Chiral dopants characteristics

The electro-optical characteristics of multiplexed STN-LCDs exhibit a significant dependence on temperature. This has to be compensated in order to avoid variations of the optical performance of the display with temperatures. This can be achieved electronically. However, this problem can also be solved by the use of optically active, chiral dopants. The capacitive threshold voltage of a chiral nematic mixture depends on the pitch of the mixture ... 

In Fig. 11.11, the chiral induction of (rac)-Poly-4 forming an N -LC phase is illustrated. The POM image of (rac)-Poly-4 shows a Schlieren texture characteristic of a lyotropic N-LC phase [52], prepared from a 10 wt % toluene solution (Fig. 11.11a). Upon addition of chiral dopant (S)-Dl at 10 wt %, the N-LC phase of (rac)-Poly-4 changes into an N -LC phase (Fig. 11.11b). The POM image of (S)-Dl/(rac)-Poly-4 reveals a fingerprint texture with a helical half-pitch of 2.0 pm. 

When an enantiomer of CSA is used in the polymerization, such as R-CSA, it is possible to create chiral polyaniline nanofibers. Figure 7.20 shows the circular dichroism (CD) spectrum of a water dispersion of as-prepared R-CSA doped polyaniline nanofibers. The positive peak at 450 nm is characteristic of chiral polyaniline [66-70], and is consistent with water s effect on the direction of the CD signals previously observed [71]. The peak at 290 nm is due to excess R-CSA in the dispersion. Recently, Wang et al. discovered that highly chiral polyaniline nanofibers can be produced by incremental addition of the oxidant, ammonium peroxydisulfate, into aniline solution with aniline oligomers and concentrated chiral dopants (>5 M R- or S-CSA) [72]. Chiral polyaniline nanofibers are very interesting for chiral recognition studies [68]. 

The N -LC to be used as an asymmetric solvent is prepared by adding a small amount of chiral compound, as a chiral dopant, into nematic LC (Figure 2.4). The formation of N -LC is recognized when a Schlieren texture characteristic of nematic LC changes into a striated Schlieren or a fingerprint texture in a polarized optical microscope (POM). The distance between the striae corresponds to a half-helical pitch of the N -LC. Note that as the degree of twist in the N -LC is larger, the helical pitch observed in POM is shorter. 

As already pointed out, one condition for achieving the fast bistable switching and all the additional characteristic properties is to get rid of the antiferroelectric helix (to use the expression employed in the 1980 paper [93]). Therefore, achieving the same properties by adding appropriate chiral dopants to the smectic C could be imagined, in order to untwist the helical structure but, as the same time, keeping a residual polarization. This is illustrated by the structure to the left in Fig. 22. However, such a hypothetical bulk structure is not stable in the chiral case. It would transform to a twisted state where the twist does not take place from layer to layer, but in the layers, in order to cancel the macroscopic polarization. For this reason, surface-stabilization requires not only that dsample thickness is sufficiently small to prevent the ap-... 

The N -LC is used as an asymmetric liquid reaction field and it is prepared by adding a small amount of a chiral dopant to a nematic liquid crystal (N-LC). The formation of the N -LC can be confirmed under the polarizing microscope (POM) by the change of the characteristic nematic Schlieren texture to a fingerprint-like or striated Schlieren texture of the chiral nematic phase. The distance between the fingerprint lines of the optical pattern is equivalent to half of the helical pitch of the N -LC phase. Therefore, the stronger twisting power of the dopant can be observed by the shorter helical pitch of the optical pattern under the POM (Fig. 9.33). 

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

Several exciting phenomena described for non-chiral nematic systems were also reported for nanoparticle-doped chiral nematic liquid crystals. We mentioned the work of Kobayashi et al., who, most notably, demonstrated a frequency modulation twisted nematic (FM-TN) mode and fast switching characteristics using metal nanoparticles as dopants [301-307, 313, 314],... 

Fig. 10 (a, b) Schematic mechanism demonstrated for a reflective color M-paper with magnetically controllable characteristics, (c, d) The intensity of magnetic field dependence on the reflection spectra of chiral nematic mixtures doped with magnetite nanoparticles that are surface modified with oleic acid and a chiral pyridine-based dopant, as well as photographs of both formulations before and after a magnetic field of 1,000 GS was applied (see photograph insets above) [364], (Copyright 2010, Taylor Francis)... 

There appear to be no reports of small molecule chiral chromonic mesogens, but it is easy to convert the nonchiral N phase into a cholesteric phase by adding a small water-soluble chiral solute such as an amino acid. Lee and Labes carried out an extensive study of the relationship between the pitch of the induced chiral phase and the concentration of various dopants [55]. They found that the twist varies linearly with concentration over large concentration ranges, and the helical twisting power is characteristic of the particular dopant. This approach offers a novel form of assay for small quantities of water-soluble chiral compounds. 

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