Highly sensitive two-photon chromophores applied to three-dimensional lithographic microfabrication: Design, synthesis and characterization towards two-photon absorption cross section

Article abstract of DOI:10.1039/b309023j

A series of D-π-A-π-D type chromophores were synthesized by the dehydration reaction of 4-R₂N-benzaldehye (R = Ph, Buⁿ, Et, Me) and diaminomaleonitrile (corresponding to the chromophores 1, 2, 3 and 4, respectively), in which a polar

Full text of DOI:10.1039/b309023j

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A R T I C L E
Highly sensitive two-photon chromophores applied to three-
dimensional lithographic microfabrication: design, synthesis and
characterization towards two-photon absorption cross section
Youmei Lu,*a Fuyuki Hasegawa,a Takamichi Goto,b Satoshi Ohkuma,b
Setsuko Fukuhara,a Yukie Kawazu,a Kenro Totani,a Takashi Yamashitab and
Toshiyuki Watanabea
aDivision of Applied Chemistry (Graduate School), 2-24-16 Naka-cho, Koganei-city
Tokyo 184-8588, Japan. Tel: 181-42-388-7289
bDepartment of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo
University of Science, 2641 Yamazaki, Noda 278-8510 Chiba, Japan.
E-mail: luym19@cc.tuat.ac.jp; toshi@cc.tuat.ac.jp; Tel: 181-47-124-9067
Received 5th August 2003, Accepted 17th September 2003
First published as an Advance Article on the web 14th October 2003
A series of D–p–A–p–D type chromophores were synthesized by the dehydration reaction of 4-R2N-
benzaldehye (R ~ Ph, Bun, Et, Me) and diaminomaleonitrile (corresponding to the chromophores 1, 2, 3 and
4, respectively), in which a polar imino double bond (–CLN–) replaced the double bond (–CHLCH–) in the
p-conjugated centers. Femtosecond laser induced fluorescence intensity was used to evaluate two-photon
absorption (TPA) cross sections, d, using a USB-2000 CCD. Results show a change of terminal groups from
Ph2N– to Me2N– influenced the d value significantly through a change of the quantum yield, Q. However, the
two-photon absorption peak position was only slightly affected. The chromophores 2 and 3 were found to
afford polymers in the presence of the functional triacrylate monomer at low laser power at 755 and 820 nm.
This demonstrated that the enhanced d value was not a main factor in the improvement of chromophore two-
photon photosensitivity. Such information can be useful in the design of more efficient two-photon
chromophores for imaging and power-limiting applications.
The molecules considered in this work have the general
Introduction
structure D–p–A–p–D, wherein D is a terminal group, p is a
phenylimine function and A is an electron-accepting cyano
group. Since cyano groups are used as the side groups, the
increase of electron transfer leads to a significant increase in the
d value relative to electron-donating methoxy groups (as in
compound 5).14 Additionally, the change of the conjugated
backbone through the introduction of the polar imino groups
will allow the tuning of the linear and nonlinear optical
responses of quadrupolar derivatives as a result of the influence
of the chromophoric two-photon absorption properties.22,23
Both polar groups (–CHLN–) will enhance the extent of the
charge transfer from the terminal groups to the center,
correlated with the two cyano-groups. Upon chromophoric
excitation, the p-conjugated center acts as an electron acceptor,
and intramolecular energy and electron transfer will occur
more readily from the terminal R2N– groups than via
intermolecular energy and electron transfer.16–21 Radical
forms can be easily generated in the terminal groups. More-
over, the solubility of the chromophore improves with the
replacement of –CHLCH– by the polar flexible –CHLN–
group. Fig. 1 shows the molecular structures 1–4 studied in this
paper. Due to the fact that they can be synthesized in one-step
with a high yield and initiate polymerization of triacrylate
monomer at a high level of sensitivity, makes application in
two-photon-induced polymerization possible.
