A Cationic Metal-Organic Framework Consisting of Nanoscale Cages: Capture, Separation, and Luminescent Probing of Cr2O72−through a Single-Crystal to Single-Crystal Process

Tunable Gas Admission via a "Molecular Trapdoor" Mechanism in a Flexible Cationic Metal-Organic Framework Featuring 1D Channels

Achieving high gas selectivity is challenging when dealing with gas pairs of similar size and physiochemical properties. The "molecular trapdoor" mechanism discovered in zeolites holds promise for highly selective gas adsorption separation but faces limitations like constrained pore volume and slow adsorption kinetics. To address these challenges, for the first time, a flexible metal-organic framework (MOF) featuring 1D channels and functioning as a "molecular trapdoor" material is intoduced. Extra-framework anions act as "gate-keeping" groups at the narrowest points of channels, permitting gas admissions via gate opening induced by thermal/pressure stimuli and guest interactions. Different guest molecules induce varied energy barriers for anion movement, enabling gas separation based on distinct threshold temperatures for gas admission. The flexible framework of Pytpy MOFs, featuring swelling structure with rotatable pyridine rings, facilitates faster gas adsorption than zeolite. Analyzing anion properties of Pytpy MOFs reveals a guiding principle for selecting anions to tailor threshold gas admission. This study not only overcomes the kinetic limitations related to gas admission in the "molecular trapdoor" zeolites but also underscores the potential of developing MOFs as molecular trapdoor adsorbents, providing valuable insights for designing ionic MOFs tailored to diverse gas separation applications.

Polyaniline-Based Cationic Porous Organic Polymers for Fast and Efficient Anion-Exchange-Driven Capture of Cr2O7 2

Efficient treatment of wastewater contaminated with carcinogenic Cr(VI) has been a long-term challenge for both academic and industrial research efforts. Removal of Cr(VI) species by ion exchange is a relatively simple and efficient method, and its combination with highly tailorable nanomaterials is promising for the treatment of such wastewater. Here, we report a type of cationic porous organic polymer (POP), namely, PTPA-PIP, which can be prepared simply by converting the corresponding aromatic polyamine PTPA to its protonated form, thereby significantly increasing its hydrophilicity and ability to disperse homogeneously in water, crucial for application in water treatment. In addition to detailed characterization of the physicochemical properties of PTPA-PIP (including using Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), and solid-state NMR techniques), adsorption experiments demonstrate that PTPA-PIP removes low-concentration dichromate anions with very high performance, including excellent exchange capacity (maximum capacity of 230 mg Cr2O7 2-/g PTPA-PIP), ultrafast removal (initial adsorption rate of 83 mg g-1 min-1), excellent selectivity (∼10% loss of adsorption capacity in the presence of 40-fold concentration of competing anions), as well as superior reusability (reusable for at least 5 cycles without compromised performance). These results demonstrate that PTPA-PIP is an outstanding candidate for application in industrial settings for the effective removal of harmful Cr(VI) pollutants in wastewater.

Three robust Cd(II) coordination polymers as bifunctional luminescent probes for efficient detection of pefloxacin and Cr2O72- in water

The accurate and rapid detection of antibiotics and heavy-metal-based toxic oxo-anions in water media employing coordination polymers (CPs) as luminescent probes has attracted a lot of attention. Three new Cd(II)-based ternary CPs derived from first-presented L ligands, including [Cd(DCTP)(L)(OH)]n (1), [Cd(TBTA)(L)(OH)]n (2), and [Cd(NPHT)(L)(H2O)]n (3) (L = 2-((1H-imidazol-1-yl)methyl)-5,6-dimethyl-1H-benzo[d]imidazole, H2DCTP = 2,5-dichloroterephthalic acid, H2TBTA = tetrabromoterephthalic acid and H2NPHT = 3-nitrophthalic acid), were successfully assembled and characterized. 1 and 2 show 2D hcb layers, which can be further extended into a 3D supramolecular framework via classic hydrogen bonding interactions. 3 features a 1D double chain that ultimately spreads into a 2D network through weak hydrogen bonding interactions. With the advantages of high stability and excellent luminescent properties, the three CPs display high sensitivity, selectivity, and good anti-interference for the sensing of pefloxacin (PEF) and Cr2O72- ions (LOD values toward PEF: 3.82 × 10-7 mol L-1 for 1, 4.06 × 10-7 mol L-1 for 2, and 1.36 × 10-8 mol L-1 for 3, and toward Cr2O72- ions: 5.97 × 10-7 mol L-1 for 1, 5.87 × 10-7 mol L-1 for 2, and 8.21 × 10-8 mol L-1 for 3). These CPs are the first examples of bifunctional luminescent sensors to detect PEF and Cr2O72- in aqueous solutions. The luminescence quenching mechanisms are explored in detail.

