Direct spectroscopic evidence for constituent heteroatoms enhancing charge recombination at a TiO₂-ruthenium dye interface

A series of three bis(tridentate) cycloruthenated sensitizers with furyl, thiophene, or selenophene units attached to the cyclometalated ligand were designed to examine how chalcogen atoms effect interfacial electron transfer events that occur following the absorption of visible light by the sensitizers when attached to mesoporous titania thin films immersed in CH₃CN electrolytes. Spectroelectrochemistry established that the Ru^III/II reduction potentials were confined to the 0.954-0.965 V vs NHE range for the series and that the density of TiO₂ acceptor states were sensitizer-independent. Pulsed light excitation into the metal-to-ligand charge transfer band of the sensitized thin films resulted in rapid excited state injection, k_inj > 10⁸ s^-1. Charge recombination (Ru^III/TiO₂(e^-) → Ru ^II/TiO₂) rate constants were insensitive to the identity of the cyclometalated compound, yet the open circuit photovoltages were markedly lower for the compound containing selenophene. These lower photovoltages appear to be a manifestation of a 4-fold-larger, second-order rate constant measured for the reaction between triiodide and TiO₂(e^-) in the case that the selenophene comprises the donor fragment of the dye. Adduct formation between oxidized iodide(s) and the selenophene moiety of the sensitizer is implicated.