Reductive Quenching of the Excited States of Ruthenium(II

Photoinduced Electron Transfer to Triplet Flavins. Correlation between the Volume Change-Normalized Entropic Term and the Marcus Reorganization Energy...
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J. Phys. Chem. 1994, 98, 11719-11726

11719

Reductive Quenching of the Excited States of Ruthenium(I1) Complexes Containing 2,2'-Bipyridine, 2,2'-Bipyrazine, and 2,2'-Bipyrimidine Ligands Hai Sun and Morton Z. Hoffman* Department of Chemistry, Boston University, Boston, Massachusetts 02215 Received: May 30, 1994; In Final Form: August 29, 1994@

The reductive quenching of the luminescent excited states of Ru(I1) complexes of the general formula Ru(bpy)3.,.,(bpm),(bpz)~+ (bpy = 2,2'-bipyridine, bpm = 2,2'- bipyrimidine, bpz = 2,2'-bipyrazine, m and z = 0,1,2,3 and m z I 3) by aromatic amines and methoxybenzenes as nonsacrificial electron donors and by C ~ 0 4 ~ EDTA, -, and TEOA as sacrificial donors in aqueous, acetonitrile, and propylene carbonate solution was examined by continuous and pulsed laser flash photolysis techniques. From kq, a value of E" for the irreversible oxidation of TEOA (-0.84 f 0.12 V vs NHE) in acetonitrile was obtained. Values of the cage escape yield of redox products (qce)showed weak or no dependencies on the driving forces of back electron transfer within the geminate redox pair in the solvent cage (AG:), suggesting that the simple model of competition between cage escape and back electron transfer may be inadequate to describe the results. A modification of the simple model, in which is introduced a kinetically important reorientation of the geminate redox pair, is proposed.

+

Introduction Much of the recent research on the synthesis and characterization of Ru(I1)-diimine complexes has been directed toward the development of electron-transfer photosensitizers that possess fine-tuned ground- and excited-state properties.' One group of complexes that has attracted a great deal of attention has been the ten compounds (RuL2+) with the general formula Ru(bpy)3.,.,(bpm),(bpz),2f (bpy = 2,2'-bipyridine, bpm = 2,2'bipyrimidine, bpz = 2,2'-bipyrazine, m and z = 0, 1, 2, 3 and m z 5 3); details of their photophysics, photochemistry, and redox chemistry, and the acid-base behavior of the groundand excited-states, have been widely explored.2-8

+

In comparison to R~(bpy)3~+, the presence of bpm and/or bpz ligands with their lower-lying acceptor orbitals renders the ground- and excited-state 3+/2+ and 2+/+ reduction potentials more p o s i t i ~ e , ~making ,~ the excited states more amenable toward reductive, rather than oxidative quenching. It has already been showng that the efficiency of release of charge-separated products into bulk solution (qce)upon the reductive quenching of *Ru(bpm)32+and *R~(bpz)3~+ by electron donors is quite high and is highest of all for sacrificial quenching. Inasmuch as one aim of the research on Ru(II) photosensitizers is to create systems that will have the most efficient electron transfer processes and provide the highest yields of charge-separated redox carriers for possible use in solar energy conversion schemes, it is appropriate to study in depth the behavior of this series of complexes toward reductive quenching under a range of solution medium and solvent conditions; the reduction of H20 and CO2 through interaction with Ru(bpz)3+ on colloidal metal surfaces has already been described.lo In this paper, we examine the reductive quenching of the luminescent excited states (*RuLZf) of members of this series of Ru(I1) complexes by electron donors (D), aromatic amines @Abstractpublished in Advance ACS Abstracts, October 1, 1994.

0022-365419412098-11719$04.50/0

and methoxybenzenes as nonsacrificial donors, and by oxalate ion (C20d2-; pKa of conjugate acids = 1.2,4.2), EDTA (pKa of conjugate acids = 0.0, 1.5, 2.0, 2.7, 6.1, 10.2), and triethanolamine (TEOA; pKa of conjugate acid = 7.8) as sacrificial donors. The one-electron-oxidizedforms (Do,') of the sacrificial donors convert irreversibly and rapidly into reducing radicals (Dr&, which are capable of transforming methylviologen (N,N '-dimethyl-4,4'-bipyridinium dication; MV2+) into M V + . The one-electron reduction of RuL2+ results in Ru(I1) species that contain a coordinated ligand radical (RuL+) capable of reducing MVZ+;the ligand that is perferentially reduced is the one possessing the lowest-lying acceptor orbital (bpz -= bpm -= bPY). The particular focus of this study was on the efficiency of cage separation of redox products into bulk solution (Q as a function of the energetics and solution medium parameters of the system. Of importance here is the fact that the homo- and heteroleptic complexes of bpy, bpm, and bpz have the same charge and geometry and would be expected to have approximately the same size when in the same redox or excited state.

Experimental Section The RuL2+ complexes, as their PF6- salts, were prepared by the methods of Rillema et a1.;2 henceforth, for simplicity, the ligand systems will be further abbreviated (bpy = y, bpm = m, bpz = z) unless more a explicit description is necessary. Methylviologen dichloride (Aldrich) was recrystallized three times from ethanol, and dried by suction and vacuum. TEOA (Fluka), N,N-dimethylaniline (Aldrich), and propylene carbonate (PC; Aldrich) were fractionally distilled three times. Acetonitrile (AN;Aldrich Optima) was used without further purification. All other materials were Aldrich reagent grade and were used without further purification. Distilled water was further purified by passage through a Millipore purification train. Aqueous solutions were buffered with 1-3 mM NaHCO3 or borate and adjusted with HC1 or NaOH to pH 8.5 for EDTA and pH 10.0 for TEOA; the C204*- solutions were unbuffered at their natural pH of -7.5. The ionic strength @) was adjusted with Na2S04. 0 1994 American Chemical Society

11720 J. Phys. Chem., Vol. 98, No. 45, 1994

Sun and Hoffman

TABLE 1: Values of E0(*2+/+) and k, for the Quenching of *RuLZ+in Aerated Aqueous Solutiona E0(*2+/+),b k,(TEOA),C &(EDTA)! &(C204*-), complex

zzz zzm Z"

ZZY

mzY mmm

V

M-I

1.68 1.63 1.43 1.44 1.33 1.34

2.5 x 1.0 x 4.2 x 4.0 x 7.6 x 2.1 x

s-I

lo8 lo8

107 107

lo6

107

M-I

4.5 x 1.7 x 5.9 x 5.1 x 8.2 x 3.1 x

s-l

lo8 lo8

M-I

TABLE 2: Values of E0(*2+/+), q,,and k, for the Quenching of *RuLZ+by TEOA in Acetonitrile and Propylene CarbonaW

s-l

Acetonitrile

1.6 x lo7 1.9 x lo6

lo6

zzz zzm mmz

107

ZZY

107 107

[RuL2+] = 50 pM, p = 1.0 M; A,, = 440-480 nm. From ref 5. At pH 10.0. At pH 8.5.

