Bent ladder-type hexaphenylene with carbazole core and spiro linkage as stable and efficient blue emitter

Article abstract of DOI:10.1021/ol901635v

A bent ladder-type hexaphenylene with a carbazole core and spiro-linkage is designed and synthesized by using the ortho-linked spirobifluorene. The design eliminates the possibility of forming a positional isomer. As a blue-emitter, the BLHPC shows good t

Full text of DOI:10.1021/ol901635v

ORGANIC
LETTERS
2009
Vol. 11, No. 18
4132-4135
Bent Ladder-Type Hexaphenylene with
Carbazole Core and Spiro Linkage as
Stable and Efficient Blue Emitter
Zuoquan Jiang,† Haiqing Yao,† Zhongyin Liu,† Chuluo Yang,*,† Cheng Zhong,†
Jingui Qin,† Gui Yu,‡ and Yunqi Liu*,‡
Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic
Materials, Wuhan UniVersity, Wuhan 430072, P. R. China, and Key Laboratory of
Organic Solids, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, P. R. China
clyang@whu.edu.cn; liuyq@iccas.ac.cn
Received July 17, 2009
ABSTRACT
A bent ladder-type hexaphenylene with a carbazole core and spiro-linkage is designed and synthesized by using the ortho-linked spirobifluorene.
The design eliminates the possibility of forming a positional isomer. As a blue-emitter, the BLHPC shows good thermal and color stability. A simple
light-emitting device fabricated from BLHPC exhibits a maximum current efficiency of 1.46 cd/A and a maximum luminance of 505 cd/m2.
In recent years, ladder-type oligomers and polymers have
attracted considerable attention in the area of organic
optoelectronic materials because their rigid coplanar con-
formations enhance π-conjugation degree, and endow them
with high luminescence efficiency, good carrier mobility, and
thermal stability.1 Mu¨llen and Scherf et al. have designed
many ladder-type materials for various optoelectronic ap-
plications during the past two decades.2 However, ladder-
type materials incorporating spiro linkage have still been very
limited.3 Bo et al. reported spiro-bridged ladder-type oligo-
(2) (a) Scherf, U.; Mu¨llen, K. Macromolecules 1992, 25, 3546. (b)
Scherf, U.; Mu¨llen, K. AdV. Polym. Sci. 1995, 123, 1. (c) Grimsdale, A. C.;
Mu¨llen, K. AdV. Polym. Sci. 2006, 199, 1. (d) Scherf, U. J. Mater. Chem.
1999, 9, 1853. (e) Grimme, J.; Scherf, U. Macromol. Chem. Phys. 1996,
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(g) Jacob, J.; Sax, S.; Gaal, M.; List, E. J. W.; Grimsdale, A. C.; Mu¨llen,
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Laquai, F.; Mishra, A. K.; Ribas, M. R.; Petrozza, A.; Jacob, J.; Akcelrud,
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M.; Mu¨llen, K. J. Org. Chem. 2008, 73, 9207. (l) Laquai, F.; Mishra, A. K.;
Mu¨llen, K.; Friend, R. H. AdV. Funct. Mater. 2008, 18, 3265. (m) Zhou,
G.; Baumgarten, M.; Mu¨llen, K. J. Am. Chem. Soc. 2008, 130, 12477. (n)
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† Wuhan University.
‡ Chinese Academy of Sciences.
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Yamaguchi, S.; Xu, C.; Tamao, K. J. Am. Chem. Soc. 2003, 125, 13662.
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10.1021/ol901635v CCC: $40.75
Published on Web 08/14/2009
 2009 American Chemical Society

