Combining photo-cleavable and hydrogen-abstracting groups in quinoxaline with thioether bond as hybrid photoinitiator

Article abstract of DOI:10.1016/j.cclet.2016.06.008

We synthesized four diphenylquinoxaline derivatives (SQs) with phenyl-thioether units, which combine photo-cleavable and hydrogen-abstracting groups in one molecule. The photochemistry and photopolymerization of SQs were investigated. SQs possess suitable

Full text of DOI:10.1016/j.cclet.2016.06.008

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Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Combining photo-cleavable and hydrogen-abstracting groups in
quinoxaline with thioether bond as hybrid photoinitiator
Ji-Yuan Yao, Hong-Hao Hou, Xiao-Dong Ma, Hong-Jie Xu, Zi-Xing Shi, Jie Yin,
*
Xue-Song Jiang
School of Chemistry and Chemical Technology, State Key Lab of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai 200240, China
A R T I C L E I N F O
A B S T R A C T
Article history:
We synthesized four diphenylquinoxaline derivatives (SQs) with phenyl-thioether units, which combine
photo-cleavable and hydrogen-abstracting groups in one molecule. The photochemistry and
photopolymerization of SQs were investigated. SQs possess suitable UVꢀvis absorption in the range
of 350ꢀ400 nm with high extinction coefficients. UVꢀvis and HPLC-MS spectra revealed that C–S bond
in phenyl-thioether group of SQs can be broken by irradiation of UV-light. Photolysis and
photopolymerization experiments showed that SQs can be used as photo-cleavable photointiators,
their photoinitiating efficiency can be enhanced by hydrogen donor. As photo-cleavable photoinitiators,
SQs could initiate hexamethylene diacrylate (HDDA) very efficiently with the double bond conversion
(DBC) of 80%. In the presence of ethyl-4-(dimethylamino) benzoate (EDB) as coinitiator, photoinitiator
systems initiated photopolymerization of commercial acrylate monomers with higher double bond
conversion than 90%. These characteristics make SQs potential photoinitiators in photo-curing field.
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Received 6 May 2016
Received in revised form 24 May 2016
Accepted 30 May 2016
Available online xxx
Keywords:
Photoinitiators
Phenylquinoxaline
Photo-cleavage
Hydrogen-abstracting
Photopolymerization
1. Introduction
design of new structures [10–14]. Yagci and Arsu’s groups have
developed a series of new photoinitiators based on modification of
The photopolymer science has witnessed the development and
application in electronic or medical apparatus industry of photo-
curing technologies, such as optical fibers, coatings, adhesives,
dental materials and tissue regeneration [1–6]. As one of the key
components in the photo-curing systems, new photoinitiators [PIs]
are still the subject of many researchers. Its rapid development is
mainly driven by the advantages of economy and environment,
such as rapid through cure, low energy requirements and solvent-
free formulations. Especially in many promising areas such as
photoimaging, microelectronics, and holographic image recording
and data storage, new photoinitiator systems with high initiation
efficiency are highly demanded. Generally, photoinitiator systems
thioxanthone or benzophenone [15–17]. Gescheidt et al. synthe-
sized new structures based on benzaldoximes and acylgermanes as
novel photointiator [18,19]. Among the most studied photoinitiator
systems are those based on PI/hydrogen or electron donor couples
(Type II). Recently, Laleve´e group have developed naphthalimide
derivatives/hydrogen donor/iodonium salt as hybrid photoinitiator
for radical and cationic polymerization [20,21].