Luminescence quenching experiments were made on airsaturated solutions of at least four different concentrations of the donors with a Perkin-Elmer MPF-2A spectrofluorimeter; the excitation wavelength (Aex) corresponded to the absorption maximum in the 400-nm region of the individual complexes, and the emission wavelength (Acm) was likewise selected. Excited-state lifetime measurements were made on Ar-purged (TO) and air-saturated solutions (rod) with a Nd:YAG pulsed laser system (532-nm excitation) that has been described before. Continuous photolyses were performed on stirred solutions contained in a sealed 1-cm cuvette at the Aex for the individual complexes, using a Bausch & Lomb high-intensity monochromator in conjunction with a 150-W quartz halogen lamp and photon counter; ferrioxalate actinometry12 was performed at every wavelength used, and the application of Beer's law for each solution gave the value of the intensity of the absorbed light (la).All experiments were conducted at the ambient temperature (-20 OC).

Results Quenching Rate Constants. In aqueous solution, the excited states of six of the ten complexes are quenched by EDTA and TEOA; no quenching by up to 0.16 M EDTA or 0.4 M TEOA was observed for R~(bpy)2(bpz)~+, Ru(bpm)2(bpy)*+,Ru(bpy)2(bpm)2+, and R~(bpy)3~+. C ~ 0 4 ~(up - to 0.33 M) quenched only the excited states of R~(bpz)3~+ and Ru(bpz)2(bpm)*+.The values of k,, shown in Table 1 for p = 1.0 M, were calculated from linear Stem-Volmer plots (Id1 vs [D]) for the luminescence of *RuL2+and a knowledge of zoai'. Values of k, were reproducible to f5%. The excited states of all the complexes except R~(bpy)3~+ are quenched by TEOA in AN and PC; values of k, obtained from the linear Stem-Volmer plots in the presence of air are given in Table 2, which also contains values of zo in Ar-purged and air-saturated solutions. The values of for aromatic amines and methoxybenzenes in AN were evaluated by the same method and are shown in Table 3. Continuous Photolysis. Values of Qmv, the quantum yield of MV'+ formation, were determined from the continuous photolysis of solutions containing RuLZ+, MV2+, and a sacnficial quencher. In aqueous solution, EDTA (pH 8.5), TEOA (pH lO.O), and C ~ 0 4 ~(pH - 7.0) were used with maximum [D] of 0.16, 0.6, and 0.67 M, respectively; in AN and PC, only TEOA served as the quencher. [MV'+] was evaluated from its absorbance at 605 nm; €605 = 1.37 x lo4 M-' cm-' in H2O and 1.39 x lo4 M-l cm-I in the organic s01vents.l~ Plots of A605 as a function of irradiation time were linear and generally passed through the origin. Replicate experiments on solutions with the same concentrations showed that the values of Qmv were reproducible to f5%. In an earlier study,14we reported that Qmv for the R~(bpz)3~+(50 pM)A4V2+EDTA(0.10M) system in aqueous solution at pH 8-9 is 1.3; our repeat of the experiment at [EDTA] = 0.10

1.74 1.67 1.50 1.53 1.41 1.27 1.41 1.35 1.20

ZYY

mmm Y" mYY

0.80 1.2 1.2 1.8 0.90 0.46 0.099 0.20 0.083

0.58 0.72 0.56 0.85 0.45 0.28 0.086 0.14 0.067

9.9 x 3.3 x 1.4 x 8.6 x 2.7 x 4.7 x 7.1 x 2.2 x 9.1 x

108 108 lo* lo7

107 106 lo7 107 lo6

Propylene Carbonate complex zzz zzm

t o , ,us

to&,

,us

4,M-'

s-*

0.66 0.54 6.1 x lo8 1.1 0.76 2.6 x lo8 Z " 1.o 0.65 1.2 x 108 ZZY 1.1 0.70 1.0 x 108 0.76 0.51 4.1 x 107 ZYY 0.28 0.18 7.6 x lo6 mmm 0.14 0.12 5.8 x 107 0.13 1.6 x 107 Y" 0.15 0.047 1.0 x 107 mYY 0.061 [RuL2+]= 50 pM,A,, = 440-485 nm. E(*2+/+) = E1,2(2+/+) Em(*RuL2+) 0.24; Eln(2+/+) and Ew(*RuL*+)are from refs 2b and 2a, respectively; the value of Em(*RuLz+)of mmz is 16 500 cm-I (Rillema, D. P., private communication).

+

+

and 0.16 M gave a value of 1.4. We also determined Qmv at varying [MV2+] for solutions containing 11.0 pM R~(bpz)3~+ and 11.O mM EDTA at pH 8.5 and p = 0.087 M; under this condition, k, = 1.9 x lo9 M-' s-l. Qmv reaches a constant plateau of 1.2 when [MV2+]2 0.5 mM; that concentration range was used in all the quantum yield measurements in this study. Qmv was determined for the following systems in aqueous solution under these specified experimental conditions: (1) Ru( ~ ~ Z ) ~ ~ + / M V ~ + /asE aDfunction TA of ionic strength (0.00341.0 M) at pH 8.5; (2) Ru(bp~)3~+ and Ru(bp~)2(bpm)~+/MV~+/ TEOA at low ionic strength (0.0040 M) at pH 10.0; (3) R~(bpz)3~+/MV~+/C204~as a function of ionic strength (0.010-2.0 M); (4) RuL2+/MV2+/EDTAor TEOA as a function of quencher concentration. In addition, Qmv was determined for RuL2+/MV2+/TEOAsystems in PC. All these data are given in the supplementary material (see paragraph at end of paper). A value of Qmv = 0.30 was obtained for the R~(bpz)2(bpm)~+(50 pM)/MV2+(4 m M ) / C ~ 0 4 ~system at p = 1.0 M; k, = 1.8 x lo6 M-' s-l, with an efficiency of quenching of the excited state of 0.30. For AN solutions, plots of A605 as a function of irradiation time were linear and generally passed through the origin when [TEOA] 1 60 mM, but a nonlinear plot was obtained at [TEOA] = 15 mM; an example is shown in Figure 1. For that reason, only [TEOA] 5 60 mM were used in AN. Values of Qmv are given in the supplementary material for the five complexes that were studied in detail; in addition, Qmv = 1.6, 1.6, 1.7, and 1.7 for R~(bpz)3~+, Ru(bpz)~(bpm)~+, Ru(bpm)z(bpz)2+, and Ru(bp~)3(bpy)~+, respectively, at high (20.15 M) [TEOA]. Pulsed Laser Flash Photolysis. When excited at 532 nm, Ar-purged acetonitrile solutions of R~(bpz)3~+ showed absorbance changes that corresponded to the bleaching of the ground state and the formation of *R~(bpz)3~+, similar to those observed in aqueous solution; l5 the absorption decayed completely via first-order kinetics. When monitored at 490 nm, the absorbance of an Ar-purged AN solution containing 23 pM R ~ ( b p z ) 3 ~and + 9 mM TEOA

