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

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 Fe₂Co₂ 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 Fe₂Co₂ system is determined experimentally.