Synthesis and photophysical properties of two-photon absorbing spirofluorene derivatives

Article abstract of DOI:10.1166/jnn.2012.5942

New spirofluorene-based quadrupolar two-photon absorbing dyes having triphenylamine and N,Ndibutylaniline as electron donors at the end of molcules were designed and synthesized. The thirdorder nonlinear optical properties of these compounds were studied using a two-photon excited fluorescence method. They were found to have high two-photon absorption cross-section owing to extended conjugation of the spirofluorene moiety. The effect of varying the donor strength could be discerned by comparing the two compounds. They were successfully used as a photosensitizers for two-photon initiated polymerization of three-dimensional micro-objects. Copyright

Full text of DOI:10.1166/jnn.2012.5942

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 4403–4408, 2012
Synthesis and Photophysical Properties of Two-Photon
Absorbing Spirofluorene Derivatives
Jea-Geon Lim1, Prém Prabhakaran1, Jin Sun Park1, Yong Son2, Tae-Dong Kim1,
Dong-Yol Yang2, and Kwang-Sup Lee1ꢀ∗
1Department of Advanced Materials, Hannam University, 461-6 Jeonmin Dong, Yuseong-Gu, Daejeon 305-811, Korea
2Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea
New spirofluorene-based quadrupolar two-photon absorbing dyes having triphenylamine and N,N-
dibutylaniline as electron donors at the end of molcules were designed and synthesized. The third-
order nonlinear optical properties of these compounds were studied using a two-photon excited
fluorescence method. They were found to have high two-photon absorption cross-section owing to
extended conjugation of the spirofluorene moiety. The effect of varying the donor strength could be
discerned by comparing the two compounds. They were successfully used as a photosensitizers
for two-photon initiated polymerization of three-dimensional micro-objects.
Keywords: Spirofluorene, Two-Photon Absorption, Stereolithography.
1. INTRODUCTION
Spiro linkage refers to the linkage of two extended
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ꢁ-systems with similar or different functions in order to
The study of nonlinear optical (NLO) phenomena in
Copyright: American Scientific Publishers
improve their morphological stability while keeping the
same electronic properties.16 The spirofluorene group can
be thought of as two fluorene moieties conjoined at their
9,10-positions. The two interconnected fluorene moieties
would align perpendicular to each other suppressing inter-
actions between their respective ꢁ-systems. This structure
assists in preventing intermolecular fluorescence quench-
ing which is common in case of dyes. As a result, these
compounds possess high TPA efficiency. They also present
an added dimension in NLO materials and are expected to
give rise to molecules with superior optical properties.16ꢀ17
The compounds were utilized as photosensitizers in two-
photon lithography for the fabrication of 3D micro-objects.
materials has generated enormous interest in recent years
because of their application in optical communication sys-
tems, medicine and defense.1–7 Studies in the past couple
of decades have focused on evolving design parame-
ters for second- and third-order NLO materials. Two-
photon absorption (TPA) is a third-order NLO property
which finds applications in three-dimensional optical data
storage, two-photon confocal microscopy, optical power
limiting, etc.4ꢀ7ꢀ8 Effective applicability of TPA materi-
als depends on achieving high TPA cross-sections. Sev-
eral systematic studies have been carried out to find
structure-property relationships in TPA chromophores.
These have involved approaches like extension of conju-
gation, substitution of powerful donors (D) or acceptors
(A) facilitating effective charge transfer, and increasing
the dimension of the molecule.9–14 In the current work,
we have designed and synthesized new spirofluorene-based
quadrupolar TPA chromophores with a D-ꢁ-D architec-
ture. The compounds Z1 and Z2 constitute triphenylamine
and N,N-dibutylaniline as electron donors, respectively.
The fluorene moiety has figured in several highly active
TPA molecules. Its rigidity and the resulting extended
conjugation make them an ideal choice as a ꢁ-center in
materials for organic electronics and NLO materials.11ꢀ14ꢀ15
2. EXPERIMENTAL DETAILS
2.1. Materials
All materials except SCR 500 were purchased from
Aldrich and used without any further purification. The
urethane-acrylate resin SCR 500 used for microfabircation
was kindly provided by JSR Japan.
