The Part and the Whole: Field‐Dependent THz‐EPR Spectroscopy Probes Lattice‐Dependent Electronic Structure in Iron(II) Spin Crossover

Frequency‐domain Fourier‐transform (FD‐FT) THz‐EPR spectroscopy provides direct access to kinetically trapped high‐spin ( HS ) states in iron(II) spin crossover (SCO) complexes. Using temperature‐induced excited spin‐state trapping (TIESST), the low‐temperature electronic structure of the HS state is accessed despite a diamagnetic low‐spin ( LS ) ground state. Field‐dependent spectra resolve the splitting of the HS ground‐state manifold and provide evidence for spin–phonon coupling, described by an avoided‐crossings Hamiltonian supported by ab initio calculations. THz‐EPR further quantifies the remaining HS fraction () and reveals shifts of magnetic transitions that depend on the surrounding spin‐state matrix. These results show that HS centers experience distinct local lattice environments in mixed HS / LS states and provide experimental access to the influence of spin‐state distribution on SCO processes. Cooperative switching scenarios, including domain formation and alternating HS / LS arrangements, can thus be distinguished. Field‐dependent THz‐EPR spectroscopy therefore provides direct access to lattice‐dependent effects in SCO materials.