High-efficient carbazole-based photo-bleachable dyes as free radical initiators for visible light polymerization

The development of dyes with both high initiating efficiency and photobleaching properties is of importance for the photopolymerization of thick materials and still an emerging challenge. Carbazole-based photo-bleachable dyes CMs (CM-1 and CM-2) and CIs (CI-1 and CI-2) as free radical initiators for visible light polymerization were designed and synthesized. Their structures were confirmed by 1 H NMR, 13 C NMR, and HR-MS. Through electron paramagnetic resonance (EPR) analysis and visible light photolysis experiments, the free radical generation mechanisms of CMs and CIs were proposed. CMs and CIs could effectively initiate the photopolymerization of 1, 6-hexanediol diacrylate (HDDA) under LED@455 nm exposure. Among these dyes, CM-2 and CI-2 with carboxylic acid in their structures showed better free radical initiating efficiencies and lower extraction rates. In addition, at a concentration of 2 × 10 −5  mol/L, CM-2/MDEA exhibited the better photopolymerization rates and double bond conversions in comparison to CQ/MDEA systems. Under the irradiation of visible light, in particular, dyes CM-2 and CI-2 could successfully initiate the polymerization of thick films of 12–13 mm to form colorless polymers. Thus, these carbazole-based photo-bleachable dyes have great potential in deep photocuring for the preparation of thick materials.