A Photoconductive Covalent Organic Framework: Self-Condensed Arene Cubes Composed of Eclipsed 2D Polypyrene Sheets for Photocurrent Generation

Covalent Organic Frameworks in Sample Preparation

Covalent organic frameworks (COFs) can be classified as emerging porous crystalline polymers with extremely high porosity and surface area size, and good thermal stability. These properties have awakened the interests of many areas, opening new horizons of research and applications. In the Analytical Chemistry field, COFs have found an important application in sample preparation approaches since their inherent properties clearly match, in a good number of cases, with the ideal characteristics of any extraction or clean-up sorbent. The review article is meant to provide a detailed overview of the different COFs that have been used up to now for sample preparation (i.e., solid-phase extraction in its most relevant operational modes-conventional, dispersive, magnetic/solid-phase microextraction and stir-bar sorptive extraction); the extraction devices/formats in which they have been applied; and their performances and suitability for this task.

Microporous Frameworks as Promising Platforms for Antibacterial Strategies Against Oral Diseases

Nowadays, the heavy burden of oral diseases such as dental caries, periodontitis, endodontic infections, etc., and their consequences on the patients' quality of life indicate a strong need for developing effective therapies. Bacterial infections played an important role in the field of oral diseases, in-depth insight of such oral diseases have given rise to the demand for antibacterial therapeutic strategies. Recently, microporous frameworks have attracted tremendous interest in antibacterial application due to their well-defined porous structures for drug delivery. In addition, intensive efforts have been made to enhance the antibacterial performance of microporous frameworks, such as ion doping, photosensitizer incorporation as building blocks, and surface modifications. This review article aims on the major recent developments of microporous frameworks for antibacterial applications against oral diseases. The first part of this paper puts concentration on the cutting-edge researches on the versatile antibacterial strategies of microporous materials via drug delivery, inherent activity, and structural modification. The second part discusses the antibacterial applications of microporous frameworks against oral diseases. The applications of microporous frameworks not only have promising therapeutic potential to inhibit bacterial plaque-initiated oral infectious diseases, but also have a wide applicability to other biomedical applications.

Covalent organic framework photocatalysts: structures and applications

In the light of increasing energy demand and environmental pollution, it is urgently required to find a clean and renewable energy source. In these years, photocatalysis that uses solar energy for either fuel production, such as hydrogen evolution and hydrocarbon production, or environmental pollutant degradation, has shown great potential to achieve this goal. Among the various photocatalysts, covalent organic frameworks (COFs) are very attractive due to their excellent structural regularity, robust framework, inherent porosity and good activity. Thus, many studies have been carried out to investigate the photocatalytic performance of COFs and COF-based photocatalysts. In this critical review, the recent progress and advances of COF photocatalysts are thoroughly presented. Furthermore, diverse linkers between COF building blocks such as boron-containing connections and nitrogen-containing connections are summarised and compared. The morphologies of COFs and several commonly used strategies pertaining to photocatalytic activity are also discussed. Following this, the applications of COF-based photocatalysts are detailed including photocatalytic hydrogen evolution, CO₂ conversion and degradation of environmental contaminants. Finally, a summary and perspective on the opportunities and challenges for the future development of COF and COF-based photocatalysts are given.

Two-Dimensional Boronate Ester Covalent Organic Framework Thin Films with Large Single Crystalline Domains for a Neuromorphic Memory Device

Despite the recent progress in the synthesis of crystalline boronate ester covalent organic frameworks (BECOFs) in powder and thin-film through solvothermal method and on-solid-surface synthesis, respectively, their applications in electronics, remain less explored due to the challenges in thin-film processability and device integration associated with the control of film thickness, layer orientation, stability and crystallinity. Moreover, although the crystalline domain sizes of the powder samples can reach micrometer scale (up to ≈1.5 μm), the reported thin-film samples have so far rather small crystalline domains up to 100 nm. Here we demonstrate a general and efficient synthesis of crystalline two-dimensional (2D) BECOF films composed of porphyrin macrocycles and phenyl or naphthyl linkers (named as 2D BECOF-PP or 2D BECOF-PN) by employing a surfactant-monolayer-assisted interfacial synthesis (SMAIS) on the water surface. The achieved 2D BECOF-PP is featured as free-standing thin film with large single-crystalline domains up to ≈60 μm² and tunable thickness from 6 to 16 nm. A hybrid memory device composed of 2D BECOF-PP film on silicon nanowire-based field-effect transistor is demonstrated as a bio-inspired system to mimic neuronal synapses, displaying a learning-erasing-forgetting memory process.

