Novel D-π3-(A)1-3 multibranched chromophores as an efficient two-photon-induced polymerization initiator

Article abstract of DOI:10.1016/j.dyepig.2010.05.001

The ability of novel branched chromophores that contained a triphenylamine group as donor, naphthalene as π-bridge and pyridyl as acceptor group, to act as two-photon absorption and two-photon induced polymerization initiators were investigated. Linear and nonlinear optical spectra revealed that chromophores with large dipolar moment change gave strong intramolecular charge-transfer characteristics that exhibited enhanced two-photon absorption cross-section. A variety of micro-objects were fabricated, using an acrylate matrix, that contained the novel chromophores as two-photon induced polymerization initiator. Molecular energy levels, electrochemistry and linear absorption spectra, as well as one-photon fluorescence quenching measurements supported the hypothesis that two-photon induced polymerization proceeds via intermolecular charge-transfer from initiator to monomer and, that the chromophore firstly generates the exciton via two-photon absorption and then transfers charge to the acrylate monomer which then polymerises.

Full text of DOI:10.1016/j.dyepig.2010.05.001

Dyes and Pigments 88 (2011) 57e64
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel De
p
3e(A)1e3 multibranched chromophores as an efficient
two-photon-induced polymerization initiator
,
c
a
b
c
,
b
Xiaomei Wang a , Feng Jin , Weizhou Zhang , Xutang Tao , Xuan-Ming Duan , Minhua Jiang
*
*
a Institute of Chemistry and Bioengineering, Suzhou University of Science and Technology, Suzhou 215009, China
b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
c Laboratory of Organic NanoPhotonics and Key Laboratory of Photochemical Conversion and Functional Materials, Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The ability of novel branched chromophores that contained a triphenylamine group as donor, naph-
Received 14 February 2009
Received in revised form
28 April 2010
Accepted 5 May 2010
Available online 20 May 2010
thalene as
p-bridge and pyridyl as acceptor group, to act as two-photon absorption and two-photon
induced polymerization initiators were investigated. Linear and nonlinear optical spectra revealed that
chromophores with large dipolar moment change gave strong intramolecular charge-transfer charac-
teristics that exhibited enhanced two-photon absorption cross-section. A variety of micro-objects were
fabricated, using an acrylate matrix, that contained the novel chromophores as two-photon induced
polymerization initiator. Molecular energy levels, electrochemistry and linear absorption spectra, as well
as one-photon fluorescence quenching measurements supported the hypothesis that two-photon
induced polymerization proceeds via intermolecular charge-transfer from initiator to monomer and, that
the chromophore firstly generates the exciton via two-photon absorption and then transfers charge to
the acrylate monomer which then polymerises.
Keywords:
Two-photon absorption
Dipolar moment change (Dmge
Two-photon-induced polymerization
De 3e(A)1e3 initiator
)
p
Fluorescence quenching
Intermolecular charge-transfer
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
counterparts [20], this being one way of enhancing the TPP efficiency
of initiators. In contrast, chromophores with branched architecture
Two-photon-induced polymerization (TPP) is an attractive tech-
nology for 3D-nanostructures and 3D-lithography for photonic
crystals, optical data storage, micro-electromechanical systems
(MEMS), optical integrated circuits and self-writing waveguides
[1e8]. These technologies take advantage of the fact that two-photon
absorption (TPA) probability depends quadratically on excitation
intensity, resulting in high resolution and deep penetration [9]. Thus,
whilst effective TPP initiators of large TPA cross-section have been the
subject of much research [1,10e15] over the past decade or so.
Improvement in initiation efficiency is required, as well as a deeper
understanding of the dependence of this efficiency on molecular
parameters. At present, many papers concern both the synthesis and
and more acceptor groups usually possess low fluorescence yield,
which might contribute to high triplet quantum yield and result in
high initiator sensitivity [21e24]. Based on these considerations,
novel, Dep3e(A)1e3 chromophores, where “D,” “p” and “A” are
a triphenylamine group, naphthalene group and pyridyl, respectively
were synthesized namely PN-1, PN-2 and PN-3 (Fig. 1), the three
compounds varting in the number of constituent pyridyl units.
