Benzothiadiazole-thiophene based conjugated Polymers: Impact of conjugated chain length on electrochromic performance

In this work, two donor-acceptor-donor (D-A-D) type electrochromic conjugated polymers were prepared upon electrochemical oxidation through their precursors employed thiophenes as the donor units and benzothiazole as the acceptor unit. The conjugated chain length effects of the polymers’ electrochemical and electrochromic properties were carefully examined. As the conjugated chain length increases, both precursors exhibited reduced the HOMO-LUMO energy gap and red-shifted UV–Vis absorption and fluorescence spectra. Both polymers displayed n-doping and p-doping process with excellent redox stability, PBT-4T retained 94.17 % of its original electroactivity, which is higher than that of PBT-2T (92.86 %). Meanwhile, both polymers exhibited reversible changes in UV–Vis absorption spectra under voltage drive of 0 V–1.8 V, accompanied by color changes from light pink to black-gray (PBT-2T) and from purple to dark brown (PBT-4T). PBT-2T showed highest optical contrast of 41 % at 750 nm and fastest response time of 0.2 s for the oxidation process at 528 nm because of porous structure, which undoubtedly facilitates the ingress and egress of ions. The varying the π-conjugated chain length from bithiophene to quaterthiophene strategy may provide a new research idea to achieve high-performance electrochromic conjugated polymers.