Developing conjugated polymers with broad-band absorption covering visible and near-infrared regions for electrochromism†

Conjugated polymers containing donor (D) and acceptor (A) units exhibit intramolecular charge transfer (ICT), which results in lower bandgaps and facilitates a red shift of the absorption peaks. Utilizing the conjugated backbone structure and ICT mechanisms, several D–A-type polymers were designed and synthesized using thiophene, strong fused-ring thiophene donors (IDT or IDTT), ultrastrong acceptor units (benzobisthiadiazole derivatives (SNT) or benzothiadiazolo[3,4-g]quinoxaline (TQX)), and acceptor benzothiadiazole (TZ) as the building blocks for constructing broad-band absorption electrochromic polymers. The incorporation of SNT (or TQX) units into the polymer chain induces the near-infrared (NIR) modulation, while the combination of TZ units fulfils long-wave visible absorption for synergistic visible and near-infrared modulation of the polymers. Consequently, the synthesized polymers and derived electrochromic devices demonstrate ultra-broad electrochromism from 350 nm to 1000 nm with high optical contrast, swift switching response, and good stability, which show great application potential in smart windows.