Influence of Specific Interactions on the Rotational Dynamics of Charged and Neutral Solutes in Ionic Liquids Containing Tris(pentafluoroethyl)trifluorophosphate (FAP) Anion

Rotational dynamics of two organic solutes, rhodamine 110 (RI ID) and 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP), has been investigated as a function of temperature in a series of 1-alkyl-3-methylimidazolium ionic liquids (alkyl = ethyl, butyl, hexyl, and 2-hydroxyethyl) containing tris(pentafluoroethyl)trifluorophosphate (FAP) anion. The present study has been essentially undertaken to examine the influence of specific interactions on the rotation of cationic (RI 10) and neutral (DMDPP) solutes in this new class of ionic liquids. Analysis of the results using the Stokes-Einstein-Debye hydrodynamic theory indicates that the rotational dynamics of R110 is closer to the stick boundary condition whereas the dynamics of DMDPP is described by the slip boundary condition. The observed slow dynamics of R110 has been rationalized on the basis of specific interactions between the cationic solute and the FAP anion of the ionic liquid. It has also been noticed that the rotational dynamics of DMDPP is slower by 30% in 1-(2-hydroxyethyl)-3-methylimidazolium FAP compared to that observed in its ethyl counterpart, which is assimilated in terms of hydrogen bonding interactions between the carbonyl groups of the solute and the hydroxyl group of the imidazolium cation.