A thioxanthone-based photocaged superbase for highly effective free radical photopolymerization

Article abstract of DOI:10.1016/j.cclet.2014.05.031

Thioxanthone-based N-phthalimidoamino acid ammonium salt (thioxanthen-DBU) as a photocaged base was synthesized and characterized. The photochemical properties and initiation mechanism were analyzed. It was found that the compound absorbs over the UV and visible region with relatively high absorption coefficients. Furthermore, the covalent binding of N-phthalimidoamino acid and type II chromophores (thioxanthone, TX) remarkably improved the photoreactivity. Specifically, in combination with a benzoyl peroxide initiator, thioxanthen-DBU was able to initiate the amine-mediated redox photopolymerization of trimethylol propane triacrylate (TMPTA), and an excellent photopolymerization profile was obtained.

Full text of DOI:10.1016/j.cclet.2014.05.031

G Model
CCLET 3012 1–5
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
2
Original article
3
4
A thioxanthone-based photocaged superbase for highly
effective free radical photopolymerization
Q1 Ming-Hui He a, Rui-Xin Xu a,c, Guang-Xue Chen a, Zhao-Hua Zeng b, Jian-Wen Yang b,
*
5
6
7
8
a State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou 510640, China
b Institute of Polymer Science, DSAPM Lab, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
c School of Media and Communication, Shenzhen Polytechnic, Shenzhen 518055, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Thioxanthone-based N-phthalimidoamino acid ammonium salt (thioxanthen-DBU) as a photocaged
base was synthesized and characterized. The photochemical properties and initiation mechanism were
analyzed. It was found that the compound absorbs over the UV and visible region with relatively high
absorption coefficients. Furthermore, the covalent binding of N-phthalimidoamino acid and type II
chromophores (thioxanthone, TX) remarkably improved the photoreactivity. Specifically, in combina-
tion with a benzoyl peroxide initiator, thioxanthen-DBU was able to initiate the amine-mediated redox
photopolymerization of trimethylol propane triacrylate (TMPTA), and an excellent photopolymerization
profile was obtained.
Received 11 February 2014
Received in revised form 13 May 2014
Accepted 14 May 2014
Available online xxx
Keywords:
Photocaged superbase
Redox initiation
Photopolymerization
ß 2014 Jian-Wen Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
9
10
1. Introduction
consideration. Firstly, the synthesis of thioxanthone (TX) 33
derivatives has recently received interest in photochemistry 34
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Recently, photopolymerization reactions have received revital-
ized interest as they congregate a wide range of economic and
ecological anticipations, while photoinitiators (PIs) or photoini-
tiating systems (PISs) have been the subject of intense studies
[1–9]. However, the light attenuation is a major issue when the
photopolymerization is used for the synthesis of photo-screened
because of their good absorption characteristics and high 35
photoinitiation efficiency in the visible region [14–19]. This is 36
significant because visible light is cheap, safe, and able to 37
penetrate formulations consisting of UV absorbing monomers, 38
pigments, and substrates. Secondly, there are significant inter- 39
actions between N-substituted maleimides and type II PIs, which 40
lead to enhanced photoefficiency [20,21], thus we would design 41
the new chemical-bonded photosensitive groups comprising of 42
the parent N-phthalimidoamino acids and type II chromophores. 43
Finally, both carboxylic acid derivatives of TXs and photolatent 44
amine have been reported to serve as hydrogen bond donors [22]. 45
In the paper, the aim is to establish the mechanism of the 46
primary photocleavage of thioxanthen-DBU and to facilitate the 47
design of future photocaged bases. In order to obtain the clear-cut 48
information on the redox photopolymerization, we extensively 49
studied the photopolymerization of trimethylol propane triacry- 50
late (TMPTA) in the presence of a latent redox initiator combina- 51
tion composed of thioxanthen-DBU and dibenzoyl peroxide (BPO), 52
and the high efficiency has been demonstrated by real-time RTIR 53
functional materials. Hence, we recently proposed
a redox
photopolymerization using a photocaged base and a peroxide,
which represents a new free radical PIS [10–13]. The main
advantages of the PISs are their longeval active centers and high
initiating capability. Remarkably, this PIS leads to significant post
conversion due to the persistent interactions of the photogener-
ated amines with the peroxide. Through this mechanism, the
problem of light attenuation that is associated with the conven-
tional free radical photopolymerization has been overcome. This
finding may enable the use of PIS as a phototrigger for self-
propagating polymerization reactions to access a larger number of
opaque composites.
