J. Phys. Chem. 1989, 93, 2445-2452
2445
Reductive Quenching of the Luminescent Excited State of Trls( 2,2'-bipyrazine)ruthenium(2+) Ion in Aqueous Solution Gilda Neshvad and Morton Z. Hoffman* Department of Chemistry, Boston University, Boston, Massachusetts 0221 5 (Received: July 6, 1988; In Final Form: September 7, 1988) The photodynamics of aqueous solutions containing Ru(bpz)?+ (bpz = 2,2'-bipyrazine) and reductive quenchers (D = EDTA (ethylenediaminetetraaceticacid), CzO$, TEOA (triethanolamine),ascorbate ion, and thiols (RS-), e g , cysteine and glutathione) in the presence and absence of MV2+(methylviologen) have been evaluated by the use of time-resolved spectrofluorimetry and pulsed laser flash photolysis as a function of pH, [D], and [MV2+];as well, we have determined @(MV'+) from the continuous photolyses of these systems. Values of kq are dependent on pH due to the acid-base properties of the quenchers and range from - 3 X lo9 M-' s-l for ascorbate ion and deprotonated thiols to 1 X lo7 M-I s-l for Cz042-.The oxidized radicals (Do;) of EDTA, Cz02-, and TEOA may undergo irreversible transformations into reducing radicals (Drd*) within the quenching solvent cage in competition with geminate-pair back electron transfer; the efficiencies of escape of the redox products ( R ~ ( b p z ) ~and + Drd*) into the bulk solution (qce) for these sacrificial quenchers are very high (10.5). Thiols are semisacrificalquenchers; their oxidized radicals (RS') must escape from the quenching cage before conversion to Dd' (RSSR') can occur; qcc I0.03. Ascorbate ion is a nonsacrificial quencher; its oxidized radical reacts rapidly and quantitatively with R ~ ( b p z ) ~in+bulk solution. In the absence of MV2+,Drd* from sacrificial and semisacrificial quenchers reacts rapidly ( k > lo9 M-' s-I) with Ru(bpz)?+, generating a second equivalent of the reduced complex. In the presence of MVZ+,both R ~ ( b p z ) ~and + Drd' produce MV" rapidly ( k = 108-109 M-' s-l). R ~ ( b p z ) ~engages + in an acid-base equilibrium (pK, = 7.1); the conjugate acid is a poorer reducing agent by -0.2 V than is the basic form and is unable to reduce MV2+. As a result, the generation of MV" in acidic solution is pH- and [MVz+]-dependent.The values of @(MV'+)from the continuous photolysis correlate very well with the rate constants and efficiencies of the various steps in the mechanism according to ~ +is, the the following expression: @(MV'+) = qqqqcc(qrcd+ qrd'), where q* is the efficiency of population of * R ~ ( b p z ) ~qq efficiency of quenching of * R ~ ( b p z ) ~and ~ + qrd , and qrd' are the efficiencies of the reactions of R ~ ( b p z ) ~and + Drd* with MV2+.remectivelv. Values of WMV") as high as 1.2 have been obtained at pH 4.5 in solutions containing 0.10 M MV2+ and 0.26 M CZO4'-.
-
-
Introduction Since the initial report by Crutchley and LeverLof the synthesis of R u ( b p ~ ) ~(bpz ~ + = 2,2'-bipyrazine) and the demonstration of the quenching of its excited state, many investigations have been devoted to the characterization of the chemical, photophysical, photochemical, radiation chemical, and electrochemical properties of that complex.2-20 Compared to R ~ ( b p y ) , ~(bpy + = 2,2'-bipyridine), the photosensitizer in the most popular and best-studied model system for the reduction of water, the lifetime ( T ~ )of *Ru(bpz):+ in water is somewhat longer (- 1 ps vs -0.6 ps) and (1) Crutchley, R. J.; Lever, A. B. P. J. Am. Chem. SOC.1980, 102,7128. (2) Balk, R. W.; Stufkens, D. J.; Crutchley, R. J.; Lever, A. B. P. Inorg. Chim. Acra 1982, 64, L49. (3) Crutchley, R. J.; Lever, A. B. P. Inorg. Chem. 1982, 21, 2276. (4) Gonzales-Velasco, J.; Rubinstein, I.; Crutchley, R. J.; Lever, A. B. P.; Bard, A. J. Inorg. Chem. 1983, 22, 822. ( 5 ) Crutchley, R. J.; Kress, N.; Lever, A. B. P. J . Am. Chem. SOC.1983, 105, 1170. (6) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D. Inorg. Chem. 1983, 22, 1617. (7) Crutchley, R. J.; Lever, A. B. P.; Poggi, A. Inorg. Chem. 1983, 22, 2647. (8) Kitamura, N.; Kawanishi, Y.; Tazuke, S.Chem. Lett. 1983, 1185. ( 9 ) Diirr, H.; Dorr, G.; Zengerle, K.; Reis, B.; Braun, A. M. Chimia 1983, 37. 245. (10) Diirr, H.; Dam, G.; Zengerle, K.; Curchod, J.-M.; Braun, A. M. Helu. Chim. Acra 1983, 66, 2652. (11) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J. Am. Chem. SOC.1984, 106, 2613. (12) Kawanishi, Y.; Kitamura, N.; Kim, Y.; Tazuke, S.Sci. Pap. Insr. Phvs. Chem. Res. (JDn.) 1984. 78. 212. 113) Diirr, H.; D&r, G.; Zengerle, K.; Mayer, E.; Curchod, J.-M.; Braun, A. M. Nouv. J. Chim. 1985, 9, 717. (14) Haga, M.-A.; Dodsworth, E. S . ; Eryavec, G.; Seymour, P.; Lever, A. B. P. Inorg. Chem. 1985, 24, 1901. (15) Prasad, D. R.; H a l e r , D.; Hoffman, M. Z.; Semne, N. Chem. Phys. Leu. 1985, 121, 61. (16) Prasad, D. R.; Hoffman, M. Z. J. Am. Chem. SOC.1986.108. 2568. (17) Kalyanasundaram, K. J. Phys. Chem. 1986, 90, 2285. (18) Venturi, M.; Mulazzani, Q. G.; Ciano, M.; Hoffman, M. Z. Inorg. Chem. 1986, 25,4493. (19) Mulazzani, Q. G.; Venturi, M.; Hoffman, M. Z. Radiat. Phys. Chem. 1988, 32, 71. (20) Barqawi, K. R.; Akasheh, T. S., Beaumont, P. C.; Parsons, B. J.; Phillips, G.0. J. Phys. Chem. 1988, 92, 291.
