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Industrialization over the past two centuries has resulted in a continuous rise in global CO2emissions. These emissions are changing ecosystems and livelihoods. Therefore, methods are needed to capture these emissions from point sources and possibly from our atmosphere. Though the amount of CO2is rising, it is challenging to capture directly from air because its concentration in air is extremely low, 0.04%. In this study, amines installed inside metal–organic frameworks (MOFs) are investigated for the adsorption of CO2, including at low concentrations. The amines used are polyamidoamine dendrimers that contain many primary amines. Chemically reversible adsorption of CO2via carbamate formation was observed, as was enhanced uptake of carbon dioxide, likely via dendrimer-amide-based physisorption. Limiting factors in this initial study are comparatively low dendrimer loadings and slow kinetics for carbon dioxide uptake and release, even at 80 °C.

Semiconductor nanocrystals (NCs) offer prospective use as active optical elements in photovoltaics, light-emitting diodes, lasers, and photocatalysts due to their tunable optical absorption and emission properties, high stability, and scalable solution processing, as well as compatibility with additive manufacturing routes. Over the course of experiments, during device fabrication, or while in use commercially, these materials are often subjected to intense or prolonged electronic excitation and high carrier densities. The influence of such conditions on ligand integrity and binding remains underexplored. Here, we expose CdSe NCs to laser excitation and monitor changes in oleate that is covalently attached to the NC surface using nuclear magnetic resonance as a function of time and laser intensity. Higher photon doses cause increased rates of ligand loss from the particles, with upward of 50% total ligand desorption measured for the longest, most intense excitation. Surprisingly, for a range of excitation intensities, fragmentation of the oleate is detected and occurs concomitantly with formation of aldehydes, terminal alkenes, H2, and water. After illumination, NC size, shape, and bandgap remain constant although low-energy absorption features (Urbach tails) develop in some samples, indicating formation of substantial trap states. The observed reaction chemistry, which here occurs with low photon to chemical conversion efficiency, suggests that ligand reactivity may require examination for improved NC dispersion stability but can also be manipulated to yield desired photocatalytically accessed chemical species.

Postsynthetic modification (PSM) of metal–organic frameworks (MOFs) enables incorporation of diverse functionalities in pores for chemical separations, drug delivery, and heterogeneous catalysis. However, the effect of PSM on molecular transport, which is essential for most applications of MOFs, has been rarely studied. In this paper, we used perfluoroalkane-functionalized Zr-MOF NU-1008 as a platform to systematically interrogate transport processes and mechanisms in solvated pores. We anchored perfluoroalkanes onto NU-1008 nodes by solvent-assisted ligand incorporation (SALI-n, with n= 3, 5, 7, and 9 denoting the number of fluorinated carbons). Transport of a luminescent molecule, BODIPY, through individual crystallites of four versions of methanol-filled SALI-nwas monitored by confocal fluorescence microscopy as a function of time and location. In comparison with the parent NU-1008, the diffusivity of the probe molecules within SALI-ndeclined by 2- to 7-fold depending on chain length and loading, presumably due to the reduction in pore diameter or adsorptive interactions with perfluoroalkyl chains. Atomistic simulations were performed to uncover the microscopic behavior of the BODIPY diffusion in SALI-n. The perfluoroalkyl chains are observed to stay close to the pore walls, instead of extending toward the pore center. BODIPY molecules, which preferably interact with linkers, were pushed to the interior of the channels as the chain length increased, resulting in solvated diffusion and minor differences in the short-time mobility of BODIPY in SALI-n. This suggested that the observed decline of transport diffusivity in SALI-nmainly stemmed from the reduction in the pore size when these flexible chains are present. We anticipate that this proof of concept will assist in understanding how pore functionalization can physically and chemically affect mass transport in MOFs and will be useful in further guiding the design of PSM to realize the optimal performance of MOFs for various applications.

