Different NIR dye scaffolds for polymerization reactions under NIR light

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Different NIR dye scaffolds for polymerization reactions

under NIR light

Aude Bonardi, Fabrice Bonardi, Guillaume Noirbent, Frederic Dumur, Céline

Dietlin, Didier Gigmes, Jean-Pierre Fouassier, Jacques Lalevee

To cite this version:

Aude Bonardi, Fabrice Bonardi, Guillaume Noirbent, Frederic Dumur, Céline Dietlin, et al.. Different NIR dye scaffolds for polymerization reactions under NIR light. Polymer Chemistry, Royal Society of Chemistry - RSC, 2019, 10 (47), pp.6505-6514. �10.1039/C9PY01447K�. �hal-02866920�

1

Different NIR dye scaffolds for polymerization

reactions under NIR light

A-H. Bonardi1,2, F. Bonardi3, G. Noirbent4, F. Dumur 4, C. Dietlin1,2, D. Gigmes4, J.P. Fouassier, and J. Lalevée*1,2

1

Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France 2_{Université de Strasbourg, France}

3_{IBISC, Univ Evry, Université Paris-Saclay, 91025, Evry, France} 4_{Aix Marseille Univ, CNRS, ICR, UMR7273, F-13397 Marseille, France}

e-mail: [email protected]

A

BSTRACT

:

In this article, near-infrared dyes of different structures have been investigated as new photoinitiators/photosensitizers for the free radical polymerization (FRP) of methacrylates upon Near Infrared (NIR) light exposure using Laser Diode @785 nm, @940 nm and @1064 nm. Interestingly, the use of squaraine, squarylium, boron-pyrromethene and porphyrin derivatives as efficient photoinitiators is clearly highlighted. These dyes are used in combination with an iodonium salt and a phosphine. Additionally, a thermal initiator can be added to the resin to take advantage of the photothermal effect i.e. to the heat release during the polymerization process, so that polymerization kinetics could be greatly improved compared to the pure photochemical mode. These dyes can be proposed as alternatives to well-established cyanine dyes, more commonly used for the photopolymerization of (meth)acrylates upon NIR light.

2

1.

I

NTRODUCTION

Photopolymerization is the transformation of a liquid monomer (or a soft film) into a solid polymer (or a solid film) upon a short light exposure. This technology is often considered as “green” because polymerization occurs at room temperature, with low energy consumption and with no or almost no release of volatile organic compounds (VOCs).[1][2][3] Because of these advantages, many products are now manufactured with the use of light: cosmetics[4], composites[5], regenerative medicine[6], inks[7], adhesives[8], 3D printing[9]… However, radical photopolymerization processes in industry often rely on high intensity UV light sources. Unfortunately, UV wavelengths are harmful both for human health and environment.[10][11] Therefore, the search for photoinitiating systems able to cure under longer wavelengths is highly desired. Lots of systems are now capable to initiate a polymerization under blue light.[12][13][14][15] The use of even longer wavelengths and in particularly Near-Infrared wavelengths is even more advantageous. Indeed, the main advantage of NIR light for polymer synthesis is a deeper penetration of light in the material thanks to a reduced diffusion.[16] Thus, in presence of fillers, composites can be cured more efficiently with NIR light than with UV or visible light. However, greater wavelengths are also synonym of less energetic photons [17] leading to safer irradiation devices. However, with longer wavelengths, the initiation is much more difficult. Different systems able to initiate

polymerization under NIR light have been reported in the

literature.[16][18][19][20][21][22][23][24][25]

The majority of the NIR photoinitiating systems are still based on cyanines as NIR photoinitiators.[16][18][20][22][26][27][28][29][30] However, NIR dyes are not restricted to cyanines. NIR dyes are components with light absorption ranging between 780 and 1400 nm and can involve several families/scaffolds such as squaraines, squaryliums, phthalocyanines… [31] Therefore, to diversify the NIR systems, we propose new photoinitiating systems based on these latter scaffolds as NIR photoinitiators. These dyes are mainly used in a four-component initiating system comprising an iodonium salt, a phosphine and a thermal initiator. This system is the merging between a pure photochemical system generating radicals through a light activated redox process (as presented in [18]) and a photothermal system based on the conversion of light to heat (NIR dye = heater) (as presented in [16]). It is crucial to combine these two systems to obtain good polymerization results i.e. the yield in initiating radicals is improved leading to faster polymerization processes. Low or no curing is observed in the photochemical system alone or in the photothermal system used alone. However, in the four-component initiating system, they are able to initiate a free radical polymerization under NIR light delivered by Laser Diodes @785 nm, 940 nm or 1064 nm.

