Blending Aryl Ketone in Covalent Organic Frameworks to Promote Photoinduced Electron Transfer

A mixed-linker strategy in covalent organic frameworks for enhanced sunlight-driven photocatalytic oxidation activity and stability

We present a simple mixed-linker strategy to enhance the photocatalytic oxidation activity and stability of covalent organic framework semiconductors. The TpTfbPa-COF, featuring an unconventional reversed AA-stacking structure with both imine and ketoamine linkages, exhibits excellent structural stability and superior photocatalytic performance compared to the fully imine-linked TfbPa-COF and the fully ketoamine-linked TpPa-COF.

A Highly Crystalline Covalent Organic Framework With Rich Redox Active Sites as Cathode Material for Lithium Batteries

Covalent organic frameworks (COFs) have been regarded as promising cathode materials for lithium batteries. However, they generally show low practical capacity. Here, the design, preparation, and battery application of a highly crystalline 2D truxenone-based COF (TRO-BT-COF) with rich redox active sites, realizing a high practical capacity is reported. In situ characterizations and theoretical calculations indicate that the lithiation reaction proceeds successively on C═O and C═N groups of TRO-BT-COF. One ring of TRO-BT-COF can theoretically store 27 Li+ ions through two stages. Consequently, the highly crystalline TRO-BT-COF displays a high discharge capacity of 435 mAh g-1 when used as a cathode material for lithium batteries. This work provides a facile approach to achieve COF electrode materials with high practical capacity for lithium batteries.

Intramolecular Charge Transfer-Regulated Isomeric Covalent Organic Frameworks for Multiple Solvent-Response

Stimulus-responsive covalent organic frameworks (COFs) own color-switching characteristics when exposed to external stimuli. However, the investigations on the multiple solvent-responsive COFs remain a challenge due to the synthetic difficulties and uncontrollable charge transfer process toward various solvents. In this contribution, two novel isomeric COFs with a regulated intramolecular charge transfer (ICT) process by modulating the distance between the donor/acceptor and the linkage are synthesized. The as-prepared two isomeric COFs exhibited significantly distinct solvatochromic behaviors in water, acid, and halogenated solvents, respectively. These multiple solvent-responsive functions are attributed to the various enhancement degrees of the ICT process by the hydrogen bond interactions, protonation interactions, and halogen/π interactions, respectively. In addition, the two isomeric COFs are employed as stimulation-responsive powder or ink, displaying excellent image and data encryption performances. The work can not only offer a novel viewpoint for the creation of multiple solvent-responsive COFs but also expand the COFs' potential applications in information encryption and anti-counterfeiting.

Viologen Covalent Organic Framework Mediates Near-Infrared Light-Induced Electron Transfer for Catalytic Oxidative Coupling Reactions

Near-infrared (NIR) light-driven photoreactions are advantageous over visible light-driven ones because NIR photons have lower energy and fewer side reactions, deeper penetration in reaction media, and high abundance in the solar spectrum. However, currently available covalent organic frameworks (COFs) absorb in the UV-vis region and catalyze photoreactions under blue or white light irradiation. Herein, we report a linker-to-linker charge transfer process in a viologen-linked porphyrin COF (Vio-COF), leading to a novel type of hyperporphyrin effect and extending the absorption into the NIR region with an absorption edge at 998 nm. Under NIR irradiation, photoinduced charge separation in Vio-COF generates a viologen radical that efficiently reduces oxygen to form superoxide radicals for catalytic oxidative coupling reactions. The proximity of porphyrin and viologen units within the framework significantly enhances the catalytic performance of Vio-COF, outperforming its homogeneous counterparts in aerobic oxidative amidation and amine coupling reactions. Vio-COF was readily recycled and used in six oxidative coupling reactions.

