Properties of novel yellow anthraquinone dichroic dyes with naphthylthio groups for guest–host liquid crystal displays

Yellow anthraquinone dichroic dyes, 1,5-bis(2-naphthylthio) anthraquinone 2 and 1-[4-(t-butyl)phenylthio]-5-(2-naphthylthio) anthraquinone 4 were synthesized. Dye 4 in addition to being highly soluble in fluorinated liquid crystals and having a very small temperature dependence of solubility, has a larger dichroic ratio (DR = 11.7), contrast ratio (CR = 10.7) and molar absorption coefficient (ε = 12,000) than a previous yellow anthraquinone dye 3 with two phenylthio groups owing to the effect of having a naphthylthio substituted at one of the substituents. In comparison with molecular structures of dyes 3 and 4, annulation of an arylthio group from a phenylthio group to a naphthylthio group is effective for improving these properties of dichroic dyes. Dye 4 fulfills the desired properties for guest–host liquid crystal displays to realize high performances.