochemical cyclization of the enones
( 3 8 ) has
been
described.
This
yields
the
tricyclic
180
Photochemistry
0
LO& (46) a :
R
3
CO,R
Me,
Me
( 4 8 ) a: n = 1 b : n = 3 c : n = 2
H
( 4 9 ) a: n
=
1
b:n= 2
181
111f2 Enone Cycloadditions and Rearrangements
& (CH 1
0 H
( 5 0 ) a: n b:n
0
= 3 = 2
(511
or 2
(53)
0 (52) n
= 1
0
182
Photochemistry
intermediates (39) which o n ring opening affords the desired products.
A study of the photochemical reactions of the naphthalenones (40) has
sought
to
establish
the
mechanism
by
which
the
products are formed. Irradiation affords the two adducts (41) and
(42). The results obtained
from this present
study have
shown that both products are formed via the intermediacy of a complex ( 4 5 ) giving a biradical
( 4 4 ) which bonds in two ways
(Scheme 4 ) to yield (41). as a primary product, and (45). This latter
compound
migration
to
possibility
is not
yield
of
a
isolated
(42).ia
and
This
3,5-migration
a photo-1,3-
undergoes
evidence
which
had
rules been
out
the
postulated
originally by Brooke et al.=+ The scope of the reaction was investigated
with
regards
to
the
substitution o n
the enone
ring and also on the chain length of the alkenyl group.iB Irradiation (Pyrex filter ( 4 6 , R = M e ) affords
the
in benzene
study was also extended
solution) of
the ester
(47) in high yield. The
cycloadduct
to cyclization of
the &-isomer
of
( 4 6 , R = M e ) and a comparison of the quantum yields of product formation quantum
was
made.
yield
reaction
is
is
The
authors*=
measured
reversible.
induction was
considered
for The
and
the
suggest,
cyclizations,
possibility
studied
since of
a
low
that
the
asymmetric
in the cyclizations of
the esters ( 4 6 , b and c). The incorporation of nitrogen into the enone system does not radically affect
the cycloaddition reactions as seen in the
(2+2)-addition reactions encountered with the enamides In
this
study
intramolecular
the
influence
cyclization
of
of
the
chain
length
enones
(48)
evaluated. Thus with a four atom chain the
crossed
chain
(48b)
addition the
(48).
on has
the been
(48a) the product
compound
(49a). With
head-to-head
adduct
a
six
(50a)
atom is
is
side
formed
exclusively while with a five atom chain ( 4 8 c ) a mixture of (49b) and
(50b) is
are obtained
produced.*.
The
(2+2)-cycloadducts
(51)
in good yield on irradiation of the enone (52).
The reaction is dependent o n the nature of the substituent o n
C-3 and when this is H or C1 no cycloaddition reaction takes place o n irradiation.i7
11112 Enone Cycloadditions and Rearrangements
183
I
(58)
.
Pr' (59)
Me
CO,Me
(60)
(611
(63) R = H or Me
Photochemistry
184
OAc
H
( 6 6 ) a: R ' = H
( 6 7 ) R2= H or Bu
b: R 1 = M e 0
hM:
RR1
Me
Me Me
R2
Me 1
(68)
(691 R
= pentyl, R 2 = H
or vice versa
COMc
(721
185
11112 Enone Cycloadditions and Rearrangements Pirrung
and
Webstersg
photocyclization
of
have
the
reported on
enones
their
(53-55).
study of
This
arises from earlier work where intramolecular were
also
studied.
In
the present
work
the
investigation cycloadditions
enones
of
the
type
shown in (53, 54) were found to be unreactive and did not show evidence in flash photolysis experiments for the generation of a
transient
which
might
have
been
the
--isomer
of
the
enone. The carbon analogue (54) did show evidence for a short lived
intermediate
products
arising
and
also
from
cycloaddition products
is
was
photoreactive
yielding
deconjugation.
The
indicative
relaxation around
that
absence
of
the enone double bond is preferred to (2+2)-cycloaddition. The diene
( 5 5 ) was
reactive and
cycloadducts (56), ( 5 7 ) , and the
solvent
was
irradiation afforded
the
three
( 5 8 ) in a ratio of 22:31:29 when
cyclohexane.
