Rational design of novel isoindigo based donor–acceptor type conjugated polymers with low bandgaps as solution-processed high-performance electrochromic materials

Isoindigo, an electron deficient heteroaromatic compound, was developed as a great potential building block in the construction of low bandgap conjugated polymers with enhanced optoelectronic performances. Herein, via the Donor–Acceptor approach, a novel range of conjugated copolymers were rationally designed and synthesized by employing isoindigo as acceptor, electron rich fused thiophenes (i.e. thieno[3,2-b]thiophene, cyclopentadithiophene) as donors. The polymeric films can be easily obtained via spray-coating method due to their excellent solubility in common solvent. The optoelectronic properties of these copolymers were detailedly studied by electrochemical and spectroelectrochemical methods. Owing to the alternating donor–acceptor structure in the polymer backbones, as-prepared copolymers showed outstanding reversible redox activity and stability as well as tunable spectroscopic properties (e.g. bandgaps, colors) depending on the composition (i.e. D–A ratio) of the copolymers. These polymeric films demonstrated low bandgaps (Eg) ranging from 1.50 to 1.53 eV. Importantly, the as-prepared films revealed distinct electrochromic switching behaviors. Highly enhanced electrochromic performance, such as high optical contrast (56.9–80.8%), fast response time (0.36–0.94 s) and high coloring efficiency (300–466.67 cm2 C−1) were found for the polymeric films in the visible or near-IR region due to the rationally designed D–A structure. Furthermore, these polymeric films showed remarkable thermal stability. All the above features make this kind of D–A typed copolymers a promising candidate for the electrochromic application. Three novel isoindigo-based D–A copolymers were synthesized, which demonstrated tunable opto-electrochemical properties and outstanding electrochromic performances, such as high optical contrast, fast switching time, and high coloration efficiency. [Display omitted] •Three novel D–A copolymers employing isoindigo as acceptor, fused thiophenes as donors were rationally synthesized.•These copolymers exhibited low bandgaps and tunable opto-electrochemical properties.•They demonstrated excellent electrochromic performances and preferable thermo and cyclic stability.