Electron Transfer in Inorganic, Organic, and Biological Systems

6 Photoinduced Charge Separation and Charge Recombination of Transient Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch006

Ion-Pair States Ultrafast Laser Photolysis Noboru Mataga Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan

The mechanisms and dynamics underlying photoinduced charge separation (CS) and charge recombination (CR) of the produced chargetransfer (CT) or ion-pair (IP) state are discussed on the basis of the results obtained by femtosecond-picosecond laser photolysis and time-resolved spectral studies on various donor-acceptor (D-A) systems combined by spacers or directly, on the uncombined fluorescer-quencher pairs, and on CT complexes. By comparing the results for these various systems concerning the effects of electronic interaction between D and A, energy gap of electron transfer, and solvent dynamics on the photoinduced CS and CR of the produced CT or IP state, a much deeper insight into the nature of the electron-transfer mechanism prevailing among those different kinds of systems has been obtained.

THE MECHANISMS AND DYNAMICS

r e g u l a t i n g the p h o t o i n d u c e d charge separation (CS) a n d charge r e c o m b i n a t i o n ( C R ) of the p r o d u c e d c h a r g e transfer (CT) or i o n - p a i r (IP) states, as w e l l as r e l a t e d p h e n o m e n a i n l i q u i d solutions, r i g i d matrices, m o l e c u l a r assemblies, a n d b i o l o g i c a l systems, are the most f u n d a m e n t a l a n d i m p o r t a n t p r o b l e m s i n the p h o t o p h y s i c a l a n d p h o t o c h e m i c a l p r i m a r y processes i n the c o n d e n s e d phase ( J - 5 ) . 0065-2393/91/0228-0091$07.25/0 © 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.

92

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

Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch006

I n g e n e r a l , the rate of the p h o t o i n d u c e d C S a n d that o f the C R of the C T or I P state are regulated b y the m a g n i t u d e of the e l e c t r o n i c i n t e r a c t i o n responsible for the e l e c t r o n transfer ( E T ) b e t w e e n D (electron donor) a n d A (electron acceptor) groups, the F r a n c k - C o n d o n ( F C ) factor ( w h i c h is r e l a t e d to the e n e r g y gap for the E T reaction), the reorganization energies o r i g i n a t i n g f r o m the v i b r a t i o n a l freedoms w i t h i n D a n d A as w e l l as the p o l a r i z a t i o n motions of the solvent molecules s u r r o u n d i n g D a n d A , a n d also the solvent-orientation d y n a m i c s i n the course of E T i n polar solvent. D e p e n d i n g u p o n the strength of the e l e c t r o n i c i n t e r a c t i o n b e t w e e n D a n d A responsible for the E T , the reaction w i l l b e nonadiabatic (a) or adiabatic (b). I f the e l e c t r o n i c i n t e r a c t i o n is sufficiently strong i n case b a n d the e n e r g y gap r e l a t i o n is also favorable, the E T process w i l l b e c o m e barrierless (c) a n d w i l l b e g o v e r n e d b y b o t h the solvent-orientation d y n a m i c s a n d i n t r a m o l e c ular vibrations of D a n d A groups (6, 7). W h e n the effect of the solvent d y n a m i c s o n the E T process is d o m i n a n t , it is b e l i e v e d that the l o n g i t u d i n a l d i e l e c t r i c relaxation t i m e , T , or solvation t i m e , T , w i l l b e i m p o r t a n t as a factor c o n t r o l l i n g the E T rate. I n a l i m i t of strong e l e c t r o n i c i n t e r a c t i o n b e t w e e n D a n d A groups c o m b i n e d r i g i d l y , its excited singlet state c a n b e r e g a r d e d as a v e r y p o l a r single m o l e c u l e , a n d a large fluorescence Stokes shift d u e to solvation i n polar solvents can b e o b s e r v e d (d). T h e first t h e o r e t i c a l f o r m u l a for the Stokes shift i n case d , g i v e n b y M a t a g a et a l . (8) a n d L i p p e r t (9), has b e e n e x t e n d e d r e c e n t l y b y B a g c h i et a l . (JO) a n d others to take i n t o c o n s i d e r a t i o n its d y n a m i c a l aspects. l

s

H o w e v e r , i n m a n y actual systems of strongly i n t e r a c t i n g D a n d A , the e l e c t r o n i c structure or the extent of the C T from D to A can change g r a d u a l l y , a c c o m p a n i e d b y some g e o m e t r i c a l change i n the course o f extensive solvat i o n . T h i s is the case b e t w e e n c a n d d ; that is, w h e n the e l e c t r o n i c i n t e r a c t i o n b e t w e e n D a n d A groups is i n c r e a s e d b e y o n d case c, the s i m p l e e l e c t r o n transfer m e c h a n i s m based o n the two-state m o d e l (locally e x c i t e d i n i t i a l state a n d final I P state d u e to one e l e c t r o n transfer) w i l l b e c o m e i n v a l i d . Because this case does not s e e m to be w e l l - r e c o g n i z e d i n g e n e r a l , w e discuss this p r o b l e m m a i n l y i n r e l a t i o n to the p h o t o i n d u c e d C S process of a strongly i n t e r a c t i n g D - A system, c o n s i d e r i n g the results of o u r f e m t o s e c o n d p i c o s e c o n d laser photolysis studies of D - A systems c o m b i n e d b y spacers o r d i r e c t l y b y single b o n d s (11-20) a n d similar studies, as w e l l as some p r e v i o u s investigations o f C T complexes b e t w e e n aromatic hydrocarbons a n d various e l e c t r o n acceptors (21-30). T h e p h o t o i n d u c e d C S b e t w e e n fluorescer a n d q u e n c h e r groups a n d excitation of the C T c o m p l e x i n s u c h strongly p o l a r solvents as acetonitrile leads to the formation of the I P or C T state. T h o s e I P s generally u n d e r g o C R a n d dissociation into free ions o r some c h e m i c a l reactions. T h e behaviors of those C T or I P states are of c r u c i a l i m p o r t a n c e from various v i e w p o i n t s ; i n m a n y cases the fate of successive reactions is d e t e r m i n e d b y i n t e r m e d i a t e C T o r I P states. F o r e x a m p l e , the v e r y r a p i d p h o t o i n d u c e d C S a m o n g the


6.

MATAGA

Separation and Recombination

of Transient Ion-Pair States

93

special d i m e r of b a c t e r i o c h l o r o p h y l l , b a c t e r i o c h l o r o p h y l l m o n o m e r , a n d baet e r i o p h e o p h y t i n , a n d the m u c h slower C R rate of p r o d u c e d I P i n the b a c t e r i a l p h o t o s y n t h e t i c reaction c e n t e r (31) are d i r e c t l y responsible for its ultrafast a n d e x t r e m e l y efficient redox reaction. O n e of the most i m p o r t a n t factors r e g u l a t i n g the C R rate of the I P l e a d i n g to the formation of the g r o u n d state seems to b e the e n e r g y gap b e t w e e n the relevant states. F o r the C R rate constant fc of I P p r o d u c e d b y the fluorescence-quenching reaction i n polar s o l u t i o n , o n l y the results for the i n v e r t e d r e g i o n w e r e available p r e v i o u s l y (32-35); no results h a d b e e n p u b l i s h e d for the n o r m a l r e g i o n . W e made systematic studies of this p r o b l e m b y d i r e c t l y o b s e r v i n g the C R deactivation of the geminate I P i n acetonitrile s o l u t i o n w i t h ultrafast laser spectroscopy. O u r results c o v e r the i n v e r t e d r e g i o n , the n o r m a l r e g i o n , a n d the top r e g i o n , e s t a b l i s h i n g a b e l l shaped r e l a t i o n s h i p (17, 36, 37).


