Monomolecular Photoinitiators for Hybrid Light-CuringSystems under UV-LEDs

Sulfonate oxime ester photoinitiators (PIs) based on a thioether-substituted 1,8-naphthalenedicarboximide backbone exhibit good absorption at the emission wavelengths of commonly used LED lamps (385, 395, and 405 nm). The presence of weak C–S and N–O bonds in the molecular structure enables them to be cleaved under light, which leads to the generation of free radicals and cations with efficient initiation properties. Electron paramagnetic resonance spectroscopy supports the mechanism of C–S bond cleavage to produce free radicals, and UV–visible real-time spectroscopy in the presence of Rhodamine B base supports the mechanism of N–O bond cleavage to produce acids. These compounds are high-performance photoinitiators without any other additives for the polymerization of double-bonded compounds and epoxides. They can be initiated at low concentrations when exposed to UV-LEDs. These photoinitiators provide important ideas for the design and synthesis of photoinitiators, which have great potential for application in the field of photopolymerization, especially in hybrid light-curing systems.