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5 C o n t r o l of Photosensitized E l e c t r o n Transfer

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Reactions i n O r g a n i z e d Interfacial Systems Vesicles, Water-in-Oil Microemulsions, and Colloidal Silicon Dioxide Particles ITAMAR WILLNER, COLJA LAANE, JOHN W. OTVOS, and MELVIN CALVIN University of California, Laboratory of Chemical Biodynamics, Department of Chemistry and Lawrence Berkeley Laboratory, Berkeley, CA 94720 The separation of photoproducts formed in photosensitized electron transfer reactions is essential for efficient energy conversion and storage. The organization of the components involved in the photoinduced process in interfacial systems leads to efficient compartmentalization of the products. Several interfacial systems, e.g., lipid bilayer membranes (vesicles), water-in-oil microemulsions and a solid SiO2 colloidal interface, have been designed to accomplish this goal. An electron transfer across a lipid bilayer membrane leading to the separation of the photoproducts at opposite sides of the membrane is facilitated by establishing a transmembrane potential and organizing the cotransport of cations with specific carriers. In the water-in-oil microemulsion the separation of photoproducts is achieved by means of the hydrophilic-hydrophobic nature of the products. A two compartment model system to accomplish the photodecomposition of water is described. Photosensitized electron transfer reactions analogous to those occurring in the two half-cells are presented. In these systems the phase transfer of one of the photoproducts into the continuous oil phase is essential to stabilize the photoproducts. The colloidal SiO particles provide a charged interface that interacts with charged photoproducts. By designing a system that results in oppositely charged photoproducts, we can produce a retardation of recombination by the charged interface. The photosensitized reduction of a neutral acceptor, propylviologen sulfonate (PVS) by positively charged sensitizers such as Ru(bipy) 3and Zn-tetramethylpyridinium porphyrin, Zu-TMPyP , is des2

o

2+

4+

0097-6156/82/0177-0071$06.00/0 © 1982 American Chemical Society Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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INORGANIC REACTIONS IN ORGANIZED MEDIA

cribed. The reactions are substantially enhanced in the SiO colloid as compared to those in the homogeneous phase. The effect of the SiO interface is attributed to a high surface potential that results in the separation of the intermediate photoproducts. The quantum yields of the photosensitized reactions are correlated to the interfacial surface potential and the electrical effects of other charged interfaces such as micelles are compared with those of SiO . The possible utilization of the energy stored in the stabilized photoproducts in further chemical reactions is discussed. Special attention is given to the photodecomposition of water as a reaction route. 2

2

2

Reactions in organized media as a means of modeling natural processes are currently an intensive subject of research (l,jO. Of particular interest is the subject of "Artificial Photosynthesis", that is, an attempt to create a synthetic apparatus that mimics the functions of the natural process (3,4). The natural photosynthetic cycle leading to the production of carbohydrates (eq.l) can be separated into two major parts (5): a photochemical part, in which visible light is captured and transformed into chemical energy, and a chemical part in which the stored energy is utilized in a sequence of dark reactions. The process is summarized in the "Z-scheme" (Figure 1). The chloroplast utilizes two photosystems composed of well organized pigment molecules. Photoexcitation of these units induces electron transfer reactions that result in the oxidation of water tp oxygen and formation of a reduced intermediate (ferredoxin). The reducing power is then used in a set of dark reactions to form carbohydrates from C0 . 2

H 0 + C0 2

2

h V

>

[CH0] + 0 2

2

(eq.l)

In this article we will discuss several approaches to the design of organized and controlled photosensitized electron transfer reactions. Special emphasis will be given to the utilization of the stored energy in the photodecomposition of water (eq.2). H0 2

h V

> H + 1/2 0 2

2

(eq.2)

A schematic cycle describing the principle of light capture and energy storage via a photosensitized electron transfer process in an artificial device is presented in Figure 2. In this system a synthetic sensitizer, S, substitutes for the natural chlorophyll as the light capturing entity. Excitation of the sensitizer, followed by an electron transfer to electron acceptor, A, results in the oxidized sensitizer and a reduced species, A". Oxidation of an electron donor, D, recycles the sensitizer and produces an

Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


Pf+AOP

Q-C550 Plostoquinone

Pi + A DP. ADP

Ferredoxin NADP reductase

Figure 1. Electron transfer (Z-scheme) in photosynthesis.

Chi a Chi b ΡββΟ

TRAP Π

TRAP I


D

D +

2

H 0

Figure 2. Cyclic photochemical scheme for water decomposition where S repre sents an artificial sensitizer that simulates the function of the natural chlorophyll and Cat represents a catalyst

^ — 1 / 4 02

— 1 / 2

Cyclic Photochemical Scheme for Decomposition of Water

S ^ S

hi/

w 5

Ο

i

ce

1

H

ο 3 s

I 1

^4

5.

WILLNER ET AL.

