Regulation of Br on molecular aggregation, film morphology and electrochromic performance of polythiophene derivatives

In this work, five polythiophene derivatives were synthesized through electrochemical copolymerization of 3,3-bis(bromomethyl)-3,4-dihydro-2H-thieno[3,4-b] (Deng et al., 2023; Li et al., 2020) [1,4]dioxepine (ProDOT-Br) and 3,3-dimethyl-3,4-dihydro-2H-thieno[3,4-b] (Deng et al., 2023; Li et al., 2020) [1,4]dioxepine (ProDOT-Me) at different mass ratios. DFT results prove that the layer spacing of conjugated blocks was increased from 5.94 Å for PProDOT-Me to 8.89 Å for PProDOT-Br due to the introduction of Br atom into side chain. And, the diverse film morphologies, such as fibre shape, cellular shape, flat shape, etc, were obtained by changing Br content in polymer. Additionally, through optimizing Br content, PBM(1,2) achieved enhanced electrochromic properties relative to two reference polymers [PProDOT-Br and PProDOT-Me], such as high optical contrast (ΔT = 76 %), fast response time (0.5 s), high transparency, and good stability. Finally, the flexible electrochromic device (8 cm × 4 cm) based on PBM(1,2) was constructed, which achieves rapid color change from purple to transparent blue. The results indicate that introducing various contents of Br atom into side chain is an effective strategy for regulating molecular microstructure and optimizing electrochromic properties of conjugated polymers.