Aspects of Artificial Photosynthesis - American Chemical Society

4 A s p e c t s of A r t i f i c i a l Photosynthesis The Role of Potential Gradients in Promoting Charge Separation in the Presence of Surfactant Vesicles

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: February 12, 1982 | doi: 10.1021/bk-1982-0177.ch004

MOHAMMAD S. TUNULI and JANOS H. FENDLER Texas A&M University, Department of Chemistry, College Station, TX 77843 Completely synthetic vesicles, prepared from dioctadecyldimethylammonium chloride, DODAC, and dihexadecylphosphate, DHP, have been used in our laboratories as media for investigating aspects of artificial photosynthesis over the past several years. Different potentials, created by the high charge densities on the surface of DODAC and DHP vesicles, and their exploitation for photochemical solar energy conversion are discussed in this presentation. Surface potential, charge separation potential, diffusion potential and Donnan potential are exploited for enhanced energy and electron transfer on charged vesicle surfaces, for the utilization of field effects for charge separation, for partitioning between the inner and outer compartments of radicals expelled from vesicle bilayers and for facilitating electron transfer across bilayers. Photochemical solar energy conversion is a vitally important and extremely active area of research (1-9). The excited state of a suitable sensitizer, produced by irradiation, is a better electron acceptor as well as a better electron donor than its ground state. Light absorption can drive, therefore, a redox reaction nonsponta-

0097-6156/82/0177-0053$05.00/0 © 1982 American Chemical Society Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

54

INORGANIC REACTIONS IN ORGANIZED MEDIA +

F u r t h e r , i f D and A~ have a p p r o p r i a t e redox p o t e n t i a l s they may d i r e c t l y reduce water t o hydrogen and o x i d i z e i t t o oxygen: 2A~ + 2H 0

•

2

2A + 20H~ + H + (2 e

4D

+

+ 2H 0

+

(4 e


reduction)

4D + 4 H + 0^

•

2

(1)

2

(2)

oxidation)

U n f o r t u n a t e l y , i n homogeneous s o l u t i o n the r a p i d recombination o f D + A" t o D + A precludes the p o s s i b i l i t y o f t h i s type of photochemical energy storage and conversion. Separation o f charges i n space provides an e f f i c i e n t method f o r d i m i n i s h i n g u n d e s i r a b l e charge recombinations. Aqueous (10) and reversed (11,12) m i c e l l e s , microemulsions (13), monolayers (14), b i l a y e r (black) l i p i d membranes (15), p o l y e l e c t r o l y t e s (16), liposomes (17) and s u r f a c tant v e s i c l e s (18-27) have been used t o a f f e c t charge s e p a r a t i o n by o r g a n i z i n g s e n s i t i z e r s , e l e c t r o n donors and acceptors i n t h e i r compartments (28)· These organized assemblies are expected t o (a) s o l u b i l i z e , concentrate, compartmentalize, organize, and l o c a l i z e r e a c t a n t s ; (b) maintain proton and/or r e a c t a n t g r a d i e n t s ; (c) a l t e r quantum e f f i c i e n c i e s ; (d) lower i o n i z a t i o n p o t e n t i a l s ; (e) change o x i d a t i o n and r e d u c t i o n p r o p e r t i e s ; ( f ) change d i s s o c i a t i o n constants; (g) a f f e c t v e c t o r i a l e l e c t r o n displacements; (h) a l t e r p h o t o p h y s i c a l pathways and r a t e s ; ( i ) a l t e r chemical pathways and r a t e s ; ( j ) s t a b i l i z e r e a c t a n t s and/or products and/or t r a n s i t i o n s t a t e s ; (k) separate charges and/or products; and (1) be chemically s t a b l e , o p t i c a l l y transparent and photochemically inactive. Completely s y n t h e t i c s u r f a c t a n t v e s i c l e s have been used i n our l a b o r a t o r i e s as media f o r examining aspects of a r t i f i c i a l photos y n t h e s i s . To-date, we have i n v e s t i g a t e d v e s i c l e s formed from dioctadecyldimethylammonium c h l o r i d e , DODAC, and dihexadecylphosphate, DHP (19-21, 23, 27): CI" CH -(CH ) 3

