DEPROTONATION AND PROTONATION OF HYDROXYPHENANTHROPERYLENES

Ground and excited state deprotonation and protonation pK(a) values of hydroxyanthraquinones, hydroxyanthrones, hydroxyphenanthroperylenes, and the natural pigments hypericin and pseudohypericin were determined by means of spectrophotometric titrations and Forster cycle calculations. It was concluded that there is a strong intramolecular excited state proton transfer in the hydroxyanthraquinones and hydroxyanthrones due to a reversion of acidity and basicity of the hydroxyl and carbonyl groups in the excited state. However, in the hydroxyphenanthroperylene and the natural pigment excited states the order of basicity and acidity of these two functional groups remain unchanged. The site of deprotonation in hypericin and pseudohypericin was deduced by comparison between the pK(a) values of suited model compounds and these pigments to be the hydroxyl group in position 3 or 4, respectively.