Naphthalimide Aryl Sulfide Derivative Norrish Type i Photoinitiators with Excellent Stability to Sunlight under Near-UV LED

Article abstract of DOI:10.1021/acs.macromol.8b02309

A series of Norrish type I photoinitiators (NASs), which are naphthalimide aryl sulfide derivatives, are prepared. The potential mechanism involved in the photolysis of NASs under UV LED at 405 nm is investigated by steady-state photolysis, nuclear magnet

Full text of DOI:10.1021/acs.macromol.8b02309

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Naphthalimide Aryl Sulfide Derivative Norrish Type I Photoinitiators
with Excellent Stability to Sunlight under Near-UV LED
Jia Yu,‡ Yanjing Gao,‡ Shengling Jiang,§ and Fang Sun*,†,‡
†State Key Laboratory of Chemical Resource Engineering, ‡College of Science, and §College of Materials Science and Engineering,
Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China
S
* Supporting Information
ABSTRACT: A series of Norrish type I photoinitiators (NASs),
which are naphthalimide aryl sulfide derivatives, are prepared. The
potential mechanism involved in the photolysis of NASs under UV
LED at 405 nm is investigated by steady-state photolysis, nuclear
magnetic resonance, electron spin resonance, fluorescence spec-
troscopy, cyclic voltammetry, and laser flash photolysis and by
calculating the bond dissociation energies of the C−S bonds of
NASs. The as-prepared photoinitiators NAS5 and NAS6 can
efficiently initiate free radical photopolymerization of (methyl)-
acrylate monomers under UV LED exposure at 405 nm. NASs/
iodonium salt systems can initiate the cationic photopolymerization of epoxide. Interestingly, NASs exhibit an excellent stability
to sunlight. Thus, the use of NASs makes the preparation, storage, and application of photocurable formulations convenient.
cine,15 and cleavable thioxanthones containing a disilylacety-
1. INTRODUCTION
lene moiety.16 These photoinitiators efficiently initiate photo-
The design and development of photoinitiators, which can
polymerization under near-UV or visible LEDs but still show
initiate photopolymerization of photocurable systems under
some shortcomings involving high price, no initiating ability
light-emitting diodes (LEDs) irradiation, have been attracting
for free radical photopolymerization of low viscosity
increasing attention in recent years because LEDs have many
monomers, and low molar extinction coefficients at wave-
advantages, including low power consumption, long lamp life,
instant on−off function, low total cost reduction of ownership,
and environmental preservation in comparison with traditional
lengths longer than 385 nm. Therefore, the design and
development of novel and versatile type I photoinitiators with
mercury UV lamps.1,2 As key components in photocurable
good absorption capacities in near-UV or visible LEDs
wavelength range are important and urgent to the photocuring
systems under LEDs irradiation, photoinitiators have the
absorption wavelength matching to the emission wavelength of
industry.
LEDs.3 Thus, photoinitiators can produce initiating species
The near-UV or visible LEDs emission wavelength is above
(e.g., radicals or cations) under LEDs irradiation.4 To achieve
365 nm (e.g., 385, 395, 405, and 455 nm). Therefore, the
photoinitiators must have good absorption capacities in the
this goal, much effort has recently been made to develop new
LEDs emission range and weak chemical bond, which can
Norrish type II photoinitiators with good absorption in the
easily break under LEDs irradiation, to produce free radicals.
LEDs emission range. Doing so obtains multicomponent
photoinitiating systems under near-UV or visible LEDs, such as
The C (aryl)−S bond of phenylthiobenzophenone can directly
naphthalimide and naphthalic anhydride derivatives,5
break under mercury lamp irradiation and consequently
dihydroxyanthraquinone derivatives,6 π-conjugated dithieno-
produce aryl and thiyl radicals, which can initiate polymer-
ization.17,18 Notably, thiyl radicals show no selectivity to
monomers and low sensitivity to oxygen.19 Meanwhile,
naphthalimide derivatives have good absorption capacities in
the near-UV and visible LEDs wavelength range.20 Hence,
molecules that combine C(aryl)−S bond and naphthalimide
groups are expected to be type I photoinitiators under near-UV
LED.
