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Proton transfer processes of organic molecules are key to charge transport and photoprotection in biological systems. Among them, excited-state intramolecular proton transfer (ESIPT) reactions are characterized by quick and efficient charge transfer within a molecule, resulting in ultrafast proton motions. The ESIPT-facilitated interconversion between two tautomers (PS and PA) comprising the tree fungal pigment Draconin Red in solution was investigated using a combination of targeted femtosecond transient absorption (fs-TA) and excited-state femtosecond stimulated Raman spectroscopy (ES-FSRS) measurements. Transient intensity (population and polarizability) and frequency (structural and cooling) dynamics of –COH rocking and –C=C, –C=O stretching modes following directed stimulation of each tautomer elucidate the excitation-dependent relaxation pathways, particularly the bidirectional ESIPT progression out of the Franck–Condon region to the lower-lying excited state, of the intrinsically heterogeneous chromophore in dichloromethane solvent. A characteristic overall excited-state PS-to-PA transition on the picosecond timescale leads to a unique “W”-shaped excited-state Raman intensity pattern due to dynamic resonance enhancement with the Raman pump–probe pulse pair. The ability to utilize quantum mechanics calculations in conjunction with steady-state electronic absorption and emission spectra to induce disparate excited-state populations in an inhomogeneous mixture of similar tautomers has broad implications for the modeling of potential energy surfaces and delineation of reaction mechanisms in naturally occurring chromophores. Such fundamental insights afforded by in-depth analysis of ultrafast spectroscopic datasets are also beneficial for future development of sustainable materials and optoelectronics.
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This content will become publicly available on March 4, 2026
Proton Pachinko: Probing Excited‐State Intramolecular Proton Transfer of St. John's Wort‐Derived Fluorescent Photosensitizer Hypericin with Ultrafast Spectroscopy**
Abstract 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 Q7,14tautomer 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.
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- Award ID(s):
- 2003550
- PAR ID:
- 10639324
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 31
- Issue:
- 22
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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