Analysis of the tautomeric equilibrium of two red monoazo dyes by UV–Visible, Raman and SERS spectroscopies

Acid Red 26 and 18 are two early synthetic dyes belonging to the monazo dye class. The molecular structure of this class is characterized by chromophoric azo group (NN) generally attached benzene or naphthalene derivatives containing electron withdrawing and/or donating groups as substituents. As both red have an OH in ortho- position respect group, they undergo azo-hydrazone tautomerism. In work, UV–Vis, Raman SERS spectroscopic analysis solutions were carried out at different pH conditions, order evaluate preponderance one tautomer over other a function pH. Different experimental conditions tested find best ones for detection dyes. Thus, spectra powder aqueous AR26 AR18 obtained natural solutions, above below that value. various values between 2 10, with excitation 442, 532 633 nm. theoretical tautomers calculated DFT methods. results used assignment vibrational modes. Finally, textile sample dyed was analyzed SERS.