Supramolecular phosphorescent polymer iridium complexes for high-efficiency organic light-emitting diodes

Article abstract of DOI:10.1021/cm400333c

The synthesis of supramolecualr phosphorescent polymers (SPPs) as a novel class of solution-processable electroluminescent (EL) emitters was presented. The SPPs were formed by utilizing the efficient nonbonding self-assembly of luminescent iridium monomer 1 and terfluorenyl -based monomers 2 and 3, tethered with either a crown ether or dibenzylammonium unit. The supramolecular assembly process was monitored and illustrated by ¹H nuclear magnetic resonance (NMR) and viscosity measurement. Moreover, the SPPs exhibit an intrinsic glass transition with a glass-transition temperature (T_g) of 72.5-81.5° C, which is absent in the monomers. The characterization of organic light-emitting diodes that consisted of these SPPs as an emitter gave an efficiency of 14.6 cd A^-1 at a luminance of 450 cd m^-2. Considering the good solution processability and catalyst-free polymerization process for the designed SPPs, combining the good device performances, the present study provide a promising alternative route to develop solution processed phosphorescent light-emitting materials for optoelectronic applications.

Full text of DOI:10.1021/cm400333c

Article
pubs.acs.org/cm
Supramolecular Phosphorescent Polymer Iridium Complexes for
High-Efficiency Organic Light-Emitting Diodes
Ai-Hui Liang, Kai Zhang, Jie Zhang, Fei Huang,* Xu-Hui Zhu,* and Yong Cao
State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China
University of Technology, Guangzhou 510640, People’s Republic of China
ABSTRACT: The synthesis of supramolecualr phosphorescent
polymers (SPPs) as a novel class of solution-processable electro-
luminescent (EL) emitters was presented. The SPPs were formed
by utilizing the efficient nonbonding self-assembly of luminescent
iridium monomer 1 and “terfluorenyl”-based monomers 2 and 3,
tethered with either a crown ether or dibenzylammonium unit. The
supramolecular assembly process was monitored and illustrated by
1H nuclear magnetic resonance (NMR) and viscosity measurement.
Moreover, the SPPs exhibit an intrinsic glass transition with a glass-
transition temperature (Tg) of 72.5−81.5 °C, which is absent in the monomers. The characterization of organic light-emitting
diodes that consisted of these SPPs as an emitter gave an efficiency of 14.6 cd A−1 at a luminance of 450 cd m−2. Considering the
good solution processability and catalyst-free polymerization process for the designed SPPs, combining the good device
performances, the present study provide a promising alternative route to develop solution processed phosphorescent light-
emitting materials for optoelectronic applications.
KEYWORDS: iridium complex, organic light-emitting diodes, phosphorescence, solution process, supramolecular polymer
advantages, such as easy control of the ratio of monomers in
the alternating copolymer and no residual metal catalyst
contamination, with respect to conventional polymers,
prepared through metal-catalyzed polymerization.9 Thus far,
there have been only occasional reports on supramolecular
polymers as EL emitters.10 Meijer et al. first utilized self-
assembled light-emitting polymers based on UPy-functionalized
fluorescent monomers (UPy = 2-ureido-4[1H]-pyrimidinone),
while the resulting luminous efficiency (LE) was <0.1 cd A−1.10a
Recently, we reported on supramolecular fluorescent polymers
by harnessing the efficient nonbonding interaction between
functionalized dibenzo-24-crown-8 and dibenzylammonium
salt, which produced a LE value of ∼4 cd A−1.10b
In this contribution, we describe a phosphorescent small-
molecule crown ether-functionalized iridium monomer and its
