Structural effects in the indanedione skeleton for the design of low intensity 300-500 nm light sensitive initiators.

Article abstract of DOI:10.1021/ma402149g

Newly synthesized indanedione derivatives combined with an iodonium salt, N-vinylcarbazole, amine, phenacyl bromide, or 2,4,6-tris(trichloromethyl)-1,3,5- triazine have been used as photoinitiating systems upon very low visible light intensities: blue lights (e.g., household blue LED bulb at 462 nm) or even a halogen lamp exposure. One of them (ID2) is particularly efficient for cationic, radical and thiol-ene photopolymerizations as well as for the synthesis of interpenetrated polymer networks (IPNs). It can be useful to overcome the oxygen inhibition. ID2 based photoinitiating systems can also be selected for the reduction of Ag⁺ and the in situ formation of Ag(0) nanoparticles in the synthesized polymers. The (photo)chemical mechanisms are studied by electron spin resonance spin trapping, fluorescence, cyclic voltammetry, laser flash photolysis, and steady state photolysis techniques.

Full text of DOI:10.1021/ma402149g

Article
pubs.acs.org/Macromolecules
Structural Effects in the Indanedione Skeleton for the Design of Low
Intensity 300−500 nm Light Sensitive Initiators.
Pu Xiao,† Frederic Dumur,‡,§,∥ Bernadette Graff,† Fabrice Morlet-Savary,† Loïc Vidal,† Didier Gigmes,*,‡
́
́
Jean Pierre Fouassier,⊥ and Jacques Lalevee*,†
́
†Institut de Science des Mater
‡Institut de Chimie Radicalaire, Aix-Marseille Universite,
́
iaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15 rue Jean Starcky, 68057 Mulhouse Cedex, France
CNRS, Institut de Chimie Radicalaire ICR, UMR 7273, F-13397 Marseille,
́
France
§Universite
́
Bordeaux, IMS, UMR 5218, F-33400 Talence, France
∥CNRS, IMS, UMR 5218, F-33400 Talence, France
S
* Supporting Information
ABSTRACT: Newly synthesized indanedione derivatives combined with an
iodonium salt, N-vinylcarbazole, amine, phenacyl bromide, or 2,4,6-tris-
(trichloromethyl)-1,3,5-triazine have been used as photoinitiating systems
upon very low visible light intensities: blue lights (e.g., household blue LED
bulb at 462 nm) or even a halogen lamp exposure. One of them (ID2) is
particularly efficient for cationic, radical and thiol−ene photopolymerizations as
well as for the synthesis of interpenetrated polymer networks (IPNs). It can be
useful to overcome the oxygen inhibition. ID2 based photoinitiating systems
can also be selected for the reduction of Ag+ and the in situ formation of Ag(0)
nanoparticles in the synthesized polymers. The (photo)chemical mechanisms
are studied by electron spin resonance spin trapping, fluorescence, cyclic
voltammetry, laser flash photolysis, and steady state photolysis techniques.
was inefficient, which shows a strong structure/reactivity
relationship for this new class of ID dyes.
INTRODUCTION
■
Dye is one of the most important components in monomer/
oligomer formulations photopolymerizable under visible lights
as it can lead, through interactions with suitable additives to
initiating radicals (for free radical polymerization (FRP) or
thiol−ene polymerization), cations or radical cations (for
cationic polymerization (CP) or free radical promoted cationic
polymerization (FRPCP)). It is of great interest to develop
novel high performance dyes for visible light sensitive
polymerization reactions for applications in radiation curing,
medicine, microelectronics, nanotechnology, imaging and
optics technologies, or material elaboration areas.1−4 The
introduction of suitable substituents into known UV light
sensitive compounds or colored structures as well as the design
of novel skeletons allows a red-shifted light absorption;
sometimes, it also significantly increases the molar extinction
coefficients ε. Recent examples of initiating systems can be seen
in refs 1 and 5−7, but relatively high light intensities are used
for the polymerization.
