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

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).

The research described in this proposal will be of great importance to both computational and experimental researchers in the Molecular Framework Materials community. Achievement of this project's aims by the data-driven development of a flexible, universal force field, and the subsequent rapid prediction of new, flexible MFMs will have a broad impact amongst researchers developing these MFMs for a wide range of applications, most notably energy storage and catalysis. Relating the dynamic behaviour of the framework to the mechanical properties of the individual building blocks will allow synthetic MFM researchers to design MFMs with specific dynamic behaviour. Development of a general-purpose force-field that accounts for flexibility will similarly have a broad impact in that it will permit both prediction of framework structures exhibiting flexible behaviour and large-scale computational screening of MFMs where even moderate amounts of flexibility in the framework structure can strongly affect the observed properties.

This two-year project signifies a unique contribution to fundamental computational materials chemistry. The results will be disseminated via two world-leading software packages, ToposPro and ADF-AuToGraFS and published in widely read journals such as Crystal Growth & Design and CrystEngComm. In the intermediate term (1-3 years) collaboration with leading synthetic groups will result in predicted MFMs published in major high impact journals such as Angew. Chem. Int. Ed., J. Am. Chem. Soc and Chem. Sci.. The data-mining methodology and database developed with the subsequent material design insights obtained will be presented at the major conferences for the European and worldwide MFM communities (EuroMOF2019 and MOF2020) by the PI and postdoc, enabling researchers in all corners of the MFM community to benefit from this research and hopefully initiate new collaborations, directly benefitting the future research of both the PI and postdoc.

While the proposed research is fundamental, and the economic impact will not be immediate, the design of Molecular Framework Materials, as efficient storage for energy (particularly methane and hydrogen), as catalysts for industrially and environmentally important reactions, such as CO2 reduction, oxidation of hydrocarbons and water splitting, and as filters - e.g. to separate helium from methane, will have a very significant long-term economic and environmental impact.