3-carboxylic acid and formyl-derived coumarins as photoinitiators in photo-oxidation or photo-reduction processes for photopolymerization upon visible light: Photocomposite synthesis and 3d printing applications

Article abstract of DOI:10.3390/molecules26061753

In this paper, nine organic compounds based on the coumarin scaffold and different substituents were synthesized and used as high-performance photoinitiators for free radical pho-topolymerization (FRP) of meth(acrylate) functions under visible light irradiation using LED at 405 nm. In fact, these compounds showed a very high initiation capacity and very good polymerization profiles (both high rate of polymerization (Rp) and final conversion (FC)) using two and three-component photoinitiating systems based on coum/iodonium salt (0.1%/1% w/w) and coum/iodonium salt/amine (0.1%/1%/1% w/w/w), respectively. To demonstrate the efficiency of the initiation of photopolymerization, several techniques were used to study the photophysical and photochemical properties of coumarins, such as: UV-visible absorption spectroscopy, steady-state photolysis, real-time FTIR, and cyclic voltammetry. On the other hand, these compounds were also tested in direct laser write experiments (3D printing). The synthesis of photocomposites based on glass fiber or carbon fiber using an LED conveyor at 385 nm (0.7 W/cm² ) was also examined.

Full text of DOI:10.3390/molecules26061753

molecules
Article
3
-Carboxylic Acid and Formyl-Derived Coumarins as
Photoinitiators in Photo-Oxidation or Photo-Reduction
Processes for Photopolymerization upon Visible Light:
Photocomposite Synthesis and 3D Printing Applications
Mahmoud Rahal 1 , Bernadette Graff 1,2, Joumana Toufaily 3 , Tayssir Hamieh 3,4 , Guillaume Noirbent 5
,
,2,3
5
5,
1,2,
Didier Gigmes , Frédéric Dumur * and Jacques Lalevée
*
1
Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France;
mahmoud-rahal@outlook.com (M.R.); bernadette.graff@uha.fr (B.G.)
Université de Strasbourg, 67081 Strasbourg, France
2
3
Laboratory of Materials, Catalysis, Environment and Analytical Methods (MCEMA) and LEADDER
Laboratory, Faculty of Sciences, Doctoral School of Sciences and Technology (EDST), Lebanese University,
Beirut 6573-14, Lebanon; joumana.toufaily@ul.edu.lb (J.T.); tayssir.hamieh@ul.edu.lb (T.H.)
SATIE-IFSTTAR, Université Gustave Eiffel, Campus de Marne-La-Vallée, 25, allée des Marronniers,
F-78000 Versailles, France
Aix Marseille Univ, CNRS, ICR UMR 7273, F-13397 Marseille, France; guillaume.noirbent@outlook.fr (G.N.);
didier.gigmes@univ-amu.fr (D.G.)
4
5
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
*
Correspondence: frederic.dumur@univ-amu.fr (F.D.); jacques.lalevee@uha.fr (J.L.)
Citation: Rahal, M.; Graff, B.;
Toufaily, J.; Hamieh, T.; Noirbent, G.;
Gigmes, D.; Dumur, F.; Lalevée, J.
Abstract: In this paper, nine organic compounds based on the coumarin scaffold and different
substituents were synthesized and used as high-performance photoinitiators for free radical pho-
topolymerization (FRP) of meth(acrylate) functions under visible light irradiation using LED at
3
-Carboxylic Acid and
Formyl-Derived Coumarins as
405 nm. In fact, these compounds showed a very high initiation capacity and very good poly-
Photoinitiators in Photo-Oxidation or
Photo-Reduction Processes for
merization profiles (both high rate of polymerization (Rp) and final conversion (FC)) using two
and three-component photoinitiating systems based on coum/iodonium salt (0.1%/1% w/w) and
coum/iodonium salt/amine (0.1%/1%/1% w/w/w), respectively. To demonstrate the efficiency of
the initiation of photopolymerization, several techniques were used to study the photophysical and
photochemical properties of coumarins, such as: UV-visible absorption spectroscopy, steady-state
photolysis, real-time FTIR, and cyclic voltammetry. On the other hand, these compounds were also
tested in direct laser write experiments (3D printing). The synthesis of photocomposites based on
Photopolymerization upon Visible
Light: Photocomposite Synthesis and
3
D Printing Applications. Molecules
021, 26, 1753. https://doi.org/
2
1
0.3390/molecules26061753
Academic Editor: Maria João Matos
2
glass fiber or carbon fiber using an LED conveyor at 385 nm (0.7 W/cm ) was also examined.
Received: 22 February 2021
Accepted: 18 March 2021
Published: 21 March 2021
Keywords: coumarin; free radical polymerization; LED; photocomposites; direct laser write
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
1
. Introduction
The development of new low-cost, environmentally friendly, and energy-efficient
polymer synthesis remains more than ever at the heart of academic and industrial concerns
and the subject of many new research strategies. In fact, thanks to technological develop-
ment, light sources which are at the same time inexpensive, efficient, and with low energy
consumption have been developed recently to induce photopolymerization reactions [
Nowadays, photopolymers are present in several fields such as coatings [ ], dentistry [
automotive [ ], cosmetics [ ], 3D printing, and holography [
industrial fields, photochemical polymerization uses ultraviolet radiation, a technique
widely known as UV curing. However, this pathway based on UV lamps (Hg lamps)
remains energy-consuming. Moreover, the ultraviolet light is harmful to human health
1
–
6
4
],
].
5
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
7
8
9
], etc. For most of these
(carcinogenic) and characterized by particularly low light penetration, which is a challenge
for the photopolymerization of thick and filled samples [10]. Therefore, alternatives to
UV lamps and the use of longer wavelengths (near UV or visible) can be advantageous.
4
.0/).
Molecules 2021, 26, 1753. https://doi.org/10.3390/molecules26061753
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 1753
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The use of light-emitting diodes (LEDs) perfectly fit this requirement for safer/cheaper,
and more efficient irradiation devices than UV lamps or UV lasers [11–14]. In parallel, it
is important to develop new photoinitiating systems able to absorb in the near UV or the
visible range where their absorption spectrum overlaps that of the LED emission. To obtain
this type of system, it is necessary to develop new organic molecules carrying chromophore
groups capable of shifting their absorption spectrum towards the near-UV-visible range.
These molecules will be called photoinitiator (PI), which can absorb the light and generate
reactive species (in combination with additives) able to initiate the photopolymerization
process.
In this paper, nine coumarin derivatives (noted Coum in Scheme 1) varying by the
substitution pattern at the 3- and 7-positions of the coumarin core were synthesized and
evaluated as photoinitiators for the FRP of acrylate and methacrylate monomers. In fact,
coumarin derivatives have already been tested as photoinitiators of FRP and they have
shown good polymerization profile (Rp and FC) as well as good photochemical and
photophysical properties [15–19].
Scheme 1. The new series of coumarins (CoumA–CoumI) examined as photoinitiators of polymer-
ization.
However, in the present work, coumarin-3-carboxylic acids, coumarin-3-aldehydes
varying by the substitution pattern of the coumarin core and a coumarin of extended
aromaticity have been studied as photoinitiators. Comparisons of the three families of
coumarins have revealed that the substitution of the 3-position by electron-withdrawing
groups such as a formyl group could improve the reactivity. The presence of a strong
electron-donating group at the 7-position, such as diethylamine or a naphthalene group,
could reinforce the electronic delocalization and the photoinitiating ability of the different
systems. An optimum situation was found when electron-donating and electron-accepting
groups were attached at both extremities of the coumarin core. Considering that the nitro
group is among the most electron-withdrawing group, a coumarin bearing this electron
acceptor was also designed and synthesized.
In fact, coumarin derivatives are usually characterized by very high fluorescence
emission and can be used as fluorescent chromophores for several applications [20]. They
are also characterized by high molar extinction coefficients in the near-UV and the visible
range [19]. These novel coumarin-based photoinitiators were tested in photopolymerization
of acrylate functions (TMPTA or TA) in both Thick (1.4 mm) and Thin sample (25
two and three-component photoinitiating systems PISs based on Coum/Iodonium salt
0.1%/1% w/w) and Coum/Iodonium salt/amine (NPG) (0.1%/1%/1% w/w/w). These
µm) using
(
systems were also used in 3D printing and photocomposite synthesis. These dyes are
characterized by very high extinction coefficients with a broad absorption extending over
the near UV/visible and high quantum yields were determined by fluorescence quenching.

