| Covalent organic frameworks (COFs) are an emerging class of fully crystalline polymers that are characterized by their high surface area and permanent porosity. COFs form as either stacks of sheets covalently bound in two-dimensions (2D) or networks with covalent bonds extending in all three-dimensions (3D). The layered structures of 2D COFs gives rise to intrinsically high charge mobilities and their synthesis as oriented films portends their use in photovoltaics and as supercapacitors. In contrast, few 3D COFs have been crystallized, and despite exhibiting exceptionally high surface areas (>4000 m2 g-1) and record low densities (0.17 cm3 g-1), these materials have no well-developed applications. Functionalizing their interior might harness these properties to furnish structurally precise platforms for catalysis, separations, and the storage and release of molecular payloads. Because these polymers are fully crystalline, structural features can be tuned to the atomic level, providing an additional level of control for materials design.;COF synthesis employs highly symmetric building blocks that are inherently devoid of additional reactive groups, impeding the synthesis of functional derivatives. We developed the first strategy to functionalize 3D COFs that avoids making inconvenient modifications to these monomers by employing a comonomer that bears a reduced number of structure-directing moieties while maintaining the geometry of the parent building block. We have defined this co-condensation as a truncated-mixed linker (TML) approach, because the additional monomer acts as a truncated derivative of the parent building block and is incorporated without modifying the framework's topology. Thus these truncated monomers are incorporated as defect sites throughout the network. Furthermore, the process of COF formation is poorly understood and is thought to rely on reversible covalent bond formation for error correction. The following observations indicate unambiguously that the rate of error correction must be slower than the rate of framework growth, a counterintuitive finding with implications for broadening the chemical scope of these polymerizations. |
