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Cationic and Thiol−Ene Photopolymerization upon Red Lights Using Anthraquinone Derivatives as Photoinitiators Pu Xiao,† Frédéric Dumur,‡ Bernadette Graff,† Jean Pierre Fouassier,§ Didier Gigmes,*,‡ 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: Anthraquinone derivatives in combination with an iodonium salt (and optionally N-vinylcarbazole) have been used as photoinitiating systems. One of them (Oil Blue N) that is particularly efficient for cationic, IPN, and thiol−ene polymerization upon red lights (laser diode at 635 nm or household red LED bulb at 630 nm) belongs to the very few systems available at this long wavelength in such experimental conditions (low light intensity in the 10−100 mW/cm2 range). Their abilities to initiate the cationic photopolymerization of epoxides or vinyl ethers under very soft halogen lamp irradiation have been also investigated. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, and electron spin resonance spin trapping techniques.

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INTRODUCTION

In the present paper, we still intend to develop PISs usable under monochromatic red light (laser diode at 635 nm and/or low intensity household red LED bulb centered at 630 nm) for CP and thiol−ene photopolymerization. The known and commercially available 9,10-anthraquinone derivatives AQD (i.e., quinizarin QUIN, disperse blue 1 DB1, disperse blue 3 DB3, and Oil Blue N OBN; Scheme 1) in combination with an iodonium salt (and optionally N-vinylcarbazole NVK) will be used here. Anthraquinones are mainly known as radical photoinitiators10 (only one paper10e was concerned with CP) working under UV light irradiation. The photochemistry and photophysics of a lot of anthraquinone derivatives are well documented (see e.g. in ref 11). The photochemical mechanisms involved in the initiation species formation will be investigated by steady state photolysis, fluorescence, cyclic voltammetry, and electron spin resonance spin trapping techniques; the use of a polychromatic light (low intensity household halogen lamp) will also be checked.

The application of promising dyes exhibiting excellent absorption spectra (especially at long wavelengths, e.g., under red light) and incorporated in high performance photoinitiating systems PISs for polymerization reactions is highly interesting and attracts an increasing attention in various fields ranging from imaging and optics technologies to medicine, microelectronics, nanotechnology, and material elaboration areas.1 Dyes play an important role in such PISs: after absorption, they interact with one or two additives to generate initiating radicals (for free radical polymerization FRP), cations, or radical cations (for cationic polymerization CP or free radical promoted cationic polymerization FRPCP). Albeit examples of red light or near-IR induced FRP are well-known (see a review in ref 1a or typical examples in ref 2), the attempts for the design of PISs that can work in red light induced CP or FRPCP are actually rather limited. Recently, we have proposed several dyes exhibiting good photoinitiating abilities for the CP of epoxides under exposure to a laser diode at 635 nm: violanthrone-79,3 2,7-di-tert-butyldimethyldihydropyrene DHP,4 perylene derivatives,5 multicomponent dyes,6 and pentacene derivatives.7 To the best of our knowledge, only one PIS (oxazine/ benzoylperoxide)8 for the thiol−ene photopolymerization of liquid crystals was shown to operate with a high intensity Kr+ laser source at 647 nm for the manufacture of holographic optical elements (in a general way, few reports8,9 have been concerned with visible light initiation of thiol−ene polymerization). © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials. The investigated anthraquinone derivatives AQD (i.e., quinizarin (QUIN), disperse blue 1 (DB1), disperse blue 3 (DB3), oil blue N (OBN)), and other chemical compounds are shown in Schemes 1 and 2. The studied anthraquinone derivatives, diphenyliodonium hexafluorophosphate (Iod), N-vinylcarbazole (NVK), methyldiethanolamine (MDEA), phenacyl bromide (R-Br), tri(ethylene Received: July 18, 2013 Revised: August 11, 2013

A

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Scheme 1. Chemical Structures of the Studied Anthraquinone Derivatives

