Dendron-jacketed electrophosphorescent copolymers: Improved efficiency and tunable emission color by partial energy transfer

Article abstract of DOI:10.1021/ma2018556

A styrene-based phosphorescent monomer VC3DbmIr(ppy)₂ with cyclometalated iridium complex was synthesized. It was then copolymerized with the first-generation dendritic carbazole monomer VC3CbzG1 through the conventional free radical polymeriza

Full text of DOI:10.1021/ma2018556

ARTICLE
pubs.acs.org/Macromolecules
Dendron-Jacketed Electrophosphorescent Copolymers: Improved
Efficiency and Tunable Emission Color by Partial Energy Transfer
Hao Jin, Wei Zhang, Dan Wang, Zengze Chu, Zhihao Shen,* Dechun Zou,* Xinghe Fan,* and Qifeng Zhou
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
ABSTRACT: A styrene-based phosphorescent monomer
VC3DbmIr(ppy)2 with cyclometalated iridium complex was
synthesized. It was then copolymerized with the first-generation
dendritic carbazole monomer VC3CbzG1 through the conven-
tional free radical polymerization to obtain a series of electro-
phosphorescent random copolymers. Characteristic vibrational
bands in the FT-IR spectra were utilized to determine the com-
positions of the copolymers. The glass transition temperatures
of the copolymers were about 90 °C, and the 5% weight loss
temperatures were all above 346 °C. All the copolymers ex-
hibited similar absorbing bands of the carbazole units in the UVꢀvis absorption spectra. In the photoluminescent spectra of the
copolymer films, energy transfer was found from the carbazole units to iridium phosphors. No obvious energy transfer was found in
the copolymer solutions. Light-emitting diodes were fabricated with two configurations. With increasing iridium content, the
electroluminescence of the copolymers shifted from blue to orange and passed through a near-white emission region. After the
optimization of the device structure, the device containing the copolymer PCbzG1Ir3 had the best performance with a maximum
luminescence of 2441 cd/m2 and a maximum external quantum efficiency of 0.520%.
’ INTRODUCTION
polymers can be processed by spin-coating or inkjet-printing
techniques to realize the flexible and large-area displays. Mean-
while, the covalent bonding between a host polymer and a guest
molecule can prevent potential phase separation in a doping
system.9 Furthermore, byintroducingdifferent kinds offunctional
units into a polymer chain, single-layer devices can be ac-
quired.10,11 A lot of dendritic materials have also been designed
recently to control the intra- or intermolecular interactions.
The strong steric hindrance effect is of benefit to inhibit the
triplet annihilation.
Dendronized polymers are a special kind of macro-
molecules,12ꢀ14 in which dendritic side groups are attached to
a linear polymer chain. They can be synthesized by the “graft
through”, “graft onto”, or “graft from” strategies.15,16 By increas-
ing the generation of dendritic side groups, the conformation of a
polymer can change from a flexible chain to a rigid chain through
a semirigid chain. This “jacketing effect” also exists in the
mesogen-jacketed liquid crystalline polymers (MJLCPs).17ꢀ19
In our previous work, we introduced the first- and second-
generation dendritic carbazole side groups into the MJLCPs
system. The two dendron-jacketed polymers, PCbzG1 and
PCbzG2, form hexatic columnar nematic phase and columnar
nematic phase, respectively.20 The ordered structures of the poly-
mer chains can inhibit the aggregation of intramolecular functional
Electrophosphorescent materials utilize both singlet and tri-
plet excitons to emit light, and using these materials can greatly
improve the device efficiencies with a 100% internal quantum ef-
ficiency in theory.1,2 Therefore, electrophosphorescent light-
emitting diodes have attracted more and more attention since
they were reported in 1998.3 When an organic molecule is ex-
cited by an electric field, it can generate a singlet exciting state
(S1) and a triplet exciting state (T1), which correspond to the
fluorescence and phosphorescence emissions, respectively. In S1
the electron-spin direction does not change, while in T1 it re-
verses. Thus, T1 is usually difficult to form in an excited molecule
because of the spin-forbidden rule. However, in some heavy
metal complexes, such as those of Ru, Re, Os, Ir, and Pt,4,5 T1 can
form under an electric field because of the strong spinꢀorbit
coupling effect. When a complex is excited, the exciting state of
the ligand converts into the “metal-to-ligand charge transfer”
(1MLCT) exciting state by the internal conversion. Then 1MLCT
3
converts into MLCT by intersystem crossing due to the over-
lapping of their potential energy curves. Finally, phosphorescence
3
emits in the radiative transition process of a triplet MLCT ex-
citing state.6
Electrophosphorescent materials can be classified as small
molecules, polymers, and dendritic materials.2,7,8 Small-molecular
phosphorescent materials are usually doped into a host material in
a device to prevent the concentration quenching. The vacuum
deposition technique is the main approach to obtain uniform
thin films for small-molecular material systems. Phosphorescent
Received: August 14, 2011
Revised:
November 14, 2011
Published: December 01, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis Route of VC3DbmIr(ppy)2
units. The carbazole units are distributed at the surface of the
stiff molecular chain, which can provide a continuous carrier-
transporting channel and favor the light-emitting process.