Photosensitized Electron Transfer Reactions

75

+

o x i d i z e d product, D . In such a way the l i g h t energy i s t r a n s formed i n t o chemical energy and s t o r e d i n the reduced and o x i d i z e d products* The reduced e l e c t r o n acceptor and o x i d i z e d e l e c t r o n donor can then be f u r t h e r used t o produce hydrogen and oxygen from water, as one example o f long term energy storage. I n t h i s way a l l the a c t i v e components o f the system a r e r e c y c l e d and the net r e s u l t i s the conversion o f water t o a p o t e n t i a l f u e l (hydrogen) · However, a b a s i c l i m i t a t i o n o f such a system i s the thermodynamically favored b a c k - 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 o f the i n t e r mediate photoproducts (eq.3 and 4 ) . By these pathways the energy s t o r e d i n the photochemical event i s degraded and an e f f i c i e n t u t i l i z a t i o n o f the photoproducts i s prevented. A" A"

+ +

S* D

>

^

A + S A + D

(eq.3) (eq.4)

In the n a t u r a l process t h i s l i m i t a t i o n i s s o l v e d by an o r g a n i z a t i o n o f the components i n a membrane. The two photosystems funct i o n a t opposite s i d e s o f a membrane and consequently the i n t e r mediates a r e separated and an e f f i c i e n t chemical u t i l i z a t i o n i s feasible. Thus, mimicking photosynthesis with the g o a l o f decomposing water must i n v o l v e three cooperative elements: (a) A l i g h t capturi n g e n t i t y that i s capable o f p h o t o s e n s i t i z i n g 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 ; (b) an i n t e r f a c i a l b a r r i e r that separates the i n t e r mediate photoproducts and prevents t h e i r recombination; and (c) s u i t a b l e redox c a t a l y s t s capable o f reducing and o x i d i z i n g water. Many s y n t h e t i c dyes such as p o r p h y r i n s , a c r i d i n e , t h i o n i n e and f l a v i n dyes have been used i n p h o t o s e n s i t i z a t i o n 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 . I n the past few years s e v e r a l promising organometallic compounds have been prepared as s u b s t i t u t e s f o r the n a t u r a l l a b i l e c h l o r o p h y l l . These organometallics i n c l u d e a v a r i e t y o f metals, c h e l a t e d t o b i p y r i d i n e o r p o r p h y r i n l i g a n d s (6). The p h o t o p h y s i c a l p r o p e r t i e s o f these s e n s i t i z e r s and t h e i r p o t e n t i a l use i n a r t i f i c i a l photosynthetic devices have been ext e n s i v e l y reviewed (_7, 9). I n p a r t i c u l a r , s e n s i t i z e r s such as R u ( I I ) - t r i s - b i p y r i d i n e , R u ( b i p y ) 5 ( 1 ) , and Zn-porphyrins, such as Zn-meso-tetraphenylporphyrins (2) o?" water-soluble d e r i v a t i v e s (3) and (4) have been widely explored. D i f f e r e n t s t r u c t u r a l modif T c a t i o n s ~ s u c h as hydrophobic s u b s t i t u e n t s and charged headgroups have a l s o been introduced. Thus, c o n t r o l o f 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 and p r e c i s e l o c a t i o n o f the s e n s i t i z e r i n h y d r o p h i l i c o r hydrophobic environments can be achieved. The storage o f energy by means o f a 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 c y c l e as presented i n F i g u r e 2 r e q u i r e s a c l o s e p r o x i m i t y of the components f o r e f f i c i e n t quenching o f the e x c i t e d s p e c i e s . However, once the photoproducts are produced, t h e i r s e p a r a t i o n must be a s s i s t e d and a b a r r i e r f o r t h e i r recombination must be introduced. S e v e r a l i n t e r f a c i a l systems such as m i c e l l e s (10,11), w a t e r - i n - o i l (12) o r o i l - i n - w a t e r microemulsions (13) and b i l a y e r +


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membranes (14,15) ( v e s i c l e s ) provide microenvironments that meet these requirements. Since these i n t e r f a c e s are u s u a l l y constructed o f charged detergents a d i f f u s e e l e c t r i c a l double l a y e r i s produced and the i n t e r f a c i a l boundary can be c h a r a c t e r i z e d by a s u r f a c e p o t e n t i a l . Consequently, e l e c t r o s t a t i c as w e l l as h y d r o p h i l i c and hydrophobic i n t e r a c t i o n s o f the i n t e r f a c i a l system can be designed. In t h i s report we w i l l review our achievements i n o r g a n i z i n g p h o t o s e n s i t i z ed 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 n d i f f e r e n t microenvironments such as b i l a y e r membranes and w a t e r - i n - o i l microemulsions.In .addition, a novel s o l i d - l i q u i d i n t e r f a c e , provided by c o l l o i d a l SiO2 par t i c l e s i n an aqueous medium w i l l be discussed as a means o f con t r o l l i n g photosensitized electron transfer reactions. Photosensitized

