Metal Complexes

Chapter 5.5

Photochemical Reactions of Transition Metal Complexes Induced by Intramolecular Electron Transfer between Weakly Coupled Redox Centers

A. Vogler

2 RESONANCE ELECTRON TRANSFER BY OPTICAL NMCT EXCITATION

3 EXCITED STATE (NON-RESONANCE) ELECTRON TRANSFER 3.1 I n t r o d u c t i o n

3.2 I r r e v e r s i b l e P h o t o r e d u c t i o n o f C o ( I I I ) 3.2.1 A r o m a t i c M o l e c u l e s as E l e c t r o n Donors 3.2.2 M e t a l Complexes as E l e c t r o n Donors 3.3 R e v e r s i b l e E x c i t e d S t a t e E l e c t r o n T r a n s f e r 3.3.1 I n t r o d u c t i o n

3.3.2 L i g a n d t o M e t a l E l e c t r o n T r a n s f e r 3.3.3 M e t a l t o M e t a l E l e c t r o n T r a n s f e r 3.3.4 L i g a n d t o L i g a n d E l e c t r o n T r a n s f e r

4 . CONCLUSION

5 REFERENCES INTRODUCTION

1 INTRODUCTION

L i g h t - i n d u c e d e l e c t r o n t r a n s f e r may t a k e p l a c e as an i n t e r - o r i n t r a m o l e - c u l a r p r o c e s s . There i s c e r t a i n l y no fundamental d i f f e r e n c e between both p o s s i b i l i t i e s . However, i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r i s g e n e r a l l y much b e t t e r d e f i n e d w i t h r e g a r d t o t h e g e o m e t r i c a l arrangement o f e l e c t r o n donor and a c c e p t o r . U n f o r t u n a t e l y , t h e i n v e s t i g a t i o n o f i n t r a m o l e c u l a r l i g h t - i n d u - ced e l e c t r o n t r a n s f e r i s hampered by o t h e r c o m p l i c a t i o n s .

In most t r a n s i t i o n metal complexes t h e e l e c t r o n i c i n t e r a c t i o n between metal and l i g a n d s cannot be n e g l e c t e d and may be r a t h e r s t r o n g . C o n s e q u e n t l y ,

i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r i n v o l v i n g metal and l i g a n d s t a k e s p l a c e between c o u p l e d redox c e n t e r s . In t h e s e c a s e s i t i s o f t e n not q u i t e c l e a r what f r a c t i o n o f c h a r g e i s t r a n s f e r r e d from t h e metal t o t h e l i g a n d and v i c e v e r s a s i n c e donor and a c c e p t o r órbita Is a r e d e l o c a l i z e d t o a c e r t a i n d e g r e e . The m a j o r i t y o f p h o t o r e d o x p r o c e s s e s o f metal complexes which have been r e - p o r t e d ( r e f s . 1-3) t a k e p l a c e upon d i r e c t o p t i c a l c h a r g e t r a n s f e r (CT) e x c i - t a t i o n i n v o l v i n g redox c e n t e r s w i t h s u b s t a n t i a l c o u p l i n g . A l t h o u g h t h i s r e - view d e a l s e s s e n t i a l l y o n l y w i t h systems c o n s i s t i n g o f w e a k l y c o u p l e d donors and a c c e p t o r s a s h o r t g e n e r a l s u r v e y o f i n t r a m o l e c u l a r o p t i c a l CT e x c i t a t i o n of t r a n s i t i o n metal complexes r e g a r d l e s s o f t h e e x t e n t o f c o u p l i n g i s g i v e n h e r e . CT t r a n s i t i o n s a r e c l a s s i f i e d a c c o r d i n g t o t h e redox s i t e s ( r e f . 4 ) . L i g a n d t o M e t a l Charge T r a n s f e r (LMCT)

LMCT i s a c l a s s i c a l o p t i c a l t r a n s i t i o n o f m e t a l c o m p l e x e s . C o r r e s p o n d i n g a b s o r p t i o n bands are o b s e r v e d at low e n e r g i e s i f t h e metal i s o x i d i z i n g and t h e l i g a n d r e d u c i n g . C o ( I I I ) and Fe(111) complexes are wel1-documented exam- p l e s . The c o l o r s of d ° o x o m e t a l l a t e s such as t h e y e l l o w C r O ^ ~ and t h e v i o l e t Mn0⁴~ are caused by LMCT b a n d s . In many c a s e s LMCT e x c i t a t i o n i s a s s o c i a t e d w i t h t h e r e d u c t i o n of t h e metal and o x i d a t i o n of t h e l i g a n d . C o ( I I I ) ammines undergo such p h o t o r e d o x r e a c t i o n s ( r e f s . 1,2).

Metal t o L i g a n d Charge T r a n s f e r (MLCT)

MLCT i s a n o t h e r c l a s s i c a l o p t i c a l t r a n s i t i o n of metal c o m p l e x e s . MLCT ab- s o r p t i o n bands appear a t long w a v e l e n g t h i f t h e metal i s r e d u c i n g and a l i -

gand p r o v i d e s low-energy empty o r b i t a l s . Complexes such as Ü F e í C N ) ^ "

and [ R u ( b i p y )3] ^+ ( b i p y = 2 , 2 " - b i p y r i d y l ) a r e t y p i c a l c a s e s ( r e f . 4 ) . In a d d i t i o n , o r g a n o m e t a l 1 i c s which c o n t a i n a metal i n a low o x i d a t i o n s t a t e and K - a c c e p t i n g l i g a n d s such as an o l e f i n o r an a r o m a t i c m o l e c u l e a r e c h a r a c t e r i - zed by low-energy MLCT bands ( r e f . 3 ) . S i n c e metal l i g a n d bonding i s not much a f f e c t e d i n t r a m o l e c u l a r p h o t o r e a c t i o n s do g e n e r a l l y not o c c u r on MLCT e x c i t a - t i o n . L i g a n d s a r e e l e c t r o n r i c h and cannot be e a s i l y reduced t o s t a b l e spe- c i e s . MLCT e x c i t a t i o n i s t h e r e f o r e o f t e n a s s o c i a t e d w i t h p h o t o o x i d a t i o n o f t h e metal w h i l e an e l e c t r o n i s t r a n s f e r r e d from t h e l i g a n d t o a n o t h e r mole- c u l e by an i n t e r m o l e c u l a r p r o c e s s . In t h e case o f [ F e ( C N ) g ]4" t h e s o l v e n t may a c t as e l e c t r o n a c c e p t o r ( r e f . 2 ) . In t h e MLCT s t a t e t h e l i g a n d can be a l s o s u s c e p t i b l e t o an e l e c t r o p h i 1 i c a t t a c k . The a d d i t i o n o f p r o t o n s t o c o o r - d i n a t e d NO ( r e f . 5) o r c a r b y n e s ( r e f . 6) i l l u s t r a t e s t h i s t y p e o f e x c i t e d s t a t e r e a c t i v i t y .

L i g a n d t o L i g a n d Charge T r a n s f e r (LLCT)

LLCT a b s o r p t i o n s were o b s e r v e d o n l y r e c e n t l y . These bands appear i f one l i g a n d i s r e d u c i n g and a n o t h e r o x i d i z i n g . LLCT bands were d e t e c t e d i n t h e s p e c t r a o f complexes o f t h e t y p e { 1 , 2 - d i i m i n e j M¹^ 1 , 2 - e t h y l e n e d i t h i o l a t e ) w i t h M = N i , P d , P t ( r e f s . 7, 8) and r e l a t e d complexes ( r e f s . 9 , 1 0 ) . The d i - imine w i t h i t s empty * o r b i t a l s i s here t h e e l e c t r o n a c c e p t o r w h i l e t h e d i - t h i o l a t e a c t s as d o n o r . LLCT t r a n s i t i o n s can be a l s o i d e n t i f i e d i n t}\e spec- t r a o f complexes c o n t a i n i n g t h e same l i g a n d i n an o x i d i z e d and reduced f o r m . [ R u ^ i b i p y ) ^ * i s a p p a r e n t l y composed o f two b i p y l i g a n d s and one i n

i t s reduced s t a t e ( b i p y " ) . The e l e c t r o n i c spectrum o f t h i s complex d i s p l a y s a b i p y " t o b i p y LLCT band i n t h e near IR ( r e f . 1 1 ) . A p h o t o r e a c t i o n o r i g i - n a t i n g from a LLCT s t a t e was r e p o r t e d r e c e n t l y ( r e f . 7 ) .

I n t r a L i g a n d Charge T r a n s f e r (ILCT)

A l i g a n d i t s e l f may c o n s i s t o f a r e d u c i n g and an o x i d i z i n g p a r t . The spec- trum o f t h e metal complex as w e l l as t h a t o f t h e f r e e l i g a n d s h o u l d show low- energy ILCT b a n d s . An example o f an o p t i c a l ILCT t r a n s i t i o n i s indeed known ( r e f . 1 2 ) .

M e t a l t o M e t a l Charge T r a n s f e r (MMCT)

L i g a n d - b r i d g e d b i n u c l e a r ( o r p o l y n u c l e a r ) complexes which c o n t a i n a redu- c i n g and an o x i d i z i n g metal a r e c h a r a c t e r i z e d by o p t i c a l f$CT t r a n s i t i o n s a t low e n e r g i e s ( r e f s . 4 , 1 3 - 1 9 ) . There a r e s e v e r a l well-documented examples o f p h o t o c h e m i c a l r e a c t i o n s i n d u c e d by i n t r a m o l e c u l a r MMCT e x c i t a t i o n ( r e f . 1 9 ) . In t h e f i r s t p a r t o f t h i s r e v i e w t h e y a r e d i s c u s s e d i n some d e t a i l as t y p i c a l c a s e s o f photoredox p r o c e s s e s i n i t i a t e d by o p t i c a l CT t r a n s i t i o n s between weakly i n t e r a c t i n g redox s i t e s .