2.1.1. Synthesis of 2,7-Dibromofluorenone (2,7-BFO)
Tetrabutylammonium hydroxide (0.39 mL, 0.36 mol)
in methanol solution was added to a solution of 2,7-
dibromofluorene in 15 mL of pyridine. The mixture was
∗Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2012.5942
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Synthesis and Photophysical Properties of Two-Photon Absorbing Spirofluorene Derivatives
Lim et al.
then stirred for 12 h at the room temperature. The resulting
mixture was added drop wise to acetic acid. The solution
was then washed with water. The product was purified by
recrystallization (solvent: chloroform and methanol). 2,7-
DFO was obtained as a yellow solid of 4 g (yield: 58%).
1H NMR (CDCl3, ppm): 7.39–7.42 (d, 2 H), 7.62–7.69
(d, 2 H), 7.78 (s, 2 H).
extracted and, washed with saturated sodium bicarbonate.
The organic layer was dried over anhydrous magnesium
sulfate. The crude product was purified by column chro-
matography on silica gel. The compound F3 was obtained
as a white solid of 2.2 g (yield: 73.8%). 1H NMR (CDCl3,
ppm): 7.33–6.93 (m, 14 H), 6.65 (d, 1 H), 5.63 (d, 1 H),
5.09 (d, 1 H).
2.1.2. Synthesis of 9,9-Bis(4-diphenyl-aminopenyl)-2,
7-Dibromofluorene(2,7-BTPF)
2.1.5. Synthesis of 4-(Dibutylamino)Benzaldehyde (G2)
ꢀ
40 mL of DMF was cooled to 0 C and 5 mL (0.053 mol)
A mixture of 2,7-DFO (3.8 g, 0.011 mol) and tripheny-
lamine (38.8 g, 0.16 mol) in trifluoromethane sulfonic acid
(1 mL) was heated for 18 h at 140 C with stirring under
of phosphorus oxychloride was added slowly to it. The
solution was stirred at 0 C for an hour and at room tem-
perature for another hour. To the stirred solution 11 mL
ꢀ
ꢀ
an inert atmosphere. The cooled mixture was extracted
with dichloromethane, and the extract was washed with
sodium carbonate solution. The organic layer was dried
over anhydrous magnesium sulfate and concentrated. The
product was purified by column chromatography on silica
gel, followed by recrystallization from acetone. 2,7-BTPF
was obtained as a white powder of 2.2 g (yield: 23.7%). 1H
NMR (CDCl3, ppm): 6.85–7.12 (m, 20 H), 7.15–7.29 (m,
8 H), 7.43–7.49 (d, 2 H), 7.49–7.52 (d, 2 H), 7.52–7.60
(d, 2 H).
(0.048 mol) N,N-dibutyaniline was added. The mixtꢀure
ꢀ
was refluxed at 110 C for 5 h and then cooled to 0 C.
A solution of 2 N NaOH in 500 mL of cold water was
added slowly with stirring. The stirring was continued
for an hour and the resulting solution was extracted with
dichloromethane. The combined extract was washed with
saturated sodium bicarbonate, and with water. The organic
layer was dried over anhydrous magnesium sulfate. The
solvent was removed under reduced pressure and the prod-
uct was purified by column chromatography on silica gel.
The compound G2 was obtained as a yellow liquid of 5.5 g
1
(yield: 48.4%). H NMR (CDCl3, ppm): 9.67 (s, 1 H), 7.7
2.1.3. Synthesis of 4-(Diphenylamino)-
Benzaldehyde (F2)
(d, 2 H), 6.6 (d, 2 H), 3.35 (t, 4 H), 1.56 (m, 4 H), 1.37
(m, 4 H), 0.95 (t, 6 H).
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6.2 mL (0.08 mol) of DMF was cooled to 0 ꢀC and
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treated dropwise with 3.8 mL (0.043 mol) of phospho-
2.1.6. Synthesis of N,N-Dibutyl-4-
Vinylbenzenamine (G3)
rus oxychloride. The solution was stirred at 0 ꢀC for
an hour, and at room temperature for another hour. To
the stirred solution 10 g (0.042 mol) of triphenylamine
in dichloromethane ꢀwas added drop wise. The mixtꢀure
was refluxed at 90 C for 5 h and then cooled to 0 C.