Two-photon absorption is attracting considerable attention
owing to its various applications in the areas of three-dimensional
imaging, optical data storage and three-dimensional lithographic
microfabrication (3DLM) through two-photon-induced poly-
merization (TPIP).1–7 This is due to the fact that simultaneous
two-photon absorption requires a very high photon flux, which is
only present at the point of the focus. Thus the TPI polymeriza-
tion is confined to the focal volume. This high spatial resolution
contributes to the ability of TPIP not only to scan the laser in the x
and y direction but also to change the focal plane (z) without
overwriting existing features. Therefore, 3DLM is obtained by a
single processing step. 3D polymeric structures include a photonic
band gap structure, waveguide structures and a micro-channel
structure.8–11 Recently, microrotator and microtweezers have
been manufactured by TPIP of less than 0.4 mm and 0.25 mm in
diameter, respectively.12,13 Efficient two-photon absorbers based
on bis(styryl)benzene constructs bearing electron-donating
moieties were synthesized by Albota14 and Bunning15 and their
co-workers, and found to undergo a presumed two-photon
induced electron transfer to highly functional free-radical
polymerizable acrylate monomer. On the other hand, Belfield
and Pitts conducted two-photon-induced polymerization under
near-IR fs-laser irradiation by using a commercially available UV
photoinitiator, consisting of the rather involved syntheses of
bis(dialkylamino)stilbene family.16–21 In this case, the acrylamine,
acrylate and acrylic acid system was induced to polymerize in the
presence of N,N-dimethylaniline, triethanolamine or an onium
salt as the co-initiator wherein the co-initiator amine acted as the
electron donor upon chromophore excitation. However, the
photosensitivity was relatively low due to the low two-photon
absorption of the UV initiator.
Experimental
1. Chemicals
Diaminomaleonitrile (98%) and 4-diphenyl- or 4-dialkyl-amino-
benzaldehydes (98%) were purchased from Aldrich. The solvents
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 , 7 5 – 8 0
7 5

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Fig. 1 The structures of the molecules studied in this work. Compounds 0 and 5 were reported by Perry’s group.10,14,22
used in the fluorescence studies were spectrophotometric grade
and were checked by fluorescence spectroscopy to make sure
they contained no fluorescent impurities.
referenced by the addition of ferrocene to the cell. Fluorescence
lifetime measurements were carried out by the single-photon
counting method. The sample was dissolved in CH3CN excited
at 337 nm by using a nitrogen laser (Nihon laser LN-100 pulse
width of 300 ps).
2. Instruments
Absorption spectra were measured with JASCO V-560 UV/Vis
spectrophotometer, accurate to ¡2 nm. Luminescence spectra
were obtained with JASCO FP-6500/6600 for windows
3. Synthesis
The synthetic route is shown in Fig. 1. The chromophores were
prepared from the dehydration reaction of 4-diphenyl- or
4-dialkyl-aminobenzaldehyde and diaminomaleonitrile. The
synthetic procedures for obtaining the chromophores 1–4 were
the same as used in the work by Yu et al.25 All quadrupolar
molecules were characterized by nuclear magnetic resonance
(1H NMR), elemental analysis (Yanako MT-5 model) and
liquid chromatography mass spectrometry (SHIMADZU),
which confirmed the assigned structures.
spectrofluorimeter. The measurements of
d values were
calibrated relative to Rhodamine-B (abbreviated as R-B)
with absolute TPA cross-section determined by Xu and
Webb in the 690–1050 nm range.24
The samples were dissolved in toluene and chloroform at a
concentration of 5 6 1025 M to reduce the effect of
reabsorption, and d values were determined through the two-
photon-induced fluorescence method using a one-arm setup
(Ti-Saphire laser: 85-fs pulses at a repetition rate of 82 MHz
(Spectra-Physics Tsunami)). The laser beam was focused over
the lens (NA ~ 0.3) at a laser power of 1.0–2.5 mW and the
fluorescence was collected at right angles with respect to the
incident beam. One used method was the conventional method
where the fluorescence was collected by a lens and passed
through a monochromator and measured by a photomultiplier
tube (PMT) as described by Rumi et al. (in toluene).22 The
second used method was measuring a spectrograph with a
CCD (in CHCl3). In the latter case, the diameter of the focused
laser beam is ca. 3.2 mm, and the diameter of the sensor of the
CCD is ca. 50 mm. The fluorescent light in the cell underfilled
the sensitive area of the detector. Thus, the fluorescence can be
measured even though the emission is weak. The measurements
were carried out in a laser intensity range for which a quadratic
dependence of the fluorescence signal on the laser was
observed. Moreover, to reduce the path length of the fluore-
scence emission in the solution under study and affect the
reabsorption, the incident beam was directed as close as
possible to the windows of the cell on the side where the light
was collected.
