Smectics and Discotics

Fig. 10. Schematic representation of nematic, smectic and discotic mesophases...

Table 2. Examples of Heats and Entropies of Transition of Nematic, Smectic, and Discotic Mesophases...

Liquid Crystallinity. The liquid-crystalline state is characterized by ori-entationally ordered molecules. The molecules are characteristically rod- or lathe-shaped and can exist in three principal structural arrangements nematic, cholesteric, smectic, and discotic (5,6). 

A hquid phase characterized by some degree of long range molecular order. (See cholesteric, nemactic, smectic and discotic liquid crystals.) Liquid crystals are usuahy anisotropic but, in principle, may be isotropic. 

As described in the previous section, both electronic and ionic conductions take place in mesophases when the material contains trace amounts of chemical impurities. These types of conduction exhibit different charge carrier transport properties, as demonstrated by the transient photocurrents the different mesophases measured under the same conditions as shown in Fig. 2.10. In these transient photocurrents, the fast transit times are shifted to shorter times as the molecular order in the mesophases is increased from SmA to SmE and from Coin to the plastic phase, while the slow transits stay in the same time range of 1,000 (xs irrespective of the mesophase in both smectics and discotics. Figiue 2.11 shows Arrhenius plots of the mobilities for fast and slow transits of the 2-phenylnaphthalene derivative 8PNP-012. The mobility for the fast transit hardly depends on the temperature, while the mobility for the slow transit does depend on temperature, with an activation energy... 

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]. 

FIG. 2. Schematic representation of different calamitic and discotic thermotropic liquid crystals (a) nematic, (b) cholesteric, (c-e) smectic, (f) columnar hexagonal, (g) columnar hexagonal tilted a-e adapted from Demus, D., and Richter, L., Textures of Liquid Crystals, Verlag Chemie, Weinheim, Germany, 1978 f,g adapted from Eidenschenk, R., Flussige Kristalle, Chem. Unserer Zeit, 18, 168-176 (1978). 

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). 

Fig. 2 Schematic representation of different calamitic and discotic thermotropic liquid crystals. (A) nematic (B) cholesteric (C-E) smectic (F) columnar hexagonal (G) columnar hexagonal tilted. (A-E Adapted from Ref F and G Adapted from Ref. l)...

Perhaps one of the most important applications of chiral induction is in the area of liquid crystals. Upon addition of a wide range of appropriate chiral compounds, the achiral nematic, smectic C, and discotic phases are converted into the chiral cholesteric (or twisted nematic), the ferroelectric smectic C and the chiral discotic phases. As a first example, we take the induction of chirality in the columns of aromatic chromophores present in some liquid-crystalline polymers. " The polymers, achiral polyesters incorporating triphenylene moieties, display discotic mesophases, which upon doping with chiral electron acceptors based on tetranitro-9-fluorene, form chiral discotic phases in which the chirality is determined by the dopant. These conclusions were reached on the basis of CD spectra in which strong Cotton effects were observed. Interestingly, the chiral dopants were unable to dramatically influence the chiral winding of triphenylene polymers that already incorporated ste-reogenic centers. 

It is interesting to study how the force profile changes by varying the temperature and approaching other phases. In the system considered in [54, 55], the micelles have a disk-like shape and the sample exhibits a second order smectic-A discotic nematic bulk transition. By lowering the temperature, the oscillations become less damped and the attractive background weakens, finally leading to the smectic force profile described in Sect. 3.3.2. 

The general, 8-diketonate template has an overall lath-like structure, and subtle structural modifications can lead to crossover between smectic and columnar mesomorphism, or to structural change between discotic to calamitic shape, simply by changing the nature of the substituent and R (Figure 33). This is an important aspect of this type of complex, which would not be appreciated fully and understood if discussed separately. For instance, symmetrical systems (R = R, D2h) can have either disk-like or X-shaped structures, whereas the unsymmetrical compounds (R R ) can possess either zigzag- or cross-shaped structures (Figure 33). For the latter, both cis (C2v) and trans (C2h) isomers are also possible, and it is reasonable to think that mixtures are always present and that transicis interconversion occurs at elevated temperatures. This study is clearly a textbook case in addition to being a perfect illustration of induction, modification, or suppression of mesomorphism on complexation. 

The term mesophase also includes ordered liquids (nematic, smectic, cholesteric and discotics), which present long-range orientational order like in a solid, but positional disorder like in a liquid [2]. In these materials, large-scale molecular motion is possible, which is a characteristic of the liquid state rather than of the solid state. The term liquid crystals is conventionally used to address them. This sub-class of mesophases will not be treated in this context. 

Figure 1 General structures of calamitic and discotic thermotropic liquid crystals. (A) Layered calamitic smectic liquid crystal. The structures of the various types depend on the local packing of the molecules, the extent of the packing, and the orientation of the long axes with respect to the layers. (B) Calamitic nematic liquid crystal. The molecules have no long-range order, and are only orientationally ordered. (C) Ordered columnar discotic liquid crystal. Disk-like molecules form ordered or disordered columns different column packings give rise to various mesophase structures. (D) Nematic-discotic liquid crystal phase. The disk-like molecules are only orientationally ordered.

Schlieren textures are very common in liquid crystals and are usually easily identified and classified. These patterns are observed in homogeneously oriented nematic phases and for smectic phases where the long axes of the molecules are tilted with respect to the layer planes and the layers are arranged parallel to the surface of the glass. Thus, schlieren textures occur for the nematic phases of both calamitic and discotic materials, and for the tilted smectic phases C, I, and F. 

Cr, crystalline state N, Sm, Ch, D, and B, nematic, smectic, cholesteric, discotic, and banana LC phases, respectively I, isotropic melt. 

The three different shapes (rodlike, disclike, boardlike) and two different mesophases (nematic and smectic) together form six different kinds of crystals. They are rodlike nematic, discotic nematic, boardlike nematic, rodlike smectic, discotic smectic, and boardlike smectic. Figure 6.3 shows these six combinations. 

FriedeP was the first who distinguished three main classes of liquid crystals, according to the different kind of orders in the mesophases nematic, smectic, and cholesteric. From the point of view of the geometrical shape of molecules, we divide the thermotropic LCs into calamitic phases (when the molecules are rodlike), sanidic phases (when the molecules are bricklike), and the discotic phases (when the molecules are disklike)... 

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