Ï-Conjugated Dithienophosphole Derivatives as High Performance

Article Cite This: Macromolecules 2018, 51, 1811−1821

π‑Conjugated Dithienophosphole Derivatives as High Performance Photoinitiators for 3D Printing Resins Assi Al Mousawi,†,‡ Patxi Garra,† Xavier Sallenave,§ Frederic Dumur,*,∥ Joumana Toufaily,‡ Tayssir Hamieh,‡ Bernadette Graff,† Didier Gigmes,∥ Jean Pierre Fouassier,† and Jacques Lalevée*,† †

Institut de Science des Matériaux de Mulhouse, IS2M−UMR CNRS 7361−UHA, 15, rue Jean Starcky, Cedex 68057 Mulhouse, France ‡ Laboratoire de Matériaux, Catalyse, Environnement et Méthodes analytiques (MCEMA-CHAMSI), EDST, Université Libanaise, Campus Hariri, Hadath, Beyrouth, Liban § Laboratoire de Physicochimie des Polymères et des Interfaces LPPI, Université de Cergy-Pontoise, 5 mail Gay Lussac, Neuville-sur-Oise, Cedex 95031 Cergy-Pontoise, France ∥ Aix Marseille Univ, CNRS, ICR UMR 7273, F-13397 Marseille, France S Supporting Information *

ABSTRACT: Photopolymerization and 3D printing applications upon near-UV or visible light are currently limited to both rather low polymerization speed and thin layer by layer productions (below 100 μm) using photoinitiating systems (PIS) mainly inherited from the 1990s. Filling the need for new PIS, two π-conjugated dithienophosphole derivatives (DTPs) are synthesized and proposed as high performance near-UV and visible light photoinitiators/photoredox catalysts for both free radical polymerization (FRP) of (meth)acrylates and cationic polymerization (CP) of epoxides (e.g., using light-emitting diode (LED) at 405 nm). Astounding polymerization initiating abilities are found, and high final reactive function conversions are obtained (for multifunctional monomers). Their utilization as materials in laser write and 3D printing experiments is especially carried out with for the first time, about 2 mm 3D printed photopolymers in a one-layer approach. A full picture of the included photochemical mechanisms is additionally given. Originally, dithienophosphole derivatives are featured as metal-free photoinitiators/photoredox catalysts.

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materials.8 One critical issue about that technology is that the PIS used are inherited from the 1990s with UV-sensitive PIS leading to rather poor light penetration. Therefore, the accessible layer thicknesses are usually below 100 μm, leading to slow 3D printing times. That is why the development of high performance PIS (i) sensitive to mild light-emitted diodes (LED),9 (ii) sensitive to higher irradiation wavelengths (safer PIS),10−13 and (iii) for thicker photopolymers14 is still ongoing. Some groups proposed the concept of photoredox catalysis in order to use lower concentrations of PIS.15 It is based on the

INTRODUCTION

Nowadays, photopolymerization reactions have more and more industrial applications from the historical field of coatings to varnishes, paints, adhesives, dentistry, graphic arts, medicine, microelectronics, microlithography, 3D machining, optics, etc.1−7 Classically, photopolymerization reactions occur when photoinitiating systems (PIS) receive actinic lights leading to the formation of active species (radicals, cations, acids, etc.) that initiate the polymerization of the surrounding resins (e.g., radical or cationic polymerization). It is an environmentally friendly strategy as photopolymerizations can occur at room temperature under aerobic media with a full temporal and spatial control. More recently, this spatial control was used in 3D printing industrial applications in order to produce layer by layer 3D printed © 2018 American Chemical Society

Received: January 6, 2018 Revised: February 8, 2018 Published: February 21, 2018 1811

DOI: 10.1021/acs.macromol.8b00044 Macromolecules 2018, 51, 1811−1821

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(PI/Iod/amine) PISs to induce the formation of reactive species (radicals or cations) and, in some cases, ensure the catalyst regeneration for free radical polymerization (FRP) and/or cationic polymerization (CP) under LED@405 nm. For DTP compounds, the additional substituents introduced on the dithienophosphole core will affect the absorption as well as the photochemical/ electrochemical properties, leading to different photopolymerization efficiencies to be discussed and compared to previous metal-free reference PIS. The use of these new high performance photosensitive systems in laser write and 3D printing experiments is also provided breaking through the 500 μm range of 3D printed samples with about 2 mm well-resolved photopolymers (per layer).

