Photo-polymerization properties of type-II photoinitiator systems based on 2-chlorohexaaryl biimidazole (o-Cl-HABI) and various N-phenylglycine (NPG) derivatives

Article abstract of DOI:10.1039/C8PP00300A

A series of p-substituted NPG derivatives (Cl-NPG, OMe-NPG and NO₂-NPG) comprising different push-pull characteristics have been synthesized and characterized. The NPG derivatives have good thermal stability and red shifted absorption when comp

Full text of DOI:10.1039/C8PP00300A

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Photobiological Sciences
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Photo-polymerization properties of type-II
photoinitiator systems based on 2-chlorohexaaryl
biimidazole (o-Cl-HABI) and various
Cite this: Photochem. Photobiol. Sci.,
019, 18, 190
2
N-phenylglycine (NPG) derivatives†
a,b
a
a
Yung-Chung Chen,
*
Yuan-Tsung Kuo and Tsung-Han Ho
A series of p-substituted NPG derivatives (Cl-NPG, OMe-NPG and NO
2
-NPG) comprising different push–
pull characteristics have been synthesized and characterized. The NPG derivatives have good thermal
stability and red shifted absorption when compared with the original N-phenyl glycine (NPG) compound.
These NPGs were selected in combination with 2-chlorohexaaryl biimidazole (o-Cl-HABI) for Type II free
radical polymerization (FRP). Commercial NPG was also mixed with o-Cl-HABI for comparison. Their
photo-polymerization properties were investigated by the gel fraction method in a nitrogen atmosphere.
Electron transfer efficiencies for those Type II packages were studied by cyclic voltammetry (CV) and free
energy change ΔG results.
Received 12th July 2018,
Accepted 26th October 2018
DOI: 10.1039/c8pp00300a
rsc.li/pps
9
,10
units.
The Type II PIs are more useful than Type I PIs due
Introduction
to better optical absorption properties in the near-visible wave-
1
1
Photopolymerization reaction is the process of converting length region.
monomers into polymers initiated by photoinitiators (PIs) or
photoinitiator packages under a suitable light irradiation.
Photoresist for printed circuit board (PCB) was one of the
industrial application using Type II PIs. These kinds of photo-
1
Owing to the change in phase or solubility, these techniques imaginable materials are also called negative photoresist or
1
2
have a wide range of industrial applications such as coating, dry films, which were developed by DuPont in the 1960s.
adhesives, 3D printing, microlithography, optics, medicine, These kinds of PIs are generally used as hexaaryl biimidazole
2
–7
and nanotechnology. Free radical polymerization (FRP) and (HABI, or lophine dimer, L ) and its derivatives such as
2
cationic polymerization (CP) are the most common procedures 2-chlorohexaaryl biimidazole (o-Cl-HABI) as hydrogen acceptor
used in photochemical reaction for different photocuring compounds. The wide use of HABI compound was due to the
8
•
applications. Among them, the FRP was paid more attention high yield of the lophyl radical (L ) generation, low sensitivity
1
3
to due to several advantages compared to CP, such as rapid in the presence of oxygen and long lifetime. Currently, the
curing ability, flexible monomer chemistry, and spatial and incorporation of substituted functional group into the HABI
temporal control. FRP processes are classified into two types backbone has received much attention due to the influence on
depending on their decompositions mechanism including (1) UV-visible absorption and photo-reactivity.
Type I or α-cleavage PIs, also called one-component photo- As for the co-initiator, which is the hydrogen donor ingredi-
9
1
4
initiator, involving generation of radicals by PIs directly via a ent, the most common hydrogen donor including amine or
homolytic cleavage process under light irradiation, and (2) thiol derivatives were selected for industrial applications.