Both Type I and Type II photoinitiators have their own
characteristics. Usually, Type I photoinitiator can produce free
radical very quickly upon irradiation of UV-light with short
wavelength. The UVꢀvis absorption of dyes as Type II photo-
initiator can be well tuned according to light sources such as high-
pressure Hg lamp and LED [22–24]. As the amount of radicals is the
key to photopolymerization, to introduce photo-cleavable groups
to Type II photoinitiators might provide more radicals upon
irradiation at the same time. So photoinitiation systems containing
Type I and Type II groups will be efficient due to the synergistic
effect.
can be divided into two types: (i) Type I, known as
a-cleavage,
produce initiating radicals by bond cleavage upon radiation of
light. (ii) Type II, undergo photo-excitation followed by an electron
or hydrogen transfer process and consequently to form initiating
species [7–9]. Two main approaches to develop new photoinitiator
currently are the modification of existing photoinitiators and the
Quinoxaline possesses a structure similar to a dye and can be
classified as type II photoinitiators [25], and diphenylquinoxaline
derivatives used as photoinitiators were investigated [26]. As our
continuous work to develop high-efficient photoinitiator systems
*
Corresponding author.
E-mail address: ponygle@sjtu.edu.cn (X.-S. Jiang).
http://dx.doi.org/10.1016/j.cclet.2016.06.008
1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J.-Y. Yao, et al., Combining photo-cleavable and hydrogen-abstracting groups in quinoxaline with
thioether bond as hybrid photoinitiator, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.008

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[27–31], in this text we explored quinoxaline containing photo-
cleavable thioether group as hybrid photoinitiator. With the
quinoxaline as a typical hydrogen abstraction group, we intro-
ethanol in 85 8C and stirred for 12 h under nitrogen protection. The
mixture was then evaporated to remove most of the solvent and
recrystallized. The precipitant was filtered from ethanol and dried
to get 2, 3-bis (4-fluorophenyl) quinoxaline (DF-Q).
duced phenyl thioether group into
a series of quinoxaline
skeletons, which was proved to be cleavable by irradiation of
UV-light. This system possesses the advantage of both Type I and
Type II mentioned above, such as the adjustable absorption
wavelength and more active or not easily quenched radicals. The
involved photochemical mechanisms were investigated by UVꢀvis
photolysis, electron spin resonance spin trapping (ESR-ST) and
high-performance liquid chromatography/mass spectrum (HPLC/
MS) techniques. Then we studied the kinetics for polymerization of
multifunctional acrylates by Real Time Fourier Transform Infrared
(FT-IR).
10 mmol of DF-Q and three times equivalent of thiophenol
(30 mmol) were put into 150 mL three-necked round bottom flask
equipped with magnetic stirrer. After the addition of 50 mL N, N-
dimethylformamide (DMF), one times equivalent of KOH and
toluene were added to accelerate reaction rate. The reaction was
under the gas protection and equipped with water separator to
remove water. The mixture was stirred at 130 8C for 24 h.
Appropriate addition of toluene should be done after the toluene
was evaporated. The mixture was slowly trickled into 10 times
ethanol. The precipitant was filtered from ethanol and dried to get
2, 3-bis (4-(phenylthio) phenyl) quinoxaline (SQ1).
2. Experimental
2.3. Instruments
2.1. Materials
1H NMR and 13C NMR spectra were recorded on a Varian
Mercury Plus-400 nuclear magnetic resonance spectrometer
(400 MHz) using CDCl3 and DMSO-d6 as solvents. FT-IR spectra
were recorded on a Thermo Scientific IS10 Fourier Transform
Infrared Spectrometer. Mass spectra were determined on ACQUI-
TYTM UPLC&Q-TOF MS Premier Ultra Performance Liquid Chro-
matography & Quadrupole-Time-of-Flight Mass Spectrometer.
UVꢀvis absorption spectra and dynamics were conducted on a
Perkin-Elmer LS-50B spectrophotometer. SQs were dissolved in
chloroform at a concentration about 5 ꢁ 10ꢀ5 mol/L. Elemental
analysis was conducted on an Elementar Vario ELIII. ESR (electron
spin resonance) experiments were carried out with a Bruker EMX
EPR spectrometer.
