Abstract
Three-dimensional (3D) covalent organic frameworks (COFs) are rare because there is a limited choice of organic building blocks that offer multiple reactive sites in a polyhedral geometry. Here, we synthesized an organic cage molecule (Cage-6-NH2) that was used as a triangular prism node to yield the first cage-based 3D COF, 3D-CageCOF-1. This COF adopts an unreported 2-fold interpenetrated acs topology and exhibits reversible dynamic behavior, switching between a small-pore (sp) structure and a large-pore (lp) structure. It also shows high CO2 uptake and captures water at low humidity (<40%). This demonstrates the potential for expanding the structural complexity of 3D COFs by using organic cages as the building units.
Highlights
Covalent organic frameworks (COFs) have emerged as a versatile class of crystalline porous solids that can be rationally designed and fine-tuned by synthesis to enhance their functionality.[1−4] The framework topology of a COF is predetermined by the connectivity and geometry of the building blocks, and, as for metal−organic frameworks (MOFs),[5,6] COF synthesis is often tolerant to changes in linker size and chemical functionality
The discovery of triangular prism-shaped building blocks was important for increasing the topological diversity of MOFs.[35−38] triangular prisms have not been used in COF synthesis because of the limited availability of suitable building blocks
Unlike planar molecular building blocks that usually lead to layered COFs, Cage-6-NH2 is decorated with 6 amine groups in a 3D trigonal prismatic arrangement that are ready to be extended in three dimensions
Summary
Covalent organic frameworks (COFs) have emerged as a versatile class of crystalline porous solids that can be rationally designed and fine-tuned by synthesis to enhance their functionality.[1−4] The framework topology of a COF is predetermined by the connectivity and geometry of the building blocks, and, as for metal−organic frameworks (MOFs),[5,6] COF synthesis is often tolerant to changes in linker size and chemical functionality. The majority of reported COFs are two-dimensional (2D) layered structures with 1D pore channels, where strong noncovalent interactions modulate the interlayer packings.[7] By contrast, COFs with 3D nets require more than one nonplanar building unit, and these have proven to be less synthetically accessible.[4,8] only eight distinct 3D nets in the Reticular Chemistry Structure Resource (RCSR)[9] have been reported for COFs: dia,[10] lon,[11] bor,[2] ctn,[2] pts,[12] rra,[13] srs,[14] and ffc.[15] The first six of these were constructed using tetrahedral building blocks with 4 points of extension, such as tetraphenylmethane, tetraphenylsilane, adamantane, and their derivatives.[4] This is in stark contrast to endeavors that have realized topologically more complex MOF nets,[16] where metal clusters with up to 24 points of extension have been reported.[17,18] organic molecules rarely have a high number (>4) of reactive sites that are spatially arranged to allow for extended interconnection This is a major reason for the difficulty in broadening the range of topologies for COFs, and it is a central challenge for expanding the functional scope of porous 3D frameworks. Article high water uptake (22 wt %) at low relative humidity (40%) and a high carbon dioxide uptake at 1 bar (204 mg/g, 273 K)
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