Effect of counter-ion on the thermotropic liquid crystal behaviour of bis(alkyl)-tris(imidazolium salt) compounds |
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Authors: | Magdalene R. Schenkel Justin B. Hooper Mark J. Moran Lily A. Robertson |
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Affiliation: | 1. Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA;2. Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT, USA |
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Abstract: | Recently, new thermotropic ionic liquid crystals (LCs) with a hexyl-linked tris(imidazolium bromide) core and two terminal alkyl chains were synthesised and characterised. To explore the effect of different counter-ions on the LC behaviour of this system, derivatives with BF4? and Tf2N? counter-ions were prepared and analysed. Five of the BF4? derivatives were found to exhibit thermotropic LC behaviour. The 12-, 14- and 16-carbon tail BF4? compounds form SmA phases. The 18- and 20-carbon tail homologues form what appears to be a smectic phase but are weakly mesogenic and harder to characterise. Only two of the Tf2N? derivatives exhibited mesogenic behaviour. The 18-carbon tail Tf2N? compound forms an as-yet unidentified, highly periodic smectic phase with positional order while the 20-carbon tail homologue forms a periodic SmA phase. The Tf2N? mesogens have much lower clearing points even though their LC phases have more order than the Br? and BF4? mesogens. X-ray diffraction showed that these mesogens have different amounts of tail interdigitation between the smectic layers depending on the counter-ion present. Atomistic molecular dynamics simulations indicated that counter-ion size plays an important role in defining the density of the ionic region, which in turn affects the amount of interdigitation in the smectic phases. |
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Keywords: | tris(imidazolium) salts counter-ions thermotropic ionic liquid crystals smectic A phase higher order smectic phase |
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