Cyano-functionalized bis-carbazole derivatives for high contrast electrochromism

Electroactive asymmetric bis-carbazole small molecules bearing cyano functional group(s) were conveniently synthesized and fully characterized by physicochemical and electrochemical methods. These carbazole-based small molecules are colorless in the neutral states and possess reversible redox processes and outstanding thermal stability. The utilization of these bis-carbazole small molecules as the electrochromic materials for the fabrication of electrochromic devices (ECDs) provided high-performance ECDs of high optical contrast up to 99% switching between colorless bleaching state and dark-red colored state and excellent coloration efficiencies of 412 cm2/C. The present work illuminates the excellent qualification of functionalized carbazole-based small molecules in developing efficient ECDs. •Cyano-modified bis-carbazole molecules show high thermal stability, colorless neutral state, and excellent redox properties.•The bis-carbazoles based electrochromic devices (ECDs) exhibit striking color change from colorless neutral states to red.•The ECDs show high optical contrast up to 94−99% and coloration efficiencies of 225−412 cm2/C.•The performances of ECDs were markedly improved by increasing of the number of cyano groups incorporated in carbazole unit.