Design of high efficiency for two-photon polymerization initiator: Combination of radical stabilization and large two-photon cross-section achieved by N-benzyl 3,6-bis(phenylethynyl)carbazole derivatives

Article abstract of DOI:10.1039/b616792f

Novel A-π-D-π-A V-shaped 3,6-bis(phenylethynyl)carbazole based chromophores were designed and synthesized as two-photon polymerization (TPP) initiators combining a large two-photon absorption cross-section with facilitated radical formation. 9-Benzyl-3,6-

Full text of DOI:10.1039/b616792f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Design of high efficiency for two-photon polymerization initiator:
combination of radical stabilization and large two-photon cross-section
achieved by N-benzyl 3,6-bis(phenylethynyl)carbazole derivatives
Jin-Feng Xing,ab Wei-Qiang Chen,*a Jie Gu,ab Xian-Zi Dong,a Nobuyuki Takeyasu,c Takuo Tanaka,*c
Xuan-Ming Duan*a and Satoshi Kawatac
Received 16th November 2006, Accepted 20th December 2006
First published as an Advance Article on the web 15th January 2007
DOI: 10.1039/b616792f
Novel A–p–D–p–A V-shaped 3,6-bis(phenylethynyl)carbazole based chromophores were
designed and synthesized as two-photon polymerization (TPP) initiators combining a large two-
photon absorption cross-section with facilitated radical formation. 9-Benzyl-3,6-bis(4-
nitrophenylethynyl)carbazole (4d) shows a strong two-photon absorption around 800 nm and
exhibits very high two-photon polymerization initiating sensitivity with a threshold power of
0.8 mW at the concentration of 0.18 mol%, which is much lower than the threshold power of
6.37 mW found for benzil. The corresponding threshold of laser exposure intensity for TPP is 3.0 6
107 mJ cm22. The lowest loading of 4d is up to 0.012 mol% with a threshold power of 3.2 mW.
large dTPA. Marder et al.18,19 have demonstrated that sym-
1. Introduction
metric stilbenes containing donor (D) or acceptor (A) groups
In the past decade, three dimensional (3D) micro-fabrication
linked by a p-conjugate bridge (D–p–D or A–p–A) exhibit
based on two-photon photopolymerization (TPP), as one of
large dTPA. The molecules with D–p–A–p–D and A–p–D–p–A
the most important applications of two-photon absorption
structures are also characterized by large dTPA. The second one
(TPA), has been attracting great interest for its various
is to optimize two component systems by changing coinitiators
with known two-photon initiators (Type II).12,20 The third one
applications to micro-electromechanical systems (MEMS),
3D optical integrated circuits and so on.1–6 Most work in this
is to fix a coinitiator unit, such as a sulfonium salt, into a large
TPA cross-section chromophore (Type III).9,10 The last one
field has involved commercial photoresist resins using conven-
tional ultraviolet (UV) active initiators. However, the conven-
would become the promising route for novel TPI since this
tional initiators typically have low TPA cross-sections (dTPA),
which result in low two-photon sensitivity.7 Thus, high
kind of photoinitiator are strong two-photon absorbers and
efficient initiators.
excitation power and long exposure time are needed to cure
the resin, which often result in damage to the structure.7,8 The
N,N-Disubstituted aniline moieties are widely used in TPA
chromphores, and N,N-dimethylaniline derivatives can act as
disadvantages of the conventional photoinitiator limit the
highly efficient coinitiators for one-photon and two-photon
photopolymerization.20 So the substituents play a key role in
versatility of three dimensional lithographic micro-fabrication
(3DLM) and its scope for widespread application. Developing
radical formation at the initiating stage, and a more stable
high-sensitivity initiators is necessary to make the process
amine may contribute to higher initiating sensitivity.
more reliable and allow structures to be patterned rapidly.9,10
Meanwhile, the charge-transfer (CT) state of a D–p–A mole-
cule can be directly produced by TPA21 and radicals can be
There are three ways to improve the sensitivity of two-
photon initiator (TPI) systems. The first one is to improve
produced in A-p–D–p–A C2v organic salts for intramolecular
change transfer (ICT),22 which may contribute to highly
the dTPA of initiators by extending the conjugate system
(Type I),9–14 which is the common way to improve TPI
efficient radical production. Here, by introducing i) a carbon–
carbon triple bond to improve the TPA cross-section,23,24 ii) a
sensitivity. Some principles for designing TPA materials have
been established. Prasad et al.15–17 have pointed out that one
carbazole moiety as a coinitiator unit, iii) an alkyl or benzyl
dimensional (1D) D–p–A type molecules containing fluorene
group for radical stabilization into an A–p–D–p–A V-shaped
or dithienothiophene as the rigid p-conjugated backbone have
molecule, we designed a series of novel Type III initiators,
rigid A–p–D–p–A V-shaped 3,6-bis(4-nitro/carbonyl-phenyl-
aLaboratory of Organic NanoPhotonics, Technical Institute of Physics
and Chemistry, Chinese Academy of Sciences, No.2, Beiyitiao,
Zhongguancun, Haidian District, Beijing, 100080, China.
