Pyrrole-based viologen derivatives with high contrast and magenta color for electrochromic-fluorescent devices

Viologen-based materials have been widely explored for various practical applications, including smart windows, information tags, energy storage, and solar conversion devices, owing to their easy color adjustment, excellent electrochemical stability, and simple synthesis route. Herein, a series of pyrrole-based viologen derivatives with narrower bandgaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) than their parent viologen, BnV are synthesized and systematically investigated. The influence of the substituent at N-benzyl on the electronic and electrochromic properties of resulting materials are thoroughly studied. The electrochromic devices containing these newly developed derivatives exhibit vivid color change (transparent to magenta), high optical contrast (up to 93%), and a long lifetime (10,000s without current attenuation). Additionally, the extension of the π system endows these compounds with unanticipated electro-fluorochromic behavior, which holds great potential for encryption-related applications. This study demonstrates the feasibility of tuning the color, electrochemical features and obtaining bifunctional devices by modifying bipyridines with pyrrole moiety and different electronegative substituents. We anticipate that our findings will stimulate further interest in exploring new viologen-based derivatives towards next-generation electrochromic-fluorescent devices.