Designing Hydrogen‐Bonded Organic Frameworks (HOFs) with Permanent Porosity

Designing organic components that can be used to construct porous materials enables the preparation of tailored functionalized materials. Research into porous materials has seen a resurgence in the past decade as a result of finding of self‐standing porous molecular crystals (PMCs). Particularly, a number of crystalline systems with permanent porosity that are formed by self‐assembly through hydrogen bonding (H‐bonding) have been developed. Such systems are called hydrogen‐bonded organic frameworks (HOFs). Herein we systematically describe H‐bonding patterns (supramolecular synthons) and molecular structures (tectons) that have been used to achieve thermal and chemical durability, a large surface area, and functions, such as selective gas sorption and separation, which can provide design principles for constructing HOFs with permanent porosity.     

Flexible Viologen‐Based Porous Framework Showing X‐ray Induced Photochromism with Single‐Crystal‐to‐Single‐Crystal Transformation

A viologen‐based Borromean entangled porous framework was found to be sensitive to both Cu_Kα and Mo_Kα X‐ray sources, showing rapid photochromic response and recovery within one minute. The X‐ray‐induced photochromic process is accompanied by a reversible single‐crystal‐to‐single‐crystal (SC‐SC) structural transformation, an unprecedented phenomenon for X‐ray sensitive materials. The complex can be further processed into portable thin films for detecting the dose of the X‐ray exposure. Moreover, the photochromism can occur over a broad temperature range of 100–333 K, both in the form of single crystals and thin films, making it a potential candidate for practical indoor and outdoor applications.     

From Discrete Molecular Cages to a Network of Cages Exhibiting Enhanced CO2 Adsorption Capacity

We have adopted the concept of “cage to frameworks” to successfully produce a Na–N connected coordination networked cage Na‐NC1 by using a [3+6] porous imine‐linked organic cage NC1 (Nanjing Cage 1) as the precursor. It is found that Na‐NC1 exhibits hierarchical porosity (inherent permanent voids and interconnected channel) and gas sorption measurements reveal a significantly enhanced CO₂ uptake (1093 cm³ g⁻¹ at 23 bar and 273 K) than that of NC1 (162 cm³ g⁻¹ under the same conditions). In addition, Na‐NC1 exhibits very low CO₂ adsorption enthalpy making it a good candidate for porous materials with both high CO₂ storage and low adsorption enthalpy.     

A Permanent Mesoporous Organic Cage with an Exceptionally High Surface Area

Recently, porous organic cage crystals have become a real alternative to extended framework materials with high specific surface areas in the desolvated state. Although major progress in this area has been made, the resulting porous compounds are restricted to the microporous regime, owing to the relatively small molecular sizes of the cages, or the collapse of larger structures upon desolvation. Herein, we present the synthesis of a shape‐persistent cage compound by the reversible formation of 24 boronic ester units of 12 triptycene tetraol molecules and 8 triboronic acid molecules. The cage compound bears a cavity of a minimum inner diameter of 2.6 nm and a maximum inner diameter of 3.1 nm, as determined by single‐crystal X‐ray analysis. The porous molecular crystals could be activated for gas sorption by removing enclathrated solvent molecules, resulting in a mesoporous material with a very high specific surface area of 3758 m² g^?1 and a pore diameter of 2.3 nm, as measured by nitrogen gas sorption.     

Epoxy‐Functionalized Porous Organic Polymers via the Diels–Alder Cycloaddition Reaction for Atmospheric Water Capture

The synthesis of highly microporous, epoxy‐functionalized porous organic polymers (ep‐POPs) by a one‐pot, catalyst‐free Diels–Alder cycloaddition polymerization is reported. The high oxygen content of ep‐POPs offer efficient hydrogen‐bonding sites for water molecules, thus leading to high water‐uptake capacities up to 39.2–42.4 wt % under a wide temperature range of 5–45 °C, which covers the span of climatic conditions and manufacturing applications in which such materials might be used. Importantly, ep‐POPs demonstrated regeneration temperatures as low as 55 °C, as well as excellent water stability, recyclability, and high specific surface areas up to 852 m² g⁻¹.     