phosphole derivatives,7 and curcumin.8 Norrish type II
photoinitiators produce initiating species by electron and/or
proton transfer, whereas Norrish type I photoinitiators
produce initiating species by the direct decomposition of
photoinitiators.9 Currently available commercial photoinitia-
tors under near-UV or visible LEDs include bisacylphosphine
oxide (BAPO), 2,4,6-trimethylbenzoyldiphenylphosphine
oxide, and oxime esters (e.g., OXE-1 and OXE-2), which are
all Norrish type I photoinitiators. However, novel and efficient
Norrish type I photoinitiators under near-UV or visible LEDs
are rarely concerned currently. There are only few examples
such as acylgermane-based photoinitiators,10−13 2-(4-methox-
ystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine,14 N-phenylgly-
In this study, we designed and synthesized a series of
naphthalimide aryl sulfide derivatives photoinitiators (NASs)
Received: October 28, 2018
Revised: January 3, 2019
© XXXX American Chemical Society
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Scheme 1. Chemical Structures of NASs
Scheme 2. Chemical Structures of the Monomers and Cationic Photoinitiator Used for Experiments
Light absorption properties of NASs were investigated by using a
Shimadzu UV-3600 UV−vis−NIR spectrophotometer.
(Scheme 1). Then, we investigated the influences of NASs
structures on the light absorption properties of NASs. We also
studied the possible photolysis mechanism of NASs through
steady-state photolysis, nuclear magnetic resonance (NMR),
electron spin resonance (ESR), fluorescence spectroscopy,
cyclic voltammetry, and laser flash photolysis and by
calculating the bond dissociation energies (BDEs) of the C−
S bonds of NASs. The photoinitiation capabilities of NASs for
the free radical polymerization of 1,6-hexanediol diacrylate
(HDDA) and methyl methacrylate (MMA) and the cationic
polymerization of 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclo-
hexane carboxylate (E-4221) under UV LED irradiation at 405
nm were investigated through real-time infrared spectroscopy
(RTIR). Then, the stability of NASs in acrylate monomers
under sunlight was explored by visually observing the changes
of photosensitive liquids and comparing their UV−vis spectra
before and after sunlight irradiation. Scheme 2 illustrates the
structures of monomers and cationic photoinitiator used in the
experiment.
Irradiation Sources. A Shenzhen Lamplic household UV LED
bulb (emission wavelength centered at 405 nm, incident light
intensity 100 mW cm−2) was used as irradiation device.
Steady-State Photolysis Experiments. NAS2 and NAS5 in
anhydrous acetonitrile and MMA, respectively, were irradiated with
UV LED at 405 nm (∼100 mW cm−2) under nitrogen, and their UV−
vis spectra were recorded with a Shimadzu UV-3600 UV−vis−NIR
spectrophotometer at different irradiation times.
ESR Experiments. ESR experiments were performed with a JEOL
JES-FA200 spectrometer (X-band) at 9.06 GHz and 100 kHz field
modulation, and a microwave power of 0.998 mW was used. The
radicals were generated at room temperature upon UV LED exposure
at 405 nm under an argon protection and trapped by DMPO.
Solutions with concentrations of 1 × 10−4 mol L−1 were obtained by
adding NASs and DMPO into tert-butylbenzene. Then, the solutions
were irradiated. The molar ratio between NASs and DMPO was 1:5.
Laser Flash Photolysis. Laser flash photolysis experiments of
NASs were performed using a Spectra-Physics Quanta-Ray Q-
switched nanosecond Nd:YAG laser at λexc = 355 nm (10 ns pulses;
energy reduced down to 10 mJ) and an analyzing system consisted of
a ceramic xenon lamp, a monochromator, a fast photomultiplier, an
Acton SpectraPro SP-2300 spectrometer, and a transient digitizer
2. EXPERIMENTAL SECTION
Materials. 4-Bromo-1,8-naphthalic anhydride, potassium thioace-
tate, bis(dibenzylideneacetone)palladium [Pd(dba)2], 1,1′-bis-
(diphenylphosphino)ferrocene (dppf), potassium phosphate
(K3PO4), iodobenzene, and 4-iodotoluene were obtained from
Beijing Ouhe Technology Co., Ltd. 2,6-Diisopropylaniline, p-
toluidine, anhydrous acetonitrile, tert-butylbenzene, and 4′-iodoace-
tophenone were purchased from J&K Scientific Ltd. 1-Fluoro-4-
iodobenzene, MMA, BAPO, and diphenyliodonium hexafluoro-
phosphate (Iod) were supplied by Energy Chemical. Isobornyl
acrylate (IBOA), HDDA, trimethylolpropane triacrylate (TMPTA),
and E-4221 were obtained from Ciba Geigy Co. 5,5-Dimethyl-1-
pyrroline N-oxide (DMPO) was obtained from Dojindo Laboratories.
