Molecular shape sorting using molecular organic cages

Introducing chirality in porous organic cages through solid-state interactions

Molecular cages contain an internal cavity designed to encapsulate other molecules, resulting in applications in molecular separation, gas storage, and catalysis. Introducing chirality in cage molecules can improve the selective separation of chiral molecules and add new functionalities due to the realisation of chiral photophysical properties. It has recently been shown that solid-state supramolecular interactions between achiral cages can result in the formation of chiral cavities. Here, we develop a computational technique to predict when achiral cages form chiral cavities in the solid-state through the combination of atomistic calculations and coarse-grained modelling to predict the crystalline phase behaviour. Our focus is on the achiral cage B11, which contains rotatable arene rings on the vertices of the cage that can form propeller-like orientations, inducing a chiral cavity. We show that by using dimer pair calculations, we can inform coarse-grained models to predict the packing of the cage. Our results reveal how the supramolecular interactions drive chirality in the achiral cages without the need for a chiral guest. These findings are a first step towards understanding how we can design chirality through supramolecular interactions by using abstract coarse-grained models to inform design principles for targeted solid-state phase behaviour.

Single-Crystal Cage Framework with High Selectivity and Reversibility in Fullerene Binding

Host-guest chemistry, a pivotal branch of supramolecular chemistry, plays an essential role in understanding and constructing complex structures through non-covalent interactions. Organic molecular cages, characterized by their intrinsic confined cavities, can selectively bind a variety of guest molecules. Their host-guest chemistry has been well studied in the solution phase, and several attempts have been made to encode well-defined molecular architectures into solid-state polymeric materials. However, only limited studies have explored their potential in the solid state, where their lack of robustness and less ordered networks significantly hinder practical applications. Herein, we report the synthesis of a single-crystal cage framework and a systematic study of its host-guest chemistry, spanning from the solution state to the solid state. Our studies reveal that the host-guest interactions inherent to the cage are successfully maintained in the solid-state polymeric material. Furthermore, the framework's robustness allows for the guest molecules (fullerene) to be released triggered by an organic acid (trifluoroacetic acid), with subsequent regeneration of the framework through an organic base (triethylamine) treatment. Our findings represent the first synthesis of a robust, single-crystal cage framework exhibiting highly selective and reversible host-guest chemistry, thus showing great potential towards molecular separation application.

From individuals to families: design and application of self-similar chiral nanomaterials

Establishing an intimate relationship between similar individuals is the beginning of self-extension. Various self-similar chiral nanomaterials can be designed using an individual-to-family approach, accomplishing self-extension. This self-similarity facilitates chiral communication, transmission, and amplification of synthons. We focus on describing the marriage of discrete cages to develop self-similar extended frameworks. The advantages of utilizing cage-based frameworks for chiral recognition, enantioseparation, chiral catalysis and sensing are highlighted. To further promote self-extension, fractal chiral nanomaterials with self-similar and iterated architectures have attracted tremendous attention. The beauty of a fractal family tree lies in its ability to capture the complexity and interconnectedness of a family's lineage. As a type of fractal material, nanoflowers possess an overarching importance in chiral amplification due to their large surface-to-volume ratio. This review summarizes the design and application of state-of-the-art self-similar chiral nanomaterials including cage-based extended frameworks, fractal nanomaterials, and nanoflowers. We hope this formation process from individuals to families will inherit and broaden this great chirality.

General and Modular Synthesis of Covalent Organic Cages for Efficient Molecular Recognition

Cage-type structures based on coordination and dynamic covalent chemistry have the characteristics of facile and efficient preparation but poor stability. Chemically stable organic cages, generally involving fragment coupling and multi-step reactions, are relatively difficult to synthesize. Herein, we offer a general and modular strategy to customize covalent organic cages with diverse skeletons and sizes. First, one skeleton (S) module with three extension (E) modules and three reaction (R) modules are connected by one- or two-step coupling to get the triangular monomer bearing three reaction sites. Then one-pot Friedel-Crafts condensation of the monomer and linking module of paraformaldehyde produces the designed organic cages. The cage forming could be regulated by the geometrical configuration of monomeric blocks. The S-E-R angles in the monomer is crucial; only 120° (2,4-dimethoxyphen as reaction module) or 60° (2,5-dimethoxyphen as reaction module) angle between S-E-R successfully affords the resulting cages. By the rational design of the three modules, a series of organic cages have been constructed. In addition, the host-guest properties show that the representative cages could strongly encapsulate neutral aromatic diimide guests driven by solvophobic interactions in polar solvents, giving the highest association constant of (2.58±0.18)×105 M-1.

Reticular Modulation of Piezofluorochromic Behaviors in Organic Molecular Cages by Replacing Non-Luminous Components

Organic piezochromic materials that manifest pressure-stimuli-responses are important in various fields such as data storage and anticounterfeiting. The manipulation of piezofluorochromic behaviors for these materials is promising but remains a great challenge. Herein, a non-luminous components regulated strategy is developed and organic molecular cages (OMCs), a burgeoning class of crystalline organic materials with structural dynamics, are first explored for the design of piezofluorochromic materials with tunable luminescence. A series of OMCs based on aggregation-induced emission (AIE) chromophores, termed Cage 1-3, are synthesized and their piezofluorochromic behaviors are investigated by diamond anvil cell technique. Due to the sufficient voids between its flexible chromophores offered by the OMC structure, Cage 1 exhibits thermofluorochromic and piezofluorochromic properties. Moreover, the piezofluorochromic performance of this OMC could be further promoted by replacing its non-luminous components with improved flexibilities, and a remarkable luminescence peak shift by 150 nm together with a response sensitivity of 13.8 nm GPa-1 was achieved upon hydrostatic compression. The cage structure plays a vital role in facilitating efficient and reversible piezofluorochromic behaviors. This study has shed light on the rational design and exploitation of OMCs as an exceptional platform to accomplish customizable piezofluorochromic behaviors and enlarge their potential applications in pressure-based luminescence.

