CORE-HALOGENATED, HELICAL-CHIRAL TRIPHENYLENE-BASED COLUMNAR LIQUID-CRYSTALS

It is shown that chlorination and even bromination of the mesogenic he xahexyloxytriphenylene 1 can take place at the sterically congested te tracyclic core delivering the respective monohalogenated, sevenfold su bstituted enantiotropic mesogens 3 and 4 in 32 and 17% yields, respect ively, giving mesophases over greater temperature ranges. Miscibility tests among all mesogenic compounds of this study prove formation of t he columnar hexagonal type of phase. The steric and electronic effects of those substituents, different from the six equal alkoxy groups, on the mesophase are discussed in detail. In our hands and to our surpri se, the sterically hindered acceptor (nitro or halogeno) substituents of 2, 3, or 4 could not be replaced by an alkylthio group in nucleophi lic aromatic substitution reactions using the aprotic polar solvent DM EU. Semi-empirical calculations on numerous hexaethers with one or two substituents, e.g. halogens or others, in one of the three bay region s of the triphenylene core reveal that these molecules are helically d eformed due to the crowding in the molecule. In this new case of atrop isomerism, the 1,12-disubstituted derivatives should offer the possibi lity of resolution into enantiomers and, therefore, of studying disc-l ike mesomorphic compounds possessing for the first time the origin of chirality within their molecular centre.