Synthesis of a polyoxadiazole containing the 4-hydroxypyridine group and photo-induced fluorescent imaging on the polymer film

Article abstract of DOI:10.1016/j.reactfunctpolym.2009.12.003

A polyoxadiazole containing the 4-hydroxypyridine group, introduced to enhance the intermolecular interaction between polymer chains as well as for photosensitization, was synthesized from a corresponding precursor polyhydrazide. The absorption and emission spectra of the polymer film exhibited red-shifted maxima compared to those of the polyoxadiazole in chloroform solution, indicative of the presence of significant intermolecular interactions. The synthesized polymer showed a unique fluorescence enhancement upon UV irradiation in solution as well as in the film, presumably due to the photosensitized oxidation of hydroxypyridine groups to produce pyridone structures having different intermolecular interactions. Thus, this phenomenon enabled us to obtain a stable patterned image with enhanced fluorescence intensity in the UV-exposed area of the polymer film without any subsequent processes such as baking or etching.

Full text of DOI:10.1016/j.reactfunctpolym.2009.12.003

Reactive & Functional Polymers 70 (2010) 223–229
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Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Synthesis of a polyoxadiazole containing the 4-hydroxypyridine group and
photo-induced fluorescent imaging on the polymer film
Young-shin Kim a,b, Dai Geun Kim c, Na Young Kwon a, Hyung Jun Kim a,d, Moon Soo Choi a,e, Minjung Lee f,
Taek Seung Lee g,
*
a Organic and Optoelectronic Materials Laboratory, Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764,
Republic of Korea
b Aekyung ChemTech Co., Ltd., R&D Center, 217-2, Shinseong-dong, Yuseong-gu, Daejeon 305-345, Republic of Korea
c Organic and Optoelectronic Materials Laboratory, Department of Nanotechnology, Chungnam National University, Daejeon 305-764, Republic of Korea
d Hyosung Corporation, 183-2 Hoge-dong, Dongan-gu, Anyang, Gyeonggi-do 431-080, Republic of Korea
e Display Materials R&D Center, Samsung Cheil Industries Inc., 332-2 Gocheon-dong, Uiwang, Gyeonggi-do 437-711, Republic of Korea
f Department of Oriental Medicine and Food Biotechnology, Joongbu University, 101 Daehack-ro, Chubu-myon, Kumsan-gun, Chungnam 312-702, Republic of Korea
g Organic and Optoelectronic Materials Laboratory, Department of Advanced Organic Materials and Textile System Engineering and Graduate School of Analytical Science
and Technology (GRAST), Chungnam National University, Daejeon 305-764, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 October 2009
Received in revised form 9 December 2009
Accepted 9 December 2009
Available online 16 December 2009
A polyoxadiazole containing the 4-hydroxypyridine group, introduced to enhance the intermolecular
interaction between polymer chains as well as for photosensitization, was synthesized from a corre-
sponding precursor polyhydrazide. The absorption and emission spectra of the polymer film exhibited
red-shifted maxima compared to those of the polyoxadiazole in chloroform solution, indicative of the
presence of significant intermolecular interactions. The synthesized polymer showed a unique fluores-
cence enhancement upon UV irradiation in solution as well as in the film, presumably due to the photo-
sensitized oxidation of hydroxypyridine groups to produce pyridone structures having different
intermolecular interactions. Thus, this phenomenon enabled us to obtain a stable patterned image with
enhanced fluorescence intensity in the UV-exposed area of the polymer film without any subsequent pro-
cesses such as baking or etching.
Keywords:
Conjugated polymers
Polyoxadiazole
Fluorescence
Fluorescent imaging
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
drawing imine nitrogen (C@N) framework and have attracted
interest because they have a high electron affinity, which facili-
In the past decades, conjugated polymers with extended
p
-con-
tates both electron injection and transport, as well as excellent
mechanical, thermal, and thermo-oxidative stabilities [12,13].
Thus, polymers containing an oxadiazole moiety have been widely
used in electronic devices such as electron-transporting materials
because the presence of the oxadiazole ring in the molecular back-
bone affects the electronic properties of the resulting polymers
[14–18].
