Synthesis, spectroscopy and photochemistry of novel branched fluorescent nitro-stilbene derivatives with benzopheonone groups

Article abstract of DOI:10.1007/s10895-010-0611-7

In this article, we presented novel nitro-stilbene derivatives with one or two benzophenone groups as photoinitiators via multi-steps synthesis. The ultraviolet/visible spectroscopy and the emission spectroscopy of the compounds were determined in various solvents. The results showed that the ultraviolet/visible absorption spectroscopy of the derivatives with benzophenone moiety displayed overlap effects of nitro-stilbene and benzophenone parts. In non-polar solvents, the derivatives exhibited strong emission, while they displayed weak emission in modest and strong polar solvents. Dyes-linked benzopheonone groups displayed stronger fluorescence emission than simple chromophore parent molecules. Visible-light photoinitiating effects of the derivatives were investigated extensively. Methyl methacrylate could be photoinitiated efficiently by the derivatives with benzophenone moieties at very low concentration, even at 1×10^-5 mol/ L. While the photopolymerization efficiency of styrene initiated by the derivatives was lower than that of methyl methacrylate. Our results showed that the dye-linked photoinitators had more efficient photoinitiating than the simple mixture of dye and photoinitator. Furthermore, the derivative with two benzophenone groups displayed more excellent phototiniatiating effects than the derivative with one benzophenone group. Thermodynamics driving for the occurrence of visible-light photoinduced intramolecular electron transfer from chromophore part to benzophenone part was evaluated. Benzopinacol moiety produced in photoreaction was confirmed by nuclear magnetic resonant spectroscopy. Thermal stability of the derivatives was analyzed. Springer Science+Business Media, LLC 2010.

Full text of DOI:10.1007/s10895-010-0611-7

J Fluoresc (2010) 20:703–712
DOI 10.1007/s10895-010-0611-7
ORIGINAL PAPER
Synthesis, Spectroscopy and Photochemistry of Novel
Branched Fluorescent Nitro-Stilbene Derivatives
with Benzopheonone Groups
Fang Gao & Jian Liu & Huayong Peng & Nvdan Hu &
Hongru Li & Shengtao Zhang
Received: 8 November 2009 /Accepted: 7 February 2010 /Published online: 23 February 2010
#
Springer Science+Business Media, LLC 2010
.
Keywords Synthesis Ultraviolet/visible absorption
Abstract In this article, we presented novel nitro-stilbene
derivatives with one or two benzophenone groups as photo-
initiators via multi-steps synthesis. The ultraviolet/visible
spectroscopy and the emission spectroscopy of the com-
pounds were determined in various solvents. The results
showed that the ultraviolet/visible absorption spectroscopy of
the derivatives with benzophenone moiety displayed overlap
effects of nitro-stilbene and benzophenone parts. In non-polar
solvents, the derivatives exhibited strong emission, while they
displayed weak emission in modest and strong polar solvents.
Dyes-linked benzopheonone groups displayed stronger fluo-
rescence emission than simple chromophore parent mole-
cules. Visible-light photoinitiating effects of the derivatives
were investigated extensively. Methyl methacrylate could be
photoinitiated efficiently by the derivatives with benzophe-
none moieties at very low concentration, even at 1×10−5 mol/
L. While the photopolymerization efficiency of styrene
initiated by the derivatives was lower than that of methyl
methacrylate. Our results showed that the dye-linked photo-
initators had more efficient photoinitiating than the simple
mixture of dye and photoinitator. Furthermore, the derivative
with two benzophenone groups displayed more excellent
phototiniatiating effects than the derivative with one benzo-
phenone group. Thermodynamics driving for the occurrence
of visible-light photoinduced intramolecular electron transfer
from chromophore part to benzophenone part was evaluated.
Benzopinacol moiety produced in photoreaction was con-
firmed by nuclear magnetic resonant spectroscopy. Thermal
stability of the derivatives was analyzed.
.
.
.
spectroscopy Fluorescence spectroscopy Photochemistry
.
Nitro-stilbene Benzophenone
Introduction
In recent years, with the development of long-wavelength
laser technology, visible-light photopolymerization has
received considerable attention due to superior advantages
such as easy fabrication of various micro-devices, rapid
dental photocuring, negligible damage to biological tissue,
less costs and so on [1–8]. Extensive efforts using
intermolecular dye-sensitization for visible-light photopo-
lymerization have been devoted to achieve high efficient
visible-light photopolymerization, which could guarantee
use of lower sample concentrations in application [9–12].
