Chiral mesophases

When the mesogenic compounds are chiral (or when chiral molecules are added as dopants) chiral mesophases can be produced, characterized by helical ordering of the constituent molecules in the mesophase. The chiral nematic phase is also called cholesteric, taken from its first observation in a cholesteryl derivative more than one century ago. These chiral structures have reduced symmetry, which can lead to a variety of interesting physical properties such as thermocromism, ferroelectricity, and so on. 

As indicated above in chiral mesophases, the introduction of a functional group in mesogenic stmctures offers the opportunity to achieve functional LCs. With this aim, mesomorphic crown-ether-isocyanide-gold(I) complexes (26) have been prepared recently [38]. The derivatives with one alkoxy chain show monotropic SmC mesophases at or close to room temperature. In contrast, the complexes with three alkoxy chains behave as monotropic (n = 4) or enantiotropic (n > 4) LCs. The structure of the mesophases could not be fully eluddated because X-ray diffraction studies in the mesophase were unsuccessful and mesophase characterization was made only on the basis of polarized optical microscopy. These complexes are luminescent not only in the solid state and in solution, but also in the mesophase and in the isotropic liquid state at moderate temperatures. The emission spectra of 26a with n=12 were... 

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. 

In Section 3.4.6 it was noted that chiral mesophases behave as chiral perturbation moieties for achiral molecules inserted to the mesophases. Here, only relations between the observed signs in LCICD and the pitch-dependent CD in liquid crystalline systems, and the helical sense of the liquid crystalline phases are described. 

Resolution attempts in cholesteric phases. Hie body of data collected to date clearly Indicates that unless specific solute-solvent Interactions occur, the stereochemistry of reactions will be little affected by chiral solvents, whether they be macrosco-pically ordered or isotropic (48-50). In fact, the low optical activity In products from irradiations in cholesteric solvents may arise from the ability of a chiral mesophase to produce circularly polarized light from normal incident radiation (51). 

Choi S-W, Fukuda K, Nakahara S, Kishikawa K, Takanishi Y, Ishikawa K, Watanahe J, Takezoe H (2006) Amplification of twisting power in chiral mesophase by introducing achiral rod-like compound with ester group. Chem Lett 35 896—897... 

Kajitani T, Masu H, Kohmoto S, Yamamoto M, Yamaguchi K, Kishikawa K (2005) Generation of a chiral mesophase by achiral molecules absolute chiral induction in the smectic C phase of 4-octyloxyphenyl 4-octyloxybenzoate. J Am Chem Soc 127 1124-1125... 

Fig. 7 Induction of chiral mesophases in achiral LCs by addition of chiral dopants...

The valency of the hydrogen bond donors and acceptors can be varied to produce chiral mesophases [85]. Mixtures of the divalent 4,4 -bipyridine with 4-[(S)-2-methylbutoxy]benzoic acid in ratios between 1 9 and 4 6 show LC behaviour, but the chiral mesophases are exhibited for only a small range of compositions [86]. Here, the cholesteric and a blue LC phase were observed. The association of the acid to the bipyridine was confirmed by a crystal structure of the 1 2 complex. 

Cholesteric liquid crystals are optically active nematic phases as a result of their gradual twist in orientational alignment. Therefore, cholesteric liquid-crystalline solvents are expected to induce enantioselectivity in chemical reactions see reference [713] for a review on photoasymmetric induction by chiral mesophases. The existing results are not very promising. So far, the maximum photoasymmetric induction reported has... 

Let us note that two homologues of the malonate addends and of the corresponding methanofullerene derivatives described above (Fig. 66) were also prepared with chiral pendant mesogenic groups, laterally attached (Fig. 69). Both the G1 [304] and G2 [57] fullerodendrimers were found to show a chiral nematic phase (Fig. 69, Gl G 26 N 69 1 G2 G 24.3 N 80.61), and are the first Ceo derivatives that possess a chiral mesophase the Gl Geo-free malonate was also mesomorphic (G 20 N 113 I). 

Hgure 4.33 Representation of the local structure of some chiral mesophases on heating. The matches describe the relative orientations of chiral molecules in space. As the temperature is raised, the system transforms from a crystalline phase (left) to a cholesteric phase (centre) characterised by a single twist, to a double-twist blue" phase (right). 

