Synthesis and nonlinear optical properties of novel multibranched two-photon polymerization initiators

Article abstract of DOI:10.1039/b403777d

Efficient Witting and Pd-catalyzed Heck coupling methodology are employed to synthesize three multibranched two-photon photopolymerization initiators diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (abbreviated to DPVPA), phenyl-bis-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]- vinyl}-phenyl)-amine (abbreviated to BPVPA) and tris-(4-{2-[4-(2-pyridin-4-yl- vinyl)-phenyl]-vinyl}-phenyl)-amine (abbreviated to TPVPA). The experimental results confirm that all these compounds are good two-photon absorbing chromophores and operative two-photon photopolymerization initiators. The calculated two-photon absorption cross sections of DPVPA, BPVPA and TPVPA for the lowest excited state are 59.3, 30.2 and 42.4 × 10^-50 cm⁴ s photon^-1, respectively. A microstructure using BPVPA as initiator has been fabricated under irradiation of 200 fs, from a 76 MHz Ti:sapphire femtosecond laser at 820 nm. The possible photopolymerization mechanism is discussed.

Full text of DOI:10.1039/b403777d

View Article Online / Journal Homepage / Table of Contents for this issue
A R T I C L E
Synthesis and nonlinear optical properties of novel multi-
branched two-photon polymerization initiators
Yun-Xing Yan,*a Xu-Tang Tao,a Yuan-Hong Sun,b Chuan-Kui Wang,b Gui-Bao Xu,a
Jia-Xiang Yang,a Yan Ren,a Xian Zhao,a Yong-Zhong Wu,a Xiao-Qiang Yua and
Min-Hua Jianga
aState Key Laboratory of Crystal Materials, Shandong University, Jinan 250100,
P. R. China. E-mail: yxyan@icm.sdu.edu.cn
bDepartment of Physics, Shandong Normal University, Jinan 250014, P. R. China
Received 11th March 2004, Accepted 17th June 2004
First published as an Advance Article on the web 13th August 2004
Efficient Witting and Pd-catalyzed Heck coupling methodology are employed to synthesize three multi-
branched two-photon photopolymerization initiators diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-
phenyl)-amine (abbreviated to DPVPA), phenyl-bis-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine
(abbreviated to BPVPA) and tris-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (abbreviated to
TPVPA). The experimental results confirm that all these compounds are good two-photon absorbing
chromophores and operative two-photon photopolymerization initiators. The calculated two-photon
absorption cross sections of DPVPA, BPVPA and TPVPA for the lowest excited state are 59.3, 30.2 and
42.4 6 10250 cm4 s photon21, respectively. A microstructure using BPVPA as initiator has been fabricated
under irradiation of 200 fs, from a 76 MHz Ti:sapphire femtosecond laser at 820 nm. The possible
photopolymerization mechanism is discussed.
also confirmed this conclusion.27 In this article, we report the
1. Introduction
synthesis and characterization of three new initiators DPVPA,
BPVPA and TPVPA with respective one-, two-, and three-
branched structures by use of an efficient Witting and Pd-
catalyzed Heck coupling methodology. All these dyes show
strong two-photon fluorescence. A microstructure has been
fabricated by use of BPVPA as initiator excited with a 820 nm,
200 fs, 76 MHz Ti:sapphire laser source and the possible inter-
pretation is discussed in terms of the charge-transfer process
when the laser irradiation is applied.
Extensive attention has been paid to the photochemical and
photophysical processes induced by two-photon absorption
due to their potential use in applications such as optical power
limiting,1–3 two-photon upconversion lasing,4,5 two-photon
fluorescence excitation microscopy,6–10 and three-dimensional
optical data storage and microfabrication.11–16 These applica-
tions take advantage of the fact that the two-photon absorp-
tion (TPA) probability depends quadratically on intensity, so
under tight-focusing conditions, the absorption is confined at
the focus to a volume of the order of l3 (where l is the laser
wavelength). This characteristic of two-photon processes has
made TPA-based photopolymerization an effective tool for
processing photonic devices, micromachines, zero-threshold
lasers and integrated optical waveguides in three dimensions
with high spatial resolution.17–20
For two-photon free radical photopolymerization, the poly-
merization rate and photo-initiation threshold is closely related
to the absorption behavior of the initiator at the excitation
wavelength that is generally twice the corresponding one-
photon absorption wavelength. Up until now, effective
initiators are still few because the design strategy, or in other
words, the relationship of the initiator structures and their
properties for initiating two-photon photopolymerization,
needs to be investigated in depth.
