J. Phys. Chem. 1984, 88, 5632-5639
5632
Quantum Yield of Formation of Methylviologen Radical Cation in the Photolysis of the Ru(bpy):+/Met hylviologen/E DTA System Krishnagopal Mandal and Morton Z. Hoffman* Department of Chemistry, Boston University, Boston, Massachusetts 0221 5 (Received: April 17, 1984; In Final Form: June 22, 1984)
The quantum yield of formation of the methylviologen radical cation, @(MV+.),upon the 450-nm continuous photolysis of dication/EDTA system in deaerated aqueous solution, has been determined as a function the R~(bpy),~+/methylviologen of [MV2+](0.5-20 mM), [EDTA] (0.05-100 mM), pH (3.7-ll.O), and temperature (6-50 "C) at constant [R~(bpy),~+] (1 .O X lo4 M). The results are discussed in terms of the mechanism of the reaction and the effect of solution medium on the rate constant of quenching of *Ru(bpy)?+ by MV2+,the cage release yield of redox products, the scavenging of Ru(bpy)?+ by EDTA, and the secondary reduction of MV2+by EDTA-derived radicals.
Introduction The splitting of water by visible light is of great importance for the photochemical conversion and storage of solar energy. A very large amount of work has been directed toward the generation of H2 from aqueous solutions' containing a photosensitizer to absorb the light and generate a long-lived excited state that can undergo electron transfer to a relay species. The most thoroughly investigated model system is the one containing R ~ ( b p y ) , ~(bpy + = 2,2/-bipyridine) as the photosensitizer and methylviologen (1,l'-dimethyl-4,4'-bipyridinium dication; MV2+) as the electron relayU2 Upon oxidative quenching of the luminescent lowest excited state of the photosensitizer, * R u ( b p ~ ) , ~ by + , MV2+,the methylviologen radical cation, MV+., is formed which, upon interaction with colloidal metal particles such as Pt, generates Hz. In order to prevent the degradative back electron transfer reaction between the redox products of the excited-state quenching reaction, a sacrificial electron donor is often employed to reduce Ru(bpy)$+ to R ~ ( b p y ) , ~so + that the energy-rich MV+. will not be destructively scavenged. One very popular sacrificial electron donor has been EDTA which, upon oxidation, undergoes irreversible degradation to the final product^.^ The mechanism of the reaction has been studied in great deand is summarized in reactions 1-7 for the system in the absence of O2 and colloidal Pt. Under those conditions, MV+builds up in the solution and can be stored as a high-energy intermediate.
5 *Ru(bpy)l+ Ru(bpy),,+ + MV+-
--
R~(bpy),~+
+ Ru(bpy)3,+ + MV'. Ru(bpy),,+ + EDTA MV+- + EDTA',,
* R ~ ( b p y ) , ~ ' MV2+
EDTA+,,
Ru(bpy)3'+
Ru(bpy),'+
--
+
(2)
(3)
(4)
+ EDTA
(5)
+ H+ + products
(6)
MV2'
EDTA'
+ MV2' + EDTA',,
(1)
EDTA' + MVZ+ MV'. (7) Despite the large number of investigations of this system, there has not been a systematic detailed study of the quantum yield of (1) "Photogeneration of Hydrogen"; Harriman, A., West, M. A,, Eds.; Academic Press: London, 1982 and references therein. (2) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159-244 and references therein. (3) Amouyal, E.; Zidler, B. Isr. J . Chem. 1982,22, 117-124 and references therein. (4) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helv. Chim. Acla 1978, 61, 2720-2730.
(5) Moradpour, A,; Amouyal, E.; Keller, P.; Kagan, H. Nouu. J . Chim. 1978, 2, 547-549. (6) Johansen, 0.; Launikonis, A,; Loder, J. W.; Mau, A. W.; Sasse, W. H. F.; Swift, J. D.; Wells, D. Aust. J . Chem. 1981, 34, 981-991, 2347-2354.
