Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Naphthalimide Aryl Sulfide Derivative Norrish Type I Photoinitiators with Excellent Stability to Sunlight under Near-UV LED Jia Yu,‡ Yanjing Gao,‡ Shengling Jiang,§ and Fang Sun*,†,‡ †
State Key Laboratory of Chemical Resource Engineering, ‡College of Science, and §College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China
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ABSTRACT: A series of Norrish type I photoinitiators (NASs), which are naphthalimide aryl sulfide derivatives, are prepared. The potential mechanism involved in the photolysis of NASs under UV LED at 405 nm is investigated by steady-state photolysis, nuclear magnetic resonance, electron spin resonance, fluorescence spectroscopy, cyclic voltammetry, and laser flash photolysis and by calculating the bond dissociation energies of the C−S bonds of NASs. The as-prepared photoinitiators NAS5 and NAS6 can efficiently initiate free radical photopolymerization of (methyl)acrylate monomers under UV LED exposure at 405 nm. NASs/ iodonium salt systems can initiate the cationic photopolymerization of epoxide. Interestingly, NASs exhibit an excellent stability to sunlight. Thus, the use of NASs makes the preparation, storage, and application of photocurable formulations convenient. cine,15 and cleavable thioxanthones containing a disilylacetylene moiety.16 These photoinitiators efficiently initiate photopolymerization under near-UV or visible LEDs but still show some shortcomings involving high price, no initiating ability for free radical photopolymerization of low viscosity monomers, and low molar extinction coefficients at wavelengths longer than 385 nm. Therefore, the design and development of novel and versatile type I photoinitiators with good absorption capacities in near-UV or visible LEDs wavelength range are important and urgent to the photocuring industry. The near-UV or visible LEDs emission wavelength is above 365 nm (e.g., 385, 395, 405, and 455 nm). Therefore, the photoinitiators must have good absorption capacities in the LEDs emission range and weak chemical bond, which can easily break under LEDs irradiation, to produce free radicals. The C (aryl)−S bond of phenylthiobenzophenone can directly break under mercury lamp irradiation and consequently produce aryl and thiyl radicals, which can initiate polymerization.17,18 Notably, thiyl radicals show no selectivity to monomers and low sensitivity to oxygen.19 Meanwhile, naphthalimide derivatives have good absorption capacities in the near-UV and visible LEDs wavelength range.20 Hence, molecules that combine C(aryl)−S bond and naphthalimide groups are expected to be type I photoinitiators under near-UV LED. In this study, we designed and synthesized a series of naphthalimide aryl sulfide derivatives photoinitiators (NASs)
1. INTRODUCTION The design and development of photoinitiators, which can initiate photopolymerization of photocurable systems under light-emitting diodes (LEDs) irradiation, have been attracting increasing attention in recent years because LEDs have many advantages, including low power consumption, long lamp life, instant on−off function, low total cost reduction of ownership, and environmental preservation in comparison with traditional mercury UV lamps.1,2 As key components in photocurable systems under LEDs irradiation, photoinitiators have the absorption wavelength matching to the emission wavelength of LEDs.3 Thus, photoinitiators can produce initiating species (e.g., radicals or cations) under LEDs irradiation.4 To achieve this goal, much effort has recently been made to develop new Norrish type II photoinitiators with good absorption in the LEDs emission range. Doing so obtains multicomponent photoinitiating systems under near-UV or visible LEDs, such as naphthalimide and naphthalic anhydride derivatives, 5 dihydroxyanthraquinone derivatives,6 π-conjugated dithienophosphole derivatives,7 and curcumin.8 Norrish type II photoinitiators produce initiating species by electron and/or proton transfer, whereas Norrish type I photoinitiators produce initiating species by the direct decomposition of photoinitiators.9 Currently available commercial photoinitiators under near-UV or visible LEDs include bisacylphosphine oxide (BAPO), 2,4,6-trimethylbenzoyldiphenylphosphine oxide, and oxime esters (e.g., OXE-1 and OXE-2), which are all Norrish type I photoinitiators. However, novel and efficient Norrish type I photoinitiators under near-UV or visible LEDs are rarely concerned currently. There are only few examples such as acylgermane-based photoinitiators,10−13 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine,14 N-phenylgly© XXXX American Chemical Society
Received: October 28, 2018 Revised: January 3, 2019
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DOI: 10.1021/acs.macromol.8b02309 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Chemical Structures of NASs
Scheme 2. Chemical Structures of the Monomers and Cationic Photoinitiator Used for Experiments
Light absorption properties of NASs were investigated by using a Shimadzu UV-3600 UV−vis−NIR spectrophotometer. Irradiation Sources. A Shenzhen Lamplic household UV LED bulb (emission wavelength centered at 405 nm, incident light intensity 100 mW cm−2) was used as irradiation device. Steady-State Photolysis Experiments. NAS2 and NAS5 in anhydrous acetonitrile and MMA, respectively, were irradiated with UV LED at 405 nm (∼100 mW cm−2) under nitrogen, and their UV− vis spectra were recorded with a Shimadzu UV-3600 UV−vis−NIR spectrophotometer at different irradiation times. ESR Experiments. ESR experiments were performed with a JEOL JES-FA200 spectrometer (X-band) at 9.06 GHz and 100 kHz field modulation, and a microwave power of 0.998 mW was used. The radicals were generated at room temperature upon UV LED exposure at 405 nm under an argon protection and trapped by DMPO. Solutions with concentrations of 1 × 10−4 mol L−1 were obtained by adding NASs and DMPO into tert-butylbenzene. Then, the solutions were irradiated. The molar ratio between NASs and DMPO was 1:5. Laser Flash Photolysis. Laser flash photolysis experiments of NASs were performed using a Spectra-Physics Quanta-Ray Qswitched nanosecond Nd:YAG laser at λexc = 355 nm (10 ns pulses; energy reduced down to 10 mJ) and an analyzing system consisted of a ceramic xenon lamp, a monochromator, a fast photomultiplier, an Acton SpectraPro SP-2300 spectrometer, and a transient digitizer (Princeton Instruments PI-MAX 4). Liquid samples with 10−5 M NASs in nitrogen-saturated acetonitrile were contained in a quartz cell with an optical path length of 1 cm. Fluorescence Experiments. The fluorescence emission spectra of NASs in acetonitrile were obtained using a Hitachi F-7000 fluorescence spectrophotometer. The concentration of each sample was 1 × 10−5 mol L−1. Redox Potentials. The oxidation potentials (Eox vs SCE) of NASs were tested in acetonitrile by cyclic voltammetry using tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte (CH Instruments Model 600E Series electrochemical workstation). The working electrode was a platinum disk, and the reference electrode was a saturated calomel electrode (SCE). Ferrocene was used as a standard, and the potentials determined from the half-peak potential were referred to the reversible formal potential of this compound (+0.38 V/SCE).
