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

Abstract

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.