Unveiling Pathway Complexity in the Growth of a Spin-Crossover MOF via Engineered Liquid-Liquid Interfacial Reactions

Coordination polymers (CPs), including metal-organic frameworks (MOFs), have recently emerged as a platform to design new materials with novel applications in fields such as electronics, magnetism, catalysis, optics and gas storage/separation. However, the pathways followed and the mechanisms underlying their formation remain largely unknown and unresolved. Accordingly, the elucidation of associated growth mechanisms remains the key obstacle in accessing new properties and functions in such materials. Herein, we demonstrate that reaction-diffusion (RD) conditions accomplished within microfluidic reaction systems can be used to uncover different crystallization pathways undertaken by spin-crossover MOFs towards their thermodynamic products. Specifically, microfluidic RD mixing (providing kinetic control) enables two peculiar nucleation-growth pathways characterized by well-defined metastable intermediates, which have never been observed in bulk environments (under thermodynamic control). Contrarily, in the latter case, crystallization by particle attachment (mesoscale assembly) is observed. These unprecedented results provide a sound basis for understanding coordination polymer growth, and open up new avenues for the engineering of advanced functional materials.