For instance, iodonium salts and sulfonium salts, which are
two sorts of typical ultraviolet photoinitators, were sensi-
tized efficiently by visible light dyes including squaraine
dyes and ketone dyes to produce free radicals for photo-
polymerization of vinyl ethers monomer and epoxides
under visible light irradiation [13, 14]. It was demonstrated
that dye molecules played the role of absorption of visible
light, and photoinduced intermolecular electron transfer
between visible light dyes and ultraviolet photoinitators
occurred. Consequently, the photosensitive initiating effi-
ciency of the dye-sensitization system was dependent on
the rate of photoinduced electron transfer, which played
significant role on photochemistry and photophysics such
as fluorescence sensors and fluorescence switches. While as
compared with intramolecular electron transfer, relative
slow intermolecular photochemical rate limited further
enhancement of visible-light photochemistry efficiency.
Accordingly, the development of dye-linked photoinitators
:
:
:
:
:
F. Gao (*) J. Liu H. Peng N. Hu H. Li S. Zhang
College of Chemistry and Chemical Engineering,
Chongqing University,
Chongqing 400044, China
e-mail: fanggao1971@gmail.com

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J Fluoresc (2010) 20:703–712
became a research focus [15, 16]. This provided great
possibility on the realization of highly efficient visible-light
photopolymerization through photoinduced intramolecular
electron transfer. However, the construction of such novel
photofunctional compounds with required and ideal photo-
chemical and photophysical properties is always challenge
for chemists. For instance, one of shortcomings is that
coupling an ultraviolet photoinitator with a known chro-
mophore always leads to the fluorescence attenuation of
chromophore part due to the interaction of chromophore
part and functional molecule [17, 18].
yields of the compounds herein. To avoid self-quenching
of fluorescence emission, the low concentration of the
compounds (1×10−6 mol/L) was prepared for the survey
of fluorescence quantum yields. The melting point was
determined using a Sichuan University 2X-1 melting point
apparatus and was uncorrected. Nuclear Magnetic Reso-
nance (NMR) was done at room temperature with a Bruker
500 MHz apparatus with tetramethylsilane (TMS) as
internal standard and CDCl3 as solvent. Element analysis
was detected by CE440 elemental analysis meter from
Exeter Analytical Inc.
Of particular interests for us is if we could build some
dye-linked photosensitive compounds, which not only
exhibit great visible-light photoinitiating efficiency, but
have excellent photoluminescent nature. Herein, we pro-
posed a new chemical strategy, which employed branched
dye-linked benzophenone as long-wavelength photoinitia-
tors. It is well-known that benzophenone is typical
ultraviolet photoinitiator [19]. If more benzophenone parts
are packed in the same dye molecule, they could provide
much more efficient visible-light photopolymerization.
Furthermore, because benzophenone has rigid structure,
the fluorescence emission of chromophore part could be
enhanced as coupling with benzophenone part via covalent
bonds. This paper is part of our endeavor on the
achievement of the above goal, which concentrated on
particularly the building branched dye-linked photoinitia-
tors. The spectroscopy, the photochemistry as well as
thermal stabilities were exclusively investigated.
The fluorescence quantum yields of the compounds in
solvents with different polarities were measured based on
the following equation.
R
n20A0 If ðlf Þdlf
Φf ¼ Φ0f
R
n2A If0ðlf Þdlf
wherein n0 and n were the refractive indices of the solvents,
A0 and A were the absorption of the reference and the
sample at excited wavelength respectively, Φf and Φ0f were
the quantum yields, and the integrals denoted the area of
the fluorescence bands for the reference and the sample
respectively.
Visible-light photochemistry
Nitro-stilbene dyes (photoinitiators), ethanolamine (as coin-
itiator) and monomer were dissolved in ethyl acetate in dark.
Photopolymerization was performed in a 10 cm pyrex tube
as irradiated by a 1KW tungsten-iodine lamp as light source
at 10 cm distance. The photochemical reaction was carried
out at room temperature via condenser water and strong fan
to remove thermopolymerization. The wavelength below
380 nm was cut off by the filter in order to remove
ultraviolet light photopolymerization. Photopolymer was
precipitated in cold methanol, and dried by vacuum, and
weighted to calculate visible-light photopolymerization
yields.
Experimental
Materials
The organic solvents were purchased from Chongqing
Oriental Chemical Corporation and further dried using
standard laboratory techniques according to published
method [20]. Other reagents were obtained from Aldrich
unless otherwise specified. C2, C3, C4, C5 and C6 (Fig. 1)
were synthesized in our laboratory (Fig. 2).
