Charge transfer driven by ultrafast spin transition in a CoFe Prussian blue analogue

Photoinduced charge-transfer is an important process in nature and technology and is responsible for the emergence of exotic functionalities, such as magnetic order for cyanide-bridged bimetallic coordination networks. Despite its broad interest and intensive developments in chemistry and material sciences, the atomic-scale description of the initial photoinduced process, which couples intermetallic charge-transfer and spin transition, has been debated for decades; it has been beyond reach due to its extreme speed. Here we study this process in a prototype cyanide-bridged CoFe system by femtosecond X-ray and optical absorption spectroscopies, enabling the disentanglement of ultrafast electronic and structural dynamics. Our results demonstrate that it is the spin transition that occurs first on the Co site within ~50 fs, and it is this that drives the subsequent Fe-to-Co charge-transfer within ~200 fs. This study represents a step towards understanding and controlling charge-transfer-based functions using light. Cyanide-bridged CoFe coordination networks exhibit photomagnetism because of coupled charge-transfer and spin transition. Now, femtosecond X-ray and optical absorption spectroscopies have enabled the electronic and structural dynamics of this light-induced process to be disentangled and show that it is the spin transition on the cobalt atom, occurring within ~50 fs, that induces the Fe-to-Co charge-transfer within ~200 fs.