Conjugated dyes carrying N, N-dialkylamino and ketone groups: One-component visible light Norrish type II photoinitiators

Article abstract of DOI:10.1016/j.dyepig.2016.10.034

This work presents new conjugated dyes carrying N, N-dialkylamino and ketone groups for using as one-component Norrish type II visible light photoinitiators. The novel dyes without ketone group were also synthesized to compose two-component visible light photoinitiators for comparisons. The target dyes show the remarkable absorption in visible light region. Photo-differential scanning calorimetry was employed to study visible light photoinitiating polymerization kinetics of methyl methacrylate by new one-component photoinitiators and two-component photoinitiators respectively. The results suggest that the one-component photoinitiators showed much more efficient photoinitiating polymerization of methyl methacrylate than the two-component photoinitiating systems under visible light irradiation. The molecular weight of visible light photopolymer produced by the new photoinitiators was determined. The visible light photoinitiating mechanism of the one-component photoinitiators was studied by absorption and emission spectra, electron spin resonance spectra and cyclic voltammograms analysis.

Full text of DOI:10.1016/j.dyepig.2016.10.034

Accepted Manuscript
Conjugated dyes carrying N, NN, N-dialkylamino and ketone groups: One-component
visible light Norrish type II photoinitiators
Ge Ding, Chuan Jing, Xiaozhuan Qin, Yulong Gong, Xueping Zhang, Shengtao
Zhang, Ziping Luo, Hongru Li, Fang Gao
PII:
S0143-7208(16)31041-5
10.1016/j.dyepig.2016.10.034
DYPI 5550
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 6 June 2016
Accepted Date: 24 October 2016
Please cite this article as: Ding G, Jing C, Qin X, Gong Y, Zhang X, Zhang S, Luo Z, Li H, Gao F,
Conjugated dyes carrying N, NN, N-dialkylamino and ketone groups: One-component visible light
Norrish type II photoinitiators, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.10.034.
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ACCEPTED MANUSCRIPT
Conjugated dyes carrying N, N-dialkylamino and ketone groups: one-component visible
light Norrish type II photoinitiators
Ge Ding, Chuan Jing, Xiaozhuan Qin, Yulong Gong, Xueping Zhang, Shengtao Zhang1,2*, Ziping Luo1,2*, Hongru Li1,2*, Fang Gao1,2*
1College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, China, 400044
2National-municipal Joint Engineering Labrotory for Chemical Process Intensification and Reaction, Chongqing University,
Chongqing, China, 400044
Tel/Fax: 86-23-65102531, Emails: stzhang@cqu.edu.cn, zpluo@126.com, hrli@cqu.edu.cn, fanggao1971@gmail.com
Abstract
This work presents new conjugated dyes carrying N, N-dialkylamino and ketone groups for using as
one-component Norrish type II visible light photoinitiators. The novel dyes without ketone group were also
synthesized to compose two-component visible light photoinitiators for comparisons. The target dyes show the
remarkable absorption in visible light region. Photo-differential scanning calorimetry was employed to study
visible light photoinitiating polymerization kinetics of methyl methacrylate by new one-component photoinitiators
and two-component photoinitiators respectively. The results suggest that the one-component photoinitiators
showed much more efficient photoinitiating polymerization of methyl methacrylate than the two-component
photoinitiating systems under visible light irradiation. The molecular weight of visible light photopolymer
produced by the new photoinitiators was determined. The visible light photoinitiating mechanism of the
one-component photoinitiators was studied by absorption and emission spectra, electron spin resonance spectra
and cyclic voltammograms analysis.
Keywords: Norrish type II photoinitiator; Visible light phtotopolymerization; Photopolymerization kinetics; Free
radical; Electron transfer; Acrylate monomers
1. Introduction

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In recent decades, the study of photoinitiating polymerization of monomers has been
receiving considerable attentions since it has wide applications such as microelectronic circuits,
printing plates, automobile coating, dental clinical and so on[1-4]. In general, photoinitiating
polymerization is finished through three steps: (a) a photoinitiator is excited by light irradiation
firstly, (2) and then the active free radicals are yielded, and (3) in the end, monomer is initiated for
photopolymerization by consuming free radicals. This process reflects that photoinitiators play
central roles in the achievement of efficient photopolymerization.
Normally, photoinitiators can be divided into bond α-cleavage type (Norrish type I) or
hydrogen abstraction type (Norrish type II) [5, 6]. Type I is a class of the molecule which yields
free radicals by direct cleavage reaction under light irradiation. For example,
2-hydoxy-2-methyl-1-phenyl-propan-1-one is representative type I photoinitaitor [7]. While
Norrish Type II is a group of the compound that produces free radicals through direct hydrogen
abstraction under irradiation of light. For instance, thioxanthones and benzil are well-known type
II photoinitaitors [8]. Considering chemical stability and blocking effect, Norrish type II
photoinitiators possess more advantages for photoinitiating polymerization of monomers.
However, the most of both Norrish type I and type II photoinitiators such as Irgacure 819 and
benzophenone only absorb 200~300 nm light, and they cannot produce free radials for
photoinitiating polymerization of monomers under visible light irradiation (≥ 400 nm light source).
Hence, some long wavelength dyes are often employed as photosensitizers to sensitize UV
photoinitiators through intermolecular electron transfer to yield active free radials for
photopolymerization of monomers under visible light irradiation [9, 10]. A number of efforts have
been devoted to develop new long-wavelength photosensitizers for visible light

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photopolymerization [11]. For example, some squaraine dyes can efficiently photosensitize
iodonium salt to produce free radicals for photopolymerization of monomers under visible light
irradiation [12].
This method is conveniently applied since a library of long wavelength organic dyes can act
as photosensitizers. However, photoinduced intermolecular electron transfer between a long
wavelength photosensitizer and a UV photoinitiator is limited by a number of factors including
compatibility, polarity as well as viscosity of the systems. In particular, with the progress of the
photopolymerization, photosensitizer molecules and UV photoinitiator molecules could be
separated by yielded photopolymeric materials. As a consequence, photoinitiating polymerization
efficiency of monomers under long wavelength irradiation by this approach needs to be improved.
Hence, photoinduced internal electron transfer is proposed to achieve more efficient long
wavelength photoinitiating polymerization [13, 14]. This requires that a photoinitiator can not only
absorb visible light efficiently, but also produce free radicals greatly through a ultrafast
photoinduced intramolecular electron transfer under visible light irradiation. However, it is a quite
challenge to prepare one-component visible light photoinitiators on the basis of rational molecular
design.
It is a useful strategy to get long wavelength photoinitiators based on molecular
preorganization of dye-attached UV photoinitiaors via covalent bonds. For instance, some
attempts have been made to synthesize chloromethyl-1,3,5-triazine-attached cyanine dyes as
visible light photoinitiators [15]. To such a molecule, cyanine dye part absorbs visible light firstly,
and then electron is transferred to chloromethyl-1,3,5-triazine segment through the linker, and the
free radicals are yielded through bond cleavage reaction for photoinitiating polymerization of

