School of Chemistry, University of Hyderabad, Hyderabad 500 046, India.
J Phys Chem B. 2020 Mar 26;124(12):2473-2481. doi: 10.1021/acs.jpcb.0c00594. Epub 2020 Mar 17.
Deep eutectic solvents (DESs) are novel environment-friendly media for a variety of applications. In order to obtain insight into the structure and dynamics of some less-explored DESs comprising ethylene glycol and tetraalkylammonium bromide salts with variable alkyl chain length, we have captured complete dynamics occurring in these solvents in a timescale of few femtoseconds to several nanoseconds by monitoring the time-dependent fluorescence Stokes shift of coumarin 153 employing a combination of time-correlated single-photon counting and fluorescence upconversion techniques. The solvent response function constructed from the measured data reveals a sub-picosecond component (∼0.8 ps, 20-35%) in addition to a slow component (180-475 ps) with a distribution of relaxation time. The slow time component is found to be strongly dependent on the viscosity of the medium, indicating that it arises from the diffusive motions of the solvent constituents into and out of the solvation shell, whereas the ultrafast time component, which is nearly independent of the solvent viscosity, arises from fast local motions of the constituents in the immediate vicinity of the solute molecule.
深共晶溶剂（DESs）是一种新型的环保介质，可用于多种应用。为了深入了解一些探索较少的 DESs 的结构和动力学，这些 DESs 由乙二醇和具有不同烷基链长的四烷基溴化铵盐组成，我们通过监测香豆素 153 的时间依赖性荧光斯托克斯位移，采用时间相关单光子计数和荧光上转换技术的组合，在几飞秒到几纳秒的时间尺度内捕获了这些溶剂中发生的完整动力学。从测量数据构建的溶剂响应函数除了慢成分（180-475 ps）外，还显示出亚皮秒成分（∼0.8 ps，20-35%），具有弛豫时间分布。发现慢时间成分强烈依赖于介质的粘度，表明它是由溶剂成分进出溶剂化壳的扩散运动引起的，而超快时间成分几乎与溶剂粘度无关，它是由溶质分子附近的成分的快速局部运动引起的。
