Design of iodonium salts for UV or near-UV LEDs for photoacid generator and polymerization purposes

Article abstract of DOI:10.3390/molecules25010149

Iodonium salts are well established photoacid generators, cationic photoinitiators, as well as additives commonly used in photoredox catalytic cycles. However, as a strong limitation, iodonium salts are characterized by low light absorption properties for λ > 300 nm so that these latter cannot be activated with cheap, safe, and eco-friendly near UV or even visible light emitting diodes (LEDs). To overcome this drawback, the covalent linkage of an iodonium salt to a chromophore absorbing at longer wavelength is actively researched. With aim at red-shifting the absorption spectrum of the iodonium salt, the synthesis of new compounds combining within a unique chemical structure both the chromophore (here the naphthalimide scaffold) and the iodonium salt is presented. By mean of this strategy, a polymerization could be initiated at 365 nm with the modified iodonium salts whereas no polymerization could be induced with the benchmark iodonium salt i.e., Speedcure 938 at this specific wavelength. To examine the effect of the counter-anion on the photoinitiating ability of these different salts, five different counter-anions were used. Comparison between the different anions revealed the bis(trifluoromethane)sulfonimide salt to exhibit the best photoinitiating ability in both the free radical polymerization of acrylates and the cationic polymerization of epoxides. To support the experimental results, molecular orbital calculations have been carried out. By theoretical calculations, the initiating species resulting from the photocleavage of the iodonium salts could be determined. The cleavage selectivity and the photochemical reactivity of the new iodoniums are also discussed.

Full text of DOI:10.3390/molecules25010149

molecules
Article
Design of Iodonium Salts for UV or Near-UV LEDs
for Photoacid Generator and Polymerization Purposes
Ségolène Villotte 1, Didier Gigmes 1 , Frédéric Dumur 1,
*
and Jacques Lalevée 2,3,
*
1
Aix Marseille Univ, CNRS, ICR UMR 7273, 13397 Marseille, France; segolene.villotte@etu.univ-amu.fr (S.V.);
didier.gigmes@univ-amu.fr (D.G.)
Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France
Université de Strasbourg, 67000 Strasbourg, France
2
3
*
Correspondence: Frederic.dumur@univ-amu.fr (F.D.); jacques.lalevee@uha.fr (J.L.);
Tel.: +33-(0)4-9128-9059 (F.D.); +33-(0)3-8960-8803 (J.L.)
Academic Editors: Frédéric Dumur and Guillaume Noirbent
Received: 16 December 2019; Accepted: 27 December 2019; Published: 30 December 2019
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Iodonium salts are well established photoacid generators, cationic photoinitiators, as well as
additives commonly used in photoredox catalytic cycles. However, as a strong limitation, iodonium
salts are characterized by low light absorption properties for
λ > 300 nm so that these latter cannot be
activated with cheap, safe, and eco-friendly near UV or even visible light emitting diodes (LEDs).
To overcome this drawback, the covalent linkage of an iodonium salt to a chromophore absorbing at
longer wavelength is actively researched. With aim at red-shifting the absorption spectrum of the
iodonium salt, the synthesis of new compounds combining within a unique chemical structure both
the chromophore (here the naphthalimide scaffold) and the iodonium salt is presented. By mean of
this strategy, a polymerization could be initiated at 365 nm with the modified iodonium salts whereas
no polymerization could be induced with the benchmark iodonium salt i.e., Speedcure 938 at this
specific wavelength. To examine the effect of the counter-anion on the photoinitiating ability of these
different salts, five different counter-anions were used. Comparison between the different anions
revealed the bis(trifluoromethane)sulfonimide salt to exhibit the best photoinitiating ability in both the
free radical polymerization of acrylates and the cationic polymerization of epoxides. To support the
experimental results, molecular orbital calculations have been carried out. By theoretical calculations,
the initiating species resulting from the photocleavage of the iodonium salts could be determined.
The cleavage selectivity and the photochemical reactivity of the new iodoniums are also discussed.
