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)(2)Ru-a(bpy-NH-CO-trpy)Ru-b(bpy)(OH2)](4+) OH2](4+;) ((PO3H(2)-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-y1)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+).