Mixtures dichroic

Field-induced change in the orientation of either dichroic dye molecules (the guest) dissolved in a mesophase (the host) or dichroic dye moieties (the guest) of polymers (the host) resulting in changes in the absorption spectrum of a mesomorphic mixture. 

The formation of furoin from furil represents the simplest and quickest demonstration of phytochemicai reduction for lecture purposes. The presence of the acyloin can be demonstrated, even in extreme dilution, by its characteristic reaction with aqueous or alcoholic sodium hydroxide (deep blue-green color with deep violet-red dichroic irridescence). This reaction can be carried out in unfiltered fermentation mixtures it gives positive results after thirty seconds. The addition of a few crystals of commercially available furil dissolved in one cc. alcohol to a fermenting sugar solution is suflBcient for demonstration. ... 

Linear-dichroic spectra of SA monolayers prepared from mixtures of OTS and a cyanine-dye surfactant established the absence of dimerization and the orientation of the chromophore parallel to the substrate [183]. In contrast, the same cyanine dye underwent sandwich-type dimer formation in LB films and had its chromophore oriented perpendicular to the water surface [192]. These results highlight an important difference between LB and SA monolayers. Parameters which determine monolayer formation on an aqueous subphase are also responsible for the orientation and organization of the surfactants therein. Furthermore, the configuration of the surfactants is retained regardless of the structure of the substrate to which the floating monolayer was subsequently transferred to by the LB technique. Conversely, in SA monolayers, surfactant organization is primarily dependent upon the nature of the substrate [183]. 

The application of an electric field between the electrodes results in a realignment of the nematic liquid crystal mixture and the dichroic dye molecules parallel to the electric field resulting in a lower optical density (absorption) and, theoretically, the disappearance of colour assuming an ideal order parameter (S = 1) of the nematic liquid crystal director and the dye molecules. A residual absorption in this state gives rise to a display with a strongly coloured background and weakly coloured information. 

Another version of the Heilmeier and Zanoni GH-LCDs with positive contrast is essentially the inverse of the Heilmeier and Zanoni GH-LCDs with negative contrast described above. A nematic mixture of negative dielectric anisotropy incorporating a dichroic dye of negative contrast is aligned parallel... 

The electrode surfaces of a normal LCD sandwich cell d 8-10 pm) are coated with an alignment layer in order to induce a planar alignment of a host (chiral) nematic mixture containing the dichroic dye of positive dichroism and a chiral dopant. Due to the absence of polarisers a very thin mirror can be incorporated within the cell on top of the rear glass plate electrode in direct contact with the guest-host mixture, see Figure 3.15. 

A suitable surface treatment results in a homeotropic orientation for a nematic mixture incorporating a dichroic dye of positive contrast and an amount of a chiral dopant insufficient to overcome the surface forces and generate a twisted structure in the nematic phase. 

The cell contains a nematic mixture with a twist of 270° and homogeneous alignment with a high pretilt angle (0), see Figure 3.16. The nematic mixture is composed of one or several dichroic dyes, a chiral dopant and a nematic host of low birefringence. 

The tetralins (181-183) collated in Table 3.23 are red liquid crystals with a transition moment orthogonal to the molecular long axis, see Figure 3.13. Since they are liquid crystalline themselves they are very soluble in nematic host mixtures. Such mixtures exhibit high negative dichroic ratios and can be used in White and Taylor GH-LCDs with positive contrast. Furthermore, mixtures containing an additional anthraquinone dye of positive dichroic ratio can be used to switch from one colour to another under the action of an electric field. 

The first nematic guest-host prototype nematic GH-LCD reported by Heilme-ier and Zanoni " contained methyl red (157) as the dichroic dye dissolved in 4-butoxybenzoic acid as the nematic liquid crystal host. Other hosts investigated later included 4-methoxycinnamic acid and 4-ethoxy-4-aminoben-zonitrile (28), see Table 3.4. The melting point of these three single components is very high. Therefore, prototype GH-LCDs had to be operated and evaluated at very high temperatures. Thermal decomposition of the mixtures led sequentially to lower contrast, homeotropic orientation due to decomposition products and finally device breakdown. However, these initial experiments were sufficient to demonstrate the feasibility of this display type. 

When a solution of the photostationary mixture of c- and t-24 was treated in the dark by a polar additive such as alcohol or amine, isomerization of t-24 to the starting Z isomer was observed. If the reaction was monitored by circular dichroism in the presence of chiral additives, a dichroic band with a maximum at 335 nm was observed. This ICD band appeared, increased with time, and then disappeared slowly within a few hours, leaving c-24 as the only product (Scheme 9). 

Fig. 4 Relationship between dichroic ratio and stretching ratio of polyacetylene films prepared by a mixture catalyst of Trihexyl aluminium and Tetradecyl titanate.

Electrical conductivity of polyacetylene films depends on orientation of polymer chains. Figure 5 shows the relationship between relative electrical conductivity and dichroic ratio of polyacetylene films prepared by a mixture catalyst of Trihexyl aluminium and Tetradecyl titanate. Conductivity of fully stretched or oriented polyacetylene films showed twenty times higher than that of as-grown films. Conductivity of polyacetylene films, prepared by other catalysts systems of trialkyl aluminium and tetraalkyl titanate, showed similar relationship, although increment of the conductivity was lower. 

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