Superacid In Situ Protected Synthesis of Covalent Organic Frameworks

Abstract

Covalent organic frameworks, as a class of fascinating crystalline porous materials, are attracting increasing attention in various fields. Synthesizing these materials to attain crystallinity and porosity is essential; however, it is time-consuming, not cost-effective, and energy-demanding as it involves extensive screenings of reaction conditions and employs undesired aromatic solvents. Despite recent progress in the synthesis, finding an efficient, convenient, low-toxicity, and widely applicable method remains a challenging goal. Here, we report an in situ-protected strategy for synthesizing imine-linked frameworks by exploring triflic acid as the catalyst to replace traditional acetic acid and deploying alcohols as a single-component reaction medium instead of aromatic solvents. We found that the function of triflic acid is threefold: it rapidly protonates amino groups of amine monomers into ammonium cations, protects formyl units of aldehyde monomers by converting them into acetals, and improves the solubilities of both monomers. The in situ-protection scheme greatly changes their concentrations and reactivities, making reactions highly controllable and reversible. This strategy is general for various monomer combinations to develop imine-linked frameworks with different topologies, including tetragonal, rhombic, pentagonal, hexagonal, kagome, dual trigonal, dual rhombic, and dual hexagonal shapes, and various pore sizes from micropores to mesopores, presenting a facile and simple way to synthesize 28 different yet high-quality frameworks in n-butanol/water. Remarkably, nine new imine-linked frameworks are synthesized for the first time, which cannot be prepared by traditional systems. The porphyrin frameworks exhibited exceptional photocatalytic activities in the activation of molecular oxygen to produce highly reactive oxygen species of singlet oxygen.