Network polydiacetylene films: Preparation, patterning, and sensor applications

Article abstract of DOI:10.1002/adfm.201002042

The synthesis, characterization, and functionalization of polydiacetylene (PDA) networks on solid substrates is presented. A highly transparent and cross-linked diacetylene film of DCDDA-bis-BA on a solid substrate is prepared first by tailoring the monom

Full text of DOI:10.1002/adfm.201002042

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Network Polydiacetylene Films: Preparation, Patterning,
and Sensor Applications
Joosub Lee, Oktay Yarimaga, Cheol Hee Lee, Yang-Kyu Choi,* and Jong-Man Kim*
blue-to-red color transition, which occurs
with environmental perturbations such
The synthesis, characterization, and functionalization of polydiacetylene
(PDA) networks on solid substrates is presented. A highly transparent and
cross-linked diacetylene film of DCDDA-bis-BA on a solid substrate is pre-
pared first by tailoring the monomers with organoboronic acid moieties
as pendant side groups and consequent drop-casting and dehydration
steps. Precisely controlled thermal curing plays a key role to obtain prop-
erly aligned diacetylene monomers that are closely packed between the
boronic acid derived anhydride structures. A second cross-linking, which
occurs by polymerization of the diacetylene monomers with UV irradia-
tion, induces a transparent to blue color shift. Accordingly, colored image
patterns are readily available by polymerization through a photomask.
The color change that takes place as a response to various organic
solvents can be simply detected by naked eyes. The thermofluorescence
change of PDA networks is demonstrated to be an effective method
by which to obtain the microscale temperature distribution of thermal
systems. The ease of film formation and stress-induced blue-to-red color
change with a simultaneous fluorescence generation features of the
network structure should find a great utility in a wide range of chemical
and thermal sensing platforms.
as heat (thermochromism),[23,24] mechan-
ical stress (mechanochromism),[25–28]
organic solvents (solvatochromism),[29–33]
and ligand–receptor interactions (affi-
nochromism).[34–45] These stress-induced
responses that generate a simultaneous
fluorescence signal have efficiently been
applied to design of bio- and chemo-
sensors,[34–45] molecular switches,[46,47]
photonic materials,[48] and information dis-
plays.[49] Recent efforts on the utilization of
thermofluorescence characteristic of PDAs
have led to the development of diverse
functional approaches to employ solution-
based and film-embedded supramolecules
as temperature indicators.[50–52]
Regardless of the sensing mechanism,
formation of uniform and stable films
of PDAs on solid substrates for most of
the as-mentioned applications has been
a common demand. However, PDAs, in
general, are quite insoluble in common
organic solvents due to their highly aggre-
gating nature, which has put a severe
1. Introduction
limitation on the fabrication of well-defined spin- and drop-
cast layers. Relatively reliable examples of the soluble PDAs
have been demonstrated by modification of side chain func-
tionalities with substituted urethanes (alkoxycarbonylmethyl-
urethanes (ACMUs) and aryl/alkyl urethanes),[53–56] carbazole
derivatives,[57] and substituted phenoxyphenyl moieties.[58] The
PDAs derived from these monomers are in general prepared
from γ-ray irradiation of crystalline or powdered diacetylene
monomers. Soluble PDAs are then obtained by extraction with
organic solvents.
