Synthesis and Electrochromic\nProperties of Ferrocene-Aryl\nDicyanovinylene-Based Donor–Acceptor Systems

The\nfavorable redox properties of ferrocene have led to the extensive\ndevelopment of ferrocene-based systems for several electrochemical\napplications but have scarcely been explored for electrochromism.\nHere, we report the synthesis and electrochromic properties of novel\nπ-conjugated ferrocene-dicyanovinylene systems (<b>M</b><sub><b>1</b></sub>-<b>M</b><sub><b>5</b></sub> and <b>D</b><sub><b>1</b></sub>-<b>D</b><sub><b>3</b></sub>). Monosubstituted (<b>M</b><sub><b>1</b></sub>-<b>M</b><sub><b>5</b></sub>) and disubstituted\n(<b>D</b><sub><b>1</b></sub>-<b>D</b><sub><b>3</b></sub>) compounds have been developed via Knoevenagel condensation\nof methyl-dicyanovinyl ferrocenes (<b>4</b> or <b>5</b>) with various aromatic aldehydes. The compounds’ optical\n(λ<sub>max</sub> = 320–515 nm) and electronic properties\n(band gap = 2.6–3.1 eV) have been tuned by the appropriate\nchoice of the aryl substituents. The strong D–A interactions\nbetween ferrocene and dicyanovinylene moieties rendered intense colors\nin the solution and thin-film state and demonstrated high-contrast\nelectrochromism. The electrochromic behavior of these compounds is\nhighly reversible (>48 cycles) and fast (0.98–1.06 s) in\nboth\nsolution and thin-film states. These are the first examples of stand-alone\nferrocene systems explored for electrochromism.