electron exchange reactions between a photoredox catalyst (PC) and additives in a catalytic cycle. In such processes, the photoredox catalysts must be regenerated, ensuring a high photosensitivity of the system. When having a look at the current systems proposed, many of the proposed PCs are metal-based PIS using expensive and/or potentially toxic iridium, ruthenium, copper, or iron based complexes.15−20 As a result, the current main trend is to develop metal-free photoredox catalysts21−24 though these examples still exhibit slow kinetics, e.g., more than 60 min for the methylene blue/diphenyliodonium/amine system.22 To the best of our knowledge, the only relevant examples of metal-free photoredox catalysts exhibiting fast enough kinetics for 3D printing were carbazole-based PCs.25,26 Even for these last references, 3D printed maximum layers remained below the millimeter range (about 600 μm for the thickest 3D resolved sample). In order to fulfill the current need for highly efficient metalfree PIS, we propose in the present paper to use two luminescent dithienophosphole derivatives (DTPs; Scheme 1),

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EXPERIMENTAL PART

Chemical Compounds. Phenyl-N-tert-butylnitrone (PBN) and ethyl 4-(dimethylamino)benzoate (EDB) were obtained from SigmaAldrich (Scheme 2). Bis(4-tert-butylphenyl)iodonium hexafluorophosphate (Iod or Speedcure 938) and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO or speedcure BPO) were obtained from Lambson Ltd. Trimethylolpropane triacrylate (TMPTA) and (3,4-epoxycyclohexane)methyl 3,4- epoxycyclohexylcarboxylate (EPOX; Uvacure 1500) were obtained from Allnex and used as benchmark monomers for radical and cationic photopolymerization. Bisphenol A-glycidyl methacrylate (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) were obtained from Sigma-Aldrich and used with the highest purity available. C2 was synthesized as previously reported in the literature, without modification and in similar yield.25 The syntheses of the different compounds (Ph-DTP and TPA-DTP) are described in the Supporting Information. Irradiation Sources. The following light-emitting diode (LED) was used as irradiation sources: LED@405 nm incident light intensity at the sample surface: I0 ≈ 110 mW cm−2. Free Radical (FRP) and Cationic (CP) Photopolymerization. The two- and three-component photoinitiating systems (PISs) are mainly based on DTP derivative/Iod (0.5%/1% w/w) for both CP and FRP and DTP/Iod/EDB (0.5%/1%/1% w/w) for FRP. Much lower concentrations of DTPs up to (0.01% w/w) were also carried out. The weight percent of the photoinitiating system is calculated from the global monomer content. The photosensitive thin formulations (∼25 μm of thickness) were deposited on BaF2 pellets under air for the CP of EPOX while for the FRP of TMPTA and BisGMA/TEGDMA, it was

Scheme 1. Two Compounds (DTPs) (Noted Ph-DTP and TPA-DTP) Investigated in This Study

possessing high degree of π-conjugation, exhibiting long wavelength absorption especially in the near-UV or visible ranges thanks to appropriated substituents, and characterized by excellent photoluminescent properties. Remarkably, these compounds were highlighted previously for organic light-emitting diodes OLEDs applications,27 and to the best of our knowledge, the family of dithienophospholes has never been used as photoinitiators (PIs) or PCs. We propose to use them in the present work where they are here incorporated into two-component (PI/Iod or PI/amine) and sometimes three-component Scheme 2. Other Used Chemical Compounds

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samples of BisGMA/TEGDMA and TMPTA were also polymerized under air. Excellent solubility of all used compounds was observed in all the monomers utilized. The evolution of the epoxy group content and the double bond content of (meth)acrylate functions (for multifunctional monomers) were continuously followed by real-time FTIR spectroscopy (JASCO FTIR 4100) at about 790 and 1630 cm−1, respectively. The evolution of the methacrylate characteristic peak for the thick samples (1.4 mm) was followed in the near-infrared range at ∼6160 cm−1. The procedure used to monitor the photopolymerization profile has been described in detail in refs 13, 14, and 20. Redox Potentials. Dithienophosphole derivatives oxidation potentials (Eox vs SCE) were measured in acetonitrile by cyclic voltammetry with tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte. The free energy change ΔGet for an electron transfer reaction was calculated from eq 1,6 where Eox, Ered, E*, and C are the oxidation potential of the electron donor, the reduction potential of the electron acceptor, the excited state energy level, and the Coulombic term for the initially formed ion pair, respectively. C is neglected as usually done in polar solvents.

