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

Modulating β-Keto-enamine-Based Covalent Organic Frameworks for Photocatalytic Atom-Transfer Radical Addition Reaction

The atom-transfer radical addition (ATRA) reaction simultaneously forges carbon-carbon and carbon-halogen bonds. However, frequently-used photosensitizers such as precious transition metal complexes, or organic dyes have limitations in terms of their potential toxicity and recyclability. Three β-ketoenamine-linked covalent organic frameworks (COFs) from 1,3,5-triformylphloroglucinol and 1,4-phenylenediamines with variable transient photocurrent and photocatalytic activity have been prepared. A COF bearing electron-deficient Cl atoms displayed the highest photocatalytic activity toward the ATRA reaction of polyhalogenated alkanes to give halogenated olefins under visible light at room temperature. This heterogeneous photocatalyst exhibited good functional group tolerance and could be recycled without significant loss of activity.

Status and Prospect of Two-Dimensional Materials in Electrolytes for All-Solid-State Lithium Batteries

Replacing liquid electrolytes and separators in conventional lithium-ion batteries with solid-state electrolytes (SSEs) is an important strategy to ensure both high energy density and high safety. Searching for fast ionic conductors with high electrochemical and chemical stability has been the core of SSE research and applications over the past decades. Based on the atomic-level thickness and infinitely expandable planar structure, numerous two-dimensional materials (2DMs) have been exploited and applied to address the most critical issues of low ionic conductivity of SSEs and lithium dendrite growth in all-solid-state lithium batteries. This review introduces the research process of 2DMs in SSEs, then summarizes the mechanisms and strategies of inert and active 2DMs toward Li+ transport to improve the ionic conductivity and enhance the electrode/SSE interfacial compatibility. More importantly, the main challenges and future directions for the application of 2DMs in SSEs are considered, including the importance of exploring the relationship between the anisotropic structure of 2DMs and Li+ diffusion behavior, the exploitation of more 2DMs, and the significance of in situ characterizations in elucidating the mechanisms of Li+ transport and interfacial reactions. This review aims to provide a comprehensive understanding to facilitate the application of 2DMs in SSEs.

Enhancing the Crystallinity of Keto-enamine-Linked Covalent Organic Frameworks through an in situ Protection-Deprotection Strategy

β-Keto-enamine-linked 2D covalent organic frameworks (COFs) have emerged as highly robust materials, showing significant potential for practical applications. However, the exclusive reliance on 1,3,5-triformylphloroglucinol (Tp aldehyde) in the design of such COFs often results in the production of non-porous amorphous polymers when combined with certain amine building blocks. Attempts to adjust the crystallinity and porosity by a modulator approach are inefficient because Tp aldehyde readily forms stable β-keto-enamine-linked monomers/oligomers with various aromatic amines through an irreversible keto-enol tautomerization process. Our research employed a unique protection-deprotection strategy to enhance the crystallinity and porosity of β-keto-enamine-linked squaramide-based 2D COFs. Advanced solid-state NMR studies, including 1D 13 C CPMAS, 1 H fast MAS, 15 N CPMAS, 2D 13 C-1 H correlation, 1 H-1 H DQ-SQ, and 14 N-1 H HMQC NMR were used to establish the atomic-level connectivity within the resultant COFs. The TpOMe -Sqm COFs synthesized utilizing this strategy have a surface area of 487 m2  g-1 , significantly higher than similar COFs synthesized using Tp aldehyde. Furthermore, detailed time-dependent PXRD, solid-state 13 C CPMAS NMR, and theoretical DFT studies shed more light on the crystallization and linkage conversion processes in these 2D COFs. Ultimately, we applied this protection-deprotection method to construct novel keto-enamine-linked highly porous organic polymers with a surface area of 1018 m2  g-1 .

Metal-organic framework-based S-scheme heterojunction photocatalysts

Photocatalysis is a promising technology to resolve energy and environmental issues, where the design of high-efficiency photocatalysts is the central task. As an emerging family of photocatalysts, semiconducting metal-organic frameworks (MOFs) with remarkable features have demonstrated great potential in various photocatalytic fields. Compared to MOF-based photocatalysts with a single component, construction of S-scheme heterojunctions can render MOFs with enhanced charge separation, redox capacity and solar energy utilization, and thus improved photocatalytic performance. Herein, an overview of the recent advances in the design of MOF-based S-scheme heterojunctions for photocatalytic applications is provided. The basic principle of S-scheme heterojunctions is introduced. Then, three types of MOF-based S-scheme heterojunctions with different compositions are systematically summarized including MOF/non-MOF, MOF-on-MOF and MOF-derived heterojunctions. Afterwards, the enhanced performances of MOF-based S-scheme heterojunctions in hydrogen production, CO2 reduction, C-H functionalization, H2O2 production and wastewater treatment are highlighted. Lastly, the current challenges and future prospects regarding the design and applications of MOF-based S-scheme heterojunctions are discussed to inspire the further development of this emerging field.

