Blue-to-red light sensitive push-pull structured photoinitiators: Indanedione derivatives for radical and cationic photopolymerization reactions

Article abstract of DOI:10.1021/ma4005082

The actual photonitiators PI can only operate in a restricted part of the visible spectrum; as a consequence, several PIs are usually necessary to harvest all the emitted visible photons. In the present paper, new dyes based on a donor-π-acceptor structur

Full text of DOI:10.1021/ma4005082

Article
pubs.acs.org/Macromolecules
Blue-to-Red Light Sensitive Push−Pull Structured Photoinitiators:
Indanedione Derivatives for Radical and Cationic
Photopolymerization Reactions
Mohamad-Ali Tehfe,† Frederic Dumur,‡ Bernadette Graff,† Didier Gigmes,‡,* Jean-Pierre Fouassier,§
́
́
and Jacques Lalevee†,*
†Institut de Science des Mater
France
́
́
iaux de Mulhouse IS2M−UMR 7361 CNRS-UHA; 15, rue Jean Starcky, 68057 Mulhouse Cedex,
‡Aix-Marseille Universite,
́
CNRS, ICR, UMR 7273, F-13397 Marseille, France
§UHA-ENSCMu, 3 rue Alfred Werner 68093 Mulhouse, France
S
* Supporting Information
ABSTRACT: The actual photonitiators PI can only operate in
a restricted part of the visible spectrum; as a consequence,
several PIs are usually necessary to harvest all the emitted visible
photons. In the present paper, new dyes based on a donor−π-
acceptor structure (1,3-indanedione derivatives) are incorpo-
rated into visible light sensitive photoinitiating systems of
polymerization. They exhibit an unusual and remarkable broad
absorption lying from the blue to the red. When employed in
the presence of an iodonium salt (Iod) and optionally N-
vinylcarbazole (NVK), these dyes can efficiently initiate the
radical photopolymerization of acrylates, the cationic photo-
polymerization of epoxide and vinylether monomers and the
hybrid cure of acrylate/epoxide blends under exposure, e.g., at
405, 457, 473, 532, and 635 nm. They partly behave as organic photocatalysts. These particular light absorption properties and
the initiation step mechanisms are investigated in detail.
the possibility of using push−pull molecules that could exhibit a
broad visible absorption and act as photoinitiators of FRP, CP
and FRPCP being able to operate under various laser lines.
As known, push−pull molecules consist in unsymmetrically
substituted D−π−A arrangements bearing electron donor (D)
and electron acceptor (A) functionalities at both ends of a
planar conjugated spacer. The push−pull effect which results
from the intramolecular donor/acceptor interaction and a
favorable orientation of the charge delocalization in the axis of
the chromophore is characterized by a strong absorption band
detected in the visible region (named intramolecular charge
transfer band ICT) whose position is determined not only by
the strength of the donor/acceptor pair but also more subtly by
the π-conjugated spacer. Push−pull molecules also relate to a
particularly interesting class of molecules that shows out-
standing transition energy shifts by changing the solvent
polarity/polarizability,8 present a remarkable ability to work
under two-photon excitations and have been developed for
potential applications, e.g., in optics, molecular electronics,
optoelectronic devices, dye-sensitized solar cells, etc.; see ref 9.
INTRODUCTION
■
Visible light induced photopolymerization reactions occur in
the presence of a colored molecule incorporated into an
absorbing photoinitiating systems PIS and being able to act as a
photoinitiator PI or a photosensitizer PS.1,2 Free radical
polymerization FRP as well as cationic polymerization CP
and free radical promoted cationic polymerization FRPCP can
be easily achieved both under high and low intensity light
sources.3 Many dyes incorporating in mono- and multi-
component PIS have been already proposed (see, e.g., an up-
to-date review in ref 1). Improving the performance attained or
meeting new or promising possibilities of applications require a
continuous search of new structures. Looking at the literature is
quite amazing: despite the fact that more than 50 years of
research have led to the possible use of a huge number of PI, PS
and PIS, the most recent analysis of the current works in this
area shows tremendous and successful efforts deployed by
numerous groups for innovative proposals of new dye
structures (see examples of recent papers in this field4−7) as
exemplified by the recent introduction of colored substituted
ketones, modified organometallic derivatives, various series of
dyes, light harvesting compounds or multicolor photoinitiators.
Nevertheless, there is still a place for the continuous disclosure
of new PISs. Following our works in this area,7 we explore here
Received: March 9, 2013
Revised: April 2, 2013
Published: April 15, 2013
© 2013 American Chemical Society
3332
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341

Macromolecules
Article
Synthesis of 2-(4-(N,N-Dimethylamino)Benzylidene)-1H-indene-
1,3(2H)-dione, D_1. 4-N,N-Dimethylaminobenzaldehyde (1 g, 6.84
mmol) and indane-1,3-dione (1 g, 6.84 mmol) were suspended in
absolute ethanol (20 mL), and a few drops of piperidine were added.
Immediately, the solution turned deep red. The solution was refluxed
for 4 h. After cooling, the solution was concentrated to 1/5th of its
initial volume. Addition of ether precipitated a red solid which was
filtered off, washed several times with water and dried under vacuum.
