Heteropolyaromatic Covalent Organic Frameworks via One-Pot Multicomponent Reactions.

Multicomponent reactions (MCRs) offer a platform to create different chemical structures and linkages for highly stable covalent organic frameworks (COFs). As an illustrative example, the multicomponent Povarov reaction generates 2,4-phenylquinoline from aldehydes and amines in the presence of electron-rich alkenes. In this study, we introduce a new domino reaction to generate unprecedented 2,3-phenylquinoline COFs in the presence of epoxystyrene. This work thus presents, for the first time, structural isomeric COFs produced by multicomponent domino and Povarov reactions. Furthermore, 2,3-phenylquinolines can undergo a Scholl reaction to form extended aromatic linkages. With this approach, we synthesize two thermally and chemically stable MCR-COFs and two heteropolyaromatic COFs using both domino and in situ domino and Scholl reactions. The structure and properties of these COFs are compared with the corresponding 2,4-phenylquinoline-linked COF and imine-COF, and their activity toward benzene and cyclohexane sorption and separation is investigated. The position of the pendant phenyl groups within the COF pore plays a crucial role in facilitating the industrially important sorption and separation of benzene over cyclohexane. This study opens a new avenue to construct heteropolyaromatic COFs via MCR reactions.

Integrating Bifunctionality and Chemical Stability in Covalent Organic Frameworks via One-Pot Multicomponent Reactions for Solar-Driven H2O2 Production.

Multicomponent reactions (MCRs) can be used to introduce different functionalities into highly stable covalent organic frameworks (COFs). In this work, the irreversible three-component Doebner reaction is utilized to synthesize four chemically stable quinoline-4-carboxylic acid DMCR-COFs (DMCR-1-3 and DMCR-1NH) equipped with an acid-base bifunctionality. These DMCR-COFs show superior photocatalytic H2O2 evolution (one of the most important industrial oxidants) compared to the imine COF analogue (Imine-1). This is achieved with sacrificial oxidants but also in pure water and under an oxygen or air atmosphere. Furthermore, the DMCR-COFs show high photostability, durability, and recyclability. MCR-COFs thus provide a viable materials' platform for solar to chemical energy conversion.

Programmable Photocatalytic Activity of Multicomponent Covalent Organic Frameworks Used as Metallaphotocatalysts.

The multicomponent approach allows to incorporate several functionalities into a single covalent organic framework (COF) and consequently allows the construction of bifunctional materials for cooperative catalysis. The well-defined structure of such multicomponent COFs is furthermore ideally suited for structure-activity relationship studies. We report a series of multicomponent COFs that contain acridine- and 2,2'-bipyridine linkers connected through 1,3,5-benzenetrialdehyde derivatives. The acridine motif is responsible for broad light absorption, while the bipyridine unit enables complexation of nickel catalysts. These features enable the usage of the framework materials as catalysts for light-mediated carbon-heteroatom cross-couplings. Variation of the node units shows that the catalytic activity correlates to the keto-enamine tautomer isomerism. This allows switching between high charge-carrier mobility and persistent, localized charge-separated species depending on the nodes, a tool to tailor the materials for specific reactions. Moreover, nickel-loaded COFs are recyclable and catalyze cross-couplings even using red light irradiation.

Solar Light Driven H2 O2 Production and Selective Oxidations Using a Covalent Organic Framework Photocatalyst Prepared by a Multicomponent Reaction.

A chemically stable 2D microporous COF (PMCR-1) was synthesized via the multicomponent Povarov reaction. PMCR-1 exhibits a remarkable and long-term stable photocatalytic H2 O2 production rate (60 h) from pure and sea water under visible light. The H2 O2 production is markedly enhanced when benzyl alcohol (BA) is added as reductant, which is also due to a strong π-π interaction of BA with dangling phenyl moieties in the COF pores introduced by the multicomponent Povarov reaction. Motivated by the concomitant BA oxidation to benzaldehyde during H2 O2 formation, the photocatalytic oxidation of various organic substrates such as benzyl amine and methyl sulfide derivatives was investigated. It is shown that the well-defined micropores of PMCR-1 enable size-selective photocatalytic oxidation.