Expanding Olefin-Linked Covalent Organic Frameworks toward Selective Photocatalytic Oxidation of Organic Sulfides

Olefin-linked covalent organic frameworks (COFs) have exhibited great potential in visible-light photocatalysis. In principle, expanding fully conjugated COFs can facilitate light absorption and charge transfer, leading to improved photocatalysis. Herein, three olefin-linked COFs with the same topology are synthesized by combining 2,4,6-trimethyl-1,3,5-triazine (TMT) with 1,3,5-triformylbenzene (TFB), 1,3,5-tris(4-formylphenyl)benzene (TFPB), and 1,3,5-tris(4-formylphenylethynyl)benzene (TFPEB), namely, TMT-TFB-COF, TMT-TFPB-COF, and TMT-TFPEB-COF, respectively. From TMT-TFB-COF to TMT-TFPB-COF, expanding phenyl rings provides only limited expansion for π-conjugation due to the steric effect of structural twisting. However, from TMT-TFPB-COF to TMT-TFPEB-COF, the insertion of acetylenes eliminates the steric effect and provides more delocalized π-electrons. As such, TMT-TFPEB-COF exhibits the best optoelectronic properties among these three olefin-linked COFs. Consequently, the photocatalytic performance of TMT-TFPEB-COF is much better than those of TMT-TFB-COF and TMT-TFPB-COF on the oxidation of organic sulfides into sulfoxides with oxygen. The desirable reusability and substrate compatibility of the TMT-TFPEB-COF photocatalyst are further confirmed. The selective formation of organic sulfoxides over TMT-TFPEB-COF under blue light irradiation proceeds via both electron- and energy-transfer pathways. This work highlights a rational design of expanding the π-conjugation of fully conjugated COFs toward selective visible-light photocatalysis.

Superhydrophilic Triazine-Based Covalent Organic Frameworks via Post-Modification of FeOOH Clusters for Boosted Photocatalytic Performance

The triazine-based covalent organic frameworks (tCOF), an intriguing subtype of COFs, are expected as highly promising photocatalysts for various photocatalytic applications owing to their fully conjugated structures and nitrogen-rich skeletons. However, the inherent hydrophobicity and fast recombination of photoexcited electron-hole pairs are two main factors hindering the application of tCOF in practical photocatalytic reactions. Here, a post-synthetic modification strategy to fabricate superhydrophilic tCOF-based photocatalysts is demonstrated by in situ growing FeOOH clusters on TaTz COF (TaTz-FeOOH) for efficient photocatalytic oxidation of various organic pollutants. The strong polar FeOOH endows TaTz-FeOOH with good hydrophilic properties. The well-defined heterogeneous interface between FeOOH and TaTz allows the photoelectrons generated by TaTz to be consumed by Fe (III) to transform into Fe (II), synergistically promoting the separation of holes and the generation of free radicals. Compared with the unmodified TaTz, the optimized TaTz-FeOOH (1%) shows excellent photocatalytic performance, where the photocatalytic degrade rate (k) of rhodamine B is increased by about 12 times, and the degradation rate is maintained at 99% after 5 cycles, thus achieving efficient removal of quinolone antibiotics from water. This study provides a new avenue for the development of COF-based hydrophilic functional materials for a wide range of practical applications.

Effect of ESIPT-Induced Photoisomerization of Keto-Enamine Linkages on the Photocatalytic Hydrogen Evolution Performance of Covalent Organic Frameworks

Photoexcitation of keto-enamine allows intramolecular proton transfer from C-NH to C=O, leading to tautomerization, while the photogenerated isomers are excluded from the study of photocatalytic applications. Herein, we demonstrate the photoisomerization of keto-enamine linkages on covalent organic frameworks (COFs) induced by excited-state intramolecular proton transfer (ESIPT). Partial enolization generates partially enolized photoisomers with a mixture of keto (C=O) and enol (OH) forms, conferring extended π-conjugation with an increase in electron density. The spatially separated D-A configuration is thus rebuilt with the enol-imine-linked branch as a donor and the keto-enamine-linked branch as an acceptor, and in turn, the photoinduced charges transfer between the two adjacent branches with a long lifetime. We further prove that the partially enolized photoisomer is a key transition instead of the keto-enamine form as an excited-state model to understand the photocatalytic behaviors. Therefore, ESIPT-induced photoisomerization must be considered for rationally designing keto-enamine-linked COFs with enhanced photocatalytic activity. Also, our study points toward the importance of controlling excited-state structures for long-lived separated charges, which is of particular interest for optoelectronic applications.

Towards high-performance nonlinear optical materials through embedding a D-A system into β-ketoenamine-linked COFs

Two covalent organic framework (COF) films supported by a glass substrate were obtained by solvothermal reaction of an electron donor with electron acceptor 1,3,5-triformylbenzene (TF) or 2,4,6-triformylphloroglucinol (TFP), respectively. The TFP-BD film exhibits a nonlinear absorption coefficient of -3.01 × 105 cm GW-1. The TFP-BD film can aggregate electrons around the connected monomer through the D-A effect due to its highly polar and electronegative carbonyl oxygen atoms, thereby modulating the electronic structure of the COFs. This work provides a novel approach for the structural modulation of optical materials with strong nonlinearity.

