Synthesis and characterization of fluorene-based conjugated polymers containing thiophene derivatives in main chain

Article abstract of DOI:10.1080/15421401003604286

Three novel alternating copolymers containing 9,9-dioctylfluorene and thiophene derivatives, poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(4,4′- bis(3″-hexyl-2″thienyl)-biphenyl-5,5″-diyl)] (P1), poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(5,5′-bis(3″-hexyl-2thienyl) -2,2′-bithiophenyl-5,5″-diyl)] (P2), and poly[(9,9-dioctylfluorene- 2,7-diyl)-alt-(5,5′-bis(3″-hexyl-2″thienyl)-2, 2′-biselenophenyl-5,5″-diyl)] (P3) were synthesized by a palladium-catalyzed Suzuki polymerization. The synthesis, thermal stability, as well as the optical and electrochemical properties of these polymers are systematically investigated. The polymers were thermally stable up to 400°C, and readily soluble in common organic solvent. The absorption and emission maxima of the copolymers varied according to the type of incorporated aromatic group (thiophene, selenophene, or phenylene).

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Synthesis and Characterization of
Fluorene-Based Conjugated Polymers
Containing Thiophene Derivatives in
Main Chain
Sang Kyu Lee a , Young Kwan Jung b , Won Suk Shin a , Hong-Ku Shim
c & Sang-Jin Moon a
a Energy Materials Research Center, Korea Research Institute of
Chemical Technology , Yuseong, Dae-jeon, Korea
b OLED Technology Development Team3, LG Display Co, Gongdan-
dong, Gumi-si , Gyungsangbuk-do, Korea
c Department of Chemistry and School of Molecular Science , Korea
Advanced Institute of Science and Technology , Dae-jeon, Korea
Published online: 19 Apr 2010.
To cite this article: Sang Kyu Lee , Young Kwan Jung , Won Suk Shin , Hong-Ku Shim & Sang-Jin Moon
(2010) Synthesis and Characterization of Fluorene-Based Conjugated Polymers Containing Thiophene
Derivatives in Main Chain, Molecular Crystals and Liquid Crystals, 520:1, 10/[286]-18/[294], DOI:
10.1080/15421401003604286
To link to this article: http://dx.doi.org/10.1080/15421401003604286
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Mol. Cryst. Liq. Cryst., Vol. 520: pp. 10=[286]–18=[294], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003604286
Synthesis and Characterization of Fluorene-
Based Conjugated Polymers Containing
Thiophene Derivatives in Main Chain
SANG KYU LEE,1 YOUNG KWAN JUNG,2
WON SUK SHIN,1 HONG-KU SHIM,3
AND SANG-JIN MOON1
1Energy Materials Research Center, Korea Research Institute of
Chemical Technology, Yuseong, Dae-jeon, Korea
2OLED Technology Development Team3, LG Display Co,
Gongdan-dong, Gumi-si, Gyungsangbuk-do, Korea
3Department of Chemistry and School of Molecular Science, Korea
Advanced Institute of Science and Technology, Dae-jeon, Korea
Three novel alternating copolymers containing 9,9-dioctylfluorene and thiophene
derivatives, poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(4,40-bis(300-hexyl-200thienyl)-
biphenyl-5’’,500-diyl)] (P1), poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(5,50-bis
(300-hexyl-2’’thienyl)-2,20-bithiophenyl-5’’,500-diyl)] (P2), and poly[(9,9-
dioctylfluorene-2,7-diyl)-alt-(5,50-bis(300-hexyl-200thienyl)-2,20-biselenophenyl-
5’’,500-diyl)] (P3) were synthesized by a palladium-catalyzed Suzuki polymeriza-
tion. The synthesis, thermal stability, as well as the optical and electrochemical
properties of these polymers are systematically investigated. The polymers were
thermally stable up to 400ꢀC, and readily soluble in common organic solvent.
The absorption and emission maxima of the copolymers varied according to the
type of incorporated aromatic group (thiophene, selenophene, or phenylene).
Keywords Electroluminescence; polyfluorene; thiophene
Introduction
Over the last decade, polymer light emitting diodes (PLEDs) have attracted much
scientific and technological research interest because of their potential utility in a
wide variety of applications such as flat-panel displays and light-emitting diodes
(LEDs) [1–3]. In particular, among all the known p-conjugated polymers for PLEDs,
poly(9,9-di-n-octylfluorene) (PFO) are the most promising candidates as blue-light-
emitting polymers because of their highly efficient photoluminescence (PL), their
Address correspondence to Hong-Ku Shim, Department of Chemistry and School of
Molecular Science, Korea Advanced Institute of Science and Technology, Dae-jeon,
305-701, Republic of Korea. E-mail: hkshim@kaist.ac.kr; Sang-Jin Moon, Energy Materials
Research Center, Korea Research Institute of Chemical Technology, Sinseongno 19, Yuseong,
Dae-jeon 305-600, Republic of Korea. E-mail: moonsj@krict.re.kr
10=[286]

