NMR Spectroscopic Studies of Cation Dynamics in Symmetrically-Substituted Imidazolium-Based Ionic Liquid Crystals

Abstract

Ionic liquid crystals (ILCs) present a new class of non-molecular soft materials with a unique combination of high ionic conductivity and anisotropy of physicochemical properties. Symmetrically-substituted long-chain imidazolium-based mesogenic ionic liquids exhibiting a smectic liquid crystalline phase were investigated by solid state NMR spectroscopy and computational methods. The aim of the study was to reveal the correlation between cation size and structure, local dynamics, and orientational order in the layered mesophase. The obtained experimental data are consistent with the model of a rod-shaped cation with the two chains aligned in opposite directions outward from the imidazolium core. The alignment of the core plane to the phase director and the restricted conformations of the chain segments were determined and compared to those in single-chain counterparts. The orientational order parameter S~0.5–0.6 of double-chain ionic liquid crystals is higher than that of corresponding single-chain analogues. This is compatible with the enhanced contribution of van der Waals forces to the stabilization of smectic layers. Increased orientational order for the material with Br− counterions, which exhibit a smaller ionic radius and higher ability to form hydrogen bonds as compared to that of BF4−, also indicated a non-negligible influence of electrostatic and hydrogen bonding interactions. The enhanced rod-shape character and higher orientational order of symmetrically-substituted ILCs can offer additional opportunities in the design of self-assembling non-molecular materials.

Ion conformation and orientational order in a dicationic ionic liquid crystal studied by solid-state nuclear magnetic resonance spectroscopy

“…This DFT-optimized structure is consistent with the experimental order parameters found in the imidazolium ring and for C1 methylenes. Other possible lower energy structures with the ring planes perpendicular to the all-trans side-chain axis or parallel to the side-chain backbone plane 26,27 are not consistent with the experimental data. Estimated S values in this and previously studied ILCs were based on certain assumptions on the model structures of the cations in mesophase [24][25][26][27] .…”

“…Other possible lower energy structures with the ring planes perpendicular to the all-trans side-chain axis or parallel to the side-chain backbone plane 26,27 are not consistent with the experimental data. Estimated S values in this and previously studied ILCs were based on certain assumptions on the model structures of the cations in mesophase [24][25][26][27] . In each case, we assumed that (i) the cation adopts on average an elongated structure, (ii) for the entire cation there exists one unique preferred direction of orientation, and (iii) the alignment of the imidazolium ring is consistent with one of the DFT-optimized low-energy structures.…”

“…Orientational order parameter S is a prime characteristic of LCs. We have previously shown that the S value in ILCs greatly varied depending on structural details [24][25][26][27] . In single-chain monocationic ILCs, a fragile balance of intermolecular forces led to low S values [24][25][26][28][29][30] .…”

“…The rest of the chain is more dynamic, however, it is forced to align on average with the director due to steric restrictions imposed by neighboring chains. Overall lower S CH values in the DILC compared to symmetric monocation analogue 27 may indicate decreasing phase order parameter S. The order parameter of the first carbon C1, having only limited conformational freedom, is primarily determined by the alignment of the imidazolium ring 26,27 . For example, a strongly reduced S CH (C1) value was found in the symmetric cation C 12 imC 12 exhibiting ring plain nearly orthogonal to the cation long axis 27 .…”

“…Smectic phases with S value as low as 0.2 have been found in single-chain imidazolium-based monocationic salts [24][25][26] , as has been also confirmed by molecular dynamics simulation 47 . We have recently shown that a modification of the cation structure by adding a second symmetrically substituted long chain altered the equilibrium between the interactions in polar and hydrophobic sublayers 27 . Due to the enhanced contribution of van der Waals forces in the double chain cations, the order parameter has increased dramatically.…”