0022-3654/89/2093-2445$01.50/0
the reduction potentials of the various excited and ground oxidation states are more than 0.5 V more positive. Because * R ~ ( b p z ) ~ ~ + , when reductively quenched by sacrificial donors such as TEOA (triethanolamine) and EDTA, yields the strongly reducing Ru(bp~)3+species, which reacts rapidly with methylviologen (1,l'dimethyl-4,4'-bipyridinium dication; MV2+)to produce MV'+ with a quantum yield ($(MV'+)) that is higher than that obtained from R ~ ( b p y ) ~R~ u+ (, b p ~ ) ~has ~ +been viewed as a superior photosensitizer. Recently, Willner and co-workers21*22demonstrated that a system consisting of R ~ ( b p z ) ~and ~ +TEOA in aqueous solution was capable of effecting the photoreduction of C 0 2to CH4 in the presence of Ru and Os metallic colloids. They accounted for their flash and continuous photolysis results in terms of the formation of R u ( b p ~ ) ~the + , catalyst-active species, in the quenching reaction; the back electron transfer in bulk solution between R ~ ( b p z ) and ~+ the one-electron oxidized form of TEOA (TEOA,') was viewed as occurring in competition with the decomposition of TEOA,'. Because of the importance of R ~ ( b p z ) as ~ ~a+photosensitizer, it is essential that the dynamics of the reactions in which it engages be well defined. Toward the goal, we have investigated the continuous and flash photolysis of the R U ( ~ ~ Z ) ~ ~ + / M V ~ + / E D T A ~ y s t e m ' ~ and , ' ~ the kinetic, acid-base, and electrochemical properties of R u ( b p ~ ) ~in+aqueous s o l ~ t i o n . ~We ~ * 'showed ~ that the superiority of R u ( b p ~ ) ~as~a+photosensitizer, beyond any inherent advantages that reductive quenching has over oxidative quenching, is due to the ability of the oxidized form of EDTA (EDTb,'), generated in the quenching reaction, to convert rapidly and irreversibly, in bulk solution and/or within the solvent cage, into a reducing radical (EDTArd'). EDTArd' is capable of producing a second equivalent of R u ( b ~ z ) ~and/or + MV'+. Another element that is of consequence when consideration is given to the dynamics of the system, is the acid-base behavior of Ru(bp&+, which is a Ru(I1) species containing a coordinated bpz'- radical: Ru(bpz)z(bpz'-)+. Its conjugate acid, Ru(bpz)z(21) Maidan, R.; Willner, I. J. Am. Chem. SOC.1986, 208,8100. (22) Willner, I.; Maiden, R.; Mandler, D.; Diirr, H.; DBrr, G.; Zengerle, K.J . Am. Chem. SOC.1987, 109, 6080.
0 1989 American Chemical Society
2446
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
(bpzH')2+ (pK, = 7,1), is a poorer reducing agent and is unreactive toward MV2+.I8 In this paper we report on our investigations of the photodynamics of aqueous solutions containing Ru(bpz)t+, in the presence and absence of MV2+, and three different classes of reductive quenchers: sacrifical (TEOA, EDTA, C2042-), semisacrifical (thiols), and nonsacrificial (ascorbate ion). The kinetic parameters (rate constants of quenching, cage escape yields, rate constants of secondary reactions involving one-electron oxidized and reduced species) of the mechanisms are shown to correlate very well with the values of @(MV'+) obtained in continuous photolysis experiments. Experimental Section Materials. Methylviologen dichloride (Aldrich) and TEOAeHCl (Aldrich) were recrystallized three times from ethanol and water/methanol, respectively; they were both dried by suction for 2 days. 2-Mercaptobenzoic acid was recrystallized from water twice and then dried by suction and vacuum. All other materials were Baker or Aldrich reagent grade and were used without further purification. R u ( b p ~ ) ~as~ +the , PF,- salt, was prepared by the method of Rillema et aL6 Solutions. Distilled water was further purified by passage through a Millipore purification train. A fresh stock solution of each quencher at the desired pH was prepared prior to use. The solutions were buffered with 5-10 mM NaH2P04, Na2HP04, potassium hydrogen phthalate, NaHC03, or Na2C03 as needed; final pH adjustments were made with HCl or NaOH. The ionic strength was adjusted to 1.0 M with Na2S04. Apparatus. Luminescence quenching experiments were made by using a Perkin-Elmer MPF-2A spectrofluorimeter set at 440 nm for excitation and 603 nm for emission. Excited-state lifetime measurements and flash photolysis experiments on nonsacrificial systems were made using a Nd:YAG pulsed laser system with excitation at 355 nm; details of the apparatus have been described before.23 Laser flash photolysis experiments on sacrificial and semisacrificial systems were performed at the Center for Fast Kinetics Research (CFKR), University of Texas, Austin; 12-ns pulses a t 355 or 532 nm were provided by Nd:YAG Q-switched lasers under computer control.24 Continuous photolyses were performed by using a Bausch & Lomb high-intensity monochromator in conjunction with a 100-W quartz halogen lamp and photon counter. All experiments were conducted at the ambient temperature (-22 "C). Procedures. Air-equilibrated solutions for luminescence quenching experiments, with at least four different concentrations of the quencher, were contained in I-cm spectrofluorimeter cuvettes. Continuous photolyses were performed on Ar-purged solutions contained in 1-cm cuvettes with septum- and plasticfilm-sealed long necks; the absorbance of the solution at 605 nm was monitored as a function of irradiation time. Solutions for the excited-state lifetime measurements and flash photolysis experiments on nonsacrificial systems were contained in a 1 X 2 cm cuvette with a long neck that could be sealed, after saturation with Ar, air, or 02,with a septum and plastic film. Solutions for laser flash photolysis at CFKR were contained in a 1 X 0.5 cm cuvette, excited along the larger dimension, and probed by the analyzing light along the smaller one; they were deaerated and continuously mixed with a stream of N,. Results and Discussion TEOA and EDTA have been extensively used as electron donors (D) in photochemical schemes. Their importance derives from the rapid and irreversible transformation of their one-electron oxidized form (Do;), in which the radical is localized on the amine moiety, into a carbon-localized reducing radical (Dd*) as the result of the loss of H+ from the carbon atom (Y to the amine group. The potential of C2042- as a sacrificial donor arises from the fact that (23) Malba, V.; Jones, G., 11; Poliakoff, E. Photochem. Phofobiol. 1985, 42, 451.