Type II PIs involving radical generation and electron transfer N-Phenylglycine (NPG), a well-known amino acid, has been
processes between hydrogen acceptor and hydrogen donor widely used due to biologically less-toxic, non-allergic and
1
5
odorless nature compared to thiol derivatives. In addition,
the NPG derivatives have gained considerable interest for
1
6
17
a
application in various fields such as drugs, reprography,
Department of Chemical and Materials Engineering, National Kaohsiung University
of Science and Technology, 415 jiangong Rd., Sanmin District, Kaohsiung City
0778, Taiwan, Republic of China. E-mail: chenyc@nkust.edu.tw
1
5
and dental restorative materials. To summarize, the photo-
decomposition of NPGs and its derivatives (NPGs) can be sen-
8
b
Photo-SMART (Photo-sensitive material advanced research and technology Center),
National Kaohsiung University of Science and Technology, 415, jiangong Rd.,
Sanmin District, Kaohsiung City 80778, Taiwan, Republic of China
sitized by lots of initiator ingredients or multicomponent
17–19
photoinitiator systems under UV-light.
For example,
Rabek et al. presented the photoreactivity of camphorquinone
(CD) and different amines. With different NPG derivatives, the
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8pp00300a
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photo-reactivity changes significantly. Among them, NPG
showed good rate of polymerization than the others.
Currently, researchers adopted NPG as coinitiators for two or
Experimental section
Materials
1
8
multicomponent CP or FRP applications. The NPG-based PI 2-Chlorohexaaryl biimidazole (o-Cl-HABI), N-phenylglycine
packages also demonstrate excellent photoreactivity under (NPG), and trimethylolpropane trimethacrylate (TMPTMA)
LED as light source and can be a good candidate for 3D print- were supplied by DuPont, Taiwan. 4-Chloroaniline, p-anisi-
2
0,21
ing or sensors.
dine, 4-nitroaniline, ethyl chloroacetate, 2-bromoacetic acid,
Previously, we had studied photo-reactivity based on and sodium acetate were purchased from Sigma-Aldrich and
different weight percentages of o-Cl-HABI and NPG PIs. It was used as received without further purification. Solvents: hexane,
found that the NPG compound had a great impact on photo- methanol, ethanol, tetrahydrofuran (THF), dimethyl-
2
2
reactivity than o-Cl-HABI.
The corresponding proposed formamide (DMF), and acetone were used as received.
mechanism and scheme of this type II PIs based on o-Cl-HABI
•
and NPG is shown below: (1) the formation of L after light Synthesis of NPG derivatives (Cl-NPG, OMe-NPG and NO -NPG)
2
irradiation, (2) the lophyl radical can abstract hydrogen atom (Scheme 1)
from hydrogen donor coinitiators (NPGs), (3) the NPG radical
was generated by the decarboxylation process, which then
Cl-NPG (N-(4-chlorophenyl)glycine). 4-Chloroaniline (20 g,
.157 mol), ethyl chloroacetate (22 mL, 0.155 mol) and sodium
0
initiated polymerization of monomers with the addition of
vinyl groups.
To continue our study on investigating o-Cl-HABI/NPG
photoinitiator package, a series of p-substituted NPG deriva-
tives with different electronics push–pull characteristics, i.e.
OMe-NPG (methoxy group), Cl-NPG (chloro group) and
acetate (15 g, 0.19 mol) were mixed together in 100 mL ethanol
used as the solvent. The solution was refluxed for 12 h with
stirring. The reaction solution was left overnight at room temp-
erature and filtered to obtain a clear solution. The clear solu-
tion was then poured into iced water. The precipitate formed
was collected by filtration and dried. The dried intermediate,
2
NO -NPG (nitro group) as shown in Fig. 1 were designed and
synthesized. In addition, their photo-reactivity, thermal pro-
perties, solubility, photo-physical and electrochemical pro-
perties are analyzed and compared with the original coinitia-
tor, NPG.
Fig. 1 Structures of N-phenyl glycine (NPG) and NPG derivatives (OMe-
Scheme 1 Synthesis route for N-phenyl glycine derivatives (OMe-NPG,
NPG, Cl-NPG and NO
2
-NPG).
