Electronic Energy Relaxation in Aromatic Vinyl Homopolymers

Chapter 18

Electronic Energy Relaxation in Aromatic Vinyl Homopolymers

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H. F. Kauffmann, W.-D. Weixelbaumer, J . Bürbaumer, and B. Mollay Institut für Physikalische Chemie, Universität Wien, Wahringerstrasse 42, A-1090 Wien, Austria

Transient fluorescence spectroscopy on nanosecond and picosecond time scales has been used in an attempt to study the pathways of excitation energy transport (EET) and trapping for poly-(l-vinylnaphthalene), p-VN, and poly-(N-vinylc arbazole), p-Ν-VCz, in dilute fluid solu tion. Excited state localization has been probed by col lecting the transient fluorescence patterns of excimer -forming-sites (EFS) in typical trapping experiments. Fluorescence r i s e - p r ο f i l e s of both mobile and static excimers have been analyzed on the premises of low -dimensional transport topology and "effective" diagonal disorder of aromatic hopping sites. A trapping function of the form k(t) = b + ct , a distribution of tran sport states {X } and a small ensemble of energy-relaxed, monomeric tail-states have been processed in kinetic schemes. The calculated profiles are nonexponential and can recover satisfactorily the experimental curves. The rationale behind - a time-dependent excited-state random walk among energy-dispersive chromophores - has been dis cussed. -1/2

1

E l e c t r o n i c e x c i t a t i o n s i n m o l e c u l a r a g g r e g a t e s have received considerable attention from t h e o r e t i s t s and experimentalists i n r e c e n t y e a r s . Much o f t h e work h a s f o c u s e d on e x c i t o n i c t r a n s p o r t i n m o l e c u l a r c r y s t a l s (J_), i n s u b s t i t u t i o n a l l y d i s o r d e r e d m i x e d c r y s t a l s (2.) a n d i n amorphous s t r u c t u r e s (3.). F u r t h e r m o r e , e x c i t a t i o n e n e r g y t r a n s f e r (EET) has been s t u d i e d i n f l u o r e s c e n c e c o n c e n t r a t i o n d e p o l a r i z a t i o n o f donor m o l e c u l e s i n s o l u t i o n (i ) as w e l l a s i n e x c i t e d - s t a t e energy t r a n s f e r o f donor a c c e p t o r dyes i n condensed p h a s e s y s t e m s (ϋ). NOTE: Dedicated to Professor Adolf Ncckel on the occasion of his 60th birthday.

0097-6156/87/0358-0220$06.50/0 © 1987 American Chemical Society

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

18.

KAUFFMANN ET AL.