CR

A s i n the case of the p h o t o i n d u c e d C S reaction, w e e x a m i n e d the k of the I P that is p r o d u c e d b y excitation of C T complexes w h e r e D a n d A interact m o r e strongly t h a n i n the fluorescence-quenching reaction i n ace t o n i t r i l e solution. W e f o u n d a k energy-gap d e p e n d e n c e q u i t e different from the b e l l - s h a p e d result (29,30). W e have c o n f i r m e d that the r e l a t i o n s h i p , l°g &CR |AG °|, is o b s e r v e d i n acetonitrile solution for a w i d e range of - A G ° values ( - A G ° is the free-energy gap b e t w e e n the I P a n d the g r o u n d states). T h i s r e m a r k a b l e difference of the I P energy-gap d e p e n d e n c e of fc seems to b e r e l a t e d to the difference i n its structure, d e p e n d i n g o n the m o d e of its f o r m a t i o n . T h e I P f o r m e d b y e x c i t i n g the C T c o m p l e x m a y have a t i g h t e r structure w i t h stronger i n t e r a c t i o n b e t w e e n D a n d A " ions i n the p a i r t h a n i n the I P p r o d u c e d b y C S at the e n c o u n t e r b e t w e e n fluorescer a n d q u e n c h e r . W e have c o m p a r e d the behaviors of these different k i n d s of IPs a n d discuss the relevant m e c h a n i s m s i n this chapter. CR

CR

α

_

ip

i p

i p

CR

+

I n a d d i t i o n , the existence of different k i n d s of I P s is c r u c i a l l y i m p o r t a n t i n some organic p h o t o c h e m i c a l reactions that p r o c e e d v i a the I P states p r o d u c e d b y p h o t o i n d u c e d C S . T h e c h e m i c a l reactivity of the I P f o r m e d b y C T c o m p l e x excitation seems to be q u i t e different from that of the I P p r o d u c e d b y the C S t h r o u g h an e n c o u n t e r b e t w e e n an excited m o l e c u l e a n d a n e l e c t r o n - d o n a t i n g o r -accepting q u e n c h e r m o l e c u l e .

Photoinduced CS and CR of the Produced IP State of Combined D-A Systems W e investigated the P n [ p - ( C H ) N - C H - ( C H ) - ( l - p y r e n y l ) , η = 1, 2, 3], A n [ p - ( C H ) N - C H - ( C H ) - ( 9 - a n t h r y l ) , η = 0, 1, 2, 3], 9 , 9 ' - b i a n t h r y l , a n d t h e i r d e r i v a t i v e s , 1,2-dianthrylethanes, w i t h f e m t o s e c o n d - p i c o s e c o n d laser photolysis a n d t i m e - r e s o l v e d transient absorption spectral m e a s u r e m e n t s i n alkanenitrile a n d viscous alcohol solvents (11-20). T h e t i m e - r e s o l v e d ultrafast 3

3

2

6

4

2

2

6

4

2

n

n


94

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

absorption spectral measurements give d i r e c t i n f o r m a t i o n o n the e l e c t r o n i c structures of the system u n d e r g o i n g p h o t o i n d u c e d E T . S u c h s t r u c t u r a l i n formation is e x t r e m e l y i m p o r t a n t for d i s c r i m i n a t i n g various cases of E T p r o c esses, as d e s c r i b e d i n the i n t r o d u c t o r y section. T h i s discussion deals m a i n l y w i t h results o b t a i n e d i n a l k a n e n i t r i l e solutions of P n a n d A n . A s an example, t i m e - r e s o l v e d absorption spectra of P n i n acetonitrile ( A C N ) are s h o w n i n F i g u r e 1 (13). T h e r a p i d rise of the characteristic sharp absorption b a n d at 500 n m indicates the i n t r a m o l e c u l a r I P state, a n d the r a p i d decay of the absorption a r o u n d 470 n m indicates the S - S ! (from the lowest excited singlet to the h i g h e r excited singlet state) transition l o c a l i z e d i n the p y r e n e part. V e r y similar t i m e - r e s o l v e d spectra w i t h s l i g h t l y slower rise a n d decay processes have b e e n o b s e r v e d also i n n - b u t y r o n i t r i l e ( B u C N ) a n d n - h e x a n e n i t r i l e ( H e x C N ) solutions. I n the case of A n (n = 1, 2, 3) i n alkanenitrile solutions, w e can observe the r a p i d rise of the characteristic absorption b a n d at 480 n m d u e to the D M A (Ν,Ν-dimethylaniline) cation of the i n t r a m o l e c u l a r I P state. I n these systems, the rise curves of the I P state c o n v e r g e to constant values. T h e s e flexible-chain c o m p o u n d s m a y have some d i s t r i b u t i o n of ground-state conformations that m i g h t affect the C S process i n the excited state. N e v e r t h e l e s s , the rise curves of the IP-state absorbance can be r e p r o d u c e d a p p r o x i m a t e l y b y an exponential function w i t h rise times o f ca. 1-10 ps, as s h o w n i n T a b l e I (13).


n

T h e T (rise t i m e of the I P state) values of P n a n d A n (n = 1 , 2 , 3) i n T a b l e I are m u c h longer than the solvent T value. T h i s c o m p a r i s o n suggests that the p h o t o i n d u c e d C S i n these systems are not d i r e c t l y c o n t r o l l e d b y the solvent-reorientation d y n a m i c s . E v e n i f w e use the solvation t i m e T (38) e s t i m a t e d f r o m the d y n a m i c fluorescence Stokes shift of the polar p r o b e m o l e c u l e , this c o n c l u s i o n is not a l t e r e d except i n the case of A w h e r e T is close to T , a result suggesting the p o s s i b i l i t y of c o n t r o l b y solvation d y n a m i c s . H o w e v e r , at ~1 ps delay t i m e after excitation of aromatic m o l ecules, the i n t r a m o l e c u l a r v i b r a t i o n a l relaxation (cooling) is not yet c o m p l e t e d (39). T h e r e f o r e , possibly the C S is t a k i n g place from the state w i t h excess v i b r a t i o n a l e n e r g y i n A c s

l

S

l 5

C S

s

P

T h e s e observations on P n a n d A n (n = 1, 2, 3) show that the photoex citation is i n i t i a l l y l o c a l i z e d i n the p y r e n e or anthracene p o r t i o n ; t h e n E T takes place to p r o d u c e the I P state. I n the case of m o d e l c o m p o u n d s u s e d p r e v i o u s l y for i n v e s t i g a t i n g the effects of solvent d y n a m i c s o n the p h o t o i n d u c e d i n t r a m o l e c u l a r E T reaction, this c r i t e r i o n was not necessarily clear (40). A s s h o w n i n T a b l e I, the T values are almost 3 orders of m a g n i t u d e l o n g e r t h a n T values. C o n t r a r y to the case of T , t h e y generally b e c o m e shorter w i t h an increase of the i n t e r v e n i n g c h a i n n u m b e r n. c r

c s

c s

It seems possible to give a satisfactory account of these results i n t e r m s of the u s u a l nonadiabatic E T m e c h a n i s m (12, 13). I n g e n e r a l , the e l e c t r o n transfer rate constant k can be w r i t t e n i n terms of the factor A , w h i c h



500

600

2.5ps

l.Ops

400

(b)

500

\

WAVELENGTH

****

/nm

600

8. Ops

Ops

5ps

l.Ops

2

3

40 0

5 00

^^^^^^

(c)

60 0

J • »uM

J ιÉ I

6.Ops

4. Ops

2. Ops

Figure 1. Time-resolved transient absorption spectra of Pi (a), P (b), and P (c) in ACN, measured with the femtosecond laser photolysis method. (Reproduced from ref 13. Copyright 1990 American Chemical Society.)

(a)


96


Table I. Rise and Decay Times of the Intramolecular IP State of Pn and An in Alkanenitrile Solutions (ps)

Tcs

Compound Pi P P 2

1.7

2.5

6.1

7.7

11

3


BuCN

0.65

A A Ao T 2

2.1

4-5

3

2.7

10

11

3.3

10

1.4

0.5

11

4.0

7.1

1.0

3.2

— —

8.0

— — —

5.2

0.7

48

5.0

HexCN

4.1

1.1

— —

2.7

0.2

L

— —

1.0

—

7.0

4.5

25

A!

(ns)

TCR

BuCN

ACN

HexCN

ACN

—

FL

(*0.7)

"Inverse of nonradiative rate constant from the (CT) fluorescent state, of which the observed lifetime was 31 ns. Data from ref. 53. SOURCE: Reproduced from ref. 13. Copyright 1990 American Chemical Society.

incorporates the t u n n e l i n g m a t r i x e l e m e n t Η

φ

a n d the F C factor, i n c l u d i n g

c o n t r i b u t i o n s from b o t h i n t r a m o l e c u l a r v i b r a t i o n s a n d solvent o r i e n t a t i o n as follows.

