Quenching of singlet photoexcited state of water soluble phthalocyanines and porphyrins by viologens interacting electrostatically

Quenching of photoexcited anionic and water-soluble phthalocyanines and 5,10,15,20-tetraarylporphyrins by viologens has been investigated. It was confirmed that the quenching of the singlet photoexcited state takes place, and that the mechanism is mostly a static one due to electrostatic interaction between the donor and the acceptor. The static mechanism was analyzed by curve-fitting of the relative emission intensity vs viologen concentration resulting in four kinds of mechanisms composed of static quenching accompanied partly by a dynamic one. The static mechanism was classified into two types: one mechanism is due to 1:n electrostatic interaction of the anionic sensitizer and the cationic acceptor, and other is a Perrin type for which the acceptor is incorporated into the quenching sphere around the sensitizer according to a Poisson distribution. The effect of micelles for the quenching was also studied including the effect of viologen with a long alkyl chain. The ionic micelles either incorporated or repulsed the ionic sensitizer and accepter resulting in either static quenching or prohibition of the quenching. Cationic phthalocyanines and porphyrins were also examined for the reaction with cationic viologens. Copyright (c) 2005 Society of Porphyrins & Phthalocyanines.