Such synthesis bottlenecks have confined COFs largely to academic curiosities rather than scalable functional materials, constraining their integration into devices or industrial processes. The reliance on solvothermal or ionothermal methodologies demands elevated temperatures sustained over several hours or days, often in sealed reactors under inert atmospheres, while intricate purification protocols are mandatory to isolate the products. These procedures not only compromise the environmental sustainability of the COF production but also impede rapid iteration and large-scale manufacturing. Crucially, efforts to circumvent these issues by developing alternative synthesis routes have frequently culminated in compromised crystallinity and pore architecture—two fundamental attributes underpinning the superior performance of COFs.

In a groundbreaking study published in Nature Chemical Engineering, Jin, Wang, Cheng, and colleagues have introduced an innovative solid-phase hot-pressing technique that promises to reconfigure the synthetic paradigm for COFs. This approach sidesteps the limitations of solvent-based reactions, enabling the fabrication of highly crystalline, porous COF platelets in mere minutes—a dramatic reduction from the conventional multi-hour protocols. Through this method, 15 different COFs encompassing diverse linkage chemistries, such as imine, hydrazone, β-ketoenamine, and imide bonds, were successfully synthesized, showcasing the technique’s versatility and broad applicability.

The essence of the solid-phase hot-pressing strategy lies in intimately mixing the monomeric powders and subjecting them to controlled heat and pressure within a solid matrix, thereby accelerating the polymerization process without necessitating solvents. This shift not only enhances the sustainability profile of COF synthesis but also yields platelet-shaped products with superior crystallinity evident through sharp diffraction peaks and enlarged surface areas as verified by nitrogen adsorption measurements. Importantly, the crystallinity and porosity are preserved or even enhanced compared to their conventionally synthesized counterparts, overcoming the historic trade-off encountered in rapid or solvent-free syntheses.

One of the most compelling advantages of this methodology is its capacity to accommodate complex COF architectures. The researchers demonstrated the fabrication of COFs with sophisticated chemical topologies, including a rare three-dimensional COF and frameworks assembled from multiple monomer components. Such complexity often bedevils traditional approaches due to difficulties in maintaining uniform reaction conditions and achieving complete polymerization. The hot-pressing technique’s ability to homogenize the reaction environment at the solid phase evidently mitigates these challenges, allowing precise control over the framework geometry.

Moreover, the process duration astonishingly spans only between 30 seconds and 5 minutes, representing an unprecedented acceleration in COF assembly. This rapid reaction kinetics stem from the synergy of heat and mechanical pressure in promoting imine condensation and other covalent bond formations at the intimate contact interfaces of monomers. Consequently, this facilitates immediate framework nucleation and growth, producing platelet morphologies that are highly suited for thin-film technologies and facile device integration.

Beyond the synthetic triumphs, the practical ramifications of this development are exemplified through a proof-of-concept application. The team assembled a β-ketoenamine-linked COF platelet directly into an atmospheric water harvesting device, demonstrating robust water absorption and collection performance. This real-world demonstration underscores the COF platelet’s enhanced surface accessibility and structural robustness—traits essential for cyclic operation under variable humidity conditions. Atmospheric water harvesting technologies benefit immensely from such materials, as their pore structures and chemical stability dictate efficiency and longevity.

The atmospheric water harvesting device exemplifies a class of applications where the morphological uniformity, high crystallinity, and porosity of COF platelets are particularly advantageous. Unlike powders or irregularly shaped aggregates, platelet structures can reliably form continuous and defect-minimized films, facilitating optimal vapor diffusion and condensate release. This tangible translation from synthetic methodology to applied technology reaffirms the hot-pressing solid-phase approach not only as an academic curiosity but also as an industrially relevant innovation.

The implications of this method extend well beyond water harvesting. The universal applicability to different COF linkage chemistries suggests potential breakthroughs in fields relying on COF-based membranes, sensors, energy storage devices, and heterogeneous catalysis. The easy scalability and rapid turnaround time reduce production costs and environmental burdens, which are critical considerations for deployment in commercial and environmental contexts. Moreover, the elimination of hazardous solvents aligns with green chemistry principles, fostering safer laboratory practices and reducing ecological footprints.

Technical characterization of the COF platelets synthesized via hot-pressing revealed exceptional crystallographic fidelity. X-ray diffraction patterns display sharp, well-defined peaks consistent with the anticipated framework topologies. Brunauer-Emmett-Teller (BET) surface areas often surpass those obtained through conventional solvothermal synthesis, indicating well-preserved or enhanced porosity. Scanning electron microscopy images illustrate uniform platelet morphology with consistent thickness and lateral dimensions, further reinforcing the high quality of the materials generated. Such detailed structural analyses validate the robustness of the synthesis protocol and provide insights into the role of solid-phase conditions in dictating framework order.

