J. Phys. Chem. 1003, 87, 3777-3782
3777
Aspects of Artificial Photosynthesis. Photosensitized Electron Transfer and Charge Separation in Redox Active Surfactant Aggregates Kame Kurlhara, Pletro Tundo, and Janos H. Fendler" Department of Chemistry, Clarkson College of Technology, Potsdam, New York 13676 (Received: December 22, 1982)
Photosensitized electron transfer and charge separation have been demonstrated by steady-state and nanosecond laser flash photolysis in the presence of redox active surfactant aggregates prepared from CH2=C(CH3)COO(CH2)ll(C5H4N+)2CH3,Br-,I(RMV2+). Tris(2,2'-bipyridine)ruthenium chloride (Ru(bp~),~+) has been used as a sensitizer. Forward electron transfer from excited R ~ ( b p y ) , ~to+RMV2+aggregates has been shown to be faster than that from R ~ ( b p y ) , ~to+methylviologen (MV2+)in homogeneous solutions. Conversely, the undesirable back-reaction between the reduced electron acceptor RMV'. and the oxidized sensitizer R~(bpy),~+ has been found to be considerably retarded compared to the reaction between MV+-and R~(bpy),~+. Further, unlike MV+., RMV+. decayed by a two-step process. These results have been rationalized in terms of a mechanism which requires most of the photosensitized forward electron transfer to occur on the surface of R M P aggregates. Subsequently,some of the oxidized sensitizer escapes the potential field of RMV2+and charge recombination is retarded by electrostatic repulsions between the positively charged aggregates and R~(bpy),~+.
Introduction Surfactant aggregates are increasingly being utilized in artificial photo~ynthesis.'-~They provide compartments of different microviscosities, charge densities, and potentials; organize sensitizers, electron donors, and acceptors; maintain electron and proton gradients; alter quantum yields, reduction potentials, photophysical pathways, and reaction rates; and facilitate charge separation.g12 Vesicles, prepared from exceedingly simple surfactants, have been found to be particularly useful.'*ls Thus, efficient electron transfer had been demonstrated from a sacrificial donor, ethylenediaminetetraacetate (EDTA), to methylviologen (MVz+)via a sensitizer, tris(2,2'-bipyridine)ruthenium chloride (Ru(bpy)p). R ~ ( b p y ) , ~was + attached to the outer surfaces and MV2+was placed on the inner surface of dihexadecyl (DHP) vesicles while EDTA was distributed in the bulk aqueous solution.ls Initially, electron transfer was considered to occur across the vesicle (1)Kiwi, J.; Kalyanasundaram, K.; Gratzel, M. 'Visible Light Induced Cleavage of Water into Hydrogen and Oxygen in Colloidal Microheterogeneous Systems"; Springer-Verlag: Heidelberg, 1981. Gerischer, H.; Katz, J. J. "Light Induced Charage Separation in Biology and Chemistry"; Verlag Chemie: New York, 1979. Claesson, S.; Engstrom, M. 'Solar Energy-Photochemical Conversion and Storate"; National Swedish Board for Energy Conversion and Storage: Stockholm, 1977. (2)Kirch, M.; Lehn, J. M.; Sauvage, J.-P.Helu. Chim. Acta 1979,62, 1384. (3)Krasna, A. I. Photochem. Photobiol. 1979,29, 267. Krasna, A. I. In "Biological Solar Energy Conversion"; San Pietro, A,, Mitaui, A., Eds.; Academic Press: New York, 1977. (4)Keller, P.; Moradpour, A,; Amouyal, E.; Kagan, H. B. Nouu.J. Chim. 1980,4,377.Keller, P.; Moradpour, A.; Amouyal, E.; Zindler, B. J. Mol. Catal. 1981, 12, 261. (5)Johnson, D.; Launikomis, A.; Loder, J. W.; Mau, A. W. H.; Sasse, W. H. F.; Swift, J. D.; Wells, D. Aust. J. Chem. 1981, 34, 1981. (6)Kalyanasundaram, K. Nouu.J. Chem. 1979,3,511. (7)Miller, D.; McLendon, G. Inorg. Chem. 1981,20,950. (8) Fendler, J. H. Acc. Chem. Res. 1980, 13, 7. (9)Fendler, J. H. J.Photochem. 1981,17, 303. (10)Fendler, J. H. 'Membrane Mimetic Chemistry"; Wiley: New York, 1982. (11)Kunitake, J. Macromol. Sci. Chem. 1979, A13, 589. (12)Fendler, J. H. J . Phys. Chem. 1980, 84, 1485. (13)Escabi-Perez, J. R.; Romero, A.; Lukac, J.; Fendler, J. H. J.Am. Chem. SOC.1979,101, 2231. (14)Nomura, T.; Escabi-Perez, J. R.; Sunamoto, J.; Fendler, J. H. J. Am. Chem. SOC.1980,102, 1484. (15)Infelta, P. P.; Gratzel, M.; Fendler, J. H. J.Am. Chem. SOC.1980, 102, 1479. (16)Monserrat, K.; Gratzel, M.; Tundo, P. J.Am. Chem. SOC.1980, 102, 5527. (17)Pileni, M. P. Chem. Phys. Lett. 71, 317. (18) Tunuli, M. S.;Fendler, J. H. J.Am. Chem. SOC.1981,103, 2507.