2. Experimental
2.1. Measurements
TPP applications of initiators of D-
Ae eDe eA architecture [1,10e19]. However, to our knowledge,
few reports have systematically studied TPP initiators with
De 3e(A)n structure. An increase in dimensionality of donor-
acceptor molecules appears as a reasonable approach to generate
improved TPA cross section in comparison with their linear
p-D, A-p-A, DepeAepeD and
Electron impact (Mode laser) mass spectra and EI mass spectra
were obtained on a 4700 Proteome Analyzer (MALDI-TOF-TOF)
produced by ABI Company and on an HP 5989 mass spectra
instrument, respectively. Hydrogen and carbon nuclear magnetic
resonance spectra were determined on a GCT-TOF NMR spec-
trometer. Element analyses were performed on Perkin 2400(II)
autoanalyzer.
p
p
p
p-
Linear absorption measurements of dilute solution were
measured with Hitachi U-3500 recording spectro-photometer from
quartz curettes of 1 cm path. Steady-state fluorescence and the
* Corresponding authors. Tel.: þ86 512 62092786; fax: þ86 512 67246786
E-mail addresses: wangxiaomei@mail.usts.edu.cn (X. Wang), xmduan@mail.ipc.
ac.cn (X.-M. Duan).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.05.001

58
X. Wang et al. / Dyes and Pigments 88 (2011) 57e64
rate of 80 MHz was used for all TPP experiments here. The beam
R1
R2
was tightly focused into a photoresist on a glass cover slip with an
oil-immersion objective lens (100ꢀ, numerical aperture ¼ 1.4,
Olympus). The acrylate resin was scanned by the focused beam in
the xey plane by a two-galvanomirror set (HurrySCAN 14, SCAN-
LAB), and along the z-axis by a piezostage (P-622.ZCL, PI), both
controlled by a computer. After laser fabrication, the unpoly-
merized monomer was washed out using ethanol. The obtained
microstructures were characterized with scanning electron
microscopy (SEM; Hitachi S-4300FEGd).
PN-1:
N
R1 =R2 = Br;
R3 =
;
N
PN-2:
PN-3:
R1 = Br; R2 = R3 =
R1 = Br; R2 = R3 =
N
N
;
.
R3
Fig. 1. Molecule structures of new De
p3e(A)1e3chromophores: PN-1, PN-2 and PN-3.
2.3. Synthesis
time-resolved decay curves were measured on an Edinburgh FLS
920 fluorophotometer equipped with the Xe lamp and a time-
correlated single-photon counting (TCSPC) card. Lifetime values
were measured by reconvolution fit analysis of the decay profiles
with the aid of nF900 software, and the fitting results were judged
by the reduced c2 value. For one-photon absorption and fluores-
cence measurements, the concentration of sample in THF solution
was 1 ꢀ 10ꢁ5 mol dmꢁ3 and the one-photon fluorescence (OPF)
2.3.1. Tris(4-(2-(4-Bromonaphthalen-1-yl)vinyl)phenyl)
amine (TBPA)
Tris-(4-formyl-phenyl) amine [27,28] (1.0 g, 3.04 mmol), ((4-
bromonaphthalen-1-yl) methyl) triphenylphosphonium (5.12 g,
9.12 mmol) and NaH (0.36 g, 15.0 mmol) in the presence of 1,4-
dioxane and triethylamine (80: 80 V/V) were refluxed for 6 h.
After cooling to room temperature, the precipitate was filtered and
the filtrate concentrated. The residue was purified using a silica gel
column (petroleum/ethyl acetate ¼ 3/1) and obtained 1.15 g of
TBPA, yield 46%. MS (MALDI) Mþ ¼ 935.0. Element analysis (%):
Cald. 69.10; H, 3.87; N, 1.49. Found. 68.73; H, 3.26; N. 1.98. 1H NMR
quantum yield (F
) was measured using fluorescein (in 0.1 mol dmꢁ3
aqueous NaOH) as a standard.
Two-photon absorption (TPA) cross-section (dTPA) of chromo-
phore solution (in THF) was determined by two-photon fluores-
cence (TPF) method. The light source is a Tsunami mode-locked
Ti:sapphire system (700e880 nm, 80 MHz, <130 fs). TPF spectra
were recorded by a fiber spectra meter (Ocean Optics USB2000
CCD). The TPA cross-section was determined based on Eq. (1) [25],
(400 MHz, CDCl3, TMS): d, ppm 6.47 (d, 6H, benze-H), 7.20 (d, 6H,
benze-H), 6.98 (d, 6H, CH]CH), 7.38e7.70 (m, 18H).
