High Performance Near-Infrared (NIR) Photoinitiating Systems

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

High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen A. H. Bonardi,† F. Dumur,‡ T. M. Grant,§ G. Noirbent,‡ D. Gigmes,‡ B. H. Lessard,§ J.-P. Fouassier,∥ and J. Lalevée*,† †

Institut de Science des Matériaux De Mulhouse, IS2M - UMR CNRS 7361 - UHA, 68057 Mulhouse, France Aix Marseille Univ, CNRS, ICR, F-13397 Marseille, France § Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada ∥ Ecole Nationale Supérieure de Chimie de Mulhouse, Mulhouse, France ‡

S Supporting Information *

ABSTRACT: Photopolymerization under near-infrared (NIR) light is challenging due to the low energy of the absorbed photon but, if successful, presents significant advantages. For example, this lower energy wavelength is safer than UV light that is currently the standard photocuring light source. Also, NIR allows for a deeper light penetration within the material and therefore resulting in a more complete curing of thicker materials containing fillers for access to composites. In this study, we report the use of threecomponent systems for the NIR photopolymerization of methacrylates: (1) a dye used as a photosensitizer in the NIR range, (2) an iodonium salt as a photoinitiator for the free radical polymerization of the (meth)acrylates, and (3) a phosphine to prevent polymerization inhibition due to the oxygen and to regenerate the dye upon irradiation. Several NIR-absorbing dyes such as a cyanine borate and a silicon−phthalocyanine are presented and studied. Systems using borate dyes resulted in methacrylate monomer conversion over 80% in air. We report three types of irradiation system: low-power LED at 660 and 780 nm as well as a higher power laser diode at 785 nm. The excellent performance reported in this work is due to the crucial role of the added phosphine. graphic industry.10 NIR photosensitizers have also been utilized in photodynamic therapy to treat cancer cells.11 NIR dyes, and more specifically borate dyes, have been also used for information recording such as xerography that is a dry photoprinting technique.9 When the cyanine borate dye is photoirradiated, electron transfer between the dye and the counterion allows a recombination of dye radical to give a colorless dye. This process facilitates the bleaching and, ultimately, the recycling of the paper multiple times. This bleaching property can also be very interesting for photopolymerization. Because of the discoloration of the sample from green to colorless while polymerization, light can penetrate further in the sample and thus thicker layer can be polymerized.9 The curing of NIR photosensitive resin using a cyanine as a dye has previously been studied.8 The cyanine acts as a photosensitizer: it absorbs the light emitted by the laser in the near-infrared range and reacts with a combination of additive (iodonium salt, triazine,2 etc.) to generate initiating radicals.

1. INTRODUCTION Light-induced free radical polymerization reactions using a photoinitiator (PI) or a photoinitiating system (PIS) have been largely studied in the past few years and find applications in many industrial fields such as coatings, paints, dentistry, medicine, and 3D printing.1 Currently, the photopolymerization of (meth)acrylate monomers is mostly performed by UVcuring.1−3 Because the UV wavelength is known to cause skin and eye damages,4 a great challenge is to develop new free radical initiating systems upon longer (safer) wavelength irradiation. While photoinitiating systems using visible light have considerably improved, only a few studies focus on the curing via a near-infrared (NIR) source.5−8 The principal advantage to cure with near-infrared source is that its wavelength induces a deeper penetration into the material. As shown in Figure 1, the greater the wavelength, the less the light is diffused by the fillers within the material. Thus, the curing of a thick and filled material can be potentially enhanced compared to curing with UV or visible light. NIR dyes have found applications in several biological and medical fields. For example, NIR fluorescent dyes are used to tag DNA,9 while polymethine carbocyanine dyes and phthalocyanine/napthalocyanine dyes are used in the photo© XXXX American Chemical Society

Received: January 9, 2018 Revised: January 26, 2018

A

DOI: 10.1021/acs.macromol.8b00051 Macromolecules XXXX, XXX, XXX−XXX

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use of a highly efficient three-component system with a dye (photosensitizer) absorbing in the NIR range, an iodonium salt (photoinitiator), and a phosphine. The phosphine which is known as an additive to reduce oxygen inhibition during the free radical polymerization of (meth)acrylate monomers20,21 was also found to exhibit a new role here as a dye regenerator. To the best of our knowledge, the development of photoinitiation for NIR lights using phosphine has never been reported. Details on the photopolymerization of 1.4 mm thick clear coatings and filler containing coatings under a 785 nm 2 light (400 mW/cm ) under air as well as on some mechanistic aspects are provided. The commercial laser dye IR-140 is mainly used. The effects of other dyes, NIR LEDs at 660 and 780 nm and various intensities up to 2550 mW/cm2, are also investigated.

