Columnar nematics

Solid I Columnar Nematic Isotropic 152QC 168QC 244QC... 

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. 

Figure 8.3 Schematic representation of the molecular arrangement in the nematic discotic (Nq), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rentangular (Coir) mesophases. A tablet represents a molecule with disk-like shape.

Cobalt(II) nitrate induces a columnar nematic phase in the 1 2 complex with 98 (Table 7) as deduced from X-ray scattering which is stable for 30 K. Also, nickel(II) nitrate turned 98 into a liquid crystal with an unknown phase. The complex between 99 and copper(II) nitrate showed the same unknown mesophase. The phase range was 140 K but the samples decompose before clearing. A possible reason for the induction of a mesophase by complexation is the stiffening of the crown and the adjacent flexibly linked groups [112]. 

Liquid crystals may be classified as nematic, cholesteric, smectic, or columnar. Nematic liquid crystals are characterized only by orientational order. Molecules tend to be aligned with a director, as illustrated in Figure 16.1. 

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]). 

Molecules of a disc-like shape form either nematic or columnar phases. Structurally the most simple mesophase is the discotic nematic phase (Nd). Just as in the case of the nematic phase formed by calamitic molecules, the molecules possess orientational order, but no positional order (Figure 2.8). The columnar nematic phase (Nc) consists of short columns of a few molecules that act like the rod-like molecules in the nematic calamitic phase (Figure 2.9). There is no organisation of the columns in a two-dimensional lattice. 

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...

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. 

Fig. 4.1 Typical LC phases, a nematic (N), b smecitc A (SmA), c smectic C (SmC), d discotic nematic, e columnar nematic, f columnar hexagnal (Colji), g columnar rectangular (Coir), h columnar oblique (Colob)...

FIGURE 6 Schematic representation of the discotic nematic phase, Nd (left) and of the columnar nematic phase, Njoi (right). 

FIGURE 3.1 Schematic representations of commonly observed discotic liquid crystalline phases. Discotic nematic (a) columnar nematic (b) columnar hexagonal (c) columnar rectangular (d). 

Discotic liquid crystals may be classified broadly into three types columnar, nematic (including its chiral modification) and lamellar. 

Figure 13. Schematic diagram of the structure of the columnar nematic phase induced hy charge transfer interactions between an electron donor, pentakisfphe-nylelhynyl)phenyialkyl ether (in hlack), and an electron acceptor, 2,4,7-trinitrofluorenone (hatched) [37-39].

As mentioned in Section 7.2.3, the observed trend of the columnar - nematic - isotropic sequence of transitions in some homologous... 

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

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