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Abstract
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.