Scheme 2. Chemical Structures of Additives and Monomers

anthraquinone derivatives, the reduction potential of Iod, the excited singlet (or triplet) state energy of the studied anthraquinone derivatives, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents):14

glycol) divinyl ether (DVE-3), trimethylolpropane tris(3-mercaptopropionate) (Trithiol), and solvents were purchased from SigmaAldrich or Alfa Aesar and used as received without further purification. The monomers (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) and trimethylolpropane triacrylate (TMPTA) were obtained from Cytec and used as benchmark monomers for radical and cationic polymerization. Irradiation Sources. Three different visible irradiation sources were used for the photopolymerization experiments: polychromatic light from a halogen lamp (Fiber-Lite, DC-950; incident light intensity: ∼12 mW cm−2 in the 370−800 nm range), red laser diode at 635 nm (100 mW cm−2), and household red LED bulb centered at 630 nm (∼10 mW cm−2). 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)12 at about 790, 1630, 1620, and 2570 cm−1, respectively. Fluorescence Experiments. The fluorescence properties of the investigated anthraquinone derivatives were studied in acetonitrile using a JASCO FP-750 spectrometer. The interaction rate constants kq between the studied anthraquinone derivatives and Iod were extracted from classical Stern−Volmer treatments13 (I0/I = 1 + kqτ0[Iod], where I0 and I stand for the fluorescent intensity of the studied anthraquinone derivatives in the absence and the presence of the Iod quencher, respectively; τ0 stands for the lifetime of the excited anthraquinone derivatives in the absence of Iod). Redox Potentials. The oxidation potentials (Eox vs SCE) of the studied anthraquinone 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 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 anthraquinone derivatives and Iod can be calculated from the classical Rehm−Weller equation (eq I, where Eox, Ered, ES (or ET), and C are the oxidation potential of the studied

ΔG = Eox − Ered − ES (or E T) + C

(I)

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 procedure15 described in detail. The ESR spectra simulations were carried out with the WINSIM software. 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 the B3LYP/6-31G* level on the relaxed geometries calculated at the UB3LYP/6-31G* level.16

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RESULTS AND DISCUSSION 1. Light Absorption Properties of the Studied Anthraquinone Derivatives. The absorption spectra of the investigated AQDs (QUIN, DB1, DB3, and OBN) in acetonitrile are given in Figure 1. As expected, the absorption

Figure 1. UV−vis absorption spectra of the studied anthraquinone derivatives in acetonitrile. B

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absorbed energy as the incident light intensities of the laser diode and the household LED bulb irradiation sources are in a 10/1 ratio (∼100 and ∼10 mW cm−2). The photopolymerization of EPOX and DVE-3 in the presence of the AQDs based photoinitiating systems (PISs) under air was also carried out at 635 nm (Iod alone cannot initiate the polymerization as it works below 300 nm).13,17 Typical conversion profiles are given in Figure 4, and the final conversions are summarized in Table 1. More interestingly, using OBN/Iod/NVK in EPOX upon the laser diode at 635 nm exposure leads to the formation of tack free coatings with a higher polymerization rate than that under the halogen lamp irradiation (Figure 4a, curve 3 vs curve 2). The polymerization efficiencies in the presence of QUIN, DB1, or DB3 based PISs are lower than that of OBN based PISs (Table 1), which is ascribed to the smaller molar extinction coefficients and the worse overlapping of the absorption spectra with the emission spectra of the irradiation sources. Interestingly, the OBN/Iod combination could also efficiently initiate the CP of DVE-3 (final conversion = 66%; Figure 4b, curve 2). The OBN/Iod, OBN/Iod/NVK, OBN/MDEA, and OBN/ MDEA/R-Br combinations can also initiate the radical polymerization of TMPTA in laminate at 635 nm, but the efficiencies are low (Figure S1 in the Supporting Information and Table 1). An interpenetrated polymer network (IPN) can also be fabricated through a concomitant cationic/radical photopolymerization of an EPOX/TMPTA blend (50%/50% w/w) using the OBN/Iod/NVK combination under air or in laminate at 635 nm (Figure 5c,d). As observed elsewhere in other systems,18 the final conversions of TMPTA are higher in laminate than under air (opposite situation when considering EPOX; Table 2), the oxygen inhibition effect playing a significant role and the radicals being predominantly consumed in the FRP of TMPTA in laminate rather than by the FRPCP of EPOX. The OBN/Iod/NVK combination could also initiate the CP of an EPOX/DVE-3 blend (50%/50% w/w) as seen in Figure S2 and Table S1. b. Soft Polychromatic Visible Light Irradiation. The AQDbased PISs can also work under air under exposure to a halogen lamp. The OBN/Iod combination efficiently initiates the polymerization of EPOX (Table 1; final conversion of 53%; Figure 4a, curve 1; tack free coating). The addition of NVK (which is known as a suitable additive19) slightly enhanced the polymerization profile (final conversion of 59%; Figure 4a, curve 2 vs curve 1; tack free coating). The OBN/Iod system for

