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Self-assembly must be transformed into multistep synthesis to create complex structures Self-assembly by intermolecular non-covalent interactions directed by self-recognition created the field of supramolecular chemistry ( 1 ). However, the word “self” appears to limit this field to mixing components in one assembly step where most of the complexity is inherent in the covalently synthesized reactants, rather than the result of a series of assembly steps that build more complex structures in reproducible procedures. The paradigm shift in supramolecular chemistry that we propose is the building of multicomponent systems following a multistep pathway—the emergence of molecular complexity (see the figure). The latter is not only directed by the information stored in the covalent framework of the components, but also controlled by the kinetics and thermodynamics of the reaction pathways selected in processing this information ( 2 ).

The dream to prepare well-defined materials drives the methodological evolution for molecular synthesis, structural control and materials manufacturing. Among various methods, chemical approaches to design, synthesize, control and engineer small molecules, polymers and networks offer the fundamental strategies. Merging covalent bonds and non-covalent interactions into one method to establish a complex structural composition for specific functions, mimicking biological systems such as DNA, RNA and proteins, is at the centre of chemistry and materials science. Covalent organic frameworks (COFs) are a class of crystalline porous polymers that enable the integration of organic units into highly ordered structures via polymerization. This polymerization system is unique as it deploys covalent bonds to construct the primary order structures of polymeric backbones via polycondensation and leverages on non-covalent interactions to create the high order structures of polymeric networks via supramolecular polymerization in a one-pot reaction system. This Primer covers all aspects of the field of COFs from chemistry to physics, materials and applications, and outlines the design principle, experimental methods, characterization and applications, with an aim to show a concise yet full picture of the field. The key fundamental issues to be addressed are analysed with an outlook on the future major directions from different perspectives.Covalent organic frameworks (COFs) are a class of crystalline porous polymers consisting of highly ordered organic structures formed by polymerization. In this Primer, Tan et al. discuss the design principle, experimental methods, characterization and applications of COFs.

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. Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts but it remains challenging to gain insight into photocatalytic active sites and reaction mechanisms. Herein, the authors 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

Precisely modulating the Ru-O covalency in RuO x for enhanced stability in proton exchange membrane water electrolysis is highly desired. However, transition metals with d -valence electrons, which were doped into or alloyed with RuO x , are inherently susceptible to the influence of coordination environment, making it challenging to modulate the Ru-O covalency in a precise and continuous manner. Here, we first deduce that the introduction of lanthanide with gradually changing electronic configurations can continuously modulate the Ru-O covalency owing to the shielding effect of 5 s /5 p orbitals. Theoretical calculations confirm that the durability of Ln-RuO x following a volcanic trend as a function of Ru-O covalency. Among various Ln-RuO x , Er-RuO x is identified as the optimal catalyst and possesses a stability 35.5 times higher than that of RuO 2 . Particularly, the Er-RuO x -based device requires only 1.837 V to reach 3 A cm −2 and shows a long-term stability at 500 mA cm −2 for 100 h with a degradation rate of mere 37 μV h −1 . Lack of stability in RuO 2 -based catalysts at industrial currents impedes their use in green hydrogen production. Here, the authors show that incorporating lanthanide elements into RuO x shields against external factors, enabling fine-tuned Ru-O covalency for durable oxygen evolution reaction electrocatalysis.

Capture of CO 2 from the air offers a promising approach to addressing climate change and achieving carbon neutrality goals 1 , 2 . However, the development of a durable material with high capacity, fast kinetics and low regeneration temperature for CO 2 capture, especially from the intricate and dynamic atmosphere, is still lacking. Here a porous, crystalline covalent organic framework (COF) with olefin linkages has been synthesized, structurally characterized and post-synthetically modified by the covalent attachment of amine initiators for producing polyamines within the pores. This COF (termed COF-999) can capture CO 2 from open air. COF-999 has a capacity of 0.96 mmol g –1 under dry conditions and 2.05 mmol g –1 under 50% relative humidity, both from 400 ppm CO 2 . This COF was tested for more than 100 adsorption–desorption cycles in the open air of Berkeley, California, and found to fully retain its performance. COF-999 is an exceptional material for the capture of CO 2 from open air as evidenced by its cycling stability, facile uptake of CO 2 (reaches half capacity in 18.8 min) and low regeneration temperature (60 °C). A polyamine-functionalized covalent organic framework, COF-999, can be used as a material for direct air capture of CO 2 from open air.

Organic semiconductors offer a tunable platform for photocatalysis, yet the more difficult exciton dissociation, compared to that in inorganic semiconductors, lowers their photocatalytic activities. In this work, we report that the charge carrier lifetime is dramatically prolonged by incorporating a suitable donor-acceptor (β-ketene-cyano) pair into a covalent organic framework nanosheet. These nanosheets show an apparent quantum efficiency up to 82.6% at 450 nm using platinum as co-catalyst for photocatalytic H 2 evolution. Charge carrier kinetic analysis and femtosecond transient absorption spectroscopy characterizations verify that these modified covalent organic framework nanosheets have intrinsically lower exciton binding energies and longer-lived charge carriers than the corresponding nanosheets without the donor-acceptor unit. This work provides a model for gaining insight into the nature of short-lived active species in polymeric organic photocatalysts. While organic semiconductors offer a tunable platform for photocatalysis, they often show worse performances. Here, authors examine how a donor-acceptor pair’s incorporation into a covalent organic framework boosts photocatalytic H 2 evolution performances with a platinum co-catalyst.