As part of our continuous interest in developing highly
effective photocaged bases, herein we design a thioxanthone-
based N-phthalimidoamino acid ammonium salt (thioxanthen-
DBU, Scheme 1), which has taken the following points into
measurements.
54
2. Experimental
55
Thisosalicyilic acid (99%), trans-4-(aminomethyl)cyclohexane- 56
*
Corresponding author.
carboxylic acid (98%), polyphosphoric acid (85%), acetic anhydride, 57
E-mail address: cesyjw@mail.sysu.edu.cn (J.-W. Yang).
http://dx.doi.org/10.1016/j.cclet.2014.05.031
1001-8417/ß 2014 Jian-Wen Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: M.-H. He, et al., A thioxanthone-based photocaged superbase for highly effective free radical
photopolymerization, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.031

G Model
CCLET 3012 1–5
2
M.-H. He et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Scheme 1. Synthesis route for thioxanthen-DBU.
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
1,8-diazabicyclo[5.4.0] undec-7-ene (DBU, 99%), and dibenzoyl
peroxide (BPO) were purchased from Aladdin-reagent (China). BPO
was purified by dissolving the commercial material in CHCl3 at
room temperature and precipitating by adding an equal volume of
MeOH. Trimethylol propane triacrylate (TMPTA, Sartomer Com-
pany), isopropyl thioxanthone (ITX, IHT Group) were used as
received. All other chemicals used were of analytical grade and
used without further purification.
0.1 mol/L) was refluxed for 90 min, and then acidified with
concentrated HCl. The mixture was refluxed with stirring for
18 h. The crude diacid c was filtered, washed with water, dried in
vacuo to give a yellow powder. Yield: 56.2%. 1H NMR (300 MHz,
DMSO-d6): d 7.66 (t, 1H), 7.85 (t, 1H), 8.01 (d, 1H), 8.10 (d, 1H), 8.48
(d, 1H), 8.90 (d, 1H). Anal. Calcd. for C15H8O5S: C, 60.12; H, 2.75; S,
10.58%; Found: C, 64.91; H, 2.81; N, 4.92; S, 11.23.
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
9-Oxo-9H-Thioxanthene-3,4-anhydride (b) [23]: A mixture of c
(0.58 g, 0.0017 mol) and acetic anhydride (8.5 mL, 0.09) in xylene
(80 mL) was heated to reflux for 90 min. The precipitated powder
was filtered, washed with water, dried in vacuo to give a yellow
The NMR spectra were obtained on
a Varian 300 MHz
spectrometer using DMSO-d6 and TMS as the solvent and internal
standard, respectively. Elemental analysis was obtained on an
Elementar Vario EL analyzer. Electrospray ionization mass spectra
(ESI-MS) were acquired on a Thermo Finnigan LCQ DECA XP ion
trap mass spectrometer, equipped with an ESI source. UV–vis
absorption spectra were obtained on a Perkin Elmer Lambda 750
UV–visible spectrophotometer. Acrylate conversions were moni-
tored by real-time Fourier transform infrared (RTIR) spectroscopy
using a modified Nicolet 5700 spectrometer. Photopolymerization
reactions were conducted in a mold from two glass plates and
spacers with 15 ꢀ 1 mm in diameter and 1.2 ꢀ 0.1 mm in thickness,
changes in the peak area from 6104 to 6222 cmꢁ1 attributed to the
stretching vibration were used to monitor the acrylate polymeriza-
tion kinetics.
powder b. Yield: 87.3%. 1H NMR (300 MHz, DMSO-d6):
d 7.67 (t,
1H), 7.86 (t, 1H), 8.02 (d, 1H,), 8.12 (d, 1H), 8.49 (d, 1H), 8.92 (d, 1H).
Anal. Calcd. for C15H6O4S: C, 63.83; H, 2.14; S, 11.36%; Found: C,
63.63; H, 2.04; S, 11.16.
