Nonconjugated hybrid of carbazole and fluorene: A novel host material for highly efficient green and red phosphorescent OLEDs

Article abstract of DOI:10.1021/ol051977h

(Chemical Equation Presented) A novel host material for efficient green and red electrophosphorescence devices is obtained by adopting the new molecular strategy of nonconjugated linkage of carbazole and fluorene moieties. The new host combines characteri

Full text of DOI:10.1021/ol051977h

ORGANIC
LETTERS
2005
Vol. 7, No. 24
5361-5364
Nonconjugated Hybrid of Carbazole and
Fluorene: A Novel Host Material for
Highly Efficient Green and Red
Phosphorescent OLEDs
Ken-Tsung Wong,*,† You-Ming Chen,† Yu-Ting Lin,‡ Hai-Ching Su,‡ and
Chung-chih Wu*,‡
Department of Chemistry and Department of Electrical Engineering, Graduate Institute
of Electro-optical Engineering and Graduate Institute of Electronics Engineering,
National Taiwan UniVersity, Taipei 106, Taiwan
kenwong@ntu.edu.tw
Received August 15, 2005
ABSTRACT
A novel host material for efficient green and red electrophosphorescence devices is obtained by adopting the new molecular strategy of
nonconjugated linkage of carbazole and fluorene moieties. The new host combines characteristics of both carbazole and fluorene, giving a
large-gap host material suitable for green and red phosphorescent OLEDs. Green and red phosphoresecent OLEDs with external quantum
efficiencies up to 10% have been achieved with this new host material.
Phosphorescent emitters containing transition metals are of
substantial interest for highly efficient organic light emitting
diodes (OLEDs).1 In devices using such materials, both
electrogenerated singlet and triplet excitons contribute to the
light emission so that emitting devices with 100% internal
quantum efficiency are made possible.2 In phosphorescent
OLEDs, to reduce quenching associated with relatively long
excited-state lifetimes of triplet emitters and triplet-triplet
annihilation etc., triplet emitters of heavy-metal complexes
are normally used as emitting guests in a host material, and
thus effective host materials are of equal importance for
efficient phosphorescent OLEDs. The development of ef-
fective host materials for triplet emitters imposes some
challenges to molecular designs. For preventing reverse
energy transfer from the guest back to the host and for
effectively confining triplet excitons on guest molecules, the
triplet level of the host must be larger than that of the triplet
(1) (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee,
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Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904. (h) Holmes, R. J.; Forrest,
S. R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M.
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Wang, J. H.; Qiu, Y. Synth. Met. 2004, 144, 249. (j) Tsuboyama, A.;
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† Department of Chemistry.
(2) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl.
Phys. 2001, 90, 5048.
‡ Department of Electrical Engineering.
10.1021/ol051977h CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/28/2005

