Characterizing structural distortions in the metastable spin states of d4–d7 transition metal ion complexes is crucial to understand the nature of their bistability and eventually control their switching dynamics. In particular, the impact of the Jahn–Teller effect needs to be assessed for any electronic configuration that could be effectively degenerate, as in e.g. the high-spin (HS) manifold of highly symmetric homoleptic FeII complexes. However, capturing its manifestations remains challenging since crystallization generally alters the molecular conformations and their interconversion. With the rapid progress of ultrafast X-ray absorption spectroscopy, it is now possible to collect data with unprecedented signal-to-noise ratio, opening up for detailed structural characterization of transient species in the homogeneous solution phase. By combining the analysis of picosecond X-ray absorption spectra with DFT simulations, the structure of the photoinduced HS state is elucidated for solvated [Fe(terpy)2]2+ (terpy = 2,2′:6′,2″-terpyridine). This species can be viewed as the average 5B structure in D2 symmetry that originates from a dynamic Jahn–Teller effect in the HS manifold. These results evidence the active role played by this particular instance of vibronic coupling in the formation of the HS state for this benchmark molecule. Ultimately, correlating the interplay between intramolecular and intermolecular degrees of freedom to conformational strain and distortions in real time should contribute to the development of advanced functionalities in transition metal ion complexes.
  • Probing the Anisotropic Distortion of Photoexcited Spin Crossover Complexes with Picosecond X‑ray Absorption Spectroscopy
    S.E. Canton, X. Zhang, L.M. Lawson Daku, A.L. Smeigh, J. Zhang, Y. Liu, C.-J. Wallentin, K. Attenkofer, G. Jennings, C.A. Kurtz, D. Gosztola, K. Wrnmark, A. Hauser and V. Sundstrm
    Journal of Physical Chemistry C, 118 (8) (2014), p4536-4545
    DOI:10.1021/jp5003963 | unige:37983 | Abstract | Article HTML | Article PDF
 
For numerous spin crossover complexes, the anisotropic distortion of the first coordination shell around the transition metal center governs the dynamics of the high-spin/lowspin interconversion. However, this structural parameter remains elusive for samples that cannot be investigated with crystallography. The present work demonstrates how picosecond X-ray absorption spectroscopy is able to capture this specifi c deformation in the photoinduced high-spin state of solvated [Fe(terpy)2 ]2+ , a complex which belongs to the prominent family of spin crossover building blocks with nonequivalent metal ligand bonds. The correlated changes in FeNAxial , Fe NDistal , and bite angle NDistal Fe NAxial  extracted from the measurements are in very good agreement with those predicted by DFT calculations in D2d  symmetry. The outlined methodology is generally applicable to the characterization of ultrafast nuclear rearrangements around metal centers in photoactive molecular complexes and nanomaterials, including those that do not display long-range order.

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