The Langmuir–Blodgett (LB) or Langmuir–Schaefer (LS)
deposition method can yield transparent PDA films.[34] How-
ever, PDA films prepared from these methods are mechani-
cally weak. Drop- or spin-casting of diacetylene monomers on
a solid substrate followed by UV irradiation tends to result
in opaque and poor quality PDA films because of the highly
aggregating nature of the diacetylene monomers. Brinker
and co-workers reported a new approach for the fabrica-
tion of transparent PDA films using alkoxysilane-containing
diacetylene monomers.[59] The method described in that work
requires acidic hydrolytic conditions and matrix monomers
to prepare the transparent PDA nanocomposite films. Fab-
rication of transparent PDA layers without employing other
Polydiacetylenes (PDAs) are conjugated polymers with alter-
nating ene–yne backbone structures. Unlike other conjugated
polymers, PDAs are uniquely prepared by UV or γ-irradiation
of molecularly assembled diacetylene monomers without
employing additional catalysts or initiators.[1–22] Undoubt-
edly, the most attractive property of PDAs is their brilliant
J. Lee, C. H. Lee, Prof. J.-M. Kim
Department of Chemical Engineering
Hanyang University
Seoul 133-791, Korea
E-mail: jmk@hanyang.ac.kr
Dr. O. Yarimaga, Prof. J.-M. Kim
Institute of Nanoscience and Technology/Asian Research Network
Hanyang University
Seoul 133-791, Korea
Prof. Y.-K. Choi
Department of Electrical Engineering
Korea Advanced Institute of Science and Technology (KAIST)
Daejeon 305-701, Korea
E-mail: ykchoi@ee.kaist.ac.kr
DOI: 10.1002/adfm.201002042
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2. Results and Discussion
O
B
HO
HO
B
HO
2.1. Structure, Stability, and Characterization
of PDA Networks
O
OH
B
OH
B
O
B
OH
B
O
OH
B
OH
HO
B
HO
OH
HO
B
OH
B
Previous studies have shown that the poly-
merization of diacetylene chains is strongly
affected by aromatic interactions, hydrogen
bonding, and hydrophobic interactions
especially between the adjacent functional
O
O
HO
B
OH
HO
B
B
B
O
HO
HO
OH
O
B
B
OH
groups.[60] Accordingly,
a
pronounceable
OH
polymerization is achievable only if the
diacetylene monomers are properly tailored,
well-aligned, and closely packed. In order to
prove the effect of the packing density of the
monomers and the functionality of boronic
acid side groups on polymerization, a set of
analog molecules of 10,12-docosadiynedioic
acid (DCDDA) derivatives (DCDDA-bis-mBzA
1, DCDDA-bis-mAPBA 2, and DCDDA-
bis-mHPBA 3) were investigated (Figure 2).
The benzoic acid terminated diacetylene
monomer DCDDA-bis-mBzA 1 was employed
to compare its film-forming property with
that of the structurally analogous phenylbo-
ronic acid containing monomer DCDDA-
bis-mAPBA 2. Results obtained with the two
diacetylene monomers, DCDDA-bis-mBzA
1 and DCDDA-bis-mAPBA 2, should reveal
the significance of the boronic acid groups in
terms of network forming ability. Addition-
ally, the ester group containing diacetylene
60oC 30min
i. Drop-casting
ii. Solvent evaporation
150oC 25min
O
O
B
O
B
OH
O
O
O
B
B
B
OH
OH
O
*
B
B
O
O
*
B
B
O
B
B
O
B
O
O
O
HO
B
O
OH
B
O
UV irradiation
iv. Polymerization
iii. Condensed packing
Figure 1. A scheme of the fabrication of a networked polydiacetylene film with DCDDA-bis-BA 1.
monomer DCDDA-bis-mHPBA 3 was selected to gain informa-
tion about the role played by amide hydrogen bonding interac-
tions in the networked PDA supramolecules.
The diacetylene monomers 1-3 were readily prepared in
one step by coupling 10,12-docosadiynedioic acid dichloride
matrix polymers or harsh conditions has been a challenging
task.
Here, we present a new approach for the preparation of
PDA networks on solid substrates. The theoretical layout
of this new method has been drawn by considering that
organoboronic acids form anhydride structures by dehy-
dration upon heating. As depicted in Figure 1, the strategy
involves three distinct steps: formation of a solid film layer
by drop-casting, a consequent thermal annealing, and
polymerization of the diacetylene networks via UV irradia-
tion. Accordingly, diacetylene monomers that are tailored
to have two boronic acids at the ends are properly aligned
during anhydride formation and closely packed into cross-
linked networks. Spectral and optical analyses reveal that
the compactness of the monomer chains is the key feature
that affects the tendency toward highly uniform polym-
erization over a large distance. Adopting the method to a
conventional photolithographic process, selectively cross-
linked PDA optical and fluorescence image patterns can
be obtained by polymerization of the monomer network
through a photomask. The chemical sensitivity test of the
samples to various organic solvents results in a distinguish-
able color contrast difference as observed by the naked-eye.
In addition, we have successfully implemented the irrevers-
ible thermofluorescence transition feature of the PDA net-
works on a microheater system to monitor the temperature
gradients on the device.
(A)
DCDDA-bis-mBzA (1)
O
H
N
OH
6
O
O
O
O
HO
(B)
HO
N
H
6
O
DCDDA-bis-mAPBA (2)
OH
B
H
N
OH
6
O
B
N
H
6
OH
(C)
DCDDA-bis-mHPBA (3)
OH
B
O
OH
6
O
HO
B
O
6
OH
Figure 2. The structures of diacetylene monomers investigated in this
study. A) DCDDA-bis-mBzA 1. B) DCDDA-bis-mAPBA 2. C) DCDDA-
bis-HPBA 3.