emitter.3 Correspondingly, the host materials must have rather
large energy gaps. To have a large energy gap, the extent of
conjugation in the molecule must be confined, which in turn
would usually impose constraints in molecular size. On the
other hand, for the molecules to form morphologically stable
and uniform amorphous thin films with typical processing
techniques, it usually requires the molecules to be bulky and
steric. Hence an exquisite and flexible molecular design is
highly desired to satisfy these conflicting demands for the
host materials of phosphorescent OLEDs.
In this communication, we report a molecular design for
large-gap materials adopting the nonconjugated linking of
large-gap moieties, such as carbazole and fluorene, that are
the usual building blocks for large-gap materials or host
materials of phosphorescent devices.3b,4 The nonconjugated
hybrid of carbazole and fluorene combines characteristics
such as high ambipolar charge carrier mobility of fluorenes5
and the low carrier injection barrier of carbazoles, giving
rise to a novel host material: difluorene substituted carbazole
(DFC) (Scheme 1) with a large triplet energy, a steric and
successfully achieved using the titled compound as a host
material.
The synthetic pathway of the novel host material DFC
through the nonconjugated hybridization of carbazole and
fluorene is depicted in Scheme 1. 9-(4-tert-Butylphenyl)-
carbazole (1) was isolated with a 92% yield by Pd-catalyzed
amination of carbazole and 1-tert-butyl-4-bromobenzene in
the presence of a catalytic amount of Pd(OAc)2 and PtBu3
in o-xylene. Bromination of the cabazole derivative 1 with
NBS in DMF at 0 °C gave 3,6-dibromocarbazole 2 with a
yield of 88%. In contrast with the unsuccessful preparation
of Grignard reagent of 2 for further reactions, the treatment
of 3,6-dibromocarbazole 2 with n-BuLi at -78 °C afforded
the dilithiated complex, which was trapped with fluorenone
to give diol compound 3 (with a yield of 82%). Carbazole
and fluorene were thus nonconjugatedly linked. The Friedel-
Crafts reaction of diol 3 with an excess amount of anisole
in the presence of sulfuric acid furnished the titled compound
DFC with a 56% yield. It is noteworthy that the flexible
molecular design here fully allows one to fine-tune the
physical properties of materials by introducing different
functional groups through either the 9 position of carbazole
or C9 of fluorene.
Scheme 1. Synthesis of the Carbazole-Fluorene
Nonconjugated Hybrid DFC
The electrochemical properties of DFC were probed by
cyclic voltammetry (CV). DFC exhibits one reversible
oxidation peak at voltage of 0.77 V (versus Fc/Fc+) and an
oxidation onset at 0.69 V, which is similar to that of
carbazole derivative 1 (with an onset at 0.71 V; see
Supporting Information, Figure S-1). Since a single fluorene
does not show reversible oxidation, it is therefore rational
to attribute the oxidation of the DFC to the oxidation
occurring on the central carbazole. The peripheral fluorene
moieties nonconjugatedly linked to the carbazole core appear
to play an insignificant role on the oxidation potential of
the whole molecule, but their addition efficiently blocks the
electrochemically active sites (C3 and C6) of carbazole and
may give the compound an extra electrochemical stability.
As shown in the optimized molecular structure of DFC
calculated with PM3 (Figure 1), linking carbazole and
fluorene through the tetrahedral C9 of fluorene and additional
bulky molecular structure, and outstanding thermal and
morphological stability. This novel host material is suitable
for green to red phosphorescent devices, and highly efficient
green and red electrophosphorescent devices have been
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M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082.
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Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (b) Holmes, R. J.; Forrest, S.
R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E.
Appl. Phys. Lett. 2003, 82, 2422. (c) Tokito, S.; Iijima, T.; Tsuzuki, T.;
Sato, F. Appl. Phys. Lett. 2003, 83, 2459. (d) Yeh, S.-J.; Wu, M.-F.; Chen,
C.-T.; Song, Y.-H.; Chi, Y.; Ho, M.-H.; Hsu, S.-F.; Chen, C. H. AdV. Mater.
2005, 17, 285.
(5) (a) Wu, C.-C.; Liu, T.-L.; Lin, Y.-T.; Hung, W.-Y.; Wong, K.-T.;
Chao, T.-C.; Hung, T.-S.; Chen, Y.-M. Appl. Phys. Lett. 2004, 85, 1173.
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R.-T.; Chen, Y.-M.; Chien, Y.-Y. J. Am. Chem. Soc. 2003, 125, 3710.
Figure 1. Optimized molecular structure of DFC calculated with
PM3.
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Org. Lett., Vol. 7, No. 24, 2005

