Abstract
Covalent organic frameworks (COFs) are excellent candidates for various applications. So far, successful methods for the constructions of COFs have been limited to a few condensation reactions based on only one type of covalent bond formation. Thus, the exploration of a new judicious synthetic strategy is a crucial and emergent task for the development of this promising class of porous materials. Here, we report a new orthogonal reaction strategy to construct COFs by reversible formations of two types of covalent bonds. The obtained COFs consisting of multiple components show high surface area and high H2 adsorption capacity. The strategy is a general protocol applicable to construct not only binary COFs but also more complicated systems in which employing regular synthetic methods did not work.
Highlights
A s an emerging class of porous crystalline materials, covalent organic frameworks (COFs) whose structures can be precisely predetermined by molecular building blocks using reticular chemistry exhibit periodic architectures, low densities, and permanent porosity,[1] rendering them good candidates for various potential applications in gas adsorption,[2] optoelectronics,[3] catalysis,[4] separation,[5] and proton conduction.[6]
Considering the documented COFs involving the formation of only one type of covalent bond at current stage, two types of organic reactions should be orthogonal in the dynamic process in order to construct predesigned COFs built from two covalent bonds
We hypothesized that this orthogonal reaction strategy, from which reversible formations of two types of covalent bonds were reported in supramolecular chemistry,[15] can be applicable to construct COFs with predetermined structures by rational choice of building blocks
Summary
Considering the generations of amorphous solids from IM-1 and IM-2, the parallel formations of imine and boronate/ boroxine groups should be concurrently accompanied by the crystallization of COFs, followed by irreversible aggregations of crystallites that was observed in COF-5.18 The proposed mechanism can well interpret the unsuccessful constructions of NTU-COF-1 and NTU-COF-2 from intermediate compounds via the step-by-step procedure. The crystallinity of both NTU-COF-1 and NTU-COF-2 was confirmed by PXRD analyses (Figures 1 and S12−S15).
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