Synthesis and Characterization of Novel D-A Type Neutral Blue Electrochromic Polymers Containing Pyrrole[3-c]Pyrrole-1,4-Diketone as the Acceptor Units and the Aromatics Donor Units with Different Planar Structures

Three soluble conjugated polymers, named BEDPP, FLDPP, and CADPP, were prepared through the Suzuki polymerized reaction, and employed benzene (BE), fluorene (FL), and carbazole (CA) as the donor units, respectively. The electron-deficient molecule 2,5-bis-(2-octyldodecyl)-3,6-bis-(5-bromo-thiophene)-pyrrole[3-c]pyrrole-1,4-diketone(DPP) was introduced and used as the acceptor unit. The properties of these three copolymers were studied by a series of detailed characterization analysis, including X-ray photoelectron spectroscopy (XPS), colorimetry, electrochemical measurements, spectroelectrochemistry, kinetics, quantitative calculation, and thermogravimetric (TG) analysis, etc. The results revealed that BEDPP displayed a blue color in the neutral state and a light brown color in the oxidized state, FLDPP exhibited a cyan color in the neutral state and a gray color in the oxidized state, while CADPP displayed pure blue color in the neutral state and a light gray color in the oxidized state. All these polymers possess narrow optical band gaps lower than 1.80 eV and satisfactory thermal stability. The kinetic characterization showed that the optical contrasts (ΔT%) in the near-infrared region were superior to the visible region. The optical contrasts of BEDPP, FLDPP, and CADPP are 41.32%, 42.39%, and 45.95% in the near-infrared region, respectively, which made them a good application prospect in the near-infrared region. Amid the three polymers, CADPP has the highest coloration efficiency (around about 288 cm(2)·C(−1)) and fast switching times (0.77 s in the coloring process and 0.52 s in the bleaching process) in the visible region, and the comprehensive performance of CADPP can be comparable to that of the reported D-A (Donor-Acceptor) type blue color polymers. In general, based on the good performances and the stable neutral blue color, the three polymers had profound theoretical significance for the development of electrochromic material and the completion of the RGB (Red, Green, Blue) color space.