Synthesis and application of new S-benzoheterocycle thiobenzoates photoinitiators

Article abstract of DOI:10.1007/s11164-020-04170-3

A series of free radical photoinitiators with thioester bonds were synthesized by the nucleophilic substitution reaction of acyl chloride and thiol or thiophenol. These compounds could be efficiently obtained under mild reaction conditions and were fully

Full text of DOI:10.1007/s11164-020-04170-3

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04170-3
Synthesis and application of new S‑benzoheterocycle
thiobenzoates photoinitiators
Ziren Zhao1 · Chen Wang1 · Feng Liu1 · Bianxiang Zhang1
Received: 14 March 2020 / Accepted: 27 April 2020
© Springer Nature B.V. 2020
Abstract
A series of free radical photoinitiators with thioester bonds were synthesized by the
nucleophilic substitution reaction of acyl chloride and thiol or thiophenol. These
compounds could be efciently obtained under mild reaction conditions and were
1
fully characterized by H NMR, 13C NMR and high-resolution mass spectrome-
try. This kind of compounds exhibited a strong ultraviolet–visible light absorption
spectrum with an extended absorption around 330 nm in DMSO. Moreover, these
compounds were utilized as free radical photoinitiators in ultraviolet curing and
the resulting flms exhibited comparable mechanical properties in comparison with
commercially available photoinitiators, such as 1-hydroxycyclohexyl phenyl ketone
(Irgacure 184) and benzophenone. We also proposed a probable photoinitiated
polymerization mechanism for S-benzoheterocycle thiobenzoates as photoinitiators.
Keywords UV-curing flm · Free radical photoinitiator · Thioester
Introduction
The ultraviolet (UV)-curing technology is a safe, green and friendly environmental
technology [1–3]. Generally, the photopolymerization system is composed of a resin
(oligomer), an active diluent (monomer), triethanolamine (auxiliary agent) and pho-
toinitiator [4, 5]. It is worth noting that the photoinitiator plays an important role in
the system as it determines the photopolymerization speed and afects the polymer
properties, such as odor, yellowing and migration. Photoinitiators can be divided
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-020-04170-3) contains supplementary material, which is available to authorized users.
*
Bianxiang Zhang
zbxthh@sxu.edu.cn
Feng Liu
liufeng112@sxu.edu.cn
1
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China
Vol.:(0123456789)
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Z. Zhao et al.
into cationic photoinitiators and free radical photoinitiators according to the pho-
tochemical mechanisms [6, 7]. Nowadays, BP and Irgacure 184 are the most com-
monly used free radical photoinitiators owing to their low cost and good solubility.
However, owing to its small molecular weight, BP can easily migrate out of the pol-
ymer network and cause yellowing. These problems limit their applications in food
and pharmaceutical packaging. On the other hand, Irgacure 184 whose maximum
absorption wavelength is around 250 nm does not match the emission wavelength of
high-pressure mercury lamp. To make them more widely used, there are two ways to
improve their optical activity: One is to add the sensitizers, and the other is to extend
conjugated degree [8, 9]. Therefore, developing long wavelength absorbed photoini-
tiators with low mobility and low benzene release is a growing area of interest for
academic research and industrial applications [10, 11].
Historically, sulfur-containing materials have received signifcant interest
both in academy and industry, optical lens materials [12], mucoadhesive materi-
als [13], drug delivery systems [14], self-assembled monolayers (SAMs) [15–17]
and thiol-containing biomolecules [18]. Thioesters are one of the most represented
S-contained functional materials [19, 20]. Tomioka et al. [21] frst reported the use
of thioester-based materials as efcient photoinitiators. However, the short light
absorption wavelength of S-phenyl thiobenzoate limited their practical applications.
Herein, we designed and synthesized a series of S-benzoheterocycle thiobenzoates,
which can be efciently obtained by the nucleophilic substitution reaction of acyl
chloride and thiol or thiophenol. The S-benzoheterocycle thiobenzoates exhibit
a strong ultraviolet–visible light absorption with an extended absorption around
330 nm in DMSO. Moreover, these compounds can be applied as efcient photoini-
tiators with high photoactivity in a thiol/vinyl carbonate formulation.
