Porous Organic Frameworks Research Articles

Systematic Study on Zirconium Chelidamates: From a Molecular Complex to a M-HOF and a MOF.

We report the synthesis and in-depth characterization of three zirconium chelidamates, a molecular complex (H8C2N)2[Zr(HL)3] (1), a porous metal-containing hydrogen-bonded organic framework (M-HOF) [Zr(H2O)2(HL)2]·xH2O (2), and a metal-organic framework (MOF) (H8C2N)2-2n[Zr(HnL)2]·x solvent (0 ≤ n ≤ 1) (3) using chelidamic acid (H3L, H5C7NO5, 4-hydroxypyridine-2,6-dicarboxylic acid) as the ligand (H8C2N+ = dimethylammonium). High-throughput investigations of the system Zr4+/H3L/HCl/DMF/H2O were carried out, which resulted in highly crystalline compounds. The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction. Single-crystal three-dimensional (3D) electron diffraction and Rietveld refinements of powder X-ray diffraction (PXRD) data had to be used to elucidate the crystal structure of 3 since only very small single crystals of about 500 nm in diameter could be obtained. In all structures, chelidamate ions act as anionic palindromic pincer ligands, and in 3, a coordinative bond is additionally formed by the aryloxy group. While dense packing of the molecular complexes is found in 1, hydrogen bonding of the molecular complexes in 2 leads to a porous network that shows flexibility depending on the water content. The three-dimensional framework structure of the Zr-MOF 3 contains a mononuclear inorganic building unit (IBU), which is very uncommon in Zr-MOF chemistry. The three compounds are stable in several organic solvents, and thermal decomposition starts above 280 °C. While the hydrogen-bonded framework 2 is only porous toward water with a water uptake of almost 3.75 mol mol-1 at p/p0 = 0.9, 3 is porous against N2, CO2, methanol, ethanol, and water with a specific Brunauer-Emmett-Teller (BET) surface area of aS,BET = 410 m2 g-1 derived from the N2 adsorption isotherm. Stability upon water adsorption covering 10 cycles between 0.5% < p/p0 < 90% for 3 is also demonstrated.

Chiral Porous Organic Frameworks: Synthesis, Chiroptical Properties, and Asymmetric Organocatalytic Applications

Chiral porous organic frameworks have emerged in the last decade as candidates for heterogeneous asymmetric organocatalysis. This review aims to provide a summary of the synthetic strategies towards the design of chiral organic materials, the characterization techniques used to evaluate their chirality, and their applications in asymmetric organocatalysis. We briefly describe the types of porous organic frameworks, including crystalline (covalent organic frameworks, COFs) and amorphous (conjugated microporous polymers, CMPs; covalent triazine frameworks, CTFs and porous aromatic frameworks, PAFs) materials. Furthermore, the strategies reported to incorporate chirality in porous organic materials are presented. We finally focus on the applications of chiral porous organic frameworks in asymmetric organocatalytic reactions, summarizing and categorizing all the available literature in the field.

Trace SO2 Gas Capture in Stable 3D Viologen Ionic Porous Organic Framework Microsphere.

Eliminating trace sulfur dioxide (SO2) using nanoporous adsorbents is industrially preferred yet of great challenge due to the competitive adsorption of CO2. Herein, we reported a highly stable 3D viologen porous organic framework (Viologen-POF) microsphere via one pot polymerization reaction of 4,4'-bipyridine and tetrakis(4-(bromomethyl)phenyl)methane. Compared to the previously reported irregular POF particles, viologen-POF microsphere shows better mass transfer uniformity. Owing to the intrinsic separated positive and negative electric charges center in viologen-POF microspheres, it exhibits excellent SO2 selective capture performance, which can be collaboratively confirmed by static single-component gas adsorption, time-dependent adsorption rate, and multicomponent dynamic breakthrough experiments. Viologen-POF exhibits high SO2 absorption capacity (1.45 mmol g-1) at ultralow pressure of 0.002 bar and high SO2/CO2 selectivity of 467 at 298 K and 100 kPa (SO2/CO2, 10/90, v/v). The theoretical calculations based on density functional theory (DFT) and DMol3 modules in Material Studio (MS) were also performed to elucidate the adsorption mechanism of viologen-POF toward SO2 at the molecular level. This study represents a new type of viologen porous framework microsphere for trace SO2 capture, which will pave the way on the applications of ionic POF for toxic gas adsorption and separation.

