Polymorph and electronic structure of methyl orange derivative

Abstract

Azo pigments are typical of the classical pigments used widely in painting and imaging industries. In azo pigments, however, there is still a pending problem associated with the azo or hydrazone structure in the solid state. As a step to clarify this, we initiated our investigation on the crystal structure of methyl orange and its derivaqtives that are typical of simple azo compounds. Then, we found a different phase of 4-(dimethylamino)azobenzene-4'-sulfonic acid (MOH) from that reported by Burke et al. Because of this, electronic structure of both phases has been studied in the present investigation on the basis of the crystal structure as well as DFT calculations. A striking difference is observed between two crystal structures, showing that the previous phase is red as characterized by a zwitterionic structure; whereas purple in the new phase. Interesting to say, DFT calculation revealed that the zwitterionic structure (that exists only in the solid state) gives an absorption band at a far longer wavelength of about 660 nm as compared with that of the ordinary MOH structure (about 420 nm). It is this longer wavelength band that gives a red color in the zwitterionic phase; whereas the absorption band lies in the shorter wavelength of the visible region in the purple phase.