Sustainable lithium extraction enabled by responsive metal-organic frameworks with ion-sieving adsorption effects

Metal-organic frameworks (MOFs) are superior ion adsorbents for selectively capturing toxic ions from water. Nevertheless, they have rarely been reported to have lithium selectivity over divalent cations due to the well-known flexibility of MOF framework and the similar physiochemical properties of Li+ and Mg2+. Herein, we report an ion-sieving adsorption approach to design sunlight-regenerable lithium adsorbents by subnanoporous MOFs for efficient lithium extraction. By integrating the ion-sieving agent of MOFs with light-responsive adsorption sites of polyspiropyran (PSP), the ion-sieving adsorption behaviors of PSP-MOFs with 6.0, 8.5, and 10.0 Å windows are inversely proportional to their pore size. The synthesized PSP-UiO-66 with a narrowest window size of 6.0 Å shows high LiCl adsorption capacity up to 10.17 mmol g-1 and good Li+/Mg2+ selectivity of 5.8 to 29 in synthetic brines with Mg/Li ratio of 1 to 0.1. It could be quickly regenerated by sunlight irradiation in 6 min with excellent cycling performance of 99% after five cycles. This work sheds light on designing selective adsorbents using responsive subnanoporous materials for environmentally friendly and energy-efficient ion separation and purification.

A Water-Stable Cationic SIFSIX MOF for Luminescent Probing of Cr2 O7 2- via Single-Crystal to Single-Crystal Transformation

The sensing and monitoring of toxic oxo-anion contaminants in water are of significant importance to biological and environmental systems. A rare hydro-stable SIFSIX metal-organic framework, SiF6 @MOF-1, {[Cu(L)2 (H2 O)2 ]·(SiF6 )(H2 O)}n , with exchangeable SiF6 2- anion in its pore is strategically designed and synthesized, exhibiting selective detection of toxic Cr2 O7 2- oxo-anion in an aqueous medium having high sensitivity, selectivity, and recyclability through fluorescence quenching phenomena. More importantly, the recognition and ion exchange mechanism is unveiled through the rarely explored single-crystal-to-single crystal (SC-SC) fashion with well-resolved structures. A thorough SC-SC study with interfering anions (Cl- , F- , I- , NO3 - , HCO3 - , SO4 2- , SCN- , IO3 - ) revealed no such transformations to take place, as per line with quenching studies. Density functional theory calculations revealed that despite a lesser binding affinity, Cr2 O7 2- shows strong orbital mixing and large driving forces for electron transfer than SiF6 2- , and thus enlightens the fluorescence quenching mechanism. This work inaugurates the usage of a SIFSIX MOF toward sensing application domain under aqueous medium where hydrolytic stability is a prime concern for their plausible implementation as sensor materials.

Design, synthesis, and application of some two-dimensional materials

Two-dimensional (2D) materials are widely used as key components in the fields of energy conversion and storage, optoelectronics, catalysis, biomedicine, etc. To meet the practical needs, molecular structure design and aggregation process optimization have been systematically carried out. The intrinsic correlation between preparation methods and the characteristic properties is investigated. This review summarizes the recent research achievements of 2D materials in the aspect of molecular structure modification, aggregation regulation, characteristic properties, and device applications. The design strategies to fabricate functional 2D materials starting from precursor molecules are introduced in detail referring to organic synthetic chemistry and self-assembly technology. It provides important research ideas for the design and synthesis of related materials.

Simultaneous detection and removal of 2,4,6-trinitrophenyl phenol and dichromate by metal-organic framework

2,4,6-trinitrophenyl phenol (TNP) and dichromate (Cr₂O₇^2-) have serious toxicological effects on environment. Therefore, it is very important to detect and remove TNP and Cr₂O₇^2- in environmental matrix. In this work, a dual-functional UiO type metal-organic framework (Zr-Sti) was synthesized for simultaneous detection and removal of those pollutants in aqueous solution. As for detection, Zr-Sti exhibited sensitive and selective fluorescence response to TNP and Cr₂O₇^2- with detection limit below μM level, and possible mechanism behind was proposed and partially supported by experiment data. In addition, adsorption capacity of the prepared Zr-Sti for TNP and Cr₂O₇^2- was further investigated to evaluate its performance in pollutant removal from aqueous solution, and the mechanism behinds the obtained high removal efficiency was proposed. These results together with the satisfied recovery for simultaneous detection of TNP and Cr₂O₇^2- in real sample, indicate the potential of the prepared Zr-Sti material in the field of environment monitoring and remediation.