Dendronized electrophosphorescent polymers have been re-
ported recently.21,22 Conventional free radical polymerization
and ring-opening metathesis polymerization (ROMP) were
utilized to synthesize two kinds of homopolymers containing an
iridium complex in the core of the dendritic side group. No
obvious decrease of the fluorescence quantum yield was found
even with high iridium content, which reflected the isolation effect
of the dendritic structure. The other achievement of their work
was that compared with the corresponding dendritic monomers,
this dendronized structure could greatly improve the solution vis-
cosity, resulting in better film qualities. However, an additional
host material was needed for these polymers.
Cyclometalated iridium complexes have been widely used in
phosphorescent light-emitting devices because of their high
quantum yields and relatively short exiting lifetime.2,8 The wave-
length of the emitting light can be adjusted by changing the
ligand structure.1 In this contribution, a series of electrophos-
phorescent random copolymers were synthesized using the
dendron-jacketed polymer PCbzG1 as the host material because
the first-generation polymer had higher efficiencies and better
electroluminescent properties.20 A new monomer containing a
cyclometalated iridium complex was synthesized and used as the
guest material. The copolymers were used as the emitting layer in
the devices without any other host materials. The dendritic side
group could isolate the triplet exciton, and the stiff molecular
conformation could reduce the intramolecular interaction of the
side groups. Furthermore, a wavelength-tunable system was ob-
tained by the intermolecular energy transfer.
’ EXPERIMENTAL DETAILS
Materials. 2,20-Azobis(isobutyronitrile) (AIBN) was recrystallized
from ethanol and dried at ambient temperature. Chlorobenzene was
washed with sulfuric acid, saturated sodium bicarbonate solution, and
water, and it was then distilled under reduced pressure. Tetrahydrofuran
(THF) was refluxed with CaH2 for 8 h and then distilled. All the other
reagents were used as received.
Characterization. All of the characterization methods such as 1H
NMR spectroscopy, mass spectrometry, elemental analysis, gel permea-
tion chromatography (GPC), thermogravimetric analysis (TGA), dif-
ferential scanning calorimetry (DSC), absorption spectra, photolumi-
nescent (PL) spectra, and cyclic voltammetric (CV) analysis used in this
study were similar to those reported previously.20
Synthesis. Synthesis of the monomer VC3DbmIr(ppy)2 was per-
formed as illustrated in Scheme 1. VC3CbzG1 had been synthesized
before.20
Synthesis of 1-(4-Hydroxyphenyl)-3-phenylpropane-1,3-
dione (1). After sodium hydride (2.3 g, 70%, 66.0 mmol) was added
into a three-necked flask, a solution of 1-(4-hydroxyphenyl)ethanone
(3.0 g, 22.0 mmol) in THF was added dropwise in an ice/water bath with
magnetic stirring. The mixture was heated slowly to reflux under a
nitrogen atmosphere. After 1 h, methyl benzoate (3.6 g, 26.0 mmol) was
added, and the mixture was refluxed for 12 h. After cooled, the mixture
was acidified in an ice/water bath. Then THF was removed by a rotary
evaporator. The residue was extracted with 50 mL of ethyl acetate and
washed with saturated sodium bicarbonate solution and water in
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Chart 1. Chemical Structures of the Dendron-Jacketed Electrophosphorescent Copolymers
sequence. After removal of ethyl acetate, the residue was washed with
CH2Cl2 to obtain the pure product as a yellow solid in 85% yield. 1H
NMR (300 MHz, CDCl3, δ, ppm): 6.79 (s, 1H, CH2), 6.90ꢀ6.94 (d,
2H, ArꢀH), 7.48ꢀ7.55 (m, 3H, ArꢀH), 7.92ꢀ7.99 (m, 4H, ArꢀH).
