Structure Design of Naphthalimide Derivatives: Toward Versatile

Article pubs.acs.org/Macromolecules

Structure Design of Naphthalimide Derivatives: Toward Versatile Photoinitiators for Near-UV/Visible LEDs, 3D Printing, and Water-Soluble Photoinitiating Systems Jing Zhang,† Frédéric Dumur,‡ Pu Xiao,*,† Bernadette Graff,† David Bardelang,‡ Didier Gigmes,‡ Jean Pierre Fouassier,§ and Jacques Lalevée*,† †

Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15, rue Jean Starcky, 68057 Mulhouse, Cedex, France ‡ Aix-Marseille Université CNRS, Institut de Chimie Radicalaire ICR, UMR7273, F-13397 Marseille, France § ENSCMu-UHA, 3 rue Alfred Werner, 68093 Mulhouse, Cedex, France S Supporting Information *

ABSTRACT: Seven naphthalimide derivatives (NDP1−NDP7) with different substituents have been designed as versatile photoinitiators (PIs), and some of them when combined with an iodonium salt (and optionally N-vinylcarbazole) or an amine (and optionally chlorotriazine) are expected to exhibit an enhanced efficiency to initiate the cationic polymerization of epoxides and the free radical polymerization of acrylates under different irradiation sources (i.e., the LED at 385, 395, 405, 455, or 470 nm or the polychromatic visible light from the halogen lamp). Remarkably, some studied naphthalimide derivative based photoinitiating systems (PIS) are even more efficient than the commercial type I photoinitiator bisacylphosphine oxide and the well-known camphorquinonebased systems for cationic or radical photopolymerization. A good efficiency upon a LED projector at 405 nm used in 3D printers is also found: a 3D object can be easily created through an additive process where the final object is constructed by coating down successive layers of material. As another example of their broad potential, a NDP compound enveloped in a cyclodextrin (CD) cavity, leads to a NDP−CD complex which appears as a very efficient water-soluble photoinitiator when combined with methyldiethanol amine to form a hydrogel. The high interest of the present photoinitiator (NDP2) is its very high reactivity, allowing synthesis in water upon LED irradiation as a green way for polymer synthesis.The structure/ reactivity/efficiency relationships as well as the photochemical mechanisms associated with the generation of the active species (radicals or cations) are studied by different techniques including steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin-trapping methods.

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INTRODUCTION One of the most challenging aspects in the field of polymerization reactions is the design and development of highperformance systems requiring low-energy consumption as well as a low ecological impact. In this direction, the use of photopolymerizable matrices adapted to irradiations with lightemitting diodes (LEDs) exhibits many advantages including eco-friendly and cheap sources, no ozone release, no harmful UV rays, low heat generation, a higher operating efficiency, lower costs, low operating and maintenance costs, and long lifetimes.1 The search for polymerization upon visible light is the subject of huge interest;1−6 controlled radical photopolymerizations were also elegantly proposed.5 Among these recently developed versatile visible light sensitive photoinitiators (PI) with novel structures,1−6 naphthalimide derivatives carrying suitable substituents have emerged as promising candidates for the initiation of cationic polymerization (CP) and free radical polymerization (FRP) reactions.7,8 Very recently, we have reported on several naphthalimide derivatives used as blue light © XXXX American Chemical Society

sensitive photoinitiators for the cationic, radical, cationic/radical, and thiol−ene photopolymerizations7 and revealed that the substituents at the nitrogen position of the naphthalimide structure have little influence on their polymerization efficiency, in contrast to derivatives where the substituents are located in the naphthalene ring. These encouraging results prompted us to explore further the possibility to prepare novel naphthalimide derivatives with various substituents in the naphthalene ring. In the present paper, we report on seven newly prepared naphthalimide derivatives (NDPs, Scheme 1) and their abilities when introduced into two- or three-component photoinitiating systems (PISs) in combination with an iodonium salt (and optionally N-vinylcarbazole) or an amine (and optionally chlorotriazine)to photochemically produce reactive species (i.e., Received: January 29, 2015 Revised: March 17, 2015

A

DOI: 10.1021/acs.macromol.5b00201 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Chemical Structures of the Studied Naphthalimide Derivatives (NDPs) and Previously Investigated ND47

