End capped polyenic structures as visible light sensitive photoinitiators for polymerization of vinylethers

Article abstract of DOI:10.1016/j.dyepig.2014.02.002

Nine new photoinitiating systems based on a combination of new and established dyes (having an end capped polyenic structure) with an iodonium salt have been synthesized and tested for the polymerization of divinylether and epoxy monomers upon a visible light exposure. They exhibit a strong absorption in the 350-650 nm range which well matches the visible light emission of different convenient visible light sources. Interestingly, very soft irradiation conditions can be employed. Excellent polymerization profiles have been obtained from selected dye iodonium salt combinations using a halogen amp or a laser diode à 457 nm. The chemical mechanisms associated with these novel photoinitiating systems are investigated by steady-state photolysis and ESR experiments.

Full text of DOI:10.1016/j.dyepig.2014.02.002

Dyes and Pigments 105 (2014) 121e129
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
End capped polyenic structures as visible light sensitive
photoinitiators for polymerization of vinylethers
,
,
Haifaa Mokbel a c, Frédéric Dumur b d, Cédric R. Mayer e, Fabrice Morlet-Savarya,
Bernadette Graff a, Didier Gigmes b, Joumana Toufaily c, Tayssir Hamieh c,
Jean-Pierre Fouassier f, Jacques Lalevée a
,
*
a Institut de Science des Matériaux de Mulhouse IS2M e UMR CNRS 7361 e UHA, 15, rue Jean Starcky, 68057 Mulhouse Cedex, France
b Aix-Marseille Université, CNRS, Institut de Chimie Radicalaire, UMR 7273, F-13397 Marseille, France
c Laboratoire de Matériaux, Catalyse, Environnement et Méthodes Analytiques (MCEMA-CHAMSI), EDST, Université Libanaise, Campus Hariri, Hadath,
Beyrouth, Liban
d Univ. Bordeaux, CNRS, IMS, UMR 5218, F-33400 Talence, France
e Laboratoire d’Ingénierie des Systèmes de Versailles LISV e EA 4048, Université de Versailles Saint Quentin en Yvelines, 10/12 avenue de l’Europe,
78140 Vélizy, France
f ENSCMu-UHA, 3 rue Alfred Werner, 68093 Mulhouse, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Nine new photoinitiating systems based on a combination of new and established dyes (having an end
capped polyenic structure) with an iodonium salt have been synthesized and tested for the polymeri-
zation of divinylether and epoxy monomers upon a visible light exposure. They exhibit a strong ab-
sorption in the 350e650 nm range which well matches the visible light emission of different convenient
visible light sources. Interestingly, very soft irradiation conditions can be employed. Excellent poly-
merization profiles have been obtained from selected dye iodonium salt combinations using a halogen
amp or a laser diode à 457 nm. The chemical mechanisms associated with these novel photoinitiating
systems are investigated by steady-state photolysis and ESR experiments.
Received 17 December 2013
Received in revised form
31 January 2014
Accepted 2 February 2014
Available online 7 February 2014
Keywords:
Dyes
Halogen lamp
Laser diode
Ó 2014 Elsevier Ltd. All rights reserved.
Cationic photopolymerization
Visible light
Photoinitiator
1. Introduction
years (see e.g. in Refs. [1,3]) in free radical polymerization (FRP) and,
to a lesser extent (indeed, until the recent years, the available
Dye photosensitized polymerization reactions (DPPR) occur in
the presence of a colored molecule (“dye”) incorporated into a
visible light absorbing photoinitiating systems (vPIS) and being
able to act as a photoinitiator (PI) or a photosensitizer (PS) [1,2].
DPPRs play an important role in many industrial applications
within e.g. the radiation curing and imaging area [1,2] as they allow
a rapid cure of a multifunctional monomer/oligomer matrix under
exposure to visible light delivered by high intensity sources or soft
irradiation devices. These DPPRs have been largely known for many
cationic (vPISs) were rather limited), in cationic polymerization.
