Polypyridine-based architectures for smart electrochromic and energy storage materials

The emerging role of metal-organic coordination compounds in the development of electrochromic materials (ECMs) fueled a growing interest in both the development of the new systems and their effective integration into functional devices. Coordination complexes of late transition metals with polypyridine ligands, like 2'-bipyridine (bpy) and 2,2':6',2''-terpyridine (tpy) often feature bright colours as a result of strong metal-to-ligand charge transfer. These colours could be reversibly bleached by oxidation of the metal centre and restored upon the metal centre reduction. The colour and its intensity can be controlled in different ways: by changing the nature of the metal centre(s) and by structural modification of ligand(s); by the construction of various metal-organic motifs and embedding them into devices, as well as by controlled layer-by-layer growth of the coordination assemblies on the surface of interest. The versatility of the available molecular building blocks allows for the systematic programming of the desired properties of the metal-organic units that could be effectively translated into the devices. This makes polypyridine metal complexes advantageous candidates for the design of effective ECMs. Here, we review the evolution of ECMs and devices based on molecularly defined bpy and tpy coordination adducts of late transition metals integrated into functional electrochromic devices over the past decade.