Fluorescent sensors: A bright future for cages

Metal Ions Trigger the Gelation of Cysteine-Containing Peptide-Appended Coordination Cages

We report a series of coordination cages that incorporate peptide chains at their vertices, prepared through subcomponent self-assembly. Three distinct heterochiral tripeptide subcomponents were incorporated, each exhibiting an L-D-L stereoconfiguration. Through this approach, we prepared and characterized three tetrahedral metal-peptide cages that incorporate thiol and methylthio groups. The gelation of these cages was probed through the binding of additional metal ions, with the metal-peptide cages acting as junctions, owing to the presence of sulfur atoms on the peripheral peptides. Gels were obtained with cages bearing cysteine at the C-terminus. Our strategy for developing functional metal-coordinated supramolecular gels with a modular design may result in the development of materials useful for chemical separations or drug delivery.

Mechanically Interlocked Water-Soluble Pd6 Host for the Selective Separation of Coal Tar-Based Planar Aromatic Molecules

Research on the synthesis of catenated cages has been a growing field of interest in the past few years. While multiple types of catenated cages with different structures have been synthesized, the application of such systems has been much less explored. Specifically, the use of catenated cages in the separation of industrially relevant molecules that are present in coal tar has not been explored before. Herein, we demonstrate the use of a newly synthesized interlocked cage 1 [C184H240N76O48Pd6] (M6L4), formed through the self-assembly of ligand L.HNO3 (tris(4-(1H-imidazole-1-yl)benzylidene)hydrazine-1-carbohydrazonhydrazide) with acceptor cis-[(tmchda)Pd(NO3)2] [tmchda = ±N,N,N',N'-tetramethylcyclohexane-1,2-diamine] (M). The interlocked cage 1 was able to separate the isomers (anthracene and phenanthrene) using a simple solvent extraction technique. Using the same technique, the much more difficult separation of structurally and physiochemically similar compounds acenaphthene and acenaphthylene was performed for the first time with 1 as the host. Other noninterlocked hexanuclear Pd6 cages having a wider cavity proved inefficient for such separation, demonstrating the uniqueness of the interlocked cage 1 for such challenging separation.

Proton-Induced Reversible Spin-State Switching in Octanuclear FeIII Spin-Crossover Metal-Organic Cages

Responsive spin-crossover (SCO) metal-organic cages (MOCs) are emerging dynamic platforms with potential for advanced applications in magnetic sensing and molecular switching. Among these, FeIII-based MOCs are particularly noteworthy for their air stability, yet they remain largely unexplored. Herein, we report the synthesis of two novel FeIII MOCs using a bis-bidentate ligand approach, which exhibit SCO activity above room temperature. These represent the first SCO-active FeIII cages and feature an atypical {FeN6}-type coordination sphere, uncommon for FeIII SCO compounds. Our study reveals that these MOCs are sensitive to acid/base variations, enabling reversible magnetic switching in solution. The presence of multiple active proton sites within these SCO-MOCs facilitates multisite, multilevel proton-induced spin-state modulation. This behavior is observed at room temperature through 1H NMR spectroscopy, capturing the subtle proton-induced spin-state transitions triggered by pH changes. Further insights from extended X-ray absorption fine structure (EXAFS) and theoretical analyses indicate that these magnetic alterations primarily result from the protonation and deprotonation processes at the NH active sites on the ligands. These processes induce changes in the secondary coordination sphere, thereby modulating the magnetic properties of the cages. The capability of these FeIII MOCs to integrate magnetic responses with environmental stimuli underscores their potential as finely tunable magnetic sensors and highlights their versatility as molecular switches. This work paves the way for the development of SCO-active materials with tailored properties for applications in sensing and molecular switching.

Encapsulation engineering of porous crystalline frameworks for delayed luminescence and circularly polarized luminescence

Delayed luminescence (DF), including phosphorescence and thermally activated delayed fluorescence (TADF), and circularly polarized luminescence (CPL) exhibit common and broad application prospects in optoelectronic displays, biological imaging, and encryption. Thus, the combination of delayed luminescence and circularly polarized luminescence is attracting increasing attention. The encapsulation of guest emitters in various host matrices to form host-guest systems has been demonstrated to be an appealing strategy to further enhance and/or modulate their delayed luminescence and circularly polarized luminescence. Compared with conventional liquid crystals, polymers, and supramolecular matrices, porous crystalline frameworks (PCFs) including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), zeolites and hydrogen-bonded organic frameworks (HOFs) can not only overcome shortcomings such as flexibility and disorder but also achieve the ordered encapsulation of guests and long-term stability of chiral structures, providing new promising host platforms for the development of DF and CPL. In this review, we provide a comprehensive and critical summary of the recent progress in host-guest photochemistry via the encapsulation engineering of guest emitters in PCFs, particularly focusing on delayed luminescence and circularly polarized luminescence. Initially, the general principle of phosphorescence, TADF and CPL, the combination of DF and CPL, and energy transfer processes between host and guests are introduced. Subsequently, we comprehensively discuss the critical factors affecting the encapsulation engineering of guest emitters in PCFs, such as pore structures, the confinement effect, charge and energy transfer between the host and guest, conformational dynamics, and aggregation model of guest emitters. Thereafter, we summarize the effective methods for the preparation of host-guest systems, especially single-crystal-to-single-crystal (SC-SC) transformation and epitaxial growth, which are distinct from conventional methods based on amorphous materials. Then, the recent advancements in host-guest systems based on PCFs for delayed luminescence and circularly polarized luminescence are highlighted. Finally, we present our personal insights into the challenges and future opportunities in this promising field.

