Mesophases discotic mesogen phases

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... 

The most obvious family of compounds able to induce columnar phases is that constituted by the macrocyclic compounds which often possess a discoid molecular shape. The ability of macrocyclic systems to stack and to self-assemble into columnar mesophases confers on these systems the properties required for potential utilization in molecular devices based on photonics, electronics, or low-dimensional ionic transport. The search for new discotic mesogens is thus still of current interest, in order to improve their chemical, thermal, and photophysical stabihty, and new families of discogenic materials appear regularly. 

A final example of induced mesogenicity in a multicomponent system is the well studied, but less well understood, carbonaceous mesophases which are comprised of a myriad of unidentified molecules which are created in situ as petroleum pitches are heated to temperatures where chemical transformations occur [163]. The processes leading to a mesophase involve decreases in both the elemental weight fraction of hydrogen and the group fraction of aliphatic carbon atoms [164]. Model studies have demonstrated that the component molecules of these phases are fused, polycyclic aromatic molecules with disk-like shapes the exact structures of the components depend upon the natures of the precursor molecules which are heated [164-167]. All of the carbonaceous mesophases somewhat resemble discotic nematic phases [168]. At least some of them probably represent another example of liquid crystallinity induced by mixing molecular components which, when separated, are not mesogenic. 

Vorlander also had the idea to look for mesogenic properties in the cases of star-like or cross-like molecules, however, his coworker did not have any success in this area [5, 6]. About 1977, Chandrasekhar et al. [7] and Billard et al. [8] were, independently, able to prove that disk-like molecules can in fact form mesophases, called columnar phases. Since then, several hundred discotic compounds have been synthesized [9-14]. 

We end this chapter with a very brief account of a new class of liquid crystal polymers, i.e. discotic polymers [118-126]. The basic monomer units are discotic mesogen-ic moieties, which are components of the polymer main chain itself or are attached to the polymer backbone as side groups. A few examples are shown as structures 14-16. Besides the columnar phase, some new types of mesophase have been identified. A... 

Fig. 1.33 These figures trace the development of the diffraction pattern of a sample of a discotic mesogen as the crystalline solid is heated tmd passes through a succession of mesophases, ending as the isotropic liquid. Note the stepwise way in which reflections broaden and become diffuse, as various types of order in the stmctures tire lost. As in the similar scheme for smectic phases given in Fig. 1.30, the edge-on view of the molecular discs drawn on the left corresponds to the alignment of the diffraction pattern shown in the centre and again, for all mesophases, the sample as a whole is taken to have a random orientation of domtiins, giving rotational symmetry around the director...

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. 

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