Aromatic Substituent Effects on the Flexibility of Metal–Organic Frameworks

Highly Transparent, Yet Photoluminescent: 2D CdSe/CdS Nanoplatelet-Zeolitic Imidazolate Framework Composites Sensitive to Gas Adsorption

In this work, thin composite films of zeolitic imidazolate frameworks (ZIFs) and colloidal two-dimensional (2D) core-crown CdSe/CdS nanoplatelet (NPL) emitters with minimal scattering are formed by a cycled growth method and yield highly transparent coatings with strong and narrow photoluminescence of the NPLs at 546 nm (FWHM: 25 nm) in a solid-state composite structure. The porous ZIF matrix acts as functional encapsulation for the emitters and enables the adsorption of the guest molecules water and ethanol. The adsorption and desorption of the guest molecules is then characterized by a reversable photoluminescence change of the embedded NPLs. The transmittance of the composite films exceeds the values of uncoated glass at visible wavelengths where the NPL emitters show no absorption (>540 nm) and renders them anti-reflective coatings. At NPL absorption wavelengths (440-540 nm), the transmittance of the thin composite film-coated glass lies close to the transmittance of uncoated glass. The fast formation of innovative, smooth NPL/ZIF composite films without pre-polymerizing the colloidal 2D nanostructures first provides a powerful tool toward application-oriented photoluminescence-based gas sensing.

Transmission Porosimetry Study on High-quality Zr-fum-MOF Thin Films

Crystalline Zr-fum-MOF (MOF-801) thin films of high quality are prepared on glass and silicon substrates by direct growth under solvothermal conditions. The synthesis is described in detail and the influence of different synthesis parameters such as temperature, precursor concentration, and the substrate type on the quality of the coatings is illustrated. Zr-fum-MOF thin films are characterized in terms of crystallinity, porosity, and homogeneity. Dense films of optical quality are obtained. The sorption behavior of the thin films is studied with various adsorptives. It can be easily monitored by measuring the transmission of the films in gas flows of different compositions. This simple transmission measurement at only one wavelength allows a very fast evaluation of the adsorption properties of thin films as compared to traditional sorption methods. The sorption behavior of the thin films is compared with the sorption properties of Zr-fum-MOF powder samples.

Radiosynthesis and Preclinical Evaluation of an 18F-Labeled Triazolopyridopyrazine-Based Inhibitor for Neuroimaging of the Phosphodiesterase 2A (PDE2A)

The cyclic nucleotide phosphodiesterase 2A is an intracellular enzyme which hydrolyzes the secondary messengers cAMP and cGMP and therefore plays an important role in signaling cascades. A high expression in distinct brain areas as well as in cancer cells makes PDE2A an interesting therapeutic and diagnostic target for neurodegenerative and neuropsychiatric diseases as well as for cancer. Aiming at specific imaging of this enzyme in the brain with positron emission tomography (PET), a new triazolopyridopyrazine-based derivative (11) was identified as a potent PDE2A inhibitor (IC50, PDE2A = 1.99 nM; IC50, PDE10A ~2000 nM) and has been radiofluorinated for biological evaluation. In vitro autoradiographic studies revealed that [18F]11 binds with high affinity and excellent specificity towards PDE2A in the rat brain. For the PDE2A-rich region nucleus caudate and putamen an apparent KD value of 0.24 nM and an apparent Bmax value of 16 pmol/mg protein were estimated. In vivo PET-MR studies in rats showed a moderate brain uptake of [18F]11 with a highest standardized uptake value (SUV) of 0.97. However, no considerable enrichment in PDE2A-specific regions in comparison to a reference region was detectable (SUVcaudate putamen = 0.51 vs. SUVcerebellum = 0.40 at 15 min p.i.). Furthermore, metabolism studies revealed a considerable uptake of radiometabolites of [18F]11 in the brain (66% parent fraction at 30 min p.i.). Altogether, despite the low specificity and the blood−brain barrier crossing of radiometabolites observed in vivo, [18F]11 is a valuable imaging probe for the in vitro investigation of PDE2A in the brain and has potential as a lead compound for further development of a PDE2A-specific PET ligand for neuroimaging.

Post-synthetic ligand cyclization in metal-organic frameworks through functional group connection with regioisomerism

A covalent connection between two orthogonal functional groups (-NH₂ and -OH) in metal-organic frameworks (MOFs) has been developed. This post-synthetic ligand cyclization (PSLC) was successfully demonstrated to synthesize a benzoxazole-functionalized MOF from a Zr-based UiO-66-2,3-(NH₂)(OH) under microwave irradiation. In contrast, the regioisomeric UiO-66-2,5-(NH₂)(OH) only produces a non-cyclized formamide-functionalized MOF.

A Computational Analysis of the Intrinsic Plasticity of Five-Coordinate Cu(II) Complexes and the Factors Leading to the Breakdown of the Orbital Directing Effect in Paddlewheel Secondary Building Units

Quantum chemical calculations on model copper paddlewheel (CPW) complexes of general formula [Cu₂ (μ₂ -O₂ CR)₄ L₂ ] establish two local coordination geometries at the metal centers depending on the balance between equatorial and axial ligand fields. When the equatorial field is stronger than the axial field (large ligand field asymmetry), d x 2 - y 2 dominates the stereochemical activity of the d⁹ shell resulting in a relatively rigid, "orbitally directed" planar or square pyramidal structure. However, if the axial field is significantly increased, or the equatorial field moderately weakened, a small ligand field asymmetry results and both d x 2 - y 2 and d z 2 are involved in the stereochemical activity. This results in a "plastic," distorted trigonal bipyramidal geometry where the former axial ligand moves into one of the original four equatorial positions. Linkers already used to synthesize zinc-dabco MOFs (dabco = 1,4-diazabicyclo[2.2.2]octane) are shown to generate plastic CPW secondary building unit analogs with potential implications for conferring breathing behavior for MOFs which would currently be assumed to be rigid. © 2019 Wiley Periodicals, Inc.