A l i g h t - i n d u c e d e l e c t r o n t r a n s f e r i n v o l v i n g weafciy c o u p l e d redox c e n t e r s

cannot o n l y t a k e p l a c e by d i r e c t o p t i c a l CT e x c i t a t i o n ( r e s o n a n c e t r a n s f e r ) . As a second p o s s i b i l i t y an i n t r a m o l e c u l a r e x c i t e d s t a t e e l e c t r o n t r a n s f e r may o c c u r (non-resonance t r a n s f e r ) . T h i s p r o c e s s does n o t r e q u i r e t h e p r e s e n c e o f CT a b s o r p t i o n s . The i n i t i a l i n t e r n a l e x c i t a t i o n o f an e l e c t r o n donor o r a c c e p t o r can be f o l l o w e d by e l e c t r o n t r a n s f e r . Examples o f t h i s t y p e a r e e l a - b o r a t e d i n t h e second p a r t o f t h i s r e v i e w .

The f u r t h e r d i s c u s s i o n i s e s s e n t i a l l y r e s t r i c t e d t o systems which a r e sub- j e c t t o a permanent p h o t o c h e m i c a l change. However, r e f e r e n c e i s a l s o made t o complexes which undergo a r e v e r s i b l e i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r . In t h e s e c a s e s t h e o c c u r e n c e o f e l e c t r o n t r a n s f e r was d e t e c t e d by e m i s s i o n o r t i m e - r e s o l v e d a b s o r p t i o n s p e c t r o s c o p y .

2 RESONANCE ELECTRON TRANSFER BY OPTICAL MMCT EXCITATION

The e l e c t r o n i c c o u p l i n g o f m e t a l l i c redox c e n t e r s i n b i n u c l e a r complexes has been s t u d i e d e x t e n s i v e l y f o r many y e a r s ( r e f s . 1 3 - 1 8 ) . The t h e o r y advan- ced by N . S . Hush p r o v i d e s t h e b a s i s f o r a d i s c u s s i o n o f t h i s i n t e r a c t i o n ( r e f s . 1 5 , 16, 18, 2 0 , 2 1 ) . A b i n u c l e a r 1 i g a n d - b r i d g e d complex o f t h e t y p e

r e d ox r e d M^a -L-M^ß i s composed o f t h e two mononuclear components M^Q -L and

ox

L-M^ß . B e s i d e s t h e b r i d g i n g l i g a n d both m e t a l s a r e c o o r d i n a t e d t o o t h e r t e r m i n a l l i g a n d s . The e l e c t r o n i c spectrum o f t h e b i n u c l e a r complex c o n s i s t s o f t h e superimposed s p e c t r a o f t h e mononuclear components i f t h e i n t e r a c t i o n between t h e r e d u c i n g and o x i d i z i n g metal i s weak. In a d d i t i o n , a new a b s o r p - t i o n band appears w h i c h b e l o n g s t o t h e o p t i c a l MMCT t r a n s i t i o n from t h e r e d u - c i n g t o t h e o x i d i z i n g metal ( r e f s . 1 3 - 1 9 ) .

With i n c r e a s i n g m e t a l - m e t a l i n t e r a c t i o n t h e i n d i v i d u a l components l o o s e t h e i r i d e n t i t y . F i n a l l y , t h e v a l e n c e o r b i t a l s o f both m e t a l s a r e c o m p l e t e l y d e l o c a l i z e d and t h e m e t a l s do not any more e x i s t i n w e l l - d e f i n e d ("trapped") o x i d a t i o n s t a t e s . As a consequence t h e s p e c t r a l f e a t u r e s o f t h e mononuclear components a r e n o t any more a p p a r e n t and MMCT bands do not o c c u r . They a r e now r e p l a c e d by e l e c t r o n i c t r a n s i t i o n s i n v o l v i n g o r b i t a l s w h i c h a r e d e l o c a l i - zed between b o t h m e t a l s . An a n a l y s i s o f t h i s a b s o r p t i o n band p r o v i d e s t h e

p i n f o r m a t i o n on t h e e x t e n t o f d e r e a l i z a t i o n a .

4.24 . 1 0 "4 • c • AO...

2 max 1/2 a -

^max • d 2

The p a r a m e t e r s a r e t h e energy o f t h e MMCT band a t i t s maximum ^m a x, t h e m o l a r e x t i n c t i o n c o e f f i c i e n t c m x> t h e h a l f - w i d t h o f t h e MMCT band ^>\/2 and t h e m e t a l - m e t a l d i s t a n c e d . A t c o m p l e t e d e l o c a l i z a t i o n a2 i s u n i t y w h i l e t h e v a l e n c i e s a r e t r a p p e d i f * i s much s m a l l e r t h a n u n i t y . O n l y i n t h e l a t t e r c a s e MMCT t r a n s i t i o n s are a s s o c i a t e d w i t h a c o m p l e t e e l e c t r o n

t r a n s f e r from one metal t o a n o t h e r . The f o l l o w i n g d i s c u s s i o n i s r e s t r i c t e d t o t h i s type o f CT i n t e r a c t i o n .

The energy o f t h e o p t i c a l MMCT t r a n s i t i o n depends on t h e p o t e n t i a l d i f f e - red/ox red/ox

rence AE between t h e redox c o u p l e s M and M. and on t h e r e o r g a - n i z a t i o n a l energy x . a p

v = AE + x max A

The parameter x c o n s i s t s o f an o u t e r - and an i n n e r - s p h e r e c o n t r i b u t i o n , x = x0 + x¿

W h i l e X j i s an i n t r i n s i c p r o p e r t y o f t h e b i n u c l e a r complex xQ depends on t h e r e o r g a n i z a t i o n o f s o l v e n t environment a c c o r d i n g t o

x =e

(i-

i - -!)(' -I)

0 2 a¹ 2 a² d n D

The parameters a^ and a² a r e t h e r a d i i o f t h e c o o r d i n a t i o n s p h e r e s o f both m e t a l s , d i s t h e m e t a l - m e t a l d i s t a n c e , n and D a r e t h e r e f r a c t i v e i n d e x and t h e s t a t i c d i e l e c t r i c c o n s t a n t o f t h e s o l v e n t . T h i s e q u a t i o n means s i m p l y t h a t xQ and hence t h e energy o f t h e MMCT t r a n s i t i o n v a r i e s w i t h t h e p o l a r i - t y o f t h e s o l v e n t . S i n c e t h e s p e c i f i c complexes d i s c u s s e d below were i n v e s t i - g a t e d o n l y i n aqueous s o l u t i o n d e t a i l s on t h e s o l v e n t dependence a r e not e l a - b o r a t e d h e r e .

The i n n e r - s p h e r e c o n t r i b u t i o n t o t h e r e o r g a n i z a t i o n a l energy i s a f r a c t i o n o f t h e Franck-Condon MMCT t r a n s i t i o n as shown i n F i g . 1. I t depends on t h e s t r u c t u r a l d i s t o r t i o n which accompanies e l e c t r o n t r a n s f e r . T h i s r e o r g a n i z a - t i o n which may be r e p r e s e n t e d by changes o f t h e m e t a l - l i g a n d bond l e n g t h o f one metal c e n t e r v a r i e s w i t h t h e o x i d a t i o n s t a t e o f t h i s m e t a l . F r e q u e n t l y r e d u c t i o n i s a s s o c i a t e d w i t h an e x t e n s i o n o f t h e m e t a l - l i g a n d d i s t a n c e .

The o p t i c a l MMCT i s a Franck-Condon t r a n s i t i o n which t e r m i n a t e s i n a v i - b r a t i o n a l l y e x c i t e d s t a t e o f t h e redox isomer ^X/ M ^e c i b e f o r e i t r e l a x e s . The e l e c t r o n t r a n s f e r cannot o n l y be a c h i e v e d by l i g h t a b s o r p t i o n but a l s o as a t h e r m a l p r o c e s s which r e q u i r e s t h e a c t i v a t i o n energy t o r e a c h t h e c r o s s i n g p o i n t o f both p o t e n t i a l c u r v e s . When t h e redox isomer e x i s t s i n i t s v i b r a t i o n a l ground s t a t e i t may undergo a t h e r m a l back e l e c t r o n t r a n s f e r by overcoming t h e a c t i v a t i o n b a r r i e r E ^ .

