A C S S Y M P O S I U M S E R I E S 307
Excited States and Reactive Intermediates
Photochemistry, Photophysics, and Electrochemistry
A. B. P. Lever, EDITOR York University
Developed from a symposium sponsored by the Divisions of Inorganic Chemistry of both the American Chemical Society
and the Chemical Institute of Canada at the 1985 Biennial Inorganic
Chemical Symposium, Toronto, Ontario,
June 6-9, 1985
American Chemical Society, Washington, DC 1986
9
Electrochemically Generated Transition Metal Complexes
Emissive and Reactive Excited States
A. Vogler, H . Kunkely, and S. Schäffl
Universität Regensburg, Institut für Anorganische Chemie, D-8400 Regensburg, Federal Republic of Germany
A v a r i e t y o f t r a n s i t i o n m e t a l complexes (A) was s u b j e c - t e d to an e l e c t r o l y s i s by an a l t e r n a t i n g c u r r e n t i n a s i m p l e u n d i v i d e d e l e c t r o c h e m i c a l c e l l . The compounds a r e r e d u c e d and o x i d i z e d a t the same e l e c t r o d e . I f the e x c i t a t i o n e n e r g y o f t h e s e compounds i s s m a l l e r than the p o t e n t i a l d i f f e r e n c e o f the r e d u c e d (A ) and o x i d i z e d
( A+) f o r m s , back e l e c t r o n t r a n s f e r may r e g e n e r a t e the complexes i n an e l e c t r o n i c a l l y e x c i t e d s t a t e ( A+ + A" -+
A + A ) . These e x c i t e d complexes may be e m i s s i v e (A •>
A + hv) and/or r e a c t i v e (A B ) . C h e m i c a l t r a n s f o r m a - t i o n s w h i c h accompany the ac e l e c t r o l y s i s do not o n l y p r o c e e d v i a e x c i t e d s t a t e s . As an i m p o r t a n t a l t e r n a t i v e the r e d u c e d o r o x i d i z e d compounds can undergo a f a c i l e c h e m i c a l change (A" B~ o r A+ -* B+) . Back e l e c t r o n t r a n s f e r m e r e l y r e s t o r e s the o r i g i n a l c h a r g e s ( A+ + B~
A + B o r A" + B+ A + B ) . T h i s mechanism and the ac e l e c t r o l y s i s w h i c h p r o c e e d s v i a the g e n e r a t i o n of e x c i - t e d s t a t e s a r e not u n r e l a t e d p r o c e s s e s . Hence the pho- t o r e a c t i o n and the ac e l e c t r o l y s i s can l e a d to the same p r o d u c t i r r e s p e c t i v e of the i n t i m a t e mechanism o f the e l e c t r o l y s i s . However, i t i s a l s o p o s s i b l e t h a t p h o t o - l y s i s and e l e c t r o l y s i s g e n e r a t e d i f f e r e n t p r o d u c t s . Examples o f ac e l e c t r o l y s e s p r o c e e d i n g by t h e s e d i f f e - r e n t mechanisms a r e d i s c u s s e d .
B i 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 r e a c t i o n s have been i n - v e s t i g a t e d e x t e n s i v e l y d u r i n g the l a s t decade ( 1 - 3 ). E l e c t r o n t r a n s - f e r i s f a v o r e d t h e r m o d y n a m i c a l l y when the e x c i t a t i o n energy E of an i n i t i a l l y e x c i t e d m o l e c u l e A* exceeds the p o t e n t i a l d i f f e r e n c e of the r e d o x c o u p l e s i n v o l v e d i n the e l e c t r o n t r a n s f e r p r o c e s s .
A + hv -> A*
+ _ E(A*) > E ° ( A / A )-E(B~/B) A* + B -> A + B"
0097-6156/ 86/0307-0120506.00/ 0
© 1986 A m e r i c a n C h e m i c a l Society
S t u d i e s o f such systems p r o v i d e d a b e t t e r u n d e r s t a n d i n g o f the mecha- n i s m o f e l e c t r o n t r a n s f e r p r o c e s s e s i n g e n e r a l . T h i s r e a c t i o n type i s a l s o the b a s i s o f a l m o s t any type o f n a t u r a l or a r t i f i c i a l p h o t o - s y n t h e s i s . Hence i t i s not s u r p r i s i n g t h a t many i n v e s t i g a t i o n s have been d e v o t e d to 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 a c t i o n s . On the c o n t r a r y , the r e v e r s a l 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 has found much l e s s a t t e n t i o n a l t h o u g h i t i s c e r t a i n l y not l e s s i n t e r e s t i n g . In the p r e s e n t paper v a r i o u s a s p e c t s o f t h i s r e a c t i o n type a r e d i s - c u s s e d . The p r o d u c t s o f a r e d o x r e a c t i o n may be g e n e r a t e d i n an e x c i t e d s t a t e i f to a f i r s t a p p r o x i m a t i o n the e x c i t a t i o n e n e r g y i s s m a l l e r than the p o t e n t i a l d i f f e r e n c e o f the a s s o c i a t e d r e d o x c o u p l e s .
A+ + B % A * + B E(A*) < E ° ( A / A+) - E ° ( B " / B )
G e n e r a l l y , t h i s energy r e q u i r e m e n t i s o n l y met when a s t r o n g o x i d a n t r e a c t s w i t h a s t r o n g r e d u c t a n t . The e x c i t e d s t a t e thus p r o d u c e d does not behave d i f f e r e n t l y from t h a t g e n e r a t e d by l i g h t a b s o r p t i o n . I t can be d e a c t i v a t e d by r a d i a t i o n or c h e m i c a l t r a n s f o r m a t i o n s . E l e c t r o n t r a n s f e r i n d u c e d e m i s s i o n ( c h e m i l u m i n e s c e n c e , c l ) i s such a p r o c e s s . W h i l e i t i s w e l l known f o r o r g a n i c systems (4) t h e r e a r e not many o b s e r v a t i o n s o f c l o r i g i n a t i n g from t r a n s i t i o n m e t a l complexes ( 5 - 1 2 ) . The r e a c t a n t s can be p r e p a r e d s e p a r a t e l y . Upon m i x i n g , 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 concoramitant e m i s s i o n o f l i g h t . W h i l e t h i s type o f e x p e r i m e n t i s c o n c e p t i o n a l l y v e r y s i m p l e i t may be d i f f i c u l t to a c c o m p l i s h due to p r a c t i c a l o r t h e o r e t i c a l l i m i t a t i o n s . F o r exam- p l e , t h i s method cannot be a p p l i e d when the r e d o x p a r t n e r s A+ and B~
a r e not v e r y s t a b l e and have o n l y a s h o r t l i f e t i m e . In t h i s c a s e the redox a g e n t s must be p r e p a r e d i n s i t u . T h i s can be done i n two d i f f e - r e n t ways. The r e d o x c a t a l y s i s r e p r e s e n t s one p o s s i b i l i t y . I t may a p p l y to h i g h l y e x o e r g i c redox r e a c t i o n s which do not p r o c e e d r a p i d l y due to l a r g e a c t i v a t i o n e n e r g i e s . A s u i t a b l e r e d o x c a t a l y s t may speed up t h i s r e a c t i o n and f i n a l l y take up the energy w h i c h i s r e l e a s e d by t h i s r e d o x p r o c e s s .
