Chiral nematic phases enantiotropic

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

In contrast, compound 53 exhibits only one enantiotropic transition by DSC, which occurs between a chiral nematic phase and the isotropic liquid, in... 

Other metallomesogens containing the optically active butadienetricarbonyliron(O) moiety, incorporated into promesogenic chiral nematic ligands derived from cholesteryl, have been reported. " Both the diastereoisomers exhibited a chiral nematic phase and a monotropic smectic phase, the chiral nematic phase being monotropic for the (-l-)-isomer (Cr 193 (S 93 N 131) I), and enantiotropic for the (—(-isomer (Cr 117 (S 99) N 133 I). 

Typical examples of chiral azobenzenes are (/f)-4-(2-methylbutyl)-4 -substituted azobenzenes (9-13) which appeared with the emergence of synthetic methods which allowed the preparation of chiral precursors, such as (-i-)-2-methylbutylbenzene [51-54]. In the case of compound 12, the presence of two chiral terminal moieties was found to increase the helical twisting power ()8) by a factor of two [51]. Both the compounds 9 and 10 show enantiotropic chiral nematic phases which have relatively low clearing points, unlike compounds 11 and 12 which are not liquid crystalline. 

The complexes bearing one chiral substituent display a smectic A mesophase when the non-chiral chain is long, or an enantiotropic cholesteric and a monotropic SmA phase for shorter alkoxy chains. A TGBA phase is observed for the derivative which contains the chiral isocyanide combined with the diethyloxy, when the SmA to cholesteric transition is studied. The compound with two chiral ligands shows a monotropic chiral nematic transition. When this compound is cooled very slowly from the isotropic liquid it exhibits blue phases BP-III, BP-II, and BP-I. 

The precursor 6 exhibits the enantiotropic nature of chiral nematic (N ), chiral smectic C (SmC ) and chiral smectic I (SmI ) phases. The shell-printed texture of the SmC phase and the rose-like texture of the SmI phase can be clearly seen in Figure 12.6. The thiophene monomers, M2 and M3, show enantiotropic N, SmA and SmC phases. The SmC phase is characteristic of ferroelectricity. The polymers show various mesophases. The phase transition temperatures are summarized in Table 12.4. PI shows an enantiotropic SmA phase. P2 shows enantiotropic SmA, SmC and SmB phases. The fan-shaped texture of the SmA phase and the striated fan-shaped texture of the SmC phase are shown in Figure 12.7. P3 shows an SmA phase in the heating process and SmA and SmX phases in the cooling process. XRD analysis suggests that the SmX phase of P3 might be a higher order smectic phase. 

Compound 120 shows a short monotropic chiral nematic and smectic A phase, whilst 121 shows a short enantiotropic chiral nematic and a monotropic smectic A phase. From this it may be concluded that the lactic acid derived ( )-2-oxypropanoy-loxy moiety is not particularly suited to sustaining liquid crystal phases this may either be partly due to poor conjugation between the liquid crystalline cores or because of the non-linear nature of molecules. 

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