E-mail: chenwq7315@mail.ipc.ac.cn; xmduan@mail.ipc.ac.cn;
Fax: +86-10-82543597; Tel: +86-10-82543596
ethynyl)carbazole derivatives (4a–d), in which the carbazole
moiety acts as a donor, phenylacetylenyl as a conjugate linker
and the nitro/carbonyl group as an acceptor. Furthermore, we
investigated one-photon/two-photon properties of the chro-
mophores, and evaluated the threshold power and the
polymerization rate for TPP. High two-photon polymerization
initiating sensitivity was characterized for these novel photo-
initiators by TPP experiments, owing to their large dTPA and
bGraduate School of Chinese Academy of Sciences, Chinese Academy of
Sciences, Zhongguancunbeiyitiao No.2, Haidian, Beijing, 100080, China
cNanophotonics Laboratory, RIKEN (The Institute of Physical and
Chemical Research), 2-1 Hirosawa, Wako-shi, Saitama, 351-0198,
Japan. E-mail: t-tanaka@riken.jp
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low fluorescence yield. The proposed concept of molecular
design here should provide a new direction for developing TPP
initiators with high initiating efficiency.
quantum yield after two-photon excitation is the same as that
after one-photon excitation. Then TPA cross-sections were
obtained by calibration against fluoroscein in aqueous NaOH
solution (pH 11) for the femtosecond measurements.29
2. Experimental
2.3 Two-photon photopolymerization
2.1 Materials
TPP experiments were performed with the irradiation of 80 fs
pulses from a mode-locked Ti-sapphire femtosecond laser
(Tsunami, Spectra-Physics) with a repetition rate of 80 MHz
and wavelength at 800 nm. The photoresist resins were
prepared by mixing methylacrylic acid (MAA) as monomer,
DEP-6A as cross-linker and dyes 4a–d as photoinitiators. In
the control experiment, commercially available benzil was used
as initiator. The lasing source was tightly focused by a 606 oil-
immersion objective lens with a high numerical aperture
(N.A. = 1.42, Olympus). The focal point was focused on the
liquid photopolymerizable resin which was placed on a cover
glass above the xyz-step motorized stage controlled by a
computer. After laser fabrication, the unpolymerized resins
were washed out with ethanol. The images of fabricated lines
1,1-Dimethyl-2-butyn-1-ol and 4-bromobenzaldehyde were
commercially available from Acros Organics. Dimethylform-
amide (DMF), sodium hydride (60% in mineral oil, NaH),
9H-carbazole, triethylamine, sodium hydroxide (NaOH),
copper iodine (CuI), triethylamine, triphenylphosphine, palla-
dium chloride (PdCl2), lithium chloride (LiCl), sodium,
benzophenone and 4-bromonitrobenzene were purchased
25
from Beijing Chemical Reagent Company. Pd(PPh3)2Cl2
and 3,6-dibromo-9-pentyl(benzyl)carbazole26 were synthesized
using literature procedures. Toluene and THF were dried over
Na and freshly distilled before use. Triethylamine was dried
over calcium hydride and freshly distilled before use. Other
solvents were used as received without further purification.
Dipentaerythritol hexaacrylate (DEP-6A, trade name: Light
Acrylate DEP-6A) was obtained from Kyoeisha Chemical Co.,
Ltd., Japan.
were observed using
a field-emission scanning electron
microscope (FE-SEM, JOEL, JSM-6330F).
2.4 Synthesis
2.2 Physical measurements
Chromophores 4a–d were synthesized in a facile way by
Sonogashira coupling reactions30 starting from 3,6-dibromo-
carbazole and 4-bromobenzaldehyde/4-bromonitrobenzene, as
shown in Scheme 1.
1H NMR and 13C NMR spectra were recorded on a Varian
Jemini-300/Bruker-400 spectrometer and all shifts are refer-
enced to TMS. Mass spectra were measured on a ZAB-HS
(Micromass, UK). UV-Visible spectra were recorded at the
concentration of 1 6 1025 M on a UV 3100PC Scanning
Spectrophotometer (Shimadzu, Japan). Infrared spectra were
recorded on a FT/IR-410 spectrophotometer (JASCO Corp.)