bilayers.ls Subsequently, electron transfer was shown to occur on the same outer surface of DHP vesicles, following photosensitized leakage of MV2+.19 This result as well as the relatively poor long-term stability of vesicles demanded alternative approaches. Covalently linked viologen moieties20 are likely to increase the efficiency of vectorial electron transfer. Electron transfer and charge separation were investigated in the present work using surfactant aggregates, prepared from CH3
u
u
RMVZ+
This surfactant provides us with an entry into the more complex polymeric21-36redox active surfactant vesicles.26 (19)Lee, L. Y. C.; Hurst, J. K.; Kurihara, K.; Politi, M.; Fendler, J. H. J. Am. Chem. SOC.,in press. (20)Baumgartner, E.; Furhop, J. H. Angew. Chem., Znt. Ed. Engl. 1980, 19, 550. (21)Day, D.; Hub, H. H.; Ringsdorf, H. Zsr. J. Chem. 1979, 18, 325. (22)Regen, S.L.; Czech, B.; Singh, A. J. Am. Chem. SOC.1980, 102, 6638. (23)Johnston, D. S.;Sanghera, S.; Pons, M.; Chapman, D. Biochim. Biphys. Acta 1980,602, 57. (24)Hub, H. H.; Hupfer, B.; Koch, H.; Ringsdorf, H. Angew. Chem., Int. Ed. Engl. 1980, 19, 938. (25)Bader, H.;Ringsdorf, H.; Skura, J. Angew. Chem., Int. Ed. Engl. 1981, 20, 91. (26)Akimoto, A,; Dom, K.; Groe, L.; Ringsdorf, H.; Schupp, H. Angew. Chem., Int. Ed. Engl. 1981,20, 90. (27)Gros, L.; Ringsdorf, H.; Schupp, H. Angew. Chem., Int. Ed. Engl. 1981, 20, 305. (28)OBrien, D. F.; Whitesides, T. H.; Klingbiel, R. T. J.Polymn. Sci., Polym. Lett. Ed. 1981, 19, 95. (29)Tundo, P.; Kippenberger, D. J.; Klahn, P. L.; Prieto, N. E.; Jao, T. C.; Fendler, J. H. J. Am. Chem. SOC.1982,104, 456. (30)Tundo, P.; Kurihara, K.; Kippenberger, D. J.; Politi, M.; Fendler, J . H. Angew. Chem., Int. Ed. Engl. 1982,21,81. (31)Tundo, P.; Kippenberger, D. J.; Politi, M.; Klahn, P. L.; Fendler, J. H. J.Am. Chem. SOC.1982, 104, 5352. (32)Kippenberger, D. J.; Rosenquist, K.; Odberg, L.; Tundo, P.; Fendler, J. H. J. Am. Chem. SOC.,in press. (33)Regen, S. L.; Singh, A.; Oehme, G.; Singh, M. J. Am. Chem. SOC. 1982,104, 791. (34)Folda, T.; Gros, H.; Ringsdorf, H. Makromol. Chem., Rapid Commun. 1982,3, 167. (35)Lopez, E.; OBrien, D. F.; Whiteside, T. H. J. Am. Chem. SOC. 1982, 104, 305.