E l e c t r o n T r a n s f e r Across B i l a y e r Membranes

With the knowledge that membranes play an important r o l e i n the n a t u r a l process, we i n i t i a t e d a study i n which b i l a y e r phos p h o l i p i d membranes ( v e s i c l e s ) serve as an a r t i f i c i a l s t r u c t u r e . For t h i s purpose an e l e c t r o n t r a n s f e r across the b i l a y e r boundary must be accomplished (14). The schematic o f our system i s present ed i n F i g u r e 3. In t h i s system an amphiphilic Ru-complex i s i n c o r porated i n t o the membrane w a l l . An e l e c t r o n donor, EDTA, i s en trapped i n the inner compartment o f the v e s i c l e , and h e p t y l v i o l o gen (HV +) as e l e c t r o n acceptor i s introduced i n t o the outer phase. Upon i l l u m i n a t i o n an e l e c t r o n t r a n s f e r process across the v e s i c l e w a l l s i s i n i t i a t e d and the reduced acceptor (HV+) i s produced. The d i f f e r e n t steps i n v o l v e d i n t h i s o v e r a l l r e a c t i o n a r e presented i n Figure 3. The e x c i t e d s e n s i t i z e r t r a n s f e r s an e l e c t r o n to HV2+ i n the primary event. The o x i d i z e d s e n s i t i z e r thus produced o x i d i z e s a Ru l o c a t e d a t the inner surface o f the v e s i c l e and thereby the separation o f the intermediate photoproducts i s a s s i s t e d (14). The f u r t h e r o x i d a t i o n o f EDTA regenerates the s e n s i t i z e r and conse quently the separation o f the reduced s p e c i e s , HV+, from the o x i d i z e d product i s achieved. In t h i s system the b a s i c p r i n c i p l e o f a v e c t o r i a l e l e c t r o n t r a n s f e r across a membrane i s demonstrated. However, the quantum y i e l d f o r the r e a c t i o n i s r a t h e r low (0 4 χ 10-4). The transmembrane e l e c t r o n t r a n s f e r was found to be the r a t e l i m i t i n g f a c t o r f o r the o v e r a l l r e a c t i o n and the o r i g i n o f the low e f f i c i e n c y . The e l e c t r o n t r a n s f e r across the membrane must be followed by cotransport o f c a t i o n s i n order to keep charge neu t r a l i t y . Since the membrane has a low p e r m e a b i l i t y t o such cations, the p h o t o s e n s i t i z e d r e a c t i o n might be l i m i t e d by t h i s e f f e c t . I n deed, f u r t h e r e l a b o r a t i o n o f the v e s i c l e system by i n c l u d i n g c a t i a i c a r r i e r s can improve the photoinduced r e a c t i o n . F o r t h i s purpose hydrophobic c a t i o n c a r r i e r s (ionophores) such as valinomycin ( s p e c i f i c f o r K+), CCCP ( s p e c i f i c f o r H+) and g r a m i c i d i n ( t r a n s p o r t agent o f K+, N a and H+) have been incorporated i n t o the hydropho b i c r e g i o n o f the v e s i c l e s (16). The p h o t o s e n s i t i z e d e l e c t r o n 2

2 +

+


00


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WILLNER E T A L .


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t r a n s f e r r e a c t i o n i n the presence o f these c a r r i e r s i s enhanced to a 3-6 f o l d extent, depending on the ionophore (Figure 4A). These r e s u l t s confirm that the cotransport o f c a t i o n s plays an important r o l e i n the p h o t o s e n s i t i z e d r e a c t i o n s . In a d d i t i o n t o t h e i r f u n c t i o n i n e s t a b l i s h i n g charge neut r a l i z a t i o n during the photochemical r e a c t i o n s , c a t i o n s might, by proper o r g a n i z a t i o n , even a s s i s t the e l e c t r o n t r a n s f e r v i a product i o n o f a transmembrane p o t e n t i a l . D i f f e r e n t concentrations o f K+ i n the opposite aqueous phases o f the l i p i d b i l a y e r were used to t e s t f o r t h i s e f f e c t . The s p e c i f i c K c a r r i e r , valinomycin, was incorporated i n t o the v e s i c l e w a l l s . Consequently, owing t o the concentration d i f f e r e n c e , a l o n g - l a s t i n g transmembrane p o t e n t i a l i s e s t a b l i s h e d . 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 r e a c t i o n appears t o be a f f e c t e d by such e l e c t r i c f i e l d s (Figure 4B). I t can be s e e i t h a t when the r a t i o o f K f / K >1,1·e·, i n t e r i o r boundary negative r e l a t i v e t o the e x t e r i o r , éne r e a c t i o n i s 2 - f o l d enhanced as compared t o the system without any a p p l i e d f i e l d . Conversely, when t h e v e s i c l e s a r e designed such that K+ /K+ 3 ( 1 ) , s o l u b l e i n the water d r o p l e t . As e l e c t r o n acceptor, benzylnTcotinamide, BNA+, was used. This amphiphilic compound i s expected t o concentrate a t the w a t e r - o i l i n t e r f a c e . 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 r e a c t i o n forms the reduced l i p o p h i l i c e l e c t r o n acceptor ΒΝΑ· which i s e j e c t e d i n t o the con tinuous organic phase and thus separated from the o x i d i z e d p r o duct. I n order t o monitor the e n t i r e phase t r a n s f e r o f the r e duced acceptor, ΒΝΑ·, a secondary e l e c t r o n acceptor, p-dimethylaminoazobenzene (dye),was s o l u b i l i z e d i n the continuous o i l phase. The photochemically induced e l e c t r o n t r a n s f e r r e a c t i o n i n t h i s system r e s u l t s i n the r e d u c t i o n o f the dye (0 = 1.3 χ 10"3). E x c l u s i o n o f the s e n s i t i z e r o r EDTA o r the primary e l e c t r o n accep t o r , BNA , from the system r e s u l t e d i n no d e t e c t a b l e r e a c t i o n . S u b s t i t u t i o n o f the primary acceptor w i t h a water s o l u b l e d e r i v a t i v e , N-propylsulfonate nicotinamide, s i m i l a r l y r e s u l t s i n no r e duction o f the dye. These r e s u l t s i n d i c a t e that to accomplish the c y c l e formulated i n Figure 6A the amphiphilic nature o f the p r i mary e l e c t r o n acceptor and i t s phase t r a n s f e r a b i l i t y i n the r e duced form are necessary requirements. S i m i l a r l y , the reduction h a l f - c e l l o f the model has been constructed. In the l a t t e r system (Figure 6B) the e l e c t r o n accep t o r dimethyl-4,4'-bipyridinium (methylviologen, MV ) and R u ( b i p y ) 5 ( 1 ) o r the water s o l u b l e Zn-porphyrins (3) and (4) were d i s s o l v e d i n the aqueous phase o f the water-i!n-oil micro emulsion with the e l e c t r o n donor, thiophenol, being concentrated at the w a t e r - o i l i n t e r f a c e . 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 the production o f the v i o l o g e n r a d i c a l c a t i o n . T h i s p h o t o s e n s i t i z ed e l e c t r o n t r a n s f e r process r e s u l t s i n the s e p a r a t i o n o f the r e duced photoproduct, MVt, from the o x i d i z e d product, d i p h e n y l d i s u l f i d e , which i s i n the toluene phase. +

