Solvent, Temperature, and Bridge Dependence of Photoinduced

7 Solvent, Temperature, and Bridge Dependence of Photoinduced Downloaded by CORNELL UNIV on November 17, 2012 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch007

Intramolecular Electron Transfer James R. Bolton , John A. Schmidt , Te-Fu Ho , Jing-yao L i u , Kenneth J. Roach , Alan C. Weedon , Mary D. Archer , Jacquin H . Wilford , and Victor P. Y. Gadzekpo 1

1

1

2

1

2

1

1

1

2

2

Photochemistry Unit, Department of Chemistry, University of Western Ontario London, Ontario Ν6Α 5B7, Canada Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EP, United Kingdom

Photoinduced intramolecular electron-transfer rate constants were determined for several PLQ (tetraarylporphine linked to p-benzoquinone) molecules. Rate constants for PAQ (porphyrin-amidequinone) vary significantly with solvent and temperature. Most results can be explained within the context of the high-temperaturelimit semiclassical Marcus equation. Analysis of the temperaturedependent data reveals that the electronic coupling energy Η varies significantly with solvent. This variability expfains the considerable scatter found in the solvent-dependent studies. Electron-transfer rate constants, determined for five other PLQ molecules, exhibit the fol lowing characteristics: (1) solvent dependence is broadly similar to that of PAQ; (2) peptide linkages are much more effective than a saturated linkage, such as bicyclobutane; (3) a phenyl linkage is the most effective, generating rate constants 100-1000 times that of a bicyclooctane linkage; and (4) the strained cyclobutane bridge is more effective than a corresponding unstrained saturated linkage. rp

THE DESIGN OF COVALENTLY LINKED

d o n o r - a c c e p t o r m o l e c u l e s to m i m i c the p r i m a r y electron-transfer process i n photosynthesis has r e c e i v e d i n creasing interest over the past decade; réf. 1 is a c o m p r e h e n s i v e r e v i e w of

0065-2393/91/0228-0117$06.00/0

© 1991 American Chemical Society

In Electron Transfer in Inorganic, Organic, and Biological Systems; Bolton, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1991.

118

E T IN INORGANIC, ORGANIC, A N D BIOLOGICAL SYSTEMS

Downloaded by CORNELL UNIV on November 17, 2012 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch007

this field u p to 1988. T h e p r i m a r y objective has b e e n to d e s i g n d o n o r - a c c e p t o r ( D - A ) molecules i n w h i c h the f o r w a r d p h o t o i n d u c e d e l e c t r o n transfer ( P E T ) process is v e r y r a p i d w h i l e the reverse electron-transfer ( E T ) rate back to the o r i g i n a l state is v e r y slow. T o the extent that this reverse reaction is i m p o r t a n t , the o v e r a l l efficiency a n d q u a n t u m y i e l d o f any e n e r g y storing process w i l l b e r e d u c e d . T h e s e m o d e l c o m p o u n d s have p r o v i d e d an excellent " l a b o r a t o r y " to study the P E T m e c h a n i s m a n d the d e p e n d e n c e of E T , rate constants o n various factors i n the m o l e c u l a r structure a n d e n v i r o n m e n t . T h e M a r c u s t h e o r y o f e l e c t r o n transfer (see C h a p t e r 2 for an exposition) has p r o v e n to b e a n excellent f r a m e w o r k for the i n t e r p r e t a t i o n o f the E T data. T h e d e p e n d e n c e factors i n c l u d e

1. E x c i t e d - s t a t e energy. G e n e r a l l y , the h i g h e r the excited-state energy, the faster the rate constant w i l l be. 2. E x e r g o n i c i t y ( - A G ) . A s p r e d i c t e d from M a r c u s t h e o r y , E T rate constants increase w i t h i n c r e a s i n g exergonicity u p to a m a x i m u m w h e r e -AG = X (the reorganization energy). T h e E T rate constants t h e n decrease for larger exergonicities i n the so-called " M a r c u s i n v e r t e d r e g i o n " . 0

