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 ([C₄C₁im]­[NTf₂]) and 1-butyl-2,3-dimethylimidazolium bis­(trifluoromethylsulfonyl)­imide ([C₄C₁C₁²im]­[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