Liquid-Crystalline and Helical Order in a Discotic Mesophase

Abstract

We have used x-ray diffraction to study oriented strands of hexa-hexylthiotriphenylene discotic liquid crystals. The low-temperature $D_{\mathrm{ho}}$ phase consists of a triangular array of columns with periodic positional and incommensurate helical intracolumn order. A three-column superlattice results from the frustration imposed by molecular interdigitation in triangular symmetry. We suggest that the $D_{\mathrm{hd}}\to D_{\mathrm{ho}}$ transition is driven by the increase in hydrocarbon tail stiffness and length with decreasing temperature.