Guanidine-Functionalized Fluorescent sp2 Carbon-Conjugated Covalent Organic Framework for Sensing and Capture of Pd(II) and Cr(VI) Ions

Palladium is a key element in fuel cells, electronic industries, and organic catalysis. At the same time, chromium is essential in leather, electroplating, and metallurgical industries. However, their unpremeditated leakage into aquatic systems has caused human health and environmental apprehensions. Herein, we reported the development of an sp² carbon-conjugated fluorescent covalent organic framework with a guanidine moiety (sp² c-gCOF) that showed excellent thermal and chemical stability. The sp² c-gCOF showed effective sensing, capture, and recovery/removal of Pd(II) and Cr(VI) ions, which could be due to the highly accessible pore walls decorated with guanidine moieties. The fluorescent sp² c-gCOF showed higher selectivity for Pd(II) and Cr(VI) ions, with an ultra-low detection limit of 2.7 and 3.2 nM, respectively. The analysis of the adsorption properties with a pseudo-second-order kinetic model showed that sp² c-gCOF could successfully and selectively remove both Pd(II) and Cr(VI) ions from aqueous solutions. The polymer also showed excellent capture efficacy even after seven consecutive adsorption-desorption cycles. Hence, this study reveals the potential of fluorescent sp² c-gCOF for detecting, removing, and recovering valuable metals and hazardous ions from wastewater, which would be useful for economic benefit, environmental safety, human health, and sustainability. The post-synthetic modification of sp² c-COF with suitable functionalities could also be useful for sensing and extracting other water pollutants and valuable materials from an aqueous system.

Synergistic Cr(VI) reduction and adsorption of Cu(II), Co(II) and Ni(II) by zerovalent iron-loaded hydroxyapatite

Multi-metal contaminated soil, such as Cr(VI), Cu(II), and Co(II), still challenge the environmental remediation. In this work, zerovalent iron-loaded hydroxyapatite (ZVI/HAP) was first applied to simultaneously adsorb multi-metal in contaminated soil. During the remediation, the co-existing Cu(II), Ni(II), and Co(II) were adsorbed and precipitated onto ZVI/HAP. This "spontaneous deposition" simultaneously achieved the adsorption of the cationic metals and improved the isoelectric point of ZVI/HAP to 4.83 from 1.59, thus significantly alleviating the electronegativity to enhance the capture and reduction efficiency of Cr(VI). The application of ZVI/HAP resulted in the reduction of more than 99% of total Cr(VI) in contaminated soil, and the almost complete adsorption of water-soluble and DTPA-extractable Cu, Ni and Co within 20 d. Based on the sequential extraction and risk reduction assessment, soil Cr, Cu, Ni, and Co speciation was transformed from an unstable state (exchangeable and carbonate-bound fractions) to a relatively stable state, reducing the risk of heavy metals in contaminated soil significantly. This study developed an efficient strategy for the remediation of multi-metal contaminated soil.

Robust DUT-67 material for highly efficient removal of the Cr(VI) ion from an aqueous solution

Robust DUT-67 was synthesized by the hydrothermal method and characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). To systematically study the removal of Cr(VI) ion by DUT-67, single-factor, competition ion, material regeneration, kinetic, and thermodynamic experiments were designed. The experimental results show that DUT-67 had a maximum removal rate of 96.1% and a maximum adsorption capacity of 105.42 mg g^-1 with material regeneration and outstanding selective adsorption. In addition, the process of removal of the Cr(VI) ion from an aqueous solution by DUT-67, which accorded with the pseudo-second-order kinetics model and Langmuir model, was studied, and its adsorption mechanism was reasonably explained by the theoretical calculation.

Selective Chromium(VI) Trapping by an Acetate-Releasing Coordination Polymer

We report the high-capacity and selective uptake of Cr(VI) from water using the coordination polymer silver bipyridine acetate (SBA, [Ag(4,4'-bipy)][CH₃CO₂]·3H₂O). Cr capture involves the release of acetate, and we have structurally characterized two of the product phases that form: silver bipyridine chromate (SBC, SLUG-56, [Ag(4,4'-bipy)][CrO₄]_0.5·3.5H₂O) and silver bipyridine dichromate (SBDC, SLUG-57, [Ag(4,4'-bipy)][Cr₂O₇]_0.5·H₂O). SBA maintains a high Cr uptake capacity over a wide range of pH values (2-10), reaching a maximum of 143 mg Cr/g at pH 4. This Cr uptake capacity is one of the highest among coordination polymers. SBA offers the additional benefits of a one-step, room temperature, aqueous synthesis and its release of a non-toxic anion following Cr(VI) capture, acetate. Furthermore, SBA capture of Cr(VI) remains >97% in the presence of a 50-fold molar excess of sulfate, nitrate, or carbonate. We also investigated the Cr(VI) sequestration abilities of silver 1,2-bis(4-pyridyl)ethane nitrate (SEN, [Ag(4,4'-bpe)][NO₃]) and structurally characterized the silver 1,2-bis(4-pyridyl)ethane chromate (SEC, SLUG-58, [Ag(4,4'-bpe)][CrO₄]_0.5) product. SEN was, however, a less effective Cr(VI) sequestering material than SBA.

