Photopolymerization under near-infrared (NIR) light is challenging due to the low energy of the absorbed photon but, if successful, presents significant advantages. For example, this lower energy wavelength is safer than UV light that is currently the standard photocuring light source. Also, NIR allows for a deeper light penetration within the material and therefore resulting in a more complete curing of thicker materials containing fillers for access to composites. In this study, we report the use of three-component systems for the NIR photopolymerization of methacrylates: (1) a dye used as a photosensitizer in the NIR range, (2) an iodonium salt as a photoinitiator for the free radical polymerization of the (meth)­acrylates, and (3) a phosphine to prevent polymerization inhibition due to the oxygen and to regenerate the dye upon irradiation. Several NIR-absorbing dyes such as a cyanine borate and a silicon–phthalocyanine are presented and studied. Systems using borate dyes resulted in methacrylate monomer conversion over 80% in air. We report three types of irradiation system: low-power LED at 660 and 780 nm as well as a higher power laser diode at 785 nm. The excellent performance reported in this work is due to the crucial role of the added phosphine.

The combination of thermally induced and photoinduced free radical polymerization of (meth)­acrylic monomers has only been scarcely presented in the literature. In this study, a two-component system with a near-infrared (NIR) dye combined with a thermal initiator is presented. The dye acts as a very efficient heat generator (heater) upon irradiation with NIR light. Several thermal initiators are presented such as an alkoxyamine (e.g., BlocBuilder-MA), azo derivatives, and (hydro)­peroxides. The heat delivered by the dye dissociates the thermal initiator, which initiates the free radical polymerization of (meth)­acrylates. Several types of heat generators are presented such as borate-based dyes and a silicon phthalocyanine derivative. For the first time, the effects of the NIR heater concentration, light intensity, and monomer structure on the heat released are studied using thermal imaging studies. NIR light curing is challenging but offers significant advantages: it is safer than shorter wavelength, and it allows a deeper penetration of the light and therefore a better curing of filled samples for a unique access to composites. Systems using a cyanine borate as a dye give high conversion rate of CC for methacrylate monomer under air. Two wavelengths of irradiation are studied: 785 and 850 nm. The presence of additives (phosphines or iodonium salts) can also improve the polymerization profiles.

This paper is devoted to the effect of a thermally activated delayed fluorescence (TADF) property in new photoinitiators/photoredox catalysts. Four carbazole derivatives A1−A4 exhibiting a TADF character are synthesized and proposed for the first time as high performance visible light photoinitiators/metal-free photoredox catalysts, in the presence of an amine or/and an iodonium salt, for both the free radical polymerization (FRP) of (meth)acrylates and the cationic polymerization (CP) of epoxides upon visible light exposure using light-emitting diodes (LEDs) at 405, 455, and 477 nm. Interestingly, the impact of the substituent effect on the excited state lifetimes and therefore on the photoinitiating ability of a series of substituted carbazoles was clearly evidenced and examined. Upon bromination of the carbazole core, clear effects on the excited state lifetimes and light absorption were demonstrated, enabling to tune the initiator performance. Excellent polymerization initiating abilities are found, and high final monomer conversions are obtained. The use of these novel carbazolebased systems in photocurable cationic formulations for LED projector 3D printing is particularly outlined. TADF molecules allow a more efficient reaction from the excited singlet state as a result of their prolonged lifetimes; i.e., this effect is well highlighted through a comparison with previously published none-TADF metal-free photoredox catalysts. A full picture of the involved photochemical mechanisms is also provided. Carbazoles exhibiting a TADF character pave the way toward metal-free photoredox catalysts active in both oxidative and reductive cycles with efficiency on par with those of the traditional metal-based photoredox catalysts/photoinitiators.