Bipolar copoly(aryl ether) containing distyrylbenzene, triphenylamine, and 1,2,4-triazole moieties: Synthesis and optoelectronic properties

Article abstract of DOI:10.1002/pola.24747

A novel copoly(aryl ether) (P1) consisting of alternate emitting segments (distyrylbenzene) and a bipolar moiety composed of directly linked electron-transporting aromatic 1,2,4-triazole and hole-transporting triphenylamine was synthesized. The copoly(aryl ether) is readily soluble in common organic solvents and exhibit good thermal stability with thermal decomposition temperature above 450 °C. The emission and the photoluminescence quantum yield of the copolymer are dominated by the emitting segments (distyrylbenzene) with longer emissive wavelength. Electron affinity of P1 is evidently enhanced after introducing the isolated bipolar unit, as confirmed by the lowered lowest unoccupied molecular orbital level (-2.77 eV) relative to P0 without bipolar unit (-2.34 eV). This results in improved emission efficiency of its polymer light-emitting diode (indium tin oxide/poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)/P1/LiF/Ca/Al) due to more balanced charges injection and transport. Blending P1 with poly(9,9-dihexylfluorene) (PF) further improves the efficiency of the device; the best performance was obtained for PF/P1 = 20/0.8 (w/w) with maximum luminance and maximum luminance efficiency being significantly enhanced to 3260 cd/m² and 1.08 cd/A, respectively, from 380 cd/m² and 0.009 cd/A of P1-based device. These results demonstrate that the bipolar moiety can be used to enhance charges injection and transport of electroluminescent polymers.

Full text of DOI:10.1002/pola.24747

Bipolar Copoly(aryl ether) Containing Distyrylbenzene, Triphenylamine,
and 1,2,4-Triazole Moieties: Synthesis and Optoelectronic Properties
CHIA-SHING WU, SHAWN-LIN LEE, YUN CHEN
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
Received 21 February 2011; accepted 23 April 2011
DOI: 10.1002/pola.24747
Published online 16 May 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A novel copoly(aryl ether) (P1) consisting of alter-
nate emitting segments (distyrylbenzene) and a bipolar moi-
ety composed of directly linked electron-transporting
aromatic 1,2,4-triazole and hole-transporting triphenylamine
was synthesized. The copoly(aryl ether) is readily soluble in
common organic solvents and exhibit good thermal stability
with thermal decomposition temperature above 450 ꢀC. The
emission and the photoluminescence quantum yield of the
copolymer are dominated by the emitting segments (distyryl-
benzene) with longer emissive wavelength. Electron affinity
of P1 is evidently enhanced after introducing the isolated
bipolar unit, as confirmed by the lowered lowest unoccupied
molecular orbital level (–2.77 eV) relative to P0 without bipo-
lar unit (–2.34 eV). This results in improved emission effi-
ciency of its polymer light-emitting diode (indium tin oxide/
poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)/P1/
LiF/Ca/Al) due to more balanced charges injection and trans-
port. Blending P1 with poly(9,9-dihexylfluorene) (PF) further
improves the efficiency of the device; the best performance
was obtained for PF/P1 ¼ 20/0.8 (w/w) with maximum lumi-
nance and maximum luminance efficiency being significantly
enhanced to 3260 cd/m2 and 1.08 cd/A, respectively, from
380 cd/m2 and 0.009 cd/A of P1-based device. These results
demonstrate that the bipolar moiety can be used to enhance
charges injection and transport of electroluminescent poly-
C
V
mers.
2011 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 49: 3099–3108, 2011
KEYWORDS: bipolar; conjugated polymers; copoly(aryl ether);
distyrylbenzene; light-emitting diodes (LED); luminescence;
triazole; triphenylamine
INTRODUCTION Since the first discovery of polymeric light-
emitting diodes (PLEDs) using poly(p-phenylene vinylene)
(PPV) as an emitting layer in 1990,1 PLEDs have attracted
much research interest because of their potential applications
in large area display and solid-state lighting.2 Polymeric mate-
rials possess many advantages owing to their solubility and
good film-forming properties, rendering them suitable for so-
lution processes such as facile fabrication via spin coating or
inkjet printing,3,4 and susceptible to structural modification.
For high luminescence efficiency to be achieved, it is not only
necessary to balance injection and transport between elec-
trons and holes but also important to decrease the energy
barriers of charges injection from the opposite electrodes.5
However, electron injection is usually more difficult than hole
injection in PPV-based conjugated polymers leading to imbal-
anced charges injection and transport, which greatly reduces
the luminescence efficiency of their PLEDs.6–10 Two strategies
have been attempted to balance charges injection and trans-
port, which is crucial in obtaining efficient electroluminescent
(EL) devices. One is to insert an additional electron injection/
transport layer between the emitter and the cathode.11–13
Nonetheless, fabrication of the multilayer device from poly-
mers, typically deposited by spin coating, is usually a difficult
task when the solution of a subsequent electron injection/
transport layer will damage the previously coated light-
emitting layer. Consequently, single-layer devices are highly
preferred from the viewpoints of process simplicity and cost
effectiveness.
