Synthesis and Electrochromic Properties of Ferrocene-Aryl Dicyanovinylene-Based Donor–Acceptor Systems

The favorable redox properties of ferrocene have led to the extensive development of ferrocene-based systems for several electrochemical applications but have scarcely been explored for electrochromism. Here, we report the synthesis and electrochromic properties of novel π-conjugated ferrocene-dicyanovinylene systems (<b>M</b>_<b>1</b>-<b>M</b>_<b>5</b> and <b>D</b>_<b>1</b>-<b>D</b>_<b>3</b>). Monosubstituted (<b>M</b>_<b>1</b>-<b>M</b>_<b>5</b>) and disubstituted (<b>D</b>_<b>1</b>-<b>D</b>_<b>3</b>) compounds have been developed via Knoevenagel condensation of methyl-dicyanovinyl ferrocenes (<b>4</b> or <b>5</b>) with various aromatic aldehydes. The compounds' optical (λ_max = 320-515 nm) and electronic properties (band gap = 2.6-3.1 eV) have been tuned by the appropriate choice of the aryl substituents. The strong D-A interactions between ferrocene and dicyanovinylene moieties rendered intense colors in the solution and thin-film state and demonstrated high-contrast electrochromism. The electrochromic behavior of these compounds is highly reversible (>48 cycles) and fast (0.98-1.06 s) in both solution and thin-film states. These are the first examples of stand-alone ferrocene systems explored for electrochromism.