Modulating electrochromic properties via thiophene donors in thieno[3,4-c]pyrrole-4,6-dione-based conjugated polymers

Abstract

Thieno[3,4-c]pyrrole-4,6-dione (TPD) is a strong imide-based electron acceptor that remains under-explored for electrochromic (EC) conjugated polymers, with existing studies limited by narrow donor scope and the use of varying device structures, hindering systematic structure–property comparisons. Herein, a series of seven TPD-based donor–acceptor conjugated polymers incorporating thiophene-derived donors, terthiophene, quaterthiophene, pentathiophene, thieno[3,2-b]thiophene (TT), 3,4-ethylenedioxythiophene (EDOT), dithieno[3,2-b:2′,3′-d]pyrrole (DTP), and cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT), are synthesised to elucidate the roles of conjugation length, backbone planarity, and donor strength in governing EC properties. Systematic modulation of donor structures enables tuning of neutral-state colours from grey to purple and dark blue, while oxidised states exhibit different grey hues. Stronger electron-donating units significantly enhance EC performance, with the EDOT-based polymer exhibiting the highest optical contrasts of 23.8% (595 nm) and 46.7% (1800 nm), while the DTP-based polymer shows the best open circuit memory retention (ca. 74% retention of initial transmittance after 10 minutes), fastest response times of 4.3 (bleaching at 608 nm) and 6.8 s (colouration at 1400 nm) and highest coloration efficiencies of 801.1 and 1358.5 cm² C⁻¹. Meanwhile, long-term switching stability appears to be more influenced by polymer molecular weight, with the pentathiophene-based polymer having the highest weight average molecular weight (M_w) of 104.5 kDa, demonstrating the highest durability, retaining 79.1% of its initial optical contrast after 3000 cycles. In addition, the electrofluorochromic (EFC) behaviour is observed for five polymers, with the terthiophene-based polymer showing the best contrasts of I_OFF/ON 11.7 for fluorescence switching at 710 nm. Overall, this work establishes a comprehensive structure–property framework for TPD-based EC polymers, providing design rules that enable predictive tuning of colour, efficiency, and stability, and supporting the rational and potentially data-driven development of future high-performance EC materials.