Nitrogen-enriched flexible metal-organic framework for CO adsorption.

A novel MOF named [Zn(L)(DMF)] was synthesized using solvothermal methods from the reaction of the new linker (4,4',4''-(4,4',4''-(benzene-1,3,5-triyltris(methylene))tris(3,5-dimethyl-1-pyrazole-4,1-diyl))tribenzoic acid) and Zn(NO)·6HO. This new MOF was characterized by means of different techniques: powder X-ray diffraction, N adsorption and desorption isotherms, thermogravimetric analysis, and scanning electron microscopy. Furthermore, suitable crystals were obtained, which allowed us to perform the X-Ray structure determination of this MOF.

Facile Low-Temperature synthesis of novel carbon nitrides for efficient conversion of carbon dioxide into Value-Added chemicals.

The interest in using carbon nitrides (CN) for CO conversion has stimulated extensive research on CN synthesis. Herein, we report the synthesis of two novel CN materials using low-cost commercially available precursors at low temperatures in a short duration of time. Two CN materials, one derived from 5-amino tetrazole (named 4NZ-CN) and the other derived from 3, 5-diamino-1, 2, 4-triazole (named 3NZ-CN) precursors, are prepared by refluxing these precursors for 2 h at 100 °C.

Framework-Topology-Controlled Singlet Fission in Metal-Organic Frameworks.

Singlet fission (SF) has been explored as a viable route to improve photovoltaic performance by producing more excitons. Efficient SF is achieved through a high degree of interchromophoric coupling that facilitates electron superexchange to generate triplet pairs. However, strongly coupled chromophores often form excimers that can serve as an SF intermediate or a low-energy trap site.

Triplet Generation Through Singlet Fission in Metal-Organic Framework: An Alternative Route to Inefficient Singlet-Triplet Intersystem Crossing.

High quantum yield triplets, populated by initially prepared excited singlets, are desired for various energy conversion schemes in solid working compositions like porous MOFs. However, a large disparity in the distribution of the excitonic center of mass, singlet-triplet intersystem crossing (ISC) in such assemblies is inhibited, so much so that a carboxy-coordinated zirconium heavy metal ion cannot effectively facilitate the ISC through spin-orbit coupling. Circumventing this sluggish ISC, singlet fission (SF) is explored as a viable route to generating triplets in solution-stable MOFs.

Metal-Carbodithioate-Based 3D Semiconducting Metal-Organic Framework: Porous Optoelectronic Material for Energy Conversion.

Solar energy conversion requires the working compositions to generate photoinduced charges with high potential and the ability to deliver charges to the catalytic sites and/or external electrode. These two properties are typically at odds with each other and call for new molecular materials with sufficient conjugation to improve charge conductivity but not as much conjugation as to overly compromise the optical band gap. In this work, we developed a semiconducting metal-organic framework (MOF) prepared explicitly through metal-carbodithioate "(-CS)M" linkage chemistry, entailing augmented metal-linker electronic communication.

Synthetic Access to a Framework-Stabilized and Fully Sulfided Analogue of an Anderson Polyoxometalate that is Catalytically Competent for Reduction Reactions.

Polyoxometalates (POMs) featuring 7, 12, 18, or more redox-accessible transition metal ions are ubiquitous as selective catalysts, especially for oxidation reactions. The corresponding synthetic and catalytic chemistry of stable, discrete, capping-ligand-free polythiometalates (PTMs), which could be especially attractive for reduction reactions, is much less well developed. Among the challenges are the propensity of PTMs to agglomerate and the tendency for agglomeration to block reactant access of catalyst active sites.

Noncovalent Surface Modification of Metal-Organic Frameworks: Unscrambling Adsorption Properties via Isothermal Titration Calorimetry.

Despite the importance of noncovalent interactions in the utilization of metal-organic frameworks (MOFs), using these interactions to functionalize MOFs has rarely been explored. The ease of functionalization and potential for surface-selective functionalization makes modification via noncovalent interactions promising for the creation of porous photocatalytic assemblies. Using isothermal titration calorimetry, photoluminescence measurements, and desorption experiments, we have explored the nature and magnitude of the interactions of [Ru(bpy)(bpy-R)]-functionalized dyes with the surface of MIL-96, where R = C, C, C, and C alkyl chains of either straight-chain or cyclic conformations.

Direct Observation of Modulated Radical Spin States in Metal-Organic Frameworks by Controlled Flexibility.

Owing to their switchable spin states and dynamic electronic character, organic-based radical species have been invoked in phenomena unique to a variety of fields. When incorporated in solid state materials, generation of organic radicals proves challenging due to aggregation. Metal-organic frameworks (MOFs) are promising candidates for immobilization and stabilization of organic radicals because of the tunable spatial arrangement of organic linkers and metal nodes, which sequesters the reactive species.

Understanding Diffusional Charge Transport within a Pyrene-Based Hydrogen-Bonded Organic Framework.

