Low Bandgap Conjugated Polymers Based on a Nature-Inspired Bay-Annulated Indigo (BAI) Acceptor as Stable Electrochromic Materials

The donor-acceptor (D-A) approach, which is to incorporate alternating electron-rich (donor) and electron deficient (acceptor) units along the conjugated polymer mainchain, has become an effective method to provide an informed search for high performance electrochromic low bandgap polymers. Herein a potent electron acceptor, namely, a much more soluble version of the nature-inspired bay annulated indigo (BAI), was employed in the synthesis of two solution-processable donor-acceptor polymers for efficient electrochromic devices (ECDs). The devices fabricated from spin-coated polymer thin films can switch reversibly between deep blue and transmissive light green hues, with high optical contrasts in the visible and near-infrared (NIR) regions, good coloration efficiency and promising ambient stability. In particular, electrochromic devices based on the copolymer containing a carbazole donor unit exhibit optical contrasts of 41% and 59% in the visible and NIR regions, respectively, and a long-term stability of more than 7500 cycles under ambient conditions with limited reduction in optical contrasts. Such longer term ambient stability underlines the great potential of BAI-derived electron acceptors for the development of practical EC materials.