Bis(bipyridinium) Salts as Multicolored Electrochromic Devices

Symmetrical and nonsymmetrical oxobis(bipyridinium) cations combined with iodide and bistriflimide anions were prepared. In the case of the bistriflimide-based bis(bipyridinium) salts, two room-temperature ionic liquids (RTILs) with a combination of linkers and terminal ethers, [(C₃O)bpy(C₄O)bpy(C₃O)][NTf₂]₄ and [(C₁₀)bpy(C₄O)bpy(C₃O)][NTf₂]₄, were obtained, with glass transition temperatures (between −6.9 and 6.0 °C) higher than those reported previously for salts of dication bipyridinium scaffolds (−17.5 to −30.0 °C). The prepared salt containing [(C₁₀)bpy(C₄O)bpy(C₁₀)][NTf₂] can form an ionic liquid crystal structure. The most promising salts were tested as efficient and reversible multicolored electrochromic devices through cyclic voltammetry studies and detailed characterizations of their electrochromic performances. In general, tetracations based on oxobis(bipyridinium) salts show higher chromatic contrast and lower coloration efficiency than similar dication salts. Exchange of the anion from iodide to [NTf₂] increases the transition times and coloration efficiencies for all the prepared di- and tetracation salts. The electrochromic device stability of [(C₃O)bpy(C₄O)bpyC₃O]I₄ was tested by on/off cycling; this device loses around 23 % of the initial color after 8640 redox cycles and shows very good cycling stability compared with previously reported diiodide salts.