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(DCDDA-Cl) with the corresponding amino or hydroxy benzoic
acids or phenylboronic acids.
A monomer solution (50 mM) in 2-methoxyethanol was
prepared before drop-casting on a glass substrate. In contrast
to the previously reported coating methods of thin PDA film
structures, such as thermal evaporation,[61] self assembly,[16,34]
and spin-coating,[62] here, drop-casting was employed for sev-
eral advantages. LB deposition is highly sensitive to substrate
and fabrication conditions. The drawback of spin-coating is
the low material efficiency due to loss of almost 95% of the
materials during spinning, while only the remaining is used to
obtain a thin 10,12-pentacosadiynoic acid (PCDA) layer. Spin-
coating also lacks processability on topographically nonplanar
surfaces, which constitutes a certain constraint on the use of
the method for device-based applications. Drop-casting, being
the simplest of all, produces very reliable and relatively thicker
layers of several micrometers. It is also cost effective and can
be applied on not only the planar substrates but also the sur-
face of microfabricated devices with varying heights. Following
the drop-casting, a two-step curing was performed at 60 °C for
30 min and 150 °C for 25 min.
Scanning electron microscopy (SEM) images of the drop-
cast and cured films are shown in Figure 3. Isolated microcrys-
tals can be clearly seen from the DCDDA-bis-mBzA 1 derived
film (Figure 3A). In addition, the photograph of the film pre-
pared from DCDDA-bis-mBzA 1 demonstrates that only an
opaque film can be achieved from the monomer (Figure 3A,
inset). The two red lines shown in the inset of the figure are
drawn on a paper and the glass substrate coated with diacety-
lene monomer is placed on the paper to test the transparency
of the film. In contrast, drop-casting and curing of the thin
film derived from DCDDA-bis-mAPBA 2 result in formation
of isolated microcrystals and yield a transparent film, as seen
in Figure 3B. The observations described above demonstrate
the significance of the boronic acid groups for the formation
of a transparent diacetylene film. Interestingly, similar to the
monomer DCDDA-bis-mBzA 1, an opaque film with grass-like
giant polymeric leafs was obtained with the ester-functionalized
boronic acid DCDDA-bis-mHPBA 3 (Figure 3C). This indicates
that the hydrogen-bondable internal amide groups, as well
as the boronic acid moieties, are necessary in the diacetylene
monomer for the preparation of a transparent film.
Fourier transform infrared (FTIR) analysis of the mole-
cule DCDDA-bis-mAPBA 2 before (a) and after (b) the curing
step, depicted in Figure 4, provides additional evidence for
the development of cross-linked sites through anhydride for-
mation. Indeed, the intensity of –OH peak of boronic acid at
around 3400 cm−1 was weakened and a new absorption peak at
734 cm−1 was generated and can be assigned to the boronic
anhydride structure.[63]
We observed even more eminent evidence showing the effect
of thermal treatment on the packing density, and hence on the
polymerization, of the network film prepared with monomer
DCDDA-bis-mHPBA 3. It should be noted that well-arranged
diacetylene monomers, in general, shift to blue color when
polymerized, whereas any disorder in the arrangement may
cause a distortion of the color transition. Figure 5 shows the
films that were obtained by UV irradiation of the samples after
annealing temperatures of 100 °C, 120 °C, and 150 °C. As
Figure 3. SEM images and photographs (inset) of drop-cast and cured
films obtained with DCDDA-bis-mBzA 1 (A), DCDDA-bis-mAPBA 2 (B),
and DCDDA-bis-mHPBA 3 (C). The two red lines shown in the inset of each
figure are drawn on a paper and the glass substrate coated with diacetylene
monomer is placed on the paper to test the transparency of the film.
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reason was considered to be that the monomers were cross-
linked through anhydride side groups, yet were not closely
packed enough to provide polymerization among the diacety-
lene backbones. Thus, a post-heating process was applied that
yielded a transparent and also polymerizable monomer net-
work structure. As shown in the visible spectra of the film in
Figure 6A, the absorption gradually increases and reaches a
maximum after ca. 10 min of irradiation with an intense blue
color at a characteristic absorption peak of around 640 nm.
