Molecular engineering of π-extended viologens with four arms for efficient and tunable electrochromic devices

In this work, five novel π-extended viologens with four arms, denoted as TPh-BMV, TMeOPh-BMV, TTh-BMV, TFPh-BMV, TBFPh-BMV, were successfully synthesized in an eco-friendly way. By introducing phenyl groups with different substituents into the pyridine ring, the optical and electrochemical properties were tuned effectively. All-in-one electrochromic devices based on the present new viologens were investigated, which showed efficient electrochromic performance involving high optical contrast, short response time, excellent cycling stability as well as high coloration efficiency. Among them, ECDs based on TBFPh-BMV exhibited an extremely high contrast of 95% at 526 nm and 96% at 596 nm, as well as good coloration efficiency over 350 cm2/C. This work provided an effective strategy to develop π-extended viologens for efficient and tunable electrochromism.