Slow Dynamics of the Spin‐Crossover Process in an Apparent High‐Spin Mononuclear Fe ^II Complex

A mononuclear Fe II complex that shows a high‐spin ( S =2) paramagnetic behavior at all temperatures (with standard temperature‐scan rates, ≈1 K min −1 ) has, in fact, a low‐spin ( S =0) ground state below 100 K. This low‐spin state is not easily accessible due to the extremely slow dynamics of the spin‐crossover process—a full relaxation from the metastable high‐spin state to the low‐spin ground state takes more than 5 h below 80 K. Bidirectional photo‐switching of the Fe II state is achieved reproducibly by two selective irradiations (at 530–590 and 830–850 nm). The slow dynamics of the spin‐crossover and the strong structural cooperativity result in a remarkably wide 95‐K hysteresis loop induced by both temperature and selected light stimuli.