Thermally activated structural phase transitions and processes in metal-organic frameworks

The structural knowledge of metal-organic frameworks is crucial to the understanding and development of new efficient materials for industrial implementation. This review classifies and discusses recent advanced literature reports on phase transitions that occur during thermal treatments on metal-organic frameworks and their characterisation. Thermally activated phase transitions and procceses are classified according to the temperaturatures at which they occur: high temperature (reversible and non-reversible) and low temperature. In addition, theoretical calculations and modelling approaches employed to better understand these structural phase transitions are also reviewed.

Flexible luminescent non-lanthanide metal-organic frameworks as small molecules sensors

Toxic, carcinogenic, and hazardous materials are omnipresent, generally obtained by anthropogenic activities, industrial activities, aerobic and anaerobic degradation of waste materials and are harmful to human health and environment. Thus, sensing, colorimetric detection, and subsequent inclusion of these chemicals are of prime importance for human health and environment. In comparison to other classes of highly porous materials, luminescent metal-organic frameworks (LMOFs) have chromophoric organic ligands, high surface area, high degree of tunability and structural diversity. They have received scientific interest as sensory materials for device fabrication to detect and sense toxic small molecules. Especially, as soft-porous materials exhibiting a degree of flexibility or dynamic behaviour, flexible LMOFs are promising for selective detection and sensing, and for encapsulation of toxic and health hazardous molecules. Such flexible LMOFs offer a potential platform for selective adsorption/separation, molecular recognition, and sensing application. In this perspective, we highlight the advantages of flexibility of LMOFs for selective detection and sensing, and inclusion of toxic small molecules (solvents, anions, halobenzenes, aromatics, aromatic amines, nitro-explosives and acetylacetone). In addition, the principles and strategies guiding the design of these MOF based materials and recent progress in the luminescent detection of toxic small molecules are also discussed. In this perspective we limit our discussion on the 'non-lanthanide' based luminescent MOFs that have flexibility in the framework and show small molecule sensing applications.

Preparation of a DNA-Tb-MOF conjugate as a time-resolved probe for the detection of SO2 derivatives through an off-on effect

Herein we synthesize a DNA-sensitized Tb-MOF conjugate (DNA-Tb-MOF) as a time-resolved luminescent probe to sensitively and selectively assay SO₂ and their derivatives (i.e., HSO₃^-) through a photoluminescence off-on effect. The charge and energy transfer mechanism enables the demonstration of the effect of the photoluminescence turn-on which results from the reaction between the amino group of the DNA-Tb-MOF conjugate and SO₂/HSO₃^-. The results demonstrate that the DNA-Tb-MOF conjugate probe can sense SO₂ and their derivatives (i.e., HSO₃^-) with a detection limit of 0.02 ppm. Moreover, the photoluminescence off-on effect can be observed even by the naked eye.

Monometallic and bimetallic Cu–Ag MOF/MCM-41 composites: structural characterization and catalytic activity

Monometallic and bimetallic MOF/MCM-41 composites (Cu, Ag and Cu–Ag) were synthesized via a solvothermal method. The synthesized composites were characterized by XRD, FTIR, SEM, EDX and BET surface area measurements. The acidity was determined through two techniques; potentiometric titration with n-butyl amine for determining the strength and the total number of acid sites and FTIR spectra of chemisorbed pyridine on the surface of MOFs for determining the type of acid sites (Brønsted and/or Lewis). All the prepared MOFs showed Lewis-acid sites and the higher acidity was observed for the bimetallic Cu–Ag MOF/MCM-41 composite. The catalytic activity was examined on the synthesis of 1-amidoalkyl-2-naphthol via the reaction of benzaldehyde, 2-naphthol and benzamide. The best yield (92.86%) was obtained in the least time (10 min) with a molar ratio 1.2 : 1.2 : 1.7 of benzaldehyde : β-naphthol : benzamide and 0.1 g bimetallic Cu–Ag MOF/MCM-41 composite under solvent-free conditions at 130 °C. Reuse of the catalysts showed that they could be used at least four times without any reduction in the catalytic activity.

Monometallic and bimetallic MOF/MCM-41 composites (Cu, Ag and Cu–Ag) were synthesized via a solvothermal method.

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.

Design of cost-efficient and photocatalytically active Zn-based MOFs decorated with Cu2O nanoparticles for CO2methanation

Here we show for the first time a MOF that is photocatalytically active for light-assisted CO₂methanation under mild conditions (215 °C), which can be further improved with the inclusion of metallic nanoparticles.