Ionic metal-organic frameworks (iMOFs): progress and prospects as ionic functional materials

Metal-organic frameworks (MOFs) have been a research hotspot for the last two decades, witnessing an extraordinary upsurge across various domains in materials chemistry. Ionic MOFs (both anionic and cationic MOFs) have emerged as next-generation ionic functional materials and are an important subclass of MOFs owing to their ability to generate strong electrostatic interactions between their charged framework and guest molecules. Furthermore, the presence of extra-framework counter-ions in their confined nanospaces can serve as additional functionality in these materials, which endows them a significant advantage in specific host-guest interactions and ion-exchange-based applications. In the present review, we summarize the progress and future prospects of iMOFs both in terms of fundamental developments and potential applications. Furthermore, the design principles of ionic MOFs and their state-of-the-art ion exchange performances are discussed in detail and the future perspectives of these promising ionic materials are proposed.

An alkali-resistant metal-organic framework as halogen bond donor for efficient and selective removing of ReO4-/TcO4. ENVIRONMENTAL SCIENCE AND POLLUTION RESEARCH INTERNATIONAL 2022;29:86815-86824. [PMID: 35794336 DOI: 10.1007/s11356-022-21870-y] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

⁹⁹Tc is one of the most problematic nuclear fuel products due to its long half-life and high environmental mobility. Direct removal of TcO₄^- from the highly alkaline solution of nuclear fuel is a serious and challenging environmental issue. In this work, the first efficient synthetic approach introducing halogens into a two-dimensional metal-organic framework, named Mn-MOF, is established using MnCl₂·4H₂O coordinating with neutral nitrogen-donor ligand, showing ultrahigh stability in alkaline aqueous even under 1 M NaOH. The luxuriant Mn-Cl bonds and ordered hydrophobic pore channels enable the Mn-MOF to have an efficient adsorption capacity for ReO₄^- with a large capacity (403 mg g^-1), which is higher than most MOF adsorbents. More importantly, the Mn-MOF shows an excellent selectivity toward ReO₄^- in high-density competitive anions, such as NO₃^- and SO₄^2-. Moreover, the outstanding performance of Mn-MOF in removing ReO₄^- endowed it successfully separated ReO₄^- from the simulated Savannah River Site (SRS) high-level waste (HLW) stream with high removal of 66.84% at the phase ratio of 10. The adsorption mechanism is further demonstrated by FT-IR, XPS analysis, and DFT calculation, showing that the ReO₄^- can selectively interact with Mn-Cl bonds and imidazole groups, forming unique halogen bonds Cl-O-Re, and a series of hydrogen bonds, respectively. This work suggests a new approach to the removal of TcO₄^- from nuclear fuel.

Preparation of new MOF-808/chitosan composite for Cr(VI) adsorption from aqueous solution: Experimental and DFT study

In this study, a series of Zirconium-based MOF and chitosan composites (MOF-808/chitosan) were synthesized as efficient adsorbent for Cr(VI) ions elimination from aqueous solution. MOF-808/chitosan structure and morphology was characterized by FE-SEM, EDX, XRD, BET, zeta potential analysis, FT-IR, XPS techniques. The kinetic studies ascertained that Cr(VI) adsorption over MOF-808/chitosan followed pseudo-second-order kinetic model. The adsorption isotherms fitted the Langmuir isotherm model, implying on homogeneously adsorption of Cr(VI) on the surface of MOF-808/chitosan. According to the Langmuir model, the maximum capacity was obtained to be 320.0 mg/g at pH 5. Thermodynamic investigation proposed spontaneous (ΔG° < 0), disordered (ΔS° > 0) and endothermic (ΔH° > 0) for adsorption process. Besides, MOF-808/chitosan displayed an appropriate reusability for the elimination of Cr(VI) ions from their aqueous solutions for six successive cycles. DFT study of the adsorption process displayed and confirmed the role of hydrogen bonding and electrostatic attraction simultaneously.

High-Quality Conjugated Polymers Achieving Ultra-Trace Detection of Cr2O72− in Agricultural Products

In view of that conjugated polymers (CPs) are an attractive option for constructing high-sensitive Cr₂O₇²⁻ sensors but suffer from lacking a general design strategy, we first proposed a rational structure design of CPs to tailor their sensing properties while validating the structure-to-performance correlation. Short side chains decorated with N and O atoms as recognition groups were instructed into fluorene to obtain monomers Fmoc-Ala-OH and Fmoc-Thr-OH. Additionally, their polymers P(Fmoc-Ala-OH) and P(Fmoc-Thr-OH) were obtained through electrochemical polymerization. P(Fmoc-Ala-OH) and P(Fmoc-Thr-OH) with high polymerization degrees have an excellent selectivity towards Cr₂O₇²⁻ in comparison to other cations and anions. Additionally, their limit of detection could achieve 1.98 fM and 3.72 fM, respectively. Especially, they could realize the trace detection of Cr₂O₇²⁻ in agricultural products (red bean, black bean, and millet). All these results indicate that short side chains decorated with N and O atoms functionalizing polyfluorene enables the ultra-trace detection of Cr₂O₇²⁻. Additionally, the design strategy will spark new ideas for the construction of highly selective and sensitive Cr₂O₇²⁻ sensors.