2.3.2. 4-((E)-2-(4-Bromonaphthalen-1-yl)vinyl)-N-(4-((E)-2-
(4- bromonaphthalen-1-yl)vinyl)phenyl)-N-(4-((E)-2-
(4-((E)-2-(pyridin-4-yl)vinyl)naphthalen-1-yl)
fref Fs nref cref
fs Fref ns cs
ds
¼
dref
(1)
vinyl)phenyl)benzenamine (PN-1)
Under a nitrogen atmosphere, a stirred mixture of 1.0 g
(1.07 mmol) of TBPA, 0.5 g (1.64 mmol) of tri-o-tolylphosphine,
0.12 mL (1.07 mmol) of vinylpyridine, 0.0048 g of palladium(II)
acetate (0.2 mmol) and 10 mL of triethylamine was refluxed for
24 h. The solvent was removed under reduced pressure and the
orange residue dissolved in dichloromethane, washed three
times with distilled water, and dried with anhydrous magnesium
sulfate, before being filtered and concentrated. The resulting
solution was purified over silica gel with ethyl acetateepetro-
leum ether (5:1) as eluent to obtain 0.125 g of PN-1 with orange
color. M. p. 88e89 ꢂC. MS (MALDI) ¼ 962.8 (Mþ). Element anal-
ysis (%): Cald. C, 76.10; H, 4.40; N, 2.91. Found. C, 76.42; H, 4.21;
where the subscripts of “ref” and “s” stand for reference and
sample, respectively. The terms n and c are the refractive index and
the concentration of the solution. The symbols
F and F represent
the OPF quantum yield and TPF integral, respectively. For elimi-
nating the disturbance from other photophysical or photochemical
processes, the intensity of input pulses was confined within
0.5e2 GW/cm2 in our experiment. The experimental uncertainty
amounts to 10%. Noted that our two-photon fluorescence signal is
picked up by an external optical fiber, different from the setup
reported previously [22]. Thus, the measured two-photon spectra
in this paper are only the approximations.
Cyclic voltammogram of chromophores in deoxygenated
dichloromethane was obtained in a three-electrode cell with
a platinum counter electrode, a glass carbon working electrode and
an Ag/AgCl (0.1 M) reference. The HOMO (highest occupied
molecular orbital) energy level relative to vacuum level was calcu-
lated using the oxidation potential (after being converted to the
potential relative to the standard hydrogen electrode potential,
N, 3.22. IR (KBr):
n
, cmꢁ1 1594.28 (m, benzene), 1507.39 (m,
benzene), 957.41 (w, trans-CH]CH), 828.06 (w, p-substituted),
758.10 (w, p-substituted). 1H NMR (400 MHz, CDCl3, TMS):
d
7.10e7.17 (m, 6H, benze-H), 7.20, 7.22 (d, 6H, J ¼ 8.0, benze-H),
7.50, 7.52 (d, 7H, ]CHe), 7.54e7.63 (m, 6H, benze-H), 7.74e7.87
(m, 5H, benze-H), 8.12, 8.16 (d, 1H, ]CHe), 8.23e8.31 (m, 6H,
benze-H), 8.63, 8.64 (d, 4H, J ¼ 4.0, pyridine-H, benze-H). 13C
NHE). The band gap (DE) was obtained from linear absorption
spectra (absorption edge) of chromophores, while the LUMO
(lowest unoccupied molecular orbital) energy level was calculated
NMR (100 MHz, CDCl3, TMS):
d 120.93, 123.24, 123.72, 124.03,
124.18, 124.24, 124.34, 126.72, 126.72, 127.17, 127.74, 128.53,
129.82, 131.51, 131.70, 131.99, 144.90, 147.01, 150.22.
from the DE value and the HOMO [26].