Keywords: iodonium salt; photoacid; photopolymerization; cationic initiator; LED
1. Introduction
The light induced cationic polymerization (CP) is already characterized by a widespread use
in the radiation curing area (see e.g., in [1–14]). Notably, photoinduced cationic polymerization is
widely used in coatings [15], adhesives [16], microelectronics [17], graphic arts [18], dentistry [19],
and 3D-printing [20,21] and these different applications can be cited as the most relevant ones.
Among cationic initiators, diaryliodonium, triarylsulfonium salts [
have been extensively studied in the literature. The chemistry of iodonium salts is notably
well-documented in the literature [ 14 22 25]. Besides, the reported structures still suffer from
poor light absorption properties for wavelengths higher than 300 nm, so that these different structures
can only be activated with UV light [26 34]. Even if some of the onium salts were proposed for
absorption at longer wavelengths (e.g., thianthrenium salts, thioxanthenium salts, modified iodonium
or sulfonium salts, pyridinium salts) [35 46], the current works are more oriented to the use of external
1–14,22–25] and others [26–34]
1– , –
–
–
Molecules 2020, 25, 149; doi:10.3390/molecules25010149
www.mdpi.com/journal/molecules

Molecules 2020, 25, 149
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photosensitizers and/or free-radical sources to extend the spectral response of these different salts [1–14].
At present, only few works have been devoted to expanding the spectral range of iodonium salts by
the covalent linkage of chromophores to the iodonium salts [47 49]. In fact, a careful survey of the
–
literature reveals these chemical modifications of iodonium salts to be limited to the replacement of
one the phenyl ring by a naphthalene or a biphenyl group what is definitely insufficient to drastically
red-shift the absorption of the corresponding iodonium salts [47–49]. A breakthrough can be achieved
by using chromophores other than phenyl or naphthalene so that the absorption spectra can be clearly
displace towards the visible region, what is possible with naphthalimides. Choice of this chromophore
was supported by previous works done by our groups evidencing nahphtalimide to be an excellent
candidate for the design of type II photoinitiators [50–60].
In this context, naphthalimide was selected as a possible candidate to act as a covalently linked
photosensitizer for diaryliodonium salts [
7
]. Therefore, in the present paper, a series of five new
photosensitive iodonium salts (noted
2–6 in Figure 1; synthesis described in Scheme 1) differing
by the counter-anions and resulting from a coupling between both a naphthalimide chromophore
and an iodonium moiety is proposed to improve the light absorption properties in the 350–380 nm
range. Interestingly, Speedcure 938 i.e., bis-(4-tert-butylphenyl)-iodonium hexafluorophosphate is
totally ineffective to initiate a polymerization at such wavelength without taking recourse to a
photosensitizer. The approach presented in this work is interesting to generate initiating species
through an intramolecular process (here a monocomponent system) instead of an intermolecular
reaction in a two-component photosensitizer/diaryliodonium system. Especially in this work, the
higher efficiency of the monocomponent system over the bicomponent system making using of the
benchmark Speedcure 938 is clearly evidenced. The polymerization of a benchmark epoxy monomer
i.e., (3,4-epoxycyclohexane)methyl 3,4-epoxycyclo-hexylcarboxylate (EPOX) is monitored by time
resolved infrared spectroscopy. The performance of the proposed iodonium salts is compared to a
commercial reference, namely Speedcure 938 (See Scheme 2), exhibiting a significant absorption only
in the 230–300 nm range. A comparison of the performances obtained while using different counter
anions is also proposed. The photochemical cleavage processes are studied by electron spin resonance
experiments and molecular orbital (MO) calculations.
Figure 1. Chemical structures of the different iodonium salts 2–6.
