Selective Anion Exchange and Tunable Luminescent Behaviors of Metal–Organic Framework Based Supramolecular Isomers

Dual stimuli-responsive phosphorescence of a Pb(II) coordination polymer to acidic vapors and thermal treatment

We present a new intensely phosphorescent Pb(II) coordination polymer (1) containing a heteroatomic ligand. It has a quantum yield of 21.62% and a lifetime of 25.46 μs. The naked-eye solid-state photoluminescence of 1 significantly changes in response to acidic vapors and thermal treatment, indicating the coexistence of acidochromic and thermochromic luminescence.

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.

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.

Humidity-Induced Structural Transformation in Pseudopolymorph Coordination Polymers

Three cadmium coordination polymers, namely, {[CdL(OAc)₂](C₂H₅OH)}_n (1), {[CdL(OAc)₂](CH₃CN)}_n (2), and [CdL(OAc)₂(H₂O)]_n (3), were synthesized by an exoditopic 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene Schiff base ligand (L) and cadmium acetate in the presence of different solvent systems. Single-crystal X-ray diffraction, powder X-ray diffraction, and thermogravimetric analysis showed that 1D ladder pseudopolymorphic compounds (1 and 2) transformed to the solvent-free 1D linear compound 3 through a rare case of water absorption from air at room temperature. Interestingly, compound 3 was transformed to compound 1 through a dissolution-recrystallization structural transformation process. The results illustrated that solvents and humidity have an important role in the formation of pseudopolymorphs with the same or different structural motifs.

Encapsulated anion-dominated photocatalytic and adsorption performances for organic dye degradation and oxoanion pollutant capture over cationic Cu(i)-organic framework semiconductors

Decontamination of industrial wastewater containing toxic organic dye molecules and oxoanions is urgently desirable for environmental sustainability and human health. Water-stable porous metal-organic frameworks (MOFs) have emerged as highly efficient photocatalysts and/or adsorbents for water purification through controllable integration of the constitutive requirements. To reveal the inclusion anion effect of microporous MOFs on wastewater treatment, two isostructural MOFs incorporating positive charge and semiconductive characteristics, {[Cu(tpt)]·3H₂O·0.5SO₄}_n (1) and {[Cu(tpt)]·2H₂O·ClO₄}_n (2, tpt = 2,4,6-tris(4-pyridyl)-1,3,5-triazine), have been synthesized and employed as dual-functional materials for both dye photodegradation and oxoanion removal. The two MOFs possess the same 3-fold interpenetrating cationic backbones but are encapsulated by highly disordered sulfate or perchlorate in the open channels. These included anions have significantly tuned the hydrophilicity of the channels, extended the visible-light absorption, optimized the bandgap and decreased the conduction band potential. Under the low-energy irradiation of a 30 W LED lamp, MOF 1 has selectively and efficiently degraded rhodamine B compared to 2 with accelerated kinetics, resulting from the stronger reduction ability and less migration resistance of the photogenerated electrons. Instead, MOF 2 can quickly capture harmful MnO₄^- and Cr₂O₇^2- by exchanging with the entrapped ClO₄^-, with maximum adsorption amounts of 557 and 168 mg g^-1, respectively, under ambient conditions. The improved decolorization of the aqueous solution over 2 benefits essentially from the shape and charge memory effect and the smaller hydration energy of ClO₄^- than SO₄^2-. These interesting observations highlight the importance of the included anions inside the porous MOF semiconductors on wastewater treatment.

Effect of pyridyl donors from organic ligands versus metalloligands on material design

This review illustrates designs and structures of various coordination frameworks constructed using assorted organic ligands and metalloligands offering pyridyl donors to evaluate the impact of flexibility versus rigidity on material design.

