Solvation dynamics in imidazolium and phosphonium ionic liquids: Effects of solute motion

Experimental and simulation results of solvation dynamics in ionic liquids have so far been explained in terms of translational motion of the ions constituting the ionic liquids under investigation. A recent theoretical study [Kashyap & Biswas, J Phys Chem B, 114 (2010) 254] has indicated that while translational motion of the constituent ions is indeed responsible for Stokes’ shift dynamics of a polar solute probe in non-dipolar ionic liquids (such as phosphonium ionic liquids), fast orientational relaxation of dipolar ions significantly affects the spectral dynamics in imidazolium ionic liquids. Herein, we investigate the effects of rotational and translational motions of a dissolved dipolar probe on the Stokes’ shift dynamics in these representative non-dipolar and dipolar ionic liquids. Theoretical results presented here suggest that while the probe-motion accelerates the solvation-rate in non-dipolar ionic liquids twice over the fixed solute values, it plays a secondary role in dipolar ionic solvents. The sensitivity to the solute’s self-motion of the rate of solvation has been found to be linked to the solute-cation size ratio and is decoupled from the initial ultrafast response in dipolar ionic liquids. This work also explains the reasons for not observing the effects of solute motion in simulation study of solvation dynamics in the imidazolium ionic liquid considered herein. Careful analyses of the effects of solute motion further support the fact that rotation of dipolar solvent species, like in conventional polar solvents, dictates the initial phase of solvation of a dipolar probe in imidazolium and other dipolar ionic liquids. In addition, probe size dependence is predicted in non-dipolar ionic liquids, such as phosphonium ionic liquids.