Ionic Liquid Dynamics Measured with 2D IR and IR Pump–Probe Experiments on a Linear Anion and the Influence of Potassium Cations

Abstract

The room-temperature ionic liquid EmimNTf2 (1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide) was studied with two-dimensional infrared (2D IR) spectroscopy and polarization selective pump–probe (PSPP) experiments using low-concentration selenocyanate (SeCN–) as the vibrational probe. SeCN– was added as EmimSeCN, which keeps the cation the same. KSeCN was also used, so K+ was added. Two 2D IR polarization configurations were employed: ⟨XXXX⟩ (all pulses have the same polarization) and ⟨XXYY⟩ (the first two pulse polarizations are perpendicular to that of the third pulse and the echo). The spectral diffusion differs for the two configurations, demonstrating that reorientation-induced spectral diffusion, in addition to structural spectral diffusion (SSD), plays a role in the observed dynamics. The SSD was extracted from the 2D IR time-dependent data. The samples with EmimSeCN have dynamics on several fast time scales; however, when KSeCN is used, both the PPSP anisotropy decay and the 2D IR decays have low amplitude offsets (nondecaying values at long times). The size of the offsets increased with increased K+ concentration. These results are explained in terms of a two-ensemble model. A small fraction of the SeCN– is located in the regions modified by the presence of K+, causing a substantial slowing of the SeCN– orientational relaxation and spectral diffusion. Having a small ensemble of SeCN– that undergoes very slow dynamics is sufficient to explain the offsets. For the major ensemble, the dynamics with and without K+ are the same.

Direct observation of dynamic crossover in fragile molecular glass formers with 2D IR vibrational echo spectroscopy

“…This model has had great success in fitting 2D IR data for a variety of systems including organic liquids, water, proteins, liquid crystals, and room-temperature ionic liquids. [55][56][57][58][59][71][72][73][74][75] The multiexponential model was chosen as our starting point, as above T m , these glass forming liquids are simple organic liquids. At these higher temperatures, the CLS(T w ) decay curves for all three samples fit well to biexponential decays.…”

“…56 However, these power laws are not unique to simple glass forming liquids, and appear also in other complex liquids, such as liquid crystals and ionic liquids, 56,57,80,81 and are conspicuously absent from 2D IR experiments on these systems. [55][56][57][58] This highlights a fundamental difference in the spectral diffusion observable from other observables, in that relaxations that have complex functional forms in other techniques appear as simple exponential relaxations in 2D IR. While a full microscopic theory of spectral diffusion is lacking, it has been shown that schematic MCT correlators can describe the multi-exponential decay in liquid crystals 56 in the same way that they can describe the power law to exponential decay in OKE spectroscopy.…”

“…The development of longlived selenocyanate probes has been an important advance, as their nitrile stretches can have lifetimes of many hundreds of picoseconds, permitting the collection of data from <1 ps to ∼1 ns. These probes have previously been used in 2D IR spectroscopy to probe dynamics in liquid crystals, 55,56 in solvation shells of water and ionic liquids, [57][58][59] and in proteins. 60 It has been shown that the spectral diffusion reported by probes of similar shape and size as the molecules that make up the bulk liquid is nearly identical to that felt by natural vibrational probes that are intrinsic to the liquid itself.…”

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

“…24 Early MD simulations noted few linear hydrogen bonds with water 25 and many arrangements of dipolar character, while more recent MD simulations of SeCN -26 found an average of 3-4 hydrogen bonds, with one of them being an axial (linear) hydrogen bond. Ultrafast infrared spectroscopy has used the ν 3 mode of pseudo-halide anions to investigate ultrafast vibrational dynamics of water and other polar solvents, 23,[27][28][29][30][31][32] concentrated ion solutions, [33][34][35] ILs, [36][37][38][39][40] supported IL membranes, 41 and colloid emulsions. 42 Furthermore, a combination of MD simulations and ultrafast infrared spectroscopy developed a spectroscopic map of SeCNin D 2 O that describes the frequency dependence of the nitrile stretch to a hydrogen bonding environment.…”

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

“…24 Early MD simulations noted few linear hydrogen bonds with water 25 and many arrangements of dipolar character, while more recent MD simulations of SeCN -26 found an average of 3-4 hydrogen bonds, with one of them being an axial (linear) hydrogen bond. Ultrafast infrared spectroscopy has used the ν 3 mode of pseudo-halide anions to investigate ultrafast vibrational dynamics of water and other polar solvents, 23,[27][28][29][30][31][32] concentrated ion solutions, [33][34][35] ILs, [36][37][38][39][40] supported IL membranes, 41 and colloid emulsions. 42 Furthermore, a combination of MD simulations and ultrafast infrared spectroscopy developed a spectroscopic map of SeCNin D 2 O that describes the frequency dependence of the nitrile stretch to a hydrogen bonding environment.…”