Novel carbazole skeleton-based photoinitiators for led polymerization and LED projector 3D printing

Article abstract of DOI:10.3390/molecules22122143

Radical chemistry is a very convenient way to produce polymer materials. Here an application of a particular photoinduced radical chemistry is illustrated. Seven new carbazole derivatives Cd1–Cd7 are incorporated and proposed as high performance near-UV photoinitiators for both the free radical polymerization (FRP) of (meth)acrylates and the cationic polymerization (CP) of epoxides utilizing Light Emitting Diodes LEDs @405 nm. Excellent polymerization-initiating abilities are found and high final reactive function conversions are obtained. Interestingly, these new derivatives display much better near-UV polymerization-initiating abilities compared to a reference UV absorbing carbazole (CARET 9H-carbazole-9-ethanol) demonstrating that the new substituents have good ability to red shift the absorption of the proposed photoinitiators. All the more strikingly, in combination with iodonium salt, Cd1–Cd7 are likewise preferred as cationic photoinitiators over the notable photoinitiator bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) for mild irradiation conditions featuring their remarkable reactivity. In particular their utilization in the preparation of new cationic resins for LED projector 3D printing is envisioned. A full picture of the included photochemical mechanisms is given.

Full text of DOI:10.3390/molecules22122143

molecules
Article
Novel Carbazole Skeleton-Based Photoinitiators for
LED Polymerization and LED Projector 3D Printing
Assi Al Mousawi 1,2, Patxi Garra 1 ID , Frédéric Dumur 3,*, Thanh-Tuan Bui 4, Fabrice Goubard 4,
Joumana Toufaily 2, Tayssir Hamieh 2, Bernadette Graff 1, Didier Gigmes 3,
Jean Pierre Fouassier 1 and Jacques Lalevée 1,
*
1
Institute of Science and Materials of Mulhouse IS2M—UMR, The National Center for Scientific
Research (CNRS) 7361—UHA, 15, Rue Jean Starcky, 68057 Mulhouse CEDEX, France;
assimoussawi@gmail.com (A.A.M.); patxi.garra@uha.fr (P.G.); bernadette.graff@uha.fr (B.G.);
jean-pierre.fouassier@uha.fr (J.P.F.)
Laboratory of Materials, Catalysts, Environment and Analytical Methods (MCEMA-CHAMSI), EDST,
Lebanese University, Campus Hariri, Hadath, Beyrouth, Lebanon; joumana.toufaily@ul.edu.lb (J.T.);
tayssir.hamieh@ul.edu.lb (T.H.)
Institut de Chimie Radicalaire—CNRS, Aix-Marseille University, UMR 7273, F-13397 Marseille, France;
dider.gigmes@univ-amu.fr
Laboratory of Physico-chemistry of Polymers and Interfaces LPPI, University of Cergy-Pontoise,
5 Mail Gay Lussac, Neuville-sur-Oise, 95031 Cergy-Pontoise CEDEX, France; tbui@u-cergy.fr (T.-T.B.);
fabrice.goubard@u-cergy.fr (F.G.)
2
3
4
*
Correspondence: frederic.dumur@univ-amu.fr (F.D.); jacques.lalevee@uha.fr (J.L.);
Tel.: +33-(0)491-282-748 (F.D.); +33-(0)389-608-803 (J.L.)
Received: 31 October 2017; Accepted: 30 November 2017; Published: 4 December 2017
Abstract: Radical chemistry is a very convenient way to produce polymer materials. Here,
an application of a particular photoinduced radical chemistry is illustrated. Seven new carbazole
derivatives Cd1–Cd7 are incorporated and proposed as high performance near-UV photoinitiators
for both the free radical polymerization (FRP) of (meth)acrylates and the cationic polymerization
(CP) of epoxides utilizing Light Emitting Diodes LEDs @405 nm. Excellent polymerization-initiating
abilities are found and high final reactive function conversions are obtained. Interestingly, these
new derivatives display much better near-UV polymerization-initiating abilities compared to a
reference UV absorbing carbazole (CARET 9H-carbazole-9-ethanol) demonstrating that the new
substituents have good ability to red shift the absorption of the proposed photoinitiators. All the
more strikingly, in combination with iodonium salt, Cd1–Cd7 are likewise preferred as cationic
photoinitiators over the notable photoinitiator bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide
(BAPO) for mild irradiation conditions featuring their remarkable reactivity. In particular their
utilization in the preparation of new cationic resins for LED projector 3D printing is envisioned. A full
picture of the included photochemical mechanisms is given.
Keywords: cationic polymerization; free radical polymerization; light emitting diodes (LEDs);
photoinitiators; 3D printing
1. Introduction
Radical (photo) chemistry plays a significant role in the production of polymer materials. Due to
their advantages, compared to thermal activation and their applications as a green technology, light
activated polymerization reactions are experiencing an ever increasing development and have inspired
a lot of work on the design of photoinitiators PIs and multicomponent photoinitiating systems
PISs (see e.g., in recent papers and books [
1–
14] and references therein). One current approach
Molecules 2017, 22, 2143; doi:10.3390/molecules22122143
www.mdpi.com/journal/molecules

