High-Spin-Locked Fe(II) Spin-Crossover Compounds: THz-EPR Investigations and Spin–Phonon Coupling

The development of spin-crossover compounds (SCO) and their modification toward suitable applications demands detailed and intimate knowledge of both their electronic structure and intermolecular interactions within the solid state. To acquire this information, frequency-domain Fourier-transform THz electron paramagnetic resonance (FD-FT THz-EPR) spectroscopy is used to trace magnetic transitions of two SCO systems that retain in the high-spin (S = 2) state at low temperatures. Two sets of transitions are observed in the range of 5 to 10 and 30 cm–1 at zero-field, which can be explained by zero-field splitting and rhombicity. To adequately simulate the dispersion of these transitions with an applied magnetic field, inclusion of spin–phonon coupling is necessary for one of the compounds.