2. EXPERIMENTAL SECTION

Figure 1. UV−vis diffusion of light for a polystyrene latex (112 nm of average diameter) and calculated penetrations of selected photons.

2.1. Synthesis of Borate Dye. IR-140 Borate. All reagents and solvents for the synthesis of IR-140 borate (Scheme 1) were purchased from Aldrich or Alfa Aesar and used as received without further purification. 5,5′-Dichloro-11-(diphenylamino)-3,3′-diethyl-10,12-ethylene−thiatricarbocyanine perchlorate (IR-140) was purchased from Aldrich. Mass spectrometry was performed by the Spectropole of AixMarseille University. ESI mass spectral analyses were recorded with a 3200 QTRAP (Applied Biosystems SCIEX) mass spectrometer. The HRMS mass spectral analysis was performed with a QStar Elite (Applied Biosystems SCIEX) mass spectrometer. Elemental analyses were recorded with a Thermo Finnigan EA 1112 elemental analysis apparatus driven by the Eager 300 software. 1H and 13C NMR spectra were determined at room temperature in 5 mm o.d. tubes on a Bruker Avance 400 spectrometer of the Spectropole: 1H (400 MHz) and 13C (100 MHz). The 1H chemical shifts were referenced to the solvent peak CDCl3 (7.26 ppm) and DMSO (2.49 ppm), and the 13C chemical shifts were referenced to the solvent peak CDCl3 (77 ppm) and DMSO (49.5 ppm). Lithium triphenylbutylborate was synthesized as previously reported in the literature, without modifications and obtained in similar yields.22 The soft salt was synthesized as previously reported in the literature by using a biphasic mixture of CHCl3/water and THF to act as a phase transfer agent. The soft salt (the organic salt) was recovered in the organic phase and the inorganic one in the aqueous phase.23 Lithium triphenylbutylborate (236 mg, 0.770 mmol, 1.2 equiv) in water (20 mL) was added to a solution of 5,5′-dichloro-11(diphenylamino)-3,3′-diethyl-10,12-ethylene−thiatricarbocyanine per-

However, the use of NIR photoinitiating systems (e.g., cyanine) is often associated with a low reactivity and requires a high light intensity.5 Other common NIR chromophores, phthalocyanines (Pcs), are conjugated macrocycles that will contain different metal or metalloid inclusions as a result of their synthetic precursors. Pcs are often simple to synthesize with relatively high yields and have been used as commercial pigments and dyes for decades.12,13 Most Pcs have a high molar absorptivity coefficient and absorb light in the red and near-infrared (NIR) region (≈ 650−700 nm).12,14 Silicon Pcs have recently attracted a great deal of interest due to their chemical versatility and optoelectronic properties and have found applications as active materials in photovoltaic devices15,16 and as chemotherapy dyes.17 The silicon atom possesses two axial bonds that can be utilized as chemical handles to tune material properties such as the solid-state arrangement16,18 and solubility19 of the resulting molecules without affecting the absorption properties that are characteristic of the Pc chromophore. It appears that the greatest polymerization conversions using NIR photoinitiation by cyanine/iodonium salt couples are reported by Schmitz et al.5 In the current study, we explore the

Scheme 1. Synthetic Pathway of Dye-Borate (Ir140 Borate and Ir780 Borate)

B


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Macromolecules Scheme 2. Chemical Structures of the NIR Photosensitizers

chlorate (500 g, 0.642 mmol, 1 equiv) in a mixture of CHCl3 (100 mL) and THF (20 mL). The solution was stirred at room temperature while being protected from light for 1 h and then set aside for 10 min. THF was removed under reduced pressure (still while protecting the solution from light), and the solution was transferred in a separating funnel (covered with aluminum foil). The organic phase was separated and dried over magnesium sulfate, and the solvent was removed under reduced pressure. Addition of THF (2 mL) followed by pentane precipitated a solid that was filtered off, washed several times with pentane, and dried under vacuum. HRMS (ESI MS) m/z: theor: 679.7428 found: 679.7426 (M+ detected). HRMS (ESI MS) m/z: theor: 299.2375; found: 299.2377 (M− detected). Anal. Calcd for C61H58BCl2N3S2: C, 74.8; H, 6.0; N, 4.3. Found: C, 74.6, H, 6.1; N, 4.4%. IR-780 Borate. All reagents and solvents for the synthesis of IR-780 borate (Scheme 1) were purchased from Sigma-Aldrich and used as received without any further purification. Lithium triphenylbutylborate was synthesized as previously reported in the literature, without modification and obtained in similar yields.22 Lithium triphenylbutylborate (45.9 mg, 0.15 mmol, 1.2 equiv) in water (20 mL) was added to a solution of 2-[2-[2-chloro-3-[(1,3-dihydro-3,3-dimethyl-1propyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]3,3-dimethyl-1-propylindolium iodide (100 mg, 0.18 mmol, 1 equiv) in a mixture of CHCl3 (100 mL) and THF (20 mL). 2-[2-[2-Chloro-3[(1,3-dihydro-3,3-dimethyl-1-propyl-2H-indol-2-ylidene)ethylidene]-

