Discotic liquid crystals mesophases

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

Since the discovery of discotic liquid crystals [121], the mesophases formed by rod-like and disc-like molecules have been considered as belonging to different liquid crystalline classes. Indeed, the conventional rod-like and disc-... 

To understand how chirality is expressed, it is important to first describe the different thermotropic mesophase assemblies which can be formed by chiral discotics. Even though expression of chirality has been observed in thermotropic mesophases, the chiral expression occurs in a rather uncontrolled manner, and systems which are suitable for applications, for example, easily switchable columns/ferroelectric discotic liquid crystals, consequently have not yet been developed. Hence, the assembly of discotics in solution has received considerable attention. Supramolecular assemblies of discotic molecules in solution are still in their infancy and have not yet found commercial application, but they are of fundamental importance since they allow a detailed and focused investigation of the specific interactions that are required to express chirality at higher levels of organization. As such, the fundamental knowledge acquired from supramolecular assemblies in solution might formulate the design criteria for thermotropic chiral discotic mesophases and provide the necessary tools for the creation of functional systems. 

Discotic liquid crystals based on carbohydrates are intrinsically chiral due to the chiral nature of their core.49,50 Numerous discotic liquid crystals derived from these mesogens have been reported however, the expression of chirality in the subsequent mesophases has only rarely been seen. An example of the latter is the a-anomer of penta- O -decanoylglucopyranosc (47), which forms a discotic mesophase in which the molecules pack helically in the columns... 

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

In contrast to calamitic mesogens, discotic liquid crystals are built from disk-like molecules that can arrange into different structures, such as the discotic nematic mesophase, the discotic columnar mesophase, or the discotic hexagonal meso-phase. 

Discotic liquid crystals arise from disc-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to that of their respective calamitic mesophases, that is, the normals... 

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. 

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. 

The first liquid crystals of disc-shaped molecules, now generally referred to as discotic liquid crystals, were prepared and identified in 1977. Since then a large number of discotic compounds have been synthesized and a variety of mesophases discovered. Structurally, most of them fall into two distinct categories, the columnar and the nematic. The columnar phase in its simplest form consists of discs stacked one on top of the other aperiodically to form liquid-like columns, the different columns constituting a two-dimensional lattice (fig. 1.1.8 (a)). The structure is somewhat similar to that of the hexagonal phase of soap-water and other lyotropic... 

Figure 1 General structures of calamitic and discotic thermotropic liquid crystals. (A) Layered calamitic smectic liquid crystal. The structures of the various types depend on the local packing of the molecules, the extent of the packing, and the orientation of the long axes with respect to the layers. (B) Calamitic nematic liquid crystal. The molecules have no long-range order, and are only orientationally ordered. (C) Ordered columnar discotic liquid crystal. Disk-like molecules form ordered or disordered columns different column packings give rise to various mesophase structures. (D) Nematic-discotic liquid crystal phase. The disk-like molecules are only orientationally ordered.

The mesophases are formed from aromatics which are produced from smaller molecules by condensation. Since the large-area molecules behave like discs, the liquid-crystal phases of pitch aromatics are referred to as discotic liquid crystals or, when arranged along a preferred axis, as discotic nematic phases. 

Discotic liquid crystals—columnar and nematic mesophases... 

Research into discotic liquid crystals has not been very extensive because of the perceived lack of applications for such materials and mesophases perhaps the lack of... 

Supramolecular discotic liquid crystals may be generated via the initial formation of disk-like supermolecules. Thus, the tautomerism-induced self-assembly of three units of the lactam-lactim form of disubstituted derivatives of phthalhydrazide yields a disk-like trimeric supermolecule (Figure 3). Thereafter, these disks self-organize into a thermotropic, columnar discotic mesophase [13]. 

Al and Dl are self-assembled groups which form Dl - -Al discotic liquid crystals. They assemble into a hexagonal columnar mesophase. 

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

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