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, approximately 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.