Molecules 2017, 22, 2143
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concerns the adaptation of PIs to LED irradiation wavelengths and, among others, a particularly
interesting application refers to 3D printing. PIs are able to produce radicals or cations under light
exposure. Suitable PISs can have dual behavior: using the same system both a radical and a cationic
photopolymerization can be carried out separately and/or together (like interpenetrating polymer
networks or IPNs).
In a recent study [15], four new carbazole derivatives exhibiting long wavelength absorption
have been proposed as PIs. As this kind of skeleton was particularly promising, seven new
carbazole derivatives (Cds) Cd1
substituents were synthesized (see Scheme 1); 9H-carbazole-9-ethanol (CARET) used previously in our
papers [16 18] was chosen here as a reference carbazole derivative. The new carbazole derivatives
–Cd7 exhibiting long wavelength absorption thanks to well selected
–
were incorporated into two-component (PI/iodonium salt Iod) and sometimes into three-component
(PI/Iod/amine) photoinitiating systems (PISs) to induce the formation of reactive species (radicals
or radical cations) for free radical polymerization (FRP) and/or cationic polymerization (CP) under
near-UV and visible light. Interestingly, the different substituents introduced on the carbazole skeleton
highly affect the absorption as well as the photochemical/electrochemical properties of Cd1–Cd7.
Obviously, this will pave the way to study the structure/reactivity/efficiency relationships of the
studied carbazole derivatives as photoinitiators in CP and FRP. The use of these new high performance
photosensitive systems in LED projector 3D printing experiments is also described. Finally, an insight
into the involved mechanisms is provided.
Scheme 1. The different carbazole derivatives (Cds) (denoted Cd1–Cd7) investigated in this study.
2. Results and Discussion
2.1. Experimental Results
2.1.1. Light Absorption Properties of Cd1–Cd7
The ground state absorption spectra of the new proposed photoinitiators in dichloromethane
(DCM) are shown in Figure 1 (see additionally Table 1). These compounds exhibit high molar extinction
coefficients in the near UV (e.g., Cd7 ~20,500 M−1
·
cm−1 @345 nm and 1500 M−1 cm−1 @405 nm).
·
Their absorptions in the 350–410 nm spectral range are interesting, guaranteeing a good overlap with
the emission spectrum of the LED@405 nm used in this work. The optimized geometries and the
frontier orbitals (Highest Occupied Molecular Orbital—HOMO and Lowest Unoccupied Molecular
Orbital—LUMO) are shown in Figure 2. Both the HOMO and LUMO are firmly delocalized all over the
π
system clearly showing a π→π* lowest energy transition. The extension of the
π framework by means
of various substituents on the carbazole moiety causes a reasonable shift in the spectrum (e.g., the
presence of 3-(1-naphthylethenyl) group causes a bathochromic shift in the spectrum of Cd7, Figure 1,
and Table 1). A significant difference in the HOMO structure of Cd3 and Cd7 is also observed. The clear

