A series of Cu(II) complexes of azo dye ligands 4-(4-hydroxy-5-(aryldiazenyl)-2-thioxothiazol-3(2H)-yl)benzenesulfonamide (HL_n) were synthesized by the coupling of 3-sulfamoylphenylrhodanine with aniline and its p-derivatives. These ligands and their [Cu(L_n)(CH₃COO)(OH₂)]H₂O complexes were characterized by elemental analyses, IR, ¹H NMR, UV–Visible and magnetic measurements as well as thermal analysis and X-ray diffraction. The molar conductance measurements proved that all the complexes are non-electrolytes. IR spectra showed that the ligands (HL_n) acts as a monobasic bidentate ligand by coordinating via the nitrogen atom of the azo group (–N

N–) and oxygen atom of the deprotonated –OH group moiety, thereby forming a six-membered chelating ring and concomitant formation of an intramolecular hydrogen bond. Spectral studies revealed that the ligand exist in an internally hydrogen bonded azo-enol form rather than the keto-hydrazone form a coordinated to the metal ion in a bidentate fashion. Analytical data revealed that all the complexes exhibited 1:1 metal-ligand ratio. Based on the ESR and electronic spectral data, the bonding parameters of these complexes have been calculated. The thermodynamic parameters of the ligands (HL_n) and their Cu(II) complexes are calculated using Coats–Redfern and Horowitz-Metzger methods. The antimicrobial activities of ligands were tested against gram negative bacteria (Escherichia coli), gram positive bacteria (Staphylococcus aureus) and fungal (Candida albicans). The cytotoxic activity of ligands (HL_n) was tested against two human cancer HePG-2 (Hepatocellular carcinoma) and MCF-7 (breast cancer). The antioxidant activities of ligands (HL_n) were performed by ABTS method.