A number of studies on fluorescence-controlled organic systems
have been undertaken to develop optical data storage [19,20]. Sys-
tems consisting of tert-butoxycarbonyl (t-Boc)-protected quiniza-
rin or coumarin moieties with a photoacid generator (PAG) are
one of the more prominent examples [21,22]. Most of the cur-
rent-generation methods for the production of fluorescent pat-
terned images in polymer films are closely connected to photo-
induced chemical transformations of substrates in exposed areas
of polymer films in the presence of PAG with the subsequent for-
mation of ionic or hydrogen-bonding interactions between newly
formed reactive groups and functional dyes. This procedure has po-
tential limitations such as the essential requirement for PAG, the
jugation have received a great deal of attention [1,2] because of
their unique properties such as electrical conductivity [3], electro-
luminescence [4], and chemical sensing [5], which have potential
applications in devices such as light-emitting diodes, photovoltaic
cells, and thin-film transistors [6,7]. A number of conjugated poly-
mers with various kinds of structures have been synthesized to
introduce functional groups into the conjugated backbone not only
to change the luminescent properties but also to endow the poly-
mers with new functions [8].
Among these, p-conjugated polymers with nitrogen-containing
fused-ring structures are one of the most promising categories of
fluorescent polymers because of their high fluorescence, high elec-
tron mobility, and easy tailoring of their chemical structure [9–11].
Aromatic oxadiazole-based compounds have an electron-with-
* Corresponding author. Tel.: +82 42 821 6615; fax: +82 42 823 3736.
E-mail address: tslee@cnu.kr (T.S. Lee).
1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2009.12.003

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Y.-s. Kim et al. / Reactive & Functional Polymers 70 (2010) 223–229
long-term instability of patterned images due to the presence of
residual PAG, and the need for subsequent steps such as baking
or etching. As described elsewhere, photolytic or thermolytic
byproducts as well as PAG remaining in the film after image pat-
terning can reduce the image resolution and deteriorate the
long-term stability of the patterned image [23]. Thus, photodegra-
dation-induced fluorescence imaging in the absence of PAG has
been reported [24].
Recently, we demonstrated a fabrication method for a patterned
image on a flexible substrate such as flexible electrospun fiber
mats [25,26] as well as on silicon wafers [27]. We also demon-
strated simple and effective fluorescent imaging on a hydroxyphe-
nylbenzoxazole polymer film by means of UV irradiation without
the aid of PAG and post-processing [28]. The key mechanism of
such patterning is related to the manipulation of intermolecular
quantum-yield measurements were carried out using a fluores-
cence photometer (QuantaMaster Photon Technology International
(PTI)) equipped with an integrating sphere.
2.3. Monomer synthesis
2.3.1. Diethyl 2,5-dihydroxyterephthalate (1)
To a solution of 2,5-dihydroxyterephthalic acid (5 g, 25 mmol)
in 200 mL of ethanol, 13 mL of sulfuric acid was added. The mix-
ture was stirred at 90 °C for 24 h and then cooled to room temper-
ature. The precipitate was filtered and recrystallized from ethanol
to afford 1 (5.07 g, 80%) as a greenish needle-like crystal. 1H NMR
(CDCl3, ppm): d 10.05 (s, 2H), 7.46 (s, 2H), 4.32 (q, 4H), 3.96 (t,
6H). 13C NMR (CDCl3, ppm): d 165.3, 150.8, 120.0, 115.0, 61.1, 13.4.
p-interaction between hydroxyphenylbenzoxazole groups in the
2.3.2. Diethyl 2,5-bis(hexadecyloxy)terephthalate (2)
backbones by UV irradiation.
To a solution of 1 (5 g, 20 mmol) and 1-bromohexadecane
(30.6 mL, 100 mmol) in 200 mL of dimethylformamide (DMF)
K2CO3 (13.6 g, 100 mmol) was added at room temperature. The
suspension was stirred at 90 °C for 96 h. After the reaction, the
reaction mixture was cooled to room temperature, and the solvent
was removed by evaporation under vacuum. The crude product
was purified twice by recrystallization from ethanol, affording
12.1 g (89%) of a white powder. 1H NMR (CDCl3, ppm): d 7.32 (s,
2H), 4.38 (m, 4H), 3.41 (t, 4H), 2.20–1.28 (m, 56H), 0.93 (br, 6H).
13C NMR (CDCl3, ppm): d 165.8, 148.4, 117.9, 114.7, 68.2, 61.4,
32.9, 28.7, 25.6, 23.0, 14.3.
