Cobalt-Porphyrin-Based Covalent Organic Frameworks with Donor-Acceptor Units as Photocatalysts for Carbon Dioxide Reduction

Regulating Conformational Locking in Covalent Organic Framework for Selective and Recyclable Photocatalytic Transformation

The exploration of new properties and functionality of covalent organic frameworks (COFs) rely mostly on the covalent modification of the starting building blocks or linkages. Noncovalent forces that guide the assembly and adhesion of layers to develop two-dimensional (2D) COFs and improve their bulk properties and functionalities, however, are rarely explored. Herein, the "conformational lock" (CL) effect in 2D hydrazine-linked COFs with intralayer F-H interaction is discovered and regulated to stabilize interlayer adhesion and develop a facile strategy to increase their stability, promote selectivity and efficiency in reactive singlet oxygen (1O2)-triggered photocatalytic transformation when acting as photocatalysts. The CL strategy endows the fluorinated COFs with an efficient intersystem crossing process for 1O2 generation and strong interlayer π-π stacking interaction. The 4F-COF with the strongest F-H noncovalent interaction exhibits the highest photocatalytic conversion and selectivity (exceeding 98%) in typical 1O2-dependent transformations, even over 7 continuous photocatalytic cycles. This work demonstrates that promoting intralayer noncovalent interaction in 2D-COFs can impart high photocatalytic activity and stability, and would vigorously inspire their developments in heterogeneous catalysis.

Donor polarization engineering of conjugated microporous polymers to boost exciton dissociation for photocatalytic degradation of tetracycline

The misuse and inevitable release of antibiotics can cause significant harm to both human health and the environment, and the use of polymeric semiconductors for photodegradation of antibiotics in aqueous environments is one of the most effective strategies to alleviate the current dilemma. Nevertheless, the inherently high exciton binding energy (Eb) and low photogenerated carrier transfer efficiency for most photocatalysts results in unsatisfactory photodegradation performance. Hence, this work proposes a donor polarization strategy to regulate the exciton dissociation of conjugated microporous polymers (CMPs) by minimizing their Eb. Results exhibited that the introduction of the strong donor unit 3,4-ethylenedioxythiophene (EDOT) not only reduces the Eb and effectively promotes exciton dissociation, but also broadens the visible light absorption of CMP. Among them, EdtTz-CMP with the lowest Eb (99 meV) delivered an efficiency of 94.6% in photocatalytic degradation of tetracycline (TC) with in 90 min, significantly higher than those of its analogues. This work provides a viable approach to design CMPs by tuning the intrinsic dipole of the donor for efficient environmental purification.

Covalent Organic Frameworks for Photocatalytic Reduction of Carbon Dioxide: A Review

Covalent organic frameworks (COFs) are crystalline networks with extended backbones cross-linked by covalent bonds. Due to the semiconductive properties and variable metal coordinating sites, along with the rapid development in linkage chemistry, the utilization of COFs in photocatalytic CO2RR has attracted many scientists' interests. In this Review, we summarize the latest research progress on variable COFs for photocatalytic CO2 reduction. In the first part, we present the development of COF linkages that have been used in CO2RR, and we discuss four mechanisms including COFs as intrinsic photocatalysts, COFs with photosensitive motifs as photocatalysts, metalated COF photocatalysts, and COFs with semiconductors as heterojunction photocatalysts. Then, we summarize the principles of structural designs including functional building units and stacking mode exchange. Finally, the outlook and challenges have been provided. This Review is intended to give some guidance on the design and synthesis of diverse COFs with different linkages, various structures, and divergent stacking modes for the efficient photoreduction of CO2.

Cyanide-based Covalent Organic Frameworks for Enhanced Overall Photocatalytic Hydrogen Peroxide Production

Photocatalytic oxygen reduction to produce hydrogen peroxide (H2O2) is a promising route to providing oxidants for various industrial applications. However, the lack of well-designed photocatalysts for efficient overall H2O2 production in pure water has impeded ongoing research and practical thrusts. Here we present a cyanide-based covalent organic framework (TBTN-COFs) combining 2,4,6-trimethylbenzene-1,3,5-tricarbonitrile (TBTN) and benzotrithiophene-2,5,8-tricarbaldehyde (BTT) building blocks with water-affinity and charge-separation. The ultrafast intramolecular electron transfer (<500 fs) and prolonged excited state lifetime (748 ps) can be realized by TBTN-COF, resulting in a hole accumulated BTT and electron-rich TBTN building block. Under one sun, the 11013 μmol h-1 g-1 yield rate of H2O2 can be achieved without any sacrificial agent, outperforming most previous reports. Furthermore, the DFT calculation and in situ DRIFTS spectrums suggesting a Yeager-type absorption of *O2⋅- intermediate in the cyanide active site, which prohibits the formation of superoxide radical and revealing a favored H2O2 production pathway.

Porphyrin-based covalent organic framework as oxidase mimic for highly sensitive colorimetric detection of pesticides

A covalent organic framework-based strategy was designed for label-free colorimetric detection of pesticides. Covalent organic framework-based nanoenzyme with excellent oxidase-like catalytic activity was synthesized. Unlike other artificial enzymes, porphyrin-based covalent organic framework (p-COF) as the oxidase mimic showed highly catalytic chromogenic activity and good affinity toward TMB without the presence of H2O2, which can be used as substitute for peroxidase mimics and H2O2 system in the colorimetric reaction. Based on the fact that the pesticide-aptamer complex can inhibit the oxidase activity of p-COF and reduced the absorbance at 650 nm in UV-Vis spectrum, a label-free and facile colorimetric detection of pesticides was designed and fabricated. Under the optimized conditions, the COF-based colorimetric probe for pesticide detection displayed high sensitivity and selectivity. Taking fipronil for example the limit of detection was 2.7 ng/mL and the linear range was 5 -500,000 ng/mL. The strategy was successfully applied to the detection of pesticides with good recovery , which was in accordance with that of HPLC-MS/MS. The COF-based colorimetric detection was free of complicated modification H2O2, which guaranteed the accuracy and reliability of measurements. The COF-based sensing strategy is a potential candidate for the sensitive detection of pesticides of interests.

Symmetrical Localized Built-in Electric Field by Induced Polarization Effect in Ionic Covalent Organic Frameworks for Selective Imaging and Killing Bacteria

Photocatalytic materials are some of the most promising substitutes for antibiotics. However, the antibacterial efficiency is still inhibited by the rapid recombination of the photogenerated carriers. Herein, we design a cationic covalent organic framework (COF), which has a symmetrical localized built-in electric field due to the induced polarization effect caused by the electron-transfer reaction between the Zn-porphyrin unit and the guanidinium unit. Density functional theory calculations indicate that there is a symmetrical electrophilic/nucleophilic region in the COF structure, which results from increased electron density around the Zn-porphyrin unit. The formed local electric field can further inhibit the recombination of photogenerated carriers by driving rapid electron transfer from Zn-porphyrin to guanidinium under light irradiation, which greatly increases the yield of reactive oxygen species. This COF wrapped by DSPE-PEG2000 can selectively target the lipoteichoic acid of Gram-positive bacteria by electrostatic interaction, which can be used for selective discrimination and imaging of bacteria. Furthermore, this nanoparticle can rapidly kill Gram-positive bacteria including 99.75% of Staphylococcus aureus and 99.77% of Enterococcus faecalis at an abnormally low concentration (2.00 ppm) under light irradiation for 20 min. This work will provide insight into designing photoresponsive COFs through engineering charge behavior.