Thermal analysis
The differential thermal analysis (DTA) and thermograving
(TGA) were conducted under nitrogen flow in Shimadzu
DTG-60H equipment at heating rate 10°C min−1.
Instruments
The UV/visible absorption spectra (1×10−5 mol/L) were
recorded with a Cintra spectrophotometer. The emission
spectra (1×10−5 mol/L) were determined with Shimadzu
RF-531PC spectrofluorophotonmeter, which was fitted
with a 150 W xenon lamp operated as a continuous wave
source, slits selectable in six steps to produce spectral
bandwidths of 1.5, 3, 5, 10, 15 and 20 nm. Rodamin 6G in
ethanol (Φ=0.94, 1×10−6−1×10−5 mol/L [21, 22]) was
used as reference to determine the fluorescence quantum
Synthesis
Synthesis route was presented in Fig. 2.
(1) C2: 4-bromomethyl benzophenone
The compound was synthesized according to a well-
known method with modified procedure [23].

J Fluoresc (2010) 20:703–712
705
Fig. 1 The chemical structures
of C1, C2, C3, C4, C5 and
C6 studied in this paper
O
O
Br
C2
C1
O
N
NO2
N
O
HO
NO2
C5
C3
C4
O
O
HO
HO
O
N
N
NO2
NO2
O
C6
(2) C3: p-N-ethyl-N-hydroxylethylamino-p′-nitro-stilbene
8.091–8.108 (d, J=8.5 Hz, 2H, Ar–H). Melting point (147–
148°C), Anal. Calcd for C18H20N2O3, C, 69.21, H, 6.45, N,
8.97, O, 15.37. Found: C, 68.83, H, 6.53, N, 8.89.
p-N-Ethyl-N-hydroxylethylamino-benzaldehyde was pre-
pared according to a reported method with Vilsmeier-Haack
formylation via protection and deprotection procedure [24].
C3 was prepared according to a similar method with
modified procedure [25]. 2 g p-Nitro-phenylacetic acid
(11 mmol) and 1.54 g p-N-ethyl-N-hydroxylethyl-amino-
benzaldehyde (8 mmol) were mixed fully, and 1.04 g
(12 mmol) piperidine was added into the mixture. The
resulting mixture was heated at 100°C for 2 h, then at 120°C
for 1 h till a black solid was formed. The product 1.5 g
(yield: 76%) was obtained as a brown solid after twice
(3) C4: p-(N, N-dihydroxyethyl)amino-p′-nitro-stilbene
p-(N,N-Dihydroxyethyl)amino-benzaldehyde was pre-
pared according to the reported method [24]. C4 was
synthesized according to a similar method with modified
procedure [25]. 2.68 g (14.8 mmol) p-Nitro-phenylacetic
acid and 2.46 g (11.8 mmol) p-(N,N-dihydroxyethyla-
mino)-p′-nitro-benzaldehyde were mixed fully and 1.5 g
piperdine (17.6 mmol) was added into the mixture. The
resultant mixture was heated at 100°C for 2 h, then at
120°C for 1 h till a black solid was formed. 2.5 g (yield:
71.4 %) C5 was obtained as a brown solid after twice
crystallization from ethanol. 1H-NMR δ: 3.450–3.475
(t, J=6.25 Hz, 4H, –O–CH2–), 3.537–3.562 (t, J=
6.25 Hz, 4H, –N–CH2–), 4.795–4.817 (t, J=5.5 Hz, 2H,
H–O), 6.713–6.730 (d, J=8.5 Hz, 2H, Ar–H), 7.060–7.093
1
recrystallization from ethanol. H-NMR, 1.125–1.154 (t, J=
7.25 Hz, 3H, –CH3), 3.419–3.462 (m, 2H, –CH2–), 3.469–
3.492 (t, J=5.75 Hz, 2H, –CH2–), 3.755–3.778 (t, J=
5.75 Hz, 2H, –CH2–), 4.248–4.270 (t, J=5.5 Hz, 1H, H–
O), 6.692 (s, 2H, Ar–H), 6.842–6.875 (d, J=16.5 Hz, 1H,
–CH=CH–), 7.101–7.134 (d, J=16.5 Hz, 1H, –CH=CH–),
7.368 (s, 2H, Ar–H), 7.479–7.496 (d, J=8.5 Hz, 2H, Ar–H),
Fig. 2 The synthesis route of
C5 and C6
O
O
HO
O
(c)
O
(b)
HO
(a)
N
CHO
N
CHO
(e)
N
N
(d)
O
O
HO
N
NO2
N
NO2
C5
C3
(1)
O
O
O
HO
HO
(c)
(b)
O
HO
HO
(a)
N
CHO
N
CHO
N
N
O
O
O
O
O
(d)
O
(e)
HO
HO
N
N
NO2
O
NO2
C4
C6
O
(2)
(1), (2) : (a) pyridine, acetic anhydride, (b) DMF/POCl3, (c) NaOH/CH3CH2OH, (d) piperidine,
phenylacetic acid, (e) 4-bromomethylbenzophenone, NaH/THF

706
J Fluoresc (2010) 20:703–712
7.411 (t, J=7.5 Hz, 4H, Ar–H), 7.483–7.520 (m, 4H, Ar–
H), 7.697–7.711 (t, J=3.5 Hz, 5H, Ar–H), 8.158–8.176 (d,
J=9.0 Hz, 2H, Ar–H). 13C-NMR: 13.124, 45.251, 69.731,
71.531, 72.409, 124.162, 126.564, 126.905, 127.249,
127.951, 128.311, 130.038, 130.285, 132.470, 132.765,
135.969, 136.908, 137.564, 140.600, 142.992, 143.709,
146.823, 196.389. Melting point (88–90°C), Anal. Calcd
for C32H30N2O4, C,75.87, H, 5.97, N, 5.53, O, 12.63.