Solvent-Triggered Reversible Phase Changes in Two Manganese-Based Metal-Organic Frameworks and Associated Sensing Events

A flexible Mn-based MOF, Mn-sdc-1, has been successfully synthesized by using the ligand 4,4'-stilbenedicarboxylic acid (H₂ sdc). Attributed to the flexibility of the framework, Mn-sdc-1 can transform into a new phase (Mn-sdc-2) with completely different structural geometry; this is induced by trace levels of H₂ O at room temperature. Reversibly, the transformation from Mn-sdc-2 to Mn-sdc-1 can be triggered by DMF upon heating beyond 100 °C. These results inspired a study of the influences of temperature and H₂ O volume in the solid-state transformation of two MOF phases and, for the first time, a phase diagram of MOFs has been depicted. This phase diagram reflects the gradual H₂ O-/temperature-dependent changes between Mn-sdc-1 and Mn-sdc-2, which is very meaningful in achieving the controllable synthesis of these two MOFs and lead to targeting the desired water-stable structure. As a result, the obtained water-stable Mn-sdc-2 can be developed as an excellent Pb²⁺ sensor in aqueous solution through the luminescence quenching effect with a limit of detection of 31.4 nm.

Guest-Induced Switchable Breathing Behavior in a Flexible Metal-Organic Framework with Pronounced Negative Gas Pressure

Flexible metal-organic frameworks (MOFs) have attracted great interest for their dynamically structural transformability in response to external stimuli. Herein, we report a switchable "breathing" or "gate-opening" behavior associated with the phase transformation between a narrow pore (np) and a large pore (lp) in a flexible pillared-layered MOF, denoted as MOF-1 as, which is also confirmed by SCXRD and PXRD. The desolvated phase (MOF-1 des) features a unique stepwise adsorption isotherm for N₂ coupled with a pronounced negative gas adsorption pressure. For comparison, however, no appreciable CO₂ adsorption and gate-opening phenomenon with stepwise sorption can be observed. Furthermore, the polar micropore walls decorated with thiophene groups in MOF-1 des reveals the selective sorption of toluene over benzene and p-xylene associated with self-structural adjustment in spite of the markedly similar physicochemical properties of these vapor molecules.

Breathing Europium-Terbium Co-doped Luminescent MOF as a Broad-Range Ratiometric Thermometer with a Contrasting Temperature-Intensity Relationship

Solvothermal reactions of lanthanide salts and a semirigid tripodal H₃tatab (4,4',4″-s-triazine-1,3,5-triyltri-p-aminobenzoic acid) ligand in mixed water and N-methyl pyrrolidone (NMP) generated novel breathing MOFs [Ln(tatab)]·solvent (Ln = Eu in 1-Eu, Tb in 1-Tb, Eu_0.015Tb_0.985 in 1-Eu_0.015Tb_0.985). The framework of 1 was contracted upon removal of guests to form partly desolvated [Ln (tatab)]·3.7H₂O·2.5NMP (1'). Single-crystal X-ray analyses demonstrated that 1 has the breathing ability to spontaneously release guests and maintain the same topology. In contrast to as-synthesized 1, the cell volume of 1' decreased markedly upon removal of the guests. Different from linear dicarboxylates, the semirigid tripodal tatab ligand is bridged to an inorganic Ln-O chain, limiting the rotation around the O-O-axis of carboxylate. The breathing mechanism is based on the flexible C-N-C angles of amide bonds in the tatab ligand, causing a change in the solvent-accessible volume in the framework. Interestingly, the luminescence color of breathing co-doped lanthanide MOF 1'-Eu_0.015Tb_0.985 is blue-shifted and turned from orange to green with an increase in temperature, which can be attributed to a change in the relative intensity of Tb and Eu emissions, and is quite different from that observed for the reported related compounds. The breathing co-doped lanthanide MOF 1'-Eu_0.015Tb_0.985 can be applied as a high-sensitivity ratiometric thermometer in a broad temperature from 90 to 300 K.

A heterobimetallic metal–organic framework as a “turn-on” sensor toward DMF

It is for the first time that a non-luminescent 3d–4f heterobimetallic CuEu organic framework (NBU-8) realized specific selective light recovery of Eu³⁺ ions toward N,N′-dimethylformamide (DMF) even in the presence of other amide molecules with similar functions.

Q-Graphene-loaded metal organic framework nanocomposites with water-triggered fluorescence turn-on: fluorimetric test strips for directly sensing trace water in organic solvents

Q-Graphene-loaded MOF nanocomposites were coated onto test strips with H₂O-triggered fluorescence turn-on for probing H₂O in organic solvents.

Controllable synthesis of isoreticular pillared-layer MOFs based on N-rich triangular prism building units: gas adsorption and luminescent properties

Isoreticular 1–4 MOFs with rare lon topologies have been rationally synthesized based on novel trigonal-prism building units.

