Pleochroic dye

From about 1930 onwards, developments in the field of naphthoquinone dyes concentrated on the use of naphthazarin and intermediates for the preparation of violet, blue, and green acid and disperse dyes [1]. More recently there has been interest in the synthesis and color and constitution properties of simple colored naphthoquinones, stimulated by the fact that such dyes have similar tinctorial properties to the anthraquinones but a smaller molecular size. The naphthoquinones provide a useful alternative to the anthraquinones for certain specialized applications, e g., as pleochroic dyes with improved solubility for liquid-crystal displays. As a result, research interest in these chromogens remains unabated, even though they have failed to make any major impact as textile dyes [2-8],... 

The first nematic guest-host prototype nematic guest-host display device contained a nematic liquid crystal (4-butoxybenzoic acid) and a pleochroic dye (methyl red or indophenol blue) sandwiched between two (Nesa) electrodes dTn 12/im) rubbed uniaxially, but with no additional orientation layer, see Figure 3.14. One polariser was fixed to the front substrate surface with its direction of maximum absorption parallel to the rubbing direction and, therefore, the nematic director. 

The nematic liquid crystal mixture containing the pleochroic dye is of positive dielectric anisotropy and is aligned parallel with the director parallel to the substrate surfaces. Therefore, plane polarised light is absorbed by the dye molecules in non-addressed areas of the display and they appear coloured. 

Modified forms of these displays have been developed - e.g., the dye display which has a pleochroic dye dissolved in the nematic material and requires the use of just one polarizer - but we shall not be discussing them here. Analytical expressions have been derived which simplify the computational effort involved in optimizing the material and device parameters, but one has to rely on numerical modelling to give a complete description of the dynamical characteristics of these devices. Certain unusual dynamical effects observed in the TN device, e.g., the reverse-... 

The electro-optical measurements were carried out using an electro-optical cell described by Talroze, et al. which was mounted on a heating stage of a polarizing microscope, MIN-8. A photo-element was used in conjunction with a recorder to monitor the transmitted light intensity. The film thickness was about 12 pm, electric field intensity was 10 V/cm. The pleochroic dye for studying the guest host effect had an absorption band at 505 nm. 

Some pleochroic dyes were contrast in electro-optical e mesogenic side groups of stru ture of low molecular mass dy tion of 1-2% by weight of dye not to change the mesophase induces an alignment of a gue which results in a partial lo film. Such optical behavior tion of dye molecules paralle dent light and by an increase s ity. 

The value of 5op ranges from unity, if the dye transition moment is uniformly parallel to the director, through zero, if the dye orientation is completely randomized to if the dye transition moment is always perpendicular to the director. Note that op depends both on the underlying orientational order of the liquid crystal molecules and on the intrinsic molecular anisotropy of the dye. Suitable dyes have been called both pleochroic dyes and dichroic dyes (the latter is more exact) we call them simply anisotropic dyes. 

As the previous sections have shown, nematic polymer liquid crystals may be oriented by surface forces and in electric fields. It has been shown recently that such field-induced changes in orientation may also be used to orient pleochroic dyes through the guest-host effect. In such an effect either guest dyes dissolved in a nematic polymer " host or side-chain dye moieties in a nematic copolymer system (where A is a nematic moiety and B is a dye in Fig. 2b) undergo a cooperative realignment as the nematic director responds to the applied field. Since the pleochroic dye has its absorption transition... 

A further advantage of the nematic follows from the incorporation of pleochroic dyes. ° The dye is oriented by the nematic and results in a polarization-dependent absorption coefficient. This can be exploited by orthogonally polarized write and read beams. The ratio of optical densities can be written. ... 

The ability of a liquid crystal to align cooperatively in an electric field can be used to orient guest pleochroic dye molecules. As the absorption spectrum of these molecules is a function of the molecular orientation with respect to the polarization of the incident light, one can electrically switch the color of transmitted light. 

Figure 36. Electrical color switching in liquid crystal cells with pleochroic dye molecules embedded in the liquid crystal layer (after reference [93]).

An interesting development beyond the FLC OASLM is the a-Si novelty filter [78]. This device uses an OASLM like structure to perform a simple motion detection to measure the difference between image frames. Rather than use the FLC material as a direct modulation medium, it uses the surface-stabilized characteristics of the FLC as a form of optic memory. The optical modulation of the FLC material is also converted from a polarization effect to an amplitude effect by the inclusion of a pleochroic dye with the FLC material. 

These parameters have been discussed in detail by the author and several other workers [14-16,42-49]. Studies have been done mostly on pleochroic dyes. In general, it has been found that the dye order parameter increases with increasing length of the dye, while it decreases with increasing breadth of the dye. The dye order parameter is heavily dependent on the host. Usually it increases with an increase in and decreases with a decrease in the host order parameter. Elongated dyes are found to have... 

For monochrome applications, one can either put a monochrome color filter inside the cell or use monochromatic back-lighting. For Heilmeier displays, there is another way to get a monochrome display by using monochromatic pleochroic dyes [26]. For full-color display, color filters must be used. Heilmeier displays exhibit better color performance than TN displays, as the chroma-ticity of the pixels does not change much at large viewing angles. 

The geometry of a double cell nematic dichroic LCD [13-16, 21, 126] is shown in Fig. 21. The liquid crystal mixture generally used is composed of pleochroic dyes in a nematic host of positive dielectric anisotropy. The first cell has a unidirectional homogeneous alignment in one direction, while the second cell has a unidirectional homo-... 

The geometry of Heilmeier cells was discussed in Sec. 3.4.8. Using a liquid crystal mixture of negative dielectric anisotropy mixed with pleochroic dye initially in ho-meotropic geometry, we can make a posi-... 

Two versions of cells using this property have been proposed using a pleochroic dye... 

Guest—host Reorientation Parallel Color colorless 1 Positive A e pleochroic dye added MBBA + PEBAB + indophenol blue... 

A third phenomenon depending solely upon dielectric forces is the guest-host or electronic color-switching interaction in which guest pleochroic dyes are incorporated within nematic host materials. The dyes have different absorption coefficients parallel and perpendicular to their optical axes. As illustrated in Fig. 6, the dye molecules can be oriented by the liquid crystal. With zero field, the liquid crystal is in the uniform parallel orientation and the dye mole-... 

White and Taylor have described a new device that combines both the guest-host effect and the cholesteric-to-nematic transition. The pleochroic dye is added to a cholesteric material and the cell transmission changes when the cholesteric undergoes the cholesteric-to-nematic transition. Because of the rotational symmetry of the long axes of the dye molecules about the cholesteric helical axes, contrast ratios of greater than 4 to 1 were obtained without the use of a polarizer. 

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