Conjugated polymers containing electron-deficient main chains and electron-rich pendant groups: Synthesis and application to electroluminescence

Article abstract of DOI:10.1016/j.orgel.2011.03.020

Two novel conjugated polymers denoted TAZF2 and TAZBTF, having electron-deficient main chains and electron-rich triphenylamine pendant groups, were designed and synthesized. While TAZF2 was constructed with fluorene and electron-deficient 1,2,4-triazole a

Full text of DOI:10.1016/j.orgel.2011.03.020

Organic Electronics 12 (2011) 1048–1062
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Conjugated polymers containing electron-deficient main chains
and electron-rich pendant groups: Synthesis and application
to electroluminescence
Kun-Li Wang a, , Man-kit Leung b, , Li-Ga Hsieh a, Chin-Chuan Chang b, Kueir-Rarn Lee c,
⇑
⇑
Chun-Lung Wu d, Jyh-Chiang Jiang e, Chieh-Yu Tseng e, Hui-Tang Wang e
a Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
b Department of Chemistry, Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10607, Taiwan
c R&D Center for Membrane Technology, Chung-Yuan Christian University, 32023 Chung Li, Taiwan
d Department of Raw Materials and Yarns, Taiwan Textile Research Institute, No. 6, Chengtian Rd., Tucheng City, Taipei County 23674, Taiwan
e Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel conjugated polymers denoted TAZF2 and TAZBTF, having electron-deficient
main chains and electron-rich triphenylamine pendant groups, were designed and synthe-
sized. While TAZF2 was constructed with fluorene and electron-deficient 1,2,4-triazole as
the major components of the main chain, fluorene, 1,2,4-triazole, and 2,1,3-benzothiadiaz-
ole were employed to form TAZBTF. The number-average molecular weights (Mw) of the
polymers fell in the range of 1.28 ꢀ 104–2.56 ꢀ 104 with polydispersity indexes
(PDI = Mw/Mn) of 1.49–1.95. The polymers had glass transition temperatures (Tg) in the
range of 153–185 °C. While TAZF2 exhibited blue photoluminescence, TAZBTF exhibited
yellow–green emission. Thermal stability of the conjugated polymers was enhanced due
to the introduction of rigid bipolar components. The incorporation of 2,1,3-benzothiadiaz-
ole into TAZBTF reduced the coplanarity of the conjugated polymer, which prohibited the
polymer from aggregation. Intramolecular and intermolecular energy transfer was clearly
evidenced by the shift in light emission to the yellow–green region. TAZBTF exhibited
maximum electroluminescence efficiency as high as 2.60 cd/A, demonstrating that a good
balance of charge carriers is injected into our devices.
Received 1 October 2010
Received in revised form 10 March 2011
Accepted 16 March 2011
Available online 31 March 2011
Keywords:
PLED
Bipolar
Fluorene
Conjugated polymer
Donor–acceptor
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
OLEDs) [1–6], photovoltaic devices [7–10], electrochromic
devices [11,12], field-effect transistors (FETs) [13–15],
Conjugated donor–acceptor (D–A) copolymers are of
growing interest for organic optoelectronic and electronic
applications, including electroluminescent materials for
polymeric or organic light emitting devices (PLEDs and
polymer memory materials [16–22], and other photoelec-
tronic devices [23,24]. In polymer light-emitting diodes,
polyfluorene (PF) derivatives have great potential for appli-
cation because of their high photoluminescence; however,
they suffer from low electroluminescence (EL) efficiency
and poor color stability [4–7]. The low EL efficiency of PF
derivatives indicates an imbalance of charge carriers [7].
To achieve high EL emission efficiency, balanced rates of
injection and transport of electrons and holes in the poly-
mers are essential. One of the most efficient ways to im-
prove the electron affinity of polymers is the insertion of
⇑
Corresponding authors. Tel.: +886 2 27712171x2558; fax: +886 2
27317117 (K.-L. Wang).
E-mail addresses: klwang@ntut.edu.tw (K.-L. Wang), mkleung@ntu.e-
du.tw (M.-k. Leung), krlee@cucu.edu.tw (K.-R. Lee), clwu.1023@ttri.org.tw
(C.-L. Wu).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.03.020

