Highly stable ion gel-based electrochromic devices: Effects of molecular structure and concentration of electrochromic chromophores

We fabricated electrochromic devices (ECDs) based on ion gels consisting of copolymers and room temperature ionic liquids (RTILs). Two electrochromic (EC) chromophores of diheptyl viologen (DHV²⁺) and monoheptyl viologen (MHV⁺) were employed in this work. A unique magenta (reddish)-colored state can be induced in contrast to typical alkyl viologens exhibiting only bluish-colored states. The ECDs with DHV²⁺ displayed various coloration behaviors according to the concentration of viologens. While only the blue-colored state was present at low concentrations, a sequential transition from blue to magenta was observed as the concentration increased. However, the resulting magenta color was not suitable for practical applications due to its instability. On the other hand, stable magenta-colored states were achieved from the MHV⁺-containing ECDs, irrespective of viologen concentration. In addition, the devices based on MHV⁺ exhibited high color purity, large transmittance contrast, and good cyclic coloration/bleaching stability. Overall, this result implies that MHV⁺ can serve as a simple but effective EC chromophore for ECDs displaying magenta-colored states.