Scheme 2. Chemical Structures of Additives and Monomers

respectively. The formation of polymer networks is expected for these multifunctional monomers. 2-Hydroxyethyl acrylate (HEA, Sigma-Aldrich) was used for polymerization in water. For 3D printing, the triethylene glycol diacrylate/tricyclodecane dimethanol diacrylate (from Sigma-Aldrich) (70%/30% w/w) blend was used as a reference for low viscosity formulation. Irradiation Sources. Different lights were used for the irradiation of the formulations: light-emitting diode (LED) at 470 nm (M470L3, ThorLabs; ∼70 mW cm−2), LED at 455 nm (M455L3, ThorLabs; ∼80 mW cm−2), LED at 405 nm (M405L2, ThorLabs; ∼110 mW cm−2), LED at 395 nm (M395L2, ThorLabs; ∼35 mW cm−2), LED at 385 nm (M385L2, ThorLabs; ∼27 mW cm−2), and polychromatic light (halogen lamp; Fiber-Lite, DC-950; incident light intensity: ∼12 mW cm−2 in the 370−800 nm range; the emission spectrum is given in Figure S1 of the Supporting Information). Photopolymerization Experiments. For CP and FRP experiments, the conditions are given in the caption of the Figures. The photocurable formulations deposited (25 μm thick) on a BaF2 pellet in laminate (the formulation is sandwiched between two polypropylene films) or under air were irradiated with different lights (see above).

radicals and cations) under various near UV/visible LEDs or a polychromatic light issued from a halogen lamp. Their efficiency in the CP of a diepoxide under air and the FRP of a low-viscosity triacrylate in laminate are checked and compared to those of a commercial type I PI (bisacylphosphine oxide; BAPO), camphorquinone CQ (well-known PI)-based PISs, and a previously studied naphthalimide derivative ND4. The possibility of these PISs to photopolymerize a very low viscosity acrylate formulation under air and create 3D objects using a 3D printer at 405 nm is tested. In a first attempt, a water-soluble NDP−CD complex (formed from the inclusion of a NDP into a cyclodextrin (CD) cavity) is also considered as a possible type II PI for the elaboration of a hydrogel from the polymerization of 2-hydroxyethyl acrylate in water. Furthermore, the photochemical mechanisms associated with the generation of the active species (i.e., radicals and cations) are investigated using steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin-trapping techniques.

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EXPERIMENTAL SECTION

Materials. The studied naphthalimide derivatives NDPs (i.e., NDP1−NDP7, Scheme 1) and NDP2/2,6-di-O-Me-β-cyclodextrin complex (NDP2-CD) were synthesized according to the procedures presented in detail in the Supporting Information. N-Vinylcarbazole (NVK), methyldiethanolamine (MDEA), 2,4,6-tris(trichloromethyl)1,3,5-triazine (R′−Cl), 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)1,3,5-triazine (R−Cl), camphorquinone (CQ), diphenyliodonium hexafluorophosphate (Iod), the other reagents, and solvents were purchased from Sigma-Aldrich or Alfa Aesar and used as received without further purification. Bisacylphosphine oxide (Irgacure 819 or BAPO) was obtained from BASF. (3,4-Epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) and trimethylolpropane triacrylate (TMPTA) were obtained from Allnex; they were used as benchmark monomers for cationic and radical photopolymerization,

Figure 1. UV−vis absorption spectra of NDP1−NDP6 in acetonitrile and NDP7 in N,N-dimethylformamide. B

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Table 1. Light Absorption Properties of the Studied NDPs: Maximum Absorption Wavelengths λmax and Extinction Coefficients at λmax and at the Maximum Emission Wavelengths of the Different Irradiation Devices NDP1 NDP2 NDP3 NDP4 NDP5 NDP6 NDP7 ND4b a

λmax (nm)

εmax (M−1 cm−1)

ε385 nm (M−1 cm−1)a

ε395 nm (M−1 cm−1)a

ε405 nm (M−1 cm−1)a

ε455 nm (M−1 cm−1)a

ε470 nm (M−1 cm−1)a

421 417 334 426 340 431 440 410

620 5600 13100 9800 17800 17400 11300 7600

560 3300

520 4300

560 5100

360 1700

500

5600

7000

8200

5700

2900

6300 7600

9100 7900

12100 8800

12500 10100

6100 7400

For different LEDs. bFrom ref 7.

