Incarceration of Iodine in a Pyrene‐Based Metal–Organic Framework

Two Metal-Organic Frameworks with a Fused Cis-Decalin Conformation for Multimedia Iodine Capture

The extensive growth of nuclear power plants has a severe detrimental effect on human health and the surroundings due to the uncontrolled and unfiltered release of radioactive wastes into the environment. One such radioactive waste is 129I which has a fatal effect when released into the air or water bodies. Hence, molecular and ionic iodine capture from multimedia has become an important area of interest in the recent past. This work is aimed at introducing two 2D metal-organic frameworks with a fused cis-decalin conformation, {[Zn2(tpbn)(fdc)2]·6H2O}n (1) and {[Cd2(tpbn)(fdc)2(H2O)2]·2H2O}n (2), synthesized at room temperature utilizing a combination of M(OAc)2·2H2O (M: Zn/Cd), a neutral flexible ligand, tpbn, and a simple commercially available furan dicarboxylate, fdc2-, for the target application. The polarizing nature of the furan moieties and the oxygen rich pores in 1 and 2 facilitate the easy capture of molecular iodine from both the vapor phase and aqueous media with high uptake values. Furthermore, their efficiency was tested for the practical application under real-world conditions using river and seawater. In addition to confirming their recyclability with the retention of structural integrity, the interaction between 1 and 2 with iodine has also been established with experimental and theoretical calculations.

Optimizing Iodine Enrichment through Induced-Fit Transformations in a Flexible Ag(I)-Organic Framework: From Accelerated Adsorption Kinetics to Record-High Storage Density

Efficient capture and storage of radioactive I2 is a prerequisite for developing nuclear power but remains a challenge. Here, two flexible Ag-MOFs (FJI-H39 and 40) with similar active sites but different pore sizes and flexibility are prepared; both of them can capture I2 with excellent removal efficiencies and high adsorption capacities. Due to the more flexible pores, FJI-H39 not only possesses the record-high I2 storage density among all the reported MOFs but also displays a very fast adsorption kinetic (124 times faster than FJI-H40), while their desorption kinetics are comparable. Mechanistic studies show that FJI-H39 can undergo induced-fit transformations continuously (first contraction then expansion), making the adsorbed iodine species enrich near the Ag(I) nodes quickly and orderly, from discrete I- anion to the dense packing of various iodine species, achieving the very fast adsorption kinetic and the record-high storage density simultaneously. However, no significant structural transformations caused by the adsorbed iodine are observed in FJI-H40. In addition, FJI-H39 has excellent stability/recyclability/obtainability, making it a practical adsorbent for radioactive I2. This work provides a useful method for synthesizing practical radioactive I2 adsorbents.

Direct Observation of Enhanced Iodine Binding within a Series of Functionalized Metal-Organic Frameworks with Exceptional Irradiation Stability

Optimization of active sites and stability under irradiation are important targets for sorbent materials that might be used for iodine (I2) storage. Herein, we report the direct observation of I2 binding in a series of Cu(II)-based isostructural metal-organic frameworks, MFM-170, MFM-172, MFM-174, NJU-Bai20, and NJU-Bai21, incorporating various functional groups (-H, -CH3, - NH2, -C≡C-, and -CONH-, respectively). MFM-170 shows a reversible uptake of 3.37 g g-1 and a high packing density of 4.41 g cm-3 for physiosorbed I2. The incorporation of -NH2 and -C≡C- moieties in MFM-174 and NJU-Bai20, respectively, enhances the binding of I2, affording uptakes of up to 3.91 g g-1. In addition, an exceptional I2 packing density of 4.83 g cm-3 is achieved in MFM-174, comparable to that of solid iodine (4.93 g cm-3). In situ crystallographic studies show the formation of a range of supramolecular and chemical interactions [I···N, I···H2N] and [I···C≡C, I-C═C-I] between -NH2, -C≡C- sites, respectively, and adsorbed I2 molecules. These observations have been confirmed via a combination of solid-state nuclear magnetic resonance, X-ray photoelectron, and Raman spectroscopies. Importantly, γ-irradiation confirmed the ultraresistance of MFM-170, MFM-174, and NJU-Bai20 suggesting their potential as efficient sorbents for cleanup of radioactive waste.

