On‐Liquid Surface Synthesis of Crystalline 2D Polyimine Thin Films

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Detachable all-carbon-linked 3D covalent organic framework films for semiconductor/COF heterojunctions by continuous flow synthesis

Bio-inspired construction of ion conductive pathway in covalent organic framework membranes for efficient lithium extraction

COFs Meet Graphene Nanoribbons

ConspectusIon-selective membranes are key components for sustainable energy devices, including osmotic power generators, electrolyzers, fuel cells, and batteries. These membranes facilitate the flow of desired ions (permeability) while efficiently blocking unwanted ions (selectivity), which forms the basis for energy conversion and storage technologies. To improve the performance of energy devices, the pursuit of high-quality membranes has garnered substantial interest, which has led to the exploration of numerous candidates, such as polymeric membranes (e.g., polyamide and polyelectrolyte), laminar membranes (e.g., transition metal carbide (MXene) and graphene oxide (GO)) and nanoporous 2D membranes (e.g., single-layer MoS2 and porous graphene). Despite impressive progress, the trade-off effect between ion permeability and selectivity remains a major scientific and technological challenge for these membranes, impeding the efficiency and stability of the resulting energy devices.Two-dimensional polymers (2DPs), which represent monolayer to few-layer covalent organic frameworks (COFs) with periodicity in two directions, have emerged as a new candidate for ion-selective membranes. The crystalline 2DP membranes (2DPMs) are typically fabricated either by bulk crystal exfoliation followed by filtration or by direct interfacial synthesis. Recently, the development of surfactant-monolayer-assisted interfacial synthesis (SMAIS) method by our group has been pivotal, enabling the synthesis of various highly crystalline and large-area 2DPMs with tunable thicknesses (1 to 100 nm) and large crystalline domain sizes (up to 120 μm2). Compared to other membranes, 2DPMs exhibit well-defined one-dimensional (1D) channels, customizable surface charge, ultrahigh porosity, and ultrathin thickness, enabling them to overcome the permeability-selectivity trade-off challenge. Leveraging these attributes, 2DPMs have established their critical roles in diverse energy devices, including osmotic power generators and metal ion batteries, opening the door for next-generation technology aimed at sustainability with a low carbon footprint.In this Account, we review our achievements in synthesizing 2DPMs through the SMAIS method and highlight their selective-ion-transport properties and applications in sustainable energy devices. We initially provide an overview of the SMAIS method for producing highly crystalline 2DPMs by utilizing the programmable assembly and enhanced reactivity/selectivity on the water surface. Subsequently, we discuss the critical structural parameters of 2DPMs, including pore sizes, charged sites, crystallinity, and thickness, to elucidate their roles in selective ion transport. Furthermore, we present the burgeoning landscape of energy device applications for 2DPMs, including their use in osmotic power generators and as electrode coating in metal ion batteries. Finally, we conclude persistent challenges and future prospects encountered in synthetic chemistry, material science, and energy device applications within this rapidly evolving field.

Covalent organic frameworks (COFs) are emerging as promising materials across diverse fields, including catalysis, gas storage, water harvesting, and energy storage. Traditionally, COF development relies on the two-component synthesis approach, where two organic monomers with specific reactive sites are strategically combined to form well-ordered frameworks in specific topologies. Although this approach has been effective, it inherently restricts the structural diversity of COFs unless new linkers are introduced, limiting the range of properties achievable. A more flexible and increasingly popular approach in COF synthesis is the use of multicomponent reactions and mixed linker strategies, incorporating three or more organic monomers into a single COF. While researchers have primarily focused on reaction pathways for synthesizing multiple-component COFs, one area that remains relatively underexplored is the use of flexible linkers within these strategies. This study addresses this gap by selecting a flexible amine linker and combining it with four distinct aldehyde linkers in various combinations. This approach allows us to successfully synthesize fifteen different highly crystalline COF materials, including two-component, along with three, four, and five-component COFs. A comparative study of photocatalytic performance between the two-component with the five-component COF demonstrated that increased structural functionalities leads to higher surface area, enhanced visible-light absorption, and consequently enhanced photocatalytic activity in the oxidative conversion of aryl phenylboronic acid to phenol.

Electroactive covalent organic frameworks: a new choice for organic electronics

Covalent organic frameworks (COFs) are a class of ordered porous organic material formed via the covalent bonding among various organic monomers. Recently, studies of chiral COFs have attracted great attention because chirality plays an important role in medicine, pharmacy and agriculture, and chiral COFs possess many fascinating characteristics like inherent porosity, tunable pore size, structural periodicity, high stability and reusability. Ascribed to the great advantages of chiral COFs, they demonstrate many potential applications in asymmetrical catalysis, asymmetrical separation and so on. One strategy to construct chiral COFs is using multifarious chiral building blocks to directly prepare chiral COFs. Various chiral COFs structures can be designed via choosing different building blocks and adding functional groups to monomers. An alternative strategy to construct chiral COFs is to incorporate metal particles or organic species into COFs via post-synthetic modifications.