Triphenylamine-containing imine-linked porous organic network for luminescent detection and adsorption of Cr(VI) in water

In this study, an imine-linked luminescent porous organic network (PON) has been successfully synthesized by the Schiff-base condensation reaction between 1,2-diphenylethylenediamine and tris(4-formylphenyl)amine. It exhibits strong fluorescence in an aqueous dispersion and can be applied as a luminescent probe for Cr(VI) (CrO₄^2- and Cr₂O₇^2-) with high selectivity and sensitivity (LOD for Cr₂O₇^2- and CrO₄^2- were below 0.35 μM and 0.4 μM, respectively) in a turn-off manner. The possible luminescence sensing mechanism and the adsorption capacity of Cr(VI) are also discussed in detail.

Preparation of Highly Stable DUT-52 Materials and Adsorption of Dichromate Ions in Aqueous Solution

Highly stable DUT-52 materials were synthesized by the hydrothermal method and well-characterized by X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). In order to systematically study the adsorption of dichromate ions in aqueous solution by the DUT-52 materials, a single factor experiment, kinetic experiment, thermodynamic experiment, competition ion experiment, and material regeneration experiment were designed. Based on the H-bond interaction between the dichromate ions and the H atoms of a NDC^2- ligand, the DUT-52 materials showed a maximum removal rate of 96.4% and a maximum adsorption capacity of 120.68 mg·g^-1 with excellent selective adsorption and material regeneration. In addition, the process of adsorption of dichromate ions by the DUT-52 materials is in accordance with the pseudo second-order kinetics and Langmuir models, and the adsorption mechanism and the important role of the H-bond interaction were reasonably explained using the XPS pattern and theoretical calculation. Accordingly, DUT-52 can be regarded as a multifunctional material for efficiently removing dichromate ions from the wastewater.

Rational Design of High-Performance Cationic Organic Network Adsorbents

Development of high-performance ionic organic network (ION) adsorbents is of great importance for water remediation. However, the research on IONs is still nascent, especially, the design philosophy regarding contaminant adsorption has rarely been explored. In this contribution, we optimized the adsorption efficiency of IONs by increasing the density of charged sites and improving their accessibility. We first produced a new cationic organic network (CON), CON-LDU4, with a high density of positive sites via synthesis from tetra(4-pyridyl)ethene. Compared to the analogue CON-LDU2 that synthesized from tetra(4-(4-pyridyl)phenyl)ethene, CON-LDU4 exhibited higher efficiency in adsorption of methyl blue, indicating that the higher ionic density results in the higher adsorption efficiency. To further improve the accessibility of the active sites, another new CON material (CON-LDU5) was synthesized by employing a hard template. CON-LDU5 exhibited a larger specific surface area than CON-LDU4, with clearly enhanced adsorption efficiency. Finally, CON-LDU5 was used to capture CrO42- ions in water with fast adsorption kinetics (k2 = 0.0328 g mg-1 min-1) and high adsorption capacity (369 mg g-1).

Monolayer Nanosheets Exfoliated from Cage-Based Cationic Metal-Organic Frameworks

The rational design and preparation of monolayer metal-organic framework (MOF) nanosheets remain great challenges. Recently, we found that monolayer MOF nanosheets can be facially exfoliated on a large scale from pristine two-dimensional (2D) MOFs with substantially reduced interlaminar interaction. By employing cage-like bicyclocalix[2]arene[2]triazine tri-imidazole as the building block, a family of cationic two-dimensional metal-organic frameworks (2D MOFs) with steric layer were designed and prepared. The single crystal structures have clearly identified that only very weak and sparse distributed C-H···π interaction exists between adjacent layers.On the basis of density functional theory calculation, the interlayer interaction of these cage-based cationic 2D MOFs was estimated to be 1/46th of that of graphite. Due to the extremely weak interaction, these cationic 2D MOFs tend to degenerate into an "amorphous" state after being soaked in other solvents; they can be readily exfoliated into 1.1 nm thick monolayer nanosheets with a high degree of thickness homogeneity, large lateral size, and significantly enlarged surface area. This work has identified that a cage-like molecule is the ideal building block for 2D cationic MOFs and ultrathin nanosheets; It was futher confirmed that weakening the interlaminar interaction is an effective strategy for facilely producing monolayer nanosheets.

