New versatile bimolecular photoinitiating systems based on amino-m-terphenyl derivatives for cationic, free-radical and thiol–ene photopolymerization under low intensity UV-A and visible light sources

Abstract

The performance of a series of 2-amino-4,6-diphenyl-benzene-1,3-dicarbonitrile derivatives (i.e., amino-meta-terphenyls) as visible light sensitizers for diphenyliodonium salt for enabling photopolymerization at different irradiation wavelengths was studied. The structural modification of a 2-amino-4,6-diphenyl-benzene-1,3-dicarbonitrile compound was carried out. This modification, in the first step, involved a substitution of the amino group for the investigation of the effect of the amine type order on the sensitizing properties of these compounds in a bimolecular photoinitiating system composed of iodonium salt. Furthermore, to improve the spectroscopic properties, which is the scope and size of the extinction coefficient, an additional modification of the 2-amino-4,6-diphenyl-benzene-1,3-dicarbonitrile compound was proposed. The second step of modification consisted of the incorporation of an electron donating group of 4-thiomethylphenyl at the para position into the phenyl ring attachment at the 4-position of the amino-meta-terphenyl moiety. This approach allowed the study of structure/reactivity/efficiency relationships of the investigated meta-terphenyl derivatives as photosensitizers of iodonium salt for the cationic ring-opening photopolymerization of epoxides, free-radical photopolymerization of acrylates in thin films and also in a thick sample of methacrylate blend and additionally for the thiol–ene photopolymerization of allyl and thiol monomers upon UV-A and visible light exposure using Light Emitting Diodes (LEDs) with maximum emission at 365 nm, 405 nm, and 420 nm. Moreover, two different initiation mechanisms were also analysed for bimolecular photoinitiating systems based on 2-amino-4,6-diphenyl-benzene-1,3-dicarbonitrile derivatives with iodonium salt or amine e.g., ethyl 4-(dimethylamino)benzoate through steady-state photolysis experiments, fluorescence experiments, theoretical calculations of molecular orbitals, and electrochemical analysis. Our results showed that 2-amino-4,6-diphenyl-benzene-1,3-dicarbonitrile derivatives have extensive applications in different types of photopolymerization processes across the photo-oxidation or photo-reduction mechanism of their excited species promoted by soft UV-A and visible light.