Exploiting a Multi-Responsive Oxadiazole Moiety in One Three-Dimensional Metal-Organic Framework for Remedies to Three Environmental Issues

Multifunctional metal-organic frameworks (MOFs) rely on the properties of metal centers (nodes) and/or linkers (struts) for their diverse applications in the emerging field of research. Currently, there is a huge demand for MOF materials in the field of capture/fixation/sensing of air pollutants, harmful chemical effluents, and nuclear waste. However, it is a challenging task to utilize one MOF for providing remedies to all these issues. On the basis of our current research activities, we have identified that an oxadiazole moiety-a five-membered ring with two different heteroatoms (O and N)-in a carboxylate linker can be the key to generating such MOF materials for its (a) inherent polarizable nature and molecular docking ability and (b) photoluminescence properties. In this work, we report a 3D MOF {[Co₂(oxdz)₂(tpbn)(H₂O)₂]·4H₂O}_n (1), self-assembled at room temperature from a three-component reaction, with an oxadiazole moiety (where H₂oxdz = 4,4'-(1,3,4-oxadiazole-2,5-diyl)dibenzoic acid and tpbn = N,N',N,"N″'-tetrakis(2-pyridylmethyl)-1,4-diaminobutane). The inherent polarizable nature of the oxadiazole moiety in 1 has been efficiently exploited for (i) multimedia iodine capture and (ii) fixation of CO₂ under solvent-free and ambient conditions. On the other hand, the luminescent nature of the framework is found to be an efficient, highly preferred turn-on sensor for the ultra-fast detection of ketones with a limit as low as parts-per-trillion (mesitylene oxide: 447 ppt; cycloheptanone: 4.7 ppb; cyclohexanone: 17.2 ppb; acetylacetone: 18 ppb).

Enhanced Thermoelectric Properties of a Semiconducting Two-Dimensional Metal-Organic Framework via Iodine Loading

We report the first result of a study in which molecular iodine has been incorporated via incipient wetness impregnation into the two-dimensional semiconducting metal-organic framework (MOF) Cu₃(2,3,6,7,10,11-hexahydroxytriphenylene)₂ Cu₃(HHTP)₂ to enhance its thermoelectric properties. A power factor of 0.757 μW m^-1 K^-2 for this MOF was obtained which demonstrates that this provides an effective route for the preparation of moderate-performance thermoelectric MOFs.

Capture of Gaseous Iodine in Isoreticular Zirconium-Based UiO-n Metal-Organic Frameworks: Influence of Amino Functionalization, DFT Calculations, Raman and EPR Spectroscopic Investigation

A series of Zr-based UiO-n MOF materials (n=66, 67, 68) have been studied for iodine capture. Gaseous iodine adsorption was collected kinetically from a home-made set-up allowing the continuous measurement of iodine content trapped within UiO-n compounds, with organic functionalities (-H, -CH₃ , -Cl, -Br, -(OH)₂ , -NO₂ , -NH₂ , (-NH₂ )₂ , -CH₂ NH₂ ) by in-situ UV-Vis spectroscopy. This study emphasizes the role of the amino groups attached to the aromatic rings of the ligands connecting the {Zr₆ O₄ (OH)₄ } brick. In particular, the preferential interaction of iodine with lone-pair groups, such as amino functions, has been experimentally observed and is also based on DFT calculations. Indeed, higher iodine contents were systematically measured for amino-functionalized UiO-66 or UiO-67, compared to the pristine material (up to 1211 mg/g for UiO-67-(NH₂ )₂ ). However, DFT calculations revealed the highest computed interaction energies for alkylamine groups (-CH₂ NH₂ ) in UiO-67 (-128.5 kJ/mol for the octahedral cavity), and pointed out the influence of this specific functionality compared with that of an aromatic amine. The encapsulation of iodine within the pore system of UiO-n materials and their amino-derivatives has been analyzed by UV-Vis and Raman spectroscopy. We showed that a systematic conversion of molecular iodine (I₂ ) species into anionic I^- ones, stabilized as I^- ⋅⋅⋅I₂ or I₃ ^- complexes within the MOF cavities, occurs when I₂ @UiO-n samples are left in ambient light.

Efficient Energy-Transfer-Induced High Photoelectric Conversion in a Dye-Encapsulated Ionic Pyrene-Based Metal-Organic Framework

The relationship between the aggregation states of pyrene-based linkers and the photoluminescence/photoelectric performance was well studied by the formation of an anionic metal-organic framework, [BMI]₂[Mg₃(TBAPy)₂(H₂O)₄]·2dioxane, which shows highly enhanced light-harvesting and photoelectric conversion efficiency by the encapsulation of D-π-A cation dyes.

Isomeric Scandium-Organic Frameworks with High Hydrolytic Stability and Selective Adsorption of Acetylene

Two solvent-controlled topological isomers of scandium-organic frameworks [Sc(Hpzc)(pzc)]·DMF·2H₂O (1·DMF·2H₂O) and [Sc(Hpzc)(pzc)]·DMA·4H₂O (2·DMA·4H₂O) were synthesized using 2,5-pyrazinedicarboxylate (pzc^2-) (DMF = dimethylformamide; DMA = dimethylacetamide). Despite the isomeric nature of the obtained metal-organic frameworks (MOFs), they possess different structural features and unique adsorption properties toward gases and iodine. The compound 1 has widely spread among MOF structures a dia topology with ultranarrow channels, which together with inner surface functionalization leads to enhanced CO₂ adsorption and high selectivity factors in CO₂/CH₄ and CO₂/N₂ gas mixtures (25.9 and 35.6, respectively, 1/1 v/v). Moreover, a rare preferable adsorption of CO₂ over C₂H₂ was demonstrated. The biporous isomeric framework 2 has a crb topology inherent in zeolites. A remarkable adsorption affinity to C₂H₂ with the ideal adsorbed solution theory selectivity factor of 127.1 for a C₂H₂/C₂H₄ mixture (1/99 v/v) was achieved for 2. Both compounds have exceptional chemical stability in a wide range of pH from acidic to basic media.

