Benzazoles and naphthazoles

1-Benzothiazolyl-5-(p-nitrophenyl)formazans, which have thermochromic properties, were synthesized. The structure of the compounds was studied by means of IR and UV spectroscopy and quantum-mechanical calculations, the state of the tautomeric equilibrium was established, and the most probable planar conformations of the formazans as a function of the character of the substituent in the 3-position of the formazan chain were found. It is shown that the thermochromism of these compounds in solution is related to dissociation of the hydrogen of the NH bond of the formazan grouping and that the deeply-colored thermally induced form is the anionic aci form.