In this work, we report a simple method for synthesizing a con-
jugated oxadiazole with hydroxypyridine groups in the main chain
and for fabricating fluorescent images in the resulting polymer
films in the absence of PAG. It is well-known that the hydroxypyr-
idine chromophore is a component of the sensitizer used for photo-
oxidation to produce pyridine-4(1H)-one during photoirradiation
[29–31]. We conjectured that the presence of hydroxypyridine
groups in the polymer chain would enable the formation of inter-
molecular hydrogen bonds between polymer chains. Upon irradia-
tion, it was expected that the hydroxypyridine moieties would be
converted into pyridone groups, which would lessen the intermo-
lecular interaction between chains. Accordingly, we expected that
the polymer would show a unique fluorescence property in solu-
tion and in film form upon UV illumination, allowing for easy
and simple fluorescent imaging. To our knowledge, this is the first
report of this type of simple fluorescent pattern imaging using the
unique phenomenon of fluorescence-intensity enhancement with-
out the aid of PAG.
2.3.3. 2,5-Bis(hexadecyloxy)terephthalohydrazide (3)
An 8.8-mL (284 mmol) quantity of hydrazine hydrate was
added to a three-necked 500-mL flask containing 100 mL of meth-
anol, and the temperature was raised to 65 °C. A 5-g (7.1 mmol)
quantity of 2 dissolved in 200 mL of methanol was slowly added
into the mixture with vigorous stirring. The reaction was continued
for 24 h. After the reaction, the mixture was evaporated to a vol-
ume of 50 mL. The mixture was cooled, and the precipitate was iso-
lated by filtration. The solid was recrystallized twice from ethanol,
affording 4.2 g (88%) of a white powder. 1H NMR (CDCl3, ppm): d
9.2 (s, 2H), 7.8 (s, 2H), 4.2 (t, 4H), 2.2–1.28 (m, 60H), 0.9 (t, 6H).
13C NMR (CDCl3, ppm): d 164.8, 147.4, 123.2, 112.7, 68.2, 61.4,
31.9, 28.9, 25.6, 23.0, 14.1.
2. Experimental
2.1. Materials and reagents
All the chemicals and reagents were purchased from Aldrich
and used without further purification unless otherwise specifically
noted. 2,5-Dihydroxyterephthalic acid, 1-bromohexadecane, 2-
ethylhexyl bromide, and chelidamic acid were purchased from Al-
drich and used as received.
2.3.4. Diethyl 2,5-bis(2-ethylhexyloxy)terephthalate (4)
To a solution of 1 (5 g, 20 mmol) and 17.6 mL of 2-ethylhexylbr-
omide (100 mmol) in DMF (200 mL) K2CO3 (13.6 g, 100 mmol) was
added at room temperature. The suspension was stirred at 90 °C
for 96 h. After the reaction, the reaction mixture was cooled to
room temperature and the solvent was removed by evaporation
under vacuum. The remaining liquid was transferred to a separato-
ry funnel, and ether was added. The mixture was washed with
water twice and dried over magnesium sulfate. After the removal
of ether under vacuum, 4 was obtained as a yellow liquid (7.8 g,
82%). 1H NMR (CDCl3, ppm): d 7.33 (s, 2H), 4.39 (m, 4H), 3.91 (t,
4H), 1.82–1.03 (br, 24H), 0.92–0.81 (br, 6H). 13C NMR (CDCl3,
ppm): d 165.8, 149.5, 118.5, 114.7, 75.2, 60.4, 40.3, 30.7, 29.8,
24.7, 22.9, 14.3, 12.1.
