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 aroma tic rings have been designed and obtained through 1:1 (molar ratio) co mplexation of 4-alkoxy- or 4-alkylbenzoic acid (nOBA or nBA; n is the carbon number of the alkyl group) and 4-octyl- or 4-undecylpyridine (8 Py or 11Py). These supramolecular H-bonded complexes exhibit stable me sophases in the vicinity of room temperature. For a series of complexe s obtained from nOBA and 8Py, only nematic phases are observed near ro om temperature. For example, the 1:1 complex obtained from an equimola r amount of 4-hexyloxybenzoic acid and 4-octylpyridine (6OBA-8Py) exhi bits a nematic phase from 38 to 48 degrees C. In contrast, for a serie s 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 3 3 degrees C. On cooling, the smectic A phase appears at 33 degrees C a nd a subsequent smectic C phase is observed from 13 to 10 degrees C. T he effects of lateral substituents on the liquid-crystalline behavior have been examined for 6OBA-8Py system using 3-chloro and 3-fluoro-4-h exyloxybenzoic acids as H-bond donors. The use of these substituents d epresses the isotropization temperature and also induces smectic phase s for the complexes. The mesophase range has been extended by the mixi ng 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.