The m a j o r i t y o f compounds which were i n v e s t i g a t e d w i t h r e g a r d t o o p t i c a l MMCT t r a n s i t i o n s a r e h o m o b i n u c l e a r complexes c o n t a i n i n g one metal i n two d i f - f e r e n t o x i d a t i o n s t a t e s . P a r t i c u l a r l y R u ^ / R u1 1* systems found much

a t t e n t i o n ( r e f s . 1 3 - 1 7 ) . These complexes a r e c a l l e d m i x e d - v a l e n c e (MV) com- pounds. I n t h i s c a s e t h e term MMCT i s a l s o known as i n t e r v a l e n c e t r a n s f e r ( I T ) . The p o t e n t i a l c u r v e s ( F i g . 1) o f both redox isomers o f such a symme- t r i c MV complex a r e l o c a t e d a t t h e same e n e r g y . They are o n l y d i s p l a c e d

Mß - ligand distance

F i g . 1. P o t e n t i a l e n e r g y v e r s u s m e t a l - l i g a n d bond l e n g t h f o r b i n u c l e a r complexes c o n t a i n i n g a r e d u c i n g and an o x i d i z i n g m e t a l

h o r i z o n t a l l y due t o t h e s t r u c t u r a l r e o r g a n i z a t i o n w h i c h accompanies t h e e l e c t r o n t r a n s f e r . U n f o r t u n a t e l y , f o r a v a r i e t y o f r e a s o n s t h e s e compounds are not w e l l s u i t e d t o o b s e r v e a p h o t o c h e m i c a l r e a c t i o n l e a d i n g t o a permanent c h e m i c a l c h a n g e . F i r s t o f a l l , i n a symmetric MV complex an e l e c t r o n exchange does not cause a r e a l c h e m i c a l c h a n g e , a l t h o u g h the

i n d i v i d u a l metal atoms have exchanged t h e i r o x i d a t i o n s t a t e and hence t h e i r e n v i r o n m e n t . But even i n most h o m o b i n u c l e a r complexes which are s l i g h t l y ' a s y m a e t r i c due t o d i f f e r e n t l i g a n d s a r a p i d t h e r m a l e l e c t r o n exchange o c c u r s . T h i s s i t u a t i o n i n t e r f e r e s w i t h t h e o b s e r v a t i o n o f l i g h t - i n d u c e d e l e c t r o n t r a n s f e r . F i n a l l y , t h e MMCT bands of t h e symmetric o r n e a r l y symmetric MV systems appear i n t h e n e a r IR which i s not e a s i l y a c c e s s i b l e by c o n v e n t i o n a l i r r a d i a t i o n s o u r c e s and l i g h t d e t e c t i o n d e v i c e s . C o n s e q u e n t l y , p h o t o a c t i v e systems s h o u l d be d e s i g n e d a c c o r d i n g t o t h e s e c o n s i d e r a t i o n s .

L i g h t - s e n s i t i v i t y w i l l be most e a s i l y o b s e r v e d f o r s t r o n g l y asymmetric b i n u c l e a r c o m p l e x e s . They may be s t a b l e towards t h e r m a l e l e c t r o n exchange

which r e q u i r e s the a c t i v a t i o n energy Et h and a r e e x p e c t e d t o d i s p l a y t h e i r MMCT bands i n t h e v i s i b l e o r UV r e g i o n (see F i g . 1 ) . An asymmetric system may be c o n s t r u c t e d i n two ways. In homobinuclear complexes d i f f e r e n t l i g a n d s at both m e t a l s can be employed. F o r example, i n a R u *¹, Ru*** complex a l a r g e redox asymmetry w i l l be a c h i e v e d i f R u1 1 i s s t a b i l i z e d by ^ - a c c e p t o r l i g a n d s and R u1 1 1 by * - d o n o r s . Much l a r g e r energy s e p a r a t i o n s a r e p o s s i b l e i n h e t e r o n u c l e a r s y s t e m s . The i n d i v i d u a l components a r e s e l e c t e d a c c o r d i n g t o t h e i r redox p o t e n t i a l s .

Another v e r y i m p o r t a n t c r i t e r i o n f o r a p r o p e r c h o i c e i s t h e a n t i c i p a t e d r e a c t i v i t y o f t h e redox isomer g e n e r a t e d by WCT e x c i t a t i o n . I t w i l l not be s t a b l e but r e t u r n r a p i d l y t o t h e s t a r t i n g p o i n t s i n c e t h e a c t i v a t i o n b a r r i e r

Et h f o r t > a c , < e l e c t r o n t r a n s f e r i s r a t h e r low ( F i g . 1 ) . An i r r e v e r s i b l e f o r m a t i o n o f s t a b l e p h o t o p r o d u c t s can o n l y be a c h i e v e d i f t h e redox isomer i s a b l e t o undergo some f u r t h e r g e o m e t r i c a l r e a r r a n g e m e n t s . These secondary pro- c e s s e s must be f a s t enough t o compete w i t h back e l e c t r o n t r a n s f e r . For exam- p l e , p h o t o a c t i v i t y i s e x p e c t e d i f C C o ( N H ^ )^A] ^⁺ i s t h e o x i d i z i n g compo-

2+

nent o f a b i n u c l e a r complex. Upon r e d u c t i o n [CofNH-jJ^] i s f o r m e d . I t i s k i n e t i c a l l y v e r y l a b i l e and undergoes a r a p i d d e c o m p o s i t i o n i n aqueous s o l u t i o n . A c c o r d i n g t o t h e s e c o n s i d e r a t i o n s i n 1975 we s t a r t e d t o e x p l o r e p h o t o c h e m i c a l r e a c t i o n s induced by MMCT e x c i t a t i o n ( r e f s . 19, 2 2 , 2 3 ) . [ ( N C )⁵R uⁿC N C o^{i n}( N H3)⁵r

The b i n u c l e a r c y a n i d e - b r i d g e d complex [ ( N C ) c R u C N C o ( N H^ ) r] " ( r e f . I l l 3+

22) may be viewed as b e i n g composed o f CCo

(^3)5^

C R u ^ f C N ) ^ ]⁴" i f t h e c o u p l i n g between Ru and Co i s weak. The assumption 3+

t h a t CCo(NH^)⁶] c a n be c o n s i d e r e d as one o f t h e components i s suppor- ted by t h e o b s e r v a t i o n t h a t b r i d g i n g c y a n i d e which c o o r d i n a t e s v i a n i t r o g e n

i s s i m i l a r t o ammonia w i t h r e g a r d t o i t s l i g a n d f i e l d s t r e n g t h . Weak c o u p l i n g i s t h e n i n d i c a t e d by t h e a b s o r p t i o n spectrum o f C(NCj^RuCNCoiNH^)^]"

which i s i n d e e d composed o f t h e s p e c t r a o f [Ru(CN),-]4~ and 3+

[ C o ( N H3)6] ( F i g . 2 ) . The b i n u c l e a r complex d i s p l a y s i t s longest-wave- l e n g t h a b s o r p t i o n a t x = 4 7 5 nm. T h i s i s t h e f i r s t l i g a n d f i e l d (LF)

«> max . band of.CCo(NH3)g] . The o t h e r component [ R u ( C N )6l does n o t

absorb above 300 nm. Independent s u p p o r t f o r a weak i n t e r a c t i o n o f both m e t a l s comes from t h e IR s p e c t r u m . B e s i d e t h e b r i d g i n g c y a n i d e which absorbs at 2135 cm"¹ t h e s t r e t c h i n g v i b r a t i o n o f t h e t e r m i n a l c y a n i d e s appears a t 2050 cm"1. T h i s i s t h e same v a l u e as t h a t o f [ R u ( C N )634~ . I t f o l l o w s t h a t t h e o x i d a t i o n s t a t e o f r u t h e n i u m i s two i n the b i n u c l e a r complex s i n c e the f r e q u e n c y o f t h i s s t r e t c h i n g v i b r a t i o n i s a s e n s i t i v e f u n c t i o n o f t h e v a l e n c y o f t h e m e t a l .

The p r e s e n c e o f t h e r e d u c i n g R u1 1 and o x i d i z i n g C o1 1 1 l e a d s t o the appearance o f a new a b s o r p t i o n ( F i g . 2) a t x = 375 nm (e = 690) which i s

F i g . 2 . A b s o r p t i o n s p e c t r a o f aqueous 3x10 M C ( N H3)5C o N C R u ( C N )5l "

and 3x10~4 M [ C o ( N H3)633 + a t room t e m p e r a t u r e , 1-cm c e l l

a s s i g n e d t o a MMCT t r a n s i t i o n ( r e f . 2 2 ) . An a n a l y s i s o f t h e band w i t h r e g a r d t o t h e Hush t h e o r y would r e q u i r e t h e knowledge o f t h e p o t e n t i a l s o f t h e redox c o u p l e s C oI I I / n and R u1 1^1 1 1 w i t h i n t h e b i n u c l e a r c o m p l e x . These p o t e n - t i a l s a r e not known. However, f o r a v e r y rough e s t i m a t e t h e p o t e n t i a l s o f t h e mononuclear components [ C o ( N H0)c]3 + / 2 + (E° = 0.11 V) ( r e f . 24) and

[ R u ( C N)6r /0 (E° = 0.86 V) ( r e f . 24) may be t a k e n . T h i s g i v e s a ^ o r g a - n i z a t i o n a l energy o f x = 59 k c a l / m o l . Such a l a r g e v a l u e i s c e r t a i n l y a s s o c i - a t e d w i t h t h e f a c t t h a t t h e MMCT t r a n s i t i o n l e a d s t o t h e p o p u l a t i o n o f an a n t i b o n d i n g eg o r b i t a l o f C o1 1 1 ( i n 0h symmetry) which c a u s e s a l a r g e d i s t o r t i o n a t C o^{1 1} i n t h e redox isomer C ( N C ) r R u^{I I I}C N C o^{I I}( N H . )^{c ;}3 " .

T Y - T I T The a c t i v a t i o n b a r r i e r f o r t h e r m a l e l e c t r o n t r a n s f e r from Ru t o Co i s then c a l c u l a t e d t o be Et h = 25 k c a l / m o l . Aqueous s o l u t i o n s o f t h e b i n u - c l e a r complex a r e s t a b l e a t room t e m p e r a t u r e but undergo a redox decomposi- t i o n upon l i g h t a b s o r p t i o n by t h e MMCT band w i t h t h e quantum y i e l d * = 0.46 at \[ r r = 366 nm.

