  Hypericin can be found in nature in Hypericum perforatum (St. John's Wort)
and has become subject of intense biochemical research. Studies report of
antidepressive, antineoplastic, antitumor and antiviral activity of hypericin.
Among the variety of potential applications hypericin can be used as
photosensitizer in photodynamic therapy (PDT), where it is brought into cancer
cells and produces singlet oxygen upon irradiation with a suitable light
source. Therefore, the photophysical properties of hypericin are crucial for a
successful application in a medical treatment. Here, we present the first
single molecule optical spectroscopy study of hypericin. Its photostability is
large enough to obtain single molecule fluorescence, surface enhanced Raman
spectra (SERS), fluorescence lifetime, antibunching and blinking dynamics.
Embedding hypericin in a PVA matrix changes the blinking dynamics, reduces the
fluorescence lifetime and increases the photostability. Single molecule SERS
spectra show both the neutral and deprotonated form of hypericin and exhibit
sudden spectral changes, which can be associated with a reorientation of the
single molecule with respect to the surface.
