Photochemistry (Volume 20)

A ~~ Volume 20 A Specialist Periodical Report Photochemistry Volume 20 A Review of the Literature published between...

Author: D. Bryce-Smith

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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

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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

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(3)

(4)R = H or Bun

(5) R’s

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(9) R=Me, Et, or Pr’

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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


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.


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


xii

CL (19)

Ph

Me H

(22)

N

(24)

(23)

“6

Me3Si -N=Si

Mee i 0H 2 P h

NHBz

=N-SSiMe3

...


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


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


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.


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


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


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


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


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


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 ~ ~ .


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


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


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.


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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3 . d e . P h v s . , 1 3 8 7 , 48, C - 7 .

2

L a s e r P i c o s e c o n d S D e c t r o s c o D v and Photochemistrv

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198,1 3 .

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


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.

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G. V.

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Less-Common

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Kwiatkowaki,

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1986, 50, 689-

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and P. S. B e l t o n ,

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Acta. 1987, _138, 241.

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1987, 70, 1807.

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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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1987, 2 3 , 2 2 5 .

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J. H. Choi, H. S . Y e o , and S. C. Shim, Ru11. Korean Chem.

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Sheretyannlkova, and M. C. Voronkov, 7.h. Ohahch. Khfm, , 1986,

56, 1830.

Photochemistry

148 66.

M. S u t o , C. Ye, R.

S.

Ram, and I..

T,ee, J . Phvs. Chem.,

C.

19137, 93, 3263.. 67.

Z.

68.

T.

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


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

4.

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

5.

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,

6.

N . J . Turro and 2. Zhang, T e t r a h e d r o n L e t t . , 1987, 2 0 ,

7.

V. Wintgens and J . C. Scaiano, C a n . J . C h e m . , 1987, 6 5 ,

8.

L. J . Johnston and J. C. Scaiano, J .

(Photochem. Org. Synth.), 256.

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5637. 2131. Am.

Chem. S O C . , 1987,

109, 5487. 9.

B. N. Rao, M. S. Syamala,

N . J . Turro. and

V . Ramamurthy,

J . O r g . C h e m . , 1987, 5 2 , 5517.

10

G. D. Redds and V . Ramamurthy. J . Org. C h e m . , 1987. 52, 5521.

1 1 . G.M. A. Bondeel, D. De Keukeleire, and M.. Verzele, J. Chem. S O C . , P e r k i n T r a n s . I , 1987, 2715. 12. A . D. Allen, A. J . Kresge, N. P. Schepp, and T. T. Tidwell, C a n . J . C h e m . , 1987, 6 5 ,

1719.

13. K . Reinholdt and P. Margaretha, J. F l u o r i n e C h e m . , 1987, 14.

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I I I I I : Photolysis of Carbonyl Compounds

171

2 1 . H. G. Henning and C. Hentschel. 2. C h e m . .

1987, 2 7 ,

339

( 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 .

2 3 . A . Nunez and

R. G. Weiss, J . A m . C h e m .

S O C . , 1987,

109,

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 .

L e t t . . 1987, 1061. 2 9 . A . Gogol and

H. J . Schaefer. L i e b i g s Ann. C h e m . ,

1987,

589.

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.

H. A . J . Carless, J. Beanland, and S. Mwesigye-Kilbende, T e t r a h e d r o n L e t t . . 1987, 2 0 , 5933.

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.

R.

38. V.

3 9 . V . P. Rao and

1987, 26,

1023.

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.,

1 9 8 7 . 60.

4489. 48.

J. K. Mogoto and J . Kossanyi, J. Ann. Chim. (Paris),

1970,

5. 4 8 1 . 49.

L. De. Cardenas, B. D. Maxwell, T. V. Singh, and €I. Morrison, 3. Ors. Chem., 1 9 8 8 , 5 3 , 2 1 9 .

50.

J. COSSY, D. Belotti, and J . P. Pete, Tetrahedron Lett.. 1987, 28, 4545.

51.

