Pyrene-Based Imidazole Derivatives as Visible-Light Photoinitiators: A Comprehensive Research and Application in Hybrid Radical/Cationic Stepwise Polymerization

The transition from traditional ultraviolet (UV) to visible-light-induced polymerization is motivated by the superior energy efficiency, environmental benefits, and biosafety of emerging light sources, such as light-emitting diodes (LEDs). Pyrene-based imidazole derivatives, including hexaarylbiimidazole (HABI), triarylimidazole, and tetraarylimidazole, exhibit outstanding visible-light absorption and ultrafast photolysis rates, making them promising photoinitiators or photosensitizers compatible with LED sources. Herein, this work first reports a comprehensive investigation into the capacity of these derivatives to initiate radical and cationic photopolymerizations under violet and blue LED irradiation. The photoinitiation mechanisms were thoroughly elucidated, revealing a secondary photoinitiation pathway that enhanced the overall polymerization efficiency. More importantly, interpenetrating polymer networks (IPNs) were successfully fabricated through hybrid radical/cationic polymerization. Furthermore, a stepwise polymerization approach was devised to achieve superior epoxide conversion by changing the wavelength of the light source, allowing selective photopolymerization of the target monomer within a mixture. Thus, pyrene-based imidazole photoinitiators offer efficient and versatile options for photopolymerization, notably advancing the synthesis of IPNs.