Figure 2. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for NDP1, NDP2, and NDP4 at the B3LYP/6-31G* level and NDP3 at the B3LYP/LANL2DZ level (isovalue = 0.04). equation: ΔG = Eox − Ered − ES (or ET) + C.15 Eox, Ered, ES (or ET), and C stand for the oxidation potential of the donor, the reduction potential of the acceptor, the excited singlet (or triplet) state energies of the studied NDPs, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents. Laser Flash Photolysis. Nanosecond laser flash photolysis (LFP) experiments were carried out using a Q-switched nanosecond Nd/YAG laser (λexc = 355 nm, 9 ns pulses; energy reduced down to 10 mJ) from Continuum (Minilite) and an analyzing system consisted of a ceramic xenon lamp, a monochromator, a fast photomultiplier, and a transient digitizer (Luzchem LFP 212).16 ESR Spin Trapping (ESR-ST) Experiments. ESR-ST experiments were carried out using an X-band spectrometer (MS 400 Magnettech). The ESR spectra simulations were carried out with the WINSIM software. The radicals were generated upon the LED at 405 nm exposure (at room temperature and under N2); these radicals were trapped by phenyl-N-tert-butylnitrone (PBN) according to a well-known procedure.17

The epoxy group content of EPOX and the double-bond content of TMPTA were continuously followed by real-time FTIR spectroscopy (JASCO FTIR 4100)11,12 at about 790 and 1630 cm−1, respectively. Thermogravimetric Analysis. The water content of the hydrogel was determined by thermogravimetric analysis (TGA) (MettlerTOLEDO TGA 851e for a weight change between RT and 200 °C). 3D Printing. The 3D polymerization was conducted using a 3D printer: ProJet 1200 from 3D systems using a LED projector at 405 nm. The 3D printing is really an additive process where the final object is constructed by coating down successive layers of material. Computational Procedure. Molecular orbital calculations were carried out with the Gaussian 03 package. The electronic absorption spectra were calculated with the time-dependent density functional theory at the B3LYP/6-31G* level on the relaxed geometries (frequency checked) calculated at the UB3LYP/6-31G* level; the molecular orbitals involved in these transition can be extracted.9,10 Fluorescence Experiments. The fluorescence properties of the investigated NDPs in acetonitrile were studied using a JASCO FP-750 spectrometer. The interaction rate constants kq between the studied NDPs and additives (i.e., Iod or MDEA) were extracted from the classical Stern−Volmer treatment:13 I0/I = 1 + kqτ0[additive]. I0 and I stand for the fluorescent intensity of the studied NDPs in the absence and the presence of the additives, respectively. τ0 stands for the lifetime of the excited NDPs in the absence of additives. Redox Potentials. The oxidation potentials (Eox vs SCE) of the studied NDPs were measured in acetonitrile by cyclic voltammetry with tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte (Voltalab 6 radiometer). The procedure has been presented in detail in ref 14. The free energy changes ΔG for the electron transfer processes (NDPs/additive) were extracted from the classical Rehm−Weller

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RESULTS AND DISCUSSION 1. Light Absorption Properties of NDPs. The absorption spectra of NDP1−NDP6 in acetonitrile and NDP7 in N,N-dimethylformamide are illustrated in Figure 1; their maximum absorption wavelengths (λmax) and extinction coefficients (ε) at λmax and at the maximum emission wavelengths of the different LEDs are summarized in Table 1. NDP3 and NDP5 were not interesting for further investigations as their absorption is located below 350 nm due to the iodo or bromo substituent in the naphthalene moiety. NDP1 bearing a nitro C

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Figure 3. Photopolymerization profiles of EPOX under air in the presence of (a) NDP1-based PISs, (b) NDP2-based PISs, (c) NDP4-based PISs, (d) NDP6-based PISs, and (e) NDP7-based PISs. NDPs/Iod upon the LED@385 nm (curve 0), LED@405 nm (curve 1), LED@455 nm (curve 2), LED@470 nm (curve 3), or halogen lamp (curve 4) exposure; NDPs/Iod/NVK upon the LED@405 nm (curve 5) or halogen lamp (curve 6) exposure (NDPs: 0.5 wt %; Iod: 2 wt %; NVK: 3 wt %).

