Probing
the Anisotropic Distortion of Photoexcited
Spin Crossover Complexes with Picosecond X‑ray Absorption Spectroscopy
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Abstract
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)<sub>2</sub>]<sup>2+</sup>, a complex which belongs to the
prominent family of spin crossover building blocks with nonequivalent
metal–ligand bonds. The correlated changes in Fe–N<sub>Axial</sub>, Fe–N<sub>Distal</sub>, and bite angle N<sub>Distal</sub>–Fe–N<sub>Axial</sub> extracted from the measurements
are in very good agreement with those predicted by DFT calculations
in <i>D</i><sub>2<i>d</i></sub> 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