Detection of a Jahn-Teller Mode as an Ultrafast Fingerprint of Spin-transition-induced Charge Transfer in CoFe Prussian Blue Analogue

Photoinduced spin transition is one of the fastest processes occurring in transition-metal-based molecules and materials, reaching down to tens of femtoseconds (1 fs = 1015 s). The underlying dynamics have been well characterized for mononuclear Fe(II) spin-crossover (SCO) complexes, where it was shown that the spin transition can induce coherent activation of metal-ligand breathing and bending vibrational modes, identified as the main reaction coordinates. While precise characterization of the femtosecond nuclear wave packet brings rich information on photoinduced spin transitions, it is scarcely explored in bimetallic Prussian Blue Analogues (PBAs). In this work, we performed transient absorption spectroscopy with sub-20 fs time resolution on the [Cs0.7Co(Fe(CN)6)0.9] PBA to unravel high-frequency coherent structural dynamics, which is interpreted as the activation of a low-symmetry CoN6 Jahn-Teller mode at 12 THz. This coherent fingerprint shows that the photoexcitation initially drives motion along the Jahn-Teller coordinate before the spin transition occurs, i.e., over less than 50 fs. These results are in stark contrast with conventional activation of fully-symmetric breathing modes reported for several SCO compounds, thus providing new insights on the ultrafast spin-transition-induced charge transfer dynamics in PBAs.