Design of Dithienopyran-Based Conjugated Polymers for High-Performance Electrochromic Devices

Numerous conjugated polymer-based electrochromic devices (ECDs) have been developed, but the correlation between electronic properties of electrochromic polymers (ECPs) and their EC performance, especially response speeds, has been rarely explored. In this work, we propose two new dithienopyran-based ECPs containing different heteroatoms with distinct electronic characteristics to elucidate the effect of the electronic structure of ECPs on EC performance. When electron-donating alkoxy groups are fused into ECPs (P(DT-P)), their highest occupied molecular orbital energy level becomes higher than the alkylthio (electron withdrawing group)-containing ECPs (P(DT-TP)) and the oxidation of P(DT-P) is easier. In addition, the P(DT-P) in the oxidized state is relatively stable due to the stabilization through the resonance of the lone-pair electron of the oxygen atom. These features contribute to the rapid bleaching of the P(DT-P) (∼0.5 s). In contrast, the alkylthio group of P(DT-TP) pulls electrons and destabilizes the radical cation in the oxidized state. Thus, when the P(DT-TP) is oxidized (bleached), the recovery to the neutral state (colored state) is preferred, leading to fast coloration (∼0.4 s) and unprecedentedly high coloration efficiency (∼1323 cm2/C). Both systems can retain more than 90% of their initial optical contrast after 5000 cyclic switchings, indicating their high feasibility for commercialization.