Synthesis and optical properties of light-emitting polyfluorene derivatives

Article abstract of DOI:10.1080/15421406.2011.566065

The emitting-polymers, Polyfluorene (PF) and Poly(fluorene- benzothiadiazole-quinoline) [PF-BT-QL], have been synthesized by the Suzuki coupling reactions. The properties of polymers were characterized using UV-Vis spectroscopy, GPC, DSC, TGA, Photoluminescence (PL), Fluoresence (FL), Electroluminescence (EL) spectroscopy. The synthetic polymers were soluble in common organic solvents and easily spin-coated onto the indium-tin oxide (ITO)-coated glass substrates. Light-emitting devices (LEDs) with ITO/PEDOT:PSS/polymer/LiF/Al configuration were fabricated, and the devices using copolymers showed red shift EL spectra relative to that of PF. The turns on voltages of copolymers were lower than that of PF. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.566065

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Synthesis and Optical Properties of
Light-Emitting Polyfluorene Derivatives
Philaiwan Aiamsen a , David L. Carroll b & Sukon Phanichphant c
a Department of Chemistry, Faculty of Science , Chiang Mai
University , Chiang Mai, Thailand
b Center for Nanotechnology and Molecular Materials, Department of
Physics , Wake Forest University , NC, USA
c Materials Science Research Center, Faculty of Science , Chiang Mai
University , Chiang Mai, Thailand
Published online: 16 May 2011.
To cite this article: Philaiwan Aiamsen , David L. Carroll & Sukon Phanichphant (2011) Synthesis and
Optical Properties of Light-Emitting Polyfluorene Derivatives, Molecular Crystals and Liquid Crystals,
539:1, 88/[428]-95/[435], DOI: 10.1080/15421406.2011.566065
To link to this article: http://dx.doi.org/10.1080/15421406.2011.566065
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Mol. Cryst. Liq. Cryst., Vol. 539: pp. 88=[428]–95=[435], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.566065
Synthesis and Optical Properties of
Light-Emitting Polyfluorene Derivatives
PHILAIWAN AIAMSEN,1 DAVID L. CARROLL,2 AND
SUKON PHANICHPHANT3
1Department of Chemistry, Faculty of Science, Chiang Mai University,
Chiang Mai, Thailand
2Center for Nanotechnology and Molecular Materials, Department of
Physics, Wake Forest University, NC, USA
3Materials Science Research Center, Faculty of Science, Chiang Mai
University, Chiang Mai, Thailand
The emitting-polymers, Polyfluorene (PF) and Poly(fluorene-benzothiadiazole-
quinoline) [PF-BT-QL], have been synthesized by the Suzuki coupling reactions.
The properties of polymers were characterized using UV-Vis spectroscopy, GPC,
DSC, TGA, Photoluminescence (PL), Fluoresence (FL), Electroluminescence
(EL) spectroscopy. The synthetic polymers were soluble in common organic sol-
vents and easily spin-coated onto the indium-tin oxide (ITO)-coated glass sub-
strates. Light-emitting devices (LEDs) with ITO=PEDOT:PSS=polymer=LiF=Al
configuration were fabricated, and the devices using copolymers showed red shift
EL spectra relative to that of PF. The turns on voltages of copolymers were lower
than that of PF.
Keywords Light-emitting polymer; polyfluorene; polymer light-emitting diode
Introduction
In polymer light-emitting diodes (PLEDs), polyfluorenes are promising candidates
as blue emitters due to their high photoluminescence quantum efficiencies (PLQEs)
as solid films [1], their excellent solubility and film-forming ability, and the ease of
controlling their properties via facile substitution in the 9,9-position of the fluorene
unit [2–4]. In addition, chemical modifications on main chain, side chain, and chain
end of PFs allow elaborate tuning of emission color covering the whole visible range
(blue, green, yellow, red, and white), enhancement of device performance, and
improvement of long-term operational stability. On the other hand, various types
of chain conformation and chain stacking of PFs permit relatively easy physical
manipulation for identical purposes as the chemical methods. In this study, new
single-polymer electroluminescent systems containing three individual emission
species polyfluorenes as a blue host, 2,1,3-benzothiadiazole unit as a green dopant
Address correspondence to Sukon Phanichphant, Materials Science Research Center,
Faculty of Science, Chiang Mai University, Chiang Mai, Thailand. E-mail: sphanichphant@
yahoo.com
88=[428]

Synthesis and Optical Properties
89=[429]
and phenylene derivative units as a red dopant on the main chain were synthesized
and characterized. The emitting polymers, polyfluorene and poly(fluorene-benzo-
thiadiazole-quinoline) [PF-BT-QL], were synthesized, characterized and fabricated
into devices.
Methods
1. Synthesis of Polymer
The synthetic route is shown in Figure 1, 1,4-dibromo-2,5-bis(8-quinolinyl)benzene
(4),
dioxaborolan-2-yl)-9,9-dioctylfluorene (7), 4,7-dibromo-2,1,3-benzothiadiazole (8)
2,7-dibromo-9,9-di-n-octylfluorene
(5),
2,7-bis(4,4,5,5-tetramethyl-1,3,2-
Figure 1. The synthetic route of monomers and polymers.

