C2v symmetrical two-photon polymerization initiators with anthracene core: Synthesis, optical and initiating properties

Article abstract of DOI:10.1039/c2cp42512b

A series of C_2v symmetrical two-photon absorption compounds with anthracene core, 2,7-bis[2-(4-substituted phenyl)-vinyl]-9,10- dipentyloxyanthracenes designated as I, II and III (the substituted groups at the 4-position of phenyl of I, II and III were dimethylamino, methyl and cyano, respectively) were designed and synthesized as initiators in two-photon induced polymerization (TPIP). The anthracene ring was modified by linking vinylphenyl groups to the 2,7-position to extend conjugation system length and two pentyloxy groups to the 9,10-position to serve as electronic donors. Two-photon absorption cross section of I was around 300 GM, which was much larger than the 10 GM of II and 29 GM of III at 800 nm. I of 0.18% molar ratio in resin composed of methacrylic acid and dipentaerythritol hexaacrylate exhibited a dramatically low threshold of 0.64 mW compared with commercial photoinitiator benzil at a scanning speed 10 μm s^-1. Moreover, the threshold of photoinitiator I was only increased to 2.53 mW at a scanning speed of 1000 μm s^-1. The dependency of threshold on the concentration and exposure time was in accordance with theoretical calculation. Finally, a reasonable mechanism of the two-photon initiating process was proposed. This study provides good prospects for developing low threshold photoinitiator in TPIP.

Full text of DOI:10.1039/c2cp42512b

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PAPER
C2v symmetrical two-photon polymerization initiators with anthracene
core: synthesis, optical and initiating properties
Jin-Feng Xing,wa Mei-Ling Zheng,*a Wei-Qiang Chen,a Xian-Zi Dong,a
Nobuyuki Takeyasu,bc Takuo Tanaka,b Zhen-Sheng Zhao,a Xuan-Ming Duan*a
and Satoshi Kawatabcd
Received 23rd July 2012, Accepted 24th September 2012
DOI: 10.1039/c2cp42512b
A series of C2v symmetrical two-photon absorption compounds with anthracene core,
2,7-bis[2-(4-substituted phenyl)-vinyl]-9,10-dipentyloxyanthracenes designated as I, II and III
(the substituted groups at the 4-position of phenyl of I, II and III were dimethylamino, methyl
and cyano, respectively) were designed and synthesized as initiators in two-photon induced
polymerization (TPIP). The anthracene ring was modified by linking vinylphenyl groups to the
2,7-position to extend conjugation system length and two pentyloxy groups to the 9,10-position to
serve as electronic donors. Two-photon absorption cross section of I was around 300 GM, which
was much larger than the 10 GM of II and 29 GM of III at 800 nm. I of 0.18% molar ratio in
resin composed of methacrylic acid and dipentaerythritol hexaacrylate exhibited a dramatically
low threshold of 0.64 mW compared with commercial photoinitiator benzil at a scanning speed
10 mm sꢀ1. Moreover, the threshold of photoinitiator I was only increased to 2.53 mW at a
scanning speed of 1000 mm sꢀ1. The dependency of threshold on the concentration and
exposure time was in accordance with theoretical calculation. Finally, a reasonable
mechanism of the two-photon initiating process was proposed. This study provides
good prospects for developing low threshold photoinitiator in TPIP.
with low two-photon sensitivity due to their small two-photon
1. Introduction
absorption (TPA) cross sections (dTPA) at a few tens of GM
(10ꢀ50 cm4 s photonꢀ1) level.3 In general, the commercial resins
Two-photon induced polymerization (TPIP) has been one
of the most attractive methods for three-dimensional (3D)
cannot fulfil the increasing requirement for TPIP rapid
lithography due to its intrinsical 3D fabrication capability and
microfabrication. Therefore, a highly sensitive TPIP initiator
the nanometric resolution.1 In the past decade, much progress
is required to achieve rapid TPIP microfabrication.
in the TPIP lithography technique has been made in the
designed as highly sensitive TPIP initiators.4 Compounds with
resolution improvement and fabrication speed. Various kinds
Varieties of novel compounds with large dTPA have been
of complicated and functional 3D structures have been fabricated
for potential applications.2 In this fabrication technique, the
both large dTPA and high initiating efficiency are required to achieve
highly sensitive TPIP initiators. So far, the structure–property
most commonly used resins are the commercial negative-tone
relations governing TPA have been studied extensively and many
photoresist containing the conventional ultraviolet initiators
molecular design strategies were proposed to provide guidelines for
the development of materials with large dTPA. It has been confirmed
a Laboratory of Organic NanoPhotonics and Key Laboratory of
Functional Crystals and Laser Technology, Technical Institute of
that dTPA increased with the donor/acceptor strength, conjugation
length, molecular dimensionality and planarity of the p-center.5 It
Physics and Chemistry, Chinese Academy of Sciences, No. 29,
is known that high initiating efficiency is also key for highly
Zhongguancun East Road, Beijing 100190, P. R. China.
sensitive TPIP initiators besides large dTPA. Naturally, optimizing
E-mail: zhengmeiling@mail.ipc.ac.cn, xmduan@mail.ipc.ac.cn;
Fax: +86-10-82543597; Tel: +86-10-82543596
conventional initiator systems to improve their dTPA is a facile and
b Nanophotonics Laboratory RIKEN (The Institute of Physical and
Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
efficient method to achieve highly sensitive TPIP initiators.6
c Photonics Center, Osaka University, 2-1 Yamadaoka,
Suita Osaka 565-0871, Japan
The derivatives of anthracene have been widely used as UV
photosensitizers or photoinitiators.7 Recently, the utility of the
anthracene as a special and efficient p-center for the two-photon
chromophores has been demonstrated by several research
groups.8 The good co-planarity of anthracene could contribute
to the improvement of dTPA.9 Basically, the anthracene unit
d Department of Applied Physics, Osaka University, 2-1 Yamadaoka,
Suita Osaka 565-0871, Japan
w Present address: Department of Polymer Science and Engineering,
School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072, China.
c
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could be linked to 9,10-, 2,6- and 2,7-linkages to form three
different conjugation pathways with different quinoid characters.