From a mechanistic standpoint, the solid-phase hot-pressing environment likely introduces unique reaction kinetics compared to solution-based methods. The absence of solvent molecules, which traditionally mediate diffusion and monomer mobility, necessitates direct contact between reacting species under pressure and heat. This enforced proximity accelerates bond formation while limiting defects and undesirable side reactions. Furthermore, the brief processing times prevent framework degradation or uncontrolled side reactions that can plague longer, high-temperature syntheses. These mechanistic advantages translate directly into the high crystallinity and pore uniformity that define the quality of COFs for functional use.

It is also notable that the newly developed methodology opens avenues for combinatorial materials science within the COF domain. By permitting multiple monomers and complex chemistries to polymerize rapidly under uniform conditions, researchers can systematically explore vast chemical space to design frameworks with tailored properties. This capability will accelerate discovery in functional COFs targeting selective adsorption, electronic properties, and catalytic activities. The integration of hot-pressing with in situ characterization techniques might further elucidate growth mechanisms and enable real-time optimization of synthetic parameters.

Given the burgeoning interest in sustainable technologies and materials, the ability to synthesize COFs rapidly, cleanly, and with outstanding structural control represents a milestone. This work bridges the gap between laboratory-scale curiosity and scalable application, potentially catalyzing a paradigm shift in the manufacturing of porous, crystalline organic frameworks. Future explorations may optimize hot-pressing parameters further, expand the library of accessible COF chemistries, and demonstrate integrated devices harnessing the full structural advantages of platelet morphologies.

In summary, the introduction of a rapid, solid-phase hot-pressing method to produce highly crystalline COF platelets signifies a powerful advancement in materials chemistry. By overcoming longstanding synthetic barriers—long reaction times, toxic solvents, and suboptimal morphologies—this strategy paves the way for the next generation of COF-enabled technologies. As researchers worldwide strive for materials solutions that are both effective and practical, such innovations promise to unlock the latent potential of COFs and inspire a new era of functional porous materials tailored for the needs of modern society.

Subject of Research: Synthesis and fabrication of highly crystalline covalent organic framework (COF) platelets via a rapid solid-phase hot-pressing method.

Article Title: Rapid solid-phase synthesis of highly crystalline covalent organic framework platelets.

Article References:
Jin, Y., Wang, H., Cheng, H. et al. Rapid solid-phase synthesis of highly crystalline covalent organic framework platelets. Nat Chem Eng (2025). https://doi.org/10.1038/s44286-025-00277-9

Image Credits: AI Generated

The significance of moiré superlattices is underscored by their rarity in organic crystal formations, in stark contrast to their presence in inorganic structures. Achieving such formations requires the materials to be ultrathin and highly crystalline—characteristics that are notoriously challenging to realize in organic substances. The research team’s focus on bilayer COFs is particularly noteworthy as it addresses the intrinsic difficulties associated with imaging organic materials using traditional microscopy techniques, which often fall short when applied to such delicate structures.

Covalent organic frameworks encapsulate a vibrant landscape of possibilities, specifically in applications like catalysis, energy storage, and gas storage. These structures are comprised of covalently bonded layers aggregated through electrostatic interactions and van der Waals forces. However, despite their utility, the transition from a monolayer to a bilayer configuration exemplifies a poorly understood aspect of their synthesis, primarily due to intermolecular bonding complexities. Information regarding the precise alignment and stacking of layers is paramount in determining the resultant material’s crystallinity and overall performance characteristics.

The research undertaken by Professor Loh Kian Ping and his team illuminates the intricate interplay of bonding forces involved in COF assembly, including van der Waals, electrostatic, and hydrogen bonds. Despite previous advancements in producing monolayers, challenges persist in synthesizing single COF crystals exceeding millimeter dimensions due to potential bonding error accumulations in both horizontal and vertical stacking processes. This misalignment can lead to significant complications regarding the crystallinity of layered materials and real-time observation of the stacking process presents an additional hurdle, particularly when dealing with the fluid dynamics involved in solution-based growth.

The research highlights that random stacking tendencies and bond formations during hydrothermal synthesis frequently hinder crystallinity, resulting in crystal domains significantly smaller than expected sizes. Gaining an in-depth understanding of the stacking mechanisms could dramatically enhance the synthesis protocols, possibly enabling the development of larger COF crystals with improved properties. The present advancements particularly in 2D polymers are exciting; however, many opportunities lie within the yet-untapped area of bilayer 2D polymer (2DP) stacks—a field promising exceptional advances through careful control of stacking and twisting of 2D materials.

