Stokes’ shift dynamics in alkylimidazolium aluminate ionic liquids: Domination of solute-IL dipole–dipole interaction

A molecular theory is employed to predict the Stokes’ shift dynamics for a dipolar solute, C153, in six different alkylimidazolium ionic liquids (ILs) containing a fixed anion, tetra(hexafluoroisopropoxy)aluminate. Calculated shifts in these ILs at ∼343 K range ∼2300–3700 cm−1, and a dominating contribution (∼75–85%) arises from the solute-IL dipole–dipole interaction. Inclusion of solvent-libration predicts ∼50% ultrafast component in the total dynamics. Although the predicted dynamics is faster than in other ILs, calculated shifts follow the same linear correlation with ion size-ratio. Furthermore, model calculations explore the solute-IL size-ratio dependence of the interaction contributions to the shift, and investigates the relative importance of solvent rotational and translational modes for IL dynamics.