2. Experimental
2.1 Chemicals
4-Vinylpyridine (95%), palladium(II) acetate (47.5% Pd) and
triphenylamine are from Acros Organics, and tri-o-tolylphos-
phine is purchased from Tokyo Kasei Kogyo Co., Ltd. The
above chemicals are used without further purification.
Dichloromethane, THF, chloroform, ethyl acetate and
petroleum benzene are anhydrous grade after further purifica-
tion. Moreover, each of the solutions used for the one-photon
fluorescence measurements is freshly prepared and kept in
the dark before measurement. The solution concentration is
1 6 1025 mol L21
.
Some approaches to improve the photosensitivity of photo-
initiators are based on the idea of increasing their TPA cross
sections.21 According to Albota et al.22 and Reinhardt et al.,23
the conjugation length, donor and acceptor strength, and
planarity of the p center are important parameters for enhan-
cing the TPA cross sections. Based on this design strategy,
some one-dimensional two-photon polymerization initiators
with charge-transfer conjugated character are synthesized.24
Recently, ab initio calculations have been carried out for
increasing dimensionality of the charge-transfer networks and
shown that the TPA cross sections of multi-branched molecules
can strongly be enhanced compared with those of their one-
branched counterparts.25,26 Experimental measurements have
2.2 Instruments
The 600 MHz H1 NMR spectra and 13C NMR are obtained on
a Bruker av600 spectrometer. Elemental analysis is performed
using a PE 2400 elemental analyzer. UV-Vis-near-IR spectra
are measured on a Hitachi U-3500 recording spectrophoto-
meter. Melting point is measured on DSC822e Mettler-Toledo
instruments. The steady-state fluorescence spectra measure-
ment is performed with use of an Edinburgh FLs920 spectro-
fluorimeter. A 450 W Xe arc lamp provides the y400 nm
excitation source. Spectra are recorded between 420 and 700 nm
using a photomultiplier tube as detector, which is operated in
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 9 5 – 3 0 0 0
2 9 9 5

View Article Online
the single photon counting mode. The spectral resolution is
0.1 nm. The quartz cuvettes are of 1 cm path length.
{4-[2-(4-Bromo-phenyl)-vinyl]-phenyl}-diphenyl-amine(abbre-
viated to BVDA). At room temperature, 2.73 g (0.01 mol)
4-(N,N-diphenylamino)benzaldehyde, 7.68
4-bromobenzyl(triphenyl)phosphonium bromide, 8.00
g (0.015 mol)
g
2.3 Synthesis
(0.2 mol) NaOH and 2 mL THF are added into a mortar.
Then the mixture is ground. It became orange after 5 min. The
mixture is poured into 200 mL distilled water, neutralized with
dilute hydrochloric acid, and extracted with dichloromethane.
The organic layer obtained is dried over anhydrous magnesium
sulfate. After filtration, the solvent is removed from the
The synthetic sequence for the preparation of three compounds
is shown in scheme 1.
4-Formyltriphenylamine, 4,4’-diformyltriphenylamine, tris-
(4-formylphenyl)amine and 4-bromobenzyl(triphenyl)phospho-
nium bromide are synthesized according to reported methods.24,28
Scheme 1 Synthesis of three compounds: (a) First step: DMF, 0 uC, second step: POCl3, 95–100 uC. (b) THF/4-bromobenzyl(triphenyl)-
phosphonium bromide, room temperature. (c) Vinylpyridine/tri-o-tolylphosphine/palladium(II) acetate/triethylamine, reflux.