For a better understanding of the ID photochemical/
chemical reactivity, four new derivatives (an indanedione
moiety linked to an alkyl phenyl moiety (ID1), a carbazole
moiety (ID2), an anthracene moiety (ID3), or two indanedione
moieties linked to each other though a phenyl ring (ID4)) have
been designed and prepared (Scheme 1). The substituents are
expected to have different and probably better effects on the
light absorption properties, the involved chemical mechanisms
and subsequently the photoinitiating abilities for the CP of
epoxide and divinyl ether monomers, the FRP of acrylates, the
hybrid cure of acrylate/epoxide blends and the thiol-
divinylether photopolymerization under very soft irradiation
conditions (∼10 mW cm−2) using a halogen lamp or a
household blue LED (a 100 mW cm−2 blue laser diode will also
be employed to check intensity effects). The ID2-based
photoinitiating systems can also be used for UV LED
irradiations (e.g., 365, 385, or 395 nm). Remarkably, these
photoinitiating systems can also be used for the in situ
formation of Ag(0) nanoparticles in the synthesized polymers
through the reduction of a silver salt. The photochemical
mechanisms involved in the initiation species formation will be
In a recent study,8 two indanedione derivatives ID
(containing an aniline or a pyrene moietynoted D_1 and
D_2 in Scheme 1, respectively) have been developed as push−
pull photoinitiators. They are based on a D-π-A structure (D is
the electron donor moiety of the molecule and A the electron
acceptor) that allowed a blue-to-red light sensitivity and an
efficient polymerization ability for D_1. On the opposite, D_2
Received: October 18, 2013
Revised: November 6, 2013
© XXXX American Chemical Society
A
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Scheme 1. Chemical Structures of the New Studied Indanedione Derivatives (ID1−ID4) and Previously Investigated
Compounds D_1 and D_2
Scheme 2. Chemical Structures of Additives and Monomers
referenced to the solvent peak CDCl3 (77 ppm), DMSO (49.5 ppm).
All these indanedione derivatives were prepared with analytical purity
up to accepted standards for new organic compounds (>98%) which
was checked by high field NMR analysis.
investigated by electron spin resonance spin trapping,
fluorescence, cyclic voltammetry, laser flash photolysis, and
steady state photolysis techniques.
Synthesis of 2-(4-Tridecylbenzylidene)-1H-indene-1,3(2H)-dione
(ID1). Methyl 4-tridecylbenzoate,9 (4-tridecylphenyl)methanol, and
(4-tridecylphenyl)methanal10 used for the synthesis of ID1 were
prepared as previously reported. 4-Tridecylbenzaldehyde (3.94 g,
13.66 mmol) and indane-1,3-dione (2 g, 13.66 mmol) were dissolved
in absolute ethanol (100 mL). A few drops of piperidine were added.
Immediately, the solution turned purple. The solution was refluxed for
4 h. After cooling, the precipitate was filtered off, washed several times
EXPERIMENTAL SECTION
■
Materials. The investigated indanedione derivatives IDn (i.e.,
ID1−ID4) and other chemical compounds are shown in Schemes 1
and 2. Diphenyliodonium hexafluorophosphate (Iod), N-vinylcarba-
zole (NVK), methyl diethanolamine (MDEA), phenacyl bromide (R−
Br), 2,4,6-tris(trichloromethyl)-1,3,5-triazine (R′−Cl), tri(ethylene
glycol) divinyl ether (DVE-3), trimethylolpropane tris(3-mercapto-
propionate) (Trithiol), and the other reagents and solvents were
purchased from Sigma-Aldrich or Alfa Aesar with the highest purity
available and used as received. The monomers (3,4-
epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX)
and trimethylolpropane triacrylate (TMPTA) were obtained from
Cytec and used as benchmark monomers for cationic and radical
photopolymerizations.
1
with ether, and dried under vacuum (5.35 g, 94% yield). H NMR
(CDCl3), δ (ppm): 0.88 (t, 3H, J = 6.6 Hz), 1.26−1.32 (m, 20H), 1.65
(qt, 2H, J = 7.0 Hz), 2.69 (t, 2H, J = 7.6 Hz), 7.33 (d, 2H, J = 8.2 Hz),
7.80−7.82 (m, 2H), 7.89 (s, 1H), 7.99−8.02 (m, 2H), 8.42 (d, 2H, J =
8.2 Hz). 13C NMR (CDCl3), δ (ppm): 14.1, 22.7, 29.31, 29.35, 29.47,
29.55, 29.64, 29.67, 31.0, 31.9, 36.3, 123.22, 123.25, 128.2, 129.0,
130.8, 134.5, 135.0, 135.2, 140.1, 142.5, 147.2, 149.7, 189.2, 190.5;
HRMS (ESI MS) m/z: theor, 417.2788; found, 417.2790 ([M + H]+
detected).
Synthesis of 2-((9-Octyl-9H-carbazol-3-yl)methylene)-1H-indene-
1,3(2H)-dione (ID2). 9-Octyl-9H-carbazole-3-carbaldehyde used for
the synthesis of ID2 was synthesized as reported without modification
and in similar yield.11
9-Octyl-9H-carbazole-3-carbaldehyde (4.21 g, 13.69 mmol) and
indane-1,3-dione (2 g, 13.69 mmol) were dissolved in absolute ethanol
(200 mL). A few drops of piperidine were added. Immediately, the
solution turned purple. The solution was refluxed for 4 h. After
cooling, the precipitate was filtered off, washed several times with ether
and dried under vacuum. A few conversion of the aldehyde was
Synthesis of the Indanedione Derivatives. 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
1
Eager 300 software. H 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).
