One-component Photoinitiators Research Articles

Push–pull biphenyl–based iodonium salts: Highly sensitive one-component photoinitiators for photopolymerization under UV–visible LEDs

Diaryliodonium salts play an important role as highly efficient photoinitiators in the field of photopolymerizations. They can rapidly photodecompose to produce free radicals and super acids; thus, can initiate highly efficient photocuring of acrylic and epoxy monomers and resins. However, the absorption of commercialized diphenyliodonium salts is poorly matched with popular LED light sources, and the participation of sensitizers is usually required to achieve efficient photoexcitation. Here, we synthesized five new phenyl biphenyl iodonium salts with different electron-pushing and electron-withdrawing substituents, e.g., methoxy and CN, at the biphenyl substituent. Our results demonstrate that all synthesized iodonium salts have strong absorption in the range (λmax = 258–352 nm; εmax = 21,300–29,900 M−1 cm−1). The photochemistry of these synthesized iodonium salt is strongly influenced by substituents on the biphenyl ring. All iodonium salts can undergo rapid photolysis under light excitation, with higher quantum yield of photoacid generation reaching 0.25 and can quickly initiate cationic photopolymerization of epoxy monomers. Furthermore, free radicals are generated during photolysis, which can successfully trigger free radical polymerization. These iodonium salts with good absorbance in the UVA band demonstrate their potential as excellent photoinitiators under 365–405 nm LED excitation.

Photochemical Synthesis of Noble Metal Nanoparticles: Influence of Metal Salt Concentration on Size and Distribution.

This paper explores the photochemical synthesis of noble metal nanoparticles, specifically gold (Au) and silver (Ag) nanoparticles, using a one-component photoinitiator system. The synthesis process involves visible light irradiation at a wavelength of 419 nm and an intensity of 250 mW/cm2. The radical-generating capabilities of the photoinitiators were evaluated using electron spin resonance (ESR) spectroscopy. The main objective of this study was to investigate how the concentration of metal salts influences the size and distribution of the nanoparticles. Proposed mechanisms for the photochemical formation of nanoparticles through photoinitiated radicals were validated using cyclic voltammetry. The results showed that the concentration of AgNO3 significantly impacted the size of silver nanoparticles, with diameters ranging from 1 to 5 nm at 1 wt% and 3 wt% concentrations, while increasing the concentration to 5 wt% led to an increase in the diameter of silver nanoparticles to 16 nm. When HAuCl4 was used instead of AgNO3, it was found that the average diameters of gold nanoparticles synthesized using both photoinitiators at different concentrations ranged between 1 and 4 nm. The findings suggest that variations in HAuCl4 concentration have minimal impact on the size of gold nanoparticles. The photoproduction of AuNPs was shown to be thermodynamically favorable, with the reduction of HAuCl4 to Au0 having ∆G values of approximately -3.51 and -2.96 eV for photoinitiators A and B, respectively. Furthermore, the photoreduction of Ag+1 to Ag0 was demonstrated to be thermodynamically feasible, with ∆G values of approximately -3.459 and -2.91 eV for photoinitiators A and B, respectively, confirming the effectiveness of the new photoinitiators on the production of nanoparticles. The synthesis of nanoparticles was monitored using UV-vis absorption spectroscopy, and their sizes were determined through particle size analysis of transmission electron microscopy (TEM) images.

Pyrrolidone based one-component photoinitiator for improving the storage stability of photocurable materials to sunlight via two beams of light excitation

The use of a light emitting diode (LED) light photoinitiator endows photopolymerization with comfortable irradiation conditions since LED owns advantages such as longer lifespan and no ozone generation. However, photoinitiator for LED irradiation (LED photoinitiator) was much more easily excited by outdoor sunlight compared to UV photoinitiator. As a result, photocurable materials containing LED photoinitiator exhibits poor storage stability to sunlight because sunlight in the visible light region exhibits wider absorption wavelength and higher penetrating ability than UV light. In this research, pyrrolidone one-component LED photoinitiator with chalcone structure(E)-1-(4-(dimethylamino)phenyl)-3-(1H-pyrrol-2-yl)prop-2-en-1-one(E-DNPO) was synthesized, and its cis isomer Z-DNPO was also obtained from the photoequilibrium state of E/Z-DNPO. All the characterization including photolysis experiments, real-time 1H NMR spectra and photopolymerization kinetics verified that cis isomer Z-DNPO cannot be stimulated by sunlight as intramolecular hydrogen bonding N-H···O = C hinders the carbonyl group from capturing hydrogen via donors to launch the polymerization. However, Z-DNPO can recover its initiating ability to polymerize different monomers upon LED@365 nm as it transforms into E-DNPO under the irradiation of LED@465 nm beforehand. This initiating system with controllable photoactivity via two beams of light excitation, directly improving the storage stability of photocurable materials, thereby making the uses, preparation and application of photocurable materials more convenient.

