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 InO:Sn (ITO) oxide were extracted from kinetic data by application of Marcus-Gerischer theory. Specifically, light excitation of the water oxidation catalyst [Ru(tpy)(4,4'-(POH)-bpy)OH] (Ru-OH), 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 > 10 s). 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 -Δ G over a 1 eV range. At pH values below the p K = 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-OH + H→ Ru-OH) occurred with smaller rate constants. Plots of the rate constants versus -Δ G provided a reorganization energy of λ = 0.9 eV and λ = 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.