Aryl and heteroaryl azo dyes derived from 6, 8‐dichloro‐4‐hydroxyquinolin‐2 (1H)‐one: synthesis,characterisation, solvatochromism and spectroscopic properties

In this research study, 6, 8‐dichloro‐4‐hydroxyquinolin‐2(1H)‐one was prepared by the thermal cyclocondensation of 2‐(2, 4‐dichlorophenylcarbamoyl) acetic acid or N, N?‐bis(2,4‐dichlorophenyl)malonamide at 140‐150°C in polyphosphoric acid, resulting in a yield of 48%. This compound was then coupled with a series of diazonium salts derived from aromatic and heteroaromatic amines for synthesis of the corresponding azo dyes. The structures of the compounds were confirmed using elemental analysis as well as ultraviolet‐visible, Fourier Transform‐infrared and proton nuclear magnetic resonance spectroscopy. The effects of organic solvents with different polarities, pH values and substituents of the diazotising components on the maximum absorption wavelength of the colorants were discussed and evaluated in detail. The acidity constants (pK_a) of the dyes were also determined using the spectrophotometric method in an ethanol‐water mixture (80:20, v/v) at 20‐23°C.