Efficient Photoinitiators Research Articles

Naphthopyrans as efficient dual color photoinitiators for volumetric 3D printing

By combining photochromic naphthopyran with a Norrish-type II benzophenone photoinitiator, a new photoswitchable photoinitiator was created that is activated by the successive absorption of two different wavelengths. This dual-colour photoinitiator was used to prepare various thin solid polymeric materials using UV and visible light LED to promote the formation of radicals that initiate polymerization of an acrylate monomer. The use of this system ensures that only the areas that receive double irradiation polymerize, providing a simple way to control polymerization using only light.

High-efficiency two-photon polymerization initiators based on coumarin derivatives for additive manufacturing with sub-wavelength resolution

Efficient photoinitiators are very important for two-photon polymerization additive manufacturing. In this paper, a series of new initiators based on coumarin groups have been designed and synthesized, and their optical properties and two-photon polymerization properties have been studied and discussed in detail, and the relationship between their structures and properties has been analyzed. The synthesized initiators can realize a two-photon polymerization process with a speed of 100,000 μm/s under a laser power of 35–50 mW, and successfully obtain a series of clear three-dimensional structures with sub-wavelength resolution. Starting from the principle of two-photon polymerization, the relationship between laser power and writing speed and resolution was discussed, two simple mathematical models were established, and a fitting formula with physical meaning was obtained, which provides a way for the prediction and control of resolution. Finally, under the conditions of 100,000 μm/s and 50 mW laser power, a low-density ring with an outer diameter of 2.4 mm and an inner diameter of 2.0 mm with sub-wavelength resolution was manufactured in just over 10 min for the fusion target experiment. This proves the possibility of using the resulting initiator to manufacture precision micro-nano devices.

Effect of substitution on light-curable properties in diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide: DFT and TD-DFT calculations

Acylphosphine oxide-based photoinitiators have been used in digital light processing (DLP) 3D printing slurry to prepare ceramic materials. However, the limitation is how to improve the absorption efficiency of photoinitiator in UV–Vis light and its initiation efficiency. In this study, through the DMol3 module of Materials Studio, the hydrogen atom is substituted by different electron-donating substituents (-NH2, -OCH3, -OH), substitution positions (ortho, meta, para) and tri-substitutions positions (2,4,6 and 3,4,5) were systematically studied respectively, including the change from the ground state to the first excited state, energy gap, ultraviolet-visible spectrum, the excitation energy, absorption wavelength, and oscillator strength. These results show that the order of electron-donating ability of these four groups is as follows: -NH2 > -OCH3 > -OH> -H. When the initiation wavelength λ≤ 425 nm, enhancing the electron-donating ability of the substituent and substitute para-substitution position can improve the oscillator strength and cause a blue-shift of the absorption peak. 4-TPO-NH2 which has the strongest electron-donating ability group substituted in para position shows the highest oscillator strength (21.5%) and blue shift (22 nm) compared with unsubstituted TPO. When the initiation wavelength λ≥ 425 nm, enhancing the electron-donating ability of the substituent, substitute para-substitution or ortho-substitution position, and tri-substituents of 2,4,6 positions, is the most beneficial to improve the oscillator strength and the red-shift of the absorption peak. Compared to unsubstituted TPO, 2,4,6-TPO-NH2 has 9.00 times increase in oscillator strength with 122 nm red-shift, and 2-TPO-NH2 has 134 nm red-shift with 2.53 times increase in oscillator strength. Furthermore, these studies can improve the development of acylphosphine oxide-based photoinitiators, and will also help to expand the application of DLP 3D printing in ceramic fields.

New water-soluble photo-initiators for two-photon polymerization based on benzylidene cyclopentanones