1H NMR (CDCl3), δ (ppm): 3.14 (s, 6H, N(CH3)2), 6.87 (d, 2H, J =
Our actual search for new initiating systems for visible light
prompts us to develop new dyes based on this D-π-A structure
(Scheme 1). They are based on difunctional structures where
Scheme 1
8.7 Hz), 7.67 (s, 1H), 7.84−7.86 (m, 4H), 8.55 (d, 2H, J = 8.7 Hz).
13C NMR (CDCl3), δ (ppm): 40.1, 111.6, 122.2, 134.7, 134.9, 139.0,
141.4, 141.4, 146.5, 154.1, 189.1, 190.5. HRMS (ESI MS) m/z: theor,
278.1176; found, 278.1178 ([M + H]+ detected)
Synthesis of 2-(Pyren-1-ylmethylene)-1H-indene-1,3(2H)-dione,
D_2. Pyrene 1-carbaldehyde (1.57 g, 6.84 mmol) and indane-1,3-
dione (1 g, 6.84 mmol) were suspended in 20 mL of absolute ethanol,
and a few drops of piperidine were added. Immediately, the solution
turned orange. The solution was refluxed for 3 h. During that time, a
precipitate formed. After cooling, the precipitate was filtered off,
washed several times with ethanol and ether, and finally dried under
an indanedione moiety is linked to an aniline or a pyrene
moiety through a CC double bond. Through the ICT,
interesting absorptions are expected. ESR experiments, laser
flash photolysis, steady state photolysis, absorption spectrum
studies and molecular orbital calculations will be carried out.
The ability of these photo initiators in FRP, CP and FRPCP
reactions upon exposure to laser diodes from 405 to 635 nm
will be checked.
1
vacuum. H NMR (DMSO-d6), δ (ppm): 7.95−8.03 (m, 2H), 8.06−
8.14 (m, 2H), 8.19−8.21 (m, 1H), 8.29−8.33 (m, 2H), 8.38−8.53 (m,
5H), 8.58−8.64 (m, 2H). HRMS (ESI MS) m/z: theor, 359.1067;
found, 359.1066 ([M + H]+ detected); Anal. Calcd for C26H14O2: C,
87.1; H, 3.9; O, 8.9. Found: C, 87.2; H, 4.0; O, 8.8.
ii. Chemical Compounds. N-Vinylcarbazole (NVK) and
diphenyliodonium hexafluorophosphate (Ph2I+ or Iod) were obtained
from Aldrich and used with the best purity available (Scheme 2). (3,4-
epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX,
Uvacure 1500) and trimethylolpropane triacrylate (TMPTA) were
obtained from Cytec (Scheme 2). Triethylene glycol divinyl ether
(DVE-3; as a representative of vinyl ether monomers) was obtained
from Aldrich and used with the best purity available (Scheme 2).
EPOX, DVE-3, and TMPTA were selected as representative
monomers: these monomers are well-known in the photopolymeriza-
tion field and represent excellent structures to evaluate the initiating
ability of new photoinitiating systems.
iii. Irradiation Sources. Several lights were used: (i) poly-
chromatic light from a halogen lamp (Fiber-Lite, DC-950; incident
light intensity = I0 ≈ 12 mW cm−2, in the 370−800 nm range); (ii)
monochromatic light delivered by laser diodes at 457, 473, 532, and
635 nm (MBL-III-BFIOPTILAS; I0 ≈ 100 mW cm−2). A laser diode at
405 nm was also used (I0 ≈ 12 mW cm−2). The emission spectra for
these several lights are given in.10
EXPERIMENTAL SECTION
■
i. Synthesis of the Different Photoinitiators. The investigated
dyes (D_1 and D_2) shown in the Scheme 1 were prepared according
to the following procedures. 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 University. ESI mass spectral analyses were recorded
with a 3200 QTRAP (Applied Biosystems SCIEX) mass spectrometer.
The HRMS mass spectral analysis was performed with a QStar Elite
(Applied Biosystems SCIEX) mass spectrometer. Elemental analyses
were recorded with a Thermo Finnigan EA 1112 elemental analysis
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
1
Avance 400 spectrometer of the Spectropole: H (400 MHz) and 13
C
1
(100 MHz). The H 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). The two dyes D_1 and D_2 were prepared by
Knoevenagel condensation of Indane-1,3-dione with the correspond-
ing aldehyde. All these dyes were prepared with analytical purity up to
accepted standards for new organic compounds (>98%) which was
checked by high field NMR analysis. The stability and storage of D_1
and D_2 are excellent; these dyes do not need special storage
conditions.
iv. Free Radical Photopolymerization (FRP) Experiments.
Trimethylolpropane triacrylate (TMPTA from Cytec) was used as a
low viscosity monomer. The experiments were carried out in
laminated conditions. The films (25 μm thick) deposited on a BaF2
pellet were irradiated (see the section Irradiation Sources). The
evolution of the double bond content was continuously followed by
real time FTIR spectroscopy (JASCO FTIR 4100) at about 1630 cm−1
Scheme 2
3333
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341

Macromolecules
Article
as in ref 11. The polymerization of TMPTA can also be observed at
about 1408 cm−1.
v. Cationic Polymerization (CP) and Free Radical Promoted
Cationic Polymerization (FRPCP). The two- and three-component
photoinitiating systems are based on dye/iodonium salt (0.3%/2% w/
w) and dye/iodonium salt/NVK (0.3%/2%/3% w/w), respectively.
