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Covalent organic framework

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers constructed from organic building units linked by strong covalent bonds, resulting in highly ordered, two- or three-dimensional structures with permanent porosity and tunable topologies.^[1] First synthesized in 2005 by A. P. Côté and colleagues under the direction of Omar M. Yaghi at the University of California, Berkeley, COFs represent an extension of reticular chemistry principles initially applied to metal-organic frameworks (MOFs), but exclusively using light-element organic linkers to form all-organic networks.^[2] These frameworks exhibit exceptional properties, including high surface areas often exceeding 4000 m²/g, low densities, and precise control over pore size and functionality due to their predesignable molecular building blocks and reversible covalent linkages that facilitate crystallization.^[3] The crystallinity of COFs arises from the directional bonding and geometric matching of rigid organic struts, enabling the formation of layered or interpenetrated lattices that maintain structural integrity under various conditions, unlike amorphous porous polymers.^[1] This structural predictability allows for the incorporation of diverse functional groups, such as amines, porphyrins, or redox-active moieties, to tailor electronic, optical, and chemical properties.^[3] COFs are primarily synthesized through polycondensation reactions, including Schiff-base (imine) formation, boronic acid condensation, and trazine linkages, often via solvothermal methods that promote reversible bond exchange for error correction and crystal growth.^[1] Advances since the initial discovery have introduced room-temperature, mechanochemical, and interfacial synthesis techniques to improve scalability and yield, addressing early challenges with low crystallinity and solubility.^[3] Notable applications of COFs span energy technologies, such as proton conduction in fuel cells, electrocatalysis for hydrogen evolution, and lithium-ion battery electrodes leveraging their ordered channels for ion transport.^[1] In environmental and separation sciences, COFs enable selective gas adsorption (e.g., CO₂ capture) and water purification through size- and affinity-based sieving.^[3] Emerging uses include photocatalysis for solar fuel production, chemical sensing via host-guest interactions, and drug delivery systems exploiting their biocompatibility and tunable pores.^[1]

History and Development

Discovery and Initial Synthesis

The discovery of covalent organic frameworks (COFs) took place in 2005, when researchers led by Omar M. Yaghi at the University of California, Berkeley, reported the first successful synthesis of these materials through condensation reactions involving boronic acids, which formed boroxine and boronate ester linkages. This breakthrough introduced a new class of porous, crystalline organic solids constructed entirely from strong covalent bonds between light elements such as boron, carbon, and oxygen. COFs were conceptualized as two- or three-dimensional, crystalline porous polymers exhibiting permanent porosity and highly ordered atomic structures, which distinguish them from traditional amorphous covalent organic polymers that lack long-range order and structural predictability. A key innovation addressed early challenges in the field, such as the difficulty of achieving crystallinity in covalent organic materials due to the typically irreversible nature of organic reactions; this was overcome by leveraging reversible condensation processes that enable dynamic error correction and self-assembly into periodic frameworks. The inaugural examples, COF-1 and COF-5, exemplified these principles. COF-1 was synthesized by dehydrative condensation of 1,4-benzenediboronic acid (BDBA) in a sealed Pyrex tube, heated solvothermally at 120°C for 72 hours in a 1:1 mixture of mesitylene and 1,4-dioxane, resulting in a 71% yield of the product featuring boroxine (B₃O₃) linkages and a pore size of approximately 7 Å. In parallel, COF-5 was prepared from BDBA and hexahydroxytriphenylene (HHTP) in a 3:2 molar ratio under similar solvothermal conditions at 100°C for 72 hours, affording a 73% yield with boronate ester (B₃O₆) linkages and larger pores around 27 Å in diameter. These syntheses established the foundational reversible chemistry for COF construction, paving the way for broader applications. In the years immediately following, the scope of COF synthesis expanded to include imine linkages, as illustrated by the development of three-dimensional imine-based frameworks like COF-300 in 2009.^[4]

Key Milestones and Recent Advances

Following the initial discovery of two-dimensional boronate ester-linked COFs in 2005, the period from 2007 to 2010 marked significant diversification in linkage chemistries and structural dimensionality. In 2007, the first three-dimensional COFs, such as COF-102, COF-103, COF-105, and COF-108, were synthesized through boronic acid self-condensation into boroxine rings, expanding the framework topology beyond planar sheets and enabling higher pore volumes for potential gas storage applications. By 2009, imine linkages were introduced in the crystalline 3D COF-300, formed via acid-catalyzed condensation of aldehyde and amine building blocks, which offered reversible bonding for potential dynamic applications while maintaining porosity.^[4] In 2010, hydrazone-linked COFs, exemplified by COF-42 and COF-43, were reported using aldehyde-hydrazide condensations, providing enhanced hydrolytic stability compared to imines. Concurrently, the first covalent triazine frameworks (CTFs), such as CTF-1, emerged in 2008 through ionothermal trimerization of aromatic nitriles at high temperatures, introducing nitrogen-rich linkages for improved electronic properties. Between 2015 and 2020, COF research advanced toward functional materials with electronic and catalytic capabilities. In 2017, the first fully π-conjugated two-dimensional COF was synthesized, enabling intrinsic electrical conductivity through extended conjugation and π-stacking, with subsequent developments like Ni-porphyrin-based COFs in 2018 achieving conductivities around 10^{-3} S cm^{-1} by incorporating redox-active units.^[5]^[6] Photocatalytic uses gained traction, with a 2014 hydrazone-linked COF demonstrating hydrogen evolution from water under visible light, a seminal example later expanded in 2018 studies showing COFs outperforming metal oxides in CO₂ reduction due to tunable bandgaps.^[7] Patents for COF-based gas storage proliferated, including a 2017 filing for boronate-linked frameworks targeting hydrogen uptake exceeding 1 wt% at 77 K, highlighting commercial potential in clean energy. From 2021 to 2025, innovations focused on scalable synthesis and hybrid functionalities, driven by the 2025 Nobel Prize in Chemistry awarded to pioneers of reticular chemistry, including Omar Yaghi, which underscored the foundational role of COFs in designing porous materials and spurred global investment in the field.^[8] Room-temperature synthesis protocols emerged, as detailed in a 2025 review, enabling rapid assembly of imine- and hydrazone-linked COF films via acid-catalyzed or mechanochemical methods without solvothermal heating, improving processability for membranes.^[9] Stimuli-responsive COFs advanced in 2025 with designs exhibiting pH- or light-triggered structural changes through dynamic imine bonds, enabling controlled release in drug delivery.^[10] Hybrid metal-organic-covalent frameworks (MOCOFs) were introduced in 2025, combining coordination bonds with covalent linkages for crystalline, stable structures with dual porosity, as reported by the Max Planck Institute.^[11] Additionally, COFs achieved high proton conductivities approaching 0.1 S cm⁻¹ at 100 °C and near 100% relative humidity in 2025, leveraging ordered channels for fuel cell applications.^[12]

