Eco-safe novel biphenyl-dimedone azo dye: synthesis, dyeing properties, computational insights, and ecotoxicological evaluation.

The sustainable design of textile dyes is essential to reduce environmental risks associated with conventional azo colorants. In this study, a novel biphenyl-dimedone-based azo dye, 2,2'-([1,1'-biphenyl]-4,4'-diylbis(diazene-2,1-diyl))bis(3-hydroxy-5,5-dimethylcyclohex-2-en-1-one) (BD), was synthesized and systematically evaluated for structural, dyeing, and ecotoxicological properties. Comprehensive characterization using FTIR, NMR, and UV-Vis spectroscopy, supported by density functional theory (DFT) calculations, confirmed the predominance of the enol tautomer, which exhibited a lower HOMO-LUMO gap (3.326 eV) than the keto form (3.671 eV), indicating enhanced electronic delocalization and fiber affinity. Cotton dyeing trials revealed that bio-mordanting with Allium cepa L. peel extract yielded superior color strength (K/S = 4.599) and fastness compared with alum mordanting, highlighting the efficacy of sustainable mordanting strategies. Ecotoxicological assessments demonstrated complete antibacterial inactivity against both Gram-positive and Gram-negative strains, minimizing risks of antimicrobial resistance, while phytotoxicity assays with Triticum aestivum showed 100% germination and only mild, reversible seedling effects, contrasting sharply with the severe phytotoxicity of conventional azo dyes such as Congo red. Collectively, these results establish BD dye as an environmentally benign, high-performance colorant and provide a design strategy for developing sustainable textile dyes that integrate strong dye-fiber interactions with ecological safety.