Generalsyntheticmethod. Thediaminomaleonitrile(0.010mol)
was mixed with slightly more than two-fold of the 4-dialkyl-
aminobenzaldehyde (0.022 mol) in glacial acetic acid (60 ml). The
product was precipitated from the reaction solution after refluxing
for 6 h. The precipitate was collected and purified by column
chromatography eluting with CHCl3. Yield: 70%.
N,N’-Bis(4-diphenylamino)benzylidenediaminomaleonitrile (1).
Calc. for C42H30N6 (617.5): C, 81.61; H, 4.86; N, 13.60. Found:
1
C, 81.00; H, 5.01; N, 13.90%. H NMR (CDCl3) (500 MHz;
TMS), dH 7.02 (4H, d, J 8.30 Hz), 7.19 (8H, d, J 8.26 Hz), 7.35
(12H, t, J 8.26 Hz), 7.80 (4H, d, J 8.37 Hz), 8.61 (2H, s); LC-
MS, m/z (%): 622 ([M 1 4H]1, 90)
N,N’-Bis(4-di-n-butylamino)benzylidenediaminomaleonitrile (2).
Calc. for C34H46N6 (538.79): C, 75.78; H, 8.61; N, 15.60.
Found: C, 76.04; H, 8.90; N, 15.08%; 1H NMR (CDCl3)
(500 MHz; TMS), dH 0.95 (12H, t, J 7.20 Hz), 1.40 (8H, m),
1.61 (8H, m), 3.38 (8H, t, J 7.56 Hz), 6.67 (4H, d, J 8.30 Hz),
7.81 (4H, d, J 8.37 Hz), 8.54 (2H, s); LC-MS, m/z (%): 539
([M 1 H]1, 80)
Cyclic voltammetry was performed under argon with
tetrahydrofuran solutions ca. 1024 M in sample and 0.1 M
in [Bun4N]1[PF6]2, using a glassy carbon working electrode, a
platinum auxiliary electrode, and a silver–silver ion electrode
which was easily constructed by putting a silver wire in a
solution of 0.01 M AgClO4 in tetrahydrofuran. Potentials were
N,N’-Bis(4-diethylamino)benzylidenediaminomaleonitrile (3).
Calc. for C26H30N6 (426.56): C, 73.20; H, 7.09; N, 19.71.
7 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 7 5 – 8 0

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Table 1 Position (1lex, 2lex) and magnitude (d in 10250 cm4 s/photon-
molecule (GM)) of TPA, and quantum yields Q for chromophores 0 and 1
in toluene
Chromphore
1lex/nm
472
2lex/nm
d/GM
Q
Qd
0a
1
810
835
810
835
3670
1940
510
0.86
0.34
3155
173
542
495
a See ref. 14.
Found: C, 73.87; H, 7.39; N, 19.18%. 1H NMR (CDCl3)
(500 MHz; TMS), dH 1.87 (12H, t, J 7.25 Hz), 3.38 (8H, qd),
6.70 (4H, d, J 8.30 Hz), 7.83 (4H, d, J 8.59 Hz), 8.55 (2H, s);
LC-MS, m/z (%): 427 ([M 1 H]1, 80)
N,N’-Bis(4-dimethylamino)benzylidenediaminomaleonitrile (4).
Calc. for C22H22N6 (370.8): C, 71.35; H, 5.94; N, 22.67. Found:
Fig. 2 Two-photon-induced fluorescence excitation spectra of chro-
mophores 1–3 in toluene at a concentration 5 6 1025 M.