Molecules 2021, 26, 1753
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It is important to note that coumarin shows a dual photo-oxidation and photo-reduction
character.
2. Results
Photoinitiation ability, the performance of photopolymerization, photophysical and
photochemical properties as well as chemical mechanisms associated with the photopoly-
merization processes will be discussed in detail.
2.1. Synthesis of the Different Dyes
As mentioned in the introduction section, three families of coumarins have been exam-
ined as photoinitiators of polymerization. The first family concerned coumarin-3-carboxylic
acids. The five dyes were prepared in solution by condensation of diethyl malonate with
ortho-hydroxyarylaldehydes [21]. After hydrolysis of esters in acidic conditions (a mixture
of hydrochloric acid and acetic acid), the solution was neutralized to provide the different
dyes with reaction yields ranging from 75% for CoumA to 86% yield for CoumE. A sim-
ilar procedure was used for CoumC except that the hydrolysis of the intermediate ester
coumarin resulted in a decarboxylation reaction, providing CoumC in 72% yield. The
presence of the dimethylamino group in CoumC is essential to activate the decarboxylation
reaction since this reaction was not observed for the other coumarins, maintaining the
acidic function on the coumarins (See Scheme 2) [22]. Using the Vilsmeier Haack reaction,
CoumC could be converted as CoumG in a 77% yield.
Scheme 2. Synthetic routes to CoumA–CoumG.
Finally, CoumH and CoumI could be prepared starting from 2-thiopheneacetic acid
and 4-diethylamino-2-hydroxybenzaldehyde. By Knoevenagel reaction, 7-(diethylamino)-
3
-(thiophen-2-yl)-2H-chromen-2-one could be obtained in 58% yield and by means of a
Vilsmeier Haack reaction, CoumH was isolated in pure form in 88% yield. Conversely,
CoumI was prepared in two steps, first by bromination of 7-(diethylamino)-3-(thiophen-
2
-yl)-2H-chromen-2-one in 86% yield, followed by a Suzuki cross-coupling reaction with
3-nitrophenylboronic acid. Using this procedure, CoumI was obtained in 67% yield (See
Scheme 3).

Molecules 2021, 26, 1753
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Scheme 3. Synthetic routes to CoumH and CoumI.
.2. Light Absorption Properties
UV-visible absorption spectra of the different coumarins in acetonitrile are depicted in
2
Figure 1 (See also Table 1). These organic compounds are characterized by a high molar
−
1
−1
extinction coefficient in both near-UV and visible range (e.g., CoumC
ε
~ 18000 M cm
-1
−1
−1
−1
at 374 nm and 3500 M cm at 405 nm, and CoumF
ε
~ 10200 M cm at 376 nm and
800 M cm at 405 nm). So, these absorption properties afford a good overlap with the
−
1
−1
4
emission spectrum of the LEDs used in this work (LED at 405 nm for FRP, LED at 375 nm
for the photolysis experiments and LED at 385 nm for the photocomposites synthesis).
Figure 1. UV-visible absorption spectra of the investigated compounds based on coumarin derivatives
in ACN: (1) CoumA, (2) CoumB, (3) CoumC, (4) CoumD, (5) CoumE, (6) CoumF, (7) CoumG, (8)
CoumH, and (9) CoumI.
In fact, the presence of different substituents on the coumarin scaffold can affect the
absorption properties (Figure 2) of these compounds and their molar extinction coeffi-
cients can be affected. For example, taking CoumA as a standard structure among these
10 compounds, we observed a shift towards higher absorption range (e.g., CoumB, CoumD,
and CoumF are strongly shifted), and towards lower absorption range (e.g., CoumE), so
a bathochromic effect is observed by introduction of electron donor group (such as OH,

Molecules 2021, 26, 1753
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OMe, and NR ) and a hypsochromic effect is observed by introduction of electron acceptor
2
group (e.g., NO in case of CoumE).
2
Table 1. Light absorption properties of coumarins at 405 nm and at λmax; singlet state energy (ES1)
determined from the crossing point of absorption and fluorescence spectra.
λmax
εmax
(M cm
ε405nm
(M cm
ES1
−
1
−1
−1
−1
(nm)
)
)
(eV)
CoumA
CoumB
CoumC
CoumD
CoumE
CoumF
CoumG
CoumH
CoumI
302
351
374
322
275
376
442
458
445
11,000
13,000
18,000
14,000
18,000
10,000
48,000
21,000
27,000
40
80
3.35
3.24
3.01
3.12
3.91
2.98
2.62
2.48
2.55
3500
100
60
4800
19,000
6500
1400
Figure 2. HOMO and LUMO frontier orbitals and their respective calculated UV-visible absorption
spectra for the different investigated compounds at the UB3LYP/6-31G* level.