0022-3654/83/2087-3777$0l .50/0 0 1983 American Chemical Society
3778
The Journal of Physical Chemistty, Vol. 87, No. 79, 7983
Kurihara et at.
Photoinduced charge separation has been demonstrated by using R ~ ( b p y ) ~asl +a sensitizer. Formation of RMV+. was followed by steady-state irradiation and electrontransfer mechanisms have been elucidated by nanosecond laser flash photolysis. Aggregate formation enhanced the photoinduced electron transfer and retarded the back-reaction.
Experimental Section Synthesis, purification, and characterization of RMV2+ have been described el~ewhere.~' Tris(2,2'-bipyridine)ruthenium chloride ( R ~ ( b p y ) , ~ + Stern), , methylviologen (MV2+,Sigma), and platinum oxide (Tridom) and Sephadex G-50 (Sigma) were used as received. RMV2+was dispersed in double-distilled water (1-2 mg/mL of HzO) by sonication at 80 O C using the microtip of a Braunsonic 1510 sonicator. Sonication turned out to be unnecessary since aggregates could also be formed by simple dissolution of RMV2+(see Results and Discussion). Weight-averaged molecular weights of the aggregates were determined by means of static light scattering using a Brice-Phoenix light scattering photometer.3s Refractive indexes of solutions were determined on a Brice-Phoenix differential refractometer Model 590. Absorption spectra were recorded on a Cary 118C spectrophotometer. Steady-state photolysis measurements were carried out by means of a 450-W Oriel Xe lamp using appropriate glass filters (350-500-nm band path). Nanosecond laser flash photolysis was carried out with a Quanta-Ray DCR Nd:YAG laser by using the second harmonic (532-nm)line, delivering approximately 8-10-ns pulses at 60 mJ per pulse. The details of the laser flash photolysis instrument were described e l ~ e w h e r e .Sam~~ ples, transferred in a 1.0-cm band-path fluorescence cell, were bubbled with Ar gas during measurements. Ru(bpy)? concentration was kept constant, 4.4 X 10" M for flash photolysis or 2.2 X M for steady-state photolysis, by additions of R ~ ( b p y ) , ~stock + solutions to RMV2+or MV2+solutions. EDTA-containingsolutions were similarly prepared. The pH of samples was adjusted to be 8.5 with a NaOH solution. Polymerization of RMV2+was carried out by exposing the samples to a 450-W Xe lamp for 6 h at room temperature. Results and Discussion Characterization of R M W Aggregates. Sonicated RMV2+solutions, unlike vesicles,g1odid not appear in the void volume on gel filtration using a Sephadex G-50 column. Weight-averaged molecular weights, M ,of RMV2+ aggregates, prepared by either sonication or simple dissolution, were determined by static light scattering. Figure 1 shows the data, plotted according to eq 1:39 KC/RB = l/M + BC (1) where C is the concentration of RMV2+(in g/mL), RBis the Rayleigh ratio, B is a term which contains the virial coefficients, and K at 8 = 90° is given by K = 2&~~(dn/dc)~/(NX~) (2) where n is the refractive index, N is Avogadro's number, X is the excitation wavelength (436 nm), and dn/dc is the (36) Paleos, C. M.; Evangelatos, G. P.; Evangelatos, G. P.; Dais, P. J.
Am. Chem. SOC.,in press. (37) Calvo-Perez, V.; Beddard, G. S.; Fendler, J. H. J. Phys. Chem. 1981,85, 2316.
(38) We are grateful to Professor J. Kratohvil for making available his instrument and for discussions on light scattering. (39) Kerker, M. 'The Scattering of Light and other Electromagnetic Radiation"; Academic Press: New York, 1969.
