The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Although the relaxation from the photoexcited electronic state during the ring-opening has been investigated in numerous studies, the accompanying changes in atomic distance have not been resolved. Here we present a direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and subångström length scale using megaelectronvolt ultrafast electron diffraction. We followed the carbon–carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by the direct measurement of time-dependent changes in the distribution of interatomic distances. We observed a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to a steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the subpicosecond timescale.
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The data that support the findings of this study are available from the corresponding authors upon reasonable request.
The codes used for the analysis of the raw experimental and simulation data and for the generation of the manuscript figures are available from the corresponding authors upon reasonable request. TeraChem is a proprietary quantum chemistry software suite developed by T. Martínez and is available via the proper license set forth by © PetaChem, LLC.
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This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. The experimental part of this research was performed at the SLAC MeV UED facility, which is supported in part by the DOE BES SUF Division Accelerator & Detector R&D program, the Linac Coherent Light Source (LCLS) Facility, and SLAC under contract nos. DE-AC02-05-CH11231 and DE-AC02-76SF00515. M.G. is funded via a Lichtenberg Professorship of the Volkswagen Foundation. D.M.S. is grateful to the NSF for a graduate fellowship. J.P.F.N. acknowledges the support of the Wild Overseas Scholars Fund of the Department of Chemistry, University of York. K.W. and M.C. are supported by the US Department of Energy Office of Science, Basic Energy Sciences under award no. DE-SC0014170. P.M.W. is supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under award no. DE-SC0017995. A.K. is supported by the Carnegie Trust for the Universities of Scotland (grant ref. CRG050414) and an RSE/Scottish Government Sabbatical Research Grant (ref. 58507).
The authors declare no competing interests.
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Supplementary Discussion, Supplementary Table, Supplementary Figures 1–18, Detailed description of the Supplementary Movies.
1,3,5-hexatriene formation through the open-ring conical intersection
1,3-cyclohexadiene formation through the open-ring conical intersection
1,3-cyclohexadiene formation through the closed-ring conical intersection
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Wolf, T.J.A., Sanchez, D.M., Yang, J. et al. The photochemical ring-opening of 1,3-cyclohexadiene imaged by ultrafast electron diffraction. Nat. Chem. 11, 504–509 (2019). https://doi.org/10.1038/s41557-019-0252-7
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