Substituted benzophenone imines for COF synthesis via formal transimination

Covalent organic frameworks (COFs) are a prominent class of organic materials constructed from versatile building blocks via reversible reactions. The quality of imine-linked COFs can be improved by using amine monomers protected with benzophenone forming benzophenone imines. Here, we present a study on substituted benzophenones in COF synthesis via formal transimination. 12 para-substituted N-aryl benzophenone imines, with a range of electron-rich to electron-poor substituents, were prepared and their hydrolysis kinetics were studied spectroscopically. All substituted benzophenone imines can be employed in COF synthesis and lead to COFs with high crystallinity and high porosity. The substituents act innocent to COF formation as the substituted benzophenones are cleaved off. Imines can be tailored to their synthetic demands and utilized in COF formation. This concept can make access to previously unattainable, synthetically complex COF monomers feasible.

Reticular framework materials for photocatalytic organic reactions

Photocatalytic organic reactions, harvesting solar energy to produce high value-added organic chemicals, have attracted increasing attention as a sustainable approach to address the global energy crisis and environmental issues. Reticular framework materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are widely considered as promising candidates for photocatalysis owing to their high crystallinity, tailorable pore environment and extensive structural diversity. Although the design and synthesis of MOFs and COFs have been intensively developed in the last 20 years, their applications in photocatalytic organic transformations are still in the preliminary stage, making their systematic summary necessary. Thus, this review aims to provide a comprehensive understanding and useful guidelines for the exploration of suitable MOF and COF photocatalysts towards appropriate photocatalytic organic reactions. The commonly used reactions are categorized to facilitate the identification of suitable reaction types. From a practical viewpoint, the fundamentals of experimental design, including active species, performance evaluation and external reaction conditions, are discussed in detail for easy experimentation. Furthermore, the latest advances in photocatalytic organic reactions of MOFs and COFs, including their composites, are comprehensively summarized according to the actual active sites, together with the discussion of their structure-property relationship. We believe that this study will be helpful for researchers to design novel reticular framework photocatalysts for various organic synthetic applications.

A 3D Covalent Organic Framework with In-situ Formed Pd Nanoparticles for Efficient Electrochemical Oxygen Reduction

Non-platinum noble metals are highly desirable for the development of highly active, stable oxygen reduction reaction (ORR) electrocatalysts for fuel cells and metal-air batteries. However, how to improve the utilization of non-platinum noble metals is an urgent issue. Herein, a highly efficient catalyst for ORR was prepared through homogeneous loading of Pd precursors by a domain-limited method in a three-dimensional covalent organic framework (COF) followed by pyrolysis. The morphology of the Pd nanoparticles (Pd NPs) was well maintained after carbonization, which was attributed to the rigid structure of the 3D COF. Thanks to the uniform distribution of Pd NPs in the carbon, the catalyst exhibited a remarkable half-wave potential of 0.906 V and a Tafel slope of 70 mV dec-1 in 0.1 M KOH, surpassing the commercial Pt/C catalyst (0.863 V and 75 mV dec-1 ). Furthermore, a maximum power density of 144.0 mW cm-2 was achieved at 252 mA cm-2 , which was significantly higher than the control battery (105.1 mW cm-2 ). This work not only provides a simple strategy for in-situ preparation of highly dispersible metal catalysts in COFs, but also offers new insights into the ORR electrocatalysis.

Embedding Hydrogen Atom Transfer Moieties in Covalent Organic Frameworks for Efficient Photocatalytic C-H Functionalization

Covalent organic frameworks (COFs) have emerged as efficient heterogeneous photocatalysts for a wide range of relatively simple organic reactions, whereas their application in complex organic transformations, like site-selective functionalization of unactivated C-H bonds, is underexplored, which can be mainly attributed to the lack of highly active organophotocatalytic cores. Herein through bonding oxygen atoms at the N-terminus of quinolines in nonsubstituted quinoline-linked COFs (NQ-COFs), we successfully realized the embedding of active hydrogen atom transfer (HAT) moieties into the skeleton of COFs. This novel designed COF (NQ-COFE5 -O), serving as both an excellent photosensitizer and HAT catalyst, exhibited much higher efficiency in C-H functionalization than the corresponding NQ-COFE5 . Specially, we evaluated the photocatalytic performance of NQ-COFE5 -O on ten different substrates, including quinolines, benzothiazole, and benzoxazole, all of which were transferred to desired products in moderate to high yields (up to 93 %). Furthermore, the as-synthesized NQ-COFE5 -O displayed excellent photostability and could be reused with negligible loss of activity for five catalytic cycles.

