Photostability of Phosphonate-Derivatized, RuII Polypyridyl Complexes on Metal Oxide Surfaces

The photostability of [Ru-II(bpy)2(4,4'-(PO3H2)(2)bpy)]Cl-2 (bpy = 4,4'-bipyridine) on nanocrystalline TiO2 and ZrO2 films was investigated using a standard measurement protocol. Stability was evaluated by monitoring visible light absorbance spectral changes, in real time, during 455 nm photolysis (30 nm fwhm, 475 mW/cm(2)) in a variety of conditions relevant to dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. Desorption (k(des)) and photochemical (k(chem)) processes were observed and found to be dependent upon solvent, anion, semiconductor, and presence of oxygen. Both processes are affected by oxygen with k(des) and k(photo) noticeably smaller in argon saturated solution. Desorption was strongly solvent and pH dependent with desorption rates increasing in the order: methanol (MeOH) approximate to acetonitrile (MeCN) < propylene carbonate (PC) < pH 1 << pH 7. Photochemistry occurred in MeOH and PC but not in aqueous, 0.1 M HClO4 and MeCN. The anion and solvent dependence of k(photo) strongly suggests the photoreaction involves ligand substitution initiated by population of metal centered d-d states. The relative stability of -PO3H2- versus -COOH-substituted [Ru-II(bpy)(3)](2+) was also quantitatively established.