Experimental section
Materials
Unless otherwise stated, all chemicals were obtained from commercial suppliers and
used without further purifcation. 1,6-Hexanediol diacrylate (HDDA) and polyure-
thane modifed acrylate resin (UV 2100) were purchased from Sanmu chemical co.,
Ltd. 1-Hydroxycyclohexyl phenyl ketone (Irgacure 184) and BP were obtained from
Beijing Yingli Technology co., Ltd (Scheme 1).
Scheme 1 Chemical structures and abbreviations of the materials used in the polymerization
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Synthesis and application of new S-benzoheterocycle…
Instrumentation
1H NMR and 13C NMR spectra were recorded in CDCl3 with a Bruker AVANCE III
600 MHz spectrometer. High-resolution mass spectra were obtained on a Thermo
Scientifc Q Exactive mass spectrometer. UV–Vis absorption spectra were recorded
in DMSO (1×10−5 M) on a Hitachi UH-5300 UV spectrophotometer (Hitachi High-
Technologies, Tokyo, Japan). The cured flms were performed on a UV-Curing
machine (ZB-T150-UV1000), and the hardness of flms was recorded by a portable
pencil scratch hardness tester (QHQ-A).
Synthesis
The substitution reaction of acyl chloride and mercapto reagent was carried out under
mild conditions, and the general procedure is as follows: Acyl chloride (2.0 mmol),
mercapto reagent (1.0 mmol) and base (2.0 mmol) were dissolved in THF. The reac-
tion mixture was stirred at 60 °C for 12 h. After cooling to room temperature, the pre-
cipitates were fltered of. The crude products were purifed by silica gel chromatog-
raphy using dichloromethane and petroleum ether as the eluent to aford compound 3.
S‑(1‑benzoyl‑6‑methoxy‑1H‑benzo[d]imidazol‑2‑yl) benzothioate 3e Pale yellow
1
crystals (75%), mp: 152–156 °C. H NMR (600 MHz, CDCl3) δ 7.97–7.90 (m,
ArH, 4H), 7.66 (dd, J=16.3, 7.5 Hz, ArH, 2H), 7.55–7.48 (m, ArH, 4H), 7.33 (dd,
J=10.6, 5.7 Hz, ArH, 1H), 6.92 (d, J=2.4 Hz, ArH, 1H), 6.88 (dd, J=8.9, 2.4 Hz,
ArH, 1H), 3.85–3.79 (m, -OCH3,3H). 13C NMR (151 MHz, CDCl3) δ 170.15,
169.00, 168.92, 157.91, 134.62, 134.38, 132.48, 132.31, 132.26, 130.72, 130.63,
128.93, 128.83, 125.41, 112.85, 112.10, 97.07, 77.25, 77.04, 76.83, 55.98. HRMS
(ESI) m/z: calcd for C22H16N2O3S: 389.0915, found: 389.0950.
S‑(6‑methoxy‑1‑(4‑methylbenzoyl)‑1H‑benzo[d]imidaz1ol‑2‑yl) 4‑methylbenzothio‑
ate 3f Pale yellow crystals (76%), mp: 239–241 °C. H NMR (600 MHz, CDCl3)
δ 7.85 (dd, J=10.6, 8.3 Hz, ArH, 4H), 7.32 (t, J=7.4 Hz, ArH, 4H), 7.27–7.24
(d, J=8.7 Hz, ArH, 1H), 6.86 (dt, J=8.7, 2.2 Hz, ArH, 2H), 3.81 (s, OCH3, 3H),
2.50–2.40 (d, J=3.1 Hz, CH3, 6H). 13C NMR (151 MHz, CDCl3) δ 169.87, 168.66,
168.58, 157.79, 146.18, 145.90, 132.35, 131.08, 131.00, 129.73, 129.63, 129.58,
129.44, 125.57, 112.60, 111.95, 96.86, 55.97, 21.99, 21.97. HRMS (ESI) m/z: calcd
for C24H20N2O3S{[M+H]+}: 417.1195, found: 417.1262.
