Multifunctional metallophosphors with anti-triplet-triplet annihilation properties for solution-processable electroluminescent devices

Article abstract of DOI:10.1039/b718653c

With the goal to provide organometallic triplet emitters with good hole-injection/hole-transporting properties, highly amorphous character for simple solution-processed organic light-emitting diodes, and negligible triplet-triplet (T-T) annihilation, a series of new phosphorescent cyclometalated Ir^III and Pt^II complexes with triphenylamine-anchored fluorenylpyridine dendritic ligands were synthesized and characterized. The photophysical, thermal, electrochemical and electroluminescent properties of these molecules are reported. The incorporation of two sterically hindered electron-rich triphenylamino groups to the 9-position of the fluorene skeleton was found not only to afford triplet emitters in the glassy state with high T_g, but also to elevate the HOMO levels and confer the hole-injection ability to the phosphorescent center. These highly amorphous metal phosphors can serve as doped emitters in a small molecular host for spin-coated emission layer in suitable OLED structures to achieve good device performance with a maximum luminance of 29380 cd m ^-2 at 23 V, a peak external quantum efficiency of 7.0%, a luminance efficiency of 21.4 cd A^-1 and a power efficiency of 2.9 lm W ^-1. Both the electrophosphorescent device characterization as well as the theoretical simulation results show that these iridium electrophosphors show negligible T-T annihilation even at high operating current densities and moderately high doping levels. Our investigations indicate that attaching the triphenylamino moieties to the fluorene ring is an effective way to overcome the T-T annihilation caused by the strong interactions among the emitting molecules. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b718653c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Multifunctional metallophosphors with anti-triplet–triplet annihilation
properties for solution-processable electroluminescent devices†
a
a
b
b
Gui-Jiang Zhou, Wai-Yeung Wong, Bing Yao, Zhiyuan Xie and Lixiang Wangb
*
*
Received 3rd December 2007, Accepted 14th February 2008
First published as an Advance Article on the web 3rd March 2008
DOI: 10.1039/b718653c
With the goal to provide organometallic triplet emitters with good hole-injection/hole-transporting
properties, highly amorphous character for simple solution-processed organic light-emitting diodes,
and negligible triplet–triplet (T–T) annihilation, a series of new phosphorescent cyclometalated IrIII
and PtII complexes with triphenylamine-anchored fluorenylpyridine dendritic ligands were
synthesized and characterized. The photophysical, thermal, electrochemical and electroluminescent
properties of these molecules are reported. The incorporation of two sterically hindered electron-rich
triphenylamino groups to the 9-position of the fluorene skeleton was found not only to afford
triplet emitters in the glassy state with high Tg, but also to elevate the HOMO levels and confer the
hole-injection ability to the phosphorescent center. These highly amorphous metal phosphors can serve
as doped emitters in a small molecular host for spin-coated emission layer in suitable OLED structures
to achieve good device performance with a maximum luminance of 29380 cd mꢀ2 at 23 V, a peak
external quantum efficiency of 7.0%, a luminance efficiency of 21.4 cd Aꢀ1 and a power efficiency of
2.9 lm Wꢀ1. Both the electrophosphorescent device characterization as well as the theoretical
simulation results show that these iridium electrophosphors show negligible T–T annihilation even at
high operating current densities and moderately high doping levels. Our investigations indicate that
attaching the triphenylamino moieties to the fluorene ring is an effective way to overcome the
T–T annihilation caused by the strong interactions among the emitting molecules.
together with the favorable triplet energy level alignment between
Introduction
the host material and dopant bring about a remarkable enhance-
ment in efficiency. Furthermore, the undesirable triplet–triplet
(T–T) annihilation can be remarkably inhibited by dispersing
emitter molecules into the host matrix. Unfortunately, this
strategy becomes unworkable at high current density as indicated
by an obvious decrease in device efficiency with increasing current
density, and such a phenomenon is very common in state-of-the-
art electrophosphorescent OLEDs.5 So, there is still much scope
in the rational design and synthesis of new complexes with special
ligands to overcome or at least largely minimize this problem.
In order to simplify the fabrication process of OLEDs and
prepare cheaper large-area displays, much research effort has
been devoted to develop OLEDs with the emitting layer prepared
by solution-processing techniques.6 In such method, the emitters
are very often mixed with a host polymer solution instead of CBP
solution because of their poor film-forming properties in the latter
host. However, the performance is usually poor for the devices
prepared by this way. While the IrIII triplet emitters anchored
with dendritic ligands represent another promising class of
metallophosphors in the advancement of OLED research,7 the
relative synthetic complication for dendrimers still urges
researchers to find alternatives for high-performance solution-
processed devices using simple molecular materials.
The pioneering success of the cyclometalated IrIII and PtII
complexes with 2-phenylpyridine (ppy) type ligands in the field
of organic light-emitting diodes (OLEDs) has triggered
substantial research attention in the design, synthesis and
electroluminescence (EL) characterization of these classes of
luminescent complexes.1 The high efficiencies of the OLEDs
derived from these complexes and their derivatives are ascribed
to the energy harvesting arising from both the singlet and triplet
excitons in the radiative emission process.2 The ease of emission
wavelength tunability over the entire visible spectrum, especially
for the IrIII phosphors, via modifying the cyclometalated ligand
structure will render them excellent emitters for efficient, full-
color displays.3 All of these unique features make heavy metal
phosphors one of the most attractive candidates for current
OLED research. In order to optimize EL efficiencies, these metal
complexes are usually doped into a layer of charge-transporting
host materials such as 4,40-N,N0-dicarbazole-biphenyl (CBP).4
By following this protocol, the ambipolar carrier transport
aDepartment of Chemistry and Centre for Advanced Luminescence
Materials, Hong Kong Baptist University, Waterloo Road, Kowloon
Tong, Hong Kong, P.R. China. E-mail: rwywong@hkbu.edu.hk; Fax:
+852-3411-7348; Tel: +852-3411-7074
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, P.R. China
In general, phosphorescent materials have some intrinsic
disadvantages, such as saturation of emissive states due to an
excessively long lifetime as well as T–T annihilation and concen-
tration quenching arising from strong bimolecular interactions
at high doping levels. However, approaches to reduce the
† Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b718653c
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J. Mater. Chem., 2008, 18, 1799–1809 | 1799

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efficiency roll-off are still rather limited. Therefore, it is highly
critical that the designed metal phosphors can avoid all or
most of these undesirable effects. Recently, it was found that
fluorene-based cyclometalating ligands substituted with hole-
transporting diarylamino units can afford phosphorescent heavy
metal complexes of IrIII and PtII for highly efficient OLEDs and
they can also block effectively the strong intermolecular inter-
actions for the cyclometalated Pt(b-diketonato) complexes.4c,g
In this contribution, we have designed and synthesized new
triplet emitters bearing fluorene-based derivatives with two
sterically bulky triphenylamino (TPA) moieties8 at the 9-position
of the fluorene ring. Not only will the TPA units with a tortured
geometry render our complexes highly amorphous to be useful in
making solution-processable OLEDs, but also these bulky
substituents hinder the strong molecular interactions among
the emitter molecules and thereby alleviate the T–T annihilation
problem at high driving currents. Furthermore, the low ioniza-
tion potential of TPA will also facilitate the hole-injection/
hole-transport (HI/HT) in OLEDs, which has aroused much
concern for emitters with large HI barriers. The promising EL
results presented herein have demonstrated the potential of our
complexes in facilitating HI, weakening the T–T annihilation
effect and fabricating simple solution-processed OLEDs even
with the use of a small molecular host only.
