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Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+

Nature Chemistry volume 7, pages 629–633 (2015)Cite this article

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Abstract

It is known that excitation by visible light of the singlet metal-to-ligand charge-transfer (1MLCT) states of Fe(II) complexes leads to population of the lowest-lying high-spin quintet state (5T) with unity quantum yield. Here we investigate this so-called spin crossover (SCO) transition in aqueous iron(II)tris(bipyridine). We use pump–probe transient absorption spectroscopy with a high time resolution of <60 fs in the ultraviolet probe range, in which the 5T state absorbs, and of <40 fs in the visible probe range, in which both the hot MLCT state and the 5T state absorb. Our results show that the 5T state is impulsively populated in less than 50 fs, which is the time we measured for the depopulation of the MCLT manifold. We propose that non-totally-symmetric modes mediate the process, possibly high-frequency modes of the bipyridine (bpy) ligand. These results show that even though the SCO process in Fe(II) complexes represents a strongly spin-forbidden (ΔS = 2) two-electron transition, spin flipping occurs at near subvibrational times and is intertwined with the electron and structural dynamics of the system.

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Figure 1: States and absorption spectra of aqueous [Fe(bpy)3]2+.
Figure 2: Ultraviolet TA results of aqueous [Fe(bpy)3]2+ on excitation at 580 nm, recorded with a time resolution of ≤60 fs.
Figure 3: Visible-range TA results of aqueous [Fe(bpy)3]2+ on excitation at 550 nm, recorded with a time resolution of ≤40 fs.
Figure 4: Summary of the photoinduced SCO in [Fe(bpy)3]2+ on photoexcitation into the MLCT band.

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Acknowledgements

This work was supported by the Swiss National Science Foundation via the National Centre of Competence in Research Molecular Ultrafast Science and Technology and by Research Equipment grant 206021_121300.

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  1. Ecole Polytechnique Fédérale de Lausanne, Laboratoire de Spectroscopie Ultrarapide, Faculté des Sciences de Base, ISIC, Lausanne, CH-1015, Switzerland

    Gerald Auböck & Majed Chergui

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  1. Gerald Auböck
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Contributions

M.C. conceived the experiments, G.A. designed and performed the experiments and analysed the data, and M.C. and G.A. co-wrote the paper.

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Correspondence to Majed Chergui.

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The authors declare no competing financial interests.

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Auböck, G., Chergui, M. Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+. Nature Chem 7, 629–633 (2015). https://doi.org/10.1038/nchem.2305

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