A Thiazole-linked Covalent Organic Framework for Lithium-Sulphur Batteries.

Lithium-sulphur (Li-S) batteries are a promising alternative power source, as they can provide a higher energy density than current lithium-ion batteries. Porous materials are often used as cathode materials as they can act as a host for sulphur in such batteries. Recently, covalent organic frameworks (COFs) have also been used, however they typically suffer from stability issues, resulting in limited and thus insufficient durability under practical conditions and applications. Herein, we report the synthesis of a crystalline and porous imine-linked triazine-based dimethoxybenzo-dithiophene functionalized COF (TTT-DMTD) incorporating high-density redox sites. The imine linkages were further post-synthetically transformed to yield a robust thiazole-linked COF (THZ-DMTD) by utilizing a sulphur-assisted chemical conversion method, while maintaining the crystallinity. As a synergistic effect of its high crystallinity, porosity and the presence of redox-active moieties, the thiazole-linked THZ-DMTD exhibited a high capacity and long-term stability (642 mAh g-1 at 1.0 C; 78.9 % capacity retention after 200 cycles) when applied as a cathode material in a Li-S battery.

Acridine-Functionalized Covalent Organic Frameworks (COFs) as Photocatalysts for Metallaphotocatalytic C-N Cross-Coupling.

Covalent organic frameworks (COFs) are structurally tuneable, porous and crystalline polymers constructed through the covalent attachment of small organic building blocks as elementary units. Using the myriad of such building blocks, a broad spectrum of functionalities has been applied for COF syntheses for broad applications, including heterogeneous catalysis. Herein, we report the synthesis of a new family of porous and crystalline COFs using a novel acridine linker and benzene-1,3,5-tricarbaldehyde derivatives bearing a variable number of hydroxy groups. With the broad absorption in the visible light region, the COFs were applied as photocatalysts in metallaphotocatalytic C-N cross-coupling. The fully ß-ketoenamine linked COF showed the highest activity, due to the increased charge separation upon irradiation. The COF showed good to excellent yields for several aryl bromides, good recyclability and even catalyzed the organic transformation in presence of green light as energy source.

Protonated Imine-Linked Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution.

Covalent organic frameworks (COFs) have emerged as an important class of organic semiconductors and photocatalysts for the hydrogen evolution reaction (HER)from water. To optimize their photocatalytic activity, typically the organic moieties constituting the frameworks are considered and the most suitable combinations of them are searched for. However, the effect of the covalent linkage between these moieties on the photocatalytic performance has rarely been studied. Herein, we demonstrate that donor-acceptor (D-A) type imine-linked COFs can produce hydrogen with a rate as high as 20.7 mmol g-1 h-1 under visible light irradiation, upon protonation of their imine linkages. A significant red-shift in light absorbance, largely improved charge separation efficiency, and an increase in hydrophilicity triggered by protonation of the Schiff-base moieties in the imine-linked COFs, are responsible for the improved photocatalytic performance.

Synthesis of Vinylene-Linked Covalent Organic Frameworks from Acetonitrile: Combining Cyclotrimerization and Aldol Condensation in One Pot.

Combining two or more consecutive reactions in one pot is a common approach for process development, as such a method involves cheap starting materials and allows in situ generation of a reactive intermediate, to undergo further reaction, without isolation. Herein, we report the synthesis of a vinylene-linked (-CH═CH-) covalent organic framework, COF-701, directly from acetonitrile, a cheap commodity solvent, by combining/telescoping two consecutive reactions-cyclotrimerization of nitrile and subsequent aldol condensation with aldehydes-in one pot. Acetonitrile is trimerized to generate protonated 2,4,6-trimethyl-s-triazine tautomers in situ, which undergo Aldol condensation with 4,4'-biphenyldicarbaldehyde in one pot to form crystalline COF-701. COF-701 is obtained as a polycrystalline powder and possesses permanent microporosity and a BET surface area (SABET) of 736 m2·g-1. This strategy can be further extended to generate other porous vinylene-linked frameworks.

