2.2. General Procedure for the Synthesis of 2-Aminobenzothiazole Derivatives

The series of novel 2-aminobenzothiazoles, which played a role of photosensitizers in two-component photoinitiating systems, i.e.,: 1,3-bis(benzothiazoleamino)squaraine (SQM1), 1,3-bis(6-bromobenzothiazoleamino)squaraine (SQM2) and 1,3-bis(6-methylbenzothiazoleamino)squaraine (SQM3), was synthesized in one-step reaction. The condensation reaction of 1 eqv. of squaric acid with 2 eqv. of 6-substituted 2-aminobenzothiazoles leads to squaraine dye formation, as presented in Scheme 2.

Method of synthesis of 2-aminobenzothiazole derivatives.

The squaraines SQM1-SQM3 were synthesized, as follows: 1,2-dihydroxycyclobuten-3,4-dione (2.5 mmol) was heated under reflux in a mixture of 1-butanol (40 mL) and toluene (20 mL). The water was distilled off azeotropically using a Dean-Stark trap. After 1 h of heating, an appropriate 6-substituted 2-aminobenzothiazole derivative (5 mmol) was added and the reaction mixture refluxed for additional 4 h. After that, the reaction mixture was cooled and the solvent removed under vacuum. The solid was dried at room temperature [46]. The 2-aminobenzothiazole derivatives were used as photosensitizers in photopolymerization experiments. The chemical structures of these compounds are depicted in Scheme 3. The ¹H and ¹³C NMR spectra of SQM1-SQM3 compounds are available in Supplementary Materials (Figures S1–S6).

The chemical structures of photosensitizers.