Electrochemically active hydrogen-bonded organic frameworks (HOFs) offer opportunities to study charge transport in supramolecular systems where the rate of movement of charges is dependent on weak electronic coupling between individual components. Here, we used potential-step chronoamperometric measurements on electrochemically active, drop-cast HOF-102 films to estimate both redox-hopping-based apparent diffusion coefficients for charge transport and rate constants for linker-to-linker charge transfer (hole transfer) in the mesoporous two-dimensional (2D) plane created by interlinker hydrogen bonding. Also present are one-dimensional columns formed by stacking pyrene units.

Superradiance and Directional Exciton Migration in Metal-Organic Frameworks.

Crystalline metal-organic frameworks (MOFs) are promising synthetic analogues of photosynthetic light-harvesting complexes (LHCs). The precise assembly of linkers (organic chromophores) around the topology-defined pores offers the evolution of unique photophysical behaviors that are reminiscence of LHCs. These include MOF excited states with photoabsorbed energy that is spatially dispersed over multiple linkers defining the molecular excitons.

Photon Upconversion in a Glowing Metal-Organic Framework.

The interaction of low-energy light with matter that leads to the production of high-energy light is known as photon upconversion. This phenomenon is of importance because of its potential applications in optoelectronics, energy harvesting, and the biomedical arena. Herein, we report a pillared-paddlewheel metal-organic framework (MOF), constructed from a tetrakis(4-carboxyphenyl)porphyrin sensitizer and a dipyridyl thiazolothiazole annihilator, designed for efficient triplet-triplet annihilation upconversion (TTA-UC).

Modulation of CO adsorption in novel pillar-layered MOFs based on carboxylate-pyrazole flexible linker.

Metal-organic frameworks (MOFs) have attracted significant attention as sorbents due to their high surface area, tunable pore volume and pore size, coordinatively unsaturated metal sites, and ability to install desired functional groups by post-synthetic modification. Herein, we report three new MOFs with pillar-paddlewheel structures that have been synthesized solvothermally from the mixture of the carboxylate-pyrazole flexible linker (HL), 4,4-bipyridine (BPY)/triethylenediamine (DABCO), and Zn(ii)/Cu(ii) ions. The MOFs obtained, namely [Zn(L)BPY], [Cu(L)BPY], and [Cu(L)DABCO], exhibit two-fold interpenetration and dinuclear paddle-wheel nodes.

Regioselective Functionalization of the Mesoporous Metal-Organic Framework, NU-1000, with Photo-Active Tris-(2,2'-bipyridine)ruthenium(II).

Solvent-assisted ligand incorporation is an excellent method for the post-synthetic functionalization of Zr-based metal-organic frameworks (MOFs), as carboxylate-derivative functionalities readily coordinate to the Zr nodes by displacing node-based aqua and terminal hydroxo ligands. In this study, a photocatalytically active ruthenium complex Ru(bpy)(dcbpy), that is, bis-(2,2'-bipyridine)-(4,4'-dicarboxy-2,2'-bipyridine)ruthenium, was installed in the mono-protonated (carboxylic acid) form within NU-1000 SALI. Crystallographic information regarding the siting of the ruthenium complex within the MOF pores is obtained by difference envelope density analysis.

Improving Energy Transfer within Metal-Organic Frameworks by Aligning Linker Transition Dipoles along the Framework Axis.

Crystalline metal-organic frameworks (MOFs) can assemble chromophoric molecules into a wide range of spatial arrangements, which are controlled by the MOF topology. Like natural light-harvesting complexes (LHCs), the precise arrangement modulates interchromophoric interactions, in turn determining excitonic behavior and migration dynamics. To unveil the key factors that control efficient exciton displacements within MOFs, we first developed linkers with low electronic symmetry (as defined by large transition dipoles) and then assembled them into MOFs.

Charge Transport in Zirconium-Based Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are a class of crystalline porous materials characterized by inorganic nodes and multitopic organic linkers. Because of their molecular-scale porosity and periodic intraframework chemical functionality, MOFs are attractive scaffolds for supporting and/or organizing catalysts, photocatalysts, chemical-sensing elements, small enzymes, and numerous other functional-property-imparting, nanometer-scale objects. Notably, these objects can be installed after the synthesis of the MOF, eliminating the need for chemical and thermal compatibility of the objects with the synthesis milieu.

A Flexible Interpenetrated Zirconium-Based Metal-Organic Framework with High Affinity toward Ammonia.

Flexible metal-organic frameworks (MOFs) are highly attractive porous crystalline materials presenting structural changes when exposed to external stimuli, the mechanism of which is often difficult to glean, owing to their complex and dynamic nature. Herein, a flexible interpenetrated Zr-MOF, NU-1401, composed of rare 4-connected Zr nodes and tetratopic naphthalenediimide (NDI)-based carboxylate linkers, was designed. The intra-framework pore opening deformation and inter-framework motions, when subjected to different solvent molecules, were investigated by single-crystal XRD.