Strongly Reducing (Diarylamino)benzene-Based Covalent Organic Framework for Metal-Free Visible Light Photocatalytic H2O2 Generation.

Photocatalytic reduction of molecular oxygen is a promising route toward sustainable production of hydrogen peroxide (H2O2). This challenging process requires photoactive semiconductors enabling solar energy driven generation and separation of electrons and holes with high charge transfer kinetics. Covalent organic frameworks (COFs) are an emerging class of photoactive semiconductors, tunable at a molecular level for high charge carrier generation and transfer. Herein, we report two newly designed two-dimensional COFs based on a (diarylamino)benzene linker that form a Kagome (kgm) lattice and show strong visible light absorption. Their high crystallinity and large surface areas (up to 1165 m2·g-1) allow efficient charge transfer and diffusion. The diarylamine (donor) unit promotes strong reduction properties, enabling these COFs to efficiently reduce oxygen to form H2O2. Overall, the use of a metal-free, recyclable photocatalytic system allows efficient photocatalytic solar transformations.

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.

3D Anionic Silicate Covalent Organic Framework with srs Topology.

The synthesis of 3D covalent organic frameworks (COFs) adopting novel topologies is challenging, and so far 3D COFs have only been reported for nets based on building blocks with tetrahedral geometry. We demonstrate the targeted synthesis of an anionic 3D COF crystallizing in a three-coordinated srs net by exploiting a recently developed linkage for the formation of anionic silicate COFs based on hypercoordinate silicon nodes. The framework, named SiCOF-5, was synthesized by reticulating dianionic hexacoordinate [SiO6]2- nodes with triangular triphenylene building blocks and adopts a two-fold interpenetrated srs-c net with an overall composition of Na2[Si(C18H6O6)] (where C18H6O6 is triphenylene-2,3,6,7,10,11-hexakis(olate)). A key requirement for the crystallization of SiCOF-5 was the careful control over the nucleation and growth rate by gradual generation of the silicon source during the course of the reaction.

Diacetylene Functionalized Covalent Organic Framework (COF) for Photocatalytic Hydrogen Generation.

Covalent organic frameworks (COFs) are crystalline, highly porous, two- or three-dimensional polymers with tunable topology and functionalities. Because of their higher chemical stabilities in comparison to their boron-linked counterparts, imine or ß-ketoenamine linked COFs have been utilized for a broad range of applications, including gas storage, heterogeneous catalysis, energy storage devices, or proton-conductive membranes. Herein, we report the synthesis of highly porous and chemically stable acetylene (-C≡C-) and diacetylene (-C≡C-C≡C-) functionalized ß-ketoenamine COFs, which have been applied as photocatalyst for hydrogen generation from water. It is shown that the diacetylene moieties have a profound effect as the diacetylene-based COF largely outperforms the acetylene-based COF in terms of photocatalytic activity. As a combined effect of high porosity, easily accessible diacetylene (-C≡C-C≡C-) functionalities and considerable chemical stability, an efficient and recyclable heterogeneous photocatalytic hydrogen generation is achieved.

A Metal-Organic Framework with Tetrahedral Aluminate Sites as a Single-Ion Li+ Solid Electrolyte.

We demonstrate the synthesis of the first anionic aluminum metal-organic framework (MOFs) constructed from tetrahedral AlO4 sites. Al-Td-MOF-1 was obtained in a simple two-step synthesis by condensation of 1,4-dihydroxybenzene and lithium aluminum hydride into an amorphous aluminate framework before applying a solvothermal treatment under basic conditions to obtain the crystalline Al-Td-MOF-1 with a chemical composition of Li[Al(C6 H4 O2 )2 ]. The overall Al-Td-MOF-1 structure consists of one-dimensional chains of alternating edge-sharing AlO4 and LiO4 tetrahedral sites describing unidirectional pore channels with a square window aperture of ≈5×5 Å2 , best described topologically as a uninodal 6-coordinated snp rod net. Al-Td-MOF-1 features the highest Li+ loading reported to date for a MOF (2.50 wt %) and proved to be an effective single-ion solid electrolyte. An ionic conductivity of 5.7×10-5  S cm-1 was measured for Al-Td-MOF-1 and the beneficial contribution of crystallinity was evidenced by an 8-fold increase in conductivity between the disordered and crystalline material.