2. Results
The different iodonium salts
cationic polymerization of epoxides in thin (See Figure 2) and thick (See Figure 3) samples upon
irradiation with a LED at 365 nm. Remarkably, it can be observed that , and lead to good
2–6 were first investigated for their photoinitiating ability for the
3,
5
6
polymerization performances that are clearly better than that of the reference Speedcure 938 for which
no polymerization occurred. These results are in agreement with their significant light absorption
properties in the 350–400 nm region (See Figure 4). On the opposite, as the reference Speedcure 938
does not absorb for
with this compound. Conversely, the maximum absorption wavelength of
λ
> 300 nm [
5
] so that no polymerization is possible upon irradiation at 365 nm
, and is close to 345 nm
3
,
5
6
with a molar extinction coefficient higher than 10,000 M−1 cm−1 so that the absorption is sufficient to
efficiently initiate a polymerization process. From the theoretical calculations (See Figure 5), it can be
observed that the naphthalimide chromophore strongly participates to the highest occupied molecular
orbital (HOMO), destabilizing the HOMO level and decreasing the HOMO–LUMO (lowest unoccupied

Molecules 2020, 25, 149
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molecular orbital) gap in the iodonium salts
2–
6. As a result of this, a bathochromic shift of the
absorption spectra for 2–6 is observed compared to that of the reference Speedcure 938.
Figure 2. Photopolymerization profiles (epoxy function conversion vs. irradiation time) measured by
RT-FTIR for the photoinduced polymerization of EPOX using the different iodoniums ( ) 1% w/w;
m) under air,
2–6
the polymerization profile obtained with the reference is noted ref; thin samples (25
upon LED@365 nm irradiation (the irradiation starts at t = 10 s, 500 mW/cm2).
µ
Figure 3. Photopolymerization profiles (epoxy function conversion vs. irradiation time) measured
by RT-FTIR for the photoinduced polymerization using the different iodoniums ( ) 1% w/w;
2–6
the polymerization profile obtained with the reference is noted ref; thick samples (1.4 mm) under air,
upon LED@365 nm irradiation (the irradiation starts at t = 10 s, 500 mW/cm2).
The light absorption properties for the different iodonium salts originating from the iodonium
salt and not from the counter anions, the difference of reactivity between 2–6 can be ascribed to the
counter anion effect. Indeed, it is well established that low nucleophilicity counter anions are more
favorable (SbF6−; PF6−; bis(trifluoromethane)sulfonamide) by preserving the cationic propagating
centers compared to higher nucleophilic counter anions (Cl−; TfO−) [1,3,5].

Molecules 2020, 25, 149
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1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
3 in ACN
5 in ACN
6 in ACN
200
300
400
500
600
700
λ (nm)
.
Figure 4. UV-vis absorption spectra for the most reactive iodonium salts (
3,
5
, and 6) in acetonitrile for
a concentration of 1 × 10−4 M.
Figure 5. Investigated cleavage processes (Cleavage A and Cleavage B); highest occupied and lowest
unoccupied molecular orbitals (HOMO and LUMO), respectively.
3. Discussion
It is well established that the iodonium salts can be cleaved (C–I bond) upon light irradiation
leading to the formation of strong acids (r1) ([
photoacid behavior:
1,3,5] and references herein) in agreement with a
R1-I+-R2/A− →→→ H+/A− (hν)
(r1)
This behavior is observed here with 3, 5, and 6. Indeed, during the irradiation of the solutions of
iodonium salts in presence of methyl red, appearance of the protonated form of this pH-indicator was
clearly observed by UV-visible spectroscopy. This photoacid ability is also in full agreement with their
initiating properties for the polymerization of epoxides [61].
To further investigate the cleavage process, molecular orbital calculations were carried out. From
the two possible cleavages (noted A and B in Figure 5), the cleavage B is more favorable than the
cleavage A due to a lower C–I bond dissociation energy (BDE): 49.7 vs. 57.4 kcal/mol for cleavage B
and A, respectively. This latter result is also in agreement with the electron spin resonance experiments.
