Nematic mesophase columnar

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.

Fig. 10 The molecules involved in the formation of hydrogen-bonded nematic and columnar phases (upper part) and a representation of the arrangement of the molecules in the mesophases (lower part)...

Note 2 The adjective diseotic is also employed to describe the nematic mesophases formed by diseotic mesogens. The mesophases formed by a columnar stacking of disc-like molecules are described as columnar mesophases. 

Note 5 If the mesogenic side-groups are rod-like (calamitic) in nature, the resulting polymer may, depending upon its detailed structure, exhibit any of the common types of calamitic mesophases nematic, chiral nematic or smectic. Side-on fixed SGPLC, however, are predominantly nematic or chiral nematic in character. Similarly, disc-shaped side-groups tend to promote discotic nematic or columnar mesophases while amphiphilic side-chains tend to promote amphiphilic or lyotropic mesophases. 

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. 

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. 

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. 

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

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... 

When fluorine groups were introduced in the 2- and 3-positions of the terminal aromatic ring of the biphenyl unit ((48) M = Cu R = OC8Hn w=l) no columnar phase was observed, but a monotropic nematic phase instead, probably due to the reduction of the molecular symmetry When the lateral chain was elongated, the columnar phase disappeared at the expense of an enantiotropic nematic phase ((48) M = Cu R = OC H2 +i, n = 6, 8, 10-12, 14 and m = 2 n=l2, m = 3 and n = 4, 8, m = 4), which became monotropic ((48) M = Cu, R = OC H2 +i, n=l2,m = 4 n = 8, m = 8 and n=12, w = 12,). Furthermore, re-entrant (Section 7.9.3.2.5) nematic phases, between two crystalline phases, were also observed for the compounds in the series m = 2. Both the temperatures of the crystal-to-mesophase (163-219 °C) and mesophase-to-isotropic liquid (175-224 °C) decreased very rapidly with increasing m and n. The stability of the nematic mesophase was also found to depend on the bulkiness of the lateral chain. For instance, using a branched side group such as HMeEt did not suppress the monotropic phase but reduced the transition temperatures considerably. The cross-over between the Coh and the nematic phase would then correspond to a decrease of the lateral interactions between molecules due to the steric hindrance of the chains. 

I.M. Saez, J.W. Goodby, R.M. Richardson, A liquid crystalline silsesquioxane dendrimer exhibiting chiral nematic and columnar mesophases. Chem. Eur. J. 7, 2758-2764 (2001)... 

A variety of systems exhibit liquid crystalline properties and a rich variety of phases that have been investigated by NMR have been reported. Particularly, molecules exhibiting unusual topologies different from the usual rod like structure such as bent-core or hockey stick-shape have been of significant interest. The biaxial nature of some of these systems has also attracted much attention. Here, reports of studies on the above systems as well as studies on molecules exhibiting nematic, smectic, columnar or lyotropic mesophases and the study of orientational order in such systems have been included. 

Discotic liquid crystals—columnar and nematic mesophases... 

Figure 1.8(a) shows a typical columnar mesophase, consisting of liquidlike columns of molecules arranged in the hexagonal lattice. This mesophase can be referred to as a unidimensional liquid and as a two-dimensional solid. The point group symmetry is Dqyi- Disc-like molecules can also form the nematic mesophase [22] shown in Fig. 1.8(b). As a rule, such a phase is optically uniaxial and negative. The optical axis coincides with the director L. 

Table 2 demonstrates the various types of the most well known and widespread mesophases consisting of rod-, board (or lath)-, disk-, and banana-shaped molecules. Depending on the orientational and positional organization of the molecules, these mesophases can roughly be divided into nematic, smectic, columnar, and bent-core (banana) LC phases. 

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]. 

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