2.2. Characterization
1H and 13C NMR spectra were collected on a Bruker DRX-300
spectrometer, with tetramethylsilane as an internal standard
(Korea Basic Science Institute). UV–Vis absorption spectra were re-
corded on a PerkinElmer Lambda 35 spectrometer. Luminescence
spectra were collected on a PerkinElmer LS 45 spectrometer, with
a xenon lamp as a light source. The molecular weight was deter-
mined by gel-permeation chromatography (GPC), with tetrahydro-
furan (THF) as an eluent with
a polystyrene standard. The
elemental analysis was determined with a CE Instruments EA-
1110 elemental analyzer. Differential scanning calorimetry (DSC)
2.3.5. 2,5-Bis(2-ethylhexyloxy)terephthalic acid (5)
Into a three-necked 250-mL flask containing 100 mL of 30 wt.%
KOH aqueous solution, 10 g (20.9 mmol) of 4 was added with vig-
orous stirring. The reaction mixture was maintained at 100 °C for
24 h. After the reaction, the mixture was cooled to 0 °C and was
neutralized with hydrochloric acid to obtain a precipitate. The pre-
cipitate was isolated by filtration, purified twice by recrystalliza-
tion from ethanol, and dried in a vacuum oven yielding 8.2 g
(94%). 1H NMR (CDCl3, ppm): d 12.80 (s, 2H), 7.28 (s, 2H), 4.30
was performed on
a DuPont model 2100 thermal analyzer
equipped with a 2910 DSC instrument at a heating rate of 10 °C/
min under a nitrogen atmosphere (temperature range: room tem-
perature to 350 °C). Thermogravimetric analysis (TGA) was con-
ducted with a PerkinElmer TGA 7 equipped with a TGA 7/3
instrument controller at a heating rate of 20 °C/min under nitrogen
(temperature range: room temperature to 900 °C). The absolute

Y.-s. Kim et al. / Reactive & Functional Polymers 70 (2010) 223–229
225
(m, 4H), 3.91 (t, 4H), 1.82–1.03 (br, 24H), 0.92–0.81 (br, 6H). 13C
NMR (CDCl3, ppm): d 169.0, 149.2, 119.7, 115.1, 75.0, 40.5, 30.9,
29.4, 25.1, 23.1, 14.3, 12.1.
24 h. Thionyl chloride was removed under reduced pressure and
the product used without further purification for the next step.
2.4. Polymer synthesis
2.3.6. 2,5-Bis(2-ethylhexyloxy)benzene-1,4-dioyl dichloride (6)
A 2.0-g (4.7 mmol) quantity of 5 and three drops of DMF were
added to a three-necked 100-mL flask containing 50 mL of thionyl
chloride. The suspension was heated to reflux for 24 h. Thionyl
chloride was removed under reduced pressure, and the product
used without further purification for the next step.
2.4.1. Precursor polymer (P1)
In a 250-mL flask, 0.7 g (3.0 mmol) of 7 in 13 mL of methylene
chloride and 1.4 g (3.0 mmol) of 6 in 17 mL chloroform were
added. A 2-mL quantity of triethylamine was added to the mixture
with stirring. A 4.0-g (5.9 mmol) quantity of 3 dissolved in 20 mL of
chloroform was added slowly, and the reaction was carried out for
2 h. After polymerization, a precipitate was obtained by pouring
the mixture into 600 mL of methanol. The polymer was washed
twice in water and finally in methanol. The polymer was redis-
solved in 50 mL of chloroform and precipitated in acetone. A pale
2.3.7. 1,4-Dihydro-4-oxopyridine-2,6-dicarbonyl dichloride (7)
A 2.5-g (11.5 mmol) quantity of chelidamic acid and 50 mL of
distilled thionyl chloride were added into a 3-necked 100 mL flask
with three drops of DMF. The suspension was heated to reflux for
OH
OH
H2SO4
EtOOC
COOEt
HOOC
HO
COOH
EtOH
HO
1
2-ethylhexylbromide
K2CO3, DMF
CH3(CH2)15Br
K2CO3, DMF
O
OC16H33
COOEt
EtOOC
COOEt
EtOOC
C16H33
O
O
4
2
KOH
H2O
NH2NH2
MeOH
O
OC16H33
HOOC
COOH
H2NHNOC
CONHNH2
O
C16H33O
3
5
SOCl2
O
ClOC
COCl
O
6
O
O
SOCl2
HOOC
N
H
COOH
ClOC
N
H
COCl
7
Scheme 1. Synthesis of monomers.

226
Y.-s. Kim et al. / Reactive & Functional Polymers 70 (2010) 223–229
yellowish powder was obtained after drying in a vacuum oven,
yielding 3.8 g (67%). 1H NMR (CDCl3, ppm): d 11.51 (s) 9.17 (s),
7.86 (d), 4.17–4.08 (m), 2.17–1.51 (m). 13C NMR (CDCl3, ppm): d
207.11, 165.67, 151.23, 123.29, 115.89, 77.65, 77.43, 77.22, 76.80,
72.43, 39.59, 32.13, 31.12, 31.01, 29.91, 29.88, 29.71, 29.57,
29.48, 29.40, 29.29, 29.19, 26.29, 24.44, 23.16, 22.89, 14.31,
14.21, 11.30. Anal. Calcd. (%) for C57.16H96.80O12.37N4.51: C, 70.92;
H, 10.00; N, 6.54. Found: C, 71.01; H, 11.05; N, 6.73.
