Understanding and engineering function in switchable molecular crystals

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.

As described under Academic Beneficiaries, this proposal maps well onto several of EPSRC's current priority areas.

Structural changes in crystalline materials impact many aspects of nature and technology. Examples include the pharmaceutical industry, where different structural forms of the same drug substance can have different efficacies, originating from their different solubilities. Alternative structural forms of conducting, magnetic and ferroelectric materials can have different physical properties, arising from differences in the arrangements of their atoms or molecules that may be quite subtle. Applications in photonics depend on switchable crystals, whose structure and refractive index can be changed rapidly and reversibly upon application of a stimulus. Finally, different forms of biominerals like CaCO3 have different morphologies and mechanical properties, that control their biological functions. An understanding of the relationship between different structure forms of crystalline compounds, and how they interconvert, is of fundamental importance for the exploitation of crystalline materials for societal needs.

A crystallographic phase change is initiated at one or more sites in the crystal (nucleation), before propagating through the bulk (growth). As well as typical phase changes, chemical reactions occurring inside crystals also proceed by the same nucleation and growth process. These range from the light-induced darkening of microcrystalline silver salts in photochromic glass, through force-induced chemical transformations, to more complicated organic photocyclisations and the post-synthetic modification of MOF materials. This proposal is a study of crystallographic phase changes, using a particular type of transition (spin-crossover, SCO) as a test-bed for our new developments. SCO transitions are unusual, in that they can be induced and monitored under thermodynamic and kinetic control using an unprecedented array of experimental techniques. New photocrystallographic methods will allow us to observe the products of SCO switching with new clarity, and measure its propagation through a crystal in real time. New simulation techniques will explain these mechanistic results, and design new switchable lattices in silico. Finally, these insights will allow us to synthesise new SCO crystals with technologically useful functionality. This represents an unprecedented program of engineering of a functional molecular crystal.

This project studies SCO switchable materials as models for more general phase change behaviour, but SCO compounds are strongly applicable in their own right. An SCO transition in a molecular material switches several of its physical properties, including its magnetic moment, colour, dielectric constant and electrical resistance. Moreover some SCO materials show pronounced hysteresis. Within the hysteresis loop, the materials are bistable switches that can be either high- or low-spin depending on their history. Several practical applications of spin-transition materials have been demonstrated, that make use of these properties. They include display and memory devices, with pixels of an SCO material whose colour or dielectric constant is switched by spot-heating and cooling; an electroluminescent device, where an SCO compound quenches light-emission through changes in its electrical resistance; and, using their switchable paramagnetism in a temperature-sensitive MRI contrast agent. Switchable SCO liquid crystals, nanoparticles and thin films have also been achieved. Nearly all these application studies have been carried out using two polymeric compounds, that show hysteresis spanning room temperature in their spin-transitions. This project aims to afford new molecular materials with comparable switching properties by de novo design. Such new materials would allow SCO devices to be prepared by different techniques (spin-coating, for example), opening up new applications of the SCO phenomenon.