Quenching of Excited Triplet State Hypericin with Energy Acceptors and Donors and Acceptors of Electrons

The quenching of the triplet state of hypericin by acceptors of electronic energy and donors and acceptors of electrons was studied by nanosecond laser photolysis. The energy of triplet hypericin is E T = 13 350 cm -1, and for acceptors with E T < 13 300 cm -1 the observed energy transfer rate constant declines with increasing exothermicity. Quenching of triplet hypericin by amines in acetonitrile and electron acceptors in DMSO occurs via electron transfer. Spectroscopic and kinetic properties and the quantum yield of hypericin radical ion formation were established. The rate constant profiles for electron transfer (in both directions) are consistent with an excited-state reaction occurring before electron transfer. The rate constant of the interaction of reduced hypericin with O 2 was estimated to be 3.8 × 10 8 M -1 s -1 in acetonitrile.