The search of colored molecules and visible light absorbing pho-
toinitiating systems (vPISs) is continuously carried out and the
proposal of novel systems still remains an exciting topic. In
particular, there is still a need in (CP) and (FRPCP) as the synthesis
of polymers based on cationic formulation can avoid the detri-
mental shrinkage of the film observed during FRP reactions.
Solvent-free formulations are based on acrylate monomers
which are known to undergo a rapid FRP [4]. Vinyl ether monomers
(VEs) (low-odor and non-irritating) have recently appeared as an
environment friendly alternative to the widely used acrylate
monomers which show a strong odor and may cause skin and eye
irritation [5]. VEs are very reactive monomers and exhibit a fast
cationic polymerization [6,7]. They are encountered in various UV-
curing applications (in particular as protective coatings) where a
* Corresponding author.
E-mail addresses: didier.gigmes@univ-amu.fr (D. Gigmes), joumana.toufaily@ul.
edu.lb (J. Toufaily), tayssir.hamieh@ul.edu.lb (T. Hamieh), jacques.lalevee@uha.fr
(J. Lalevée).
http://dx.doi.org/10.1016/j.dyepig.2014.02.002
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

122
H. Mokbel et al. / Dyes and Pigments 105 (2014) 121e129
high speed and an extensive cure are desired, together with low
shrinkage, great impact strength and high adhesion of the UV-
cured polymer. They can also be used to reduce the formulation
viscosity.
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 ana-
lyses were recorded with a Thermo Finnigan EA 1112 elemental
analysis apparatus driven by the Eager 300 software. 1H and 13C
NMR spectra were determined at room temperature in 5 mm o.d.
tubes on a Bruker Avance 400 spectrometer of the Spectropole: 1H
(400 MHz) and 13C (100 MHz). The 1H chemical shifts were
referenced to the solvent peak DMSO-d6 (2.49 ppm), CDCl3
(7.26 ppm) and the 13C chemical shifts were referenced to the
solvent peak DMSO-d6 (39.5 ppm), CDCl3 (77 ppm). (E,E)-2,5-
Dioctyloxy-1,4-bis(2-[4]pyridyl-ethenyl)benzene M-1, 2,20,200,2000-
(((2,5-bis(hexyloxy)-1,4-phenylene)bis(methanylylidene))bis(1H-
indene-3,2,1 (2H)-triylidene))tetramalononitrile M-2, 1,4-bis[2-(4-
([4]pyridyl-ethenyl))-2,5-dioctyloxy-phenyl)ethenyl]-2,5-bis(octy-
loxy)benzene M-3, oligo(p-phenylene)vinylenes M-4 and M-6,
2,20-((2Z,20Z)-((2,5-bis(hexyloxy)-1,4-phenylene)bis(methanylyli-
dene))bis(3-oxo-2,3-dihydro -1H-indene-2,1-diylidene))dimalono-
nitrile M-7 were synthesized as previously reported [8]. 4,40-
((1E,10E)-(2,5-bis(octyloxy)-1,4-phenylene)bis(ethene-2,1-diyl))bis-
(2,5-bis(octyloxy)benzaldehyde) M-5 was synthesized by a two-
step formylation of 1,4-bis[2-(4-bromo-2,5-dioctyloxyphenyl)
ethenyl]-2,5-bis(octyloxy)benzene [9] consisting in the lithiation
In this laboratory, we have recently launched a series of work
aiming at designing vPIS for radical and cationic polymerizations. In
the present paper, we look for compounds presenting an extended
delocalization and being able, as part of vPISs, to initiate the cationic
polymerization of vinyl ethers and epoxides upon visible light
exposure under a soft irradiation source (e.g. household halogen
lamp) or laser diodes. Nine new dyes (based on an end capped
polyenic structure containing one or more 2,5-dioctyloxyphenyl
units (M-1 to M-8) or a pyridinium unit (M-9) in the main chain;
see Scheme 1) will be synthesized and their polymerization initia-
tion ability checked. The chemical mechanisms will be investigated
by steady state photolysis and ESR experiments.
