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Nitrogen oxide (NOx ) is a family of poisonous and highly reactive gases formed when fuel is burned at high temperatures during anthropogenic behavior. It is a strong oxidizing agent that significantly contributes to the ozone and smog in the atmosphere. Thus, NOx removal is important for the ecological environment upon which the civilization depends. In recent decades, metal-organic frameworks (MOFs) have been regarded as ideal candidates to address these issues because they form a reticular structure between proper inorganic and organic constituents with ultrahigh porosity and high internal surface area. These characteristics render them chemically adaptable for NOx adsorption, separation, sensing, and catalysis. In additional, MOFs enable potential nitric oxide (NO) delivery for the signaling of molecular NO in the human body. Herein, the different advantages of MOFs for coping with current environmental burdens and improving the habitable environment of humans on the basis of NOx adsorption are reviewed.

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RESUMEN

By using tritopic and ditopic organic linkers derived from the same 2,4,6-triphenylpyridine core, copper(II) metal-organic frameworks with different three-dimensional structures have been successfully synthesized under ambient conditions. The crystalline framework, PTB MOF ([Cu3 (PTB)2 (H2 O)3 ]n , where H3 PTB=4,4',4''-(pyridine-2,4,6-triyl)tribenzoic acid, was observed to be mesoporous in nature and exhibited dual functionality in the removal of organic dyes. While cationic dyes such as methylene blue and malachite green, which are of different sizes, were adsorbed by PTB MOF; anionic dyes such as tartrazine could be effectively degraded in a photo-Fenton-like reaction catalyzed by the MOFs under irradiation with visible light.

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Organic dyes are heavily used in industries for the manufacture of colored goods. This has eventually resulted in the generation of contaminated wastewater which is hard to be purified. Recent studies have demonstrated that metal-organic frameworks (MOFs), a class of supramolecular materials of immense interest, are useful in the adsorption of organic dye molecules because of their modifiable porous structures. In this mini review, the recent advances in the use of MOFs for the adsorption of organic dyes will be summarized.

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The photocatalytic generation of an NADH synthetic analogue, i.e. 1-benzyl-1,4-dihydronicotinamide (1,4-BNAH), has been studied using the cobalt diimino-dioxime complexes and the BF2-bridged derivative as catalysts. 1,4-BNAH was produced in both aqueous and organic media at unprecedented turnover numbers with metal and organic photosensitizers, respectively.

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A series of blue-green emitting RuII diisocyano complexes containing 2-benzoxazol-2-ylphenolate (PBO) have been prepared. The complexes were isolated under varied reaction conditions in two isomeric forms, i.e., trans,trans,trans- (1) and cis,trans,cis- (2), with varied ligand coordination geometry above the RuII center. The photoluminescence of the isomeric complexes has been compared and tuned by the systematic variation of the electronic properties of the isocyanides. The cis,trans,cis- isomers exhibit structureless emission in the blue-green region (471-517 nm) upon excitation at λex > 400 nm in dichloromethane solution at room temperature. Both isomeric forms show similarly structured greenish emission at 499-523 nm on excitation at λex > 355 nm in a methanol/ethanol (4:1) glassy medium at 77 K. On careful comparison with the corresponding absorption and electrochemical data, it is suggested that the solution emission of the cis,trans,cis- isomers (2) at room temperature is originated from the metal-to-ligand charge transfer (MLCT), while a ligand-centered (LC) parentage is assigned for the emission in a glassy state for both isomeric forms. In line with the above experimental results, DFT calculation demonstrates the change in the nature and relative energy of the HOMOs and LUMOs with respect to the varied ligand coordination geometry and π-accepting ability of the isocyanides.

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A layered metal-organic framework (MOF) with a porous kagomé lattice, kgmSMe, was synthesized by complexation of 5-methylthioisophthalate (SMe-ip) with Cu2+ in MeOH. As observed by powder XRD, kgmSMe (state I), when immersed in aprotic polar solvents such as THF, underwent stepwise interlayer expansion into a monolayer-expanded state (state III) through a bilayer-expanded state (state II). We successfully obtained the single-crystal structures of states I-III. Of interest, when further immersed in appropriate solvents, state II and III crystals preferentially exfoliated into bilayer and monolayer MOF nanosheets, respectively. The stepwise expansion followed by exfoliation, thus developed, may enable a nonstochastic approach to the selective synthesis of ultrathin porous nanosheets from layered MOF crystals.

