Photoionization of [Ru(bpy)3]2+:: A catalytic cycle with water as sacrificial donor

The photoionization of the tris-2,2'-bipyridyl ruthenium(II) ion [Ru(bpy)(3)](2+) in water was investigated by laser-flash photolysis at 308, 355, and 532 nm up to very high excitation intensities I-exc (500...900 mJ/cm(2)). By single-pulse absorption and two-pulse luminescence measurements, it was established that the mechanism is cyclic and comprises three steps: excitation of [Ru(bpy)(3)](2+) to the metal-to-ligand charge-transfer (MLCT) excited state *[Ru(bpy)(3)](2+) ionization of *[Ru(bpy)(3)](2+)by a second photon to give a hydrated electron e(aq)(.-) and the oxidized complex [Ru(bpy)(3)](3+), and photoreduction of [Ru(bpy)(3)](3+) by water. No other species is involved in the reaction. Experiments on chemically generated [Ru(bpy)(3)](3+) yield direct evidence for the photoreduction step and show that it is completed in less than 30 ns. The concentrations of [Ru(bpy)(3)](2+), *[Ru(bpy)(3)](2+) [Ru(bpy)(3)](3+), and e(aq)(.-) at the end of the laser pulse were measured as functions of I-exc. Closed- form expressions for these dependences were derived by a kinetic treatment. Very good simultaneous fits to the concentrations of all species were obtained with three global kinetic parameters only. A decision as to whether the photoreduction leads back to [Ru(bpy)(3)](2+) or to *[Ru(bpy)(3)](2+) is kinetically impossible in that system, but evidence pointing to a delayed generation of *[Ru(bpy)(3)](2+) by this reaction step is presented, which would also explain short-time anomalies of the luminescence reported in the literature. The quantum yield phi(2) of the photoionization step is the same at 308 and 355 nm, is independent of pH, and is an order of magnitude higher (0.016 +/- 0.001) than previously thought. The quantum yield phi(3) of the photoreduction is comparable to phi(2) at pH 7 but lower by a factor of 14 at pH 0.