(24) Foyt, D. C. Comput. Chem. 1981, 5, 49.
Neshvad and Hoffman upon its one-electron oxidation to C204*-, clean and rapid decarboxylation yields C02', a simple one-electron reducing agent.25 Thiols (RS-) are semisacrificial reductive quenchers inasmuch as their oxidation yields RS' radicals that do not engage in irreversible transformations but, rather, establish an equilibrium with RS-,26 forming strongly reducing RSSR'- radical^.^' Of importance in model systems for the photosensitized reduction of water is the fact that cysteine and reduced glutathione diminish markedly the extent of hydrogenation of M V " upon its interaction with colloidal metal catalysts for the generation of dihydrogen.28 Ascorbate ion, a nonsacrificial donor, has found use as a reductive quencher of * R ~ ( b p y ) ~ ~*+R, ~ ( p h e n ) ~(phen ~ + = 1,lOphenanthroline), and their analogues.DqB Its one-electron oxidized form does not transform, decaying instead via bimolecular disporp~rtionation.~' On the basis of what is known about the reductive quenching of R ~ ( b p z ) ~in~the + absence and presence of MV2+ and the chemistry of sacrificial and nonsacrificial electron donors, the generic mechanism of the photosensitization reaction can be written as reactions 1-1 1.
hu
R u ( ~ P z ) ~ ~ +* R u ( ~ P z ) ~ ~ +
+
2
* R ~ ( b p z ) ~ ~ +R ~ ( b p z ) ~ ~hv' + * R ~ ( b p ~ ) 3+~D+
k,
Ru(~Pz)~ ++Dox*
-
Ru(bpz)3+ + Dox*
Ru(~Pz)+ ~ ~D+
kd
kU
Dox' Drd*
Drei'
+ R ~ ( b p z ) ~ ~ +R ~ ( b p z ) ~++products
kP
R ~ ( b p z ) ~ + protonation Ru(bpz)3+
-
+ MV2+
+
Dred* MVZ+
Drd* MV'+
kd
kdl
kk
MV"
+ products
products
-
+ Dox*
+ RU(~PZ)~'+
MV"
kd'
MV2+
+D
The generation of the lowest energy luminescent excited state with an efficiency of 7. via reaction 1 is followed by quenching reaction 3, in competition with the natural decay of *Ru(bpz)?+ (reaction 2), forming R ~ ( b p z ) ~and + D,' in bulk solution. Back electron transfer reaction 4 between the strong oxidant and the strong reductant would annihilate the redox pair were it not for reaction 5, the irreversible transformation of D,' into a reducing radical (Drd*). In the absence of MV2+, Drd* can reduce Ru( b p ~ ) , ~via + reaction 6 to produce a second equivalent of the reduced photosensitizer. In the presence of MVZ+(Eldo= -0.44 V vs NHE, the formation of MV" can occur via reactions 8 and 9. Reaction 7 represents the protonation of R ~ ( b p z ) ~ to+form unreactive products; the extent of this reaction would, of course, be pH dependent. The possibility that Drd' can undergo further (25) Mulazzani, Q. G.; D'Angelantonio, M.; Venturi, M.; Hoffman, M. 2.;Rodgers, M. A. J. J . Phys. Chem. 1986, 90, 5341. (26) Hoffman, M. Z.; Hayon, E. J . Phys. Chem. 1973, 77, 990. (27) Surdhar, P. S.; Armstrong, D. A. J . Phys. Chem. 1986, 90, 5915. (28) Johansen, 0.;Launikonis, A.; Loder, J. W.; Mau, A. W.-H.;Sasse, W. H. F.; Swift, J. D.; Wells, D. Aust. J . Chem. 1981, 34, 2347. (29) Brown, G. M.; Brunschwig, B. S.; Creutz, C.; Endicott, J. F.; Sutin, N . J . Am. Chem. SOC.1979, 101, 1298. (30) Krishnan, C. V.; Creutz, C.; Mahajan, D.; Schwarz, H.; Sutin, N. Isr. J . Chem. 1982, 22, 98. ( 3 1 ) Bielski, B. H. J. Adu. Chem. Ser. 1982, 200, 81.
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2447
Quenching of * R ~ ( b p z ) ~in~Aqueous + Solution
-+-
1.0-
$ 0.8-
n
#4-
+
9-
7
-0 0
2
'u)
I
0.6 -
I
U
e
r" m
- 0.4-
-0
8
E
8-
I, # f
7-
O . 4
0.0
2
I
I
3
4
5
I
I
!
,
6
7
8
9
6
10
6 4
5
6
PH Figure 1. 1, and T,, (relative to plateau values at pH > 6 ) for *Ru(bpz)t+ as a function of pH. 1, (m) measured in air-saturated solutions; T~ ( 0 ) measured in Ar-purged solutions.