The experimental conditions are given in the Figure captions. The
residual weight content is related to the monomer. The photosensitive
formulations (25 μm thick) were deposited on a BaF2 pellet under air
(or in laminated conditions for DVE-3; DVE-3 is a very low viscosity
monomer: polymerization must be carried out in laminated conditions
to avoid a change of thickness). 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 and 1619 cm−1, respectively.
vi. 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.
vii. ESR Spin Trapping (ESR-ST) Experiments. The ESR-ST
experiments were carried out using an X-Band spectrometer (MS 400
Magnettech). The radicals were produced at RT under a halogen lamp
exposure under N2 and trapped by phenyl-N-tert-butylnitrone (PBN)
according to a procedure described in detail in refs 12 and 13. The
ESR spectra simulations were carried out with the PEST WINSIM
program.
viii. Fluorescence Experiments. The fluorescence properties of
the compounds were studied using a JASCO FP-750 spectrometer.
ix. Redox Potentials. The redox potentials were measured in
acetonitrile by cyclic voltammetry with tetrabutyl-ammonium
hexafluorophosphate 0.1 M as a supporting electrolyte. The free
energy change ΔGet for an electron transfer reaction is calculated from
the classical Rehm−Weller equation (eq 1)14 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.15
Figure 1. (A) UV−visible absorption spectra of the different new dyes
(D_1 and D_2) in acetonitrile; (B) HOMO and LUMO for D_1 (at
B3LYP/6-31G* level).
that the lowest energy transition involves strongly delocalized
HOMO and LUMO for both dyes (Figure 1B) (π → π*
character). For D_1, the HOMO and LUMO are mainly
localized on amine and indanedione fragment, respectively.
This highlights a charge transfer character in agreement with
the push−pull character of this structure
ΔGet = Eox − Ered − ET + C
(1)
x. Laser Flash Photolysis. Nanosecond laser flash photolysis
(LFP) experiments were carried out using a Qswitched nanosecond
Nd/YAG laser (λexc = 355 nm, 9 ns pulses; energy reduced down to 10
mJ) from Continuum (Minilite) and an analyzing system consisting of
a ceramic xenon lamp, a monochromator, a fast photomultiplier and a
transient digitizer (Luzchem LFP 212).10
In order to support the presence of this charge transfer
character, their solvatochromic behaviors were investigated in
seven solvents of different polarities. Indeed, push−pull
molecules represent a particularly interesting class of molecules
that shows outstanding transition energy shifts by changing the
solvent polarity/polarizability.8 Results are summarized in the
Table 1. Over the years, several empirical polarity scales were
developed to investigate the solvatochromism, and different
empirical solvent polarity scale, i.e., Dimroth−Reichardt’s,16
Kamlet−Taft’s,17 Catalan’s,18 Lippert−Mataga’s,19 Bakh-
shiev’s,20 Kawski−Chamma−Viallet’s,21 McRae’s,22 and Sup-
pan’s scales,23 among others, were reported. Intramolecular
nature of the observed charge transfer can be easily
demonstrated by realizing successive dilutions, intermolecular
charge transfer disappearing at low concentration. If an increase
of the Stoke shift can be observed for D_2 with the solvents
polarity, an irregular behavior was evidenced for D_1 and no
correlation could be established in first approximation (see
Table 1). Noticeably, halogenated solvents gave larger
absorption wavelengths than nonhalogenated solvents of
similar ε values for the two dyes and this unusual trend in
halogenated solvents is well-reported in the literature.8b,24
While plotting the Stoke’s shift values against the Dimroth−
Reichardt solvent parameters or the empirical Kamlet−Taft
RESULTS AND DISCUSSION
■
1. Absorption Properties of the New Dyes (D_1 and
D_2). The ground state absorption spectra of the proposed
indanedione derivatives (the best one being D_1) allow a large
and efficient matching with the light source emission spectra
(halogen lamp and laser diodes at 405, 457, 473, 532, or 635
nm). Remarkably, high molar extinction coefficients are
determined (about 38 000 and 21 000 M−1cm−1 for D_1 and
D_2 at ∼478 and ∼464 nm, respectively; ∼ 8000 M−1cm−1 for
D_1 at ∼635 nm; Figure 1) compared to absorption only in the
UV range for the indanedione, aniline and pyrene moieties.
This shows that an important charge transfer occurs in the
electronic transition. The spectrum of D_1 is unchanged with
the concentration even for very low concentrations (Figure S1
in the Supporting Information); i.e., the presence of absorption
bands associated with dimers can be ruled out.
Molecular orbital MO calculations (using the time-depend-
ent density functional theory at B3LYP/6-31G* level on the
relaxed geometries calculated at UB3LYP/6-31G* level) show
3334
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341

Macromolecules
Article
Table 1. Absorption and Emission Maxima (nm) Obtained Experimentally by UV−Visible Spectroscopy and Fluorescence
Spectroscopy for Dyes D_1 and D_2
solvent
dielectric constant (ε)
refractive index (n)
λmax (D_1)
λem (D_1)
Stoke shift
λmax (D_2)
λem (D_2)
Stoke shift
toluene
2.4
4.8
1.497
1.446
1.407
1.424
1.359
1.328
1.344
600
620
607
616
610
629
616
662
700
685
700
62
80
78
84
−
482
487
471
478
466
472
463
521
534
530
540
546
560
554
39
47
59
62
80
88
91
chloroform
THF
7.4
CH2Cl2
acetone
8.9
a
20.6
32.7
37.5
n.e.
methanol
acetonitrile
710
n.e.
