Anna G. Slater

Function-led design of new porous materials

Its all about the holes From kitchen sieves and strainers to coffee filters, porous materials have a wide range of uses. On an industrial scale, they are used as sorbents, filters, membranes, and catalysts. Slater and Cooper review how each application will limit the materials that can be used, and also the size and connectivity of the pores required. They go on to compare and contrast a growing range of porous materials that are finding increasing use in academic and industrial applications. Science, this issue 10.1126/science.aaa8075 BACKGROUND Porous materials are important in established processes such as catalysis and molecular separations and in emerging technologies for energy and health. Porous zeolites have made the largest contribution to society so far, and that field is still developing rapidly. Other porous solids have also entered the scene in the past two decades, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers. No single class of porous material is ideal for all purposes. For example, crystallinity and long-range order might enhance selectivity for a molecular separation while also reducing mechanical stability or processability with respect to less ordered structures. To have an impact on real applications, porous materials must be scalable and must satisfy multiple functional criteria such as long-term stability, selectivity, adsorption kinetics, and processability, all within a viable cost envelope. This presents a broad design challenge, and it requires us to be able to control structure and to understand multiple structure-property relationships at a detailed level. ADVANCES In addition to MOFs, COFs, and porous polymer networks, other classes of molecular porous solids have emerged in the past 10 years, such as polymers of intrinsic microporosity and porous organic cages. The range of possible functions for porous solids is thus much broader than before. For example, conjugated microporous polymers and some COFs have extended, conjugated structures that are not present in zeolites or MOFs and have led to porous organic photocatalysts and electronic materials. The crystal engineering approaches developed for zeolites, MOFs, and COFs cannot be applied directly to amorphous solids such as porous polymers, but analogous modular strategies have allowed functions such as porosity and electronic band gap to be controlled by choosing the appropriate molecular building blocks. Rapid advances in the computational prediction of structure and function offer a strategy for identifying the best porous materials for specific applications, for example, via large-scale screening of gas adsorption in hypothetical MOFs. OUTLOOK Advances in synthesis have produced new classes of functional porous solids as well as fundamental breakthroughs in areas such as selective carbon dioxide capture, molecular separations, and catalysis. As yet, these rapid developments in basic understanding are unmatched by large-scale commercial implementation, but enhanced functions (such as enzyme-like CO2 selectivity) and new processing options (such as soluble porous solids) present exciting opportunities. A general challenge will be to reengineer porous materials where scale-up is prohibited by cost, retaining the advanced function but using cheaper and more sustainable building blocks. It is therefore important to develop structure-property relationships to understand how promising materials work. Not all future opportunities for porous solids involve improving on existing materials or the development of more scalable preparation routes. For example, porous photocatalysts that can perform direct solar water splitting might provide a completely new platform for energy production. As we seek increasingly complex functions for porous materials, the use of in silico computational design to guide experiment will become more important. Porous materials can be defined by type or by function, but it is function that will determine the scope for practical applications. Our ability to design functions in porous solids has advanced markedly in the past two decades as a result of developments in modular synthesis, materials characterization, and (more recently) computational structure-property predictions. This figure is based on the pore channels, shown in yellow, for an organic cage molecule, a new type of solution-processable porous solid developed over the past 6 years. Porous solids are important as membranes, adsorbents, catalysts, and in other chemical applications. But for these materials to find greater use at an industrial scale, it is necessary to optimize multiple functions in addition to pore structure and surface area, such as stability, sorption kinetics, processability, mechanical properties, and thermal properties. Several different classes of porous solids exist, and there is no one-size-fits-all solution; it can therefore be challenging to choose the right type of porous material for a given job. Computational prediction of structure and properties has growing potential to complement experiment to identify the best porous materials for specific applications.

Surface-based supramolecular chemistry using hydrogen bonds.

CONSPECTUS: The arrangement of molecular species into extended structures remains the focus of much current chemical science. The organization of molecules on surfaces using intermolecular interactions has been studied to a lesser degree than solution or solid-state systems, and unanticipated observations still lie in store. Intermolecular hydrogen bonds are an attractive tool that can be used to facilitate the self-assembly of an extended structure through the careful design of target building blocks. Our studies have focused on the use of 3,4,9,10-perylene tetracarboxylic acid diimides (PTCDIs), and related functionalized analogues, to prepare extended arrays on surfaces. These molecules are ideal for such studies because they are specifically designed to interact with appropriate diaminopyridine-functionalized molecules, and related species, through complementary hydrogen bonds. Additionally, PTCDI species can be functionalized in the bay region of the molecule, facilitating modification of the self-assembled structures that can be prepared. Through a combination of PTCDI derivatives, sometimes in combination with melamine, porous two-dimensional arrays can be formed that can entrap guest molecules. The factors that govern the self-assembly processes of PTCDI derivatives are discussed, and the ability to construct suitable target arrays and host-specific molecular species, including fullerenes and transition metal clusters, is demonstrated.

Two-dimensional supramolecular chemistry on surfaces

Self-assembly of two-dimensional supramolecular arrays on surfaces represents a significant challenge to chemists, materials scientists and physicists. This article highlights advances in using supramolecular interactions, particularly hydrogen bonding, to self-assemble such two-dimensional arrays on surfaces. Scanning-probe microscopies, particularly scanning tunnelling microscopy (STM), can be used to determine the precise molecular arrangement of the self-assembled structures allowing insight into the self-assembly process at the molecular level. The use of such supramolecular assemblies to trap guest species, mimicking host–guest chemistry in the solution phase, will also be discussed. Such images provide great insight into the advantages and restrictions of working in two dimensions in comparison to the solution phase or the solid state.

