Optically Transparent Porous Salt Thin Films Enable In Situ Characterization of Structure- and Depth-Dependent Post-Synthetic Metalation

Post-synthetic metalation and metathesis chemistry are central to rational synthesis of metal-organic frameworks (MOFs) that are unavailable by direct self-assembly. The rate, extent, and distribution of post-synthetically modified sites is challenging to characterize due to the microcrystallinity and heterogeneity of many materials. 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 the thickness of the obtained films. The obtained thin films are optically transparent monoliths that retain the permanent porosity of the corresponding porous salts. Postsynthetic metalation of these films with Mn(HMDS)2 affords the corresponding Mn(II) porphyrin-based materials (HMDS = hexamethyldisilazide). In situ spectroscopic monitoring, made possible by the optical transparency and synthetically controlled optical density of the films, enables metalation kinetics and extent 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.