0022-3654/84/2088-5632$01.50/0
MV+. formation $(MV+.), in the absence of Pt as a function of the critical solution medium parameters; in comparison, a relatively large number of studies involving the optimization of the H2yield in the presence of Pt has been p u b l i ~ h e d . ~ - 'Reported ~ values of @(MV+.) from the continuous photolysis of the R ~ ( b p y ) , ~ + / MVZ+/EDTA~ y s t e m ~range , ' ~ from ~ ~ 0.07 to 1.15 and show great disparity for studies performed under ostensibly similar conditions. One factor that has only recently been appreciated is the effect of ionic interactions among the species in the system, especially when relatively high concentrations of solutes are employed to effect extensive excited-state quenching (reaction 2) and redox scavenging (reaction 4); ion-pair interaction and aggregation among MV2+, EDTA, and R ~ ( b p y ) , ~ +have been established. 17,2 1-23 In this paper, we report the results of our systematic examination of the quantum yield of MV+- formation as a function of pH and the concentrations of the various species in solution in the absence of a colloidal metal catalyst and 02.In the presence of catalyst, the formation of H2 from MV+- is described by stoichiometric reaction 8; inasmuch as @(H2) = 1/2@(MVe*) Mi'+-
+ H+
-
+
MVZ+ '/zH,
(8)
according to reaction 8, the discovery of those parameters that affect the quantum yield of formation of MV+. will be of great importance to the optimization of the yield of H2. (7) Kiwi, J.; Gratzel, M. J . Am. Chem. SOC.1979, 101, 7214-7217. (8) Lehn, J.-M.; Sauvage, J.-P.; Ziessel, R. Nouu. J . Chim. 1981, 5, 291-295. (9) Keller, P.; Moradpour, A. J. Am. Chem. SOC.1980, 102, 7193-7196. (10) Keller, P.; Moradpour, A.; Amouyal, E.; Kagen, H. B. Nouu. J . Chim. 1980, 4, 377-384. (11) Brugger, P.; Cuendet, P.; Gratzel, M. J . Am. Chem. SOC.1981,103, 2923-2927. (12) Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visca, M.; Gratzel, M. J. Am. Chem. SOC.1981, 103, 6324-6329. (13) Harriman, A.; Mills, A. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 21 11-2124. (14) Miller, D.; McLendon, G. Inorg. Chem. 1981, 20, 950-953. (15) Maestri, M.; Sandrini, D. N o w . J . C@m. 1981, 5 , 637-641. (16) Amouyal, E.; Grand, D.; Moradpour, A,; Keller, P. Nouu. J . Chim. 1982, 6, 241-244. (17) Kennelly, T.; Gafney, H. D. "Abstracts of Papers", 178th National Meeting of the American Chemical Society, Washington, DC, Sept 1979; American Chemical Society; Washington, DC, 1979; INOR 97. (18) Frank, A. J.; Stevenson, K. L. J . Chem. SOC.,Chem. Commun. 1981, 593-594. (19) Xu,J.; Porter, G. B. Can. J . Chem. 1982, 60,2856-2858. (20) NenadoviE, M. T.; MiEiE, 0. I.; Rajh, T.; SaviE, D. J . Photochem. 1983, 21, 35-44. (21) Ebbesen, T. W.; Ohgushi, M. Photochem. Photobiol. 1983, 38, 251-252. (22) Hoffman, M. Z.; Prasad, D. R.; Jones, G., 11; Malba, V. J . Am. Chem. SOC.1983, 105, 6360-6362. (23) Mandal, K.; Hoffman, M. 2. J . Phys. Chem. 1984, 88, 185-187.
0 1984 American Chemical Society
Quantum Yield of Formation of MV+. 80
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5633 80
A 0I
60
60
I
I
A
=L
+>
n 40
+ n 40
> I
I
Y
U
20
0
20
I
I
30
40
I
10
20
0
Figure 1. Typical plots of the formation of MV+- as a function of [EDTA]: (0), 100 mM; (O), 5 mM; (0). 1 mM; (m), 0.05mM. [Ru( b ~ y ) ~ *= + ]1.0 X lo4 M, [MV2+] = 20 mM, pH 11.0,p = 0.5 M except for 0.10 M EDTA where p = 1.0 M. Variations in Io between runs have not been normalized.
