Photoinitiation Activity Research Articles

Two-Orders-of-Magnitude Enhancement of Photoinitiation Activity via a Simple Surface Engineering of Metal Nanoclusters.

Development of high-performance photoinitiator is the key to enhance the printing speed, structure resolution and product quality in 3D laser printing. Here, to improve the printing efficiency of 3D laser nanoprinting, we investigate the underlying photochemistry of gold and silver nanocluster initiators under multiphoton laser excitation. Experimental results and DFT calculations reveal the high cleavage probability of the surface S-C bonds in gold and silver nanoclusters which generate multiple radicals. Based on this understanding, we design several alkyl-thiolated gold nanoclusters and achieve a more than two-orders-of-magnitude enhancement of photoinitiation activity, as well as a significant improvement in printing resolution and fabrication window. Overall, this work for the first time unveils the detailed radical formation pathways of gold and silver nanoclusters under multiphoton activation and substantially improves their photoinitiation sensitivity via surface engineering, which pushes the limit of the printing efficiency of 3D laser lithography.

Two‐Orders‐of‐Magnitude Enhancement of Photoinitiation Activity via a Simple Surface Engineering of Metal Nanoclusters

AbstractDevelopment of high‐performance photoinitiator is the key to enhance the printing speed, structure resolution and product quality in 3D laser printing. Here, to improve the printing efficiency of 3D laser nanoprinting, we investigate the underlying photochemistry of gold and silver nanocluster initiators under multiphoton laser excitation. Experimental results and DFT calculations reveal the high cleavage probability of the surface S−C bonds in gold and silver nanoclusters which generate multiple radicals. Based on this understanding, we design several alkyl‐thiolated gold nanoclusters and achieve a more than two‐orders‐of‐magnitude enhancement of photoinitiation activity, as well as a significant improvement in printing resolution and fabrication window. Overall, this work for the first time unveils the detailed radical formation pathways of gold and silver nanoclusters under multiphoton activation and substantially improves their photoinitiation sensitivity via surface engineering, which pushes the limit of the printing efficiency of 3D laser lithography.

Construction of Photoinitiator Functionalized Spherical Nanoparticles Enabling Favorable Photoinitiating Activity and Migration Resistance for 3D Printing.

A straight-forward method was exploited to construct a multifunctional hybrid photoinitiator by supporting 2-hydroxy-2-methylpropiophenone (HMPP) onto a nano-silica surface through a chemical reaction between silica and HMPP by using (3-isocyanatopropyl)-triethoxysilane (IPTS) as a bridge, and this was noted as silica-s-HMPP. The novel hybrid-photoinitiator can not only initiate the photopolymerization but also prominently improve the dispersion of nanoparticles in the polyurethane acrylate matrix and enhance the filler-elastomer interfacial interaction, which results in excellent mechanical properties of UV-cured nanocomposites. Furthermore, the amount of extractable residual photoinitiators in the UV-cured system of silica-s-HPMM shows a significant decrease compared with the original HPMM system. Since endowing the silica nanoparticle with photo-initiated performance and fairly lower mobility, it may lead to a reduction in environmental contamination compared to traditional photoinitators. In addition, the hybrid-photoinitiator gives rise to an accurate resolution object with a complex construction and favorable surface morphology, indicating that multifunctional nanosilica particles can be applied in stereolithographic 3D printing.