2+

+

Photosensitized

E l e c t r o n T r a n s f e r Reactions i n SiO^ C o l l o i d s

C o l l o i d a l S1O2 p a r t i c l e s i n an aqueous suspension p r o v i d e a s o l i d - l i q u i d i n t e r f a c e . The s i l a n o l groups on the p a r t i c l e sur face a r e i o n i z e d a t a pH £ 6 . Consequently, the s u r f a c e o f the par t i c l e i s n e g a t i v e l y charged and a d i f f u s e e l e c t r i c a l double l a y e r i s produced i n the v i c i n i t y o f the s o l i d i n t e r f a c e (17,18). Be cause o f the negative charges on the p a r t i c l e s they r e p e l one another and t h e i r agglomeration i s prevented. The p a r t i c l e s can be used t o exert e l e c t r o s t a t i c r e p u l s i v e and a t t r a c t i v e i n t e r a c t i o n s with the components i n v o l v e d i n p h o t o s e n s i t i z e d r e a c t i o n s .


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Interphase

83

Toluene

Figure 6. Cyclic mechanism for photoinduced electron transfer across the interface of a water-in-toluene microemulsion. Key: A, oxidative half-cell; and B, reductive half-cell


84


By means of these i n t e r a c t i o n s a component can be s e l e c t i v e l y adsorbed to the i n t e r f a c e and i t s recombination w i t h an o p p o s i t e l y charged photoproduct can be retarded* To achieve such an o r g a n i z a t i o n i n the system the d i f f e r e n t components have to be f u n c t i o n a l ! z e d . Two p o s i t i v e l y charged sens i t i z e r s . R u ( b i p y ) § ( ] L ) or Zn-meso-tetramethylpyridinium porphyrin, Zn-TMPyP*+ (_3), that aire adsorbed to the Si02 i n t e r f a c e are used (19). The z w T t t e r i o n i c d i 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 (5) ( p r o p y l v i o l o g e n s u l f o n a t e , PVS°) i s used as e l e c t r o n acceptor", and triethanolamine, TEA, i s introduced as e l e c t r o n donor. Photosensit i z e d e l e c t r o n t r a n s f e r i n these systems r e s u l t s i n a r a p i d prod u c t i o n of the v i o l o g e n r a d i c a l , PVST. The r a t e s of PVST formation i n the c o l l o i d a l Si02 systems u s i n g the d i f f e r e n t s e n s i t i z e r s are shown i n F i g u r e 7, and compared w i t h the analogous r e a c t i o n s i n a homogeneous phase. I t can be seen that the e l e c t r o n t r a n s f e r react i o n s i n the Si02 c o l l o i d are ca. 1 0 - f o l d enhanced r e l a t i v e to the homogeneous phase and u s i n g Zn-TMPyP4+ as s e n s i t i z e r a h i g h quantum y i e l d (0 = 0.35) i s obtained. The enhanced quantum y i e l d s i n the SiO^ c o l l o i d s are a s c r i b e d to the c o n t r o l of the e l e c t r o n t r a n s f e r r e a c t i o n by means o f 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 (eq. 5 and 6 and F i g u r e 8). The e l e c t r o n t r a n s f e r from the e x c i t e d sen+ s i t i z e r , Ru(bipy)3 , to the n e u t r a l e l e c t r o n acceptor l e s u l t s i n two o p p o s i t e l y charged photoproducts. The charged i n t e r f a c e i n t e r acts w i t h these intermediate photoproducts; the o x i d i z e d s e n s i t i zer i s adsorbed at the i n t e r f a c e w h i l e the reduced, n e g a t i v e l y charged e l e c t r o n acceptor i s r e p e l l e d . Consequently, the 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 t r o d u c e a b a r r i e r to the degradative geminate recombination of the photoproducts. As a r e s u l t , the f u r t h e r e f f e c t i v e u t i l i z a t i o n o f the o x i d i z e d product, R u ( b i p y ) ^ , i n o x i d i z i n g the e l e c t r o n donor, TEA, i s f a c i l i t a t e d and h i g h quantum y i e l d s are obtained. +

