Proton Pachinko: Probing Excited-State Intramolecular Proton Transfer of St. John's Wort-Derived Fluorescent Photosensitizer Hypericin with Ultrafast Spectroscopy.

Hypericin from St. John's wort has been used as a potent photosensitizer, but its working mechanism remains elusive which hinders its rational design for improved functionality. We implement ultrafast spectroscopy and quantum calculations to track the excited-state dynamics in an intricate hydrogen-bonding network of hypericin in solution. Using femtosecond transient absorption (fs-TA), we track excited state intramolecular proton transfer (ESIPT) via a previously unreported blueshift of a long-wavelength stimulated emission (SE) band with excitation-dependent dynamics in various solvents, owing to the dominant Q7,14 tautomer that undergoes bidirectional ESIPT. This finding is corroborated by ground-state femtosecond stimulated Raman spectroscopy (GS-FSRS) and density functional theory (DFT) calculations. Moreover, contrasting the neutral and anionic forms of hypericin enables us to reveal an intramolecular charge transfer step underlying ESIPT. We demonstrate UV and visible excitations as an integral platform to provide direct insights into the photophysics and origin for phototoxicity of hypericin. Such mechanistic insights into the excited state of hypericin will power its future development and use.