Synthesis of New Fluorene-Based Copolymers
11=[287]
good thermal, chemical, and electrochemical stability, and their convenient solubility
in common organic solvents [4–8]. Moreover, many research groups have studied
various electroluminescent fluorene-based polymers emitting in the region of the
three elementary colors of light (red, green, and blue). The optical and electrolumi-
nescent properties of the polymer may be tuned by introducing into PF of comono-
mers with a lower band-gap than fluorene monomer [9–11].
A serious problem associated with PFOs is an imbalance in charge carriers due
to large hole injection barriers and different charge carrier mobilities [12]. Several
approaches have been tried in efforts to improve the injection of charges from the
electrode [13–15]. To promote the charge-transporting properties, it is necessary to
design and synthesize conjugated polymers that possess a hole-transporting unit.
Here we report the synthesis and characterization of new fluorene-based copolymers
containing thiophene derivatives in the main chain. To tune the optical and EL
properties of the polymer, the type of incorporated aromatic group (thiophene,
selenophene, or phenylene) into thiophene derivative was modified. In this study,
three novel alternating copolymers containing 9,9-dioctylfluorene and thiophene
derivatives, poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(4,40-bis(300-hexyl-200thienyl)-biphenyl-
5’’,500-diyl)] (P1), poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(5,50-bis(300-hexyl-200thienyl)-
2,20-bithiophenyl-5’’,500-diyl)] (P2), and poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(5,50-
bis(300-hexyl-200thienyl)-2,20-biselenophenyl-5’’,500-diyl)] (P3) were synthesized by a
palladium-catalyzed Suzuki polymerization. We systematically investigated the
synthesis, thermal stability, as well as the optical and electrochemical properties of these
polymers.
Experimental
The monomers and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,90-di-n-
octylfluorene were prepared according to a literature procedure [16,17]. All copoly-
mers were synthesized by a polycondensation of 2,7-bis(4,4,5,5-tetramethyl-1,
3,2-dioxaborolan-2-yl)-9,90-di-n-octylfluorene and corresponding dibromonated
dihexylthiophene units through Pd-catalyzed Suzuki coupling reactions (Scheme 1) [18].
The chemical structures of the polymers were verified with 1H NMR and
elemental analysis. UV-visible analysis was performed with a Shimadzu Jasco
V-530 UV=vis spectrometer. The number- and weight-average molecular weights
of the polymers were determined by gel permeation chromatography (GPC) on a
Waters GPC-150C instrument, using tetrahydrofuran (THF) as the eluent and poly-
styrene as the standard. Thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) were performed under a nitrogen atmosphere at a heating
rate of 10ꢀC=min with a Dupont 9900 analyzer. The thicknesses of the polymer films
were measured using an Alpha step 200 profilometer. Photoluminescence (PL) spec-
tra of the polymers were obtained with a Spex Fluolog-3 (Model FL3-11) spectro-
fluorometer. Electroluminescence spectra were obtained with a Minolta CS-1000.
The current-voltage (I-V) and luminescence-voltage (L-V) characteristics of the
devices were measured with a Keithley 238 as the voltage and current source and
a Minolta LS-100 as the luminance detector. All the measurements were performed
in air at room temperature.
In this study, the devices were fabricated with ITO=PEDOT:PSS=polymer=Ca=
Al structures. The procedure for cleaning the ITO surface included sonication
and rinsing in deionized water, methanol, and acetone. The hole-transporting

