Crystal and electronic structure of methyl orange

Abstract

Azo pigments are typical of the classical pigments as characterized by the azo group (−N=N−) and 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, structure analysis of methyl orange (MO: the simplest azo compound) has been carried out together with its electronic structure. MO is found to crystallize in the space group of P212121 which includes three molecules in the asymmetric unit. The three N/N bonds are typical of the azo structure and the three Na atoms form two kinds of complexes: two seven-coordinate and one five-coordinate complexes. These are connected through the sulfonic anions and form one dimensional polymer along the a-axis. Furthermore, solvated MO is found to give an additional band around 580 nm on going from solution to the solid state. This makes the color orange.