Among photoactive materials, Prussian blue analogues (PBA) are of specific interest, as light can address a broad variety of functions: photoexcitation of the diamagnetic CoIII(S=0)FeII(S=0) system generates the magnetic CoII(S=3/2)FeIII(S=1/2) state, with a charge-transfer coupled to a spin transition on the Co.
However, questions about the photoinduced processes were unanswered. Does charge-transfer induce spin-transition or vice versa, which is the way?
In our paper entitled "Charge-transfer driven by ultrafast spin-transition in a CoFe Prussian blue analogue" published in Nature chemistry (https://www.nature.com/articles/s41557-020-00597-8) we reveal the process, thanks to the element-specific X-ray absorption data, measured at both the Fe and Co K-edges, allowed for monitoring the ∼50 fs dynamics of the spin transition around the Co and the ∼200 fs Charge-transfer dynamics.
This study, including also optical spectroscopy data and DFT calculations, demonstrates that it is the elongation of the Co-N bonds, induced by the optical excitation, which drives the charge-transfer.