Redox c a t a l y s i s l e a d i n g to c l i s i l l u s t r a t e d by two e x a m p l e s . The o x i d a t i o n o f o x a l a t e by P b ( I V ) does n o t p r o c e e d r e a d i l y a l t h o u g h i t i s s t r o n g l y f a v o r e d t h e r m o d y n a m i c a l l y . T h i s r e a c t i o n i s c a t a l y z e d by R u ( b p y ) ß 2 + w i t h b i p y - 2 , 2 ' b i p y r i d i n e a c c o r d i n g to the f o l l o w i n g mechanism ( 1 3 ) ;
2 R u ( b i p y ) ^+ + P b 02 + 4H+ -> 2 R u ( b i p y ) ^+ + P b2 + + 2H20 Ru(bipy)^*" + C20^~ + R u ( b i p y ) ^+ + C 02 + C0~
R u ( b i p y ) 3+ + C0~ + [ R u ( b i p y ) ^+] * + C 02
[ R u ( b i p y ) ^+] * + R u ( b i p y ) ^+ + hv 2+
The r e a c t i o n o f R u ( b i p y ) ^ w i t h C02 i s the energy r e l e a s i n g e l e c t r o n t r a n s f e r s t e p l e a d i n g to the f o r m a t i o n o f the e l e c t r o n i c a l l y e x c i t e d
(*) complex. I t cannot be c a r r i e d out s e p a r a t e l y . The s t r o n g o x i - dant CO" must be p r e p a r e d i n s i t u s i n c e i t i s a s h o r t - l i v e d r a d i c a l .
The c a t a l y z e d d e c o m p o s i t i o n o f e n e r g y - r i c h o r g a n i c p e r o x i d e s i s a n o t h e r t y p i c a l r e a c t i o n o f t h i s t y p e . I t was c a l l e d " c h e m i c a l l y i n i t i a t e d e l e c t r o n - e x c h a n g e l u m i n e s c e n c e " (CIEEL) by S c h u s t e r , who used o r g a n i c compounds as redox c a t a l y s t s ( 1 4 ) . However, t r a n s i t i o n
m e t a l complexes work as w e l l . The complex Re(o-phen) (CO)~C1 (R) (o-phen = o - p h e n a n t h r o l i n e ) c a t a l y z e s the d e c o m p o s i t i o n or t e t r a l i n e - h y d r o p e r o x i d e (T) to the k e t o n e a - t e t r a l o n e (K) and water a c c o r d i n g to the mechanism ( 1 5 ) :
R + T -* R+ + T~
T" -> K~ + H20 R+ + K~ + R* + K R* -> R + hv
The r e a c t i o n o f the k e t y l r a d i c a l a n i o n w i t h the o x i d i z e d rhenium complex i s the e n e r g y - r e l e a s i n g e l e c t r o n t r a n s f e r s t e p . T h i s r e a c t i o n cannot be c a r r i e d out s e p a r a t e l y . W h i l e k e t y l r a d i c a l a n i o n s are s t a b l e s p e c i e s , the o x i d i z e d complex i s not s t a b l e and must be gene- r a t e d as s h o r t - l i v e d i n t e r m e d i a t e .
E l e c t r o l y s i s r e p r e s e n t s a n o t h e r , v e r y e l e g a n t method to p r e p a r e s u i t a b l e r e d o x p a i r s i n s i t u w h i c h a r e g e n e r a t e d by c a t h o d i c r e d u c t i o n and a n o d i c o x i d a t i o n . By a p p l i c a t i o n of an a l t e r n a t i n g c u r r e n t the r e d o x p a i r i s g e n e r a t e d a t the same e l e c t r o d e . 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 from the e l e c t r o g e n e r a t e d r e d u c t a n t to the o x i d a n t near the e l e c t r o d e s u r f a c e . At an a p p r o p r i a t e p o t e n t i a l d i f f e r e n c e t h i s a n n i h i l a t i o n r e a c t i o n l e a d s to the f o r m a t i o n o f e x c i t e d p r o d u c t s . As a r e s u l t an e m i s s i o n ( e l e c t r o g e n e r a t e d c h e m i l u m i n e s c e n c e , e e l ) may be o b s e r v e d ( 1 6 ) . Redox p a i r s o f l i m i t e d s t a b i l i t y can be i n v e s t i g a t e d by ac e l e c t r o l y s i s . The f r e q u e n c y o f the ac c u r r e n t must be a d j u s t e d to the l i f e t i m e of the more l a b i l e r e d o x p a r t n e r . Many o r g a n i c com- pounds have been shown to undergo e e l ( 1 7 - 1 9 ) . Much l e s s i s known about t r a n s i t i o n m e t a l c o m p l e x e s . Most o f the o b s e r v a t i o n s i n v o l v e R u ( b i p y ) ^+ and r e l a t e d complexes w h i c h p o s s e s s e m i s s i v e charge t r a n s - f e r (CT) m e t a l - t o - l i g a n d (1**L) e x c i t e d s t a t e s (13,20-31). The organo- m e t a l l i c compound R e ( o - p h e n ) ( C O ) ^ C l i s a f u r t h e r example o f t h i s c a t e - gory ( 3 2 ) . P a l l a d i u m and p l a t i n u m p o r p h y r i n s w i t h e m i t t i n g i n t r a - l i g a n d e x c i t e d s t a t e s a r e a l s o e e l a c t i v e ( 3 3 ) . Under s u i t a b l e con- d i t i o n s e e l was a l s o o b s e r v e d f o r C r ( b i p y )3 * ~ ( 2 7) . In t h i s c a s e the e m i s s i o n o r i g i n a t e s from a l i g a n d f i e l d (LF) e x c i t e d s t a t e . Almost a l l o f the e e l a c t i v e t r a n s i t i o n m e t a l complexes c o n t a i n b i p y o r r e - l a t e d l i g a n d s . I t was t h e r e f o r e o f i n t e r e s t to see i f e e l c o u l d be extended to o t h e r t y p e s o f t r a n s i t i o n m e t a l compounds which have e m i t t i n g s t a t e s of d i f f e r e n t o r i g i n .