2
Cl-NPG and NO -NPG).
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ethyl ester of Cl-substituted glycine, was used for the next step Measurements
without further purification. Aqueous sodium hydroxide solu-
tion (2.25 M, 50 mL) and ethyl ester of Cl-substituted glycine
were mixed together and refluxed for 1 h. After cooling, the
reaction mixture was acidified using aqueous hydrochloric
acid to pH = 2. The precipitated light yellow Cl-substituted
glycine (Cl-NPG) solid was filtered and washed thoroughly
1
H-NMR spectra were recorded using an Agilent Unity plus-400
spectrometer. High-Resolution Mass spectra (HR-MS, FAB) and
Electron Ionization spectra (EI) were recorded using a Bruker
APEX II mass spectrometer and a JEOL JMS-700 mass spectro-
meter, respectively. Ultraviolet–visible (UV-vis) spectra were
recorded using
Spectrometer. Cyclic voltammetry (CV) was conducted using a
BioLogic SP-150 model at a scan rate of 100 mV s . Melting
Point (T ) was determined using TA Instruments DSC 2920
Modulated at a heating rate of 10 °C min from 30 to 250 °C
in a nitrogen atmosphere. The photopolymerization was con-
ducted using a Philips 16 W lamp (type Actinic BL) with a total
a
PerkinElmer Lambda 35 UV/VIS
1
with cold water (3.6 g, 49.1%) (m.p. = 146 °C). H-NMR
(
400 MHz, CDCl
J = 8.8 Hz, 2H, H
culated for C H ClNO , 186.03163 found: 186.03159.
3
, δ, ppm): 7.16 (d, J = 9.2 Hz, 2H, H
a
), 6.55 (d,
−1
+
b
), 3.96 (s, 2H, –CH , H ). HRMS m/z [M] cal-
2
c
m
−1
8
8
2
OMe-NPG (N-(4-methoxyphenyl)glycine). The procedure
used for this synthesis was similar to that described for Cl-
NPG. p-Anisidine (8 g, 0.07 mol), ethyl chloroacetate (6 mL,
−2
light around 605 mJ cm for 120 s exposure time, which was
tested using a radiometer.
0
.04 mol) and sodium acetate (5.6 g, 0.07 mol) were mixed
together in 50 mL ethanol used as the solvent to obtain the
2
3
OMe-NPG (2.3 g, 43%). (m.p. = 165 °C) (m.p. = 142 °C).
1
H-NMR (400 MHz, DMSO-d
6
, δ, ppm): 8.8 (1H, –COOH, H
d
),
Results and discussion
6
2
.71 (d, J = 9.2 Hz, 2H, H
a
), 6.5 (d, J = 9.2 Hz, 2H, H ), 3.72 (s,
b
+
H, –CH , H ), 3.63 (s, 3H, –OCH , H ). HRMS m/z [M] calcu- The synthetic procedures for the series of NPG derivatives (Cl-
2
c
3
e
lated for C
NO -NPG (N-(4-nitrophenyl)glycine). 4-Nitrobenzenamine NPG and OMe-NPG were synthesized according to the method
3 g, 0.02 mol), 2-bromoacetic acid (3 g, 0.02 mol) and sodium reported previously in the literature. First, the p-substituted
acetate (3.5 g, 0.04 mol) were mixed together in 40 mL H O used anilines were reacted with ethyl chloroacetate in the presence
9
H11NO
3
, 182.08117 found: 182.08128.
2
NPG, OMe-NPG and NO -NPG) are shown in Scheme 1. The Cl-
2
2
3
(
2
as the solvent. The solution was refluxed for 24 h with stirring. of base as catalyst to afford the intermediates 1 and 2.
