Determination Of Proton-Coupled Electron Transfer Reorganization Energies With Application To Water Oxidation Catalysts

The reorganization energy, lambda, for interfacial electron transfer (ET) and for proton-coupled electron transfer (PCET) between a water oxidation catalyst and a conductive In2O3:Sn (ITO) oxide were extracted from kinetic data by application of Marcus-Gerischer theory. Specifically, light excitation of the water oxidation catalyst [Ru-II(tpy)(4,4'-(PO3H2)(2)-bpy)OH2](2+) (Ru-II-OH2), where tpy is 2,2':6',2 ''-terpyridine and bpy is 2,2'-bipyridine, anchored to a mesoporous thin film of ITO nanocrystallites resulted in rapid excited-state injection (k(inj) > 10(8) s(-1)). The subsequent reaction of the injected electron (ITO(e(-))) and the oxidized catalyst was quantified spectroscopically on nanosecond and longer time scales. The metallic character of ITO allowed potentiostatic control of the reaction free energy change -Delta G degrees over a 1 eV range. At pH values below the pK(a) = 1.7 of the oxidized catalyst, ET was the primary reaction. Within the pH range 2 <= pH <= 5, an interfacial PCET reaction (ITO(e(-)) + Ru-III-OH + H+ -> Ru-II-OH2) occurred with smaller rate constants. Plots of the rate constants versus -Delta G degrees provided a reorganization energy of lambda(PCET) = 0.9 eV and lambda(ET) = 0.5 eV. A second water oxidation catalyst provided similar values and demonstrated generality. The utilization of conductive oxides is shown to be a powerful tool for quantifying PCET reorganization energies at oxide surfaces for the first time.