Mesogens calamitic/discotic

Fig. 3.1 Building blocks of thermotropic and lyotropic liquid crystalline phases. The upper part of the figure shows two examples of typical thermotropic mesogens. Calamitic mesogens, such as terephthal-bis-(p-butylaniline) (TBBA) [2], can be represented by prolate ellipsoids or rigid rods, while discotic mesogens, such as benzene-hexa-n-octanoate (BH8) [4], are usually described by oblate ellipsoids or discs. The lower part of the figure shows the typical surfactant molecule sodium dodecyl sulfate (SDS), which forms lyotropic phases with water [5], Such a surfactant molecule is basically composed of a polar head group and a flexible hydrophobic tail. These amphiphilic molecules aggregate into different types of micelles, which are the actual mesogens of lyotropic liquid crystals. The shape of the micelles depends mainly on the solvent concentration...

In the case of copolymer LCs there is speculation that the alternating primary structure. .. core-spacer-core-spacer... restricts inter-core translations and thereby stabilizes smectic phases. While we remain cognizant of the potentially important role that primary and secondary molecular structure of mesogens can play, in the remainder of this chapter, however, we will focus on aspects of mesomorphism that, for the most part, can he described in terms of the idealized prolate (oblate) shape of calamitic (discotic) mesogens (Fig. 5.1). We consider the nature and implications of the local molecular order in the nematic state after briefly reviewing the molecular crystal and the isotropic liquid. 

Thennotropic liquid crystal phases are fonned by anisotropic molecules witli long-range orientational order and in many types of stmcture witli some degree of translational order. The main types of mesogen are Arose tlrat are rodlike or calamitic and Arose Arat are disclike or discotic. 

Liquid crystals may be divided into two broad categories, thermotropic and lyotropic, according to the principal means of breaking down the complete order of the soHd state. Thermotropic Hquid crystals result from the melting of mesogenic soHds due to an increase in temperature. Both pure substances and mixtures form thermotropic Hquid crystals. In order for a mixture to be a thermotropic Hquid crystal, the different components must be completely miscible. Table 1 contains a few examples of the many Hquid crystal forming compounds (2). Much more is known about calamitic (rod-Hke) Hquid crystals then discotic (disk-like) Hquid crystals, since the latter were discovered only recendy. Therefore, most of this section deals exclusively with calamities, with brief coverage of discotics at the end. 

Most polymeric Hquid 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. 

It is not possible to predict from the related crystal structure alone whether the compound will melt to a liquid crystalline phase or not, because the anisotropic molecules (calamitic and discotic ones) form in favourable anisotropic packing. As a rule long shaped rod-like molecules quite often possess a layered arrangement in the solid state regardless of whether the compound is mesogenic or not. 

Coco, S Espinet, P Marb n-Alvarez, J.M. and Levelut, A.M. (1997) Effects of Isonitrile Substituents on Mesogenic Properties of Halogoldiisonitrile Complexes Calamitic and Discotic Liquid Crystals. Journal of Materials Chemistry, 7, 19-23. 

There are now three major shape classifications of low molar mass liquid crystals - rod-like (calamitic), disc-like (discotic) and bent-core. The last of these is the most recent, and while examples of bent mesogens have been known for some years, it is only since the mid-1990s that the area has attracted widespread attention [2],... 

Mesogen eomposed of moleeules incorporating one or more metal atoms. Note yistdiWomesogens may be either calamitic or discotic mesogens. 

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. 

Thermotropic liquid crystals come in two types calamitic and discotic. Calamitic phases (from the Greek for tube ) are all those that are caused by rod-like mesogens. The more recently characterised discotic phases are caused by disc-like species. Calamitic phases may be either nematic (from the Greek for thread ), smectic (from the Greek for soap ) or cholesteric (named after the cholesterol derivatives such as 13.4, which exhibit this behaviour). 

To some extent, the design criteria for discotic mesogens are somewhat simpler than those for their calamitic counterparts. In many cases, it is possible to choose a favorite disk-like molecule, then add 6-8 peripheral alkyl chains to generate a mesomorphic (i.e., hquid-crystalline) material. Thus, phthalocyanines, triphenylenes, truxenes, and many other systems will generate mesogenic (i.e., liquid-crystal-like) materials with appropriate substitution these examples are shown in Fig. 24. Note that it is not necessary for the mesogen to have a planar core. 

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

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. 

This formation of nematic, lamellar and columnar phases within a single class of molecule leads to the idea that they can be considered as a missing link between calamitic and discotic mesogens. 

Fig. la-e. Sketches of the possible molecular architectures for liquid crystal dimers a symmetric calamitic dimer b non-symmetric calamitic dimer c symmetric discotic dimer d non-symmetric discotic-calamitic dimer e laterally linked symmetric dimer. Ellipses represent rod-like mesogenic groups, circles disc-like mesogenic units and wavy lines denote the flexible spacers... 

Double and triple hydrogen bonds can also be used to design the supramolecular mesogens. Complexes 33 [49] and 34 [21] have shapes between rodlike and dis-cotic mesogens. Complex 33 exhibits monotropic calamitic or discotic phases. Triple hydrogen bonded complex 34 is a discotic mesogen with a columnar phase from 200 °C to room temperature on cooling. 

Figure 2 The rod- (a) and disk-like (b) motifs of calamitic and discotic mesogens, respectively.

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