2 9 9 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 9 5 – 3 0 0 0

View Article Online
solution at reduced pressure and the residue dissolved in
dichloromethane. The solution obtained is chromatographed
on silica gel using petroleum ether as eluent. The green
compound is crystallized in ethyl acetate with a yield of 75%
and m.p. 179.6 uC. The enthalpy is 289.8 mJ mg21. Anal.
Calcd for C26H20NBr: C, 73.24; H, 4.69; N, 3.29. Found: C,
73.31; H, 4.75; N, 3.34. 1H NMR (600 MHz, CDCl3) [ppm] d:
7.46 (d, J ~ 8.3 Hz, 2H), 7.36 (m, 4H), 7.27 (t, J ~ 7.7 Hz, 4H),
7.12 (d, J ~7.9 Hz, 4H), 7.04 (m, 5H), 6.92 (d, J ~ 16.2 Hz,
1H). 13C NMR (600 MHz, CDCl3) [ppm] d: 147.78, 147.61,
136.77, 131.78, 131.18, 129.33,129.09, 127.79, 127.47, 125.81,
124.68, 123.48, 123.21, 120.93.
Table 1 The absorption data, one- and two-photon fluorescence
spectra with solvent effects of three compounds
Compounds Solvents
lmð1aaxÞ/nm e/104 lðm1afÞx/nm t/ns lmð2afÞx/nm
DPVPA
BPVPA
TPVPA
DMF
CH2Cl2
Benzyl alcohol 409
397
399
5.76 570
4.80 540
4.28 558
5.39 536
4.66 520
7.13 578
6.39 546
4.41 564
6.21 542
5.03 526
8.35 582
7.86 550
6.83 568
7.69 546
7.23 528
2.44 559
2.19 542
1.01 574
1.93 538
1.74 523
1.43
1.22
1.12
1.19
1.13 529
1.35
1.17
1.39
1.17
THF
398
401
412
416
CHCl3
DMF
CH2Cl2
Benzyl alcohol 427
THF
CHCl3
DMF
CH2Cl2
Benzyl alcohol 434
THF
CHCl3
415
419
419
423
Diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-
amine (DPVPA). 2.13 g (5 mmol) {4-[2-(4-bromo-phenyl)-
421
425
vinyl]-phenyl}-diphenyl-amine,
0.65
g
(2.15
mmol)
1.10 534
tri-o-tolylphosphine, 1.16 mL (10.75 mmol) vinylpyridine,
0.06 g palladium(II) acetate (0.27 mmol) and 100 mL redistilled
triethylamine under nitrogen, are added to a three-necked flask
equipped with a magnetic stirrer, a reflux condenser, and a
nitrogen input tube. The reaction mixture is refluxed in an oil
bath under nitrogen. An orange product is obtained after
heating and stirring for 24 h. Then the solvent is removed under
reduced pressure and the residue is dissolved in methylene
chloride, washed three times with distilled water, and dried
with anhydrous magnesium sulfate. Then it is filtered and
concentrated. The resulting solution is chromatographed on
silica gel using ethyl acetate/petroleum ether (1:1) as eluent. The
orange compound is crystallized using ethyl acetate with a yield
ð1aÞ
ð1fÞ
ð2fÞ
and l are one-photon absorption, one-photon fluores-
max
a l , l
max
max
cence and two-photon fluorescence maxima peaks, respectively. t is
the one-photon fluorescence lifetime.
3. Results and discussion
3.1 One- and two-photon fluorescence
The photophysical properties of these compounds are sum-
marized in Table 1. The linear absorption spectra of these
compounds are measured in solvents of different polarity at a
concentration of c ~ 1 6 1025 mol L21, in which the solvent
influence is not included. The one-photon fluorescence spectra
are measured with the same concentration as that of the linear
absorption spectra. From Table 1, one can see that the
maximum absorption peaks show a slightly blue-shift, while
the maximum fluorescence peaks clearly show a red-shift, and
the fluorescent lifetime are lengthened except for benzyl alcohol
with the increase of the polarity of the solvent for each
compound. This can be explained by the fact that the excited
state of these compounds may possess higher polarity than the
ground state, since the solvatochromism is associated with the
energy level lowering. Increasing dipole–dipole interaction
between the solute and solvent leads to lowering the energy
level greatly.29,30 Fig. 1 and Fig. 2 are the linear absorption and
one-photon fluorescence in chloroform of these compounds,
respectively. The maximum excited wavelengths of DPVPA,
BPVPA and TPVPA are 400, 410 and 420 nm, respectively.