Found: C, 75.78, H, 6.13, N, 5.45.
(5) C6, 4-N, N-di[(2-(p-benzoyl-phenylmethoxyl)ethyl]-4′-
Fig. 3 Typical UV/visible spectroscopy of C1, C3 and C5 in
acetonitrile (c=1×10−5 mol/L)
nitro-stilbene
The title compound was synthesized by a similar
procedure as C5. Crude NaH was washed by dry
cyclohexane twice to remove mineral oil. C2 5.5 g
(20 mmol) and C4 (1.64 g, 5 mmol) were dissolved in
NaH/50 ml dry THF, the mixture was stirred at room
temperature under argon for 20 h. The solid was got rid of
solution by filtration and THF was removed fully by
evaporation. The resulting mixture was dissolved in
chloroform and washed by water for three times. The
organic layer was dried with anhydrous sodium sulfate and
then concentrated. The product was purified by column
chromatography. Further purification was carried out with
twice recrystallization from benzene to yield 1.22 g
(d, J=16.5 Hz,1H, –CH=CH–), 7.377–7.410 (d, J=
16.5 Hz,1H, –CH=CH–), 7.457–7.474 (d, J=8.5 Hz, 2H,
Ar–H), 7.737–7.754 (d, J=8.5 Hz, 2H, Ar–H), 8.160–8.177
(d, J=8.5 Hz, 2H, Ar–H). Melting point (168–1169°C),
Anal. Calcd for C18H20N2O4, C, 65.84, H, 6.14, N, 8.53, O,
19.49. Found: C, 65.69, H, 6.21, N, 8.67.
(4) C5, 4-[N-ethyl-N-(2-(p-benzoyl-phenylmethoxyl)
ethyl]-4′-nitro-stilbene
Crude sodium hyride was washed by dry cyclohexane
twice to remove mineral oil. C2 22.75 g (10 mmol) and
C3 (1.56 g, 5 mmol) were dissolved in NaH/50 ml dry
THF, the mixture was stirred at room temperature under
argon for 24 h. The solid was got rid of solution by
filtration and THF was removed fully by evaporation. The
resulting mixture was dissolved in chloroform and washed
by water for three times. The organic layer was dried with
anhydrous sodium sulfate and then concentrated. The
product was purified by column chromatography. Further
purification was carried out with twice recrystallization
from benzene to yield 0.8 g (1.6 mmol) deep red solid of
1
(1.7 mmol) deep red solid of C6 (yield34.2%). H-NMR
δ: 3.736 (s, 8H, –CH2–), 4.590 (s, 4H, –O–CH2–Ar),
6.747–6.763 (d, J=8.0 Hz, 2H, Ar–H), 6.893–6.926 (d, J=
16.5 Hz, 1H, –CH=CH–), 7.161–7.194 (d, J=16.5 Hz, 1H,
–CH=CH–), 7.400–7.416 (d, J=8.0 Hz, 6H, Ar–H), 7.448
(s, 1H, Ar–H), 7.463 (s, 2H, Ar–H), 7.479 (s, 1H, Ar–H),
7.533 (s, 1H Ar–H), 7.551–7.559 (m, 2H, Ar–H),7.572
(s, 1H, Ar–H), 7.765–7.780 (t, J=3.5 Hz, 8H, Ar–H),
8.158–8.176 (d, J=9.0 Hz, 2H, Ar–H). 13C-NMR: 51.947,
67.880, 77.348, 111.834, 121.634, 124.143, 125.334,
126.222, 127.075, 128.319, 128.599, 130.009, 130.271,
132.462, 133.222, 136.900, 137.587, 142.850, 144.772,
145.999, 196.304. Melting point (102–103°C), Anal. Calcd
for C46H40N2O6, C, 77.08, H, 5.62, N, 3.91, O, 13.39.
