Guest control of structure in porous organic cages

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.

Tunable Molecular Sieving by Hierarchically Assembled Porous Organic Cage Membranes with Solvent-Responsive Switchable Pores

Molecular separations involving solvents and organic impurities represent great challenges for environmental and water-intensive industries. Novel materials with intrinsic nanoscale pores offer a great choice for improvement in terms of energy efficiency and capital costs. Particularly, in applications where gradient and ordered separation of organic contaminants remain elusive, smart materials with switchable pores can offer efficient solutions. Here, we report a hierarchically networked porous organic cage membrane with dynamic control over pores, elucidating stable solvent permeance and tunable dye rejection over different molecular weights. The engineered cage membrane can spontaneously modulate its geometry and pore size from water to methanol and DMF in a reversible manner. The cage membrane exhibits ≥585.59 g mol-1 molecular weight cutoff preferentially in water and is impeded by methanol (799.8 g mol-1) and DMF (≈1017 g mol-1), reflecting 36 and 73% change in rejection due to self-regulation and the flexible network, respectively. Grazing incidence X-ray diffraction illustrates a clear peak downshift, suggesting an intrinsic structural change when the cage membranes were immersed in methanol or DMF. We have observed reversible structural changes that can also be tuned by preparing a methanol/DMF mixture and adjusting their ratio, thereby enabling gradient molecular filtration. We anticipate that such cage membranes with dynamic selectivity could be promising particularly for industrial separations and wastewater treatment.

Fascinating isomeric covalent organic frameworks

Isomeric covalent organic frameworks possessing the same chemical constitutions but different atomic arrangement structures and physicochemical properties are fascinating branches of covalent organic frameworks (COFs). However, the rational design and targeted synthesis of isomeric COFs remain conundrums, so the investigation of isomeric COFs is still in a fledging period. According to the diversity of frameworks, positional isomers with similar structures and framework isomers having distinct constructions are the main existing subspecies of isomeric COFs. In this review, we focus on the research progress and substantial achievements in this fascinating embranchment and systematically summarize and highlight the design principles of both positional isomeric and framework isomeric COFs, which will potentially facilitate further exploitation and investigation of novel isomeric COFs. The application and structure-property relationship of these isomeric COFs have been briefly introduced. Moreover, key constraints of current isomeric COFs and further advancement of this promising field are proposed and anticipated.

Pore topology analysis in porous molecular systems

Porous molecular materials are constructed from molecules that assemble in the solid-state such that there are cavities or an interconnected pore network. It is challenging to control the assembly of these systems, as the interactions between the molecules are generally weak, and subtle changes in the molecular structure can lead to vastly different intermolecular interactions and subsequently different crystal packing arrangements. Similarly, the use of different solvents for crystallization, or the introduction of solvent vapour, can result in different polymorphs and pore networks being formed. It is difficult to uniquely describe the pore networks formed, and thus we analyse 1033 crystal structures of porous molecular systems to determine the underlying topology of their void spaces and potential guest diffusion networks. Material-agnostic topology definitions are applied. We use the underlying topological nets to examine whether it is possible to apply isoreticular design principles to porous molecular materials. Overall, our automatic analysis of a large dataset gives a general insight into the relationships between molecular topologies and the topological nets of their pore network. We show that while porous molecular systems tend to pack similarly to non-porous molecules, the topologies of their pore distributions resemble those of more prominent porous materials, such as metal-organic frameworks and covalent organic frameworks.

Coarse-grained modelling to predict the packing of porous organic cages

How molecules pack has vital ramifications for their applications as functional molecular materials. Small changes in a molecule's functionality can lead to large, non-intuitive, changes in their global solid-state packing, resulting in difficulty in targeted design. Predicting the crystal structure of organic molecules from only their molecular structure is a well-known problem plaguing crystal engineering. Although relevant to the properties of many organic molecules, the packing behaviour of modular porous materials, such as porous organic cages (POCs), greatly impacts the properties of the material. We present a novel way of predicting the solid-state phase behaviour of POCs by using a simplistic model containing the dominant degrees of freedom driving crystalline phase formation. We employ coarse-grained simulations to systematically study how chemical functionality of pseudo-octahedral cages can be used to manipulate the solid-state phase formation of POCs. Our results support those of experimentally reported structures, showing that for cages which pack via their windows forming a porous network, only one phase is formed, whereas when cages pack via their windows and arenes, the phase behaviour is more complex. While presenting a lower computational cost route for predicting molecular crystal packing, coarse-grained models also allow for the development of design rules which we start to formulate through our results.