2-bromofluorene (Scheme 1 and see ESI†). 2-Bromofluorenone
was obtained in high yield by oxidation of 2-bromofluorene in
pyridine in the presence of Bu4NOH under an air flow.9 The
n
introduction of TPA groups to the 9-position of fluorene was
easily achieved by the reaction of 2-bromofluorenone with a large
ꢁ
excess of triphenylamine in methanesulfonic acid at ca. 140 C
for 12 h.10 A large excess of the amine was employed for the
sake of preventing the simultaneous reaction of its free para-
positions with fluorenone to give a bridged species. As the key
synthon, the fluorene-based organic ligand HL1 was prepared
via the Stille coupling of 9,9-bis(4-diphenylaminophenyl)-2-
bromofluorene with 2-(tributylstannyl)pyridine, while HL2 was
obtained through a Suzuki-type coupling of its boronic acid
derivative with 1-chloroisoquinoline. The homoleptic IrIII
complexes Ir-1 and Ir-3 were obtained by direct thermal reaction
of [Ir(acac)3] with the corresponding ligands in refluxing glycerol.
The heteroleptic IrIII compound Ir-2 was synthesized in two steps
from the cyclometalation of IrCl3$nH2O with HL1 to form
initially the chloride-bridged dimer, followed by treatment with
acetylacetone in the presence of Na2CO3.11 The PtII complex
Pt-1 was synthesized using a similar protocol to that for Ir-2
but instead K2PtCl4 was employed as the reactant.12 Purification
of the crude complexes by silica chromatography gave the target
complexes as air-stable powders in high purity. The relatively
low yields of these metal complexes are presumably caused by
the limited solubility of the ligands HL1 and HL2 in the reaction
medium but the ligands can be easily recovered for making more
batches of metal complexes in the next run. All of the new metal
compounds were fully characterized by NMR spectroscopy
and mass spectrometry (by either matrix-assisted laser desorp-
tion ionization time-of-flight (MALDI-TOF) or fast-atom
bombardment (FAB) technique). However, all attempts to get
Results and discussion
Synthesis and characterization
The new ligands required for the preparation of new phospho-
rescent IrIII and PtII emitters were synthesized by the well-estab-
lished organic synthetic procedures from commercially available
Scheme 1 Synthesis of new iridium(III) and platinum(II) cyclometalated complexes.
1800 | J. Mater. Chem., 2008, 18, 1799–1809
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good-quality crystals for X-ray structural analyses have met with
little success presumably because of their poor crystallinity and
highly amorphous nature (vide infra).
cyclometalated IrIII and PtII complexes in the literature,4,12,13
the room-temperature phosphorescence should emanate
3
predominantly from a ligand-centered (LC) p–p* excited state
for Ir-1, Ir-2 and Pt-1 on the basis of the observed vibrational
fine structure and large Stokes shifts (>100 nm) of the PL
spectra. The vibronic progression of ca. 1166–1277 cmꢀ1 (n0–1
Photophysical and thermal properties
)
The UV–visible and photoluminescence (PL) spectra of the four
metallophosphors are displayed in Fig. 1 and the corresponding
data are shown in Table 1. There is a close resemblance of their
absorption behavior with those found in other related complexes
reported in the literature.4–6 The intense absorption band (log 3 ꢂ
4.9–5.3) at ca. 309 nm can be assigned to the spin-allowed 1p–p*
transitions associated with the organic ligands. While the
weaker, lower energy features can extend toward the visible
region (ca. 380–520 nm for Ir-1 and Ir-2, ca. 400–590 nm for
Ir-3, and 390–460 nm for Pt-1), they can be ascribed to the
in the emission profile of Ir-1, Ir-2 and Pt-1 corresponds to the
aromatic stretching of the cyclometalated ligands which is
diagnostic of the substantial involvement of the ligand-centered
p–p* transitions in the PL. We also anticipate that there is
a weakening of the spin–orbit coupling and hence an increasing
amount of the 3p–p* character in the lowest energy excited state
as the cyclometalating organic group becomes larger.6a,13b Such
vibrational fine structures plus the molecular size factor suggest
3
an insignificant contribution from the MLCT states for Ir-1,
Ir-2 and Pt-1. However, the MLCT states contribute greatly to
the deep red emission of Ir-3 in light of a broad and featureless
spectral pattern displayed in its PL spectrum. All of them are
also intensely phosphorescent in the glass matrices at 77 K.
The hypsochromic shifts observed at 77 K due to the rigido-
chromic effect are due to the solvent reorganization in a fluid
solution at low temperature that can stabilize the charge transfer
states prior to emission.13c This process is significantly impeded
in a rigid matrix at low temperature, giving phosphorescence
at a higher energy. At 77 K, the PL spectra for each of them
display more structured spectral profiles. All of the phosphors
show relatively high phosphorescent quantum yields (18–26%).
