Origin and Mechanisms of Flexibility in Molecular Framework Materials: A Data-driven, Graph Theoretical Approach

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Molecular framework materials (MFMs) are an emerging materials class that combines the immense richness of functionalities of molecules with the advantages of regular solid materials. High surface area, tuneable pore size and functional groups offer applications in fields such as gas adsorption and separation, catalysis and sensing. They are synthesised by joining metal clusters (nodes) with organic linkers. There are several dozen possible nodes of different shapes, and almost any organic molecule can be used as a linker resulting in a huge "Molecular Meccano" set for the creation of a vast variety of porous MFMs. Several interesting phenomena have been observed in these materials. Flexibility in the linkers themselves and their attachment to each node leads to breathing and gating behaviour in the materials, without destroying their crystallinity. Framework breathing, for example, can admit guest molecules that would not otherwise fit through pore gates. In a similar manner, small rotations of linkers can create / destroy ideal pockets for absorption of gases such as CO2 and H2.While fundamental to the behaviour of MFMs, this flexibility poses an inherent challenge to the development of these materials and as yet the fundamental atomistic understanding of the breathing phenomena is not at the stage where it can be employed to design these materials. This project will create a database of all known building blocks for MFMs and then use that database to determine degree and type of flexibility inherent in each building block. The flexibility in each building block can then be related back to the overall framework structures and used to design flexible MFM materials tailored for specific applications - e.g. to store energy (hydrogen or methane) or to separate and purify gas mixtures (such as helium in natural gas).