substituent absorbed at 421 nm, but the ε values are too low. On the opposite, NDP2, NDP4, NDP6, and NDP7 exhibited an excellent visible light absorption (due to the presence of an amino substituent as in refs 7 and 8 and similar to that of the previously studied ND47) which perfectly matched the emission spectra of the used LEDs (385, 395, 405, 455, and 470 nm) and the halogen lamp (see in Figure 1, Figure S1 in the Supporting Information, and Table 1). The incorporation of the NDPs in the cyclodextrin cavity slightly changes the absorption maxima e.g. from 417 nm for NDP2 in acetonitrile to 407 nm for NDP2−CD in water. Therefore, the NDP2−CD complex exhibited a good solubility in water and was well adapted for an irradiation with the LED at 405 nm (see below the polymerization in water). The molecular orbitals of the different NDPs mainly involved in their lowest energy absorption band (HOMO → LUMO) are depicted in Figure 2. For compounds exhibiting a significant visible light absorption (NDP2, NDP4), a charge transfer transition from the amino group (for the highest occupied molecular orbital, HOMO) to the naphthalimide moiety (for the lowest unoccupied molecular orbital, LUMO) is found.

This charge transfer transition is in agreement with the enhanced light absorption properties of these derivatives compared to the unsubstituted parent compounds7 or the derivatives containing nitro, bromo, or iodine groups (NDP1 and NDP3). 2. Photoinitiating Ability of the Investigated Naphthalimide Derivatives. 2a. Cationic Photopolymerization of Epoxides. 2a-1. Efficiency of the Different NDPs. According to our previous study on other photoinitiators,7,8,18 cations (i.e., NDPs•+ and Ph-NVK+) are expected to be generated from the interaction between the studied NDPs and the additives (reactions 1−5) under the different irradiations to initiate the cationic photopolymerization. NDPs → 1NDPs (hν) 1,3

1

NDPs → 3 NDPs

NDPs + Ph 2I+ → NDPs•+ + Ph 2I• •

D

and

•

(1) (2)

Ph 2I → Ph + Ph−I

(3)

Ph• + NVK → Ph−NVK•

(4)

Ph−NVK• + Ph 2I+ → Ph−NVK+ + Ph• + Ph−I

(5)

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Table 2. EPOX Conversions (in %) Obtained under Air upon Exposure to Different Light Sources at t = 800 s in the Presence of NDPs/Iod (0.5%/2%, w/w) or NDPs/Iod/NVK (0.5%/2%/3%, w/w/w), CQ/Iod (0.5%/2%, w/w), and CQ/Iod/NVK (0.5%/2%/3%, w/w/w) as References LED (385 nm) NDP1/Iod NDP1/Iod/NVK NDP2/Iod NDP2/Iod/NVK NDP2-CD/Iod NDP2-CD/Iod/NVK NDP4/Iod NDP4/Iod/NVK NDP6/Iod NDP6/Iod/NVK NDP7/Iod NDP7/Iod/NVK ND4/Iod7 CQ/Iod CQ/Iod/NVK

61 61

LED (395 nm)

LED (405 nm)

LED (455 nm)

27 67 59 62|62a 49 66 62 60 63 58 28 52

18

62 65

LED (470 nm)

halogen lamp

62

59

59 59

56

60

62

61

55b 62 53 54

13 14 npc npc

a

Measured after 1 week of storage at room temperature. bTack-free coatings can be obtained for those having conversions >55%. cnp: no polymerization.

As seen in Figure 3 and Table 2, NDP1/Iod (Figure 3a, curves 1 and 2) or NDP7/Iod (Figure 3e, curves 1 and 2) initiated the CP of EPOX under air at 405 or 455 nm but with a low efficiency (EPOX conversions 50% (Figure 3b−d, Table 2). They are better than the previously studied ND4/Iod system7 (under the halogen lamp exposure). Interestingly, tack-free coatings were obtained in any case except with NDP6/Iod exposed to the halogen lamp. These two-component systems being already very efficient and the final conversions being limited by the high cross-linking degree of the polymer networks as in ref 19, the addition of NVK has almost no effect (e.g., at 405 nm for NDP2 based PISs: Figure 3b curve 5 vs curve 1). For NDP7/Iod, a rather low efficiency is found (Figure 3e); this can be explained by the rather low efficiency of the radical cation (NDP7•+) to initiate the cationic polymerization. A better behavior is found in the presence of NVK. Furthermore, it can be outlined that the studied NDPs based PISs are well adapted to a UV LED@385 nm irradiation (e.g., EPOX conversion of 61% with NDP2/Iod/NVK, tack-free coatings). For comparison, the CQ/Iod or CQ/Iod/ NVK reference systems did not work in the present experimental conditions, e.g., under the halogen lamp irradiation. The CQ/Iod initiating system was proposed as an efficient system for other conditions in ref 2c. 2a-2. Possible Use of NDP-CDs as Photoinitiating Systems. The photoinitiating ability of NDP2-CD-based PIS for the CP of EPOX under air was also investigated. As illustrated in Figure 4 and Table 2, the NDP2-CD/Iod system exhibited a lower efficiency (lower polymerization rate and final conversion) than the NDP2/Iod combination. This might be ascribed to the fact that in NDP2-CD/Iod the generated NDPs•+ cation was confined inside the cyclodextrin cavity, its