Indeed, the ESR-ST spectrum obtained upon irradiation of 3 in the presence of PBN as a spin-trap agent

Molecules 2020, 25, 149
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in tert-butylbenzene showed the formation of an aryl (Ar•)/PBN radical adduct (hyperfine coupling
constants hfcs: aN = 14.3 G and aH = 2.2 G; reference values in [62]).
The wavelength 405 nm is important in 3D printing and laser write experiments [63], however,
the light absorption properties of the new proposed iodoniums is not high sufficient to initiate
a polymerization process at this wavelength. Recently, the possibility to create charge transfer
complexes (CTC) between electron rich amines (e.g., trimethylaniline (TMA) in Scheme 2) and electron
poor iodonium salts has been reported in the literature (r2) [63]. These CTC can be photolyzed
at 405 nm, leading to an efficient release of aryl radicals Arꢀ (here Tolꢀ) (r3) to initiate radical
polymerization processes.
Amine + Iodonium → [Amine-Iodonium]CTC
(r2)
[Amine-Iodonium]CTC →→→ Arꢀ (hν)
(r3)
This approach has notably been used here in laser write experiments @405 nm for the
5/TMA
system in trimethylolpropane triacrylate (TMPTA) used as a trifunctional acrylic monomer (Figure 6).
No polymerization was observed while using
combination through a CTC.
5 or TMA alone, showing the requirement of this
Figure 6. Numerical optical microscope observation of patterns “ICR” written from the polymerization
of trimethylolpropane triacrylate (TMPTA) with the iodonium in combination with trimethylaniline
5
(TMA) (1%/1% w/w): 3D overall appearance. A writer using a laser diode at 405 nm (size of the spot
around 50 µm) with the intensity of 100 mW was used for the spatially controlled irradiation.
4. Materials and Methods
4.1. Synthesis of the New Iodonium Salts
The general synthetic pathway for the iodonium salts
2–6 is depicted in Scheme 1. The procedure
is described in detail below. The procedure has been inspired from [64].

Molecules 2020, 25, 149
6 of 13
Scheme 1. Synthetic pathways to iodonium salts 2–6.
All reagents and solvents were purchased from Aldrich (St. Louis, MO, USA) or Alfa Aesar
(Haverhill, MA, USA) and used as received without further purification. Mass spectroscopy and
NMR were performed by the Spectropole of Aix-Marseille University (Marseille, France). HRMS
mass spectral analysis was performed with a SYNAPT G2 HDMS (Waters) mass spectrometer (Waters
Corporation, Manchester, UK). 1H and 13C NMR spectra were recorded at room temperature in 5 mm
o.d. tubes on a Bruker AC 400 spectrometer of the Spectropole (Billerica, MA, USA): 1H (400 MHz), 19
F
(376 MHz) and 13C (100 MHz). The 1H chemical shifts were referenced to the solvent peak (CD3)2CO
(2.05 ppm), (CD3)2SO (2.49 ppm), CDCl3 (7.26 ppm) and the 13C chemical shifts, determined by
13C-APT experiment, were referenced to the solvent peak Acetone-D6 (29.80 ppm and 206.3 ppm),
DMSO-D6 (39.52 ppm), CDCl3 (77.0 ppm).
N-(p-iodophenyl)-1,8-naphthalimide (
M = 198.17 g
mol−1), 4-iodoaniline (4.70 g, 21.46 mmol, 2.eq., M = 219.02 g
20.81 mmol, 20 eq., M = 68.08 g
mol−1) in chloroform (70 mL) was refluxed for 24 h. After cooling,
1
): A mixture of 1.8-naphthalic anhydride (2.10 g, 10.60 mmol,
·
·
mol−1) and imidazole (14.17 g,
·
the solvent was removed under reduced pressure. The crude product was taken up in absolute ethanol
and the resulting suspension was sonicated for 15 min. The suspension was filtered off and washed
with ethanol to afford the pure product as a grey solid (3.87 g, 9.69 mmol, 91% yield). 1H NMR
(CDCl3)
δ
(ppm): 7.05–7.10 (m, 2H), 7.80 (t, 2H, J = 7.8 Hz), 7.85–7.90 (m, 2H), 8.28 (dd, 2H, 3J = 8.3 Hz,
4J = 0.8 Hz), 8.64 (dd, 2H, 3J = 7.3 Hz, 4J = 0.9 Hz). 13C NMR (CDCl3)
δ(ppm): 94.5 (Cq), 122.7 (Cq, 2C),
127.2 (CH, 2C), 128.6 (Cq), 130.8 (CH, 2C), 131.9 (CH, 2C), 131.8 (Cq), 134.6 (CH, 2C), 135.3 (Cq), 138.7
(CH, 2C), 164.2 (C=O, 2C). HRMS (ESI MS) m/z: theor: 399.9829 found: 399.9829 ((M + H)+ detected).

