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.

Photopolymerization, or the use of light to trigger polymerization, is one of the most exciting technologies for advanced manufacturing of polymers. One of the key components in the photopolymerization processes is the photoactive compound that absorbs the light, generating the active species that promotes the polymerization and largely determines the final properties of the material. The field of photopolymerization has been dominated by photoradical generators to mediate radical reactions. In the last decade, to expand the number of polymers that can be prepared by photopolymerization, intensive research has been devoted to the synthesis and utilization of photoactive molecules that are able to generate a base or an acid upon irradiation. These organic compounds are known to promote not only the ring‐opening polymerization of various heterocyclic monomers such as lactones, carbonates, or epoxides but also to trigger the step‐growth synthesis of polyurethanes. This Minireview highlights the recent advances in the development of organic photobase and photoacid generators, with the aim of encouraging the wider application of these photoactive compounds in the photopolymerization area and to expand the use of these polymers in advanced manufacturing processes.

International audienceZinc tetraphenylporphyrin (ZnTPP) is proposed as a high performance visible light photoinitiator for both the free radical polymerization (FRP) of methacrylates (thick films) and the cationic polymerization (CP) of epoxides (thin films) upon visible light exposure using light emitting diodes (LEDs) at 405, 455, 477, and 530 nm. ZnTPP combined with an iodonium salt shows excellent polymerization initiating abilities and high final conversions were obtained. Remarkably, for the ligand alone (tetraphenylporphyrin derivative, H2TPMP) used as photoinitiator, no polymerization occurs, indicating the importance of the metal in the initiating complex for an efficient process. A full picture of the involved chemical mechanisms is given. The high performance of ZnTPP as cationic initiating system is also well shown for new cationic 3D printing resins upon exposure to LED projector at 405 nm. © 2017 American Chemical Society

Four new carbazole derivatives (C1–C4) are synthesized and proposed as high performance visible light photoinitiators/photoredox catalysts 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. Excellent polymerization initiating abilities are found, and high final reactive function conversions are obtained. Interestingly, these new derivatives exhibit much better visible light polymerization initiating ability compared to a reference UV-absorbing carbazole (CARET, 9H-carbazole-9-ethanol) showing that the new substituents are of great interest to red-shift the absorption of the proposed photoinitiators. More remarkably, in combination with an iodonium salt, C1–C4 are also better than the well-known bis­(2,4,6-trimethyl­benzoyl)­phenyl­phosphine oxide (BAPO) photoinitiator for mild irradiation conditions highlighting their outstanding reactivity. Their use in new cationic resins for LED projector 3D printing is particularly outlined. A full picture of the involved photochemical mechanisms is provided. Originally, these carbazoles behave as metal-free photoredox catalysts active in both oxidative and reductive cycles.

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.

Copper complexes are synthesized and evaluated as new photoredox catalysts/photoinitiators.

Photoinitiated polymerization has been the subject of continued research efforts due to the numerous applications in which this polymerization technique is involved (coatings, inks, adhesives, optoelectronic, laser imaging, stereolithography, nanotechnology, etc.). More recently, photopolymerization has received renewed interest due to the emergence of 3D-printing technologies. However, despite current academic and industrial interest in photopolymerization methodologies, a major limitation lies in the slow rates of photopolymerization. The development of new photoinitiating systems aimed at addressing this limitation is an active area of research. Photopolymerization occurs through the exposure of a curable formulation to light, generating radical and/or cationic species to initiate polymerization. At present, photopolymerization is facing numerous challenges related to safety, economic and ecological concerns. Furthermore, practical considerations such as the curing depth and the competition for light absorption between the chromophores and other species in the formulation are key parameters drastically affecting the photopolymerization process. To address these issues, photoinitiating systems operating under low intensity visible light irradiation, in the absence of solvents are highly sought after. In this context, the use of photoredox catalysis can be highly advantageous; that is, photoredox catalysts can provide high reactivities with low catalyst loading, permitting access to high performance photoinitiating systems. However, to act as efficient photoredox catalysts, specific criteria have to be fulfilled. A strong absorption over the visible range, an ability to easily oxidize or reduce as well as sufficient photochemical stability are basic prerequisites to make these molecules desirable candidates for photoredox catalysis. Considering the similarity of requirements between organic electronics and photopolymerization, numerous materials initially designed for applications in organic electronics have been revisited in the context of photopolymerization. Organic electronics is a branch of electronics and materials science focusing on the development of semiconductors devoted to three main research fields; organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells (OSCs). The contribution of organic electronics to the field of electronics is important as it paves the way toward cheaper, lighter, and more energy efficient devices. In the present context of photopolymerization, materials that were investigated as photocatalysts were indifferently organic semiconductors used for transistors, charge-transport materials, and light-emitting materials used in electroluminescent devices or conjugated polymers and small molecule dyes for solar cells. In this Account, we summarize our latest developments in elaborating on photocatalytic systems based on these new classes of compounds. Through an in-depth understanding of the parameters governing their reactivities and ou...

Efficient redox initiating systems for radical polymerizations under air usually show some severe drawbacks such as handling and toxicity of the oxidizing agent (peroxides, persulfates, peroxidiphosphates, disulfides, etc.), toxicity of the reducing agent (aromatic amines such as 4-N,N-trimethylaniline), and more generally a low top surface conversion due to high oxygen inhibition. The originality of the proposed paper relies on the development of new redox initiating systems for the polymerization of (meth)­acrylate monomers that does not contain any hazardous compound (no peroxide) and is amine-free (low toxicology issues). And these new systems that possess the advantages of redox-initiated polymerization (possibility to cure thick samples) can also be photoactivated in order to have a faster and better curing of the surface of the sample (advantages of the photopolymerization to overcome the oxygen inhibition). The light activations presented in this work have been done with light-emitting diodes (LEDs) as they have a low energy consumption and are safer to the user than UV light. Two wavelengths have been used: 405 nm (in the blue range of the spectrum) and 780 nm (in the near-infrared) showing the versatility of these new photo/redox initiating systems. In particular, the reactivity in the near-IR (not related to any heating of the sample) is a very original result. In addition with the presentation of the performance of the new systems in polymerization and photoactivated polymerization, a full analysis of the chemical mechanisms and species involved has been performed. Remarkably, the new proposed Cu­(acac)2/2dppba system is also able to overcome the oxygen inhibition for free radical redox (photo)­polymerization.