Zirconium-based metal–organic frameworks (MOFs) are candidate materials for effective nerve agent detoxification due to their thermo- and water stability as well as high density of catalytic Zr sites. However, as high-porosity materials, most of the active sites of Zr-MOFs can only be accessed by diffusion into the crystal interior. Therefore, the transport of nerve agents in nanopores is an important factor in the catalytic performance of Zr-MOFs. Here, we investigated the transport process and mechanism of a vapor-phase nerve agent simulant, dimethyl methyl phosphonate (DMMP), through a representative Zr-MOF, NU-1008, under practical conditions of varying humidity. Confocal Raman microscopy was used to monitor the transport of DMMP vapor through individual NU-1008 crystallites, where the relative humidity (RH) of the environment was tuned to understand the impact of water. Counterintuitively, water in the MOF channels, instead of blocking DMMP transport, assists DMMP diffusion; indeed, the transport diffusivity (Dt) of DMMP in NU-1008 is one order of magnitude higher at 70% than 0% RH. To understand the mechanism, magic angle spinning NMR and molecular dynamics simulations were performed and suggested that high water content in the channels prevents DMMP from hydrogen-bonding with the nodes, allowing for faster diffusion of DMMP in the channels. The simulated self-diffusivity (Ds) of DMMP is observed to be concentration-dependent. At low loading of DMMP, Dsis higher at 70% RH than 0% RH, while at high loadings the trend reverses due to the DMMP aggregation in water and the reduction of free volume in channels.

Starting with ferrocyanide ions in acidic aqueous solution, cyano-ferrate(II) species are post-synthetically grafted to the nodes of a mesoporous zirconium-based MOF, NU-1000. As indicated by single-crystal X-ray crystallography, grafting occurs by substitution of cyanide ligands by node-based hydroxo and oxo ligands rather than by substitution of node aqua ligands by cyanide ligands as bridges between Fe(II) and Zr(IV). The installed moieties yield a broad absorption band that is tentatively ascribed to iron-to-zirconium charge transfer. Consistent with Fe(III/II) redox activity, a modest fraction of the installed iron complexes are directly electrochemically addressable.

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. Nevertheless, the pervasive presence of transition metal sulfur clusters metalloenzymes or cofactors that catalyze reduction reactions and the justifiable proliferation of studies of two-dimensional (2D) metal-chalcogenides as reduction catalysts point to the promise of well-defined and controllable PTMs as reduction catalysts. Here, we report the fabrication of agglomeration-immune, reactant-accessible, capping-ligand-free CoIIMo6IVS24n–clusters as periodic arrays in a water-stable, hierarchically porous Zr-metal–organic framework (MOF; NU1K) by first installing a disk-like Anderson polyoxometalate, CoIIIMo6VIO24m–, in size-matched micropores where the siting is established via difference electron density (DED) X-ray diffraction (XRD) experiments. Flowing H2S, while heating, reduces molybdenum(VI) ions to Mo(IV) and quantitatively replaces oxygen anions with sulfur anions (S2–, HS–, S22–). DED maps show that MOF-templated POM-to-PTM conversion leaves clusters individually isolated in open-channel-connected micropores. The structure of the immobilized cluster as determined, in part, by X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) analysis, and pair distribution function (PDF) analysis of total X-ray scattering agrees well with the theoretically simulated structure. PTM@MOF displays both electrocatalytic and photocatalytic competency for hydrogen evolution. Nevertheless, the initially installed PTM appears to be a precatalyst, gaining competency only after the loss of ∼3 to 6 sulfurs and exposure to hydride-forming metal ions.