Loh’s team employed a significant methodological leap that allowed them to synthesize large-area bilayer 2D COFs directly at the liquid-substrate interface. By utilizing a direct condensation technique during synthesis, they adhered to the layered structure’s integrity. Their implementation of scanning tunneling microscopy (STM) in solution was revolutionary, as it permitted real-time observation of the molecular assembly during bilayer formation. This method was crucial in revealing how solvent composition and molecular structure influenced bilayer stacking modes, leading to the spectacular emergence of large-area moiré superlattices.

The technical challenges posed by COFs, given their organic and highly porous nature, complicate imaging under traditional conditions. The scenarios necessitating ultra-high vacuum (UHV) or air-exposed conditions often contribute to the degradation of quality essential for atomic-scale imaging. However, by adapting their imaging methods to directly observe COFs while they remain in solution, the research team was able to circumvent many of these obstacles. Prof. Loh expressed the advantage of conducting STM in a liquid medium, remarking that it creates cleaner surfaces than those typically seen when materials are subjected to air.

In pursuit of characterizing the fundamental aspects of twisted bilayers, the research team dedicated significant attention to comparing different isomers, namely pyrene-2,7-diboronic acid (27-PDBA) and pyrene-1,6-diboronic acid (16-PDBA). They discovered that the second layer’s stacking behavior was influenced considerably by the variations in the precursor molecular architecture. Specifically, with 27-PDBA, the stacking could result in either an AA-stacked configuration or a twisted formation, showcasing the potential scalability of tunable properties. Conversely, 16-PDBA yielded a consistent moiré structure without the emergence of dwellings eliciting twist differences, demonstrating the complexity arising from the distinct electrostatic properties of the constituent molecules.

The implications of this research are far-reaching and suggest profound potential applications across various fields. With a foundation built upon controlled synthesis and the ability to manipulate twist angles, the opportunities for tailored materials are vast. The enhancement of ultra-thin porous structures paves the way for innovations in nanofiltration technologies—they could serve as functional barriers and frameworks with tuned channel geometries. Moreover, opportunities for developments that enable optimized light propagation, including manipulation of phase and polarization, are emerging as a critical avenue for further exploration.

Looking towards the future, the research group aims to leverage their foundational knowledge to elaborate upon a broader array of molecular precursors characterized by diverse linkage chemistries. Achieving deterministic control over the twist angles in subsequent bilayer COF systems could unlock previously unimagined applications, further contributing to the rapidly evolving field of organic electronics and nanomaterials. This ambitious initiative signals a promising horizon for researchers and industries alike, as they pursue novel applications driven by understanding and manipulating the molecular architecture of layered organic frameworks.

The intersection of advanced materials science and innovative imaging technologies heralds exciting prospects in the development of next-generation materials. With substantial evidence showcasing the practical applications and a clarified framework for future endeavors, the research conducted at the National University of Singapore establishes itself as a cornerstone in the ongoing quest toward functionalized, smart materials that blur the lines between traditional chemistry and advanced engineering.

Participants in this collaborative research included notable figures from various institutions, extending the impact of their findings across the global scientific community. The collective effort underscores the importance of cross-institutional collaboration in tackling complex challenges and pushing the frontiers of what is achievable in the field of materials science.

The research findings were disseminated through a formal publication in the esteemed journal, "Nature Chemistry," currently heralding significant interest in the scientific community. The implications of these discoveries are poised to inspire an extensive array of future studies exploring the intricate properties and applications of twisted bilayers in diverse scientific domains.

Given the evolving landscape of materials science and the potential for novel innovations to emerge, this research not only contributes to the current body of knowledge but also ignites curiosity for unexplored avenues in bilayer COFs and moiré superlattices. As researchers continue to unravel the complexities within these organic frameworks, we anticipate further revelations and advancements that could redefine technological applications and foster sustainable solutions within our increasingly material-driven world.

Subject of Research: Covalent Organic Frameworks and Moiré Superlattices
Article Title: Moiré two-dimensional covalent organic framework superlattices
News Publication Date: 20-Feb-2025
Web References: Link to Nature Chemistry
References: DOI 10.1038/s41557-025-01748-5
Image Credits: National University of Singapore

Keywords

Superlattices, Discovery Research, Two Dimensional Materials, Covalent Organic Frameworks, Scanning Tunneling Microscopy, Molecular Structure.