Unprecedented photochromism of ferrocene-aryl dicyanovinylenes

An unprecedented photochromism of dicyanovinylene-functionalised ferrocenes has been explored. Photochromic systems (M1-M3) have been developed by the Knoevenagel condensation of dicyanovinylmethyl ferrocene with various aromatic aldehydes. M1-M3 exhibit strong donor-acceptor interactions leading to intramolecular charge transfer in the ground state. As a result, the compounds exhibit intense colours and strong visible light absorption (λ_abs1 = 362-485 nm, λ_abs2 = 590-600 nm). The ferrocene systems (M1-M3) exhibit photochromism in the solution state when irradiated with ultraviolet light, with a high-contrast visual colour change from blue, yellow and green to yellow, purple and yellow, respectively. The process is reversible and takes place either by chemical or electrochemical reduction. In addition, M2 exhibits turn-on fluorescence upon light irradiation, with a strong emission signal appearing in the red region (λ_max = 675 nm). This transformation is rapid and turns on fluorescence within 200 s. The photochromic behaviour of M1-M3 has been ascribed to an unprecedented light-induced oxidation of ferrocene to ferrocenium, supported by EPR, NMR and FT-IR studies. This study is highly novel, and ferrocene-based systems have never been explored before for redox-switching-based photochromism.