Characterization. NMR spectra were recorded on a Bruker
AV400 unity spectrometer (400 MHz). The high-resolution mass
spectra in dichloromethane were obtained with an Agilent 6540 Q-
ToF mass detector equipped with an electrospray ionization source.
(Princeton Instruments PI-MAX 4). Liquid samples with 10−5
M
NASs in nitrogen-saturated acetonitrile were contained in a quartz cell
with an optical path length of 1 cm.
Fluorescence Experiments. The fluorescence emission spectra
of NASs in acetonitrile were obtained using a Hitachi F-7000
fluorescence spectrophotometer. The concentration of each sample
was 1 × 10−5 mol L−1.
Redox Potentials. The oxidation potentials (Eox vs SCE) of NASs
were tested in acetonitrile by cyclic voltammetry using tetrabutyl-
ammonium hexafluorophosphate (0.1 M) as a supporting electrolyte
(CH Instruments Model 600E Series electrochemical workstation).
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 referred to the reversible formal potential of this
compound (+0.38 V/SCE).
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Scheme 3. Synthesis Route of NASs
The free energy changes ΔG for the electron transfer between
NASs and Iod can be calculated from the classical Rehm−Weller
equation:21 ΔG = Eox − Ered − ES (or ET) + C, where Eox, Ered, ES (or
ET), and C are the oxidation potential of the electron donor, the
reduction potential of the electron acceptor, the excited singlet (or
triplet) state energies of NASs, and the electrostatic interaction energy
for the initially formed ion pair, generally considered as negligible in
polar solvents.
3. RESULTS AND DISCUSSION
Light Absorption Property of NASs. Figure 1 illustrates
the UV−vis spectra of NASs in anhydrous dichloromethane.
Computational Procedure. The Gaussian 09 package was used
in the molecular orbital calculations. All molecular structures of NASs
were optimized in the ground state by the density functional theory
(DFT) at B3LYP/6-31G**. Whether optimized geometries had
energy minima (zero imaginary frequency) was determined through
frequency calculations. Frontier molecular orbital properties were
analyzed and visualized with Gaussview 5.0 software.22 DFT
calculations of the dissociation energy of C−S bonds of NASs were
performed with Gaussian 09 software. All geometry optimizations
were performed without imposing any geometry constraints by using
a B3LYP hybrid density functional with a 6-311G* basis set.23 The
results were analyzed through GaussView 5.0. In all molecules,
frequency was calculated at the same level of theory to confirm that
structures had energy minima. Reported energies were zero point
corrected.
Figure 1. UV−vis absorption spectra of NAS1−6 in anhydrous
dichloromethane (concentration = 1 × 10−5 mol L−1).
Photopolymerization Kinetics. The photopolymerization ki-
netics of monomers (i.e., HDDA, MMA, and E-4221) in the presence
of NASs or NASs/Iod photoinitiating systems were investigated by
employing a Nicolet 5700 FT-IR spectroscope. Photosensitive liquids
were coated on a KBr plate under air or clamped in laminate between
two KBr plates (∼30 μm) for irradiation with UV LED at 405 nm
(∼100 mW cm−2). The conversions of acrylate, methacrylate, and
epoxy functional groups were assessed by calculating the proportion
of the peak areas of 831−792, 831−796, and 756−738 cm−1,
respectively.
The maximum absorptions of NAS1−6 are located at 389, 385,
340, 387, 391, and 395 nm, respectively. The maximum
absorption peaks of NAS2, NAS3, and NAS4 have a blue shift
compared with that of NAS1, which is attributed to the
presence of the electron-withdrawing group (the acetyl for
NAS2, nitro for NAS3, and fluoro group for NAS4) at the para
position of the benzene ring in their molecules. Particularly,
the blue shift for NAS3 is very great due to the strong electron-
withdrawing effect of the nitro. Contrarily, the maximum
absorption peaks of NAS5 and NAS6 show a red shift due to
the presence of the electron-donating group (methyl group) at
the para position of the benzene ring in their molecules. Table
1 presents the molar extinction coefficients of NAS1−6 in
anhydrous dichloromethane at typical UV LED emission
wavelengths (385, 395, and 405 nm). The molar extinction
Each sample was repeated three times, and errors on the reported
double bond and epoxy conversions as a function of polymerization
time were <2%. In most cases, the errors were <1%.
