Study of the absorption and emission spectroscopy of "A-B" type photosensitive compounds including two-photon chromophore and benzophenone moiety

Article abstract of DOI:10.1016/j.saa.2007.07.064

"A-B" type photosensitive compounds including two-photon chromophore and benzophenone moiety have been designed, synthesized and characterized. The UV-vis absorption and fluorescence emission of the compounds have been extensively studied in various solvents. The results show that the absorption of "A-B" type compounds displays obvious double absorption bands, one of which at short-wavelength is related to the benzophenone moiety, the other at long-wavelength is mainly contributed by chromophore. The emission of "A-B" type compounds at 500-700 nm shows an "unexpected" blue-shift comparing with that of the sole chromophore. The photosensitive compounds with amino group display strong emission in apolar solvents and have a low fluorescence quantum yields in polar solvents. In contrast, the compounds without amino group exhibit strong fluorescence emission in polar solvents, and low fluorescence quantum yields in apolar solvents. The fluorescence quantum yields of "A-B" type compounds are higher than those of the sole chromophore. The discoveries suggest that charge redistribution induced by the introduction of benzophenone moiety plays a key role on the absorption and emission spectroscopy.

Full text of DOI:10.1016/j.saa.2007.07.064

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 1006–1012
Study of the absorption and emission spectroscopy of “A–B” type
photosensitive compounds including two-photon chromophore and
benzophenone moiety
Fang Gao∗, Nvdan Hu, Ting Xie, Zhibin Cheng, Long Yang, Xiaojiao Liu,
Zedong Li, Xinfu Wang, Hongru Li∗, Xueming Li∗
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China
Received 11 April 2007; received in revised form 19 July 2007; accepted 25 July 2007
Abstract
“A–B” type photosensitive compounds including two-photon chromophore and benzophenone moiety have been designed, synthesized and
characterized. The UV–vis absorption and fluorescence emission of the compounds have been extensively studied in various solvents. The results
show that the absorption of “A–B” type compounds displays obvious double absorption bands, one of which at short-wavelength is related to the
benzophenone moiety, the other at long-wavelength is mainly contributed by chromophore. The emission of “A–B” type compounds at 500–700 nm
shows an “unexpected” blue-shift comparing with that of the sole chromophore. The photosensitive compounds with amino group display strong
emission in apolar solvents and have a low fluorescence quantum yields in polar solvents. In contrast, the compounds without amino group exhibit
strong fluorescence emission in polar solvents, and low fluorescence quantum yields in apolar solvents. The fluorescence quantum yields of “A–B”
type compounds are higher than those of the sole chromophore. The discoveries suggest that charge redistribution induced by the introduction of
benzophenone moiety plays a key role on the absorption and emission spectroscopy.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Absorption; Emission; Chromophore; Photoinitiator; Fluorescence quantum yield
1. Introduction
large two-photon absorption cross-section and high-efficient
UV-photoinitiators. When the dyes are excited by two-photon,
an intermolecular electron transfer occurs between dyes and
photoinitiators, free radicals are then formed, and monomers are
initiated. This pathway has been confirmed by several research
groups [7–10]. It is convenient because numerous sensitiz-
ers and UV-photoinitiators can be chosen from compounds’
library. On the other hand, because free radical is produced from
two-photon intermolecular electron transfer, the possibility of
one to two orders of magnitude improvement of photopoly-
merization efficiency decreases. (2) Monomer is initiated by
two-photon absorption dyes. Marder and co-workers firstly uti-
lized “D-␲-A” molecules as two-photon initiators to obtain
photopolymer [1], and a substantial number of other researchers
have also explored such molecules as two-photon initiators
[11–13], and the recent feather article of Marder is especially
comprehensive review on the advance of this field [14]. While,
the issue of this approach is that the mechanism of two-photon
polymerization is not clear [11]. (3) Common UV photoinitia-
tors are directly used as two-photon initiators. This method,
Sincethefirsttimerealizationoftwo-photonfabrication[1], it
hasshownanimportantroleinmanyfields, forinstancedatastor-
age [1], micro-fabrication of medical device [2], microstructure
of hydrogel [3], microstructure of protein [4], DNA loading [5].
As compared to one-photon polymerization, two-photon micro-
fabrication offers many advantages, such as space resolution
and deep fabrication [1]. However, the resolution of two-photon
fabrication still needs to be further increased for the more exten-
sive application, especially for nanoscale materials fabrication
[6].
Till now, three approaches have been established to achieve
two-photon polymerization: (1) Free radical for photopoly-
merization is produced from common UV-photoinitiators via
two-photon dye-sensitization. This requires suitable dyes with
∗
Corresponding authors.
E-mail address: alicegao@gmail.com (F. Gao).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.07.064

