Phenylglycine as a Versatile Photoinitiator under Near-UV LED

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

N‑Phenylglycine as a Versatile Photoinitiator under Near-UV LED J. Zhang,†,‡,⊥,§ J. Lalevée,‡,⊥ X. Mou,§ F. Morlet-Savary,‡,⊥ B. Graff,‡,⊥ and P. Xiao*,†,‡,⊥,§ †

Research School of Chemistry, Australian National University, Canberra, ACT 2601, Australia Université de Haute-Alsace, CNRS, IS2M UMR 7361, Cedex F-68100 Mulhouse, France ⊥ Université de Strasbourg, France § School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia ‡

S Supporting Information *

ABSTRACT: N-Phenylglycine is normally used as a coinitiator and can be photodecomposed by different dyes under UV or visible light irradiation to generate radicals for the initiation of free radical photopolymerization. However, the photochemistry and photoinitiation ability of N-phenylglycine alone upon exposure to near-UV light (e.g., UV LED at 392 nm), to the best of our knowledge, have not been investigated. In this research, the photochemistry of N-phenylglycine under the UV LED at 392 nm is studied using various approaches, and it reveals that radicals (PhNHCH2•) can be produced from the direct photodecomposition of N-phenylglycine. In addition, N-phenylglycine can also interact with iodonium salt under the UV LED at 392 nm to generate phenyl radicals and cations. These formed active species exhibit high performance to initiate the free radical photopolymerization of acylates and cationic photopolymerization of epoxides and divinyl ethers.

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INTRODUCTION

even though much effort has been devoted to the investigation of novel multicomponent photoinitiating systems (PISs).1,14−21 N-Phenylglycine (NPG), an amino acid, has been reported as a substituent of widely used co-initiators tertiary amines with type II photoinitiators (e.g., camphorquinone22) due to its advantages including nonallergic and biologically less toxic.23 Moreover, NPG derivatives were used in multicomponent photoinitiating systems with dyes (e.g., coumarin derivatives,24 xanthene dyes,25,26 azomethine dyes,27 pyrazoloquinoxaline dyes,28 and oxime esters29) for free radical photopolymerization (FRP) under visible light irradiation. Interestingly, the photodecomposition of NPG sensitized by polycyclic aromatic hydrocarbons (e.g., pyrene) was investigated under the irradiation of UV light (366 nm) and can be utilized as a photoinitiating system for the free radical photopolymerization of acrylate monomers.30 In addition, tetraalkylammonium salts of amino acids and sulfur-containing amino acids have also been reported as effective co-initiators of free radical polymerization in the presence of aromatic ketones.31 Even though NPG exhibits the UV light absorption, to the best of our knowledge, there is no report on the investigation of its inherent photochemistry and photoinitiation ability of polymerization under near-UV light. Herein, the photochemistry of NPG is studied using molecular orbital calculations, fluorescence, cyclic voltammetry,

The development of photocurable resins which can be polymerized under the irradiation of light-emitting diodes (LEDs) has been attracting increasing attention recently due to higher operating efficiency, better light output, safer usage, and lower cost of LEDs compared to the traditional mercury UV lamps.1−3 Photoinitiator (PI) is one of the most important components of photocurable resins to realize the LED-induced photopolymerization, as it is essential for it to absorb the certain light delivered from the LEDs and generated active species (e.g., radicals or cations) to induce the photopolymerization reactions.1,4 Several free radical photoinitiators are commercially available and applicable to polychromatic UV light. For instance, 2-hydroxy-2-methyl-1-phenylpropan-1-one (HAP; Irgacure 1173), 1-hydroxycyclohexylphenyl ketone (HCAP; Irgacure 184), bisacylphosphine oxide (BAPO), 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO), and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1 (BDMB; Irgacure 369) are well-known Type I cleavable PIs which mainly work under UV light in industrial applications.1,5−7 Interestingly, some of these commercial PIs exhibit longer wavelength of light absorption (e.g., up to 440 nm for BAPO) and have been explored for the application under UV LED at 395 nm.3 But the choices of type I cleavable PIs are rare. Specifically, the development of novel cleavable photoinitiators is still in infancy (only few examples such as acylgermane-based PIs,8−11 cleavable thioxanthones,12 and 2(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine13) © XXXX American Chemical Society