Supramolecular engineering of intrinsic and extrinsic porosity in covalent organic cages

Control over pore size, shape, and connectivity in synthetic porous materials is important in applications such as separation, storage, and catalysis. Crystalline organic cage molecules can exhibit permanent porosity, but there are few synthetic methods to control the crystal packing and hence the pore connectivity. Typically, porosity is either 'intrinsic' (within the molecules) or 'extrinsic' (between the molecules)--but not both. We report a supramolecular approach to the assembly of porous organic cages which involves bulky directing groups that frustrate the crystal packing. This generates, in a synthetically designed fashion, additional 'extrinsic' porosity between the intrinsically porous cage units. One of the molecular crystals exhibits an apparent Brunauer-Emmett-Teller surface area of 854 m(2) g(-1), which is higher than that of unfunctionalized cages of the same dimensions. Moreover, connectivity between pores, and hence guest uptakes, can be modulated by the introduction of halogen bonding motifs in the cage modules. This suggests a broader approach to the supramolecular engineering of porosity in molecular organic crystals.     

3D‐Printing of Functionally Graded Porous Materials Using On‐Demand Reconfigurable Microfluidics

Tailoring the morphology of macroporous structures remains one of the biggest challenges in material synthesis. Herein, we present an innovative approach for the fabrication of custom macroporous materials in which pore size varies throughout the structure by up to an order of magnitude. We employed a valve‐based flow‐focusing junction (vFF) in which the size of the orifice can be adjusted in real‐time (within tens of milliseconds) to generate foams with on‐line controlled bubble size. We used the junction to fabricate layered and smoothly graded porous structures with pore size varying in the range of 80–800 μm. Additionally, we mounted the vFF on top of an extrusion printer and 3D‐printed constructs characterized by a predefined 3D geometry and a controlled, spatially varying internal porous architecture, such as a model of a bone. The presented technology opens new possibilities in macroporous material synthesis with potential applications ranging from tissue engineering to aerospace industry and construction.     

From Concept to Crystals via Prediction: Multi‐Component Organic Cage Pots by Social Self‐Sorting

We describe the a priori computational prediction and realization of multi‐component cage pots, starting with molecular predictions based on candidate precursors through to crystal structure prediction and synthesis using robotic screening. The molecules were formed by the social self‐sorting of a tri‐topic aldehyde with both a tri‐topic amine and di‐topic amine, without using orthogonal reactivity or precursors of the same topicity. Crystal structure prediction suggested a rich polymorphic landscape, where there was an overall preference for chiral recognition to form heterochiral rather than homochiral packings, with heterochiral pairs being more likely to pack window‐to‐window to form two‐component capsules. These crystal packing preferences were then observed in experimental crystal structures.     

Investigation of an Unusual Crystal Habit of Hydrochlorothiazide Reveals Large Polar Enantiopure Domains and a Possible Crystal Nucleation Mechanism

The observation of an unusual crystal habit in the common diuretic drug hydrochlorothiazide (HCT), and identification of its subtle conformational chirality, has stimulated a detailed investigation of its crystalline forms. Enantiomeric conformers of HCT resolve into an unusual structure of conjoined enantiomorphic twin crystals comprising enantiopure domains of opposite chirality. The purity of the domains and the chiral molecular conformation are confirmed by spatially revolved synchrotron micro‐XRD experiments and neutron diffraction, respectively. Macroscopic inversion twin symmetry observed between the crystal wings suggests a pseudoracemic structure that is not a solid solution or a layered crystal structure, but an unusual structural variant of conglomerates and racemic twins. Computed interaction energies for molecular pairs in the racemic and enantiopure polymorphs of HCT, and the observation of large opposing unit‐cell dipole moments for the enantiopure domains in these twin crystals, suggest a plausible crystal nucleation mechanism for this unusual crystal habit.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Dihalobenzene Shape Sorting by Nonporous Adaptive Crystals of Perbromoethylated Pillararenes

The separation of dihalobenzene isomers, such as dichlorobenzene isomers and difluorobenzene isomers, has a high practical value in both synthetic chemistry and industrial production. Herein we provide a simple to operate and energy‐efficient adsorptive separation method using nonporous adaptive crystals of perbromoethylated pillar[5]arene ( BrP5 ) and pillar[6]arene ( BrP6 ). BrP6 crystals show a preference towards the ortho isomer of dichlorobenzene in isomer mixtures, but cannot discriminate difluorobenzene isomers. Single‐crystal structures reveal that this selectivity is derived from the stability of the new host–guest crystal structure of BrP6 after uptake of the preferred guest and the binding strength of the host–guest interactions. Furthermore, because of the reversible transition between guest‐free and guest‐loaded structures, BrP6 crystals are recyclable.     

