Solvation dynamics 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 solvation response functions of the solute coumarin 153 (C153) in mixtures of 1-butyl-3- methylimidazolium tetrafluoroborate ([Im₄₁][BF₄]) + acetonitrile (CH₃CN) at room temperature (20.5 C) are reported. Available density, shear viscosity, and electrical conductivity data at 25 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 S₀ â" 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 Im₄₁⁺ and CH₃CN as well as C153 rotation times (τ) are approximately related to solution viscosity (η) as D, τ η^p with values of p = -0.88, -0.77, and +0.90, respectively. Spectral/solvation response functions (S _ν(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 (〈τ _solv〉 = ∫0∞S_ν(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 ∼ 1 predicted values of 〈τ_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.