Fluorescence spectra were recorded at the concentrations
of 1 6 1026 M on a RF5300PC spectrofluorophotometer
(Shimadzu, Japan) with coumarin 307 in acetonitrile as a
reference standard (Wf = 0.58).27,28 TPA spectra were obtained
by the two-photon-induced excited fluorescence (TPEF)
method reported by Xu and Webb,29 TPEF spectra were
recorded on a SD2000 spectrometer (Ocean Optical) with the
excitation by a mode-locked Ti-sapphire femtosecond laser
(Tsunami, Spectra-Physics) for which the oscillating wave-
length, pulse width and repetition rate were 780 nm, 80 fs
and 82 MHz, respectively. It is assumed that the fluorescence
Synthesis of 1a–b. To a flask containing NaH (60% in
mineral oil, 2.00 g, 50 mmol) in DMF (20 mL), a solution
of 3,6-dibromo-9-H-carbazole (13.00 g, 40 mmol) in DMF
(80 mL) was added dropwise; after one hour, 1-bromopentane
(5.4 mL, 40 mmol) was added slowly. The resulting suspension
was stirred for another 6 h and carefully poured into water.
After filtration and washing with 95% ethanol (2 6 30 mL),
3,6-dibromo-9-pentyl-carbazole (1a) was obtained as a grey
solid, yield: 94%. Mp 75–76 uC. 1H NMR (300 MHz, CDCl3, d
(ppm)): 0.87 (t, 3 H, J = 6.9 Hz), 1.33 (m, 4 H), 1.84 (m, 2 H),
4.25 (t, 2 H, J = 6.9 Hz), 7.28 (d, 2 H, J = 8.3 Hz), 7.55 (d, 2 H,
J = 8.3 Hz), 8.15 (s, 2 H).
3,6-Dibromo-9-benzyl-carbazole (1b): the same procedure
as 1a, yield: 91%. Mp 162–163 uC. 1H NMR (300 MHz,
Scheme 1 Synthetic route of 4a–d.
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CDCl3, d (ppm)): 5.47 (s, 2 H), 7.07 (m, 2 H), 7.25 (m, 5 H),
2 H), 4.33 (t, J = 7.2 Hz, 2 H), 7.43 (d, J = 8.4 Hz, 2 H),
7.69 (d, J = 8.4 Hz, 2 H), 7.72 (d, J = 7.8 Hz, 4 H), 7.90 (d,
J = 7.8 Hz, 4 H), 8.33 (s, 2 H), 10.04 (s, 2 H); 13C NMR
(100 MHz, CDCl3, d (ppm)): 13.9, 22.4, 28.7, 29.4, 43.4, 87.5,
95, 109.2, 109.5, 113.3, 122.5, 124.5, 129.6, 130.0, 130.2, 131.0,
131.9, 132.7, 135.1, 140.8, 191.4; IR (KBr, cm21): 2965, 2927,
2202, 1694, 1592,1482, 1205, 826; HRMS (C35H27NO2): Calcd.:
493.20418; Obsd.: 493.20491 (1.5 ppm).
7.53 (m, 2 H), 8.18 (s, 2 H).
Synthesis of 2a–b. To a 250 mL three-necked flask, 3,6-
dibromo-9-pentyl-carbazole (1a) (3.120 g, 8.0 mmol), CuI
(76 mg, 0.4 mmol), PdCl2(PPh3)2 (140 mg, 0.2 mmol),
PPh3 (104 mg, 0.4 mol), 1,1-dimethyl-2-butyn-1-ol (2.50 mL,
30 mmol) and triethyl amine (100 mL) were added and
bubbled with N2 for 10 min. The resulting mixture was allowed
to reflux for 24 h. After cooling, the solvent was removed.
The residue was poured into water and extracted with ethyl
acetate (3 6 50 mL). Organic layers were combined, dried over
MgSO4, concentrated and purified by flash column chromato-
graphy using ethyl acetate–petroleum ether (1 : 3 v/v) as eluent
to give a pale yellow oil.
