Crystal nonchiral smectics

A further element of formalism in this argument is the arbitrary insistence on right-handed coordinates. Unless the molecules are enantiomorphic (i.e., chiral) or enantio-morphically arranged, this condition may also be relinquished. This is the case provided the phase is composed of nonchiral molecules. Thus we can cancel further terms contributing to the elastic moduli in both nematic and nonchiral smectic liquid crystals. 

The synthesis of nonchiral smectic liquid crystals is a broad topic for discussion, however, it can be divided into subsections in two different ways. For example, smectic systems can be split into metallomesogens and nonmetallomesogens, alternatively, they can be divided into materials for (1) meso-phase structure elucidation and classification [ 1 ], (2) property-structure correlations [2] and (3) host systems for ferroelectric and antiferroelectric mixtures. In the following sections template structures used for the synthesis of smectic materials will be described, followed by discussions of the syntheses of materials that have extensive histories in the elucidation of smectic phase structures, and finally of the syntheses of smectogens that are useful in applications. 

When we apply an electric field across an FLC cell there will always be a dielectric torque acting on the director, in addition to the ferroelectric torque. This is of course the same torque as is present in all liquid crystals and in particular in nematics, but its effect will be slightly different here than it is in a nematic. This is because in the smectic C phase the tilt angle 0 is a hard variable, as earlier pointed out. It is rigid in the sense that it is hardly affected at all by an electric field. This is certainly true for a nonchiral smectic C and also for the chiral C, except in the immediate vicinity of Tc a. where we may not neglect the electroclinic effect. Hence we will consider the tilt angle 6 uninfluenced by the electric field, and this then only controls the phase variable <(> of Fig. 60. 

Nishiyama, I., Goodby, J. A nonchiral swallow-tailed liquid crystal exhibiting a smectic C structure that has an antiferroelectric structure. J. Mater. Chem. 2, 1015-1023 (1992)... 

The symmetry approach to ferroelectricity in liquid crystals can be realized not only for individual substances but also for multicomponent systems. For low-molar-mass ferroelectric liquid crystals, most applications use LC mixtures with two main components a nonchiral matrix providing the tilted smectic structure and a chiral dopant [7]. As for the preparation of FLCPs, mixing of a smectic C polymer with a chiral dopant also results in a ferroelectric chiral smectic system [74]. Japanese authors [75,76] have carried out systematic studies on mixing tilted smectic polymers with low-molar-mass ferroelectric liquid crystals. 

It is important to note that also nonchiral molecules are capable of forming chiral mesophases. In particular, molecules with a bent core ( bananashaped molecules) can build polar, and even chiral liquid crystal structures [75]-[78]. Bent-core molecules form a variety of new phases (B1-B7, Table 1.3) which differ from the usual smectic and columnar phases (see also Chapter 8). As a consequence of the polar arrangement, antiferroelectric-like switching was observed in the B2 phase formed by bent-core molecules, and second harmonic generation was found in both the B2 phase and the B4 phase. The latter phase is probably a solid crystal. It consists of two domains showing selective reflection with opposite handedness. In the liquid crystalline B2 phase, the effective nonlinear susceptibility can be modulated by an external dc field [79] (Figure 1.15). 

If a smectic-C phase is formed by chiral molecules—regardless of whether a chiral compound exhibits a smectic-C phase by itself or a smectic-C phase of nonchiral molecules is doped with a chiral additive—a helical structure appears which is in some aspects similar to the helical structure in a cholesteric liquid crystal. The helical structure of the chiral smectic-C phase had been recognized in the early 1970s [13], [14], [15], well before the ferroelectric properties of this phase were realized. 

The ferroelectric properties of the chiral smectic-C phase and the electroclinic effect of the smectic- phase appeared as a result of the symmetry breaking caused by the presence of chiral molecules. One can think of smectic phases in which nonchiral molecules arrange themselves in a polar order [86], and it seems that such phases were recently observed, indeed experimentally. The molecules which establish these phases are not chiral but possess a bent core resembling a bow- or banana-like shape [87] a second class of nonchiral liquid crystals showing polar ordering consists of certain polymer-monomer mixtures [88],... 

The antiferroelectric SmC structure (see Fig. 17) can also occur in racemates [94] or in nonchiral compounds such as symmetric dimers [136, 137], nonsymmetric dimers [133], and main chain liquid crystal polymers [138], where its formation is driven by steric and/or conformational effects. Antiferroelectric ordering has been shown to increase the smectic order parameters in ferroelectric liquid crystals [94, 95]. 

In the case of low-molar-mass LCX the phenomenon of induced spontaneous polarization is well known and established in coaunerdal FLC mixtures. By doping smectic C materials with chiral dopants, spontaneous polarization can be induced. Advantages over pure Sr liquid crystals are tunable ferroelectric properties and high switching speed achievable by using a nonchiral. low-viscous Sc basic material and doping with small amounts of Ugh-polarization Sc substances. 

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