A fluorescent paramagnetic Mn metal-organic framework based on semi-rigid pyrene tetra-carboxylic acid: sensing of solvent polarity and explosive nitroaromatics

Highly Efficient Detection of Pd2+ in Aqueous Medium by an Elusive Mn(II) Coordination Polymer

Herein, we report the synthesis of a Mn(II)-based coordination polymer (CP); and its structure, phase consistency and thermal stability have been established by single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD) and thermalgravimetric analysis (TGA) respectively. This is the first example of paramgnetic Mn(II)-based CP that acts as pH-dependent emitting material [λem=525 nm (pH=2.0-4.0) and 450 nm (pH=5.0-12.0)]. Its emission is quenched by Pd2+ in aqueous medium in presence of other thirteen cations with reasonably low pH-dependent limits of detection (LODs) [21.178 ppb (pH=3), 15.005 ppb (pH=7.0) and 59.940 ppb (pH=10.0)] as described by well-established mechanism. Therefore, urgency of such stable sensor remains high in regard to the environmental pollution.

Mechanoluminescence and aggregation-enhanced emission (AEE) of an In-MOF based on a 9,9'-diphenyl-9H-fluorene tetraacid linker

A water-stable In-MOF, constructed based on a conformationally-flexible tetraacid linker, i.e., 2,7-bis(3,5-dicarboxyphenyl)-9,9'-diphenyl-9H-fluorene, i.e., H4DPF, is shown to exhibit a significantly enhanced solid-state fluorescence quantum yield (φf) of 23% in comparison with that of the linker (φfca. 4%) as a consequence of rigidification of the latter by metalation. Application of external stimulus in the form of grinding of the In-MOF leads to a drastic enhancement by 29%, φf from 23 to 52%. Solid-state absorption and emission spectra show that the absorption in the region of 368-550 nm gets diminished with a concomitant change in the emission maximum with a blue shift upon grinding. Fluorescence enhancement with grinding is correlated with a gradual reduction in the size of the particles, as established by SEM analysis. MOF particle aggregation has been invoked to account for the observed fluorescence enhancement in addition to a subtle conformational change in the structure of the linker upon grinding. Intriguingly, the ground MOF particles exhibit aggregation behaviour in the DMF-water solvent system with the emission further increasing up to 75% for the increase in the water fraction (fw) from 0 to 60%; hydrophobic aggregation of particles evidently leads to a change in the conformation of the linker and particle aggregation-enhanced emission (AEE). De-aggregation of particles ensues for fw = 70-90%, as reflected by a gradual decrease in the emission intensity. It is shown that the suspension of ground In-MOF particles in water permits sensing of metal ions, in particular Al3+ ions, by fluorescence quenching with detection at a sub-ppb level. The observed results comprise first demonstration of both mechanoluminescence and AEE of MOF particles.

Confinement-guided photophysics in MOFs, COFs, and cages

In this review, the dependence of the photophysical response of chromophores in the confined environments associated with crystalline scaffolds, such as metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and molecular cages, has been carefully evaluated. Tunability of the framework aperture, cavity microenvironment, and scaffold topology significantly affects emission profiles, quantum yields, or fluorescence lifetimes of confined chromophores. In addition to the role of the host and its effect on the guest, the methods for integration of a chromophore (e.g., as a framework backbone, capping linker, ligand side group, or guest) are discussed. The overall potential of chromophore-integrated frameworks for a wide-range of applications, including artificial biomimetic systems, white-light emitting diodes, photoresponsive devices, and fluorescent sensors with unparalleled spatial resolution are highlighted throughout the review.

Pyrene-based metal organic frameworks: from synthesis to applications

Pyrene is one of the most widely investigated aromatic hydrocarbons given to its unique optical and electronic properties. Hence, pyrene-based ligands have been attractive for the synthesis of metal-organic frameworks (MOFs) in the last few years. In this review, we will focus on the most important characteristics of pyrene, in addition to the development and synthesis of pyrene-based molecules as bridging ligands to be used in MOF structures. We will summarize the synthesis attempts, as well as the post-synthetic modifications of pyrene-based MOFs by the incorporation of metals or ligands in the structure. The discussion of promising results of such MOFs in several applications; including luminescence, photocatalysis, adsorption and separation, heterogeneous catalysis, electrochemical applications and bio-medical applications will be highlighted. Finally, some insights and future prospects will be given based on the studies discussed in the review. This review will pave the way for the researchers in the field for the design and development of novel pyrene-based structures and their utilization for different applications.

