Pushed and pulled by competing interactions, molecules can self-assemble into complex structures. Using supramolecular self-assembly, we can synthesise materials with unique structures and function. However, predicting how molecules will assemble themselves, and controlling the reaction conditions to nudge them into forming a desirable structure, is challenging. Using porous cage molecules as building blocks for larger structures is an attractive prospect. Using a hierarchical approach embeds cage molecules and their useful properties into more complex structures with new functions. However, predicting and controlling the synthesis process becomes increasingly difficult. A team of researchers from the University of Liverpool, Herriot-Watt University, Imperial College London, the University of Southampton and East China University of Science and Technology has developed a hierarchical cage molecule that can adsorb other molecules, like carbon dioxide and sulfur hexafluoride. A key aspect of the project was using computer modelling to accurately predict how the precursor molecules would self-assemble into a new material. Their work, recently published in Nature Synthesis, suggests that computational predictions backed up by experimental studies could be a successful strategy for yielding more complex and interesting materials in the future.
“The computer modelling work was done by Prof Graeme Day at the University of Southampton and Prof Kim Jelfs at Imperial College London,” says Dr Marc Little at Herriot-Watt University.
One of the advantages of these computational methods is that they can give you new areas to explore experimentally. Our study has highlighted a new type of material that has useful properties and may warrant further investigation. But we’ve also shown that by working together cooperatively, and having computational predictions backed up by experimental studies, we can develop more complex and more interesting materials in the future. The ‘cage of cages’ material that we’ve developed is a new level of complexity, but we’ve been able to validate the accuracy of the computer predictions, and that is really key.
Although there are a number of experimental techniques for characterising molecules, including Nuclear Magnetic Resonance (NMR), the size and complexity of the structure of this material means that synchrotron diffraction was crucial.
Prof Andrew Cooper, from the University of Liverpool, explains,
These are very large molecules, with cavities that create very diffuse scattering. That can lead to weak diffraction, particularly early on in the discovery process, when we’re still optimising the crystallisation processes, and so working with smaller crystals. In this particular case, the molecule has high symmetry, meaning that other characterisation techniques were unable to offer much insight into the structure of the molecule.
The Cooper group at the University of Liverpool has had Block Allocation Group (BAG) access to Diamond’s I19 and I11 beamlines for over 10 years. This means that members of the team can collect data regularly, without having to wait for a standard access beamtime allocation.
Dr Little explains,
That continuous beam access to world-class facilities has been really important. It enables researchers to frequently visit or send samples remotely to Diamond to collect X-ray diffraction (XRD) data and solve the structure of complex new materials. Qiang Zhu, the PhD student who led the experimental work, was able to analyse crystals at I19 very soon after growing them. So very early on, we were able to characterise this new molecule, and have that feedback straight away for the project.
This hierarchical cage molecule has a high storage capacity for gas molecules like carbon dioxide and sulfur hexafluoride. In the future, complex hierarchical structures could be used to perform challenging molecular separation, such as filtering toxic volatile organic compounds (VOCs) from the air, or have applications in medical science.
Predicting how those molecules may form, however, is an ongoing challenge. Prof Cooper added,
We’ve moved the bar with what we can predict. However, the complex materials we’re developing now are so large that – computationally speaking – they’re a million times more difficult. For the moment, at least, there are still new materials that we can crystallise, and analyse, but not predict.
To find out more about the I19 beamline or discuss potential applications, please contact Principal Beamline Scientist Dave Allan: dave.allan@diamond.ac.uk.
Zhu Qiang et al. Computationally guided synthesis of a hierarchical [4 [2+ 3]+ 6] porous organic ‘cage of cages’." Nature Synthesis 3, 825–834 (2024). DOI:10.1038/s44160-024-00531-7
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
Copyright © 2022 Diamond Light Source
Diamond Light Source Ltd
Diamond House
Harwell Science & Innovation Campus
Didcot
Oxfordshire
OX11 0DE
Diamond Light Source® and the Diamond logo are registered trademarks of Diamond Light Source Ltd
Registered in England and Wales at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom. Company number: 4375679. VAT number: 287 461 957. Economic Operators Registration and Identification (EORI) number: GB287461957003.
Science