M(II) effect on encapsulation of guests into a series of M3L2 chiral cages: enantio-recognition

Self-assembly of M(ClO4)2 (M2+ = Ni2+, Cu2+, and Zn2+) with (1S,1'S,1''S,2R,2'R,2''R)-(benzenetricarbonyltris(azanediyl))tris(2,3-dihydro-1H-indene-2,1-diyl) trinicotinate (s,r-L) and the corresponding enantiomer (r,s-L) as a pair of chiral tridentate donors gives rise to the chiral cage pairs [M3(s,r- and r,s-L)2](ClO4)6. For the two pairs of [(Me2CO)(H2O)@M3(r,-s and s,r-L)2](ClO4)6 (M2+ = Ni2+ and Zn2+), the inner cavity is occupied by both an acetone and a single water molecule, whereas for the copper(II) pair of [Me2CO@Cu3(r,s- and s,r-L)2](ClO4)6 under the same conditions, the cavity is filled by only one acetone molecule. Thus, the encapsulation of guest molecules into the cages during self-assembly shows significant metal(II) ion effects. These chiral cages are effective for the enantio-recognition of chiral (S)-2-butanol and (R)-2-butanol via the shifts of the electrochemical oxidation potentials obtained by the linear sweep voltammetry (LSV) technique, density functional theory (DFT) calculations, and the chiral 2-butanol adsorption in the single-crystal-to-single-crystal (SCSC) mode.

Biomass-derived carbon dots as emerging visual platforms for fluorescent sensing

Biomass-derived carbon dots (CDs) are non-toxic and fluorescently stable, making them suitable for extensive application in fluorescence sensing. The use of cheap and renewable materials not only improves the utilization rate of waste resources, but it is also drawing increasing attention to and interest in the production of biomass-derived CDs. Visual fluorescence detection based on CDs is the focus of current research. This method offers high sensitivity and accuracy and can be used for rapid and accurate determination under complex conditions. This paper describes the biomass precursors of CDs, including plants, animal remains and microorganisms. The factors affecting the use of CDs as fluorescent probes are also discussed, and a brief overview of enhancements made to the preparation process of CDs is provided. In addition, the application prospects and challenges related to biomass-derived CDs are demonstrated.

Tailoring Metallosupramolecular Glycoassemblies for Enhancing Lectin Recognition

Multivalency is a fundamental principle in nature that leads to high-affinity intermolecular recognition through multiple cooperative interactions that overcome the weak binding of individual constituents. For example, multivalency plays a critical role in lectin-carbohydrate interactions that participate in many essential biological processes. Designing high-affinity multivalent glycoconjugates that engage lectins results in systems with the potential to disrupt these biological processes, offering promising applications in therapeutic design and bioengineering. Here, a versatile and tunable synthetic platform for the synthesis of metallosupramolecular glycoassemblies is presented that leverages subcomponent self-assembly, which employs metal ion templates to generate complex supramolecular architectures from simple precursors in one pot. Through ligand design, this approach provides precise control over molecular parameters such as size, shape, flexibility, valency, and charge, which afforded a diverse family of well-defined hybrid glyconanoassemblies. Evaluation of these complexes as multivalent binders to Concanavalin A (Con A) by isothermal titration calorimetry (ITC) demonstrates the optimal saccharide tether length and the effect of electrostatics on protein affinity, revealing insights into the impact of synthetic design on molecular recognition. The presented studies offer an enhanced understanding of structure-function relationships governing lectin-saccharide interactions at the molecular level and guide a systematic approach towards optimizing glyconanoassembly binding parameters.

Guest-Responsive Near-Infrared-Luminescent Metal-Organic Cage Organized by Porphyrin Dyes and Yb(III) Complexes

Metal-organic cages (MOCs) with luminophores have significant advantages for the facile detection of specific molecules based on turn-on or turn-off luminescence changes induced by host-guest complexation. One important challenge is the development of turn-on-type near-infrared (NIR)-luminescent MOCs. In this study, we synthesized a novel MOC consisting of two porphyrin dyes linked by four Yb(III) complexes, which exhibit bimodal red and NIR fluorescence signals upon photoexcitation of the porphyrin π system. Single-crystal X-ray structural analysis and computational molecular modeling revealed that planar aromatic perfluorocarbons were intercalated into the MOC. The tight packing between the MOC and guests enhanced the NIR fluorescence of Yb(III) by suppressing energy transfer from the photoexcited porphyrin to oxygen molecules. Guest-responsive turn-on NIR fluorescence changes in an MOC were successfully demonstrated.

Naphthalene Diimide-Based Metallacage as an Artificial Ion Channel for Chloride Ion Transport

Developing synthetic molecular devices for controlling ion transmembrane transport is a promising research field in supramolecular chemistry. These artificial ion channels provide models to study ion channel diseases and have huge potential for therapeutic applications. Compared with self-assembled ion channels constructed by intermolecular weak interactions between smaller molecules or cyclic compounds, metallacage-based ion channels have well-defined structures and can exist as single components in the phospholipid bilayer. A naphthalene diimide-based artificial chloride ion channel is constructed through efficient subcomponent self-assembly and its selective ion transport activity in large unilamellar vesicles and the planar lipid bilayer membrane by fluorescence and ion-current measurements is investigated. Molecular dynamics simulations and density functional theory calculations show that the metallacage spans the entire phospholipid bilayer as an unimolecular ion transport channel. This channel transports chloride ions across the cell membrane, which disturbs the ion balance of cancer cells and inhibits the growth of cancer cells at low concentrations.

