Near-infrared electrochromic materials for optical attenuation based on transition-metal coordination complexes

Redox-active transition-metal complexes which show strong changes in their electronic spectra in the near-infrared (NIR) region in different oxidation states are attracting increasing interest as NIR electrochromic dyes for electro-optic switching at the wavelengths used for silica-based optical telecommunications networks. This brief review surveys recent work in the field based on ruthenium(II)-dioxolene, tris(pyrazolyl)borato-molybdenum(V) and mixed-valence Ru(II)-Ru(III) complexes, which all show strong NIR electrochromism and have been incorporated into prototypical devices such as the variable optical attenuator by attachment to transparent electrodes, either by adsorption or polymerisation.