2.

E

XPERIMENTAL

S

ECTION

2.1.

3

The blend “Mix-MA”, used as a benchmark monomer, has been prepared with 33.3 wt% of (hydroxypropyl)methacrylate (HPMA), 33.3 wt% of 1,4-butanediol dimethacrylate (1,4-BDMA) and 33.3 wt% of a urethane dimethacrylate monomer (UDMA), obtained from Sigma Aldrich and represented in Scheme 1.

Scheme 1. Mix-MA resin mix

Additives

The phosphine 4-(diphenylphosphino)benzoic acid (4-dppba, Scheme 2) has been purchased from Sigma-Aldrich. The iodonium salt bis(4-tert-butylphenyl)iodonium hexafluorophosphate (Ar2I+/PF6-; Scheme 2) was obtained from Lambson Ltd. The thermal initiator BlocBuilder®MA (Scheme 2) has been obtained from Arkema.

Scheme 2. Chemical structures of additives

NON

-

The NIR dyes were prepared according to the procedures presented in detail in supporting information. Squaraine dyes used in this work are represented in the Scheme 3, Squarylium dyes in Scheme 4, Boron-pyromethenes (often called Bodipy) in Scheme 5 and Porphyrins in Scheme 6.

4

Scheme 3. Squaraine dyes

Scheme 4. Squarylium dyes

Scheme 5. Bodipy dyes

5

2.2.

Laser Diode@785nm with an incident light intensity at the sample surface I = 400 mW/cm², Laser Diode @940nm with selectable irradiance from 0 W to 4 W/cm² and Laser Diode @1064 nm with selectable irradiance from 0 W to 3 W/cm² were selected for the present work. These diodes have been purchased from Changchun New Industries (CNI).

2.3.

The photoinitiating systems (PISs) for free radical polymerization are mainly based on NIR dye/iodonium salt/phosphine/thermal initiator (0.1w%/3w%/2w%/2w%). The weight percentages were calculated from the monomer content. An excellent solubility of all NIR dyes was observed in Mix-MA monomer. The photosensitive formulation is deposited in a mold (1.4 mm high) under air and irradiated with NIR Laser Diodes. The evolution of the content of methacrylate functions is continuously followed by Real Time Fourier Transform Infrared (RT-FTIR) spectroscopy (a Jasco 4600) through the band at 6100-6220 cm-1.

For all experiments, the NIR light is switched on 17 seconds after the first spectrum measurement in order to have a baseline. Details for each experiment are given in the figure captions of the concerned experiment. The procedure has been described more in details in [32].

2.4.

The UV-vis spectroscopy has been performed with a Varian Cary 3 spectrometer in order to investigate the absorption properties of the different compounds. Thanks to the Beer Lambert Law [33], the molar extinction coefficients were calculated. Measurements were performed at room temperature in a cell with 1 cm optical pathlength. For measurement in acetonitrile, a quartz cell is used. For measurement in monomer, a plastic cell is used. More details about the experimental procedure are given in the caption of the concerned figures.

3.

R

ESULTS AND DISCUSSION

As different classes of dyes were investigated as NIR photoinitiators/photosensitizers, the results will be presented successively for the different families.

3.1.

S

6

Absorption spectra for the investigated squaraines are reported in Figure 1. The extinction coefficients at the specific emission wavelengths of the different Laser Diodes used for photopolymerization are also reported in Table 1.

The optical properties of squaraine dyes have already been largely described in the literature.[34] These compounds exhibit a sharp and intense absorption band with a maximum for the molar extinction coefficient higher than 105 L.mol-1.cm-1: this property is clearly observed on the squaraines presented in this work. The squaraine dyes are also reported in the literature for their tunable character i.e. their absorption maxima which can cover the visible or infrared region depending of their structures.[35] Indeed, the maximum of absorption of these compounds is strongly affected by the donor moieties and the π-conjugated bridge. [36] i.e. with different moieties, different absorptions are observed. In particularly, for SQ_4, a clear bathochromic shift is observed vs. SQ_1, SQ_2 and SQ_3.