Photochemically generated ionic species for cationic and step-growth polymerizations

Developing innovative photochemical methods for the purpose of attaining green chemistry and mitigating global warming has recently become a very important concept. While we have to move towards a more sustainable world, daily life is becoming increasingly dependent on the use of polymeric materials. Conventional thermal polymerization systems fail to fulfill the requirements of sustainable chemistry, resulting in an increase in energy demand. This article outlines cationic and step-growth polymerizations of various monomers initiated by different photochemically generated ionic species and their advantages over conventional polymerization systems. Typical examples of photoinduced cationic and step-growth polymerizations performed under UV, visible and near-infrared (NIR) light are summarized under two main categories; direct and indirect initiating systems. In direct initiating systems the use of commercially available and structurally modified onium salts as one-component photoinitiators for both polymerizations are summarized. Indirect initiating systems, proceeding through i) photoinduced electron/energy transfer (PET), ii) charge-transfer complexation and iii) free-radical promoted cationic polymerization (FRPCP), are summarized with a focus on the recent step-growth methods developed in our laboratory.

Symmetric Iodonium Salts Based on Benzylidene as One-Component Photoinitiators for Applications in 3D Printing

We present a group of seven new advanced symmetric iodonium salts bearing two benzylidene-based chromophores. This is the first example of the use of a double bond in the structure of a symmetric iodonium salt. Such an improvement ensures excellent absorbing properties in the UV-A and visible range, representing a significant advance over the commonly used diaryliodonium salts. New symmetric iodonium derivatives effectively photoinitiate cationic polymerization using light-emitting diodes (LEDs) with no additives. Thus, they can act as one-component photoinitiators even in visible light, where common diaryliodonium salts remain inactive. They can be used in advanced applications such as three-dimensional (3D) VAT printing using an LCD printer. In addition, it was possible to study in detail substituent effects on the properties of the new iodonium salts due to the easily modifiable structure of the benzylidene chromophore. The research described represents a significant advance in the development of iodonium salts.

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.

Synthesis of novel one-component photoinitiators based on thioxanthone derivatives

Synthesis of novel one-component photoinitiators based on thioxanthone derivatives

Synthesis and free radical photopolymerization of one-component type II photoinitiator based on benzophenone segment

In this study, novel visible light one-component photoinitiators (BPN-D, BPN-Ph, BPN-N, BPN-An and BPN-Py), based on benzophenone scaffold have been synthesized. Compared to the reference samples (BPN and EMK), these new photoinitiators not only contribute to red-shifted UV–Vis absorption regions but also strongly enhance the thermal properties. The free radical generation in one-component Type II photoinitiators were also confirmed by the electron spin resonance (ESR) and steady-state photolysis techniques, and their chemical reaction mechanisms were also discussed. Finally, a photopolymerization studied using those novel and reference photoinitiators as one-component Type II photoinitiator was performed. The photopolymerization of TMPTMA monomer initiated by various photoinitiators was monitored by photo-DSC experiment. Under UV lamp irradiation, the BPN-Ph compound could be promoted with the highest double bond conversion efficiency up to 30%, which was higher than the reference BPN (25%) and EMK (22%) based photoinitiating systems. In addition, the BPN-An based formulation exhibited highest double conversion efficiency (22%) upon LED@405 nm. The experimental results show that the synthesized compounds can effectively improve photosensitivity as well as enhanced their optical and thermal properties.

One-Component Cationic Photoinitiators from Tunable Benzylidene Scaffolds for 3D Printing Applications

This paper describes a series of novel benzylidene scaffold-based iodonium salt one-component photoinitiators containing double bonds and dialkylamino groups synthesized in one step via classical aldol condensation reactions. Systematic investigations of structure–activity relationships were performed via quantum-chemical calculations and experimental tests. New salts can efficiently decompose under irradiation from UV to visible regions of the spectrum. The design of new iodonium salts is described, followed by a description of properties of photoinitiators prepared in this way. All presented salts were characterized in terms of their spectral properties (UV–vis spectroscopy), ability to photodecompose (steady-state photolysis), cationic polymerization of the vinyl monomer, and epoxide monomer photoinitiation (quantum efficiency of acid generation and real-time Fourier-transform infrared measurements) as well as their proposed application in 3D printing. New benzylidene iodonium salts can photoinitiate the polymerization of vinyl ethers and epoxy monomers under LED@365 and LED@405 irradiation. Investigated compounds can simultaneously initiate and monitor the polymerization process according to the change of fluorescence during the photocuring process. Formulations before photopolymerization do not exhibit fluorescence, during the photopolymerization process fluorescence is “turning on”. This phenomenon might be used to monitor photopolymerization in an “online” manner.