K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
1049
electron-deficient moieties into the polymer backbone
[4,25,26].
Aromatic 1,2,4-triazole shows good electron affinity
and hole-blocking properties. However, its high rigidity
may lead to low solubility in common organic solvents
[4,20,27,28]. On the other hand, triphenylamine exhibits
good hole affinity and solubility properties [25,26,29–
31]. These complementary properties make 1,2,4-triazole
and triphenylamine couple well, and they have been
incorporated as electron- and hole-transporting moieties
in organic- and polymeric-emitting materials. The incor-
poration of electron-rich and electron-deficient compo-
nents into a single polymer is expected to enhance the
hole-transporting and electron-transporting properties
of that polymer. Barriers to charge injection from both
electrodes into the polymer matrix would therefore be
reduced. Moreover, the bulky pendant side chain substitu-
ent triphenylamine is also expected to prevent the poly-
mer from aggregating. Self-quenching would therefore
be reduced, and the light-emitting efficiency would be
enhanced.
Some studies [26,29,30] had reported that polymers
containing donor–acceptor main chains show a reduction
in the band gap of the polymers due to the backbone
charge transfer from the donor to the acceptor. Based on
the aforementioned considerations, our research group
previously reported the synthesis and electroluminescent
properties of donor–acceptor conjugated polymers TPAF
and TPABTF with triphenylamine main chains and 1,2,4-
triazole side chains [4]. The presence of the bipolar moiety
significantly improves charge injection and charge trans-
port in the polymer matrix.
In this paper, contrary to the previous donor–acceptor
main chain-pendant design, we report the synthesis and
light-emitting properties of a series of copolymers contain-
ing electron-deficient main chains with electron-rich pen-
dant groups. The electron-deficient main chains contain
1,2,4-triazole and 2,1,3-benzothiadiazole as their major
components. The electron-rich side chains are mainly com-
posed of triphenylamine groups. The thermal, PL, and EL
properties of the polymers are discussed.
2. Experimental
2.1. Materials
N-Bromosuccinimide (Acros), 4-bromobenzoyl chloride
(Fluka), phosphorus pentachloride (Riedel-de Haën), 4-
nitrofluorobenzene (Acros), tin (II) chloride anhydrous
(Showa), hydrazine monohydrate (Alfa Aesar), heptanoyl
chloride (Acros), tetrakis(triphenylphosphine)palladium
(0) (Acros), aluminum chloride (Acros), and 9,9-dioctylflu-
orene-2,7-diboronic acid bis(1,3-propanediol) ester (Al-
drich) were used as received. N-Methyl-2-pyrrolidinone
(NMP), N,N-dimethylacetamide (DMAc) and dimethyl sulf-
oxide (DMSO) were purchased from TEDIA, and tetrahy-
drofuran (THF), N,N-dimethylformamide (DMF) were
purchased from ECHO. These solvents were distilled after
drying with appropriate agents and stored over 4 Å molec-
ular sieves. The surfactant trioctylmethylammonium chlo-
ride (Aliquat336Ò) was purchased from TCI and used as
received.
C8H17 C8H17
N
n
N
TPAF
N
N
S
C8H17 C8H17
N
N
C8H17 C8H17
N
N
q
p
2.2. Measurements
TPABTF
N
N
Infrared (IR) spectra were recorded on a PerkinElmer GX
FT-IR spectrometer. NMR spectra were recorded using a
BRUKER DRX-500 NMR (1H at 500 MHz and 13C at
125 MHz). Elemental analyses were made using a Perkin-
Elmer 2400 instrument. Weight-average (Mw) and num-
ber-average (Mn) molecular weight were determined by
gel permeation chromatography (GPC). Five Waters
(Ultrastyragel) columns 300 ꢀ 7.7 mm (guard, 500, 103,
104, 105 in a series) were chained with a refractive index
detector (RI 2000), using tetrahydrofuran (THF)
(1 mL minꢁ1) as the eluent and polystyrene as the standard
for GPC analysis. Thermogravimetric data were obtained
It is well-known that the molecular architectures
importantly affect physical properties of polymers, such