Experimental Section Materials. MVZ+(BDH) as the C1- salt and Ru(bpy),Cl, (G.F. Smith) were recrystallized three times from water upon addition of methanol and dried. NazEDTA and NaZSO4were used as received (Baker Analyzed Reagents). Triply distilled water was further treated by passage through a Millipore purification train. Apparatus. Photolyses were performed using a Bausch and Lomb high-intensity monochromator and associated 200-W super-pressure mercury lamp. Solutions were contained in thermostated (f0.2 "C) 1-cm cuvettes provided with a greaseless vacuum stopcock and a Teflon stirring bar. Determinations of the yield of MV+. were made on Cary 118 and 212 spectrophotometers. Whenever possible, absorbance measurements were made within 10 s of the end of a photolysis period. No evidence was ever found for the post-irradiation decay of MV+- under any of the conditions used. The quenching of the R ~ ( b p y ) ~lu-~ + minescence by MVZ+was examined with a Perkin-Elmer MPF-2A spectrofluorometer. Procedures. Solutions were rigorously degassed by 3-4 freeze-pump-thaw cycles at i10-3 torr on a vacuum line; additional cycles up to eight had no effect on the results. Solutions were photolyzed at 450 f 10 nm while being stirred with a small Teflon-coated magnetic bar in order to ensure homogeneity. The intensity of light incident on the solutions was 1 X einstein L-' min-' as determined relative to the ferrioxalate a ~ t i n o r n e t e r ~ ~ and did not vary more than f20% from run to run; the intensity of absorbed light was established for each sample from Beer's law. Values of @(MV+.) were determined from the linear growth of [MV+.] as a function of irradiation time; [MV+.] was established from the absorbance of the solution at 605 nm by taking €605 = 1.37 X lo4 M-' cm-1.25
-
Results In order to evaluate systematically the parameters that affect the value of @(MV+.), the concentration of Ru(bpy),2+ was maintained constant at 1.0 X 10" M. As a result, the absorbance of the photolyzed solutions at 450 nm was always the same with >90% of the incident light absorbed. The concentration of Ru( b ~ y ) must ~ ~ +be controlled inasmuch as @(MV+.) is a function of that parameter at high [Ru(bpy)?+], [MV2+], and [EDTA] in alkaline solutionz3where EDTA (pK, = 0.0, 1.5, 2.0, 2.8, 6.1, 10.2)26is present as 3-/4- anions; however, this functionality does not appear to be operative in acidic solution nor is it of any major (24) Calvert, J. G.; Pitts, J. N., Jr. "Photochemistry";Wiley: New York, 1966; p 784. (25) Watanabe, T.;Honda, K. J. Phys. Chem. 1982, 86, 2617-2619. (26) "Critical Stability Constants"; Martell, A. E., Smith, R. M., Eds.; Plenum Press: New York, 1974; Vol. 1, p 204.
30
20
10
IRRADIATION TIME , MIN
40
IRRADIATION TIME MIN Figure 2. Typical plots of the formation of MV+. as a function of [MV*+]: (O),20 mM; (O), 5 mM; (0), 1 mM; (M), 0.5 mM. [Ru(bpy)?] = 1.0 X 10" M, [EDTA] 0.10 M, pH 11.0, p = 1.0 M. Variations in Zo between runs have not been normalized.
t
p'
0
5
10
5,
20
IRRADIATION TIME MIN Figure 3. Typical plots of the formation of MV'. as a function of pH: (0), 11.0; (O), 4.7; (O),3.7. [Ru(bpy)?+] = 1.0 X 10" M,[MVzt] = 0.020 M, [EDTA] = 0.10 M;p I?: 1.0 M at pH 11.0 and 0.6 M at pH 4.7 and 3.7. Variations in Zo between runs have not been normalized,
consequence in alkaline solution at the concentrations used in this study. Because of the ionic nature of the species involved in the individual steps of this photoreaction, the ionic strength of the solutions was controlled with EDTA and Na2S04. Determination of Quantum Yield. Irradiation of the Ru( ~ ~ Y ) ~ * + / M V ~ + / Esystem D T A leads to the photosensitized formation of MV+. characterized by the appearance of the MV+* absorption bands at -395 and -605 nm.25 In the absence of air, MV+. is very stable in alkaline, neutral, and mildly acidic solution; in more acidic (pH
I
W
e4
0.10-
0.05
0.00
1
I
I
I
I
I
0
5
10
15
20
25
[ MV2+
1, mM
Figure 5. Quantum yield of MV'. formation as a function of [MVZt] at pH 4.7 (m) and 11.0 (0);[Ru(bp~)~~'] = 1.0 X lo4 M, [EDTA] = 0.10 M; p = 1.0 M at pH 11.0 and 0.7-0.9 M at pH 4.7. Points represent averages of 3-4 individual runs. TABLE I: Effect of Temperature on the Quantum Yield of MV'. Formation" 0 T. OC VH4.7 VH 11.0 6.0 0.13 0.19 22.0 0.14 0.21 50.0 0.16 0.25
