Temperature-dependent ultrafast solvation dynamics of choline chloride-based deep eutectic solvent (DES) and hydroxyl functionalized room temperature ionic liquids (RTILs): Exploring the difference in solvent response between DES and RTILs

The present study aims to provide a comprehensive understanding of the difference in solvent relaxation behavior between DESs and RTILs. For this purpose, dynamics of solvation have been investigated in a choline chloride-based DES and three hydroxyls functionalized ILs, keeping the hydroxyl functionalities same in both classes of media, so that the role of various inter/intra molecular interactions, including hydrogen bonding interaction on the solvent relaxation behavior can be understood. Fluorescence up-conversion spectroscopy (FLUPS) techniques coupled with time-correlated single-photon counting (TCSPC) techniques have been exploited to study the complete Stokes shift dynamics of a dipolar probe, coumarin 153 (C153) dissolved in these media. The solvent response functions generated from the complete dynamic Stokes shift data reveal a bimodal solvent relaxation behaviour having a very fast sub-picosecond and a relatively slower picosecond to sub-nanosecond solvation time component for both DES and hydroxyl ILs. The relatively slower solvation time component, which correlates with bulk viscosity of the concerned medium, is found to arise due to the diffusional motion of the constituents of both DES and hydroxyl ILs. However, temperature-dependent solvent relaxation measurements have revealed that at nearly isoviscous conditions, the solvent relaxation is much faster in the DES as compared to that in hydroxyl IL indicating the differences in solvent relaxation behaviour at the later part of the solvent response. Interestingly, when the early part of the dynamics is monitored, the amplitude associated with the ultrafast component for the DES increases significantly with the increase in temperature, whereas the same remains almost unchanged for the hydroxyl IL. This essentially indicates that even at shorter time scale, the process of solvent relaxation is considerably different for both DES and hydroxyl ILs. All these investigations have essentially demonstrated that despite having similar functionalities, different motions related to the solvent relaxation, operating at microscopic level in both these classes of solvents, are significantly different from each other.