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Porosity is a rare property for molecular materials but, surprisingly, porous solids built from discrete organic cage molecules have emerged as a versatile functional-materials platform. From modest beginnings less than a decade ago, there are now organic cage solids with surface areas that can rival extended metal–organic frameworks. In contrast to network polymers and frameworks, these organic cages are synthesized first and then assembled in the solid state in a separate step. This offers solution-processing options that are not available for insoluble organic and inorganic frameworks. In this Review, we highlight examples of porous organic cages and focus on the unique features that set them apart, such as their molecular solubility, their increased tendency to exhibit polymorphism and the scope for modular co-crystallization. The surface areas of molecular organic cage solids now rival those of metal–organic frameworks. In this Review, the synthesis and structures of various porous organic cages are outlined together with a discussion of the characteristics — such as solubility, polymorphism and modular co-crystallization — that distinguish these cages from their inorganic or hybrid counterparts.

Inverse vulcanization, a sustainable platform, can transform sulfur, an industrial by-product, into polymers with broad promising applications such as heavy metal capture, electrochemistry and antimicrobials. However, the process usually requires high temperatures (≥159 °C), and the crosslinkers needed to stabilize the sulfur are therefore limited to high-boiling-point monomers only. Here, we report an alternative route for inverse vulcanization—mechanochemical synthesis, with advantages of mild conditions (room temperature), short reaction time (3 h), high atom economy, less H 2 S, and broader monomer range. Successful generation of polymers using crosslinkers ranging from aromatic, aliphatic to volatile, including renewable monomers, demonstrates this method is powerful and versatile. Compared with thermal synthesis, the mechanochemically synthesized products show enhanced mercury capture. The resulting polymers show thermal and light induced recycling. The speed, ease, versatility, safety, and green nature of this process offers a more potential future for inverse vulcanization, and enables further unexpected discoveries. Inverse vulcanization is a process that enables to convert sulfur, a by-product of the petroleum industry, into polymers. Here the authors report a synthetic method of inverse vulcanization via mechanochemical synthesis; compared to thermal routes, a broader range of monomers can be used, and the protocol yields materials with enhanced mercury capture capacity.

The discovery of inverse vulcanization has allowed stable polymers to be made from elemental sulfur, an unwanted by-product of the petrochemicals industry. However, further development of both the chemistry and applications is handicapped by the restricted choice of cross-linkers and the elevated temperatures required for polymerisation. Here we report the catalysis of inverse vulcanization reactions. This catalytic method is effective for a wide range of crosslinkers reduces the required reaction temperature and reaction time, prevents harmful H 2 S production, increases yield, improves properties, and allows crosslinkers that would be otherwise unreactive to be used. Thus, inverse vulcanization becomes more widely applicable, efficient, eco-friendly and productive than the previous routes, not only broadening the fundamental chemistry itself, but also opening the door for the industrialization and broad application of these fascinating materials. Inverse vulcanization allows stable polymers to be made from elemental sulfur, but development is restricted by cross-linkers and the elevated temperatures required. Here the authors report a catalytic method for a wide range of cross-linkers and found a reduced reaction temperature and reaction time is required.

Molecular crystals cannot be designed in the same manner as macroscopic objects, because they do not assemble according to simple, intuitive rules. Their structures result from the balance of many weak interactions, rather than from the strong and predictable bonding patterns found in metal–organic frameworks and covalent organic frameworks. Hence, design strategies that assume a topology or other structural blueprint will often fail. Here we combine computational crystal structure prediction and property prediction to build energy–structure–function maps that describe the possible structures and properties that are available to a candidate molecule. Using these maps, we identify a highly porous solid, which has the lowest density reported for a molecular crystal so far. Both the structure of the crystal and its physical properties, such as methane storage capacity and guest-molecule selectivity, are predicted using the molecular structure as the only input. More generally, energy–structure–function maps could be used to guide the experimental discovery of materials with any target function that can be calculated from predicted crystal structures, such as electronic structure or mechanical properties. Energy–structure–function maps that describe the possible structures and properties of molecular crystals are developed, and these maps are used to guide the experimental discovery of porous materials with specific functions. Energy–structure–function maps of crystals It can be difficult to predict and optimize the structure and properties of crystalline molecular compounds without resorting to a 'wet' synthesis. Rather than crystallizing according to simple rules, their structures result from the combination of many weak interactions. Computational calculations and screening can help, but are limited by the computational power available and often rely on prior knowledge and assumptions about the structure. Here, Graeme Day and colleagues use structure prediction maps to computationally scan various packings for small molecular crystals, and assign functions, such as porosity or gas storage capacity, to each plausible structure found. These energy–structure–function maps can guide synthetic work by ruling out seemingly promising structures that are calculated to have undesirable properties. To validate the method, the authors synthesize a molecule that crystallizes as a porous solid with a record low density for a porous organic compound.

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. Proton conduction is a fundamental process for fuel cell development, but three-dimensional proton conduction in crystalline porous solids is rare. Here, the authors report organic molecular cages in which the structure imposes three-dimensional proton conductivity competing with metal-organic frameworks.