From Fig. 1 and Fig. 2, one can see that the linear absorption
peaks and one-photon fluorescence peaks are all red-shifted in
the same solvent with increasing molecular dimensionality.
of 60% and m.p. 229.0 uC. The enthalpy is 293.43 mJ mg21
.
Anal. Calcd for C33H26N2: C, 88.00; H, 5.78; N, 6.22%. Found:
C, 88.15; H, 5.81; N, 6.26%. 1H NMR (600 MHz, CDCl3)
[ppm] d: 8.59 (d, J ~ 4.0 Hz, 2H), 7.54 (s, 4H), 7.41 (d,
J ~ 8.9Hz, 4H), 7.30 (m, 4H), 7.14 (d, J ~ 7.7 Hz, 4H), 7.11(s,
1H), 7.07 (t, J ~ 7.2 Hz, 4H), 7.03(s, 2H), 7.00 (s, 1H). 13C
NMR (600 MHz, CDCl3) [ppm] d: 149.55, 147.65, 147.46,
145.25, 138.41, 134.88, 133.33, 131.13, 129.31, 128.9 8, 127.48
(CH), 127.46, 126.72, 126.18, 125.27, 124.62, 123.36, 123.17,
120.90.
Phenyl-bis-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-
amine (abbreviated to BPVPA). It is obtained by a similar
method to DPVPA. The red compound was purified by column
chromatography on silica gel using ethyl acetate/alcohol (1:1)
as eluent with a yield of 47% and m.p. 300.2 uC. The enthalpy is
232.61 mJ mg21. Anal. Calcd for C48H37N3: C, 87.94; H, 5.65;
1
N, 6.41%. C, 87.57; H, 5.41; N, 6.25%. H NMR (600 MHz,
CDCl3) [ppm] d: 8.59 (d, J ~ 4.0 Hz, 4H), 7.53 (t, J ~ 4.9 Hz,
6H), 7.44 (m, 4H), 7.41(s, 1H), 7.38(d, J ~ 4.3 Hz, 3H), 7.36 (d,
J ~ 5.0 Hz, 2H), 7.33 (d, J ~ 3.2 Hz, 1H), 7.29 (m, 3H), 7.19 (t,
J ~ 8.5 Hz, 2H), 7.15 (t, J ~ 6.4 Hz, 2H), 7.10 (m, 5H), 7.04
(m, 3H), 6.99 (m, 1H). 13C NMR (600 MHz, CDCl3) [ppm] d:
150.17, 147.19, 147.13, 144.69, 138.16, 135.14, 132.78, 129.88,
129.37, 129.29, 127.41, 126.94, 126.74, 125.74, 125.59, 124.98,
123.90, 123.23, 120.79.
Tris-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine
(abbreviated to TPVPA). It is obtained by a similar method
to DPVPA. The red compound was purified by column
chromatography on silica gel using ethyl acetate/alcohol (1:1)
as eluent with a yield of 41% and m.p. 358.5 uC. The enthalpy is
228.37 mJ mg21. Anal. Calcd for C63H48N4: C, 87.91; H, 5.58;
1
N, 6.51%. C, 87.68; H, 5.43; N, 6.33%. H NMR (600 MHz,
CDCl3) [ppm] d: 8.59 (d, J ~ 4.0 Hz, 6H), 7.55 (s, 10H), 7.46 (t,
J ~ 8.2 Hz, 6H), 7.33 (m, J ~ 10 Hz, 10 H), 7.14 (t, J ~ 8.6
Hz, 8H), 7.05 (m, 8H). 13C NMR (600 MHz, CDCl3) [ppm] d:
151.68, 148.29, 146.16, 139.57, 136.72, 134.25, 130.79, 130.14,
129.10, 128.92, 128.43, 128.27, 127.15, 125.81, 122.30.
