EXCITED-STATE PHOTOPHYSICS OF HYPERICIN AND ITS HEXAMETHOXY ANALOG - INTRAMOLECULAR PROTON-TRANSFER AS A NONRADIATIVE PROCESS IN HYPERICIN

The excited-state photophysics of the light induced antiviral agent, h ypericin, are compared with those of its methylated analog, hexamethox yhypericin. This comparison is instructive in understanding both the g round- and the excited-state properties of hypericin. That the hexamet hoxy analog has no labile protons that can be transferred, that it can not protonate its own carbonyl groups, that it has a reduced fluoresce nce quantum yield and lifetime with respect to hypericin, and that it exhibits no stimulated emission or, more specifically, rise time in st imulated emission completely support our emerging model of the hyperic in photophysics. The results are consistent with the presence of intra molecular excited-state proton transfer in hypericin but not in its me thylated analog.