2.3.3. N-(4-((E)-2-(4-Bromonaphthalen-1-yl)vinyl)phenyl)-4-((E)-
2-(4-((E)-2-(pyridin-4-yl) vinyl)naphthalen-1-yl)vinyl)-N-(4-((E)-
2-(4-((E)-2-(pyridin-4-yl)vinyl)naphthalen-1-yl)vinyl)phenyl)
benzenamine (PN-2)
Under a nitrogen atmosphere, 1.0 g (1.07 mmol) of TBPA, 1.0 g
(3.2 mmol) of tri-o-tolylphosphine, 0.25 mL (2.25 mmol) of vinyl-
pyridine, 0.0096 g of palladium(II) acetate (0.4 mmol) and 20 mL of
triethylamine was refluxed using the procedure outlined for the
preparation of PN-1. The red colored, two-branched PN-2 was
obtained (yield. 46%). M.p. 115 ꢂC. MS (MALDI) ¼ 907.5 (MþeBr,
100%). Element analysis (%): Cald. C, 82.75; H, 4.90; N, 4.26. Found.
2.2. Two-photon initiated photopolymerization
Photocurable acrylate monomer was prepared by mixing methyl
acrylate (monomer) and dipentaerythritol hexaacrylate (cross-
linker) in almost equal quantity (48/52, W/W). Then, 0.2 wt% of
Dep3e(A)nechromophore was added as a photoinitiator and
a little chloroform was used in order to control the viscosity and
increase the compatibility. A laser beam from a mode-locked Ti:
sapphire laser system (Tsunami, Spectra-Physics) with the wave-
length of 800 nm, a pulse width duration of 80 fs and a repetition

X. Wang et al. / Dyes and Pigments 88 (2011) 57e64
59
Scheme 1. The synthetic routes of multibranched chromophores PN-1, PN-2 and PN-3.
C, 82.36, H, 4.28; N, 4.58. IR (KBr):
n
, cmꢁ1 1594.13 (m, benzene),
124.29,124.42,126.20,126.42,126.80,127.25,127.82,128.73,129.89,
130.30, 131.57, 144.90, 147.01, 150.26.
1504.81 (m, benzene), 959.99 (m, trans-CH]CH), 825.89 (m, p-
substituted), 760.12 (m, p-substituted). 1H NMR (400 MHz, CDCl3,
TMS):
d
, ppm 7.11e7.19 (m, 6H, benze-H), 7.22, 7.24 (d, 6H, J ¼ 8.0,
2.3.4. Tris(4-(2-(4-(2-(Pyridin-4-yl)vinyl)naphthalen-1-yl)vinyl)
phenyl) amine (PN-3)
Under a nitrogen atmosphere, 1.0 g (1.07 mmol) of TBPA, 1.5 g
(5.08 mmol) of tri-o-tolylphosphine, 0.37 mL (3.3 mmol) of vinyl-
pyridine, 0.01 g of palladium(II) acetate (0.6 mmol) and 35 mL of
benze-H), 7.49,7.50 (d, 8H, ]CHe), 7.55e7.64 (m, 10H, benze-H),
7.73e7.89 (m, 6H, benze-H), 8.13, 8.17 (d, 2H, ]CHe), 8.25e8.33 (m,
6H, benze-H), 8.64, 8.66 (d, 4H, J ¼ 8.0, pyridine-H). 13C NMR
(100 MHz, CDCl3, TMS):
d
121.01, 123.24, 123.80, 124.03, 124.18,
1.2
1.0
0.8
0.6
0.4
0.2
0.0
70000
a: PN-1(413nm)
a
a: PN-1 (590 nm)
b: PN-2 (642 nm)
c: PN-3 (645 nm)
60000
50000
40000
30000
20000
10000
0
b: PN-2(426 nm)
b
c: PN-3(434 nm)
c
b
a
c
250 300 350 400 450 500 550 600 650 700
450 500
550
600 650
700
750 800
wavelength/nm
wavelength (nm)
Fig. 2. Linear absorption (left) and fluorescence (right) spectra of De
p
3e(A)1e3 chromophores in DMF (c ¼ 1 ꢀ 10ꢁ5 mol dmꢁ3).

60
X. Wang et al. / Dyes and Pigments 88 (2011) 57e64
triethylamine was refluxed using the procedure outlined for the
preparation of PN-1. The red colored three-branched chromophore
(PN-3) (yield. 40%). M.p. 158 ꢂC. MS (MALDI): Mþ ¼ 1010.4(Mþ,
100%), 907.3 (M- vinylpyridine, 20%). Element analysis of (%): Cald.