Two-dimensional amine and hydroxy functionalized fused aromatic covalent organic framework

Ordered two-dimensional covalent organic frameworks (COFs) have generally been synthesized using reversible reactions. It has been difficult to synthesize a similar degree of ordered COFs using irreversible reactions. Developing COFs with a fused aromatic ring system via an irreversible reaction is highly desirable but has remained a significant challenge. Here we demonstrate a COF that can be synthesized from organic building blocks via irreversible condensation (aromatization). The as-synthesized robust fused aromatic COF (F-COF) exhibits high crystallinity. Its lattice structure is characterized by scanning tunneling microscopy and X-ray diffraction pattern. Because of its fused aromatic ring system, the F-COF structure possesses high physiochemical stability, due to the absence of hydrolysable weak covalent bonds.

Covalent organic frameworks for separation applications

Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with highly tuneable structures and functionalities. COFs have been proposed as ideal materials for applications in the energy-intensive field of molecular separation due to their notable intrinsic features such as low density, exceptional stability, high surface area, and readily adjustable pore size and chemical environment. This review attempts to highlight the key advancements made in the synthesis of COFs for diverse separation applications such as water treatment or the separation of gas mixtures and organic molecules, including chiral and isomeric compounds. Methods proposed for the fabrication of COF-based columns and continuous membranes for practical applications are also discussed in detail. Finally, a perspective regarding the remaining challenges and future directions for COF research in the field of separation has also been presented.

Ultrahigh Responsivity Photodetectors of 2D Covalent Organic Frameworks Integrated on Graphene

2D materials exhibit superior properties in electronic and optoelectronic fields. The wide demand for high-performance optoelectronic devices promotes the exploration of diversified 2D materials. Recently, 2D covalent organic frameworks (COFs) have emerged as next-generation layered materials with predesigned π-electronic skeletons and highly ordered topological structures, which are promising for tailoring their optoelectronic properties. However, COFs are usually produced as solid powders due to anisotropic growth, making them unreliable to integrate into devices. Here, by selecting tetraphenylethylene monomers with photoelectric activity, elaborately designed photosensitive 2D-COFs with highly ordered donor-acceptor topologies are in situ synthesized on graphene, ultimately forming COF-graphene heterostructures. Ultrasensitive photodetectors are successfully fabricated with the COF_ETBC-TAPT -graphene heterostructure and exhibited an excellent overall performance with a photoresponsivity of ≈3.2 × 10⁷ A W^-1 at 473 nm and a time response of ≈1.14 ms. Moreover, due to the high surface area and the polarity selectivity of COFs, the photosensing properties of the photodetectors can be reversibly regulated by specific target molecules. The research provides new strategies for building advanced functional devices with programmable material structures and diversified regulation methods, paving the way for a generation of high-performance applications in optoelectronics and many other fields.

Anatomy of On-Surface Synthesized Boroxine Two-Dimensional Polymers

Synthetic two-dimensional polymers (2DPs) obtained from well-defined monomers via bottom-up fabrication strategies are promising materials that can extend the realm of inorganic 2D materials. The on-surface synthesis of such 2DPs is particularly popular, however the pathway complexity in the growth of such films formed on solid surfaces is poorly understood. In this contribution, we present a straightforward experimental protocol which allows the synthesis of large-area, defect-free 2DPs based on boroxine linkages at room temperature. We focus on unravelling the multiple pathways available to the polymerizing system for the spatial extension of the covalent bonds. Besides the anticipated 2DP, the system can evolve into self-assembled monolayers of partially fused monodisperse reaction products that are difficult to isolate by conventional synthetic methods or remain in the monomeric state. The access to each pathway can be controlled via monomer concentration and the choice of the solvent. Most importantly, the unpolymerized systems do not evolve into the corresponding 2DP upon annealing, indicating the presence of strong kinetic traps. Using high-resolution scanning tunneling microscopy, we show reversibility in the polymerization process where the attachment and the detachment of monomers to 2DP crystallites could be monitored as a function of time. Finally, we show that the way the 2DP grows depends on the choice of the solvent. Using UV-vis absorption and emission spectroscopy, we reveal that the dominant pathway for 2DP growth is via in-plane self-condensation of the monomers, whereas in the case of an aprotic solvent, the favored growth mode is via π stacking of the monomers.

