Single-Crystalline Hydrogen-Bonded Crosslinked Organic Frameworks and Their Dynamic Guest Sorption


Porous organic framework materials constructed by periodically aligned molecular entities offer chemically tailored microenvironments to absorb molecules and ions for various applications. Fundamentally understanding the microenvironments of these porous organic materials─from pore size, shape, and dynamics to potential substrate binding sites─is critical for the rational design of porous organic materials. The solid-state structures of these porous organic materials, such as covalent organic frameworks (COFs), can provide unambiguous atomic-level structural details. However, it remains challenging to synthesize these materials as single crystals that can be fully characterized by single-crystal X-ray diffraction (SCXRD) or rotational electron diffraction (RED) analysis. In addition, the balance of single crystallinity, permanent porosity, and good chemical stability requires delicate control of the assembly of the molecular building blocks and covalent crosslinking during synthesis. In this Account, we discuss the development of hydrogen-bonded crosslinked organic frameworks (HCOFs) possessing balanced single crystallinity and high chemical stability. HCOFs are obtained through covalently crosslinking molecular crystals that are preorganized via hydrogen bonding. Due to the dual hydrogen-bonded network and covalent crosslinking, HCOFs can deform upon guest adsorption by breaking the hydrogen bonds and subsequently restore their original form through the desorption of guests by re-establishing the hydrogen-bonded networks. Thus, HCOFs can dynamically adjust their pore sizes according to the framework–substrate interactions. In the discussion, we link HCOFs with COFs and single-crystalline 2D polymers by comparing their synthetic approaches to accessing high crystallinity. The method to synthesize HCOFs allows for the employment of various flexible building blocks and linking motifs that are largely avoided in the current design regimes of COFs and 2D polymers. We also draw the connections between HCOFs and hydrogen-bonded organic frameworks (HOFs) by highlighting their shared design principles for constructing hydrogen-bonding networks with large voids. Compared to their hydrogen-bonded precursor crystals, reinforcing the hydrogen-bonded networks with covalent linkages endows HCOFs with enhanced chemical and structural stability. In addition, we emphasize that the structure elucidation of HCOFs often requires combined SCXRD analysis and experimental evidence, with the methods and challenges thoroughly discussed. The details are presented in the following sequence: (1) synthesizing single-crystalline COFs by matching the polycondensation rate to the nucleation rate and their subsequent analyses by SCXRD/RED; (2) obtaining single-crystalline polymers and networks through topochemical reactions; (3) constructing HOFs with designed voids using highly directional hydrogen bonding building blocks; and (4) developing HCOFs via monomer crystal engineering followed by single-crystal to single-crystal (SCSC) synthesis and studying their unique dynamic guest sorption behaviors. We hope this Account will inspire researchers to expand the synthetic methods for advancing HCOFs with detailed solid-state structures, as well as designing porous organic framework materials with dynamic sorption capabilities to enhance their performance for applications in molecular storage, separation, catalysis, etc.

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Accounts of Materials Research


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