as molecular skeleton, side chains, functional groups, bond
connections and stereo structures. Therefore, it is interest-
ing to study the relationship between the physical proper-
ties and chemical structure of polymers as shown below.
The LUMO and HOMO of the polymers will be significantly
different due to the different moiety connection; and the
physical properties such as solubility, thermal, photophys-
ical properties and emitting properties as an emitting
material will be different.

1050
K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
with a PerkinElmer Pyris 6 TGA under nitrogen flow at a
rate of 20 cm3 minꢁ1 and a heating rate of 20 °C minꢁ1. Dif-
ferential scanning calorimetric analysis was performed
using a PerkinElmer Pyris DSC 6 under nitrogen flow at a
box under nitrogen atmosphere (oxygen and moisture
<1 ppm), inside which the device was then encapsulated
with glass covers using UV-cured epoxy glue. Current–
voltage characteristics were measured on a programmable
source-meter (Keithley 2400). Luminance was recorded
with a BM-8 luminance meter (Topcon).
rate of 20 cm3 minꢁ1 and a heating rate of 10 °C minꢁ1
.
Absorption spectra were recorded with a UV–Visible spec-
trometer (UV 500) at room temperature in air. Photolumi-
nescence (PL) spectra were performed using a PerkinElmer
LS 55 Luminescence spectrometer. Cyclic voltammetry
(CHI model 619A) was conducted with the use of a three-
electrode cell in which ITO was used as the working elec-
trode, a platinum wire was used as the auxiliary electrode,
and an Ag/AgCl couple was used as the reference electrode.
Ferrocene was used as the internal standard for potential
calibration.
2.4. Synthesis of monomers
The synthetic pathways are summarized in Scheme 1.
Compounds 2, 3, 4, and 8 were prepared and characterized
as described in the literatures [3,4,32]. The syntheses of 5,
6, and the polymers TAZF1, TAZF2, and TAZBTF are sum-
marized as follows.
2.4.1. 3,5-Bis-(4-bromophenyl)4-([4-(N,N-
2.3. EL device fabrication and characterization
di(phenyl)amino)]phen-1-yl)-1,2,4-triazole (5)
Compounds
3 (2.70 g, 6.2 mmol) and 4 (3.40 g,
PLEDs were fabricated into an ITO glass/PEDOT:PSS/
polymer/LiF/Al structure. ITO-coated glass, with a sheet
13 mmol) and xylene (80 mL) were added to a three-
necked flask fitted with a stirrer. The mixture was refluxed
with stirring at 160 °C for 72 h. After cooling, the solvent
was evaporated to collect the crude product, which was
further washed with hot ethanol and recrystallized from
a cosolvent of ethanol and tetrahydrofuran (1:1) to yield
5 as gray–white crystals (65%): mp 294 °C (by DSC); FT-
IR (KBr, cmꢁ1): 1072 (C–Br); 1509 (C@N). 1H NMR (CDCl3,
ppm): d 6.94–6.96 (d, 2H, Ar–H, J = 9.04 Hz), 7.00–7.02 (d,
2H, Ar–H, J = 8.82 Hz), 7.11–7.14 (m, 6H, Ar–H), 7.31–
7.34 (t, 4H, Ar–H, J = 7.84 Hz), 7.38–7.40 (d, 2H, Ar–H,
J = 8.41 Hz), 7.48–7.50 (d, 2H, Ar–H, J = 8.49 Hz). 13C NMR
(CDCl3, ppm): d 121.5, 124.6, 124.7, 125.2, 125.6, 126.2,
128.2, 129.7, 130.2, 131.7, 146.3, 149.4, 154.0. Anal. Calcd.
resistance of 15
X/sq, was purchased from Applied Film
Corp. Glass substrates with patterned ITO electrodes were
washed well and cleaned by an oxygen plasma treatment.
A thin film (850 Å) of hole-transporting material poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonate) (PED-
OT:PSS; Bayton PVPAI 8000) was spin-cast on the ITO glass,
followed by spin-coating of the polymer film from the fil-
tered (0.45-lm filter) dioxane solution (20 mg/mL). The
films were dried at 80 °C for 1 h in a glove box. Subse-
quently, the metallic cathode was thermally deposited in
a high-vacuum chamber (5 ꢀ 10ꢁ6 torr). Then, the PLED
was transferred from the evaporation chamber to a glove
O
C
O
C
PCl5
Cl
C
Cl
C
NH2NH2H2O
Br
COCl
Br
Br
Br
Br
N N
N3N
1
H
H
2
N
N
Cl
C
Cl
C
N
Br
Br
+
N3N
NH2
Br
Br
N
N
4
5
C6H13OC
COC6H13
N
N
AlCl3, C6H13COCl
Br
Br
N
N
6
Scheme 1. Synthetic route for dipolar dibromo monomers.