Revealing the Local Electronic Structure of a Single-Layer Covalent Organic Framework through Electronic Decoupling

Covalent organic frameworks (COFs) are molecule-based 2D and 3D materials that possess a wide range of mechanical and electronic properties. We have performed a joint experimental and theoretical study of the electronic structure of boroxine-linked COFs grown under ultrahigh vacuum conditions and characterized using scanning tunneling spectroscopy on Au(111) and hBN/Cu(111) substrates. Our results show that a single hBN layer electronically decouples the COF from the metallic substrate, thus suppressing substrate-induced broadening and revealing new features in the COF electronic local density of states (LDOS). The resulting sharpening of LDOS features allows us to experimentally determine the COF band gap, bandwidths, and the electronic hopping amplitude between adjacent COF bridge sites. These experimental parameters are consistent with the results of first-principles theoretical predictions.

Covalent Organic Frameworks: Design, Synthesis, and Functions

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

Building Block Design for Minimizing Defects in the Construction of Two-Dimensional Covalent Organic Frameworks

Polymerization of monomers into two-dimensional covalent organic frameworks with precise porous structures exhibits desired catalytic, gas separation, and optoelectronic properties. However, the defects arising from covalent bonding in a polymerization process always result in amorphous films with small crystalline domains or polycrystalline powders. It is still a tremendous challenge to synthesize high-quality crystalline products, even single crystals with a large size over the micrometer scale. In this work, we propose a general strategy of building block design to reduce the defects during growth of two-dimensional covalent organic frameworks. We demonstrate that the building block with a hexagonal pore unit, i.e., a hexamer, could greatly decrease defects by directional uniform growth in polymerization, while monomer, dimer, and trimer building blocks form more defects due to linear growth. Our work provides a new strategy to construct superlarge single crystals in practical applications by combining building block design and growing dynamics control.

Rapid Synthesis of High Surface Area Imine-Linked 2D Covalent Organic Frameworks by Avoiding Pore Collapse During Isolation

Imine-linked 2D covalent organic frameworks (COFs) form more rapidly than previously reported under Brønsted acid-catalyzed conditions, showing signs of crystallinity within a few minutes, and maximum crystallinity within hours. These observations contrast with the multiday reaction times typically employed under these conditions. In addition, vacuum activation, which is often used to isolate COF materials significantly erodes the crystallinity and surface area of the several isolated materials, as measured by N₂ sorption and X-ray diffraction. This loss of material quality during isolation for many networks has historically obscured otherwise effective polymerization conditions. The influence of the activation procedure is characterized in detail for three COFs, with the commonly used 1,3,5-tris(4-aminophenyl)benzene-terephthaldehyde network (TAPB-PDA COF), the most prone to pore collapse. When the networks are activated carefully, rapid COF formation is general for all five of the imine-linked 2D COFs studied, with all exhibiting excellent crystallinity and surface areas, including the highest surface areas reported to date for three materials. Furthermore, to simplify the workup of COF materials, a simple nitrogen flow method provides high-quality materials without the need for specialized equipment. These insights have important implications for studying and understanding how 2D COFs form.

A simple molecular design for tunable two-dimensional imine covalent organic frameworks for optoelectronic applications

We systematically study the electronic structure, carrier mobility and work function of imine based 2D-COFs. The bandgaps of these semiconducting materials can be tailored by doping with nitrogen for tunable electronic/optoelectronic properties.

Discrete boronate ester ladders from the dynamic covalent self-assembly of oligo(phenylene ethynylene) derivatives and phenylenebis(boronic acid)

Reversible boronate ester chemistry enables the controlled, dynamic self-assembly of olig(phenylene ethynylene)s into highly conjugated ladder frameworks.

Function-oriented synthesis of two-dimensional (2D) covalent organic frameworks – from 3D solids to 2D sheets

This review provides guidelines for the function-oriented synthesis of 2D COFs from 3D solids to 2D sheets.

The luminescent and photophysical properties of covalent organic frameworks

This review highlights the luminescent and unique photophysical properties of covalent organic frameworks. Their potential use in applications related to chemical sensing, photocatalysis, and optoelectronics are discussed.

An ionic covalent organic polymer toward highly selective removal of anionic organic dyes in aqueous solution

A novel ionic COP was prepared for the highly selective removal of anionic organic dyes in aqueous solution.