(FC)

k - I -

(1)

w h e r e ( F C ) is r e l a t e d to the free-energy gap - A G , ( ω ) is the average angular 0

frequency

of the i n t r a m o l e c u l a r v i b r a t i o n a l m o d e , a n d h is P l a n c k ' s constant

d i v i d e d b y 2ττ. The

free-energy

gap for C S ( - A G

C S

° ) o f the D - A pairs o f these systems

i n A C N , for e x a m p l e , is ~ 0 . 5 eV, w h e r e the k

(T

cs

c s

- 1

) VS. - A G

c s

°

rela

t i o n s h i p is i n the n o r m a l r e g i o n a n d rather close to the top r e g i o n . T h i s p l a c e m e n t agrees w i t h b o t h t h e o r e t i c a l p r e d i c t i o n s (33, 41-45) a n d t h e ex p e r i m e n t a l e s t i m a t i o n of k

values b y means of the transient effect i n the

cs

bimolecular of k

cs

fluorescence-quenching

reaction (46). T h e r e f o r e , the difference

values a m o n g c o m p o u n d s w i t h v a r y i n g c h a i n n u m b e r s m a y b e a s c r i b e d

not o n l y to the s m a l l difference i n - A G

C S

° b u t j i l s o to t h e factor A c o n t a i n i n g

the tunneling matrix element Η . B y taking A = 1 0 φ

e s t i m a t e d to be 1 0 - 1 0 U

1 2

s

1

1 2

-10

(33). O n the o t h e r h a n d , - A G

1 3

C S

s~\ k

cs

can b e

° w i l l decrease

s l i g h t l y i n the o r d e r of A C N > B u C N > H e x C N because o f the decrease of solvation e n e r g y o f the I P state. Because the fe

cs

is i n t h e n o r m a l r e g i o n at - A G i n a slight decrease of k . cs

C S

vs. - A G

C S

° relationship

° — 0 . 5 eV, this decrease of - A G

C S

° results

N e v e r t h e l e s s , as the solvent r e o r g a n i z a t i o n e n e r g y

also decreases i n the same o r d e r , the effect w i l l be rather s m a l l . T h e m u c h s m a l l e r rate o f C R i n the i n t r a m o l e c u l a r I P state can b e w e l l u n d e r s t o o d as a r e s u l t o f the o v e r w h e l m i n g i n f l u e n c e of t h e F C factor. A s


6.

MATAGA


97


already discussed, w e have established b y ultrafast laser photolysis m e a s u r e m e n t s the b e l l - s h a p e d fc

CR

(T

c r

_ 1

) VS. - A G ° (free-energy gap b e t w e e n i p

the I P a n d the n e u t r a l g r o u n d state) r e l a t i o n s h i p for the I P p r o d u c e d fluorescence-quenching

by

reaction i n acetonitrile solution (17, 36, 37). F o r

b o t h P n a n d A n , the fc

CR

vs. - A G ° r e l a t i o n s h i p is i n the i n v e r t e d r e g i o n i p

at the large e n e r g y gap a r o u n d - A G ° ~ 2.8 e V a n d the e n e r g y gap for A n i p

is s l i g h t l y s m a l l e r t h a n that for P n . T a k i n g A c o m p o u n d s y i e l d s fc

CR

=

10

1 3

s"

1

for the η =

1

= 1 0 s" , i n agreement w i t h observation. F o r (n = 8

1

3) c o m p o u n d s , configurational change to s a n d w i c h f o r m can take place; this change increases A a n d decreases - A G ° ,


i p

fc

CR

~ 10

9

l e a d i n g to the faster C R w i t h

s" . 1

Because the τ

c s

o f Αχ is rather close to the solvation t i m e T

s

i n the

a l k a n e n i t r i l e solvent, the p h o t o i n d u c e d C S of this system m a y be c o n t r o l l e d a p p r o x i m a t e l y b y solvent d y n a m i c s . I n A , the D a n d A groups are d i r e c t l y 0

c o m b i n e d b y a single b o n d , a n d the e l e c t r o n i c i n t e r a c t i o n b e t w e e n w i l l b e m u c h stronger t h a n i n A

v

The photoinduced C S i n A

0

them

w i l l be truly

c o n t r o l l e d b y solvent o r i e n t a t i o n d y n a m i c s or, i n strongly i n t e r a c t i n g D a n d A groups, its C S process cannot b e d e s c r i b e d b y the s i m p l e two-state m o d e l , A * - D —» A - D , b u t w i l l b e e x p l a i n e d b y a s s u m i n g a g r a d u a l change o f +

e l e c t r o n i c structure t o w a r d i n c r e a s i n g p o l a r i t y , a c c o m p a n i e d b y some geo m e t r i c a l change a n d extensive solvation. T h e spectra i n F i g u r e 2 show a gradual change f r o m an absorption b a n d that is somewhat analogous to b u t b r o a d e r t h a n that o f the S state of anthracene to one that indicates the C T x

state, w i t h its characteristic D M A cation b a n d a r o u n d 450 n m . T h e a p p r o x i m a t e decay t i m e o f the i n i t i a l state o r rise t i m e o f the C T state can b e e s t i m a t e d as s h o w n i n T a b l e I. T h e s e values are m u c h l o n g e r t h a n the corresponding T

c s

values o f

T h e s e results o n A

0

A

v

i m p l y that A

0

is the case b e t w e e n c a n d d , as

discussed i n the o p e n i n g section. F o r s u c h systems w i t h v e r y strong e l e c t r o n i c i n t e r a c t i o n b e t w e e n D a n d A groups, the p h o t o i n d u c e d C S is not r e a d i l y d e t e r m i n e d b y the solvent d y n a m i c s . S i m i l a r l y , these results cannot b e i n t e r p r e t e d s i m p l y b y the two-state m o d e l based o n the u s u a l E T theories that assume weak i n t e r a c t i o n . A possible i n t e r p r e t a t i o n for this fact m a y b e that the C S i n s u c h a strongly i n t e r a c t i n g system b e c o m e s a little slower because of i n t r a m o l e c u l a r g e o m e t r i c a l rearrangements a n d that extensive solvation is necessary to p r e v e n t the e l e c t r o n i c d e r e a l i z a t i o n i n t e r a c t i o n i n the I P state. P h o t o i n d u c e d C S can take place not o n l y i n systems w i t h definite D and A groups c o m b i n e d b y spacer or d i r e c t l y [as i n the i n t r a m o l e c u l a r exc i p l e x c o m p o u n d s already discussed (11-13, tems w i t h i d e n t i c a l halves (14-16,

17, 18)], b u t also i n some sys

19, 20). T h e most w e l l - k n o w n e x a m p l e

is the s o l v a t i o n - i n d u c e d b r o k e n s y m m e t r y (19) of e x c i t e d 9 , 9 ' - b i a n t h r y l (BA) i n p o l a r solvent (14, 19, 47). T h e p i c o s e c o n d t i m e - r e s o l v e d transient a b sorption spectra of B A i n 1-pentanol, for e x a m p l e , can b e r e p r o d u c e d a p -


98


(a)

0.2ps

(b) -0.2ps


^ ^ K ^

400

500

600

400

0.2ps

500

600

WAVELENGTH/nm

Figure 2. Time-resolved transient absorption spectra of A in BuCN (a) and HexCN (b), measured with the femtosecond laser photolysis method. (Repro duced from ref 13. Copyright 1990 American Chemical Society.) 0

p r o x i m a t e l y as a l i n e a r c o m b i n a t i o n of the spectra i n hexane (nonpolar S state a little d e l o c a l i z e d o v e r two anthracene rings) a n d acetonitrile (intra m o l e c u l a r C T state), a n d the t i m e - d e p e n d e n t change of the spectra converges to an e q u i l i b r i u m (14). F r o m such analysis, the p h o t o i n d u c e d C S of B A i n 1-pentanol has b e e n c o n f i r m e d to take place w i t h T ~ 1 7 0 - 1 8 0 ps at 23 °C (14). A s i m i l a r result, v e r y close to T = 174 ps, has b e e n o b t a i n e d b y p i c o s e c o n d t i m e - r e s o l v e d fluorescence m e a s u r e m e n t (14). l

c s

l

A l t h o u g h the t i m e - r e s o l v e d transient absorption spectra of Β A i n 1p e n t a n o l can be r e p r o d u c e d approximately b y l i n e a r c o m b i n a t i o n of the spectra i n hexane a n d acetonitrile, the s p e c t r u m i n acetonitrile is m u c h b r o a d e r a n d peak positions are shifted c o m p a r e d w i t h the s u p e r p o s i t i o n of the absorption bands of anthracene cation a n d anion radicals i n acetonitrile


6.