K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
1051
for C32H22Br2N4: C, 61.76; N, 9.00; H, 3.56; found: C, 61.59;
N, 8.90; H, 3.61.
lowish solid: mp 167 °C (by DSC); FT-IR (KBr, cmꢁ1): 1074
(C–Br); 1506 (C@N); 1685 (C@O); 2852, 2930 (alkyl group).
1H NMR (CDCl3, ppm): d 0.84–0.89 (m, 6H, aliphatic H),
1.22–1.36 (m, 12H, aliphatic H), 1.67–1.73 (m, 4H, aliphatic
H), 2.88–2.91 (t, 4H, aliphatic H, J = 7.38 Hz), 7.09–7.17 (m,
8H, Ar–H), d 7.31–7.32 (d, 4H, Ar–H, J = 8.34 Hz), 7.45–7.46
(d, 4H, Ar–H, J = 8.24 Hz), 7.90–7.91 (d, 4H, Ar–H,
J = 8.55 Hz). 13C NMR (CDCl3, ppm): d 13.9, 22.4, 24.2,
28.9, 31.5, 38.3, 123.5, 124.5, 125.3, 125.4, 128.9, 129.7,
129.8, 130.0, 131.6, 132.7, 147.5, 149.8, 153.9, 198.8. Anal.
Calcd. for C46H46Br2N4O2: C, 65.25; N, 6.62; H, 5.48; found:
C, 65.49; N, 6.52; H, 5.56.
2.4.2. 3,5-Bis-(4-bromophenyl)4-([4-(N,N-di(4-
heptanoylphen-1-yl)amino)]phen-1-yl)-1,2,4-triazole (6)
AlCl3 (1.41 g, 11 mmol),
5 (1.00 g, 1.6 mmol), and
CH2Cl2 (50 mL) were added to a three-necked flask fitted
with a stirrer. The mixture was stirred in an ice bath for
5 min under nitrogen atmosphere. A solution of heptanoyl
chloride (1.8 mL, 12 mmol) in CH2Cl2 was added drop-wise
into the mixture. The mixture was then allowed to further
react for 4 h. The resulting mixture was quenched by pour-
ing into water. The product was extracted into CH2Cl2 and
washed with water several times. The extracts were then
dried over anhydrous MgSO4, concentrated, and purified
with liquid column chromatography (silica gel, ethyl ace-
tate/hexane (1/1) as eluent) to yield 6 (0.80 g, 59%) as a yel-
2.4.3. Preparation of TAZF1, TAZF2 and TAZBTF
The polymers were prepared via the facile Suzuki cou-
pling reaction (Scheme 2). The standard procedure for
TAZF2 is described below.
R
R
N
C8H17
C8H17
O
n
n
B
+
O
N
7
Br
Br
N
N
5 or 6
R
R
N
N
Pd(PPh3)4
TAZF1 R=-H
THF/2M K2CO3
TAZF2 R= -COC6H13
C8H17 C8H17
N
N
n
C6H13OC
COC6H13
N
N
S
C8H17
C8H17
N
N
O
O
n
n
2n
B
B
+
+
O
O
Br
Br
7
8
Br
Br
N
N
6
C6H13OC
COC6H13
N
N
Pd(PPh3)4
S
C8H17 C8H17
N
N
C8H17 C8H17
THF/2M K2CO3
N
N
y
x
TAZBTF
Scheme 2. Preparation of conjugated polymers.