The stress-induced color change of the network structure
was investigated in the next phase of our studies. The ther-
mochromic behavior of a polymerized blue-colored film of
monomer DCDDA-bis-mAPBA 2 is shown in Figure 6B. The
film undergoes an irreversible blue-to-red color transition
as a response to thermal stress, which generates a shift of
the absorption peak from 640 nm to 540 nm. As illustrated
in Figure 6B and in the inset picture, there is a temperature
margin of about 70 °C from 50 °C to 120 °C over which the red
transition occurs.
a
a
b
b
3400
3400
775566
500 1000 1500 2000 2500 3000 3500 4000
Wavenumber (cm-1)
Figure 4. FTIR spectra of the monomer DCDDA-bis-mAPBA 2 before
(a) and after (b) thermal curing.
confirmed by the different responses of the samples, the best
arrangement of the monomers occurred at 150 °C with the
blue transition, and the other two samples were proved to be
in disordered states with purple and red colors. The results pre-
sented in Figure 5 also provide some useful information about
color control of the PDA film. It is interesting that red-, purple-,
or blue-colored PDA films can be readily achieved by simply
manipulating the annealing temperature of the monomer film.
To the best of our knowledge, this is the first example of PDA
color control based on the annealing temperature strategy.
Intriguingly, although the preheating of the drop-cast solu-
tion of monomer DCDDA-bis-mAPBA 2 afforded a transparent
solid film as a result of solvent evaporation, the sample did not
show any polymerization phenomena via UV irradiation. The
2.2. Fabrication of Patterned Images
We also made an effort to fabricate colored micropatterns on the
film by UV irradiation via a photomask.[62,64] Figure 7 shows
the obtained optical and corresponding fluorescence images of
the film with sequential irradiation and heating steps. Depicted
in Figure 7A are the images taken after the first irradiation
through a photomask. As a weak fluorescence was observed
from the monomer film, the blue phase did
not emit any fluorescence, which meant
polymerization phenomenon quenched the
original fluorescence from the transparent
diacetylene film. As expected, heating the film
at 130 °C caused a transition of blue images
to red with an intense fluorescence signaling
from the image patterns (Figure 7B). At first,
it seems that the red fluorescence from the
non-irradiated transparent part in Figure 7B
was quenched due to heating. However, this
is simply due to the fact that intense red fluo-
rescence of the optically red patterns screens
relatively weak fluorescence of the monomer
region. Additionally, a second maskless UV
irradiation of the overall film produced dual-
colored optical patterns with clear-edged
fluorescence images because polymerization
of the monomer part quenched the weak
background fluorescence of that region
(Figure 7C). As a result, both positive and
negative fluorescence images were obtain-
able using a single photomask.
2.3. Application of PDA Networks for
Differentiation of Organic Solvents
Figure 5. Photographs of the films prepared with monomer DCDDA-bis-mHPBA 3 on glass by
drop-casting and different thermal curing conditions and the corresponding color responses
after 254 nm UV irradiation for 10 min. Note that perfect polymerization with the blue color
shift occurs after curing at 60 °C for 30 min and 150 °C for 25 min.
Fabrication of efficient chemosensors to detect
various materials that have environmental,
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biological, or diagnostic significance has a great impact
among many scientists. We have recently reported a new
strategy for colorimetric detection of volatile organic
compounds (VOCs) based on PDA-embedded electrospun
microfibers.[30,65] Accordingly, differentiation of the organic
solvents had been achieved by arising color patterns that were
distinguishable with respect to several VOCs, such as chloro-
form, hexane, ethyl acetate (EA), and tetrahydrofuran (THF).
Here we anticipated that if the PDA network film exhibits a
selective solvatochromic feature in such organic solvents then
it would lead to the development of a simple strategy for a color
contrast-based chemosensor. Figure 8 shows the colorimetric
responses of the PDA film samples prepared with monomer
DCDDA-bis-mAPBA 2 that were exposed to common organic
solvents. Preserving the morphological stability, the samples
displayed vivid color changes, which were detectable by eye,
in varying and combinational tones of blue and red. THF was
apparently superior to other solvents in disturbing the blue-
phase PDA network.