F u r t h e r m o r e , e x c i t e d s t a t e s g e n e r a t e d e l e c t r o c h e m i c a l l y may be not o n l y e m i s s i v e but a l s o r e a c t i v e . The p o s s i b i l i t y o f such an
" e l e c t r o p h o t o c h e m i s t r y " (epc) has been c o n s i d e r e d b e f o r e ( 3 4 ) . But r e a l examples were d i s c o v e r e d o n l y q u i t e r e c e n t l y and w i l l be d i s - c u s s e d l a t e r ( 3 5 , 3 6 ) . However, c h e m i c a l t r a n s f o r m a t i o n s i n d u c e d by ac e l e c t r o l y s i s may not o n l y p r o c e e d v i a e x c i t e d s t a t e s . Other me- chanisms can be a l s o c o n s i s t e n t w i t h t h e s e o b s e r v a t i o n s . W h i l e t h i s e x t e n d s the range o f r e a c t i o n t y p e s o f ac e l e c t r o l y s i s , i t c o m p l i c a - t e s the e l u c i d a t i o n o f the r e a l mechanism. Examples o f the v a r i o u s r e a c t i o n t y p e s are p r e s e n t e d i n the f o l l o w i n g s e c t i o n s .
E l e c t r o g e n e r a t e d Chemiluminescence
F o r our e e l s t u d i e s a v e r y s i m p l e t e c h n i q u e was employed. A 1-cm s p e c t r o p h o t o m e t e r c e l l was used as an u n d i v i d e d e l e c t r o c h e m i c a l c e l l . It was e q u i p p e d w i t h two p l a t i n u m f o i l e l e c t r o d e s which were d i r e c t l y c o n n e c t e d to a s i n e wave g e n e r a t o r as an ac v o l t a g e s o u r c e . Much more s o p h i s t i c a t e d methods have been d e s c r i b e d i n the l i t e r a t u r e (16) but t h i s s i m p l e d e s i g n p e r m i t t e d the o b s e r v a t i o n o f e e l which appears a t b o t h e l e c t r o d e s .
R e c e n t l y we o b s e r v e d e e l o f the b i n u c l e a r p l a t i n u m complex t e t r a - k i s ( d i p h o s p h o n a t o ) d i p l a t i n a t e ( I I ) (Pt2(pop)^~) ( 3 7 ) . T h i s a n i o n has a t t r a c t e d much a t t e n t i o n due to i t s i n t e n s e g r e e n l u m i n e s c e n c e i n room t e m p e r a t u r e s o l u t i o n (38-40) - 0.52) ( 4 1 ) . The e x c i t e d s t a t e of t h i s complex undergoes o x i d a t i v e (42) and r e d u c t i v e q u e n c h i n g ( 4 1 ) . From the q u e n c h i n g e x p e r i m e n t s the r e d o x p o t e n t i a l s were e s t i m a t e d to be E ° = -1.4 V v s . SCE f o r the r e d u c t i o n and E ° ~ 1 V f o r the o x i d a - t i o n o f P t2( p o p ) ^ ~ ( 4 1 ) . The p o t e n t i a l d i f f e r e n c e o f 2.4 V a l m o s t matches the energy o f the p h o s p h o r e s c i n g t r i p l e t (~ 2.5 eV) o f P t « -
( p o p ) ^ ~ . C o n s e q u e n t l y , i t s h o u l d be p o s s i b l e to o b s e r v e e e l o f t h i s complex. However, the reduced ( P t « ( p o p ) | ~ ) (43) and o x i d i z e d ( P t ^ -
(pop)|~) (44,45) forms a r e not s t a b l e , but decay r a p i d l y i n s o l u t i o n . Hence an e e l o f P t ^ i p o p ) ^ " w i l l o n l y take p l a c e i f the subsequent g e n e r a t i o n o f b o t h redox p a r t n e r s o c c u r s b e f o r e they undergo a d e c a y .
The e e l e x p e r i m e n t was c a r r i e d out i n a s o l u t i o n o f a c e t o n i t r i l e w i t h Bu^NBF^ as s u p p o r t i n g e l e c t r o l y t e ( 3 7 ) . At an ac v o l t a g e o f 4 V, a f r e q u e n c y o f 280 Hz, and a c u r r e n t o f 13 mA a g r e e n e m i s s i o n appea- r e d a t the e l e c t r o d e s . I t was i d e n t i c a l w i t h the p h o s p h o r e s c e n c e
( X ^ = 5 1 7 nm) o f P t2( p o p ) ^ ~ . T h i s o b s e r v a t i o n i s c o n s i s t e n t w i t h t h e P f o l l o w i n g r e a c t i o n sequence:
4- - 5-
P t ^ í p o p ) ^ + e -> P t2( p o p ) ^ c a t h o d i c c y c l e 4- - 3-
P t ^ í p o p ) ^ - e P t ^ i p o p ) ^ a n o d i c c y c l e
P t2( p o p ) ^ " + P t2( p o p ) ^ ~ - [ P t2( p o p ) £ ~ ] * + P t2( p o p ) £ "
[ P t2( p o p ) £ ~ ] * P t2( p o p ) £ ~ + hv 4-
The r e d u c t i o n and o x i d a t i o n o f P t2( p o p ) ^ takes p l a c e a t the same e l e c t r o d e . Back e l e c t r o n t r a n s f e r g e n e r a t e s one o f the s t a r t i n g i o n s i n the e x c i t e d t r i p l e t s t a t e which undergoes p h o s p h o r e s c e n c e . I n t e - r e s t i n g l y , the f l u o r e s c e n c e o f the complex w h i c h appears on photo- e x c i t a t i o n a t X = 407 nm, i s not o b s e r v e d i n the e e l e x p e r i m e n t . T h i s i s not s u r p r i s i n g s i n c e the back e l e c t r o n t r a n s f e r does not p r o - v i d e enough energy (~ 2.4 V) to p o p u l a t e the e m i t t i n g s i n g l e t (~ 3.3 V ) .
