Synthesis, characterization, and non-linear optical properties of two new symmetrical two-photon photopolymerization initiators

Article abstract of DOI:10.1071/CH04111

Two new symmetrical two-photon free-radical photopolymerization initiators, 1,4-bis-{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-2,5-bisdimethoxybenzene 6 and 1,4-bis-{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-2,5-bisdodecyloxybenzene 7, were synthesized using

Full text of DOI:10.1071/CH04111

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2005, 58, 29–33
Synthesis, Characterization, and Non-Linear Optical Properties of Two
New Symmetrical Two-Photon Photopolymerization Initiators
YunxingYan,A,C Xutang Tao,A Guibao Xu,A Huaping Zhao,A Yuanhong Sun,B
Chuankui Wang,B Jiaxiang Yang,A Xiaoqiang Yu,A Xian Zhao,A and Minhua JiangA
A State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China.
B Department of Physics, Shandong Normal University, Jinan 250014, China.
C Corresponding author. Email: yxyan@icm.sdu.edu.cn
Two new symmetrical two-photon free-radical photopolymerization initiators, 1,4-bis-{2-[4-(2-pyridin-
4-ylvinyl)phenyl]vinyl}-2,5-bisdimethoxybenzene 6 and 1,4-bis-{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-2,5-
bisdodecyloxybenzene 7, were synthesized using an efficient Wittig and Pd-catalyzed Heck coupling methodology.
One-photon fluorescence, one-photon fluorescence quantum yields, one-photon fluorescence lifetimes, and two-
photon fluorescence have been investigated. Experimental results show that both compounds were good two-photon
absorbing chromophores and effective two-photon photopolymerization initiators. Two-photon polymerization
microfabrication experiments have been studied and the possible photopolymerization mechanism is discussed.
Manuscript received: 26 June 2004.
Final version: 9 November 2004.
Introduction
of high transmission in the visible spectrum (400–800 nm).
Although symmetrical molecules (D-π-D) show very small
dipole moments in the ground state, quantum chemical calcu-
lations have confirmed that there is substantial change in both
the transition dipole moment and quadrupole moment[10]
upon excitation. Such changes assist intermolecular charge
transfer, which increases the two-photon absorptivity. There-
fore, the synthesis of this type of symmetric chromophore
has practical application. Here we report two new sym-
metrically substituted stilbene-type chromophores 6 and 7
synthesized by means of an efficient Wittig and Pd-catalyzed
Heck coupling methodology. Experimental results show
that the two compounds are good two-photon absorbing
chromophores and effective two-photon photopolymeriza-
tion initiators. Both chromophores can be used as initiators
to fabricate three-dimensional polymerized structures under
irradiation of a Ti:sapphire femtosecond laser at 830 nm. The
possible photopolymerization mechanism is discussed.
Organic materials possessing large two-photon absorption
(TPA) cross-sections have a variety of technical applications,
such as optical power limiting,[1,2] two-photon fluorescence
microscopy,[3] frequency-upconverted lasing,[4] three-
dimensional (3D) optical data storage, and microfabrica-
tion.[5–8] These applications take advantage of the fact that
the TPA depends quadratically on the excitation intensity,
so under tight-focussing conditions, absorption at the focus
is confined to a volume in the order of λ3 (where λ is the
excitation wavelength). Three-dimensional microfabrication
has been demonstrated using two-photon-initiated polymeri-
zation of resins incorporating conventional ultraviolet-
absorbing initiators. However, the TPA cross-sections, δ, of
most initiators including commercial dyes are typically very
small, and as a result they exhibit low two-photon sensitivity.
Resins containing these initiators can be polymerized only
by means of long exposure times and high excitation inten-
sities that frequently result in damage to the structure of the
polymerized resin.
Some approaches towards improving the photosensitivity
of photointiator molecules are based on the idea of increasing
their two-photon absorption cross-sections.[9] Several chro-
mophores with large two-photon absorption cross-sections
have been identified by theoretical and experimental
studies.[10–14] Symmetrically substituted stilbene-type chro-
mophores (D-π-D) are the most widely investigated com-
pounds in these studies, because their two-photon absorption
cross-section values are larger than their respective asym-
metric counterparts,[14] and because they have the advantage
Experimental
Chemicals
4-Vinylpyridine (95%), palladium(ii) acetate (47.5% Pd), and
4-bromobenzaldehyde were purchased from Acros Organics. Tri-o-
tolylphosphine and 4-(N,N-diphenylamino)benzaldehyde were pur-
chased fromTokyo Kasei Kogyo Co.These chemicals were used without
further purification.
Instrumentation
1H NMR and 13C NMR spectra were obtained on a BrukerAV600 spec-
trometer. Elemental analysis was carried out on a Perkin–Elmer PE 2400
elemental analyzer. UV-Vis–near-IR spectra were measured on a Hitachi
© CSIRO 2005
10.1071/CH04111
0004-9425/05/010029