The reaction solution was filtered hot to remove the impurity, and Subsequently, the relative acetic ester groups were converted
the filtrate was cooled to precipitate the crude product. Then, the into acid groups and the target compounds were obtained (Cl-
product was subjected to column chromatography using ethyl NPG and OMe-NPG). On the other hand, the NO
2
-NPG com-
acetate as the eluent to give the yellow solid (0.7 g, 20.1%) (m.p. = pound was obtained directly from p-substituted aniline with
1
1
2
–
38 °C). H-NMR (400 MHz, DMSO-d
COOH, H ), 8.00 (d, J = 9.2 Hz, 2H, H ), 7.76 (1H, –NH, H
d, J = 9.2 Hz, 2H, H ), 3.99 (3H, –CH , H ). EI-Mass m/z [M] calcu- of NPG derivatives. Fig. 2 presents the H-NMR spectra of Cl-
6
, δ, ppm): 12.83 (1H, 2-bromoacetic acid in the presence of base as catalyst. H-NMR
e
a
c
), 6.66 and HR-MS were used to confirm the structural characteristics
1
(
b
2
d
lated for C
8
H
8
N
2
O
4
, 196.16, found: 196.0483.
NPG, OMe-NPG and NO
three compounds resonated in the region of 6.55–7.16,
.5–6.71, and 6.66–8.00 ppm, and their methyl protons (–CH2)
appeared in the region of 3.96, 3.72, and 3.99 ppm, respect-
The gel fraction method was selected in this study to obtain the ively. A slightly downfield shift of the aromatic protons for
2
-NPG. The aromatic protons for these
6
Degree of conversion (DC), gel fraction
24
DC value and calculated by the following eqn (1). Gel fraction is NO -NPG compound was attributed to the electron-withdraw-
2
a convenient method to measure the insoluble fractions, such as ing characteristic of the nitro group. In addition, the carboxyl
the cross-linked or network polymers. Typical formulations group of the OMe-NPG and NO -NPG could be measured in
2
(TMPTMA/o-Cl-HABI/NPGs = 97.95/2/0.05 wt%) were used in this regions of 8.88 and 12.83 ppm, respectively. Furthermore, the
study for gel fraction testing. First, TMPTMA, o-Cl-HABI and rela- HR-MS spectroscopies were confirmed with the proposed
tive NPG derivatives were mixed together at weight ratios of structures. Thus, these results are in good agreement with the
2 2
2
97.95 : 2 : 0.05 using CH Cl as the solvent. After completely dis- expected structures of Cl-NPG, OMe-NPG and NO -NPG.
solution, the solvent was removed and poured into the plate with
The melting points of these NPG derivatives were evaluated
fixed area and weight (around 0.5 g). Subsequently, the formu- by DSC. In addition, the o-Cl-HABI and NPG was also con-
lation was exposed for 120 s under nitrogen atmosphere. Finally, ducted in this study. Fig. 3 shows the DSC curves of these com-
the cured sample was washed by Soxhlet extraction using hexane pounds and their relevant data are summarized in Table 1.
as the solvent for 6 h. After that, the sample was filtered out and DSC experiments were carried out at a heating rate of
10 °C min− in a nitrogen atmosphere. The melting points of
these compounds are related to the relative ease or difficulty of
ð1Þ overcoming the cohesive forces between the molecules. The
melting point of o-Cl-HABI was 210 °C. Moreover, the melting
1
dried to get the final polymerized polymer.
Wt ðgÞ
Gel fraction ¼
ꢀ 100%
W0 ðgÞ
0 t 2
where W is the original weight and W is the weight after points of the NPGs were in the order of NO -NPG (238 °C) >
drying. To ensure the results, the average of at least three sep- OMe-NPG (165 °C) > Cl-NPG (146 °C) > NPG (127 °C). All the
arate determinations was used.