Chemically functionalized series of metal–organic frameworks (MOFs), with subtle differences in local structure but divergent properties, provide a valuable opportunity to explore how local chemistry can be coupled to long-range structure and functionality. Using in situsynchrotron X-ray total scattering, with powder diffraction and pair distribution function (PDF) analysis, we investigate the temperature dependence of the local- and long-range structure of MOFs based on NU-1000, in which Zr6O8nodes are coordinated by different capping ligands (H2O/OH, Cl–ions, formate, acetylacetonate, and hexafluoroacetylacetonate). We show that the local distortion of the Zr6nodes depends on the lability of the ligand and contributes to a negative thermal expansion (NTE) of the extended framework. Using multivariate data analyses, involving non-negative matrix factorization (NMF), we demonstrate a new mechanism for NTE: progressive increase in the population of a smaller, distorted node state with increasing temperature leads to global contraction of the framework. The transformation between discrete node states is noncooperative and not ordered within the lattice, i.e., a solid solution of regular and distorted nodes. Density functional theory calculations show that removal of ligands from the node can lead to distortions consistent with the Zr···Zr distances observed in the experiment PDF data. Control of the node distortion imparted by the nonlinker ligand in turn controls the NTE behavior. These results reveal a mechanism to control the dynamic structure of MOFs based on local chemistry.

Materials-based approaches are needed to achieve high volumetric density for the storage and release of hydrogen. In this work, we investigate metal oxides for their ability to store hydrogen, based on the idea that the known reduction potentials of proton-coupled electron transfer in metal oxides are in a range that suggests these materials could have suitable energetics for hydrogen storage and release at near-ambient temperature. We hypothesize that the more positive (or less negative) is the reduction potential, the greater is the favorability to absorb hydrogen. To test this idea, the absorption of atomic hydrogen (which can be derived from molecular hydrogen with a suitable catalyst) in six metal oxides (MnO2, MoO3, SnO2, TiO2,WO3, and ZrO2) is studied using density functional theory at the PBE-D3(BJ), PBE-D3(BJ)+U, and PBE0 levels of theory. A correlation between reduction potentials and the energies of absorption (relative to free H2) is found, and three of the metal oxides are predicted to absorb hydrogen at storage-relevant temperatures and pressures, with MoO3showing the most promise.

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)2(bpy-R)]2+-functionalized dyes with the surface of MIL-96, where R = C3, C8, C12, and C18alkyl chains of either straight-chain or cyclic conformations. The orientation of the dyes appears to be flat against the surface with respect to the long alkyl chains, and the surface concentration approaches a monolayer at high initial concentrations of dye. Strangely, the dodecyl-functionalized dye, despite having a smaller interaction energy and larger footprint than either octyl-functionalized dye, achieves the highest maximum surface concentration. Based on photoluminescence spectra, desorption experiments, and ITC data, we believe this is due to the core of the dye being lifted from the surface as the chain length increases. Our understanding of these interactions is important for further utilization of this method for the functionalization of the internal and external surface areas of MOFs.

Effective heterogeneous photocatalysts capable of detoxifying chemical threats in practical settings must exhibit outstanding device integrity. We report a copolymerization that yields robust, porous, processible, chromophoric BODIPY (BDP; boron-dipyrromethene)-containing polymers of intrinsic microporosity (BDP-PIMs). Installation of a pentafluorophenyl at the meso position of a BDP produced reactive monomer that when combined with 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane (TTSBI) and tetrafluoroterephthalonitrile (TFTPN) yields PIM-1. Postsynthetic modification of these polymers yields Br-BDP-PIM-1aand -1b─polymers containing bromine at the 2,6-positions. Remarkably, the brominated polymers display porosity and processability features similar to those of H-BDP-PIMs. Gas adsorption reveals molecular-scale porosity and Brunette–Emmet–Teller surface areas as high as 680 m2g–1. Electronic absorption spectra reveal charge-transfer (CT) bands centered at 660 nm, while bands arising from local excitations, LE, of BDP and TFTPN units are at 530 and 430 nm, respectively. Fluorescence spectra of the polymers reveal a Förster resonance energy-transfer (FRET) pathway to BDP units when TFTPN units are excited at 430 nm; weak phosphorescence at room temperature indicates a singlet-to-triplet intersystem crossing. The low-lying triplet state is well positioned energetically to sensitize the conversion of ground-state (triplet) molecular oxygen to electronically excited singlet oxygen. Photosensitization capabilities of these polymers toward singlet-oxygen-driven detoxification of a sulfur-mustard simulant 2-chloroethyl ethyl sulfide (CEES) have been examined. While excitation of CT and LEBDPbands yields weak catalytic activity (t1/2> 15 min), excitation to higher energy states of TFTPN induces significant increases in photoactivity (t1/2≅ 5 min). The increase is attributable to (i) enhanced light collection, (ii) FRET between TFTPN and BDP, (iii) the presence of heavy atoms (bromine) having large spin–orbit coupling energies that can facilitate intersystem crossing from donor–acceptor CT-, FRET-, or LE-generated BDP singlet states to BDP-related triplet states, and (iv) polymer triplet excited-state sensitization of the formation of CEES-reactive, singlet oxygen.