Effect of π-Linkages in Covalent Organic Framework-Catalyzed Light-Harvesting Thioesterification Reaction

Covalent organic frameworks (COFs) serve as an outstanding platform for heterogeneous photocatalysis. We synthesized two analogous pyrene-based two-dimensional COFs with π-conjugated networks, one linked by C═N bonds and the other by C═C bonds, through Schiff base and Knoevenagel condensation reactions, respectively. We investigated the impact of these linkages on the photocatalytic activity of these COFs, using visible-light-mediated thioesterification as a model reaction. It was found that the olefin-linkage COF outperformed the imine-linkage COF as a photocatalyst. The developed protocol demonstrated a broad substrate scope, including 35 diverse carboxylic acids, 14 drug molecules, and several disulfide coupling partners, achieving up to a 95% yield of thioesters. The practical utility of this strategy is further demonstrated by its successful application in gram-scale reactions. The photocatalyst is robust and was successfully reused for multiple cycles without any loss of catalytic activity. The COF backbone facilitated enhanced electron transfer upon light irradiation, enabling the cross-coupling of carboxylic acid and disulfide through a reductive photocatalytic cycle.

Visible-Light-Mediated Aerobic α-Oxygenation of Tetrahydroisoquinolines and Isoindolines Without External Photocatalysts

A visible-light-mediated strategy for the direct oxygenation of N-substituted tetrahydroisoquinolines and isoindolines to the corresponding benzo-fused lactams under clean conditions without using any external photocatalysts has been developed. The reaction was performed in the presence of a catalytic amount of base and oxygen. Mechanistic studies reveal that the reaction is initiated by the substrates themselves as photosensitizers. Additionally, BHT could be used as a buffer-like additive to improve reaction selectivity and product yield in this photo-oxidation process.

A vinylene-linked diketopyrrolopyrrole-based covalent organic framework for photocatalytic oxidation reactions

A vinylene-linked DPP-COF with an ultra-narrow bandgap of 1.06 eV was reported. This COF demonstrates high chemical stability and significant charge transfer properties, and was applied to the photooxidation of sulfides and tetrahydroisoquinolines, exhibiting exceptional photoactivities.

Regulating the surface chemistry of covalent organic frameworks for enhancement cationic dye removal and identification

Simultaneous removal and identification of trace-level cationic dye pollutants from water is both important and challenging owing to their highly polar and complex sample matrices. In this study, three covalent organic frameworks (COFs) were synthesized using 2, 4, 6-triformylphloroglucinol with ethidium bromide (EB) containing positively charged groups, 3, 5-diaminobenzoic acid (DABA) containing negatively charged groups, and p-phenylenediamine (Pa) lacking charged groups. These were named EB-COFs, TpPa-1, and DP-COFs, respectively, and were employed as adsorbents for the extraction and identification of cationic dyes. The adsorption performance of the three COFs toward methylene blue (MB) and crystal violet (CV) was investigated. By incorporating carboxyl groups into DP-COFs, the surface chemistry of the adsorbent was effectively tailored, enabling complete exploitation of selective cationic sites. This facilitated dynamic interactions with cationic dyes through multiple adsorption mechanisms, including electrostatic, π-π, and H-bonding interactions. DP-COFs exhibited high adsorption capacities for MB and CV, achieving 383 and 326 mg g-1, respectively. The adsorption behavior was further analyzed using adsorption isothermals, kinetics, and thermodynamics. Moreover, DP-COFs were employed as a matrix in laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) to adsorb and directly identify both cationic dyes without the need for an elution process. This approach demonstrated high sensitivity, high reproducibility, low background interference, and excellent salt tolerance. The limits of detection for MB and CV were 0.12 and 0.04 ng mL-1, respectively, representing improvements of 166-fold and 225-fold compared with using DP-COFs solely as a matrix. Recovery rates of both dyes in spiked industrial wastewater and lake water samples ranged from 81.4 to111.1% with RSDs of 1.9-6.3%. These results highlight the high reliability of the proposed method.

