Hexaalkoxytricycloquinazolines: New Discotic Liquid Crystals

Tricycloquinazoline (TCQ), a molecule of both biological and physical interest, has been found to function as the core fragment for a new family of discotic mesogens, which have C₃ symmetry and six aliphatic side chains. Six representative homologous 2,3,7,8,12,13-hexaalkoxy-TCQ derivatives 5 were synthesized, with alkyl side chain lengths varying from 3 to 16 carbon atoms. These highly fluorescent, heteroaromatic compounds are conveniently obtained by alkylation of hexahydroxy-TCQ, 10. The latter is prepared by trimerization of 5,6-dimethoxyanthranil, 8, followed by demethylation of the resultant hexamethoxy-TCQ, 9. All compounds, 5, are mesogenic within a very broad temperature range. The nature of these new mesophases was studied by DSC and by optical microscopy with polarized light and X-ray diffraction. DSC measurements show highly reversible behavior at both melting and clearing points, indicating high chemical stability. Photomicroscopic pictures show mosaic textures, characteristic of an ordered hexagonal, columnar phase (D_ho), that are very similar to those we reported earlier for hexakis(thioalkoxy)-TCQ derivatives 4. The diffraction spacings of two representative compounds (having 5 and 11 carbon atoms in the side chain) in the mesophase are consistent with a two-dimensional hexagonal lattice having unit cell sides of a = 21.2 and 28.7 Å, respectively, and containing one molecule per unit cell.