Discotic cores, large

There are relatively few reports of liquid crystal dimers containing discotic mesogenic units [105-112] and this has been due in large part to the difficulties in preparing monofunctionalised discotic precursors. Without such monofunctional precursors the preparation of discotic dimers often involves laborious separation procedures involving several isomers of the discotic core. In recent years, however, there have been many advances in the synthesis of triphenylene-... 

Discotic architectures that use a central triphenylene core with six peripheral triphenylene units also exhibit colunmar mesophases, and these are commonly called star-like liquid crystals. Compoimd 28 is a very large molecule that uses flexible spacers to attach peripheral triphenylene units to a central discotic core in a star-like manner. The hexagonal columnar phase of compound 28 has been identified as hexagonal. Such stractmes are ohgomeric and could almost be considered polymeric (see Chapter 5). 

Debije MG, Piris J, de Haas MP, Warman JM, Tomoviae Z, Simpson CD, Watson MD, Mullen K. (2004) The optical and charge transport properties of discotic materials with large aromatic hydrocarbon cores. J Am Chem Soc 126 4641-4645. 

In discotics, the interdisk stacking interaction is several orders of magnitude stronger than the inter-columnar interactions, because of the phase separation induced by the side-chains whose van der Waals interactions are much weaker. Generally, long polymers are obtained via discotics that have strong and specific intermolecular interactions, either via a large aromatic core or via additional specific intermolecular... 

Recently, discotic liquid crystal mesomoiphism has been found for a range of materials based on the tricycloquinazohne core, which is a fairly large polyarorrratic heterocyclic core (20). 

It is generally aeeepted that a fairly large, planar, and rigid core that has six or eight peripheral moieties attached is required for the generation of discotic mesophases. However, the phase is exhibited, albeit monotropically, by one of the very few... 

Compound 18 exhibits solely a chiral nematic discotic phase (N ) phase because the steric effect of the branched chains at the chiral centre disrupts the ability of the molecules to pack in columns. The large size of the planar aromatic core ensures a high clearing point, but the liquid crystal tendency depends critically on the type of chiral peripheral chain. Hexa-substituted phenylacetylenes were discussed in chapter 3 and exhibited the Np phase. Perhaps not surprisingly, when one of the peripheral acetylene units is chiral, the N phase is exhibited (compound 19). 

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