Disk-like mesogen

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... 

Figure 22 The discotic ordered hexagonal (Dho) phase of a main-chain liquid-crystalline polymer containing disk-like mesogenic groups (a) viewed from the side of the columns (b) viewed from the top of the columns (c) top disk layer of the columns (d) top two disk layers of the columns. (From Ref. 76.)...

While it is in discotic liquid crystals that the formation of columnar phases is most readily recognized supra), there exists a family of non-disk-like mesogens, the polycatenar liquid crystals, where these mesophases are also formed extensively. As will be seen from their shape, this observation is of some interest, but it is the fact that certain polycatenar materials can, within a homologous series, show mesomorphism characteristic of both rod-like and disklike mesogens that makes them particularly interesting. 

While it is in discotic liquid crystals that the formation of columnar phases is most readily recognized (vide supra), there exists a family of non-disk-like mesogens, the polycatenar liquid... 

Scheme 4.8 Chemical structure of disk-like mesogen capping agents 19...

All these cases represent overall examples of hierarchical self-assembly, a conditional process where the initial assembly of molecular components into a disk-like mesogenic supermolecule is a prerequisite for the subsequent formation of a discotic columnar architecture by stacking of the disks (Figure 4 also see Section II.B). 

In order to investigate the gap between dis-cotic and calamitic liquid crystals and in an attempt to obtain biaxial nematics, Fletcher and Luckhurst [235] synthesized hybrids of rod-like and disk-like mesogens, by linking two units to dimers. Among the several compounds of this type is one example that shows nematic behaviour (21) however, the... 

Figure 20. Structures of molecules of some liquid crystal polymers. The rod-like and disk-like mesogenic groups can be included as main groups in rigid or flexible polymeric chains, or can be attached as side groups of flexible chains. Cross-linking gives the liquid crystal elastomers or thermosets. (By courtesy of Adamczyk [355]).

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