4-((1,3,6-Trioxothiochromeno[2,3-e]isoindol-2(1H,3H,6H)-
yl)methyl)cyclohexanecarboxylic acid (a):
A solution of b
(5 mmol) and trans-4-(aminomethyl)-cyclohexanecarboxylic acid
(0.785 g, 5 mmol) in acetic acid (20 mL) was refluxed for 3 h. The
reaction mixture was cooled to room temperature, poured into ice
cooled water (50 mL), and stirred for 15 min. A white crystalline
product was obtained, filtered and dried. Yield: 66.1%. ESI-MS
79
80
(negative mode): m/z 420.1 (MꢁHꢁ) (calcd. for MꢁHꢁ: 420.1). 1
H
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
Synthesis route for thioxanthen-DBU was given in Scheme 1.
2-(1,3-Dioxoisoindolin-4-ylthio)benzoic acid (e) [23]: Thiosa-
licylic acid (15.4 g, 0.1 mol) in NaOH aqueous solution (200 mL,
1 mol/L) was warmed until the solid dissolved, and then
continually warmed to remove water. Ethanol was added into
the sticky mixture, and then the white precipitates were collected
by filtration, washed with ethanol, and dried in vacuo to obtain
sodium thiosalicylate (f). A mixture of f (18.215 g, 0.1034 mol),
3-nitrophthalimide (15.892 g, 0.0827 mol), and DMF (200 mL) was
stirred at 80 8C for 8 h. A HCl solution (200 mL, 2 mol/L) was added.
The precipitate was filtered, washed with water, dried in vacuo,
and then recrystallized from dioxane to give a yellow powder.
NMR (300 MHz, DMSO-d6): d 1.05 (m, 2H), 1.23 (m, 2H), 1.73 (m,
3H), 1.90 (m, 2H), 2.14 (m, 1H), 3.44 (d, 2H), 7.63 (t, 1H), 7.82 (t,
1H), 7.95 (t, 2H), 8.44 (d, 1H), 8.80 (d, 1H). Anal. Calcd. for
C23H19NO5S: C, 65.54; H, 4.54; N, 3.32; S, 7.61%; Found: C, 64.34; H,
4.59; N, 3.27; S, 7.46.
4-((1,3,6-Trioxothiochromeno[2,3-e]isoindol-2(1H,3H,6H)-
yl)methyl)cyclohexanecarboxylic acid DBU salt (thioxanthen-
DBU): To a (5 mmol) in dioxane (30 mL) was slowly added excess
DBU (10 mmol) in dioxane (20 mL) and the mixture was stirred at
room temperature for 24 h. The mixture was poured into water
(100 mL), filtered, washed thrice with water, and dried to give
the white powder. The product can be further purified by silica
gel column chromatography using hexane: EtOAc (1:5) to give a
pure product. Yield: 85%. ESI-MS (negative mode): m/z 420.1
(M ꢁHꢁ) (calcd. for MꢁHꢁ: 420.5); (positive mode): m/z 153.4
(M +H+) (calcd. for M+H+: 153.14). 1H NMR (300 MHz, DMSO-d6):
Yield: 95.4%. 1H NMR (300 MHz, DMSO-d6):
d 7.29–7.31 (m, 2H),
7.47 (m, 2H), 7.66 (m, 2H), 7.86 (m, 1H). Anal. Calcd. for
C15H9NO4S: C, 60.19; H, 3.03; N, 4.68; S, 10.71; Found: C, 59.48;
H, 3.20; N, 4.54; S, 10.51.
Thiochromeno[2,3-e]isoindole-1,3,6(2H)-trione (d) [23]:
A
d 1.04 (m, 2H), 1.26 (m, 2H), 1.61 (m, 9H), 1.77 (m, 4H), 2.13 (m,
1H), 2.66 (d, 2H), 3.24–3.55 (m, 6H), 3.62 (m, 2H), 7.64 (t, 1H),
7.83 (t, 1H), 7.96 (t, 2H), 8.45 (d, 1H), 8.81 (d, 1H). Anal. Calcd. for
suspension of e (6 g, 0.02 mol) in polyphosphoric acid (100 g,
0.30 mol) was stirred at 150 8C for 90 min, and then the mixture
was diluted to 500 mL with ice water. The precipitate was filtered,
washed several times with H2O, and dried in vacuo. Recrystallisa-
tion from xylene gave a yellow powder. Yield: 56.2%. 1H NMR
C
32H35N3O5S: C, 66.99; H, 6.15; N, 7.32; S, 5.59%; Found: C, 65.67;
H, 6.28; N, 7.22; S, 5.47.