81
−
a
n.e.: non-evaluated (too weak to be measured).
Figure 2. Bakhshiev’s (a), Kawski−Chamma−Viallet’s (b), Lippert−Mataga (c), McRae’s (d), and Suppan’s (e) solvatochromic correlation plots for
D_2 in seven solvents of different polarities and R2 of the corresponding linear regressions; F is the solvent parameter.
parameters, no reasonable linear correlations could be obtained
for the two dyes. On the other hand, Lippert−Mataga’s,
Bakhshiev’s, Kawski−Chamma−Viallet’s, McRae’,s and Sup-
pan’s polarity scales all based on both the dielectric constant
and the refractive index of the solvents gave remarkable
correlations for D_2 (see Figure 2). In particular, the strong
negative slopes obtained with the different plots are indicative
of significant charge redistribution upon photoexcitation. As
anticipated, the largest magnitude of the Stokes shift was
observed in the most polar solvents and this variation is
indicative of a major change in the dipole moment value
referring to a geometry of the excited state different to that of
the ground state. Surprisingly, the solvatochromic behavior of
D_1 was markedly less linear, as illustrated in the Figure 3. This
unexpected behavior can be assigned to the dynamical solute−
solvent interactions that take place in solution and to the strong
donating ability of the dimethylaminobenzene moiety render-
ing to some extend this chromophore more sensitive to the
polarity/polarizability of the solvents than D_2 only possessing
a weak electron donor. In particular, for a given solvent,
competing effects of polarity/polarizability and hydrogen bond
donating/accepting abilities of opposite forces can also be
observed rendering the solvatochromism of D_1 more difficult
to anticipate (see Figure 4).
2. Cationic Photopolymerization. a. Photopolymeriza-
tion of EPOX. Using the dye/Iod system, a ring-opening
polymerization ROP of EPOX under air is clearly observed
(e.g., with D_1/Iod under the laser diode irradiation at 473 nm
3335
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341

Macromolecules
Article
Figure 3. Bakhshiev’s (a), Kawski−Chamma−Viallet’s (b), Lippert−Mataga (c), McRae’s (d), and Suppan’s (e) solvatochromic correlation plots for
D_1 in seven solvents of different polarities and R2 of the corresponding linear regressions.
characterized by an excellent light absorption in the visible
range (Figure 1).
The efficiency of D_1/NVK/Iod under the selected laser
diode irradiations decreases in the series (405 nm <635 nm
∼532 nm< 457 nm< 473 nmsee Figure 6). This trend of
reactivity is in agreement with the light absorption properties of
D_1 at the different laser diode irradiations (Table 2; see the
respective absorbed light intensity Iabs), i.e., the best polymer-
ization profiles are observed at 473 and 457 nm in agreement
with very high extinction coefficients and high diode light
intensity. At 405 nm, a slower polymerization process is found;
this is ascribed to the low light intensity of the laser diode
Figure 4. Competitive solvatochromic effects affecting the solvato-
chromism of D_1.
(Table 2). Interestingly for the different irradiation wave-
lengths, final conversions >40% are always found.
b. Photopolymerization of DVE-3. The photopolymeriza-
tion of DVE-3 was carried out in laminated conditions at 473
nm. D_1 or Iod alone does not lead to any polymerization
(Figure 7 curves 1 and 2). In the presence of the two-
component D_1/Iod photoinitiating system, an excellent
polymerization is already noted with a conversion about 95%
after 30 s of irradiation. The difference between the DVE-3 vs
the EPOX profiles should be ascribed to the faster polymer-
ization reactions usually observed with vinyl ethers rather than
to a specific effect of the photoinitiating systems (as in1,2).
3. Free Radical Photopolymerization. The best typical
conversion−time profiles of trimethylolpropane triacrylate
(TMPTA) in laminated conditions under, e.g., a laser diode
and the halogen lamp; Figure 5; no ROP in the presence of Iod
or the dye alone). A very fast photopolymerization and very
high final conversions are reached upon addition of NVK
(Figure 5A,B, curve 1 vs curve 2; presence of D_1/NVK/Iod);
final tack free coatings are obtained, e.g., within 6 min of
irradiation. A high NVK consumption is noted (i.e., Figure 5A);
the formation of the polyether network (as characterized by its
absorption band at 1080 cm−1 i.e. Figure 5B) is evidenced. On
the opposite, no ROP occurs in the presence of D_2/Iod/NVK
(Figure 5A, curve 3). This trend of reactivity does not follow
the light absorption properties of the two dyes as D_2 is also
3336
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341

Macromolecules
Article
Figure 5. Photopolymerization profiles of EPOX under air. (A) Upon a laser diode exposure at 473 nm in the presence of (1) D_1/Iod (0.3%/2%
w/w), (2) D_1/NVK/Iod (0.3%/3%/2% w/w), and (3) D_2/NVK/Iod (0.3%/3%/2% w/w). Inset: NVK conversion during the
photopolymerization of (2). (B) Upon a halogen lamp irradiation in the presence of (1) D_1/Iod (0.3%/2% w/w) and (2) D_1/NVK/Iod
(0.3%/3%/2% w/w). Inset: IR spectra recorded during the photopolymerization of 2.
Figure 6. Photopolymerization profiles of EPOX under air in the
presence of D_1/NVK/Iod (0.3%/3%/2% w/w): upon different laser
diodes exposure at (1) 457, (2) 473, (3) 532, (4) 635, and (5) 405
nm.