J. COSSY. D. Belotti. and J . P. P e t e , Tetrahedron Lett.. 1 9 8 7 , 28,

4547.

5 2 . E. P. Serebryakov. G.

K. Nalbandyan, and S. A. Vartanyan.

I z v . Akad. Neuk SSSR, Ser. Khim., 1 9 8 6 . 1 6 0 7 ( C h e m .

Abstr.. 1 9 8 7 . 1 0 7 , 7 7 6 6 4 ) . 53.

H. Cerfontain and J . A. J . Geenevassen, Recl. Trav. Chim. Pays-Bas, 1 9 8 6 , 1 0 5 , 3 8 6 ( C h e m . Abstr.. 1 9 8 7 , 106, 213442).

5 4 . A. Takuna.

H. Tagana, H. Iwamoto, 0. Soga, and It.

Yaruyama, Chem. L e t t . , 1 9 8 7 . 1 0 9 1 . 55.

Y. Ito, Y. Oozu, and T. Matsuura, J. Chem. SOC., Chem. Commun.. 1 9 8 8 , 5 6 2 .

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


&

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


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


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


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


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


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


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


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 cyclized product (160). However, when irradiation of either nm

(159a) or (159b) is carried out at 405

in methanol with an electron acceptor the three products

(160),

that

(161), and

all

of

(162) are produced. The authorsma suggest

these

are

formed

from

the

common

intermediete

(163). Celas-Mialhe ~t el.== report the efficient cyclization of the enones (164) to afford the derivatives shown in Scheme 6 . The authorsmP

suggest that product

formation arises by a

conrotatory cyclization followed by a 1.4 suprafacial hydrogen transfer

within

the

ylide

(165). A

detailed

study

of

intramolecular photochemical cyclizations of the enones

the

(166-

168) has been reported.ma The results obtained indicate that the enones (166) and (167) produce radical intermediates which revert

to

measurement

can

of

starting material the

change

in

a

fact

alkene

established by

geometry

in

the

recovered

202

Photochemistry

-A hv

0-0

(170)

R'

+

@o

c (172 1

(173 1

+ +

R

(175)

R ' = H, vice versa

C02H

(176 1

Scheme 7 Me

Me

0

Me R

Me0 (177) R = OEt or NHEt R'

CHO

(138) R ' = R 2 = H

R'= H ,

R2= Me

R'= Me, R

2

= H

go (174 1

R*= CHO o r

Me

CHO

203


H 11821

(180 1

vo II

PhC 0 (185)a:R

(183)

1

= H, R 2 = Ph

b: R1= Me, R 2 = Me o r Ph

Ph

Y

Ph

Ph

phYph

OCOPh

(187)

(186)

0 (188 1

m = n = l m = 2 . n =I

m : n

= 3

R = Me, Ph, p - McOC6H4, m

- MeO%H4, p - CF3C6H4. a-naphthyl

Or

p-ClC6Hb

Photochemistry

204

starting material. This is not the case for the enones (168) which apparently cyclize to product without any decay back to starting material. The

enone

epoxide

irradiation shown

in

in

(169) is photochemically

benzene

Scheme

solution

7. The

afforded

formation of

reactive the

six

products

and

on

products

involves

the

formation of two intermediates ( 1 7 0 ) and (171).

photochemical

These thermally intermediate

rearrange to the observed (172 -

(169) yielding furan

(176).04

products

175) while The

with the

intermediate

(171) affords

the

reaction paths

is to be contrasted with the simpler reaction

complexity

of

these

path described earlier by Mukai et a l . = = A

study

aromatic

the

of

sensitized

retinoids

photochemical

(177)

has

shown

( 4 + 2 ) - ~ y c l o d i m e r i s a t i o n s . ~ The ~

behaviour that

photo

they

of

the

undergo

reactions

of

the

pentaenals (178) has been reported.m7 Wagner and NahmeD have observed the photoaddition of a remote double bond

to the benzene ring of

affording ( 1 7 9 ) and

acetophenone derivatives

(180) from the irradiation of

(182)

In

another

publication

(181) and they

have

reported further on the process and have demonstrated that the from the reaction is a secondary photoproduct.

final product Thus

the

irradiation

(181)

of

has

been

shorn

spectroscopy to afford an initial photoproduct

by

n.m.r.