3'0
t
0
0.5 [RMV"]
1.0 x
1.5
lo3 (9/ml )
Figure 1. Plot of statlc light scattering data for RMV2+ in aqueous solution at 25.0 O C and 8 = 90' according to eq 1. A h = 395 nm
1 psec H
C A = 450 nm
c (
400 nsec
B
D
h = 395 nm
A = 610 nm
Flgure 2. Transient absorbances at 395 nm (A and B), bleaching at 450 nm (C), and luminescence at 610 nm (D) of an aqueous 4.5 X R~(bpy)~ solution ~+ contalning 1.27 X M RMV2+ at pH 8.5.
differential index of refraction (determined to be 0.2085 cm3/g). Taking n = 1.340 (refractive index of water at 25 "C) we calculated a value of 1.187 X for K. This, in combination with the data presented in Figure 1, leads to M = 3.97 X lo4. Dividing by the molecular weight of MRV2+leads to an average aggregation number of 70. These sized aggregates are commonly associated with micelles.1° The negative slope and curvature, shown in Figure 1, are indicative of a type of self-association in which the aggregate size increases with increasing monomer concentration. Laser Flash Photolysis of Ru(bpy)z+ in the Presence of R M P Aggregates. Excitation of an aqueous 4.4 X M solution of Ru(bpy)t+in the presence of RMV2+aggregates by 532-nm nanosecond high-energy laser pulses resulted in the bleaching of the ground-state absorbance of Ru(bpy)?+ at 450 nm, in the concomitant luminescence, and in the appearance of a transient absorbing at 395 and 600 nm (Figure 2). Following its formation, this transient decayed on the 100-c~~ time scale (Figure 2). These results are explicable in terms of a photosensitized electron transfer from R ~ ( b p y ) ~ to~RMV2+ + and a subsequent back-reaction between RMV+. and the oxidized Ru( b p ~ ) ~R~~ + ( b,p y ) , ~ +Kinetics . of these processes can be analyzed in a manner analogous to that developed for homogeneous solutions; excitation of R ~ ( b p y ) results ~~+ in the formation of a metal-to-ligand charge-transfer ex-
The Journal of Physical Chemistry, Vol. 87, No. 19, 1983 3779
Aspects of Artificial Photosynthesis
A
2 usec
4.3
I
li'AA = 0 . 0 0 5
1 0 0 p sec
2 usec
B
H
P
1
0.005
AA=
2
C
IhA=
sec IL
0.005
H 1 0 0 u sec
7 1 I 0.01 A =
1.0
2 psec
D
H
1
0
100 psec
1.0 133
[Mv2+l or 133 I P S . ~ V ~ + I81,
Flgure 4. Plots of Ru2+' lifetimes as functions of MV2+ or RMV2+ Concentrations in aqueous 4.4 X M Ru(bpyh2- solutions containing the indicated added solutes; [EDTA] = 0.01 M, [KCI] = 0.1 M.
11
AA = 0 . 0 3 5
Figure 3. Laser flash photolysis traces of the buildup and decay of transient absorbances at 395 nm of aqueous 4.4 X M Ru(bpy):+ M MV2+ (A), 1.74 X solutions at pH 8.5 Containing 1.56 X M RMV2+ (E), 1.54 X IO4 M RMV2+and 8.7 X lo3 M EDTA (C), and 1.74 X M RMV2+and 0.10 M KCI (D).
cited state (eq 3), followed by the formation of a cage complex and electron transfer within the cage (eq 4), and diffusion out of the cage prior (eq 5 ) and subsequent (eq 6) to back electron transfer. Back electron transfer can also occur subsequent to diffusion out of the cage (eq 7 ) : k
&Ru~+*
(3)
[Ru3+. .MV+.]
.
(4)
[R$+ ...MV+.] k", Ru3+ + MY+.
(5)
Ru2f
k-3
Ru2+* + MV2+
k4
k0
+ Ru2++ MV2+
[Ru~+*-.MV+.] -L Ru2+ MV2+
+
2.3
k7
(6)
Ru3+ MV+* (7) Differences in the behavior between R W + aggregates and MV2+in homogeneous solution are clearly seen by comparing the transient behavior of the radical cations formed from these species (Figure 3). Differences between these two systems can be treated more qualitatively by considering processes which deactivate Ru2+*: d[Ru2*]/dt = -(l/~)[Ru'+*] = -(k-3 + k4([MV2+]or [RMV2+]))[Ru2+*](8) where T is the luminescence lifetime of Ru2+*. Figure 4 shows plots of the reciprocal Ru2+*lifetimes against the concentrations of MV2+or RMV2+. Faster forward electron transfer to RMV2+ than to MV2+ is clearly seen. Values for k4 were calculated to be 3.16 x lo8 and 6.43 x lo8 M-l s-l for MV2+in the absence and in the presence of KCl or EDTA. Similarly, values of 6.16 X los and 1.5
x 109 M-' s-1 were obtained for k, in RMV2+aggregates in the absence and in the presence of KC1 or EDTA, respectively. The intercept of the plots in Figure 4 yielded a value of 6.2 X s for the natural lifetime of Ru*+*, in good agreement with that given in the literature (6.2 X 10-7 s).40 Rate constants for the forward electron transfer can also be compared by considering the concentrations of RMV+. and MV+. radicals produced in the laser flash. Figure 5 shows plots of reciprocal cation radical concentrations, = 3.9 X 104 M-' cm-l, for RMV+. calculated by taking or MV+. against the reciprocal concentrations of their parent dications according to 1 [MV+.] or [RMV'.]