Effect of ring rotation upon gas adsorption in SIFSIX-3-M (M = Fe, Ni) pillared square grid networks

Dynamic and flexible metal-organic frameworks (MOFs) that respond to external stimuli, such as stress, light, heat, and the presence of guest molecules, hold promise for applications in chemical sensing, drug delivery, gas separations, and catalysis. A greater understanding of the relationship between flexible constituents in MOFs and gas adsorption may enable the rational design of MOFs with dynamic moieties and stimuli-responsive behavior. Here, we detail the effect of subtle structural changes upon the gas sorption behavior of two "SIFSIX" pillared square grid frameworks, namely SIFSIX-3-M (M = Ni, Fe). We observe a pronounced inflection in the Xe adsorption isotherm in the Ni variant. With evidence from X-ray diffraction studies, density functional theory, and molecular simulations, we attribute the inflection to a disordered to ordered transition of the rotational configurations of the pyrazine rings induced by sorbate-sorbent interactions. We also address the effect of cage size, temperature, and sorbate on the guest-induced ring rotation and the adsorption isotherms. The absence of an inflection in the Xe adsorption isotherm in SIFSIX-3-Fe and in the Kr, N2, and CO2 adsorption isotherms in SIFSIX-3-Ni suggest that the inflection is highly sensitive to the match between the size of the cage and the guest molecule.

An uncommon 3D 3,3,4,8-c Cd(ii) metal–organic framework for highly efficient luminescent sensing and organic dye adsorption: experimental and theoretical insight

A stable 3D Cd(ii) MOF used as chemosensor and adsorbent for dyes in wastewater, which exhibited excellent adsorption performance and selectivity.

Metal–organic frameworks: functional luminescent and photonic materials for sensing applications

This review summarizes the diverse routes to derive sensing applications from suitably functionalized and crystal-engineered metal–organic framework (MOF) materials, either by fluorometric responses, or based on photonic crystal-based signal transduction.

Chemical Sensors Based on Metal-Organic Frameworks

Metal-organic frameworks (MOFs) as chemical sensors have developed rapidly in recent years. There have been many papers concerning this field and interest is still growing. The reason is that the specific merits of MOFs can be utilized to enhance sensitivity and selectivity by various energy/charge transfers occurring among different ligands, ligand, and metal centers, such as from ligands to metal centers or metal centers to ligands, as well as from MOF skeletons to guest species. This review intends to provide an update on recent progress in various applications of different MOF-based sensors on the basis of their luminescent and electrochemical responses towards small molecules, gas molecules, ions (cations and anions), pH, humidity, temperature, and biomolecules. MOF-based sensors function by utilizing different mechanisms, including luminescent responses of "turn-on" and "turn-off", as well as electrochemical responses.

A series of Zn(ii) and Cd(ii) coordination compounds based on 4-(4H-1,2,4-triazol-4-yl)benzoic acid: synthesis, structure and photoluminescence properties

Seven d¹⁰ coordination compounds based on a triazolate–carboxylate bifunctional ligand have been synthesized and structurally characterized. Their thermal stability and photoluminescence properties were also investigated.

A luminescent metal–organic framework for highly selective sensing of nitrobenzene and aniline

A luminescent metal–organic frameworkgea-MOF-1, can act as a fluorescent sensor for sensitive and selective detection of nitrobenzene and aniline. More importantly, the detection limit of nitrobenzene is on the 5–60 ppm scale.

A Zn based coordination polymer exhibiting long-lasting phosphorescence

A coordination polymer consisting of Zn and 6-aminoisonicotinate shows an initial bright blue fluorescence that turns into persistent green phosphorescence.

Double-step CO2 sorption and guest-induced single-crystal-to-single-crystal transformation in a flexible porous framework

A new 2D highly flexible and breathing porous framework [CuL(Me2NH)]·DMF·H2O () (H3L = 5-(4'-carboxylphenoxy)nicotinic acid) has been synthesized using a tritopic linker with a flexible joint. The desolvated framework, [CuL(Me2NH)] (), undergoes structural contraction, and exhibits selective and double-step hysteretic adsorption for CO2. Furthermore, on exposure to CH2Cl2 at room temperature, a unique single-crystal-to-single-crystal transformation occurred between and [Cu2L2(Me2NH)2(H2O)2]·5H2O ().

Isoreticular synthesis of 2D MOFs with rotating aryl rings

A series of isoreticular two-dimensional metal–organic frameworks (MOFs) was synthesized with group 11 metals and semirigid ligands that bind in a syn conformation.

A series of homonuclear lanthanide coordination polymers based on a fluorescent conjugated ligand: syntheses, luminescence and sensor for pollutant chromate anion

Series of homonuclear lanthanide coordination polymers incorporating conjugated ligand have been fabricated successfully and characterized systematically.

Multiple single-crystal-to-single-crystal guest exchange in a dynamic 1D coordination polymer

We report on the multiple single-crystal-to-single-crystal exchange of chlorinated volatile organic compounds in a remarkably stable 1D coordination polymer possessing 1D channels.

Controlling the flexibility and single-crystal to single-crystal interpenetration reconstitution of metal–organic frameworks

By controlling the guest accessibility to open metal sites, MOFs can select interpenetration reconstitution or not.

Solvent modulated assembly of two Zn metal–organic frameworks: syntheses, luminescence, and gas adsorption properties

Two new MOFs based on H₂btca and zinc – [Zn(btca)(2,2′-bipy)]_n (1) and [Zn₂(btca)₂(bpy)(H₂O)]_n·n(DMA) (2) – have been designed and synthesized by modulating the solvent.