Molecules 2017, 22, 2143
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bathochromic shift caused by the different substituents in Cd1
–
Cd7 is consummately anticipated and
it follows the following order: Cd7~Cd1 > Cd3 > Cd5 > Cd4 > Cd2 > Cd6. For the reference carbazole
derivative (CARET), with no substituent on the phenyl rings, a shorter wavelength absorption is found
−1
−1
)
as well as lower extinction coefficients (e.g., no absorption for CARET@405 nm vs.
ε
~1500 (M
·
cm
for Cd7 see in Table 1). Surprisingly, contrasted with the well known photoinitiator (BAPO), the new
proposed structures Cd1
–
Cd7 exhibit much better light absorption properties in all the 350–410 nm
range (Figure 1).
cd1
cd2
35? 00
35,000
cd1
cd2
cd3
cd4
12? 00
12,000
cd3
cd4
cd5
cd6
30? 00
30,000
10? 00
10,000
25? 00
25,000
cd7
BAPO
CARET
cd5
cd6
cd7
CARET
BAPO
8? 00
8000
20? 00
20,000
6? 00
6000
15? 00
15,000
4? 00
4000
10? 00
10,000
2? 00
2000
5? 00
5000
0
380
0
300
390
400
 (nm)
410
420
350
400
 (nm)
(a)
(b)
Cd7 vs. CARET and BAPO. (a) For short wavelength (300–400
Figure 1. Absorption properties for Cd1
–
nm); (b) for extended wavelengths, particularly between 380 and 415 nm.
Table 1. Absorption properties for Cd1–Cd7 vs. CARET and BAPO (@405 nm).
Cd1
Cd2
Cd3
Cd4
Cd5
Cd6
Cd7
BAPO
CARET
ε 1
n.a 2
n.a 2
n.a 2
1000
180
90
140
1500
500
1
@405 nm (M−1·cm−1); 2 no absorption.
LUMO
HOMO
Cd1
Cd2
Cd3
Cd4
Figure 2. Cont.

Molecules 2017, 22, 2143
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Cd5
Cd6
Cd7
Figure 2. Contour plots of HOMOs and LUMOs for the Cd structures optimized at the B3LYP/6-31G*
level of theory.
2.1.2. Cationic Photopolymerization (CP) of Epoxides
Upon LED@405 nm illumination, the cationic polymerization (CP) of epoxides (e.g., EPOX) under
air using two-component photoinitiating systems based on Cd/Iod blends (0.5%/1% w/w) displays
high effectiveness in term of final epoxy function conversion (FC) (e.g., FC 55% with Cd7; at t = 800 s;
Figure 3a, band 1). The polyether network formation can clearly be observed at ~1080 cm−1 (Figure 3b)
in the FTIR spectra. Iod alone does not initiate the polymerization, obviously demonstrating the role
of Cd1
rates of polymerization (Rp) were obviously achieved with the Cd1
Cd2/Iod and Cd6/Iod system contrasted with the CARET/Iod system set as a reference for which no
polymerization happens. The correlation of Cd1 Cd7 with CARET is important as all photoinitiators
–Cd7 in inducing the iodonium salt disintegration upon near UV light LED. Amazingly, high
–
Cd7/Iod systems aside from
–
are characterized by having of a carbazole moiety. Likewise, astoundingly, in comparison with the
well-known BAPO/Iod (0.5%/1% w/w) system, both Cd1 and Cd7, aside from Cd2 and Cd6, are
discovered much preferable photoinitiators over BAPO (Figure 3a, curve 8 for BAPO contrasted with
curve 1 for Cd7). This information demonstrates the outstanding predominance of the new carbazoles
as far as efficiencies over the well-known BAPO (i.e., the polyether peaks are not observed neither
for CARET/Iod e.g., in Figure 3c, nor for BAPO/Iod contrasted with the Cd7/Iod system as e.g.,
in Figure 3b). The efficiency trend for CP using LED@405 nm follows the following order: Cd7
~Cd1
>
Cd3 Cd5 Cd4 > BAPO > Cd6 Cd2 > CARET (Table 2). RT-FTIR monitoring for curves 4, 7, 8
>
>
~
and 9 were stopped before 800 s as very poor kinetics were observed (conversion below 10% after
200 s), characteristics of very poor PIS efficiencies (strongly inhibited by oxygen). Clearly, this is
straightforwardly related to the absorption properties of the carbazole derivatives as Cd7 and Cd1 are
the leaders in the efficiency trend and having higher extinction coefficients at 405 nm contrasted with
the others (see Table 1).

