Imidazolium salt end-capped arylene diimides for good solubility, low driving voltage and excellent cycling stability in semi-solid hydrogel-state electrochromic devices

Abstract

Arylene diimide derivatives possess good redox activity owing to their rigid planar conjugated structure and strong electron-deficient aromatic nuclei, but poor solubility and high working voltage limit their practical applications in electrochromic field. In this paper, benzene and naphthalene-based arylene diimides end-capped with imidazolium salts, PMDI-M-C4 and NDI-M-C4, were synthesized and used as the electrochromic materials in both solution- and semi-solid hydrogel-state electrochromic devices. The introduction of imidazolium salts greatly improved their solubility in polar solvents including water, methanol, dimethyl sulfoxide, etc. In solution-state electrochromic devices, both of them showed a moderate working voltage, high optical contrasts (>50 %), high coloration efficiencies (>300 cm²/C), fast switching times (<2 s) and good cycling stability. They showed different colored state due to different aromatic cores, and NDI-M-C4 with a larger conjugation degree core of naphthalene exhibited better cycling stability. In semi-solid carboxymethyl cellulose sodium hydrogel-state electrochromic devices, similar colored states were observed whereas the working voltage was markedly reduced. It's noteworthy that semi-solid hydrogel-state electrochromic device based on NDI-M-C4 performed best with the lowest working voltage of −0.8 V, a high optical contrast of 48.3 %, the best cycling stability and satisfactory coloration efficiency. Finally, the applications of the semi-solid hydrogel-state electrochromic devices in various electronic displays were demonstrated, and its design concept and successful exploration have widely promising applications in displays, adaptive camouflage, bionics, and so on.