Colloidal semiconductor nanocrystals, or quantum dots (QDs), show great promise as light absorbers and emitters in next-generation optoelectronic devices, but deleterious charge and energy-transfer processes that occur in solid-state thin films of QDs hinder their wide-scale utilization. Several classes of materials have been used previously to encapsulate QDs in the solid state but have failed to fully prevent charge and energy transfer in densely packed thin films of QDs relevant for device applications, necessitating the exploration of other materials. This paper describes the use of a metal–organic framework (MOF), zeolitic imidazolate framework-8 (ZIF-8), as a novel matrix encapsulation material to regulate the inter-QD spacing of core–shell QDs in a crystalline host, thus enhancing the photoluminescence quantum yield (PL QY) and irradiance of the films. The brightest composite films have PL QYs of 7.4–9.8% (whereas the PL QY of the QD-only film is 2.1%) and volume-normalized irradiances that are at least a factor of 5 higher than that of the QD-only film. Spectrally resolved PL lifetime measurements indicate that ZIF-8 improves the emission of the QDs by inhibiting inter-QD energy transfer, primarily by preventing the formation of large aggregates of QDs that are present in the film of poly(vinylpyrrolidone) (PVP)-coated QDs without ZIF-8. This demonstration of control over QD spacing within the crystalline MOF matrix reveals new pathways toward mesoscale QD structures with controlled electronic coupling and dielectric environments. Furthermore, this study is one of the first investigations of the photophysics of QD@MOF composite materials.

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. Herein, a flexible, redox-active tetracarboxylic acid linker bearing two imidazole units was chosen to construct a new Zr6-MOF, NU-910, with scutopology. By exploiting the structural flexibility of NU-910, we successfully modulate the dynamics between an isolated organic radical species and an organic radical π-dimer species in the MOF system. Single-crystal X-ray diffraction analysis reveals that through solvent exchange from N,N-diethylformamide to acetone, NU-910undergoes a structural contraction with interlinker distances decreasing from 8.32 Å to 3.20 Å at 100 K. Organic radical species on the bridging linkers are generated via UV light irradiation. Direct observation of temperature-induced spin switches from an isolated radical species to a magnetically silent radical π-dimer in NU-910after irradiation in the solid state was achieved via variable-temperature single-crystal X-ray diffraction and variable-temperature electron paramagnetic resonance spectroscopy. Ultraviolet–visible–near infrared spectroscopy and density functional theory calculations further substantiated the formation of a radical cation π-dimer upon irradiation. This work demonstrates the potential of using flexible MOFs as a platform to modulate radical spin states in the solid phase.