MATAGA



99


solution. T h i s means that c o m p l e t e C S seems to b e difficult, e v e n i n acet o n i t r i l e solution, because of the strong i n t e r a c t i o n b e t w e e n two anthracene moieties. M o r e o v e r , the t i m e - d e p e n d e n t change of the spectra converges to an e q u i l i b r i u m , a n d a considerable p r o p o r t i o n of the n o n p o l a r state seems to b e p o p u l a t e d i n this e q u i l i b r i u m state. T h e r e f o r e , e v e n t h o u g h the two anthracene planes i n B A are t w i s t e d , w h i c h decreases the d e r e a l i z a t i o n i n t e r a c t i o n b e t w e e n the two m o i e t i e s , a considerable a m o u n t of i n t e r a c t i o n still exists. O w i n g to this i n t e r a c t i o n , the p h o t o i n d u c e d C T process of B A i n polar solvents is d e e m e d a gradual change of e l e c t r o n i c structure f r o m n o n p o l a r to polar, p r o b a b l y a c c o m p a n i e d b y a s m a l l change of t w i s t i n g angle i n the course of solvation. A n a p p r o x i m a t e l y quantitative d e s c r i p t i o n of such change of electronic structure along the solvation coordinate was g i v e n p r e v i o u s l y (47) w i t h o u t consideration of the change of t w i s t i n g angle b y means of the g e n e r a l i z e d L a n g e v i n e q u a t i o n . W e have e x a m i n e d also the p h o t o i n d u c e d C S of 1 0 - c h l o r o - 9 , 9 ' - b i a n t h r y l (BAC1), i n w h i c h the s y m m e t r y of B A is slightly p e r t u r b e d b y s u b s t i t u t i o n at the 10 p o s i t i o n . I n this case, T has b e e n c o n f i r m e d to b e 140 ps, w h i c h is c o n s i d e r a b l y shorter than that of B A . T h i s result means that the i n t r a m o l e c u l a r C S of this slightly p e r t u r b e d B A is not d e t e r m i n e d easily b y the solvent r e o r i e n t a t i o n , b u t the slightly p r e s o l v a t e d state for this s y m m e t r y d i s t u r b e d c o m p o u n d w i l l facilitate the C S process (14). c s

If two anthracene moieties are separated b y m e t h y l e n e chains, an almost c o m p l e t e l y charge-separated state may b e r e a l i z e d i n strongly p o l a r solvents, as w e o b s e r v e d i n the P n a n d A n (n = 1, 2, 3) systems. A c t u a l l y , w e have o b s e r v e d b y means of picosecond laser photolysis such a s o l v a t i o n - i n d u c e d C S i n the e x c i t e d state of 1 , 2 - d i - l - a n t h r y le thane [ D ( 1 - A ) E ] a n d l , 2 - d i - 9 a n t h r y l e t h a n e [ D ( 9 - A ) E ] , b o t h of w h i c h seem to have partially a n d w e a k l y o v e r l a p p e d configurations b e t w e e n two anthracene rings, as i n d i c a t e d i n F i g u r e 3 (25, 16, 20). I n the case of D ( 1 - A ) E i n acetonitrile, for e x a m p l e ,

Figure 3. Schematic diagram of partially and weakly overlapped of 1,2-di-l-anthrylethane (left) and 1,2-di-9-anthrylethane

configurations (Hght).


100

E T IN INORGANIC, O R G A N I C , A N D BIOLOGICAL SYSTEMS

the formation o f the c o m p l e t e l y charge-separated state w i t h i n —30 ps has b e e n o b s e r v e d b y p i c o s e c o n d t i m e - r e s o l v e d absorption spectral measu r e m e n t s (15, 16). T h i s rise t i m e of the C S state m a y not b e unreasonable i n v i e w o f the r e l e v a n t energy gap - A G ° ~ 0.33 e V (15, 16).


C S

T h e p h o t o i n d u c e d C S i n t h e b a c t e r i a l p h o t o s y n t h e t i c reaction c e n t e r seems to start at the special p a i r (SP) of b a c t e r i o c h l o r o p h y l l s a n d l e a d to a series o f redox processes. I n the SP, two b a c t e r i o c h l o r o p h y l l c h r o m o p h o r e s interact w e a k l y a n d overlap o n l y partially w i t h each other. T h e p h o t o i n d u c e d C S i n the S P m i g h t b e i n d u c e d b y some e n v i r o n m e n t a l effect i n p r o t e i n s , o n w h i c h the c h r o m o p h o r e s are h e l d . T h i s c i r c u m s t a n c e is different from the p h o t o i n d u c e d C S i n composite systems w i t h t w o aromatic groups i n p e r p e n d i c u l a r configuration, as i n B A a n d T I C T (twisted i n t r a m o l e c u l a r charge transfer) c o m p o u n d s . R a t h e r , it is v e r y s i m i l a r to the example p r e sented o f the p h o t o i n d u c e d C S o f 1,2-dianthrylethanes i n a c e t o n i t r i l e . O n the o t h e r h a n d , - A G ° = 2.85 e V for the C R of the i n t r a m o l e c u l a r I P state o f D ( 1 - A ) E i n a c e t o n i t r i l e , w h i c h leads to £ R ~ 1 0 - 1 0 s o n the basis o f the o b s e r v e d b e l l - s h a p e d energy-gap d e p e n d e n c e o f the C R reaction of the geminate I P (37). H o w e v e r , the o b s e r v e d l i f e t i m e o f the I P state b e c o m e s m u c h shorter t h a n 10 ns as a result o f c o n v e r s i o n to the e x c i m e r state (15, 16). T h e e x c i m e r f o r m a t i o n m a y b e facilitated b y a slight m u t u a l a p p r o a c h of two c h r o m o p h o r e s i n the I P state. I f the m u t u a l a p p r o a c h o f t h e c h r o m o p h o r e s i n the p r e s e n t system is p r e v e n t e d b y fixing t h e m w i t h a r i g i d spacer, the l i f e t i m e of the I P state m a y b e c o m e m u c h longer. i p

C

7

8

_ 1

CS Processes in the Excited CT Complexes A n o t h e r e x t r e m e case of p h o t o i n d u c e d C S i n the strongly i n t e r a c t i n g D - A system is p r o v i d e d b y the e x c i t e d C T complexes. A b r i e f discussion follows o f the results of f e m t o s e c o n d - p i c o s e c o n d laser photolysis a n d t i m e - r e s o l v e d a b s o r p t i o n spectral studies o n aromatic h y d r o c a r b o n - T C N B (1,2,4,5-tetracyanobenzene), - a c i d a n h y d r i d e s , - T C N Q (tetracyanoquinodinomethane), a n d - T C N E (tetracyanoethylene) complexes. W e c o m p a r e the p h o t o i n d u c e d C S processes of these various C T complexes w i t h D a n d A of different strengths. I n a d d i t i o n , w e c o m p a r e the results o n C T complexes w i t h those o f the D - A systems c o m b i n e d d i r e c t l y b y single b o n d o r b y spacer. P r e v i o u s l u m i n e s c e n c e m e a s u r e m e n t s , nanosecond laser photolysis studies o n the T C N B - t o l u e n e s y s t e m , a n d some M O (molecular orbital) theoretical investigations o n its electronic structures i n the g r o u n d a n d exc i t e d states i n d i c a t e d a large change of g e o m e t r i c a l structure w i t h i n the c o m p l e x a n d the s u r r o u n d i n g solvents i n the course o f relaxation from the e x c i t e d F C to the e q u i l i b r i u m C S state (21-24,48). N e v e r t h e l e s s , the details o f this change w e r e still unclear. W e o b s e r v e d it d i r e c t l y w i t h femtosecond laser spectroscopy (12, 25-27). I n g e n e r a l , the rate of the C S i n the e x c i t e d state of C T complexes w i l l


6.