Reductive Quenching of Ru(I1) Complexes

J. Phys. Chem., Vol. 98, No. 45, I994 11721

TABLE 3: Rate Constants for the Quenching of *RuLZ+ by Aromatic Amines and Methoxybenzenes in Aerated Acetonitrile NJV-dimethylaniline (0.78 V)b 1,4-dimethoxybenzene(1.34 V)' NJV-diphenylaniline(0.83 V)c 1,2,4-trimethoxybenzene(1.12 V)' complex AGO, eVd ka, M-I AGO, eVd k,M-I s-l AGO, eVd k,,, M-' s-' AGO, eVd k.,, M-I s-l zzz 9.8 x 109 -0.16 1.3 x 109 -0.72 6.2 x 109 zzm -0.09 3.0 x lo8 -0.65 4.9 x 109 mmz 0.08 5.5 x 107 -0.48 4.5 x 109 0.05 4.6 x 107 -0.51 ZZY 4.4 x 109 0.17 3.1 107 -0.39 W Y 2.1 x 109 -0.25 ZYY --0.05 m" 0.17 1.9 x 107 -0.34 3.1 109 9.8 x 108 4.3 x 109 -0.39 3.9 x 109 2.0 x 109 0.01 -0.28 1.5 x lo8 -0.33 Y" -0.13 4.2 x los 1.4 x 109 0.16 6.7 x lo6 -0.18 mYY a

[RuLZ+]= 50 pM, de, = 440-485 nm. EII~(D+/D) vs SCE from ref 45. Eln(D+/D)vs SCE from ref 46. AGO = EI/z(D+/D)- E1/2(*2+/+).

TABLE 4: Pulsed Laser Flash Photolysis Results for the Quenching of *Ru(bpz)32+by TEOA in Acetonitrile

0.8-

0.6

[TEOAI, mM

AA49dA&43

4a

3.0 3.8 6.0 12.0

-0.36 -0.42 -0.49 -0.53

0.72 0.77 0.84 0.91

-

0.4a vce

@N

%ea

0.44 0.50

0.61

0.59 0.64

0.70 0.70

0.65

= @mIqq.

concentration of R~(bpz)3~+ in the absence of TEOA; the results are given in Table 4.

0.2-

Discussion 0

20

40

60

80

100

120

140

t, sec.

Figure 1. Plots of A605 as a function of irradiation time for Ru(bpz)32+(50 pM)/MV2+(2mM)lTEOA systems in AN at room temperature. For [TEOA] = 15 mM, qs = 0.90; for [TEOA] = 150 mM, qs = 0.99. The lines are computer-generated fits of eq 22, taking I, = 3.97 x einstein L-l s-l and qce= 0.77.

The reductive quenching of *R~(bpz)3~+ and *Ru(bpm)32+ by sacrificial donors has been extensively ~ t u d i e d .On ~ the basis of what is known about those systems, the general behavior of RuL2+ as photosensitizers upon the reductive quenching of their excited states by D can be described generically by reactions 1-12.

+

R ~ L + D,; 0.02-

'

2

4

6

+D

(4)

+ products

(6)

RUL~+

I +

Drei RuL2+ k,RuL' t Figure 2. Pulsed laser flash photolysis (d = 532 nm) of a sample containing 23 mM Ru(bpz)3*+and 9 mM TEOA in AN (monitored at 490 nm): 500 ns for each data point for the upper curve and 10 ns for each data point for the lower curve. Total number of points measured for each curve is 1000. The solid curve is a computer-generated firstorder kinetic fit. increased and reached a plateau (lower curve; Figure 2) instantaneously upon excitation. After about 15 ps, the absorbance started to rise via fist-order kinetics (k = 3.3 x lo4 sT1) and reached a second plateau (top curve; Figure 2 ) ; AA490 up to 10 ps is about one-half that at 500 ps. Values of A A 4 9 d A A 4 3 were measured for Ar-purged AN solutions containing 35 pM Ru(bpz)s2+ and varying [TEOA], where AA490 is the absorbance increase at 490 nm immediately after the laser pulses due to the formation of Ru(bpz)3+ and A A 4 3 is the differential absorbance at 443 nm extrapolated to the midpoint of the flash for a solution containing the same

+

R ~ L + H+ 5 R ~ L H ~ +

+ + MV*+ %' RULH~+ +MV~+ R ~ L+ ~ MV*+ + + H+ h' Drei+ MV2+ - MV" + products R ~ L + MV~+ k..R U L ~ +

(7)

(8) (9) (10)

The generation of the lowest-energy luminescent excited state with an efficiency of p via reaction 1 is followed by quenching reaction 3, in competition with the natural decay of *RuL2+

11722 J. Phys. Chem., Vol. 98, No. 45, I994

Sun and Hoffman

(reaction 2), forming RuL+ and Do; in bulk solution. Electron transfer reaction 4 between the strong oxidant and the strong reductant would annihilate the redox pair were it not for the irreversible transformation of Do; into D& (reaction 5) in the case of the sacrificial donors. In the absence of MV2+, Dr& can reduce RuL2+ via reaction 6 to produce a second equivalent of the reduced photosensitizer. In the presence of MV2+ ( E d o = -0.44 V vs NHE), the formation of MV'+ occurs via reactions 8-10. Reaction 7 represents the protonation of RuL+; the position of this equilibrium would, of course, be dependent on pH. In nonsacrificial systems, where reactions 5 , 6, and 10 are not operative, reaction 11, in competition with reaction 4, would destroy the MV+. The efficiency of the quenching reaction, qs, is the result of the competition between reactions 2 and 3 and is expressed by eq 12. The quenching reaction is conventionally viewed as

occurring via the formation of a geminate redox pair within the solvent cage as a result of the transfer of one electron (reaction 13), with qce,the efficiency of cage escape of the redox pair, being a measure of the competition between back electron transfer (reaction 14) and diffusional escape of the redox products (reaction 15) into the bulk solution.16 From this model, eq 16 is easily derived; the experimental determination of qce and the application of the equations that describe kce permit the dependence of the energetics of electron transfer and solution medium on kbt to be evaluated.