2. Experimental section
i) Synthesis of the different photoinitiators:
All reagents and solvents were purchased from Aldrich or Alfa
Aesar and used as received without further purification. Mass
spectroscopy was performed by the Spectropole of Aix-Marseille
Scheme 1. Investigated compounds.

H. Mokbel et al. / Dyes and Pigments 105 (2014) 121e129
123
Scheme 2. Synthetic procedure.
with n-BuLi followed by treatment with DMF. (E)-2,5-bis(octy-
2.2. Synthesis of (E)-2-(2,5-bis(octyloxy)-4-(2-(pyridin-4-yl)vinyl)
loxy)-4-(2-(pyridin-4-yl)vinyl)benzaldehyde used for the synthe-
sis of M-8 was synthesized as previously reported [10]. [[50-
(Bromomethyl)-2,20-bipyridin-5-yl]methyl]phosphonium bromide
[11] and (E)-N,N-dimethyl-4-(2-(pyridin-4-yl)vinyl)aniline [12]
used as reagents for the synthesis of M-9 was synthesized as
previously reported, without modifications and in similar yields
(Scheme 2).
benzylidene)malononitrile
(E)-2,5-bis(octyloxy)-4-(2-(pyridin-4-yl)vinyl)benzaldehyde
(2 g, 4.29 mmol) and malononitrile (1.42 g, 21.47 mmol, 5 eq.) were
dissolved in acetonitrile (30 mL) and a few drops of piperidine were
added. The reaction was refluxed overnight. After cooling, the sol-
vent was removed under reduced pressure. The residue was dis-
solved in
a minimum of acetone and addition of ethanol
precipitated the product. It was filtered off, washed several times
with ethanol and dried under vacuum (1.85 g, 84% yield). 1H NMR
2.1. Synthesis of 4,40-((1E,10E)-(2,5-bis(octyloxy)-1,4-phenylene)
bis(ethene-2,1-diyl))bis(2,5-bis(octyloxy)benzaldehyde) M-5
(CDCl3)
d
(ppm): 0.91 (t, 6H, J ¼ 6.9 Hz), 1.29e1.51 (m, 20H), 1.79e
1.84 (m, 4H), 4.06 (t, 2H, J ¼ 6.6 Hz), 4.11 (t, 2H, J ¼ 6.6 Hz), 7.16 (d,
1H, J ¼ 16.5 Hz), 7.17 (s, 1H), 7.34 (s, 1H), 7.39 (d, 2H, J ¼ 6.0 Hz), 7.66
(d, 1H, J ¼ 16.5 Hz), 8.26 (s, 1H), 8.61 (d, 2H, J ¼ 6.0 Hz); 13C NMR
5,50-((1E,1’E)-(2,5-bis(octyloxy)-1,4-phenylene)bis(ethene-2,1-
diyl))bis(2-bromo-1,4-bis (octyloxy)benzene) (3 g, 2.48 mmol) was
suspended in dry THF (100 mL) and the solution was cooled
at ꢀ78 ꢁC. n-BuLi (3.1 mL, 4.96 mmol, 2 eq.) was added and the
reaction mixture was stirred at that temperature for 15 min. Dry
DMF (1 mL) was added and the reaction mixture was allowed to
warm to room temperature and stirred overnight. After addition of
water, DCM was added. The water layer was washed several times
with DCM. The organic phase was combined, dried over magne-
sium sulfate, filtered and the solvent removed under reduced
pressure. The residue was chromatographed on silica gel (DCM/
Hexanes as the eluent). Several fractions corresponding to the
starting materials (0.51 g, 17% recovered starting materials) the
monoaldehyde (0.7 g, 29%) and the dialdehyde (1 g, 43%) were
separated. The last fraction was evaporated and addition of
(CDCl3)
d (ppm): 14.0, 22.5, 26.0, 26.1, 29.07, 29.12, 29.16, 29.22,
29.55, 29.6, 31.7, 31.8, 69.0, 69.2, 110.1, 111.1, 113.6, 125.0, 127.4,
129.3, 132.5, 151.0, 155.9; HRMS (ESI MS) m/z: theor: 113.3355
found: 113.3353 (Mþ. detected).