Cationic metal-organic frameworks constructed from a trigonal imidazole-containing ligand for the removal of Cr2O72- in water

Recently, cationic metal-organic frameworks (MOFs) have drawn considerable attention in the treatment of wastewater containing toxic anions via anion exchange due to the presence of exchangeable anions in their pores....

Construction and two-dimensional assembly of double-shell Na@Sn6L6@Sn3L3 clusters through tetrahedral citrate ligands

Herein we report a double-shell Na@Sn6L6@Sn3L3 cluster and their further assembly into 2D layer, which belongs to rare Sn-oxo coordination cage based extended structure. Tetrahedral citrate ligands with multiple coordination...

Two porous three-dimensional (3D) metal–organic frameworks based on diverse metal clusters: selective sensing of Fe3+ and Cr2O72−

Using a rigid 3,5-di(2′,5′-dicarboxylphenyl)benzoic acid, two porous 3D MOFs have been synthesized and characterized, and the luminescent properties have been studied.

A cationic thorium-organic framework with triple single-crystal-to-single-crystal transformation peculiarities for ultrasensitive anion recognition

Single-crystal-to-single-crystal transformation of metal-organic frameworks has been met with great interest, as it allows for the creation of new materials in a stepwise manner and direct visualization of structural transitions when subjected to external stimuli. However, it remains a peculiarity among numerous metal-organic frameworks, particularly for the ones constructed from tetravalent metal cations. Herein, we present a cationic thorium-organic framework displaying unprecedented triple single-crystal-to-single-crystal transformations in organic solvents, water, and NaIO3 solution. Notably, both the interpenetration conversion and topological change driven by the SC-SC transformation have remained elusive for thorium-organic frameworks. Moreover, the single-crystal-to-single-crystal transition in NaIO3 solution can efficiently and selectively turn the ligand-based emission off, leading to the lowest limit of detection (0.107 μg kg-1) of iodate, one of the primary species of long-lived fission product 129I in aqueous medium, among all luminescent sensors.

Cationic coordination polymers with thirteen-fold interpenetrating dia networks: selective coloration and ion-controlled photochromism

A novel Cd-based cationic coordination polymer (Cd-CCP) constructed using viologen derivatives, which exhibits an unusual thirteen-fold interpenetrating diamondoid network, has been synthesized. Notably, Cd-CCP displays selective and naked-eye distinguished coloration and ion-controlled photochromism towards halide anions.

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.

The Synthesis and Properties of TIPA-Dominated Porous Metal-Organic Frameworks

Metal-Organic Frameworks (MOFs) as a class of crystalline materials are constructed using metal nodes and organic spacers. Polydentate N-donor ligands play a mainstay-type role in the construction of metal-organic frameworks, especially cationic MOFs. Highly stable cationic MOFs with high porosity and open channels exhibit distinct advantages, they can act as a powerful ion exchange platform for the capture of toxic heavy-metal oxoanions through a Single-Crystal to Single-Crystal (SC-SC) pattern. Porous luminescent MOFs can act as nano-sized containers to encapsulate guest emitters and construct multi-emitter materials for chemical sensing. This feature article reviews the synthesis and application of porous Metal-Organic Frameworks based on tridentate ligand tris (4-(1H-imidazol-1-yl) phenyl) amine (TIPA) and focuses on design strategies for the synthesis of TIPA-dominated Metal-Organic Frameworks with high porosity and stability. The design strategies are integrated into four types: small organic molecule as auxiliaries, inorganic oxyanion as auxiliaries, small organic molecule as secondary linkers, and metal clusters as nodes. The applications of ratiometric sensing, the adsorption of oxyanions contaminants from water, and small molecule gas storage are summarized. We hope to provide experience and inspiration in the design and construction of highly porous MOFs base on polydentate N-donor ligands.

"In-situ synthesized" iron-based bimetal promotes efficient removal of Cr(VI) in by zero-valent iron-loaded hydroxyapatite

Anionic Cr(VI) and cationic heavy metals generally co-exist in industrial effluents and threaten the public health. Zero-valent iron (ZVI) particles tent to passivate rapidly, which results in a gradual drop in its reactivity. In this work, a strategy of "in-situ synthesized" iron-based bimetal was first developed to stimulate the self-activation of passivated ZVI. During this process, ZVI-loaded hydroxyapatite (ZVI/HAP) was prepared to enhance the affinity for co-existing Cu²⁺, which promoted the in-situ Cu⁰ deposition on ZVI/HAP to form a Fe-Cu bimetal. The deposited Cu⁰ significantly decreased the activation energy (E_a) of Cr(VI) reduction by 24.9%, and its corresponding Cr(VI) removal (96.53%) was much higher that of single Cr(VI) system (68.67%) within 9 h. More importantly, the removal of Cr(VI) and Cu²⁺ were synchronously achieved. Systematical electrochemical characterizations were first introduced to explore the galvanic behaviors of iron-based bimetal. The charge transfer resistance and the negative open circuit potential of ZVI/HAP significantly decreased with the Cu⁰ deposition, thereby accelerating the electron transfer from Fe⁰ to Cu²⁺. The enhanced electron transfer further facilitated the Fe(II) release to promote Cr(VI) reduction. This "in-situ synthesized" iron-based bimetal strategy provides a novel pattern for ZVI activation and exhibits practical application in remediation of combined contaminant.

