Temperature-dependent electron injection from Ru(II) polypyridyl compounds with low lying ligand field states to titanium dioxide

The Ru(II) compounds Ru(bpy)(2)(ina)(2)(PF6)(2), Ru(deeb)(2)(py)(2)(PF6)(2), Ru(deeb)(bpy)(2)(PF6)(2), and Ru(dcb)(bpy)(2)(PF6)(2), where py is pyridine, bpy is 2,2'-bipyridine, ina is isonicotinic acid, deeb is 4,4'-(CO2Et)2,2'-bipyridine, and dcb is 4,4'-(CO2H)(2)-2,2'-bipyridine, have been prepared, characterized, and anchored to colloidal ZrO2 and TiO2 thin films for excited state and interfacial electron-transfer studies. In neat acetonitrile at 22 +/- 2 degrees C, Ru(bpy)(2)(ina)(2)(PF6)(2) is photochemically unstable and nonemissive with a short excited-state lifetime, tau < 10 ns. When anchored to ZrO2, the lifetime of Ru(bpy)(2)(ina)(2)(PF6)(2) increases to 60 ns at 22 +/- 2 degrees C and is highly temperature dependent due to the population of a low-lying state(s) that are proposed to be ligand field (LF) state(s). The LF state(s) can be populated directly from hot vibrational excited states and from the thermally equilibrated metal-to-ligand charge-transfer excited state. On TiO2 the excited-state behavior of Ru(bpy)(2)(ina)(2)(PF6)(2) is very similar to that on ZrO2, except that fast interfacial electron injection occurs, k(inj) > 10(8) s(-1) Ru(bpy)(2)(ina)(2)/TiO2 displays temperature-dependent electron injection, intersystem crossing,and emission quantum yields. A model is proposed where the direct population of LF states competes kinetically with electron injection. This behavior is contrasted with other sensitizers that have excited states localized on a surface-bound ligand.