Polypyrene Porous Organic Framework for Efficiently Capturing Electron Specialty Gases.

The polypyrene polymer with an extended π-conjugated skeleton is attractive for perfluorinated electron specialty gas (F-gas) capture as the high electronegativity of fluorine atoms makes F-gases strongly electronegative gases. Herein, a polypyrene porous organic framework (termed as Ppy-POF) with an extended π-conjugated structure and excellent acid resistance was constructed. Systematic studies have shown that the abundant π-conjugated structures and gradient electric field distribution in Ppy-POF can endow it exceptional adsorption selectivity for high polarizable F-gases and xenon (Xe), which has been collaboratively confirmed by single-component gas adsorption experiments, time-dependent adsorption rate tests, dynamic breakthrough experiments, etc. Electrostatic potential distribution and charge density difference based on Grand Canonical Monte Carlo simulations and density functional theory calculations demonstrate that the selective adsorption of F-gases and Xe in Ppy-POF is attributed to the strong charge-transfer effect and polarization effect between Ppy-POF and gases. These results manifest that the POF with an extended π-conjugated structure and gradient electric field distribution has great potential in efficiently capturing electron specialty gases.

Emulsion interfacial synthesis of hierarchically porous covalent organic framework microcapsules with multilayered boronic acid binding sites for specific molecular separation

Although capsule-like materials as matrix for separation have been a hot research in recent years, creating abundant recognizing sites for enhanced adsorption performance is still a formidable challenge. Herein, we created a two-step continuous Pickering emulsion interfacial assembly to fabricate boronate affinity covalent organic framework microcapsule (BA-TAPB-BTAC-BPTA MC) for selective separation of Naringin (NRG). The COF particles were stabilized at water–oil interface during the first emulsion interfacial assembly. The other COFs species were then grown using low concentration of catalyst in the multiphase emulsion interface. As expected, the highly reactive alkynylation COFs particles and COFs species shell could effectively stabilize the pre-Pickering W/O emulsion. The azide-phenylboronic acid were introduced in confine space of alkynylation COFs MC via “click” modification. The sorbents presented promising adsorption capacity (20.36 µmol g−1) in 360 min under the phosphate buffer/methanol solution (pH = 8.0, v/v, 9/1). Additionally, sorbents possess excellent stability after 5 cycles of adsorption–desorption experiments (93.57%). Moreover, the commercially NRG could be effectively extracted to 93.07%. Our findings provided an idea for constructing affinity ligands COFs MC with fast penetrability and multilayered binding sites have great potential for separation and purification of industrial samples.

Construction of palladium porphyrins and triptycene photo-activated nanomaterial for enhanced colorimetric detection and inactivation of bacteria

In the face of increasing bacterial resistance, design of high-performing and dual-functional nanomaterials to satisfy the requirements for both detecting and eradicating bacteria is of immense importance, but still remains a great challenge. Herein, a hierarchically three-dimensional (3D) porous organic frameworks (PdPPOPHBTT) was rationally designed and fabricated for the first time to realize ideal simultaneous detection and eradication of bacteria. PdPPOPHBTT covalently integrated palladium 5,10,15,20-tetrakis-(4′-bromophenyl) porphyrin (PdTBrPP, an excellent photosensitizer) with 2,3,6,7,12,13-hexabromotriptycene (HBTT, a 3D building module). The resulting material had outstanding NIR absorption, narrow bad gap and robust singlet oxygen (1O2) production capacity, which is responsible for the sensitive detection and effective removal of bacteria. We successfully realized the colorimetric detection of S. aureus and the efficient removal of S. aureus and E. coli. The first-principles calculations found at the highly activated 1O2 derived from the 3D conjugated periodic structures and ample palladium adsorption site in PdPPOPHBTT. The bacterial infection wound model revealed that PdPPOPHBTT possesses good disinfection ability and negligible side effect to normal tissue in vivo. This finding provides an innovative strategy for designing individual porous organic polymer (POPs) with multi-function and also broaden the applications of POPs as powerful nonantibiotic type of antimicrobials.

