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Epilogue R. A . Marcus Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, C A 91125
THE FIELD OF ELECTRONT -RANSFER REACTIONS
has e x p a n d e d dramatically since the early days i n the late 1940s a n d the 1950s w h e n the rates of m a n y such reactions (isotopic-exchange reactions) w e r e s t u d i e d b y u s i n g isotopic l a b e l i n g t e c h n i q u e s . T h e chapters i n this v o l u m e demonstrate some of the m o r e recent d e v e l o p m e n t s . T h e y speak e l o q u e n t l y for themselves, a n d m y s u m m a r y w i l l be r e l a t i v e l y brief. T h e excellent prefactory chapters b y B o l t o n et a l . a n d b y B o l t o n a n d A r c h e r i n t r o d u c e the electron-transfer field. T h e latter surveys k e y concepts u n d e r l y i n g the t h e o r y for these reactions, together w i t h r e l e v a n t equations u s e d i n comparisons w i t h e x p e r i m e n t a l results. A m o n g the topics treated i n some d e t a i l i n the s y m p o s i u m a n d i n this v o l u m e are 1. factors i n f l u e n c i n g the effect of d o n o r - a c c e p t o r separation distance o n e l e c t r o n transfer ( E T ) reaction rates, the distance i n f l u e n c i n g the rate constant v i a b o t h electronic a n d reorganizational factors, 2. e l e c t r o n transfers i n p r o t e i n s , i n c l u d i n g p h o t o s y n t h e t i c systems, 3. the " i n v e r t e d effect" for E T rates, 4. comparisons of p h o t o i n d u c e d charge separation a n d charge r e c o m b i n a tion, 5. solvent a n d / o r t e m p e r a t u r e a n d m o l e c u l a r b r i d g e effects o n E T rates, 6. charge-transfer states i n p o r p h y r i n - c h l o r o p h y l l systems a n d t h e i r e n h a n c e m e n t of the q u e n c h i n g of locally excited states i n p o l a r solvent b y m i x i n g w i t h C T states, the role of p e r p e n d i c u l a r i t y i n f a v o r i n g the C T state,
0065-2393/91 /0228-0277$06.00/0 © 1991 American Chemical Society
In Electron Transfer in Inorganic, Organic, and Biological Systems; Bolton, James R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1991.
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E T IN INORGANIC, ORGANIC, A N D BIOLOGICAL SYSTEMS
7. solvent d y n a m i c s a n d i n t r a m o l e c u l a r E T , 8. the effect of a p p l i e d e l e c t r i c fields o n the long-range B C h l
2
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b i n a t i o n i n a photosynthetic system, a n d
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9. i n t r a m o l e c u l a r transfers i n v o l v i n g triplets.
T h i s v o l u m e thus embraces a b r o a d range of topics i n the e l e c t r o n transfer field. It is, of course, not possible to be a l l - i n c l u s i v e i n a r e l a t i v e l y s m a l l p u b l i c a t i o n . T h e r e are a n u m b e r of other active areas i n the e l e c t r o n transfer field: e l e c t r o n transfers at m e t a l - l i q u i d , s e m i c o n d u c t o r - l i q u i d , a n d l i q u i d - l i q u i d interfaces; E T o n s e m i c o n d u c t o r c o l l o i d a l particles, m i c e l l e s , a n d at m o d i f i e d m e t a l electrodes; E T w i t h b o n d r u p t u r e ; salt effects o n E T rates; c o m p u t e r simulations o f reorganizational a n d d y n a m i c a l aspects o f e l e c t r o n transfers; a n d the d e t a i l e d r e l a t i o n to charge transfer a n d p h o t o e l e c t r i c e m i s s i o n spectra. S e v e r a l recent results from o u r laboratory o n e l e c t r o n transfer i n l i q u i d - l i q u i d a n d s e m i c o n d u c t o r - l i q u i d systems a n d o n e l e c t r o n i c m a t r i x elements i n d o n o r - a c c e p t o r r i g i d m o l e c u l a r b r i d g e a n d p r o t e i n systems are d e s c r i b e d elsewhere ( I , 2). T h e w i d e s p r e a d g r o w t h i n t o n e w areas m a y strike m a n y observers of the electron-transfer field. N e w p r o b l e m s , n e w questions, a n d challenges c o n t i n u e to arise a n d are b e i n g addressed, b o t h e x p e r i m e n t a l l y a n d t h e o retically. Systems of i n c r e a s i n g c o m p l e x i t y , or systems at a greater l e v e l of m o l e c u l a r d e t a i l , are b e i n g e x a m i n e d . I n the early days of m o d e r n electron-transfer study (namely, i n the few decades after 1945) a p r i m a r y focus was d e l i n e a t i o n of the m a i n features of s i m p l e i n o r g a n i c e l e c t r o n transfers, the i m p a c t of the standard free e n e r g y of reaction o n t h e i r rate, a n d the effect of an " i n t r i n s i c reorganizational p a r a m e t e r " r e l a t e d to b o n d - l e n g t h changes a n d to m o l e c u l a r size. S u c h concepts as the extent of adiabaticity vs. nonadiabaticity w e r e also i m p o r t a n t , as was the r e l a t i o n s h i p b e t w e e n h o m o g e n e o u s a n d heterogeneous e l e c t r o n transfer rates. N o w , w i t h the a i d of the information a n d m e t h o d o l o g y d e r i v e d i n these e a r l i e r studies, m a n y areas are b e i n g e x p l o r e d , i n c l u d i n g those at various o t h e r interfaces a n d i n biological systems. C o m p u t e r simulations p r o v i d e a useful a d d e d s u p p l e m e n t to the e a r l i e r analytical-type (equations) d e v e l o p m e n t , a n d recent calculations of electronic effects have s u p p l e m e n t e d c u r r e n t e x p e r i m e n t a l w o r k o n long-range e l e c t r o n transfer. F u r t h e r c o m m u n i c a t i o n a m o n g researchers w o r k i n g i n rather different aspects of the E T field continues to be desirable. S o m e m a y not b e familiar, for e x a m p l e , w i t h the r e l a t i v e l y recent w o r k b y Iwasita et a l . (3) s h o w i n g adiabaticity for e l e c t r o n transfer b e t w e e n R u ( N H ) and metal electrodes. T h e y s h o w e d that the electron-transfer rate b e t w e e n i o n a n d e l e c trode (the "exchange c u r r e n t " ) was constant for metals h a v i n g w i d e l y different density of e l e c t r o n i c states. T h i s example may also serve to show 3
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In Electron Transfer in Inorganic, Organic, and Biological Systems; Bolton, James R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1991.
MARCUS
Epilogue
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h o w i n f o r m a t i o n c a n be r e a d i l y o b t a i n e d i n one area, whereas c o m p a r a b l e information for homogeneous b i m o l e c u l a r E T s is less r e a d i l y d e r i v e d .
References
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 19, 2016 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch018
1. Marcus, R. A . / . Phys. Chem. 1990, 94, 1050, 4152. 2. Siddarth, P.; Marcus, R. A . / . Phys. Chem. 1990, 94, 2985, 8430. Iwasita, T.; Schmickler, W.; Schultz, J . W. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 138. R E C E I V E D May 17, 1990.
In Electron Transfer in Inorganic, Organic, and Biological Systems; Bolton, James R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1991.
