A mononuclear FeII 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 FeII 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.

No abstract

Three polymorphs of a new Fe(ii) complex have been selectively prepared in their pure forms. They display different SCO properties: the α-form displays a hysteretic one-step spin transition centered at 134 K, the β-form undergoes a hysteretic two-step spin transition (T1/2 = 153 and 144 K) and the γ-form is paramagnetic over the whole temperature region.

The temperature of spin crossover was systematically tuned by replacing the NCE− (E = S, Se, BH3) co-ligands in a family of mononuclear complexes.