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| - Solvation and Rotational Dynamics of Coumarin 153 in Ionic Liquids: Comparisons toConventional Solvents
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| - Steady-state and time-resolved emission spectroscopy with 25 ps resolution are used to measure equilibriumand dynamic aspects of the solvation of coumarin 153 (C153) in a diverse collection of 21 room-temperatureionic liquids. The ionic liquids studied here include several phosphonium and imidazolium liquids previouslyreported as well as 12 new ionic liquids that incorporate two homologous series of ammonium and pyrrolidiniumcations. Steady-state absorption and emission spectra are used to extract solvation free energies andreorganization energies associated with the S0 ↔ S1 transition of C153. These quantities, especially the solvationfree energy, vary relatively little in ionic liquids compared to conventional solvents. Some correlation isfound between these quantities and the mean separation between ions (or molar volume). Time-resolvedanisotropies are used to observe solute rotation. Rotation times measured in ionic liquids correlate with solventviscosity in much the same way that they do in conventional polar solvents. No special frictional couplingbetween the C153 and the ionic liquid solvents is indicated by these times. But, in contrast to what is observedin 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 responsefunctions in ionic liquids are also nonexponential and can be reasonably represented by stretched-exponentialfunctions of time. The solvation times observed are correlated with the solvent viscosity, and the much slowersolvation 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 solventviscosity. Only liquids incorporating the largest phosphonium cation appear to follow a distinctly differentcorrelation.
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