Synthesis and characterization of electrophosphorescent jacketed conjugated polymers

Article abstract of DOI:10.1002/pola.26189

A monomer containing bent side chains with oxadiazole unit was synthesized. And it was copolymerized with polyfluorene at different ratios. The photophysical and electrochemical properties of the copolymers were characterized. The results show that the introduction of the oxidiazole-containing side chains into the polymer reduces the lowest unoccupied molecular orbital level. And the steric hindrance of the side groups can effectively suppress the aggregation of the polymer backbones. Electroluminescent devices were fabricated with a configuration of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene (PEDOT):PSS/Sample/Ca/Al. All of the devices emit blue light. The device of the copolymer PFOXD50 shows the best performance with the maximum luminance of 1033 cd/m² and the maximum current efficiency of 0.29 cd/A. Then a cyclometalated iridium complex monomer (ppy)₂Ir(BrPhPyBr) was copolymerized with PFOXD50 at different ratios. The devices with the same configuration emit orange light. The efficiency generally increases with the increasing Ir content. Among them, the device of the copolymer PFOXDIr7 shows the best performance with the maximum luminance of 846 cd/m² and the maximum current efficiency of 0.61 cd/A.

Full text of DOI:10.1002/pola.26189

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Synthesis and Characterization of Electrophosphorescent Jacketed
Conjugated Polymers
Wei Zhang, Hao Jin, Feng Zhou, Zhihao Shen, Dechun Zou, Xinghe Fan
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Correspondence to: X. H. Fan (E-mail: fanxh@pku.edu.cn), Z. H. Shen (E-mail: mailto:zshen@pku.edu.cn)
Received 14 April 2012; accepted 22 May 2012; published online
DOI: 10.1002/pola.26189
ABSTRACT: A monomer containing bent side chains with oxa-
diazole unit was synthesized. And it was copolymerized with
polyfluorene at different ratios. The photophysical and electro-
chemical properties of the copolymers were characterized. The
results show that the introduction of the oxidiazole-containing
side chains into the polymer reduces the lowest unoccupied
molecular orbital level. And the steric hindrance of the side
groups can effectively suppress the aggregation of the polymer
backbones. Electroluminescent devices were fabricated with a
configuration of indium tin oxide (ITO)/poly(3,4-ethylenedioxy-
thiophene (PEDOT):PSS/Sample/Ca/Al. All of the devices emit
blue light. The device of the copolymer PFOXD50 shows the
best performance with the maximum luminance of 1033 cd/m2
and the maximum current efficiency of 0.29 cd/A. Then a cyclo-
metalated iridium complex monomer (ppy)2Ir(BrPhPyBr) was
copolymerized with PFOXD50 at different ratios. The devices
with the same configuration emit orange light. The efficiency
generally increases with the increasing Ir content. Among
them, the device of the copolymer PFOXDIr7 shows the best
performance with the maximum luminance of 846 cd/m2 and
C
V
the maximum current efficiency of 0.61 cd/A.
2012 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 000: 000–000,
2012
KEYWORDS: conjugated polymers; light-emitting diodes (LED);
liquid-crystalline polymers (LCP)
INTRODUCTION Recently, polymer light-emitting diodes
(PLEDs) have attracted a lot of attention because of their
good thermal stabilities, easy processing methods, such as
spin-coating and inkjet-printing, and applications in flexible
and large-area displays. The first polymer light-emitting mate-
rial is poly(1,4-phenylene vinylene), discovered in 1990.1
Most PLED materials emit fluorescent light from singlet excit-
ing states. However, the maximum theoretical internal quan-
tion.12,13 Besides, most PF materials are p-types, which have
better abilities to transport holes than electrons. However,
the balance of carrier transportation greatly affects the
device efficiencies. A common way to solve this problem is
to build mutilayered devices or to dope with other materials.
However, problems, such as phase separation or crystalliza-
tion, may occur. Another way is to introduce other moieties
that have a different carrier-transporting property into the
molecules to form bi-polar structures.14–17 Oxadiazole deriv-
atives are well known electron-transporting and hole-block-
ing materials.18,19 The introduction of oxadiazole moieties in
this work is expected to balance the carrier transportation.