Synergistic Tri-efficiency Enhancement Utilizing Functionalized Covalent Organic Frameworks for Photocatalytic H2O2 Production

The overall maximization of photocatalytic H2O2 production efficiency urgently requires the comprehensive optimization of each step in multiplex photocatalysis. Despite numerous endeavors, isolated researches focusing on single efficiencies hinder further advancements in overall catalytic activity. In this work, a series of imine-linked COFs (TT-COF-X), incorporating electronically tunable functional groups (X = ─H, ─OMe, ─OH, ─Br), are rationally fabricated for visible-light-driven H2O2 production via a dual-channel pathway involving 2e- water oxidation and 2e- oxygen reduction. Combined simulations and characterizations reveal that the synergistic modification of functional groups for electronic conjugation and locally intramolecular polarity collectively enhanced light absorption, charge separation and transfer, and interface water-oxygen affinity efficiency. Notably, femtosecond time-resolved transient absorption (fs-TA) reveals that the polarity-induced built-in electric field play a crucial role in facilitating exciton dissociation by reacting BIEF-mediated shallow trapping state. The simultaneously optimal tri-efficiency ultimately results in the highest H2O2 production rate of 3406.25 µmol h-1 g-1 and apparent quantum yields of 8.1% of TT-COF-OH. This study offers an emerging strategy to rational design of photocatalysts from the comprehensive tri-efficiency-oriented perspective.

Ketone-functionalized conjugated organic polymers boost red-light-driven molecular oxygen-mediated oxygenation

Photocatalytic molecular oxygen activation has emerged as a valuable tool for organic synthesis, environmental remediation and energy conversion. Most reported instances have relied on high-energy light sources. Herein, 9-fluorenone-functionalized porous organic polymers (POPs) were reported to enable red-light-excited photocatalysis for the organic oxygenation reaction. Notably, this modification extends the conjugated backbone, allowing the capture of lower-energy light. Incorporating ketone groups into POPs also facilitates charge separation and enhances carrier concentration, thereby promoting catalytic efficiency. The new POP photomaterials exhibit high activity for the direct α-oxygenation of N-substituted tetrahydroisoquinolines (THIQs) using O2 as a green oxidant under 640 nm light irradiation, achieving high yield in short reaction times. Detailed mechanistic investigations clearly showed the role of oxygen and the photocatalyst. This work provides valuable insights into the potential of ketone-modified POPs for superior photocatalytic activation of molecular oxygen under low-energy light conditions.

Triazine-Carbazole-Based Covalent Organic Frameworks as Efficient Heterogeneous Photocatalysts for the Oxidation of N-aryltetrahydroisoquinolines

Covalent organic frameworks (COFs) have attracted growing interests as new material platform for a range of applications. In this study, a triazine-carbazole-based covalent organic framework (COF-TCZ) was designed as highly porous material with conjugated donor-acceptor networks, and feasibly synthesized by the Schiff condensation of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tr ianiline (TAPB) and 9-(4-formylphenyl)-9H-carbazole-3,6-dicarbaldehyde (CZTA) under the solvothermal condition. Considering the effect of linkage, the imine-linked COF-TCZ was further oxidized to obtain an amide-linked covalent organic framework (COF-TCZ-O). The as-synthesized COFs show high crystallinity, good thermal and chemical stability, and excellent photoactive properties. Two π-conjugated triazine-carbazole-based COFs with tunable linkages are beneficial for light-harvesting capacity and charge separation efficiency, which are empolyed as photocatalysts for the oxidation reaction of N-aryltetrahydroisoquinoline. The COFs catalyst systems exhibit the outstanding photocatalytic performance with high conversion, photostability and recyclability. Photoelectrochemical tests were employed to examine the behavior of photogenerated charge carriers in photo-illumination system. The control experiments provide further insights into the nature of photocatalysis. In addition, the current research also provided a valuable approach for developing photofunctional COFs to meet challenge in achieving the great potential of COFs materials in organic conversion.

