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THE EXCITED STATE OF CIS-THIOINDIGO
A. Corval, H. Trommsdorff
To cite this version:
JOURNAL
DE
PHYSIQUE
Colloque C7, supplkment a u nOIO, Tome 46, o c t o b r e 1985 page C 7 4 4 7
T H E EXCITED STATE OF CIS-THIOINDIGO
A. C o r v a l and H.P. TrommsdorffLaboratoire de Spectrome'trie physique+, Universite' Scientifique e t Me'dicaZe
de GrenobZe,
B.P. 8 7 , 38402Saint-Martin drHe'res Cedex, France
RBsumb - Dans l e c a d r e de l ' b t u d e d e l a p h o t o i s o m Q r i s a t i o n c i s - t r a n s r Q v e r - s i b l e du t h i o i n d i g o , d e s m e s u r e s s p e c t r o s c o p i q u e s B b a s s e t e m p e r a t u r e d e s i s o m e r e s c i s e t t r a n s i s o l b s en m a t r i c e o n t Q t d r b a l i s d e s . C ' e s t l a p r e m i s r e o b s e r v a t i o n , p o u r l l i s o m Q r e c i s , d e s p e c t r e s a y a n t une s t r u c t u r e v i b r a t i o n - n e l l e r B s o l u e . D i f f d r e n t e s e v a l u a t i o n s du rendement q u a n t i q u e d e f l u o r e s - c e n c e de c e composb i n d i q u e n t que l a d u r 6 e de v i e d e l ' b t a t e x c i t d e s t dans l e domaine s u b p i c o s e c o n d e . A b s t r a c t - S p e c t r o s c o p i c measurements a t low t e m p e r a t u r e of m a t r i x i s o l a t e d c i s - a n d t r a n s - t h i o i n d i g o a r e p r e s e n t e d and a r e d i s c u s s e d i n r e l a t i o n t o t h e p h o t o i n d u c e d r e v e r s i b l e i s o m e r i z a t i o n o f t h i s m o l e c u l e . For t h e c i s i s o m e r t h e s e d a t a r e p r e s e n t t h e f i r s t v i b r a t i o n a l l y r e s o l v e d s p e c t r a . E s t i m a t e s o f t h e e m i s s i o n quantum y i e l d f o r t h i s s p e c i e s show t h a t t h e e x c i t e d s t a t e d e c a y s on a s u b p i c o s e c o n d t i m e s c a l e . I - INTRODUCTIDI The r e v e r s i b l e t r a n s - c i s p h o t o i s o m e r i z a t i o n of some i n d i g o d y e s i s a p r o t o t y p e example of l i g h t i n d u c e d c o n f o r m a t i o n a l changes i n p o l y a t o m i c m o l e c u l e s and h a s been e x t e n s i v e l y s t u d i e d .
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While t h e t r a n s i s o m e r js w e l l c h a r a c t e r i z e d and t h e t r a n s + c i s r e a c t i o n i s g e n e - r a l l y t h o u g h t t o p r o c e e d v i a t h e t r i p l e t s t a t e /1/2/3/, l i t t l e i s known a b o u t t h e i n v e r s e r e a c t i o n /4/. The m e t a s t a b l e c i s i s o m e r [ l i f e t i m e i n t h e r a n g e o f m i n u t e s i n v a r i o u s s o l v e n t s a t room t e m p e r a t u r e ) i s n o t w e l l c h a r a c t e r i z e d : n o v i b r a t i o - n a l l y r e s o l v e d s p e c t r a e x i s t , no e m i s s i o n has been d e t e c t e d , and t h e e x c i t e d s t a t e p o t e n t i a l i s h y p o t h e t i c a l .We have s t a r t e d an e x t e n s i v e i n v e s t i g a t i o n of t h i o i n d i g o : f o r t h e s t a b l e t r a n s i s o m e r t h e a b s o r p t i o n and f l u o r e s c e n c e s p e c t r a a r e v e r y r e g u l a r , t h e v i b r a t i o n a l f r e q u e n c i e s change o n l y l i t t l e upon e l e c t r o n i c e x c i t a t i o n and r e m a i n harmonic up t o s e v e r a l q u a n t a i n b o t h t h e ground and e x c i t e d e l e c t r o n i c s t a t e , i n d i c a t i n g t h a t t h e changes i n t h e p o t e n t i a l a r e s m a l l .
The p r e s e n t work i s c o n c e r n e d w i t h t h e c i s i s o m e r : s t a b i l i z e d by m a t r i x i s o l a t i o n , v i b r a t i o n a l s p e c t r a i n t h e ground and e x c i t e d s t a t e have been o b t a i n e d . Emission was d e t e c t e d and e s t i m a t e s of t h e e m i s s i o n quantum y i e l d c o u l d b e made.