1
C, 71.80; H, 6.13; N, 23.10%. H NMR (CDCl3) (500 MHz;
TMS), dH 3.11 (12H, s), 6.72 (4H, d, J 8.59 Hz), 7.85 (4H, d, J
8.59 Hz), 8.58 (2H, s); LC-MS, m/z (%): 371 ([M 1 H]1, 100)
d value was enhanced significantly in the apolar solvent,
toluene, rather than the polar solvent, CHCl3. By contrast, the
experimental results show that the d values of chromophores 2
and 3 were large in CHCl3 relative to those in toluene as seen in
Tables 2 and 3 and Figs. 2 and 3. In the case of the
chromophore 1, four terminal phenyl groups decrease the
molecular polarity through the extended p-conjugation. As a
result, two-photon absorption and excited fluorescence was
enhanced in toluene even in the presence of the strong electron-
accepting groups –CHLN– and –CN. The enhancement of two-
photon absorption of the chromophores 2 and 3 in the polar
solvent was attributed to the increase of the molecular polarity
from the –CHLN– and –CN groups in the center. It was thus
concluded that the two-photon proprieties were dependent
upon the solvent polarity in a significant way.
Results and discussion
1 Core effects
The role of p-conjugation on the dmax value can be seen in
Table 1. For the chromophore 1, the change of p-conjugation,
–CHLN– replacing –CHLCH–, led to dmax decreasing five-fold
with the quantum yield Q reduced to half the value relative to
compound 0,14 while its UV absorption peak red-shifted
around 70 nm. This is indicative of improved electronic
conjugation in 1. However the experimental results showed
that the increase in the molecular polarity is unfavorable to the
enhancement of two-photon absorption and the quantum yield
as seen from Table 1.
3 Terminal groups effects
2 Solvent effects
As listed in Table 3, the change of the terminal group from
Ph2N– to Me2N– produced different effects on the d value. The
A summary of optical data and parameters of chromophores
1–4 in toluene and CHCl3 are listed in Tables 2 and 3. Since
chromophore 4 did not dissolve in toluene, its two-photon
properties were not investigated in that solvent.
1
positions of lex were slightly influenced as shown in Fig. 4.
Based on the experimental results that the Qd value was similar
from Ph2N to Me2N–, the significant difference in the d value is
mainly attributed to the change of the quantum yield. That is to
say, the terminal groups modified the TPA through the effects
on Q. In particular, the Ph2N– terminal groups led to a red-shift
of the two photoabsorption peaks, relative to Bu2N–, Et2N–
and Me2N– with an obvious reduction in two-photon
absorption and an increase in the quantum yield. This is
mainly attributed to the increase in the extent of electronic
For chromophore 1, with Ph2N– as the terminal group, the
Table 2 Position (1lex, 2lex) and magnitude (d in 10250 cm4 s/photon-
molecule (GM)) of TPA, and quantum yields Q for the chromophores 1–3
in toluene
Chromphore
1lex/nm
542
2lex/nm
d/GM
Q
Qd
1
2
3
810
835
800
830
800
820
510
495
395
430
330
340
0.34
173
541
0.06
0.03
26
12
537
Table 3 Position (1lex, 2lex) and magnitude (d in 10250 cm4 s/photon-
molecule (GM)) of TPA, and quantum yields Q for the chromophores
1–4 in chloroform
Chromphore
1lex/nm
552
2lex/nm
d/GM
Q
Qd
1
2
3
4
840
885
820
840
820
840
820
840
180
265
490
480
545
520
2050
1910
0.46
0.1
120
49
553
547
0.05
0.02
29
31
536
Fig. 3 Two-photon-induced fluorescence excitation spectra of chro-
mophores 1–4 in CHCl3 at a concentration 5 6 1025 M.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 7 5 – 8 0
7 7

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cast on to a glass substrate at a thickness controlled by a Baker
Film Applicator, and allowed to set by slow evaporation of the
solvent. The thickness of the cured films could be varied from
100 to 200 mm. In microfabrication experiments the laser beam
was passed through a microscope (Olympus, IX50/IX70), and
the samples were exposed by irradiation with tightly focused
laser pulses (numerical aperture, NA ~ 1.2, y0.39 mm radial
spot size at 755 nm and y0.45 mm at 820 nm) from a
Ti:sapphire laser. The exposures were patterned under
computer-control by shuttering the beam and translating the
film relative to the focal point using a three-axis micropois-
tioner (Sutter, MP-285). Following exposure, the film was
immersed in dimethylformamide to dissolve the non-irradiated
resin leaving behind the cross-linked polymeric structure.
Chromophore 4 was not easily dissolved in the resin. As for
chromophore 1, it readily dissolved in to the solution of the
resin, but the dye was partly crystallized after the film dried.
