J . Phys. Chem. 1988, 92, 3458-3464
3458
Cage Escape Ylelds from the Quenching of * Ru(bpy),*+ by Methylvlologen in Aqueous Solution Morton Z. Hoffman Department of Chemistry, Boston University, Boston, Massachusetts 0221 5 (Received: November 30, 1987)
The primary quantum yield of formation of Ru(bpy),,+ and methylviologen radical cation (MV"), 4, resulting from the oxidative quenching of * R ~ ( b p y ) by ~ ~methylviologen + (MV") in aqueous solution, has been determined by the use of pulsed-laser flash photolysis (A,,, = 532 nm;, , ,X = 450 nm) as a function of [MV2+],ionic strength, and pH in the absence and presence of Na2S04,C2042-(Na' or K+ salt), and ethylenediaminetetraacetate (EDTA). Inasmuch as 4 = vqqcc,where qq is the efficiency of the quenching of *Ru(bpy)?+ (=kq[MV2+]/(ko+ kq[MV2+]))and ve is the cage escape efficiency of the geminate are a complex function of the solution medium parameters, redox pair, values of )te are easily calculated. The values of tCe reflecting the self-aggregation of MV2+,the presence of ion-pair aggregates of MV2+and EDTA, and the effect of the ionic strength of the solution on the rate constant of the cage escape reaction. Values range from -0.45 in the limit of zero ionic strength at [MV2+] 3 5 mM to -0.10 at high ionic strength (3.0 M) in the presence of Na2S04and C20d2-; vC, follows an empirical relationship as a function of p: ,7 = (2.22 + 3.81p'/2)-'. For solutions containing 0.010-0.10 M EDTA, vcc is generally lower than that predicted by the empirical relationship, indicating the existence of a specific effect for that reagent due, perhaps, to the presence of ion-paired aggregates of MV2+and EDTA. The results obtained here compare very well with values of 7, reported by other investigators for similar solution medium conditions, when due account is taken of the differences in the e-values of the various species used in the calculations. The implications of the dependence of 7% on solution medium are explored for the model photochemical system containing R ~ ( b p y ) ~MV2+, ~ + , and a sacrificial donor.
Introduction The model system for the photosensitized reduction of water that serva as the prototype against which all others are compared is the one containing R ~ ( b p y ) , ~(bpy + = 2,2'-bipyridine) as the = 450 nm; e450 = 1.46 X lo4 M-' cm-' ) and photosensitizer (A, methylviologen (l,l'-dimethyl-4,4'-bipyridiniumdication; MVZ+) as the electron relay.' The generation of the lowest energy luminescent excited state of the photosensitizer, * R ~ ( b p y ) , ~ via +, reaction 1 with an efficiency (7,) of approximately unity2 is followed by its natural decay (reaction 2; kz = k,) in competition with its oxidative quenching by MVZ+(reaction 3; k3 = k,) to yield the methylviologen radical cation (MV"; A,, = 396, 605 e605 = 4.21 X lo4, 1.37 X lo4 M-' cm-I , re~pectively).~ nm; In the absence of a sacrificial electron donor, such as EDTA or triethanolamine (TEOA), back electron transfer of the solventseparated redox pair occurs (reaction 4). In the presence of the donor (D), R ~ ( b p y ) is ~ ~scavenged + competitively (reaction 5);4 irreversible secondary reactions of the oxidized donor radical (D") can eventually lead to the generation of a second equivalent of MV". R ~ ( b p y ) , ~5 + *R~(bpy),~+
-
(1)
* R ~ ( b p y ) ~ ~ R+ ~ ( b p y ) , ~++ hv'
(2)
+
-
*R~(bpy)+ ~ ~MV2+ +
R ~ ( b p y ) , ~ + MV"
(3)
R ~ ( b p y ) ~+~MV" +
R ~ ( b p y ) , ~++ MV2+
(4)
+
R~(bpy),~+ D
R ~ ( b p y ) , ~++ D"
(5)
The primary quantum yield of MV'+ and Ru(bpy)?+ formation,
4, from reactions 1-3 is equal to qqqq,, where qq is the efficiency
of the quenching reaction (=k,[MV2+]/(ko + k,[MV2+])) and q, is the efficiency of the escape of the redox pair from within the solvent cage wherein they were generated (reactions 3a-c); vCe= k 3 b / ( k 3 b + The overall quantum yield of MV'+,
( 1 ) (a) Kalyanasundaram, K. Coord. Chem. Reu. 1982, 46, 159. (b) Photogeneration ofHydrogen; Harriman, A,, West, M. A,, Eds.; Academic: London, 1982. (c) Energy Resources through Phofochemisrry and Catalysis;
GrBtzel, M., Ed.;Academic: New York, 1983. (2) Demas, J. N.; Taylor, D. G. Inorg. Chem. 1979, 18, 3177. (3) Watanabe, T.; Honda, K. J . Phys. Chem. 1982,86, 2617. ( 4 ) Amouyal, E.; Zidler, B. Isr. J . Chem. 1982, 22, 117.
