All-in-one electrochromic devices with high optical contrast based on novel mono-substituted extended viologens

In this study, three novel monoheptyl-substituted extended viologens (EDOT-C7, O-C7, and BTD-C7) were synthesized by introducing ethylenedioxythiophene, furan, and benzothiadiazole moieties between the two pyridine rings as the chromophores, respectively. Their all-in-one ECDs were prepared, which were named ECD-EDOT, ECD-O and ECD-BTD, respectively. The devices’ performance was investigated systematically, including electrochemical properties, photoelectric properties and electrochromic properties. Due to the different electronic properties of the chromophores, the devices exhibited different colored states, including orange (ECD-EDOT), magenta (ECD-O) and blue-violet (ECD-BTD). All the devices exhibit good coloration efficiency of 71.1 cm²/C (ECD-EDOT), 107.7 cm²/C (ECD-O) and 91.6 cm²/C (ECD-BTD). In addition, the devices displayed relatively high optical contrast and cycling stability. The optical contrast of ECD-EDOT, ECD-O and ECD-BTD reached 62.3 %, 71.1 % and 70.0 %, respectively. Among them, ECD-BTD exhibited the most excellent stability, which retains 82.0 % optical contrast of its initial state after continuous 1000 working cycles. Additionally, a variety of multicolor (including blue, black, brown, and red, etc) electrochromic devices based on the synthesized monoheptyl-substituted extended viologen (O-C7) and a reported diheptyl viologen (DHBP) have been successfully obtained by the regulation of the molar ratios of the two chosen compounds, which shows the bright application prospect of mono-substituted extended viologen in the multicolor electrochromic field.