Do Dipolar Cosolvents Mitigate Microheterogeneity in Deep Eutectic Solvents? Solvation Dynamics and Solute Rotations in Glyceline/Methanol Solutions.

Steady-state and time-resolved fluorescence was used to investigate the solvation and rotational dynamics of coumarin 153 (C153) and coumarin 343 (C343) in binary solutions of methanol and glyceline, a deep eutectic solvent (DES) composed of a 1:2 molar ratio of choline chloride and glycerol. Time-resolved Stokes shifts were used to quantify the solvation dynamics and were found to be biexponential for both C153 and C343 with nearly identical integral solvation times. The solvation times were also found to be weakly dependent upon solution viscosity, and a power law exponent of p = 0.18 was found for mole fractions of glyceline greater than xDES = 0.2. Solute rotational dynamics were explored using time-resolved fluorescence anisotropy. The reorientation times of C153 and C343 were found to be single exponential at all mixture compositions but did not follow a power-law dependence on solution viscosity. Instead, there was evidence for preferential solvation of the probes by components of glyceline.