12=[288]
S. K. Lee et al.
Scheme 1. Synthetic scheme for the monomers and copolymers.
PEDOT:PSS layer was spin-coated onto each ITO anode from a solution purchased
from Bayer Co. Each polymer solution was then spin-coated onto the PEDOT:PSS
layer. The polymer solution for spin-coating was prepared by dissolving the polymer
(1 wt%) in chlorobenzene. Ca and aluminum contacts were formed by vacuum depo-
sition at pressures below 10ꢁ6 Torr, giving an active area of 0.04 cm2.
Results and Discussion
The synthesized copolymers are soluble in common organic solvent such as tetrahy-
drofuran (THF), chloroform, or toluene. The molecular weight were determined by
GPC against polystyrene standards in THF eluent and were found to be in the range
of 11,000–45,000 with a polydispersity index of 2.2–2.3. As shown in Figure 1, the
copolymers have good thermal stability up to ca. 400ꢀC. The physical properties
of the polymers are summarized in Table 1.
The UV-vis absorption spectra of the polymers in chloroform solution and thin
films are shown in Figures 2 and 3, respectively, and the corresponding absorption
and emission properties are summarized in Table 2. The absorption and emission

Synthesis of New Fluorene-Based Copolymers
13=[289]
Figure 1. TGA thermograms of the copolymers.
maxima of the copolymers varied according to the type of incorporated aromatic
group (thiophene, selenophene, or phenylene). The polymers, P3 containing
biselenophene units in solution exhibited red shifted spectra compared to the
thiophene-based polymer, P2. The optical band gap obtained from the extrapolation
of absorption edges of film are in the order of P3 (2.08 eV) < P2 (2.16 eV) < P1
(2.70 eV). The band-gap of selenophene-based copolymer showed a decrease of ca.
0.1 eV compared to that of thiophene-based copolymer. Such a decrease in the band
gap can be explained by the fact that the ease of p-electron delocalization for
selenophene unit is greater than that for thiophene, and that the bandwidth of the
selenophene unit is larger than that of thiophene unit, which may contribute to
the observed red shift [19].
To investigate the redox behavior of the copolymers and determined the energy
levels of their highest occupied molecular orbitals (HOMO) and lowest unoccupied
molecular orbitals (LUMO), the electrochemical properties of the polymers were
investigated by using cyclic voltammetry (CV). All measurements were calibrated
using ferrocene as a standard. In the anodic scan, the onset of oxidation of P1,
P2, and P3 was found to occur at 0.91, 0.58, and 0.50 V (vs. SCE), respectively,
corresponding to the ionization potential values of ꢁ5.61, ꢁ5.28, and ꢁ5.24 eV,
respectively (Figure 4). Unfortunately, it was difficult to obtain the LUMO energies
Table 1. Physical properties of the polymers
PDIa
Tg (ꢀC)b
a
a
Mn
Mw
P1
P2
P3
14,000
45,000
11,000
32,000
101,000
24,000
2.3
2.2
2.2
65
60
61
aMn, Mw, and PDI of the polymers were determined by gel permeation chromatography
using polystyrene standards.
bGlass transition temperature determined by differential scanning calorimetry (DSC).

14=[290]
S. K. Lee et al.
Figure 2. UV-vis absorption and PL spectra of the copolymers in the chloroform
solution.
Figure 3. UV-vis absorption and PL spectra of P1, P2, and P3 as films.
Table 2. Spectral data and energy levels of the polymers
Film, kmax (nm)
HOMOb=LUMOc (eV)
Eogpt (eV)d
a
Absorption
Emission
UPL
P1
P2
P3
398
473
514
474
596
623
1.00
0.41
0.20
ꢁ5.61=ꢁ2.91
ꢁ5.28=ꢁ3.12
ꢁ5.24=ꢁ3.16
2.70
2.16
2.08
aThe film fluorescence quantum yields measured on quartz plates relative to P1 assuming
1.00 for comparison.
bCalculated from E1=2 (energy level of ferrocene of 4.8 eV under vacuum).
cCalculated from the HOMO level and optical band gap.
dThe optical band gap was determined from the UV-vis. absorption onset in the solid state.