Among those three types of anthracene derivatives, 9,10- and
2,6-linkages were widely synthesized and studied due to their high
fluorescence quantum yield and large dTPA. Those two types of
anthracene derivatives have potential applications in the
investigation of two-photon excited fluorescence (TPEF)
and photo-induced electron transfer. However, the study on
2,7-linkage type anthracene derivatives with C2v symmetrical
conjugated system has been ignored. So far, only one 2,7-
linkage type anthracene derivative, 2,7-bis(4-dihexylamino-
styryl)-9,10-dicyanoanthracene, has been synthesized and
investigated. It possessed much lower fluorescence quantum
yield (Ffl) compared with its linear analog, 2,6-bis(4-dihexyl-
aminostyryl)-9,10-dicyanoanthracene.9 Considering the competi-
tion in the relaxation processes of the excited state, it is clear that
a lower Ffl is favorable to achieve higher initiating efficiency
for photopolymerization, since the proportion of relaxation
through non-irradiation processes would increase in this case.
Our previous work also demonstrated that chromophores with
C2v symmetrical conjugated systems possessed dramatically
low Ffl and presented good TPIP initiating properties.10
In this study, we focused on designing and synthesizing a
series of 2,7-bis[2-(4-substituted phenyl)-vinyl]-9,10-dipentyl-
oxyanthracenes, I, II and III (the substituted groups at
4-position of phenyl for I, II and III are dimethylamino,
methyl and cyano groups, respectively) with C2v symmetry
structure through the Wittig reaction. We modified the anthracene
ring by linking vinylphenyl groups to the 2,7-position instead of
the 2,6-position by reducing the symmetry of TPA molecules to
avoid the high Ffl. The modification of the 2,7-position in the
anthracene ring with vinylphenyl groups resulted in a large
conjugation system with C2v symmetry. The substituents at the
4-position of benzene of the vinylphenyl moieties worked as
electronic donor or acceptor according to their electron-
donating capability. Two pentyloxy groups at the 9,10-position
of the anthracene ring served as electronic donors and played an
important role in the solubility improvement in resin. These
anthracene derivatives with a large conjugation system would
improve the p-conjugated intramolecular charge transfer
(ICT), and their C2v symmetric structures would reduce the
symmetry of these molecules and lead to a lower Ffl. This
concept of molecular design could contribute to a large dTPA
and high TPIP initiating activities. The one-photon absorption
and two-photon absorption properties as well as TPIP
initiating properties of these compounds have been investigated,
and the TPA initiating mechanism of 9,10-dialkoxyanthracene
has been proposed.
sulfate (MgSO4), petroleum ether, ethyl acetate, triphenyl
phosphine (PPh3), sodium hydride (NaH), methacrylic acid
(MA) and benzil were of reagent grade and purchased from
Beijing Chemical Reagent Company. Dichloromethane (CH2Cl2,
spectrophotometric grade) was also obtained from Beijing
Chemical Reagent Company. 4-Dimethylamino-benzaldehyde,
4-methylbenzaldehyde, 4-cyanobenzaldehyde and 2-methyl
phthalic anhydride were obtained from J&K Chemicals
Company. Dipentaerythritol hexaacrylate (DPE-6A, trade
name: light acrylate DPE-6A) was purchased from Kyoeisha
Chemical Co., Ltd., Japan. Silica gel (200–300 mesh, Beijing
Chemical Reagent Company) was used in column chromatography
1
as the stationary phase. The H NMR spectra were recorded
on a Varian-300 or Bruker AMX-400. Infrared spectra were
recorded on a FTIR-410 spectrophotometer (JASCO Corp.)
in the range 4000–400 cmꢀ1 with a resolution of 1.0 cmꢀ1
.
High resolution mass spectra (HRMS) were recorded on a
JEOL JMS-700 instrument.
2.2 Measurements
The UV-vis absorption spectra were recorded on UV-2550
Shimadzu UV-vis spectrophotometer at a concentration of
1 ꢁ 10ꢀ5 M in CH2Cl2 and fluorescence spectra were recorded
on a Hitachi F-4500 fluorescence spectrophotometer at a
concentration of 1 ꢁ 10ꢀ6 M in CH2Cl2. The fluorescence
quantum yields were measured using coumarin 307 in acetonitrile
as reference standard (Ffl = 0.58).11 The one-photon excited
fluorescence (OPEF) lifetimes were measured using a streak
scope camera (Hamamatsu, C4334) with the excitation of
femtosecond laser pulse at 400 nm. The time resolution of the
system was limited by that of the streak camera of about 10 ps.
TPEF spectra were recorded on SD2000 spectrometer (Ocean
Optics) at a concentration of 1 ꢁ 10ꢀ4 M in CH2Cl2 with
excitation by using a femtosecond laser (Tsunami, Spectra-
Physics) with a pulse width of 80 fs and repetition rate of
80 MHz. dTPA was determined by the two-photon induced
fluorescence method.12
2.3 Synthesis and characterization of compounds
2,7-Dibromomethyl-anthraquinone (B). 2,7-Dimethyl-anthra-
quinone (5.834 g, 24.6 mmol) and NBS (11.413 g, 64.1 mmol)
were added to a three-necked flask containing CCl4 (400.0 mL).
After the suspension was heated to 70 1C, benzoyl peroxide
(0.760 g, 3.30 mmol) was added. Then, the suspension was
heated at reflux for 24 h. When the mixture was cooled down to
room temperature, the precipitate was filtered, washed with
water (200.0 mL ꢁ 3), dried under vacuum, and recrystallized
from toluene to give B (5.838 g, 60%) as a light yellow solid.
Mp 271–273 1C; dH (300 MHz, CDCl3): 8.33 (s, 2H), 8.30
(d, J = 8.1 Hz, 2H), 7.83 (d, J = 8.1 Hz, 2H), 4.60 (s, 4H).
