Out-of-equilibrium MOF nucleation in microfluidics combined with molecular dynamics simulations unveils new crystallization pathways
Metal-organic frameworks (MOFs) and coordination polymers (CPs) are widely studied for their remarkable structure-property relationships. Some of these species, such as spin-crossover MOFs, can reverse their spin under an external stimulus, which in turn changes their magnetism. This unusual property could be particularly useful in switching of optical displays or data storage devices. However, the ability to fine tune such properties requires a deep mechanistic understanding of their nucleation and growth during synthesis. Unfortunately, such studies are extremely challenging when using conventional (macroscale) synthetic reactors, where the various pathways leading to the thermodynamically favored product stay largely undisclosed.
To address this problem, Néstor Calvo Galve, members of the deMello group and collaborators at the universities of Valencia and Rome have used continuous-flow microfluidic devices to uncover novel crystallization pathways of CCP-4, a magnetically active MOF. The researchers leverage laminar flows to create reaction-diffusion regions, where mass-transport of reactive species can be held constant throughout an experiment. Using simple microfluidic reactors and transmission electron microscopy analysis at different time points, the authors successfully identify two out-of-equilibrium pathways (termed A and B) for the growth of CCP-4 that eventually lead to the same thermodynamic product. Data indicate that pathway A (hexagon vertices growth) prevails at early stages of the synthesis, with pathway B (hexagon edges growth) dominating at later times. Significantly, a competition study and molecular dynamics simulations provide evidence that in this case pathway A was kinetically favored.
This work displays how controlled reaction-diffusion conditions in microfluidic devices can be leveraged to understand the growth of CPs and MOFs, and subsequently used to engineer novel porous materials via the use of out-of-equilibrium conditions.
Written by Thomas Moragues
Read the accepted manuscript here