Another strategy is to fabricate single-layer LEDs that
blend charge injection/transport molecules such as oxadia-
zole, 1,2,4-triazole, triphenylamine (TPA) or carbazole
derivatives. Nevertheless, the drawback of this strategy is
the crystallization-induced degradation and thermal break-
down of the small molecules during device operation,
which unavoidably lead to low emission efficiency and
poor stability if their glass transition temperatures (Tg) are
low.14–16 Therefore, the development of emitting copoly-
mers chemically bonded with both electron and hole injec-
tion/transport moieties is an ideal strategy in material
design of PLEDs. Several research groups prepared copoly-
mers carrying both electron- and hole-transporting
moieties in an attempt to improve their device perform-
ance.17–23 Song et al.17(e) synthesized a copolymer contain-
ing both carbazole and oxadiazole units; it exhibited 36
times higher EL quantum efficiency as compared to PPV. Li
et al.18(c) synthesized a copolymer with both electron-
Additional Supporting Information may be found in the online version of this article. Correspondence to: Y. Chen (E-mail: yunchen@mail.ncku.edu.tw)
C
V
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3099–3108 (2011) 2011 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
withdrawing cyano and electron-rich TPA groups to
enhance performance of PPV-based device. Zhang et al.19(d)
synthesized an EL polymer containing hole-deficient TPA
and electron-deficient oxadiazole to enhance performance of
PPV-based device, with a maximum luminescent efficiency
of 0.65 cd/A and a maximum brightness of 2100 cd/m2.
Wu et al.20(a) synthesized a fluorene-based copolymer con-
taining TPA moieties and oxadiazole pendent groups; an
organic light-emitting device with this copolymer as the
emitting layer exhibited maximum brightness and luminance
efficiency of 7128 cd/m2 and 2.07 cd/A, respectively.
calibration standards. The thermogravimetric analysis (TGA)
of polymers was performed under nitrogen atmosphere at a
heating rate of 20 C/min, using a PerkinElmer TGA-7 ther-
ꢀ
mal analyzer. Thermal transition properties of polymers
were investigated using a differential scanning calorimeter
(DSC), PerkinElmer DSC 7, under nitrogen atmosphere at a
heating rate of 10 ꢀC/min. Absorption and photolumines-
cence (PL) spectra were measured with a Jasco V-550 spec-
trophotometer and a Hitachi F-4500 fluorescence spectro-
photometer, respectively. Cyclic voltammograms were
recorded with a voltammetric analyzer (model CV-50W from
Bioanalytical Systems) at room temperature under nitrogen
atmosphere. The measuring cell consisted of a polymer-
coated glassy carbon as the working electrode, an Ag/AgCl
electrode as the reference electrode, and a platinum wire as
the auxiliary electrode. The electrodes were immersed in ace-
tonitrile containing 0.1 M (n-Bu)4NClO4 as electrolyte. The
energy levels were calculated from onset oxidation or onset
TPA and its derivatives have attracted considerable interest
as hole-transporting materials for use in organic EL devices
due to their relatively high hole mobility and low ionization
potentials.24 On the contrary, aromatic 1,2,4-triazole and
related derivatives are electron-deficient materials, which
possess good thermal and chemical stabilities, leading to
their common use as electron-transport and hole-blocking
materials in LEDs.14,25,26 They are two commonly used hole
and electron transporters, respectively, in organic and poly-
meric light-emitting diodes. In general, hole- and electron-
transporting materials also demonstrate simultaneously
enhanced hole and electron injection, respectively.