2.3. Synthesis of (E)-4-(4-(2,2-dicyanovinyl)-2,5-bis(octyloxy)
styryl)-1-(4-methylbenzyl)pyridin-1-ium bromide M-8
(E)-2-(2,5-bis(octyloxy)-4-(2-(pyridin-4-yl)vinyl)benzylidene)
malononitrile (1 g, 1.95 mmol) and
a-bromo-p-xylene (360 mg,
1.95 mmol) were dissolved in acetonitrile (10 mL) and the reaction
mixture was refluxed overnight. After cooling, addition of diethyl
ether precipitated the product which was filtered off and washed
with a mixture of ethanol/ether (1/5). The product was isolated as a
ethanol precipitated the dialdehyde. 1H NMR (CDCl3)
d (ppm):
0.87e0.91 (m, 18H), 1.27e1.60 (m, 60H), 1.75e1.90 (m, 12H), 4.03e
4.15 (m, 12H), 7.18 (s, 2H), 7.22 (s, 2H), 7.35 (s, 2H), 7.54 (d, 2H,
J ¼ 16.3 Hz), 7.62 (d, 2H, J ¼ 16.3 Hz), 10.5 (s, 2H, CHO); 13C NMR
powder (1.29 g, 95% yield). 1H NMR (CD3CN)
d (ppm): 0.87 (t, 6H,
J ¼ 6.5 Hz), 1.29e1.44 (m, 20H), 1.50e1.57 (m, 4H), 2.36 (s, 3H), 4.16
(t, 4H, J ¼ 6.7 Hz), 5.59 (s, 2H), 7.30e7.37 (m, 4 H), 7.54 (d, 1H,
J ¼ 16.5 Hz), 8.00 (d, 2H, J ¼ 5.4 Hz), 8.05 (d, 1H, J ¼ 16.5 Hz), 8.57 (d,
(CDCl3)
d (ppm): 14.5, 23.1, 26.6, 26.7, 26.8, 29.64, 29.68, 29.70,
29.73, 29.76, 29.79, 31.2, 31.2, 32.21, 32.26, 32.27, 69.47, 69.55,
69.80, 110.5, 110.8, 111.2, 123.7, 124.5, 127.4, 127.9, 135.4, 151.1, 151.8,
156.6; HRMS (ESI MS) m/z: theor: 1106.8514 found: 1106.8513 (Mþ.
detected).
2H, J ¼ 7.1 Hz); 13C NMR (CD3CN)
d (ppm): 14.4, 21.2, 23.4, 26.9,
29.8, 29.9, 30.0, 32.6, 64.2, 70.4, 113.3, 125.3, 125.8, 128.2, 130.0,
131.0, 131.4, 136.7, 141.0, 144.8, 153.3, 155.3; HRMS (ESI MS) m/z:
theor: 618.4054 found: 618.4050 (Mþ. detected).

124
H. Mokbel et al. / Dyes and Pigments 105 (2014) 121e129
Scheme 3. Additives and Monomers.
2.4. Synthesis of (E)-4-(4-(dimethylamino)styryl)-1-((50-
((triphenylphosphonio)methyl)-[2,20-bipyridin]-5-yl)methyl)
pyridin-1-ium bromide M-9
iv) Cationic Polymerization (CP) and Free Radical Promoted
Cationic Polymerization (FRPCP):
50-(Bromomethyl)-2,20-bipyridin-5-yl]methyl]phosphonium
bromide (0.6 g, 0.993 mmol) and (E)-N,N-dimethyl-4-(2-(pyridin-
4-yl)vinyl)aniline (0.27 g, 1.20 mmol) were suspended in acetoni-
trile (20 mL) and the reaction mixture was refluxed overnight. After
cooling, the solvent was partially evaporated and addition of ether
precipitated a solid which was filtered off and washed several times
with ether. 750 mg of product were isolated (91% yield). 1H NMR
The two-component photoinitiating systems are based on dye/
iodonium salt (0.3%/2% w/w). The experimental conditions are
given in the Figure captions. The residual weight content is related
to the monomer. The photosensitive formulations (25 mm thick)
were deposited on a BaF2 pellet under air (or in laminate for DVE-3;
DVE-3 is a very low viscosity monomer: polymerization must be
carried out in laminate to avoid a change of thickness).