F. Gao et al. / Spectrochimica Acta Part A 70 (2008) 1006–1012
1007
of course, is easy to be used. However, two-photon absorp-
tion cross-section of UV photoinitiators generally is small and
their absorption wavelength is short, consequently, their appli-
cation future in two-photon polymerization becomes gloomy
[15].
oil) was purchased from Chongqing Yiyao Corp. 4-Methyl-
benzophenone (Scheme 1, compound 1) was a gift from
Zhengjiang Zhengdan Chemical Corp. and recrystallization
from benzene/cyclohexane before use. N-Bromosuccinimide
(NBS) was obtained from Chongqing Yiyao Corp. and used
as received. 18-Crown-6 was obtained from Zhejiang Xianju
Chemical Corp. The organic solvents were purchased from
Chongqing Oritental Chemical Corp. and dried using stan-
dard laboratory techniques according to methods published
[18]. The compounds 2–6 (Scheme 1) were synthesized in our
laboratory.
The UV–vis absorption spectra were recorded with a Cin-
tra spectrophotometer. The emission spectra were checked with
Shimadzu RF-531PC spectrofluorophotometer. Rodamin 6G in
ethanol (φ = 0.94, 1 × 10−5 mol/L [19]) was used as reference
to determine the fluorescence quantum yields of the compounds
herein. The melting point was determined on a Sichuan Univer-
sity 2X-1 melting point apparatus and was uncorrected. Nuclear
magnetic resonance (NMR) was done with a Bruker 500 MHz
apparatus with tetramethylsilane (TMS) as internal standard.
Element analysis was detected by CE440 elemental analysis
meter from Exeter Analytical Inc.
In a word, controlling the two-photon polymerization at the
molecular level has not been realized. In 2002, Marder and
co-workers have covalently attached sulfonium moieties to the
two-photon dyes and achieved high-efficient significant two-
photon polymerization [16]. It is well known that compounds
with sulfonium moieties are typical cationic photoinitiator [17],
therefore, the “Marder Molecule” has a “twin” nature, which
has an absorption cross-section and is a typical cationic pho-
toinitiator at the same time. The results indicate that it is
feasible to increase two-photon polymerization efficiency and
achieve the goal of the molecular-based controlling two-photon
polymerization through the molecular design and construc-
tion. This paper is a part of our endeavour of realization the
above target, of which particular interest concentrates on the
design and synthesis of “A–B” type compounds including chro-
mophore and common UV photoinitiator, and on the extensive
study of their absorption and emission spectroscopy in vari-
ous solvents. The two-photon chemistry and physics, and more
“A–B”, “A–2B” and “A–nB” type compounds will be reported
soon.
2.1. Synthesis
2.1.1. Compound 2: 4-bromomethyl benzophenone
The compound was synthesized according to a well-known
method with modified procedure [20].
2. Experimental
p-Nitro-phenylacetic acid was purchased from Jiang-
shu Jinguan Chemical Corp. and further purification by
recrystallization from ethanol/methylene chloride before use.
N-Methyl-N-hydroxyethylamino benzaldehyde was a gift from
Zhejiang Le’an Chemical Corp. and further purification
by recrystallization from benzene/hexane before use. p-
Hydroxylbenzaldehyde was purchased from Chongqing Yiyao
Corp. and used as received. NaH (60% dispersion in mineral
2.1.2. Compound 3: p-hydroxyl-pꢀ-nitro-stilbene
The compound was synthesized according to a similar
method with modified procedure [21]. Two p-nitro-phenylacetic
acid (11 mmol) and 1.0 g p-hydroxy-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.
Scheme 1. The structure of compounds studied in this paper.

