Thermotropic nematic phases columnar

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]. 

Discotic LC are formed by disk-like molecules with aromatic cores and side chains that are either hydrophobic (i.e., thermotropic) or hydrophilic (i.e., lyotropic). The discotic nematic (No) phase behaves like a normal nematic phase formed by rod-like molecules, and the disk-like molecules are oriented with their short molecular axes parallel to the director but show no positional order. More ordered columnar phases are commonly formed by thermotropic discotics. The two-dimensional structure can pack the columns into a hexagonal or rectangular columnar phase, while within the columns, disks can be... 

Fig. 3 Hekates with flexible spacers. Compounds 1, 2 Thermotropic properties as a function of spacer length. Compounds 3, 4 LC properties with unchanging spacer length and variable peripheral chains. Compounds 5, 6 Nematogens claimed to form biaxial nematic phases. Cr crystal, g glass, N nematic, N cholesteric, SmX unidentified smectic phase, Colh columnar hexagonal phase. All temperature are given in °C...

Liquid crystalline phases can show not only long-range orientational order as nematic phases do but also long-range positional order. When this positional order is one-dimensional, the mesophase is called lamellar or smectic when it is two-dimensional, it is called columnar. The latter case is often found with thermotropic liquid-crystal disk-like molecules. Such molecules stack in columns that assemble on a 2-D lattice of hexagonal, rectangular, or oblique symmetry. The molecules in a given column only show 1-D liquid-Hke order and the uncorrelated columns are free to slide past each other, which ensures the mesophase fluidity [73]. 

Molecules which combine the features of the rod and the disc may be expected to form new types of mesophases. An example is the biaxial nematic phase reported in thermotropic systems (see 6.6). Malthete et a/. have prepared an interesting series of mesogens shaped like stick insects called phasmids (fig. 6.1.5(n)). Some of them form columnar mesophases the structure proposed for the hexagonal phase is shown schematically in fig. 6.1.5( >). 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

It is now certain that metal carboxylates were the first metal-containing liquid crystals, reported in 1855 with Heinz s work on magnesium tetradecanoate. Then, many other mesomorphic mono-, di-, and tri-valent carboxylate complexes, with the general formula [M(02CC H2 +i)J (x=l, 2, 3) or [M2(02CC H2 +i)4] were prepared. Some of them were described in 1910 by Vorlander." These materials may show thermotropic nematic, smectic, cubic, and columnar mesophases, but also, when dissolved in water or alkanes, lyotropic mesophases. While not all of the compounds described in this section show columnar phases, it was decided to keep these materials together. 

A second noteworthy series is that of the dirhodium tetraalkanaotes and tetrabenzoates which have been studied extensively as thermotropic mesogens (vide supra). In solution in alkane, columnar nematic phases were formed as evidenced by the schlieren textures observed. 

This section pertains to reports on oriented molecules in which phases other than the usual thermotropic nematics have been used. Studies in chiral, smectic, columnar, lyotropic and polymeric liquid crystals as well as other unusual phases have been presented. The use of carbon-proton heteronuclear selective refocusing 2D NMR experiment designed for the spectral analysis of enantiomers dissolved in weakly ordering chiral liquid crystal solvents has been proposed." The method permits the extraction of carbon-proton residual dipolar couplings for each enantiomer from a complex or unresolved proton-coupled... 

Low molecular mass semiconductors are particularly appealing for computational scientists because the timescales of their rearrangements are relatively fast and often accessible to atomistic simulations. It is then becoming possible to reproduce both the equilibrium structure and the self-assembly process of crystalline, liquid crystalline, and amorphous organic semiconductors. Among those, thermotropic LCs are certainly the most fascinating and challenging materials from the structural point of view because of the variety of different phases (nematic, smectic, columnar, and many more [124]) they exhibit in relatively small temperature windows. 

In addition to the above lamellar, columnar and optically isotropic phases, there are also lyotropic nematic phases, which usually involve mixtures of a charged amphiphilic, such as simple soap, with an alkanol (a weaker amphiphilic where the head group is an alcohol), together with water and a simple salt. They are termed nematic because, like thermotropic nematics, their optical axes are easily oriented by external magnetic fields. In contrast to thermotropic nematics, the basic units of lyotropic nematics are molecular aggregates with dimensions of about 2-10 nm. In lyotropics, the nematic phase is much less usual than in thermotropic liquid crystals. Lyotropic nematics... 

Rod like poly(p-biphenylene terephathalates) having long n-alkyl side chains with different lengths exhibit well-defined thermotropic LC phase behavior as elucidated by DSC and X-ray diffraction [29, 30]. These polymers form the nematic LC phase at high temperatures and two ordered mesophases at low temperatures. The stmcture of the mesophases in the low temperature range depends on the carbon number of the n-alkyl side chain, n. One of the mesophases is the hexagonal columnar phase in the polymers with n of 8 to 12. Another is the layered phase in the polymer with n = 18. In the polymers with n of 13 to 16, the hexagonal columnar phase and layered phase coexist. 

In this new type of nematic phase reported about ten years ago [18,19], its columnar superstructure acts as calamitic molecules do in their nematic phase and also displays a Schlieren texture, which is responsible for its name. X-ray diffraction studies [18,19] have proved the nematic array of columns. This method of mesophase induction constitutes an attractive and novel principle [9,14,17,18] for the design of tailor-made thermotropic nematogens from discotic starting materials by spontaneous supramolecular self-organisation. 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

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