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

Abstract

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.