Electronic Energy Relaxation

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The e f f e c t o f d i s o r d e r on e x c i t a t i o n e n e r g y t r a n s p o r t in polychromophoric arrays i s a formidable theoretical p r o b l e m and has b e e n i n v e s t i g a t e d e x t e n s i v e l y . S p e c i a l e m p h a s i s has been g i v e n t o t h e d y n a m i c a l n a t u r e o f m i g r a t i v e m o t i o n (6.). F o r i n f i n i t e s y s t e m s o f d o n o r ensembles, r a n d o m l y d i s t r i b u t e d and e n e r g e t i c a l l y e q u i v a l e n t (posi tional disorder), i n c o h e r e n t e v o l u t i o n of electronic t r a n s p o r t has been f o r m u l a t e d and a n a l y z e d i n t e r m s o f s y m m e t r i c P a u l i m a s t e r e q u a t i o n s (PME) ( X ) . F r o m d i a g r a m m a t i c (8.) a n d p e r t u r b a t i v e a p p r o a c h e s (2.), b u t a l s o from c o n t i n u o u s t i m e r a n d o m w a l k a n a l y s e s ( C T R W ) (±Q_ ) a n d f r a c t a l arguments ( J J J t h e r e i s a g r e e m e n t now that elec t r o n i c t r a n s p o r t i s n o n d i f f u s i v e , i . e. t i m e - v a r i a n t , a n d b e c o m e s d i f f u s i v e , i . e. t i m e - i n d e p e n d e n t , a f t e r a r e l a t i v e l y l o n g t i m e c o m p a r a b l e t o an a v e r a g e h o p p i n g t i m e o f e x c i t a t i o n . I n t r i n s i c e x c i t e d s t a t e d i s p e r s i o n of random, p o s i t i o n a l d i s o r d e r - e v i d e n c e d by t y p i c a l n o n e x p o n e n t i a l t r a n s i e n t f l u o r e s c e n c e - has been e x p e r i m e n t a l l y v e r i f i e d i n dye s o l u t i o n s ( 1 2 - 1 4 ) , w h e r e e n e r g e t i c f l u c t u a t i o n o f m o l e c u l a r s e l f e n e r g i e s a r e s m e a r e d o u t by t h e r m a l a c t i vation. The s i t u a t i o n i s e v e n m o r e c o m p l e x f o r m o l e c u l a r ag g r e g a t e s f o r w h i c h s p a t i a l , random f l u c t u a t i o n s of i n t e r molecular potentials significantly exceed kT. Such systems are s a i d t o posess d i a g o n a l d i s o r d e r where p o s i t i o n a l d i s o r d e r causes a pronounced spread of s i t e ener g i e s and, t h u s , i n a d d i t i o n , e n e r g e t i c d i s o r d e r ( 15. ). D i s p e r s i o n as a c o n s e q u e n c e o f d i s t r i b u t i o n o f s e l f - e n e r g i e s i s w e l l d o c u m e n t e d by t h e i n h o m o g e n e o u s ( G a u s s i a n ) b r o a d e n i n g o f o p t i c a l S ^ *— S transitions in vapor con densed o r g a n i c 7r-systems (16). F u r t h e r e x p e r i m e n t a l mani f e s t a t i o n s of e n e r g e t i c d i s o r d e r are f l u o r e s c e n c e line narrowing ( H ) , the i d e n t i f i c a t i o n of t y p i c a l mobility e d g e s (J_8_*. 12.) a s w e l l a s e n e r g e t i c n a r r o w i n g o f t i m e gated, t r a n s i e n t e m i s s i o n s p e c t r a (20). The s p r e a d o f s i t e e n e r g i e s i n s y s t e m s o f d i a g o n a l d i s o r d e r i s o f c o n s i d e r a b l e i m p a c t on t h e s p a c e - t i m e e v o l u t i o n o f an e l e m e n t a r y e x c i t a t i o n . Ε η e r g y - d i s ρ e r s i ν e excited-state walks are involved in a hierarchy of s e q u e n t i a l steps of g r a d u a l l y d e c r e a s i n g event times which have been r e c o g n i z e d to produce n o n d i f f u s i v e , i . e., t i m e - d e p e n d e n t phenomena, i n g e n e r a l . A t r a n s i e n t o b s e r v a b l e i s e x p e c t e d , t h e r e f o r e , t o show a nonexpo n e n t i a l r e l a x a t i o n p r o f i l e on r e l e v a n t t i m e s c a l e s . None x p o n e n t i a l decay a n a l y s i s i n random s y s t e m s w i t h e n e r g e t i c d i s o r d e r has been a c h i e v e d , i n p a r t i c u l a r , for t r i p l e t - e x c i t a t i o n s ( 2 1 , 22.) w h i c h a r e - b e c a u s e o f t h e i r r e d u c e d h o p p i n g t i m e s and t h e i r l a r g e i n h e r e n t l i f e t i m e s conveniently a c c e s s i b l e to experimental i n v e s t i g a t i o n s . F o r an a s s e m b l y o f d o n o r m o l e c u l e s c o v a l e n t l y f i x e d i n close, l o c a l proximity to a macromolecular environment, s i m i l a r s u c c e s s f u l t h e o r e t i c a l t r e a t m e n t s - as outlined above f o r r a n d o m i z e d a r r a y s o f i n d e p e n d e n t donors ( 8 - 1 1 ) h a v e n o t b e e n p e r f o r m e d , s o f a r . A m o n g ρο 1 y c h r ο m ο h ο r i c s y s t e m s s e t up by c o r r e l a t e d d o n o r s i t e s a r o m a t i c v i n y l Q