Molecules 2021, 26, 1753
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The electron-donating effect of these substituents is presented by ascending order:
CoumH > CoumI > CoumG > CoumF > CoumC > CoumD > CoumB.
2.3. Free Radical Photopolymerization
2.3.1. Photopolymerization of Methacrylate Function of Mix-MA
The FRP profiles of methacrylate functions using Mix-MA as the benchmark monomer
was performed in thick sample and in the presence of two or three-component PISs
based on Coum/Iod (or NPG) (0.1%/1% w/w) or Coum/Iod/NPG (0.1%/1%/1% w/w/w)
respectively, upon visible light irradiation with a LED at 405nm are given in Figure 3 (See
also Table 2).
80
60
40
20
0
90
(
3)
10)
(8)
(10)
(4)
(
A)
(B) 75
(
(
2)(6)
60
(7)
45
(
7)
(
(
5)
30
(
9)
3)(8)
(9)
(
(
(5)
6)
2)
15
0
(1)
(4)
(1)
0
100
200
300
400
0
100
200
300
400
Time (s)
Time (s)
9
0
(
3)(4)(10)
1)(2)(9)
8)
(
C) 75
(
(
(
5)(6)
60
45
(7)
30
15
0
0
100
200
300
400
Time (s)
Figure 3. Photopolymerization profiles of methacrylate functions (conversion vs. irradiation time) using MIX-MA in thick
sample (1.4 mm) under visible light irradiation using a LED at 405 nm: ( ) Coum/Iod (0.1%/1% w/w), ( ) Coum/NPG
0.1%/1% w/w) and (C) Coum/Iod/NPG (0.1%/1%/1% w/w/w): (1) CoumA, (2) CoumB, (3) CoumC, (4) CoumD,
A
B
(
(
5) CoumE, (6) CoumF, (7) CoumG, (8) CoumH, (9) CoumI and (10) Iod/NPG (1%/1% w/w). Irradiation starts at t = 10 s.
The obtained results show that Coum/Iod (or NPG) is less reactive than three-
component PISs (Coum/Iod/NPG), this result can be explained by a higher yield of
reactive species (radicals) in the presence of Iod/NPG which is not able, alone, to initi-
ate the FRP (e.g., FC = 24% for CoumI/Iod vs. 76% for CoumI/Iod/NPG, and FC = 0%
for CoumA/Iod vs. 76% for CoumA/Iod/NPG; show Figure 3A,C curve 1). In fact,
CoumB, CoumD, CoumF and CoumG showed a photoreduction process rather than a
photo-oxidation process (e.g., FC = 0% for CoumB/Iod vs. 67% for CoumB/NPG, and
FC = 0% for CoumD/Iod vs. 75% for CoumD/NPG), but CoumC and Coum8 show an
opposite behavior with a photo-oxidation process probably more favorable than the pho-
toreduction (FC = 70% for CoumC/Iod vs. 28% for CoumC/NPG; Figure 3A,B curve 3).
The FRP profiles also show a low rate of polymerization, this can be due to the high oxygen
inhibition effect.

Molecules 2021, 26, 1753
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Table 2. Final reactive functions conversion (FC%) for different monomers and different PISs upon
visible light irradiation using a LED at 405 nm (400 s of irradiation and thickness = 1.4 mm).
Two-Component PISs
Coum/Additives (0.1%/1% w/w)
Three-Component PISs
Coum/Iod/NPG (0.1%/1%/1% w/w/w)
TMPTA
TA
Mix-MA
TMPTA
80%
TA
Mix-MA
76%
a
a
a
n.p
n.p
30%
n.p
n.p
n.p
n.p
67%
70%
28%
n.p
75%
n.p
30%
13%
63%
35%
55%
62%
29%
CoumA
CoumB
CoumC
CoumD
CoumE
CoumF
CoumG
CoumH
CoumI
85%
b
b
b
6
1%
a
a
a
n.p
78%
80%
86%
73%
80%
50%
81%
70%
88%
88%
92%
75%
88%
40%
87%
75%
76%
80%
79%
64%
64%
46%
75%
76%
b
b
a
b
a
b
b
7
8%
0%
2%
3%
3%
81%
80%
56%
74%
90%
n.p
a
b
a
b
a
6
4
3
8
b
a
b
a
a
a
n.p
b
a
b
a
b
a
b
a
b
b
a
b
a
b
a
b
a
b
b
3
6%
0%
1%
0%
5%
5%
3%
8%
0%
25%
86%
88%
80%
82%
84%
59%
78%
31%
a
6
8
7
7
6
5
5
6
b
a
b
a
b
a
24%
b
18%
a
Coum/Iod (0.1%/1% w/w); b Coum/NPG (0.1%/1% w/w).
2
.3.2. Photopolymerization of Acrylates (TMPTA or TA)
In fact, iodonium salt or NPG alone cannot initiate the FRP of acrylate at 405 nm
due to their absorption in the UV range [17 23]. Therefore, the coumarins derivatives are
,
introduced in order to improve the absorption properties of photosensitive formulations.
Firstly, the most of Coumarin derivatives show high extinction coefficients at 405 nm.
The photopolymerization profiles of acrylate functions in thick (1.4 mm) or thin (25 µm)
samples (conversion vs. irradiation time) using TMPTA (or TA) as benchmark monomers
are depicted in Figure 4 (see also Tables 2 and 3). The obtained results show that the two-
component PISs based on Coum/Iod (0.1%/1% w/w) (or Coum/NPG) are able to strongly
initiate the FRP, but a very higher performance [Final conversion (FC) and polymerization
rate (Rp)] was acquired using the three-component PISs based on Coum/Iod/NPG which
is quite efficient in the FRP of acrylate functions upon LED at 405 nm (e.g., FC = 60%
for CoumC/Iod (0.1%/1% w/w) vs. 80% for CoumC/Iod/NPG (0.1%/1%/1% w/w/w),
Figure 4A and B curve 3).
Moreover, the Iod/NPG (1%/1% w/w) couple weakly initiates the FRP (FC = 47%).
This is ascribed to the formation of a charge transfer complex (CTC) between Iod and
NPG [24] which is able to generate reactive species when it absorbs light. Clearly, the
presence of Coumarin as photoinitiator is improving the performance of the photopoly-
merization processes.
Some of the coumarins can show both photoreduction (electron transfer from NPG to
Coumarin) and photoxidation (electron transfer from Coumarin to Iod) processes, while
other derivatives show only photoreduction process, such as CoumA, CoumB and CoumE
(
e.g., FC = 0% for CoumB/Iod (0.1%/1% w/w) vs. FC = 78% for CoumB/NPG (0.1%/1%
w/w)).

