Multicolored Cathodically Coloring Electrochromism and Electrofluorochromism in Regioisomeric Star-Shaped Carbazole...

In this work, a series of fluorescent cathodically coloring electrochromic (EC) small molecules , , and with 3,5-di(9H-carbazol-9-yl)benzene (DCz) linked to dibenzofuran (DBF) at different substitutional positions were synthesized and fully characterized. These compounds are electroactive and undergo quasi-reversible two-step single-electron reduction generating radical anions and dianions. The absorptions of , , and in the neutral states lie in the UV region (λ_onset ≈ 350 nm), showing high transparency, while the absorption of their reduced states can be largely tuned across the visible region through driving voltage and substitutional positions. Initially generated spectroelectrochemically radical anions show absorption in the short-wavelength region of ∼380-500 nm with weak broad absorptions at longer wavelengths. On further reduction, these bands disappear on the cost of growing intense bands from dianions at longer wavelengths of 500-700 nm with some tail absorptions in the shorter-wavelength region. This renders the colors of the EC devices based on these materials, which are changed from green to red, yellow to magenta, and light to deep blue for , , and , respectively, covering four legs of the L*a*b* color space. Besides excellent optical contrast (>90%) and high coloration efficiency (up to 504 cm² C^-1), the fluorescence observed in solution of neutral , , and can be modulated between the fluorescence and quenched states by direct electrochemical redox reactions. Both EC and electrofluorochromic (EFC) processes are reversible on cycling. This research demonstrates the feasibility of developing multifunctional EC/EFC materials with multicolored electrochromism through exploiting electrochemical properties of traditional fluorescent small molecules.