Furthermore, they possess short phosphorescent lifetimes,
especially for the IrIII complexes. The room temperature lifetimes
(tP) of the complexes are in the microsecond range (tP ¼ 1.3, 1.1,
1.0 and 8.2 ms for Ir-1, Ir-2, Ir-3 and Pt-1, respectively), which
together with the large Stokes shift of the PL band are indicative
of the phosphorescent origin for the excited states in each case.
Accordingly, the radiative lifetimes (tr) of the triplet state
deduced from tr ¼ tP/FP are as short as 4.7–5.3 ms for the Ir
phosphors. It is well documented that tP is a key factor to cause
T–T annihilation for the Ir dopant in the OLED operation.14 The
longer tP of the phosphorescent material usually leads to a more
significant T–T annihilation. The relatively short lifetimes of our
complexes, which play an important role in weakening the T–T
annihilation effect, can increase spin state mixing and reduce
the chance of device efficiency decay (vide infra). The tP values
at 77 K are longer than those at 293 K. In general, the emission
lifetimes of the complexes are determined by nonradiative
decay rates which usually decrease with lowering temperature,
1
3
3
excitation of MLCT, MLCT and p–p* states, indicative of
the strong spin–orbital coupling between the singlet and triplet
manifolds. All of our complexes exhibit strong triplet emissive
bands at 293 K (lem in CH2Cl2 ¼ 544, 551, 649 and 540 nm
for Ir-1, Ir-2, Ir-3 and Pt-1, respectively). The slightly red-shifted
emission observed for Ir-2 relative to Ir-1 may be associated with
the comparatively higher ligand field strength of the acac anion.
The addition of two TPA groups does not seem to alter the
emission of the complexes much relative to their non-TPA
coordinated congeners, as would be expected due to the
conjugation-interrupting effect of the sp3-hybridized carbon
atom.4c,g,h With reference to the spectroscopic data for other
Fig. 1 Absorption and PL spectra of the complexes in CH2Cl2 at 293 K.
Table 1 Photophysical and thermal properties of the complexes
e
Complex
Ir-1
labs (nm)a (293 K)
lem (nm)a (293 K/77 K)
544, 583/541, 586
551, 589/544, 590
649/641, 695
VP (%)b
25.6
tp (ms)b (293 K/77 K)
1.3/5.5
DT5%/Tg (ꢁC)c
476/225
tr (ms)d
5.1
kr/knr (105 sꢀ1
1.97/5.72
)
309 (5.30), 390
(4.14), 475 (3.49)
309 (5.12), 395
(3.72), 483 (3.08)
309 (5.22), 375 (4.51),
455 (4.15), 559 (3.47)
309 (4.88), 364 (3.95),
406 (3.71), 422 (3.71)
Ir-2
20.9
1.1/6.3
370/205
5.3
1.90/7.19
Ir-3
21.1
1.0/2.9
454/250
4.7
2.11/7.89
Pt-1
540, 580/539, 584, 625
18.3
8.2/12.7
321/183
44.8
0.22/1.00
a
Measured in CH2Cl2. Loge values are shown in parentheses. b Measured in degassed toluene. c DT5% ¼ 5% weight-reduction temperature. Tg ¼ glass
d
e
transition temperature. The radiative lifetimes tr are deduced from tP/FP. kr ¼ FP/tP and knr ¼ (1 ꢀ FP)/tP.
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justifying a lengthening of the lifetime at lower temperatures.
Assuming that the emitting state of a heavy metal phosphor is
formed with unit efficiency, one can calculate the radiative (kr)
and nonradiative (knr) rate constants using the relationships FP
¼ FISC{kr/(kr + knr)} and tP ¼ (kr + knr)ꢀ1 (FISC ¼ 1).15 The
calculated radiative and nonradiative rate constants are listed
in Table 1. The three Ir complexes have similar kr values
(1.9–2.1 ꢃ 105 sꢀ1) and knr has the same order of magnitude as
kr. However, because of the longer tP, the kr and knr values for
Pt-1 are smaller than those for the Ir congeners.
help in facilitating the HI properties, rendering them versatile
bifunctional phosphors. The second oxidation process in each
case can be assigned to the oxidation of the metal center. For
the Ir complexes, the second oxidation potential is located at
ca. 0.39–0.42 V, which correlates well with the reported anodic
wave of the IrIII core in other similar complexes.4i The second
oxidation wave for Pt-1 appears at a much higher potential
(ca. 0.49 V), which is consistent with the fact that PtII is generally
electrochemically more stable than the IrIII analogue.4g
The thermal properties of the complexes were characterized by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). The thermal stability data reveal good
stability of the complexes and the 5% weight-reduction tempera-
tures (DT5%) of these complexes are all higher than 320 ꢁC.
Thermally induced phase transition behavior of the molecules
was also investigated with the DSC technique, which indicated
no crystallization and melting processes but only a glass transi-
tion. All of them showed a very high Tg value in excess of 180
ꢁC and thus they exist as highly amorphous solids and are
resistant to crystallization. While the parent unsubstituted
complex [Ir(Flpy)3] (H(Flpy) ¼ 2-(9,9-diethylfluoren-2-yl)pyri-
Electrophosphorescent OLED characterization
In order to test the electrophosphorescent ability and light-emit-
ting performance of our complexes, doped OLEDs using these
triplet emitters were prepared at different doping levels. The
emitting layers for all of the devices were prepared by using
the relatively cheaper solution-processed spin-coating technique.
It should be noted that CBP itself normally cannot be spin-
coated from solution to form good-quality thin films. When
the dopant concentration is very low (e.g. <3 wt.-%), the CBP
host is prone to crystallization in the as-prepared films and the
devices short easily. This illustrates that the good film-forming
properties of our bulky metal phosphors can ensure a good
chemical compatibility with the CBP host, resulting in a homo-
geneous distribution of each Ir dopant in CBP. The OLEDs
were fabricated in the configuration of ITO/PEDOT : PSS/x%
dopant : CBP/BCP/Alq3/LiF/Al (Fig. 2). The composite polymer
solution of PEDOT : PSS (poly(3,4-ethylene-dioxythiophene)-
poly(styrene sulfonate)) was first spin-coated on the ITO surface
as the HI layer but the conventional hole-transporting NPB layer
was not necessary in fabricating our devices. This can, to a certain
extent, corroborate with the improved HI/HT properties of our
materials from the PEDOT : PSS layer. Then the emitting layer
composed of the triplet dopant and CBP host was deposited.