Visible-Light Amine Thioxanthone Derivatives as Photoredox Catalysts for Photopolymerization Processes

Herein, mono- and disubstituted thioxanthones have been synthesized and introduced as photosensitizers of iodonium salt for cationic and free-radical photopolymerization of epoxy and an acrylate monomer, respectively. Photoinitiation activity of these molecules was verified during photopolymerization processes performed upon exposure to visible LEDs of 420, 455, and 470 nm. The kinetics of the photopolymerization processes was followed using FT-IR spectroscopy. Moreover, the developed compounds were proposed as strongly reducing metal-free photoredox catalysts in organocatalyzed atom transfer radical polymerization (ATRP). Thioxanthone derivatives (with alkyl halide) efficiently activate the ATRP of methyl methacrylate using a visible LED as an irradiation source. The formed polymers in the ATRP process were characterized by SEC. Apart from studying the photopolymerization processes with the participation of obtained thioxanthones, absorption, fluorescence, as well as electrochemical properties of thioxanthone derivatives were investigated. Furthermore, the performance of studied compounds in 3D printing applications was also examined. The most important result of this study is the design and possible application of photosensitizers/photocatalysts for photopolymerization processes.

Synthesis and Characterization of a Trifunctional Photoinitiator Based on Two Commercial Photoinitiators with α-Hydroxyl Ketone Structure.

A trifunctional photoinitiator based on commercial photoinitiators 2-hydroxy-2-methylpropiophenone (Irgacure1173) and 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure2959) was synthesized by an esterization reaction. Its structure was characterized by UV-Vis spectrometry, Infrared Transformed Fourier, Proton Nuclear Magnetic Resonance Spectra, 13 Carbon Nuclear Magnetic Resonance Spectra, Mass Spectrometry, and Thermogravimetry. In addition, its photoinitiating activity was investigated. The results showed that the novel photoinitiator had good photoinitiating activity and thermal stability compared to commercial photoinitiators. The migration of the residual photoinitiator in the cured film was lower than that of 1173 and 2959.

Synthesis and Application of Asymmetry Diphenylketone Photoinitiators

AbstractIn this study, a series of novel functionalized benzophenone derivatives were elaborately designed and efficiently synthesized with low‐cost raw materials through a simple esterification reaction of heteroatoms/heterocycles substituted hydroxybenzophenone with acyl chloride. The chemical structures of the compounds were carefully characterized by proton nuclear magnetic resonance (1H NMR), carbon nuclear magnetic resonance (13C NMR), and high‐resolution mass spectrometry (HRMS). All compounds exhibited a strong and broad UV‐visible light absorption spectrum in comparison with BP. Moreover, owing to the breaking symmetry and higher molecular weight, these compounds displayed efficient photoinitiating activity and low mobility and self‐quenching as free radical photoinitiators. The resulting films exhibited much better mechanical properties compared with commercially available BP and Irgacure 184, implying potential application in industrial UV curing.

Preparation and photochemical properties of PEG based alpha-hydroxyalkylphenone photoinitiator

A novel emulsified photoinitiator (named as OP10-IPDI-2959) was prepared by the reaction of isophorone diisocyanate (IPDI) with 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Ir-2959) and nonylphenol polyoxyethylene ether (OP-10). The structure of OP10-IPDI-2959 was characterized by FT-IR, 1H NMR, UV–vis and high-resolution mass spectrometry. The photochemical behavior of OP10-IPDI-2959 was investigated through the photolysis and polymerization of neopentyl glycol diacrylate (NPGDA), mixture of epoxy acrylate resin and 1,6-hexanediol diacrylate (HDDA). Under irradiation of UV light, the OP10-IPDI-2959 has similar photochemical properties with the Ir-2959. Combined with benzophenone, the photoinitiated rate of OP10-IPDI-2959 is higher than that of Ir-2959. The polyoxyethylene fragment of OP10-IPDI-2959 is not only an alternative to toxic amine as hydrogen donor, but also a hydrophilic unit for interface between oil and water. The OP10-IPDI-2959 can effectively emulsify the epoxy acrylate resin and 1,6-hexanediol diacrylate mixture to form low viscosity emulsion. Compared with Ir-2959, OP10-IPDI-2959 has good photoinitiation activity, lower migration and good water emulsification performance.

