Achieving black-to-transmissive conjugated copolymers enabling ultrabroad spectrum modulation based on benzobisthiadiazole derivatives†

Black-to-transmissive electrochromic (EC) materials, possessing superior electrochromic properties, still face limitations in smart windows, military camouflage, etc., due to their narrow spectrum modulation (below 800 nm). In this work, a strong acceptor benzobisthiadiazole derivative (SN) is incorporated into the copolymer backbone to enhance the near-infrared (NIR) transition, and several novel “black-to-transmissive” copolymers are designed and synthesized via Stille coupling of fused thiophene (IDTT), benzo[c][1,2,5]thiadiazole (BTD), 3,4-propylenedioxythiophene (ProDOT), and SN monomers, while the incorporation of ProDOT and BTD units compensates for the spectrum control between 500 nm and 700 nm. As a result, the resulting copolymers feature relatively ultrabroad spectrum modulation above 900 nm compared to other black electrochromic polymers (ECPs) and the assembled electrochromic devices (ECDs) based on these ECPs seamlessly transition between black and transmissive with ultrabroad spectrum modulation with excellent properties, prompting further investigation of ultra-low band ECPs to overcome the deficiency of conventional ECPs with the inverse visible and NIR modulation for application in smart windows.