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monomers. It is obvious that these molecules are regarded as Norrish type I visible light
photoinitators.
While so far, there are few studies on Norrish type II visible light photoinitiators due to the
more difficulty in the molecular preconstruction. Arsu etal reported a thioxanthone-based fused
multi aromatic ring as a visible photoinitiator [16]. However, it required the other component
tertiary alkyl amine as the coinitiator to photoinitiate acrylate monomers together under visible
light irradiation. If without the coinitiators, visible light photoinitiating of acrylate monomers is
hard to occur. In addition, the rigid fused aromatic ring of the target photoinitiator lowers the
solubility and the compatibility with the other components in formulation.
Inspired by these efforts, this study proposes to develop well-conjugated photoinitiators
carrying N, N-dialkylamino and ketone groups. The presence of phenyl keto moiety means that
the target molecules could be used as Norrish type II photoinitiators. N, N-Dialkylamino groups
play the followings roles: (1) a tertiary amine group in target dyes act as a hydrogen donor to
produce active free radicals for visible photoinitiating polymerization of monomers [17, 18]; (2) it
favors the extension of the absorption wavelength of the target photoinitiators due to
electron-donating properties; (3) it increases photoinduced internal electron transfer, which is
favorable for the yields of free radicals [19]; and (4) it can improve the compatibility of target
visible photoinitators with the other formulation components [20, 21].
Furthermore, the good π conjugated structure of the target molecules could lead to the
red-shift of absorption wavelength as well as the improvement of photoinduced intramolecular
electron transfer. Hence, the more efficient visible light photoinitiating polymerization of
acrylated monomers by the new target dyes can be expected.

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In this study, photo-differential scanning calorimetry (photo-DSC) was employed to study
visible light photoinitiating polymerization kinetics of monomers by the target photoinitiators.
Meanwhile, the corresponding new photosensitizers without ketone group were also prepared as
the references to compose two-component visible light photoinitiators for the comparisons. The
results show that the new one-component visible light photoinitiators could photoinitiate
polymerization of acrylate monomers more efficiently than two-component ones. This study can
guide us to develop new visible light Norrish type II one-component photoinitiators for free
radical polymerization of acrylate monomers through rational molecular preorganization.
2. Experimental
2.1. Materials and characterization
The organic solvents supplied by Aldrich Corporation were dried according to standard
laboratory procedures [22]. The monomer methyl methylacetate (MMA) was redistilled in vacuum
prior to use, which was purchased from Aldrich. The new visible photoinitiators including
4-benzoyl-4’-dimethylamino-diphenylethylene
4-benzoyl-4’-diethylamino-diphenylethylene
(C1),
(C2),
and
4-benzoyl-4’-dibutylamino-diphenylethylene (C3) were prepared in our laboratory (Scheme 1).
The
novel
visible
light
photosensitizers
consisting
of
(C4),
and
dimethyl-(4-(2-(4-styryl-phenyl)-vinyl)-phenyl)-amine
diethyl-(4-(2-(4-styryl-phenyl)-vinyl)-phenyl)-amine
(C5)
dibutyl-(4-(2-(4-styryl-phenyl)-vinyl)-phenyl)-amine (C6) were also synthesized in our laboratory
(Scheme 1). The starting materials for the synthesis of these new target molecules were obtained

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from Acros Corporation. Nuclear magnetic resonance (NMR), elemental analysis as well as X-ray
single crystal diffraction were used to characterize chemical structures of the target molecules.
Insert Scheme 1
1H- & 13C- NMR spectra of the samples were determined in deuterated solvents by a Bruker
500 MHz spectrometer at room temperature. Tetramethysilance (TMS) was used as an internal
standard to identify NMR chemical shifts of the samples. Elemental analysis of the samples was
performed by a CE440 from Exeter Analytical Inc apparatus. The melting points of the samples
were studied by a X-5 apparatus from Beijing Focus Instrument Co. Ltd.
Single crystals of the target dyes were obtained by a slow volatilization of organic solvents in
NMR tube. X-ray diffraction data were acquired by a Bruker-AXS CCD area detector equipped
with diffractometer with Mo Ka (λ = 0.71073 Å) at 298 K. A suitable single crystal was put inside
a glass fiber capillary to determine the structure through direct methods which were refined by
full-matrix least squares on F2. All the hydrogen atoms were labeled in calculated positions
respectively and all the non-hydrogen atoms were introduced by anisotropic temperature factors.
2.2. Static and transient absorption and emission spectra
A Cintra spectrophotometer was employed to determine the static UV/visible absorption
spectra of the samples. The fluorescence emission spectra of the samples (2×10-6 mol/L) were
determined by F-531PC spectrofluorophotonmeter. Quinine sulfate in 0.5 mol/L H2SO4 (Φ, 0.546)
was used as the reference to measure the fluorescence quantum yields of the samples [23].
A 355 nm nanosecond laser was used to perform laser flash photolysis measurements, which
were conducted in a rectangular quartz cell with a path length of 5 mm along the monitoring light
path. In a dark room, the samples were deaerated by bubbling with argon for one hour before laser