Owing to the highly amorphous character of the dopant, the
inclination for crystallization of CBP was significantly hindered
and good-quality emission layer was obtained. 2,9-Dimethyl-
4,7-diphenyl-1,10-phenanthroline (BCP) is used as the hole-
blocking material and tris(8-hydroxyquinoline) (Alq3) as the
electron-transporter. LiF acts as the electron-injection (EI) layer
and aluminium as the cathode. The key performance data of the
devices are listed in Table 3 and Fig. 3 shows the corresponding
EL spectra of the best-performing devices for each dopant
(namely, devices D, G, I and O). Here, the EL performance is still
impressive even for solution-processed devices. In each case, no
obvious emission from the CBP host was observed, indicating
an efficient energy transfer from CBP to the triplet emitters as
has been largely described and explained in the literature.1f,4e
The EL spectra closely match their corresponding PL peaks.
Even at the driving voltage of 16 V, the maximum EL peak for
all of the devices is independent of the dopant concentration
(see ESI†). The relatively high turn-on voltages of the devices
may be due to the thicker emissive layer of CBP:Ir dopant used
and the large hole barrier at the PEDOT:PSS/CBP interface
(HOMOs of CBP ꢂ 6.2 eV4f versus PEDOT ꢂ5.0 eV). The
OLEDs with the Pt-1 dopant show an obvious advantage in
anti-aggregation as compared to those reported for cyclo-
metalated Pt(b-diketonato) complexes with 2-phenylpyridine-
type ligands17 which are typically characterized by strong
dine) only possesses a Tg of 118 C,4c,d,g our results demonstrate
ꢁ
the pivotal role of TPA moieties in improving the amorphous
nature of the phosphor molecules, leading to high-Tg materials.
This would afford new complexes with improved compatibility
between the phosphorescent dopant and the small-molecule
organic host that would be useful for highly efficient electrophos-
phorescent OLEDs. Although the new phosphors are not easily
vacuum-sublimable presumably because of their large molecular
weights, these amorphous materials are very good candidates for
use in solution-processed OLEDs.
Electrochemical and electronic properties
The electrochemical properties of the complexes were characte-
rized by cyclic voltammetry (Table 2). Ir-1, Ir-2 and Pt-1 show
reduction waves between ꢀ2.42 and ꢀ2.97 V, which should be
assigned to the reduction of the pyridyl moiety.6f Ir-3 shows
a less negative reduction of ꢀ2.32 V, which is consistent with
the reduction potential of the isoquinolinyl group. The first
oxidation wave for all of them appears in the narrow range of
0.29–0.35 V, attributable to the oxidation of the electron-rich
TPA group in the organic ligands because the anodic potential
is close to the ionization potential of triphenylamine.4c,11,16 These
results suggest that the emitters are better hole acceptors and can
Table 2 The redox properties and frontier orbital energy levels of the
triplet emitters
Complex E1/2ox/Va E1/2red/Va
HOMO/eV LUMO/eV T1/eVc
Ir-1
Ir-2
Ir-3
Pt-1
0.35, 0.42 ꢀ2.60b, ꢀ2.96 ꢀ5.15
ꢀ2.20
ꢀ2.30
ꢀ2.48
ꢀ2.38
2.29
2.28
1.93
2.30
0.34, 0.42 ꢀ2.50b, ꢀ2.97 ꢀ5.14
0.29, 0.39 ꢀ2.32
ꢀ5.09
0.35, 0.49 ꢀ2.42
ꢀ5.15
a
0.1 M [Bu4N]PF6 in THF, scan rate 100 mV sꢀ1, vs. Fc/Fc+ couple.
b
Irreversible wave. c Determined from the PL spectrum in frozen toluene
at 77 K.
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Fig. 2 The OLED device configuration and the molecular structures of the relevant compounds used in these devices.
Table 3 Device performance of the electrophosphorescent OLEDs
Device
Phosphor dopant
Vturn-on/V
Lmax /cd mꢀ2
hext (%)a
lmaxb/nm
a
a
hL /cd Aꢀ1
a
hp /lm Wꢀ1
A
B
C
D
E
F
G
H
I
Ir-1 (3 wt.-%)
Ir-1 (6 wt.-%)
Ir-1 (9 wt.-%)
Ir-1 (12 wt.-%)
Ir-2 (3 wt.-%)
Ir-2 (6 wt.-%)
Ir-2 (9 wt.-%)
Ir-2 (12 wt.-%)
Ir-3 (3 wt.-%)
Ir-3 (6 wt.-%)
Ir-3 (9 wt.-%)
Ir-3 (12 wt.-%)
Pt-1 (3 wt.-%)
Pt-1 (6 wt.-%)
Pt-1 (10 wt.-%)
Pt-1 (15 wt.-%)
11.0
11.0
11.0
11.0
10.0
10.5
10.6
10.6
11.0
11.0
11.0
11.0
8.0
26160 (23)
15540 (23)
15330 (24)
25660 (24)
26180 (21)
29380 (23)
20100 (25)
26960 (27)
6471 (21)
4669 (21)
4512 (23)
3898 (23)
10810 (18)
10070 (18)
16070 (17)
12490 (18)
13.93 (21)
16.90 (23)
14.88 (24)
15.07 (22)
18.38 (21)
21.40 (23)
20.42 (23)
17.72 (16)
1.69 (20)
1.53 (21)
1.13 (23)
1.17 (22)
7.79 (14)
8.31 (11)
9.55 (11)
8.93 (13)
4.34 (21)
5.26 (23)
4.60 (24)
4.63 (22)
6.02 (21)
7.04 (23)
6.43 (23)
5.54 (16)
4.23 (20)
4.00 (21)
3.03 (23)
3.20 (22)
1.68 (14)
2.88 (14)
3.36 (14)
3.42 (14)
2.08 (21)
2.31 (23)
1.95 (24)
2.36 (17)
2.75 (21)
2.92 (23)
3.05 (23)
3.64 (15)
0.27 (20)
0.23 (21)
0.15 (23)
0.17 (22)
1.75 (14)
2.13 (14)
2.31 (14)
2.10 (14)
544 (0.45, 0.54)
544 (0.45, 0.54)
544 (0.45, 0.54)
544 (0.45, 0.54)
552 (0.48, 0.52)
552 (0.48, 0.52)
552 (0.47, 0.52)
552 (0.47, 0.52)
648 (0.70, 0.30)
648 (0.70, 0.30)
648 (0.70, 0.30)
648 (0.70, 0.29)
540 (0.46, 0.53)
540 (0.46, 0.53)
540 (0.47, 0.52)
540 (0.48, 0.51)
J
K
L
M
N
O
P
9.0
8.0
8.0
a
b
Maximum values of the devices. Values in parentheses are the voltages at which they were obtained. CIE coordinates [x, y] in parentheses.
