Chiral: axis dopant

Figure 7.15 Chirality transfer from dopant to solvent. Chiral inducer with an (M)-helidty aligned with its biaryl axis parallel to biphenyl axis of solvent can have close contact only with molecules of solvent having same helicity Chirality is therefore transferred from dopant to near solvent molecule and from this to next near one and so on, via chiral conformations.

When a discotic liquid crystal is sandwiched between two substrates (or exposed to air), the direction of the uniaxial axis can be controlled by alignment layers, external electric fields, and chiral dopants [46,47]. It is therefore possible to develop discotic compensation films with spatially varied uniaxial axis orientations. For example, Fuji Photo Film Co. developed discotic compensation films for TN LCDs. In both the TN display and discotic compensation film, the liquid crystal directors vary in the vertical direction. Each layer of nematic liquid crystal with a certain director orientation is compensated by a layer of discotic liquid crystal with the same director orientation. 

The experimental investigation of the chiral induction of the amino-anthraquinones and binaphthyls showed, on the one hand, that the induction by a dopant molecule depends strongly on the orientation of the orientation axis with respect to its skeleton and, on the other hand, on the orientation of a chiral group with respect to the principal axes of the order tensor of the molecule. It became clear that the quantitative contribution to the HTP of a chiral ligand of a molecule of class A depends on the orientation of this chiral ligand with respect to the skeleton. These properties are typical for the behavior of tensorial quantities and thus a tensorial description has to be developed. 

A property that may be admitted by noncen-trosymmetry may very well be ruled out by one of the other symmetry operations of the medium. As an example we will finally consider whether some of the properties discussed so far would be allowed in the cholesteric liquid crystals, which lack a center of inversion. A cholesteric is simply a chiral version of a nematic, abbreviated N, characterized by the same local order but with a helical superstructure, which automatically appears if the molecules are chiral or if a chiral dopant is added (see Fig. 30). Could such a cholesteric phase be spontaneously polarized If there were a polarization P, it would have to be perpendicular to n, because of the condition of Eq. (6), and thus along the helical axis direction m. However, the helical N phase has an infinity of twofold rotation axes perpendicular to m and the symmetry operation represented by any of these would invert P. Hence P=0. A weaker requirement would be to ask for piezoelectricity. (Due to the helical configuration, the liquid has in fact some small... 

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