Michael E. Briggs

Separation of rare gases and chiral molecules by selective binding in porous organic cages

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.

Chemical and Structural Stability of Zirconium‐based Metal–Organic Frameworks with Large Three‐Dimensional Pores by Linker Engineering

The synthesis of metal–organic frameworks with large three-dimensional channels that are permanently porous and chemically stable offers new opportunities in areas such as catalysis and separation. Two linkers (L1=4,4′,4′′,4′′′-([1,1′-biphenyl]-3,3′,5,5′-tetrayltetrakis(ethyne-2,1-diyl)) tetrabenzoic acid, L2=4,4′,4′′,4′′′-(pyrene-1,3,6,8-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid) were used that have equivalent connectivity and dimensions but quite distinct torsional flexibility. With these, a solid solution material, [Zr6O4(OH)4(L1)2.6(L2)0.4]⋅(solvent)x, was formed that has three-dimensional crystalline permanent porosity with a surface area of over 4000 m2 g−1 that persists after immersion in water. These properties are not accessible for the isostructural phases made from the separate single linkers.

Hyperporous Carbons from Hypercrosslinked Polymers

Porous carbons with extremely high surface areas are produced through the carbonization of hypercrosslinked benzene, pyrrole, and thiophene. Such carbons show largely microporous and mesoporous domains and exhibit Brunaeur-Emmett-Teller surface areas up to 4300 m2 g-1 . The best performing material also displays exceptionally high CO2 and H2 uptakes.

Side-chain control of porosity closure in single- and multiple-peptide-based porous materials by cooperative folding

Porous materials are attractive for separation and catalysis—these applications rely on selective interactions between host materials and guests. In metal–organic frameworks (MOFs), these interactions can be controlled through a flexible structural response to the presence of guests. Here we report a MOF that consists of glycyl–serine dipeptides coordinated to metal centres, and has a structure that evolves from a solvated porous state to a desolvated non-porous state as a result of ordered cooperative, displacive and conformational changes of the peptide. This behaviour is driven by hydrogen bonding that involves the side-chain hydroxyl groups of the serine. A similar cooperative closure (reminiscent of the folding of proteins) is also displayed with multipeptide solid solutions. For these, the combination of different sequences of amino acids controls the frameworks response to the presence of guests in a nonlinear way. This functional control can be compared to the effect of single-point mutations in proteins, in which exchange of single amino acids can radically alter structure and function. A family of dipeptide-based metal–organic frameworks has been shown to respond to the presence of guests in a cooperative manner controlled by one amino acid residue. When the linker features a serine residue, guest removal enables the formation of hydrogen bonds between the residues side-chains, causing a conformational change that closes the MOFs porous domain.

Porosity-engineered carbons for supercapacitive energy storage using conjugated microporous polymer precursors

Conjugated microporous polymers (CMPs) are considered an important material, combining aspects of both microporosity and extended π-conjugation. However, pristine CMP electrodes suffer from poor electrical conductivity which limits the material in electrochemical applications. In this work, direct carbonisation of conjugated microporous polymers (CMPs) yields porosity-engineered carbons, important for the flow of ions through the electrode. These conductive carbonised CMPs show specific capacitance as high as 260 F g−1, excellent rate capability and no loss in performance after 10 000 charge/discharge cycles. This study provides a procedure to enhance the performance of CMP-based materials, opening up a new source of electroactive materials.

Shape Selectivity by Guest‐Driven Restructuring of a Porous Material

A flexible metal-organic framework selectively sorbs para- (pX) over meta-xylene (mX) by synergic restructuring around pX coupled with generation of unused void space upon mX loading. The nature of the structural change suggests more generally that flexible structures which are initially mismatched in terms of fit and capacity to the preferred guest are strong candidates for effective molecular separations.

Trapping virtual pores by crystal retro-engineering

Stable guest-free porous molecular crystals are uncommon. By contrast, organic molecular crystals with guest-occupied cavities are frequently observed, but these cavities tend to be unstable and collapse on removal of the guests—this feature has been referred to as ‘virtual porosity’. Here, we show how we have trapped the virtual porosity in an unstable low-density organic molecular crystal by introducing a second molecule that matches the size and shape of the unstable voids. We call this strategy ‘retro-engineering’ because it parallels organic retrosynthetic analysis, and it allows the metastable two-dimensional hexagonal pore structure in an organic solvate to be trapped in a binary cocrystal. Unlike the crystal with virtual porosity, the cocrystal material remains single crystalline and porous after removal of guests by heating. Organic molecular crystals with guest-occupied cavities are often observed, but the cavities tend to collapse when the guests are removed. Now, the porous domain of a crystalline solvate has been stabilized by formation of a cocrystal with a second molecule whose size and shape matches those of the unstable voids.

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.

Swellable functional hypercrosslinked polymer networks for the uptake of chemical warfare agents

The need for porous materials to function as sorbents in order to allow for bulk uptake (and potential deactivation) of chemical warfare agent (CWA) stockpiles is of significant importance in the world today. Hypercrosslinked polymers (HCPs) represent a class of such sorbents being produced using the facile and tuneable so-called “knitting” procedure. Several HCPs are reported and their properties including apparent Brunauer–Emmett–Teller surface areas (SABET) and swellability (Q) against CWA simulants are examined using two reliable swelling methods which we have developed. The HCP derived from fluorobenzene showed the greatest potential for using such materials for CWA uptake and was tested against real agents including isopropyl methylphosphonofluoridate (sarin, GB) and bis(2-chloroethyl)sulfane (sulfur mustard, HD) revealing uptakes close to 20 mL g−1.

Sponge-Like Behaviour in Isoreticular Cu(Gly-His-X) Peptide-Based Porous Materials.

We report two isoreticular 3D peptide-based porous frameworks formed by coordination of the tripeptides Gly-l-His-Gly and Gly-l-His-l-Lys to CuII which display sponge-like behaviour. These porous materials undergo structural collapse upon evacuation that can be reversed by exposure to water vapour, which permits recovery of the original open channel structure. This is further confirmed by sorption studies that reveal that both solids exhibit selective sorption of H2O while CO2 adsorption does not result in recovery of the original structures. We also show how the pendant aliphatic amine chains, present in the framework from the introduction of the lysine amino acid in the peptidic backbone, can be post-synthetically modified to produce urea-functionalised networks by following methodologies typically used for metal–organic frameworks built from more rigid “classical” linkers.