*M

+ D - [M- ...D"]

[M- ...D"]

(13)

kbl M + D

(14)

+ D"

(15)

[M- ...Do+]k,,M-

For the sacrificial donors, transformation reaction 5 , in which the reducing radicals are formed, is rapid and irreversible.

-

c20,'- k; co;++ co, EDTA,,.

k,".EDTq,,'

+ TEOA -TEOq,,' + TEOA + H+

(5a) (5b)

kw"'

TEOA,,"

(5c)

The one-electron oxidation of C Z O ~ produces ~a radical (C,O4*-), which undergoes very rapid unimolecular decarboxylation, probably within the solvent cage, transforming into the strongly reducing COZ- radical.17 For EDTA in alkaline solution, reaction 5b is believed to occur unimolecularly, probably within the solvent cage, by proton elimination from the carbon atom a to the amine and carboxylate groups.'* For TEOA, transformation occurs via bimolecular reaction 5c in bulk solution, for which k,"' is reported to be 3.3 x lo6 M-' s-' at pH 7-9.19 The values of &k and & are of the order of 108-109 M-' s-1?,5,9,18,20 making the scavenging of all the reductive species rapid and quantitative in the presence of millimolar concentrations of MV2+. The efficiency of transformation of Do; to Dd*(qW)reflects the competition of reaction 5 in competition with annihilation reactions 4 and 11 (eq 17).

qtr= kJ(k,

+ k,,[RuL+] + k,t'[MV'+])

(17)

From the general mechanism, it is clear that the value of Qmv in the RuL2+/MV2+/Dsystem will be a complex function of the nature of D, the concentration of the solutes, the pH, and the rate constants of the various reactions. Under continuous photolysis, Qmv would be expected to range from zero in nonsacrificial systems to potentially significant values in sacrificial systems. The relationship between Qmv and the efficiencies of the various reactions under the specified experimental conditions is given by eq 18. @mv

-

= 2V*VqVceVtr

(18)

It is known that q* 1 for R~(bpy)3,+.~'On the basis of the similarities of their excited-state properties, it is reasonable to assume that q* 1 for all these tris and mixed ligand Ru(I1) complexes; in fact, a unitary value of q* has been used earlier in studies of the photophysica12,22 and photochemicalgproperties of these complexes. For C ~ 0 4 ~ -EDTA, , and TEOA at sufficiently high concentrations, qW 1. In the presence of sufficient MV2+, the conversion of all the reducing equivalents to MV'+ is quantitative. Under those conditions of continuous photolysis, Qmv = 217ceqq. When total quenching is achieved, qq = 1 and Qmv has reached its limiting value (Qh). Generation of *Ru(bpz)J*+. The differential absorption spectrum obtained upon the excitation of Ru(bpz)3,+ in AN is similar to that of *Ru(bpz)3,+ observed in aqueous s o l ~ t i o n ~ ~ ~ ~ ~ and AN-H20 (1:l by volume).25 By using *Ru(bpy)3,+ as the actinometric standard with A6450 = -1.0 x lo4 M-' cm-1,26 we obtained values of A6443 for *R~(bpz)3~+ at various pulse energies with an average of -1.2 x lo4 M-' cm-' for both aqueous and AN solutions. Kalyana~undaram'~reported a value of A6 = -5.7 x lo3 M-I cm-' at 440 nm for *Ru(bpz)3,+ in aqueous solution, which was evaluated by monitoring AA as a function of laser intensity toward the total conversion of the ground state to the MLCT state; this value differs considerably from ours. Ohno et aLZ4 reported that A€ = -1.2 x lo4 M-' cm-' at 443 nm in ANH2O (9:l v/v), obtained via energy transfer from *Ru(bpz)3,+ to anthracene by monitoring the formation of the wellcharacterized anthracene triplet excited state; this value is in excellent agreement with the one obtained here. It is likely that the low value of -A€ reported by Kalyana~undaram'~ was due to an error in the determination of the extent of conversion achieved; incomplete conversion results in less negative values of A€. The absorption spectra of the excited states of the ten complexes have recently been p~blished.,~ Generation of Ru(bpz)3+. The fist equivalent of Ru(bpz)3+, generated upon the quenching of *R~(bpz)3~+ by TEOA in the absence of MV2+ and monitored at 490 nm, is described by the formation curve in the shorter time frame of Figure 2; transformation reaction 5c is followed by reaction 6 with the second equivalent of Ru(bpz)3+ generated via fist-order kinetics in the longer time frame. Since E490(RU2+) = 2.0 x lo3 M-' cm-l, A6490 = ~490(Ru+)- c490(Ru2+)= 9.9 x lo3 M-' cm-'; the concentration of the first equivalent of redox products can be estimated from AA = 0.035 to be [Ru(bpz)3+1= [TEO&;+I = 3.5 pM. The concentration of TEOA,d' (3.5 pM),generated through reaction 5c, is much smaller than that of R ~ ( b p z ) 3 ~ + (20 pM); therefore, reaction 6 is a pseudo first-order reaction ( k = 3.3 x lo4 s-'). The value of k, for TEOA in AN can be calculated from w[R~(bpz)3~+] to be 1.7 x lo9 M-' s-'. However, when reaction 6 is much faster than reaction 5c at higher [R~(bpz)3~+] and lower [TEOA], the rate of the formation

-

-

J. Phys. Chem., Vol. 98, No. 45, 1994 11723

Reductive Quenching of Ru(I1) Complexes of the second equivalent of Ru(bpz)3+ is dictated by reaction 5c. In such a case we find k,"' = 3.7 x lo6 M-' s-' , which agrees very well with its reported value in aqueous ~olution.'~ Inasmuch as the initial quantum yield of formation of Ru(bpZ)3+ (aru) is equal to rsrce, the A A 4 9 d M 4 4 3 data can easily be converted into rcevalues, which are given in Table 4. Quenching of *RuL*+. The kinetic processes for reductive quenching can be described by scheme 19, where kd and k-d are the diffusional encounter and dissociation rate constants for the initial reactants, respectively. According to this scheme the quenching rate constant is given by eq 20.