Effects of pore modification on the templating of guest molecules in a 2D honeycomb network

1,7-Diadamantanethioperylene-3,4:9,10-tetracarboxylic diimide, (Ad-S)2–PTCDI, adsorbed on Au(111) from solution was investigated by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). (Ad-S)2–PTCDI forms a well-ordered monolayer whose structure is described by a (2√63 × √19)R19.1° chiral unit cell containing four molecules. Codeposition of (Ad-S)2–PTCDI with 1,3,5-triazine-2,4,6-triamine (melamine) yields a honeycomb network whose (7√3 × 7√3)R30° unit cell is identical to the unsubstituted PTCDI/melamine analogue. The effect of the adamantyl thioether moieties on the adsorption of guest molecules is investigated using adamantane thiol and C60. While the thioether units do not affect the packing of adamantane thiol molecules a pronounced influence is seen in the case of fullerene. Pore modification involving different combinations of enantiomers of (Ad-S)2–PTCDI give rise to distinctly different arrangements of C60 molecules. The diversity of patterns is further increased by the presence of unsubstituted PTCDI molecules.

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.

Fullerenes as adhesive layers for mechanical peeling of metallic, molecular and polymer thin films.

Summary We show that thin films of C60 with a thickness ranging from 10 to 100 nm can promote adhesion between a Au thin film deposited on mica and a solution-deposited layer of the elastomer polymethyldisolaxane (PDMS). This molecular adhesion facilitates the removal of the gold film from the mica support by peeling and provides a new approach to template stripping which avoids the use of conventional adhesive layers. The fullerene adhesion layers may also be used to remove organic monolayers and thin films as well as two-dimensional polymers which are pre-formed on the gold surface and have monolayer thickness. Following the removal from the mica support the monolayers may be isolated and transferred to a dielectric surface by etching of the gold thin film, mechanical transfer and removal of the fullerene layer by annealing/dissolution. The use of this molecular adhesive layer provides a new route to transfer polymeric films from metal substrates to other surfaces as we demonstrate for an assembly of covalently-coupled porphyrins.

A solution-processable dissymmetric porous organic cage

Two dissymmetric racemic analogues of the chiral porous organic cage, CC3, were isolated and unambiguously characterised as a racemate pair of the R,R,R,S,S,S and S,S,S,R,R,R-diastereomers (CC3-RS and CC3-SR). CC3-RS/CC3-SR equals the highest porosity measured for CC3 but is an order of magnitude more soluble, making it an excellent candidate for incorporation into a membrane for separation applications.

Ultra-Fast Molecular Rotors within Porous Organic Cages

Abstract Using variable temperature 2H static NMR spectra and 13C spin‐lattice relaxation times (T1), we show that two different porous organic cages with tubular architectures are ultra‐fast molecular rotors. The central para‐phenylene rings that frame the “windows” to the cage voids display very rapid rotational rates of the order of 1.2–8×106 Hz at 230 K with low activation energy barriers in the 12–18 kJ mol−1 range. These cages act as hosts to iodine guest molecules, which dramatically slows down the rotational rates of the phenylene groups (5–10×104 Hz at 230 K), demonstrating potential use in applications that require molecular capture and release.

Computer-guided porous materials design: from rationalization to prediction

Organic molecules will tend to pack in dense crystal structures, avoiding the formation of voids. However, the generally less energetically favourable arrangement of molecules into porous crystal structures show important advantages in applications such as gas storage, separation or catalysis. Weak ‒electrostatic and dispersive‒ intermolecular interactions dominate molecular packing and are the origin of the unpredictability that seems to surround their crystallization. Thus, the rational design of new materials for technological applications will be limited by the ability to reliably anticipate: (i) the final crystal structure formed and (ii) the physico-chemical properties of such a material.

Crystal Structure Prediction (CSP) datasets of homochiral and racemic TCC1-3 crystals

Dataset supporting: Slater, A. G. et al (2016) Reticular synthesis of porous molecular 1D nanotubes and 3D networks. Nature Chemistry.Zip file containing: 1) Six cif files with the sets of predicted crystal structures for homochiral (-R) and racemic (-R/-S) TCC1-3 systems: TCC1R_CSP.cif, TCC1RS_CSP.cif, TCC2R_CSP.cif TCC2RS_CSP.cif, TCC3R_CSP.cif and TCC3RS_CSP.cif.2) Six text files with a summary of structural and energetic properties for each set set of predicted crystal structures for homochiral (-R) and racemic (-R/-S) TCC1-3 systems: TCC1R_CSP.txt, TCC1RS_CSP.txt, TCC2R_CSP.txt TCC2RS_CSP.txt, TCC3R_CSP.txt and TCC3RS_CSP.txt.3) A text file (Match_to_experimental_structures.txt) with the name of predicted crystal structures matching the experimentally observed crystal structure.Funded by EPSRC (Chemical Synthesis of Transformative Extended Materials; EP/H000925/1; 2009 to 2015).