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This paper aims to synthesize 6-ethyl-4-hydroxyquinolin-2(1H)-one as a new enol-type coupling component in the preparation of some 3-arylazo-4-hydroxyquinolin-2(1H)-one dyes and evaluate the solvent effects on their absorption in ultraviolet-visible spectra.

6-Ethyl-4-hydroxyquinolin-2(1H)-one was synthesized by thermal cyclocondensation reaction of N, N′-bis(4-ethylphenyl) malonamide at 130–140°C in polyphosphoric acid. This compound was then applied in the azo-coupling reaction with some aniline-based diazonium salts, so as to prepare seven new mono-heterocyclic azo dyes. The structures of the compounds were confirmed using mass spectroscopic technique. Fourier transform infra red (FT-IR) and 1H proton nuclear magnetic resonance (1H NMR) and carbon-13 nuclear magnetic resonance (13 C NMR) studies on the structure of the azo compounds revealed that they exist as two E- and Z-isomers of hydrazone tautomer both in solid and solution state. The effects of acid and base on the visible absorption spectra of the dyes were also evaluated and discussed.

Ultra violet-visible UV-vis absorption spectra of the dyes didn’t show significant variation by changing of solvents because of intramolecular H-bonding between proposed hydrazone forms and 2- and 4-keto functions in their structures. The spectra of the dyes were not sensitive to the addition of acid but were very sensitive to base.

The synthesized 3-arylazo-4-hydroxyquinolin-2(1H)-one dyes are new members in the 4-hydroxyquinolin-2(1H)-one azo dyes family, where very few details regarding the synthesis of such dyes are reported before in the literature. They are unique in terms of synthesis and spectral properties.

The purpose of this study, 3-aminopyridine, 8-aminoquinoline and some new synthesized 2-aminobenzothiazoles were diazotized with nitrosyl sulfuric acid and subsequently coupled with 5-chloro-8-hydroxy quinoline to synthesize the corresponding heteroarylazo dyes 6–13.

The structures of dyes were characterized by mass, Fourier transform infra red, ¹H proton nuclear magnetic resonance and ultra violet-visible spectroscopic techniques. Absorption spectra of the dyes were measured in acetic acid, ethanol, chloroform, acetonitrile, dimethyl formamide and dimethyl sulfoxide and correlated with the nature of the solvents and substituents. The effects of varying pH on the absorption wavelengths of the azo dyes were also studied. In addition, the acidity constants (pKa) of the dyes were determined using the spectrophotometric method in an ethanol-water mixture (80:20, v/v) at 20–23°C. Besides, density functional theory (DFT) calculations were carried out to compare the energies of proposed azo and hydrazone tautomers of the dyes.

The results showed that the withdrawing chloro groups on the diazo moiety have significant influence (red shift) on the electron absorption spectra of these dyes. In addition, introducing electron withdrawing chloro groups into the benzothiazoles moiety increased the acidic character of dyes.

The synthesized 7-hetroarylazo-5-chloro-8-hydroxy quinoline dyes are new members in the 8-hydroxyquinoline azo dyes family, where very few details regarding the synthesis of such dyes are reported before in the literature. They are unique in terms of synthesis, spectral properties and DFT calculations.