Figure 1. Absorption spectra of DTPs and C2 in acetonitrile and BAPO in DCM.

Table 1. Absorption Properties for DTPs vs C2 and BAPO absorption properties λmax (nm)/ε@λmax (M−1 cm−1) ε@λ = 405 nm (M−1 cm−1)

Ph-DTP

TPA-DTP

C2

420/∼12600

465/∼30000

374/∼11200

∼12000

∼12800

∼5200

ΔGet = Eox − Ered − E* + C

(1)

ESR Spin Trapping (ESR-ST) Experiments. The ESR-ST experiments were carried out using an X-band spectrometer (Bruker EMXPlus). LED@405 nm was used as irradiation source for triggering the production of radicals at room temperature (RT) under N2 saturated tert-butylbenzene and trapped by phenyl-N-tert-butylnitrone (PBN) according to a procedure described elsewhere in detail.13,14,20 The ESR spectra simulations were carried out with the PEST WINSIM program.

BAPO

∼500

done in laminate (the formulation is sandwiched between two polypropylene films to reduce the O2 inhibition). The 1.4 mm thick

Figure 2. (A) Contour plots of HOMOs and LUMOs for the DTPs structures optimized at the B3LYP/6-31G* level of theory. (B) Calculated UV−vis spectra for DTPs (MPW1PW91/6-31G* level of theory). 1813


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Macromolecules Absorption Experiments. The UV−vis absorption properties of the compounds were studied using a JASCO V730 spectrometer. Fluorescence Experiments. The fluorescence properties of the compounds were studied using a JASCO FP-6200 spectrometer. Computational Procedure. Molecular orbital calculations were carried out with the Gaussian 03 suite of programs.28,29 The electronic absorption spectra for the different compounds were calculated with the time-dependent density functional theory at the MPW1PW91/ 6-31G* level of theory on the relaxed geometries calculated at the UB3LYP/6-31G* level of theory. 3D Printing Experiments. For laser write and 3D printing experiments, a laser diode at 405 nm with an intensity of 100 mW cm−2 (spot size ∼50 μm) was used for the spatially controlled irradiation. The photosensitive resin (2 mm thickness) deposited onto a microscope slide was polymerized under air and the generated patterns analyzed by a numerical optical microscope (DSX-HRSU from OLYMPUS corporation) or by profilometry.

are characterized by high extinction coefficients in the near-UV but also the visible range (e.g., TPA-DTP ∼30 000 M−1 cm−1 at 465 nm). Remarkably, their absorptions are excellent in the 350−600 nm. An excellent overlap with the emission spectrum of the LED@405 nm used in this work is achieved (see in Table 1 the extinction coefficient at 405 nm). On the one hand, in comparison, TPA-DTP is characterized by highly favorable absorption properties over Ph-DTP (redshifted absorption and higher extinction coefficients in visible region: for TPA-DTP, ε@λmax(465 nm) = 30 000 M−1 cm−1; for Ph-DTP ε@λ max(420 nm) = 12 600 M−1 cm−1). In fact, TPA-DTP brings two additional diarylamine groups as substituents, causing a red-shift in the spectrum and giving rise to preferable absorption properties of TPA-DTP over Ph-DTP. On the other hand, DTPs (both TPA-DTP and Ph-DTP) are characterized by more favorable absorption properties in the near-UV and visible region than the previously used carbazole derivative C2 as metal-free PC and for the commercially used photoinitiator BAPO (see Table 1). The optimized geometries as well as the frontier orbitals [(highest occupied molecular orbital (HOMO) and lowest