The evolution of the functional groups of the monomers e
conversion C (%) e is determined by measuring the peak area A of
(DMSO-d6)
d
(ppm): 2.41 (s, 3H, Me), 5.47 (d, 2H, J ¼ 14.8 Hz), 5.87
(s, 2H), 6.76 (d, 2H, J ¼ 8.7 Hz), 7.22 (d, 1H, J ¼ 16.0 Hz), 7.54 (d, 2H,
J ¼ 8.7 Hz), 7.57 (d, 2H, J ¼ 8.9 Hz), 7.75e7.79 (m, 12H), 7.90e7.96
(m, 4H), 8.10e8.15 (m, 2H), 8.22 (d, 1H, J ¼ 8.3 Hz), 8.32 (d, 2H,
J ¼ 3.3 Hz), 8.64 (d, 1H, J ¼ 6.2 Hz), 8.92 (s, 1H), 9.03 (d, 2H,
the corresponding characteristic bands at each time:
C
(%) ¼ (A0 ꢀ At)/A0 ꢂ 100 where C is the conversion at t time, A0 the
initial peak area before irradiation and At the peak area of the
functional groups at t time. The evolution of the epoxy group (for
EPOX) and the double bond (for vinyl ether monomer) contents
were continuously followed by real time FTIR spectroscopy (JASCO
FTIR 4100) at about 790 cmꢀ1 and 1619 cmꢀ1, respectively.
J ¼ 6.5 Hz); 13C NMR (DMSO-d6)
(ppm): 25.2 (d, J ¼ 47.2 Hz), 39.5,
d
40.0, 58.7, 111.9, 117.0, 118.4, 120.5 (d, J ¼ 27 Hz), 121.4, 122.7, 129.6,
130.2 (d, J ¼ 12 Hz), 130.4, 134.0 (d, J ¼ 10.5 Hz), 135.3, 137.7, 139.4
(d, J ¼ 5.3 Hz), 142.8, 143.2, 143.5, 149.6, 151.0 (d, J ¼ 6 Hz), 151.9,
154.1 (d, J ¼ 3.8 Hz), 154.3, 154.9 (d, J ¼ 2.2 Hz); HRMS (ESI MS) m/z:
theor: 334.1529 (Mþ/2) found: 334.1524 (Mþ. detected).
v) Computational Procedure:
Molecular orbital calculations were carried out with the
Gaussian 03 suite of programs. The electronic absorption spectra
for the different compounds were calculated with the time-
dependent density functional theory at B3LYP/6-31G* level on the
relaxed geometries calculated at UB3LYP/6-31G* level.
ii) Chemical compounds:
Diphenyliodonium hexafluorophosphate (Ph2Iþ or Iod) and
Triethyleneglycol divinyl ether (DVE-3; as a representative of vinyl
ether monomers) were obtained from Aldrich and used with the
best purity available (Scheme 3). Trimethylolpropane triacrylate
vi) ESR spin trapping (ESR-ST) experiments:
(TMPTA)
and
(3,4-epoxycyclohexane)methyl
3,4-
epoxycyclohexylcarboxylate (EPOX; Uvacure 1500) were obtained
from Cytec (Scheme 3). TMPTA, EPOX, and DVE-3 were selected as
benchmark monomers: these monomers are well known in the
photopolymerization field and represent excellent structures to
evaluate the initiating ability of new photoinitiating systems.
The ESR-ST experiments were carried out using an X-Band
spectrometer (MS 400 Magnettech). The radicals were produced at
RT upon a halogen lamp exposure under N2 and trapped by phenyl-
N-tbutylnitrone (PBN) according to a procedure described in detail
in Refs. [14,15]. The ESR spectra simulations were carried out with
the PEST WINSIM program.
iii) Irradiation sources:
vii) Fluorescence experiments:
Several lights were used: i) polychromatic light from a halogen
lamp (Fiber-Lite, DC-950
e
incident light intensity:
I0 z 12 mW cmꢀ2; in the 370e800 nm range); ii) monochromatic
light delivered by a laser diode at 457 nm (MBL-III-BFIOPTILAS;
I0 z 100 mW cmꢀ2). The emission spectra of these several lights
were already given in Ref. [13].
