J. Photochem. Photobiol. B: BioZ., 20 (1993) 133-137 133 Lowest excited triplet states of hypericin and isohypericin A. Angerhofer 3. Physikalisches Institut, Universitiit Stuttgart, pfaffenwaldring 57, D-70550 Stuttgart (Germany) H. Falk, J. Meyer and G. Schoppel Johannes Kepler Universitiit Linz, Znstimt fiir Chemie, Altenbergerstr. 69, A-4040 Linz (Au&a) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR (Received February 26, 1993; accepted June 1, 1993) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Abstract 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 O-O transition, as expected. The O-O vibronic lines of the phosphorescence are found at 13190 and 13622 cm-‘, 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. Key words: Hypericin; Isohypericin; Triplet state; Phosphorescence; Fluorescence 1. Introduction Hypericin (1) is a natural pigment which is found in plants, e.g. Hypericum pe$oratum (St. John’s Wort) from which it derives its name, the protozoa Blephatima and Stentor, Fagopyrum es- culentum (buckwheat) and certain Australian in- sects [l-7]. The richest source of hypericin is the genus Hypericum [S]. It is known to induce hy- pericism, a severe state of skin photosensitivity in animals feeding on hypericin-containing plants [l, 71. This is due to its strong photodynamic action, i.e. the sensitization of singlet oxygen by the excited triplet state of the pigment [7, 9, lo]. It is thus an interesting candidate for photodynamic therapy. Recently, it has been found that hypericin also has a photodynamic effect on retro viruses to which the human immune deficiency virus HIV belongs [ll-131. Due to the efficient singlet oxygen sensitization by triplet hypericin, the molecule can be used to study the photoprotective role of carotenoids in photosynthesis. Carotenoids are known to quench singlet oxygen in organic solvents (see refs. 14-16 and references cited therein). The chemical synthesis of hypericin and some of its derivatives has been reported by Brockmann and coworkers [17-191, Rodewald et al. [20] and Falk et al. [21-231. It may also be conveniently isolated from plant material [24]. Isohypericin (2) was also suspected to occur in natural materials, but was only recently prepared by total synthesis [22]. The photophysical characteristics of hypericin have been studied in depth by Gautron and co- workers [9, 10, 25, 261. They measured an inter- system crossing yield from the first excited singlet into the triplet state of 0.71 f0.04. The energy of the triplet state was estimated from triplet transfer experiments to be 13700 cm-’ [25]. The triplet decay rate was measured from ground state bleach- ing experiments to be about 1.1 ms at room temperature and from phosphorescence decay at 77 K‘to lie between 1 and 10 ms [25, 261. The maximum of the very weak phosphorescence (quan- OH 0 OH +i$ “@OH OH 0 OH OH 0 OH I 2 loll-1344/93/$6.00 0 1993 - Elsevier Sequoia. All rights reserved