Fluorinated-Squaramide Covalent Organic Frameworks for High-Performance and Interference-Free Extraction of Synthetic Cannabinoids

Synthetic cannabinoids (SCs), one of the largest groups of new psychoactive substances (NPSs), have emerged as a significant public health threat in different regions worldwide. Analyzing SCs in water samples is critical to estimate their consumption and control. However, due to their low background concentration and the coexistence of complex matrix, the selective and effective enrichment of SCs is still challenging. In this study, a series of fluorinated-squaramide-based covalent organic frameworks (COF: FSQ-2, FSQ-3, and FSQ-4) are synthesized, and the as-prepared FSQ-4 exhibits strong affinity to different SCs. The proper pore size (1.4 nm) and pre-located functional groups (hydrogen-bond donors, hydrogen-bond acceptors, and fluorophilic segments) work synergistically for efficient SCs capture. Remarkably, when coupled FSQ-4 with solid-phase microextraction (SPME), trace-level (part per trillion, 10-9 ) determination of 13 SCs can be easily achieved, representing one of the best results among NPS analyses, and the excellent extraction performance can be maintained under various interfering conditions.

Diaryl Dihydrophenazine-Based Porous Organic Polymers Enhance Synergistic Catalysis in Visible-Light-Driven Organic Transformations

Porous organic polymers (POPs) have emerged as a novel class of porous materials that are synthesized by the polymerization of various organic monomers with different geometries and topologies. The molecular tunability of organic building blocks allows the incorporation of functional units for photocatalytic organic transformations. Here, we report the synthesis of two POP-based photocatalysts via homopolymerization of vinyl-functionalized diaryl dihydrophenazine (DADHP) monomer (POP1) and copolymerization of vinyl-functionalized DADHP and 2,2'-bipyridine monomers (POP2). The fluorescence lifetimes of DADHP units in the POPs significantly increased, resulting in enhanced photocatalytic performances over homogeneous controls. POP1 is highly effective in catalysing visible-light-driven C-N bond forming cross-coupling reactions. Upon coordination with Ni2+ ions, POP2-Ni shows strong synergy between photocatalytic and Ni catalytic cycles due to the confinement effect within the POP framework, leading to high efficiency in energy, electron, and organic radical transfer. POP2-Ni displays excellent activity in catalysing C-P bond forming reactions between diarylphosphine oxides and aryl iodides. They increased the photocatalytic activities by more than 30-fold in C-N and C-P cross-coupling reactions. These POP catalysts were readily recovered via centrifugal separation and reused in six catalytic cycles without loss of activities. Thus, photosensitizer-based POPs provide a promising platform for heterogeneous photocatalytic organic transformations.

Green and Rapid Synthesis of Acridine-Functionalized Covalent Organic Polymers for Photocatalysis by Combining Sonochemistry and Ion Induction

Covalent organic polymers (COPs) are powerful candidates for achieving the visible-light-driven degradation of organic pollutants by virtue of structural designability, but their synthesis relies on harmful reagents and high temperatures, which weakens their associated green merits. Here, we report a novel strategy for combining sonochemistry with ion induction for the rapid preparation of acridine-functionalized COPs in green and mild aqueous solutions with tunable high yields of 80 to 90%. Photochemical studies reveal the ability of these COPs to harvest visible light and their sufficient conduction potentials for generating superoxide radicals. Furthermore, the photodegradation of methylene blue confirms the good photocatalytic activity and reusability of the zinc ion-based acridine-functionalized COP, which achieves 90.8% removal in 150 min and retains 82.5% activity after 5 reuse cycles, with a rate constant of up to 3.2 times that of commercial titanium dioxide nanoparticles. This strategy paves the way for the green, rapid, and mild synthesis of acridine-functionalized COPs, enabling visible light photocatalytic degradation for water treatment and energy conversion to advance in a thoroughly environmentally friendly and cost-effective manner.

A π-conjugated covalent organic framework enables interlocked nickel/photoredox catalysis for light-harvesting cross-coupling reactions

Covalent organic frameworks (COFs) are an outstanding platform for heterogeneous photocatalysis. Herein, we synthesized a pyrene-based two-dimensional C[double bond, length as m-dash]C linked π-conjugated COF via Knoevenagel condensation and anchored Ni(ii)-centers through bipyridine moieties. Instead of traditional dual metallaphotoredox catalysis, the mono-metal decorated Ni@Bpy-sp2c-COF interlocked the catalysis mediated by light and the transition metal. Under light irradiation, enhanced energy and electron transfer in the COF backbone, as delineated by the photoluminescence, electrochemical, and control experiments, expedited the excitation of Ni centers to efficiently catalyze diverse photocatalytic C-X (X = B, C, N, O, P, S) cross-coupling reactions with efficiencies orders of magnitude higher than the homogeneous controls. The COF catalyst tolerated a diverse range of coupling partners with various steric and electronic properties, delivering the products with up to 99% yields. Some reactions were performed on a gram scale and were applied to diversify pharmaceuticals and complex molecules to demonstrate the synthetic utility.