Imidazole-containing photoinitiators for fabrication of sub-micron structures by 3D two-photon polymerization

Three imidazole-containing compounds with a phenanthroline and phenanthrene fragment were for the first time applied as initiators of two-photon polymerization individually and in combination with amines. The characterization of the novel two-photon photoinitiators was performed both theoretically and experimentally. The conducted DFT calculations revealed a correlation between nonlinear optical properties (dipole moment, mean polarizability, anisotropy of polarizability, first hyperpolarizability) and the photoinitiating activity of the compounds in one- and two-photon polymerization. A photocomposition based on phenanthroline-containing imidazole and triethanolamine demonstrated the widest fabrication window, the highest degree of conversion, as well as the two-photon polymerization threshold two times lower than the commonly used commercial IP-L photoresist. Consequently, the most effective photocomposition was proposed as imidazole photoresist for 3D two-photon polymerization and used for the fabrication of sub-micron and cell-like polymer structures.

Novel Norrish type I flavonoid photoinitiator for safe LED light with high activity and low toxicity by inhibiting the ESIPT process

In this study, flavonoid with N,N-diphenylamino group, 2-(4-(diphenylamino)phenyl)-3-hydroxy-4H-chromen-4-one (3HF–OH) was designed and synthesized as low-toxicity photoinitiator for photopolymerization under safe visible LED sources, however, low photoinitiation activity of 3HF–OH was found due to the excited state proton transfer (ESIPT) process. Hence, by replacing the 3-position hydroxyl group of 3HF–OH with benzoyl chloride and benzenesulfonyl chloride, flavonol carboxylate (3HF–C) and flavonol sulfonate (3HF–S) were synthesized, respectively. 3HF–C and 3HF–S showed not only one-component photoinitiating activity but also two-component photoinitiating activity when they were used with triethanolamine (TEOA) and iodonium salt (ONI) under safe, long-wavelength LED soft light sources. Photopolymerization kinetics reveals that photoinitiator 3HFs at low concentration exhibits the high polymerization rate and conversion. Results also showed that 3HF–S has much higher photoinitiation activity during photopolymerization compared with 3HF–OH and 3HF–C. The high extinction coefficient and aggregation-induced emission (AIE) characteristics of 3HF–S were observed, which proved that reducing the photoinitiator usage can effectively progress the photopolymerization activity. At the same time, the low usage of 3HF–S further reduces the toxicity of the cured material. Through electron spin resonance (ESR) experiment, the photoinitiation and sensitization mechanisms of these photoinitiators were observed. The school badge structure of hydrogels could be successfully 3D printed with 3HF–S and the stiffness of the structure was sufficient for further processing without breaking the structure. Moreover, the use of the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) method confirmed the lower toxicity for the final photopolymer provided.

Photopolymerization of acrylate resin and ceramic suspensions with benzylidene ketones under blue/green LED

Finding novel photoinitiators with photobleaching properties suitable for long-wavelength LED light sources is important for their potential use in the photocuring ceramic suspensions containing acrylate resin. Two D-π-A-π-D conjugated benzylidene ketones with visible light absorption (TPAK and CZK) were synthesized and evaluated as photoinitiators in the photocuring of acrylate resin and ceramic suspension under blue (460 nm)/green (520 nm) LED sources. Results showed that the two photoinitiators have high photoinitiating activity and can effectively produce thick-layer curing in acrylate resin and ceramic suspension due to their quick photobleaching properties under 460 nm and 520 nm LED light sources. The curing depth of the ceramic suspensions was measured as a function of the concentration of TPAK and CZK and the energy level of the LED light source. Jacob's equation was used to determine the critical energy Ec and depth sensitivity Dp of every suspension with different concentrations of benzylidene ketones.