and poly(p-phenylene)s and demonstrated that the spiro-
linkage could suppress the photo/thermal oxidation of the
9-position of the fluorene unit to the undesired ketonic
defect.4 On the other hand, carbazole derivatives have been
widely applied in optoelectronic devices due to their good
hole-transporting properties and high triplet energy.5 In this
context, we were intrigued to design a carbazole-cored and
spiro-bridged ladder-type molecule. We anticipate that the
carbazole implantation could improve the hole injection and
transportation of ladder-type materials, and the spiro-
configuration could effectively impede the intermolecular
π-π interactions and consequently lead to amorphous
morphology of a ladder-type molecule. The above-mentioned
two factors are crucial for OLEDs.6
cyclization. In our design, the molecule is featured with the
4-position of spirobifluorene substituents at the C2 and C7
positions of the carbazole units. The molecular plane is
subsequently flattened at the 3-position of spirobifluorene. The
synthetic route is presented in Scheme 2.
Scheme 2. Synthesis of BLHPC
Up to now, all ladder-type structures have shown a linear
backbone, and the established classic synthetic strategies
involve acid-catalyzed intramolecular ring closure between
a tertiary alcohol and adjacent aryl ring in the final step.
From a statistical view, such cyclization may occur on either
side of the terphenyl backbone and should lead to positional
isomers. The separation of the isomers remains very difficult,
which should hamper material applications because a mixture
of isomers may cause the uncertainty and inconsistencies in
the devices.7
The two reactants,10 4-(9,9′-spirobifluorenyl)pinacol bo-
ronate for constructing bent configuration and 2,7-dibromo-
N-(2-ethylhexyl)-3,6-bis(benzoyl)carbazole in which the alkyl
chain ensures the solubility of the rigid molecule and the
benzoyl group acts as precursor to bridge-link the carbazole
and spirobifluorene, were coupled by Suzuki reaction to give
the diketone in 69% yield. The addition of a slight excess
of 4-methylphenyllithium, generated in situ by treatment
4-bromotoluene with n-butyllithium at -10 °C, to the
carbonyl group of the diketone produced the corresponding
tertiary alcohol as intermediate. Without further purification,
the alcohol was subsequently subjected to intramolecular
annulation under acidic conditions to afford a bent ladder-
type hexaphenylene (BLHPC) in a good isolated yield (82%).
As expected, the last cyclization occurs only on the C3
position of spirobifluorene, since the C1 position is blocked.
The target compound is easily purified due to the elimination
of the positional isomer. The compound was fully character-
ized by 1H NMR, 13C NMR, mass spectrometry, and
elemental analysis (see the Supporting Information).
For example, Lee et al. reported 2,7-dibromo-3′,6′-bis(octy-
loxy)-9,9′-spirobisfluorene and its geometrical isomer, 2,7-
dibromo-1′,6′-bis(octyloxy)-9,9′-spirobisfluorene, formed in the
last cyclization (Scheme 1).8 Most recently, Poriel et al. also
Scheme 1
.
Positional Isomers Generated in Ring-Closure
Reaction
As shown in the optimized molecular structure of BLHPC
calculated with PM3 (Figure 1), the molecule exhibits a
crablike profile with a pair of spirobifluorenes as the pincers.
The rigid molecular structure of BLHPC imparted by the
spiro-fused orthogonal linkages is very beneficial to the
reported two dispiro positional isomers though the bulkiness
of the tert-butyl substituents favor the less sterically hindered
positional isomer (Scheme 1).9 To avoid the problem, we report
a strategy to eliminate the possibility of forming positional
isomers by remaining only one reactive site in the final
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Org. Lett., Vol. 11, No. 18, 2009
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deduced from the difference between the HOMO level and
the optical band gap is 2.41 eV.
The UV-vis absorption and PL spectra of BLHPC in
CH2Cl2 solution are shown in Figure 3. The symmetric
Figure 1. Optimized molecular structure of BLHPC calculated with PM3.
thermal stability,11 as manifested by the high thermal
decomposition temperatures (Td, corresponding to 5%
weight loss in the thermogravimetric analysis) of 510 °C
and a very high glass-transition temperature (Tg) of 319
°C determined through differential scanning calorimetry
(DSC).The incorporation of both carbazole cores with a
long alkyl chain and spiro-configuration leads to good
amorphous morphology, which is highly important to
improve the lifetime of OLEDs.
The electrochemical property of BLHPC is studied by
cyclic voltammetry, and the compound shows two reversible
oxidation processes in CH2Cl2 solution. The first oxidation
occurs at 1.10 V and the second occurs at 1.51 V, which
can be attributed to carbazole and spirobifluorene moieties,
respectively. The electrochemical behavior of BLHPC is
similar to those reported polymers containing fluorene and
carbazole.12 The HOMO energy level determined from the
onset of the oxidation is 5.30 eV (relative to vacuum energy
level), which is higher than that of other ladder-type
p-phenylenes due to the introduction of the carbazole moiety
and means a lower barrier for hole injection.13 This is also
consistent with the DFT-calculated results. As shown in the
inset of Figure 2, the HOMO orbital mainly locates at the
hole-transporting carbazole moiety. The LUMO energy level
Figure 3. Normalized electronic absorption and photoluminescent
spectra of BLHPC in CH2Cl2 solution.
mirror images of the absorption and emission spectra, as
well as the small Stokes shift (462 cm-1), coincide with
the rigid molecular structure of BLHPC.14 The almost
100% fluorescence quantum yield of BLHPC in CH2Cl2
solution measured by an integrating sphere indicates that
nonradiative decay pathways are completely suppressed
by restricting bond rotation in such confined and rigid
backbones.4 Noticeably, BLHPC reveals a very stable
emission upon heat treatment. As shown in Figure 4,
Figure 4. PL spectra of BLHPC in film before and after annealing
at 150 °C in air.
almost no PL spectral change was observed after heating
for 12 and 36 h at 150 °C in air. The result clearly
indicates the enhancement of stability of the compound
with respect to aggregate formation and/or thermo-
oxidation, which should be attributed to the rigid and
Figure 2. Cyclic voltammogram of BLHPC (inset: distribution of
the HOMO and LUMO levels from DFT calculation of BLHPC).
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Org. Lett., Vol. 11, No. 18, 2009