Some
for
evidence
dependency was detected. Intramolecular
solvent
cyclization has been
reported following the irradiation of the enone (59). This o n irradiation
in pentane
under
an inert atmosphere yields the
(2+2)-cycloadduct (60).19
Intermolecular.photochemical the
Reviews
have
described
some
of
the
(2+2)-cycloaddition reactions of enones used in
synthesis of natural
p r o d u c t s . e o * e % Other
reports have
also focussed on such additions as key steps in the design of natural
products.
Thus
the
photoadducts
obtained
by
the
cycloaddition of the enone ( 6 1 ) to the alkene ( 6 2 ) have been used a s the starting materials for the synthesis of terpenoid intermediates.Pe The
cycloaddition
of
the
allenes
(63)
to
cyclopentenone
results in the formation of the two adducts ( 6 4 ) and (65) in a ratio of the
4:l. The addition reaction occurs primarily
less
substituted
double
bond
Cycloaddition of acetylene and but-1-yne can be
brought
about
using
of
the
across
allene.='
to the enones
left-circularly
polarised
(66)
light.
The resultant cyclobutane derivatives are readily converted to the optically active enones (67).er The photoaddition of hex1-yne
to
the
lactone
(68) affords the two isomeric
adducts
(69) resulting from both modes of addition. The isolation of
(70) from the reaction mixture suggests that the cycloaddition
186
Photoch ernistry
C02Et
4fo H
K
O
H
(761
OH
(771
(78)
OH
OH ( 7 9 ) R = H or Me
b... (81 1
OH
(801 R = H or Me
Me0
(82)
b., (831
11112 Enone Cycloadditions and Rearrangements
187
OR2 OMe
( 8 5 ) R2 = ( - ) - 3 - m e n t h y l
6
H
C0,R'
Ph 1
(86) R
= ( - 1 -3-menthyl
(87)
R' = ( +) -3 -ment hyl
H
Me
H
Me
(90) n
=
Ph
Ph
(88)
(89)
5,7,8 or 9
(91)
Me
188
Photochemistry
occurs by a from
the
two-step mechanism. This affords biradical
head-to-tail
,
undergoing compound
a hydrogen
addition
mode
abstraction
to
is
capable
afford
the
open
(70).es Pentyne, also, adds photochemically
steroidal
enone
( 7 2 ) to give
the
isomeric
study of
A
(74) in a ratio of 10:l.em
(71)
which
adducts
of
chain to the
( 7 3 ) and
the photoaddition of
alkenes to the dione ( 7 5 ) has been r e p o r t e d e e 7 Saligenin
(76)
is
in
basic
irradiated
photochemically
reactive
(MeOH/HeO).
media
The
(254 n m )
when
reaction affords
phenol/formaldehyde type resins in reasonable yield. The route to
the
enone
condensation
process
involves
( 7 7 ) by photochemically
the
formation of
the
induced expulsion of hydroxide
from the phenolic anion generated from (76). This enone then reacts with another
anion ultimately
to build
up oligomers.
Evidence for this process comes from the minor products formed during
the
produced enone
by
reaction. the
These
are
addition of
the
methanol
ether
( 7 8 ) which
(the solvent)
to
is the
(77). Furthermore the diphenslmethane derivatives (79)
and (80) are also formed by the condensation of two substrate molecules
either
with
or
without
the
addition
solvent.
of
Products of this type are considered as good evidence f o r the condensation reaction proposed.em
(2+2)-Photocycloaddition of ethoxyethene to the enone (81) at 254 nm
in methanol affords the adduct
(82). Addition of
the
a cyclobutane
same alkene to the enone ( 8 3 ) also proceeds
intermediate but this is unstable and ring opens to afford the cyclo-octane
derivative
(84)."