Strategy for establishing sensitive fluorescent sensor toward p-nitrophenol integrating magnetic molecularly imprinted materials and carbon dots

In this work, a sensitive fluorescent sensor toward p-nitrophenol (4-NP) integrating magnetic molecularly imprinted materials and carbon dots (CDs) was proposed. Magnetic material and CDs derived from K3 [Fe(CN)6] and glucose were simultaneously obtained through simple one-step hydrothermal process. Introducing of molecularly imprinted materials based magnetic solid phase extraction (MSPE) endowed the constructed fluorescent sensor with higher sensitivity and selectivity. The significant factors affecting the sensitivity of the sensor toward 4-NP were optimized. Good linearity was obtained between fluorescent intensity of CDs and different concentration of 4-NP from 0.08 to 62.5 μg L-1. The sensitivity of constructed sensor was very low with detection limit of 0.02 μg L-1. Reliable applicability was also proved by the well-pleasing recoveries of 94.2-97.8% with different spiked concentrations of 4-NP in real environmental waters.

Non-Templated Assembly of D5h-Symmetric Pd5L10 Rings by Precise Ligand Angle Adjustment

We report a series of Pd(II)nL2n coordination rings for which nuclearity is controlled by the binding angle of the corresponding bis-monodentate bridging ligands. Judicious choice of the angle within a family of rather rigid ligands allowed for the first-time to synthesize a homoleptic five-membered Pd5L10 ring that does not require any template to form. We demonstrate that control over the ring size is maintained both in the solid-, solution-, and gas-phase. Two X-ray structures of five-membered rings from ligands with ideal angles (yielding a perfect pentagonal ring) vs. suboptimal angles (resulting in a highly distorted structure) illustrate the importance of the correct ligand geometry. A mathematical model for estimating the expected ring size based on the ligand angle was derived and DFT computations show that ring-strain is the major factor determining the assembly outcome.

Chiral Truxene-Based Self-Assembled Cages: Triple Interlocking and Supramolecular Chirogenesis

Incorporating chiral elements in host-guest systems currently attracts much attention because of the major impact such structures may have in a wide range of applications, from pharmaceuticals to materials science and beyond. Moreover, the development of multi-responsive and -functional systems is highly desirable since they offer numerous benefits. In this context, we describe herein the construction of a metal-driven self-assembled cage that associates a chiral truxene-based ligand and a bis-ruthenium complex. The maximum separation between both facing chiral units in the assembly is fixed by the intermetallic distance within the lateral bis-ruthenium complex (8.4 Å). The resulting chiral cavity was shown to encapsulate polyaromatic guest molecules, but also to afford a chiral triply interlocked [2]catenane structure. The formation of the latter occurs at high concentration, while its disassembly could be achieved by the addition of a planar achiral molecule. Interestingly the planar achiral molecule exhibits induced circular dichroism signature when trapped within the chiral cavity, thus demonstrating the ability of the cage to induce supramolecular chirogenesis.

Non-statistical assembly of multicomponent [Pd2ABCD] cages

Self-assembled hosts, inspired by biological receptors and catalysts, show application potential in sustainable synthesis, energy conversion and medicine. Implementing multiple functionalities in the form of distinguishable building blocks, however, is difficult without risking narcissistic self-sorting or a statistical mess. Here we report a systematic series of integratively self-assembled heteroleptic cages in which two square-planar PdII cations are bridged by four different bis-pyridyl ligands, A, B, C and D, via synergistic effects to exclusively form a single isomer-the lantern-shaped cage [Pd2ABCD]. This self-sorting goal-forming just one out of 55 possible structures-is reached under full thermodynamic control and can be realized progressively (by combining progenitors, such as [Pd2A2C2] with [Pd2B2D2]), directly from ligands and PdII cations or by mixing all four corresponding homoleptic cages. The rational design of complex multicomponent assemblies that enables the modular incorporation of diverse chemical moieties will advance their applicability in functional nanosystems.

54 K Spin Transition Temperature Shift in a Fe6L4 Octahedral Cage Induced by Optimal Fitted Multiple Guests

Spin-crossover (SCO) coordination cages are at the forefront of research for their potential in crafting next-generation molecular devices. However, due to the scarcity of SCO hosts and their own limited cavities, the interplay between the SCO host and the multiple guests binding has remained elusive. In this contribution, we present a family of pseudo-octahedral coordination cages (M6L4, M = ZnII, CoII, FeII, and NiII) assembled from a tritopic tridentate ligand L with metal ions. The utilization of FeII ion leads to the successful creation of the Fe6L4-type SCO cage. Host-guest studies of these M6L4 cages reveal their capacity to encapsulate four adamantine-based guests. Notably, the spin transition temperature T1/2 of Fe6L4 is dependent on the multiple guests encapsulated. The inclusion of adamantine yields an unprecedented T1/2 shift of 54 K, a record shift in guest-mediated SCO coordination cages to date. This drastic shift is ascribed to the synergistic effect of multiple guests coupled with their optimal fit within the host. Through a straightforward thermodynamic cycle, the binding affinities of the high-spin (HS) and low-spin (LS) states are separated from their apparent binding constant. This result indicates that the LS state has a stronger binding affinity for the multiple guests than the HS state. Exploring the SCO thermodynamics of host-guest complexes allows us to examine the optimal fit of multiple guests to the host cavity. This study reveals that the T1/2 of the SCO host can be manipulated by the encapsulation of multiple guests, and the SCO cage is an ideal candidate for determining the multiple guest fit.

