Tuning photoluminescent wavelength of water-soluble oligothiophene/polymer complex film by proton bonding

Article abstract of DOI:10.1246/cl.2011.264

We synthesized a new oligothiophene derivative with pyridine end groups and investigated their optical properties. Upon adding an aqueous polystyrene sulfonic acid solution, it forms homogeneous polymer complex films with protonated structure. Furthermore

Full text of DOI:10.1246/cl.2011.264

doi:10.1246/cl.2011.264
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Published on the web February 5, 2011
Tuning Photoluminescent Wavelength of Water-Soluble
Oligothiophene/Polymer Complex Film by Proton Bonding
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Mizuho Kondo,* Jun-ichi Miyake, Kazuya Tada, and Nobuhiro Kawatsuki*
Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo,
167 Shosha, Himeji, Hyogo 671-2280
Division of Electrical Engineering, Graduate School of Engineering, University of Hyogo,
167 Shosha, Himeji, Hyogo 671-2280
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Received December 3, 2010; CL-101026; E-mail: mizuho-k@eng.u-hyogo.ac.jp)
We synthesized a new oligothiophene derivative with pyridine
end groups and investigated their optical properties. Upon adding an
aqueous polystyrene sulfonic acid solution, it forms homogeneous
polymer complex films with protonated structure. Furthermore,
reversible color change in photoluminescence in response to a base
due to reversible protonation of pyridyl end groups in oligothio-
phene was demonstrated, which can be applied to acid sensors.
Figure 1. Chemical structure of oligothiophene used in this study.
a) (b)
(
Recently, controlling the electron status of ³-conjugated
materials to optimize optoelectrical properties by external
stimuli has been intensively investigated. In particular, using
hydrogen bonding (H-bonding) or protonation at basic atoms in
³-conjugated linkages has attracted attention because it rever-
sibly varies dynamic features, morphology, and processing
characteristics as well as electrooptical properties of the
materials without synthetic modification.1 This technique has
been applied in photovoltanic devices for tuning such optical
properties as absorption (Abs.) and photoluminescent (PL)
spectra with improvement of dispersity. Lin and co-workers
used liquid-crystalline phenylene­pyridilene derivatives, and
developed supramolecular structure from them and tuned the
optical properties by using such various acidic compounds as
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Figure 2. UV­vis absorption (Abs., (a)) and photoluminescent
spectra upon exposure to 405-nm light (PL, (b)) of 3Th-MP/acid
complexes (3Th-MP/acid = 1/4(mol/mol)) dissolved in THF. The
inset in (b) shows photographs of the complex solution upon
exposure to 365-nm light.
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acid-modified gold nanoparticles, monofunctional acid, and
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bifunctional acid.
chloroform and tetrahydrofuran (THF) due to improvement of
solubility by introducing methyl side branches at pyridyl rings.
Figure 2 shows Abs. and PL spectra of 3Th-MP in THF
solution containing various acids. It was clearly observed that
maximum wavelength shifted from 400 to 430 nm in Abs. and
from 500 to 560 nm in PL spectrum on adding a strong acid (10-
camphorsulfonic acid (CSA)) while both spectra showed no
change on adding other weak acids: 4-decyloxybenzoic acid
(10BA) and crotonic acid (CRA). In addition, the chemical shift
In the previous work, we explored tunability of the PL
wavelength of fluorene (FL) derivatives composed of pyridyl-
cyanovinyl groups by doping various acids. The maxima
wavelength of PL varied according to acidity of the dopants,
which causes H-bonding or protonation.5 Furthermore, we
succeeded in simultaneous light-driven patterning of molecular
alignment and photoluminescent behavior of FL derivatives
by using hydrogen-bonded photocrosslinkable liquid-crystalline
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polymer films as an alignment layer. In this work, we used
of H NMR of around pyridyl rings was observed in 3Th-MP/
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oligothiophene with pyridiyl ring alternatively to achieve
longer-wavelength emission with simple structure. Oligothio-
phenes are promising ³-conjugated materials for organic semi-
conductors, photovoltanic devices, as well as photoluminescent
devices though tunability of optoelectrical properties by acidity
have not yet been much explored because of poor solubility in
CSA complex in comparison to other weak acids (see SI ). The
inset in Figure 2b displays photographs of THF solution of 3Th-
MP and 3Th-MP/CSA complex exhibiting greenish and yellow-
ish photoluminescence upon exposure to 365-nm light. The
maximum shift of PL was a little longer though the shift range
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was smaller than our previous report of FL derivatives. These
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water. We synthesized terthiophene containing pyridine rings
with a methyl side branch at both molecular termini (3Th-MP,
Figure 1) to enhance solubility and explored their photolumi-
nescent behavior.
results indicate that 3Th-MP easily forms a protonated structure
with stable, soluble, and long-wavelength emissive structure by
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adding strong acids while no gradient change occurred in
maximum wavelength due to formation of H-bonding as we
previously reported. Next, aqueous polystyrene sulfonic acid
(PSS) solution was used to prepare protonated 3Th-MP.
Figure 3a shows PL spectra of the aqueous complex solution
containing 3Th-MP with different concentration. A peak at
The synthetic route of the compounds and experimental
details are written in Supporting Information (SI).9