Programmable synthesis of organic cages with reduced symmetry

Integrating symmetry-reducing methods into self-assembly methodology is desirable to efficiently realise the full potential of molecular cages as hosts and catalysts. Although techniques have been explored for metal organic (coordination) cages, rational strategies to develop low symmetry organic cages remain limited. In this article, we describe rules to program the shape and symmetry of organic cage cavities by designing edge pieces that bias the orientation of the amide linkages. We apply the rules to synthesise cages with well-defined cavities, supported by evidence from crystallography, spectroscopy and modelling. Access to low-symmetry, self-assembled organic cages such as those presented, will widen the current bottleneck preventing study of organic enzyme mimics, and provide synthetic tools for novel functional material design.

Design and assembly of porous organic cages

Porous organic cages (POCs) represent a notable category of porous materials, showing remarkable material properties due to their inherent porosity. Unlike extended frameworks which are constructed by strong covalent or coordination bonds, POCs are composed of discrete molecular units held together by weak intermolecular forces. Their structure and chemical traits can be systematically tailored, making them suitable for a range of applications including gas storage and separation, molecular separation and recognition, catalysis, and proton and ion conduction. This review provides a comprehensive overview of POCs, covering their synthesis methods, structure and properties, computational approaches, and applications, serving as a primer for those who are new to the domain. A special emphasis is placed on the growing role of computational methods, highlighting how advanced data-driven techniques and automation are increasingly aiding the rapid exploration and understanding of POCs. We conclude by addressing the prevailing challenges and future prospects in the field.

Recent progress in iodine capture by macrocycles and cages

The effective capture of radioiodine is vital to the development of the nuclear industry and ecological environmental protection. There is, therefore, a continuously growing research exploration in various types of solid-state materials for iodine capture. During the last decade, the potential of using macrocycle and cage-based supramolecular materials in effective uptake and separation of radioactive iodine has been demonstrated. Interest in the application of these materials in iodine capture originates from their diversified porous characteristics, abundant host-guest chemistry, high iodine affinity and adsorption capacity, high stability in various environments, facile modification and functionalization, and intrinsic structural flexibility, among other attributes. Herein, recent progress in macrocycle and cage-based solid-state materials, including pure discrete macrocycles and cages, and their polymeric forms, for iodine capture is summarized and discussed with an emphasis on iodine capture capacities, mechanisms, and design strategies.

Highly Selective p-Xylene Separation from Mixtures of C8 Aromatics by a Nonporous Molecular Apohost

High and increasing production of separation of C8 aromatic isomers demands the development of purification methods that are efficient, scalable, and inexpensive, especially for p-xylene, PX, the largest volume C8 commodity. Herein, we report that 4-(1H-1,2,4-triazol-1-yl)-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (TPBD), a molecular compound that can be prepared and scaled up via solid-state synthesis, exhibits exceptional PX selectivity over each of the other C8 isomers, o-xylene (OX), m-xylene (MX), and ethylbenzene (EB). The apohost or α form of TPBD was found to exhibit conformational polymorphism in the solid state enabled by rotation of its triazole and benzene rings. TPBD-αI and TPBD-αII are nonporous polymorphs that transformed to the same PX inclusion compound, TPBD-PX, upon contact with liquid PX. TPBD enabled highly selective capture of PX, as established by competitive slurry experiments involving various molar ratios in binary, ternary, and quaternary mixtures of C8 aromatics. Binary selectivity values for PX as determined by 1H NMR spectroscopy and gas chromatography ranged from 22.4 to 108.4, setting new benchmarks for both PX/MX (70.3) and PX/EB (59.9) selectivity as well as close to benchmark selectivity for PX/OX (108.4). To our knowledge, TPBD is the first material of any class to exhibit such high across-the-board PX selectivity from quaternary mixtures of C8 aromatics under ambient conditions. Crystallographic and computational studies provide structural insight into the PX binding site in TPBD-PX, whereas thermal stability and capture kinetics were determined by variable-temperature powder X-ray diffraction and slurry tests, respectively. That TPBD offers benchmark PX selectivity and facile recyclability makes it a prototypal molecular compound for PX purification or capture under ambient conditions.

Deep generative design of porous organic cages via a variational autoencoder

Porous organic cages (POCs) are a class of porous molecular materials characterised by their tunable, intrinsic porosity; this functional property makes them candidates for applications including guest storage and separation. Typically formed via dynamic covalent chemistry reactions from multifunctionalised molecular precursors, there is an enormous potential chemical space for POCs due to the fact they can be formed by combining two relatively small organic molecules, which themselves have an enormous chemical space. However, identifying suitable molecular precursors for POC formation is challenging, as POCs often lack shape persistence (the cage collapses upon solvent removal with loss of its cavity), thus losing a key functional property (porosity). Generative machine learning models have potential for targeted computational design of large functional molecular systems such as POCs. Here, we present a deep-learning-enabled generative model, Cage-VAE, for the targeted generation of shape-persistent POCs. We demonstrate the capacity of Cage-VAE to propose novel, shape-persistent POCs, via integration with multiple efficient sampling methods, including Bayesian optimisation and spherical linear interpolation.

