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 ( γ HS ) 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.