Design of High Performance Photoinitiators at 385–405 nm: Search

Article pubs.acs.org/Macromolecules

Design of High Performance Photoinitiators at 385−405 nm: Search around the Naphthalene Scaffold Pu Xiao,† Frédéric Dumur,‡ Bernadette Graff,† Fabrice Morlet-Savary,† 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 ‡ CNRS, Institut de Chimie Radicalaire, UMR 7273, Aix-Marseille Université, F-13397 Marseille, France S Supporting Information *

ABSTRACT: Novel naphthalene derivatives have been designed to be used as versatile photoinitiators upon a laser diode (405 nm), a polychromatic halogen lamp, or an UV LED (385 nm) exposure. The reactive species produced from photoinitiating systems based on one particular naphthalene derivative (NA3) and an iodonium salt, N-vinylcarbazole, an amine or 2,4,6tris(trichloromethyl)-1,3,5-triazine were particularly efficient for cationic, radical, IPN and thiol−ene photopolymerizations upon low light intensity exposure. The best proposed systems exhibit a higher efficiency than references systems for visible lights (i.e., camphorquinone CQ-based photoinitiating systems). The mechanisms for the photochemical generation of reactive species (i.e., radicals and cations) were studied by electron spin resonance spin-trapping, fluorescence, cyclic voltammetry, laser flash photolysis, and steady state photolysis techniques.

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INTRODUCTION Free radical polymerization (FRP), thiol−ene polymerization (TEP), cationic polymerization (CP) or free radical promoted cationic polymerization (FRPCP) and interpenetrated polymer networks (IPN) synthesis are well-known for the formation of highly cross-linked networks allowing applications in various fields.1−3 The photoinitiating systems PISs play an important role. In industrial application, most of the PISs normally operate under UV light irradiation.1−4 Some systems are already used in industrial applications under visible lights, e.g., camphorquinone (CQ) (a reference photoinitiator PI in the 450−470 nm range), bis(acylphosphine) oxides, dyes, and titanocenes. Polychromatic visible lights and monochromatic purple, blue, green, yellow, and red lights are particularly attractive as household halogen lamps and LEDs as well as laser diodes are by now available and cheap sources. The challenge still remains open to develop versatile PISs that can efficiently initiate various types of polymerization reactions under such low light intensity visible sources. A continuous flow of papers is noted in the PI and PIS area (see a review in ref 1 and some examples of recent papers5,6). In the particular blue range, huge efforts have been done using a lot of ketone structures and modified ketones (e.g., benzoinethers, benzophenones, thioxanthones, diketones, ketocoumarins, indanediones, acridinediones...). To leave the traditional ketone area, the modification of hydrocarbon scaffolds has also been checked. Anthracene7,8 or pyrene9 © 2014 American Chemical Society

moiety based PIs have led to successful results. Incorporation of a naphthalene ring into a larger planar structure was recently achieved in the naphtalimide series.10 In the present paper, we synthesize nine new naphthalene derivatives NA (Scheme 1 and Scheme S1, Supporting Information) exhibiting a planar geometry where a strong electron delocalization is expected. Their role in soft visible light sensitive versatile PISs for CP, FRP, IPN and thiol−ene polymerization will be studied and compared to that of the reference camphorquinone CQ based PISs. The production of reactive species (radicals and cations) from the NAs in the presence of an iodonium salt and optionally N-vinylcarbazole will be investigated by electron spin resonance spin-trapping (ESR-ST), fluorescence, cyclic voltammetry, laser flash photolysis, and steady state photolysis techniques.

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EXPERIMENTAL SECTION

Materials. The investigated naphthalene derivatives NAn (i.e., NA1−NA9) and other chemical compounds are shown in Schemes 1, 2 and S1 in the Supporting Information. NA1−NA9 were prepared according to the procedures presented in detail in the Supporting Information. For NA2 and NA4, the isomers also existed in the products as presented in the Supporting Information. DiphenyliodoReceived: December 23, 2013 Revised: January 28, 2014 Published: January 31, 2014 973