Figure 7. Photopolymerization profiles of DVE-3 in laminate. Upon
the laser diode exposure at 473 nm in the presence of (1) D_1 (0.3%
w/w), (2) Iod (2% w/w), and (3) D_1/Iod (0.3%/2% w/w).
Table 2. Molar Extinction Coefficients ε of D_1 at Selected
Laser Diode Emission Wavelengths
a
The same holds true when using a halogen lamp irradiation
with D_1/NVK/Iod (Figure S2 in the Supporting Information:
final conversion >45%; tack-free coating). As previously for
ROP process, D_2 is not found to be an efficient sensitizer for
FRP of acrylates; the reactivity of D_2 vs D_1 will be discussed
below.
4. Hybrid Cure: Photopolymerization of TMPTA/EPOX
Blends. A concomitant radical/cationic polymerization of a
TMPTA/EPOX blend (50%/50% w/w) readily occurs in a
one-step hybrid cure procedure (e.g., upon exposure at 473 nm;
Figure 9; D_1/NVK/Iod system). A tack free coating is also
obtained after only 6 min of irradiation. Under laminated
conditions, the acrylate and epoxide conversions are ∼70% and
∼40% at t = 400 s, respectively. Tack free coatings can also be
obtained under air.
λ (nm)
ε (M−1 cm−1
)
I0 (mW/cm2)
Iabs
405
457
473
532
635
7200
29000
38000
4600
12
100
100
100
100
4
81
88
23
36
8000
a
In acetonitrile. Emission light intensity of the laser diodes used in
this work (I0); absorbed light intensity (Iabs in a.u.) for D_1 (1% w/w)
under the selected polymerization conditions (evaluated from Iabs
=
I0(1 − 10−ε[D_1]l)).
irradiation at 473 nm are shown in Figure 8. A relatively low
final conversion is obtained when using D_1/Iod (Figure 8
curve 1) whereas a better polymerization profiles is reached
upon addition of NVK (final conversion ∼50% with D_1/Iod/
NVK; Figure 8 curve 2; final tack-free coating within 4 min).
5/. Chemical Mechanisms for D_1 and D_2. When
exposed to the laser diode at 473 nm, a very fast bleaching of
the D_1/Iod solution is found (Figure 10). It results (r1) from
3337
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341

Macromolecules
Article
clearly observed: the hyperfine coupling constants hfc (aN
=
14.2 G; aH = 2.2 G) agree with the known data for the PBN/
Ph• adduct;12,13 the radical cation is characterized by a distinct
singlet signal with g ∼ 2.003 in agreement with ref 25. Upon
identical light absorbed intensity, D_1 is more reactive than
D_2: indeed, the photolysis of the D_1/Iod solution is much
faster than that of D_2/Iod (Figure 10B vs Figure 10A). The
lower photolysis of D_2/Iod can be explained by a back
electron transfer in r1 regenerating the starting compounds.
Such a deactivation pathway decreases the yield in Ph• and
D_1•+ in line with the very low efficiency of the D_2 based
systems in polymerization experiments (see below).
1
The free energy change ΔGet for the dye/Ph2I+ electron
transfer reaction is favorable according to the redox properties
of the reactants (e.g., Eox = 0.92 and 1.34 V for D_1 and D_2,
respectively [from this work; see Figure 12A]; Ered(Ph2I+) ∼
Figure 8. Photopolymerization profiles of TMPTA in laminated
conditions upon a laser diode exposure at 473 nm in the presence of
(1) D_1/Iod (0.3%/2% w/w) and (2) D_1/NVK/Iod (0.3%/3%/2%
w/w).
1
−0.2 V ; E(1D_1) ∼ 1.85 eV (Figure 12B) and E(1D_2) ∼
2.47 eV − from fluorescence experiments; free energy change
ΔG = −0.73 eV and −0.93 eV for D_1 and D_2, respectively).
In laser flash photolysis experiments, no transient ascribable to
a triplet state can be observed for D_1 in agreement with a S1
pathway.
As usually,26 these phenyl radicals (Ph•) are easily converted
into NVK radicals (NVK•) by an addition to the double bond
of NVK (r3). The formation of the NVK radicals (NVK•) upon
irradiation of D_1/Ph2I+/NVK (aN = 14.3 G; aH = 2.5 G in
agreement with13,26) is also observed (Figure 11C). As
previously discussed in other related systems,26,27 the NVK+
cations are easily formed (r4, r5) and will initiate the ROP of
the epoxide. Reaction r5 allows a regeneration of the dye and
brings an interesting photocatalyst character to D_1. For
cationic polymerization, dye•+ and Ph-NVK+ are the initiating
species whereas Ph• and Ph-NVK• can initiated free radical
polymerization.
dye → 1dye (hν)
Figure 9. Photopolymerization profiles of a TMPTA/EPOX blend
(50%/50%) in laminated conditions using D_1/NVK/Iod (0.3%/
3%/ 2% w/w). The evolutions of the acrylate and epoxide functions
were followed as presented in ref 10.
