Design and synthesis of novel D-A copolymers based on fused thienothiophene for efficient broad absorption electrochromism

Developing conjugated polymers with broad spectrum modulation is highly desirable in electrochromic field. Herein, three novel polymers based on fused thienothiophene derivative (tris(thienothiophene) (TTT) or indacenodithieno[3,2-b]thiophene (IDTT)) and two acceptors (5,8-bis(4-(2-butyloctyl)thiophen-2-yl)-dithieno[3′,2':3,4; 2″,3'':5,6]benzo[1,2-c][1,2,5]thiadiazole (TBT) and 2,1,3-Benzothiadiazole (BT)) are synthesized via direct arylation polymerization (DArP). The neutral copolymers exhibit visibly broadband absorption due to the combination of the π-π* transition and intramolecular charge transfer absorption, which can be tuned by the feed ratio of the donor and acceptor monomers. As a result, the copolymer synthesized by IDTT, TBT and BT with a feed ratio of 3/2/1 presents a black color in the neutral state and high transparency in the oxidized state, which gives an average optical contrast of 34% in the visible band (400 nm–700nm), good stability and high coloration efficiency (370 cm2/C). Additionally, an electrochromic device with excellent electrochromic performance is fabricated based on the synthesized polymers. All of these confirm the synthesized polymers are attractive candidates for broad absorption electrochromism.