Synthesis. Scheme 3 displays the synthetic route of NASs.24 Aryl
bromide 2 synthesized on the basis of 4-bromo-1,8-naphthalic
anhydride 1 and arylamine (2 mmol), aryl iodides (2 mmol),
potassium thioacetate (228 mg, 2 mmol), Pd(dba)2 (115 mg, 0.2
mmol), 1,1′-bis(diphenylphosphino)ferrocene (155 mg, 0.28 mmol),
and K3PO4 (1.02 g, 4.8 mmol) were added into a round-bottomed
flask. Toluene (4.0 mL) and acetone (2.0 mL) were then added. The
reaction mixture was stirred for 3 h at 70 °C under nitrogen. Then,
the reaction temperature was increased to 110 °C, and the mixture
continued to be stirred for 12 h under nitrogen. Subsequently, the
mixture was cooled to room temperature, poured into 10 mL of
saturated aqueous ammonium chloride, and extracted with dichloro-
methane (3 × 10 mL). Organic layers were combined and washed
with brine (3 × 15 mL), dried over anhydrous sodium sulfate, and
filtered. Then, the solvent was removed under vacuum, and the
resulted crude product was purified by flash chromatography on silica
gel to obtain the pure product. The specific synthesis methods and
data are listed in the Supporting Information.
Table 1. Molar Extinction Coefficients (ε) of NAS1−6 in
Anhydrous Dichloromethane at Typical UV LED Emission
Wavelengths
ε385 nm × 104
ε395 nm × 104
ε405 nm × 104
NASs
(M−1 cm−1
)
(M−1 cm−1
)
(M−1 cm−1
)
NAS1
NAS2
NAS3
NAS4
NAS5
NAS6
1.23
1.37
1.29
1.81
1.43
1.27
1.23
1.27
0.99
1.75
1.46
1.33
1.01
0.93
0.61
1.28
1.26
1.17
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Figure 2. HOMO and LUMO of NAS1−6 at the B3LYP/6-31G** level (isovalue = 0.02).
Figure 3. Steady-state photolysis of (a) NAS2 and (b) NAS5 in anhydrous acetonitrile upon UV LED at 405 nm (concentration = 1.0 × 10−5 mol
L−1).
coefficients of NASs at 385 and 395 nm are all higher than 1 ×
104 M−1 cm−1, except the molar extinction coefficient of NAS3
at 395 nm. Moreover, the molar extinction coefficients of
NAS1, NAS4, NAS5, and NAS6 at 405 nm are higher than 1 ×
104 M−1 cm−1, whereas those of NAS2 and NAS3 at 405 nm
are lower than 1 × 104 M−1 cm−1. Figure 2 depicts the
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Figure 4. 1H NMR spectra of (a) NAS2 and (b) NAS5 under different times of UV LED irradiation at 405 nm (400 MHz, dichloromethane-d2 as
solvent).
Figure 5. Photolysis of (a) NAS2 and (b) NAS5 in MMA upon UV LED at 405 nm (concentration = 1.0 × 10−5 mol L−1).
molecular orbitals of different NASs mainly involved in their
lowest energy absorption band (highest occupied molecular
orbital [HOMO] → lowest unoccupied molecular orbital
[LUMO]). It is found that the electron-withdrawing nitro
group in NAS3 makes the electron cloud in HOMO mainly
distribute on the benzene ring connected with nitrogen atom
of naphthalimide moiety, whereas the electron-donating
methyl and isopropyl groups in NAS6 make the electron
cloud in HOMO mainly distribute on the benzene ring
connected with the sulfur atom. For compounds exhibiting a
significant visible light absorption (NAS1, NAS2, NAS4,
NAS5, and NAS6), a charge transfer transition from the sulfur
atom or the phenyl (for NAS6) linked to the sulfur atom (for
HOMO) to the naphthalimide moiety (for LUMO) is found.
This charge transfer transition agrees with the improved light
absorption properties of these derivatives compared to the
derivative containing nitro group (NAS3).20 Here, we selected
UV LED at 405 nm as the light source to study properties of
NASs because 405 nm is the longest emission wavelength
matching the absorption of NASs.
