Aryliodonium ylides as novel and efficient additives for radical chemistry: Example in Camphorquinone (CQ)/Amine based photoinitiating systems

Article abstract of DOI:10.3390/molecules24162913

Diaryliodonium salts are well-established compounds in free radical chemistry and are already used as photoinitiators (free radical or cationic polymerization), but the presence of counter anions is a strong drawback. Indeed, a counter anion is always req

Full text of DOI:10.3390/molecules24162913

molecules
Article
Aryliodonium Ylides as Novel and Efficient Additives
for Radical Chemistry: Example in Camphorquinone
(CQ)/Amine Based Photoinitiating Systems
Julie Kirschner 1, Julien Paillard 1, Mariem Bouzrati-Zerelli 1, Jean-Michel Becht 1,
Joachim E. Klee 2, Saloua Chelli 3, Sami Lakhdar 3 and Jacques Lalevée 1,
*
1
Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15 rue Jean Starcky,
68057 Mulhouse CEDEX, France
Dentsply Sirona, De-Trey-Straβe 1, 78462 Konstanz, Germany
Normandie Univ., LCMT, ENSICAEN, UNICAEN, CNRS, 6, Boulevard Maréchal Juin, 14000 Caen, France
Correspondence: jacques.lalevee@uha.fr; Tel./Fax: +33-3-89-60-88-03
2
3
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: John C. Walton
Received: 22 July 2019; Accepted: 9 August 2019; Published: 11 August 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Diaryliodonium salts are well-established compounds in free radical chemistry and are
already used as photoinitiators (free radical or cationic polymerization), but the presence of counter
anions is a strong drawback. Indeed, a counter anion is always required (e.g., SbF6−) leading to
potential toxicity issues or release of HF. In the present paper, counter anion-free and fluoride-free
aryliodonium salts are proposed, that is, aryliodonium ylides (AY) are studied here as new and
efficient additives for radical chemistry and an example is provided for the camphorquinone
(CQ)/amine based photoinitiating systems (PISs) for the polymerization of thick (1.4 mm) and thin
(20–13 µm) methacrylates under air and blue light irradiation. The newly proposed PISs, for example,
CQ/amine/AY, presented excellent polymerization performances and good bleaching properties were
obtained after polymerization. Real-time Fourier transform infrared spectroscopy (RT-FTIR) was used
to monitor the photopolymerization profiles. The chemical mechanisms involved were investigated
using electron spin resonance (ESR).
Keywords: photoinitiator; iodonium salt; radical initiation; photopolymerization
1. Introduction
Photoinitiated polymerization has been widely used in the past decades and has found many
industrial applications, such as 3D printing, coatings, inks, adhesives, and so on. Two types of
polymerization can be distinguished depending on the nature of the photoinitiator used; free
radical photopolymerization and cationic polymerization [
advantages compared to the former, such as insensitivity to oxygen, low shrinkage during curing,
dark polymerization after the removal of the light source, and so on [ ] Two families of cationic
photoinitiators are generally used: iodonium and sulfonium salts [ ]. As iodonium salts present
better solubility in non-polar monomers than the corresponding sulfonium salts, they are the most
widespread cationic photoinitiators [ ]. Iodonium salts are also usually very efficient, but absorb at
shorter wavelengths. Diphenyliodonium salts associated with BF4−, PF6−, AsF6−, SbF6− as counter
anions are the simplest and the most frequently used [ ]. The cationic part of the diphenyliodonium
salt is responsible for light absorption whereas the nature of the counter anion governs the strength
of the acid formed during photolysis [ ]. Nevertheless, the diaryliodonium photoinitiators used
currently all exhibit absorption properties in the UV absorption range between 220 and 280 nm while
there are no light sources efficient enough operating in this spectral range [ ]. To overcome their
1,2]. The latter presents numerous
3
1,4,5
6
1,6
1
1,6–9
Molecules 2019, 24, 2913; doi:10.3390/molecules24162913
www.mdpi.com/journal/molecules