reduction potentials [EHOMO ¼ –(Eox,FOC þ 4.8) eV; ELUMO
¼
ꢁ(Ered,FOC þ 4.8) eV], using ferrocene (FOC) as standard
(ꢁ4.8 eV with respect to vacuum level).27
Materials
4-Fluorobenzoyl chloride (TCI), hydrazine monohydrate
(Showa), phosphorus pentachloride (PCl5; Riedel-dehaen),
diphenylamine (Fluka), 4-fluoronitrobenzene (Acros), palla-
dium on activated carbon (Pd/C; Aldrich), 1-bromohexane
(Acros), hydroquinone (Lancaster), iodine (I2; Riedel-
deha¨en), iodic acid (HIO3; Lancaster), 4-acetoxystyrene
(Aldrich), palladium (Pd) acetate (Acros), triethylamine
(Merck), tri-(o-toly)phosphine (Acros), phenol (Merck), N,N-
dimethyaniline (Acros), 1,3-dimethyl-3,4,5,6-tetrahydro-2-
(1H)-pyrimidinone (DMPU; Alfa), dimethyl sulfoxide (DMSO;
Tedia), N,N-dimethylformamide (DMF; Tedia), chloroform
(CHCl3) (Tedia), THF (Tedia), and other solvents were from
commercial source and used without purification. 1,2-Bis
(chloro(4-fluorophenyl)methylene)hydrazine (4) was pre-
pared from 4-fluorobenzoyl chloride by reacting consecu-
tively with hydrazine monohydrate and PCl5 (Scheme 1).
N0,N0-Diphenylbenzene-1,4-diamine (8) was synthesized from
diphenylamine (5) and 1-fluoro-4-nitrobenzene (6) to obtain
N-(4-nitrophenyl)-N-phenylbenzenamine (7), followed by
reduction with hydrazine monohydrate. The synthesis of
monomers M2 and 15 were reported previously.28
In this study, we synthesized a novel copoly(aryl ether) (P1)
consisting of alternate light-emitting segment (distyrylben-
zene derivatives) and bipolar moiety. The bipolar moiety
comprises directly linked hole-transporting TPA and elec-
tron-transporting aromatic 1,2,4-triazole. The copoly(aryl
ether) exhibits not only good thermal stability but also bal-
anced hole and electron transport relative to homo-poly(aryl
ether) (P0) due to the presence of the bipolar moieties.
Moreover, the aryl ether functional group was introduced to
enhance its solubility in common organic solvents. Finally, to
reduce the barrier height for electron (0.4 eV) between emit-
ting distyrylbenzene segments and bipolar moiety, P1 was
blended with poly(9,9-dihexylfluorene) (PF) and used as
emitting layer. The blending effectively improved the balance
between holes and electrons injection, resulting in greatly
enhanced maximum luminance and maximum luminance
efficiency. These results reveal that the bipolar copoly(aryl
ether) (P1) is applicable as emitting dopant; it is also effec-
tive in balancing charges injection and transport.
EXPERIMENTAL
Synthesis of 3,5-Bis(4-fluorophenyl)-4-
triphenylamine-1,2,4-triazole (9)
Measurements
All newly synthesized compounds were identified by NMR
spectra, Fourier transform infrared (FTIR) spectra, and elemen-
tal analysis. 1H NMR and 13C NMR spectra were obtained on a
Bruker AMX-400 MHz or an AV500 MHz FT-NMR spectrometer,
with the chemical shifts reported in ppm using tetramethylsi-
lane as an internal standard. The FTIR spectra were measured
as KBr disk on a FTIR spectrometer, model 7850 from Jasco.
Elemental analysis was carried out on a Heraus CHN-Rapid ele-
mental analyzer. The weight-average molecular weight (Mw)
and polydispersity index (PDI) of polymers were measured
with a gel permeation chromatograph (GPC), using tetrahydro-
furan (THF) as eluent and monodisperse polystyrenes as
To a 100-mL two-necked glass reactor, 4 (0.10 g, 3 mmol), 8
(0.84 g, 3 mmol), and DMF (50 mL) were added. The mix-
ture was stirred at 135 C for 24 h under a nitrogen atmos-
ꢀ
phere. The precipitates, obtained after pouring the mixture
into aqueous solution of HCl (500 mL, 3 N), were collected,
dried, and recrystallized from ethanol to provide white
sheet-like crystals of 9 (59%).
mp > 240 ꢀC 1H NMR (CDCl3, ppm): d 7.5–7.47 (m, 4H),
7.33 (t, J ¼ 8.6 Hz, 4H), 7.26 (t, J ¼ 8.6 Hz, 4H), 7.23–7.21
(d, J ¼ 8.7 Hz, 2H), 7.10 (t, J ¼ 8.4 Hz, 2H), 7.04–7.02 (d, J ¼
8.0 Hz, 4H), 6.90–6.88 (d, J ¼ 8.8 Hz, 2H). FTIR (KBr, pellet,
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SCHEME 1 Synthesis of bipolar monomer
9, bipolar model M1 and emitting model
M2 and emitting monomer 15.
cm–1): m 1512 (C¼¼N), 1093 (CAF). Anal. Calcd. (%) for:
(Scheme 1). The mixture was precipitated from distilled
water; the precipitates were collected by filtration, dried
in vacuo, and recrystallized from ethanol to afford M1
(83%). mp >240 ꢀC. 1H NMR (CDCl3, ppm): d 7.5–7.47 (m,
8H), 7.35–7.32 (m, 4H), 7.29–7.25 (m, 4H), 7.23 (m, 2H),
7.12–7.09 (m, 8H), 7.04 (m, 4H), 6.90 (m, 2H). FTIR (KBr,
pellet, cm–1): m 1509 (C¼¼N), 1264 (CAN), 1232 (CAOAC).