Size Determination of Organic Cages by Diffusion NMR Spectroscopy

Reliable structure elucidation of covalent organic cage compounds remains challenging as routine analysis might leave ambiguities. Diffusion-ordered NMR spectroscopy (DOSY) allows insight into the molecular size and mass of the species present in solution, but a systematic evaluation of the diffusion behavior for cage assemblies is rarely considered. Here we report the synthesis of four series of covalent organic cages based on tribenzotriquinacenes and diboronic acids with varying geometry and exohedral substituents. We provide a guideline for the consistent measurement of diffusion coefficients from 1 H-DOSY NMR spectroscopy, which was utilized to study the diffusion behavior for the whole set of cages and selected examples from the literature. For structurally similar cages, a linear correlation between the solvodynamic volume and the molecular mass allows precise size determination. For more complex systems, multiple parameters, such as window size or rigid exohedral functionalization. further modulate cage diffusion in solution.

Picking the lock of coordination cage catalysis

The design principles of metallo-organic assembly reactions have facilitated access to hundreds of coordination cages of varying size and shape. Many of these assemblies possess a well-defined cavity capable of hosting a guest, pictorially mimicking the action of a substrate binding to the active site of an enzyme. While there are now a growing collection of coordination cages that show highly proficient catalysis, exhibiting both excellent activity and efficient turnover, this number is still small compared to the vast library of metal-organic structures that are known. In this review, we will attempt to unpick and discuss the key features that make an effective coordination cage catalyst, linking structure to activity (and selectivity) using lessons learnt from both experimental and computational analysis of the most notable exemplars. We will also provide an outlook for this area, reasoning why coordination cages have the potential to become the gold-standard in (synthetic) non-covalent catalysis.

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.

Porous organic materials for iodine adsorption

The nuclear industry will continue to develop rapidly and produce energy in the foreseeable future; however, it presents unique challenges regarding the disposal of released waste radionuclides because of their volatility and long half-life. The release of radioactive isotopes of iodine from uranium fission reactions is a challenge. Although various adsorbents have been explored for the uptake of iodine, there is still interest in novel adsorbents. The novel adsorbents should be synthesized using reliable and economically feasible synthetic procedures. Herein, we discussed the state-of-the-art performance of various categories of porous organic materials including covalent organic frameworks, covalent triazine frameworks, porous aromatic frameworks, porous organic cages, among other porous organic polymers for the uptake of iodine. This review discussed the synthesis of porous organic materials and their iodine adsorption capacity and reusability. Finally, the challenges and prospects for iodine capture using porous organic materials are highlighted.

Industrial Separation Challenges: How Does Supramolecular Chemistry Help?

The chemical industry and the chemical processes underscoring it are under intense scrutiny as the demands for the transition to more sustainable and environmentally friendly practices are increasing. Traditional industrial separation systems, such as thermally driven distillation for hydrocarbon purification, are energy intensive. The development of more energy efficient separation technologies is thus emerging as a critical need, as is the creation of new materials that may permit a transition away from classic distillation-based separations. In this Perspective, we focus on porous organic cages and macrocycles that can adsorb guest molecules selectively through various host-guest interactions and permit molecular sieving behavior at the molecular level. Specifically, we summarize the recent advances where receptor-based adsorbent materials have been shown to be effective for industrially relevant hydrocarbon separations, highlighting the underlying host-guest interactions that impart selectivity and permit the observed separations. This approach to sustainable separations is currently in its infancy. Nevertheless, several receptor-based adsorbent materials with extrinsic/intrinsic voids or special functional groups have been reported in recent years that can selectively capture various targeted guest molecules. We believe that the understanding of the interactions that drive selectivity at a molecular level accruing from these initial systems will permit an ever-more-effective "bottom-up" design of tailored molecular sieves that, in due course, will allow adsorbent material-based approaches to separations to transition from the laboratory into an industrial setting.

Potential of nonporous adaptive crystals for hydrocarbon separation

Hydrocarbon separation is an important process in the field of petrochemical industry, which provides a variety of raw materials for industrial production and a strong support for the development of national economy. However, traditional separation processes involve huge energy consumption. Adsorptive separation based on nonporous adaptive crystal (NAC) materials is considered as an attractive green alternative to traditional energy-intensive separation technologies due to its advantages of low energy consumption, high chemical and thermal stability, excellent selective adsorption and separation performance, and outstanding recyclability. Considering the exceptional potential of NAC materials for hydrocarbon separation, this review comprehensively summarizes recent advances in various supramolecular host-based NACs. Moreover, the current challenges and future directions are illustrated in detail. It is expected that this review will provide useful and timely references for researchers in this area. Based on a large number of state-of-the-art studies, the review will definitely advance the development of NAC materials for hydrocarbon separation and stimulate more interesting studies in related fields.