Covalent Organic Framework Cladding on Peptide-Amphiphile-Based Biomimetic Catalysts.

Peptide-based biomimetic catalysts are promising materials for efficient catalytic activity in various biochemical transformations. However, their lack of operational stability and fragile nature in non-aqueous media limit their practical applications. In this study, we have developed a cladding technique to stabilize biomimetic catalysts within porous covalent organic framework (COF) scaffolds. This methodology allows for the homogeneous distribution of peptide nanotubes inside the COF (TpAzo and TpDPP) backbone, creating strong noncovalent interactions that prevent leaching. We synthesized two different peptide-amphiphiles, C10FFVK and C10FFVR, with lysine (K) and arginine (R) at the C-termini, respectively, which formed nanotubular morphologies. The C10FFVK peptide-amphiphile nanotubes exhibit enzyme-like behavior and efficiently catalyze C-C bond cleavage in a buffer medium (pH 7.5). We produced nanotubular structures of TpAzo-C10FFVK and TpDPP-C10FFVK through COF cladding by using interfacial crystallization (IC). The peptide nanotubes encased in the COF catalyze C-C bond cleavage in a buffer medium as well as in different organic solvents (such as acetonitrile, acetone, and dichloromethane). The TpAzo-C10FFVK catalyst, being heterogeneous, is easily recoverable, enabling the reaction to be performed for multiple cycles. Additionally, the synthesis of TpAzo-C10FFVK thin films facilitates catalysis in flow. As control, we synthesized another peptide-amphiphile, C10FFVR, which also forms tubular assemblies. By depositing TpAzo COF crystallites on C10FFVR nanotubes through IC, we produced TpAzo-C10FFVR nanotubular structures that expectedly did not show catalysis, suggesting the critical role of the lysines in the TpAzo-C10FFVK.

Removal of iodine by dry adsorbents in filtered containment venting system after 10years of Fukushima accident.

Radioactive iodine is a hazardous fission product and a major concern for public health. Special attention is paid to iodine out of 80 fission products because of its short half-life of 8.02days, high activity, and potential health hazards like its irreversible accumulation in thyroid gland and ability to cause thyroid cancer locally. Radioactive iodine can get released in the form of aerosols (cesium iodide), elemental iodine, and organic iodide after a nuclear accident and can cause off-site and on-site contamination. Filtered containment venting system (FCVS) is a safety system whose main objective is mitigation of severe accidents via controlled venting and removal of different forms of iodine to ensure safety of people and environment. After nuclear accidents like Fukushima, extensive research has been done on the removal of iodine by using dry scrubbers. This review paper presents research status of iodine removal by dry adsorbents especially after 10years of Fukushima to assess the progress, research gap, and challenges that require more attention. A good adsorbent should be cost-effective; it should have high selective adsorption towards iodine, high thermal and chemical stability, and good loading capacity; and its adsorption should remain unaffected by aging and the presence of inhibitors like CO, NO2, CH3Cl, H2O, and Cl2 and radiation. Research on different dry adsorbents was discussed, and their capability as a potential filter for FCVS was reviewed on the basis of all the above-mentioned features. Metal fiber filters have been widely used for removal of aerosols especially micro- and nanoscale aerosols. For designing a metal fiber filter, optimal size or combination of sizes of fibers, number of layers, and loading capacity of filter should be decided according to feasibility and requirement. Balance between flow resistance and removal efficiency is also very important. Sand bed filters were successful in retention of aerosols, but they showed low trapping of iodine and no trapping of methyl iodide at all. For iodine and methyl iodide removal, many adsorbents like activated carbon, zeolites, metal organic frameworks (MOFs), porous organic frameworks (POPs), silica, aerogels, titanosilicates, etc. have been used. Impregnated activated carbon showed good results but low auto-ignition temperature and decline in adsorption due to aging and inhibitors like NOx made them less suitable. Silver zeolites have been very successful in methyl iodide and iodine removal, but they are expensive and affected by presence of CO. Titanosilicates, macroreticular resins, and chalcogels were also studied and they showed good adsorption capacities, but their thermal stability was low. Other adsorbents like silica, MOFs, aerogels, and POPs also showed promising results for iodine adsorption and good thermal stability, but very limited or no research is available on their performance in severe accident conditions. This review will be very helpful for researchers to understand the merits and demerits of different types of dry adsorbents, the important operating parameters that need optimization for designing an efficient scrubber, margin of research, and foreseeable challenges in removal of different forms of iodine.

