Tailoring of the electronic property of Zn-BTC metal–organic framework via ligand functionalization: an ab initio investigation

Mapping the correlations between bandgap, HOMO, and LUMO trends for meta substituted Zn-MOFs

Bandgap is a key property that determines electrical and optical properties in materials. Modulating the bandgap thus is critical in developing novel materials particularly semiconductors with improved features. This study examines the bandgap, highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) energy level trends in a metal organic framework, metal-organic framework 5 (MOF-5), as a function of Hammett substituent effect (with the constant σm in the meta-position of the benzene ring) and solvent dielectric effect (with the constant ε). Specifically, experimental design and response surface methodologies helped to assess the significance of trends and correlations between these molecular properties with σm and ε. While the HOMO and LUMO decrease with increasing σm, the LUMO exhibits greater sensitivity to the substituent's electron withdrawing capability. The relative difference in these trends helps to explain why the bandgap tends to decrease with increasing σm.

Tuning Electrical and Mechanical Properties of Metal-Organic Frameworks by Metal Substitution

Metal-organic frameworks (MOFs), synthesized by the self-assembly of organic ligands and metal centers, are structurally designable materials. In the current study, first-principles calculation based on density functional theory (DFT) was performed to investigate the intrinsic mechanical and electrical properties and mechanical-electrical coupling behavior of MOF-5. To improve the conductivity of MOF-5, homologous elements of Cu, Ag, and Au were adopted to replace the Zn atom in MOF-5, reducing the band gap and improving its electrical performance. Cu-MOF-5 and Au-MOF-5, with stable structures, exhibit better conductivity. The intrinsic mechanical properties such as independent elastic constants of MOF-5 and M-MOF-5 (M = Cu, Ag, Au) were obtained. MOF-5 and Cu-MOF-5 were experimentally synthesized to demonstrate the reduction in the band gap after metal substitution. The study of the strain effect of MOF-5 and Cu-MOF-5 proves that strain engineering is an effective method to regulate the band gap and this modulation is repeatable. This study clarifies the tunability of the band gap of MOF-5 with metal substituents and provides an efficient strategy for the development of new types of MOFs with desired physical properties using the combination of theoretical prediction and experimental synthesis and validation.

Zn(II)-Based Mixed-Ligand-Bearing Coordination Polymers as Multi-Responsive Fluorescent Sensors for Detecting Dichromate, Iodide, Nitenpyram, and Imidacloprid

Coordination polymers (CPs) are organo-inorganic porous materials consisting of metal ions or clusters and organic linkers. These compounds have attracted attention for use in the fluorescence detection of pollutants. Here, two Zn-based mixed-ligand-bearing CPs, [Zn₂(DIN)₂(HBTC^2-)₂] (CP-1) and [Zn(DIN)(HBTC^2-)]·ACN·H₂O (CP-2) (DIN = 1,4-di(imidazole-1-yl)naphthalene, H₃BTC = 1,3,5-benzenetricarboxylic acid, and ACN = acetonitrile), were synthesized under solvothermal conditions. CP-1 and CP-2 were characterized by single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, elemental analysis, and powder X-ray diffraction analysis. Solid-state fluorescence analysis revealed an emission peak at 350 nm upon excitation at 225 and 290 nm. Fluorescence sensing tests showed that CP-1 was highly efficient, sensitive, and selective for detecting Cr₂O₇^2- at 225 and 290 nm, whereas I^- was only detected well at an excitation of 225 nm. CP-1 detected pesticides differently at excitation wavelengths of 225 and 290 nm; the highest quenching rates were for nitenpyram at 225 nm and imidacloprid at 290 nm. The quenching process may occur via the inner filter effect and fluorescence resonance energy transfer.

A new rapid synthesis of hexagonal prism Zn-MOF as a precursor at room temperature for energy storage through pre-ionization strategy

In this paper, a new hexagonal prismatic Zn-MOF is rapidly synthesized at room temperature through a one-step precipitation method as precursor for the preparation of porous carbon. The SEM and GCD tests indicate that the pre-ionization process of BTC greatly accelerates the reaction speed between BTC and Zn ions, and only 0.5 h is required for the preparation of Zn-MOF with orderly morphology at room temperature, far less than 3-24 h of the existing hydrothermal synthesis. The derived porous carbon (BTCC) is provided with a considerable specific surface area of 1,464 m² g^-1 and suitable pores of 3.9 nm in size. Its richly porous structure offers a superior supercapacitor performance. The BTCC electrode offered a high specific capacitance and an excellent cycle stability. Furthermore, the assembled two symmetrical supercapacitors, C|1 M Na₂SO₄|C and C|6 M KOH|C, provide high energy density of 22.4 Wh kg^-1 and 13.7 Wh kg^-1, respectively. Their energy retention rates were 80.0% and 89.4%, respectively after 10,000 cycles at 20 A g^-1. The proposed pre-ionization strategy is a facile, convenient and easy-to-industrial method for the preparation of new MOFs, thereby significantly reducing the manufacturing cost of porous carbon for energy storage.