Molecules 2020, 25, 149
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(4-(1,3-Dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl)(p-tolyl)iodonium trifluoro-methanesulfonate (
mixture of N-(para-iodophenyl)-1,8-naphthalimide (0.67 g, 1.68 mmol, M = 399.18 g
mol−1), m-CPBA
(77%) (0.39 g, 1.74 mmol, 1.04 eq., M = 172.57 g
mol−1), toluene (0.35 mL, 30.28 mg, 3.29 mmol, 1.95 eq.,
d = 0.865, M = 92.14 g
mol−1), and trifluoromethanesulfonic acid (0.35 mL, 59.36 mg, 3.96 mmol,
2.4 eq., d = 1.696, M = 150.08 g
mol−1) was stirred at room temperature for 2 h. The mixture was
2): A
·
·
·
·
poured into diethyl ether and the resulting precipitate was filtered off and washed with diethyl ether
1
to afford a pure product as a light yellow solid. (0.64 g, 1.02 mmol, 61% yield). H NMR (Acetone-d6)
3
4
δ
(ppm): 2.43 (s, 3H), 7.44 (d, 2H, J = 8.1 Hz), 7.71 (dt, 2H, J = 8.8 Hz, J = 2.4 Hz) 7.87 (dd, 2H,
3J = 8.2 Hz, 4J = 7.3 Hz), 8.31 (dt, 3J = 8.2 Hz, 4J = 2.4 Hz), 8.42–8.53 (m, 6H). 19F NMR (Acetone-d6)
13C NMR (Acetone-d6)
(ppm): 21.4 (CH3), 111.5 (Cq), 114.2 (Cq), 121.2 (q, CF3,
δ
(ppm): −78.77 (s, 3F)
.
δ
1JC-F = 320 Hz) 123.6 (Cq, 2C), 128.0 (CH, 2C), 129.2 (Cq), 131.8 (CH, 2C), 132.8 (Cq), 133.9 (CH, 2C),
134.1 (CH, 2C), 135.5 (CH, 2C), 136.8 (CH, 2C), 136.9 (CH, 2C), 141.4 (Cq), 144.9 (Cq), 164.5 (C=O, 2C).
HRMS (ESI MS) m/z: theor: 490.0299 found: 490.0302 ((M-TfO−)+ detected), theor 1129.0123, found
1129.0128 (((M+TfO−)M+) detected).
