Solvation Dynamics of Several Choline Chloride-based Deep-Eutectic Solvents

Deep-eutectic solvents (DES), formed by mixing two or more components of pure materials, show deeply suppressed melting points and are in the liquid state at room temperature. Intermolecular hydrogen bonding, crystal lattice frustration, and entropy gain upon mixing have been considered responsible for this phenomenon. Dynamic solvation responses of coumarin 153 in a range of choline chloride-based DES were investigated with time-resolved fluorescence emission spectroscopy at room temperature. Time-resolved Stokes shifts were used to quantify dynamic solvation, with the solvation response function in DES found to be a biexponential function of time in contrast to the single exponential commonly observed in molecular solvents. This biexponential character of the solvation response is similar to that previously observed for ionic liquids, while solvation times are similar or slightly faster in DES than in ionic liquids of comparable viscosity. Average solvation times obtained from the dynamic Stokes shifts showed a partial correlation with viscosity suggesting that a model that only takes viscosity into account is not suitable for describing DES solvation.