Anal. Calcd. (%) for: C44H32N4O2: C, 81.46; H, 4.97; N, 8.64.
Found: C, 81.24; H, 5.00; N, 8.60.
C
32H22F2N4: C, 76.79; H, 4.43; N, 11.19. Found: C, 76.72; H,
4.51; N, 11.23.
Synthesis of N-(4-(3,5-Bis(4-phenoxyphenyl)-4H-1,2,4-
triazol-4-yl)phenyl)-N-phenylbenzenamine (M1)
A mixture of 9 (0.391 g, 1.5 mmol), phenol (0.298 g, 3.15
mmol), K2CO3 (0.442 g, 3.2 mmol), and 6 mL of DMF was
stirred at 160 ꢀC for 20 h under a nitrogen atmosphere
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SCHEME 2 Synthesis of bipolar copoly
(aryl ether) (P1) and poly(aryl ether) (P0).
Synthesis of Bipolar Copoly(aryl ether) (P1) and
Poly(aryl ether) (P0)
pellet, cm–1): m 1598 (AC¼¼NA), 1240 (CAOAC), 964 (C¼¼C).
Anal. Calcd. (%) for P1: C, 81.28; H, 6.80; N, 5.67. Found: C,
79.34; H, 6.41; N, 5.83.
Bipolar copoly(aryl ether) (P1) and poly(aryl ether) (P0)
were prepared simultaneously for comparative study. The
poly(aryl ether)s P1 or P0 were prepared from bisphenol
(15) and bipolar monomer (9) or dibromo monomer (16),
respectively, as shown in Scheme 2. A two-necked glass reac-
tor was charged with monomers (16 or 9; 0.50 mmol),
bisphenol 15 (0.50 mmol), 10 mL of toluene, 5 mL of DMPU,
and an excess of K2CO3 (0.166 g, 1.20 mmol). The mixture
was stirred at 160 ꢀC for 48 h (P0) or 24 h (P1). After re-
moval of toluene, the reaction mixture was first diluted with
2 mL of N-methylpyrrolidone and then dropped into 250 mL
of a mixture of methanol and distilled water (v/v ¼ 9/1).
The appearing precipitates were collected by filtration and
further purified by extraction with isopropyl alcohol for 24 h
using a Soxhlet extractor.
Fabrication of Polymer Light-Emitting Diodes
To evaluate the applicability of the bipolar copoly(aryl ether)
(P1) in enhancing emission efficiency, double-layer PLEDs
[indium tin oxide/poly(3,4-ethylene dioxythiophene):poly(s-
tyrene sulfonate) (ITO/PEDOT:PSS)/polymer/Ca/Al] were
fabricated by the solution process. First, a transparent ITO
glass substrate was successively cleaned with neutraler rein-
iger/deionized water (v/v ¼ 1/3) mixture, deionized water,
acetone, and 2-propanol using an ultrasonic bath, and then
dried in vacuo overnight. Aqueous dispersion of PEDOT:PSS
was immediately spin coated on top of the cleaned ITO glass
ꢀ
as hole-injection layer and dried at 150 C for 15 min. Emit-
ting layer was then deposited by spin coating (1000 rpm)
onto the PEDOT:PSS layer from a polymer solution in 1,2-
dichlorobenzene (10 mg/mL). Finally, calcium and aluminum
were consecutively vacuum deposited as cathodes using a
vacuum coater at a pressure of 2 ꢂ 10–6 Torr. Luminance
versus voltage and current density versus voltage character-
istics of the devices were measured using a combination of a
Keithley power source (model 2400) and an Ocean Optics
USB2000 fluorescence spectrophotometer. The optoelectronic
measurements were conducted in a glove box filled with
nitrogen to avoid possible degradation under device
operation.