Then a solution of 2 N NaOH in 500 mL of cold water
was added slowly with stirring. The reaction mixture was
stirred for an additional hour and the resulting solution
was extracted with dichloromethane. The combined extract
was washed with saturated sodium bicarbonate, followed
by washing with water. The organic layer was dried over
anhydrous magnesium sulfate and concentrated. The prod-
uct was purified by column chromatography on silica gel.
The compound F2 was obtained as a yellow solid of 4.5 g
A mixture of sodium hydride (6.2 g, 0.28 mol) and methyl-
triphenyl phosphonium bromide (27.6 g, 0.077 mol) in dry
THF solution was added to the solution of G2 (6.0 g,
0.026 mol) in dry THF. The mixture was refluxed at 60 ꢀC
for 12 h, cooled to room temperature and added to MeOH.
The combined extract was washed with saturated sodium
bicarbonate followed by water. The organic layer was dried
over anhydrous magnesium sulfate. The product was puri-
fied by column chromatography on silica gel. The com-
pound G3 was obtained as a yellow liquid of 3.2 g (yield:
1
53.7%). H NMR (CDCl3, ppm): 7.25 (d, 2 H), 6.6 (m,
2 H), 3.2 (t, 4 H), 1.56 (m, 4 H), 1.37 (m, 4 H), 0.95
(t, 6 H).
1
(yield: 40.3%). H NMR (CDCl3, ppm): 9.97 (s, 1 H),
7.67 (d, 2 H), 7.32 (t, 4 H), 7.14 (d, 6 H) 6.98 (d, 4 H).
2.1.7. Synthesis of TPA Dye Z1
2.1.4. Synthesis of N,N-Diphenyl-4-
Vinylbenzenamine (F3)
The mixture of 2,7-BTPF (0.1 g, 0.12 mmol), F3 (0.08 g,
0.29 mmol), bis(triphenylphosphine) palladium (II) chlo-
ride (0.1 g, 0.15 mmol), and tri-o-tolyphosphine (0.1 g,
0.33 mmol) were added along with 5 mL of triethy-
A mixture of sodium hydride (3 g, 0.13 mol) and methyl-
triphenyl phosphenium bromide (10 g, 0.028 mol) in dry
THF solution was added to the solution of F2 (3 g,
0.011 mol) in dry THF. The mixture was refluxed at 60 ꢀC
for 12 h and then cooled to room temperature. The solution
was added drop wise to MeOH. The resultant mixture was
ꢀ
lamine to 10 mL of DMF at 60 C/under argon. The solu-
tion was then refluxed at 110 ꢀC for 48 h and cooled
to room temperature. The solution was extracted with
dichloromethane. The extract was washed with sodium
4404
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Lim et al.
Synthesis and Photophysical Properties of Two-Photon Absorbing Spirofluorene Derivatives
carbonate solution, and dried over anhydrous magnesium
sulfate and concentrated. The product was purified by col-
umn chromatography on silica gel. The compound Z1 was
1
obtained as a yellow powder of 0.03 g (yield: 20.4%). H
NMR (CDCl3, ppm): 7.7 (d, 2 H), 7.5 (t, 4 H), 7.39 (d,
4 H), 7.29–7.19 (m, 20 H), 7.12–6.9 (m, 40 H).
2.1.8. Synthesis of TPA Dye Z2
The mixture of 2,7-BTPF (0.1 g, 0.12 mmol), G3 (0.06 g,
0.29 mmol) and bis(triphenylphosphine)palladium (II)
chloride (0.1 g, 0.15 mmol), tri-o-tolyphosphine (0.1 g,
0.33 mmol) were dissolved in 5 mL of triethylamine
and 10 mL of DMF at 60 ꢀC under argon. The mix-
ture was then refluxed at 110 ꢀC for 48 h and cooled
to room temperature. The solution was extracted with
dichloromethane, and the extract was washed with sodium
carbonate solution. The organic layer was dried over anhy-
drous magnesium sulfate and concentrated. The product
was purified by column chromatography on silica gel. The
compound Z2 was obtained as a yellow powder of 0.07 g
(yield: 51.0%). 1H NMR (CDCl3, ppm): 7.7 (d, 2 H), 7.49
(t, 4 H), 7.37 (d, 4H), 7.29–7.19 (m, 8 H), 7.19–7.02
(m, 12 H), 7.0–6.82 (m, 12 H), 6.6 (d, 4 H), 3.26 (t, 8 H),
1.56 (m, 8 H), 1.37 (m, 8 H), 0.95 (t, 12 H). (NMR data
of Z1 and Z2 are available as supplementary data).