Purely Covalent Molecular Cages and Containers for Guest Encapsulation

Cage compounds offer unique binding pockets similar to enzyme-binding sites, which can be customized in terms of size, shape, and functional groups to point toward the cavity and many other parameters. Different synthetic strategies have been developed to create a toolkit of methods that allow preparing tailor-made organic cages for a number of distinct applications, such as gas separation, molecular recognition, molecular encapsulation, hosts for catalysis, etc. These examples show the versatility and high selectivity that can be achieved using cages, which is impossible by employing other molecular systems. This review explores the progress made in the field of fully organic molecular cages and containers by focusing on the properties of the cavity and their application to encapsulate guests.

A smart and responsive crystalline porous organic cage membrane with switchable pore apertures for graded molecular sieving

Membranes with high selectivity offer an attractive route to molecular separations, where technologies such as distillation and chromatography are energy intensive. However, it remains challenging to fine tune the structure and porosity in membranes, particularly to separate molecules of similar size. Here, we report a process for producing composite membranes that comprise crystalline porous organic cage films fabricated by interfacial synthesis on a polyacrylonitrile support. These membranes exhibit ultrafast solvent permeance and high rejection of organic dyes with molecular weights over 600 g mol^-1. The crystalline cage film is dynamic, and its pore aperture can be switched in methanol to generate larger pores that provide increased methanol permeance and higher molecular weight cut-offs (1,400 g mol^-1). By varying the water/methanol ratio, the film can be switched between two phases that have different selectivities, such that a single, 'smart' crystalline membrane can perform graded molecular sieving. We exemplify this by separating three organic dyes in a single-stage, single-membrane process.

Enabling Technology for Supramolecular Chemistry

Supramolecular materials-materials that exploit non-covalent interactions-are increasing in structural complexity, selectivity, function, stability, and scalability, but their use in applications has been comparatively limited. In this Minireview, we summarize the opportunities presented by enabling technology-flow chemistry, high-throughput screening, and automation-to wield greater control over the processes in supramolecular chemistry and accelerate the discovery and use of self-assembled systems. Finally, we give an outlook for how these tools could transform the future of the field.

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.

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.

The Ionic Organic Cage: An Effective and Recyclable Testbed for Catalytic CO2 Transformation

Porous organic cages (POC) are a class of relatively new molecular porous materials, whose concept was raised in 2009 by Cooper’s group and has rarely been directly used in the area of organic catalysis. In this contribution, a novel ionic quasi-porous organic cage (denoted as Iq-POC), a quaternary phosphonium salt, was easily synthesized through dynamic covalent chemistry and a subsequent nucleophilic addition reaction. Iq-POC was applied as an effective nucleophilic catalyst for the cycloaddition reaction of CO2 and epoxides. Owing to the combined effect of the relatively large molecular weight (compared with PPh3+I−) and the strong polarity of Iq-POC, the molecular catalyst Iq-POC displayed favorable heterogeneous nature (i.e., insolubility) in this catalytic system. Therefore, the Iq-POC catalyst could be easily separated and recycled by simple centrifugation method, and the catalyst could be reused five times without obvious loss of activity. The molecular weight augmentation route in this study (from PPh3+I− to Iq-POC) provided us a “cage strategy” of designing separable and recyclable molecular catalysts.