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F. Gao et al. / Spectrochimica Acta Part A 70 (2008) 1006–1012
The product 1.5 g (yield: 76%) was obtained as a brown
solid after twice recrystallization from ethanol. H NMR: δ,
6.788–6.818 (d, 2H, 3, 15), 7.125–7.157 (d, 1H, 6), 7.373–7.390
(d, 1H, 13), 7.502–7.515 (d, 2H, 4, 14), 8.125–8.179 (d, 4H,
7, 8, 10, 11), 9.236 (s, 1H, 1), melting point (196.5–
200.5 ◦C).
ing point (136–137 ◦C), Anal. Calcd for C28H21NO4: C, 77.23;
H, 4.86; N, 3.22; O, 14.70. Found: C, 77.13; H, 4.86; N,
3.23.
1
2.1.4. Compound 5:
p-(N-methyl-N-hydroxyethyl)-pꢀ-nitro-stilbene
2.68 g (14.8 mmol) p-nitro-phenylacetic acid and
2.0 g (11.8 mmol) p-(N-methyl-N-hydroxyethyl)-pꢀ-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%) compound 5 was obtained as a
brown solid after twice crystallization from ethanol. 1H NMR:
δ, 2.098 (s, 1H, 7), 3.049 (s, 1H, 6), 3.520–3.570 (m, 1H, 8),
3.803–3.823 (t, 1H, 9), 6.935–6.968 (d, 2H, 5, 10), 7.066–7.172
(q, 2H, 3, 12), 7.563–7.581 (d, 2H, 4, 11), 8.094–8.165 (d, 4H,
1, 2, 13, d14). Melting point (143–146 ◦C).
2.1.3. Compound 4:
(p-nitro-stilbene)-(methylenebenzophenone) ether
The compound was synthesized according to Scheme 2.
Compounds 2 (1.37 g, 5 mmol) and 3 (1 g, 4 mmol) were
dissolved in 0.05 g 18-C-6/1.45 g K2CO3/50 ml dry acetone
solution, the mixture was stirred at room temperature under
argon for 24 h. The solid was got rid of solution by filtration and
acetone was removed fully by evaporation. The resultant mixture
was dissolved in CHCl3 and washed by water for 3 times. The
organic layer was dried with anhydrous sodium sulfate and then
concentrated. The product was purified by column chromatog-
raphy. Further purification was done by twice recrystallization
from benzene to yield bright yellow 0.6 g (1.4 mmol, 30% yield)
compound 4. 1H NMR: δ, 5.122 (s, 1H, 8), 6.934–6.966 (t, 2H,
6, 22), 6.966–7.139 (q, 2H, 3, 24), 7.405–7.448 (m, 2H, 10,
21), 7.482–7.544 (m, 3H, 16, 17, 18), 7.729–7.782 (m, 6H,
5, 11, 15, 19, 20, 23), 8.125–8.143 (d, 4H, 1, 2, 26, 27); 13C
NMR, δ, 70.172, 115.981, 124.863, 127.254, 127.668, 129.043,
129.194, 129.928, 130.233, 130.780, 131.217, 138.006,
138.234, 142.028, 144.855, 147.196, 159.763, 197.267. Melt-
2.1.5. Compound 6: [p-(N-methyl-N-hydroxyethyl)-pꢀ-
nitro-stilbene]-(methylenebezophone)
ether
The compound was synthesized according to Scheme 3.
0.24 g (60%, 10 mmol) NaH was washed by dry cyclohex-
ane twice to remove mineral oil. 2.77 g (10 mmol) compounds
2 and 5 (1.5 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 fil-
tration and THF was removed fully by evaporation. The
resulting mixture was dissolved in CHCl3 and washed by
water for 3 times. The organic layer was dried with anhy-
drous 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 compound 6 (yield 32.3%).
1H NMR: δ, (3.030, s, 1H, 10), 3.579–3.602 (t, 1H, 25),
3.369–3.700 (t, 1H, 26), 4.521 (s, 1H, 11), 6.861–6.893 (d,
2H, 8, 27), 7.107–7.210 (q, 2H, 5, 29), 7.287–7.316 (t, 2H,
12, 23), 7.376–7.410 (m, 3H, 18, 19, 20), 7.480–7.517 (m,
4H, 13, 17, 21, 22), 7.694–7.711 (t, 2H, 7, 28), 8.098–8.116
(d, 2, 3, 30, 31), 13C NMR: δ, (18.442, 58.472, 68.582,
72.738, 124.160, 126.142, 126.978, 128.293, 128.515, 130.013,
130.272, 132.436, 136.877, 137.606, 196.377). Melting point
Scheme 2. The synthesis route of compound 4.

