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

Abstract

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)₂Ru_a(bpy-NH-CO-trpy)­Ru_b(bpy)­(OH₂)]⁴⁺ ([TiO₂–Ru_a^II–Ru_b^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)-<i>N</i>-((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^–)–Ru_a^II–Ru_b^III–OH₂]⁴⁺→[TiO₂–Ru_a^II–Ru_b^II–OH₂]⁴⁺