1
The H chemical shifts were referenced to the solvent peak CDCl3
(7.26 ppm), DMSO (2.49 ppm) and the 13C chemical shifts were
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detected. Indane-1,3-dione (2 g, 13.69 mmol) was added to the former
precipitate; 100 mL absolute ethanol was added followed by a few
drops of piperidine. The solution was refluxed overnight. The solvent
was removed under reduced pressure. The residue was purified by
column chromatography (SiO2) using DCM:pentane 1:1 as the eluent.
The product was eluted as the first fraction. After evaporation of the
volatiles, dissolution in ether followed by addition of pentane
precipitated a light yellow solid which was filtered off, washed several
Redox Potentials. The oxidation potentials (Eox vs SCE) of the
IDs were measured in acetonitrile by cyclic voltammetry with
tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting
electrolyte (Voltalab 6 Radiometer). The working electrode was a
platinum disk and the reference electrode was a saturated calomel
electrode (SCE). Ferrocene was used as a standard, and the potentials
determined from the half peak potential were calibrated to the
reversible formal potential of this compound (+0.44 V/SCE). The free
energy change ΔG for an electron transfer between the studied IDs
and Iod can be easily calculated from the classical Rehm−Weller
equation (eq I, where Eox, Ered, ES (or ET), and C are the oxidation
potential of the studied indanedione derivatives, the reduction
potential of Iod, the excited singlet (or triplet) state energy of the
studied indanedione derivatives, and the electrostatic interaction
energy for the initially formed ion pair, generally considered as
negligible in polar solvents):16
1
times with pentane and dried under vacuum (5.12 g, 86% yield). H
NMR (CDCl3), δ (ppm): 0.85 (t, 3H, J = 6.8 Hz), 1.23−1.33 (m,
10H), 1.86 (qt, 2H, J = 6.7 Hz), 4.26 (t, 2H, J = 7.1 Hz), 7.28−7.50
(m, 4H), 7.72−7.75 (m, 2H), 7.93−7.97 (m, 2H), 8.03 (s, 1H), 8.21
(d, 1H, J = 7.7 Hz), 8.62 (d, 1H, J = 8.6 Hz), 9.39 (s, 1H). 13C NMR
(CDCl3), δ (ppm): 14.1, 22.6, 27.3, 28.9, 29.1, 29.3, 31.8, 43.4, 108.9,
109.4, 120.5, 121.0, 122.8, 123.2, 123.6, 124.9, 125.4, 126.6, 128.7,
133.3, 134.5, 134.8, 139.9, 141.0, 142.4, 143.7, 148.7, 189.7, 191.1.
HRMS (ESI MS) m/z: theor, 435.2198; found, 435.2199 (M+•
detected).
ΔG = Eox − Ered − ES (or ET) + C
(I)
Synthesis of 2-(Anthracen-9-ylmethylene)-1H-indene-1,3(2H)-
dione (ID3). 9-Anthracenealdehyde (2.82 g, 13.67 mmol) and
indane-1,3-dione (2 g, 13.67 mmol) were dissolved in absolute
ethanol (100 mL). A few drops of piperidine were added, and the
reaction mixture was refluxed for 3 h. The solvent was removed under
reduced pressure. The residue was purified by column chromatog-
raphy (SiO2) using DCM as the eluent. The product was eluted as the
first band (3.74 g, 82% yield). 1H NMR (CDCl3), δ (ppm): 7.48−7.53
(m, 4H), 7.81−7.85 (m, 3H), 7.97−8.00 (m, 2H), 8.05−8.08 (m, 2H),
8.12−8.15 (m, 1H), 8.58 (s, 1H), 8.91 (s, 1H). 13C NMR (CDCl3), δ
(ppm): 123.5, 123.6, 125.4, 125.5, 126.8, 129.1, 129.9, 130.4, 131.0,
134.0, 135.4, 135.6, 140.8, 141.7, 142.7, 187.5, 189.3. HRMS (ESI MS)
m/z: theor, 334.0994; found, 334.0997 (M+• detected).
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.17,18
Laser Flash Photolysis. Nanosecond laser flash photolysis (LFP)
experiments were carried out using a Q-switched nanosecond Nd/
YAG laser at λexc = 355 nm (9 ns pulses; energy reduced down to 10
mJ; minilite Continuum); the analyzing system (Luzchem LFP 212)
consisted of a ceramic xenon lamp, a monochromator, a fast
photomultiplier and a transient digitizer.19
Transmission Electron Microscopy (TEM). Observations were
conducted on a Philips CM200 microscope operating at an
acceleration voltage of 200 kV. For metal nanoparticles in acetonitrile,
the suspension was deposited on a carbon coated copper grid. Once
the acetonitrile was evaporated at room temperature, it left a
distribution of particles on the support.