A water-soluble Pd4 molecular tweezer for selective encapsulation of isomeric quinones and their recyclable extraction

Quinones (QN) are one of the main components of diesel exhaust particulates that have significant detrimental effects on human health. Their extraction and purification have been challenging tasks because these atmospheric particulates exist as complex matrices consisting of inorganic and organic compounds. In this report, we introduce a new water soluble Pd4L2 molecular architecture (MT) with an unusual tweezer-shaped structure obtained by self-assembly of a newly designed phenothiazine-based tetra-imidazole donor (L) with the acceptor cis-[(tmeda)Pd(NO3)2] (M) [ tmeda = N,N,N',N'-tetramethylethane-1,2-diamine]. The molecular tweezer encapsulates some quinones existing in diesel exhaust particulates (DEPs) leading to the formation of host-guest complexes in 1 : 1 molar ratio. Moreover, MT binds phenanthrenequinone (PQ) more strongly than its isomer anthraquinone (AQ), an aspect that enables extraction of PQ with a purity of 91% from an equimolar mixture of the two isomers. Therefore, MT represents an excellent example of supramolecular receptor capable of selective aqueous extraction of PQ from PQ/AQ with many cycles of reusability.

A rhodamine coumarin-derived fluorescence probe that selectively detects Fe3+ and measures radiation doses

We synthesized a fluorescence ratiometric probe by combining coumarin and rhodamine B with ethylenediamine to sense Fe3+ and measure ionizing radiation doses. The presence of Fe3+ caused rhodamine to transition from a closed helical structure to an open-ring structure. Additionally, fluorescence resonance energy transfer (FRET) occurred between coumarin and rhodamine B. As a result, the fluorescence intensity at 405 nm (I405) due to coumarin was decreased, whereas that at 585 nm (I585) derived from open-ring structure rhodamine B was increased. The ratio of I585 and I405 (I585/I405) linearly increased as the Fe3+ concentration increased. The probe sensed Fe3+ in a 0-110 μM range, with a lower limit of detection (LOD) of 0.226 μM. Inspired by Fricke dosimeters, we extended the probe to measure X-ray doses using the fluorescence methodology. The probe measured X-ray doses in a 0-30 Gy range with a lower LOD of 0.5 Gy. Additionally, the dosing capability was independent of the dosing rates. Our probe showed potential for detecting Fe3+ and measuring ionizing radiation doses.

Diastereoselective Self-Assembly of Low-Symmetry Pdn L2n Nanocages through Coordination-Sphere Engineering

Metal-organic cages (MOCs) are popular host architectures assembled from ligands and metal ions/nodes. Assembling structurally complex, low-symmetry MOCs with anisotropic cavities can be limited by the formation of statistical isomer libraries. We set out to investigate the use of primary coordination-sphere engineering (CSE) to bias isomer selectivity within homo- and heteroleptic Pdn L2n cages. Unexpected differences in selectivities between alternative donor groups led us to recognise the significant impact of the second coordination sphere on isomer stabilities. From this, molecular-level insight into the origins of selectivity between cis and trans diastereoisomers was gained, highlighting the importance of both host-guest and host-solvent interactions, in addition to ligand design. This detailed understanding allows precision engineering of low-symmetry MOC assemblies without wholesale redesign of the ligand framework, and fundamentally provides a theoretical scaffold for the development of stimuli-responsive, shape-shifting MOCs.

Recent applications of organic cages in sensing and separation processes in solution

Organic cages are three-dimensional polycyclic compounds of great interest in the scientific community due to their unique features, which generally include simple synthesis based on the dynamic covalent chemistry strategies, structural tunability and high selectivity. In this feature article, we present the advances over the last ten years in the application of organic cages as chemosensors or components in chemosensing devices for the determination of analytes (pollutants, analytes of biological interest) in complex aqueous media including wine, fruit juice, urine. Details on the recent applications of organic cages as selective (back-)extractants or masking agents for potential applications in relevant separation processes, such as the plutonium and uranium recovery by extraction, are also provided. Over the last ten years, organic cages with permanent porosity in the liquid and solid states have been highly appreciated as porous materials able to discriminate molecules of different sizes. These features, combined with good solvent processability and film-forming tendency, have proved useful in the fabrication of membranes for gas separation, solvent nanofiltration and water remediation processes. An overview of the recent applications of organic cages in membrane separation technologies is given.

Analytical Tools and Methods for Explosive Analysis in Forensics: A Critical Review

This review summarizes (i) compositions and types of improvised explosive devices; (ii) the process of collection, extraction and analysis of explosive evidence encountered in explosive and related cases; (iii) inter-comparison of analytical techniques; (iv) the challenges and prospects of explosive detection technology. The highlights of this study include extensive information regarding the National & International standards specified by USEPA, ASTM, and so on, for explosives detection. The holistic development of analytical tools for explosive analysis ranging from conventional methods to advanced analytical tools is also covered in this article. The most important aspect of this review is to make forensic scientists familiar with the challenges during explosive analysis and the steps to avoid them. The problems during analysis can be analyte-based, that is, interferences due to matrix or added molding/stabilizing agents, trace amount of parent explosives in post-blast samples and many more. Others are techniques-based challenges viz. specificity, selectivity, and sensitivity of the technique. Thus, it has become a primary concern to adopt rapid, field deployable, and highly sensitive techniques.