2. Experimental section
2,7-Diacetoxymethyl-9,10-anthraquinone (C). 2,7-Dibromo-
methyl-anthraquinone (4.030 g, 10.2 mmol) was added into a
solution of anhydrous sodium acetate (11.893 g, 145 mmol) in
acetic acid (60.0 mL) and acetic anhydride (60.0 mL). The
mixture was heated at reflux for 14 h. Then, the solvent was
removed by distillation. The residue was cooled down and
poured into water. The precipitate was collected and washed
with water until the acid was free. The crude product was
2.1 Materials and characterization
N-bromosuccinimide (NBS), benzoyl peroxide (BPO), toluene,
anhydrous sodium acetate, acetic acid, acetic anhydride,
methanol, sodium hydroxide (NaOH), tetrabutyl ammonium
bromide (TBAB), tetrahydrofuran (THF), thionyl chloride,
sodium dithionite (Na2S2O4), 1-bromo-pentane, chloroform
(CHCl3), chloroform-D (CDCl3), anhydrous magnesium
c
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purified by recrystallization from toluene to give C (2.000 g,
58%) as a white solid. Mp 145–146 1C; dH (300 MHz, CDCl3)
8.21 (d, J = 7.8 Hz, 2H), 8.17 (s, 2H), 7.90 (d, J = 7.8 Hz,
2H), 5.28 (s, 4H), 2.15 (s, 6H).
2,7-Bis[2-(4-dimethylamino-phenyl)vinyl]-9,10-dipentyloxy-anthra-
cene (I). 2,7-Bis(triphenylphosphonium)-9,10-dipentyloxy-anthra-
cene (0.308 g, 0.31 mmol) and 4-dimethylamino-benzaldehyde
(0.093 g, 0.06 mmol) were added to a two-necked flask containing
absolute THF (40.0 mL) under N2, then NaH (0.140 g, 5.83 mmol)
was added into the mixture. The reaction mixture was allowed to
reflux for 15 h. Water (20.0 mL) was added slowly after the
solution was cooled to room temperature. The mixture was
extracted with chloroform (20.0 mL ꢁ 3). The combined extract
was washed with water (10.0 mL ꢁ 2) and dried over anhydrous
MgSO4. After filtration, the filtrate was concentrated and purified
by silica gel chromatography (eluent: petroleum ether and ethyl
acetate) to give I (0.080 g, 40%) as a yellow solid. dH (400 MHz,
CDCl3) 8.14 (m, 4H), 7.67 (m, 2H), 7.44 (d, J = 10.6 Hz, 4H), 7.12
(m, 8H), 6.69 (d, J = 10.6 Hz, 4H), 4.11 (m, 4H), 3.41 (s, 4H), 2.94
(m, 12H), 2.10 (m, 4H), 1.65 (m, 4H), 1.46–1.19 (m, 8H), 1.01–0.81
(m, 6H); HRMS: calcd for C44H52N2O2: 640.4029, found:
640.3991 (ꢀ5.9 ppm).
2,7-Dihydroxymethyl-9,10-anthraquinone (D). 2,7-Diacetoxy-
methyl-9,10-anthraquinone (0.102 g, 0.28 mmol) was added to
methanol (15.0 mL), then the solution of NaOH (0.050 g,
1.25 mmol) in water (5.0 mL) was added. The mixture was
heated at reflux for 17 h. After the mixture was cooled down to
room temperature, it was poured into water. The precipitate was
collected and dried in vacuum. The crude product was dissolved
into chloroform and purified by silica gel chromatography (eluent:
petroleum ether and ethyl acetate) to give D (0.040 g, 53%) as a
white solid. Mp 203–205 1C; dH (300 MHz, CDCl3) 8.17 (s, 4H),
7.85 (d, J = 8.0 Hz, 2H), 5.59 (s, 2H), 4.70 (s, 4H).
2,7-Dihydroxymethyl-9,10-dipentyloxy-anthracene (E). 2,7-Dihy-
droxymethyl-9,10-anthraquinone (0.206 g, 0.76 mmol) and TBAB
(0.241 g, 0.74 mmol) were dissolved in THF (15.0 mL) and
water (15.0 mL), and stirred at room temperature, followed by
the addition of a solution of sodium dithionite (0.529 g,
3.04 mmol) in water (10.0 mL). After 10 min, a solution of
NaOH (0.744 g, 18.6 mmol) in water (3.0 mL) was added. The
reaction mixture immediately turned into dark red. 1-Bromo-
pentane (2.0 mL) was added and the reaction mixture was
heated at reflux for 30 h. After the mixture was cooled down to
room temperature, the mixture was extracted with chloroform
(15.0 mL ꢁ 3). The extract was washed with water and dried
over anhydrous MgSO4. After filtration, the filtrate was
concentrated. The crude product was purified by silica gel
chromatography (eluent: petroleum ether and ethyl acetate) to
give E (0.140 g, 47%) as a light yellow solid. Mp 131–133 1C;
dH (300 MHz, CDCl3) 8.35–8.19 (m, 4H), 7.54 (t, 2H), 4.92
(s, 4H), 4.15 (t, 2H), 3.53 (t, 2H), 2.06 (s, 2H), 1.81 (m, 4H),
1.53 (m, 8H), 1.03 (m, 6H).
2,7-Bis-[2-(p-tolyl)-vinyl]-9,10-dipentyloxy-anthracene (II).
Synthesized by the same procedure as for I; yellow solid, yield
65%. dH (400 MHz, CDCl3) 8.21 (m, 4H), 7.78 (m, 2H), 7.90
(d, J = 8.1 Hz, 4H); 7.22 (m, 4H), 6.95 (d, J = 8.1 Hz, 4H),
4.18 (m, 4H), 3.85 (s, 6H), 3.50 (m, 4H), 2.10 (m, 4H), 1.80
(m, 4H), 1.7 (m, 4H), 1.56 (m, 4H), 1.51 (m, 4H), 1.30 (m, 4H),
1.03 (m, 6H), 0.97 (m, 6H); HRMS: calcd for C42H46O2:
582.3498, found: 582.3477 (ꢀ3.6 ppm).
2,7-Bis[2-(4-cyanophenyl)vinyl]-9,10-dipentyloxy-anthracene
(III). Synthesized by the same procedure as for I; orange solid,
yield 42%. dH (400 MHz, CDCl3) 8.41 (m, 2H), 8.32 (m, 2H),
7.91 (m, 2H), 7.86–7.35 (m, 12H), 4.21 (m, 4H), 3.01 (s, 4H),
2.14 (m, 4H), 1.80 (m, 4H), 1.71–1.20 (m, 8H), 1.05 (m, 6H),
0.80 (m, 6H); HRMS: calcd for C42H40N2O2: 604.3090, found:
604.3087 (ꢀ0.5 ppm).