S‑(6‑nitro‑1H‑benzo[d]imidazol‑2‑yl) benzothioate 3g Colorless oil liquid (62%),
1H NMR (600 MHz, CDCl3) δ 8.22–8.17 (m, ArH, 3H), 7.71 (t, J=7.5 Hz, ArH,
2H), 7.56 (t, J=7.9 Hz, ArH, 4H).
S, S’‑(thiobis(4,1‑phenylene)) dibenzothioate 3h White Crystals (81%), mp:
1
112–114 °C. H NMR (600 MHz, CDCl3) δ 8.05 (t, J=8.6 Hz, ArH, 2H), 7.64 (t,
J=7.3 Hz, ArH, 1H), 7.49 (d, ArH, 6H). 13C NMR (151 MHz, CDCl3) δ 189.86,
137.24, 136.47, 135.78, 133.83, 131.52, 128.82, 127.5, 126.45, 77.26, 77.05, 76.83.
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Z. Zhao et al.
S, S’‑(thiobis(4,1‑phenylene)) bis(4‑methylbenzothioate) 3i White crystals (71%),
1
mp: 147–149 °C. H NMR (600 MHz, CDCl3) δ 2.50 (s, CH3, 3H), 7.28–7.30 (m,
ArH, 2H), 7.43–7.49(m, ArH, 4H)), 7.91–7.93 (m, ArH, 2H). 13C NMR (75 MHz,
CDCl3) δ: 22.7, 77.5, 77.9, 78.3, 127.5, 128.5, 130.3, 132.4, 134.8, 136.7, 138.0,
145.7, 190.3; HRMS (ESI) m/z: calcd for C28H22O2S3: 487.0815, found 487.0852.
The preparation of cured flms
The polymeric formulation is composed of 45 wt% monomer (HDDA), 50 wt%
polyurethane modifed acrylate resin (UV 2100), 2 wt% triethanolamine and 3 wt%
photoinitiator. The photoinitiator and HDDA were mixed at room temperature under
stirring, and then, triethanolamine and resin were successively added. A thin layer
(ca. 75 um) of the resulting mixture was coated on the pre-cleaned glass substrates
and exposed to UV irradiation for a certain time to aford cured flms.
The gel content was determined by solvent extraction method. The cured flms baked
at 50 °C for 3 h and weighed M1. After soaking in acetone solution for 24 h, the fltrated
solids were dried and weighted M2. The gel content was calculated by the equation:
Gel content (%) = M2∕M1 × 100%.
Results and discussion
Optimization of the reaction conditions
Firstly, taking the reaction of 2-mercapto imidazole (1a) and benzoyl chloride (2a) as
an example, we carefully studied the reaction conditions, including the reaction solvent,
temperature, time and base. All the detailed results are summarized in Table 1. Initial
Table 1 Optimization of the reaction conditions
Entry
Temp (°C)
Time (h)
Solvent
Base
Yield (%)
1
2
3
4
5
6
7
8
9
r.t
6
6
DMF
DMF
DMF
DMF
DMF
DMF
DMF
THF
NaOH
NaOH
NaOH
NaOH
NaOH
K2CO3
Et3N
NaOH
K2CO3
Et3N
35
43
52
53
55
52
60
65
67
75
60
60
60
80
60
60
60
60
60
12
24
12
12
12
12
12
12
THF
THF
10
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Synthesis and application of new S-benzoheterocycle…
investigations focused on the reaction of 1a with 2.0 equiv of 2a in the presence of
NaOH (2.0 equiv) as base in DMF at room temperature for 6 h. It was found that the
expected product 2-phenylthiobenzimidazole 3a was obtained in a low isolated yield
(35%) (Table 1, entry 1). Further investigations revealed that the most efcient tempera-
ture was 60 °C (Table 1, entries 1–5). In addition, when the reaction time was increased
from 6 h to 12 h, the yield of the product 3a increased from 43 to 52%. To further
optimize the reaction conditions, various bases and solvents were examined (Table 1,
entries 6–10). Finally, 3a can be obtained with a high yield of 75% in THF at 60 °C for
12 h in the presence of Et3N as base (Table 1, entry 10).