+ A * R ~ L ~ + . .. D R ~ L... +D*+

*R~L~+

k-d

k l

(19)

TABLE 5: Values of AGO for *RuL2+-TEOA and RUL+-TEOA,'~ Redox Pairs complex AG,,", eVa AGto,eVb zzz -0.90 -1.28 ~

zzm "z

ZZY ZYY m"

Y" mYY

-0.83 -0.66 -0.69 -0.57 -0.36 -0.57 -0.5 1 -0.36

-1.35 -1.39 - 1.39 -1.43 -1.51 -1.51

- 1.55 -1.62

a For *RuL2+-TEOA couples: AG," = E"(TEO&,,'+/TEOA) Eo(*2+/+); Eo(TEOA,,*+/TEOA) = 0.84 V; E"(*2+/+) is from Table 2. For RuL+-TEOAOxcouples: AG? = Eo(2+/+) - Eo(TEO&,,'+/ TEOA); Eo(2+/+) is from ref 2b.

is the same. By using the RW and MAL equations and applying the parameters obtained for the aromatic amines and methoxybenzenes to the TEOA system, the values of AG*(O) and E"(TEO&,,'+/ TEOA) (AGO = E"(TEO&,'+mOA) - Eo(*2+/+)) can be estimated. The values of log k, vs E0(*2+/ +) for TEOA are shown in the supplementary material with the best fits for the following parameters: AG*(O) = 38 and 41 kJ mol-' and Eo(TEOA,,'+ITEOA) = 0.72 and 0.96 V with the RW and MAL treatments, respectively; the two treatments generated nearly identical curves. Thus, the average values of AG*(O)and EO(TEO&;+mOA) are 40 f 2 kJ mol-' and 0.84 f 0.12 V, respectively. The intrinsic barrier of the overall electron-transfer reaction, AG*(O),can be treated as the average of the two intrinsic barriers for the self-exchange reactions of the donor and the acceptor. AG*(O) is composed of two terms: AG*(O), for the inner coordination sphere contribution and AG*(O), for the outer solvation shell contribution. As was done for the equivalent *RuL2+/RuL3+couple,34it can be assumed that the inner term for the *RuL2+/RuL+couple is probably very small because the distortions are spread over many bonds. As a consequence, the value of AG*(O) associated with the *RuL2+/RuL+ couple is mainly due to the solvent reorganizational energy; for the equivalent *RuL2+/RuL3+couple, AG*(O) = 18 kJ By using this value for the *RuL2+/RuL+couple, AG*(O) = 18 kJ mol-' for aromatic amines and methoxybenzenes and 62 kJ mol-' for the TEO&,'+/TEOA couple were calculated. The much higher value of AG*(O) for TEOA,,'+/TEOA, compared to those of the aromatic amines and methoxybenzenes, results from large contributions from both the outer- and inner-sphere reorganizational energies. As was pointed out by Ballardini et al.,30 the greater electron localization in the case of aliphatic amines results in a more extended geometrical change upon oxidation than is the case for the aromatic amines and methoxybenzenes and contributes to a larger value of AGt(O), for TEOA. On the other hand, the greater electron localization for TEOA promotes stronger interactions with a polar solvent, which also contributes to a value of AG*(O), that is larger than that of the aromatic amines and methoxybenzenes. Because of the rapid transformation reaction that the TEO&,,'+ radical undergoes, its reduction potential cannot be obtained directly by cyclic voltammetric measurements. The value of E"(TEOA,,'+/TEOA) = 0.84 f 0.12 V obtained here is less positive than the value of 1.14 V (0.90 V vs SCE) reported by Tazuke et al.36 in AN, but it is very similar to the value of 0.82 V reported by Kalyanasundaram et al.37 in aqueous solution. The value of 0.84 V in AN was used to calculate AGO for the *RuL2+-TEOA and RuL+-TEO&,*+ redox pairs; the results are given in Table 5 . Similar data-fitting techniques have been used to estimate the reduction potentials and the

KV,

The classical expression for kt (=Kv,, exp(AG*/RT))can now be substituted into eq 20. AG* is a function of AGO, with the relationship expressed by two similar equations: the RehmWeller (RW)27 and the Marcus-Agmon-Levine (MAL)28*29 equations, in which resides AG*(O), the intrinsic barrier for electron transfer. By using best-fitting computer procedures on plots of log k, vs AGO, it is possible to evaluate AG$(O) and the potentials for ground- and excited-state reaction^.'^^^^^^^^^^ Because the Ru(II) complexes used in this study have the same or very similar size, shape, charge, and electronic structure, it can be reasonably assumed that kd, k-d, K , vn, and AG*(O) have constant values when quenchers with similar properties are used. In such a case of an homologous series, k, is only a function of the free energy change of the quenching reaction. The plot of log kq vs AGO for the quenching of *RuL2+ by aromatic amines and methoxybenzenes in AN is shown in the supplementary material. The best fit of the data allows, in principle, the values of kd, k-d/Kv,,, and AG$(O)to be evaluated. However, we could not achieve a satisfactory fit for the three adjustable parameters in the rather complex equations. Now, the values of log k, appr0ach.a limiting value where k, = kd as AGO becomes more negative. Indeed, other authors have reported that the value of k, on the diffusional plateau is about 1 x 1Olo M-' s-' for the reductive quenching of R~(bpy)3~+ by aromatic amines and m e t h o x y b e n ~ e n e sand ~ ~the ~ ~oxidative ~ quenching of Ru(bpy)32f by neutral organic acceptors in AN.33 Therefore, kd was taken here as that value. As a result, the best fit to the data was made with the following parameters: k-d/KVn = 0.032 and AG$(O) = 20 kJ mol-' by using the RW treatment; k-d/Kvn = 0.20 and AG*(O) = 16 kJ mol-' by using the MAL equation. The curves generated by the two treatments (supplementary material) are very similar, but the deviation in the values of k-&vn is quite large; however, the value of dG*(O) (18 kJ mol-') does not show much variation. This is understandable inasmuch as the k-d/Kv, term is much more sensitive to the data-fitting than is AG*(O), which is in the exponential term in the equations. As a consequence, the values of AG*(O) should be more reliable and accurate than are those of k-d/KvUn. A similar analysis can be performed with the k, data for TEOA in AN. Since the size of TEOA is very similar to the aromatic amines and methoxybenzenes, it can be reasonably assumed that kd and k-d for TEOA in the same solvent are the same as those other quenchers. Also, since the atoms (nitrogen, for example, in TEOA and aromatic amines) and interacting orbitals involved in the electron transfer are similar, the frequency of nuclear vibration and electronic coupling can be assumed to be similar. Hence, it can be assumed as well that

