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

Abstract

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.

Construction of SCO-Active Fe(II) Mononuclear Complexes from the Thio-pybox Ligand

“…The new phase possessing hysteresis was stable in the continuous measurement cycles and returned to its initial phase at ambient temperature for a long time (Figure S46b). Changing the temperature-scan rate from 1 to 5 K·min –1 did not significantly influence the hysteresis width, indicating that the bistable states are well maintained (Figure S45). The dilution experiment is a favorable tool for investigating the relationship between the elastic interaction of Fe II centers and the thermal hysteresis.…”

Manipulating the spin crossover behaviour in a series of cyanide-bridged {FeIII2FeII2} molecular squares through NCE− co-ligands

“…It has been known that the SCO process of the mononuclear complexes of [(L)Fe II (NCE) 2 ] may be engineered in a predictable way via the substitution of NCE − (E = S, Se, BH 3 ) co-ligands. 19 Generally, T 1/2 increases with the increase of ligand strength (NCS − < NCSe − < NCBH 3 − ). However, this strategy has not been adopted in the homometallic cyanometallate (Fe–Fe) system featuring electronic versatility, which may exhibit SCO and/or MMCT behaviours.…”

Impact of counter anions on spin-state switching of manganese(iii) complexes containing an azobenzene ligand

“…[46][47][48][49] In this regard, apart from well-studied thermo-induced SCO, the photo-switched SCO phenomenon, particularly ligand-driven light-induced spin change (LDLISC), has recently offered an excellent opportunity to study SCO under ambient conditions. [50][51][52][53] Several photo-active ligands have been used by incorporating various photo-active motifs e.g., styryl, azobenzene, spiropyran, and diarylethene units to explore the LDLISC effect, [54][55][56][57][58][59][60][61][62][63] where the photochemical reaction between two photo-isomers produces significantly different structural and electronic changes which significantly alter the ligand field strength around the SCO metal center. It is most important to note that these studies have mainly concentrated on iron(II), iron(III), and cobalt(II) complexes; 49,52,53 however, incorporation of a photo-active motif for SCO in manganese(III) has not been explored yet and remains challenging.…”

“…46–49 In this regard, apart from well-studied thermo-induced SCO, the photo-switched SCO phenomenon, particularly ligand-driven light-induced spin change (LDLISC), has recently offered an excellent opportunity to study SCO under ambient conditions. 50–53 Several photo-active ligands have been used by incorporating various photo-active motifs e.g. , styryl, azobenzene, spiropyran, and diarylethene units to explore the LDLISC effect, 54–63 where the photochemical reaction between two photo-isomers produces significantly different structural and electronic changes which significantly alter the ligand field strength around the SCO metal center.…”