Synthesis,Characterization, and Theoretical Calculation of New Azo Dyes Derived from [1,5‐a]Pyrimidine‐5‐one Having Solvatochromic Properties

7‐Amino‐3‐phenylazo‐2‐methyl‐4H‐pyrazolo1,5‐a]pyrimidine‐5‐one ( 3 ) was synthesized by the reaction of 5‐amino‐3‐methyl‐4‐phenylazo‐1H‐pyrazole and 2‐aminobenzothiazole with ethyl cyanoacetate in acetic acid at 150°C. Four novel heterocyclic azo disperse dyes were obtained by the coupling of heterocyclic amines‐based diazonium chloride with compound 3 . They were purified and characterized by elemental analysis, FTIR, and ¹H NMR. Furthermore, solvatochromic behaviors of related dyes were studied in detail by using ultraviolet–visible absorption spectrometer. The experimental data were supported by density functional theory using b3lyp/cc‐pvtz level calculations, and a detailed analysis of predicted tautomeric structures was made.