the optically active alkene
A
study of the
( 8 5 ) to the enone
four cyclobutane adducts two from
head-to-head
addition of (86) affords addition and
two f r o m head-to-tail addition. These cycloadditions lead to a double
induction
giving
diastereoselectivity.ao ethylene
to the enone
either
The
increased
intermolecular
( 8 7 ) yields
or
decreased
cycloaddition
the two adducts
of
( 8 8 ) and
(89). The photoadducts are apparently susceptible to secondary irradiation and the maximum yields of the adducts was obtained at 50% consumption of the starting material conditions
(88)
respectively. The steps
to
afford
and
(89)
isomer
were
obtained
(88) was
ultimately
taken o n
racemic
(87). Under these
in
71
through
sterpuric
and
23%
several acid
a
11112 Enone Cycloadditions and Rearrangements
189
Me OTMS
OTMS
a qMe OT MS
(92)
(93)
(941
0
I (95 1
(961
(97)
(98) a: R = Ac
OH
b: R
= Me
0
OAc (991
%
OMe CH,
(100)
190
Photoch emisrry
(102 1
(101)
(1031 R
= H
o r Me
OR
R'
A
0
(105 1
(104)
R4= H
R ' = R3= R'= H , R
R2
(1061
R' = M e , R2= OCH2CH,0, R3 = R4= H R' = R 2 = H, R3 = OCH2CH20 , R4 = Me R'-
Q
2
z
OH
R ' = R2= R4= H, R3= OH
191
11112 Enone Cycloadditions and Rearrangements sesquiterpenoid
A
fungal metabolite.aa
route to 1.3-bridged
cyclooctatetraenes has been developed using the photochemical addition of process
trans-1,2-dichloroethene to the enones (90). This
affords
transformed
the
to
the
(91)
adducts desired
which
products.mg
are
The
chemically
generality
cycloaddition of cyclohexenones ( e . g . 92) to the alkene has been
360 nm
studied. This photocycloaddition, brought light,
Thermolysis
pentane
in
of
these
the
affords
yields
the
adducts (95)
diketones
of
(93)
about
by
94).
(e.g.
which
are
useful compounds for the synthesis of the perhydroazulenes This
specific
product
example
( 9 5 ) was
daucene ( 9 6 ). m a
converted
Otherss4
photocycloaddition of the enone
have
into
also
the
.
natural
described
the
(92) to the cyclobutene ( 9 3 )
to afford the (2+2)-adduct ( 9 4 ) in 2 4 X yield. The adduct was transformed triplet
into
state
the
of
natural
conjugated
product enones
balanitol
can
be
(97).
The
photochemically
reduced by sodium borohydride.== High regioselectivity
is found in the photochemical addition
of allene to the enones (98). With enone (98a) cycloaddition affords
(99)
compound
was
while used
(98b)
for
yields
subsequent
the
adduct
reactions
(100).
This
in syntheses of
taxane-like molecules.ms Cycloaddition of the enol and
2-naphthol
or
the
form of acetylacetone (101) to 1corresponding
methyl
ethers
gives
products dependent on the naphthol used. Thus addition to the 2-naphthol system affords the diketone (102) presumably v i a a non-isolated cyclobutane adduct
(103). Such addition and ring
opening is well established as the de Mayo reaction. Addition to the 1-naphthol
system yields two products (104) and (105).
The first of these arises by addition of the enol to the 1,2bond
of the naphthalene skeleton in the same fashion as for
the formation of (102). The second arises by (2+2)-addition to the 3,4-bond of the naphthalene again followed by cyclobutane ring opening to yield the diketone
(105)
In this instance
aromatization does not occur. An earlier report dealt with the synthesis of hirsutene utilising the de Yayo cyclization. This approach involved the cycloaddition of the alkene (106) to the dione (107). The resultant cycloadduct (108) readily undergoes ring
opening
to
afford
the
new
diketone
(109). This
was
Photochemistry
I92
Cl Cl
do
OMe
d-l,
H
H
(110 )
CJ:xo
(111)
R'
(112) X = NMe or NEt, R' = Me X = NMe, R
1
(113)
= H,Bu,penti or Ph
X = NEt, R 1 = Ph
X = NPh, R ' = Me X
= 0, R 1 = Me, Bu,penti or Ph
(115 1
(115) R ' = H, Me, CO,Me, O,CCH=CH,, CO,CH=CH2. R 2 = H or Me. X = 0 or NH
(116) X
=
N H , NMt or
0
(117)
193
11112 Enone Cycloadditions and Rearrangements
0
o*Rf / OMe 11181
(119)
R
= deoxy-p- 0-ribofuranosyl
Me
OMe (12 0 1
(121 1
&1
0
PXo1
H *’
0’
(122 1
Ar
(123)
CN
&OMe
CO, Et
H
(121)
C0,Et
Ar
(125)
(126)
Photochemistry
194
0
YI .