However, solvatochromic effects as well as the formation of charge transfer complexes are still possible when squaraines are combined with other components of the photoinitiating system or the monomer. Therefore, in Table 1, we report the molar extinction coefficients of different NIR dyes (one for each family) in the Mix-MA monomer in the same conditions than that used during polymerization i.e. in the presence of Ar2I+/PF6- (3w%), 4-dppba (2w%), BlocBuilder®MA (2w%)).

7

Figure 1. Absorption spectra of (1) SQ_1, (2) SQ_2,(3) SQ_3 and (4) SQ_4 in acetonitrile

Comparison of the absorption spectra in ACN vs. Mix-MA (in the presence of the different polymerization additives) (see Table 1) shows that the absorption is different in acetonitrile and in Mix-MA. More precisely, in the NIR range, the absorption is broader in monomer with all additives than in the pure solvent. Even if the molar extinction coefficients are not very high, a none-negligible absorption is found in agreement with the possible NIR light activation (see the polymerization kinetics below for rather thick samples: 1.4 mm). Indeed, even if the molar extinction coefficients of the studied squaraines at the wavelengths of irradiation of the different NIR sources are quite low, a photoinitiation can occur when we used rather thick samples (1.4mm).

(1)

(2)

(3)

(4)



(L

.mo

l

.cm

)

Wavelength (nm)

8

Table 1. Maximum absorption wavelength (visible and NIR range) and associated extinction coefficient and molar extinction coefficients for the investigated compounds at the wavelengths of the different Laser Diodes used in acetonitrile or in Mix-MA in presence of the polymerization additives.

λmax (nm) ɛ(λmax) (L.mol-1._cm-1₎ ɛ(785nm) in ACN (L.mol-1. cm-1 ) ɛ(940nm) in ACN (L.mol-1.cm-1) ɛ(785nm)a in Mix-MA (L.mol-1. cm-1 ) ɛ(940nm)a in Mix-MA (L.mol-1.cm-1) SQ_1 521 66890 70 10 120 80 SQ_2 514 91700 70 20 n.d. n.d. SQ_3 525 78610 470 290 n.d. n.d. SQ_4 610 53730 150 150 n.d. n.d.

a: in presence of the polymerization additives: in the presence of Ar2I + /PF6 (3w%), 4-dppba (2w%), BlocBuilder®MA (2w%). n.d. not determined.

A

The four investigated squaraines have been used as photoinitiators/photosensitizers in the four-component initiating systems described above combining photochemical (NIR-dye/iodonium/phosphine) and photothermal (NIR-dye/thermal initiator) systems described in [37] i.e. 0.1w% of photoinitiator, 2w% of 4-dppba, 3w% of Ar2I+/PF6- and 2w% of BlocBuilder®MA. The good performance of this system for initiation of a benchmarked methacrylate resin for thick samples (1.4 mm) is shown in Figure 2. Final conversion higher than 75% are clearly observed with the four squaraines dyes proposed upon Laser Diode @785 nm (Figure 2, (A)) and low light intensity (only 400 mW/cm² are necessary to obtained tack-free sample within 200 seconds of irradiation). Upon Laser Diode @940 nm, solid and tack-free polymer is obtained with the 4 squaraines, with final conversion close to 80% for SQ_1, SQ_3 et SQ_4 as represented in Figure 2 (B).

9

Figure 2. Photopolymerization profiles for Mix-MA under air (methacrylates function conversion vs. irradiation time) in the presence of Ar2I+/PF6- (3w%), 4-dppba (2w%), BlocBuilder®MA (2w%) and: (1) SQ_1

(0.1w%), (2) SQ_2 (0.1w%), (3) SQ_3 (0.1w%)and (4) SQ_4 (0.1w%) upon (A) Laser Diode@785nm and (B) Laser Diode@940nm, thickness=1.4mm, the irradiation starts for t = 17s.

Time necessary to reach the final conversion and final conversion are summarized in Table 2. In terms of final conversions, we can see that SQ_3 and SQ_4 are the best dyes at both wavelength of irradiation. However, the lowest inhibition time is obtained with SQ_3 but with afinal conversion a bit lower than for SQ_4.