Flourescence sensors for heavy metal detection: major contaminants in soil and water bodies

Due to the increasing consumption of heavy metals, there is a rising need for specific and useful methods that are employed for the detection of heavy metals. Fluorescence sensing is a highly selective, rapid and biosensing technique that is employed in the determination of some heavy metals in any sample of soil or water, any other living person, the food being consumed or any other substance which are being used daily. These fluorescent methods are a type of analytical technique and they are mainly based on detection. Many types of metal conjugated molecules have been used of the detection of these heavy metals with various mechanisms. We have taken into account some specific sensor molecules as they were more suitable and easily accessible. These techniques that were employed in the detection of various heavy metals such as copper, lead and mercury have been discussed in the following review article.

Formation Process of SiF6@Cu2L4 Chiral Cage Pairs in a Glass Vessel: Catechol Oxidation Catalysis and Chiral Recognition

Self-assembly of CuX2 (X- = BF4-, PF6-, and SbF6-) with a pair of chiral bidentate ligands, (1R,2S)-(+)- and (1S,2R)-(-)-1-(nicotinamido)-2,3-dihydro-1H-inden-2-yl-nicotinate (r,s-L or s,r-L), in a mixture solvent including ethanol in a glass vessel gives rise to SiF62--encapsulated Cu2L4 chiral cage products. The SiF62- anion from the reaction of X- with SiO2 of the glass-vessel surface acts as a cage template or cage bridge. One of the products, [SiF6@Cu2(SiF6)(s,r-L)4]·3CHCl3·4EtOH, is one of the most effective heterogeneous catalysts for the oxidation of 3,5-di-tert-butylcatechol. Furthermore, an l-DOPA/d-DOPA pair is recognizable by the cyclic voltammetry (CV) signals of its combination with chiral cages [SiF6@Cu2(BF4)2(s,r- or r,s-L)4]·4CHCl3·2EtOH pair and [SiF6@Cu2(SiF6)(s,r- or r,s-L)4]·3CHCl3·4EtOH pair.

Dynamic Cages-Towards Nanostructured Smart Materials

The interest in capsular assemblies such as dynamic organic and coordination cages has blossomed over the last decade. Given their chemical and structural variability, these systems have found applications in diverse fields of research, including energy conversion and storage, catalysis, separation, molecular recognition, and live-cell imaging. In the exploration of the potential of these discrete architectures, they are increasingly being employed in the formation of more complex systems and smart materials. This Review highlights the most promising pathways to overcome common drawbacks of cage systems (stability, recovery) and discusses the most promising strategies for their hybridization with systems featuring various dimensionalities. Following the description of the most recent advances in the fabrication of zero to three-dimensional cage-based systems, this Review will provide the reader with the structure-dependent relationship between the employed cages and the properties of the materials.

Unexpected structural complexity of d-block metallosupramolecular architectures within the benzimidazole-phenoxo ligand scaffold for crystal engineering aspects

Design of metallosupramolecular materials encompassing more than one kind of supramolecular interaction can become deceptive, but it is necessary to better understand the concept of the controlled formation of supramolecular systems. Herein, we show the structural diversity of the bis-compartmental phenoxo-benzimidazole ligand H3L1 upon self-assembly with variety of d-block metal ions, accounting for factors such as: counterions, pH, solvent and reaction conditions. Solid-state and solution studies show that the parent ligand can accommodate different forms, related to (de)protonation and proton-transfer, resulting in the formation of mono-, bi- or tetrametallic architectures, which was also confirmed with control studies on the new mono-compartmental phenoxo-benzimidazole H2L2 ligand analogue. For the chosen architectures, structural variables such as porous character, magnetic behaviour or luminescence studies were studied to demonstrate how the form of H3L1 ligand affects the final form of the supramolecular architecture and observed properties. Such complex structural variations within the benzimidazole-phenoxo-type ligand have been demonstrated for the first time and this proof-of-concept can be used to integrate these principles in more sophisticated architectures in the future, combining both the benzimidazole and phenoxide subunits. Ultimately, those principles could be utilized for targeted manipulation of properties through molecular tectonics and crystal engineering aspects.

Gas Permeation through Mechanically Resistant Self-Standing Membranes of a Neat Amorphous Organic Cage

The synthesis and characterization of a novel film-forming organic cage and of its smaller analogue are here described. While the small cage produced single crystals suitable for X-ray diffraction studies, the large one was isolated as a dense film. Due to its remarkable film-forming properties, this latter cage could be solution processed into transparent thin-layer films and mechanically stable dense self-standing membranes of controllable thickness. Thanks to these peculiar features, the membranes were also successfully tested for gas permeation, reporting a behavior similar to that found with stiff glassy polymers such as polymers of intrinsic microporosity or polyimides. Given the growing interest in the development of molecular-based membranes, for example for separation technologies and functional coatings, the properties of this organic cage were investigated by thorough analysis of their structural, thermal, mechanical and gas transport properties, and by detailed atomistic simulations.

