Exploring Ultrafast Photoswitching Pathways in RbMnFe Prussian Blue Analogue.

We study by femtosecond optical pump-probe spectroscopy the photoinduced charge transfer (CT) in the RbMnFe Prussian blue analogue. Previous studies evidenced the local nature of the photoinduced MnIII FeII → MnII FeIII process, occurring within less than 1 ps. Here we show experimentally that two photoswitching pathways exist, depending on the excitation pump wavelength, which is confirmed by band structure calculations. Photoexcitation of α spins corresponds to the Mn(d-d) band, which drives reverse Jahn-Teller distortion through the population of antibonding Mn-N orbitals, and induces CT within ≈190 fs. The process launches coherent lattice torsion during the self-trapping of the CT small-polaron. Photoexcitation of ß spins drives intervalence Fe→Mn CT towards non-bonding states and results in a slower dynamic.

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.