Table 2. Photopolymerization results in Mix-MA under air in the presence of dye/Ar2I+PF6-

/4-dppba/BlocBuilder®MA (0.1w%/3w%/2w%/2w%) under exposure to Laser Diode@785nm or Laser Diode @940nm; thickness=1.4mm.

785 nm 940 nm

Final conversion Time to reach the final conversion

Final conversion Time to reach the final conversion

SQ_1 75% 233 s 86% 362 s

SQ_2 84% 155 s 48% 288 s

SQ_3 83% 140 s 85% 153 s

SQ_4 89% 262 s 88% 342 s

The four-component initiating system used here combines the photochemical mechanisms (as presented by us in [18]) and the photothermal initiation (also proposed by us in [16]). Remarkably, when these squaraines have been tried in the pure photochemical or photothermal systems, no polymerization has been obtained within 500 s of irradiation. We concluded that it was necessary to use the combined mode to obtain good polymerization profiles. Through the combined mode, the yield in initiating radicals is increased and efficient polymerization processes are observed.

0 100 200 300 400 0 20 40 60 80 100 Co nv ers io n (%) Time (s) 0 100 200 300 400 500 0 20 40 60 80 100 C o n v e rsi o n (% ) Time (s) (1) (2) (3) (4) (1) (2) (3) (4) (A) (B)

10

3.2.

S

LIGHT ABSORPTION PROPERTIES

Squaryliums are rather similar to squaraines. Both families are characterized by an aromatic four membered ring system. For squaraine dyes, the two oxygens are attached to carbons on alpha position from each other whereas for squarylium, the oxygens are attached to carbons on beta position from each other. The second difference is that one oxygen of the squarylium dye exhibits a negative charge. Their light absorption properties are rather comparable: both exhibits a narrow but very intense band with position of the absorption maxima higher than 500 nm.[38] Very high molar absorption coefficients are observed (superior to 250 000 L.mol-1.cm-1) in the case of SQm_4 for example (Table 3) but associated with narrower absorption bands. The difference of absorption maxima between the different structures is due to different electron-donating properties of the moieties attached to the π-conjugated bridge.

Visible absorption data for the dyes are summarized in the Table 3 and depicted in Figure 3. As previously mentioned, the absorption spectrum of a representative dye (here SQm_4) has been checked in the monomer with all components used in the initiating system. An absorption shoulder in the near-infrared region is observed (better absorption than in acetonitrile – see Table 3).

11

Figure 3. Absorption spectrum of (1) SQm_1, (2) SQm_2,(3) SQm_3 and (4) SQm_4 in acetonitrile

Table 3. Maximum absorption wavelengths (visible and NIR range) and associated extinction coefficients and molar extinction coefficients for the investigated compounds at the wavelengths of the different Laser Diodes used (in acetonitrile or in Mix-MA in presence of the polymerization additives).

λmax (nm) ɛ(λmax) (L.mol-1._cm-1₎ ɛ(785nm) in ACN (L.mol-1._cm-1₎ ɛ(940nm) in ACN (L.mol-1.cm-1) ɛ(785nm)a in Mix-MA (L.mol-1._cm-1₎ ɛ(940nm)a in Mix-MA (L.mol-1.cm-1) SQm_1 663 120538 130 30 n.d. n.d. SQm_2 645 219297 220 10 n.d. n.d. SQm_3 457 59800 60 70 n.d. n.d. SQm_4 632 272970 120 120 1750 1410

a: in presence of the polymerization additives: in the presence of Ar2I+/PF6- (3w%), 4-dppba (2w%),

BlocBuilder®MA (2w%). n.d. not determined.

A

400 500 600 700 800 900 1000 0 50000 100000 150000 200000 250000 300000



(m

o

l.

L

.cm

)

Wavelength (nm)

(1)

(2)

(3)

(4)



(L

.mo

l

.cm

)

Wavelength (nm)

12

The same trends are observed with squarylium dyes than with squaraine dyes: despite the rather low light absorption properties of the different squaryliums at the NIR irradiation devices, the photopolymerization is possible.