Porous Organic Cages

Porous organic cages (POCs) are a relatively new class of low-density crystalline materials that have emerged as a versatile platform for investigating molecular recognition, gas storage and separation, and proton conduction, with potential applications in the fields of porous liquids, highly permeable membranes, heterogeneous catalysis, and microreactors. In common with highly extended porous structures, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), POCs possess all of the advantages of highly specific surface areas, porosities, open pore channels, and tunable structures. In addition, they have discrete molecular structures and exhibit good to excellent solubilities in common solvents, enabling their solution dispersibility and processability─properties that are not readily available in the case of the well-established, insoluble, extended porous frameworks. Here, we present a critical review summarizing in detail recent progress and breakthroughs─especially during the past five years─of all the POCs while taking a close look at their strategic design, precise synthesis, including both irreversible bond-forming chemistry and dynamic covalent chemistry, advanced characterization, and diverse applications. We highlight representative POC examples in an attempt to gain some understanding of their structure-function relationships. We also discuss future challenges and opportunities in the design, synthesis, characterization, and application of POCs. We anticipate that this review will be useful to researchers working in this field when it comes to designing and developing new POCs with desired functions.

Competitive aminal formation during the synthesis of a highly soluble, isopropyl-decorated imine porous organic cage

The synthesis of a new porous organic cage decorated with isopropyl moieties (CC21) was achieved from the reaction of triformylbenzene and an isopropyl functionalised diamine. Unlike structurally analogous porous organic cages, its synthesis proved challenging due to competitive aminal formation, rationalised using control experiments and computational modelling. The use of an additional amine was found to increase conversion to the desired cage.

An Organic Molecular Nanobarrel that Hosts and Solubilizes C60

Organic cages have gained increasing attention in recent years as molecular hosts and porous materials. Among these, barrel-shaped cages or molecular nanobarrels are promising systems to encapsulate large hosts as they possess windows of the same size as their internal cavity. However, these systems have received little attention and remain practically unexplored despite their potential. Herein, we report the design and synthesis of a new trigonal prismatic organic nanobarrel with two large triangular windows with a diameter of 12.7 Å optimal for the encapsulation of C60 . Remarkably, this organic nanobarrel shows a high affinity for C60 in solvents in which C60 is virtually insoluble, providing stable solutions of C60 .

A tetrahedron from homooxacalix[3]arene, the fifth Platonic polyhedron from calixarenes and uranyl

A self-assembled tetrahedral cage results from two C ₃-symmetry building blocks, namely, homooxacalix[3]arene tricarboxylate and uranyl cation, as demonstrated by X-ray crystallography. In the cage, four metals coordinate at the lower rim with the phenolic and ether oxygen atoms to shape the macrocycle with appropriate dihedral angles for tetrahedron formation, whereas four additional uranyl cations further coordinate at the upper-rim carboxylates to finalize the assembly. Counterions dictate the filling and porosity of the aggregates, whereas potassium induces highly porous structures, and tetrabutylammonium yields compact, densely packed frameworks. The tetrahedron metallo-cage complements our previous report (Pasquale et al., Nat. Commun., 2012, 3, 785) on uranyl-organic frameworks (UOFs) from calix[4]arene and calix[5]arene carboxylates (octahedral/cubic and icosahedral/dodecahedral giant cages, respectively) and completes the assembly of all five Platonic solids from just two chemical components.

Identifying porous cage subsets in the Cambridge Structural Database using topological data analysis

As rationally designable materials, the variety and number of synthesised metal-organic cages (MOCs) and organic cages (OCs) are expected to grow in the Cambridge Structural Database (CSD). In this regard, two of the most important questions are, which structures are already present in the CSD and how can they be identified? Here, we present a cage mining methodology based on topological data analysis and a combination of supervised and unsupervised learning that led to the derivation of - to the best of our knowledge - the first and only MOC dataset of 1839 structures and the largest experimental OC dataset of 7736 cages, as of March 2022. We illustrate the use of such datasets with a high-throughput screening of MOCs and OCs for xenon/krypton separation, important gases in multiple industries, including healthcare.

[2+3] Amide Cages by Oxidation of [2+3] Imine Cages – Revisiting Molecular Hosts for Highly Efficient Nitrate Binding

The pollution of groundwater with nitrate is a serious issue because nitrate can cause several diseases such as methemoglobinemia or cancer. Therefore, selective removal of nitrate by efficient binding to supramolecular hosts is highly desired. Here we describe how to make [2+3] amide cages in very high to quantitative yields by applying an optimized Pinnick oxidation protocol for the conversion of corresponding imine cages. By NMR titration experiments of the eight different [2+3] amide cages with nitrate, chloride and hydrogen sulfate we identified one cage with an unprecedented high selectivity towards nitrate binding vs. chloride (S=705) or hydrogensulfate (S>13500) in CD₂Cl₂/CD₃CN (1 : 3). NMR experiments as well as single‐crystal structure comparison of host‐guest complexes give insight into structure‐property‐relationships.

Challenges and opportunities for chiral covalent organic frameworks

As highly versatile crystalline porous materials, covalent organic frameworks (COFs) have emerged as an ideal platform for developing novel functional materials, attributed to their precise tunability of structure and functionality. Introducing chiral functional units into frameworks produces chiral COFs (CCOFs) with chiral superiorities through chirality conservation and conversion processes. This review summarises recent research progress in CCOFs, including synthetic methods, chiroptical characterisations, and their applications in asymmetric catalysis, chiral separation, and enantioselective recognition and sensing. Challenges and limitations are discussed to uncover future opportunities in CCOF research.