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poor absorption properties in the visible range, diaryliodonium salts can be used in combination
with a photosensitizer (PS) and/or a visible free radical photoinitiator [10]. As proposed by Crivello,
diaryliodonium salts can be sensitized by the PS through an electron or energy transfer process [11].
After excitation of the PS and reduction of the diaryliodonium salt, aryl radicals are generated through
the cleavage of the C–I bond [11–15].
On the other hand, two types of photoinitiator (PI) are used in free radical photopolymerization:
Type I and Type II PI [1]. Due to its good visible absorption properties, camphorquinone is the most
common Type II PI used for the free radical photopolymerization of (meth-)acrylates under blue light
irradiation. To be efficient, camphorquinone has to be used in combination with a hydrogen donor,
for example, an amine [16
also be improved in the presence of well-chosen additives, such as diaryliodonium salts, as they
generate initiating aryl radicals upon light irradiation [ ]. Diaryliodonium hexafluorophosphates
(e.g., diphenyliodonium hexafluorophosphate, Speedcure 938, and so on) are frequently used
diaryliodonium salts, due to their commercial availability [19 20]. Nevertheless, in certain conditions,
–18]. The performances of the camphorquinone (CQ)/amine system can
1
,
the hexafluorophosphate counter anion may cause some issues due to the release of HF [21,22].
The idea of this paper is to develop new counter anion-free and fluoride-free aryliodonium
salts to avoid the formation of side products, for example, formation of HF and to enhance their
solubility in monomers. In this paper, we describe two aryliodonium ylides (S5 and S6) as new efficient
aryliodonium salts and additives for CQ/amine based systems for the free radical polymerization
of methacrylates under visible light irradiation. Aryliodonium ylides (AY) were studied for the
replacement of traditionally used diaryliodonium salts (diphenyliodonium salt, Speedcure 938, and so
on) in the CQ/amine system. The performances of the newly proposed CQ/amine/AY systems for the
free radical photopolymerization of thick and thin samples of methacrylates were evaluated using
real-time FTIR experiments. Electron spin resonance (ESR) spectroscopy was used to identify the
radicals generated upon light irradiation. A chemical mechanism between CQ and aryliodonium ylide
is proposed.
2. Materials and Methods
2.1. Materials
S5 and S6 (Scheme 1) were prepared according to a synthetic procedure already described
in the literature [23]. Camphorquinone was obtained from Sigma Aldrich (St. Louis, MO, USA)
and used as a representative Type II PI (Scheme 2). Ethyl-4-(dimethylamino)benzoate (EDB) and
4-(dimethylamino)benzonitrile (DMABN) were used as additives in multicomponent systems and
obtained from Sigma Aldrich or TCI Chemicals (Scheme 2). Speedcure938 (SC938) was obtained from
Lambson Ltd. (Wetherby, UK, Scheme 2). Phenyl-N-tert-butylnitrone (PBN, Scheme 2) was used as
spin trapping agent in the ESR experiments.
Scheme 1. Structures of the aryliodonium ylides (AY) studied: S5 and S6.

