Engineering Porosity and Functionality in a Robust Twofold Interpenetrated Bismuth-Based MOF: Toward a Porous, Stable, and Photoactive Material.

Defect engineering in metal-organic frameworks (MOFs) has gained worldwide research traction, as it offers tools to tune the properties of MOFs. Herein, we report a novel 2-fold interpenetrated Bi-based MOF made of a tritopic flexible organic linker, followed by missing-linker defect engineering. This procedure creates a gradually augmented micro- and mesoporosity in the parent (originally nonporous) network. The resulting MOFs can tolerate a remarkable extent of linker vacancy (with absence of up to 60% of linkers per Bi node) created by altering the crystal-growth rate as a function of synthesis temperature and duration. Owing to the enhanced porosity and availability of the uncoordinated Lewis acidic Bi sites, the defect-engineered MOFs manifested improved surface areas, augmented CO2 and water vapor uptake, and catalytic activity. Parallel to this, the impact of defect engineering on the optoelectronic properties of these MOFs has also been studied, offering avenues for new applications.

The Application of Porous Organic Polymers as Metal Free Photocatalysts in Organic Synthesis.

Concerns about increasing greenhouse gas emissions and their effect on our environment highlight the urgent need for new sustainable technologies. Visible light photocatalysis allows the clean and selective generation of reactive intermediates under mild conditions. The more widespread adoption of the current generation of photocatalysts, particularly those using precious metals, is hampered by drawbacks such as their cost, toxicity, difficult separation, and limited recyclability. This is driving the search for alternatives, such as porous organic polymers (POPs). This new class of materials is made entirely from organic building blocks, can possess high surface area and stability, and has a controllable composition and functionality. This review focuses on the application of POPs as photocatalysts in organic synthesis. For each reaction type, a representative material is discussed, with special attention to the mechanism of the reaction. Additionally, an overview is given, comparing POPs with other classes of photocatalysts, and critical conclusions and future perspectives are provided on this important field.

Exploring the Charge Storage Dynamics in Donor-Acceptor Covalent Organic Frameworks Based Supercapacitors by Employing Ionic Liquid Electrolyte.

Two donor-acceptor type tetrathiafulvalene (TTF)-based covalent organic frameworks (COFs) are investigated as electrodes for symmetric supercapacitors in different electrolytes, to understand the charge storage and dynamics in 2D COFs. Till-date, most COFs are investigated as Faradic redox pseudocapacitors in aqueous electrolytes. For the first time, it is tried to enhance the electrochemical performance and stability of pristine COF-based supercapacitors by operating them in the non-Faradaic electrochemically double layer capacitance region. It is found that the charge storage mechanism of ionic liquid (IL) electrolyte based supercapacitors is dependent on the micropore size and surface charge density of the donor-acceptor COFs. The surface charge density alters due to the different electron acceptor building blocks, which in turn influences the dense packing of the IL near its pore. The micropores induce pore confinement of IL in the COFs by partial breaking of coulomb ordering and rearranging it. The combination of these two factors enhance the charge storage in the highly microporous COFs. The density functional theory calculations support the same. At 1 A g-1 , TTF-porphyrin COF provides capacitance of 42, 70, and 130 F g-1 in aqueous, organic, and IL electrolyte respectively. TTF-diamine COF shows a similar trend with 100 F g-1 capacitance in IL.

Super-Oxidizing Covalent Triazine Framework Electrocatalyst for Two-Electron Water Oxidation to H2 O2.

Electrochemical two-electron water oxidation (2e WOR) is gaining surging research traction for sustainable hydrogen peroxide production. However, the strong oxidizing environment and thermodynamically competitive side-reaction (4e WOR) posit as thresholds for the 2e WOR. We herein report a custom-crafted covalent triazine network possessing strong oxidizing properties as a proof-of-concept metal-free functional organic network electrocatalyst for catalyzing 2e WOR. As the first-of-its-kind, the material shows a maximum of 89.9 % Faradaic Efficiency and 1428 µmol/h/cm2 H2 O2 production rate at 3.0 V bias potential (vs reversible hydrogen electrode, RHE), which are either better or comparable to the state-of-the-art electrocatalysts. We have experimentally confirmed a stepwise 2e WOR mechanism which was further computationally endorsed by density functional theory studies.

Pyrene-Based Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Production.