conversion to unreactive products, perhaps via pH-dependent pathways, necessitates the inclusion of reaction 10; bimolecular disproportionation and combination reactions of the radical species, which are not expected to be of consequence under continuous photolysis conditions, have not been included. In nonsacrificial systems, where reactions 5,6,9, and 10 are not operative, reaction 11 would destroy any MV'+ that was formed in the presence of MV2+. From the generic mechanism, it is clear that the value of 6(MV") in the R ~ ( b p z ) ~ ~ + / M v system ~ + / D will be a complex function of the nature of D, the concentrations of the solutes, the pH, and the rate constants of the various reactions. +(MV'+) would be expected to range from zero in nonsacrificial systems to potentially significant values in sacrificial systems; we have already shown that O(MV'+) = 1.3 in alkaline solution for D = EDTA.'$ The dynamics of the individual reactions of the mechanism and the yields of products from continuous photolysis are now examined with an eye toward the further evaluation of the utility of Ru(bpz)j2+ as a photosensitizer. Quenching of * R ~ ( b p z ) ~The ~ + .lifetime of *Ru(bpz)$+ in ) natural pH Ar-purged H 2 0in the absence of quenchers ( T ~ at (6.5) was 0.95 ps; in air- and 02-saturated solutions, T was 0.84 and 0.56 ps, respectively. From these values, k, = 5.5 X lo8 M-l s-l for the quenching of * R ~ ( b p z ) ~by~02. + In comparison, the corresponding value of k , for *Ru(bpy)32+and O2 is 2.8 X lo9 M-I s-l under the same experimental conditions ( T = ~ 0.62 ps). In both cases, energy-transfer quenching, possibly to form IO2, is implicated.j2 The difference in the values of k, for these complexes, which have almost identical excited-state energies, may reflect differences in the contribution of charge transfer to the processes.33 Both T~ and fl are functions of pH in acidic solution, as shown in Figure 1. The "titration curve", due to the protonation of * R ~ ( b p z ) ~is~ in + ,agreement with that presented by Crutchley et al.,5 who assigned a pK, of 2.0 (3.8 calcd) to the conjugate acid of the excited state. It is important to note that the strong sensitivity of T~ to pH requires that solutions be carefully buffered in acidic medium; in the absence of buffer, the control of pH may not be sufficient to maintain T~ constant as solute concentrations are varied in a series of experiments. This factor may be the explanation of a discrepancy in our results with those in the literature regarding quenching by MV2+. Kitamura et al.* reported that k,, as determined from f , , is 8.6 X lo6 M-l s-I; the pH was not given. Upon repeating the luminescence quenching experiments with unbuffered solutions, we obtained an apparent k, of about that magnitude but found that (32) Tanielian, C.; Esch, M. Abstracts of Sixth International Conference on Photochemical Conversion and Storage of Solar Energy, Paris, 1986, No. F-7. (33) Olmsted, J., 111, personal communication.
7
8
0
1 0 1 1 1 2 1 3
PH Figure 2. log kq vs pH for EDTA (A),TEOA (+), (22042- (O), cysteine (m), and ascorbate ion (+) as a function of pH at p = 1 M (Na2S04).
the pH was 4.8 (doubtlessly due to the presence of small amounts of persistent acidic impurities in the recrystallized MV2+),which . fact, in buffered solutions at pH results in a diminution in T ~ In > 5 with [MV2+] = 0.1 M, no quenching was observed. The effect of pH on the decay of an emission from the excited state is also undoubtedly the explanation for the nonlinear Stern-Volmer behavior we reported earlier for the quenching by EDTA in acidic solution.16 An increase in [EDTA] in unbuffered solutions results in a decrease in the pH; the pH of 4.7 that was quoted is a nominal value within the range exhibited. When the solutions are buffered, the Stern-Volmer plots are perfectly linear. The following compounds at concentrations up to 0.1 M did not quench the emission from * R ~ ( b p z ) ~ ~HOCH2C02H +: (pH 7.2), ( C ~ H S ) ~ C ( O H ) C O(PH ~ H 7.0), C ~ H S C H ( O H ) C O ~(PH H 7.0), H2NCH2C02H (PH 11.5), CH3CH(NH2)C02H (PH 11.5). Values of k, were calculated from linear Stern-Volmer plots (fp/Zl vs [D]) for the following quenchers in buffered solutions: EDTA, TEOA, C20d2-,ascorbate ion, cysteine, reduced glutathione, mercaptoacetate ion, HSCH2CH2NH2,HSCH2CH2N(CH3)2, 4-aminobenzoic acid, and 2-mercaptobenzoic acid. The values range, in alkaline solution, from at or near the diffusioncontrolled limit ( lo9 M-' s-' ) f or the thiols and ascorbic acid, to -IO8 M-' s-I for EDTA and TEOA and -10' M-' s-l for C204*-. Plots of log k, as a function of pH for EDTA (pK, = 0.0, 1.5, 2.0, 2.7, 6.1, 10.2), TEOA (pK, = 7.8), CzOz- (pK, = 1.2, 4.2), cysteine (pK, = 1.9, 8.2, 10.3), and ascorbate ion (pK, = 4.1) are presented in Figure 2. As expected, the reducing abilities of the quenchers diminish markedly upon their protonation, although cysteine and the other thiols still retain significant reducing ability in mildly acidic medium. It is interesting to note that k, for C2042-,which is independent of pH across a very wide range, does exhibit an increase in its value by a factor of -3 as protonation occurs. If the values of k, for the reaction between a common excited state and a series of simple molecules and anions, albeit with different charges, were dependent only on the energetics of the quenching reaction, with no appreciable intrinsic barrier to the adiabatic (or nearly so) electron transfer, one would expect to see the diffusion-controlled limit reached for systems with an overall driving force (E?) greater than -0.2 V; rate constants of IO8 and lo7 M-I s-l would be expected to occur for E? or -0 and - 4 . 1 V, re~pectively.~~ The reduction potential of * R ~ ( b p z ) ~ ~ + can be estimated from a knowledge of the excited-state emission energy (2.1 eV)6 and Edo of the ground state (-0.50 V vs NHE);'" Erd0(*Ru(bpz)?+) 1.6 V vs NHE. From k , we can estimate the Eo: values for the one-electron oxidation of the quenchers: ascorbate ion and completely deprotonated cysteine, more positive than -1.4 V; TEOA and EDTA, --1.5 V; C20d2-, --1.7 V. In
-
-
-
-
(34) Balzani, V.; Scandola, F. In Energy Resources through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic: New York, 1983; pp 1-48.
2448
Neshvad and Hoffman
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
TABLE I: Values of qn for tbe Reductive Quenching of *Ru(bpz)32ta
[quencher] 0.1 M C20d20.1 M EDTA 0.05 M TEOA 2.0 mM cysteine 2.0 mM mercaptoacetate ion 2.0 mM glutathione 2.0 mM (CH,)2NCH2CH2S-
PH 11 4.7 12 12 12 12 12
ArlAo 0.22 0.18 0.21 -0.016 -0.015 -0.0032 -0.053
4
tlo
tl-
0.45 0.38 0.43 0.021 0.021 0.0044 0.072
0.60 0.82 0.92 0.85 0.61 0.84 0.84
0.75 0.46 0.47 0.03 0.03 0.005 0.09
' [ R ~ ( b p z ) , ~=~ ]50 pM; p = 1.0 M (Na,SO,); average of 5-10 shots per sample, 2-3 replicate runs. comparison, a value of Eo: for generic RS- species has been calculated from thermodynamic data to be -0.84 V;27 the corresponding value for ascorbate ion is -0.68 V.3s Because the one-electron oxidation products of TEOA, EDTA, and C20d2-are very unstable radicals toward irreversible transformation, reversible potentials for these species are not available from conventional electrochemical measurements. Cage Escape Efficiency. Electron-transfer quenching reactions, such as reaction 3, do not usually occur quantitatively; the efficiency of formation of the redox products in the bulk solution is less than unity. In the most elementary sense, this is due to the competition between the reactions involving the solvent-caged species formed in the quenching act (reaction 3a): back electron
quenching of *Ru(bpz)32t, )tee can be calculated:
4 = Af(C* - t2)/A0(61 - €2) '7q
vce
(35) Creutz, C. Inorg. Chem. 1981, 20, 4449. (36) Hoffman, M. Z. J . Phys. Chem. 1988, 92, 3458.