F. Gao et al. / Spectrochimica Acta Part A 70 (2008) 1006–1012
1009
Fig. 1. A typical absorption of compounds 1,
5
and
6
in CH3CN
(1 × 10−5 mol/L).
3.2. The UV–vis absorption spectroscopy
As shown in Fig. 1, compound 4 or 6 shows obvious double
absorption bands, one of which locates around 225–300 nm, the
other is at long-wavelength (350–550 nm). Benzophenone moi-
ety makes major contribution to the short-wavelength absorption
band, and the long-wavelength absorption band is related to the
absorption of chromophore. The absorption of compound 4 or
6 shows an overlap effect of that of compound 1 and compound
3 or 5, respectively. While, the molar extinction coefficient of
compound 6 or 4 at long-wavelength generally is less than that
of the sole chromophore, which indicates that the introduction
Scheme 3. The synthesis route of compound 6.
(110–110.5 ◦C), Anal. Calcd for C31H28N2O4: C, 75.59; H,
5.73; N, 5.69; O, 12.29. Found: C, 75.58; H, 5.83; N,
5.65.
of benzophenone to the chromophore has an influence on charge
distribution of compound 4 or 6, and the electron transmission
process thus is affected.
3. Results and discussion
3.1. Construction of the compounds 4 and 6
As compared to compound 3 or 4, the absorption of com-
pound 5 or 6 at long-wavelength shows a red-shift (Fig. 2), which
suggests that the long-wavelength absorption is induced by
It was demonstrated that p-nitro-stilbene has significant non-
linear optical (NLO) nature [22] and benzophenone is a free
radical type photoinitiator [23–26], they were chosen as chro-
mophore and photoinitiator moieties to build the compounds for
two-photon polymerization in this paper.
Compound 2, which play a role of the introduction of ben-
zophenone moiety to the chromophore backbone, was obtained
from bromide reaction of compound 1 and NBS. Reference
compounds 3 and 5 were obtained by condensation reaction of
p-nitro-phenylacetic acid and benzaldehyde in the presence of
piperidine as catalyst at high temperature. Compounds 4 and
6 have been constructed by compound 2 and compounds 3,
5, respectively. The preparation of compound 4 was achieved
by Williason reaction of compound 2 and compound 3 in the
presence of weak base K2CO3 as catalyst, and 18-C-6 as phase
transfer reagent. Compound 6 was prepared from compound 2
and compound 5 in the presence of strong base NaH.
Fig. 2. A typical absorption of compounds 4 and 6 in CH2Cl2 (1 × 10−5 mol/L).

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F. Gao et al. / Spectrochimica Acta Part A 70 (2008) 1006–1012
intramolecular charge transfer (ICT). With a stronger electron-
donor group, compound 5 or 6 shows obvious absorption in
visible light region till 550 nm. Moreover, compared to apolar
solvents, for instance CCl4 and benzene, the absorption of the
four compounds at long-wavelength show obvious red-shift in
polar solvent, for example CH2Cl2 and CH3CN. It is further
evident that the absorption at long-wavelength is induced from
intramolecular charge transfer (ICT).
3.3. The fluorescence spectroscopy
It is interesting to observe that compounds 3 and 4 display
obvious emission in polar solvents, for example in CH2Cl2 and
CH3CN, while very weak or none emission in apolar solvents,
such as in CCl4 and benzene, at 500–700 nm while excited at
350 nm. In contrast, compounds 5 and 6 display a strong emis-
sion in apolar solvents, for example in CCl4 and benzene, while
very weak or none emission in polar solvents, such as in CH2Cl2
and CH3CN, at 500–700 nm when excited at 350 nm as well.
A typical example was shown in Fig. 3. The results suggest
that stronger intramolecular charge transfer causes the fluores-
cence quenching of compounds 5 and 6 at 500–700 nm in polar
solvents.
A fluorescence quenched and red-shift phenomenon at
500–700 nm was observed with the increasing of CH2Cl2 ratio in
benzene/CH2Cl2 mixed solvents when compound 6 was excited
at 350 nm. In contrast, with the increasing of benzene ratio in
benzene/CH2Cl2 mixed solvents, the fluorescence of compound
4 at 500–700 nm was quenched continuously and a blue-shift
phenomenon took place when it was excited at 350 nm. On
the other hand, no change of the emission band at 360–440 nm
was observed with the change of CH2Cl2 or benzene ratio in
benzene/CH2Cl2 mixed solvents. A typical example was shown
in Fig. 4. This suggests that: (1) The emission of compounds 4
and6at500–700 nmisrelatedtotheintramolecularchargetrans-
fer. As compared to compound 4, compound 6 has a stronger
intramolecular charge transfer nature, therefore, only in apolar
solvents it can have a high fluorescence yield at 500–700 nm. (2)
The emission band at 360–440 nm is produced from a similar
excited state of compounds 4 and 6, respectively.
Fig. 4. The fluorescence quenching of compound 6 with the increasing CH2Cl2
ratio to CH2Cl2/benzene mixed solvents (Ex: 350 nm, slit width—Ex: 5 nm, Em:
5 nm, C = 1 × 10−5 mol/L).
with those of compounds 3 and 4 in modest polar solvents, such
as in tetrahydrofuran or ethyl acetate, when excited at 350 nm.
This gives further solid evidence that compounds 5 and 6 have
a nature of a stronger intramolecular charge transfer because of
the better electron-donor group. As compared to compounds 3
and 5, an “unexpected” blue-shift of emission at 500–700 nm of
compounds 4 and 6 is observed. A charge redistribution induced
by the introduction of benzophenone moiety to chromophore
causes “the abnormal” emission behavior [27].
3.4. The fluorescence quantum yields
In order to further assess the effect of benzophenone moi-
ety on the emission of compounds 4 and 6, the fluorescence
quantum yields of compounds 4, 6 and reference compounds
3, 5 have been studied. As shown in Table 1, compounds 3, 4
have low fluorescence quantum yields in apolar solvents and
high fluorescence quantum yields in polar solvents. On the con-
trary, compounds 5 and 6 display strong fluorescence emission
in apolar solvents, and low fluorescence quantum yields in polar
solvents. As discussed above, “the contrary” is related to the
intramolecular charge transfer.
For compounds 3 and 4, the emission at 360–500 nm makes a
major contribution to the fluorescence quantum yields in apolar
solvents when they are excited at 350 nm. And for compounds 5
and 6, the major contribution to the fluorescence quantum yield
As shown in Fig. 5, the emission of compounds 5 and 6 dis-
play remarkable red-shift (approximately 100 nm) comparing
Fig. 5. The fluorescence spectra of compounds 3–6 in tetrahydrofuran (Ex:
350 nm, for convenient observation, the slit width of excitation and emission
was not the same, C = 1 × 10−5 mol/L).
Fig. 3. The typical fluorescence spectra of compound 4 in benzene and acetoni-
trile (Ex: 350 nm, silt width—Ex: 5 nm, Em: 5 nm, C = 1 × 10−5 mol/L).