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detection. The transmitted monitoring light from the samples was collected and focused onto a
monochromator, which was processed by a computer system.
2.3. Photopolymerization kinetics study by Photo-DSC
Photo-DSC-204 F1 phoneix from Netzsch Corporation of Germany was used to evaluate the
visible light photoinitating polymerization kinetics of MMA photoinitiated by the studied systems.
A high pressure mercury lamp was used as the light source. The wavelength above 400 nm light
was allowed to go through by cutting UV wavelength light through a filter.
An open aluminum pan with a diameter of 6.6 mm contained the formulation components
for visible light photoinitiaited polymerization, and an empty aluminum pan was employed as the
blank. The photoinitiating polymerization mixture was cooled in a nitrogen flow of 60 mL/ min
under visible light irradiation. The average power of visible light irradiation was 20 mW/cm2
measured by a Coherent Model Fieldmaster power meter (Germany). Visible light
photopolymerization kinetics of monomer MMA was analyzed by the exothermal plots.
By integrating the area under the exothermic peak, the conversion percent of acrylate double
bond (C%):
C%=∆Ht / ∆H0theory ×100%
(1)
wherein
is the reaction heat at time t, and ∆H0theory shows the theoretical reaction heat for
∆Ht
theory
100% conversion. ∆H0
is 86 kJ/mol for an acrylic double bond [24]. The rate of
polymerization (Rp) is directly related to the heat flow dH/dt by the following equation:
Rp =dC/dt=(dh/dt)/∆H0theory
(2)
2.4. Cyclic voltammograms determination

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Shanghai Chenhua electrochemical work station was used to determine cyclic
voltammograms of the samples. A representative three-electrode system including two platinum
work electrodes and an Ag/AgCl reference electrode were put in a cell for measurements of cyclic
voltammograms. A 0.05 mol/L solution of tetra-n-butyl-ammonium hexafluorophosphate in
dichloromethane containing the samples was bubbled with argon for 30 min.
2.5. Gel permeation chromatography (GPC) analysis
Gel permeation chromatography (GPC) was used to determine molecular weights of
photopolymer PMMA produced in this work. Variations of molecular weights (number average
molecular weight, Mn, weight average molecular weight, Mw) and polydispersity index (PDI) of
polymethyl methacrylate (PMMA) were acquired by a Shimadzu GPC instrument with
tetrahydrofuran (HPLC grade, Sigma-Aldrich Co, USA) as the mobile phase at a low rate of 1
mL/min.
The photopolymer PMMA was yielded by the studied visible light photoinitiating systems,
which was precipitated in cold methanol, and dried in vacuum. The polydispersity of the above
PMMA based on molecular weight according to the following equation:
Mw/Mn=1+P
(3)
wherein P represents the extent of the polymerization reaction.
2.6. Electron spin resonance (ESR) analysis
Bruker EMX EPR spectrometer at 9.5 GHz with a modulation frequency of 200 KHz was
employed to perform electron spin resonance (ESR) with 5, 5-dimethyl-pyrroline-N-oxide
(DMPO) as the radical capturing agent. A high-pressure mercury lamp was used as the light
source in ESR spectrometer cavity. The concentrations of the components in organic solvents

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were 5×10-4 mol/L, and then 0.5 mL of each sample was placed into a quartz ESR tube and then
purged with nitrogen for half an hour to remove oxygen.
2.7. Photobleaching measurements
Photobleaching of the studied visible light photoinitiating polymerization systems was
conducted in organic solvents, which were contained in purex tube. The pure argon was bubbled
for half an hour to remove oxygen. The 400 W high pressure mercury lamp was used as the light
source, and the light below 400 nm wavelength was cut off by a filter to avoid ultraviolet light.
Photobleaching ratios could be calculated by the following equation:
w%=∆A/A0 =(A0 - A )/A0
(4)
t
wherein A0 is the optical density at the maximal absorption wavelength before visible light
irradiation, and At represents the absorption at 400 nm after (t) time irradiation of visible light.
2.8. Preparation of the target molecules
Synthesis of C1~C3
Intermediate 1, (4-bromomethyl-phenyl)-phenyl-methanone
4-Methylbenzophenone 10.0 g (0.05 mol) and azodiisobutyronitrile (AIBN, 0.075 g) was
added to 150 ml CCl4 in a round bottom flask with a condenser and a magnetic stirrer, and then
N-bromosuccinimide (NBS, 9.0 g, 0.05 mol) was added. The mixture was keep refluxing for 24 h,
then was cooled to room temperature, the solvent was evaporated under vacuum in the end. The
product was further purified through recrystallization from benzene and cyclohexane to yield
white solid (yield, ~59%, m.p., 103.0~105.0 °C). 1H-NMR (500 MHz, CDCl3) δ (ppm)：4.456 (s,
-CH2, 2H), 7.18-7.20 (d, Ar-H, 2H), 7.532-7.534 (d, Ar-H, 2H), 7.425-7.434 (d, Ar-H, 2H),
7.505-7.519 (t, Ar-H, 1H), 7.711-7.725 (d, Ar-H, 2H). 13C-NMR (125 MHz, CDCl3) δ (ppm):

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38.323, 124.652, 126.863, 129.832, 130.083, 133.843, 135.083, 135.326, 139.093, 187.083.
Elementary analysis, Anal. Calcd for C14H11BrO: C, 61.12; H, 4.03. Found: C, 61.02; H, 3.97.
Intermediate 2, (4-benzoyl-benzyl)-phosphonic acid diethyl ester
In a three-necked flask equipped with a magnetic stirrer, (4-bromomethyl-phenyl)-phenyl-
methanone (1.1 g, 4 mmol) and triethylphosphite (6.6 ml) were added with molar ratio of 1:10.
The mixture was heated to 130-160 °C for 5 hours under stirring, After the reaction, The
triethylphosphite was removed under reduced pressure to provide the crude product (yield , ~95%).
1H-NMR (500 MHz, CDCl3) δ (ppm)：1.046-1.336 (t, -CH3, 6H), 3.062 (s, -CH2, 2H), 3.824-3.930
(m, -OCH2, 4H), 7.210-7.245 (m, Ar-H, 2H), 7.245-7.281 (d, Ar-H, 2H), 7.338-7.385 (d, Ar-H,
3H), 7.530-7.584 (m, Ar-H, 2H). 13C-NMR (125 MHz, CDCl3) δ (ppm): 14.293, 31.038, 61.937,
127.093, 127.973, 130.274, 130.285, 131.093, 133.072, 135.528, 138.049, 187.938. Elementary
analysis, Anal. Calcd for C18H21O4P: C, 65.04; H, 6.38. Found: C, 65.12; H, 6.25.
C1, 4-benzoyl-4’-dimethylamino-diphenylethylene
Diethyl-4-benzoylbenzylphosphonate and sodium methoxide (4 mmol, 0.216 g) was added to
40 ml THF in a round bottom flask. 4-(Dimethylamino)-benzaldehyde (3.5 mmol) was added into
the mixture, and then kept stirring for 18~24 h at room temperatures. After the reaction was
finished, the mixture was cooled to room temperature. After the solvent was evaporated under
vacuum, the mixture was further purified by column chromatography (benzene/ethyl acetate, 30/1,
v/v) and recrystallization from benzene and cyclohexane to give yellow solid (yield, ~66%, m.p.,
1
186~187 °C). H-NMR (500 MHz, CDCl3) δ (ppm): 3.00 (s, -CH3, 6H), 6.79 (s, Ar-H, 2H), 6.96
(d, J=16 Hz, Ar-CH=CH, 1H), 7.18 (d, J=16 Hz, Ar-CH=CH, 1H), 7.47(m, Ar-H, 4H), 7.56 (m,
Ar-H, 3H), 7.80 (m, Ar-H, 4H). 13C-NMR (125 MHz, CDCl3) δ (ppm): 40.413, 112.569, 123.070,

