Chemical Structure, Photostability, and Molecular Engineering

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 

Dye Commercial No. Code Structure ax (nm) Order parameter Host (solubility) 

In a mixture of dyes, the situation is more complex due to synergetic effects. This interaction may increase the light-fastness of the dyes, but may also have the opposite effect [58]. Since it is not easy to forecast how a photoinduced reactivity will develop, the correct photostability of the dye can be found only experimentally in the liquid crystal host. 

The excellent photostability of the anthraquinone dyestuffs are related to the introduction of a proton donor in the a-posi-tion of the anthraquinone molecule [58], The anthraquinone molecule itself has no absorption band in the visible spectrum [46], so at least one of the 1,4, 5, or 8 (or 2, 

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- 

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