Photopolymerization: PIs (3 ꢂ 10ꢁ5 mol) composed of thiox-
anthen-DBU, BPO, or ITX were dissolved in DMSO (0.5 mL) under
ultrasonication, and then TMPTA (1 g) was added to this solution.
The mixture was injected into a mold, and irradiated with
an optical cable-directed UV lamp in the 200–400 nm range
(300 MHz, DMSO-d6):
d 7.63 (t, 1H), 7.83 (t, 1H), 7.91 (d, 1H), 7.95
(d, 1H), 8.47 (d, 1H), 8.80 (d, 1H). Anal. Calcd. for C15H7NO3S: C,
64.05; H, 2.51; N, 4.98; S, 11.40%; Found: C, 64.91; H, 2.81; N, 4.92;
S, 11.23.
9-Oxo-9H-thioxanthene-3,4-dicarboxylic acid (c) [23]: Com-
pound d (0.55 g, 1.95 mmol) in NaOH aqueous solution (60 mL,
(RW-UVA-
F200U, Runwing Co., China). The light intensity at the
surface level of the cured samples was measured to be 20 mW/cm2
Please cite this article in press as: M.-H. He, et al., A thioxanthone-based photocaged superbase for highly effective free radical
photopolymerization, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.031

G Model
CCLET 3012 1–5
M.-H. He et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
Table 1
1.5
1.0
0.5
0.0
Photopolymerizations of TMPTA initiated by BPO, thioxanthen-DBU, and thiox-
anthen-DBU&BPO combination.
Entry
Initiator
Tstart
Rpmax
Final
b
(min)a
(minꢁ1
)
conversion
(%)
1
2
3
4
5
BPO
–
–
–
0c
BPO
–
0
Thioxanthen-DBU
Thioxanthen-DBU&BPO
ITX
0.3
0.03
0.1
378
580
30
83.3
85.6
40.8
a
The induction period, can be expressed by irradiation duration required for
initiating the photopolymerization.
b
The maximum initiation rate.
c
Irradiation time is 0 min, reaction time is 30 min.
300
350
400
450
500
(DBU). The obtained thioxanthen-DBU was characterized by 1H 180
NMR, elemental analysis, and ESI-MS spectra.
Wavelength (nm)
181
Fig. 1. UV–vis absorption spectra of thioxanthen-DBU (1 ꢂ 10ꢁ5 mol/L) in DMSO.
TX derivatives have extended absorption up to 420 nm 182
depending upon the type of substitutions [22]. Fig. 1 demonstrates 183
the absorption spectrum of thioxanthen-DBU. It exhibits good 184
absorption characteristics with a maximum at 285 nm and 418 nm 185
and surprisingly a tail over 480 nm. A clear red shift was observed 186
compared with TX due to the amide substituents on the TX 187
160
161
162
163
164
165
166
by a UV-radiometer (type UV-A, Photoelectric Instrument Factory,
Beijing Normal University).
Photo-generated DBU detected by ESI-MS: A thioxanthen-DBU
solution in DMSO (0.01 mmol/L) was irradiated for 30 min, and
then isometric water was added, nonphotolytic thioxanthen-DBU
was filtered off. ESI-MS spectrum of the filtrated solution was then
measured.
skeleton. The n!
p* absorption band appears in the visible region 188
and high molar absorptivity may allow this photocaged base to be 189
used in day light polymerization applications (instead of medium 190
pressure mercury lamps). Furthermore, photogenerated protonat- 191
ed DBU (m/z 153.4) can be detected by ESI-MS spectra after 30 min 192
of irradiation (Fig. 2), which was the direct evidence for the 193
167
3. Results and discussion
generation of DBU from irradiated thioxanthen-DBU.