90=[430]
P. Aiamsen, D. L. Carroll, and S. Phanichphant
were synthesized. Suzuki coupling polymerization reaction with (4), (5), (7) and (8)
were conducted in biphasic system (toluene=aqueous Na2CO3) with Pd(PPh3)4 as a
catalyst precursor. The reaction were kept stirring at reflux temperature for 3 days.
Then, phenyl boronic was added to reduce the heavy atom affecting which quenched
fluorescence [5].
2. Fabrication of Light-Emitting Devices
In this study, the devices were fabricated with ITO=PEDOT:PSS=polymer=LiF=Al
structures. Indium tin oxide (ITO)-coated glass was ultrasonically cleaned sequen-
tially for 15 min in 2% sonicating solution, water, acetone, and 2-propanol. The
hole-transporting PEDOT:PSS layer was spin-coated onto each ITO anode Each
polymer solution was then spin-coated onto the PEDOT:PSS layer. The polymer sol-
ution for spin-coating was prepared by dissolving the polymer (5 wt%) in tetrahydro-
furan. LiF and aluminum contacts were formed by vacuum deposition at pressures
below 10ꢀ6 Torr. The active sections of the devices were then covered with an epoxy
resin and allowed to dry so that the active regions were shielded from oxygen
exposure during device testing. The devices were tested using photospectrometer
to a Keithley 238 and Minolta LS-100 as the luminance detector.
Results and Discussion
All polymers could be fully dissolved in common organic solvents, such as chloro-
form, toluene, and THF. The crude polymers were redissolved in a certain amount
of THF, which was added drop-wise to methanol so as to precipitate the high molar
mass polymers. The fractioned high molar mass polymers were separated by
filtration and dried in vacuum oven. The molecular weights determined by gel
permeation chromatography (GPC) are summarized in Table 1.
The optical properties of polymers, absorption (UV-vis), fluorescence (FL) and
photoluminescence (PL) are summarized in Figure 2. A broad absorption band was
observed with kmax value at 390 nm, which can be attributed to the p-pꢁ transition of
the conjugated backbone; correspondingly, a slight shoulder observed above 400 nm
can be attributed to the fluorene unit [6]. The optical band gaps of polymers were
2.95, and 2.50 eV corresponding to PF, and PF-BT-QL, respectively, which were
calculated from the onset of UV-vis spectra.
From the FL spectra obtained upon excitation at 390 nm of polymers, the PF
has a strong emission band at 404 nm and flat after 430 nm. It is possible that the
Table 1. Average molecular weights for Polyfluorene [PF] and
Poly(fluorene-benzothiadiazole-quinoline) [PF-BT-QL]
a
a
Mw (g molꢀ1
)
Mn (g molꢀ1
)
PDIb
PF
PF-BT-QL
40000
1600
20000
1100
2.0
1.4
aMw ꢂ weight-averaged molecular weight; Mn ꢂ number-averaged
molecular weight.
bPolydispersity index ðPDIÞÞ ¼ Mw=Mn.

Synthesis and Optical Properties
91=[431]
Figure 2. Optical properties of polymers.
shoulder arises earlier due to the coupling reaction between the fluorene and vinylene
units to form a new electronic state with a lower energy [7], while, PF-BT-QL exhib-
ited a broad peak at about 520 nm.
From the PL spectra of the polymers, the major emission band of PF appeared
at 436 nm with a strong shoulder at a slightly longer wavelength (þ30 nm). Again,
the shoulder likely arose from electronic coupling between the fluorene and vinylene
moieties, while PF-BT-QL exhibited a broad peak at about 540 nm. As the results,
copolymer showed red-shifted emission in comparison to the homopolymer.
The electrochemical properties and HOMO and LUMO energy levels of PF and
PF-BT-QL were investigated using cyclic voltammetry (CV). The measurement was
performed in a solution of n-Bu4NPF6 (0.1 M) and polymers (20 mg) in dried CH2Cl2
at a scan rate of 50 mV=s at room temperature under the protection of argon. A
glassy carbon electrode, a platinum electrode and an Ag=Agþ electrode were used
as the working electrode, the counter electrode and the quasi-reference electrode
respectively. As shown in Figure 3, PF and PF-BT-QL exhibit quasi-irreversible oxi-
dation processes. The EHOMO was calculated according to an empirical equation as
follows [8]:
EHOMO ¼ ꢀð4:44 þ EonsetÞ ðeVÞ
and the Egap was estimated from absorption onset of the polymers, thus, ELUMO was
calculated using the equation,
ELUMO ¼ EHOMO þ Egap