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Scheme 2. Structures of the photoinitiator (camphorquinone; CQ) and additives/co-initiators used.
Spectrum® TPH®3 resin was received from Dentsply Sirona (Konstanz, Germany) and used as
a representative matrix of dental materials, consisting of a mixture of modified BisGMA, TEGDMA
(Scheme 3) and other methacrylate monomers. PrimeBond Active® was received without photoinitiator
from Dentsply Sirona and used as a representative formulation for the preparation of dental adhesives.
Scheme 3. Structures of the monomers used.
2.2. Synthesis of Aryliodonium Ylides S5 and S6
Synthesis of S5: iodobenzene diacetate (0.20 g, 0.62 mmol, 1 equiv) was dissolved in ethanol
96 % (1 M, 0.62 mL) under heating and added quickly to a dimedone (0.087 g, 0.62 mmol, 1 equiv)
dissolved in a saturated aqueous solution of sodium carbonate (0.3 M, 1.90 mL). The resulting mixture
was stirred for 30 min at RT. Cold water (2 mL) was then added and the aqueous phase was extracted
3 times with DCM (3
×
10 mL). The organic phase was dried with anhydrous MgSO4 and concentrated
1
under reduced pressure to give S5 (0.12 g, 0.35 mmol, 57 % isolated yield) as a white solid. H NMR
(CDCl3, 300 MHz) δ (ppm): 7.77 (d, J = 6 Hz, 2H), 7.47 (m, 1H), 7.31 (m, 2H), 2.45 (s, 4H), 1.02 (s, 6H);
13C NMR (CDCl3, 50 MHz)
δ (ppm): 188.4, 133.9, 131.5, 112.0, 94.6, 50.8, 32.1, 28.2; HRMS (ESI MS) m/z:
calculated: 343.0190 found: 343.0188 ((M + H)+ detected).
Synthesis of S6: dimedone (0.19 g, 1.37 mmol, 1 equiv) was dissolved in a saturated solution of
sodium carbonate (0.3 M, 1.70 mL). Iodomesitylene diacetate (0.50 g, 1.37 mmol, 1 equiv) was dissolved
in ethanol 96 % (1 M, 1.37 mL) under heating. The solution of iodomesitylene was quickly added to
the solution containing the conjugated base of dimedone. The resulting mixture was stirred for 30 min
at RT. Cold water (2 mL) was then added and the aqueous phase was extracted 3 times with DCM
(
3 × 10 mL). The organic phase was dried with anhydrous MgSO4 and concentrated in vacuo to give
1
S6 (0.34 g, 0.88 mmol, 64 % isolated yield) as a light yellow powder. H NMR (CDCl3, 300 MHz)
(ppm): 6.96 (s, 2H), 2.72 (s, 6H), 2.42 (s, 4H), 2.28 (s, 3H), 1.02 (s, 6H); 13C NMR (CDCl3, 50 MHz)
δ
δ
(ppm): 187.9, 142.6, 129.5, 93.4, 50.9, 32.0, 28.1, 27.3; HRMS (ESI MS) m/z: calculated: 385.0659 found:
385.0660 ((M + H)+ detected).
2.3. Irradiation Sources
A blue LED@477nm representative of dental materials usage (SmartLite Focus from Dentsply
Sirona ~300 mW
at 455 nm (M455L3-ThorLabs; ~80 mW
·
cm−2 in the selected conditions; see emission spectrum in Figure 1) and an LED centered
·
cm−2) were used for the irradiation of the photocurable samples.

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1,0
0,8
0,6
0,4
0,2
0,0
380
400
420
440
460
480
500
λ
(nm)
Figure 1. The emission spectrum of a blue LED centered at 477 nm (SmartLite Focus from Dentsply
Sirona Germany).
2.4. Absorption Experiments
The UV-visible spectra were recorded using a JASCO V-730 UV/vis spectrophotometer
(JASCO, Japan).
2.5. Photopolymerization Experiments
The photosensitive formulations were deposited on a polypropylene film and the sample thickness
was controlled using a mold (1.4 mm thick samples). The samples were polymerized under air and
irradiation with the LED source. The evolution of the methacrylate functions was continuously
followed by real time FTIR spectroscopy (JASCO FTIR 6600, JASCO, Japan) at about 6165 cm−1 for
thick (1.4 mm) methacrylate samples and at 1220 cm−1 for thin (13–20 µm) ones.
2.6. ESR Spin Trapping Experiments:
ESR experiments were carried out using a Bruker EMX-plus spectrometer (X-band, Bruker
Company, Rheinstetten, Germany). The radicals were generated at room temperature upon the blue
LED exposure (SmartLite Focus from Dentsply Sirona) under N2. The radicals were trapped by
phenyl-N-tert-butylnitrone (PBN, Scheme 2) according to a procedure already described [24]. The ESR
spectra simulations were carried out using WINSIM software.
3. Results and Discussion
3.1. Photochemical Mechanism
In the presence of a spin trap agent (PBN) for the photolysis of CQ/S5 under N2 upon exposure to
the blue LED centered at 477 nm, the Ph•/PBN radical adducts were clearly observed (i.e., characterized
by hyperfine coupling constants (hcfs) aN = 14.0 G, aH = 2.1 G in agreement with hfcs known in
the literature [25] (Figure 2)). Therefore, ESR spin trapping experiments showed the formation of
phenyl radicals generated from the CQ/S5 interaction upon blue light. This result clearly highlights the
cleavage of the C–I central bond of S5 and the possible release of benzene by the decomposition of S5
(through hydrogen abstraction process with the generated phenyl radical in S5). As benzene release
must be clearly avoided in industrial applications for safety reasons, a derivative of S5 was prepared
where the phenyl group was replaced by a 2,4,6-trimethylphenyl (mesityl) group: S6.