The backbones are expected to transport holes while the
side groups transport electrons. This di-channel structure20
should enhance the device performance. Besides, the oxadia-
zole side chains attached to the backbone can be viewed as
a first-generation dendron. Dendronized polymers can self-
assemble into interesting structures.21,22 By tuning the struc-
ture of the side group, such as changing the generation,
different supermolecular structures occur.23–25 This will
probably also benefit the electro-optic properties.26
€
tum efficiencies are limited to 25%. In 1998, Forrest discov-
ered the first electrophosphorescent material.2 Some heavy
metal complexes can use the triplet exciting states, which
account for the greater proportion in theory, to emit phos-
phorescent light. Electrophosphorescent materials make use
of the triplet excitons, which allows for the theoretical internal
quantum efficiencies to reach as high as 100%.3 Because of
their relatively high quantum yields and short exciting life-
times, iridium complexes are intensively researched.4,5
Polyfluorene (PF) and its derivatives are highly efficient
blue-light materials6,7 and good host materials.8,9 However,
the planar structures of PFs may easily form intermolecular
aggregation, leading to the undesirable changes of emitting
colors.10,11 Therefore, some side groups with large steric hin-
drances are often introduced to suppress the aggrega-
Mesogen-jacketed liquid crystalline polymers (MJLCPs) are
first proposed as a special kind of side-chain liquid crystalline
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SCHEME 1 Syntheses of the monomer OXDBr2 and PFOXD series of polymers.
EXPERIMENTAL
polymers with very short spacers or without spacers between
the main chain and the side-on mesogen.27,28 Recently, jack-
eted structures are also applied to rigid, conjugated poly-
mers.29,30 And researches on the functionalization of MJLCPs
have been done in the past several years. The studies of
MJLCPs in electroluminescent devices have proven that the
jacketed structure can suppress the long-wavelength emis-
sions, benefiting the electroluminescent properties.31–33
Materials
HC3OXD-G1 was obtained using the previously reported
method.17 2,5-Dibromobenzene-1,4-diol and (ppy)2Ir(BrPh-
PyBr) were synthesized according to the published proce-
dures.34,35 All the other reagents were used as received.
Characterization and Device Fabrication
This article focuses on a series of electroluminescent jack-
eted conjugated polymers. First, a series of PF copolymers
containing different contents of bent side chains with oxadia-
zole moiety were synthesized. The polymer in this series
with the best balance of carrier transportation was chosen
as the host material and copolymerized with a monomer
containing the iridium complex at different ratios to obtain
phosphorescent polymers. The jacketed side groups are
expected to prevent the iridium complexes attaching to the
backbone from aggregating. To the best of our knowledge,
these are the first jacketed conjugated polymers with iridium
attached to the conjugated backbone.
The characterization methods such as gel permeation chro-
1
matography, H NMR spectroscopy, thermogravimetric analy-
sis, differential scanning calorimetry, UV-vis spectrum, photo-
luminescent (PL) spectrum, and cyclic voltammetry (CV),
and the device fabrication process were similar to those we
used previously.32
Synthesis
The synthetic routes of the PFOXD and PFOXDIr series of
polymers are outlined in Schemes 1 and 2, respectively.
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SCHEME 2 Synthesis of PFOXDIr series of polymers.
Synthesis of TsC3OXD-G1 (1)
containing 100 mL of tetrahydrofuran and 200 mL of N,N-
dimethyl formamide. K2CO3 (1.9 g, 13.8 mmol) and KI (0.2ꢀg,
1.4 mmol) were added, and the mixture was heated at 70 C
under an Ar atmosphere for 8 h. After cooled to ambient tem-
perature, the mixture was filtered, and the filtrate was con-
centrated. The pure product was obtained through silica gel
column chromatography (CHCl3:EtOAc ¼ 8:1) as a white solid
HC3OXD-G1 (7.0 g, 10.7 mmol) and TsCl (2.7 g, 14.2 mmol)
were dissolved in CH2Cl2 (200 mL). Et3N (10 mL) was added.