Effect of the Metal within Regioisomeric Paddle-Wheel-Type Metal-Organic Frameworks

The effect of metal on the degree of flexibility upon evacuation of metal-organic frameworks (MOFs) has been revealed with positional control of the organic functionalities. Although Co-, Cu-, and Zn-based DMOFs (DMOF = DABCO MOF, DABCO = 1,4-diazabicyclo[2.2.2]octane) with ortho-ligands (2,3-NH₂ Cl) have frameworks that are inflexible upon evacuation, MOFs with para-ligands (2,5-NH₂ Cl) showed different N₂ uptake amounts after evacuation by metal exchange. Considering that the structural analyses were not fully sufficiently different to explain the drastic changes in N₂ adsorption after evacuation, quantum chemical simulation was explored. A new index (η) was defined to quantify the regularity around the metal based on differences in the oxygen-metal-oxygen angles. Within 2,5-NH₂ Cl, the η value becomes larger as the metal are varied from Co to Zn. A large η value means that the structures around the metal center are less ordered. These results can be used to explain flexibility changes upon evacuation by altering the metal cation in this regioisomeric system.

Understanding structural flexibility of the paddle-wheel Zn-SBU motif in MOFs: influence of pillar ligands

Structural parameters and energies of secondary building units (SBUs) in Zn2(C8H4O4)2·C6H12N2 have been examined with DFT and CCSD(T) methods. The SBU structure exhibits flexibility due to close energies of formation of its D4h, D4, and D2d forms. The potential energy surfaces (PESs) calculated for these systems testify that the presence of pillar ligands with negatively charged nitrogens (NCH or DABCO) can lead to almost barrier-free transitions between structural SBU forms. The mobility regions of the SBU motif have been determined.

Three Component Controls in Pillared Metal-Organic Frameworks for Catalytic Carbon Dioxide Fixation

Three components of pillared metal-organic frameworks (MOFs, three components = metal ion, carboxylic acid ligand, and N-chelating ligand) were controlled for CO2 cycloaddition catalysts to synthesize organic cyclic carbonates. Among the divalent metals, Zn2+ showed the best catalytic activity, and in DABCO (1,4-diazabicyclo[2.2.2]octane)-based MOFs, hydroxy-functionalized DMOF-OH was the most efficient MOF for CO2 cycloaddition. For the BPY (4,4’-bipyridyl)-type MOFs, all five prepared BMOFs (BPY MOFs) showed similar and good conversions for CO2 cycloaddition. Finally, this pillared MOF could be recycled up to three times without activity and crystallinity loss.

New Azo-DMOF-1 MOF as a Photoresponsive Low-Energy CO2 Adsorbent and Its Exceptional CO2/N2 Separation Performance in Mixed Matrix Membranes

A new generation-2 light-responsive metal-organic framework (MOF) has been successfully synthesized using Zn as the metal source and both 2-phenyldiazenyl terephthalic acid and 1,4-diazabicyclo[2.2.2]octane (DABCO) as the ligands. It was found that Zn-azo-dabco MOF (Azo-DMOF-1) exhibited a photoresponsive CO₂ adsorption both in static and dynamic condition because of the presence of azobenzene functionalities from the ligand. Further application of this MOF was evaluated by incorporating it as a filler in a mixed matrix membrane for CO₂/N₂ gas separation. Matrimid and polymer of intrinsic microporosity-1 (PIM-1) were used as the polymer matrix. It was found that Azo-DMOF-1 could enhance both the CO₂ permeability and selectivity of the pristine polymer. In particular, the Azo-DMOF-1-PIM-1 composite membranes have shown a promising performance that surpassed the 2008 Robeson Upper Bound.

Engineering design toward exploring the functional group substitution in 1D channels of Zn–organic frameworks upon nitro explosives and antibiotics detection

Three MOFs (1, 2, 3) with distinct functional groups were prepared. 2 exhibited moderate sensitivity for the detection of specific explosives/antibiotics at the same concentration, which was lower and higher than that of 3 and 1, respectively.

Three ligand-originated MOF isomers: the positional effect of the methyl group on structures and selective C2H2/CH4 and CO2/CH4 adsorption properties

The positional effect of the methyl group on structures and gas adsorption properties was explored in a copper-based MOF platform constructed from bent diisophthalate ligands bearing the methyl group at different positions.

Bicyclo[2.2.2]octane-1,4-dicarboxylic acid: towards transparent metal–organic frameworks

The preparation of transparent porous materials can offer a different access towards the study of molecules under solid confined space.

Flexibility in metal–organic frameworks derived from positional and electronic effects of functional groups

The position of functional groups and the subsequent e-density of the benzene rings in a bi-/tri-functionalized zinc-based metal–organic framework (MOF) have been controlled to reveal structural differences.