[ ( N C )5R uUC N C oi n( N H3)⁵r ^ [ ( N C )⁵R ui nC N C on( N H3)⁵r [ ( N C )⁵R u^{I I I}C K C o^{I I}( N H³)⁵] ~ - [ R u^{I U}( C N )⁶]³~ + C o^{2 +} + 5 NH³

Because o f the s t r o n g d i s t o r t i o n o f t h e v i b r a t i o n a l l y r e l a x e d MMCT s t a t e , t h e r m a l back e l e c t r o n t r a n s f e r i s now a s s o c i a t e d w i t h a s u b s t a n t i a l a c t i v a t i o n b a r - r i e r o f E|^H = 5 k c a l . The d e c o m p o s i t i o n o f the k i n e t i c a l l y l a b i l e Co¹* i s a p p a r e n t l y f a s t enough t o compete w i t h charge r e c o m b i n a t i o n .

An attempt t o p r e p a r e t h e c o r r e s p o n d i n g b i n u c l e a r i r o n complex [ ( N H3)5C oI I IN C F eI I( C N )6] " f a i l e d . Upon m i x i n g s o l u t i o n s o f

[ C o ( N H³)⁵H²0 ]^{3 +} and CFe(CN)⁶3⁴~ a r a p i d t h e r m a l o u t e r - s p h e r e

e l e c t r o n t r a n s f e r from F e ( I I ) t o C o ( I I I ) t a k e s p l a c e ( r e f . 25) b e f o r e t h e f o r m a t i o n o f t h e b i n u c l e a r complex o c c u r s . [ F e ( C N)⁶3⁴~ ( E ° = 0.36 V v s SCE) i s more r e d u c i n g than [ R u ( C N)⁶3⁴~ by 0.5 V ( r e f . 2 4 ) . Assuming t h e same r e o r g a n i z a t i o n a l energy o f x = 58 k c a l / m o l f o r both b i n u c l e a r complexes [ ( N H³)⁵C o^{I I I}N C M^{I I}( C N )⁵] " w i t h M = Fe and Ru t h e a c t i v a t i o n bar-

r i e r would now be o n l y E ^ = 17 k c a l / m o l f o r t h e r m a l e l e c t r o n t r a n s f e r w i t h i n t h e h y p o t h e t i c a l complex [ ( N H3)5C oI I IN C F eI I( C N)⁵3 " ' i n an

aqueous s o l u t i o n . [ ( N C )5F eI IC N C rI I I( N H3)⁵3 "

The b i n u c l e a r i o n C ( N C )⁵F e^{I I}C N C r ( N H³)⁵r c o n s i s t s o f t h e components [ F eH( C N )6l4" and C C r ( N H3)⁶3^{3 +} which a r e weakly c o u p l e d ( r e f . 2 6 ) .

The MLCT band o f [ F e i C N ) . ] a t xo max q v = 210 nm can be r e c o g n i z e d i n t h e spectrum o f t h e b i n u c l e a r c o m p l e x . The t y p i c a l LF bands o f [Cr(NH3)g3

are a p p a r e n t l y o b s c u r e d by t h e much more i n t e n s e a b s o r p t i o n a t x = 376 max

nm (e = 2400) which i s a s s i g n e d t o t h e MMCT t r a n s i t i o n from t h e r e d u c i n g F e1 1 t o t h e o x i d i z i n g C r1 1 1.

The p o t e n t i a l d i f f e r e n c e between t h e redox c o u p l e s [Fe(CN)^J and C C r ( N H³)⁶]^{3 + / 2 +} i s AE = 1.19 V. Taking t h i s as a rough e s t i m a t e f o r A E o f t h e b i n u c l e a r complex t h e e l e c t r o n t r a n s f e r i s a s s o c i a t e d w i t h a r e o r g a n i - z a t i o n a l energy o f x = 49 k c a l . T h i s l a r g e v a l u e s u p p o r t s t h e assumption t h a t t h e a n t i b o n d i n g eg o r b i t a l s a t C r ( I I I ) ( i n 0h symmetry) are t h e a c c e p t o r o r b i t a l s o f t h e MMCT t r a n s i t i o n . The b i n u c l e a r complex i s q u i t e s t a b l e i n aqueous s o l u t i o n w i t h r e g a r d t o thermal e l e c t r o n t r a n s f e r ( E^{t h} = 29 k c a l ) w h i l e an e f f i c i e n t p h o t o l y s i s t a k e s p l a c e upon MMCT e x c i t a t i o n (* =0.1 nm a t

i r r = 366 ( r e f . 2 6 ) :

[ ( N C )5F e1 1C N C r1 1 1( N H3)5l " ¿ U [ ( N C ^ F e1 1 'CNCr1 1 ( N H3)5 3"

C ( N C )⁵F e^{I I I}C N C r^{I I}( N H³)⁵3 ^ C F e^{I I I}( C N )⁶3³" + C r^{2 +} + 5 NH³

The Franck-Condon s t a t e reached by t h e o p t i c a l MMCT t r a n s i t i o n undergoes a r a p i d v i b r a t i o n a l r e l a x a t i o n t o t h e t h e r m a l l y e q u i l i b r a t e d redox isomer Fe / C r^{1 1}. The l a r g e d i s t o r t i o n a t C r^{1 1} slows down back e l e c t r o n t r a n s f e r ( E |K ~ 2 k c a l ) . The k i n e t i c a l l y l a b i l e C r1 1 undergoes a l i g a n d

t n 2+

d i s p l a c e m e n t b e f o r e charge r e c o m b i n a t i o n can o c c u r . F i n a l l y , C r i s o x i - d i z e d by o x y g e n .

[ ( N C )5MI IC N C oI Í Í( C N )5]6" w i t h M = F e , Ru, and Os

The IR s p e c t r a o f t h e a n i o n s [(NC)RM**CNCo***(CN) J6~ ( r e f s . 2 3 , I I 26) w i t h M = F e , Ru, and Os a r e i n d i c a t i v e o f weak c o u p l i n g between M and C o * *1. The a b s o r p t i o n s o f t h e t e r m i n a l c y a n i d e s appear a t n e a r l y t h e same wavenumbers as t h o s e o f mononuclear cyano complexes o f M** and Co***. The e l e c t r o n i c s p e c t r a o f t h e b i n u c l e a r complexes a r e l e s s i n s t r u c t i v e s i n c e w e l 1 - s e p a r a t e d bands do not a p p e a r . However, t h e r e i s much e v i d e n c e t h a t t h e a b s o r p t i o n f e a t u r e s a t xm i V 385 nm (e = 630) f o r M = F e , 312 nm (460) f o r

max T i Ru, and 360 nm (734) f o r 0s c a n be a s s i g n e d t o MMCT t r a n s i t i o n s from M t o

Co*** ( r e f s . 2 3 , 2 6 ) . The energy o f t h e MMCT band v a r i e s w i t h t h e r e d u c i n g s t r e n g t h o f [ M ( C N )6]4" (M = F e , E ° = +0.36 V; Ru 0.86 V and 0s 0.56 V) ( r e f s . 24, 2 7 ) .

The MMCT t r a n s i t i o n s o f t h e b i n u c l e a r complexes t e r m i n a t e i n e_ o r b i t a l s o f Co***. The t h e r m a l l y e q u i l i b r a t e d redox isomers [ ( N C )5M *nC N C o - ( C N ) , J ^ " a r e e x p e c t e d t o undergo l a r g e d i s t o r t i o n s a t Co * . R e o r g a n i - z a t i o n a l e n e r g i e s o f more t h a n 45 k c a l a r e e s t i m a t e d .

A l l t h r e e a n i o n s [(NC^M**CNCo**1 ( C N )536" a r e s t a b l e i n s o l u t i o n ( r e f s . 2 3 , 2 6 ) . The a c t i v a t i o n e n e r g i e s f o r t h e r m a l e l e c t r o n t r a n s f e r may exceed 30 k c a l . However, i n a l l c a s e s t h e aqueous complexes undergo photo- redox r e a c t i o n s upon MMCT e x c i t a t i o n . The redox isomers

C ( N C )5M * * * C N C o *I( C N )5]6" d i s s o c i a t e i n t h e p r i m a r y p h o t o c h e m i c a l s t e p :

[ ( N C )5MI IC N C oI I I( C N )5]6" í ^ [ ( N C )5MI I IC N C oI I( C N )5]6' [ ( N C )5MI I IC N C oI I( C N )5l6" - C MI I I( C N )613" + [ C o1 1( C N )5]3~

In t h e absence o f oxygen a c o m p l e t e r e g e n e r a t i o n o f t h e b i n u c l e a r complexes o c c u r s by a t h e r m a l i n n e r - s p h e r e e l e c t r o n t r a n s f e r w h i c h i s s i m p l y a r e v e r s a l o f t h e p h o t o r e a c t i o n . The a c t i v a t i o n b a r r i e r f o r t h i s back e l e c t r o n t r a n s f e r was e s t i m a t e d t o be around E £h - 2 k c a l . The r e g e n e r a t i o n o f t h e b i n u - c l e a r complexes by t h i s i n n e r - s p h e r e redox p r o c e s s i s not s u r p r i s i n g s i n c e a l l t h r e e c y a n i d e - b r i d g e d a n i o n s a r e s y n t h e s i z e d by t h i s r e a c t i o n . The i r o n complex was p r e p a r e d by Haim and W i l m a r t h i n 1961 a c c o r d i n g t o t h i s p r o c e d u r e ( r e f . 2 8 ) . In d i s t i n c t i o n t o ammine complexes o f Co** which decay i r r e v e r - s i b l y i n aqueous s o l u t i o n t h e complex [ C o * * ( C N ) , J3~ does n o t decompose.