Structure and Design Principles

Basic Architecture and Topology

Covalent organic frameworks (COFs) are extended, porous, crystalline materials constructed through covalent bonding of organic building blocks, resulting in either two-dimensional (2D) layered sheets or three-dimensional (3D) networks. These frameworks exhibit a high degree of structural order, with pores formed by the precise arrangement of molecular linkers. In 2D COFs, covalent bonds create planar layers that stack along the third dimension primarily through weak π-π interactions between aromatic units or, in some cases, interpenetration of layers, leading to one-dimensional channels aligned perpendicular to the planes. This architecture imparts permanent porosity and crystallinity, distinguishing COFs from amorphous porous polymers. Recent advances have enabled the synthesis of single-crystalline COFs (scCOFs), achieving micrometer-sized crystals with long-range order for improved structural resolution via single-crystal X-ray diffraction.^[13]^[14] The topology of COFs is classified based on their connectivity and geometric arrangement, with 2D variants typically adopting a hexagonal honeycomb lattice, as exemplified by COF-1, the first reported COF, which features boroxine linkages forming uniform hexagonal pores. In contrast, 3D COFs extend covalent connectivity into all three dimensions, enabling more complex topologies such as the diamond (dia) net or the srs (augmented srs) net, where tetrahedral or octahedral building units create interconnected pore networks. For instance, early 3D COFs like COF-102 adopt the bor topology with tetrahedral nodes, while later designs utilize srs nets for enhanced interconnectivity and larger void spaces. These topological designs are predetermined by the geometry and functionality of the organic precursors, ensuring predictable framework formation.^[15]^[16] Pore size in COFs is primarily controlled by the length and rigidity of the linkers used in their construction, allowing for tunable apertures that influence guest molecule accommodation and transport. In 2D COFs, shorter linkers like those in COF-1 yield small pores of approximately 0.7 nm in diameter, suitable for gas separation, while elongated linkers in expanded designs can produce apertures exceeding 5 nm, as seen in frameworks incorporating extended aromatic units such as anthracene derivatives or rigid polyaromatic scaffolds. This tunability arises from the linear extension of linkers within the lattice, directly scaling the distance between nodes and thus the pore dimensions, without altering the overall topology.^[16] The theoretical pore volume of a COF can be estimated from its unit cell geometry using a simplified model that approximates pores as cylindrical voids. For a 2D COF, the pore volume V_p (in cm³/g) is given by:

V_p = \frac{\pi r^2 h N}{M}

where r is the pore radius, h is the interlayer spacing (typically 0.34–0.7 nm for π-stacked layers), N is the number of pores per unit cell, and M is the mass of the unit cell. This formula derives from calculating the total void volume within the unit cell—approximating each pore as a cylinder with cross-sectional area \pi r^2 and height h, multiplied by N—and normalizing by the unit cell mass M to obtain volume per unit mass. Such calculations provide an upper-bound estimate of accessible porosity, assuming complete crystallinity and no framework collapse, and are validated against experimental data from nitrogen adsorption isotherms.

Secondary Building Units and Linkages

Covalent organic frameworks (COFs) are constructed from secondary building units (SBUs), which are rigid organic linkers that serve as the molecular nodes and struts defining the framework's geometry and topology. These SBUs typically include boronic acids, aldehydes, and amines, selected for their ability to form strong covalent bonds while maintaining structural rigidity. For instance, tetrahedral geometries, such as those derived from tetraphenylmethane or adamantane-based linkers, enable the formation of three-dimensional (3D) COFs, whereas planar linkers like 1,3,5-tris(4-aminophenyl)benzene facilitate two-dimensional (2D) layered structures. Recent designs incorporate expanded SBUs, such as octatopic porphyrins, for complex 3D topologies.^[17]^[18] The linkages in COFs arise from condensation reactions between these SBUs, creating the covalent connections that extend the structure. Common linkages include boroxine and boronate esters (\ce{B-O-B}), formed from boronic acids; imine (\ce{C=N}), from aldehydes and amines; hydrazone (\ce{C=NN}), from aldehydes and hydrazines; azine (\ce{C=N-N=C}), from dialdehydes and hydrazines; and spiroborate, from borates and diols. More recent irreversible linkages, such as sp²c (vinyl, –C=C–) formed via Knoevenagel condensation, offer exceptional chemical and hydrolytic stability for demanding applications. Boroxine and boronate ester linkages were among the earliest used but suffer from sensitivity to humidity and hydrolysis. In contrast, nitrogen-based linkages like imine and hydrazone offer improved robustness, with hydrazone bonds demonstrating greater resistance to hydrolysis than imine bonds due to the stabilizing effect of the additional nitrogen atom. Spiroborate linkages provide even higher stability in aqueous environments compared to boronate esters.^[19]^[17]^[18] Design principles for SBUs and linkages emphasize geometric compatibility to ensure defect-free assembly and crystallinity. The angular disposition of functional groups in linkers must align with the target lattice geometry; for example, linkers with 120° angles are ideal for hexagonal nets, as in many 2D COFs, to prevent strain and promote ordered stacking. A representative case is the Tp-Bd COF, where the triangular aldehyde triformylphloroglucinol (Tp) connects to the linear diamine benzidine (Bd) through imine linkages, yielding a stable hexagonal framework stabilized further by keto-enamine tautomerism.^[17] Reversible linkages, such as imine and boroxine, play a crucial role in COF synthesis by enabling dynamic error correction and self-healing during assembly, which enhances crystallinity through thermodynamic equilibration. Irreversible linkages, like certain spiroborate, vinyl, or post-formed bonds, offer superior long-term stability but demand highly precise SBU design to avoid kinetic trapping of defects.^[19]

