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12 Photosensitized Electron-Transfer Reactions in Organized Systems

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The Role of Synthetic Catalysts and Natural Enzymes in Fixation Processes I. WILLNER Department of Organic Chemistry, T h e Hebrew University of Jerusalem, Jerusalem 91904, Israel

Charged colloids and water-in-oil microemulsions provide organized environments that control photosensitized electron transfer reactions. Effective charge separation of the primary encounter cage complex, and subsequent stabilization of the photoproducts against back electron transfer reactions is achieved by means of electrostatic and hydrophobic interactions of the photoproducts and the organized media. Chemical utilization of the photoproducts has been accomplished by the introduction of synthetic catalysts or natural enzymes into the photochemical systems. With Ru(NH ) Cl as electron acceptor and catalyst the photocleavage of acetylene (C H ) to methane is observed. This process is a 6-electron reduction process and offers a model for the N -fixation reaction. Also, by the introduction of the enzyme ferredoxin reductase, a photochemical NADPH regeneration cycle has been established. The NADPH has been utilized in the reduction of ketones to alcohols in the presence of a secondary enzyme, alcohol dehydrogenase. Induced disproportionation of photochemical single electron transfer products to two electron charge relays occurs in water-oil two phase systems. This process is a result of opposite solubility properties of the comproportionation products in the two phases. 2+

3

5

2

2

2

M i m i c k i n g p h o t o s y n t h e s i s by means o f a r t i f i c i a l systems seems a p r o m i s i n g r o u t e f o r s o l a r energy c o n v e r s i o n and s t o r a g e (l_-2) . One p o s s i b l e c y c l e t h a t i s b e i n g e x t e n s i v e l y examined i n r e c e n t y e a r s (_3-4) i s d i s p l a y e d i n F i g u r e 1. I t i n v o l v e s a l i g h t a b s o r b e n t , S, t h a t upon e x c i t a t i o n i n d u c e s a t r a n s f e r o f an e l e c t r o n t o an e l e c t r o n a c c e p t o r , A, l e a d i n g t o t h e p h o t o p r o d u c t s S and A . Subsequent o x i d a t i o n o f an e l e c t r o n donor, D, r e c y c l e s t h e s e n s i t i z e r , and r e s u l t s i n t h e c o n v e r s i o n o f l i g h t energy t o c h e m i c a l p o t e n t i a l , s t o r e d i n t h e p r o d u c t s A and D (eq. 1 ) . +

0097-6156/85/0278-0191 $06.00/0 © 1985 A m e r i c a n C h e m i c a l Society

Fox; Organic Phototransformations in Nonhomogeneous Media ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

192

ORGANIC PHOTOTRANSFORMATIONS IN NONHOMOGENEOUS

h

A + D

V

< , , > back reaction

A

+ D

+

MEDIA

(1)

+

The p h o t o p r o d u c t s A and D c a n then be u t i l i z e d i n c h e m i c a l r o u t e s , e.g. t h e reduced photoproduct A"" c a n be used f o r t h e r e d u c t i o n o f water t o hydrogen, and f i x a t i o n o f carbon d i o x i d e o r n i t r o g e n t o o r g a n i c f u e l s o r ammonia. The o x i d i z e d photoproduct D might be u t i l i z e d i n o x i d a t i o n p r o c e s s e s such as e v o l u t i o n o f oxygen from w a t e r . Thus, one might e n v i s a g e a v a r i e t y of c o u p l e d p h o t o c h e m i c a l c h e m i c a l p r o c e s s e s t h a t d r i v e e n d o e r g i c r e a c t i o n s c o n v e r t i n g abundant m a t e r i a l s t o f u e l s o r u s e f u l c h e m i c a l s (eq. 2 - 4 ) . +

H

0

2 ""*

C0 N

2

2

H

2

+

h

+ H 0 —• 2

+ 3H 0 -» 2

( 2 )

°2 CH 0 + 0 2

(3)

2

2NH + lh0 3

(4)

2

The d e s i g n of such a r t i f i c i a l p h o t o s y n t h e t i c systems s u f f e r s from some b a s i c l i m i t a t i o n s : a) The r e c o m b i n a t i o n o f t h e p h o t o p r o d u c t s A and S o r D i s a t h e r m o d y n a m i c a l l y f a v o u r e d p r o c e s s . These d e g r a d a t i v e pathways p r e v e n t e f f e c t i v e u t i l i z a t i o n o f t h e p h o t o p r o d u c t s i n c h e m i c a l r o u t e s , b) The p r o c e s s e s o u t l i n e d i n eq. 2-4 a r e m u l t i 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 , w h i l e the photochemical r e a c t i o n s a r e s i n g l e e l e c t r o n t r a n s f o r m a t i o n s . Thus, t h e d e s i g n o f c a t a l y s t s a c t i n g as charge r e l a y s i s c r u c i a l f o r t h e accomplishment o f subsequent c h e m i c a l f i x a t i o n p r o c e s s e s . S i g n i f i c a n t p r o g r e s s i n t h e development o f such a r t i f i c i a l p h o t o s y n t h e t i c systems, p a r t i c u l a r l y aimed a t t h e p h o t o l y s i s o f w a t e r , has been r e p o r t e d i n r e c e n t y e a r s . S e v e r a l approaches t o r e s o l v e t h e problems i n v o l v e d i n c o n t r o l l i n g t h e photoinduced e l e c t r o n t r a n s f e r p r o c e s s as w e l l as t h e development o f c a t a l y s t s f o r m u l t i - e l e c t r o n f i x a t i o n p r o c e s s e s w i l l be d i s c u s s e d i n t h i s paper. +

+

C o n t r o l of charge s e p a r a t i o n o f t h e p h o t o p r o d u c t s . The p h o t o s e n s i t i z e d e l e c t r o n t r a n s f e r p r o c e s s i n v o l v e s two s u c c e s s i v e s t e p s (eq. 5 ) : I n t h e p r i m a r y event an encounter cage complex o f t h e p h o t o p r o d u c t s i s formed. T h i s c a n e i t h e r recombine t o y i e l d t h e o r i g i n a l r e a c t a n t s o r d i s s o c i a t e i n t o s e p a r a t e d p h o t o p r o d u c t s . The s e p a r a t e d p h o t o p r o d u c t s c a n then recombine by a d i f f u s i o n a l back e l e c t r o n t r a n s f e r r e a c t i o n t o form t h e o r i g i n a l r e a c t a n t s . We have i n t r o d u c e d two c o n c e p t i o n a l approaches as a means f o r a s s i s t i n g t h e s e p a r a t i o n o f t h e encounter cage complex and f o r t h e s t a b i l i z a t i o n S + k ^ — t

[ S * - - A"]

-* S

+

+ A~

(5)

of t h e p h o t o p r o d u c t s a g a i n s t t h e d e g r a d a t i v e r e c o m b i n a t i o n p r o c e s s e s ( 5 - 6 ) . These two approaches i n v o l v e t h e o r g a n i z a t i o n o f t h e photoc h e m i c a l system i n i n t e r f a c i a l systems t h a t c o n t r o l charge s e p a r a t i o n by means o f e l e c t r o s t a t i c o r hydrophobic i n t e r a c t i o n s of t h e i n t e r face w i t h the photoproducts (Figure 2). E l e c t r o s t a t i c i n t e r a c t i o n s t h a t c o n t r o l charge s e p a r a t i o n a r e e x e m p l i f i e d i n F i g u r e 2(a) u s i n g a n e g a t i v e l y charged i n t e r f a c e as o r g a n i z a t i o n medium. I n t h i s


12.