1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-propylindolium iodide is abbreviated “IR-780 iodide” in the present paper. The solution was stirred at room temperature during 1 h in a round-bottom flask covered with aluminum foil. The organic phase was then removed under pressure. CHCl3 (10 mL) was added in the round-bottom flask. The solution was transferred in a separating funnel (covered with aluminum foil) to separate the organic phase which was then dried over magnesium sulfate. The solvent left was removed under reduced pressure. HRMS (ESI MS) m/z: theor: 539.3188. found: 539.3184 (M+ detected). HRMS (ESI MS) m/z: theor: 299.2375 found: 299.2379 (M− detected). Anal. Calcd for C58H68BClN2: C, 83.0; H, 8.2; N, 3.3. Found: C, 82.9, H, 8.1; N, 3.4%. 2.2. Synthesis of Bis(octanoate)silicon Phthalocyanine(OCT)2-SiPc. All reagents and solvents for the synthesis of (OCT)2SiPc were purchased from Sigma-Aldrich and used as received without any further purification. Dichlorosilicon phthalocyanine (Cl2-SiPc)25 was synthesized as described in the literature. 1H NMR spectra was determined at room temperature in CDCl3 on a Bruker Avance II spectrometer operating at 400 MHz. The 1H chemical shifts (δ) are reported in parts per million (ppm) referenced to tetramethylsilane (0 ppm). Octanoic acid (1.3 mL, 8.2 mmol) was added to a solution of Cl2SiPc (0.5 g, 0.82 mmol) and toluene (30 mL) in a three-neck roundbottom flask. The solution was stirred overnight at 110 °C and then let cool to room temperature. The reaction mixture was precipitated in C


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Macromolecules methanol and filtered to recover the product as a fine blue powder (0.46 g, yield = 68%). 1H NMR (400 MHz, CDCl3) δ 9.68−9.66 (m, 8H), 8.36−8.34 (m, 8H), 7.29−7.27 (m, 4H), 6.65−6.61 (m, 4H), 6.38−6.34 (m, 4H), 4.87−4.85 (d, 4H), 3.65−3.54 (m, 4H). 2.3. Commercial Photoinitiators. IR-780 iodide, IR-140 perchlorate, and IR-813 p-toluenesulfonate (Scheme 2) were purchased from Sigma-Aldrich. Indocyanine Green was purchased from TCI Chemicals (Scheme 2). S2265 and S0991 (Scheme 2) were obtained from Few Chemicals GmbH and used without further purification. 2.4. Other Chemical Compounds. The mix of monomers used in this study, referred to as “Mix-MA” (Scheme 3), is a mixture of 33.3

Scheme 3. Chemical Structures of the Monomers (Mix-MA)

Figure 2. RT-FTIR spectra of methacrylate resin (Scheme 3) between 4500 and 7500 cm−1 (1) before polymerization and (2) after polymerization. Circled in purple the peak representative of the double bond CC conversion used to calculate the photopolymerization profile (methacrylate function conversion vs irradiation time). 140 borate alone in acetonitrile (ACN, Sigma-Aldrich); IR-140 borate and 4-dppba (11.2 mM) in ACN; IR-140 borate and Ar2I+PF6− (7.9 mM) in ACN; IR-140 borate, Ar2I+PF6− (12 mM), and 4-dppba (15 mM) in ACN. During irradiation, spectra were registered every 5 s. 2.8. Electron Spin Resonance Spin-Trapping (ESR-ST) Experiments. ESR-ST experiments were carried out using an Xband spectrometer (Bruker EMX-Plus) at room temperature. The radicals were created under N2 upon the laser diode at 785 nm, 2.55 W exposure in tert-butylbenzene and trapped with phenyl-N-tertbutylnitrone (PBN). Additional details with regards to the procedure have previously been reported in ref 26. The PEST WINSIM program was used to simulated ESR spectra.27 2.9. Thermal Imaging. An infrared thermal imaging camera (Fluke TiX500) was used to follow the temperature propagation over 1 cm of the sample. Thermal resolution is about 1 °C, and spatial resolution is 1.31 mrad. The software Fluke SmartView4.1 has been used to characterize recorded images.

wt % of (hydroxypropyl)methacrylate (HPMA), 33.3 wt % of 1,4butanediol dimethacrylate (1,4-BDDMA), and 33.3 wt % of a urethane dimethacrylate monomer (Scheme 3). Mix-MA will be used as a benchmarked resin in photopolymerization reaction. The iodonium salt bis(4-tert-butylphenyl)iodonium hexafluorophosphate (Ar2I+/PF6− or also Speedcure 938) was obtained from Lambson (Scheme 4). 4-(Diphenylphosphino)benzoic acid (4-dppba,