Molecules 2021, 26, 1753
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7
0
(7)
8)
(3)
9)
6)
100
(B)
(
A)60
(
(
(
(4)
(6)
2)
80
60
40
(
50
(7)
40
(1)
9)
(8)
(
(4)
30
(3)
20
(5)
1
0
(
5)
20
0
(
1)
0
(2)
-
10
0
100
200
300
400
0
50
100
4)
150
200
250
300
350
Time (s)
Time (s)
100
(
C)
(
(8)
80
(
(6)
(2)(5)
(9)
1)(3)
60
(7)
(10)
40
20
0
0
50
100
150
200
250
300
Time (s)
Figure 4. Photopolymerization profiles of acrylate functions (conversion vs irradiation time) using TMPTA in thick sample
1.4 mm) under visible light irradiation using a LED at 405 nm: ( ) Coum/Iod (0.1%/1% w/w), ( ) Coum/NPG (0.1%/1%
w/w) and ( ) Coum/Iod/NPG (0.1%/1%/1% w/w/w): (1) CoumA, (2) CoumB, (3) CoumC, (4) CoumD, (5) CoumE, (6)
CoumF, (7) CoumG, (8) CoumH, (9) CoumI and (10) Iod/NPG (1%/1% w/w). The molar concentrations for 0.1 % w/w are
.0055, 0.0051, 0.0049, 0.0048, 0.0045, 0.0044, 0.0043, 0.0032, and 0.0025 M for CoumA, CoumB, CoumC, CoumD, CoumE,
CoumF, CoumG, CoumH, CoumI, respectelievly. The irradiation starts at t = 10 s.
(
A
B
C
0
Table 3. Final reactive functions conversion (FC%) for different monomers and different PISs upon
visible light irradiation using a LED at 405 nm (150 s of irradiation and thickness = 25 µm).
Two-Component PISs
Coum/Additives (0.1%/1% w/w)
Three-Component PISs
Coum/Iod/NPG (0.1%/1%/1% w/w/w)
TMPTA
TA
Mix-MA
TMPTA
25%
TA
Mix-MA
36%
a
a
a
n.p
n.p
n.p
n.p
22%
14%
15%
n.p
17%
17%
22%
n.p
n.p
18%
70%
43%
64%
36%
48%
CoumA
CoumB
CoumC
CoumD
CoumE
CoumF
CoumG
CoumH
CoumI
38%
b
b
b
3
4
2%
9%
a
b
a
b
a
b
a
a
32%
24%
53%
15%
42%
45%
n.p
28%
50%
42%
30%
51%
55%
46%
42%
44%
75%
70%
40%
73%
81%
68%
58%
52%
72%
57%
n.p
b
a
b
a
b
b
1
3%
a
2
2
4
3
5%
5%
2%
4%
b
a
b
a
a
a
n.p
n.p
b
b
b
n.p
a
a
a
1
1%
7%
8%
8%
9%
42%
65%
68%
67%
43%
45%
37%
45%
79%
74%
74%
66%
b
a
b
a
b
a
b
b
a
b
a
b
a
b
b
4
3
4
1
a
b
a
b
1
1
2
4%
5%
5%
a
12%
b
29%
a
Coum/Iod (0.1%/1% w/w); b Coum/NPG (0.1%/1% w/w).

Molecules 2021, 26, 1753
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2
.4. D Printing Experiments Using Coum/Iod/amine PISs and Optical Microscopy
Characterization
New 3D patterns were obtained by direct laser write experiments of Coum/Iod/amine
PISs using a laser diode at 405 nm and characterized by optical microscopy. These
3
(
D patterns were obtained under air using different PISs based on Coum/Iod/TMA
0.05%/0.5%/0.235% w/w/w) in TA or TMPTA (See Figure 5). In fact, the high photosensi-
tivity of this resin allowed an efficient polymerization process in the irradiated area so a
high spatial resolution is observed. Markedly, a great thickness is obtained (~2090 m) and
µ
these patterns were carried out in a very short irradiation time (~2–3 min). Using a well-
established Type I photoinitiator (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide—TPO)
in similar direct laser write conditions; similar performances can be reached but requiring
a higher content (0.5% w/w). This latter result demonstrates the interest in using Coum
derivatives. It is important to note that the 3D patterns based on CoumC exhibit a blue
fluorescence when these structures are characterized by the light of the microscope.
Figure 5. Characterization of 3D patterns by numerical optical microscopy obtained by free radical photopolymeriza-
tion experiment (using TA or TMPTA as benchmark monomer) using a diode laser at 405 nm: ( ) CoumC/Iod/4,N-
N-TMA (0.05%/0.5%/0.235% w/w/w) in TA, ( ) CoumH/Iod/TMA (0.05%/0.5%/0.19% w/w/w) in TMPTA and (C)
CoumD/Iod/4,N-N-TMA (0.05%/0.5%/0.275% w/w/w) in TMPTA.
A
B
2
.5. Near-UV Conveyor Experiments for the Synthesis of Photocomposites Using Coum/Iod/NPG
(
0.1%/1%/1% w/w/w)
Generally, photocomposites are materials composed of at least two components:
matrix and reinforcement. The mixture of these two components leads to new interesting
properties that the two components separately do not have. The production of composites
in the last decades and until today represents a very dynamic market in different fields
such as aeronautics, automotive, wind power, and buildings. So, due to their very high
mechanical resistance and chemical resistance, the glass fibers are used in this work as a
matrix for the photocomposite synthesis.
In this work, the proposed coumarin derivatives were tested for access to photocom-
2
posites upon near-UV light using a LED conveyor at 385 nm (0.7 W/cm ). The curing
results obtained are summarized in Figure 6. Firstly, photocomposites were prepared by
impregnation of glass fibers with an acrylic resin (TMPTA) (50% glass fibers/50% acrylic
resin) and irradiated upon a LED at 385 nm. Remarkably, a very fast polymerization was
observed using Coum/Iod/NPG (0.1%/1%/1% w/w/w), where both the surface and the
bottom are tack-free after some passes.