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saturated calomel electrode (SCE). Ferrocene was used as a standard, and the potentials determined from the half peak potential were referred to the reversible formal potential of this compound (+0.44 V/ SCE). The free energy change ΔG for an electron transfer between the studied naphthalene derivatives and Iod can be calculated from the classical Rehm−Weller equation (eq 1, where Eox, Ered, ES (or ET), and C are the oxidation potential of the studied naphthalene derivatives, the reduction potential of Iod, the excited singlet (or triplet) state energy of the studied naphthalene derivatives, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents):12

Scheme 1. Chemical Structures of NA1−NA9

ΔG = Eox − Ered − ES (or E T) + C

(1)

Computational Procedure. Molecular orbital calculations were carried out with the Gaussian 03 suite of programs. The electronic absorption spectra for the different compounds were calculated with the time-dependent density functional theory at B3LYP/6-31G* level on the relaxed geometries calculated at UB3LYP/6-31G* level.13,14 Laser Flash Photolysis. Nanosecond laser flash photolysis (LFP) experiments were carried out using a Q-switched nanosecond Nd/ YAG laser at λexc = 355 nm (9 ns pulses; energy reduced down to 10 mJ; minilite Continuum) and the analyzing system (for absorption measurements) consisted of a ceramic xenon lamp, a monochromator, a fast photomultiplier and a transient digitizer (Luzchem LFP 212).15 Photopolymerization Experiments. For photopolymerization experiments, the conditions are given in the figure captions. The photosensitive formulations were deposited on a BaF2 pellet under air or in laminate (25 μm thick) for irradiation with different lights. The evolution of the epoxy group content of EPOX, the double bond content of TMPTA, the double bond content of DVE-3 and the thiol (S−H) content of Trithiol were continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100)16,17 at about 790, 1630, 1620, and 2580 cm−1, respectively.

Scheme 2. Chemical Structures of Additives and Monomers

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nium hexafluorophosphate (Iod), N-vinylcarbazole (NVK), methyl diethanolamine (MDEA), 2,4,6-tris(trichloromethyl)-1,3,5-triazine (R′−Cl), tri(ethylene glycol) divinyl ether (DVE-3), trimethylolpropane tris(3-mercaptopropionate) (Trithiol), and the other reagents and solvents were purchased from Sigma-Aldrich or Alfa Aesar from the highest purity available and used as received without further purification. The monomers (3,4-epoxycyclohexane)methyl 3,4epoxycyclohexylcarboxylate (EPOX) and trimethylolpropane triacrylate (TMPTA) were obtained from Cytec and used as benchmark monomers for cationic and radical photopolymerization. Irradiation Sources. Three different low intensity visible light sources were used for the irradiation of samples: polychromatic light from a 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 the Supporting Information), a low intensity 385 nm LED (ML385-L2 − ThorLabs, ∼9 mW cm−2), and laser diode at 405 nm (∼8 mW cm−2). ESR Spin Trapping (ESR-ST) Experiments. ESR-ST experiments were carried out using an X-Band spectrometer (MS 400 Magnettech). The radicals were generated at RT upon the halogen lamp exposure under N2 and trapped by phenyl-N-tert-butylnitrone (PBN) according to a procedure11 described in detail. The ESR spectra simulations were carried out with the WINSIM software. Fluorescence Experiments. Fluorescence properties of the investigated naphthalene derivatives in acetonitrile were studied using a JASCO FP-750 spectrometer. The interaction rate constants kq between the studied naphthalene derivatives and Iod were extracted from classical Stern−Volmer treatments1 (I0/I = 1 + kqτ0[Iod]; where I0 and I stand for the fluorescent intensity of the studied naphthalene derivatives in the absence and the presence of the Iod quencher, respectively; τ0 stands for the lifetime of the excited naphthalene derivatives in the absence of Iod). Redox Potentials. The oxidation potentials (Eox vs SCE) of the studied naphthalene derivatives were measured in acetonitrile by cyclic voltammetry with tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte (Voltalab 6 Radiometer). The working electrode was a platinum disk and the reference electrode was a

RESULTS AND DISCUSSION 1. Light Absorption Properties of the Studied Naphthalene Derivatives. Compared to naphthalene itself (λmax ∼ 275 nm, molar extinction coefficient ε ∼ 6000 M−1cm−1 in cyclohexane),18 the ground state maximum absorptions of NA1 and NA3 are significantly red-shifted with much higher ε values (Figure 1 and Table 1; λmax = 381

Figure 1. UV−vis absorption spectra of the studied NA1 and NA3 in acetonitrile.