As mentioned in the introduction, chiral compounds can exhibit chiral mesophases and these are important due to the important physical properties that they may exhibit, including thermochroism, ferroelectric and electroclinic effects [15], In 1975, Meyer predicted the existence of a spontaneous polarization (Pg) in chiral, tilted smectic phases [86], and the existence of such polar order within a liquid crystal phase has important implications both scientifically and industrially [19]. The asymmetry associated with the chirality may also produce a beneficial lowering of transition temperatures. 

There have also been a number of examples reported of supermolecular materials composed of different mesogenic units. For example, Yelamaggad et al. [48,49] reported a supermolecular material, 8, that was composed of three different mesogenic groups linked together in a linear fashion, see Fig. 6. This material possesses a chiral steroidal unit, a linear biphenyl moiety and a photoactive azobenzene unit, and remarkably it exhibits unusual frustrated chiral mesophases. 

The ease of forming the smectic mesophase by this class of side-group type liquid crystalline polymers has rendered a great possibility in synthesizing polymeric chiral smectic materials useful in non-linear optics, transducers, pyroelectric detectors and display devices (Chapter 6). The first polymer forming a chiral smectic-C phase was synthesized by Shibaev et al. (1984). It has a polymethacrylate main chain, a long polymethylene spacer, and a mesogenic unit attached at the end with a chiral moiety (polymer (3.60)). Since then, a lot of polymers with chiral mesophases have been synthesized and studied (Le Barny and Dubois, 1989). 

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). 

Other heteroligand palladium complexes (249) with Y-X being either an non-chiral iV-[4 -(dode-cyloxy)resorcylidene]-4-alkylaniline or a chiral iV-[4 -(dodecyloxy)resorcylidene]-4-alkoxyanilines ((249) M = Pd, n = 14, OC H2m+i = (7J)-( )-2-octanol, (5)-(—)-y8-citronellol) have been prepared. They showed chiral mesophases over narrow temperature ranges. ... 

Usually, it is possible to observe chiral mesophases exhibiting ferroelec-tricity in mesogens made from optically active molecules, but in some particular cases, achiral bent-core molecules can form polar ordered smectic phases... 

In the chiral crystal smectic mesophases the helix is in effect unwound by the crystal structure and the form chirality is suppressed. There are also other chiral mesophases of the smectic type (e.g., TGBA and S anti phases) which will be discussed later. 

Identification of Liquid Crystal Phases Mesophase Characterisation 195 93.3.3 Other chiral mesophases... 

There is a wide range of other chiral mesophases that are somewhat unconventional in terms of their structure and many of these are rather recent developments. Blue phases are cubic mesophases that have a double twist stracture. They are exhibited by highly chiral materials and appear at temperatures above the chiral nematic phase. These phases give characteristic platelet type textures that are often blue (hence the name) but they can also appear green, yellow and red. The twist grain boundary (TGBA ) phase also has a fmstrated complex helical stmcture. In this case the normal lamellar stmcture is... 

Addition of CSDAs and silica sources to a solution of folic acid or guano-sine monophosphate allows replicating their chiral mesophases to form ordered mesoporous materials with hexagonal phases. These materials have been denoted nanoporous folic acid material (NFM-1 Atluri et al. 2009). The chiral arrangement of the template within the pores was recently verified through circular dichroism and electron diffraction (Atluri et al. 2013). 

When molecules are chiral, that is, they do not possess the mirror symmetry, a variety of chiral mesophases can be observed. Examples of such molecular models are shown in Fig. l.l(i, j). As a rule, chiral mesophases manifest modulated helical structures with the pitch of the helix incommensurate with the molecular size. 

In general, the difference in structure and thermodynamic properties between a chiral mesophase and its racemic counterpart is small, and we will not discuss chiral G, I, F, etc., liquid crystals. 

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). 

Even the first thermotropic liquid crystals (cholesterol benzoate and cholesterol acetate) were chiral molecules, and their chiral mesophases were observed [2]. Here, the chirality is taken from the chiral pool. Because liquid crystals are needed in gram scales for physical research and technical applications, they should be prepared by short synthetic pathways using easily available starting materials. Thus, synthetic strategies based on the chiral pool are more often used than asymmetric synthesis or chiral separation techniques. 

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