successful incorporation into a luminescent supramolecular
polymer (see Scheme 1). The optical, morphological, and EL
properties of the resulting polymers, as well as the supra-
molecular assembly process, were discussed. The supra-
molecualr phosphorescent polymers (SPPs) exhibit an intrinsic
glass transition with a glass-transition temperature (Tg) of
72.5−81.5 °C. A high LE of 14.6 cd A−1 at a current density of
3.1 mA cm−2, and a maximal brightness of 2863 cd m−2 were
obtained based on the single-emissive-layer polymer light-
emitting diodes with SPP50 as the emitter. This work opens a
new avenue in the development of solution-processable
INTRODUCTION
■
Phosphorescent organic light-emitting diodes (OLEDs) have
been recognized as one of the most promising candidates for
flat-panel displays and solid-state lighting.1 In order to obtain
high-efficiency OLEDs, phosphorescent complexes have been
extensively developed as the emissive materials, since they can
harvest both singlet and triplet excitons, which can potentially
ensure nearly 100% internal quantum efficiency.2 In particular,
cyclometalated iridium complexes show short lifetimes in
excited states (on the order of microseconds) and high
luminescence efficiencies.3 Recently, solution-processable elec-
troluminescent (EL) emitters have been the subject of
increasing interest for low-cost and large-area OLED displays
and lighting.4 In this context, electrophosphorescent polymers,
where the iridium complexes are incorporated into the
polymeric main chain5 or side chain6 via covalent bonds,
have attracted considerable interest. Compared with the
physical blends of polymer hosts and phosphors of iridium
complexes, this strategy can effectively prohibit phase
segregation. However, some challenges still exist in developing
high-performance solution-processed phosphorescent organic
light-emitting diodes (PHOLEDs). For example, a trace of
metal catalyst remaining in the polymers would inevitably
impede the improvement of device performances.7 In addition,
it is difficult to control the distribution and exact content of the
iridium complex in the polymer, which plays an important role
in the properties of the resulting PHOLEDs.4b,8
The programmed supramolecular assembly of functional
components might open up a new avenue toward well-
controlled supramolecular polymers, which exhibit many
Received: January 26, 2013
Revised: March 4, 2013
Published: March 4, 2013
© 2013 American Chemical Society
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K2CO3 solution (20.0 mL) in toluene (30.0 mL) and ethanol (10.0
mL). The mixture was degassed and heated to 80 °C with vigorously
stirring for 48 h under N2. After being cooled to room temperature,
the mixture was poured into water. The organic phase was separated
and the aqueous phase was extracted with dichloromethane (3 × 30
mL). The combined organic layers were dried over MgSO4 and
evaporated to remove the solvents under vacuum. The residue was
purified by column chromatography (ethyl acetate/dichloromethane =
2/1 (v/v)) to provide monomer 1 (617 mg) as an orange solid in 48%
yield. 1H NMR (300 MHz, CDCl3, TMS) δ (ppm): 8.90−8.88 (d, J =
5.82 Hz, 1H), 8.34−8.31 (d, J = 7.92 Hz, 1H), 8.02−8.00 (d, J = 8.43
Hz, 1H), 7.95−7.93 (d, J = 8.52 Hz, 1H), 7.88−7.82 (t, J = 7.96 Hz,
1H), 7.80−7.74 (m, 2H), 7.70−7.68 (d, J = 5.25 Hz, 1H), 7.58 (s,
2H), 7.55−7.53 (d, J = 5.82 Hz, 2H), 7.42 (s, 1H), 7.35 (s, 1H), 7.31−
7.07 (m, 9H), 6.95−6.91 (d, J = 7.23 Hz, 1H), 6.87 (s, 10H), 6.73 (s,
1H), 6.55 (s, 1H), 4.23 (m, 16H), 3.93 (s, 16H), 3.84 (s, 16H), 2.00−
1.84 (m, 8H), 1.25−0.98 (m, 40H), 0.82−0.68 (m, 20H). Anal. Calcd.
for C122H156IrN3O18: C, 68.32; H, 7.33; N, 1.96; Found: C, 68.39; H,
7.35; N, 1.92. MALDI-TOF (m/z): calcd for [M]+ 2142.10, found [M
+ Na]+ 2164.20.
