Stokes Shift Dynamics in Ionic Liquids: Temperature Dependence

A recently developed molecular theory is used to investigate the temperature dependence of the dynamic fluorescence Stokes shift of a dipolar solute in seven imidazolium ionic liquids. The temperature range considered is 278.15-338.15 K, for which experimental dielectric relaxation data (frequency range of 0.2 <= v/GHz <= 89) are available. The theory used here explores and substantiates the relation between fluorescence spectral dynamics and dielectric relaxation in ionic liquids. The slope of the temperature-dependent change in the calculated total dynamic Stokes shift is predicted to follow an inverse-linear correlation with that (slope) of the experimentally measured temperature dependence of the static dielectric constant of these liquids. This explains the experimentally observed decrease of polarity parameter, ET(30), with temperature for several different ionic liquids. A significant part of the stabilization energy of a dissolved excited dipolar solute is found to arise from the reorientational dynamics of the dipolar ions (mainly imidazolium cations) of these liquids. The separated solute-solvent dipole-ion interaction contribution to the shift exhibits a stronger temperature dependence than the dipole-dipole interaction component. Calculations predict bimodal Stokes shift dynamics for all of these liquids with a fast initial component arising from rapid angular adjustment of the dipolar ions. The slow, stretched-exponential component shows a systematic temperature dependence and is linked to an environment rearrangement through the center-of-mass motion of the ions. Subsequently, calculated solvation activation energies are found to be closely related to those observed in the corresponding conductivity and viscosity measurements for these ionic liquids.