Reductive Electron Transfer Quenching of MLCT Excited States Bound To Nanostructured Metal Oxide Thin Films

Abstract

The ruthenium compounds Ru(deeb)(bpz)2(PF6)2, Ru(deeb)2(bpz)(PF6)2, and Ru(deeb)2(dpp)(PF6)2, where deeb is 4,4‘-(CO2CH2CH3)2-2,2‘-bipyridine, bpz is 2,2‘-bipyrazine, and dpp is 2,3-bis(2-pyridyl)pyrazine, have been prepared, characterized, and anchored to mesoporous nanoparticle thin films comprised of the wide band gap semiconductor TiO2 or the insulator ZrO2. The metal-to-ligand charge-transfer (MLCT) excited states of these compounds are potent photooxidants (E°(RuII*/+) > +1.0 V vs SCE) with long lifetimes (τ > 1 μs) that efficiently oxidize iodide and phenothiazine with rate constants that approach the diffusion limit in acetonitrile. Photogalvanic cells based on the sensitized TiO2 materials yield photocurrent action spectra that agree well with the Ru(II) absorptance spectra. The photocurrent efficiency was very low, φ < 10-4. Transient absorption data show that neither the excited nor the reduced state of the ruthenium compounds efficiently inject electrons into the TiO2 particles. The cage escape yields following excited-state electron transfer are approximately 2/3 lower in the mesoporous thin films than in fluid solution. Intermolecular energy transfer across the nanoparticle surfaces is manifest in a second-order component to the excited-state relaxation kinetics.