Novel Low-Cytotoxic and Highly Efficient Type I Photoinitiators for Visible LED-/Sunlight-Induced Photopolymerization and High-Precision 3D Printing

The development of photoinitiators (PIs) combining high initiation ability, low-toxicity, and availability for high-precision 3D printing is a key challenge and an urgent problem to be solved nowadays in photopolymerization. In this study, carbazole chalcone glyoxylate oxime ester derivatives (denoted as Cs, C1-C5) containing both glyoxylate and oxime ester moieties with good light absorption properties in the visible range have been designed as type I PIs for the free radical photopolymerization (FRP) of trimethylolpropane triacrylate (TMPTA) and ethoxylated trimethylolpropane triacrylate (ETPTA) under 405 nm and 450 nm light-emitting diodes (LEDs) as well as sunlight irradiation. The reaction properties and mechanism of Cs are firstly predicted by molecular modeling/molecular design, which anticipate that C5 could exhibit the best photoinitiation ability, this structure being more prone to decarboxylation. Subsequent experimental results clearly show that the photoinitiation ability of C5 outperforms that of the benchmark commercial PIs (methyl benzoylformate (MBF), diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO)), and phenylbis (2,4,6-trimethylbenzoyl)-phosphine oxide (BAPO) under the same conditions. Compared to TPO, the photoinitiation ability of C5 improved by 40 %, 132 %, and 47 % exposed to LED@405 nm, LED@450 nm, and sunlight. In addition, C5 is successfully applied to 3D printing for the manufacture of large-scale and high-resolution object. The photochemical mechanism of C5 is systematically and comprehensively analyzed using a combination of steady state photolysis, decarboxylation reaction, fluorescence experiments, and electron spin resonance-spin trapping (ESR-ST) technology. It is found that both glyoxylate and oxime ester in C5 are highly active and capable of undergoing decarboxylation reactions to produce CO and free radicals, which is consistent with the results predicted by molecular modeling. Furthermore, the low-toxicity of C5 is evidenced by cytotoxicity assays. The comprehensive molecular modeling and experimental approach adopted in this research has led to the development of novel PIs that are highly efficient and low-toxic, and can be used for high-precision 3D printing, which offers broad application prospects in the fields of environmental sustainability, visible light curing, and biomedical science.