Reticular Synthesis Concepts

Reticular chemistry represents the foundational paradigm for designing covalent organic frameworks (COFs) through the precise linking of molecular building units into extended, crystalline structures with predetermined topologies. This approach, pioneered in the context of porous materials, emphasizes the modular assembly of secondary building units (SBUs) and organic linkers via strong covalent bonds to achieve predictable architectures.^[20] By treating SBUs—such as tetrahedral or planar nodes—and linkers as geometric primitives, reticular synthesis enables the rational construction of frameworks where the connectivity and geometry dictate the overall topology, much like assembling a lattice from standardized components.^[21] A key application of reticular chemistry in COFs is the creation of isoreticular (IR) series, where systematic variations in linker length or substituents maintain the underlying topology while tuning pore size and functionality. For instance, in IR-COFs derived from imine linkages, extending the linker from benzene to biphenyl derivatives expands pore apertures from microporous (~1 nm) to mesoporous regimes (up to 5 nm), enhancing guest molecule accessibility for applications like gas storage or separation.^[22] This modularity allows for targeted property optimization without altering the crystalline scaffold, exemplifying how reticular design translates molecular precision to mesoscale control.^[11] Central to successful reticular synthesis is the principle of reversibility, leveraging dynamic covalent chemistry to facilitate error correction and achieve thermodynamic equilibrium during assembly. Reversible reactions, such as imine formation, permit bond breaking and reformation (e.g., imine exchange), enabling the system to self-correct defects and crystallize into ordered structures rather than amorphous polymers.^[23] This dynamic process contrasts with irreversible polymerizations, providing the necessary mobility for nuclei to grow into extended lattices under controlled conditions like solvothermal heating.^[24] Topological prediction underpins reticular design by simulating possible frameworks from input building units, using computational tools to forecast viable structures before synthesis. Software packages like TOPOSPro analyze geometric and topological features of crystal nets, identifying compatible topologies such as the pts (primitive tetragonal) net for 3D COFs assembled from tetrahedral SBUs and linear linkers.^[25] For example, reticular tools have guided the synthesis of pts-topology 3D COFs, where tetrahedral nodes connect via alkyne linkages to form interpenetrated diamond-like networks with tunable pore volumes. These predictions, often validated by powder X-ray diffraction modeling, ensure synthetic feasibility and structural accuracy, with recent extensions to single-crystal structures.^[26]^[27]^[14] Despite these advances, reticular synthesis of COFs faces challenges, particularly defect formation arising from incomplete reversibility or mismatched kinetics, which can disrupt long-range order and reduce crystallinity. Strategies like kinetic trapping—employing mild conditions or modulators to slow nucleation—help mitigate these issues by promoting selective growth of defect-free domains over disordered aggregates. Addressing such defects remains crucial for scaling up high-quality COFs, as they directly impact mechanical stability and performance in practical devices.^[28]^[29]

Synthesis Methods

Traditional Condensation Reactions

Traditional condensation reactions represent the foundational approaches to synthesizing covalent organic frameworks (COFs), primarily relying on solvothermal methods that involve heating precursor molecules in sealed vessels under moderate temperatures to facilitate reversible bond formation and crystallization. These reactions typically occur in solvent mixtures such as mesitylene and 1,4-dioxane, at temperatures ranging from 80°C to 120°C for durations of 3 to 7 days, enabling the self-assembly of building units into ordered porous structures. Solvothermal synthesis promotes high crystallinity, as confirmed by powder X-ray diffraction (PXRD), and yields often reach around 70% for benchmark materials. A prototypical example is the synthesis of COF-1 through the self-condensation of 1,4-benzenediboronic acid (BDBA), where dehydration leads to boroxine linkages, resulting in a two-dimensional layered framework with pore sizes of approximately 0.7 nm. Imine condensation, another cornerstone of traditional COF synthesis, involves the reaction between aldehydes and amines to form Schiff base linkages, catalyzed by acidic conditions such as acetic acid. This reversible process allows for error correction during assembly, yielding crystalline frameworks with tunable topologies. The general reaction is represented as:

\mathrm{RCHO + R'NH_2 \rightleftharpoons RCH=NR' + H_2O}

Early demonstrations include COF-300, synthesized solvothermally from trialdehyde and triamine precursors in dioxane with acetic acid at 120°C for 3 days, achieving high crystallinity and a Brunauer-Emmett-Teller (BET) surface area exceeding 1000 m²/g. These imine-linked COFs exhibit enhanced chemical stability compared to boronic acid-based analogs due to the robustness of the C=N bond. Boron-based condensations further exemplify traditional methods, where diboronic acids undergo dehydration to form either boroxine rings (trimeric B-O-B units) or boronate esters, depending on the co-reactants and conditions. In COF-5, for instance, BDBA reacts with 1,3,5-triformylbenzene in a 1:1.5 ratio under solvothermal conditions at 100°C for 72 hours, producing a framework with hexagonal pores and a surface area of about 711 m²/g. Triazine trimerization, often via cyanuric chloride as a precursor in Friedel-Crafts reactions with aromatic monomers and Lewis acids like AlCl₃, generates nitrogen-rich CTFs at elevated temperatures (up to 150°C) in solvents such as nitrobenzene, yielding porous materials with triazine cores for applications requiring basic sites.^[30] To enhance crystallinity and orientation in these syntheses, templated approaches incorporate salts like NaCl as scaffolds, which direct the growth of COF layers by providing a confined environment during condensation. For example, NaCl crystals serve as templates in solvothermal reactions of boronic acids or imines, promoting epitaxial growth and resulting in oriented polycrystalline films after template removal by washing; this method has been shown to improve powder diffraction peak intensities, indicating higher order. Such templating has enabled the scalable production of monolithic COF foams with self-floating properties for practical use.^[31]

