Dynamic Solvation in Room-Temperature Ionic Liquids

Abstract

The dynamic solvation of the fluorescent probe, coumarin 153, is measured in five room-temperature ionic liquids using different experimental techniques and methods of data analysis. With time-resolved stimulatedemission and time-correlated single-photon counting techniques, it is found that the solvation is comprised of an initial rapid component of ∼55 ps. In all the solvents, half or more of the solvation is completed within 100 ps. The remainder of the solvation occurs on a much longer time scale. The emission spectra of coumarin 153 are nearly superimposable at all temperatures in a given solvent unless they are obtained using the supercooled liquid, suggesting that the solvents have an essentially glassy nature. The physical origin of the two components is discussed in terms of the polarizability of the organic cation for the faster one and the relative diffusional motion of the cations and the anions for the slower one. A comparison of the solvation response functions obtained from single-wavelength and from spectral-reconstruction measurements is provided. Preliminary fluorescence-upconversion measurements are presented against which the appropriateness of the single-wavelength method for constructing solvation correlation functions and the use of stimulated-emission measurements is considered. These measurements are consistent with the trends mentioned above, but a comparison indicates that the presence of one or more excited states distorts the stimulated-emission kinetics such that they do not perfectly reproduce the spontaneous emission data. Fluorescence-upconversion results indicate an initial solvation component on the order of ∼7 ps. December 2, 2003; In Final Form: April 20, 2004 The dynamic solvation of the fluorescent probe, coumarin 153, is measured in five room-temperature ionic liquids using different experimental techniques and methods of data analysis. With time-resolved stimulatedemission and time-correlated single-photon counting techniques, it is found that the solvation is comprised of an initial rapid component of ∼55 ps. In all the solvents, half or more of the solvation is completed within 100 ps. The remainder of the solvation occurs on a much longer time scale. The emission spectra of coumarin 153 are nearly superimposable at all temperatures in a given solvent unless they are obtained using the supercooled liquid, suggesting that the solvents have an essentially glassy nature. The physical origin of the two components is discussed in terms of the polarizability of the organic cation for the faster one and the relative diffusional motion of the cations and the anions for the slower one. A comparison of the solvation response functions obtained from single-wavelength and from spectral-reconstruction measurements is provided. Preliminary fluorescence-upconversion measurements are presented against which the appropriateness of the single-wavelength method for constructing solvation correlation functions and the use of stimulated-emission measurements is considered. These...

Heterogeneity in a room-temperature ionic liquid: Persistent local environments and the red-edge effect

“…In their work, they show that the intermediate power law of mode coupling theory spans a duration from 1 ps to several hundred picoseconds or even longer in the case of 1-ethyl-3-methylimidazolium nitrate. In a recent time-resolved study using coumarin 153, Petrich and coworkers (20) found that at least 50% of the solvation response happens within 100 ps, and the rest occurs on a much longer time scale.…”

Experimental and Theoretical Investigations of Solvation Dynamics of Ionic Fluids: Appropriateness of Dielectric Theory and the Role of DC Conductivity

“…For electrolyte solutions and molten salts, this point of view has been used fruitfully to interpret the experimental dielectric spectra. [24][25][26][27][28] Our recent work on the dynamic aspects of solvation by RTILs using Stokes shift data of the fluorescent probe, coumarin 153, from the picosecond to the nanosecond time regimes 20 already suggested such a connection. In that work, we investigated four RTILs based on the 1-butyl-3-methylimidazolium cation, BMIM + .…”

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