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

## TL;DR

A new photomagnet material undergoes phase transitions triggered by light, enabling control of its electronic states through charge transfer processes.

## Contribution

Demonstrates homometallic intervalence charge transfer in a heterometallic photomagnet induced by phase transitions.

## Key findings

- A two-step thermal phase transition occurs due to heterometallic charge transfer-induced spin transitions.
- Photoirradiation at specific wavelengths induces distinct photoinduced phases with homometallic IVCT bands.
- Defects in the structure enable pyrazine bridging and homometallic CoII–CoIII interactions.

## Abstract

A photomagnet, Co8[W(CN)8]5Cl·(pyrazine)11·21H2O, exhibiting Class II mixed valency due to homometallic intervalence charge transfer (IVCT) between CoII and CoIII 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 [CoII–pyrazine–CoIII] 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 [CoII–pyrazine–CoIII] 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.

A photoinduced phase transition via heterometallic intervalence charge transfer (IVCT) between Co and W induces homometallic IVCT between CoII and CoIII in a cyanido‐bridged Co–W photomagnet.© 2026 WILEY‐VCH GmbH