Synthesis of tris-, tetrakis-, and pentakisazo dyes and their application to guest-host liquid crystal displays

Tris-, tetrakis-, and pentakisazo dyes have been prepared by the oxidation of 4-(arylazo)arylamines. Iodobenzene diacetate was the preferred oxidant for preparation of 4,4'-bis(arylazo)azobenzenes and 4,4'-bis(arylazo)-2,2',3,3',5,5',6,6'-octafluoroazobenzenes, whereas silver(II) oxide proved a satisfactory oxidant for synthesis of 4,4'-(arylazo)(methyl-substituted)azobenzenes. Unsymmetrical tris- and tetrakisazo dyes were also obtained by cross oxidation. The absorption bands of these azo dyes were observed in the range of 381-472 nm in liquid crystals. The solubility of the trisazo dyes in hexane was the highest, followed by the tetrakis- and pentakisazo dyes. The unsymmetrical derivative having methyl groups in the central p-phenylene moieties was most soluble among the trisazo dyes. The order parameters (S) of the tris-, tetrakis-, and pentakisazo derivatives were in the range 0.66-0.81, sufficiently high for the practical application of these dyes in liquid crystal displays.