Transformation of Porous Organic Cages and Covalent Organic Frameworks with Efficient Iodine Vapor Capture Performance

The reaction of 5,5'-([2,2'-bipyridine]-5,5'-diyl)diisophthalaldehyde (BPDDP) with cyclohexanediamine and [benzidine (BZ)/[2,2'-bipyridine]-5,5'-diamine (BPDA)], respectively, affords a nitrogen-rich porous organic cage BPPOC and two two-dimensional (2D) covalent organic frameworks (COFs), USTB-1 and USTB-2 (USTB = University of Science and Technology Beijing), under suitable conditions. Interestingly, BPPOC with a single-crystal X-ray diffraction structure is able to successfully transform into USTB-1 and USTB-2 (newly converted COFs denoted as USTB-1c and USTB-2c, respectively) upon exchange of the imine unit of cyclohexanediamine in the cage by BZ and BPDA. Such a transformation also enables the isolation of analogous COFs (USTB-3c and USTB-4c) on the basis of an isostructural organic cage, BTPOC, which is derived from 5,5'-([2,2'-bithiophene]-4,4'-diyl)diisophthalaldehyde (BTDDP) and cyclohexanediamine. However, the conventional solvothermal reaction between BTDDP and BPDA leads to an impure phase of USTB-4 containing incompletely converted aldehyde groups due to the limited solubility of the building block. The newly prepared COFs have been characterized by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. In particular, BPPOC is able to absorb the iodine vapor with an uptake of 5.64 g g^-1, breaking the porous organic cage's (POC's) record value of 3.78 g g^-1. Nevertheless, the cage-derived COFs exhibit improved iodine vapor adsorption capability in comparison with the directly synthesized counterparts, with the highest uptake of 5.80 g g^-1 for USTB-1c. The mechanism investigation unveils the superiority of nitrogen atoms to sulfur atoms for POCs in iodine vapor capture with the assistance of definite crystal structures. This, in combination with porosity, synergistically influences the iodine vapor capture capacity of COFs.

Enantioselective assembly and recognition of heterochiral porous organic cages deduced from binary chiral components

Chiral recognition and discrimination is not only of significance in biological processes but also a powerful method to fabricate functional supramolecular materials. Herein, a pair of heterochiral porous organic cages (HPOC-1), out of four possible enantiomeric products, with mirror stereoisomeric crystal structures were cleanly prepared by condensation occurring in the exclusive combination of cyclohexanediamine and binaphthol-based tetraaldehyde enantiomers. Nuclear magnetic resonance and luminescence spectroscopy have been employed to monitor the assembly process of HPOC-1, revealing the clean formation of heterochiral organic cages due to the enantioselective recognition of (S,S)-binaphthol towards (R,R)-cyclohexanediamine derivatives and vice versa. Interestingly, HPOC-1 exhibits circularly polarized luminescence and enantioselective recognition of chiral substrates according to the circular dichroism spectral change. Theoretical simulations have been carried out, rationalizing both the enantioselective assembly and recognition of HPOC-1.

Guest-induced amorphous-to-crystalline transformation enables sorting of haloalkane isomers with near-perfect selectivity

The separation of haloalkane isomers with distillation-free strategies is one of the most challenging research topics in fundamental research and also gave high guiding values to practical industrial applications. Here, this contribution provides a previously unidentified solid supramolecular adsorption material based on a leggero pillararene derivative BrP[5]L, which can separate 1-/2-bromoalkane isomers with near-ideal selectivity. Activated solids of BrP[5]L with interesting amorphous and nonporous features could adsorb 1-bromopropane and 1-bromobutane from the corresponding equal volume mixtures of 1-/2-positional isomers with purities of 98.1 and 99.0%, respectively. Single-crystal structures incorporating theoretical calculation reveal that the high selectivity originates from the higher thermostability of 1-bromoalkane-loaded structures compared to its corresponding isomer-loaded structures, which could be further attributed to the perfect size/shape match between BrP[5]L and 1-bromoalkanes. Moreover, control experiments using its counterpart macrocycle of traditional pillararene demonstrate that BrP[5]L has better adsorptive selectivity, benefiting from the intrinsic free-rotation phenylene subunit on its backbone.

One-step removal of alkynes and propadiene from cracking gases using a multi-functional molecular separator

Refineries generally employ multiple energy-intensive distillation/adsorption columns to separate and purify complicated chemical mixtures. Materials such as multi-functional molecular separators integrating various modules capable of separating molecules according to their shape and chemical properties simultaneously may represent an alternative. Herein, we address this challenge in the context of one-step removal of alkynes and propadiene from cracking gases (up to 10 components) using a multi-functional and responsive material ZU-33 through a guest/temperature dual-response regulation strategy. The responsive and guest-adaptive properties of ZU-33 provide the optimized binding energy for alkynes and propadiene, and avoid the competitive adsorption of olefins and paraffins, which is verified by breakthrough tests, single-crystal X-ray diffraction experiments, and simulation studies. The responsive properties to different stimuli endow materials with multiple regulation methods and broaden the boundaries of the applicability of porous materials to challenging separations.

Separating mixtures of hydrocarbons of low molecular weight is desirable but challenging. Here, the authors report a porous material with responsive and self-adaptive properties that enables one-step removal of alkynes and propadiene from cracking gases.

