Structures nonchiral smectics

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. 

Figure 22. Director configuration for (a) a nonchiral smectic C and (b) a chiral smectic C in its natural (helicoidal) structure (after [97]). The first could also be imagined as the bulk structure for a chiral smectic C with infinite pitch, which would then correspond to a macroscopic polarization P pointing towards (or from) the reader. However, such a structure is not stable.

The method described by Jones et al. [161] (long pitch method) was developed for materials with essentially infinite pitch. It may be used for long pitch materials in cells where the helical structure is unwound by the surface interactions and a uniform director profile is established. The weakness of the method is that it requires the director profile to be known. On the other hand, it works for the nonchiral smectic C. It also works for materials with 0=45° (typically materials with N SmC transitions, lacking the SmA phase), which is a singular case where the short pitch method fails. 

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). 

Essentially, the structural features described above apply to both non-chiral and chiral compounds. However, the presence of chiral molecules in smectic- and C phases results in additional properties and structures not present in phases of nonchiral substances. These are the ferroelectric properties and the electroclinic effect, which will be discussed in detail in Sections 8.3 and 8.4, and the helical structure in the smectic-C phase. 

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 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]. 

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