Ultrafast dynamics of pyrrolidinium cation ionic liquids

Abstract

We have investigated the ultrafast molecular dynamics of five pyrrolidinium cation room temperature ionic liquids using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids studied are N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P14+/NTf2-), N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bromide P1EOE+, and N-ethoxyethyl-N-methylpyrrolidinium dicyanoamide P1EOE+/DCA-). For comparing dynamics among the five ionic liquids, we categorize the ionic liquids into two groups. One group of liquids comprises the three pyrrolidinium cations P14+, P1EOM+, and P1EOE+ paired with the NTf2- anion. The other group of liquids consists of the P1EOE+ cation paired with each of the three anions NTf2-, Br-, and DCA-. The overdamped relaxation for time scales longer than 2 ps has been fit by a triexponential function for each of the five pyrrolidinium ionic liquids. The fast ( approximately 2 ps) and intermediate (approximately 20 ps) relaxation time constants vary little among these five ionic liquids. However, the slow relaxation time constant correlates with the viscosity. Thus, the Kerr spectra in the range from 0 to 750 cm(-1) are quite similar for the group of three pyrrolidinium ionic liquids paired with the NTf2- anion. The intermolecular vibrational line shapes between 0 and 150 cm(-1) are fit to a multimode Brownian oscillator model; adequate fits required at least three modes to be included in the line shape.

Spotlight on ionic liquids

“…7. Another illustration of the effect of the nature of the anion on intermolecular vibrational dynamics was reported by Shirota et al 98 among the series of ethoxyethyl-methyl-pyrrolidinium ͓N͑CN͒ 2 ͔ − , Br − , and ͓NTf 2 ͔ − ILs, where the interionic interactions of the ͓NTf 2 ͔ − anion were clearly weaker than the other two, consistent with the observed viscosities and orientational relaxation.…”

“…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.…”

Ultrafast Solvation Dynamics of Coumarin 153 in Imidazolium-Based Ionic Liquids

“…The 4 -] has been measured using fluorescence up-conversion techniques with a response function of 230 fs. The better time-resolution with respect to the majority of other studies on imidazolium-based RTILs allows us to address directly the early part of the emission.…”

Experimental and Theoretical Investigations of Solvation Dynamics of Ionic Fluids: Appropriateness of Dielectric Theory and the Role of DC Conductivity

“…We are cautious, however, to extend this conclusion to the NTf 2 -anion, which is polar, polarizable, large, and flexible. 29 In this report the solvation relaxation measured from fluorescence upconversion experiments is compared with that calculated from the dielectric spectra of the neat ionic liquids.…”