[1,10-Biphenyl]-3,30,4,40-tetraamine,1,2-bis(4-fluorophenyl)
ethane-1,2-dione, naphthalene-2,3-diamine were obtained from
TCI (Shanghai) Development Co., Ltd. Benzene-1,2-diamine,
[1,10:20,100-terphenyl]-40,50-diamine were purchased from Meryer
(Shanghai) Chemical Technology Co., Ltd. 2-Mercaptobenzothia-
zole (MBO) was obtained from J&K Chemical Co., (Shanghai, China).
N-methyldiethanolamine (MDEA) and ethyl, 4-(dimethylamino)-
benzoate (EDB) were purchased from Alfa Aesar. ((2-((2-(Acry-
loyloxy)propoxy)methyl)propane-1,3-diyl)bis(oxy))bis(propane-
2,1-diyl) diacrylate (G3POTA) and 2-phenoxyethyl acrylate
(EM2102) were provided by Eternal Chemical Co., CTD. 2, 2-Bis
(4-(acryloxypolyethoxy) phenyl) propane (A-BPE-10) was provid-
ed by Shin-Nakamura Chemical Co., Ltd. Other chemicals were
obtained from China National Pharmaceutical Group Co. (Shang-
hai, China). All reagents were used as received except as noted.
2.4. ESR spin trapping (ESR-ST) experiment
ESR (electron spin resonance) experiments were carried out
with a Bruker EMX EPR spectrometer at 9.5 GHz with a modulation
frequency of 200 kHz with 5, 5-dimethyl-1- pyrroline-N-oxide
(DMPO) as radical capturing agent. A high-pressure mercury lamp
with a cut-off filter (365 nm) was used for irradiation in the ESR
spectrometer cavity. The concentrations of SQ4 dissolved in
mL of each sample
was transformed into a quartz ESR tube and then purged with
nitrogen to get rid of oxygen.
2.2. Synthesis of 2, 3-diphenylquinoxaline derivatives
Four phenylquinoxalines with thioether group (SQ), 2, 3-bis-
(4-(phenylthio)phenyl)quinoxaline (SQ1), 2, 20, 3, 30-tetrakis- (4-
(phenylthio)phenyl) -6, 60-biquinoxaline (SQ2), 2, 3-bis (4-(phe-
nylthio)phenyl) benzo[g] quinoxaline (SQ3), and 2, 3-diphenyl-5, 6-
bis (4-(phenylthio)phenyl) pyrazine (SQ4) were synthesized accord-
ing to Scheme 1, as well as phenyl quinoxalines (Q) as reference. Take
SQ1 as example, 10 mmol of benzene-1, 2-diamine and 10 mmol 1,
2-bis(4-fluorophenyl)- ethane-1, 2-dione were dissolved in 20 mL of
dichloromethane were 1 ꢁ 10ꢀ4 mol/L. 25
2.5. Radical photopolymerization with real time Fourier transform
Infrared spectra
Photopolymerization of different monomers (Scheme 2)
initiated by SQs was carried out while acquiring real time
Fourier Transform Infrared Spectroscopy (FT-IR, Thermo Scien-
tific IS10) with a high-pressure mercury lamp as the light source.
The intensity of light is about 10 mW/cm2. Here, the sample
consists of 4 mmol/L SQs and without any solvent. An
approximately 2 mg film sample of 50
mm thickness was held
on KBr for 3 min while irradiated under the protection of
nitrogen. The change of Peak area in about 1640 cmꢀ1 is directly
proportional to the amount of acrylate reacted in the system. By
integrating the area under the characteristic peak, the double
bond conversion (DBC) of the HDDA was determined according
to Eq. (1) [32,33]:
D
ðS0ꢀStÞ
DBC ¼
(1)
St
Scheme 1. Process for the synthesis of SQs based on quinoxalines and their
structures. (i, 85 8C ethanol as solvent. ii, DMF as solvent, toluene to take away
moisture, KOH as catalyzer in 130 8C).
