Engineering the building blocks in metal–organic materials is an effective strategy for tuning their dynamical properties and can affect their response to external guest molecules. Tailoring the interaction and diffusion of molecules into these structures is highly important, particularly for applications related to gas separation. Herein, we report a vanadium-based hybrid ultramicroporous material, VOFFIVE-1-Ni, with temperature-dependent dynamical properties and a strong affinity to effectively capture and separate carbon dioxide (CO₂) from methane (CH₄). VOFFIVE-1-Ni exhibits a CO₂ uptake of 12.08 wt % (2.75 mmol g^–1), a negligible CH₄ uptake at 293 K (0.5 bar), and an excellent CO₂-over-CH₄ uptake ratio of 2280, far exceeding that of similar materials. The material also exhibits a favorable CO₂ enthalpy of adsorption below −50 kJ mol^–1, as well as fast CO₂ adsorption rates (90% uptake reached within 20 s) that render the hydrolytically stable VOFFIVE-1-Ni a promising sorbent for applications such as biogas upgrading.

The contamination of water resources by organic pollutants presents significant environmental and health challenges. Therefore, it is urgent to develop highly efficient and green approach for treating organic water pollutants. Bismuth oxybromide (BiOBr) has gained attention due to its high photoactivity. In this work, we report a modification to improve its photocatalytic activity. BiOBr were prepared using a capping agent, benzene-1,3,5-tricarboxylic acid, to tune the morphology of the compound. The resulting BiOBr shows nanosheet morphology, which provides a high surface-to-volume ratio and a larger conduction band compared to bulk BiOBr. As a result, the BiOBr nanosheets show the highest efficiency for photodegradation of Rhodamine B, compared to benchmark TiO2 and bulk BiOBr catalysts.

The structures of covalent organic frameworks (COFs) are typically determined through modeling based on powder X-ray diffraction. However, the intrinsically limited crystallinity of COFs often results in structural determinations of low fidelity. Here, we present real-space imaging of an extensively studied two-dimensional imine-based COF. Contrary to the conventional understanding that this COF features uniform hexagonal pores, our observations reveal the presence of two distinct sets of pores with differences in shape and size. Motivated by this finding, we conducted reciprocal-space characterizations, complemented by solid-state nuclear magnetic resonance spectroscopy and density functional theory calculations, to reevaluate this seemingly simple structure. The collective results allow for the establishment of a new structural model for this landmark COF and its derivatives, differing from the conventional model in both intra- and interlayer configurations. Furthermore, we identified various previously unrecognized defective structures through real-space imaging, which have significant implications for COF applications in separation and catalysis. Our study demonstrates the complexity and heterogeneity of COF structures, while also highlighting the imperative for structural reevaluation using advanced characterization techniques.

Two-dimensional metal–organic nanosheets (MONs) have emerged as attractive alternatives to their three-dimensional metal–organic framework (MOF) counterparts for heterogeneous catalysis due to their greater external surface areas and higher accessibility of catalytically active sites. Zr MONs are particularly prized because of their chemical stability and high Lewis and Brønsted acidities of the Zr clusters. Herein, we show that careful control over modulated self-assembly and exfoliation conditions allows the isolation of the first example of a two-dimensional nanosheet wherein Zr₆ clusters are linked by dicarboxylate ligands. The hxl topology MOF, termed GUF-14 (GUF = Glasgow University Framework), can be exfoliated into monolayer thickness hns topology MONs, and acid-induced removal of capping modulator units yields MONs with enhanced catalytic activity toward the formation of imines and the hydrolysis of an organophosphate nerve agent mimic. The discovery of GUF-14 serves as a valuable example of the undiscovered MOF/MON structural diversity extant in established metal–ligand systems that can be accessed by harnessing the power of modulated self-assembly protocols.

Conspectus In the development of 2D metal-organic frameworks (MOFs) and 2D covalent organic frameworks (COFs), obtaining structural details at the atomic level is crucial to understanding their properties and related mechanisms in potential applications. However, since 2D-MOFs and COFs are composed of layered structures and often exhibit sheet-like morphologies, it is challenging to grow large crystals suitable for single-crystal X-ray diffraction (SCXRD). Therefore, ab initio structure determination, which refers to solving the structure directly from experimental data without using any prior knowledge or computational input, is extremely rare for 2D-MOFs and COFs. In contrast to SCXRD, three-dimensional electron diffraction (3DED) only requires crystals sized in tens or hundreds of nanometers, making it an ideal method for single-crystal analysis of 2D-MOFs and COFs and obtaining their fine structural details. In this Account, we describe our recent development of the 3DED method and its application in structure determination and property studies of 2D-MOFs and COFs. A key development is the establishment of a continuous 3DED data collection protocol. By collecting electron diffraction (ED) patterns continuously while performing crystal tilting, the electron dose applied to the target nanocrystal is greatly reduced. This allows the acquisition of high-resolution 3DED data from 2D-MOFs and COFs by minimizing their damage under the electron beam. We have also developed an approach to couple 3DED with real-space structure solution methods, i.e., simulated annealing (SA), for single-crystal structural analysis of materials that do not have high crystallinity. We successfully determined two 2D-COF structures by combining 3DED with SA. We provide several examples demonstrating the application of 3DED for the ab initio structure determination of 2D-MOFs and COFs, revealing not only their in-plane structures but also their stacking modes at the atomic level. Notably, the obtained structural details serve as the foundation for further understanding the properties of 2D-MOFs and COFs, such as their electronic band structures, charge mobilities, etc. Beyond structure determination, we describe our work on using 3DED as a high-throughput method for the discovery of new materials. Using this approach, we discovered a novel MOF that was present only in trace amounts within a multiphasic mixture. Through this discovery, we were able to tune the synthesis conditions to obtain its pure phase. We detail how 3DED can be used to probe different levels of molecular motions in MOFs through the analysis of anisotropic displacement parameters (ADPs). Additionally, we show that 3DED can provide accurate information about intermolecular weak interactions such as hydrogen bonding and van der Waals (vdW) interactions. Our studies demonstrate that 3DED is a valuable method for the structural analysis of 2D-MOFs and COFs. We envision that 3DED can accelerate research in these fields by providing unambiguous structural models at the atomic level.

