Chiral mesogens

In this section, we will neglect the crystal structures of the mesogenic perfluorinated phenyl benzoates [23-27], benzoic acids [6, 28-31], cinnamic acids [7, 32, 33], dicarboxylic acids [34, 35], and cinnamate compounds [8, 36-40]. The single crystal X-ray analyses of chiral mesogenic carboxylates are described in Sect. 6. 

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. 

Table 18. Crystal data and phase sequences of chiral mesogenic compounds (see Fig. 27)... 

Similar to cholesteric liquid crystals, the nematics have an orientational long-range order with the deviation that the direetion of the preferred orientation does not rotate (Fig. 2a). If, however, a chiral mesogen is dissolved in a nematic liquid crystal, the latter will be transformed into a eholesterie liquid crystal. 

It is possible for chiral mesogens to produce essentially achiral mesophases. For instance, in certain ranges of concentration and molecular weight, DNA will form an achiral line hexatic phase. A curious recent observation is of the formation of chiral mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes called banana LCs) have been shown to form liquid crystal phases that are chiral. In any particular sample, various domains will have opposite handedness, but within any given domain, strong chiral ordering will be present. 

R = -CH3, -CmH2m+-, functional group, functional molecule (e.g., chiral, mesogenic, pro-mesogenic) TOAB = tetraoctylammonium bromide... 

The terminus of chirality induction is used for processes, in which the structural information of a chiral molecule is transferred to an initially achiral collective which then will form a superstructural chiral phase. One of the most prominent examples can be found in the field of liquid crystals The doping of a nematic LC phase with chiral mesogenes (dopants) can lead to a twisted, helical cholesteric phase. Noteworthy is the fact that the length scales of the chiral information that characterizes the involved species can differ by several orders of magnitude a few Angstrpms in a single chiral molecule, but the pitch of a helical cholesteric phase amounts typically a few microns. 

The role of supramolecular chemistry in materials is perhaps expressed most impressively in liquid crystals, in which slight variations of chiral content can lead to dramatic influences in the properties of the mesophases. The helical sense of these mesophases is determined not only by intrinsically chiral mesogens but also by the use of dopants which more often than not interact with achiral host LCs to generate chiral phases (Fig. 7). These phenomena are important both scientifically and technologically, most notably for the chiral smectic and cholesteric liquid crystal phases [68-71]. These materials—as small molecules and as polymers [72,73]—are useful because their order... 

Table 6 Mesomorphic behavior of the carbosilane dendrimers bearing chiral mesogens (H, I and J)...

It is possible to conceive of situations where the chemical linking of molecular components around a template is not as crucial as the formation of defined, non-covalent interactions during templating. This may be exemplified by the polymerisation of a nematic liquid crystalline crosslinker in the presence of a template, a non-polymerisable cholesteric mesogen [23]. The chiral dopant forces the crosslinker to form a cholesteric phase. After polymerisation of the crosslinker, the polymer still exhibits a helicoidal structure which is stable over a wider temperature range than the initial cholesteric phase. It is not reported in this work whether extraction of the chiral mesogen has been attempted or not. 

In general, cholesteric liquid crystals are found in optically active (chiral) mesogenic materials. Nematic liquid crystals containing optically active compounds show cholesteric liquid crystalline behavior. Mixtures of right-handed and left-handed cholesteric liquid crystals at an adequate proportion give nematic liquid crystals. From these results cholesteric liquid crystals are sometimes classified into nematic liquid crystals as twisted nematics . On the other hand, cholesteric liquid crystals form batonnet and terrace-like droplets on cooling from isotropic liquids. These behaviors are characteristic of smectic liquid crystals. Furthermore, cholesteric liquid crystals correspond to optically negative mono-axial crystals, different from nematic... 

These two synthetic approaches have been applied to the cooligomers shown in Scheme 1, employing a chiral mesogen I and an achiral methacrylate-bearing mesogen III. 

Scheme 2. Crossiinkable cyclic LC siloxane. x is the molar fraction of the chiral mesogen. n represents the ring size of the siloxane backbone.

Terminally appended chiral mesogens have also been investigated through the deployment of cholesteryl substituents [97]. For example, material 46 has been prepared and shown to exhibit a smectic A phase, thus it has properties... 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

The chiral nematic state, first observed in cholesteryl derivatives, was later detected in other chiral mesogens, and can also he induced hy adding small chiral molecules to a host nematic TCP. Chiral nematic LCPs have thus heen synthesized as side-chain polymers hy introducing a chiral unit in the tail moiety of mesogens or hy copolymerizing cholesterol-containing monomers with another potential mesogenic monomer. Examples [4] of these types are shown as structures (V) and (VI) ... 

Various models proposed may not account for all these experimental facts. The Keating [20] and Bottcher [21] evaluation does not account for such a variety of behavior. Goossens [22] proposed the chiral nematic structure as the result of an anisotropic dispersion energy between chiral mesogens, and predicts for thermotropic LCs a pitch that is essentially independent of temperature. Lin-Liu et al. [23] developed a theory that accounts for all the above-stated temperature effects. The temperature dependence of the pitch is determined by the shape and position of the intermolecular potential as a function of the intermolecular twist... 

Chiral LC phases can be observed either in chiral mesogenic compounds or in achiral LCs doped with chiral molecules. The supramolecular chirality in the former systems originates from the intrinsic chirality of mesogens and forms chiral... 

CLCs can be fabricated by two main methods. The first and simplest way is based on chiral mesogens, which can exhibit the cholesteric phase at a certain temperature range. The cholesteric phase produced by this method consists of pure materials and thus may exhibit advantages such as good uniformity, enhanced stability, and fast response time to stimuli. However, they are typically produced at... 

As chiral LCs can be formed by chiral mesogens or induced by chiral guest molecules, photoresponsive chiral LCs can be derived from these two systems by photosensitization. One strategy is to photosensitize the existing constituent molecules in chiral LCs, i.e. LC host or chiral dopant. The other strategy is to dope additional photoresponsive agents to the existing chiral LC systems. These two... 

Photoresponsive molecules are required for the fabrication and photomodulation of photoresponsive CLC materials regardless of being employed as chiral mesogens, achiral LC host, or chiral/achiral dopants. The photoisomerization of molecules leads to change in molecular shape (geometry/conformation) and alter the bulk properties of LC material, which constitutes the basis for the photomodulation in chiral LCs [28, 29]. There are many types of photoresponsive molecules and some examples are shown in Fig. 5.4. 

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