Multiple electrochemically accessible colour states in surface-confined metal–organic monolayers: stepwise embedding of individual metal centres†
Sequential covalent embedding of cobalt, osmium, and iron complexes of 4′-(pyridin-4-yl)-2,2′:6′,2′′-terpyridine to surface-enhanced supports pre-functionalized with a templating layer results in hetero-bimetallic (Os–Fe and Co–Fe) and hetero-trimetallic (Co–Os–Fe) monolayer materials. During sequential embedding, each subsequent metal complex deposits onto unoccupied sites of the templating layer leading to densely packed functional materials. Electrochemical and XPS results show that the packing density of Fe(II) species on the surface of the resulting materials is higher than those of pre-deposited Co(II) and Os(II) isostructural complexes. Moreover, according to Raman spectroscopy results, Fe-containing molecules experience steric hindrance when anchored to sequentially deposited Os– or Co–Os materials. The oxidation states of the metal centres in these materials can be successively electrochemically changed resulting in distinct colour-to-colour transitions. Importantly, monolayer materials formed on the conductive surface-enhanced indium-tin-oxide support can operate as entire working electrodes, which, in combination with a polymer gel electrolyte and an indium tin oxide/glass counter electrode, result in two-electrode electrochromic devices (ECDs). Upon operation these devices demonstrate some deterioration of the change in optical density (ΔOD), however, this deterioration is mostly due to gel electrolyte decomposition. The nature of the material allows easy replacement of the electrolyte and the counter electrode. Re-assembly of Co–Fe ECD using a new gel electrolyte and an ITO-glass counter electrode results in almost complete ΔOD recuperation.