In situ growth of porous organic framework on iron wire for microextraction of polycyclic aromatic hydrocarbons

In this work, a novel spherical metal organic framework (MOF) was first in situ grown on the surface of iron wire (IW), in which IW served as the substrate and metal source for MOF (type NH2-MIL88) growth without adding additional metal salts in the process, while spherical NH2-MIL88 provided more active sites for further construction of multifunctional composites. Subsequently, a covalent organic framework (COF) was covalently bonded to the surface of the NH2-MIL88 to obtain the IW@NH2-MIL88@COF fibers, which were used for headspace solid-phase microextraction (HS-SPME) of polycyclic aromatic hydrocarbons (PAHs) in milk samples prior to determination by gas chromatography-flame ionization detection (GC-FID). Compared with the fiber prepared by physical coating, the IW@NH2-MIL88@COF fiber prepared by in situ growth and covalent bonding exhibits better stability and possesses more uniform layer. The extraction mechanism of the IW@NH2-MIL88@COF fiber for PAHs was discussed, which mainly owed to π-π interactions and hydrophobic interactions. After optimization of the primary extraction conditions, the SPME-GC-FID method was established for five PAHs with a wide linear range (1–200 ng mL−1), good linearity coefficient (0.9935–0.9987) and low detection limits (0.017–0.028 ng mL−1). The relative recoveries for PAHs detection in milk samples ranged from 64.69 to 113.97%. This work not only provides new ideas for the in situ growth of other types of MOF, but also provides new methods for the construction of multifunctional composites.

Urea-based magnetic porous organic frameworks as novel adsorbent for the enrichment of phenylurea herbicides in foods

A novel urea-based magnetic porous organic frameworks Fe3O4@UPOFs (ETTA-PPDI) was synthesized by a simple polymerization reaction under mild conditions. The adsorbent displayed desirable adsorption performance for phenylurea herbicides (PUHs) with optimized adsorption time of only 4 min. The adsorption capacities of the adsorbent for PUHs ranged from 47.30 to 111.93 mg g−1. A magnetic solid-phase extraction based on Fe3O4@UPOFs combined with high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was established for the efficient determination of six PUHs in food samples (wheat, edible oil and cucumber), with determination coefficient (R2) ≥ 0.9972. The LODs of the method were in the range of 0.003–0.07 μg kg−1 and recoveries ranged from 82.00 to 112.53%. The relative standard deviations were lower than 6.7%. The newly prepared adsorbent displayed great application prospects for the efficient enrichment of trace phenylurea herbicides in complex food matrices.

From Supramolecular Organic Cages to Porous Covalent Organic Frameworks for Enhancing Iodine Adsorption Capability by Fully Exposed Nitrogen-Rich Sites.

In order to overcome the limitations of supramolecular organic cages for their incomplete accessibility of active sites in the solid state and uneasy recyclability in liquid solution, herein a nitrogen-rich organic cage is rationally linked into framework systems and four isoreticular covalent organic frameworks (COFs), that is, Cage-TFB-COF, Cage-NTBA-COF, Cage-TFPB-COF, and Cage-TFPT-COF, are successfully synthesized. Structure determination reveals that they are all high-quality crystalline materials derived from the eclipsed packing of related isoreticular two-dimensionalframeworks. Since the nitrogen-rich sites usually have a high affinity toward iodine species, iodine adsorption investigations are carried out and the results show that all of them display an enhancement in iodine adsorption capacities. Especially, Cage-NTBA-COF exhibits an iodine adsorption capacity of 304 wt%, 14-fold higher than the solid sample packed from the cage itself. The strong interactions between the nitrogen-rich sites and the adsorbed iodine species are revealed by spectral analyses. This work demonstrates that, utilizing the reticular chemistry strategy to extend the close-packed supramolecular organic cages into crystalline porous framework solids, their inherent properties can be greatly exploited for targeted applications.