Synthesis and preliminary photopolymerization evaluation of novel photoinitiators containing phototrigger to overcome oxygen inhibition in the UV- curing system

In this work, two types of novel photoinitiaors containing phototrigger were prepared to overcome oxygen inhibition in the UV- curing system in the absence of hydrogen donor. The structures of prepared novel photoinitiators were determined by nuclear magnetic resonance (NMR) and high resolution MS (HRMS) spectra data. The photo chemical behavior and photo-reactivity were also evaluated by ultraviolet-visible (UV–vis) spectroscopy and real-time Fourier transform infrared spectroscopy (RT-FTIR), respectively. The results show the prepared photoinitiators exhibit remarkable redshift compared to the commercial BP (benzophenone) and Irgacure 907 (2-methyl-1-(4-methylsulfanylphenyl)-2-morpholin-4-ylpropan-1-one), fast photolysis by CS bond, good photo initiation and significant overcoming oxygen inhibition for some compounds, which can be used as one-component photoinitiator candidates.

Cucurbit[7]uril-Carbazole Two-Photon Photoinitiators for the Fabrication of Biocompatible Three-Dimensional Hydrogel Scaffolds by Laser Direct Writing in Aqueous Solutions.

We have introduced a novel water-soluble two-photon photoinitiator based on the host-guest interaction between 3,6-bis[2-(1-methyl-pyridinium)vinyl]-9-pentyl-carbazole diiodide (BMVPC) and cucurbit[7]uril (CB7) because most of the commercial photoinitiators have poor two-photon initiating efficiency in aqueous solutions. The binding ratio of BMVPC and CB7 was determined as 1:1 by isothermal titration calorimetry and quantum chemical calculation. The formation of the host-guest complex increases the two-photon absorption cross-section about five times, and improves the water solubility required as the photoinitiator for hydrogel fabrication. The BMVPC-CB7 inclusion complex was used as the one-component photoinitiator, and the polyethylene glycol diacrylate with promising biocompatibility was used as the hydrogel monomer to form the aqueous-phase photoresist system applied to two-photon polymerization microfabrication. A relatively low laser threshold of 4.5 mW, a high fabricating resolution of 180 nm, and the true three-dimensional (3D) fabricating capability in the aqueous solution have been obtained by using the as-prepared photoresist system. Finally, 3D engineering hydrogel scaffold microstructures with low toxicity and good biocompatibility have been fabricated and cocultured with living HeLa cells, which demonstrates the potential for further application in tissue engineering.

One‐component photoinitiator based on benzophenone and sesamol

A new benzodioxole derivative, 4‐(1,3‐benzodioxol‐5‐yloxy) benzophenone (BPBDO), based on benzophenone and sesamol was precisely synthesized, and it can be used as a 1‐component type II photoinitiator. Elementary analysis, atmospheric pressure chemical ionization mass spectrometry, 1H nuclear magnetic resonance, and 13C nuclear magnetic resonance studies revealed that the molecular structure of BPBDO consisted of both benzophenone (BP) and benzodioxole (BDO) structures. The laser flash photolysis experiments and electron spin resonance test indicated that the process of radicals generated from BPBDO after irradiation was similar to 3 processes of ethyl 4‐dimethylaminobenzoate and BP. The kinetics of photopolymerization of the photoinitiator was also studied by real‐time infrared spectroscopy. The oxygen content, light intensity, and viscosity of the monomer affected the decomposition (Rd) and polymerization rate, and the final double bond conversion was also studied. All the results suggest that BPBDO is a 1‐component photoinitiator that is an efficient photoinitiator for free radical polymerization. In contrast to typical dual‐component photoinitiators, eg, BP/ethyl 4‐dimethylaminobenzoate or BP/BDO, BPBDO does not require an additional amine coinitiator for the initiation and is applicable in nonamine resin systems.

A one-component photoinitiator based on 4-methylbenzophenone and morpholine

4-Benzoylbenzyl-3-morpholinopropanoate was synthesized, characterized and used as a one-component photoinitiator of Type II. Real-time near-IR spectroscopy was used to study the kinetics of conversion of double bonds of monomers vs. irradiation time. In this initiator, the radicals could be generated through hydrogen abstraction based on intermolecular reaction. Light intensity, concentration of initiator, and the functionality of acrylates affected the polymerization rate and the final monomer conversion.

Conjugated dyes carrying N, N-dialkylamino and ketone groups: One-component visible light Norrish type II photoinitiators

This work presents new conjugated dyes carrying N, N-dialkylamino and ketone groups for using as one-component Norrish type II visible light photoinitiators. The novel dyes without ketone group were also synthesized to compose two-component visible light photoinitiators for comparisons. The target dyes show the remarkable absorption in visible light region. Photo-differential scanning calorimetry was employed to study visible light photoinitiating polymerization kinetics of methyl methacrylate by new one-component photoinitiators and two-component photoinitiators respectively. The results suggest that the one-component photoinitiators showed much more efficient photoinitiating polymerization of methyl methacrylate than the two-component photoinitiating systems under visible light irradiation. The molecular weight of visible light photopolymer produced by the new photoinitiators was determined. The visible light photoinitiating mechanism of the one-component photoinitiators was studied by absorption and emission spectra, electron spin resonance spectra and cyclic voltammograms analysis.