+

u

Zn-TMPyP

4+

. Ru(bipy)3

+

PVS°

*

Zn-TMPyP

Ru(bipy)^

+

PVS

+

PVS-

7

(eq.5)

(eq.6)

The f u n c t i o n of the SiO^ c o l l o i d i n r e t a r d i n g b a c k - 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 has been confirmed by s e v e r a l methods: (a) The quantum y i e l d i n the S1O2 c o l l o i d depends s t r o n g l y on the i o n i c s t r e n g t h of the medium. By i n c r e a s i n g the i o n i c s t r e n g t h the i n t e r f a c i a l s u r f a c e p o t e n t i a l i s decreased. As a r e s u l t , the 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 with the i n t e r f a c e are reduced and the quantum y i e l d i s decreased. (b) S u b s t i t u t i o n of the p o s i t i v e s e n s i t i z e r with one that i s neg a t i v e l y charged y i e l d s two photoproducts that are r e p e l l e d by the i n t e r f a c e . Thus, the f u n c t i o n of the i n t e r f a c e i n s e p a r a t i n g the a c t i v e intermediates i s l o s t . Indeed, w i t h a n e g a t i v e l y charged s e n s i t i z e r , Zn-meso-tetraphenylporphyrin s u l f o n a t e , Zn-TPPS^- ( 4 ) ,



4

4

4

7

x Ι Ο einsteins absorbed

4

g 4

1

1

4

2

s

Figure 7. Propylviologen radical, PVS-, formation as a function of light adsorbed, monitored by the increase of absorbance at λ = 602 nm fc = 12,500 M" cm";. Key: in A, Ru(bipy) * as sensitizer; a, SiO» system; b, homogeneous system; c, micellar system; d, NaLS micellar system with 0.1 M NaCl; and in B, Zn-TMPyP + and Zn-TPPS ' as sensitizers; a, SiO* system with ZnTMPyP *; b, homogeneous system with Zn-TMPyP *; c, SiO system with Zn-TPPS '; d, homo geneous system with Zn-TPPS '.

5

χ Ι Ο emsteins absorbed

^

g

86



5.

WILLNER E T A L .


87

there i s no enhancement o f quantum y i e l d i n the Si(>2 system (Figure 7B). (c) The back-electron t r a n s f e r r e a c t i o n o f the intermediate photoproducts (eq. 5 and 6) has been d i r e c t l y followed i n the S1O2 c o l l o i d by means o f f l a s h p h o t o l y s i s and compared t o the s i m i l a r process i n a homogeneous phase. A s i g n i f i c a n t r e t a r d a t i o n o f backe l e c t r o n t r a n s f e r i s observed.With Zn-TMPyP^* as s e n s i t i z e r , the recombination r a t e constant (eq.5) i n the Si02 c o l l o i d i s r e duced by a f a c t o r o f 100 r e l a t i v e t o the value i n a homogeneous phase. S i m i l a r l y , with Ru(bipy>3 as s e n s i t i z e r the recombination r a t e i s c a . 9 0 - f o l d retarded i n the Si02 c o l l o i d . The extent t o which back-electron t r a n s f e r r e a c t i o n s a r e r e tarded i n the Si02 c o l l o i d can be improved by i n t r o d u c t i o n o f m u l t i n e g a t i v e l y charged e l e c t r o n acceptors such as Fe(CN>5 that increase the r e p u l s i v e i n t e r a c t i o n s with the i n t e r f a c e (20). However, with such e l e c t r o n acceptors the primary e l e c t r o n t r a n s f e r event i s expected t o be r a t h e r i n e f f i c i e n t because they cannot approach the i n t e r f a c e . I n order t o keep the balance o f e f f i c i e n t quenching o f the e x c i t e d s t a t e , together with a s u b s t a n t i a l r e t a r d a t i o n o f the recombination r a t e , two coupled e l e c t r o n acceptors can be used. F o r t h i s purpose, a c o l l o i d a l Si02 system has been designed i n which Ru(bipy)§ is the s e n s i t i z e r , PVS° (5) the p r i mary e l e c t r o n acceptor and triethanolamine, TEA, the e l e c t r o n donor. A secondary e l e c t r o n acceptor, K3Fe(CN>6, i s introduced i n t o the system to provide a s i n k f o r the e l e c t r o n (Figure 9 ) . The complete 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 r e s u l t s i n the r e d u c t i o n o f Fe(CN)$~ t o Fe(CN)£~ (Figure 10). I t appears that the p h o t o s e n s i t i z e d r e a c t i o n i s a t l e a s t 6 0 - f o l d enhanced r e l a t i v e to the r e a c t i o n i n a homogeneous phase (20). The sequence o f events o c c u r r i n g i n t h i s p h o t o s e n s i t i z e d e l e c t r o n t r a n s f e r have been followed by f l a s h p h o t o l y s i s . The reduced primary e l e c t r o n acceptor PVS i s produced by the quenching o f the e x c i t e d s e n s i t i z e r adsorbed t o the Si02 i n t e r f a c e (eq.6). The reduced species i s e j e c t e d i n t o the continuous aqueous phase where Fe(CN)$~ i s reduced (eq. 7) i n a "dark" r e a c t i o n . The intermediate photoproducts thus created, R u ( b i p y ) ^ and Fe(CN)$"" , tend t o back-react (eq.8). I n a homogeneous system t h i s process i s d i f f u s i o n cont r o l l e d ( k f c * 1 0 M - l . s e c - l ) ( 2 1 ) . However, i n the Si02 c o l l o i d a s u b s t a n t i a l i n h i b i t i o n o f the recombination r a t e i s observed Xkb - 1 0 - 1 0 M - l - s e c " ) . These r e s u l t s i n d i c a t e that the d i f f e r ent f u n c t i o n s r e q u i r e d f o r an e f f i c i e n t e l e c t r o n t r a n s f e r process can be achieved by c o u p l i n g two o r more e l e c t r o n acceptors. +