The fluorescence properties of the compounds were studied
using a JASCO FP-750 spectrometer.
viii) Redox potentials:

H. Mokbel et al. / Dyes and Pigments 105 (2014) 121e129
125
Fig. 1. (a) UVevisible absorption spectra of the different dyes in acetonitrile or toluene; (b) HOMO and LUMO for M-3 and M-5 at B3LYP/6-31G* level.
The redox potentials (Eox vs. SCE) were measured in acetonitrile
by cyclic voltammetry with tetrabutyl-ammonium hexa-
fluorophosphate 0.1 M as a supporting electrolyte. The free energy
matching with different light source emission spectra (e.g. halogen
lamp and laser diodes at 405, 457, 473, 532 or 635 nm; see Table 1).
Remarkably, high molar extinction coefficients are determined at
457 nm (e.g. 21 200, 37 500 Mꢀ1 cmꢀ1 for M-3 and M-5,
respectively).
change DGet for an electron transfer reaction is calculated from the
classical RehmeWeller equation (Eq. (1)) [16] where Eox, Ered, ET and
C are the oxidation potential of the donor, the reduction potential of
the acceptor, the excited state energy and the coulombic term for
the initially formed ion pair, respectively. C is neglected as usually
done in polar solvents [17].
Molecular orbital MO calculations, using the time-dependent
density functional theory at B3LYP/6-31G* level on the relaxed
geometries calculated at UB3LYP/6-31G* level, show that the
lowest energy transition involves strongly delocalized MOs (see e.g.
the highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) of M-3 and M-5 in Fig. 1(b)).
Remarkably for M-3, the most efficient dye in the photo-
initiating system, the maximum absorption wavelength (lmax) is
red-shifted in the presence of the iodonium salt (Fig. 2); the
change of lmax for different [Iod] is given in Table 2. This shift can
be likely explained by the formation of a ground state complex
between M-3 and Iod. For M-5/Iod, such a behavior is not
observed (the UVevis spectrum of M-5 is affected by the presence
of Iod).
DGet ¼ Eox ꢀ Ered ꢀ ET þ C
(1)
3. Results and discussion
3.1. Absorption properties of the new dyes
The ground state absorption spectra of the new proposed dyes
are depicted in Fig. 1. These dyes allow a large and efficient

126
H. Mokbel et al. / Dyes and Pigments 105 (2014) 121e129
Table 1
Maximum absorption wavelengths for the new proposed dyes.
3.2. Cationic photopolymerization
The photopolymerization of DVE-3 was carried out in laminate
upon visible lights (457 nm laser diode and halogen lamp). In the
presence of the M-3/Iod two-component system, very high rates
of polymerization and high final conversions are found (e.g.
conversions of about 95% and 94% after 300 s of irradiation with
the 457 nm laser diode and halogen lamp, respectively; Fig. 3(A)).
The low intensity of the halogen lamp (12 mW cmꢀ2) being
already enough (almost total conversion), a w8 fold increase
(using the laser diode) cannot obviously change anything in the
polymerization profile. Tack free coatings are obtained. The
bleaching of the polymer film under irradiation is quite remark-
able (see below for the bleaching of the dye in presence of Iod
upon irradiation); this can be useful for applications requiring
colorless coatings. When using M-5/Iod (Fig. 3(B)), a final con-
version of w50% (laser diode) and w80% (halogen lamp) is
reached after 10 min irradiation; the polymerization rate is faster
with the laser diode but a lower final conversion is reached, this
can be ascribed to a too fast polymer network synthesis pre-
venting a full conversion with the laser diode. The other dye/Iod
systems are much less efficient (low final conversions are
obtained < 15%) (Fig. 1 in supporting information).