Hydrogels, mostly known for their use in biomedical applications, are of particular interest in recent research fields. Beside their usage in biomedical applications, 3D-structured hydrogels possess properties which are interesting for other fields of research like the fabrication of lenses, sensors or electronics.Two-photon polymerization (2PP) emerged as a suitable technique to process hydrogels, due to its high spatio-temporal resolution. In order to obtain precise 3D structues via 2PP from aqueous solutions, efficient water-soluble two-photon photo-initiators (2PIs) are required. In this contribution, several 2PIs based on benzylidene cyclopentanone structures bearing carboxyl or amino groups were synthesized via aldol condensation. These 2PIs were converted into sodium carboxylate or ammonium chloride salts to increase water solubility. Also, 2PIs equipped with oligo(2-ethyl-2-oxazoline) chains to enhance water solubility were investigated concerning their suitability for two-photon polymerization.Z-scan measurements of the synthesized 2PIs revealed two-photon cross-sections of 80 to 253 GM at a wavelength of 800 nm in water. Cytotoxicity tests showed that six of the initiators featured low or no cytotoxicity up to a concentration of 1 mg mL−1. Five out of the nine tested 2PIs revealed good initiation efficiencies at a concentration of 0.5 wt% in aqueous formulations containing 50 wt% of poly(2-ethyl-2-oxazoline) diacrylate (PEtOx-DA, DP = 10), showing broad fabrication windows at a writing speed of up to 10 mm s−1.

Curing Behavior of UV-Initiated Surface-Modified Nano-TiO2/Epoxy Resin Prepolymers and the Properties of Cured Composites.

Nano-titanium dioxides (nano-TiO2) surface modified with isopropyl tri(dioctylpyrophosphate) titanate (NDZ-201), a titanate coupling agent, and 3-glycidoxypropyltrimethoxysilane (KH-560), a silane coupling agent, were separately mixed with bisphenol A epoxy resin (DEGBA) prepolymer and then cured using a UV-normal temperature synergistic curing process. Then, the isothermal curing process of the system was investigated by differential scanning calorimetry (DSC). The relationship between the organization structures, mechanical properties, and heat resistance properties of the cured composites and material formulation was studied, and the DSC results showed that the addition of nano-TiO2 reduced the curing reaction rate constant k1 and increased the k2 of the prepolymer, while the activation energy of the curing reaction after UV irradiation Ea1 decreased, and the activation energy in the middle and later periods Ea2 increased. The characterization results of the composite material showed that nano-TiO2 as a scattering agent reduced the photoinitiation efficiency of UV light, and due to its obvious agglomeration tendency in the epoxy resin, the mechanical properties of the composite material were poor. The dispersibility of the coupling-agent-modified nano-TiO2 in the epoxy resin was greatly enhanced, and the mechanical and heat resistance properties of the composite material improved remarkably. The comparison results of the two coupling agents showed that NDZ-201 had better performance in increasing the impact strength by 6.8% (minimum value, the same below) and the maximum thermal decomposition rate temperature by 4.88 °C of the composite, while KH-560 improved the tensile strength by 7.3% and the glass transition temperature (Tg) by 3.34 °C of the composite.

Inclusion Complexes of Photosensitizers with Cyclodextrins for Enhancing the Fabrication of Volume Grating

A supramolecule photosensitizer (supra-photosensitizer) based on the host-guest complexation of cyclodextrins and a bis-chalcone dye (BDEA) was prepared. Methyl-β-cyclodextrin (Me-β-CD), with the highest inclusion rate, was confirmed as the best host among the four cyclodextrins. The host-guest properties of Me-β-CD and BDEA were characterized by FTIR, XRD, 1H NMR, PLQY, SEM, Job's plot, Benesi-Hildebrand plot, and others. Compared to the conventional photosensitizers, the supra-photosensitizers showed higher fluorescence emission and longer fluorescence lifetime whether as a powder or distributed in a film. The improvement in fluorescence coincided with improvement in photoinitiation efficiency and was proven to enhance the generation of volume grating. It is expected that the supra-photosensitizer may open a new avenue for the design of high-performance photoinitiation systems.

Strategies to develop α-aminoketone derivatives photoinitiators with low migration ability for UV–vis LED photopolymerization

In this work, a polymerizable vinyl sulfide modified 907 derivative (PI-1) and four macromolecular photoinitiators (PI-2–5) based on PI-1 were designed and synthesized to realize low migration together with a high photoactivity. The structures of these photoinitiators were confirmed by 1H NMR, 13C NMR, HRMS, and GPC analysis. Time-resolved 1H NMR, photolysis, and quantum yield experiments illustrated that the rate of the photogenerated free radicals from PI-1 was slower than that of Irgacure 907. The macromolecular strategies of PI-1 successfully improved the photoactivity of PI-2–5 compared with PI-1. Moreover, PI-4 prepared by copolymerization between PI-1 and methyl methacrylate demonstrated a higher quantum yield of free radical generation than Irgacure 907 and better photoinitiating performance as evaluated by bulk and solution photopolymerization. Furthermore, the migration stabilities of these photoinitiators in the photocured films were better than those of 907. The high efficiency and low migration of the above photoinitiators make them potential applications in food packaging and biological materials.