1052
K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
To a mixture of 6 (1 g, 1.61 mmol) and 2,7-bis(4,4,5,5-
correct integral ratios for the aliphatic protons indicated
that the acylation was completely carried out as proposed.
In the 13C NMR spectrum, observation of 6 peaks of the ali-
phatic carbons, 13 peaks of the aromatic carbons, and 1
peak of carbonyl carbon at 198.8 ppm again supported
the proposed structure.
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene
(7) (0.68 g, 1.22 mmol), several drops of Aliquat336Ò and
Pd(PPh3)4 (0.027 g, 23 mmol) and aqueous K2CO3 (2 M,
6.7 mL) and toluene (20 mL) were added under argon.
The mixture was heated to 110 °C and stirred for 96 h
and then quenched by pouring into a mixture of deionized
water and methanol (1:10). The precipitate was collected
by filtration and dried. The precipitate was further purified
by dissolving in CHCl3 and re-precipitated into a mixture of
deionized water and methanol several times. The polymer
was further washed by Soxhlet extraction with methanol
for 24 h. The polymer (TAZF2) was obtained after drying
under vacuum at 60 °C overnight (0.708 g, 56%). FT-IR
(KBr, cmꢁ1): 1507 (C@N); 1677 (C@O); 2852, 2924 (alkyl
group). 1H NMR (CDCl3, ppm): d 0.76–0.80 (m, 10H),
0.87–0.89 (t, 6H), 1.08–1.17 (m, 20H), 1.25–1.37 (m,
12H), 1.71–1.74 (t, 4H), 2.08 (s, 4H), 2.89–2.92 (m, 4H),
7.05–7.25 (m, 8H), 7.59–7.85 (m, 14H), 7.85–7.95 (m,
4H). Anal. Calcd. for C75H88N4O2: C, 83.60; N, 5.20; H,
8.23; found: C, 83.53; N, 5.06; H, 8.14.
Fluorene-based conjugated polymers were readily pre-
pared by facile Suzuki cross-coupling of 7 with 5/6 and 8
as shown in Scheme 2. The polymerization was carried
out in a mixture of aqueous NaHCO3 and THF with freshly
prepared Pd(PPh3)4 as the catalyst. Unfortunately, the sol-
ubility of TAZF1 obtained by this approach was so poor
that TAZF1 could only be identified by FT-IR and elemental
analysis, which supported the proposed structure. In con-
trast, TAZF2 and TAZBTF are soluble in several different or-
ganic solvents and, therefore, could be further identified by
NMR. In the FT-IR spectra, the observation of the character-
istic bands of 1,2,4-triazole at 1507 cmꢁ1 and of the alkyl
chains at 2852 and 2925 cmꢁ1 strongly suggested poly-
meric structures. Moreover, disappearance of the charac-
teristic stretching C–Br band of the dibromo monomers
at 1074 cmꢁ1 is consistent with the assumption of conden-
sation polymerization. The 1H NMR spectra of TAZF2 and
TAZBTF are illustrated in Fig. 2. The spectra are assigned
by matching the chemical-shift data with those of the cor-
responding dibromo and fluorene monomers. For TAZBTF,
the copolymerized segment ratio (x/y) of 5/6 was
determined (Fig. 2B) on the basis of the peak integrals of
The yield of TAZF1 was 53%. Because TAZF1 is highly
insoluble in organic solvents, a good quality NMR is diffi-
cult to obtain. Therefore, TAZF1 was characterized by FT-
IR and elemental analysis. FT-IR (KBr, cmꢁ1): 1508
(C@N); 1676 (C@O); 2852, 2925 (alkyl group). Anal. Calcd.
for C61H62N4: C, 86.08; N, 6.58; H, 7.34; found: C, 86.56; N,
6.44; H, 7.16.
TAZBTF copolymer (98% yield) was prepared by react-
He (a-protons adjacent to the C@O groups) against the to-
tal aliphatic protons attached to triphenylamine and fluo-
rene moieties.
ing equal moles of 6 and 8. FT-IR (KBr, cmꢁ1): 1507
1H NMR
(C@N); 1678 (C@O); 2853, 2925 (alkyl group).
(CDCl3, ppm): d 0.75–0.88 (m, 25H), 1.12–1.16 (m, 41H),
1.25–1.38 (m, 14H), 1.73 (t, 5H), 2.10 (d, 5H), 2.92 (s,
4H), 7.19–7.25 (m, 10H), 7.63–8.09 (m, 24H). Anal. Calcd.
for C110H130N6O2S: C, 82.56; N, 5.25; H, 8.19; found: C,
81.56; N, 5.44; H, 8.16.
The molecular weights and thermal properties of the
conjugated polymers are summarized in Table 1. The
weight-average molecular weights (Mw) of the polymers
were 1.28 ꢀ 104 and 2.56 ꢀ 104 for TAZF2 and TAZBTF,
respectively, as measured by gel permeation chromatogra-
phy (GPC) relative to the polystyrene standards. The ther-
mal properties of the conjugated polymers were evaluated
by differential scanning calorimetry (DSC) and thermo-
gravimetric analysis (TGA). The glass transition tempera-
tures (Tg) of the polymers were found to be 153–185 °C,
which are much higher than that of poly(9,9-dioctylfluo-
rene) (POF; Tg = 75 °C) [28]. The thermal properties
reported in the literature [4] for polymers TPAF and
TPABTF containing triphenylamine main chains and
1,2,4-triazole pendant groups were collected in the table
for comparison. It is evident that incorporation of the rigid
1,2,4-triazole and triphenylamine subunits into the poly-
mer enhances the rigidity of the chain and increases Tg.
Obviously, TAZF1 and TAZF2, with their 1,2,4-triazole
main chains, show much higher Tg than previously re-
ported for TPAF (135 °C) [4], in which the triphenylamine
is in the main chain. However, TPABTF and TAZBTF con-
taining 2,1,3-benzothiadiazole (BTDZ) moiety have similar
Tgs. In contrast, the thermal properties of these polymers
with 1,2,4-triazole main chains exhibit slightly lower deg-
radation temperatures than the reported polymers with
triphenylamine main chains due to the lower molecular
weight and higher content of aliphatic segments.
3. Results and discussion
Monomers 5 and 6 containing the electron-donating tri-
phenylamine and the electron-accepting 1,2,4-triazole
moieties were prepared as shown in Scheme 1. Bromoben-
zoyl chloride (1) was reacted with hydrazine hydrate to
give hydrazide 2, which was further chlorinated in the
presence of PCl5 to give chlorinated azine 3. Cycloconden-
sation of compounds 3 and 4 gave compound 5, which was
further acetylated through Friedel–Crafts acylation to give
6. Formation of the 1,2,4-triazole ring on 5 could be mon-
itored by the change of a characteristic band in the FT-IR
spectra; the characteristic band of the chlorinated azine 3
at 1571 cmꢁ1 gradually disappeared as a new characteris-
tic band of the 1,2,4-triazole ring appeared at 1509 cmꢁ1
.
Formation of 6 was evidenced by the appearance of a band
at 1685 cmꢁ1 for the carbonyl group in the IR spectrum.
The NMR spectra and 2D C–H COSY of 5 are shown in the
supplementary material (Fig. S1). Fig. 1 shows the 1H
NMR and 13C NMR spectra of 6 in CDCl3. In the 1H NMR
spectrum of 6, the Hg shifted to d 7.90 ppm due to incorpo-
ration of the electron-withdrawing carbonyl groups at the
para-positions. The peaks appearing at d 0.8–2.9 ppm with
The results of the qualitative solubility test (1 mg/
10 mL) of the polymers are summarized in Table 2. The