Photochemistry of NASs. Figure 3 reports the steady-
state photolysis of NAS2 and NAS5 in anhydrous acetonitrile.
After 960 s of UV LED irradiation at 405 nm, the absorption
peaks of NAS2 and NAS5 do not decrease. We also performed
NMR tests on two compounds before and after irradiation.
1
Figure 4 reveals that H NMR spectra of NAS2 and NAS5 do
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Figure 6. ESR spectra of the radicals generated by (a) NAS1, NAS2, NAS3 and (b) NAS4, NAS5, NAS6 upon UV LED exposure at 405 nm for 5
min and trapped by DMPO in tert-butylbenzene.
Table 2. BDEs of C−S Bonds in NASs Calculated at the B3LYP/6-311G* Level of Theory
BDEs (kJ mol−1
)
bond
NAS1
NAS2
NAS3
NAS4
NAS5
NAS6
1
2
332.57
329.56
329.85
331.75
329.52
333.03
291.96
330.94
292.06
330.91
292.46
331.01
Scheme 4. Potential Mechanism of Photolysis of NASs
not change before and after irradiation of UV LED at 405 nm.
Two possible causes contribute to this phenomenon. The first
reason is the cage effect of the solvent in the low-concentration
solution,25 which results in the recombination of free radicals
generated by the decomposition of photoinitiators to form
original molecules. The second reason is that NAS2 and NAS5
do not decompose under UV LED irradiation at 405 nm.
However, subsequent photolysis experiments of NAS2 and
NAS5 in MMA (Figure 5) show that the ground-state
absorption at 344 nm for NAS2 and the absorption at 386
nm for NAS5 decreased with increasing of irradiation time. It
suggests that NAS2 and NAS5 were consumed to produce free
radicals which can react with MMA to new products. To
further determine the photodegradation mechanism of NASs,
we explored ESR spectra of NASs under such UV LED
irradiation at 405 nm. Figure 6 reveals that all photoinitiators
produce free radicals in the presence of DMPO as a trapping
reagent. Then, we calculated the BDEs of the C−S bonds of
NASs, and Table 2 presents the results. The BDEs of the C−S
bonds of NAS1, NAS2, and NAS3 are all about 330 kJ mol−1,
indicating that such bonds at positions 1 and 2 for NAS1,
NAS2, and NAS3 have a similar fracture probability. The BDEs
of the C−S bonds at position 1 for NAS4, NAS5, and NAS6
are all about 292 kJ mol−1, whereas such bonds at position 2
are all about 330 kJ mol−1, indicating that C−S bonds at
position 1 more likely break than those at position 2 for NAS4,
NAS5, and NAS6. The results are consistent with the ESR test
results that the ESR spectra of NAS1/DMPO, NAS2/DMPO,
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and NAS3/DMPO are similar. By contrast, the ESR spectra of
NAS4/DMPO, NAS5/DMPO, and NAS6/DMPO are similar.
Moreover, ESR spectra of NAS1, NAS2, and NAS3 systems are
more complex than those of NAS4, NAS5, and NAS6 systems.
This finding is attributed to the fact that C−S bonds at
positions 1 and 2 for NAS1, NAS2, and NAS3 have a similar
fracture probability.
Based on the above information, the potential mechanism of
photolysis of NASs is proposed (Scheme 4). These photo-
initiators produce free radicals in two ways under UV LED
irradiation at 405 nm. NAS1, NAS2, and NAS3 have a similar
probability of generating free radicals in both ways. NAS4,
NAS5, and NAS6 should mainly produce free radicals in the
first way. If no monomers or spin trapping reagents are found
in the system, then these free radicals recombine into original
molecules. Conversely, if monomers exist in the systems, then
these free radicals can initiate monomers to polymerize,
thereby driving the balance of photolysis reaction to the right.