Four highly porous covalent organic frameworks (COFs) containing pyrene units were prepared and explored for photocatalytic H2 O2 production. The experimental studies are complemented by density functional theory calculations, proving that the pyrene unit is more active for H2 O2 production than the bipyridine and (diarylamino)benzene units reported previously. H2 O2 decomposition experiments verified that the distribution of pyrene units over a large surface area of COFs plays an important role in catalytic performance. The Py-Py-COF though contains more pyrene units than other COFs which induces a high H2 O2 decomposition due to a dense concentration of pyrene in close proximity over a limited surface area. Therefore, a two-phase reaction system (water-benzyl alcohol) was employed to inhibit H2 O2 decomposition. This is the first report on applying pyrene-based COFs in a two-phase system for photocatalytic H2 O2 generation.

Engineering Covalent Organic Frameworks as Heterogeneous Photocatalysts for Organic Transformations.

Covalent organic frameworks (COFs) are an emerging category of organic polymers with highly porous crystalline structures. In the last decade, reports on the use of COFs as heterogeneous photocatalysts for organic transformations have shown significant progress. Still, comprehensive reviews on the mechanisms of the photocatalytic organic transformations using COFs are lacking. This Review provides a comprehensive and systematic overview of COF-based photocatalysts for organic transformations. Firstly, we discuss the photophysical properties and the characterization methods of COF-based photocatalysts. Then, the general photocatalytic mechanism, the advantages, and the strategies to improve the photocatalytic efficiency of COF-based photocatalysts are summarized. After that, advanced examples of COF-based photocatalysts for organic transformations are analyzed with regard to the underlying mechanisms. The Review ends with a critical perspective on the challenges and prospects.

Overview of N-Rich Antennae Investigated in Lanthanide-Based Temperature Sensing.

The market share of noncontact temperature sensors is expending due to fast technological and medical evolutions. In the wide variety of noncontact sensors, lanthanide-based temperature sensors stand out. They benefit from high photostability, relatively long decay times and high quantum yields. To circumvent their low molar light absorption, the incorporation of a light-harvesting antenna is required. This Review provides an overview of the nitrogen-rich antennae in lanthanide-based temperature sensors, emitting in the visible light spectrum, and discusses their temperature sensor ability. The N-rich ligands are incorporated in many different platforms. The investigation of different antennae is required to develop temperature sensors with diverse optical properties and to create a diverse offer for the multiple application fields. Molecular probes, consisting of small molecules, are first discussed. Furthermore, the thermometer properties of ratiometric temperature sensors, based on di- and polynuclear complexes, metal-organic frameworks, periodic mesoporous organosilicas and porous organic polymers, are summarized. The antenna mainly determines the application potential of the ratiometric thermometer. It can be observed that molecular probes are operational in the broad physiological range, metal-organic frameworks are generally very useful in the cryogenic region, periodic mesoporous organosilica show temperature dependency in the physiological range, and porous organic polymers are operative in the cryogenic-to-medium temperature range.

Identification of vanadium dopant sites in the metal-organic framework DUT-5(Al).

Studying the structural environment of the VIV ions doped in the metal-organic framework (MOF) DUT-5(Al) ((AlIIIOH)BPDC) with electron paramagnetic resonance (EPR) reveals four different vanadium-related spectral components. The spin-Hamiltonian parameters are derived by analysis of X-, Q- and W-band powder EPR spectra. Complementary Q-band Electron Nuclear DOuble Resonance (ENDOR) experiments, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), X-Ray Diffraction (XRD) and Fourier Transform InfraRed (FTIR) measurements are performed to investigate the origin of these spectral components. Two spectral components with well resolved 51V hyperfine structure are visible, one corresponding to VIV[double bond, length as m-dash]O substitution in a large (or open) pore and one to a narrow (or closed) pore variant of this MOF. Furthermore, a broad structureless Lorentzian line assigned to interacting vanadyl centers in each other's close neighborhood grows with increasing V-concentration. The last spectral component is best visible at low V-concentrations. We tentatively attribute it to (VIV[double bond, length as m-dash]O)2+ linked with DMF or dimethylamine in the pores of the MOF. Simulations using these four spectral components convincingly reproduce the experimental spectra and allow to estimate the contribution of each vanadyl species as a function of V-concentration.

