Complete solvation response of coumarin 153 in ionic liquids.

The dynamic Stokes shift of coumarin 153, measured with a combination of broad-band fluorescence upconversion (80 fs resolution) and time-correlated single photon counting (to 20 ns), is used to determine the complete solvation response of 21 imidazolium, pyrrolidinium, and assorted other ionic liquids. The response functions so obtained show a clearly bimodal character consisting of a subpicosecond component, which accounts for 10-40% of the response, and a much slower component relaxing over a broad range of times. The times associated with the fast component correlate with ion mass, confirming its origins in inertial solvent motions. Consistent with many previous studies, the slower component is correlated to solvent viscosity, indicating that its origins lie in diffusive, structural reorganization of the solvent. Comparisons of observed response functions to the predictions of a simple dielectric continuum model show that, as in dipolar solvents, solvation and dielectric relaxation involve closely related molecular dynamics. However, in contrast to dipolar solvents, dielectric continuum predictions systematically underestimate solvation times by factors of at least 2-4.