Antiferroelectric liquid

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... 

Extensive research has already been carried out on incorporation of fluorine into molecules which can lead to profound and unexpected results on biological activities and/or physical properties [ 1 - 5]. In particular, optically active fluorine-containing molecules have been recognized as a relatively important class of materials because of their interesting characteristics and potential applicability to optical devices such as ferroelectric or antiferroelectric liquid crystals [6-11]. Recent investigations in this field have opened up the possibility for the... 

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 ... 

FIGURE 42. Chemical structure of an antiferroelectric liquid crystal (R1 alkoxy, R2 = 4-methylheptyloxycarboxyl)... 

Lagerwall ST (1999) Ferroelectric and antiferroelectric liquid crystals. Wiley-VCH, Weinheim... 

Takezoe H, Gorecka E, Cepic M (2010) Antiferroelectric liquid crystals interplay of simplicity and complexity. Rev Mod Phys 82 897-937... 

Fig. 3 Change in transmittance of an antiferroelectric liquid crystal containing an azobenzene (a) in the dark ...

Fig. 4 Structure of azobenzene dopant and azobenzene antiferroelectric liquid crystal.

In the following sections, we shah demonstrate that the observed behavior of electro-optic activity with chromophore number density can be quantitatively explained in terms of intermolecular electrostatic interactions treated within a self-consistent framework. We shall consider such interactions at various levels to provide detailed insight into the role of both electronic and nuclear (molecular shape) interactions. Treatments at several levels of mathematical sophistication will be discussed and both analytical and numerical results will be presented. The theoretical approaches presented here also provide a bridge to the fast-developing area of ferro- and antiferroelectric liquid crystals [219-222]. Let us start with the simplest description of our system possible, namely, that of the Ising model [223,224]. This model is a simple two-state representation of the to-... 

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-... 

Moriyama T, Kajita J, Takanashi Y, Ishikawa K, Takezoe H, Fukuda A. 1993. Optically addressed spatial light modulator using an antiferroelectric liquid crystal doped with azobenzene. Jpn J Appl Phys 32 L589 L592. 

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. 

S.T. LagerwaU, Ferroelectric and Antiferroelectric Liquid Crystals, Wiley-VCH, Wernheim, 1999. 

The chapter is organized as follows The second section discusses the prototype polar smectics the ferroelectric liquid crystals. We discuss the structure of the ferroelectric phase, the theoretical explanation for it and we introduce the flexoelectric effect in chiral polar smectics. Next we introduce a new set of chiral polar smectics, the antiferroelectric liquid crystals, and we describe the structures of different phases found in these systems. We present the discrete theoretical modelling approach, which experimentally consistently describes the phases and their properties. Then we introduce the discrete form of the flexoelectric effect in these systems and show that without flexoelectricity no interactions of longer range would be significant and therefore no structures with longer periods than two layers would be stable. We discuss also a few phenomena that are related to the complexity of the structures, such as the existence of a longitudinal, i.e. parallel to the... 

Antiferroelectric liquid crystals were discovered in 1989.xheir antiferroelectric properties were considered as a surprise as, at that time, nobody believed that significant changes of the tilt from layer to layer were possible. In addition to the surprising antiferroelectric properties of one of the phases, several additional phases within a rather narrow temperatm-e region were found. Why such a rich variety of phases occurs within a narrow temperature region and what their structures are, has been a hot experimental and theoretical problem for a long time. The main difference between the microscopic structm-es of the phases was the period of the basic structure. Various phases will be described in more detail later but here we only mention the periods. The SmC phase is defined by the structure of a single... 

The antiferroelectric phase is present in all materials that are called antiferroelectric liquid crystals, as the lowest temperature phase of the tilted... 

In the previous section flexoelectric interactions were not considered in the free energy. We have also seen that only three of the structures found in antiferroelectric liquid crystals can be explained with the form of the free energy presented in the previous section. Let us first consider the discrete form of the flexoelectric effect and its influences on the theoretical description of the structures. We shall see that the flexoelectric effect is a source of indirect interactions between more distant layers and consequently the reason for all structures that cannot be expressed by the single phase difference. 

In more complex chiral polar smectics, antiferroelectric liquid crystals, there are many consequences of the flexoelectric effect. It influences interlayer interactions and causes indirect interactions between more distant layers to appear and become important. The phenomenon is the reason for the appearance of commensurate structures that extend up to six layers. In addition, longitudinal polarization, i.e. the polarization that has a component parallel to the tilt, exists in more complex structures such as the SmCpi2, the SmC jj and the SmC phases. Unfortunately it seems that flexoelectric polarization cannot be detected separately from other phenomena by simple means. A way of measuring the flexoelectric contribution in tilted polar smectics still seems to be an open question. 

Musevic, R. Blinc and B. Zeks, The Physics of Ferroelectric and Antiferroelectric Liquid Crystals, World Scientific, Singapore, London, 2000. 

---