Influence of the Organized Structure of 1‑Alkyl-3-methylimidazolium Tetrafluoroborates on the Rotational Diffusion of Structurally Similar Nondipolar Solutes

To understand how the organized structure of the ionic liquids influences the location and mobility of nondipolar solutes, rotational diffusion of 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) has been examined in 1-alkyl-3-methylimidazolium (alkyl = ethyl, butyl, hexyl, and octyl) tetrafluoroborates. Both the solutes are structurally similarthe sole difference being the two NCH₃ groups of DMDPP are replaced by two NH groups in DPP. The rotational diffusion of DPP is found to be significantly slower than DMDPP due to specific interactions between the NH groups of the solute and the anion of the ionic liquid. It has been observed that for a given viscosity and temperature, the rotational diffusion of DMDPP becomes progressively faster with an increase in the length of the alkyl chain on the imidazolium cation. DMDPP resides in the nonpolar domains of these ionic liquids whose sizes increase with an increase in the length of the alkyl chain, and as a result it experiences microviscosity that is lower than the bulk viscosity. However, an increase in the length of the alkyl chain has no apparent effect on the rotational diffusion of DPP because specific interactions with tetrafluoroborate necessitate the solute to be located in the vicinity of the anion. The results of this work exemplify that despite having similar size and shape, the rotational diffusion of DMDPP and DPP is quite contrasting as their sites of solubilization and the nature of interactions with the surroundings are vastly different owing to subtle variations in their chemical structures