Rigid Nanoporous Urea-Based Covalent Triazine Frameworks for C2/C1 and CO2/CH4 Gas Separation.

C2/C1 hydrocarbon separation is an important industrial process that relies on energy-intensive cryogenic distillation methods. The use of porous adsorbents to selectively separate these gases is a viable alternative. Highly stable covalent triazine frameworks (urea-CTFs) have been synthesized using 1,3-bis(4-cyanophenyl)urea. Urea-CTFs exhibited gas uptakes of C2H2 (3.86 mmol/g) and C2H4 (2.92 mmol/g) at 273 K and 1 bar and is selective over CH4. Breakthrough simulations show the potential of urea-CTFs for C2/C1 separation.

A Ru-Complex Tethered to a N-Rich Covalent Triazine Framework for Tandem Aerobic Oxidation-Knoevenagel Condensation Reactions.

Herein, a highly N-rich covalent triazine framework (CTF) is applied as support for a RuIII complex. The bipyridine sites within the CTF provide excellent anchoring points for the [Ru(acac)2(CH3CN)2]PF6 complex. The obtained robust RuIII@bipy-CTF material was applied for the selective tandem aerobic oxidation-Knoevenagel condensation reaction. The presented system shows a high catalytic performance (>80% conversion of alcohols to α, ß-unsaturated nitriles) without the use of expensive noble metals. The bipy-CTF not only acts as the catalyst support but also provides the active sites for both aerobic oxidation and Knoevenagel condensation reactions. This work highlights a new perspective for the development of highly efficient and robust heterogeneous catalysts applying CTFs for cascade catalysis.

Hydrogenative Ring-Rearrangement of Furfural to Cyclopentanone over Pd/UiO-66-NO2 with Tunable Missing-Linker Defects.

Upgrading furfural (FAL) to cyclopentanone (CPO) is of great importance for the synthesis of high-value chemicals and biomass utilization. The hydrogenative ring-rearrangement of FAL is catalyzed by metal-acid bifunctional catalysts. The Lewis acidity is a key factor in promoting the rearrangement of furan rings and achieving a high selectivity to CPO. In this work, highly dispersed Pd nanoparticles were successfully encapsulated into the cavities of a Zr based MOF, UiO-66-NO2, by impregnation using a double-solvent method (DSM) followed by H2 reduction. The obtained Pd/UiO-66-NO2 catalyst showed a significantly better catalytic performance in the aforementioned reaction than the Pd/UiO-66 catalyst due to the higher Lewis acidity of the support. Moreover, by using a thermal treatment. The Lewis acidity can be further increased through the creating of missing-linker defects. The resulting defective Pd/UiO-66-NO2 exhibited the highest CPO selectivity and FAL conversion of 96.6% and 98.9%, respectively. In addition, the catalyst was able to maintain a high activity and stability after four consecutive runs. The current study not only provides an efficient catalytic reaction system for the hydrogenative ring-rearrangement of furfural to cyclopentanone but also emphasizes the importance of defect sites.

Quantifying the Likelihood of Structural Models through a Dynamically Enhanced Powder X-Ray Diffraction Protocol.

Structurally characterizing new materials is tremendously challenging, especially when single crystal structures are hardly available which is often the case for covalent organic frameworks. Yet, knowledge of the atomic structure is key to establish structure-function relations and enable functional material design. Herein, a new protocol is proposed to unambiguously predict the structure of poorly crystalline materials through a likelihood ordering based on the X-ray diffraction (XRD) pattern. Key of the procedure is the broad set of structures generated from a limited number of building blocks and topologies, which is submitted to operando structural characterization. The dynamic averaging in the latter accounts for the operando conditions and inherent temporal character of experimental measurements, yielding unparalleled agreement with experimental powder XRD patterns. The proposed concept can hence unquestionably identify the structure of experimentally synthesized materials, a crucial step to design next generation functional materials.

Luminescent Ratiometric Thermometers Based on a 4f-3d Grafted Covalent Organic Framework to Locally Measure Temperature Gradients During Catalytic Reactions.

