Effect of Low Viscous Nondipolar Solvent on the Rotational Diffusion of Structurally Similar Nondipolar Solutes in an Ionic Liquid

Fluorescence anisotropies of two structurally similar nondipolar solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo­[3,4-<i>c</i>]­pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo­[3,4-<i>c</i>]­pyrrole (DPP), have been measured in 1-methyl-3-octylimidazolium hexafluorophosphate–dibenzyl ether ([MOIM]­[PF₆]–DBE) mixtures to understand how the addition of a low viscous nondipolar solvent influences solute rotation. The data when analyzed with Stokes–Einstein–Debye hydrodynamic theory reveals that the measured reorientation times of DMDPP are closer to the predictions of slip boundary condition, whereas those of DPP follow stick hydrodynamics. This outcome arises due to specific interactions between DPP and the solvent medium. Nevertheless, the important result of this study is that the rotational diffusion of DMDPP becomes gradually slower with an increase in the mole fraction of DBE (<i>x</i>_DBE) for a given viscosity and temperature. In contrast, such a trend is not noticed for the hydrogen-bond donating solute DPP. Instead, two sets of reorientation times have been obtained, one corresponding to <i>x</i>_DBE = 0.0–0.2 and the other <i>x</i>_DBE = 0.4–1.0. The results for DMDPP have been rationalized on the basis of the organized structure of [MOIM]­[PF₆], which attains homogeneity at the microscopic level with an increase in <i>x</i>_DBE. In case of DPP, however, the propensity of the solute to be in the neighborhood of DBE, as a consequence of its stronger hydrogen bond accepting ability compared to the ionic liquid, appears to be the reason for the observed behavior