And the mixture was stirred at ambient temperature for 8 h.
Then the solvent was removed by a rotary evaporator, and the
residue was purified through silica gel column chromatogra-
phy (CH2Cl2:EtOAc ¼ 10:1) to give a white solid (6.5 g, 7.6
mmol) in a 72% yield. 1H NMR (400 MHz, CDCl3, d, ppm):
0.86–0.89 (t, 6H, CH3), 1.27–1.35 (m, 16H, CH2), 1.45–1.47 (m,
4H, CH2), 1.78–1.84 (m, 7H, OCH2CH2, phCH3), 2.19–2.23 (m,
2H, OCH2CH2CH2O), 4.02–4.05 (t, 4H, OCH2), 4.12–4.15 (t, 2H,
OCH2), 4.28–4.31 (t, 2H, OCH2), 7.01–7.05 (d, 4H, ArAH), 7.24
(m, 2H, ArAH), 7.67–7.68 (d, 2H, ArAH), 7.76–7.77 (m, 2H,
ArAH), 8.08–8.12 (m, 4H, ArAH), 8.38–8.39 (t, 1H, ArAH).
Mass spectroscopy (MS) (electrospray ionization (ESI)): m/z
851.4 [M þ H]þ. Anal. Calcd. For C48H58N4O8S: C, 67.74; H,
6.87; N, 6.58. Found: C, 67.78; H, 6.86; N, 6.61.
1
(5.0 g, 3.1 mmol) in a 88% yield. H NMR (400 MHz, CDCl3,
d, ppm): 0.88–0.91 (t, 12H, CH3), 1.30–1.39 (m, 32H, CH2),
1.44–1.50 (m, 8H, CH2), 1.79–1.86 (m, 8H, OCH2CH2), 2.35–
2.38 (m, 4H, OCH2CH2CH2O), 4.03–4.06 (t, 8H, OCH2), 4.20–
4.23 (t, 4H, OCH2), 4.39–4.42 (t, 4H, OCH2), 7.02–7.04 (d, 8H,
ArAH), 7.16 (s, 2H, ArAH), 7.83 (m, 4H, ArAH), 8.08–8.11
(m, 8H, ArAH), 8.41 (t, 2H, ArAH). MS (matrix assisted laser
desorption ionization (MALDI)-time of flight (TOF)): m/z
1647.6 [M þ Na]þ. Anal. Calcd. For C88H104N8O12Br2:C, 65.02;
H, 6.45; N, 6.89. Found: C, 65.08; H, 6.46; N, 6.89.
Synthesis of OXDBr2 (2)
General Procedure of Polymerizations
TsC3OXD-G1 (6.2 g, 7.2 mmol) and 2,5-dibromobenzene-1,4-
diol (0.93 g, 3.5 mmol) were dissolved in a mixed solvent
Typical Suzuki polycondensation was used in this work.
Using PFOXD10 as an example, 9,9-dioctylfluorene-2,7-
TABLE 1 Molecular Weights and Thermal Properties of PFOXD Series
Oxadiazole-Containing Monomer
Content
Polymer
Yield (%)
In feed (%)
In polymer (%)b
Mn (g/mol)a
PDIa
Td (ꢀC)c
Tg (ꢀC)d
PF
73
84
82
94
0
0
8,000
4.29
4.38
1.65
1.71
428
413
417
417
60
67
74
77
PFOXD10
PFOXD30
PFOXD50
a
10
30
50
9
9,100
32
55
d
10,800
10,600
Determined by GPC using polystyrene standards.
Calculated from 1H NMR data.
The temperature of 5% weight loss of the polymer determined from
Glass transition temperature determined by DSC in the second heat-
ing process at a heating rate of 20 ꢀC/min.
b
c
TGA under a nitrogen atmosphere at a heating rate of 20 ꢀC/min.