2

1 7

v^

^ C H

3

CH -(CH ) -0 3

2

0

15

T» CH - (CH ) 3

^ CH

2

DODAC

3

CH - (CH ) 3

2

^

f

_

TT""

^-0'

DHP

D i s p e r s a l of DODAC or DHP i n water by u l t r a s o n i c i r r a d i a t i o n r e s u l t s i n the formation o f f a i r l y uniform s i n g l e compartment v e s i c l e s (Figure 1 ) . DODAC and DHP v e s i c l e s a r e s t a b l e f o r weeks

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

4. TUNULI

AND

FENDLER

55

Aspects of Artificial Photosynthesis


I° I ^1 ν § 1^ s sJ SSI

il* 1·Ι ι •m nil if} 3.S s: Cc« '*·» 5 ,*» < Q

1feI

p i


INORGANIC REACTIONS IN ORGANIZED MEDIA

56

i n pH 2-12 range, o s m o t i c a l l y a c t i v e , undergo thermotropic phase t r a n s i t i o n s and, most importantly entrap and r e t a i n molecules i n t h e i r compartments. Advantages of s u r f a c t a n t v e s i c l e s over other systems are that they are able t o organize l a r g e numbers of s e n s i t i z e r s , e l e c t r o n donors and acceptors per aggregate and that they are amenable to e l e c t r o s t a t i c m o d i f i c a t i o n and chemical f u n c t i o n a l ization. Importantly, u n l i k e n a t u r a l membranes which are composed mostly of z w i t t e r i o n i c l i p i d s , s u r f a c t a n t v e s i c l e s are h i g h l y charged and have h i g h charge d e n s i t i e s on t h e i r s u r f a c e s . These charges create a p p r e c i a b l e p o t e n t i a l s . The types of p o t e n t i a l s a s s o c i a t e d with s u r f a c t a n t v e s i c l e s and t h e i r e x p l o i t a t i o n i n photochemical s o l a r energy conversion are the subject of t h i s presentation.


Types of P o t e n t i a l s A s s o c i a t e d w i t h S u r f a c t a n t V e s i c l e s In a d d i t i o n to the s u r f a c e p o t e n t i a l , Ψ , present at the outer and i n n e r surfaces of charged v e s i c l e s s e v e r a l a d d i t i o n a l p o t e n t i a l s can be c r e a t e d . Of these, the charge s e p a r a t i o n p o t e n t i a l , the d i f f u s i o n p o t e n t i a l and the Donnan p o t e n t i a l w i l l be b r i e f l y d i s c u s s e d . 0

Surface P o t e n t i a l . The presence of i o n i z e d head groups on a s p h e r i c a l v e s i c l e w i t h r a d i u s r and charge q on the v e s i c l e leads to a s u r f a c e p o t e n t i a l Ψ : v

ο

Ψ

2

0

= q e /er v

(3)

where ε i s the d i e l e c t r i c constant at the i n t e r f a c e . The s u r f a c e p o t e n t i a l decreases with i n c r e a s i n g d i s t a n c e s from the charged s u r f a c e . At a d i s t a n c e x, from the surface the p o t e n t i a l i s given by (see F i g u r e 2): Ψ

χ

= Ψ βχρ(-βχ) ο

(4)

s

where β (distance from s u r f a c e ) / ( d i s t a n c e from the outer Helmholtz plane)· In the Gouy-Chapman d i f f u s e l a y e r the c o n c e n t r a t i o n - d i s t a n c e p r o f i l e i s given by the Boltzmann d i s t r i b u t i o n :

C

C

e

x- o *>(-

( 5 )

where C and C are the concentration of ions at d i s t a n c e χ from the s u r f a c e and at the bulk, r e s p e c t i v e l y , z^ i s the number of u n i t s of e l e c t r o n i c charge on i o n i , F and R are the Faraday and u n i v e r s a l gas constants and Τ i s the absolute temperature. The Poisson equation r e l a t e s the p o t e n t i a l p r o f i l e to the charge d e n s i t y by: x

Q


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

Λ

Figure 2. Potential profile at the interface and across the bilayer of positively charged surfactant vesicles. The potential at r -» oo was taken as reference, that is, Φ ^ = 0.

