Covalent organic frameworks (COFs) are a fascinating class of crystalline porous materials that have gained more prominence in recent years. These ordered structures are constructed from organic building blocks connected through strong covalent bonds, forming extended two-dimensional (2D) or three-dimensional (3D) networks with predictable topologies and exceptional structural control.
Other types of porous materials have made headlines recently, notably metal-organic frameworks (MOFs), which were the subject of the 2025 Nobel Prize in Chemistry. MOFs are promising for their applications in gas separation, catalysis, energy storage, and biomedical uses like sensors. COFs can also be used in these applications because of their similar functionality. However, unlike MOFs or other porous materials such as zeolites, COFs are composed entirely of light elements such as carbon, hydrogen, nitrogen, oxygen, and boron.
This metal-free composition makes COF materials lighter, less susceptible to hydrolysis, and endows them with outstanding chemical and thermal stability. Moreover, while MOFs have metal nodes that can be toxic and may pose environmental concerns, COFs offer a more sustainable alternative, with the potential for recyclability and reduced toxicity.
COFs also exhibit ultra-high surface areas (often exceeding 2000 m²/g), and tunable pore sizes which range from microporous to mesoporous scales. They offer several key advantages including permanent porosity, low density, and facile surface functionalization, along with the ability to undergo pre- and post-synthetic modifications.
The unique combination of crystalline pores, tunable architecture, and structural precision has positioned COFs at the forefront of applications such as gas storage and separation, catalysis, sensing, and optoelectronics. As research in this field continues to expand, COFs are poised to play more important roles in next-generation technologies ranging from carbon capture and clean energy storage to advanced drug delivery systems and environmental remediation.
Publication data verifies the growing interest in COFs. By analyzing the CAS Content Collection™, the largest human-curated repository of scientific information, we confirmed that this field has grown exponentially since the discovery of COFs in 2005 by Yaghi et. al (see Figure 1).
Publication activity remained relatively modest for the first decade, reflecting the early exploratory phase of this field. Starting around 2016, there was a sharp, sustained increase in journal articles and patent families — a trend that signals a transition from foundational research to broader application-driven studies. This growth reflects the expanding scientific interest in COFs and their increasing relevance in real-world innovations.
How COF structures work
The exceptional versatility of COFs is attributed to their modular synthesis, wherein carefully selected organic monomers are systematically linked to form extended crystalline networks. These monomers are combined under solvothermal or mechanochemical conditions to form specific linkages, such as imine (C=N), β-ketoenamine, boronate ester, hydrazone, azine, and triazine.
The choice of monomers — their geometry, functionality, and linkage type — directly determines whether the COF adopts a 2D layered structure or a 3D framework. In 2D COFs, planar sheets stack via π–π interactions, while in 3D COFs, tetrahedral or C₃-symmetric building blocks lead to interconnected polyhedral networks. In addition, 2D COFs offer high in-plane conductivity, while 3D COFs provide higher surface areas and interconnected pore networks.
Leveraging tools including CAS SciFinder® and CAS STNext® with the data contained in the CAS Content Collection, we identified the most widely used monomers found in COF structures (see Figure 2a). These monomers included aldehydes such as 1,3,5-Triformylphloroglucinol (Tp), 1,3,5-Benzenetricarboxaldehyde (TFB), 2,5-Dimethoxy-1,4-benzenedicarboxaldehyde (DMTA) and amines such as p-Phenylenediamine (Pa-1 or PDA) , 1,3,5-Tris(4-aminophenyl)benzene (TAPB) and 4,4′,4′′-(1,3,5-Triazine-2,4,6-triyl)tris[benzenamine] (TAPT), each offering distinct reactivity, strong conjugation, and structural influence, which support 2D and 3D architectures.
For instance, Tp is valued for forming chemically robust β-ketoenamine linkages when reacting with aromatic amines, leading to COFs that maintain crystallinity even under acidic or humid conditions. This COF structure is therefore ideal for catalysis, sensing, and energy storage.
In addition, more complex, C₃-symmetric amines such as TAPB and TAPT introduce extended π-systems and electron-deficient triazine cores, respectively, which improve crystallinity and chemical resilience. Figure 2b showcases a set of representative COF structures made up of these monomers, like TpPa-1, TpTAPT, and TAPB-DMTA, illustrating the diversity in topology, pore geometry, and dimensionality. These examples demonstrate how variations in monomer geometry (e.g., linear vs. trigonal) and linkage type directly influence material properties such as porosity, crystallinity, and chemical robustness.