(183). This is

thermally labile a n d , i s converted into the triene (184) which undergoes

a

photochemical

cyclization

to

yield

the

stable

product (179):The enol esters

(185) show photochemical

be

in terms

rationalised

N.C-3

bond

in

the

undergoes typical

diene.

of

the

Thus

a methyl

molecule

is

now

conjugated. The photochemistry dominated (187)."O

by

1.3-acyl

behaviour which can

the degree of the

flat

tranq-&-isomerization

(186). However, with part

of

twisted

migrations

these to

around ester

the

(185)

to afford the diene

substituent of

twist enol

on C-3

and

is

the diene no

longer

compounds (185b) is

afford

the

diketones

A detailed report of the photochemical conversion of

the enamides

(188) into

the spiro

compounds

(189) has been


205

0

C02Me

CO R'

c0,Me

(wavelengths

c&-but-2-ene.

identified as presumed

The

(337). s p 7

irradiation

320

afforded

(339) and

the addition of

(338) was

nm) two

in

the

products

(340). These

are

the alkene to the

anhydride in two modes affording the intermediates (341) and (342). Collapse of publication

these

cites

yields

results

the observed

obtained

from

products.

the

The

addition

of

several alkenes . * * I Irradiation of the dione (343) affords the two products ( 3 4 4 , 50%)

(345. 15%).

and

follow

the

The formation of these is presumed

path

monodecarbonylation

outlined affords

in

the

to

Scheme

11

where

diene

(346)

which

keto

undergoes a n intramolecular Diels-Alder addition affording the (344). A second decarbonyletion of (346) yields

minor product the

tetraene

(347)

which

photocyclizes

by

(4t4)-

a

cycloaddition yielding the polycyclic compound ( 3 4 5 ) . s u s

6

Quinones

A review has highlighted the kinetic studies of electron and hydrogen transfer processes in quinone photochemical

reactions

of

the

The

quinones

(348 -

350)

with

amines has been studied using time resolved techniques. This approach has allowed the assignment of which triplet state is involved

in

such

systems.*ss

The

laser

irradiation

of

the

quinones, 3,5- and 3,6-di-t-butylbenzo-1,2-quinone, has shown that the triplet state is produced. These states are rapidly quenched

by

the

semiquinones

The

photochemically Pyrex

pyrocatechols

p-quinone

to

yield (351)

derivatives

the are

reactive and on irradiation in benzene using

filtered

methylenedioxy

corresponding

light

they

are

converted

into

the

derivatives (352). The route preferred by the

authorssmm involves a Norrish Type I 1 process affordinE the biradical

( 3 5 3 ) which

observed products.

subsequently

cyclizes

to

yield

the


233

:R+O

(360)

( 3 6 11

aR2 0

R'

R3

0

(362)

(363 1

f y 4 R 4

f y 4 4 R 4 R5

R5

0 (364 1

OAc

R 1 = OMc or H R2=

H, R3= Br or c[

R 3 = H , R2= Br or C[ R 4 = h, R5=

R6co

R 5 = H, R4= R'CO

WNH' 0 (366)

(365 1

Photochemistry

234

Q

0 (368)

@co2H \ 0

(369)

R2 R@ '

0

R'

t

R2= H 2

R ' = Me, R = H R ' = R 2 = Me

RR21*

R ' = OMe, R 2 = H

0 (371 1

R'-R2

=

C6H4


OPh 0

(375)

235

0

OPh

(376)

Photochemistry

236 A

detailed

study

benzoquinone

of

(354)

the

photochemical

with

arenes

has

behaviour been

of

the

reported.%**

Photoreaction of the quinone ( 3 5 5 ) with morpholine affords the amine substituted occur

with

quinone

several

(356). This reaction was found to

amines . * I s

Sunlight

irradiation

of

an

ethereal solution of the diquinone (357) is reported to afford the light stable intramolecular cyclized product (358).amm at