From 1D to 2D Cd(II) and Zn(II) Coordination Networks by Replacing Monocarboxylate with Dicarboxylates in Partnership with Azine Ligands: Synthesis, Crystal Structures, Inclusion, and Emission Properties

Eight mixed-ligand coordination networks, [Cd(2-aba)(NO₃)(4-bphz)_3/2]_n·n(dmf) (1), [Cd(2-aba)₂(4-bphz)]_n·0.75n(dmf) (2), [Cd(seb)(4-bphz)]_n·n(H₂O) (3), [Cd(seb)(4-bpmhz)]_n·n(H₂O) (4), [Cd(hpa)(3-bphz)]_n (5), [Zn(1,3-bdc)(3-bpmhz)]_n·n(MeOH) (6), [Cd(1,3-bdc)(3-bpmhz)]_n ·0.5n(H₂O)·0.5n(EtOH) (7), and [Cd(NO₃)₂(3-bphz)(bpe)]_n·n(3-bphz) (8) were obtained by interplay of cadmium nitrate tetrahydrate or zinc nitrate hexahydrate with 2-aminobenzenecarboxylic acid (H(2-aba)), three dicarboxylic acids, sebacic (decanedioic acid, H₂seb), homophthalic (2-(carboxymethyl)benzoic acid, H₂hpa), isophthalic (1,3-benzenedicarboxylic acid, H₂(1,3-bdc)) acids, bis(4-pyridyl)ethane (bpe) and with four azine ligands, 1,2-bis(pyridin-4-ylmethylene)hydrazine (4-bphz), 1,2-bis(1-(pyridin-4-yl)ethylidene) hydrazine (4-bpmhz), 1,2-bis(pyridin-3-ylmethylene)hydrazine (3-bphz), and 1,2-bis(1-(pyridin-3-yl) ethylidene)hydrazine (3-bpmhz). Compounds 1 and 2 are 1D coordination polymers, while compounds 3–8 are 2D coordination polymers. All compounds were characterized by spectroscopic and X-ray diffraction methods of analysis. The solvent uptakes and stabilities to the guest evacuation were studied and compared for 1D and 2D coordination networks. The de-solvated forms revealed a significant increase of emission in comparison with the as-synthesized crystals.

One-pot concomitant preparation of two copper(II) coordination polymers with different configurations of bridging bithiophene ligands

By employing the conjugated bithiophene ligand 5,5'-bis(1H-imidazol-1-yl)-2,2'-bithiophene (bibp), which can exhibit trans and cis conformations, two different Cu^II coordination polymers, namely, poly[[μ-5,5'-bis(1H-imidazol-1-yl)-2,2'-bithiophene-κ²N:N'](μ₂-4,4'-oxydibenzoato-κ²O:O')copper(II)], [Cu(C₁₄H₈O₅)(C₁₄H₁₀N₄S₂)]_n or [Cu(bibp)(oba)]_n, (I), and catena-poly[μ-aqua-bis[μ-5,5'-bis(1H-imidazol-1-yl)-2,2'-bithiophene-κ²N:N']bis(μ₃-4,4'-oxydibenzoato)-κ³O:O':O'';κ⁴O:O',O'':O'-dicopper(II)], [Cu₂(C₁₄H₈O₅)₂(C₁₄H₁₀N₄S₂)(H₂O)]_n or [Cu₂(bibp)(oba)₂(H₂O)]_n, (II), have been prepared through one-pot concomitant crystallization and characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis, powder X-ray diffraction (PXRD) and thermogravimetric (TG) analysis. Single-crystal X-ray diffraction indicates that the most interesting aspect of the structure is the existence of sole trans and cis conformations of the bibp ligand in a single net of (I) and (II), respectively. Compound (I) displays a threefold interpenetrating three-dimensional framework with a 4-connected {6⁵.8} cds topology, whereas (II) features a one-dimensional chain structure. In the crystal of (II), the polymeric chains are further extended through C-H...O hydrogen bonds and C-H...π interactions into a three-dimensional supramolecular architecture. In addition, strong intramolecular O-H...O hydrogen bonds formed between the bridging water molecules and the carboxylate O atoms improve the stability of the framework of (II). Furthermore, solid-state UV-Vis spectroscopy experiments show that compounds (I) and (II) exhibit optical band gaps which are characteristic for optical semiconductors, with values of 2.70 and 2.26 eV, respectively.

