On the role of the metal‐to‐ligand charge transfer states in the light‐induced spin crossover in FeII (bpy)₃

The light‐induced spin crossover in Fe II complexes is studied with complete active space second‐order perturbation theory. The potential energy surfaces of the low‐spin and high‐spin states are determined as function of the Fe‐‐ligand distance interpolating between the high‐spin and low‐spin equilibrium geometries. In addition, we calculate the relative energies of the lowest excited Fe-3 d⁶ states along this coordinate. To address the mechanism of spin crossover, the metal‐to‐ligand charge transfer states with singlet, triplet, and quintet spin coupling were also treated. The curves of the singlet and triplet MLCT states nearly coincide, but the quintet MLCT curve is higher in energy and the equilibrium distance displaced to larger distances. The possible deactivation mechanisms of the 1 MLCT state are discussed considering the relative energies and spin‐orbit coupling matrix elements of the relevant electronic states.