A Spin-Crossover Module for Monolayers and Supramolecular Architectures - Cooperativity in Two Dimensions

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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 or magnetic materials.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 is a fundamental project, whose main aim is to study spin-crossover in two-dimensional lattices, formed from monolayers of functional iron centres bonded to a gold surface. Under these conditions we can measure the propagation of the transition in the monolayer as a whole, or in close-up by individually monitoring small clusters of molecules. By measuring the transition at different positions of the layer, we can map how the transition proceeds at the atomic level. It has recently been proposed, that a spin-crossover event is initiated at flaws in the lattice structure, before propagating into the bulk. We hope to be able to observe that experimentally.A second goal of the grant, is to prepare a new type of switchable surfactant compound, that assembles itself into nanostructures in solution. These structures might be vesicles, or membranes. The molecular design we are using for the monolayer chemistry also lends itself to being used in surfactants, so we will also examine this aspect during the grant. Our aim is to make weakly associating hollow spheres or tubes that reversibly assemble and disassemble, or change their size or shape, as their molecules undergo spin-crossover. Structures like this, that change their aggregation at different temperatures or pHs, can be made to release a chemical payload following their structural transformation. As such, they are being heavily studied as vehicles for drug delivery. We will not achieve a new drug delivery agent during this grant, but a new method of switching micellar structures could lead to comparable applications down the line. One advantage our new micelles could have over conventional designs, is that their structural rearrangement will be accompanied by a change in colour from the metal head group, that can be monitored with the naked eye.