Role of specific interactions on the rotational diffusion of organic solutes in a protic ionic liquid-propylammonium...

Rotational diffusion of two pairs of structurally similar organic solutes has been examined in a protic ionic liquid, n-propylammonium nitrate (PAN), to understand the influence of specific interactions on solute rotation. It has been observed that the rotation of the nondipolar solute, 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) in PAN is 30-50% slower compared to its structurally similar counterpart 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP). Analysis of the data using Stokes-Einstein-Debye hydrodynamic theory indicates that the measured reorientation times of DMDPP and DPP are between the stick and slip limits. Furthermore, the rotation of the hydrogen bond accepting solute DMDPP was found to be 60% slower compared to the predictions of slip hydrodynamics, which has been rationalized on the basis of specific interactions between the solute and n-propylammonium cation of the ionic liquid. DPP, on the other hand, experiences specific interactions with both the anion and the cation of the ionic liquid due to the presence of hydrogen bond donating as well as accepting groups, resulting in slower rotation compared to DMDPP. The reorientation times of the ionic solutes fluorescein (FL) and rhodamine 110 (R110) are almost identical and closer to the predictions of stick hydrodynamics. The observed behavior is a consequence of the anionic solute FL and the cationic solute R110 experiencing hydrogen bonding interactions with n-propylammonium cation and nitrate anion, respectively. An attempt has also been made to rationalize these trends in terms of hydrogen bond acidity and basicity of PAN with the aid of existing scales, such as Kamlet-Taft and the Abraham model.