Observation of Interpenetrated Topology Isomerism for Covalent Organic Frameworks with Atom-Resolution Single Crystal Structures

Regulating Electron Distribution in Regioisomeric Covalent Organic Frameworks for Efficient Solar-Driven Hydrogen Peroxide Production

Covalent organic frameworks (COFs) are emerging as a transformative platform for photocatalytic hydrogen peroxide (H2O2) production due to their highly ordered structures, intrinsic porosity, and molecular tunability. Despite their potential, the inefficient utilization of photogenerated charge carriers in COFs significantly restrains their photocatalytic efficiency. This study presents two regioisomeric COFs, α-TT-TDAN COF and β-TT-TDAN COF, both incorporating thieno[3,2-b]thiophene moieties, to investigate the influence of regioisomerism on the excited electron distribution and its impact on photocatalytic performance. The β-TT-TDAN COF demonstrates a remarkable solar-to-chemical conversion efficiency of 1.35%, outperforming its α-isomeric counterpart, which is merely 0.44%. Comprehensive spectroscopic and computational investigations reveal the critical role of subtle substitution change in COFs on their electronic properties. This structural adjustment intricately connects molecular structure to charge dynamics, enabling precise regulation of electron distribution, efficient charge separation and transport, and localization of excited electrons at active sites. Moreover, this finely tuned interplay significantly enhances the efficiency of the oxygen reduction reaction. These findings establish a new paradigm in COF design, offering a molecular-level strategy to advance COFs and reticular materials toward highly efficient solar-to-chemical energy conversion.

3D Cluster-Based Covalent Organic Framework for Efficient CO2 Photoreduction

Copper clusters exhibit superior catalytic activity for the CO2 reduction reaction (CO2RR), and their defined structures endow them with unique advantages for modeling the catalytic mechanism at the atomic level. Additionally, the construction of highly stable and regularly structured covalent organic frameworks (COFs) based on copper clusters still presents significant challenges. Herein, we reported two highly stable and reactive cluster-based COFs (termed Cu4COF-1 and Cu4COF-2) constructed via a stepwise assembly strategy. The epitaxially amino-modified Cu4 cluster (Cu4-NH2) was initially synthesized based on coordination bonds. Then, Cu4COFs were obtained by the covalent linkage of Cu4-NH2 clusters and organic linkers. Compared with isolated Cu4 clusters, the Cu4COFs exhibit greater stability, a narrower band gap, a larger specific surface area, and better charge transfer ability, which endow them with superior photocatalytic CO2RR performance under visible light. In-situ infrared spectroscopy and density functional theory (DFT) calculations revealed that the covalently linked Cu4COFs could efficiently lower the energy barrier for the formation of the critical *COOH intermediate, thereby enhancing the photocatalytic activity. This study offers a solid basis for the atomically precise construction of novel metal-cluster-based COF catalysts.

Regulating the Isomerization Geometry and Energy State of Covalent Organic Frameworks for Enhanced Oxygen Reduction Activity

Embedding isomer entities onto crystalline frameworks with precisely defined spatial distributions represents a promising approach to enhancing the efficiency of oxygen reduction reaction (ORR) in fuel cells. However, accurately constructing covalent organic frameworks (COFs) to regulate energy state effectively remains a significant challenge. Herein, an innovative geometric isomerization strategy aimed at minimizing the rotational barrier energy (ΔE), average local ionization energy (ALIE), and Gibbs free energy (ΔG) for ORR within COFs is proposed. Based on this strategy, isomeric Py-COF-αα with 2,2-substitution, Py-COF-ββ with 3,3-substitution, and Py-COF-αβ with 2,3-substitution on the mainchain frameworks have been obtained. The electronic states and intermediate adsorption capabilities are finely tuned through isomer modification, yielding a precisely controllable chemical activity. Notably, Py-COF-αβ with lower ΔE between thiophenes achieves remarkable performance, evidenced by a half-wave potential of 0.77 V vs reversible hydrogen electrode (RHE), surpassing most reported metal-free electrocatalysts. Combined with theoretical prediction and in situ Raman spectra, it is revealed that the increased dipole moment and non-uniform charge distribution caused by isomer endows pentacyclic-carbon (thiophene β-position) far from sulfur atoms with efficient catalytic activity. This work has opened up a novel paradigm for the isomerization of COFs and underscores the pivotal role of charge regulation in facilitating efficient catalysis.