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A new class of donor-acceptor type luminescent bis(alkynyl)gold(iii) N⁁C complexes has been synthesized and characterized. These gold(iii) complexes not only exhibit high photoluminescence quantum yields of up to 0.81, but also interesting mechanochromic luminescence behaviors that are reversible. Upon grinding, a dramatic luminescence color change from green to red can be observed in solid samples of the gold(iii) complexes, and the mechanochromic luminescence can be readily tuned via a judicious selection of substituents on the pyridine ring. In addition, solution-processable OLEDs based on this class of complexes with EQE values of up to 4.0% have been realized, representing the first demonstration of bis(alkynyl)gold(iii) N⁁C complexes as emissive materials in solution-processable OLEDs.

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A series of luminescent bis-cyclometalated gold(III) complexes with different nuclearities and various alkynyl ligands derived from hexaphenylbenzene (HPB) and hexabenzocoronene (HBC) have been synthesized. The energies of the low-energy metal-perturbed intraligand (IL) π-π*(R-C^N^C) absorptions of the HPB-alkynyl gold(III) complexes have been fine-tuned by attaching various substituent groups to the bis-cyclometalating ligands. Similarly, the metal-perturbed 3 IL [π→π*(R-C^N^C)] emissions of the complexes show energy shifts according to the electronic nature of the bis-cyclometalating ligands. On the contrary, the absorption and emission spectra of the HBC-alkynyl gold(III) complex have been assigned as dominated by the IL transitions of the HBC-alkynyl unit, as supported by transient absorption studies. The supramolecular assembly morphologies of the gold(III) complexes have been studied by TEM and SEM, along with comparisons across different complexes based on the molecular structures, and their assembly processes monitored by concentration-dependent and variable-temperature 1 H NMR spectroscopy studies.

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A series of luminescent bis-cyclometalated gold(III) complexes containing bridging alkynyl ligands of different natures has been synthesised and characterised. The photophysical properties of the complexes have been investigated through electronic absorption spectroscopy and emission studies. The vibronic emission bands are found to originate from the triplet intraligand (IL) π-π* excited states of the bis-cyclometalating ligands with some mixing of 3 IL π-π* character of the alkynyl ligands. The electrochemical study of a nonsymmetric dinuclear complex shows two successive reduction processes originating from the reductions of the two different cyclometalating ligands. The complexes are found to undergo supramolecular self-assembly processes driven by π-π stacking and hydrophobic/hydrophilic interactions to give honeycomb nanostructures, as revealed from the SEM images. Solvent-dependent morphological transformations have also been observed, which have been studied by SEM and 1 H NMR spectroscopy.

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A series of triarylamine-containing Zn(ii) diimine bis-thiolate complexes, [Zn(N^N)(SC6H4Me-4)2] (N^N = 5,5'-bis(N,N-diaryl-4-[ethen-1-yl]-aniline)-2,2'-bipyridine or 1,10-bis(N,N-diaryl-4-[ethen-1-yl]-aniline)-phenanthroline), were synthesized and characterized by (1)H NMR spectroscopy, FAB mass spectrometry and satisfactory elemental analysis. Some of the complexes exhibited intense emissions in dichloromethane solution with maxima at 611-685 nm, which originated from both ligand-to-ligand charge transfer [pπ(SR(-)) → π*(diimine)] and intraligand charge transfer [π(triarylamine) → π*(diimine)] excited states. The emission maxima were tuned by variation of the donor or acceptor moieties. Thin film emission studies were also carried out on the complexes. All these complexes showed similar Gaussian-shaped emission bands with emission energies similar to those observed in dichloromethane solution at 298 K. In addition, the emission bands displayed concentration-dependent properties in thin-film emission studies.