I t s h o u l d be mentioned h e r e t h a t the p r o c e s s e s w h i c h a r e i n v o l - ved i n the appearance of an e e l o f P t2( p o p ) ¿ ~ a r e a s s o c i a t e d w i t h changes i n the m e t a l - m e t a l b o n d i n g o f t h i s b i n u c l e a r complex (38-40, 42,44,46,47). The P t - P t bond o r d e r which i s z e r o i n the ground s t a t e i s i n c r e a s e d to 0.5 by o x i d a t i o n as w e l l as by r e d u c t i o n . The a n n i - h i l a t i o n r e a c t i o n l e a d s to the f o r m a t i o n o f P t2( p o p ) ^ ~ as the ground
(bond o r d e r = 0) and e x c i t e d s t a t e (bond o r d e r = 1 ) . A r e l a t e d case which was r e p o r t e d q u i t e r e c e n t l y i s the e e l o f Mo^-Cl 9^ ~ . The m e t a l -
m e t a l b o n d i n g o f the c l u s t e r i s i n v o l v e d i n the r e d o x p r o c e s s e s w h i c h a r e a s s o c i a t e d w i t h the e e l ( 4 8 ) .
E l e c t r o g e n e r a t i o n o f E x c i t e d Complexes U n d e r g o i n g E m i s s i o n and R e a c t i o n The e l e c t r o c h e m i c a l g e n e r a t i o n o f e x c i t e d s t a t e s may not o n l y l e a d to an e m i s s i o n . In a d d i t i o n o r as an a l t e r n a t i v e the e x c i t e d s t a t e can undergo a c h e m i c a l r e a c t i o n ( " e l e c t r o p h o t o c h e m i s t r y " , epc) as i t would o c c u r upon l i g h t a b s o r p t i o n ( p h o t o c h e m i s t r y ) . In the e e l e x p e r i - ments the o b s e r v a t i o n o f l u m i n e s c e n c e i s by i t s e l f a p r o o f f o r the g e n e r a t i o n o f e x c i t e d s t a t e s . But the f a c t t h a t e l e c t r o l y s i s and p h o t o l y s i s b o t h l e a d to the f o r m a t i o n o f the same p r o d u c t does not prove the e l e c t r o c h e m i c a l g e n e r a t i o n o f an e x c i t e d s t a t e (see b e l o w ) . F o r t h i s r e a s o n i t i s an advantage to study compounds w h i c h a r e s i m u l - t a n e o u s l y p h o t o e m i s s i v e and p h o t o r e a c t i v e . A p o s i t i v e c o r r e l a t i o n between e e l and the e l e c t r o c h e m i c a l r e a c t i o n i s a good i n d i c a t i o n t h a t the c h e m i c a l t r a n s f o r m a t i o n i s i n d e e d a s s o c i a t e d w i t h an e x c i t e d s t a t e . In t h i s case the e l e c t r o c h e m i c a l r e a c t i o n i s a t r u e e p c . Upon ac e l e c t r o l y s i s the complex R u ( b i p y ) 2+ undergoes s i m u l t a n e o u s l y e e l and
epc ( 4 9 ) . 2 +
The well-known p h o t o l u m i n e s c e n c e o f R u ( b i p y ) ^ o c c u r s from the lowest e x c i t e d s t a t e w h i c h i s o f the CT (Ru+bipy) type ( 5 0 , 5 1 ) . The e m i s s i o n a p p e a r s i n aqueous as w e l l as i n non-aqueous s o l u t i o n s . W h i l e the complex i s h a r d l y l i g h t - s e n s i t i v e i n water (52) i t can under- go an e f f i c i e n t p h o t o s u b s t i t u t i o n o f a b i p y l i g a n d i n non-aqueous s o l v e n t s (50,51,53-56). The r e a c t i v e e x c i t e d s t a t e seems to be a LF s t a t e w h i c h l i e s a t s l i g h t l y h i g h e r e n e r g i e s but can be p o p u l a t e d
t h e r m a l l y f r o m the e m i t t i n g CT s t a t e (50-52,55-58). A c c o r d i n g to 2 + these o b s e r v a t i o n s the e l e c t r o c h e m i c a l g e n e r a t i o n o f e x c i t e d R u ( b i p y ) ^ i n non-aqueous s o l u t i o n s s h o u l d not o n l y be accompanied by the w e l l - known e e l but a l s o by an e p c . M o r e o v e r , the e f f i c i e n c y o f b o t h p r o - c e s s e s s h o u l d show a p o s i t i v e c o r r e l a t i o n . P r e l i m i n a r y e x p e r i m e n t s i n d e e d p r o v i d e e v i d e n c e f o r a s i m u l t a n e o u s o c c u r a n c e o f e e l and epc of R u ( b i p y ) 2 + ( 4 9 ) .
An ac e l e c t r o l y s i s o f [ R u ( b i p y ) ~ ] C 1 « was c a r r i e d out i n a s p e c t r o - photometer c e l l as an u n d i v i d e d e l e c t r o c h e m i c a l c e l l e q u i p p e d w i t h p l a t i n u m f o i l e l e c t r o d e s . A c e t o n i t r i l e was used as s o l v e n t and Bu.NBF, s e r v e d as s u p p o r t i n g e l e c t r o l y t e . The e l e c t r o l y s i s l e d to the t y p i c a l e e l o f R u ( b i p y ) ^+ (20,21,23,25). S i m u l t a n e o u s l y , the complex underwent a c h e m i c a l c h a n g e . The s p e c t r a l v a r i a t i o n s w h i c h accompa- n i e d the e l e c t r o l y s i s ( F i g u r e 1) were v e r y s i m i l a r to those o b s e r v e d d u r i n g the p h o t o l y s i s o f the same s o l u t i o n (X. > 335 nm). The p r o - d u c t o f e l e c t r o l y s i s and p h o t o l y s i s was n o t y e ?ri d e n t i f i e d d e f i n i t e l y , but a c c o r d i n g to a p r e l i m i n a r y c h a r a c t e r i z a t i o n i t seems to be
[ R u ( b i p y )2( C H3C N ) C 1 ] . However, i t i s i m p o r t a n t to n o t e t h a t a l l changes or the e x p e r i m e n t a l c o n d i t i o n s ( e . g . v a r i a t i o n s o f the ac f r e q u e n c y , s t i r r i n g o f the s o l u t i o n ) w h i c h l e a d to a change o f the e e l i n t e n s i t y a l s o c a u s e d a c o r r e s p o n d i n g change o f the e f f i c i e n c y o f the e l e c t r o c h e m i c a l r e a c t i o n . These o b s e r v a t i o n s a r e good i n d i c a t i o n t h a t b o t h p r o c e s s e s p r o c e e d v i a the g e n e r a t i o n o f e x c i t e d R u ( b i p y ) ^+. I t i s s u g g e s t e d t h a t the ac e l e c t r o l y s i s can be d e s c r i b e d by the f o l l o - wing mechanism:
R u ( b i p y ) ^ + e 2 +
R u ( b i p y ) 2+
R u ( b i p y ) R u ( b i p y ) 3 3+
R u ( b i p y ) * + R u ( b i p y ) ^+
[ R u i b i p y ) * * ] *
[ R u ( b i p y ) 3+] * + R u ( b i p y ) 3+
R u ( b i p y ) ¡ ?+ + hv
c a t h o d i c c y c l e a n o d i c c y c l e
a n n i h i l a t i o n e e l
[Ruibipy)***"]* + C l " + CI13CN
R u ( b i p y )2( C H3C N ) C l
+ b i p y epc
The c o n c l u s i o n t h a t the e l e c t r o c h e m i c a l r e a c t i o n o f R u ( b i p y ) ^ t a k e s 2+
p l a c e v i a an e x c i t e d s t a t e i s a l s o s u p p o r t e d by o t h e r o b s e r v a t i o n s . A c c o r d i n g t o+e l e c t r o c h e m i c a l s t u d i e s the r e d u c e d and o x i d i z e d comple- xes R u ( b i p y ) ^ and R u ( b i p y )3 + are f a i r l y s t a b l e and not e x p e c t e d to undergo r a p i d c h e m i c a l t r a n s f o r m a t i o n s (21,23,25,50).