Optimizing Propylene/Propane Sieving Separation through Gate-Pressure Control within a Flexible Organic Framework

The separation of propylene (C3H6) and propane (C3H8) is of great significance in the chemical industry, which poses a challenge due to their almost identical kinetic diameters and similar physical properties. In this work, we synthesized an ultramicroporous flexible hydrogen-bonded organic framework (named HOF-FJU-106) by using molecule 2,3,6,7-tetra (4-cyanophenyl) tetrathiafulvalene (TTF-4CN). The formation of the dimer causes the TTF-4CN molecular to bend and weaken π-stacked interactions, coupled with the flexibility of C≡N ⋯ ${\cdots }$ H-C hydrogen bonds, which leads to reversible conversion between open and closed frameworks through the mutual slip of adjacent layers/columns under activation and stimulation of gas molecules. Through gas adsorption isotherms and adsorption enthalpy, HOF-FJU-106a exhibited adaptive adsorption and stronger binding affinity for C3H6, and presented a recorded gas uptake ratio of C3H6/C3H8 (23.77) among presentative HOF materials at room temperature to date. Importantly, the flexible HOF-FJU-106a shows an interesting phenomenon about the reversible gate pressure control under variable temperature, which realized the gas adsorption and separation performance enhancement for the binary C3H6/C3H8 mixtures. This strategy through designing HOFs with thermoregulatory gating effect is a powerful way to maximize the performance of materials.

Single-Crystal Dynamic Covalent Organic Frameworks for Adaptive Guest Alignments

Dynamic 3D covalent organic frameworks (COFs) have shown a concerted structural transformation upon adaptive guest inclusion. However, the origin of the conformational mobility and the host-guest adaptivity remain conjecture of the pedal motions of revolving imine linkages, often without considering the steric hindrance from the interwoven frameworks. Here, we present atomic-level observation of the rotational and translational dynamics in single-crystal COF-300 upon adaptive guest inclusion of various organic molecules, featuring multiple rotamers of covalent linkages and switchable interframework noncovalent interactions. Specifically, we developed a diffusion gradient transimination protocol to facilitate the growth of COF single crystals, enabling a high-resolution X-ray diffraction structural analysis. We uncovered metastable and low-symmetry intermediate phases from contracted to expanded phases during structural evolution. We identified torsion angles in the terephthalaldehyde diimine motifs that switch from anti-periplanar to syn-periplanar/anticlinal conformations. Moreover, the rotational dynamics of the imine linkage were concurrent with the translational dynamics of tetraphenylmethane units, which tend to form the translational quadruple phenyl embrace. Such conformational mobility allows the frameworks to adapt to various guest molecules, such as alcohols, esters, phenols, and diols, featuring double linear, herringbone, zigzag chains, triple helix, and tubular alignments. Quantitative energy analyses revealed that such dynamic structure transformations are not arbitrary but follow specific pathways that resemble protein folding. The work is paving the way to developing robust, dynamic, and crystalline molecular sponges for studying the condensed structure of liquids without the need for further crystallization.

Single-Crystalline 3D Covalent Organic Frameworks with Exceptionally High Specific Surface Areas and Gas Storage Capacities

Single-crystalline covalent organic frameworks (COFs) are highly desirable toward understanding their pore chemistry and functions. Herein, two 50-100 μm single-crystalline three-dimensional (3D) COFs, TAM-TFPB-COF and TAPB-TFS-COF, were prepared from the condensation of 4,4',4″,4‴-methanetetrayltetraaniline (TAM) with 3,3',5,5'-tetrakis(4-formylphenyl)bimesityl (TFPB) and 3,3',5,5'-tetrakis(4-aminophenyl)bimesityl (TAPB) with 4,4',4″,4‴-silanetetrayltetrabenzaldehyde (TFS), respectively, in 1,4-dioxane under the catalysis of acetic acid. Single-crystal 3D electron diffraction reveals the triply interpenetrated dia-b networks of TAM-TFPB-COF with atom resolution, while the isostructure of TAPB-TFS-COF was disclosed by synchrotron single-crystal X-ray diffraction and synchrotron powder X-ray diffraction with Le Bail refinements. The nitrogen sorption measurements at 77 K disclose the microporosity nature of both activated COFs with their exceptionally high Brunauer-Emmett-Teller surface areas of 3533 and 4107 m2 g-1, representing the thus far record high specific surface area among imine-bonded COFs. This enables the activated COFs to exhibit also the record high methane uptake capacities up to 28.9 wt % (570 cm3 g-1) at 25 °C and 200 bar among all COFs reported thus far. This work not only presents the structures of two single-crystalline COFs with exceptional microporosity but also provides an example of atom engineering to adjust permanent microporous structures for methane storage.