C, 89.08; H, 5.38; N, 5.54. Found. C, 88.58; H, 5.69; N, 5.31. IR (KBr):
n
, cmꢁ1 1591.70 (s, benzene), 1504.81 (s, benzene), 957.41 (w, trans-
CH]CH), 828.06 (m, p-substituted), 763.25 (s, p-substituted). 1H
NMR (400 MHz, CDCl3, TMS): , ppm 7.10e7.14 (m, 6H, benze-H),
d
7.22, 7.24 (d, 6H, J ¼ 8.0, benze-H), 7.50,7.51 (d, 9H, ]CHe),
7.57e7.63 (m, 12H, benze-H), 7.78e7.88 (m, 6H, benze-H), 8.13, 8.17
(d, 3H, ]CHe), 8.24e8.33 (m, 6H, benze-H), 8.63, 8.65 (d, 6H,
J ¼ 8.0, pyridine-H). 13C NMR (100 MHz, CDCl3, TMS):
d 121.09,
123.28, 124.06, 124.25, 124.46, 124.55, 126.25, 126.48, 127.87,
128.64, 130.58, 131.57, 132.62, 133.31, 136.27, 145.23, 147.02, 149.98.
3. Results and discussion
3.1. Synthesis
We have successfully synthesized three two-photon absorbing
chromophores, denoted as PN-1, PN-2 and PN-3, which contain tri-
phenylamine as electron donor, pyridyl ring as electron acceptor, and
naphthalene as p-conjugation unit. The synthetic routes of PN-1, PN-
2 and PN-3 are shown in Scheme 1. To achieve synthesis of these
chromophores, Vilsmier reaction, Heck-type and Wittig-type
condensations have been explored. With triphenylamine (1) as the
startingmaterial, 4,40-diformyl triphenylamine (2) and tris-(4-formyl-
phenyl) amine (3) were obtained under the condition of Vilsmier
reaction. Then, compound 6 was produced in high yield (80%) by
initial bromination and following Wittig reaction, with 1-bromo-4-
methylnaphthalene (4) as the starting material. Also, by Wittig
reaction of 3 with 6, TBPA was obtained in the bright yellow powders
(40%). Finally, the reaction of TBPA and vinylpyridine in the presence
of palladium(II) acetate and tri-o-tolylphosphine treated in Heck
condition produces the object products PN-1, PN-2 and PN-3.
Fig. 3. Two-photon absorption cross-sections (a) and two-photon-induced fluores-
3.2. One-photon and two-photon optical properties
cence spectra (b) of PN-1, PN-2 and PN-3 in DMF (1 ꢀ 10ꢁ4 mol dmꢁ3
)
Linear absorption spectra of chromophores (see Fig. 2, left),
show that the ICT (intramolecular charge-transfer) bands have
13 nm of red-shift from PN-1 (at 413 nm) to PN-2 (at 426 nm) and
8 nm of red-shift from PN-2 to PN-3 (at 434 nm); meanwhile, the
one-photon fluorescence (OPF) spectra, presented in Fig. 2 (right),
show the red-shift of 52 nm from PN-1 (at 590 nm) to PN-2 (at
642 nm) and of 3 nm from PN-2 to PN-3 (at 645 nm). These imply
that PN-2 possesses stronger intramolecuar charge-transfer and
delocalized electron-coupling, which have contribution to TPA
cross-section [29]. As a result (see Fig. 3a), PN-2 (162 GM) exhibits
rapid enhancement in TPA cross-section, relative to PN-1 (52 GM)
and PN-3 (165 GM). Moreover, two-photon fluorescence of PN-2
shows almost as the same as that of PN-3, which are the obvious
bathochromic shift relative to PN-1 (see Fig. 3b). We reported that
the enhancement in TPA cross-section for triphenylamine-
branching chromophore was originated from larger dipole moment
change (Dmge) and strong intramolecular charge-transfer [29]. Thus,
the Dmge values between the ground- and excited-states for
In this equation, Stoke’s shift (nabs
moment change (Dmge). The symbols
and the light speed, respectively. The symbol Z is Plank constant
divided by 2 . Orientation polarizability ( f) of solvent is defined by
Eq. (3), wherein and n are the dielectric constant and the refractive
ꢁ
nem) is related to dipole
and c are the molecular size
a
p
D
3
index of the solvent, respectively. The relationship between Stoke’s
8200
8000
7800
7600
7400
7200
7000
6800
6600
De
p3e(A)n-chromophores were testified according to Lip-
perteMataga relationship (Eq. (2)).
