An isomerism strategy to optimize electrochromic performance: a small orientation change of S atoms in thiophene makes a big difference

Abstract

The effect of isomerism in polymers is appealing as a means of optimization of molecular configurations but has not been well investigated. Two isomers, benzo[1,2-b:4,5-c′]dithiophene-4,8-dione (TTDO) and benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) with different orientations of their fused thiophene rings, were used to design and synthesize two novel donor–acceptor–donor (D–A–D) type polymers PETTD and PEBDD to study the influence of isomerization in polymers on their electrochromic performance. The monomer ETTD exhibited stronger absorption and a lower initial oxidation potential (0.7 V) than EBDD. Both PETTD and PEBDD displayed excellent redox stability and could still maintain about 82% electroactivity after 1000 cycles. PEBDD displayed a color change from green to gray with a higher coloration efficiency (CE) value of 310.8 cm² C⁻¹ and a superior optical contrast of 50.9% in the near-infrared region at 830 nm in comparison with PETTD, indicating that PEBDD has potential applications in electrochromic devices used in the near infrared (NIR) region. This suggests that fine tuning the orientation of S atoms in fused thiophene can result in a large difference in molecular performance, and also provides the design principle of such isomeric systems for superior electrochromic performance.