Columnar nematic phase hexagonal

However, the area was further studied by Usol tseva [205] who synthesized several peripherally carboxylated phthalocyanines and successfully characterized their phase behaviour as lyotropic columnar. The complexes in Figure 111 with M = 2H, Cu, Zn or Co(II) and X = H, Y = COOH and Z H or COOH were found to show columnar nematic and hexagonal phases in aqueous ammonia, but this was suppressed when the phthalocyanine ring was substituted according to X = COOH and Y = Z = H or when the central metal ion was Al(III). In this latter case, suppression of the mesomorphism was due to the formation of p-oxo dimers ([PcAl-O-AlPc]). 

Fig. 1.1.10. Mesophases of discotic polymers (a) the hexagonal columnar phase. The diagram illustrates intercolumnar binding as well as intracolumnar back-folding of the main chain (Herrmann-Schonherr, Wendorff and Ringsdorf ) (b) the sanidic nematic phase composed of boards stacked parallel to one another (Herrmann-Schonherr et a/. ) (c) the columnar nematic phase (Ringsdorf...

Figure 33. Mesophases of discotic polymers (a) hexagonal columnar phase (b) the board-like, or sanidic, nematic phase (c) the columnar nematic phase.

McMillan s model [71] for transitions to and from tlie SmA phase (section C2.2.3.2) has been extended to columnar liquid crystal phases fonned by discotic molecules [36, 103]. An order parameter tliat couples translational order to orientational order is again added into a modified Maier-Saupe tlieory, tliat provides tlie orientational order parameter. The coupling order parameter allows for tlie two-dimensional symmetry of tlie columnar phase. This tlieory is able to account for stable isotropic, discotic nematic and hexagonal columnar phases. 

The discotic mesophases are classified in two types columnar, and nematic discotic. The structure of the nematic discotic mesophase (Np, Figure 8.3, left) is similar to that of rod-like molecules, but constituted by disk-like units. In columnar mesophases, the molecules are stacked in a columnar disposition and, depending on the type of columnar arrangement, several columnar mesophases are known. The most common lattices of the columnar phases are nematic discotic (No), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rectangular (Col,) mesophases. 

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... 

Frenkel [412] also examined the behaviour of systems consisting of hard spheres in which the top and bottom have been cut off to form disclike structures. Here it is possible to demonstrate the existence of both discotic nematic and hexagonal columnar discotic phases. 

In discotic phases the orientation of the molecules is perpendicular to the molecular plane. Here, the columns can be arranged in a nematic or columnar manner. In the nematic phase the molecules possess a centre of gravity randomly ordered, but with the short molecular axis of each molecule more or less parallel. In the columnar phase, beside the preferable orientation of the short molecular axes, the disc-like molecules are ordered forming columns. Depending on the correlation strength between he columns these phases can be subdivided into ordered or disordered. A third possibility is to have a thermodynamically preferable position of the columns in the mesophase, like in a hexagonal cell. Additionally, a tilt of the columns is also possible. 

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...

Here, a paper by Smalyukh et al. stands out, which reports on nanorod alignment using likely more suitable lyotropic liquid crystals [6], The authors demonstrated spontaneous, long-range orientational ordering of CTAB-capped GNRs dispersed in lyotropic nematic and hexagonal columnar liquid crystalline phases formed by... 

However, more remarkable was the discovery that in alkane solvents, large, orrfiometallated macrocyclic complexes of palladium shown in Figure 92 would form lyotropic columnar phases [171], These remarkable materials have been shown to form columnar hexagonal phases and, in suitable solvents, lyotropic nematic phases derived from columnar organization. Further, in certain non-mesomorphic examples, mesophases can be induced by the addition of an electron acceptor such as trinitrofluorenone chiral phases are introduced when the acceptor is resolved 2 -(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)propionic acid (know as TAPA). 

Compound 43 was found to exhibit a chiral nematic phase (see the texture in Fig. 48), hexagonal disordered columnar (see Fig. 49) and rectangular disordered columnar phases, in the sequence g 5.4 Col d 30 Col 102.3 N 107.7 °C Iso Liq [94]. Thus the increase in the number density of the meso-gens bound to the central scaffold transforms the situation from the octamers 41 and 42, which exhibit calamitic phases, to one where the hexadecamer 43 exhibits columnar phases. However, the formation of columnar phases when the dendrimer possesses rod-like mesogenic groups is not easy to vi-... 

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]. 

The number and arrangement of the chains has a profound effect on the mesomorphism, and, while it is not the intention here to describe this in too much detail, some explanation is required. The mesomorphism of most isomers is, in fact, rather straightforward as they form columnar phases. This is true for hexa-, penta-, and tetracatenar (Zmm)(Zmm) systems, and the phases observed are normally either of hexagonal and/or rectangular (e.g., cZmm or symmetry. Tricatenar (Zmp)-V(p) species tend to form smectic (usually SmC) and nematic phases, whereas... 

In addition to those formed by surfactant amphiphiles, two other types of lyotropic mesophases are generally recognized, neither of which exhibits a cmc. The first of these are lyotropic phases of rigid-rod polymers that can form mesophases in both aqueous and non-aqueous solvents " these mesophases are of the nematic or hexagonal type. Examples include polymeric metal acetylide complexes and DNA." The other type is usually formed from flat and largely aromatic molecules which stack to give lyotropic columnar phases, also referred to as chromonic phases." " This latter class is formed from systems with ionic or strongly hydrophilic peripheral functions, and forms mesophases... 

Most of what has been aid so far applies to rod-like molecules, but similar phases exist for disk-shaped molecules, such as porphyrins and phthalo-cyanines. These tend to form phases in which the director is now orthogonal to the plane of the molecule. Thus, where the planes of the molecules are more or less parallel, but with no positional ordering into columns, the phase is a nematic discotic. If, however, the macrocycles form themselves into columns as well, we have a columnar smectic phase. A loose analogy of this phase is that it is like stacked coins, with the stacks arranged in a hexagonal lattice. 

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( >). 

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