Synthesis and electrochromic properties of electrochromic polymers based on propylenedioxythiophene, diketopyrrolopyrrole and benzodithiophene units

Two novel soluble polymers, PD-1 and PD-2, were synthesized based on three kinds of monomers, including propylenedioxythiophene (ProDOT), diketopyrrolopyrrole (DPP) and benzodithiophene (BDT), via Stille coupling reaction with the feed ratios (ProDOT:BDT:DPP) of 3:4:1, and 1:2:1, respectively. Wherein, propylene-dioxythiophene (ProDOT) and benzodithiophene (BDT) exist as the donor unit, and diketopyrrolopyrrole (DPP) as the acceptor units. Both of the two copolymers were characterized via H-1 NMR, FT-IR spectra, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), spectroelectrochemistry, switching kinetics, colorimetry and thermogravimetric analysis. Electrochemical studies demonstrate that both copolymers have the ability of rapid doping and dedoping and excellent stability. PD-1 displays a brown color in the neutral state, and a light green color in the oxidized state, while PD-2 exhibits gray green color in the neutral state and a transparent light green color in the oxidized state. As the content of the DPP remains constant, the changes in the content of the ProDOT unit can effectively control the color changes of the copolymers without apparent changes in bandgap (E-g) values, and subsequently lead to the fine tune of the other properties of the copolymers, such as optical contrast, response time and coloring efficiency. The E-g values of PD-1 and PD-2 are 1.55 eV and 1.52 eV, respectively. The optical contrasts are 42.36% (1600 nm) for PD-1 and 67.74% (at 1620 nm) for PD-2, both are ideal candidate materials for the fabrication of near infrared devices. They also have moderate optical contrast in the visible region, with fast response time and high coloring efficiency.