K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
1053
Fig. 1. (A) 1H NMR, (B) 13C NMR spectra of the dibromo monomer 6.
previously reported TPAF exhibited good solubility due to
the propeller triphenylamine structure in the main chain
[4]. However, TAZF1 has much lower solubility in compar-
ison to TPAF due to its relatively rigid 1,2,4-triazole main
chain structure. However, TAZF2 was reasonably soluble
or partially soluble in some organic solvents; the aliphatic

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K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
Fig. 2. 1H NMR spectrum of conjugated polymers: (A) TAZF2 and (B) TAZBTF.
side chains attached on the triphenylamine help TAZF2 to
increase the solubility. The TAZBTF shows much better sol-
ubility due to the higher flexible aliphatic contents.
The optical properties of the conjugated polymers are
summarized in Table 3. The optical properties of the poly-
mers were investigated by UV–Vis and photoluminescence
spectroscopy in the solid state and in chloroform (1 mg/
10 mL). The UV–Vis and photoluminescence spectroscopy
of the polymers are shown in Fig. 3. In chloroform, TAZF2
UV
max
exhibits absorption with k
at 355 nm (Fig. 3A). By com-
paring this spectrum with the absorption spectra of
poly(9,9-dioctylfluorene) (POF), one might ascribe the

K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
1055
at 435 nm is attributed to the p–
p⁄ transitions of the ben-
zothiadiazole–fluorene component of the main chain. The
Table 1
Molecular weights and thermal properties of the conjugated polymers.
Polymer
GPC dataa
Thermal propertiesb
second absorption band at 345 nm might arise from the
p–
p⁄ transition of the conjugated polymer backbone. How-
Mw
Mn
PDI
Tg
Td5
Td10
ever, a red shift by 10 nm relative to that was observed for
c
c
c
TAZF1
TAZF2
TAZBTF
TPAF [4]
TPABTF [4]
–
–
–
178
185
153
135
155
383
411
406
428
433
427
436
434
455
456
PL
TAZF2. TAZBTF fluoresces at k
of 538 nm in chloroform.
max
12,800
25,600
32,500
38,000
8500
1.49
1.95
1.58
1.60
Because the emission from the TAZ subunit at 425 nm has
a good overlap with the spectral absorption of the BTDZ
subunit at 435 nm, Förster energy transfer [34] from TAZ
to BTDZ is expected so that light emission at 425 nm is
13,000
20,500
24,000
a
The molecular weights are relative to polystyrene standard.
b
By DSC and TGA measurements, respectively. Td5 and Td10 are 5% and
quenched and transferred to the emission at 538 nm. Sim-
PL
10% degradation temperature, respectively.
ilarly, emission with k
at 541 nm was found in solid
max
c
The polymer is insoluble in THF.
film. The emission is red-shifted by 113 and 128 nm in
comparison to the spectra observed for TAZF2 in solution
and in the film state, respectively (Table 3). The high quan-
tum yield of TAZBTF is over 0.9 in solution [35] indicating
that the Förster energy transfer from the TAZ to BTDZ unit
is efficient. Although the quantum yield of 0.4 in the film
state is higher than that of 0.08 for TAZF2 in solid film, this
value is relatively low in comparison to that in solution as
well as to other BTDZ-based polymers reported in the liter-
ature [35], indicating that fluorescence quenching would
occur in solid films.
Careful study of the PL spectra of TAZBTF in CHCl3 re-
vealed that some residue emission from the TAZ compo-
nent exists at 425 nm. Although this detectable emission
was concentration-dependent, the corresponding PL inten-
sity was very weak even under high dilution (<2%) (Fig. 4).
Because the ratio of the integrated intensities of 550 nm
peaks to 425 nm peaks can be considered a measure of
internal energy transfer efficiency from TAZ subunits to
BTDZ subunits [35], this result indicates that the internal
Föster energy transfer is efficient and may play a major
role in the energy transfer process. However, the energy
transfer might still operate intermolecularly at high con-
centration or in the condensed phase and cannot be simply
absorption at 355 nm to a p–
p⁄ transition derived from the
conjugated polymer backbone. However, a blue shift by
36 nm relative to that observed for POF (at 391 nm) [33]
is due to the twist (31–35°) between 1,2,4-triazole and
adjacent phenyl in the main chain (see below), which
shortens the conjugated length of the polymer. Thin-film
TAZF2 shows an absorption peak at 356 nm, which is close
to that in solution. The cut-off for absorption was 404 nm
for the film state, and the optical energy band gap (Eg)
was calculated to be 3.1 eV. TAZF2 emits blue fluorescence
PL
with k
at 425 nm in chloroform, while in solid film the
max
PL
emission is slightly blue-shifted and exhibits k
at
max
413 nm (Fig. 3A). The charge-transfer properties in the ex-
cited-state emission in chloroform are apparently signifi-
cant so that a broad and red-shifted emission band was
observed. Low PL quantum yield (Uf) of 0.29 in chloroform
and 0.08 in the film state for TAZF2, as excited at 355 nm,
were recorded by comparing against the fluorescence stan-
dard 9,10-diphenylanthrancene (Uf = 0.9).
For TAZBTF, two major bands are observed at 435 and
345 nm in chloroform (Fig. 3B). The first absorption band
Table 2
Solubilities of the conjugated polymers.a
Polymer
CH2Cl2
CHCl3
THF
Toluene
Dichlorobenzene
1,4-Dioxane
TAZF1
TAZF2
TAZBTF
TPAF [4]
TPABTF [4]
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
+ꢁ
++
++
++
++
++
++
++
+ꢁ
++
++
++
+ꢁ
+ꢁ
+ꢁ
+ꢁ
+ꢁ
++
++
++
+ꢁ
+ꢁ
++
+ꢁ
a
The solubility was measured in the concentration of 1 mg/mL. ++: soluble at room temperature; +ꢁ: partially soluble at room temperature; ꢁ: insoluble
on heating.
Table 3
Optical properties of the conjugated polymers.
Uf (%)b
k
(nm)
k
(nm)a
UV
max
PL
max
Polymer
Chloroform
Film
Chloroform
Toluene
Film
Chloroform
Film
TAZF2
TAZBTF
TPAF [4]
TPABTF [4]
355
356
425
538
421, 442
543
–
522
–
413
541
435
546
28.5
>90
>90
>90
8.3
39.9
18.4
33.1
345, 435
275, 371
346, 438
347, 450
280, 364
346, 453
526
a
The maximum wavelength (nm) of photoluminescence was excited by the maximum wavelength (nm) of absorption.
The PL quantum yields (Uf) were measured relative to a 9,10-diphenylanthrancene (Uf = 0.9) standard.
b

1056
K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
Fig. 3. Absorption and PL spectra of the conjugated polymers: (A) TAZF2 and (B) TAZBTF.
excluded [36,37]. The Stokes shifts for polymer films are
greater than 57 nm (Table 3), indicating that the spectral
overlap between PL and UV spectra is not significant.
Therefore, the interference in the emissive spectral
properties of TAZF2 and TAZBTF in the solid phase through
self-absorption in luminescence should be insignificant.
Theoretical studies provide insights into polymers and
have been making a great contribution to support experi-
mental results [38–40]. To better understand the absorp-
tion behavior of the polymers, we used density functional
theory (DFT) and semiempirical ZINDO calculation to study
the electronic and optical properties of the polymers. All
theoretical calculations in this study are carried out using
quantum mechanical package Gaussian 03. The electronic
states for the polymers are determined using DFT calcula-
tion with the B3LYP functional and the 6-31G⁄ basis set.
The calculated excitation energies with the largest oscilla-
tor strength (f) are compared to the maximum absorption
in UV–Vis spectrum. The simulated results with two
repeating units (n = 2) are shown in Fig. 5. Fig. 5A shows
that the S0 ? S5 electronic transition has the largest oscil-
lator strength for TAZF2 with maximum absorption at