Fig. 1 Linear absorption spectra of three compounds in chloroform
with a concentration of 1 6 1025 mol L21
.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 9 5 – 3 0 0 0
2 9 9 7

View Article Online
Fig. 2 One-photon fluorescence spectra of three compounds in
chloroform with a concentration of 1 6 1025 mol L21
Fig. 4 Two-photon induced emission spectrum of three compounds in
chloroform with a concentration of 1 6 1022 mol L21
.
.
Fig. 3 is the one-photon fluorescence spectra of the three
solid compounds. The measurement apparatus and the
excitation wavelengths are the same as those of one-photon
fluorescence in solution. From Fig. 3, one can see that the
maximum peaks of the DPVPA, BPVPA and TPVPA are 564,
588 and 596 nm, respectively. One can see that the maximum
fluorescence peak in Fig. 3 shows red-shifted compared to the
corresponding counterpart in Fig. 2. The maximum fluores-
cence peak in solid form also shows red-shifted with increasing
molecular dimensionality as is the case in solution.
Fig. 4 shows the two-photon fluorescence of three com-
pounds in chloroform with a concentration of c ~ 0.01 mol
L21. The two-photon induced fluorescence spectra can be
observed with a certain laser beam from a mode-locked
Ti:sapphire laser (Coherent Mira 900 F) as the pump source
with a pulse duration of 200 fs, a repetition rate of 76 MHz, and
a single-scan streak camera (Hamamatsu Model C5680-01)
together with a monochromator as the recorder. The excitation
wavelengths of DPVPA, BPVPA and TPVPA are 800, 820 and
830 nm, respectively. From Fig. 4, one can see that the
maximum peaks of the two-photon fluorescence also show red-
shifted with increasing molecular dimensionality.
one- and two-photon cases. This can be explained by the effect
of reabsorption such that the linear absorption band has a
slight overlap with the emission band and our two-photon
fluorescence experiments were carried out in concentrated
solutions that made reabsorption significant.
In addition to the similarities between one-photon and two-
photon fluorescence, the positions of two-photon fluorescence
peaks 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 via internal conversion and/or vibrational
relaxation.
3.2 One- and two-photon absorption
The transition intensity for one-photon absorption (OPA) is
described by an oscillator strength
X
2vf
2
dop~
jS0jꢀaj f Tj
(1)
3
a
where ma is the electric dipole moment operator, vf denotes the
excitation energy of the excited state |fm, |0m denotes the ground
state, and the summation is performed over the molecular x, y
and z axes.
From Table 1, one can see that the peak position of the two-
photon fluorescence is slightly red-shifted compared to that of
the one-photon fluorescence in chloroform for each compound.
As an example, we present the absorption and fluorescence
spectra of TPVPA in Fig. 5. From Fig. 5, one can clearly see
the red-shifted phenomenon for the fluorescence spectra of
In terms of sum-over-state formula, the two-photon matrix
Fig. 5 Absorption and fluorescence spectra of TPVPA (a) linear
absorption with c ~ 1 6 1025 mol L21 in chloroform, (b) one-photon
fluorescence spectra with c ~ 1 6 1025 mol L21 in chloroform, (c) two-
photon fluorescence spectra with c ~ 1 6 1022 mol L21 in chloroform.
Fig. 3 One-photon fluorescence spectra of three compounds in solid
form.
2 9 9 8
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 9 5 – 3 0 0 0

View Article Online
element for the two-photon resonant absorption of identical
energy is written as
From the theoretical results reported above, it should be
noted that both one-photon and two-photon absorptions exist
for the first excited state of all molecules, which indicates that
the electronic states have mixed symmetry. In measurements,
the two-photon excitation wavelengths are simply taken as
twice those of the one-absorption maxima and the two-photon
fluorescence spectra are observed. In such a point of view, the
computational work corresponds highly with the observation.
