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

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 $[\mathrm{bmim}{]}^{+}$ containing the anions trifluoromethanesulfate $[\mathrm{TfO}{]}^{-},$ bis(trifluoromethanesulfonyl)imide $[{\mathrm{Tf}}_2\mathrm{N}{]}^{-},$ and hexafluorophosphate $[{\mathrm{PF}}_6{]}^{-}.$ The second series is based on $[{\mathrm{Tf}}_2\mathrm{N}{]}^{-}$ salts containing the three cations 1-butyl-2,3-dimethylimidazolium $[\mathrm{bmmim}{]}^{+},$ 1-methyl-3-octylimidazolium $[\mathrm{omim}{]}^{+},$ and $[\mathrm{bmim}{]}^{+}.$ 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 $\text{100\hspace{0.5em}}{\mathrm{cm}}^{\text{−1}}$ corresponding to three local configurations of the anion with respect to the cation.