Greener Protocol for the Synthesis of NIR Fluorescent Indolenine-Based Symmetrical Squaraine Colorants

An environmentally benign synthesis of NIR fluorescent indolenine-based symmetrical squaraines is accomplished by using choline chloride:urea as a biorenewable deep eutectic solvent (DES). This ammonium DES is biodegradable and serves a dual role as a catalyst as well as the reaction medium. Furthermore, DES has been recycled up to four consecutive cycles without significant loss in its activity. The developed method has many advantages compared to those reported in the literature including being environmentally more benign and having operational simplicity, a low reaction temperature, ease of product isolation, a low environmental factor (E-factor), and higher yield for the synthesis of symmetrical squaraine colorants in a short reaction time. In addition, the present method could be implemented on a multigram scale. Thus, it provides a good alternative route for the industrial synthesis of squaraine colorants.