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Hypericin is one of the most efficient photosensitizers used in photodynamic tumor therapy (PDT). The reported treatments of this drug reach from antidepressive, antineoplastic, antitumor and antiviral activity. We show that hypericin can be optically detected down to a single molecule at ambient conditions. Hypericin can even be observed inside of a cancer cell, which implies that this drug can be directly used for advanced microscopy techniques (PALM, spt-PALM, or FLIM). Its photostability is large enough to obtain single molecule fluorescence, surface enhanced Raman spectra (SERS), fluorescence lifetime, antibunching, and blinking dynamics. Sudden spectral changes can be associated with a reorientation of the molecule on the particle surface. These properties of hypericin are very sensitive to the local environment. Comparison of DFT calculations with SERS spectra show that both the neutral and deprotonated form of hypericin can be observed on the single molecule and ensemble level.
Direct observation of structural heterogeneity and tautomerization of single hypericin molecules
(2021)
Tautomerization is a fundamental chemical reaction which involves the relocation of a proton in the reactants. Studying the optical properties of tautomeric species is challenging because of ensemble averaging. Many molecules, such as porphines, porphycenes, or phenanthroperylene quinones, exhibit a reorientation of the transition dipole moment (TDM) during tautomerization, which can be directly observed in single-molecule experiments. Here, we study single hypericin molecules, which is a prominent phenanthroperylene quinone showing antiviral, antidepressive, and photodynamical properties. Observing abrupt flipping of the image pattern combined with time-dependent density functional theory calculations allows drawing conclusions about the coexistence of four tautomers and their conversion path. This approach allows the unambiguous assignment of a TDM orientation to a specific tautomer and enables the determination of the chemical structure in situ. Our approach can be applied to other molecules showing TDM reorientation during tautomerization, helping to gain a deeper understanding of this important process.