2.4 Two-photon induced polymerization
2,7-Dichloromethyl-9,10-dipentyloxy-anthracene (F). 2,7-Dihy-
droxymethyl-9,10-dipentyloxy-anthracene (0.908 g, 2.21 mmol)
was dissolved in dry CHCl3 (35.0 mL). The solution was cooled
to below 5 1C. A solution of thionyl chloride (14.400 g, 121 mmol)
in CHCl3 (10.0 mL) was added. The reaction mixture was
stirred at room temperature for 3 h. The mixture was washed
with water, and dried over anhydrous MgSO4. The filtrate was
concentrated and the residue was purified by silica gel
chromatography (eluent: petroleum ether and ethyl acetate)
to give F (0.520 g, 72%) as a light yellow solid. Mp 75–77 1C;
dH (300 MHz, CDCl3) 8.27 (d, J = 8.8 Hz, 2H), 8.23 (s, 2H),
7.50 (d, J = 8.8 Hz, 2H), 4.82 (s, 4H), 4.15 (m, 4H), 2.07
(m, 4H), 1.62 (m, 4H), 1.47 (m, 4H), 1.00 (m, 6H).
TPIP experiments were performed by resins exposed in the
pattern of a target structure with a tightly focused (60ꢁ /1.42
NA oil immersion objectives) laser pulse from a Ti : sapphire
laser with a wavelength of 800 nm, pulse width of 80 fs and
repetition rate of 80 MHz. The resins were prepared by mixing
the monomer and photoinitiator. For example, MA (102.0 mg,
66.47 wt% in resin), multifunctional acrylic compound DPE-
6A (50.0 mg, 32.59 wt% in resin) as crosslinker, I (1.45 mg,
0.94 wt% or 0.18 mol% in resin) and THF (0.1 mL) were mixed
to give resin 1 (R1). Resins 2–8 (R2–R8) were prepared in the
same way. The exposure time was controlled by a shutter. Final
structures were obtained after development using ethanol. The
images of fabricated lines were observed using a field-emission
scanning electron microscope (FE-SEM, JOEL, JSM-6330F). The
working voltage and current were 5 kV and 8 mA, respectively.
2,7-Bis(triphenylphosphonium)-9,10-dipentyloxy-anthracene
dichloride (G). 2,7-Dichloromethyl-9,10-dipentyloxy-anthracene
(0.198 g, 0.44 mmol) and PPh3 (1.08 g, 4.12 mmol) were dissolved
in toluene (40.0 mL). The reaction mixture was heated at reflux
for 24 h. Then, the precipitate was filtered, washed with toluene
(2.0 mL ꢁ 3) and dried in vacuum to give G (0.208 g, 72%) as a
yellow solid. Mp 4300 1C; dH (300 MHz, CDCl3) 7.93–7.61
(m, 36H), 3.95 (t, J = 6.7 Hz, 4H), 3.40 (m, 4H), 1.90 (m, 4H),
1.42 (m, 4H), 0.99 (m, 6H).
3. Results and discussion
3.1 Synthesis of designated anthracene derivatives
2,7-Dimethyl-anthraquinone (A) was prepared from 2-methyl
phthalic anhydride according to reference 13 in two steps.
Starting from 2,7-dibromomethyl-anthraquinone (B), compound D
c
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Table 1 The photophysical properties of I–III in CH2Cl2 at 25 1Ca
(1)
lmax
(OPF)
lmax
(TPF)
lmax
(2)
lmax
ST Ffl t(ns)
dmax
I
396
532
484
510
136 0.17 3.46 ꢂ 0.16 538
137 0.36 2.79 ꢂ 0.11 490
155 0.22 3.24 ꢂ 0.05 524
750
700
700
466
78
121
II 347
III 355
a
(1)
(OPF)
lmax is the one-photon absorption maximum. lmax
is the
wavelength of one-photon fluorescence peak. Ffl is fluorescence
quantum yield. ST is the Stokes shift; ST = (lmax(OPF)) – (lmax(1)). t is
fluorescence lifetime excited at a wavelength of 400 nm. lmax(TPF) is the
(2)
wavelength of two-photon fluorescence peak. lmax is the wavelength
of TPA maximum. dmax is the maximum value of TPA cross section.
1 GM = 10ꢀ50 cm4 s photon–1
.
summarized in Table 1. The absorption maximum appeared at
396 nm for I, 347 nm for II and 355 nm for III, respectively. The
absorption band showed a red-shift of around 40 nm when a
strong electron-donating (I) end group was used, indicating an
enlargement of the ICT extent.14 According to reference 9, it is
known that two-photon allowed states for quadrupoles are at a
higher energy than the Frank–Condon states. For the end groups
with weak electron-donating (II) or electron-withdrawing (III)
strength, although the absorption maximum is shorter than
360 nm, there are several peaks in the absorption spectra. This
indicates an increased number of one-photon-allowed states,
demonstrating that the structures of II and III are probably
distorted and the ICT is significantly inhibited, which would in
turn increase the probability of the localized absorption.15
The fluorescence emission spectra (Fig. 1b) of II showed
two emission peaks with a maximum at 484 nm and another
at 458 nm, indicating the existence of more than one emitting
state.15 There was a large Stokes shift for these three com-
pounds (Table 1). Compared to II, the emission bands of I and
III was red-shifted 48 nm and 26 nm, respectively. This result
indicated that molecules typically underwent a large change in
dipole moment upon electronic excitation.16 None of the
fluorescence spectra of I and III have multiple peaks, which
indicates that the emission occurs from the lowest excited state.9
Fluorescence is a process competing with other deactivation
pathways like intersystem crossing from singlet excited state
(S1) to the lowest triplet state (T1). It was extensively
demonstrated that the main route of most photoreaction
occurs via the T1 level which is related to the efficiency of
photoinitiator as it leads to bond breaking and the formation
of radicals.17 Thus, it can be assumed that high Ffl is indicative
of low efficiency in radical formation. The Ffl of II was more
than twice as high as that of I (Table 1), indicating a lower
efficiency of II in radical formation. The shorter fluorescence
lifetime of II compared with I also implies that the C2v
symmetric chromophores with strong donors are advantageous
to improve the efficiency of radical formation. The lower Ffl and
longer fluorescence lifetime for I and III were attributed to
dominant nonradiative decay mechanisms.18 The irradiative
decay constants (kf, kf = Ffl/t) of I, II and III are 4.91 ꢁ
107 sꢀ1, 1.29 ꢁ 108 sꢀ1 and 6.79 ꢁ 107 sꢀ1, respectively. The
nonradiative decay constant (knr) can be calculated from kf
and Ffl according to the equation:19
Scheme 1 Synthetic route for I–III. Reagents and conditions:
(i) NBS/BPO/CCl4, 60%; (ii) anhydrous sodium acetate/acetic acid/acetic
anhydride, 58%; (iii) NaOH/methanol/H2O, 53%; (iv) Na2S2O4/NaOH/
TBAB/CH3(CH2)4Br/H2O/THF, 47%; (v) thionyl chloride/CHCl3, 72%;
(vi) PPh3/toluene, 72%; (vii) p-R-C6H4CHO/NaH/THF, 40–65%.