Promoting Photocatalytic Direct C-H Difluoromethylation of Heterocycles using Synergistic Dual-Active-Centered Covalent Organic Frameworks

Difluoromethylation reactions are increasingly important for the creation of fluorine-containing heterocycles, which are core groups in a diverse range of biologically and pharmacologically active ingredients. Ideally, this typically challenging reaction could be performed photocatalytically under mild conditions. To achieve this separation of redox processes would be required for the efficient generation of difluoromethyl radicals and the reduction of oxygen. A covalent organic framework photocatalytic material was, therefore, designed with dual reactive centers. Here, anthracene was used as a reduction site and benzothiadiazole was used as an oxidation site, distributed in a tristyryl triazine framework. Efficient charge separation was ensured by the superior electron-donating and -accepting abilities of the dual centers, creating long-lived photogenerated electron-hole pairs. Photocatalytic difluoromethylation of 16 compounds with high yields and remarkable functional group tolerance was demonstrated; compounds included bioactive molecules such as xanthine and uracil. The structure-function relationship of the dual-active-center photocatalyst was investigated through electron spin resonance, femtosecond transient absorption spectroscopy, and density functional theory calculations.

Bimodal Oxidation Electrochemiluminescence Mechanism of Coreactant-Embedded Covalent Organic Frameworks via Postsynthetic Modification

Electrochemiluminescence (ECL) efficiency is determined by charge transfer between coreactants and emitters in coreactant systems, which are usually limited by their slow intermolecular charge transfer. In this study, a covalent organic framework (COF) with aldehyde residue was synthesized, and then coreactants were covalently integrated into the skeleton through the postsynthetic modification strategy, resulting in a crystalline coreactant-embedded COF nanoemitter (C-COF). Compared to the pristine COF with an equivalent external coreactant, C-COF exhibited an extraordinary 1008-fold enhancement of ECL intensity due to the rapid intrareticular charge transfer. Significantly, with the pH increase, C-COF shows protonation-induced ECL enhancement for the first ECL peaked at +1.1 V and an opposite trend for the second ECL at +1.4 V, which were attributed to the antedating oxidation of coreactant in framework and COF self-oxidation, respectively. The resulting bimodal oxidation ECL mechanism was rationalized by spectral characterization and density functional theory calculations. The postsynthetic coreactant-embedded nanoemitters present innovative and universal avenues for advancing ECL systems.

Metal-Free Activation of Molecular Oxygen by 9-Fluorenone-Based Porous Organic Polymers for Selective Aerobic Oxidation

Oxygen activation is a critical step in heterogeneous oxidative processes, particularly in catalytic, electrolytic, and pharmaceutical applications. Among the various catalysts available for photocatalytic O2 activation, homogeneous aryl ketones are at the forefront. To avoid the degradation and deactivation of aryl ketones, 9-fluorenone-based porous organic polymers were designed and regulated by doping them with co-monomers. The obtained heterogeneous photocatalyst showed good performance in O2 activation, and its performance was better than that of homogeneous 9-fluorenone. The obtained heterogeneous photocatalyst showed good reusability. We believe that the presented method and findings represent an important step toward designing catalysts tailored for specific tasks.

Nonadiabatic Molecular Dynamics with Non-Condon Effect of Charge Carrier Dynamics

Nonradiative multiphonon transitions play a crucial role in understanding charge carrier dynamics. To capture the non-Condon effect in nonadiabatic molecular dynamics (NA-MD), we develop a simple and accurate method to calculate noncrossing and crossing k-point NA coupling in momentum space on an equal footing and implement it with a trajectory surface hopping algorithm. Multiple k-point MD trajectories can provide sufficient nonzero momentum multiphonons coupled to electrons, and the momentum conservation is maintained during nonvertical electron transition. The simulations of indirect bandgap transition in silicon and intra- and intervalley transitions in graphene show that incorporation of the non-Condon effect is needed to correctly depict these types of charge dynamics. In particular, a hidden process is responsible for the delayed nonradiative electron-hole recombination in silicon: the thermal-assisted rapid trapping of an excited electron at the conduction band minimum by a long-lived higher energy state through a nonvertical transition extends charge carrier lifetime, approaching 1 ns, which is about 1.5 times slower than the direct bandgap recombination. For graphene, intervalley scattering takes place within about 225 fs, which can occur only when the intravalley relaxation proceeds to about 50 fs to gain enough phonon momentum. The intra- and intervalley scattering constitute energy relaxation, which completes within sub-500 fs. All the simulated time scales are in excellent agreement with experiments. The study establishes the underlying mechanisms for a long-lived charge carrier in silicon and valley scattering in graphene and underscores the robustness of the non-Condon approximation NA-MD method, which is suitable for rigid, soft, and large defective systems.

