Darren Bradshaw

Porous organic cages

Porous materials are important in a wide range of applications including molecular separations and catalysis. We demonstrate that covalently bonded organic cages can assemble into crystalline microporous materials. The porosity is prefabricated and intrinsic to the molecular cage structure, as opposed to being formed by non-covalent self-assembly of non-porous sub-units. The three-dimensional connectivity between the cage windows is controlled by varying the chemical functionality such that either non-porous or permanently porous assemblies can be produced. Surface areas and gas uptakes for the latter exceed comparable molecular solids. One of the cages can be converted by recrystallization to produce either porous or non-porous polymorphs with apparent Brunauer-Emmett-Teller surface areas of 550 and 23 m2 g(-1), respectively. These results suggest design principles for responsive porous organic solids and for the modular construction of extended materials from prefabricated molecular pores.

A Guest-Responsive Fluorescent 3D Microporous Metal−Organic Framework Derived from a Long-Lifetime Pyrene Core

The carboxylate ligand 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy)-based on the strongly fluorescent long-lifetime pyrene core-affords a permanently microporous fluorescent metal-organic framework, [In(2)(OH)(2)(TBAPy)].(guests) (1), displaying 54% total accessible volume and excellent thermal stability. Fluorescence studies reveal that both 1 and TBAPy display strong emission bands at 471 and 529 nm, respectively, upon excitation at 390 nm, with framework coordination of the TBAPy ligands significantly increasing the emission lifetime from 0.089 to 0.110 ms. Upon desolvation, the emission band for the framework is shifted to lower energy: however, upon re-exposure to DMF the as-made material is regenerated with reversible fluorescence behavior. Together with the lifetime, the emission intensity is strongly enhanced by spatial separation of the optically active ligand molecules within the MOF structure and is found to be dependent on the amount and chemical nature of the guest species in the pores. The quantum yield of the material is found to be 6.7% and, coupled with the fluorescence lifetime on the millisecond time scale, begins to approach the values observed for Eu(III)-cryptate-derived commercial sensors.

An Adaptable Peptide-Based Porous Material

Swelling Pores Porosity is a key parameter when selecting materials for catalysts, chemical separations, gas storage, host-guest interactions, and related chemical processes. In most cases the porosity of a material is fixed. Rabone et al. (p. 1053; see the Perspective by Wright) have described a molecular material in which the size of the pores changed during the sorption process. The porosity increased because a dipeptide linker between metal centers reoriented during uptake of some gases, thus improving the capacity of the material to adsorb. Conformational changes in a porous material during the sorption of small molecules lead to a dynamic increase in porosity. Porous materials find widespread application in storage, separation, and catalytic technologies. We report a crystalline porous solid with adaptable porosity, in which a simple dipeptide linker is arranged in a regular array by coordination to metal centers. Experiments reinforced by molecular dynamics simulations showed that low-energy torsions and displacements of the peptides enabled the available pore volume to evolve smoothly from zero as the guest loading increased. The observed cooperative feedback in sorption isotherms resembled the response of proteins undergoing conformational selection, suggesting an energy landscape similar to that required for protein folding. The flexible peptide linker was shown to play the pivotal role in changing the pore conformation.

MOF‐Polymer Composite Microcapsules Derived from Pickering Emulsions

Hollow composite microcapsules are prepared by the assembly of pre-formed nanocrystals of metal-organic frameworks (MOFs) around emulsion droplets, followed by interfacial polymerisation of the interior. The micropores of the MOF crystals embedded within a semipermeable hierarchically structured polymeric membrane are an effective combination for the retention of encapsulated dye molecules. Release can be triggered however by acid dissolution of the MOF component.

Hydrogen adsorption in microporous hypercrosslinked polymers

A microporous hypercrosslinked polymer resin was synthesized and shown to adsorb 3.04 wt.% hydrogen at 77 K and 15 bar; this represents the highest level of hydrogen adsorption yet observed for an organic polymer.