bulky molecular configuration, as well as the inhibition
of ketonic defects by fully aryl-substituted and spiro-
carbons at the 9-position of fluorene units.15,4a
ladder-type polymer containing carbazole units reported by
Mu¨llen et al. exhibits a maximum current efficiency of 0.1
cd/A with CIE coordinates (0.19, 0.19) in a device of ITO/
PEDOT/ladder-type polymer/Ca/Al;2g a ladder-type hexaphe-
nylene containing a spirostructure recently reported by Poriel
et al. shows a maximum current efficiency of 0.16 cd/A with
CIE coordinates (0.30, 0.27) in a device of ITO/PEDOT/
ladder-type hexaphenylene /BCP/Ca.3
To evaluate the potential of BLHPC as a blue emitter in
OLED applications, a double-layer EL device was fabricated
with the simple configuration: ITO/BLHPC (40 nm)/1,3,5-
tris(N-phenylbenzimidizol-2-yl)benzene (TPBI, 30 nm)/LiF
(1 nm)/Al (100 nm). TPBI is utilized as electron-transporting
and hole-block material; LiF serves as the electron-injecting
layer. Because of the low hole injection barrier of BLHPC,
it can act as emission layer as well as hole-transport layer.
The device exhibits blue emission with 1931 Commission
International de L’Eclairage x, y coordinates of (0.21, 0.18).
Current-voltage-luminance (J-V-L) characteristics and
current efficiency versus current density of the device are
shown in Figure 5. A maximum current efficiency of 1.46
In summary, we have synthesized a novel ladder-type
molecule with carbazole core and spiro linkage. This is the
first report of ladder-type hexaphenylene with bent confor-
mation. The judicious design removes the possibility of
forming geometrical isomers by using the ortho-linked
spirobifluorene, which are usually encountered in the syn-
thesis of conventional ladder-type molecules. As a blue-
emitter, the BLHPC shows good thermal and color stabilities.
The incorporation of the central carbazole core significantly
reduces the ionization potential, and therefore the OLED
device configurations could be simplified without using
PEDOT:PSS as hole-injection layer. To our knowledge, the
blue emitter represents one of the best blue fluorescent
ladder-type materials to date.
Acknowledgment. We thank the National Natural Science
Foundation of China (Project Nos. 50773057 and 20474047)
and the Ministry of Science and Technology of China
through the 973 program (Grant Nos. 2009CB623602) for
financial support.
Supporting Information Available: General experimental
information; synthesis and characterization of BLHPC. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 5. J-V-L curves of the device. (Inset: the current
efficiency-current density curve of the device.)
OL901635V
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