194
The catalytic behaviors of the photocaged redox initiator 195
combination (thioxanthen-DBU&BPO), and its parent compounds, 196
thioxanthen-DBU and BPO, were compared using real-time FTIR 197
spectroscopy at room temperature (Fig. 3). As depicted in Table 1, 198
control experiments (entries 1 and 2) indicate that both the 199
photocaged base and photoirradiation are essential to trigger this 200
redox photopolymerization. The photopolymerization of TMPTA in 201
the presence of thioxanthen-DBU was fast (Fig. 3, curve b), 80% 202
final conversion was achieved in 1 min. As described in the 203
previous work [20,24], planar N-aromatic maleimides do not 204
generate radicals and initiate photopolymerization. Hence, the 205
photopolymerization initiated by thioxanthen-DBU alone should 206
be attributed to the structure of TX. On the other hand, BPO alone 207
168
169
170
171
172
173
174
175
176
177
178
179
Synthesis of the photocaged base thioxanthen-DBU was
achieved according to the procedures given in Scheme 1. Firstly,
the NO2 group in 3-nitrophthalimide was displaced by salicylic
acid sodium salt (f), followed by an intramolecular acylation of the
obtained intermediate e in polyphosphoric acid (PPA). Secondly,
the hydrolysis of
d in alkaline aqueous solution gave the
dicarboxylic acid c, which reacted with acetic anhydride to
produce the 9-oxo-9H-thioxanthene-3,4-anhydride b. Finally,
thioxanthone-based N-phthalimidoamino acid ammonium salt
(thioxanthen-DBU) was synthesized from trans-4-(aminomethyl)-
cyclohexanecarboxylic acid and the anhydride b, followed by a salt
formation reaction with 1,8-diazabicyclo[5.4.0] undec-7-ene
100
80
100
153.4
80
Thioxanthen-DBU&BPO
60
60
40
a
Thioxanthen-DBU
BPO
ITX
b
40
20
0
d
After irradiation
20
c
0
100
150
200
250
300
0
1
2
3
4
5
Irradiation time (min)
m/z
Fig. 3. Conversion vs time for the photopolymerization of TMPTA in the presence of:
Fig. 2. Positive ion ESI-MS spectra of irradiated thioxanthen-DBU in DMSO
(a) thioxanthen-DBU&BPO, (b) thioxanthen-DBU, (c) BPO, and (d) ITX.
(0.01 mol/L).
Please cite this article in press as: M.-H. He, et al., A thioxanthone-based photocaged superbase for highly effective free radical
photopolymerization, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.031

G Model
CCLET 3012 1–5
4
M.-H. He et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Scheme 2. Proposed mechanism of photoinduced radical generation.
References
256
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
cannot initiate a clear polymerization (Fig. 3, curve c). Interesting-
ly, the thioxanthen-DBU&BPO combination can invoke the
newborn amine-mediated redox radical polymerization (Fig. 3,
curve a), and 82.5% yield was obtained in 0.22 min. Compared to
thioxanthen-DBU, the induction period (Tstart) of the two-compo-
nent redox system was obviously shortened from 0.3 min to
0.03 min and the maximum initiation rate (Rpmax) dramatically
increased from 378 minꢁ1 to 580 minꢁ1, indicating this two-
component catalytic system proceeded rapidly and efficiently.
Due to the one-component structure of the thioxanthen-DBU, both
the photosensitizer and the hydrogen bond donor are combined
together therefore this type of PI does not require an additional co-
initiator. Hence, compared to the conventional PI ITX (Fig. 3, curve
d), the superiority of thioxanthen-DBU can be clearly observed.
The involvement of a decarboxylation process during the
photolysis of N-phthalimidoamino acid derivative was described
in the literature [24,25]. Furthermore, as proposed in the
literatures [26–28], this kind of acetic acid-based thioxanthone
derivative that self-quenching from the triplet state could lead to
the formation of the initiating radicals during the decarboxylation
process. After extrusion of CO2, the corresponding methyl radical is
possibly responsible for the initiation of the polymerization of
TMPTA. Moreover, photogenerated DBU can transiently activate
the oxidation-reduction polymerization in conjunction with the
oxidant BPO. Taking the foregoing points into consideration, the
principal photoreaction pathways of thioxanthen-DBU and BPO
are laid out in Scheme 2.
