Evidence of a tilted columnar structure for mesomorphic phases of benzene-hexa-n-alkanoates

Abstract

Optical observations on the mesomorphs of benzene-hexa-n-alkanoates, and their admixtures with benzene, show that the planes of these discotic molecules are not perpendicular to the columns in which they are stacked, as was supposed originally. This evidence comes principally from the occurrence in texture domains of circular bending, between crossed polarizer and analyser, of oblique extinction brushes (i.e., brushes not parallel or perpendicular to polarizer and analyser directions). The obliquity varies from specimen to specimen up to a maximum of about 35°. A tilted stacking, with a tilt angle (for the optic axes of the molecules) of about 35°, will fit all the observations. Four alternative variants of such structures, two with the same tilt axes for all molecules, two with a herring-bone alternation between two orientations of tilt axis, all of which give rectangular pseudo-hexagonal lattices, are discussed. It is shown that the departure from hexagonal axial ratio required by these tilted structures is small enough not to be incompatible with the X-ray diffraction evidence previously interpreted as indicating hexagonal symmetry. These structures allow several different twinning modes, and complex optical features in some domains of circular bending are explicable in terms of these twinnings.