Photoinduced Stepwise Oxidative Activation of a Chromophore–Catalyst Assembly on TiO2

To probe light-induced redox equivalent separation and accumulation, we prepared ruthenium polypyridyl molecular assembly [(dcb)2Ru(bpy-Mebim2py)Ru(bpy)(OH2)]4+ (Rua II–Rub II–OH2) with Rua as light-harvesting chromophore and Rub as water oxidation catalyst (dcb = 4,4′-dicarboxylic acid-2,2′-bipyridine; bpy-Mebim2py = 2,2′-(4-methyl-[2,2′:4′,4″-terpyridine]-2″,6″-diyl)bis(1-methyl-1H-benzo[d]imidazole); bpy = 2,2′-bipyridine). When bound to TiO2 in nanoparticle films, it undergoes MLCT excitation, electron injection, and oxidation of the remote −Rub II–OH2 site to give TiO2(e–)–Rua II–Rub III–OH2 3+ as a redox-separated transient. The oxidized assembly, TiO2–Rua II–Rub III–OH2 3+, similarly undergoes excitation and electron injection to give TiO2(e–)–Rua II–Rub IVO2+, with Rub IVO2+ a known water oxidation catalyst precursor. Injection efficiencies for both forms of the assembly are lower than those for [Ru(bpy)2(4,4′-(PO3H2)2bpy)]2+ bound to TiO2 (TiO2–Ru2+), whereas the rates of back electron transfer, TiO2(e–) → Rub III–OH2 3+ and TiO2(e–) → Rub IVO2+, are significantly decreased compared with TiO2(e–) → Ru3+ back electron transfer.