Scheme 4. Chemical Structures of the Additives

3. RESULTS AND DISCUSSION 3.1. Photoinitiation Ability of the Three-Component System Using IR-140 Borate. 3.1.1. Low NIR Light Intensity. IR-140 borate has been used in this study to initiate the free radical polymerization upon irradiation at 785 nm (400 mW/ cm2) with a laser diode. The three-component formulation containing IR-140 borate (as a dye)/Ar2I+PF6−/4-dppba leads to a high polymerization rate (Figure 3). The final monomer conversion is roughly 50−60%, and the final polymer is tackfree at the surface. When omitting one of the two additives (Ar2I+PF6− or 4-dppba) or when irradiating the monomer alone, no photopolymerization took place. The polymerization efficiency of the different control formulations is outlined in Table 1. When increasing the light source energy from LED at 400 mW/cm2 (experiment 3, in Figure 3) to laser diode at 2.55W/cm2 (experiment 5, in Figure 3), the conversion is twice greater. The initiator developed here absorbs both red and NIR light and can be used for red or NIR light activation. 3.1.2. Higher NIR Light Intensity. The irradiance of the laser diode at 785 nm can easily be tuned between 0 and 2.55 W/ cm2. The effect of the laser diode power on the polymerization rate has been evaluated for the formulation with IR-140 borate, 4-dppba, and Ar2I+PF6− (Figure 4 and Table 2). By increasing the power of the laser diode, the polymerization rate and final

Scheme 4) was purchased from Sigma-Aldrich. No further purification was made. In this study, the fillers used were silica beads of 400 nm diameter. 2.5. Irradiation Sources. Several types of irradiation sources were used throughout the study: NIR LED at 660 nm (with an irradiance of 80 mW/cm2) and NIR LED at 780 nm (with an irradiance of 130 mW/cm2), both from Thorlabs, and a NIR laser diode at 785 nm with tunable irradiance (from 0 to 2.55 W/cm2) from Changchun New Industries (CNI). 2.6. Photopolymerization Experiments. The photosensitive formulations were deposited on a polypropylene film; the sample thickness was controlled using a mold (1.4 mm) and polymerized under air by irradiation with LED or laser diode. The conversion is followed by real-time Fourier transform infrared spectroscopy (RTFTIR) using a JASCO 4600 spectrometer. The double bond CC content of Mix-MA is measured at 6100−6220 cm−1 (Figure 2). Specific reaction conditions for each photopolymerization experiments are given in the figure captions. For all experiments, recording of the FTIR spectra is initialized a minimum of 10 s prior to the irradiation to obtain a baseline for comparison. 2.7. Photolysis Experiments. The photolysis of IR-140 borate is accomplished by irradiation with laser diode at 785 nm (2.55 W/cm2 or 400 mW/cm2) and followed by UV−vis measurements using a Varian Cary 3 spectrophotometer. Four solutions were prepared: IRD


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Figure 4. Photopolymerization profiles of Mix-MA under air (methacrylate functions conversion vs irradiation time) in the presence of IR-140 borate (0.1 wt %)/Ar2I+PF6− (3 wt %)/4-dppba (2 wt %); laser diode at 785 nm; thickness = 1.4 mm; (1) 0.4 W/cm2, (2) 1.37 W/cm2, (3) 1.82 W/cm2, (4) 2.08 W/cm2, (5) 2.34 W/cm2, and (6) 2.55 W/cm2. The irradiation starts at t = 17 s.

Figure 3. Photopolymerization profiles of Mix-MA under air (methacrylate functions conversion vs irradiation time) in the presence of (1) IR-140 borate (0.1 wt %), (2) IR-140 borate/Ar2I+PF6− (0.1 wt %/3 wt %), (3) IR-140 borate/Ar2I+PF6−/4-dppba (0.1 wt %/3 wt %/2 wt %); (4) monomer alone; (1−4) under exposure to laser diode at 785 nm, 400 mW/cm2; and (5) IR-140 borate/Ar2I+PF6−/4-dppba (0.1 wt %/3 wt %/2 wt %); laser diode at 785 nm, 2.55 W/cm2; thickness = 1.4 mm. The irradiation starts at t = 17 s.