3,6-Bis[2-(4-carbonylphenyl)ethynyl]-9-benzyl-carbazole (4b),
1
yield: 53%. Mp 244–245 uC. H NMR (300 MHz, CDCl3, d
(ppm)): 5.55 (s, 2H), 7.15 (m, 2H), 7.29 (m, 3H), 7.39 (d, J =
8.3 Hz, 2H), 7.65 (d, J = 8.3 Hz, 2H), 7.72 (d, J = 8.1 Hz, 4H),
7.88 (d, J = 8.1 Hz, 4H), 8.36 (s, 2H), 10.04 (s, 2H); 13C NMR
(100 MHz, CDCl3, d (ppm)): 46.9, 87.5, 94.7, 109.4, 113.7,
122.6, 124.4, 126.2, 127.8, 128.9, 129.5, 130.0, 130.1, 135.0,
135.9, 140.9, 191.2; IR (KBr, cm21): 2834, 2738, 2201, 1695,
1592, 1479, 1208, 830, 728; HRMS (C37H23NO2): Calcd.:
513.17288; Obsd.: 513.17344 (1.1 ppm).
3,6-Bis(3,3-dimethyl-3-hydroxy-1-butynyl)-9-pentyl-carbazole
(2a), yield: 82%. 1H NMR (300 MHz, CDCl3, d (ppm)): 0.86 (t,
3 H, J = 6.6 Hz), 1.30 (m, 4 H), 1.68 (s, 12H), 1.81 (m, 2 H),
2.37 (bs, 2 H), 4.21 (t, 2 H, J = 6.9 Hz), 7.28 (d, 2 H, J =
8.4 Hz), 7.52 (d, 2 H, J = 8.4 Hz), 8.11 (s, 2 H).
3,6-Bis[2-(4-nitrophenyl)ethynyl]-9-pentyl-carbazole
(4c),
1
yield: 68%. Mp 202–204 uC. H NMR (300 MHz, CDCl3, d
(ppm)): 0.90 (t, J = 6.6 Hz, 3 H), 1.38 (m, 4 H), 1.90 (m, 2 H),
4.32 (t, 2 H, J = 7.1 Hz), 7.41 (d, J = 8.7 Hz, 2 H), 7.67 (d, J =
9.0 Hz, 6 H), 8.22 (d, J = 9.0 Hz, 4 H), 8.32 (s, 2 H); 13C NMR
(75 MHz, CDCl3, d (ppm)): 13.9, 22.4, 28.7, 29.4, 46.0, 86.7,
96.3, 109.4, 112.9, 122.5, 123.7, 124.7, 130.1, 130.8, 132.0,
141.0, 146.7; IR (KBr, cm21): 2962, 2922, 2201, 1585, 1512,
1339, 1105, 854, 749; HRMS (C27H25N3O4): Calcd.:
527.18451; Obsd.: 527.18474 (0.4 ppm).
3,6-Bis(3,3-dimethyl-3-hydroxy-1-butynyl)-9-benzyl-carbazole
1
(2b), yield: 78%. H NMR (300 MHz, CDCl3, d (ppm)): 1.87
(s, 12 H), 2.13 (s, 2 H), 5.47 (s, 2 H), 7.07 (m, 2 H), 7.25 (m,
5 H), 7.48 (d, 2 H, J = 8.4 Hz), 8.16 (s, 2 H); 13C NMR
(100 MHz, CDCl3, d (ppm)): 31.7, 46.7, 65.8, 83.0, 92.3, 109.1,
110.0,113.8, 122.5, 124.2, 126.3, 127.7, 128.9, 129.9, 136.3,
140.6.
3,6-Bis[2-(4-nitrophenyl)ethynyl]-9-benzyl-carbazole
(4d),
Synthesis of 3a–b. To a solution of 3,6-bis(3-methyl-3-
hydroxy-1-butynyl)-9-pentyl-carbazole (2a) (2.58 g, 6.4 mmol)
in toluene (60 mL), was added sodium hydroxide (3.10 g,
77.5 mmol). Then the resulting mixture was refluxed overnight.
After filtration and concentration, the dark brown residue was
purified by flash column chromatography using ethyl acetate–
petroleum ether (1 : 9–1 : 20 v/v) to give a light yellow oil.
3,6-Diethynyl-9-pentyl-carbazole (3a), yield: 62%, 1H NMR
(300 MHz, CDCl3, d (ppm)): 0.88 (t, 3 H, J = 6.6 Hz), 1.34 (m,
4 H), 1.84 (m, 2 H), 3.1 (s, 2 H), 4.24 (t, 2 H, J = 7.2 Hz), 7.32
(d, 2 H, J = 8.4 Hz), 7.62 (d, 2 H, J = 8.4 Hz), 8.22 (s, 2 H).
3,6-Diethynyl-9-benzyl-carbazole (3b), yield: 81%, 1H NMR
(300 MHz, CDCl3, d (ppm)): 3.09 (s, 2 H), 5.50 (s, 2 H), 7.10
(m, 2 H), 7.28 (m, 3 H), 7.30 (d, 2 H, J = 8.4 Hz), 7.58 (d, 2 H,
J = 8.4 Hz), 8.25 (s, 2 H).
