Order 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. 

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],... 

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. 

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. 

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. 

Incorporation oFdyes with high order parameter and dichroic ratio can improve the color and contrast of a PDLC film. These dyes must be incorporated in the liquid crystal droplets of the PDLC to be effective. Utilizing highly crosslinked polymer binders, it is possible to achieve high segregation of the dichroic dye in the liquid crystal droplets (West, J. L. Ondris, R. Erdmann, M. A. SPIE. to appear, 1257). 

The values quoted above for the domain order parameter refer to the alignment of the mesogenic groups. It is known, for instance, that the order parameter of the spacer methylene units in smectic one-comb PLCs is only 0.3-0.4 whereas the mesogenic group order parameter is close to 0.9 [30]. Finkelmann et al. [31] reported the following data on the order parameter of absorbing dichroic dyes, chemically bonded to a nematic, one-comb PLC with a siloxane backbone, close to the isotropization temperature Tjt 0.48 (T/Tj = 0.90) 0.40 (T/Tj = 0.95) 0.30 (T/Tj = 0.99) 0.2 T/T. = 1.0). 

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. 

The Maier-Saupe theory is very useful in considering liquid crystal systems consisting of more than one type of molecules, such as mixtures of nematic liquid crystals and dichroic dyes. The interactions between different molecules are different and the eonstituent molecules have different order parameters. 

Consider a molecule ndye of the guest dye whose long molecular axis forms an angle 6 with the director nLc of a liquid crystal host. Let the absorption oscillator O be located at an angle / with respect to the long molecular axis of the dye (Fig. 2.13). Then the order parameter of the dichroic dye 5dye, /3, and the dichroic ratio N = are known to... 

The difference between 6 and (j) is one of the reasons for the difference in the order parameters between the dye and the liquid crystal host. The main criterion for dichroic dye efficiency is the order parameter of the transition moment of the dye absorption 5, which is responsible for the color and absorption of the dye [14, 16, 46]. It must be clearly understood that the value of Sj-may differ from the order parameter of the long molecular axis of the dye Sj. This is the case when the transition moment of the dye makes an angle with its long molecular axis. If the transition moment makes an angle (not shown in Fig. 2) with the director of the liquid crystal, then by analogy with... 

To make a black mixture, many dichroic dyes covering the whole visible spectrum are required [55-58]. Besides the desired Amax and color, the three important criteria for choosing the dyes are their stability, solubility, and order parameter. Broad spectral band dyes are preferred over narrow band dyes. 

Dyes that dissolve in the liquid crystal and have high extinction coefficients at the laser emission wavelength are required. If the device is for projection applications, the dye should not absorb at the wavelengths that may be used to project the final image-infrared dyes and lasers are well suited for this. The dye will experienee very high temperatures for very short time periods when addressed by the laser and should therefore be stable to heat and light. Dichroic dyes appear to give smaller spot widths than isotropic dyes, and so dyes with a modest order parameter (5=0.5-0.6) are desirable [23]. 

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