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Electron-Transfer

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193

system, a p o s i t i v e l y charged s e n s i t i z e r , S , t h a t i s adsorbed onto the i n t e r f a c e , and a n e u t r a l e l e c t r o n a c c e p t o r , s e r v e as t h e p h o t o r e a c t a n t s . 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 r e s u l t s i n t h e encounter cage complex of t h e p h o t o p r o d u c t s . A l t h o u g h the p h o t o p r o d u c t s have o p p o s i t e e l e c t r i c charges and e x h i b i t mutual a t t r a c t i o n s , t h e h i g h l y charged i n t e r f a c e i s expected t o r e p e l t h e n e g a t i v e c o u n t e r p a r t of t h e cage complex. C o n s e q u e n t l y , charge s e p a r a t i o n of t h e i n t e r m e d i a t e encounter complex i s a s s i s t e d . The r e c o m b i n a t i o n of the s e p a r a t e d p h o t o p r o d u c t s v i a t h e d i f f u s i o n a l mechanism i s a l s o r e t a r d e d owing t o t h e r e p u l s i o n of t h e n e g a t i v e p h o t o p r o d u c t , A" from t h e o x i d i z e d s p e c i e s w h i c h i s a s s o c i a t e d w i t h the charged i n t e r f a c e . S i m i l a r l y , h y d r o p h o b i c - h y d r o p h y l i c b o u n d a r i e s c a p a b l e of c o n t r o l l i n g the charge separation process are exemplified i n F i g u r e 2(b). Using t h i s approach, t h e photosystem i s s o l u b i l i z e d i n t h e aqueous media, w h i l e t h e reduced p h o t o p r o d u c t i s d e s i g n e d t o e x h i b i t h y d r o p h o b i c c h a r a c t e r . C o n s e q u e n t l y , e x t r a c t i o n of t h e reduced p h o t o p r o d u c t from the water phase i n t o t h e o i l medium i s a n t i c i p a t e d t o a s s i s t charge s e p a r a t i o n and t o r e t a r d t h e r e c o m b i n a t i o n p r o c e s s e s . We have examined two t y p e s of o r g a n i z e d media t h a t e f f e c t i v e l y c o n t r o l t h e charge s e p a r a t i o n and back r e a c t i o n s of t h e i n t e r m e d i a t e p h o t o p r o d u c t s . These i n c l u d e , (a) charged c o l l o i d s i . e . S1O2 and ZrÛ2 c o l l o i d s t h a t i n t r o d u c e e l e c t r o s t a t i c i n t e r a c t i o n s between t h e p h o t o p r o d u c t s and i n t e r f a c e ( 7 - 1 0 ) , and (b) w a t e r - i n - o i l m i c r o e m u l s i o n s t h a t p r o v i d e a q u e o u s - o i l two phase systems c a p a b l e of c o n t r o l l i n g t h e r e a c t i o n s by proper d e s i g n of the h y d r o p h o b i c - h y d r o p h i l i c b a l a n c e of the p h o t o p r o d u c t s ( 6 ) . The s i l a n o l groups of t h e S1O2 c o l l o i d a r e i o n i z e d i n b a s i c media (pH > 7.5). C o n s e q u e n t l y , a d i f f u s e d o u b l e l a y e r i s produced i n t h e v i c i n i t y of t h e c o l l o i d p a r t i c l e s , and t h e n e g a t i v e l y charged c o l l o i d i s c h a r a c t e r i z e d (11) by an e l e c t r i c a l s u r f a c e p o t e n t i a l of c a . -170 mV. S i m i l a r l y , ZrU2 c o l l o i d s a r e p o s i t i v e l y charged i n aqueous a c i d i c environments (pH = 4.2-4.5). We have s y n t h e s i z e d two n e u t r a l , w a t e r - s o l u b l e , e l e c t r o n a c c e p t o r s : d i - ( 3 - p r o p y l s u l f o n a t e ) 4 , 4 - b i p y r i d i n i u m , PVS°, ( 1 ) , and d i - ( 2 - p r o p y l s u l f o n a t o ) - 2 , 2 b i p y r i d i n i u m , DQS°, ( 2 ) . The p h o t o s e n s i t i z e d r e d u c t i o n of t h e s e two e l e c t r o n a c c e p t o r s w i t h Ru(bipy)§ as s e n s i t i z e r and t r i e t h a n o l a m i n e , TEOA, as e l e c t r o n donor, has been examined i n aqueous S1O2 c o l l o i d s (pH = 9 . 8 ) and compared t o t h e s i m i l a r r e a c t i o n s i n a homogeneous aqueous phase ( T a b l e I ) (7 ,_9) . The quantum y i e l d of PVS° r e d u c t i o n ,

T

+

i n t h e p r e s e n c e of t h e e l e c t r o n donor i s 8 - f o l d i n c r e a s e d i n t h e S1O2 c o l l o i d as compared t o t h a t i n a homogeneous phase. S i m i l a r l y , t h e r e d u c t i o n of DQS° proceeds i n t h e S1O2 c o l l o i d i n t h e p r e s e n c e of TEOA ( 2 x 1 0 " M) w i t h a quantum y i e l d of φ=2.4χ10" w h i l e no r e d u c t i o n 3

2


194

O R G A N I C P H O T O T R A N S F O R M A T I O N S IN N O N H O M O G E N E O U S

F i g u r e 1. C o n v e r s i o n of l i g h t energy t o c h e m i c a l p o t e n t i a l by means of p h o t o s e n s i t i z e d 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 .

F i g u r e 2. Charge s e p a r a t i o n and s t a b i l i z a t i o n of p h o t o p r o d u c t s i n o r g a n i z e d environments: a) A p p l i c a t i o n of t h e e l e c t r o s t a t i c i n t e r a c t i o n s w i t h charged S1O2 c o l l o i d s , b) Use of hydrophobic-hydrophilic interactions i n w a t e r - i n - o i l microemulsions. Fox; Organic Phototransformations in Nonhomogeneous Media ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

MEDIA

12.

WILLNER

Photosensitized

Electron-Transfer

Reactions

195

T a b l e I . E f f e c t s o f S i 0 c o l l o i d on charge s e p a r a t i o n o f t h e p r i m a r y encounter cage complex and r e c o m b i n a t i o n r a t e s o f P V S and DQS 2

T

T

Horn. 0.38

.s 1

,s- )

d

pH=9.8 0.38

7.9xl0

9

T

(SiO?)

PVST

5.7xl0

DQS (SiO?) [NaCl] = [NaCl] = 0.1M pH= pH= G «4M pH=9.8 pH=9.8 Horn? 9.8 7.8 0.38 0.26 0.20 0.16

pH=8.2 0.37 7

3.9xl0

c

8

4.3xl0

8

-° l x l O

7

2xl0

8

8xl0

8

a. Homogeneous phase. No S1O2 c o l l o i d i n c l u d e d , pH=9.8. b. Quantum y i e l d f o r t h e s e p a r a t i o n o f t h e encounter cage complex o f p h o t o p r o d u c t s ( e q u a t i o n 7 ) . Determined by t h e absorbance o f P V S a t λ=602 nm and b l e a c h i n g o f Ru(bpy)§ a t λ=460 nm f o r DQS . c. No s e p a r a t i o n o f t h e encounter cage complex c o u l d be observed i n a homogeneous aqueous phase. T h e r e f o r e , no d i r e c t d i f f u s i o n a l r e c o m b i n a t i o n r a t e c o n s t a n t c o u l d be e s t i m a t e d . Note however, t h a t a t h i g h i o n i c s t r e n g t h and l o w pH media, where t h e S i 0 2 c o l l o i d e l e c t r i c surface p o t e n t i a l i s low, the recombination r a t e c o n s t a n t i s almost d i f f u s i o n c o n t r o l l e d ( k ^ - 1 0 M . s ~ ) . d. Determined by f o l l o w i n g t h e b l e a c h i n g o f photoproduced P V S a t λ=602 nm ( ε , * 12500 M ~ l cm"-'-) and f o l l o w i n g t h e r e c o v e r y o f T

+

T

9

T

b l e a c h e d Ru(bpy)§

a t λ=452 nm, f o r DQS' ( ε ^ £ 14500 M

cm" ) .

of DQS° o c c u r s i n homogeneous media ( 9 ) . To account f o r t h e i n c r e a s e d quantum y i e l d s under steady s t a t e i l l u m i n a t i o n i n t h e presence o f S1O2 c o l l o i d , we have c h a r a c t e r i z e d t h e p r o c e s s e s i n v o l v e d i n t h e p h o t o s e n s i t i z e d 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 : i ) quenching o f t h e e x c i t e d s p e c i e s ( e q u a t i o n 6 ) , i i ) quantum y i e l d s f o r t h e s e p a r a t i o n of t h e p r i m a r y encounter cage complex ( e q u a t i o n 7) and i i i ) t h e r e c o m b i n a t i o n r a t e o f t h e s e p a r a t e d p h o t o p r o d u c t s ( e q u a t i o n 8 ) . The p h y s i c a l d a t a f o r these r e a c t i o n s a r e summarized i n T a b l e 1. *Ru(bpy)3 [Ru(bpy)3

q

+ A +

>

Τ

· · ·Α ]

Ruibpy)^ + A

T

> Ru(bpy)^ b

Τ

[Ru(bpy)3

>

···Α ]

+

Ru(bpy)^

(6)

+ A~ +

(7)

+ A

(8)

