Dye Excited States Oriented Relative to TiO2 Surface Electric Fields

Abstract

Two ruthenium compounds were anchored to mesoporous nanocrystalline TiO2 thin films to probe local electric fields generated by TiO2 reduction or cation adsorption in acetonitrile electrolytes. The metal-to-ligand charge-transfer (MLCT) excited states were well formulated as [RuIII(dtb)2(dcb–)]2+* and [RuIII(btfmb)(dcb)(btfmb–)]2+*, where dtb is 4,4′-(tert-butyl)2-2,2-bipyridine, dcb is 4,4′-(CO2H)2-2,2-bipyridine, and btfmb is 4,4′-(CF3)2-2,2-bipyridine. The MLCT excited state was orientated toward the TiO2 surface linker (antiparallel to the field) for Ru(dtb)2(dcb–)*/TiO2 and away from the surface for Ru(btfmb)(dcb)(btfmb–)*/TiO2. Reduction of the TiO2 in an electrochemical cell resulted in a blue shift of the Ru(dtb)2(dcb–)*/TiO2 photoluminescence spectrum, while the Ru(btfmb)(dcb)(btfmb–)*/TiO2 spectrum was either unchanged or was red-shifted. Accompanying these spectral shifts were bimodal changes in the PL intensity and excited-state lifetime that first increased and then decreased as the quasi-Fer...