[ R ~ ( b p y ) ~=~ + 1.0] X M, [MV2+]= 20 mM, [EDTA] = 0.10 M, p = 0.7-0.9 M at pH 4.7 and 1.0 M at pH 11.0. Effect of Temperature on Quantum Yield. Values of Q(MV+.) were determined at 6, 22, and 50 OC for solutions at p H 4.7 and 11.0 containing 20 mM MVZ+and 0.10 M EDTA. The data are shown in Table I. Determination of Quenching Rate Constants. The quenching of the luminescence at 608 nm from *Ru(bpy)?+ by MVZ+was examined on air-saturated solutions containing 5.0 X M Ru(bpy)32+;the excitation wavelength was 450 nm. Figure 7 shows Stern-Volmer plots of the data in acidic and alkaline medium and in the absence and presence of EDTA. The slopes of the plots (Ksv) are presented in Table 11. The intensity of the 608-nm emission for solutions free of MVZ+was the same in the presence (0.10 M) or absence of EDTA irrespective of pH. Deviations from linear Stern-Volmer behavior have been reported for solutions containing high concentrations of Ru(bpy)?+,
..- , 0
1
2
[ MV2+
3
3, mM
I
I
4
5
Figure 7. Stern-Volmer plot of the quenching of the luminescence from [R~(bpy)~~'] = 5.0 X 10" M. Experimental * R ~ ( b p y ) by ~ ~MV2'; + conditions: pH 4.7, 0.10 M EDTA, p = 0.6 M (0);pH 11.0 in the absence of EDTA, p = 0.84 M ( 0 ) ;pH 4.7 in the absence of EDTA, p = 0.5 M (0); pH 11.0, 0.10 M EDTA, p = 1.0 M (m). TABLE II: Stern-Volmer Quenching Datae PH [EDTA], M Ksv, M-I ambienta 163 ambientb 563 4.7= 0.0010 475 0.010 563 0.10 625 11.0d 631 5.0 X 530 0.0010 48 3 0.10 390
k,, M-l s-I 3.5 x 10s 1.2 x 109 1.0 x 109 1.2 x 109 1.4 x 109 1.4 x 1.2 x 1.1 x 8.4 X
109 109 109
lo8
"In the absence of EDTA and Na2S04;no ionic strength control. the absence of EDTA and presence of Na2S04;b = 0.5 M. c b = 0.5-0.6 M. d p = 1.0 M. '[Ru(bpy),2'] = 5.0 X lo6 M, air-saturated solutions; A,, = 450 nm, A,,iss = 608 nm.
MV2+, and EDTA in alkaline solution.23 However, in acidic solution and in alkaline solution under the photolysis conditions employed here, such deviations are not significant. Discussion The mechanism of the R~(bpy)~~+-photosensitized formation of MV+- is summarized by reactions 1-7 with the symbols EDTA, EDTA',,, and EDTA' representing all the forms of those sub-
Quantum Yield of Formation of MV+-
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5635
stances present in the solution at the pH of interest. Reaction 1 represents the formation of the luminescent lowest excited state of Ru(bpy):+ with an efficiency (7.) of unity;28the natural lifetime of *Ru(bpy):+ in air-free aqueous solution at room temperature, as determined by laser flash photolysis, is 600 ns (reaction -1). + MV2+; Reaction 2 represents the quenching of * R ~ ( b p y ) , ~by the efficiency of quenching (7,) is given by k2[MV2+]/(k2[MV2+] k-l). Reaction 3 represents the rapid back electron transfer annihilation of the energy-rich products of the quenching process. The oxidized form of EDTA, EDTA',,, produced in reaction 4, can be represented as R,$J+CH,-; deprotonation of the carbon atom adjacent to the nitrogen radical site in competition with bimolecular radical reaction 5, which is likely to be near the diffusion-controlledlimit given the exergicity of the reaction, yields EDTA' (R2NCH-) according to reaction 6. This step can be described in detail by reaction 9. The EDTA' radical is a strong
+
RZN'CH2-
RZNCH-
+ H+
(9) reducing agent; reaction 7 reflects the reaction of EDTA' with MVZ+to yield a second equivalent of MV+.. In competition with reactions 6 and 7 will be the bimolecular decays of the EDTA',, and EDTA' radicals to yield two-electron reduced and two-electron oxidized EDTA species and their hydrolysis and decomposition products; presumably, cross-reactions between EDTA',, and EDTA' would also occur. However, these reactions could only be competitive under conditions of very low [MVz+]despite their having rate constants that are probably at or near the diffusion-controlled limit; in all the experiments described here, a relatively high concentration of MVZ+is used in order to quench * R ~ ( b p y ) efficiently ~~+ and initiate the photochemical process. Under the conditions of continuous irradiation, the concentrations of EDTA+,, and EDTA' radicals can never become high enough to allow them to decay via bimolecular radical-radical reactions in competition with unimolecular processes or bimolecular reactions involving stable species at high concentrations. In flash photolysis it is possible to achieve high instantaneous concentrations of R ~ ( b p y ) and ~ ~ +MV+., enabling reaction 3 to be competitive with reaction 4; under continuous photolysis conditions in the presence of a sufficiently high concentration of EDTA, the rate of reaction 4 can be significantly greater than the rate of reaction 3 depending, of course, on the values of the respective rate constants. Consideration of reactions 1-7 and application of the steadystate approximation for *Ru(bpy),,+, R ~ ( b p y ) , ~ + EDTA+,, , and EDTA' yield eq A; at this point, the assumption is made tha +