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JOURNAL
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I1 - EXPERIMENTAL Sample p r e p a r a t i o n :-
F o r t h e a b s o r p t i o n s p e c t r a : a s o l u t i o n o f t h i o i n d i g o i n n-nonane i s i r r a d i a t e d d u r i n g 1 m i n u t e w i t h a 150 W i o d i n e lamp, u s i n g a f i l t e r t o e l i m i n a t e A >.500 nm, and t h e n q u i c k l y p l u n g e d i n t o l i q u i d n i t r o g e n .-
F o r t h e e m i s s i o n s p e c t r a : t h e sample o f t h i o i n d i g o i n l i q u i d 2 , 5 - d i p h e n y l o x a z o l e ( ; M.P. : 7Z°C) i s i r r a d i a t e d and r a p i d l y c o o l e d by s p u t t e r i n g on a g l a s s p l a t e . I n b o t h s o l u t i o n s i t was v e r i f i e d t h a t t h e p h o t o i n d u c e d s p e c i e s , a s c r i b e d t o t h e c i s i s o m e r , r e v e r t s i n t h e d a r k t o t r a n s - t h i o i n d i g o . The i r r a d i a t e d samples s t i l l c o n t a i n a l a r g e amount o f t r a n s - t h i o i n d i g o .I11 - ABSORPTION SPECTRA
F i g . 1 shows a b s o r p t i o n s p e c t r a o f t r a n s a n d c i s - t h i o i n d i g o i n n-nonane. The non- i r r a d i a t e d sample c o n t a i n s t r a n s - t h i o i n d i g o o n l y . The amount o f t h e c i s i s o m e r i n t h e i r r a d i a t e d sample i s e s t i m a t e d t o be a t l e a s t 15% o f t h e t o t a l amount o f t h i o i n d i g o .
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(A) F i g . 1 - A b s o r p t i o n s p e c t r a o f t r a n s - and c i s - t h i o i n d Z g a I n a n-nanane m a t r i x a t 4 K. T h i s i s t h e f i r s t v i b r a t i o n a l l y r e s o l v e d a b s o r p t i o n s p e c t r u m o f c i s - t h i o i n d i g o . I t shows one p r o g r e s s i o n w i t h a s p a c i n g of 500 cm-': t h e FWHM o f t h e l i n e s v a r i e s f r o m 300 t o 400 cm-'. The t r a n s i s o m e r under i d e n t i c a l c o n d i t i o n s e x h i b i t s l i n e s o f 15 cm-'width. The inhomogeneous c o n t r i b u t i o n s t o t h e w i d t h o f t h e c i s s p e c t r u m a r e t h u s t h o u g h t t o be n e g l i g i b l e . The o b s e r v e d l i n e w i d t h c o u l d be due t o a s t r o n g e l e c t r o n - p h o n o n c o u p l i n g and/or t o a v e r y r a p i d decay ( 2 0 f s l o f t h e e x c i t e d s t a t e[see b e l o w ) .
I V - EMTSSION SPECTRA
I n a d d i t i o n t o t h e b r o a d e m i s s i o n background, s h a r p l i n e s a r e o b s e r v e d , most o f w h i c h c o r r e s p o n d t o Raman l i n e s o f t h e m a t r i x ; some Raman l i n e s o f b o t h t h i o i n d i g o i s o m e r s c o u l d a l s o b e i d e n t i f i e d .
F i g . 2 - F l u o r e s c e n c e s p e c t r a o f c i s - and t r a n s - t h i o i n d i g o i n a DPO m a t r i x a t 1 . 6 K . E x c i t a t i o n = 4 8 8 0
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e x p a n s i o n i n f i g . 3.These measurements a r e used t o make two i n d e p e n d e n t e v a l u a t i o n s o f t h e e m i s s i o n q u a n - tum y i e l d o f t h e c i s i s o m e r .
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We can e s t i m a t e t h a t a t t h e w a v e l e n g t h of e x c i t a t i o n [ 4 8 8 0a)
b o t h i s o m e r s a b s o r b e q u a l amounts o f l i g h t ; t h e r a t i o o f t h e i n t e g r a t e d e m i s s i o n s a t t r i b u t e d t o t h e two s p e c i e s t h e r e f o r e e q u a l s t h e r a t i o o f t h e e m i s s i o n quantum y i e l d s . A s t h e f l u o r e s c e n c e quantum y i e l d o f t r a n s - t h i o i n d i g o i s known t o be +trans = U.7 / 5 / ,F we o b t a i n f o r t h e c i s - i s o m e r < 7 * 1 0 - ~ .
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81 A second e s t i m a t e o f t h i s quantum y i e l d can b e made u s i n g t h e i n t e n s i t y o f a Raman l i n e o f t h e m a t r i x f o r t h e c a l i b r a t i o n /6/ :
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m o l e c u l e s , and t h e i n t e n s i t y o f t h e c o n s i d e r e d m a t r i x Rarnan l i n e . oabs and I a r e F r e s p e c t i v e l y t h e a b s o r p t i o n c r o s s s e c t i o n and t h e f l u o r e s c e n c e i n t e n s i t y of t h e c i s i s o m e r , and N i s t h e number of c i s m o l e c u l e s .