Molecules 2021, 26, 1753
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2
Figure 6. Photocomposites manufactured upon near-UV irradiation at 385 nm (0.7 W/cm ) using
glass fiber/resin (50%/50% w/w) in the presence of three-component PISs based on Coum/Iod/NPG
(
0.1%/1%/1% w/w/w): (1) CoumC, (2) CoumD, (3) CoumF, (4) CoumG, and (5) CoumH.
. Discussion
For a better understanding of the photoinitiation ability, the photochemical properties
3
of the studied coumarins were investigated. More particularly, their photolysis behaviors,
fluorescence quenchings, and redox properties were investigated in the presence of addi-
tives (amine/iodonium salt), allowing to establish the photochemical mechanisms (see
Scheme 4 below).
Scheme 4. Proposed chemical mechanisms.
3.1. Steady-State Photolysis of Coumarins
Steady-state photolysis of coumarins derivatives in ACN and under irradiation light
using a LED at 375 nm have been performed to explain the obtained results in FRP. So,
the photolysis of one of these compounds (CoumC) is presented in Figure 7. First of
all, the photolysis of CoumC alone upon irradiation at 375 nm is very slow compared
to that obtained with Iod, which is very fast. In fact, the appearance of a weak peak

Molecules 2021, 26, 1753
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between 425 and 500 nm and the evolution of the absorption peak of CoumC shows that a
high interaction between CoumC and Iod took place by an electron transfer process, this
process induced, during the irradiation, a photolysis of the CoumC and generation of new
photoproducts. On the other hand, the photolysis of CoumC with Iod/NPG couple was
very slow (Figure 7D curve 3) and poor consumption was obtained; these results can be
explained by a high regeneration of CoumC in three-component PISs.
−
Figure 7.
ACN, (
(
A
) Photolysis of CoumC alone in ACN, (
B
) photolysis of CoumC with Iod (10 2 M) in
−
2
C
) photolysis of CoumC with Iod/NPG (10 M) couple, (
D
) percentage of consumption
of CoumC (1) without Iod, and with (2) with Iod (10 M), and (3) with Iod/NPG vs. irradiation
time—upon exposure to the LED@375 nm in ACN.
−
2
3.2. Fluorescence Quenching and Cyclic Voltammetry Experiments for the Coumarins
Fluorescence quenching and emission spectra of the different coumarins (e.g., CoumC)
have been carried out in ACN and reported in Figure 8. Firstly, where the emission intensity
1
of CoumC decreases when we added Iod or NPG, so an interaction between Coum-C and
Iod (or NPG) occurs, this result is in full agreement with FRP and photolysis experiments
shown above. To compare the reactivity of different coumarin with Iod or NPG, the Stern-
Volmer coefficient (Ksv) have been calculated according to Equation (1). For example, a
very high quenching of CoumF with NPG and poor quenching of CoumC with NPG were
−
1
observed, so Ksv for CoumF is higher than that of CoumC (Ksv = 44 M for CoumC
−
1
and 400 M for CoumF), therefore a high electron transfer quantum yield is obtained for
CoumF φ = 0.9) compared to that obtained for CoumC (φ = 0.6) (Table 4)
φ
S1
= KSV[Iod]/(1 + KSV[Iod])
(1)
The free energy change (
or NPG is an important parameter to evaluate the feasibility of this process.
extracted from the ES1 and the electrochemical properties (Eox and Ered) (using Equation (1)
e.g., G = 2.39 eV for CoumF/Iod which is more reactive in FRP of acrylate functions
TA monomer) (FC = 86%). All these data are gathered in Table 4.
Finally, the FRP results of acrylate functions can be explained by a global mechanism
∆
G) for the electron transfer between coumarins and Iod
∆
G can be
)
∆
−
(
based on the different results obtained by the characterization techniques (steady-state
photolysis, Fluorescence quenching and cyclic voltammetry). First of all, the photoinitiator
(Coumarin) goes to its excited state once it absorbs suitable light energy, and as it is not
able to give reactive species alone, the Iod salt (or NPG), therefore, interacts with its excited
state and will be able to dissociate and give reactive species responsible to initiate the
FRP (r1–r2). The addition of NPG to the photosensitive formulation is very important

Molecules 2021, 26, 1753
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because of the formation of a charge-transfer complex between Iod salt and NPG [Iod-
NPG]CTC able to generate reactive species (r3–r4). Moreover, a hydrogen transfer process
•
from NPG to Coumarins can occur which generates two types of radicals (Coum-H ,
•
•
NPG(-H) ) (r5). In fact, a decarboxylation of NPG
can take place and leads to the
), which react with Iod salt to produce reactive species
(-H)
•
radical formation (NPG
(
-H, -CO2)
•
+
•
•
(
Ar and NPG
) (r6–r7). Ar and NPG
(r1–r9) radicals are assumed as
(
-H, CO2)
(-H,-CO2) )
the reactive species responsible to the FRP of the (meth)acrylate functions. The coumarins
consumption is reduced in three-component PIS (Figure 6); this can be explained by a
regeneration of the photoinitiator, which is in agreement on r8-r9 (See Scheme 4).
Figure 8.
(A) Fluorescence quenching of CoumC by Iod, (B) ES1 determination, (C) determination of
KSV (Stern–Volmer coefficient), and (D) oxidation potential (Eox) determination of CoumC.
1
1
Table 4. Parameters characterizing the chemical mechanisms associated with Coum/Iod or Coum/NPG interaction in
acetonitrile. For Iod and NPG, a reduction and oxidation potential of
calculation.
−
0.2 and 1.03 eV were used respectively for the ∆Get
−
1
−1
Eox
Ered
(V)
∆G (eV)
∆G (eV)
KSV
M
KSV
M
Φ
Φ
(V)
(Coum/Iod) (Coum/NPG) (Coum/Iod) (Coum/NPG) (Coum/Iod)
(Coum/NPG)
CoumA
CoumB
CoumC
CoumD
CoumE
CoumF
CoumG
CoumH
CoumI
-
-
1
0.6
-
0.39
0.46
0.96
-
−0.61
-
-
−1.71
-
-
7.25
21
44
14
5297
9
6
48
4
104
174
44
46.5
773
400
16
0.35
0.39
0.5
0.3
0.972
0.3
0.2
0.6
0.25
0.78
0.82
0.6
0.5
0.9
0.9
0.5
0.6
-
-
-
−1.81
−2.32
-
−2.39
−1.96
−1.32
-
−0.98
−1.11
-
-
−1.01
−1.35
−1.40
−1.06
−0.94
−0.24
−0.05
−0.460
22
-
The photoinitiation ability is a strong interplay between these different reactions
r1-r9), but their light absorption properties and intersystem crossing behavior (singlet
(
vs. triplet state pathways, lifetimes) must also be taken into account. Therefore, a deeper
characterization of their structure/reactivity/efficiency relationship is beyond the scope of
the present work.