Advanced and Room-Temperature Techniques

Advanced synthesis techniques for covalent organic frameworks (COFs) have emerged since 2020 to address the limitations of energy-intensive traditional methods, emphasizing low-temperature, rapid, and scalable processes that enhance crystallinity and yield while minimizing solvent use. These approaches leverage catalysts, alternative media, and external stimuli to facilitate efficient condensation reactions, such as imine formation, under milder conditions.^[9] Room-temperature synthesis represents a key advancement, enabling the production of high-quality COF powders and films without heating, which reduces energy consumption and improves compatibility with sensitive substrates. Mechanochemical methods, particularly ball-milling, involve grinding precursors in the presence of minimal liquid additives to drive solid-state reactions, yielding crystalline imine-linked COFs like TpMA in just 1 hour with iodine adsorption capacities exceeding 200 wt%. This solvent-sparing technique has been applied to boroxine-linked COFs, achieving room-temperature assembly in minutes via shear forces that promote reversible bond formation. Solution-based room-temperature strategies often employ acid catalysts, such as acetic acid, for open-air synthesis of hydrazone or imine COFs. A notable example is interfacial polymerization at 25°C, where immiscible phases confine reactions to form ultrathin COF films with yields up to 88%, as detailed in recent reviews on scalable film production.^[32] Ionothermal synthesis utilizes ionic liquids as green solvents to conduct reactions at lower temperatures and pressures than traditional solvothermal methods, accelerating nucleation and growth through ion templating effects. For instance, 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate enables the formation of 2D keto-enamine COFs in open containers at 120°C for 3 days, while incorporating ionic components for enhanced conductivity. Microwave-assisted techniques further expedite synthesis by providing uniform heating, reducing reaction times dramatically; imine-linked COFs, such as those from terephthalaldehyde and p-phenylenediamine, can be prepared in as little as 1 hour with high crystallinity and porosity suitable for iodine capture. These methods build on traditional imine condensation but operate under far less stringent conditions.^[33]^[34] Modulator-solvent induced polymerization has gained traction for fabricating thin-film COFs, particularly for device integration. A 2025 report demonstrates large-area synthesis (up to 60 cm²) of oriented COF films via this solution-based process at 80°C, producing asymmetric structures with controllable thicknesses for applications in membranes.^[35] To achieve practical scalability, continuous flow reactors have been adapted for gram-scale COF production, integrating mechanochemical or solution-based reactions in modular setups. For example, flow systems using acetic acid catalysis produce imine COF single crystals at rates of 1 g/hour, with post-synthetic modification capabilities for tuned properties, marking a shift toward industrial viability.^[36]^[37]

Post-Synthetic Modifications

Post-synthetic modification (PSM) of covalent organic frameworks (COFs) involves the covalent attachment of functional groups to pre-formed frameworks, enabling precise tuning of properties without requiring redesign of the initial structure. A prominent strategy is the exchange of reversible imine linkages with amine or other groups, allowing the introduction of targeted functionalities such as amines or metal-binding sites while leveraging the dynamic nature of these bonds. For instance, imine-to-amine conversion via reduction maintains the framework's topology, as demonstrated in the transformation of imine-linked COFs to amine-linked variants.^[38] Non-covalent doping complements this by encapsulating guest species, such as metal ions or nanoparticles, within the pores through coordination or host-guest interactions, yielding hybrid materials with enhanced catalytic or conductive properties.^[38] Linkages like imines, amenable to exchange from the synthesis phase, facilitate these modifications under controlled conditions.^[39] Representative examples highlight PSM's efficacy in applications like gas capture. Amine-functionalized COFs, achieved through post-assembly imine reduction or click chemistry, exhibit enhanced CO₂ affinity due to the introduced basic sites, with selectivity improvements over N₂. In one case, post-synthetic amine installation on an imine-COF platform retained full crystallinity and porosity, as confirmed by powder X-ray diffraction and nitrogen sorption analyses.^[40] Yields for such modifications often exceed 80% crystallinity retention when using mild reductants like sodium cyanoborohydride. Recent advances, including enzyme-catalyzed click reactions, have further enabled stability-enhanced PSM by implanting groups like hydroxyethylthio under aqueous, ambient conditions, preserving structural integrity for photocatalytic uses.^[41] Challenges in PSM include potential framework collapse from harsh reagents or steric hindrance, which can disrupt crystallinity and porosity. Strategies to mitigate this involve mild, reversible conditions—such as acid-catalyzed exchanges or biocatalytic methods—to minimize defects and retain over 80% of original surface area in many cases.^[38] These approaches, categorized into post-functionalization and post-metalation, ensure high-fidelity modifications while addressing compatibility issues inherent to direct synthesis.^[39]

