Photoredox Catalysts

Article pubs.acs.org/Macromolecules

Carbazole Scaffold Based Photoinitiator/Photoredox Catalysts: Toward New High Performance Photoinitiating Systems and Application in LED Projector 3D Printing Resins Assi Al Mousawi,†,‡ Frederic Dumur,*,§ Patxi Garra,† Joumana Toufaily,‡ Tayssir Hamieh,‡ Bernadette Graff,† Didier Gigmes,§ Jean Pierre Fouassier,† and Jacques Lalevée*,† †

IS2M−UMR CNRS 7361−UHA, Institut de Science des Matériaux de Mulhouse, 15, rue Jean Starcky, 68057 Mulhouse, Cedex, France ‡ Laboratoire de Matériaux, Catalyse, Environnement et Méthodes analytiques (MCEMA-CHAMSI), EDST, Université Libanaise, Campus Hariri, Hadath, Beyrouth, Liban § Aix Marseille Univ, CNRS, ICR, UMR 7273, F-13397 Marseille France S Supporting Information *

ABSTRACT: Four new carbazole derivatives (C1−C4) are synthesized and proposed as high performance visible light photoinitiators/photoredox catalysts for both the free radical polymerization (FRP) of (meth)acrylates and the cationic polymerization (CP) of epoxides upon visible light exposure using light-emitting diodes (LEDs) at 405, 455, and 477 nm. Excellent polymerization initiating abilities are found, and high final reactive function conversions are obtained. Interestingly, these new derivatives exhibit much better visible light polymerization initiating ability compared to a reference UV-absorbing carbazole (CARET, 9H-carbazole-9-ethanol) showing that the new substituents are of great interest to red-shift the absorption of the proposed photoinitiators. More remarkably, in combination with an iodonium salt, C1−C4 are also better than the well-known bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) photoinitiator for mild irradiation conditions highlighting their outstanding reactivity. Their use in new cationic resins for LED projector 3D printing is particularly outlined. A full picture of the involved photochemical mechanisms is provided. Originally, these carbazoles behave as metal-free photoredox catalysts active in both oxidative and reductive cycles.

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INTRODUCTION The development of polymerization processes triggered by light is the subject of intense research efforts.1−4 A lot of applications

photosensitive systems are often associated with the use of noxious UV light (e.g., from Hg lamp). Also, UV light is characterized with a particularly low light penetration13 which is problematic for the photopolymerization of different samples (thick, filled, pigmented, or dispersed). Therefore, the development of photosensitive systems upon longer and safer wavelengths was simulated13,14 and experimentally tested.15−20 There is also a huge challenge for the use of less harmful (i.e., longer) wavelengths. This is particularly supported by the development of visible light-emitting diode (LED) technology21−24 which is characterized by clear advantages compared to UV lamps or UV lasers (better light penetration but also lower energy consumption). Altogether, there is a need for new versatile photoinitiators or photoredox catalysts (PCs) capable to operate in the visible region upon LED irradiation and to initiate fast photopolymerization (radical, anionic, or cationic). 3D printing is an example of photopolymerization application which is particularly looking for very efficient photoinitiating systems under mild conditions25,26(LED irradiation,

Scheme 1. Different Carbazole Derivatives (Cs) (Noted C1−C4) Investigated in This Study

are related to radical photopolymerization1−5 but also cationic and anionic photopolymerization6,7 and even controlled radical (photo)polymerization.8−12 Indeed, light-activated processes are characterized by important advantages compared to the traditional thermal activation; i.e., fast reactions occur even at low temperature (e.g., room temperature) and with a possible spatial resolution (the polymerization proceeds only in the light irradiated area). It can also be described as a green technology with a low energy consumption and a strong decrease in volatile organic compounds (VOCs) emissions. However, many © XXXX American Chemical Society

Received: January 27, 2017 Revised: March 6, 2017

A

DOI: 10.1021/acs.macromol.7b00210 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 2. Other Used Chemical Compounds