An anionic two dimensional covalent organic framework from tetratopic borate centres pillared by lithium ions.

Non-covalent interactions play an important role for the framework formation of two-dimensional covalent organic frameworks. Until now, π-π interactions and hydrogen bonding are the main reported forces facilitating the stacking of framework layers. Here, we present a two-dimensional anionic covalent organic framework based on tetratopic borate linkages, where layers are connected by ionic interactions between the linkage site and counter cations. The crystalline covalent organic framework is accessed through the formation of an amorphous borate-based polymer and subsequent solvothermal treatment. The progress of crystallization is investigated, revealing the crystallite growth and morphological change from agglomerated dense particles to hollow crystallite spheres. Due to the pillared nature, the crystallites can be exfoliated into nanosheets by sonication of the material in the presence of methanol. The crystallization and ordered arrangement of the lithium ions in the interlayer space is shown to benefit the conductivity tenfold compared to the amorphous material.

Finding the Sweet Spot of Photocatalysis─A Case Study Using Bipyridine-Based CTFs.

Covalent triazine frameworks (CTFs) are a class of porous organic polymers that continuously attract growing interest because of their outstanding chemical and physical properties. However, the control of extended porous organic framework structures at the molecular scale for a precise adjustment of their properties has hardly been achieved so far. Here, we present a series of bipyridine-based CTFs synthesized through polycondensation, in which the sequence of specific building blocks is well controlled. The reported synthetic strategy allows us to tailor the physicochemical features of the CTF materials, including the nitrogen content, the apparent specific surface area, and optoelectronic properties. Based on a comprehensive analytical investigation, we demonstrate a direct correlation of the CTF bipyridine content with the material features such as the specific surface area, band gap, charge separation, and surface wettability with water. The entirety of these parameters dictates the catalytic activity as demonstrated for the photocatalytic hydrogen evolution reaction (HER). The material with the optimal balance between optoelectronic properties and highest hydrophilicity enables HER production rates of up to 7.2 mmol/(h·g) under visible light irradiation and in the presence of a platinum cocatalyst.

Constitutional isomerism of the linkages in donor-acceptor covalent organic frameworks and its impact on photocatalysis.

When new covalent organic frameworks (COFs) are designed, the main efforts are typically focused on selecting specific building blocks with certain geometries and properties to control the structure and function of the final COFs. The nature of the linkage (imine, boroxine, vinyl, etc.) between these building blocks naturally also defines their properties. However, besides the linkage type, the orientation, i.e., the constitutional isomerism of these linkages, has rarely been considered so far as an essential aspect. In this work, three pairs of constitutionally isomeric imine-linked donor-acceptor (D-A) COFs are synthesized, which are different in the orientation of the imine bonds (D-C=N-A (DCNA) and D-N=C-A (DNCA)). The constitutional isomers show substantial differences in their photophysical properties and consequently in their photocatalytic performance. Indeed, all DCNA COFs show enhanced photocatalytic H2 evolution performance than the corresponding DNCA COFs. Besides the imine COFs shown here, it can be concluded that the proposed concept of constitutional isomerism of linkages in COFs is quite universal and should be considered when designing and tuning the properties of COFs.

Hydrothermal polymerization of porous aromatic polyimide networks and machine learning-assisted computational morphology evolution interpretation.