The quenching r a t e c o n s t a n t s ( k ) 07,9) o f t h e e x c i t e d s e n s i t i z e r by PVS° and DQS° i n t h e S i 0 c o l l o i d a r e 1 . 5 x l 0 and 4 x l 0 M ^ r e s p e c t i v e l y . I n a homogeneous aqueous phase t h e quenching r a t e c o n s t a n t s c o r r e s p o n d t o 1 . 5 x l 0 M^.s f o r PVS° and 5 . 9 x l 0 M .s"" f o r DQS°. Thus, t h e h i g h e r quantum y i e l d s i n t h e S1O2 c o l l o i d cannot be a t t r i b u t e d t o t h e quenching p r o c e s s . T a b l e 1 shows t h a t t h e escape y i e l d s o f p h o t o p r o d u c t s from t h e p r i m a r y cage s t r u c t u r e as w e l l as t h e r e c o m b i n a t i o n r a t e s o f t h e s e p a r a t e d p h o t o p r o d u c t s a r e s t r o n g l y a f f e c t e d by t h e Si02 c o l l o i d . I t i s e v i d e n t t h a t t h e i n i t i a l encounter complex o f DQS i s n o n - s e p a r a b l e i n a homogeneous aqueous phase and t h e p h o t o p r o d u c t s degrade i n t h e cage s t r u c t u r e . However, t h e S1O2 c o l l o i d a s s i s t s t h e s e p a r a t i o n o f p h o t o p r o d u c t s from t h e cage s t r u c t u r e (φ =0.26). q

9

8

- 1

- 1

2

9

8

1

T

3


196

ORGANIC PHOTOTRANSFORMATIONS

IN N O N H O M O G E N E O U S

MEDIA

A second s i g n i f i c a n t e f f e c t of t h e S1O2 c o l l o i d i s observed on the back r e a c t i o n r a t e c o n s t a n t s . I n a homogeneous phase t h e recombina t i o n r e a c t i o n of P V S and DQS w i t h t h e o x i d i z e d s p e c i e s , R u ( b p y ) ^ , i s d i f f u s i o n c o n t r o l l e d . I n the p r e s e n c e of t h e S1O2 c o l l o i d t h i s back e l e c t r o n t r a n s f e r p r o c e s s i s s u b s t a n t i a l l y r e t a r ded and c a . 2 0 0 - f o l d s l o w e r t h a n i n t h e homogeneous phase. The f u n c t i o n s of t h e S1O2 c o l l o i d i n charge s e p a r a t i o n and r e t a r d a t i o n of back r e a c t i o n s a r e a t t r i b u t e d to e l e c t r o s t a t i c i n t e r a c t i o n s of the p h o t o p r o d u c t s and the charged c o l l o i d i n t e r f a c e ( F i g u r e 3 ) . The p o s i t i v e l y charged s e n s i t i z e r i s bound t o t h e c o l l o i d i n t e r f a c e . The i n i t i a l encounter c a t e complex formed upon e l e c t r o n t r a n s f e r i s s t i l l p o s i t i v e l y charged and a s s o c i a t e d w i t h the p a r t i c l e . Y e t , t h e n e g a t i v e l y charged component, DQS , i s r e p e l l e d by t h e i n t e r f a c e and e j e c t e d from t h e cage s t r u c t u r e . The s e p a r a t e d p h o t o p r o d u c t s a r e s t a b i l i z e d a g a i n s t d i f f u s i o n a l back 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 s i n c e DQS" o r P V S i s r e p e l l e d by t h e charged c o l l o i d t o w h i c h the o x i d i z e d p h o t o p r o d u c t , Ru(bpy)§ , i s bound. The e f f e c t i v e c o n t r o l of t h e d e g r a d a t i v e pathways of the p h o t o p r o d u c t s , i n t h e S1O2 c o l l o i d a l l o w s the e f f i c i e n t subsequent o x i d a t i o n o f TEOA, by R u ( b p y ) 3 , and c o n s e q u e n t l y h i g h quantum y i e l d s a r e observed under s t e a d y s t a t e i l l u m i n a t i o n . The e l e c t r o s t a t i c f u n c t i o n s o f the S1O2 c o l l o i d i n t h e s e r e a c t i o n s have been c o n f i r m e d by a l t e r i n g t h e pH and i o n i c s t r e n g t h of t h e c o l l o i d media. A c i d i f i c a t i o n of t h e c o l l o i d e n v i r o n ment r e s u l t s i n p a r t i a l n e u t r a l i z a t i o n o f t h e s i l a n o l g r o u p s , and d e c r e a s e of t h e c o l l o i d s u r f a c e p o t e n t i a l . S i m i l a r l y , i n c r e a s e of the i o n i c s t r e n g t h r e d u c e s the d i f f u s i o n a l d o u b l e l a y e r s u r f a c e p o t e n t i a l (7_,9) . A c c o r d i n g l y , r e d u c t i o n i n the charge s e p a r a t i o n y i e l d s of the cage complex as w e l l as enhanced back r e a c t i o n r a t e s a r e observed a t l o w e r pH v a l u e s and h i g h i o n i c s t r e n g t h c o n d i t i o n s of t h e S i 0 c o l l o i d ( T a b l e I ) . The a p p l i c a t i o n of t h e S i 0 c o l l o i d i n c o n t r o l l i n g the p h o t o s e n s i t i z e d e l e c t r o n t r a n s f e r process i s l i m i t e d t o b a s i c aqueous s o l u t i o n . We s h o u l d , however, n o t e t h a t we have used p o s i t i v e l y charged Zr02 c o l l o i d s f o r e f f e c t i n g s i m i l a r e l e c t r o s t a t i c i n t e r a c t i o n s i n a c i d i c environments (pH=4.5) ( 1 0 ) . Hydrophobic i n t e r a c t i o n s as a means f o r c o n t r o l l i n g charge s e p a r a t i o n and back 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 demon s t r a t e d i n a two phase system of a w a t e r - i n - o i l m i c r o e m u l s i o n ( 6 ) . We have examined the p h o t o s e n s i t i z e d r e d u c t i o n of a s e r i e s of , 4 , 4 - b i p y r i d i n i u m s a l t s , C V , Q ) , (where n=l-16) w i t h Ru(bpy)§ as s e n s i t i z e r and (NHi+^EDTA as e l e c t r o n donor i n a w a t e r - i n - t o l u e n e m i c r o e m u l s i o n media. Under s t e a d y s t a t e i l l u m i n a t i o n t h e quantum y i e l d of C V~i* f o r m a t i o n s t r o n g l y depends on t h e a l k y l c h a i n l e n g t h of t h e e l e c t r o n a c c e p t o r ( F i g u r e 4 ) . I t improves as t h e h y d r o T

T

+

T

T

+

+

2

2

l

2 +

n

n

(a) R

CH

(d) R

3

Ν -C H n 2 n + l (b) R

C H

9

(e) R

(c) R

C H

1 3

(f) R

M

4

6

3


12.

WILLNΕR

Photosensitized

Electron- Transfer

197

Reactions

p h o b i c i t y of CV~!" i s i n c r e a s e d and r e a c h e s an o p t i m a l v a l u e f o r n=8-16. The f u n c t i o n s of the w a t e r - i n - t o l u e n e m i c r o e m u l s i o n i n c o n t r o l l i n g the p r o c e s s were v e r i f i e d by f o l l o w i n g the quenching r e a c t i o n , cage s t r u c t u r e s e p a r a t i o n and back e l e c t r o n t r a n s f e r ( T a b l e I I ) . I t can be seen t h a t f o r n=l no charge s e p a r a t i o n o c c u r s . F o r n=4 charge s e p a r a t i o n i s i n e f f i c i e n t and t h e r e c o m b i n a t i o n r a t e i s v e r y r a p i d . W i t h the l o n g c h a i n e l e c t r o n a c c e p t o r (η ^ 8) the s e p a r a t i o n of the cage s t r u c t u r e i s e f f e c t i v e and the back r e a c t i o n r a t e c o n s t a n t i s c a . 1 0 - f o l d s l o w e r as compared to Ci+V"*". The s o l u b i l i t y p r o p e r t i e s of the reduced p h o t o p r o d u c t C V"t* i n w a t e r - o r g a n i c two phase systems depends on the a l k y l c h a i n l e n g t h : w h i l e Ci-Ci+V*" n

n

T a b l e I I . Charge s e p a r a t i o n y i e l d s and r e c o m b i n a t i o n r a t e s i n the p h o t o s e n s i t i z e d r e d u c t i o n of C V i n water-in-toluene m i c r o e m u l s i o n s (6) 2

n

c

v

2 +

1

4

6

8

14

18

0

6

36

40

50

54

26

8

0.7

0.33

1.2

2.5

7.5

8.1

7.2

η Φ

«_. xlO separation

3

a

k^^ ^xlÔ^ sec ^ mole ^ T

steady

a.

ίο"

state

0.8

5

Determined by f o l l o w i n g the d i s a p p e a r a n c e of C V+ a t λ=602 nm (ε=12.500 M" cm- ). n

1

1

1

q

b.

