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 (hexagonal: D h , rectangular: D r , oblique: D ob ), and secondly the order or disorder within the column. Several examples of disc-like compounds which form phases D ho , D hd , D rd(P2 1/a) , D rd(P2/a) , D rd(C2/m) , D od.d are described and a discussion of the specificity of the corresponding optical textures is given. Then 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, based on the shape of the structural element (finite or infinite cylinder, disc, infinite sheet, globule).