Photoredox process induced polymerization reactions: Iridium complexes for panchromatic photoinitiating systems

The photoredox catalysis applied to the field of polymers and more particularly to the design of photoinitiating systems is briefly reviewed. Two novel phenylisoquinoline-based iridium complexes with fluorine substituents (bis[5-fluoro-2-(1-isoquinolinyl-κN)phenyl-κC](2,2,6,6-tetramethyl-3,5-heptanedionato-κO3,κO5)-iridium (III) (Ir_b) and bis[3,5-difluoro-2-(1-isoquinolinyl-κN)phenyl-κC](2,2,6,6-tetramethyl-3,5-heptanedionato-κO3,κO5)-iridium (III) (Ir_c) are proposed as photoredox catalysts (also called photoinitiator catalysts) and incorporated into suitable photoinitiating systems for cationic and radical polymerization. (3,4-Epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate (EPOX) and trimethylolpropane triacrylate (TMPTA) were used as benchmark monomers for cationic and radical photopolymerization. These new catalysts are compared to our very recently proposed unsubstituted catalyst compound: bis[2-(1-isoquinolinyl-κN)phenyl-κC](2,2,6,6-tetramethyl-3,5-heptanedionato-κO3,κO5)-iridium (III) (Ir_a). Remarkably, these catalysts exhibit improved light absorption properties and are characterized by a panchromatic behavior which ensures the photosensitivity of the polymerizable films to blue, green and red lights. The photochemical properties as well as the chemical mechanisms associated with these catalysts are investigated by ESR spin-trapping, laser flash photolysis, steady state photolysis, cyclic voltammetry and luminescence experiments. The structure/reactivity relationships as well as the substitution effect (by the fluorine) are discussed.