Solvation Dynamics In Ionic Liquids Research Articles

Nonlinear measurements of kinetics and generalized dynamical modes. II. Application to a simulation of solvation dynamics in an ionic liquid.

Solvation dynamics in ionic liquids show features that are often associated with supercooled liquids, including "stretched" nonexponential relaxation. To better understand the mechanism behind the stretching, the nonlinear mode-correlation methods proposed in Paper I [S. R. Hodge and M. A. Berg, J. Chem. Phys. 155, 024122 (2021)] are applied to a simulation of a prototypical ionic liquid. A full Green's function is recovered. In addition, specific tests for non-Gaussian dynamics are made. No deviations from Gaussian dynamics are found. This finding is incompatible with rate heterogeneity as a cause of the nonexponential relaxation and appears to be in conflict with an earlier multidimensional analysis of the same data. Although this conflict is not resolved here, this work does demonstrate the practicality of mode-correlation analysis in the face of finite datasets and calculations.

Observations of probe dependence of the solvation dynamics in ionic liquids.

Solvation and rotational dynamics of 4-aminophthalimide (4AP) in four ionic liquids (ILs) are measured using a combination of fluorescence upconversion spectroscopy and time-correlated single photon counting. These data are compared with previously reported data for coumarin 153 (C153) to investigate the probe dependence of solvation dynamics. No fast component (<15 ps) in the fluorescence anisotropy is observed with 4AP. The differences between the solvation response functions of 4AP and C153 are significant in all four ILs, but these differences can be reduced by applying a correction for solute rotation using measured emission anisotropies. Response functions of other probes available in the literature are used to further examine the validity of this correction. The corrected data are also compared to predictions of dielectric continuum models of solvation. By replacing the measured static conductivity of the ILs with an estimated value, such predictions show good agreement with the observed spectral response functions, especially when the anion size is small.

Conductivity and Solvation Dynamics in Ionic Liquids.

It was shown recently that a simple dielectric continuum model predicts the integral solvation time of a dipolar solute ⟨τsolv⟩ to be inversely proportional to the electrical conductivity σ0 of an ionic solvent or solution. In this Letter, we provide a more general derivation of this connection and show that available data on coumarin 153 (C153) in ionic liquids generally support this prediction. The relationship between solvation time and conductivity can be expressed by ln(⟨τsolv⟩/ps) = 4.37 - 0.92 ln (σ0/S m(-1)) in 34 common ionic liquids.

Steady-state and time-resolved fluorescence behavior of coumarin-153 in a hydrophobic ionic liquid and ionic liquid–toluene mixture

Steady-state and time-resolved fluorescence behavior of C-153 in neat ultra hydrophobic 1-(2-methoxyethyl)-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and its mixture with toluene has been investigated. Steady-state absorption and emission spectral behavior of C-153 is found not to be affected by the nonpolar cosolvent. Time-resolved fluorescence anisotropy measurements show that the rotational diffusion of the probe becomes faster in presence of toluene. Solvation dynamics in ionic liquid–toluene mixtures is found to be biphasic, and the average solvation time is found to decrease with the addition of nonpolar cosolvent to the ionic liquid. A significant blue shift of the time-zero maximum of fluorescence spectrum upon addition of toluene indicates a more nonpolar environment around C-153 at the initial stage of dynamics in the mixed solvent system. A comparison of the dynamics of solvation data in ionic liquid and ionic liquid–toluene mixture suggests that toluene can effectively penetrate into the ionic liquid-rich cybotactic region of the probe. This is explained in terms of favorable hydrophobic interaction between ionic liquid and toluene.

ChemInform Abstract: Solvation Dynamics in Ionic Liquids: What We Have Learned from the Dynamic Fluorescence Stokes Shift Studies

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Solvation Dynamics in Ionic Liquids: What We Have Learned from the Dynamic Fluorescence Stokes Shift Studies

Ionic liquids have generated tremendous excitement as a new class of solvent systems having the potential to serve as an environmentally friendly alternative to the traditionally used volatile organic compounds and have emerged as specialty media or materials for many advanced processes and devices. Solvent reorganization in ionic liquids is a complex process, which differs significantly from that in conventional polar molecular solvents. This Perspective highlights the present understanding of the mechanism and time scale of the dynamics of solvation in ionic liquids obtained from the dynamic fluorescence Stokes shift studies on dipolar molecules taking the related theoretical results into consideration.

1 ∕ f spectrum and memory function analysis of solvation dynamics in a room-temperature ionic liquid

To understand the nonexponential relaxation associated with solvation dynamics in the ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate, we study power spectra of the fluctuating Franck–Condon energy gap of a diatomic probe solute via molecular dynamics simulations. Results show 1∕f dependence in a wide frequency range over 2–3 decades, indicating distributed relaxation times. We analyze the memory function and solvation time in the framework of the generalized Langevin equation using a simple model description for the power spectrum. It is found that the crossover frequency toward the white-noise plateau is directly related to the time scale for the memory function and thus the solvation time. Specifically, the low crossover frequency observed in the ionic liquid leads to a slowly decaying tail in its memory function and long solvation time. By contrast, acetonitrile characterized by a high crossover frequency and (near) absence of 1∕f behavior in its power spectra shows fast relaxation of the memory function and single exponential decay of solvation dynamics in the long-time regime.