Metal-Organic Framework Based Sensors for Benzene Vapor

Sensing of benzene vapor is a hot spot due to the volatile drastic carcinogen even at trace concentration. However, achieving convenient and rapid detection is still a challenge. As a sort of functional porous material, metal-organic frameworks (MOFs) have been developed as detection sensors by adsorbing benzene vapor and converting it into other signals (fluorescence intensity/wavelength, chemiresistive, weight or color, etc.). Supramolecular interaction between benzene molecules and the host framework, aperture size/shape and structural flexibility are influential factors in the performance of MOF-based sensors. Therefore, enhancing the host-guest interactions between the host framework and benzene molecules, or regulating the diffusion rate of benzene molecules by changing the aperture size/shape and flexibility of the host framework to enhance the detection signal are effective strategies for constructing MOF-based sensors. This concept highlights several types of MOF-based sensors for the detection of benzene vapor.

Quasi-open Cu(I) sites for efficient CO separation with high O2/H2O tolerance

Carbon monoxide (CO) separation relies on chemical adsorption but suffers from the difficulty of desorption and instability of open metal sites against O2, H2O and so on. Here we demonstrate quasi-open metal sites with hidden or shielded coordination sites as a promising solution. Possessing the trigonal coordination geometry (sp2), Cu(I) ions in porous frameworks show weak physical adsorption for non-target guests. Rational regulation of framework flexibility enables geometry transformation to tetrahedral geometry (sp3), generating a fourth coordination site for the chemical adsorption of CO. Quantitative breakthrough experiments at ambient conditions show CO uptakes up to 4.1 mmol g-1 and CO selectivity up to 347 against CO2, CH4, O2, N2 and H2. The adsorbents can be completely regenerated at 333-373 K to recover CO with a purity of >99.99%, and the separation performances are stable in high-concentration O2 and H2O. Although CO leakage concentration generally follows the structural transition pressure, large amounts (>3 mmol g-1) of ultrahigh-purity (99.9999999%, 9N; CO concentration < 1 part per billion) gases can be produced in a single adsorption process, demonstrating the usefulness of this approach for separation applications.

Fluorescence Enhancement of a Metal-Organic Framework for Ultra-Efficient Detection of Trace Benzene Vapor

Indoor detection of volatile organic compounds (VOCs) concentration is necessary due to the serious toxicity hazards even at trace level. However, physisorbents usually exhibit weak interactions especially in the presence of trace concentrations of VOCs, thus exhibiting poor responsive signal. Herein, we report a new flexible metal-organic framework (MOF) that exhibits interesting pore-opening behavior after immersing in H2O. The pore-opening phase shows significant (~116 folds) and extremely fast (< 1 minute) fluorescence enhancement after being exposed to saturated benzene vapor. The limit of detection concentration for benzene vapor can be calculated as 0.133 mg L-1. Thus this material represents the first MOF to achieve visual detection of trace benzene vapor by the naked eyes. Theoretical calculations and single-crystal structure reveal that the special "bilateral π-π stacking" interactions between the host and guest, which facilitate electron transfer and greatly enhance the intensity of fluorescence.

A series of naphthalenediimide-based metal-organic frameworks: synthesis, photochromism and inkless and erasable printing

Three new three-dimensional metal-organic frameworks were synthesized based on a naphthalenediimide derivative ligand, all of which exhibit photochromic behaviour due to the presence of the naphthalenediimide core. Interestingly, two of them possess significant colour changes under light, excellent stability, and appropriate photochromic lifetimes, thus showing potential for application in inkless and erasable printing media.

Photochromic Metal-Organic Framework for High-Resolution Inkless and Erasable Printing

Inkless and erasable printing as a new technology has received intense attention in reducing paper waste and environmental hazards caused by the use of large amounts of ink. However, achieving high-resolution printing by inkless and erasable printing for practical applications remains a huge challenge. Herein, a new metal-organic framework (MOF) has been synthesized, which exhibits a reversible photochromic behavior. None of the unpaired electrons of metal ions and a unique three-dimensional network hinder electron transfer between the ligands and metal nodes, as well as between the ligands themselves, which are conducive to prolonging the photo-generated color lifetime and suitable for inkless and erasable printing. By virtue of the proper photo-generated color lifetime, strong contrast color before and after light irradiation, and reversible color transformation, a high-resolution printing content for inkless and erasable printing can be achieved by light irradiation. Notably, the paper coated with this MOF can be used for printing not only simple patterns such as pictures but also even texts for practical applications, surpassing other photochromic MOF materials for inkless and erasable printing, and almost comparable to ink and laser printing in terms of practicality and resolution. In addition, the MOF-coated paper can be reused for multiple cycles without significant deterioration.

A porphyrin-based metal-organic framework with highly efficient adsorption and photocatalytic degradation of organic dyes

Organic dye pollution has become an urgent issue due to their toxicity to humans and potential for damage to the environment. However, achieving highly efficient adsorption and degradation materials for...

