Effect of counter-ion on the thermotropic liquid crystal behaviour of bis(alkyl)-tris(imidazolium salt) compounds

<div><p>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.</p>