This paper describes the development of new coumarin chromophore-based iodonium salts as efficient one-component cationic photoinitiators upon LEDs irradiation with maximum emission under the UV-A region at 365 nm and under visible light at 405 nm.

New 2-(diethylamino)-4,6-diphenyl-benzene-1,3-dicarbonitrile derivatives were synthesized and proposed as high-performance photoinitiators in conjunction with iodonium salts for: (i) the cationic polymerization of epoxides, (ii) the free-radical polymerization of acrylates, and (iii) the synthesis of interpenetrating polymer networks (IPNs) from acrylate/epoxy blends. Investigated photoinitiating systems exhibit initiating efficiency upon exposure to soft irradiation conditions of visible light-emitting diodes (LEDs) with emission maxima at 405 nm and 420 nm. Excellent polymerization profiles are obtained during photopolymerization processes. The studied photoinitiating systems were also used for photosensitive resins for laser writing/3D printing experiments upon exposure to a laser diode at 405 nm. The photophysical and photochemical properties, i.e., absorption, steady-state photolysis and fluorescence spectroscopy, of terphenyl derivatives were studied. A photoinitiation mechanism related to the production of cations and radicals was studied using fluorescence quenching experiments, laser flash photolysis, cyclic voltammetry and electron spin resonance.

In the present paper, novel thioxanthone-based compounds were synthesised and evaluated as a component of photoredox catalysts/photoinitiating systems for the free-radical polymerisation (FRP) of acrylates and the ring-opening cationic polymerisation (CP) of epoxy monomers. The performance of the obtained thioxanthones in two- and three-component photoinitiating systems, in combination with amines, iodonium or sulphonium salt, as well as with alkyl halide, for photopolymerisation processes upon exposure to light emitting diodes (LEDs) with a maximum emission of 405 nm and 420 nm, was investigated. The studied compounds act also as one-component free-radical photoinitiators. Fourier transform real-time infrared spectroscopy was used to monitor the kinetics of disappearance of the functional groups of the monomers during photoinitiated polymerisation. Excellent photoinitiating efficiency and high final conversions of functional groups were observed. Moreover, the influence of thioxanthone skeleton substitution on photoinitiating efficiency was discussed. The photochemical mechanism was also investigated through cyclic voltammetry. It was discovered that thioxanthone derivatives can be used as a metal-free photoredox catalyst active for both oxidative and reductive cycles. Furthermore, a photopolymerizable system based on novel thioxanthone derivatives in a stereolithography three-dimensional (3D) printing technology under visible sources of light was used. The effects of photoinitiator type system and monomer type in photoresins during 3D printing processes were explored. The outcome of this research is the development of high-performance visible photosensitive resins with improved photosensitivity obtained thanks to the development of entirely novel photoinitiating systems specifically adapted for this application.

The performance of a series of 2-amino-4,6-diphenyl-pyridine-3-carbonitrile derivatives as fluorescent molecular sensors for monitoring photopolymerization processes of different monomers by the Fluorescence Probe Technique (FPT) was studied. It has been shown that the new derivatives are characterized by much higher sensitivity than the commercially available 7-diethylamino-4-methylcoumarin (Coumarin 1) and trans-2-(2′,5′-dimethoxyphenyl)ethenyl-2,3,4, 5,6-pentafluorobenzene (25ST) probes. It has been discovered that the 2-amino-4,6-diphenyl-pyridine-3-carbonitrile derivatives accelerate the cationic photopolymerization process initiated with diphenyliodonium photoinitiators at the wavelength where the photoinitiator alone does not work. They are particularly efficient for the photoinitiation of cationic photopolymerization of an epoxide and vinyl monomers. Consequently, the application of the 2-amino-4,6-diphenyl-pyridine-3-carbonitrile derivatives in a dual role: (a) as fluorescent sensors for monitoring the free-radical, thiol-ene and cationic polymerization progress, and (b) as long-wavelength co-initiators for diphenyliodonium salts initiators, is proposed.