Phenothiazine Derivative-Based Photoinitiators for Ultrafast Sunlight-Induced Free Radical Polymerization and Rapid Precision 3D Printing

In this work, we introduce twenty-six phenothiazine derivatives (PTZs) that were designed and synthesized as visible light photoinitiators. These compounds, in combination with an amine [ethyl 4-(dimethylamino)benzoate (EDB)] and an iodonium salt [di-<i>tert</i>-butylphenyl iodonium hexafluorophosphate (Iod)], could furnish high-performance three-component (PTZs/EDB/Iod) photoinitiating systems that were employed for the free radical polymerization of thick films of a low-viscosity model acrylate resin, namely, trimethylolpropane triacrylate (TMPTA) under visible light and sunlight exposure. A commercial thioxanthone, i.e., isopropylthioxanthone (ITX) was selected to design a reference ITX/EDB/Iod photoinitiating system. Double bond conversions of 87% and 76% were measured for the developed and synthesized photoinitiating systems under 405 and 450 nm light-emitting diode irradiation, respectively, and a conversion as high as 70% could be determined under sunlight irradiation─about 23 times higher than the conversion obtained with the comparable system prepared with the commercial photoinitiator. The relevant photoinitiation abilities and photochemical mechanisms are comprehensively investigated by a combination of techniques including real-time Fourier transform infrared spectroscopy, UV-visible absorption spectroscopy, fluorescence spectroscopy, steady-state photolysis, cyclic voltammetry, and electron paramagnetic resonance. Notably, the exceptional performance of the photoinitiators enabled the fabrication of 3D objects with precise morphology and superior resolution through 3D printing and direct laser write techniques. These findings not only provide opportunities for efficient polymerization under artificial and natural light conditions but also pave the way for scalable, cost-effective, environmentally sustainable, and green chemistry-driven curing applications.