Proton-Controlled Electron Injection from Molecular Excited States to the Empty States in Nanocrystalline TiO2

Abstract

The excited-state and redox properties of Ru(deeb)(bpy)2(PF6)2, Ru(dcb-H2)(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-H2 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-pret...