Dynamics in tris(pentafluoroethyl)trifluorophosphate (FAP) anion based ionic liquids: A 2D-IR study with tungsten hexacarbonyl

Abstract

Ultrafast two-dimensional infrared spectroscopy (2D-IR) measurements for tungsten hexacarbonyl, W(CO)6, has been performed in two fluoroalkylphosphate ([FAP])-based ultrahigh hydrophobic, ionic liquids, namely, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate ([EMIM][FAP]) and 1-(hexyl)-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIM][FAP]) to probe their local structure and dynamics. To understand the effect of FAP anion on the dynamics, a relatively less hydrophobic [PF6-] anion has been introduced in the form of [HMIM][PF6]. While the linear IR spectra records some influence of anion in terms of spectral shifts, the alkyl chain length of the cation seems to have no significant influence on the spectral peak position; and even the spectral broadening is found to be in the similar range. Further, vibrational population relaxation dynamics and vibrational anisotropy decay also does not show any significant variation due to difference in constituent of these ionic liquids as well as its solvent properties, such as, viscosity. Finally, 2D-IR spectra show large variation in the spectral shapes with waiting times, which qualitatively remains similar in all the tested ionic liquids. The frequency-fluctuation correlation function measured from 2D-IR spectra displays a bi-exponential decay with the shorter time-constant dependent on the identity of the anion, and has been attributed to the ion-rattling motion, whereas, the longer component has been found to be invariant with the identity of ionic liquid, and has been attributed to ion-cage motions which is in line with previous MD simulations as well as previous experimental measurements, using ionic vibrational probe. Overall, 2D-IR spectroscopy, in combination with this neutral and hydrophobic probe, W(CO)6 provides a unique opportunity to understand the local structural dynamics of this ultrahigh hydrophobic ionic liquid which are otherwise not accessible by hitherto explored ionic vibrational probes.