Received: April 8, 2018 Revised: April 28, 2018

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DOI: 10.1021/acs.macromol.8b00747 Macromolecules XXXX, XXX, XXX−XXX

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orbitals involved in the lowest energy transition can be extracted.32,33 The geometries were frequency checked. Fluorescence Experiments. The fluorescence property of NPG in acetonitrile was investigated using the Varian Cary Eclipse fluorescence spectrophotometer. The fluorescence quenching of NPG by Iod were investigated from the classical Stern−Volmer treatment7 (I0/I = 1 + kqτ0[Iod], where I0 and I stand for the fluorescent intensity of NPG in the absence and the presence of Iod, respectively; τ0 stands for the lifetime of NPG in the absence of Iod). Redox Potentials. The oxidation potential (Eox vs SCE) of NPG was measured in acetonitrile by cyclic voltammetry with tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte (Voltalab 6 Radiometer). The procedure has been presented in detail in ref 2. The free energy changes ΔG for the electron transfer between NPG and Iod can be calculated from the classical Rehm−Weller equation:34 ΔG = Eox − Ered − ES + C, where Eox, Ered, ES, and C are the oxidation potential of the electron donor, the reduction potential of the electron acceptor, the excited singlet state energies of NPG, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents. Steady-State Photolysis Experiments. NPG (and optionally with Iod) in acetonitrile was irradiated with the LED at 392 nm, and the UV−vis spectra were recorded using the Lambda 950 UV/vis/NIR spectrophotometer (PerkinElmer) at different irradiation times. Nuclear Magnetic Resonance (NMR) Spectrometry. All NMR spectra were conducted using a Bruker Avance III 300 spectrometer (300 MHz). Chemical shifts are recorded in ppm (δ) relative to tetramethylsilane (δ = 0 ppm), referenced to the chemical shifts of residual solvent resonances (1H). Electron Spin Resonance Spin Trapping (ESR-ST) Experiments. ESR-ST experiments were carried out using a Bruker EMXplus spectrometer (X-band). The radicals were generated at room temperature upon the UV LED exposure under N2 and trapped by phenyl-N-tert-butylnitrone (PBN) according to a procedure35 described elsewhere in detail. The ESR spectra simulations were carried out using the WINSIM software. Photopolymerization Experiments. The photopolymerization reactions of the multifunctional monomers (i.e., EB605, TPGDA, DPGDA, TMPTA, EPOX, and DVE-3) in the presence of NPG or NPG-based photoinitiating systems (PISs) upon exposure to the household UV LED bulb (392 nm) were monitored using the ATR-IR (BRUKER, IFS 66/s). Specifically, a layer of liquid formulation (∼20 μm thick) was coated on the surface of the ATR horizontal crystal, and the ATR-IR spectra of the samples were recorded at different time intervals during the household LED irradiation. The evolution of the double bond content of EB605, TPGDA, DPGDA, and TMPTA were all followed by the ATR-IR spectroscopy using the bands at about 1635 cm−1. The double bond content of DVE-3 and the epoxide group content of EPOX were followed using the bands at about 1620 and 790 cm−1, respectively.36 The photopolymerization of acrylates and DVE-3 were conducted in laminate, while the cationic photo-

steady state photolysis, nuclear magnetic resonance (NMR) spectrometry, and ESR spin trapping experiments. In addition, the photoinitiation ability of NPG alone and NPG-based photoinitiating systems for the FRP of acrylates and the cationic photopolymerization of epoxides and divinyl ethers under the UV LED at 392 nm is investigated using Infrared spectroscopy.

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EXPERIMENTAL SECTION

Materials. The investigated N-phenylglycine (NPG), diphenyliodonium hexafluorophosphate (Iod), ethyl 4-(dimethylamino)benzoate (EDB), trimethylolpropane triacrylate (TMPTA), (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX), and tri(ethylene glycol) divinyl ether (DVE-3) were purchased from Sigma-Aldrich, while tripropylene glycol diacrylate (TPGDA), dipropylene glycol diacrylate (DPGDA), and EBECRYL 605 (EB605; it is the bisphenol A epoxy diacrylate, EBECRYL 600, diluted 25 wt % with TPGDA) were obtained from Allnex. Their chemical structures are illustrated in Scheme 1.