Porous shape-persistent rylene imine cages with tunable optoelectronic properties and delayed fluorescence

A simultaneous combination of porosity and tunable optoelectronic properties, common in covalent organic frameworks, is rare in shape-persistent organic cages. Yet, organic cages offer important molecular advantages such as solubility and modularity. Herein, we report the synthesis of a series of chiral imine organic cages with three built-in rylene units by means of dynamic imine chemistry and we investigate their textural and optoelectronic properties. Thereby we demonstrate that the synthesized rylene cages can be reversibly reduced at accessible potentials, absorb from UV up to green light, are porous, and preferentially adsorb CO2 over N2 and CH4 with a good selectivity. In addition, we discovered that the cage incorporating three perylene-3,4:9,10-bis(dicarboximide) units displays an efficient delayed fluorescence. Time-correlated single photon counting and transient absorption spectroscopy measurements suggest that the delayed fluorescence is likely a consequence of a reversible intracage charge-separation event. Rylene cages thus offer a promising platform that allows combining the porosity of processable materials and photochemical phenomena useful in diverse applications such as photocatalysis or energy storage.

Exploring cooperative porosity in organic cage crystals using in situ diffraction and molecular simulations

A porous organic cage crystal, α-CC2, shows unexpected adsorption of sulphur hexafluoride (SF₆) in its cage cavities: analysis of the static crystal structure indicates that SF₆ is occluded, as even the smallest diatomic gas, H₂, is larger than the window of the cage pore. Herein, we use in situ powder X-ray diffraction (PXRD) experiments to provide unequivocal evidence for the presence of SF₆ inside the 'occluded' cage voids, pointing to a mechanism of dynamic flexibility of the system. By combining PXRD results with molecular dynamics simulations, we build a molecular level picture of the cooperative porosity in α-CC2 that facilitates the passage of SF₆ into the cage voids.

Investigations of CO2 Capture from Gas Mixtures Using Porous Liquids

Porous liquids, a new porous material with fluidity, can be applied in numerous fields, such as gas storage and/or separation. In this work, the separation of binary gas mixtures CO₂/N₂ and CO₂/CH₄ with porous liquids was examined by molecular dynamics (MD) simulations. The pure gas adsorption capacity was analyzed with different concentrations of porous liquids. The dependence of the separation effect of a gas mixture on the total pressure and temperature was investigated. Meanwhile, for both CO₂/N₂ and CO₂/CH₄ systems, the adsorption and separation effects of porous liquids with a cage:solvent ratio of 1:12 are better than those of 1:91 and 1:170. The results of the spatial distribution function and/or trajectories indicated that porous liquids prefer CO₂, leading to the location of CO₂ in the channels formed in porous liquids. However, N₂ and CH₄ are hardly adsorbed into the bulk. The diffusion of gas molecules follows the order of CO₂ > N₂ (for CO₂/N₂) and CH₄ > CO₂ (for CO₂/CH₄) in the bulk and N₂ > CO₂ (for CO₂/N₂) and CH₄ > CO₂ (for CO₂/CH₄) at the interface of porous liquids. Upon increasing the concentrations of porous liquids, the working capacities of CO₂ show small decreases in CO₂/N₂ and CO₂/CH₄ systems, but the sorbent selection parameters are higher in pressure- and temperature-swing adsorption processes. The porous liquid with a cage:solvent ratio of 1:12 is more suitable for the separation of CO₂/N₂ and CO₂/CH₄ systems than ratios of 1:91 and 1:170.

Adsorption and sequential thermal release of F2 , Cl2 , and Br2 molecules by a porous organic cage material (CC3-R): Molecular dynamics and grand-canonical Monte Carlo simulations

The adsorption-desorption behavior of fluorine, chlorine, and bromine molecules onto a crystalline porous organic cage, namely CC3-R was calculated at different temperatures using molecular dynamics (MD) and grand-canonical Monte Carlo (GCMC) simulations. Self-diffusion coefficients, radial distribution functions (RDF), and adsorption isotherms were calculated for this purpose. The results show that CC3-R has varied capacities to capture these halogens at ambient and high temperatures, so that the thermal release of fluorine is completed with increasing temperature up to around 70°C and chlorine molecules remain at the CC3-R surface up to 100°C and all bromine molecules are removed from the CC3-R surface at 200°C. We found that bromine self-diffusion was almost independent of temperature between 0 and 100°C in contrast to fluorine and chlorine. Among different diffusion regimes, Knudsen diffusion appears to have an important role in the adsorption of heavy halogens at higher temperatures.