was prepared after esterification and deprotection. Then, the
quinone moiety of D was reduced and protected by a pentyloxy
group. E reacted with thionyl chloride followed by the treat-
ment of triphenylphosphine to give the salt G. Further Wittig
reaction gave compounds I–III (Scheme 1).
3.2 Linear optical properties
The normalized one-photon absorption and fluorescence
spectra in CH2Cl2 of I–III were shown in Fig. 1a and Fig. 1b,
respectively. The results of the photophysical measurements were
Fig. 1 (a) Normalized UV-vis spectra of I (solid), II (dot) and III
(dash dot) in CH2Cl2; (b) Normalized fluorescence spectra of I (solid),
II (dot) and III (dash dot) in CH2Cl2 obtained at the excitation
wavelength of 400 nm.
ꢀ
ꢁ
1
knr
¼
ꢀ 1 kf
Ffl
c
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The knr of I, II and III are 2.40 ꢁ 108 sꢀ1, 2.29 ꢁ 108 sꢀ1 and
2.41 ꢁ 108 sꢀ1, respectively. Therefore, the nonradiative decay
of I and III are faster than that of II.
3.4 Two-photon induced polymerization
The threshold energy of TPIP is one of the most effective
parameters to evaluate initiating efficiency of a photoinitiator.
Usually, the threshold energy of TPIP is defined as the lowest
average laser power which can produce the solid polymer lines
from a photoresist resin.21 Considering the fact that photo-
resist resins usually contain monomers and crosslinkers, the
polymer after photopolymerization is supposed to be a single
huge polymer molecule including a network. The threshold
energy of TPIP means the lowest laser power which can
generate enough radicals for constructing a polymer network,
and still has the resistance to not develop. The capability for
forming polymer network is influenced by many factors, such
as the components of monomers, crosslinkers, photoinitiators,
and laser exposure parameters.
3.3 TPA property
The two-photon fluorescence spectra of I–III were obtained at
the excitation wavelength of 800 nm and the emission maxima
appeared at wavelengths ranging from 491 to 538 nm (Fig. 2a).
The maxima in the two-photon fluorescence spectra of I–III
showed slight red-shift compared to their one-photon fluores-
cence spectra. It is well known that two-photon excitation
process induces different excited states for molecules compared
with single photon excitation, and the different excited states by
one- and two-photon absorption leads to the differences in
relaxing and emitting processes.20
Here, we focus our investigation on the evaluation of the
initiating efficiencies of photoinitiators based on the discussion
of TPIP mechanism. The polymerization threshold (Pth) is
defined as the average power before the objective lens, below
which the polymer line cannot be fabricated using a linear
scanning speed of 10 mm sꢀ1. Therefore, the TPIP threshold of
exposure intensity (Eth) can be calculated as the lowest laser
intensity. In order to evaluate the TPIP initiating efficiency of
I–III in the aspects of threshold intensity, we have designed
three experimental samples including (1) resins designated as
R1, R2, R3 and R4 containing the same molar ratio of I–III and
To demonstrate the applicability of the molecules designed
for TPIP, we have evaluated the dTPA of compounds I–III in
the range of 700–880 nm. The two-photon absorption spectra
of I–III were shown in Fig. 2b. The values of dmax in the range
of 700–880 nm were listed in Table 1. The largest value of dTPA
obtained was 466 GM for I at 750 nm, which was much larger
than those of II and III. The enhancement of dTPA for I
compared with those of II and III was mainly attributed to the
contribution from the strong electronic donating capability of
dimethylamino groups at the 4-position of benzene rings, since
the strong electronic donors led to large ICT. The
value of dTPA for I at 800 nm, the mostly used wavelength
in TPIP microfabrication, remained at a high level of
around 300 GM, which was much larger than those of
the commercial photoinitiators.3 It indicates that the
resin using I as photoinitiator may exhibit higher two-
photon photosensitivity.
benzil at a fixed laser scanning speed of 10 mm sꢀ1
,
respectively; (2) resins designated as R1, R5, R6, R7 and R8
containing different molar ratio of I at a fixed laser scanning
speed of 10 mm sꢀ1, respectively, and (3) R1 containing 0.18%
molar ratio of I at different laser scanning speeds. The
components and polymerization threshold of R1–R4 are
summarized in Table 2. The SEM images on the TPIP
results of R1–R4 are shown in Fig. 3. The Pth for R1, R2, R3
and R4 at a linear scanning speed of 10 mm sꢀ1 were 0.64, 1.60,
8.37 and 6.37 mW, respectively. As we know, polymerization
rate can be denoted by number of monomer changing into
polymer per second. Polymer volume per second is enlarged
when the monomers change into polymer per second.