The preservation of sarin and O,O'-diisopropyl fluorophosphate inside coordination cage hosts

The host-guest chemistry of O,O'-diisopropyl fluorophosphate (DFP), a phosphonofluoridate G-series chemical warfare agent simulant, was investigated in the presence of a number of octanuclear cubic coordination cage hosts. The aim was to demonstrate cage-catalysed hydrolysis of DFP at near neutral pH: however, two octanuclear coordination cages, HPEG (containing water-solubilising PEG groups) and HW (containing water-solubilising hydroxymethyl groups), were actually found to increase the lifetime of DFP in aqueous buffer solution (pH 8.7). Crystallographic analysis of DFP with a structurally related host cage revealed that DFP binds to windows in the cage surface, not in the internal cavity. The phosphorus-fluorine bond is directed into the cavity rather than towards the external environment, with the cage/DFP association protecting DFP from hydrolysis. Initial studies with the chemical warfare agent (CWA) sarin (GB) with HPEG cage in a buffered solution also showed a drastically reduced rate of hydrolysis for sarin when bound in the host cage. The ability of these cages to inhibit hydrolysis of these P-F bond containing organophosphorus guests, by encapsulation, may have applications in forensic sample preservation and analysis.

Anionic Templates Drive Conversion between a ZnII9L6 Tricapped Trigonal Prism and ZnII6L4 Pseudo-Octahedra

This work introduces the use of 8-aminoquinoline subcomponents to generate complex three-dimensional structures. Together with a tris(formylpyridine), 8-aminoquinoline condensed around Zn^II templates to produce a tris(tridentate) ligand. This ligand is incorporated into either a tricapped trigonal prismatic Zn^II₉L₆ structure or a pair of pseudo-octahedral Zn^II₆L₄ diastereomers, with S₄ and D₂ symmetries. Introduction of a methyl group onto the aminoquinoline modulated the coordination sphere of Zn^II, which favored the Zn^II₉L₆ structure and disfavored the Zn^II₆L₄ assembly. The tricapped trigonal prismatic Zn^II₉L₆ architecture converted into a single Zn^II₆L₄ cage diastereomer following the addition of a dianionic 4,4'-dinitrostilbene-2,2'-disulfonate guest. Four of these guests clustered tightly at the four windows of the Zn^II₆L₄ cage, held in place through electrostatic interactions and hydrogen bonding, stabilize a single diastereomeric configuration with S₄ symmetry.

Triphenylamine-Functionalized Coordination Cage as a Supramolecular Fluorescence Sensor for Sequential Detection of Aluminum Ions and Nitrofurantoin

Coordination cages with a well-defined nanocavity are a class of promising supramolecular materials for molecular recognition and sensing. However, their applications in sequential sensing of multiple types of pollutants are highly desirable yet extremely limiting and challenging. Herein, we demonstrate a convenient strategy to develop a supramolecular fluorescence sensor for sequentially detecting environmental pollutants of aluminum ions and nitrofurantoin. A coordination cage (Ni-NTB), adopting an octahedral structure with triphenylamine chromophores occupying on the faces, is weakly emissive in solution due to the intramolecular rotations of the phenyl rings. Ni-NTB exhibits sensitive and selective fluorescence "off-on-off" processes during consecutive sensing of Al³⁺ and nitrofurantoin, an antibacterial drug. These sequential detection processes are highly interference-tolerant and visually observable with the naked eye. Mechanism studies reveal that the fluorescence switch is controllable by tuning the degree of intramolecular rotations of the phenyl rings and the pathway of intermolecular charge transfer, which is associated with the host-guest interaction. Moreover, the fabrication of Ni-NTB on test strips enabled a quick naked-eye sequential sensing of Al³⁺ and nitrofurantoin in seconds. Hence, this novel supramolecular fluorescence "off-on-off" sensing platform provides a new approach to developing supramolecular functional materials for monitoring environmental pollution.

Selective Supramolecular Recognition of Nitroaromatics by a Fluorescent Metal-Organic Cage Based on a Pyridine-Decorated Dibenzodiaza-Crown Macrocyclic Co(II) Complex

Two isomorphous fluorescent (FL) lantern-shaped metal-organic cages 1 and 2 were prepared by coordination-directed self-assembly of Co(II) centers with a new aza-crown macrocyclic ligand bearing pyridine pendant arms (L_py). The cage structures were determined using single-crystal X-ray diffraction analysis, thermogravimetric, elemental microanalysis, FT-IR spectroscopy, and powder X-ray diffraction. The crystal structures of 1 and 2 show that anions (Cl^- in 1 and Br^- in 2) are encapsulated within the cage cavity. 1 and 2 bear two coordinated water molecules that are directed inside the cages, surrounded by the eight pyridine rings at the "bottom" and the "roof" of the cage. These hydrogen bond donors, π systems, and the cationic nature of the cages enable 1 and 2 to encapsulate the anions. FL experiments revealed that 1 could detect nitroaromatic compounds by exhibiting selective and sensitive fluorescence quenching toward p-nitroaniline (PNA), recommending a limit of detection of 4.24 ppm. Moreover, the addition of 50 μL of PNA and o-nitrophenol to the ethanolic suspension of 1 led to a significant large FL red shift, namely, 87 and 24 nm, respectively, which were significantly higher than the corresponding values observed in the presence of other nitroaromatic compounds. The titration of the ethanolic suspension of 1, with various concentrations of PNA (>12 μM) demonstrated a concentration-dependent emission red shift. Hence, the efficient FL quenching of 1 was capable of distinguishing the dinitrobenzene isomers. Meanwhile, the observed red shift (10 nm) and quenching of this emission band under the influence of a trace amount of o- and p-nitrophenol isomers also showed that 1 could discriminate between o- and p-nitrophenol. Replacement of the chlorido with a bromido ligand in 1 generated cage 2 which was a more electron-donating cage than 1. The FL experiments showed that 2 was partially more sensitive and less selective toward NACs than 1.

