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.

The complete solvation response of coumarin 153 (C153) has been determined over the range 10(-13)-10(-8) s in a variety of ionic liquids by combining femtosecond broad-band fluorescence upconversion and picosecond time-correlated single photon counting measurements. These data are used together with recently reported dielectric data in eight ionic liquids to test the accuracy of a simple continuum model for predicting solvation dynamics. In most cases the features of the solvation response functions predicted by the dielectric continuum model are similar to the measured dynamics of C153. The predicted dynamics are, however, systematically faster than those observed, on average by a factor of 3-5. Computer simulations of a model solute/ionic liquid system also exhibit the same relationship between dielectric predictions and observed dynamics. The simulations point to spatial dispersion of the polarization response as an important contributor to the over-prediction of solvation rates in ionic liquids.

Dielectric and solvation data on mixtures of 1-butyl-3-methylimidazilium tetrafluoroborate ([Im41][BF4]) + water are reported and used to examine the utility of dielectric solvation models. Dielectric permittivity and loss spectra (25 °C) were recorded over the frequency range 200 MHz to 89 GHz at 17 compositions and fit to a 4-Debye form. Dynamic Stokes shift measurements on the solute coumarin 153 (C153), made by combining fluorescence upconversion (80 fs resolution) and time-correlated single photon counting data (20 ns range), were used to determine the solvation response at 7 compositions (20.5 °C). All properties measured here were found to depend upon mixture composition in a simple continuous manner, especially when viewed in terms of volume fraction. Solvation response functions predicted by a simple dielectric continuum model are similar to but ∼7-fold faster than the spectral response functions measured with C153. The solvation data are in better agreement with the recently published predictions of a semimolecular model of Biswas and co-workers [J. Phys. Chem. B 2011, 115, 4011], but these latter predictions are systematically slow by a factor of ∼3.

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 ↔ S1 transition of C153 are slightly (≤15%) larger in neat [Im41][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 (Sν(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)(∞)Sν(t) dt) follow a power law on viscosity for mixturecompositions 0.2 ≤ x(IL) ≤ 1 with p = 0.79. With recent broad-band dielectric measurements [J. Phys. Chem. B 2012, 116, 7509] asinput, a simple dielectric continuum model provides predictions for solvation response functions that correctly capture thedistinctive 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.

Steady-state absorption and emission and femtosecond time-resolved emission spectroscopy of two benzylidene malononitriles, 2-[4-(dimethylamino)benzylidene])malononitrile (DMN) and julolidinemalononitrile (JDMN), are reported in a variety of room-temperature solvents. Solvatochromic shifts of these molecules are consistent with dielectric continuum descriptions and an S(1)-S(0) dipole moment change of 8.5 D. Time-resolved spectra show modest dynamic Stokes shifts of approximately 1000 cm(-1) occurring independently of fluorescence decay, which takes place in 0.5-5 ps in most room-temperature solvents. Absorption transition moments and fluorescence decay times are used to determine radiative rate constants: k(rad) = 0.32 +/- 0.02 ns(-1) in DMN and 0.28 +/- 0.02 ns(-1) in JDMN, assumed to be independent of solvent. Quantum yield data together with these radiative rates provide the reaction rate constants k(rxn) associated with the internal conversion process of these molecules in 33 representative solvents at 298 K and in several solvents as functions of temperature. Reaction rates of JDMN are systematically lower than those of DMN by a factor of 2.0. Values of k(rxn) in series of homologous solvents or in a single solvent at different temperatures are correlated to solvent viscosity eta and temperature T in the manner k(rxn)/T proportional to eta(-p) with exponents 0.2 < or = p < or = 0.8. Solvent polarity appears to influence these reactions such that for a given viscosity reaction in high polarity solvents is significantly slower than in nonpolar solvents. However, this conclusion is predicated on the assumption that reactive friction is identical in solvents of the same viscosity, which is unlikely to be quantitatively correct. The observed reaction rates and their solvent dependence are discussed in terms of isomerization about the C=C bond occurring on a shelf-like potential.

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Ionic liquids with cyano anions have long been used because of their unique combination of low-melting temperatures, reduced viscosities, and increased conductivities. Recently we have shown that cyano anions in ionic liquids are particularly interesting for their potential use as electron donors to excited state photo-acceptors [B. Wu et al., J. Phys. Chem. B 119, 14790-14799 (2015)]. Here we report on bulk structural and quantum mechanical results for a series of ionic liquids based on the 1-ethyl-3-methylimidazolium cation, paired with the following five cyano anions: SeCN(-), SCN(-), N(CN)2 (-), C(CN)3 (-), and B(CN)4 (-). By combining molecular dynamics simulations, high-energy X-ray scattering measurements, and periodic boundary condition DFT calculations, we are able to obtain a comprehensive description of the liquid landscape as well as the nature of the HOMO-LUMO states for these ionic liquids in the condensed phase. Features in the structure functions for these ionic liquids are somewhat different than the commonly observed adjacency, charge-charge, and polarity peaks, especially for the bulkiest B(CN)4 (-) anion. While the other four cyano-anion ionic liquids present an anionic HOMO, the one for Im2,1 (+)/B(CN)4 (-) is cationic.