Covalent Organic Frameworks (COFs), an emerging class of crystalline porous materials, are a new type of support for grafting lanthanide ions (Ln3+ ), which can be employed as ratiometric luminescent thermometers. In this work we have shown that COFs co-grafted with lanthanide ions (Eu3+ , Tb3+ ) and Cu2+ (or potentially other d-metals) can synchronously be employed both as a nanothermometer and catalyst during a chemical reaction. The performance of the thermometer could be tuned by changing the grafted d-metal and solvent environment. As a proof of principle, the Glaser coupling reaction was investigated. We show that temperature can be precisely measured during the course of the catalytic reaction using luminescence thermometry. This concept could be potentially easily extended to other catalytic reactions by grafting other d-metal ions on the Ln@COF platform.

A Visible-Light-Harvesting Covalent Organic Framework Bearing Single Nickel Sites as a Highly Efficient Sulfur-Carbon Cross-Coupling Dual Catalyst.

Covalent Organic Frameworks (COFs) have recently emerged as light-harvesting devices, as well as elegant heterogeneous catalysts. The combination of these two properties into a dual catalyst has not yet been explored. We report a new photosensitive triazine-based COF, decorated with single Ni sites to form a dual catalyst. This crystalline and highly porous catalyst shows excellent catalytic performance in the visible-light-driven catalytic sulfur-carbon cross-coupling reaction. Incorporation of single transition metal sites in a photosensitive COF scaffold with two-component synergistic catalyst in organic transformation is demonstrated for the first time.

Strongly Reducing (Diarylamino)benzene-Based Covalent Organic Framework for Metal-Free Visible Light Photocatalytic H2O2 Generation.

Photocatalytic reduction of molecular oxygen is a promising route toward sustainable production of hydrogen peroxide (H2O2). This challenging process requires photoactive semiconductors enabling solar energy driven generation and separation of electrons and holes with high charge transfer kinetics. Covalent organic frameworks (COFs) are an emerging class of photoactive semiconductors, tunable at a molecular level for high charge carrier generation and transfer. Herein, we report two newly designed two-dimensional COFs based on a (diarylamino)benzene linker that form a Kagome (kgm) lattice and show strong visible light absorption. Their high crystallinity and large surface areas (up to 1165 m2·g-1) allow efficient charge transfer and diffusion. The diarylamine (donor) unit promotes strong reduction properties, enabling these COFs to efficiently reduce oxygen to form H2O2. Overall, the use of a metal-free, recyclable photocatalytic system allows efficient photocatalytic solar transformations.

Engineering a Highly Defective Stable UiO-66 with Tunable Lewis- Brønsted Acidity: The Role of the Hemilabile Linker.

The stability of metal-organic frameworks (MOFs) typically decreases with an increasing number of defects, limiting the number of defects that can be created and limiting catalytic and other applications. Herein, we use a hemilabile (Hl) linker to create up to a maximum of six defects per cluster in UiO-66. We synthesized hemilabile UiO-66 (Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate (PSBA) as the hemilabile linker. The PSBA acts not only as a modulator to create defects but also as a coligand that enhances the stability of the resulting defective framework. Furthermore, upon a postsynthetic treatment in H2SO4, the average number of defects increases to the optimum of six missing BDC linkers per cluster (three per formula unit), leaving the Zr-nodes on average sixfold coordinated. Remarkably, the thermal stability of the materials further increases upon this treatment. Periodic density functional theory calculations confirm that the hemilabile ligands strengthen this highly defective structure by several stabilizing interactions. Finally, the catalytic activity of the obtained materials is evaluated in the acid-catalyzed isomerization of α-pinene oxide. This reaction is particularly sensitive to the Brønsted or Lewis acid sites in the catalyst. In comparison to the pristine UiO-66, which mainly possesses Brønsted acid sites, the Hl-UiO-66 and the postsynthetically treated Hl-UiO-66 structures exhibited a higher Lewis acidity and an enhanced activity and selectivity. This is further explored by CD3CN spectroscopic sorption experiments. We have shown that by tuning the number of defects in UiO-66 using PSBA as the hemilabile linker, one can achieve highly defective and stable MOFs and easily control the Brønsted to Lewis acid ratio in the materials and thus their catalytic activity and selectivity.

Effect of Building Block Transformation in Covalent Triazine-Based Frameworks for Enhanced CO2 Uptake and Metal-Free Heterogeneous Catalysis.

Invited for the cover of this issue is the group of Pascal Van Der Voort at the University of Ghent and colleagues at Technische Universität Berlin. The image depicts the covalent triazine frameworks reported in the manuscript for the sorption of CO2 and also in metal-free catalysis. Read the full text of the article at 10.1002/chem.201903926.