Where St is the characteristic peak area in about 1640 cmꢀ1
evolved at time t and S0 is the peak area before irradiation.
Please cite this article in press as: J.-Y. Yao, et al., Combining photo-cleavable and hydrogen-abstracting groups in quinoxaline with
thioether bond as hybrid photoinitiator, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.008

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3
Scheme 2. Structure of coinitiators (MDEA, MBO, EDB) and multi-functional monomers (TMPTA, HDDA, EM2102, PPGDA-540, G3POTA, A-BPE-10).
3. Results and discussion
The UVꢀvis spectra of the obtained SQs are presented in Fig. 2,
and the detailed data of their photochemical properties are
summarized in Table 1. Conjugate group plays an important rule in
absorption wavelength. The maximal absorption wavelengths
3.1. Synthesis and photochemical properties of quinoxalines
Firstly, we synthesized phenyl thioether containing quinox-
alines (SQs) according to Scheme 1. Fluorine substituted quinoxa-
line skeletons were synthesized through the condensation of
benzil compounds and aromatic diamines compounds. Thioether
group was formed through the reaction between phenyl fluorine
and thiophenol. The resulting SQs were confirmed by NMR spectra
(Fig. 1 and Fig. S1 in Supporting information), elemental analysis,
ES/MS and FT-IR spectra (Fig. S2, Table S1 in Supporting
information). Taking SQ1 as example, 1H NMR and 13C NMR
spectra of SQ1 and its intermediate (DF-Q1) are shown in Fig. 1. As
for 1H NMR of DF-Q1, the peak between 7.9 ppm to 8.2 ppm in the
downfield can be assigned to the strong electron withdrawing
effect of quinoxaline group. Two peaks at 7.25 ppm and 7.55 ppm
can be assigned to phenyl fluorine. Compared with DF-Q1, the
introduction of thioether group increased the electron density of
phenyl ring. As shown in Fig. 1a, the appearance of three peaks
between 7.2 ppm and 7.5 ppm corresponding to –C6H4–S indicates
the successful introduction of thioether bond into quinoxaline
group, which was also confirmed by the intensity ratio of signals.
The structure of SQ1 was further determined by 13C NMR
spectrum. The chemical shifts from 127 ppm to 153 ppm are well
assigned to the labeled carbon atom in Fig. 1b and there are no
excess miscellaneous peaks. The other three SQs were also
confirmed as SQ1 (refer to Fig. S1). Purities higher than 90% for
the five complexes were determined from these results.
(
l
max) of the four SQs are in the range of 350ꢀ450 nm. Due to the
p-p* absorption of quinoxaline ring and large conjugate structure,
their extinction coefficients (
the introduction of the conjugate groups, the phenomenon of red
e
) are all above 1 ꢁ 104. Along with
4
SQ1
SQ2
SQ3
3
SQ4
Q
2
1
0
300
400
500
Wavelength (nm)
Fig. 2. UVꢀvis absorption spectra of five quinoxalines in CHCl3.
Fig. 1. Partial NMR spectra of SQ1 and its intermediate product in DMSO-d6 (1H NMR, a) and CDCl3 13C NMR, b).
(
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thioether bond as hybrid photoinitiator, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.008

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Table 1
(b)
UVꢀvis spectroscopic parameters of quinoxalines in chloroform.
(a)
Quinoxaline
Absorption
lmax
(nm)
lcutoff
(nm)
e
at lmax
E at 365 nm
(L molꢀ1cmꢀ1
)
(L molꢀ1 cmꢀ1
)
SQ1
SQ2
SQ3
SQ4
Q
370
402
410
350
342
413
470
476
409
380
17790
54113
18868
32051
7334
16014
32468
13722
29487
4147
10 Gs
Fig. 4. ESR spin trapping spectra of the radicals generated in SQ4 (a) and SQ4/EDB
(b) trapped by DMPO in CH2Cl2 upon the exposure to high-pressure mercury lamp
with a cut-off filter (365 nm). Photolysis time is 5 min.
shift has occurred. For example, SQ2 has an obvious red shift
compared with SQ1. As the conjugate group in SQ2 gives more
space to let the
SQ1, resulting in around 40 nm red-shift of
p
-electrons further away from the domain than
max. Because the
l
most commonly utilized irradiated wavelength of UV light is
around 365 nm, the satisfactory UV absorption properties make
SQs potential photoinitiators.
