Covalent organic frameworks (COFs) are an emerging class of two- and three-dimensional polymers linked entirely by the covalent bonds of light elements such as carbon, nitrogen, boron, and oxygen. These materials can be predictably designed to form highly organized, crystalline networks with high-surface area and porosity, and are suitable candidates for a variety of applications such as gas storage, charge storage, and photovoltaic cells. COFs are often constructed from planar aromatic monomers, providing two-dimensional polymer sheets that stack into a layered crystal. Although the mechanism of COF formation is not fully understood, the reversible bond formation during the polymerization process is thought to be necessary for the formation of crystalline networks. Dynamic bond formation provides a mechanism for the network to correct defects as it crystallizes. Tailoring COF design to this requirement constrains the types of linkages, and expected properties of these materials. As a result, COFs that can achieve crystallinity under non-reversible reaction conditions based on the rigidity of their monomers, or have limited aromatic surface areas available for stacking, will diversify the potential designs and applications of future COFs. Two such approaches for COF formation will be discussed. (1) A non-reversible, nucleophilic aromatic substitution is a potential candidate for the formation of a crystalline COF from a non-reversible reaction. Computational modeling has shown this linkage to be rigid and planar, which suggests that the polymer network would be forced into the typical, planar COF sheet. (2) The formation of a COF as a single-layer 2D polymer might be promoted by reducing the high surface-area contacts between adjacent sheets. Triptycene is a three-fold symmetric monomer whose aromatic groups are arranged in a 3-dimensional fan, rather than in a flat plane, and has been incorporated into COFs previously. Condensation reactions of triptycene-based monomers orient the π orbitals into the pore of the COF, which limits interlayer interactions and might facilitate the isolation of single-layer structures. Progress toward the syntheses of these two COF networks will be presented, including the optimized solvents, temperatures, and catalytic conditions.

Diversifying covalent organic framework designs through nonreversible linkages and non-planar aromatic monomers

249th National Meeting (2015)

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