Synthesis and characterization of cationic monoazo dyes incorporating quaternary ammonium salts

Nine (three series) antimicrobial cationic monoazo dyes were prepared by a diazotization–coupling reaction involving two aromatic amino compounds (featuring different chain length alkyl quaternary ammonium salts) as diazo components, and N,N-dimethyl-benzeneamine and 1-phenyl-3-methyl-5-pyrazolone as coupling components. Structures of the nine dyes were determined by FTIR, 1H NMR, 13C NMR, and ESI-MS analysis, and have not been previously published. Wavelengths of maximum absorption (λmax) and absorbance of three series dyes were tested by UV/vis spectrometer. It was found that alkyl chain length did not significantly affect λmax, but did affect absorbance. Minimum inhibitory concentration (MIC) data showed the efficacy of these dyes against both Staphylococcus aureus and Escherichia coli. Nine dyes were especially sensitive to S. aureus. The aqueous solution of dye series 1 with N,N-dimethyl-benzeneamine as the coupling component had better antimicrobial efficacy than did the aqueous solution of dye series 2 and 3 with 1-phenyl-3-methyl-5-pyrazolone as coupling component. Antimicrobial effect increased as hydrocarbon chain length increased, with other factors held constant.