Macro-/microporous MOF composite beads

We have prepared a series of metal–organic framework (MOF) composite materials by depositing crystalline MOFs into mm-sized macroporous polyacrylamide (PAM), oxide or oxide–PAM beads using solvothermal methods or immersion in mother solutions. The facile nature of the composite preparation is in part due to the inherent hydrophilicity and processability of the PAM, which bears suitable functionality to interact with the MOF. The resulting composites display hierarchical porosity across the macro- and microporous length scales; the MOF loading level, and hence degree of microporosity in the composites, can be tuned by modulating the concentration of the reactants in the MOF synthesis. The MOF@PAM beads display enhanced mechanical stability and handling over bulk MOF phases in heterogeneous reaction systems, clearly indicating that such materials can improve recovery and recyclability of crystalline MOFs. This is an important step for future applications.

Silica SOS@HKUST-1 composite microspheres as easily packed stationary phases for fast separation

Metal–organic frameworks (MOFs) have been investigated for separations including chromatography. Typically, MOF particles are directly packed into columns for the separations. The irregular shapes and wide size distributions of MOF particles have led to difficulty in column packing and low column efficiency or high back pressure. We describe here the preparation of MOF–silica microspheres as packing materials for fast and efficient liquid chromatography. Spheres-on-sphere (SOS) silica particles are prepared, modified with –COOH and –NH2 groups, and then used as support to grow HKUST-1. HKUST-1 nanocrystals and films are formed and attached firmly onto the SOS particles with adjustable porosity. The composite microspheres, showing core–shell properties, are directly packed into columns to offer separation capability of MOFs and efficient packing and support of silica microspheres. These columns show separation of toluene/ethylbenzene/styrene and toluene/o-xylene/thiophene within 1.5 minutes. Although HKUST-1 is not good for separating xylene isomers, the separation can be achieved in 5 minutes using the composite microspheres after conditioning the column with dichloromethane or toluene. Remarkably, it is observed that conditioning with DCM can change retention time and selectivity (elution order) of xylene isomers. It is also possible to produce other types of MOFs (e.g., ZIF-8) on the SOS particles, indicating the potential of this method for wider applications.

CO2 selectivity of a 1D microporous adenine-based metal–organic framework synthesised in water

The coordination of adenine to Ni(2+) forms a dimer unit which can be linked by 3,5-pyrazoledicarboxylic acid into chains which assemble under hydrothermal conditions via hydrogen bonding into a robust porous network. The material displays selectivity for CO(2) over CH(4) and an isosteric heat which increases with guest loading.

A colloidal water-stable MOF as a broad-range fluorescent pH sensor via post-synthetic modification

We report for the first time the pH-dependent fluorescence of UiO-66-NH2 across the wide range from 1 to 9. By application of a post-synthetic modification (PSM) diazotisation strategy, we synthesized a new material, UiO-66-N=N-ind, which shows increased chemical stability and enhanced sensing up to pH 12.

Control of Porosity Geometry in Amino Acid Derived Nanoporous Materials

Substitution of the pillaring ligand in the homochiral open-framework [Ni(2)(L-asp)(2)(bipy)] by extended bipy-type ligands leads to a family of layer-structured, homochiral metal-organic frameworks. The 1D channel topology can be modified by the nature of the organic linker, with shape, cross-section and the chemical functionality tuneable. In addition, the volume of these channels can be increased by up to 36 % compared to the parent [Ni(2)(L-asp)(2)(bipy)]. The linker 1,4-dipyridylbenzene (3rbp) gives access to a new layered homochiral framework [Ni(2)(L-asp)(2)(3rbp)] with channels of a different shape. In specific cases, non-porous analogues with the linker also present as a guest can be activated to give porous materials after sublimation. Their CO(2) uptake shows an increase of up to 30 % with respect to the parent [Ni(2)(L-asp)(2)(bipy)] framework.