-
1 k5/kG[Ru2+*]
(L +
k-3
k4[MV2+]or [RMV2+]
The intercept of these plots yielded values for k5/k6, which in combination with k3 (the natural lifetime of Ru2+*, calculated to be 6.2 X lO-'s, vide supra) provided values for k,. These k4 values differed from those calculated from the luminescence data since the bulk back-reaction (eq 7 ) had been neglected in deriving eq 9. Table I summarizes the various rate constants obtained by luminescence and flash photolysis. Differences between RMV2+aggregates and MV2+in homogeneous solutions are evident. Forward electron transfer (k4) as well as escape from the cage (k5)is more efficient in the RMV2+aggregates than those in the homogeneous solutions containing MV2+. The close proxim(40) Brugger, P. A.; Infelta, P. P.; Braun, A. M.; Gratzel, M.J.Am. Chem. SOC.1981,103,320. (41)DeLaive, P.J.; Giannotti, C.; Whitten, D. G. J.Am. Chem. SOC. 1978,100,7413.
The Journal of Physical Chemistty, Vol. 87,No. 19, 1983
3700
Kurihara et al.
T A B L E I: Calculated R a t e Constants for Photosensitized Electron-Transfer Processesa
MV2+ 3.16 X 10' 7.4 X 10'
k,,b M-' k,,C M-I s-'
k j / k , [ R u Z + *lo,' M k m . dM - ' s - '
RMVZ+
0.38 X 1.3 X 10"
M V Z ++ 0.01 M EDTA
6.16 X l o 8 2.3 x 109
6 . 4 3 X 10' 2.2 x 109
0.67
1.39 X
X
RMV2+ + 0.01M EDTA 1.49 x 109 1 . 3 5 x 109 1.39 x
R M V a ++ 0.10 M KCl 1.49 x 10-9 0.62 X 10"j
(0.81X 3.5 x 103 2.9 X 10'
k,,,? s - ' k 7 , i , g M" s-l
3.7 x 103
a See e q 3-7 for t h e description of t h e different reactions. Taking advantage of luminescence quenching, using e q 8. Taking advantage of transient absorption measurements ( a n d neglecting e q 7), using e q 9. See e q 10. e T a k e n f r o m ref 41. f See e q 7', calculated f r o m e q 10. g See e q 7 " , calculated f r o m eq 10.
0
-0.1
+2
-0.2
-
-0.3
-
I a -. -0.4 +> E a
--
Y
-0.5
-
-0.6
-
-0.7
-
C
L
/I
I
1
I
- 2
2+
I
I
2
1
lr 11-
13,
i-
1
I
1
I
I
100
200
300
400
I
Time, psec
h
,
F W e 5. Plots of l/[MV+.] or l/[RMV+.] against [MV2+] or [RMV"]. [R~(bpy),~+] = 4.4 X
1 0
M.