(CARET)34) but not as photoinitiators or photoredox catalysts. Very recently, another carbazole derivative (1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene) was elegantly proposed as a photoredox catalyst for atom transfer radical polymerization (ATRP).35 In the present paper, four new carbazole derivatives (Cs) exhibiting absorptions at longer wavelength than the aforementioned carbazole derivatives thanks to well selected substituents are synthesized (C1−C4 in Scheme 1); 9Hcarbazole-9-ethanol CARET used previously in our papers34,36 is set here as a reference carbazole derivative. They are incorporated into two-component (PI/iodonium salt Iod or PI/ amine) and sometimes into three-component (PI/Iod/amine) photoinitiating systems PISs to induce the formation of reactive species (radicals or radical cations) for free radical polymerization (FRP) and/or cationic polymerization (CP) upon nearUV or visible light. The different substituents introduced on the carbazole scaffold will affect nonetheless the absorption but also the photochemical/electrochemical properties of C1−C4. This will pave the way to study the structure/reactivity/efficiency relationships of the studied carbazole derivatives as photoinitiators or photoredox catalysts in CP and FRP. The use of these new high performance photosensitive systems in LED projector 3D printing experiments is also provided.

room temperature, under air). For example, cheap 3D printers based on LED technology27 (e.g., LED projector) were recently introduced in the market. This technology requires the development of highly active photoinitiating systems (PIS) developed for visible LED wavelengths and intensities. In order to extend the type of materials produced, these visible light PISs should be able to access to thick or filled samples that are not currently polymerized with the UV technology. In parallel, the cationic polymerization is particularly interesting as it is characterized by very low shrinkage compared to radical polymerization. Nevertheless, its application for 3D printings remains limited as many cationic PIS are slower than radical PIS.6,7 In this context, novel photosensitive systems were proposed; among them, photoinitiators exhibiting a photoredox catalyst behavior (the photoinitiator is now called a photoredox catalyst PC). Indeed, the regeneration of PC ensures outstanding performances.28,29 These recent metal-based PIS were found to be suitable for application in 3D printing29 but also to avoid the use of metals (potential toxicity, bioaccumulation, low storage stability, dependence to the market costs of the metals, etc.); there is thus a need for new metal-free photoredox catalysts. In previous works, different carbazole derivatives were proposed as additives in cationic polymerization under visible light (e.g., N-vinylcarbazole (NVK)30−33 or 9H-carbazole-9-ethanol

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EXPERIMENTAL PART

Chemical Compounds. 9H-Carbazole-9-ethanol (CARET), phenyl-N-tert-butylnitrone (PBN), and ethyl 4-(dimethylamino)benzoate (EDB) were obtained from Sigma-Aldrich (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. 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

Table 1. Absorption Properties for C1−C4 vs CARET and BAPOa

C1 C2 C3 C4 CARET BAPO

Figure 1. Absorption spectra of the carbazole derivatives in acetonitrile, CARET in methanol, and BAPO in DCM.

a

B

λmax (nm)/ε (M−1 cm−1)

ε at 405 nm (M−1 cm−1)

ε at 455 nm (M−1 cm−1)

ε at 470 nm (M−1 cm−1)

364/∼11750 374/∼11180 364/∼14000 388/∼6000 343/∼4100 370/∼950

∼2600 ∼5200 ∼2450 ∼5200 n.a. ∼500

∼160 ∼100 ∼50 ∼1190 n.a. n.a.

∼100 ∼50 ∼50 ∼500 n.a. n.a.

n.a.: no absorption of light. DOI: 10.1021/acs.macromol.7b00210 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 2. Contour plots of HOMOs and LUMOs for the Cs structures optimized at the B3LYP/6-31G* level of theory. 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.

photopolymerization. Bisphenol A−glycidyl methacrylate (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) were obtained from Sigma-Aldrich and used with the highest purity available. For the synthesis of C1−C4, the procedures are fully presented in the Supporting Information as well as their characterization NMR spectra. Irradiation Sources. The following Light-Emitting Diodes (LEDs) were used as irradiation sources: (i) LED at 375 nm; incident light intensity at the sample surface: I0 ≈ 40 mW cm−2; (ii) LED at 405 nm (I0 ≈ 110 mW cm−2); (iii) LED at 455 nm (I0 ≈ 80 mW cm−2); (iv) LED at 477 nm (I0 ≈ 300 mW cm−2). Free Radical (FRP) and Cationic (CP) Photopolymerization. The two-component photoinitiating systems (PISs) are mainly based on Cs/iodonium salt (0.5%/1% w/w) for both CP and FRP. The weight percent of the photoinitiating system is calculated from the 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 done in laminate (the formulation is sandwiched between two polypropylene films to reduce the O2 inhibition). The 1.4 mm thick samples of BisGMA/TEGDMA were polymerized under air. An excellent solubility was observed in EPOX monomer for all the carbazole derivatives examined in this study. The evolution of the epoxy group content and the double bond content of (meth)acrylate functions was 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 is described in detail in refs 37 and 38. Redox Potentials. The oxidation or the reduction potentials (Eox or Ered vs SCE) for the different carbazole derivatives were measured in acetonitrile by cyclic voltammetry using tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte. The free energy change ΔGet for an electron transfer reaction was calculated from the Rehm−Weller equation (eq 1)39 where Eox, Ered, E*, and C

