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 isoni cotinic acid, bound to nanocrystalline TiO2 and colloidal ZrO2 films have b een studied in acetonitrile at room temperature as a function of the interf acial proton concentration. High surface proton concentrations favor a "car boxylic acid" type linkage(s) where low proton concentrations favor "carbox ylate" type binding mode(s) for Ru(II) compounds with ethyl ester or carbox ylic acid functional groups. The "carboxylic acid" linkages are unstable wh en 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-p retreated samples. The spectroscopic properties are consistent with ester h ydrolysis by the base-pretreated metal oxide surfaces. The efficiency for i ntermolecular Ru-III/II electron "hopping" between surface bound compounds approaches zero when the proton concentration is low. Protons or lithium ca tions promote rapid and reversible oxidation-reduction of all the surface b ound compounds. The origin of this cation effect is speculative but may ref lect the translational mobility of the surface bound compounds. Small chang es in the Ru-III/II formal reduction potentials, < 100 mV, were observed wi th pH pretreatment. High proton concentrations favor interfacial electron i njection where low proton concentrations favor the formation of long-lived excited states.