Orientation of Discotic Mesophases

Abstract

The orientation of some fluid (D_F) and optically uniaxial columnar (D) discophases by surfaces is examined by optical microscopy. The test materials are 2,3,6,7,10,11 hexa-substituted triphenylenes, including two new compounds exhibiting mesophases at room temperature, and a binary mixture with a D_F, phase stable between 57 and 100°C. The alignment with the directors perpendicular to the walls can be obtained on glass surfaces coated with flat molecules possessing six polar side-functions. Similar homeotropic alignments for the DF phase and total normal orientations for the D phase are observed on faces of cleaved apophyllite and muscovite crystals. For the D_F phase, the uniform alignment with the directors parallel to the surfaces is obtained using glass surfaces coated with silicon oxides deposited at oblique incidence. Mutual orientations of the D and D_F phases are noted. Between two slides and with the parallel alignment, left or right handed twisted fluid discophases with only an infinite-fold screw symmetry axis can be obtained. Displays using optically negative mesophases can be envisaged.