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125.288, 127.686, 127.831, 29.483, 130.405, 131.063, 131.714, 134.967, 137.659, 142.073,
150.305, 195.702. Elementary analysis, Anal. Calcd for C23H21NO: C, 84.37; H, 6.46; N, 4.28.
Found: C, 84.25; H, 6.37; N, 4.17.
C2, 4-benzoyl-4’-diethylamino-diphenylethylene
The synthesis of C2 followed the same procedure as described that of C1, in which
4-(dimethylamino)-benzaldehyde was replaced by 4-(diethylamino)-benzaldehyde in the last step.
1
The product of yellow solid was obtained (yield, ~69%, m.p., 110~111 °C). H-NMR (500 MHz,
CDCl3) δ (ppm): 1.19 (t, J=7 Hz, -CH3, 6H), 3.39 (m, Ar-CH2, 4H), 6.67 (d, J=9 Hz, Ar-H, 2H),
6.92 (d, J=16.5 Hz, Ar-H, 1H), 7.18 (d, J=16.5 Hz, Ar-CH=CH, 1H), 7.42 (m, Ar-CH=CH, 2H),
7.49 (m, Ar-CH=CH, 2H) 7.56 (m, Ar-H, 3H), 7.80 (m, Ar-H, 4H). 13C-NMR (CDCl3, 125 MHz):
12.702, 44.449, 111.633, 122.284, 123.994, 125.575, 128.259, 128.400, 129.912, 130.860,
131.850, 132.094, 135.090, 138.186, 142.878, 147.950, 196.103. Elementary analysis, Anal.
Calcd for C25H25NO: C, 84.47; H, 7.09; N, 4.50. Found: C, 84.39; H, 7.16; N, 4.61.
C3, 4-benzoyl-4’-dibutylamino-diphenylethylene
The synthesis of C3 followed the same procedure basically as described that of C1, in which
4-(dimethylamino)-benzaldehyde was replaced by 4-(dibutylamino)-benzaldehyde in the last step.
1
The solid pale yellow product was obtained (yield, ~60%, m.p., 108.9~110.6 °C). H-NMR (500
MHz, CDCl3) δ (ppm): 1.03 (t, J= 7.25 Hz, -CH3, 6H), 1.42 (m, -CH2-, 4H), 1.64 (m, -CH2-, 4H),
3.35 (t, J=7.75 Hz, Ar-CH2-, 4H), 6.69 (d, J=8.5 Hz, Ar-H, 2H), 6.97 (d, J=16.0 Hz, Ar-CH=CH,
1H), 7.23 (d, J=16.5Hz, Ar-CH=CH, 1H), 7.47 (d, J=8.5 Hz, Ar-H, 2H), 7.53 (m, Ar-H, 2H), 7.60
(m, Ar-H, 3H), 7.86 (d, J=8.0 Hz, Ar-H, 4H). 13C-NMR (CDCl3, 125 MHz): 14.085, 20.382,
29.515, 50.776, 111.617, 122.160, 123.809, 125.552, 128.237, 128.333, 129.892, 130.838,

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131.871, 132.065, 135.023, 138.170, 142.886, 148.350, 195.988. Elementary analysis, Anal.
Calcd for C29H33NO: C, 84.65; H, 8.02; N, 3.40. Found: C, 84.56; H, 7.93; N, 3.49.
Synthesis of C4~C6
Intermediate 3, 1-methyl-4-styryl-benzene
In a three-necked flask equipped with a magnetic stirrer, 4-methyl-benzaldehyde, phenyl
acetic acid and piperidine were added with the molar ratio of 1:3:3. The mixture was heated to
100 °C for 3 hours with stirring, then the reaction was kept at 130 °C for 5 hours. After the
reaction, the product was then extracted with CH2Cl2 three times. The combined organic layer was
dried by anhydrous MgSO4. The solvent was removed under reduced pressure to leave the crude
product, which was further purified by column chromatography (n-hexane/CH2Cl2, 2/1, v/v) to
afford the desired product as the yellow needle solid (yield, ~65%, m.p., 146.0~2148.0 °C).
1H-NMR (500 MHz, CDCl3) δ (ppm): 2.353 (s, -CH3, 3H), 6.933 (d, J=16.0 Hz, CH=CH, 1H),
7.023 (d, J=16.5 Hz, CH=CH, 1H), 7.153 (d, Ar-H, 2H), 7.231 (t, Ar-H, 1H), 7.278 (d, Ar-H, 2H),
7.294 (d, Ar-H, 2H), 7.324 (m, Ar-H, 2H). 13C-NMR (125 MHz, CDCl3) δ (ppm): 20.438, 121.323,
121.673, 123.634, 124.076, 126.339, 127.032, 127.964, 131.093, 132.532, 135.025. Elementary
analysis, Anal. Calcd for C15H14: C, 92.74; H, 7.26. Found: C, 92.81; H, 7.13.
Intermediate 4, 1-bromomethyl-4-styryl-benzene
The synthesis of intermediate 4 followed essentially the same procedure as described that of
intermediate 1, where 4-methylbenzophenone was replaced by 1-methyl-4-styryl-benzene in
1
synthetic process. (yield, ~55%, m.p. 106.0～107.8 °C). H-NMR (500 MHz, CDCl3) δ (ppm)：
4.476 (s, -CH2, 2H), 6.934 (d, J=16.5 Hz, CH=CH, 1H), 7.032 (d, J=8.0 Hz, CH=CH, 1H), 7.128
(d, Ar-H, 2H), 7.298 (t, Ar-H, 1H), 7.334 (d, Ar-H, 2H), 7.385 (d, Ar-H, 2H), 7.403 (d, Ar-H, 2H).