Iodine Capture Using Zr-Based Metal-Organic Frameworks (Zr-MOFs): Adsorption Performance and Mechanism

The effective capture of radioiodine, produced or released from nuclear-related activities, is of paramount importance for the sustainable development of nuclear energy. Here, a series of zirconium-based metal-organic frameworks (Zr-MOFs), with a Zr₆(μ₃-O)₄(μ₃-OH)₄ cluster and various carboxylate linkers, were investigated for the capture of volatile iodine. Their adsorption kinetics and recyclability were investigated in dry and humid environments. The structural change of Zr-MOFs during iodine trapping was studied using powder X-ray diffraction and pore structure measurements. Experimental spectra (Raman and X-ray photoelectron spectroscopy) and density functional theory (DFT) calculations for the linkers and Zr clusters were performed to understand the trapping mechanism of the framework. When interacting with iodine molecules, MOF-808, NU-1000, and UiO-66, with highly connected and/or rigid linkers, have better structural stability than UiO-67 and MOF-867, which have flexible linkers with less connectivity. Particularly, MOF-808, with a rigid and tritopic benzenetricarboxylate linker, has the highest iodine adsorption capacity (2.18 g/g, 80 °C), as well as the largest pore volume after iodine elution. In contrast, UiO-67, with long linear ditopic linkers, exhibits the weakest stability and lowest adsorption capacity (0.53 g/g, 80 °C) because of its most serious collapse of pore structures. After incorporating with strong electron-donating imidazole/pyridine ligands, both the stability and adsorption capacity of MOF-808/NU-1000 decrease. DFT calculations verify that the N-heterocycle groups could enhance the affinity toward iodine by strong charge transfer. DFT calculations also suggest that the terminal -OH in MOF-808 has a strong affinity toward iodine (-54 kJ/mol I₂) and water (-63 kJ/mol H₂O) and a weak affinity toward NO₂ (-27 kJ/mol NO₂). With high adsorption capacity and excellent stability, MOF-808 shows great potential for the sustainable removal of radioiodine.

Cucurbit[7]uril-Based Metal-Organic Rotaxane Framework for Dual-Capture of Molecular Iodine and Cationic Potassium Ion

Metal-organic rotaxane frameworks (MORFs) attracted much attention in the past years for construction of intelligent functional materials. Herein, a one-pot synthesis is reported of a three-dimensional (3D) cucurbit[7]uril (Q[7])-based MORF under hydrothermal conditions, namely Q[7]-MORF-1, formed by encapsulating the anionic benzoate moieties of the tricarboxylate ligand into the cavity of Q[7]. Furthermore, Q[7]-MORF-1 shows dual-capture capacity for iodine and K⁺ selectively among the alkali metal ions. The captured molecular iodine is included in the cavity of Q[7] through halogen-bonding interactions and the K⁺ cations are positioned at the carbonyl port of the Q[7] through K-O coordination interactions.

Temperature-dependent interchromophoric interaction in a fluorescent pyrene-based metal-organic framework

Compounds exhibiting tuneable fluorescence emission upon heating or cooling are considered smart materials as their optical properties can be exquisitely controlled by adjusting the external temperature. Herein, we report such a material, which is a porous pyrene-based metal-organic framework with a chemical formula of [Mg1.5(HTBAPy)(H2O)2]·3DMF (H4TBAPy = 1,3,6,8-tetrakis(p-benzoic acid)pyrene), named SION-7. The bulk solid material of SION-7 can display either monomer or excimer fluorescence emission due to the temperature-dependent extent of interchromophoric interactions between the HTBAPy3- ligands within the framework. Consequently, the fluorescence emission colours gradually change from blue at low temperature (80 K) to yellow-green at high temperature (450 K). Interestingly, while kept in a relatively wide temperature range of 80-200 K, SION-7 displays a structured monomer-like spectrum and its colour changes reversibly from deep to light blue. Ex situ variable-temperature (100-350 K) single-crystal X-ray diffractometry studies revealed the impact of solvent content on the optical properties of SION-7, and illustrated the correlation between the position of the phenylene groups of the HTBAPy3- ligands at different temperatures and the interchromophoric interaction. Our study demonstrates a step forward towards the design of tuneable thermofluorochromic materials sought by optoelectronics.