=
4/qq
The values of the molar absorptivities of Ru(bpz)32+and Ru(bpz)3+at 440 nm are well established: t2 = 1.39 X lo4 M-I cm-' reported and t I = 1.11 X lo4 M-' cm-' .l8 K alyana~undaram'~ that At = t* - c2 = -5700 f 800 M-' cm-', as determined by the intensity saturation method. Thus, we take e* = 8.2 X lo3 M-' cm-', while recognizing the magnitude of the uncertainty in the value. The products of the oxidation of C20:-, EDTA, and TEOA and the reducing radicals (C02'-, EDTArd', and TEOArd*, respectively) that result from their irreversible transformations (reactions 5a-c) do not absorb at 440 nm and do not interfere C204*-
-+
EDTA,,TEOA,,'
C02'- + C 0 2
(5a)
EDTA,,d'
(5b)
TEOA,d'
(5c)
+
+
in this analysis. Thus, 4 = 2.03(Af/A0). The values of qq and qcc are given in Table I. It should be noted that qcc has been estimated to be -0.7 for EDTA in alkaline solution from the continuous photolysis of the Ru(bpz);+/EDTA/MV2+ system.15 The result of the oxidation of thiols is the RS' radical, which However, absorbs very weakly at 440 nm (e C 100 M-' RS' does not undergo irreversible transformation; rather, it reacts very rapidly ( k lo9 M-I s-') with RS- to establish an equilibrium with RSSR'- (reaction 5d; KSd IO3 M-1).26 Thus, RSSR'- is
-
transfer between the geminate redox pair (reaction 3b), and escape of the species into the bulk (reaction 3c). From this scheme the efficiency of escape of R ~ ( b p z ) ~out + of the solvent cage into the kJc). bulk solution can be written qcc = k3c/(k3biThe value of qce is the critical parameter governing the quantum yield of formation of Ru(bpz)3+ and other high-energy radical species. In this study, pulsed-laser flash photolysis was used to evaluate qa from the quenching of *Ru(bpz)32+by the reductive quenchers. When excited at 532 nm, deaerated solutions containing 50 pM Ru(bpz)?+ exhibited the characteristic absorption (Amx 375 nm) 603 nm; monitored at 720 nm for reasons of and emission (A,, instrumental sensitivity) of *Ru(bpz)?+, and the bleaching of the ground state (A,, 440 nm). The decay of these features back to the base line occurred via first-order kinetics with a lifetime (70 = l/ko) of 0.75-0.91 ps, depending on the temperature of the nonthermostated solutions. In the presence of C20d2-, EDTA, and TEOA, the decay of the excited state occurred more rapidly (Tobsd = I/k,bd) due to the quenching reaction. In the course of the quenching reaction, a large fraction of the absorption at 440 nm was recovered. Extrapolation of the first-order recovery of the 440-nm absorption to the midpoint of the laser pulse yielded the value of the initial absorbance, which is due to the presence of *Ru(bpz)32+and the absence of an equivalent amount of Ru(bpz)32+(Ao,a negative number); the absorbance of the solution before the flash was taken as the zero absorbance base line. At the end of the quenching reaction the absorbance of the solution reaches a plateau; this absorbance, due to the presence of Ru(bpz),+ and the absence of an equivalent amount of R ~ ( b p z ) ~ was ~ + , Af (a negative number). The ratio A f / A ois given in Table I for the systems examined. As described previously,36the determination of A f / A oat a single wavelength permits an evaluation of the quantum yield of formation of the redox products in bulk solution (4) to be made. The value of 4 is obtained from APIAoand the molar absorptivities of R ~ ( b p z ) , ~ +* ,R ~ ( b p z ) ~and ~ + ,R ~ ( b p z ) ~(e2, + e*, and t i , respectively). Then, through knowledge of qq, the efficiency of
+ ko) = 1 - (kO/kOM)
= k,[Dl/(k,[Dl
-
RS'
+ RS- e RSSR'-
(54
formed and stabilized in the presence of millimolar concentrations of RS- within the time frame of the quenching and cage escape reactions; its absorption must be taken into account in the evaluation of 4 via reactions 1-3 and 5d. In the case where D = RS-, 4 = Ad€* - tz)/Ao(tl tR - t2), where tR is the molar absorptivity of RSSR'-at 440 nm. Although at 400-420 nm) the absorption spectra of RSSR'- species (A,, are well-known, tmsx values are not known with pre~ision.~' Nevertheless, for the various RSSW- species involved in this study, E,*, is of the order of 9 ( f l ) X lo3 M-' cm-I; depending on the shape of the spectral band, tR at 440 nm is less than tmaxby an unknown factor but probably not more than 10-20%. Thus, it is probably a reasonable approximation to take cR as 7 X lo3 M-I cm-'. From the t values involved, 4 = -( 1.4)Af/Ao. The quenching of * R ~ ( b p z ) ~at~pH + 12 by cysteine (charge = 2-), reduced glutathione (3-), mercaptoacetate ion (2-), and (CH3),NCH2CH2S (1-) displays very low values of qcc (Table I) compared to those for the sacrificial quenchers; the values of RS-are given to a lower precision because of the small Af/Aoratio and the uncertainties in the values of e* and tR. For these thiols with similar redox potential^,^^ kjb can be regarded as being constant for the series. The variation in qa, then, is governed by the dependence of kk on the charges of the oxidized species within the solvent cage (one unit more positive than the parent). As expected, the rate of cage escape of Ru(bpz)3+and RS' into bulk solution is significantly diminished upon the increase in the electrostatic attraction between the redox pair. In fact, the relative rates of k,, for the oxidized species with charges of 2-, 1-, and 0 are 1:6:18, respectively. If kJbhas a value of 1O'O s-', k3, is predicted to be 107-108 s-', a value that may be amenable to direct determination by appropriate fast kinetics techniques. Unless k3b for the oxidized forms of the sacrificial donors is unusually small, the high values of qoefor C2042-,TEOA (charge = 0), and EDTA (3-/4-) in alkaline solution, and EDTA (2-) in acidic solution can occur if irreversible transformation (reaction 3d), followed by the cage escape of Ru(bpz)3+and Drd' (reaction 3e), occurs within the solvent cage in competition with back
+
-
(37) Hoffman, M . Z.; Hayon, E. J . Am. Chem. SOC.1972, 94, 7950.