Heteroaromatic molecular building blocks can be assembled in the highly ordered spatial environment of covalent organic frameworks (COFs), forming two-dimensional (2D) and 3D crystalline architectures. For example, photoactive molecular moieties such as porphyrins and other chromophores can be spatially integrated into their crystalline lattice, allowing us to create models for organic bulk heterojunctions, chemical sensors and porous electrodes for photoelectrochemical systems.Here, we will discuss different strategies aimed at creating electroactive networks capable of light-induced and electrochemical charge transfer. In earlier work, we have developed COF-based heterojunctions containing stacked thienothiophene-based building blocks acting as electron donors with a 3 nm open pore system, which show light-induced charge transfer to an intercalated fullerene acceptor phase.[1] We now create interpenetrated donor-acceptor COF phases with novel dibenzochrysene-based building blocks that can be viewed as constrained propeller-shaped tetraphenylethylene with reduced curvature engaging in very tight π-stacking.[2] Thienothiophene- (TT) and benzodithiophene-2,6-dicarboxaldehyde based 2D kagome COFs were synthesized in situ with a fullerene derivative to create interpenetrated electron-donor/acceptor host-guest systems showing efficient charge transfer within ps from the COF to the fullerene guest.Contrasting the above approach, one can design COF integrated heterojunctions consisting of alternating columns of stacked donor and acceptor molecules, promoting the photo-induced generation of mobile charge carriers inside the COF network.[3] Synthetic efforts have led to several COFs integrating extended chromophores capable of efficient harvesting of visible and near infrared light, for example.[4] Notably, heterocyclic regioisomers that can be embedded in the same COF crystal structure allow for fine-tuning of optical absorption and luminescence.[5]Extending thin film growth methodology to create a solvent-stable oriented 2D COF photoabsorber structure, COF films can serve in photoelectrochemical water splitting systems.[6] The detailed mechanism of excited state dynamics in light-harvesting conjugated COFs has been revealed by means of transient absorption spectroscopy,[7] while ongoing work establishes efficient excited state diffusion – even across grain boundaries – in 2D COF thin films. Many optoelectronic applications of COFs depend on significant electrical conductivity. Here, Wurster-type structural motifs are attractive building blocks for imparting high conductivity in the corresponding COFs,[8] which feature tunable optical properties upon integrating donor-acceptor moieties. COF films can also act as ultrafast solvatochromic chemical sensors,[9] as photodetectors,[10] and show very efficient electrochromic response.[11]While guest-responsive (breathing) MOFs have been studied for some time, to date few guest-responsive COF structures have been described. We have now developed dynamic two-dimensional COFs that can open and close their pores upon uptake or removal of guests while fully retaining their crystalline long-range order.[12] Here, a wine rack design based on rigid, π-stacked columns of perylene diimides (PDIs) interconnected by non-stacked, flexible bridges shows stepwise phase transformations between contracted pore and open pore forms with up to 40% increase in unit cell volume. By reversibly tuning π-interactions via guest sorption, we modulate excitonic coupling within the COFs, where the PDI moieties can be switched from excimer-forming H-aggregates to null-aggregates with monomer-like absorption and emission characteristics. These findings open new pathways towards designing stimuli-responsive optical, electronic, or spintronic materials.Ongoing work focuses on the design of ultra-large pore donor-acceptor COFs with extended light-harvesting abilities and optimized charge separation, illustrating their intriguing structural diversity leading to enhanced optoelectronic functionality.[13]References Dogru et al., Chem. Int. Ed. 2013, 52, 2920.Xue et al., to be submitted Calik et al., J. Am. Chem. Soc. 2014, 136, 17802.Keller et al., J. Am. Chem. Soc. 2017, 139, 8194.Guntermann et al., 2024, in revision. Sick et al., J. Am. Chem. Soc. 2018, 140, 2085.Jakowetz et al., J. Am. Chem. Soc. 2019, 141, 11565.Rotter et al., Chem. Sci. 2020, 11, 12843.Ascherl et al., Nature Commun. 2018, 9, 3802.Bag et al., J. Am. Chem. Soc. 2023, 145, 1649.Muggli et al., ACS Nanosci. Au 2023, 3, 153.Auras et al., Nature Chem. 2024, 16, 1373. Blätte, Ortmann, Bein, J. Am. Chem. Soc., 2024, 146, 32161.

Toward miniaturizing microelectronics using covalent organic framework dielectric

Covalent organic frameworks (COFs) are promising materials for a variety of applications, including membrane-based separations, thin-film electronics, and as separators for electrochemical devices. Robust mechanical properties are critical to these applications, but there are no reliable methods for patterning COFs or producing free-standing thin films for direct mechanical testing. Mechanical testing of COFs has only been performed on films supported by a rigid substrate. Here, we present a method for patterning, transferring, and measuring the tensile properties of free-floating nanoscale COF films. We synthesized COF powders by condensation of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehyde (PDA) and prepared uniform thin films by spin casting from a mixture of trifluoroacetic acid and water. The COF films were then reactivated to recover crystallinity and patterned by plasma etching through a poly(dimethylsiloxane) (PDMS) mask. The films were transferred to the surface of water, and we performed direct tensile tests. We measured a modulus of approximately 1.4 GPa for TAPB-PDA COF and a fracture strain of 2.5%, which is promising for many applications. This work advances the development of COFs for thin-film applications by demonstrating a simple and generally applicable approach to cast, pattern, and transfer COF thin films and to perform direct mechanical analysis.

