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

► Calculated dynamic Stokes' shifts for these ILs range between ∼2300 and 3700 cm−1. ► Suggests a total domination (∼75-85%) of probe-IL dipolar interaction. ► Ion-size crucial for the ion-dipole interaction contribution to the total shift. ► Predicts a presence of sub-50 fs component with substantial amplitude. ► Ultrafast response originates from the collective orientational relaxation.