Cation and anion substitution effects on the ultrafast dynamics of interionic interaction in imidazolium based ionic liquids

Abstract

Room-temperature Ionic Liquids (ILs) have numerous unique properties that differ from those of conventional molecular solvents. Although the unique properties of ILs have been suggested to origin from their microscopic interionic interaction, detailed dynamics of interionic interaction of ILs has not been fully understood. Here, with the Femtosecond Optical Heterodyne-Detected Raman Induced Kerr Effect Spectroscopy (fs-OHD-RIKES), we measured the ultrafast dynamics of the interionic interaction of three typical imidazolium based ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), and 1-decyl-3-methylimidazolium tetrafluoroborate ([dmim][BF4]). We observed several periods of subpicosecond oscillation in their fs-OHD-RIKES signals. Through decomposing their fs-OHD-RIKES signals into four Brownian oscillators in time domain, we explored the cation and anion substitution effects on the ultrafast dynamics of interionic interaction of ILs. We found that the cation substitution affected all the low frequency motions we observed, while the anion substitution only affected the two higher low frequency motions.