Photochromic and Electrochromic Hydrogels Based on Ammonium- and Sulfonate-Functionalized Thienoviologen Derivatives

Ammonium cations and sulfonate anions have been introduced as end-caps for alkyl viologens with thiophene-derived bridges. When the as-prepared thienoviologen derivatives are dispersed in polyacrylamide (PAAm) hydrogels, photochromic (PC) and electrochromic (EC) bifunctional hydrogels can be simply realized. The incorporated thiophene or ethylenedioxylthiophene bridge not only expands the photoresponse range but also stabilizes the photoinduced radical intermediate. Therefore, reversible PC and EC behaviors can be achieved for hydrogels containing thienoviologens N,N'-di(3-(trimethylammonio)propyl)-4,4'-(thien-2,5-diyl)bispyridinium tetrabromide (ATV), N,N'-bis(3-sulfonatopropyl)-4,4'-(thien-2,5-diyl)bispyridinium (STV), N,N'-di(3-(trimethylammonio)propyl)-4,4'-(3,4-ethylenedioxylthien-2,5-diyl)bispyridinium tetrabromide (AETV), and N,N'-bis(3-sulfonatopropyl)-4,4'-(3,4-ethylenedioxylthien-2,5-diyl)bispyridinium (SETV). On the contrary, no photochromism can be observed for PAAm hydrogels based on N,N'-di(3-(trimethylammonio)propyl)-4,4'-bipyridinium tetrabromide (AV) and N,N'-bis(3-sulfonatopropyl)-4,4'-bipyridinium (SV) without thiophene bridges. Furthermore, no significant coloration difference can be observed between the hydrogels containing ammonium- and sulfonate-functionalized viologens. However, during repetitive cycles, the transmittance contrast losses of electrochromic devices (ECDs) based on the hydrogels containing ammonium-modified viologens are lower than those for sulfonate-substituted viologens probably due to their larger number of cation-anion pairs and thus higher solubility in aqueous media. Typically, no observable difference can be found for unsealed ECDs after 15 days in ambient conditions. Additionally, a large-area ECD with a diameter of 10 cm has been facilely fabricated by simply sandwiching the EC hydrogels, and the transparency can be finely tuned upon applying different potentials. Overall, our findings may provide a new path to design multifunctional hydrogels with PC and EC responses.