Molecules 2017, 22, 2143
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60
50
40
30
before polymerization 1
after polymerization 2
3
2
1
0
2
1081 cm-1
790 cm-1
1
1
6
3
7
5
20
8
10
2
9
4
0
0
750
1000
200
400
600
800
(cm-1)
Time(s)
(a)
(b)
before polymerization 1
after polymerization 2
2
1
0
790 cm-1
1081 cm-1
2
1
750
1000
 (cm-1)
(c)
Figure 3. (a) Polymerization profiles of EPOX (epoxy function conversion vs. irradiation time) upon
exposure to the LED@405 nm under air in the presence of different photoinitiating systems: (1) Cd7/Iod
(0.5%/1% w/w); (2) Cd1/Iod (0.5%/1% w/w); (3) Cd3/Iod (0.5%/1% w/w); (4) Cd2/Iod (0.5%/1%
w/w); (5) Cd4/Iod (0.5%/1% w/w); (6) Cd5/Iod (0.5%/1% w/w); (7) Cd6/Iod (0.5%/1% w/w);
(8) BAPO/Iod (0.5%/1% w/w); (9) CARET/Iod (1%/1% w/w); IR spectra recorded before and after
polymerization using: (b) Cd7/Iod (0.5%/1% w/w); (c) CARET/Iod (1%/1% w/w). The irradiation
starts at t = 10 s.
Table 2. Function conversions (FC): epoxy for EPOX; acrylate for TMPTA and methacrylate for
BisGMA/TEGDMA using different photoinitiating systems; LED@405 nm for irradiation.
Cd1
Cd2
Cd3
Cd4
Cd5
Cd6
Cd7
BAPO
CARET
EPOX 1
TMPTA 2
44%
46%
57%
0%
0%
0%
41%
34%
27%
34%
30%
43%
37%
28%
44%
57%
60%
10%
1%
60%
BisGMA/TEGDMA 3
1
Epoxy function Conversion FC (%) for EPOX (at t = 100 s) (0.5% PI + 1% Iod); 2 Acrylate function conversion FC
for TMPTA (%) (at t = 100 s) (0.5% PI + 1% Iod); excepted (1% CARET + 1% Iod); 3 Methacrylate conversion FC for
BisGMA/TEGDMA (%) (at t = 100 s) (0.5% PI + 1% Iod).
2.1.3. Free Radical Photopolymerization
•
Photopolymerization of Acrylates
The FRP of TMPTA thin films in laminate initiated from diverse Cd/Iod couples is very effective
in some cases using the LED@405 nm, while Iod alone obviously does not initiate the polymerization.
Typical acrylate function conversion-time profiles are given in Figure 4 and the FCs are outlined in
Table 2. Best FCs are achieved when using Cd7/Iod and Cd1/Iod (~57% and 46% respectively at
t = 100 s), that’s why these structures will be of more deeply studied in the rest of the manuscript.
Strikingly, the reactivity trend follows the order in-line with what observed for the CP process:
Cd7 > Cd1 > Cd5 > Cd3 > Cd4 > Cd6~Cd2.