Tsuji

a1.a*7

have

previously

reported

the photochemical

behaviour of the quinones (359a) and (360a). In an extension of the work they have examined

the behaviour of the quinone

derivatives (359b and c ) and (360b and c ) . These derivatives behave in the same manner and afford

the products

(361) and

(362) on irradiation in THF-methanol.*sm The

halo

afford

quinones

the acyl

(363) undergo

(364) and

photochemical

the quinol

acylation

derivatives

to

(365).*=*

The sunlight irradiation of acetic anhydride solutions of the quinone

(366)

Irradiation

affords

of

(366) in

the

triquinone

acetone

product

also affords

(367).

(367) but

in

addition the diquinone (368) is formed by a free radical path initiated by the abstraction of hydrogen from the amino group by excited state acetone.lao The quinone (369) photochemically adds to a variety of alkenes by a (2+2)-addition to afford the adducts (370).1** A kinetic study of the photohydroxylation of the quinone (371)

in potassium hydroxide and perchloric acid solution. hes been reported.'*=

The

phototautomerism

.of the quinone

(372) has

been studied and the effect of pH and acidity evaluated.**g The

quinones

( 3 7 3 ) are

photochemically

reactive

and

are

converted into the quadricyclane derivatives (374) by use of light of wavelength

>

( > 63

quantum

X )

and

the

410 nm. The yields are generally high efficiency

reaction appears to be unaffected subs t i tut ion,

by

is

about

the methoxy

0.21.

The

and methyl

4

A triplet excited

state has been shown to be involved in the

isomerization of (375) into (376).%*5

237

11112 Enone Cycloadditions and Rearrangements 7

References

R . B. Bates, V . V.,Kane.

1.

A.

R. Martin, R. B:Mujumdar, R and Y. Kanaoka. J .

Ortega. Y. Hatanaka. K . San-nohe, O r g . Chem., 1987, 5 2 , 3178.

B. B. Snider, E. Ron, and B. W . Burbaum, J . O r g . Chem.,

2.

1987, 5 2 , 5413.

M . C. Pirrung and S. A . Thomson. T e t r a h e d r o n L e t t . , . 2 7 . 2703.

3.

1986,

M . C . Pirrung and S. A . Thornson. J . Org. Chem., 1 9 8 8 , 5 3 ,

4.

227.

A . R. Matlin, T. C. Leckta, D. J. McGarvey, P. I. Jacob,

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T. S . Fang. H. T. Chang, and S. J . Shih-Chen, J. C h i n . Chem. S O C . , 1 9 8 5 . 3 2 . 4 5 7 ( C h e m . A b s t r . , 1 9 8 7 , 106, 193342 1 .

1 4 2 . N. A . Vysotskaya and N.

A . Ogurtsov. U k r . K h i m . Z h .

( R u s s . E d . ) , 1 9 8 6 , 5 2 . 1 1 4 1 ( C h e m . A b s t r . , 1 9 8 7 , 107, 153723). 143.

F . Salinas, A . Wunoz do la Pena, and J . A. Yurillo,

144.

T. Suzuki, Y. Yamashita, T. Yukai, and T. Yiyaohi,

T a l a n t a , 1986, 33, 923 (Chem. A b s t r . . 1 9 8 7 . 107, 9 6 2 1 1 ) . T e t r a h e d r o n L e t t . , 1 9 8 8 . 29, 1406.

P. Strokach, V . A. Barachevskii. N. T. Sokolyuk. and Yu. E. Gerasimenko, Khim. F i z . , 1 9 8 7 . 6 . 3 2 0 ( C h e r . A b s t r . . 1 9 8 7 , 107, 2 3 5 7 9 6 ) .

1 4 5 . Yu.