+

7

+

10

D

6

BVS

7

7

1

+ Fe(CN)j?"

*

PVS°

+

Fe(CN)*~

(eq.7)

^b Ru(bipy)^

+

+

Fe(CN)^"

•> R u i b i p y ) ^ +

Fe(CN)^"


(eq.8)

88


t I

f •δ»

!

* 8. ρ

! •2. .ο •δ Q

Ι Os

I


WILLNER ET A L .


Figure 10. Reduction of K Fe(CN) as a function of light adsorbed. Key: SiOft system including PVS°; b, SiO system; arrow, time of PVS° addition; homogeneous system; and d, NaLS micellar system (20). s

e

t


90

INORGANIC REACTIONS IN

ORGANIZED MEDIA

C o r r e l a t i o n of Quantum Y i e l d s w i t h I n t e r f a c i a l P o t e n t i a l s The f u n c t i o n of the Si02 c o l l o i d i n 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 o r i g i n a t e s from s e l e c t i v e i n t e r a c t i o n s of the components w i t h the i n t e r f a c e . The e l e c t r i c a l p r o p e r t i e s of the i n t e r f a c e and the b i n d i n g c h a r a c t e r i s t i c s of the p o s i t i v e l y charg ed s e n s i t i z e r , R u ( b i p y ) ^ , have been examined by means of flow dialysis ( K . = 1.1 x 1 0 M ~ l ) ( 2 2 ) . The number of b i n d i n g s i t e s on each S1O2 p a r t i c l e has been determined to be 65. These i o n i c s i t e s e s t a b l i s h an i n t e r f a c i a l s u r f a c e p o t e n t i a l of ca. -170 mV. The quantum y i e l d f o r 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 PVS° (using R u ( b i p y ) ? as s e n s i t i z e r ) has been c o r r e l a t e d with the i n t e r f a c i a l s u r f a c e p o t e n t i a l of the S1O2 c o l l o i d ( c o n t r o l l e d by v a r y i n g the i o n i c s t r e n g t h o f the medium) (22). The c o r r e l a t i o n curve (Figure 11) shows that up to an i n t e r f a c i a l p o t e n t i a l of ca. -40 mV the quantum y i e l d i s not a f f e c t e d . Increasing the p o t e n t i a l above t h i s apparent threshold value r e s u l t s i n a sharp i n c r e a s e i n the quantum y i e l d . A s i m i l a r c o r r e l a t i o n curve was obtained when Zn-TMPyp4 was used as s e n s i t i z e r i n s t e a d of R u ( b i p y ) ^ . The o r g a n i z a t i o n o f components i n the Si02 c o l l o i d s and the 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 could, i n p r i n c i p l e , be designed with other n e g a t i v e l y charged i n t e r f a c e s such as m i c e l l e s . The photo s e n s i t i z e d r e d u c t i o n of PVS° using R u ( b i p y ) $ as s e n s i t i z e r and triethanolamine» TEA, as e l e c t r o n donor has been i n v e s t i g a t e d i n the presence of n e g a t i v e l y charged NaLS m i c e l l e s and compared to the r e s u l t s i n the Si02 c o l l o i d (Figure 7A) (22). The s i z e of the NaLS m i c e l l e s i s s i m i l a r to that of the Si02 p a r t i c l e s . The sen s i t i z e r , R u ( b i p y ) ^ , appears to bind f i r m l y to the m i c e l l a r i n t e r face ( K s . = 3.5 χ 103 M" ). Yet,the quantum y i e l d f o r the PVST formation i s 4 - f o l d l e s s e f f i c i e n t than that observed i n the Si02 c o l l o i d . T h i s r e s u l t i s a t t r i b u t e d to the d i f f e r e n c e i n the surface p o t e n t i a l of the two i n t e r f a c e s . Flow d i a l y s i s measurements (22) i n d i c a t e that the NaLS m i c e l l a r i n t e r f a c e has a surface p o t e n t i a l of only -85 mV, s i g n i f i c a n t l y lower than the v a l u e determined f o r the Si02 i n t e r f a c e (-170 mV). The experimental quantum y i e l d s i n the NaLS m i c e l l a r system f i t n i c e l y i n t o the c o r r e l a t i o n curve shown i n F i g u r e 11. T h i s i n d i c a t e s that due to the r e l a t i v e l y low s u r f a c e p o t e n t i a l of the m i c e l l e s the 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 are not as e f f e c t i v e . The comparison of photoinduced r e a c t i o n s i n the Si02 c o l l o i d to that o c c u r r i n g i n the NaLS m i c e l l e s i m p l i e s that both i n t e r f a c e s are capable o f e x e r t i n g 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 i s can be used f o r o r g a n i z i n g the components i n v o l v e d i n the photochemical r e a c t i o n . However, the p h y s i c a l c h a r a c t e r i s t i c s of the e l e c t r i c f i e l d of the d i f f e r e n t i n t e r f a c e s i s r a t h e r impor tant i n c o n t r o l l i n g the r e a c t i o n . In the NaLS m i c e l l a r system, d e s p i t e the o r g a n i z a t i o n o f the components, the s u r f a c e p o t e n t i a l i s r e l a t i v e l y low and l i m i t s the a b i l i t y to r e t a r d back r e a c t i o n . 2