~~

11724 J. Phys. Chem., Vol. 98,No. 45,1994

Sun and Hoffman

reorganization energies for the excited states of naphtholate type anions upon quenching by organic acceptors31aand for thymine derivatives upon their oxidative quenching of organic photosensitizer~.~~~ It has been d e m o n ~ t r a t e dthat ~ ~the values of E” for R u L ~ + / ~ + and MV2+’+ in 4:l H20/AN mixed solvent are more negative than those in neat AN by 0.23 V, a quantity that represents the junction potentials between the solvent systems. However, the values of AGO for RuL3+-MV’+ redox pairs in the mixed solvent are essentially the same as those in neat AN. By analogy, it can be assumed that the values of AGO for the *RuL2+-TEOA and RuL+-TEO&,,’+ redox pairs are the same in H20 and PC as in AN (Table 5). Plots of log k, vs AGO for TEOA in AN, H20, and PC are shown in the supplementary material; similar data for the EDTA system in H20 are also shown for comparison. The values of AGO are not negative enough for k, to reach the diffusion limit, while they are not positive enough for k, to be completely activation controlled; k, is controlled by both the diffusion of the two reactants and the activation of the electron transfer. The similarity of the values of k, for EDTA to those for TEOA in H20 suggests that the values of EO(EDT&,,’/EDTA) and AG$(O) at pH 8.5 may be very similar to those of TEOA at pH 10.0. Quantum Yields. Because the values of Qmv depend on the extent to which *RuL2+ is quenched (v,), a comparison of 7;lce for the yields of MV’+ among the complexes is only meaningful when vs = 1 and Qmv = Qlim. However, that condition often cannot be reached experimentally due to limitations in k,, the solubility of D, and the control of ionic strength. Nevertheless, Qmv is a linear function of with a slope = a b m ; indeed, plots of Qmv vs v, were linear for all the systems studied. Values of Qlim and vce obtained from those plots are given in the supplementary material. The values of vcefor R~(bpy)2(bpm)~+ in both AN and PC are very small compared to those for the other complexes. Since t and k, are also small for R~(bpy)2(bpm)~+, the values of v c e that were measured are less reliable than for the other complexes and are not discussed further. The nonlinear plot of A605 vs irradiation time (Figure 1) observed for 15 mM TEOA in AN is due to the less-than-unitary value of vtr at low [TEOA]. For TEOA, v, is given by eq 21. At sufficiently high [TEOA], k,”’[TEOA] >> ket[RuL+] ke([MV’+], and vu 1. However, as [MV’+] builds up upon continuous photolysis, reaction 11 will dominate reactions 4 and 5c, causing vu to become less than unity.

v,

+

-

ktr”’[TEOA] rtr

=

k/[TEOA]

+ k,,[RuL+] + ke{[MV*+]

(21)

Equations 18 and 21 can be combined, taking tp = 1. Inasmuch as Qmv = [MV+]/Zat, where laand t are the rate of light absorption and irradiation time, respectively, the concentration of MV’+ formed as a function of t can be described by eq 22. By taking the value of k,”’ to be 3.7 x lo6 M-’ s-l in CH3CN,36 the best fit of the data in Figure 1 was made with k{ = 1.9 x lo8 M-’ s-l.

0.2

-

0.0

-

A

2 \

g -Oe23 -0.4f - O d I

-1.6

I

I

-1.5

,:%A

I

I

-1.3

-1.4

-1.2

eV

Figure 3. Dependence of log kb&, on AG? for C Z O ~ ~EDTA, -, and TEOA in aqueous solution. LO+

“4L--A 0 acetonilrile

0 propylene U I ~ O M I ~

0.0

-1.6

-1.5

-1.4

-1.3

-1.2

AG/, eV Figure 4. Dependence of qceon A%? for TEOA in AN and PC.

dependence, with a plateau for the more easily reduced complexes (less negative AG?) and somewhat increasing values of qccas the reducing strength of RuL+ increases (more negative AGto). Although there are only two points, the behavior of God2- appears to be very similar to that of EDTA, with increasing values of vceas the strength of the RuL+ reductant increases in the region of less negative A&:. Equation 16 can be rearranged into eq 23; because of the logarithmic dependence of rate constants and free energy terms, it is customary to plot lOg(vce-l - 1) vs AGIO. Figure 3 shows such a plot for God2-, EDTA, and TEOA in aqueous solution. The EDTA system exhibits a well-defined “bell-shaped” curve, suggesting that kbt traverses the normal and inverted Marcus regions based on the conventional solvent cage model of reactions 13-15. On the other hand, the TEOA system appears to exhibit somewhat different behavior. A reliable assessment cannot be made for the C2Od2- system due to the limited number of data points. -1

vce

- = kbdkce

(23)

The values of 7;lce for the TEOA system in AN and PC as a function of AGIO are shown in Figure 4. There is no evident [MV“] = dependence in PC and a very weak, if any, dependence in AN, -k,”’[TEOA] + {(k,”’[TEOA])2 8~ce~~ak~~k,”’[TEOA]t}1’2indicating that kbt is virtually independent of driving force, according to the conventional model. The bell-shaped curve %,‘ observed for the EDTA system in aqueous solution is qualita(22) tively in agreement with this model, although the curve occurs Cage Escape Efficiencies. The values of vcefor EDTA in over a rather narrow energy range of less than 250 mV. aqueous solution as a function of AGt0 clearly describe a Certainly, the apparent difference between the EDTA and TEOA U-shaped dependence; the data for TEOA show a much weaker systems in aqueous solution requires a further explanation.

+

J. Phys. Chem., Vol. 98, No. 45, 1994 11725

Reductive Quenching of Ru(I1) Complexes

I reorientation

ground yalc

diffuse apart out of the cage. Unless kbt were anomalously small, one would expect vceto be much smaller; the same analysis also applies to the C2042- system. It had been proposed,20ain connection with the photoexcitation of MV2+/ EDTA ion pairs, that EDTA undergoes its unimolecular transformation within the solvent cage. The results here clearly show that the values of vce for EDTA and C*04*- are independent of ionic strength. Because heis dependent on ionic strength for charged species, the lack of an ionic strength dependence strongly suggests that the process competing with back electron transfer and leading to cage escape for EDTA and C ~ o 4 ~ systems is not diffusional and that the unimolecular irreversible transformation within the solvent cage (reactions 5a and 5b) is the dominant process. If this is true, vcefor EDTA and C2Od2- systems should be expressed by eq 24, and the plots in Figure 3 should represent, for EDTA and C204*-, the dependence of log kbt/kt, on the reaction driving force.