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

1 Addition styrenes medium

Reactions of Alkenes

Eeactions.and

.*

phenyl

The

photohydration

acetylenes

(1)

Diyne

reactive(irradiati0n at

has

is

been a1 so

of

substituted

studied

in

acid

photochemically

300 nm) with methanol affordins the

Z and E isomers. The ( 2 ) are photochemically labile and are interconverted on prolonged irradiation affording a photostationary mixture.e (2)

addition product isomeric

as a mixture of

products

Interest in CdS mediated surface photochemistry has continued. The irradiation (wavelength

>

aerated

results

420 nm) of the alkene ( 3 ) in an in the formation of the products shown in Scheme 1. Under deaerated conditions all the products shown apart from the ketone ( 4 ) are formed. The results obtained are interpreted as involving electron suspension of

CdS

transfer from the alkene to the CdS affording a radical cation (5).

Subsequent

cyclization

and

back

electron

transfer

or

disproportionation yields (6) and (7). Deprotonation followed by back electron transfer or disproportionation yields (8) and (9).r

The

reactions

described

were

all

quenched

when

irradiation w a s carried out in the presence of the electron donor, 1.2.4,5-tetramethoxybenzene. CdS samples was carried out.

A

comparison

of

various

Ysriano and his coworkers have continued their study of the photoreactions of iminiui salts. Thus the xanthone-sensitized irradiation of the pyrrolinium perchlorates (10) brings about cyclization yielding the products (111..

&-trans

1aorerization.- &-trrnq-Isomerization of alkenee has been discussed in a review lecture. Laboratory experiments 245

Photochemistry

Ar Ar

A Ar

Ar

hV

CdS, h >420nrn

Ar

( 3 ) Ar = p

- MeOC6H,

( 41

(91

(8)

+ Ar

Ar

+ Me0

Ar Ar

(6)

Scheme 1

ArnAr Ar

Ar

(5)

(72) R

=

H or Me

Me0

@ Ar

(7)

Ar

I I I N : Photochemistry of Alkenes, Alkynes and Related Compounds

0

**

247

\

\

(13) R

= H. Me or Pr'

'Xk

R'O

Me0

R

R

(161 a: R' = Me, R = 9 - a n t h r y l b:R = H

(171

(18)

248

Photochemistry Ph

R2 ph+R1 Ph ( 2 1 1 a: R ' = R 2 = Me b: R' = Me, R 2 = Et

Ph (231

(22)

c: R' = Me, R2= Pr'

Scheme 2

x\ 'CHMc

(281

H2s '%"Me

(29)

x2cHM"' H

(301

(311

Illl3: Photochemistry of Alkenes, Alkynes and Related Compounds illustrating

d . = have

isomerization also

photochemical

were

described

also

249

mentioned.=

laboratory

--&-isomerization

Levine

experiment of

stilbenes

for

the

and

the

of

the

of

the

chromatographic separation of the products. Physical

details

stilbenes

(12)

relating have

photophysical properties'of high

the

isomerization

A

from

study

the styrylstilbenes ( 1 3 ) has shown

trans-&

that quantum yields for singlet but

to

been

isomerism are low from the

the triplet

stat0.O The photochemical

isomerization of the alkene ( 1 4 ) in an ethanol glass affords the --isomer

with high

temperatures.)

1,2-di-l-naphthylethylene crystalline

of

The

( 1 4 ) has

A

phase

isomeriz&ion reported.

efficiency

even at

liquid helium

Photochemical &-trans-isomerization

a

study

series of

mechanism

also been of

styryl

of

the

the

a-

of

studied

in

the

photochemical

phenanthrenes has reaction

been

involved

was

discussed.%% An

investigation

dianthryl

of

the

(15,

ethylenes

photochemical 16)

has

behaviour

been

the

of

carried

out.%P

Irradiation of the 2-isomer ( 1 5 ) brings about the formation of the (4+4)-addition product ( 1 7 ) with a quantum yield of 0.16. Irradiation

of

the

E-isomer

cycloaddition product tren~-&-isomerization much

lower.

different

The

also

affords

this

quantum yield, 0.002, is

the overall

irradiation

route and

(16a)

( 1 7 ) but because of the need for prior

yields

of

the

enol

(16b) follows

( 1 8 ) following an

a

intramolecular

hydrogen transfer process. Hydrogen Abstraction.-

Xanthone-sensitized irradiation of the

(19) affords

dihydroisoquinoline

a

6%

yield

of

the

spiro

The reaction is akin to a Norrish Type I 1

derivative (20).'O

process but in this instance hydrogen abstraction by the irine nitrogen

is

involved.

