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1. Excited State Intramolecular Proton Transfer with Nuclear-Electronic Orbital Ehrenfest Dynamics

   https://doi.org/10.1021/acs.jpclett.1c00564  

   Zhao, Luning; Wildman, Andrew; Pavošević, Fabijan; Tully, John C.; Hammes-Schiffer, Sharon; Li, Xiaosong (April 2021, The Journal of Physical Chemistry Letters)

   null (Ed.)
2. Characterizing excited-state intramolecular proton transfer in 3-hydroxyflavone with ultrafast transient infrared spectroscopy

   https://doi.org/10.1039/d4cc03427a  

   Winkler, Valerie S; Fournier, Joseph A (October 2024, Chemical Communications)

   Excited-state intramolecular proton transfer dynamics are characterized in 3-hydroxyflavone derivatives using ultrafast transient infrared spectroscopy. 

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3. Chemical bond reorganization in intramolecular proton transfer revealed by ultrafast X-ray photoelectron spectroscopy

   https://doi.org/10.1073/pnas.2321343121  

   Gu, Yonghao; Yong, Haiwang; Gu, Bing; Mukamel, Shaul (April 2024, Proceedings of the National Academy of Sciences)

   Time-resolved X-ray photoelectron spectroscopy (TR-XPS) is used in a simulation study to monitor the excited state intramolecular proton transfer between oxygen and nitrogen atoms in 2-(iminomethyl)phenol. Real-time monitoring of the chemical bond breaking and forming processes is obtained through the time evolution of excited-state chemical shifts. By employing individual atomic probes of the proton donor and acceptor atoms, we predict distinct signals with opposite chemical shifts of the donor and acceptor groups during proton transfer. Details of the ultrafast bond breaking and forming dynamics are revealed by extending the classical electron spectroscopy chemical analysis to real time. Through a comparison with simulated time-resolved photoelectron spectroscopy at the valence level, the distinct advantage of TR-XPS is demonstrated thanks to its atom specificity. 

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

   https://doi.org/10.1002/chem.202500639  

   Johnson, Seth_L; Krueger, Taylor_D; Solaris, Janak; Chen, Cheng; Fang, Chong (March 2025, Chemistry – A European Journal)

   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 Q^7,14tautomer 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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5. Photogrammetry of Ultrafast Excited-State Intramolecular Proton Transfer Pathways in the Fungal Pigment Draconin Red

   https://doi.org/10.3390/molecules28083506  

   Solaris, Janak; Krueger, Taylor D.; Chen, Cheng; Fang, Chong (April 2023, Molecules)

   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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