"Spin-crossover" compounds undergo a change in magnetic moment upon application of heat, light or pressure. This is caused by a high-spin-to-low-spin transition at a transition metal centre, and is accompanied by a colour change. Materials like these, whose colours can be reversibly and rapidly switched, have applications in display, memory and sensor devices and in optical computing .
We have projects in various aspects of this chemistry, involving organic and inorganic synthesis; crystallography; solid and solution-phase magnetic measurements; and other techniques as appropriate .
Spin-crossover in solid materials can occur abruptly, but is also often observed as a more gradual thermal equilibrium. A fraction of spin-crossover materials also exhibit thermal hysteresis in their transitions ; these are most suitable for the applications mentioned above. Whether a compound undergoes spin-crossover gradually or abruptly, with or without hysteresis, is controlled by the crystal packing in the bulk material rather than the molecule itself. One of our current goals is to understand the relationship between structure and function, so we can design new spin-crossover materials from scratch .
In another project, we are pursuing different ways to incorporate new functionality into spin-crossover materials . Our current goal is to find a new spin-crossover molecule that is suitable to use in molecular devices and nanoscience, which will widen the range of applications that spin-crossover materials can be used for. Such a compound should have the following attributes:
- It should be easily modified at just one position on the periphery of its ligand(s), so it can be cleanly tethered to other components in a molecular device, or to a surface;
- It should be soluble and stable in water and organic solvents, so it can be used for as many applications as possible;
- It should work at room temperature.
In forty years of spin-crossover research, no one has yet made a compound that meets all these criteria, which pose a challenging problem of molecular design and synthesis.
 M. A. Halcrow, ‘The foundation of modern spin-crossover’, Chem. Commun., 2013, 49, 10890–10892.
 T. D. Roberts, F. Tuna, T. L. Malkin, C. A. Kilner, M. A. Halcrow, ‘An iron(II) complex exhibiting five anhydrous phases, two of which interconvert by spin-crossover with wide hysteresis’, Chem. Sci., 2012, 3, 349–354.
 R. Kulmaczewski, H. J. Shepherd, O. Cespedes, M. A. Halcrow, ‘A homologous series of [Fe(H2Bpz2)2(L)] spin-crossover complexes with annelated bipyridyl co-ligands’, Inorg. Chem. 2014, 53, 9809−9817.
 L. J. Kershaw Cook, R. Mohammed, G. Sherborne, T. D. Roberts, S. Alvarez, M. A. Halcrow, ‘Spin state behaviour of iron(II)/dipyrazolylpyridine complexes. New insights from crystallographic and solution measurements’, Coord. Chem. Rev., 2015, 289–290, 2–12