Department of Physics, College of Physical Science, Nankai University, Tianjin, China.
J Phys Chem B. 2013 Apr 25;117(16):4291-304. doi: 10.1021/jp305430a. Epub 2012 Aug 31.
The dynamic Stokes shift of coumarin 153, measured with a combination of broad-band fluorescence upconversion (80 fs resolution) and time-correlated single photon counting (to 20 ns), is used to determine the complete solvation response of 21 imidazolium, pyrrolidinium, and assorted other ionic liquids. The response functions so obtained show a clearly bimodal character consisting of a subpicosecond component, which accounts for 10-40% of the response, and a much slower component relaxing over a broad range of times. The times associated with the fast component correlate with ion mass, confirming its origins in inertial solvent motions. Consistent with many previous studies, the slower component is correlated to solvent viscosity, indicating that its origins lie in diffusive, structural reorganization of the solvent. Comparisons of observed response functions to the predictions of a simple dielectric continuum model show that, as in dipolar solvents, solvation and dielectric relaxation involve closely related molecular dynamics. However, in contrast to dipolar solvents, dielectric continuum predictions systematically underestimate solvation times by factors of at least 2-4.
香豆素 153 的动态斯托克斯位移，通过宽带荧光上转换（80fs 分辨率）和时间相关单光子计数（20ns）的组合进行测量，用于确定 21 种咪唑鎓、吡咯烷鎓和其他各种离子液体的完全溶剂化响应。由此得到的响应函数显示出明显的双峰特征，包括占响应的 10-40%的亚皮秒分量和在很宽的时间范围内弛豫的慢分量。与快速分量相关的时间与离子质量相关，证实了其起源于惯性溶剂运动。与许多先前的研究一致，较慢的分量与溶剂粘度相关，表明其起源在于溶剂的扩散、结构重组。将观察到的响应函数与简单介电连续体模型的预测进行比较表明，与偶极溶剂一样，溶剂化和介电弛豫涉及密切相关的分子动力学。然而，与偶极溶剂相反，介电连续体预测系统地低估了溶剂化时间，至少低估了 2-4 倍。