Contiguous layer based metal-organic framework with conjugated π-electron ligand for high iodine capture

Herein, a novel 3-dimensional (3D) Cu(II) metal-organic framework (MOF), [Cu₃(μ₂-O)₂(p-tr₂Ph)₂(HCOO)][NO₃]·3DMF·3H₂O (compound 1), which is constructed by directly interlocking regionalized hollow two-dimensional (2D) layers, has been conceived and solvothermally synthesized. Such a distinctive regionalized pore system effectively maintains the uniformity of the pore structure, isolates the counterions and bridging ligands in the partition layer, and endows compound 1 with high porosity. In consequence, compound 1 exhibits excellent adsorption ability of iodine in cyclohexane. The removal efficiency in cyclohexane solution (0.01 mol L^-1) can reach up to 80% in 8 min, and the absorption ability towards iodine can reach about 1.15 g g^-1. Moreover, iodine can also be controllably released in ethanol. The release rate was up to 4 × 10^-5 mol L^-1 min^-1. Furthermore, compound 1 also showed prominent recyclability due to the high stability, and the maximum sorption amount could be retained after 3 cycles. This study paves a new way towards opening up MOFs' potential application in capturing radioactive iodine to protect the environment.

Neutral Nitrogen Donor Ligand-based MOFs for Sensing Applications

Neutral nitrogen donor (N-donor) ligand-based MOFs, with their enticing features inclusive of facile synthesis, labile metal-ligand bond, framework flexibility, atomic level tunability renders them appealing in molecular recognition-based studies. Intriguingly, the flexibility in such systems (owing to weaker metal-nitrogen bonds) promote maximization of host-analyte interactions, which is critical for the manifestation of a signaling response. Such host-analyte interactions can be tapped by discerning any change in the physical properties associated with the system, such as optical, fluorometric, chemiresistive, magnetic, dielectric constant, mass. This minireview presents a brief discussion on the various types of signal transduction pathways unveiled hitherto using neutral N-donor ligand-based MOFs and the fundamental insight into the signal's origin. Moreover, an elaborate compilation of the recent examples in this field has been presented. Also, the untapped prospects have been highlighted, which may serve as a beacon to drive future research.

Coordination networks for the recognition of oxo-anions

Crystalline coordination networks with an infinite extended framework, also known as coordination polymers (CPs) or metal-organic frameworks (MOFs), have attracted significant attention owing to their tremendous performance in an extensive range of applications. The unique structural features and high stability of coordination networks are responsible for their utilization and potential in diverse fields especially in the area of sensing using luminescent CPs/MOFs. The recognition of toxic oxo-anions present in water is a crucial and first step towards environmental remediation, mainly in the case of water pollution. Accordingly, the utilization of luminescent coordination networks has received significant interest, particularly for the recognition of various toxic oxo-anions, which are considered a threat to aquatic life and the environment. In this frontier article, we summarize recent reports on luminescent CPs/MOFs, their utilization for the sensing of oxo-anions and the chemistry of the interaction of oxo-anions with CPs/MOFs, which is responsible for tuning their optical signals.

Tunable Metal-Organic Frameworks Based on 8-Connected Metal Trimers for High Ethane Uptake

Metal trimers [M₃ (O/OH)](OOCR)₆ are among the most important structural building blocks. From these trimers, a great success has been achieved in the design of 6- or 9-connected framework materials with various topological features and outstanding gas-sorption properties. In comparison, 8-connected trimer-based metal-organic frameworks (MOFs) are rare. Given multiple competitive pathways for the formation of 6- or 9-connected frameworks, it remains challenging to identify synthetic or structural parameters that can be used to direct the self-assembly process toward trimer-based 8-connected materials. Here, a viable strategy called angle bending modulation is revealed for creating a prototypical MOF type based on 8-connected M₃ (OH)(OOCR)₅ (Py-R)₃ trimers (M = Zn, Co, Fe). As a proof of concept, six members in this family are synthesized using three types of ligands (CPM-80, -81, and -82). These materials do not possess open-metal sites and show excellent uptake capacity for various hydrocarbon gas molecules and inverse C₂ H₆ /C₂ H₄ selectivity. CPM-81-Co, made from 2,5-furandicarboxylate and isonicotinate, features selectivity of 1.80 with high uptake capacity for ethane (123 cm³ g^-1 ) and ethylene (113 cm³ g^-1 ) at 298 K and 1 bar.

