Concept of triphenylamine side chains with four electroactive nitrogen centers toward record-high stable electrochromic polyamides

Abstract

Addressing the challenge of long-term electrochromic stability for polymer-based materials is the fundamental key to practical application. To date, numerous examples of electrochromic triarylamine-based polymers have been reported. However, the structure-electrochromic stability relationships are still poorly understood and mostly unexplored. Herein, the side-chain engineering concept of multiple electroactive nitrogen atoms by designing a new triphenylamine-based diamine monomer (4) is successfully incorporated into a series of novel polyamides (PAs) (5) and demonstrated the remarkable enhancement in electrochromic stability. Compared to all other triarylamine-based polymers reported to date, the studied polymer, 5a, presents the highest electrochromic stability of only 8.8% and 9.9% decay of its coloration efficiency (CE) at 1404 nm and 440 nm after 24 000 and 20 000 switching cycles at the first oxidation stage, respectively, as well as multiple color changes, and a high contrast of the optical transmittance change. The record-high electrochromic stability of 5a could be ascribed to the capability of the stabilizing polaron, originating from a charge of the cation radical dispersed by resonance in between the different redox states and evidenced clearly by the more extended wavelength absorption in the near-infrared (NIR) region. Our described concept confirms that the key to determining the electrochromic stability of the materials is the resonance by the electrons delocalized over all the redox centers rather than the electronic coupling between the different redox centers.