Dinuclear iron(II) spin crossover compounds: singular molecular materials for electronics

Dinuclear spin crossover molecules can adopt three different spin-pair states: a fully diamagnetic low spin state, [LS - LS], with both iron(II) atoms in the LS state; a paramagnetic mixed spin-pair state [ LS - HS]; and an antiferromagnetically coupled [ HS - HS] state. Stabilisation of the [ LS - HS] state depends on a subtle balance between intra- and inter-molecular interactions in the solid state, consequently, the thermal dependence of the physical and structural properties can present one-step or two-step spin transitions. The former case involves the [ LS - LS] <-> [ HS - HS] transformation while in the latter case the intermediate stage responsible for the plateau, at 50% conversion between the two steps, is observed. It may be due to the formation of a 50% mixture of [ HS - HS] and [ LS - LS] or to the existence of 100% [ LS - HS] species. In some cases switching between the three spin-pair states has been observed upon the action of temperature, pressure or light, which implies competition between magnetic coupling and spin crossover phenomena. The results here reviewed represent a first step in the search for polynuclear spin crossover molecules of nanometric dimensions, which could be used as multi-stepped molecular switches.