(A)
UV
0 min
2 min
4 min
6 min
8 min
10 min
254nm
1.0
0.5
0.0
400
500
600
Wavelength (nm)
700
800
(B)
50oC
60oC
Heat
120 o
70oC
C
1.0
80oC
90oC
100oC
110oC
120oC
0.5
0.0
400
500
600
700
800
2.4. Temperature-Monitoring System Using the
Thermofluorescence Feature of PDA Networks
Wavelength (nm)
Figure 6. Visible absorption spectra of the networked film prepared with
monomer DCDDA-bis-mAPBA 2 and irradiated with UV (254 nm) for var-
ying expose times (A) and the visible absorption change as the blue phase
film was heated from 50 °C to 120 °C for 10 s at each step (B). The insets
show photographs of the films after UV irradiation for 10 min (inset of (A))
and heat treatment of 120 °C for 10 min (inset of (B)).
To date, the thermochromic and thermofluorescence features of
PDAs have been extensively investigated and proposed as tem-
perature sensors in the forms of solutions, mono- and multi-
layer films, and even vesicles that are embedded in polymer
matrices.[50–52,66] However, the feasibility of the reported features
still remains at the primitive state due to the technical chal-
lenges coming from the film-forming methods, which are usu-
ally substrate selective, and the lack of detectable color contrast.
Quantification of the color contrast is another constraint on the
utilization of PDAs as efficient temperature-sensitive materials.
In addition to optical color transitions, a thermally induced
non-fluorescence to fluorescence transition of PDAs can be
harnessed as a very useful tool for temperature-monitoring
systems. In parallel efforts aimed at the functionalization of
PDAs as temperature indicators, we successfully implemented
the thermofluorescence property of PDA networks to obtain
the temperature profile of a solid surface. A simple heat source
was designed and fabricated as an array of serpentine-type Au
microheaters on a silicon substrate as shown in Figure 9. A
film of monomer DCDDA-bis-mAPBA 2 was then prepared on
the device and polymerized by UV irradiation to obtain a blue-
phase PDA network structure (Figure 9). The microheaters were
supplied with varying current levels to generate heat deviations
on the substrate surface, and the corresponding fluorescence
images were acquired and are shown in Figure 10 (left column).
The figures indicate that due to continuous heat generation from
the microheaters, the network film exhibits an increasing trend
of fluorescence intensity on the overall surface. This result is in
good agreement with the well-known thermofluorescence fea-
ture of PDAs. However, more precise temperature-dependent
fluorescence profiles of the surface can be extracted by
quantification of the images with a software program. As illus-
trated in Figure 10 (right column), the fluorescence inten-
sity diagrams reveal the mechanism of heat conduction from
microheaters to the silicon substrate. While the fluorescence
Figure 7. The optical (left) and fluorescence (right) images of the
micropatterns fabricated on the network film of monomer DCDDA-bis-
mAPBA2.A)AfterUV(254nm)irradiationthroughaphotomaskfor10min.
B) After heating the film in (A) at 130 °C for 10 s. C) After maskless UV
(254 nm) irradiation of the film in (B) for 10 min. Exposure times of the flu-
orescence images in (A–C) were 1/5 s, 1/25 s, and 1/12 s, respectively.
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3. Conclusions
Using a new concept for tailoring diacety-
lene monomers with boronic acid pendant
side groups, we successfully prepared drop-
cast and photopolymerizable diacetylene
network films on solid substrates. The com-
Figure 8. Photographs of the PDA network films after exposure to various organic solvents
for 30 s.
pactness of the monomers forming the net-
work was particularly arranged by thermal
annealing, which was shown to be a crucial
intensity peaks in the regions where the heat sources are popu-
lated in Figure 10A, increasing the current level on the micro-
heaters causes heat transfer from the center to corners through
silicon and creates thermal contours, as depicted in Figure 10B.
Figure 10C shows that a further increase in the current level
produces new contours, but with smaller fluorescence inten-
sity differences due to good thermal conductivity of silicon
(149 W m−1 K−1 ).
step for proper polymerization. Using the difference of the film
structure before and after polymerization, a photolithographic
process was adopted to generate colored images. The intriguing
chromatic and fluorescence features of the polymer network
afforded the construction of efficient chemo- and thermal sen-
sors. Selective color transition in response to various organic
solvents was employed to develop a color-contrast-based dif-
ferentiation of the solvents. In addition, in contrast to previous
reports on the use of PDA films as thermochromic indicators,
the irreversible non-fluorescence to fluorescence switching fea-
ture of the network film provided a simple method by which to
monitor heat deviations of a thermal system. Accordingly, the
fluorescence intensity variations of the PDA film were corre-
lated with the temperature gradients of the underlying surface.