Postsynthetic functionalization of covalent organic frameworks

Covalent organic frameworks (COFs) have been at the forefront of porous-material research in recent years. With predictable structural compositions and controllable functionalities, the structures and properties of COFs could be controlled to achieve targeted materials. On the other hand, the predesigned structure of COFs allows fruitful postsynthetic modifications to introduce new properties and functions. In this review, the postsynthetic functionalizations of COFs are discussed and their impacts towards structural qualities and performances are comparatively elaborated on. The functionalization involves the formation of specific interactions (covalent or coordination/ionic bonds) and chemical reactions (oxidation/reduction reaction) with pendant groups, skeleton and reactive linkages of COFs. The chemical stability and performance of COFs including catalytic activity, storage, sorption and opto-electronic properties might be enhanced by specific postsynthetic functionalization. The generality of these strategies in terms of chemical reactions and the range of suitable COFs places them as a pivotal role for the development of COF-based smart materials.

Dibenzochrysene enables tightly controlled docking and stabilizes photoexcited states in dual-pore covalent organic frameworks

Covalent organic frameworks (COFs), consisting of covalently connected organic building units, combine attractive features such as crystallinity, open porosity and widely tunable physical properties. For optoelectronic applications, the incorporation of heteroatoms into a 2D COF has the potential to yield desired photophysical properties such as lower band gaps, but can also cause lateral offsets of adjacent layers. Here, we introduce dibenzo[g,p]chrysene (DBC) as a novel building block for the synthesis of highly crystalline and porous 2D dual-pore COFs showing interesting properties for optoelectronic applications. The newly synthesized terephthalaldehyde (TA), biphenyl (Biph), and thienothiophene (TT) DBC-COFs combine conjugation in the a,b-plane with a tight packing of adjacent layers guided through the molecular DBC node serving as specific docking site for successive layers. The resulting DBC-COFs exhibit a hexagonal dual-pore kagome geometry, which is comparable to COFs containing another molecular docking site, namely 4,4',4'',4'''-(ethylene-1,1,2,2-tetrayl)-tetraaniline (ETTA). In this context, the respective interlayer distances decrease from about 4.6 Å in ETTA-COFs to about 3.6 Å in DBC-COFs, leading to well-defined hexagonally faceted single crystals sized about 50-100 nm. The TT DBC-COF features broad light absorption covering large parts of the visible spectrum, while Biph DBC-COF shows extraordinary excited state lifetimes exceeding 10 ns. In combination with the large number of recently developed linear conjugated building blocks, the new DBC tetra-connected node is expected to enable the synthesis of a large family of highly correlated and ordered 2D COFs with promising optoelectronic properties.

Covalent Organic Frameworks: Chemical Approaches to Designer Structures and Built-In Functions

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

Pore surface engineering of covalent organic frameworks: structural diversity and applications

Connecting molecular building blocks by covalent bonds to form extended crystalline structures has caused a sharp upsurge in the field of porous materials, especially covalent organic frameworks (COFs), thereby translating the accuracy, precision, and versatility of covalent chemistry from discrete molecules to two-dimensional and three-dimensional crystalline structures. COFs are crystalline porous frameworks prepared by a bottom-up approach from predesigned symmetric units with well-defined structural properties such as a high surface area, distinct pores, cavities, channels, thermal and chemical stability, structural flexibility and functional design. Due to the tedious and sometimes impossible introduction of certain functionalities into COFs via de novo synthesis, pore surface engineering through judicious functionalization with a range of substituents under ambient or harsh conditions using the principle of coordination chemistry, chemical conversion, and building block exchange is of profound importance. In this review, we aim to summarize dynamic covalent chemistry and framework linkage in the context of design features, different methods and perspectives of pore surface engineering along with their versatile roles in a plethora of applications such as biomedical, gas storage and separation, catalysis, sensing, energy storage and environmental remediation.