Synthesis and preliminary photopolymerization envaluation of photopolymerizalbe type II photoinitiators BRA and TXRA

Two novel kinds of low migration photoinitiators BRA and TXRA based on benzophenone (BP) and thioxanthone (ITX) were synthesized and used as Type II polymerizable photoinitiators. The structures were confirmed by proton nuclear magnetic resonance (1H NMR), carbon nuclear magnetic resonance (13C NMR), FT-IR and high resolution MS. The spectral properties were studied by Ultraviolet–Visible (UV–vis) spectroscopy. The photo-initiating activity of the initiators was evaluated by real-time RT-IR in the presence of a hydrogen donor such as triethylamine. The results show that they exhibited almost similar initiating activity as the reference BP or ITX, while exhibiting more low migration than the non-functionalized BP or ITX. These results suggest that BRA or TXPA is an efficient photoinitiator for reducing migration in photo-curing system.

Acylstannanes: Cleavable and Highly Reactive Photoinitiators for Radical Photopolymerization at Wavelengths above 500 nm with Excellent Photobleaching Behavior.

Within this work, a novel acylstannane-based photoinitiator (PI) is presented. Tetrakis(2,4,6-trimethylbenzoyl)stannane (1) displays outstanding properties compared to state-of-the-art acylgermane-based initiators. Most importantly, the initiator shows absorption up to 550 nm, which allows higher penetration depths, especially in highly filled photopolymers. Besides that, 1 shows extremely high photoinitiating activity towards (meth)acrylic double bonds, as well as very fast photobleaching. Furthermore, unlike many organotin compounds, 1 shows surprisingly low cytotoxicity.

Regulating photochemical behavior and property of imidazolium-based water soluble polysiloxane macromolecular photoinitiators by anions

Five polysiloxane macromolecular photoinitiators containing different anion structures with ionic liquid character ([Si-di2959Im][X] (X = Br−, Ph4B−, TsO−, NO3−, CF3COO−)) were designed and synthesized. The effect of different anion structures on photochemical behavior, phoinitiating mechanism, the water solubility, thermal property and volatility of the photoinitiators was systematically explored. [Si-di2959Im][X] have high extinction coefficient that reaches up to 40,210 M−1 cm−1, and similar photoinitiating activity and mechanism to 2-hydroxy-1-[4-(2-hydroxyethoxy) phenyl]-2-methylpropan-1-one (Irgacure 2959), while the photopolymerization induction periods of the TPGDA systems initiated by [Si-di2959Im][X] (X = Ph4B−, CF3COO- and NO3) are slightly shorter than that by Irgacure 2959. In addition, [Si-di2959Im][X] have not only good water solubility, but also desirable thermal property and low saturated vapor pressure. The order of water-solubility of [Si-di2959Im][X] is Ph4B− <Br− <TsO− <NO3− <CF3COO−, the order of the saturated vapor pressure is CF3COO− < Ph4B−< NO3−< Br− < TsO−. High-performance liquid chromatography chromatograms reveal that the migration of the photolysis fragments of [Si-di2959Im][X] from the cured material is mitigated significantly by their high molecular weights.

A multifunctionalized macromolecular silicone-naphthalimide visible photoinitiator for free radical polymerization

Abstract Multifunctionalization strategy in the photoinitiator design keeps in step with the requirement of the green chemistry in photopolymerization techniques. Therefore, a multifunctional macromolecular silicone-naphthalimide visible photoinitiator (SND) has been designed and prepared by a convenient thermal polymerization process of 4-(1,3-dioxo-6-(piperidin-1-yl)-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl acrylate (ND-PA) and 3-bis(trimethylsilyoxy)methylsiyl-propylmethacrylate (TSMSPM). Its structure has been confirmed by 1H NMR, 13C NMR, FT-IR and GPC analysis. On one hand, SND has been evidenced as a visible photoinitiator by its successfully initiating the free radical polymerization of acrylate monomers in the presence or absence of a hydrogen donor under the irradiation of different LEDs (405 nm, 455 nm and 470 nm). Furthermore, SND/MDEA (N-methyldiethanolamine) system shows higher photoinitiating activity than that of the commercial CQ (camphorquinone)/MDEA system. On the other hand, this multifunctionalization strategy also brings about some desired modification for cured-materials. After employing only 5 wt% SND as the photoinitiator, for example, a photo-cured film of urethane diacrylate prepolymer (PUA) increases its water contact angles from 70.1° to 101.2°, implying that SND could change the surface property of the PUA cured film from hydrophilic to hydrophobic. Meanwhile, the use of SND as an initiator could significantly improve the resistance against water and ethanol. When the mass fraction of SND increases to 15%, the water and ethanol absorption ratios of the cured PUA films decrease from 4.4% to 1.0% and 8.9% to 2.8%, respectively.