2.4.2. Oxadiazole polymer (P2)
In a 50-mL flask, 0.3 g of P1 and 10 mL of phosphorous oxychlo-
ride were heated to 110 °C for 24 h. The reaction mixture was
cooled to room temperature, and then it was poured into 500 mL
of ice water. The precipitate was isolated by filtration and washed
sequentially with water and acetone. The polymer was redissolved
in 50 mL of chloroform and precipitated in 500 mL of acetone. A
dark yellowish powder was obtained after drying in a vacuum
oven. 1H NMR (CDCl3, ppm): d 10.51 (s), 8.03–7.63 (m), 4.43–3.87
(m), 2.16–1.39 (m). 13C NMR (CDCl3, ppm): d 207.11, 166.24,
163.35, 161.03, 156.50, 152.34, 151.03, 124.59, 124.14, 117.03,
73.83, 73.55, 70.30, 70.21, 70.19, 70.13, 69.91, 61.57, 52.49,
39.40, 32.12, 31.10, 29.90, 29.56, 26.11, 25.71, 23.76, 23.21,
22.88, 14.30, 10.82. Anal. Calcd. (%) for C54.89H95.32O5.48N3.87: C,
72.78; H, 10.53; N, 5.99. Found: C, 71.02; H, 10.28; N, 5.99.
Fig. 1. TGA thermograms of the precursor polymer P1 (dotted) and P2 (solid) under
a nitrogen atmosphere.
P1, as expected, in which the thermally stable oxadiazole unit
played a role in its thermal stability. However, the glass-transition
temperatures (Tg) of the polymers were not found even in the sec-
ond scan in the DSC investigation.
The number-average molecular weights (Mn) of the precursor
polymers P1 and P2 were measured at 25,350 and 24,210, respec-
tively, according to the GPC analysis. The weight-average molecu-
lar weights (Mw) of P1 and P2 were found to be 49,710 and 41,410,
2.5. Fluorescence patterning
Thin films of P2 were obtained via spin-casting from a 0.5 wt.%
chloroform solution onto a glass slide or a silicon wafer. After dry-
ing in vacuo at 50 °C for 12 h, the film was exposed through a pho-
tomask to a monochromatic 254 nm UV light with an intensity of
630 l
W/cm2 from a hand-held UV lamp (the distance from the
UV lamp to the film was 5 cm). The fluorescent image photographs
were taken by a fluorescence microscope equipped with a cooled
CCD camera (Meta Imaging Series 4.6, Universal Imaging Corp.).
3. Results and discussion
The synthetic routes for the diacid chlorides 6 and 7 and dihy-
drazide monomer 3 are shown in Scheme 1. Monomers 3, 6, and 7
were prepared with good yields according to previously reported
methods with slight modifications [32,33]. The copolyhydrazide
P1 was easily obtained by reactions with 3, 6, and 7 in chloroform
and methylene chloride. For oxadiazole ring formation, P1 was re-
acted in phosphorous oxychloride for 24 h. The copolymer compo-
sition of each polymer was calculated from elemental analysis
data. It was found that the molar composition (as a mole fraction)
x was calculated to be 0.64 in the polymer backbone. The copolyox-
adiazole P2 as well as the precursor copolyhydrazide P1 showed
good solubility in common organic solvents such as chloroform,
THF, DMF, and DMSO, but they were not soluble in acetone and
methanol. The molar composition (x) was found to be 0.67 from
the integral calculation from the 1H NMR spectrum of P2. The com-
positions calculated from the elemental analysis and 1H NMR data
were in reasonable agreement. During the polymerization reaction,
the pyridone group was converted into a hydroxypyridine unit,
with a chemical shift around 10 ppm (for hydroxyl group) in the
NMR spectrum similar to the previous reports [33,34]. It is plausi-
ble that such a downfield-shifted peak of the hydroxyl group re-
sulted from a weakening of OAH bond in the presence of
intermolecular hydrogen bonding.
TGA and DSC were employed to investigate the thermal behav-
iors of the polymers. As shown in Fig. 1, the onset decomposition
temperatures (Td) of P1 and P2 were found to be 398.5 and
415.2 °C, respectively. The onset Td of P2 was higher than that of
Fig. 2. Absorption (dotted, 9.2 Â 10À6 M) and emission spectra (solid, 7.4 Â 10À9 M)
of (a) P2 solution in chloroform and (b) P2 film.

