AZO-QUINONEHYDRAZONE TAUTOMERISM IN 2-PHENYLAZO-1-NAPHTHOL

The tautomeric equilibrium of 2-phenylazo-1-naphthol in 13 different solvents is analysed using a previously developed quantitative approach. The spectral characteristics of both tautomeric forms and the tautomeric constants K-T are determined and compared with those of the isomers 1-phenylazo-4-naphthol and 1-phenylazo-2-naphthol. The long wavelength band at about 500 nm, deter mining the colour, corresponds to the hydrazone form and consists of three vibrational sub-bands. The low sensitivity towards changes in solvent polarity and the highest content of hydrazone form within the three isomeric phenylazonaphthols explains the wide use of dyes containing a 2-phenylazo-1-naphthol moiety. PPP-MO calculations were in good agreement with the experimental spectral data for both tautomeric forms. The redistribution of the pi-electronic density during the S-0 --> S-1 transition is calculated for both tautomeric forms, and its influence on substituent effects, dye-solvent interaction and colour strength is discussed.