30
Y. Yan et al.
(a)
BrH2C
Br
BrPh3PH2C
Br
1
OCH3
CH2PPh3Cl
O(CH2)11CH3
CHO
ClPh3PH2C
H3CO
OHC
H3C(H2C)11
O
2
3
(b)
OCH3
(c)
O(CH2)11CH3
Br
Br
Br
Br
H3C(H2C)11
O
H3CO
4
5
(d )
(d)
O(CH2)11CH3
OCH3
N
N
N
N
H3C(H2C)11
O
H3CO
6
7
Scheme 1. Synthesis of two compounds: (a) PPh3/CCl4/BPO, reflux; (b) THF/4-bromobenzaldehyde,
grind, room temp.; (c) THF/1/grind, room temp.; (d) vinylpyridine/tri-o-tolylphosphine/palladium(ii)
acetate/triethylamine, reflux.
U-3500 recording spectrophotometer. Melting points were measured
on a Mettler-Toledo DSC822e differential calorimeter. Steady-state
fluorescence spectra were measured on an Edinburgh FLS920 spec-
trofluorimeterwitha450WXearclampasthe400 nmexcitationsource.
Spectra were recorded between 420 and 700 nm at a resolution of 0.1 nm
using a photomultiplier tube detector in single-photon counting mode.
The quartz cuvettes had a 1 cm path length.
resulting solution was chromatographed on silica gel using ethyl acetate
as eluent to produce a red compound in 34% yield. Found: C 84.0,
H 6.0, N 5.2. Calc. for C38H32N2O2: C 83.2, H 5.8, N 5.1. δH (CDCl3,
600 MHz) 8.58 (4H, q, J 5.2), 7.56 (3H, q, J 10.9), 7.44 (3H, t, J 7.6),
7.36 (7H, m), 7.15 (1H, t, J 9.1), 7.02 (4H, d, J 16.2), 6.80 (2H, d, J 7.3),
6.75 (2H, t, J 12.3), 6.66 (2H, t, J 12.8), 3.90 (3H, s), 3.54 (3H, s).
δC (600 MHz, CDCl3) 152.3, 151.6, 146.3, 146.1, 139.9, 139.6, 136.4,
134.5, 134.3, 131.3, 131.2, 131.0, 129.9, 128.9, 128.5, 128.3, 128.3,
127.8, 127.5, 127.4, 127.2, 127.1, 125.4, 122.3, 122.3, 114.8, 114.3,
110.3, 57.7, 57.4.
Synthesis
The synthetic sequence for the preparation of our target molecules
is shown in Scheme 1. Triphosphonium chloride 2 was synthesized
in our laboratory. 4-Bromobenzyl(triphenyl)phosphonium bromide 1
and 2,5-bisdodecyloxybenzene-1,4-dicarbaldehyde 3 were synthesized
according to reported methods.[15,16]
1,4-Bis{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-2,5-
bisdodecyloxybenzene 7
The compound was obtained by a similar synthetic method as for
compound 6 in 39% yield. Found: C 84.8, H 9.0, N 3.4. Calc. for
C60H76N2O2: C 84.1, H 8.9, N 3.3. δH (CDCl3, 600 MHz) 8.59 (4H, d,
J 4.0), 7.57(10H, m), 7.41 (4H, d, J 4.7), 7.36 (1H, s), 7.33 (1H, s), 7.19
(1H, s), 7.15 (3H, d, J 2.2), 7.05 (2H, d, J 16.2), 4.09 (4H, t, J 6.4), 1.91
(4H, m), 1.58 (4H, m), 1.41 (32H, m), 0.88 (6H, t, J 6.8). δC (600 MHz,
CDCl3) 151.3, 149.6, 138.7, 135.1, 133.3, 128.2, 127.5, 127.0, 125.4,
124.3, 120.9, 114.6, 114.4, 110.7, 69.6, 32.0, 29.7, 29.7, 29.6, 29.6,
29.5, 29.5, 29.4, 26.3, 22.7, 14.1.
1,4-Bis-{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-2,5-
bisdimethoxybenzene 6
At room temperature, 4-bromobenzaldehyde (1.85 g, 0.01 mol),
triphosphonium chloride 2 (3.16 g, 0.004 mol), NaOH (8.00 g, 0.2 mol),
and tetrahydrofuran (2 mL) were placed in a mortar. After being ground
for about 5 min, the mixture was poured into 200 mL distilled water,
then neutralized with dilute hydrochloric acid, and extracted with
dichloromethane. The organic layer thus obtained was dried over anhy-
drous magnesium sulfate. After filtration, the solvent was removed
from the solution at reduced pressure and the residue dissolved in
dichloromethane. This solution was separated by chromatography on
silica gel using ethyl acetate/petroleum ether (1 : 20) as eluent.The green
crystalsobtainedwereaddedtoamixtureoftri-o-tolylphosphine(1.96 g,
6.44 mmol), vinylpyridine (3.48 mL, 32.24 mmol), palladium(ii) acetate
(0.18 g, 0.8 mmol), and redistilled triethylamine (100 mL). The mix-
ture was refluxed in an oil bath under nitrogen. An orange product
was obtained after heating and stirring for 36 h. The solvent was then
removed under reduced pressure and the residue was dissolved in
dichloromethane, washed three times with distilled water, dried with
anhydrous magnesium sulfate, and then filtered and concentrated. The
Results and Discussion
Linear Absorption and Fluorescence Spectra
The photophysical properties of compounds 6 and 7 are
summarized in Table 1. Linear absorption spectra were mea-
sured in solvents of different polarity at a concentration c of
10−5 mol L−1. At this concentration, there is no influence of
solvent on the spectra. The one-photon fluorescence spectra
were measured at the same concentrations as for the linear
absorption spectra. The maximum excitation wavelengths of
6 and 7 were 420 and 430 nm, respectively. Fig. 1 shows the