1
yield: 77%. Mp 258–261 uC. H NMR (300 MHz, CDCl3, d
(ppm)): 5.57 (s, 2H), 7.15 (m, 2H), 7.32 (m, 3H), 7.41 (d, J =
8.4 Hz, 2H), 7.65 (d, J = 8.4 Hz, 2H), 7.72 (d, J = 8.9 Hz, 4H),
7.88 (d, J = 8.9 Hz, 4H), 8.36 (s, 2H); 46.9, 86.7, 96.1, 109.5,
113.4, 122.6, 123.6, 124.6, 126.2, 127.8, 128.9, 130.2, 130.6,
131.9, 135.2, 141.1, 146.5; IR (KBr, cm21): 2921, 2201, 1586,
1508, 1338, 1105, 854, 748; HRMS (C35H21N3O4): Calcd.:
547.15321; Obsd.: 547.15208 (22.1 ppm).
3. Results and discussion
3.1 One-photon absorption and fluorescence
The normalized one-photon absorption and fluorescence
spectra for 4a–d are presented in Fig. 1, these two pairs of
compounds showed very similar absorption and emission
spectra. In the same conjugated system, N-benzyl derivatives
showed a blue shift compared to the N-pentyl ones. The
absorption maximum in chloroform for 4a appeared at 383 nm,
for 4b at 378 nm with a 5 nm blueshift. The same phenomenon
was observed for 4c and 4d: the absorption maximum for 4d
appears at 385 nm with a 11 nm blueshift compared to that
of 4c at 396 nm. 4c and 4d showed a dual fluorescence in
chloroform, the emission at 450 nm corresponding to a typical
p–p* transition, delocalized over the whole conjugated
p-electronic system; and the emission at 560 nm representing
a typical charge transfer fluorescence.31
Synthesis of 4a–d. To a 50 mL three-necked flask, 3,6-
diethynyl-9-pentyl-carbazole (3a) (577 mg, 2.0 mmol), CuI
(19 mg, 0.1 mmol), PdCl2(PPh3)2 (35 mg, 0.05 mmol), PPh3
(26 mg, 0.1 mol), 4-bromobenzaldehyde (1.11 g, 6.0 mmol) and
triethyl amine (30 mL) were added and bubbled with N2 for
10 min. The resulting mixture was allowed to reflux for 20 h.
After cooling, the solvent was removed. The residue was
poured into water and extracted with chloroform (3 6 30 mL).
Organic layers were combined, dried over MgSO4, concen-
trated and purified by flash column chromatography using
chloroform–petroleum ether (1 : 3 v/v) as eluent to give 4a as a
yellow powder (653 mg, 65% yield).
However, 4a and 4b exhibited only one emission peak at
about 460 nm. The Stokes shifts of 4c and 4d are larger than
those of 4a and 4b, the Stokes shift of 4d is 8053 cm21 larger
than that of 4c (7522 cm21). These results indicated that strong
3,6-Bis[2-(4-carbonylphenyl)ethynyl]-9-pentyl-carbazole (4a),
1
yield: 65%. Mp 197–198 uC. H NMR (300 MHz, CDCl3, d
(ppm)): 0.90 (t, J = 6.3 Hz, 3 H), 1.38 (m, 4 H), 1.91 (m,
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Fig. 1 Normalized UV-Vis and fluorescence spectra of 4a (solid line),
4b (dashed line), 4c (dotted line) and 4d (dot-dashed line) in
chloroform.
Fig. 2 TPA cross-sections of 4a–d in range 720–880 nm: 4a–b
measured in chloroform and 4c–d measured in THF.
3.3 Two-photon photopolymerization
intramolecular charge transfer existed in 4c and 4d compared
to 4a and 4b, which might contribute to a larger dTPA. At the
same time, 4c and 4d exhibited low fluorescence quantum
yields (Wf) compared to 4a and 4b, which might contribute to
improvement of non-irradiative decay and higher initiating
efficiency. The fluorescence quantum yields of 4c and 4d are
much lower in chloroform than in THF, which probably
results from the difference in contact charge transfer to
the solvent. Photophysical data of UV-Vis absorption and
fluorescence of 4a–d are presented in Table 1.
In order to understand the TPP behavior of chromophores
4a–d, two-photon polymerization experiments were per-
formed. Usually, the threshold energy of polymerization is
evaluated as the lowest average laser power with which the
polymer lines are produced by the translating resin.14 Here, the
polymerization threshold is defined as the average power
before being induced into the objective lens, below which the
polymer line can not be fabricated using a linear scan speed of
10 mm s21. The resin components and polymerization thresh-
old powers of the resins are summarized in Table 2.