Vinylene-Linked Covalent Organic Frameworks by Base-Catalyzed Aldol Condensation

Two 2D covalent organic frameworks (COFs) linked by vinylene (-CH=CH-) groups (V-COF-1 and V-COF-2) are synthesized by exploiting the electron deficient nature of the aromatic s-triazine unit of C3 -symmetric 2,4,6-trimethyl-s-triazine (TMT). The acidic terminal methyl hydrogens of TMT can easily be abstracted by a base, resulting in a stabilized carbanion, which further undergoes aldol condensation with multitopic aryl aldehydes to be reticulated into extended crystalline frameworks (V-COFs). Both V-COF-1 (with terepthalaldehyde (TA)) and V-COF-2 (with 1,3,5-tris(p-formylphenyl)benzene (TFPB)) are polycrystalline and exhibit permanent porosity and BET surface areas of 1341 m2  g-1 and 627 m2  g-1 , respectively. Owing to the close proximity (3.52 Å) of the pre-organized vinylene linkages within adjacent 2D layers stacked in eclipsed fashion, [2+2] photo-cycloadditon in V-COF-1 formed covalent crosslinks between the COF layers.

Fused Aromatic Network Structures as a Platform for Efficient Electrocatalysis

Fused aromatic network (FAN) structures are a category of ordered porous polymers that permit the specific fusion of building blocks into extended porous network structures with designed skeletons and pores. One significant feature of FANs is that their structures can be tailorable with fused aromatic rings without rotatable single-bond connectivity. As a result, the geometry and space orientation of the building blocks are easily incorporated to guide the topological expansion of the architectural periodicity. The variety of building units and fused linkages make FANs a promising materials platform for constitutional outline and functional design. The stably confined spaces of FAN architectures can be extended for the exchange of photons, ions, electrons, holes, and guest molecules, and exhibit customized chemical, electrochemical and optical properties. Herein, the main progress and advances in the field of 2D and 3D FANs and their utilization as a platform to develop efficient electrocatalysts for energy conversion and storage applications are summarized.

Luminescent Molecularly Imprinted Polymers Based on Covalent Organic Frameworks and Quantum Dots with Strong Optical Response to Quinoxaline-2-Carboxylicacid

Three-dimensional molecularly imprinted polymers (MIPs) based on quantum dots-grafted covalent organic frameworks (QDs-grafted COFs) are reported in this study. The compound 1,3,5-triformylphloroglucinol-P-phenylenediamine was used as COF material to react with the amino-modified CdSe/ZnS QDs by Schiff-base reactions. The amino-derived QDs reacted with quinoxaline-2-carboxylicacid (QCA) via a non-covalent interaction. The system combines the advantages of MIPs, COFs, and QDs for highly sensitive and selective QCA detection. The MIPs based on QDs-grafted COFs showed good chemical selectivity and thermal stability, as well as consistency in QCA optosensing. Under optimal conditions, the detection limit for QCA in meat and feed samples was 0.85 μmol L⁻¹, over a linear concentration range of 1–50 μmol L⁻¹. The current findings suggest a potential application of MIPs based on QDs-grafted COFs for the detection of trace levels of hazardous chemicals for food safety and environmental control.

Fabrication of Hydrazone-Linked Covalent Organic Frameworks Using Alkyl Amine as Building Block for High Adsorption Capacity of Metal Ions

There are several researches on the preparation and application of hydrazone-linked covalent organic frameworks (COFs), and all of them generally necessitate rigid aromatic amines. Herein, we report a strategy for design and synthesis of COF with flexible alkyl amine as a building block and intramolecular hydrogen bonding as a knot in the network. The proof-of-concept design was demonstrated by exploring 1,3,5-triformylphloroglucinol and oxalyldihydrazide (ODH) as precursors to synthesize a novel COF material (TpODH), in which different organic building units are combined through hydrazone bonds to form two-dimensional porous frameworks. It should be pointed that irreversible enol-to-keto tautomerism and intramolecular N-H···O═C hydrogen bonding of TpODH would enhance the crystallinity and chemical stability, leading to large specific surface area of 835 m² g^-1. However, another COF synthesized with 1,3,5-triformylbenzene and ODH exhibited less crystallinity and low special surface area (94 m² g^-1). Representatively, the resulting TpODH afforded Cu(II) and Hg(II) capacities of 324 and 1692 mg g^-1, respectively, which exceeded that of most COFs previously reported. Moreover, the Fourier-transform infrared and X-ray photoelectron spectroscopy spectra analyses were taken to demonstrate the adsorption mechanism. These results suggested that the materials could be applied to the removal of metallic ions in the future.