—I

n

— _1_

L i g h t i n t e n s i t y 7.56x10"° e i n s t e i n s . 5 , · min

a r e r a t h e r s o l u b l e i n water and i n s o l u b l e i n t o l u e n e , the a m p h i p h i l i c e l e c t r o n a c c e p t o r s CsV* - C i g V * a r e e x t r a c t e d from the aqueous en v i r o n m e n t i n t o the o r g a n i c phases. T h e r e f o r e , the enhanced quantum y i e l d s of the l o n g c h a i n p h o t o p r o d u c t s , C V"Î" (n=8-16) , a r e a s s i g n e d to h y d r o p h o b i c i n t e r a c t i o n s of the i n t e r m e d i a t e p h o t o p r o d u c t s w i t h the w a t e r - o i l m i c r o e m u l s i o n medium ( F i g u r e 4). The p r i m a r y encounter cage complex f o r C V"t" where η ^ 8 i s a s s o c i a t e d w i t h the hydropho b i c i n t e r f a c e boundary. E x t r a c t i o n of the h y d r o p h o b i c component, C V+, i n t o the o i l phase a s s i s t s the s e p a r a t i o n of the cage complex and the s e p a r a t e d p h o t o p r o d u c t s a r e s u b s e q u e n t l y s t a b i l i z e d a g a i n s t the d i f f u s i o n a l back r e a c t i o n s by means of the two phases. These two e f f e c t s r e s u l t i n h i g h quantum y i e l d s under s t e a d y s t a t e i l l u m i n a t i o n . n

n

n

M u l t i - E l e c t r o n Charge R e l a y s T r a n s f o r m a t i o n of s i n g l e e l e c t r o n t r a n s f e r p r o d u c t s i n t o m u l t i e l e c t r o n charge r e l a y s i s a b a s i c r e q u i r e m e n t f o r a c c o m p l i s h i n g complex f i x a t i o n r e a c t i o n p r o c e s s e s ( e q u a t i o n s 3 and 4). A p o s s i b l e way to a c h i e v e such t r a n s f o r m a t i o n s i s the d i s p r o p o r t i o n a t i o n of a s i n g l e e l e c t r o n t r a n s f e r p r o d u c t to the c o r r e s p o n d i n g d o u b l y reduced s p e c i e s ( e q u a t i o n 9). The c o m p r o p o r t i o n a t i o n e q u i l i b r i u m c o n s t a n t (K^) i s determined by the r e d u c t i o n p o t e n t i a l s of the two s p e c i e s i n v o l v e d i n the p r o c e s s ( e q u a t i o n 10). U s u a l l y , E < E l and c o n s e q u e n t l y the d i s p r o p o r t i o n a t i o n e q u i l i b r i u m l i e s o v e r w h e l m i n g l y towards the s i n g l e e l e c t r o n t r a n s f e r p r o d u c t . Y e t , t h i s s i t u a t i o n i s v a l i d i n a homogeneous phase o n l y , and might be r a t h e r a l t e r e d i n a two phase system ( F i g u r e 5 ) . Assuming t h a t the e l e c t r o n 2


198

ORGANIC PHOTOTRANSFORMATIONS

I NN O N H O M O G E N E O U S

MEDIA

l/2H

2

TEOA*

DOS -* 0

SlOa

Γ

[Ru(biiy^—DOS*"]

Pt

^Ru(bipy)J

•DOS

7

TEOA

F i g u r e 3. F u n c t i o n s o f t h e S i 0 c o l l o i d i n c o n t r o l l i n g t h e p h o t o s e n s i t i z e d e l e c t r o n t r a n s f e r p r o c e s s and H - e v o l u t i o n . 2

2

F i g u r e 4. F u n c t i o n s o f t h e w a t e r - i n - o i l m i c r o e m u l s i o n i n charge s e p a r a t i o n and s t a b i l i z a t i o n o f p h o t o p r o d u c t s a g a i n s t b a c k - r e a c t i o n s .

2Mt Organic Phase

Water

Τ

_

y

M

2 + ^

S

F i g u r e 5. Induced c o m p r o p o r t i o n a t i o n o f an a m p h i p h i l i c e l e c t r o n a c c e p t o r i n a w a t e r - o i l two phase system. Fox; Organic Phototransformations in Nonhomogeneous Media ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12.

WILLNER

Photosensitized

Electron-Transfer

Reactions

199

a c c e p t o r e x h i b i t s a d e l i c a t e h y d r o p h i l i c - h y d r o p h o b i c b a l a n c e , where t h e o x i d i z e d form A i s s o l u b l e i n an aqueous media, w h i l e the mono-reduced product A*, i s h y d r o p h o b i c i n n a t u r e and e x t r a c t e d from the aqueous s o l u t i o n i n t o o r g a n i c phases. Under such c o n d i t i o n s d i s p r o p o r t i o n a t i o n of A* i n t h e o r g a n i c phase i s accompanied by r e e x t r a c t i o n of A to the aqueous phase. C o n s e q u e n t l y , a two phase system and the proper d e s i g n of h y d r o p h i l i c - h y d r o p h o b i c b a l a n c e of t h e d i s p r o p o r t i o n a t i o n p r o d u c t s p r o v i d e s an o r g a n i z e d environment f o r induced c o m p r o p o r t i o n a t i o n of a s i n g l e e l e c t r o n t r a n s f e r p r o d u c t to t h e d o u b l y reduced c h a r g e r e l a y . I t i s also e v i d e n t from t h i s c y c l e , t h a t c o n t i n u o u s r e d u c t i o n of A will u l t i m a t e l y form the d o u b l y reduced s p e c i e s , Α , t h a t f u n c t i o n s as an e l e c t r o n s i n k . 2 +

2 +

2 +

Σ

A

2A+

+ A

2 +

2+

K

d

=

ΓΑ1 ΓΑ 1 +2 ' [ A

(9) -

5

K

d

=

1 0

]

n F A E

°

R T

( 1 0 )

(where ΔΕ°=Ε°-Ε°) f

?

We have found (12) t h a t the e l e c t r o n a c c e p t o r , N , N - d i o c t y l - 4 , 4 b i p y r i d i n i u m , ( 3 d ) , C s V , and i t s r e d u c t i o n p r o d u c t s meet t h e c o r r e c t h y d r o p h o b i c - h y d r o p h i l i c b a l a n c e f o r an induced d i s p r o p o r t i o n a t i o n of CsV"!", (4) t o N j N ' - d i o c t y l b i p y r i d i n y l i d e n e ( 5 ) , i n o r g a n i c - w a t e r two phase systems ( e q u a t i o n 1 1 ) . The e l e c t r o n a c c e p t o r ( 2 d ) , undergoes two s u c c e s s i v e o n e - e l e c t r o n r e d u c t i o n p r o c e s s e s t o N , N - d i o c t y l - 4 , 4 ' - b i p y r i d i n i u m r a d i c a l c a t i o n , CsV"!", and N,N d i o c t y l - 4 , 4 - b i p y r i d i n y l i d e n e , C V, ( 5 ) , ( e q u a t i o n 1 2 ) , (Ei=-0.47V and E2=-0.90V, v s . NHE r e s p e c t i v e l y ) . The c o m p r o p o r t i o n a t i o n c o n s t a n t f o r CQV^ i n homogeneous aqueous phase i s K ^ = 5 . 5 x l 0 . Thus, the d i s p r o p o r t i o n a t i o n e q u i l i b r i u m o f CQV^ i n a homogeneous aqueous phase l i e s overwhelmingly towards the s i n g l e e l e c t r o n r e d u c t i o n product. The e l e c t r o n a c c e p t o r , C 8 V , i s s o l u b l e i n water and i n s o l u b l e i n o r g a n i c media such as t o l u e n e o r e t h y l a c e t a t e . I n t u r n , t h e one e l e c t r o n r e d u c t i o n p r o d u c t , CsV*, i s h y d r o p h o b i c i n n a t u r e and e x t r a c t e d i n t o o r g a n i c phases from aqueous e n v i r o n m e n t s . The consequence of t h e o p p o s i t e s o l u b i l i t y p r o p e r t i e s of C 8 V and i t s r e d u c t i o n p r o d u c t s i n t h e o r g a n i z e d two phase system on the d i s p r o D o r t i o n a t i o n of CQV' i s shown i n F i g u r e 6. The e l e c t r o n a c c e p t o r , CsV , i s photoreduced i n water u s i n g Ru(bpy)§ as s e n s i t i z e r and (ΝΗ^)3EDTA as e l e c t r o n donor. The p h o t o p r o d u c t , C s V t , i s e x t r a c t e d from t h e aqueous phase t o the o r g a n i c phase and t h e a b s o r p t i o n s p e c t r a of the p h o t o p r o d u c t s i n t h e o r g a n i c phase i s d i s p l a y e d i n F i g u r e 6. A f t e r a s h o r t i l l u m i n a t i o n time of the aqueous phase, t h e a b s o r p t i o n spectrum of t h e p h o t o p r o d u c t i n the o r g a n i c phase r e s e m b l e s t h a t of CQV^. However, when t h e a b s o r p t i o n spectrum of 2 +

