Anthraquinone, 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. 

The photostability of dichroic dyes is one of the most important considerations for their use in LCDs [16, 43 -46,49, 62 -67]. The photostability of many classes of dye is not sufficient for display applications [46, 64-66], this being particularly true for monoazo dyes with triazole and benztria-zole rings, as well as bis(azo)dyes with the-ophelline and naphthalene fragments [46]. Monoazo and bis(azo) dyes without triazole and benztriazole rings have a sufficiently long lifetime [46]. Anthraquinone dyes are much more stabe than azo compounds. Naphthoquinone dyes have a photostability... 

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 auxochromic groups not only influence the color but also the angle of the transition moment in the dye molecule [46, 57]. Whereas for a symmetrical substituted pattern the transition moment will be parallel (or perpendicular) to the long axis of the anthraquinone skeleton, the situation becomes quite complicated for an asymmetric substitution [74]. As the axis of alignment normally will not coincide with the axis of the skeleton, an oblique orientation of the transition moment with the skeleton axis may even be an advantage, and could provide an explanation for the high dichroic ratios ob-... 

With regard to the dye-related parameter measurements, the dye concentration is usually kept at about 0.5-2%, depending on its absorption coefficient. For azo dyes the dye concentration is about 0.25-1% for dye doped TN, about 2-4% for Heilmeier displays, and about 2-5% for phase change displays. The cell thickness is usually about 5 pm for dye-doped TN at the first Gooch Tarry minimum [ 15,16,88], about 7-10 pm for dye-doped TN at the second Gooch Tarry minimum [15, 16, 88], about 5-15 pm for Heilmeier, and about 10-20 pm for phase change dichroic displays. Usually, the dye concentration is increased by a factor of about 1.5-2 when anthraquinone dyes are used instead of azo dyes, as anthraquinone dyes have less absorbance than azo dyes. Sometimes combinations of azo and anthraquinone dyes are used [16, 54]. 

It is worth mentioning that Osman et al. [74] found theoretically that, in planar dyes, such as anthraquinone, the order tensor deviates strongly from cylindrical symmetry. The direction of the optical transition moment does not, in general, coincide with one of the principal axes of the order tensor. Based on their calculations they found that only those anthraquinone dyes that have a small angle between the transition moment and the molecular axis show a good dichroic ratio [74]. 

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