Rotational Diffusion of Nondipolar and Charged Solutes in Alkyl-Substituted Imidazolium Triflimides: Effect of C2 Methylation on Solute Rotation

Rotational diffusion of a nondipolar solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo­[3,4-c]­pyrrole (DMDPP) and a charged solute rhodamine 110 (R110) has been investigated in 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([BMIM]­[Tf₂N]) and 1-butyl-2,3-dimethylimidazolium bis­(trifluoromethylsulfonyl)­imide ([BMMIM]­[Tf₂N]) to understand the influence of the C2 methylation on solute rotation. The measured reorientation times of the nondipolar solute DMDPP are similar in both the ionic liquids and follow Stokes–Einstein–Debye hydrodynamic theory with slip hydrodynamics. In contrast, rotational diffusion of the charged solute R110 in [BMIM]­[Tf₂N] obeys stick hydrodynamics due to specific interactions with the anion of the ionic liquid. Nevertheless, the intriguing result of this study is that the reorientation times of R110 in [BMMIM]­[Tf₂N] deviate significantly from the predictions of stick hydrodynamics, especially at ambient temperatures. The solute–solvent boundary condition parameter C_obs, which is defined as the ratio of the measured reorientation time to the one calculated using the SED theory with stick boundary condition, for R110 is lower by a factor of 2 in [BMMIM]­[Tf₂N] compared to [BMIM]­[Tf₂N] at 298 K. Upon increasing the temperature, C_obs gradually increases and eventually matches with that obtained in [BMIM]­[Tf₂N] at 348 K. It has been well established that methylation of the C2 position in [BMMIM]­[Tf₂N] switches off the main hydrogen-bonding interaction between the anion and the cation, but increases the Coulombic interactions. As a consequence of the enhanced interionic interactions between the cation and anion of the ionic liquid, specific interactions between R110 and [Tf₂N] diminish leading to the faster rotation of the solute. However, such an influence is not apparent in case of DMDPP as it does not experience specific interactions with either the cation or the anion of these ionic liquids.