MS (HRꢀESI): m/z 263.06751 [M + Na]+. Anal. Calcd for C15H12O3:
C 74.99, H 5.03. Found: C 74.81, H 5.23.
Synthesis of 1-[4-(3-Hydroxypropoxy)phenyl]-3-phenyl-
propane-1,3-dione (2). 1-(4-Hydroxyphenyl)-3-phenylpropane-1,3-
dione (2.0 g, 8.3 mmol), 3-bromopropan-1-ol (1.4 g, 10.0 mmol), and
potassium carbonate (1.7 g, 12.5 mmol) were added into 100 mL of ace-
tone, and the mixture was refluxed overnight. The hot mixture was filtra-
ted, and the solvent in the filtrate was evaporated by a rotary evaporator.
The residue was dissolved in 50 mL of ethyl acetate and washed with a
saturated sodium chloride solution (50 mL ꢁ 3). The pure product was
obtained by silica gel column chromatography (CH2Cl2:ethyl acetate =
Figure 1. FT-IR spectra of the copolymers.
1
15:1, v:v) as a white solid in 72% yield. H NMR (400 MHz, CDCl3,
δ, ppm): 1.77 (br, 1H, ꢀOH), 2.05ꢀ2.11 (m, 2H, CH2CH2CH2),
3.84ꢀ3.89 (t, 2H, HOCH2), 4.16ꢀ4.21 (t, 2H, CH2O), 6.79 (s, 1H,
CH2), 6.92ꢀ6.99 (d, 2H, ArꢀH), 7.46ꢀ7.58(m, 3H, ArꢀH), 7.96ꢀ8.02
(m, 4H, ArꢀH), 16.99 (br, 1H, CH2). MS (HRꢀESI): m/z 321.10973
[M + Na]+. Anal. Calcd for C18H18O4: C 72.47, H 6.08. Found: C 72.30,
H 6.10.
Synthesis of the Chloro-Bridged Iridium Dimer [Ir(ppy)2-μ-
Cl]2 (4). Iridium chloride trihydrate (1.0 g, 2.8 mmol) and 2-phenylpyr-
idine (1.2 g, 7.8 mmol) were added into a mixed solvent of 2-ethoxy-
ethanol (24 mL) and deionized water (8 mL). The mixture was refluxed
at 130 °C for 24 h. Then the precipitate was filtrated and washed with de-
ionized water, acetone, ethanol, and hexane in sequence to afford the
crude product as a yellow powder, which was used directly in the fol-
lowing reaction after dried in a vacuum oven.