NPG derivatives showed higher melting point than the refer-
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1
Fig. 2 H-NMR spectra of OMe-NPG, Cl-NPG and NO
2
-NPG.
ence NPG sample, which indicated that all the samples polar solvent, DMF, and were also soluble in low-boiling
showed higher thermal stability than NPG. Moreover, NO2- solvents such as THF and acetone. However, the NO -NPG and
2
m 2 2
NPG showed the highest T due to the presence of the strong OMe-NPG samples showed poor solubility in CH Cl and dis-
electron-withdrawing nitro group, resulting in larger dipole solved in MeOH after heating. The overall solubilities of the
2
5,26
moment.
For ensuring well-dissolved for the formulation (TMPTMA/ OMe-NPG ≈ NO
o-Cl-HABI/NPGs), the solubility in various organic solvents NO -NPG was due to the stronger intramolecular strength than
NPG derivatives were in the following order: NPG > Cl-NPG >
2
-NPG. The poor solubility of OMe-NPG ≈
2
is very important. The solubility for the o-Cl-HABI and other samples. This is also consistent with the melting point
NPGs samples was determined at a concentration of 1 mg of results mentioned above. Thus, to ensure the well-dispersed
compound in 1 mL of solvent and the results are listed in formulation for DC testing, we used THF as the solvent in
Table 2. All the samples showed good solubility in aprotic latter formulation.
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Table 2 Solubility behavior of o-Cl-HABI and NPG derivatives in
a
various organic solvents
Compound
DMF
2
CH Cl
2
THF
CH
3
OH
Acetone
o-Cl-HABI
NPG
Cl-NPG
OMe-NPG
○
○
○
○
○
○
○
Δ
X
○
○
○
○
○
○
○
○
Δ
Δ
○
○
○
○
○
2
NO -NPG
X
a
Solubility was determined with 1 mg of compound in 1 mL of
solvent. ○ = soluble at room temperature; Δ = soluble on heating; X =
insoluble even on heating.
Fig. 3 DSC curves of o-Cl-HABI and NPG derivatives.
The UV-visible absorption spectra of all the samples of o-Cl-
HABI and NPGs in THF solution are shown in Fig. 4, and their
corresponding data are listed in Table 1. The o-Cl-HABI
showed broader and higher absorption than NPG ranging
from 250–325 nm. All the NPGs have the absorption bands
shorter than 300 nm due to the localized π–π* transition. In
addition, p-substituted NPG derivatives show that the long
absorption at >300 nm are from more delocalized π–π* tran-
sition with charge transfer character. Thus, the absorption
Fig. 4 UV-vis absorption of o-Cl-HABI and NPG derivatives in THF
solutions.
bands decrease in the order of NO
NPG depending on the push–pull characteristics. In
addition, the original NPG exhibited smaller absorption at
300 nm, which indicated the insignificant charge transfer for
this molecule.
2
-NPG > OMe-NPG > Cl-NPG
>
>
action energy for the initially formed ion pair, generally con-
2
7
To investigate the DC values with o-Cl-HABI/NPGs packages, sidered as negligible in polar solvents. Thus, Eox of NPGs,
the free energy changes, ΔG, were calculated. The ΔG values Ered of o-Cl-HABI and E of NPGs can be measured by cyclic
g
illustrated the electron transfer phenomenon between o-Cl- voltammetry and the cutoff of the absorption spectra. The oxi-
HABI and NPGs, and these values were calculated from the dation spectra of the NPGs, reduction spectra of the o-Cl-HABI
2
7
classical Rehm–Weller equation, i.e. ΔG = Eox − Ered − E
g
+ C, and their corresponding results are summarized in Fig. 5,
where E , E , and E are the oxidation potential of the elec- Fig. S1 and Table 1.† The values of the relative ΔG were also
ox
red
g
tron donor (NPGs), the reduction potential of the electron obtained based on the Rehm–Weller equation mentioned