Figure 4. Photopolymerization profiles of EPOX under air in the presence of NDP2/Iod, NDP2/Iod/NVK, NDP2-CD/Iod, and NDP2-CD/Iod/ NVK upon the LED@405 nm exposure (NDP2:16.5 μmol/g or 0.5 wt %; NDP2-CD: 16.5 μmol/g or 2.7 wt %; Iod: 2 wt %; NVK: 3 wt %).

reactivity being thus reduced by a steric effect. Interestingly, in the presence of NVK, even with a lower polymerization rate, NDP2-CD/Iod/NVK and NDP2/Iod/NVK led to quite similar (or slightly better) EPOX conversion (66% vs 62%): the formation/ reactivity of the Ph-NVK+ cation is likely not affected by the cyclodextrin. 2b. Free Radical Photopolymerization of Acrylates. 2b-1. Efficiency of the Different NDPs. The radicals (Ph• or Ph-NVK•) generated from the NDPs/Iod or NDPs/Iod/NVK PISs (reactions 1−5) act as initiating species for the FRP of TMPTA in laminate. As illustrated in Figure 5a−c and Table 3, NDP1/Iod was not so efficient due to its poor absorption and its low electron donating character (TMPTA conversion = 19%, LED@405 nm exposure) while NDP2/Iod and NDP4/ Iod exhibited an excellent photoinitiating ability at 405 nm (TMPTA conversion = 52%). Interestingly, NDP2/Iod and NDP4/Iod also worked at 385 nm and NDP2/Iod at 455 and 470 nm as well as under the halogen lamp. The relatively higher polymerization efficiency of NDP2/Iod at 405 nm (Figure 5b, curve 2 vs curves 1, 3, 4, and 5; Table 3) may be ascribed to the higher light intensity of the LED@405 (∼110 mW cm−2) and the better matching of the LED emission/NDP2 absorption. Moreover, the addition of NVK significantly improved the final conversions (53%, 63%, and 58% with NDP1/Iod/ NVK, NDP2/Iod/NVK, and NDP4/Iod/NVK respectively) E

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Figure 5. Photopolymerization profiles of TMPTA in laminate in the presence of: (a) NDP1/Iod/NVK upon the LED@405 nm exposure (curve 1); NDP1/MDEA/R−Cl upon the LED@385 nm (curve 2), LED@405 nm (curve 3) or halogen lamp (curve 4) exposure; (b) NDP2/Iod upon the LED@385 nm (curve 1), LED@405 nm (curve 2), LED@455 nm (curve 3), LED@470 nm (curve 4), or halogen lamp (curve 5) exposure; NDP2/ Iod/NVK upon the LED@385 nm (curve 6), LED@405 nm (curve 7) or halogen lamp (curve 8) exposure; (b′) NDP2/MDEA upon the LED@ 405 nm exposure (curve 1); NDP2/MDEA/R-Cl upon the LED@385 nm (curve 2), LED@405 nm (curve 3) or halogen lamp (curve 4) exposure; NDP2/MDEA/R′-Cl upon the LED@405 nm exposure (curve 5); (c) NDP4/Iod upon the LED@385 nm (curve 1) or LED@405 nm (curve 2) exposure; NDP4/Iod/NVK upon the LED@385 nm (curve 3), LED@405 nm (curve 4) or halogen lamp (curve 5) exposure; (c′) NDP4/MDEA upon the LED@405 nm exposure (curve 1); NDP4/MDEA/R−Cl upon the LED@385 nm (curve 2), LED@405 nm (curve 3) or halogen lamp (curve 4) exposure; CQ/Iod and CQ/MDEA upon the halogen lamp, or BAPO upon the LED@405 nm exposure are used as references (NDPs, CQ, or BAPO: 0.5 wt %; Iod or MDEA: 2 wt %; NVK, R−Cl or R′−Cl: 3 wt %).