Synthesis of VC3DbmIr(ppy)2 (5). Bis{3-[4-(3-oxo-3-phenyl-
propanoyl)phenoxy]propyl} (381.8 mg, 0.5 mmol), [Ir(ppy)2-μ-Cl]2
(706.6 mg, 0.7 mmol), and anhydrous sodium carbonate (537.4 mg, 5.1
mmol) were added into 60 mL of acetonitrile. After being stirred at 80 °C
for 48 h under a nitrogen atmosphere, the mixture was filtrated, and the
filtrate was concentrated by a rotary evaporator. The residue was passed
through a silica gel column (petroleum:acetone = 3:2, v:v) to afford the
pure product as a red powder in 65% yield. 1H NMR (400 MHz, CDCl3,
δ, ppm): 2.17ꢀ2.16 (m, 2H, CH2CH2CH2), 4.07ꢀ4.11 (m, 2H, CH2O),
4.48ꢀ4.53 (m, 2H, COOCH2), 5.33ꢀ5.36 (d, 1H, CH2d), 5.66ꢀ5.71
(d, 1H, CH2d), 6.35ꢀ6.37 (d, 4H, ArꢀH), 6.53 (s, 2H, CH2),
6.70ꢀ6.86 (m, 12H ArꢀH), 6.96ꢀ7.01 (m, 4H, ArꢀH), 7.27ꢀ7.38
(m, 7H, ArꢀH), 7.55ꢀ7.65 (m, 8H, ArꢀH), 7.78ꢀ7.90 (m, 14H,
ArꢀH), 8.19 (s, 1H, ArꢀH), 8.54ꢀ8.58 (q, 4H, ArꢀH). MS
Synthesis of Bis{3-[4-(3-oxo-3-phenylpropanoyl)phenoxy]-
propyl}-2-vinylterephthalate (3). 2-Vinylterephthalic acid (1.4 g,
7.4 mmol), 1-[4-(3-hydroxypropoxy)phenyl]-3-phenylpropane-1,3-
dione (4.9 g, 16.3 mmol), N,N0-dicyclohexylcarbodiimide (DCC, 3.9 g,
18.6 mmol), and 4-(dimethylamino)pyridine (DMAP, 0.2 g, 1.8 mmol)
were added into 80 mL of CH2Cl2. The mixture was stirred for 8 h at
ambient temperature and filtrated. The filtrate was concentrated by a
rotary evaporator. The residue was washed with acetone first and then
purified through a silica gel column using CH2Cl2 as eluent to afford the
pure product as a white solid in 75% yield. 1H NMR (400 MHz, CDCl3, δ,
ppm): 2.27ꢀ2.35 (m, 2H, CH2CH2CH2), 4.18ꢀ4.22 (m, 2H, CH2O),
4.54ꢀ4.59 (m, 2H, COOCH2), 5.38ꢀ5.41 (d, 1H, CH2d), 5.71ꢀ5.75
(d, 1H, CH2d), 6.79 (s, 1H, CH2), 6.92ꢀ6.98 (d, 4H ArꢀH), 7.36ꢀ7.43
(q, 1H, ArꢀH), 7.45ꢀ7.55 (m, 6H, ArꢀH), 7.89ꢀ8.01 (m, 10H, ArꢀH),
8.23 (s, 1H, ArꢀH), 17.00 (br, 1H, CH2). MS (LRꢀESI): m/z 753.0
[M]+. Anal. Calcd forC46H40O10: C 73.39, H 5.36. Found: C 73.31, H 5.37.
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Table 1. Composition, Molecular Weight, Molecular Weight Distribution, and Thermal Properties of the Copolymers
iridium content (mol %)
b
copolymer
in feed
in copolymera
0
yield (%)
Mn (g/mol)b
Mw/Mn
Td (°C)c
Tg (°C)d
PCbzG1
0
76
79
70
81
75
78
78
52 000
51 000
58 000
51 000
49 000
50 000
53 000
2.01
2.04
1.91
1.96
2.05
2.01
1.99
368
373
356
352
347
352
346
88
88
89
89
91
92
94
PCbzG1Ir05
PCbzG1Ir1
PCbzG1Ir3
PCbzG1Ir5
PCbzG1Ir7
PCbzG1Ir10
0.5
1.0
3.0
5.0
7.0
10.0
2.1
4.0
5.6
9.2
a Determined by the characteristic absorbing bands in FT-IR spectra. b Determined by GPC in THF using polystyrene standards. 5% weight loss
temperatures determined by TGA at a heating rate of 20 °C under a nitrogen atmosphere. d Evaluated by DSC during the second heating cycle at a rate of
20 °C/min.
c
Figure 2. TGA (a) and DSC (b) curves of the copolymers.
(MALDIꢀTOF): m/z 1752.6 [M]+. Anal. Calcd for C90H72O10N4Ir2:
C 61.70; H 4.03; N 3.20. Found: C 61.29; H 4.29; N 3.30.
Figure 3. UVꢀvis spectra of VC3DbmIr(ppy)2 and PL spectra of
PCbzG1 in THF (a) and films (b).
