Azo dyes and pigments are widely used in the imaging industry because of their versatile colors as well as their low price. However, it is often pointed out that the color in the solid state is sometimes drastically different from that in solution. An attempt has been therefore made in the present investigation to elucidate the electronic structure of two classical simple azo dyes on the basis of the crystal structure: methyl orange [Na(I)-(dimethylamino)azobenzene-4-sulfonate (MO)] and its derivative [4-(dimethylamino)azobenzene-sulfonic acid (MOH)]. Accordingly the authors found a new trinuclear Na complex (pale red) of MO, as well as a new polymorph (purple) of MOH in addition to the previously reported phase (vivid red). In MO and also in the purple phase of MOH, excitonic interactions are basically responsible for the color change on going from solution to the solid state. On the other hand, the authors found a zwitterionic structure in the vivid-red phase of MOH as formed by NH···O intermolecular hydrogen bonds. The vivid-red color can be interpreted as arising mainly from the zwitterionic structure.
H. Shibata, K. Sato, J. Mizuguchi, "Crystal Structure of Methyl Orange Derivatives and Their Electronic Spectra" in Journal of Imaging Science and Technology, 2009, pp 50302-1 - 50302-7, https://doi.org/10.2352/J.ImagingSci.Technol.2009.53.5.050302