We report on the hydrothermal polymerization (HTP) of polyimide (PI) networks using the medium H2O and the comonomers 1,3,5-tris(4-aminophenyl)benzene (TAPB) and pyromellitic acid (PMA). Full condensation is obtained at minimal reaction times of only 2 h at 200 °C. The PI networks are obtained as monoliths and feature thermal stabilities of >500 °C, and in several cases even up to 595 °C. The monoliths are built up by networks of densely packed, near-monodisperse spherical particles and annealed microfibers, and show three types of porosity: (i) intrinsic inter-segment ultramicroporosity (<0.8 nm) of the PI networks composing the particles (∼3-5 µm), (ii) interstitial voids between the particles (0.1-2 µm), and (iii) monolith cell porosity (∽10-100 µm), as studied via low pressure gas physisorption and Hg intrusion porosimetry analyses. This unique hierarchical porosity generates an outstandingly high specific pore volume of 7250 mm3 g-1. A large-scale micromorphological study screening the reaction parameters time, temperature, and the absence/presence of the additive acetic acid was performed. Through expert interpretation of hundreds of scanning electron microscopy (SEM) images of the products of these experiments, we devise a hypothesis for morphology formation and evolution: a monomer salt is initially formed and subsequently transformed to overall eight different fiber, pearl chain, and spherical morphologies, composed of PI and, at long reaction times (>48 h), also PI/SiO2 hybrids that form through reaction with the reaction vessel. Moreover, we have developed a computational image analysis pipeline that deciphers the complex morphologies of these SEM images automatically and also allows for formulating a hypothesis of morphology development in HTP that is in good agreement with the manual morphology analysis. Finally, we upscaled the HTP of PI(TAPB-PMA) and processed the resulting powder into dense cylindrical specimen by green solvent-free warm-pressing, showing that one can follow the full route from the synthesis of these PI networks to a final material without employing harmful solvents.

Donor-acceptor covalent organic frameworks for visible light induced free radical polymerization.

Covalent organic frameworks (COFs) are promising materials for applications in photocatalysis, due to their conjugated, porous and chemically stable architectures. Alternating electron donor-acceptor-type structures are known to enhance charge carrier transport mobility and stability in polymers and are therefore also interesting building units for COFs used as photocatalysts but also as photoinitiator. In this work, two donor-acceptor COFs using electron deficient 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline and electron rich thiophene-based thieno[3,2-b]thiophene-2,5-dicarbaldehyde or [2,2'-bithiophene]-5,5'-dicarbaldehyde linkers are presented. The resulting crystalline and porous COFs have been applied as photoinitiator for visible light induced free radical polymerization of methyl methacrylate (MMA) to poly-methyl methacrylate (PMMA). These results pave the way to the development of robust and heterogeneous systems for photochemistry that offers the transfer of radicals induced by visible light.

Versatile and efficient functionalisation of multiallylic dendronised polymers: can dense packing be reached?

Surface modification of a multiallylic dendronised polymer was performed using hydrosilylation, hydroboration or radical addition of thiols to cover the polymer with various functional moieties; surface congestion is expected to occur when bulky groups are grafted.

Anionic silicate organic frameworks constructed from hexacoordinate silicon centres.

Crystalline frameworks composed of hexacoordinate silicon species have thus far only been observed in a few high pressure silicate phases. By implementing reversible Si-O chemistry for the crystallization of covalent organic frameworks, we demonstrate the simple one-pot synthesis of silicate organic frameworks based on octahedral dianionic SiO6 building units. Clear evidence of the hexacoordinate environment around the silicon atoms is given by 29Si nuclear magnetic resonance analysis. Characterization by high-resolution powder X-ray diffraction, density functional theory calculation and analysis of the pair-distribution function showed that those anionic frameworks-M2[Si(C16H10O4)1.5], where M = Li, Na, K and C16H10O4 is 9,10-dimethylanthracene-2,3,6,7-tetraolate-crystallize as two-dimensional hexagonal layers stabilized in a fully eclipsed stacking arrangement with pronounced disorder in the stacking direction. Permanent microporosity with high surface area (up to 1,276 m2 g-1) was evidenced by gas-sorption measurements. The negatively charged backbone balanced with extra-framework cations and the permanent microporosity are characteristics that are shared with zeolites.