Copolymer Synthesis. A series of random copolymers PCbzG1-
co-PDbmIr(ppy)2 (Chart 1) were synthesized from the corresponding
styrene-based monomers VC3DbmIr(ppy)2 and VC3CbzG1. The co-
polymers were named according to the iridium contents as PCbzG1,
PCbzG1Ir05, PCbzG1Ir1, PCbzG1Ir3, PCbzG1Ir5, PCbzG1Ir7, and
PCbzG1Ir10. Take the synthesis of PCbzG1Ir10 as an example to illu-
strate the polymerization process. VC3CbzG1 (200 mg), VC3DbmIr-
(ppy)2 (26.5 mg), chlorobenzene (2.0 g), and 50 μL of a chlorobenzene
solution containing 0.5 mg of AIBN were transferred into a precleaned
glass tube. After three freezeꢀpumpꢀthaw cycles, the tube was sealed
off under vacuum. The polymerization was performed at 60 °C for 24 h,
and then the reaction was quenched in liquid nitrogen. The tube was
opened, and the solution was diluted with 5 mL of THF. The target poly-
mer was precipitated in 200 mL of acetone. To eliminate the unreacted
monomers, the precipitation process was repeated three times. After
filtrated and dried, the target copolymer was obtained.
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Monomers and Copoly-
mers. In order to synthesize the target phosphorescent monomer,
a β-diketone ligand 1 was synthesized by the Claisen condensation
reaction. Excess sodium hydride was used in this reaction because
of the existence of the phenolic hydroxyl group. Then a Williamson
etherification reaction was performed to synthesize compound 2.
The amount of 3-bromopropan-1-ol was in slight excess to avoid
the side reaction between the bromine and methylene group in
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Figure 4. UVꢀvis spectra of the copolymers in THF (a) and films (b).
Figure 5. PL spectra of the copolymers in THF (0.1 mg/mL) (a) and
films (b).
compound 1. An esterification reaction was followed to obtain
compound 3. Finally, the coordination reaction was carried out in
acetonitrile at 80 °C to afford the target monomer in a relatively
high yield.23 Compound 4 was synthesized according to the re-
ported method.8
The number-average molecular weights (Mn’s) of the copoly-
mers, the polydispersity indexes, and the thermal properties
are also summarized in Table 1. The Mn’s were all about
50 000 g/mol, and the polydispersity indexes were ∼2.00. All
the copolymerizations afforded high yields of more than 70%.
These results indicated that the iridium feed ratio played a
minor role on the polymerization process. In the two mono-
mers, a short spacer of three methylene units was introduced
into the molecules between the polymerizable sites and the
side groups. This special design was based on three reasons.
First, an introduction of a flexible spacer could reduce the huge
steric hindrance of the dendritic side groups during the poly-
merization. Second, the spacer decoupled the polymerizable sites
and the side groups,9 resulting in similar polymerization reactiv-
ities for the two monomers, which benefited the synthesis of
random copolymers instead of homopolymers or block copoly-
mers. Third, if the dendritic side groups were attached to the main
chain of some conjugated polymers, the flexible spacer could
inhibit the energy back-transfer.23,24 The polymerization results
in Table 1 suggest that the flexible spacer was indeed effective in
these aspects.
All of the copolymers were synthesized under similar condi-
tions by the conventional free radical polymerization in chloro-
benzene using AIBN as the initiator. The unreacted monomers
could be easily removed in the precipitation process because both
monomers could be readily dissolved in acetone. FT-IR spectra
(Figure 1) of these copolymers were used to determine the com-
positions of the copolymers and the differences between the real
iridium contents and those in feed. With increasing iridium
content in feed, the intensity of the characteristic vibrational
absorbing bands of CdN in VC3DbmIr(ppy)2 at 1540 cmꢀ1 in-
creased. According to the LambertꢀBeer law, the concentration of
functional groups is proportional to the absorbing intensity. On the
other hand, the characteristic band of 1,3,5-trisubstituted phenyl in
VC3CbzG1 at 1600 cmꢀ1 could be chosen as the internal reference
to calculate the real iridium content. After the determination of
the relationship between the molar ratio of the two monomers and
the ratio of the areas of the characteristic bands, the compositions
of the copolymers were obtained. The results are summarized in
Table 1. For the copolymer PCbzG1Ir05 and PCbzG1Ir1, the
compositions were difficult to calculate because the iridium con-
tents were so low that the characteristic bands of CdNcouldnotbe
observed in the FT-IR spectra. For the other copolymers, the real
iridium contents were a little lower than those in feed, which was
probably due to the steric hindrance effect of the dendritic side
groups in the copolymerization.