At first, 4a–d were used as initiators in resins with a molar
ratio of 0.18%. When 4b was used as initiator, the threshold
power (Pth) of R2 is found to be 1.6 mW which is much lower
than that of R1, Pth = 10.11 mW. Similarly, when 4d was used
as initiator in R4, Pth = 0.80 mW, which is lower than that of
R3, Pth = 2.02 mW. In the same conditions, when benzil was
used, Pth increased to 6.37 mW for R9. Selected SEM images
of R4 and R9 after polymerization are shown in Fig. 3. From
Fig. 3a, the line width in R4 declined to 250 nm from 710 nm
with the laser power decreasing to 0.80 mW from 2.53 mW.
The results indicated that 4b–d exhibited much higher
sensitivity than benzil. The low threshold powers of R2 and
R4 are due to the benzyl groups in 4b and 4d that may stabilize
the formed radical in the initiator stage. The thresholds of R1
3.2 Two-photon absorption and fluorescence
TPA cross-sections, dTPA, were measured to understand their
TPA behaviour using the two-photon fluorescence method and
the TPA spectra of 4a–d are presented in Fig. 2. The dTPA
values of 4a–d are also listed in Table 1. Chromophores 4a and
4b had TPA cross-section maxima around 300 GM at the
wavelength of 730 nm, and 4c and 4d exhibited dTPA more than
500 GM in the wavelength range 780–820 nm, a range in which
laser light is widely used for two-photon microfabrication.
This is preferred for two-photon polymerization together with
low Wf, which might contribute to high triplet quantum yield
and usually leads to high initiating sensitivity.32–35 Therefore,
4c and 4d should be promising candidates as photoinitiators.
Table 1 Photophysical data of 4a–d at room temperature
Compd.
Solvent
labsmax/nm (emax/M21cm21 a
)
lemmax/nmb
STc/cm21
Wfd
dTPAmaxe/GM
4a
4b
4c
CHCl3
CHCl3
CHCl3
THF
CHCl3
THF
383 (54800)
378 (55700)
396 (42700)
390 (49400)
385 (38700)
384 (42100)
468
460
564
546
558
544
b
4742
4715
7522
7326
8053
7659
0.63
0.66
0.005
0.065
0.007
0.065
337 (730)
308 (730)
—
645 (770)
—
916 (780)
4d
a
c
Peak wavelength of one-photon absorption and molar extinction coefficient. Peak wavelength of one-photon fluorescence. ST stands for
d
Stokes shift, which is equal to (1/labs 2 1/lem). Fluorescence quantum yield determined relative to coumarin 307 (W = 0.58 in acetonitrile).
e
The maximum of two-photon absorption cross-section. 1 GM = 10250 cm4 s photon21. The numbers in parentheses are the wavelengths of
maximum TPA.
1436 | J. Mater. Chem., 2007, 17, 1433–1438
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Table 2 Components and polymerization thresholds of resins
Resin component (wt%)
No. Initiator MAA Initiator DEP-6A M.R. (%)a Pth/mWb
R1 4a
R2 4b
R3 4c
R4 4d
R5 4d
R6 4d
R7 4d
R8 4d
R9 benzil
a
66.62 0.72
66.60 0.75
66.59 0.77
66.57 0.80
66.84 0.40
66.97 0.20
67.04 0.10
67.07 0.05
66.90 0.31
32.66
32.65
32.64
32.63
32.76
32.83
32.86
32.88
32.79
0.18
0.18
0.18
0.18
0.09
0.045
0.023
0.012
0.18
10.11
1.60
2.02
0.80
1.01
1.60
2.53
3.20
6.37
b
Molar ratio of dyes in resin. Threshold power of dye in resin.
Fig. 4 The dependence of polymerization rate upon laser power for
R2, R3, R4 and R9.
presented in Fig. 4. The polymerization rates of R2–4 are
larger than that of R9 when the same laser power is used. For
example, when the laser power is 8.0 mW, the Rp of R2 and R3
are 11.5 and 12.9 mm3 s21, respectively, which are one order
larger than that of R9, Rp = 0.65 mm3 s21. This demonstrates
that the TPP initiating sensitivities of 4b–d are higher than that
of benzil. The Rp of R2 is smaller than that of R3, although R2
exhibited a lower threshold power than R3. One reason for the
lower polymerization rate of 4b is its high fluorescence
quantum yield (Wf = 0.66), which results in fast radiative
deactivation and leads to a low intersystem crossing yield. The
Rp of R4 is larger than those of R2 and R3 at the same laser
power. These results showed the rank for higher initiating
efficiency is 4d . 4c . 4b. Therefore, low Wf, large TPA cross-
section and the presence of a benzyl group contributed to the
high initiating efficiency for compound 4d.