Discerning Detection of Mutagenic Biopollutant TNP from Water and Soil Samples with Transition Metal-Containing Luminescence Metal-Organic Frameworks

Two luminescent MOFs, Mn@MOF and Cd@MOF, have been reported herein, which are capable of selectively detecting 2,4,6-trinitrophenol (TNP), one of the potent organic water pollutants in the class of mutagenic explosive nitroaromatic compounds (epNACs). It is perceived that the d¹⁰-based Cd(II)-constituting MOF shows a better response in the realm of TNP-like nitroaromatic sensing in comparison to the d⁵-based Mn@MOF which may possess lower electron density over the conjugated building blocks. The sensing competences of these chemosensors have been explored by means of various spectroscopic experimentations, and it is observed that for both d⁵ and d¹⁰-containing MOFs, the initial fluorescence intensity is significantly quenched in response to an aqueous solution of TNP. However, Cd@MOF is more selective and sensitive toward TNP over several other epNACs than Mn@MOF. The high chemical stability of the MOF samples, as well as its amusing sensing efficiency of Cd@MOF, further instigated to investigate the sensing ability in various environmental specimens like soil and water culled from several zones of West Bengal, India.

Temperature-Dependent Emission and Turn-Off Fluorescence Sensing of Hazardous "Quat" Herbicides in Water by a Zn-MOF Based on a Semi-Rigid Dibenzochrysene Tetraacetic Acid Linker

A zinc metal-organic framework, i.e., Zn-MOF (Zn-DBC), with ca. 27% solvent-accessible void volume was synthesized from a rationally designed tetraacid based on sterically insulated dibenzo[g,p]chrysene core; the latter inherently features concave shapes. Due to rigidification of the fluorophore in the MOF, Zn-DBC exhibits a respectable fluorescence quantum yield of ca. 30% in the solid state. The fluorescent and water-stable Zn-DBC MOF was found to display intriguing temperature-dependent emission behavior with an activation barrier of 1.06 kcal/mol for radiationless deactivation from the singlet-excited state. It is shown that the Zn-MOF can be employed as an efficient sensory material for detection of hazardous "quat" dicationic herbicides in water by diffusion-limited "turn-off" fluorescence. Due to confinement of the cationic guest analytes within the pores of the MOF, the fluorescence quenching via excited-state charge transfer mechanism is shown to depend on the molecular size of the analyte in addition to the redox potentials. Remarkably, Zn-DBC permits sensing of DQ, a well-known toxic "quat" herbicide, with a detection limit as low as 2.8 ppm in water. The unique structural attributes of the Zn-MOF for highly efficient fluorescence sensing of toxic herbicides in water are thus exemplified for the first time.

Recent Progress on Luminescent Metal-Organic Framework-Involved Hybrid Materials for Rapid Determination of Contaminants in Environment and Food

The high speed of contaminants growth needs the burgeoning of new analytical techniques to keep up with the continuous demand for monitoring and legislation on food safety and environmental pollution control. Metal-organic frameworks (MOFs) are a kind of advanced crystal porous materials with controllable apertures, which are self-assembled by organic ligands and inorganic metal nodes. They have the merits of large specific surface areas, high porosity and the diversity of structures and functions. Latterly, the utilization of metal-organic frameworks has attracted much attention in environmental protection and the food industry. MOFs have exhibited great value as sensing materials for many targets. Among many sensing methods, fluorometric sensing is one of the widely studied methods in the detection of harmful substances in food and environmental samples. Fluorometric detection based on MOFs and its functional materials is currently one of the most key research subjects in the food and environmental fields. It has gradually become a hot research direction to construct the highly sensitive rapid sensors to detect harmful substances in the food matrix based on metal-organic frameworks. In this paper, we introduced the synthesis and detection application characteristics (absorption, fluorescence, etc.) of metal-organic frameworks. We summarized their applications in the MOFs-based fluorometric detection of harmful substances in food and water over the past few years. The harmful substances mainly include heavy metals, organic pollutants and other small molecules, etc. On this basis, the future development and possible application of the MOFs have prospected in this review paper.

