(725d) Micro-, Meso-, and Macro-Scale Defects in Porous Organic Cages

Porous organic cages (POCs) are single molecules with permanent porosity built from organic linkers. They have attracted significant interest in adsorption and separation studies because of their modularity, solubility, and processability. In this study, we analyzed a variety of defect behaviors inside imine-based POCs, including defects that arise from packing frustration, grain boundaries, and molecular defects. POC molecules can pack in the solid state to form crystalline interconnected porous systems. The accessible surface area of these crystals arises from contributions of both the internal and external pores. By controlling the removal rate of the solvents, one type of POC, CC3-R, showed total amorphization and a 2.5-fold increase in surface area, which is attributed to external pore contributions from random packing. The amorphization of cage crystals was also achievable via vertex functionalization—i.e., by introducing mixed diamine linkers into the cages, a variety of amorphous scrambled porous organic cages (ASPOCs) were synthesized, which can be used as molecular porous substrates and membrane fillers. Conventional CC3-R/CC3-S crystals were examined with confocal fluorescence microscopy and the mesoscale grain boundaries were revealed in these as-synthesized crystals. We utilized inter-cage chiral recognition with stronger packing affinity to eliminate the grain boundaries, and achieved increased acid stability of the resulting mixed-chirality cage crystals. Lastly, we explored (both experimentally and computationally) the likelihood of molecular point defects in pristine POCs. To study the effects of the point defects on the material’s properties, we applied non-solvent induced crystallization to introduce a non-trivial amount of defects for magnified detection of the resulting properties. We also designed and synthesized a “missing-linker” cage to introduce extra functionality.