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RESULTS AND DISCUSSION Light Absorption Properties of DTPs. The UV−vis absorption spectra of the new proposed photoinitiators in acetonitrile are reported in Figure 1 (see also Table 1). These compounds

Figure 3. (A) Polymerization profiles of EPOX (epoxy function conversion vs irradiation time) upon exposure to the LED@405 nm under air in the presence of different photoinitiating systems: (1) Ph-DTP/Iod (0.5%/1% w/w); (2) TPA-DTP/Iod (0.5%/1% w/w); (3) BAPO/Iod (0.5%/1% w/w). IR spectra recorded before and after polymerization using (B) TPA-DTP/Iod (0.5%/1% w/w) and (C) BAPO/Iod (0.5%/1% w/w). The irradiation starts at t = 10 s. 1814


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or DTP alone does not activate the polymerization. This shows that dithienophosphole derivatives are quite efficient in a photooxidation process (electron transfer from DTP to Iod). This efficient behavior will be discussed later in detail in the mechanistic part. Typical acrylate function conversion−time profiles are given in Figure 4A, and the FCs are summarized in Table 2. High FCs are reached in all DTP/Iod systems (e.g., TPA-DTP/ Iod, FC= 47% at t = 100 s; Figure 4A curve 2). Under air, FC drops down to 7% due to well-known oxygen inhibition.30 The initiating ability in FRP using LED@405 nm is rather better for TPA-DTP compared to Ph-DTP. This improved behavior of TPA-DTP can be partly ascribed to the better light absorption properties in the visible region (Table 1). a. Effect of Concentration of PI. The dithienophosphole derivatives are also quite efficient PIs for the FRP polymerization of thick acrylate films (1.4 mm) under air. Upon LED@ 405 nm irradiation, using (0.5% TPA-DTP/Iod w/w), a highly dark thick polymer was obtained under air with tack free surface but still tacky in its deep bulk due to light penetration issue (inner filter effect, i.e., light cannot penetrate anymore because it is filtered by the surface of the polymerized sample). To overcome this issue, it is found with experiments that as the concentration of photoinitiator decreases, the bleaching properties are changed (photo of curve 3 in Figure 4B), and less dark green color is obtained (photo of curve 2 in Figure 4B), allowing light to still penetrate during the polymerization process. Surprisingly, the best photoinitiating system for FRP thick samples (1.4 mm) was obtained with (0.01% TPA-DTP/ 1% Iod in TMPTA; FC ∼ 30% at t = 200 s; Figure 4B curve 2), which can be considered as a lowest threshold limit value of a photoinitiator concentration to initiate FRP. b. Effect of Amine in Photoredox Catalytic Behavior. In the same context, for a thick sample (1.4 mm) under air, the efficiency in terms of FC and Rp (rate of polymerization) increases when an amine (EDB) is incorporated into the photoinitiating system. Thus, TPA-DTP is incorporated herein into three-component photoinitiating system e.g. using TPA-DTP/ Iod/EDB (0.01%/1%/1% w/w) in TMPTA resin (Figure 4C) under air using LED@405 nm as irradiation source. Clearly, the EDB addition shows a huge influence on the polymerization

