Determination of Proton-Coupled Electron Transfer Reorganization Energies with Application to Water Oxidation Catalysts

The reorganization energy, λ, 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 [RuII(tpy)(4,4′-(PO3H2)2-bpy)OH2]2 (RuII-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 (kinj > 108 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-ΔGo over a 1 eV range. At pH values below the pKa = 1.7 of the oxidized catalyst, ET was the primary reaction. Within the pH range 2 ≤ pH ≤ 5, an interfacial PCET reaction (ITO(e-) RuIII-OH H → RuII-OH2) occurred with smaller rate constants. Plots of the rate constants versus-ΔGo provided a reorganization energy of λPCET = 0.9 eV and λET = 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.