The

resultant

biradical

cyclizes

to

yield the observed product (20). A

study

alkenes

of

the photochemical behaviour

(21-24)

in

the

presence

sensitizers 1.4-dicyanobenzene in

deconjugation

to

of

of

the

the

1.1-diphenyl

electron-accepting

and 1-cyanonaphthalene results

afford products

isomeric with

starting

250

Photochemistry

(33 1

(32)

SPh

JY

phXF

SO2Ph

Ph

R

(341

136) R

Ph

Cl

=

Me, E t , a l l y 1 o r benzyl

F

X

L

Ar

A

Ph

H

Ph

(37)

(39)

(381

Ar

t

(40) Ph o r p-ClC,H,

R'

GR2 Me

(411a: n = 1 b:n= 2

( 4 2 1 a: R' = Me, R 2 = H 1 b: R = H, R 2 = Me

[:AN CN

(431

CN

r

11113: Photochemistry of Alkenes, Alkynes and Related Compounds material. Typically isomers

(25-27)

the alkene

251

(21b) is converted

into

the

shown in Scheme 2. The reaction involves a n

electron

transfer

acceptor

and

process

from

rearrangement

the

alkene

within

the

the

electron

resultant

to

radical

cat ion.% a Sensitized irradiation of the allene (28) in xylene-methanol afforded

the photochemically

(29)

stable reduced compound

as

the only monomeric product. In this instance excitation of the alkene moiety

results.

The

allene

(30) reacted

differently

under the s a m e conditions affording the isomeric trienes (31). The

formation

excitation

of

of

these

products

is

presumed

the allene. This gives

rise

to

arise

by

to the biradical

(32) which cyclizes to yield (33) in a process analogous to a Cope reaction.%" Collapse of the biradical ( 3 3 ) yields (31). Miscellaneous reported

in

A

Reactions.the

photochemical

conversion of

the

1.3-migration

thiophenyl

is

sulphone

(34)

into the isomer (35).*= Irradiation of the fluoroalkene

(36)

results in the formation of the reduced-dechlorinated (37)

and

the

migration.a7

product The

(381,

compounds

the

result

produced

of

in

a

this

product

1.2-phenyl

reaction

are

influenced by solvent and by the nature of the second halogen. Radical and cationic intermediates are thought to be involved. Irradiation of

the

tetracyanoethylene

the

cyclobutene

affords

propose

that

irradiation of

cation

of

the

the

( 3 9 ) as

diene

(40).

the complex

cyclobutene

and

a

complex with

The

yields

that

this

authorsag

the

radical

undergoes

stereospecific electrocycloreversion to the diene. Irradiation of the alkene (41a) at 185 nrn in isooctane affords cyclohexa1.3-diene. cyclopentene and acetylene. The larger ring alkene (41b) is also photochemically reactive at 185 nm yielding

cis-cycloocta-1,3-diene,

&-

cis-trans-cycloocta-1.3-diene,

cyclohexene, acetylene, and the bicyclic alkene (43). The ring opening

of

the

alkene

(41)

is

not

stereospecific.

