Chiral chains, antiferroelectrics

Chiral chains play a critical role in determining the stability of the antiferro-electricity. The variety of chains is rather limited. The most common chiral chain is - CH(CF3)C H2 +i, which stabilizes the antiferroelectricity much more than does - CH(CH3)C H2 +i, as illustrated in Figure 28 the odd-even effect is clearly observed in - CH(CH3)C H2 +i, while only the SmC phase occurs in - CH(CF3) C H2 +1. Trifluoromethylalk-oxyalkyl chains, - CH(CF3)Q H2mOC H2 +i, give a low threshold field and fast response... 

Zareba et al. [165] described the crystal structure of the chiral 4-(l-methyl-heptyloxycarbonyl)-phenyl 4-heptyloxytolane-4 -carboxylate (C7-tolane) which shows monotropic antiferroelectric and ferroelectric phases. The single-crystal X-ray analysis of this compound shows that the crystal has a smectic-like layer structure composed of largely bent molecules where the chain of the chiral group is almost perpendicular (86°) to the core moiety. Within the layers, the molecules are tilted. The central tolane group of the molecule is roughly planar. 

Fig. 30 Selected examples of chiral rod-like mesogens with one fluorinated chain (77° C) one enantiomer is shown as example (SmCA = antiferroelectric SmC phase SmC = ferroelectric SmC phase SmCpi = ferrielectric SmC phase SmCa = helical SmC phase SmI = chiral tilted low temperature phase) [197-199]...

The photochemical control of phase change of antiferroelectric LCs wi achieved either by using azobenzenes possessing chiral side chains as d< (A-6) in an antiferroelectric LC or by using azobenzene-containing chiral roelectric LCs (A-7, A-8) [63,64]. 

Boemelburg, J., Heppke, G., and Hollidt, J., Evidence for an antiferroelectric smectic phase in a chiral side-chain polymer, Makromol. Chem. Rapid Commun., 12, 483-488 (1991). 

Polymer 21 is a typical ferroelectric liquid crystal poly(acrylate) with a rather simple chiral terminal chain commonly seen in chiral liquid crystals. A range of chiral liquid crystal phases are exhibited by polymer 21, complicated by the reported presence of two phases. The recent interest in the antiferroelectric phase (see above) has been... 

New materials are discovered sometimes intentionally and sometimes accidentally. The former case can be exemplified by ferroelectric liquid crystals (FLCs), wherein the chirality was introduced into the lateral chain to reduce the symmetry, leading to a noncentrosymmetric system [1]. In the latter case, it can be referred to as antiferroelectric liquid crystals (AFLCs) which were discovered accidentally. Actually, compounds which exhibit the antiferroelectric phase had been synthesized several years before they were proven as AFLCs. The hitherto known AFLCs include three materials, as shown in Figure 9.1 (1) MHPOBC, (2) MHTAC, and (3) R) and (6 )-l-methylpentyl 4 -(4"-n-decyloxybenzoyloxy)bipheny-l-4-carboxylates. 

Monofluoro- and difluoromethylalkyl chains do not stabilize antiferroelectricity. Racemates do not exhibit antiferroelectricity, but they do frequently have the herringbone structure. Moreover, Nishiyama and coworkers [53, 54] have shown that chirality is not essential for the appearance of the herringbone structure the swallow-tailed compounds (e.g. 9),... 

Chirality can also be introduced when one or more chiral carbons are incorporated in the molecules, for example in the hydrocarbon terminal chains [67, 68], within the bent-core [69], or by addition of chiral dopants [6, 70], It was noted during the early research that the handedness of the homochiral structures is very sensitive to chiral dopants [6], or even on chiral surfaces [71]. On the other hand, it was observed that banana-smectics made of enantiomeric chiral molecules form synclinic - antiferroelectric [44] and anticlinic ferroelectric [67] domains. This combination of tilt and polar order implies that the phase is racemic, with a rigid alternation of right- and left-handed chiral layers. This shows that the molecular chirality has no or minor effect on deciding about anticlinic or synclinic packing (which is mainly determined by entropic reasons), but it can bias the otherwise degenerate tilt directions. 

Experiments by Binet et al. [72], carried out on chiral bent core materials containing biphenyl (BP) cores with S or R hydrocarbon chains, and on achiral biphenyl core molecules with chiral dopants, reveal the effect of the molecular chirality on the polarization stmcture. In addition the polarization Pb due to the closed packing of the bent-shape molecules, another polarization, P is introduced due to the chiral and tilted molecular structure. It was found [72] that in the antiferroelectric racemic domains at low fields, of the synclinic - racemic... 

Quite recently, Prof. Fukuda has determined that the chiral alkyl chain makes a bent angle >54.7° (the magic angle) with the core axis and that the carbonyl group near the chiral center lies on the tilt plane in antiferroelectric SmCA on the basis of the polarized infrared spectroscopy. Jin, B. Ling, Z. Takanishi, Y. Ishikawa, K. Takezoe,... 

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