• Mapping the Ultrafast Changes of Continuous Shape Measures in Photoexcited Spin Crossover Complexes without Long-Range Order
    S.E. Canton, X. Zhang, L.M. Lawson Daku, Y. Liu, J. Zhang and S. Alvarez
    The Journal of Physical Chemistry C, 119 (6) (2015), p3322-3330
    DOI:10.1021/jp5117189 | unige:46831 | Abstract | Article HTML | Article PDF
 
Establishing a tractable yet complete reaction coordinate for the spin-state interconversion in d4–d7 transition metal complexes is an integral aspect of controlling the dynamics that govern their functionality. For spin crossover phenomena, the limitations of a single-mode approximation that solely accounts for an isotropic increase in the metal–ligand bond length have long been recognized for all but the simple octahedral monodentate FeII compounds. However, identifying the coupled deformations that also impact on the unimolecular rate constants remains experimentally and theoretically challenging, especially for samples that do not display long-range order or when crystallization profoundly alters the dynamics. Owing to the rapid progress in ultrafast X-ray absorption spectroscopy (XAS), it is now possible to obtain transient structural information in any physical phase with unprecedented details. Using picosecond XAS and DFT modeling, the structure adopted by the photoinduced high-spin state of solvated [Fe(terpy)2]2+ (terpy: 2,2′:6′,2″-terpyridine) has been recently established. Based on these results, the methodology of the continuous shape measure is applied to classify and quantify the short-lived distortion of the first coordination shell. The reaction coordinate of the spin-state interconversion is clearly identified as a double axial bending. This finding sets a benchmark for gauging the influence of first-sphere and second-sphere interactions in the family of FeII complexes that incorporate terpy derivatives. Some implications for the optimization of related photoactive FeII complexes are also outlined.
  
  • 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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