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13C-NMR (125 MHz, CDCl3) δ (ppm): 37.983, 122.083, 122.487, 124.083, 124.756, 125.932,
128.672, 129.083, 130.483, 130.562, 132.984. Elementary analysis, Anal. Calcd for C15H13Br: C,
65.94; H, 4.81. Found: C, 66.02; H, 4.73.
Intermediate 5, (4-styryl-benzyl)-phosphonic acid diethyl ester
The synthesis of crude intermediate 5 followed essentially the same procedure as described
that of intermediate 2, in which (4-bromomethyl-phenyl)-phenyl-methanone was replaced by 1-
bromomethyl-4-styryl-benzene in synthetic process. (yield, ~92%). 1H-NMR (500 MHz, CDCl3) δ
(ppm): 1.134 ( t, J=8.0 Hz, -CH3, 6H), 3.038 (s, -CH2, 2H), 3.794 (m, -OCH2, 4H), 6.932 (d,
J=16.0 Hz, CH=CH, 1H), 7.011 (d, J=16.5 Hz, CH=CH, 1H), 7.021 (d, Ar-H, 2H), 7.112 (s, Ar-H,
1H), 7.238 (d, Ar-H, 2H), 7.242 (m, Ar-H, 2H), 7.315 (m, Ar-H, 2H). 13C-NMR (125 MHz, CDCl3)
δ (ppm): 14.632, 30.324, 63.283, 123.943, 124.032, 126.847, 127.043, 127.956, 128.043, 128.746,
129.283, 132.094, 133.184. Elementary analysis, Anal. Calcd for C19H23O3P: C, 69.07; H, 7.03.
Found: C, 69.15; H, 6.96.
C4, dimethyl-(4-(2-(4-styryl-phenyl)-vinyl)-phenyl)-amine
(4-Styryl-benzyl)-phosphonic acid diethyl ester (3 mmol) and sodium methoxide (3.6 mmol)
in 40 ml dry THF were added into 100 ml round bottom flask and stirred at 0 for 30 min,
4-(dimethylamino)-benzaldehyde (3.5 mmol, 2.35 g) was added into the mixture as the color was
changed to orange. Then, the mixture was stirred for overnight at room temperature. After the
reaction was finished, THF was removed under reduced pressure to leave the crude product, which
was then purified by column chromatography (benzene/ethyl acetate, 30/1, v/v) and
recrystallization from benzene and cyclohexane gave yellow solid (yield, ~65%, m.p.,
1
183.5~184.8 °C). H-NMR (500 MHz, CDCl3) δ (ppm): 2.94 (s, -CH3, 6H), 6.59 (s, Ar-H, 2H),

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6.96 (d, J=16.0 Hz, CH=CH, 1H), 7.08 (d, J=8.0 Hz, CH=CH, 1H), 7.11 (d, J=8.0 Hz, CH=CH,
1H), 7.13 (d, J=16 Hz, CH=CH, 1H), 7.24 (m, Ar-H, 2H), 7.28 (m, Ar-H, 2H), 7.33 (m, Ar-H, 1H),
7.40 (m, Ar-H, 2H), 7.63 (m, Ar-H, 4H). 13C-NMR (125 MHz, CDCl3) δ (ppm): 43.423, 114.387,
124.302, 126.273, 126.835, 127.532, 127.873, 128.841, 129.854, 134.632, 134.763, 143.782.
Elementary analysis, Anal. Calcd for C24H23N: C, 88.57; H, 7.12; N, 4.30. Found: C, 88.61; H,
7.05; N, 4.37.
C5, diethyl-(4-(2-(4-styryl-phenyl)-vinyl)-phenyl)-amine
The synthesis of C5 followed essentially the same procedure as described that of C4, in which
4-(dimethylamino)-benzaldehyde was replaced by 4-(diethylamino)-benzaldehyde in the last step.
Recrystallization from CH2Cl2 and cyclohexane gave pure material as yellow solid (yield, ~ 43%,
1
m.p. 131.3~133.5°C). H-NMR (500 MHz, CDCl3) δ (ppm): 1.13 (t, J=7.0 Hz, -CH3, 6H), 3.41
(m, -CH2, 4H), 6.63 (d, J=9.0 Hz, Ar-H, 2H), 6.97 (d, J=16.0 Hz, CH=CH, 1H), 7.11 (d, J=16.5
Hz, CH=CH, 1H), 7.17 (d, J=16.5 Hz, CH=CH, 1H), 7.19 (d, J=16.0 Hz, CH=CH, 1H),7.24 (d,
J=16.0 Hz, Ar-H, 2H), 7.212 (t, Ar-H, 1H), 7.33 (m, Ar-H, 2H), 7.42 (m, Ar-H, 2H, 3H), 7.68 (m,
Ar-H, 3H). 13C-NMR (CDCl3, 125 MHz) : 13.311, 46.433, 112.754, 123.432, 124.764, 126.126,
126.782, 128.265, 128.922, 133.860, 134.893, 143.932. Elementary analysis, Anal. Calcd for
C26H27N: C, 88.33; H, 7.64; N, 3.96. Found: C, 88.42; H, 7.57; N, 3.88.
C6, dibutyl-(4-(2-(4-styryl-phenyl)-vinyl)-phenyl)-amine
The synthesis of C6 followed essentially the same procedure as described that of C4, in which
4-(dimethylamino)-benzaldehyde was replaced by 4-(dibutylamino)-benzaldehyde in the last step.
1
Yield, ~63%, m.p., 116.2~117.6 °C. H-NMR (500 MHz, CDCl3) δ (ppm): 0.99 (t, J=7.25 Hz,
-CH3, 6H), 1.38 (m, -CH2-, 4H), 1.53 (m, -CH2-, 4H), 3.34 (t, J=7.75 Hz, Ar-CH2-, 4H), 6.59 (d,