(4-(1,3-Dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl)(p-tolyl)iodonium
): mixture of (4-(1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl)(p-tolyl)iodonium
trifluoromethanesuflonate (205 mg, 0.33 mmol, M = 625.35 g
mol−1), and NaPF6 (331 mg, 0.197 mmol,
6 eq., M = 167.95 g
mol−1) in CH3CN (4.5 mL) was stirred for 24 h. Then, the solvent was removed
hexafluorophosphate
(3
A
·
·
under reduced pressure. Water was introduced and the resulting precipitate was filtered off and
washed with water to afford a pure compound as a light yellow solid (130 mg, 0.20 mmol, 62%
1
yield). H NMR (Acetone-d6)
δ
(ppm): 2.45 (s, 3H), 7.47 (d, 2H, J = 8.3 Hz), 7.73 (d, 2H, 3J = 8.7 Hz),
7.85–7.94 (m, 2H), 8.33 (d, 2H, J = 8.4 Hz), 8.47–8.55 (m, 6H). 19F (Acetone-d6)
δ
(ppm):
−
72.36
3
1
(d, 6F, JP-F = 706.9 Hz). 13C NMR (Acetone-d6)
δ
(ppm): 21.4 (CH3), 111.2 (Cq), 114.0 (Cq), 123.7
(Cq, 2C), 128.0 (CH, 2C), 129.2 (Cq), 131.8 (CH, 2C), 132.9 (Cq), 134.1 (CH, 2C), 134.3 (CH, 2C),
135.5 (CH, 2C), 136.8 (CH, 2C), 136.9 (CH, 2C), 141.7 (Cq), 145.3 (Cq), 164.5 (C=O, 2C). HRMS (ESI
MS) m/z: theor: 490.0299, found 490.0299 ((M-PF6−)+ detected), theor: 1125.0244 found: 1125.0243
(((M+PF6−)M+) detected).
(4-(1,3-Dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl)(p-tolyl)iodonium chloride (
N-(p-iodophenyl)-1,8-naphthalimide (1.80 g, 4.51 mmol, M = 399.18 g
mol−1), m-CPBA (77%) (1.12g,
5.00 mmol, 1.12 eq., M = 172.57 g
mol−1), toluene (0.95 mL, 82.18 mg, 8.92 mmol, 1.98 eq., d = 0.865,
M = 92.14 g
mol−1), and trifluoromethanesulfonic acid (0.35 mL, 59.36 mg, 3.96 mmol, 2.4 eq., d = 1.696,
M = 150.08 g
mol−1) was stirred in dichloromethane (30 mL) at room temperature for 2 h. Then
4): A mixture of
·
·
·
·
dichloromethane was removed under vacuum and the crude product was washed in 70 mL of diethyl
ether. The solid was filtered off, dissolved in acetic acid. Brine is added until precipitation. The mixture
is stirred at 80 ◦C during 24 h. The light-yellow solid is filtered, washed with 3
×
70 mL of demineralized
1
water (1.24 g, 2.36 mmol, 52%). H NMR (DMSO-d6)
δ
(ppm): 2.36 (s, 3H), 7.35 (d, 2H, 3J = 8.2 Hz), 7.54
(d, 2H, 3J = 8.6 Hz), 7.91 (t, 2H, 3J = 8.2 Hz), 8.17 (d, 2H, 3J = 8.3), 8.31 (d, 2H, 3J = 8.6 Hz), 8.45–8.57 (m,
4H). 13C NMR (DMSO-d6)
(ppm): 20.9 (CH3), 115.3 (Cq), 118.4 (Cq), 122.4 (Cq, 2C), 127.3 (CH, 2C),
δ
127.8 (Cq), 130.8 (CH, 2C), 131.5 (Cq), 132.2 (CH, 2C), 132.3 (CH, 2C), 134.7 (CH, 2C), 135.1 (CH, 2C),
135.4 (CH, 2C), 139.0 (Cq), 142.2 (Cq), 163.5 (C=O, 2C). HRMS (ESI MS) m/z: theor: 490.0299, found
490.0299 ((M-Cl−)+ detected).
(4-(1,3-Dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl)(p-tolyl)iodonium bis((trifluoromethyl) sulfonyl)amide
(
5
): A mixture of (4-(1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl)(p-tolyl)iodonium chloride
(0.50 g, 0.95 mmol, M = 527.77 g
mol−1) and bis(trifluoromethane)sulfonimide lithium salt (1.70 g, 5.92
mmol, 6.23 eq., M = 287.09 g
mol−1) was stirred in CH3CN (6 mL) during 24 h at 40 ◦C. Then, the
·
·
solvent was removed under vacuum. The crude product was washed with demineralized water to
1
afford a pure product as light yellow solid (m = 0.60 g, n = 0.78 mmol, 81% yield). H NMR (400 MHz,
DMSO-d6): 2.37 (s, 3H), 7.37 (d, 2H, 3J = 8.2Hz), 7.56 (d, 2H, 3J = 8.5 Hz) 7.90 (t, 2H, 3J = 7.8 Hz), 8.19
(d, 2H, 3J = 8.2 Hz), 8.34 (d, 2H, 3J = 8.5 Hz), 8.47–8.53 (m, 4H). 19F NMR (DMSO-d6)
δ
(ppm) 78.70 (s).