Two New Symmetrical Two-Photon Photopolymerization Initiators
31
Table 1. Solvent effects on the absorption spectra and one- and two-photon fluorescence spectra of compounds 6 and 7
(1f)
(2)
max
λ(m1aax), λmax , and λ
are one-photon absorption, one-photon fluorescence, and two-photon fluorescence maxima peaks, respectively.
Φ is the one-photon quantum yield determined using coumarin 307 as the standard;[21] τ is the one-photon fluorescence lifetime;
ε is the relative permittivity at 20◦C;[22] εres is the corresponding molar absorption coefficient
(1f)
max
(1a)
max
(2)
Cpds
Solvent
ε
λ
[nm]
εres/104
λ
[nm]
τ [ns]
Φ
λ [nm]
max
6
DMF
Benzyl alcohol
THF
CH2Cl2
CHCl3
DMF
Benzyl alcohol
THF
CH2Cl2
CHCl3
37.6
13.1
7.58
9.1
4.806
37.6
13.1
7.58
9.1
4.806
415
4.36
5.51
6.62
6.02
5.78
3.52
4.87
5.54
5.38
5.00
518
1.67
1.42
1.52
1.56
1.41
1.65
1.52
1.38
1.43
1.35
0.66
0.76
0.60
0.63
0.66
0.65
0.75
0.58
0.61
0.63
429
420
419
423
421
433
426
425
428
520
510
512
506
520
522
512
514
508
525
7
529
linear absorption and one-photon fluorescence spectra of the
two compounds in chloroform. Table 1 is organized such
that the polarity of solvents decreases gradually from DMF
(3.82 D) to CHCl3 (1.01 D). Table 1 shows that for both com-
pounds the maximum absorption peaks show a slight blue
shift, the maximum fluorescence peaks show a clear red shift,
andthefluorescencelifetimesarelengthenedwithanincrease
in solvent polarity, with the exception of benzyl alcohol. This
effect can be explained by the fact that the excited state of
two compounds may produce a higher polarity than that of
the ground state, because the solvatochromism is associated
with energy level lowering.An increased dipole–dipole inter-
action between the solute and solvent leads to a lowering of
the energy level.[17,18] For benzyl alcohol, hydrogen bond-
ing between the solvent and solute molecules decreases the
excitation energy.[19] It is should noted that both the largest
one-photon oscillator strength and the maximum two-photon
absorption cross-section occur in the same excited state,
the charge-transfer state, in the lower-energy region because
of the symmetrical characteristics of the present molecules.
Therefore, we studied the two-photon fluorescence spectra,
as described in the following Section.
0.8
a
b
c
d
e
f
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
300
400
500
600
700
800
900
1000
Wavelength (nm)
Fig. 1. Absorption and fluorescence spectra of 6 and 7. The lin-
ear absorption and one-photon fluorescence spectra were measured in
chloroform at a concentration of 1 × 10−5 mol L−1. Two-photon fluo-
rescence spectra were measured in chloroform with the concentration of
1 × 10−5 mol L−1. Spectra a, c, and e are linear absorption and one- and
two-photon fluorescence spectra of 6, respectively; spectra b, d, and f
are linear absorption and one- and two-photon fluorescence spectra of
7, respectively.
Two-Photon Fluorescence Spectra
dodecyl group linked to the O atom is larger than that of
methyl linked to O.
The two-photon fluorescence spectra of 6 and 7 in chlo-
roform are also shown in Fig. 1 at concentrations c of the
order of 10−3 mol L−1. Spectra were recorded with a mode-
locked Ti:sapphire laser (Coherent Mira 900F) pump source
with a 200 fs pulse duration, a repetition rate of 76 MHz,
and a single-scan streak camera (Hamamatsu C5680-01) and
monochromator. Excitation wavelengths for 6 and 7 were
all 830 nm. Table 1 shows that the maximum peak of the
two-photon fluorescence has a slight red shift compared with
that of one-photon fluorescence in chloroform for each com-
pound. This can be ascribed to the effect of reabsorption for
the linear absorption band having a slight overlap with the
emission band, and our two-photon fluorescence experiments
were carried out on concentrated solutions that made reab-
sorption significant.[23] Table 1 also shows that the maximum
peak positions in the absorption and fluorescence spectra
of 7 show a red shift compared with those of 6. This can
be explained by the fact that the electron donor strength of
In addition to the similarities between one-photon flu-
orescence and two-photon fluorescence, the two-photon flu-
orescence peak positions are also independent of the laser
wavelength used.Thus, although the electrons can be pumped
to the different excited states by linear absorption or two-
photon absorption due to the different selection rules, they
would finally relax to the same lowest excited state by internal
conversion and/or vibrational relaxation.
Two-Photon Photopolymerization
Our two-photon initiating photopolymerization experiments
have been accomplished using 6 or 7 as an initiator. The
two-dimensional lattice was fabricated with negative resins
system, which contained oligomers (bisphenyl A epoxide
dimethylacylate) and 6 or 7 (5%) as an initiator and a small
amount of dichloroethane (increasing the compatibility and