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 EMX-Plus). LED at 405 nm was used as irradiation source for triggering the production of radicals at room temperature (RT) for N2-saturated toluene solutions and trapped by phenyl-N-tert-butylnitrone (PBN) according to a procedure described by us in detail.40 The ESR spectra simulations were carried out with the PEST WINSIM program. Absorption Experiments. The UV−vis absorption properties of the compounds were studied using a JASCO V730 spectrophotometer. Fluorescence Experiments. The fluorescence properties of the compounds were studied using a JASCO FP-6200 fluorometer. Computational Procedure. Molecular orbital calculations were carried out with the Gaussian 03 suite of programs.41,42 The electronic absorption spectra for the different compounds were calculated with the time-dependent density functional theory at the MPW1PW91/ 6-31g(d) level of theory on the relaxed geometries calculated at the UB3LYP/6-31G* level of theory. 3D Printing Experiments. For 3D printing experiments, a LED projector at 405 nm (Thorlabs) was used. The photosensitive cationic resin was polymerized under air and the generated patterns analyzed by a numerical optical microscope (DSX-HRSU from OLYMPUS corporation) or by profilometry. The procedure is presented in refs 27 and 29. Laser Flash Photolysis (LFP). Nanosecond laser flash photolysis (LFP) experiments were carried out using Luzchem LFP 212 spectrometer. For the excitation, a Q-switched nanosecond Nd/YAG laser (λexc = 355 nm, ∼6−8 ns pulses; an energy reduced to 10 mJ) from Continuum (Minilite) was used.40 C


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Figure 3. (A) Polymerization profiles of EPOX (epoxy function conversion vs irradiation time) upon exposure to the LED at 405 nm under air in the presence of different photoinitiating systems: (1) C1/Iod (0.5%/1% w/w); (2) C2/Iod (0.5%/1% w/w); (3) C3/Iod (0.5%/1% w/w); (4) C4/Iod (0.5%/1% w/w); (5) CARET/Iod (1%/1% w/w); (6) BAPO/Iod (0.5%/1% w/w). IR spectra recorded before and after polymerization using (B) C1/Iod (0.5%/1% w/w), (C) CARET/Iod (1%/1% w/w). The irradiation starts at t = 10 s.

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RESULTS AND DISCUSSION

group is strongly involved in the LUMO showing this charge transfer (HOMO−LUMO) transition. The clear red-shift caused by the different substituents in C1−C4 is perfectly predicted in the theoretical calculations; i.e., the bathochromic shift follows the following order CARET < C1 ∼ C3 < C2 ≪ C4 in both cases (experimental and theoretical; see Figures S1 and S2 in the Supporting Information). For the reference carbazole derivative (CARET), without any substituent on the phenyl rings, an absorption at shorter wavelengths is found as well as lower extinction coefficients (e.g., λmax = 343 nm for CARET vs 364 nm for C1 and 388 nm for C4; see Table 1). Conversely, the contribution of the aldehyde, the nitro, or the amine substituent of the carbazole in C1−C4 to the electronic delocalization modifies the energies of the HOMO/LUMO levels, resulting in absorptions at longer wavelength. Remarkably, compared to a well-known photoinitiator (BAPO), the new proposed structures C1−C4 are characterized by much better light absorption properties in all the 350−500 nm range (Figure 1). Cationic Photopolymerization (CP) of Epoxides. Upon irradiation with a LED at 405 nm, the CP of epoxides (e.g., EPOX) under air using two-component photoinitiating systems based on C/Iod combinations (0.5%/1% w/w) exhibits a very high efficiency in terms of final epoxy function conversion (FC) (e.g., FC 76% with C1; Figure 3A, curve 1, Table 2).

