Synthesis, Spectroscopic Characterization, Solvent Effect, and Acid–Base Indicator Properties of Novel Niclosamide-Derived Azo Dyes

Abstract

Three new azo dyes (1–3) were effectively synthesized by use of a route starting from the anthelmintic drug niclosamide, with reduction of the nitro group followed by diazotization and coupling reaction with 1-naphthol, 2-naphthol, and 2-hydroxybenzoic acid, respectively. FT-IR, ^1H NMR, mass, and UV-visible spectroscopy confirmed the chemical structures of the dyes. The dyes showed maximum absorption wavelengths around 330 nm. It was found that the dye’s absorption spectrum is influenced by the solvent polarity significantly. Systematic studies of their acid–base properties over the pH range 2–12 showed spectral shifts in the absorption maxima and isosbestic points associated with protonation/deprotonation. Protonation/ionization constants of the studied compounds were estimated by the half-height method; thereby, the acid–base behavior of the compounds was established. It should be noted that the synthesized dyes were stable in solution for 24 hours and changed their color reversibly depending on the acidity of the medium. Titration of the solutions of these compounds in acidic/alkaline media with HCl and NaOH solutions was performed. It was shown that their color change is appropriate for an acid–base indicator, and it was similar to that of phenolphthalein. These findings demonstrate that the synthesized niclosamide-derived azo dyes are stable, readily prepared, and potentially useful as novel acid–base indicators for analytical applications.