Scheme 1. Chemical Structures of N-Phenylglycine (NPG), Investigated Additives (Iod and EDB), and Monomers (TPGDA, DPGDA, TMPTA, EPOX, and DVE-3)

Irradiation Sources. Household UV LED bulb (emission wavelength centered at 392 nm; incident light intensity: 100 mW cm−2) was used as irradiation device. Computational Procedure. Molecular orbital calculations were carried out with the Gaussian 03 package. The electronic absorption spectrum for NPG was calculated with the time-dependent density functional theory at the MPW1PW91/6-31G* level on the relaxed geometry calculated at the UB3LYP/6-31G* level; the molecular

Figure 1. (a) UV−vis absorption spectrum of NPG in acetonitrile and (b) emission spectrum of UV LED at 392 nm and its overlap with absorption spectrum of NPG. B


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Macromolecules polymerization of EPOX was carried out exposed to the air. The degree of double bond or epoxide group conversion C at time t during the photopolymerization is calculated from C = (A0 − At)/A0 × 100% (where A0 the initial peak area before irradiation and At the peak area of the functional groups at time t). The conversions C measured here are not throughout the whole sample thickness as the penetration depth of infrared beam into the sample is ca. 0.5−3 μm for the ATRIR spectroscopy. The conversions measured here can be used to evaluate the photoinitiation efficiency of the relevant photoinitiating systems (PISs).

between the excited singlet state of NPG and Iod (Figure 3b; fluorescence lifetime of NPG τ0 ∼ 6.2 ns). More specifically, the electron transfer quantum yield [ΦeT = kqτ0[Iod]/(1 + kqτ0[Iod]),1 where [Iod] = 4.7 × 10−2 M in formulations] of 1 NPG/Iod was determined to be 0.83, which indicates the high reactivity of the system. Moreover, the free energy change ΔGs for the 1NPG/Iod electron transfer reaction was determined to be −2.12 eV [calculated from the classical Rehm−Weller equation34 by using Eox(NPG) = 1.14 V (vs SCE) measured by cyclic voltammetry, Ered(Iod) = −0.2 V (vs SCE),1 and singlet state energy ES = 3.29 eV extracted from the UV−vis absorption and fluorescence emission spectra of NPG as usually done37], and the highly negative value can ensure the process is favorable. In the process, NPG can be oxidized by Iod to generate NPG•+, while Iod can be reduced to produce phenyl radicals Ph•. Steady-state photolysis experiments were carried out to investigate the overall photochemical behavior of NPG alone or NPG/Iod in acetonitrile upon the LED at 392 nm exposure. As illustrated in Figure 4a, the ground-state absorption of NPG (345 nm) decreased while the absorption at 280 nm increased during the light irradiation. It indicates the consumption of NPG and the generation of new products. Interestingly, an isosbestic point was observed at 323 nm, demonstrating that no side/secondary reactions occurred in the process. In addition, a faster decrease of NPG absorption was observed in the process of steady-state photolysis of NPG/Iod (Figure 4b), which indicates the relatively higher reactivity of NPG/Iod combination than that of NPG alone. Furthermore, the photochemical reactions of NPG alone or NPG/Iod system upon exposure to UV LED were investigated using nuclear magnetic resonance (NMR) spectrometry. Specifically, 1H NMR spectra of NPG with or without the light irradiation are shown in Figure 5a. It illustrates that the characteristic peak (2.81 ppm) belonging to N-methylaniline appeared and increased with the light irradiation, which can be ascribed to the cleavage of C−COOH bond of NPG. In addition, with the light irradiation of NPG/Iod combination, the characteristic peak of Iod (8.21 ppm) decreased while the peaks of iodobenzene (7.17, 7.34, and 7.71 ppm) increased (Figure 5b). It can be attributed to the fact that NPG absorbed the light delivered from the UV LED and then interact with Iod to lead to the formation of iodobenzene which cannot be generated directly from the cleavage of Iod under this irradiation condition (i.e., Iod alone can be cleaved directly only below 300 nm of light irradiation).

