Photomagnetism in Cyano-Bridged Bimetal Assemblies

Abstract

The study of photoinduced phase-transition materials has implications for the fields of inorganic chemistry, solid-state chemistry, and materials science. Cyano-bridged bimetal assemblies are promising photomagnetic materials. Because cyano-bridged bimetal assemblies possess various absorption bands in the visible light region, their electronic and spin states can be controlled by visible light irradiation. Moreover, the selection of magnetic metal ions and organic ligands provide a way of controlling spin–spin interactions through a cyano bridge. In this Account, we describe cyano-bridged bimetal assemblies developed in our laboratory. Cu^II₂[Mo^IV(CN)₈]·8H₂O (CuMo), Rb^IMn^II[Fe^III(CN)₆] (RbMnFe), and Co^II₃[W^V(CN)₈]₂·(pyrimidine)₄·6H₂O (CoW) induce photomagnetism via photoinduced metal-to-metal charge transfers (MM′CT), while Fe^II₂[Nb^IV(CN)₈]·(4-pyridinealdoxime)₈·2H₂O (FeNb) exhibits a photoinduced magnetization via a photoinduced spin crossover. Irradiation with 473 nm light causes the CuMo system to exhibit a spontaneous magnetization with a Curie temperature (T_C) of 25 K, but irradiation with 532, 785, and 840 nm light reduces the magnetization. In this reversible photomagnetic process, excitation of the MM′CT from Mo^IV to Cu^II produces a ferromagnetic mixed-valence isomer of Cu^ICu^II[Mo^V(CN)₈]·8H₂O (CuMo′). CuMo′ returns to CuMo upon irradiation in the reverse-M′MCT band. RbMnFe shows a charge transfer (CT)-induced phase transition from the Mn^II–Fe^III phase to the Mn^III–Fe^II phase. Irradiation with 532 nm light converts the Mn^III–Fe^II phase into the Mn^II–Fe^III phase, and we observe photodemagnetization. In contrast, irradiation of the Mn^II–Fe^III phase with 410 nm light causes the reverse phase transition. A CT-induced Jahn–Teller distortion is responsible for this visible light-induced reversible photomagnetic effect. In the CoW system, a CT-induced spin transition causes the thermal phase transition from the Co^II–W^V phase to the Co^III–W^IV phase. Irradiation of the Co^III–W^IV phase with 840 nm light causes ferromagnetism with a T_C of 40 K and magnetic coercive field (H_c) of 12 000 Oe, but excitation of the back M′MCT (Co^II → W^V) with 532 nm light leads to the reverse phase transition. These examples of the photomagnetic effect have occurred by exciting MM′CT bands. In the fields of inorganic chemistry and materials science, researchers have studied extensively the photoinduced phase transitions between low-spin (LS) and high-spin (HS) transition metal ions. Recently, we have observed the first example of photoinduced spin crossover ferromagnetism with a FeNb system (T_C = 20 K and H_c = 240 Oe), in which a strong superexchange interaction between photoproduced Fe^II(HS) and neighboring paramagnetic Nb^IV operates through a CN bridge. The optical switching magnets described in this Account may lead to novel optical recording technologies such as optomagnetic memories and optical computers.