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JOURNALDE PHYSIQUE
The c o n c e n t r a t i o n o f t h i o i n d i g o was m e a s u r e d by t a k i n g a b s o r p t i o n s p e c t r a i n s o l u - t i o n , a n d t h e r e l a t i v e c o n c e n t r a t i o n and t h e a b s o r p t i o n c r o s s s e c t i o n o f t h e c i s i s o - mer w e r e e v a l u a t e d f r o m o u r a n d l i t t e r a t u r e s p e c t r a l d a t a /7/. T h i s d e t e r m i n a t i o n y i e l d e d % F B o t h e v a l u a t i o n s o f t h i s q u a n t i t y s u f f e r f r o m t h e f a c t t h a t t h e m e a s u r e m e n t s a r e m a d e on s t r o n g l y s c a t t e r i n g p o l y c r y s t a l l i n e m a t e r i a l and t h a t d i f f e r e n t a m o u n t s o f e a c h i s o m e r may n o t b e m a t r i x i s o l a t e d b u t a r e r e j e c t e d f r o m t h e m a t r i x i n t h e p r e p a r a - t i o n p r o c e s s . Measurements o n room t e m p e r a t u r e s o l u t i o n s , w h i c h a v o i d t h e s e p r o b l e m s , a r e l i m i t e d i n t h e p r e s e n t c a s e by t h e f a c t t h a t , a t h i g h t e m p e r a t u r e s , t h e v e r y w e a k c i s e m i s s i o n becomes masked by t h e s t r o n g e r t r a n s . e m i s s i o n . F i g . 3 i s a blow-up o f t h e s p e c t r a l r e g i o n A i n F i g . 2 , w h e r e s t r o n g Raman l i n e s o f b o t h i s o m e r s p e c i e s c a n b e 7 o b s e r v e d . 4-
cis Aexc=5145 4 5 0 490 530 F i g . 3 - R e s o n a n c e Raman s p e c t r a o f t h i o i n d i g o i n a DPO m a t r i x a t 1 . 6 K .F o r c i s - t h i o i n d i g o a 490 cm-I mode g i v e s r i s e t o t h e s t r o n g e s t Raman l i n e and i s ob- s e r v e d a l s o i n two q u a n t a ; v e r y l i k e l y i t c o r r e l a t e s w i t h t h e mode a p p e a r i n g i n t h e s t r o n g 500 cm-I p r o g r e s s i o n i n a b s o r p t i o n . The c r o s s s e c t i o n o f t h i s l i n e was m e a s u r ed t o b e 2.10-31 cm2 m o l e c u l e - I ( h e x c = 4880 A ) . A l l o t h e r Raman l i n e s a r e c o n s i d e - r a b l y w e a k e r and a r e a t o r b e l o w o u r d e t e c t i o n l i m i t . V
-
DISCUSSION The q u a n t i t a t i v e e v a l u a t i o n o f v e r y weak e m i s s i o n i s s u b j e c t t o g r e a t u n c e r t a i n t i e s ; i n t h e p r e s e n t s t u d y i t becomes e v e n more d i f f i c u l t d u e t o t h e s t r o n g f l u o r e s c e n c e o f t h e a l w a y s p r e s e n t t r a n s i s o m e r . Our r e s u l t s do n e v e r t h e l e s s c l e a r l y i n d i c a t e t h a t t h e e m i s s i o n quantum y i e l d o f c i s - t h i o i n d i g o i s v e r y low, l y i n g i n t h e r a n g e o f t o T h e l i f e t i m e o f t h e f i r s t e x c i t e d s t a t e t h e r e f o r e i s v e r y s h o r t (6 200 f s l . a s t h e r a d i a t i v e l i f e t i m e c a n b e e s t i m a t e d t o b e s i m i l a r f a r t h e two i s o m e r s and i s 1known t o b e - = 2 0 n s /5/ f o r t r a n s - t h i o i n d i g o . S u c h a s h o r t l i f e t i m e would make a kr
t o r s i o n a l mode. We s u g g e s t t h a t t h i s r a p i d decay i s i n d i c a t i v e o f an u n s t a b l e n u c l e a r c o n f i g u r a t i o n o f t h e e x c i t e d s t a t e o f t h e c i s i s o m e r a l o n g t h i s c o o r d i n a t e . The ex- a c t pathway o f t h i s r a p i d e l e c t r o n i c r e l a x a t i o n p r o c e s s r e m a i n s however t o b e d e t e r - mined. ACKNOWLEDGEMENT We a r e g r a t e f u l t o W. L t t t k e , f o r many i n t e r e s t i n g d i s c u s s i o n s r e g a r d i n g i n d i g o d y e s . and f o r p r o v i d i n g us w i t h t h e t h i o i n d i g o samples. REFERENCES
/I/ Grellmann, K. H. and H e n t z s c h e l , P., Chem. Phys. L e t t .
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(19781 545. /2/ GGrner, H. and S c h u l t e - F r o h l i n d e , 0.. Chem. Phys. L e t t .66
(19791 363./3/ K a r s t e n s , T., Kobs, K. and Mernming, R., Ber. Bunsenges. Phys. Chern.
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(19791 504./4/ Memming, R. and Kobs, K., Ber. Bunsenges. Phys. Chem.