Converting commonly-used paper into nano-engineered fluorescent biomass-based platform for rapid ClO- quantitative detection in living cells and water sources

Hypochlorous acid (HClO) and derivative ionic form (ClO^-) are significant components of reactive oxygen species, and thus various diseases are correlatively related to the concentration of ClO^-. Recently, paper-based indicators have been confirmed to be efficient strategy for sensing hazardous and noxious substances. However, most of these materials can only achieve qualitative detection of the substrates. Herein, an extremely simple manufacturing strategy was proposed to convert commonly-used paper into nano-engineered fluorescent biomass-based platform (CMJL-FP) integrated with on-demand self-assembled colorimetric and ratiometric fluorescence sensor (CMJL) for rapid ClO^- quantitative detection in organisms or water sources using smartphones. The CMJL exhibited a highly selective and sensitive ratiometric response to ClO^- at a low detection limit (LOD = 92.6 nM). The associating interactions between the fluorescence nano-particles and micro-nano fibers of CMJL-FP ensure good-stability during ClO^- detection. It has been experimentally demonstrated that CMJL-FP allows one to realize the rapid quantitative detection of ClO^- ions in living cells and large-scale water sources by using color recognition software as part of a simple smartphone. Therefore, integrating the proposed fluorescent paper with smartphones provides an effective, sustainable, cheap and conceptual strategy for quantitative detection of hazardous and noxious substances in organisms and environments.

Solvent Induced Conversion of a Self-Assembled Gyrobifastigium to a Barrel and Encapsulation of Zinc-Phthalocyanine within the Barrel for Enhanced Photodynamic Therapy

A rare gyrobifastigium architecture (GB) was constructed by self-assembly of a tetradentate donor (L) with Pd^II acceptor in DMSO. The GB was converted to its isomeric tetragonal barrel (MB) upon treatment with water. The hydrophobic cavity of MB has been explored for the encapsulation of zinc-phthalocyanine (ZnPc), which is an excellent photosensitizer for photodynamic therapy (PDT). However, the poor water-solubility and aggregation tendency are the main reasons for the suboptimal PDT performance of free ZnPc in the aqueous medium. Effective solubilization of ZnPc in an aqueous medium was achieved by encapsulating it in the cavity of MB. The inclusion complex (ZnPc⊂MB) showed enhanced singlet oxygen generation in water. Higher cellular uptake and anticancer activity of the ZnPc⊂MB compared to free ZnPc on HeLa cells indicate that encapsulation of ZnPc in an aqueous host is a potential strategy for enhancement of its PDT activity in water.

Modular enhancement of circularly polarized luminescence in Pd2A2B2 heteroleptic cages

Metal-mediated assembly allows us to combine an achiral emissive ligand A with different chiral ligands (such as B) in a non-statistical fashion, obtaining Pd₂A₂B₂ heteroleptic cages showing circularly polarized luminescence (CPL). By using the 'shape complementary assembly' (SCA) strategy, the cages are exclusively obtained as cis-Pd₂A₂B₂ stereoisomers, as confirmed by NMR, MS and DFT analyses. Their unique chiroptical properties derive from the synergy of all the building blocks. Ligand B imparts the chiral information of its aliphatic backbone, comprising two stereogenic sp³ carbon centres, to the overall structure, causing CD and CPL signal induction for the chromophore on ligand A. The heteroleptic cage shows CPL with a |g_lum| value of 2.5 × 10^-3, which is 3-times higher than that for a progenitor based on aromatic helical building block H, thus opening a rational route towards optimizing the CPL properties of self-assembled nanostructures in a modular way.

Metallic–Organic Cages (MOCs) with Heterometallic Character: Flexibility-Enhancing MOFs

The dichotomy between metal–organic frameworks (MOFs) and metal–organic cages (MOCs) opens up the research spectrum of two fields which, despite having similarities, both have their advantages and disadvantages. Due to the fact that they have cavities inside, they also have applicability in the porosity sector. Bloch and coworkers within this evolution from MOFs to MOCs manage to describe a MOC with a structure of Cu2 paddlewheel Cu4L4 (L = bis(pyrazolyl)methane) with high precision thanks to crystallographic analyses of X-ray diffraction and also SEM-EDX. Then, also at the same level of concreteness, they were able to find the self-assembly of Pd(II)Cl2 moieties on the available nitrogen donor atoms leading to a [Cu4(L(PdCl2))4] structure. Here, calculations of the DFT density functional allow us to reach an unusual precision given the magnitude and structural complexity, explaining how a pyrazole ring of each bis(pyprazolyl)methane ligand must rotate from an anti to a syn conformation, and a truncation of the MOC structure allows us to elucidate, in the absence of the MOC constraint and its packing in the crystal, that the rotation is almost barrierless, as well as also explain the relative stability of the different conformations, with the anti being the most stable conformation. Characterization calculations with Mayer bond orders (MBO) and noncovalent interaction (NCI) plots discern what is important in the interaction of this type of cage with PdCl2 moieties, also CuCl2 by analogy, as well as simple molecules of water, since the complex is stable in this solvent. However, the L ligand is proved to not have the ability to stabilize an H2O molecule.