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. However, because the HOF-102 crystallites containing these columns are oriented parallel to an underlying electrode, dynamics of charge transport (hole-transport) along the column axis, in contrast to the plane, are not directly probed by the electrochemical measurements. Furthermore, we employed electrochemical impedance spectroscopy to measure the electrical conductivity of the as-deposited films biased at various potentials. We found that both the neutral/singly oxidized and the singly oxidized/doubly oxidized pyrene linker redox couples of HOF-102 can engender hopping-based film conductivity within the 2D plane of HOF-102. Consistent with the radical cation and radical dication nature of the singly and doubly oxidized linkers, respectively, HOF-102 films are electrochromic. The measured values of in-plane charge-diffusion coefficients (∼10–10to 10–11cm2s–1) and electrical conductivity (∼10–6to 10–8S cm–1) compare favorably with those for related redox-conductive MOFs and suggest that the transport and conductivity parameters for HOF-102 are sufficiently large to support electrocatalysis by subsequently installed catalysts in films─specifically, films of micron or greater thickness, corresponding to the equivalent hundreds of monolayers of closely packed (i.e.,face-to-face-packed) pyrene-derivatives, but with solution access (solvent, ion, and reactant access) still readily provided by channels oriented parallel to an underlying planar electrode.

The inherent crystallinity of metal–organic framework (MOF) catalysts offers the possibility to understand the structure of the active site at the molecular level. This property is often lacking in traditional amorphous supports. Cr-SIM-NU-1000, a MOF-supported Cr3+heterogeneous catalyst, is shown to be competent for ethylene polymerization after activation with AlEt2Cl (DEAC), producing crystalline linear polyethylene (PE). The polymer produced has a low polydispersity (= 2.0), in marked contrast to the Phillips supported chromium catalyst, Cr@SiO2(∼ 8–65). Cr-SIM-NU-1000 achieves a turnover frequency of 2.6 × 103h−1under 40 bar ethylene pressure at room temperature, with corresponding PE productivity of 1.3 × 105g PE mol−1Cr per h. Single crystal X-ray diffraction (SC-XRD) of the pre-catalyst was conducted by collecting a structure of alkyl aluminum (DEAC) co-catalyst treated Cr-SIM-NU-1000. This crystal structure provides insight into the interactions between DEAC co-catalyst and Cr active site, revealing a Cr–C bond after treatment with DEAC. Furthermore, DEAC is crystallographically resolved at the terminal oxy-ligands of the node and likely also exists within the window pores of the framework between nodes, based on electron density mapping. Cr-SIM-NU-1000 offers the opportunity to study a structurally well-defined olefin polymerization system, with atomically precise characterization of the pre-catalyst structure. This allows the proposal of a mechanism and feeds into future development of next-generation heterogeneous catalyst systems.

How molecules approach, bind at, and release from catalytic sites is key to heterogeneous catalysis, including for emerging metal–organic framework (MOF)-based catalysts. We use in situsynchrotron X-ray scattering analysis to evaluate the dominant binding sites for reagent and product molecules in the vicinity of catalytic Ni-oxo clusters in NU-1000 with different surface functionalization under conditions approaching those used in catalysis. The locations of the reagent and product molecules within the pores can be linked to the activity for ethylene hydrogenation. For the most active catalyst, ethylene reagent molecules bind close to the catalytic clusters, but only at temperatures approaching experimentally observed onset of catalysis. The ethane product molecules favor a different binding location suggesting that the product is readily released from the active site. An unusual guest-dependence of the framework negative thermal expansion is documented. We hypothesize that reagent and product binding sites reflect the pathway through the MOF to the active site and can be used to identify key factors that impact the catalytic activity.

Solvothermal deposition in metal–organic frameworks (MOFs) can be used to mount single-metal-atom catalytic species at chemically reactive sites on hexa-Zr(IV)–oxy(oxo,hydroxo,aqua) nodes in nanoscale crystallites of the MOF NDC–NU-1000 in a self-limiting fashion. Upon photoexcitation of the 1,3,6,8-tetrakis(p-benzoato)pyrene chromophores of the parent framework, charge transfer may occur between the chromophores and the installed heterometal sites. Extended X-ray absorption fine structure studies revealed the single-atom nature of the installed species. A combination of steady-state and ultrafast optical spectroscopy was used to uncover evidence of a charge-separated (CS) state arising in the metalated samples. The relevant dynamics were characterized with transient photoluminescence and femtosecond transient absorption spectroscopy. We find that a titanium–oxy single-atom site gives rise to the longest lived CS species compared to cobalt and nickel in a similar arrangement. This study provides guidance in designing MOF-based catalytic systems for photocatalysis and solar fuel production.