Scheme 1. Schematic Representation of the Construction of
SPPs from Monomers 1, 2, and 3
Synthesis of 3F-2CHO. A mixture of 9,9-dioctylfluorene-2,7-
bis(boronic acid pinacol ester) (1.6 g, 2.5 mmol), 7-bromo-9,9-
dioctylfluorene-2-carbaldehyde (2.8 g, 5.6 mmol), potassium carbonate
(7.7 g, 56.0 mmol), and Pd(PPh3)4 (130 mg) in toluene (50 mL) and
deionized water (28 mL) was refluxed for 24 h under argon
atmosphere. After being cooled to the room temperature, the mixture
was poured into brine and extracted twice with dichloromethane. The
combined organic layers were dried over MgSO4 and the solvent was
removed. The crude product was purified with column chromatog-
raphy (petroleum ether/dichloromethane = 4/1 (v/v)) to afford a
light yellow solid in 72% yield (2.2 g). 1H NMR (CDCl3, 300 MHz) δ
(ppm): 10.08 (s, 2H), 8.11 (d, J = 8.4 Hz, 2H), 7.88 (s, 4H), 7.86−
7.85 (d, J = 2.22 Hz, 2H), 7.82 (s. 2H), 7.73−7.72 (d, J = 1.44 Hz,
2H), 7.69 (s, 2H), 7.66−7.64 (d, J = 4.62 Hz, 4H), 2.12−2.07 (m,
12H), 1.25−1.07 (m, 60H), 0.81−0.77 (m, 30H).
Synthesis of 3F-2Boc. Compound 3F-2CHO (2.2 g, 1.8 mmol)
and benzylamine (0.48 g, 4.5 mmol) were heated together in refluxing
toluene (40 mL) for 20 h under argon. The reaction mixture was
cooled to room temperature, and the solvent was evaporated off under
vacuum. The residue was redissolved in tetrahydrofuran (THF) (20
mL) and added in a 100-mL two-necked round-bottomed flask.
Methanol (MeOH) (60 mL) and NaBH4 (0.28 g, 7.2 mmol) were
added, and the mixture was heated to reflux for 8 h. The reaction
mixture was quenched with aqueous NaHCO3 and extracted with
dichloromethane. The two phases were separated, and the water phase
was extracted twice with dichloromethane. The combined organic
extracts were washed three times with water, dried over MgSO4, and
filtered. The filtrate was evaporated to dryness. The obtained oil was
stirred together with (Boc)2O (1.04 g, 4.7 mmol) and catalytic amount
of DMAP in dry dichloromethane (50 mL) for 12 h at room
temperature. The solution was evaporated and the residue was purified
by column chromatography (petroleum ether/ethyl acetate = 20/1 (v/
v)) to afford a colorless liquid in 86.7% yield (3.0 g). 1H NMR
(CDCl3, 300 MHz, tetramethylsilane (TMS)) δ (ppm): 7.82−7.80 (d,
J = 7.86 Hz, 2H), 7.78−7.75 (d, J = 7.86 Hz, 2H), 7.69−7.61 (m, 8H),
7.35−7.24 (m, 16H), 4.52−4.34 (m, 8H), 2.12−2.07 (m, 12H), 1.54
(s, 18H), 1.25−1.07 (m, 60H), 0.81−0.77 (m, 30H).
electrophosphorescent polymers for organic light-emitting
diodes.
EXPERIMENTAL SECTION
■
Materials. 2-(7-Bromo-9,9-dioctylfluoren-2-yl)pyridine (BrFPy),11
4-(boronic acid pinacol ester)-dibenzo-24-crown-8,10b 7-bromo-9,9-
dioctylfluorene-2-carbaldehyde,12 9,9-dioctylfluorene-2,7-bis(boronic
acid pinacol ester),13 and monomer 310b were prepared according to
the reported procedures. All reactions were performed under nitrogen.
All solvents were carefully dried and distilled from appropriate drying
agents prior to use. Commercially available reagents were used without
further purification, unless otherwise stated.
Synthesis of (BrFPy)2Ir(Pic). A mixture of iridium trichloride
hydrate (0.35 g, 1.0 mmol) and 2-(7-bromo-9,9-dioctylfluoren-2-
yl)pyridine (BrFPy) (1.36 g, 2.50 mmol) in 2-ethoxyethanol (24 mL)
and water (8 mL) was heated and refluxed for 24 h under nitrogen
atmosphere. After being cooled to room temperature, the resulting
precipitate was collected, and then washed respectively with water,
ethanol, and petroleum ether to afford 1.18 g of chlorine-bridged
iridium dimer [(BrFPy)2IrCl]2 as an orange powder. The dimer was
directly used in the following procedure.