Dye
lmax (nm) [ 3 ]
in Mꢀ1 cmꢀ1
absorption
M-1
M-2
M-3
M-4
M-5
M-6
M-7
M-8
M-9
395
534
446 [22 322]
536
438 [41 983]
495
557
460
487
1
The M-3/Iod system is also suitable for the ring opening poly-
merization of EPOX under air (w50% of conversion at t ¼ 800 s;
laser diode irradiation; Fig. 4). Therefore, M-3 appears to be an
efficient photosensitizer for the iodonium salt decomposition. Us-
ing the other dyes, poorer polymerization profiles are obtained (e.g.
20% conversion with M-5/Iod and <15% for the other dyes). The
difference between the DVE-3 vs. the EPOX profiles should be
ascribed to the better propagation rate constants of vinyl ethers
rather than to a specific effect of the photoinitiating systems (as in
Refs. [1,2]).
0
400
500
600
700
λ (nm)
Although the irradiated M-3/Iod or M-5/Iod systems generate
radicals (see below), no photopolymerization of TMPTA signifi-
cantly occurs (i.e. final conversions <15%).
Fig. 2. Absorption spectra for M-3 for different [Iod] (see Table 2) in acetonitrile/
toluene.
Table 2
lmax for M-3 for different [Iod] in acetonitrile/
toluene (50%/50%).
3.3. Chemical mechanisms observed in the dye/Iod systems
[Iod] M
lmax (nm)
Because of their high efficiency in photopolymerization, the
mechanistic study was focused on M-3 and M-5. For the other dyes,
a very low reactivity was found for polymerization, but also for
photolysis (see Fig. 2 in supporting information). This can be
ascribed to the low reactivity of theses dyes with Iod to generate
initiating species for the polymerization reaction (see below for the
0.5 10ꢀ3
446
446
0.15 10ꢀ2
0.37 10ꢀ2
0.58 10ꢀ2
0.79 10ꢀ2
1.00 10ꢀ2
1.27 10ꢀ2
1.53 10ꢀ2
446.8
446.9
449.3
454.9
460.5
469.3
structure/reactivity relationship). The free energy change
DGet for
the expected M-3/Ph2Iþ and M-5/Ph2Iþ electron transfer eT
100
1
2
100
80
60
40
20
0
A
B
90
2
80
70
60
1
50
40
30
20
10
0
0
200
400
600
800
0
200
400
600
800
Time (s)
Time (s)
Fig. 3. Photopolymerization profiles of DVE-3 in laminate; in the presence of (A) M-3/Iod (0.3%/2%w/w) and (B) M-5/Iod (0.3%/2%w/w) upon exposure to: (1) laser diode at 457 nm;
(2) halogen lamp.

H. Mokbel et al. / Dyes and Pigments 105 (2014) 121e129
127
dye / 1dye (h
n)
40
30
20
10
0
1dye þ Ph2Iþ / dyeꢃþ þ Ph2Iꢃ
dyeꢃþ þ Ph2Iꢃ / dye þ Ph2Iþ
Ph2Iꢃ / Phꢃ þ Ph-I
(r1a)
(r1b)
(r2)
Phꢃ þ DVE-3 / Ph-DVE-3ꢃ
(r3)
Ph-DVE-3ꢃ þ Ph2Iþ / Ph-DVE-3þ þ Phꢃ þ Ph-I
(r4)
0
200
400
600
800
Time (s)
Fig. 4. Photopolymerization profiles of EPOX under air; upon a laser diode exposure at
457 nm in the presence of M-3/Iod (0.3%/2%w/w).
3.4. Structure/reactivity for the different investigated dyes
The efficiency of the dyes decreases in the series M-3 > M-
5 >> M-1, M-2, M-4, M-6 to M-9: this trend can be ascribed to a
strong interplay of different factors:
Table 3
Maximum emission wavelength experimentally obtained by fluorescence spec-
troscopy for M-3 and M-5; singlet state energy level (ES1) from Fig. 5(B); oxidation
potential (Eox) and D
G for the electron transfer (1dye/Iod).
i) the respective light absorption properties of these dyes; this is
probably not the driving factor i.e. the extinction coefficient
is higher for M-5 than for M-3 at 457 nm despite a lower
reactivity for M-5 (36 700 vs. 20 700 Mꢀ1 cmꢀ1 for M-5 and
M-3, respectively).