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RESULTS AND DISCUSSION Light Absorption Property of NPG. The UV−vis absorption spectrum of NPG in acetonitrile is shown in Figure

Figure 2. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of NPG at the UB3LYP/631G* level (isovalue = 0.02).

1a. Specifically, it exhibits an absorption maximum at 345 nm, and the relevant corresponding molar extinction coefficient ε345 nm is ∼4370 M−1 cm−1. Interesting, it also demonstrates overlap with the emission spectrum of the UV LED at 392 nm (Figure 1b), and the extinction coefficient at the maximum emission wavelength of the UV LED (392 nm) ε392 nm is ∼60 M−1 cm−1. The HOMO−LUMO transition (Figure 2) is characterized by a π−π* character. Photochemistry of NPG. NPG can absorb light delivered from the UV LED at 392 nm (Figure 1b), and the related photochemistry during the light irradiation is investigated using various approaches. The fluorescence emission of NPG, which can be associated with its excited singlet states, was observed during the light excitation (Figure S1). The wavelength of maximum fluorescence emission is 408 nm, and the fluorescence quantum yield is determined to be 0.446 in this study. Interestingly, the intensity of NPG fluorescence can be quenched by the addition of Iod into its acetonitrile solution as shown in Figure 3a, and the interaction rate constant of 1NPG/Iod (i.e., kq ∼ 1.67 × 1010 M−1 s−1) was determined from the Stern−Volmer treatment, which indicates that the process was diffusioncontrolled and the electron transfer reaction proceeded

Figure 3. (a) Fluorescence spectra of NPG as a function of [Iod] in acetonitrile and (b) the relevant Stern−Volmer plot (λex = 340 nm). C


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Figure 4. Steady-state photolysis of (a) NPG alone and (b) NPG/Iod in acetonitrile ([Iod] = 22 mM). UV−vis spectra recorded at different irradiation times; UV LED at 392 nm irradiation (100 mW cm−2).

Figure 5. 1H NMR spectra of (a) NPG and (b) NPG/Iod combination under different times of UV LED irradiation (300 MHz, d6-DMSO as solvent).

Figure 6. ESR spectra of the radicals generated in (A) NPG and (B) NPG/Iod combination upon the near-UV LED exposure and trapped by PBN in tert-butylbenzene: (a) experimental and (b) simulated spectra.

Photoinitiation Ability of NPG-Based Photoinitiating Systems under UV LED at 392 nm. Free Radical Photopolymerization. The radicals generated from NPG alone or NPG/Iod (Figure 6) can be expected to initiate free radical photopolymerization (FRP) of acrylates under the irradiation of UV LED at 392 nm. As presented in Figure 7, NPG (0.5 wt %) alone was efficient to initiate FRP of EB605 (i.e., the bisphenol A epoxy diacrylate, EBECRYL 600, diluted 25 wt % with TPGDA; viscosity: 6000−9000 cP at 25 °C, Allnex Technical Data) under the UV LED irradiation, and 66% of double-bond conversion of EB605 was attained after 300 s polymerization reaction. The increase of NPG concentration (2 wt %) in the formulation can enhance both the polymerization

In addition, ESR spin trapping experiments were conducted to detect the radicals produced from the NPG alone or the electron transfer in the NPG/Iod system under the UV LED irradiation, and the hyperfine splitting constants (hfcs) for both the nitrogen (aN) and the hydrogen (aH) of the PBN/radical adducts can be used to determine the types of radicals. As shown in Figure 6A, aN = 14.3 G and aH = 2.7 G were measured in NPG alone upon exposure to the UV LED, which can be assigned to PBN/PhNHCH2• radical adducts formed from the direct photodecomposition of NPG. For the NPG/ Iod combination, aN = 14.3 G and aH = 2.2 G were observed which are the typical values of PBN/phenyl radical adducts.38,39 D


