The general procedure for the synthesis of 2-acetylphenothiazine from 1-(3-phenylaminophenyl)-acetophenone was as follows: first, aniline and m-acetylphenol were added to the reaction vessel in a mass ratio of 1.2:1 and heated until dissolved. Subsequently, trifluoromethanesulfonic acid (the amount added was 0.025 times the mass of m-acetylphenol) was added and the dehydration reaction was carried out at 190 °C. The water generated was separated during the reaction. The reaction was terminated after 6 hours. Excess aniline was removed by distillation under reduced pressure to give a dark red solid, 3-acetyl diphenylamine (Compound A). Next, Compound A was added to a reaction vessel with sulfur (molar ratio of Compound A to sulfur is 1:2.3) and 250 mL of acetone (total mass of sulfur and monomers is 1 mole) and heated to dissolve to form a clear liquid. Then, iodine was added (the amount added was 0.0125 times the mass of compound A), and the reaction was stirred and refluxed at 170 °C. After 25 min, the reaction was essentially complete, the stirring was stopped, and the reaction was gradually cooled down to 120 °C, when a yellow and a black delamination was visible. The upper layer of yellow liquid was carefully decanted and a yellow solid was precipitated after cooling at room temperature. The crude product was recrystallized by ethanol-water mixed solvent to give a light yellow solid, i.e., the target product 2-acetylphenothiazine. The total yield in this embodiment was 90.5% and the HPLC purity was 99.7%.