Dye
lmax (nm) emission
ES1 (eV)
Eox (V vs. SCE)
D
G (1dye/Iod)(eV)
M-3
M-5
522
516
2.53
2.54
0.78
0.9
ꢀ1.55
ꢀ1.44
ii) the reactivity of the dye for (r1a); with two favorable
DGet
for r1a for M-3/Iod and M-5/Iod, a very different reac-
tivity was found. This process is probably not the crucial
one.
reaction is favorable (
D
G ¼ ꢀ1.55 and ꢀ1.44 eV, respectively;
Ered(Ph2Iþ)wꢀ0.2 V1; used parameters in Table 3; see Fig. 5(A) and
iii) the yield in Phꢃ leading to a FRPCP process through r2-r4.
Indeed, a back electron transfer reaction (r1b) can decrease
the yield in Phꢃ; this latter parameter can hardly be
extracted but the intensity of the free radical signals in
ESR-spin trapping experiments roughly follows the reac-
tivity trend (M-3 > M-5 >> M-1, M-2, M-4, M-6 to M-9).
Almost no radical was observed for the irradiation of M-1
(or M-2, M-4, M-6 to M-9)/Iod solutions in full agreement
with the lack of reactivity of these dyes. These data suggest
a strong influence of the yield in Phꢃ in the overall reac-
tivity of the system.
Fig. 5(B)).
When exposed to the halogen lamp, a very fast bleaching of the
M-3/Iod and M-5/Iod solution is found (Fig. 6). The radicals
generated in these systems can be observed in ESR-spin trapping
experiments. For the irradiation of M-3/Iod and M-5/Iod solutions
upon a halogen lamp exposure under N2, a phenyl radical Phꢃ
(originating from the known r21) is clearly observed (Fig. 7). It
obviously results (r1a) from this eT process (oxidation of the dye by
Iod) leading to PhI, dye ꢃþ and Phꢃ. For the other investigated dyes,
this bleaching is much slower or even not observed.
In the presence of radical inhibitors (oxygen, PBN), no poly-
merization occurs for the M-3/Iod and M-5/Iod initiating systems.
This is in agreement with a free radical promoted cationic poly-
merization reaction (FRPCP) (r3, r4), Ph-DVE-3þ being the main
initiating structure. Indeed, Ph-DVE-3ꢃ is the free radical gener-
ated though the addition of Phꢃ onto the vinyl ether double bond
(r3). This latter radical is electron rich and can be oxidized by
Ph2Iþ leading to Ph-DVE-3þ (r4) as polymerization initiating
structure.
4. Conclusion
In this paper, two-component photoinitiating systems contain-
ing an iodonium salt and various dyes involving a novel end capped
polyenic skeleton have been proposed for the cationic polymeri-
zation of a vinylether (DVE-3) and a diepoxide under air either
upon a very soft irradiation (halogen lamp) or a laser diode
A
B
1
1.0
0.5
0.0
10
0
400
500
600
0.0
0.4
0.8
1.2
E (V)
(nm)
λ
Fig. 5. (A) Cyclic voltammogram for M-3 in acetonitrile/toluene (50%/50%) (B) (a) Absorption and (b) fluorescence spectra for M-3 in acetonitrile/toluene (50%/50%).

128
H. Mokbel et al. / Dyes and Pigments 105 (2014) 121e129
(A)
(B) 0 5
0 min
0,5
0 4
0 3
0 2
0 1
0 0
0 min
15 min
400
2 min
400
0,0
500
600
300
500
600
700
(nm)
(nm)
λ
λ
Fig. 6. Photolysis of M-3/Iod (A) and M-5/Iod (B) in acetonitrile/toluene (50%/50% w/w); under air; halogen lamp exposure; [Iod] ¼ 0.02 M.
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.02.002

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