Therefore, the concentration of chromophores 1 or 4 in the
resin was low relative to that of chromophores 2 and 3. As a
result their two-photo polymerization photosensitivity was not
investigated. There may be the similar reason why the
bis(styryl)benzene chromophore 0 with bisphenylamino term-
inal groups with a large d value was not used as a two-photon
initiator as widely as that of 5 with bis-n-butylamino groups.14
Considering the chromophores 2 and 3 have a significant
absorption peak at 755 nm (Fig. 3), the microfabrications
shown in Fig. 5(a) and (b) and Fig. 6(a) and (b) were obtained
at 755 and 820 nm, respectively. Here, we define the two-
photon polymerization rates (Rp) based on the equation
p(d/2)2vs, where d is the width of the written protruding line
and the scanning speed vs is 50 mm s21. The results are listed in
Table 4. It was found that the sensitivity of the chromophores 2
and 3 increased at a peak of two-photon absorption, 820 nm,
Fig. 4 UV/Vis absorption spectrum recorded for chromophores 1–4
in CHCl3.
delocalization by attaching four phenyl rings on both ends. As
for chromophore 4, its large d value was due to its relatively
small quantum value.
4 Microfabrication and sensitivity by TPIP
To demonstrate the utility of these two-photon chromophores,
we attempted to prepare two-photon cross-linkable semi-solid
acrylate resins, to include the chromophores 1–5 at molar
concentration of 5 6 1024 M. The resins consisted, by weight,
of 70% multifunctional acrylate monomer (SR9008 : SR368 ~
1 : 1, purchased from Sartomer Co. Ltd.) and 29.9% poly-
(styrene-co-acrylonitrile) as a polymer binder, as described in
the literature.26 The resin mixture was dissolved in dioxane,
Fig. 5 Optical micrograph of the grating with 20 mm spacing using
chromophore 2. (a) Excitation wavelength: 755 nm; laser power:
5.0 mW with 1.8 mm width; (b) excitation wavelength: 820 nm; laser
power: 3.0 mW with 2.4 mm width.
Fig. 6 Optical micrograph of the grating with 20 mm spacing using
chromophore 3. (a) Excitation wavelength: 755 nm; laser power:
5.0 mW with 2.0 mm width; (b) Excitation wavelength: 820 nm; laser
power: 3.0 mW with 3.4 mm width.
7 8
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 7 5 – 8 0

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Table 4 Position (2lex) and magnitude (d in 10250 cm4 s/photon-molecule (GM)) of TPA of the chromophores 2 and 3 and compound 5 under the
applied conditions: the oxidation potential (ED1/D), free energy DG of the electron transfer reaction, and fluorescence lifetime (t) for the
chromophores are also listed. The polymerization rates (Rp) were monitored every second from the width of the written protruding line (d)
Chromophore
ED1/D/V
0.05
DG/eV
0.58
t/ns
0.3
0.3
1
2lex
d/GM
Q
d/mm
Rp/mm3 s21
2
3
5
755
820
755
820
730
315
490
390
545
900
0.1
0.05
0.88
1.8
2.4
2
3.4
3
120
230
160
450
395
0.18
0.71
20.01
20.14
relative to that at 755 nm. Though the laser power at 755 nm
(5.0 mW) is higher than that at 820 nm (3.0 mW), a higher
efficiency of the polymerization occurred at 820 nm (cf. Fig. 5(b),
6(b) to Fig. 5(a), 6(a), respectively). However, at the same laser
wavelength and power, the ratio of Rp of the chromophores 2 and
3 was not consistent with the ratio of d, as listed in Table 4.
The analysis of the oxidation potential for chromophores 2
and 3 (0.050 and 0.18 V, respectively) showed that Bu2N– is a
strong donor relative to Et2N– in the ground state. This indicates
that the highest occupied molecular orbital (HOMO) of the
chromophore 2 is located at a higher energy than that of the
chromophore 3. The free energy, DG, of the electron transfer
reaction was evaluated from the relation DG ~ ED1/D 2
EAcc/Acc2 2 Eex, as cited in the literature,26 where ED1/D is the
electrode potential of the chromophore 2 or 3, the electrode
potential of the monomer acrylate, EAcc/Acc2, is 22.77 V, and
Eex is the energy difference between the ground state and the
lowest-lying excited state (S1) of the chromophore 2 or 3. Both
the free energy values have been found to be positive as listed in
Table 4.
indicates that they may be a differing main factor influencing
the TPIP sensitivity rather than the reaction free energy value
and the excited energy level.
