Discotic mesogens based on triphenylene-fused benzimidazole or perimidine: facile synthesis, mesomorphism, optical properties and self-assembly

Four new mesomorphic triphenylene-fused nitrogen-containing heterocyclic systems, including three benzimidazole and one perimidine derivatives with extended triphenylene core of different sizes are synthesised from the condensation reaction between aromatic diamines and our developed triphenylene dicarboxylic anhydrides. These discotic mesogens self-assemble into supramolecular columnar hexagonal mesophases with high clearing transition temperatures, demonstrating a strong tendency for pi-stacking aggregation in the LC state owing to their good planar molecular structures. Four triphenylene-fused mesogens exhibit strong absorption bands in the range of 262 nm to 450 nm and corresponding emission bands between 468 nm and 615 nm, respectively. Both the experimental results and computational calculations show that the mesomorphic and electronic properties of four mesogens are strongly affected by the expansion of triphenylene core and the electronic nature of the nitrogenated benzimidazole or perimidine ring. Besides, these mesogens also possess higher HOMO energies and narrower HOMO-LUMO gaps than parent anhydride. Furthermore, the solution of TDPI or TDP in THF at a concentration as low as 0.45 wt % exhibited organogel phases due to strong intermolecular pi-pi interactions between the large aromatic cores. All these properties qualify this new type of discotic liquid crystals as potential applications as organic semiconductor materials and sensors.