Light Absorption Properties of C1−C4. The UV−vis absorption spectra of the new proposed photoinitiators in acetonitrile are reported in the Figure 1 (see also Table 1). These compounds are characterized by high extinction coefficients in the near-UV but also in the visible range (e.g., C2 ∼ 11 100 M−1 cm−1 at 375 nm and C2 ∼ 5100 M−1 cm−1 at 405 nm). Remarkably, their absorptions are excellent in the 350−450 nm spectral range ensuring a good overlap with the emission spectra of the LEDs used in this work (e.g., at 375, 405, 455, and 477 nm). The optimized geometries as well as the frontier orbitals (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) are shown in Figure 2. Both the HOMO and LUMO are strongly delocalized all over the π system clearly showing a π → π* lowest energy transition. The presence of various substituents on the carbazole moiety causes a clear shift in the absorption spectrum; more precisely for C4, the presence of a secondary amine group N(C8H17)2 as an electron donating group and a nitro group NO2 as an electron accepting group gives rise to a push−pull effect, resulting in a red-shift as well as in a clear broadening of the peaks in the visible region. For C4, the participation of the amine group to the HOMO is clearly noted whereas the nitro D


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49 C4

58%; 50% [C3/Iod(1%/ 1% w/w)] 70

C1 C2

C3

43

49

49

57

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, clearly showing the crucial role of C1−C4 for the sensitization of the iodonium salt decomposition upon near-UV to visible light LEDs exposure. Remarkably, very high rates of polymerization (Rp) were clearly achieved with the C1−C4/Iod systems compared to the CARET/Iod system set as a reference for which almost no polymerization occurs. The comparison of C1−C4 with CARET is relevant as both compounds are characterized by the presence of a carbazole moiety. Also, remarkably, in comparison to the well-known BAPO/Iod1 (0.5%/1% w/w) system, C1−C4 are found to be much better photoinitiators than BAPO (Figure 3A, curve 6 for BAPO compared to curve 1 for C1). These data show the extreme superiority of the new carbazoles in terms of efficiency over the well-known BAPO (i.e., the polyether peaks are not observed neither for CARET/ Iod, e.g., in Figure 3C, nor for BAPO/Iod compared to C1/Iod system as e.g. in Figure 3B). The efficiency trend for the CP using LED at 405 nm follows the order: C1 > C4 > C3 > C2 ≫ BAPO > CARET. Obviously, this is not directly related to the absorption properties of the carbazole derivatives as C1 is the leader in the efficiency trend but exhibits a lower extinction coefficient at 405 nm compared to the others (see Table 1). The structure/reactivity relationship will be discussed below. The performance of the new proposed systems increases when decreasing the photoinitiator concentration (e.g., FC ∼ 50% with 1% C3 vs FC ∼ 58% with 0.5% C3; at t = 800 s; LED at 405 nm; Table 2). This result is probably the consequence of an internal filter effect;1,2 i.e., upon increasing the PI concentration, the penetration of the light decreases. In the same context, the performance of the system also decreases when using a higher wavelength (LED at 455 nm instead of LED at 405 nm) for the same C3/Iod (1%/1% w/w) system: FC ∼ 50% vs 43%, respectively (Figure 4, curve 2 vs curve 1, Table 2). This is also ascribed to the higher light intensity of the LED at 405 nm vs LED at 455 nm (see the Experimental Part). Free Radical Photopolymerization. Photopolymerization of Acrylates (TMPTA). The FRP of TMPTA in thin films

50

thick sample (1.4 mm): 65% LED at 455 nm (at t= 300 s); 44% LED at 477 nm (at t = 100 s) thin sample (25 μm): 49%; 43% (at t = 100 s) LED at 405 nm; LED at 455 nm, respectively thick sample (1.4 mm): 43% (at t = 300 s) LED at 455 nm; 18% LED at 477 nm (at t = 100 s) 57

C/Iod/EDB (0.5%/1%/1% w/w)

methacrylate conversion FC for BisGMA/TEGDMA (%)

C/Iod (0.5%/1% w/w) LED at 405 nm C/Iod/EDB (0.5%/1%/1.5% w/w)

LED at 405 nm C/EDB (0.5%/1% w/w) 50 50 LED at 405 nm C/Iod (0.5%/1% w/w) 46 56 LED at 455 nm C/Iod(1%/1% w/w) LED at 405 nm C/Iod(0.5%/1% w/w) 76 50

acrylate conversion FC for TMPTA (%) (at t = 100 s)

Figure 4. Polymerization profiles of EPOX (epoxy function conversion vs irradiation time) under air in the presence of the C3/Iod (1%/1% w/w) two component photoinitiating system upon exposure to different LEDs: (1) at 405 nm; (2) at 455 nm. The irradiation starts at t = 10 s.