Porous Liquids: Computational Design for Targeted Gas Adsorption

In this Perspective, we present the unique gas adsorption capabilities of porous liquids (PLs) and the value of complex computational methods in the design of PL compositions. Traditionally, liquids only contain transient pore space between molecules that limit long-term gas capture. However, PLs are stable fluids that that contain permanent porosity due to the combination of a rigid porous host structure and a solvent. PLs exhibit remarkable adsorption and separation properties, including increased solubility and selectivity. The unique gas adsorption properties of PLs are based on their structure, which exhibits multiple gas binding sites in the pore and on the cage surface, varying binding mechanisms including hydrogen-bonding and π-π interactions, and selective diffusion in the solvent. Tunable PL compositions will require fundamental investigations of competitive gas binding mechanisms, thermal effects on binding site stability, and the role of nanoconfinement on gas and solvent diffusion that can be accelerated through molecular modeling. With these new insights PLs promise to be an exceptional material class with tunable properties for targeted gas adsorption.

Molecular Cavity for Catalysis and Formation of Metal Nanoparticles for Use in Catalysis

The employment of weak intermolecular interactions in supramolecular chemistry offers an alternative approach to project artificial chemical environments like the active sites of enzymes. Discrete molecular architectures with defined shapes and geometries have become a revolutionary field of research in recent years because of their intrinsic porosity and ease of synthesis using dynamic non-covalent/covalent interactions. Several porous molecular cages have been constructed from simple building blocks by self-assembly, which undergoes many self-correction processes to form the final architecture. These supramolecular systems have been developed to demonstrate numerous applications, such as guest stabilization, drug delivery, catalysis, smart materials, and many other related fields. In this respect, catalysis in confined nanospaces using such supramolecular cages has seen significant growth over the years. These porous discrete cages contain suitable apertures for easy intake of substrates and smooth release of products to exhibit exceptional catalytic efficacy. This review highlights recent advancements in catalytic activity influenced by the nanocavities of hydrogen-bonded cages, metal-ligand coordination cages, and dynamic or reversible covalently bonded organic cages in different solvent media. Synthetic strategies for these three types of supramolecular systems are discussed briefly and follow similar and simplistic approaches manifested by simple starting materials and benign conditions. These examples demonstrate the progress of various functionalized molecular cages for specific chemical transformations in aqueous and nonaqueous media. Finally, we discuss the enduring challenges related to porous cage compounds that need to be overcome for further developments in this field of work.

Rigidity and Flexibility in Rotaxanes and Their Relatives; On Being Stubborn and Easy-Going

Rotaxanes are an emerging class of molecules composed of two building blocks: macrocycles and threads. Rotaxanes, and their pseudorotaxane and polyrotaxane relatives, serve as prototypes for molecular-level switches and machines and as components in materials like elastic polymers and 3D printing inks. The rigidity and flexibility of these molecules is a characteristic feature of their design. However, the mechanical properties of the assembled rotaxane and its components are rarely examined directly, and the translation of these properties from molecules to bulk materials is understudied. In this Review, we consider the mechanical properties of rotaxanes by making use of concepts borrowed from physical organic chemistry. Rigid molecules have fewer accessible conformations with higher energy barriers while flexible molecules have more accessible conformations and lower energy barriers. The macrocycles and threads become rigidified when threaded together as rotaxanes in which the formation of intermolecular interactions and increased steric contacts collectively reduce the conformational space and raise barriers. Conversely, rotational and translational isomerism in rotaxanes adds novel modes of flexibility. We find that rigidification in rotaxanes is almost universal, but novel degrees of flexibility can be introduced. Both have roles to play in the function of rotaxanes.

An Ultrathin Functional Layer Based on Porous Organic Cages for Selective Ion Sieving and Lithium-Sulfur Batteries

Thin films with effective ion sieving ability are highly desired in energy storage and conversion devices, including batteries and fuel cells. However, it remains challenging to design and fabricate cost-effective and easy-to-process ultrathin films for this purpose. Here, we report a 300 nm-thick functional layer based on porous organic cages (POCs), a new class of porous molecular materials, for fast and selective ion transport. This solution processable material allows for the design of thin films with controllable thickness and tunable porosity by tailoring cage chemistry for selective ion separation. In the prototype, the functional layer assembled by CC3 can selectively sieve Li⁺ ions and efficiently suppress undesired polysulfides with minimal sacrifice for the system's total energy density. Separators modified with POC thin films enable batteries with good cycle performance and rate capability and offer an attractive path toward the development of future high-energy-density energy storage devices.

An Adaptive Hydrogen-Bonded Organic Framework for the Exclusive Recognition of p-Xylene

Separation of xylene isomers is one of the most important but most challenging and energy-intensive separation processes in the petrochemical industry. Here, we report an adaptive hydrogen-bonded organic framework (HOF-29) constructed from a porphyrin based organic building block 4,4',4'',4'''-(porphyrin-5,10,15,20-tetrayl) tetrabenzonitrile (PTTBN), exhibiting the exclusive molecular recognition of p-xylene (pX) over its isomers of o-xylene (oX) and m-xylene (mX), as clearly demonstrated in the single crystal structure transformation and ¹ H NMR studies. Single crystal structure studies show that single-crystal-to-single-crystal transformation from the as-synthesized HOF-29 to the pX exclusively included HOF-29⊃pX is triggered by the encapsulation of pX molecules, accompanied by sliding of the 2D layers and local distortion of the ligand, which provides multiple C-H⋅⋅⋅π interactions.

