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Abstract
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.
Acknowledgements
Primary financial support for this research from the National Science Foundation (DMR-0820579) is gratefully acknowledged. Support for M.R.S. in the form of matching funds from the University of Colorado Graduate School is also gratefully acknowledged. We also thank M. Springer for performing the Spartan computational molecular modelling and Dr. N. D. Urban for ionic column chromatography purification of some of the prepared compounds.