(77d) Core-Shell/Yolk-Shell/Hollow Covalent Organic Framework Nanostructures with Size-Selective Permeability

Covalent organic frameworks (COFs) is an emergent area in materials research. COFs are organic solids possessing precise and rigid nanoporous structures. Many methods exist for their synthesis but precise morphology control allowing the tailoring of core-shell and yolk-shell nanostructures has been elusive, mainly due to poor control of reaction-diffusion processes. Herein, we propose precise regulation of the COF generation and decomposition rates and their relationship with the diffusion rate of COF species during the synthesis, which enables a successful preparation of core-shell, yolk-shell, hollow-spherical, and multiple yolk-shell COF structures with tunable sizes ranging from 200 to 1400 nm. The core-shell and yolk-shell COFs containing thin shells of small pores are mechanically robust enough to prevent the common collapse issue for large-pore COF cores during solvent evaporation and possess surface areas of 669-834 m²/g compared to 62 m²/g for the same large-pore COFs without shells. Formed COF structures can potentially serve as nanoreactors or nanocontainers with the shell layer providing molecular-size selectivity determined by the pore size demonstrated here using Suzuki coupling reactions with phenylboronic acid, which produced a size-cutoff efficiency for halogenated aromatics approaching 100%. The precise morphology design and control of COF particles and hybrids will help enhance application efficiencies and give access to new functionalities for COF materials.