[1] Y. Yagci, S. Jockusch, N.J. Turro, Photoinitiated polymerization: advances, chal-
lenges, and opportunities, Macromolecules 43 (2010) 6245–6260.
[2] L. Gonsalvi, M. Peruzzini, Novel synthetic pathways for bis (acyl) phosphine oxide
photoinitiators, Angew. Chem. Int. Ed. 51 (2012) 7895–7897.
[3] J.V. Crivello, E. Reichmanis, Photopolymer materials and processes for advanced
technologies, Chem. Mater. 26 (2014) 533–548.
[4] Y.L. Xu, H.J. Xu, X.S. Jiang, J. Yin, Versatile functionalization of the micropatterned
hydrogel of hyperbranched poly(ether amine) based on ‘‘thiol-yne’’ chemistry,
Adv. Funct. Mater. 24 (2014) 1679–1686.
[5] M. Tehfe, F.E.D.E. Dumur, P. Xiao, et al., Chalcone derivatives as highly versatile
photoinitiators for radical, cationic, thiol-ene and IPN polymerization reactions
upon exposure to visible light, Polym. Chem. 5 (2014) 382–390.
[6] J.L. Yang, S.Q. Shi, F. Xu, J. Nie, Synthesis and photopolymerization kinetics of
benzophenone sesamol one-component photoinitiator, Photochem. Photobiol.
Sci. 12 (2013) 323–329.
[7] H. Tar, D. Sevinc Esen, M. Aydin, et al., Panchromatic type II photoinitiator for free
radical polymerization based on thioxanthone derivative, Macromolecules 46
(2013) 3266–3272.
[8] H. Chen, Z.L. Zou, S.L. Tan, et al., Efficient synthesis of water-soluble calix[4]arenes
via thiol-ene ‘‘click’’ chemistry, Chin. Chem. Lett. 24 (2013) 367–369.
[9] Y.Y. Cui, Y.E. Ren, X.X. Liu, Synthesis of methyl methacrylate star-branched
polymer with divinylbenzene as a linking agent via controlled/living photopoly-
merization, Chin. Chem. Lett. 23 (2012) 985–988.
[10] M.H. He, X. Huang, Y.G. Huang, Z.H. Zeng, J.W. Yang, Photoinduced redox initiation
for fast polymerization of acrylaytes based on latent superbase and peroxides,
Polymer 53 (2012) 3172–3177.
[11] M.H. He, X. Huang, Z.H. Zeng, J.W. Yang, Photo-triggered redox frontal polymeri-
zation: a new tool for synthesizing thermally sensitive materials, J. Polym. Sci. A:
Polym. Chem. 51 (2013) 4515–4521.
[12] M.H. He, X. Huang, Z.H. Zeng, J.W. Yang, Phototriggered base proliferation: a
highly efficient domino reaction for creating functionally photo-screened mate-
rials, Macromolecules 46 (2013) 6402–6407.
[13] M.H. He, S. Jiang, R.X. Xu, et al., Domino free radical photopolymerization based on
phototriggered base proliferation reaction and redox initiation, J. Polym. Sci. A:
Polym. Chem. 52 (2014) 1560–1569.
[14] G. Yilmaz, B. Aydogan, G. Temel, et al., Thioxanthone-fluorenes as visible light
photoinitiators for free radical polymerization, Macromolecules 43 (2010)
4520–4526.
[15] D.K. Balta, G. Temel, G. Goksu, et al., Thioxanthone-diphenyl anthracene: visible
light photoinitiator, Macromolecules 45 (2011) 119–125.
[16] D. Tunc, Y. Yagci, Thioxanthone-ethylcarbazole as a soluble visible light photo-
initiator for free radical and free radical promoted cationic polymerizations,
Polym. Chem. 2 (2011) 2557–2563.
[17] G. Yilmaz, S. Beyazit, Y. Yagci, Visible light induced free radical promoted cationic
polymerization using thioxanthone derivatives, J. Polym. Sci. A: Polym. Chem. 49
(2011) 1591–1596.
[18] M.A. Tehfe, F. Dumur, B. Graff, et al., Design of new Type I and Type II photo-
initiators possessing highly coupled pyrene-ketone moieties, Polym. Chem. 4
(2013) 2313–2324.
