Effect of carboxyl moiety on the electrochromic properties of 1,1′-dibenzyl-4,4′-bipyridinium salt

Viologen exhibits excellent comprehensive electrochromic properties including optical contrast, coloration efficiency, response time and cycling stability. Herein, the electrochromic properties of 1,1′-bis(4-carboxyl-benzyl)-4,4′-bipyridinium dibromide were examined via introducing carboxyl moiety onto benzyl group. The solutions of this compound and benzyl viologen were sandwiched into electrochromic devices and their electrochromic properties were compared. We found that carboxylic moiety facilitates benzyl viologen to exhibit increased coloration efficiency up to 165.9 cm²/C with comparison to that of 131.6 cm²/C without this group. The optical contrast, ΔT, reaches up to 74.6 % for the carboxylic product while that of 61.3 % was obtained without this group. The optical contrast of carboxylic product monitored at 607 nm for 1000 cycles retained 93.3 % of its initial value, which is larger than that of 69.8 % without modification, indicating that the cycling stability is enhanced by carboxylic moiety. This work demonstrated that carboxylic group can improve the overall electrochromic performance of benzyl viologen.