Discotic liquid crystals molecular structure

Fig. 12. Molecular structure and phases of a typical discotic liquid crystal.

Chandrasekhar S 1998 Columnar, discotic, nematic and lamellar liquid crystals Their structures and physical properties Handbook of Liquid Crystals Vol 2B. Low Molecular Weight Liquid Crystals I ed D Demus, J Goodby, G W Gray, H-W Spiess and V Vill (New York Wiley-VCH)... 

Fig. la-d. Molecular structures of rod-like (calamitic) and disc-like (discotic) liquid crystals a) Low molecular weight liquid crystals, b) liquid crystal main chain polymers, c) liquid crystal side chain polymers, d) combined main-chain/side-chain polymers... 

The two volumes on Low Molecular Weight Liquid Crystals are divided into parts dealing with calamitic liquid crystals (containing chapters about phase structures, nematics, cholesterics, and smectics), discotic liquid crystals, and non-conventional liquid crystals. 

Fig. 7.8 Magnetic alignment of discotic liquid crystals, a Molecular structure of a discotic compound and b an arrangement of columnar stacks perpendicular to the magnetic field on a substrate. Reproduced with permission from [142]. Copyright 2006 Elsevier...

There are several different phases in thermotropic liquid crystals. The structural nature of mesophases is influenced by the molecular shape and therefore depends on whether the liquid crystal is formed by rod-like or disc-like molecules. Thermotropics of rod-like molecules may be divided into two main categories nematic and smectic phases. There exist many types of smectic phases, labeled as 5, 5b, S /. When an ordered solid of a liquid crystal melts (see Fig. 1.1), it may melt into a nematic phase or a smectic A phase. Upon further heating, it eventually turns into an isotropic liquid. First, classical thermotropic liquid crystals are described, and then a group of more exotic liquid crystals like discotic thermotropics, lyotropics, and liquid crystalline polymers. 

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

We start by reminding ourselves that columnar discotic liquid crystals are comprised of disordered stacks (1-dimensional fluids) of disc-shaped molecules arranged on a two-dimensional lattice (Fig. 1) [1]. This structure imparts novel properties to these materials from which applications are likely to stem. One such property is the transport of charge along the individual molecular stacks [2-7]. The separation between the aromatic cores in, for example, the hexa-alkoxytriphenylenes (HATn), the archetypal columnar discotic mesogen, is of the order of 0.35 nm, so that considerable overlap of n orbitals of adjacent aromatic rings is... 

Another class of liquid crystal materials that has been studied extensively is the discotic liquid crystal. As the name implies, these phases form from disk-shaped molecules. A material showing this behavior was first identified and studied in 1977 by Chandasekar. Examples of some typical discotic molecular structures are shown in Figure 2.19. There are two main classes of discotic phase, the nematic and columnar discotic phases schematics of these phases can be seen in Figure 2.20. The nematic discotic... 

FIGURE 2.19 Molecular structures of some early discotic liquid crystal... 

Substances that show a liquid crystalline phase, or mesophase, are called mesogens. Several thousands of compounds, both with low molecular mass and polymeric, are now known to form mesophases. They are mainly highly geometrically anisotropic in shape, rodlike or disclike (hence the terms calamitic and discotic liquid crystals), or they are anisotropic in solubility properties, like amphiphilic molecules and, depending on their detailed molecular structure, they can exhibit one or more mesophases between the crystalline solid and the isotropic liquid. Transitions to these intermediate states may be induced by purely thermal processes (thermotropic liquid crystals) or by the action of solvents (lyotropic liquid crystals). Each of these two categories can be further divided according to the structure of the mesophases and/or molecules Scheme 1 shows the classification of thermotropic mesophases. 

As discussed in Chaps. 3 and 4, (columnar) discotic liquid crystals are oriented in columns separated by molten aliphatic chains and, consequently, they can conduct charge efficiently along the channels in one dimension. The organization of the different phases is described elsewhere [19, 20] and the efficiency of charge transport can be directly related to the short intermolecular spacing and order of different types of mesophase, with few exceptions [21]. For example, hole mobility is higher in ordered, rather than disordered, columnar phases and even higher in helically-ordered phases where molecular rotation is suppressed about the columnar axis [22], Some mesomorphic derivatives of hexabenzocoronene, for example hexaphenyl-substituted hexabenzocoronene (HBCn, see Table 8.2 for chemical structures of all discotic materials discussed here) have hole mobilities... 

Liquid crystals (LCs) are organic liquids with long-range ordered structures. They have anisotropic optical and physical behaviors and are similar to crystal in electric field. They can be characterized by the long-range order of their molecular orientation. According to the shape and molecular direction, LCs can be sorted as four types nematic LC, smectic LC, cholesteric LC, and discotic LC, and their ideal models are shown in Fig. 23 [52,55]. 

Liquid-crystalline phases are characterized to some degree by the shape of the molecules and by their packing arrangements and ordering in the mesomorphic state. Typically, molecules can have cither disc- or rod-like shapes and can form discotic or calamitic mesophases, respectively. Ferrocene liquid crystal systems that have so far been synthesized tend to have molecular structures that are lath- or rod-like in shape, and consequently the phases observed are calamitic. However, this does not preclude the possibility that a polysubstituted ferrocene could be prepared where the molecular shape is disc-like, thereby holding out the prospect of possibly producing discotic/columnar phases. 

For example, the 815 cm-l peak shifted to 825 cm i and the doublet at 1730/1750 cm-l, indicative of crystal-like structures has disappeared at the highest shear rate. The spectral changes near 815 cm-1 of Fig. 15a could conceivably correspond to those of Fig. 13 which contained the spectra at different temperatures around the discotic/ isotropic liquid transition. On the other hand, no disappearance of the carbonyl structures occurred with simple phase transition. TTie disappearance of the carbonyl bands at 1730/1750 cm at the highest shear rate must therefore corresponds to a molecular realignment in which the dipole moment changes causing the infixed bands have been reoriented along the direction of observation. 

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