0

3. D i s t a n c e b e t w e e n d o n o r D a n d acceptor A . E T rate constants have b e e n f o u n d to fit w e l l to an e x p o n e n t i a l d e p e n d e n c e o n the edge-to-edge distance b e t w e e n D a n d A , w i t h the rates decreasing about lie for e v e r y - 1 - Â increase i n distance. 4. O r i e n t a t i o n o f D w i t h respect to A . A n o r i e n t a t i o n effect has b e e n f o u n d i n some r i g i d m o d e l c o m p o u n d s , b u t not e n o u g h is k n o w n yet to p r o v i d e a f u l l u n d e r s t a n d i n g o f this effect. 5. N a t u r e of the linkage. T h e m o l e c u l a r structure o f the linkage b e t w e e n D a n d A has b e e n f o u n d to play a v e r y i m p o r t a n t role i n m e d i a t i n g E T from D to A . I n most cases E T occurs t h r o u g h the b o n d s o f the b r i d g e a n d not t h r o u g h the s u r r o u n d i n g m e d i u m . O f t e n E T appears to b e m e d i a t e d b y a superexchange m e c h a n i s m i n v o l v i n g the a n t i b o n d i n g orbitals of the b r i d g e . A r o m a t i c a n d unsaturated bridges are therefore e x p e c t e d to b e m o r e effective t h a n saturated b r i d g e s , a l t h o u g h there are some surprises. 6. Solvent. T h e s u r r o u n d i n g solvent changes two parameters that can m a r k e d l y alter E T rates. F i r s t , the exergonicity can b e a l t e r e d t h r o u g h different solvation of the p r o d u c t i o n p a i r D - A ~ . S e c o n d , the external c o n t r i b u t i o n to the r e o r g a n i zation e n e r g y X is a l t e r e d t h r o u g h changes i n e a n d e , the optical a n d static d i e l e c t r i c constants (see e q 10, C h a p t e r 2). +

o u t

o p

s


7.

BOLTON ET AL.

Solvent, Temperature,

119

and Bridge Dependence

7. T e m p e r a t u r e . A s w i t h any rate constant, E T rate constants usually e x h i b i t a n A r r h e n i u s t e m p e r a t u r e d e p e n d e n c e , b u t the activation energies are usually small.


D u r i n g the past several years o u r groups have b e e n s t u d y i n g the i n t r a m o l e c u l a r P E T rates i n a series of P L Q (tetraarylporphine l i n k e d to p - b e n z o quinone) molecules. C h a r t 1 illustrates the structures o f the various P L Q

bridge

symbol

bridge

symbol

PCBQ

PBOQ

PPhQ

Chart

1. Structures

of porphyrin-quinone bridges.

molecules containing

various


120


m o l e c u l e s . T h i s chapter is i n t e n d e d to r e v i e w this w o r k , a n d i n so d o i n g w e shall attempt to c o m e to some conclusions r e g a r d i n g the influence of the factors l i s t e d , p a r t i c u l a r l y factors 5 - 7 . T h e data for this r e v i e w c o m e from several p u b l i s h e d papers (2-8) a n d u n p u b l i s h e d w o r k (9-11).


Fluorescence Lifetimes T h e c o m p o u n d that w e have s t u d i e d most is the p o r p h y r i n - a m i d e - q u i n o n e m o l e c u l e P A Q (see C h a r t 1 for structure). A l t h o u g h the b r i d g e i n this m o l ecule has some flexibility, m o l e c u l a r m o d e l i n g (6) has s h o w n that the c e n t e r to-center distance varies o n l y f r o m 12.7 Â i n the most c o m p a c t s t r u c t u r e to 14.3 Â i n the most e x t e n d e d structure. T h e fluorescence decay, as d e t e r m i n e d f r o m the t i m e - c o r r e l a t e d single p h o t o n c o u n t i n g t e c h n i q u e (2, 7, 8), is b i e x p o n e n t i a l w i t h a short a n d a l o n g c o m p o n e n t . T h e l o n g c o m p o n e n t is of l o w a m p l i t u d e ( 0.025 e V ) , w h e r e e q 2 is no longer v a l i d . 0

o u t

φ

A c o m p a r i s o n of P A Q w i t h P G Q demonstrates that the E T rates are not strongly attenuated b y increasing the l e n g t h of the c h a i n i n this p e p t i d e linkage. E v e n t h o u g h P G Q has three m o r e atoms i n its p e p t i d e c h a i n t h a n

Table III. Bridge Dependence of Porphyrm-Quinone Rate Constants in Various Solvents Solvent

PAQ

1,2-Dimethoxyethane E t h y l acetate 2-Methyltetrahydrofuran Acetone Acetonitrile Propionitrile Anisole 1-Butanol Benzonitrile Methylene chloride Chloroform

0.20 0.22 0.23 0.29 0.48 0.56 3.30 2.39 3.90 8.00 24.0

a

NOTE:

All values are

"From From From From From ^From

ref. ref. ref. ref. ref. ref.

