Novel Low-Cytotoxic and Highly Efficient Type I Photoinitiators for Visible LED-/Sunlight-Induced Photopolymerization and High-Precision 3D Printing

The development of photoinitiators (PIs) combining high initiation ability, low-toxicity, and availability for high-precision 3D printing is a key challenge and an urgent problem to be solved nowadays in photopolymerization. In this study, carbazole chalcone glyoxylate oxime ester derivatives (denoted as Cs, C1-C5) containing both glyoxylate and oxime ester moieties with good light absorption properties in the visible range have been designed as type I PIs for the free radical photopolymerization (FRP) of trimethylolpropane triacrylate (TMPTA) and ethoxylated trimethylolpropane triacrylate (ETPTA) under 405 nm and 450 nm light-emitting diodes (LEDs) as well as sunlight irradiation. The reaction properties and mechanism of Cs are firstly predicted by molecular modeling/molecular design, which anticipate that C5 could exhibit the best photoinitiation ability, this structure being more prone to decarboxylation. Subsequent experimental results clearly show that the photoinitiation ability of C5 outperforms that of the benchmark commercial PIs (methyl benzoylformate (MBF), diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO)), and phenylbis (2,4,6-trimethylbenzoyl)-phosphine oxide (BAPO) under the same conditions. Compared to TPO, the photoinitiation ability of C5 improved by 40 %, 132 %, and 47 % exposed to LED@405 nm, LED@450 nm, and sunlight. In addition, C5 is successfully applied to 3D printing for the manufacture of large-scale and high-resolution object. The photochemical mechanism of C5 is systematically and comprehensively analyzed using a combination of steady state photolysis, decarboxylation reaction, fluorescence experiments, and electron spin resonance-spin trapping (ESR-ST) technology. It is found that both glyoxylate and oxime ester in C5 are highly active and capable of undergoing decarboxylation reactions to produce CO2 and free radicals, which is consistent with the results predicted by molecular modeling. Furthermore, the low-toxicity of C5 is evidenced by cytotoxicity assays. The comprehensive molecular modeling and experimental approach adopted in this research has led to the development of novel PIs that are highly efficient and low-toxic, and can be used for high-precision 3D printing, which offers broad application prospects in the fields of environmental sustainability, visible light curing, and biomedical science.

Naphthalene-Based Oxime Esters as Type I Photoinitiators for Free Radical Photopolymerization

In order to discuss the polymerization effect from the substituted position and methoxy group of Type I photinitiators, a series of naphthalene-based oxime esters was designed and synthesized. Compared to the 2-naphthalene-substituted compound, the UV absorption region of the 1-naphthalene-based compound was greatly improved. In addition, the methoxy substitution exhibited longer absorption characteristics than did the methoxy-free one. The photochemical reaction behavior of these novel compounds was also studied by photolysis, cyclic voltammetry (CV), and electron spin resonance (ESR) experiments. Finally, the initiation abilities of naphthalene-based oxime esters toward trimethylolpropane triacrylate (TMPTA) monomer were conducted through the photo-DSC instrument under UV and a 405@nm LED lamp. Remarkedly, the naphthalene-based oxime ester (NA-3) that contains 1-naphthalene with o-methoxy substituent showed the rather red-shifted absorption region with the highest final conversion efficiency under UV (46%) and 405@nm LED (41%) lamp irradiation.

From Light to Structure: Photo Initiators for Radical Two-Photon Polymerization

Two-photon polymerization (2PP) represents a powerful technique for the fabrication of precise three-dimensional structures on a micro- and nanometer scale for various applications. While many review articles are focusing on the used polymeric materials and their application in 2PP, in this review the class of two-photon photo initiators (2PI) used for radical polymerization is discussed in detail. Because the demand for highly efficient 2PI has increased in the last decades, different approaches in designing new efficient 2PIs occurred. This review summarizes the 2PIs known in literature and discusses their absorption behavior under one- and two-photon absorption (2PA) conditions, their two-photon cross sections (σTPA ) as well as their efficiency under 2PP conditions. Here, the photo initiators are grouped depending on their chromophore system (D-π-A-π-D, D-π-D, etc.). Their polymerization efficiencies are evaluated by fabrication windows (FW) depending on different laser intensities and writing speeds.