To further explore the photoinitiation ability of NASs for
photopolymerization, we performed photopolymerization
experiments of the monomers, including HDDA, MMA, and
E-4221, initiated by NASs or NASs/Iod.
a biomaterial, including bone cements and dental materi-
als.28,29 However, the slow polymerization kinetics involving
the conversion of MMA to PMMA at ambient temperature is a
major disadvantage for in situ polymerizations.30 Many works
have been conducted to solve this problem, such as using
anionic polymerization31 or using a mixture of MMA/
oligomeric PMMA for PMMA preparation.32 In this study,
we investigated the photopolymerization kinetics of MMA
initiated by NAS5 and NAS6, which are efficient for initiating
photopolymerization of HDDA. Figure 8 shows that NAS5
Photopolymerization Kinetics. Free Radical Photo-
polymerization of Acrylate. Figure 7 reveals that all NASs
Figure 8. Photopolymerization profiles of MMA in laminate in the
presence of NAS5 (0.5 wt %), NAS6 (0.5 wt %), BAPO (0.5 wt %),
and BAPO (2 wt %) upon UV LED exposure at 405 nm.
(0.5 wt %) is more effective than BAPO for initiating free
radical polymerization of MMA under UV LED irradiation at
405 nm. The final conversion of the double bond of MMA
initiated by NAS5 (0.5 wt %) after 200 s of irradiation is 40%,
which is significantly higher than those of MMA initiated by
BAPO (0.5 wt %, conversion = 10% and 2 wt %, conversion =
20%). Interestingly, NAS5 (0.5 wt %) is more effective than
NAS6 (0.5 wt %, conversion = 13%) for initiating free radical
polymerization of MMA, whereas NAS6 is more effective than
NAS5 for initiating free radical polymerization of HDDA. The
phenomenon is ascribed to the steric effects of MMA and
NAS6. The methyl group attached to the carbon−carbon
double bond of MMA and two isopropyl groups in NAS6
molecule all increase the steric hindrance; therefore, such
effects have a greater impact on the polymerization of MMA
initiated by NAS6 than that of MMA initiated by NAS5.
Cationic Photopolymerization of Epoxide. Naphthalimide
derivatives/Iod systems20 can initiate the cationic photo-
polymerization of epoxide. Therefore, we also investigated the
cationic photopolymerization of epoxide initiated by NASs/
Iod systems.
Figure 7. Photopolymerization profiles of HDDA in laminate in the
presence of NAS1 (curve 1), NAS2 (curve 2), NAS3 (curve 3), NAS4
(curve 4), NAS5 (curve 5), NAS6 (curve 6), and BAPO (curve 7)
upon UV LED exposure at 405 nm (concentration of NASs and
BAPO: 0.5 wt %).
can initiate free radical photopolymerization of HDDA under
UV LED irradiation at 405 nm. The time to reach the highest
photopolymerization rates of systems initiated by NAS1,
NAS4, NAS5, and NAS6 is shorter than those of systems
initiated by NAS2 and NAS3 (see Figure S1). The result is
attributed to the fact that the molar extinction coefficients at
405 nm of NAS1, NAS4, NAS5, and NAS6 are greater than
those of NAS2 and NAS3. NAS6 can efficiently initiate free
radical photopolymerization of HDDA under UV LED
irradiation at 405 nm, which can be attributed to the fact
that the electron-rich radicals produced by the photolysis of
NAS6 with isopropyl and methyl groups are easy to react with
the electron-deficient double bond of acrylate.26 Moreover, the
double bond conversion of HDDA initiated by NAS6 after 200
s of irradiation reaches 83%, which is higher than that of
HDDA initiated by BAPO (75%).
Figure 9 shows that NASs/Iod systems can initiate cationic
photopolymerization of epoxy monomer (E-4221) under UV
LED irradiation at 405 nm irradiation. The free energy changes
1
for the NASs/Iod electron transfer reaction were calculated
and are listed in Table 3. The free energy changes ΔGS for
1NASs/Iod electron transfer reactions are negative and ensure
the process is favorable. Moreover, the NASs transient
absorption spectra were obtained after excitation at 355 nm
(Figure S3). As shown in Figure S3, there are two broad
absorption bonds in the 420−500 and 600−650 nm regions
for NAS1−NAS3, whereas there are two broad absorption
bonds in the 460−500 and 600−675 nm regions as well as a
shoulder in the range between 425 and 460 nm for NAS4−
NAS6. These findings indicate that NASs have an excited
triplet state.33 Hence, the possible mechanism for NASs/Iod to
initiate cationic polymerization of E-4221 is suggested to be a
Free Radical Photopolymerization of Methacrylate, MMA.
