Synthesis and characterisation of ionic liquid crystals based on substituted pyridinium cations

Novel ionic liquid crystals based on pyridinium and methylpyridinium cations with a dodecyl chain and bromide as anion have been prepared and characterised. [C₁₂Pyr][Br] and [C₁₂-n-Pic][Br] (n = 2, 3 or 4) were studied in order to understand the influence of the methyl group position in the mesomorphic behaviour. Neat microwave-assisted reactions were carried out to synthesised [C₁₂Pyr][Br], [C₁₂-3-Pic][Br] and [C₁₂-4-Pic][Br] as a sustainable approach, leading to significantly reduced reaction times (35 min vs. 48 h in conventional method). All prepared compounds were characterised by spectroscopic techniques (¹H and ¹³C-NMR; ATR-FTIR), Elemental Analysis, Differential Scanning Calorimetry (DSC), Polarised Optical Microscopy (POM) and X-Ray Powder Diffraction (XRD). It was observed that the presence of water highly influences the phase transformations. Nevertheless, thermotropic smectic phases were detected for all materials with a layer spacing between 20 and 35 Å, compatible with a bilayer interdigitated or tilted lamellar arrangement of the cation’s alkyl chain. This work also indicates new routes for preparing ionic liquid crystals with various functionalities ruled by the synthesis design.