Table 1. Light Absorption Properties of NA1 to NA3 (for NA4 to NA9 See the Supporting Information) λmax (nm) [ε (M−1cm−1)] NA1 NA2 NA3 974

381 [8800] 376 [>9200] 378 [25700] dx.doi.org/10.1021/ma402622v | Macromolecules 2014, 47, 973−978

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Figure 2. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of NA1−NA4 at UB3LYP/6-31G* level (isovalue = 0.04).

nm, ε381 nm ∼ 8800 M−1cm−1 and λmax = 378 nm, ε378 nm ∼ 25700 M−1 cm−1 for NA1 and NA3, respectively). This redshift is attributed to the enhanced π electron delocalization in the structures. The absorption of NA3 is better than that of NA1 at 405 nm (ε405 nm ∼ 16700 vs 5300 M−1cm−1). NA3 is also particularly attractive for UV LED@385 nm (ε385 nm ∼ 23000 M−1cm−1). NA2 that presents a low solubility in usual organic solvents exhibits an absorption maximum at 376 nm (ε > 9200 M−1cm−1 in acetonitrile). The solubility of NA4−NA9 is pretty poor (almost zero) due to the bulky and symmetric chemical structures; moreover, their absorption in the visible range is too low (Figure S1 in Supporting Information). The enhanced light absorption properties of NA1−NA4 compared to those of naphthalene can be ascribed to the high delocalization of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) involved in the lowest energy transition (π→ π* character: Figure 2). 2. Photochemistry and Photoinitiating Ability of the Investigated Naphthalene Derivatives. 2a. The NAs in the Cationic Photopolymerization of Epoxides. The photopolymerization of EPOX in the presence of NAn/Iod or NAn/Iod/NVK PISs under air were carried out using the very soft halogen lamp (emission spectrum: Figure S2 in the Supporting Information) or the laser diode at 405 nm (conversion profiles are depicted in Figure 3 and conversions at t = 800 s summarized in Table 2). NA2 (or NA4)/Iod, NA2

Table 2. EPOX Conversions Obtained under Air upon Exposure to Different Visible Light Sources for 800 s in the Presence of NAn/Iod (0.5%/2%, w/w) or NAn/Iod/NVK (0.5%/2%/3%, w/w/w) NAn

halogen lamp

laser diode 405 nm

NA1 NA2 NA3 NA4−NA9

npa|27%b npa,b npa|59%b npa,b

50%b − 52%b −

a NAn/Iod (0.5%/2%, w/w). np: no polymerization. bNAn/Iod/NVK (0.5%/2%/3%, w/w/w).

(or NA4)/Iod/NVK and NA1 (or NA3)/Iod cannot initiate the polymerization of EPOX. The addition of NVK (as in other systems19) significantly improves the polymerization profile under (i) the halogen lamp irradiation (conversion =27% and 59% for NA1/Iod/NVK and NA3/Iod/NVK, respectively; tack free coatings were obtained after 800 s of irradiation for the NA3/Iod/NVK containing formulation), (ii) the laser diode at 405 nm (conversion = 50% and 52% with NA1/Iod/NVK and NA3/Iod/NVK systems), or (iii) the UV LED (385 nm) (conversion ∼40% for NA3/Iod/NVK system). NA3/Iod combination is also able to initiate a cationic polymerization of DVE-3 (see below the thiol−ene process). NA5−NA9 do not work. In the same conditions, camphorquinone CQ/Iod or CQ/Iod/NVK systems (CQ is a well-known visible light photosensitizer)1 cannot operate. In line with the behavior of other related PISs,1,6,8−10 the following plausible set of reactions (1-3) accounts for the generation of radicals and cations upon light exposure of the NA/Iod and NA/Iod/NVK systems. NA3 and NA1 yields a fluorescence emission (fluorescence lifetime: ∼ 20 ns and fluorescence quantum yield: ∼0.40 for NA3, using anthracene (0.27) as a standard20). The free energy change ΔG for the corresponding NA/Iod electron transfer reaction 2a in the singlet state is negative (ΔG = −1.04 eV and −1.33 eV for NA3/Iod and NA1/Iod, respectively, using the following parameters: oxidation potentials Eox = 1.61 and 1.45 V as measured by cyclic voltammetry in this work; reduction potential Ered = −0.2 V1 for Iod; singlet state energy ES = 2.85 and 2.98 eV as extracted from the UV−vis absorption and fluorescence emission spectra as usually done21). The interaction rate constants are high (kq = ∼3.0 × 109 M−1 s−1 for 1NA3/Iod). Phenyl radicals (2) are observed in ESR spin trapping experiments on NA3/Iod (Figure 4). In laser flash