Non-

stereospecific ring-opening is also shorn by the cyclobutenes ( 4 2 ) on irradiation. This suggests that both disrotatory and conrotatory ring opening is involved in the reactions.am

Photochemistry

252

CN

HRN

Me

:C

CN

/

\

Ph

CN

‘R

( 4 8 ) a: R = OCOMC

(47 1

(46)

Ph

b:R

Ph

ph%, Ph

= OH

C ZN

Ph

N- OAC

(491

(50)

Me Me

Ph

Ph

ph&CHO Ph

‘Ar ‘Ar

(52 1

(51) Ar

(53)

= P h . 4 - MtOC6H4, 4-CIC6H4, 3

- MeOC6H4,

L

- CNC&

An

IIll3: Photochemistry of Alkenes, Alkynes and Related Compounds The alkene ( 4 4 ) is photochemically

253

reactive on irradiation in

a matrix at low temperature. An analysis of the i.r. spectrum of

the

photolysate

showed

acetaldehyde are formed.*O media

failed

to

perpendicular thermal

give

alkene

of

had

of

ketene

the

been

the

( 4 4 ) in argon the

(45)

singlet

led

formation

predicted

and

carbene

of

a

following

state.

Extended

to the formation of a new

identified as the oxirane

compound which was addition

the

for

evidence which

equilibration

irradiation of

that

Other experiments in a variety of

(47),

( 4 6 ) formed by

produced

by

secondary

irradiation of (45). to acetaldehyde.

2 Armesto

Reactions involving Cyclopropane Rings

et

have

a1

isomerization

of

irradiation

the

using

is

the

acetate

acetophenone

(49). This

derivative

reported

enol

affords

thermally

photochemical

(48a).

Sensitized-

the

cyclopropyl

unstable

and

during

isolation undergoes loss of acetic acid to afford the nitrile

(50). The reaction is a further example of the acyclic aza-diz-methane process and results by photochemical the

1.1-diphenyl

feature

in

this

irradiation under

alkene work

is

moiety that

in

the

excitation of

(48a). The oxime

interesting

(48b) is

the same conditions. I t

inert

is suggested

on

that

the presence of the acetoxy group minimises the influence of an

electron

transfer

process

which

prevents

the

aza-di-x-

methane rearrangement. In another study theyPP have reported on the photochemical reactivity of

B.

-unsaturated imines. In

this work Armesto, Horspool and coworkerszP

have studied the

influence of

substituents on the aryl group attached to the

nitrogen

compounds

in

(51).

Irradiation using

acetophenone

sensitization converts all of the compounds studied into the corresponding cyclopropylimine ( 5 2 ) which were then hydrolysed and

isolated

yields

showed

as

the

that

aldehyde the

most

(53). Measurement

of

quantum

efficient -cyclization in

the

series arose with a p-cyano substituent. The results obtained suggest that the aza-di-x-methane process is more efficient in these examples when the lone pair on the nitrogen is tied up by

overlap

with

the aryl

substituted function. This, also,

suggests that the lone pair o n the nitrogen must

in some w a y

254

Photochemistry

An

+Y

An

+ An

An

(58)

(57)

An &An An

(60)

(59 1 An = p

- MeOC6H, Cl

&

R a t i o 33: 67

(61)

OMe Scheme 3

&N

+

Ratio

25~75

R a t& io

6 0 : 10

7

-

+

cL*

a: X = C I

X

R a t i o 8 3 : 17

(62)

x

h

h

cL&)

/

Scheme 4

N

IIIN: Photochemistry of Alkenes, Alkynes and Related Compounds

255

Ql+ 4ridget-t;::

(63)a: R' = RR22= MeR' b: R' = R 2 = S i M q c: R' d: R'

=

Me,

= Me,

R 2 = Br R2=CN

&R2

R'

( 6 6 ) a: CN b: Me c: M e 0 d: C02Me e: C l f: C N 9: C02Mc

(65) V i n y l ic control

R'

R2 H H

H H H CN C02Mc

(681 a: 100 b: 70 c: 100 d: 100

R2

R'

(67) a: CN b: Me C:

d:

c: f:

g: h: i:

CF3 Me CN OMc CL CN C02Me

'control

( 6 9 ) a: b: c: d:

30

-

R'

~1

R2 H H

H Me Me H H CN C02Me

(701 a: 100 b: 73

86 d : 50 c: 100 C:

( 7 1 ) a:

b: c: d: e:

