Dichroic dyes

In order to develop the dyes for these fields, characteristics of known dyes have been re-examined, and some anthraquinone dyes have been found usable. One example of use is in thermal-transfer recording where the sublimation properties of disperse dyes are appHed. Anthraquinone compounds have also been found to be usehil dichroic dyes for guest-host Hquid crystal displays when the substituents are properly selected to have high order parameters. These dichroic dyes can be used for polarizer films of LCD systems as well. Anthraquinone derivatives that absorb in the near-infrared region have also been discovered, which may be appHcable in semiconductor laser recording. 

Other schemes involving dichroic dyes with heat and electrical fields are also possible. Each of the possibilities could use the plastic structure of the substrates, its durability, or both. This approach would recycle the material for carrying the printed messages at the point of use, eliminating handling and distribution costs, and would require a fraction of the enormous amount of paper now consumed in delivering news and other literary material. The newspaper or periodical would have the familiar size and appearance and would present little change to the reader. The convenience of real on time home delivery and other built in aspects of the system would make it a useful successor to the present one. (This is just a point to discuss and amuse oneself but it could happen.)... 

Polarizing filter (analyzer), 24 675 Polarizer filter dichroic dyes, 9 340 Polar monomers, polymerization of, 26 113-114... 

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. 

Electrically switchable host for dichroic dyes Guest-host and dyed phase change displays... 

Figure 5.10 Schematic of a guest-host (GH) LCD using dichroic dyes and a chiral nematic...

In the case of LC polymers, the polymeric matrix performs as a host, while the guest is a dye, whose molecules are elongated in shape, and the absorption oscillator is parallel (or perpendicular) to the big axis of the molecule 65,163-165>. The experiments investigating guest-host effect in nematic polymers with dichroic dyes covalently attached to the polymer 163) (type I) and mechanically incorporated65) (type II) reveal the possibility to obtain regulated color indicators (see page 60). 

Many relatively simple amino- and hydroxyanthraquinone derivatives show a high degree of order in liquid-crystalline systems and are therefore suitable as dichroic dyes for guest-host displays. Several new dyes for this application have been described. For example, 9 is a blue dichroic dye [4],... 

A closely related phenomenon induced by linearly polarized light was found independently by Gibbons et al., who employed a polyimide (PI) film doped with azobenzene molecules as a dichroic dye and showed that the direction of homogeneous... 

Figure 2. Schematic structure of side chain liquid crystal polymers with dichroic dyes (imparting nonlinear optical properties) and mesogenic side groups.

Guest-host (GH) LCDs are coloured displays, whereby the colour and changes in colour are attributable to the absorption of incident light by a dichroic guest dye dissolved in a liquid crystalline host material, usually nematic, and the co-operative reorientation of these dichroic dyes in an electric field, i.e. electro-optical devices. 

Table 3.19 Molecular structure, peak absorption (k ax .abs nm), colour and order parameter for the dichroic dyes (156-165)...

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. 

Incident plane polarised light after traversing the polariser is absorbed by the dichroic dye, whose transition moment is parallel to the electric vector of the light. The internal reflector reflects the coloured light back through the polariser. Thus, the non-addressed background appears coloured. 

Dichroic dyes of positive and negative dichroism are required for various configurations of the Heilmeier and Zanoni as well as the White and Taylor GH-LCDs with positive and negative contrast. They should be chemically, photochemically and electrochemically stable, as well as exhibit a high order parameter, a high dichroic ratio and good solubility in the host nematic matrix. 

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. 

Katoh [1] prepared a liquid crystal composition for use as electronic paper consisting of one dual-frequency switchable smectic liquid crystal, (1), and at least one dichroic dye. 

The E/Z-isomerization process is characterized by angular-dependent excitation and leads, therefore, to the photoselection of a preferred azobezene dye orientation. In other words, the dichroic dye units choose an orientation where the electronic transition moment is perpendicular to the light electric vector. It promotes, in turn, the cooperative reorientation of neighboring moieties, which include other fragments of the macromolecule, such as the main chain or photochemically inactive comonomer units, and low molar mass additives. Thus, a macroscopic orientation of the sample arises, and it remains long after the illumination is stopped and all the dye moieties return to the thermodynamically equilibratory -state. 

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