92=[432]
P. Aiamsen, D. L. Carroll, and S. Phanichphant
Figure 3. Cyclic voltammogram of polymers in CH2Cl2 with 0.1 M of n-Bu4NPF6 at a scan
rate of 50 mV=s.
The onset oxidation potential (Eonset), EHOMO, band gap energy (Egap), and ELUMO
are summarized in Table 2. So the oxidation process at about 1.21 V corresponds
to the removal of electrons from fluorene core generating radical dications.

Synthesis and Optical Properties
93=[433]
Table 2. The onset oxidation potential (Eonset), EHOMO, band gap energy (Egap), and
ELUMO of polymers
Eonset (V)
EHOMO (eV)
Egap (eV)
ELUMO (eV)
PF
PF-BT-QL
1.20
1.26
5.64
5.70
2.95
2.50
2.69
3.20
EHOMO decreased and ELUMO increased for copolymers, which indicated more dif-
ficulty in hole or electron (charge) injection.
Light-emitting diode devices were fabricated in order to test the current-voltage-
luminescence and lifetime properties of the polymers to determine their viability for
use in organic optoelectronic devices. On the basis of the band gap=energy level of
the polymers, an ITO(150 nm)=PEDOT:PSS(40 nm)=polymer(80 nm)=LiF(0.5 nm)=
Al(150 nm) device configuration was found suitable.
Figure 4 shows EL spectra of PLEDs device. The EL spectra of PF devices
exhibited blue peaks at 440 nm and extra peaks at about 465 nm. This indicated
that, at 440 nm, it was originated from a radiative decay process of singlet excitons
in PF molecules. The EL spectra of copolymers were similar to the PL spectra,
which indicated that the same energy transfer was involved in EL and PL. It
was reasonable to propose that the result was due to an energy transfer from high
energy state fluorene moiety to the low energy state quinoline and benzothiadiazole
moieties. As the results, the transferred excitons decayed through radiative recom-
bination in quinoline and benzothiadiazole and exhibited the red-shifted emission
in comparison to the homopolymer. The EL intensity of PF was higher than that
of PF-BT-QL copolymer at the same voltage applied. The lower luminance in the
copolymer is probably due to the efficient energy transfer from PF to quinoline
Figure 4. Electroluminescence spectra of polymers which have ITO=PEDOT:PSS=polymer=
LiF=Al configuration.

94=[434]
P. Aiamsen, D. L. Carroll, and S. Phanichphant
Table 3. The performances of PLEDs
Device
kem (nm)
CIE (x,y)
Turn on voltage (V)
PF
PF-BT-QL
440,465
517
0.17, 0.11
0.28, 0.44
10
5
and benzothiadiazole unit. Thus, because of the red shift and the relatively lower
luminance in PF-BT-QL, the luminance of copolymer was lower than the real blue
emission with high luminance.
The performances of PLED are shown in Table 3. The CIE coordinates of
devices and their emission color of the devices are shown in Figure 5. By changing
the molar ratios of these monomer units, the electroluminescence spectra can be
adjusted to white-light emission.
Though the difference in turn-on voltages will lead to a disparity in current den-
sity and brightness at a particular voltage, another attribute to consider would be the
slopes of the I-V and L-V curves. These are indicative of the rates at which the
current and brightness are increasing. Figures 6(a) and 6(b), both current and elec-
troluminescence vs voltage data exhibit readily observable increases in the slopes of
PF-BT-QL from PF. A higher slope indicates either efficient charge injection, higher
charge mobility, or a combination of both. The threshold voltage of PF-BT-QL was
lower than that of PF.
&
Figure 5. CIE coordinates of PF( ), and PF-BT-QL (ꢃ) devices.

Synthesis and Optical Properties
95=[435]
Figure 6. (a) Current-voltage (I-V) (b) luminance-voltage (L-V) characteristics of polymers
with ITO=PEDOT:PSS=polymer=LiF=Al configuration.
Acknowledgment
The Royal Golden Jubilee Ph.D. Program, the National Research University Project
under Thailand’s Office of the Higher Education Commission, the Center for Nano-
technology and Molecular Materials, Wake forest University, NC, USA, the Gradu-
ate School; Department of Chemistry, Faculty of Science, Chiang Mai University,
Thailand are gratefully acknowledged.
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