Light actuated single-chain magnet with magnetic coercivity

Abstract

In this study, a pair of cyanide-bridged {Fe₂Co}-based coordination polymers, {[(^PzTp)Fe(CN)₃]₂Co(L)₂}·4H₂O (1) and {[(Tp)Fe(CN)₃]₂Co(L)₂}·3H₂O·CH₃OH (2), was synthesized by assembling Co^II with [(^PzTp)Fe^III(CN)₃]⁻ (^PzTp: tetrakis(pyrazolyl)borate) and the less hindered [(Tp)Fe^III(CN)₃]⁻ (Tp: tris(pyrazolyl)borate) in the presence of the asymmetric ditopic ligand L (4-(1H-imidazol-5-ylmethylene-amino)-4H-1,2,4-triazole). Both compounds exhibited thermally induced metal-to-metal electron transfer (MMET) accompanied by the dielectric anomaly, resulting in the transformation between a high-temperature (HT) phase of {Fe^III_LS–Co^II_HS–Fe^III_LS} and a low-temperature (LT) phase of {Fe^II_LS–Co^III_LS–Fe^III_LS}. Contributed by the light-actuated electron transfer from Fe^II to Co^III ions, the LT phase can be converted into the HT phase by 946 nm light irradiation. Interestingly, compound 1 exhibited the stepwise transition of the dielectric constant with the electron transfer process, presenting a rare example showing the synergetic switching of dielectric and magnetic properties. Moreover, compound 1 showed the photo-induced slow relaxation of magnetization and a coercive field of 400 Oe, being a single-chain magnet. In contrast, compound 2 exhibited metamagnetization behavior after light irradiation.