Star-shaped monosubstituted 2,6-diphenyl-4,4′-bipyridinium salts with good electrochromic switching stability

In this study, a series of novel star-shaped monosubstituted 2,6-diphenyl-4,4′-bipyridinium salts with various alkyl, alkenyl, and aryl groups were designed and synthesized, and their electrochemical, electro-optical, and electrochromic properties were investigated. The electrochromic properties such as different colored states (sky blue versus purple), reasonable contrast, coloration efficiency, and, in particular, the switching stabilities of electrochromic devices based on star-shaped monosubstituted 2,6-diphenyl-4,4′-bipyridinium salts were better than those of devices based on linear monosubstituted pyridinium salts. All the compounds with alkyl, alkenyl, and aryl groups showed good switching stability and the change in the optical contrast after 1000 cycles was small (<5%). However, N-substituents significantly affected the switching stability of electrochromic devices based on linear monosubstituted 4,4′-bipyridine derivatives.