The p h o t o l y s i s o f a l l t h r e e complexes i n d u c e d by MMCT e x c i t a t i o n l e a d s t o a permanent c h e m i c a l change o n l y i n t h e p r e s e n c e o f a i r . The complex

3-

[ C o ( C N )5l c a n be i n t e r c e p t e d by 02:

2 [ C o ( C N )5l3- + o2 - [ ( N C )5C oI I I( 02 2- ) C oI I I( C N )536-

In a c i d i c s o l u t i o n t h e peroxo complex decomposes t o y i e l d H ^ and 2 C C o * * * ( C N) r( H?0 ) I2" w h i l e i n b a s i c s o l u t i o n t h e peroxo complex i s

f u r t h e r o x i d i z e d t o t h e superoxo complex [ ( N C )5C oI I i( 02" ) C o1 1 1- ( C N)515\ The quantum y i e l d s f o r t h e f o r m a t i o n o f [ MH I( C N)613~ a r e s l i g h t l y wavelength-dependent due t o the o v e r l a p o f the MMCT bands w i t h a b s o r p t i o n s o f o t h e r o r i g i n . The quantum y i e l d s may exceed u n i t y because [M(CN)c] i s not o n l y produced i n the p r i m a r y p h o t o c h e m i c a l r e a c t i o n b u t 3

A- • • a l s o by t h e o x i d a t i o n o f [M(CN)g] by H ^ . The quantum y i e l d s a r e

f a i r l y l a r g e : $ = 1.6 a t xi f r = 4 0 5 nm f o r M = Fe; $ = 0.39 a t xi r r = 313 nm f o r M = Ru, and $ = 0.32 a t x. = 366 nm f o r M = Os ( r e f s . 23, 2 6 ) .

irr

3 EXCITED STATE (NON-RESONANCE) ELECTRON TRANSFER 3.1 I n t r o d u c t i o n

A p h o t o i n d u c e d e l e c t r o n t r a n s f e r does not o n l y o c c u r by d i r e c t o p t i c a l e x c i - t a t i o n ( r e s o n a n c e t r a n s f e r ) . As an a l t e r n a t i v e an e l e c t r o n i c a l l y e x c i t e d m o l e c u l e may undergo an e l e c t r o n t r a n s f e r t o o r from another m o l e c u l e . I n t e r - m o l e c u l a r e x c i t e d s t a t e e l e c t r o n t r a n s f e r o f t h i s t y p e has been s t u d i e d i n - t e n s i v e l y ( r e f s . 14, 29, 3 0 ) . P a r t i c u l a r l y the complex [ R u ( b i p y )³]^{2 +} was used as e x c i t e d s t a t e e l e c t r o n donor o r a c c e p t o r ( r e f . 3 1 ) . In s o l u t i o n

i n t e r m o l e c u l a r e l e c t r o n t r a n s f e r can t a k e p l a c e i f the e x c i t e d m o l e c u l e has a d i f f u s i o n a l e n c o u n t e r w i t h a s u i t a b l e redox p a r t n e r b e f o r e i t r e t u r n s t o t h e ground s t a t e .

An e x c i t e d s t a t e e l e c t r o n t r a n s f e r may t a k e p l a c e a l s o as i n t r a m o l e c u l a r p r o c e s s . An e x c i t e d c h r o m o p h o r i c group can undergo an e l e c t r o n t r a n s f e r t o o r from a n o t h e r p a r t o f the same m o l e c u l e . S i n c e donor and a c c e p t o r are a l r e a d y i n c o n t a c t p r i o r t o e l e c t r o n i c e x c i t a t i o n a long l i f e t i m e o f t h e e x c i t e d s t a t e i s n o t r e q u i r e d . Even h i g h e r e x c i t e d s t a t e s whicn c a n not p a r t i c i p a t e i n b i m o l e c u l a r r e a c t i o n s due t o t h e i r s h o r t l i f e t i m e s may undergo i n t r a m o l e - c u l a r e l e c t r o n t r a n s f e r . W h i l e i n b i m o l e c u l a r redox p r o c e s s e s t h e s t r u c t u r a l arrangement o f donor and a c c e p t o r i n t h e e n c o u n t e r complex i s not known i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r o c c u r s i n a b e t t e r d e f i n e d e n v i r o n m e n t . A l - though t h e s e f e a t u r e s make i t a t t r a c t i v e t o s t u d y i n t r a m o l e c u l a r e x c i t e d s t a t e e l e c t r o n t r a n s f e r t h i s s u b j e c t has been l a r g e l y n e g l e c t e d u n t i l a few y e a r s a g o .

The r e c e n t i n t e r e s t i n i n t r a m o l e c u l a r e x c i t e d s t a t e e l e c t r o n t r a n s f e r i s as- s o c i a t e d w i t h a t t e m p t s t o u n d e r s t a n d t h e p r i m a r y e v e n t s o f p h o t o s y n t h e s i s and t o d e s i g n model systems f o r t h e p h o t o s y n t h e s i s ( r e f . 3 2 ) . In t h e f i r s t s t e p an e x - c i t e d s t a t e e l e c t r o n t r a n s f e r o c c u r s which must be u p h i l l w i t h r e g a r d t o t h e ground s t a t e i n o r d e r t o c o n v e r t l i g h t i n t o c h e m i c a l e n e r g y . In s i m p l e systems t h i s f i r s t s t e p i s f o l l o w e d by a r a p i d d o w n h i l l charge r e c o m b i n a t i o n . I n t h e p h o t o s y n t h e s i s a c h a r g e s e p a r a t i o n i s a c h i e v e d by i n t r o d u c i n g a b a r r i e r f o r back e l e c t r o n t r a n s f e r . R e c e n t l y model compounds have been d e s i g n e d t o s t u d y t h e c h a r g e s e p a r a t i o n i n d e t a i l . A system which found much a t t e n t i o n c o n s i s t s o f a p o r p h y r i n as e x c i t e d s t a t e e l e c t r o n donor which i s l i n k e d c o v a l e n t l y t o a q u i -

none as e l e c t r o n a c c e p t o r . In a d d i t i o n , a c a r o t e n e may be a t t a c h e d as a donor

" t o a c c o m p l i s h c h a r g e s e p a r a t i o n over l a r g e d i s t a n c e s ( r e f . 3 2 ) . However, i n t h i s r e v i e w the d i s c u s s i o n i s r e s t r i c t e d t o t y p i c a l t r a n s i t i o n metal comple- x e s .

Two d i f f e r e n t c a s e s w i l l be d i s t i n g u i s h e d . F i r s t , the p r i m a r y e l e c t r o n t r a n s f e r i s f o l l o w e d by r a p i d secondary p r o c e s s e s which compete s u c c e s s f u l l y w i t h back e l e c t r o n t r a n s f e r . As a r e s u l t , the l i g h t a b s o r p t i o n l e a d s t o a permanent c h e m i c a l c h a n g e . S e c o n d l y , c h a r g e r e c o m b i n a t i o n i s r a p i d and a net p h o t o l y s i s i s not o b s e r v e d . Luminescence and t i m e - r e s o l v e d a b s o r p t i o n s p e c - t r o s c o p y were a p p l i e d t o g a i n i n s i g h t i n t o t h e c h a r g e s e p a r a t i o n and recom- b i n a t i o n p r o c e s s e s .

The compounds d i s c u s s e d below do not d i s p l a y CT a b s o r p t i o n bands i n v o l v i n g the e l e c t r o n donor and a c c e p t o r . In some c a s e s t h i s i s a good i n d i c a t i o n t h a t e l e c t r o n i c c o u p l i n g i s weak. In o t h e r c a s e s t h e e x t e n t o f c o u p l i n g i s more d i f f i c u l t t o a s s e s s s i n c e CT bands o f i n t e r e s t may be o b s c u r e d by a b s o r p t i o n s o f d i f f e r e n t o r i g i n .

3.2 IRREVERSIBLE PHOTOREDUCTION OF C o ( I I I ) 3.2.1 A r o m a t i c M o l e c u l e s as E l e c t r o n Donors

In 1969 Adamson e t a l . o b s e r v e d a p h o t o r e d u c t i o n o f Co*** upon IL e x c i - t a t i o n o f aqueous [ C o * * * ( N H3)5T S C ]2 + w i t h TSC" = t r a n s - 4 - s t i l b e n e -

c a r b o x y l a t e ( r e f . 3 3 ) . In a l a t e r s t u d y a d e t a i l e d a n a l y s i s o f the p h o t o r e d o x p r o d u c t s of t h i s complex was c a r r i e d out ( r e f . 3 4 ) .

The s t i l b e n e m o i e t y i s an i s o l a t e d chromophore o f t h e complex s i n c e i t s a b s o r p t i o n spectrum d i d not change upon c o o r d i n a t i o n v i a t h e c a r b o x y l i c g r o u p . Any bands w h i c h c o u l d be a s s i g n e d t o a CT t r a n s i t i o n from t h e s t i l b e n e group t o Co*** do not a p p e a r .