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FIGURE 1 1H NMR spectra of the PFOXD series of polymers.
bis(ethyleneborate) (0.13 g, 0.25 mmol), 2,7-dibromo-9,9-
dioctylfluorene (0.11 g, 0.20 mmol), OXDBr2 (0.08 g, 0,05
mmol), and Pd(PPh3)4 (0.02 g, 0.002 mmol) were added in
a schlenk tube. Toluene (5 mL) and an aqueous solution of
K2CO3 (3 mL, 1 mol/L) were used as the mixed solvent. Af-
ter three times of freeze-pump-thaw cycles, the tube was
heated at 80 ꢀC under an Ar atmosphere for 72 h. A small
amount of bromobenzene was added to remove the boric
acid ester end groups. After 4 h, a small amount of phenyl-
boronic acid was added to remove the bromine end groups.
The reaction mixture was diluted by CHCl3 (150 mL) and
then washed with water (100 mL ꢁ 3). The organic layer
was concentrated after dried by MgSO4. The residual solu-
tion was passed through an alumina column and precipi-
tated in acetone. The same precipitation process was done
two more times.
1H NMR (400 MHz, CDCl3, d, ppm)
PF: 0.75–1.25 (m, CH3, CH2), 1.95–2.30 (br, fluorene–CH2),
7.32–8.04 (m, ArAH).
FIGURE 2 Normalized UV-vis absorption spectra of PFOXD series in CHCl3 (a) and in films (b).
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TABLE 2 UV-vis and PL Spectra Data of PFOXD Series
In CHCl3
In Film
UV-vis
(nm)a
PL
(nm)b,c
UV-vis
(nm)d
PL
(nm)c,e
PL
(nm)f
PF
377
422, 441 375
424, 446 439, 466
425, 449 440, 465
PFOXD10 375, 310 420, 439 378, 300
PFOXD30 366, 307 417
PFOXD50 365, 305 415
376, 301
451
450
382, 330, 302 430, 440 442
a
Solution concentration: 10ꢂ5mol/L.
b
Solution concentration: 0.1 mg/ml.
c
Excited at 300 nm.
d
Films were spin-coated from CHCl3 solutions (0.1 mg/ml) and dried at
ambient temperature.
e
Films were cast from CHCl3 solutions (10 mg/ml) and dried at ambient
temperature.
f
Annealed at 100 ꢀC for 24 h.
PFOXD30: 0.70–1.50 (m, CH3, CH2), 1.70–1.90 (br, fluorene–
CH2), 1.90–2.45 (m, OCH2CH2CH2O, fluorene–CH2), 3.85–4.45
(m, OCH2), 6.90–8.50 (m, ArAH).
PFOXD50: 0.65–1.50 (m, CH3, CH2), 1.70–1.85 (br, fluorene–
CH2), 1.85–2.45 (m, OCH2CH2CH2O, fluorene–CH2), 3.85–4.45
(m, OCH2), 6.85–8.45 (m, ArAH).
PFOXDIr1: 0.65–1.50 (m, CH3, CH2), 1.70–1.85 (br, fluorene–
CH2), 1.85–2.45 (m, OCH2CH2CH2O, fluorene–CH2), 3.85–4.50
(m, OCH2), 6.88–8.45 (m, ArAH).
PFOXDIr3: 0.65–1.50 (m, CH3, CH2), 1.70–1.85 (br, fluorene–
CH2), 1.85–2.45 (m, OCH2CH2CH2O, fluorene–CH2), 3.90–4.52
(m, OCH2), 6.90–8.45 (m, ArAH).
FIGURE 3 Normalized PL spectra of PFOXD series in CHCl3 (a)
PFOXDIr5: 0.65–1.50 (m, CH3, CH2), 1.70–1.85 (br, fluorene–
CH2), 1.85–2.45 (m, OCH2CH2CH2O, fluorene–CH2), 3.92–4.52
(m, OCH2), 6.91–8.44 (m, ArAH).
and in films (b).
PFOXD10: 0.70–1.50 (m, CH3, CH2), 1.70–1.90 (br, fluorene–
CH2), 1.90–2.40 (m, OCH2CH2CH2O, fluorene–CH2), 3.85–4.45
(m, OCH2), 6.90–8.45 (m, ArAH).