A mixture of [(BrFPy)2IrCl]2 (1.18 g, 0.447 mmol), picolinic acid
(138 mg, 1.12 mmol), Na2CO3 (572 mg, 5.4 mmol) in 2-
ethoxyethanol (20 mL) was refluxed for 24 h under nitrogen
atmosphere, then was allowed to cool to room temperature and
mixed with water (40 mL). The mixture was extracted with
dichloromethane (3 × 30 mL). The resulting organic layers were
dried over MgSO4 and evaporated to remove the solvents under
vacuum. The residue was purified by column chromatography (ethyl
acetate/dichloromethane = 1/1 (v/v)) to obtain (BrFPy)2IrPic as an
1
Synthesis of Monomer 2. TFA (3.0 mL, 39.0 mmol) was added
to a solution of 3F-2Boc (1.57 g, 1.0 mmol) in dichloromethane (30
mL), and the mixture was stirred for 12 h at room temperature. A
saturated aqueous solution of NH4PF6 was added to the reaction
mixture. Upon removal of the volatile solvents, a precipitate was
formed which was filtered off and dissolved in acetonitrile. After the
acetonitrile solution was also treated with the aqueous NH4PF6
solution to afford a white solid in 85% yield (1.44 g). 1H NMR
(DMSO, 300 MHz, TMS) δ (ppm): 9.17 (s, 4H), 7.94−7.92 (d, J =
7.26 Hz, 6H), 7.79 (s, 8H), 7.57 (s, 2H), 7.48 (s, 2 H), 7.46 (s, 10 H),
4.27 (s, 4H), 4.15 (s, 4H), 1.98−1.90 (m, 12H), 1.10−1.02 (m, 60H),
0.72−0.64 (m, 30H). MALDI-TOF (m/z): calcd for [M − 2PF6]+
1406.12, found [M − 2PF6]+ 1405.90.
orange solid in 67% yield (842 mg). H NMR (300 MHz, CDCl3,
TMS) δ (ppm): 8.87−8.86 (d, J = 4.8 Hz, 1H), 8.34−8.31 (d, J = 7.47
Hz, 1H), 8.01−7.98 (d, J = 8.16 Hz, 1H), 7.94−7.92 (d, J = 8.04 Hz,
1H), 7.89−7.85 (t, J = 6.71 Hz, 1H), 7.80−7.74 (m, 2H), 7.67−7.65
(d, J = 4.77 Hz, 1H), 7.58−7.52 (m, 3H), 7.39 (s, 1H), 7.35 (s, 1H),
7.31−7.14 (m, 5H), 7.10−7.07 (d, J = 8.07 Hz, 1H), 6.95−6.91 (t, J =
6.57 Hz, 1H), 6.73 (s, 1H), 6.55 s, 1H), 2.10−1.91 (m, 8H), 1.25−
1.02 (m, 40H), 0.85−0.63 (m, 20H). Anal. Calcd. for
C74H90Br2IrN3O2: C, 63.23; H, 6.45; N, 2.99; Found: C, 63.29; H,
6.35; N, 2.93.
Synthesis of Monomer 1. Pd(PPh3)4 (105 mg, 0.09 mmol) was
added to a mixture of (BrFPy)2IrPic (842 mg, 0.6 mmol), 4-(boronic
acid pinacol ester)-dibenzo-24-crown-8 (1.03 g, 1.8 mmol), and 2.0 M
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Scheme 2. Synthetic Route to Monomers 1 and 2
Synthesis of Supramolecular Phosphorescent Polymers.
Monomers 1, 2, and 3 are dissolved at a certain ratio in a solution
of CHCl3−CH3CN (1/1 (v/v)). The resulting solution was left for
evaporation at room temperature. The obtained supramolecular
phosphorescent polymers are denoted as SPP5, SPP10, SPP25 and
SPP50, corresponding to the monomer molar ratios of 1:2:3 as
0.5:5:4.5, 1:5:4, 2.5:5:2.5, and 5:5:0, respectively, without any further
purification.
lammonium hexafluorophosphate (Bu4NPF6) (0.1 M) in acetonitrile
at a scan rate of 50 mV/s at room temperature, under the protection of
argon. A platinum electrode was used as the working electrode. A
platinum wire was used as the counter electrode, and a saturated
calomel electrode (SCE) was used as the reference electrode. The PL
quantum yields of SPPs and monomer 1 in neat film were measured
by an absolute method.15 The measurements were carried out on a
SENS-9000 Fluorosens System with an integrating sphere at ambient
temperature, using a 380-nm excitation wavelength. The photo-
luminescent lifetime was investigated on a Model FLS920
spectrometer demonstration system.
