Evidence for Conduction Band-Mediated Two-Electron Reduction of a TiO2-Bound Catalyst Triggered by Visible Light Excitation of Co-Adsorbed Organic Dyes

Abstract

Here, we report direct spectroscopic evidence of a visible light-induced two-electron-transfer reaction to a molecular model catalyst, facilitated by conduction band mediation. Using a simple, yet remarkably reproducible, co-sensitization approach, we anchored dyes and catalysts to mesoporous TiO2 thin films, at dye/catalyst ratios of 30:1, 15:1, and 8:1. The distinct optical spectroscopic signatures of different redox states of the catalyst used, Co-protoporphyrin IX, allows for transient absorption detection of intermediate and final photoproducts in the photoexcited co-sensitized dye-TiO2-catalyst assemblies. We show that by tuning the dye/catalyst ratios, control over whether photoexcitation results in one or two electrons being transferred to the catalyst is achieved. The two-electron-transfer process is favored by a large excess of photosensitizer compared to catalyst.