LED/PIS

epoxy function conversion FC (%) for EPOX (at t = 800 s)

Table 2. Final Reactive Function Conversions (FC): Epoxy for EPOX, Acrylate for TMPTA, and Methacrylate for BisGMA/TEGDMA Using Different Photoinitiating Systems; Different LEDs for Irradiation

Macromolecules

E


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Figure 5. (A) Polymerization profiles of TMPTA (acrylate function conversion vs irradiation time) in laminate upon exposure to LED at 405 nm in the presence of (1) C1/Iod (0.5%/1% w/w); (2) C2/Iod (0.5%/1% w/w); (3) C3/Iod (0.5%/1% w/w); and (4) C4/Iod (0.5%/1% w/w). (B) Polymerization profiles of TMPTA (acrylate function conversion vs irradiation time) in laminate upon exposure to LED at 405 nm in the presence of (1) C1/EDB (0.5%/1% w/w); (2) C2/EDB (0.5%/1% w/w); (3) C3/EDB (0.5%/1% w/w); and (4) C4/EDB (0.5%/1% w/w). (C) Polymerization profiles of BisGMA/TEGDMA (methacrylate function conversion vs irradiation time) in the presence of C2/Iod (0.5%/1% w/w) laminate (25 μm thin sample) upon exposure to (1) LED at 405 nm and (2) LED at 455 nm. (D) Polymerization profiles of BisGMA/TEGDMA (methacrylate function conversion vs irradiation time) under air (1.4 mm thick sample) in the presence of two-component photoinitiating system: C2/Iod (0.5%/1% w/w) upon exposure to (1) LED@455 nm; (3) LED@477 nm; and in the presence of three-component photoinitiating systems: C2/Iod/EDB (0.5%/1%/1% w/w) upon exposure to (2) LED at 455 nm and (4) LED at 477 nm. The irradiation starts at t = 10 s.

EDB is incorporated into the three-component photoinitiating systems, e.g., using C2/Iod/EDB and C4/Iod/EDB (0.5%/1%/ 1.5% w/w) in a TMPTA resin (Figure S3). Clearly, EDB shows a higher influence in the case of C4 rather than for C2. For instance, FC increases from 50% to 57% using C4/Iod (0.5%/ 1% w/w) and C4/Iod/EDB (0.5%/1%/1.5% w/w), respectively, whereas a very low increase (from 56% to 57%) is found using C2/Iod (0.5%1% w/w) vs C2/Iod/EDB (0.5%/1%/1.5% w/w), respectively (Table 2). Consequently, it will be seen below that C4 is a better photocatalyst than C2 (7% increase of FC is case of C4 compared to only 1% increase of FC in C2 when EDB is added). Photopolymerization of Methacrylates. Interestingly, the C2/Iod (0.5%/1%) couple efficiently initiates the FRP of a blend of methacrylates (Bis-GMA/TEGDMA 70%/30% w/w) both in laminate (25 μm thick sample) (Figure 5C) and under air (1.4 mm thick sample) (Figure 5D) upon irradiation with LED at 455 nm or LED at 477 nm. Remarkably, tack-free polymers are obtained for thick samples under air, but with very poor bleaching properties. The addition of an amine (EDB) as an electron donor leads to an increase of the performance, i.e., a FC up to 65% is obtained with C2/Iod/EDB (0.5%/1%/1%) instead of only 43% for the C2/Iod (0.5%/1%) system (using LED at 455 nm;

(laminate) in the presence of the different C/Iod or C/EDB couples is quite efficient using LED at 405 nm, whereas Iod alone, EDB alone, or C alone is not capable to initiate the polymerization of the monomer. This clearly evidence that the carbazole derivatives examined in this work are quite efficient in both photooxidation processes (electron transfer from C to Iod) or photoreduction processes (electron transfer from EDB to C) but also to initiate a FRP in combination with Iod or EDB. The chemical mechanisms involved in the polymerization process will be discussed in the Photochemical Mechanisms section. Typical acrylate function conversion−time profiles are given in Figure 5A, and the FCs are summarized in Table 2. High FCs are reached in all C/Iod or C/EDB systems (e.g., C2/Iod FC = 56% at t = 100 s; Figure 5A, curve 2). The efficiency in FRP using LED at 405 nm is rather similar for the different C/Iod systems albeit C2 and C4 exhibit a slightly higher performance than the other derivatives (C1 and C3). The improved photoinitiating ability of C2 and C4 can be probably partly ascribed to the better light absorption properties of these derivatives at 405 nm (Table 1). Similarly, for the different C/EDB systems, the polymerization initiating ability is rather similar (see Figure 5B and Table 2). In the same context, for a thin sample (25 μm), the efficiency in terms of FC and Rp (rate of polymerization) increases when F