A zeolite-like MOF based on a heterotritopic linker of imidazolyl, carboxyl and pyridine with a long-sought uks net on Schwarz's D-surface

The title MOF features a potential uninodal zeolite net, uks, which has never been found for real zeolites or zeolite-like MOFs. This rare structure exhibits specific anion binding of N3- over SCN- in water, which reverses the Hofmeister series.

Four Mixed-Ligand Zn(II) Three-Dimensional Metal-Organic Frameworks: Synthesis, Structural Diversity, and Photoluminescent Property

Assemblies of four three-dimensional (3D) mixed-ligand coordination polymers (CPs) having formulas, {[Zn₂(bdc)₂(4-bpdh)]·C₂H₅OH·2H₂O}_n (1), [Zn(bdc)(4-bpdh)]_n (2), {[Zn₂(bdc)₂(4-bpdh)₂]·(4-bpdh)}_n (3), and {[Zn(bdc)(4-bpdh)]·C₂H₅OH}_n (4) (bdc²⁻ = dianion of 1,4-benzenedicarboxylic acid, 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) have been synthesized and structurally characterized by single-crystal X-ray diffraction method. Structural determination reveals that the coordination numbers (geometry) of Zn(II) ions in 1, 2, 3, and 4 are five (distorted square-pyramidal (SP)), six (distorted octahedral (O_h)), five (trigonal-bipyramidal (TBP)), and four (tetrahedral (T_d)), respectively, and are bridged by 4-bpdh with bis-monodentate coordination mode and bdc²⁻ ligands with bis-bidentate in 1, chelating/bidentate in 2, bis-monodentate and bis-bidentate in 3, and bis-monodentate in 4, to generate two-fold interpenetrating 3D cube-like metal-organic framework (MOF) with pcu topology, non-interpenetrating 3D MOF, two-fold interpenetrating 3D rectangular-box-like MOF with pcu topology and five-fold interpenetrating diamondoid-like MOF with dia topology, respectively. These different intriguing architectures indicate that the coordination numbers and geometries of Zn(II) ions, coordination modes of bdc²⁻ ligand, and guest molecules play important roles in the construction of MOFs and the formation of the structural topologies and interpenetrations. Thermal stabilities, and photoluminescence study of 1–4 were also studied in detail. The complexes exhibit ligands based photoluminescence properties at room temperature.

Guest-Responsive Metal-Organic Frameworks as Scaffolds for Separation and Sensing Applications

Metal-organic frameworks (MOFs) have evolved to be next-generation utility materials because of their serviceability in a wide variety of applications. Built from organic ligands with multiple binding sites in conjunction with metal ions/clusters, these materials have found profound advantages over their other congeners in the domain of porous materials. The plethora of applications that these materials encompass has motivated material chemists to develop such novel materials, and the catalogue of MOFs is thus ever-escalating. One key feature that MOFs possess is their responsiveness toward incoming guest molecules, resulting in changes in their physical and chemical properties. Such uniqueness generally arises owing to the influenceable ligands and/or metal units that govern the formation of these ordered architectures. The suitable host-guest interactions play an important role in determining the specific responses of these materials and thus find important applications in sensing, catalysis, separation, conduction, etc. In this Account, we focus on the two most relevant applications based on the host-guest interactions that are carried out in our lab, viz., separation and sensing of small molecules. Separation of liquid-phase aromatic hydrocarbons by less energy-intensive adsorption processes has gained attention recently. Because of their tailored structures and functionalized pore surfaces, MOFs have become vital candidates in molecular separation. Prefunctionalization of MOFs by astute choice of ligands and/or metal centers results in targeted separation processes in which the molecular sieving effect plays a crucial role. In this view, separation of C₆ and C₈ liquid aromatic hydrocarbons, which are essential feedstock in various chemical industries, is one area of research that requires significant attention because of the gruesome separation techniques adopted in such industries. Also, from the environmental perspective, separation of oil/water mixtures demands significant attention because of the hazards of marine oil spillage. We have achieved successful separation of such by careful impregnation of hydrophobic moieties inside the nanochannels of MOFs, resulting in unprecedented efficiency in oil/water separation. Also, recognition of small molecules using optical methods (fluorescence, UV, etc.) has been extended to achieve sensing of various neutral species and anions that are important from environmental point of view. Incorporation of secondary functional groups has been utilized to sense nitroaromatic compounds (NACs) and other small molecules such as H₂S, NO, and aromatic phenols. We have also utilized the postfunctionalization strategy via ion exchange to fabricate MOFs for sensing of environmentally toxic and perilous anionic species such as CN^- and oxoanions. Our current endeavors to explore the applicability of MOFs in these two significant areas have widened the scope of research, and attempts to fabricate MOFs for real-time applications are underway.

