Synthesis and thermotropic liquid-crystalline properties of N-alkylpyridinium bromides substituted with a terphenylene moiety

Abstract

Molecules containing a terphenylene core, two alkyl chains and a pyridinium ring associated with its bromine counterion were synthesised and their liquid crystalline properties were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The results were compared with those of chemical intermediates, which also develop a liquid crystalline behaviour. Both intermediates and pyridinium salts showed a rich polymorphism at temperatures ranging from around 100 to 200°C and 115 to 220°C, respectively. X-ray results indicate that both intermediates and pyridinium salts develop tilted smectic mesophases with molecules stacked in single and double layers, respectively. The tilt angle of some of these compounds decreases so markedly upon cooling that molecules attain almost an orthogonal position. The stacking of molecules in the smectic layers was explained in terms of the mutual repulsion interactions between the terphenylene core, the alkyl chains and the ionic species (the pyridinium ring associated with its counterion) and it was proposed that the π–π interactions between the long aromatic cores counterbalance the strong forces between the ionic species, leading to a full segregation of these molecular parts in periodic sublayers. A molecular arrangement model is proposed for these salts.

Keywords:

• terphenylene
• pyridinium salts
• mesomorphic
• smectic

Acknowledgements

We thank the CONACYT of Mexico for supporting this work (project 43741-Y) and for the grant (ref. 202220) given to R. G. Santos-Martell during her MSc studies. We also wish to thank Guadalupe Méndez for her helpful assistance in the thermal characterisation.