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.
The authors declare no competing financial interests.
Extended data figures and tables
a, The calculated Kβ fluorescence spectra of iron complexes: triplet Fe(ii) in square planar crystal field (red) (calculation parameters based on Fe(ii)phthalocyanine), and triplet excited state in an octahedral crystal field (blue) (calculation parameters based on [Fe(2,2′-bipyridine)3]2+). b, The experimental Kβ fluorescence difference spectrum (red) obtained by subtracting the singlet [Fe(2,2′-bipyridine)3]2+ spectrum from the triplet Fe(ii)phthalocyanine spectrum, and the calculated Kβ fluorescence difference spectrum (blue) generated by subtracting the spectrum of the singlet state in an octahedral crystal field from the triplet state in a square planar crystal field.
Extended Data Figure 2 Time-dependent Kβ fluorescence spectra and fit using the sequential kinetic model with a triplet transient.
a, Experimental transient fluorescent amplitude difference spectra plotted with arbitrary units, and b, fit using the sequential kinetic model with a triplet transient. c, Residuals for the best fit, with the colour-scale maximum and minimum set to one-fifth of the value used in a and b. d, The excited state populations extracted from the best fit.
Extended Data Figure 3 Time-dependent Kβ fluorescence spectra and fit using the direct kinetic model without a triplet transient.
a, Experimental transient fluorescent amplitude difference spectra plotted with arbitrary units, and b, fit using the direct kinetic model without a triplet transient. c, Residuals for the best fit with the colour scale maximum and minimum set to one-fifth of the value used in a and b. d, The excited state populations extracted from the best fit.
Extended Data Figure 4 The 50 fs time delay normalized Kβ fluorescent amplitude difference spectrum (ΔI) and kinetic model fit plotted as a function of X-ray emission energy.
The measured data (black circles and line), along with the best global fit from the sequential kinetic model with a transient triplet state (red line).
a, The ultraviolet–visible absorption spectrum of [Fe(2,2′-bipyridine)3]2+ in water. b, Power (fluence) dependence of the change in probe transmission measured at 520 nm, following excitation of an aqueous solution of [Fe(2,2′-bipyridine)3]Cl2 with a 520 nm pump pulse. The figure shows the change in transmission (ΔT) measured at a 10 ps time delay, a time long compared to the spin crossover and vibrational cooling timescales, but short compared to the lifetime of the high-spin excited state.
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Zhang, W., Alonso-Mori, R., Bergmann, U. et al. Tracking excited-state charge and spin dynamics in iron coordination complexes. Nature 509, 345–348 (2014). https://doi.org/10.1038/nature13252
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