General Interfacial Self‐Assembly Engineering for Patterning Two‐Dimensional Polymers with Cylindrical Mesopores on Graphene

Free‐standing 2D porous nanomaterials have attracted considerable interest as ideal candidates of 2D film electrodes for planar energy storage devices. Nevertheless, the construction of well‐defined mesopore arrays parallel to the lateral surface, which facilitate fast in‐plane ionic diffusion, is a challenge. Now, a universal interface self‐assembly strategy is used for patterning 2D porous polymers, for example, polypyrrole, polyaniline, and polydopamine, with cylindrical mesopores on graphene nanosheets. The resultant 2D sandwich‐structured nanohybrids are employed as the interdigital microelectrodes for the assembly of planar micro‐supercapacitors (MSCs), which deliver outstanding volumetric capacitance of 102 F cm^?3 and energy density of 2.3 mWh cm^?3, outperforming most reported MSCs. The MSCs display remarkable flexibility and superior integration for boosting output voltage and capacitance.     

Mechanochemical Friedel–Crafts Alkylation—A Sustainable Pathway Towards Porous Organic Polymers

This study elucidates an innovative mechanochemical approach applying Friedel–Crafts alkylation to synthesize porous covalent triazine frameworks (CTFs). Herein, we pursue a counterintuitive approach by utilizing a rather destructive method to synthesize well‐defined materials with intrinsic porosity. Investigating a model system including carbazole as monomer and cyanuric chloride as triazine node, ball milling is shown to successfully yield porous polymers almost quantitatively. We verified the successful structure formation by an in‐depth investigation applying XPS, solid‐state NMR and FT‐IR spectroscopy. An in situ study of pressure and temperature developments inside the milling chamber in combination with two‐dimensional liquid‐state NMR spectroscopy reveals insights into the polymerization mechanism. The versatility of this mechanochemical approach is showcased by application of other monomers with different size and geometry.     

2D Organic Photonics: An Asymmetric Optical Waveguide in Self‐Assembled Halogen‐Bonded Cocrystals

Anisotropic organic molecular construction and packing are crucial for the optoelectronic properties of organic crystals. Two‐dimensional (2D) organic crystals with regular morphology and good photon confinement are potentially suitable for a chip‐scale planar photonics system. Herein, through the bottom‐up process, 2D halogen‐bonded DPEpe‐F₄DIB cocrystals were fabricated that exhibit an asymmetric optical waveguide with the optical‐loss coefficients of R_Backward=0.0346 dB μm^?1 and R_Forward=0.0894 dB μm^?1 along the [010] crystal direction, which can be attributed to the unidirectional total internal reflection caused by the anisotropic molecular packing mode. Based on this crystal direction‐oriented asymmetric photon transport, these as‐prepared 2D cocrystals have been demonstrated as a microscale optical logic gate with multiple input/out channels, which will offer potential applications as the 2D optical component for the integrated organic photonics.     

MOF‐on‐MOF: Oriented Growth of Multiple Layered Thin Films of Metal–Organic Frameworks

The precise alignment of multiple layers of metal–organic framework (MOF) thin films, or MOF‐on‐MOF films, over macroscopic length scales is presented. The MOF‐on‐MOF films are fabricated by epitaxially matching the interface. The first MOF layer (Cu₂(BPDC)₂, BPDC=biphenyl‐4,4′‐dicarboxylate) is grown on an oriented Cu(OH)₂ film by a “one‐pot” approach. Aligned second (Cu₂(BDC)₂, BDC=benzene 1,4‐dicarboxylate, or Cu₂(BPYDC)₂, BPYDC=2,2′‐bipyridine‐5,5′‐dicarboxylate) MOF layers can be deposited using liquid‐phase epitaxy. The co‐orientation of the MOF films is confirmed by X‐ray diffraction. Importantly, our strategy allows for the synthesis of aligned MOF films, for example, Cu₂(BPYDC)₂, that cannot be grown on a Cu(OH)₂ surface. We show that aligned MOF films furnished with Ag nanoparticles show a unique anisotropic plasmon resonance. Our MOF‐on‐MOF approach expands the chemistry of heteroepitaxially oriented MOF films and provides a new toolbox for multifunctional porous coatings.     

