Rod-like mesogenic molecule

Figure 8.1 Typical shapes (a) rod-like mesogenic molecule ...

Achiral smectic materials with anticUnic molecular packing are very rare [40] and their antiferroelectric properties have unequivocally been demonstrated only in 1996 [41]. The antiferroelectilc properties have been observed in mixtures of two achiral components, although no one of the two manifested this behaviour. In different mixtures of a rod like mesogenic compound (monomer) with the polymer comprised by chemically same rod-like mesogenic molecules a characteristic antiferroelectric hysteresis of the pyroelectric coefficient proportional to the spontaneous polarization value has been observed for an example see Fig. 13.27a. Upon application of a low voltage the response is linear, at a higher field a field-induced AF-F transition occurs. 

Rowan, Mather et al. [63—65] have examined the use of complementary nu-cleobase interactions to allow the formation of supramolecular LC materials of the main type shown in Fig. 3a and b. In these cases, a rod-like mesogen-bis(phenylethynyl)benzene-was functionalized symmetrically (homoditopi-cally) with thymine (T) or N6-anisoyl protected adenine (AAn) using alkoxy spacers of varying length to yield molecules of the type Bp-1-Bp, where Bp depicts the variable nucleobase moiety, AAn or T (Fig. 5). 

Many molecules with rod-like mesogens have been analyzed in the liquid crystalline phase, but only few of the low-temperature pramrsor crystals have been studied. Among them one might find many conformationally disordered crystals A survey... 

In another approach to introduce liquid crystallinity, aliphatic tails were connected to the convex side of a glycoluril-based host (23. Fig. 6a).It was reasoned that in the solid state, this host would form dimers, in a similar way as the parent compound 3b. As can be seen in Fig. 6c. such a dimer resembles the structure of a typical rod-like mesogen. Although the material formed by 23 exhibited the typical features of a liquid crystal, i.e., birefringent optical textures and a high degree of malleability, calorimetric and powder diffraction measurements showed that it was a plastic crystal composed of lamellae of dimers (Fig. 6c). Strong Ti-n interactions between the aromatic surfaces of adjacent molecules are the reason the material is not liquid crystalline. The... 

When azobenzene groups possess flexible alkyl substituents in their phenyl rings, they often self-assemble into ordered LC phases, as shown in Figure 7.4. In this case, azobenzene moieties may play both the role of photoresponsive molecule and rod-Uke mesogen. Upon irradiation with unpolarized UV light, trans-to-cis photoisomerization occurs, which converts the rod-like mesogen into a more globular molecule... 

The results above show that the Frank moduli are determined mainly by the structure of mesogenic units which are similar for conventional nematics and thermotropic polymers (the situation changes considerably for the lyotropic solutions of long rod-like polymeric molecules, see the next section). On the other hand, the dynamics of reorientation are strongly influenced by the backbone. Field response and relaxation times depend dramatically on the molecular mass of a polymer though, in the first approximation, obey the same equations (4.30, 4.31). Figure 4.42 shows field-response times as a function of temperature for a comb-like acryl polymer H... 

The first group includes rod-like mesogens substituted by long-chain groups at any position, e.g. laterally alkyl substituted and swallow-tailed mesogens. In such compounds the liquid crystalline behaviour is largely determined by the repulsion forces of the surrounding molecules, which press... 

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