Therefore, polymer volume per second can be used to
express the polymerization rate indirectly. Polymer volume
per second of TPIP (Rp) can be calculated by the equation
Rp = p(d/2)2ns, where d is the width of fabricated line and ns is
linear scanning speed.21 As shown in Fig. 4, the results show
that the sensitivity of resins is R1 4 R2 4 R4 4 R3,
corresponding to the photoinitiating efficiency of compounds
that is I 4 II 4 benzyl 4 III. According to the reference, the
Table 2 TPIP threshold of resins
c
d
MA
(wt%)
DPE-6A
(wt%)
MRa
(%)
nsb
Pth
(mm sꢀ1) (mW)
Eth
(mJ cmꢀ2
)
R1 66.47
R2 66.53
R3 66.50
R4 66.89
32.59
32.61
32.61
32.80
0.18
0.18
0.18
0.18
10
10
10
10
0.64
1.60
8.37
6.37
2.77 ꢁ 107
6.93 ꢁ 107
3.56 ꢁ 108
2.82 ꢁ 108
a
b
c
Molar ratio of initiator in resin. Linear scanning speed. Threshold
d
power of dye in resin. Exposure intensity at threshold power.
Fig. 2 (a) Normalized two-photon fluorescence spectra of I–III in
CH2Cl2; (b) two-photon absorption spectra of I–III in CH2Cl2.
c
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Table 3 TPIP threshold resins with different concentrations
MRa
(%)
nsb
Pth
(mW)
Eth
(mJ cmꢀ2
)
c
d
MA
(wt%)
DPE-6A
(wt%)
(mm sꢀ1
)
R1
R5
R6
R7
R8
66.47
66.78
66.94
67.02
67.06
32.59
32.75
32.82
32.86
32.88
0.18
0.09
0.045
0.023
0.012
10
10
10
10
10
0.64
1.01
1.60
2.02
5.06
2.77 ꢁ 107
4.37 ꢁ 107
6.93 ꢁ 107
8.74 ꢁ 107
2.19 ꢁ 108
a
b
c
Molar ratio of initiator in resin. Linear scanning speed. Threshold
d
power of dye in resin. Exposure intensity at threshold power.
much larger than that of R4, demonstrating that resin R1 with
I
as two-photon photoinitiator exhibited much higher
sensitivity than benzil.
High initiating efficiency of I encouraged us to further
investigate its initiating property with lower loading in resin.
The components and TPIP threshold of R1 and R5–R8 are
summarized in Table 3. When the molar ratio of I was reduced
from 0.18% in resin R1 to 0.012% in resin R8, the threshold
power increased from 0.64 to 5.06 mW. Correspondingly, the
threshold laser exposure intensity increased from 2.77 ꢁ 107 to
2.19 ꢁ 108 mJ cmꢀ2. The dependence of threshold power P2th
on the concentration of I was shown in Fig. 5. There is a
relationship as shown in the equation:
Fig. 3 SEM images of R1 (a), R2 (b), R3 (c) and R4 (d) after TPIP,
respectively. The linear scanning speed was 10 mm sꢀ1
.
amount of photogenerated radicals can be expressed with the
following equation:22
Z
dTPA
½Aeꢃ ¼ F
½Aꢃ Ipulse2dt ꢁ Npulse
2
ln(Pth2) = ln N ꢀ ln[I]
2ðhnÞ
where, Pth is the threshold laser power, [I] is the molar ratio of
the photoinitiator I in resin, here N is considered to be
constant decided by dTPA, the quantum yield of radical
generation and the exposure time. According to this
equation, when the concentration of photoinitiator in the
resin decreased, the amount of photogenerated radicals at
the threshold power would decrease in resins R5–R8 and R1.
Therefore, high threshold laser power would be obtained with
low loading concentration of photoinitiator, which was in
agreement with the data as shown in Table 3.
where, F is quantum efficiency of radical generation, and [A] is
the concentration of the photoinitiator. I, hn and Npulse are the
pulse intensity, photon energy and the number of pulses,
respectively.
If the solidified polymer formed at the threshold was treated
as a molecule and its formation only depended on the amount
of photogenerated radicals, the photoinitiator initiating effi-
ciency would depend on dTPA and F. Therefore, the larger
dTPA at 800 nm and lower Ffl of I contributed to the higher
sensitivity of R1 compared with those of II and III. This result
can be attributed to the stronger electron donating ability of
dimethylamino group in I compared with methyl group in II
and cyano group in III, which would stabilize radicals
produced by two-photon excitation.23 The threshold
exposure intensity Eth of R1 was 2.77 ꢁ 107 mJ cmꢀ2, which
was ten times lower than 2.82 ꢁ 108 mJ cmꢀ2 for R4 that used
benzil as photoinitiator. The polymerization rate of R1 was
When the linear scanning speed increased from 10 to
1000 mm sꢀ1 for R1, the exposure time decreased from 40
to 0.4 ms and the threshold laser power increased from
0.64 to 2.53 mW. The components and polymerization
threshold of R1 were summarized in Table 4. The square of
the threshold power increased with the decrease of the
exposure time (Fig. 6). In this case, the Eth decreased from
Fig. 4 TPIP rates of R1, R2, R3 and R4, respectively. The linear
Fig. 5 The dependence of threshold power on the concentration of
photoinitiator I.
scanning speed was 10 mm sꢀ1
.
c
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Table 4 The dependence of threshold exposure intensity on scanning
speed
species M3 initiate the polymerization of methyl acrylate
derivatives. In our study, the stability of formed radicals is
supposed to be I 4 II 4 III since the electron donating ability
is I 4 II 4 III. The larger dTPA at 800 nm and the stronger
electron donating ability of dimethylamino group in I
compared with methyl group in II and cyano group in III
contribute to the higher sensitivity of I. Therefore, higher
sensitivity and more rapid polymerization speed were
observed when I was used as TPIP initiator.
c
d
MA DPE-6A
(wt%) (wt%)
MRa
(%)
nsb
Pth
(mW) (mJ cmꢀ2
)
Eth
(mm sꢀ1
)
R1 66.47
32.59
32.59
32.59
32.59
0.18
0.18
0.18
0.18
10
100
500
1000
0.64
1.01
1.60
2.53
2.77 ꢁ 107
R1 66.47
R1 66.47
R1 66.47
4.37 ꢁ 106
1.39 ꢁ 106
1.09 ꢁ 106
a
b
c
Molar ratio of initiator in resin. Linear scanning speed. Thresh-
d
old power of dye in resin. Exposure intensity at threshold power.