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2.2. Spectroscopic Measurements
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UV-vis absorption and fluorescence spectra were recorded
on Shimadzu 310 pc spectrophotometer and
a
a
Scheme 1. Synthetic routes for Z1 and Z2.
Horiba/Jobin-Yvon spectrofluorometer (SPEX 270 M) in
toluene, THF and DMF solution (1×10−5 M for all com-
pounds). The TPA cross-sections were measured with
two-photon excited fluorescence method employing a Ti-
Sapphire femtosecond laser with a pulse width of 85 fs.
Fluorescein (in 0.1 N NaOH) was used as a reference dye
for the TPA measurements.
coupled with triphenylamine or N,N-dibutylaniline at the
end of molecules, were synthesized by the reaction of
9,9-bis(4-diphenylaminopenyl)-2,7-dibromofluorene (2,7-
BTPF) with N,N-diphenyl-4-vinylbenzenamine (F3) or
N,N-dibutyl-4-vinylbenz-enamine (G3). After purification
of final products, the yields of TPA dyes Z1 and Z2 were
20.4 and 51.0%, respectively. The structures of all interme-
diates and the final compounds were confirmed by spec-
tral analysis. Both compounds are well soluble in common
organic solvents like DMF, and O-chlorobenzene.
2.3. Two-Photon Microfabrication Setup
A titanium sapphire laser mode-locked at 80 MHz and a
780 nm wavelength with pulses of less than 100 fs were
utilized as the light source for two-photon based micro-
fabrication. A set of two galvano mirrors was used to
move the focused laser beam in the horizontal plane, and
a piezoelectric stage was used for the vertical alignment
of the beam. The femtosecond laser source is focused on
the volume of the resin through a microscope with a high
numerical aperture lens.18–20
3.2. One-Photon Absorption and
Fluorescence Emission
The one-photon absorption and emission studies of Z1 and
Z2 were carried in toluene, THF and DMF (Fig. 1). The
one-photon absorption maxima for the Z1 and Z2 in the
solutions do not vary much. The maxima was found to
be slightly red-shifted with the increasing solvent polarity
(toluene < THF < DMF). However, the fluorescent spectra
showed greater variations depending on the nature of the
solvent. This trend in fluorescent emission is well known
in literature and is associated with the rearrangement of
3. RESULTS AND DISCUSSION
3.1. Synthesis of TPA Dyes
As shown in Scheme 1, the two-photon absorbing dyes
(Z1 and Z2), in which spirofluorene ꢁ-centers are
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Synthesis and Photophysical Properties of Two-Photon Absorbing Spirofluorene Derivatives
Lim et al.
(a)
3.3. Two-Photon Absorption
TPA cross-section (ꢅ2ꢆ of the spirofluorene derivatives
were measured using the two-photon induced fluorescence
method.10 The graphs for two-photon induced fluorescence
excitation (TPEF) spectra of Z1 and Z2 are given in
Figure 2. And the TPA values are summarized in Table I.
The compound Z2 was found to have a greater ꢅ2 than Z1
in 10−5 M of toluene solution. This can again be attributed
to the greater charge separation in the Z2 substituted with
a butylamine moiety when compared to the Z1 which con-
tains with a triphenylamine moiety. It is well known that an
increase in charge transfer would lead to an increase in the
ꢅ2 value for centrosymmetric quadrupolar molecules.10ꢀ14
For the compound Z1, the interaction of a lone pair in
the amine units with surrounding aromatic rings decreases
its electron donating ability. This effect is absent in the
case of dibutylamine. The lone pair electrons in the butyl-
substituted amino group in Z2 interact to greater extent to
the core of the molecule than the lone pair electrons on
the triphenyl amine moiety in Z1. This would result in the
increase of charge transfer in Z2.