The Journal of Physical Chemistry, Vof. 93, No. 6, 1989 2449
Quenching of *Ru(bpz)32+in Aqueous Solution
TABLE 11: Summary of the Rate Constants of the Reactions Involving Radicals and R ~ ( b p z ) ~ + "
---
reaction R u ( ~ P z )+~C02'~+ R u ( ~ P z )+ ~ +C02 Ru(bpz)j2+ EDTArd' Ru(bpz),+ + products
(6a) (6b)
+
(64
Ru(bpz),*+ + TEOArd' Ru(bpz),+ + products R ~ ( b p z ) ~+~RSSR'* Ru(bpz),+ + RSSR
(8)
Ru(bpz),+ + MV2+
(6c)
Ru(bpz),+ + MV"
----
+ MV2+ MV'+ + C02 + MV2+ MV" + products TEOArdd'+ MV2+ MV" + products RSSR'- + MV2+ MV'+ + RSSR R u ( b p ~ ) ~++As' Ru(bpz)?+ + AsMV*+ + As' MV2++ AsC0;-
(94 (9b)
EDTArd'
(94 (94 (4b) (lib)
PH
k , M-' s-' 8.8 X 108b*c no
11 4.7 8.7. 11.0 12 12
-4 x 109d.c 1.2 x 1096~ 2.8 x 1096~8 1.5 x 1096-h 2.5 x 109b.i 3.5 x 109bj
no reactiond,c 4.5 x 108d.e,k 1.9 x 109c 3.8 X lo8/ 5.0 x 109' 6.4 x 109' 5.9 x 10Sd-e 1.5 x 109d' 2.1 x 109f 6.7 X lo8' 1.5 x 109m 1.4 x 109m
4.7 8.7 11 12 12 11
4.7 8.7, 11.0 12 12 4.5-9.5
4.5-9.5
a p = 1.0 M (Na2S04)unless otherwise indicated. "From pseudo first-order kinetics, taking [R~(bpz),~+] = 50 pM. c I n the presence of 0.10 M C20;-. dReference 16. eIn the presence of 0.10 M EDTA. 'In the presence of 0.050 M TEDA. g I n the presence of 2.0 mM cysteine. *In the presence of 2.0 mM reduced glutathione. 'In the presence of 2.0 mM (CH3)2NCH2CH2S-.]In the presence of 2.0 mM mercaptoacetate ion. k p = 0.60 M; no Na2S04present. 'In the presence of 2.0 mM RS-. '"In the presence of 3.0 mM As-.
+
electron transfer. Then, qce= k3d/(k3+, k3d). Furthermore, if k3d,representing the rate constants for the irreversible intramolecular decarboxylation and deprotonation reactions that convert Do; to Dd*, has a value of 109-1010S-I, reaction 3d will compete favorably with both reactions 3b and 3c.
-
[Ru(~~z)~+...D,,'] [Ru(bpz)3+*.*Dr,d']
(3d)
[ R U ( ~ P Z ) ~ + * * . D ~Ru(bpz)3+ ~*] +D,d'
(3e)
-
The values of qcc for sacrificial quenchers in Table I reveal that the ratio k3d/k3b(or k3c/k3b) is -2.5 for C2042-and EDTA in alkaline solution and 1 for EDTA in acidic solution of TEOA in alkaline solution. Inasmuch as the Do,' species are likely to have different kinetic properties for their redox and sacrificial transformations, it is not possible at this time to establish trends in the values of the rate constants. Nevertheless, it is clear that C204" and EDTA in alkaline solution are the quenchers of choice for the maximum yield of redox products to be obtained. We have suggested before15J6and reiterate here the view that reaction 3c may be inoperative for sacrificial reductive quenchers; as will be noted in the following section, no evidence of reaction 4 upon pulsed-laser excitation is seen. Thus, for sacrificial reductive quenching, reactions 3a, b, and c (or d and e) describe the formation and destruction of the solvent-caged species. A set of chemical equations analogous to reactions 3a-c can be written for the oxidative quenching of Ru(I1)-polypyridyl complexes ( R U ( N N ) ~ ~by + )an electron acceptor (A), involving R u ( N N ) ~ ~and + Ard* as the species in the solvent cage. While it may be true that the difference in the electronic spins involved in the geminate pair back electron transfer reactions in oxidative and reductive quenching affect the values of the rate constants of the corresponding reactions,38 the higher values of qccfor reductive quenching involving sacrificial electron donors cannot be used as evidence. Care must be taken to make comparisons only for systems in which the same mechanism of reactions within the solvent cage is operative. Fast Dynamics in the Absence of MP'. The quenching of *Ru(bpz)32+in alkaline solution by C2O4*-, EDTA, and TEOA results in residual bleaching at 440 nm and the formation of an absorption at 490 nm, due to the generation of Ru(bpz)3+;in the case of RS-, quenching results in a small positive absorption at 440 nm, due to the presence of RSSR'-, and the absorption at 490 nm. Subsequently to the quenching reaction, another equivalent of Ru(bpz),+ is formed via first-order kinetics in the
-
(38) Olmsted, J., 111; Meyer, T. .I. J . Phys. Chem. 1987, 91, 1649.
microsecond time frame; the bleaching of the absorption at 440 nm and the formation of additional absorption at 490 nm occur with the same kinetics. The reaction, which can be attributed to steps 6 a 4 , has a second-order rate constant of (1.5-3.5) X lo9 M-' s-l (based on [ R ~ ( b p z ) ~=~ +50] p M ) for the various Drd*. In the one case in which it was tested, k6 was independent of [D] (2, 10, and 50 mM TEOA).