P0: 15 (0.258 g, 0.5 mmol) and 16 (0.132 g, 0.5 mmol). 1H
NMR (CDCl3, ppm): d 7.45 (m, 8H), 7.27 (m, 4H), 7.06–6.88
(m, 8H), 5.06 (s, 4H), 4.01 (m, 4H), 1.83–1.49 (m, 12H), 1.33
(m, 4H), 0.88 (m, 6H). FTIR (KBr, pellet, cm–1): m 1240
(CAOAC), 968 (C¼¼C). Anal. Calcd. (%) for P0: C, 81.78; H,
7.84. Found: C, 80.74; H, 7.69.
P1: 15 (0.258 g, 0.5 mmol) and 9 (0.250 g, 0.5 mmol). 1H
NMR (CDCl3, ppm): d 7.5–7.47 (m, 4H), 7.35–7.32 (m, 4H),
7.32–7.28 (m, 8H), 7.23–7.20 (m, 8H), 7.12–7.09 (m, 2H),
7.04 (m, 4H), 6.90 (m, 2H) 6.76 (m, 4H), 4.03 (m, 4H), 1.78
(m, 4H), 1.50 (m, 4H), 1.34 (m, 8H), 0.87 (m, 6H). FTIR (KBr,
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TABLE 1 Polymerization Results and Characterization of
phenyl)-N-phenylbenzenamine (M1) containing bipolar moi-
eties were synthesized according to the synthetic routes out-
lined in Scheme 1.23(a),28(b) The 1H NMR spectra of P0 and
P1 are shown in Supporting Information Figure. S1. The
Polymers
a
a
b
c
Yield
(%)
Mn
Mw
Tg
Td
(ꢂ104)
(ꢂ104)
PDIa
(ꢀC)
(ꢀC)
1
Polymer
main H NMR spectral difference between P0 and P1 lies in
chemical shift regions at 6.7–7.7 ppm, which originate from
incorporated bipolar residues. Bipolar copoly(aryl ether)
(P1) and poly(aryl ether) (P0) were synthesized by the
nucleophilic displacement reaction of 1,4-bis(4-hydroxys-
tyryl)-2,5-dihexyloxyphenylene (15) with functionalized aro-
matic monomers 9 and 16, respectively (Scheme 2).
P0
P1
a
79
98
2.74
1.35
8.49
2.05
3.10
1.52
136
–
389
450
Mn, Mw, and PDI of the polymers were determined by gel permeation
chromatography using polystyrene standards in THF.
b
Glass transition temperature was measured by DSC at a scan rate of
20 ꢀC/min under nitrogen. The glass transition temperature of P1 was
not detectable up to 300 ꢀC.
c
The poly(aryl ether)s (P0 and P1) are readily soluble in
common organic solvents such as toluene, chloroform, chlor-
obenzene, and THF. The weight-average molecular weights
(Mw) of P0 and P1 are 8.49 ꢂ 104 and 2.05 ꢂ 104, respec-
tively, with PDIs being 3.10 and 1.52 (Table 1), as
determined by GPC using monodisperse polystyrenes as cali-
bration standards. Clearly, the Mw of P1 is much lower than
that of P0. This has been attributed to reduced reactivity of
bipolar monomer 9 than 1,4-bis(bromomethyl)benzene (16)
due to its steric hindranceꢀand electron-deficient characteris-
tic. The Tg of P0 was 136 C, as determined from the second
heating DSC scans, while no noticeable Tg was observed for
P1 (Supporting Information Fig. S2). Thermal decomposition
temperatures (Td: at 5 wt % loss) of the poly(aryl ether)s
The temperature at 5 wt % loss under nitrogen atmosphere.
RESULTS AND DISCUSSION
Characterization of Bipolar Monomer (9) and Model
(M1) and Copoly(aryl ether) (P1) and Poly(aryl ether)
(P0)
In this study, difluoro bipolar monomer (9), with directly
linked TPA and 1,2,4-triazole as core, was used to synthesize
bipolar copoly(aryl ether) (P1). N-(4-(3,5-Bis(4-fluoro-
phenyl)-4H-1,2,4-triazol-4-yl)phenyl)-N-phenylbenzenamine
(9) and N-(4-(3,5-bis(4-phenoxyphenyl)-4H-1,2,4-triazol-4-yl)
ꢀ
were evaluated with TGA. The residual weights at 800 C of
both P0 and P1 were above 30% under nitrogen atmosphere
(Supporting Information Fig. S3). Moreover, the Td of P0 and
P1 were 389 and 450 ꢀC (Table 1), respectively, indicating
that their thermal stability is above moderate.
