Singlet oxygen production by hypericin in various solvents

The quantum yield of singlet oxygen production by hypericin was measured by means of an IR laser fluorimeter : Φ3 = 0.37, 0.36, 0.26, 0.17 , 0.23, 0.23 and < 0.02 inmethanol, ethanol, acetonitrile, acetone, dimethylformamide, tetrahydrofuran and D2O or H2O, respectively. The Φ3value decreases with the decrease of a the Taft's parameter describing the ability of solvent to form hydrogen bonds with a solute. It was explained by the formation of the weak intramolecular hydrogen bonds between CO groups and the nearest OH groups in hypericin, that should lead to the efficient radiationless deactivation of the [math] state by the mechanism of an intramolecular proton-transfer. In water hypericin exists in the form of aggregates and the low value of Φ3 can be explained by the low value of the triplet quantum yield for hypericin aggregates. The superoxide anion formation was not observed by pulsed conductivity method upon laser excitation of hypericin aggregates in D2O and H2O and hypericin monomers in other solvents investigated. Quantum yields of fluorescence are given