Synthesis of 2,2′-(1,4-Phenylenebis(methanylylidene))bis(1H-in-
dene-1,3(2H)-dione) (ID4). Terephthaldehyde (0.91 g, 6.78 mmol)
and indane-1,3-dione (2 g, 13.56 mmol, 2 equiv) were dissolved in
absolute ethanol (100 mL). A few drops of piperidine were added.
Immediately, the solution turned bordeaux red. The solution was
refluxed for 4 h. After cooling, the yellow precipitate was filtered off,
washed several times with ether, and dried under vacuum (1.95 g, 74%
RESULTS AND DISCUSSION
■
1. Light Absorption Properties of the Studied
Indanedione Derivatives. The absorption spectra of the
investigated IDn (ID1-ID3) in acetonitrile are given in Figure
1. The difunctional derivative ID4 whose absorption maxima is
1
yield). H NMR (DMSO-d6), δ (ppm): 7.95−8.06 (m, 8H), 8.63 (s,
4H). HRMS (ESI MS) m/z: theor, 390.0892; found, 390.0888 (M+•
detected). Anal. Calcd for C26H14O4: C, 80.0; H, 3.6; O, 16.4. Found:
C, 80.1; H, 3.4; O, 16.7.
Irradiation Sources. Different visible irradiation sources were
used for the photopolymerization experiments: (i) polychromatic light
from a halogen lamp (Fiber-Lite, DC-950; incident light intensity ∼ 12
mW cm−2 in the 370−800 nm range), (ii) laser diode at 457 nm (100
mW cm−2), (iii) a low intensity 385 nm LED (ML385-L2 −
ThorLabs; ∼9 mW cm−2), and (iv) a household blue LED bulb
centered at 462 nm (∼10 mW cm−2).
Photopolymerization Experiments. For photopolymerization
experiments, the conditions are given in the figure captions. The
photosensitive formulations were deposited on a BaF2 pellet under air
or in laminate (25 μm thick) for irradiation with different lights. The
evolution of the epoxy group content of EPOX, the double bond
content of TMPTA, the double bond content of DVE-3 and the thiol
(S−H) content of Trithiol were continuously followed by real time
FTIR spectroscopy (JASCO FTIR 4100)12,13 at about 790, 1630,
1620, and 2580 cm−1, respectively. The conversions of NVK were not
measured in TMPTA.
ESR Spin Trapping (ESR-ST) Experiments. ESR-ST experiments
were carried out using a X-Band spectrometer (MS 400 Magnettech).
The radicals were generated at room temperature upon visible light
exposure (e.g., halogen lamp) under N2 and trapped by phenyl-N-tert-
butylnitrone (PBN) according to a procedure14 described elsewhere in
detail. The ESR spectra simulations were carried out with the
WINSIM software.
Figure 1. UV−vis absorption spectra of the studied indanedione
derivatives in acetonitrile.
located at 389 nm (ε > 26900 M−1 cm−1) has a poor solubility
in organic solvents (e.g., acetonitrile, acetone, etc.) probably
due to its bulky and symmetric chemical structure. The
absorption of ID1 is mainly located in the UV range (λmax
=
351 nm, ε351 nm = 29900 M−1 cm−1) and exhibits a little
overlapping with the emission spectra of the halogen lamp.
Interestingly, ID2 and ID3 show a better absorption in the
visible wavelength range (λmax = 448 nm, ε448 nm = 42200 M−1
Fluorescence Experiments. The fluorescence properties of the
investigated IDs were studied in acetonitrile using a JASCO FP-750
spectrometer. The interaction IDs/Iod can be investigated through
classical Stern−Volmer treatments.15
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Figure 2. Highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) for ID1−ID4 at UB3LYP/6-31G*
level (isovalue = 0.04).
cm−1 and λmax = 467 nm, ε467 nm = 7700 M−1 cm−1 for ID2 and
ID3, respectively) than that of an indanedione, a carbazole or
an anthracene moiety, which is due to the important charge
transfer occurring in the electronic transition.8 Compared to
our previously studied D_1 derivative (λmax = 478 nm, ε478 nm
=
=
38000 M−1 cm−1),8 the ε of ID2 is even higher (e.g.: ε448 nm
42200 M−1 cm−1). The ID2 absorption spectrum allows an
interesting matching with the emission spectra of the different
light sources: the halogen lamp, the laser diodes at 457 nm
(ε457 nm = 38600 M−1 cm−1) and 405 nm (ε405 nm = 18 600 M−1
cm−1), the household blue LED bulb centered at 462 nm
(ε462 nm = 33900 M−1 cm−1) and UV LEDs at 385 nm (ε385 nm
= 9600 M−1 cm−1) or 395 nm (ε395 nm = 13 300 M−1 cm−1). As
to ID3, an acceptable matching is also observed but to a lesser
extent (e.g.: ε457 nm = 7500 M−1 cm−1).