Molecules 2017, 22, 2143
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•
Photopolymerization of Methacrylates
Interestingly, the Cd7/Iod (0.5%/1% w/w) couple effectively initiates the FRP of a mix
of methacrylates (BisGMA/TEGDMA 70%/30% w/w) under air (1.4 mm thick example) upon
illumination with the LED@405 nm, as shown in Figure 4d. Remarkably, a straightforward tack
free polymer is produced even in thick examples (Figure 4d).
60
70
7
60
50
1
50
5
40
40
3
30
6
30
20
10
0
20
10
0
4
2
100
0
20
40
60
80
100
0
20
40
60
80
time(s)
time(s)
(a)
(b)
1
before polymerization (1)
after polymerization (2)
6160 cm-1
0
6150
6200
 (cm-1)
(c)
(d)
Figure 4. (a) Polymerization profiles of TMPTA (acrylate function conversion vs. irradiation time) in
laminate upon exposure to LED@405 nm in the presence of: (1) Cd1/Iod (0.5%/1% w/w); (2) Cd2/Iod
(0.5%/1% w/w); (3) Cd3/Iod (0.5%/1% w/w); (4) Cd4/Iod (0.5%/1% w/w); (5) Cd5/Iod (0.5%/1%
w/w); (6) Cd6/Iod (0.5%/1% w/w); (7) Cd7/Iod (0.5%/1% w/w); (b) Polymerization profiles of
BisGMA/TEGDMA (70%/30%) (methacrylate function conversion vs. irradiation time) (1.4 mm
film thick sample) under air upon exposure to LED@405 nm in the presence of Cd7/Iod (0.5%/1%
w/w); (c) Near IR spectra recorded before and after polymerization of BisGMA/TEGDMA using
Cd7/Iod (0.5%/1% w/w); (
d) Photos of BisGMA/TEGDMA film (1.4 mm thick) upon irradiation
with the LED@405 nm for 100 s in the presence of Cd7/Iod (0.5%/1% w/w) under air before and
after polymerization.
2.1.4. Synthesis of Inter-Penetrated Networks (IPNs) through Photopolymerization
The Cd/Iod two-component PIS can jointly create both free radicals (Ph•) and radical cations
(Cd•+) simultaneously upon visible light exposure using LED@405 nm and hence can be utilized for
the synthesis of IPNs from acrylate/epoxy combinations (Figure 5a). Effectively, the synthesis of a thick
1.4 mm tack free coating was performed under air (Figure 5b). Very high final function conversion of
acrylate functional group compared to the epoxy functional group (FCs
≈
76% and 23% for the acrylate
and the epoxide, band 1 and band 2 in Figure 5a, respectively). In reality, this behaviour which can
be credited to the more receptive radicals (Ph•) in the event of free radical polymerization contrasted
with cationic polymerization (where cations are starting species) is reported in the literature [19].

Molecules 2017, 22, 2143
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80
70
60
50
40
30
20
10
(1)
(2)
0
0
100
200
300
400
time(s)
(a)
(b)
Figure 5.
vs. irradiation time) under air thick sample 1.4 mm upon exposure to LED@405 nm in the presence
of Cd7/Iod (0.5%/1% w/w): (1) TMPTA; (2) EPOX; ( ) Photos of TMPTA/EPOX film (1.4 mm thick)
(a) Polymerization profiles of TMPTA/EPOX blend (acrylate and epoxy function conversions
b
upon irradiation with the LED@405 nm for 400 s in the presence of Cd7/Iod (0.5%/1% w/w) under air
before and after polymerization.
2.1.5. Surface Patterning or 3D Printing Using Cd1–Cd7 Based System upon LED@405 nm Projector
For 3D printing tests, a LED projector @405 nm (110 mW/cm2, Thorlabs, Gmbh, Germany) was
utilized. A similar intensity on the surface of the specimen and a similar emission spectrum for
the LEDs used in 3D printing and in monitoring FTIR tests were used for purpose of comparison.
In Figure 6, some 3D printing tests were done using the LED projector illumination using the
Cd7/Iod (0.5%/1% w/w) PIS, which is extremely responsive in the cationic polymerization of
epoxides (see above) under air. Remarkably, the high photosensitivity of this resin permits an effective
polymerization process in the illuminated area. This LED projector tests demonstrate a favorable
element over other laser based 3D printing techniques as the whole layer is illuminated at one time.
The fast cationic process upon exposure to the LED projector @405 nm likely outperform the radical
process by diminishing the shrinkage generally observed in the latter. Thin 3D objects (25–50 µm) and
in addition thicker ones (0.45 mm) can be effectively generated through the LED projector technique
(Figure 6).
(a)
(b)
(c)
(d)
Figure 6. Cationic photopolymerization experiments using LED projector @405 nm: (
) Profilometry characterization for a thin pattern obtained (thin samples 0.02 mm); ( ) observation
(numerical optical microscopy) of pattern written in 3D for a thicker sample (0.5 mm).
a,b) patterns
(
c
d