32
Y. Yan et al.
controlling the viscosity). The 830 nm laser source from the
mode-locked Ti:sapphire was tightly focussed through an
objective lens (×40, NA 0.65) onto the sample film mounted
on the xy-step monitorized stage controlled by a computer.
The pulse energy after being focussed by the objective lens
was approx. 10 mW. Unreacted liquid was washed out to
obtain the polymerized grating shown in the micrograph in
Fig. 2.
The photopolymerization mechanism of the two new ini-
tiators is still unclear. According to Cumpston et al.,[5] strong
donor substituents would make the conjugated system elec-
tron rich, so that electrons may be transferred to relatively
weak acceptor molecules after one- or two-photon excita-
tion. This process may activate the polymerization reaction.
In order to demonstrate this process, we performed ab initio
calculations at time-dependent hybrid density functional the-
ory B3LYP level in the Gaussian package for 6, showing that
the first excited state is the charge-transfer (CT) state with
the excited energy λ of 493 nm. When the molecule is irra-
diated by 830 nm laser, it can be expected that the molecule
will simultaneously absorb two photons and is excited to the
first excited state (the CT state). For a better understanding
of the charge-transfer process, we have plotted the charge
density difference between the ground state and the CT state
for molecule 6 in the gas phase (see Fig. 3) by the Molekel
program.[25] It can be seen that upon excitation, charges are
mainly transferred from the acceptor side (pyridine side) to
the donor side (the central parts) of the molecule. In the CT
state, there are more electrons on the central parts of the
molecule, indicating that an electron may be more readily
released from the molecule to the surroundings. This picture
seems to support the conclusion of Cumpston et al.[5] How-
ever, whether the photo-induced electron-transfer reaction is
energetically feasible needs further theoretical investigation.
Conclusions
Two new symmetrical two-photon photopolymerization ini-
tiators, 6 and 7, have been synthesized and characterized.The
one-photon fluorescence quantum yields, lifetimes, and sol-
vent effects of these initiators are presented in detail. Both
compounds exhibit large delocalized π-electron conjugated
systems. Experimental results show that the two compounds
are good two-photon absorbing chromophores and effec-
tive two-photon photopolymerization initiators. A possible
photopolymerization mechanism involving a charge-transfer
process under laser irradiation is discussed.
Acknowledgments
This work was supported by grants from the state National
Natural Science Foundation of China (grant nos 50323006,
50325311, and 10274044) and the Swedish International
Development Cooperation Agency (SIDA).
6␮m
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