First, we explored change in optical properties of 3Th-MP
by adding various acids. We found that 3Th-MP can dissolve in
Chem. Lett. 2011, 40, 264­265
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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(
a)
(
b)
Figure 4. IR spectra of 3Th-MP combined with PSS in a different
ratio. The highlights indicate absorbance around 1517, 1440, and
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348 cm of the samples.
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0.2 mol%
Figure 3. Change in PL spectra of aqueous solution (a) and
photographs of 3Th-MP/PSS complex film (b) containing 3Th-MP
at different ratio. The films in (b) were irradiated with 365-nm light.
Figure 5. Photographs of the complex film containing 0.24 mol %
Th-MP irradiated with 365-nm light under air (left) and ammonia
right). The inset of the photograph indicates plausible relevant
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67 nm originating from PSS decreased while a broad absorp-
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tion around 560 nm increased with an increase in 3Th-MP
content. The PSS peak also disappeared on adding ammonium
solution, suggesting that the decrease in PL spectra of PSS
resulted from energy transfer from PSS to 3Th-MP and
diacidification. Furthermore, it can form a homogeneous and
transparent film without aggregation of chromophore when
the complex solution is cast onto a glass substrate. The films
prepared with the complex solution exhibited photolumines-
cence upon exposure to 365-nm light with relevant emission
wavelength (Figure 3b). It changed from blue to orange through
whitish emission with increase in thiophene content. In addition,
the melting point of 3Th-MP (240 °C) was erased upon binding
to PSS, and no obvious peak was observed before the
decomposition temperature of PSS (<270 °C, see SI suggesting
that 3Th-MP dispersed in PSS homogeneously and that thermal
stability was enhanced by protonation).
Figure 4 displays FT-IR spectra of PSS, 3Th-MP, and the
complexes to evaluate formation of a proton bonding between
PSS and 3Th-MP. PSS and the complex films were powdered to
compare to 3Th-MP powder. 3Th-MP shows strong peaks at
structure.
times. Dissolving the complex films in water changed the color
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when the solution became basic (see SI ). This suggests that the
color change in the complex films is due to the deprotonation of
3Th-MP under the basic atmosphere.
In summary, we investigated photoluminescent behavior of
PSS/3Th-MP complexes by absorption, FT-IR, and PL spec-
troscopy. They can dissolve in water, and their absorption and
photoluminescent spectra can be tuned by acid resulting from
reversible diprotonation of pyridyl rings in 3Th-MP. This can be
applied to fluorescent acid sensors. Furthermore, water-soluble
conductive polymer complex can be applied by spray-coating as
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well as spin-coating at low temperature to prepare a thin film
due to high processability. The effect of protonation and binding
in PSS on conductance are now under investigation.
This work was supported in part by a Grant-in-Aid for
Scientific Research (B, No. 21225006) from the Japan Society
for the Promotion of Science, and a research grant from The
Hyogo Science and Technology Association.
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594 and 1548 cm assigned to a typical vibration of C=N and
C=C at a free pyridine ring, respectively. In contrast to pure
Th-MP, these peaks disappeared upon biding to a large amount
of PSS (3Th-MP/PSS = 1/20 (mol/mol)) while new peaks at
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References and Notes
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T. Kato, N. Mizoshita, K. Kishimoto, Angew. Chem., Int. Ed. 2006,
5, 38.
D. Gonzalez-Rodriguez, A. P. H. J. Schenning, Chem. Mater., in
press. doi:10.1021/cm101817h
D. Sahu, H. Padhy, D. Patra, D. Kekuda, C.-W. Chu, I.-H. Chiang,
H.-C. Lin, Polymer 2010, 51, 6182.
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517, 1440, and 1348 cm appeared which were not detected in
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pure PSS. These peaks are indicative of formation of protonated
pyridyl ring in 3Th-MP at both molecular terminals. Further-
more, these peaks increased when the feed ratio of chromophore
reached 20 mol %; however, the peaks of pure 3Th-MP appeared
again when the content of 3Th-MP increased in 33 mol %. The
protonated oligothiophene easily reverted to its original structure
in the film under basic condition. Figure 5 shows photographs
of 3Th-MP/PSS complex film under air and ammonia upon
exposure to 365-nm light. The complex contained 0.24 mol %
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T.-C. Liang, H.-C. Lin, J. Polym. Sci., Part A 2009, 47, 2734.
N. Kawatsuki, Y. Minami, J. Lee, Chem. Lett. 2009, 38, 298.
N. Kawatsuki, R. Ando, R. Ishida, M. Kondo, Y. Minami, Macromol.
Chem. Phys. 2010, 211, 1741.
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J. Hock, A. M. W. C. Thompson, J. A. McCleverty, M. D. Ward,
J. Chem. Soc., Dalton Trans. 1996, 4257.
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R. Nakajima, H. Iida, T. Hara, Bull. Chem. Soc. Jpn. 1990, 63, 636.
Supporting Information is available electronically on the Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
0 P. M. Beaujuge, C. M. Amb, J. R. Reynolds, Adv. Mater. 2010, 22,
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Th-MP to clarify the change in color. When the film was kept
under basic atmosphere, the emission immediately changed
greenish from orange, while it reverted to the original color
when kept under air, and the change can be repeated over three
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Chem. Lett. 2011, 40, 264­265
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/