Phenothiazine Derivative-Based\nPhotoinitiators for\nUltrafast Sunlight-Induced Free Radical Polymerization and Rapid Precision\n3D Printing

In this work, we introduce twenty-six phenothiazine derivatives\n(PTZs) that were designed and synthesized as visible light photoinitiators.\nThese compounds, in combination with an amine [ethyl 4-(dimethylamino)benzoate\n(EDB)] and an iodonium salt [di-<i>tert</i>-butylphenyl\niodonium hexafluorophosphate (Iod)], could furnish high-performance\nthree-component (PTZs/EDB/Iod) photoinitiating systems that were employed\nfor the free radical polymerization of thick films of a low-viscosity\nmodel acrylate resin, namely, trimethylolpropane triacrylate (TMPTA)\nunder visible light and sunlight exposure. A commercial thioxanthone,\ni.e., isopropylthioxanthone (ITX) was selected to design a reference\nITX/EDB/Iod photoinitiating system. Double bond conversions of 87%\nand 76% were measured for the developed and synthesized photoinitiating\nsystems under 405 and 450 nm light-emitting diode irradiation, respectively,\nand a conversion as high as 70% could be determined under sunlight\nirradiationabout 23 times higher than the conversion obtained\nwith the comparable system prepared with the commercial photoinitiator.\nThe relevant photoinitiation abilities and photochemical mechanisms\nare comprehensively investigated by a combination of techniques including\nreal-time Fourier transform infrared spectroscopy, UV–visible\nabsorption spectroscopy, fluorescence spectroscopy, steady-state photolysis,\ncyclic voltammetry, and electron paramagnetic resonance. Notably,\nthe exceptional performance of the photoinitiators enabled the fabrication\nof 3D objects with precise morphology and superior resolution through\n3D printing and direct laser write techniques. These findings not\nonly provide opportunities for efficient polymerization under artificial\nand natural light conditions but also pave the way for scalable, cost-effective,\nenvironmentally sustainable, and green chemistry-driven curing applications.