We also analyzed the top 10 most frequently utilized COFs in the CAS database (see Figure 3), which revealed TpPa-1 is the most dominant framework, followed by TAPB-DMTA and TFB-PDA. TpPa-1 forms a 2D layered architecture with β-ketoenamine linkages, which confers exceptional stability under harsh chemical and thermal conditions.
In contrast, TAPB-DMTA and TFB-PDA incorporate imine linkages that facilitate high crystallinity, superior hydrolytic stability, and well-defined pore structures, making them particularly suitable for applications requiring precise molecular organization. Notably, all ten COFs presented in Figure 3 feature either imine or β-ketoenamine linkages, reflecting the widespread use of these linkages in COF design. These linkages offer a unique combination of synthetic accessibility, structural versatility, and chemical robustness. Their ability to form stable, crystalline frameworks with tunable properties makes them the preferred choice for numerous COF research and applications.
COF applications in energy, environmental remediation, and biotechnology
COFs are gaining prominence for their versatility across various scientific and technological domains, including catalysis, biomedicine, sensing, energy and gas storage, and electronics (see Figure 4A). The distribution of relevant publications shows catalysis leading, followed by energy storage and biomedical applications. COFs are also widely employed in environmental remediation due to their non-toxic nature, biocompatibility, and tunable pore sizes tailored to specific pollutants.
Figure 4B focuses on some of the most promising applications of COFs across these domains. For instance, photoactive units (e.g., triazine, porphyrin, benzothiadiazole) and extended π-conjugation in COFs enable visible light absorption, charge separation, and transport. This makes them suitable for photocatalytic and electronic applications such as semiconductors and optoelectronic devices, including light-emitting diodes (LEDs), field-effect transistors (FETs), and photodetectors.
In the energy storage sector, COFs are primarily used in batteries, largely as electrode materials, owing to their ordered porous channels for fast ion diffusion (e.g., Li⁺ or Na⁺) and redox-active sites (e.g., carbonyls, imines, azo units) for reversible charge storage.
In the biomedical field, COFs’ biocompatibility, and modularity support drug delivery and therapeutic uses such as photodynamic therapy, photothermal therapy, and combination therapy. Their high surface area, ordered crystalline porosity, and tunable chemical functionality also enable water purification and desalination; selective sensing of biomolecules, gases, and ions; and efficient adsorption and separation of gases such as CO₂, H₂, and CH₄.
Returning to the top 10 most frequently cited COFs in the literature, we can see how these are used across seven major application domains (see Figure 5). This analysis reveals that nearly all COFs are employed in catalysis, owing to their high surface area, facile functional modification, and excellent recyclability, making them ideal platforms for efficient and selective catalytic processes.
Moreover, while TpPa-1 is also employed for environmental remediation, TAPB-DMTA is widely used for biomedical and sensor applications. Additionally, TpPa-SO3H is prominently used in the energy storage domain. This highlights how the structural and chemical diversity of COFs enables their tailored use across many advanced technologies.
Future outlook for COFs
Despite their promising properties, COFs face several critical challenges that have limited their widespread adoption and commercialization. One of the primary issues is the lack of scalability and reproducibility in synthesis. Many COFs require specific conditions such as solvothermal treatments, long reaction times, or harsh solvents, making large-scale production difficult. Additionally, while COFs are often valued for their crystallinity, achieving high crystallinity and structural uniformity remains a significant challenge, and poor crystallinity can compromise performance.
Limited electrical conductivity in many COFs also restricts their direct use in electronic and energy devices without further modification. Their integration into practical devices remains challenging due to difficulties in processing them into thin films or composites and their susceptibility to degradation in humid, acidic, or oxidative environments.
To address these challenges, researchers are pursuing several strategic approaches. Scalable and greener synthesis methods including mechanochemical, microwave-assisted, and room-temperature reactions are being developed to replace traditional solvothermal processes, making production more efficient and environmentally friendly.
Recent advances in topology prediction and AI-assisted design have significantly accelerated the discovery of novel COF structures. The development of hybrid COFs — by incorporating MOFs, polymers, or even nanomaterials — is also opening multifunctional capabilities, bridging the advantages of COFs and traditional materials. These developments suggest that COFs are moving beyond the limitations of complex synthesis and are being designed with specific applications in mind.
Thanks to their modularity, crystallinity, and tunable nature, COFs have extensive potential to improve a broad spectrum of fields, ranging from energy and gas storage to biomedicine. As advances in synthetic methodologies, functional design, and computational screening continue to improve, COFs can make the transition from promising laboratory materials to key enablers in sustainable and high-performance technologies.