This result encouraged us to further investigate lower
loadings of 4d in the resin. As listed in Table 2, when the
molar ratio of 4d in the resin was reduced from 0.18% (R5) to
0.09% (R6), the threshold power of the resin increased slowly
to 1.01 mW from 0.8 mW. When 0.012% 4d was used in the
resin its threshold energy increased to 3.2 mW for R8. The
threshold power of a polymerization system is strongly
dependent on many factors, including concentration of dyes,
monomer, cross-linker, the width of pulses, the size of the
focus point, and so on. Since exposure energy per unit area
directly reflected the TPP initiating sensitivity, we calculated
the threshold exposure. The thresholds of laser exposure
intensity for 4d at 800 nm at the molar ratios of 0.18% and
0.012% are Eth = 3.0 6 107 and 1.2 6 108 mJ cm22
,
Fig. 3 SEM images of R4 (a) and R9 (b) after polymerization. The
laser powers used to fabricate lines are shown on the SEM images. The
scan speed is 10 mm s21
respectively, which are comparable with the values reported
for a two-component initiator system.14 The above results
show that 4d exhibited high initiating efficiency even at very
low loadings.
.
and R2 were higher than those of R3 and R4, respectively, due
to larger dTPA of 4c–d compared with those of 4a–b.
Furthermore, we investigated the substituent effect by
calculating the quasi-polymerization rate (Rp) using the
equation Rp = p(d/2)2ns, where d is the width of the line
fabricated and ns is the linear scan speed.14 The plots of
polymerization rate vs. laser power for R2–4 and R9 are
Conclusions
We have designed a series of novel two-photon polymerization
initiators with high sensitivity combining large dTPA with
facilitated radical formation, and successfully synthesized
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 1433–1438 | 1437

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S. R. Marder and J. W. Perry, J. Photopolym. Sci. Technol., 2001,
14, 657–688.
11 J. Wu, Y. X. Zhao, X. Li, M. Q. Shi, F. P. Wu and X. Y. Fang,
New J. Chem., 2006, 30, 1098.
12 Y. X. Zhao, X. Li, F. P. Wu and X. Y. Fang, J. Photochem.
Photobiol., A: Chem., 2006, 177, 12.
13 Y. X. Yan, X. T. Tao, G. B. Xu, H. P. Zhao, Y. H. Sun,
C. K. Wang, J. X. Yang, X. Q. Yu, X. Xian and M. H. Jiang, Aust.
J. Chem., 2005, 58, 29.
14 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.
15 B. A. Reinhardt, L. L. Brott, S. J. Clarson, A. G. Dillard,
J. C. Bhatt, R. Kannan, L. Yuan, G. S. He and P. N. Prasad,
Chem. Mater., 1998, 10, 1863.
16 R. Kannan, G. S. He, L. Yuan, F. Xu, P. N. Prasad,
A. G. Dombroskie, B. A. Reinhardt, J. W. Baur, R. A. Vaia and
L.-S. Tan, Chem. Mater., 2001, 13, 1896.
17 O.-K. Kim, K.-S. Lee, H. Y. Woo, K.-S. Kim, G. S. He,
J. Swiatkiewicz and P. N. Prasad, Chem. Mater., 2000, 12, 284.
18 M. Rumi, J. E. Ehrlich, A. A. Heikal, J. W. Perry, S. Barlow,
Z. Hu, D. McCord-Maughon, T. C. Parker, H. Ro¨ckel,
S. Thayumanavan, S. R. Marder, D. Beljonne and J.-L. Bre’das,
J. Am. Chem. Soc., 2000, 122, 9500.
novel 3,6-bis(ethynyl)carbazole derivatives (4a–d) in a facile
way by a Sonogashira coupling reaction. 4c and 4d exhibit
large dTPA from 780–820 nm and low fluorescence quantum
yields. Two-photon polymerization experiments verified that
the low quantum yield, large dTPA and benzyl group con-
tributed the high efficiency of 4d in acrylate resins. The
threshold power of 4d at 0.18 mol% in R4 is 0.8 mW, which is
much lower than the value of 6.37 mW for benzil, and the
lowest loading of 4d is up to 0.012 mol% with a threshold
power of 3.2 mW. When 4d was used as TPP initiator, the
corresponding thresholds of laser exposure intensity for R4
and R8 are 3.0 6 107 and 1.2 6 108 mJ cm22, respectively. It
showed extremely high sensitivity as a TPP initiator. The
concept of combination of radical stabilization and large
two-photon cross-section within a molecular structure is
significant for developing highly efficient two-photon poly-
merization initiators.