6400
PN-1
PN-2
ꢀ
ꢁ
6200
6000
PN-3
me Dmge
nabs
ꢁ
nem
¼
D
f þ const:
(2)
(3)
Zca3
5800
0.20
0.22
0.24
0.26
0.28
0.30
0.32
3
ꢁ 1
n2 ꢁ 1
with
D
f ¼
ꢁ
2n2 þ 1
2
3
þ 1
Fig. 4. Relationship between
Df and Stokes shift (Dn) of Dep3e(A)1e3 chromophores.

X. Wang et al. / Dyes and Pigments 88 (2011) 57e64
61
Table 1
One-photon/two-photon properties, oxidation potential and energy gap of De
p
3e(A)1e3 chromophores.
TPF
max
Compd.
lOmPaAx(nm)
lOmPaFx(nm)
Ff
s0 (ns)
knf (10ꢁ10 sꢁ1
)
HOMO (eV)
LUMO (eV)
DE (eV)
l
(nm)a
dTPA (GM)b
PN-1
PN-2
PN-3
413
426
434
590
642
645
0.375
0.102
0.093
2.42
2.42
2.43
2.5
3.7
3.7
ꢁ5.61
ꢁ5.59
ꢁ5.59
ꢁ3.07
ꢁ3.12
ꢁ3.14
2.54
2.47
2.45
583
628
628
800 (720)
800 (720)
800 (720)
52 (96)
162 (256)
175 (230)
a
800 nm laser excitation.
b
Data in bracket is obtained under the excitation of 720 nm.
shift (nabs
ꢁ
nem) and orientation polarizability (
D
f) is presented in
their Ff value decrease. On the other hand, calculated nonradiative
Fig. 4, wherein the solid lines represent the fitting results and the
slope (k) is in the order of k3 (PN-3) z k2 (PN-2) > k1 (PN-1). Because
the size (a) of PN-1, PN-2 and PN-3 is similar (see Fig. 1), the dipole
moment change (Dmge) should follow the same sequence: Dmge (PN-
3) z Dmge (PN-2) > Dmge (PN-1). It was confirmed again that the TPA
cross-section is strongly correlative to dipole moment change
decay rate constants (knr
)
showed the order of PN-1
(2.5 ꢀ 10ꢁ10 sꢁ1) < PN-2 (3.7 ꢀ 10ꢁ10 sꢁ1) ¼ PN-3 (3.7 ꢀ 10ꢁ10 sꢁ1),
which might contribute to high triplet quantum yield and usually
leads to high initiating sensitivity [23,24].
3.3. Two-photon induced polymerization
(Dmge) and strong intramolecular charge-transfer.
The fluorescence quantum yields (Ff), lifetimes (
s
) and the
As mentioned above, there are two obvious TPA regions beyond
720 nm and 800 nm, and the two-photon-induced polymerization
(TPP) experiments were performed with the mode-locked Ti-sapphire
femtosecond laser at the wavelength of 800 nm. According to the
principle of TPP, the polymerization only occurred near the vicinity of
focal point of a beam and the polymerization reaction did not take
place in the part out of the laser focus, where the resin could be easily
washed out by ethanol. As shown in Fig. 5, the acrylate monomer can
be successfully polymerized using our chromophore as initiator, and
nonradiative (knr ¼ kf (1 ꢁ Ff)/Ff) decay rate constants of chro-
mophores are presented in Table 1. As observed, the fluorescence
quantum yields reduce from PN-1 (Ff ¼ 0.375) to PN-2 (Ff ¼ 0.102)
and to PN-3 (Ff ¼ 0.093). From the point of view of molecular
structure, multibranched molecule has more tendency to twisted
configuration and might consume more excited energy, which
certainly will decrease molecular Ff value. Although all chromo-
phores in this study have three branches, the increasing vinyl-
pyridine group, the electron acceptor, on the end of each branch
extends molecular chain from PN-1 to PN-2, and to PN-3 and makes
a variety of micro-objects including micro-ox (15
size) and microstereolithography (20
m ꢀ 20
m
m ꢀ 10
mm bulk
m
mm bulk size and
Fig. 5. SEM images of R1 after TPP at the scan speed is 10 m
m sꢁ1 a and b: micron ox; c and d: Logpile structure and the corresponding magnified image, respectively.