Guest transport through metal–organic frameworks (MOFs) is a critical process in the application of MOFs for catalysis. Understanding the interplay between transport behavior and a MOF’s structure is of fundamental importance to further tailor MOFs for optimal catalysis. Here, we investigated dye transport processes through two solvent-filled Zr-MOFs, NU-600 and NU-1008, which are compositionally the same but display different topologies, i.e., sheand csq, respectively. Dye transport through individual MOF crystallites was monitored spatially and temporally by confocal fluorescence microscopy. In both MOF crystals, dye molecules permeated the external-surface barrier first, then diffused along channels. Transport in NU-600 is three dimensional due to orthogonal channels, while diffusion in NU-1008 is primarily one dimensional owing to parallelly aligned channels. Quantitatively, the diffusivity of dye molecules in NU-600 is smaller than in NU-1008, which is attributed to the narrower channels and tortuous pore network of NU-600. However, comparing crystals of the same volume, macroscopic uptake of dye in NU-600 is significantly more efficient than in NU-1008, highlighting that the she-net NU-600, which features intersecting channels, affords efficient pathways for substrate transport. Additionally, for NU-600 and NU-1008, the nanoscale topologies of the compounds qualitatively govern the resulting macroscopic crystallite morphologies, including aspect ratios. The morphology difference is crucial to conferring a transport efficiency advantage on NU-600. Atomistic simulations of solvated dye diffusion in the two MOFs indicate energetically favorable interaction between the linkers and dye. Molecular dynamics trajectories reveal that the dye molecule spends more time on the linkers in NU-600 relative to NU-1008, which supports the smaller diffusivity in NU-600 measured by experiments. In this work, we combined experiments and simulations to demonstrate the interplay between MOF structure and guest transport behavior both microscopically and macroscopically, which provides insights for selecting or designing MOF topologies to enhance guest transport through MOFs intended, for example, for chemical catalysis.

We report uptake capacities and transport diffusivities, D, for each of eight linear alkanes (ranging from C5to C16) in quartz crystal-supported films of solvent-evacuated ZIF-8. Analyses of the alkane uptake profiles revealed that the transport dynamics are governed by guest diffusion through metal–organic framework (MOF) (ZIF-8) crystallites rather than by rates of entry into films at the MOF/vapor interface. The obtained diffusivities range from just over 10–18m2/s to just under 10–14m2/s. Notably, minimum cross-sectional widths for all guests exceed the crystallographically measured width of ZIF-8’s largest apertures and imply consistently with previous experimental and computational studies that apertures expand to accommodate guest uptake. On average, each additional carbon decreases the transport diffusivity of an alkane by twofold. Closer examination, however, reveals an odd–even effect such that linear alkanes having even numbers of carbons diffuse more rapidly than alkanes featuring one more or one less carbon atom. Thus, ZIF-8’s differentiation of transport diffusivities for pairs of alkanes differing in length by only one carbon atom can be significantly greater than the aforementioned factor of 2. Elucidation of the microscopic basis for the odd–even behavior, however, awaits the outcome of molecular dynamics calculations that are beyond the scope of the present study. For compact, solvothermally prepared films, guest transport is dominated by 1D diffusion from the film/vapor interface and toward the underlying quartz crystal. For much lower density, electrophoretically deposited (EPD) films, crystallites behave nearly independently, and guest transport can be adequately modeled by assuming rapid permeation of macroscopic voids between crystallites, followed by entry and rate-limiting radial diffusion into isolated crystallites. One consequence is that EPD films can be much more rapidly infiltrated by molecular guests than can compact, solvothermally grown films. The combined results have potentially favorable implications for the development of kinetic separation schemes for closely related analytes.