Hierarchical Corannulene-Based Materials: Energy Transfer and Solid-State Photophysics

We report the first example of a donor-acceptor corannulene-containing hybrid material with rapid ligand-to-ligand energy transfer (ET). Additionally, we provide the first time-resolved photoluminescence (PL) data for any corannulene-based compounds in the solid state. Comprehensive analysis of PL data in combination with theoretical calculations of donor-acceptor exciton coupling was employed to estimate ET rate and efficiency in the prepared material. The ligand-to-ligand ET rate calculated using two models is comparable with that observed in fullerene-containing materials, which are generally considered for molecular electronics development. Thus, the presented studies not only demonstrate the possibility of merging the intrinsic properties of π-bowls, specifically corannulene derivatives, with the versatility of crystalline hybrid scaffolds, but could also foreshadow the engineering of a novel class of hierarchical corannulene-based hybrid materials for optoelectronic devices.

Efficient and Selective Uptake of TcO4- by a Cationic Metal-Organic Framework Material with Open Ag+ Sites

⁹⁹Tc is one of the most problematic radioisotopes in used nuclear fuel owing to its combined features of high fission yield, long half-life, and high environmental mobility. There are only a handful of functional materials that can remove TcO₄^- anion from aqueous solution and identifying for new, stable materials with high anion-exchange capacities, fast kinetics, and good selectivity remains a challenge. We report here an 8-fold interpenetrated three-dimensional cationic metal-organic framework material, SCU-100, which is assembled from a tetradentate neutral nitrogen-donor ligand and two-coordinate Ag⁺ cations as potential open metal sites. The structure also contains a series of 1D channels filled with unbound nitrate anions. SCU-100 maintains its crystallinity in aqueous solution over a wide pH range from 1 to 13 and exhibits excellent β and γ radiation-resistance. Initial anion exchange studies show that SCU-100 is able to both quantitatively and rapidly remove TcO₄^- from water within 30 min. The exchange capacity for the surrogate ReO₄^- reaches up to 541 mg/g and the distribution coefficient K_d is up to 1.9 × 10⁵ mL/g, which are significantly higher than all previously tested inorganic anion sorbent materials. More importantly, SCU-100 can selectively capture TcO₄^- in the presence of large excess of competitive anions (NO₃^-, SO₄^2-, CO₃^2-, and PO₄^3-) and remove as much as 87% of TcO₄^- from the Hanford low-level waste melter off-gas scrubber simulant stream within 2 h. The sorption mechanism is well elucidated by single crystal X-ray diffraction, showing that the sorbed ReO₄^- anion is able to selectively coordinate to the open Ag⁺ sites forming Ag-O-Re bonds and a series of hydrogen bonds. This further leads to a single-crystal-to-single-crystal transformation from an 8-fold interpenetrated framework with disordered nitrate anions to a 4-fold interpenetrated framework with fully ordered ReO₄^- anions. This work represents a practical case of TcO₄^- removal by a MOF material and demonstrates the promise of using this type of material as a scavenger for treating anionic radioactive contaminants during the nuclear waste partitioning and remediation processes.

Recent advances on supramolecular isomerism in metal organic frameworks

This review highlights selected examples of supramolecular isomerism in the area of metal organic frameworks and major factors of their formation are also discussed.

Solvent-induced construction of two zinc supramolecular isomers: synthesis, framework flexibility, sensing properties, and adsorption of dye molecules

Solvent-induced construction of two zinc supramolecular isomerism, sensing properties and adsorption of dye molecules.