aggregative emissions in their EL spectra even at low doping
concentrations. For Pt-1, the aggregation effect is much less
serious in its EL spectrum even as the dopant concentration
increases up to 15 wt.-% (see ESI†). Our results reveal very
weak bimolecular interactions in the presence of two substituted
TPA moieties on the ligand that efficiently block close approach
among the emitting molecules. In other words, the sterically
hindered TPA moieties in the phosphor molecule can consider-
ably reduce concentration-quenching, akin to the previous results
using a pinene group as the spacer in the framework of ppy.14b
Because the TPA groups in these complexes can play a crucial
role in promoting hole transport and reducing self-quenching,
the EL performance is still impressive even for solution-processed
Fig. 3 EL spectra of the devices D, G, I and O.
devices (Table 3). Essentially, devices A–D with the Ir-1 dopant
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show a strong yellow EL peak at about 544 nm and the Commis-
sion Internationale de L’Eclairage (CIE) color coordinates of
(0.45, 0.54). Devices E–H containing Ir-2 are characterized by
a prominent yellow-orange EL emission at 552 nm and the CIE
color coordinates at (0.47, 0.52). Devices I–L doped with Ir-3
emit a deep-red light with the maximum peak at ca. 648 nm and
the CIE coordinates at (0.70, 0.30) in the pure red region. The
devices M–P prepared from Pt-1 display a main EL peak at 540
nm, accompanied by shoulder peaks at ca. 580 and 626 nm.
Devices M–P turned on at 8.0–9.0 V and give the CIE color
coordinates at around (0.47, 0.52). Fig. 4 presents current density
(J) and luminance (L) versus bias voltage (V) (J–V–L) curves of
the best devices D, G, I and O in each category of dopant (see
ESI† for other devices). Generally, the luminance increases with
increasing driving voltage for the devices since a larger amount
of carriers were injected into the devices at a higher driving
voltage. Device D achieves the highest luminance (Lmax) of
25660 cd mꢀ2 at 24 V, a maximum external quantum efficiency
(hext) of 4.63%, a luminance efficiency (hL) of 15.07 cd Aꢀ1 and
a power efficiency (hp) of 2.36 lm Wꢀ1. Device G furnishes Lmax
of 20100 cd mꢀ2 at 25 V, peak hext of 6.43%, hL of 20.42 cd Aꢀ1
and hp of 3.05 lm Wꢀ1. The pure red emitting device I can attain
Lmax of 6471 cd mꢀ2 at 21 V, peak hext of 4.23%, hL of 1.69 cd Aꢀ1
and hp of 0.27 lm Wꢀ1. The lower luminosity for devices I–L is not
surprising and should be due to the characteristic pure red
emitting nature of Ir-3 in a spectral region where the eye has
poor sensitivity. Gratifyingly, the CIE color of (0.70, 0.30) for
I–L corresponds to the true red region and is close to the National
Television Standards Committee recommended red (standard red
specification) at x ¼ 0.670 and y ¼ 0.330 for a video display. The
performance of device I is not far from those devices made from
the first-generation red-emitting Ir dendrimers with meta-bonded
phenylene blocks in terms of EL efficiency (5.70 and 4.25%) but
its color purity ((0.70, 0.30) for device I) appears better than the
latter known dendrimers ((0.64, 0.36) and (0.67, 0.30)).18 Device
O prepared with Pt-1 also shows an encouraging performance
with Lmax of 16070 cd mꢀ2 at 17 V, maximum hext of 3.36%, hL
of 9.55 cd Aꢀ1 and hp of 2.31 lm Wꢀ1. The electrophosphorescence
efficiencies of our yellow or orange devices A–H are some of the
best reported for solution-processed devices and do not differ
much from those of the evaporated OLEDs obtained from the
vacuum-sublimable Ir dopants.4g,h Our results show a great
potential of the dendritic complexes as emitters for robust
solution-processed OLEDs. The TPA units may also be acting
as traps for the holes, thus also lowering the PL and EL efficiency
relative to the unsubstituted counterpart.11 Further tuning the
oxidation potential of the triarylamine by substitution should
enable us to optimize the EL efficiency by minimizing the amount
of traps, and by a better match of the phosphor oxidation level to
the HOMO of PEDOT : PSS or the work function of ITO.
The hext versus J curves for all of the devices are depicted in
Fig. 5 (also see ESI† for the hL and hp curves). Excitingly, hext
generally increases with increasing J values within the usual
operating range ($100 mA cmꢀ2) in many cases, especially for
the devices doped with the Ir emitters, and this suggests that
saturation of the phosphorescence sites is not severe. Even at
the higher doping level, the roll-off of hext is only gentle with
increasing J. These interesting results are in contrast to most
of the literature data, in which the decrease in hext with increasing
J usually takes place rapidly due to the severe T–T annihilation
Fig. 4 J–V–L characteristics of the electrophosphorescent OLED devices D, G, I and O.
1804 | J. Mater. Chem., 2008, 18, 1799–1809
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Fig. 5 External quantum efficiency as a function of current density for all of the OLEDs studied.
caused by the strong bimolecular quenching effect.19 Further-
more, it is typical of the electrophosphoresence in OLEDs to
show peak efficiency at low J (<0.1 mA cmꢀ2).20 Instead, the J
values corresponding to peak hext for devices D, G, I and O
appear higher at 52.1, 37.7, 230.9 and 33.9 mA cmꢀ2, respec-
tively. These results show the T–T annihilation effect to be
much relieved by a careful design of the metal phosphor
structure. Negligible or only very weak aggregate interaction is
present because the sterically demanding TPA units prevent
the dopant phosphorescent molecules from approaching
sufficiently to allow close contacts, which is similar to the other
reports by the attachment of bulky aryl side chains at the
9-position of the fluorene.21 The work can certainly provide
a useful alternative to solve the T–T annihilation problem
commonly encountered in OLEDs. It should be noted that the
decrease of hext at a lower J for device A (3 wt.-%) should be
attributed to the saturation of the emissive sites but not the
T–T annihilation since the latter event did not occur even in
devices B and C with higher doping levels within a similar
J range. The reason for the observed T–T annihilation effect in
devices M–P with Pt-1 is probably due to its longer tP (8.2 ms
at 293 K), which increases the chance for triplet–exciton
quenching.
thus resulting in a loss of an exciton. Thus, such a process
depends on the concentration of excitons; excitons will meet
more frequently at high concentrations, leading to faster decay.