MATAGA



101


d e p e n d o n the configurations of D a n d A i n the c o m p l e x i n the g r o u n d state, e x c i t e d F C state, a n d relaxed I P state; the strength of interactions b e t w e e n D a n d A ; a n d the nature of the e n v i r o n m e n t , T h e d y n a m i c s a n d m e c h a n i s m s of the changes from the excited F C to the relaxed I P state can b e d e m o n strated b y t i m e - r e s o l v e d absorption spectral m e a s u r e m e n t s , as s h o w n i n F i g u r e s 4 a n d 5 for T C N B i n toluene s o l u t i o n . I m m e d i a t e l y after excitation, a slight change of absorption i n t e n s i t y a c c o m p a n i e d b y a slight s h a r p e n i n g of the b a n d shape t o w a r d the free T C N B a n i o n b a n d takes place w i t h a t i m e constant of 1.5 ps. T h i s spectral change can b e a s c r i b e d to the configuration change of D a n d A w i t h i n the 1:1 c o m p l e x from the F C excited state w i t h a s y m m e t r i c a l configuration t o w a r d a m o r e s y m m e t r i c a l o v e r l a p p e d s a n d w i c h - t y p e configuration as i n d i c a t e d i n F i g u r e 6, w h i c h increases the extent of C S a c c o r d i n g to the p r e v i o u s M O p r e d i c t i o n s (22-24). T h i s structural change w i t h i n the 1:1 c o m p l e x does not l e a d to the c o m p l e t e C S , b u t f u r t h e r i n t e r a c t i o n w i t h d o n o r a n d formation of the 1:2 c o m p l e x ( A " * D ) * is of c r u c i a l i m p o r t a n c e for it. A s s h o w n i n F i g u r e 5b, the s p e c t r u m at 170 Κ is v e r y s i m i l a r (even at 100 ps delay time) to that at 4.5 ps at r o o m t e m p e r a t u r e . T h e positive h o l e created b y r e m o v i n g an 1

2

+

Wavelength/η m

Time/ps

Figure 4. (a) Time-resolved transient absorption spectra of TCNB in toluene solution measured with the femtosecond laser photolysis method. (b,c) Time profiles of absorbance at 465 nm; c is an enlargement of the first section of b. (Reproduced from ref. 25. Copyright 1989 American Chemical Society.)


102


(α)

• Q

"Ό φ Ν


Ο ε I

J

I

•

«0.5ps

1 l\

°A.5ps • 100ps

1

1

1

1

Φ ο c

ο

χ»

Figure 5. (a) Transient absorption spec tra at several delay times corrected for the chirping of the monitoring white pulse. The absorption intensity is nor malized at 465 nm. (b) Transient absorp tion spectra at 100 ps delay time ob served at 170 K. (Reproduced from ref. 25. Copyright 1989 American Chemical Society.)

Ο ω

Χ)

ι

(

Α

-

.

D

2

+

) *

( 3 )

w h e r e δ, δ ' , a n d δ " represent the degree of partial charge transfer, T — 2 ps, 1.5 ps, a n d 550 fs, a n d T (time constant of the 1:2 c o m p l e x formation) = 20 ps, 30 ps, a n d 4 0 p s , r e s p e c t i v e l y , for b e n z e n e , t o l u e n e , a n d m e s i t y l e n e solutions (25, 26). W e also e x a m i n e d the p h o t o i n d u c e d C S of the T C N B complexes i n p o l a r solvents (25, 27). T h e results s h o w n i n F i g u r e 7 for the T C N B - t o l u e n e c o m p l e x i n A C N indicate that the solvent r e o r i e n t a t i o n can i n d u c e C S w i t h a t i m e constant shorter than 1 ps to a considerable extent, b u t not c o m p l e t e l y . D


R

Α •J

Ό

Φ Ν

U

Ρ

no η < 400

400

500

600

700

Wavelength / nm

(b)

1

·

Γ\

ο 35ps

450

1ps

500

Wavelength / nm

40

60

Time / ps

Figure 7. (a) Time-resolved transient absorption spectra of the TCNB-toluene complex in acetonitrile measured with the femtosecond laser photolysis method, (b) Transient absorption bands at 1- and 35-ps delay times, corrected for the chirping of the monitoring white pulse. The intensity is normalized at 462 nm (peak wavelength in acetonitrile). {j) Time profile of absorbance of corrected spectra at 462 nm. (Reproducedfrom ref. 25. Copyright 1989 American Chem ical Society.)


104


F o r the c o m p l e t e C S l e a d i n g to I P formation, f u r t h e r i n t r a c o m p l e x s t r u c t u r a l change a n d solvation, w h i c h take place w i t h a t i m e constant ( T ) of ~ 2 0 ps, s e e m to be necessary. cs

(A" " · D 8

+

8

V *

structural change and further solvation

^ - - D + V

(4)

The T value i n A C N has b e e n c o n f i r m e d as b e c o m i n g shorter w i t h l o w e r i n g of the oxidation p o t e n t i a l of the d o n o r (that is, T = 4 1 , 20, 13, 12, — 7 - 8 , a n d — 5 - 6 ps, respectively, for the T C N B complexes w i t h b e n zene, toluene, mesitylene, p-xylene, durene, and hexamethylbenzene donors, w i t h oxidation p o t e n t i a l decreasing i n this order). T h e s e T values are m u c h l o n g e r t h a n T of A C N . A c c o r d i n g l y , this C S process seems to i n v o l v e a considerable i n t r a c o m p l e x s t r u c t u r a l change w i t h i n the 1:1 complex. P r e s u m a b l y , this change i n c l u d e s a slight increase i n the distance b e t w e e n the c h a r g e d D a n d A , assisted b y strong solvation. T h e extent of this s t r u c t u r a l change w i l l be smaller i n the case of a d o n o r w i t h l o w e r oxidation p o t e n t i a l . W e have also c o n f i r m e d that the T value b e c o m e s shorter w i t h a n increase i n the solvent d i e l e c t r i c constant. T h e I P f o r m e d b y a r e o r i e n t a t i o n of the s u r r o u n d i n g solvent a n d a n i n t r a c o m p l e x structural change i n the course of relaxation f r o m the F C exc i t e d state of the c o m p l e x , as already discussed, seems to be a C I P (contact IP) w i t h o u t i n t e r v e n i n g solvent b e t w e e n D a n d A " i n the pair. T h e form a t i o n of the C I P f r o m the F C excited state becomes faster w i t h a n increase of the strength of the d o n o r (i.e., w i t h a decrease of the oxidation p o t e n t i a l of the donor, i n the case of the T C N B complexes). W e have o b s e r v e d a s i m i l a r effect w h e n the r e d u c t i o n p o t e n t i a l of the acceptor becomes h i g h e r , as follows (29, 30). c s


c s

c s

l

c s

+

C o m p a r e d w i t h T C N B , P M D A ( p y r o m e l l i t i c d i a n h y d r i d e ) is a little stronger e l e c t r o n acceptor. B y d i r e c t observation of C I P formation from the e x c i t e d F C state of the P M D A - t o l u e n e c o m p l e x i n acetonitrile w i t h f e m tosecond laser photolysis, w e f o u n d that T = 7 ps (30), c o m p a r e d w i t h T = 20 ps for the T C N B - t o l u e n e complex. S i m i l a r measurements o n the P M D A - h e x a m e t h y l b e n z e n e c o m p l e x i n acetonitrile solution a n d analysis of the results have i n d i c a t e d a few picoseconds as the f o r m a t i o n t i m e o f C I P (30). W h e n w e use stronger e l e c t r o n acceptors, such as T C N E a n d T C N Q , it has b e e n c o n f i r m e d that the formation of the C I P state b e c o m e s m u c h faster. F o r example, i n the case of the p y r e n e - T C N E a n d p e r y l e n e - T C N E c o m p l e x e s i n acetonitrile s o l u t i o n , the o b s e r v e d t i m e profiles of the transient absorbance of the C I P state can be r e p r o d u c e d b y c o n v o l u t i o n of the e x c i t i n g f e m t o s e c o n d laser pulse a n d the decay c u r v e of C I P w i t h a short decay t i m e of a few h u n d r e d femtoseconds, w i t h o u t t a k i n g into account the finite form a t i o n t i m e of C I P (30). C S

c s


6.

MATAGA



105

B y s u m m a r i z i n g these results o n the C S processes of r e l a t i v e l y w e a k C T complexes l i k e T C N B - b e n z e n e a n d T C N B - t o l u e n e systems, one m a y c o n c l u d e that the p h o t o i n d u c e d C S process is m u c h slower t h a n the solvent reorientation d y n a m i c s because of the i n t r a c o m p l e x configurational change. T h i s change seems to be necessary to cut the strong d e r e a l i z a t i o n i n t e r a c t i o n b e t w e e n ions i n the I P state, analogous to the case of A discussed i n the


0

c o m b i n e d D a n d A systems. T h e extent of the i n t r a c o m p l e x configurational change i n the course of the relaxation f r o m the F C e x c i t e d state of the c o m p l e x to the C I P b e c o m e s smaller i n the case of the stronger C T c o m p l e x . Specifically, the character o f the C I P state changes, d e p e n d i n g o n the nature of D a n d A . T h e e l e c t r o n i c a n d g e o m e t r i c a l structures of C I P w i l l b e c o m e closer to those of the excited C T state of the c o m p l e x itself w i t h an increase i n the strengths o f D a n d A (30).