aryl substitution on C9 of fluorene render the molecule rather
steric, rigid, and bulky. The rigid linkage through C9 of
fluorene is strongly beneficial to the thermal stability of DFC,
as indicated by the high decomposition temperature (Td,
corresponding to 10% weight loss) of 446 °C in the
thermogravimetric analysis. The steric and bulky molecular
conformation also gives the material a rather high glass
transition temperature (Tg) of 180 °C as determined by
differential scanning calorimetry (DSC; see Supporing
Information, Figure S-2), which is essential for morphologi-
cal stability of thin films. Due to the thermal and morpho-
logical stability of DFC, thin films of DFC could be prepared
by vacuum deposition and vacuum-deposited DFC forms
homogeneous and stable amorphous films.
Figure 2 compares the absorption and fluorescence spectra
Figure 3. Normalized phosphorescence (77 K) spectra of DFC in
thin films (∼100 nm), the photoluminescence spectra of the green
triplet emitter Ir(ppy)3 and the red triplet emitter Btp2Ir(acac) in
CHCl3 slution, and the electroluminescence spectra of devices A
and B.
triplet energy gap indicates that DFC is suitable as a host
material for triplet emitters with emitting wavelength > 490
nm or energy gap < 2.53 eV (i.e., green and red). The
photophysical data (absorption onset energy) together with
electrochemical data (oxidation potential) allow us to estimate
the HOMO and LUMO energy levels of DFC to be -5.57
eV and -2.20 eV (relative to vacuum level), respectively.
We have investigated the use of DFC as the host material
for green and red phosphorescent devices with the device
structures of: Device A: ITO/PEDT:PSS (300 Å)/TCTA
(400 Å)/8 wt % Ir(ppy)3 doped in DFC (200 Å)/BCP (100
Å)/Alq3 (300 Å)/LiF (5 Å)/Al; Device B: ITO/PEDT:PSS
(300 Å)/TCTA (300 Å)/8 wt % Btp2Ir(acac) doped in DFC
(300 Å)/BCP (100 Å)/Alq3 (600 Å)/LiF (5 Å)/Al. The
conducting polymer polyetheylene dioxythiophene:polystrene
sulfonate (PEDT:PSS) was used as the hole injection layer,
4,4′,4”-tri(N-carbazolyl)triphenylamine (TCTA) as the hole-
transport layer. The emitting layer (EML) consists of the
large energy gap DFC doped with 8 wt % of the common
green phosphor fac-tris(2-phenylpyridine)iridium (Ir(ppy)3)
or red phosphor bis(2-(2′-benzo[4,5-R]thienyl) pyridinato-
N,C3′) (Btp2Ir(acac)).7 A thin layer of 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline (BCP) was inserted between
the EML and the electron-transport tris-(8-hydroxyquino-
line)aluminum as the hole-blocking and exciton-blocking
layer to provide exciton and carrier confinement. A 5 Å LiF
serves as the electron-injection layer (see Supporting Infor-
mation, Figure S-3 for device structures and relative HOMO-
LUMO alignment of active layers).
Figure 2. Normalized absorption and fluorescence spectra of
carbazole 1, fluorene, and DFC in CHCl3.
of carbazole 1, fluorene, and DFC in CHCl3. All three
compounds show UV emission in the range of 308-366 nm.
A single fluorene shows substantially larger transition
energies in either absorption or fluorescence than carbazole
1 and DFC. The features of the lowest absorption band and
fluorescence of DFC are very similar to those of carbazole
1, except for a ∼15-nm red shift in both spectra. Such a red
shift is similar to that occurring on a carbazole when
introducing quarternary substitutions at 3 and 6 positions.6
Thus, the lowest absorption band and fluorescence of DFC
can be unambiguously attributed mainly to the lowest π-π*
transition of the central carbazole chromophore of DFC.
Thus, in terms of absorption and fluorescence of DFC, the
tetrahedral C9 carbon of fluorene seemingly serves as a
spacer blocking the π-conjugation of the carbazole core from
extending to the peripheral substitution. For evaluating the
capability of DFC as a host for electrophosphorescent
devices, phosphorescence of DFC in thin films was also
detected at 77 K. Figure 3 shows the phosphorescence
spectrum of DFC, which exhibits a maximum at 490 nm,
corresponding to a triplet energy gap of 2.53 eV. Such a
As shown in Figure 3, the triplet energy of DFC (∼2.53
eV) is higher than that of Ir(ppy)3 (∼2.42 eV) and Btp2Ir-
(acac) (∼2.0 eV), ensuring that the triplet energy transfer
from DFC to Ir(ppy)3 or to Btp2Ir(acac) is exothermic and
that the transferred triplet excitons are well confined on Ir-
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B.; Hameurlaine, A.; Dehaen, W. J. Am. Chem. Soc. 2003, 125, 5356.
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M. E.; Kwong, R. C. Appl. Phys. Lett. 2001, 78, 1622.
Org. Lett., Vol. 7, No. 24, 2005
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Figure 4 shows the current-voltage-brightness and
efficiency characteristics of devices A and B. High brightness
of ∼176 000 cd/m2 and ∼5340 cd/m2 are obtained for
devices A and B (Figure 4a,b), respectively. Both devices
possess a rather low turn-on voltage of ∼2.0-2.5 V (defined
as the voltage at which emission becomes detectable),
indicating effective carrier injection into the doped emitting
layer from adjacent hole-transporting and hole-blocking
layers. The maximal EL quantum efficiencies are ∼10.5%
(at 0.5 mA/cm2) for device A and ∼9% (at 0.01 mA/cm2)
for device B (Figure 4c). The maximal power efficiencies
reach ∼25 lm/W for device A and 4.0 lm/W for device B.
Both devices show a gradual decrease in the EL quantum
efficiency with current, which is often observed in phos-
phorescent OLEDs and is usually attributed to triplet-triplet
annihilation.9 Nevertheless, at a practical brightness of 100
cd/m2, the EL quantum efficiencies remain above 10% for
device A and 6.5% for device B, which are still substantially
higher than that of fluorescent green and red OLEDs (3-
5%) used in current OLED displays. When compared with
green or red phosphorescent OLEDs using the same triplet
emitter in a common host 4,4′-bis(9-carbazoly)-2,2′-biphenyl
(CBP), the devices using DFC as the host show substantially
improved performance. For instance, the maximum EL
quantum efficiency of device B (∼9%) obtained here is
substantially higher than that obtained with the CBP host
(∼7%).7 Moreover, these devices also show a less efficient
decay at high current densities.
In summary, a novel host material with nonconjugated
linkage of carbazole and fluorene moieties has been synthe-
sized and characterized. Green and red electrophosphorescent
devices using such a host material show remarkable external
quantum efficiencies and less efficiency decay at high current
densities.
Acknowledgment. This work was financially supported
by the National Science Council of Taiwan.
Supporting Information Available: Details of synthetic
procedures and spectroscopic characterization of new com-
pounds, details of device fabrication and characterization,
cyclic voltammograms of compound 1 and DFC, DSC
analysis of DFC, and relative energy levels alignments of
active layers of devices A and B. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 4. (a) I-V-L characteristics of device A, (b) I-V-L
characteristics of device B, (c) EL quantum efficiency as a function
of current density for devices A and B.
(ppy)3 or Btp2Ir(acac).8 As a result, the electrophosphores-
cence devices A and B exhibit pure green EL from Ir(ppy)3
and pure red EL from Btp2Ir(acac) (Figure 3), respectively.
Furthermore, no emission from DFC is observed, indicative
of complete energy transfer from DFC to the green phosphor
or the red phosphor.
OL051977H
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