A review on Fluoran compounds as widely used leuco dyes

Leuco dyes are important commercial and useful compounds capable of switching between two different chemical structures, one of which is commonly a pale color or colorless substance. Fluoran compounds are a category of leuco dyes that can switch between colorless spirolactone (closed-ring) and zwitterionic open-ring forms, and depending on their substituents, would provide a wide variety of colors from black and dark violet to orange and red. This property of fluoran molecules to often alternate reversibly between two colors, distinguishable by naked eyes, has encouraged organic chemists to synthetically manipulate fluoran structures to tailor them for desired applications in various fields of science and technology. In this review, after presenting a short literature survey on fluoran compounds, recent advancements in the synthesis, encapsulation, and characterization of fluoran dyes are summarized. In the following, various applications of fluorans as thermo-, hydro-, and electrochromic compounds or as chemosensors and fluorescent probes is discussed. Other uses of these compounds such as in radiation detection and radiotherapy are also pointed out.