Energetics and dynamics of solvent reorganization in charge-transfer excited states

Abstract

The dynamics of solvation of the Ru(bpy) 2(CN) 2 metal-to-ligand charge-transfer excited state have been examined in a series of aliphatic alcohols. Steady-state emission spectra recorded at low temperature (≈ 10 K) and at room temperature were used to resolve internal-mode and solvent contributions to the emission bandshape. Time-resolved emission spectra were fit to a model that takes into account internal-mode distortions as well as time-dependent broadening and shifts in emission maxima. A single-exponential solvent relaxation function does not adequately describe the temporal development of the emission profile of Ru(bpy) 2(CN) 2 in alcohols. The evolution of the emission spectrum is clearly biphasic, and can be reasonably fit with a biexponential function. The slower of the two relaxation times is comparable to the longest longitudinal relaxation time reported for the bulk solvent. These slower components, however, represent less than half of the overall approach to equilibrium. Local heating due to above-threshold excitation, and local solvent relaxation are two likely sources of the faster dynamics.