Rotational diffusion of neutral and charged solutes in ionic liquids: is solute reorientation influenced by the nature of the cation?

To assess the influence of the nature of cation on solute reorientation, fluorescence anisotropies of two organic solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and rhodamine 110 (R110), have been measured in four ionic liquids containing tris(pentafluoroethyl)trifluorophosphate (FAP) anion over the temperature range 308-348 K. The ionic liquids used in the study are 1-butyl-1-methylpyrrolidinium FAP (BMPL FAP), 1-(2-methoxyethyl)-1-methylpyrrolidinium FAP (MOEMPL FAP), 1-(2-methoxyethyl)-1-methylpiperidinium FAP (MOEMPIP FAP), and N-(2-methoxyethyl)-N-methylmorpholinium FAP (MOEMMO FAP). Analysis of the data carried out with the aid of Stokes-Einstein-Debye hydrodynamic theory reveals that the rotation of the neutral solute DMDPP in all the four ionic liquids is essentially governed by the viscosity of the medium and the reorientation times follow slip boundary condition. In contrast, the results obtained for the cationic solute, which experiences specific interactions with the FAP anion, are somewhat different. The reorientation times of R110 are in between stick and slip limits and found to be independent of the nature of the cation of the ionic liquid except in case of highly viscous MOEMMO FAP, wherein it has been observed that the rotation of R110 is faster by a factor of 1.5. The observed behavior has been rationalized on the basis of highly associative nature of the MOEMMO cation, which precludes R110 from experiencing strong specific interactions with the FAP anion.