+
R u ( b p ~ ) ~ ~C02'+
-
-.,
Ru(bpz)3+ + C 0 2
(6a)
Ru(bpz)32++ EDTAred*
Ru(bpz)3+ + products (6b)
Ru(bpz)32++ TEOArd'
Ru(bpz)3+ + products (6c)
+
Ru(bpz)32+ RSSR'-
-.,
Ru(bpz)3+
+ RSSR
(6d)
In the case of C2042-, EDTA, and TEOA, the amount of Ru(bpz)3+ formed additionally is approximately equal to the amount formed in the quenching reaction; bimolecular reaction of EDTAd', TEOArd', and C0;- occurs to a small extent under the pulsed-laser conditions. Thus, 2 equiv of Ru(bp&+ are formed as the result of the absorption of one photon. For EDTA at pH 4.7, the secondary formation of Ru(bpz)3+ via reaction 6 is not observed; in this case, EDTArd' does not have the capacity to reduce R ~ ( b p z ) , ~ +so, that the H+-promoted transformation and/or bimolecular decay of the radical predominate. With RS-, the secondary formation of Ru(bpz)3+ is 7.
+,
50
40
60
M-'
Figure 5. I/@(MV'+) vs l/[C,O?-] for solutions at ambient pH (7.5-7.8) containing 5.0 or 20 mM MV2+;p = 1 M (Na2S04).
.
1.2
1.0-
0.8-
r v
e
I
*
* 0.6-
W
0.6
0.2
-.-
0.41 0.0 !
, 0
0
I
100
50
[MV"],
150
mM
Figure 4. @(MV'+)as a function of [MV2+]for Ar-purged solutions containing 50 pM Ru(bpz)j2' and 0.26-0.30 M C2042-at p = 1 M (Na2S04): m, pH 4.5; +, pH 5.3; A, pH 6.1; 0 , pH >7.
the decay of MV" should mirror the disappearance of Ru( b p ~ ) ~ ( b p z H ' ) ~experiments +; are in progress to examine that aspect of the overall mechanism. Our results for EDTA (Figure 3) at pH 4.3-11.0show that values of O(MV'+) approaching those in alkaline solution can be obtained in acidic medium in the presence of a high concentration of MV2+,suggesting that even in acidic solution, reactions 8 and 9 can be made to be competitive with reactions 7 and 10, respectively, in the generic mechanism. The results for C20d2-(Figure 4) at pH 4.5-11.0support this conclusion. Here, too, instability of MV" and curvature of the plots of [MV"] vs irradiation time ( t ) toward a plateau are observed for decreasing values of [MV2+] and pH. At pH 4.5, the plot is linear only for t < 2 min for [MV2+]= 2.0 and 10 mM; the linearity continues to 3 min for 0.10 M MV2+. At pH 5.3, linearity exists to 3 min for 2.0 and 10 mM MV2+; at 0.10M MV2+,the entire 5-min irradiation time results in a linear plot. By pH 6.1,only vestiges of nonlinearity are observed at the lowest [MV2+]. Thus, almost equally high quantum yields of MV'+ can be obtained in both alkaline and acidic solutions under the conditions of efficient quenching of the excited state of the photosensitizer and efficient scavenging of Ru(bpz)3+and the secondary radicals from the sacrificial reductive quencher. The dependence of a(MV'+) on [C204*-]a t ambient pH (7.5-7.8)is a direct result of the variation of qq. By taking the generic mechanism and considering reactions 6,7, 10,and 1 1 as being inoperative under the conditions of these continuous photolysis experiments, it is easy to see that a(MV'+) = 2qqqce. From this, a plot of l / a -
,
,
I
50
"
'
I
100
"
I
I
150
200
i 0
[TEOA], mM Figure 6. @(MV'+)as a function of [TEOA) for Ar-purged solutions containing 50 pM Ru(bpz)32+and 12 mM MV2+at pH 9.9; p = 1 M (Na2S04).
( M Y + ) vs 1/[C2042-]is predicted to be linear with an intercept = 1/2qw and intercept/slope = k,/ko. Figure 5 shows the result, from which qw = 0.71and kq/ko = 1 1 is calculated. A comparison of these values with those obtained directly (0.75and 12, respectively; vide supra) demonstrates the validity of the choice of the t-values in the determination of qw, the generic mechanism, and the specific steps relating to C2042-as the quencher. Earlier,15 we had reported that a similar plot for D = EDTA is linear, giving equally good correlation of calculated and observed rate constants; 03 was obtained. a value of O(MV'+) = 1.4 at [EDTA] There are some reports in the literature of values of O(MV'+) from the continuous photolysis of the Ru(bpz)32+/MV2+/TEOA system. Crutchley and Lever' obtained a(MV'+) = 0.77for 20 mM MV2+and 0.6 M TEOA for solutions purged with N2;they took ebo5 for MV" as 1.07 X lo4 M-' cm-l. Kitamura et a1.* reported a value of 0.75under the same experimental conditions but 0.98when the solutions were deaerated by freeze-pumpthaw cycles.42 More recently, Diirr et aI.l3 gave a value of 0.30 for Ar-purged solutions containing 2 mhf MV2+ and 0.05 M TEOA at pH 8; e602 for MV" was taken as 1.1 X lo4 M-' cm-I. Figure 6 shows that the values of a(MV'+) for solutions containing 12 mM MV2+ at pH 9.9 in the presence of varying concentrations of TEOA reaches a plateau at 1 .O (standard deviation 0.09) for [TEOA] 1 0.10M; they agree well with those obtained
-
(42) These authors43 erred in their claim that their value of @(MY+) = 0.98 demonstrates that nearly 100% photon efficiency was achieved; they failed to take into account the formation of the second equivalent of MV'+ from TEOAd'. (43) Tazuke, S.; Kitamura, N.; Kawanishi, Y . J . Photochem. 1985, 29,
123.