Y.-s. Kim et al. / Reactive & Functional Polymers 70 (2010) 223–229
227
0.5
0.4
0.3
0.2
0.1
0.0
500
400
300
200
100
0
0.20
0.15
0.10
0.05
0.00
600
400
200
0
300
400
500
600
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 3. Absorption (9.2 Â 10À6 M) and emission (7.4 Â 10À9 M) spectra of the P2
solution in chloroform upon UV irradiation (time: 0, 15, and 20 min for absorption;
0, 5, 10, 15, 20, 25, and 30 min for emission).
Fig. 4. Absorption and emission spectra of the P2 film upon UV irradiation (time: 0,
10, 20, and 30 min for absorption; 0, 5, 10, 15, 20, 25, and 30 min for emission).
the shapes of the spectra. A large red shift of the emission maxi-
mum (about 18 nm) and the formation of a new emission shoulder
about 470 nm were shown in the film state. We found that the
excitation spectra from the emission at 450 nm and at 470 nm
were the same as the absorption spectrum in the solid state. It is
presumed that the large red shift in absorption and emission indi-
cates the presence of intermolecular interactions between molecu-
lar backbones, which is in accordance with our synthetic strategy
[35,36].
respectively. The molecular weight difference between P1 and P2
is ascribed to the difference in the hydrodynamic volume resulting
from the difference in chain flexibility between hydrazide and the
oxadiazole ring.
Fig. 2 shows the absorption and emission spectra of P2 in chlo-
roform solution at the concentrations of 9.2 Â 10À6 M and
7.4 Â 10À9 M, respectively (Fig. 2a), and in the solid film (Fig. 2b).
The absorption maxima of P2 in the solutions and in the film were
located at 382 nm, 314 nm, and 397 nm, respectively. Besides
showing a new absorption band at a short wavelength (314 nm),
a red shift of the absorption (382 nm to 397 nm) was observed
by comparing the kmax values of the solutions and the solid. The
photoluminescence of the P2 film shown in Fig. 2b is different from
that of the solution (Fig. 2a) in both emission-band positions and
In our previous report, we showed that the intermolecular
interaction between polymer backbones can be manipulated by
UV irradiation, which induced fluorescence quenching at exposed
areas of the polymer film [28]. In that system, intermolecular
p-
interaction played a crucial role in fluorescence manipulation. In
O
OC16H33
CONHNH2
O
+
+
H2NHNOC
C16H33
ClOC
COCl
ClOC
N
H
COCl
O
O
7
6
3
O
OC16H33
CONHNHCO
O
OC16H33
CONHNHCO
CONHNHCO
CONHNHCO
N
H
x
1-x
C16H33
O
O
C16H33O
P1
OH
N
OC16H33
O
O
OC16H33
O
POCl3
O
O
x
N
N
N
N
N
N
N
1-x
N
C16H33
O
O
C16H33
O
P2
Scheme 2. Synthesis of polymer.

228
Y.-s. Kim et al. / Reactive & Functional Polymers 70 (2010) 223–229
contrast to such a fluorescence-quenching system, the fluorescence
intensity of the P2 solution was greatly enhanced as UV irradiation
time increased (more than twofold), as shown in Fig. 3. Considering
that there was a negligible change in UV absorption during UV irra-
diation, possible chemical degradation or photobleaching by UV
irradiation was not expected; the photochemically stable oxadiaz-
ole ring might have provided this advantage [18]. With UV irradi-
ation, a gradual increase in fluorescence intensity as well as slight
red shift of the emission maximum (from 432 nm to 439 nm) was
observed, as shown in Fig. 3, indicative of the possible photo-oxi-
dation of the hydroxypyridine ring to form a keto tautomer such
as a pyridone ring [37,38].
Photomask
Spin-cast P2 film (dark-blue fluorescence)
on silicon wafer
UV irradiation
This unique phenomenon regarding the fluorescence enhance-
ment by UV irradiation was also observed in the film state, as
shown in Fig. 4. Similarly to the case of the solution, UV absorption
did not change upon UV irradiation even after a prolonged time. In
a manner similar to the P2 solution, the P2 film exhibited fluores-
cence enhancement upon UV illumination, which was likely due to
the same photo-oxidation mechanism to form the keto tautomer
[39]. The degree of fluorescence intensity increase was propor-
tional to the UV irradiation time without emission-maximum
change. Although there was no shift in absorption and emission,
we believe that the much larger fluorescence enhancement in the
P2 film compared to that of the solution resulted from the facili-
tated photochemical conversion of 4-hydroxypyridine groups to
4-pyridone groups that may have been influenced by the closer
proximity of the molecular chains in the solid.
