Panchromatic photochromic push–pull dyes featuring a ferrocene donor group

Photochromic molecules are light-responsive compounds that undergo reversible structural changes when exposed to light, producing isomers with different absorption spectra. Their ability to switch between molecular states with different optical properties makes them valuable for use in smart materials, anti-counterfeiting systems, optical data storage and optoelectronic devices. Diphenyl-naphthopyrans are a type of photochromic system that has attracted particular interest due to their tunable absorption spectra, fast response times and good fatigue resistance. However, their relatively narrow and selective absorption in the visible spectrum limits their use in applications requiring neutral colouration, such as smart windows and ophthalmic lenses. To address this limitation, we investigated which structural modifications could be employed to adjust the key optical and photochromic properties, such as the absorption range, colouring ability, and isomerisation kinetics. In this study, we present a strategy for obtaining novel push-pull photochromic dyes with wide, panchromatic absorption. Our approach involves replacing a phenyl unit with a ferrocene unit within the diphenyl-naphthopyran framework, while also adding an anchoring acceptor group to create a push-pull structure. We present the synthesis of five new dyes, detailing their optical and electrochemical properties. We investigated their photochromic behaviour in both solution and the solid state by grafting them onto metal oxide surfaces or dispersing them in a polymer matrix. Our results demonstrate that these dyes can be used to effectively produce panchromatic photochromic coatings. Furthermore, we show that some of these compounds act as efficient photosensitisers in dye-sensitised solar cells (DSSCs).