3.1.3. Photopolymerization of Formulations with Fillers: Access to Composites Using NIR Light. Fillers are often used in the polymerization of composites. However, the access to composites is very difficult by photopolymerization as the light penetration is usually highly restricted for UV light (Figure 1); i.e., when added to the formulation, the fillers decrease the penetration of the light through a light diffusion process. Using the unique advantage of the better NIR light penetration, the new proposed photoinitiating system has been explored here for the preparation of composites. Using the three-component initiating system (IR-140 borate/4-dppba/Ar2I+PF6−), the photopolymerization has been followed by RT-FTIR (Figure 6A). Weight percentage of fillers ranging from 25 to 75 wt % resulted in tack-free polymers and final monomer conversion between 60% and 80%. Remarkably, the polymerization was not significantly slower with the addition of fillers. Interestingly, the photopolymerization of very thick samples with 75 wt % fillers was also possible (Figures 6B,C). A sample of 1 cm high was successfully polymerized from the irradiation of its surface in 30 s, and a tack-free surface has been obtained. The performance obtained for the very high filler content (75%) is remarkable, i.e., using UV or blue light initiators, the maximal depth of cure being only 2−4 mm. 3.2. Chemical Mechanisms for IR-140 Borate. The three-component system (IR-140 borate/4-dppba/Ar2I+PF6−) has also been studied by steady state photolysis to gain an insight into the chemical mechanisms. The decrease of the IR140 borate absorption at 800 nm was followed during the irradiation at 785 nm (400 mW/cm2) (Figure 7; condition details in the caption; the photolysis results for an irradiation at 785 nm and 2.55 W/cm2 are also given in the Supporting Information (Figures S1 and S2)). Four different conditions were examined: IR-140 borate alone, IR-140 borate in the presence of Ar2I+PF6−, IR-140 borate in the presence of 4-dppba, and IR-140 borate in the presence of Ar2I+PF6−and 4-dppba. In the four scenarios, it is clearly observed that the dye is quickly decomposed (Figure 8, curve 1), in agreement with the reaction 1 already reported for

Table 1. Photopolymerization Results of Mix-MA under Air in the Presence of Different Photoinitiating Systems under Exposure to Laser Diode at 785 nm (400 mW/cm2) for 1000 sa system monomer (Mix-MA) alone IR-140 borate IR-140 borate/4dppba IR-140 borate/ Ar2I+PF6− IR-140 borate/4dppba/Ar2I+PF6−

light irradiance (on the surface of the sample) (W/cm2)

polymerization

0.4

−

0.4 0.4

− −

0.4

−

0.4

+

a

Thickness = 1.4 mm: (+) efficient polymerization or (−) no polymerization observed.

conversion both increase. Regardless of the power, the final polymers were tack-free. However, the inhibition time at the beginning of the polymerization, credited to the oxygen inhibition, is reduced when increasing the power. It is important to note that at maximum irradiance, 2.55 W/ cm2, the polymerization starts almost as soon as the laser diode is turned on, and the final monomer conversion is greater than 80%. This result is as expected: by increasing the power, more photons reach the sample and a higher percentage of dye can be excited and will initiate the polymerization. Figure 5 illustrates (using thermal imaging camera) that the temperature reaches for the monomer blend in the presence of the NIR dye upon 400 mW/cm2 does not exceed 35 °C for 100 s of polymerization. Therefore, the observed initiation is clearly ascribed to a photochemical effect and not to a thermal effect for 400 mW/cm2; i.e., IR-140 borate behaves as a photosensitizer upon NIR light without any heating effect. E


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Figure 5. (A) Temperature profiles of Mix-MA under air (temperature maximum of the sample vs irradiation time) recorded using a thermal imaging camera in the presence of IR-140 borate (0.1 wt %); laser diode at 785 nm and 0.4 W/cm2; thickness = 10 mm; thermal imaging pictures before irradiation (0 s) and after different times of irradiation; the temperature range is given in (B).

excited state of the photosensitizer (IR-140 borate) through a redox process leading to a faster degradation of the dye (reaction 2). 6 IR-140 borate being a nonfluorescence compound, fluorescence quenching experiments can hardly be used for reaction 2. However, this latter process is in full agreement with the observation of aryl radicals (generated from the reduction of Ar2I+) in ESR experiments (see below and Figure 9; Scheme 5). In the presence of 4-dppba, the decomposition is slower (Figure 8, curve 4). The slower degradation of the dye in the presence of the phosphine is due to the catalytic nature of the reaction between the phosphine and the dye. The oxidation potential of the 4-dppba has been measured by cyclic voltammetry: Eox = 1.2 V, which means that it is a good electron-donating component. This phenomenon allows the regeneration of the dye and explains the slower degradation of the dye in the presence of 4-dppba. Thus, reaction 5 in Scheme 5 is proposed as the pathway to regenerate the dye in the presence of phosphine, leading to a slower consumption of the dye in the three-component system compared to the dye/Ar2I+ system.