Selective signalling of alcohols by a molecular lattice and mechanism of single-crystal-to-single-crystal transformations

A molecular material undergoes spin-switching as it exchanges MeOH, EtOH or ⁿPrOH with acetone from the lattice. The subsequent thermal single-crystal-to-single-crystal desorption of ⁿPrOH is followed by single crystal X-ray diffraction snapshots.

Shape-persistent porous organic cage supported palladium nanoparticles as heterogeneous catalytic materials

Porous Organic Cages (POCs) are an emerging class of self-assembling, porous materials with novel properties. They offer a key advantage over other porous materials in permitting facile solution processing and re-assembly. The combination of POCs with metal nanoparticles (NPs) unlocks applications in the area of catalysis. In this context, POCs can function as both the template of ultra-small NPs and a porous, but reprocessable, heterogeneous catalyst support. Here, we demonstrate the synthesis of ultra-small Pd NPs with an imine linked POC known as 'CC3', and show that hydrogen gas can be used to form metallic NPs at ∼200 °C without the reduction of the organic cage (and the accompanying, unwanted loss of crystallinity). The resulting materials are characterized using a range of techniques (including powder diffraction, scanning transmission electron microscopy and synchrotron X-ray absorption spectroscopy) and shown to be recrystallizable following dissolution in organic solvent. Their catalytic efficacy is demonstrated using the widely studied carbon monoxide oxidation reaction. This demonstration paves the way for using ultra-small NPs synthesized with POCs as solution-processable, self-assembling porous catalytic materials.

Solvothermal synthesis of porous organic cage CC3 in the presence of dimethylformamide as solvent

Morphology, and crystal product of porous organic cage CC3, was modified by the use of a novel and non-traditional high dielectric constant solvent dimethyl formamide.

pywindow: Automated Structural Analysis of Molecular Pores

Structural analysis of molecular pores can yield important information on their behavior in solution and in the solid state. We developed pywindow, a python package that enables the automated analysis of structural features of porous molecular materials, such as molecular cages. Our analysis includes the cavity diameter, number of windows, window diameters, and average molecular diameter. Molecular dynamics trajectories of molecular pores can also be analyzed to explore the influence of flexibility. We present the methodology, validation, and application of pywindow for the analysis of molecular pores, metal-organic polyhedra, and some instances of framework materials. pywindow is freely available from github.com/JelfsMaterialsGroup/pywindow .

Central metal dependent modulation of induced-fit gas uptake in molecular porphyrin solids

The induced-fit accommodation of a variety of gaseous molecules including non-polar molecules has been demonstrated in porphyrin-based supramolecular architectures for the first time. Moreover, the gas uptake behaviour can be modulated by changing the central cation of porphyrin.

Computationally-Guided Synthetic Control over Pore Size in Isostructural Porous Organic Cages

The physical properties of 3-D porous solids are defined by their molecular geometry. Hence, precise control of pore size, pore shape, and pore connectivity are needed to tailor them for specific applications. However, for porous molecular crystals, the modification of pore size by adding pore-blocking groups can also affect crystal packing in an unpredictable way. This precludes strategies adopted for isoreticular metal-organic frameworks, where addition of a small group, such as a methyl group, does not affect the basic framework topology. Here, we narrow the pore size of a cage molecule, CC3, in a systematic way by introducing methyl groups into the cage windows. Computational crystal structure prediction was used to anticipate the packing preferences of two homochiral methylated cages, CC14-R and CC15-R, and to assess the structure-energy landscape of a CC15-R/CC3-S cocrystal, designed such that both component cages could be directed to pack with a 3-D, interconnected pore structure. The experimental gas sorption properties of these three cage systems agree well with physical properties predicted by computational energy-structure-function maps.

A Perspective on the Synthesis, Purification, and Characterization of Porous Organic Cages

Porous organic cages present many opportunities in functional materials chemistry, but the synthetic challenges for these molecular solids are somewhat different from those faced in the areas of metal-organic frameworks, covalent-organic frameworks, or porous polymer networks. Here, we highlight the practical methods that we have developed for the design, synthesis, and characterization of imine porous organic cages using CC1 and CC3 as examples. The key points are transferable to other cages, and this perspective should serve as a practical guide to researchers who are new to this field.