2452
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
by the other workers. In all cases, the plots of [MV"] vs irradiation time were linear. On the basis of the qce value and the fast dynamics of the reactions for D = cysteine, one would expect to obtain rather low values of @(MV'+); in fact, the yield is -0.1 for 20 mM MV2+ and 12 mM cysteine at pH 10.0. The behavior of this system is complicated by the fact that MV2+and cysteine form photoactive ion-pair electron donor-acceptor complexes that can absorb competitively with R ~ ( b p z ) ~ ~The + . photolysis ~~ of systems in which D = RS- is under further investigation. Summary and Implicationsfor Solar Energy Model Systems. The rate constants of the reactions involving radicals and Ru(bp~)3+in the systems discussed in this paper are summarized in Table 11. Efficient reductive quenching of *Ru(bpz)?+ by electron donors (D) can be achieved across a wide range of pH; EDTA and C2042can be used to almost pH 4. For sacrificial donors (EDTA, TEOA, C2042-), the one-electron oxidized forms of which (Dox*) undergo irreversible transformations, possibly within the quenching cage to form reducing radicals (Dred') in competition with geminate-pair back electron transfer, Ru(bpy),+ is efficiently generated in bulk solution from the quenching reaction (qce = 0.5-0.75). Although RS- compounds, such as cysteine and reduced glutathione, are effective quenchers, their cage escape efficiencies are low (7, < 0.03); the released RS' radicals combine with R S and establish an equilibrium with strongly reducing RSSR' radicals. A second equivalent of R ~ ( b p z ) ~is+formed rapidly from the reaction of the photosensitizer with Drd*, except for EDTA in acidic solution where the radical does not have sufficient reducing ability, and undergoes competitive H+-promoted degradative reactions; @ ( R ~ ( b p z ) ~=+ )q.qqqce(1 qrx), where qrx is the efficiency of the reaction of Drd* with R ~ ( b p z ) ~ Inasmuch ~+. as 7. 1 and qrx = 1 in neutral-alkaline solution, @(Ru(bpz),+) = 2qqq, under those conditions; as qq 1, @ ( R ~ ( b p z ) ~ + )271%:
+
-
-
-
+ kdec) In the presence of MV2+, R ~ ( b p z ) ~and + Drd' rapidly produce two equivalents of MV"; for D = RS-, MV" is unstable toward oxidation by RS'. In acidic solution, the protonation of Ru(bpz),+ (pK, = 7.1) generates a species that does not reduce MV2+ and is unstable toward disproportionation. Nevertheless, it is possible to approach the maximum value of @(MV'+) in acidic solution by utilizing high concentrations of MV2+. The values of +(MV'+) from the continuous photolyses of the R~(bpz)~~+/MV~+/sacrificial donor system correlate very well with the efficiencies of the various steps in the mechanism according to the following expression: @(MV'+) = qtqqqce(qrd + qrd'), where qrd and qrd' are the efficiencies of the reactions of R u ( b p ~ ) ~and + Drd' with MV2+, respectively. Inasmuch as qrd and qrd' are unity in neutral-alkaline solution, @(MV'+)= 27?,; as qq 1, @(MV'+) 2qce. Even in acidic solution at high [MV2+]with D = C2042- and EDTA, values of @(MV'+) approaching those in alkaline solution can be obtained: qrx
-
= krx[Ru(bPz)32'l /(krx[Ru(bPz)32'l
-
(44) Prasad, D. R.; Hoffman, M. 2. J . Phys. Chem. 1984, 88, 5660.
Neshvad and Hoffman
Because MV" reduces H 2 0 to H2 and R ~ ( b p z ) reduces ~+ C02 to CHI in the presence of catalysts, the R~(bpz)~~+/sacrificial donor system, in the absence or presence of MV2+, serves as a very important model system for the photochemical conversion and storage of solar energy. Inasmuch as high quantum yields of the reactive species can be obtained, significant optimized yields of the storage substances can be achieved. In comparison, for the classical R u ( ~ ~ ~ ) ~ ~ + / M V ~ + / E D T A model system, @(MV'+) = q.qqqceqsc(1 qrd'), where qscis the efficiency of scavenging of R ~ ( b p y ) , ~by + EDTA (- 1 in acidic and alkaline solution). It is easy to see that in the presence of 1, the sufficient MV2+and EDTA, such that qq, qsc,and qrd' parameter that limits the value of @(MV'+) is qce. However, for solutions containing 20 mM MV2+and 0.1 M EDTA ( p = 1 M) at pH 4.7, 8.7, and 1 1.O, qcc is only 0.12.33 Although qce can be higher (up to 0.27) at lower concentrations of MV2+ and EDTA due to the lessening of the extent of formation of ion-pair aggregates of these species,4547qq, qsc, and qrd are correspondingly lower, causing @(MV'+) to be undesirably small. In conclusion, as long as sacrificial donors are necessary to use in the model photochemical systems, the reductive quenching of *Ru(bpz)32+is to be preferred to the oxidative quenching of * R ~ ( b p y ) ~for ~ +the production of high yields of energy-rich species. All other things being equal, the larger values of qcefor the reductive quenching by sacrificial donors ensure that the quantum yields for systems utilizing R ~ ( b p z ) ~as~the + photosensitizer can be significantly higher than for systems containing Ru(bPY),2+.
+
-
Acknowledgment. This research was supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, U S . Department of Energy. CFKR is supported jointly by the Biomedical Research Technology Program of the Division of Research Resources of N I H (RR 00886) and by The University of Texas. We thank Dr. S. J. Atherton (CFKR) for technical assistance and his continued interest in this work. Registry No. MV2+, 4685-14-7; EDTA, 60-00-4; TEOA, 102-71-6; MV", 25239-55-8; EDTAd', 118459-24-8; TEOAd., 91 184-93-9; RU(bpZ)j'+, 75523-96-5; C204'-, 338-70-5; (CH9)ZNCHZCH&, 118459-23-7; CO;-, 14485-07-5; Ru(bpz)3+, 75523-97-6; ascorbate ion, 299-36-5; cysteine, 104170-20-9; mercaptoacetate ion, 16561-17-4; glutathione, 118459-22-6.
Supplementary Material Available: Quenching rate constants and quantum yield values (5 pages). Ordering information is given on any current masthead page. (45) Prasad, D. R.; Hoffman, M. Z.; Mulazzani, Q.G.; Rodgers, M. A. J. J . Am. Chem. SOC.1986, 108, 5135.
(46) Prasad, D. R.; Hoffman, M. Z. J . Chem. SOC.,Faraday Trans. 2 1986.82, 2275. (47) Hoffman, M. Z.; Prasad, D. R. In Supramolecular Photochemistry; Balzani, V., Ed.; D. Reidel: Dordrecht, Holland, 1987; pp 153-165.