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Figure 6. Cationic photopolymerization experiments using a LED projector at 405 nm: (A) number (1); (B) letter (X); (C) pattern, and (D) “M” letter as easily observed by using a numerical optical microscope; (E) profilometry characterization for a thin pattern obtained; (F) observation (numerical optical microscopy) of “mt” written in 3D for a thicker sample (0.45 mm); (G) thick objects written in 3D (0.23 mm) obtained through LED projector technique and observed by numerical optical microscopy.

Figure 7. (A) C2/Iod photolysis upon exposure to LED at 375 nm and (B) photolysis of C2 in the absence of Iod.

in 3D printing and the kinetic FTIR experiments were used in order the comparison to be possible. In Figure 6, some 3D printing experiments were carried out upon the LED projector irradiation using the C2/Iod (0.5%/1% w/w) system, which is very reactive in the cationic polymerization of epoxides (see above) under air. Remarkably, the high photosensitivity of this resin allowed an efficient polymerization process in the irradiated area. This LED projector experiments showed an advantageous feature among the other laser-based 3D printing strategies as the entire layer is projected at one time. The fast cationic

Figure 5D, curve 2 vs curve 1; thickness = 1.4 mm; under air). The same trend is found while using the higher wavelength LED at 477 nm (the FC increases from 18% to 44% in the presence of EDB; Figure 5D, curve 3 vs curve 4; thickness = 1.4 mm; under air; Table 2). Surface Patterning or 3D Printing Using C1−C4 Based System upon LED at 405 nm Projector. To perform the 3D printing experiments, a LED projector at 405 nm (Thorlabs, 110 mW/cm2) was used. A similar intensity on the surface of the sample and a similar emission spectrum for the LEDs used G


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Figure 8. (A) Singlet state energy determination. (B) Cyclic voltammetry for the C2 oxidation. (C) Cyclic voltammetry for the C3 oxidation. (D) Cyclic voltammetry for the C4 oxidation. (E) Decay kinetics of 3C2 in presence or absence of Iod at 440 nm in acetonitrile (under N2); laser excitation at t = 10 μs. (F) Stern−Volmer treatment of the lifetime quenching of 3C2 in the presence of Iod.

Fluorescence Quenching, Laser Flash Photolysis (LFP), Cyclic Voltammetry, and ESR Experiments. C/Iod Interaction. The fluorescence experiments on C2 in acetonitrile are shown in Figure 8. The crossing point of the absorption and the fluorescence spectra allows the determination of the first singlet excited state energies (ES1) (Table 3). Fast fluorescence quenching processes of C1−C4 by Iod are noted (high value of the Ksv Stern−Volmer coefficients; Table 3) except for C4. In fact, an increase of the fluorescence intensity upon addition of increasing amounts of Iod is observed in the case of C4. This may be attributed to the formation of a C4/Iod charge transfer complex in the excited state as already observed in other donor/acceptor couples.43

process upon the LED projector at 405 nm should likely surpass the radical process by reducing the shrinkage usually observed. Different thin 3D objects (25−50 μm) as well as thicker objects (0.45 mm) can be easily obtained through the LED projector technique (Figures 6F,G). Photochemical Mechanisms. Steady State Photolysis. The steady state photolysis of C1−C4/Iod in acetonitrile is very fast compared to the high photostability of Cs alone (e.g., C2/ Iod in Figure 7A vs C2 alone in Figure 7B). A new photoproduct (characterized by a significant new absorption for λ > 450 nm) is formed in all cases which, accordingly, is due to the C2/Iod interaction. Two clear isosbestic points are detected as shown in the Figure 7A demonstrating that no by-side reaction occurs. H


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Table 3. Parameters Characterizing the Chemical Mechanisms Associated with the C1−C4/Iod Systems in Acetonitrile Eoxa [V] C1/Iod C2/Iod C3/Iod C4/Iod C1/EDB C2/EDB C3/EDB C4/EDB