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J=8.5 Hz, Ar-H, 2H), 6.98 (d, J= 16.5 Hz, CH=CH, 1H), 7.02 (d, J= 16.5 Hz, CH=CH, 1H), 7.13
(d, J= 8.0 Hz, CH=CH, 1H), 7.19 (d, J= 16.0 Hz, CH=CH, 1H), 7.23 (d, J=8.5, Ar-H, 1H), 7.36 (m,
13
Ar-H, 2H), 7.38 (m, Ar-H, 2H), 7.44 (m, Ar-H, 3H), 7.63 (m, Ar-H, 3H). C-NMR (CDCl3, 125
MHz): 13.885, 20.125, 31.263, 53.482, 113.617, 124.863, 124.856, 126.522, 126.673, 127.783,
128.073, 128.973, 134.036, 134.733, 144.732. Elementary analysis, Anal. Calcd for C30H35N: C,
87.96; H, 8.55; N, 3.42. Found: C, 88.05; H, 8.46; N, 3.49.
3. Results and discussion
3.1 1H/13C-NMR spectra of the new target photoinitistors
The synthesis of the target molecules was according to Wittig-Horner reaction through
multi-step routes shown in Scheme 1. The synthesis strategies provided the advantages such as
satisfied yields as well as convenient operations, which produced coplanar trans C-C double
bonds.
The representative 1H-NMR and 13C-NMR spectra of C3 were shown in Figure 1. It is found
that the chemical shift of –CH2 peaked at b, c and d as well as -CH=CH- peaked at g and h all
display remarkably separated peaks because they are not equivalent hydrogen atoms. Triplet peaks
at a and d are shown due to the effects of neighboring protons. Furthermore, the chemical shift of
H (e) moves to more upfield compared to the other H atoms in phenyl rings since N, N-alkylamino
groups possess strong electron donating effects.
Insert Figure 1
1
In contrast, the electron withdrawing effect of carbonyl group leads to H-NMR shift of its
adjacent H atoms (j) in phenyl rings to the downfield. Meanwhile, the typical 13C-NMR chemical
shift of ketone group is found (Figure 1(b), 195.988 ppm). In addition, C-C double bonds show

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1
123.909 and 122.160 ppm (f, g) chemical shifts in 13C-NMR spectra. In summary, the H-NMR
13
and C-NMR spectra suggest that benzophenone was successfully covalently attacted to the dye
1
by C-C double bonds. The other tagret photoinitistors C1 and C2 show the similar H-NMR and
13C-NMR spectra (typically shown in，Figure S1, Supplementary data).
As a matter of fact, the other multi-step synthesis routes using Wittig-Horner reaction have
also been successful established to prepare the target dyes in our lab (Scheme 2). Hence, it is
feasible to obtain a large scale preparation of the target dyes for the future application.
Insert Scheme 2
The single crystal of C1 was prepared by a slow evaporation of mixed organic solvents.
X-ray single crystal diffraction analysis and molecular π-π stacking structure demonstrate that C-C
double bond in C1 shows neat trans-configuration (Figure 2). In addition, the small dihedral angle
(∠C28-C31-C32-C33) around C-C double bond (3.02 °) indicates that π conjugation part in C1
possess the great coplanarity (see Supplementary data).
Insert Figure 2
3.2 Static and transient UV/visible absorption and emission
The representative UV/visible absorption spectra of the target molecules C1~C6 in
methylene chloride are given in Figure 3. It is shown that the maximal absorption wavelengths of
these molecules are located in visible light region and the absorption is extended to 500 nm.
Furthermore, C1~C3 exhibit a dramatic absorption band around 275 nm, which could be
attributed to phenyl keto segment. In contrast, there is absence of remarkable absorption band
around 275 nm for C4~C6.
Insert Figure 3

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The typical absorption parameters of these molecules in various solvents are presented in
Table 1. The data suggest that these molecules yield remarkable absorption in visible light region
(εmax at 400 nm, 105 magnetitude, cm-1·mol-1·L). The absorption maxima of the target molecules
show the red-shift with the increase of carbon chain length of N, N-dialkylamino group, which
could be ascribed to the enhancement of electron-donating ability with the extended carbon chain.
Insert Table 1
As compared with C4~C6, C1~C3 show the smaller fluorescence quantum yields (such as in
CH2Cl2, C3, Φ, 0.302, C6, Φ, 0.735). This indicates that phenyl keto part could increase the yields
of the triplet states of C1~C3, in which the initiating radicals are produced. Laser flash photolysis
experiments were further conducted to survey the triplet states of photoinitiators. The typical
transient absorption spectra of C3 after the irradiation of laser pulses are provided in Figure 4.
Insert Figure 4
It is found that the transients could be efficiently quenched by oxygen, demonstrating that
they are ascribed to triplet-triplet absorption. The transient peak around 530 nm is observed under
the laser irradiation, which is similar with typical characterization of triplet-triplet spectra of
benzophenone [25]. The results further demonstrate that triplet state yields of C1~C3 are
increased by phenyl keto group.
3.3. Visible light photoinitiating polymerization studied by photo-DSC
In this study, two sorts of visible light photoinitiating polymerization systems were studied.
The first kind of system included only the new photoinitiator (C1 or C2 or C3), namely it is a
one-component photoinitiating system. While the second type of system was consisted of the new
photosensitizer (C4 or C5 or C6) and the UV photoinitiator benzophenone, meaning it is a

