Thermotropic liquid crystals from N-alkylpyridinium halides ω-substituted with a mesogenic group

Abstract

A series of N-alkylpyridinium halides ω-substituted with a 4-methoxybiphen-ylyloxy mesogenic group were synthesized and characterized. The molecules of these compounds contain three distinct parts: a flexible aliphatic chain, a rigid polarizable aromatic core, and a positively charged pyridinium ring associated with a negatively charged counterion. Their thermotropic liquid-crystalline behaviour was studied by differential scanning calorimetry and optical microscopy. Three types of smectic mesophase, namely A, B, and E, were identified by X-ray diffraction. Their structure consists of single layers of upright molecules laterally arranged head to tail. Segregated from the non-ionic parts of the molecules, the ionic end groups are set in double layers with the oppositely charged species facing each other and equally distributed between the two sub-layers. For the ordered smectic E phases, the anions are arranged in rows along the rectangular two dimensional unit cell diagonals with the pyridinium rings sandwiched between them in a chevron-type structure. As for smectic mesophases obtained with soaps, the smectic ordering described in the present work relies primarily on the electrical interactions of the ionic endgroups. Although present, the van der Waals repulsions of the aliphatic and aromatic moieties turn out to be ineffectual.