Physical and Chemical Properties

Porosity and Surface Area

Covalent organic frameworks (COFs) exhibit permanent porosity, characterized by well-defined void spaces that range from micropores (<2 nm) to mesopores (2-50 nm), enabling high accessibility for guest molecules. This porosity arises from the precise arrangement of organic building units linked covalently into extended crystalline lattices, resulting in Brunauer-Emmett-Teller (BET) surface areas typically spanning 500 to 5000 m²/g, with recent examples reaching up to approximately 4400 m²/g as of 2024.^[42] For instance, the three-dimensional COF-103, synthesized via boronate ester condensation, achieves a BET surface area of 4210 m²/g with a pore size of approximately 1.25 nm, while COF-108 demonstrates mesoporous characteristics at 3.0 nm and a BET surface area of 4800 m²/g. These values highlight the potential for tailoring pore dimensions through structural design, contributing to the materials' exceptional capacity for hosting species within their frameworks. The porosity and surface area of COFs are quantitatively assessed using nitrogen (N₂) adsorption isotherms measured at 77 K, which provide data on gas uptake as a function of relative pressure. The BET model is applied to the low-pressure region (typically P/P₀ = 0.05-0.3) to calculate the monolayer capacity (V_m) and derive the specific surface area (S_BET), assuming multilayer adsorption on a homogeneous surface. For more detailed pore size distribution, density functional theory (DFT) models, such as non-local DFT (NLDFT), analyze the entire isotherm to deconvolute contributions from micropores and mesopores, revealing narrow distributions centered around 1-4 nm in many COFs. Complementarily, the Langmuir surface area (S_Langmuir) offers an estimate based on monolayer adsorption assumptions, calculated via the equation S = \frac{V_m \cdot N_A \cdot \sigma}{M}, where V_m is the monolayer volume, N_A is Avogadro's number, \sigma is the molecular cross-sectional area of the adsorbate (e.g., 0.162 nm² for N₂), and M is the molar mass of the gas; this often yields higher values than BET, as seen in COF-103 with S_Langmuir at 5350 m²/g. These techniques confirm the fully accessible and reversible nature of COF pores, essential for their functional performance.^[43] Key factors influencing the porosity and surface area in COFs include the rigidity of linkers and the underlying topological network. Rigid aromatic linkers, such as tetra(4-boronic acid)silane in COF-103, prevent framework collapse and maintain open pore channels by minimizing flexibility-induced stacking defects. The topology, dictated by the connectivity of building units (e.g., ctn or bor nets in 3D COFs), determines pore shape, size, and interconnectivity, with higher connectivity often leading to larger accessible voids and enhanced surface areas. Crystallinity plays a crucial role in preserving these open pores against collapse during activation or exposure to solvents. In comparison to metal-organic frameworks (MOFs), COFs' all-organic composition imparts superior hydrolytic stability in aqueous environments, where metal nodes in MOFs may degrade, allowing COFs to retain porosity under conditions challenging for hybrid frameworks.^[44]

Crystallinity and Stability

Covalent organic frameworks (COFs) exhibit crystallinity characterized by long-range molecular order, which is typically confirmed through sharp peaks in powder X-ray diffraction (PXRD) patterns, reflecting the periodic arrangement of their building units and linkages.^[45] This ordered structure arises from the precise reticular synthesis, enabling the formation of eclipsed or slipped-parallel stacking in two-dimensional (2D) layers or more complex topologies in three-dimensional (3D) frameworks. The crystallite domain sizes in COFs, estimated using the Scherrer equation D = \frac{K\lambda}{\beta \cos\theta} where K is the shape factor, \lambda is the X-ray wavelength, \beta is the full width at half maximum, and \theta is the Bragg angle, typically range from 50 to 500 nm, indicating nanoscale coherence lengths that contribute to their structural integrity.^[46] Such crystallinity is essential for enabling uniform porosity through aligned pore channels in the stacked layers.^[47] The thermal and chemical stability of COFs varies significantly depending on the linkage type, with imine-linked COFs demonstrating robust performance under harsh conditions. Imine-based frameworks maintain structural integrity up to approximately 300°C, as evidenced by thermogravimetric analysis showing minimal weight loss before decomposition.^[48] These materials also exhibit excellent chemical resilience across a broad pH range of 2 to 12, resisting hydrolysis and degradation in acidic or basic environments due to the reversible yet stable nature of the C=N bonds under controlled synthesis conditions.^[49] In contrast, boron-based COFs, such as those with boronate ester or boroxine linkages, are more susceptible to hydrolysis, particularly in aqueous or humid conditions, where nucleophilic attack at the electron-deficient boron centers leads to bond cleavage and framework disassembly.^[50] Mechanical stability in 2D COFs is highlighted by their Young's modulus values, which typically fall in the range of 30 to 120 GPa for single-layer or multilayer sheets, reflecting a balance between covalent in-plane rigidity and interlayer interactions.^[51] This modulus, measured via nanoindentation or atomic force microscopy on exfoliated flakes, underscores the frameworks' ability to withstand tensile stresses without fracturing, akin to other 2D materials but tuned by organic composition. Recent advances in 2025 have introduced stimuli-responsive COFs capable of switchable crystallinity, where external triggers like pH or light induce reversible transitions between crystalline and amorphous states, enhancing adaptability for dynamic applications.^[10]

Electronic and Optical Properties

Covalent organic frameworks (COFs) exhibit semiconducting electrical conductivity typically ranging from 10^{-6} to 10^{0} S/cm, enabled by the incorporation of conjugated linkers that facilitate charge transport through extended π-systems.^[52] This conductivity arises from the ordered two-dimensional layers and interlayer stacking, which minimize defects and promote delocalized electron pathways, distinguishing COFs from amorphous organic semiconductors.^[52] A representative example is the BTT-Tz COF, which achieves a conductivity of 3.5 \times 10^{-2} S/cm due to its benzotrithiophene-triazine linkage enhancing electron mobility.^[52] The optical properties of COFs are characterized by tunable bandgaps spanning 1.5 to 3 eV, modulated by donor-acceptor linker designs that control the highest occupied molecular orbital-lowest unoccupied molecular orbital energy levels.^[53] For instance, imine-linked COFs display fluorescence with quantum yields of 20-50%, attributed to restricted intramolecular rotations and efficient energy transfer within the crystalline lattice.^[53] These photophysical traits stem from the precise control over conjugation via linkages, allowing emission wavelengths to shift from blue to red depending on the building units.^[53] Proton conductivity in COFs reaches up to 10^{-2} S/cm under hydrated conditions, particularly in azine-linked variants where water molecules form hydrogen-bonded networks facilitating the Grotthuss mechanism. This process involves rapid proton hopping along azine nitrogen sites and hydrated pores, yielding values like 1.82 \times 10^{-2} S/cm at 70°C and 90% relative humidity.^[54] Recent 2025 advances include stimuli-responsive optical switching in dynamic COFs, where reversible bond formations enable fluorescence modulation under external triggers such as light or pH, opening pathways for adaptive optoelectronic materials.^[10]