The TGA and DSC curves of the copolymers are shown
in Figure 2. The 5% weight loss temperatures (Td’s) of the
copolymers were all above 346 °C (Figure 2a), and only one
weight loss step was found in each TGA curve, implying good
thermal stabilities, which was in favor of device fabrication. The
glass transition temperatures (Tg’s) were ∼90 °C. No other
phase transitions were found in the DSC curves (Figure 2b).
With increasing iridium contents, a small increase in Tg was also
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found probably because there were less flexible units in the
phosphorescent monomer.
produced by the energy transfer between the host and the guest or
by the carrier-trapping mechanism. For an efficient F€orster energy
transfer, the degree of overlapping between the PL spectrum of the
host and the UVꢀvis absorption spectrum of the guest is of great
importance.25,26 Therefore, in the present work, the PL spectrum
of PCbzG1 and the UVꢀvis absorption spectrum of VC3DbmIr-
(ppy)2 were measured. The results of THF solutions and films are
shown in Figures 3a and 3b, respectively.
Photophysical Properties. In order to prevent the triplet
annihilation caused by the high current density or the high con-
centration of triplet excitons, an effective way is to build up the
hostꢀguest system. Guest materials are attached to the host
polymers by covalent bonds, and the phosphorescent emission is
Whether in solutions or in films, the MLCT absorbing bands
1
of the iridium complex were all observed. The singlet MLCT
Figure 6. Cyclic voltammogram of VC3Dbm(ppy)2.
Figure 7. Energy level diagram of the materials used in this study.
Figure 9. EL spectra (a) and CIE coordinate (b) of the copolymers.
Figure 8. Device configuration and auxiliary materials used in this study.
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Table 2. EL Properties of Device a
samples
Uonset (V)a
Lmax (cd/m2)b
ηI,max (cd/A)c
ηL,max (lm/W)d
ηext,max (%)e
CIE coordinatef
λ (nm)g
PCbzG1
4.5
4.8
4.6
5.0
5.5
5.3
635
639
0.122
0.117
0.134
0.281
0.140
0.211
0.062
0.067
0.079
0.116
0.057
0.083
0.119
0.079
0.060
0.101
0.050
0.058
(0.18, 0.16)
(0.20, 0.23)
(0.24, 0.37)
(0.37, 0.51)
(0.41, 0.52)
(0.47, 0.50)
428, 483
429, 495
439, 503
430, 524
432, 565
576
PCbzG1Ir1
PCbzG1Ir3
PCbzG1Ir5
PCbzG1Ir7
PCbzG1Ir10
680
1522
1150
2096
a Turn-on voltage at 1 cd/m2. b Maximum luminance. c Maximum current efficiency. d Maximum luminescence efficiency. e Maximum external quantum
efficiency. f 1931 CIE coordinate. g Emission wavelength at 6 V.
Table 3. EL Properties of Device b
samples
Uonset (V)a
Lmax (cd/m2)b
ηI,max (cd/A)c
ηL,max (lm/W)d
ηext,max (%)e
CIE coordinatef
PCbzG1
6.1
6.7
6.6
6.2
7.4
7.4
1132
1613
2441
3044
2257
2693
0.191
0.694
0.880
0.783
0.350
0.413
0.093
0.323
0.409
0.394
0.147
0.173
0.186
0.467
0.520
0.280
0.126
0.160
(0.18, 0.16)
(0.20, 0.23)
(0.24, 0.37)
(0.37, 0.51)
(0.41, 0.52)
(0.47, 0.50)
PCbzG1Ir1
PCbzG1Ir3
PCbzG1Ir5
PCbzG1Ir7
PCbzG1Ir10
a Turn-on voltage at 1 cd/m2. b Maximum luminance. c Maximum current efficiency. d Maximum luminescence efficiency. e Maximum external quantum
efficiency. f 1931 CIE coordinate.
