Solvation Dynamics in a Prototypical Ionic Liquid plus Dipolar Aprotic Liquid Mixture: 1-Butyl-3-methylimidazolium Tetrafluoroborate plus Acetonitrile

Solvation energies, rotation times, and 100 fs to 20 ns solvation response functions of the solute coumarin 153 (C153) in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([Im(41)][BF4]) + acetonitrile (CH3CN) at room temperature (20.5 degrees C) are reported. Available density, shear viscosity, and electrical conductivity data at 25 degrees C are also collected and parametrized, and new data on refractive indices and component diffusion coefficients presented. Solvation free energies and reorganization energies associated with the S-0 <-> S-1 transition of C153 are slightly (<= 15%) larger in neat [Im(41)][BF4] than in CH3CN. No clear evidence for preferential solvation of C153 in these mixtures is found. Composition-dependent diffusion coefficients (D) of Im(41)(+) and CH3CN as well as C153 rotation times (tau) are approximately related to solution viscosity (eta) as D, tau proportional to eta(p) with values of p = -0.88, -0.77, and +0.90, respectively. Spectral/solvation response functions (S-nu(t)) are bimodal at all compositions, consisting of a subpicosecond fast component followed by a broadly distributed slower component extending over ps-ns times. Integral solvation times ( = integral S-infinity(0)nu(t) dt) follow a power law on viscosity for mixture compositions 0.2 <= x(IL) <= 1 with p = 0.79. With recent broad-band dielectric measurements [J. Phys. Chem. B 2012, 116, 7509] as input, a simple dielectric continuum model provides predictions for solvation response functions that correctly capture the distinctive bimodal character of the observed response. At x(IL) similar to 1 predicted values of (tau(solv)) are smaller than those observed by a factor of 2-3, but the two become approximately equal at x(IL) = 0.2. Predictions of a recent semimolecular theory [J. Phys. Chem. B 2011, 115, 4011] are less accurate, being uniformly slower than the observed solvation dynamics.