Proton-controlled electron injection from molecular excited states to the empty states in nanocrystalline TiO2

The excited-state and redox properties of Ru(deeb)(bpy)(2)(PF6)(2), Ru(deb-H-2)(bpy)(2)(PF6)(2), Ru(bpy)(2)(ina)(2)(PF6)(2), and Ru(dpbp)(bpy)(2)(PF6)(2), where bpy is 2,2 ' -bipyridine, deeb is 4,4 '-(CO2Et)(2)-bpy, dcb-H-2 is 4,4 '-(CO2H)(2)-bpy, dpbp is 4,4 '-(PO(OEt)(2))(2)-bpy, and ina is isonicotinic acid, bound to nanocrystalline TiO2 and colloidal ZrO2 films have been studied in acetonitrile at room temperature as a function of the interfacial proton concentration. High surface proton concentrations favor a carboxylic acid type linkage(s) where low proton concentrations favor carboxylate type binding mode(s) for Ru(II) compounds with ethyl ester or carboxylic acid functional groups. The carboxylic acid linkages are unstable when Lewis acids such as Li+ are present in acetonitrile, while desorption is absent for the carboxylate binding under the same conditions. The kinetics for binding are faster when the interfacial proton concentration is high; however, the saturation surface coverage is about 1/3 lower than for base-pretreated samples. The spectroscopic properties are consistent with ester hydrolysis by the base-pretreated metal oxide surfaces. The efficiency for intermolecular Ru-III/II electron hopping between surface bound compounds approaches zero when the proton concentration is low. Protons or lithium cations promote rapid and reversible oxidation-reduction of all the surface bound compounds. The origin of this cation effect is speculative but may reflect the translational mobility of the surface bound compounds. Small changes in the Ru-III/II formal reduction potentials, < 100 mV, were observed with pH pretreatment. High proton concentrations favor interfacial electron injection where low proton concentrations favor the formation of long-lived excited states.