Core chiral nematics

The mesogenic behavior of 43 emphasizes the role played by the oc-tasilsesquioxane core when comparing with the related side-chain liquid-crystalline polysiloxanes [95,96]. It is remarkable that the thermal stability of the chiral nematic phase is similar in both the polymer and the dendrimer, suggesting that the cubic core does not perturb significantly the associations between the mesogens necessary to support the chiral nematic phase. 

When the terminal ester functionality in the phenyl propiolates is replaced with a carbonyl group, i.e., a keto function, TGB phases disappear altogether indicating that the increased size of the lateral dipole at terminus to the core has detrimental effects on the stability of TGB phases [35]. These small variations in structure show how delicate the stability of the TGB phase is, nevertheless, most chiral nematic to smectic phase transitions are accompanied by a TGB phase no matter how transient it is. 

Two sets of chiral-nematic hybrid are chemically bonded to the same dithienylethene core as for the photoswitchable nematic GLC (30), resulting in a smectic C -cholesteric GLC as depicted in Fig. 6.10. A sample of (31) was placed between two alignment-coated fused silica substrates for heating to 230 °C followed by cooling to 207 °C, where shearing was applied to induce alignment. The... 

Compound 18 exhibits solely a chiral nematic discotic phase (N ) phase because the steric effect of the branched chains at the chiral centre disrupts the ability of the molecules to pack in columns. The large size of the planar aromatic core ensures a high clearing point, but the liquid crystal tendency depends critically on the type of chiral peripheral chain. Hexa-substituted phenylacetylenes were discussed in chapter 3 and exhibited the Np phase. Perhaps not surprisingly, when one of the peripheral acetylene units is chiral, the N phase is exhibited (compound 19). 

Fig. 14, then it is found that the number of atoms (n) that the chiral center is removed from the rigid central core determines the handedness of the helical structure of the chiral nematic phase. As the atom count by which the chiral center is removed from the core (n) switches from odd (o) to even (e) (parity), so the handedness of the helix alternates from left to right or vice versa. Sim-... 

Despite the wide variety of chiral nematic liquid crystals (and of course low molar mass chiral liquid crystals in general), it is possible to sub-divide them into three class types, according to the relationship of the chiral moiety and the liquid-erystalline core [29]. Firstly, we consider type I here the chiral centre (or multiplieity of chiral centres) is situated in a terminal alkyl chain attached to the effective liquid-crystalline core. As will be seen later, it is entirely possible that the compound can have two or more chiral terminal groups associated with the molecule. Type I materials are probably the most frequently encountered chiral nematic liquid-crystalline compounds, often... 

The esters with 4 -pentylbiphenyl cores were somewhat disappointing in their phase behaviour all materials showed smectic A and crystal B phases and only the meth-oxy compound 89 displayed a short chiral nematic phase (3.8 C). 

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. 

The first two steps, that is the stereoselective cyclization in the presence of (S)-(-)-proline to give the bicyclic diketone (124) and the hydrogenation of the double bond formation during the condensation step to give compound 125, are crucial in setting up the appropriate stereochemistry necessary for the acid-catalysed cyclization step which results in the formation of the basic twistance core (126). Of the five materials synthesized in this way, only three show chiral nematic liquid crystal phases, as listed in Table 10. 

M ribbons, chromonics 996 Mach-Zehnder interferometer 132 macrocyeles, hydrocarbon cores 719 f macrocyclic complexes, metallomesogens 926 macroscopic flow, chiral nematics 379 macroscopic polarization, ferroelectrics 541 ff macroscopic properties... 

Table 2.1 The Pitch Length of a Chiral Nematic Liquid Crystai increases as the Chiral Center Is Moved away from the Molecular Core...

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