In situ characterization of post-synthetic metalation in porous salt thin films

Abstract

Post-synthetic metalation and metathesis chemistry are central to rational synthesis of metal-organic frameworks (MOFs) that are unavailable by direct self-assembly. The inherent microcrystallinity and heterogeneous nature of many MOFs renders characterization of the rate, extent, and distribution of post-synthetic modifications challenging. Here we describe the deposition of optically transparent, permanently porous thin films comprised of a peripherally carboxylated free-base porphyrin and a cationic porous molecular cage. The films are assembled via layer-by-layer growth controlled by Coulombic charge pairing, which allows for systematic control over film thickness. The obtained thin films are optically transparent monoliths that retain the permanent porosity of the corresponding porous salts. Post-synthetic metalation of these films with Mn(HMDS)2 affords the corresponding Mn(II) porphyrin-based materials (HMDS = hexamethyldisilazide). Access to thin films with systematically varied thickness (and thus optical density), combined with in situ spectroscopy, enables the kinetics and extent of metalation to be directly monitored. We demonstrate both structure- and thickness-dependence on metalation kinetics. These results provide a unique window into the molecular-scale mechanisms that underpin materials synthesis.