Acknowledgements
19 M. Albota, D. Beljonne, J.-L. Bre’das, J. E. Ehrlich, J.-Y. Fu,
A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder,
D. McCord-Maughon, J. W. Perry, H. Ro¨ckel, M. Rumi,
G. Subramaniam, W. W. Webb, X.-L. Wu and C. Xu, Science,
1998, 281, 1653.
20 K. D. Belfield, X. Ren, E. W. V. Stryland, D. J. Hagan,
V. Dubikovsky and E. J. Miesak, J. Am. Chem. Soc., 2000, 122,
1217.
We acknowledge financial support from the ‘‘One Hundred
Overseas Talents Program’’ of Chinese Academy of Sciences
(CAS) and the ‘‘Nonlinear Nanophotonics’’ CREST Project of
Japan Science and Technology Agency (JST).
21 C. K. Wang, K. Zhao, Y. Su, Y. Ren, X. Zhao and Y. Luo,
J. Chem. Phys., 2003, 119, 1208.
References
22 X.-M. Duan, T. Wada, S. Okada, H. Oikawa, H. Sasabe and
H. Nakanishi, Mater. Res. Soc. Symp. Proc., 2000, 581, B.3.31-1-6.
23 K. Kamada, K. Ohta, Y. Iwase and K. Kondo, Chem. Phys. Lett.,
2003, 372, 386.
24 Y. Iwase, K. Kamada, K. Ohta and K. Kondo, J. Mater. Chem.,
2003, 13, 1575.
25 L. Brandsma, S. F. Vasilevsky and H. D. Verkruijsse, Application
of Transition Metal Catalysts in Organic Synthesis, Springer,
Berlin, 1998, p. 25.
1 S. Kawata and Y. Kawata, Chem. Rev., 2000, 100, 1777.
2 K. D. Belfield, K. J. Schafer, Y. Liu, J. Liu, X. B. Ren and
E. W. Van Stryland, J. Phys. Org. Chem., 2000, 13, 837.
3 W. Denk, J. H. Strickler and W. W. Webb, Science, 1990, 248, 73.
4 T. C. Lin, S. J. Chung, K. S. Kim, X. P. Wang, G. S. He,
J. Swiatkiewicz, H. E. Pudavar and P. N. Prasad, Adv. Polym.
Technol., 2003, 161, 175.
5 S. Kawata, H.-B. Sun, T. Tanaka and K. Takada, Nature, 2001,
412, 697.
26 K. R. J. Thomas, J. T. Lin, Y.-T. Tao and C.-W. Ko, J. Am. Chem.
Soc., 2001, 123, 9404.
6 B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer,
J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler,
I. Y. S. Lee, D. McCord-Maughon, J. Q. Qin, H. Ro¨ckel, M. Rumi,
X. L. Wu, S. R. Marder and J. W. Perry, Nature, 1999, 398, 51.
7 K. J. Schafer, Joel M. Hales, Mihaela Balu, K. D. Belfield,
E. W. Van Stryland and D. J. Hagan, J. Photochem. Photobiol., A:
Chem., 2004, 162, 497.
27 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
28 R. A. Velapoldi and H. H. Tonnesen, J. Fluoresc., 2004, 14, 465.
29 C. Xu and W. W. Webb, J. Opt. Soc. Am. B, 1996, 13, 481.
30 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett.,
1975, 4467.
8 Tommaso Baldacchini, Christopher N. LaFratta and Richard
A. Farrer, J. Appl. Phys., 2004, 95, 6072.
9 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: Chem., 2003, 158, 163.
31 Z. R. Grabowski and K. Rotkiewicz, Chem. Rev., 2003, 103, 3899.
32 Y. J. Hua and J. V. Crivello, Macromolecules, 2001, 34, 2488.
33 M. Nag and W. S. Jenks, J. Org. Chem., 2004, 69, 8177.
34 T. Y. Ohulchanskyy, D. J. Donnelly, M. R. Detty and P. N. Prasad,
J. Phys. Chem. B, 2004, 108, 8668.
10 S. M. Kuebler, M. Rumi, T. Watanabe, K. Braun, B. H. Cumpston,
A. A. Heikal, L. L. Erskine, S. Thayumanavan, S. Barlow,
35 M. N. Berberan-Santos and J. M. Garcia, J. Am. Chem. Soc., 1996,
118, 9391.
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