Reactivity of the photo excited forms of Hypericin, Hypocrellin A, Hypocrellin B and methylated Hypericin towards molecular oxygen: The role of charge transfer interaction

Abstract

This work concerns the comparison of four structurally close photosensitizers (PSS) towards their abilities to react with molecular oxygen. Indeed, Hypocrellin’s A (HAH 2) and B (HBH 2) and Hypericin’s (HypH 2) (native and permethylated forms) present interesting medicinal properties which can be due to their abilities to produce singlet oxygen (type II) or in parallel of superoxide anion (type I). Both mechanism implies reaction between the triplet state of the photosensitizer and O 2, meanwhile these processes are quiet different since only the second one involved electron exchange (redox process). In order to identify the nature of the interaction between these two species, the photosensitizers have been characterized by means of photophysic and electrochemical investigation. Singlet oxygen and triplet yields have been determined in organic medium, in both neutral and basic condition. For Hypocrellin’s the yields of singlet oxygen and triplet are almost quantitative whereas for Hypericin these values are lower. The discrepancy towards these yield values between the studied photosensitizers does not appear to be thermodynamic ruled. Indeed, although the excited form of Hypericin’s present reducing properties towards molecular oxygen (whereas such redox process is not thermodynamically favored), the electron transfer step has not been displayed since transient absorption experiments at short time do not allow the observation of radical species of Hypericin’s. The very poor efficiency of O 2 − generation seems to be linked with the very slow kinetic of electron transfer between the excited form of the photosensitizer and molecular oxygen. As a result, high efficiency of O 2 − production should be obtained for the four studied photosensitizers in the presence of additional electron donor.