PLEDs Fabrication and Measurements. The ITO-coated glass
substrates were ultrasonically cleaned with deionized water, acetone,
detergent, deionized water, and isopropyl alcohol. A layer of 40-nm-
thick poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid)
(PEDOT:PSS) (H.C. Starck, 4083) then was spin-coated onto the
precleaned and O2-plasma-treated ITO substrates. After that, the
PEDOT:PSS layer was baked at 150 °C for 20 min to remove residual
water, and then the devices were moved into a glovebox under the
argon-protected environment. SPPs + PBD (30 mg mL−1 in o-DCB)
were spin-coated onto PEDOT:PSS at a speed of 2000 rpm to yield
emitting layers 90 nm thick. The samples were transferred into a
chamber and kept under vacuum (3.0 × 10−4 Pa) for 2 h. Cesium
fluoride (CsF) with a thickness of 1.5 nm and aluminum with a
thickness of 100 nm, then were subsequently deposited on top of the
EML to form the cathode. The current density (J) and brightness (L)
versus voltage (V) data were collected using a Keithley Model 236
Measurements and Characterization. 1H spectra were recorded
on a Bruker 300 or 400 MHz spectrometer operating at 300 or 400
MHz at room temperature. Chemical shifts were reported as δ values
(ppm), relative to an internal TMS standard. Time-of-flight mass
spectrometry (TOF-MS) was performed in the positive-ion mode with
a matrix of dithranol using a Bruker-Autoflex III Smartbeam system.
Differential scanning calorimetry (DSC) measurements were carried
out with a Netzsch Model DSC 204 system under N2 flow at heating
and cooling rates of 10 °C min−1. The Tg value was determined from
the second heating scan. The viscosity was measured with a digital
viscometer from Brookfield Engineering Laboratories, Inc. (Model
LVDV-I+). Ultraviolet-visible light (UV−vis) absorption spectra were
measured on a Hewlett−Packard Model HP 8453 spectrophotometer.
Photoluminescence (PL) spectra were recorded on a Hitachi Model F-
4500 fluorescence spectrophotometer. The phosphorescence quantum
yield of monomer 1 was determined in CH2Cl2 solution at 293 K
against fac-[Ir(ppy)3] (Hppy = 2-phenylpyridine) as a reference (Φp =
0.90).14 Cyclic voltammetry was carried out on a CHI Instruments
Model 660A electrochemical workstation in a solution of tetrabuty-
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source meter and silicon photodiode. After typical encapsulation with
UV epoxy and cover glass, the devices were taken out from the drybox
and the luminance was calibrated by a PR-705 SpectraScan
Spectrophotometer (Photo Research) with simultaneous acquisitions
of the EL spectra.
RESULTS AND DISCUSSION
Synthesis and Characterization. The synthesis of
monomers 1 and 2 is presented in Scheme 2. Monomer 3
■
Figure 3. Differential scanning calorimetry (DSC) plots of SPP5,
SPP10, SPP25, and SPP50.
Table 1. Ultraviolet (UV), Photoluminenscence (PL), and
Thermal Performances of SPP5, SPP10, SPP25, and SPP50
monomer
ratio (1/2/3) λabs (nm)
ΦPL
a
Tg
(°C)
a
a
SPP
λPL (nm)
(%)
SPP5
0.5:5:4.5
1:5:4
358, 456
416, 438, 562, 604
416, 438, 562, 604
416, 562, 604
562, 604
17
17
20
21
72.5
73.4
75.0
81.5
SPP10
SPP25
SPP50
358, 456
358, 456
358, 456
1
2.5:5:2.5
5:5:0
Figure 1. Stacked H NMR spectra (400 MHz, CDCl3−CD3CN, 1/1
(v/v), 22 °C) of solutions of 1 and 2 at different concentrations: (a) 1,
(h) 2, and equimolar solutions of 1 and 2 at concentrations of (b) 2,
(c) 5, (d) 10, (e) 20, (f) 30, and (g) 50 mM L−1. Peaks of linear
polymer, cyclic dimer, and uncomplexed monomer are designated as
lin, cyc, and uc, respectively.
a
Measured in the neat film at room temperature.