Table 2. Photopolymerization Results of Mix-MA under Air in the Presence of Different Photoinitiating Systems under Exposure to Laser Diode at 785 nm (2.55 W/cm2) for 1000 sa photoinitiating system monomer (Mix-MA) alone IR-140 borate IR-140 borate/4dppba IR-140 borate/ Ar2I+PF6− IR-140 borate/4dppba/Ar2I+PF6−


polymerization

2.55

−

2.55 2.55

− −

2.55

−

2.55

+

a

Thickness = 1.4 mm; (+) polymerization observed or (−) polymerization not observed.

other dye/borate salts.6 Interestingly, in the presence of Ar2I+PF6−, the decomposition is clearly accelerated (Figure 8, curve 3). We surmise that the Ar2I+PF6−is able to quench the

Figure 6. Photopolymerization of Mix-MA under air in the presence of IR-140 borate/Ar2I+PF6−/4-dppba (0.1 wt %/3 wt %/2 wt %) and different fillers contents; laser diode at 785 nm, 2.55 W/cm2; (A) profiles (methacrylate functions conversion vs irradiation time) for (1) 0 wt % of fillers, (2) 25 wt % of fillers, (3) 50 wt % of fillers, and (4) 75 wt % of fillers; thickness = 1.4 mm; the irradiation starts at t = 17 s. (B) Picture of a composite obtained from the photopolymerization with 75 wt % fillers: thickness = 1 cm. (C) Picture of a composite of same composition obtained for a thickness = 1.4 mm. F


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Figure 7. (A) Photolysis of IR-140 borate in ACN upon laser diode at 785 nm, 400 mW/cm2: UV−vis spectra for different irradiation times. (B) Photolysis of IR-140 borate + 4dppba (10−2 mol L−1) in ACN upon laser diode at 785 nm, 400 mW/cm2: UV−vis spectra for different irradiation times. (C) Photolysis of IR-140 borate + Ar2I+PF6 (10−2 mol L−1) −in ACN upon laser diode at 785 nm, 400 mW/cm2: UV−vis spectra for different irradiation times. (D) Photolysis of IR-140 borate + 4-dppba (10−2 mol L−1) + Ar2I+PF6−(10−2 mol L−1) in ACN upon laser at 785 nm, 400 mW/ cm2: UV−vis spectra for different irradiation times.

Figure 9. ESR spin trapping spectrum of IR-140 borate/Ar2I+PF6−; in tert-butylbenzene; under laser diode at 785 nm (2.55 W/cm2) exposure; under N2; experimental (a) and simulated (b) spectra. Ntert-Butyl-α-phenylnitrone (PBN) is used as a spin trap. The hyperfine coupling constants hfc (aN = 14.2 G; aH = 2.2 G) agree with the known data for the radical adduct PBN/Ph•.

Figure 8. Photolysis (% decomposition of the peak at 800 nm) upon laser diode at 785 nm, 400 mW/cm2 of (1) IR-140 borate, (2) IR-140 borate + 4dppba, (3) IR-140 borate + Ar2I+PF6−, and (4) IR-140 borate + Ar2I+PF6−+ 4-dppba in ACN.

the phosphine (reaction 4, Scheme 5), preventing their further attack onto the dye to avoid its accelerated decomposition. From the polymerization data obtained above (only the IR140 borate/4-dppba/Ar2I+PF6− three-component being able to initiate efficiently the polymerization), it can be expected that reactions 2−5 are the most important ones. Indeed, IR-140 borate being inefficient alone, reaction 1 can probably be neglected. The ESR-ST spectrum obtained upon irradiation of IR-140 borate (at 785 nm, 2.55 W/cm2) in the presence of the N-tertbutyl-α-phenylnitrone (PBN) spin-trap agent, in tert-butylbenzene, revealed the formation of a phenyl (Ph•)/PBN radical adduct (Figure 9) with the hyperfine coupling constants hfc (aN = 14.2 G; aH = 2.2G), in full agreement with the reference values given in ref 28.

As seen previously, the 4-dppba is essential for the system to polymerize effectively under our specified conditions. Performing the radical polymerization of methacrylates in air can be problematic due to their extreme sensitivity to oxygen. There is creation of peroxyl radicals (ROO•) that are too stable for the polymerization to proceed (Scheme 5). The 4-dppba helps to overcome the inhibition by the oxygen by reacting with the peroxyl radicals to create less stable radicals (RO•), which can continue the polymerization (as previously reported in ref 20). This role of 4-dppba is also shown by the photolysis of IR-140 borate/4-dppba that is slower than that of IR-140 borate alone (Figure 8, curve 1 vs curve 2); i.e., the peroxyls are trapped by G


