The effects of anion and cation substitution on the ultrafast solvent dynamics of ionic liquids: A time-resolved optical Kerr-effect spectroscopic study

Abstract

Ultrafast solvent dynamics of room-temperature ionic liquids have been investigated by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy ~OHD-RIKES! by studying the effects of cation and anion substitution on the low frequency librational modes. The spectra of two series of imidazolium salts are presented. The first series is based on the 1-butyl-3-methylimidazolium salts @bmim#1 containing the anions trifluoromethanesulfate @TfO#2, bis~trifluoromethanesulfonyl!imide @Tf2N#2, and hexafluorophosphate @PF6#2. The second series is based on @Tf2N#2 salts containing the three cations 1-butyl-2,3-dimethylimidazolium @bmmim#1, 1-methyl-3-octylimidazolium @omim#1, and @bmim#1. It is found in all five samples that the signal is due to libration of the imidazolium ring at three frequencies around 30, 65, and 100 cm21 corresponding to three local configurations of the anion with respect to the cation

Spotlight on ionic liquids

“…7 are typical for ILs. 96 As with polar solvents, strong electrostatic caging effects lead to molecular librations of the ions with peak frequencies near 50 cm −1 . Spectral features with maxima near 15 cm −1 are often attributed to translational dynamics for both molecular solvents and ILs.…”

“…Spectral features with maxima near 15 cm −1 are often attributed to translational dynamics for both molecular solvents and ILs. 94,[96][97][98] If the cation in a series of ILs is held constant, changes in the low-frequency spectrum upon anion substitution illustrate the differences in the strength and specificity of anion-cation interactions, as shown in Fig. 7.…”

Low-frequency Raman spectra and fragility of imidazolium ionic liquids

“…On the basis of the quality of the fit, it is difficult to distinguish among many different functions that have been used to fit low-frequency Raman spectra and OKE spectra of molecular or ionic liquids. For instance, overdamped Lorentzian function for the fast relaxation on 1-3 ps time scale, 14 and Brownian-oscillator model, 14,16 Ohmic function, 17 BucaroLitovitz function, 15 or antisymmetrized Gaussian function 15,17 for the intermolecular vibrations have been used. In this work, a Lorentzian band centered at zero wavenumber represents the QES contribution, whose intensity ͑I QES ͒ is depends on temperature, but its bandwidth is essentially the same, within the 5.0-6.0 cm −1 range, irrespective of the ionic liquid and temperature.…”

“…2 The short-time intermolecular dynamics of ionic liquids has been the subject of many investigations by optical Kerr effect ͑OKE͒ spectroscopy. [13][14][15][16][17][18] In OKE spectroscopy, the original data in time domain are Fourier transformed to frequency domain covering a frequency range similar to Raman spectroscopy. The main issue in these OKE spectroscopy studies was the investigation of librational modes usually assigned to the imidazolium ring.…”

“…The main issue in these OKE spectroscopy studies was the investigation of librational modes usually assigned to the imidazolium ring. [13][14][15][16][17][18] This work is not concerned with such librational dynamics, instead we focus here on the relative contribution of fast relaxational and vibrational motions in low-frequency Raman spectra of ionic liquids. In order to probe the slow relaxational dynamics, ionic liquids have been investigated by dielectric relaxation spectroscopy, 19,20 which covers a frequency range several orders of magnitude below the terahertz range that is available by low-frequency Raman spectroscopy.…”