Capturing Ring Opening in Photoexcited Enolic Acetylacetone upon Hydrogen Bond Dissociation by Ultrafast Electron Diffraction

Photoinduced biological and chemical reactions are often based on key structural transformations of a molecule driven across multiple electronic states. Acetylacetone (AcAc) is a prototypical system for complex chemical pathways involving several conical intersections (CI) and singlet-triplet intersystem crossings (ISC) characterized by distinct geometries. In the gas phase, AcAc is predominantly in a planar ring-like enolic form stabilized by a strong intramolecular O-H···O hydrogen bond. Following excitation into the S<sub>2</sub> (ππ*) state at 266 nm, acetylacetone undergoes rapid internal conversion followed by intersystem crossing. Such relaxation pathways are associated with structural changes including ring opening, deplanarization, and bond elongation. In this work, ultrafast electron diffraction (UED) at the SLAC MeV-UED setup is employed as a direct structural probe with a time resolution of 160 fs. Together with trajectory surface hopping simulations, analysis of the UED data provides a new perspective on the early time nuclear dynamics in acetylacetone. Specifically, AcAc is observed to undergo ring opening, deplanarization, and bond elongation all within the first 700 fs after photoexcitation. The monitored dynamics is associated mainly with the nuclear motion on the S<sub>1</sub> potential energy surface, formed after very rapid transfer from S<sub>2</sub> to S<sub>1</sub>, allowing AcAc to reach the conical intersection to intersystem crossing. Such time scales of nuclear motion are contrasted with the time scales of electronic transitions in AcAc that were previously characterized with spectroscopic methods, specifically internal conversion (<100 fs) and intersystem crossing (∼1.5 ps).

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Publication Details

Journal
The Journal of Physical Chemistry Letters
Published
2025-05-14
DOI
https://doi.org/10.1021/acs.jpclett.5c00285
Citations
3
Primary Topic
Advanced Chemical Physics Studies
Type
article
Field-Weighted Citation Impact
3.74

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article

Capturing Ring Opening in Photoexcited Enolic Acetylacetone upon Hydrogen Bond Dissociation by Ultrafast Electron Diffraction

Elio G. Champenois, Stephen Weathersby, Yen-Cheng Lin, Melanie Schnell et al.
3 citations
The Journal of Physical Chemistry Letters
Advanced Chemical Physics Studies
3.74
article

Capturing Ring Opening in Photoexcited Enolic Acetylacetone upon Hydrogen Bond Dissociation by Ultrafast Electron Diffraction

Elio G. Champenois, Stephen Weathersby, Yen-Cheng Lin, Melanie Schnell, Eric A. Haugen, Valeriu Scutelnic, Fuhao Ji, Yury V. Vishnevskiy, Sonia Marggi Poullain, Ming‐Fu Lin, Xiaozhe Shen, Duan Luo, Denis S. Tikhonov, Andrew Attar, Kristina F. Chang, James D. Gaynor, Stephen R. Leone, Ashley P. Fidler, Lou Barreau, Thomas Wolf, Xijie Wang, Jie Yang
article en
3 citations

Abstract

Photoinduced biological and chemical reactions are often based on key structural transformations of a molecule driven across multiple electronic states. Acetylacetone (AcAc) is a prototypical system for complex chemical pathways involving several conical intersections (CI) and singlet-triplet intersystem crossings (ISC) characterized by distinct geometries. In the gas phase, AcAc is predominantly in a planar ring-like enolic form stabilized by a strong intramolecular O-H···O hydrogen bond. Following excitation into the S<sub>2</sub> (ππ*) state at 266 nm, acetylacetone undergoes rapid internal conversion followed by intersystem crossing. Such relaxation pathways are associated with structural changes including ring opening, deplanarization, and bond elongation. In this work, ultrafast electron diffraction (UED) at the SLAC MeV-UED setup is employed as a direct structural probe with a time resolution of 160 fs. Together with trajectory surface hopping simulations, analysis of the UED data provides a new perspective on the early time nuclear dynamics in acetylacetone. Specifically, AcAc is observed to undergo ring opening, deplanarization, and bond elongation all within the first 700 fs after photoexcitation. The monitored dynamics is associated mainly with the nuclear motion on the S<sub>1</sub> potential energy surface, formed after very rapid transfer from S<sub>2</sub> to S<sub>1</sub>, allowing AcAc to reach the conical intersection to intersystem crossing. Such time scales of nuclear motion are contrasted with the time scales of electronic transitions in AcAc that were previously characterized with spectroscopic methods, specifically internal conversion (<100 fs) and intersystem crossing (∼1.5 ps).

The Journal of Physical Chemistry LettersVol. 16(20)
Universidad Complutense de Madrid (ES), United States Naval Research Laboratory (US), Lawrence Berkeley National Laboratory (US), TU Dortmund University (DE), Bielefeld University (DE), Deutsches Elektronen-Synchrotron DESY (DE), SLAC National Accelerator Laboratory (US), University of California System (US), Christian-Albrechts-Universität zu Kiel (DE), Health Research Alliance (US), University of Duisburg-Essen (DE)
Arnold and Mabel Beckman Foundation, Deutsche Forschungsgemeinschaft, Division of Chemistry, Basic Energy Sciences, HORIZON EUROPE Marie Sklodowska-Curie Actions
Openalex Percentile: Top 7%
Advanced Chemical Physics Studies
3.74
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