Understanding and engineering function in switchable molecular crystals

Lead Research Organisation: University of Oxford
Department Name: Oxford Chemistry

Abstract

The physical properties of a crystalline material depend on the spacial arrangement of its atoms or molecules, as much as on the molecules themselves. Quite often the same molecules can generate two or more different kinds of crystal, by packing together in different ways, leading to materials that are physically distinct but with the same chemical composition (polymorphs). A compound can often prefer to adopt different crystal polymorph structures under different conditions of temperature or pressure. Thus, when the temperature is changed, the crystal lattice can rearrange itself into a new three-dimensional structure - a phase transition. This is important, for example, in the pharmaceutical industry, for example, where different crystal polymorphs of drug compounds can have different solubilities, with the less soluble form being less active. Crystal phase transitions can also have drastic effects on the properties of conducting, magnetic and photonic materials, where small rearrangements of the atoms in a material have large consequences for how their electrons behave.

One type of phase change that we have been studying for some time is spin-crossover, which is a rearrangement of the electrons in an atom in response to a change in temperature. This is common in some types of transition metal compound, being particularly prevalent in iron chemistry. While the molecules in a material undergo spin-crossover individually, it leads to large changes in their size and shape which are propagated through the material in the solid state. As one molecule undergoes the transition and changes its size, it causes a change in pressure in the crystal lattice that in turn promotes the transition in its nearest neighbours. These effects are transmitted through a crystal lattice at differing rates, depending on the strength of the interactions between molecules. Hence, whether a particular material undergoes spin-crossover abruptly or gradually, with temperature or with time, is controlled by its crystal packing. Spin-crossover is a rather extreme example of a crystallographic phase change, in terms of the changes involved to the structure of the material. But it can serve as a model for other, more general types of crystal phase behaviour.

This project represents a concerted program to improve our understanding of phase changes in crystalline materials, using spin-crossover compounds as a test-bed. We will establish new fundamental principles for engineering phase changes into molecular crystal, that occur under pre-defined conditions (of temperature and/or light irradiation), at different rates, and with the property of hysteresis. As well as synthesising these new materials, this apparently simple objective requires state-of-the-art methods for measuring these structure changes. This will be achieved using new X-ray diffraction techniques, for inducing phase changes in crystals in high yields under controlled conditions, and for interpreting the data that result from these experiments (to deconvolute contributions from the starting and product phases of the material, for example). We will also develop improved methods for simulating the phase change events using computer models, to provide new insight into how the design of the crystal affects the propagation of the phase change through its bulk.

The combination of expertise in our consortium will achieve real advances towards solving a problem, that has only been successfully addressed up to now by trial-and-error.

Publications

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Cooper RI (2016) Absolute structure determination using CRYSTALS. in Acta crystallographica. Section C, Structural chemistry

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Parois P (2018) An enhanced set of displacement parameter restraints in CRYSTALS in Journal of Applied Crystallography

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Watkin DJ (2016) Why direct and post-refinement determinations of absolute structure may give different results. in Acta crystallographica Section B, Structural science, crystal engineering and materials

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Zaiter S (2019) Heteroatom substitution effects in spin crossover dinuclear complexes. in Dalton transactions (Cambridge, England : 2003)

 
Description Methods for analysing time-resolved X-ray diffraction data sets of crystalline materials. Developments to our analytical tools enable study of in-situ changes to crystalline materials, which can help to understand and improve the behaviour of materials which change properties when exposed to light, chemicals or heat.
Exploitation Route Our methods are implemented in an open source crystallographic software package and document in publications and the software manual. They can be used for general purpose non-standard experiments where there is an scientific advantage in combining multiple diffraction data sets or crystallographic models in a single analysis.
Sectors Energy,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

 
Title Development of custom crystallographic displacement parameters restraints for modelling excited state structures 
Description Development and implementation of new mathematical restraints for linking atoms in a structure, or between multiple structures (e.g. ground state / excited state) now allows refinement of more realistic crystallographic models. The work is implemented in the refinement package CRYSTALS, developed in this group, and is available over the web. A publication describing the new tools is under review at the time of writing. 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? Yes  
Impact None so far as we are still awaiting final acceptance of the publication (expected this week). 
URL http://www.xtl.ox.ac.uk/tag/crystals-release.1.html
 
Description Nancy group 
Organisation University of Lorraine
Country France 
Sector Academic/University 
PI Contribution Ongoing development of strategy software and optimal experimental setup for measuring rate data in pump-probe time-resolved experiments.
Collaborator Contribution The group and Universite de Lorraine is leading a time-resolved diffraction project and undertakes the experimental work. They have developed and integrated the components of the data collection platform, pump-probe laser and additional spectroscopic apparatus.
Impact There is a paper submitted with Dr Pascal Parois as co-author describing the equipment developed.
Start Year 2013
 
Title CRYSTALS structure analysis software 
Description The core CRYSTALS package has been developed for many years. Recent work has focussed on providing tools for both analysing and designing better experiments for measuring small in-situ differences in crystalline samples. These include measuring photo-excitation rates in single crystal samples and the determination of absolute structure using small signals from resonant X-ray scattering. 
Type Of Technology Software 
Year Produced 2015 
Open Source License? Yes  
Impact Tools for experimental optimisation by analysis of preliminary structure results are included in the CRYSTALS software package. We are currently writing up the theory and applied examples with recommendations to allow other groups to follow the methods. 
URL http://www.xtl.ox.ac.uk/crystals.1.html
 
Title Strategy determination tool 
Description An online implementation of a tool for determining fast data collection strategies for area detector diffractometers which allows prioritisation of a number of data points. Supported controller / instrument combinations are CrysalisPro on Agilent Supernova, CrysalisPro on Xpad (U. of Nancy), CrysalisPro on Diamond Light Source, beamline I19, hutch 2 goniometer. 
Type Of Technology Software 
Year Produced 2015 
Open Source License? Yes  
Impact Existing software tools for designing crystallographic experiments are often closed-source and are limited to standard experiments. Scientists are left with great challenges to exploit new instrument technology for non-standard data collections like photo­crystallography. The tools presented here have been used to create experiment strategies in-house and at Diamond Light Source, by Oxford researchers and by beamline staff. The software has also been used to support developments of a time-resolved crystallography instrument at University of Nancy. 
URL http://blog.debroglie.net/strategies/