Rotation and translation dynamics of coumarin 153 in choline chloride-based deep eutectic solvents

The equilibrium and dynamic solvation responses of coumarin 153 (C153) in a range of deep eutectic solvents (DESs) based on choline chloride with either urea (molar ratio 1:2, ChCl:U), glycerol (1:2, ChCl:G), ethylene glycol (1:2, ChCl:E), or malonic acid (1:1, ChCl:Mal) were investigated using both steady-state and time-resolved fluorescence emission spectroscopy at room temperature (298 K). From steady-state fluorescence data, "red-edge effects" were observed in all the DESs studied, attributed to spatial heterogeneity of the DES matrix. 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, which were used to obtain average solvation times (⟨τ ⟩) which are generally faster in DES than in ionic liquids of comparable viscosity. Average solvation times showed a partial correlation with viscosity between different DESs. The choline chloride-glycerol DES showed deviation from the viscosity trend observed in the other DES for both dynamic and steady-state results. Rotational reorientation times obtained from dynamic anisotropy (r(t)) measured for these DESs showed a partial correlation with viscosity between different DESs. Determination of the DES rotational coupling with C153 showed more "slip"-like behavior than the previously reported ionic liquids and dipolar solvents.