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 Cobs,
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, Cobs 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.