4. Conclusions
In summary, we have designed and synthesized a series of new
C2v symmetrical 2,7-bis[2-(4-substituted phenyl)-vinyl]-9,10-
dipentyloxyanthracenes (I, II and III) by Wittig reaction as
TPIP photoiniators. I possesses a relatively lower fluorescence
quantum yield, a larger two-photon absorption cross section
and higher two-photon polymerization initiating efficiency
compared with II and III in the range of 700–850 nm. In
TPIP experiments, low thresholds of 0.64 mW and 2.53 mW
have been achieved with 0.18% molar ratio loading of I in
resin at the scanning speed of 10 mm sꢀ1 and 1000 mm sꢀ1
,
respectively. When the molar ratio loading of I decreased to
0.01% in resin, the threshold increased to 5.06 mW. The
high TPIP photoinitiating sensitivity of I was attributed to
the C2v symmetrical structure, dialkoxyanthracene core and
strong electron donating ability of dimethylamino group. The
stability of formed radicals is supposed to be I 4 II 4 III since
the electron donating ability is I 4 II 4 III. Therefore, the
photoinitiator efficiency followed a simple electron-donating
effect trend (I 4 II 4 III).
Fig. 6 The dependence of threshold power on the exposure time.
2.77 ꢁ 107 mJ cmꢀ2 to 1.09 ꢁ 106 mJ cmꢀ2. The Eth value is
obviously lower than that reported previously in reference 24.
The results further indicate that I has extremely high
sensitivity as TPIP initiator.
In order to clarify initiating efficiency difference of I, II and
III, the TPIP initiating mechanism has been proposed based
on the mechanism deduced from single-photon photoinitiating
chemical studies as shown in Scheme 2.23 When excited by a
two-photon absorption process, anthracene derivatives absorb
two photons to produce activated species M1. The species M1
undergoes an electron photooxidation process to generate
cationic radical species M2 in the 9,10-position. Those with
electron donor groups could effectively increase the reduction
potential and stabilize the cationic radical M2. Then, the
cationic radical M2 undergoes a C–O cleavage to form a
radical M3 at 9,10-position. Consequently, the free radical
Acknowledgements
We are grateful for 973 (2010CB934103), ICP programs of
MOST (2010DFA01180), the National Natural Science Foun-
dation of China (Grant No. 50973126, 60907019, 61077028,
91123032), and Japan Science and Technology Agency (JST).
Notes and references
1 S. Kawata, H. B. Sun, T. Tanaka and K. Takada, Nature, 2001,
412, 697–698; D. F. Tan, Y. Li, F. J. Qi, H. Yang, Q. H. Gong,
X. Z. Dong and X. M. Duan, Appl. Phys. Lett., 2007, 90, 071106;
X. Z. Dong, Z. S. Zhao and X. M. Duan, Appl. Phys. Lett., 2008,
92, 091113; M. Pawlicki, H. A. Collins, R. G. Denning and
H. L. Anderson, Angew. Chem., Int. Ed., 2009, 48, 3244–3266;
J. E. Koskela, S. Turunen, L. Yla-Outinen, S. Narkilahti and
M. Kellomaki, Polym. Adv. Technol., 2011, 23, 992–1001;
Y. Y. Cao, N. Takeyasu, T. Tanaka, X. M. Duan and
S. Kawata, Small, 2009, 5, 1144–1148.
2 W. H. Zhou, S. M. Kuebler, K. L. Braun, T. Y. Yu,
J. K. Cammack, C. K. Ober, J. W. Perry and S. R. Marder,
Science, 2002, 296, 1106–1109; T. Watanabe, M. Akiyama,
K. Totani, S. M. Kuebler, F. Stellacci, W. Wenseleers, K. Braun,
S. R. Marder and J. W. Perry, Adv. Funct. Mater., 2002, 12,
611–614; K. Kaneko, H. B. Sun, X. M. Duan and S. Kawata,
Appl. Phys. Lett., 2003, 83, 2091–2093; C. N. LaFratta,
J. T. Fourkas, T. Baldacchini and R. A. Farrer, Angew. Chem.,
Int. Ed., 2007, 46, 6238–6258; X. Z. Dong, Z. S. Zhao and
X. M. Duan, Appl. Phys. Lett., 2007, 91, 124103; X. Z. Dong,
Q. Ya, X. Z. Sheng, Z. Y. Li, Z. S. Zhao and X. M. Duan, Appl.
Phys. Lett., 2008, 92, 231103; Z. B. Sun, X. Z. Dong, W. Q. Chen,
S. Shoji, X. M. Duan and S. Kawata, Adv. Mater., 2008, 20,
Scheme 2 The mechanism of two-photon photoinitiated polymeriza-
tion at 800 nm.
c
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Phys. Chem. Chem. Phys., 2012, 14, 15785–15792 15791

View Article Online
914–919; W. K. Wang, Z. B. Sun, M. L. Zheng, X. Z. Dong,
Z. S. Zhao and X. M. Duan, J. Phys. Chem. C, 2011, 115,
11275–11281; X. M. Duan, H. B. Sun, K. Kaneko and
S. Kawata, Thin Solid Films, 2004, 453–454, 518–521.
W. J. Yang, M. S. Seo, X. Q. Wang, S. J. Jeon and B. R. Cho,
J. Fluoresc., 2008, 18, 403–411; Y. L. Zhang, P. H. Ren,
H. C. Zhang, E. Q. Guo and W. J. Yang, J. Fluoresc., 2010, 130,
527–530.
3 K. J. Schafer, J. M. Hales, M. Balu, K. D. Belfield, E. W. Van
Stryland and D. J. Hagan, J. Photochem. Photobiol., A, 2004, 162,
497–502.