Highly efficient carbon dot-based photoinitiating systems for 3D-VAT printing

In this work, different types of carbon dots (CDs) based on citric acid as a precursor were synthesized using an efficient procedure to purify these materials from low molecular by-products and fluorophores.

Carbazole-fused coumarin based oxime esters (OXEs): efficient photoinitiators for sunlight driven free radical photopolymerization

The use of sunlight to initiate free radical polymerization under air is a key challenge.

Preparation of amino-substituted anthraquinone: study of the intersystem crossing and application as efficient photoinitiators for photopolymerization

Intersystem crossing was observed for diaminoanthraquinone via transient absorption spectroscopy and the compounds were used as novel initiators for photopolymerization.

The optimization of photoinitiation system for holography by receiving the protection from dithiothreitol

Thiols have been investigated to play multiple roles in photoinitiation systems (PSs), but their static quenching occurred with photosensitizers and their dimerization remain major challenges. At first, adjusting thiols with different structures proved less successful in tackling the challenges. Inspired from the biomedical field, a new attempt was designed to provide a reducing environment for thiols via introducing reducing agents. Dithiothreitol (DTT) was selected from several reducing agents owing to its great cleavage of disulfides and little damage to the PS. Kinds of characterizations indicated that the introduction of DTT not only protected thiols in the PS from oxidation to disulfide and from static quenching with photosensitizers, but also enhanced the activity of thiols. The results provided strong evidence for the superiority of DTT in promoting photoinitiation efficiency of PSs.

Coumarin Ketoxime Ester with Electron-Donating Substituents as Photoinitiators and Photosensitizers for Photopolymerization upon UV-Vis LED Irradiation.

High-performance photoinitiators (PIs) are essential for ultraviolet-visible (UV-Vis) light emitting diode (LED) photopolymerization. In this study, a series of coumarin ketoxime esters (COXEs) with electron-donating substituents (tert-butyl, methoxy, dimethylamino and methylthio) were synthesized to study the structure/reactivity/efficiency relationships for substituents for the photoinitiation performance of PIs. The introduction of heteroatom electron-donating substituents leads to a redshift in the COXE absorption of more than 60 nm, which matches the UV-Vis LED emission spectra. The PIs also show acceptable thermal stability via differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The results from real-time Fourier transform infrared (RT-FTIR) measurements indicate that COXEs show an excellent photoinitiation efficiency for free radical polymerization under UV-Vis LED irradiation (365-450 nm); in particular, the conversion efficiency for tri-(propylene glycol) diacrylate (TPGDA) polymerization initiated by COXE-O and COXE-S (4.8 × 10-5 mol·g-1) in 3 s can reach more than 85% under UV-LED irradiation (365, 385 nm). Moreover, the photosensitization of COXEs in the iodonium hexafluorophosphate (Iod-PF6) and hexaarylbiimidazole/N-phenylglycine (BCIM/NPG) systems was investigated via RT-FTIR. As a coinitiator, COXEs show excellent performance in dry film photoresist (DFR) photolithography. This excellent performance of COXEs demonstrates great potential for UV-curing and photoresist applications, providing a new idea for the design of PIs.

Covalent Organic Frameworks as Efficient Photoinitiators and Cross-Linkers To Fabricate Highly Stretchable Hydrogels.

In this work, two kinds of imine-type covalent organic framework (COF) nanoparticles are demonstrated as efficient photocatalytic initiators to trigger the free-radical polymerization of acrylamide (AM) to prepare polyacrylamide (PAM) hydrogels under visible light irradiation, without any assistance from the co-initiator. Simultaneously, the COF nanoparticles bearing vinyl side groups (COF-V) promote covalent cross-linking of the polymer chains, which significantly reinforces the mechanical properties of the nanocomposite hydrogel. The obtained PAM/COF-V hydrogel is highly stretchable with an extraordinary elongation up to 3300% strain. On the other hand, the COF nanoparticles modified with methoxy moieties (COF-OMe) endow the resulting PAM/COF-OMe hydrogel with a promising fluorescence feature. In addition, this strategy provides a visible-light-regulated photocatalytic polymerization approach with a simplified recipe to fabricate COF-based nanocomposite hydrogels or resins with diverse functions.