a s s

+

+

+

+

+

1

a S


wiLLNER E T A L .


3

2 M

Ο

Χ -θI

0 L—ι

1 -40

1

I -80

ι

I -120

ι

I -160

ι -200

Surface potential (mV) Figure 11. Quantum yield for propylviologen, PVS*, formation as a function of the surface potential of negatively charged interfaces. Key: O, Si0 system; and · , NaLS micellar system (22). 2


92

INORGANIC REACTIONS I N ORGANIZED M E D I A

Chemical U t i l i z a t i o n o f the Photoproducts i n the Photodecomposit i o n o f Water The d i f f e r e n t i n t e r f a c i a l systems d e s c r i b e d i n t h i s paper represent supramolecular assemblies f o r the 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 photoproducts. These photoproducts generated i n the 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 are an o x i d i z e d s e n s i t i z e r and a r e duced s p e c i e s ( a c c e p t o r ) . I n a l l systems that have been d e s c r i b e d here a s a c r i f i c a l e l e c t r o n donor (EDTA o r TEA) has been used. For any p r a c t i c a l c o n f i g u r a t i o n , t h i s s a c r i f i c i a l component must be excluded and water i t s e l f should be the compound o x i d i z e d . The o x i d i z e d i n t e r m e d i a t e s , R u ( b i p y ) ^ and Zn-TMPyP , have the potent i a l f o r o x i d i z i n g water t o oxygen ( E ( R u ( b i p y ) 3 / R u ( b i p y ) | ) « 1.26 v o l t ; E (Zn-TMPyp5+/Zn-TMPyP4+) - 1 . 2 volt) ,but s i n c e the r e a c t i o n r e q u i r e s a concerted f o u r - e l e c t r o n process w h i l e the photoproducts a r e s i n g l e e l e c t r o n oxidants, a mediating chargestorage c a t a l y s t i s needed. In recent years t r a n s i t i o n metal oxides and, i n p a r t i c u l a r , Ru(>2 and Pt02, have been reported t o a c t as oxygen e v o l u t i o n c a t a l y s t s w i t h Ru(bipy)§ as oxidant (23,24). In an analogous way, the reduced s p e c i e s produced i n the photosens i t i z e d r e a c t i o n should be coupled to hydrogen e v o l u t i o n . Reduced b i p y r i d i n i u m s a l t s (viologen r a d i c a l s ) are capable o f reducing water t o hydrogen (25, 26). For t h i s r e a c t i o n c o l l o i d a l platinum has been found to be an e f f i c i e n t charge-storage c a t a l y s t . A schematic view o f one p o s s i b l e complete system i s shown i n F i g u r e 12. Since t h e . o x i d i z e d photoproduct, e.g., Ru(bipy)^+, i s a s s o c i a t e d with one c o l l o i d a l p a r t i c l e , i t s i n t e r f a c e should be coated w i t h an oxygen-evolving c a t a l y s t . An a d d i t i o n a l c o l l o i d a l s i t e i s introduced by supporting platinum on a n e g a t i v e l y charged polymer. The e l e c t r o s t a t i c r e p u l s i o n s o f the two n e g a t i v e l y charged i n t e r f a c e s would prevent agglomeration. By u s i n g a polymer w i t h a low enough s u r f a c e p o t e n t i a l the approach o f the reduced photoproduct, P V S T , t o the hydrogen e v o l u t i o n c a t a l y s t would be permitted w h i l e i t s recombination w i t h R u ( b i p y ) ^ on the other, more h i g h l y charged c o l l o i d , would be prevented. I n t h i s way, the vect o r i a l c h a r a c t e r o f the e l e c t r o n t r a n s f e r process could be used f o r an e f f i c i e n t cleavage of water. Acknowledgement : The work was supported, i n p a r t , by the D i r e c t o r , O f f i c e o f Energy Research, O f f i c e o f B a s i c Energy Sciences, Chemical Science D i v i s i o n , o f the U.S. Department of Energy under Contract W-7405-ENG-48 and by the Netherlands Organi z a t i o n f o r the Advancement o f Pure Research (Z.W.O.). +

5+

+

0

0

+

+


f

WILLNER E T AL.