Intramolecular charge transfer ampholytes with water-induced pendulum-type fluorescence variation

Triphenylimidazole-based ampholytes with intramolecular charge transfer were designed with the introduction of carboxyl groups. In solution, the synergistic solvent and ionization effects on the ampholytes led to a unique pendulum-type fluorescence variation during the water content increasing process. Among them, 4-(4,5-bis(4-hydroxyphenyl)-1H-imidazol-2-yl)benzoic acid showed the most prominent three-step fluorescence switching property.

Flexibility of Metal-Organic Framework Tunable by Crystal Size at the Micrometer to Submillimeter Scale for Efficient Xylene Isomer Separation

Understanding, controlling, and utilizing the flexibility of adsorbents are of great importance and difficulty. Analogous with conventional solid materials, downsizing to the nanoscale is emerging as a possible strategy for controlling the flexibility of porous coordination polymers (or metal-organic frameworks). We report a unique flexibility controllable by crystal size at the micrometer to submillimeter scale. Template removal transforms [Cu₂(pypz)₂]·0.5p-xylene (MAF-36, Hpypz = 4-(1H-pyrazol-4-yl)pyridine) with one-dimensional channels to α-[Cu₂(pypz)₂] with discrete small cavities, and further heating gives a nonporous isomer β-[Cu₂(pypz)₂]. Both isomers can adsorb p-xylene to give [Cu₂(pypz)₂]·0.5p-xylene, meaning the coexistence of guest-driven flexibility and shape-memory behavior. The phase transition temperature from α-[Cu₂(pypz)₂] to β-[Cu₂(pypz)₂] decreased from ~270°C to ~150°C by increasing the crystal size from the micrometer to the submillimeter scale, ca. 2-3 orders larger than those of other size-dependent behaviors. Single-crystal X-ray diffraction showed coordination bond reconstitution and chirality inversion mechanisms for the phase transition, which provides a sufficiently high energy barrier to stabilize the metastable phase without the need of downsizing to the nanoscale. By virtue of the crystalline molecular imprinting and gate-opening effects, α-[Cu₂(pypz)₂] and β-[Cu₂(pypz)₂] show unprecedentedly high p-xylene selectivities of 16 and 51, respectively, as well as ultrafast adsorption kinetics (<2 minutes), for xylene isomers.

Intermediate-sized molecular sieving of styrene from larger and smaller analogues

Molecular sieving can lead to ultrahigh selectivity and low regeneration energy because it completely excludes all larger molecules via a size restriction mechanism. However, it allows adsorption of all molecules smaller than the pore aperture and so separations of complicated mixtures can be hindered. Here, we report an intermediate-sized molecular sieving (iSMS) effect in a metal-organic framework (MAF-41) designed with restricted flexibility, which also exhibits superhydrophobicity and ultrahigh thermal/chemical stabilities. Single-component isotherms and computational simulations show adsorption of styrene but complete exclusion of the larger analogue ethylbenzene (because it exceeds the maximal aperture size) and smaller toluene/benzene molecules that have insufficient adsorption energy to open the cavity. Mixture adsorption experiments show a high styrene selectivity of 1,250 for an ethylbenzene/styrene mixture and 3,300 for an ethylbenzene/styrene/toluene/benzene mixture (orders of magnitude higher than previous reports). This produces styrene with a purity of 99.9%+ in a single adsorption-desorption cycle. Controlling/restricting flexibility is the key for iSMS and can be a promising strategy for discovering other exceptional properties.

Partially Fluorinated Cu(I) Triazolate Frameworks with High Hydrophobicity, Porosity, and Luminescence Sensitivity

Solvothermal reactions of 3-methyl-5-trifluoromethyl-1,2,4-triazole (Hfmtz) with Cu(CH₃COO)₂ at 120 °C in the presence of Cl^- generate two partially fluorinated coordination polymers: i.e., [Cu₄Cl(fmtz)₃] (1 or MAF-51) and [Cu₇Cl(fmtz)₆] (2 or MAF-52). Single-crystal X-ray diffraction revealed 1 to have a three-dimensional (3D) nonporous structure with pcu topology consisting of 6-connected Cu₄(μ₄-Cl) clusters and 2 to possess a highly porous (void ratio 48%) 3D bnn network consisting of 5-connected Cu₅(μ₅-Cl) clusters. Benefiting from the hydrophobic pendant groups, complete coordination of the ligand N atoms, and strong M-N coordination bonds, 1 and 2 possess high water stability (exposed to water for at least 1 year) and hydrophobicity (water contact angles of 141° and 148°, respectively). The N₂ sorption isotherm of activated 2 gave Langmuir/BET surface areas of 1023/848 m² g^-1 and a pore volume of 0.365 cm³ g^-1. Moreover, 2 can adsorb large amounts of benzene and methanol but barely adsorb water. Both 1 and 2 show phosphorescence of Cu(I) complexes, but only that of porous 2 is sensitive to O₂, showing a linear Stern-Volmer response below 1 mbar with an ultrahigh K_sv value of 5234 bar^-1 and ultralow limit of detection of 1.9 ppm.

