Spectroscopic studies of Solophenyl red 3BL polyazo dye tautomerism in different solvents using UV–visible, 1H NMR and steady-state fluorescence techniques

Azo–hydrazone tautomerism of Solophenyl red 3BL (C.I. Direct 80) polyazo dye in dimethylsulfoxide, methanol, propionic acid, formamide, dimethylformamide and water was investigated using UV–visible, 1H NMR and steady-state fluorescence techniques. The results showed that 3BL dye molecules exist as an equilibrium mixture of azo and hydrazone tautomer in all solvents, but azo form was dominant. In the UV–visible absorption spectra a broad intense band at 570 nm appeared which is related to azo (at 520 nm) and hydrazone (at 560 nm) tautomer bands and UV–visible band contour analyses further confirmed this statement. UV–visible absorption spectra of the two tautomric forms were completely distinguished in DMSO and DMF solvents while in water and propionic acid they were slightly distinguished. Furthermore, in methanol only azo tautomer was dominant. By increasing solvent polarity from water (1.82 D) to DMSO (3.96 D), the splitting of azo and hydrazone tautomers was increased. 1H NMR studies also confirmed the azo and hydrazone tautomerization in D2O and DMSO–d6 solvents. Finally, steady-state fluorescence spectroscopy results showed that 3BL dye molecules did not exhibit any appreciable light emission in acidic, neutral and basic aqueous solutions or in other solvents.