Characterization Techniques

Structural Analysis Methods

Powder X-ray diffraction (PXRD) serves as the cornerstone technique for elucidating the structural geometry and topology of covalent organic frameworks (COFs), particularly for polycrystalline samples that predominate in synthesis outcomes. By indexing diffraction peaks, researchers simulate possible framework models, often assuming layered or 3D arrangements based on building unit connectivity. Pawley refinement, a profile-fitting method, refines these models against experimental patterns to extract unit cell parameters without requiring atomic positions, enabling precise determination of lattice dimensions and space group symmetry. For instance, in the seminal COF-5, Pawley refinement of the PXRD data yielded a hexagonal unit cell with parameters a = b = 29.70(1) Å and c = 3.460(2) Å, confirming an eclipsed stacking of boronate-linked layers with AA alignment. This approach has been pivotal in validating reticular designs across diverse COFs, though challenges arise from peak broadening in low-crystallinity samples, necessitating complementary techniques for full structural resolution. Transmission electron microscopy (TEM) provides direct nanoscale visualization of COF morphology, revealing pore apertures, layer stacking, and domain sizes that corroborate diffraction data. High-resolution TEM images often display ordered hexagonal pore arrays or sheet-like structures at resolutions below 1 nm, highlighting the framework's periodic topology. Selected area electron diffraction (SAED), integrated with TEM, assesses local crystallinity by producing spot or ring patterns that match simulated diffraction from PXRD-derived models, thus confirming long-range order in individual crystallites. Recent advances include three-dimensional electron diffraction (3D-ED), which enables ab initio structure determination of nanocrystalline COFs at atomic resolution, even for beam-sensitive or low-crystallinity samples. For example, 3D-ED has been used to solve the structure of COF-300, revealing degradation processes during synthesis and providing insights into framework integrity. In single-crystal-like COF domains, SAED has resolved lattice fringes corresponding to interlayer distances of approximately 0.34 nm, as observed in imine-linked frameworks grown via modulated polymerization. These imaging methods are particularly valuable for amorphous or defect-rich COFs where PXRD signals are weak.^[55] Solid-state nuclear magnetic resonance (NMR) spectroscopy complements diffraction by probing atomic connectivity within the framework, focusing on chemical shifts indicative of linkage formation. In particular, 13C solid-state NMR identifies carbonyl or imine carbons through distinct resonance peaks; for example, the imine carbon (C=N) in Schiff-base COFs typically appears at around 160 ppm, shifting from precursor aldehyde signals near 190 ppm and confirming reversible condensation. Cross-polarization magic-angle spinning enhances sensitivity for these low-abundance nuclei, allowing detection of linkage-specific environments in powder samples. This technique has been essential in verifying the integrity of boroxine or imine bonds in early COFs like COF-1 and LZU-1. Computational modeling, especially density functional theory (DFT), predicts and refines COF structures by optimizing geometries of proposed building unit assemblies under periodic boundary conditions. DFT calculations evaluate stacking motifs (e.g., AA vs. AB) and pore topologies by minimizing energies, often predicting lattice parameters within 5% of experimental PXRD values. For instance, DFT has forecasted stable 2D and 3D COF variants with tunable band gaps, guiding synthetic targets like those with strong optical absorption from spiroborate linkers. These simulations integrate with experimental data to resolve ambiguities in PXRD indexing, such as distinguishing eclipsed from slipped layers. Porosity metrics, like pore volumes, can be derived from these modeled structures to estimate accessible space.

Spectroscopic and Thermal Methods

Fourier-transform infrared (FTIR) spectroscopy is widely employed to confirm the formation of covalent linkages in COFs by identifying characteristic bond vibrations. For imine-linked COFs, the appearance of a C=N stretching band around 1620–1625 cm⁻¹, coupled with the disappearance of precursor signals such as aldehyde C=O at approximately 1700 cm⁻¹ and amine N-H at 3300–3500 cm⁻¹, indicates successful condensation reactions.^[56]^[57] Similarly, in β-ketoenamine-linked frameworks like TpPa-1, the C=N stretch shifts to lower wavenumbers (e.g., 1580 cm⁻¹), reflecting the tautomeric equilibrium.^[58] These spectral changes allow real-time monitoring of synthesis completion and post-modification efficacy. Raman spectroscopy complements FTIR by probing vibrational modes sensitive to framework ordering and defects. In aromatic or graphitic COFs, such as azine-linked variants, the D band at ~1350 cm⁻¹ (disorder) and G band at ~1600 cm⁻¹ (graphitic sp² carbon) are observed, with I_D/I_G ratios (e.g., 1.01–1.11) quantifying crystallinity levels that correlate briefly with powder X-ray diffraction patterns.^[58] X-ray photoelectron spectroscopy (XPS) provides elemental composition and bonding insights, particularly for nitrogen-containing COFs; imine N 1s peaks appear at ~398 eV, distinct from amine (~400 eV) or pyridinic (~399 eV) nitrogens, enabling differentiation of linkage types.^[59] UV-Vis spectroscopy assesses electronic properties, revealing absorption onsets that define bandgaps (e.g., 1.8–2.4 eV in donor-acceptor COFs), with valence band edges probed via photoemission for photocatalytic applications.^[60] Thermogravimetric analysis (TGA) evaluates thermal stability through decomposition profiles under inert atmospheres. Many COFs exhibit onset temperatures of 300–400 °C, with imine-linked examples like COF-5 showing multi-step weight loss (e.g., 5–10% below 300 °C from solvent residues, followed by framework decomposition at 400–500 °C, leaving 40–60% char yield).^[61]^[58] Differential scanning calorimetry (DSC) detects phase transitions and enthalpic changes, such as endothermic peaks for layer sliding in 2D COFs or glass transitions in amorphous variants, typically in the 100–300 °C range.^[62] Nitrogen sorption isotherms at 77 K, often following thermal activation (e.g., outgassing at 150–200 °C), yield Brunauer-Emmett-Teller (BET) surface areas up to 4000 m²/g, linking thermal robustness to accessible porosity.^[58]