"
#
X
S0jꢀajjTSjjꢀbjfT S0jꢀbjjTSjjꢀajfT
ꢀ
ꢀ
Sab
~
z
(2)
vj{vf
2
vj{vf
2
j
Where |0m and |fm denote the ground state and the final state,
respectively. |jm means all the intermediate states, including the
ground state. vj is the excitation energy of the excited state |jm.
The TPA cross section is given by orientational averaging over
the two-photon absorption probability31
3.3 Two-photon photopolymerization
h
i
The wavelengths for initiating two-photon polymerization
reactions of DPVPA, BPVPA and TPVPA are 800, 820 and
830 nm, respectively. A three dimensional lattice is created by a
two-photon polymerization of an acrylic ester oligomer using
BPVPA as initiator. A film of the initiator and oligomer
mixture with a weight ratio of y4 to y5% (a little
dichloroethane is added to make a solution of the initiator
and oligomer) is prepared by spin-coating onto glass plates.
The same mode-locked Ti:sapphire laser as that used in the
two-photon fluorescence measurements is applied for two-
photon microfabrication. The 820 nm lasing source is tightly
focused via an objective lens (640, NA ~ 0.65), and the focal
point is focused on the sample film on the xy-step motorized
stage controlled by computer. The pulse energy after being
focused by the objective lens is y20 mW. The polymerized
solid skeleton is obtained after the unreacted liquid mixture has
been washed out. The fabricated lattice is observed through a
polarization microscope (Olympus, BX-51). Its photograph is
illustrated in Fig. 6.
X
dtpa
~
F|SaaSbꢀbzG|SabSaꢀbzH|SabSbꢀa
(3)
ab
where the coefficients F, G and H are related to the incident
radiation. For the linearly polarized light, F, G and H are 2, 2
and 2, but for the circular case, they are 22, 3, and 3,
respectively. In the present work, we only consider the results
with the linearly polarized laser beam. The summation goes
over the molecular axes a, b ~ {x, y, z}.
The TPA cross section directly comparable with experi-
mental measurements is defined as32
4p2a50a v2gðvÞ
stpa
~
dtpa
(4)
15c0
Cf
where a0 is the Bohr radius, c0 is the speed of light, a is the fine
structure constant, hv is the photon energy of the incident
light, g(v) denotes the spectral line profile, here it is assumed to
be a d function. Cf is the lifetime broadening of the final state,
which is assumed to be 0.1 eV.22
The photopolymerization mechanisms of these new initiators
are still unknown. According to Cumpston et al.,11 strong
donor substitutes would make the conjugated system electron
rich, and after one- or two-photon excitation, these chromo-
phores would be able to transfer an electron even to relatively
weak acceptors, and this process could be used to activate the
polymerization reaction. In order to demonstrate this process,
we attempted to make a theoretical investigation. Our ab initio
calculation at time-dependent hybrid density functional theory
B3LYP level coded in GAUSSIAN package33 for the BPVPA
molecule shows that the first excited state is the charge-transfer
(CT) state with the excited energy l ~ 503 nm. When the
molecule is irradiated by a 820 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 and the CT
states for BPVPA in the gas phase (see Fig. 7), which is
visualized by the use of the MOLEKEL program.36 It can be
seen that upon excitation, charges are mainly transferred from
We utilized the GAUSSIAN-98 program package33 to
optimize the molecular geometrical structure with the hybrid
density functional theory (DFT/B3LYP) and a basis set
6–31G. The excited energies and the OPA intensities for the
three molecules are then calculated by the time-dependent
density functional theory. The numerical results show that the
OPA spectra are mainly dominated by absorption to the first
excited state for all three molecules. The lowest excited states
for DPVPA, BPVPA, and TPVPA are situated at 466 nm,
503 nm, and 511 nm, respectively, with the respective oscillator
strengths 1.25, 1.76, 2.99. The excited energies show a red-shift
as the dimensionality of the molecules is increased. The reason
can be ascribed to the electronic coupling becoming stronger
with increasing molecular branches. The stronger electronic
coupling would push down the lowest excitation energy. It is
clearly seen that the simulation for OPA gives a similar trend as
the measurement shown by Fig. 1 for the three molecules.