Distance from Interface

BIFACE



58

which upon i n t e g r a t i o n (using appropriate boundary gives an expression f o r the e l e c t r i c f i e l d :

Χ -

± . C _ p ( - - f c H H -


χ

conditions)

1\Y

(7)

Charge Separation P o t e n t i a l . An e l e c t r o n can be t r a n s f e r r e d across the b i l a y e r of s u r f a c t a n t v e s i c l e s from a donor to an acceptor. This t r a n s f e r renders the donor s i d e of the v e s i c l e surface to be more p o s i t i v e than the acceptor s i d e , thus i t creates a p o t e n t i a l , r e f e r r e d to as charge separation p o t e n t i a l , c.s. T h i s , by analogy to charging a p a r a l l e l p l a t e condenser, i s given by: q

δ S_ Αεε

=

(8) (8)

1

c.s

where q' i s the a d d i t i o n a l charge deposited on the v e s i c l e due to the p h o t o i n i t i a t e d e l e c t r o n t r a n s f e r , δ i s the thickness of the s u r f a c t a n t v e s i c l e , A i s i t s surface area, ε denotes the d i e l e c t r i c constant of the i n t e r f a c i a l r e g i o n and ε i s the p e r m i t t i v i t y of f r e e space. I f Ψ i s known, q can be obtained from: 1

f

where C i s the b u l k c o n c e n t r a t i o n of e l e c t r o n donor/acceptor. Negative s i g n i s used i f i s p o s i t i v e and vice versa. D i f f u s i o n P o t e n t i a l . D i f f u s i o n p o t e n t i a l a r i s e s from a c o n c e n t r a t i o n gradient, VC , across the v e s i c l e b i l a y e r . Addition of an e l e c t r o l y t e whose component ions d i f f e r i n t h e i r m o b i l i t i e s to already formed v e s i c l e s w i l l g i v e r i s e to s p a c i a l segregation of i o n s . Sodium c h l o r i d e provides an example f o r such a behavior. At 298.15 K the m o b i l i t i e s of Na+ and CI" are 5 . 2 x l 0 " c m 2 s e c - V " and 7.9x10-4 c m s e c " v " . For a v e s i c l e of thickness δ, immersed i n a stagnant ( i . e . no convection) d i l u t e e l e c t r o l y t e s o l u t i o n the transport of species across the b i l a y e r can be formulated by the f o l l o w i n g s e t of equations (29): e

4

2

J

i

β

1

z

" iW C W 1

1

1

1

migration

-

D1VC1 diffusion


( 1 0 )

4. TUNULi AND FENDLER

59


dC - d f -" i

"

V

J

i

( i o n current) = F j ] z J . i 1

β


£ z i

1

c. *

(12)

1

ο ( p r i n c i p l e of e l e c t r o n e u t r a l i t y )

(13)

where i s the i o n i c f l u x under the j o i n t i n f l u e n c e of e l e c t r i c a l p o t e n t i a l and c o n c e n t r a t i o n g r a d i e n t s , μ^, i s the m o b i l i t y of the i t h s p e c i e s , and D^ i s the d i f f u s i o n c o e f f i c i e n t of the i t h s p e c i e s . Equation 10 can be i n t e g r a t e d n u m e r i c a l l y without much d i f f i c u l t y . A simpler approach has been provided by Goldman who assumed the constancy of the f i e l d across the membrane (30). The problem then becomes analogous to that of e l e c t r i c conduction i n the copper - copper oxide r e c t i f i e r (31). Using t h i s assumption, i n t e g r a t i o n of equation 10 leads t o i F

U7 ή - - ^ * 6

1

Ru(bpy) * + MV

61


Ru(bpy) *

2+

(19)

