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

Abstract

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 α-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 α-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, α-nitration increases the mesophase range (relative to the simple aryl ether), and for the systems with –OC₁₂H₂₅, –OC₁₄H₂₉, and –OC₁₆H₃₃, α-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 175 °C.