The highest occupied molecular orbitals (HOMO) are
mainly located on the carbazole and anthracence moieties for
ID2 and ID3, respectively; the lowest unoccupied molecular
orbitals (LUMO) lie on the indanedione skeleton. Therefore,
the excellent visible light absorption properties of ID2 and ID3
are associated with the high charge transfer character of the
lowest energy transition; this π → π* band is associated with
the HOMO → LUMO transition (Figure 2).
2. Photoinitiating Ability of the Investigated Indane-
dione Derivatives. 2a. Cationic Photopolymerization of
Epoxides. Typical conversion−time profiles for the photo-
polymerization of EPOX in the presence of IDn/Iod or IDn/
Iod/NVK PISs under air are displayed in Figure 3 (halogen
lamp or 457 nm laser diode; conversions for 800 s of irradiation
summarized in Table 1; Iod alone is not efficient as it works
below 300 nm15,20). In the presence of ID1 (or ID3)/Iod or
ID1 (or ID3)/Iod/NVK combinations, no polymerization of
EPOX was observed (too low absorption, see Figure 1). The
ID4/Iod does not work (halogen lamp irradiation), but the
addition of NVK (as elsewhere in other systems21) significantly
enhances the polymerization profile but the conversion remains
low (33% at t = 800 s; Figure 3, curve 4). Interestingly, ID2/
Iod and ID2/Iod/NVK exhibit a very good efficiency and lead
to the formation of tack free coatings with conversion of 56%
Figure 3. Photopolymerization profiles of EPOX under air in the
presence of ID2/Iod (0.5%/2%, w/w) upon the halogen lamp (curve
1); ID2/Iod/NVK (0.5%/2%/3%, w/w/w) upon the halogen lamp
(curve 2) and laser diode at 457 nm (curve 3) exposure; ID4/Iod/
NVK (0.5%/2%/3%, w/w/w) upon the halogen lamp exposure (curve
4).
Table 1. EPOX Conversions Obtained under Air upon
Exposure to Different Visible Light Sources for 800 s in the
Presence of IDn/Iod (0.5%/2%, w/w) or IDn/Iod/NVK
a
(0.5%/2%/3%, w/w/w)
IDn
halogen lamp
bc
laser diode 457 nm
,
ID1
ID2
ID3
ID4
np
−
59%
b
c
c
56%, 64%
bc
c
,
np
np
b
c
np, 33%
−
c
a
b
np: no polymerization. IDn/Iod (0.5%/2%, w/w). IDn/Iod/NVK
(0.5%/2%/3%, w/w/w).
and 64% at t = 800 s (halogen lamp; Figure 3, curve 1 and
curve 2). A higher polymerization rate with a little lower
conversion (59%; Figure 3, curve 3 vs curve 2) is noted with
ID2/Iod/NVK at 457 nm as resulting from the higher light
intensity of the diode laser (100 mW cm−2). The new ID2/Iod
and ID2/Iod/NVK combinations are better than the previously
D
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Figure 4. Photopolymerization profiles of TMPTA in laminate in the presence of (i) ID2/Iod (0.5%/2%, w/w) upon the halogen lamp (curve 1) or
the laser diode at 457 nm (curve 2) exposure; ID2/Iod/NVK (0.5%/2%/3%, w/w/w) at 457 nm exposure (curve 3); (ii) ID2/MDEA (0.5%/2%, w/
w) upon the halogen lamp (curve 4) or at 457 nm (curve 5); ID2/MDEA/R−Br (0.5%/2%/3%, w/w/w) at 457 nm (curve 6); (iii) ID2/R′−Cl
(0.5%/1%, w/w) (curve 7); ID2/MDEA/R′−Cl(1) (0.5%/2%/1%, w/w/w) (curve 8); ID2/MDEA/R′−Cl(3) (0.5%/2%/3%, w/w/w) (curve 9) at
457 nm; (iv) CQ/Iod (0.5%/2%, w/w) or CQ/MDEA (0.5%/2%, w/w) as references at 457 nm.
reported D_1/Iod or D_1/Iod/NVK systems (halogen lamp;
conversion <35% at t = 800 s).8 Remarkably, in the same
conditions, the known CQ (camphorquinone)/Iod1 or even
CQ/Iod/NVK systems are not efficient. ID2/Iod/NVK can
also be used for UV LED exposure at 385 nm leading to tack
free coatings (61% of conversion for 1000s of irradiation); this
can be ascribed to the excellent light absorption properties of
ID2 in the 300−400 nm (λ > 4200 M−1 cm−1; Figure 1). The
ID2/Iod system can also initiate the cationic polymerization of
DVE-3 (see below the thiol−ene process).
