Supramolecular Liquid-Crystalline Complexes Exhibiting Room-Temperature Mesophases and Electrooptic Effects. Hydrogen-Bonded Mesogens Derived from Alkylpyridines and Benzoic Acids

Hydrogen-bonded liquid-crystalline complexes containing only two aromatic rings have been designed and obtained through 1:1 (molar ratio) complexation of 4-alkoxy- or 4-alkylbenzoic acid (rcOBA or nBA; n is the carbon number of the alkyl group) and 4-octylor 4-undecylpyridine (8Py or 11Py). These supramolecular H-bonded complexes exhibit stable mesophases in the vicinity of room temperature. For a series of complexes obtained from nOBA and 8Py, only nematic phases are observed near room temperature. For example, the 1:1 complex obtained from an equimolar amount of 4-hexyloxybenzoic acid and 4-octylpyridine (60BA-8Py) exhibits a nematic phase from 38 to 48 °C. In contrast, for a series of complexes from nBA and 8Py, only smectic phases are seen at room temperature. For example, 6BA-8Py shows a smectic A phase from 29 to 33 °C. On cooling, the smectic A phase appears at 33 °C and a subsequent smectic C phase is observed from 13 to 10 °C. The effects of lateral substituents on the liquid-crystalline behavior have been examined for 60BA-8Py system using 3-chloro and 3-fluoro-4-hexyloxybenzoic acids as H-bond donors. The use of these substituents depresses the isotropization temperature and also induces smectic phases for the complexes. The mesophase range has been extended by the mixing of complexes because of the resulting depression of melting points. A mixture of nematogenic complexes can be aligned on a rubbed surface and shows electrooptic effects in a twisted nematic cell.