Hlin increased while that of Hcyc decreased. Meanwhile, the
broadening of the Hlin signal may imply the formation of high-
molecular-weight polymeric structures.
The linear self-assembly process was further evidenced by
viscosity measurements. The double logarithmic plot of specific
viscosity versus the initial concentrations of equimolar solutions
of monomers 1 and 2 is shown in Figure 2. At low
concentrations, the curve has a slope of 0.31, which is
characteristic for noninteracting assemblies of constant size.18
As the concentration increases, the curve exhibits a sharp rise
with a slope of 1.55, indicating the emergence of linear
supramolecular polymers. The critical polymerization concen-
tration at ∼18 mM L−1 indicates a transition from cyclic species
to linear supramolecular polymers. Furthermore, the viscosity
measurements reveal that the resulting SPPs possess excellent
solution processability for optoelectronics.
The differential scanning calorimetry (DSC) measurements
show that the as-prepared phosphorescent polymers SPP5−
SPP50 are inherently amorphous with a glass-transition
temperature of Tg = 72.5−81.5 °C (see Figure 3 and Table
1), which is absent in the monomers.10b This amorphous
morphology is desirable for EL purposes.19
Figure 2. Specific viscosity of equimolar mixtures of 1 and 2 in
CHCl3−CH3CN (1/1 (v/v)) solution.
was reported by us recently.10b The new phosphorescent
monomer 1 was obtained by a bis-Suzuki coupling of bis(2-(7-
bromo-9,9-dioctylfluoren-2-yl)pyridine)Ir(picolinate) with 4-
(boron acid pinacol ester)-dibenzo-24-crown-8. The bis-
ammonium monomer 2 that contains a linear-conjugated
terfluorenyl unit was prepared by a modified procedure, starting
from 3F-2CHO.10b,16
Photophysical Properties. The absorption and photo-
luminescence (PL) spectra of monomers 1−3 in solution (1
and 3 in CH2Cl2 and 2 in CH3CN) and those of the SPPs as
films are shown in Figure 4, and relevant data are presented in
Tables 1 and 2. The iridium monomer gives a weak absorption
band above 400 nm, which corresponds to an admixture of
1H NMR studies show that the formation of linear
supramolecular polymers is favored at high monomer
concentrations, in contrast to cyclic structures. For an
equimolar solution of monomers 1 and 2, the H1 and H2 of
monomer 2 undergo a downfield shift at low concentrations,
splitting into two sets of signals, namely, Hcyc and Hlin (Figure
1), which is attributed to the coexistence of both cyclic and
linear assemblies.17 With increasing concentration, the signal of
3
3
1MLCT, MLCT, and π−π* excited states, caused by the
heavy-metal effect of Ir center, leading to strong spin−orbital
coupling.20 The PL spectrum shows a maximal phosphorescent
peak at 566 nm at ambient temperature, excited at a wavelength
of 380 nm. The triplet emission displays a large Stokes shift
from the lowest energy absorption peak. The phosphorescence
quantum yield (ΦP), recorded in degassed CH2Cl2 solution
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Figure 4. Normalized UV−vis absorption spectra of monomer 1−3 (a) in solution and (b) SPPs in neat film; normalized PL spectra of monomer
1−3 (c) in solution and (d) SPPs in neat film at room temperature.
