Self-organisation of liquid crystal diphenyl acetylenes

The diphenyl acetylene unit has been incorporated into a series of oligomers, chiral dimers and ultimately supermolecular Janus liquid crystals. In this way a family of Janus liquid crystals featuring incompatible cyanobiphenyl and perfluoroalkyl mesogenic units have been synthesised and characterised. The dominant mesophase organisation of these materials is smectic A due to microphase segregation of the perfluoroalkyl and cyanobiphenyl units. A dendritic structure derived from either pentaerythritol or 1,1,1-tris(hydroxymethyl)ethane has been used to incorporate branching on either side of the diphenyl acetylene core, and in this way the relative volumes of the two faces of the Janus materials have been altered. When the mismatch in volume between the two faces is increased the molecular shape becomes tapered and the curvature is expressed into the mesophase structure for example by the formation of columnar mesophases.

A family of chiral dimers featuring either ester or ether linkages between a terminal chiral chain – (S)-2-methylbutyl or 1-methylheptyl – and the diphenyl acetylene unit are reported. The ether linkage increases the polarizability of the diphenyl acetylene core and thus strength of the intermolecular interactions, therefore favouring the formation of smectic A or smectic C* phases. Contrastingly the ester linkage favours the formation of chiral nematic phases and these dimers provide a greater helical twisting power when incorporated as chiral dopants into a nematic host, E7.

Compounds containing the diphenyl acetylene unit can undergo cyclotrimerisation reactions to provide hexaphenyl benzene materials providing a rapid increase in molecular complexity in a single step. The reaction of mixtures of symmetrical diphenyl acetylenes to provide Janus hexaphenyl benzene liquid crystals has been investigated.