High-Field EPR Spectroscopy of a Mn³⁺ Spin-Crossover Complex

When stimulated by changes in temperature, pressure, electric field, magnetic field, or optical irradiation, spin-crossover (SCO) complexes change their spin states and exhibit large atomic displacements, making them appealing for use in magnetoelectric applications. Coupling between the molecular magnetic properties and the charge/elastic degrees of freedom of the lattice drives cooperativity via local lattice strains, resulting in large changes in magnetic susceptibility with relatively small perturbations. To investigate the potential of molecular metal-organics as magnetoelectric candidates, a study utilizing continuous-wave high-field powder EPR spectroscopy was conducted on a Mn³⁺ complex that undergoes a sharp thermal transition (T_1/2 = 51 K; with < 10 K hysteresis) from a pure high-spin (HS) S = 2 state to a pure low-spin (LS) S = 1 state. Spin-orbit coupling dominates the magnetic anisotropy in d-block transition metals, which admixes crystal field states. Indeed, significant zero-field splitting (D and E) parameters were obtained for both the LS (D = +21.23 cm^-1, E = +2.275 cm^-1) and HS states (D = + 5.66 cm^-1, E = + 1.31 cm^-1) in this study. Although positive D parameters are unusual for octahedral Mn³⁺ complexes, these observations are rationalized based upon the coordination environment of the compound and the associated spin state changes.