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(a)
(b)
3320
3330
3340
3350
3360
3370
3380
3390
B (G)
Figure 2. Electron spin resonance (ESR) spectra of the radicals generated in CQ/S5 (0.05 M/0.01 M)
and trapped by phenyl-N-tert-butylnitrone (PBN) (0.01M) in tert-butylbenzene upon exposure to a
LED@477 nm (SmartLite Focus): N2 saturated medium Ph•/PBN hyperfine coupling constants: aN
13.96 G, aH = 2.06 G; (a) experimental and (b) simulated spectra.
=
A general chemical mechanism for aryliodonium ylides can be proposed in agreement with the
result obtained for the ESR spin trapping experiment for CQ/S5. Aryl radicals were generated according
to reaction 1 (r1) through an electron transfer between camphorquinone and the aryliodonium ylide
(Ar–I–R).
*CQ + Ar-I-R → CQꢀ− + Arꢀ + R = I+
(r1)
Aryl radicals could initiate the free radical polymerization of (meth)acrylate monomers in
agreement with the good initiating ability of CQ/EDB/S5 vs. CQ/EDB found in polymerization
experiments (see below, Figures 3 and 4).
100
(3)
(4)
(1)
80
(2)
60
40
20
0
0
20
40
60
80
100
120
Time (s)
Figure 3. Photopolymerization profiles (methacrylate function conversion vs. irradiation time) for
Spectrum® TPH®3 resin (under air; 1.4 mm thick films) upon exposure to the LED@477 nm (I0 =
300 mW
cm−2) using different photoinitiating systems. (1) CQ/ethyl-4-(dimethylamino)benzoate (EDB)
·
(0.2/0.5 % w/w); (2) CQ/EDB/SC938 (0.2/0.5/1 % w/w); (3) CQ/EDB/S5 (0.2/0.5/1 % w/w); (4) CQ/EDB/S6
(0.2/0.5/1 % w/w). The irradiation starts for t = 5 s.

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100
90
80
70
60
50
40
30
20
10
0
(3)
(4)
(2)
(1)
0
20
40
60
80
100
120
Time (s)
Figure 4. Photopolymerization profiles (methacrylate function conversion vs. irradiation time) for a
Spectrum® TPH®3 resin blend (under air; 1.4 mm thick films) upon exposure to the LED@455 nm (I0 =
80 mW
cm−2) using different photoinitiating systems. (1) CQ/EDB (0.2/0.5 % w/w); (2) CQ/EDB/SC938
·
(0.2/0.5/1 % w/w); (3) CQ/EDB/S5 (0.2/0.5/1 % w/w); (4) CQ/EDB/S6 (0.2/0.5/1 % w/w). The irradiation
starts for t = 5 s.
In the presence of amine (EDB, DMABN), additional reactions were expected [
additional initiating radicals.
1
] r2–r4 leading to
*CQ + Me2NAr → CQ-H• + Me(CH2•)NAr
(r2)
CQ-H• + Ar-I-R → → CQ-H− + Ar•
CQ-H• + Ar-I-R → → CQ + H+ + Ar•
(r3)
(r4)
3.2. Absorption Properties
The absorption properties of diaryliodonium salts are well known in the literature, for example,
for diphenyliodonium the maximum absorption wavelength is located at ~230 nm [ ]. The UV-visible
1
spectra of S5 and S6 present a broad band in the 300–400 nm region (Figure 5). Contrary to the
well-known diphenyliodonium, S5 and S6 present shifted absorption spectra to the near UV region
with maximum absorption wavelengths located at ~330 nm. Therefore, compared to diphenyliodonium
salt, the aryliodonium ylides S5 and S6 lead to a shift of the absorption spectra to longer wavelengths.
Remarkably, the newly proposed aryliodonium ylides present rather good absorption properties at
365 nm (ε
: 532 M−1.cm−1 for S5 and 367 M−1.cm−1 for S6) matching well with the emission spectra of
common mercury lamps used in the coatings industry, and could therefore be used as photoinitiators
in this application. For industrial fields using blue light irradiation, for example, dentistry, S5 and
S6 must be sensitized by camphorquinone to form initiating radicals under visible light irradiation.
Accordingly, no radicals were observed in ESR experiments upon blue light without camphorquinone.

