Synthesis, characterization and theoretical study of new hetarylazopyrazolone dyes and investigation of their absorption spectra

• Six new hetarylazopyrazolone dyes were synthesized and characterized.
• The solvatochromic properties of the dyes were investigated.
• The structure, tautomerism and absorption spectra of dyes were investigated theoretically by DFT.

A series of hetarylazopyrazolone dyes were synthesized by coupling 1-(2-benzothiazolyl)-3-methylpyrazol-5-one with six heterocylic amines prepared in nitrosyl sulphuric acid. The dyes were characterized by spectral methods and elemental analysis. The solvatochromic properties of the dyes were investigated in various solvents. Additionally, acid–base, concentration, temperature and substituent effects on the visible absorbtion spectra were also examined.The ground-state geometry of the synthesized compounds were studied by using density functional theory (DFT) calculations at the B3LYP/6-31+G(d) level of theory. The absorption spectra of the dyes are obtained by using time-dependent density functional theory (TD-DFT) method associated with the integral equation formalism polarizable continuum model (IEFPCM). Tautomerism of 1-(2-benzothiazolyl)-3-methyl-4-(2-thiazolylazo)pyrazol-5-one has been examined by means of B3LYP/6-31+G(d) method. Calculated absorption maxima are evaluated via comparison with experimental values.

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