Found: C, 76.92, H, 5.71, N, 3.83.
1
C5 (yield 31.5%). H-NMR δ: 1.150 (s, 3H, –CH3), 3.438
(s, 2H, –N–CH2–), 3.263 (s, 2H, –N–CH2–), 3.705 (s, 2H,
–O–CH2–), 4.517 (s, 2H, –O–CH2–Ar), 6.865–6.896
(d, J=15.5 Hz, 1H, –CH=CH–), 7.104–7.136 (d, J=
14.0 Hz, 1H, –CH=CH–), 7.313 (s, 2H, Ar–H), 7.381–
ꢀ
ꢁ
Table 1 The absorption spectral
data of C3, C4, C5 and C6 in
various solvents
" : 1 ꢀ 105 L=mol ꢁ cm
Solvents
λmax (nm)
C3
C4
C5
C6
C3
C4
C5
C6
Cyclohexane
Benzene
Ethyl acetate
THF
419
436
440
444
443
439
–
423
436
437
442
446
439
420
431
433
436
440
439
–
–
–
–
431
435
443
443
436
0.2952
0.4614
0.3741
0.3355
0.4078
0.2875
0.4526
0.3555
0.3058
0.3680
0.2944
0.3607
0.3282
0.2968
0.3529
0.2832
0.3334
0.3113
0.2738
0.3112
CH2Cl2
CH3CN

J Fluoresc (2010) 20:703–712
707
Results and discussion
Synthesis
4-Bromethyl-benzophenone, which was prepared from
bromide reaction of 4-methyl-benzophenone and N-
bromosuccinimide, played the role of the introduction of
benzophenone moiety to the nitro-stilbene backbone.
Owing to the electron-withdrawing effect of nitro group
of p-nitro-phenylacetiac acid and the electron-donating of
N, N-diethylamino group of benzaldehyde derivatives, C3
and C4 were produced by the condensation in the presence
of weak base piperidine, which was carried out at high
temperature to ensure full decarboxylation. C5 or C6 was
prepared by C2 and C3 or C4 respectively for the
formation of ether bond using sodium hydride as base in
mild conditions, and an acceptable yield was obtained. The
combination use of flash column chromatography and
recrystallization was efficient on the purification of C5
and C6.
Fig. 5 The fluorescence change of C6 with the increasing THF in
THF/benzene binary solvents (Ex: 350 nm, Slit Width: Ex: 5 nm, Em:
3 nm, c=1×10−6 mol/L)
increasing of the solvent polarity (approximate 20 nm).
This indicated that the long-wavelength absorption could be
ascribed to strongly allowed (π, π*) transition with internal
charge transfer nature [26]. Interesting, we observed that
the optical density of maximal absorption wavelength of C5
or C6 was a little less than that of C3 or C4 respectively in
various solvents. This implied that the benzophenone part
had some influence on the molecular electron density
distribution of chromophore part, and the electron trans-
mission process of the compound was thus altered [27].
Mirror symmetry was observed for the excitation and
temission spectroscopy of C5 and C6, as shown in Fig. 4.
C3, C4, C5 and C6 showed strong fluorescence emission
in non-polar solvents such as in benzene. While, the
photoluminescence became weak in polar solvents such as
in THF and ethyl acetate. Typical fluorescence spectrosco-
py of C6 in benzene/THF binary solvents was presented in
Fig. 5. It clearly showed that with the increasing THF
volume ratio in benzene/THF binary solvents, the fluores-
cent intensity of C6 decreased, and the maximal fluores-
cence emission wavelength displayed red-shift gradually.
This confirmed the existence of internal charge transfer at
the excited state. Because diarylaminostilbene derivatives
were typically characterized with twisted intramolecular
UV/Visible absorption and fluorescence spectroscopy
Typical ultraviolet/visible spectroscopy of C1, C3 and C5
in acetonitrile was presented in Fig. 3. Clearly, the
absorption in the short-wavelength region came from
benzophenone part mainly. While, the absorption in the
visible-light region could be assigned to nitro-stilbene part.
