Nomenclature discotics

Figure C2.2.7. Schematic illustrating tire classification and nomenclature of discotic liquid crystal phases. For tire columnar phases, tire subscripts are usually used in combination witli each otlier. For example, denotes a rectangular lattice of columns in which tire molecules are stacked in a disordered manner (after [33])...

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

A few words of clarification about the nomenclature are relevant here. Columnar phases have been known for many years they were evident, for example, in the work of Spegt and Skoulios (28) on metal soaps, although these are not classical disk-shaped molecules. In 1977, however, a hexasubstituted benzene derivative was reported (29), this derivative was the first example of a properly disk-like mesogen, and the term discotic was coined to describe the mesophases it formed. Thus, for example, the discotic hexagonal phase was labeled Dh- The introduction of this nomenclature has actually caused confusion as disk-like molecules are not alone in their capacity to form columnar phases (indeed, some... 

Discotic liquid crystals came to prominence in the late 1970s when Chandrasekhar, Sadashiva, and Suresh reported the discovery of this new class of liquid-crystalline molecules, which were found to form columnar phases. The first of these, a hexaalkanoate of benzene, is shown in Figure 9. There then followed a rather unfortunate confusion of nomenclature in which the phases formed by discotic molecules were themselves referred to as discotic, carrying the abbreviation D. A liquid-crystal phase must be characterized by its symmetry and organization and not the shape of the molecules of which it is composed this is particularly important in columnar systems as many non-discotic molecules exhibit columnar phases. Indeed, columnar mesophases have been recognized for many years and studies date back to at least the 1960s with the work of Skoulios with various metal soaps. " Therefore, columnar phases take the abbreviation Col followed by some descriptor that describes the symmetry of the phase. 

The nomenclature of discotic liquid crystals depends on the arrangement of the compounds (molecules) in columnar or nematic structures. Within a short spatial range and in the temporal and spatial average the longitudinal axes of nematic molecules are oriented parallel to each other. 

Figure 1. Schematic diagram illustrating the basis for the classification and nomenclature of the discotic me-sophases. For the columnar phases the subscripts are often used in combination with each other. Hence refers to a (two dimensional) rectangular lattice of columns within which the discs are stacked in a disordered manner...

Figure 4. Schematic phase diagrams of members of the radial hexayne series 1,2, and 3 as electron donors with TNF as the electron acceptor [26] Cr crystalline, Nd nematic discotic, Col, columnar rectangular, and Col), columnar hexagonal mesophase (o intracolumnar ordered). Iso isotropic liquid (see footnote 1 in Sec. 3 commenting on the nomenclature of columnar mesophases).

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