The p h o t o l y s i s t a k e s p l a c e a c c o r d i n g t o the f o l l o w i n g r e a c t i o n scheme (*

e x c i t e d s t a t e ) :

[ C o * * * ( N H3)5( T S C " ) ]2 + ^ [ C o * * * ( N H3)5( T S C " ) * J2 +

[ C o * * * { N H³)⁵( T S C " ) * ]^{2 +} - C C o^{I I}( N H³)⁵( T S C ° ) ]^{2 +} [ C o * * ( N H3)5( T S C ° ) ]2 + - C o2 + + 5NH3 + T S C0

C C o * * * ( N H3)5( T S C " ) ]2 + + TSC0 - C o2 + + 5NH3 + TSC" + o x i d . TSC TSC⁰ + 0² - b e n z a l d e h y d e + o t h e r p r o d u c t s

Upon l i g h t a b s o r p t i o n a t 313 nm the f i r s t e x c i t e d n* s i n g l e t o f t h e TSC"

l i g a n d i s p o p u l a t e d . T h i s IL s t a t e i s s t r o n g l y r e d u c i n g ( E ^2 ~ -2 V vs SCE) w h i l e t h e Co*** c e n t e r i s a weak o x i d a n t ( E^{1 / 2} = -0.03 V) ( r e f . 3 5 ) . E x c i t e d s t a t e e l e c t r o n t r a n s f e r from t h e IL s t a t e o f the s t i l b e n e group t o Co*** has o b v i o u s l y a l a r g e d r i v i n g f o r c e and i s a p p a r e n t l y v e r y r a p i d . W h i l e the f r e e l i g a n d shows a s t r o n g f l u o r e s c e n c e and undergoes a t r a n s / c i s

p h o t o í s o m e r i z a t i o n t h e s e p r o c e s s e s are not o b s e r v e d i n the c o o r d i n a t e d s t a t e . The e l e c t r o n t r a n s f e r i s thus much f a s t e r than o t h e r d e a c t i v a t i o n p r o c e s s e s of the i n * s i n g l e t . In the o r i g i n a l study ( r e f . 33) i t was suggested t h a t the e x c i t e d IL s t a t e undergoes an energy t r a n s f e r t o a n o n - s p e c t r o s c o p i c LMCT e x c i t e d s t a t e of the complex. T h i s e x p l a n a t i o n seems t o be e q u i v a l e n t t o an e x c i t e d s t a t e e l e c t r o n t r a n s f e r i f the LMCT s t a t e can be d e s c r i b e d as a s t i l - bene r a d i c a l c a t i o n c o o r d i n a t e d t o Co** by a c a r b o x y l i c group ( r e f . 3 4 ) .

The Co** complex g e n e r a t e d by e x c i t e d s t a t e e l e c t r o n t r a n s f e r i s k i n e t i - c a l l y l a b i l e and decomposes b e f o r e an e f f i c i e n t charge r e c o m b i n a t i o n t a k e s p l a c e . A TSC r a d i c a l i s r e l e a s e d and undergoes f u r t h e r r e a c t i o n s a c c o r d i n g t o the scheme ( r e f . 3 4 ) .

I n t e r e s t i n g l y , t h e lowest n* t r i p l e t o f t h e TSC" l i g a n d which can be p o p u l a t e d by i n t e r m o l e c u l a r energy t r a n s f e r from b i a c e t y l and o t h e r s e n s i t i - z e r s i n d u c e s o n l y the t r a n s / c i s i s o m e r i z a t i o n of the l i g a n d but not the r e d u c t i o n o f Co*** ( r e f . 3 4 ) . The redox p o t e n t i a l o f t h e IL t r i p l e t i s o n l y

E1/2 ~ " ° "3 V v s S C E" E l e c t r o n t r a n s f e r i s now a p p a r e n t l y not f a s t enough t o compete w i t h o t h e r d e a c t i v a t i o n p r o c e s s e s such as the p h o t o i s o m e r i z a t i o n .

A v a r i e t y o f o t h e r complexes of the t y p e [Co***(NH3)5OOCR32 + w i t h R = a r o m a t i c group such as 1- and 2 - n a p h t h a l e n e , 9 - a n t h r a c e n e , 4 - b i p h e n y l shows q u a l i t a t i v e l y t h e same b e h a v i o r as the TSC complex ( r e f s . 37, 3 8 ) . The quantum y i e l d s o f C o2 + p r o d u c t i o n was dependent v e r y much on the n a t u r e o f R. A s i m p l e c o r r e l a t i o n was not a p p a r e n t .

The complexes [ 2 - n a p h t h y l - C 0 N H - ( C H2)n- C 0 0 C o * * * ( N H3)⁵3^{2 +} w i t h n

= 1-5 were s t u d i e d i n o r d e r t o l e a r n more about t h e s t r u c t u r a l r e q u i r e m e n t s f o r e x c i t e d s t a t e e l e c t r o n t r a n s f e r ( r e f . 3 9 ) . The naphthy! grcirp *s e x c i t e d s t a t e donor and Co*** as a c c e p t o r are connected by a p e p t i d e l i n k a g e . In d i s t i n c t i o n t o the complexes d i s c u s s e d above Co*** and the a r o m a t i c group are now not o n l y s e p a r a t e d by a c a r b o x y l i c group but a l s o by s a t u r a t e d and hence e l e c t r o n i c a l l y i n s u l a t i n g methylene g r o u p s . The b a s i c o b s e r v a t i o n s were the same as t h o s e f o r the r e l a t e d complexes w i t h o u t i n t e r v e n i n g CH^ g r o u p s . However, a d d i t i o n a l d a t a were o b t a i n e d s i n c e f l u o r e s c e n c e q u e n c h i n g ' o f the n a p h t h y l m o i e t y by Co*** was e f f i c i e n t but not c o m p l e t e . The o b s e r v a t i o n s can be d e s c r i b e d by the f o l l o w i n g r e a c t i o n scheme (N = 2 - n a p h t h y l , B = pep- t i d e b r i d g e ) :

C N - B - C oI I I( N H3)5]2 + + h v - [ N * - B - C o * * * ( N H3)5]2 +

[ N * - B - C om( N H3)⁵3^{2 +} ^ [N-3-Co***(NH³)⁵3^{2 + +} hv

C N * - B - C o^{I I I}( N H3)⁵3^{2 +} ^ > [N-B-Co***(NH3)⁵3^{2 +} + heat

I I I /

[N*-B-Co ( N H3)5] > ^K3 ^rSlCN - B - C o+ I I / ^{1 1}( N H³)⁵] ⁿ2+

C N⁺- B - C o^H( N H³)⁵]^{2 +} -I» C N - B - C o^{I H}( N H³)⁵] 2+

k

[ N⁺- B - C o^H( N H³)⁵]^{2 +} J+ C o^{2 +} + 5 NH³ + o x i d . N

The « i * s i n g l e t o f the n a p h t h y l group i s p o p u l a t e d by i r r a d i a t i o n w i t h x = 313 nm. The e x c i t e d s i n g l e t of the f r e e l i g a n d has a l i f e t i m e o f approxima- t e l y 10 s and undergoes an e f f i c i e n t f l u o r e s c e n c e , which i s s t r o n g l y quenched i n the complex due t o e l e c t r o n t r a n s f e r t o C o^{1 1 1}. From the r e l a - t i v e quantum y i e l d s o f f l u o r e s c e n c e the r a t e c o n s t a n t s k3 and quantum y i e l d s o f e x c i t e d s t a t e e l e c t r o n t r a n s f e r were o b t a i n e d . An i n c r e a s e f r o m k3 = 4 . 9 x 1 09 s"1 and $E T = 0.98 f o r n = 1 t o 9 . 2 x 1 09 s"1 and 0.99 f o r n = 4 was o b s e r v e d . At n = 5 k³ and dropped t o 6 . 0 x 1 0⁹ s "¹ and 0.98.

From and t h e e x p e r i m e n t a l quantum y i e l d s o f C o2 + f o r m a t i o n r e l a - t i v e r a t e c o n s t a n t s f o r back e l e c t r o n t r a n s f e r k i were c a l c u l a t e d a s s u -

2+

ming t h a t t h e r a t e of Co f o r m a t i o n k^ i s independent o f n . I t was found t h a t t h e r a t e of back e l e c t r o n t r a n s f e r r e a c h e d a l s o a maximum a t n = 4.

On t h e b a s i s o f t h e s e r e s u l t s i t i s c o n c l u d e d t h a t t h e a c t u a l d i s t a n c e o f e x c i t e d s t a t e and back e l e c t r o n t r a n s f e r d e c r e a s e s 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 o f the p e p t i d e from n = 1 t o 4.

F i g . 3. Suqgested s t r u c t u r e o f aqueous C2-naphthyl-C0NH-(CH2)4-C00Co I I I - ( N H3)53 2+

I t i s assumed t h a t donor and a c c e p t o r come t o a c l o s e r approach by an a p p r o p r i a t e b e n d i n g o f the f l e x i b l e p e p t i d e l i n k a g e . T h i s back f o l d i n g may be f a v o r e d by hydrogen bonding between c o o r d i n a t e d ammonia and c a r b o n y l groups of the p e p t i d e ( F i g . 3 ) . At n = 5 e l e c t r o n t r a n s f e r becomes l e s s e f f i c i e n t . The d o n o r - a c c e p t o r d i s t a n c e may now i n c r e a s e by an e x t e n s i o n of the p e p t i d e .

F i n a l l y , i t s h o u l d be mentioned h e r e t h a t e x c i t e d s t a t e e l e c t r o n t r a n s f e r from an a r o m a t i c m o l e c u l e t o C o ( I I I ) ammines t a k e s p l a c e a l s o as an i n t e r - m o l e c u l a r r e a c t i o n ( r e f s . 38, 4 0 ) . F i r s t o b s e r v a t i o n s were e x p l a i n e d by the a s s u m p t i o n t h a t an energy t r a n s f e r o c c u r s t o r e a c t i v e LMCT s t a t e s of t h e com- p l e x ( r e f . 4 0 ) . However, more r e c e n t i n v e s t i g a t i o n s have shown t h a t t h e a r o - m a t i c m o l e c u l e s are indeed o x i d i z e d and a l l o b s e r v a t i o n s can be e x p l a i n e d b e s t by an e x c i t e d s t a t e e l e c t r o n t r a n s f e r mechanism ( r e f . 3 8 ) .

