Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure−function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

This work demonstrates a platform for designing a photocatalyst to promote lightdriven production of hydrogen from water. The newly developed photocatalyst consists of all sp 2 carbon frameworks that are fully p conjugated to promote exciton migration and offer a narrow band gap to harvest visible and near-infrared light. Engineering the lattice periphery with electron-deficient units tunes the band structure and constitutes built-in interfaces to generate electrons. The resulting frameworks enable efficient, continuous, and stable hydrogen production under irradiation.

A new approach has been developed to design organic polymers using topology diagrams. This strategy enables covalent integration of organic units into ordered topologies and creates a new polymer form, that is, covalent organic frameworks. This is a breakthrough in chemistry because it sets a molecular platform for synthesizing polymers with predesignable primary and high‐order structures, which has been a central aim for over a century but unattainable with traditional design principles. This new field has its own features that are distinct from conventional polymers. This Review summarizes the fundamentals as well as major progress by focusing on the chemistry used to design structures, including the principles, synthetic strategies, and control methods. We scrutinize built‐in functions that are specific to the structures by revealing various interplays and mechanisms involved in the expression of function. We propose major fundamental issues to be addressed in chemistry as well as future directions from physics, materials, and application perspectives.

Covalent organic frameworks enable the topological connection of organic chromophores into π lattices, making them attractive for creating light-emitting polymers that are predesignable for both the primary- and high-order structures. However, owing to linkages, covalent organic frameworks are either unstable or poor luminescent, leaving the practical synthesis of stable light-emitting frameworks challenging. Here, we report the designed synthesis of sp2 carbon-conjugated frameworks that combine stability with light-emitting activity. The C=C linkages topologically connect pyrene knots and arylyenevinylene linkers into two-dimensional all sp2 carbon lattices that are designed to be π conjugated along both the x and y directions and develop layer structures, creating exceptionally stable frameworks. The resulting frameworks are capable of tuning band gap and emission by the linkers, are highly luminescent under various conditions and can be exfoliated to produce brilliant nanosheets. These results suggest a platform based on sp2 carbon frameworks for designing robust photofunctional materials.

Progress in chemistry over the past four decades has generated a variety of porous materials for removing iodine-a radioactive emission accompanying nuclear fission. However, most studies are still based on the notion that entangled pores together with specific binding sites are essential for iodine capture. Here, an unraveled physical picture of iodine capture that overturns the preconception by exploring 1D channeled porous materials is disclosed. 2D covalent organic frameworks are constructed in a way so that they are free of interpenetration and binding sites but consist of 1D open channels. As verified with different channels shaping from hexagonal to tetragonal and trigonal and ranging from micropores to mesopores, all the 1D channels enable a full access to iodine, generalizing a new paradigm that the pore volume determines the uptake capacity. These results are of fundamental importance to understanding iodine uptake and designing materials to treat coagulative toxic vapors.

Covalent organic frameworks (COFs) with ordered one-dimensional channels could offer a predesigned pathway for ion motion. However, implanting salts into bare channels of COFs gives rise to a limited ion conductivity. Here, we report the first example of polyelectrolyte COFs by integrating flexible oligo(ethylene oxide) chains onto the pore walls. Upon complexation with lithium ions, the oligo(ethylene oxide) chains form a polyelectrolyte interface in the nanochannels and offer a pathway for lithium ion transport. As a result, the ion conductivity was enhanced by more than 3 orders of magnitude compared to that of ions across the bare nanochannels. The polyelectrolyte COFs promoted ion motion via a vehicle mechanism and exhibited enhanced cycle and thermal stabilities. These results suggest that the strategy for engineering a polyelectrolyte interface in the 1D nanochannels of COFs could open a new way to solid-state ion conductors.

Transformation of carbon dioxide to high value‐added chemicals becomes a significant challenge for clean energy studies. Here a stable and conductive covalent organic framework was developed for electrocatalytic carbon dioxide reduction to carbon monoxide in aqueous solution. The cobalt(II) phthalocyanine catalysts are topologically connected via robust phenazine linkage into a two‐dimensional tetragonal framework that is stable under boiling water, acid, or base conditions. The 2D lattice enables full π conjugation along x and y directions as well as π conduction along the z axis across the π columns. With these structural features, the electrocatalytic framework exhibits a faradaic efficiency of 96 %, an exceptional turnover number up to 320 000, and a long‐term turnover frequency of 11 412 hour−1, which is a 32‐fold improvement over molecular catalyst. The combination of catalytic activity, selectivity, efficiency, and durability is desirable for clean energy production.

Covalent organic frameworks (COFs) offer ordered π structures that are useful for developing light-emitting materials. However, most COFs are weak in luminescence. Here we report the conversion of less emissive COFs into light-emitting materials via a pinpoint surgery on the pore walls. Deprotonation of the N-H bond to form an anionic nitrogen species in the hydrazone linkage can eliminate the nitrogen-related fluorescence quenching pathway. The resulting COF enhances the fluorescence in a linear proportion to the progress of deprotonation, achieving a 3.8-fold improved emission. This pinpoint N-H cleavage on the pore walls can be driven only by the fluoride anion while other halogen anions, including chloride, bromide, and iodide, remain inactive, enabling the selective fluorescence switch-on sensing of the fluoride anion at a ppb level.