Porous crystals made to order Controlling and predicting the structural properties of porous molecular crystals would have important implications in gas adsorption, separation and catalysis applications, but remain an unmet goal. This paper introduces a new concept of modular assembly at the molecular level for the formation of porous crystalline solids. Different large chiral molecules with intrinsic nanosize pores, or porous modules, self-assemble through chiral recognition during co-crystallization to produce solid porous frameworks. The three-dimensional structure of the final material can be predicted theoretically. The paper explores four different, albeit analogous, porous modules, which form four different porous solids. Nanoporous molecular frameworks 1 , 2 , 3 , 4 , 5 , 6 , 7 are important in applications such as separation, storage and catalysis. Empirical rules exist for their assembly but it is still challenging to place and segregate functionality in three-dimensional porous solids in a predictable way. Indeed, recent studies of mixed crystalline frameworks suggest a preference for the statistical distribution of functionalities throughout the pores 7 rather than, for example, the functional group localization found in the reactive sites of enzymes 8 . This is a potential limitation for ‘one-pot’ chemical syntheses of porous frameworks from simple starting materials. An alternative strategy is to prepare porous solids from synthetically preorganized molecular pores 9 , 10 , 11 , 12 , 13 , 14 , 15 . In principle, functional organic pore modules could be covalently prefabricated and then assembled to produce materials with specific properties. However, this vision of mix-and-match assembly is far from being realized, not least because of the challenge in reliably predicting three-dimensional structures for molecular crystals, which lack the strong directional bonding found in networks. Here we show that highly porous crystalline solids can be produced by mixing different organic cage modules that self-assemble by means of chiral recognition. The structures of the resulting materials can be predicted computationally 16 , 17 , allowing in silico materials design strategies 18 . The constituent pore modules are synthesized in high yields on gram scales in a one-step reaction. Assembly of the porous co-crystals is as simple as combining the modules in solution and removing the solvent. In some cases, the chiral recognition between modules can be exploited to produce porous organic nanoparticles. We show that the method is valid for four different cage modules and can in principle be generalized in a computationally predictable manner based on a lock-and-key assembly between modules.

The main strategy for constructing porous solids from discrete organic molecules is crystal engineering, which involves forming regular crystalline arrays. Here, we present a chemical approach for desymmetrizing organic cages by dynamic covalent scrambling reactions. This leads to molecules with a distribution of shapes which cannot pack effectively and, hence, do not crystallize, creating porosity in the amorphous solid. The porous properties can be fine tuned by varying the ratio of reagents in the scrambling reaction, and this allows the preparation of materials with high gas selectivities. The molecular engineering of porous amorphous solids complements crystal engineering strategies and may have advantages in some applications, for example, in the compatibilization of functionalities that do not readily cocrystallize. The construction of porous solids from discrete organic molecules usually involves the formation of regular porous crystals. In this study, a covalent scrambling reaction gives molecules with a range of shapes that do not pack effectively — manipulation of the reagent ratio allows fine control of porosity.

Inverse vulcanization has emerged as a popular strategy for transforming the waste material, elemental sulfur, into functional polymers with high sulfur content (>50 wt.%, normally). Inverse vulcanized polymers are intrinsically processable and recyclable, and have been demonstrated as promising for applications in many fields. However, the mechanical properties of inverse vulcanized polymers are currently underdeveloped. If this kind of material is to be widely used in some scenarios to replace some traditional plastics, it is necessary to make them with appropriate thermal and mechanical properties that meet basic application requirements. Here, we report a series of terpolymers copolymerized from two distinct organic comonomers and elemental sulfur to obtain polymers with a wide range of glass transition temperatures (−43 °C to 45 °C) that exhibit good mechanical properties, by blending crosslinkers with varying feed monomer ratio and chain length of linear sections, which expands the application opportunities of inverse vulcanization. The thermal and mechanical properties of inverse vulcanized polymers are currently underdeveloped. Here, a series of terpolymers copolymerized from two distinct organic comonomers and elemental sulfur yield polymers with a wide range of glass transition temperatures and show good mechanical properties.

The inverse vulcanization (IV) of elemental sulfur to generate sulfur-rich functional polymers has attracted much recent attention. However, the harsh reaction conditions required, even with metal catalysts, constrains the range of feasible crosslinkers. We report here a photoinduced IV that enables reaction at ambient temperatures, greatly broadening the scope for both substrates and products. These conditions enable volatile and gaseous alkenes and alkynes to be used in IV, leading to sustainable alternatives for environmentally harmful plastics that were hitherto inaccessible. Density functional theory calculations reveal different energy barriers for thermal, catalytic and photoinduced IV processes. This protocol circumvents the long curing times that are common in IV, generates no H 2 S by-products, and produces high-molecular-weight polymers (up to 460,000 g mol −1 ) with almost 100% atom economy. This photoinduced IV strategy advances both the fundamental chemistry of IV and its potential industrial application to generate materials from waste feedstocks. Inverse vulcanization (IV) generates sulfur-rich functional polymers from elemental sulfur and organic crosslinkers, but the harsh reaction conditions required limit the scope of suitable crosslinkers. Now, a photoinduced IV has been shown to proceed at ambient temperatures, enabling the use of volatile and gaseous alkenes and alkynes as crosslinkers and broadening the range of products.

The separation of molecules with similar size and shape is an important technological challenge. For example, rare gases can pose either an economic opportunity or an environmental hazard and there is a need to separate these spherical molecules selectively at low concentrations in air. Likewise, chiral molecules are important building blocks for pharmaceuticals, but chiral enantiomers, by definition, have identical size and shape, and their separation can be challenging. Here we show that a porous organic cage molecule has unprecedented performance in the solid state for the separation of rare gases, such as krypton and xenon. The selectivity arises from a precise size match between the rare gas and the organic cage cavity, as predicted by molecular simulations. Breakthrough experiments demonstrate real practical potential for the separation of krypton, xenon and radon from air at concentrations of only a few parts per million. We also demonstrate selective binding of chiral organic molecules such as 1-phenylethanol, suggesting applications in enantioselective separation. A porous organic-cage molecule is shown to exhibit unprecedented performance for the separation of rare gases, with selectivity arising from a precise size match between the rare gas and the organic-cage cavity.