−

Molecules 2020, 25, 149
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13C NMR (101 MHz, DMSO-d6) 21.6 (CH3), 113.2 (Cq), 116.4 (Cq), 120.1 (q, CF3, 1JC-F = 320 Hz) 122, 6
(Cq, 2C), 128.1 (CH, 2C), 128.4 (Cq), 131.9 (CH, 2C), 132,1 (Cq), 133.4 (CH, 2C), 133.4 (CH, 2C), 135.7
(CH, 2C), 136.0 (CH, 2C), 136.6 (CH, 2C), 140.0 (Cq), 144.0 (Cq), 164.5 (C=O, 2C). HRMS (ESI MS) m/z:
theor: 490.0299, found 490.0296 ((M-TFSI−)+ detected).
(4-(1,3-Dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl)(p-tolyl)iodonium hexafluoroantimonate (V) (
A mixture of (4-(1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl)(p-tolyl)iodonium chloride
(195.5 mg, 0.37 mmol, M = 527.77 g
mol−1) and sodium hexafluoroantimonate (V) (590.0 mg, 2.28 mmol,
M = 258.74 g
mol−1) was refluxed in CH3CN (20 mL) for 7 days. After cooling, the solvent was removed
6):
·
·
under vacuum. The crude product was washed with water and filtered to afford a pure product as a
1
light yellow solid (m = 0.14 g, 0.19 mmol, 51% yield). H NMR (400 MHz, Acetone-d6)
δ
2.42 (s, 3H),
7.45 (d, 2H, 3J = 8.3 Hz), 7.69 (d, 2H, 3J = 8.6 Hz), 7.91 (t, 2H, 3J = 7.7 Hz), 8.29 (d, 2H, 3J = 8.3 Hz),
8.45–8.51 (m, 4H), 8.55 (d, 2H, 3J = 7.2 Hz). F NMR (Acetone-d6) δ −110.47–
−
136.42 (m, 6F). 13C NMR
9
(DMSO-d6) δ(ppm): 20.9 (CH3), 113.4 (Cq), 116.5 (Cq), 122.4 (Cq, 2C), 127.3 (CH, 2C), 127.8 (Cq), 130.7
(CH), 130.8 (CH), 131.5 (Cq), 132.4 (CH, 2C), 132.6 (CH, 2C), 134.5 (CH), 134.7 (CH), 135.3 (CH, 2C),
135.6 (CH, 2C), 139.4 (Cq), 142.7 (Cq), 163.5 (C=O, 2C). HRMS (ESI MS) m/z: theor: 490.0299, found
490.0298 ((M-SbF6−)+ detected).
4.2. Other Materials
(3,4-Epoxycyclohexane)methyl-3,4-epoxycyclo-hexylcarboxylate (EPOX) and trimethylolpropane
triacrylate (TMPTA) were selected as benchmark monomers for cationic and radical polymerization,
respectively. These monomers were obtained from Allnex. The reference iodonium salt (Speedcure
938, Iod) was obtained from Lambson Ltd. (Wetherby, UK). Trimethylaniline (TMA) was purchased
from Sigma-Aldrich (St. Louis, MO, USA). Their corresponding molecular structures are shown in
Scheme 2.
Scheme 2. Chemical structures of Iodonium salt (Speedcure 938), amine (TMA) and benchmark
monomers used.
