Synthesis, acidity constants and tautomeric structure of 7-arylhydrazono[1,2,4]triazolo[3,4-b][1,3,4]thiadiazines in ground and excited states

7-Arylhydrazono[1,2,4]triazolo[3,4-b] [1,3,4]thiadiazines 4 were synthesized from the reactions of 4-amino-5-phenyl-4H-[1,2,4]triazole-3-thiol 2 and 2-(2-naphthyl)-2-oxoethanehydrazonoyl bromides 1 and their acid dissociation constants pK and pK*, in the ground and excited states, respectively, were determined. Both pK and pK* constants were correlated by Hammett equation. The pK and the spectral data presented indicate that the title compounds exist predominantly in the hydrazone tautomeric form.