Synthesis and photoinitiation properties of lignin model compounds

Lignin could generate active free radicals under light irradiation and be directly used as photoinitiator (PI). However, the photoinitiation unit is unclear due to its complex molecular structure. Some previous studies indicate that the photoyellowing of lignin is due to the photosensitivity of Cα‑carbonyl β-O-4 unit in its molecular structure. Herein, four lignin model compounds D2, S3, G4 and F4 containing Cα‑carbonyl β-O-4 structure unit were synthesized and characterized. Among them, F4 was obtained by introducing co-initiator 4-(dimethylamino) benzoic acid ethyl ester (EDAB) to the lignin structural unit. The results demonstrated that under 365 nm LED irradiation, the four model compounds all could directly initiate the polymerization of acrylate monomer PEGDA but their photoinitiation efficiency was quite different. F4 exhibited the best photoinitiation performance compared with the other three PIs. As a one-component photoinitiator (PI), the photoinitiation efficiency of F4 was equal to that of a commercial two-component system benzophenone (BP)/EDAB. In addition, when combined with a hydrogen donor triethanolamine (TEOA), the photoinitiation efficiency of the two-component system F4/TEOA was higher than that of BP/TEOA. Furthermore, the photolysis mechanism of F4 was clarified through identifying the generated active species under light irradiation by electron spin resonance and laser flash photolysis.

Cover Feature: The Road to Bisacyldigermanes: A New Compound Class Suitable as Visible Light Photoinitiators (ChemPhotoChem 9/2022)

The Cover Feature illustrates the road towards the new compound class bisacyldigermanes. These derivatives are highly efficient photoinitiators (PIs) which, among others, exhibit excellent photobleaching properties. In the image, the passengers journey from their starting point (widely used acylphosphine oxides, depicted in the rear-view mirror) and drive, with the aid of a roadmap, towards their destination, bypassing toxicity and mutagenicity to reach an oasis with the new compound class bisacyldigermanes. Cover design by Ilva Leinich. More information can be found in the Research Article by Michael Haas and co-workers.

Trace amounts of mercaptans with key roles in forming an efficient three-component photoinitiation system for holography

Investigating the photoinitiation mechanisms will help to find new directions for high-efficiency photoinitiation systems (PS). This article aimed to explore other possible roles of mercaptans in a PS where mercaptans were widely regarded as chain transfer agents. A bis-chalcone dye (BDEA) and a biimidazole (HABI) were employed as photosensitizer and co-initiator, and 2-mercaptobenzoxazole (MBO) was chosen as a representative of mercaptans. Trace amounts of mercaptans have been shown a negligible chain transfer effect in previous research studies, but in this work, they were found a miraculous ability to significantly promote the diffraction efficiency. Besides, the diffraction efficiency showed an interesting trend of first increasing and then decreasing with the increase of MBO content. The diffraction behavior was subsequently confirmed to be closely correlated with photoinitiation efficiency by monomer polymerization kinetics. Therefore, we made many efforts to detect the photoinitiation mechanism of PS and the roles of mercaptans. Combining the thermodynamic feasibility analysis with the spin trapping of free radicals, the presence of mercaptans could make the PS evolve a double-cycle photoreaction, which was considered to be an important contribution to the obvious promotion of photoinitiation efficiency. Meanwhile, considering the results of the fluorescence and absorption spectra, the static quenching between BDEA and MBO was presumed to be the factor of the decline in photoinitiation efficiency. Finally, the optimized photopolymers exhibited good performance in recording and reproducing reflection holograms.

