Iron Complexes in Visible-Light-Sensitive Photoredox Catalysis: Effect of Ligands on Their Photoinitiation Efficiencies

International audienceThe novel role of metal-based complexes as photoinitiator catalysts fits well into the concept of green chemistry, as it realizes the activation of polymer synthesis processes by visible light that are abundant in the solar light and allows the marked reduction of photoinitiator amount in the systems. In the present paper, a series of iron complexes (FeC_x) with various ligands have been proposed as new photoinitiator catalysts to initiate the cationic polymerization of epoxides or the free radical polymerization of acrylates upon a near-UV or visible-light LED exposure. The ligands play an important role on the light absorption properties and the photoinitiation ability of the iron complexes. In combination with one or two additives, FeC_x are capable to efficiently generate radicals, cations, and radical cations through an oxidative or a reductive path. Two of the newly developed FeC_x-based photoinitiating systems exhibited comparable photoinitiation efficiency with the commercial Type I photoinitiator bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (BAPO). Owing to the photocatalytic effect, remarkable photoinitiation efficiencies have been achieved by using very low concentration of iron complexes (0.02 wt %) in the systems. The involved photochemical mechanisms have been studied using electron spin resonance spin trapping, steady state photolysis, cyclic voltammetry, and laser flash photolysis technique