unoccupied molecular orbital (LUMO)] are shown in Figure 2A. The predicted spectra are in good agreement with the experimental ones (Figure 2B). Both the HOMO and LUMO are strongly delocalized all over the π system, clearly showing a π → π* lowest energy transition. For TPA-DTP, a higher charge transfer character for this electronic transition is found in agreement with the bathochromic shift observed, i.e., the triphenylamine moieties being strongly involved in the HOMO and the central dithienophosphole core being involved in the LUMO. The predictions of absorption wavelengths but also the active centers from the frontier molecular orbitals (HOMO and LUMO) are important for further development of DTP derivatives. Cationic Photopolymerization (CP) of Epoxides. Upon irradiation with the LED@405 nm, the CP of epoxides (e.g., EPOX) under air using two-component photoinitiating systems based on DTP/Iod combinations (0.5%/1% w/w) exhibits a very high efficiency in terms of final epoxy function conversion (FC) (e.g., FC 53% with TPA-DTP; Figure 3A, curve 2, and Table 2). A new peak ascribed to the polyether network arises at ∼1080 cm−1 (Figures 3B) in the FTIR spectra. Iod alone does not activate the polymerization (FC ∼ 0% using Iod alone), showing the role of DTP for the sensitization of the iodonium salt decomposition upon near-UV to visible light LEDs. Remarkably, very high rates of polymerization (Rp) were clearly achieved with the proposed DTP/Iod systems compared to the BAPO/Iod system set as a reference for which a very poor polymerization occurs (Figure 3A, curve 3); the polyether peak is only slightly observed for BAPO/Iod in Figure 3C. The efficiency trend for CP using LED@405 nm follows the order TPA-DTP > Ph-DTP ≫ BAPO. Obviously, this is directly related to the absorption properties of the DTP as TPA-DTP is the leader in the efficiency trend and has the best extinction coefficients at 405 nm compared to the others (see Table 1). The structure/reactivity relationship will be discussed below. Free Radical Photopolymerization (FRP). Photopolymerization of Acrylates (TMPTA). The FRP of TMPTA in thin films (25 μm), in laminate, in the presence of the DTP/Iod couples is quite efficient using LED@405 nm, while Iod alone

Table 2. Functional Group Conversions (FC): Epoxy for EPOX, Acrylate for TMPTA, and Methacrylate for BisGMA/ TEGDMA Using Different Photoinitiating Systems; LED@405 nm for Irradiation resin/PIS EPOX

TMPTA

bisGMA/TEGDMA

conditionsa 1 2a 2b 2c 2d 2e 2f 3a 3b 3c

Ph-DTP (%)

TPA-DTP (%)

C2 (%)

BAPO (%)

41 41

53 47 5 17 30 0 80 54 0 63

50 56

15

0 0

43

a

Condition 1: epoxy function conversion FC (%) for EPOX (at t = 800 s under air) (0.5% PI/1% Iod w/w); 2a: acrylate function conversion FC for TMPTA (%) (at t = 100 s in laminate) (0.5% PI/1% Iod w/w ; thickness = 25 μm); 2b: FC for TMPTA (%) (at t = 100 s under air) (0.5% PI/1% Iod w/w; thickness = 1.4 mm); 2c: FC for TMPTA (%) (at t = 100 s under air) (0.1% PI/1% Iod w/w; thickness = 1.4 mm); 2d: FC for TMPTA (%) (at t = 100 s under air) (0.01% PI/1% Iod w/w; thickness = 1.4 mm); 2e: FC for TMPTA (%) (at t = 100 s under air) (0.01% PI/1% EDB w/w; thickness = 1.4 mm); 2f: FC for TMPTA (%) (at t = 100 s under air) (0.01% PI//1% Iod/1% EDB w/w; thickness = 1.4 mm); 3a: methacrylate conversion FC for BisGMA/TEGDMA (%) (at t = 200 s) (0.01% PI/1% Iod w/w under air; thickness = 1.4 mm); 3b: FC for BisGMA/TEGDMA (%) (at t = 200 s) (0.01% PI/1% EDB w/w under air; thickness = 1.4 mm); 3c: FC for BisGMA/TEGDMA (%) (at t = 200 s) (0.01% PI/1% Iod/1% EDB w/w under air; thickness = 1.4 mm). 1815


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Figure 4. (A) Polymerization profiles of TMPTA (acrylate function conversion vs irradiation time) in laminate (thickness = 25 μm) upon exposure to LED@405 nm in the presence of (1) Ph-DTP/Iod (0.5%/1% w/w) and (2) TPA-DTP/Iod (0.5%/1% w/w). (B) Polymerization profiles of TMPTA (acrylate function conversion vs irradiation time) under air (thickness = 1.4 mm) upon exposure to LED@405 nm in the presence of various concentrations of TPA-DTP: (1) TPA-DTP/Iod (0.1%/1% w/w); (2) TPA-DTP/Iod (0.01%/1% w/w); (3) TPA-DTP/Iod (0.5%/1% w/w). (C) Polymerization profiles of TMPTA (acrylate function conversion vs irradiation time) under air (thickness = 1.4 mm) upon exposure to LED@ 405 nm in the presence of (1) TPA-DTP/Iod (0.01%/1% w/w), (2) TPA-DTP/Iod/EDB (0.01%/1%/1% w/w), and (3) TPA-DTP/EDB (0.01%/ 1% w/w). (D) Polymerization profiles of BisGMA/TEGDMA (methacrylate function conversion vs irradiation time) under air (1.4 mm thick sample) in the presence of (1) TPA-DTP/Iod (0.01%/1% w/w), (2) TPA-DTP/Iod/EDB (0.01%/1%/1% w/w), and (3) TPA-DTP/EDB (0.01%/ 1% w/w). The irradiation starts at t = 10 s.