Synthesis and performances of polysiloxane-modified 5-arylthianthrenium salt cationic photoinitiators with self-floating capability

Currently cationic photoinitiators possess undesirable characteristics of poor solubility in epoxy resins and the release of odorous and toxic by-products. These shortcomings greatly limit the applications of the photoinitiators. Here, the synthesis and initiating cationic photopolymerization properties of three new polysiloxane-modified 5-arylthianthrenium salt cationic photoinitiators (1187-Si-A/B/C) whose polysiloxane-modified cations could be considered as environmentally friendly are reported. The photoinitiators are synthesized based on 9-(4-hydroxyethoxyphenyl) thianthrenium hexaflorophosphate (Esacure 1187) and polysiloxane. The cationic photoinitiators not only exhibit excellent solubility in epoxy resins, but also do not release odour and toxic by-products under UV irradiation, which is demonstrated by liquid chromatography-electrospray ionization-mass spectrometry [LC-ESI(+)-MS]. The photoinitiating activity of 1187-Si-A/B/C is investigated by real-time infrared spectroscopy (RTIR), and it is found that 1187-Si-A/B/C can effectively initiate cationic photopolymerization by the photosensitization of 9,10-dibutoxyanthracene (DBA). More importantly, the polysiloxane chains offer 1187-Si-A/B/C good self-floating capability, and thus the photoinitiators are promising for the preparation of gradient polymers by cationic photopolymerization.

N‐alkoxypyridinium salts as coinitiators in radical polymerization: Synthesis and Photochemical Properties

ABSTRACTA few N‐alkoxypyridinium salts are developed as photoinitiators for efficient polymerization reactions. They are characterized by absorption properties below 300 nm, and generate alkoxy radicals on UV‐Vis light exposure. The squarylium dye was used as a blue‐light photosensitizer. Polymerization results are correlated with the photochemistry of N‐alkoxypyridinium salts. The quenching of the excited singlet state of squarylium dye by pyridinium salt and the formation of the semioxidized species of squaraine suggests an electron transfer from an excited dye to a coinitiator, and that the resulting oxygen‐centered radical initiates the polymerization process. The chemical mechanism was investigated by steady state photolysis and nanosecond laser flash photolysis experiments. Photoinitiating activity of new photoinitiators for initiation of polymerization of trimethylolpropane triacrylate in the UV‐blue light region was compared with photoinitiating ability of selected commercially available initiators. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2840–2850

Structure-reactivity relationships of novel monomeric photoinitiators

Abstract Two different groups of methacrylates containing Type I or Type II photoinitiating moieties were synthesized and evaluated for use in UV curing applications. The first, a novel group of monomeric photoinitiators (MPIs) were synthesized by reactions of tert -butyl α-bromomethacrylate (TBBr) with 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone (Irgacure 2959), 1-hydroxy-cyclohexyl-phenyl-ketone (Irgacure 184) and 4-hydroxyacetophenone to give monomers MPI 1 , MPI 3 and MPI 4 respectively; and conversion of MPI 1 to MPI 2 by cleavage of tert -butyl ester groups with trifluoroacetic acid. The second group of photoinitiators were synthesized by reaction of 2-isocyanatoethyl methacrylate (IEM) with Irgacure 2959 (MPI 5 ) and 4-hydroxybenzophenone (MPI 6 ). All these MPIs’ absorption range in the UV region was found to be similar to their nonmonomeric analogs. Their photoinitiating abilities in the polymerizations of hexane-1,6-diol diacrylate (HDDA), 2-hydroxyethyl methacrylate (HEMA) and trimethylolpropane triacrylate (TMPTA) were studied using photodifferential scanning calorimeter and the kinetic parameters were correlated with the structures of the photoinitiating systems. Photoinitiating activities of investigated Type I photoinitiators, including small molecule commercial analogs, during polymerizations of HDDA and TMPTA are very slightly influenced by MPIs structure. However, the MPI structure is found to be important for the curing of HEMA, and some of the synthesized MPIs have better efficiency than commercial ones tested. The MPIs based on Irgacure 2959, Irgacure 184 and benzophenone (BP), with similar or increased photoinitiating activity compared to their commercial precursors, appear to be promising photoinitiators. An extra advantage of MPIs is their incorporation into the final formulations, reducing undesired migration of small molecules.