ity of viologens is expected to result in electron transfer from one RMV+. molecule to its neighbor (RMV2+).42 Enhanced forward electron transfer rates upon the addition of electrolytes (Table I) are explicable in terms of reducing the repulsive forces between R ~ ( b p y ) , ~and + RMV2+or MV2+. In homogeneous solution of Ru(bpy)g2+and MV2+,in the absence of additives, the MV+. formed is reoxidized (eq 7 ) by a second-order process governed by l/[MV+.]o - l/[MV+-] = -k,t (10) where [MV+.], represents the initial cation radical concentration. Plots of l/[MV+.] against time gave good straight lines (not shown) from which the value for It, was calculated. It is in good agreement with the literature value (Table I). Conversely, the decay of RMV+-is more com(42) Takuma, K.; Sakamoto, T.; Nagamura, T.; Matuso, T. J. Phys Chem. 1981,85, 619
Figure 6. Logarithmic decrease of [RMV'.] concentrations as a function of time (first-order plots) at 2.1 X (A), 4.6 X (B), 8.1 X lo-' (C), 1.3 X (D),and 1.74 X lo-, (E)M RMV2+ concentrations.
plex. Flash photolytic traces for the decay of RMV+. indicated a two-step process: one on a faster and one on a shorter time scale. Back electron transfer can occur on the surface of RMV+ aggregates (eq 7') or in the bulk solution (eq 7") subsequent to the diffusion of the oxidized sensitizer from the aggregate surface to the bulk (eq 11): b
R u , ~ ++ RMV+--Ru2+ RU?+
---f
+ RMV2+
Rub3+
+
Rub3+ RMV+. -% Ru2++ RMV2+
(7') (11)
(7")
where the subscripts s and b refer to the reaction sites (surface of aggregates and bulk). The observed two-step decay of RMV+. is explicable in terms of reaction 7' and 7". The fast decay, a first-order process, occurs on the surface of RMV2+aggregates and is governed by eq 12, d[RMV+.]/dt = -k,,[RMV+.]
(12)
The Journal of Physical Chemistry, Vol. 87,No. 79, 7983 3781
Aspects of Artificial Photosynthesis
2.0
f l A a
r
0.8
w
1
1.0
1 I
a 0.5
ri
I
0
G
395 nm
5
10
15
Time, min.
\ r-
I
n
x
16 min.
0 1
0
I
I
I
100
I
300
203
400
0.2
Time, u s e c Flgure 7. Plots of l/[RMV+.] against time (second-order plots) at 2. X lo4 (A), 4.6 X lo4 (B), 8.1 X (C), and 1.T4 X (D) I RMV+ concentrations.
0 400
500
600
700
Wavelength, nm
Flgure 9, Buildup of RMV'. in steady-state photolysis. Plots of absorbances against wavelength are shown at different irradiation times (450-W xenon lamp) of an aqueous 2.2 X M R~(bpy),~+solution, containing 1.1 X M RMV2+ and 3.5 X M EDTA at pH 8.5. The insert shows the buildup of absorbances as a function of time.
- 0.2 - 0.4
- 0.6 - 0.8 - 1.0
- 1.2 L
I
I
I
I
0
100
200
300
400
Time, psec Flgure 8. Logarithmic decrease of [RMV'.] concentrations as function of time in the resence of 0.10 M KCI at 2.0 X (4), 4.1 X l o 4 (B), 7.2 X 10- (C), 1.13 X (D), and 1.55 X (E) M RMV*+ concentrations.
P
which is followed by a slower second-order process occurring in the bulk solution and is governed by eq 13. 1/ [RMV+.]o - 1/ [RMV+.] = -kyt
(13)
Treatments of the data for the decrease of RMV+- con-
centration as functions of time in terms of first- and second-order processes are shown in Figures 6 and 7, respectively. Breaks in these plots are indicative of the multistep recombination. It is interesting to note that increasing the RMV2+concentration results in increasing proportions of the reaction following first-order kinetics (notice the increasing linearity of lines on going from A to E in Figure 6). Addition of 0.10 M KC1 has also increased the proportion of the reaction following first-order kinetics (Figure 8). Further, the rate of RMV+ decay in the presence of the 0.10 M electrolyte was found to be rapid (more than 60% of the RMV+. decayed within 300 ps) and independent of the concentration of RMV2+. These facts indicate that the majority of back-reaction occurs on the surface of the aggregates prior to the escape of R ~ ( b p y )to ~ +the bulk solution. The rate constant for k, in the presence of 0.10 M KC1 was found to be 3.7 X lo3 s-l. Bulk back-reaction is faster in homogeneous solution than that in the presence of surfactant aggregates (compare values of k, with k7,,in Table I) indicating, once again, the beneficial effect of organized assemblies in favoring charge separation. Steady-State Photolysis. Steady-state irradiation of 2.2 X M Ru(bpy),2+ in the presence of RMV2+aggregates M EDTA by a 450-W xenon lamp (using and 3.5 X a 350-600-nm band-path filter) resulted in the buildup of absorbances with maxima a t 395 and 600 nm (Figure 9). These absorbances correspond, of course, to the formation of RMV+. cation radical. The apparent rate of initial cation radical buildup is 2.9 X M/s. Interestingly, RMV+. dimer formation was observed in irradiated solutions of polymeric RMV2+(Figure 10) suggesting stronger interactions of the redox active surfactant head groups than that in their nonpolymeric counterparts. The ap-
3782
The Journal of Physical Chemistty, Vol. 87, No. 19, 1983
-
Kurihara et ai.