Anisotropic Redox Conductivity within a Metal-Organic Framework Material.

Engendering electrical conductivity in otherwise insulating metal-organic framework (MOF) materials is key to rendering these materials fully functional for a range of potential applications, including electrochemical and photo-electrochemical catalysis. Here we report that the platform MOF, NU-1000, can be made electrically conductive via reversible electrochemical oxidation of a fraction of the framework's tetraphenylpyrene linkers, where the basis for conduction is redox hopping. At a microscopic level, redox hopping is akin to electron self-exchange and is describable by Marcus' well-known theory of electron transfer.

Torsion Angle Effect on the Activation of UiO Metal-Organic Frameworks.

We present a systematic investigation of the factors influencing the surface area of zirconium-based UiO-type metal-organic frameworks (MOFs), revealing an important relationship between factors including the conformation of the organic linker in the MOF, surface tension of the guest molecules (solvent), and the stability of MOFs toward activation (removal of guest molecules). The results obtained demonstrate how the structure of the linkers forming the isostructural series of UiO MOFs with fcu topology could alter the resistance and stability of the MOF frameworks toward capillary force-driven structural degradation governed by the solvent during activation.

Stabilization of an Unprecedented Hexanuclear Secondary Building Unit in a Thorium-Based Metal-Organic Framework.

The crystal structures of thorium clusters are important for understanding the formation and transformation mechanisms of actinide species in solution, which can in turns benefit nuclear waste processing and management. However, stabilizing thorium clusters in aqueous solution is quite challenging because of their fast olation and oxolation reactions. Here, we report a thorium-based metal-organic framework, NU-905, with the formula [Th(μ-O)(HCOO)(HO)(TCPP)] [TCPP = tetrakis(4-carboxyphenyl)porphyrin], synthesized by a solvothermal reaction in N, N-dimethylformamide and water at 120 °C.

Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series.

Porous materials, including metal-organic frameworks (MOFs), are known to undergo structural changes when subjected to applied hydrostatic pressures that are both fundamentally interesting and practically relevant. With the rich structural diversity of MOFs, the development of design rules to better understand and enhance the mechanical stability of MOFs is of paramount importance. In this work, the compressibilities of seven MOFs belonging to two topological families (representing the most comprehensive study of this type to date) were evaluated using in situ synchrotron X-ray powder diffraction of samples within a diamond anvil cell.

Oxygen-Assisted Cathodic Deposition of Zeolitic Imidazolate Frameworks with Controlled Thickness.

Processing metal-organic frameworks (MOFs) as films with controllable thickness on a substrate is increasingly crucial for many applications to realize function integration and performance optimization. Herein, we report a facile cathodic deposition process that enables the large-area preparation of uniform films of zeolitic imidazolate frameworks (ZIF-8, ZIF-71, and ZIF-67) with highly tunable thickness ranging from approximately 24 nm to hundreds of nanometers. Importantly, this oxygen-reduction-triggered cathodic deposition does not lead to the plating of reduced metals (Zn and Co).

Boosting Transport Distances for Molecular Excitons within Photoexcited Metal-Organic Framework Films.

Herein, we describe the fabrication of porphyrin-containing metal-organic framework thin films with 1,4-diazabicyclo[2.2.2]octane (DABCO) pillaring linkers and investigate exciton transport within the films.

Improving the Efficiency of Mustard Gas Simulant Detoxification by Tuning the Singlet Oxygen Quantum Yield in Metal-Organic Frameworks and Their Corresponding Thin Films.

The photocatalytically driven partial oxidation of a mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES), was studied using the perylene-based metal-organic framework (MOF) UMCM-313 and compared to the activities of the Zr-based MOFs: PCN-222/MOF-545 and NU-1000. The rates of CEES oxidation positively correlated with the singlet oxygen quantum yield of the MOF linkers, porphyrin (PCN-222/MOF-545) < pyrene (NU-1000) < perylene (UMCM-313). Subsequently, thin films of UMCM-313 and NU-1000 were solvothermally grown on a conductive glass substrate to minimize catalyst loading and prevent light scattering by suspended MOF particles.

A porous, electrically conductive hexa-zirconium(iv) metal-organic framework.

Engendering electrical conductivity in high-porosity metal-organic frameworks (MOFs) promises to unlock the full potential of MOFs for electrical energy storage, electrocatalysis, or integration of MOFs with conventional electronic materials. Here we report that a porous zirconium-node-containing MOF, NU-901, can be rendered electronically conductive by physically encapsulating C, an excellent electron acceptor, within a fraction ( 60%) of the diamond-shaped cavities of the MOF. The cavities are defined by node-connected tetra-phenyl-carboxylated pyrene linkers, species that are excellent electron donors.