Thioxanthone dicarboxamide derivatives as one-component photoinitiators for near-UV and visible LED (365–405 nm) induced photopolymerizations

A series of thioxanthone dicarboxamide derivatives containing hydrogen donors as one-component LED photoinitiators have been prepared.

Synthesis and characterization of polymerizable one-component photoinitiator based on sesamol

Migration stability and biocompatibility are the crucial features for a photoinitiator or coinitiator used in UV curable formulations, especially when the cured product is in direct contact with food or human skin. To reduce the migration issues and increase the biocompatibility, a polymerizable one-component photoinitiator, 1-(1,3-benzodioxol-5-yloxy)-3-(4-benzoylphenoxy)propan-2-yl acrylate (BDOBPAc), based on sesamol and benzophenone has been synthesized. The photopolymerization induced by BDOBPAc was monitored by real-time infrared spectroscopy. The rate of decomposition and the migration stability of photoinitiators were studied by UV-Vis spectroscopy. The results showed that BDOBPAc is an effective free radical photoinitiator with good migration stability, which has great potential to be widely used in the food packing or biomedical fields.

Synthesis and photopolymerization kinetics of benzophenone sesamol one-component photoinitiator

A benzodioxole derivative, 4-(2-(benzodioxol-5-yloxy)ethoxy)benzophenone (BPC2BDO), based on 4-hydroxybenzophenone and sesamol was synthesized, and used as a one-component Type II photoinitiator. The structure of BPC2BDO was characterized by elementary analysis, APCI-MS, (1)H NMR, and (13)C NMR. The rate of decomposition (R(d)) of BPC2BDO in acetonitrile was studied by UV-Vis spectroscopy and found that R(d) was proportional to light intensity. Real-time near-IR was used to study the kinetics of photopolymerization of the photoinitiator. As the benzophenone (BP) moiety and hydrogen donor were introduced into one molecule in BPC2BDO, radicals could be generated through intra-molecular reaction due to the close vicinity of the hydrogen donor and BP, which might be faster than inter-molecular reaction. The results also showed that the rate of polymerization of acrylates was significantly higher than that of methacrylates at the same polymerization conditions; the functionality of acrylates, concentration of BPC2BDO, and light intensity affected the polymerization rate and the final conversion.

The investigation of photoinitiated polymerization of multifunctional acrylates with TX-BT by Photo-DSC and RT-FTIR

Photopolymerization of multifunctional acrylates with 3H-5-Thia-1,2,3-triaza-cyclopenta[b]anthracene-10-one (TX-BT) as the one-component photoinitiator was investigated using real-time Fourier transform infrared spectroscopy (RT-FTIR). Photo-differential scanning calorimetric (Photo-DSC) technique was used to study the gelation of trimethylolpropanetriacrylate (TMPTA), P-3038 (epoxydiacrylate (EA) 75% and tripropyleneglycoldiacrylate (TPGDA) 25%) and tripropyleneglycol diacrylate (TPGDA) + P-3038 in the presence of TX-BT. Photopolymerization reactions were performed under different conditions of initiator concentration and UV light intensity.

Determination of photoinitiated polymerization of multifunctional acrylates with acetic acid derivatives of thioxanthone by RT-FTIR

Photopolymerization of multifunctional acrylates with 2-thioxanthone-thioacetic acid (TXSCH 2COOH) and 2-(carboxymethoxy) thioxanthone (TXOCH 2COOH) as the one-component photoinitiator has been investigated by real-time Fourier transform infrared (RT-FTIR) spectroscopy. The photobleaching of these one-component nature initiators was performed in air. The irradiation time for total bleaching was 240 s for TXSCH 2COOH and 540 s for TXOCH 2COOH.

Hazards of fluorinated contaminants: Bedford, C. T., Blair, D. & Stevenson, D. E. (1977). Toxic fluorinated compounds as by-products of certain BF 3-catalysed industrial processes. Nature, Lond.267, 335

Photopolymerization of multifunctional acrylates with 2-thioxanthone-thioacetic acid (TXSCH2COOH) and 2-(carboxymethoxy) thioxanthone (TXOCH2COOH) as the one-component photoinitiator has been investigated by real-time Fourier transform infrared (RT-FTIR) spectroscopy. The photobleaching of these one-component nature initiators was performed in air. The irradiation time for total bleaching was 240 s for TXSCH2COOH and 540 s for TXOCH2COOH.