Bimetallic Ag-Zn-BTC/GO composite as highly efficient photocatalyst in the photocatalytic degradation of reactive yellow 145 dye in water

Ag_x-Zn_100-x-BTC/GO composites (BTC: benzene-1,3,5-tricarboxylic, GO: graphene oxide) with different Ag/Zn molar ratios were synthesized using microwave-assisted hydrothermal treatment. The Ag_x-Zn_100-x-BTC/GO exhibited excellent photocatalytic performance in the reactive yellow 145 dye (RY-145) degradation under irradiation of visible light with nearly 100% of RY-145 removal after 35 min, as compared to Zn-BTC/GO and Ag-BTC/GO. Reactive oxygen species scavenging assays have shown that the holes (h⁺) and superoxide radical anion (O₂^-•) play a primary role in RY-145 degradation. Based on the band structure of materials, the Z-scheme photocatalytic mechanism was suggested. The effect of catalyst dosage, pH and dye concentration on the efficiency of photocatalytic activity of bimetallic Ag₅₀-Zn₅₀-BTC/GO was also investigated. The improvement in photocatalytic activity of bimetallic Ag₅₀-Zn₅₀-BTC/GO could be given by the synergism of (i) absorption of visible light confirmed by UV-Vis diffuse reflectance spectra; (ii) the increased lifetime as evidenced by photoluminescence spectra and transient photocurrent response; (iii) the increased oxygen vacancy defects as confirmed by X-ray photoelectron spectroscopy results. The degradation pathway of RY-145 dye was also predicted based on liquid chromatography-mass spectrometer analysis. The removed chemical oxygen demand, biological oxygen demand, total organic carbon outcomes indicated the high mineralization ability for RY-145 degradation over Ag₅₀-Zn₅₀-BTC/GO.

Electronic Structure Modeling of Metal-Organic Frameworks

Owing to their molecular building blocks, yet highly crystalline nature, metal-organic frameworks (MOFs) sit at the interface between molecule and material. Their diverse structures and compositions enable them to be useful materials as catalysts in heterogeneous reactions, electrical conductors in energy storage and transfer applications, chromophores in photoenabled chemical transformations, and beyond. In all cases, density functional theory (DFT) and higher-level methods for electronic structure determination provide valuable quantitative information about the electronic properties that underpin the functions of these frameworks. However, there are only two general modeling approaches in conventional electronic structure software packages: those that treat materials as extended, periodic solids, and those that treat materials as discrete molecules. Each approach has features and benefits; both have been widely employed to understand the emergent chemistry that arises from the formation of the metal-organic interface. This Review canvases these approaches to date, with emphasis placed on the application of electronic structure theory to explore reactivity and electron transfer using periodic, molecular, and embedded models. This includes (i) computational chemistry considerations such as how functional, k-grid, and other model variables are selected to enable insights into MOF properties, (ii) extended solid models that treat MOFs as materials rather than molecules, (iii) the mechanics of cluster extraction and subsequent chemistry enabled by these molecular models, (iv) catalytic studies using both solids and clusters thereof, and (v) embedded, mixed-method approaches, which simulate a fraction of the material using one level of theory and the remainder of the material using another dissimilar theoretical implementation.

Three new coordination polymers based on bis(4-(4H-1,2,4-triazol-4-yl)phenyl)methane: syntheses, structures, multiresponsive luminescent sensitive detection for antibiotics and pesticides, and antitumor activities

Three novel coordination polymers (CPs), namely, {[Ag2(L)2(Mo4O13)·(CH3CN)]} n (1), {[Zn(L)(1,4-bdc)2·2(1,4-H2bdc)]} n (2), {[Cd(L)(1,4-bdc)0.5]} n (3) have been synthesized under solvothermal conditions by the reaction of bis(4-(4H-1,2,4-triazol-4-yl)phenyl)methane (L) and varied metal salts. Their structures are determined by single X-ray crystal diffraction, and further characterized by elemental analysis, IR, TGA and PXRD. CP 1 with ammonium molybdate as a secondary ligand displays a 2D network with (2,3,3,3,4)-connected net topology and the point symbol of {4·82}6{4·84·10}2{8}, CP 2 and CP 3 with 1,4-H2bdc as a secondary ligand demonstrate 3D structures with different topologies. CP 2 exhibits high sensibility and low detection limit in the recognition of antibiotics (NZF, NFT and FZD) and pesticide (DCN) identification. CP 1 demonstrates good anti-tumor activity toward the tested glioma cells. The possible luminescent sensitivity and anti-tumor mechanisms are also discussed.