Photoinduced Electron Transfer in a Chromophore–Catalyst Assembly Anchored to TiO2

Photoinduced formation, separation, and buildup of multiple redox equivalents are an integral part of cycles for producing solar fuels in dye-sensitized photoelectrosynthesis cells (DSPECs). Excitation wavelength-dependent electron injection, intra-assembly electron transfer, and pH-dependent back electron transfer on TiO2 were investigated for the molecular assembly [((PO3H2-CH2)-bpy)2Rua(bpy-NH-CO-trpy)Rub(bpy)(OH2)]4+ ([TiO2–Rua II–Rub II–OH2]4+; ((PO3H2-CH2)2-bpy = ([2,2′-bipyridine]-4,4′-diylbis(methylene))diphosphonic acid); bpy-ph-NH-CO-trpy = 4-([2,2′:6′,2″-terpyridin]-4′-yl)-N-((4′-methyl-[2,2′-bipyridin]-4-yl)methyl) benzamide); bpy = 2,2′-bipyridine). This assembly combines a light-harvesting chromophore and a water oxidation catalyst linked by a synthetically flexible saturated bridge designed to enable long-lived charge-separated states. Following excitation of the chromophore, rapid electron injection into TiO2 and intra-assembly electron transfer occur on the subnanosecond time scale followed by microsecond–millisecond back electron transfer from the semiconductor to the oxidized catalyst, [TiO2(e–)–Rua II–Rub III–OH2]4+→[TiO2–Rua II–Rub II–OH2]4+.