Molecules 2017, 22, 2143
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2.2. Photochemical Mechanisms
2.2.1. Steady State Photolysis
The steady state photolysis of Cd1
–
Cd7/Iod in acetonitrile is quick compared to the photolysis of
Cds alone (e.g., Cd7/Iod in Figure 7a versus Cd7 alone in Figure 7b). A new photo-product (described
by a noteworthy new absorption for
λ
> 550 nm; Figure 7a) is formed and expected due to the Cd7/Iod.
3
3
0s
0s
2
1
0
2
1
200s
60s
200s
0s
0
400
500
600
700
400
500
600
700
(nm)
 (nm)
(a)
Cd7/Iod photolysis upon exposure to LED@405 nm and (
(b)
Figure 7.
(a
)
b
) Photolysis of Cd7 in the
absence of Iod.
2.2.2. Fluorescence Quenching, Laser Flash Photolysis (LFP), Cyclic Voltammetry and
ESR Experiments
Fluorescence experiments in acetonitrile are shown in Figure 8 for Cd7. The intersection point
of the absorption and fluorescence spectra permits the determination of the first singlet excited state
energies (ES1) (Table 3; e.g., ES1 for Cd7 Figure 8a). Ideal fluorescence quenching processes were
proven by high estimations of the Stern Volmer coefficients leading to
Figure 8c,d). Highly favorable free energy changes for the electron transfer reaction were noted
between carbazole derivatives and Iod (e.g., Get = 1.95 eV for Cd7; the oxidation potential for Cd7
was determined by cyclic voltammetry in Figure 8b), electron transfer quantum yields from S1 ( et)
were computed by Equation (1): et ~0.43 with Cd7). Efficient processes can also be expected from the
triplet state of Cd ( Get (T1) < 0 eV; Table 3); but the singlet state pathway is probably more favorable
due to the high value of Φet from S1 found above:
Φet in (Equation (1)) (Table 3,
∆
−
Φ
Φ
∆
Φet = Ksv [Iod]/(1 + Ksv[Iod])
(1)
The favorable quenching process of carbazole derivatives by Iod is depicted by the reactions r1
and r2:
Cd→ *Cd (hυ)
*Cd + Ar2I+ → Cd•+ + Ar2I• → Cd•+ + Ar• + ArI
(r1)
(r2)
The presence of Ar• (r2) is fully confirmed by ESR results. Indeed, the phenyl radicals (Ar•)
were easily detected as PBN/Ar• radical adducts in the irradiation of a Cd7/Iod solution in ESR-ST
experiments (Figure 9) i.e., the simulation of the experimental ESR spectrum yields the hyperfine
coupling constants (hfcs): aN = 14.13 G and aH = 2.21 G typical for the PBN/Ph• radical adducts.

Molecules 2017, 22, 2143
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(1) blank
(1) absorption
(2) emission ( ex= 370 nm)
40
30
20
10
0
(2) with Cd7
1
1
(2)
(2)
(1)
(1)
0
0
600
-10
0
300
400
500
1
2
Potential [V vs. SCE]
 (nm)
(a)
(b)
[Iod]= 0 M
80
60
40
y= 1.003 + 32.767 x
R2= 0.999
2
[Iod]= 0.018 M
20
0
1
0
2
4
6
8
10
12
14
16
18
20
400
450
500
550
600
[Iod] mM
 (nm)
(c)
(d)
Figure 8.
(a) Singlet state energy determination; (b) Cyclic voltammetry for the Cd7 oxidation;
(
c) Fluorescence quenching of Cd7 with Iod; (
d) Stern-Volmer treatment of the lifetime quenching of
*Cd7 in the presence of Iod.
simulated
experimental
3300
3320
3340
3360
3380
3400
B (G)
Figure 9. ESR-ST spectra obtained upon irradiation (LED@405 nm) of a Cd7/Iod solution, experimental
(lower spectrum), and simulated (upper spectrum) in tert-butyl benzene as a solvent.
Table 3. Parameters characterizing the chemical mechanisms associated with the Cd1
–Cd7/Iod systems
in acetonitrile.
2
Eox (eV) 1
ES1 (eV)
∆GetS1 (eV)
ET1 (eV) 2
∆GetT1 (eV)
Φet (S1)
Cd1
Cd2
Cd3
Cd4
Cd5
Cd6
Cd7
0.92
1.06
0.97
0.96
0.99
1.49
0.98
3.17
3.5
3.2
3.35
3.25
3.5
−2.05
−2.22
−2.02
−2.19
−2.06
−1.81
−1.95
1.97
1.99
1.97
1.99
1.97
3.04
1.89
−0.85
−0.73
−0.81
−0.83
−0.79
−1.36
−0.72
0.34
0.5
0.41
0.5
3.13
0.43
1
2
Oxidation potentials for Cd1
of theory).
–
Cd7 (this work); from molecular orbital calculations (uB3LYP/6-31G* level