E l e c t r o g e n e r a t i o n of R e a c t i v e E x c i t e d S t a t e s
Most compounds w h i c h undergo a p h o t o c h e m i c a l r e a c t i o n do not s i m u l - t a n e o u s l y show p h o t o l u m i n e s c e n c e . I t i s then more d i f f i c u l t to prove t h a t a r e a c t i o n i n d u c e d by ac e l e c t r o l y s i s p r o c e e d s v i a the i n t e r m e - d i a t e f o r m a t i o n o f e x c i t e d s t a t e s . A d i f f e r e n t mechanism may be i n o p e r a t i o n . In t h i s c a s e the c h e m i c a l t r a n s f o r m a t i o n o c c u r s i n the r e d u c e d and/or o x i d i z e d f o r m . The back e l e c t r o n t r a n s f e r m e r e l y r e g e n e r a t e s the c h a r g e s o f the s t a r t i n g compound:
A + e
A - e"
A "
A+ + B"
A c a t h o d i c c y c l e A+ a n o d i c c y c l e -> B c h e m i c a l r e a c t i o n
•> A + B a n n i h i l a t i o n
N e v e r t h e l e s s , the r e s u l t o f the e l e c t r o l y s i s may be the same as t h a t o f the p h o t o l y s i s , because the o r i g i n o f the r e a c t i v i t y i s s i m i l a r i n b o t h c a s e s . F o r example, a bond weakening may o c c u r upon r e d u c t i o n o r o x i d a t i o n s i n c e an e l e c t r o n i s added to an a n t i b o n d i n g TT* o r b i t a l or removed from a b o n d i n g IT o r b i t a l . The same changes take p l a c e upon TTTT* e x c i t a t i o n .
A c a s e i n q u e s t i o n i s the ac e l e c t r o l y s i s o f the complex R e ( t r a n s - S P )2( C 0 ) C l (SP = 4 - s t y r y l p y r i d i n e ) ( 5 9 ) . I t was shown b e f o r e t h a t the c o o r d i n a t e d l i g a n d SP undergoes a p h o t o c h e m i c a l t r a n s / e i s i s o m e r i - z a t i o n ( 6 0 ) . The r e a c t i v e e x c i t e d s t a t e i s the l o w e s t TTTT* i n t r a l i g a n d
(IL) s t a t e , w h i c h i s not l u m i n e s c e n t . The ac e l e c t r o l y s i s l e a d s a l s o to the t r a n s / c i s i s o m e r i z a t i o n o f the c o o r d i n a t e d l i g a n d ( 5 9 ) . Hence i t i s a r e a s o n a b l e a s s u m p t i o n t h a t the e l e c t r o l y s i s p r o c e e d s v i a the g e n e r a t i o n o f the TTTT* IL s t a t e :
R e ( t r a n s - S P )2( C O )3C l + e Re ( t r a n s - S P )2( C 0 )3C 1 ~ Re ( t r a n s - S P )2( C O )3C 1 - e~ ->- R e ( t r a n s - S P ) ( C O )3C 1+
R e ( t r a n s - S P )2( C O )3C 1+ + R e ( t r a n s - S P ) ( C O )3C l "
+ R e ( t r a n s - S P )2( C O )3C l * + Re ( t r a n s - S P )2 (CO) C l R e ( t r a n s - S P )2( C O )3C l * -> Re ( c i s - S P )2 (C0>3C1
However, as an a l t e r n a t i v e the i s o m e r i z a t i o n may take p l a c e i n the r e d u c e d and/or o x i d i z e d form:
R e ( t r a n s - S P )2( C O )3C l " •> Re ( c i s - S P ) (CO) C l "
Re ( t r a n s - S P ) 2 (CO) 3C 1+ -> Re ( c i s - S P ) (CO) 3C1 +
R e ( c i s - S P )2( C O )3C l+ + R e ( c i s - S P ) (CO) C l ~ -> 2 R e ( c i s - S P )2( C O )3C l
I n s p e c t i o n o f some a d d i t i o n a l d a t a does n o t l e a d to a d i s t i n c t i o n between t h e two p o s s i b i l i t i e s . The p o t e n t i a l d i f f e r e n c e o f the r e - duced and o x i d i z e d complex (2.94 V) exceeds the e l e c t r o n i c e x c i t a - t i o n energy o f the n e u t r a l complex (~ 2.1 eV) ( 5 9 ) . On e n e r g e t i c grounds t h e e l e c t r o c h e m i c a l g e n e r a t i o n o f e x c i t e d s t a t e s i s c e r t a i n l y p o s s i b l e . The r e l a t e d complex Re(o-phen)(CO)3C1 i s n o t l i g h t s e n s i - t i v e b u t i s p h o t o l u m i n e s c e n t and a l s o e e l a c t i v e ( 3 2 ) . By a n a l o g y one might assume t h a t t h e e l e c t r o l y s i s o f b o t h complexes p r o c e e d s by the same mechanism. On the o t h e r s i d e , c y c l i c voltammetry shows t h a t the o x i d i z e d form o f R e ( t r a n s - S P ) (CO) C l i s f a i r l y s t a b l e b u t the r e d u c e d complex decays i r r e v e r s i b l y ( 5 9 ) . Only a t l a r g e scan r a t e s (100 Vs~^) the r e d u c t i o n wave shows b e g i n n i n g r e v e r s i b i l i t y . I t i s then n o t u n r e a s o n a b l e t o assume t h a t t h e l i g a n d i s o m e r i z a t i o n takes p l a c e i n the r e d u c e d complex. The f i n a l back e l e c t r o n t r a n s f e r would m e r e l y r e s t o r e the n e u t r a l complex. Of c o u r s e , i n the absence o f e e l any d i r e c t p r o o f o f the e l e c t r o c h e m i c a l g e n e r a t i o n o f e x c i t e d s t a t e s i s d i f f i c u l t t o o b t a i n . N e v e r t h e l e s s , i n d i r e c t b u t c o n c l u s i v e e v i - dence showed i n d e e d t h a t an e x c i t e d s t a t e mechanism l e d to the e l e c - t r o c h e m i c a l i s o m e r i z a t i o n o f the complex.
