Thiocyanate as a Local Probe of Ultrafast Structure and Dynamics in Imidazolium-Based Ionic Liquids: Water-Induced Heterogeneity and Cation-Induced Ion Pairing

Ultrafast two-dimensional infrared spectroscopy (2D-IR) of thiocyanate ([SCN]-) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C(4)C(1)im][NTf2]) and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C(4)C(1)C(12)im][NTf2]) ionic liquids probes local structure and dynamics as a function of the water content, solute counterion, and solute concentration. The 2D-IR spectra of the water-saturated ionic liquids resolve two distinct kinds of dynamics. This dynamical heterogeneity is explained as two subensembles, one with and one without a water molecule in the first solvation shell. When the countercation is K+, ion pairs between K+ and [SCN]- that persist for >100 ps are detected by long-lasting vibrational frequency correlations. The observed dynamics are invariant to [SCN]-concentration, which indicates that the [SCN]- does not cluster in ionic liquid solution. Taken together, these results are consistent with a picture of thiocyanate as a local probe that can interrogate ultrafast structure and dynamics at a small spatial scale in ionic liquids.