The analytical response theory34 is used to study the two-
photon absorption cross sections of the three molecules. The
implementation has been coded as the Dalton35 program
package. The TPA cross sections of the three molecules are
calculated in the random phase approximation with a 6–31G
basis set. For the first excited state, the TPA cross sections s
(10250 cm4 s photon21) for molecules DPVPA, BPVPA, and
TPVPA lit by the linearly polarized laser are 59.3, 30.2, and
42.4, respectively, which indicates that all three molecules have
relatively large two-photon absorption cross sections under the
experimental conditions. Therefore, these three molecules can
be used as two-photon polymerization initiators. We should
note that our theoretical calculations do not show enhance-
ment of the two-photon absorption cross section with
increasing molecular dimensionality. The reason is that here
we only consider the electronic contribution to the two-photon
absorption process. However, according to the result given by
Macak et al.,26 the vibronic contributions can play an
important role for multi-branched molecules, which definitely
indicates that a full description of the two-photon absorption
properties of multi-branched molecules needs to consider
vibronic contributions. The corresponding work is under way.
Fig. 6 Optical micrograph of the lattice fabricated via two-photon
polymerization of an acrylic ester oligomer using BPVPA as initiator.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 9 5 – 3 0 0 0
2 9 9 9

View Article Online
2
3
4
5
G. S. He, J. D. Bhawalkar, C. F. Zhao and P. N. Prasad, Appl.
Phys. Lett., 1995, 67, 2433.
J. E. Ehrlich, X. L. Wu, L.-Y. Lee, Z.-Y. Hu, H. Roeckel,
S. R. Marder and J. Perry, Opt. Lett., 1997, 22, 1843.
J. D. Bhawalkar, G. S. He and P. N. Prasad, Rep. Prog. Phys.,
1996, 59, 1041.
C. F. Zhao, G. S. He, J. D. Bhawalkar, C. K. Park and
P. N. Prasad, Chem. Mater., 1995, 7, 1979.
6
7
8
W. Denk and K. Svoboda, Neuron, 1997, 18, 351.
W. Denk, Proc. Natl. Acad. Sci. USA, 1994, 91, 6629.
C. Xu, W. R. Zipfel, J. B. Shear, R. M. William and W. W. Webb,
Proc. Natl. Acad. Sci. USA, 1996, 93, 10763.
9
W. Denk, J. H. Strickler and W. W. Webb, Science, 1990, 248, 73.
10 R. H. Ko¨hler, J. Cao, W. R. Zipfel, W. W. Webb and
M. R. Hansen, Science, 1997, 276, 2039.
11 B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer,
J. E. Ehrlich, L. L. Erskine, A. A. Keikal, S. M. Kuebler,
I.-Y. S. Lee, D. M. Maughon, J. Qin, H. Ro¨ckel, M. Rumi,
X. L. Wu, S. R. Marder and J. W. Perry, Nature, 1999, 398, 51.
12 J. H. Strickler and W. W. Webb, SPIE Proc., 1990, 1398, 107.
13 S. Maruo, O. Nakamura and S. Kawata, Opt. Lett., 1997, 22, 132.
14 A. S. Dvornikov and P. M. W. Rentzepis, Opt. Commun., 1995,
119, 341.
Fig. 7 Density difference between the charge-transfer and ground
states of BPVPA in the gas phase. Areas with criss-cross marking and
dots represent electron loss and gain, respectively, upon excitation.
15 D. A. Parthenopoulos and P. M. Rentzepis, Science, 1989, 245,
843.
16 H. -B. Sun, S. Matsuo and H. Misawa, Appl. Phys. Lett., 1999, 74,
786.