Photomask removal
Exposed area
(enhanced bright-blue fluorescence)
Unexposed area
(pristine dark-blue fluorescence)
(a)
We measured the absolute quantum yields of P2 in solution and
in the solid state to test for possible photochemical degradation.
The fluorescence quantum yields of P2 in chloroform solution
(4.21 Â 10À6 M) and in a thin film were 35% and 0.95%, respec-
tively. The thin film was exposed to UV irradiation to enhance
the fluorescence up to 3.5%. The irradiated polymer film was redis-
solved in chloroform for quantum-yield measurement. The fluores-
cence quantum yield of the solution recovered to 32%, close to that
of the pristine polymer solution. This recovery of quantum yield
strongly indicates that there was no new formation of small fluo-
rescent molecular species by photochemical degradation.
We carried out the same experiment of UV irradiation on the
film of a polymer (P3), i.e., x = 1 in P2 in Scheme 2.
(b)
Fig. 5. (a) Schematic diagram of the fluorescent patterning process and (b)
fluorescence patterned image through a photomask on a silicon wafer by UV
irradiation for 30 min. The polymer film was spin-cast from 0.5 wt.% chloroform
solution without PAG. The bright blue fluorescent region was exposed to UV. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
OC16H33
O
O
O
N
N
N
N
Finally, latent fluorescence images were fabricated in a spin-
cast film of P2 in the absence of PAG by simple irradiation from
a 254 nm hand-held UV lamp through a photomask for 30 min,
for which the process is illustrated in Fig. 5a. As shown in
Fig. 5b, the irradiated P2 film was highly emissive, with a bright
blue fluorescence. The dark blue fluorescent image was made on
the unexposed area through the photomask. Fine patterning could
C16H33
O
O
P3
be accomplished, with an emissive line width as low as 1 lm
The fluorescence intensity of the P3 film was not changed by UV
irradiation. Thus, it can be concluded that the photosensitized
hydroxypyridine units were essential for the photo-induced fluores-
cence enhancement in this system. We employed spectroscopic
methods such as IR, NMR and X-ray photoelectron spectroscopy
(XPS) to investigate the structural changes in P2 before and after
UV irradiation; unfortunately, we were not able to determine any
differences in their chemical structures due to irradiation. It is likely
that the fluorescence-enhancement phenomenon upon UV irradia-
tion is related to the conversion of the hydroxypyridine unit to a
pyridone group upon photoirradiation, as has been reported previ-
ously [29–31].
depending onto the photomask used. Latent patterns could not
be observed under ambient light, while highly resolved fluores-
cence patterns were clearly seen under UV light. The patterned im-
age was stable for more than a month, maintaining its enhanced
fluorescence intensity and resolution at ambient conditions. After
that time, the intensity decreased gradually for about a month.
4. Conclusions
We have synthesized a new conjugated polymer containing
hydroxypyridine groups linked with oxadiazole units for the

Y.-s. Kim et al. / Reactive & Functional Polymers 70 (2010) 223–229
229
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purpose of introducing the combination of facile photosensitiza-
tion (hydroxypyridine) with thermal and photochemical stability
(oxadiazole). Both the polymer solution and the film showed fluo-
rescence enhancement upon UV irradiation, likely due to the con-
version of hydroxypyridine to pyridone groups, which influenced
the intermolecular interactions between polymer backbones. The
initial weak fluorescence intensity could be recovered by redissolv-
ing the polymer film with enhanced fluorescence, suggesting that
the fluorescence enhancement was not due to a photochemical
degradation by UV irradiation. By exploiting the unique optical
characteristics of this system, we prepared a novel example of
fluorescence imaging of a fluorescence pattern with enhanced
intensity by simple UV irradiation.
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Acknowledgments
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(2007) 4430.
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Macromolecules 38 (2005) 9427.
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Financial support from the Korea Science and Engineering
Foundation (KOSEF) funded by the Korea Government (MEST) is
gratefully acknowledged (No. R01-2007-000-10740-0 and 2009-
008146).
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