Covalent organic cage-framework hybrids as functional porous materials

Covalent organic frameworks (COFs) and porous organic cages (POCs) represent complementary classes of porous materials, but their integration into hierarchical structures remains challenging. Herein, we report the synthesis of the first fully organic Cage@COF hybrid materials via a host-in-host strategy driven by electrostatic interactions. An anionic sulfonate-functionalised COF (NaSO₃COF) was designed to capture a cationic ammonium cage derived from a chiral POC, CC3, from aqueous solution, enabling post-synthetic incorporation through wet impregnation in the bulk framework material. The resulting Cage@COF materials retain crystallinity while exhibiting controlled pore size reduction and modulation of the internal confined chemical environment. Systematic adsorption studies reveal high cage loadings governed by chemisorption and strong host-guest affinity. This structural tuning translates into pronounced molecular sieving effects, leading to suppressed CH₄ uptake and enhanced CO₂ selectivity. Notably, the Cage@NaSO₃COF (1 : 1) composite displays significantly improved CO₂/CH₄ selectivity compared to the parent frameworks, highlighting the synergistic interplay. These findings establish Cage@COF host-in-host systems made from COFs and POCs as a versatile platform for precise pore engineering and advanced gas adsorption in organic materials.