Poly(methyl methacrylate) (PMMA) is one of the most widely
used thermoplastic materials because of its excellent
mechanical properties, outstanding biocompatibility, and
relatively easy process.27 PMMA has been broadly applied as
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3
NAS → 1NAS (hν) and 1NAS → NAS
(1)
1,3NAS + Ph2I+ → NAS•+ + Ph2I•
(2)
(3)
•
Ph2I• → Ph + Ph−I
It is also found from Figure 9 that the final epoxy conversions
of NAS2/Iod and NAS3/Iod systems are low, and they are
only 33% and 6%, which is due to their low molar extinction
coefficients (ε) at 405 nm (0.93 M−1 cm−1 for NAS2 and 0.61
M−1 cm−1 for NAS3). Interestingly, the final epoxy conversion
of NAS5/Iod (46%) is lower than that of NAS6/Iod (56%),
but the ε at 405 nm for NAS5 (1.26 M−1 cm−1) is higher than
that for NAS6 (1.17 M−1 cm−1). It is suggested that the
reactivity of NAS6•+ is higher than that of NAS5•+.
Stability of NASs. Because photoinitiators are very
sensitive to light, photocurable formulations containing
photoinitiators need to be protected from light during their
preparation, storage, transportation, and use, which brings
great inconvenience to application of UV-curing technology.
We also investigated the sensitivity of NASs to sunlight. Three
frequently used acrylate monomers with different function-
alities (IBOA, HDDA, and TMPTA) and NAS6, which is the
most effective photoinitiator among NASs for initiating the
photopolymerization of HDDA, were selected as models.
Figure 10 illustrates that after an hour of sunlight irradiation all
monomers were completely cured in the presence of BAPO
(0.5 wt %). However, after sunlight irradiation for 5 days, all
monomers almost did not polymerize in the presence of NAS6
(0.5 wt %) through visual observation. Nevertheless,
monomers polymerized in the presence of NAS6 under UV
LED irradiation at 405 nm (∼100 mW cm−2) for about 15 min
(see Figure S4). UV−vis absorption tests were conducted to
confirm whether NAS6 can produce free radicals to initiate
photopolymerization of monomers under sunlight irradiation,
and Figure 11 reveals the results. The UV−vis spectra of the
three systems almost did not change before and after sunlight
irradiation for 5 days. Thus, nearly no free radical was
produced by NAS6 to initiate photopolymerization under
sunlight. This phenomenon may be because C−S bonds of
Figure 9. Photopolymerization profiles of E-4221 under air in the
presence of NASs/Iod (0.5 wt %/2 wt %,) upon UV LED exposure at
405 nm: NAS1/Iod (curve 1), NAS2/Iod (curve 2), NAS3/Iod
(curve 3), NAS4/Iod (curve 4), NAS5/Iod (curve 5), and NAS6/Iod
(curve 6).
Table 3. Parameters Characterizing the Reactivity of NASs
with Iod
a
b
c
Eox (V vs SCE)
ES (eV)
ΔGS (eV)
NAS1
NAS2
NAS3
NAS4
NAS5
NAS6
1.77
1.81
1.92
1.75
1.65
1.67
2.92
2.95
3.05
2.96
2.92
3.01
−0.95
−0.94
−0.93
−1.01
−1.07
−1.14
a
b
Eox values measured by cyclic voltammetry for NASs. Singlet state
energies ES extracted from the UV−vis absorption and fluorescence
c
emission spectra of NASs (see Figure S2). ΔGS = free energy change
for the NASs/Iod singlet interaction; reduction potential Ered = −0.2
V for Iod.19
photosensitization cationic polymerization and is depicted in
the following reactions.
Figure 10. Stability of NAS6 and BAPO in IBOA, HDDA, and TMPTA under sunlight with different irradiation times: (1) BAPO in IBOA, (2)
NAS6 in IBOA, (3) BAPO in HDDA, (4) NAS6 in HDDA, (5) BAPO in TMPTA, and (6) NAS6 in TMPTA (concentration of photoinitiators =
0.5 wt %).
H
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Article
Figure 11. UV−vis spectra of (a) NAS6/IBOA, (b) NAS6/HDDA, and (c) NAS6/TMPTA (concentration of NAS6 in monomers is 0.5 wt %)
systems before and after sunlight irradiation for 0 s and 5 days.