Liquid Metal Nanoparticles as a Highly Efficient Photoinitiator to Develop Multifunctional Hydrogel Composites

Liquid metal (LM) composites are a class of emerging soft multifunctional materials that are promising for a variety of applications, yet the chemistry properties of LM have not been fully understood. Here, we report that LM nanoparticles can directly perform as a photoinitiator for radical polymerization and the in situ development of highly tough and multifunctional LM hydrogel composites. It is revealed that the photocatalytic activity of LM nanoparticles originates from the oxide layer on LM. Significantly, positively charged metal-organic framework (MOF) nanoparticles are used to stabilize LM nanoparticles in aqueous solutions, where the MOF can anchor on the surface of LM nanoparticles by electrostatic interaction while helping to preserve the unshielded oxide layer, therefore realizing the highly efficient photoinitiation and polymerization. The LM nanoparticle-initiated photopolymerization is shown to develop hydrogel composites featuring excellent stretchability, stimuli responsiveness, and sustained photocatalytic activity. The photocatalytic polymerization initiated by LM nanoparticles not only deepens the understanding on the semiconductor properties of the oxide skin on LM but also broadens the application scenarios of multifunctional LM/polymer composites in smart materials, wearable electronics, and soft robotics.

Real-time and in situ monitoring of Irgacure 907 penetration into food plastic packaging through surface-enhanced Raman spectroscopy

Irgacure 907 is a highly efficient photoinitiator and is widely used in food packaging materials. Understanding the penetration behavior of photoinitiators in food plastic packaging is crucial for reducing the exposure of food to photoinitiators. However, a method for in situ monitoring of photoinitiator penetration is lacking. Therefore, a method for rapid, real-time, and in situ monitoring penetration behaviors of Irgacure 907 with common solvents for inks use into three types of plastics was developed based on surface-enhanced Raman spectroscopy. The results showed that the penetration ability of Irgacure 907 with different solvents on all three plastics was in the following order: acetone > acetonitrile > methanol > ethyl acetate > xylene. The effectiveness of polyethylene terephthalate and polypropylene in blocking Irgacure 907 penetration was higher than that of polyethylene. The findings of this study may provide guidance for safe application of photoinitiators on packages.

Phenothiazine-based charge-transfer complexes as visible-light photoinitiating systems for acrylate and thiol-ene photopolymerization

Owing to their unique photodynamics, phenothiazine (PTZ) can be used to form charge-transfer complexes (CTCs), which serve as effective visible-light photoinitiating systems (PIS) for acrylate and thiol-based click polymerization. In this study, seven PTZ structures and their corresponding CTCs with iodonium salt (Iod) were evaluated by calculated spectra and polymerization kinetics. The effectiveness of the initiator decreased in the order of N-substituted polycyclic aromatic hydrocarbons > electron-donating aromatics > electron-withdrawing aromatics ≈ aliphatic alkanes. Substituted dimethylaminophenyl derivatives exhibited a lower energy gap and higher photoinitiation efficiency in CTCs. Moreover, PTZ-based CTCs as PIS rendered increased thermal stability to the photosensitive resin systems due to the mild free-radical quenching of PTZ. Such characteristic demonstrates potential versatility in the 3D printing of high-performance and high-precision materials.

High-efficient lignin-based polymerizable macromolecular photoinitiator with UV-blocking property for visible light polymerization

Developing high-efficient visible light macromolecular photoinitiator (macro PI) with excellent initiation performance, low migration, high biosafety and multi-function is beneficial to broaden the application of photopolymer. Lignin contains chromophores which could generate free radicals under light irradiation. In this study, a lignin-based polymerizable macro PI (DAL-11ene-amine) was designed and synthesized through covalent grafting 10-undecenoyl chloride (11ene) and hydrogen donor 4-(dimethylaminobenzoic acid) ethyl ester (EDAB) into dealkaline lignin (DAL) skeleton. The structure of DAL-11ene-amine was characterized by UV–vis, FTIR, 1H NMR, GPC, and 31P NMR spectra. Under the irradiation of a 405 nm LED, DAL-11ene-amine can directly produce active species and initiate the polymerization of acrylate monomers or thiol-ene click reaction. The photoinitiation efficiency of DAL-11ene-amine is higher than that of DAL-11ene or the two-component combination of DAL-11ene and EDAB. Using DAL-11ene-amine as PI, the prepared polymer films exhibit excellent UV-blocking property. With only 0.5 wt% addition of DAL-11ene-amine, nearly 100% of UVB + UVC and the most of UVA can be blocked by the films. Moreover, DAL-11ene-amine exhibits higher migration stability and biosafety because it can be covalently linked into polymer cross-linking networks. The results indicate that DAL-11ene-amine has great application potentials in preparing environmentally friendly UV-blocking films and biosafety coatings.