Antiferroelectric liquid crystal materials

Antiferroelectric Liquid Crystal Materials with Unusual Chemical Structures... 

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... 

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

The molecular interaction between each lajrer is weak and the molecular directors in one layer are not strongly bound by molecular directors in the adjacent layers. As a result, neither a ferroelectric liquid crystal state nor an antiferroelectric liquid crystal state can be realized in forming this random orientation. In the early stages, these materials were called thresholdless antiferroelectric liquid crystals (TLAF) . However, for these materials antiferroelectric order or antiferroelectricity cannot be observed. Recently, the terms frustoelectricity and frustrated electricity have been proposed [36]. These terms imply that the formation of both ferroelectricity and antiferroelectricity are prohibited and neither phase can be formed. 

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. 

The applications of liquid crystals have unquestionably added incentive to the quest for new liquid crystal materials with superior properties such as viscosity, elastic constants, transition temperatures, and stability. In recent years this has catalyzed work on chiral materials as dopants for ferroelectric displays and for antiferroelectric materials with structures avoiding the number of potentially labile ester groups that were present in the original materials in which... 

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. 

Actually, the first solid antiferroelectric material had been discovered at Tokyo Institute of Technology [40]. The discovery of the first antiferroelectric liquid crystal at the very same university was quite unintentional, but can be considered quite fortunate. In this chapter, I will introduce some details of what led to the discovery of antiferroelectric liquid crystals from the study of ferroelectric liquid crystals and also discuss the history of the discovery of the ferroelectric phase. 

Thiophenes of type 31 (X-Y = CH) were generated via Lawesson s reagent-mediated cyclization of 1,4-dicarbonyl compounds 30 under microwave irradiation in the absence of solvent [37]. The reaction was carried by mixing the two solid reagents in a glass tube inserted inside a household microwave apparatus and irradiating until the evolution of H2S ceased. An interesting application of this method is the preparation of liquid crystals and other ferro- and antiferroelectric material such as compound 33 (Scheme 10). 

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.

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... 

Chirality is also an important aspect of liquid crystals. The introduction of chiral moieties into the chiral smectic phases induces functions such as ferroelectricity and antiferroelectricity. A few of the unconventional chiral liquid crystals are described in Chapter 1. The blue phase is one of the exotic chiral liquid crystalline phases. In Chapter 3, Kikuchi introduces the basic aspects and recent progress in research of the blue phase. Recently, the materials exhibiting the blue phases have attracted attention because significant photonic and electro-optic functions are expected from the materials. 

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. 

Table A..5-1 The 72 families of ferroelectric materials. The number assigned to each family corresponds to the number used in LB III/36. The numbers in parentheses (A sub>. f+a ) after the family name serve the purpose of conveying some information about the size and importance of the family. The numbers indicate the following A sub the number of pure substances (ferroelectric, antiferroelectric, and related substances) which are treated as members of this family in LB III/36 A f+A the number of ferroelectric and antiferroelectric substances which are treated as members of this family in LB III/36 n, the number of representative substances from this family whose properties are surveyed in Sect. 4.5.4. For some of these families, additional remarks are needed for instance, because the perovskite-type oxide family has many members and consists of several subfamilies because the liquid crystal and polymer families have very specific properties compared with crystalline ferroelectrics and because the traditional names of some families are apt to lead to misconceptions about their members. Such families are marked by letters a-m following the parentheses, and remarks on these families are given under the corresponding letter in the text in Sect. 4.5.3.1...

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. 

So far, four display modes have been proposed in ferroelectric and antiferroelectric display applications, as shown in Figure 9.34. A bistable switching in surface stabilized ferroelectric liquid crystals (SSFLCs) has been manufactured as a passive matrix liquid crystal display (PM-LCD). The counterpart of AFLC is a tristable switching, which is also a promising candidate for PM-LCD. In addition to these PM-LCDs, active matrix displays (AM-LCDs) are also proposed in FLC and AFLC materials, i.e., deformed helix FLCD (DHFLC) and V-shaped LCD (VLCD). In this section, PM-AFLCD and AM-VLCD will be described. 

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. 

It was the Harvard physicist Meyer who in 1974 first recognized that the symmetry properties of a chiral tilted smectic would allow a spontaneous polarization directed perpendicular to the tilt plane [61]. In collaboration with French chemists, he synthesized and studied the first such materials [62]. These were the first polar liquid crystals recognized and as such something strikingly new. As mentioned before, substances showing a smectic C phase had been synthesized accidentally several times before by other groups, but their very special polar character had never been surmised. Meyer called these liquid crystals ferroelectric. In his review from 1977 [43] he also discussed the possible name antiferroelectric, but came to the conclusion that ferroelectric was more appropriate. 

As we have seen, most liquid crystals have too high a symmetry to be macroscop-ically polar if they obey the n - -n invariance (which all civilized liquid crystals do, that is, all liquid crystal phases that are currently studied and well understood). The highest symmetry allowed is C2 (monoclinic), which may be achieved in materials which are liquid-like at most in two dimensions. Even then external surfaces are required. Generally speaking, a polar liquid crystal tends to use its liquid translational degrees of freedom so as to macroscopical-ly cancel its external field, i.e., achieve some kind of antiferroelectric order. For more liquid-like liquids, piezo-, pyro-, ferro-, and antiferroelectricity are a fortiori ruled out as bulk properties. These phenomena... 

Another important observation is that in liquid crystals the antiferroelectric phase always seem to occur only from the SmC phase and not from the SmA phase. This means that the antiferroelectric phases appear at lower temperatures than of the ferroelectric ones. This is the opposite of the situations in solid states, where the ferroelectric state is the lower temperature phase. This clearly indicates that the cause of the ferroelectric and antiferroelectric orders is different in the fluid states than in the solid materials. 

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