Ultrafast Vibrational Dynamics and Energy Transfer in Imidazolium Ionic Liquids

Abstract

Femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) is used as a probe for monitoring the vibrational dynamics of room temperature ionic liquids (ILs). The experiments are performed on a series of 1,3-dialkylimidazolium ILs containing the bis(trifluoromethylsulfonyl)imide [NTf2] anion. The effect of methylation of the cationic C2 position on the dephasing time is studied analyzing [NTf2]-ILs of 1-ethyl-3-methylimidazolium [EMIM], 1-ethyl-2,3-dimethylimidazolium [EMMIM], 1-butyl-3-methylimidazolium [BMIM], and 1-butyl-2,3-dimethylimidazolium [BMMIM]. Raman coherences are excited around ∼1400 cm(-1), and the vibrational dephasing of the modes in the fingerprint region is monitored as a function of time. The results indicate that vibrational energy transfer occurs governed by the interionic interactions. This is suggested by mode beating involving vibrations beyond the excitation spectrum as well as systematic differences in the temporal dephasing behavior. In contrast, the length of the cationic alkyl side chain has a negligible impact on the vibrational dynamics.

Effects of C(2) Methylation on Thermal Behavior and Interionic Interactions in Imidazolium-Based Ionic Liquids with Highly Symmetric Anions

“…To study the effects of the C(2) position, it has commonly been analyzed in its protonated and methylated forms [4,8,19,[25][26][27]. The methylation at C(2) basically removes the possibility to form hydrogen bonds with the anion at this site.…”

Dynamics of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide studied by nuclear magnetic resonance dispersion and diffusion

“…2,3 The microscopic properties of some selected ILs have been investigated recently. [4][5][6][7] A common theoretical description is not fully available though, in part because of the asymmetric charge distribution in the molecular ions that determines the electrostatic interaction potential and Coulomb forces. ILs are in their majority glass-forming systems.…”

Structure of protic (HCnImNTf2, n=0–12) and aprotic (C1CnImNTf2, n=1–12) imidazolium ionic liquids: A vibrational spectroscopic study

“…Furthermore, its overall intensity is decreased compared to the C 4,5 -H. We have to note here that the assignment of this band is a matter of debate, since it is well established that there is Fermi resonance with the ring stretching vibrational mode. This make the interpretation of this spectral area quite complicated, as we already mentioned in a recent publication and we will not offer any new discussion here [36][37][38][39][40][41][42][43]. No more shifts are observed with increasing n from 2 to 12.…”