In this way the thermal fluctuations on a device surface were
promptly monitored and quantified using the thermal sensitivity
of the PDA network. This method should find great utility in
the thermal systems, especially where the detection of regional
Figure 10. Fluorescence images (left) and the corresponding fluores-
cence intensity diagrams (right) of the microheater array coated with PDA
network film. The fluorescence intensity contours appeared depending on
the surface temperature variations after supplying the microheater array
with the current levels of A) 0.150 A, B) 0.175 A, and C) 0.200 A for 10 s
at each step.
Figure 9. Optical microscopy images of the microheater array fabricated
on a Si substrate before (A) and after (B) coating the substrate with a PDA
network film prepared with monomer DCDDA-bis-mAPBA 2.
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conventional lithography and wet-etching processes. The resultant
resistance of the microheater array was measured to be 45.6 Ω.
Preparation and Thermal Monitoring of the Microheater System: A
PDA network film was coated on a microheater array using the same
method of film forming as on glass. The microheaters were supplied
with a potential through a power supply while being monitored with
a fluorescence microscope, (Olympus BX51W/DP70). The array was
supplied with three current levels of 0.150 A, 0.175 A, and 0.200 A, each
for 10 s. The fluorescence measurements after each step were performed
after cooling the substrate to room temperature.
temperature elevations is critical such as power semiconductor
chips and thermal microelectromechanical (MEMS) devices.
4. Experimental Section
Materials: 10,12-Docosadiyndioic acid (DCDDA) was purchased
from GFS Chemicals. DCDDA-bis-mBzA 1 was prepared according to
the published protocol.[67] Oxalyl chloride, 3-aminophenylboronic acid,
3-hydroxyphenyl boronic acid, and 4-hydroxyphenyl boronic acid were
purchased from Aldrich.
Scanning Electron Microscopy (SEM): SEM images of the PDA films
were obtained by using a JEOL (JSM-6330F) microscope. Samples were
coated with Pt for 30 s before analysis.
Synthesis of Docosa-10,12-Diynedioyl Dichloride (DCDDA-Cl): Oxalyl
chloride (612.8 mg, 4.8 mM) was added dropwise to the methylene
chloride solution containing DCDDA (500 mg, 1.4 mM). After 30 min
of stirring, one drop of dimethylformamide (DMF) was added to the
solution and the resulting mixture was stirred for another 4 h. Removal
of the solvent in vacuo afforded the desired DCDDA-Cl in 100% yield and
the product was used for the next reaction without further purification.
Synthesis of DCDDA-bis-mAPBA 2: 3-Aminophenylboronic acid
hemisulfate salt (717.9 mg, 4.1 mM) and triethylamine (1. 4 g, 13.8 mM)
were dissolved in methylene chloride and DCDDA-Cl (551 mg, 1.4 mM)
was dissolved in a small amount of THF. The THF solution containing
DCDDA-Cl was added dropwise into the methylene chloride solution.
The resultant solution was stirred at room temperature overnight and
concentrated in vacuo. The crude residue was dissolved in a small
amount of methanol and added dropwise to water. The precipitates
formed were collected and subjected to column chromatography on a
silica gel with 1:1 ethyl acetate:hexane as an eluent to give the desired
product (yield = 60%). Melting point (m.p): 178-179 °C; 1H NMR
(300 MHz, DMSO-d6): δ = 1.20–1.62 (m, 24H), 2.18 (t, 4H), 2.21–2.38
(m, 4H), 7.22 (t, 2H), 7.45 (d, 2H), 7.72 (d, 2H), 7.82 (s, 2H), 8.00 (s,
4H), 9.79 (s, 2H); 13C NMR (75 MHz, DMSO-d6): δ = 19.00, 25.88, 28.43,
28.93, 29.06, 29.39, 34.37, 37.04, 66.06, 78.71, 121.85, 125.88, 128.26,
129.47, 139.19, 165.60, 171.84, 175.19; IR (KBr) 3487, 2947, 2846, 2158,
1682, 1527, 1489, 1018, 962, 903, 800 cm−1 .
Acknowledgements
J.L. and O.Y. contributed equally to this work. The authors are grateful
to the National Research Foundation of Korea (NRF) for financial
support through Basic Science Research Program (20100018438 and
20100009903), Center for Next Generation Dye-sensitized Solar cells
(20100001844), and International Research & Development Program
(K20901000006-09E0100-00610).
Received: September 28, 2010
Revised: November 9, 2010
Published online: February 8, 2011
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