T

f

f

8

-8

2 +

2 +

+

i n a homogeneous phase, where d i s p r o p o r t i o n a t i o n i s n e g l i g i b l e , i s s u b t r a c t e d from t h e e x p e r i m e n t a l a b s o r p t i o n spectrum of t h e p h o t o p r o d u c t s p r e s e n t i n t h e o r g a n i c phase, an a b s o r p t i o n p a t t e r n of a second component a t λ=400 nm i s observed ( F i g u r e 6(b) ) . T h i s a b s o r p t i o n band i s i d e n t i c a l t o t h a t of N j N ' - d i o c t y l b i p y r i d i n y l i dene, CQV, t h a t i s produced e l e c t r o c h e m i c a l l y ( 1 3 ) . (The sub t r a c t i o n p r o c e d u r e i s based on t h e f a c t t h a t C8V does not absorb a t λ=602 nm (ε=12.500 Μ Γ c m ) . T h e r e f o r e , t h e a b s o r p t i o n of C V1

- 1

8


200


can be s u b t r a c t e d from, the c o m p o s i t e spectrum u s i n g concentration f a c t o r ) (14). 2C V R

t

g—-» C V + C V

MEDIA

the respective

2 +

(11) (12)

Thus i t i s e v i d e n t t h a t i n t h e two phase system t h e d o u b l y reduced p h o t o p r o d u c t , C V, i s formed i n c o n j u n c t i o n w i t h t h e p h o t o s e n s i t i z e d one e l e c t r o n t r a n s f e r p r o c e s s (12, 14). I n a d d i t i o n , p r o l o n g e d i l l u m i n a t i o n r e s u l t s i n t h e a c c u m u l a t i o n o f t h e doubly reduced p r o d u c t , C V, a t t h e expense o f t h e s i n g l e e l e c t r o n t r a n s f e r p r o d u c t , CQV~^ ( F i g u r e 6(c) ) . These r e s u l t s a r e c o n s i s t e n t w i t h t h e induced d i s p r o p o r t i o n a t i o n mechanism o u t l i n e d i n F i g u r e 5. Due t o t h e opposite s o l u b i l i t y properties of the d i s p r o p o r t i o n a t i o n products i n t h e two phases, C V i s r e e x t r a c t e d i n t o t h e aqueous phase and the d o u b l y reduced c o m p r o p o r t i o n a t i o n p r o d u c t , C V, i s accumulated i n t h e o r g a n i c phase. The q u a n t i t a t i v e s p e c t r o s c o p i c e s t i m a t i o n o f [C V"Î~] and [ C V ] i n t h e o r g a n i c phase a l l o w e d us t o e s t i m a t e t h e c o m p r o p o r t i o n a t i o n c o n s t a n t s o f C V"t i n v a r i o u s o r g a n i c - w a t e r two phase systems ( 1 4 ) . F o r example, i n e t h y l a c e t a t e we have e s t i m a t e d a v a l u e o f Κ^=3χ10 M f o r t h e d i s p r o p o r t i o n a t i o n e q u i l i b r i u m o f CV"*~. T h i s v a l u e i s c a . 10? h i g h e r than t h e c o m p r o p o r t i o n a t i o n c o n s t a n t o f t h e s i m i l a r p r o c e s s i n an homogeneous aqueous medium. I t i s evident that a s i n g l e e l e c t r o n t r a n s f e r photoproduct i s t r a n s f o r m e d i n t o a d o u b l y reduced charge r e l a y i n two phase systems. The p r i m a r y p r o c e s s e s i n t h e n a t u r a l p h o t o s y n t h e t i c a p p a r a t u s i n v o l v e s i n g l 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 t h a t proceed i n h y d r o p h o b i c - h y d r o p h i l i c c e l l u l a r m i c r o e n v i r o n m e n t . Thus, we suggest s i m i l a r induced d i s p r o p o r t i o n a t i o n mechanisms as p o s s i b l e r o u t e s f o r the f o r m a t i o n o f m u l t i - e l e c t r o n charge r e l a y s , e f f e c t i v e i n t h e f i x a t i o n o f CO2 o r N . The subsequent c h e m i c a l u t i l i z a t i o n o f t h e two e l e c t r o n charge r e l a y has a l s o been a c c o m p l i s h e d (12, 1 4 ) . The e l e c t r o c h e m i c a l r e d u c t i o n o f C V , Q d ) by means o f c y c l i c voltammetry shows two r e v e r s i b l e , one e l e c t r o n , r e d u c t i o n waves a t Ei=-0.47V and E = -0.90V ( v s . NHE) c o r r e s p o n d i n g t o t h e f o r m a t i o n o f C v t and C V r e s p e c t i v e l y ( e q u a t i o n 12). A d d i t i o n o f m e s o - 1 , 2 - d i b r o m o s t i l b e n e (6) does n o t a f f e c t t h e r e v e r s i b i l i t y o f t h e f i r s t r e d u c t i o n wave. Y e t , t h e r e o x i d a t i o n wave o f C V i s d e p l e t e d upon a d d i t i o n o f £ implying a chemical r e a c t i o n of C V with the dibromides. Introduc t i o n o f m e s o - 1 , 2 - d i b r o m o s t i l b e n e , (§) , i n t o t h e o r g a n i c phase o f an 8

8

2 +

8

8

8

8

8

-1

8

2

2 +

8

2

8

8

8


8

12.

WILLNER

Photosensitized

Electron-Transfer

201

Reactions

e t h y l a c e t a t e - w a t e r two phase system, t h a t i n c l u d e s Ru(bpy)§ a s sensitizer, C V as e l e c t r o n a c c e p t o r and (NHi+) 3EDTA as e l e c t r o n donor, r e s u l t s upon i l l u m i n a t i o n i n t h e q u a n t i t a t i v e f o r m a t i o n o f t r a n s - s t i l b e n e (7) i n t h e o r g a n i c phase ( e q u a t i o n 13). 2 +

8

Br

Br

H + 2Br

2e

(13)

Ph

Ph 6

The e l e c t r o c h e m i c a l s t u d i e s r e v e a l t h a t t h e a c t i v e s p e c i e s i n d e b r o m i n a t i o n i s t h e two e l e c t r o n r e d u c t i o n p r o d u c t , CsV. Y e t , t h e primary photochemical process i s a s i n g l 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 t h a t y i e l d s CsVÎ. I n v i e w o f o u r p r e v i o u s d i s c u s s i o n , we suggest the c y c l e p r e s e n t e d i n F i g u r e 7 as the m e c h a n i s t i c r o u t e f o r d e b r o m i n a t i o n o f 6. I n t h i s c y c l e p h o t o r e d u c t i o n o f C V i n the aqueous s o l u t i o n i s accompanied by e x t r a c t i o n o f CV"*" from t h e aqueous phase i n t o t h e o r g a n i c phase. Induced d i s p r o p o r t i o n a t i o n o f C V"t" y i e l d s t h e d o u b l y reduced s p e c i e s , CsV, t h a t i s t h e a c t i v e r e d u c t a n t i n t h e d e b r o m i n a t i o n p r o c e s s . The d i s c u s s i o n s u g g e s t s t h a t a s i m i l a r p r o c e s s s h o u l d be p r e v e n t e d i n a homogeneous phase s i n c e t h e f o r m a t i o n o f C V i s n o t f a v o u r e d . Indeed, i l l u m i n a t i o n of ê and t h e p r e v i o u s l y d e s c r i b e d photosystem i n a homogeneous d i m e t h y l f o r m a m i d e s o l u t i o n does n o t l e a d t o t h e f o r m a t i o n o f t r a n s s t i l b e n e ( 7 ) , d e s p i t e t h e e f f e c t i v e formation of C V . 2 +

