Intermolecular dynamics of deep eutectic solvents probed via dynamic optical Kerr effect spectroscopy

Deep eutectic solvents (DESs) are room-temperature liquid-state mixtures composed of two or more components that exhibit a significant depression in their melting points compared with those of their pure components. Owing to their facile preparation and broad range of component combinations, DESs are gaining attraction in materials chemistry and engineering. The intermolecular interactions in DESs are often more diverse than those in ionic liquids or hydrogen-bonding liquids, because they include different kinds of species, which are key to the strong eutectic effect. Thus, DESs are an attractive research target in chemical physics and physical chemistry from both dynamic and static perspectives. This article focuses on the intermolecular dynamics of DESs, particularly those explored using dynamic optical Kerr effect spectroscopy (OKES) and other spectroscopic techniques. OKES is a robust spectroscopic technique for observing molecular dynamics in the low-frequency (or terahertz to subterahertz) region, as it captures both intermolecular vibrations and collective orientational dynamics. Notably, this technique enables detecting dynamics without the need for a probe molecule. This article summarizes key findings from existing research, including characteristic vibrational modes and relaxation processes, as well as their dependence on DES constituents and the effects of water and temperature. Finally, potential directions for future research on DES dynamics are discussed. We hope that this Perspective will help not only to understand the current research situation in DES dynamics, but also to drive further progress and development of the DES and its related research area.