&ET

fe c

d e

PPAQ

0.15 0.16 0.11 0.08 0.20 0.18 0.71 1.20 1.2 2.10 3.90

0.29 0.68 0.33 0.89 0.99

b

C

PCBQ

d

1.0

PBOQ

e

0.016 0.024 0.000 0.330

1.30 1.2 3.00 3.90

PPhQf 38 37 33 48 57 81

8.4 8.4 8.4 27.0

0.150

S

(10 s ) measured at 295 K. 8

2. 8. 10. 11. 3. 9.

PGQ

_1


226 27 12

7.

BOLTON ET AL.

Solvent, Temperature,

129

and Bridge Dependence

P A Q , the rate constants decrease b y o n l y a factor of ca. 2 - 5 . T h e r e p l a c e m e n t of a h y d r o g e n atom i n t h e c e n t r a l m e t h y l e n e group o f the p e p t i d e l i n k a g e i n P G Q b y the p h e n y l g r o u p i n P P A Q enhances t h e rate constant b y a factor o f ca. 1-4. T h i s e n h a n c e m e n t c o u l d b e d u e to the i n t e r p o s i t i o n of an a r o m a t i c g r o u p i n the b r i d g e , b u t i t c o u l d also b e d u e to a change i n the conformation of the b r i d g e . I s i e d a n d co-workers (16-18) s t u d i e d ground-state E T i n a series o f R u - C o b i n u c l e a r complexes, i n w h i c h t h e t w o m e t a l centers are l i n k e d v i a a m i n o acids, i n v o l v i n g three atoms i n t h e l i n k a g e , o r b y p e p t i d e groups w i t h t w o o r m o r e l i n k e d a m i n o acids. T h e y f o u n d that k drops b y a factor o f ~ 1 0 for each a d d i t i o n a l p r o l i n e i n t h e p e p t i d e b r i d g e . F o r a t h r e e - a t o m increase i n t h e linkage l e n g t h , this factor is c o n s i d e r a b l y larger t h a n i n o u r case. ET


2

I n contrast, Schanze a n d Sauer (19), i n a study o f P E T i n a series o f p o l y p y r i d y l Ru(II) complexes l i n k e d to p - b e n z o q u i n o n e b y p r o l i n e p e p t i d e bridges (n = 0 - 4 ) , f o u n d that each a d d i t i o n a l p r o l i n e decreased t h e rate constant b y ~ 1 0 . C a b a n a a n d Schanze (20) f o u n d s i m i l a r results i n a study of P E T i n a series o f p o l y p y r i d y l Re(I) complexes l i n k e d to 4 - ( N , A - d i m e t h ylamino)benzoate b y p r o l i n e p e p t i d e bridges (n = 0-2). T h e results o f these latter t w o studies are closer to o u r findings. T h e series P C B Q , P B O Q , a n d P P h Q reveals some i m p o r t a n t insights into t h e role o f b r i d g e orbitals i n m e d i a t i n g i n t r a m o l e c u l a r E T . T h e rate constants i n P P h Q are ca. 1 0 - 1 0 times greater t h a n i n P B O Q . T h i s dif ference almost c e r t a i n l y arises f r o m t h e availability o f l o w - l y i n g e m p t y a n t i b o n d i n g levels i n t h e p h e n y l g r o u p , w h i c h are n o t p r e s e n t i n t h e bicyclooctane b r i d g e . T h e results for P C B Q are n o t e w o r t h y . T h e cyclobutane b r i d g e i n P C B Q has one fewer b o n d t h a n the bicyclooctane b r i d g e i n P B O Q , 7