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4). The IR spectra of EB605 before and after photopolymerization upon exposure to UV LED further confirmed the reaction of the double bond (∼1635 cm−1) of EB605 (Figure S2 in the Supporting Information). NPG cannot initiate FRP of TMPTA, DPGDA, and TPGDA upon exposure to the UV LED at 392 nm. It can be attributed to the fact that these acrylates possess lower viscosities (80− 135, 5−15, and 10−15 cP at 25 °C, respectively, Allnex Technical Data) than that of EB605, and the oxygen in the atmosphere can be much easier to be diffused into the formulations leading to the more significant oxygen inhibition effect. Interestingly, as illustrated in Figure 8, NPG/Iod PIS, a more reactive system than NPG alone, is capable of initiating FRP of these low-viscosity acrylates, and 53%, 79%, and 85% of double-bond conversions can be obtained for TMPTA, DPGDA, and TPGDA, respectively, after 300 s of photopolymerization reactions. Moreover, the photopolymerization reactions of these low-viscosity acrylates (e.g., TPGDA) can also be observed in the IR spectra; i.e., the double bond signal (∼1635 cm−1) decreased after the photopolymerization (Figure S3). The final polymerization conversion of TMPTA is much lower than those of DPGDA and TPGDA, which can be ascribed to the effect of functionality of acrylates; i.e., the higher functionality can result in the higher cross-link density at the early stage of photopolymerization and thus set a limit to the extent of conversion.40 Cationic Photopolymerization. NPG/Iod PIS is also efficient for the cationic photopolymerization due to the generation of NPG•+ under the light irradiation. As shown in Figure 9, both EPOX and DVE-3 can be polymerized in the presence of NPG/Iod PIS under the UV LED at 392 nm irradiation. Specifically, 65% of epoxide group conversion can be attained for the photopolymerization of EPOX. Moreover, from the IR spectra before and after the cationic photopolymerization of EPOX, the reaction can also be confirmed by the consumption of epoxide group (∼790 cm−1) and the concomitant formation of polyether (∼1070 cm−1) and hydroxyl (∼3430 cm−1) group (Figure S4). In addition, NPG/Iod is very efficient for the cationic photopolymerization of DVE-3 upon exposure to the UV LED at 392 nm, and 92% of double-bond conversion can be attained after 300 s of reaction. It can also be confirmed by the decrease of double bond signal (∼1620 cm−1) of DVE-3 (Figure S5).

Figure 7. Photopolymerization profiles of EB605 (conversion of acrylate functions vs irradiation time) in laminate in the presence of NPG-based PISs upon exposure to UV LED at 392 nm (100 mW cm−2).

Figure 8. Photopolymerization profiles (conversion of acrylate functions vs irradiation time) of different monomers (i.e., TMPTA, DPGDA, and TPGDA) in laminate in the presence of NPG/Iod (0.5%/2%, wt) PIS upon exposure to UV LED at 392 nm (100 mW cm−2).

rate and conversion (74% of conversion was obtained). Interestingly, the addition of EDB (which is normally used as a co-initiator with hydrogen-abstraction type photoinitiators) did not affect the photopolymerization efficiency. Markedly, the NPG/Iod photoinitiating system (PIS) exhibited significantly enhanced photoinitiation ability compared to NPG alone; i.e., polymerization rate and final conversion (82%) were much higher than those of NPG alone, which is in line with the relevant results of steady-state photolysis experiments (Figure

Figure 9. Photopolymerization profiles of (a) EPOX (conversion of epoxy functions vs irradiation time) under air and (b) DVE-3 (vinyl ether function conversion vs irradiation time) in laminate in the presence of NPG/Iod (0.5%/2%, wt) PIS upon exposure to UV LED at 392 nm (100 mW cm−2). E


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CONCLUSIONS This research demonstrates that N-phenylglycine (NPG) can photodecompose directly under the irradiation of UV LED at 392 nm and act as an efficient type I cleavable photoinitiator for free radical polymerization of acrylates. Moreover, the NPG/ Iod two-component photoinitiating system exhibits higher photoinitiation ability than that of NPG alone. Interestingly, the NPG/Iod system is also capable of initiating the cationic photopolymerization of epoxides and divinyl ethers upon exposure to the UV LED at 392 nm. It reveals that NPG is actually a versatile photoinitiator under UV light.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00747. UV−vis absorption, fluorescence excitation, and fluorescence emission spectra of NPG; IR spectra recorded before and after the photopolymerization of various monomers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.X.). ORCID

J. Lalevée: 0000-0001-9297-0335 P. Xiao: 0000-0001-5393-7225 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.X. acknowledges funding from the Australian Research Council Future Fellowship (FT170100301). REFERENCES

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