CR Deactivation of Geminate IP A s discussed, the C R process o f the i n t r a m o l e c u l a r I P state p r o d u c e d b y p h o t o i n d u c e d C S o f P n a n d A n (n = 1, 2, 3) is almost 3 orders of m a g n i t u d e slower t h a n the p h o t o i n d u c e d C S itself i n alkanenitrile solutions. W e i n t e r p r e t e d this large difference b e t w e e n the C S a n d C R rate constants as d u e to t h e i r energy-gap d e p e n d e n c e . T h e C R s i n these systems are i n the i n v e r t e d r e g i o n a n d are q u i t e slow because of the large energy gap b e t w e e n the I P a n d g r o u n d state (13). A s m e n t i o n e d , w e have m a d e systematic studies of the energy-gap d e p e n d e n c e of the C R rate of geminate I P p r o d u c e d b y C S at the e n c o u n t e r b e t w e e n fluorescer a n d q u e n c h e r i n a strongly p o l a r solvent. O u r studies i n v o l v e d d i r e c t l y o b s e r v i n g the C R deactivation process c o m p e t i n g w i t h the dissociation b y means of ultrafast laser spectroscopy a n d m o n i t o r i n g the t i m e d e p e n d e n c e of the absorbance of geminate IP, ( A ~ - - * D ) . 1

1

A * + D o r A + * D * - » (A ~ l

s

··· D

s

+

)

S

S

+

A " + s

D

s

+

A + D ± - A ··· D T h i s investigation established the b e l l - s h a p e d energy-gap d e p e n d e n c e this t y p e of C R process of I P (17, 36, 37, 49), as i n d i c a t e d i n F i g u r e 8.

(5) of

O n the other h a n d , v e r y few systematic studies such as this have b e e n made o n the C R processes of the I P s f o r m e d b y excitation of the C T c o m plexes i n strongly p o l a r solutions. W e discussed the C S processes i n the e x c i t e d state o f various aromatic h y d r o c a r b o n - e l e c t r o n acceptor C T c o m plexes i n the section " C S Processes i n the E x c i t e d C T C o m p l e x e s " . W e c o n c l u d e d that the i n t r a c o m p l e x configuration change i n the e x c i t e d state is m o r e or less necessary for the C S of these strongly i n t e r a c t i n g e l e c t r o n


106


13 10

12

.

^7

16

/ °

5 ο 3

TO

9

ο /

ο \


θ 7

ι 2.0

I

0

1.0

o

1 3.0

-2dG?p/eV Figure 8. Energy-gap dependence of the CR rate constant of IP produced by CT complex excitation (Φ, A J compared with that formed by fluorescencequenching reaction (O, data taken from ref 8c) in acetonitrile solution. (1) Py -PA~, (2)An -PA~, (3)Per -PA~, (4)Naph -PMDA~, (5)Chr -PMDA-, (6) Py +-PMDA-, (7) Per -PMDA~, (8) Naph -TCNQ~, (9) Py -TCNE~, (10) Per -TCNE~, (11) Bz -PMDA~, (12) Tol -PMDA~, (13) m-XyV-PMDA~, (14) v-Xyl -PMDA~, (15) Du -PMDA-, (16) HMB+-PMDA~. Py: pyrene; An: anthracene; Per: perylene; Naph: naphthalene; Chr: chrysene; Bz: benzene; Toi: toluene; m-Xyl: m-xylene; p-Xyl: p-xylene; Du: durene; HMB: hexamethylbenzene; PA: phthalic anhydride; PMDA: pyromellitic dianhydride; TCNE: tetracyanoethylene; TCNQ: tetracyanoquinodimethane. (Reproducedfrom ref. 29. Copyright 1989 American Chemical Society.) +

+

+

+

+

+

+

+

+

+

+

+

+

d o n o r - a c c e p t o r systems (25-27, 30). T h e extent of the configuration change d e p e n d s o n the strengths of D a n d A . C S a c c o m p a n i e d b y configuration change has b e e n s h o w n to be a rather slow process i n the r e l a t i v e l y weak C T complexes s u c h as T C N B - b e n z e n e a n d T C N B - t o l u e n e systems. H o w e v e r , it is an ultrafast process i n s u c h strong C T complexes as T C N E - p y r e n e a n d T C N E - p e r y l e n e systems (30). S u c h C S processes taking place i n the strongly i n t e r a c t i n g D - A systems are q u i t e different from t h e s i m p l e r one that usually occurs i n w e a k l y i n teracting D - A systems. T h e d i s t i n c t i o n suggests a difference i n the s t r u c t u r e o f g e m i n a t e I P b e t w e e n the t w o cases o f C T c o m p l e x excitation a n d fluo r e s c e n c e - q u e n c h i n g reaction b y diffusional e n c o u n t e r . T h e structural dif ference i n the I P i n these t w o cases w i l l p r o f o u n d l y affect the behaviors o f the IP, s u c h as C R deactivation a n d dissociation into free ions. A c t u a l l y , w e p r e v i o u s l y o b s e r v e d that the k vs. - A G ° r e l a t i o n s h i p o f p y r e n e - T C N E I P p r o d u c e d b y the fluorescence-quenching reaction i n acetonitrile was i n t h e n o r m a l r e g i o n a n d fc = 2.6 Χ 1 0 s" . I n contrast, C R o f the same system b u t w i t h I P p r o d u c e d b y C T c o m p l e x excitation was m u c h faster; fc = 2 Χ 1 0 s" (17, 29, 30, 36, 37, 49). CE

CR

CR

1 2

i p

9

1

1


6.

MATAGA



107

A s w e discussed i n the previous section, the I P p r o d u c e d b y C T c o m p l e x excitation w i l l b e the C I P w i t h o u t i n t e r v e n i n g solvent m o l e c u l e s b e t w e e n D a n d A " i n the pair. T h e I P f o r m e d b y the f l u o r e s c e n c e - q u e n c h i n g r e action b y diffusional e n c o u n t e r w i l l be the so-called S S I P (solvent-separated IP) w i t h solvent molecules i n t e r v e n i n g b e t w e e n D and A " . The much stronger e l e c t r o n i c i n t e r a c t i o n b e t w e e n D a n d A " i n the C I P seems to result i n r e m a r k a b l y different fc values i n the p y r e n e - T C N E system. +

+

+


CR

I n v i e w of these results, w e have m a d e a systematic study of the C R decay of the I P f o r m e d b y excitation of various C T complexes a n d o b t a i n e d the results i n d i c a t e d i n F i g u r e 8 (29, 30), together w i t h the results for the I P f o r m e d b y the f l u o r e s c e n c e - q u e n c h i n g reaction. A s an e x a m p l e , results of the P M D A ( p y r o m e l l i t i c d i a n h y d r i d e ) c o m p l e x i n a c e t o n i t r i l e , m e a s u r e d w i t h the femtosecond laser photolysis m e t h o d , are s h o w n i n F i g u r e 9 (29, 30). I n F i g u r e 8, the energy-gap d e p e n d e n c e of the C R rate constant o f the I P f o r m e d b y the excitation of C T complexes w i t h D a n d A systems of various strengths is d e m o n s t r a t e d , together w i t h that of the I P p r o d u c e d b y the C S at e n c o u n t e r i n the f l u o r e s c e n c e - q u e n c h i n g reactions b e t w e e n s i m i l a r D a n d A systems i n the same solvent, acetonitrile. T h e - A G ° values are e v a l u a t e d i n b o t h cases b y the same c o n v e n t i o n a l m e t h o d . T h e - A G ° values for C I P i p

i p

500

600

700

λ/nm

t/ps

Figure 9. Femtosecond time-resolved absorption spectra of PMDA-HMB complex excited at 355 nm (A) in acetonitnle solution, and time profiles of absorbance at 665 nm observed for the PMDA-HMB (B) and PMDA-Naph (C) systems. (Reproduced from ref 29. Copyright 1989 American Chemical Society.)


