Disubstituted phenyl azo derivatives of 4-hydroxy-6-nitro-coumarin: synthesis, characterization, spectroscopic properties, antibacterial evaluation and DFT chemical calculation

Purpose This paper aims to utilize 6-nitro-4-hydroxycoumarin as an enol-type coupling component for the preparation of 3-phenylazo-6-nitro-4-hydroxycoumarin, along with two 3-(o,p-disubstituted phenylazo)-6-nitro-4-hydroxycoumarin dyes, and to evaluate their spectroscopic properties and antibacterial activity. Design/methodology/approach 6-Nitro-4-hydroxycoumarin was prepared by reacting 4-hydroxycoumarin with potassium nitrate in concentrated sulfuric acid while cooling the mixture in an ice bath. The synthesized compound was subsequently employed in azo-coupling reactions with the diazonium salts of aniline and two o, p-disubstituted anilines to produce three corresponding azo dyes. The molecular structures of the resulting azo compounds were elucidated through mass spectrometry. Findings Fourier Transform Infrared (FT-IR), Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy confirmed the presence of two distinct azo-enol and hydrazone-keto isomers of the proposed tautomeric forms, both in the solid state and in solution. The UV-vis absorption spectra of the dyes remained largely unaffected by solvent changes, likely due to intramolecular hydrogen bonding within their molecular structures. The dyes were also subjected to density functional theory (DFT) quantum mechanical calculations to evaluate the stability of the proposed azo and hydrazone structures and compare them with experimental data. The results were consistent with the experimental results, validating the stability of the hydrazone isomeric products. Originality/value The synthesized 3-disubstitutedarylazo-4-hydroxy-6-nitro-coumarin dyes are new members of the 4-hydroxycoumarin-based azo dyes family, with very few details regarding the synthesis of such dyes reported in the literature before. They are unique in terms of synthesis, spectral properties and density functional theory calculation (DFT) studies.