Viologen-Based Cationic Metal-Organic Framework for Efficient Cr2O72- Adsorption and Dye Separation

A novel cationic metal-organic framework composed of {Cu₂(COO)₄} paddle-wheel units and a tetracarboxylic viologen derivative, namely, {[Cu₂(bdcbp)(H₂O)₂]·2NO₃·2H₂O}_n (Cu-CMOF, H₄bdcbpCl₂ = 1,1'-bis(3,5-dicarboxyphenyl)-4,4'-bipyridinium dichloride), has been successfully synthesized and structurally characterized. In Cu-CMOF, the {Cu₂(COO)₄} unit and viologen derivative both act as four-connected nodes forming an ssb-type cationic network with 4².8⁴ topology, in which the positive charges are distributed on the organic viologen moieties. Deeper insight of the structure indicates that the 3D architecture of Cu-CMOF can be seen as packing of a 26-faceted polyhedral cage and two cuboid cages. Notably, Cu-CMOF displays a highly efficient anion exchange ability for capture and removal of anionic pollutants. UV-vis absorption spectra and digital images demonstrate that Cu-CMOF is capable of adsorbing the dichromate anion and anionic dyes effectively, such as methyl orange (MO^-), Congo red (CR^2-), and New Coccine (NC^3-). Meaningfully, anionic dyes (MO^-, CR^2-, and NC^3-) can be efficiently and selectively removed by Cu-CMOF in the presence of cationic dye methylene blue (MLB⁺). Such behaviors of anionic pollutant adsorption and dye separation are mainly caused by an ion-exchange process facilitated by the large cavity and decentralized distribution of positive charge in Cu-CMOF.

3D Metallo-organic coordination assembly-based anion-enriched supramolecular material for fast and efficient removal of Cr2O72

Chromium(VI), especially dichromate (Cr₂O₇^2-) contamination in wastewater due to rapid industrialization with uncontrolled effluent management is still a serious concern which needs focused attention. Multiprong approaches are practiced such as chemical precipitation, reverse osmosis, ion-exchange, adsorption by granular activated carbon etc. to capture and separate this "Group A" human carcinogenic effluent from water. However, low capture capacity, non-reusability, poor selectivity, pH-limited performance are some major limitations of these techniques. Recently, metal organic frameworks (MOFs), metal organic cages (MOCs), porous organic polymers (POPs) or covalent organic frameworks (COFs), covalent organic networks (CONs) etc. emerged as new-generation materials to overcome such limitations. However, the development is still in initial stage and issues related to structural stability and integrity of many MOFs in water and in wide pH range, as well as reusability need to be addressed. At this juncture, herein we report a novel [Zn(terpyridine)₂]²⁺-templated trisimidazolium-based highly cationic three-dimensional metal-organic coordination assembly (3D MOCA), serving as a new class of efficient, fast, robust and recyclable dichromate-removal material. Not only the highly cationic assembly is enriched with a high density of Br^- anions, but its three-dimensional propagation and flexibility also exposes the exchangeable Br^- ions for facile anion-metathesis with Cr₂O₇^2-. By virtue of the benefits of these attributes, the presented supramolecular material exhibits a high capture capacity (469 mg g^-1), fast exchange kinetics (0.028 g mg^-1 min^-1), wide working pH range (pH 2-12) and reusability up to a minimum of 10 cycles without much loss of efficiency. Key mechanistic examinations highlight the evidences in favor of ion-exchange-based chemistry to be responsible for dichromate removal with the present material.

A novel multichromic Zn(II) cationic coordination polymer based on a new flexible viologen ligand exhibiting aniline detection in the solid state

In this work, a novel multichromic cationic coordination polymer, named [Zn4(BTC)3(bcbpy)2]·5H2O (1), based on a new flexible viologen ligand 1,1'-bis(3-cyanobenzyl)-[4,4'-bipyridine]-1,1'-diium (H2bcbpy·2Cl), Zn(NO3)2·6H2O and pyromellitic acid (H4BTC) was synthesized. Compound 1 has good photosensitive activity and can respond to sunlight at room temperature. The colour of compound 1 changes rapidly in response to UV light and blue ray irradiation within 5 s. We rarely obtained the crystal structures after irradiation under UV light and blue ray. At the same time, compound 1 shows the hydrochromism phenomenon when heated at 120 °C, and it also shows the ability of detecting aniline and NO2- under low-concentration conditions (10-4 M).

Rapid simultaneous removal of cationic dyes and Cr(VI) by boron cluster polyaniline with a target site

The stable physicochemical properties of polyaniline/closo-[B12H12]2- (PA/B12) obtained by an ion exchange technique combined with polyaniline (PA) and closo-[B12H12]2- (B12) can realize rapid kinetic adsorption and complete removal of Cr(vi) and cationic dye pollutants at low concentrations. The reversible adsorption/desorption process of pollutants represents that PA/B12 has practical industrial use value.