8

8

8

8

8

C a t a l y s t s f o r C h e m i c a l U t i l i z a t i o n o f the P h o t o p r o d u c t s Heterogeneous and Homogeneous C a t a l y s i s For chemical u t i l i z a t i o n of the e l e c t r o n t r a n s f e r photoproducts i n c l u s i o n o f c a t a l y s t s seems t o be e s s e n t i a l . These c a t a l y s t s might f u n c t i o n as charge s t o r a g e s i t e s f o r t h e complex m u l t i - e l e c t r o n t r a n s f e r f i x a t i o n p r o c e s s e s , and/or might p a r t i c i p a t e i n t h e a c t i v a t i o n o f t h e s u b s t r a t e s towards t h e c h e m i c a l r e a c t i o n s . I n t h e n a t u r a l p h o t o s y n t h e t i c system enzymes f u n c t i o n as c a t a l y t i c s i t e s f o r t h e complex f i x a t i o n p r o c e s s e s . We might e n v i s a g e two a l t e r n a t i v e approaches i n d e v e l o p i n g c a t a l y s t s f o r t h e c h e m i c a l u t i l i z a t i o n o f t h e photoinduced e l e c t r o n t r a n s f e r p r o d u c t s : ( i ) One p o s s i b i l i t y i n v o l v e s t h e development o f s y n t h e t i c c a t a l y s t s t h a t mimic. t h e f u n c t i o n s o f enzymes w i t h r e s p e c t t o charge s t o r a g e and s u b s t r a t e a c t i v a t i o n c a p a b i l i t i e s , ( i i ) The second approach might i n v o l v e t h e i n t r o d u c t i o n o f n a t u r a l enzymes i n t o a r t i f i c i a l c h e m i c a l systems ( p r o v i d e d t h a t t h e enzymes a r e s t a b l e i n t h i s a r t i f i c i a l environment). Most o f t h e p a s t e f f o r t s f o r t h e u t i l i z a t i o n o f t h e p h o t o p r o d u c t s i n subsequent c h e m i c a l r e a c t i o n s were d i r e c t e d towards t h e p h o t o l y s i s o f water t o hydrogen and oxygen (15-19). I n t h e c o u r s e of our s t u d i e s u s i n g charged c o l l o i d s as a means f o r c o n t r o l l i n g the charge s e p a r a t i o n and r e c o m b i n a t i o n r e a c t i o n s , we have d e s i g n e d an a d d i t i o n a l ^ - e v o l u t i o n system (_9,20). As s t a t e d p r e v i o u s l y t h e n e g a t i v e l y charged S1O2 c o l l o i d a s s i s t s t h e s e p a r a t i o n o f t h e photoproduced N , N - d i p r o p y l - 2 , 2 - b i p y r i d i n i u m r a d i c a l a n i o n , DQS*. and s t a b i l i z e s t h e i n t e r m e d i a t e p h o t o p r o d u c t s , DQS , and Ru(bpy)§ ,

f

7


202

ORGANIC

340

400

PHOTOTRANSFORMATIONS

450

500

550

600

IN N O N H O M O G E N E O U S

650

MEDIA

700

WAVELENGTH /nm

F i g u r e 6. A b s o r p t i o n s p e c t r a o f t h e components i n t h e o r g a n i c phase o b t a i n e d a f t e r e x t r a c t i o n o f p h o t o g e n e r a t e d CgV+ from t h e aqueous phase: a) Composite spectrum o f CgV+ and C 3 V i n e t h y l a c e t a t e a f t e r i l l u m i n a t i o n o f t h e system f o r 10 m i n u t e s ; b) Spectrum o f C3V i n e t h y l a c e t a t e o b t a i n e d a f t e r s u b t r a c t i o n o f CgV*.spectrum from t h e composite spectrum ( a ) . I n s e r t i s e n l a r g e d a b s o r p t i o n spectrum o f CgV. c) Spectrum o f t h e p h o t o p r o d u c t s when t o l u e n e i s used a s t h e o r g a n i c phase i n t h e two phase system. Spectrum r e c o r d e d a f t e r i l l u m i n a t i o n f o r 15 minutes c o r r e s p o n d s t o CgV as major p r o d u c t .

F i g u r e 7. P h o t o s e n s i t i z e d d e b r o m i n a t i o n o f meso-1,2-dibromos t i l b e n e , ( 6 ) , i n a two phase system. Fox; Organic Phototransformations in Nonhomogeneous Media ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12.

WILLNER

Photosensitized

Electron-Transfer

203

Reactions

a g a i n s t t h e b a c k - e l e c t r o n t r a n s f e r p r o c e s s ( F i g u r e 2 ) . The reduced p h o t o p r o d u c t , DQS , (E°=-0.75V v s . NHE) m e d i a t e s t h e e v o l u t i o n o f hydrogen i n t h e p r e s e n c e o f P t c o l l o i d . I t s h o u l d be noted t h a t i n t h i s system H i s e v o l v e d from t h e b a s i c aqueous S i 0 c o l l o i d (pH=8.5-10.2). T h i s might f a c i l i t a t e t h e complementary 0 - e v o l u t i o n process. The s i m i l a r H - e v o l u t i o n cannot be accomplished i n a homogeneous phase s i n c e no s e p a r a t e d p h o t o p r o d u c t s a r e o b t a i n e d . Thus, t h e S i 0 - c o l l o i d p r o v i d e s an o r g a n i z e d medium t h a t s t a b i l i z e s the e l e c t r o n t r a n s f e r p r o d u c t s and a l l o w s t h e subsequent e v o l u t i o n of hydrogen ( F i g u r e 2 ) . R e c e n t l y we have attempted t o pursue m u l t i - e l e c t r o n f i x a t i o n p r o c e s s e s as models f o r N o r C 0 f i x a t i o n . I n n a t u r e , t h e N2f i x a t i o n enzyme, n i t r o g e n a s e , e x h i b i t s n o n - s p e c i f i c i t y p r o p e r t i e s , and a c e t y l e n e competes f o r n i t r o g e n a s t h e f i x a t i o n s u b s t r a t e ( 2 1 ) . The f i x a t i o n p r o c e s s o f a c e t y l e n e t o methane and o f n i t r o g e n t o ammonia ( e u q a t i o n s 14 and 15) have s e v e r a l common f e a t u r e s : ( i ) b o t h i n v o l v e t h e c l e a v a g e o f a t r i p l e bond; ( i i ) t h e two r e a c t i o n s i n v o l v e 6 e l e c t r o n s i n t h e f i x a t i o n mechanism. Thus, i t seems t h a t t h e p h o t o c l e a v a g e o f a c e t y l e n e t o methane might o f f e r a good model f o r development o f N - f i x a t i o n c y c l e s ( 2 2 ) . T

2

2

2

2

2

2

2

2

N

+

2

+ 6e + 6 H — »

2NH

(14)

3

HC=CH + 6 i + 6H —*+> 2CH 4 +

(15)

L

2

Ruthenium ( I I ) pentamine, R u ( N H 3 ) 5 H 0 i s known t o b i n d a c e t y l e n e (23) a s w e l l as n i t r o g e n ( 2 4 ) . We have t h e r e f o r e examined t h e p h o t o s e n s i t i z e d r e d u c t i o n o f a c e t y l e n e u s i n g Ru(bpy)§ as s e n s i t i z e r , t r i e t h a n o l a m i n e , TEOA, a s e l e c t r o n donor and R u ( N H 3 ) s C l as e l e c t r o n a c c e p t o r and c a t a l y s t f o r t h e f i x a t i o n p r o c e s s ( 2 2 ) . I l l u m i n a t i o n o f t h i s system w i t h v i s i b l e l i g h t r e s u l t s i n t h e f o r m a t i o n o f methane ( F i g u r e 8 ) . S i m i l a r l y , m e t h y l a c e t y l e n e (C^CECH) and e t h y l a c e t y l e n e (CH3CH -C^CH) a r e p h o t o c l e a v e d t o methane and ethane o r propane r e s p e c t i v e l y . I n t u r n d o u b l y s u b s t i t u t e d a c e t y l e n e s u b s t r a t e s e.g. d i m e t h y l a c e t y l e n e a r e n e i t h e r p h o t o c l e a v e d n o r reduced. S e v e r a l mechanistic aspects i n v o l v e d i n the photocleavage of a c e t y l e n e t o methane have been e l u c i d a t e d (Scheme 1 ) . The p r i m a r y event i n v o l v e s t h e p h o t o r e d u c t i o n o f R u ( N H 3 ) 5 C l by *Ru(bpy)§ f o l l o w e d by a q u a t i o n o f t h e reduced p r o d u c t t o R u ( N H ) 5 ( H 0 ) * ( e q u a t i o n s 16 and 1 7 ) . T h i s p h o t o p r o d u c t adds a c e t y l e n e i n a " s i d e on" complex t h a t has been i s o l a t e d and c h a r a c t e r i z e d ( e q u a t i o n 1 8 ) . N e v e r t h e l e s s , t h e π-acetylene complex appears t o be i n e r t towards r e d u c t i o n o r c l e a v a g e t o methane i n t h e p r e s e n c e o f r e d u c i n g a g e n t s . Y e t , upon i l l u m i n a t i o n o f t h e π-acetylene c o o r d i n a t i o n compound i t undergoes a t r a n s f o r m a t i o n t o a new complex t h a t i s a c t i v e i n methane e v o l u t i o n i n t h e p r e s e n c e o f r e d u c i n g a g e n t s i n c l u d i n g R u ( N H 3 ) ( H 0 ) . The photoinduced a c t i v a t i o n o f t h e π-bonded a c e t y l e n e complex has been f o l l o w e d s p e c t r o s c o p i c a l l y and i s a t t r i b u t e d t o a π-σ a c e t y l e n e l i g a n d rearrangement ( e q u a t i o n 1 9 ) . The σ-acetylene complex undergoes subsequent r e d u c t i v e c l e a v a g e t o methane ( e q u a t i o n 20). The photoinduced σ-π a c e t y l e n e bond rearrangement c l e a r l y e x p l a i n s t h e l a c k o f r e d u c t i v e c l e a v a g e o f d i m e t h y l a c e t y l e n e s i n c e t h i s t r a n s f o r m a t i o n i s n o t p o s s i b l e . The 2