Two-dimensional covalent organic frameworks (2D COFs) are formed by the polycondensation of geometrically specific monomers to grow covalently connected 2D polygonal polymers over the a–b plane and supramolecular polymerization and/or crystallization of 2D sheets along the c direction to constitute layer architectures. Despite various efforts, the synthesis of single-crystal 2D COFs remains a challenging goal. Here, we report the synthesis of single-crystal 2D COFs, by taking the representative imine-linked TPB-DMTP-COF as an example, to reveal the key synthetic parameters that control the crystallization of 2D COFs. We systematically tune the synthetic conditions including the glassware setup, the degas method, the solvent, the temperature, the modulator, and the reaction time and observed that all these parameters greatly affect the polymerization and crystallization processes, controlling the crystal quality. We found that a homogeneous system with all components dissolved and the presence of a suitable modulator at a temperature of 50–70 °C allows the growth of TPB-DMTP-COF single crystals as isolated individual rods, with tunable diameters of 200 nm to 3 μm and a length of 1–20 μm. The single-crystal structure was characterized by three-dimensional electron diffraction (3DED), which revealed two conformations of trans and cis for the linker in the 2D polymer sheets, which stack in an antiparallel mode to shape the frameworks with double-sized unit cells. These results uncover these hidden structural features which have been overlooked in polycrystalline and single-crystal studies and provide new insights into the synthesis of high-quality single crystals of 2D COFs.

Developing the topochemical polymerization of metal-organic frameworks (MOFs) is of pronounced significance for expanding their functionalities but is still a challenge on third-order nonlinear optics (NLO). Here, we report diacetylene MOF (CAS-1-3) films prepared using a stepwise deposition method and film structural transformation approach, featuring dynamic structural diversity. The MOF structures were determined by the three-dimensional electron diffraction (3D ED) method from nanocrystals collected from the films, which provides a reliable strategy for determining the precise structure of unknown MOF films. We demonstrate the well-aligned diacetylene groups in the MOFs can promote topological polymerization to produce a highly conjugated system under thermal stimulation. As a result, the three MOF films have distinct NLO properties: the CAS-1 film exhibits saturable absorption (SA) while CAS-2 and CAS-3 films exhibit reverse saturable absorption (RSA). Interestingly, due to the topochemical polymerization of the MOF films, a transition from SA to RSA response was observed with increasing temperatures, and the optical limiting effect was significantly enhanced (∼46 times). This study provides a new strategy for preparing NLO materials and thermally regulation of nonlinear optics.

Metal-organic frameworks (MOFs) have emerged as highly promising materials for hosting functional biomolecules. Here, a 1,2,4-benzenetricarboxylate ligand with a flat asymmetric shape is applied to infuse an unusual behavior to a 3D europium MOF (SLU-1). Solvent addition results in the 3D MOF splitting into a 2D one (SLU-2), and in the presence of excess water, gets cross-linked into a different 3D MOF (SLU-3) prone to spontaneous exfoliation. SLU-3 features a combination of highly hydrophilic and hydrophobic spots and serves as an attractive host for incorporating large active species. As a representative demonstration, horseradish peroxidase (HRP) is incorporated into the exfoliated 3D-layered structure by simple mixing, and secured by an outer silica layer in the form of core-shell structures. The resulting HRP-based biocatalyst exhibited enhanced stability and reusability, effectively degrading phenol. This work showcases the potential of reconfigurable MOFs, offering upheld applications through the controlled uptake and retention of biocatalytic agents. (Figure presented.).

Metal–organic frameworks (MOFs) are widely used as precursors to generate derivatives for electrocatalysis. However, two-dimensional (2D) MOFs often suffer from the collapse of their 2D structures after being treated at high temperature. Herein, we used the dense ZIF-EC1 as a precursor and doped Co as a secondary metal. The content of Co in ZIF-EC1 can be tuned without changing the crystalline structure. After pyrolysis, the derived carbon-based material maintains the 2D morphology from the parental precursor. The derived ZIF-EC1(ZnCo)-20-900 exhibits the best activity toward the ORR, which is even better than that of Pt/C. This work demonstrates the potential of using a nonporous dense MOF as a precursor and optimizing electrocatalytic ORR activity by tuning the Co content.

Two-dimensional conjugated metal–organic frameworks (2D c-MOFs) are emerging as a unique class of electronic materials. However, 2D c-MOFs with band gaps in the Vis-NIR and high charge carrier mobility are rare. Most of the reported conducting 2D c-MOFs are metallic (i.e. gapless), which largely limits their use in logic devices. Herein, we design a phenanthrotriphenylene-based, D_2h-symmetric π-extended ligand (OHPTP), and synthesize the first rhombic 2D c-MOF single crystals (Cu₂(OHPTP)). The continuous rotation electron diffraction (cRED) analysis unveils the orthorhombic crystal structure at the atomic level with a unique slipped AA stacking. The Cu₂(OHPTP) is a p-type semiconductor with an indirect band gap of ≈0.50 eV and exhibits high electrical conductivity of 0.10 S cm⁻¹ and high charge carrier mobility of ≈10.0 cm² V⁻¹ s⁻¹. Theoretical calculations underline the predominant role of the out-of-plane charge transport in this semiquinone-based 2D c-MOF.