One-step synthesis of a benzothiadiazole-based nonbranching functionalized covalent organic framework and its application in efficient removal of Hg2

In recent years, a variety of adsorbents have been developed for Hg2+ removal. However, these adsorbents are unsatisfactory for adsorption due to narrow and irregular pore channels or poor adsorption capacity and low stability. Therefore, it is worth exploring a porous Hg2+ adsorbent material with high adsorption performance and stability. In this study, a benzothiadiazole-based nonbranching functionalized covalent organic framework (COF) material (TPS-COF) by one-step synthesis was reported, which exhibited a high specific surface area of 1564 m2 g-1, high crystallinity and stability attributed to its high conjugated linkage structure of benzothiadiazole. In addition, due to the rich S and N elements of the benzothiadiazole unit, it exhibited excellent adsorption performance on Hg2+, including excellent adsorption amount (1040 mg g-1), high initial adsorption rate (448 mg g-1 min-1) and very short adsorption equilibrium time (10 min), with an efficient removal rate of Hg2+ in the pH range of 2-8. After desorption, the TPS-COF still retained good pore stability, adsorption capacity, and reusability. Such a one-step synthetic unbranched functionalization strategy provides further insights to achieve a good balance between the high crystallinity, functionality and stability of COFs.

Metallaphotocatalytic Amination of Aryl Chlorides Enabled by Highly Crystalline Acetylene-Based Hydrazone-Linked Covalent Organic Frameworks

Amination of aryl chlorides by metallaphotocatalysis is highly desired but remains practically challenging. Meanwhile, relying on soluble noble-metal photocatalysts suffers from resource scarcity and structural instability which limit their practical application. Here in, a highly crystalline acetylene-based hydrazone-linked covalent organic framewok-1 (AC-COF-1) is reported that enables metallaphotocatalytic amination of aryl chlorides. The non-planar effect of hydrazone linkage and weak interlayer attraction of acetylene bond are minimized by intralayer hydrogen-bonding. As a result, the COF shows not only improved crystallinity and porosity, but also enhanced optical and electronic properties compared to a COF analog without hydrogen-bonding. Notably, dual AC-COF-1/Ni system affords CN coupling products from broad aryl chloride substrates in excellent yields (up to 99%) and good functional tolerance. Furthermore, AC-COF-1 is recoverable and reusable for seven times photocatalysis cycles. This report demonstrates simple approach to tune the structure-activity relationship in COFs at molecular level.

Hexatopic Vertex-Directed Approach to Vinylene-Linked Covalent Organic Frameworks with Heteroporous Topologies

A D_3h-symmetric hexatopic monomer was first prepared by attaching the three-fold ditopic moiety 2,6-dimethylpyridine to the meta-positions of a phenyl ring. It was further condensed at its six pyridylmethyl carbons with linear ditopic aromatic dialdehydes, resulting in two vinylene-linked COFs with heteroporous topologies, as revealed by powder X-ray diffraction (PXRD), nitrogen sorption, and pore-size distribution analyses, as well as transmission electron microscopy (TEM) image. The linear- and cross-conjugations, respectively, arising from the 2,6-linked pyridines and meta-linked phenylenes in the hexatopic nodes rendered the resultant COFs with well-patterned π-delocalization, allowing for efficiently catalyzing the bromination of aromatic derivatives with the pore-size-dependent conversion yields and regioselectivity under the irradiation of green light.

Benzothiadiazole covalent organic framework photocatalysis with an electron transfer mediator for selective aerobic sulfoxidation

Covalent organic frameworks (COFs) are promising visible light photocatalysts for aerobic oxidation reactions. However, COFs usually suffer from the assault of reactive oxygen species, leading to hindered electron transfer. This scenario could be addressed by integrating a mediator to promote photocatalysis. Starting with 4,4'-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline (BTD) and 2,4,6-triformylphloroglucinol (Tp), TpBTD-COF is developed as a photocatalyst for aerobic sulfoxidation. Adding an electron transfer mediator 2,2,6,6-tetramethylpiperidine-1‑oxyl (TEMPO), the conversions are radically accelerated, over 2.5 times of that without TEMPO. Moreover, the robustness of TpBTD-COF is preserved by TEMPO. Remarkably, TpBTD-COF could endure multiple cycles of sulfoxidation, even with higher conversions than the fresh one. TpBTD-COF photocatalysis with TEMPO implements diverse aerobic sulfoxidation by an electron transfer pathway. This work highlights that benzothiadiazole COFs are an avenue for tailor-made photocatalytic transformations.