108


can b e e s t i m a t e d e m p i r i c a l l y b y u s i n g e x p e r i m e n t a l results of the fluorescent C T c o m p l e x of t e t r a c h l o r o p h t h a l i c a n h y d r i d e ( T C P A ) a n d h e x a m e t h y l b e n z e n e (50). B y extrapolating the solvent-polarity effect o n the fluorescence Stokes shift (50), the wave n u m b e r of the C T fluorescence b a n d peak Çv J) of the T C P A - h e x a m e t h y l b e n z e n e c o m p l e x i n acetonitrile, w h e r e this c o m p l e x is p r a c t i c a l l y nonfluorescent, can be o b t a i n e d . F r o m the wave n u m b e r of the C T absorption b a n d peak ( v ) and v v a l u e , w e can estimate the s u m (Δν) of the F C destabilization energies i n the excited a n d g r o u n d state by ma


max

v

m a x

a

-

a

m a x

vJ m

f

Δν

=

(6)

A s s u m i n g this Δ ν value to b e t y p i c a l of the C I P p r o d u c e d b y e x c i t i n g a r e l a t i v e l y strong C T c o m p l e x i n acetonitrile, a r o u g h a p p r o x i m a t i o n of the value of -àG ° for the C I P state f o r m e d b y e x c i t i n g the nonfluorescent c o m p l e x w i t h the lowest C T absorption peak at v may be given b y ip

m a x

-àG °~hc ip

^v

m a x

a

-

a

I AvJ

(7)

T h e - A G ° values estimated b y e q 7 are shifted about 0.2 e V to the h i g h e r e n e r g y side as a w h o l e , c o m p a r e d w i t h the values o b t a i n e d b y the c o n v e n t i o n a l m e t h o d of u s i n g the oxidation p o t e n t i a l of D a n d r e d u c t i o n p o t e n t i a l of A i n acetonitrile. Because the -AG ° values shift as a w h o l e , the f u n c t i o n a l f o r m of the energy-gap d e p e n d e n c e of fc m a y not be seriously affected b y the m e t h o d for evaluation of -àG °. i p

ip

CR

ip

T h e reaction scheme analogous to e q 5 i n the case of the excitation of the C T c o m p l e x m a y b e g i v e n b y e q 8 or 9.

'(A" ' 8

vD

+ 8

')s

F C

A s already discussed, the C I P f o r m e d b y excitation of the C T c o m p l e x i n acetonitrile solution generally shows a q u i t e different C R deactivation rate, c o m p a r e d w i t h that of the S S I P of the same D , A p a i r p r o d u c e d b y C S at diffusional e n c o u n t e r i n the fluorescence-quenching reaction i n the same


6.

MATAGA


109


solvent. T h i s difference can b e ascribed to the s t r u c t u r a l difference

between

these I P s . I n the course of the relaxation from the F C e x c i t e d state o f the c o m p l e x to the I P a n d finally to dissociated ions c o m p e t i n g w i t h the C R d e a c t i v a t i o n f r o m t h e I P state, an S S I P state s i m i l a r to that f o r m e d b y diffusional e n c o u n t e r i n the


reaction m a y be p r o d u c e d f r o m C I P

as the p r e c u r s o r state of the i o n i c dissociation. H o w e v e r , t h e r e was p r a c t i c a l l y no d i r e c t observation of the reaction s c h e m e of e q 9, except o u r

recent

results o f p i c o s e c o n d laser spectroscopic studies o n T C N B - t o l u e n e , - b e n


z e n e , a n d - x y l e n e s i n acetonitrile (28).

M a n y o t h e r systems do not s h o w

s u c h b e h a v i o r , b u t the o b s e r v e d result can be r e p r o d u c e d w e l l b y t h e r e action s c h e m e of e q 8

(28-30).

M o r e o v e r , i n almost a l l cases e x a m i n e d i n F i g u r e 8, & larger t h a n its dissociation rate constant

(fcd

i s s

),

of C I P is m u c h

C R

w h i c h leads to the n e g l i g i b l e

dissociation y i e l d . I n the P M D A c o m p l e x e s i n F i g u r e 9, fc R

=

C

s" a n d k 1

diss

a n d fc

CR

3.8 Χ

10

1 0

= 2 Χ 1 0 s" for the P M D A - n a p h t h a l e n e C I P i n a c e t o n i t r i l e 9

= 1.9 Χ 1 0

1 1

1

s" a n d fc 1

diss

= 2 Χ 10 s 9

for the P M D A - H M B C I P

1

i n a c e t o n i t r i l e . T h e dissociation y i e l d i n the latter system is p r a c t i c a l l y z e r o . A l t h o u g h w e assume a loose structure w i t h i n t e r v e n i n g solvent m o l e cules b e t w e e n A " a n d D

ions for the geminate I P f o r m e d b y the

+

fluores

c e n c e - q u e n c h i n g reaction i n acetonitrile s o l u t i o n , this g e m i n a t e I P w i l l b e q u i t e different f r o m the c a t i o n - e l e c t r o n pairs i n n o n p o l a r solvents that are f r e q u e n t l y i n v e s t i g a t e d i n r a d i a t i o n c h e m i s t r y . I n s u c h loose g e m i n a t e p a i r s , t h e y can u n d e r g o a w i d e range of t h e r m a l motions before C R a n d dissocia t i o n . T h e geminate S S I P f o r m e d b y


r e a c t i o n i n ace

t o n i t r i l e solution w i l l have a r a t h e r definite structure. T h e fact that t h e

fc

CR

vs. - A G ° r e l a t i o n s h i p for those S S I P s shows a r a t h e r t y p i c a l b e l l shape i p

indicates strongly that the S S I P s of various D - A systems have a definite s t r u c t u r e w i t h s i m i l a r i n t e r i o n i c distance a n d s i m i l a r solvent r e o r g a n i z a t i o n energies. T h i s s i m i l a r i t y suggests that the i n t e r a c t i o n b e t w e e n each i o n i n the I P a n d the s u r r o u n d i n g polar solvents is strong a n d r a t h e r specific.

Energy-Gap Dependence ofk

CIP

CR

C o n t r a r y to the b e l l - s h a p e d energy-gap d e p e n d e n c e of the fc

CR

&CR

v s

of S S I P , t h e

- - A G ° r e l a t i o n s h i p for C I P of s i m i l a r D - A systems is q u i t e different i p

a n d can b e g i v e n b y fc R C

CIP

= aexp[- |AG 7

i p

°|]

w h e r e a a n d 7 are constants i n d e p e n d e n t of A G ° . T h e energy-gap i p

(10) depen

d e n c e of e q 10 is q u a l i t a t i v e l y analogous to that of the radiationless t r a n s i t i o n p r o b a b i l i t y i n the so-called " w e a k c o u p l i n g " l i m i t (51). I n this respect, e x a m i n e d the effect of d e u t e r a t i o n o n the &

C R

we

by using perdeuterated toluene

a n d b e n z e n e . W e d e t e c t e d no effect of d e u t e r a t i o n . It also seems difficult to give a reasonable i n t e r p r e t a t i o n for the o b s e r v e d 7 v a l u e of e q 10, the


110


slope i n the p l o t o f l o g k ° against |AG °|, o n the basis o f the u s u a l t h e o r y of radiationless transition. A n e w theoretical i n t e r p r e t a t i o n of this C R process is n e e d e d . CB

lF

ip

W e give h e r e a tentative i n t e r p r e t a t i o n o f this k ° vs. - A G ° r e l a t i o n s h i p o n the basis of the i d e a d e r i v e d f r o m o u r p r e v i o u s studies o n exciplexes a n d e x c i t e d C T complexes (1,21, 52), as w e l l as the recent studies o n the ultrafast d y n a m i c s of the excited C T complexes a n d C I P s (12-15, 25-30). T h e e l e c t r o n i c a n d g e o m e t r i c a l structures of these strongly i n t e r acting D - A systems v a r y w i t h the strengths o f D a n d A , as w e l l as the solvent polarity. Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch006

cv

lF

i p

I n the p r e v i o u s sections, w e discussed the facts that, as the s t r e n g t h o f D a n d A increases, the formation of C I P from the e x c i t e d F C state of the C T c o m p l e x b e c o m e s faster a n d the absorption s p e c t r u m of the transient C I P state b e c o m e s b r o a d e r c o m p a r e d w i t h that of dissociated ions a n d o f the S S I P i n acetonitrile solutions. T h i s result indicates that the extent o f the e l e c t r o n i c a n d g e o m e t r i c a l structure change, i n c l u d i n g s u r r o u n d i n g solvent i n the C I P formation process, is smaller i n the stronger C T complexes. I n o t h e r w o r d s , the stronger the c o m p l e x , the closer is the p o s i t i o n o f the p o t e n t i a l m i n i m u m o f the C I P state o n the reaction coordinate i n c l u d i n g the g e o m e t r i c a l configuration o f D a n d A , as w e l l as the extent of solvation to that o f the C T c o m p l e x itself, as s h o w n i n F i g u r e 10. F i g u r e 10 shows that observation of the n o r m a l r e g i o n is difficult i n the energy-gap d e p e n d e n c e of & for s u c h a s m a l l h o r i z o n t a l shift of the p o t e n t i a l m i n i m u m o f the C I P state relative to the g r o u n d state. W i t h a m u c h l a r g e r h o r i z o n t a l shift of the p o t e n t i a l m i n i m u m o f the I P state along the reaction coordinate, observation of the n o r m a l r e g i o n w i l l b e c o m e possible at s m a l l - A G ° values, as i n d i c a t e d i n F i g u r e 11. T h e large h o r i z o n t a l shift represents a large structural change o f the C I P , i n c l u d i n g the solvation state. T h e shift corresponds to the formation o f the S S I P state for s u c h D - A systems w i t h s m a l l - A G ° values as p y r e n e - T C N E a n d p e r y l e n e - T C N E i n acetonitrile solutions. A c t u a l l y , w e have o b s e r v e d the r e l a t i v e l y s m a l l k values o f the S S I P o f these systems f o r m e d b y diffiisional e n c o u n t e r i n the fluorescenceq u e n c h i n g reaction i n the n o r m a l r e g i o n ; the fc values of C I P o f the same system are e x t r e m e l y large. C R