The results suggested that the ultraviolet/visible absorption
of dye-linked photoinitiators exhibited overlap effect of the
chromophore and photoinitiator parts. This further demon-
strated that benzophenone part was linked covalently with
nitro-stilbene part.
The ultraviolet/visible spectral data of C3, C4, C5 and
C6 were listed in Table 1. It showed that the absorption
maxima of C3 and C4 displayed bathchromic shift with the
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
C5 Excitation
C5 Emission
C6 Excitation
C6 Emission
690
325 350
400
450
500
550
600
650
Wavelength ( nm )
Fig. 6 Typical comparison of fluorescence spectroscopy of C4 and
C6 in benzene (Ex: 350 nm, Slit Width: Ex: 3 nm, Em: 3 nm, c=1×
10−6 mol/L)
Fig. 4 Normalized excited and emission spectroscopy of C5 and C6
in benzene (Ex: 350 nm, c=5×10−6 mol/L)

708
J Fluoresc (2010) 20:703–712
Table 2 The maxima fluores-
cence emission wavelength
and fluorescence quantum yields
of C3, C4, C5 and C6 in
various solvents
Solvents
λf, max (nm)
Ф
C3
C4
C5
C6
C3
C4
C5
C6
Benzene
Ethyl Acetate
THF
551
601
602
555
602
601
551
606
607
556
608
609
0.233
0.236
0.355
0.471
0.0330
0.0193
0.0332
0.0198
0.0533
0.0525
0.0629
0.0710
charge transfer state in polar solvents [28], the derivatives
exhibited strong fluorescence emission in apolar solvents
and weak fluorescence emission in strong solvents.
absorption in the visible-light region for benzophenone
and triethanolamine and thus no occurrence of photochem-
istry between them. We have carried out four photosensitive
initiating systems for visible light polymerization in this
study, which included P1 (C1, C3, triethanolamine), P2
(C1, C4, triethanolamine), P3 (C5, triethanolamine), and
P4 (C6, triethanolamine).
The photopolymeric efficiency ratios of P3 to P1, P4 to
P2 were investigated. The ratio kept approximately con-
stant with irradiation time at the fixed concentration of the
photoinitiators. Typical results were presented as Fig. 7.
The results suggested that P1 and P2 systems were able to
photoinitiate the polymerization of monomer as they were
irradiated by visible light, but the photopolymerization
yields were lower than those of P3 and P4 respectively. The
ratio was approximately 1.3 to 1.5 with irradiation time,
indicating that the visible-light photosensitized initiating
efficiency of P3 and P4 was much higher than that of P1
and P2 respectively. This demonstrated that free radicals
produced from intramolecular photochemical reaction were
much more than those from intermolecular photochemical
reaction.
IC5 and C6 exhibited stronger fluorescence emission
than C3 and C4 in various solvents. Typical comparison of
fluorescence spectroscopy of C4 and C6 in benzene was
presented in Fig. 6. This showed that the fluorescence
quantum yields of C5 to C6 were higher than those of C3
and C4 in various solvents, as shown in Table 2. The data
indicated that the emission of the derivatives was increased
by benzophenone parts. This could be understood from two
aspects: (1) phenylmethyloxyl should have more stronger
electron-donating effects that hydroxyl groups, thus the
electron density distribution of chromophore of C5 and C6
was strengthened and their π→π* electron transition could
be enhanced, (2) the rigidity and stiffness of overall
molecule were enhanced by benzophenone group. These
might result in the enhancement of charge transfer, the
limitation of cis-trans twist, the decreasing vibration of
molecules, and the reduction of interaction of solute
molecules, and accordingly the fluorescence emission of
C5 and C6 became stronger [29].
