Ligand Driven, Light-Induced Spin-Crossover

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The synthesis, and solid state chemistry and physics, of spin-crossover compounds is of great international interest at present. These are materials that can undergo a reversible change in magnetic moment upon application of heat, light or some other physical stimulus. This most commonly corresponds to a high-spin-to-low-spin d-electron transition at a transition metal centre, and so is accompanied by a colour change. Materials like these, whose colours can be reversibly and rapidly switched, have potential applications in display devices and in optical computing, among other things. We have been studying the iron chemistry of 2,6-dipyrazolylpyridines for some time. Iron(II) complex compounds of these ligands often undergo spin-crossover transitions near room temperature or under laser irradiation at low temperatures. Unusually, we can control the temperature of their temperature-induced colour change ( thermochromism ) fairly reliably, by appropriate substitution of our organic ligands. Conversely, we have recently found that substitution of the pyridine ring in our iron complexes has very little effect on their spin-crossover behaviour. That is a useful result that allows us to append other groups to our iron centres without compromising their thermochromism, giving us a route into multifunctional molecular devices. We now wish to apply our system to the phenomenon of light-driven, light-induced spin-crossover. That is, using mechanical motion at a light-sensitive group on the periphery of our molecules to drive a spin-state change at their iron centres. This idea has been demonstrated before, in a rather limited range of model systems. Although the switching achieved by this route up to now has generally been incomplete, it has the advantage of working at room temperature. Other ways of switching iron compounds using laser light are more efficient, but usually only work below -130 degrees Centigrade.We will make new versions of our iron(II) complexes containing two types of light-sensitive trigger. First, are groups that undergo cis/trans isomerisations about a double bond under UV light. Second, is a group that undergoes a reversible ring-opening reaction upon UV irradiation. Using the latter we hope to produce a light-driven conformational spin-crossover switch that works as a (poly)crystalline solid, which has not been achieved before.