Long chiral nematics

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

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. 

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. 

Just because a molecule is long, narrow, and meets the requirement of geometric anisotropy docs not ensure that it will have a liquid crystal phase. The particular phase structure that occurs in a compound, i.e.. smectic, nematic, or chiral nematic, not only dc >cnds on the molecular shape hut is intimately connected with the strength and position of the polar or polarizable groups within the molecule, the overall polarizability of the molecule, and the presence ol chiral centers. 

In most cases, the addition of a rod-shaped molecule to a crown ether leads to the formation of a nematic phase. In the case of addition of a chiral unit, such as cholesterol, chiral nematic phases can be observed. Some molecules can also show smectic phases when the attached rod shaped unit is long and inflexible. 

New chiral phases, called smectic blue phases (BPSm), have been discovered in a small temperature range with the following phase sequence TGB-BPsml-BPsm2-BPsm3-Iso, without any intermediate chiral nematic phase between the BPSm and TGB phases [27,28]. X-ray scattering studies showed that unlike the traditional blue phases, the smectic blue phases possess the quasi-long-range smectic order in addition to three-dimensional orienta-... 

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... 

If a chiralized nematic with a long pitch ( 27r/g) and positive Ae is... 

Depending on its chemical stmcture, the pitch of a cholesteric liquid crystal could take any value in the region from a few tenths of a micron to infinitely long. The periodicity of a cholesteric liquid crystal with the pitch Pg is PJ2, because n and - n are equivalent. Cholesteric liquid crystals are also called chiral nematic liquid crystals and denoted as N. Nematic liquid crystals can be considered as a special case of cholesteric liquid crystals with an infinitely long pitch. 

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