The plot of photopolymeric ratio of MMA of P4 to P3
with irradiation time was presented in Fig. 8. It showed that
visible-light photoinitiating efficiency of P4 was higher
than that of P3, which suggested that two benzophenone
parts induced a more efficient visible-light photopolymeri-
zation than one benzophenone part. The results suggested
that branch effect played a significant role on the
enhancement of visible-light photopolymerization efficien-
Visible light photochemistry
We firstly investigated the possibility of visible-light
photoinitiating of MMA with the system of benzophenone
and triethanolamine. The results showed clearly that there
was no production of PMMA, which was due to no
1.6
1.55
1.5
3.5
3.1
1.45
1.4
line 1
line 2
2.7
line 1
line 2
2.3
1.35
1.3
1.9
1.5
1.25
1.2
0
1
2
3
4
5
6
7
8
Irradiation Time (hours)
0
1
2
3
4
5
6
7
8
Irradiation Time (hours)
Fig. 8 The ratio of the photoinitiating polymerization yields of MMA
to that of styrene with irradiation time at the concentration 1×
10−5 mol/L of the photoinitiators. Where in: monomer: 4.7 mol/L,
triethanolamine: 2.65 mol/L (line 1: MMA to styrene of P4 system,
line 2: MMA to styrene of P3 system)
Fig. 7 The ratio of the photoinitiating polymerization yields of MMA
with the irradiation time at the concentration 1×10−5 mol/L of the
photoinitiators. Wherein MMA: 4.7 mol/L, triethanolamine: 2.65 mol/
L, (line 1: P4 to P2, line 2: P3 to P1)

J Fluoresc (2010) 20:703–712
709
2.2
2
line 1
line 2
line 3
line 4
line 5
1.8
1.6
1.4
1.2
1
0
2
4
6
8
10
12
Irradiation Time (hours)
Fig. 9 The ratio of the photoinitiating polymerization yields of MMA
(P4 to P3) with irradiation time. Wherein: the concentration of
compounds 5 and 6 for different lines: line 1: 1×10−5 mol/L, line 2:
2×10−5 mol/L, line 3: 3×10−5 mol/L, line 4: 4×10−5 mol/L, line 5:
5×10−5 mol/L, MMA: 4.7 mol/L triethanolamine: 2.65 mol/L
Fig. 10 The variation of 1H-NMR of C5/triethanolamine system
before and after irradiation
cy. With the increasing visible-light irradiation time, the
radical coupling rate might increase due to more free
radicals produced, and this could lead to decrease gradually
the ratio of visible-light photoinitiating efficiency of P4 to
P3 with irradiation time.
thermodynamics. Accordingly, these compounds might
undergo rapid visible light photochemistry during irradiation.
Photoinitiating mechanism of benzophenone/triethanol-
amine system was investigated well. Initiating free radicals
for photopolymerization were formed via intermolecular
hydrogen transfer from triethanolamine to benzophenone in
the exciplex, which was formed through the triplet state of
benzopheone [33, 34]. In the present study, the exciplex
between photoiniator and triethanolamine could be formed
at the excited state of photoinitator via intermolecular
electron transfer as C5 or C6 was excited by the visible
light. As discussed, the fluorescence quantum yields of C5
and C6 were higher than those of C3 and C4 respectively.
Furthermore, visible light photopolymerization efficiency
of styrene was bad. Thus, we assumed that photoinduced
intramolecular electron transfer could occur through the
The visible-light photopolymerization of MMA and
styrene photoinitiated by P3 and P4 was also studied. The
plot of the photopolymeric ratio of MMA to that of styrene
with the concentration of the photoinitiators was shown in
Fig. 9. It showed clearly that the photopolymerization of
MMA was more efficient than that of styrene, which was
mostly due to the efficient quenching of excited triplet
states of the photoinitiztors by styrene [30]. The ratio
increased and reached approximately constant with irradi-
ation time at the fixed concentration of the photoinitiators,
which suggested that visible light photochemistry reaction
kept relative stable with irradiation time. The effects of the
coinitiators on the visible-light photopolymerization were
also investigated. Triethanolamine displayed much more
excellent photoinitiating effects than diethanolamine,
monoethanolamine and triethylamine.
The value of free energy change ΔG for the electron
transfer reaction between nitro-stilbenzene dyes and benzo-
phenone was estimated according to the Rohm–Weller
equation [31] and the results were listed in Table 3. Large
negative ΔG suggested that photoinduced electron transfer
between nitro-stilbenzene and benzophenone was allowed on
Table 3 The estimated values of free energy change ΔG of electron
transfer between nitro-stilben dyes and benzopheone
Eox
(*)
ΔE0,0
ΔG
ðD=DþÞ
C3 0.512 ev
C4 0.716 ev
2.03 ev
2.01 ev
−235.6 KJ/mol
−214.0 KJ/mol
Fig. 11 The variation of the maximum absorption spectroscopy of P3
system with visible-light irradiation time. Wherein: MMA: 4.7 mol/L,
triethanolamine: 2.65 mol/L, the concentrations of C5: 4.5×10-6 mol/
L, solvent: ethyl acetate
Ered
for benzopheone 0.925 ev [32]; estimation from the
ðA=AꢂÞ
fluorescence emission*

710
J Fluoresc (2010) 20:703–712
visible light
Fig. 12 A proposed photoiniti-
ating polymerization
processing for C5
O
O
N
NO2
HO
OH
N
HO
charge transfer
HO
O
O
N
OH
N
NO2
HO
exciplex
HO
HO
O
OH
N
N
NO2
initiating radical
methyl acrylate
+
semibenzopinacol
termating radical
HO
PMMA
excited triplet state of the chromophore part and benzophe-
none part.
photobleaching than P1 and P2 systems, which demon-
strated that they had more efficient visible-light photo-
chemistry. According to experimental observation and the
references [35, 36], a possible photoinitating mechanism
was proposed as presented in Fig. 12.