Adsorption-Assisted Redox Center in Porous Organic Frameworks for Boosting Lithium Storage.

Due to the designable structure and capacity, organic materials are promising candidates for lithium-ion batteries. Herein, we report a novel type of porous organic frameworks (POFs) based on the coupling reaction of diazonium salt as the anodes for lithium ion storage. The active center containing an azo group and the adjacent lithium-philic adsorption site is constructed to investigate the electrochemical behaviors and reaction mechanism. As synthesized POF material (named as POF-AN) exhibits high reversible lithium storage capacities of 523 mAh g-1 at 0.5 A g-1 and 445 mAh g-1 at 2.0 A g-1 after 1500 cycles, showing excellent cycle stability and rate performance. The detailed characterizations reveal that the azo group can act as an electrochemical active site that reversibly bonds with Li-ions, and the adjacent oxygen atoms can electrostatically adsorb with Li-ions to promote the lithium storage reaction. This adsorption-assisted three-atom redox center is beneficial to synergistically enhance the adsorption and intercalation of lithium ions, which can further improve the capacity and cycle stability. By replacing the precursor, it is also facile to synthesize more similar structure types. The reversible redox chemistry of the adsorption-assisted three-atom active center provides new opportunities for the development of long lifespan and high-rate organic anodes.

Experimental Confirmation of a Predicted Porous Hydrogen-Bonded Organic Framework.

Hydrogen-bonded organic frameworks (HOFs) with low densities and high porosities are rare and challenging to design because most molecules have a strong energetic preference for close packing. Crystal structure prediction (CSP) can rank the crystal packings available to an organic molecule based on their relative lattice energies. This has become a powerful tool for the a priori design of porous molecular crystals. Previously, we combined CSP with structure-property predictions to generate energy-structure-function (ESF) maps for a series of triptycene-based molecules with quinoxaline groups. From these ESF maps, triptycene trisquinoxalinedione (TH5) was predicted to form a previously unknown low-energy HOF (TH5-A) with a remarkably low density of 0.374 g cm-3 and three-dimensional (3D) pores. Here, we demonstrate the reliability of those ESF maps by discovering this TH5-A polymorph experimentally. This material has a high accessible surface area of 3,284 m2 g-1 , as measured by nitrogen adsorption, making it one of the most porous HOFs reported to date.

A multi-channel responsive AuNP@COF core-shell nanoprobe for simultaneous subcellular profiling of multiple cancer biomarkers

The abnormal change in the expression profile of multiple cancer biomarkers is closely related to tumor progression and therapeutic effect. Due to their low abundance in living cells and the limitations of existing imaging techniques, simultaneous imaging of multiple cancer biomarkers has remained a significant challenge. Here, we proposed a multi-modal imaging strategy to detect the correlated expression of multiple cancer biomarkers, MUC1, microRNA-21 (miRNA-21) and reactive oxygen (ROS) in living cells, based on a porous covalent organic framework (COF) wrapped gold nanoparticles (AuNPs) core-shell nanoprobe. The nanoprobe is functionalized with Cy5-labeled MUC1 aptamer, a ROS-responsive molecule (2-MHQ), and a miRNA-21-response hairpin DNA tagged by FITC as the reporters for different biomarkers. The target-specific recognition can induce the orthogonal molecular change of these reporters, producing fluorescence and Raman signals for imaging the expression profiles of membrane MUC1 (red fluorescence channel), intracellular miRNA-21 (green fluorescence channel), and intracellular ROS (SERS channel). We further demonstrate the capability of the cooperative expression of these biomarkers, along with the activation of NF-κB pathway. Our research provides a robust platform for imaging multiple cancer biomarkers, with broad potential applications in cancer clinical diagnosis and drug discovery.