Indeed, as shown in Figure 4(A), upon irradiation with a Laser Diode @ 785 nm and 400 mW/cm², it is possible to reach high final methacrylate function conversions with SQm1 and SQm_2 within 100s of irradiation in the four-component initiating system. At 940 nm, SQm_2 absorbs almost no light and no polymerization is observed. SQm_1 and SQm_3 give also lower final conversions at 940 nm than SQm_4 as observed in Figure 4(B). SQm_4 absorbs enough light at this wavelength of irradiation to give a good polymerization profile. This squarylium dye was not able to initiate a polymerization with our Laser Diode @ 785 nm and SQm_3 gives a late polymerization. It can be explained by the power of the Laser Diodes we used: the one at 785 nm has an irradiance of 400 mW/cm² when the one at 940 nm has an irradiance of 3 W/cm² and the oxygen inhibition (the polymerization is carried out under air).

Figure 4. Photopolymerization profiles for Mix-MA under air (methacrylates function conversion vs. irradiation time) in the presence of Ar2I

+

/PF6

(3w%), 4-dppba (2w%), BlocBuilder®MA (2w%) and: (1) SQm _1 (0.1w%), (2) SQm_2 (0.1w%), (3) SQm_3 (0.1w%) and (4) SQm_4 (0.1w%) upon (A) Laser Diode@785nm and (B) Laser Diode@940nm, thickness=1.4mm, the irradiation starts for t = 17s.

The different photopolymerization data (final methacrylate function conversion and time necessary to reach it) are summarized in Table 4. SQm_1 is the dye which reached the final conversion the fastest at both wavelengths (785 or 940 nm). At 785 nm, it is also the dye with the highest final conversion. At 940 nm, the highest final conversion is reported for SQm_4.

Table 4. Photopolymerization results of Mix-MA under air in the presence of NIR dye/Ar2I+PF6-

/4-dppba/BlocBuilder®MA (0.1w%/3w%/2w%/2w%) under exposure to Laser Diode@785nm or Laser Diode @940nm; thickness=1.4mm. 785 nm 940 nm 0 200 400 0 20 40 60 80 100 C on ve rs io n (% ) Time (s) 0 100 200 300 400 500 0 20 40 60 80 100 Co nv er si on (% ) Time (s) (1) (2) (3) (4) (1) (2) (3) (4) (A) (B)

13

Final conversion Time to reach the final conversion

Final conversion Time to reach the final conversion SQm_1 84% 118 s 60% 332 s SQm_2 67% 211 s 12% at 500s >500 s SQm_3 31% at 500s >500 s 60% 416 s SQm_4 2% at 500s >500 s 75% 417 s

3.3.

B

LIGHT ABSORPTION PROPERTIES

Visible absorption data for the BODIPY are presented in Table 5 with related spectra are depicted in Figure 5. Many photophysical properties of BODIPY have already been described in the literature.[39] High intensities of absorption and fluorescence are observed due to the “BF2” moiety [39]. Tunability of the absorption properties for this type of compounds is easy by changing the substituents at different positions of the Bodipy core. They are particularly described for their absorption in the visible range and are suitable for applications in photodynamic therapy. The two boron-pyrromethenes presented here exhibit a strong absorption around 500 nm (Figure 5) and a low absorption at our wavelengths of interest (NIR) (Table 5). Absorption of Bodipy_1 can be compared in acetonitrile and in the monomer with all components of the initiating system together: the absorption is higher for longer wavelengths in monomer than in acetonitrile (Table 5).

14

Figure 5. Absorption spectrum of (1) Bodipy_1 and (2) Bodipy_2 in acetonitrile

400 500 600 700 800 900 1000 0 10000 20000 30000 40000 50000 60000 70000



(L

.m

ol

.c

m

)

Wavelength (nm)

(1)

(2)

15

Table 5. Maximum absorption wavelengths (visible and NIR range) and associated extinction coefficient and molar extinction coefficients for the investigated compounds at the wavelengths of the different Laser Diodes used (in acetonitrile or in Mix-MA in presence of the polymerization additives).

λmax (nm) ɛ(λmax) (L.mol-1._cm-1₎ ɛ(785nm) in ACN (L.mol-1. cm-1 ) ɛ(940nm) in ACN (L.mol-1.cm-1) ɛ(785nm)a in Mix-MA (L.mol-1. cm-1 ) ɛ(940nm)a in Mix-MA (L.mol-1.cm-1) Bodipy_1 500 18980 300 250 840 510 Bodipy_2 492 68080 170 100 n.d. n.d.

a: in presence of the polymerization additives: in the presence of Ar2I+/PF6- (3w%), 4-dppba (2w%),

BlocBuilder®MA (2w%). n.d. not determined.