Figure 3. Photopolymerization profiles of EPOX under air in the presence of NA3/Iod/NVK (0.5%/2%/3%, w/w/w) upon the halogen lamp (curve 1) and the laser diode at 405 nm (curve 2) exposure; NA1/Iod/NVK (0.5%/2%/3%, w/w/w) at 405 nm (curve 3). 975

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Figure 4. ESR spectra of the radicals generated in NA3/Iod upon the halogen lamp exposure and trapped by PBN in tert-butylbenzene: (a) experimental and (b) simulated spectra. PBN/phenyl radical adducts obtained in NA3/Iod: aN = 14.1 G, aH = 2.1 G. Reference values.22,23

Figure 6. Photopolymerization profiles of TMPTA in laminate in the presence of NA3/Iod (0.5%/2%, w/w) (curve 1), NA3/Iod/NVK (0.5%/2%/3%, w/w/w) (curve 2), NA3/MDEA (0.5%/2%, w/w) (curve 3) NA3/MDEA/R′-Cl (0.5%/2%/0.5%, w/w/w) (curve 4) and CQ/MDEA (0.5%/2%, w/w); halogen lamp exposure.

photolysis experiment, a transient absorption following the laser excitation of NA3 in acetonitrile at 355 nm is recorded and ascribed to a triplet state (this transient is quenched by oxygen) (Figure S3 in the Supporting Information). The low intensity of this transient probably indicates a rather limited triplet state quantum yield. As a consequence, the singlet route predominates in part 2. The NA3/Iod and NA1/Iod electron transfer quantum yields ΦeT calculated according to ΦeT = kqτ0 [Iod]/(1+ kqτ0 [Iod]) are 0.74 and 0.26 (for [Iod] = 4.7 × 10−2 M), respectively. This indicates that the reactivity of NA3/Iod is much higher than that of NA1/Iod: this is in line with the fast bleaching of NA3/Iod compared to the slow bleaching of NA1/ Iod (Figure 5). The NA•+ and Ph-NVK+ are the cationic initiating species. The reactivity of NA•+ may partly explain the observed differences when using the different NA/Iod systems.

respectively) with conversions =50% and 28%, respectively; NA3/Iod and NA3/MDEA lead to conversions =14% and 6%. Interestingly, the NA3/Iod/NVK combination is better than that the traditional CQ/MDEA system (conversion =35%). The NA3/MDEA and NA3/MDEA/R′-Cl PISs should work according to reactions 4−8 by analogy with other systems.17,24−26 The Ph•, Ph-NVK•, aminoalkyl (MDEA(‑H)•) and R′• radicals are the initiation species for the FRP of TMPTA (reactions 1−8).

NA → 1NA (hν) and 1NA → 3 NA

(1a)

NA + Ph 2I+ → NA•+ + Ph 2I•

(2a)

1,3

Ph 2I• → Ph• + Ph−I

(2b)

Ph• + NVK → Ph−NVK•

(3a)

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

(3b)

1,3

1,3

NA3 + MDEA → NA3•− + MDEA•+ → NA3− H• + MDEA (‐H)•

(4)

NA3 + R′−Cl → NA3•+ + (R′−Cl)•−

(5)

NA3•− + R′−Cl → NA3 + (R′−Cl)•−

(6)

NA3−H• + R′−Cl → NA3 + H+ + (R′−Cl)•−

(7)

(R′−Cl)•− → R′• + Cl−

(8)

2c. The NAs in the IPN synthesis: Photopolymerization of EPOX/TMPTA Blends. A simultaneous cationic/radical photopolymerization of an EPOX/TMPTA blend (50%/50% w/w) using the NA3/Iod/NVK combination is feasible (formation of an interpenetrated polymer network; Figure 7). As in other systems,10,27−29 the final conversions of TMPTA are much higher in laminate than under air (Table 3). Tack free coatings are obtained after only 5 min of the halogen lamp irradiation in laminate.