Based on the assumption that only the dopant triplet excitons
participate in the T–T annihilation process and the fact that
there is strong spin–orbit coupling in the dopant molecules
induced by the heavy-atom perturbation of Ir and Pt, the depen-
dence of hext on J can be expressed by the following equation:19a
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ
ꢁ
hext J0
¼
J
1 þ 8 ꢀ 1
J0
h0
4J
4qd
J0 ¼
2
kTTtP
Here, h0 is the quantum efficiency in the absence of T–T
annihilation, q is the electron charge, d is the thickness of the
exciton formation zone, and tP is the phosphorescent recombina-
tion lifetime. Based on the theoretical fitting of the experimental
hext data using the equation above (Fig. 6), we can calculate the
numeric value of J0, which is the onset current density at hext
¼
h0/2 where biexcitonic triplet interactions (T–T annihilation)
become significant. This parameter can be employed to quantify
the relative merits of different triplet emitters in OLEDs. The
simulation curves for the devices with high doping levels are
shown in Fig. 6 (for devices D, H and P) and in the ESI†.
From the curves, it can be seen that the experimental data fit
Exciton–exciton annihilation is a crucial issue in phosphores-
cent materials because of the longer exciton lifetime, and it leads
to a decrease of EL efficiency at high current densities.19 It occurs
when two excitons meet and form a single higher excited state,
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Fig. 6 Experimental hext versus current density data fitted by a theoretical equation involving T–T annihilation for devices (a) D, (b) H and (c) P.
well the theoretical prediction, indicating the validity of the
T–T annihilation theory in our cases. The values of J0 for devices
D, H and P obtained by the theoretical fitting are 3856 ꢄ 140,
2534 ꢄ 176 and 360 ꢄ 85 mA cmꢀ2, respectively. These results
reveal that our Ir complexes do not suffer from severe T–T
annihilation problems and the detrimental intermolecular
interactions between the emissive cores are largely controlled
with such a well-designed dendritic skeleton. This can also be
supported from the neat film PL spectra of the Ir metallo-
phosphors, in which there is a negligible shift in the emission
wavelength in each case from the corresponding solution PL
spectrum and we also observe no emission from the long-wave-
length emitting aggregate states, while the PL emission from
aggregate states can be seen clearly from the neat film of a similar
Ir complex without the dendritic structure (see ESI†). Further-
more, even for Pt-1 with a longer lifetime (ca. 8.2 ms) at a high
doping level, the T–T annihilation only plays its role at a much
higher J0 (360 ꢄ 85 mA cmꢀ2) than that of [Ir(ppy)3] possessing
a much shorter lifetime of ca. 0.5 ms (J0 ꢂ 100 mA cmꢀ2) and
2,3,7,8,12,13,17,18-octaethylporphine platinum (PtOEP) with
a longer lifetime of ca. 30 ms (J0 ꢂ 1 mA cmꢀ2).19a The more
pronounced T–T annihilation for Ir(ppy)3 has been discussed
in terms of the clustering of the phosphor in the CBP blend.22
So the significance of introducing bulky TPA moieties on a
fluorene-based ligand to solve the T–T annihilation problem is
well supported by theoretical data. Furthermore, even other Ir
complexes with much more bulky dendritic ligands cannot fulfil
such behavior within all operational J values.7,18 All of the above
results agree with the strong anti-T–T annihilation ability of
these novel triplet emitters. Encouragingly, the molecular design
strategy here opens a new avenue for designing organometallic
triplet emitters with minimal T–T annihilation that usually
depend strongly on the molecular size factor.
high doping levels. Inclusion of the pendant hole-trapping
TPA moieties in these phosphors has been shown to offer
advantages in terms of improving the hole-injection as well as
suppressing the crystallinity and aggregation of the materials
as compared to the neat 2-pyridinylfluorene congener, which
makes them good multi-component phosphorescent materials.
This enables efficient stable OLEDs to be obtained without the
need for a hole-transporting layer. In addition, the T–T annihi-
lation behavior in our devices fits well to the well-established
theory in the field. The high limiting current density (J0), above
which T–T annihilation would dominate, also confirms the
successful exploitation of our approaches to relieve a bottle-
neck problem in phosphorescent OLEDs, i.e. to reduce the
efficiency roll-off due to T–T annihilation by weakening the
strong interactions among the triplet emitters.
Experimental
General information
All reactions were performed under a nitrogen atmosphere.
Solvents were carefully dried and distilled from appropriate
drying agents prior to use. Commercially available reagents
were used without further purification unless otherwise stated.
All reactions were monitored by thin-layer chromatography
(TLC) with Merck pre-coated glass plates. Flash column
chromatography and preparative TLC were carried out using
silica gel from Merck (230–400 mesh). Fast atom bombardment
(FAB) mass spectra were recorded on a Finnigan MAT SSQ710
spectrometer and MALDI-TOF mass spectra were obtained on
a Autoflex Bruker MALDI-TOF system. Proton NMR spectra
were measured in CDCl3 on a Varian Inova 400 MHz or
a JEOL EX270 FT-NMR spectrometer; chemical shifts were
1
quoted relative to tetramethylsilane for H and 13C{1H} NMR
data.
