Triphenylene discotic liquid crystals: biphenyls, synthesis, and the search for nematic systems

The development of the cyanobiphenyls for display applications was one of the most important technological advances of the 20th century. Here, we present a personal account of how advances in biphenyl science (synthesis and application) influenced the development of discotic liquid crystals. We focus on those based on triphenylene, the most widely studied discotic core. The search for nematic discotics proved to be challenging with relatively few early examples being reported. In examples of symmetrically hexasubstituted triphenylenes, a ring of peripheral aryl ester or alkynylaryl substituents provides effective steric blocking to columnar assembly - the face-to-face molecular arrangement that is observed in the majority of triphenylene discotics. The synthetic challenges associated with early triphenylene synthesis have now been largely overcome. Efficient oxidative trimerisation of dialkoxybenzenes gave direct access to materials with high purity and at scale. The intermediates can be manipulated through deprotection and elaboration, a pathway that allowed synthesis of lyotropic (Nc) systems among others. The combination of modern cross-coupling chemistry with oxidative processes has proved versatile, leading to synthesis of unsymmetrical monomeric and dimeric systems. Dimeric systems linked by rigid bridges have been identified as a separate class of materials that show strong tendency for nematic mesophase formation.