2

3

a n d t h e 56-fold increase o b s e r v e d i n fc i n m e t h y l e n e c h l o r i d e is larger than m i g h t b e expected for such a modest b o n d decrease. s

ET

H

w

is e x p e c t e d to d e p e n d o n t h e center-to-center distance r a c c o r d i n g

to

tf p(r) I

= H

r

p

(r )exp[" 0

P

(

r

2

"

r

o

)

]

(7)

w h e r e r is t h e center-to-center distance w h e n d o n o r a n d acceptor are at the v a n d e r Waals contact distance, β has b e e n f o u n d to b e ~ 1 A from a n u m b e r o f studies (J). T h e edge-to-edge distance of the bicyclooctane b r i d g e is ca. 1 A greater t h a n that o f the cyclobutane b r i d g e . T h u s i f distance w e r e the o n l y factor, t h e rate constant s h o u l d increase b y a factor o f o n l y ca. e = 2.7 o n c h a n g i n g t h e b r i d g e f r o m bicyclooctane to cyclobutane. It m a y b e that t h e strained aliphatic orbitals i n t h e cyclobutane b r i d g e m e d i a t e E T m o r e effectively, o r that this b r i d g e orients Ρ a n d Q m o r e favorably. S u p p o r t for the s t r a i n e d - o r b i t a l concept is p r o v i d e d f r o m a r e c e n t study 0

-

1


130


b y Sakata et a l . (21) of two rigid p o r p h y r i n - q u i n o n e m o l e c u l e s . T h e y f o u n d that the E T rate constant for a strained spiro[4.4]nonane b r i d g e is about 5 times faster than a i r a n s - d e c a l i n b r i d g e , a l t h o u g h each b r i d g e has the same n u m b e r of saturated bonds b e t w e e n the d o n o r a n d acceptor. M o r e o v e r , O n u c h i c a n d B e r a t a n (22) p r e d i c t e d theoretically that a spiro c y c l o b u t a n e linkage s h o u l d e x h i b i t h i g h e r rate constants than those w i t h other types of aliphatic h y d r o c a r b o n linkages w i t h the same n u m b e r of b r i d g e bonds.


Conclusions T h e data a n d analyses i n this r e v i e w a l l o w the f o l l o w i n g conclusions to b e drawn: 1. T h e E T rate-constant data as a function of solvent for P A Q fit tolerably w e l l (with a considerable scatter i n the points) to the h i g h - t e m p e r a t u r e l i m i t of the M a r c u s e q u a t i o n (eq 2), p r o v i d e d that X a n d A G are d e t e r m i n e d i n each solvent. o u t

0

2. W h e n the P A Q rate constants are e x a m i n e d as a f u n c t i o n of t e m p e r a t u r e , a reasonable fit w i t h the h i g h - t e m p e r a t u r e l i m i t of the M a r c u s e q u a t i o n is again o b t a i n e d ; h o w e v e r , the elec tronic c o u p l i n g energy varies significantly w i t h solvent. T h i s v a r i a b i l i t y explains most of the scatter f o u n d i n the s o l v e n t - d e p e n d e n c e analysis. 3. A p e p t i d e b r i d g e is a v e r y effective linkage b e t w e e n the p o r p h y r i n a n d q u i n o n e , the rate constant d r o p p i n g o n l y slowly w i t h i n c r e a s i n g c h a i n l e n g t h . T h e i n t r o d u c t i o n of an aromatic side g r o u p increases the rate slightly, b u t not d r a m a t i c a l l y . 4. A n unsaturated b r i d g e (e.g., phenyl) allows E T at rates 100-1000 times faster t h a n a saturated b r i d g e (e.g., b i c y c l o o c tane); h o w e v e r , a cyclobutane b r i d g e allows E T faster t h a n other saturated bridges of s i m i l a r d i m e n s i o n s . T h i s chapter has p r o v i d e d strong e v i d e n c e that the solvent a n d t e m p e r a t u r e a n d the nature of the b r i d g e are v e r y i m p o r t a n t factors i n m e d i a t i n g i n t r a m o l e c u l a r E T i n m o d e l c o m p o u n d s . T h i s a n d other studies s h o u l d p r o v i d e considerable insight into u n d e r s t a n d i n g the mechanisms of i n t r a m o l e c u l a r e l e c t r o n transfer i n natural a n d artificial systems.