Figure 5. Cartoon of the reductive quenching of 'RuL2+ by TEOA.

The Marcus quadratic equation16predicts that the bell-shaped curve becomes narrower as the reorganization energy becomes In the case of TEOA, vceat p = 0.004 M is higher than at 1 smaller and the maximum of the curve moves to less negative M, which is consistent with the fact that k,, is larger at lower AGto. In fact, it has been demonstrated e~perimentally~~ that ionic strength for like-charged species and that diffusional cage the driving force dependence of the back electron transfer rate escape must occur before reaction 5c takes place. is much stronger (but the solvent reorganization energy is much Modification of the Cage Model. The lack of an energy smaller) for contact ion paired donor-acceptor systems than gap dependence of vcefor TEOA in AN and PC suggests that for the solvent-separated donor-acceptor system that involves the description of the events in the solvent cage by reactions electron transfer between geminate radical pairs of anthracenes 13-15 may be inadequate. In order to account for these and and substituted benzenes. Understandably, if the molecular other observations in which vceis a weak or negligible function dimensions and the charges on the reactants are the same, the of A G I , ~ ~ ,we ~ ~proposed43 ,~~ a modified model in which a rateinteraction between the reactants and solvent molecules is determining electronic or geometrical reorientation process of stronger for solvent-separated ion pairs than for contact ion pairs, the redox pair within the solvent cage is operative; if the resulting in a higher solvent reorganization energy upon electron reorientation step is slower than electron transfer between the transfer due to extended exposure of the reactants to solvent initial geminate radical pair, it will dictate the overall rate of molecules. back electron transfer. There is no doubt that the ground and excited states of the The concept can be described as follows. The approach of photosensitizers are ion paired in the presence of the relatively the electron-rich reductive quencher toward the excited complex, high concentrations of EDTA needed to effect quenching. In which has an electron localized on one of the ligands, is an earlier work,@the existence of R U ( ~ ~ Y ) ~ ~ + - M V ~ + - E D T Avisualized as occurring with the reduced ligand and the electron aggregates was suggested in order to explain the dependence donor as far from each other as possible; that is, the quencher of vceon [Ru(bpy)3*+]in the quenching of the excited state by would preferentially approach the excited complex on the side MV*+ in the presence of EDTA. The electron density redisopposite to the reduced ligand so as to effect the transfer of an tribution upon electron transfer in the highly charged ion paired electron into the t2g acceptor orbital of the metal center, which EDTA system may require much less reorientation of the solvent is oriented toward the interligand pockets. On the other hand, and/or ionic atmosphere than in those systems with a neutral back electron transfer would most efficiently occur between the molecule, such as TEOA, as one of the reactants or products. one-electron-occupied x* orbital of the reduced ligand and the As a consequence, the reorganization energy for the EDTA orbitals of the oxidized quencher. However, as the result of system may be small and, therefore, the bell-shaped curve very the quenching step, these orbitals may be well separated and narrow. This analysis requires that EO(EDTA,,'/EDTA) be less could be in a sterically unfavorable position for rapid transfer positive than that estimated for TEOA (0.84 V), so that the bellto occur. The random reorientation of the geminate pair would shaped curve would occur in a less negative AGt0region than lead to favorable electronic overlap (or coupling) between the that shown in Figure 3. Unfortunately, a quantitative analysis donor and acceptor orbitals. If the steric and/or orbital cannot be made due to the limited number of data points, the reorientation were slower than the subsequent rapid back variation of Eo(RuL2+'+), and the uncertainty about the value electron transfer, the overall rate of that process would be of E"(EDTA,,'/EDTA). established by the reorientation and would be independent of TEOA is an analogue to the neutral amines used in previous the energy gap of the reaction. In that case, vcewould be a s t ~ d i e s ~and ~ , ~might ' be expected to describe a bell-shaped measure of the competition between cage escape and reorientacurve for a similar back-electron-transfer reaction. The weak tion, as described by eq 25, where k, is the rate constant for or negligible dependencies for TEOA shown in Figures 3 and the reorientation process. A cartoon of the possible quenching and back electron transfer processes for TEOA is given in Figure 4 may be a portion of such a curve near or at the maximum. The much higher values of rcein AN than in PC and aqueous 5. solutions reflect the fact that k,, is significantly higher in AN than in the other two solvents. Finally, regarding the values of vcefor EDTA, it was noted In aqueous solution the situation is much more complicated, earlier9 that they were surprisingly large, considering the especially for ionic species, due to the structure and the high opposite charges on the RuL+ and EDTA'*- species that must

11726 J. Phys. Chem., Vol. 98, No. 45, 1994 dielectric constant of the solvent. Recent resultsu have shown that quenching and cage escape behavior are very sensitive to the existence of ion pairing, especially the size and charge of the added electrolyte ions. The impact of these factors on the values of qceand their dependencies on AGtO are currently being explored.

Acknowledgment. This research was supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, US.Department of Energy. Supplementary Material Available: Tables of amy as a function of quencher concentration, ionic strength, and solution medium; tables of @” and qce;plots of log vs AGO for the systems examined (12 pages). Ordering information is given on any current masthead page. References and Notes (1) (a) Juris, A.; Barigelletti, F.; Campagna, S.; Balzani, V.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988,84,85. (b) Balzani, V.; Ballardini, R. Photochem. Photobiol. 1990,52,409. (c) Kalyanasundaram, K. Photochemistrv of Polvovridine and Pomhvrin Comolexes: Academic * , Press: New York, 1592. (2) (a) Allen, G. H.: White, R. P.: Rillema. D. P.; Mever. T. J. J. Am. Chem. SOC. 1984,106,2613. (b) Ross, H. B.; Boldaji, M.{Rillema, D. P.; Blanton, C. B.; White, R. P. Inorg. Chem. 1989,28,1013. (3) Sun, H.; Neshvad, G.; Hoffman, M. Z. Mol. Cryst. Liq. Cryst. 1991, 194,141. (4) Venturi, M.; Mulazzani, Q.G.; D’Angelantonio, M.; Ciano, M.; Hoffman, M. Z. Radiar. Phys. Chem. 1991,37, 449. (5) D’Angelantonio, M.; Mulazzani, Q. G.; Venturi, M.; Ciano, M.; Hoffman, M. Z. J. Phys. Chem. 1991,95,5121. (6) Sun, H.; Hoffman, M. Z. J. Phys. Chem. 1993,97,5014. (7) Sun, H.; Hoffman, M. Z. J. Phys. Chem. 1993,97, 11956. (8) Mulazzani, Q.G.; Sun, H.; Hoffman, M. Z.; Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994,98, 1145. (9) (a) Neshvad, G.; Hoffman, M. Z. J. Phys.Chem. 1989,93, 2445. (b) Neshvad, G.; Hoffman, M. Z.; Mulazzani, Q.G.; Venturi, M.; Ciano, M.; D’Angelantonio, M. J. Phys. Chem. 1989,93, 6080. (10) (a) Maidan, R.; Willner, I. J. Am. Chem. SOC. 1986,108,8100. (b) Willner, I.; Maidan, R.; Mandler, D.; Dum, H.; Dorr, G.; Wngerle, K. J. Am. Chem. SOC. 1987,109,6080. (11) (a) Malba, V.; Jones, G., Q Poliakoff, E. Photochem. Photobiol. 1985,42,451. (b) Jones, G., II; Oh, C. J. Phys. Chem. 1994,98,2367. (12) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, 1966; p 783. (13) Watanabe, T.; Honda, K. J. Phys. Chem. 1982,86,2617. (14) Prasad, D. R.; Hessler, D.; Hoffman, M. 2.; Serpone, N. Chem. Phys. Lett. 1985,121,61. (15) Kalyanasundaram, K. J. Phys. Chem. 1986,90,2285. (16) Balzani, V.; Scandola, F. Energy Resorces Through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic Press: New York, 1983; pp