4.3. Photopolymerization Reactions Monitored by Real Time Fourier Transformed Infrared Spectroscopy
(RT-FTIR)
The weight percent of the photoinitiator is calculated from the monomer content (w/w).
The photosensitive thin formulations (~25 µm of thickness) were deposited on a BaF2 pellet under air
for the CP of EPOX. The 1.4 mm thick samples of EPOX were also polymerized under air into a rounded
plastic mold of ~7 mm diameter and 1.4 mm of thickness. For thin and thick samples, the evolution of
the epoxy group content of EPOX was continuously followed by real time FTIR spectroscopy (JASCO
FTIR 4100, Tokyo Japan) at about 790 and 3700 cm−1, respectively. The procedure used to monitor the
photopolymerization profiles has been described in detail in refs [65,66].
4.4. UV-Visible Absorption Spectroscopy
The UV-Visible absorbance properties of the different iodoniums were studied by JASCO V730
UV–visible spectrometer (JASCO, Tokyo Japan). To demonstrate the formation of acid upon irradiation,
methyl red was added as pH indicator as reported in [67].

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4.5. Electron Spin Resonance (ESR) Spin Trapping (ESR-ST)
Electron spin resonance-spin trapping experiments were carried out using an X-band spectrometer
(Magnettech MS400, Magnettech, Berlin, Germany). The radicals were observed under nitrogen
saturated media at room temperature. N-tert-Butylnitrone (PBN) was used as a spin trap agent in
acetonitrile [66]. ESR spectra simulations were carried out using PEST WINSIM software.
4.6. Laser Write Experiments
A computer programmed laser diode with spot size around 50
µm was utilized for spatially
controlled irradiation to produce specific 3D patterns using /TMA in TMPTA monomer. Finally,
5
the printed patterns were analyzed through a numerical optical microscope (DSX-HRSU, OLYMPUS
corporation, Tokyo, Japan) [66].
4.7. Molecular Modeling
Frontier Molecular Orbital calculations were performed by the Gaussian 03 suite of programs
(Pittsburgh, PA, USA) [68,69]. The simulation of UV absorption spectra and the calculations of the C–I
bond dissociation energies (BDE) were done at the density functional theory level (UB3LYP/LANL2DZ)
on the relaxed geometries calculated at this level.
5. Conclusions
In the present paper, the design and the synthesis of a new series of five iodonium salts combining
within a unique chemical structure the naphthalimide chromophore and the iodonium scaffold are
presented. Compared to the reference structure Speedcure 938, the proposed iodoniums are more
reactive at 365 nm to initiate the cationic polymerization of epoxides. Bis(trifluoromethane)sulfonimide
was found as the most interesting counter anion, the best photoinitiating abilities in both radical
and cationic polymerizations being determined for the iodonium salt possessing this counteranion.
The development of new 3D printing resins @405 nm based on the cationic polymerization of epoxides
and based on the elaboration of a CTC between an iodonium salt and an amine has allowed to
polymerize at longer wavelength than what is theoretically possible on the simple basis of the
absorption spectrum of the iodonium salt. The formation of CTC to red-shift the absorption spectrum
of a photoinitiator and to allow the polymerization at longer wavelength is an efficient tool that
deserves to be further investigated. Especially, by mean of this innovative approach, benchmark UV
photoinitiators that do not exhibit an absorption in the visible range could be revisited in the context of
this recent approach.
Author Contributions: Conceptualization, J.L., F.D. and D.G.; methodology, J.L., F.D.; validation, J.L., F.D. and
D.G.; formal analysis, S.V., J.L., F.D.; investigation, S.V.; supervision, J.L., F.D. and D.G.; project administration,
J.L., F.D. and D.G. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Region Grand Est, grant name “Mippi-4D”.
Acknowledgments: In this section you can acknowledge any support given which is not covered by the author
contribution or funding sections. This may include administrative and technical support, or donations in kind
(e.g., materials used for experiments).
Conflicts of Interest: The authors declare no conflict of interest.
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2019 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 (http://creativecommons.org/licenses/by/4.0/).