+

2 +

2

2 +

+

2

3

2

2 +

5

2



204

MEDIA

Scheme I Ru(bipy)

2 +

+ Ru(NH ) Cl 3

Ru(NH ) Cl 3

+

2 +

Ru(bpy)

5

2 +

+ H 0 — * Ru(NH ) (H 0)

5

2

3

5

+

+ Ru(NH ) Cl

3

3

+

(16)

5

+ Cl"

2

(17)

H Ru(NH ) (H 0) 3

5

+ HCECH — * R u ( N H ) ( fij )

2 +

2

2

3

+

(18)

5

H

H ι R u ( N H ) ( jjj ) 3

2

+

(NH ) Ru-CECH

5

3

1+

+ H

5

• Η

+

(19)

Η 2

[(HN)

Ru=C=C

] Η

5Ru(NH ) (H 0) 3

5

2

2 +

+ (NH ) - R u - C 5 C H 3

+1

5

+

+ 6H ~*6Ru(NH ) ( H 0 ) 3

5

+ 2CH

3 +

2

(20)

4

p h o t o c l e a v a g e o f a c e t y l e n e t o methane i n v o l v e s 6 e l e c t r o n s and p r o t o n s . Thus, t h e f e a s i b i l i t y o f a c c o m p l i s h i n g t h e i s o e l e c t r o n i c f i x a t i o n o f n i t r o g e n t o ammonia by s i m i l a r means i s c o n c e i v a b l e and remains a c h a l l e n g e . Enzymatic c a t a l y s i s A d i f f e r e n t approach f o r u t i l i z a t i o n o f t h e p h o t o p r o d u c t s i n c h e m i c a l r o u t e s i n v o l v e s t h e i n t r o d u c t i o n o f n a t u r a l enzymes as c a t a l y s t s i n t h e p h o t o c h e m i c a l system. I n n a t u r e , d i h y d r o n i c o t i n a m i d e adenine d i n u c l e o t i d e (NADH) and d i h y d r o n i c o t i n a m i d e d i n u c l e o t i d e phosphate (NADPH) p a r t i c i p a t e a s r e d u c i n g c o f a c t o r s i n a v a r i e t y o f enzymatic r e d u c t i o n p r o c e s s e s . Thus, t h e development o f p h o t o c h e m i c a l NADH and NADPH r e g e n e r a t i o n c y c l e s i s a n t i c i p a t e d t o a l l o w a v a r i e t y o f r e d u c t i o n p r o c e s s e s by i n c l u s i o n o f s u b s t r a t e s p e c i f i c NAD(P)H dependent enzymes. S e v e r a l c h e m i c a l r o u t e s have been developed f o r t h e r e g e n e r a t i o n o f NADH and NADPH (25, 2 6 ) . We have developed (27) a photo c h e m i c a l system f o r r e g e n e r a t i o n o f NADPH ( F i g u r e 9 ) . 4,4 -Bip y r i d i n i u m r a d i c a l c a t i o n s reduce NADP t o NADPH i n t h e presence of t h e enzyme f e r r e d o x i n r e d u c t a s e , FDR. I l l u m i n a t i o n o f a system composed o f t h e s e n s i t i z e r Ru(bpy)§ , e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d , t h e e l e c t r o n a c c e p t o r s d i m e t h y l - 4 , 4 - b i p y r i d i n i u m , (methyl v i o l o g e n , M V ) and NADP^" and t h e enzyme f e r r e d o x i n r e d u c t a s e (FDR) l e a d s t o t h e q u a n t i t a t i v e f o r m a t i o n o f NADPH. A d d i t i o n o f 2-butanone and t h e second enzyme a l c o h o l dehydrogenase (from T. B r o c k i i ) , ALDH, r e s u l t s i n t h e f o r m a t i o n o f (-)2-butanol and NADP . Continuous i l l u m i n a t i o n o f t h i s system r e s u l t s i n t h e a c c u m u l a t i o n f

+

+

f

2 +

+


WILLNER

Photosensitized

Electron-Transfer

205

Reactions

50 100 Illumination time

150 200 (minutes)

F i g u r e 8. R a t e of p h o t o c l e a v a g e o f a c e t y l e n e (C2H2) and m e t h y l a c e t y l e n e (C3Hi4) as a f u n c t i o n o f i l l u m i n a t i o n t i m e , a) Methane (CH^) from a c e t y l e n e and c) ethane (C2Hg) from methylacetylene using meso-Zn-tetramethylpyridinium porphyrin as s e n s i t i z e r , b) Methane (CH4) from a c e t y l e n e and d) ethane (C2H6) from m e t h y l a c e t y l e n e w i t h Ru(bpy)§ as s e n s i t i z e r . +

r-r

ν

Ί

OH (NH ) E D T A ^ V _ 4

R u

(bpy)^

M

A

M

£CH CH V*^NADP*^CH3Ô 2

FDR

jj I

lALOH

3

"

F i g u r e 9. P h o t o r e d u c t i o n of 2-butanone u s i n g two c o u p l e d enzymes and t h e NADPH r e g e n e r a t i o n c y c l e .


206


MEDIA

of (-)2-butanol a t t h e expense o f t h e s a c r i f i c i a l e l e c t r o n donor EDTA ( F i g u r e 1 0 ) . I n t h i s p r o c e s s two enzyme c a t a l y z e d s u b - c y c l e s a r e o p e r a t i v e : The p r i m a r y c y c l e r e p r e s e n t s t h e p h o t o - r e g e n e r a t i o n of NADPH, w h i l e t h e subsequent c y c l e d e s c r i b e s t h e c h e m i c a l u s e o f NADPH i n t h e f i x a t i o n o f a ketone t o an a l c o h o l . The n e t r e a c t i o n accomplished i n t h i s c y c l e i s t h e r e d u c t i o n o f 2-butanone by EDTA ( e q u a t i o n 20). T h i s p r o c e s s i s e n d o e r g i c by c a . 33 k c a l m o l ~ l o f EDTA consumed. CH CO " ° R-N^ + CH CCH CH N^CO" 5 1 5

+ 2H 0 1

RNHCH C0

+ CH 0 + HCO

9

I

I

I

+

(20)

5

OH CH^—C—CH CH 2

3

Η A major a s p e c t t o c o n s i d e r i n such enzyme-catalyzed photochemical systems i s t h e s t a b i l i t y o f enzymes i n t h e a r t i f i c i a l c h e m i c a l environments. T a b l e I I I summarizes t h e t u r n o v e r (TN) numbers f o r the d i f f e r e n t enzymes and c o f a c t o r s i n v o l v e d i n t h e r e d u c t i o n o f 2-butanone. I t i s e v i d e n t t h a t t h e enzymes e x h i b i t h i g h s t a b i l i t y Table I I I .

Turnover Numbers (TN) o f t h e components i n v o l v e d i n t h e p h o t o s e n s i t i z e d r e d u c t i o n o f 2-butanone.

Ru(bpy) TN a. b. c.

