Cooperativity of Electron Transfer Coupled Spin Transitions in a Tetranuclear Fe/Co Prussian Blue Analogue Revealed by Ultrafast Spectroscopy

Abstract Cooperative intermolecular interactions, usually observed in solid state, can confer useful properties to stimuli–responsive spin transition materials. Here, we demonstrate for the first time intramolecular cooperativity between the two Fe–Co subunits of a molecular cyanido‐bridged square Fe 2 Co 2 Prussian blue analogue (PBA) in solution, which upon single photon excitation sequentially undergo electron transfer coupled spin transition (ETCST) from a diamagnetic low‐spin (LS) to a paramagnetic high‐spin (HS) state. Ultrafast UV–vis and IR pump‐probe spectroscopies show that irradiation into the IVCT band of the LS state induces electron transfer within one Fe–Co subunit followed by fast (360 fs) SCO to an intermediate HS/LS species and a further ETCST event in the other Fe–Co subunit then occurs on a ns timescale. Kinetic analysis reveals that this cooperative switching of the two Fe–Co subunits is caused by two coupled equilibria favouring the second ETCST step, and the free energy landscape for the square Fe 2 Co 2 system is determined experimentally.