PFOXDIr7: 0.65–1.50 (m, CH3, CH2), 1.70–1.85 (br, fluorene–
CH2), 1.85–2.45 (m, OCH2CH2CH2O, fluorene–CH2), 3.85–4.45
(m, OCH2), 6.80–8.50 (m, ArAH).
FIGURE 4 Normalized PL spectra of PFOXD series in films after
annealing.
FIGURE 5 Cyclic voltammograms of PF and PFOXD30.
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TABLE 3 Energy Levels of PFOXD Series
HOMO (eV)a
LUMO (eV)a
LUMO (eV)b
PF
ꢂ5.87
ꢂ5.90
ꢂ6.07
ꢂ6.19
–
ꢂ3.06
ꢂ3.09
ꢂ3.16
ꢂ3.23
PFOXD10
PFOXD30
PFOXD50
a
–
ꢂ2.51
ꢂ2.51
Calculated from CV.
b
Calculated from optical band gap.
RESULTS AND DISCUSSION
PFOXD Series
The number-average molecular weights (Mns) and polydispersity
indexes (PDIs) are summarized in Table 1. Their Mn values are
8000-10,800 g/mol, with PDIs of 1.65-4.38. The contents of the
FIGURE 6 EL spectra of PFOXD series.
1
oxadiazole in the polymers can be obtained from H NMR data
Highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) levels are important
properties for organic light-emitting diode materials. They
can be determined from the onset reduction potential and
onset oxidation potential in CV spectra according to the for-
mulae EHOMO ¼ ꢂe[Uonset(ox) þ (4.8 VꢂU1/2,FOC)] and ELUMO
¼ ꢂe[Uonset(red) þ (4.8 VꢂU1/2,FOC)].32 In the CV spectra of
the polymers (Fig. 5, showing PF and PFOXD30 as examples),
the onset reduction potentials of PF and PFOXD10 are not
observed. This is probably because of the low oxadiazole
contents. Therefore, for these two samples, their LUMO levels
were calculated from the onset absorption in their UV-vis
spectra. The results are listed in Table 3. By introducing the
oxadiazole moieties into the polymers, the LUMO levels
decrease, which indicates the improvement of the electron-
transporting property. The backbones may act as the hole-
transporting part, while the side groups may act as the elec-
tron-transporting part. Such a bi-channel structure may be
beneficial for balancing the carrier transportation in
devices.20
(Fig. 1). The chemical shift at around 4 ppm is attributed to the
OCH2 in the oxadiazole monomer. The chemical shift at around
2 ppm is attributed to both the OCH2CH2 in the oxadiazole-con-
taining monomer and CH2 directly connecting the fluorene.
Therefore, the ratio of the two monomers can be calculated by
the integral area. The results (shown in Table 1) are close to the
designed values. And all the polymers exhibit good thermal
stabilities with the 5% weight loss temperatures (Tds) higher
than 400 ꢀC. The glass transition temperature (Tg) increases
slightly because of the rigid structure of oxadiazole side groups.
Figure 2 is the UV-vis absorption spectra of the polymers in
solution and in films. With the increase of the oxadiazole
content, the absorption peak at about 300 nm attributed to
oxadiazole36 increases in intensity, and the peak at about
380 nm attributed to the backbone29 decreases in intensity.
This result is as expected.
Excited at 300 nm, all the polymers emit typical blue light of
the backbone (Fig. 3).29 The most remarkable is that the PL
spectra of PF, PFOXD10, and PFOXD30 change dramatically
ꢀ
after annealed at 100 C for 24 h. However, the spectrum of
Electroluminescent devices were fabricated with a configura-
tion of indium tin oxide (ITO)/poly(3,4-ethylenedioxythio-
phene (PEDOT):PSS/Sample/Ca/Al. All of the devices emit
blue light (Table 4). The efficiencies generally increase with
the increase of the oxadiazole content. Because the introduc-
tion of oxadiazole moiety enhances the electron-transporting
abilities, which makes the carrier transportation more bal-
anced, the device with PFOXD50 shows the best performance
PFOXD50, which had the highest oxadiazole content in the
series, almost remains the same as before annealing (Fig. 4).