1
•
dye + Ph2I+ → dye•+ + Ph + Ph − I
(r1)
•
Ph2I• → Ph + Ph − I
(r2)
(r3)
•
the oxidation of D_1 by Iod leading to PhI, (D_1)•+ and a
phenyl radical Ph• (originating from the subsequent very fast
cleavage of Ph2I• in eq r2)1 as supported by ESR-ST and ESR
experiments. Ph• (Figure 11A) and D_1•+ (Figure 11B) are
Ph + NVK → Ph − NVK•
Ph − NVK• + Ph2I+ → Ph − NVK+ + Ph − I + Ph
•
(r4a)
Figure 10. Photolysis of D_1/Iod (A) or D_2/Iod (B) in acetonitrile, under air; laser diode exposure at 473 nm (identical light absorption
intensity); [Iod] = 0.04 M. The UV−visible spectra were recorded at the same irradiation times; for curve B, all the spectra are superposed.
3338
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341

Macromolecules
Article
Figure 11. (A) ESR-spin trapping spectrum of D_1/Iod; [Iod] = 0.01 M. (B) ESR spectrum of the cation radical (D_1)●+ at room temperature. (C)
ESR-spin trapping spectrum of D_1/Iod/NVK in tert-butylbenzene. Laser diode exposure at 473 nm. Experimental (a) and simulated (b) spectra.
Phenyl-N-tert-butylnitrone (PBN) is used as spin trap, under N2.
Figure 12. (A) Cyclic voltammogram for D_1 in acetonitrile. (B) (a) Absorption and (b) fluorescence spectra for D_2.
Ph − NVK• + dye•+ → Ph − NVK+ + dye
hybrid cure of acrylate/epoxide blends are feasible. The
synthesis of these dyes is relatively easy, their shell life stability
(r4b)
Ph − NVK+ + M → Ph − NVK − M+
is good and their solubility in organic formulations is excellent.
These push−pull structurally tailor-made compounds probably
open a new route for the design of multicolor photoinitiators
and the development of high performance panchromatic
monomer/oligomer films: works are under progress.
(r5)
CONCLUSION
■
In the present paper, original difunctional dyes exhibiting a
push−pull molecular character have been proposed as a new
series of photoinitiators. Their own unusual broad absorption
lying from the blue to the red allows polymerization reactions
under various laser diodes between 405 and 635 nm or low
light intensity visible light sources (for example, a halogen lamp
or a LED bulb). The free radical polymerization of a low
viscosity acrylate monomer (TMPTA), the cationic polymer-
ization of vinyl ethers and epoxides and the radical/cationic
ASSOCIATED CONTENT
■
S
* Supporting Information
Figure S1, UV−visible absorption spectra of D_1 for different
concentrations, and Figure S2, photopolymerization profiles of
TMPTA in laminated conditions upon a halogen lamp
irradiation in the presence of D_1/Ph2I+ or D_1/NVK/
3339
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341

Macromolecules
Article
Ph2I+. This material is available free of charge via the Internet at
http://pubs.acs.org/.
(6) (a) Kabatc, J.; Jurek, K. Polymer 2012, 53, 1973−1980.
(b) Czech, Z.; Butwin, A.; Kabatc, J. J. Appl. Polym. Sci. 2011, 120,
3621−3627. (c) Jedrzejewska, B.; Urbanski, S. J. Appl. Polym. Sci. 2010,
118, 1395−1405. (d) Kabatc, J.; Krzyzanowska, E.; Jedrzejewska, B.;
Pietrzak, M.; Paczkowski, J. J. Appl. Polym. Sci. 2010, 118, 165−172.
AUTHOR INFORMATION
■
Corresponding Author
(7) (a) Lalevee
initiators; Ackrine, W. J., Ed.; Nova Science Publishers, Inc.: chap. 8,
2010. (b) Lalevee, J.; Tehfe, M. A.; Blanchard, N.; Morlet-Savary, F.;
́
, J.; Tehfe, M. A.; Allonas, X.; Foussier, J. P. In Polymer
*E-mail: (J.L.) jacques.lalevee@uha.fr; (D.G.) didier.gigmes@
univ-amu.fr.
́
Fouassier, J. P. In Radical Polymerization: New Developments;
Notes
Pauloskas, I. O.; Urbonas, L. A. Eds., Nova Science Publishers, Inc.:
The authors declare no competing financial interest.
Hauppauge, NY, 2011; Chapter 7. (c) Fouassier, J. P.; Lalevee
́
, J. Adv.
Chem. 2012, 2, 2621−2629. (d) Lalevee, J.; Foussier, J. P. Overview of
́
radical initiation in Encyclopedia of Radicals in Chemistry, Biology and
Materials; Studer, A., Chatgilialoglou, C., Eds.; Wiley: New York, 2012;
Vol. 1, Chapter 2.
ACKNOWLEDGMENTS
■
This work was partly supported by the “Agence Nationale de la
Recherche” Grant ANR 2010-BLAN-0802. J.L. thanks the
Institut Universitaire de France for the financial support.
(8) (a) Guerlin, A.; Dumur, F.; Dumas, E.; Miomandre, F.; Wantz,
G.; Mayer, C. R. Org. Lett. 2010, 12, 2382−2385. (b) Dumur, F.;
́
Mayer, C. R.; Dumas, E.; Miomandre, F.; Frigoli, M.; Secheresse, F.
Org. Lett. 2008, 10, 321−324. (c) Dumur, F.; Mayer, C. R.; Hoang-
REFERENCES
(1) Fouassier, J. P.; Lalevee
Scope, Reactivity and Efficiency; Wiley-VCH: Weinheim, Germany,
2012.
