Jin Hui, Baker Gary A, Arzhantsev Sergei, Dong Jing, Maroncelli Mark
Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, Pennsylvania 16802, USA.
J Phys Chem B. 2007 Jun 28;111(25):7291-302. doi: 10.1021/jp070923h. Epub 2007 May 27.
Steady-state and time-resolved emission spectroscopy with 25 ps resolution are used to measure equilibrium and dynamic aspects of the solvation of coumarin 153 (C153) in a diverse collection of 21 room-temperature ionic liquids. The ionic liquids studied here include several phosphonium and imidazolium liquids previously reported as well as 12 new ionic liquids that incorporate two homologous series of ammonium and pyrrolidinium cations. Steady-state absorption and emission spectra are used to extract solvation free energies and reorganization energies associated with the S0 <--> S1 transition of C153. These quantities, especially the solvation free energy, vary relatively little in ionic liquids compared to conventional solvents. Some correlation is found between these quantities and the mean separation between ions (or molar volume). Time-resolved anisotropies are used to observe solute rotation. Rotation times measured in ionic liquids correlate with solvent viscosity in much the same way that they do in conventional polar solvents. No special frictional coupling between the C153 and the ionic liquid solvents is indicated by these times. But, in contrast to what is observed in most low-viscosity conventional solvents, rotational correlation functions in ionic liquids are nonexponential. Time-resolved Stokes shift measurements are used to characterize solvation dynamics. The solvation response functions in ionic liquids are also nonexponential and can be reasonably represented by stretched-exponential functions of time. The solvation times observed are correlated with the solvent viscosity, and the much slower solvation in ionic liquids compared to dipolar solvents can be attributed to their much larger viscosities. Solvation times of the majority of ionic liquids studied appear to follow a single correlation with solvent viscosity. Only liquids incorporating the largest phosphonium cation appear to follow a distinctly different correlation.
采用具有25皮秒分辨率的稳态和时间分辨发射光谱，来测量香豆素153（C153）在21种不同室温离子液体中的溶剂化平衡和动力学方面。这里研究的离子液体包括之前报道过的几种鏻盐和咪唑盐液体，以及12种新的离子液体，它们含有两个同系系列的铵阳离子和吡咯烷鎓阳离子。利用稳态吸收光谱和发射光谱来提取与C153的S0<-->S1跃迁相关的溶剂化自由能和重组能。与传统溶剂相比，这些量，尤其是溶剂化自由能，在离子液体中的变化相对较小。在这些量与离子间的平均间距（或摩尔体积）之间发现了一些相关性。利用时间分辨各向异性来观察溶质的旋转。在离子液体中测量的旋转时间与溶剂粘度的相关性，与在传统极性溶剂中的情况大致相同。这些时间并未表明C153与离子液体溶剂之间存在特殊的摩擦耦合。但是，与在大多数低粘度传统溶剂中观察到的情况相反，离子液体中的旋转相关函数是非指数型的。利用时间分辨斯托克斯位移测量来表征溶剂化动力学。离子液体中的溶剂化响应函数也是非指数型的，并且可以用时间的拉伸指数函数合理地表示。观察到的溶剂化时间与溶剂粘度相关，与偶极溶剂相比，离子液体中溶剂化慢得多可归因于它们大得多的粘度。所研究的大多数离子液体的溶剂化时间似乎与溶剂粘度遵循单一的相关性。只有含有最大鏻阳离子的液体似乎遵循明显不同的相关性。
