Variable-temperature emission studies of solvation dynamics: Evidence for coupling of solvation to chromophore structural dynamics in the evolution of charge-transfer excited states

Variable-temperature emission data over the range 90-298 K have been collected for a series of bipyridyl complexes of Ru-II. Spectra obtained for [Ru(dmb)(3)](2+) (dmb = 4,4'-dimethyl-2,2'-bipyridine), [Ru(dpb)(3)](2+) (dpb = 4,4'-diphenyl-2,2'-bipyridine), [Ru(dotb)(3)](2+) (dotb = 4,4'-di-o-tolyl-2,2'-bipyridine), and [Ru(dmesb)(3)](2+) (dmesb = 4,4'-dimesityl-2,2'-bipyridine) in 4:1 EtOH/MeOH show similar trends in terms of both the red shift of the emission spectrum and the thermal breadth of the solvent response as the temperature is increased through the glass-to-fluid transition. In contrast, data collected in 2-MeTHF show a strong dependence on the identity of the chromophore, the details of the spectral evolution qualitatively correlating with changes in the steric demands of the system. The most dramatic effect is observed for [Ru(dmesb)(3)](2+), in which there is an apparent change in the nature of the emitting species with increasing temperature. These observations suggest a strong coupling of solvation dynamics and solute structure in the low-temperature regime as well as at intermediate temperatures where the structure of the chromophore is evolving in the course of excited-state relaxation. The results underscore the potential importance of specific solvent-solute interactions in the dynamics of solvation for cases in which large amplitude molecular motion of the chromophore accompanies thermalization of the excited state.