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Acta Cryst. (2014). A70, C632
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Intensive research into microporous materials has been driven by potential applications in areas such as catalysis, gas separation, storage, and sensing. Recently, a new class of purely organic molecular cage materials has emerged, which can exhibit significant porosity arising from the internal molecular cavity as well as extrinsic porosity from packing in the crystal structure [1]. Unlike extended frameworks, porous molecular materials lack strongly directional interactions to drive their assembly, complicating the crystal engineering possible for isoreticular metal-organic frameworks [2], for example. Our work has focused on covalent imine-linked cages, which exhibit diverse crystal chemistry. The connectivity of the pore network is derived from the cage packing: Therefore, the crystal structure directly affects the observed porosity. The imine cages synthesised so far lack strongly hydrogen bonding groups. Thus, the solid state supramolecular assembly of cage molecules is governed by the aggregate of weak interactions, such as van der Waals forces. By identifying robust `tectons', that is, regularly occurring supramolecular motifs, progress toward designing the crystal structure and therefore controlling the physical properties of organic cage materials becomes possible. Here, we report exploiting robust supramolecular motifs, comprising either cage modules or host and guest molecules to gain control over the porosity of the bulk material. We demonstrate how formation of a desired void network topology can be driven by hosting a specific guest in preferred sites which maximise weak host-guest interactions [3]. Subsequent guest removal can produce stable polymorphs, one of which exhibited double the Brunauer-Emmett-Teller surface area with respect to the originally observed polymorph. We also examine how the interaction between gas phase guests and cage host is important in the application of porous organic cages in rare gas separation.

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Acta Cryst. (2014). A70, C667
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Among microporous materials, there has been an increasing recent interest in porous organic cage (POC) crystals, which can display permanent intrinsic (molecular) and extrinsic (crystal network) porosity. These materials can be used as molecular sieves for gas separation and potential applications as enzyme mimics have been suggested since they exhibit structural response toward guest molecules[1]. Small structural modifications of the initial building blocks of the porous organic molecules can lead to quite different molecular assembly[1]. Moreover, the crystal packing of POCs is based on weak molecular interactions and is less predictable that other porous materials such as MOFs or zeolites.[2] In this contribution, we show that computational techniques -molecular conformational searches and crystal structure prediction- can be successfully used to understand POC crystal packing preferences. Computational results will be presented for a series of closely related tetrahedral imine- and amine-linked porous molecules, formed by [4+6] condensation of aromatic aldehydes and cyclohexyl linked diamines. While the basic cage is known to have one strongly preferred crystal structure, the presence of small alkyl groups on the POC modifies its crystal packing preferences, leading to extensive polymorphism. Calculations were able to successfully identify these trends as well as to predict the structures obtained experimentally, demonstrating the potential for computational pre-screening in the design of POCs within targeted crystal structures. Moreover, the need of accurate molecular (ab initio calculations) and crystal (based on atom-atom potential lattice energy minimization) modelling for computer-guided crystal engineering will be discussed.
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