■
Thi, K.; Ledoux-Rak, I.; Miomandre, F.; Clavier, G.; Dumas, E.;
́
, J.; Photoinitiators for Polymer Synthesis:
́ ́
Meallet-Renault, R.; Frigoli, M.; Zyss, J.; Secheresse, F. Inorg. Chem.
2009, 48, 8120−8133.
(9) (a) Li, H.; Ming Koh, T.; Hagfeldt, A.; Gratzel, M.; Mhaisalkar, S.
̈
(2) (a) Fouassier, J. P.; Photoinitiation, Photopolymerization and
Photocuring: Fundamental and Applications; Hanser Publishers: New
York, 1995. (b) Crivello, J. V. Photoinitiators for Free Radical, Cationic
and Anionic Photopolymerization, 2nd ed.; Bradley, G., Ed.; New York,
1998. (c) Dietliker, K. A Compilation of Photoinitiators commercially
available for UV today; Sita Technology Ltd: Edinburgh, London,
2002. (d) Photoinitiated Polymerization Belfied, K. D.; Crivello, J. V.;
Eds., ACS Symposium Series 847; American Chemical Society:
Washington DC, 2003. (e) Photochemistry and UV Curing; Fouassier,
J. P., Ed.; Research Signpost: Trivandrum, India, 2006; (f)
Photochemistry and photophysics of polymer materials; Allen, N. S.,
Ed.; Wiley: New York, 2010. (g) Basics of photopolymerization
reactions; Fouassier, J. P.; Allonas, X., Eds.; Researchsignpost:
Trivandrum India, 2010. (h) Green, W. A. Industrial Photoinitiators;
CRC Press: Boca Raton, FL, 2010.
(3) (a) Oster, G. K.; Oster, G.; Pratti, G. J. Am. Chem. Soc. 1957, 79,
595−599. (b) Timpe, H. J. In Radiation Curing in Polymer Science and
Technology; Fouassier, J. P., Ed.; Elsevier: Barking, U.K., 1993,; Vol. 2,
pp 529−554. (c) Urano, T. J. Photopolym. Sci. Technol. 2003, 16, 129−
156. (d) Eaton, D. Photogr. Eng. Sci 1979, 23, 150−154. (e) Monroe,
B. M.; Weed, G. C. Chem. Rev. 1993, 93, 435−448. (f) Cunningham,
A. F.; Desobry, V.; in “Radiation Curing in Polymer Science and
Technology”, Fouassier, J. P.; Rabek, J. F.; Eds., Elsevier: Barking (UK),
1993, 2, 323−374; (g) Fouassier, J. P.; Allonas, X.; Burget, D. Prog.
Org. Coating 2003, 47, 16−36. (h) Yagci, Y.; Jockusch, S.; Turro, N. J.
Macromolecules 2010, 65, 6245−6260. (i) Roloff, A.; Meier, K.;
Riediker, M. Pure Appl. Chem. 1986, 58, 1267−1273.
G.; Grimsdale, A. C. Chem. Commun. 2013, 49, 2409−2411.
(b) Oyama, Y.; Shimada, Y.; Inoue, S.; Nagano, T.; Fujikawa, Y.;
Komaguchi, K.; Imae, I.; Harima, Y. New J. Chem. 2011, 35, 111−118.
(c) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A.
J.; Bazan, G. C. Nat. Mater. 2007, 6, 497−500. (d) He, G. S.; Tan, L.
S.; Zheng, Q.; Prasad, P. N. Chem. Rev. 2008, 108, 1245−1330.
́
(10) Tehfe, M. A.; Lalevee, J.; Dumur, F.; Zein-Fakih, A.; Telitel, S.;
Gigmes, D.; Contal, E.; Bertin, D.; Morlet-Savary, F.; Graff, B.;
Fouassier, J. P. Eur. Polym. J. 2013, http://dx.doi.org/10.1016/j.
eurpolymj.2013.01.023.
(11) (a) Tehfe, M. A.; Lalevee
Gigmes, D.; Bertin, D.; Nechab, M.; Graff, B.; Morlet-Savary, F.;
Fouassier, J. P. Macromolecules 2012, 45, 4454−4460. (b) Lalevee, J.;
El-Roz, M.; Allonas, X.; Fouassier, J. C. J. Polym. Sci., Part A: Polym.
Chem. 2008, 46, 2008−2014. (c) Tehfe, M. A.; Lalevee, J.; Gigmes, D.;
Fouassier, J. P. Macromolecules 2010, 43, 1364−1370. (d) Tehfe, M.
A.; Lalevee, J.; Gigmes, D.; Fouassier, J. P. J. Polym. Sci., Part A: Polym.
Chem. 2010, 48, 1830−1837. (e) Souane, R.; Tehfe, M. A.; Lalevee, J.;
Gigmes, D.; Fouassier, J. P. Macromol. Chem. Phys. 2010, 211, 1441−
1445. (f) Lalevee, J.; Tehfe, M. A.; Gigmes, D.; Fouassier, J. P.
Macromolecules 2010, 43, 6608−6615.
(12) Lalevee, J.; Blanchard, N.; Tehfe, M. A.; Morlet-Savary, F.;
́
, J.; Telitel, S.; Contal, E.; Dumur, F.;
́
́
́
́
́
́
Fouassier, J. P. Macromolecules 2011, 43, 10191−10195.
