Quinone-Quinoneimine Tautomerism of 5, 8-Bis(donor) substituted 1, 4-Naphthoquinonoid Dyes

Quinone-quinoneimine tautomerism of 5, 8-bis(donor) substituted 1, 4-naphthoquinones have been investigated by means of their visible absorption spectra. Tautomerism was found to be influenced mainly by the nature of the substituents and the polarity of the solvents; the equilibrium shifted to the quinone form in the more polar solvents. The quinone tautomer absorbed visible light at much longer wavelengths (about 160–180 nm) than did the quinoneimine tautomer. Isosbestic points of the tautomerism were observed. The calculated absorption maximum of each tautomer, obtained by means of the PPP MO method, was correlated with the observed values to predict the principal tautomer in solution.