Synthesis of New Fluorene-Based Copolymers
15=[291]
Figure 4. Cyclic voltammograms of the copolymers.
for the copolymers using this technique, so these values were estimated from the
optical band gap and the HOMO energies. The energies of the LUMO levels of
P1, P2, and P3 are listed in Table 2.
To investigate the EL properties of the polymers, we fabricated devices with
the configuration: ITO=PEDOT:PSS (50 nm)=polymers (80 nm)=Ca (50 nm)=Al
(100 nm) and characterized them as a function of the applied voltage. The result-
ing polymer was spin-coated from a 1.0 wt% solution in chlorobenzene onto an
ITO substrate coated with PEDOT. As shown in Figure 5, the EL spectra of
the copolymers are nearly identical to the PL spectra of the spin-coated films,
Figure 5. EL spectra of the copolymers devices with the configuration of ITO=PEDOT:PSS=
polymer=Ca=Al under 10 V.

16=[292]
S. K. Lee et al.
Table 3. Summary of the device characteristics
kmax
CIE coordinates
(x,y)b
Maximum
luminance (cd=m2)
Turn-on
voltage (V)c
(nm)a
P1
P2
P3
466
596
625
(0.20, 0.30)
(0.59, 0.41)
(0.63, 0.37)
700
1100
340
4.5
3.0
3.0
aMaximum of the EL spectrum at a voltage of 10 V.
bDetermined from EL spectrum (Figure 5).
cVoltage needed for brightness of 1 cd=m2.
and this indicates that the energy transfer involved in EL is similar to that in PL.
P1, P2, and P3 emitted blue, orange, and red light with a maximum emission at
466, 596, and 625 nm, respectively. Thiophene or selenophene has a narrower
band gap energy than phenylene, and hence the emission from copolymers (P2
or P3) containing thiophene or selenophene is red-shifted with respect to that
of the copolymer (P1) containing phenylene. In particular, P3 (which contains
biselenophene units) showed almost pure red emission with Commission
Internationale de l’Eclairage (CIE) coordinates of (0.63, 0.37) as shown in
Table 3. Figure 6 shows the characteristics of the voltage-luminance (V-L) and
Figure 6. (a) V-L and (b) V-current efficiency characteristics of the copolymers.

Synthesis of New Fluorene-Based Copolymers
17=[293]
the voltage-current efficiency, and Table 3 summarized the EL spectra and
related performance data. The turn-on voltages of the polymers ranged from
about 3.0 to 4.5 V. The luminance reached 700 cd=m2 at 9.6 V in P1, 1100cd=m2
at 11.6 V in P2, and 340 cd=m2 at 12V in P3. We are currently endeavoring to
further improve the performance of PLEDs by incorporating a third monomer into
the polymer backbone. A detailed study aimed at improving the efficiency of these
devices and further research into the new applications in organic photovoltaic cells
(OPVs) of these narrow energy band gap polymers are currently underway.
Conclusion
We synthesized new conjugated fluorene-based copolymer containing thiophene
derivatives with a variety of aromatic group through palladium-catalyzed Suzuki
polymerization. The absorption and emission maxima of the copolymers varied
according to the type of incorporated aromatic group (thiophene, selenophene, or
phenylene). P1 (which contains biphenylene units), P2 (which contains bithiophene
units), and P3 (which contains biselenophene units) emitted blue, orange, and red
light with a maximum emission at 466, 596, and 625 nm, respectively. Further modi-
fication of the polymer structure and further research into the new application in
OPV of these materials are currently underway.
Acknowledgments
This work was supported by BK21 of the Ministry of Education and Human
Resources Development, by a grant (M2009010025) from the Fundamental R&D
Program for Core Technology of Materials funded by the Ministry of Knowledge
Economy (MKE) and by a grant from the cooperative R&D Program funded by
the Korea Research Council Industrial Science and Technology, Republic of Korea.
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