Lowest excited triplet states of hypericin and isohypericin.

The fluorescence and phosphorescence of hypericin and isohypericin were studied in an ethanol matrix at 1.2 K. The prompt fluorescence spectra are mirror images of the absorption around the 0-0 transition, as expected. The 0-0 vibronic lines of the phosphorescence are found at 13,190 and 13,622 cm-1, and the phosphorescence decay times are 2.79 and 6.6 ms at 1.2 K for hypericin and isohypericin respectively. The fluorescence excitation spectrum of isohypericin reveals a small trace of a different pigment, possibly a tautomeric form of isohypericin. The decay of the phosphorescence of isohypericin is biexponential. The slow component is attributed to an unknown contaminant with a high quantum yield of phosphorescence.