A

Bodipy_1 and Bodipy_2 have been incorporated into the four-component initiating system for photopolymerization of methacrylates upon Laser Diode@785 nm and Laser Diode@940 nm exposure. Both components have strong abilities to initiate the photopolymerization as shown in Figure 6 despite their relatively low absorptions (see Table 5). At 785 nm, final conversions measured are higher than 80% and tack-free samples are obtained. For Bodipy_1, photoinitiation is even better at 940 nm: the same final conversion is reached faster than at 785 nm, this can be explained by the higher power of the Laser Diode at 940 nm compared to the power at 785 nm. For Bodipy_2, the final conversion observed at 940 nm is lower than at 785 nm. Photopolymerization results using the two boron-pyrromethenes are summarized in Table 6. In both case, tack-free samples are obtained within 500 seconds of irradiation.

16

Figure 6. Photopolymerization profiles of Mix-MA under air (methacrylates function conversion vs. irradiation time) in the presence of Ar2I+/PF6- (3w%), 4-dppba (2w%), BlocBuilder®MA (2w%) and: (1)

Bodipy_1 (0.1w%) and (2) Bodipy_2 (0.1w%) upon (A) Laser Diode@785nm and (B) Laser Diode@940nm, thickness=1.4mm, the irradiation starts for t = 17s.

Table 6. Photopolymerization results of Mix-MA under air in the presence of dye/Ar2I+PF6-

/4-dppba/BlocBuilder®MA (0.1w%/3w%/2w%/2w%) under exposure to Laser Diode@785nm or Laser Diode @940nm; thickness=1.4mm;

785 nm 940 nm

Final conversion Time to reach the final conversion

Final conversion Time to reach the final conversion

Bodipy_1 86% 196s 84% 85s

Bodipy_2 92% 356s 60% 405s

3.4.

P

LIGHT ABSORPTION PROPERTIES

Porphyrins are macrocyclic compounds comprising 18 π-electrons. They have two characteristic bands of absorption [40]: a strong absorption band around 400 nm called the Soret band and a second one in the 500-700 nm range called the Q-band. For the three porphyrins investigated here, localization and molar extinction coefficients related to these two bands are reported in Table 7. The absorption spectrum of the porphyrins is also influenced by the ligands and the central metal.[41] In our case, for the three porphyrins, ligands are the same but Porph_1 has no central atom, Porph_2 bears a copper as central atom and Porph_3 bears a palladium. As observed in Figure 7 and in Table 8, there are no influence of the central atom on the wavelength of the Soret band. A bathochromic shift of the Q-band is observed due to the introduction of a central metal.

0 100 200 300 400 500 600 0 20 40 60 80 100 C on ve rsi on (% ) Time (s) 0 100 200 300 400 500 0 20 40 60 80 100 C on ve rsi on (% ) Time (s) (1) (2) (1) (2) (A) (B)

17

At the wavelength of irradiation of the different Laser Diodes, the molar extinction coefficients of the investigated porphyrins are reported as being very low. However, we observed the same phenomenon than for SQ_1: the shoulder of absorption is clearly higher in the monomer in combination with all the additives than in acetonitrile (Table 8).

Figure 7. Absorption spectra of: (1) Porph_1, (2) Porph_2 and (3) Porph_3 in acetonitrile

Table 7. Absorption wavelengths and associated extinction coefficient of the Soret band (SB) and the Q-band (QB) for the investigated compounds (in acetonitrile).

Porphyrin λSB (nm) ɛ(λSB) (L.mol-1. cm-1 ) λQB (nm) ɛ(λQB) (L.mol-1. cm-1 ) Porph_1 413 48820 512 2000 Porph_2 412 8080 555 9500 Porph_3 412 15720 520 1530 500 600 700 0 500 1000 1500 2000 2500  (L .m o l -1.cm -1) Wavelength (nm) 400 500 600 700 800 900 1000 0 10000 20000 30000 40000 50000  (L .m o l -1 .c m -1 ) Wavelength (nm) (1) (3) (2)

18

Table 8. Molar extinction coefficients for the investigated compounds at the wavelengths of the different Laser Diodes used (in acetonitrile or in Mix-MA in presence of the polymerization additives).