Modular assembly of porous organic cage crystals: isoreticular quasiracemates and ternary co-crystal

Co-crystallisation of helically chiral porous organic cage molecules has enabled the formation of isoreticular quasiracemates and a rare porous organic ternary co-crystal.

Three-dimensional protonic conductivity in porous organic cage solids

Proton conduction is a fundamental process in biology and in devices such as proton exchange membrane fuel cells. To maximize proton conduction, three-dimensional conduction pathways are preferred over one-dimensional pathways, which prevent conduction in two dimensions. Many crystalline porous solids to date show one-dimensional proton conduction. Here we report porous molecular cages with proton conductivities (up to 10(-3) S cm(-1) at high relative humidity) that compete with extended metal-organic frameworks. The structure of the organic cage imposes a conduction pathway that is necessarily three-dimensional. The cage molecules also promote proton transfer by confining the water molecules while being sufficiently flexible to allow hydrogen bond reorganization. The proton conduction is explained at the molecular level through a combination of proton conductivity measurements, crystallography, molecular simulations and quasi-elastic neutron scattering. These results provide a starting point for high-temperature, anhydrous proton conductors through inclusion of guests other than water in the cage pores.

Predicting solvent effects on the structure of porous organic molecules

A computational approach for the prediction of the open, metastable, conformations of porous organic molecules in the presence of solvent is developed.

Chiral metal-macrocycle frameworks: supramolecular chirality induction and helicity inversion of the helical macrocyclic structures

Porous molecular solids composed of discrete macrocycles/cages have great potential for catalysis, separation and sensing techniques. Dynamic structural transformation of the host building blocks, especially a helicity inversion responsive to chemical triggers, is central to upgrading the spatial functions. Here we have achieved the syntheses of homochiral porous molecular solids composed of helical metal macrocycles through supramolecular chirality induction to both enantiomorphic forms with the aid of two different enantiopure sugar-derived lactones in the crystallization process. Moreover, we found that the helicity of the macrocyclic skeletons can be inverted in the crystalline state only by changing the type of solvent. This finding would lead to dynamic control of space chirality in connection with optical resolution, chiral amplification and asymmetric reactions.

Porous Organic Cages for Sulfur Hexafluoride Separation

A series of porous organic cages is examined for the selective adsorption of sulfur hexafluoride (SF6) over nitrogen. Despite lacking any metal sites, a porous cage, CC3, shows the highest SF6/N2 selectivity reported for any material at ambient temperature and pressure, which translates to real separations in a gas breakthrough column. The SF6 uptake of these materials is considerably higher than would be expected from the static pore structures. The location of SF6 within these materials is elucidated by X-ray crystallography, and it is shown that cooperative diffusion and structural rearrangements in these molecular crystals can rationalize their superior SF6/N2 selectivity.

Crystal Structures of a Molecule Designed Not To Pack Tightly

Organic molecules of intrinsic microporosity (OMIMs) are structurally constructed to not pack tightly. Consequently, only weak interactions between OMIM molecules can occur, which is the reason that almost all OMIMs have been described and investigated in their amorphous states. For the same reason it is very difficult to grow single crystals of OMIMs for X-ray structural analysis. Here we describe four different polymorphs of an OMIM that was before only described in the amorphous state.

Synthesis of a rigid C3v -symmetric tris-salicylaldehyde as a precursor for a highly porous molecular cube

The development of a synthetic approach to a C3v -symmetric tris-salicylaldehyde based on triptycene is presented. The tris-salicylaldehyde is a versatile precursor for porous molecular materials, as demonstrated in the [4+4] condensation reaction with a triptycene triamine to form a molecular shape-persistent porous cube. The amorphous material of the molecular porous cube shows a very high surface area of 1014 m(2)  g(-1) (BET model) and a high uptake of CO2 (18.2 wt % at 273 K and 1 bar). Furthermore, during the multistep synthesis of the tris-salicylaldehyde precursor, a relatively rare (twofold) addition of the aryne to the anthracene in the 1,4- and 1,4,5,8-positions have been found during a Diels-Alder reaction, as proven by X-ray structure analysis.