(Scheme 1). Then, we investigated the influences of NASs structures on the light absorption properties of NASs. We also studied the possible photolysis mechanism of NASs through steady-state photolysis, nuclear magnetic resonance (NMR), electron spin resonance (ESR), fluorescence spectroscopy, cyclic voltammetry, and laser flash photolysis and by calculating the bond dissociation energies (BDEs) of the C− S bonds of NASs. The photoinitiation capabilities of NASs for the free radical polymerization of 1,6-hexanediol diacrylate (HDDA) and methyl methacrylate (MMA) and the cationic polymerization of 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexane carboxylate (E-4221) under UV LED irradiation at 405 nm were investigated through real-time infrared spectroscopy (RTIR). Then, the stability of NASs in acrylate monomers under sunlight was explored by visually observing the changes of photosensitive liquids and comparing their UV−vis spectra before and after sunlight irradiation. Scheme 2 illustrates the structures of monomers and cationic photoinitiator used in the experiment.
2. EXPERIMENTAL SECTION Materials. 4-Bromo-1,8-naphthalic anhydride, potassium thioacetate, bis(dibenzylideneacetone)palladium [Pd(dba)2], 1,1′-bis(diphenylphosphino)ferrocene (dppf), potassium phosphate (K3PO4), iodobenzene, and 4-iodotoluene were obtained from Beijing Ouhe Technology Co., Ltd. 2,6-Diisopropylaniline, ptoluidine, anhydrous acetonitrile, tert-butylbenzene, and 4′-iodoacetophenone were purchased from J&K Scientific Ltd. 1-Fluoro-4iodobenzene, MMA, BAPO, and diphenyliodonium hexafluorophosphate (Iod) were supplied by Energy Chemical. Isobornyl acrylate (IBOA), HDDA, trimethylolpropane triacrylate (TMPTA), and E-4221 were obtained from Ciba Geigy Co. 5,5-Dimethyl-1pyrroline N-oxide (DMPO) was obtained from Dojindo Laboratories. Characterization. NMR spectra were recorded on a Bruker AV400 unity spectrometer (400 MHz). The high-resolution mass spectra in dichloromethane were obtained with an Agilent 6540 QToF mass detector equipped with an electrospray ionization source. B
DOI: 10.1021/acs.macromol.8b02309 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 3. Synthesis Route of NASs
The free energy changes ΔG for the electron transfer between NASs and Iod can be calculated from the classical Rehm−Weller equation:21 ΔG = Eox − Ered − ES (or ET) + C, where Eox, Ered, ES (or ET), and C are the oxidation potential of the electron donor, the reduction potential of the electron acceptor, the excited singlet (or triplet) state energies of NASs, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents. Computational Procedure. The Gaussian 09 package was used in the molecular orbital calculations. All molecular structures of NASs were optimized in the ground state by the density functional theory (DFT) at B3LYP/6-31G**. Whether optimized geometries had energy minima (zero imaginary frequency) was determined through frequency calculations. Frontier molecular orbital properties were analyzed and visualized with Gaussview 5.0 software.22 DFT calculations of the dissociation energy of C−S bonds of NASs were performed with Gaussian 09 software. All geometry optimizations were performed without imposing any geometry constraints by using a B3LYP hybrid density functional with a 6-311G* basis set.23 The results were analyzed through GaussView 5.0. In all molecules, frequency was calculated at the same level of theory to confirm that structures had energy minima. Reported energies were zero point corrected. Photopolymerization Kinetics. The photopolymerization kinetics of monomers (i.e., HDDA, MMA, and E-4221) in the presence of NASs or NASs/Iod photoinitiating systems were investigated by employing a Nicolet 5700 FT-IR spectroscope. Photosensitive liquids were coated on a KBr plate under air or clamped in laminate between two KBr plates (∼30 μm) for irradiation with UV LED at 405 nm (∼100 mW cm−2). The conversions of acrylate, methacrylate, and epoxy functional groups were assessed by calculating the proportion of the peak areas of 831−792, 831−796, and 756−738 cm−1, respectively. Each sample was repeated three times, and errors on the reported double bond and epoxy conversions as a function of polymerization time were