3.2.2 Metal Complexes as E l e c t r o n Donors

I n t r a m o l e c u l a r e x c i t e d s t a t e e l e c t r o n t r a n s f e r between weakly c o u p l e d redox c e n t e r s t a k e s a l s o p l a c e i n complexes o f t h e t y p e

[ ( N H3)5C o1 1 1N H2- ( C H2)n- C H = C H2C u * 34 +

The Cu* o l e f i n «-complex i s now an i s o l a t e d chromophore which i s c h a r a c t e - r i z e d by an o p t i c a l Cu1 t o * * ( o l e f i n ) MLCT t r a n s i t i o n ( r e f . 4 1 ) . The i n t e r a c t i o n o f t h i s chromophore w i t h Co i s c e r t a i n l y weak due t o t h e i n t e r - v e n i n g s a t u r a t e d methylene g r o u p s . L i g h t a b s o r p t i o n by the Cu* o l e f i n c h r o - mophore i s f o l l o w e d by t h e r e d u c t i o n of C o1 1 1 and o x i d a t i o n of Cu*. In aqueous s o l u t i o n s t a b l e redox p r o d u c t s are formed a c c o r d i n g t o t h e e q u a t i o n : C ( N H3)5C oI I IN H2- ( C H2)n- C H = C H2C uI]4 + - C o2 + + C u2 + +

+ 5NH³ + NH²-(CH²)^p-CH=CH²

The MLCT s t a t e o f t h e copper complex a c t s h e r e as e x c i t e d s t a t e e l e c t r o n d o n o r . In the e x c i t e d s t a t e i t i s the reduced o l e f i n which undergoes e l e c t r o n t r a n s f e r t o C o1 1 1:

{ ( N H3)5C oI I I- N H2- ( C H2)n- C ( C H = C H2) C uI I] * }4 + -

[ ( N H³)⁵C o^{1 1}- N H²- ( C H²}ⁿ- C H r C H²C u^{1 1}3^{4 +}

The e l e c t r o n t r a n s f e r t o Co**¹ i s f a s t enough t o compete w i t h i n t e r n a l d e a c t i v a t i o n i n t h e copper c o m p l e x . The r a t e c o n s t a n t f o r e l e c t r o n t r a n s f e r

7 fi was e s t i m a t e d by t i m e - r e s o l v e d a b s o r p t i o n s p e c t r o s c o p y t o be k > 10 -10 s "¹. F i n a l l y , the k i n e t i c a l l y l a b i l e C o^{1 1} complex undergoes a l i g a n d d i s p l a c e m e n t . The quantum y i e l d of C o2 + f o r m a t i o n d e c r e a s e s w i t h an i n c r e a - s i n g number n o f CH² g r o u p s . Beyond f o u r t o f i v e methylene groups e l e c t r o n t r a n s f e r i s no l o n g e r c o m p e t i t i v e w i t h i n t e r n a l d e a c t i v a t i o n w i t h i n t h e Cu chromophore. The a c t u a l d i s t a n c e between donor and a c c e p t o r seems h e r e t o i n c r e a s e w i t h t h e l e n g t h of t h e c o n n e c t i n g CH² c h a i n . At a f u l l y extended

c h a i n a maximum c . s t a n c e o f about 7 A f o r e l e c t r o n t r a n s f e r was e s t i m a t e d . S i m i l a r o b s e r v a t i o n s were made w i t h b i n u c l e a r Co***/Cu* complexes which c o n t a i n a p y r i d i n e o r c a r b o x y l i c group i n s t e a d o f an amine c o o r d i n a t e d t o C o1 1 1 ( r e f . 4 1 ) .

i n t r a m o l e c u l a r e x c i t e d s t a t e e l e c t r o n t r a n s f e r from a m e t a l complex as donor t o Co*1* as a c c e p t o r o c c u r s a l s o i n t h e n e u t r a l complex:

[ ( N C )5F e * * p y r a z i n e C oI* * ( N H3)5]

T h i s b i n u c l e a r confound i s composed o f t h e chromophore [Fe**(CN ^ p y r a - z i n e ]3" and [ C o *1 ¿' N H3)5p y r a z i n e ]3 + ( r e f . 4 2 ) . The metal c e n t e r s do

not seem t o be s t r o n g l y c o u p l e d . The i r o n c o n t a i n i n g chromophore i s c h a r a c t e - r i z e

nm).

r i z e d by an o p t i c a l Fe** t o i* ( p y r a z i n e ) MLCT t r a n s i t i o n ( xm a v = 630 max

Upon l i g h t a b s o r p t i o n i n t o t h i s CT band an e f f i c i e n t r e d u c t i o n o f Co***

took p l a c e a c c o r d i n g t o t h e e q u a t i o n :

[ ( N C )5F e * * p y r a z i n e C o * * * ( N H3)5] ^ [ F e * * * ( C N )5p y r a z i n e ]2"

4 C o2* + 5NH3

The p r o c e s s e s f o l l o w i n g l i g h t a b s o r p t i o n s h o u l d be q u i t e a n a l o g o u s t o t h o s e o f t h e Co***/Cu* complex d e s c r i b e d a b o v e . The MLCT e x c i t e d s t a t e o f t h e i r o n chromophore t r a n s f e r s an e l e c t r o n from t h e reduced p y r a z i n e b r i d g e t o Co*** b e f o r e i t undergoes an i n t e r n a l d e a c t i v a t i o n .

I n t e r e s t i n g l y , t h e complex [ ( N C )l- F e *I Ip y r a z i n e C o * * ( N H J , - ] w h i c h i f I T T i s t h e p r i m a r y p r o d u c t o f t h e e l e c t r o n t r a n s f e r i s a Fe t o Co MMCT s t a t e o r redox isomer o f t h e s t a r t i n g c o m p l e x . T h i s MMCT s t a t e l i e s c e r t a i n l y w e l l below t h e Fe t o p y r a z i n e MLCT s t a t e . The e x c i t e d s t a t e e l e c t r o n t r a n s f e r can t h e n be a l s o c o n s i d e r e d as an e n e r g y t r a n s f e r f r o m a MLCT t o a WCT e x c i - t e d s t a t e . In t h e a b s o r p t i o n spectrum o f t h e b i n u c l e a r Fe**/Co*** complex • a MMCT band was n o t i d e n t i f i e d . However, i t may be o b s c u r e d by t h e MLCT ab- s o r p t i o n . I t i s a l s o p o s s i b l e t h a t a MMCT band a p p e a r s i n t h e n e a r IR r e g i o n which may have n o t been i n v e s t i g a t e d . The e x p e c t a t i o n t h a t a WCT s t a t e o c c u r s a t r a t h e r low e n e r g i e s i s s u p p o r t e d by a n o t h e r o b s e r v a t i o n . T h i s WCT s t a t e c a n be p o p u l a t e d even t h e r m a l l y . The redox d e c o m p o s i t i o n o f t h e b i n u - c l e a r complex does n o t t a k e p l a c e o n l y as a p h o t o r e a c t i o n b u t a l s o as a t h e r - mal p r o c e s s ( r e f . 4 2 ) . Analogous r e s u l t s were o b t a i n e d w i t h b i n u c l e a r Fe**/Co*** complexes which c o n t a i n b r i d g i n g l i g a n d s s i m i l a r t o p y r a z i n e ( r e f . 4 2 ) .

3.3 R e v e r s i b l e E x c i t e d S t a t e E l e c t r o n T r a n s f e r 3.3.1 Introduction

I n t r a m o l e c u l a r e x c i t e d s t a t e e l e c t r o n t r a n s f e r between w e a k l y c o u p l e d redox c e n t e r s d e s c r i b e d i n t h e p r e v i o u s s e c t i o n r e s u l t s i n a permanent chemi-

c a l change. In a d d i t i o n , t h e r e are many systems which undergo an e f f i c i e n t back e l e c t r o n t r a n s f e r r e g e n e r a t i n g t h e s t a r t i n g compound. These m a t e r i a l s appear t o be not l i g h t - s e n s i t i v e . However, by means o f t i m e - r e s o l v e d absorp- t i o n o r e m i s s i o n s p e c t r o s c o p y i t i s p o s s i b l e t o demonstrate the e x i s t e n c e of s h o r t - l i v e d i n t e r m e d i a t e s . Many o f such s t u d i e s were c a r r i e d out i n o r d e r t o get more i n f o r m a t i o n on the charge s e p a r a t i o n p r o c e s s . I t was assumed t h a t back e l e c t r o n t r a n s f e r c o u l d be r e t a r d e d i f t h e e l e c t r o n / h o l e p a i r can be s e p a r a t e d over i n c r e a s e d d i s t a n c e s . However, t h i s may not ahways be t r u e . In s u i t a b l e c a s e s through-bond i n t e r a c t i o n seems t o p r o v i d e some c o u p l i n g o f donor and a c c e p t o r which may f a c i l i t a t e an e f f i c i e n t charge r e c o m b i n a t i o n over l a r g e r d i s t a n c e s . T h i s assumption i s s u p p o r t e d by r e c e n t o b s e r v a t i o n s t h a t o p t i c a l CT t r a n s i t i o n s o c c u r a l s o between remote redox c e n t e r s . The spectrum o f C ( N H3)5R uI Ip y r a z i n e R uI I( N H3)4p y r a z i n e R uI I I( N H3)537 +

d i s p l a y s a near-IR a b s o r p t i o n which was a s s i g n e d t o an end-to-end WCT t r a n - s i t i o n ( r e f . 4 3 ) . An a b s o r p t i o n band o f t h e i o n [(NH3)5Ru**NCRu**-

( b i p y )²C N R u^{I I}( N H³)⁴l^{4 +} was i d e n t i f i e d as MLCT t r a n s i t i o n from

Ru¹* c o o r d i n a t e d t o ammonia t o b i p y o f t h e a d j a c e n t ruthenium atom ( r e f . 4 4 ) . As a f u r t h e r example M** t o b i p y MLCT bands were d e t e c t e d i n t h e spec- t r a o f [ ( b i p y ) ( H²0 ) P t^{I I}N C M^{I I}( C N )⁴C N P t^{I I}( H²0 ) ( b i p y ) ] w i t h M = Fe,

Ru, and Os ( r e f . 4 5 ) .