4

• Ruftpy)^ " + MV*

(20)

on the outer and Inner surfaces of a n i o n i c DHP s u r f a c t a n t v e s i c l e s (Systems I I I and IV, r e s p e c t i v e l y i n F i g u r e 3) (23)· The apparent r a t e constant f o r r e a c t i o n 20, (4-5) 1 0 * M ^ s e c - T T i n Systems I I I and IV are three orders o f magnitude g r e a t e r than that found i n water ( 2 x l 0 M ^ s e c " ! ) . E l e c t r o n t r a n s f e r i s l i k e l y t o occur by d i f f u s i o n or hopping on the v e s i c l e s u r f a c e . Under t y p i c a l condit i o n s , approximately 60 molecules o f R u ( b p y ) ^ and 300 molecules o f MV a s s o c i a t e w i t h each DHP v e s i c l e . Taking charge r e p u l s i o n s i n t o c o n s i d e r a t i o n , average areas f g r R u ( b p y ) ^ and MV molecules are estimated t o be 400 A and 200 A , r e s p e c t i v e l y . Since the surface area o f a DHP v e s i c l e i s 1.2x10 A (18) the maximum area the r e a c t i v e partners need t o cover p r i o r t o c o l l i s i o n i s onjy 200 A . This value i s orders of magnitude smaller than 1 0 A estimated f o r d the square o f the mean d i f f u s i v e displacement o f R u ( b p y ) and M V (12). Thus, the r e a c t i v e partners can r e a d i l y f i n d each other on the s u r f a c e o f the DHP v e s i c l e s w i t h i n t h e i r l i f e t i m e s . Unfortunately, the c l o s e proximity a l s o r e s u l t s i n much enhanced back r e a c t i o n : 1

8

+

2+

+


2

2

2

7

2

2

5

2

2

2 +

2+

+

Ru(bpy)3 +

MV*

•

Ru(bpy)

2 +

+

MV

2+

(21)

D i f f e r e n t o r g a n i z a t i o n i s needed, t h e r e f o r e , t o accomplish the d e s i r e d e f f i c i e n c y i n energy conversion; i . e . , t o enhance the r a t e of the forward e l e c t r o n t r a n s f e r ( r e a c t i o n 20, f o r example) and a t the same time reduce the back r e a c t i o n ( r e a c t i o n 21, f o r example). E x p l o i t a t i o n o f p o t e n t i a l s t o accomplish t h i s goal w i l l be i l l u s t r a t e d i n the following sections. Influence o f F i e l d E f f e c t . Since e l e c t r o n t r a n s f e r r a t e s are d i r e c t l y r e l a t e d to the f i e l d , a j u d i c i o u s manipulation o f the d i s t a n c e o f a s e n s i t i z e r and an e l e c t r o n acceptor (or donor) from a h i g h l y charged surface across the Stern l a y e r (Figure 2, equation 7) i s expected t o r e s u l t i n a l t e r e d e f f i c i e n c i e s . This expectation has been r e a l i z e d i n a c h i e v i n g e f f e c t i v e charge separat i o n under the i n f l u e n c e o f a p o s i t i v e e l e c t r i c f i e l d , generated by DODAC v e s i c l e s (35). Rate constant f o r e l e c t r o n t r a n s f e r from L - c y s t e i n e t o the e x c i t e d s t a t e of Ru(bpy)| : +

2+

R u ( b p y ) * + L-cysteine —

• Ru(bpy)* + L - c y s t i n e

+

(22)

has been determined by l a s e r f l a s h p h o t o l y s i s (35). S a t i s f a c t o r y agreement has been obtained between the experimentally observed r a t e constants, kggg, and those c a l c u l a t e d , k g g | , on the b a s i s o f the presence o f an e l e c t r i c f i e l d (Table I ) . c



System IV

System III

Z

Figure 3. Schematics of the different arrangements of the sensitizer, tris(2,2'-bipyridine)ruthenium cation (Ru(bpy)i**), and acceptor, methylviologen (MV *), on DHP surfactant vesicle surfaces.

System II

System I


4.