k2[MV2+])so that the formation of MV+. will be linear with irradiation time as long as the concentration of MV+- remains sufficiently low and the concentration of EDTA is sufficiently high. As [MV'.] increases, the rates of reactions 3 and 5 will increase so that eventually d[MV+.]/dt will leave the linear domain and decrease, approaching zero. In the experiments reported here, linear formation of MV+. was observed in the initial phases of the reaction for all the solute concentrations and for all the pH values used from which the values of @(MV+.) were calculated. When [MV'.] becomes sufficiently high and/or when insufficient [EDTA] is present initially, the formation of MV+-is clearly and unequivocally nonlinear. At EDTA concentrations below the threshold for rate4 >> rate3, no MV+. is observed at all. The observed initial values of @(MV+.) are dependent, according to eq A, on the relative values of the rate constants of the reactions of the mechanism, the concentrations of MV2+and EDTA, and the pH of the solution. We will now examine reactions 2, 4, and 7 in detail and derive values of the efficiency of cage release of redox products in the excited-state electron-transfer quenching reaction from the observed values of @(MV+-). The ramifications for the generation of H, from the R ~ ( b p y ) , ~ + / MVZ+/EDTAsystem will then be explored. Quenching of * R ~ ( b p y ) by ~ ~M + P . Values of k, (= k,) can be obtained from the Stern-Volmer data of Table I1 by dividing Ksv by the value of the lifetime of * R ~ ( b p y ) under ~ ~ + the conditions of the quenching experiments, Le., air-saturated solution. Pulsed laser flash photolysis shows T* = 460 ns; Table I1 includes values of k, for the specific experimental conditions. In the absence of any species except R ~ ( b p y ) , ~and + MV2+at ambient pH, k, = 3.5 X lo8 M-' s-l , in very good agreement with the recently published value of Amouyal and Zidler3 and consistent with earlier determinations at low ionic ~ t r e n g t h . ~This , ~ ~value ~~ is approximately 1 order of magnitude lower than the diffusioncontrolled limit for neutral species but is not unreasonable for the interaction of relatively large dipositive ions. It is well established23,O that k, increases as the ionic strength of the solution increases, approximately following the Debye-Brernsted law; deviations are attributed31 to the existence of specific ionic interactions at high ionic strengths. For solutions containing 0.28 M Na2S04( p = 0.84 M), we find that k, = 1.2 X lo9 and 1.4 X lo9 M-' s-l at pH 7.0 and 11.0, respectively, representing the reaction of cationic species primarily ion paired to SO:- (reaction 10). Now, with increasingly greater EDTA and lower Na2S04
+
+
k3 [MV'.]
Z,k2[ MVZ+] k-l
+ k2[MVZ+]
k3[MV+.]
k4 [EDTA] k3[MV+.]
+ kd[EDTA]
[
+ kq[EDTA] +
k6
- k5 [Mv+*I
k6
+ k5[MV+-]
]]
the reactions are stoichiometric as written. Because of the acid-base nature of EDTA, EDTA',,, and EDTA', the concentrations of the various forms of these species will be dictated by the pH of the solution; as a reasonable first approximation, the pK, values of EDTA',, and EDTA' can be taken to be the same as those of EDTA. In addition, it is to be expected that k4, k5, k6, and k7 will be functions of pH. Thus, the rate of reaction 4, for example, can be expressed in terms of k4[EDTA] with the recognition that this term represents C,ki[EDTAi]. The issue of the pH dependence of the rates of the reactions in the mechanism lies at the heart of the discussion that follows. According to eq A, in the limit where rate, >> rate4, the rate of formation of MV+. is zero. If rate4 >> rate, but rate5 >> rate6, d[MV+.]/dt = 0. However, when rate4 >> rate, and rate6 >> rate5, eq A predicts that d[MV+.]/dt = 21,k2[MVz+]/(k-, + (28) Demas, J. N.; Taylor, D. G. Inorg. Chem. 1979, 18, 3177-3179.