Conclusions
Physical measurements
A new series of cyclometalated IrIII and PtII triplet emitters
decorated with triphenylamine-substituted fluorene have been
successfully synthesized. By meticulous selection of the cyclo-
metalating ligand, these new complexes not only provide outlets
to dendronized triplet emitters (with no aggregate/excimer
emission for the Ir derivatives) for high-performance solution-
processed OLEDs, but also present new examples of metallo-
phosphors with negligible or reduced T–T annihilation even at
UV-visible spectra were obtained on a HP-8453 spectrophoto-
meter. The photoluminescent properties of the compounds
were probed on a Photon Technology International (PTI)
Fluorescence Master Series QM1 system. The phosphorescence
quantum yields were determined in toluene solutions at 293 K
against fac-[Ir(ppy)3] standard (FP ¼ 0.40).23 Phosphorescence
lifetimes were measured in degassed toluene with a Lecroy
1806 | J. Mater. Chem., 2008, 18, 1799–1809
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Wave Runner 6100 digital oscilloscope using the third harmonic
of a Nd : YAG laser (l ¼ 355 nm, pulse width ¼ 8 ns) as the
excitation source (Spectra-Physics Quantum-Ray). The decay
curves were analyzed using a Marquardt-based nonlinear least-
squares fitting routine and were shown to follow a single-
room temperature and water (50 mL) was added. An orange
precipitate was collected, dried and then purified with TLC
plates using CH2Cl2–hexane (3 : 1, v/v) as eluent. The target
product was obtained as an orange solid (31 mg, 15%). 1H
NMR (270 MHz, CDCl3): d (ppm) 8.56 (d, J ¼ 5.7 Hz, 2 H,
Ar), 7.83 (d, J ¼ 8.4 Hz, 2 H, Ar), 7.73 (t, J ¼ 7.0 Hz, 2 H,
Ar), 7.60 (s, 2 H, Ar), 7.29–6.64 (m, 68 H, Ar), 5.22 (s, 1 H,
acac), 1.80 (s, 6 H, acac). 13C NMR (68 MHz, CDCl3):
d (ppm) 184.50 (acac), 168.35, 151.99, 148.15, 147.63, 146.48,
145.62, 144.32, 144.11, 140.96, 140.74, 140.21, 139.85, 136.54,
128.94, 127.07, 126.62, 125.90, 124.11, 123.11, 122.98, 122.35,
121.55, 121.05, 120.16, 118.81 (Ar), 100.44 (acac), 63.87 (quat.
C), 28.96 (acac). MALDI-TOF MS (m/z): 1749 [M]+. Anal.
Calcd for C113H83N6O2Ir: C, 77.59; H, 4.78; N, 4.80. Found:
C, 77.60; H, 4.56; N, 4.67%.
exponential function in each case according to I ¼ I0
+
Aexp(ꢀt/t). Electrochemical measurements were made using
a Princeton Applied Research model 273A potentiostat at
a scan rate of 100 mV sꢀ1. A conventional three-electrode
configuration consisting of a platinum or glassy carbon working
electrode and Pt wires as both the counter and reference
electrodes was used. The supporting electrolyte was 0.1 M
[Bu4N]PF6 in THF. Ferrocene was added as a calibrant after
each set of measurements, and all potentials reported were
quoted with reference to the ferrocene–ferrocenium (Fc/Fc+)
couple. The first oxidation (Eox) and reduction (Ered) potentials
were used to determine the HOMO and LUMO energy levels
Ir-3. [Ir(acac)3] (38 mg, 0.080 mmol) and HL2 (0.22 g,
using the equations EHOMO ¼ ꢀ(Eox + 4.8) eV and ELUMO
¼
0.28 mmol) were mixed in glycerol (15 mL) under a N2 atmos-
ꢁ
ꢀ(Ered + 4.8) eV which were calculated using the internal
standard ferrocene value of ꢀ4.8 eV with respect to the
vacuum.6f Thermal analyses were performed with Perkin-Elmer
Pyris Diamond DSC and Perkin-Elmer TGA6 thermal
analyzers.
phere and the reaction mixture was heated to 220 C overnight.
The mixture was cooled to room temperature and water (50 mL)
was added. The resulting mixture was extracted with CH2Cl2
(3 ꢃ 100 mL) and the organic phase was dried over MgSO4.
Upon solvent removal under vacuum, the residue was purified
by column chromatography using CH2Cl2–hexane (2 : 1, v/v)
as eluent to afford the title compound as a dark red solid (24
Preparation of metal complexes
1
mg, 12%). H NMR (400 MHz, CDCl3): d (ppm) 8.46 (d, J ¼
Ir-1. [Ir(acac)3] (96 mg, 0.20 mmol) and HL1 (0.50 g, 0.69
mmol) were mixed in glycerol (18 mL) under a N2 atmosphere.
The reaction mixture was heated to 220 ꢁC for 18 h, after which
time the mixture was cooled to room temperature and water
(50 mL) was added. The resulting mixture was extracted with
CH2Cl2 (3 ꢃ 100 mL) and the organic phase was dried over
MgSO4. Upon solvent removal under vacuum, the residue was
purified by column chromatography using CH2Cl2 as eluent to
8.8 Hz, 3 H, Ar), 8.20 (s, 3 H, Ar), 7.66 (d, J ¼ 8.4 Hz, 3 H,
Ar), 7.55 (t, J ¼ 7.6 Hz, 3 H, Ar), 7.46 (s, 3 H, Ar), 7.40–7.36
(m, 6 H, Ar), 7.31–7.29 (m, 6 H, Ar), 7.22–7.16 (m, 33 H, Ar),
7.09–7.05 (m, 30 H, Ar), 7.02–6.94 (m, 27 H, Ar), 6.43 (t, J ¼
7.4 Hz, 3 H, Ar). 13C NMR (100 MHz, CDCl3): d (ppm)
168.27, 163.35, 151.92, 147.85, 146.04, 145.77, 144.84, 143.42,
141.83, 141.24, 140.31, 139.55, 136.61, 130.01, 129.16, 128.80,
128.27, 127.86, 127.18, 127.09, 126.40, 125.61, 124.36, 123.91,
123.75, 123.30, 122.37, 120.99, 119.91 (Ar), 63.96 (quat. C).
MALDI-TOF MS (m/z): 2528 [M]+. Anal. Calcd for
C174H120N9Ir: C, 82.63; H, 4.78; N, 4.98. Found: C, 82.55; H,
4.89; N, 5.04%.
