Halogen bonding matters: visible light-induced photoredox catalyst-free aryl radical formation and its applications

Photo-involving aryl halide activation plays a pivotal role in organic synthesis and materials science. Revealing the mechanism and understanding the photophysical and photochemical processes in the activation is of great importance. Here, we found that aryl halides could be directly activated to form aryl radicals via halogen bonding under visible light irradiation without using photocatalysts or high power light. The interaction between the aryl halide and Lewis base (Et3N), as well as the triplet state formation, play crucial roles. Halogen bonding between aryl halide and Et3N facilitates intersystem crossing (ISC) and leads to an abundance of the triplet aryl halide/Et3N complex, where the carbon-halogen bond is much lengthened and prone to fracture. Therefore, visible light-driven photoredox catalyst-free C-C coupling reaction and radical-initiated polymerization were achieved. Avoiding the use of a catalyst not only brings convenience for low-cost operation, but also facilitates further purification. Meanwhile, the wide scope of aryl halide tolerance provides opportunities for chemical synthesis and polymerization.