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performance in terms of both polymerization rate and final conversion. Indeed, at 785 nm and 400 mW/cm2, only Indocyanine Green and IR-140 borate lead to a polymer formation. However, with Indocyanine Green, the final conversion is low, and the polymer is liquid at the surface whereas with IR-140 the polymer is tack-free and the polymerization much faster. With the other dyes, no polymerization is observed. The energy of the photon is too low to initiate the polymerization with IR-813 p-toluenesulfonate and IR-780 iodide. At 785 nm and under 2.55 W/cm2, IR-140 borate still ensures a faster polymerization (Figure 10C). However, Indocyanine Green and IR-813 p-toluenesulfonate lead to the same final conversion as IR-140 borate. With IR-780 borate, the final conversion is slightly lower than with the three other cyanines. For these four dyes, the polymer is tack-free at the surface. No polymerization at all is observed using (OCT)2SIPC: this is likely due to its negligible absorption above 700 nm. Polymerization above 700 nm with a phthalocyanine is still possible using other derivatives absorbing at longer wavelengths such as manganese(II) phthalocyanine (Mn-Ph, chemical structure of Mn-Ph in the Supporting Information (Scheme S1)). Tack-free polymers have been observed using this phthalocyanine. Other types of dyes such as porphyrin (chlorophyll B, chlorophyllin copper salt) or nickel dithiolene can also be used but the performance remains always lower than for the cyanine presented above. Structures of these dyes are given in the Supporting Information (Scheme S1). 3.4. Influence of the Counterion in Cyanine Dyes. Two dyes have been compared with two different counterions: IR780 iodide/IR-780 borate and IR-140 perchlorate/IR-140 borate. Using IR-780 iodide, no polymerization was observed, whereas with IR-780 borate, a tack-free polymer is obtained with a final conversion of ∼70%. With IR-140, there is a polymerization with both counterions, but as observed for IR780, the polymerization is much more efficient with the borate than with the perchlorate (both higher polymerization rate and final conversion) (Figure 11). This result clearly shows that the decomposition of the borate moiety (reaction 3 in Scheme 5) is important for the formation of initiating radicals. The borate counterion acts as an electron-donating component with dye•+, and therefore the dye/borate system is able to generate free radicals in reaction 3. Therefore, the borate counteranion can reduce the probability of back electron transfer from Ph2I• to dye•+ (reaction 2), increasing the yield of initiating radicals as also observed in other systems.6 To summarize, three advantages for the use of borate as counteranion can be invoked: first, the solubility of the dye in the monomer is better, second, the initiating radicals yield is increased (reactions 2 and 3, Scheme 5), and third, an electron transfer from the borate anion to the cyanine cation leads to a recombination between the cyanine radical and the alkyl radical (which alkylates the cyanine). The alkylated cyanine has no color thereby improving the bleaching properties.29 3.5. IR-140 Borate vs a Reference Cyanine for NIR Photopolymerization. In a recent study, commercial cyanines were elegantly used to successfully polymerize methacrylates monomer under NIR radiation.5 In this latter work, cyanines in combination with an iodonium salt (noted RI in Scheme 6) allowed the creation of initiating radicals by electron transfer. Two cyanines were used in this work: S2265 and S0991 (Scheme 2). With these dyes, the polymerization of

Scheme 5. Proposed Chemical Mechanisms for the Photochemical Reactivity of IR-140 Borate in ThreeComponent Systems

3.3. Comparison with Other Red−NIR Dyes. A variety of dyes absorbing in the red−NIR range have been checked in dye/Ar2I+PF6−/4-dppba three-component formulations. Commercially available dyes were compared to borate dyes and a novel silicon Pc-based dye. Three different irradiation configurations were used: laser diode at 785 nm at 400 mW/ cm2 and at 2.55 W/cm2 and LED at 660 nm. All polymerization results are tabulated in Table 3. Table 3. Photopolymerization Results of Mix-MA under Air in the Presence of Dye/Ar2I+PF6−/4-dppba (0.1 wt %/3 wt %/2 wt %) under Exposure to Laser Diode at 785 nma dye IR-140 borate (OCT)2-SiPC Indocyanine Green IR-780 borate manganese(II) phthalocyanine IR813 ptoluenesulfonate IR-140 borate (OCT)2-SIPC Indocyanine Green IR-780 borate manganese(II) phthalocyanine IR813 ptoluenesulfonate


polymerization

2.55 2.55 2.55 2.55 2.55

+ − + + +

2.55

+

0.4 0.4 0.4 0.4 0.4

+ − + + −

0.4

−

a

Thickness = 1.4 mm; (+) efficient polymerization or (−) no polymerization observed.

