OBSERVATION OF EXCITED-STATE TAUTOMERIZATION IN THE ANTIVIRAL AGENT HYPERICIN AND IDENTIFICATION OF ITS FLUORESCENT SPECIES

The absorption spectra, fluorescence spectra, and fluorescence lifetim es of hypericin, an analog lacking hydroxyl groups, mesonaphthobianthr one, and hexamethylhypericin are obtained in aprotic and protic solven ts. In aprotic solvents, mesonaphtobianthrone is nonfluorescent. In st rong acids such as sulfuric or triflic acids, it becomes fluorescent. Furthermore, its spectrum is very similar to that of hypericin. Simila rly, only in sulfuric acid does hexamethylhypericin afford absorption and emission spectra resembling those of hypericin. We therefore concl ude that the fluorescent species of hypericin has one or both of its c arbonyl groups protonated. The protonation equilibrium in both the gro und and the excited state is discussed. The first detailed measurement s of the primary processes in the antiviral agent, hypericin, are perf ormed with picosecond resolution and a white-light continuum. Transien t absorption measurements of hypericin with similar to 1-ps resolution indicate that upon optical excitation a new species is created that a bsorbs in the range of roughly 580-640 nm. This species exhibits a 6-1 2-ps decay, depending on the solvent. It is also observed that the sti mulated emission signal, which arises from the fluorescent state, grow s in with a time constant of 6-12 ps. Based upon the identification of the fluorescent species as hypericin with one or both carbonyl groups protonated, the rise time for the appearance of the stimulated emissi on signal is attributed to excited-state tautomerization.