Through molecular engineering, single diarylethenes were covalently sandwiched between graphene electrodes to form stable molecular conduction junctions. Our experimental and theoretical studies of these junctions consistently show and interpret reversible conductance photoswitching at room temperature and stochastic switching between different conductive states at low temperature at a single-molecule level. We demonstrate a fully reversible, two-mode, single-molecule electrical switch with unprecedented levels of accuracy (on/off ratio of ~100), stability (over a year), and reproducibility (46 devices with more than 100 cycles for photoswitching and ~10⁵ to 10⁶ cycles for stochastic switching).

Radioactive molecular iodine (I 2 ) and organic iodides, mainly methyl iodide (CH 3 I), coexist in the off-gas stream of nuclear power plants at low concentrations, whereas few adsorbents can effectively adsorb low-concentration I 2 and CH 3 I simultaneously. Here we demonstrate that the I 2 adsorption can occur on various adsorptive sites and be promoted through intermolecular interactions. The CH 3 I adsorption capacity is positively correlated with the content of strong binding sites but is unrelated to the textural properties of the adsorbent. These insights allow us to design a covalent organic framework to simultaneously capture I 2 and CH 3 I at low concentrations. The developed material, COF-TAPT, combines high crystallinity, a large surface area, and abundant nucleophilic groups and exhibits a record-high static CH 3 I adsorption capacity (1.53 g·g −1 at 25 °C). In the dynamic mixed-gas adsorption with 150 ppm of I 2 and 50 ppm of CH 3 I, COF-TAPT presents an excellent total iodine capture capacity (1.51 g·g −1 ), surpassing various benchmark adsorbents. This work deepens the understanding of I 2 /CH 3 I adsorption mechanisms, providing guidance for the development of novel adsorbents for related applications. Radioactive molecular iodine (I 2 ) and methyl iodide (CH 3 I) coexist in the off-gas stream of nuclear power plants at low concentrations and only few adsorbents can effectively adsorb low-concentration I 2 and CH 3 I simultaneously. Here, the authors demonstrate simultaneous capture of I 2 and CH 3 I at low concentrations by exploiting different adsorptive sites in a covalent organic framework.

Covalent organic frameworks have recently gained increasing attention in photocatalytic hydrogen generation from water. However, their structure-property-activity relationship, which should be beneficial for the structural design, is still far-away explored. Herein, we report the designed synthesis of four isostructural porphyrinic two-dimensional covalent organic frameworks (MPor-DETH-COF, M = H 2 , Co, Ni, Zn) and their photocatalytic activity in hydrogen generation. Our results clearly show that all four covalent organic frameworks adopt AA stacking structures, with high crystallinity and large surface area. Interestingly, the incorporation of different transition metals into the porphyrin rings can rationally tune the photocatalytic hydrogen evolution rate of corresponding covalent organic frameworks, with the order of CoPor-DETH-COF < H 2 Por-DETH-COF < NiPor-DETH-COF < ZnPor-DETH-COF. Based on the detailed experiments and calculations, this tunable performance can be mainly explained by their tailored charge-carrier dynamics via molecular engineering. This study not only represents a simple and effective way for efficient tuning of the photocatalytic hydrogen evolution activities of covalent organic frameworks at molecular level, but also provides valuable insight on the structure design of covalent organic frameworks for better photocatalysis. Covalent organic frameworks (COFs) present well-defined materials for constructing structure-property-activity relationships. Herein, authors explore isostructural porphyrinic two-dimensional COFs with tunable of photocatalytic H 2 production rates arising from tailored charge-carrier dynamics.

Covalent organic frameworks (COFs) are an emerging type of crystalline and porous photocatalysts for hydrogen evolution, however, the overall water splitting activity of COFs is rarely known. In this work, we firstly realized overall water splitting activity of β -ketoamine COFs by systematically engineering N-sites, architecture, and morphology. By in situ incorporating sub-nanometer platinum (Pt) nanoparticles co-catalyst into the pores of COFs nanosheets, both Pt@TpBpy-NS and Pt@TpBpy-2-NS show visible-light-driven overall water splitting activity, with the optimal H 2 and O 2 evolution activities of 9.9 and 4.8 μmol in 5 h for Pt@TpBpy-NS, respectively, and a maximum solar-to-hydrogen efficiency of 0.23%. The crucial factors affecting the activity including N-sites position, nano morphology, and co-catalyst distribution were systematically explored. Further mechanism investigation reveals the tiny diversity of N sites in COFs that induces great differences in electron transfer as well as reaction potential barriers. Covalent organic frameworks (COFs) are an emerging type of crystalline and porous photocatalysts for hydrogen evolution. Here, the authors report a β-ketoamine COF by systematically engineering N-sites, architecture, and morphology for improved water splitting activity.