Probing the Anisotropic Distortion of Photoexcited Spin Crossover Complexes with Picosecond X-ray Absorption Spectroscopy

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/low-spin 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 specific 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 Fe–N Axial, Fe–N Distal, and bite angle N Distal –Fe–N Axial extracted from the measurements are in very good agreement with those predicted by DFT calculations in D 2 d 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.