Synthesis and design of hypercrosslinked porous organic frameworks containing tetraphenylpyrazine unit for high‐performance supercapacitor

AbstractHuman society faces significant environmental challenges due to the rapid production and consumption of fossil fuels. Therefore, developing effective and environmentally friendly methods of generating and storing energy is essential. Hypercrosslinked polymers (HPPs) have become increasingly popular due to their diverse preparation methods, simple functionalization, large specific surface area, low cost, mild reaction conditions, high chemical and thermal stability, and small size. We synthesized two hypercrosslinked porous organic polymers using Friedel‐Crafts reactions of 2,3,5,6‐tetraphenyl pyrazine (Pyra) with 4,4′‐bis(chloromethyl)biphenyl (BP), and dimethoxymethane (DDM), resulting in Pyra‐BP‐HPP and Pyra‐DDM‐HPP, respectively. The chemical structures and thermal stability of these polymers were confirmed through solid‐state 13C NMR, FTIR, and TGA. The surface area of Pyra‐BP‐HPP and Pyra‐DDM‐HPP was determined to be 984 and 435 m2 g−1, respectively, with micro and mesoporous structures present. Pyra‐BP‐HPP displayed a high specific capacitance of 94 F g−1 at 1 A g−1, with a capacity retention of 95% after 2000 cycles, which can be attributed to its larger surface area, microporosity, and abundance of electron‐rich phenyl rings when compared to Pyra‐DDM‐HPP.

Carbon molecular sieve membranes derived from hydrogen-bonded organic frameworks for CO2/CH4 separation

Carbon molecular sieve (CMS) membranes are ideal candidates for natural gas separation due to their rigid pore structures and stability. The chemical composite and spatial configuration of the used precursors for deriving CMS membranes can greatly influence the final microporous structure and separation performance of the membranes. Most CMS membranes are transformed from amorphous precursors such as polymers nowadays. Here, for the first time, we report a new type of CMS membrane derived from a crystalline porous hydrogen-bonded organic framework membrane (named HCMS). The conversion process has been studied with the characterization of TGA, FTIR, XRD, SEM, and pore size distribution. The effect of pyrolysis temperature on the membrane structure has been investigated to achieve optimized CO2/CH4 separation performance. On the one hand, as the pyrolysis temperature increases from 550 to 650 °C, the pore size distribution of HCMS membrane is narrowed, which is conducive to improving the molecular sieving effect of the membrane. On the other hand, higher ratios of graphitic N and pyrrolic N, which show better affinity for CO2 molecules than pyridinic N, are obtained on the HCMS membranes, leading to more favorable adsorption of CO2. The optimized HCMS membrane pyrolyzed at 600 °C shows a remarkable CO2/CH4 selectivity of 128 and excellent separation stability at varying temperatures and pressures. The HCMS membranes derived from the crystalline HOF precursor can also maintain a stable separation performance against physical aging. The results reported in this work may open the door to constructing CMS membranes with the precursor of crystalline porous membranes.

Green construction of hydroxyl-functionalized magnetic porous organic framework for effective extraction of triazine herbicides from environmental water and watermelon juice samples

Triazine herbicides have been widely detected in water resources and food, which poses a potential hazard to both ecosystem and human health. Due to their high polarity, conventional adsorbents have limitations for their extractions. Herein, for the effective magnetic extraction of triazine herbicides, a novel and effective magnetic adsorbent was prepared with a satisfactory extraction performance. In the experiments, five porous organic frameworks (POFs) with hydroxyl functional groups were synthesized by diazo-coupling reactions in aqueous solution with β-cyclodextrin (β-CD) as a green monomer. After evaluation of the five POFs, the DDM-CD-POF, which was synthesized with 4′4-diaminodiphenylmethane (DDM) and β-CD, showed the largest specific surface area and the best adsorption capacity for the five triazine herbicides. Then, it was magnetized by introducing Fe3O4@SiO2 into it to prepare a magnetic adsorbent (M-DDM-CD-POF) to facilitate separation and recycling. Finally, the M-DDM-CD-POF-based magnetic solid-phase extraction in combination with high performance liquid chromatographic detection method was established for the quantitative determination of the triazine herbicides in environmental water and watermelon juice samples. The current strategy showed low limits of detection of 0.03–0.11 ng mL−1 for environmental water and 0.07–0.22 ng mL−1 for watermelon juice sample. The method recoveries for spiked samples ranged from 84.0% to 113.0% with the relative standard deviations ≤8.8%. This work provides a new approach for the detection of the triazine herbicides with good application prospect.