Ereda [V]

ES1 [eV]

ΔGetS1b [eV]

−1.32 −1.38 −1.4 −1.37

2.78 2.8 2.9 2.6 2.78 2.8 2.9 2.6

−1.12 −1.1 −1.00 −1.30 −0.36 −0.32 −0.4 −0.13

1.46 1.5 1.7 1.1

ET1c [eV]

ΔGetT1b [eV]

2.53 2.52 2.56

−0.83 −0.62 −1.26

2.53 2.52 2.56

−0.05 −0.02 −0.9

Ksv (M−1)

Φet(S1)

4.7 4.55 2.65

0.1 0.1 0.05

6.4

Oxidation or reduction potentials for C1−C4. bReduction potential of −0.2 V1 is used for Iod; oxidation potential of 1.1 V1 is used for EDB in eq 1. From molecular orbital calculations (uB3LYP/6-31G* level of theory).

a c

experimental ESR spectrum yields the hyperfine coupling constants (hfcs): aN = 14.3 G and aH = 2.3 G typical for the PBN/Ph• radical adducts.32 The free energy changes ΔGet1 for (r2) arising from S1 or T1 were calculated from the classical Rehm−Weller equation (eq 1); herein the triplet state energies were calculated on the basis of the uB3LYP/6-31G level of theory. The processes from S1 or T1 are both found favorable e.g. for C2, ΔGetT1 = −0.83 eV and ΔGetS1 = −1.1 eV (Table 3). However, rather low electron transfer quantum yields in the excited singlet state ϕet(S1) were calculated (eq 2, Table 3). ϕet(S1) = K sv[Iod]/(1 + K sv[Iod])

Laser flash photolysis experiments were carried out on C2 chosen as a model compound. The C2 triplet state lifetime in acetonitrile was measured as being ∼10 μs under nitrogen (Figure 8E) and 330 ns under air, thereby outlining a strong oxygen quenching (kO2 ∼ 1.7 × 109 M−1 s−1). In addition, the 3 C2/Iod electron transfer rate constant in reaction 2 was also determined (∼8 × 108 M−1 s−1; Figure 8F). Therefore, a high electron transfer quantum yield was calculated: ϕet(T1) = ∼0.9 which is noticeably higher than ϕet(S1) (∼0.1; see above). This clearly shows that C2 mainly reacts through a triplet state pathway. The oxidation or reduction potentials of C1−C4 were determined by cyclic voltammetry CV (Figure 8). From the cyclic voltammograms, it is clear that C4 and C3 show a rather good reversibility (Figure 8; the reversibility can be evaluated from the ratio of cathodic and anodic currents). For C2 (Figure 8B), only a partial reversibility is observed. This reversibility can affect the behavior of carbazoles as photocatalysts as a part of C2 is unstable when oxidized. This is clearly demonstrated when a better performance of C4 over C2 is obtained while using EDB to regenerate the photocatalyst according to Scheme 3. C/EDB Interaction. The C/EDB interaction corresponds to an electron transfer reaction between EDB and C followed by the formation of EDB•(−H) (r3) capable to initiate the FRP in C/EDB PIS. This interaction is well proved by fluorescence quenching experiments (e.g., C2/EDB fluorescence quenching, Figure 10A). This is in full agreement with the favorable free energy changes ΔGet from both S1 and T1 calculated from the classical Rehm−Weller equation (eq 1); the reduction potentials of C1−C4 were determined by cyclic voltammetry (e.g., for C2 Figure 10B; Table 3). Rather high values of the Stern−Volmer coefficients (Ksv) for 1C/EDB interaction (Table 3) are determined. Remarkably, the Ksv for the 1C/ EDB interaction is in the same range than that of the 1C/Iod interactions.

Figure 9. ESR-ST spectra obtained upon irradiation (LED at 420 nm) of a C2/Iod solution, experimental (lower spectrum), and simulated (upper spectrum) in toluene as a solvent.

