Rapid Cathodic Coloration in Solution-Processable Electrochromic Polymers of Intrinsic Microporosity

Abstract

This study describes four solution-processable polymers of intrinsic microporosity with rapid electrochemical switching in solid-state two-terminal devices. Conventional electrochromic polymers are often designed to be conformationally coplanar to maximize π-orbital overlap, which is presumed to increase electronic conductivity and electrochemical redox accessibility. However, this reduces the void space and polymer chain mobility needed to facilitate electrolyte ion penetration and exchange, which typically reduces switching speed. Here, we pursue a polymer design that incorporates spirobisindane units to intentionally install nonplanarity and create permanent void space in polymer thin films, which would typically be considered antithetical to electrochromic polymer design. The four n-type polymers we produce have four distinct chromophores that each host two electrochemically accessible reduced states, for a total of 12 distinct optical profiles. In complete electrochemical devices with a gel electrolyte, we achieve >90% optical contrast (ΔT %) with a 1-s switching time (t₉₅). This rapid electrochromic switching is attributed to the large accessible surface areas (>250 m² g^–1) of these polymers, as determined by CO₂ sorption isotherms and electrochemical impedance spectroscopy measurements. Additionally, high coloration efficiencies (CE) up to 450 cm² C^–1 are achieved with >95% retention over 200 cycles. Together, these findings highlight that electrochemical redox accessibility and porosity are not mutually exclusive and provide macromolecular structural design guidelines for the next generation of organic mixed ionic-electronic conductors.