With the optimized conditions in hand, we further examined the scope of the sub-
strates for the reaction of 2-mercaptoheterocycles (1b–1h) with chloride (2a–2b)
(Table 2).
Table 2 Synthesis of thioester compounds
entry sulfhydryl compound
aryl chloride
product
λabs
(nm)
ε (L·mol-1·cm-
1
1
2
3 (%)
)
1
2
3
305
306
311
3.93×104
7.56×104
7.38×104
2a
2a
4
5
323
7.31×104
2b
311
309
7.39×104
7.45×104
6
7
2a
2a
265
266
7.58×104
7.84×104
8
9
2b
2a
258
5.18×104
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Z. Zhao et al.
As shown in Table 2, the reactions of acyl chlorides with 2-mercaptobenzoxa-
zole and 2-mercaptobenzothiazole exhibited moderate yields (<70%), compared to
2-mercaptobenzimidazole (78%, Table 2, entries 1–3). In addition, we found that
methoxy-substituted 2-mercaptobenzimidazole gave the disubstituted products in
high yields (>70%), while 2-mercaptobenzimidazole with an electron-withdrawing
group (nitro) on the benzene ring only gave S-benzimidazole thiobenzoate, which
may ascribe to the activation of the N–H bond by the methoxy group (Table 2,
entries 4–5). On the other hand, we also synthesized other thioester compounds
from the corresponding substrates (Table 2, entries 7–9). Note that the reaction
of 4-methoxybenzenethiol and benzoyl chloride gave the corresponding product
S-(4-methoxyphenyl) benzothioate (3j) with the highest isolated yield of 98%.
UV–Vis absorption studies
Figure 1 depicts the UV absorption spectra of compounds 3 in DMSO solution.
These spectra were measured at an identical molar concentration (1×10−5 M), so
that a direct comparison of absorbance could be made. As shown in Fig. 1, com-
pounds 3 exhibit stronger ultraviolet absorption intensity due to the introduction
of benzoheterocycles and sulfur atoms in comparison with BP, and the maximum
absorption wavelength has a signifcant redshift.
Compared with BP, it can be seen that 3a, 3c and 3d possess higher ultravio-
let absorption intensity and redshift of maximum absorption wavelength due to
the introduction of benzoheterocycles. In addition, the absorption wavelengths of
compounds 3 with diferent benzoheterocycles are quite diferent. When connected
with benzoxazole, compound 3a presents the most signifcant red-shifted absorp-
tion spectrum with a maximum peak at 325 nm. As shown in Fig. 1b, the absorp-
tion wavelength of compounds 3e and 3f has a strong redshift with the increase in
carbonyl structure and benzene ring, which is attributed to n–π* or π–π* electron
transition. As shown in Fig. 1c, compounds 3h, 3i and 3j have better ultraviolet
absorption properties than BP due to the introduction of sulfur atoms. The chemi-
cal structures and absorption spectra of 3h and 3i are similar. Compared with BP,
3j exhibited stronger light absorption intensity and red-shifted spectrum due to the
formation of n-π* transition between heteroatoms and benzene ring structure, which
leads to the enhancement of the conjugation degree.
Photopolymerization studies
To evaluate the photopolymerization properties, all the synthesized thioester com-
pounds were utilized as free radical photoinitiators in polyurethane modifed acrylate
resin (UV 2100) and the activated monomer hexane-1,6-diyl diacrylate (HDDA)
with triethanolamine as additive. The experiments of UV-curable surface coating
were realized under commercially available UV-lamp (main peak at 365 nm) illumi-
nation. All the mechanical properties of the cured flm are summarized in Table 3.
