LED-Induced Polymerization (385, 405, and 455 nm) Using Star-Shaped Tris(4-(thiophen-2-yl)phenyl)amine Derivatives as Light-Harvesting Photoinitiators

Two star-shaped tris(4-(thiophen-2-yl)phenyl)amine derivatives, namely tris(4-(5-(3-pentylthieno[3,2-b]thiophen-5-yl)thiophen-2-yl)phenyl) amine and tris(4-(5-(3-pentyl-2-(thiophen-2-yl)thieno[3,2-b]thiophen-5-yl) thiophen-2-yl) phenyl) amine, are developed as photoinitiators for radical and cationic polymerizations under near-UV and visible light-emitting diodes (LEDs) (e.g., 385, 405, and 455 nm). When used in combination with an iodonium salt (and optionally N-vinyl carbazole) or an amine/alkyl halide couples, they lead to excellent photoinitiating abilities for the polymerization of epoxides or (meth) acrylates under air. Compared with commercial photoinitiators, i.e., camphorquinone-based systems or bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide, the novel photoinitiators exhibit noticeably higher polymerization efficiencies under air (epoxide conversions = 41-57% vs approximate to 0%, halogen lamp exposure; methacrylate conversions = 50-55% vs 44%, LED at 405 nm exposure; methacrylate conversions = 34-42% vs 0-8%, LED at 455 nm exposure). These systems are also interesting in overcoming oxygen inhibition. The photochemical mechanisms are studied by steady-state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry, and laser flash photolysis techniques.