+
[ * R ~ ( b p y ) 3 ~ + * * 8 0 4 ~[MVZ+***S042-] -] [R~(bpy)3~+--SO,"] [MV+*.**SO,2-] (10)
+
[ * R u ( ~ ~ ~ ) ~ ~ + * - E D[MV2+-EDTA] TA] [ R U ( ~ P ~ ) , ~ + * * * E D T[MV+....EDTA] A] (1 1)
concentrations at constant ionic strength, the variations in k, should reflect the specific effect of EDTA. At pH 4.7, the ambient pH for EDTA-containing solutions, where dinegative SO:- is replaced by dinegative EDTA as the ion-pairing agent, k is essentially constant with an average value of 1.2 X lo9 M-] s-?. At pH 11.O, where EDTA exists mainly as 3-/4- anions, ion pairing of EDTA with both R ~ ( b p y ) , ~and + MV2+ (reaction 11) causes a significant decrease in the value of kq (8.4 X los M-' s-l ) due to the increased electrostatic repulsion of the similarly charged species. Under conditions of constant pH, [EDTA], and [ R ~ ( b p y ) ~ ~ + ] , the value of @(MV+.) as a function of [MVz+]reflects vq and the changing competition between reactions -1 and 2. The kinetics treatment predicts that the data from Figure 5 , when l/@(MV+.) is plotted vs. l/[MVz+], should yield a straight line with an intercept of I/@,,, where a0is the quantum yield under conditions of the complete quenching of * R ~ ( b p y ) , ~at + infinite MVZ+ concentration, and an intercept/slope ratio of k2/k-1. Figure 8 shows the treatment of the data at pH 4.7 and 11.O; values of (29) Lee, P. C.; Meisel, D. J . Am. Chem. SOC.1980, 102, 5477-5481. (30) Rodgers, M. A. J.; Becker, J. C. J. Phys. Chem. 1980,84,2762-2768. (31) Gaines, G. L., Jr. J. Phys. Chem. 1979, 83, 3088-3091.
5636
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Mandal and Hoffman
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984
'r O f
0
R ~ ( b p y ) ~0.002 ~ + , M MV2+,0.05 M EDTA, pH 5), Ru(bpy)?+ should be reduced by EDTA in a time scale shorter than that of reaction 3. Instead, they observed a decay of MV+- in the microsecond time scale corresponding to a loss of -25% of its initial concentration which was attributed to the competition of reaction 3; however, they did not evaluate k4 from their data. Recently, Lee et ala3,reported that, in the laser flash photolysis of 5 X M Ru(bpy)z+, 5 X lo4 M MV2+,and 0.01 M EDTA at pH 4.8, the recovery of the absorption of R ~ ( b p y ) , ~through + reaction 4 predominates and reaction 3 does not interfere. The inference can be drawn from these reports that under laser flash photolysis conditions, where as much as 10 pM MV+. is formed very rapidly, the presence of >0.01 M EDTA at pH -5 renders rate4 2 rate3. From the description of the experimental conditions in these papers, one can conclude that k4 has a lower limit of lo6 M-I s-' under those circumstances. Because EDTA becomes a stronger reducing agent as it is successively deprotonated (EO, = -0.56 V at pH 2 and -0.13 V at pH 9),35the value of k4 would be expected to increase upon increasing pH. Miller and McLendon14 used the stopped-flow technique to determine directly the rate of the reaction between R ~ ( b p y ) and ~ ~ +EDTA as a function of pH. They reported values ranging from 8 X lo3 M-' s-' a t pH 4 to 2 X lo6 M-'s-l a t PH 7 with the rate profile describing a titration curve with an approximate pK of 5.5. Presumably, the growth of absorbance of Ru(bpy)?+ was monitored although, unfortunately, that feature of the experimental procedures, as well as the concentrations of the materials used, was not specified. Sutin also has reported36 a value of 2 X lo6 M-' s-' a t pH 8.2, but without specific experimental details. In the absence of the necessary information, it is very difficult to understand the reasons for the obvious discrepancy between the laser and stopped-flow experiments. Perhaps the problem results from the nature of the solution media used in the experiments involving the two techniques. If the stopped-flow studies utilized EDTA concentrations considerably lower than those used in the laser flash experiments because of differences in the time resolution, then the extent or" ion-pair complexation between the ruthenium cations and EDTA could be significantly different. In the absence of extensive ion pairing, reaction 4 would involve the bimolecular interaction of the reactants to form a collision complex (reaction 12); the electron-transfer act and the escape of the redox
-
I
I
I
I
I I
500
1000
1500
2000
2500
1 / [ MV2+],
M-'
Figure 8. Reciprocal plot of the data from Figure 5.
TABLE III: Values of On(MV+.)and k,/k-,"
pH
a0(MV+.)
kzlk-I, M-' (obsd)
4.7 11.0
0.14 0.22
875 642
kz/k-,, M-' (calcd) 808 503
"Values of ao(MV+.)and k 2 / k - , (obsd) are obtained from l/intercept and intercept/slope, respectively, in Figure 5. Calculated values of kz/k-, are obtained from k, values (Table 11) and k , = 1.67 X lo6 s-1.