We investigated the photo-cleavage behavior of SQs under
irradiation of UV-light with Q as reference (Fig. 3, Fig. S3 in
Supporting information). Indeed, upon irradiation by UV-light, SQs
were cleavable, while Q kept almost unchanged. The absorption
peak of SQs decreased with the increasing irradiation time. We
supposed that the photo-cleavage behavior of SQs might be
ascribed to the weak bond of C–S, which might leads to generation
of free radicals. To confirm this point, we observed the DMPO/
radical adduct in an irradiated SQ4 or SQ4/EDB solution by ESR spin
trapping (Fig. 4), which supports the generation of desired radicals
through the SQ4 photo-cleavage and H-abstraction with EDB. In
the presence of 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) as
radical capturing agent, Fig. 4 shows that ESR signals had seven line
Fig. 5. Total ion flow chart (TIC) of extracts of SQ4 in acetonitrile using High
performance liquid chromatography mass spectrometry (HPLC-MS) before (a) and
after UV irradiation at 365 nm for an hour (b).
spectrum, which is explained by a triplet with
a
-nitrogen and a
thioether bond is ꢂ310 kJ/mol, which can be weakened by the
electron withdrawing group. Therefore, the cleavage of thioether
bond might be resulted from the energy transferred by the excited
quinoxaline group [35–38].
further split into a doublet with a -proton. The stronger intensity
b
of ESR peak for SQ4/EDB suggests the higher amount of radicals
generated after addition of EDB (Fig. 4b). The value of the magnetic
parameter, factor-g, was 2.0081, thus indicating that the thioether
or amine radicals had been released and trapped by DMPO [34].
To further investigate its cleavage mechanism, we used HPLC/
MS to analyze SQ4 in acetonitrile before and after irradiation. As
shown in Fig. 5, the total ion flow chart (TCI) spectra indicate the
mass corresponding to quinoxaline structure before or after
irradiation. Before irradiation, there is only one peak with a
molecular weight of 602.1835 g/mol in HPLC-MS, which is
corresponding to SQ4 (Fig. 5a). After irradiation, a peak with
molecular weight of 386.1760 g/mol appears in HPLC-MS spec-
trum, which is corresponding to quinoxaline group (Fig. 5b).
Comparing the two spectra and the different functional group
weights in SQ4, it can be confirmed that the C–S bond close to
quinoxaline group is photo-cleavable. The cleavage energy of
3.2. Photopolymerization of monomers initiated by quinoxalines
Since SQs possess satisfactory characteristics as photoinitiator
such as good absorption at UVꢀvisible range with high molar
extinction coefficient and the potential synergistic reaction of
photo-cleavable and hydrogen-abstracting groups, we investigat-
ed photopolymerization of a series of multifunctional acrylate
monomers initiated by SQs. To test the possibility of SQs as
cleavable photoinitiators, we first studied the photopolymeriza-
tion of HDDA initiated by SQs without hydrogen donor as
coinitiator. As shown in Fig. 6, the real-time FT-IR profiles reveal
that the photopolymerization behavior of HDDA appears to be
similar to the other bifunctional monomers. The final conversion of
1.5
1.5
(b)
(a)
0
0
5
5min
10min
20min
40min
10
1.0
1.0
15
25 min
0.5
0.5
0.0
0.0
300
400
500
300
400
500
Wavelength (nm)
Wavelength (nm)
Fig. 3. Photolysis experiments carried out in chloroform under the LED at 365 nm with gas protection. Evolution of the absorption spectrum as a function of time for SQ1 (a)
and Q (b).