Functional π-Conjugated Two-Dimensional Covalent Organic Frameworks

Fingerprints of π-conjugated compounds are ubiquitous in nature and play a crucial part in human existence. For instance, cis-retinal, an endogenous π-conjugated molecule present in the eye, performs a vital role in the function of visual perception. π-Conjugated molecules have also received a great deal of attention owing to their intriguing optical properties and created a surge in optoelectronics. Varieties of π-conjugated molecules/oligomers have been developed and explored for a number of applications such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and sensors, among others. While the extended π-delocalization in one-dimensional (1D) polymers versus oligomers produce superior optical and electronic properties, further extension of π-delocalization to the second dimension (2D) is expected to give rise even more intriguing properties as revealed by theoretical studies. As a matter of fact, graphene is the best example of 2D-conjugated polymers, but its zero-band-gap behavior is a major impediment for semiconducting applications. In contrast, it was challenging to prepare 2D crystalline polymers until the discovery of boroxine/boronate ester linked covalent organic frameworks (COFs) by Yaghi and co-workers. COFs are a new class of porous crystalline polymers in which organic building blocks are held together by covalent bonds. These polymers exhibit potential applications in gas storage, energy storage, photocatalyst, heterogeneous catalysis, sensors, etc. However, the first π-conjugated COF was realized in 2009 via the introduction of imine linker (-C═N-) between the building blocks. Since then, wide varieties of COFs with various π-delocalization promoting spacers have been developed and explored their electronic and optical properties and pertinent applications. In this review, we will highlight the importance of 2D π-conjugated COFs and their achievements in developing novel functionalities.

Molecular Insights into Carbon Dioxide Sorption in Hydrazone-Based Covalent Organic Frameworks with Tertiary Amine Moieties

Tailorable sorption properties at the molecular level are key for efficient carbon capture and storage and a hallmark of covalent organic frameworks (COFs). Although amine functional groups are known to facilitate CO₂ uptake, atomistic insights into CO₂ sorption by COFs modified with amine-bearing functional groups are scarce. Herein, we present a detailed study of the interactions of carbon dioxide and water with two isostructural hydrazone-linked COFs with different polarities based on the 2,5-diethoxyterephthalohydrazide linker. Varying amounts of tertiary amines were introduced in the COF backbones by means of a copolymerization approach using 2,5-bis(2-(dimethylamino)ethoxy)terephthalohydrazide in different amounts ranging from 25 to 100% substitution of the original DETH linker. The interactions of the frameworks with CO₂ and H₂O were comprehensively studied by means of sorption analysis, solid-state NMR spectroscopy, and quantum-chemical calculations. We show that the addition of the tertiary amine linker increases the overall CO₂ sorption capacity normalized by the surface area and of the heat of adsorption, whereas surface areas and pore size diameters decrease. The formation of ammonium bicarbonate species in the COF pores is shown to occur, revealing the contributing role of water for CO₂ uptake by amine-modified porous frameworks.

Hollow Microspherical and Microtubular [3 + 3] Carbazole-Based Covalent Organic Frameworks and Their Gas and Energy Storage Applications

Covalent organic frameworks (COFs) are a family of crystalline porous networks having applications in various fields, including gas and energy storage. Despite respectable progress in the synthesis of such crystalline materials, examples of the use of template-free methods to construct COFs having hollow nano- and microstructures are rare. Furthermore, all reported methods for synthesizing these hollow structural COFs have involved [4 + 2] and [3 + 2] condensations. Herein, we report the synthesis of hollow microspherical and microtubular carbazole-based COFs through template-free, one-pot, [3 + 3] condensations of the novel triamine 9-(4-aminophenyl)-carbazole-3,6-diamine (Car-3NH₂) and triformyl linkers with various degrees of planarity. Depending upon the monomer's planarity, a unique morphological variety was observed. A time-dependent study revealed that each COF formed through an individual mechanism depended on the degree of planarity of the triformyl linker; it also confirmed that the hollow structures of these COFs formed through inside-out Ostwald ripening. Our COFs exhibited high Brunauer-Emmett-Teller surface areas (up to ca. 1400 m² g^-1), excellent crystallinity, and high thermal stability. Moreover, the CO₂ uptake capacities of these COFs were excellent: up to 61 and 123 mg g^-1 at 298 and 273 K, respectively. The high surface areas facilitated greater numbers of strong interactions with CO₂ molecules, leading to high CO₂ uptake capacities. Moreover, the prepared COFs exhibited redox activity because of their redox-active triphenylamine and pyridine groups, which can be utilized in electrochemical energy storages. Accordingly, such hollow COFs having high surface areas appear to be useful materials for industrial and biological applications.