Mesoporous Graphitic Carbon Nitride as a Heterogeneous Visible Light Photoinitiator for Radical Polymerization.

The use mesoporous graphitic carbon nitride (mpg-C3N4) in conjunction with tertiary amines as initiators in visible-light-induced free radical polymerization is described. The initiation mechanism involves photoinduced free radical generation by scavenging holes with amines and subsequent hydrogen abstraction. The efficiency of the photoinitiation is controlled by the nature of the amines and specific surface area of the carbon nitride powder. Apparently, amines with higher basicity and available hydrogens provide more favorable conditions for the photoinitiation process. Due to its heterogeneous nature, the photoinitiator preserves its photoinitiation activity after the polymerization and can easily be separated and used for further polymerizations.

Synthesis and UV Curing Properties of A Photo-active Cardanol Derivative

A cardanol derivative(1) with photo-initiating activity has been synthesized in 86% yield using cardanol and 4-bromomethyl benzophenone as precursors.The photo-initiating performance of the derivative 1 is better than common benzophenone according to the results of the controlled experiments.The derivative 1 can be completely cured within 30 s under UV irradiation without the presence of any photoinitiator.Furthermore,the mixtures of the derivative 1 and cardanol with various ratios can also be cured under UV irradiation that the UV-curing time is prolonged up to dozens of minutes with the increase of cardanol content.The resulting cured films show relatively high hardness(3H-5H),excellent solvent,acid and base resistance,which have potential application in anti-corrosion coatings.

UV‐Irradiation Cured Organic‐inorganic Hybrid Nanocomposite Initiated by Ethoxysilane‐modified Multifunctional Polymeric Photoinitiator through Sol‐gel Process

AbstractThe UV‐cured organic‐inorganic hybrid nanocomposite (nano‐Si‐m‐PI) was prepared through the photopolymerization of acrylic resin initiated by ethoxysilane‐modified multifunctional oligomeric photoinitiator (Si‐m‐PI). The esterification reaction of 2‐hydroxy‐4′‐(2‐hydroxyethoxy)‐2‐methylpropiophenone (Irgacure 2959) with thioglycolic acid, and the following addition reactions with dipentaerythritol hexaacrylate and then 3‐aminopropyltriethoxysilane were carried out for preparing the Si‐m‐PI. The Si‐m‐PI exhibits the similar UV absorption and molar extinction coefficient with Irgacure 2959. The photoinitiating activity study by photo‐DSC analysis showed that the Si‐m‐PI possesses high photopolymerization rate at the peak maximum (Rpmax) and final unsaturation conversion (Pf) in the cured hybrid films. From the scanning electron microscope (SEM) observation, the SiO2 nanoparticles dispersed uniformly in the formed nano‐Si‐m‐PI, whereas the aggregation of nanoparticals occurred in nano‐Irg, which was prepared through the photopolymerization of acrylic resin initiated by Irgacure 2959. Moreover, compared with the UV‐cured pure polymer and nano‐Irg, the nano‐Si‐m‐PI showed remarkably enhanced thermal stability and mechanical properties.