SolvationDynamics in a Prototypical Ionic Liquid+ Dipolar Aprotic Liquid Mixture:1-Butyl-3-methylimidazolium Tetrafluoroborate + Acetonitrile.

Solvation energies, rotation times,and 100 fs to 20 ns solvationresponse functions of the solute coumarin 153 (C153) in mixtures of1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) + acetonitrile (CH3CN) at room temperature (20.5°C) are reported. Available density, shear viscosity, and electricalconductivity data at 25 °C are also collected and parametrized,and new data on refractive indices and component diffusion coefficientspresented. Solvation free energies and reorganization energies associatedwith the S0↔ S1transition of C153 areslightly (≤15%) larger in neat [Im41][BF4] than in CH3CN. No clear evidence for preferential solvationof C153 in these mixtures is found. Composition-dependent diffusioncoefficients (D) of Im41+andCH3CN as well as C153 rotation times (τ) are approximatelyrelated to solution viscosity (η) as D, τ∝ ηpwith values of p= −0.88, −0.77, and +0.90, respectively.Spectral/solvation response functions (Sν(t)) are bimodal at all compositions, consistingof a subpicosecond fast component followed by a broadly distributedslower component extending over ps-ns times. Integral solvation times(⟨τsolv⟩ = ∫0∞Sν(t) dt) followa power law on viscosity for mixture compositions 0.2 ≤ xIL≤ 1 with p= 0.79.With recent broad-band dielectric measurements [J. Phys. Chem. B2012, 116, 7509] as input, a simple dielectric continuum model provides predictionsfor solvation response functions that correctly capture the distinctivebimodal character of the observed response. At xIL∼ 1 predicted values of ⟨τsolv⟩ are smaller than those observed by a factor of 2–3,but the two become approximately equal at xIL= 0.2. Predictions of a recent semimolecular theory [J. Phys. Chem. B2011, 115, 4011] are lessaccurate, being uniformly slower than the observed solvation dynamics. [ABSTRACT FROM AUTHOR]