At 660 nm, only IR-140 borate and (OCT)2-SiPc allowed an efficient polymerization. In both cases, the polymer obtained is tack-free at the surface. The polymerization is slower for (OCT)2-SIPC than for IR-140 borate (Figure 10A). Likely the low polymerization rate is due to the mismatch between the narrow absorption of silicon phthalocyanines (λmax = 685 nm) and the light source wavelength. At 785 nm, the result is correlated to the laser diode intensity. For 400 mW/cm2 (Figure 10B), IR-140 has better H


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Figure 10. Photopolymerization profiles of Mix-MA under air (methacrylate functions conversion vs irradiation time) in the presence of 3 wt % Ar2I+PF6−, 2 wt % 4-dppba, and 0.1 wt % (1) Indocyanine Green, (2) IR-140 borate, (3) IR-813 p-toluenesulfonate, (4) (OCT)2-SIPC, and (5) IR780 borate under exposure to (A) LED at 660 nm; (B) laser diode at 785 nm, 400 mW/cm2; and (C) laser diode at 785 nm, 2.55 W/cm2; thickness = 1.4 mm. The irradiation starts at t = 17 s.

the polymerization was less efficient but a cross-linking was observed. Cyanine S2265 can be considered as a reference and has been used here for the first time with a phosphine in a threecomponent system (S2265/Ar2I+PF6−/4-dppba) to initiate the free radical polymerization of Mix-MA upon irradiation at 785 nm (400 mW/cm2). Without the phosphine, no polymerization occurs, showing the huge effect of the 4-dppba to improve the NIR initiating ability of S2265. In the presence of 4-dppba, the polymerization is fast (Figure 12, curve 2), resulting in a final conversion of ∼80% (tack-free polymer obtained). Remarkably, we notice, however, that IR-140 borate leads to a better performance than S2265 with faster and higher conversion than S2265 (Figure 12, curve 1 vs curve 2). An inhibition time is observed with S2265 but not with IR-140 borate. It is also noted that the color of the resin changes before and after polymerization: the formulation is green before irradiation and becomes brown after irradiation. All these observations suggest that the combination of IR-140 borate with iodonium salts and phosphines result in high performance NIR photosensitive systems. Other elegant NIR dyes were reported in refs 30 and 31 for photocontrolled or photoregulated polymerizations.

Figure 11. Photopolymerization profiles of Mix-MA under air (methacrylate functions conversion vs irradiation time) in the presence of Ar2I+PF6−(3 wt %), 4-dppba (2 wt %), and IR-140 (0.1 wt %): (1) perchlorate; (2) borate; under exposure to laser diode at 785 nm, 2.55 W/cm2; under air; thickness = 1.4 mm. The irradiation starts at t = 17 s.

4. CONCLUSION In the present study, new three-component systems for NIR curing of (meth)acrylates have been proposed. Radical photopolymerization of methacrylates using two wavelengths of irradiation (660 and 785 nm) was successfully initiated by several types of dyes, including novel silicon phthalocyanine-

UDMA (Scheme 6) was very efficient under exposure to irradiation at 808 nm. When S2265 was used without S 0991,

Scheme 6. Chemical Structures of Monomer and Additives Used in Ref 5

I


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ACKNOWLEDGMENTS

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REFERENCES

The authors thank Dr. Jérémie Fournier and Gwendoline Lejeune from SATT Conectus for fruitful discussion on NIR initiating systems. The authors thank the ANR agency for the financial support for the ANR project “FastPrinting” (no. ANR15-CE08-0012). We are very grateful for financial support from the NSERC Discovery Grant to B. L. The authors thank Dr. Emmanuel Lacote and Dr. Fréderic Le Quemener (University of Lyon) for the latex preparation for Figure 1,.

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Figure 12. Photopolymerization profiles of Mix-MA under air (methacrylate functions conversion vs irradiation time) in the presence of Ar2I+PF6− (3 wt %), 4-dppba (2 wt %) and (1) IR-140 borate (0.1 wt %) and (2) S2265 (0.1 wt %) under exposure to laser diode at 785 nm, 2.55 W/cm2; thickness = 1.4 mm. The irradiation starts at t = 17 s.

based dyes and borate dyes, and the best performance was obtained when using IR-140 borate. The use of a phosphine and an iodonium salt has been proved as essential for the photopolymerization to occur. The system successfully polymerized with a 400 mW/cm2 irradiation, but the choice of the dye is crucial for the polymerization in this experimental condition. A larger range of dyes can be used for a tack-free polymerization in the NIR range; however, a higher power is required. Other NIR photoinitiating systems for longer wavelengths (>850 nm) will be proposed in forthcoming studies; such systems can be very useful for the access to composites by photopolymerization of filled samples. Currently, NIR dyes have found applications in several biological and medical fields. However, a careful investigation of the toxicology profile of these compounds will be interesting.

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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.8b00051. Figure S1: photolysis of IR-140 borate in different conditions; Figure S2: photolysis of the dye upon laser diode at 785 nm in the presence of phosphine; Scheme S1: chemical structures of other dyes which gave successful polymerization (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.L.). ORCID

B. H. Lessard: 0000-0002-9863-7039 J. Lalevée: 0000-0001-9297-0335 Notes

The authors declare no competing financial interest. J


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High Performance Near-Infrared (NIR) Photoinitiating Systems

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