Stable low-bandgap isoindigo-bisEDOT copolymer with superior electrochromic performance in NIR window

High-performance electrochromic materials with high optical contrast, coloration efficiency, and excellent stability have always been critical for polymer electrochromics, and will boost diverse applications including flexible displays and wearable electronics. Herein, we develop a donor-acceptor-donor type copolymer from strong electron-withdrawing acceptor unit isoindigo and electron-donating moiety bis-EDOT by synthesizing and electropolymerizing its precursor (E)-1,1′-dihexyl-6,6′-bis(2,2′,3,3′-tetrahydro-[5,5′-bithieno[3,4-b][1,4]dioxin]-7-yl)-[3,3′-biindolinylidene]-2,2′‑dione (BisE−IDOH−BisE). Such polymer exhibits a low optical bandgap of 1.02 eV with stable and reversible electrochromism from kelly-green to sky blue. Further kinetic studies demonstrate that this polymer displays intriguing overall electrochromic performances throughout the NIR window with simultaneously high optical contrast up to 90%, high coloration efficiency of 1025 cm2 C−1, fast switching time of 1.1 s, and excellent reversible stability (< 4% loss upon 5000 cycling), which presents one of the best overall performances among the reported electrochromic polymers so far. With these amazing electrochromic properties, isoindigo-EDOT based polymers can be further developed towards device patterning and real applications like flexible indoor decoration displays and near infrared camouflage.