Photophysical Properties of Bipolar Copoly(aryl ether)
(P1) and Poly(aryl ether) (P0)
Optical properties of the poly(aryl ether)s were investigated
by absorption and PL spectra, both in solution and film
states, to evaluate the influence of the bipolar groups. Figure
1 illustrates the absorption and PL spectra of M1, M2, P0,
and P1 in CHCl3 and those of P0 and P1 films, with the
characteristic optical data summarized in Table 2. The
absorption maximum of bipolar model M1 in CHCl3 appears
at 308 nm, whereas those of fluorophore model M2, P0,
TABLE 2 Optical Properties of M1, M2, P0, and P1
UV–Vis
kmax
PL kmax
PL kmax
Molecule/ Solution UV–Vis kmax Solution Film
Polymer (nm)a
Film (nm)
(nm)b
(nm)b
UPL
c
M1
M2
P0
P1
a
308
–
–
390
–
–
0.71
0.57
331, 397
445, 463s
335, 401 337, 402
310, 403 319, 413
446, 470s 486, 510s 0.53
448, 471s 488s, 514 0.54
FIGURE 1 Absorption and PL spectra of M1, M2, P1, and P0: (a)
in THF (10–5 M; kex ¼ 308 nm for M1, 331 nm for M2, 401 nm
for P0, and 403 nm for P1); (b) in film state (kex ¼ 402 nm for
P0 and 413 nm for P1) and PL spectrum of PF in film state.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
In THF (1 ꢂ 10ꢁ5 M).
b
c
s: Wavelength of shoulder.
U
PL: quantum yield determined in CHCl3, relative to quinine sulfate in
1 N H2SO4(aq) at a concentration of 10ꢁ5 M (UPL ¼ 0.55).
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Electrochemical Investigations
Hole and electron affinities of M1, P0, and P1 were eval-
uated by their onset oxidation and reduction potentials
determined by electrochemical methods. Their cyclic voltam-
mograms (CVs) were first measured to investigate the elec-
trochemical properties. The highest (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels were
estimated by the equations: EHOMO (eV) ¼ –(Eox,FOC þ 4.8)
and ELUMO (eV) ¼ –(Ered,FOC þ 4.8), where Eox,FOC and
Ered,FOC are the onset oxidation and reduction potentials,
respectively, with respect to the FOC/ferrocenium couple.
The CVs are shown in Figure 2, with the representative elec-
trochemical data summarized in Table 3. In this study, copo-
ly(aryl ether) (P1) contains bipolar residues derived from
electron-transporting triazole and hole-transporting TPA. The
estimated HOMO energy levels of M1, P0, and P1 are ꢁ5.35,
ꢁ5.30, and ꢁ5.32 eV, whereas the estimated LUMO levels
are ꢁ2.74, ꢁ2.34, and ꢁ2.77 eV, respectively. In addition, the
HOMO and LUMO energy levels of M2 should be close to
ꢁ5.15 and ꢁ2.59 eV, respectively, of our previous polymer,29
because they possess the same distyrylbenzene residue. The
HOMO level of P1 (–5.32 eV) is almost equal to that of P0 (–
5.30 eV) after incorporating the bipolar residues, suggesting
that holes are mostly injected into the emitting segments
(M2 residues). However, the LUMO level of P1 (–2.77 eV) is
much lower than that of P0 (–2.34 eV) after introducing the
bipolar residues, meaning that energy barrier of electron
injection from cathode is lower for P1. Therefore, electron
affinity of P1 is enhanced due to the incorporation of the
bipolar residues, resulting in improved balance in charges
injection and transport. Moreover, the LUMO level of PF (–
2.53 eV) lies between those of P0 (–2.34 eV) and bipolar M1
(–2.74 eV). Accordingly, blending P1 with PF will enhance
electron transport from the bipolar residues to emitting seg-
ments (distyrylbenzene derivatives). This might be an effec-
tive strategy to improve balance in charges injection and
transport for P1 and enhance charges recombination in emit-
ting segments.
FIGURE 2 Cyclic voltammograms of M1, P0, and P1 coated on
glassy carbon electrode; scan rate: 100 mV/s.
and P1 are around 397–403 nm. The main absorption
bands around 397–403 nm can be attributed to the p–p*
transitions of distyrylbenzene derivatives in M2, P0, and P1.
Both emitting segments (distyrylbenzene derivatives) and
bipolar residues contribute to the absorption spectrum of
P1, which are comparable to the absorption spectra of model
compounds. For instance, the longer major absorption of P1
(ꢃ403 nm) can be attributed to emitting segments (M2 resi-
dues), whereas the shorter wavelength absorption (ca. 310
nm) has definitely originated from bipolar M1 residues.
Solid-state absorption maxima of P0 and P1 are slightly red
shifted (1–10 nm) relative to solution state ones, originating
in aggregate formation via intrachain or interchain
interactions.
