Chlorinated benzothiadiazole-based donor-acceptor polymers with tunable optoelectronic performances

Abstract

Chlorinated benzo[c][1,2,5]thiadiazole, a new analogue of benzo[c][1,2,5]thiadiazole (BT), exhibits strong electron-withdrawing ability nature, while also offering advantages of lower cost and easier accessibility compared to its fluorinated analogues. This inspires us to investigate donor-acceptor-type (D-A) conjugated polymers based on chlorinated BT. Therefore, in this work, three D-A-D type monomers (0Cl, 1Cl, and 2Cl) were successfully synthesized with BT and chlorinated BT as acceptor units and EDOT as donor units by Stille coupling reaction. Subsequently, the corresponding D-A polymers (P0Cl, P1Cl, and P2Cl) were obtained via electrochemical polymerization. The results show that the two chlorinated monomers have higher initial oxidation potentials of 1.08 V (1Cl) and 1.09 V (2Cl), which are significantly higher than their non-chlorinated analogue (0.72 V). The two chlorinated polymers demonstrate good redox activity but with relatively poor stability (only 65.7 % and 20.7 % electroactivity are preserved after 100 cycles). Due to the decreased effective conjugation length caused by the deviations from planarity, both absorption spectra of the chlorinated polymers were blue-shifted when compared with P0Cl, leading to an increase in the optical band gap (1.64–1.68 eV). Introducing the Cl atom substitution on the BT unit resulted in the improvement of both the switching time and coloration efficiency. Specifically, the monochlorinated polymer (P1Cl) showed light gray in the neutral state and obvious electrochromic properties in the near-infrared region (optical contrast: 24 %, coloration efficiency: 195.6 cm2 C−1, and quick response time: 0.4 s). This study further expands the selection of chlorinated BT-based acceptor units and the application prospect of D-A electrochromic polymers.