Cooperative interactions are the origin of useful properties of spin crossover (SCO) materials, with hybrid framework materials exhibiting the greatest cooperativity and hysteresis. However, little is known about the chemical origin of cooperativity in frameworks. Here, we the origin of cooperativity in the metal–organic frameworks that exhibit the largest known hysteresis window of all SCO materials and unusually high transition temperatures. Variable-temperature vibrational spectroscopy provides evidence that “soft modes” associated with dynamic metal–linker bonding trigger the cooperative SCO transition, while thermodynamic analysis also confirms a cooperativity magnitude much larger than those of other SCO systems. The SCO behavior becomes strongly size-dependent when these MOFs are varied in size between 5 – 200 nm. This combined experimental–computational study provides a basis for understanding cooperative magnetism and highlights the important role of dynamic bonding and size dependent structures in the functional behavior of framework materials.

Dynamic bonding in nanoscale MOFs

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