predominant site of forward electron transfer is, therefore, within the Stern layer of RM@+ aggregates. The linear increase of Ru(bpy)glL+* quenching rate with RMV2+concentrations (eq 4, Figure4) supports this postulate and is iscompatible with a mechanism which requires bulk solution to be the site of RMV2+reduction. The oxidized tris(2,2'-bipyridine)ruthenium cation (Ru3+)which escapes the cage complex (eq 5) is expelled from the potential field of R M F . Back electron transfer (eq 7) is retarded by electrostatic repulsions between the positively charged aggregates and Ru3+ The observed
" ' : @
IW
400
iw
IW
700
WAYELENCTH "m
Figure 10. Buildup of RMV'. in the photolysis (450-W lamp) of potymerized RMV~+.plots of absorbances against wavelength are shown at different times for a 2.2 X 10" M Ru(bpyh2+, 1.1 X lo4 M RMV2+, and 3.5 X M EDTA at pH 8.5. The insert shows the buildup of absorbances as a function of time.
parent initial rate of cation radical buildup in polymeric M/s. vesicles is 2.5 X Electron-Transferand Charge-SeparationMechanism. Presence of redox active surfactant aggregates, RMV2+, facilitates the forward electron transfer and retards the undesirable back-reaction. Experimental data support a mechanism in which the photosensitized electron transfer occurs on the charged aggregate surface: A priori one
would expect electrostatic repulsions to hinder the approach of R ~ ( b p y ) , ~to+ the surface of R M P aggregates. This expectation is quite realistic: most of the sensitizer is distributed in the bulk solution. A small amount of it is localized, however, within the potential field of RMV2+ aggregates. The hydrophobic pyridine moieties provide the driving force for this association. Conversely, aggregate formation of R M P is sufficiently strong to minimize the concentration of monomeric RMV2+present in bulk solution. The overall effect of distributions of Ru(bpy),2+ and RMV2+between the aggregates and bulk solution is such that greater concentrations of donor-acceptor pairs are present in the aggregates than that in the bulk. The
two-step kinetics support this contention. Effects of electrolytes on the forward reaction and the back-reaction are also in accord with the postulated mechanism. Addition of 0.1 M salt to RMV2+ (2.0 X 10*1.5 X lW3 M) aggregates results in considerable charge neutralization. Consequently, both the photosensitized reduction of RMV2+and the subsequent recombination are faciliated and occur on the screened surfaces of the aggregates. A different behavior has been reported previously for photosensitized electron transfer to micellarized viologen surfactants from R ~ ( b p y ) , ~ + , 4The ~ ? ~forward ~ electron transfer was considered to take place in the bulk solutions. Subsequent to reduction, the amphilitic methylviologen cation radical (a more hydrophobic species than its parent divalent cation) entered the micelle. Recombination between the oxidized sensitizer, located in bulk aqueous solution, and reduced electron acceptor, anchored into the micelle, was, once again, retarded by electron static repulsions. Differences between these two systems caution against generalizations and point out how finely organized assemblies can be tuned to different requirements in artificial photosynthesis.
Acknowledgment. Support of this work by the Department of Energy is gratefully acknowledged. Registry No. Ru(bpy)gz+,15158-62-0; RMV2+, 82797-85-1; RMV'., 86632-49-7; MV2+, 1910-42-5; EDTA, 60-00-4. (43) Infelta, P. P.; Brugger, P. A. Chem. Phys. Lett. 1981,82,462-8.
