Chiral nematic phase materials forming

The chiral nematic assembly is formed by materials that have chiral centers and that form a nematic phase. Here, a chiral-imposed twist is imparted to the linear chains composing each layer, resulting in a three-dimensional helical arrangement. 

All interest in the unique optical properties of the chiral nematic phase stems from the two-fold optical activity of the phase (5, 6-9]. That is, the mesophase displays (1) molecular optical activity - the phase being composed of optically active molecules and (2) macromolecular optical activity - arising from the macroscopic helical twist induced by the chiral molecules in the phase (sometimes termed form chirality). These features are of immense importance technologically, as many of the applications of such materials depend on one if not both of these phenomena. 

The cholesteric phase in hquid crystals is analogous to the nematic phase but it is formed by materials that contain a chiral centre, initially derivatives of cholesterol (5.3), hence the name cholesteric LCs. Since synthetic chiral molecules can also be used on their own or as dopants for nematic LCs, e.g. (5.4), chiral nematic is probably a more appropriate term for these materials. 

Optically active bis-imine-functionalized diarylethene (2-4 %) (Scheme 13) was used as a chiral, photoresponsive dopant in the nematic LC materials K15 and ZLI-389, resulting in stable cholesteric phases. For the open form of 26a, [5m values of 11 [tm-1 (K15) and 13 xm 1 (ZLI-389) were measured, while the closed form 26b did not show any helical twisting power. Irradiation at 300 nm (30-50 s) resulted in the closed form and disappearance of the cholesteric phase. Irradiation with visible light restored the cholesteric phase. The gradual decrease in pitch, representing a multi-... 

However, more remarkable was the discovery that in alkane solvents, large, orrfiometallated macrocyclic complexes of palladium shown in Figure 92 would form lyotropic columnar phases [171], These remarkable materials have been shown to form columnar hexagonal phases and, in suitable solvents, lyotropic nematic phases derived from columnar organization. Further, in certain non-mesomorphic examples, mesophases can be induced by the addition of an electron acceptor such as trinitrofluorenone chiral phases are introduced when the acceptor is resolved 2 -(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)propionic acid (know as TAPA). 

When the nematic phase is composed of optically active materials (either a single component or a multicomponent mixture made up of chiral compounds or chiral compounds mixed with achiral materials), the phase itself becomes chiral and has reduced environmental space symmetry. The structure of the chiral nematic (or cholesteric) modification is one where the local molecular ordering is identical to that of the nematic phase, but in the direction normal to the director the molecules pack to form a helical macrostructure, see Fig. 5. As in the nematic phase the molecules have no long-range positional order, and no layering exists. The pitch of the helix can vary from about 0.1 x 10 m to almost infinity, and is dependent on optical purity and the degree of molecular... 

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