DISC-LIKE MESOGEN POLYMORPHISM

Among the various mesogenic polyaromatic cores, only benzene, triphenylene, truxene and anthraquinone hexasubstituted derivatives exhibit one or several D columnar phases and/or a N//D lenticular nematic phase. A simple scheme of nomenclature is suggested for the six different types of columnar phase requiring only two structural parameters: firstly the symmetry of the lattice, and secondly the order or disorder within the column. Several examples of disc-like compounds which form phases are described and a discussion of the specificity of the corresponding optical textures is given. The complex polymorphism of these new mesogenic materials (normal, inverse, reentrant columnar-nematic sequences) is reviewed. Finally, a simple classification of the various types of lyotropic and thermotropic phase is proposed.