Table 2. Electrochemical and Photophysical Data of Monomers 1, 2, and 3
a
b
b
monomer
Eox (V)
Ered (V)
HOMO (eV)
LUMO (eV)
Eg (eV)
λabs (nm)
λPL (nm)
1
2
3
0.85
1.4
−1.69
−1.75
−1.88
−5.25
−5.8
−5.56
−2.71
−2.65
−2.52
2.54
3.15
3.04
358, 454
355
566, 605
400, 417
411, 432
1.16
365
a
b
Measured in a solution of Bu4NPF6 (0.1 M) in acetonitrile at a scan rate of 50 mV/s at room temperature. Measured in CH2Cl2 solution at room
temperature.
than that of [Ir(Flpy)3] (2.8 μs).20c Accordingly, the radiative
lifetime (τr) of the triplet excited state was 4.1 μs, deduced from
τr = τp/Φp. The triplet radiative and nonradiative rate constant
(kr and knr, calculated from Φp and τp, using the expressions Φp
= ΦISC[kr/(kr + knr)] and τp = (kr + knr)−1, are 2.4 × 105 and 3.5
× 105 s−1, respectively, assuming that the ΦISC is unity.
The PL spectra of the SPPs show a similar intense emission
at 562 nm with a shoulder at 604 nm, characteristic of the
iridium monomer. Nevertheless, the presence of an additional
emission peak at ca. 430 nm indicates an incomplete energy
transfer from the terfluorenyl to iridium units at a low iridium
content (for instance, in SPP5, SPP10, and SPP25). On the
other hand, the PL quantum efficiencies (ΦPL) of the SPPs in
neat film enhanced from 17% to 21% with increasing iridium
content from 5 mol % to 50 mol % (Table 1). By contrast, the
ΦPL of the iridium monomer in neat film is 7.7%.
Figure 5. Electroluminescent (EL) spectra of SPP5, SPP10, SPP25,
Electrochemical Properties. The electrochemical behav-
iors of monomers 1, 2, and 3 were studied by cyclic
voltammetry in order to estimate the potential charge
injection/transport properties of the SPPs (see Table 2). The
iridium monomer 1 shows a reversible anodic wave at 0.85 V vs
SCE, attributed to the oxidation of the iridium(III). The onset
and SPP50 in the OLEDs (ITO/PEDOT/EMLs/CsF/Al).
excited at 380 nm, is 0.41, relative to a fac-[Ir(ppy)3] standard
(ΦP = 0.9, Hppy = 2-phenylpyridine).14 The emission lifetime
(τp) in CH2Cl2 was well fitted to a single-exponential decay at
room temperature and measured as 1.7 μs, which is shorter
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Electroluminescent Devices. As a first attempt, we
introduce the SPPs into OLEDs as a phosphorescent emitter
(ITO/PEDOT:PSS/SPPs(70 wt %) + PBD(30 wt %)/CsF/
Al). Poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate)
(PEDOT:PSS) was used as a hole-injection layer. The electron-
transporting/hole-blocking material 2-(4-biphenyl)-5-(4-tert-
butylphenyl)-1,3,4-oxadiazole (PBD) was blended with the
SPPs in order to balance electron/hole injection in the emitting
layer. A vacuum-deposited cesium fluoride (CsF) layer was
used as an electron-injection layer.
The EL spectra of the SPPs are identical with a maximal
emission peak at 562 nm and a shoulder at 605 nm, arising
from the iridium monomer (Figure 5). Relative to PL spectra,
the EL emission from monomers 2 and 3 is completely
quenched with the iridium content as low as 5 mol % in the
supramolecular polymer. The substantial difference of energy
transfer efficiencies between PL and EL spectra of the SPPs
implies that different mechanisms are involved.22 Under
photoexcitation, the singlet excited states are created on
monomers 2 and 3 and then transferred to the iridium
monomer by Forster energy transfer.23 In contrast, under
̈
electrical excitation, charge trapping is considered as dominant,
since the iridium monomer is a more efficient hole trap, as
indicated by the electrochemical experiments.24 Consequently,
holes and electrons are likely to recombine directly on the
iridium unit, leading to pure electrophosphorescent emission.