.
# I
II
H
H
q
Me02C Ph
0
H
Ph C02Me
(129)
(128)
(127 1
Me
A
+
Ph
+
Me
(O-LM1
(0.02M1
52h
1
hV
Me02C Me
B,: I
Me02C H
B-1:
+
1
1 :I mixture
hA
bh h e
Scheme
5
R0 S
II
- P ( 0 E t l2
Me
.Cl
0 (1301
(131 1
195
11112 Enone Cycloadditions and Rearrangements subsequently
transformed
in
a
few
steps
to
the
desired
product.ao Full details of this have been published.as Photocycloaddition (110) affords
of
1.1-dichloroethene
the adduct (lll).40
to
the
The triplet
quinolone
excited
state
of the enones (112) are photoreactive and undergo addition to alkenes
to
afford
(113).41 Both
reasonable
elect,ron
photochemically
add
rich
to
yields
and
the
enone
the
azetidines
(114)
to
alkenes
afford
the
the C = N is unreactive
cyclobutane adducts (115). Normally (2+2)-cycloadditions but
of
electron deficient
the authors4=
believe
that
to
in this
case the C=N system is activated by the trifluoromethyl group. The
azetidine-2-ones
(116)
can
be
readily
prepared
by
irradiation of the enones (114) in the presence of ketene.*m A study of the photochemical addition of 2,3-dimethylbut-2-ene
to
the
furocoumarin
establish
an
alkenes to
order
of
derivatives
(117-119)
reactivity
for
the enone double bond.4*
the
The
has
sought
cycloaddition
(2t2) adduct
to of
(120)
exhibits temperature dependent fluorescence.4s
Photochemical
addition
affords
two
the
of
furan
to Pummerer’s ketone
(4+2)-cycloadducts
(122)
and
(121)
(123).
The
structures of both of these products has been established by
X-ray
crystallography.
Spectroscopic
studies
on
the
enone
(121) suggest that the triplet state is highly twisted and the authors4*
suggest that
formation
of
two
the addition to furan results in the
biradicals
products. No evidence for
which
lead
the involvement
to of
the a
observed
trans-ground
state enone was ~ b t a i n e d . ~ ’
Direrization.- A reinvesti6ation of the dimerization of
(124)
in the solid phase has identified (125) as the product.47 The irradiation of microcrystalline methyl cinnamate ( 1 2 6 ) affords a
complex mixture
of
products. However,
the
preparation
of
crystalline complexes of this ester with boron trifluoride or stannic chloride followed by of
the a-truxillate dimer
complexes
with
crystalline
the
same
complexes
irradiation affords high yields
(127). Ethyl reagents
affords
a
but
cinnamate also forms irradiation
mixture
of
of
dirners
these
in
lor
Ph orochemistry
196
%
0O
W
0
o0
0
O-r
O H H O (133)
(132 1
@ I )
0
0
OH (134)
OH (1351
Me
Yp
(1381 R'
t
CECH OCH2Ph
R
(136 1
(137) R
H , Me or Et
(139)
R'=
RL
= Me, Me0 o r C l
(140) H or Me H, MePh,SMc,OMe
Pri o r p - Cl C,H, R3 = Me, Pri, PhCH,
197
11112 Enone Cycloadditions and Rearrangements
yields.
Dimerization
solution
phase
in
of
methyl
the
presence
cinnamate
also
of
acids
Lewis
occurs
in
affording
truxinete and truxillate dimers as well as E-2 isomerization. Crossed additions of methyl cinnamate and alkenes can also be brought
about
presence
of
in
dichloromethane
Lewis
Photodimerization
acids
of
as
by
irradiation
illustrated
cyclopentenone
is
in
also
in
the 6:.