Switching photorelease to singlet oxygen generation by oxygen functionalization of phenothiazine photocages

A phenothiazine-based photoremovable protecting group (PRPG) for single and dual release of carboxylic acids was developed. The change in the oxidation state of the sulfur atom of the phenothiazine PRPG resulted in singlet oxygen generation, rather than photorelease. The difference in the photochemistry between oxygen-free and oxygen-functionalized phenothiazine was investigated and supported by DFT calculations.

Effect of the Steric Hindrance and Branched Substituents on Visible Phenylamine Oxime Ester Photoinitiators: Photopolymerization Kinetics Investigation through Photo-DSC Experiments

In this work, free radical photopolymerization (FRP) kinetics for series of different phenylamine oxime ester structures (DMA-P, DEA-P, DMA-M, TP-2P, TP-2M and TP-3M) was investigated. Steric hindrance and branched substituents were prepared to realize the corresponding electronic and photopolymerization effects. The photophysical, electrochemical, thermal properties and radical concentration were investigated by UV-visible spectroscopy, cyclic voltammetry (CV), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR). Furthermore, the structure-reactivity relationships were also studied in detail through photo-DSC experiment. We demonstrate that the introduction of alkyl chains and/or numbers of oxime esters affects significantly the photoreactivity. Under the same weight ratio of formulation and irradiated condition, TP-3M containing three oxime esters in its structure and methyl group in the periphery exhibits the highest double-bond conversion efficiency. TP-3M-based formulation also shows a wide operation window under different contents and light intensities. Importantly, the photoreactivity of the TP-3M-based system was found to be better than the commercial photoinitiator (OXE-01) under LED@405 nm at a low concentration. This work could provide some significance to the design of oxime esters with enhanced photoreactivity.

Synthesis and free radical photopolymerization of triphenylamine-based oxime ester photoinitiators

Four visible light triphenylamine-based oxime ester photoinitiators (TP-1–4) were synthesized successfully. Photochemical reaction, photoreactivity and 3D pattern experiments were also conducted.

NIR-Sensitized Activated Photoreaction between Cyanines and Oxime Esters: Free-Radical Photopolymerization

Cyanines comprising either a benzo[e]‐ or benzo[c,d]indolium core facilitate initiation of radical photopolymerization combined with high power NIR‐LED prototypes emitting at 805 nm, 860 nm, or 870 nm, while different oxime esters function as radical coinitiators. Radical photopolymerization followed an initiation mechanism based on the participation of excited states, requiring additional thermal energy to overcome an existing intrinsic activation barrier. Heat released by nonradiative deactivation of the sensitizer favored the system, even under conditions where a thermally activated photoinduced electron transfer controls the reaction protocol. The heat generated internally by the NIR sensitizer promotes generation of the initiating reactive radicals. Sensitizers with a barbiturate group at the meso‐position preferred to bleach directly, while sensitizers carrying a cyclopentene moiety unexpectedly initiated the photosensitized mechanism.

Cleavable coumarin-based oxime esters with terminal heterocyclic moieties: photobleachable initiators for deep photocuring under visible LED light irradiation

Oxime ester photoinitiators with terminal heterocyclic moieties can efficiently initiate acrylate and thiol–ene photopolymerization under 450 nm LED light irradiation to prepare materials of thickness up to 10 mm.

Substituted stilbene-based oxime esters used as highly reactive wavelength-dependent photoinitiators for LED photopolymerization

Different substituents on stilbene-based oxime esters play an important role in the relationship between their structure and properties.