62
X. Wang et al. / Dyes and Pigments 88 (2011) 57e64
Table 2
1
m
m ꢀ 1
mm grid size) have been fabricated with spatial resolution of
Components and two-photon-induced polymerization thresholds of resins.
micron and even submicron scale.
Resin
Initiator
Resin component (wt%)
mol (%)a
Pth/mWb
In order to investigate the two-photon photosensitivity of
chromophores, the threshold power (Pth) (defined as the average
power before being induced into the objective lens, below which
the polymer line cannot be fabricated using a linear scan speed of
MMA
DEPꢁ6A
Initiator
R1
R2
R3
R4
PN-1
PN-2
PN-3
Benzil
48.22
48.16
48.30
48.35
51.58
51.64
51.50
51.45
0.20
0.20
0.20
0.20
0.03
0.03
0.03
0.17
1.4
1.3
1.3
7.5
10
m
m sꢁ1) of polymerization was evaluated by analyzing the
polymer lines. The results obtained are presented in Table 2. As can
be seen that the Pth values of PN-1, PN-2 and PN-3 are 1.4, 1.3 and
1.3 mW, respectively. Usually, 1e5 wt% of initiator in acrylate
monomer have been used for photopolymerization [23]. Here, only
0.2 wt% (0.03 mol%) of chromophores were added in the monomer
a
Molar ratio of initiator in resin.
b
Threshold power of initiator in resin. The lowest average laser power, measured
before the objective lens, that can guarantee fabricating a solid line with a linear scan
speed of 10
m
m/s was defined as the threshold of two-photon polymerization (TPP).
140000
A
B
e
[acrylate] V %
[acrylate] V %
500000
f
e
e
75 %
66 %
120000
e: 75 %
d 66 %
c 33 %
b 15 %
a 0
d
d
d
PN-1
c
b
a
33 %
15 %
0
PN-1
g
400000
300000
200000
100000
0
100000
80000
60000
40000
20000
0
c
f
h
e 80 %
f 80 %
g 85 %
h 95 %
i 99 %
e 75 %
f 80 %
g 85 %
h 95 %
i 99 %
c
g
i
b
b
a
a
h-i
400
450
500
550
600
650
700
750
300
350
450
500
Wavelength (nm)
Wavelength (nm)
400000
350000
300000
250000
200000
150000
100000
50000
0
C
D1200000
e
[acrylate] V %
[acrylate] V %
e
e
d
c
b
a
75 %
66 %
33 %
15 %
0
e: 75 %
d 66 %
c 33 %
b 15 %
a 0
d
PN-2
1000000
800000
600000
400000
200000
0
PN-2
f
f
d
e 75 %
f 80 %
g 85 %
h 95 %
i 99 %
g
e 75 %
f 80 %
g 85 %
h 95 %
i 99 %
c
c
g
b
b
h
h
i
i
a
a
300
350
400
450
500
550
450
500
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
160000
140000
120000
100000
80000
60000
40000
20000
0
400000
F
e
E
[acrylate] V %
[acrylate] V %
e
f
e 75 %
d 66 %
c 33 %
b 15 %
a 0
e 75 %
d 66 %
c 33 %
b 15 %
a 0
PN-3
320000
240000
160000
80000
0
PN-3
d
g
e 75 %
f 80 %
g 85 %
h 95 %
i 99.0 %
d
e 75 %
f 80 %
g 85 %
h 95 %
i 99 %
f
c
g
h
c
h
b
b
a
i
i
a
300
350
400
450
500
550
450
500
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
Fig. 6. Fluorescence emission (A, C, E) and fluorescence excitation (B, D, F) spectra of PN-1, PN-2 and PN-3 in the presence of acrylate monomer with different concentrations [23,24].