0022-3654/88/2092-3458$01.50/0
(P(MV'+), is dictated by the value of 4, the kinetic competition of reactions 4 and 5, and the efficiency of formation of additional equivalents of MV". *R~(bpy)+ ~ ~MV2+ +
-
[R~(bpy),~+.-MV'+l (3a)
[R~(bpy)3~+.-MV'+] Ru(bpy)J3+ -+
[R~(bpy)3~+.-MV'+] Ru(bpy),'+ -+
+ MV" + MV2+
(3b) (3~)
Inasmuch as the efficiency of quenching can be controlled easily from a knowledge of k2 and k3, and the choice of [MV2+],it is clear that qcc is the critical parameter governing the value of 4 and hence the efficacy of the photochemical system as a model for the storage and conversion of solar energy. From both practical and theoretical points of view, it is necessary to understand, in a quantitative way, the conditions of solution medium that control the value of qcc. In a number of reviews on the s ~ b j e c tit, ~is~stated ~ that 7, has a value of 0.25 in aqueous solution at room temperature; in others, the value given is 0.30.'9* Interestingly, although secondary literature references for values of qce are often cited by authors of papers in this field, there are only relatively few primary references in which explicit experimental details are given. The general technique, using pulsed-laser flash photolysis, involves the determination of the extent of bleaching of R ~ ( b p y ) , ~and/or + the formation of * R ~ ( b p y ) , ~immediately + after the flash in one set of pulses, and the extent of formation of MV'+ and/or Ru( b ~ y ) , ~in+another set of pulses, all at wavelengths appropriate to the species monitored. For example, the bleaching of the solution at -450 nm immediately after the flash is identified with the loss of substrate and the formation of * R ~ ( b p y ) , ~in+reaction 1; MV'+ is formed in quenching reaction 3. The ratios [MV"] / [ * R ~ ( b p y ) , ~ +and ] [ R u ( b ~ y ) ~ ~ [' *] /R ~ ( b p y ) ~ are ~+] identified as 4; qcc = 4/qq. Values of 4 can also be obtained by determining the amount of MV" formed and the number of photons absorbed by the solution with the use of a photochemical actinometer. (5) Balzani, V.; Scandola, F. In Energy Resources through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic: New York, 1983; p 1 . (6) Sutin, N.; Creutz, C. Pure Appl. Chem. 1980, 52, 2717. (7) Kiwi, J.; Kalyanasundaram, K.; Gratzel, M . Struct. Bonding 1982.49,
39. (8) Amouyal, E. Sci. Pap. Inst. Phys. Chem. Res. (RIKEN) 1984,78,220.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3459
Quenching of *Ru(bpy)32+by MV2+ TABLE I: Literature Values of
qec from
the Quenching of *Ru(bpy)3*' by MVZ+in Aqueous Solution"
t-values used, X10-4 M-I cm-' (A, nm) Ru(I1) *Ru(II) Ru(II1) MV"
[MV*+], s o h medium (salt to control p )
mM 10 nr
1.43 (452) 5 mM Pbuf, pH 6.9;
p
= 0.02 M
nr 2 nr
nr
0.1-2
0.05 M EDTA 0.1 M TEOA, pH 7, or 0.2 M TEOA, pH 8; p = 0.5 M
0.54 (360) 2.6 (360)e6 0.2 (452) 1.47 (452) 1.47 (452) 0.45 (452vh 1.465 (453) 0.77 (453)'
(Na2SO4)
0.24 (452)
0.1 M ACbuf; pH 5 0.5 M NaCl 1.0 M NaCl 2.0 M NaCl 1.0 M Na2S04 1.0 M Na2S04,0.1 M ACbuf; pH 5 p = 0.01 M (NaCI) p = 0.16 M (NaCI) p = 0.52 M (NaCI) p = 1.6 M (NaC1)
0.536 (393)
0.15 (452)
vCc
1.13 (600)b O.3Oc 0.17d 0.4 f 0.18
ref 9 10
1.20 (603)' 0.51 11 1.13 (603)' 0.22 12 0.25 f 0.0Y 13 1.1 (602)
0.148 (393) 3.75 (393) 1.13 (600)
0.25 0.25 0.24 0.20 0.19 0.16 0.10 0.38 0.32 0.25 0.22
14
15
-
"Abbreviations: nr, not reported; Pbuf, phosphate buffer; ACbuf, acetate buffer. bTrudinger, P. A. Anal. Biochem. 1970, 36, 222. CIf e(*Ru(bpy)32')
