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 <i>D</i>_2<i>d</i> 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