As shown in Table 3, compounds 3a, 3b, 3c, 3e, 3h, 3j have been transformed
into smooth and transparent flms, indicating that they can be used as efcient radical
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Synthesis and application of new S-benzoheterocycle…
Fig. 1 Absorption spectra of
compounds 3 and BP
photoinitiators in the ultraviolet curing system. Moreover, we found that compounds
3e, 3f, 3h and 3i need shorter curing time to transform into flms. Compared with
BP and Irgacure 184, all the UV-cured coating flms show better adhesion. We also
found that some flms (3e, 3f, 3g and 3h) exhibited a better pencil hardness. Gel
content is an important parameter of photoinitiators, and 3e and 3h give the highest
gel content. Accordingly, compounds 3e and 3h are the most efcient photoinitia-
tors, which may ascribe to the introduction of multi-ketone segments in the chemical
structures.
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Z. Zhao et al.
Table 3 Properties of UV-curable flms
Entry Photoinitiators Gloss
Curing Adhesionb Pencil hardness Gel content (%)
timea
(s)
1
3a
Smooth and transpar- 20
ent
0
0
0
0
0
0
0
0
0
0
1
1
4H
4H
4H
4H
5H
5H
5H
5H
4H
4H
4H
4H
95.6
94.3
93.6
90.1
97.8
97.2
92.3
97.8
97.7
96.3
96.8
96.7
2
3b
Smooth and transpar- 20
ent
3
3c
Smooth and transpar- 25
ent
4
3d
Smooth and trans-
20
lucent
5
3e
Smooth and transpar- 15
ent
6
3f
Smooth and trans-
15
lucent
7
3g
Frosted and trans-
lucent
30
8
3h
Smooth and transpar- 15
ent
9
3i
Smooth and trans-
lucent
15
10
11
12
3j
Smooth and transpar- 20
ent
Irgacure 184
Smooth and transpar- 20
ent
BP
Smooth and transpar- 25
ent
aThe curing time of the UV-curing coating was measured by the fnger contact method
bThe determination of curing flm adhesion by the grid method. The levels are classifed into 0–5, level 0
is the best, and level 5 is the worst
Photoinitiation mechanism
Generally, the free radical photoinitiators [22–24] can be divided into pyrolytic
free radical photoinitiators and hydrogen-scavenging free radical photoinitia-
tors. Tomioka et al. considered that homolytic α-cleavage of C-S bond in S-phenyl
thiobenzoates is the initiation mechanism of photopolymerization under UV irradia-
tion. [21, 25–27] However, we found that the resulting flms without triethanolamine
as additive needed repeated UV-lamp illumination under the same curing conditions.
Thus, as shown in Scheme 2, we took the compound 3a as a model molecular and
proposed the possible photoinitiation mechanism [28–32]. Under UV illumination,
the sensitized 3a and the coinitiator triethanolamine can form an exciplex, and then,
two radical sources are produced through a photoinduced electron transfer process.
Both of them can participate in the following polymerization, thus accelerating the
curing process.
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Synthesis and application of new S-benzoheterocycle…
Scheme 2 Possible mechanism of polymerization
Conclusion
A series of S-benzoheterocycle thiobenzoate photoinitiators have been synthesized,
1
and their chemical structures were characterized by H NMR and 13C NMR, and
high-resolution mass spectrometry (HRMS). By introducing the benzoheterocycle
into the backbone, S-benzoheterocycle thiobenzoate photoinitiators exhibited strong
and red-shifted ultraviolet–visible light absorption spectrum. Moreover, these com-
pounds could be utilized as efcient free radical photoinitiators. They exhibited
excellent photoinitiating activity. The resulting flms possessed comparable/better
mechanical properties in comparison with BP and Irgacure 184, implying potential
applications in industrial production.
Acknowledgements We thank the National Natural Science Foundation of China (No. 21971154) and
the Shanxi Province Talent Project (No. 2018043) for fnancial support. We are also grateful for the plat-
form provided by the Scientifc Instrument Center of Shanxi University.
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