Structural and spectral characterizations of C.I. Disperse Blue 148 having a new crystalline form

Abstract

Single-crystal structures of the best high-temperature trichromatic blue azo dye C.I. Disperse Blue 148 and its diazonium component 3-amino-5-nitro-[2,1]- benzisothiazole are described herein. C.I. Disperse Blue 148 exhibits an essentially coplanar molecular structure and a dimeric packing mode between adjacent phenyl and benzisothiazole rings. Particularly, PXRD measurement reveals the existence of a new crystalline form of C.I. Disperse Blue 148 having the highest melting point and the best thermal stability, which is obviously different from all the known α, β, and γ forms. A one-phase recrystallization method, which can guarantee the complete transformation, is used to prepare the new crystalline form instead of the conventional heating of an aqueous suspension of α form. Furthermore, TGA–DSC and pH titration experiments have been carried out to reveal the thermal and pH stabilities of this heterocyclic dye. To our knowledge, this is the first structural example on benzisothiazole involved azo dyes.