530

2 +

MV 40

2 +

FDR

b

24000

NADP 40

+

ALDH° 6000

No l o s s o f system a c t i v i t y c o u l d be d e t e c t e d a f t e r d e t e r m i n a t i o n of these t u r n o v e r numbers. F.W. ^ 40,000; C f . M. S h i n , Methods i n Enzymology 23, 441 (1971). F.W. - 40,000; C f . R . J . Lamed and G. Z e i k u s , Biochem. J . , 195, 183 (1981).

towards d e n a t u r a t i o n and t h a t t h e r a t e o f product f o r m a t i o n i s n o t a f f e c t e d even a f t e r prolonged i l l u m i n a t i o n t i m e s . The s t a b i l i t y of the system and t h e o p t i c a l p u r i t y o f t h e product (-)2-butanol (100%) demonstrate an e f f e c t i v e method f o r p r o d u c t i o n o f o p t i c a l l y a c t i v e a l c o h o l s . C e r t a i n l y , such p h o t o s e n s i t i z e d f i x a t i o n c y c l e s based on the r e g e n e r a t i o n o f NAD(P)H- might be g e n e r a l i z e d . By proper s u b s t i t u t i o n o f t h e secondary NAD(P)H-dependent enzyme r e d u c t i o n of v a r i o u s o t h e r s u b s t r a t e s i s c o n c e i v a b l e . Some o f t h e s e p o s s i b i l i t i e s e.g. p r o d u c t i o n of amino a c i d s and C 0 2 ~ f i x a t i o n a r e now b e i n g examined i n o u r l a b o r a t o r y . Conclusions D i f f e r e n t aspects i n v o l v e d i n the design of a r t i f i c i a l photosynthe t i c systems have been d i s c u s s e d . Charged c o l l o i d s and w a t e r - o i l m i c r o e m u l s i o n s p r o v i d e e f f e c t i v e o r g a n i z e d media f o r c o n t r o l l i n g p h o t o s e n s i t i z e d 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 . Development o f c a t a l y s t s c a p a b l e of u t i l i z i n g t h e photoproducts i n c h e m i c a l r o u t e s , p a r t i c u l a r l y i n m u l t i - e l e c t r o n f i x a t i o n p r o c e s s e s i s of major


12.

WILLNER

Photosensitized

Electron-Transfer

Reactions

10 20 30 Illumination time ( hours)

40

F i g u r e 10. Rate o f 2 - b u t a n o l f o r m a t i o n a t d i f f e r e n t t i m e i n t e r v a l s of i l l u m i n a t i o n . I n i t i a l (NHi+) 3EDTA c o n c e n t r a t i o n 2x10 M a) A d d i t i o n o f (NH4)3EDTA, 2 x l 0 " M , b) and c) A d d i t i o n o f (NH ) EDTA, 1.7xlO- M. 2

2

1+

3


208

O R G A N I C P H O T O T R A N S F O R M A T I O N S I NN O N H O M O G E N E O U S

MEDIA

i m p o r t a n c e . The induced d i s p r o p o r t i o n a t i o n o f a s i n g l e e l e c t r o n t r a n s f e r p r o d u c t t o a two e l e c t r o n charge r e l a y i n a w a t e r - o i l two phase system seems a g e n e r a l p r i n c i p l e w o r t h d e v e l o p i n g . Two d i f f e r e n t approaches i n d e v e l o p i n g c a t a l y s t s have been d i s c u s s e d . One approach i n v o l v e s t h e i n t r o d u c t i o n o f homogeneous and h e t e r o geneous s y n t h e t i c c a t a l y s t s . The o t h e r a l t e r n a t i v e suggested t h e i n t r o d u c t i o n o f n a t u r a l enzymes i n t o t h e p h o t o c h e m i c a l systems. The p h o t o c l e a v a g e o f a c e t y l e n e t o methane u s i n g t h e homogeneous catalyst Ru(NH3) Cl i m p l i e s t h e f e a s i b i l i t y o f d e s i g n i n g homo geneous c a t a l y s t s f o r m u l t i e l e c t r o n f i x a t i o n p r o c e s s e s . Other r e a c t i o n s , such a s Ν 2 and 00 -ίixations r e m a i n f u t u r e c h a l l e n g e s . S i m i l a r l y , i n t r o d u c t i o n o f n a t u r a l enzymes a s s p e c i f i c c a t a l y s t s i n a r t i f i c i a l p h o t o s y n t h e t i c systems seems t o be o f broad a p p l i c a bility. I n t r o d u c t i o n o f NADH and NADPH dependent enzymes f o r t h e p r e p a r a t i o n o f amino a c i d s , C 0 2 ~ f i x a t i o n i n t o sugars a s w e l l a s r e d u c t i o n o f k e t o s u b s t i t u t e d n a t u r a l p r o d u c t s seems f e a s i b l e . We thus r e a l i z e t h a t t h e d u p l i c a t i o n o f f u n c t i o n s o f n a t u r a l p h o t o s y n t h e s i s by means o f a r t i f i c i a l p r o c e s s e s i s c o n c e i v a b l e . The p r o g r e s s i n r e c e n t y e a r s i s e n c o u r a g i n g us t o c o n t i n u e pursuing these challenging goals. 2 +

5

2

Acknowledgment The a u t h o r w i s h e s t o e x p r e s s h i s deep g r a t i t u d e t o h i s c o l l a b o r a t o r s : Y. Degani, Z. Goren, P. Dan, D. M a n d l e r , R. Maidan,and E. P o r whose i n s p i r e d and e n t h u s i a s t i c e f f o r t s have made p o s s i b l e t h e s u c c e s s o f t h i s work. Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Sutin, N . ; Creutz, C. Pure Appl. Chem., 1980, 52, 2717-38. Gratzel, M. Acc. Chem. Res., 1981, 14, 276-84. Kalyanasundaram, K. Coord. Chem. Rev., 1982, 46, 159-244. Whitten, D.G.; Russel, J . C . ; Schnell, R.H. Tetrahedron, 1982, 38, 2455-87; Acc. Chem. Res., 1980, 13, 83-90. Willner, I . ; Laane, C . ; Otvos, J.W.; Calvin, M. in "Inorganic Reactions in Organized Media"; Holt, S . L . , Ed.; Advances in Chemistry Series, No. 177, American Chemical Society, Washington, D.C. 1982, p. 71. Mandler, D.; Degani, Y . ; Willner, I. J . Phys. Chem. in press 1984. Willner, I . ; Yang, J . - M . ; Otvos, J.W.; Calvin, M. J. Phys. Chem. 1981, 85, 3277. Willner, I . ; Otvos, J.W.; Calvin, M. J. Am. Chem. Soc., 1981, 103, 3203. Degani, Y . ; Willner, I. J . Am. Chem. Soc., 1983, 105, 6228. Willner, I . ; Degani, Y. Isr. J . Chem., 1982, 22, 163. Laane, C . ; Willner, I . ; Otvos, J.W.; Calvin, M. Proc. Natl. Acad. S c i . , U.S.A., 1981, 78, 5829. Goren, Z . ; Willner, I. J. Am. Chem. Soc., 1983, 105, 7764. Watanabe, T . ; Honda, K. J. Phys. Chem., 1982, 86, 2617. Maidan, R.; Goren, Z . ; Becker, J . V . ; Willner, I. J . Am. Chem. Soc., 1984, in press. Keller, P.; Moradpour, Α . ; Amouyal, E., Kagan, H.B. Nouv. J. Chim.,1980, 4, 377-84; J. Am. Chem. Soc., 1980, 102, 7193-96.


12.

WILLNER

Photosensitized

Electron-Transfer

Reactions

209

16. Harriman, Α.; Porter, G . ; Richoux, M.-C. J. Chem. Soc. Faraday Trans. 2, 1981, 77, 1939. 17. Kalyanasundaram, K.; Gratzel, Μ., Helv. Chim. Acta, 1980, 63, 478. 18. Kirch, M.; Lehn, J . - M . ; Sauvage, J . - P . Helv. Chim. Acta, 1979, 62, 1345. 19. Lehn, J . - M . ; Sauvage, J . - P . ; Zeissel, R. Nouv. J . Chim., 1980, 4, 623-27. 20. Degani, Y . ; Willner, I. J . Chem. Soc. Chem. Commun., 1983, 710-12. 21. Schrauzer, G.N. in "New Trends in the Chemistry of Nitrogen Fixation", Chatt, J.; da Camara Pina,L.M.; Richards, R.L. Eds., Academic Press, London, 1980, p. 105. 22. Degani, Y . ; Willner, I . , submitted for publication. 23. Lehman, H . ; Schenk, K . J . ; Chapuis, G . , Ludi, A. J. Am. Chem. Soc., 1979, 101, 6197. 24. Harrison, D.F.; Weissberger, E . ; Taube, H . , Science, 1968, 159, 320. 25. Wong, C.-H.; Daniels, L . ; Orme-Johnson, W.H.; Whitesides, G.M.; J . Am. Chem. Soc., 1981, 103, 6227-28. 26. Wong, C.-H.; Whitesides, G.M. J. Org. Chem., 1982, 104,1552-54. 27. Mandler, D.; Willner, I. J . Am. Chem. Soc., 1984, 106,5352-53. RECEIVED January 10, 1985


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