K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
1057
Fig. 4. Emission spectra of the polymer TAZBTF in chloroform solution at different concentrations.
357 nm, which corresponds to the maximum absorption at
355 nm in the UV–Vis spectrum. The HOMO ? LUMO+2
transition mainly dominates the S0 ? S1 transition, and
the absorption is mainly distributed by the charge transfer
from the conjugated main chain to the triphenylamine side
chain. For TAZBTF polymer (Fig. 5B), the S0 ? S1, S0 ? S7
and S0 ? S9 electronic transitions have the larger oscillator
strengths with maximum absorption at 499, 360 and
352 nm, respectively. The HOMOꢁ1 ? LUMO transition
mainly dominates the S0 ? S1 transition, and the absorp-
tion is mainly distributed by the charge transfer from the
conjugated main chain to the 2,1,3-benzothiadiazole
(BTDZ) moiety. The HOMO ? LUMO+6 and HOMO-
3 ? LUMO+1 transitions mainly dominate, respectively,
the S0 ? S7 and S0 ? S9 transitions, and the absorptions
LUMO values of TAZF2 and TAZBTF were found to be
ꢁ5.72 and ꢁ2.62 eV, and ꢁ5.78 and ꢁ3.40 eV, respectively.
According to the above results, we first conclude that the
HOMO and LUMO levels of TAZF2 and TAZBTF, with
1,2,4-triazole main chains and triphenylamine pendant
groups, are much lower than those of TPAF and TPABTF
with triphenylamine main chains and 1,2,4-triazole pen-
dant groups. However, the energy gaps for the similar
polymer pairs (TAZF2 vs TPAF and TAZBTF vs TPABTF)
are almost the same. As simulated results shown in
Fig. 5, the HOMOs and LUMOs were controlled by the con-
jugated backbone. Therefore, the polymers with triazole
acceptor moieties in the main chain will lower the HOMOs
and LUMOs comparing to those polymers with triphenyl-
amine donor moieties in the main chain. Because the sim-
ilar polymers have the same functional moieties in the
polymer structure, the HOMOs and LUMOs decrease in
the similar values that makes the band gaps are almost
the same for the polymer pairs. The polymers with BTDZ
in the main chain (TAZBTF and TPABTF) have lower
HOMOs and LUMOs than those (TAZF2 and TPAF) without
BTDZ due to the same reason. The second conclusion from
the experimental results is that the polymers with BTDZ in
the main chain (TAZBTF and TPABTF) have lower energy
gaps than those (TAZF2 and TPAF) without BTDZ moieties.
As sown in the simulation, the BTDZ involves in the main
chain conjugation. Therefore, the BTDZ increases the con-
jugation lengths and results in smaller energy gaps of the
polymers (TAZBTF and TPABTF).
are mainly distributed by the combination of p–
p⁄ transi-
tions in the main chain. The UV–Vis spectra for the poly-
mers are also calculated based on the electronic states as
shown in Fig. 6. The calculated results are comparative to
the experimental results in Fig. 3, especially for TAZF2.
For TAZBTF, the S0 ? S7 and S0 ? S9 electronic transitions
with maximum absorption at 360 and 352 nm, respec-
tively, are converse to an absorption peak corresponding
to the actual absorption at 345 nm. The S0 ? S1 electronic
transition with maximum absorption at 499 nm corre-
sponds to the actual absorption at 435 nm.
The electrochemical behavior of the polymers was
investigated by cyclic voltammetry (CV) (Fig. 7). From the
oxidation potential relative to ferrocene/ferrocenenium,
which refers to an energy level of ꢁ4.8 eV below the vac-
uum level, we can estimate the HOMO energy level of
the conjugated polymers, although the LUMO level of the
polymers could, in principle, be estimated according to
the equation: LUMO = HOMO + Eg, where Eg is the value
of the optical band gap. The HOMO and LUMO values of
the polymers are summarized in Table 4. The HOMO and
To further understand the optoelectronic properties of
the polymers, electroluminescence (EL) devices were fabri-
cated with the configuration ITO/HTL/polymer/cathode, in
which poly(3,4-ethylenedioxythiophene):poly(styrene sul-
fonic acid) (PEDOT:PSS) or poly(vinyl carbazole) (PVK) was
used as the hole-injection layer. The hole-transporting PVK
was blended with the emissive polymer due to good solu-

1058
K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
Fig. 5. Electron distribution in Frontier molecular orbitals for (A) TAZF2 and (B) TAZBTF with two repeating units (n = 2).
Fig. 6. Calculated UV–Vis spectra for TAZF2 and TAZBTF.

K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
1059
Fig. 7. CV diagrams of (A) TAZF2 and (B) TAZBTF.
Table 4
of all devices obtained from the current density (J)–voltage
(V)–luminance (L) characteristics is summarized in Table 5.
The relative energy level diagrams for the devices are
sketched in Fig. 8.
Electrochemical properties of the conjugated polymers.
Polymer
Eg (eV)a
Eonset (V)b
HOMO (eV)c
LUMO (eV)c
TAZF2
TAZBTF
POF [33]
TPAF [4]
TPABTF [4]
3.10
2.38
2.95
2.91
2.36
1.52
1.58
1.40
1.11
1.18
ꢁ5.72
ꢁ5.78
ꢁ5.80
ꢁ5.31
ꢁ5.38
ꢁ2.62
ꢁ3.40
ꢁ2.85
ꢁ2.39
ꢁ3.02
Among the devices we fabricated, the EL devices of
TAZF2 are non-emissive. However, devices I and II, using
TAZBTF as the emissive layer, exhibit green–yellow emis-
sion with kmax (EL) at 534–542 nm, similar to the PL emis-
sion. This result suggests that the BTDZ subunit functions
as the major core of light emission; all excitons formed
within the polymer matrix will have their energy trans-
ferred to the BTDZ subunit to emit. It should be noted that
the devices using the same PEDOT:PSS as the hole-injec-
tion layer but using different metallic cathodes of Mg and
Ca show the same switch-on voltage at 8 V. This result im-
plies that the electron-injection could not be improved
even using a metal with a lower work function. This phe-
a
b
c
Energy gap (Eg) = 1240/konset (nm).
Initial oxidation potential in film state.
HOMO ¼ ꢁðEox;onset ꢁ EFocx;onsetÞ ꢁ 4:8; LUMO = Eg ꢁ HOMO.
bility in chloroform; it is likely that the interface between
the PVK and polymer layer is diffused rather than being
flat and well defined. Two layers of Mg or Ca followed by
Ag (150 nm) were used as the cathode. The EL performance