@,,(MV+.) and k2/k-' from the quantum yield measurements and from direct calculation are given in Table 111. The comparison of the k2/k-1values shows good agreement (within k23. One can readily see that the rate of reaction 22, the static scavenging of R ~ ( b p y ) by ~ ~solvent-caged + EDTA, is greater than twice the rate of reaction 23. This ratio is a function of [R~(bpy),~+]; we ~+] have shown23that vcr is even larger at higher [ R ~ ( b p y ) ~indicating the formation of ground-state triple aggregates among Ru(bpy)?+, MV2+,and EDTA which, upon excitation, yield the same caged species as from the quenching reaction (reactions 25-27).
Ru(bpy)?+
+ MV2+ + EDTA F? [ Ru(bpy)32+...EDTA.,.MV2+] (25) hv
[ R U ( ~ ~ ~ ) ~ ~ + . . . E D T A . . Tt .MV~+] [ *Ru(bpy)32+...EDTA...MV2+] (26) [* R u ( ~ ~ ~ ) ~ ~ + . . . E D T A . . . M V ~ + ] +
[RU(~P~)~~+...EDTA...MV+.] (27) The dependence of @(MV+.)on pH (Figure 6) shows the same trends for EDTA concentrations of O.OOl(M.10 M. The lowered yield of MV+- in acidic solutions will arise from the interference of reactions 15 and 16. As a result, eq B cannot be applied in acidic solution and a direct relationship between r)n and @(MV+.) ceases to exist. However, as Figure 4 shows, @(MV+.) exhibits similar functionality toward [EDTA] in both acidic and alkaline solution. At pH 4.7, a plateau value of @(MV+.) of 0.14 is obtained at high [EDTA]; a lower plateau at -0.04 can be discerned at low [EDTA]. Further reduction in [EDTA] results in the generation of no MV+. at all. The sudden mechanistic switch occurring below 0.001 M EDTA supports the suggestion made above that the value of k4 in acidic solution is a function of [EDTA]. Extrapolation of the data in Figure 6 shows that the lowest pH for the formation of MV'. is -3.5, regardless of the concentration of EDTA used. If the reactions that cause the lowering of the yield of MV+. in acidic solution are not affected by [EDTA] at constant pH, then the ratio of @(MV+.) values will equal the ratio of the vor values for the specific experimental conditions employed. Thus, irrespectivt of the absolute values of vcr at pH 4.7, the efficiency of cage release of the redox products at high [EDTA] is -3 times greater than the efficiency at low [EDTA], the same ratio as found at pH 1 1 .O. The fact that complexation by EDTA to cations is weaker in acidic than alkaline solution is manifested by the differences in the onset of the plateaus; apparently, the presence of EDTA within the solvent cage of the quenching pair has ap-
J. Phys. Chem. 1984, 88, 5639-5643 proximately the same effect on qcr independent of the state of protonation of EDTA. At pH 4.7, the temperature dependence of the quantum yield is very weak and is barely outside experimental error; the apparent activation energy is somewhat less than in alkaline solution. Optimization of the System. From the point of view of the photosensitized generation of MV+-, the optimum conditions for the formation of MV+- are those in which q and qcrare maximized and destructive reactions involving MV'. and EDTA-radical species are minimized. The use of high concentrations of MV2+ and EDTA ensures that reactions 2 and 4 predominate over competing reactions; the use of a solution medium of pH 2 7 ensures that reactions 6 and 7 are quantitative. Alkaline solution and a high concentration of EDTA cause qcr to be maximized through the static scavenging of R ~ ( b p y ) ~by~ion-paired + EDTA. The presence of a high concentration of R ~ ( b p y ) makes ~ ~ + certain that all the incident light (at the appropriate wavelength) is absorbed; even higher concentrations of R ~ ( b p y ) , ~ensure + that triple aggregates of Ru(bpy)32+,MV2+, and EDTA can form, undergo excitation, and generate even higher yields of redox products.23 Unfortunately, for the catalytic generation of H2from MV+. on colloidal Pt, acidic solution is required because of the energetics of redox reaction 8; in fact, the efficiency of H2 formation from MV+. becomes greater with increasingly higher acidity.* Because the yield of MV+. is greater in alkaline solution, the combination of these effects leads to the well-established3relationship between H2 yield and pH that exhibits a rather sharp maximum at pH 5 and falls off dramatically at higher or lower pH's. It is instructive to compare the results obtained here with those reported by Harriman and Mills13for the optimization of the H2 yield from the same system in the presence of colloidal Pt by continuous photolysis. These authors determined the rates of H2 (46) Venturi, M.; Mulazzani, Q. G.; Hoffman, M. Z. J . Phys. Cbem. 1984, 88, 912-918.
