Efficient Light-Driven Oxidation of Alcohols Using an Organic Chromophore–Catalyst Assembly Anchored to TiO2

The ligand 5-PO3H2-2,2′:5′,2″-terthiophene-5-trpy, T3 (trpy = 2,2′:6′,2″-terpyridine), was prepared and studied in aqueous solutions along with its metal complex assembly [Ru­(T3)­(bpy)­(OH2)]2+ (T3-Ru-OH2, bpy = 2,2′-bipyridine). T3 contains a phosphonic acid group for anchoring to a TiO2 photoanode under aqueous conditions, a terthiophene fragment for light absorption and electron injection into TiO2, and a terminal trpy ligand for the construction of assemblies comprising a molecular oxidation catalyst. At a TiO2 photoanode, T3 displays efficient injection at pH 4.35 as evidenced by the high photocurrents (∼350 uA/cm2) arising from hydroquinone oxidation. Addition of [Ru­(bpy)­(OTf)]­[OTf]2 (bpy = 2,2′-bipyridine, OTf– = triflate) to T3 at the free trpy ligand forms the molecular assembly, T3-Ru-OH2, with the oxidative catalyst fragment: [Ru­(trpy)­(bpy)­(OH2)]2+. The new assembly, T3-Ru-OH2, was used to perform efficient light-driven oxidation of phenol (230 μA/cm2) and benzyl alcohol (25 μA/cm2) in a dye-sensitized photoelectrosynthesis cell.