Isomeric orientation of the S atom in thiophene of benzodithiophene-4,8-dione to achieve a high-performance electrochromic polymer

In this work, two pairs of D–A–D type isomeric monomers, ETTD, EBDD and EETTD, EEBDD, were designed and synthesized by modifying the orientation of the thiophene ring and the conjugation length of the molecular backbone based on benzodithiophene-4,8-dione acceptor units. We also successfully synthesized polymers PEETTD and PEEBDD via electrochemical polymerization, but failed to obtain PETTD and PEBDD from ETTD and EBDD. These subtle structural changes significantly impacted the optical and electrochemical properties and polymerization of the monomers. Notably, EEBDD exhibited the lowest onset oxidation potential (0.44 V) and the strongest absorption at 486 nm, while EETTD displayed the most red-shifted absorption edge. PEEBDD demonstrated impressive electrochromic properties, including a better optical contrast of 39.9% and a coloration efficiency of 320.7 cm² C⁻¹ at 660 nm when compared to PEETTD. These results reveal the importance of thiophene ring orientation and conjugation length in tuning the electrochromic performance of polymers, offering a promising approach for designing high-performance electrochromic materials.