E x p e r i m e n t s were c a r r i e d o u t to d e t e r m i n e i f d u r i n g t h e ac e l e c - t r o l y s i s the l i g a n d i s o m e r i z a t i o n r e q u i r e s t h e f o r m a t i o n o f the r e d u - ced and o x i d i z e d form ( 5 9 ) . T h i s would i n d i c a t e an e x c i t e d s t a t e mechanism. I f the i n t e r m e d i a t e f o r m a t i o n o f the reduced o r o x i d i z e d complex i s s u f f i c i e n t to i n d u c e the i s o m e r i z a t i o n , e x c i t e d s t a t e s a r e not r e q u i r e d . F i r s t s u p p o r t i n f a v o r o f a t r u e epc was o b t a i n e d by the r e s u l t s o f the ac e l e c t r o l y s i s o f R e ( t r a n s - S P )2( C O )3C 1 i n the p r e s e n c e o f r e d o x b u f f e r s . T e t r a m e t h y 1 - p - p h e n y l e n e d i a m m e (TMPD) was used as r e d u c t a n t and the p a r a q u a t c a t i o n (PQ^+) s e r v e d as o x i d a n t . In t h e p r e s e n c e o f an e x c e s s o f TMPD the complex i s s t i l l r e d u c e d , but TMPD i s o x i d i z e d d u r i n g the e l e c t r o l y s i s . S i n c e the o x i d a t i o n p o t e n t i a l o f TMPD i s much lower than t h a t o f t h e complex, the a n n i h i - l a t i o n r e a c t i o n o f the complex a n i o n and TMPD+ does n o t p r o v i d e enough energy t o g e n e r a t e the complex i n t h e e x c i t e d s t a t e . Q u i t e an a n a l o -
gous s i t u a t i o n a p p l i e s to the e l e c t r o l y s i s i n the p r e s e n c e of PQ Now the complex i s o x i d i z e d but PQ^+ r e d u c e d . A g a i n , the p o t e n t i a l d i f f e r e n c e of the complex c a t i o n and PQ i s s m a l l e r than the e x c i t a - t i o n e n e r g y o f R e ( t r a n s - S P ) ( 0 0 ) ^ 0 1 . In b o t h e x p e r i m e n t s the l i g a n d i s o m e r i z a t i o n was e s s e n t i a l l y s u p p r e s s e d . C o n s e q u e n t l y the i n t e r m e - d i a t e f o r m a t i o n o f the complex c a t i o n o r a n i o n a l o n e cannot be respon- s i b l e f o r the i s o m e r i z a t i o n .
A d d i t i o n a l e v i d e n c e i n s u p p o r t o f an e x c i t e d s t a t e mechanism was o b t a i n e d by c o n t i n u o u s p o t e n t i a l s t e p c h r o n o c o u l o m e t r i c e x p e r i m e n t s
( 5 9 ) . When the e l e c t r o d e p o t e n t i a l was s t e p p e d o n l y o v e r the o x i d a - t i o n p o t e n t i a l o f the complex a t a f r e q u e n c y o f 10 Hz a slow net o x i - d a t i o n took p l a c e . P o t e n t i a l s t e p s i n v o l v i n g o n l y the r e d u c t i o n wave l e d to r a p i d net r e d u c t i o n but no l i g a n d i s o m e r i z a t i o n . The i s o m e r i - z a t i o n o c c u r r e d o n l y when the p o t e n t i a l s t e p s i n c l u d e d b o t h r e d u c t i o n and o x i d a t i o n o f the complex. S i n c e the voltammograms of R e ( t r a n s - S P ^ ( C 0 )3C 1 and R e ( c i s - S P )2( 0 0 ) ^ 0 1 a r e v i r t u a l l y i n d i s t i n g u i s h a b l e , the l i g a n d i s o m e r i z a t i o n was not accompanied by a p o t e n t i a l c h a n g e . No net F a r a d a i c p r o c e s s was o b s e r v e d .
The c o n c l u s i o n t h a t the ac e l e c t r o l y s i s o f R e ( t r a n s - S P )2( C 0 )3C 1 p r o c e e d s v i a e x c i t e d s t a t e s i s a l s o s u p p o r t e d by the d i r e c t i o n o f i s o m e r i z a t i o n . In t h e r m a l r e a c t i o n s o f s t i l b e n e d e r i v a t i v e s and r a d i - c a l s e i s to t r a n s c o n v e r s i o n s a r e g e n e r a l l y o b s e r v e d ( 6 1 ) . C o n t r a r y to t h i s b e h a v i o r the p h o t o l y s i s and ac e l e c t r o l y s i s l e a d to e n e r g e t i - c a l l y u p h i l l t r a n s t o e i s i s o m e r i z a t i o n .
AC E l e c t r o l y s i s W i t h o u t G e n e r a t i o n o f E x c i t e d S t a t e s
As d i s c u s s e d a b o v e , a c h e m i c a l t r a n s f o r m a t i o n which o c c u r s d u r i n g the ac e l e c t r o l y s i s does not r e q u i r e the i n t e r m e d i a t e f o r m a t i o n of e x c i - ted s t a t e s . The c h e m i c a l r e a c t i o n may take p l a c e i n the r e d u c e d and/
or o x i d i z e d form o f a compound. N e v e r t h e l e s s , i n t h i s c a s e the e l e c - t r o l y s i s may s t i l l l e a d to the same p r o d u c t s as t h o s e of the photo- l y s i s due to the o b v i o u s r e l a t i o n s h i p between e l e c t r o n i c e x c i t a t i o n and r e d o x p r o c e s s e s . I t w i l l be then q u i t e d i f f i c u l t to e l u c i d a t e the mechanism of e l e c t r o l y s i s . T h i s r e a c t i o n t y p e may a p p l y to the e l e c t r o c h e m i c a l s u b s t i t u t i o n of C r ( C 0 )6 ( 5 9 ) .