Applications

Gas Storage and Separation

Covalent organic frameworks (COFs) have emerged as promising materials for gas storage due to their high porosity and tunable pore environments, which facilitate physisorption through van der Waals interactions. For hydrogen storage, COF-102 demonstrates an excess uptake of 7.2 wt% at 77 K and 35 bar, achieved via weak physisorptive binding in its large pores (up to 3.6 nm diameter), positioning it among the top performers for cryogenic storage applications.^[63] This capacity is enabled by the framework's exceptional Brunauer-Emmett-Teller surface area exceeding 4000 m²/g, allowing efficient packing of H₂ molecules without chemisorption.^[63] Isosteric heats of adsorption for H₂ in such COFs typically range from 5 to 15 kJ/mol, determined through virial expansion analysis of isotherms, reflecting the reversible nature of physisorption suitable for storage and release cycles. Methane storage in COFs benefits similarly from their ordered porosity, with representative examples achieving volumetric uptakes of 5-10 v/v at 298 K and 65 bar, comparable to isoreticular metal-organic frameworks (IRMOFs) due to structural analogies in pore design. For instance, COF-102 exhibits a total methane uptake of approximately 180 v/v at 298 K and 35 bar, driven by van der Waals forces in its interconnected pore network.^[63] These capacities meet or approach U.S. Department of Energy targets for vehicular natural gas storage, highlighting COFs' potential as lightweight, high-density adsorbents.^[63] In gas separation, COFs excel through selective adsorption mechanisms, particularly for CO₂/N₂ mixtures relevant to flue gas capture. Amine-appended COFs, such as those with post-synthetic grafting of ethylenediamine, achieve CO₂/N₂ selectivities exceeding 40 at 298 K and 1 bar, attributed to strong quadrupole interactions between CO₂'s polarizable electron cloud and the electron-donating amine groups within the pores.^[64] Computational screening of imine-linked 3D COFs further identifies structures with mixture selectivities up to 215 for 15/85 CO₂/N₂ feeds, leveraging pore sizes around 1.0 nm for size- and affinity-based discrimination.^[65] Recent advances in 2025, including fluorinated COF variants, have enhanced CO₂/N₂ separation efficiencies with selectivities over 25 while maintaining high permeabilities, though bulk powder forms emphasize adsorption over permeation.^[66]

Catalysis and Energy Conversion

Covalent organic frameworks (COFs) have emerged as versatile platforms for catalysis and energy conversion due to their tunable porosity, high surface area, and ability to incorporate active sites such as metal centers or functional linkers. These features enable efficient substrate diffusion, site isolation, and enhanced charge transfer, making COFs superior to traditional catalysts in many reactions. In catalysis, COFs confine reactants within ordered pores, promoting selectivity and recyclability, while in energy devices, their structural stability supports sustained performance under operational stresses.^[67] In photocatalysis, COFs facilitate CO₂ reduction by leveraging metal or non-metal active sites to drive multi-electron processes. For instance, cobalt-integrated COFs like COF-367-Co achieve CO production rates of over 10,000 μmol·g⁻¹·h⁻¹ with turnover numbers exceeding 100 and selectivity above 75% for CO and CH₄, attributed to enhanced charge separation via donor-acceptor architectures. Similarly, rhenium-based COFs demonstrate CO formation with near 100% selectivity and turnover numbers >100, highlighting the role of metal sites in stabilizing intermediates. Non-metal sites, such as nitrogen-rich frameworks, enable CH₃OH production, though at lower rates, underscoring the versatility of COF designs for sustainable fuel synthesis.^[67]^[68]^[69] For electrocatalysis, COFs doped with transition metals exhibit low overpotentials in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Platinum single atoms anchored on asymmetric nitrogen-coordinated COFs deliver an HER overpotential of just 13 mV at 10 mA·cm⁻² in acidic media, benefiting from optimized hydrogen adsorption energies. Cobalt-doped imine-based COFs, such as Co/CoO@COF, show an OER overpotential of 278 mV at 10 mA·cm⁻² in alkaline conditions, with a Tafel slope of 80 mV·dec⁻¹, outperforming benchmarks like RuO₂ due to synergistic nanoparticle-framework interactions. These metrics reflect COFs' capacity to disperse active sites uniformly, minimizing aggregation and boosting kinetics.^[70] In general catalysis, the porosity of COFs enables confinement of enzymes or organocatalysts, enhancing stability and activity in organic transformations. Enzyme@COF biocomposites, formed via in situ encapsulation, retain over 90% activity post-immobilization, leveraging the framework's biocompatibility and pore channels to prevent denaturation while allowing substrate access. For organocatalysis, imine-linked COFs catalyze Knoevenagel condensations with yields exceeding 90%, as seen in N-heterocyclic carbene-functionalized variants that achieve >99% conversion in short times through basic site activation. This confinement strategy isolates catalytic units, suppressing side reactions and enabling facile recovery. For energy storage, COFs serve as electrodes in supercapacitors and batteries, exploiting redox-active linkers for charge storage. Pyridine-rich COFs deliver specific capacitances up to 546 F·g⁻¹ at 0.5 A·g⁻¹ in acidic electrolytes, driven by pseudocapacitive nitrogen sites and high conductivity. In lithium-ion batteries, COFs with anthraquinone or triazine linkers act as anodes, providing capacities over 500 mAh·g⁻¹ via reversible multi-electron redox, with cycling stability exceeding 500 cycles due to the framework's mechanical robustness. These applications highlight COFs' role in bridging molecular design with device-level performance.