Similarly, P0 and P1 exhibit major PL peaks at 486–514 nm
in solid state which are red shifted about 40–43 nm relative
to those in solution state (Table 2), implying the formation
of aggregate in film state. Moreover, the PL spectra of P1
were almost identical whether they were excited with 308
nm (major absorption of bipolar model M1) or 402 nm
(major absorption of emitting M2), suggesting efficient
Fo¨rster energy transfer from bipolar residues to emitting
segments (distyrylbenzene derivatives) under the photoexci-
tations. Partial overlap between emission spectrum of bipo-
lar M1 and absorption spectrum of P0 contributes greatly to
the energy transfer.18(e),20,21(a) In addition, it is noteworthy
that the emission spectrum of PF partially overlaps with the
absorption spectra of P0–P1 (Fig. 1), indicating that energy
transfer from PF to P0–P1 can be expected as well. Based
on this characteristic, efficient PLEDs were obtained using
blends of P0–P1 and PF as emitting layer, which will be dis-
cussed later. The PL quantum yields (UPL) of M1, M2, P0,
and P1 in THF (at a concentration of 10–5 M) are 0.71, 0.57,
0.53, and 0.54, respectively. The UPLs of P0 and P1 are
almost the same and close to that of M2, suggesting that
they are mainly determined by the emitting segments (dis-
tyrylbenzene derivatives).28(a)
Electroluminescent Properties of Bipolar Copoly(aryl
ether) (P1) and Poly(aryl ether) (P0)
Double-layer PLEDs [ITO/PEDOT:PSS/P1 or P0/LiF(0.5 nm)/
Ca(50 nm)/Al (100 nm)] were fabricated to investigate their
device performances. Figure 3 shows the current density and
luminance versus bias characteristics of the EL devices, with
TABLE 3 Electrochemical Properties of M1, P0, and P1
Eox
Ered
Molecule/ versus
versus
EHOMO
ELUMO
(eV)b
Polymer
FOC (V)a FOC (V)a (eV)b
Eg (eV)c
el
M1
P0
P1
a
0.55
0.50
0.52
ꢁ2.06
ꢁ2.46
ꢁ2.03
ꢁ5.35
ꢁ5.30
ꢁ5.32
ꢁ2.74
ꢁ2.34
ꢁ2.77
2.61
2.96
2.55
EFOC ¼ 0.47 V versus Ag/AgCl.
b
c
EHOMO ¼ ꢁ(Eox,FOC þ 4.8) eV; ELUMO ¼ ꢁ(Ered, FOC þ 4.8) eV.
el
Electrochemical band gap: Eg ¼ LUMO – HOMO.
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ARTICLE
FIGURE 3 Brightness versus bias and current density versus
bias characteristics of PLEDs using P0 and P1 as emitting layer.
Device structure: ITO/PEDOT:PSS/P0 or P1 (90–110 nm)/LiF(0.5
nm)/Ca(50 nm)/Al(100 nm). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
the characteristic performance data summarized in Table 4.
The turn-on voltage of P1-based device is 8.9 V, which is
much lower than that of P0-based one (17.7 V) This is
clearly attributed to the presence of bipolar moieties in P1
that promotes charges injection and transport. Moreover,
maximum luminance and maximum luminance efficiency of
the P1-based device were improved to 380 cd/m2 and 0.009
cd/A, respectively, from 30 cd/m2 and 0.001 cd/A of the P0-
based device. The performance improvement is attributable
to more balanced charges injection and transport (Fig. 3).
The inferior device performance is probably caused by the
high energy barrier for electron transport (0.4 eV), between
the emitting segments (distyrylbenzene derivatives) and the
bipolar moieties, than that for hole transport (0.05 eV). This
leads to imbalance in the charges transport and reduction in
the recombination ratio of electrons and holes. Moreover, the
FIGURE 4 (a) Brightness versus bias and (b) current density
versus bias characteristics of PLEDs using blends of PF and P1
(0–1.6 mg) as emitting layer. Device structure: ITO/PEDOT:PSS/
PF þ P1 (90–110 nm)/ LiF(0.5 nm)/Ca(50 nm)/Al(100 nm). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
LUMO level of PF (–2.53 eV) lies between those of P0 (–2.34
eV) and bipolar M1 (–2.74 eV). Accordingly, blending PF
with P1 might facilitate electron transport from the bipolar
moieties to the emitting segments and thus enhance the de-
vice performance.
TABLE 4 Optoelectronic Performance of Polymer Light-emitting
Diodesa
Emitting
Maximum
Layer
Luminance Maximum CIE
Efficiency Luminance Coordinates
Composition Turn-on
(w/w)b
Voltage (V)c (cd/A)
(cd/m2)
(x, y)d
PF/P1 (20/0.2) 5.5
PF/P1 (20/0.4) 6.0
PF/P1 (20/0.8) 6.2
PF/P1 (20/1.6) 7.2
0.72
0.88
1.08
0.61
0.009
0.33
0.001
0.49
1700
2400
3260
1810
380
(0.20, 0.22)
(0.20, 0.41)
(0.21, 0.49)
(0.21, 0.50)
(0.21, 0.50)
(0.18, 0.18)
(0.18, 0.35)
(0.18, 0.29)
P1
PF
P0
8.9
5.5
1160
30
17.7
PF/P0 (20/0.8) 7.2
1210
a
Device structure: ITO/PEDOT:PSS/emitting layer/LiF/Ca/Al.