This suggests that the steric hindrance of the side groups
can effectively suppress the aggregation, which leads to
changes of the spectra, of the polymers during annealing.
And it also implies that PFOXD50 may be the best polymer
to build high-performance devices. The photophysical data
are summarized in Table 2.
TABLE 4 Electroluminescent Properties of PFOXD Series
Uonset (V)
Lmax (cd/m2)
gImax (cd/A)
gLmax (lm/W)
CIE Coordinate
k (nm)a
PF
5.6
5.5
5.7
4.9
488
864
549
1033
0.0685
0.2050
0.1400
0.2890
0.0279
0.0879
0.0528
0.1140
0.19, 0.13
0.18, 0.13
0.20, 0.12
0.18, 0.10
423, 438, 481
421, 442, 481
420
PFOXD10
PFOXD30
PFOXD50
418
a
At low voltage.
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TABLE 5 Molecular Weights and Thermal Properties of
PFOXDIr Series
Polymer
Yield (%) Mn (g/mol)a PDIa Td (ꢀC)b Tg (ꢀC)c
PFOXDIr1 90
PFOXDIr3 95
PFOXDIr5 95
PFOXDIr7 97
a
8,400
8,300
8,600
11,700
1.28 412
1.35 406
1.42 401
1.51 405
77
75
76
82
Determined by GPC using polystyrene standards.
b
The temperature of 5% weight loss of the polymer determined from
TGA under a nitrogen atmosphere at a heating rate of 20 ꢀC/min.
c
Determined by DSC in the second heating process at a heating rate of
20ꢀC/min.
with the maximum luminance of 1033 cd/m2 and the maxi-
mum current efficiency of 0.29 cd/A. And in the electrolumi-
nescent (EL) spectra (Fig. 6), the shoulder peaks at about
500 nm decrease with the increase of the oxadiazole content.
This is in agreement with the PL spectra. All these results
reveal that PFOXD50 has the best properties among this se-
ries of copolymers and is the most possible one as the host
material for the phosphorescent guest in the next step.
FIGURE 8 Normalized PL spectra of PFOXDIr series in CHCl3
(a) and in films (b).
PFOXDIr Series
On the basis of the above study, PFOXD50 is the best among
the PFOXD series of copolymers. We introduced iridium com-
plexes into this polymer by copolymerizing with (ppy)2Ir(BrPh-
PyBr) at different ratios. As listed in Table 5, the Mns of the
TABLE 6 UV-vis and PL Spectra Data of PFOXDIr Series
In CHCl3
In Film
UV-vis
(nm)a
UV-vis
(nm)d
PL (nm)b,c
PL (nm)c,e
PFOXDIr1
PFOXDIr3
PFOXDIr5
PFOXDIr7
a
304
304
304
304
418
418
418
418
300
306
305
299
445, 568
451, 571
461, 574
463, 579
Solution concentration: 10ꢂ5mol/L.
Sulution concentration: 0.1 mg/mL.
Excitation wavelength was 300 nm.
b
c
d
Films were spin-coated from CHCl3 solutions (0.1 mg/mL) and dried at
ambient temperature.
Films were cast from CHCl3 solutions (10 mg/mL) and dried at ambient
e
FIGURE 7 Normalized UV-vis absorption spectra of PFOXDIr
series in CHCl3 (a) and in films (b).
temperature.
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TABLE 7 Electroluminescent Properties of the PFOXDIr Series
Uonset
Lmax
gImax
gLmax
CIE
(V)
(cd/m2) (cd/A)
(lm/W) Coordinate k (nm)a
PFOXDIr1 5.9
619
352
931
846
0.4046 0.1414 0.46, 0.46
0.3682 0.1456 0.48, 0.47
416, 569
PFOXDIr3 6.0
PFOXDIr5 5.7
PFOXDIr7 5.4
417, 577
580
0.4886 0.1792 0.51, 0.48
0.6118 0.2394 0.53, 0.47
585
a
At low voltage.