Cagearenes: synthesis, characterization, and application for programmed vapor release

Here, we announce the establishment of a new family of organic molecular cages, named cagearenes, by taking advantage of a versatile strategy. These cagearenes were prepared via the Friedel–Crafts reaction by condensing two equivalents of a precursor bearing three 1,4-dimethoxybenzene groups and three equivalents of formaldehyde. Two cages, namely cagearene-1 and cagearene-2, are obtained and well characterized. The cagearene-1 solid exhibits the ability to adsorb benzene vapour from an equimolar benzene/cyclohexane mixture with a purity of 91.1%. Then, the adsorbed benzene molecules can be released from the cage at a relatively lower temperature, namely 70 °C, as a consequence of which, cyclohexane with a high purity was left within the cage solid. Heating the cage solid further at 130 °C led to the production of cyclohexane with a purity up to 98.7%. As inferred from the single crystal structures and theoretical calculations, the ability of the cage in programmed release of benzene and cyclohexane results from the different binding modes of these two guests.

Two organic cages, cagearene-1 and cagearene-2, are prepared. The cagearene-1 solid selectively absorbs benzene vapor from a benzene/cyclohexane mixture and is used to achieve temperature-controlled programmed vapor release.

Explainable graph neural networks for organic cages

The development of accurate and explicable machine learning models to predict the properties of topologically complex systems is a challenge in materials science. Porous organic cages, a class of polycyclic molecular materials, have potential application in molecular separations, catalysis and encapsulation. For most applications of porous organic cages, having a permanent internal cavity in the absence of solvent, a property termed “shape persistence” is critical. Here, we report the development of Graph Neural Networks (GNNs) to predict the shape persistence of organic cages. Graph neural networks are a class of neural networks where the data, in our case that of organic cages, are represented by graphs. The performance of the GNN models was measured against a previously reported computational database of organic cages formed through a range of [4 + 6] reactions with a variety of reaction chemistries. The reported GNNs have an improved prediction accuracy and transferability compared to random forest predictions. Apart from the improvement in predictive power, we explored the explicability of the GNNs by computing the integrated gradient of the GNN input. The contribution of monomers and molecular fragments to the shape persistence of the organic cages could be quantitatively evaluated with integrated gradients. With the added explicability of the GNNs, it was possible not only to accurately predict the property of organic materials, but also to interpret the predictions of the deep learning models and provide structural insights for the discovery of future materials.

We report the development of explainable Graph Neural Networks to predict shape persistence of organic cages. Integrated gradient analysis identifies collapse-inducing molecular fragments and helps chemists design more shape persistent structures.

FeII 4L4 tetrahedron binds and aggregates DNA G-quadruplexes

Since the discovery of the G-quadruplex (G4) structure in telomeres in 1980s, studies have established the role it plays in various biological processes. Here we report binding between DNA G4 and a self-assembled tetrahedral metal-organic cage 1 and consequent formation of aggregates, whereby the cage protects the DNA G4 from cleavage by S1 nuclease. We monitor DNA–cage interaction using fluorescence spectroscopy, firstly by quenching of a fluorescent label appended to the 5′ end of G4. Secondly, we detect the decrease in fluorescence of the G4-selective dyes thioflavin-T and Zn-PPIX bound to various DNA G4 sequences following the addition of cage 1. Our results demonstrate that 1 interacts with a wide range of G4s. Moreover, gel electrophoresis, circular dichroism and dynamic light scattering measurements establish the binding of 1 to G4 and indicate the formation of aggregate structures. Finally, we find that DNA G4 contained in an aggregate of cage 1 is protected from cleavage by S1 nuclease.

We find Fe^II₄L₄ binds to G-quadruplex and forms aggregates. G-quadruplex in the aggregates is protected from digestion by S₁ nuclease.

Hollow and highly diastereoselective face-rotating polyhedra constructed through rationally engineered facial units

Molecular face-rotating polyhedra (FRP) exhibit complex stereochemistry, rendering it challenging to manipulate their assembly in a stereoselective manner. In our previous work, stereocontrolled FRP were gained at the cost of losing the confined inner space, which hampers their host–guest interactions and potential applications. Through a rational design approach, herein we demonstrate the successful construction of hollow FRP with high diastereoselectivity. Whereas the [4 + 4] imine condensation of meta-formyl substituted C_3h-symmetric TAT-m and C₃-symmetric Tri-NH2 led to the formation of all feasible FRP-12 diastereoisomers; the para-substituted constitutional isomer, TAT-p, exclusively assembled into a pair of homo-directional enantiomeric FRP-13-CCCC/AAAA with a cavity size larger than 600 ų. Detailed structural characterizations and theoretical investigations revealed the thermodynamic landscape of FRP assembly can be effectively shaped by modulating the van der Waals repulsive forces among the facial building blocks. Our work provided a novel strategy towards stereospecific assembly of pure organic cages, opening up new opportunities for further applications of these chiral materials.

The rationally engineered facial units, TAT-m and TAT-p, resulted in distinct diastereoselectivity of face-rotating polyhedra (FRP).