In t h e f o l l o w i n g d i s c u s s i o n o f r e v e r s i b l e i n t r a m o l e c u l a r e x c i t e d s t a t e e l e c t r o n t r a n s f e r i n d i v i d u a l complexes a r e p r e s e n t e d . They a r e c l a s s i f i e d a c c o r d i n g t o t h e donor and a c c e p t o r s i t e o f e x c i t e d s t a t e e l e c t r o n t r a n s f e r f o l l o w i n g d i r e c t l y o p t i c a l e x c i t a t i o n o f ene c h r c c o p h c r e . Evidence of t h i s p r i m a r y e x c i t e d e l e c t r o n t r a n s f e r was o b t a i n e d by t h e i d e n t i f i c a t i o n o f sub- sequent s l o w e r p r o c e s s e s .

3.3.2 L i g a n d t o M e t a l E l e c t r o n T r a n s f e r

In t h e b i n u c l e a r i o n C ( N H³)⁵R u * * p y r a z i n e C u * * ( a q ) ]^{4 +} t h e o p t i c a l

MLCT t r a n s i t i o n g e n e r a t e s t h e e x c i t e d s t a t e [ ( N H³)⁵R u^{I I I}( p y r a z i n e " ) C u^{I I}]^{4 +}* ( r e f . 4 6 ) . T h i s i s f o l l o w e d by a r a p i d e l e c t r o n t r a n s f e r from t h e reduced b r i d g i n g l i g a n d t o Cu1* p r o d u c i n g C ( N H3)5R u * * * p y r a z i n e C u * 34 +

which undergoes r e g e n e r a t i o n of t h e s t a r t i n g c o m p l e x . The m e t a l - t o - m e t a l back e l e c t r o n t r a n s f e r t a k e s p l a c e w i t h a f i r s t o r d e r r a t e c o n s t a n t o f k =

7.8x103 s "1.

The m i x e d - v a l e n c e compound [ ( N H³)⁵R u * * p y r a z i n e R u * * * ( e d t a ) ]⁺ can be e x c i t e d t o C ( N H3)5R u * * * ( p y r a z i n e " ) R uI I I( e d t a ) ]+* by l i g h t

a b s o r p t i o n i n t o t h e c o r r e s p o n d i n g MLCT band ( r e f . 4 7 ) . In t h i s e x c i t e d s t a t e e l e c t r o n t r a n s f e r t a k e s p l a c e from t h e reduced p y r a z i n e l i g a n d t o Ru*** co- o r d i n a t e d t o e d t a w i t h k > 1 0^{1 1} s "¹. The complex [{NH^)cRu***pyra-

II +

z i n e R u ( e d t a ) ] which i s a h i g h - e n e r g y redox isomer o f t h e ground s t a t e o r a MMCT e x c i t e d s t a t e r e g e n e r a t e s t h e s t a r t i n g complex by m e t a l - t o - m e t a l

e l e c t r o n t r a n s f e r (k = 0 . 8 x 1 01 0 s "1) . S i m i l a r r e s u l t s were o b t a i n e d w i t h

" r e l a t e d m i x e d - v a l e n c e compounds ( r e f . 4 7 ) .

The complex [ ( b i p y í R e ^ í C O J ^ p y - P T Z ) ]⁺ c o n t a i n s a c o o r d i n a t e d p y r i d i - ne (py) which i s l i n k e d a t i t s 4 p o s i t i o n t o t h e r e d u c i n g p h e n o t h i a z i n e (PTZ) m o l e c u l e v i a an i n s u l a t i n g methylene group ( r e f . 4 8 ) . An o p t i c a l MLCT t r a n - s i t i o n l e a d s t o t h e e x c i t e d s t a t e [ ( b i p y " ) R e^{I J}( C O )³( p y - P T Z ) 3⁺* which undergoes r a p i d e x c i t e d s t a t e e l e c t r o n t r a n s f e r from PTZ t o t h e o x i d i z e d Re c e n t e r . In c o n t r a s t t o t h e p r e v i o u s examples t h i s e l e c t r o n t r a n s f e r i s not an i n n e r - s p h e r e p r o c e s s s i n c e t h e donor s i t e i s i s o l a t e d by t h e i n t e r v e n i n g CH^ g r o u p . The r e s u l t o f t h i s e x c i t e d s t a t e e l e c t r o n t r a n s f e r i s a LLCT e x c i t e d s t a t e w i t h t h e e l e c t r o n d i s t r i b u t i o n [ ( b i p y ' ) R e1 ( C O M p y - P T Z * ) ] * * .

8 1 I t r e v e r t s t o t h e ground s t a t e w i t h k = 4x10 s~ . 3.3.3 Metal to Metal Electron Transfer

The b i n u c l e a r i o n [ ( b i p y )2( C 0 ) 0 sHL 0 sn (o-phen) ( d p p e ) C l 33 + w i t h L = 4,4'-b and dppe = cis-Ph²PCH=ChPPhp undergoes o p t i c a l MLCT e x c i t a t i o n t o form

[ ( b i p y ) ( b i p y-) ( C 0 ) 0 sI I IL 0 s1 I( o - p h e n ) ( d p p e) c n^{3 +}* w h i c h i s a mixed-

v a l e n c e compound e x i s t i n g o n l y i n t h e e x c i t e d s t a t e ( r e f . 4 9 ) . The o p t i c a l t r a n s i t i o n i s f o l l o w e d by an e x c i t e d s t a t e m e t a l - t o - m e t a l e l e c t r o n t r a n s f e r from 0 s1 1 t o 0 sI H w i t h k ~ 1 07 s "1 y i e l d i n g [ ( b i p y ) ( b i p y " ) ( C 0 ) -

O s ^ L O s^{1 1} ^ o - p h e n ) ( d p p e ) C l 3^{3 +}. Subsequent e l e c t r o n t r a n s f e r from b i p y "

t o t h e remote 0 s1 1 1 r e g e n e r a t e s t h e s t a b l e ground s t a t e . In t h e h e t e r o b i m e t a l l i c c a t i o n C ( b i p y )²R u^{I I}b i p y m R e^I( C 0 )³C l 3^{2 +}

w i t h bipym = 2 . 2 ' - b i p y r i m i d i n e an o p t i c a l MLCT t r a n s i t i o n t e r m i n a t e s i n t h e e x c i t e d s t a t e [ ( b i p y ) ~ R uI I( b i p y m" ) R eI I( C O ) ~ C 1 32 +* ( r e f . 5 0 ) . T h i s

II

MLCT s t a t e undergoes m e t a l - t o - m e t a l e l e c t r o n t r a n s f e r from Ru t o t h e o x i - d i z e d rhenium t o form [ ( b i p y )²R u^{I I I}( b i p y m " ) R e^I( C O )³C l 3^{2 +}* . The

p r o d u c t i o n o f t h i s MLCT e x c i t e d s t a t e was d e t e c t e d by i t s e m i s s i o n t o t h e ground s t a t e .

3.3.4 Ligand t o Ligand Electron Transfer

The complexes [ ( b i p y )2R uI I( M e Q+)234 + ( r e f . 51) and [ ( b i p y ) R e1- ( C O )³( M e Q⁺) 3^{2 +} ( r e f . 52) c o n t a i n t h e o x i d i z i n g l i g a n d NCH³-4.4'-bipy- r i d i n i u m c a t i o n (MeQ⁺) w h i c h i s weakly c o u p l e d t o t h e m e t a l s . The o p t i c a l t r a n s i t i o n t o t h e MLCT s t a t e i n v o l v i n g t h e p r o m o t i o n o f an e l e c t r o n from t h e metal t o t h e b i p y l i g a n d i s f o l l o w e d by t h e r e v e r s e p r o c e s s as e m i s s i o n b u t o n l y a t low t e m p e r a t u r e s i n a r i g i d m a t r i x . In f l u i d s o l u t i o n a t room tempe- r a t u r e t h e MLCT s t a t e s [ ( b i p y ) ( b i p y " ) R u^{I H}( M e Q⁺)²3^{4 +}* and

[ ( b i p y " ) R e^{I I}( C O )³( M e Q '^,' ) 3^{2 +}* undergo e x c i t e d s t a t e e l e c t r o n t r a n s f e r from t h e reduced b i p y " to t h e o x i d i z i n g MeQ+ l i g a n d . T h i s p r o c e s s genera- t e s t h e MLCT s t a t e s [ { b i p y U R u1 1 1 ( M e Q ° ) ( M e Q+) 34 +* and [ ( b i p y ) R en( C O k -

(MeQ°)3 * which were i d e n t i f i e d by t h e i r e m i s s i o n s p e c t r a . The e x c i t e d