5639
formation, but not absolute quantum yields, under various experimental conditions. They found that, at pH 4.7 with [RuM and [MVz+] = 1 X M, the rate of ( b p ~ ) ~ ~=' ]7 X H2 production fell off from a plateau at [EDTA] I 5 X M which was attributed to increased competition of back electron transfer reaction 3. This dependence is similar to our observations in Figure 4 although, as pointed out above, we cannot accept that explanation inasmuch as it is not consistent with the observed linear buildup of MV'.. More interestingly, they also observed that the rate of H2 formation paralleled qq up to [MV2+] 1 X M (at pH 4.7 with 0.05 M EDTA) but then plateaued at vq E 0.3 with increasing [MV2+]. They were surprised by this low value which they attributed to increased competition from reaction 3, presumably due to the higher steady-state concentrations of MV+. and R ~ ( b p y ) at ~ ~higher + quenching efficiencies. However, as seen in Figure 5, @(MV+.) mirrors qq at pH 4.7 and MV+- builds up in a linear manner; thus, we cannot accept this explanation either. It is possible that the low yield of H2 at higher [MVz+] that Harriman and Mills13 observed results from the aggregation of MV2+on the colloidal metal particle, perhaps mediated by the poly(viny1 alcohol) stabilizer that must be used to prevent the coagulation of the catalyst. It should be noted that MV2+exhibits the onset of aggregation at -10 mM in water but at lower concentrations in 0.1 M phosphate buffer or in methanol.21 The effect of the presence of EDTA or colloidal metal particles on aggregation is not known. It is possible that such facile aggregation on the metal surface decreases its ability to catalyze the formation of H2; we have shown46 that a diminution in H2 formation is mirrored by an increase in the ultimate hydrogenation, and deactivation, of MV+.. Acknowledgment. This research was supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, U S . Department of Energy. Registry No. MVZ+,4685-14-7; MVt., 2523945-8; EDTA, 60-00-4;
Ru(bpy)9'+, 15158-62-0; H20, 7732-18-5.
Carbonyl Ylldes Photogenerated from Isomeric Stilbene Oxides. Temperature Dependence of Decay Kinetics'" C. V. Kumar,lb S. K. Chattopadhyay,lb and P. K. Das* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: May 10, 1984; In Final Form: July 5, 1984)
Upon 266-nm laser excitation in fluid solutions, trans- and cis-stilbene oxides form isomeric carbonyl ylides (Xmsn)s = 470 and 490 nm) that are spectrally and kinetically distinct from one another. Red shifted in absorption and considerably shorter-lived, the ylide photogenerated from the cis oxirane is assigned the cis-exo,exo structure on the basis of symmetry rules for electrocyclic ring opening; its counterpart from the trans oxirane is given the trans-exo,endo structure. The short lifetimes of the ylides arise in part from moderately small activation enthalpies (6-7 kcal mol-') and in part from small and negative activation entropies (-5 to -7 eu). Laser flash photolysis in acetonitrile gives rise to a second, longer-lived transient species (A, = 335 nm) assignable as the nitrile ylide derived from phenylcarbene and an acetonitrile molecule. Bimolecular rate constants in the range 106-109 M-I s-l are presented for the reactions of carbonyl and nitrile ylides with several dipolarophiles.
Introduction Carbonyl ylides from stilbene oxides are prototypical of 1,3dipolar species derived from thermal and photochemical ring opening of 2,3-diaryloxiranes in general, and have received con(1) (a) The work described herein was supported by the Office of Basic Energy Sciences, Department of Energy. This is document no. NDRL-2586 from the Notre Dame Radiation Laboratory. (b) Present addresses: S.K.C., Chemistry Department, Georgetown University, Washington, DC 20057; C.V.K., Department of Chemistry, Columbia University, New York, NY 10027.
siderable attention as far as absorption-spectral chara~terization**~ under solid-state constraints and dipolarophilic trappingG in fluid (2) Becker, R. S.; Bost, R. 0.;Kolc, J.; Bertoniere, N. R.; Smith, R. L.; Griffin, G. W. J . Am. Cbem. SOC.1970, 92, 1302-1311. (3) (a) Do-Minh, T.; Trozzolo, A. M.; Griffin, G. W. J . Am. Cbem. SOC. 1970, 92, 1402-1403; (b) Trozzolo, A. M.; Leslie, T. M.; Sarpotdar, A. S.; Small, R. D.; Ferraudi, G. J. Pure Appl. Cbem. 1979, 51, 261-270. (4) Lee, G. A. J. Org. Chem. 1976,41, 2656-2658. ( 5 ) Wong, J. P. K.; Fahmi, A. A.; Griffin, G. W.; Bhacca, N. S. Tetrabedron 1981, 37, 334553355, (6) Albini, A.; Arnold, D. R. Can. J. Cbem. 1978, 56, 2985-2993.
0022-3654/84/2088-5639$01.50/00 1984 American Chemical Society