,.

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(17) Prasad, D. R.; Hoffman, M. Z.; Mulazzani, Q.G.; Rodgers, M. A. J. J. Am. Chem. SOC. 1986,108, 5135. (18) Mulazzani, Q.G.; Venturi, M.; Hoffman, M. Z. J. Phys. Chem. 1985,89,722.

Sun and Hoffman (19) Chan, S.-F.; Chou, M.; Creutz, C.; Matsubara, T.; Sutin, N. J. Am. Chem. SOC. 1981,103,369. (20) (a) Prasad, D. R.; Hoffman, M. Z. J. Am. Chem. Chem. 1986,108, 2568. (b) Venturi, M.; Mulazzani, Q. G.; Ciano, M.; Hoffman, M. Z. Inorg. Chem. 1986,25,4493. (21) (a) Demas, J. N.; Taylor, D. G. Inorg. Chem. 1979,18,3177. (b) Bolletta, F.; Juris, A.; Maestri, M.; Sandrini, D. lnorg. Chim. Acta 1980, 44, L175. (c) Demas, J. N.; Crosby, G. A. J. Am. Chem. SOC. 1971,93, 2841. (d) Bolletta, F.; Maestri, M.; Balzani, V. J. Phys. Chem. 1976,80, 2499. (22) Rillema, D. P.; Blanton, C. B.; Shaver, R. J.; Jackman, D. C.; Boldaii, M.; Worl, S. L. A.; Mever, T. J. horn. Chem. 1992,31, 1600. (2i) Sun, H.; Hoffman, M. Z:; Mulazzani, Q.G. Res. Chem. Intermed. 1994,20,735. (24) Barqawi, K. R.; Akasheh, T. S.; Beaumont, P. C.; Parsons, B. J.; Philips, G. 0. J. Phys. Chem. 1988,92,291. (25) Ohno,T.; Yoshimura, A,; Mataga, N.; Tazuke, S.; Kawanishi, Y.; Kitamura,N. J. Phys. Chem. 1989,93, 3546. (26) Yoshimura, A.; Hoffman, M. Z.; Sun,H. J. Photochem. Photobiol., A: Chem. 1993,70, 29. (27) (a) Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969,73, 834. (b) Rehm, D.; Weller, A. Isr. J. Chem. 1970,8, 259. (28) Marcus, R. A. J. Phys. Chem. 1968,72,891. (29) Agmon, N.; Levine, R. D. J. Phys. Chem. 1979, 71, 3034. (30) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V. J. Am. Chem. SOC. 1978,100,7219. (31) (a) Legros, B.; Vandereecken, P.; Soumillion, J. P. J. Phys. Chem. 1991,95,4752.(b) Yeh, S. R.; Falvey, D. E. J. Am. Chem. SOC. 1992,114, 7313. (32) Kitamura, N.; Kim, H. B.; Okano, S.; Tazuke, S. J. Phys. Chem. 1989,93,5750. (33) Kim, H. B.; Kitamura, N.; Kawanishi, Y.; Tazuke,S. J. Phys. Chem. 1989,93,5757. (34) Creutz, C.; Keller, A. D.; Sutin, N.; Zipp, A. P. J. Am. Chem. SOC. 1982,104, 3618. (35) Sabbatini, N.; Dellonte, S.; Bonazzi, A.; Ciano, M.; Balzani, V. Inorg. Chem. 1986,25, 1738. (36) Tazuke, S.; Kitamura, N.; Kim, H.-B. in Photochemical Energy Conversion; Noms, J. R., Jr.; Meisel, D., Eds.; Elsevier: New York, 1989; pp 96- 110. (37) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helv. Chim. Acta 1978, 61,2720. (38) Ohno,T.; Yoshimura, A.; Prasad, D. R.; Hoffman, M. Z. J. Phys. Chem. 1991,95,4723. (39) Gould, I. R.; Young, R. H.; Moody, R. E.; Farid, S. J. Phys. Chem. 1991,95, 2068. (40) Mandal, K.; Prasad, D. R.; Hoffman, M. Z. Coord. Chem. Rev. 1985,64, 175. (41) (a) Ohno,T.; Yoshimura, A.; Mataga, N. J. Phys. Chem. 1986,90, 3295. (b) Ohno,T.; Yoshimura, A.; Shioyama, H.; Mataga, N. J. Phys. Chem. 1987,91,4365. (c) Ohno,T.; Yoshimura, A.; Mataga, N. J. Phys. Chem. 1990,94, 4871. (42) (a) Yonemoto, E. H.; Riley, R. L.; Kim, Y. I.; Atherton, S. J.; Schmehl, R. H.; Mallouk, T. E. J. Am. Chem SOC. 1992,114, 8081. (b) Ohno, T.; Yoshimura, A.; Prasad, D. R.; Hoffman, M. Z.; Sun, H. Manuscript in preparation. (43) Sun, H.; Hoffman, M. Z. J. Photochem. Photobiob, A: Chem., in press. (44)Clark, C. D.; Hoffman, M. Z. Manuscript in preparation. (45) Hino, T.; Akazawa, H.; Mataga, N. J. Phys. Chem. 1976,80, 33. (46) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous Systems; Marcel Dekker: New York, 1970.