Photochromic and Electrochromic Hydrogels based on Electron Donor and Acceptor Bridged Viologen Derivatives for Multifunctional Signage Applications

Inserting a conjugated bridge between the two pyridinium units of the viologen is an effective approach to tuning the colorations of the viologen derivatives. However, the relationship between the charge density on the viologen bridge and the related photochromic (PC) and electrochromic (EC) properties has been rarely studied. Herein, three conjugated bridges with different charge densities, i.e., thienyl, phenyl, and quinoxalinyl groups, are incorporated between the two pyridinium units of ammonium-functionalized viologens. The coloration scope of the viologens before and after chromism is expanded due to the adjusted intramolecular charge transfer interactions. Moreover, the introduction of an electron-withdrawing bridge is greatly beneficial to the chromic properties of the conjugation-extended viologens in polyacrylamide hydrogels. The EC device based on AQV hydrogel exhibits the highest optical contrast of 85% at 580 nm, the fastest switching time (1.0 and 0.9 s for coloration and bleaching processes, respectively), and no decay in optical contrast after 1000 EC cycles. Furthermore, multifunctional and multicolored signages as sports icons and traffic signs are fabricated and demonstrated. Variable traffic signs presenting Permitted, Warning, and Prohibited cautions for pedestrians and vehicles can be realized in a single device with only one piece of ITO glass.

Automated summary

Inserting a conjugated bridge between the two pyridinium units of the viologen is an effective approach to tuning the colorations of the viologen derivatives. The relationship between the charge density on the viologen bridge and the related photochromic and electrochromic properties has been rarely studied. Three conjugated bridges with different charge densities were incorporated between the two pyridinium units of ammonium-functionalized viologens, expanding the coloration scope and improving the chromic properties. An EC device based on AQV hydrogel exhibited high optical contrast, fast switching time, and no decay in optical contrast after 1000 cycles.