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 pi-conjugated ferrocene-dicyanovinylene systems (M(1)-M(5) and D(1)-D(3)). Monosubstituted (M(1)-M(5)) and disubstituted (D(1)-D(3)) compounds have been developed via Knoevenagel condensation of methyl-dicyanovinyl ferrocenes (4 or 5) with various aromatic aldehydes. The compounds' optical (lambda(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.