Photooxidation of sodium sulfide and sodium thiosulfate under irradiation with visible light catalyzed by water soluble polynuclear phthalocyanine complexes

Abstract

The catalytic and photocatalytic activities of polynuclear water soluble phthalocyanine complexes, [CoPc(COOH) n ] poly, [ZnPc(COOH) n ] poly, [AlPc(COOH) n ] poly, and [Pc(COOH) n ] poly, have been investigated in the oxidation of Na 2S and Na 2S 2O 3. The studied complexes represent two-dimensional polymers, formed of condensed phthalocyanine macrocycles. The average number of the phthalocyanine units in them is, respectively: [CoPc(COOH) n ] poly≈4, [ZnPc(COOH) n ] poly≈4, [AlPc(COOH) n ] poly≈3, [Pc(COOH) n ] poly≈4. No visible bathochromic 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 low π–π electron delocalization in excited state between the phthalocyanine units of the polynuclear complexes. The catalytic and photocatalytic activity per mole of the polynuclear complexes is 2–5 times higher than that of the respective mononuclear phthalocyanine complexes. In the dark or upon irradiation with visible light the cobalt(II) phthalocyanine complexes catalyze only the oxidation of Na 2S, which is proceeding incompletely to thiosulfate. The zinc, aluminum and non-metal polynuclear phthalocyanine complexes are active catalysts only in the case of irradiation with visible light. The photooxidation of substrates like Na 2S and Na 2S 2O 3 with the previously mentioned photosensitizers is proceeding completely, whereupon the valence state of the sulfur in the final products is S 6+. In this case the photooxidation process involves singlet oxygen. The high photocatalytic activity of the polymers might be explained by the proceeding of a intra-molecular triplet–triplet energy transfer between the phthalocyanine macrocycles in accordance with the mechanism of Dexter.