Table 2. TMPTA Conversions Obtained in Laminate upon
Exposure to Different Visible Light Sources for 400 s in the
Presence of ID2/Iod (0.5%/2%, w/w); ID2/Iod/NVK
(0.5%/2%/3%, w/w/w); ID2/MDEA (0.5%/2%, w/w); ID2/
MDEA/R−Br (0.5%/2%/3%, w/w/w); ID2/R′−Cl (0.5%/
1%, w/w); ID2/MDEA/R′−Cl(1) (0.5%/2%/1%, w/w/w);
ID2/MDEA/R′−Cl(3) (0.5%/2%/3%, w/w/w); CQ/Iod
(0.5%/2%, w/w); or CQ/MDEA (0.5%/2%, w/w)
% conversion
halogen lamp
laser diode (457 nm)
2b. Free Radical Photopolymerization of Acrylates. The
ID2/Iod or ID2/MDEA combinations slowly initiate the FRP
of TMPTA in laminate using the low intensity halogen lamp
(12 mW cm−2) as shown in Figure 4, curve 1 and curve 4 for
ID2/Iod (conversion = 8%) and ID2/MDEA (conversion =
23%), respectively. Under the higher intensity laser diode at
457 nm (100 mW cm−2), the polymerization profiles are
improved (Figure 4, curve 2 vs curve 1 for ID2/Iod; curve 5 vs
curve 4 for ID2/MDEA). The ID2/Iod two-component system
is better than CQ/Iod (conversion = 45% vs 35%) under the
laser diode at 457 nm. The ID2/Iod/NVK and ID2/MDEA/
R−Br (or R′−Cl) three-component systems are still more
efficient (see Figure 4 and Table 2); the ID2/R′−Cl system can
also work but to a lesser extent (conversion =16%; Figure 4,
curve 7). Interestingly, R′−Cl is a better additive than R−Br
(Figure 4, curve 9 vs curve 6) and an increase of [R′−Cl]
improves the polymerization profile (Figure 4, curve 9 vs curve
8). In addition, the ID2/MDEA/R′−Cl combination (Figure 4,
curve 8 and 9) exhibits a better efficiency than CQ/MDEA
reference.
2c. Hybrid Cure: Photopolymerization of EPOX/TMPTA
Blends. The ID2/Iod/NVK combination allows the formation
of an interpenetrated polymer network (IPN) through a
concomitant cationic/radical photopolymerization of an
EPOX/TMPTA blend (50%/50% w/w) upon the household
blue LED bulb exposure at 462 nm (Figure 5). Tack free
coatings can be obtained after only 5 min. Although the final
conversions (Table 3) of TMPTA (as observed in other
systems6,7,22) are higher in laminate than under air, the
observed high efficiency under air upon very low light intensity
highlights the ability of the ID2 based systems to overcome the
oxygen inhibition in a FRP process.
CQ/Iod
18
8
35
45
58
46
35
42
16
52
60
ID2/Iod
ID2/Iod/NVK
CQ/MDEA
−
35
23
−
−
−
−
ID2/MDEA
ID2/MDEA/R−Br
ID2/R′−Cl
ID2/MDEA/R′−Cl(1)
ID2/MDEA/R′−Cl(3)
2d. Thiol−Ene Photopolymerization. Thiol−ene photo-
polymerizations were mainly based on UV light. Remarkably,
excellent conversion−time profiles are here obtained for the
Trithiol/DVE-3 thiol−ene photopolymerization using the ID2/
Iod combination under the household blue LED and the blue
laser diode exposure (Figure 6). Even though the incident light
intensity of the LED is 10 times lower than that of the laser
diode, the polymerization profiles are quite similar (vinyl
double bond conversions ∼98% in both cases). The S−H
conversions (46% and 49%) are lower which probably reflects a
concomitant CP polymerization of DVE-3. Remarkably, the
polymerization of thick samples (from 1 mm to 5 mm) can also
be achieved with the ID2/Iod system upon LED irradiation;
this can be highly useful for applications requiring deep curing.
3. Photochemical Mechanisms of the Investigated
Indanedione Derivatives. 3a. ID/Iod and ID/Iod/NVK
Systems. As demonstrated in other related PISs,1,6−8 the
expected reactions in the ID based systems (1−4) are likely the
following:
ID → 1ID (hν) and 1ID → ID
3
(1)
E
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Figure 5. Photopolymerization profiles of an EPOX/TMPTA blend (50%/50%, w/w) in the presence of ID2/Iod/NVK (0.5%/2%/3%, w/w/w)
under air (a) and in laminate (b) upon the blue LED bulb exposure at 462 nm.