New perspective on the fluorescence and sensing mechanism of TNP chemosensor 2-(4,5-bis(4-chlorophenyl)-1H-imidazol-2-yl)-4-chlorolphenol

For TNP chemosensor 2-(4,5-Bis(4-Chlorophenyl)-1H-Imidazol-2-yl)-4-Chlorolphenol (HPICI), previous thought with no theoretical basis was that excited-state intramolecular proton transfer (ESIPT) process and the ground-state HPICI-TNP complex are mainly responsible for its fluorescence emission and the detection of TNP. However, this interpretation has been proved to be wrong by the present theoretical DFT/TDDFT explorations. Actually, the strong fluorescence of HPICI is mainly induced by the local excitation of the enol form HPICI(E) without ESIPT, and the fluorescence quenching by TNP is due to the photo-induced electron transfer (PET) process together with the cooperative effect of hydrogen-bonding interaction and π-π stacking interaction coexisting in the HPICI-TNP complex. The strengthened excited-state hydrogen bond promotes the PET process, thus facilitates the fluorescence quenching. This mechanism is proposed on the basis of the theoretical analyses on molecule geometry, binding energy, Gibbs free energy, electronic transitions, and frontier molecular orbitals (FMOs).

In Situ Growth of Metal-Organic Frameworks in Three-Dimensional Aligned Lumen Arrays of Wood for Rapid and Highly Efficient Organic Pollutant Removal

Organic contaminants in water have become one of the most serious environmental problems worldwide. Adsorption is one of the most promising approaches to remove organic pollutants from water. However, the existing adsorbents have relatively low removal efficiency, complex preparation processes, and high cost, which limit their practical applications. Here, we developed three-dimensional (3D) zirconium metal-organic frameworks (MOFs) encapsulated in a natural wood membrane (UiO-66/wood membrane) for highly efficient organic pollutant removal from water. UiO-66 MOFs were in situ grown in the 3D low-tortuosity wood lumens by a facile solvothermal strategy. The resulting UiO-66/wood membrane contains the highly mesoporous UiO-66 MOF structure as well as many elongated and open lumens along the direction of the wood growth. Such a unique structural feature improves the mass transfer of organic pollutants and increases the contact probability of organic contaminants with UiO-66 MOFs as the water flows through the membrane, thereby improving the removal efficiency. Furthermore, the integrated multilayer filter consisting of three pieces of UiO-66/wood membranes exhibits a high removal efficiency (96.0%) for organic pollutants such as rhodamine 6G, propranolol, and bisphenol A at the flux of 1.0 × 10³ L·m^-2·h^-1. The adsorbed capacity of UiO-66/wood for Rh6G (based on the content of UiO-66 MOFs) is calculated to be 690 mg·g^-1. We believe that such low-cost and scalable production of the UiO-66/wood membrane has broad applications for wastewater treatment and other related pollutant removal.

Fluorescent 2D metal-organic framework nanosheets (MONs): design, synthesis and sensing of explosive nitroaromatic compounds (NACs)

2D metal-organic framework nanosheets (MONs) lie at the heart of contemporary research on metal-organic materials. We have rationally designed and synthesized a fluorescent 6-connecting hexaacid linker H₆TPA based on the 1,3,5-triphenylbenzene core to access layered MOFs by metal-assisted self-assembly. Treatment of H₆TPA with In³⁺ salt does indeed lead to a layered porous MOF, i.e.In-TPA. It is shown that the ultrasonication-induced liquid phase of exfoliation (UILPE) of the latter in a top-down fashion affords few-layer 2D metal-organic nanosheets (MONs). The delamination of 2D MONs in ethanol occurs with 'turn-on' fluorescence, which is otherwise suppressed in the bulk material. The exfoliated MONs in ethanol exhibit blue fluorescence with a respectable quantum yield of 0.15 and serve as efficient sensory materials for 'turn-off' fluorescence detection of explosive nitroaromatic compounds (NACs). While all nitroaromatics are found to quench the fluorescence of MONs, the most electron-deficient trinitrotoluene (TNT) exhibited the highest efficiency; at 2 mM concentration of TNT, the fluorescence of 2D MONs was found to be quenched with an efficiency of 80% (permitting the detection of TNT at ca. 11 ppm level), while that of the unmetallated linker, i.e.H₆TPA, was quenched with only 5% efficiency. The unique attributes of MONs, namely the rigidity of the linker upon metallation and porosity that facilitates guest confinement within the pores, for efficient sensing of nitroaromatics are thus demonstrated for the first time.

Bicomponent β-sheet assembly of dipeptide fluorophores of opposite polarity and sensitive detection of nitro-explosives

Fluorescent hydrogels formed by the bicomponent β-sheet co-assembly of dipeptide–pyrene amphiphiles of opposite polarity provide a 3D microenvironment to detect toxic nitro-explosives.

Synthesis, structure and characterization of two solvatochromic metal–organic frameworks for chemical-sensing applications

Two isostructural Zr and Hf metal–organic frameworks with doubly interpenetrated fcu-c topology, exhibits obvious solvatochromic behaviour for solvent sensing application.