Thermally Induced Persistent Covalent-Organic Frameworks Radicals

Persistent covalent-organic framework (COF) radicals hold important applications in magnetics and spintronics; however, their facile synthesis remains a daunting challenge. Here, three p-phenylenediacetonitrile-based COFs (named CityU-4, CityU-5, and CityU-6) were synthesized. Upon heat treatment (250 °C for CityU-4 and CityU-5 or 220 °C for CityU-6), these frameworks were brought into their persistent radical forms (no obvious changes after at least one year), together with several observable factors, including color changes, red-shifted absorption, the appearance of electron spin resonance (ESR) signals, and detectable magnetic susceptibility. The theoretical simulation suggests that after heat treatment, lower total energy and nonzero spin density are two main factors to guarantee persistent COFs radicals and polarized spin distributions. This work provides an efficient method for the preparation of persistent COF radicals with promising potentials.

Construction of Thiadiazole-Linked Covalent Organic Frameworks via Facile Linkage Conversion with Superior Photocatalytic Properties

The establishment of facile synthetic routes to engineer covalent organic frameworks (COFs) with fully conjugated structure and excellent stability is highly desired for practical applications in optoelectronics and photocatalysis. Herein, a novel linkage conversion strategy is reported to prepare crystalline thiadiazole-linked COFs via thionation, cyclization, and oxidation of N-acylhydrazole bonds with Lawesson's reagent (LR). The as-prepared thiadiazole-linked COFs not only remain porosity and crystallinity, but enhance its chemical stability. Furthermore, thiadiazole-linked COFs are more favorable to lower exciton binding energy and promote π-electron delocalization over the whole reticular framework than N-acylhydrazone-linked COFs. Notably, the extended π-conjugation structure and decent crystallinity of the resulting TDA-COF are reflected by its higher photocatalytic H2 evolution rate (61.3 mmol g-1 in 5 h) in comparison with that (7.5 mmol g-1 ) of NAH-COF.

Site Engineering of Covalent Organic Frameworks for Regulating Peroxymonosulfate Activation to Generate Singlet Oxygen with 100 % Selectivity

Singlet oxygen (1 O2 ) is an excellent reactive oxygen species (ROSs) for the selective conversion of organic matter, especially in advanced oxidation processes (AOPs). However, due to the huge dilemma in synthesizing single-site type catalysts, the control and regulation of 1 O2 generation in AOPs is still challenging and the underlying mechanism remains largely obscure. Here, taking advantage of the well-defined and flexibly tunable sites of covalent organic frameworks (COFs), we report the first achievement in precisely regulating ROSs generation in peroxymonosulfate (PMS)-based AOPs by site engineering of COFs. Remarkably, COFs with bipyridine units (BPY-COFs) facilitate PMS activation via a nonradical pathway with 100 % 1 O2 , whereas biphenyl-based COFs (BPD-COFs) with almost identical structures activate PMS to produce radicals (⋅OH and SO4 .- ). The BPY-COFs/PMS system delivers boosted performance for selective degradation of target pollutants from water, which is ca. 9.4 times that of its BPD-COFs counterpart, surpassing most reported PMS-based AOPs systems. Mechanism analysis indicated that highly electronegative pyridine-N atoms on BPY-COFs provide extra sites to adsorb the terminal H atoms of PMS, resulting in simultaneous adsorption of O and H atoms of PMS on one pyridine ring, which facilitates the cleavage of its S-O bond to generate 1 O2 .