Table 3. EPOX and TMPTA Final Conversions Obtained in
the Polymerization of an EPOX/TMPTA Blend (50%/50%,
w/w) under Air or in Laminate upon Exposure to the
Household LED at 462 nm (t = 800 s) in the Presence of
ID2/Iod/NVK (0.5%/2%/3%, w/w/w)
EPOX conversion (%)
TMPTA conversion (%)
under air
71
44
52
73
in laminate
1,3ID + Ph2I+ → ID•+ + Ph2I•
(2a)
•
ID•+ + Ph2I• → ID•+ + Ph + Ph−I
(2b)
(3a)
•
Ph + NVK → Ph−NVK•
Figure 7. ESR spectra of the radicals generated in ID2/Iod and
trapped by PBN in tert-butylbenzene: (a) experimental and (b)
simulated spectra. PBN/phenyl radical adducts obtained in ID2/Iod:
aN = 14.1 G; aH = 2.1 G; reference values.23,24
•
Ph−NVK• + Ph2I+ → Ph−NVK+ + Ph + Ph−I
(3b)
(4a)
•
Ph + RS − H → Ph−H + RS•
RS• + R′−CHCH2 → R′−CH•−CH2SR
(4b)
potentials Eox of the ID2 = 0.64 V, as measured by cyclic
voltammetrythis work; reduction potential Ered = −0.2 V1 for
Iod; singlet state energy ES =2.55 eV for ID2 as extracted from
the UV−vis absorption and fluorescence emission spectra as
usually done25). The results of the fluorescence quenching
experiments were not reported due to the low fluorescence
quantum yield of ID2 (∼0.001). As in the previously studied
indanedione derivatives,8 no triplet state absorption is observed
R′−CH•−CH2SR + RSH → R′−CH2−CH2SR + RS•
(4c)
In line with reaction 2, phenyl radicals are clearly observed in
ESR spin trapping experiments on ID2/Iod (Figure 7).
The calculations of the free energy change ΔG for the
corresponding electron transfer reaction 2a support a favorable
singlet state process (ΔG = −1.71 eV using: oxidation
3
in laser flash photolysis experiments. However, a ID2/Iod
Figure 6. Photopolymerization profiles of Trithiol/DVE-3 blend (40%/60%, n/n; 57%/43%, w/w) in laminate in the presence of ID2/Iod (0.5%/
2%, w/w) upon the (a) blue laser diode at 457 nm and (b) blue LED bulb at 462 nm exposure. Curve 1: DVE-3 (vinyl double bond) conversion.
Curve 2: trithiol (S−H) conversion.
F
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Figure 8. Steady state photolysis of (a) ID2/Iod and (b) ID3/Iod in acetonitrile upon the halogen lamp exposure. [Iod] = 45 mM. UV−vis spectra
recorded at different irradiation times.
electron transfer in the triplet state should be feasible as ΔG is
negative (−1.34 eV; the triplet state energy level is 2.18 eV as
calculated at UB3LYP/6-31G* level). Contrary to ID2, no
fluorescence is detected in ID1, ID3 and ID4.
The steady state photolysis of ID2/Iod and ID3/Iod in
acetonitrile (Figure 8; halogen lamp exposure; under air)
confirms a noticeable reactivity difference (very fast bleaching
of ID2/Iod and tiny bleaching of ID3/Iod) which is responsible
for the relative polymerization abilities. Similarly, the very little
bleaching of ID1/Iod and the increase of the light absorption at
386 nm in ID4/Iod (Figure S1 in the Supporting Information)
are also consistent with their poor efficiency.
higher yield in reactive species (Ph• and ID•+; reaction 2) and
(iii) the higher reactivity of the radical cation (ID2•+) in CP
and FRPCP processes. In ID2 based PISs, the initiating species
are: ID2•+ and Ph-NVK+ (reactions 1−3)21 for the CP/FRPCP
of EPOX; the Ph•, Ph-NVK•, aminoalkyl (MDEA (‑H)•) and R′•
(or R•) radicals for the FRP of TMPTA (reactions 1−3, 5-9);
the thiyl radicals (reactions 1−2, 4) for the thiol−ene
photopolymerization.