Analysis of COF-300 synthesis: probing degradation processes and 3D electron diffraction structure

Although COF-300 is often used as an example to study the synthesis and structure of (3D) covalent organic frameworks (COFs), knowledge of the underlying synthetic processes is still fragmented. Here, an optimized synthetic procedure based on a combination of linker protection and modulation was applied. Using this approach, the influence of time and temperature on the synthesis of COF-300 was studied. Synthesis times that were too short produced materials with limited crystallinity and porosity, lacking the typical pore flexibility associated with COF-300. On the other hand, synthesis times that were too long could be characterized by loss of crystallinity and pore order by degradation of the tetrakis(4-aminophenyl)methane (TAM) linker used. The presence of the degradation product was confirmed by visual inspection, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As TAM is by far the most popular linker for the synthesis of 3D COFs, this degradation process might be one of the reasons why the development of 3D COFs is still lagging compared with 2D COFs. However, COF crystals obtained via an optimized procedure could be structurally probed using 3D electron diffraction (3DED). The 3DED analysis resulted in a full structure determination of COF-300 at atomic resolution with satisfying data parameters. Comparison of our 3DED-derived structural model with previously reported single-crystal X-ray diffraction data for this material, as well as parameters derived from the Cambridge Structural Database, demonstrates the high accuracy of the 3DED method for structure determination. This validation might accelerate the exploitation of 3DED as a structure determination technique for COFs and other porous materials.

Dynamic Entwined Topology in Helical Covalent Polymers Dictated by Competing Supramolecular Interactions

Naturally occurring polymeric structures often consist of 1D polymer chains intricately folded and entwined through non-covalent bonds, adopting precise topologies crucial for their functionality. The exploration of crystalline 1D polymers through dynamic covalent chemistry (DCvC) and supramolecular interactions represents a novel approach for developing crystalline polymers. This study shows that sub-angstrom differences in the counter-ion size can lead to various helical covalent polymer (HCP) topologies, including a novel metal-coordination HCP (m-HCP) motif. Single-crystal X-ray diffraction (SCXRD) analysis of HCP-Na revealed that double helical pairs are formed by sodium ions coordinating to spiroborate linkages to form rectangular pores. The double helices are interpenetrated by the unreacted diols coordinating sodium ions. The reticulation of the m-HCP structure was demonstrated by the successful synthesis of HCP-K. Finally, ion-exchange studies were conducted to show the interconversion between HCP structures. This research illustrates how seemingly simple modifications, such as changes in counter-ion size, can significantly influence the polymer topology and determine which supramolecular interactions dominate the crystal lattice.

A review of recent advancement in covalent organic framework (COFs) synthesis and characterization with a focus on their applications in antibacterial activity

The primary objective of this review is to present a comprehensive examination of the synthesis, characterization, and antibacterial applications of covalent organic frameworks (COFs). COFs represent a distinct category of porous materials characterized by a blend of advantageous features, including customizable pore dimensions, substantial surface area, and adaptable chemical properties. These attributes position COFs as promising contenders for various applications, notably in the realm of antibacterial activity. COFs exhibit considerable potential in the domain of antibacterial applications, owing to their amenability to functionalization with antibacterial agents. The scientific community is actively exploring COFs that have been imbued with metal ions, such as copper or silver, given their observed robust antibacterial properties. These investigations strongly suggest that COFs could be harnessed effectively as potent antibacterial agents across a diverse array of applications. Finally, COFs hold immense promise as a novel class of materials for antibacterial applications, shedding light on the synthesis, characterization, and functionalization of COFs tailored for specific purposes. The potential of COFs as effective antibacterial agents beckons further exploration and underscores their potential to revolutionize antibacterial strategies in various domains.

Encoding ordered structural complexity to covalent organic frameworks

Installing different chemical entities onto crystalline frameworks with well-defined spatial distributions represents a viable approach to achieve ordered and complex synthetic materials. Herein, a covalent organic framework (COF-305) is constructed from tetrakis(4-aminophenyl)methane and 2,3-dimethoxyterephthalaldehyde, which has the largest unit cell and asymmetric unit among known COFs. The ordered complexity of COF-305 is embodied by nine different stereoisomers of its constituents showing specific sequences on topologically equivalent sites, which can be attributed to its building blocks deviating from their intrinsically preferred simple packing geometries in their molecular crystals to adapt to the framework formation. The insight provided by COF-305 supplements the principle of covalent reticular design from the perspective of non-covalent interactions and opens opportunities for pursuing complex chemical sequences in molecular frameworks.