| Abstract
| - [{CoII(pyrimidine)2}2{CoII(H2O)2}{WV(CN)8}2]·4H2O exhibits a photoreversible magnetic effect upon irradiating with two different lights. The forward process is caused by the photoinduced charge transfer from WIV (S = 0) to CoIII (S = 0), and the back process is due to the photoinduced reverse charge transfer from CoII (S = 3/2) to WV (S = 1/2).
- This paper describes the crystal structure, magnetic properties, and photoreversible magnetic properties of CoII3[WV(CN)8]2(pyrimidine)4·6H2O. We found that complexes of this formula had two types of crystal structures ([{CoII(pyrimidine)2}2{CoII(H2O)2}{WV(CN)8}2]·4H2O (1) and [{CoII(pyrimidine)(H2O)}2{CoII(H2O)2}{WV(CN)8}2](pyrimidine)2·2H2O (2)). These two structures had similar metal−organic frameworks but differed in the coordination environment around Co1, i.e., Co1(NC)4(pyrimidine)2 in 1 and Co1(NC)4(pyrimidine)(H2O) in 2. In 1, a temperature-induced phase transition from the CoII (S = 3/2)−NC−WV (S = 1/2) [high-temperature (HT)] phase to the CoIII (S = 0)−NC−WIV (S = 0) [low-temperature (LT)] phase was observed due to a charge-transfer-induced spin transition. However, 2 did not exhibit such a phase transition. When the LT phase of 1 was irradiated by 840 nm light, ferromagnetism with a Curie temperature of 40 K and magnetic coercive field of 12 kOe were observed. UV-vis reflectance and infrared measurements suggested that the LT phase optically transited to the photoinduced (PI) phase, which had a similar valence state as the HT phase, through the metal-to-metal charge-transfer (WIV → CoIII) band. In contrast, when the back metal-to-metal charge transfer (CoII → WV) band of the PI phase was excited by 532 nm light, the reverse phase transition from the PI phase to the LT phase occurred, and the spontaneous magnetization decreased.
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