Molecular Insight into Assembly Mechanisms of Porous Aromatic Frameworks

The structural and dynamic factors governing porosity in porous aromatic frameworks (PAFs) are investigated using coarse-grained molecular dynamics simulations. PAFs form amorphous, porous networks with potential for gas storage and separation applications. We focus on a series of four PAFs - PAF-1, PPN-1, PPN-2, and PPN-3 - which exhibit an unexpected trend in porosity as the structure of the PAF monomer is varied. The simulations suggest that nonbonding dispersion interactions that stabilize misbound monomer configurations play an essential role in the formation of porosity-reducing interpenetrated frameworks in PAFs comprising the larger PPN-1 and PPN-2 monomers; on the other hand, the simulations indicate that the steric bulk of a key reaction intermediate acts to limit interpenetration in PAFs made up of the smaller PAF-1 and PPN-3 monomers. The simulations also show that the rate of cluster growth, which depends largely on the monomer concentration used in the experimental synthesis, is significantly higher for PPN-1 and PPN-2, which would exacerbate the kinetic trapping of interpenetrated misbound configurations. This work provides design rules for synthesizing highly porous amorphous networks through the choice of monomer structure and reaction conditions that limit framework interpenetration.