1
afford the title compound as an orange solid (48 mg, 10%). H
NMR (400 MHz, CDCl3): d (ppm) 7.79 (d, J ¼ 8.8 Hz, 3 H,
Ar), 7.67–7.64 (m, 3 H, Ar), 7.47 (t, J ¼ 7.0 Hz, 3 H, Ar), 7.27
(d, J ¼ 7.2 Hz, 3 H, Ar), 7.25–6.75 (m, 99 H, Ar), 6.39 (t, J ¼
7.4 Hz, 3 H, Ar). 13C NMR (100 MHz, CDCl3): d (ppm)
166.50, 161.06, 152.12, 147.79, 147.73, 147.66, 147.00, 145.60,
143.54, 141.55, 140.35, 135.71, 129.16, 129.08, 128.87, 126.86,
125.48, 124.51, 124.22, 122.93, 122.78, 122.53, 121.43, 120.44,
119.15 (Ar), 63.82 (quat. C). MALDI-TOF MS (m/z): 2378
[M]+. Anal. Calcd for C162H114N9Ir: C, 81.79; H, 4.83; N, 5.30.
Found: C, 81.85; H, 4.77; N, 5.34%.
Pt-1. Under a N2 atmosphere, ligand HL1 (0.78 g, 1.07 mmol)
and K2PtCl4 (0.28 g, 0.67 mmol) were added to 2-ethoxyethanol–
water (20 mL, 3 : 1, v/v). The reaction mixture was stirred at
80 ꢁC for 16 h. After it was cooled to room temperature, the reac-
tion mixture was poured into water (50 mL). A yellow precipitate
appeared which was collected and dried under vacuum to give
a chloro-bridged dimer that was used for the next step without
further purification. The Pt dimer (0.56 g, 0.29 mmol),
Na2CO3 (0.60 g, 5.66 mmol) and acetylacetone (0.2 mL) were
added to 2-ethoxyethanol (10 mL) under a N2 atmosphere.
Ir-2. Ligand HL1 (0.50 g, 0.69 mmol) and IrCl3$nH2O (64 mg,
54 wt.-% Ir content) were added to a mixture of 2-ethoxyethanol
and ꢁwater (15 mL, 3 : 1, v/v). The reaction mixture was stirred at
110 C for 18 h under a N2 atmosphere. After cooling to room
temperature, an orange precipitate was obtained. The precipitate
was collected and washed with ethanol (20 mL) and hexane
(10 mL). Subsequently, the chloride-bridged Ir dimer was dried
under vacuum and it was finally isolated as an orange solid
that was used for the next step without further purification.
The Ir dimer (0.20 g, 0.059 mmol), Na2CO3 (63 mg,
0.59 mmol) and acetylacetone (0.2 mL) were combined in
2-ethoxyethanol (16 mL) and the reaction mixture was heated
to 110 ꢁC overnight. Afterwards, the mixture was cooled to
ꢁ
The reaction temperature was kept at 100 C for 16 h. Then, it
was cooled to room temperature and water (50 mL) was added.
The yellow precipitate was collected and dried. It was then
purified by column chromatography with CH2Cl2–hexane
(3 : 2, v/v) as eluent. The product was obtained as a bright yellow
solid (95 mg, 16%). 1H NMR (400 MHz, CDCl3): d (ppm) 8.98–
8.97 (m, 1 H, Ar), 8.02 (s, 1 H, Ar), 7.85 (d, J ¼ 7.6 Hz, 1 H, Ar),
7.76–7.72 (m, 1 H, Ar), 7.57 (d, J ¼ 8.0 Hz, 1 H, Ar), 7.45 (s, 1 H,
Ar), 7.40 (d, J ¼ 7.2 Hz, 1 H, Ar), 7.38–7.35 (m, 1 H, Ar), 7.30–
7.28 (m, 1 H, Ar), 7.22–7.18 (m, 8 H, Ar), 7.13–7.10 (m, 4 H, Ar),
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4 H, Ar), 5.51 (s, 1 H, acac), 2.09 (s, 3 H, acac), 2.02 (s, 3 H,
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167.94, 152.38, 147.68, 147.33, 147.16, 145.95, 143.98, 141.26,
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I. D. W. Samuel, Adv. Mater., 2007, 19, 1675; (e) G.-J. Zhou,
W.-Y. Wong, B. Yao, Z.-Y. Xie and L.-X. Wang, Angew. Chem.,
Int. Ed., 2007, 46, 1149.
8 (a) P. J. Low, M. A. J. Paterson, H. Puschmann, A. E. Goeta,
A. K. Howard, C. Lambert, J. C. Cherryman, D. R. Tackley,
S. Leeming and B. Brown, Chem. Eur. J., 2004, 10, 83; (b)
P. J. Low, M. A. J. Paterson, A. E. Goeta, D. S. Yufit,
J. A. K. Howard, J. C. Cherryman, D. R. Tackley and B. Brown,
J. Mater. Chem., 2004, 14, 2516; (c) R. D. Hreha, C. P. George,
OLED fabrication and measurements
Commercial indium tin oxide (ITO) coated glass with sheet resis-
tance of 20–30 U ,ꢀ1 was used as the starting substrates. Before
device fabrication, the ITO glass substrates (anode) were cleaned
by ultrasonic baths in organic solvents followed by ozone treat-
ment for 20 min. Then 40 nm thick PEDOT : PSS film was first
deposited on the ITO glass substrates, and cured at 120 ꢁC for 30
min in air. The emitting layer (70 nm) was obtained by spin-
coating a chloroform solution of each phosphorescent dopant
(x%) in CBP at various concentrations. Then, the sample was
dried in a vacuum oven at 50 ꢁC for 15 min and it was transferred
to the deposition system for organic and metal deposition. BCP
(15 nm), Alq3 (40 nm), LiF (1 nm) and Al cathode (100 nm) were
successively evaporated at a base pressure less than 10ꢀ6 Torr.
The EL spectra and CIE coordinates were measured with
a PR650 spectra colorimeter. The J–V–L curves of the devices
were recorded by a Keithley 2400/2000 source meter and the
luminance was measured using a PR650 SpectraScan spectro-
meter. All the experiments and measurements were carried out
under ambient conditions.
Acknowledgements
Financial support from a CERG grant from the Research
Grants Council of the Hong Kong SAR, People’s Republic of
China (Project No. HKBU202106) and the Hong Kong Baptist
University is gratefully acknowledged for this work.
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