Novel discotic liquid crystals created by electrophilic aromatic substitution

Abstract

Penta-, hexa- and hepta-alkoxy derivatives of triphenylene can be smoothly mono-nitrated in high yield to give novel discotic liquid crystals with enhanced mesophase properties. We have investigated the regiochemistry of these reactions which appear to be controlled by a combination of steric and electronic factors. Nitration of 2,3,6,7,10,11-hexahexyloxytriphenylene and of 2,3,6,7,10,11-hexakis-[2-(2-chloroethoxy)ethoxy]triphenylene results in the introduction of a single nitro group in the 1-position. Nitration of 1,4,6,7,10,11-hexahexyloxytriphenylene gives the mono-nitrated product with the vitro group at the 2-position (an example of β-nitration). When 2,3,6,7-tetrahexyloxy-9,10,11-trimethoxytriphenylene is nitrated the vitro group is substituted on the ring which contains the three methoxy groups giving a fully substituted ring. Nitration of 2,3,6,7,10-pentahexyloxytriphenylene produces 2,6,7,10,11-pentahexyloxy-1-nitrotriphenylene and 1,3,6,7,10,11-hexahexyloxytriphenylene gives the mono-nitrated 2,4,6,7,10,11-hexahexyloxy-1-nitrotriphenylene. The vitro group can be modified using standard aromatic chemistry to give amino, amido, azido, etc. substituents. These reactions normally result in systems with increased mesophase ranges. In two of the cases examined nitration converts a non-mesogenic substrate into one exhibiting a monotropic columnar phase and in another it converts a monotropic into a enantiotropic system. Furthermore, these reactions allow fundamental properties such as dipole moment, redox potential and bandgap to be engineered at a molecular level.