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7000
6000
5000
4000
3000
2000
1000
0
(2)
(1)
300
325
350
375
400
425
450
475
500
λ (nm)
Figure 5. UV-visible spectra of (1) S5 and (2) S6 in acetonitrile.
3.3. CQ/EDB/S5 and CQ/EDB/S6 Initiating Ability
The polymerization profiles of thick films (1.4 mm) of Spectrum® TPH®3 resin upon exposure to
the blue LED at 477 nm (SmartLite Focus) using different photoinitiating systems are presented in
Figure 3. Under air, using a 300 mW
cm−2 light intensity, the systems CQ/EDB/S5 and CQ/EDB/S6
·
exhibited very good polymerization ability in thick samples. High polymerization rates Rp and
high final methacrylate conversions (FCs) up to 80 % were obtained for the systems CQ/EDB/S5
and CQ/EDB/S6 (curves 3–4 Figure 3). Remarkably, similar or better performances were obtained
for the two PISs investigated, compared to the reference systems CQ/EDB and CQ/EDB/SC938
(curves 1–2 Figure 3).
Similar results were obtained using an LED centered at 455 nm for the polymerization of thick
(1.4 mm) samples of Spectrum® TPH®3 resin under air. The polymerization profiles of thick films
(1.4 mm) of Spectrum® TPH®3 resin upon exposure to the LED at 455 nm using different photoinitiating
systems are presented in Figure 4. Remarkably, the CQ/EDB/S5 and CQ/EDB/S6 systems lead to higher
final conversions and polymerization rates than the reference system CQ/EDB.
Remarkably, good bleaching properties were obtained with the systems CQ/EDB/S5 and
CQ/EDB/S6 compared to the references CQ/EDB and CQ/EDB/SC938 after 115s of irradiation with
the blue LED centered at 477 nm. Similar or better bleaching properties are obtained with the newly
developed PISs compared to those achieved with the reference PISs (Figure 6).
3.4. CQ/EDB/S5 and CQ/EDB/S6 for the Polymerization of Thin Samples of Methacrylates and Adhesives
under Air
The newly proposed PISs CQ/EDB/S5 and CQ/EDB/S6 showed excellent polymerization
performances for the polymerization of thick (1.4 mm) samples of methacrylates under air upon
exposure to a blue LED centered at 477 nm and an LED centered at 455 nm. The performances of these
systems were also investigated for the preparation of thin (20 µm) samples of methacrylates under
exposure to the LED centered at 477 nm (Figure 7). Remarkably, the system CQ/EDB/S5 presented an
enhanced polymerization rate and final methacrylate function conversion (~35 % vs. 25 %) compared
to the reference system CQ/EDB for polymerization under air. The lower conversion in Figure 7
compared to Figures 3 and 4 was ascribed to the oxygen inhibition that is stronger for thin samples
compared to thick ones.