Evaluation of packing single and multiple atoms and molecules in the porous organic cage CC3-R

The absorption of multiple atoms and molecules, including Kr, Xe, CH₄, CO₂, C₂H₂, H₂O, and SF₆, within CC3-R, a Porous Organic Cage (POC), was calculated and analyzed. The CC3-R molecule has one central cavity and four window sites. Most adsorbents were modeled with either one unit in the central cavity, four units in the window sites, or with five units in both sites. For Xe, the most favorable site was the central one. The CO₂ molecule binds about 3 kcal mol^-1 in free energy more strongly than CH₄ in the central cavity of CC3-R at 300 K which may be enough to allow useful discrimination. Four C₂H₂ units and four CO₂ units are calculated to bind similarly inside CC3-R (ΔH(298 K) = -8.6 and -7.7 kcal mol^-1 per unit, respectively). Since H₂O is smaller, more waters can easily fit inside. For twelve water molecules, the binding enthalpy per water is ΔH(298 K) = -16.4 kcal mol^-1. For comparison, the binding enthalpy of (H₂O)₁₂ at the same level of theory (B3LYP/6-31G(d,p)-D3BJ//M06-2X/6-31G(d)) is predicted to be -12.3 kcal mol^-1 per water. Finally, the dimerization of CC3-R and the association of CC3-R with CC3-S was studied as well as 3 to 9 iodine atoms enclosed in CC3-R.

Molecular recognition and adsorptive separation of m-xylene by trianglimine crystals

The separation of xylene isomers is one of the most challenging tasks in the petrochemical industry. Herein, we developed an efficient adsorptive molecular sieving strategy using crystalline trianglimine macrocycle (1) to separate the elusive m-xylene isomer from an equimolar xylenes mixture with over 91% purity. The selectivity is attributed to the capture of the preferred guest with size/shape selectivity and C-H⋯π interactions. Moreover, the trianglimine crystals are readily recyclable due to the reversible transformation between the guest-free and guest-loaded structures.

Porous organic cage for enantiomeric fluorescence recognition of amino acid and hydroxy acid

A new method based on the enantioselective recognition of porous organic cages CC3-R was established for the first time. Porous organic cages are widely used for separation, adsorption and host-guest interaction sensing, but are rarely used for fluorescence sensing. Based on the inherent chiral environment of CC3-R and the inherent fluorescence properties of the organic ligands constituting the cage, when different chiral monomers diffuse into the cage, different effects occur to produce changes in fluorescence. We found for the first time that the fluorescence of CC3-R can be enhanced and quenched by tyrosine and mandelic acid, respectively, and that different chiral monomers are enhanced or quenched differently at the same concentration. Unlike the chiral recognition of other composite luminescent materials, the chiral porous organic cage not only utilizes its own host-guest effect for chiral recognition, but also utilizes the organic ligands constituting the cage for luminescence recognition. This work provides an alternative method to accomplish chiral recognition other than chromatography, that is using porous organic cages (POC), but it can show the advantages of simplicity, low cost and high sensitivity. We believe this work could provide valuable thoughts in the exploration of POC in chiral recognition as new FL probes for the future.

Facile room temperature synthesis of ultra-small sized porous organic cages for fluorescent sensing of copper ion in aqueous solution

Facile synthesis of nano porous organic cages with small size and good fluorescence property is highly desirable, but still challenging and scarce for their sensing applications. Here we report a rapid room-temperature recrystallization method for the preparation of nano porous organic cages with ultra-small size as a fluorescent probe for copper ion. The prepared nano porous organic cages gave the diameter of 2.49 ± 0.04 nm, and exhibited stable emission at 535 nm with absolute quantum yield of 0.68%. On the basis of the coordination interaction and charge transfer between the nano porous organic cages and copper ion, a simple fluorescent probe for copper ion in aqueous solution was developed. The developed method gave a calibration function of QE = 0.4815lg[Cu²⁺] + 0.5847 (where QE is the quenching efficiency; [Cu²⁺] in μM) (R² = 0.9987) in a concentration range of 0.1-2 μM, the limit of detection (3s) of 8 nM, and the relative standard deviation of 0.36% for 10 replicate determinations of 0.5 μM copper ion. The recoveries of spiked copper ion in tap water samples ranged from 96.8% to 103.0%. The proposed method possesses good sensitivity, selectivity and accuracy.

Isoreticular Crystallization of Highly Porous Cubic Covalent Organic Cage Compounds*

Modular frameworks featuring well-defined pore structures in microscale domains establish tailor-made porous materials. For open molecular solids however, maintaining long-range order after desolvation is inherently challenging, since packing is usually governed by only a few supramolecular interactions. Here we report on two series of nanocubes obtained by co-condensation of two different hexahydroxy tribenzotriquinacenes (TBTQs) and benzene-1,4-diboronic acids (BDBAs) with varying linear alkyl chains in 2,5-position. n-Butyl groups at the apical position of the TBTQ vertices yielded soluble model compounds, which were analyzed by mass spectrometry and NMR spectroscopy. In contrast, methyl-substituted cages spontaneously crystallized as isostructural and highly porous solids with BET surface areas and pore volumes of up to 3426 m2  g-1 and 1.84 cm3  g-1 . Single crystal X-ray diffraction and sorption measurements revealed an intricate cubic arrangement of alternating micro- and mesopores in the range of 0.97-2.2 nm that are fine-tuned by the alkyl substituents at the BDBA linker.

Cucurbitimines - imine cages with concave walls

The variety of shape-persistent organic cages by imine bond formation has tremendously enlarged in recent years by using different building blocks (aldehydes and amines) in the condensation reactions. Here, we describe the use of a kinked tetraldehyde to generate pumpkin-shaped cages with concave walls, similar to cucurbiturils.

Kinked tetraaldehyde building blocks lead in condensation reactions with diamines to pumpkin shaped cages – the cucurbitimines.