Synthesis and tautomeric structures of some novel thiophene-based bis-heterocyclic monoazo dyes

In this study, ethyl 2-amino-5-methyl-4-(phenylcarbamoyl)thiophene-3-carboxylate (1) was prepared using Gewald’s methodology. This 2-aminothiophene derivative was diazotised and coupled with, 3-methyl-1H-pyrazolin-5-one, 3-methyl-1-phenylpyrazolin-5-one, 3-amino-5-hydroxy-1H-pyrazole, 3-amino-5-hydroxy-1-phenylpyrazole, 3-cyano-6-hydroxy-4-methyl-2-pyridone, barbituric acid and 4-hydroxycoumarin, respectively (2–8). The newly synthesized bis-heterocyclic monoazo dyes based on thiophene ring were characterised by elemental analysis and spectral methods. The solvatochromic behaviour and tautomeric structures of these bis-heterocyclic monoazo dyes in various solvents was evaluated. Acid and base effects on the visible absorption maxima of the dyes are also reported.

► A series of novel monoazo dyes based on thiophene ring was synthesised. ► The solvatochromic behaviours and tautomeric structures of dyes were evaluated in detail. ► Acid and base effects on λmax of the dyes were also examined in detail.