The weight ratios are given in parentheses.
The voltage required for the luminance of 10 cd/m2.
The CIE coordinates at maximum luminance.
b
c
d
FIGURE 5 Energy band diagrams of M1, P0, P1, and PF.
SYNTHESIS OF BIPOLAR COPOLY(ARYL ETHER), WU, LEE, AND CHEN
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 6 AFM images of blend films (scan size: 5 ꢂ 5 lm2): (a) PF/P1 ¼ 20/0.4, RMS roughness ¼ 1.31 nm, (b) PF/P1 ¼ 20/1.6,
RMS roughness ¼ 3.38 nm.
Figure 4 shows the current density and luminance versus
bias characteristics of the PLEDs using blends of PF and P1
as emitting layer and the representative data are summar-
ized in Table 4. The turn-on voltages were raised from 5.5 V
to 7.2 V when the weights of P1 were increased from 0.2 mg
to 1.6 mg. This is reasonably attributed to the bipolar resi-
dues in P1, which act as charge trapping center in the blend
layer. Because the LUMO and HOMO levels of P1 are
enclosed within those of PF (Fig. 5), maximum luminance
and maximum luminance efficiency of the blend device (PF/
P1 ¼ 20 mg/0.8 mg) were enhanced to 3260 cd/m2 and
1.08 cd/A, respectively, superior to those of P1-based device.
The performance improvement has been ascribed to more
balanced charges injection and transport in the blend device.
However, further increase in P1 contents (PF/P1 ¼ 20 mg/
1.6 mg) leads to quick performance degradation, suggesting
that there exists an optimal content of bipolar residues in
balancing the charges injection and transport. Moreover,
morphology of active layer also plays role in determining the
device performance. The atomic force microscopy (AFM)
images of active layer (Fig. 6) can also reasonably explain
this performance reversion at higher P1 contents. The root-
mean-square (RMS) surface roughness of the blend films
increases drastically from 1.31 nm to 3.38 nm as the PF/P1
ratio is increased from 20 mg/0.4 mg to 20 mg/1.6 mg. High
surface roughness leads to performance degradation of the
blend device. Wang and coworkers28(c) prepared two bipolar
fluorene-based conjugated copolymers chemically doped
with benzothiadiazolyl chromophore, which were applied as
emitting layers to obtain a optimal device efficiency of 2.02
FIGURE 7 Emission spectra of PLEDs using blends of PF and
P1 as emitting layer. Device structure: ITO/PEDOT:PSS/PF þ P1
(90–110 nm)/LiF(0.5 nm)/Ca(50 nm)/Al(100 nm). [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 8 CIE coordinates of the EL devices (at maximum
brightness) based on blends of PF and P1. Device structure:
ITO/PEDOT:PSS/PF þ P1 (90–110 nm)/LiF(0.5 nm)/Ca(50 nm)/
Al(100 nm). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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ARTICLE
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We have successfully synthesized a novel copoly(aryl ether)
(P1) consisting of alternate emitting segment (distyrylben-
zene derivatives) and bipolar moiety (linked TPA and aro-
matic 1,2,4-triazole). The copoly(aryl ether) was soluble in
common organic solvents and thermally stable (Td at 5 wt %
loss: 450 ꢀC). The PL spectra of P1 were almost identical
whether they were excited with 308 nm or 402 nm, because
of efficient Fo¨rster energy transfer. Estimated HOMO and
LUMO levels of bipolar model M1 were ꢁ5.35 and ꢁ2.74 eV,
respectively. The HOMO levels of P0 and P1 were ꢁ5.30 and
ꢁ5.32 eV, respectively, while their LUMO levels were ꢁ2.34
and ꢁ2.77 eV. Maximum luminance and maximum luminance
efficiency of P1-based device (380 cd/m2, 0.009 cd/A) were
superior to those of P0-based device (30 cd/m2, 0.001 cd/
A). Furthermore, blending the bipolar copoly(aryl ether)
(P1) with PF resulted in significant performance enhance-
ment. The performance enhancements had been attributed
to improved carriers injection and transport. Blend device
(PF/P1 ¼ 20/0.8) showed the best performance (maximum
luminescence efficiency: 1.08 cd/A; maximum luminescence:
3260 cd/m2). Current results indicate that the bipolar copo-
ly(aryl ether) (P1) is potentially applicable not only as
charges injection/transport promoter but also as emitting
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