maxima of DB1, DB3, and OBN (around 635 nm) are redshifted compared with that of QUIN (477 nm) due to the presence of the 1,4-diamino substituents on the anthraquinone skeleton which ensures, especially for OBN, a better overlapping with the emission spectra of the halogen lamp, the red LED bulb, and the laser diode at 635 nm. Molar extinction coefficients at 635 nm are ∼2100, 4100, and 14 400 M−1 cm−1 for DB1, DB3, and OBN, respectively. Molecular orbital calculations confirm this red-shift e.g. for OBN (λmax = 548 nm, f = 0.228) vs QUIN (λmax = 452 nm, f = 0.175), f being the oscillator strength characterizing the intensity of the transition. The red-shifted transitions are ascribed to a strong participation of the amino substituent to the HOMO and LUMO (see Figure 2 for OBN).

Figure 2. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the QUIN and OBN at the UB3LYP/6-3G* level.

2. Photoinitiating Ability of the Investigated Anthraquinone Derivatives. a. Monochromatic Red Light Irradiation. Excellent polymerization profiles are obtained for the thiol−ene (Trithiol/DVE-3) photopolymerization using the OBN/Iod combination under a red light irradiation (Figure 3). The vinyl double bond conversions (98% and 68% with the red laser diode and red LED, respectively) are higher than those of S−H (35% and 20%), which could be attributed to the fact that DVE-3 can also be polymerized by a CP process initiated by the cations generated from the OBN/Iod combination in DVE-3 (see Figure 4b and reactions 1−4 below) or through the thiol− ene process (see reactions 5−7 below). The low final conversion of the trithiol indicates that (i) the cationic photopolymerization of DVE-3 probably predominates and (ii) the thiol−ene process occurs to a lesser extent (Figure 4b curve 2 vs Figure 3a curve 2). The monomer and S−H conversion profiles are obviously affected by the amounts of

Figure 3. Photopolymerization profile of Trithiol/DVE-3 blend (40%/60%, n/n; 57%/43%, w/w) in laminate in the presence of OBN/Iod (0.5%/ 2%, w/w) upon the (a) red laser diode at 635 nm and (b) red LED bulb at 630 nm exposure; curve 1: DVE-3 (vinyl double bond) conversion; curve 2: trithiol (S−H) conversion. C

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Figure 4. Photopolymerization profile of (a) EPOX under air in the presence of (1) OBN/Iod (0.5%/2%, w/w) and (2) OBN/Iod/NVK (0.5%/ 2%/3%, w/w/w) upon the halogen lamp exposure; (3) OBN/Iod/NVK (0.5%/2%/3%, w/w/w) upon the laser diode at 635 nm exposure; (b) DVE-3 in laminate in the presence of OBN/Iod (0.5%/2%, w/w) upon (1) the halogen lamp and (2) the laser diode at 635 nm exposure.

AQD•+ + Ph 2I• → AQD•+ + Ph• + Ph−I

Table 1. EPOX or TMPTA Final Conversions (%) Obtained upon Exposure to the Halogen Lamp and the Laser Diode at 635 nm in the Presence of Anthraquinone Derivative Based PISs laser diode at 635 nm

halogen lamp a

PISs

EPOX

Iod QUIN/Iod (0.5%/2%) QUIN/Iod/NVK (0.5%/2%/ 3%) DB1/Iod (0.5%/2%) DB1/Iod/NVK (0.5%/2%/3%) DB3/Iod (0.5%/2%) DB3/Iod/NVK (0.5%/2%/3%) OBN/Iod (0.5%/2%) OBN/Iod/NVK (0.5%/2%/3%) OBN/MDEA (0.5%/2%) OBN/MDEA/R-Br (0.5%/2%/ 3%) OBN (2%) OBN/R-Br (2%/3%) a