Moreover, the highly efficient PHOLEDs were almost all
obtained when the guest is doped into the host materials with a
low concentration (usually lower than 10 wt %), because of the
concentration quenching and triplet−triplet (T-T) annihilation
of the guest, which leads to lower efficiency when the
concentration of the guest is too high.25 In contrast, it was
found that the EL performances of the SPPs seem to improve
as the iridium complex content in the active layer blend
increases from ca. 4 wt % to 39 wt % (see Figure 6 and Table
3). For instance, the device that contained SPP50 showed a
lowest turn-on voltage of 6.6 V (defined at a luminance of 1 cd
m−2) and a highest luminous efficiency (LE) of 14.6 cd A−1 at a
luminance of 450 cd m−2, corresponding to an external
efficiency of 6.9%. At a current density of 20 mA cm−2, LE
remained at 10.8 cd A−1. This observation further suggests that
the incorporated “iridium monomer” in the SPPs efficiently
traps hole charges and thereby serves as hole carriers. It is also
worthy of note that the present supramolecular polymer
structure considerably shields the iridium luminescent centers
from concentration quenching.
Figure 6. (a) Current density (J) and brightness (L) versus voltage
(V) characteristics (J−L−V) and (b) luminous efficiency−current
density−luminance (LE−J−L) characteristics of SPP5, SPP10, SPP25,
and SPP50 in the devices with configuration of ITO/PEDOT:PSS/
EMLs/CsF/Al).
of irreversible reduction potential located at −1.69 V, while the
reduction occurs primarily on the ligands. On the basis of the
redox data and the formulas of EHOMO = −(Eox + 4.4) eV,
ELUMO = −(Ered + 4.4) eV, and ELUMO = EHOMO + Eg,21 the
EHOMO and ELUMO values of monomer 1 are calculated as −5.25
and −2.71 eV, respectively. The reversible oxidation waves of 2
and 3 were observed at 1.4 and 1.16 V, respectively,
corresponding to EHOMO values of −5.8 and −5.56 eV. Based
on their absorption onset, ELUMO = −2.65 eV for 2 and −2.52
eV for 3. Thus, the iridium monomer shows a substantially
higher HOMO level by 0.3−0.55 eV than monomers 2 and 3,
implying that the iridium unit in the SPPs could be a strong
hole trap in OLEDs.
CONCLUSIONS
■
In summary, we have developed the first supramolecular
phosphorescent polymers (SPPs) for organic light-emitting
diodes. These SPPs are formed through efficient nonbonding
interactions between designed small-molecule monomers. It
Table 3. EL Performances of SPP5, SPP10, SPP25, and SPP50
turn-on voltage,
maximum external quantum
maximum luminous efficiency, maximum brightness, Lmax
c
a
b
SPP
dopant (wt %)
λEL (nm)
Vturn‑on (V)
efficiency, EQE (%)
LEmax (cd A−1
)
(cd m−2
)
SPP5
4
8
563, 608
563, 608
563, 608
563, 608
7.8
7.7
7.7
6.6
5.4
6.1
6.5
6.9
10.7
11.7
12.3
14.6
936 (270)
967 (187)
1556 (330)
2863 (450)
SPP10
SPP25
SPP50
20
39
a
b
c
Weight ratio of monomer 1 in the active layer blend. Turn-on voltage at a brightness of 1 cd m−2. Data in parentheses were recorded at LEmax
.
1018
dx.doi.org/10.1021/cm400333c | Chem. Mater. 2013, 25, 1013−1019

Chemistry of Materials
Article
(d) Yasuda, T.; Yamaguchi, I.; Yamamoto, T. Adv. Mater. 2003, 15,
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was shown that incorporation of an iridium monomer can
afford promising electroluminescent (EL) properties. Further-
more, the high photoluminescence (PL) and EL efficiencies of
the SPP with 50 mol % contents indicate that this type of self-
assembly process may afford a new class of high-performance
solution-processable EL emitters.
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AUTHOR INFORMATION
Corresponding Author
*E-mail addresses: msfhuang@scut.edu.cn (F.H.), xuhuizhu@
scut.edu.cn (X.-H.Z.).
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was financially supported by the Ministry of Science
and Technology (Nos. 2009CB623601, 2009CB930604, and
2011AA03A110), the Natural Science Foundation of China
(Nos. 21125419, 50990065, 51010003, and 51073058) and
Guangdong Natural Science Foundation (Grant No.
S2012030006232).
(11) Bettington, S.; Tavasli, M.; Bryce, M. R.; Beeby, A.; Al-Attar, H.;
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