TRIPLET-STATE REACTIONS OF HYPERICIN - TIME-RESOLVED LASER PHOTOLYSIS AND ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY

Hypericin (Hyp), a naphthodiantrone derivative carrying hydroxyl and methyl substituents, lately attracted much attention because of its possible use a therapeutic agent-especially against retroviral-induced diseases. Here, the excited triplet state of the molecule, 3Hyp, is being studied by laser photolysis and by EPR spectroscopy both in ethanol and in a nematic liquid crystal. It was found that, in addition to its first-order decay (k1 = 780 s-1) 3Hyp loses its excitation energy through quenching by ground-state Hyp (k2 = 3.5 x 10(7) M-1 s-1), by triplet-triplet annihilation (k3 = 2 X 10(9) M-1 s-1), by energy transfer to another Hyp molecule, by electron transfer to a suitable electron acceptor (duroquinone, k(ET) = 4.5 X 10(9) M-1 s-1), and by energy transfer to an aromatic porphyrinoid acceptor (porphycene, k(EnT) = 2.7 X 10(9) M-1 s-1). The rise and decay of 3Hyp as well as its quenching by various processes were followed by laser photolysis. The energy transfer was studied by laser photolysis, and the electron transfer was followed both by laser photolysis and by EPR. In addition, the EPR measurements enabled us to establish the orientation of 3Hyp, as a guest, in the liquid crystal host, where it was found to exist in the monomeric form. Analysis of magnetophotoselection EPR experiments in ethanol, in which Hyp is illuminated with laser light polarized in parallel and perpendicularly to the magnetic field, showed that at low concentrations (10(-4) M) the molecule in its monomeric form has its optical transition moment in the molecular plane. At higher concentrations (3.5 x 10(-3) M), a dimer is formed. The angles between the planes of the dimers were estimated to be 25-degrees.