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4. Materials and Methods
4.1. Synthesis of the Coumarins
All reagents and solvents were purchased from Aldrich, Alfa Aesar, or TCI Europe
and used as received without further purification. Mass spectroscopy was performed by
the Spectropole of Aix-Marseille University. ESI mass spectral analyses were recorded with
a 3200 QTRAP (Applied Biosystems SCIEX) mass spectrometer. The HRMS mass spectral
analysis was performed with a QStar Elite (Applied Biosystems SCIEX) mass spectrometer.
Elemental analyses were recorded with a Thermo Finnigan EA 1112 elemental analysis
1
13
apparatus driven by the Eager 300 software. H and C NMR spectra were determined
at room temperature in 5 mm o.d. tubes on a Bruker Avance 400 spectrometer and on a
1
Bruker Avance 300 spectrometer of the Spectropole: The H chemical shifts were referenced
13
to the solvent peak CDCl (7.26 ppm), and the C chemical shifts were referenced to
3
the solvent peak CDCl (77 ppm). 7-(Diethylamino)-3-(thiophen-2-yl)-2H-chromen-2-one
3
and 3-(5-bromothiophen-2-yl)-7-(diethylamino)-2H-chromen-2-one used as intermediates
of reaction have been synthesized according to procedures previously reported in the
literature, without modifications and in similar yields [18].
Synthesis of 2-oxo-2H-chromene-3-carboxylic acid (CoumA)
Dimethyl malonate (2.64 g, 20 mmol, M = 132.11 g/mol) and piperidine (1 mL,
1
0 mmol) were mixed and added to a solution of salicylaldehyde (1.22 g, 10 mmol,
M = 122.12 g/mol) dissolved in absolute ethanol (30 mL). After stirring and heating to
reflux for 6 h, the solvent was removed under reduced pressure. Then, concentrated HCl
(
20 mL) and acetic acid (20 mL) were added for hydrolysis and the solution was refluxed
overnight. After cooling, the solution was poured onto ice and aq. 40% NaOH was added
until pH = 5. A pale precipitate formed. After stirring for another 30 min, the mixture
1
was filtered, washed with water, pentane, and dried under vacuum (1.43 g, 75% yield). H
NMR (400 MHz, DMSO-d6)
.42 (dd, J = 15.4, 7.9 Hz, 2H). Analyses were consistent with those previously reported in
the literature [25].
δ 8.75 (s, 1H), 7.91 (dd, J = 7.7, 1.2 Hz, 1H), 7.80–7.68 (m, 1H),
7
Synthesis of 7-hydroxy-2-oxo-2H-chromene-3-carboxylic acid (CoumB),
Dimethyl malonate (2.64 g, 20 mmol, M = 132.11 g/mol) and piperidine (1 mL,
0 mmol) were mixed and added to the solution of 2,4-dihydroxybenzaldehyde (1.38 g,
0 mmol, M = 138.12 g/mol) dissolved in absolute ethanol (30 mL). After stirring and heat-
1
1
ing to reflux for 6 h, the solvent was removed under reduced pressure. Then, concentrated
HCl (20 mL) and acetic acid (20 mL) were added for hydrolysis, and the solution was
refluxed overnight. After cooling, the solution was poured onto ice and aq. 40% NaOH
was added until pH = 5. A pale precipitate formed. After stirring for another 30 min, the
mixture was filtered, washed with water, pentane, and dried under vacuum (1.61 g, 78%
1
yield). H NMR (400 MHz, DMSO-d )
δ 8.68 (s, 1H), 7.75 (d, J = 8.6 Hz, 1H), 6.85 (dd,
6
J = 8.6, 2.3 Hz, 1H), 6.74 (d, J = 2.1 Hz, 1H). Analyses were consistent with those previously
reported in the literature [25].
Synthesis of 7-(diethylamino)-2H-chromen-2-one (CoumC)

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Dimethyl malonate (2.64 g, 20 mmol, M = 132.11 g/mol) and piperidine (1 mL,
0 mmol) were mixed and added to the solution of 4-(Diethylamino)-salicylaldehyde
1
(
1.93 g, 10 mmol) dissolved in absolute ethanol (30 mL). After stirring and heating to
reflux for 6 h, the solvent was removed under reduced pressure. Then, concentrated HCl
20 mL) and acetic acid (20 mL) were added for hydrolysis and the solution was refluxed
(
overnight. After cooling, the solution was poured onto ice and aq. 40% NaOH was added
until pH = 5. A pale precipitate formed. After stirring for another 30 min, the mixture was
1
filtered, washed with water, pentane, and dried under vacuum (1.56 g, 72% yield). H
NMR (400 MHz, CDCl3)
δ 7.53 (d, J = 9.3 Hz, 1H), 7.24 (d, J = 8.8 Hz, 1H), 6.60–6.44 (m,
2H), 6.03 (d, J = 9.3 Hz, 1H), 3.41 (q, J = 7.1 Hz, 4H), 1.21 (t, J = 7.1 Hz, 6H). Analyses were
consistent with those previously reported in the literature [26].
Synthesis of 8-methoxy-2-oxo-2H-chromene-3-carboxylic acid (CoumD)
Dimethyl malonate (2.64 g, 20 mmol, M = 132.11 g/mol) and piperidine (1 mL,
1
1
0 mmol) were mixed and added to the solution of 3-methoxysalicylaldehyde (1.52 g,
0 mmol, M = 152.15 g/mol) dissolved in absolute ethanol (30 mL). After stirring and
heating to reflux for 6 h, the solvent was removed under reduced pressure. Then, concen-
trated HCl (20 mL) and acetic acid (20 mL) were added for hydrolysis and the solution was
refluxed overnight. After cooling, the solution was poured onto ice and aq. 40% NaOH
was added until pH = 5. A pale precipitate formed. After stirring for another 30 min,
the mixture was filtered, washed with water, pentane, and dried under vacuum (1.80 g,
1
8
2% yield). H NMR (400 MHz, DMSO-d )
δ
8.50 (s, 1H), 7.42–7.26 (m, 3H), 3.91 (s, 3H).
6
Analyses were consistent with those previously reported in the literature [27].
Synthesis of 6-nitro-2-oxo-2H-chromene-3-carboxylic acid (CoumE)
Dimethyl malonate (2.64 g, 20 mmol, M = 132.11 g/mol) and piperidine (1 mL,
1
1
0 mmol) were mixed and added to the solution of 2-hydroxy-5-nitrobenzaldehyde (1.67 g,
0 mmol, M = 167.12 g/mol) dissolved in absolute ethanol (30 mL). After stirring and
heating to reflux for 6 h, the solvent was removed under reduced pressure. Then, concen-
trated HCl (20 mL) and acetic acid (20 mL) were added for hydrolysis and the solution was
refluxed overnight. After cooling, the solution was poured onto ice and aq. 40% NaOH
was added until pH = 5. A pale precipitate formed. After stirring for another 30 min,
the mixture was filtered, washed with water, pentane, and dried under vacuum (2.02 g,
1
8
7
6% yield). H NMR (400 MHz, DMSO-d )
δ
8.81 (d, J = 2.8 Hz, 1H), 8.51–8.33 (m, 2H),
6
.59 (d, J = 9.1 Hz, 1H) Analyses were consistent with those previously reported in the
literature [24].
Synthesis of 3-oxo-3H-benzo[f]chromene-2-carboxylic acid (CoumF)
Dimethyl malonate (2.64 g, 20 mmol, M = 132.11 g/mol) and piperidine (1 mL,
0 mmol) were mixed and added to the solution of 2-hydroxy-1-naphthaldehyde (1.72 g,
0 mmol, M = 172.18 g/mol) dissolved in absolute ethanol (30 mL). After stirring and
1
1