Recently, semibenzopinacol moiety was successfully
confirmed via the analysis of nuclear magnetic resonnan
spectroscopy for ultraviolet light photopolymerization of
benzophenone and triethanolamine photoinitiating system
1
[35]. In this study, we performed the comparison of H-
Thermal analysis
NMR of the C5/triethanolamine system before and after
irradiation to confirm the production of benzopinacol
moiety during visible light photochemical reaction. As
shown in Fig. 10, two new nuclear magnetic resonant peaks
were detected during irradiation, which could be assigned
to benzopinacol moiety.
Photobleaching could be observed for these photoinitiat-
ing systems only if sufficient polymer was produced,
suggesting that semibenzopinacol might be attached to
polymer tail. A typical variation of UV/visible spectra of
C5/triethanolamine system was shown in Fig. 11. Because
the percent of chromophore moiety in the polymer line was
remarkably low, the color of the chromophore moiety could
not be observed. P3 and P4 systems displayed more rapid
Thermal property of the derivatives was analyzed by the
measurement of differential thermal analysis (DTA) and
thermograving (TGA) under a nitrogen atmosphere. The
onset decomposition temperatures of C3, C4, C5 and C6
determined by TGA were 335.77, 322.51, 328.10 and
331.81°C respectively as presented in Fig. 12, which
suggested that the substitution of benzopheonone part did
not play negative effect on the thermal stability of the
compounds. The derivatives could be regarded as robust
molecules. The weight loss of C3 (MW=312.26) was
31.629% at 335.77°C, which was close to the loss of N-
ethyl-N-hydroethyl group (MW=88.13). Interestingly, the
weight loss of C4 (MW=328.36) was 21.306% at 322.51°C,

J Fluoresc (2010) 20:703–712
711
which was also close to the loss of two C–OH parts (MW=
64.08). The weight loss of C5 (MW=522.59) was 17.450%
at 328.10°C, which was close to the loss of phenyl ring
(MW=78.11). Remarkably, C6 (MW=702.79) displayed
12.355% weight loss at 331.81°C, which was perfectly same
as that of C5, indicating the possible loss of phenyl ring as
well. The analysis showed that: (1) the decomposition of
the C3, C4, C5 and C6 did occur at high temperatures, and
(2) thermal stabilities of dye-linked photoinitiators was not
much influenced by the substitution of photoinitator parts
(Fig. 13).
Conclusions
To summarize, novel fluorescent photoinitiators of dye-
linked benzophenone via ether bond for visible-light
photochemistry were synthesized and characterized. Re-
markable solvatochromic effect was observed for the
absorption and fluorescence spectroscopy of the deriva-
tives. The occurrence of rapid visible light photoinduced
intramolecular electron transfer of C5 and C6 could lead to
much higher efficient long-wavelength photopolymeriza-
tion, which was superior over those simple combinations
(P1 and P2 systems). We have presented strong evidences
that the branch effect was an efficient chemical strategy on
the enhancement of visible-light photoinitating polymeriza-
tion of acrylate ester. Novel photoinitators displayed almost
the same thermal decomposed temperatures as the chromo-
phore parent molecules, which demonstrated that the
thermal stabilities of novel photoinitators were not reduced
by the benzophenone groups.
Acknowledgements The authors appreciate financial support from
National Natural Science Foundation of China (Nos. 20776165,
20702065, 20872184). We would thank the “Foundation of Chongq-
ing Science and Technology Commission” (CSTC2008BA4020,
CSTC2009BB4216). H. Li thanks “A Foundation for the Author of
National Excellent Doctoral Dissertation of PR China (200735)” for
financial support. This paper is partly sponsored by the Scientific
Research Foundation for the Returned Overseas Chinese Scholars,
State Education Ministry as well (Nos. 20071108-1, 20071108-5). The
support from Innovative Talent Training Project, the Third State of
“211 Project”, Chongqing University (S-09103) is greatly appreciated.
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