Photooxidation of phenols in aqueous solution, catalyzed by mononuclear and polynuclear metal phthalocyanine complexes

Abstract The photocatalytic oxidation of phenols by oxygen has been studied in alkaline aqueous medium upon irradiation with visible light. Metal Zn(II) and Al(III) mono- and polynuclear phthalocyanine complexes, soluble in water, have been used as photocatalysts. The studied polynuclear complexes represent two-dimensional polymers, formed of condensed phthalocyanine macrocycles. The number of the phthalocyanine units in them is 3 or 4. No visible batochromic shifting is observed in the Q-band electron transition ( λ =665–680 nm) of the polynuclear complexes with respect to their mononuclear analogues. This fact is an indication of a low π–π electron delocalization between the phthalocyanine units of the polynuclear complexes in the excited state. The photocatalytic activity per mole of the polynuclear complexes at pH=7 or 13 is much higher than that of the respective mononuclear phthalocyanine complexes. The high photocatalytic activity of the polymers might be explained by the proceeding of an intra-molecular triplet–triplet energy transfer between the phthalocyanine macrocycles in accordance with the mechanism of Dexter. The singlet oxygen, obtained by photon-induced energy transfer from the excited mono- and polynuclear photosensitizers, dominates the initial steps of photooxidation. The rate of photocatalytic oxidation of phenols depends on the pH of the medium and on the phthalocyanine complex aggregation degree. The products of the oxidation of phenols in alkaline medium are the same in the both cases of using mono- and polynuclear phthalocyanine complexes as catalysts. The formation of quinones is an intermediate step during the photocatalytic oxidation of phenols at pH=13 within the process of destruction of the substrate. Maleic and fumaric acids as well as carbon dioxide have been registered as final products of the photooxidation.