Recent Advances in the Use of Covalent Organic Frameworks as Heterogenous Photocatalysts in Organic Synthesis

Organic photochemistry is intensely developed in the 1980s, in which the nature of excited electronic states and the energy and electron transfer processes are thoroughly studied and finally well-understood. This knowledge from molecular organic photochemistry can be transferred to the design of covalent organic frameworks (COFs) as active visible-light photocatalysts. COFs constitute a new class of crystalline porous materials with substantial application potentials. Featured with outstanding structural tunability, large porosity, high surface area, excellent stability, and unique photoelectronic properties, COFs are studied as potential candidates in various research areas (e.g., photocatalysis). This review aims to provide the state-of-the-art insights into the design of COF photocatalysts (pristine, functionalized, and hybrid COFs) for organic transformations. The catalytic reaction mechanism of COF-based photocatalysts and the influence of dimensionality and crystallinity on heterogenous photocatalysis performance are also discussed, followed by perspectives and prospects on the main challenges and opportunities in future research of COFs and COF-based photocatalysts.

Covalent Organic Frameworks as Porous Pigments for Photocatalytic Metal-Free C-H Borylation

Covalent organic frameworks (COFs) are highly promising as heterogeneous photocatalysts due to their tunable structures and optoelectronic properties. Though COFs have been used as heterogeneous photocatalysts, they have mainly been employed in water splitting, carbon dioxide reduction, and hydrogen evolution reactions. A few examples in organic synthesis using metal-anchored COF photocatalysts were reported. Herein, we report highly stable β-keto-enamine-based COFs as photocatalysts for metal-free C-B bond formation reactions. Three different COFs have been availed for this purpose. Their photocatalysis performances have been monitored for 12 different substrates, like quinolines, pyridines, and pyrimidines. All the COFs showcase moderate-to-high yields (up to 96%) depending upon the substrate's molecular functionality. High crystallinity, a large surface area, a low band gap, and a suitable band position result in the highest catalytic activity of TpAzo COF. The thorough mechanistic investigation further highlights the crucial role of light-harvesting capacity, charge separation efficiency, and current density during catalysis. The light absorbance capacity of the COF plays a critical role during catalysis as yields are maximized near the COF's absorption maxima. The high photostability of the as-synthesized COFs offers their reusability for several (>5) catalytic cycles.

A Tailored COF for Visible-Light Photosynthesis of 2,3-Dihydrobenzofurans

Heterogeneous photocatalysis is considered as an ecofriendly and sustainable approach for addressing energy and environmental persisting issues. Recently, heterogeneous photocatalysts based on covalent organic frameworks (COFs) have gained considerable attention due to their remarkable performance and recyclability in photocatalytic organic transformations, offering a prospective alternative to homogeneous photocatalysts based on precious metal/organic dyes. Herein, we report Hex-Aza-COF-3 as a metal-free, visible-light-activated, and reusable heterogeneous photocatalyst for the synthesis of 2,3-dihydrobenzofurans, as a pharmaceutically relevant structural motif, via the selective oxidative [3+2] cycloaddition of phenols with olefins. Moreover, we demonstrate the synthesis of natural products (±)-conocarpan and (±)-pterocarpin via the [3+2] cycloaddition reaction as an important step using Hex-Aza-COF-3 as a heterogeneous photocatalyst. Interestingly, the presence of phenazine and hexaazatriphenylene as rigid heterocyclic units in Hex-Aza-COF-3 strengthens the covalent linkages, enhances the absorption in the visible region, and narrows the energy band, leading to excellent activity, charge transport, stability, and recyclability in photocatalytic reactions, as evident from theoretical calculations and real-time information on ultrafast spectroscopic measurements.

Enhanced Energy Transfer in A π-Conjugated Covalent Organic Framework Facilitates Excited-State Nickel Catalysis

Covalent organic frameworks (COFs) have received broad interest owing to their permanent porosity, high stability, and tunable functionalities. COFs with long-range π-conjugation and photosensitizing building blocks have been explored for sustainable photocatalysis. Herein, we report the first example of COF-based energy transfer Ni catalysis. A pyrene-based COF with sp² carbon-conjugation was synthesized and used to coordinate Ni^II centers through bipyridine moieties. Under light irradiation, enhanced energy transfer in the COF facilitated the excitation of Ni centers to catalyze borylation and trifluoromethylation reactions of aryl halides. The COF showed two orders of magnitude higher efficiency in these reactions than its homogeneous control and could be recovered and reused without significant loss of catalytic activity.

Tuning excited state electronic structure and charge transport in covalent organic frameworks for enhanced photocatalytic performance

Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts. However, their complicated structures lead to indeterminacy about photocatalytic active sites and reaction mechanisms. Herein, we use reticular chemistry to construct a family of isoreticular crystalline hydrazide-based COF photocatalysts, with the optoelectronic properties and local pore characteristics of the COFs modulated using different linkers. The excited state electronic distribution and transport pathways in the COFs are probed using a host of experimental methods and theoretical calculations at a molecular level. One of our developed COFs (denoted as COF-4) exhibits a remarkable excited state electron utilization efficiency and charge transfer properties, achieving a record-high photocatalytic uranium extraction performance of ~6.84 mg/g/day in natural seawater among all techniques reported so far. This study brings a new understanding about the operation of COF-based photocatalysts, guiding the design of improved COF photocatalysts for many applications.

