Molecules with Disk-Like Mesogens

Benzene-hexa-n-hexanoate (n = 5 in Fig. 5.5) has no discotic liquid crystal phase, but three condis states (phases III, II, I) The thermodynamic transition parameters are listed in Table 5.5. According to Eq. (1), one expects for fusion of the rigid crystal (phase IV) besides the ASp and 24 conformational contributions 

The heptanoate and octanoate show liquid crystalline discotic phases over the narrow temperature ranges, 354 to Wd and 355 to 357 K, respectiwly The main 

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A final example of induced mesogenicity in a multicomponent system is the well studied, but less well understood, carbonaceous mesophases which are comprised of a myriad of unidentified molecules which are created in situ as petroleum pitches are heated to temperatures where chemical transformations occur [163]. The processes leading to a mesophase involve decreases in both the elemental weight fraction of hydrogen and the group fraction of aliphatic carbon atoms [164]. Model studies have demonstrated that the component molecules of these phases are fused, polycyclic aromatic molecules with disk-like shapes the exact structures of the components depend upon the natures of the precursor molecules which are heated [164-167]. All of the carbonaceous mesophases somewhat resemble discotic nematic phases [168]. At least some of them probably represent another example of liquid crystallinity induced by mixing molecular components which, when separated, are not mesogenic. 

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. 

Similarly to the molecular engineering of calamitic molecules to produce ferroelectric smectic C phases [129], disk-like molecules with chiral peripheral chains tilted with respect to the columnar axis were predicted to lead to ferroelectric columnar mesophases [130]. Indeed, as it is the case with all flat disk-shaped mesogenic molecules, the tilt is mainly associated with the flat rigid aromatic cores of the molecules, the side-chains being in a disordered state around the columnar core. Thus, the nearest part of the chains from the cores makes an angle with the plane of the tilted aromatic part of the molecules. If the chiral centre and the dipole moment are located close to the core, then each column possesses a non-zero time averaged dipole moment, and therefore a spontaneous polarization. For reasons of symmetry, this polarization must be, on average, perpendicular to both the columnar axis and to the tilt direction in other words, the polarization is parallel to the axis about which the disk-shaped molecules rotate when they tilt as shown in Fig. 29. 

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