Cooperative Capture of Uranyl Ions by a Carbonyl‐Bearing Hierarchical‐Porous Cu–Organic Framework

To efficiently capture the toxic uranyl ions (UO₂²⁺), a new hierarchical micro‐macroporous metal–organic framework was prepared under template‐free conditions, featuring interconnected multi‐nanocages bearing carbonyl groups derived from a semi‐rigid ligand. The material exhibits an unusually high UO₂²⁺ sorption capacity of 562 mg g^?1, which occurs in an intriguing two‐steps process, on the macropore‐based crystal surface and in the inner nanocages. Notably, the latter is attributed to the cooperative interplay of the shrinkage of the host porous framework induced by uranyl accommodation and the free carbonyl coordination sites, as shown by both single‐crystal X‐ray diffraction and a red‐shift of the infrared [O=U^VI=O]²⁺ antisymmetric vibration band.     

Chiral Self‐Sorting of [2+3] Salicylimine Cage Compounds

An inherently chiral C₃‐symmetric triaminotribenzotriquinacene was condensed in racemic and enantiomerically pure form with a bis(salicylaldehyde) to form [2+3] salicylimine cage compounds. Investigations on the chiral self‐sorting revealed that while entropy favors narcissistic self‐sorting in solution, selective social self‐sorting can be achieved by exploiting the difference in solubility between the homochiral and heterochiral cages. Gas sorption measurements further showed that seemingly small structural differences can have a significant impact on the surface area of microporous covalent cage compounds.     

Encapsulation of Isolated Luminophores within Supramolecular Cages

The sequestration of luminophores within supramolecular polyhedral compartments of a crystalline zeolite‐like hydrogen‐bonded framework illustrates a unique approach to limiting the self‐quenching ordinarily exhibited at the high concentrations achievable in this framework. A range of differently sized luminescent guests, namely coumarin 1, coumarin 4, fluorescein, [Ru(bpy)₃]Cl₂, and rhodamine B, can be encapsulated in amounts of up to one molecule per cage, equivalent to a concentration of 0.175 m , which is significantly higher than the concentration at which aggregation‐induced quenching occurs in other media. The luminescence spectra of the encapsulated guests are consistent with the presence of isolated monomers and the absence of self‐quenching. The emission color of the single crystals can be tuned readily from blue to red through the choice of guest molecules. These observations promise an approach to organic solid‐state lasing compounds if crystals of sufficient size and quality can be prepared.     

Unravelling Crystal Structures of Covalent Organic Frameworks by Electron Diffraction Tomography

Featuring the art of covalent chemistry on 2D and 3D with molecular precision, covalent organic frameworks (COFs) have attracted immense interests from inorganic, organic, polymer, materials and energy chemistry. However, due to the synthetic challenge of “crystallization problem”, structural determination of COFs has been the bottle‐neck in speeding up their discovery and design, as well as building up their structure‐ property relation. Electron diffraction tomography (EDT) has been developed to determine crystal structures of COFs with only sub‐micrometer sized single crystals, which enabled the ab initio determination of crystal structure, molecular connectivity, pore metrics, and host‐guest interaction at the atomic level. In this review, we summarized the recent developments of EDT for addressing challenges in structure determinations of such e‐beam sensitive, organic porous crystals, covering comprehensively automatic data collection, low dose, cryogenic protocols, structural solution method, powder X‐ray diffraction refinement, and high‐resolution transmission electron microscopy (HRTEM) imaging techniques. We do believe the EDT will propel this field into the new era of COF chemistry with atomic precision, and we envision the wide application of artificial intelligence will promote the structural determination and particle analysis of COFs and related materials.     

Creating Coordination‐Based Cavities in a Multiresponsive Supramolecular Gel

Creating cavities in varying levels, from molecular containers to macroscopic materials of porosity, have long been motivated for biomimetic or practical applications. Herein, we report an assembly approach to multiresponsive supramolecular gels by integrating photochromic metal–organic cages as predefined building units into the supramolecular gel skeleton, providing a new approach to create cavities in gels. Formation of discrete O‐Pd₂L₄ cages is driven by coordination between Pd²⁺ and a photochromic dithienylethene bispyridine ligand (O‐PyFDTE). In the presence of suitable solvents (DMSO or MeCN/DMSO), the O‐Pd₂L₄ cage molecules aggregate to form nanoparticles, which are further interconnected through supramolecular interactions to form a three‐dimensional (3D) gel matrix to trap a large amount of solvent molecules. Light‐induced phase and structural transformations readily occur owing to the reversible photochromic open‐ring/closed‐ring isomeric conversion of the cage units upon UV/visible light radiation. Furthermore, such Pd₂L₄ cage‐based gels show multiple reversible gel–solution transitions when thermal‐, photo‐, or mechanical stimuli are applied. Such supramolecular gels consisting of porous molecules may be developed as a new type of porous materials with different features from porous solids.