The addition of an amine (EDB) as an electron donor leads to an increase of the performance, i.e., FC up to 65% is obtained with TPA-DTP/Iod/EDB (0.01%/1%/1%) instead of only 53% with the TPA-DTP/Iod (0.01%/1% w/w) (using LED@ 405 nm; Figure 4D, curve 2 vs curve 1; thickness = 1.4 mm; under air). This 65% conversion (with only 0.01% of TPA-DTP with 1% Iod in two-component PIS) outranks the current fourcomponent references such as commercial PIS implemented with additives (50% with a four-component system camphorquinone (CQ)/Am3/Iod1/TPP under a 80 mW/cm2 LED@470 nm irradiation,30 same setup and resin) though it was necessary to use 1.2 wt % CQ in the latter four component PIS. Next, the behavior already observed with acrylates above holds true; i.e., TPADTP/EDB (0.01%/1% w/w) is not efficient for FRP using LED@ 405 nm (Figure 4D, curve 3; thickness = 1.4 mm; under air). Laser Write Experiments Using TPA-DTP/Iod System. Laser write 3D polymerization experiments were carried out to generate printing of 3D patterns easily. In Figure 5, some 3D printing examples are shown using the TPA-DTP/Iod

profile of FRP: FC increases to 80% using three-component PIS TPA-DTP/Iod/EDB (0.01%/1%/1% w/w) (Figure 4C, curve 2) compared to FC ∼ 30% using the two-component TPA-DTP/Iod (0.01%/1% w/w) PIS (Figure 4C, curve 1; Table 2); the two-component system TPA-DTP/EDB (0.01%/ 1% w/w) shows no efficiency in FRP (Figure 4C, curve 3). Consequently, a photoredox catalytic behavior can be claimed, and the EDB role is to regenerate TPA-DTP through a catalytic cycle (this will be discussed later). Compared to the previous carbazole derivative (C2) used as PC, the new dithienophosphole derivatives are clearly much better to initiate the FRP of acrylates as very low amount of PC (∼0.01% w/w) can be used. Photopolymerization of Methacrylates. Interestingly, the TPA-DTP/Iod (0.01%/1% w/w) couple efficiently initiates the FRP of a blend of methacrylates (BisGMA/TEGDMA 70%/ 30% w/w) under air (1.4 mm thick sample) (Figure 4D) upon irradiation with LED@405 nm. Remarkably, tack free polymer is obtained for thick samples under air. 1816


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Figure 5. Free radical photopolymerization experiments for laser write (A) “CNRS” logo. (B) Characterization in 3D by profilometry of the thick sample (∼2 mm).

(0.01%/1% w/w) system, which is very reactive in the radical polymerization of methacrylates (see above) under air. Remarkably, the high photosensitivity of this resin allows an efficient polymerization process in the irradiated area. Thick samples up to ∼2 mm were obtained with high spatial resolution (Figure S1) and for short printing time ( 550 nm) is formed due to the TPA-DTP/Iod interaction. Three clear isosbestic points are shown, suggesting no by-side reactions. The above results show a high tendency of DTPs to react with Iod salt. However, another photolysis experiment is carried