(b)
3.4. Two-Photon Lithography with Z1 and Z2
In contrast to conventional lithography involving a series
of masking, exposing and developing stages; two-photon
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lithography offers a simpler procedure which involves the
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direct writing of the required structures in a photoactive
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medium. The TPA behavior has an inverse dependence
on the intensity of the incident radiation. Because of this
dependence it has an inherent spatial selectivity in induc-
ing chemical changes in a very small region around the
focus of the radiation used.24–28 The TPA dyes are used
as photosensitizers in two-photon lithography. Achieving
high resolution structures by this technique depends on
carrying out fabrication at very low powers of the incident
radiation. Chromophores with high TPA activities give
Fig. 1. UV/vis absorption and emission spectra of (a) Z1 and (b) Z2
in toluene, THF and DMF. “Three traces with dense symbols to the left
represent the absorption, and three traces with spaced symbols to the
right represent emission.”
solvent shell around the excited state molecules.21–23 The
excited state leads to a greater charge separated state which
is stabilized by the polar solvent causing a red-shift in
the fluorescent emission. The absorption and fluorescence
properties of Z1 and Z2 are summarized in Table I. No
one-photon absorption was found for these compounds
above 530 nm, which makes them potential candidates for
two-photon studies in the NIR region.
Table I. One and two-photon photophysical properties of Z1 and Z2
chromophores in toluene.
Compound
ꢂa
ꢃabs (nm)
ꢃflu (nm)
ꢄb
ꢅ2 (GM)c
Z1
Z2
5.1
4.1
414
417
457
461
0.59
0.55
1140
2596
Notes: amolar extinction coefficient (10−5 M−1 cm−1ꢆ; bfluorescent quantum yield;
cTPA cross-section (1 GM = 1×10−50 cm4/photon·molecule); experimental uncer-
tainty 15%.
Fig. 2. Two-photon-induced fluorescent excitation (TPEF) spectra of
Z1 and Z2.
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Lim et al.
Synthesis and Photophysical Properties of Two-Photon Absorbing Spirofluorene Derivatives
flexibility over the laser powers that can be used for fabri-
cation. Ideally the TPA sensitizer having a large ꢅ2 value
should be easily dispersible in photopatternable resins.
The femtosecond laser source is focused on points inside
the photopolymerizable resin to induce very specific poly-
merization reactions. Complex structures can be directly
written using the technique simply by manipulating the
position of the focus. We tested both Z1 and Z2 as photo-
sensitizers in two-photon lithography. Both chromophores
were mixed well (0.1 wt%) into SCR 500 resin used for
fabricating microstructures. Scanning electron microscope
(SEM) images of three-dimensional structures fabricated
at the same power by two-photon lithography of resins
containing Z1 and Z2 are shown in Figure 3. It is obvious
from the SEM images that Z2 shows greater TPA sensiti-
zation than Z1 for a given power. This can be discerned
from the greater line thickness of the structures fabricated
in case of Z2 because of its greater TPA cross-section. At
a fabrication power of 100 mW the respective line widths
of structures fabricated by Z1 and Z2 were 450 nm and
525 nm, respectively. The higher TPA sensitivity of Z2
allows us to lower fabrication power to achieve smaller
dimensions. Microstructures fabricated from resins sensi-
tized with Z2 at 30 mW are showed in the figures(f) and
(f-1 to f-3). The fabrication was not possible with resins
sensitized with Z1 at the same power.
4. CONCLUSION
We have designed and synthesized highly active spiro-
fluorene based chromophores Z1 and Z2. N,N-dibutyl-
4-vinylbenzenamine was found to be a better electron
donating moiety compared N,N-diphenyl-4-vinylbenzen-
amine resulting in a higher TPA cross-sections. Both
compounds were found to be useful as sensitizers in two-
photon lithography. Because of the higher TPA cross-
section of Z2 it could be used at very low powers for
fabrication of microstructures.
Acknowledgment: This research was supported by the
National Research Foundation of Korea (2010-000499 and
No. R11-2007-050-00000-0).
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Received: 9 May 2010. Accepted: 21 July 2011.
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