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)(2)Ru(bpy-Mebim(2)py)Ru(bpy)-(OH2)(4+) (Ru-a(II)-Ru-b(II)-OH2) with Ru-a as light-harvesting chromophore and Ru-b as water oxidation catalyst (dcb = 4,4'-dicarboxylic acid-2,2'-bipyridine; bpy-Mebim(2)py = 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 - Ru-b(II) - OH2 site to give TiO2(e(-)) - Ru-a(II)-Ru-b(III)-OH23+ as a redox-separated transient. The oxidized assembly, TiO2-Ru-a(II)-Ru-b(III)-OH23+, similarly undergoes excitation and electron injection to give TiO2-(e(-))-Ru-a(II)-Ru-b(IV)=O2+, with Ru-b(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)(2)bpy)](2+) bound to TiO2 (TiO2-Ru2+), whereas the rates of back electron transfer, TiO2(e(-)) -> Ru-b(III)-OH23+ and TiO2(e(-)) -> Ru-b(IV)=O2+, are significantly decreased compared with TiO2(e(-)) -> Ru3+ back electron transfer.