References 1. Connolly, J. S.; Bolton, J. R. In Photoinduced Electron Transfer; Fox, Μ. Α.; Chanon, M., E d s . ; Elsevier: N e w York, 1989; pp 303-393. 2. Schmidt, J. Α.; Siemiarczuk, Α.; Weedon, A. C.; Bolton, J. R. J. Am. Chem. Soc. 1985, 107, 6112.


7.

A. A.


A.

M.

M.

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Solvent, Temperature, and Bridge

Dependence

131

3. Bolton, J. R.; H o , T.-R.; Liauw, S.; Siemiarczuk, Α.; Wan, C. S. K.; Weedon, J. Chem. Soc., Chem. Commun. 1985, 559. 4. Wilford, J. H.; Archer, M. D.; Bolton, J. R.; H o , T.-F.; Schmidt, J. Α.; Weedon, C. J. Phys. Chem. 1985, 89, 5395. 5. Archer, M. D.; Gadzekpo, V. P. Y.; Bolton, J. R.; Schmidt, J. Α.; Weedon, C. J. Chem. Soc., Faraday Trans. 2 1986, 82, 2305. 6. Schmidt, J. Α.; McIntosh, A. R.; Weedon, A. C.; Bolton, J. R.; Connolly, J. S.; Hurley, J. K.; Wasielewski, M. R. J. Am. Chem. Soc. 1988, 110, 1733. 7. Schmidt, J. Α.; L i u , J.-Y.; Bolton, J. R.; Archer, M. D.; Gadzekpo, V. P. Y. J. Chem. Soc, Faraday Trans. 1 1989, 85, 1027. 8. Schmidt, J. Α., P h . D . Thesis, University of Western Ontario, 1986. 9. H o , T . - R ; Bolton, J. R., unpublished work. 10. L i u , J.-Y.; Bolton, J. R., unpublished work. 11. Roach, K. J.; Bolton, J. R.; Weedon, A. C., unpublished work. 12. Siemiarczuk, Α.; McIntosh, A. R.; H o , T.-R.; Stillman, M. J.; Roach, K. J.; Weedon, A. C.; Bolton, J . R.; Connolly, J. S. J. Am. Chem. Soc. 1983, 105, 7224. 13. Gouterman, M.; Khalil, G.-E. J. Mol. Spectrosc. 1974, 53, 88. 14. Marcus, R. Α.; Sutin, N . Biochim. Biophys. Acta 1985, 811, 265. 15. Gunner, M. R.; Robertson, D. E.; Dutton, P. L. J. Phys. Chem. 1986, 90, 3783. 16. Isied, S. S.; Vassilian, A. J. Am. Chem. Soc. 1984, 106, 1726 and 1732. 17. Isied, S. S. Prog. Inorg. Chem. 1984, 32, 443. 18. Isied, S. S. In Electron Transfer in Biology and the Solid State; Johnson, K.; King, R. B.; Kurtz, D. M., Jr.; Kutal, C . ; Norton, M. L.; Scott, R. Α., Eds.; Advances in Chemistry 226; American Chemical Society: Washington, D C , 1990; p 91. 19. Schanze, K. S.; Sauer, K. J. Am. Chem. Soc. 1988, 110, 1180. 20. Cabana, L. Α.; Schanze, K. S. In Electron Transfer in Biology and the Solid State; Johnson, M. K.; King, R. B.; Kurtz, D. M., J r . ; Kutal, C.; Norton, L.; Scott, R. Α., E d s . ; Advances in Chemistry 226; American Chemical Society: Washington, D C , 1990; p 101. 21. Sakata, Y.; Nakashima, S.; Goto, Y.; Tatemitsu, H.; M i s u m i , S.; Asahi, T.; H a gihara, M.; Nishikawa, S.; Okada, T.; Mataga, N . J. Am. Chem. Soc. 1989, 111, 8979. 22. Onuchic, J. N.; Beratan, D. N. J. Am. Chem. Soc. 1987, 109, 6771. C.

R E C E I V E D for review A p r i l 27, 1990. A C C E P T E D revised manuscript September 11, 1990.


Solvent, Temperature, and Bridge Dependence of Photoinduced

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