Hetarylazoindoles 2. Spectroscopic and structural investigation of new benzothiazolylazo indole dyes

Synthesis of new benzothiazolylazo indole dyes were carried out by diazotization of 2-aminobenzothiazole, 6- and 5,6-disubstituted 2-aminobenzothiazoles using with nitrosyl sulfuric acid and coupling with various 2- and 1,2-disubstituted indole derivatives. The structures of the dyes were characterized by UV–vis, FT-IR, 1H NMR, mass spectroscopic techniques and elemental analysis. Their solvatochromic properties in different solvents were investigated and their absorption spectra were found to be solvent dependent. The azo-hydrazone tautomerism was discussed in relation to solvent and substituent effects. In addition, acid–base effect on this equilibrium was also evaluated. To determine the most tautomeric form of the prepared dyes in solid state, X-ray data for 2-Methyl-3-(6-methoxybenzothiazol-2-yldiazenyl)-1H-indole were recorded in different solvents. The X-ray results showed that the dye was stable as azo tautomer in the solid state.