Isoindigo as an electron-deficient unit for high-performance polymeric electrochromics

Isoindigo has been found to be an excellent electron-accepting building block towards electroactive conjugated polymers with remarkable optoelectronic properties for organic electronics due to its ease of scalable synthesis with high yields. Isoindigo-based conjugated polymers have been successfully applied in organic photovoltaics and organic field effect transistors most recently, yet have attracted surprisingly scant attention in polymeric electrochromics despite their potential advantages. Herein, to evaluate the electrochromic performances of isoindigo-based materials, we synthesized two donor-acceptor-donor (D-A-D) type precursors of isoindigo moiety end-capped with 3,4-ethylenedioxythiophene (EDOT), (E)-6,60-bis(2,3-dihydrothieno[3,4-b] [1,4] dioxin-5-yl)-1,1'-dihexyl-[3,30-biindolinylidene]-2,20-dione (IDOHE) and (E)-6,6'-bis(2,3-dihydrothieno[3,4-b] [1,4] dioxin-5-yl)-1,1'-didodecyl-[3,3'-biindolinylidene]-2-2,2'-dione (IDODE) by Stille coupling, and electropolymerized them to get the corresponding polymer films. These p and n-dopable low band-gap polymers (similar to 1.26 eV) revealed electrochromic nature from green color in the neutral state to blue upon oxidation. Further kinetic investigation demonstrated isoindigo-EDOT polymers exhibited outstanding electrochromic performances especially in the NIR region, including high coloration efficiency of 362 cm(2) C-1 at 1050 nm, fast switching time of 0.5 s, and high optical contrast of 59% at 1500 nm, and excellent redox stability (<8% after 4000 cycling). Based on these findings, isoindigo-EDOT polymers possessed competitive electrochromic parameters suffice for device fabrication and commercial applications and can be further explored towards indoor electrochromic products and flexible NIR displays. Overall, isoindigo and its derivatives can be regarded as a series of promising acceptors for synthesizing high-performance D-A type NIR electrochromic polymers. (C) 2017 Elsevier Ltd. All rights reserved.