Molecules that can be switched between two contrasting coloured forms by a reversible chemical reaction have potential applications in light transmissive/reflective devices (windows, protective eyewear) and information storage. One of the simplest chemical reactions is a single-electron oxidation or reduction.

It is often the case that the colour of a coordination complex is dramatically altered on changing the oxidation state of the metal. We have recently identified a system that fulfils the above requirements of chemical reversibility and high contrast ratio between the oxidised and reduced form. The bridged dinuclear Co^III-Fe^II complex shown above (view crystal structure) exhibits a prominent metal-to-metal charge transfer (MMCT) transition (Fe^II to Co^III) at 530 nm, giving the complex an intense maroon colour. One-electron oxidation of this compound generates the bright yellow Fe^III-Co^III analogue, with loss of the MMCT transition (see spectra). This process can be cycled back and forth many times (P.V. Bernhardt and M. Martinez, Inorganic Chemistry, 1999, 38, 424).

We are currently elaborating a system similar to the one above into one that exhibits stability over a wider range of oxidation states. For example, a dinuclear Co/Fe complex might be stable in three different oxidation states (and colours).

This will require that the ligand systems of the two metal ions are appropriate in order that we can characterise these species over a range of oxidised or reduced forms. There are a number of known macrocyclic ligands (other than the one shown above) that might lead to stable Co^III-Fe^II and Co^II-Fe^II systems. We hope to incorporate these complexes with optically transparent electrodes to generate an electrochromic device.