Spin Crossover Induced by Changing the Identity of the Secondary Metal Ion from PdII to NiII in a Face-Centered FeII 8 MII 6 Cubic Cage

Discrete spin crossover (SCO) heteronuclear cages are a rare class of materials which have potential use in next-generation molecular transport and catalysis. Previous investigations of cubic cage [Fe₈ Pd₆ L₈ ]²⁸⁺ constructed using semi-rigid metalloligands, found that Fe^II centers of the cage did not undergo spin transition. In this work, substitution of the secondary metal center at the face of the cage resulted in SCO behavior, evidenced by magnetic susceptibility, Mössbauer spectroscopy and single crystal X-ray diffraction. Structural comparisons of these two cages shed light on the possible interplay of inter- and intramolecular interactions associated with SCO in the Ni^II analogue, 1 ([Fe₈ Ni₆ L₈ (CH₃ CN)₁₂ ]²⁸⁺ ). The distorted octahedral coordination environment, as well as the occupation of the CH₃ CN in the Ni^II axial positions of 1, prevented close packing of cages observed in the Pd^II analogue. This led to offset, distant packing arrangements whereby important areas within the cage underwent dramatic structural changes with the exhibition of SCO.

Circularly Polarized Luminescent Eu4()4 Cage for Enantiomeric Excess and Concentration Simultaneous Determination of Chiral Diamines

Undoubtably, it is challenging to simultaneously determine the identity, enantiomeric excess (ee), and total concentration of an enantiomer by just one optical measurement. Herein, we design a chiral tetrahedron Eu₄(L^R)₄ with circularly polarized luminescence (CPL), which presents highly selective/stereoselective, rapid, and "turn-on" CPL response to chiral diamines, rather than the monoamino compounds, such as monoamines or amino alcohols. By recording the left- and right-CPL intensities of the Eu³⁺ ion at 591 nm, the enantiomeric composition and concentration of chiral diamines can be simultaneously determined by monitoring the g_lum value and total emission intensity (I_L + I_R), respectively. Spectroscopy analyses demonstrate that the variations of g_lum depend on the inversion and maintenance of configuration around the Eu³⁺ ion (Δ ↔ Λ), while the "turn-on" response arises from the raising of the T₁ state of the ligand. The molecule/electron structural variations are proposed from the synergetic supramolecular interactions of NH₂ groups with pendant diols and trifluoroacetyl groups.

Complementarity and Preorganisation in the Assembly of Heterometallic–Organic Cages via the Metalloligand Approach—Recent Advances

The design of new metallocage polyhedra towards pre-determined structures can offer both practical as well as intellectual challenges. In this mini-review we discuss a selection of recent examples in which the use of the metalloligand approach has been employed to overcome such challenges. An attractive feature of this approach is its stepwise nature that lends itself to the design and rational synthesis of heterometallic metal–organic cages, with the latter often associated with enhanced functionality.

Guest-Modulated Circularly Polarized Luminescence by Ligand-to-Ligand Chirality Transfer in Heteroleptic PdII Coordination Cages

Multicomponent metallo-supramolecular assembly allows the rational combination of different building blocks. Discrete multifunctional hosts with an accessible cavity can be prepared in a non-statistical fashion. We employ our shape-complementary assembly (SCA) method to achieve for the first time integrative self-sorting of heteroleptic PdII cages showing guest-tunable circularly polarized luminescence (CPL). An enantiopure helicene-based ligand (M or P configuration) is coupled with a non-chiral emissive fluorenone-based ligand (A or B) to form a series of Pd2 L2 L'2 assemblies. The modular strategy allows to impart the chiral information of the helicenes to the overall supramolecular system, resulting in CPL from the non-chiral component. Guest binding results in a 4-fold increase of CPL intensity. The principle offers potential to generate libraries of multifunctional materials with applications in molecular recognition, enantioselective photo-redox catalysis and information processing.

Assembly of a Heterometallic Cu(II)-Pd(II) Cage by Post-assembly Metal Insertion

Porous structures based on multi-metallic motifs are receiving growing interest, but their general preparation still remains a challenge. Here, we report the self-assembly and structure of a Cu^II metal-organic cage (MOC) that is functionalized with free bis(pyrazolyl)methane sites. The homometallic Cu₄L₄ cage is isolated as a water-stable crystalline solid, and its formation is dependent on metal-ligand stoichiometry and the pre-organization of the Cu₂ paddlewheel. We show by X-ray diffraction and SEM-EDX that Pd^II chloride can be quantitatively inserted into the free chelating sites of the MOC to yield a [Cu₄(L(PdCl₂))₄] structure. Moreover, the solvent employed in the metalation dictates the solid-state isomerism of the heterometallic cage─a further handle to control the MOC's structural diversity and permanent porosity.

Metal-organic cages against toxic chemicals and pollutants

The continuous release of toxic chemicals and pollutants into the atmosphere and natural waters threatens, directly and indirectly, human health, the sustainability of the planet, and the future of society. Materials capable of capturing or chemically inactivating hazardous substances, which are harmful to humans and the environment, are critical in the modern age. Metal-organic cages (MOCs) show great promise as materials against harmful agents both in solution and in solid state. This Highlight features examples of MOCs that selectively encapsulate, adsorb, or remove from a medium noxious gases, toxic organophosphorus compounds, water pollutant oxoanions, and some emerging organic contaminants. Remarkably, the toxicity of interacting contaminants may be lowered by MOCs as well. Specific cases pertaining to the use of these cages for the chemical degradation of some harmful substances are presented. This Highlight thus aims to provide an overview of the possibilities of MOCs in this area and new methodological insights into their operation for enhancing their activity and the engineering of further remediation applications.