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Figure 6. Photos of the samples before and after polymerization (under air; thickness = 1.4 mm;
SmartLite Focus (300 mW
cm−2); 115 s irradiation): (1) CQ/EDB (0.2/0.5 % w/w); (2) CQ/EDB/SC938
·
(0.2/0.5/1 % w/w); (3) CQ/EDB/S5 (0.2/0.5/1 % w/w); (4) CQ/EDB/S6 (0.2/0.5/1 % w/w) in Spectrum®
TPH®3.
60
50
40
(2)
30
(1)
20
10
0
0
50
100
150
200
Time (s)
Figure 7. Photopolymerization profiles (methacrylate function conversion vs. irradiation time) for
a BisGMA/TEGDMA (70/30 % w/w) resin blend (under air; 20 m thin films) upon exposure to the
LED@477 nm (I0 = 300 mW
cm−2) using different photoinitiating systems. (1) CQ/EDB (1/1 % w/w);
µ
·
(2) CQ/EDB/S5 (1/1/1 % w/w). The irradiation starts for t = 5 s.
The newly proposed systems were also investigated for the preparation of very thin (~13 µm)
samples of adhesives by polymerization under air under exposure to the blue dental LED. The
polymerization profiles of thin adhesive films (~13
m) of Prime&Bond Active® formulation upon
exposure to the blue LED at 477 nm (SmartLite Focus) using different photoinitiating systems are
µ

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depicted in Figure 8. Remarkably, better polymerization performances (i.e., higher final methacrylate
function conversions and higher polymerization rates) were achieved for the systems CQ/DMABN/S5
and CQ/DMABN/S6 compared to the reference system CQ/DMABN. The new systems proposed
presented excellent performances for the polymerization in difficult conditions, for example, for very
thin samples and under air.
100
80
(3)
60
(2)
(1)
40
20
0
0
20
40
60
80
100
120
Time (s)
Figure 8. Photopolymerization profiles (methacrylate function conversion vs. irradiation time) for
Prime&Bond Active® resin (under air; 13
m thin films) upon exposure to the LED@477 nm (I0
= 300 mW
cm−2) using different photoinitiating systems. (1) CQ/4-(dimethylamino)benzonitrile
µ
·
(DMABN) (1.5/0.6 % w/w); (2) CQ/DMABN/S5 (1.5/0.6/0.75 % w/w); (3) CQ/DMABN/S6 (1.5/0.6/0.75 %
w/w). The irradiation starts for t = 5 s.
4. Conclusions
In the present paper, aryliodonium ylides (AY) were reported as new high performance iodonium
salts and efficient additives for CQ/amine based systems for the polymerization of methacrylates
under blue light irradiation. Enhanced polymerization performances were achieved for the system
CQ/amine/AY compared to the reference system CQ/amine. Good bleaching properties of the final
polymers were also obtained after polymerization. Interestingly, the newly developed CQ/amine/AY
systems presented excellent initiating ability in strongly oxygen-inhibited conditions (thin samples,
under air), for example, for the preparation of adhesives. Aryliodonium ylides therefore represent a
new class of iodonium salts for photoinitiated polymerization and an interesting alternative to the
well-known diaryliodonium salts. Remarkably, as the counter anion is located on the iodonium moiety,
enhanced solubility in monomers is expected, as well as reduced formation of side products, compared
to the classical diaryliodonium salts, especially diaryliodonium hexafluorophosphate.
Author Contributions: Conceptualization and methodology, J.E.K., J.L.; Investigation, J.K., J.P., M.B.-Z.; Data
curation, J.K., J.P., M.B.-Z.; Writing—original draft preparation, J.K., M.B.-Z.; Writing—review and editing, J.-M.B.,
J.E.K., S.C., S.L. and J.L.
Funding: This research was funded by Dentsply Sirona.
Acknowledgments: IS2M authors thank Dentsply Sirona and the Grand Est region for the funding of this work.
The authors also thank Valérie Monnier (Spectropole, Aix Marseille University) for the MS experiments.
Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds S5 and S6 are available from the authors.
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