Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons1,2,3,4. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics5 and the flux limitations of ultrafast X-ray sources6,7. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2′-bipyridine)3]2+, where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy6,7,8,9,10,11,12,13,14,15. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

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We thank P. Frank, B. Lin and S. DeBeer for discussion, S. DeBeer for some model iron complex X-ray fluorescence spectra, and D. Stanbury for providing some iron complexes. Experiments were carried out at LCLS and SSRL, which are National User Facilities operated for DOE and OBES respectively by Stanford University. W.Z., R.W.H., H.W.L., D.A.M., Z.S. and K.J.G. acknowledge support from the AMOS programme within the Chemical Sciences, Geosciences and Biosciences Division of the Office of Basic Energy Sciences, Office of Science, US Department of Energy. E.I.S. acknowledges support from the NSF (CHE-0948211). R.G.H. acknowledges a Gerhard Casper Stanford Graduate Fellowship and the Achievements Rewards for College Scientists (ARCS) Foundation. T.K. acknowledges the German Research Foundation (DFG), grant KR3611/2-1. K.S.K., M.M.N. and T.B.v.D. acknowledge support from the Danish National Research Foundation and from DANSCATT. K.K. thanks the Volkswagen Foundation for support under the Peter Paul Ewald fellowship program (I/85832). G.V. acknowledges support from the European Research Council (ERC-StG-259709) and the Lendület Programme of the Hungarian Academy of Sciences. C.B., W.G. and A.G. thank the DFG (SFB925), as well as the European XFEL, for financial support.

Author information


  1. PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, USA

    • Wenkai Zhang
    • , Robert W. Hartsock
    • , Huiyang W. Liang
    • , Drew A. Meyer
    • , Carola Purser
    • , Zheng Sun
    •  & Kelly J. Gaffney
  2. LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • Roberto Alonso-Mori
    • , Uwe Bergmann
    • , Matthieu Chollet
    • , Henrik T. Lemke
    • , Joseph S. Robinson
    •  & Diling Zhu
  3. European XFEL, D-22761 Hamburg, Germany

    • Christian Bressler
    • , Andreas Galler
    •  & Wojciech Gawelda
  4. Department of Chemistry, Stanford University, Stanford, California 94305, USA

    • Ryan G. Hadt
    • , Robert W. Hartsock
    • , Thomas Kroll
    • , Huiyang W. Liang
    • , Drew A. Meyer
    •  & Edward I. Solomon
  5. Centre for Molecular Movies, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark

    • Kasper S. Kjær
  6. Centre for Molecular Movies, Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark

    • Kasper S. Kjær
    • , Martin M. Nielsen
    •  & Tim B. van Driel
  7. Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany

    • Katharina Kubiček
  8. Deutsches Elektronen Synchrotron, Notkestraße 85, 22607 Hamburg, Germany

    • Katharina Kubiček
  9. SSRL, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • Edward I. Solomon
    • , Dimosthenis Sokaras
    •  & Tsu-Chien Weng
  10. Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, Hungary

    • György Vankó


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W.Z., R.A.-M., U.B., R.W.H., D.A.M., T.-C.W. and K.J.G. designed the experiment. W.Z., R.A.-M., U.B., M.C., R.W.H., K.S.K., K.K., H.T.L., H.W.L., C.P., J.S.R., Z.S., D.S., T.B.v.D., T.-C.W., D.Z. and K.J.G. did the experiment. W.Z., T.K., K.S.K., T.B.v.D., G.V. and T.-C.W. analysed the data. W.Z., R.A.-M., U.B., C.B., W.G., A.G., R.G.H., R.W.H., T.K., K.S.K., K.K., D.A.M., M.M.N., E.I.S., D.S. and K.J.G. wrote the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Kelly J. Gaffney.

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