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.

A nanoporous Ag(i) coordination polymer for selective adsorption of carcinogenic dye Acid Red 26

This work presents a unique Ag(i) nanoporous coordination polymer, showing selective and complete removal of carcinogenic dye Acid Red 26 from aqueous solution upon the size-exclusion and charge-matching effect.

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.

Neutral N-donor ligand based flexible metal–organic frameworks

This short review focuses on the flexibility aspect of MOFs based on neutral N-donor ligands with representative examples concerning the structural aspects and the subsequent properties induced by the reorganization of the frameworks.

Exploring the crystallization landscape of cadmium bis(N-hydroxyethyl, N-isopropyldithiocarbamate), Cd[S2CN(iPr)CH2CH2OH]2

Crystallization of Cd[S2CN(iPr)CH2CH2OH]2 from ethanol yields the coordination polymer [{Cd[S2CN(iPr)CH2CH2OH]2}·EtOH]∞ (1) within 3 h. When the solution is allowed to stand for another hour, the needles begin to dissolve and prisms emerge of the supramolecular isomer (SI), binuclear {Cd[S2CN(iPr)CH2CH2OH]2}2·2EtOH (2). These have been fully characterized spectroscopically and by X-ray crystallography. Polymeric 1 has 2-fold symmetry and features dithiocarbamate ligands coordinating two octahedral Cd atoms in a μ 2 κ 2-tridentate mode. Binuclear 2 is centrosymmetric with two ligands being μ 2 κ 2-tridentate as for 1 but the other two being κ 2-chelating leading to square pyramidal geometries. The conversion of the kinetic crystallization product, 1, to thermodynamic 2 is irreversible but transformations mediated by recrystallization (ethanol and acetonitrile) to related literature SI species, namely coordination polymer [{Cd[S2CN(iPr)CH2CH2OH]2}3·MeCN]∞ and binuclear {Cd[S2CN(iPr)CH2CH2 OH]2}2·2H2O·2MeCN, are demonstrated, some of which are reversible. Three other crystallization outcomes are described whereby crystal structures were obtained for the 1:2 co-crystal {Cd[S2CN(iPr)CH2CH2OH]2}2:2[3-(propan-2-yl)-1,3-oxazolidine-2-thione] (3), the salt co-crystal [iPrNH2(CH2CH2OH)]4[SO4]2{Cd[S2CN(iPr)CH2CH2OH]2}2 (4) and the salt [iPrNH2(CH2CH2OH)]{Cd[S2CN(iPr)CH2CH2OH]3} (5). These arise as a result of decomposition/oxidation of the dithiocarbamate ligands. In each of 3 and 4 the binuclear {Cd[S2CN(iPr)CH2CH2OH]2}2 SI, as in 2, is observed strongly suggesting a thermodynamic preference for this form.

Single-crystal-to-single-crystal transformation of an anion exchangeable dynamic metal–organic framework

A 3D cationic Metal–Organic Framework has been fabricated with a neutral N-donor ligand and Cd(ClO₄)₂ which shows guest triggered dynamic behaviour at room temperature. This structural dynamism has been demonstrated from the SCSC transformation experiment.

Periodic tiling of triangular and square nanotubes in a cationic metal–organic framework for selective anion exchange

A unique cationic metal–organic framework formed by connecting the neutral chain-like secondary building units with positively charged ligands shows charge- and size-dependent ion-exchange of anion dyes.

Guest-containing supramolecular isomers of silver(i) 3,5-dialkyl-1,2,4-triazolates: syntheses, structures, and structural transformation behaviours

Using benzene as a template, a series of guest-containing silver(i) 3,5-dialkyl-1,2,4-triazolate isomers showing different structural transformation behaviours have been synthesized.

Tuned synthesis of two coordination polymers of Cd(ii) using substituted bent 3-pyridyl linker and succinate: structures and their applications in anion exchange and sorption properties

Two new coordination polymers of Cd(ii) have been synthesized using disodium succinate and a substituted bent N,N'-donor ligand where the substitution on N,N'-donor ligand controlled the formation of the product.