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K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
Table 5
EL performance of devices with configuration of ITO/HTL/polymer/metal.
HTLa
Polymer
Cathode
Von (V)b
Lmax (cd/m2)c
Emax (cd/A)d
CIE (x, y)
EL
max
Device
k
(nm)
I
II
III
PEDOT
PEDOT
–
TAZBTF
TAZBTF
TAZBTF
PVK (10%)
Mg:Ag
Ca:Ag
8
8
15
147
172
392
2.07
2.60
0.208
538
542
534
0.340, 0.615
0.339, 0.624
0.328, 0.628
Ca:Ag
I0 [4]
II0 [4]
III0 [4]
PEDOT
PEDOT
–
TPABTF
TPABTF
TPABTF
PVK (5%)
Mg:Ag
Ca:Ag
Ca:Ag
7
5
14
516
696
629
1.24
2.02
0.06
544
542
538
0.355, 0.618
0.345, 0.625
0.306, 0.628
a
b
c
Hole transporting layer.
Turn-on voltage observed at 1 cd/m2.
Maximum luminance.
d
Maximum luminance efficiency.
Fig. 8. Relative energy level diagrams for the devices.
nomenon can be rationalized by the low-lying LUMO level
of ꢁ3.40 eV for TPABTF; electron injection either from Mg
or Ca cathode into the LUMO of the polymer matrix should
be energetically favorable (Fig. 8C). Nevertheless, the
switch-on voltage of 8 V is relatively high in comparison
to the devices of TPABTF. We attribute this to the lower-ly-
ing HOMO level of the triphenylamine units on TAZBTF.
The large energy gap of 0.58 eV between the PEDOT:PSS
and TAZBTF (Fig. 8D) might become a major factor in caus-
ing a high Fowler–Nordheim barrier for tunneling injec-
tion. Furthermore, the presence of the barrier may also
prevent high current injection into the device, limiting
the value of the highest brightness. Nevertheless, the lumi-
nance efficiencies of the EL devices are as high as 2.07 and
2.60 cd/A, indicating a good balance of charge carriers
within the device.
dized to form a radical cation under applied electrical
potentials. Luminescence quenching can be by two basic
mechanisms: (1) normal diffusional or dynamic quenching
of Stern–Volmer type and (2) associational or static
quenching where the donor and quencher form a nonlumi-
nescent association pair which reduces the amount of exci-
tation energy absorbed by potentially luminescent donors
[42]. Brudvig et al. reported that oxidized chlorophyll (a re-
dox-active site) is a potent quencher of chlorophyll fluores-
cence with 15% of oxidized chlorophyll sufficient to quench
70% of the fluorescence intensity, which involves electron
donation to the electron donor [43]. The situation is not
the same but similar to this system. Therefore, we suppose
that formation of the triphenylaminyl radical cation during
hole-injection may lead to a quenching effect that prohib-
its electroluminescence in the TAZ group.
Jen has recently found that conjugated polymers having
electron-deficient benzothiadiazole main chains show very
poor electron-injection ability compared to polymers with
electron-rich polyfluorene main chains [41]. This phenom-
enon is quite different from that obtained from conven-
tional designs of electron-injection materials. Similar to
Jen’s observations, the low brightness with high efficiency
strongly indicates in our EL devices the difficulty of charge
injection through both electrodes, which limits the carrier
density inside the polymer matrix.
Blending 10% hole-transporting PVK with a TAZBTF
emissive layer, device III exhibits electroluminescence
with higher brightness but much lower efficiency. Because
the introduction of PVK in the blending approach plays a
role in reducing the hole-injection barrier (Fig. 8E), higher
current density is allowed, which leads to higher bright-
ness. Because PVK is unable to function effectively as an
electron block, charge recombination within the PVK ma-
trix may cause the low efficiency.
Comparing the structural designs of TPABTF and
TAZBTF, the electron-deficient TAZ group was incorpo-
rated into the main chain while the electron-rich TPA
group was grafted as the pendant group in TAZBTF. The re-
The reasons for the failure of the EL devices using TAZF2
are still unclear. In our device, the triphenylamine group is
also the major hole-transport component, which is oxi-

K.-L. Wang et al. / Organic Electronics 12 (2011) 1048–1062
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Conjugated polymers containing electron-deficient
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phenylamine pendant groups were successfully synthe-
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Intramolecular and intermolecular energy transfers were
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Acknowledgements
Financial support by National Science Council of Taiwan
(NSC98-2221-E-027-002; NSC98-2119-M-002-006-MY3)
and Thematic Project, Academia Sinica is gratefully
acknowledged. The authors would also like to thank Ms.
S.-H. Wang (Taipei Medical University) for NMR technical
support.
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