Antiferroelectric Switching

Along with the prediction and discovery of a macroscopic dipole in the SmC phase and the invention of ferroelectric liquid crystals in the SSFLC system, the discovery of antiferroelectric liquid crystals stands as a key milestone in chiral smectic LC science. Antiferroelectric switching (see below) was first reported for unichiral 4-[(l-methylheptyloxy)carbonyl]phenyl-4/-octyloxy-4-biphenyl carboxylate [MHPOBC, (3)],16 with structure and phase sequence... 

Figure 8.8 Structure and phase sequence of (R)-MHPOBC is shown. One of most famous smectic LCs, antiferroelectric switching in SSFLC cells was first discovered with this material.

Figure 8.12 Longitudinal sheets with antiparallel polar symmetry are illustrated for achiral SmCA and SmC phases. Since it is not possible to switch to ferroelectric state in such system upon application of electric field, these structure should not be considered antiferroelectric.

Figure 8.13 Hypothetical smectic mesogen with hinge in center of core is illustrated. Such material could in principal switch to ferroelectric state, which we term the SmAPp, upon application of electric field in plane of layers. If this state exists in well on configurational hypersurface, then ground-state structure is antiferroelectric, denoted SmAPA.

Using this method, the M6R8/PM6R8 blend showed precisely the behavior expected for the achiral SmAPA structure. Specifically, the optical properties of the films were consistent with a biaxial smectic structure (i.e., two different refractive indices in the layer plane). The thickness of the films was quantized in units of one bilayer. Upon application of an electric field, it was seen that films with an even number of bilayers behaved in a nonpolar way, while films with an odd number of bilayers responded strongly to the field, showing that they must possess net spontaneous polarization. Note that the electric fields in this experiment are not strong enough to switch an antiferroelectric to a ferroelectric state. Reorientation of the polarization field (and director structure) of the polar film in the presence of a field can easily be seen, however. 

Figure 8.17 Structure and phase sequence of first banana-phase mesogen, reported by Vorlander in 1929, is given. Liquid crystal phase exhibited by this material (actually Vorlander s original sample) was shown by Pelzl et al.36a to have B6 stmeture, illustrated on right, in 2001. Achiral B6 phase does not switch in response to applied fields in way that can be said to be either ferroelectric or antiferroelectric.

Apparently this switching mode is disfavored since, in fact, the chirality of the layers does not change upon switching to the ferroelectric state rather the layer interface clinicity changes. This occurs when the molecules in alternate layers simply precess about the tilt cone in a manner exactly analogous to antiferroelectric to ferroelectric switching in the chiral SmC phase. As shown in Figure 8.25, the ferroelectric state obtained from the ShiCsPa antiferroelectric phase is a ShiCaPf structure, an achiral macroscopic racemate with anticlinic layer interfaces. 

The highly complex and unusual textures observed for B7 materials is complemented by unusual X-ray diffraction behavior. While the beautiful mystery of the B7 texture is not understood in detail, MHOBOW shows EO behavior, which allows some definitive statements regarding its nature. Thus, while some B7 materials are reported to be EO-inactive (no EO switching),54 and some are reported to exhibit antiferroelectric EO behavior,59 MHOBOW exhibits a unique texture change upon application of electric fields. 

P, which can be switched by an electric field, E, as illustrated in the P vs. E hysteresis loops in Figure 2) in favor of the non-ferroelectric cubic and antiferroelectric (AFE) phases. At a 65/35 ratio of PbZr03 to PbTi03, a concentration of 9.5% lanthanum is sufficient to reduce the rhombohedral-cubic phase transi-... 

Antiferroelectric LC s with tristable switching have been known for the last ten years120,121. The chemistry of ferroelectric and antiferroelectric liquid crystals follows the rules valid for calamitic LC s, but with some additional constraints ... 

SmB SmC SmC SmCA SmCPA SmCPp SmCo, SmIA SmX UCST XRD Smectic B phase Smectic C phase (synclinic tilted smectic C phase) Chiral (synclinic tilted) smectic C phase Chiral anticlinic tilted (antiferroelectric switching) SmC phase Antiferroelectric switching polar smectic C phase Ferroelectric switching polar smectic C phase Chiral smectic C alpha phase Chiral antiferroelectric switching smectic I phase Smectic phase with unknown structure Upper critical solution temperature X-ray diffraction... 

There are numerous properties which make fluorinated LC attractive for applications. Short Rp-segments lead to de Vries phases and to 90° tilted anti-ferroelectric SmCA phases, useful for orthoconic switching in new display applications. Fluorination could also lead to enhanced polarization in ferroelectric and antiferroelectric LC phases. 

As noted earlier, the incorporation of chiral groups in the liquid crystal moieties can have the effect of inducing non-linear properties, which include thermochromism, ferroelectricity, antiferroelectricity, electrostriction, and flexoelectricity. In a now classical study, Hult [82] demonstrated that it was possible for supermolecular material 34 to exhibit two-state ferroelectric switching. The remarkable material he investigated, shown in Fig. 30, was found to exhibit two hitherto unclassified mesophases between the smectic... 

A possible model for the structure of the columnar phases is sketched below. Since two optical isomers (A and A) are present in a 50 50 ratio, they must be randomly distributed in a column. Also, it is possible to consider a stacking of the complexes in such a way that a net polarisation is induced in the column direction (Fig. 6a stacking of a same enantiomer). In columnar phases, the symmetry is such that the net dipole of every column cannot be cancelled out by the one of the neighbouring column, that is a perfectly antiferroelectric arrangement of the columns is impossible the system is frustrated (Fig. 6b). Thus, a net polarisation must result in the columnar phase whose direction it should be possible to switch. 

The main features of the antiferroelectric switching in FLCPs are a third state, which shows an apparent tilt angle of zero, a less marked threshold between the three states when compared to the low molecular weight antiferroelectric liquid crystals, a hardly observed hysteresis, and an anomalous behavior of the spontaneous polarization with temperature (Fig. 24), which is not encoun-... 

Negishi M, Kanie K, Ikeda T, Hiyama T. 1996b. Synthesis and photochemical switching of the antiferroelectric liquid crystals containing a diazenediyl group. Chem Lett 25 583 584. 

Shirota K, Yamaguchi I. 1997. Optical switching of antiferroelectric liquid crystal with azo dye using photochemically induced SmC A SmC phase transition. Jpn J Appl Phys 36 L1035 L1037. 

R. Amaranatha Reddy, M.W. Schroder, M. Bodyagin, H. Kresse, S. Diele, G. Pelzl and W. Weissflog, Field-induced switching of chirality in undulated ferroelectric and antiferroelectric SmCP phases formed by bent-core meso-gens, Angew. Chem. 117(5), 784-788, (2005). doi 10.1002/ange.200461490... 

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