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

Abstract

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.

Squaraine Dyes for Photovoltaic and Biomedical Applications

“…also reported an environmentally friendly synthetic method of indolenine‐based symmetrical SQs in a mixed choline chloride/urea deep eutectic solvent, showing many advantages including low reaction temperature, avoidance of organic solvents and bases, and enhanced yield within a short reaction time. [ 18 ]…”

Solvent‐free Reactions for the Synthesis of Indolenine‐based Squaraines and Croconaines: Comparison of Thermal Heating, Mechanochemical Milling, and IR Irradiation

“…((3,3‐Dimethyl‐3 H ‐indol‐1‐ium‐2‐yl)methylene)‐2‐((( E )‐3,3‐dimethylindolin‐2‐ylidene)methyl)‐3‐oxocyclobut‐1‐en‐1‐olate (4) : [5] Compound 4 was prepared from squaric acid (30 mg, 0.26 mmol), 2,3,3‐trimethylindolenine (0.083 ml, 0.52 mmol), and anhydrous K10 montmorillonite (30 mg) in accordance with the general protocol P3. The crude product was purified by column chromatography on silica gel ( n‐ hexane/ethyl acetate=1 : 1), affording 71.1 mg of compound 4 (69 % yield) as a blue metallic solid (Table 5, entry 3).…”

“…[4] Shankarling and co-workers developed an environmentally benign synthesis of indolenine-based symmetric squaraines using choline chloride/urea mixture acting as both the reaction solvent and the catalyst. [5] Solvent is the main component by weight of a chemical reaction, but it also represents the majority of waste in synthetic processes. Furthermore, solvents represent a serious concern as most of them are synthesized from non-renewable raw material, consuming energy at all the different synthesis steps and negatively influencing the atmospheric CO 2 balance.…”

Squaraine-Based Optical Sensors: Designer Toolbox for Exploring Ionic and Molecular Recognitions

“…However, the most commonly used solvents are as follows: (a) mixtures of aromatic hydrocarbons (typically toluene or benzene) and alcohols (typically n-butanol), with the azeotropic removal of water (this is, by far, the most prevalent system) [12][13][14]; (b) alcohols, in the presence of trialkylorthoformates, which are used as dehydrating agents [15][16][17]; (c) typical molecular organic solvents, such as dimethylformamide [18][19][20]. Non-conventional types of media, such as deep eutectic solvents, have been recently shown to be suitable for the synthesis of squaraine dyes [21]. Furthermore, solvent-free approaches to some squaraine dyes have been introduced [22].…”