The synthesis of highly crystalline covalent organic frameworks via the monomer crystal induction for the photocatalytic asymmetric α‐alkylation of aldehydes

AbstractCovalent organic frameworks (COFs) are ordered porous organic frameworks, which found wide applications in gas sorption and separation, catalysis, sensoring, and many others owing to their permanent porosity and designable structural motifs. To take full advantage of the well‐defined porous structure, COFs need to be synthesized with high crystallinity. However, the synthesis of COFs with high crystalline is a general challenge, which requires dedicated linker design and reaction condition optimization. To achieve this, we developed a monomer crystal‐induced strategy to construct crystalline COFs. The synthesized COFs have higher crystallinity, surface area, and thermal stability than those prepared without monomer crystal induction. Furthermore, the high crystalline COFs exhibit excellent performance in the photocatalytic asymmetric alkylation of aldehydes. The monomer crystal‐induced method not only represents a new route for the synthesis of crystalline COFs but also sheds light on the mechanism of the formation of COFs.

Title: Cr(III) Incorporated Melamine‐Terephthalaldehyde Porous Organic Framework Nanosheet Catalyst for Carbon Dioxide Fixation Reaction

AbstractCr(III) centers have been incorporated into melamine‐terephthalaldehyde porous organic framework (MT‐POF) through N‐donor site coordination. Morphological studies by SEM give evidence of the transformation of spherulitic texture of non‐metalated MT‐POF to aggregated platelike structure in chromium metalated POF, MT‐Cr(III) POF (bulk). The flattening in the surface morphology of bulk MT‐Cr(III) POF might be a consequence of conformational change in POF caused by the template effect of the Cr(III) coordination. On sonication, bulk MT‐Cr(III) POF transformed into a distinguishable stacked nanosheet structure in MT‐Cr(III) POFN as revealed by FE‐SEM and TEM images. MT‐Cr(III) POFN could catalyse the carbon dioxide cycloaddition reaction to epoxides to yield cyclic carbonates in presence of tetrabutylammonium bromide (TBAB) co‐catalyst. It was found that bulkier epoxide substrates showed lesser conversions to their corresponding cyclic carbonates (styrene oxide, 41 %; allyl glycidyl ether, 70 %; butyl glycidyl ether, 63 % conv.), whereas smaller substrates showed much higher conversions (epichlorohydrin, 96 %; epibromohydrin, 95 % conv.) under the reaction conditions [MT‐Cr(III) POFN, 10 mg; TBAB, 2 mol%; 5 bar CO2; 60 °C] giving a clear evidence of the substrate discrimination. The reason for the lesser conversions of the bigger substrates can be attributed to the steric clash rendered by the POF network.

Electrostatic Design of the Nanoscale Internal Surfaces of Porous Covalent Organic Frameworks.

It is well established that the collective action of assemblies of dipoles determines the electronic structure of surfaces and interfaces. This raises the question, to what extent the controlled arrangement of polar units can be used to also tune the electronic properties of the inner surfaces of materials with nanoscale pores. In the present contribution, state-of-the-art density-functional theory calculations are used to show for the prototypical case of covalent organic frameworks (COFs) that this is indeed possible. Decorating pore walls with assemblies of polar entities bonded to the building blocks of the COF layers triggers a massive change of the electrostatic energy within the pores. This, inevitably, also changes the relative alignment between electronic states in the framework and in guest molecules and is expected to have significant impacts on charge separation in COF heterojunctions, on redox reactions in COFs-based electrodes, and on (photo)catalysis.