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A number of adamantane-containing ruthenium(II) and rhenium(I) complexes have been synthesized, characterized, and noncovalently functionalized with ß-cyclodextrin-capped gold nanoparticles (ß-CD-GNPs) through the host-guest interaction between cyclodextrin and adamantane. The resultant nanoconjugates have been characterized by transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDX), and 2D ROESY (1) H NMR experiments. The Förster resonance energy transfer (FRET) properties of the nanoconjugates can be modulated by both esterase-accelerated hydrolysis and competitive displacement of steroid, by monitoring the emission intensity and luminescence lifetime. The FRET efficiencies are found to vary with the nature of the chromophores and the length of the spacer between the transition metal complexes and the GNPs. This work constitutes a "proof-of-principle" assay method for the dual-functional detection of important classes of biomolecules, such as enzymes and steroids.

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A bis-cyclometalated alkynylgold(III) complex, [Au((t)BuC^N^C(t)Bu)(C≡C-C6H4N(C6H5)2-p)] ((t)BuHC^N^CH(t)Bu = 2,6-bis(4-tert-butylphenyl)pyridine), has been synthesized and characterized. The complex was found to exhibit rich photophysical and electrochemical properties. More interestingly, the complex has been employed in the fabrication of organic memory devices. The as-fabricated memory devices exhibited good performances with low operating voltage, high ON/OFF ratio, long retention time, and good stability.

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A series of discrete decanuclear gold(I) µ(3)-sulfido complexes with alkyl chains of various lengths on the aminodiphosphine ligands, [Au(10){Ph(2)PN(C(n)H2(n+1))PPh(2)}(4)(µ(3)-S)(4)](ClO(4))(2), has been synthesized and characterized. These complexes have been shown to form supramolecular nanoaggregate assemblies upon solvent modulation. The photoluminescence (PL) colors of the nanoaggregates can be switched from green to yellow to red by varying the solvent systems from which they are formed. The PL color variation was investigated and correlated with the nanostructured morphological transformation from the spherical shape to the cube as observed by transmission electron microscopy and scanning electron microscopy. Such variations in PL colors have not been observed in their analogous complexes with short alkyl chains, suggesting that the long alkyl chains would play a key role in governing the supramolecular nanoaggregate assembly and the emission properties of the decanuclear gold(I) sulfido complexes. The long hydrophobic alkyl chains are believed to induce the formation of supramolecular nanoaggregate assemblies with different morphologies and packing densities under different solvent systems, leading to a change in the extent of Au(I)-Au(I) interactions, rigidity, and emission properties.

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A novel class of alkynylgold(III) complexes of the dianionic ligands derived from 2,6-bis(benzimidazol-2'-yl)pyridine (H2bzimpy) derivatives has been synthesized and characterized. The structure of one of the complexes has also been determined by X-ray crystallography. Electronic absorption studies showed low-energy absorption bands at 378-466 nm, which are tentatively assigned as metal-perturbed π-π* intraligand transitions of the bzimpy(2-) ligands. A computational study has been performed to provide further insights into the nature of the electronic transitions for this class of complexes. One of the complexes has been found to show gelation properties, driven by π-π and hydrophobic-hydrophobic interactions. This complex exhibited concentration- and temperature-dependent (1)H NMR spectra. The morphology of the gel has been characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

RESUMEN

A number of ruthenium(II) and rhenium(I) bipyridine complexes functionalized with lipoic acid moieties have been synthesized and characterized. Functionalization of gold nanoparticles with these chromophoric ruthenium(II) and rhenium(I) complexes has resulted in interesting supramolecular assemblies with Förster resonance energy transfer (FRET) properties that could be modulated via esterase hydrolysis. The luminescence of the metal complex chromophores was turned on upon cleavage of the ester bond linkage by esterase to reduce the efficiency of FRET quenching. The prepared nanoassembly conjugates have been characterized by transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), UV-visible spectroscopy, and emission spectroscopy. The quenching mechanism has also been studied by transient absorption and time-resolved emission decay measurements. The FRET efficiencies were found to vary with the nature of the chromophores and the length of the spacer between the donor (transition metal complexes) and the acceptor (gold nanoparticles).

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