James T. A. Jones

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.

Modular and predictable assembly of porous organic molecular crystals

Nanoporous molecular frameworks 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 rather than, for example, the functional group localization found in the reactive sites of enzymes. 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. 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, allowing in silico materials design strategies. 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.

Nanoporous Organic Polymer/Cage Composite Membranes

Organic?organic composite membranes are prepared by in?situ crystallization of cage molecules in a polymer of intrinsic microporosity. This allows a direct one-step route to mixed-matrix membranes, starting with a homogeneous molecular solution. Extremely high gas permeabilities are achieved, even after ageing for more than a year, coupled with good selectivity for applications such as CO2 recovery.

On–Off Porosity Switching in a Molecular Organic Solid

Pulling the old switcheroo: Microporosity can be switched “on” and “off” in a crystalline molecular organic solid composed of cage molecules (see scheme). The switch is facilitated by conformational flexibility in the soft organic crystal state.

Porous organic molecular solids by dynamic covalent scrambling

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.

On crystal versus fiber Formation in dipeptide hydrogelator systems

Naphthalene dipeptides have been shown to be useful low-molecular-weight gelators. Here we have used a library to explore the relationship between the dipeptide sequence and the hydrogelation efficiency. A number of the naphthalene dipeptides are crystallizable from water, enabling us to investigate the comparison between the gel/fiber phase and the crystal phase. We succeeded in crystallizing one example directly from the gel phase. Using X-ray crystallography, molecular modeling, and X-ray fiber diffraction, we show that the molecular packing of this crystal structure differs from the structure of the gel/fiber phase. Although the crystal structures may provide important insights into stabilizing interactions, our analysis indicates a rearrangement of structural packing within the fibers. These observations are consistent with the fibrillar interactions and interatomic separations promoting 1D assembly whereas in the crystals the peptides are aligned along multiple axes, allowing 3D growth. This observation has an impact on the use of crystal structures to determine supramolecular synthons for gelators.

Large Self‐Assembled Chiral Organic Cages: Synthesis, Structure, and Shape Persistence

Keep the cage filled: Two large organic cages (see example) with void diameters of 1.2 nm were synthesized through [8+12] imine condensation reactions. The materials become amorphous upon solvent removal and show little permanent porosity. Molecular dynamics simulations give an insight into the mechanism of these processes, suggesting strategies for synthesizing larger shape-persistent organic cages in the future.

A soft porous organic cage crystal with complex gas sorption behavior.

Big softy! A soft porous molecular crystal composed of organic cages exhibits complex multistep gas sorption isotherms (see figure), analogous to those observed in soft porous metal–organic frameworks. Softness is induced by frustrated packing of the cages and structural flexibility leads to kinetic guest trapping.

Acid- and Base-Stable Porous Organic Cages: Shape Persistence and pH Stability via Post-synthetic “Tying” of a Flexible Amine Cage

Imine cage molecules can be reduced to amines to improve their chemical stability, but this introduces molecular flexibility. Hence, amine cages tend not to exhibit permanent solid-state porosity. We report a synthetic strategy to achieve shape persistence in amine cages by tying the cage vertices with carbonyls such as formaldehyde. Shape persistence is predicted by conformer stability calculations, providing a design basis for the strategy. The tied cages show enhanced porosity and unprecedented chemical stability toward acidic and basic conditions (pH 1.7-12.3), where many other porous crystalline solids would fail.