Ultra-low-band gap thienoisoindigo-based ambipolar type neutral green copolymers with ProDOT and thiophene units as NIR electrochromic materials

Abstract In this paper, thiophene and alkyl substituted propylenedioxythiophene (ProDOT) are taken as the donor units and the alkyl substituted thienoisoindigo (TIIG) unit is adopted as the electron deficient unit. The three donor-acceptor (D-A) type polymers were synthesized via Stille coupling polymerization at the molar feed ratio of 3:2:1, 2:1:1 and 3:1:2 (Thiophene:TIIG:ProDOT), namely as PTIIG-1, PTIIG-2, and PTIIG-3, respectively. The electrochemical band gaps are 0.96, 0.95 and 0.99 eV as the TIIG content decreases. The optical band gaps of the polymers are obtained by the UV–Vis–NIR analysis, and the data are 0.92, 0.94 and 0.98 eV as the TIIG content decreases, suggesting that all polymers belong to the ultra-low-band gap polymers. PTIIG-1 and PTIIG-2 showed neutral green color and oxidized blue color. Polymer PTIIG-3 is an exception, which exhibited a brown color in neutral state and a blue color in oxidized state. At the n-doping state, three polymers exhibited a greyish-white color. As electrochromic polymers, they not only have multicolor properties, but also have excellent kinetic characteristics. Three polymers all exhibited fast response time lower than 1 s in a 4 s time interval, stable optical contrast of 18–25% in 440–870 nm and 58–72% in NIR, large coloration efficiency of 102–254 cm2C-1 in 440–870 nm and 404–612 cm2C-1 in NIR. The results of electrochemical and optical curves, spectroelectrochemical and electrochromic behaviors showed that the three TIIG-based polymers had potential application in NIR electrochromic areas.