[19] H.Y. Wang, J. Wei, X.S. Jiang, J. Yin, Highly efficient, polymerizable, sulfur-contain-
ing photoinitiator comprising a structure of planar N-phenylmaleimide and
benzophenone for photopolymerization, J. Polym. Sci. A: Polym. Chem. 44
(2006) 3738–3750.
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
235
4. Conclusion
236
237
238
239
240
241
242
243
244
245
246
247
In the paper, a photocaged base (thioxanthen-DBU) using
Thioxanthen as the peripheral chromophore of N-phthaloyltra-
nexamic acid ammonium salt is proposed. In combination with a
benzoyl peroxide initiator, its ability to initiate free radical
photopolymerization of TMPTA was demonstrated and compared
with that of the parent thioxanthen-DBU, BPO, and conventional PI
ITX. It is found that thioxanthen-DBU&BPO is a highly effective
photoinitiator combination. The mechanism of the radical forma-
tion to initiate the redox photopolymerization is proposed. This
photodecarboxylation reaction is of particular interest as it is can
facilitate the design of new photocaged bases with very promising
properties.
[20] H. Wang, J. Wei, X. Jiang, et al., Novel chemical-bonded polymerizable sulfur-
containing photoinitiators comprising the structure of planar N-phenylmaleimide
and benzophenone for photopolymerization, Polymer 47 (2006) 4967–4975.
[21] S.K. Dogruyol, Z. Dogruyol, N. Arsu, A thioxanthone-based visible photoinitiator, J.
Polym. Sci. A: Polym. Chem. 49 (2011) 4037–4043.
[22] W. Fischer, Aromatic nucleophilic substitution. Part 3. Preparation of novel 9-oxo-
9H-thioxanthene-and 9-oxo-9H-xanthenedicarboximides and-dicarboxylates,
Helv. Chim. Acta 74 (1991) 1119–1126.
[23] C.W. Miller, E.S. Jo¨nsson, C.E. Hoyle, K. Viswanathan, E.J. Valente, Evaluation of N-
aromatic maleimides as free radical photoinitiators: a photophysical and photo-
polymerization characterization, J. Phys. Chem. A 105 (2001) 2707–2717.
[24] Y. Takahashi, T. Miyashi, U.C. Yoon, et al., Mechanistic studies of the azomethine
ylide-forming photoreactions of N-(silylmethyl) phthalimides and N-phthaloyl-
glycine, J. Am. Chem. Soc. 121 (1999) 3926–3932.
248
Acknowledgments
249 Q2
250
251
252
253
This research was financially supported by National Natural
Science Foundation of China (Grant No. 20974127, 21374135),
China Postdoctoral Science Foundation (2013M542178), the Open
Foundation of the State Key Laboratory of Pulp and Paper
Engineering in South China University of Technology
(C713043z), and the Fundamental Research Funds for the Central
Universities (2013ZB0025).
254
255
Please cite this article in press as: M.-H. He, et al., A thioxanthone-based photocaged superbase for highly effective free radical
photopolymerization, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.031

G Model
CCLET 3012 1–5
M.-H. He et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
5
324
325
326
327
328
329
[25] H.G.O. Rner, A.G. Griesbeck, T. Heinrich, et al., Time-resolved spectroscopy of
[27] M. Aydin, N. Arsu, Y. Yagci, One-component bimolecular photoinitiating systems, 330
sulfur-and carboxy-substituted N-alkylphthalimides, Chem. Eur. J.
1530–1538.
7
(2001)
2, Macromol. Rapid Commun. 24 (2003) 718–723.
331
[28] D.K. Balta, G. Temel, M. Aydin, N. Arsu, Thioxanthone based water-soluble 332
photoinitiators for acrylamide photopolymerization, Eur. Polym. J. 46 (2010) 333
[26] M. Aydin, N. Arsu, Y. Yagci, S. Jockusch, N.J. Turro, Mechanistic study of photo-
initiated free radical polymerization using thioxanthone thioacetic acid as one-
component type II photoinitiator, Macromolecules 38 (2005) 4133–4138.
1374–1379.
334
Please cite this article in press as: M.-H. He, et al., A thioxanthone-based photocaged superbase for highly effective free radical
photopolymerization, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.031