Effect of Building Block Transformation in Covalent Triazine-Based Frameworks for Enhanced CO2 Uptake and Metal-Free Heterogeneous Catalysis.

Covalent triazine frameworks (CTFs) have provided a unique platform in functional material design for a wide range of applications. This work reports a series of new CTFs with two new heteroaromatic building blocks (pyrazole and isoxazole groups) through a building-block transformation approach aiming for carbon capture and storage (CCS) and metal-free catalysis. The CTFs were synthesized from their respective building blocks [(4,4'-(1H-pyrazole-3,5-diyl)dibenzonitrile (pyz) and 4,4'-(isoxazole-3,5-diyl)dibenzonitrile (isox))] under ionothermal conditions using ZnCl2 . Both of the building blocks were designed by an organic transformation of an acetylacetone containing dinitrile linker to pyrazole and isoxazole groups, respectively. Due to this organic transformation, (i) linker aromatization, (ii) higher surface areas and nitrogen contents, (iii) higher aromaticity, and (iv) higher surface basicity was achieved. Due to these enhanced properties, CTFs were explored for CO2 uptake and metal-free heterogeneous catalysis. Among all, the isox-CTF, synthesized at 400 °C, showed the highest CO2 uptake (4.92 mmol g-1 at 273 K and 2.98 mmol g-1 at 298 K at 1 bar). Remarkably, these CTFs showed excellent metal-free catalytic activity for the aerobic oxidation of benzylamine at mild reaction conditions. On studying the properties of the CTFs, it was observed that organic transformations and ligand aromatization of the materials are crucial factor to tune the important parameters that influence the CO2 uptake and the catalytic activity. Overall, this work highlights the substantial effect of designing new CTF materials by building-block organic transformations resulting in better properties for CCS applications and heterogeneous catalysis.

N-Rich Porous Polymer with Isolated Tb3+ -Ions Displays Unique Temperature Dependent Behavior through the Absence of Thermal Quenching.

The challenge of measuring fast moving or small scale samples is based on the absence of contact between sample and sensor. Grafting lanthanides onto hybrid materials arises as one of the most promising accurate techniques to obtain noninvasive thermometers. In this work, a novel bipyridine based porous organic polymer (bpyDAT POP) was investigated as temperature sensor after grafting with Eu(acac)3 and Tb(acac)3 complexes. The bpyDAT POP successfully showed temperature-dependent behavior in the 10-310 K range, proving the potential of amorphous, porous organic frameworks. We observed unique temperature dependent behavior. More intriguingly, instead of the standard observed change in emission as a result of a change in temperature for both Eu3+ and Tb3+ , the emission spectrum of Tb3+ remained constant. This work provides framework- and energy-based explanations for the observed phenomenon. The conjugation in the bpyDAT POP framework is interrupted, creating energetically isolated Tb3+ environments. Energy transfer from Tb3+ to Eu3+ is therefore absent, nor energy back transfer from Tb3+ to bpyDAT POP ligand (i.e. no thermal quenching) is detected.

Structural and Photophysical Properties of Various Polypyridyl Ligands: A Combined Experimental and Computational Study.

Covalent triazine frameworks (CTFs) with polypyridyl ligands are very promising supports to anchor photocatalytic complexes. Herein, we investigate the photophysical properties of a series of ligands which vary by the extent of the aromatic system, the nitrogen content and their topologies to aid in selecting interesting building blocks for CTFs. Interestingly, some linkers have a rotational degree of freedom, allowing both a trans and cis structure, where only the latter allows anchoring. Therefore, the influence of the dihedral angle on the UV-Vis spectrum is studied. The photophysical properties are investigated by a combined computational and experimental study. Theoretically, both static and molecular dynamics simulations are performed to deduce ground- and excited state properties based on density functional theory (DFT) and time-dependent DFT. The position of the main absorption peak shifts towards higher wavelengths for an increased size of the π-system and a higher π-electron deficiency. We found that the position of the main absorption peak among the different ligands studied in this work can amount to 271 nm; which has a significant impact on the photophysical properties of the ligands. This broad range of shifts allows modulation of the electronic structure by varying the ligands and may help in a rational design of efficient photocatalysts.