Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers that have received much attention for their potential in heterogeneous catalysis, gas adsorption and separation, sensing, energy storage, and optoelectronics. Three-dimensional (3D) COFs, in which the molecular building blocks are covalently linked into a 3D network, can possess unique structural characteristics (e.g. high porosity, hierarchical nanopores, numerous open sites, etc.) and have recently been considered as promising platforms for a wide range of applications. However, the structural diversity of 3D COFs is severely limited due to some long-standing issues such as challenging synthesis and complicated structure determination, which greatly hinders the exploration of their applications. In fact, it is possible to build 3D frameworks in reticular chemistry by increasing the dimensionality through two-dimensional (2D) planar sheets entanglement, which can also be used to expand the family of 3D COF structures.
Herein, we report the introduction of steric hindrance in molecular building blocks to prevent π-π stacking, thus allowing 2D covalent organic sheets to form 3D COFs through entanglement. Starting from the rationally designed precursors containing a bulky anthracene unit in the vertical direction, a highly crystalline COF (3D-An-COF) was successfully synthesized. Very interestingly, 3D-An-COF was determined as an entangled 2D square net (sql) structure, and the high-resolution data obtained by the continuous rotation electron diffraction technique allowed us to directly locate all non-hydrogen atoms. Structurally, the presence of an anthracene group outside the C_2h symmetry plane strongly reduces the π-π interactions and promotes the formation of square entanglements. In addition, 3D-An-COF is fluorescent and can be used as a sensor to detect the trace amount of antibiotics in water.

Bo Gui

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