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two-component photoinitiating system. The new visible light photoinitiators and photosensitizers
exhibited the good compatibility with the other formulations such as monomers in the systems due
to the presence of N, N-dialkylamino groups.
The relationships between the conversion percents of double bond of MMA and visible light
irradiation time at different concentrations of the photoinitiators C1, C2 and C3 was investigated
by photo-DSC respectively, which was further used to determine the dependence of the
photopolymerization rates of MMA on visible light irradiation time.
Figure 5 shows that the conversion percents of double bond of MMA gradually increase with
the increase of visible light irradiation time at a fixed concentration of the photoinitiator (such as
at 0.05 mol% of C3, C%, 6.82%, for 2 s irradiation, C%, 32.3%, for 5 s irradiation). While, the
conversion percents of double bond of MMA tend be stable with the further increase of irradiation
time (for example at 0.05 mol% of C3, C%, 50.3%, for 10 s irradiation, C%, 55.7%, for 15 s
irradiation). Meanwhile, the photopolymerization rates of MMA show the increase with the
increase of visible light irradiation time (for instance at 0.05 mol% of C3, Rp (s-1), 0.0312, for 2 s
irradiation, Rp (s-1), 0.117 for 5 s irradiation), but they decrease from the peak values with further
extension of irradiation time (for example at 0.05 mol% of C3, Rp (s-1) , 0.0345, for 10 s
irradiation, Rp (s-1) , 0 for 15 s irradiation). This is mostly due to the large consumption of the
photoinitiating formulations with further irradiation time [26].
As shown in Figure 5, the photoinitiating polymerization rates of MMA increase greatly with
the increase of the concentrations of the photoinitiators at the same visible light irradiation time
firstly (for example C3, at 0.05 mol%, Rp (s-1), 0.116, 0.10 mol%, Rp (s-1), 0.196, for 5 s
irradiation). However, the photoinitiating polymerization rates of MMA show small increase as

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the concentrations of the photoinitiators are further enhanced (such as C3, at 0.15 mol%, Rp (s-1),
0.233, 0.20 mol%, Rp (s-1), 0.278, for 5 s irradiation). As a consequence, the percents of double
bond conversion of MMA increase with the increase of the concentrations of photoinitiators firstly
(such as C3, at 0.05 mol%, C%, 31.9%, 0.10 mol%, C%, 42.8% for 5 s irradiation), and then they
show the light increase with further enhancement of the concentrations of the photoinitiators at
fixed visible light irradiation time (for example C3, at 0.15 mol%, C%, 45.8%, at 0.20 mol%, C%,
48.9% for 5 s irradiation). This could be caused by radical annihilations at the higher
concentrations of the photoinitiators [27].
Insert Figure 5
Figure 5 also suggests that the maximal conversion percents of double bond of MMA and
the peak photopolymerization rates of MMA increase with the extension of carbon chain length of
N, N-dialkylamino groups at the same conditions (such as at 0.20 mol%, C1, Rpmax (s-1), 0.181,
C%, 82.2%, C2, Rpmax (s-1), 0.213, C%, 84.3%, C3, Rpmax (s-1), 0.283, C%, 85.4%). The longer
alkyl group means the more intensive electron donating effect, yielding the stronger photoinduced
intramolecular charge transfer in the photoinitiators. Hence, the free radicals could be produced by
C3 more efficiently under visible light irradiation. It suggests that the electron-donating groups
can effectively improve the visible light photoinitiating performance of the photoinitiators.
Insert Figure 6
We also studied visible light photoinitiaing efficiencies of the two-component systems
composed by the photosensitizer (C4, or C5 or C6) and benzophenone. Figure 6 shows that the
peak conversion percents of double bond of MMA and the maximal photopolymerization rates of
MMA produced by the one-component photoinitiators are remarkably greater than those yielded

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by the corresponding two-component systems (such as at 0.25 mol% of C3, C%, 91.9%, Rpmax
(s-1), 0.356; at 0.25 mol% of C6 and 0.25 mol% benzophenone, C%, 57.3%, Rpmax (s-1), 0.119).
The results demonstrate that the free radicals could be produced by the photoinduced internal
electron transfer systems more efficiently.
3.4. Molecular weight and polydispersity of photopolymer MMA
The lower value of polydispersity P of the produced polymer means the greater extent of the
polymerization reaction. For instance, PMMA yielded by C3 showed the small P value (0.569) as
its concentration was 1×10-5 mol/L under visible light irradiation (Mw, 7.631×105, Mn, 4.865×105),
which is lower than that produced by the corresponding two-component photoinitiating system
(C6 plus benzophenone, P, 0.797) at the same experimental conditions. Furthermore, the other
one-component photoinitaitors C1 and C2 showed the similar results as C3. The study suggests
that the new one-component photoinitiators (C1, C2 and C3) could efficiently photoinitiate
polymerization of MMA with a large molecular weight as well as a good polydispersity under
visible light irradiation.
3.5. ESR spectra analysis
ESR spectra of the new photoinitiators were determined to analyze visible photoinitiation
mechanisms. The active free radicals with the short lifetime formed upon the irradiation could be
trapped by DMPO in spin-trapping experiments. Scheme 3 shows the trapping mechanism of the
radicals yielded by the new one-component photoinitiators.
Insert Scheme 3
Typical ESR spectra of C3 and its corresponding two-component photoinitiating system (C6
plus benzophenone) are given in Figure 7. It shows that the active free radical signals produced

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by C3 system are similar to those yielded by C6 system. According to Scheme 3, α-amine radicals
could be recognized by DMPO [28]. It is further found that C3 shows the greater ESR signal
intensities than the two-component C6 system. As a consequence, the acrylate monomer MMA
could be photoinitiated by these one-component photoinitiators more efficiently.
Insert Figure 7
It is considered that N, N-dialkylamino-stilbene part in C3 undergoes photoinduced
intramolecular electron transfer to phenyl keto part under visible light irradiation. Then, an
exciplex between the negatively charged transferred species and its ground state could be yielded.
In the end, the active α-aminoalkyl free radicals are produced through an intermediate by
abstracting hydrogen reaction (Scheme 4) [16].
Insert Scheme 4
3.6. Photobleaching studies
The UV/visible absorption of one-component or two-component visible light photoinitiating
polymerization systems showed the decreases in organic solvents or solid phase state, which
demonstrates photoinduced internal or inter molecular electron transfer leads to the
decomposition of the photoinitiators. The representative absorption spectral variations of C3
system with the visible light irradiation time in CH2Cl2 are presented in Figure 8, suggesting the
occurrence of photobleaching.
Insert Figure 8
The plots of the photobleaching ratios of one-component or two-component photoinitiating
systems with the irradiation time are shown in Figure 9. The results suggests that C1 and C3
systems produce the greater photobleaching rates than C4 and C6 systems respectively, which