out for three-component mixture (TPA-DTP/Iod/EDB). The results show slower photolysis kinetics of TPA-DTP/Iod/EDB compared to TPA-DTP/Iod (Figure 6D). This slower consumption of TPA-DTP in the three-component system might be a significance of partial photoredox catalytic cycle allowing the regeneration of TPA-DTP. Fluorescence Quenching, Laser Flash Photolysis (LFP), Cyclic Voltammetry, and ESR Experiments. 1. DTPs/Iod Interaction. The crossing point of the absorption and fluorescence spectra allows the determination of the first singlet excited state energies (ES1) (e.g., TPA-DTP shown in Figure 7A; Table 3). Fluorescence experiments on DTPs in acetonitrile are carried out. Fast fluorescence quenching processes of DTPs by Iod is noted (high value of the Ksv Stern−Volmer coefficient of TPADTP/Iod interaction ∼92.9 M−1 vs Ph-DTP/Iod interaction ∼66.2 M−1; Table 3). This is in full agreement with photopolymerization kinetics results, which show better performance of TPA-DTP over Ph-DTP. In fact, as in other related systems, the DTP/Iod interaction corresponds to an electron transfer reaction finally leading to an aryl radical Ar• (r1 and r2). Ar• and DTP•+ can be considered as the initiating species for 1817


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Figure 6. (A) TPA-DTP/Iod photolysis upon exposure to LED@405 nm and (B) photolysis of TPA/EDB. (C) Photolysis of TPA in the absence ofIod or EDB. (D) Photolysis kinetic comparison of TPA-DTP, TPA-DTP/Iod, TPA-DTP/EDB, and TPA-DTP/Iod/EDB solutions at chosen wavelength 465 nm characterizing TPA-DTP.

The photo-oxidation process corresponding to the DTP/Iod interaction is highly favorable with the superiority of 1TPA-DTP/ Iod interaction compared to that of 1Ph-DTP/Iod (ΔGetS1 = −1.35 eV for TPA-DTP vs ΔGet(S1) = −1.26 eV for Ph-DTP/ Iod; Table 3). In addition, the electron transfer quantum yields from the excited singlet state ϕet(S1) were calculated (according to eq 2; with higher ϕet(S1) ∼ 0.63 of TPA-DTP/Iod compared to Ph-DTP/Iod ∼0.55; Table 3). These results are in agreement with photopolymerization kinetics results, which show better performance of TPA-DTP over Ph-DTP. From these high values of electron transfer quantum yields from S1, the triplet state pathway can be probably neglected.

the radical polymerization and the cationic polymerization, respectively. DTP → 1,3DTP (hv) 1,3

(r1)

DTP + Ar2I+ → DTP•+ + Ar2I• → DTP•+ + Ar• + PhI (r2) •

The presence of Ar (Figure 8B) and the probable detection of DTP•+ radical cation (Figure 8A) are confirmed by ESR results. Indeed, the phenyl radicals (Ar•) were easily detected as PBN/Ar• radical adducts in the irradiated DTP/Iod system in ESR-ST experiments (e.g., TPA-DTP/Iod in Figure 8B). Indeed, the simulation of the experimental ESR spectrum yields the hyperfine coupling constants (hfc’s): aN = 14.1 G and aH = 2.0 G typical for the PBN/Ar• radical adducts.16,25,26 The free energy changes ΔGet for r2 arising from S1 were calculated from the classical equation (eq 1); herein, the oxidation potentials of DTPs were determined by cyclic voltammetry CV (e.g., TPA-DTP in Figure 7B). From CV, it is clear that DTPs shows high reversibility. This reversibility allows DTP compounds to behave as photoredox catalysts, i.e., the oxidized form (DTP•+) is stable and can be reduced in the presence of amine (EDB) to regenerate DTP.

ϕet(S1) = K sv[Iod]/(1 + K sv[Iod])

(2)

2. DTPs/EDB Interaction. The DTPs/EDB interaction corresponds to an electron transfer reaction between EDB and DTP followed by formation of EDB(−H•) (r3) which must be capable of initiating FRP in DTP/EDB photoinitiating system. This interaction is proved by fluorescence quenching experiments. However, polymerization kinetic studies show no ability of TPADTP/EDB system to initiate FRP contrasted to the TPA-DTP/ Iod system. For this reason, a comparative fluorescence quenching study of both systems (TPA-DTP/Iod and TPA-DTP/EDB) 1818


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Figure 7. (A) Singlet state energy determination. (B) Cyclic voltammetry for the TPA-DTP oxidation. (C) Stern−Volmer treatment of the fluorescence quenching of TPA-DTP in the presence of Iod and EDB.

its very low ability to interact through photoreductive process (TPA-DTP/EDB).

