Antiferroelectric liquid crystal structure

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

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

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... 

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. 

T. Nakai, S. Miyajima, Y. Takanishi, S. Yoshida, A. Pukuda, High-resolution NMR study of an antiferroelectric liquid crystal verification of the bent chain structure, J. Phys. Chem. B 103 (1999) 406. 

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). 

It seems that finally this kind of phenomenon may have been observed in liquid crystals [135]. However, in contrast to the cases of Si02 and NaC103, the helical structures are probably possible on a molecular level as well as on a supermolecular level. Thus we may expect domains of nonchiral molecules in different conformations, right and left-handed, which behave as if they belong to different enantiomeric forms. The possibility that a space-filling flexoelectric deformation will be spontaneous for certain molecular shapes and thereby create a chiral structure out of nonchiral molecules will be much enhanced if the deformation can take place in the layer rather than in the interlayer twist-bend structure of Fig. 52, and may then lead to antiferroelectric (rather than helielectric) order similar to that in antiferroelectric liquid crystals made of chiral compounds. The polarization may very... 

Antiferroelectric Liquid Crystal Materials with Unusual Chemical Structures... 

Very soon it was recognized that antiferroelectric liquid crystal (AFLC) materials exhibit not only the anticlinic antiferroelectric phase, but also three other subphases. 1 One of them, called SmC. phase, clearly has so-called "ferrielectric" characteristics, which structure is shown in Figure 8.24. 

Otaracteristic for antiferroelectric liquid crystals are three stales of switching emerging between the zero field state with zigzag structure layer orienbtion and the two known... 

FIGURE 2.15 The structure of the antiferroelectric liquid crystal phase (center) and electric-field-induced switching states. 

Fig. 9.4 Local molecular orientation structure of antiferroelectric liquid crystal...

The tilting of molecules in the B2 phase is clearly confirmed from the observation that the spherulites emerging from the isotropic phase show an electric field dependence of the position of the optical extinction lines (Fig. 9.26). Because of the tilting of banana molecules to the layer, chirality is spontaneously generated in addition to the polarity this fact sounds shocking but is so simple to be understood [132, 133]. If the molecule is rotated around their polar axis (the orientation of the bent in the molecules), which is akin to tilting the molecules in the layer, the rotation operation cannot be achieved by a simple translation (see Fig. 9.27). That is, these two states are in a mirror relation with left-handed and right-handed chirality. This is called the layer chirality. When the chirality couples with the polarity of the molecules, one would consider various smectic liquid crystal structures. There are two homochiral phases in which either (—) or (-I-) chiral molecules stack in the layers and a racemic phase in which layers are alternately stacked with layers of (—) and (-I-) chiral molecules. Each of those phases can be either ferroelectric or antiferroelectric, so that in total six different phases are present... 

In this section, we will present the crystal structures of chiral mesogenic compounds exhibiting ferroelectric liquid crystalline phases which are listed in Table 18 [152-166]. Moreover, four compounds of the list show antiferroelectric properties and two compounds form only orthogonal smectic phases. The general chemical structures of the investigated chiral compounds are shown in Fig. 27. 

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.

The flexoelectric effect is a phenomenon where a space variation of the order parameter induces polarization. Chiral polar smectics are liquid crystals formed of chiral molecules and organized in layers. All phases in tilted chiral polar smectic liquid crystals have modulated structures and they are therefore good candidates for exhibiting the flexoelectric effect. The flexoelectric effect is less pronounced in the ferroelectric SmC phase and in the antiferroelectric SmC. The flexoelectric effect is more pronounced in more complex phases the three-layer SmCpu phase, the four-layer SmCFi2 phase and the six-layer SmCe a phase. 

Thermal polarized light microscopy of liquid crystal systems still primarily involves the identification of phase types. Recently, however, a number of novel phases with complex structures have been discovered and detailed examinations of the configurations of their defects are required in order to provide a basis for future phase classification. Thermal microscopy is also used extensively in examination of the alignment processes of liquid crystals, and, in a related context, electric-field studies on meso-phases are carried out in aligned cells. Electric-field studies are now used as adjuncts to phase classification, e.g., antiferroelectric phases are sometimes identified in the microscope with the aid of electric-field studies. 

Liquid crystal ferri and antiferroelectrics have many features discovered for years of comprehensive studies of corresponding crystalline substances. Thus, it would be convenient and instructive to begin with a short introduction in the structure and properties of antiferroelectric crystals. A difference between ferro-, ferri and antiferroelectrics is schematically shown in Fig. 13.15, where the three very simplified... 

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)... 

In the Landolt-Bdmstein data collection, ferroelectric and antiferroelectric substances are classified into 72 families according to their chemical composition and their crystallographic structure. Some substances which are in fact neither ferroelectric nor antiferroelectric but which are important in relation to ferroelectricity or anti-ferroelectricity, for instance as an end material of a solid solution, are also included in these families as related substances. This subsection surveys these 72 families of ferroelectrics presented in Landolt-Bornstein Vol. III/36 (LB III/36). Nineteen of these families concern oxides [5.1,2], 30 of them concern inorganic crystals other than oxides [5.3], and 23 of them concern organic crystals, liquid crystals, and polymers [5.4]. Table 4.5-1 lists these families and gives some information about each family. Substances classified in LB 111/36 as miscellaneous crystals (outside the families) are not included. 

In the early development of liquid crystals, for the most part, the study of small molecular systems dominated the field because of the close link between molecular design and commercial applications. However, it is only in the last 20 years that materials with unusual, and often hybrid structures have been investigated for their liquid-crystalline behavior. As noted, phasmidic materials, which have molecular structures that are part-disc part-rod, were found to exhibit both columnar and smectic phases. More recently, molecular systems having bent-rod-like structures have been investigated and found to exhibit a wide range of novel phases, many of which were found to be ferroelectric or antiferroelectric (without molecular chirality) due to the reduced symmetry of their mesophase structures. 

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