The ac e l e c t r o l y s i s o f C r ( C 0 )6 i n CH^CN was accompanied by the same s p e c t r a l changes ( F i g u r e 2) as those o b s e r v e d i n the p h o t o l y s i s o f the same s o l u t i o n w i t h 333-nm l i g h t . In b o t h c a s e s C r ( C O ) , was c o n v e r t e d to C r ( C 0 )5( C H3C N ) ( 5 9 ) . A c c o r d i n g to P i c k e t t and P l e t c h e r
(62) C r ( C 0 )6 shows a r e v e r s i b l e o x i d a t i o n wave a t 1.52 V v s . SCE;
the r e d u c t i o n wave a t -2.66 V i s i r r e v e r s i b l e and was a t t r i b u t e d to a r a p i d o r even c o n c e r t e d l o s s of CO f r o m C r ( C 0 ) ~ to g i v e C r ( C 0 ) ~ . A r e v e r s e peak i n the c y c l i c voltammogram a t -2.1 V shows the r e o x i - d a t i o n o f the l a t t e r s p e c i e s to the c o o r d i n a t i v e l y u n s a t u r a t e d Cr(CO) which can be s t a b i l i z e d by the a d d i t i o n of a s o l v e n t m o l e c u l e as a s i x t h l i g a n d . C o n s e q u e n t l y , the ac e l e c t r o l y s i s may p r o c e e d a c c o r d i n g to the f o l l o w i n g r e a c t i o n scheme w i t h o u t i n v o k i n g an e l e c t r o n i c a l l y e x c i t e d s t a t e i n the back e l e c t r o n t r a n s f e r (59):
C r ( C O )6 - e~ + C r ( C O ) * a n o d i c c y c l e C r ( C O ) . + e~ + Cr(C0)7 c a t h o d i c c y c l e
D D
Cr(C0)7 + Cr(C0)7 + CO l i g a n d d i s s o c i a t i o n
b d
C r ( C O ) * + C r ( C 0 ) ~ •+ C r ( C O ) . + C r ( C O )c a n n i h i l a t i o n O D D J
C r ( C O )5 + CH3CN + Cr ( C 0 )5 (CH3CN) l i g a n d a d d i t i o n
T h i s mechanism and the p h o t o l y s i s have i n common t h a t the a d d i t i o n of an e l e c t r o n to the a n t i b o n d i n g e o r b i t a l s i n d u c e s the d i s s o c i a t i o n
of a CO l i g a n d . g
As a f u r t h e r p o s s i b i l i t y the ac e l e c t r o l y s i s may l e a d to o t h e r p r o d u c t s than t h o s e o f the p h o t o l y s i s . In t h i s c a s e an e x c i t e d s t a t e mechanism i s , o f c o u r s e , e x c l u d e d . A l t h o u g h t h e r e i s a c e r t a i n s i m i - l a r i t y between the e l e c t r o n i c s t r u c t u r e o f an e x c i t e d s t a t e and the r e d u c e d or o x i d i z e d f o r m o f a m o l e c u l e , they a r e not i d e n t i c a l . Con- s e q u e n t l y , i t i s not s u r p r i s i n g when p h o t o l y s i s and e l e c t r o l y s i s do not y i e l d the same p r o d u c t . A n o t h e r r e a s o n f o r such an o b s e r v a t i o n may be the d i f f e r e n t l i f e t i m e s . An e x c i t e d s t a t e can be e x t r e m e l y s h o r t - l i v e d . N o n - r e a c t i v e d e a c t i v a t i o n c o u l d then compete s u c c e s s - f u l l y w i t h a p h o t o r e a c t i o n . The compound i s not l i g h t - s e n s i t i v e . On the c o n t r a r y , the r e d u c e d and o x i d i z e d i n t e r m e d i a t e s g e n e r a t e d by ac e l e c t r o l y s i s s h o u l d have comparably l o n g l i f e times which may p e r m i t a r e a c t i o n . The ac e l e c t r o l y s i s o f N i ( I I ) ( B A B A ) ( M N T ) (BABA = b i a c e t y l - b i s ( a n i l ) and MNT^~ = d i s u l f i d o m a l e o n i t r i l e ) i s an example of t h i s r e a c t i o n type ( 6 3 ) .
The complex Ni(BABA)(MNT) (64) i s not l i g h t s e n s i t i v e (X. >
400 nm) i n s o l u t i o n s o f a c e t o n i t r i l e but undergoes an ac e l e c t r o l y s i s which i s accompanied by s p e c t r a l changes as shown i n F i g u r e 3. A c c o r - d i n g to a p r e l i m i n a r y a n a l y s i s o f the p r o d u c t s the e l e c t r o l y s i s l e a d s to a l i g a n d exchange:
2 N i1 1 (BABA) (MNT) -> N i1 1 (BABA)^+ + N i ^ i M N T )2"
The e l e c t r o c h e m i s t r y o f Ni(BABA)(MNT) has been i n v e s t i g a t e d r e c e n t l y ( 6 4 ) . The f i r s t r e d u c t i o n o c c u r s r e v e r s i b l y a t E ¿ = -0.7 V v s . SCE.
However, the o x i d a t i o n i s i r r e v e r s i b l e (Ep/2 = v) • F o r t n e r e l a - ted complex N i ( o - p h e n )( S ^ C ^ P h ^ ) i t was shown t h a t the c a t i o n
N i ( o - p h e n ) ( S « C 2 P h 2 )+ g e n e r a t e d by p h o t o o x i d a t i o n i n h a l o c a r b o n s o l v e n t s undergoes a f a c i l e l i g a n d exchange to y i e l d the symmetric complexes N i ( o - p h e n ) |+ and Ni(S2C2Ph2)2 ( 6 5 ) . A c c o r d i n g to these c o n s i d e r a t i o n s the ac e l e c t r o l y s i s can be r a t i o n a l i z e d by the f o l l o w i n g r e a c t i o n scheme:
Ni(BABA)(MNT) + e" + Ni(BABA)(MNT)~ c a t h o d i c c y c l e N i (BABA) (MNT) - e" •> Ni (BABA) (MNT)+ a n o d i c c y c l e 2 N i ( B A B A ) ( M N T )+ Ni(BABA)^* + N i ( M N T )2 l i g a n d exchange 2 Ni (BABA) (MNT)"" + Ni (MNT) -» 2 Ni (BABA) (MNT) e l e c t r o n
+ N i ( M N T )2-2 t r a n s f e r
1.2
0.8 -
0.4 -
F i g u r e 2. S p e c t r a l changes d u r i n g ac e l e c t r o l y s i s o f 6.5x10 " M .-4 C r ( C 0 )6 i n a c e t o n i t r i l e / 0 . 0 5 m Bu^NBF a t (a) 0 and ( f ) 300-min e l e c t r o l y s i s time a t 2.5 V/10 Hz and 5 mA, 1-cm c e l l .