Molecules 2021, 26, 1753
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heating to reflux for 6 h, the solvent was removed under reduced pressure. Then, concen-
trated HCl (20 mL) and acetic acid (20 mL) were added for hydrolysis and the solution was
refluxed overnight. After cooling, the solution was poured onto ice and aq. 40% NaOH
was added until pH = 5. A pale precipitate formed. After stirring for another 30 min,
the mixture was filtered, washed with water, pentane, and dried under vacuum (1.82 g,
1
7
6% yield). H NMR (400 MHz, DMSO-d )
δ
9.37 (s, 1H), 8.60 (d, J = 8.3 Hz, 1H), 8.31
6
(d, J = 9.0 Hz, 1H), 8.09 (d, J = 7.8 Hz, 1H), 7.77 (ddd, J = 8.3, 7.0, 1.3 Hz, 1H), 7.65 (dt,
J = 12.9, 2.9 Hz, 1H), 7.60 (d, J = 9.1 Hz, 1H). Analyses were consistent with those previously
reported in the literature [28].
Synthesis of 7-(diethylamino)-2-oxo-2H-chromene-3-carbaldehyde (CoumG)
◦
2
0 mL of POCl was added dropwise to 20mL of dry DMF at 0 C under Argon
3
◦
and stirred during 30 minutes at 50 C. Then 7-(diethylamino)-2H-chromen-2-one (15.0 g,
6
9.1 mmol, M = 217.27g/mol) in 100 mL of DMF was added to the mixture and the mixture
◦
was heated to 60 C overnight. Afterward, the mixture was poured into 500 mL of ice
water and a solution of NaOH 20% was added. The precipitate was filtered and washed
1
with water. (13.12 g, 77% yield). H NMR (300 MHz, CDCl )
δ
10.08 (s, 1H), 8.21 (s, 1H),
3
7
4
.46–7.32 (m, 1H), 6.61 (dd, J = 9.0, 2.5 Hz, 1H), 6.45 (d, J = 2.3 Hz, 1H), 3.46 (q, J = 7.1 Hz,
H), 1.23 (t, J = 7.1 Hz, 6H). Analyses were consistent with those previously reported in the
literature [26].
Synthesis of 5-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)thiophene-2-carbaldehyde
CoumH)
(
7
-(Diethylamino)-3-(thiophen-2-yl)-2H-chromen-2-one (3.00 g, 10 mmol, M = 299.39 g/mol)
◦
was dissolved in DMF (7 mL) and POCl (1.8 mL, 20 mmol) was slowly added at 0 C. The
mixture was heated up to 80 C overnight. After cooling, the solution was quenched with water.
3
◦
The mixture was extracted with DCM several times. The organic phases were combined, dried
over magnesium sulfate and the solvent removed under reduced pressure. It was used without
1
any further purification (2.88 g, 88% yield). H NMR (400 MHz, CDCl )
δ
9.90 (s, 1H), 8.02 (s,
3
1H), 7.77 (d, J = 4.1 Hz, 1H), 7.73 (d, J = 4.1 Hz, 1H), 7.38 (d, J = 8.9 Hz, 1H), 6.69 (dd, J = 8.9,
2.5 Hz, 1H), 6.56 (d, J = 2.4 Hz, 1H), 3.46 (q, J = 7.1 Hz, 4H), 1.25 (t, J = 7.1 Hz, 6H). Analyses
were consistent with those previously reported in the literature [29].
Synthesis of 7-(diethylamino)-3-(5-(3-nitrophenyl)thiophen-2-yl)-2H-chromen-2-one
(
CoumI)
Tetrakis(triphenylphosphine)palladium (0) (0.46 g, 0.744 mmol, M = 1155.56 g.mol 1)
−
was added to a mixture of 3-(5-bromothiophen-2-yl)-7-(diethylamino)-2H-chromen-2-one
−
1
(
2.31 g, 6.11 mmol, M = 378.28 g.mol ), 3-nitrophenylboronic acid (1.53 g, 9.16 mmol,
−
1
M = 166.93 g.mol ), toluene (54 mL), ethanol (26 mL) and an aqueous potassium carbonate
solution (2 M, 6.91 g in 25 mL water, 26 mL) under vigorous stirring. The mixture was

Molecules 2021, 26, 1753
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◦
stirred at 80 C for 48 h under a nitrogen atmosphere. After cooling to room temperature,
the reaction mixture was poured into water and extracted with ethyl acetate. The organic
layer was washed with brine several times, and the solvent was then evaporated. The
residue was purified by filtration on a plug of silicagel using a mixture of DCM/ethanol as
1
the eluent (67% yield, 1.72 g). H NMR (400 MHz, CDCl )
δ 8.49 (t, J = 1.9 Hz, 1H), 8.10
3
(
ddd, J = 8.2, 2.2, 1.0 Hz, 1H), 7.98–7.92 (m, 2H), 7.64 (d, J = 4.0 Hz, 1H), 7.56 (d, J = 8.0 Hz,
1
H), 7.43 (d, J = 4.0 Hz, 1H), 7.35 (d, J = 8.9 Hz, 1H), 6.64 (dd, J = 8.8, 2.5 Hz, 1H), 6.55 (d,
J = 2.4 Hz, 1H), 3.45 (q, J = 7.1 Hz, 4H), 1.24 (t, J = 7.1 Hz, 6H); HRMS (ESI MS) m/z: theor:
+.
4
20.1144 found: 420.1147 (M detected); Anal. calc. for C H N O S: C, 65.7, H, 4.8, O,
23 20 2 4
15.2; found: C 65.5, H 4.7, O 15.4.
4
.2. Other Chemical Compounds
All the other chemicals (Figure 9) were selected with the highest purity available and
used as received. Di-tert-butyl-diphenyliodonium hexafluorophosphate (Iod) and TMA
4,N,N-Trimethylaniline) were obtained from Lambson Ltd. (Wetherby, UK). Trimethy-
(
lolpropane triacrylate (TMPTA), di(trimethylolpropane) tetraacrylate (TA), Mix-MA, N-
Phenylglycine (NPG) were obtained from Allnex or Sigma Aldrich (Darmstadt, Germany).
TMPTA, TA, and Mix-MA were selected as benchmark monomers for the radical polymer-
izations.
Cationic initiator
Sacrifical amines
PF6
N
O
H
N
OH
Iod
NPG
4,N,N-TMA
Monomers
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
TMPTA
TA
O
O
O
H
N
O
O
R
O
O
N
H
O
33 wt%
O
O
O
O
OH
O
O
HPMA 33 wt%
1,4DBMA 33 wt%
Mix-MA
Figure 9. Other chemical compounds used in this paper.
4
.3. Irradiation Light Sources
Weuseddifferentlight-emittingdiodes(LEDs)aslightsources: (1)at405nm(I = 110 mW/cm2)
2
for the photopolymerization experiments, (2) at 375 nm (I = 40 mW/cm ) for the photolysis
of Coumarins and (3) at 385 nm (I = 0.7 W/cm ) for the photocomposite synthesis.
2
4
.4. Real-Time Fourier Transform Infrared Spectroscopy (RT-FTIR): Kinetic Followed and Final
Conversion (FC) Determination for the Photopolymerisation
In this research, the ability of coumarins to initiate the photopolymerization of
(meth)acrylate functions (FRP) was studied using two and three-component photoini-