Sensing and Biomedical Uses

Covalent organic frameworks (COFs) have emerged as effective platforms for sensing applications, leveraging their tunable optical properties and porous structures to detect hazardous substances with high sensitivity. In explosive detection, fluorescent COFs enable the identification of trinitrotoluene (TNT) through fluorescence quenching mechanisms, such as photo-induced electron transfer (PET), where nitro groups in TNT interact with the COF's emissive sites. For instance, the DL-COF material achieves detection of nitroaromatics, including TNT, at parts-per-billion (ppb) levels, corresponding to limits of detection (LOD) around 1 ppm, demonstrating selectivity in complex environments.^[71] Similarly, heavy metal ions are sensed via chelation with functional groups incorporated into COF frameworks, such as thioether or amine moieties that form coordination bonds, leading to fluorescence quenching or enhancement. The COF-LZU8, functionalized with thioether groups, detects Hg²⁺ with an LOD of 25 ppb and supports removal efficiencies exceeding 98% at low concentrations, highlighting the role of uniform pore distribution in enhancing binding site accessibility.^[72]^[73] In biomedical applications, COFs facilitate controlled drug delivery, particularly for anticancer agents, by exploiting their high surface area and stimuli-responsive linkages. A notable example is the one-pot synthesis of DOX@COF, where doxorubicin (DOX) achieves a loading capacity of 32.1 wt%, with pH-responsive release that accelerates in acidic tumor microenvironments (pH ~5.0), enabling targeted chemotherapy while minimizing off-target effects. Antibacterial COFs have gained traction as alternatives to traditional antibiotics amid rising resistance, with porphyrin-based frameworks generating reactive oxygen species (ROS) under light to disrupt bacterial membranes. A 2025 review underscores their rising potential, emphasizing strategies like pore engineering for enhanced ROS production against pathogens such as Staphylococcus aureus and Escherichia coli.^[74]^[75] Fluorescent COFs further advance bioimaging by providing biocompatible probes for cellular tracking, benefiting from their photostability and adjustable emission. TpPy covalent organic nanosheets (CONs), with sizes below 200 nm, enable high-resolution imaging of tumor cells and deep tissue penetration up to 150 μm, allowing real-time monitoring of cellular processes without significant cytotoxicity. For targeted therapy, stimuli-responsive COFs respond to tumor-specific cues like acidic pH or elevated glutathione, promoting on-demand drug release and combination modalities. These frameworks support chemotherapy integration with photodynamic therapy (PDT), where porphyrin-based COFs enhance ROS generation for precise cancer cell ablation while reducing aggregation issues in traditional photosensitizers.^[76] Recent 2025 developments extend COF utility to vaccine adjuvants, where large-pore 2D frameworks like PyCOFamide serve as self-adjuvanting platforms. By adsorbing antigens such as ovalbumin, these COFs act as reservoirs that promote antigen-presenting cell trafficking to lymph nodes, eliciting robust humoral and cell-mediated immunity upon degradation, which releases adjuvant monomers to activate immune responses. This approach outperforms free antigens, offering a stable, metal-free alternative for subunit vaccines.^[77]

Membranes and Filtration

Covalent organic frameworks (COFs) have emerged as promising materials for membrane-based filtration due to their tunable nanopores and high porosity, which enable selective molecular transport while maintaining structural integrity under operational pressures. In water filtration applications, COF membranes leverage their ordered pore structures for efficient desalination, where stacked two-dimensional (2D) layers act as molecular sieves with pore cutoffs around 0.5 nm to reject hydrated ions while permitting rapid water permeation. For instance, hourglass-shaped nanochannels in COF membranes, formed by integrating amino-cyclodextrin nanoparticles with 2D COF nanosheets, achieve water fluxes of 98 L m⁻² h⁻¹ at 2 bar with over 90% salt rejection for NaCl and Na₂SO₄, demonstrating superior performance compared to pristine COF membranes (45 L m⁻² h⁻¹ under identical conditions) and highlighting the role of hydrophilic entrances and hydrophobic spouts in enhancing selectivity and flux. These membranes also exhibit pH-responsive behavior and long-term stability over 7 days, making them suitable for practical seawater desalination. In gas separation, thin-film COF membranes fabricated via interfacial synthesis or layer-by-layer assembly offer high permeance and selectivity, capitalizing on their rigid frameworks for kinetic sieving of gases. A notable example is an ionic COF membrane with confined mobile carriers, which delivers a CO₂ permeance of 2347 gas permeation units (GPU) at 90 °C and a CO₂/N₂ selectivity of 191, surpassing traditional polymeric membranes by enabling efficient post-combustion capture from flue gases. Such thin films, often tens of nanometers thick, benefit from the inherent porosity of COFs to achieve permeances exceeding 1000 GPU while maintaining selectivities above 50 for CO₂ over N₂, as reported in recent advancements. For carbon capture specifically, amine-functionalized COF-based mixed-matrix membranes enhance CO₂ affinity through facilitated transport mechanisms, targeting post-combustion flue gas streams with low CO₂ concentrations. Amine groups, such as those in COF-300-Amide integrated into Pebax matrices, promote selective CO₂ binding and diffusion, yielding improved CO₂/N₂ separation factors and stability under humid conditions compared to unmodified COFs. These membranes facilitate CO₂ fluxes on the order of enhanced permeance values, supporting scalable capture processes with reduced energy penalties. COF coatings have also been applied to oil-water separation, where superhydrophobic surfaces repel water while allowing oil passage. Trifluoromethyl-containing COF composites form robust superhydrophobic membranes with water contact angles exceeding 150°, enabling efficient separation of oil-water emulsions with efficiencies over 99% across multiple cycles and resistance to mechanical abrasion. This approach utilizes the chemical stability and tunable wettability of COFs to address industrial wastewater challenges without compromising flux rates.

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