222

PHOTOPHYSICS OF POLYMERS

homopolymers a r e a s p e c i a l c l a s s o f s y n t h e t i c , linear macromolecules i n which the a r o m a t i c subsystem, s u c h as benzene, naphthalene or c a r b a z o l e i s c h e m i c a l l y l i n k e d to t h e m a i n c h a i n and p e r i o d i c a l l y s e p a r a t e d by t h r e e s a t u r a t e d c a r b o n a t o m s . Ρ ο 1 y - ( s t y r e n e ) , p-S, poly-(vinyln a p h t h a l e n e s ) , p-VN, o r p o l y - ( N - v i n y l c a r b a z o l e ) , p-N-VCz, are prominent examples. Because of the i n c o r p o r a t i o n of dyads of d i f f e r e n t t a c t i c i t y these polymers are stereo c h e m i c a l ^ not pure. In t o t o , they r e p r e s e n t p r e d o m i nantly disordered finite-size, many-ρartic1e systems superimposed by i n t e r f e r i n g local domains of slight short-range order. Consequently, t h i s r e s u l t s i n s i g n i f i cant p o s i t i o n a l h e t e r o g e n e i t y l e a d i n g to s p a t i a l distri b u t i o n o f c h r o m o p h o r e s w h i c h i s n e i t h e r random nor p e r i o d i c a l f o r the m a j o r i t y of e x p e r i m e n t a l c o n d i t i o n s . Therefore, for a single excitation statistically c r e a t e d a m i d s t a swarm of polymer-bound (identical) aro m a t i c s i t e s i n a h o m o p o l y m e r , g e o m e t r i c and energetic i n h o m o g e n e i t y must i n d u c e even more c o m p l e x t r a n s p o r t d y n a m i c s as c o m p a r e d t o r a n d o m , a m o r p h o u s m a t e r i a l s , and t h e r e does not yet e x i s t - f o r low d i m e n s i o n a l t r a n s p o r t media, i n p a r t i c u l a r , a comprehensive u n d e r s t a n d i n g on a microscopic level. S t a t i s t i c a l correlation effects, local semi-coherent evolutions, a distribution of v a r i o u s transport dimensionalities, e n e r g e t i c d i s o r d e r and nonequilibrium as w e l l as t h e f i n i t e v o l u m e f o r p o l y m e r s , i n g e n e r a l , a r e s e r i o u s p r o b l e m s w h i c h make t h e a n a l y s e s o f s t o c h a s t i c m a s t e r e q u a t i o n s p r a c t i c a l l y i m p o s s i b l e . So f a r , o n l y i s o e n e r g e t i c h o p p i n g p r o c e s s e s b a s e d on a p p r o x i m a n t s o f GAFa n d L A F - e x p a n s i o n s (8.) h a v e b e e n a n a l y z e d i n p o l y m e r i c m a t e r i a l s , p r o v i d e d the chromophores a r e r a n d o m and c h r o m o p h o r e d e n s i t y i s s m a l l ( 2 ^ - 2 6 ) .

TRANSIENT FLUORESCENCE IN AROMATIC

VINYLPOLYMERS

While m i c r o s c o p i c t h e o r i e s designed to model the dyna mics of e l e c t r o n i c t r a n s p o r t i n the h i g h d e n s i t y l i m i t of polymer-bound chromophores are r a t h e r underdeveloped u n t i l r e c e n t l y , the phenomenological aspects of e l e c t r o n i c e n e r g y d i f f u s i o n and e x c i t o n i c s t a t e s i n a r o m a t i c vinylpolymers h a v e now been a t o p i c of fundamental i n t e r e s t o v e r a p e r i o d o f t w e n t y y e a r s (2.2.)· E x c i t a t i o n t r a n s f e r was c l a i m e d f r o m s t u d i e s o f f l u o r e s c e n c e d e p o l a r i z a t i o n (2.8.) a n d i t was h y p o t h e t i s i z e d f r o m e x p e r i m e n t s b a s e d u p o n e n e r g y t r a n s f e r ( 2SL ) ) , d y n a m i c a l quenching (3_0) a n d e x c i m e r f o r m a t i o n (3J_). F r o m l i t e r a t u r e o f p o l y mer p h o t o p h y s i c s t h e n a t u r e o f p o l y m e r - i n h e r e n t t r a p s h a s been d i s c u s s e d a l m o s t e x c l u s i v e l y i n terms o f w e l l e s t a b lished s t e r e o c h e m i c a l c o n c e p t s of physical-organic c h e m i s t r y . I n t h e l i m i t o f b i c h r o m o p h o r i c i n t e r a c t i o n an e l e c t r o n i c t r a p c a n be c o n s i d e r e d t o be a l o c a l i z e d s t a t e o f an e x c i m e r o r a n e x c i m e r - f o r m i n g - s i t e , E F S , (3_2). E F S r e f e r t o ρο 1 y m e r - i n h e r e n t , i n t r i n s i c c o n f o r m a t i o n s o f d i s t i n c t p a i r s of chromophores, with different geometries


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K A U F F M A N N ET AL.