Table 3. Parameters Characterizing the Chemical Mechanisms Associated with the DTP/Iod or EDB Systems in Acetonitrile Eoxa [V] ES1 [eV] TPA-DTP/Iod Ph-DTP/Iod TPA-DTP/EDB a

0.8 1.1

2.35 2.64

ΔGet(S1)b

Ksv (M−1)

Φet(S1)

−1.35 −1.26

92.9 66.2 6.5

0.63 0.55 0.25

[eV]

Oxidation potentials for TPA-DTP and Ph-DTP. potential of −0.2 V6 is used for Iod in eq 1.

b

DTP + EDB ↔ DTP•− + EDB•+

(r3)

ϕet(S1) = K sv[EDB]/(1 + K sv[EDB])

(3)

1,3

3. Three-Component DTPs/EDB/Iod System. Actually, when adding EDB in three-component DTP/Iod/EDB PIS, regeneration of TPA-DTP could take place in the photo-oxidative cycle (Scheme 3): DTP•+ can be reduced by EDB as the reduction potential of DTP•+ (1.1 V for Ph-DTP•+ and 0.8 V for TPA-DTP•+ in Table 3) is quite close to the oxidation potential of EDB (1.0 V6). As seen above, no polymerization was observed with DTP/amine, excluding the hypothesis of TPA-DTP to react in photoreduction process and confirming its behavior as partial photoredox catalyst in an oxidative cycle. Therefore, the regeneration of DTP ensures an improved reactivity (Scheme 3). This is in full agreement with the experimental results which show that the performance of the DTP/Iod/EDB three-component PIS is better than that of DTP/Iod.

Reduction

is carried out (Figure 7C). According to results, rather high values of the Stern−Volmer coefficients (Ksv) for 1TPA-DTP/ Iod interaction vs 1TPA-DTP/EDB (92.9 M−1 vs 6.5 M−1, respectively; Table 3) and thus higher electron transfer quantum yields with Iod (∼0.63) compared to that with EDB (∼0.25 according to eq 3) (Table 3). In fact, with no photolysis observed with EDB (Figure 6B), and with the value of ∼0.25 of electron transfer quantum yield, we could claim the possibility of back electron transfer occurrence in the TPADTP/EDB system. These different elements confirm our polymerization results and prove the higher ability of DTP compounds to react through photo-oxidative process (TPA-DTP/Iod) and

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CONCLUSION In the present paper, two DTP derivatives are proposed as high performance photoinitiators for 3D printing upon near-UV 1819


Article

Macromolecules

Figure 8. ESR-ST spectra obtained upon irradiation (LED@405 nm) of a TPA-DTP/Iod/PBN solution in tert-butylbenzene as a solvent: (A) just after irradiation corresponding to the formation of radical cation (DTP•+); (B) after irradiation corresponding to aryl radical detection: experimental (lower spectrum) and simulated (upper spectrum).

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Scheme 3. DTP Photoredox Catalytic Cycle

AUTHOR INFORMATION

Corresponding Authors

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

Jacques Lalevée: 0000-0001-9297-0335 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the “Agence Nationale de la Recherche” (ANR) for the grant “FastPrinting”. The Lebanese group thanks “The Association of Specialization and Scientific Guidance” (Beirut, Lebanon) for funding and supporting this scientific work.

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or visible light (e.g., LED@405 nm). In the presence of iodonium salt and amine, an oxidative catalytic cycle occurs leading to a partial regeneration of DTP. Remarkably, these new photoinitiators can be used for both radical and cationic polymerizations. The access to both high polymerization rate and thick materials appears as possible with these structures. The development of other photosensitive resins for 3D printing of composites (in the presence of high content of fillers) is under investigation. The development of specific photoinitiators (as DTP here) will be important to improve the depth of cure in composites.

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REFERENCES

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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.8b00044. Scheme S1: synthesis of Ph-DTP and TPA-DTP molecules; Figure S1: step dimensions of the 3D printing by profilometry of thick sample (1800 μm) by using a numerical optical microscope (PDF) 1820


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