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further interprets that C1 and C3 undergo the more efficient visible light photoinitiating than C4
and C6. Figure 9 also shows that C3 system exhibits the more rapid photobleaching than C1 one,
which further suggests that the stronger electron-donating of N, N-dibutylamino group favors the
occurrence of photoinduced electron transfer, and the more efficient visible light photoinitiating
polymerization of MMA by C3 is obtained accordingly.
Insert Figure 9
3.7. The cyclic voltammograms analysis
The cyclic voltammograms of C1~C3 were performed at various scan rates in CH2Cl2. The
representative cyclic voltammograms of C3 at the sweeping rates 0.08, 0.1, 0.12, 0.15 and 0.20
V/s is shown in Figure 10. It is shown that with the increase of the scan rates, the anodic to
cathodic peak current is different from the unity and ipa is not equaled to ipc, which suggests that
the redox processes of C3 are irreversible under all the sweeping rates. In addition, the linear
increasing of the peak potentials with the square roots of the scan rates shows that the redox
process of C3 is dominated by the diffusion-controlled electron transfer reactions [29].
Insert Figure 10
The intramolecular electron transfer number of C3 based on cyclic voltammograms is further
calculated. According to Laviron equation [30], the intramolecular electron transfer numbers
could be calculated:
Ep = E0 + (1− ln
ν )RT /α nF
(5)
wherein α shows the cathodic electron transfer coefficient (irreversible process α=0.5), n
represents the electron transfer number. R shows the gas constant (R, 8.314 J·mol-1·K-1), T is the
temperature in Kelvin (T=298K) and F stands for the Faraday constant (F, 96493 C·mol-1).

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Insert Figure 11
Figure 11 shows the relationship between the peak potentials (Epa and Epc) and the nature
logarithm of the sweeping rate (lnv). It is shown that the peak potentials are proportionally
changed to lnv with a linear regression equation. Hence, npa=0.736 and npc=0.730 are obtained,
which indicates C3 could undergo intramolecular negative charge transfer. The study also
suggests that an internal charge transfer occurs in C1 and C2 as well respectively (C1, npa, 0.696,
npc, 0.692 C2, npa, 0.714, npc, 0.711). The greater intramolecular electron transfer numbers in C3
means the more possibility to produce free radicals under visible light irradiation.
4. Conclusions
To be closing, a variety of new conjugated dyes bearing N, N-dialkylamino and ketone
groups as Type II one-component visible light photoinitiators are synthesized. The excellent
conplanarity of trans C-C double bonds in new target dyes is demonstrated by NMR spectra and
X-ray single crystal diffraction. Visible light photoinitiating polymerization of MMA by new
one-component photoinitiators are studied by photo-DSC. The results demonstrate that the
one-component photoinitiators show much more efficient visible light photoinitiating
polymerization of MMA than the corresponding two-component photoinitiating systems. The
carbon chain increase of N, N-dialkylamino groups favors the improvement of visible light
photoinitiating polymerization of MMA. The presence of phenyl keto segment in molecular
skeleton increases the triplet-triplet absorption of the new photoinitiators. The yielded active free
radicals by the excited photoinitiators are verified by ESR spectra. The cyclic voltammograms
demonstrate that the new photoinitators could undergo intramolecular electron transfer. The

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results would guide us to develop new one-component visible light type II photoinitiators as
required.
Acknowledgments
The authors greatly thank Natural Science Foundation of Chongqing Projects numbered by
CSTC2012jjB50007 and CSTC2010BB0216. H. Li is grateful to the China Postdoctoral Science
Foundation for financial supporting (Grant No. 22012T50762 & 2011M501388). We deeply
appreciate the support on optical measurements from Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, TIPC, Chinese Academy of Sciences.
Supplementary data
X-ray diffraction single crystal data of C1 (Table S1~S4) and Figure S1 associated with this
work are shown in the Supplementary data, which are available free of charge from website of
this journal. Cif check report of C1 is also attached to the Supplementary data.
The supplementary crystallographic data for this work were also contained in CCDC number
713269,
which
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html or from Cambridge crystallographic data center,
12, Union Road, Cambridge CB2 1EZ, UK; fax: 44-1223-336033.
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Captions of Schemes, Tables and Figures
Captions of Schemes 1-4
Scheme 1 Synthesis and chemical structures of the target dyes studied in this work
Scheme 2 The other multi-step synthesis routes of the target dyes studied in this work
Scheme 3 The trapping mechanisms of free radicals by DMPO
Scheme 4 The visible light photoinitiated polymerization processing of photoinitiator C3
Caption of Table
Table 1 The maximal absorption wavelength of the target molecules in various solvents, the
maximal molar extinction coefficient (105·cm-1·mol-1·L), Φ, the fluorescence quantum yield
Captions of Figures 1-11
1
Figure 1 The representative H and 13C NMR spectra of C3 determined by 500 MHz NMR
apparatus in CDCl3
Figure 2 (a) X-ray single crystal diffraction of C1 ORTEP drawing with 30% possibility of
thermal ellipsoids, (b) Single crystal π-π stacking structure of C1
Figure 3 Typical UV/visible spectra of C1~C6 studied in ethyl acetate, the concentration is 1×10-5
mol/L respectively
Figure 4 Transient absorption spectra of C3 in CH2Cl2, the concentration is 5×10-5 mol/L
Figure 5 (Left/a1, a2, a3) The kinetic curves of visible light photopolymerization of MMA
photoinitiated by C1, C2, C3 respectively at room temperature. and (right/b1, b2, b3) the
relationship between the photopolymerization conversion percents of double bond of (-CH=CH-)
of MMA and the irradiation time. Irradiation intensity, Ia, 20mW/cm2.
Figure 6 (Left/a1, a2, a3) The kinetic curves of visible light photopolymerization of MMA

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photoinitiated by one-component and two-component photoinitiating systems respectively. And
(right/b1, b2, b3) the relationship between the photopolymerization conversion percent of double
bond (-CH=CH-) of MMA and visible light irradiation time. The concentrations of the
photoinitiators, the photosensitizers were 0.25 mol% respectively. Irradiation intensity,
20mW/cm2
Figure 7 Typical ESR spectra of C3 (a) and C6 (b) in ethyl acetate.
Figure 8 The variations of UV/visible absorption spectra of C3 photoinitiating polymerization
system with visible light irradiation time, wherein the concentration of C3 is 1×10-5 mol/L, the
concentration of MMA is 1×10-2 mol/L.
Figure 9 The plots of the photobleaching ratios of various photoinitiating polymerization systems
versus the irradiation time
Figure 10 Cyclic voltammetry curves in CH2Cl2 of C3
Figure 11 The linear relationship between Epa/Epc and lnv of C3 in methylene chloride