acceptor (o-Cl-HABI), and the excited singlet state energies of above and also summarized in Table 1. The ΔG values of o-Cl-
NPGs, respectively. In addition, C is the electrostatic inter- HABI/NPG, o-Cl-HABI/Cl-NPG, o-Cl-HABI/OMe-NPG and o-Cl-
Table 1 Characteristic parameters of o-Cl-HABI and NPG derivatives
a
abs (ε) (M−1 cm−1) nm
b
c
d
e
f
Compound
λ
E
ox (V)
E
red (V)
E
g
(eV)
ΔG (V)
m
T (°C)
o-Cl-HABI
NPG
Cl-NPG
OMe-NPG
287 (17 460)
280 (4681), 289 (4344)
278 (2580), 308 (2080)
280 (1094), 291 (1570), 314 (2162)
280 (2208), 289 (1767), 371 (11 251)
—
−1.71
—
—
—
—
—
—
210
127
146
165
238
0.69
0.82
0.44
1.23
3.94
3.75
3.64
2.99
−1.54
−1.22
−1.49
−0.05
2
NO -NPG
a
Recorded in THF solutions at 10−5 M. Recorded in THF solutions. Scan rate, 100 mV s−1; electrolyte, Bu
b
n
4
NPF
6
. The oxidation potentials were
, was derived from the
. Eox: oxidation potential of relative NPGs; Ered: reduction
c
d
measured by cyclic voltammetry. The reduction potential was also measured by cyclic voltammetry. The bandgap, E
offset of the absorption spectra. From the Rehm–Weller equation. ΔG = Eox − Ered − E
g
potential of o-Cl-HABI; E : band gap of the NPGs. Baseline shift in the second heating DSC traces, at a heating rate of 10 °C min .
g
e
g
f
−1
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The degree of conversion (DC %) was also tested by gel frac-
tion method mentioned above (eqn (1)). Fig. 6 exhibits gel frac-
tion results under a similar exposure time (120 s) with the
weight composition of o-Cl-HABI : NPGs at 2 : 0.05. The
exposed environment is a critical issue to determine the con-
2
2
version efficiency of the PI package. In order to correctly and
systematically realize the DC values of different packages, we
eliminated the effects from the uncertainty of atmosphere.
Thus, the gel fraction results were tested in a nitrogen atmo-
sphere for the reduction of the NPGs diluted effects by air
during the exposed period. The DC % decreases in the order
of o-Cl-HABI/NPG (51.7%) > o-Cl-HABI/OMe-NPG (41.8%) >
o-Cl-HABI/Cl-NPG (35.5%)
2
> o-Cl-HABI/NO -NPG (0%). A
higher DC value implies good electron transfer from hydrogen
acceptor to hydrogen donor, that is, the o-Cl-HABI/NPG has
the best electron transfer efficiency than others. These results
are also consistent with the free energy changes mentioned
above. In addition, the o-Cl-HABI/NO -NPG showed nearly no
2
reaction after exposure for 120 s, which might be due to no
electron transfer between o-Cl-HABI and NO -NPG.
2
Conclusions
Fig. 5 Cyclic voltammograms of the o-Cl-HABI and NPG derivatives in
In conclusion, a series of p-substituted NPG derivatives (Cl-
THF solutions.
NPG, OMe-NPG and NO -NPG) have been synthesized and
2
characterized. The NPG derivatives have good thermal stability
and red shift absorption than the pristine NPG analogue.
However, the DC value of those o-Cl-HABI/NPGs package is
still lower than that of o-Cl-HABI/NPG. We created the electron
transfer correlation between o-Cl-HABI and NPGs, which
should be useful for structural designing of these kinds of type
II photoinitiator package.
HABI/NO -NPG were −1.54, −1.22, −1.49 and −0.05 V, respect-
ively. The ΔG value of the o-Cl-HABI/NPG package is lower
2
than other packages, suggesting that this package has the
2
8
most
spontaneous electron
transfer characteristic.
Furthermore, the ΔG value of the o-Cl-HABI/NO -NPG package
2
is close to zero, which indicates nearly no electron transfer
relative to this package.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Financial support from (1) Ministry of Science and
Technology, Taiwan ROC and (2) DuPont, Taiwan is gratefully
acknowledged.
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