Geometry and Chemistry: Influence of Pore Functionalization on Molecular Transport and Diffusion in Solvent-Filled Zirconium Metal–Organic Frameworks

Post-synthetic modification (PSM) of metal–organic frameworks (MOFs) enables incorporation of diverse func-tionalities in pores for chemical separations, drug delivery, and heterogeneous catalysis. However, the effect of PSM on molecular transport, which is essential for most applications of MOFs, is rarely studied. In this paper, we used the perfluoroalkane-functionalized Zr-MOF NU-1008 as a platform to systematically interrogate transport processes and mechanisms in solvated pores. We anchored perfluoroalkanes onto NU-1008 nodes by solvent-assisted ligand incorporation (SALI-n, with n = 3, 5, 7, 9 denoting the number of fluorinated carbons). Transport of a luminescent molecule, BODIPY, through individual crystallites of four versions of methanol-filled SALI-n was monitored by confocal fluorescence microscopy as a function of time and location. In com-parison with the parent NU-1008, the diffusivity of the probe molecules within SALI-n declined by 2 to 7-fold depending on chain length and loading, presumably due to steric effects or adsorptive interactions with per-fluoroalkyl chains. Atomistic simulations were performed to uncover the microscopic behavior of BODIPY dif-fusion in SALI-n. BODIPY molecules, which preferably interact with linkers, were pushed to the interior of the channels as chain length increased, resulting in solvated diffusion and minor differences in the short-time mobility of BODIPY in SALI-n. This suggested that the observed decline of transport diffusivity in SALI-n mainly stemmed from the steric effect from these flexible chains. We anticipate that this proof-of-concept will assist in understanding how pore functionalization can physically and chemically affect mass transport in MOFs and will be useful in further guiding the design of PSM to realize the optimal performance of MOFs for various applications.