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.
Highlights
Ionic liquid crystals (ILCs) are nanostructural soft materials which combine the orientational ordering behavior of liquid crystals with the properties of ionic liquids (ILs) [1]
ILCs have already been used as templates to synthesize mesostructured porous materials [4,5,6] and as orientationally-ordered ionic solvents in catalysis [7,8]
The long-range-ordered structures exhibited by ILCs offer a model environment to explore anisotropic dynamic properties of ions self-organized in two-dimensional (2D) and three-dimensional (3D) space
Summary
Ionic liquid crystals (ILCs) are nanostructural soft materials which combine the orientational ordering behavior of liquid crystals with the properties of ionic liquids (ILs) [1]. ILC-based devices for low-dimensional transport of ions and electrons hold great potential for applications in electrochemistry [2,3]. ILCs have already been used as templates to synthesize mesostructured porous materials [4,5,6] and as orientationally-ordered ionic solvents in catalysis [7,8]. The long-range-ordered structures exhibited by ILCs offer a model environment to explore anisotropic dynamic properties of ions self-organized in two-dimensional (2D) and three-dimensional (3D) space. A better understanding of self-assembling and nanosegregation behavior of ILs contributes to improved designs towards the development of ILC-based devices for specific applications
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