TUNULi AND FENDLER

63

Aspects of Artificial Photosynthesis TABLE I

C a l c u l a t e d and Observed Rate Constants f o r the E l e c t r o n T r a n s f e r Ruibpy)^"* + L - c y s t e i n e ·* Ru(bpy)$ + L-cysteine+ i n DODAC V e s i c l e s

M

PH 3.0 8.4 11.5 Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: February 12, 1982 | doi: 10.1021/bk-1982-0177.ch004

, calc obs »

obs* - l -1 M sec 3.4xl0

7

8.2xl0

7

1.2xl0

8

cale. AG, _ K c a l mole

M sec

a

AGT, _ K c a l mole

±

6.7xl0

7

-25.8

1.4

1.3xl0

8

-36.4

1.0

1.7xl0

8

-44.3

0.8

C a l c u l a t e d by means o f equation 31. f o r d e t a i l s on the c a l c u l a t i o n s .

±

See the ensuing d i s c u s s i o n

calc D e t a i l s o f o b t a i n i n g the k ^ values are as f o l l o w s . The f i e l d operates through L - c y s t e i n e , whose charge and hence whose d i s t a n c e from the DODAC v e s i c l e surface can be a l t e r e d by changing the b u l k hydrogen i o n concentration o f the s o l u t i o n . D i s s o c i a t i o n constant o f L - c y s t e i n e i n the presence o f s u r f a c t a n t v e s i c l e s , pKa, i s given by: Q

pKa

s

= pKa° - ζ Ρ Ψ

(23)

χ

where pKa° i s the d i s s o c i a t i o n constant i n water and Ψ i s d e f i n e d by equation 4. The redox p o t e n t i a l f o r t h i s process can be written as: χ

V/D

E

+

" D /D - ° ·

0 5 9

{

1 0

*

~

" (24)

ζΈΊ!

where α i s a constant (0.509x2.303) and μ i s the i o n i c s t r e n g t h , r e s p e c t i v e l y . The f r e e energy change f o r r e a c t i o n 22 i s given by: AG = - 2 3 . 0 6 [ E

and

R u ( b p y )

2 * +

/ R u ( b p y ) +

-

E

D + / D

]

(25)

that f o r the r a t e by: k

= Aexpi-AGt/RT)


(26)

64


Assuming that the mechanism involves three steps; the diffusion of the reacting partners:

2+

Ru(bpy) 3

+ L-cysteine

Ru(bpy)

2+*

| L-cysteine

(27)

1-

electron transfer: k 2+

Ru(bpy) *

L-cysteine

Ru(bpy)* | L-cysteine

(28)

-et Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: February 12, 1982 | doi: 10.1021/bk-1982-0177.ch004

charge separation: Ru(bpy)*

L-cysteine

+

+

(29)

+ L-cysteine

(30)

* Ru(bpy)* | L-cysteine

and thermal recombination: Ru(bpy)*

L-cysteine

+

Ru(bpy) c

2+

c

and the observed overall rate, k f l * , i s described by: * obs * • calc obs

where k from:

kD "

D

1 +

K

kD

k

+

k

, (31)

[ e x p G / R T ) + exp (AG/RT) ]

i s the association constant (K - k / k ) , obtainable 3 K

=

A

exp (μ (a) A T )

(32)

2 z

z

e

a e

w

n

e

r

e

a

with μ(a) i 2 / i s the distance of closest approach, 8 i s the s t a t i c d i e l e c t r i c constant and z- and z are the charges on the reacting species. The diffusion race constant k i n equation 31 has been calculated from: 8

S

2

1

- 4ir(D + D )afN/1000

where the diffusion coefficient D factor are given by: Di

(33)

2

=

(D = D1 or D2) and electrostatic

kT/6nr\ri

and


(34)

4.

TUNULI AND FENDLER

65


where η i s the viscosity of the medium and ri i s the radius of the species· AG has been estimated from (36):


>

-

4

[

-

-

+

]

(

-

t

(where AGt(o) i s the activation free energy for isoenergetic (AG =» 0) electron transfer situation and ε i s the optical d i e l e c t r i c constant of the medium using the adjusted parameter K ( k + k ) = 6.9x10 M-lsec" (37). - Considering the assumptions involved the agreement between obs oSi ( *>

Aspects of Artificial Photosynthesis - American Chemical Society

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