Please cite this article in press as: J.-Y. Yao, et al., Combining photo-cleavable and hydrogen-abstracting groups in quinoxaline with
thioether bond as hybrid photoinitiator, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.008

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5
100
80
60
40
20
PIs
efficiency of SQs as photoinitiator. Fig. 7 presents the photo-
polymerization profiles of HDDA initiated by four SQs as well as Q
in. The final conversion of double bond for all four SQs is higher
than 90%, indicating that SQ/EDB is very efficient photoinitiator
system. Compared with SQs as photoinitiator, the higher efficiency
of SQ/EDB might be ascribed to synergistic effect of photo-
cleavable thioether groups and hydrogen-abstraction of quinoxa-
line groups, which can produce more free radicals under
irradiation of UV-light. In the presence of EDB as hydrogen donor,
the polymerization induction period for SQ2 become shorter, and
the final conversion of double bond for SQ1, SQ3 and SQ4 is higher
than 95%, much higher than that initiated by Q/EDB. Besides,
photoinitiation system combining with EDB shows an obvious
increase in the rate of polymerization (The rate of polymerization
reaction and relative parameters please refer to Table S2). This
indicated that the enhancement of initiating efficiency by EDB
should be ascribed to the hydrogen-abstracting reaction between
SQs and EDB, resulting in generation of more free radicals.
Compared with Q/EDB reference, the much higher final conversion
of double bond for SQs/EDB should be resulted from both photo-
cleavage and hydrogen-abstraction.
SQ1
SQ2
SQ3
SQ4
Q
0
0
1
2
3
Time (min)
Fig. 6. Photopolymerization profiles of HDDA in the presence of SQs under a
20 mL/min nitrogen flow, [SQs] = 10 mmol/L.
PIs/EDB
100
80
60
SQ1
Based on the photolysis and photopolymerization results
reveled by UVꢀvis absorption, HPLC/MS and RT-FTIR spectra,
the possible photoinitiation mechanism of SQ is proposed in
Scheme 3. Upon irradiation of UV light, the excited SQs can cause
cleavage of C–S bond to produce phenyl thiol radicals through
type- I approach, or abstract hydrogen from hydrogen donor
through type-II approach, resulting in formation of amino and
quinoxaline radicals. Owing to the steric hindrance and delocali-
zation of the unpaired electron, the quinoxaline radicals are
usually not active toward vinyl monomers. The photopolymeriza-
tion of vinyl monomers is usually initiated by phenyl thiol and
amino radicals, which are rather active. Therefore, both thioether
groups and hydrogen donor play an important role in SQ
photoinitiator systems.
To find the best hydrogen donor (coinitiator) for SQs, we
investigated the photopolymerization of HDDA initiated by SQ1 in
the presence of three coinitiators, EDB, MBO and MDEA. As shown
in Fig. 8a, the final conversion of HDDA double bond for three SQ1
systems is higher than 90%, suggesting that EDB, MBO and MDEA
are all efficient hydrogen donors for SQ1. Among these coinitiators,
EDB is the most efficient one, while MBO is the preference in other
detail [32]. Maybe aromatic secondary amine group have some
interaction with the photolysis products and EDB radicals were
more soluble in this system. Coinitiator content is also the key to
photopolymerization. Less coinitiator could not achieve enough
radicals, while excessive one may have some negative influences
on curing product, such as molecule migration, increased cost and
so on. Fig. 8b shows the real-time FT-IR profiles of photopolymer-
ization of HDDA initiated by SQ1/EDB systems with different
content of EDB. The final conversion of double bond was enhanced
by the increasing content of EDB. The optimum proportion of SQ1
and EDB is 1:4 and increased content of EDB has no positive effect
on photopolymerization.