Soheme
influenced
by
Lewis acids. In the presence of such species the dimerization affords head-to-head
(128)
dimer
predominates while in their
absence the head-to-tail dimer ( 1 2 9 ) is the major product.ag The
enone
(1301,
the
pesticide
regioselective dimerization 313
nm. The
Irradiation methanol
product of
Coumaphos,
irradiation at
identified
was
the
on
furocourarin,
as
the
undergoes
wavelen6ths dimer
(132),
Imperatorin
affords a low yield of the (2+2)-direr
>
(131).m0
(133).
in This
product is accompanied by the dealkylated compound ( 1 3 4 ) and the rearrangement product ( 1 3 5 ) . This last compound is formed by a photo-Claisen rearrangement. The influence of solvent was studied and the dimer was obtained only in methanol. The other two products were formed in a variety of polar and non polar solvents. In any solvent
the dealkylated compound
(135)
was
the main
2
Rearrangement Beautions
a,@-Unsaturated
A
Systems.-
review
has
surveyed
the
photorearrangements of enones and dienones.ER A study of tbe photodeconjugation reaction of the ester ( 1 3 6 )
in the prmsence of a variety of optically active amines such as
(1R,2S)-l-phenyl-2-isopropylamino
the product about
70
alcohols
Irradiation
yields
of
(methanol. ethanol
derivatives path..*
propanol has shorn that
can be obtained with an enantiomeric excess of
(138)
Excitation
by of
a
free the
the cyclized products
were verified by X-ray
the
or
alkynylketones
propanol) affords radical
hydrogen
cyclohexenone (140)
(137)
the
in
furan
abstraction
derivatives
(139)
the structures of which
crystallography. The formation of the
amide products is reminiscent of a Norrish Type I 1 process.
198
Photochemistry
(142 1
(141)
OMc
H
CHMe 0 (144 1
(143)
eCNWC &CN
(1461
(145)
PCN &cN
(148)
(147)
CN
199
11112 Enone Cycloadditions and Rearrangements
CN
CN
(151 1
O
(153 1
C
N
(154)
(156)
y
CN CN
+':o -
0 (159)
(158)
(157)
0:
R
= H
b: R = 4',6'-diMeO
Rqp 0
0
(161 1
(160)
.wPh WPh OMe
\
0 (162)
R
0 (163 1
Photochemistry
200
R3 R4h
N
JH f
(164)
-
2
+
R3&R2
R4
3&R2
H
R'
R3
R4
R2
H
H
H
COOCH(CH,
H
R4
l2
H
Me
Me
COOCH( C H 3 12
Me
H
H
CO,E t
Me
Me
Me
CN
Me
H
H
CN
Me
Me
Me
CN
Scheme 6
R2
R3&
R4 (166 1
(165)
H
dR 0
(167)
0
(168) R
= 0 or
Me
201
IIll2 Enone Cycloadditions and Rearrangements The hydrogen abstraction, in this instance, presumably occurs at
the P-carbon of
ring
closes
excited enone
to
triplet
state
(141) into
involves
the enone
afford is
the
hydrogen
cyclization
the
(139). The resultant biradical observed
involved
spiroketone
abstraction
within
the
products.6s
in
(142). The
by
The
IC--L*
the conversion of
the
6-carbon
biradical.==
the
reaction again followed
by
Photocyclization
of
enamides such as (143) has led to a new synthetic approach to the
yohimbine
an4
photocyclization
reserpine
the enone
of
type
alkaloids . s 7
The
( 1 4 4 ) provides a route for the
synthesis of 3,10-dimethoxyprotoberberines.so Previous work by
Jeger
and his
coworkerssg
has studied
the
photoreactivity of 8,6-epoxy enones. More recent work by Ishii et
has
al.=O
examined
related nitriles
(145) 'brings about
via
formed
by
the
a
irradiation
by
ring
nitrile
intermediate.
epoxide
reactivity
the major
cyclopropenyl (145)
isomerization. The direct
(147) as
carbene
of
photochemical
(146). The direct
isomerization
cyclopropane to afford accompanied
the
(145) and
only
the
opening
of
the
product. This
is
(148) presumably
Triplet
sensitized
about
trans-cis
brings
(149) is
of
irradiation of
also
photoreactive
irradiation affords the products (150)
-
and
(156) either
by the intermediacy of an ylide ( 1 5 7 ) or a carbene (158).=* Irradiation of the chalcone (159) at wavelengths
>
365 nm in
methanol yields only the cycl
