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 similar to 343 K range similar to 2300-3700 cm(-1), and a dominating contribution (similar to 75-85%) arises from the solute-IL dipole-dipole interaction. Inclusion of solvent-libration predicts similar to 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. (C) 2011 Elsevier B. V. All rights reserved.