SpinCrossover

Spin crossover is a class of molecular and solid-state phenomena in which a transition metal ion can reversibly switch between two electronic spin states, typically a low-spin (LS) and a high-spin (HS) configuration, in response to external stimuli. It is most commonly observed in octahedral coordination complexes of iron(II) and cobalt(II) and in related materials. The transition arises from a competition between crystal-field splitting, Δ_oct, and the pairing energy, P. When Δ_oct is larger than P, electrons pair in the lower-energy t2g set (LS). If Δ_oct is comparable to or smaller than P, electrons occupy higher-energy eg orbitals to maximize spin (HS). The HS state often has a larger ionic radius, leading to measurable changes in structure, color, and magnetic properties.

In solids, cooperativity among neighboring metal centers can produce abrupt transitions and hysteresis, enabling memory-like behavior. Transitions can occur with changing temperature, pressure, or light. A light-induced excited spin state trapping (LIESST) effect can transfer the system into a metastable HS or LS state at low temperature by irradiation with appropriate wavelengths, with relaxation back to the ground state upon warming.

Applications are exploratory and include molecular switches, sensors, displays, and memory devices. Spin crossover materials span discrete coordination compounds, polymers, MOFs, and nanoparticles. Research focuses on tuning transition temperatures, hysteresis width, and the stability of photoinduced states through ligand design and intermolecular interactions.