SQ2
SQ3
40
SQ4
Q
20
0
0
1
2
3
Time (min)
Fig. 7. Photopolymerization profiles of HDDA in the presence of SQs/EDB under a
20 mL/min nitrogen flow. [SQs] = 10 mmol/L, [EDB] = 40 mmol/L.
HDDA for all four SQs is higher than 80%, indicating that SQs can be
used as the efficient photointiator. On the contrast, photoinitiating
efficiency of Q without thioether as reference photointiator is very
low in the absence of coinitiator. These results further confirm that
C–S bond of SQs can be broken to produce free radicals by
irradiation of UV-light. It should be noted that the final conversion
for SQ2 were more than 85%, while the polymerization induction
period is longer than the other SQs (The rate of polymerization
reaction relative parameters please refer to Table S2 in Supporting
information). This might be ascribed to large conjugate structure
with four cleavage thioether groups. The star-like structured SQ2
could provide more free radicals in the case of an equivalent to the
other SQs, might leading to high conversion. On the other hand, the
larger conjugate structure of SQ2 might lead to less solubility in
HDDA, resulting in the less diffusion rate of free radicals and longer
induction period.
The quinoxaline ring can abstract hydrogen from hydrogen
donor to generate radicals due to its strong electron-accepting
ability and excellent charge-transfer characteristics [39]. By
initiating photopolymerization of HDDA in the presence of EDB
hydrogen donor as coinitiator, we investigated photo-initiating
Scheme 3. Proposed mechanism for photoinitiators based on quinoxaline.
Please cite this article in press as: J.-Y. Yao, et al., Combining photo-cleavable and hydrogen-abstracting groups in quinoxaline with
thioether bond as hybrid photoinitiator, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.008

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Fig. 8. Photopolymerization profiles of HDDA in the presence of SQs/EDB, MBO, MDEA upon the high-pressure mercury lamp at 365 nm exposure under a 20 mL/min nitrogen
flow. [SQ1] = 10 mmol/L, [Co-i] = 40 mmol/L (a). Photopolymerization profiles of HDDA in the presence of SQs/EDB upon the LED at 365 nm exposure under a 20 mL/min
nitrogen flow. [SQ1] = 10 mmol/L, [EDB] = 20, 30, 40, 50 mmol/L (b).
100
80
60
40
20
0
with higher double bond conversion than 90%. It is believed that
these characteristics provide SQs as potential photoinitiators in
photo-curing field.
Acknowledgment
PH3EOA
G3POTA
A-BPE-10
PPGDA
The authors thank the National Basic Research Program (No.
2013CB834506) and National Nature Science Foundation of China
(Nos. 51373098 and 21522403) for their financial support. X. Jiang
thanks Mr. Kaji for discussions and the financial support from
Hitachi-Chemical.
TMPTA
0
1
2
3
Time (min)
Appendix A. Supplementary data
Fig. 9. Photopolymerization profiles of PH3EOA, G3POTA, A-BPE-10, PPGDA-540
and TMPTA in the presence of SQ1/EDB under
[SQ1] = 10 mmol/L, [EDB] = 40 mmol/L.
a 20 mL/min nitrogen flow.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.06.
008.
Motivated by the high photoinitiating efficiency of SQs in the
presence of coinitiator, we studied the photopolymerization of
various commercial acrylate monomers. Fig. 9 presents the real-
time FT-IR profiles of photopolymerization of various commercial
acrylate monomers EM2012, G3POTA, TMPTA, A-BPE-10 and
PPGDA initiated by SQ1/EDB. The final conversion of double bond
for all monomers is higher than 95%, except TMPTA with 55%
conversion, indicating that SQs are very potential photoinitiators.
As the UVꢀvis spectrum of SQ2 shows absorption peaks in the
visible region, we investigated its initiation efficiency at the
wavelength of 450 nm and the conversion of HDDA could be above
70% initiated by SQ2/EDB.
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