Figure 12. Photopolymerization profiles of HDDA in laminate in the presence of (a) NAS6 upon UV LED exposure at 405 nm of different light
intensities (in mW cm−2): 1 (curve 1), 5 (curve 2), 10 (curve 3), 20 (curve 4), 45 (curve 5), 60 (curve 6), 75 (curve 7), and 100 (curve 8) and (b)
BAPO upon UV LED exposure at 405 nm of different intensities (concentration of NAS6 and BAPO: 0.5 wt %).
NAS6 hardly break under sunlight with the low intensity in the
near-UV−vis wavelength range.19 We used RTIR to verify this
explanation and selected HDDA as a model monomer. Figure
12 shows that with the decrease of light intensity from 75 to 1
mW cm−2, the initiating efficiency of NAS6 evidently
decreases. When light intensity is 1 and 5 mW cm−2, NAS6
hardly initiates the photopolymerization of HDDA. However,
BAPO can efficiently initiate photopolymerization of HDDA,
even if the light intensity is 1 mW cm−2. The high stability of
NASs to sunlight makes the preparation, storage, and
application of photocurable formulations convenient because
avoiding sunlight is no longer necessary.
such radicals recombine into original molecules in the absence
of acrylate monomers. Moreover, NASs/Iod systems can
initiate the cationic photopolymerization of E-4221 under UV
LED irradiation at 405 nm, and NAS6/Iod is the most
efficient. Compared with BAPO, NAS5 exhibits a more
excellent initiating capability for free radical photopolymeriza-
tion of MMA under the same irradiation. Interestingly, the
stability of NASs in IBOA, HDDA, and TMPTA under
sunlight is remarkable, thereby endowing NASs with
tremendous potential application in the photocuring field.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b02309.
■
S
4. CONCLUSIONS
Six NASs were successfully designed and synthesized. NASs
exhibit excellent absorption at different UV LED emission
wavelengths (385, 395, and 405 nm). NAS5 with methyl and
NAS6 with methyl and isopropyl show longer absorption
wavelength. The NASs can produce free radicals under UV
LED irradiation at 405 nm, and these free radicals can
efficiently initiate acrylate monomers to polymerize. However,
Photopolymerization rate of HDDA in laminate initiated
by NASs and BAPO upon exposure to LED at 405 nm,
the absorption and emission spectra for NASs in
acetonitrile, the results of laser flash photolysis experi-
ments of NASs, the photograph of NAS6/monomer
I
DOI: 10.1021/acs.macromol.8b02309
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systems after LED irradiation at 405 nm, and the specific
Article
Based Photoinitiator in the Presence of Onium Salts. Macromolecules
2008, 41, 6714−6718.
synthesis methods and NMR/HRMS data of NASs and
intermediate products (PDF)
(12) Ganster, B.; Fischer, U. K.; Moszner, N.; Liska, R. New
Photocleavable Structures. Diacylgermane-Based Photoinitiators for
Visible Light Curing. Macromolecules 2008, 41, 2394−2400.
́
AUTHOR INFORMATION
Corresponding Author
*E-mail: sunfang60@yeah.net (F.S.).
(13) Tehfe, M. A.; Blanchard, N.; Fries, C.; Lalevee, J.; Allonas, X.;
■
Fouassier, J. P. Bis(germyl)ketones: Toward a New Class of Type I
Photoinitiating Systems Sensitive Above 500 nm? Macromol. Rapid
Commun. 2010, 31, 473−478.
ORCID
(14) Zhang, J.; Xiao, P.; Morlet-Savary, F.; Graff, B.; Fouassier, J. P.;
́
Lalevee, J. A Known Photoinitiator for a Novel Technology: 2-(4-
Jia Yu: 0000-0003-3588-4582
Fang Sun: 0000-0003-4064-1921
Notes
Methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine for near UV or
Visible LED (385 nm, 395 or 405 nm) Induced Polymerization.
Polym. Chem. 2014, 5, 6019−6026.
The authors declare no competing financial interest.
́
(15) Zhang, J.; Lalevee, J.; Mou, X.; Morlet-Savary, F.; Graff, B.;
Xiao, P. N-Phenylglycine as a Versatile Photoinitiator under Near-UV
LED. Macromolecules 2018, 51, 3767−3773.
ACKNOWLEDGMENTS
■
́
(16) Lalevee, J.; Blanchard, N.; Tehfe, M. A.; Fries, C.; Morlet-
The financial support from the National Key Research and
Development Program of China (2017YFB0307800) and the
National Natural Science Foundation of China (Grant No.
51273014) is gratefully acknowledged.
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