Single-side and double-side swing behaviours of a flexible porous coordination polymer with a rhombic-lattice structure

A rhombic porous coordination polymer can show special reversible–irreversible structural transformations with single-side and double-side swing, as well as sophisticated bond reconstitution behaviours.

Direct synthesis of an aliphatic amine functionalized metal–organic framework for efficient CO2 removal and CH4 purification

An aliphatic amine functionalized MOF was directly synthesized for CO₂ adsorption and CH₄ purification.

Mesoporous Metal-Organic Frameworks with Exceptionally High Working Capacities for Adsorption Heat Transformation

Pore size is one of the most important parameters of adsorbents, and mesoporous materials have received intense attention for large guests. Here, a series of mesoporous coordination polymers underlying a new framework prototype for fast expansion of pore size is reported and the profound effect of pore size on adsorption heat transformation is demonstrated. Three isostructural honeycomb-like frameworks are designed and synthesized by combining ditopic linear metal oxalate chains and triangular tris-pyridine ligands. Changing the ligand bridging length from 5.5 to 8.6 and 9.9 Å gives rise to effective pore diameter from 20 to 33 and 37 Å, surface area from 2096 to 2630 and 2749 m² g^-1 , and pore volume from 1.19 to 1.93 and 2.36 cm³ g^-1 , respectively. By virtue of the unique and tunable isotherm shape of mesopores, exceptionally large working capacity up to 1.19 g g^-1 or 0.38 g cm^-3 for adsorption heat transformation can be achieved using R-134a (1,1,1,2-tetrafluroethane) as a working fluid.

Cobalt(II) Magnetic Metal-Organic Framework with an Effective Kagomé Lattice, Large Surface Area, and High Spin-Canted Ordering Temperature

To make a porous material with high magnetic ordering temperature is challenging because the low density of the material is adverse to the dense magnetic moment, a prerequisite to high-performance magnets. Herein, we report a hollow magnetic metal-organic framework (MMOF) [Co₃(bpdc)₃(tpt)_0.66] 1 (H₂bpdc = 4,4'-biphenyldicarboxylic acid) with a Langmuir surface area of 1118 m²/g and spin-canted long-range magnetic ordering up to 22 K. Such a high performance is owing to the unique antiferromagnetic Kagomé lattice made of ferromagnetic Co₃ clusters and conjugated 2,4,6-tri(4-pyridinyl)-1,3,5-triazine (tpt) ligands, which is closely coupled with each other via double-interpenetration of the porous networks. Moreover, a parameter defined as the product of magnetic ordering/blocking temperature and the surface area for measuring the performance of porous molecular magnets is proposed.

Mixed-Lanthanide Porous Coordination Polymers Showing Range-Tunable Ratiometric Luminescence for O2 Sensing

Luminescent porous coordination polymers (PCPs) are emerging as attractive oxygen-sensing materials, but they are mostly based on single-wavelength luminometry. Here, we report a special mixed-lanthanide strategy for self-referenced ratiometric oxygen sensing. A series of isostructural, pure-lanthanide, or mixed-lanthanide PCPs, MCF-53(Tb/Eu_x), were synthesized by solvothermal reactions. Single-crystal X-ray diffraction revealed that MCF-53(Tb/Eu_x) is composed of complicated two-dimensional coordination networks, which interdigitate to form a three-dimensional supramolecular structure retaining one-dimensional ultra-micropores. MCF-53(Tb/Eu_x) can undergo multiple single-crystal to single-crystal structural transformations upon desorption/adsorption of coordinative and lattice guest molecules, and the lanthanide metal ions are partially exposed on the pore surface at the guest-free state. Tb(III) ions are not luminescent and only act as separators between Eu(III) ions, and the Tb(III)/Eu(III) mixing ratio can tune the relative emission intensities, luminescence lifetimes of the Eu(III) phosphorescence, and the ligand fluorescence, giving rise to not only ratiometric photoluminescence oxygen sensing but also tunable emission-color-changing ranges.

Diverse coordination polymers from a new bent dipyridyl-type ligand 3,6-di(pyridin-4-yl)-9H-carbazole

In the presence of different anions, Zn(ii) and a new bent dipyridyl-type ligand self-assemble to form unique structures.

High-symmetry hydrogen-bonded organic frameworks: air separation and crystal-to-crystal structural transformation

A columnar supramolecular structure can reversibly transform to a porous HOF, which can selectively adsorb O₂ over Ar and N₂.