Homometallic Intervalence Charge Transfer Band of Co(II/III) Induced by Phase Transitions in a Heterometallic Co–W Charge Transfer Photomagnet

A photomagnet, Co₈[W(CN)₈]₅Cl·(pyrazine)₁₁·21H₂O, exhibiting Class II mixed valency due to homometallic intervalence charge transfer (IVCT) between Co^II and Co^III centers, is synthesized. The compound features a 3D cyanido-bridged CoW coordination network composed of two crystallographically independent Co sites (Co1 and Co2) and one W site. Rectangular tubular structures formed by Co1-W bridges are further crosslinked by the Co2 sites. Defects in the [W(CN)₈] sites enable the formation of pyrazine bridges between the Co1 sites. Upon cooling, the compound undergoes a two-step thermal phase transition, attributed to heterometallic charge-transfer-induced spin transitions between high- and low-spin electronic states. The first-step charge transfer (CT) phase transition leads to the formation of homometallic [Co^II-pyrazine-Co^III] bridges, producing a near-infrared IVCT band at 2300 nm. Photoirradiation at 785 nm at 3 K induces a transition to a photoinduced (PI1) phase. The PI1 phase also shows a homometallic IVCT band due to the emergence of the [Co^II-pyrazine-Co^III] state. Subsequent photoirradiation at 532 nm to the PI1 phase induces a transition to the paramagnetic photoinduced (PI2) phase. This study demonstrates the modulation of electronic states in a phase transition material and a photomagnet, enabled by homo- and heterometallic CT processes.