TMPTA

0 0 0

•

b

0

a

EPOX 0

TMPTA

b

0

(3b)

Ph• + VE → Ph−VE•

(4a)

Ph−VE• + Ph 2I+ → Ph−VE+ + Ph• + Ph−I

(4b) (5)

•

•

(6) •

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

(7) 12 24 27 28 22 25

AQD →1 AQD (hν) and 1AQD → 3AQD AQD + Ph 2I → AQD

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

RS + R′−CH=CH 2 → R′−CH −CH 2SR

0 28 0 38 53 59

•+

(3a)

•

54

Fluorescence quenching experiments (Figures S3 and S4) of DB3 and 1OBN by Iod and calculations of the free energy changes ΔG for the corresponding electron transfer reaction 2a supports a fast process: very high interaction rate constants kq = ∼8.3 × 109 and ∼2.8 × 109 M−1 s−1 for DB3 and OBN, respectively; ΔG = −1.16 and −1.15 eV for 1DB3/Iod and 1 OBN/Iod, respectively (oxidation potentials Eox of the DB3 and OBN = 0.55 and 0.56 V, respectively, as measured by cyclic voltammetrythis work (Figure S5); reduction potential Ered = −0.2 V1a for Iod; singlet state energy Es = 1.91 and 1.91 eV for DB3 and OBN as extracted from the UV−vis absorption and fluorescence emission spectra as usually done20). The DB3/Iod and OBN/Iod electron transfer quantum yields ΦeT calculated according to eq II are 0.66 and 0.35, respectively (for [Iod] = 4.7 × 10−2 M). As a consequence, the singlet route plays an important role for reaction 2. These relative yields (ΦeT) do not follow the experimental efficiency (i.e., OBN ≫ DB3), showing that the much better light absorption properties of OBN counterbalance its lower reactivity. In addition, the triplet process of DB3/Iod or OBN/Iod is also favorable and can participate to a minor part (ΔG = −0.34 and −0.31 eV for DB3/Iod and OBN/Iod, respectively, using the calculated triplet state energy ET (1.09 and 1.07 eV for DB3 and OBN, respectively, at the B3LYP/6-31G* level)).

10 13 22 25

1

6 8

the polymerization of DVE-3 in laminate is less efficient (Figure 4b, curve 1; lower polymerization rate, final conversion of 43%). The radical polymerization of TMPTA in laminate using the OBN/Iod, OBN/Iod/NVK, OBN/MDEA, and OBN/ MDEA/R-Br combinations is also feasible, but the efficiencies are low under the halogen lamp irradiation (Figure S1 and Table 1). An interpenetrated EPOX/TMPTA polymer network (IPN) can also be fabricated using the OBN/Iod/NVK combination under air or in laminate upon exposure to the halogen lamp (Figure 5a,b). 3. Photochemistry of the Investigated Anthraquinone Derivatives. DB1, DB3, and OBN are expected to exhibit low fluorescence and intersystem crossing quantum yields as already observed e.g. for the 1,4-dimethylamino-9,10-anthraquinone parent compound (∼0.0211a and 0.01511e in toluene, respectively), its first excited singlet state mostly decaying through internal conversion (∼96%). According to the studies of other related PISs,1a the following reaction sequence holds true as confirmed by the experiments reported below:

+

Ph + NVK → Ph−NVK

Ph• + RS−H → Ph−H + RS•

Under air for 800 s. bIn laminate for 400 s.