Porphyrin ɛ(940 nm) in ACN (L.mol-1. cm-1 ) ɛ(1064 nm) in ACN (L.mol-1.cm-1) ɛ(940 nm)a in Mix-MA (L.mol-1. cm-1 ) ɛ(1064 nm)a in Mix-MA (L.mol-1.cm-1) Porph_1 70 50 n.d. n.d. Porph_2 150 170 1320 1410 Porph_3 190 200 n.d. n.d.

a: in presence of the polymerization additives: in the presence of Ar2I+/PF6- (3w%), 4-dppba (2w%),

BlocBuilder®MA (2w%). n.d. not determined

A

The same phenomenon is observed with porphyrins than with the other NIR absorbing dyes presented before: despite the low absorption of the porphyrins at the wavelengths of irradiation of both Laser Diodes, photopolymerization in the four component initiating system is very successful despite an initial inhibition time. Indeed, as observed in Figure 8, high conversions are observed within 500 seconds of irradiations.

Figure 8. Photopolymerization profiles for Mix-MA under air (methacrylates function conversion vs. irradiation time) in the presence of Ar2I

+

/PF6

(3w%), 4-dppba (2w%), BlocBuilder®MA (2w%) and: (1) Porph_1 (0.1w%), (2) Porph_2 (0.1w%) and (3) Porph_3 (0.1w%) upon (A) Laser Diode@940nm and (B) Laser Diode@1064nm, thickness=1.4mm; the irradiation starts for t= 17s.

19

Final conversions and polymerization times needed to reach it are summarized in the Table 9. It is clearly observed that, in both cases, the porphyrin bearing a copper can reach more rapidly the final conversion. At 1064 nm, the three porphyrins lead to same final conversions. However, at 940 nm, the highest final conversion is obtained for the metal-free porphyrin.

We also tried the photopolymerization using a porphyrin bearing a zinc cation. However, the formulation was not stable: spontaneous polymerization was observed within 1 hour after mixing all components.

Table 9. Photopolymerization results of Mix-MA under air in the presence of dye/Ar2I+PF6-

/4-dppba/BlocBuilder®MA (0.1w%/3w%/2w%/2w%) under exposure to Laser Diode@940nm or Laser Diode @1064nm; thickness=1.4mm;

940 nm 1064 nm

Final conversion Time to reach the final conversion

Final conversion Time to reach the final conversion

Porph_1 88% 432s 75% 597s

Porph_2 65% 307s 76% 530s

Porph_3 84% 499s 76% 566s

4.

C

ONCLUSION

Herein, new NIR dyes are investigated as NIR photoinitiators/photosensitizers. Their initiating abilities in combination with iodonium salt, phosphine and thermal initiator are reported. Remarkably, despite the low absorption coefficient in the near infrared range, these structures are able to initiate the free radical polymerization of methacrylate functions. It exists other NIR scaffolds (for example quinone or push-pull dyes) which have not been tested for photopolymerization under Near-Infrared light and could be a good perspective for this work. For the access to composites (carbon fibers, glass fibers), NIR light can be very

0 200 400 600 800 0 20 40 60 80 100 C o n v e rs io n ( % ) Time (s) 0 200 400 600 0 20 40 60 80 100 Conve rsion (%) Time [sec] 0 200 400 600 800 0 20 40 60 80 100 Conve rsion (%) Time (s) (A) (B) (1) (2) (3) (1) (2) (3) 0 200 400 600 0 20 40 60 80 100 Conve rsion (%) Time [sec] 0 200 400 600 800 0 20 40 60 80 100 Conve rsion (%) Time (s) (A) (B) (1) (2) (3) (1) (2) (3) 0 200 400 600 0 20 40 60 80 100 Conve rsion (%) Time [sec] 0 200 400 600 800 0 20 40 60 80 100 Conve rsion (%) Time (s) (A) (B) (1) (2) (3) (1) (2) (3)

(A)

(B)

(2)

(1)

(3)

20

promising, therefore, the use of such dyes for composites preparation will be presented in forthcoming papers.

A

CKNOWLEDGEMENTS

The authors thank the "Agence Nationale de la Recherche" (ANR) for the grant "FastPrinting".

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R

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