0 27 14 50 0

R2

PhoI ochemistry

256

adversely affect the efficiency of the cyclization. The actual nature

of

this

is unknown

at

this have

sensitized

dianisyl

about

irradiation

irradiation

direct

the

These

and

other

of

the

( 5 4 ) brings

formation

of

the

rearrangement. Direct

hand

compounds

all-carbon

the acetone-

alkene

the

afforded

are

also

the cyclopropane ( 5 5 ) and i t

irradiation

the

shown that

di-z-methane

(54) on

of

(56-59).

irradiation of the

the

( 5 5 ) via a

cyclopropane products

of

isomerization

trans-cis

In

time.

systems Zimmerman and Kamathes

alkene

the

four

produced

on

is presumed that

( 5 4 ) proceeds

to

the

cyclopropane followed by secondary irradiation. Thus the four products arise from the biradical cyclopropyl affords

ring

bond.

product

( 6 0 ) formed on fission of a

Hydrogen

( 5 9 ) while

the

abstraction other

within

three,

this

(56-58).

are

formed by a 1.2-migration of the methoxy cumyl radical. Some evidence was obtained

to indicate that

these

three products

can arise by a disproportionation-recombination path.

The

triplet

state

methanoquinoline

di-z-methane

(61, 6 2 )

systems

reactivity have

been

of

the

studied.

The

results of the irradiations are shown in Schemes 3 and 4 where

it

can

be

seen

that

each

compound

usually

affords

two

products. The results indicate that the pyridine nitrogen does not

manifestly

alter

the

photochemical

behaviour

of

the

compounds. A detailed analysis of the reasons for the observed regioselectivity theory

.=.

is made

The

benzonorbornadienes Paquette

and

on

the basis

di-z-methane

of

molecular

reactivity

orbital

of

the

( 6 3 ) still excites considerable interest.

Burkegs

have

studied

the

triplet

state

reactivity of the derivatives ( 6 3 ) in an attempt to establish the influence of bridgehead substitution. Thus the irradiation of

(63a) results

in the formation of both

isomers

(64a) and

(65a) in 4 2 and 58 X

respectively. Similar yields are shown

for

of

the

irradiation

(63b)

when

the

two

products

are

obtained in similar yields to the above. However, irradiation of the derivative (63c) results in the formation of only the cyclized

product

(64c)

while

the

derivative

with

the

bridgehead cyano (63d) yields both (64d) and (65d) but in 10 and 90 X yields respectively. The authorsLm present arguments to explain the observed

specificities leading to products of

either bridgehead control or vinylic control of the biradical

257

11113: Photochemistry of Alkenes, Alkynes and Related Compounds

CO2BuS

R2 R'

R3

c

H

A

Ar2

C02Me

Ar' (75)

CO, Me

CO, Me I

C0,Me Ph

Ph phJ%: C02Me

(76)

Ph

C02Me

(77 1

Ph

(78)

R' = CO2Mc, R 2 = R' =

H

H, R 2 = C 0 , M

R

Meofrph \

(79)

Ph

(80) R' t CN, C0,Me R 2 = Ph or 2-pyridyl

(81) R X

= =

C N or C 0 2 M e CH or N

Photochemistry

258 intermediates

involved in the rearrangements. Other workerse'

have also studied the influence of substitution and ring size on the photochemical and

(67).

Both

reactivity of the bicyclic compounds ( 6 6 )

acetone-

or

acetophenone-sensitization

and

direct irradiation were used to effect the conversions and the triplet process w a s established

a s being more efficient. T h e

(68. 6 9 ) and ( 7 0 ,

regiospecificity of the reactions affording

7 1 ) were

studied

appropriate bicyclic

and the yields obtained are shown under the

structures. T h e authorsP6

compounds

( 6 6 e-g)

and

rearrange o r the rearrangement results were discussed

The

irradiation of

also observed that the

( 6 7 f-i)

either

was extremely

the optically active dibenzbarrelene isomeric products

ratio

(73)(atb):(73)(c+d)

products

to

in some detail.

in solution affords the four of

failed

inefficient. T h e

formed

(72)

( 7 3 a-d).

The

55:45.

is

Irradiation of a single crystal affords the same four products but

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