1,3

(2b)

•

•

+ Ph 2I

ΦeT = kqτ0[Iod]/(1 + kqτ0[Iod])

(II)

In line with reaction 2, phenyl radicals are also observed in ESR spin trapping experiments on DB3/Iod (Figure 6A) and OBN/Iod (Figure 6B). As expected from the efficient singlet state quenching, a very fast bleaching (within 30 s) was observed during the irradiation of DB1/Iod, DB3/Iod, or OBN/Iod in acetonitrile (Figure 7;

(1) (2a) D

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Figure 5. Photopolymerization profile of an EPOX/TMPTA blend (50%/50%, w/w) in the presence of OBN/Iod/NVK (0.5%/2%/3%, w/w/w) under air (a, c) and in laminate (b, d); halogen lamp exposure (a, b); laser diode exposure at 635 nm (c, d).

properties, the inhibition effect of the hydroxyl groups in DB3 for the ring-opening cationic polymerization, and the possible back electron transfer reactions or other by-side reactions following the reaction 2a. In OBN-based PISs, the initiating species are OBN•+ and Ph-NVK+ (reactions 1−3)19 for the CP/FRPCP of EPOX, the Ph-VE+ cations (reactions 1, 2, and 4) for the CP of DVE-3, and the thiyl radicals (reactions 5−7) for the thiol−ene photopolymerization.

Table 2. EPOX and TMPTA Final Conversions (%) Obtained from the Photopolymerization of an EPOX/ TMPTA Blend (50%/50%, w/w) under Air or in Laminate upon Exposure to the Halogen Lamp or the Laser Diode at 635 nm for 800 s in the Presence of OBN/Iod/NVK (0.5%/ 2%/3%, w/w/w) EPOX conversion (%) halogen lamp laser diode at 635 nm

TMPTA conversion (%)

under air

in laminate

under air

in laminate

46 45

40 22

19 15

49 39

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CONCLUSION The commercial anthraquinone derivative Oil Blue N (OBN) in combination with an iodonium salt Iod and optionally Nvinylcarbazole NVK can be used as a high performance red light sensitive PIS to efficiently initiate the cationic polymerization of EPOX and DVE-3, the TMPTA/EPOX blend IPN polymerization, and the thiol−ene polymerization as well. Even if the design and the synthesis of original compounds is the royal way

halogen lamp exposure; under air) in contrast to the low bleaching noted with QUIN/Iod. On the basis of the above investigations, the lower polymerization efficiency of DB1 (DB3, QUIN)/Iod compared to that of OBN/Iod may be associated with the light absorption

Figure 6. ESR spectra of the radicals generated in (A) DB3/Iod and (B) OBN/Iod and trapped by PBN in tert-butylbenzene: (a) experimental and (b) simulated spectra. PBN/phenyl radical adducts obtained in DB3/Iod and OBN/Iod: aN = 14.1 G, aH = 2.2 G and aN = 14.1 G, aH = 2.2 G, respectively; reference values.21 E

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Figure 7. Steady state photolysis of (a) QUIN/Iod, (b) DB1/Iod, (c) DB3/Iod, and (d) OBN/Iod in acetonitrile upon the halogen lamp exposure; [Iod] = 23.5 mM. UV−vis spectra recorded at different irradiation times. The isosbestic points are observed at 520 and 523 nm for DB3/Iod and OBN/Iod, respectively, which clearly indicate that no secondary reactions occur during the considered time range. Interestingly, a new band at 706 nm appeared immediately upon addition of Iod into the DB1 solution (b), which may be due to the formation of DB1/Iod complex in the solution.

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

to make great scientific and technical strides in the photopolymerization area, this present research reveals that a careful screening of various known and commercial dyes can ever remain an easy and convenient way to discover promising and rare compounds that can operate in particular soft experimental conditions (low intensity visible lights).

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ASSOCIATED CONTENT

S Supporting Information *

Photopolymerization profile of TMPTA in the presence of OBN based PISs (Figure S1); photopolymerization profile of an EPOX/DVE-3 blend in the presence of OBN/Iod/NVK (Figure S2); examples of fluorescence spectra for DB3 or OBN in presence of Iod (Figure S3); examples of fluorescence quenching of DB3 or OBN by Iod (Figure S4); cyclic voltammogram of OBN (Figure S5); EPOX and DVE-3 final conversions obtained from the photopolymerization of an EPOX/DVE-3 blend in the presence of OBN/Iod/NVK (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address §

J.P.F.: Formerly ENSCMu-UHA, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France. Notes

The authors declare no competing financial interest. F

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dx.doi.org/10.1021/ma401513b | Macromolecules XXXX, XXX, XXX−XXX