Anionic merocyanine dyes based on thiazol-2-hydrazides: reverse solvatochromism, preferential solvation and multiparametric approaches to spectral shifts

Reverse solvatochromism, preferential solvation and multiparametric approaches to spectral shifts of (4-nitro/cyanophenyl)-substituted thiazol-2-hydrazide colored merocyanine dyes are presented.

Synthesis, structure, and fluorescence properties of a calcium-based metal–organic framework

The solvothermal reaction of a mixture of calcium acetylacetonate and 1,4-naphthalenedicarboxylic acid (H₂NDC) in a solution containing ethanol and distilled water gave rise to a metal–organic framework (MOF), {(H₃O⁺)₂[Ca(NDC)(C₂H₅O)(OH)]}₄·1.1H₂O. This MOF possesses a new structure composed of calcium clusters and H₂NDC linker anions and shows a unique fluorescence property; it exhibits a fluorescence peak at 395 nm (λ_ex = 350 nm) at room temperature, which is blue-shifted compared with that exhibited by the free H₂NDC ligand. One of the possible mechanisms for this fluorescence is likely attributable to a ligand-to-metal charge transfer (LMCT) transition and is the first example of a calcium-based MOF exhibiting blue-shifted fluorescence due to LMCT.

The solvothermal reaction of a mixture of calcium acetylacetonate and 1,4-naphthalenedicarboxylic acid (H₂NDC) in a solution containing ethanol and distilled water gave rise to a metal–organic framework (MOF), {(H₃O⁺)₂[Ca(NDC)(C₂H₅O)(OH)]}₄·1.1H₂O.

Luminescent Zn(ii) coordination polymers as efficient fluorescent sensors for highly sensitive detection of explosive nitroaromatics

Zn CPs 1–3 exhibit strong blue/cyan fluorescence emissions in toluene suspension for highly selective and sensitive detection of explosive nitroaromatics through remarkable fluorescence quenching responses. In addition, the framework Zn²⁺ ions in 3 were partially substituted by Cu²⁺ ions via a single-crystal to single-crystal (SCSC) transformation.

Selective Sensing of Metal Ions and Nitro Explosives by Efficient Switching of Excimer-to-Monomer Emission of an Amphiphilic Pyrene Derivative

An amphiphilic pyrene derivative exhibiting unusually stable excimer emission due to strong aggregation is presented. The aggregated system served as an intelligent sensor for metal ions and nitro explosives in aqueous media. The excimer displayed excellent selectivity toward Cu²⁺ among the tested cations. The observation was interpreted on the basis of chelation of metal ions involving the hydroxyl and amino groups of two molecules, leading to the ligand-to-metal charge-transfer (CT) process. The excimer was further applied for the cell imaging of Cu²⁺ ions. Also, while treating the excimer with various nitro explosives, it displayed efficient 2,4,6-trinitrophenol sensing, corroborating mainly the CT process from pyrene to the analyte due to intercalation of the analyte within pyrene.

Remarkably selective and enantiodifferentiating sensing of histidine by a fluorescent homochiral Zn-MOF based on pyrene-tetralactic acid

A highly luminescent and water-stable homochiral Zn-MOF, i.e., Zn-PLA, developed based on pyrene-tetraacetic, selectively senses histidine amongst all other amino acids and also differentiates d and l isomers, as revealed by quenching of the fluorescence of its aqueous suspension.

A highly luminescent and water-stable homochiral Zn-MOF, i.e., Zn-PLA, has been developed based on a pyrene-tetralactic acid, which inherently features concave shapes for guest inclusion, to explore sensing of amino acids by fluorescence quenching; the solid-state fluorescence quantum yield of the MOF was found to be 46%. The fluorescence of an aqueous suspension of Zn-PLA was shown to be quenched specifically by histidine amongst all the other amino acids. Selective sensing of histidine is of prime importance due to its relevance in a variety of biological functions. The lack of quenching of fluorescence of Zn-PLA by all the amino acids other than histidine has been rationalized based on the exchange of the cationic dimethylammonium species in the MOF crystals with histidine that is protonated in water; the latter is envisaged to quench the fluorescence via charge transfer in the excited state. Furthermore, the homochiral crystals of Zn-PLA were found to permit enantiodiscrimination in the quenching by the d- and l-forms such that the ratio of enantioselectivity, i.e., K_d/K_l, is 1.8, as determined by Stern–Volmer quenching plots. The highly selective as well as enantiodifferentiating sensing of amino acids by MOFs is unprecedented for any sensor type.