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and d e n s i t y n u m b e r s . T h e y a r e i m m o b i l i z i n g t h e h o p p i n g p r o c e s s by a b s o r b i n g t h e e x c i t e d s t a t e e n e r g y i n t h e i r p o t e n t i a l s i n k s , and a f t e r w a r d s , they a r e d i s s i p a t i n g t h e excitation via significant red shifted fluorescence i n a subsequent step. Because of i t s m i g r a t i v e nature the t r a p p i n g p r o c e s s i s a v e r y e f f i c i e n t e v e n t i n an a r o m a t i c homopolymer. Since t h e quantum y i e l d s o f r a d i a t i o n a r e considerably high for excimers, the dominant fluorescence in a CW-experiment i s mostly e x c i m e r i c , i n nature. Pulsed e x c i t a t i o n techniques have been a p p l i e d t o s y n t h e t i c p o l y m e r s o v e r a p e r i o d o f 10 y e a r s now w i t h t h e o b j e c t i v e t o s t u d y t h e r e p o n s e - l a w s o f monomer- and e x c i mer s t a t e s . S p e c i a l emphasis has been g i v e n t o t i m e resolved fluorescence of aromatic polymers i n dilute, fluid s o l u t i o n . The p r i m a r y aspect of early investi g a t i o n s was c o n c e r n e d w i t h t h e e v a l u a t i o n o f segmental r o t a t i o n on t h e b a s i s o f t r a n s i e n t B i r k s k i n e t i c s . F o r the m a j o r p a r t o f t h e s e p o l y m e r s , h o w e v e r , a more a c c u r a t e d a t a a n a l y s i s h a s r e v e a l e d a more c o m p l e x e x c i t e d state dynamics. I t i s g e n e r a l l y observed i n these poly m e r s , t h a t t h e l o w - i n t e n s i t y , m o n o m e r f l u o r e s c e n c e c a n be f i t t e d more s a t i s f a c t o r i l y by a sum o f t h r e e e x p o n e n tials, while f o r the consecutive profiles of excimer f l u o r e s c e n c e , q u i t e o b v i o u s l y , more t h a n two e x p o n e n t i a l s are r e q u i r e d t o recover t h e data. T h i s i s i n d i c a t i v e o f a d i s c r e p a n c y between e x p e r i m e n t and k i n e t i c scheme w h i c h , q u i t e o b v i o u s l y , shows that - even i n t h e absence o f energy m i g r a t i o n - r o t a t i o n a l motion between c o r r e l a t e d c h r o m o p h o r e s c a n n o t be m o d e l e d i n a n a l o g y t o c o l l i s i o n a l l y induced, t r a n s l a t o r y d i f f u s i o n o f i n d e p e n d e n t and r a n d o m l y d i s t r i b u t e d , 1οw-mο 1ecu1 ar c h r o m o p h o r e s . The s i g n i f i c a n t d e v i a t i o n s from conventional Birks kinetics h a v e l e d t o an i n c r e a s e d d i s c u s s i o n as t o t h e n a t u r e o f p o l y m e r i c S-j-states and t h e i r i n t r i n s i c r e l a x a t i o n chan n e l s . I n t h e meantime, these k i n e t i c c o m p l i c a t i o n s have been p a r t i a l l y o v e r c o m e by s t r a t e g i e s w h i c h c o r r e l a t e t h e number o f e x p o n e n t i a l s n u m e r i c a l l y e x t r a c t e d from reconv o l u t i o n p r o c e d u r e s (3-1 ) t o t h e n u m b e r o f k i n e t i c t r a n s i e n t s i n l i n e a r c o u p l e d s c h e m e s . The r o l e o f p h y s i c a l t r a n s i e n t s h a v e b e e n d i s c u s s e d i n t e r m s o f a) p a r t i a l o v e r l a p h i g h - e n e r g y e x c i m e r s (3_4) > b) r o t a t i o n a l i s o m e r i c states of bichromophor i c , excited conformations (15.) a n d c) monomer-like excited states i n s t a t i s t i c a l t r i a d s of aromatic c o p o l y m e r s (1_6). Typical m u 11ieχ ρο ηeηtia 1 f l u o r e s c e n c e convolutes s t u d i e d , more r e c e n t l y , i n o u r l a b o r a t o r y a r e g i v e n i n F i g u r e s 1 a - 1 c . F i g u r e 1a r e f e r s t o t h e l o w - e n e r g y e x c i m e r f l u o r e s c e n c e o f p-1-VN (5.10 base molar i n benzene, 25°C) a t e m i s s i o n w a v e l e n g t h s a r o u n d i * = 4 7 0 nm. T h e n s p r o f i l e h a s b e e n a c u m u l a t e d b y means" o f c o n v e n t i o n a l f l a s h - l a m p e x c i t a t i o n ( ?ι = 2 9 5 nm, t y p i c a l pulsewidth o f 1.5 n s FWHM) a n d s i n g l e - p h o t o n - t i m i n g d e t e c t i o n (SPT). N u m e r i c a l r e c o n v o l u t i o n y i e l d s a b e s t - f i t three expo n e n t i a l form according to p

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PHOTOPHYSICS OF POLYMERS

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AUTOCORRELATION OF R E S I D U A L S

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K A U F F M A N N E T AL.

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Electronic Energy Relaxation in Aromatic Vinyl Homopolymers

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