X. Wang et al. / Dyes and Pigments 88 (2011) 57e64
63
(R1eR3) for TPP reaction. Furthermore, in the control experiment,
0.2 wt% (0.17 mol%) of commercial benzil was added in resin (R4)
and the threshold power of 7.5 mW was obtained. Evidently,
chromophores are efficient TPP photoinitiators in comparison with
commercial benzil.
study have three branches, the increasing vinylpyridine groups
from PN-1, PN-2 and to PN-3 decrease the molecular fluorescence
quantum yield, which is desired for photoinitiation. The threshold
powers of chromophores (w1.3 mW) were much lower than that of
commercial benzil (7.5 mW) under identical experimental condi-
It was reported that the two-photon induced polymerization of
acrylate monomer proceeds to a radical mechanism via either
energy-transfer or electron-transfer between initiator and mono-
mer [10]. Fluorescence emission and excitation spectra of PN-1, PN-
2 and PN-3 with and without acrylate monomer are presented in
Fig. 6(AeF), wherein the changes of intensity and position distinctly
expressed the intermolecular interaction existence. One can see
that with increasing [acrylate] from step a to e [30], the fluores-
cence intensities of PN-1 are enhanced concomitant redshifted
peaks from 532e576 nm (see Fig. 6A). Noted that the concentra-
tions of PN-1 are the same (3.3 ꢀ 10ꢁ6 mol dmꢁ3) while those of
acrylate are increased from step a to e. The fact of the increasing
fluorescence from a to e can be interpreted the molecular rotation
suppressing and the complanation resulting from the solvent
viscosity. Continuing addition of acrylate into the solution from
step e to i [31], however, the fluorescence quenching was observed,
with the position blueshifted, implying that the interactions within
steps (eei) and (aee) are different. These can be confirmed by the
corresponding fluorescence excitation spectra (see Fig. 6B). Firstly,
one can see that PN-1 displayed dual-fluorescence excitation peaks
locating at w350 nm and w450 nm, similar to the absorption
spectra (Fig. 2, left). From step a to e (Fig. 6B), the intensities in
fluorescence excitation spectra were increased up to the maximum;
and then rapidly decreased from e to i. Meantime, the dual-fluo-
rescence spectra locating at w350 nm and w450 nm meta-
morphose to the single fluorescence located at 420 nm. Evidently,
the amount of acrylate resin have markedly different influence
upon the chromophore: that is, the fluorescence of PN-1 shows
enhancement when [acrylate] is less than 75% (V%); further
increasing [acrylate], the fluorescence decreases, accompanying the
changes of maximum position and shape. Similar results of PN-2
and PN-3 have been presented in Fig. 6CeF. Although the emission
quenching does not imply the photoinitiation, emission quenching
is the consequence of strong interaction between monomer and
initiator, such as photoinitiation [10,32]. Since the HOMOeLUMO
gap of chromophores (2.45e2.54 eV) (see Table 1) is less than that
of MMA monomer (4.1 eV) [33] the energy-transfer from the
former to monomer is forbidden. Considering that two-photon
absorbing chromophores usually exhibit large dipolar moment
change (Dmge), according to LipperteMataga relationship (Eq. (2)),
and strong intramolecular charge-transfer [29]. they would possess
strong intermolecular charge-transfer. This is reasonable since the
tions. Microstructures of micro-ox size of 15
microstereolithography with 20
m ꢀ 20
m ꢀ 1 m grid size were fabricated. Intermolecular charge-
transfer from De 3e(A)n initiator to acrylate monomer was
deduced within the two-photon polymerization reaction, proved
by the fluorescence quenching, electrochemistry measurements
and LipperteMatag equation
m
m ꢀ 10
mm and
m
mm bulk size and
1
m
m
p
Acknowledgements
The authors are grateful to the National Natural Science Foun-
dation of China (Grant Nos. 50673070, 50973077 and 50773091),
Key Project of Chinese Ministry of Education (No.
204053),The National Basic Research Program of China
(2010CB934103), and Science and technology development Project
of Suzhou (SYJG0931) for financial supports.
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¼
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ꢀ
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;
g: [acrylate]
¼
85%, [PN- n]
¼
1.4
ꢀ
10ꢁ6
; h:
[acrylate] ¼ 95%, [PN- n] ¼ 4.0 ꢀ 10ꢁ7; i: [acrylate] ¼ 99%, [PN- n] ¼ 1.0 ꢀ 10ꢁ7
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