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 (E-g) 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 cm(2) 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.