i p

i p

CR

CR

T h i s i n t e r p r e t a t i o n of the C R mechanisms f r o m the C I P a n d S S I P states is somewhat analogous to the radiationless transition m e c h a n i s m s i n the w e a k c o u p l i n g a n d strong c o u p l i n g l i m i t (51), respectively. H o w e v e r , it seems to b e difficult to i n t e r p r e t q u a n t i t a t i v e l y the energy-gap d e p e n d e n c e o f k c o v e r i n g a w i d e energy-gap range a n d w i t h a v e r y m i l d slope i n the p l o t against - A G ° , i n terms of the m e c h a n i s m of a " w e a k c o u p l i n g " l i m i t . I n spite o f this it seems possible, at least qualitatively, to i n t e r p r e t the e n e r g y gap d e p e n d e n c e of fc b y taking into consideration the change of the s t r u c t u r e of C I P as it d e p e n d s o n the strength of D a n d A (i.e., d e p e n d i n g o n the - A G ° v a l u e , as d e m o n s t r a t e d i n F i g u r e 10). T h e analogy b e t w e e n the CE

i p

CR

i p



6.

MATAGA



111

Reaction Coordinate Figure 10. Free-energy curves for the IP states and the ground state (GS) of D-A systems vs. reaction coordinate. Change of the position of the potential minimum of the CIP depends on the change of the - A G ° value, a result illustrating that the CR reaction of CIP is in the inverted region for all - A G ° values. (Reproduced from ref 30. Copyright 1991 American Chemical Society.) ip

fp

theories o f radiationless transition a n d o f electron-transfer reaction has b e e n r e c o g n i z e d for a l o n g t i m e . T h e result s h o w n i n F i g u r e 8 m a y b e a n e x p e r i m e n t a l c o u n t e r p a r t for this analogy, illustrating t h e t w o cases i n e l e c t r o n transfer qualitatively c o r r e s p o n d i n g to the " w e a k c o u p l i n g " a n d " s t r o n g c o u p l i n g " cases i n radiationless transition.

Concluding Remarfa W e have discussed t h e m e c h a n i s m s o f p h o t o i n d u c e d C S a n d C R o f t h e p r o d u c e d I P o r C T state o n t h e basis o f o u r results o b t a i n e d b y f e m t o s e c o n d - p i c o s e c o n d laser photolysis studies, m a i n l y o n t h e strongly i n t e r -


112



Reaction Coordinate Figure 11. Relationship between the free-energy curves between the ground state and the S SIP state corresponding to the small -AG ° value, where the large horizontal shift of the potential minimum of S SIP against ground state brings the CR rate constant of S SIP to the normal region. (Reproduced from ref 30. Copyright 1991 American Chemical Society.) ip

acting D - A systems c o m b i n e d b y spacers o r d i r e c t l y b y s i n g l e - b o n d a n d C T complexes. T h e most i m p o r t a n t conclusions are as follows: 1. P h o t o i n d u c e d C S processes i n such strongly i n t e r a c t i n g D - A systems cannot b e d e s c r i b e d b y t h e s i m p l e two-state m o d e l , ( D * ··· A o r D ··· A * ) —» D ··· A " . T h i s m o d e l assumes that weak i n t e r a c t i o n is responsible for the e l e c t r o n transfer, as is the case i n the c o n v e n t i o n a l electron-transfer theories. I n s u c h systems, t h e C S proceeds b y gradual change o f e l e c t r o n i c structure d u e to t h e extensive solvation o f the D - A system, a c c o m p a n i e d b y some change o f its g e o m e t r i c a l s t r u c t u r e , w h i c h decreases t h e electronic d e r e a l i z a t i o n i n t e r a c t i o n b e t w e e n D a n d A that facilitates t h e C S . S u c h b e h a v i o r i n t h e C S process of the i n t r a m o l e c u l a r exciplexes a n d C T complexes w i t h D a n d A i n t e r a c t i n g strongly is i n accordance w i t h t h e i d e a I p r o p o s e d m a n y years ago that the e l e c t r o n i c a n d geom e t r i c a l structures o f exciplexes a n d excited C T complexes vary w i t h t h e strengths o f D a n d A a n d w i t h t h e solvent polarity (21, 52). T h a t is, as t h e oxidation p o t e n t i a l o f D d e creases a n d t h e r e d u c t i o n p o t e n t i a l o f A a n d t h e solvent p o larity increase, the electronic d e r e a l i z a t i o n interaction b e t w e e n D a n d A decreases a n d the structure o f the exciplex comes close to that o f the i o n p a i r . +

2. D i r e c t observation o f t h e C I P formation process f r o m t h e e x c i t e d F C state o f the C T c o m p l e x i n acetonitrile solution has r e v e a l e d that the extent o f the electronic a n d g e o m e t r i c a l structural changes, i n c l u d i n g the s u r r o u n d i n g solvent configurations i n the course o f C I P f o r m a t i o n , is s m a l l e r for D w i t h


6.

MATAGA



113


the s m a l l e r oxidation p o t e n t i a l a n d A w i t h the h i g h e r r e d u c t i o n p o t e n t i a l . T h i s r e l a t i o n s h i p also means that the extent o f the g e o m e t r i c a l structural change necessary to decrease the d e localization i n t e r a c t i o n i n the course of C I P f o r m a t i o n is smaller i n the case of stronger D a n d A , i n agreement w i t h the reasoning g i v e n i n paragraph 1. 3. T h e C I P f o r m e d b y excitation of the C T c o m p l e x i n acetonitrile solution undergoes C R deactivation. T h e C R rate constant shows a p e c u l i a r energy-gap d e p e n d e n c e [i.e., a monotonous (exponential) increase of the C R rate w i t h decrease o f the freeenergy gap -àG ° b e t w e e n the C I P a n d the g r o u n d state]. T h i s d e p e n d e n c e contrasts w i t h the C R rate of S S I P f o r m e d b y C S at d i f f u s i o n a l e n c o u n t e r b e t w e e n f l u o r e s c e r a n d q u e n c h e r , w h i c h shows a b e l l - s h a p e d energy-gap d e p e n dence. T h e p e c u l i a r energy-gap d e p e n d e n c e of the C R rate of C I P has b e e n i n t e r p r e t e d o n the basis of the reasoning g i v e n i n paragraph 2. I n this v i e w , the s t r u c t u r a l changes f o l l o w i n g the excitation o f the C T c o m p l e x affect the p o s i t i o n of the p o t e n t i a l m i n i m u m of the C I P state o n the reaction coordinate, w h i c h involves the D , A configuration a n d solvat i o n . T h u s the C R rate of S S I P comes closer to that of the C T c o m p l e x itself w i t h increase of the strength o f D a n d A , w h i c h makes it difficult to observe the n o r m a l r e g i o n i n the e n e r g y gap d e p e n d e n c e o f the C R rate. ip

4. T h e s i m i l a r i t y b e t w e e n the energy-gap d e p e n d e n c e of the C R rate of the C I P a n d that of the radiationless transition p r o b ability i n the so-called " w e a k c o u p l i n g " l i m i t has b e e n p o i n t e d out.

Acknowledgments T h e studies discussed i n this chapter w e r e s u p p o r t e d b y a G r a n t - i n - A i d ( N o . 6265006) from the Japanese M i n i s t r y of E d u c a t i o n , S c i e n c e , a n d C u l t u r e a n d w e r e made i n collaboration w i t h f o l l o w i n g i n d i v i d u a l s , whose c o n t r i b u t i o n I appreciate: S. N i s h i k a w a , S. O j i m a , T. A s a h i , H . M i y a s a k a , a n d T. Okada.

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Electron Transfer in Inorganic, Organic, and Biological Systems

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