Scheme 3. Carbazole Photoredox Catalyst

As in other related systems, the C1−C4/Iod interaction corresponds to an electron transfer reaction finally leading to an aryl radical Ar• (r1 and r2). Ar• and C•+ can be considered as the initiating species for the radical polymerization and the cationic polymerization, respectively. C→

1,3

C(hv)

(r1)

C + Ar2I+ → C•+ + Ar2I• → C•+ + Ar • + PhI

1,3

(2)

(r2)

•

The presence of Ar is fully confirmed by ESR results. Indeed, the phenyl radicals (Ar•) were easily detected as PBN/ Ar• radical adducts in the irradiation of a C2/Iod solution in ESR-ST experiments (Figure 9); i.e., the simulation of the I


Article

Macromolecules

Figure 10. (A) Fluorescence quenching experiments of 1C2 by EDB. (B) Cyclic voltammetry for the C2 reduction.

C + EDB → C•− + EDB•+ → EDB•(−H) + H+

1,3

in a more efficient way compared to C3. Similarly, the steric hindrance of C4 is more important than that of C1.

(r3)

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Three-Component C/EDB/Iod System. Remarkably, when incorporating EDB in three-component C/Iod/EDB PIS, a reaction regenerating C takes place in the oxidative cycle (Scheme 3): C•+ is reduced by EDB as the reduction potential of C•+ (e.g., 1.5 V for C2•+ in Table 3) is higher than the oxidation potential of EDB (1.1 V1,2). The regeneration of C ensures a photoredox catalyst behavior in line with an improved reactivity (Scheme 3). This is in full agreement with the experimental results which show that the performance of the C/Iod/EDB three-component PIS is better than that of C/Iod. As shown above, the 1C/EDB (C reduction process) and the 1 C/Iod (C oxidation process) are characterized by rather similar rate constants; i.e., both reductive and oxidative catalytic cycles occur (Scheme 3). Structure/Reactivity/Efficiency Relationship. In FRP, all the carbazole derivatives exhibit a relatively good and similar efficiency in terms of final conversion of the acrylate function group. Therefore, the efficiency trend of the carbazole derivatives follows more or less their absorption trend. This is in contrast with the behavior found for the CP of EPOX. Indeed, the efficiency trend (C1 > C4 > C3 > C2) is not in line with the absorption of the Cs, e.g., ε(C1) ∼ 2600 M−1 cm−1 compared to ε(C4) ∼ 5200 M−1 cm−1 (Table 1). For the CP, all the carbazoles showed relatively similar free energy of electron transfer with Iod salt (Table 3) and react through the reaction 2. The ΔGs of electron transfer can be excluded for being the key factor for the structure/ reactivity/efficiency relationships for these derivatives as they are rather similar in the C1−C4 series. Therefore, as the yield of formation of the initiating species is comparable in the C1−C4 series, only the reactivity of the initiating radical cation (C•+) in CP can explain their differences of reactivity. Therefore, C•+ radical cation’s structure can probably play a key role in CP. For example, C1 is much better than C3 as a PI. From the chemical structure point of view, C1 and C3 differ only by the alkyl group on the nitrogen atom; i.e., C3 carries the much bulkier octyl group compared to the methyl group carried by C1. As the CP is initiated and proceeds, the bulk becomes more and more solid and the diffusion of the radical cation capable to initiate the CP becomes more and more difficult. Thus, C1 being less bulky than C3 is probably capable to initiate the CP

CONCLUSION In the present paper, carbazole is proposed as an interesting scaffold for the development of new high performance photoinitiator or photoredox catalysts upon near-UV or visible LEDs for the photoinitiation of both the cationic polymerization of epoxides and the free radical polymerization of (meth)acrylates. High final conversions and polymerization rates are obtained. Examples of the use of these new initiating systems in cationic photocurable resins for LED projector 3D printing are provided. Development of other carbazole derivatives for different irradiation wavelengths and 3D printing resins will be presented in forthcoming papers.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00210. Scheme S1: synthesis of the carbazole derivatives (C1− C4); Figure S1: calculated UV−vis absorption spectra of carbazole derivatives (MPW1PW91/6-31g(d) level of theory); Figure S2: UV−vis maximum absorption wavelengths of the carbazole derivatives experimentally in acetonitrile vs calculated (MPW1PW91/6-31g(d) level of theory); Figure S3: polymerization profiles of TMPTA upon exposure to LED at 405 nm in the presence of different initiating systems (PDF)

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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 Lebanese group thanks “The Association of Specialization and Scientific Guidance” (Beirut, Lebanon) for funding and J


Article

Macromolecules supporting this scientific work. The authors thank the “Agence Nationale de la Recherche” (ANR) for the grant “FastPrinting”.

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