Synthesis of a near-infrared emitting squaraine dye in an undergraduate organic laboratory

Squaraines are a class of organic fluorophores that possess unique photophysical properties, including strong near-infrared absorption and emission. The synthesis of many squaraines involves the condensation of an electron-rich aromatic ring with squaric acid. These reactions are generally refluxed overnight in a benzene-butanol solvent mixture. Reported herein are modifications to the synthesis of a squaraine dye that allowed the experiment to be performed in an undergraduate laboratory setting. The desired compound was formed after a 2-h reflux, using toluene as a co-solvent rather than the more toxic benzene. Moreover, the photophysical properties of the synthesized squaraine compound were analyzed in a separate laboratory period and led to important pedagogical opportunities about the absorption and fluorescence of organic compounds.