PHOTOPHYSICS OF RU(II)-DIIMINE COMPLEXES IN H2O-CH3CN MIXED-SOLVENTS

The photophysical properties of Ru(bpy)(3)(2+), Ru(bpz)(3)(2+), and Ru(bpz)(2)(bpm)(2+) (bpy = 2,2'-bipyridine, bpz = 2,2'-bipyrazine, bpm = 2,2'-bipyrimidine) have been studied in neat and mixed CH3CN-H2O solutions. The temperature-dependent lifetimes, room-temperature emission spectra, and quantum yields were determined for the complexes; the rate constants and activation parameters for the various photophysical pathways were calculated from the data. Shifts of the emission energy (E(cm)) as a function of solvent composition suggest that the excited states are preferentially solvated by water molecules. Although radiative decay is relatively insensitive to the change of solution medium, nonradiative decay (k(nr)) and the temperature-dependent thermal population of the metal centered d-d state, which account for more than 90% of the decay of the emitting MLCT state, are strongly dependent on solvent and temperature; the observed phenomena can be well understood on the basis that the energy level of the MLCT state, which possesses a static dipole moment, is sensitive to the polarity change of the solution medium. At lower temperatures, nonradiative decay is the dominant process, and the overall deactivation of the emitting state is faster in water-rich than in acetonitrile-rich solutions; at higher temperatures, the thermal population of the d-d state is the dominant process, and the overall decay is slower in water-rich solvents. The nonlinear correlations between In k(nr) and E(cm) in these solvents are qualitatively explained by the energy gap law for the radiationless transition and support the argument that water molecules provide an unusually high solvent reorganization energy for the decay of the MLCT excited states.