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

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.

This is a fundamental study in self-assembly, an area that the EPSRC has signposted this year for priority funding. The most important goal is to observe a thermal spin-state transition in a two-dimensional lattice, and to measure how it progresses at (close to) the molecular level. Since spin-crossover in the solid state is a crystal phase transition, our results will have a wide additional impact beyond the spin-crossover and nanoscience communities. We will obtain the first experimental observations of the progress of a phase transition in a molecular lattice, which will greatly improve our understanding of phase changes and chemical reactivity in crystalline materials. This is relevant, for example, to problems of polyporphism in the pharmaceutical industry; to the design of metal-organic framework (MOF) materials for gas storage, gas purification and catalysis applications; and, to molecule-based conductors, magnets and fluorophores, where a change in crystal packing caused by polymorphism or a phase transition can have a drastic effect on the properties of the bulk material. Our work may also be of help to the emerging fields of spintronics and quantum computing. It has been proposed that information can be stored in the spin of the 57Fe nucleus, which is measurable from the hyperfine interaction with the surrounding electrons. Hyperfine splitting is only present in the high-spin state of the iron atoms, so each bit of information can be turned on and off individually using a spin-transition. That would greatly simplify the problem of addressing spins individually, by turning them off (without losing the nuclear information) when they are not needed. Our aim of inducing and measuring transitions between spin-states in small clusters of molecules would be the first step towards making that idea a reality. Our other goal, of producing switchable micellar or bilayer structures using spin-crossover amphiphiles, is also at the proof-of-principle stage but has more immediate potential applications. Switchable micelles can be used to store a chemical cargo, then release it during the switching process. Thus, they are being heavily developed as drug delivery agents in molecular medicine, and as transfection agents in synthetic biology for delivering DNA into cells (effectively, as artificial viruses). Most such work up to now has been done with block copolymer amphiphiles, but incorporation of spin-crossover centres into these structures could have some benefit for laboratory studies, at least. For one thing, it would allow the micelle switching to be monitored with the naked eye, from the associated colour change; or, by NMR, using the paramagnetic/diamagnetic transformation in an iron(II) spin-crossover head group. The surface science and amphiphile parts of the project both require a spin-crossover module , that undergoes a spin-transition reliably and predictably and can be easily functionlised with just one tail or tether. Such a module would have a much broader use within the supramolecular and nanochemistry communities, since it could be easily used to incorporate switching functionality into sensors and molecular devices. For example, it could also be used as a reporter group for biochemical probes, that change colour and/or magnetic moment (measurable by NMR) when bound to a hydrophobic enzyme active site or membrane. They could be used as temperature-dependent switches to reversibly quench emission from a photochemical donor/acceptor array, since the high and low-spin states should interfere with energy transfer between chromophores to differing degrees. Alternatively, they could be incorporated into molecular wires to modulate their conductivity, since the high- and low-spin states of a metal complex have different conductivities. Or, they could be attached to the side-chains of polymers or the capping groups of dendimers, yielding new types of thermochromic organic material.