(13) (a) Tordo, P. Spin-trapping: recent developments and applications;
Atherton, N. M.; Davies, M. J., Gilbert, B. C., Eds.; Electron Spin
Resonance 16; The Royal Society of Chemistry: Cambridge, U.K.,
1998. (b) Landolt Bornstein: Magnetic Properties of Free Radicals;
Fischer, H., Ed.; Springer Verlag: Berlin, 2005; Vol. 26d. (c) Chandra,
H.; Davidson, I. M. T.; Symons, M. C. R. J. Chem. Soc. Faraday Trans.
1 1983, 79, 2705−2711. (d) Alberti, A.; Leardini, R.; Pedulli, G. F.;
Tundo, A.; Zanardi, G. Gazz. Chim. Ital. 1983, 113, 869−871.
(14) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259−271.
(4) (a) Yilmaz, G.; Iskin, B.; Yilmaz, F.; Yagci, Y. ACS Macro Lett.
2012, 1, 1212−1215. (b) Karaka-Balta, D.; Temel, G.; Okal, N.; Yagci,
Y. Macromolecules 2012, 45, 119−125. (c) Yilmaz, G.; Acik, G.; Yagci,
Y. Macromolecules 2012, 45, 2219−2224. (d) Kumbaraci, V.; Aydogan,
B.; Talinli, N.; Yagci, Y. J. Polym. Sci., Part A: Polym. Chem. 2012, 50,
2612−2618. (e) Keskin Dogruyol, S.; Dogruyol, Z.; Arsu, N. J. Polym.
Sci., Part A: Polym. Chem. 2011, 49, 4037−4040. (f) Korkut, S. E.;
́
(15) Lalevee, J.; Dirani, A.; El Roz, M.; Graff, B.; Allonas, X.;
Fouassier, J. P. Macromolecules 2008, 41, 2003−2010.
(16) Reichardt, C. Chem. Rev. 1994, 94, 2319−2358.
(17) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J.
Org. Chem. 1983, 48, 2877−2887.
Temel, G.; Karaca Balta, D.; Arsu, N.; Kasım Şener, M. J. Lumin. 2013,
136, 389−394. (g) Muftuogli, A. E.; Tasdelen, M. A.; Yagci, Y. In
Handbook on photochemistry and photophysics of polymer materials;
Allen, N. S., Ed.; Wiley: New York, 2010; pp 509−540.
(18) Catalan, J. J. Org. Chem. 1997, 62, 8231−8234.
(19) Lippert, E. Z. Naturforsch. 1955, 10a, 541−545.
(20) Bakshiev, N. G. Opt. Spektrosk. 1964, 16, 821−832.
(21) Kawski, A. Acta Phys. Polon. 1966, 29, 507−518.
(22) McRae, E. G. J. Phys. Chem. 1957, 61, 562−572.
(23) Suppan, P. J. Chem. Soc. A 1968, 3125−3133.
(24) (a) Ooyama, Y.; Kushimoto, K.; Oda, Y.; Tokita, D.; Yamaguchi,
N.; Inoue, S.; Nagano, T.; Harima, Y.; Ohshita, J. Tetrahedron 2012,
68, 8577−8580. (b) Ooyama, Y.; Asada, R.; Inoue, S.; Komaguchi, K.;
(5) (a) Asvos, X.; Siskos, M. G.; Zarkadis, A. K.; Hermann, R.; Brede,
O. J. Photochem. Photobiol. A: Chem. 2011, 219, 255−264. (b) Santos,
W. G.; Schmitt, C. C.; Neumann, M. G. J. Photochem. and Photobiol. A:
Chem. 2013, 252, 124−130. (c) Kitano, H. K.; Ramachandran, K.;
Bowden, N. B.; Scranton, A. B. J. Appl. Polym. Sci. 2013, 128, 611−
618. (d) Han, J.; Wang, J.; Shen, K.; Wang, G.; Li, Y.; Zhao, D. J. Appl.
Polym. Sci. 2012, 126, 30−37. (e) Ivan, M. G.; Scaiano, J. C. In
Handbook on photochemistry and photophysics of polymer materials;
Allen, N. S., Ed.; Wiley: New York, 2010, 479−508.
3340
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341

Macromolecules
Article
Imae, I.; Harima, Y. New J. Chem. 2009, 33, 2311−2316. (c) Huang,
Y.; Cheng, T.; Li, F.; Luo, C.; Huang, C.; Cai, H. Z.; Zeng, X.; Zhon, J.
J. Phys. Chem. B 2002, 106, 10031−10040. (d) Abdel-Halim, S. T.; M
Awad, K. J. Mol. Struct. 2009, 920, 332−341. (e) Reichardt, C. Solvents
and Solvent Effects in Organic Chemistry: Wiley-VCH: Weinheim,
Germany, 2003.
(25) Kim, S. J.; Jang, H.-G.; Lee, J. K.; Min, H.-K.; Hong, S. B.; Seo,
G. Chem. Commun. 2011, 47, 9498−9500.
́
(26) Lalevee, J.; Dumur, F.; Mayer, C. R.; Gigmes, D.; Nase, G.;
Tehfe, M. A.; Telitel, S.; Morlet-Savary, F.; Graff, B.; Fouassier, J. P.
Macromolecules 2012, 45, 4134−4141.
(27) Hua, Y.; Crivello, J. V. J. Polym. Sci. A: Polym. Chem. 2000, 38,
3697−3709.
3341
dx.doi.org/10.1021/ma4005082 | Macromolecules 2013, 46, 3332−3341