NDPs•− + R′−Cl → NDPs + (R′−Cl)•−

compared to those of the corresponding NDPs/Iod twocomponent systems (e.g in Figure 5b, curve 7 vs curve 2; Table 3). Remarkably, when exposed to the LED@405 nm, the NDP2/Iod/NVK or NDP4/Iod/NVK systems were even more efficient than BAPO (see in Figure 5b, curve 7 vs curve BAPO or Figure 5c, curve 4 vs curve BAPO; Table 3). This can be ascribed to the lower light absorption properties of BAPO for this latter wavelength (ε405 nm ∼ 800 M−1 cm−1). Different radicals can also be produced from the NDPs/ amine (and optionally chlorotriazine) combinations to initiate the FRP of TMPTA in the same way as indicated in reactions 6−10 derived from experiments on other systems.12,20−24 1,3

1,3

NDPs−H• + R′−Cl → NDPs + H+ + (R′−Cl)•−

(R′−Cl)•− → R′ • + Cl−

NDPs•− + MDEA•+ → NDPs + MDEA

(8) (9) (10)

Figure 5a,b′,c′ and Table 3 show that NDP1/MDEA, NDP2/MDEA, or NDP4/MDEA led to TMPTA conversions of 17, 43, and 45% in laminate (LED@405 nm exposure). As expected, the addition of chlorotriazine (R−Cl or R′−Cl) dramatically enhanced the final conversions (62−64% with NDP1 (or NDP2, NDP4)/MDEA/chlorotriazine; LED@405 nm); although R−Cl can additionally absorb light at 405 nm (see in Figure S2 of the Supporting Information)25 and behave as a type I photoinitiator, its effect as an additive was similar to that of R′−Cl, which is transparent at this wavelength (Figure 5b′, curve 5 vs curve 3). These conversions are even

NDPs + MDEA → NDPs•− + MDEA•+ → NDPs−H• + MDEA•(−H)

NDPs + R′−Cl → NDPs•+ + (R′−Cl)•−

(7)

(6a) (6b) F

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Table 3. TMPTA Conversions Obtained in Laminate upon Exposure to Different Light Sources for 400 S in the Presence of NDPs Based PISs (NDPs: 0.5 wt %; Iod or MDEA: 2 wt %; NVK, R−Cl or R′−Cl: 3 wt %); CQ/Iod (0.5%/2%, w/w), CQ/MDEA (0.5%/2%, w/w), or BAPO (0.5 wt %) as References LED (385 nm) NDP1/Iod NDP1/Iod/NVK NDP1/MDEA NDP1/MDEA/R-Cl NDP2/Iod NDP2/Iod/NVK NDP2/MDEA NDP2/MDEA/R-Cl NDP2/MDEA/R′-Cl NDP4/Iod NDP4/Iod/NVK NDP4/MDEA NDP4/MDEA/R-Cl ND4/Iod/NVK7 ND4/MDEA/R′-Cl7 CQ/Iod CQ/MDEA BAPO a

63 44 59 59 44 52 50%) upon exposure to the LED at 405, 455, or 470 nm or the halogen lamp, even much better than camphorquinone-based combinations. The NDP2/Iod/NVK or NDP2/MDEA/chlorotriazine threecomponent systems were also more efficient in the radical polymerization of acrylates under irradiation of light 405 nm than the CQ-based systems or BAPO. The present NDP-based PISs also allowed the manufacture of 3D objects using a 3D printer. The possibility to insert NDP2 in a cyclodextrin cavity has led to the design of an efficient water-soluble photoinitiator for the production of a hydrogel. Specific development of other water-soluble photoinitiating systems will be proposed in forthcoming papers.

experiments have been carried out to support the expected mechanisms (reactions 1−10). As observed in steady state photolysis experiments, the fast bleaching of NDP1 (or NDP2, NDP4)/Iod (Figure 8a−c; in acetonitrile; LED@405 nm exposure) and NDPs/MDEA/chloro triazine as well as the slow bleaching of NDP1 (or NDP2, NDP4)/MDEA (Figure 8a′−c′) were in line with their performance in polymerization. The lower reactivity of these last systems could reflect the possible occurrence of a back-electron-transfer reaction 6b. Table 4 gathered the characteristics of the fluorescence emission (lifetimes, quantum yields) and the interaction rate constants kq between the NDPs and the additives (Iod or MDEA). The kq values are almost diffusion-controlled. The free energy changes ΔG of the reactions in the NDP singlet states are highly negative (