17 S. Kawata, H. Sun, T. Tanaka and K. Takada, Nature, 2001, 412,
597.
18 H. Sun, V. Mizeikis, Y. Xu, S. Juodkazis, J. Ye, S. Matsuo and
H. Misawa, Appl. Phys. Lett., 2001, 79, 1.
19 M. P. Joshi, H. E. Pudavar, J. Swiatkiewicz, P. N. Prasad and
B. A. Reianhardt, Appl. Phys. Lett., 1999, 74, 170.
20 P. Galajda and P. Ormos, Appl. Phys. Lett., 2001, 78, 249.
21 S. M. Kuebler, B. H. Cumpston, S. Ananthavel, S. Barlow,
J. E. Ehrlich, L. L. Etsine, A. A. Heikal, D. McCord-Maughon,
J. Qin, H. Rocel, M. Rumi, S. R. Marder and J. W. Perry, SPIE-
Int. Soc. Opt. Eng., 2000, 3937, 97.
the acceptor side to the donor side of the molecule. In the CT
state, there are more electrons on average at the donor side,
indicating that the molecule could be ready to give away its
electron to its surroundings. This picture seems to support the
Cumpston discussion. However, whether the photoinduced
electron-transfer reaction can be energetically feasible needs to
be further investigated theoretically. The systematic study of
the microstructure created by two-photon free-radical photo-
polymerization using BPVPA as initiator will be reported in
detail elsewhere.
4. Conclusion
22 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.
23 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.
In summary, three new two-photon photopolymerization
initiators have been synthesized using Witting and Heck
reactions. These dyes produce strong one-photon fluorescence
both in the solid state and in solution. The experimental results
confirm that all these compounds are good two-photon
absorbing chromophores and effective two-photon photopoly-
merization initiators. The wavelengths for initiating two-
photon polymerization reactions of DPVPA, BPVPA and
TPVPA are 800, 820 and 830 nm, respectively. A micro-
structure has been fabricated of BPVPA as a two-photon
photopolymerization initiator and a possible mechanism via
the charge-transfer process when laser irradiation is applied is
discussed.
24 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.
˚
25 P. Norman, Y. Luo and H. Agren, J. Chem. Phys., 1999, 111, 7758.
˚
26 P. Macak, Y. Luo, P. Norman and H. Agren, J. Chem. Phys.,
2000, 113, 7055.
27 S. J. Chung, K. S. Kim, T. C. Lin, G. S. He, J. Swiatkiewitz and
P. N. Prasad, J. Phys. Chem. B., 1999, 103, 10741.
28 G. Lai, X. R. Bu, J. Santos and E. A. Mintz, Synlett, 1997, 1275.
29 P. Fromherz, J. Phys. Chem., 1995, 99, 7188.
30 U. Narang, C. F. Zhao, J. D. Bhawalkar, F. V. Bright and
P. N. Prasad, J. Phys. Chem., 1996, 100, 4521.
31 P. R. Monson and W. M. McClain, J. Chem. Phys., 1970, 53, 29.
Acknowledgements
This work was financially supported by grants from the state
National Natural Science Foundation of China (grant
no. 50323006, 50325311, 10274044) and the Swedish Interna-
tional Development Cooperation Agency (Sida).
˚
32 Y. Luo, P. Norman and H. Agren, Nonlinear Opt., 2000, 26, 153.
33 GAUSSIAN-98, references in http://www.gaussian.com.
34 J. Olsen and P. Jørgensen, J. Chem. Phys., 1985, 82, 3235.
35 T. Helgaker et al., DALTON, an ab initio electronic structure
program, Release 1.0 (1997). See http://www.kjemi.uio.no/software/
dalton/dalton.html.
References
36 MOLEKEL, references in http://www.cscs.ch/molekel/.
1
P. A. Fleitz, M. C. Brant, R. L. Sutherland, F. P. Strohkendl,
J. R. Larson and L. R. Dalton, SPIE Proc., 1998, 91, 3472.
3 0 0 0
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 9 5 – 3 0 0 0