The HOMO and LUMO energy levels of the polymers calcu-
lated from CV spectra (CV curve of PFOXDIr7 is shown in
Fig. 9 as an example) are also quite close. The energy dia-
gram (Fig. 10) shows that the HOMO and LUMO levels of
Ir(ppy)3 are in between those of PFOXD50. This indicates
that the complexes may act as the traps of the carriers in
light-emitting devices.
FIGURE 9 Cyclic voltammogram of PFOXDIr7.
polymers are 8300-11,700 g/mol, with PDIs of 1.28-1.51. They
have similarly good thermal properties as PFOXD50.
Devices with the same configuration for the PFOXD series
were fabricated. They emit orange light (Table 7). The
change of the EL spectra (Fig. 11) is similar to that of the PL
spectra. Emissions from both the host and the guest can still
be observed in devices with PFOXDIr1 and PFOXDIr3 that
have relatively low Ir contents. However, in devices with
PFOXDIr5 and PFOXDIr7, there are only single peaks at
around 580 nm. The device efficiencies generally increase
from PFOXDIr1 to PFOXDIr7. No obvious decrease of the effi-
ciencies caused by quenching of triplet excitons at the high
Ir concentration is observed. This can be attributed to the
‘‘jacketing’’ effect. The big side groups can efficiently sepa-
rate the iridium complexes from each other and suppress
the triplet annihilation. The device with PFOXDIr7 has the
best performance with the maximum luminance of 846 cd/
m2 and the maximum current efficiency of 0.61 cd/A.
Because the contents of the iridium complexes are small in
the copolymers, their absorption spectra are similar (Fig. 7).
For PL emission in solution, all the polymers are similar to
PFOXD50 with the maximum emission peaks at around 420
nm (Fig. 8). No obvious emission peaks from the guest are
observed, which indicates that there is no energy transfer
from the host to the guest. However, the situation in films is
quite different (Table 6). As the Ir content increases, the in-
tensity of the emission peak from the guest at about 570 nm
increases, while that of the emission peak from the host at
about 450 nm decreases. This suggests that energy transfers
from the host to the guest. For PFOXDIr7, there is only one
emission peak at 579 nm, indicating nearly complete energy
transfer.
CONCLUSIONS
A series of electroluminescent jacketed conjugated polymers
with different contents of oxadiazole moeity were
FIGURE 10 Energy levels of the host and guest materials.
FIGURE 11 EL spectra of PFOXDIr series.
8
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13 Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.;
Mullen, K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am.
Chem. Soc. 2001, 123, 946–953.
successfully synthesized. The contents of oxadiazole deter-
mined by H NMR are close to the designed values. And vari-
1
ation in the absorption spectra is expected. With the increase
of the oxadiazole content, the LUMO levels decrease, and the
performance of the simple device with a configuration of
ITO/PEDOT:PSS/Sample/Ca/Al enhances, with the device
containing PFOXD50 having the best properties. And
PFOXD50 was chosen as the host material and copolymer-
ized with (ppy)2Ir(BrPhPyBr) at different ratios. The absorp-
tion spectra and the CV spectra do not change much at dif-
ferent Ir contents. However, the intensity of the orange-light
emission of the PL spectra in films from the guest increases,
while the emission from the host decreases. There is an
energy transfer between the host and the guest. With the
same configuration as with the PFOXD series, they all emit
orange light. The efficiencies generally increase with increas-
ing Ir content, which indicates that the bulky side groups
containing oxidiazole can not only balance the carrier trans-
portation but also separate the iridium complexes and sup-
press the triplet annihilation because of the ‘‘jacketing’’
effect. All of these devices have higher current efficiencies
than that of the device with PFOXD50. Among them, the de-
vice with PFOXDIr7 shows the best performance with the
maximum luminance of 846 cd/m2 and the maximum cur-
rent efficiency of 0.61 cd/A.
14 Tao, Y. T.; Wang, Q.; Yang, C. L.; Wang, Q.; Zhang, Z. Q.;
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Foundation of China (Grants 20974002 and 21134001) and the
973 National Basic Research Program (2011CB606004).
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