9 W. J. Yang, D. Kim, M. Y. Jeong, H. Kim, Y. Lee, X. Fang,
S. J. Jeon and B. R. Cho, Chem.–Eur. J., 2005, 11, 4191–4191.
10 J. F. Xing, W. Q. Chen, X. Z. Dong, T. Tanaka, X. Y. Fang,
X. M. Duan and S. Kawata, J. Photochem. Photobiol., A, 2007,
189, 398–404; W. E. Lu, X. Z. Dong, W. Q. Chen, Z. S. Zhao and
X. M. Duan, J. Mater. Chem., 2011, 21, 5650–5659.
4 S. M. Kuebler, K. L. Braun, W. H. Zhou, J. K. Cammack, T. Yu,
C. K. Ober, S. R. Marder and J. W. Perry, J. Photochem. Photobiol.,
A, 2003, 158, 163–170; Y. M. Lu, F. Hasegawa, S. Ohkuma, T. Goto,
S. Fukuhara, Y. Kawazu, K. Totani, T. Yamashita and T. Watanabe,
J. Mater. Chem., 2004, 14, 1391–1395; J. Gu, W. Yulan, W. Q. Chen,
X. Z. Dong, X. M. Duan and S. Kawata, New J. Chem., 2007, 31,
63–68; X. M. Wang, F. Jin, Z. G. Chen, S. Q. Liu, X. H. Wang,
X. M. Duan, X. T. Tao and M. H. Jiang, J. Phys. Chem. C, 2011, 115,
776–784; X. M. Wang, F. Jin, W. Z. Zhang, X. T. Tao, X. M. Duan
and M. H. Jiang, Dyes Pigm., 2011, 88, 57–64.
11 G. Jones, W. R. Jackson, C. Y. Chol and W. R. Bergmark, J. Phys.
Chem., 1985, 89, 294–300.
12 C. Xu and W. W. Webb, J. Opt. Soc. Am. B, 1996, 13, 481–491.
13 G. T. Morgan and E. A. Coulson, J. Chem. Soc., 1929, 2551.
14 J. Brunel, O. Mongin, A. Jutand, I. Ledoux, J. Zyss and
M. Blanchard-Desce, Chem. Mater., 2003, 15, 4139–4148.
15 S. K. Lee, W. J. Yang, J. J. Choi, C. H. Kim, S. J. Jeon and
B. R. Cho, Org. Lett., 2005, 7, 323–326.
5 E. Zojer, D. Beljonne, P. Pacher and J. L. Bredas, Chem.–Eur. J.,
´
2004, 10, 2668–2680; G. S. He, L. S. Tan, Q. Zheng and
P. N. Prasad, Chem. Rev., 2008, 108, 1245–1330; H. C. Zhang,
E. Q. Guo, Y. L. Zhang, P. H. Ren and W. J. Yang, Chem. Mater.,
2009, 21, 5125–5135; F. Todescato, I. Fortunati, S. Carlotto,
C. Ferrante, L. Grisanti, C. Sissa, A. Painelli, A. Colombo,
C. Dragonetti and D. Roberto, Phys. Chem. Chem. Phys., 2011,
13, 11099–11109; E. Collini, Phys. Chem. Chem. Phys., 2012, 14,
3725–3736.
16 K. D. Belfield, A. R. Morales, B. S. Kang, J. M. Hales,
D. J. Hagan, E. W. Van Stryland, V. M. Chapela and J. Percino,
Chem. Mater., 2004, 16, 4634–4641.
17 J. P. Malval, F. Morlet-Savary, H. Chaumeil, L. Balan,
D. L. Versace, M. Jin and A. Defoin, J. Phys. Chem. C, 2009,
113, 20812–20821.
18 C. Toro, L. De Boni, S. Yao, K. D. Belfield and F. E. Hernandez,
J. Phys. Chem. B, 2008, 112, 12185–12190.
6 K. D. Belfield, X. Ren, E. W. Van Stryland, D. J. Hagan,
V. Dubikovsky and E. J. Miesak, J. Am. Chem. Soc., 2000, 122,
1217–1218; P. J. Campagnola, D. M. Delguidice, G. A. Epling,
K. D. Hoffacker, A. R. Howell, J. D. Pitts and S. L. Goodman,
Macromolecules, 2000, 33, 1511–1513; C. Li, L. Luo, S. Wang,
W. Huang, Q. Gong, Y. Yang and S. Feng, Chem. Phys. Lett.,
2001, 340, 444–448.
7 E. W. Nelson, T. P. Carter and A. B. Scranton, Macromolecules,
1994, 27, 1013–1019; K. Tanaka, Y. Koizumi, T. Igarashi and
T. Sakurai, Macromolecules, 2006, 39, 8556–8558.
8 W. J. Yang, D. Kim, M. Y. Jeong, H. Kim, S. J. Jeon and
B. R. Cho, Chem. Commun., 2003, 2618–2619; W. J. Yang,
C. H. Kim, M. Y. Jeong, S. K. Lee, M. J. Piao, S. J. Jeon and
B. R. Cho, Chem. Mater., 2004, 16, 2783–2789; S. Kim,
Q. D. Zheng, G. S. He, D. J. Bharali, H. E. Pudavar, A. Baev
and P. N. Prasad, Adv. Funct. Mater., 2006, 16, 2317–2323;
19 C. A. Zoto and R. E. Connors, J. Mol. Struct., 2010, 982, 121–126.
20 K. D. Belfeld, K. J. Schafer, Y. Liu, J. Liu, X. Ren and E. W. Van
Stryland, J. Phys. Org. Chem., 2000, 13, 837–849; S. Ohira,
I. Rudra, K. Schmidt, S. Barlow, S. J. Chung, Q. Zhang,
´
J. Matichak, S. R. Marder and J. L. Bredas, Chem.–Eur. J.,
2008, 14, 11082–11091.
21 Y. Ren, X. Q. Yu, D. J. Zhang, D. Wang, M. L. Zhang, G. B. Xu,
X. Zhao, Y. P. Tian, Z. S. Shao and M. H. Jiang, J. Mater. Chem.,
2002, 12, 3431–3437.
22 J. F. Xing, X. Z. Dong, W. Q. Chen, X. M. Duan, N. Takeyasu,
T. Tanaka and S. Kawata, Appl. Phys. Lett., 2007, 90, 131106.
23 J. V. Crivello and W. Jang, J. Photochem. Photobiol., A, 2003, 159,
173–188.
24 J. F. Xing, W. Q. Chen, J. Gu, X. Z. Dong, N. Takeyasu,
T. Tanaka, X. M. Duan and S. Kawata, J. Mater. Chem., 2007,
17, 1433–1438.
c
15792 Phys. Chem. Chem. Phys., 2012, 14, 15785–15792
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