Molecules 2017, 22, 2143
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2.2.3. Structure/Reactivity/Efficiency Relationship
In both CP and FRP, the studied carbazole derivatives follow the same trend of efficiency in terms
of final conversion of both acrylate functional groups as well as epoxy functional groups, respectively.
In fact, the efficiency trend of the carbazole derivatives more or less follows their absorption trend.
Clearly, both Cd7 and Cd1 are the best photoinitiators in terms of efficiency under LED@405 nm
irradiation (see above in parts 2 and 3), and at the same time they are characterized by relatively higher
absorption ability at this range of wavelengths. On the other hand, the ∆Gs of electron transfer of all
Cds as well electron transfer quantum yields (Table 3) are relatively similar and thus can be excluded
for being the key factor for the Structure/Reactivity/Efficiency relationships of these derivatives as
they are rather similar in the Cd1–Cd7 series. Therefore, the key factor affecting the reactivity and
efficiency of these carbazole derivatives as PIs is their absorption properties since as better absorption
of Cd7 and Cd1 of the light from LED@405 nm is observed, these carbazole derivatives exhibit the
best performance. The following table (Table 4) summarizes the trends in magnitude of absorption,
efficiencies in both CP and FRP.
Table 4. Trends of bathochromic shift, Cationic polymerization efficiency, and Free radical
polymerization efficiency.
Trend of bathochromic shift
Trend of CP efficiency
Trend of FRP efficiency
Cd7~Cd1 > Cd3 > Cd5 > Cd4 > Cd2 > Cd6
Cd7~Cd1 > Cd3 > Cd5 > Cd4 > Cd6~Cd2
Cd7 > Cd1 > Cd5 > Cd3 > Cd4 > Cd6~Cd2
3. Materials and Methods
3.1. Chemical Compounds
9H-Carbazole-9-ethanol (CARET) and phenyl-N-tert-butylnitrone (PBN) (Scheme 2) were obtained
from Sigma Aldrich (St. Louis, MO, USA). Bis-(4-tert-butylphenyl)iodonium hexafluorophosphate
(Iod or Speedcure 938) and bis(2,4,6-trimethylbenzoyl) phenylphosphine oxide (BAPO or speedcure
BPO) were obtained from Lambson (Lambson Ltd., Wetherby, UK). Trimethylolpropane triacrylate
(TMPTA) and 3,4-epoxycyclohexylmethyl 3,4-epoxycyclo-hexylcarboxylate (EPOX; Uvacure 1500)
were obtained from Allnex (Frankfurt, Germany) and used as benchmark monomers for radical and
cationic photopolymerization. Bisphenol A-glycidyl methacrylate (BisGMA) and triethyleneglycol
dimethacrylate (TEGDMA) were obtained from Sigma Aldrich and used with the highest purity available.
The full procedures for the synthesis of Cd1–Cd7, are presented in the Supporting Information.
Scheme 2. Other used chemical compounds.
3.2. Irradiation Sources
The following Light Emitting Diodes (LEDs) were used as irradiation sources: (i) LED@375 nm—
incident light intensity at the sample surface: I0
≈
40 mW cm−2; (ii) LED@405 nm (I0
≈
110 mW cm−2).