Tuning excited state electronic structure and charge transport in covalent organic frameworks for enhanced photocatalytic performance

Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts. However, their complicated structures lead to indeterminacy about photocatalytic active sites and reaction mechanisms. Herein, we use reticular chemistry to construct a family of isoreticular crystalline hydrazide-based COF photocatalysts, with the optoelectronic properties and local pore characteristics of the COFs modulated using different linkers. The excited state electronic distribution and transport pathways in the COFs are probed using a host of experimental methods and theoretical calculations at a molecular level. One of our developed COFs (denoted as COF-4) exhibits a remarkable excited state electron utilization efficiency and charge transfer properties, achieving a record-high photocatalytic uranium extraction performance of ~6.84 mg/g/day in natural seawater among all techniques reported so far. This study brings a new understanding about the operation of COF-based photocatalysts, guiding the design of improved COF photocatalysts for many applications.

Antiaromatic Covalent Organic Frameworks Based on Dibenzopentalenes

Despite their inherent instability, 4n π systems have recently received significant attention due to their unique optical and electronic properties. In dibenzopentalene (DBP), benzanellation stabilizes the highly antiaromatic pentalene core, without compromising its amphoteric redox behavior or small HOMO-LUMO energy gap. However, incorporating such molecules in organic devices as discrete small molecules or amorphous polymers can limit the performance (e.g., due to solubility in the battery electrolyte solution or low internal surface area). Covalent organic frameworks (COFs), on the contrary, are highly ordered, porous, and crystalline materials that can provide a platform to align molecules with specific properties in a well-defined, ordered environment. We synthesized the first antiaromatic framework materials and obtained a series of three highly crystalline and porous COFs based on DBP. Potential applications of such antiaromatic bulk materials were explored: COF films show a conductivity of 4 × 10^-8 S cm^-1 upon doping and exhibit photoconductivity upon irradiation with visible light. Application as positive electrode materials in Li-organic batteries demonstrates a significant enhancement of performance when the antiaromaticity of the DBP unit in the COF is exploited in its redox activity with a discharge capacity of 26 mA h g^-1 at a potential of 3.9 V vs. Li/Li⁺. This work showcases antiaromaticity as a new design principle for functional framework materials.

One-pot cascade construction of nonsubstituted quinoline-bridged covalent organic frameworks

Irreversible locking of imine linkages into stable linkages represents a promising strategy to improve the robustness and functionality of covalent organic frameworks (COFs). We report, for the first time, a multi-component one-pot reaction (OPR) for imine annulation to construct highly stable nonsubstituted quinoline-bridged COFs (NQ-COFs), and that equilibrium regulation of reversible/irreversible cascade reactions by addition of MgSO₄ desiccant is crucial to achieve high conversion efficiency and crystallinity. The higher long-range order and surface area of NQ-COFs synthesized by this OPR than those of the reported two-step post-synthetic modification (PSM) facilitate charge carrier transfer and photogeneration ability of superoxide radicals (O₂˙^-), which makes these NQ-COFs more efficient photocatalysts for O₂˙^- mediated synthesis of 2-benzimidazole derivatives. The general applicability of this synthetic strategy is demonstrated by fabricating 12 other crystalline NQ-COFs with a diversity of topologies and functional groups.

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.

Highly Crystalline Polyimide Covalent Organic Framework as Dual-Active-Center Cathode for High-Performance Lithium-Ion Batteries

Polyimide covalent organic framework (PI-COF) materials that can realize intrinsic redox reactions by changing the charge state of their electroactive sites are considered as emerging electrode materials for rechargeable devices. However, the highly crystalline PI-COFs with hierarchical porosity are less reported due to the rapid reaction between monomers and the poor reversibility of the polyimidization reaction. Here, we developed a water-assistant synthetic strategy to adjust the reaction rate of polyimidization, and PI-COF (COF_TPDA-PMDA) with kgm topology consisting of dual active centers of N,N,N',N'-tetrakis(4-aminophenyl)-1,4-benzenediamine (TPDA) and pyromellitic dianhydride (PMDA) ligands was successfully synthesized with high crystallinity and porosity. The COF_TPDA-PMDA possesses hierarchical micro-/mesoporous channels with the largest surface area (2669 m²/g) in PI-COFs, which can promote the Li⁺ ions and bulky bis(trifluoromethanesulfonyl)imide (TFSI^-) ions in organic electrolyte to sufficiently interact with the dual active sites on COF skeleton to increase the specific capacity of cathode materials. As a cathode material for lithium-ion batteries, COF_TPDA-PMDA@50%CNT which integrated high surface area and dual active center of COF_TPDA-PMDA with carbon nanotubes via π-π interactions gave a high initial charge capacity of 233 mAh/g (0.5 A/g) and maintains at 80 mAh/g even at a high current density of 5.0 A/g after 1800 cycles.