Molecules 2021, 26, 1753
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tiating systems based on Coum/Iod salt (or NPG) (0.1%/1% w/w) and Coum/Iod/NPG
(0.1%/1%/1% w/w/w). The percentage of the different chemical compounds is calcu-
lated according to the monomer weight. Kinetic study, as well as the reactive function
conversion, were monitored by the evolution of the double bond vs. time. In fact, the
polymerization experiments were performed in both thick (1.4 mm) and thin (25
µm)
samples, they were obtained by deposition of the formulation into the mold (1.4 mm) or
between two propylene films in order to reduce O inhibition, respectively. In addition,
2
excellent solubility of all coumarin derivatives (excluding the CoumE) were observed. For
the thick and thin samples, the evolution of the (meth)acrylate functions for TMPTA or
−
1
Mix-MA were followed by RT-FTIR spectroscopy (JASCO FTIR 6600) at about 6150 cm
−
1
and 1630 cm , respectively. The procedure used to monitor the photopolymerization
profile was described in detail in [30,31].
4.5. Redox Potentials
The reduction (Ered) or oxidation (Eox) potentials for the different coumarin derivatives
were determined by cyclic voltammetry in ACN using tetrabutylammonium hexafluo-
rophosphate as the supporting electrolyte (potential vs. saturated calomel electrode–SCE).
The free energy change (∆Get) for an electron transfer reaction was calculated using equa-
tion (2) [27], where Eox, Ered, E*, and C represent the oxidation potential of the electron
donor, the reduction potential of the electron acceptor, the excited state energy level (deter-
mined from luminescence experiments) and the coulombic term for the initially formed
ion pair, respectively. Here, C was neglectedm as usually done for polar solvents.
∆
Get = Eox - Ered - E* + C
(2)
4
.6. UV-Vis Absorption and Photolysis Experiments
The absorption properties (UV-visible absorption spectrum and molar extinction
coefficient) as well as the steady state photolysis of the investigated Coumarin derivatives
CoumA–CoumI) in acetonitrile have been investigated using a JASCO V730 spectrometer.
(
4.7. Fluorescence Experiments
The fluorescence properties of these organic compounds in ACN were studied using
1
a JASCO FP-6200 spectrofluorimeter. The fluorescence quenching of coumarin by Iod
or NPG were examinated from the classical Stern-Volmer treatment [32] (I /I = 1 + kq
0
τ
0
[Q], where I and I stand for the fluorescent intensity of coumarin in the absence and the
0
presence of Iod or NPG, respectively;
τ stands for the lifetime of coumarin in the absence
0
of Iod and [Q] stand for the concentration of quencher, in our study Iod or NPG).
4.8. Computational Procedure
Molecular orbital calculations were carried out with the Gaussian 03 suite of pro-
grams [33,34]. Electronic absorption spectra for the different compounds were calculated
with the time-dependent density functional theory at the MPW1PW91-FC/6-31G* level of
theory on the relaxed geometries calculated at the UB3LYP/6-31G* level of theory.
4.9. Near-UV Conveyor for Photocomposite Synthesis
Photocomposites have been achieved using glass fibers for the reinforcement and
an organic resin based on acrylates (50%/50% w/w). The photosensitive resin has been
deposited on glass fibers, then the mixture was irradiated using a LED conveyor at 385 nm
2
(
0.7 W/cm ). A Dymax-UV conveyor was used, the distance between the belt and the LED
was fixed to 15 mm, and the belt speed was fixed to 2 m/min.

Molecules 2021, 26, 1753
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4
.10. Laser Writing and 3D patterns Characterization
For the direct laser write experiments, a computer-controlled laser diode at 405 nm
spot size = 50 m) was used, and the 3D patterns obtained were characterized by a
numerical optical microscope (DSX-HRSU from OLYMPUS Corporation) [35].
(
µ
5. Conclusions
Nine coumarins varying by the substitution pattern at the 3- and 7-positions of the
coumarin core have been tested and proposed as highly efficient photoinitiators for the
FRP of meth(acrylates) functions under visible light irradiation using a LED at 405 nm.
Remarkably, these photoinitiators can be used in 3D printing experiments and these dyes
showed a very high efficiency in the photocomposite synthesis (significant curing of the
surface and the bottom) using a LED conveyor at 385 nm. The challenge remains, therefore,
to develop new coumarins absorbing at longer wavelengths e.g., in the near-infrared range
for a better penetration of light into thick/filled samples.
Author Contributions: Conceptualization, J.L. and F.D.; methodology, J.L. and F.D.; software, B.G.;
validation, all authors; formal analysis, J.L.; investigation, M.R. and G.N.; resources, J.L., F.D., J.T.,
T.H., and D.G.; data curation, J.L.; writing—original draft preparation, M.R., B.G., F.D., and J.L.;
writing—review and editing, all authors; visualization, J.L.; supervision, J.L.; project administration,
J.L.; funding acquisition, J.L. All authors have read and agreed to the published version of the
manuscript.
Funding: This research was funded by The Association of Specialization and Scientific Guidance,
the Centre National de la Recherche Scientifique, Aix Marseille Université and the Université de
Haute Alsace. This research was also funded by the Agence Nationale de la Recherche (ANR agency)
through the PhD grant of Guillaume Noirbent (ANR-17-CE08-0054 VISICAT project).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Acknowledgments: The Lebanese group would like to thank “The Association of Specialization and
Scientific Guidance” (Beirut, Lebanon) for funding and supporting this scientific work.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
Sample Availability: Samples of the compounds are not available from the authors.
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