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3.3. Free Radical (FRP) and Cationic (CP) Photopolymerization
The two-component photoinitiating systems (PISs) are mainly based on Cds/Iodonium salt
(0.5%/1% w/w) for both CP and FRP. The weight percent of the photoinitiating system is calculated
from the monomer content. The photosensitive thin-film formulations (~25
µm of thickness)
were deposited on BaF2 pellets under air for the CP of EPOX while for the FRP of TMPTA
and BisGMA/TEGDMA, it was done in laminate (the formulation is sandwiched between two
polypropylene films to reduce the O2 inhibition). The 1.4 mm thick samples of BisGMA/TEGDMA
were polymerized under air.
Excellent solubility of all carbazole derivatives except for Cd6 were observed in EPOX monomer.
The decrease of the epoxy group content and the double bond content of (meth)acrylate functions were
continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100, Oklahoma City, OK, USA)
at about 790 cm−1 and 1630 cm−1, respectively. The evolution of the methacrylate characteristic peak
for the thick samples (1.4 mm) was followed in the near infrared range at ~6160 cm−1. The procedure
used to monitor the photopolymerization profile has been described in detail in [20,21].
3.4. Redox Potentials
The oxidation potentials (Eox vs. SCE) for the different carbazole derivatives were measured
in acetonitrile by cyclic voltammetry with tetrabutylammonium hexafluorophosphate 0.1 M as the
supporting electrolyte. The free energy change ∆Get for an electron transfer reaction was calculated
from the Rehm-Weller equation (Equation (2)) [22] where Eox, Ered, E* and C are the oxidation
potential of the electron donor, the reduction potential of the electron acceptor, the excited state energy
level and the coulombic term for the initially formed ion pair, respectively. C is neglected as usually
done in polar solvents:
∆Get = Eox − Ered − E* + C
(2)
3.5. ESR Spin Trapping (ESR-ST) Experiments
The ESR-ST experiments were carried out using an X-Band spectrometer (EMX-Plus, Bruker,
Billerica, MA, USA). LED@405 nm was used as irradiation source for triggering the production
of radicals at room temperature (RT) for N2 saturated toluene solutions and trapped by
phenyl-N-tert-butylnitrone (PBN) according to a procedure described by us in details [23]. The ESR
spectra simulations were carried out with the PEST WINSIM program.
3.6. Absorption Experiments
The UV-vis absorption properties of the compounds were studied using a JASCO
V730 spectrophotometer.
3.7. Fluorescence Experiments
The fluorescence properties of the compounds were studied using a JASCO FP-6200 fluorimeter.
3.8. Computational Procedure
Molecular orbital calculations were carried out with the Gaussian 03 suite of programs [24,25].
The electronic absorption spectra for the different compounds were calculated with the time-dependent
density functional theory at the MPW1PW91/6-31g(d) level of theory on the relaxed geometries
calculated at the UB3LYP/6-31G* level of theory.
3.9. 3D Printing Experiments
For 3D printing experiments, a LED projector @405 nm (Thorlabs) was used. The photosensitive
cationic resin was polymerized under air and the generated patterns analyzed by an opto-digital

Molecules 2017, 22, 2143
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microscope (DSX-HRSU from Olympus Corporation, Tokyo, Japan) or by profilometry. The procedure
was presented in [26].
4. Conclusions
In the present paper, the carbazole scaffold is proposed as an excellent precursor for the
development of new photoinitiators using near-UV and visible wavelength LEDs for the photoinitiation
of both the cationic polymerization of epoxides and the free radical polymerization of (meth)acrylates.
High final conversions and polymerization rates are achieved. Examples of using these new initiating
frameworks in cationic photocurable resins for LED projector 3D printing are given. Basically,
the primary processes involve the simultaneous production of radicals and radical cations that can
be applied both in radical chemistry and/or cationic chemistry. New photoinitiating systems based
on carbazole derivatives capable to be operated at various illumination wavelengths or that can be
incorporated in 3D printing resins will be reported in future papers.
Supplementary Materials: Supplementary Materials are available online, the synthesis of Cd1–Cd7, the
procedures are fully presented in supporting information.
Acknowledgments: The Lebanese group would like to thank ‘The Association of Specialization and Scientific
Guidance’ (Beirut, Lebanon) for funding and supporting this scientific work. The authors thank the “Agence
Nationale de la Recherche” (ANR) for the grant “FastPrinting”.
Author Contributions: Assi Al Mousawi and Patxi Garra performed the polymerization experiments and the
photophysical characterizations of the different photoinitiators, analyzed the whole data sets and wrote the
first draft of the paper. Frédéric Dumur and Thanh-Tuan Bui did the synthesis of the different photoinitiators
reported in this work and characterized the chemical structures of the different molecules by usual techniques.
Didier Gigmes and Fabrice Goubard contributed to the molecular design of the photoinitiators. Bernadette Graff
did the theoretical calculations and interpreted the data. Jean Pierre Fouassier and Jacques Lalevée conceived
the project and contributed to all aspects of the study and writing of the final manuscript. Joumana Toufaily
and Tayssir Hamieh contributed to the supervision of Assi Al Mousawi as co-advisors, assisted in obtaining the
Ph.D. grants and editing the manuscript and contributed to the project with fruitful discussions. All authors
discussed the contents of the manuscript and approved the submission.
Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
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