Figure 12. Utilization of Si0 colloids in the photodecomposition of water. 2


94


Literature Cited 1. Dolphin, D., McKenna, C, E., Murakami, Y., and Tabushi, I., Eds.; "Biomimetic Chemistry"; American Chemical Society, Washington, DC, 1981. Advances in Chemistry Series No. 191. 2. Breslow, R. Acc. Chem. Res. 1980, 13, 170; Isr. J. Chem. 1979, 18, 187. 3. Bolton, J. R. Science, 1978, 202, 105; Bard, A. J. Science, 1980, 207, 139; Porter, G. Pure and Appl. Chem. 1978, 50, 263. 4. Calvin, M. Acc. Chem. Res. 1978, 10, 369; Willner, I.; Ford, W. E.; Otvos, J. W.; Calvin, M. in "Bioelectrochemistry", Keyzer, H. and Gutmann, F. Eds.; Plenum Press, New York, 1980, pp. 558-581. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Gibbs, M. "Structure and Functions of Chloroplasts"; Springer-Verlag, Berlin, 1971; Calvin, M. and Bassham, J. A. "The Path of Carbon in Photosynthesis"; Prentice-Hall, Englewood Cliffs, 1957. Balzani, V.; Boletti, M. T.; Gandolfi, M. T.; Maestri, M Top. Curr. Chem. 1978, 75, 1; Balzani, V.; Bolleta, F.; Scandola, F.; Ballardini, R. Pure and Appl. Chem. 1979, 51, 299. Whitten, D. G. Rev. Chem. Intermediates, 1979, 2, 107; Hopf, R.; Whitten, D. G. in "Porphyrins and Metalloporphyrins", Smith, K. M. Ed.; Elsevier, New York, 1975, pp. 667-700. Whitten, D. G. Acc. Chem. Res. 1980, 13, 83. Sutin, N. J. Photochem. 1979, 10, 19. Kalyanasundaram, K. Chem. Soc. Rev. 1978, 7, 453; Turro, N. J . ; Grätzel, M.; Brown, A. M. Angew. Chem. Internat. Ed. 1980, 19, 573. Brugger, P. Α.; Grätzel, M. J. Amer. Chem. Soc. 1980, 102, 2461. Jones, C. Α.; Weaner, L. E.; Mackay, R. A. J. Phys. Chem. 1980, 84, 1495. Willner, I.; Ford, W. E.; Otvos, J. W.; Calvin, M. Nature (London), 1979, 280, 823. Ford, W. D.; Otvos, J. W. ; Calvin, M. Nature (London), 1978 274, 507; Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 3590. Infelta, P.P.; Grätzel, M.; Fendler, J. H. J. Amer. Chem. Soc. 1980, 102, 1479; Matsuo, T.; Itoh, K.; Takuma, K.; Hashimoto, K.; Nagamura, T. Chem. Lett. 1980, 8, 1009. Laane, C.; Ford, W. E.; Otvos,J. W.; Calvin, M. Proc. Natl. Acad. Sci. U.S.A., 1981, 78, 0000. Iler, R. K. "The Colloid Chemistry of Silica and Silicates"; Cornell University Press, Ithaca, New York (1955); Loeb, A. L.; Overbeek, J.Th.G. Wiersema, P. H. "The Electrical Double Layer Around a Spherical Colloid Particle", M.I.T. Press, Cambridge, Massachusetts (1961).


5. 18· 19. 20. 21. 22. 23. 24.

25. 26.

WILLNER ET AL.


James, A. M. Chem. Soc. Rev. 1979, 8, 289. Willner, I.; Otvos, J. W.; Calvin, M. J . Amer. Chem. Soc. 1981, 103, 0000. Willner, I.; Yang, J.-M.; Otvos, J . W.; Calvin, M. J . Amer. Chem. Soc. submitted for publication. Pellizzetti, E.; Pramaura, E. Inorg. Chem. 1979, 18, 882. Laane, C.; Willner, I.; Otvos, J . W.; Calvin, M. J . Amer. Chem. Soc. submitted for publication. Lehn, J.-M.; Sauvage, J . P.; Ziessel, R. Nouv. J . Chim. 1979, 3, 423; 1980, 4, 623. Borgarello, E.; Kiwi, J.; Pellizzetti, M. V.; Grätzel, M. Nature (London), 1981, 289, 158; Kalyanasundaram, K.; Micic, O.; Promauro, E.; Grätzel, M. Helv. Chim. Acta, 1979, 62, 2432. Moradpour, Z.; Amoruyal, E.; Keller, P.; Kagan, H. Nouv. J . Chim. 1978, 2, 547; J . Amer. Chem. Soc. 1980, 102, 7193. Kalyanasundaram, K.; Kiwi, J.; Grätzel, M. Helv. Chim. Acta, 1978, 61, 2720; DeLaive, D. J.; Sullivan, B. P.; Meyer, T.T.; Whitten, D. G. J . Amer. Chem. Soc. 1979, 101, 4007.

RECEIVED May 27,

1981.


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Inorganic Reactions in Organized Media - ACS Publications

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