3
absorption was at about 400 nm, and the triplet one, MLCT,
was at about 465 nm.8 The two characteristic absorbing bands
were effectively overlapped with the emission band of PCbzG1,
which provided a necessary condition for the energy transfer
from PCbzG1 to the iridium complex. Therefore, it was possible
to synthesize electrophosphorescent copolymers using PCbzG1
as the host material.
Electrochemical Properties. The electrochemical properties
of the host material PCbzG1 had been investigated previously.
In this work, the CV behavior of the monomer VC3DbmIr(ppy)2
was studied, as shown in Figure 6. The onset oxidation potential
(0.76 V) and the onset reduction potential (ꢀ1.53 V) were ob-
tained directly from the curve, and then the HOMO (ꢀ5.11 eV)
and LUMO (ꢀ2.82 eV) levels were calculated from the equations
The UVꢀvis absorption spectra of the copolymers in THF and
films are shown in Figure 4. All the copolymers exhibited similar
absorbing bands as PCbzG1 in solutions and films. Only the
characteristic bands of the carbazole units were observed in the
spectra. The absorption of the iridium complexwas notclearly ob-
served, which was probably due to its low content. In addition, the
energy of the coordination bond was lower, and itsabsorption was
mainly centered in the infrared region.
The PL spectra of these copolymers in solutions and films
were quite different (Figure 5). In THF solutions, all the copo-
lymers exhibited similar emitting bands as PCbzG1. Their peak
emissions were all centered at about 440 nm, indicating that the
efficient energy transfer from PCbzG1 to the iridium complex did
not take place. In contrast, in films, with increasing iridium con-
tent, the peak emission of the host became weaker, while a new
emission at about 550 nm appeared, with its intensity increasing
gradually, which suggested the energy transfer occurred in these
films. Compared with the peak emission of the host material in
solutions, a 10 nm blue shift was also found in films, which agreed
with the results we reported previously.20
For the F€orster energy transfer process, a certain distance be-
tween the host and the guest is also required in addition to the
overlap of their PL and UVꢀvis absorption spectra. The suitable
distance is 5ꢀ10 nm. In solutions, the spectra matched well, and
both the carbazole and iridium units could readily dissolve. How-
ever, the distance between them might exceed the limit, and the
energy transfer still could not take place. In films, the molecules
were packed more tightly, and carbazole and iridium complexes
were close to each other. Therefore, the energy transfer occurred.
EHOMO = ꢀe[Uonset(ox) + (4.8 V ꢀ U1/2,FOC)] and ELUMO
=
ꢀe[Uonset(red) + (4.8 V ꢀ U1/2,FOC)]. The energy levels of the
materials used in this study are summarized in Figure 7.
As shown in Figure 7, the monomer VC3DbmIr(ppy)2 pos-
sessed a higher HOMO level and a lower LUMO level. Mean-
while, the two levels were in between the corresponding levels
of PCbzG1. In this case, the guest played a trapping role,
which suggested the possibility of carrier-trapping mechanism in
the electroluminescent device based on this guestꢀhost system.
Electroluminescent Properties. The light-emitting devices
were fabricated in two configurations (Figure 8) to investigate
the electroluminescent properties of the copolymers. Device a had
a configuration of ITO/PEDOT:PSS/copolymer/TPBI(15 nm)/
LiF(1 nm)/Al(100 nm) while device b ITO/PEDOT:PSS/copo-
lymer/TPBI(15 nm)/AlQ(30 nm)/Ca(20 nm)/Al(100 nm).
The electroluminescent (EL) spectra and the CIE coordinates
of the copolymers at 6 V are illustrated in Figure 9.
In Figure 9a, with increasing iridium content of the copoly-
mers, energy transfer from the host to the guest occurred. The
blue emission of the host weakened, and the orange emission of
the guest gradually increased. Compared with the PL spectra, the
energy transfer in the EL spectra was more complete. For the
copolymer PCbzG1Ir7 and PCbzG1Ir10, the emission from the
host almost disappeared, indicating that the energy transfer tended
to be complete. The difference of the PL spectra between the
copolymer solutions and the films (Figure 5) had already
indicated that the energy transfer might take place in an inter-
molecular way because no obvious energy transfer was observed
in the PL spectra of the copolymer solutions.