1.0
0.8
0.6
0.4
0.2 -
400 500 600 X [nm]
-4 M F i g u r e 3. S p e c t r a l changes d u r i n g ac e l e c t r o l y s i s o f 1.5x10 Ni(BABA)(MNT) i n a c e t o n i t r i l e / O . 1 M Bu^NBF^ a t (a) 0 and (d) 30- rain e l e c t r o l y s i s time a t 3 V/20 Hz and 40 mA, 1-cm c e l l .
The l i g a n d exchange p r o d u c e s NiCMNT)^ which i s n o t s t a b l e b u t a s t r o n g o x i d a n t ( 6 6 ) . I t o x i d i z e s a p p a r e n t l y the r e d u c i n g a n i o n Ni(BABA)(MNT) i n two subsequent e l e c t r o n t r a n s f e r s t e p s .
R e a c t i o n s R e l a t e d to the AC E l e c t r o l y s i s
There a r e o t h e r r e a c t i o n s o f t r a n s i t i o n m e t a l complexes which a r e r e - l e v a n t t o o u r o b s e r v a t i o n s on the ac e l e c t r o l y s i s . R e c e n t l y , new mechanisms o f l i g a n d s u b s t i t u t i o n r e a c t i o n s have been r e p o r t e d which a r e c h a r a c t e r i z e d by e l e c t r o n t r a n s f e r r e a c t i o n s as key s t e p s a l t h o u g h the o v e r a l l r e a c t i o n s a r e n o t r e d o x p r o c e s s e s , e.g.,
ML + e~ -* ML~
ML" + L* -> MLf" + L ML1" - e~ -+ ML1
o v e r a l l : ML + L1 ML* + L
The s u b s t i t u t i o n a l ^ l a b i l e complex may be g e n e r a t e d n o t o n l y by r e - d u c t i o n but by o x i d a t i o n as w e l l . An immediate r e l a t i o n s h i p o f such a r e a c t i o n to the ac e l e c t r o l y s i s p r o c e e d i n g w i t h o u t g e n e r a t i o n o f e x c i t e d s t a t e s c a n be r e c o g n i z e d . The i n i t i a l p r o d u c t i o n o f the sub- s t i t u t i o n a l ^ l a b i l e o x i d a t i o n s t a t e o f ML c a n be a c h i e v e d e l e c t r o - c h e m i c a l l y ( 6 7 - 7 6 ) , c h e m i c a l l y (75-77) o r p h o t o c h e m i c a l l y ( 7 8 ) . In the e l e c t r o c h e m i c a l e x p e r i m e n t s r e d u c t i o n o r o x i d a t i o n was a c c o m p l i s h e d by a d i r e c t c u r r e n t . In most c a s e s t h e s e p r o c e s s e s a r e c a t a l y t i c c h a i n r e a c t i o n s w i t h F a r a d a i c e f f i c i e n c i e s much l a r g e r than u n i t y . E l e c t r o - c h e m i c a l s u b s t i t u t i o n o f M(CO). w i t h M = C r , Mo, W was c a r r i e d out by c a t h o d i c r e d u c t i o n to M(CO)~ which d i s s o c i a t e s i m m e d i a t e l y t o y i e l d M(CO)~. Upon a n o d i c r e o x i d a t i o n a t t h e o t h e r e l e c t r o d e c o o r d i n a t i v e l y u n s a t u r a t e d M(CO) i s formed and s t a b i l i z e d by a d d i t i o n o f a l i g a n d L to g i v e M ( C O )5L ( 6 8 ) .
P h o t o c h e m i c a l s u b s t i t u t i o n v i a a l a b i l e o x i d a t i o n s t a t e may o c c u r 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 f the m e t a l complex has a l o n g - l i v e d e x c i t e d s t a t e , i t c a n undergo an e l e c t r o n exchange w i t h a r e d u c - t a n t o r o x i d i d a n t i n a b i m o l e c u l a r r e a c t i o n . The l a b i l e r e d u c e d o r o x i d i z e d complex thus produced i s s u s c e p t i b l e to a l i g a n d s u b s t i t u t i o n . A c a t a l y t i c c h a i n r e a c t i o n takes p l a c e when t h e s u b s t i t u t e d complex i n the l a b i l e o x i d a t i o n s t a t e undergoes a f u r t h e r e l e c t r o n exchange w i t h a n o t h e r u n s u b s t i t u t e d complex. The c h a i n t e r m i n a t e s by back e l e c t r o n t r a n s f e r between the l a b i l e o x i d a t i o n s t a t e and the e x t e r n a l r e d o x p a r t n e r w h i c h was g e n e r a t e d i n i t i a l l y . The c a t i o n Re(o-phen)(CO)^~
(CH^CN)"1" undergoes t h i s new type o f p h o t o s u b s t i t u t i o n ( 7 8 ) . The o c c u r - r e n c e o f a c h a i n r e a c t i o n was c o n f i r m e d by t h e quantum y i e l d s which were as l a r g e as <j> = 24 depending on the e x p e r i m e n t a l c o n d i t i o n s . Of c o u r s e , the e f f i c i e n c y o f the u s u a l p h o t o s u b s t i t u t i o n s w h i c h o r i g i n a t e from LF e x c i t e d s t a t e s o f m e t a l complexes do n o t exceed u n i t y .
C o n c l u s i o n
The use o f ac e l e c t r o l y s i s i n a l l i t s v a r i a t i o n s i s c e r t a i n l y an i n t e r e s t i n g and v a l u a b l e t e c h n i q u e f o r s t u d y o f t h e mechanism o f e l e c t r o n t r a n s f e r r e a c t i o n s . The g e n e r a t i o n o f a s h o r t - l i v e d redox p a i r as c h e m i c a l i n t e r m e d i a t e s i s an i m p o r t a n t f e a t u r e o f t h e ac e l e c t r o l y s i s . In the f u t u r e i t may even be d e v e l o p e d to s y n t h e t i c a p p l i c a t i o n s i r r e s p e c t i v e o f the m e c h a n i s t i c d e t a i l s . In some c a s e s i t c o u l d be a c o n v e n i e n t a l t e r n a t i v e t o p h o t o c h e m i c a l r e a c t i o n s . In o t h e r c a s e s i t r e p r e s e n t s a new r e a c t i o n type w h i c h has no p r e c e d e n t . Acknowledgments
We thank P r o f e s s o r Andreas Merz f o r h e l p f u l d i s c u s s i o n s . F i n a n c i a l s u p p o r t o f t h i s work by the Deutsche F o r s c h u n g s g e m e i n s c h a f t and t h e Fonds d e r Chemischen I n d u s t r i e i s g r a t e f u l l y a c k n o w l e d g e d .
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