3d. ID2/Iod/NVK System: A Photoinitiating System for the
Ag(0) Nanoparticles Formation. Interestingly, upon a low
intensity household blue LED exposure, the ID2/Iod/NVK
system in the presence of a silver salt (AgSbF6) is also able to
generate Ag(0) nanoparticles NP (Figure 9). Remarkably, the
3b. ID/Amine/Alkyl Halide System. ID2/MDEA, ID2/
MDEA/R−Br, and ID2/MDEA/R′−Cl systems can work
according to reactions 5−9 as already proposed in other dye
systems.13,26−28 Reactions 5 and 6 are probably in competition
as their free energy changes ΔG for the corresponding electron
transfer reaction are favorable: −1.13 eV for 1ID2/R−Br; −0.76
3
1
eV for ID2/R−Br; −0.73 eV for ID2/MDEA; −0.36 eV for
3ID2/MDEA. As the reduction potentials of R−Br29 and R′−
Cl30 are similar (−0.78 V), reactions 6−8 probably occur for
R−Br and R′−Cl. The higher efficiency of ID2/MDEA/R′−Cl
compared to ID2/MDEA/R−Br in FRP probably results from
a better reactivity of the generated radicals (R′• vs R•) toward
the initiation of the polymerization reaction.
•+
1,3ID2 + MDEA → ID2•− + MDEA
→ ID2 − H• + MDEA(−H)
•
Figure 9. Observed TEM micrograph for the irradiation of a ID2/Iod/
NVK/AgSbF6 solution in acetonitrile (blue LED bulb at 462 nm;
exposure 10 min).
(5)
1,3ID2 + R′−Cl (or R−Br)
→ ID2•+ + (R′−Cl)•− [or (R−Br)•−]
(6)
ID2/Iod/NVK/AgSbF6 system is able to both initiate the
polymerization reactions (see above) and generate the Ag(0)
NPs, i.e., the synthesized Ag(0) NPs are in situ embedded in
the synthesized polymers. As shown recently,31 the reduction of
the silver salt (10) can be associated with the easy oxidation of
Ph-NVK• (generated in 3a) as no Ag(0) NP formation is
observed when using the ID2/Iod/AgSbF6 system.
ID2•− + R′−Cl (or R−Br)
→ ID2 + (R′−Cl)•− [or (R−Br)•−]
(7)
ID2−H• + R′−Cl (or R−Br)
→ ID2 + H+ + (R′−Cl)•− [or (R−Br)•−]
(R′−Cl)•− [or (R−Br)•−] → R′• + Cl− [or R• + Br−]
(8)
Ph−NVK• + Ag+ → Ph−NVK+ + Ag(0)
(10)
(9)
CONCLUSION
■
3c. Initiation Mechanisms. On the basis of the above
investigations, the higher polymerization efficiency of ID2/Iod
compared to that of ID1 (or ID3, ID4)/Iod may be associated
with (i) the better light absorption properties of ID2, (ii) the
The newly developed indanedione derivative ID2 in combina-
tion with an iodonium salt Iod and optionally N-vinylcarbazole
NVK can be used as a high performance visible light or blue
light sensitive PIS to efficiently initiate the cationic polymer-
G
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2013, 54, 3182−3187. (p) Podsiadły, R.; Strzelczyk, R. Dyes Pigm.
2013, 97, 462−468. (q) Shen, K.; Li, Y.; Liu, G.; Li, Y.; Zhang, X. Prog.
Org. Coat. 2013, 76, 125−130. (r) Xiao, Q.; Ji, Y.; Xiao, Z.; Zhang, Y.;
Lin, H.; Wang, Q. Chem. Commun. 2013, 49, 1527−1529.
ization of EPOX, the radical polymerization of TMPTA, the
TMPTA/EPOX blend IPN polymerization, and the thiol−ene
polymerization as well under low light intensities. In addition,
the ID2/amine/phenacyl bromide or the ID2/amine/2,4,6-
tris(trichloromethyl)-1,3,5-triazine combinations are also effi-
cient for the radical polymerization of TMPTA. Moreover, ID2
is more efficient than the well-known visible light sensitizer CQ
and can be useful to overcome the oxygen inhibition. The
proposed structure is also very efficient for the in situ formation
of Ag(0) nanoparticles. This present research confirms that the
design of push−pull photoinitiators is a promising way to
develop highly absorbing compounds in the blue wavelength
range which ensures polymerization reactions under low
intensity light sources.
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ASSOCIATED CONTENT
■
Fouassier, J. P.; Lalevee
2201.
́
, J. Macromol. Chem. Phys. 2013, 214, 2189−
S
* Supporting Information
Figure S1, steady state photolysis of ID1/Iod and ID4/Iod in
acetonitrile. This material is available free of charge via the
Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Authors
* E-mail: jacques.lalevee@uha.fr (J.L.).
* E-mail: didier.gigmes@univ-amu.fr (D.G.).
́
■
́
Present Address
⊥Formerly at: ENSCMu-UHA, 3 rue Alfred Werner, 68093
Mulhouse Cedex, France.
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Notes
The authors declare no competing financial interest.
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.;
Stratmann, J. R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford,
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ACKNOWLEDGMENTS
J.L. thanks the Institut Universitaire de France for the financial
support.
■
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