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aluminum, AlQ) was introduced on top of the hole-blocking layer
(1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl, TPBI), and
then a calcium layer was deposited to enhance the electron-
injecting properties. Compared with device a, device b had better
overall properties, which could be attributed to the better
electron-transporting property of AlQ and improved carrier-
transporting balance. With increasing iridium content, the device
efficiencies of the two configurations increased first and then
dropped. For the optimized device b, the addition of a small
amount of iridium complex resulted in a significant improvement
of the device performance, which reflected the higher efficiency
of the phosphorescent emission.
The device b with the copolymer PCbzG1Ir3 had the best
properties. Its luminescence and efficiency curves are shown in
Figure 10. The maximum luminescence, current efficiency, and
external quantum efficiency were 2441 cd/m2, 0.880 cd/A, and
0.520%, respectively. As mentioned above, the generation of the
dendritic side group was low. Therefore, the isolation effect for
the iridium complex was limited, and the triplet annihilation
might still exist to some extent, which led to the decrease of device
performance at higher iridium contents. The red-shift of EL
spectra at high iridium contents also supported this conjecture.
’ CONCLUSIONS
In this contribution, a series of electrophosphorescent ran-
dom copolymers PCbzG1-co-PDbmIr(ppy)2 were synthesized
through the conventional free radical polymerization. The irid-
ium feed ratio played a minor role on the copolymerization pro-
cess. FT-IR spectra were utilized to determine the compositions
of the copolymers, and the real iridium contents were a little
lower than those in feed. The copolymers had good thermal
stabilities with the 5% weight loss temperatures all above 346 °C.
The PL spectrum of PCbzG1 effectively overlapped with the
UVꢀvis absorption spectrum of the iridium monomer, which
provided a necessary condition for the energy transfer from the
host to the guest. All the copolymers exhibited a single blue emis-
sion peak in THF, while in films the energy transfer occurred and
the emission from the guest was found. In the EL spectra, more
complete energy transfer was observed. With increasing iridium
content, the emission color shifted from blue to orange, passing
through a near-white region. The emission wavelength could be
controlled through the copolymer composition by the intermo-
lecular energy transfer. Two series of electroluminescent devices
were fabricated. An introduction of an AlQ layer resulted in
better carrier-transporting balance and thus improved the device
properties. With increasing iridium content, the device proper-
ties passed through a maximum value and then decreased, which
was attributed to the limited isolation effect of the dendritic
side group. The device b of the copolymer PCbzG1Ir3 had the
best performance, with the maximum luminescence, current
efficiency, and external quantum efficiency of 2441 cd/m2, 0.880
cd/A, and 0.520%, respectively. In order to take full advantage of
the site-isolation effect of the dendritic structure, we are currently
investigating polymers with dendritic side groups of higher gen-
eration and electrophosphorescent dendron-jacketed conjugated
polymers.
Figure 10. Electroluminescent properties of copolymer PCbzG1Ir3 and
PCbzG1Ir7 (device b): current densityꢀvoltageꢀluminescence curve
(a); current efficiencyꢀcurrent densityꢀpower efficiency curve (b); ex-
ternal quantum efficiencyꢀcurrent density curve (c).
When the iridium content was higher, the peak emission of the
guest red-shifted to a certain extent. This phenomenon could be
attributed to the aggregation of the guest units because the site-
isolation effect of the low-generation host polymer PCbzG1 was
not strong enough. On the other hand, the intermolecular in-
teractions of the iridium phosphors would also lead to the red
shift of the EL spectra. In the CIE coordinate, with increasing
iridium content, the emitting color of the copolymers shifted
from blue to orange and passed through a near-white region,
which suggested that the application of this kind of copolymer
might realize the fabrication of a white organic light-emitting
diode (WOLED) from a single polymer chain by the intramo-
lecular partial energy transfer. And the emission color could be
adjusted by the composition of the copolymer.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: fanxh@pku.edu.cn (X.F.); zshen@pku.edu.cn (Z.S.);
dczou@pku.edu.cn (D.Z.).
The device properties are summarized in Tables 2 and 3.
In device b, an electron-transporting layer (8-hydroxyquinoline
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’ ACKNOWLEDGMENT
Financial support from the National Natural Science Founda-
tion of China (Grants 20634010, 20974002, and 50833001) is
gratefully acknowledged.
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