Annealed Covalent Organic Framework Thin Films for Exceptional Absorption of Ultrabroad Low-Frequency Electromagnetic Waves

Different from harvesting of ultraviolet and visible lights via electronic transitions, absorption of low-frequency electromagnetic waves is sophisticated in mechanism and poor in efficiency, imposing the structural design arduous and challenging. Here, the first example of exploring covalent organic frameworks for highly efficient absorption of low-frequency electromagnetic waves is reported. Three pyrene frameworks are synthesized and annealed into porous networks, which upon mixture with paraffin are processed into thin films with tunable thickness. The films absorb ultrabroad low-frequency electromagnetic waves covering S, C, X, and Ku bands and achieve exceptional efficiency of 99.999% with a thickness of only 2.5 mm and a loading content of only 20%. This result originates from a synergistic effect of conductivity, heteroatoms, and pores and outperforms the state-of-the-art polymers, carbons, and metals. This approach opens a way to electromagnetic wave absorption.

Shining Visible Light on Reductive Elimination: Acridine-Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids

Despite the recent tremendous progress on transition-metal/photoredox dual catalysis in organic synthesis, single transition-metal catalysis under visible-light irradiation, which can utilize light energy more efficiently, is still underdeveloped. Herein, we report the design of photosensitizing phosphinoacridine bidentate ligands for visible-light-induced transition-metal catalysis, expecting that the electron-accepting acridine moiety would create a highly reactive electron-deficient metal center toward reductive elimination via metal-to-ligand charge transfer (MLCT). Using these ligands, we have achieved a palladium-catalyzed cross-coupling reaction of aryl halides with carboxylic acids under visible-light irradiation. Electronic tuning of the phosphinoacridine ligands not only enabled the use of a variety of aryl halides as the coupling partner, including less reactive aryl chlorides, under blue light irradiation, but also realized the employment of lower-energy green and red light for the cross-coupling. Experimental mechanistic studies have proved that the reductive elimination of aryl esters is induced by photoirradiation of phosphinoacridine-ligated arylpalladium(II) carboxylate complexes. The theoretical calculation suggests that the reductive elimination in the excited state is promoted by decreasing the electron density of the Pd center through photoinduced intramolecular electron transfer, i.e., MLCT, in the transition state owing to the electron-deficient acridine scaffold. This is a very rare example of photoinduced reductive elimination on palladium(II) complexes.

Electroactive Covalent Organic Framework Enabling Photostimulus-Responsive Devices

Two-dimensional covalent organic frameworks (2D COFs) feature graphene-type 2D layered sheets but with a tunable structure, electroactivity, and high porosity. If these traits are well-combined, then 2D COFs can be applied in electronics to realize functions with a high degree of complexity. Here, a highly crystalline electroactive COF, BDFamide-Tp, was designed and synthesized. It shows regularly distributed pores with a width of 1.35 nm. Smooth and successive films of such a COF were fabricated and found to be able to increase the conductivity of an organic semiconductor by 10³ by interfacial doping. Upon encapsulation of a photoswitchable molecule (spiropyran) into the voids of the COF layer, the resulted devices respond differently to light of different wavelengths. Specifically, the current output ratio after UV vs Vis illumination reaches 100 times, thus effectively creating on and off states. The respective positive and negative feedbacks are memorized by the device and can be reprogrammed by UV/Vis illumination. The reversible photostimulus responsivity and reliable memory of the device are derived from the combination of electroactivity and porosity of the 2D COF. This work shows the capability of 2D COFs in higher-level electronic functions and extends their possible applications in information storage.

Direct Construction of Isomeric Benzobisoxazole-Vinylene-Linked Covalent Organic Frameworks with Distinct Photocatalytic Properties

Vinylene/olefin-linked two-dimensional covalent organic frameworks (v-2D-COFs) have emerged as advanced semiconducting materials with excellent in-plane conjugation, high chemical stabilities, and precisely tunable electronic structures. Exploring new linkage chemistry for the reticular construction of v-2D-COFs remains in infancy and challenging. Herein, we present a solid-state benzobisoxazole-mediated aldol polycondensation reaction for the construction of two novel isomeric benzobisoxazole-bridged v-2D-COFs (v-2D-COF-NO1 and v-2D-COF-NO2) with trans and cis configurations of benzobisoxazole. Interestingly, the isomeric benzobisoxazole linkers endow the two v-2D-COFs with distinct optoelectronic and electrochemical properties, ranging from light absorption and emission to charge-transfer properties. When employed as the photocathode, v-2D-COF-NO1 exhibits a photocurrent of up to ∼18 μA/cm² under AM 1.5G irradiation at -0.3 V vs reversible hydrogen electrode (RHE), which is twice the value of v-2D-COF-NO2 (∼9.1 μA/cm²). With Pt as a cocatalyst, v-2D-COF-NO1 demonstrates a photocatalytic hydrogen evolution rate of ∼1.97 mmol h^-1 g^-1, also in clear contrast to that of v-2D-COF-NO2 (∼0.86 mmol h^-1 g^-1) under identical conditions. This work demonstrates the synthesis of v-2D-COFs via benzobisoxazole-mediated aldol polycondensation with isomeric structures and distinct photocatalytic properties.