Cyanide-bridged Fe-Co complex [Fe(Tp)(CN)3]2Co(bpe) ×5H2O (1×5H2O; Tp = hydro-tris(pyrazolyl) borate; bpe = 1,2-bis(4-pyridyl)ethane) shows temperature- and light- induced metal-to-metal charge transfer (MMCT) involving spin state changes between magnetic FeIIILS-CoII HS(HS = high spin, LS = low spin) state and nonmagnetic Fe IILS-CoIIIHS state, while the dehydrated material 1 does not show any MMCT and holds Fe IIILS-CoIIHS state. We have investigated the magnetic properties of each spin state in 1 and 1×5H2O by means of magnetization and ESR measurement under pulsed high magnetic field. At low temperature below T N, in both 1 and 1×5H2O, the saturation magnetization in the induced ferromagnetic phase is well explained by S and g values derived from the magnetic susceptibility study. In the ESR of 1, we observed characteristic modes corresponding to a spin excitation in the induced ferromagnetic phase where its temperature dependence shows an evolution of spin correlation in the Fe IIILS-CoIIHS state at low temperature. We further found that the similar ESR modes grow in the light-induced state of 1×5H2O. The results strongly suggest that the light-induced magnetization in 1×5H2O is driven by a light-induced MMCT, which involves transition of spin multiplicity from the nonmagnetic FeIILS-CoIII HS to the magnetic FeIIILS-Co IIHS pair.
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