Discotic liquid crystals flat discotics

Discotic liquid crystals on the other hand are based around a fairly flat core structure and are generally surrounded by six or eight peripheral alkyl(oxy) chains. Examples are given in Fig. 3. 

Most polymeric liquid crystals are based on stiff rod-like molecular units which are called calamitic mesogens. There are some unusual polymers (which are not discussed here) that contain flat disk-like molecular units called discotic mesogens in which the disks form columnar arrays like stacks of poker chips. 

A few years later Chandrasekhar (1977) successfully synthesized this kind of discotic liquid crystal. There is a rigid flat aromatic core connected with 3, 4, 6, 8, 9 flexible side chains. Some examples of the discotic liquid crystal molecules are shown in Figure 1.2. Generally, the... 

This group has a characteristically flat disk shape, which is responsible for the discotic liquid crystal phase. The nickel bis (dithiolene) core has high thermal and photochemical stability. The substituted R groups impart the solubility in nonpolar solvents. Also, if the R groups are sufficiently long, they can form a nematic liquid crystal, as shown in Figure 5 and 6. 

The structure of liquid crystals can broadly be classified as nematic, cholesteric and smectic, see Fig. 1. None of them have full three-dimensional (3-D) positional order, but some degree of orientational order. Most often the constituent molecules are elongated, as indicated in Fig. 1, but distinctly flat molecules make up the socalled discotic liquid crystals. The nematic phase has only orientational ordering of the molecules. The collection of molecules have one symmetry axis called the director n. The cholesteric phase has only orientational order, formed by the constituent chiral molecules. The director twists with a pitch comparable to the wavelength of light. 

For example, a discotic ferroelectric liquid crystal was synthesized by introducing an asymmetric center in a discotic liquid crystal, which is a flat platelike molecule [34]. 

Two basic types of discotic mesophases have been widely recognized these are columnar and nematic. In general, discotic liquid crystals consist of flat and rigid aromatic cores surrounded by six or eight flexible chains. These flexible chains are coimected to the core by different linking imits, such as ether, ester, benzoate, alkyne, etc (Kumar 2010). 

In the discotic nematic (No) phase (Fig. 16.3a), molecules have orientationally ordered arrangement of discs with no long-range translational order. This is the least order (usually high temperature) in the disc-like molecules (Kumar 2009). The nematic mesophase can be assimilated to a lamellar nematic liquid crystal, in which the director vector (an optical axis) is perpendicular to the average direction along which the flat molecules are aligned, as illustrated in Fig. 16.3a. 

---