Helical geometry and liquid crystalline properties of 2,3,6,7,10,11-hexaalkoxy-1-nitrotriphenylenes

The single-crystal X-ray structure of 2,3,6,7,10,11-hexaethoxy-1-nitrotriphenylene confirms earlier calculations of the molecular geometry and shows that the alpha-nitro substituent imparts a helical twist to the triphenylene nucleus. Within the crystal, the molecules are arranged in tilted columns on an oblique lattice, and along the columns the molecular dipoles are arranged antiferroelectrically, with a molecule-molecule dipole-dipole interaction of ca. -5.5 kJ mol(-1). It is suggested that this interaction may be important in stabilising the mesophase. Eleven other 2,3,6,7,10,11-hexaalkoxytriphenylenes and their mononitro derivatives (with side chains ranging from -OC2H5 to -OC16H33) have been prepared and characterised. Most of the alpha-nitrated compounds give enantiotropic columnar mesophases that show wider mesophase ranges than the precursor aryl ethers. For the systems with side chains from -OC4H9 to -OC11H23, alpha-nitration increases the mesophase range (relative to the simple aryl ether), and for the systems with -OC12H25, -OC14H29, and -OC16H33, alpha-nitration induces liquid crystal behaviour (the simple aryl ethers analogues being non-mesogenic). Although it had previously been claimed that 2,3,6,7,10,11-hexapropoxytriphenylene is not mesogenic, we found that it exhibits a plastic columnar phase between 100 and 175degreesC.