How Does the Structural Organization of a Cyclic Ammonium Ionic Liquid Differ from That of Acyclic Branched Ammonium Ionic Liquids?

This study explored the structural organization of cyclic and acyclic ammonium-based ionic liquids (ILs). For this purpose, we selected one cyclic IL, <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMPyr-NTf₂), and two acyclic ILs, methyl-tributylammonium (TBMA-NTf₂) and methyl-trioctylammonium (TOMA-NTf₂) bis(trifluoromethanesulfonyl)imide. Thermophysical properties, such as viscosity and density, were measured to evaluate the physicochemical behavior of the ILs. We complemented these measurements with time-resolved fluorescence anisotropy (TRFA), density functional theory (DFT) calculations, and fluorescence correlation spectroscopy (FCS). Viscosity and density data revealed notable differences between the cyclic IL (BMPyr-NTf₂) and its acyclic counter IL (TBMA-NTf₂), suggesting that anion-cation interactions play a key role in governing physicochemical properties. DFT results displayed that the binding energy of the cyclic IL is more than that of structurally analogous acyclic ILs, with a corresponding increase in anion-cation distance in the acyclic IL. TRFA measurements indicated that the cyclic IL exhibited significantly higher rotational correlation times (τ_r) than its acyclic counter IL under iso-viscous conditions. This resulted in a smaller hydrodynamic radius for acyclic ILs, likely due to weaker anion-cation interactions and reduced ion aggregation. FCS measurements supported these observations. Overall, our findings demonstrate that the structural organization of cyclic ammonium-based IL differs markedly from their acyclic counter ILs.