Strategies enabling solution processing of covalent organic framework (COF) thin films will become increasingly important as these versatile materials are integrated into a wide range of electronic and optical devices. This work highlights an approach to yield thin film synthesis of TAPA‐PDA (TAPA: tris(4‐aminophenyl)amine, PDA: terephthalaldehyde) and TAPB‐PDA (TAPB: 1,3,5‐tris(4‐aminophenyl)benzene) imine COFs using azomethine compounds which can be drop cast onto a variety of substrates. High crystalline COF films are shown to form on various electronically‐relevant substrates. Grazing incidence wide angle X‐ray scattering characterization reveals COF films with a preferred horizontal orientation in the case of TAPA‐PDA COF and a more mixed/vertical orientation in the TAPB‐PDA COF film regardless of the substrate. As this exciting class of crystalline organic materials becomes more relevant for various device applications, solution processing techniques will be vital to take advantage of the properties of thin film COFs.

Enhancing Fenton-like reaction mediating performance of covalent organic frameworks through porosity modification

The added flexibility in architecture and multidimensional applications make metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) compelling candidates for the development of thin-film sensors. This review examines the on-surface synthesis of thin films of MOFs and COFs and focuses on advanced deposition methods, including chemical vapor deposition (CVD) and interface-assisted synthesis (IAS), as well as multifunctional sensing with MOF and COF thin films. Due to the high porosity, tunable structures, and excellent electronic/photoelectric properties, COF and MOF thin films have shown promising applications in gas sensing, environmental monitoring, and in wearable sensors. Additionally, the review discusses challenges related to long-term stability, film uniformity, and integration under real-world conditions. Some of the uniformity, longitudinal stability, and real-world conditions span deep time durability issues in practical use as well as long-term severe environment exposure alongside other prospective solutions for stilled gaps, which the review analyses. This overview aims to provide a roadmap for the development of next-generation sensors based on structurally robust and functional MOF/COF thin films.

Covalent organic frameworks (COFs) materials are crystalline polymer networks constructed by organic building blocks. Their tunable structure, low density, well-defined pores, high surface areas and high thermal stability render them have promising applications in gas storage and separations, energy storage, catalysis, sensing and optoelectronic. This review focuses on the design principle of COF based on the structure and functionality, and summarizes the recent synthesize method of COF bulk powder, single layer COFs and COF films. The design principle of COFs is classified into the structural-oriented and functional-oriented protocols. For structural design, the basic principle is reticular chemistry. The geometry of the building units and the symmetry of the reaction group determine the dimension, topology, structure, and the pore size and shape of COFs. The reaction types of COF are usually based on thermodynamic controlled reversible reaction. Moreover, the functionality of COFs can be easily modulated via three strategies, including directly using functionalized building blocks to construct COFs, post-synthetic modification of the pre-prepared framework, and encapsulation of functional molecules into the channel of COFs. Along with the structural and functional design, various COF materials have been reported and greatly boosted the developments of this field. Then we summarized the synthetic methodologies of COFs. In general, the developed synthetic methodologies follow three aspects: (1) fast and efficient; (2) for large-scale industrial production; (3) for processing shaped-body of COF. Recently, there are several available methods to fabricate COF into free-standing films including the liquid-liquid interfacial polymerization, vapor-assisted conversion and baking the molecular precursors. These approaches indicate a way forward for accessing COF into films and enable applying to real industrial applications. Although there has been a significant expansion of COF chemistries over the past 15 years, yet this field is far from mature, the article ends with a perspective on the future developments in this growing field and discusses the challenges remain for these materials. To date, it is still hard to obtain single crystals, thus advanced characterization techniques and theoretical calculation methods are highly demanded to be developed. The mechanisms for the nucleus formation and growth are still needed to be deeply investigated. In addition, exploring novel topologies and linkages of COF are necessary from the chemistry perspective. Moreover, in order to precisely construct and tune the functions of COF materials, further studies on the structure-property correlation via the directly design functional building blocks and encapsulation of functional molecules into the pores are still required. In terms of the processing COF from the particle to shaped bodies and to devices would be critically important in many practical applications, like gas separation, catalysis and organic light-emitting diodes, thus further development towards the efficient synthesis of COF films or membranes with large area, high orientation, no defect, and uniform thickness would be of great interest. For COF materials, advances in the fast, efficient and economic fabrication methods and the exploration of new functionalization would make them become the commercialized material for industrial application in the fields of energy, health, environment and so on.