New ionic organic compounds containing a linear tris(imidazolium) core and their thermotropic liquid crystal behaviour

Imidazolium-containing thermotropic ionic liquid crystals (TILCs) are of interest because of their structural similarity to imidazolium-based ionic liquids (ILs), allowing them to exhibit some IL-like properties in addition to the LC order. More imidazolium units are important for increasing the IL character; however, the effect of multiple imidazolium units on LC behaviour is not well known. Most reported imidazolium TILCs contain one imidazolium unit; only a handful containing multiple imidazolium units is known. The only examples of TILCs with multiple imidazolium units sequentially linked with a flexible spacer are based on an alkyl- or oligo(ethylene oxide)-bridged bis(imidazolium) core with an n-alkyl tail at each end. Herein, a series of 10 new symmetric compounds containing three hexyl-bridged imidazolium bromide units as the core and two terminal n-alkyl chains was synthesised and analysed. Thermotropic LC phases were generally not observed for homologues with even-numbered tails less than 16 carbons. However, after initial annealing, the homologues containing 16-, 18- and 20-carbon tails, all form a smectic A phase in the 25–137°C range, with the latter two also forming a higher-order smectic phase. This indicates that with sufficiently long n-alkyl tails to counterbalance the poly(ion) core, these tris(imidazolium) salts can exhibit thermotropic mesomorphism.