In our case, the TPIP sensitivities of the chromophores 2 and 3
were demonstrated through increasing the scanning speed at an
identical laser power and wavelength. Because of changes of laser
power resulting from a slight change of the focus position in the
z-axis direction it is difficult to investigate the threshold power at
which the weakly polymerized resins become set following
developing. As shown in Fig. 7(a) and (b), both microstructures
were observed after development at a scanning speed of
290 mm s21. However, comparing Fig. 7(a) and (b), we found
the strength of the fabricated structures initiated by chromo-
phore 3 is stronger than that by chromophore 2 at 290 mm s21
.
In addition, Fig. 8 shows the microstructure obtained using
compound 5 at a laser power of 3.0 mW, excitation wave-
length 730 nm and with a scanning speed at 50 mm s21
.
Taking into account a slight difference of the radial spot size
of the focused beam at the different excitation wavelengths, the
Rp value of the compound 5 is similar to that of the chro-
mophore 3 at the identical laser power, 3.0 mW, as listed in
Table 4.
Fig. 7 Optical micrographs with 10 mm spacing: (a) using chromophore
2 with 2.4 mm width; (b) using the chromophore 3 with 3.4 mm width.
Excitation wavelength: 820 nm; laser power: 3.0 mW. The scan speed was
increased from 50 to 290 mm s21 with a step of 30 mm s21 from the left.
Compound 5 has been demonstrated to be a very efficient
two-photon initiator among the bis(dialkylamino)stilbene
family synthesized by Perry’ group. For compound 5, it was
reported that the bimolecular quenching rate with SR9008 was
large (up to 3.7 6 108 M21 s21 along with a negative free
energy, DG). Even though the experimental results showed that
bimolecular quenching with SR9008 was not observed for
chromophores 2 and 3, the high two-photon sensitivity of
chromophores 2 and 3 means that electron-transfer occurred
easily between the initiator and the monomer despite their
positive DG and ED1/D relative to the compound 5. Provided
that TPIP is a process of the free radical polymerization, the
rate of gerenation of excited states is mainly influenced by TPA
cross section, but the radical-generation quantum yield may be
related to the quantum yield and the fluorescence lifetime of the
chromophore. Otherwise, we observed that the chromophore 3,
with the lowest quantum yield and the shortest fluorescence
lifetime, can still initiate acrylate to polymerize with the highest
sensitivity among the chromophores 2, 3 and compound 5. This
Fig. 8 Optical micrograph of the grating with 20 mm spacing using
compound 5. Excitation wavelength: 730 nm; laser power: 3.0 mW with
3.0 mm width.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 7 5 – 8 0
7 9

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For the series of chromophores studied here, the presence of
strong electron acceptors in the center of p-conjugation led to a
significant decrease in the values of Q and d, compared to
bis(dialkylamino)stilbene. It was demonstrated that the
improvement of two-photon photosensitivity was not mainly
consistent with high two-photon absorption efficiency. Based
on the fact that chromophores 2 and 3 can be synthesized in
one-step and can induce polymerization of acrylate for a wide
range of laser wavelength (755 and 820 nm) suggests the
practicality of broader use in 3DLM technology. Based on the
fact that the chromophore 3 has a high sensitivity of TPIP,
similar to that of compound 5, suggests it may replace
compound 5, which requires a rather involved synthesis. In
the development of 3DLM technology, in order to optimize the
sensitivity of the two-photon initiators and monomer, such
information may be useful in searching for chromophores with
ideal properties for energy transfer and electron transport for
imaging and power-limiting applications.
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We are grateful to Mrs Y. Takizawa and N. Okamura for the
NMR measurements, and Dr H. Yamazawa and Dr
T. Hasimoto for their discussion of LC-MS. This research
was also supported by the Ministry of Education, Science,
Sports and Culture, Japan, in order to fulfill the Grant-in-Aid
for Scientific Research on Priority Areas (Molecular synchro-
nization), 11167220, 2001–2003, Grant-in-Aid for Scientific
Research (B), 14350128, 2003 and the 21st Century COE
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