Abstract: Cyclopentadithiophene (CPDT)-based polymers have emerged as promising research platforms for multicolor electrochromic materials due to their favorable color tunability. However, insufficient cyclic stability has hindered their translation into practical applications. In this study, two CPDT-based conjugated polymers with distinct substituent groups were designed and synthesized: PCPDT-Ph (copolymerized with unsubstituted benzene units) and PCPDT-PhOMe (copolymerized with dimethoxy-substituted benzene units). The influence of dimethoxy substitution on the electrochromic properties and stability of the polymers was systematically investigated. Electrochemical and electrochromic characterizations demonstrated that the electron-donating ability of the dimethoxy groups not only effectively regulated the polymer’s intrinsic properties but also significantly enhanced its cycling stability. Compared with PCPDT-Ph, PCPDT-PhOMe exhibited a reduced onset oxidation potential from 0.66 V (vs. Ag/AgCl) to 0.46 V, an upshifted highest occupied molecular orbital (HOMO) energy level, and a narrowed optical band gap (calculated theoretically) from 1.73 eV to 1.61 eV. The PCPDT-PhOMe film showed magenta in the neutral state and transparency in the oxidized state, with a color difference (ΔEab?) of 46.36. The coloring/bleaching response times were measured as 0.7/0.6 s, and the optical contrast retention reached 84% after 1000 cycles, outperforming the PCPDT-Ph film (79.5% retention after 500 cycles). Additionally, it exhibited a coloration efficiency of 543.9 cm2/C, demonstrating favorable comprehensive electrochromic performance. Electrochromic devices assembled with PCPDT-PhOMe achieved reversible switching between magenta and transparent states, with a response time of ≤ 1.0 s and a contrast retention of 71% after 30,000 cycles, indicating good stability. This work clarifies the role of substituent electronic effects in regulating the electrochromic properties of CPDT-based polymers, providing experimental basis and theoretical support for the molecular design of solution-processable thiophene-based electrochromic materials. Furthermore, it validates the potential application of PCPDT-PhOMe in smart windows, electronic displays, and other related fields.

Key words: Electrochromism; Cyclopentadithiophene, Electrochromic device