2b. The NAs in the Free Radical Photopolymerization of Acrylates. The NA3/Iod/NVK or NA3/MDEA/R′-Cl systems allow the free radical polymerization (FRP) of TMPTA in laminate upon exposure to the halogen lamp (Figure 6, curve 2 and curve 4 for NA3/Iod/NVK and NA3/MDEA/R′-Cl,

Figure 5. Steady state photolysis of (a) NA1/Iod and (b) NA3/Iod in acetonitrile upon the halogen lamp exposure. [Iod] = 28 mM. UV−vis spectra recorded at different irradiation times. 976

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Figure 7. Photopolymerization profiles of an EPOX/TMPTA blend (50%/50%, w/w) in the presence of NA3/Iod/NVK (0.5%/2%/3%, w/w/w) under air (a) and in laminate (b) upon halogen lamp exposure.

Table 3. EPOX and TMPTA Conversions Obtained during the Polymerization of an EPOX/TMPTA Blend (50%/50%, w/w) under Air or in Laminate upon Exposure to the Halogen Lamp (t = 800 s) in the Presence of NA3/Iod/NVK (0.5%/2%/3%, w/w/w) under air in laminate

EPOX conversion (%)

TMPTA conversion (%)

35 33

9 70

(9a)

RS• + R′−CHCH 2 → R′−CH•−CH 2SR

(9b)

CONCLUSION

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ASSOCIATED CONTENT

Radicals and cations can be photochemically generated using naphthalene derivatives (NA3) in combination with an iodonium salt Iod and optionally N-vinylcarbazole NVK under low intensity of visible light irradiation. In addition, the NA3/amine and NA3/amine/2,4,6-tris(trichloromethyl)-1,3,5triazine systems are also able to produce radicals. The cationic polymerization of EPOX, the radical polymerization of TMPTA, the EPOX/TMPTA blend interpenetrated polymer network polymerization and the thiol−ene polymerization as well can be achieved. These compounds extend the range of available PIs working under purple/blue lights. Other works are under progress.

2d. The NAs in Thiol−Ene Photopolymerization. The NA3/Iod combination is also capable of initiating a thiol−ene (Trithiol/DVE-3) polymerization (reaction 9) under the halogen lamp (Figure 8). After 400 s of light irradiation, the vinyl ether double bond of DVE-3 and the thiol (S−H) group of Trithiol at 1620 and 2580 cm−1, respectively, decreased (Figure 8b) due to the consumption of these two functional groups during the thiol−ene photopolymerization. The different vinyl double bond vs S−H conversions (81% and 36% after 400 s of light exposure) suggests a significant contribution of the cationic polymerization of DVE-3 (see reactions 1−2). Ph• + RS−H → Ph−H + RS•

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S Supporting Information *

General details of the syntheses, chemical structures of NA5NA9 (Scheme S1), syntheses of naphthalene derivatives NA1− NA9 (Scheme S2); UV−vis absorption spectra of NA5-NA9 (Figure S1), emission spectrum for the halogen lamp (Figure S2), and transient absorption spectrum of NA3 in the laser flash photolysis experiment (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

R′−CH•−CH 2SR + RSH → R′−CH 2−CH 2SR + RS•

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

(9c)

Figure 8. (a) Photopolymerization profiles of Trithiol/DVE-3 blend (40%/60%, n/n; 57%/43%, w/w) in laminate in the presence of NA3/Iod (0.5%/2%, w/w) upon the halogen lamp exposure: curve 1, DVE-3 (vinyl double bond) conversion; curve 2, trithiol (S−H) conversion. (b) IR spectra of Trithiol/DVE-3 blend (40%/60%, n/n; 57%/43%, w/w) in laminate in the presence of NA3/Iod (0.5%/2%, w/w) before and after 400 s. 977

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Notes

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The authors declare no competing financial interest. § Former address: ENSCMu-UHA, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France.

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ACKNOWLEDGMENTS J.L. thanks the Institut Universitaire de France for financial support.

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dx.doi.org/10.1021/ma402622v | Macromolecules 2014, 47, 973−978