F. Gao et al. / Spectrochimica Acta Part A 70 (2008) 1006–1012
1011
Table 1
The fluorescence quantum yields of the compounds 3–6
Solvents
Compound 3
Compound 4
Compound 5
Compound 6
Carbon tetrachloride
Benzene
Ethyl acetate
Tetrahydrofuran
Methylene chloride
Acetonitrile
6.86 × 10−4
3.86 × 10−3
0.091
0.17
0.12
2.96 × 10−3
6.82 × 10−3
0.023
0.034
0.20
0.15
0.18
0.032
0.017
0.013
6.4 × 10−4
0.12
0.36
0.054
0.056
0.019
2.6 × 10−4
0.03
0.03
also comes from the emission at 360–440 nm in polar solvents
when they are excited at 350 nm. The emission at 500–700 nm
makes a very minor contribution to the fluorescence quantum
yields of these compounds at the above situation. It is interesting
to observe that the emission at 360–440 nm of these compounds
always can be detected in solvents, despite sometimes it is very
small comparing with the emission at 500–700 nm. This further
demonstrates that the emission band at 360–440 nm is produced
from the similar excited state of all compounds. Seen from the
chemical structures, this may be related to the p-nitro-benzene
moiety.
for the Author of National Excellent Doctoral Dissertation of PR
China (200735)”and “Initiative Funding of Ministry of Person-
nal of PR China (2006164)” for part of financial support. This
paper is part financially sponsored by the Scientific Research
Foundation for the Returned Overseas Chinese Scholars, State
Education Ministry as well (Nos. 20071108-1, 20071108-5).
The authors appreciate Mr. Qingping Feng and Mr. Guangzhi
Li for their warm support on this project. Fang Gao also thanks
Prof. Dr. Richard M. Pagni for his helpful discussion on the
spectroscopy.
Moreover, the fluorescence yields of compounds 4 and 6
(compound 4: φ = 0.20 in CH2Cl2, compound 6: φ = 0.36 in ben-
zene) are higher than those of compounds 3 and 5 (compound
3: φ = 0.17 in THF, compound 5: φ = 0.18 in benzene). It is well
known that benzophenone is a typical triplet molecule with an
intersystem crossing (ISC) efficiency of 100% [23], and dis-
plays typical spectroscopy of ketone [28]. While, as compared
to compounds 3 and 5, an occurring of charge redistribution
of compounds 4 and 6 induced by the introduction of ben-
zophenone moiety causes the reduction of the molar extinction
coefficients, an “unexpected” blue-shift of fluorescence emis-
sion, and an “abnormal” increasing of fluorescence quantum
yields.
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Acknowledgements
The authors thank Natural Science Foundation of CQ CSTC
(Nos. CSTC 2007BB4175, 2007BB4179), and National Nat-
ural Science Foundation of China (NSFC, Nos. 20776165,
20702065) for financial supports. We also thank “Chongqing
University Postgraduates Science and Innovation Funds (Nos.
200701Y1A0010187, 200707A1A0140251)” and “Foundation

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