Ferroelectric host materials

Ferroelectric host materials must be carefully designed to provide a range of optimum physical properties and the material requirements for a good ferroelectric host mixture are as follows. 

The chiral dopant need not be mesogenic bnt should have a mesogenic-like stracture, preferably similar to the stracture of the host materials (to help maintain the properties of the host mixture). Consequently, some of the most interesting chiral materials in liquid ciystal mixtures are not liquid ciystalline or at least their hquid crystal phases are less important than their effect on the physical properties of the ferroelectric mixture. 

Ferroelectric host systems are mixtures composed of materials that are achiral, and when blended together they exhibit a nematic-smectic A-smectic C phase sequence for the purposes of ease of alignment with the smectic C phase being available over a temperature range that includes room tem-... 

The properties required of ferroelectric hosts therefore means that materials such as esters, Schiff s bases, and azo compounds are unsuitable because of their slow response times and poor stability. The best materials discovered so far, rely on removal of any functional or linking groups which might increase the viscosity or lower the stability. Figure 39 shows a variety of families of host materials [31,33,48-51 ]. These materials have a number of attributes in common. First, they are devoid of linking groups, thus the aromatic or heterocyclic rings in the core are directly linked. Second, only alkyl or alkoxy terminal chains are used in order to maintain as low a viscosity as possible. Third, some materials carry lateral fluoro substituents in order to increase... 

Figure 39. Achiral host materials for ferroelectric liquid mixtures.

The synthesis of the first achiral host material that exhibits a smectic Cau phase [36] (achiral version of the antiferroelectric phase) is shown in Fig. 42. It can be seen that this material has three aromatic rings and two ester groups, but in addition it has a swallow-tail unit. This unit (for no apparent reason at the moment) stabilizes the formation of smectic Cai, phases. The material like a ferroelectric host can be doped with a chiral dopant to give an antiferroelectric mixture. 

J.C. Jones and E.P. Raynes, Measurement of the biaxial permittivities for several smectic C host materials used in ferroelectric liquid crystal devices, Liq. Cryst, 11, 199-217 (1992). 

The paper is organized as follows. Sect. 2 describes the general principles and recent results for the synthesis of cyclotriveratrylenes Sect. 3 describes some applications of these compounds to host-guest chemistry. This latter section is devoted principally to the cryptophanes, and to host molecules containing one CTV unit that have recently been described. Sec. 4 presents some prospective work in the field of material sciences, i.e., ferroelectric liquid crystals and organic three-dimensional charge transfer salts. 

Main group oxides with three-dimensional stmctures or transition metal oxides with d° or d ° configurations are wideband gap materials and are colorless when pure. As such they may serve as transparent optical materials or hosts for such applications as lasers or luminescent materials when properly doped. Others that lack a center of symmetry may have ferroelectric or ferroic properties that make them useful for a variety of device applications. Some of these may have nonlinear optical properties so important to modem communication networks (see Sections 6.3 and 6.5 and see Luminescence and see Ferroelectricity). 

The properties of zeotype host-guest composites described above - i e, spatial organization, protection and stabilization of guest species - will become more important as molecules exhibit further properties that are essential for their use as materials. The well-developed synthetic methods for molecular compounds allow the preparation of designed molecular entities that possess predictable properties. However, no such thoroughly elaborated synthetic methods are available for the construction of organized arrays of functional molecules in their solid structures [36]. This is cumbersome since often the arrangement of the molecules in their solid compounds is detrimental to the effects (e g. non-linear optical, ferroelectric, electro-optical) that are to be exploited in materials. For example, many structures of molecules in the solid state are centrosymmetric. Also, molecular... 

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