A R T I C L E S
Brunner et al.
resulting in maximum internal quantum efficiencies of 60 to
80%.2,4-9,17-19
of the coupling site connecting the monomers to form oligomers
determines the triplet energy. Furthermore, the HOMO level
can be engineered by substitution at the 3, 6, and 9 position of
the carbazoles (see compound 1 in Figure 1 for the numbering
of the ring system) while at the same time a high triplet energy
is maintained. Finally, electroluminescent devices are con-
structed to test two carbazole derivatives in a host-guest system
with a phosphorescent metal complex as guest.
Since the exchange energy20 of many small molecules is
found to be in the range of 0.7 to 1 eV21 a large triplet energy
is usually found for compounds with a very large singlet energy.
In stable systems, a large singlet energy generally means a high
oxidation potential and, consequently, a high barrier for hole
injection from anodes such as indium tin oxide. Additional hole
injection and transporting layers have to be introduced thereby
complicating the device architecture. Moreover, the triplet
energy of CBP (2.56 eV) is too small to yield highly efficient
devices with blue triplet emitters so that hosts with even higher
triplet energies (g2.75 eV) are desired.10
Experimental Section
Synthesis. All reagents and solvents were used as received or purified
using standard procedures. NMR spectra were recorded on a Varian
Mercury Vx at frequencies of 400 and 100 MHz for 1H and 13C nuclei
1
or Varian Gemini 2000 at frequencies of 300 and 75 MHz for H and
Next to the luminescent behavior, the charge transporting
properties of a molecule define its applicability as host in an
electroluminescent device. Many carbazole derivatives are
known to predominantly transport positive charge carriers. For
example, PVK is often described in the literature as being a
unipolar hole transporter.22 Consequently, in a polymer OLED
electron transporting compounds have to be admixed to the PVK
layer to increase the charge carrier balance and shift the emission
zone away from the cathode,13,14 or extra layers (hole blocking
and electron transporting) have to be used.11,12,16 CBP is reported
to have a more bipolar transport character.23 Nevertheless, also
in small-molecule OLEDs employing carbazole derivatives extra
layers are often introduced to confine the positive charge carriers
in the emission zone.
13C nuclei, respectively. Details on the synthesis are provided in the
Supporting Information section.
OLEDs. OLEDs were prepared by vacuum evaporation at a base
pressure of about 1-2 × 10-6 mbar. The substrates were cleaned with
a detergent and 2-propanol and subsequently subjected to a O2-plasma
treatment for 10 min. The device layout is shown in an inset of Figure
5. N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (R-NPD), fac-tris-
(2-phenylpyridine)-iridium Ir(ppy)3, 2,9-dimethyl-4,7-diphenyl-phenan-
throline (BCP) and tris(8-hydroxyquinoline)aluminum (Alq3) were
obtained from Sensient/Syntec and used as received. Voltage-current-
luminance (U-I-L) curves were measured in nitrogen atmosphere by
a HP 3245A universal source, Keithley 2000 multimeter and a LMT
1006 luminance meter.
Phosphorescence Measurements. The phosphorescence spectra
were obtained on highly diluted (about 1 mg/l) solutions in methyl-
THF, which gives a clear glass at 77 K. The emission spectra at 77 K
were recorded with an Edinburgh 900 spectrofluorometer. Nongated
and gated spectra were recorded to discriminate the phosphorescence
from fluorescence. The gate delay was 500 µs with a gate width of 9
ms. The highest energy peak in the phosphorescence spectrum was
taken for the Sν0)0 r T1ν)0 transition.
Cyclic Voltammetry (CV). Cyclic voltammetry measurements were
recorded in dichloromethane, with 1 M tetrabutylammonium hexafluo-
rophosphate as supporting electrolyte. The working electrode was a
platinum disk (0.2 cm2), the counter electrode was a platinum plate
(0.5 cm2), and a saturated calomel electrode (SCE) was used as reference
electrode, calibrated against a Fc/Fc+ couple. As the oxidation potential
of SCE relative to the vacuum level is known (4.74 V29), the measured
oxidation potentials can be converted into ionization potentials. In this
report, we use the ionization potential as a measure for the energy of
the highest occupied molecular orbital (HOMO). Although care has to
be taken to compare HOMO levels obtained in this way to those
obtained by other techniques (such as UPS), the HOMO levels of the
oligomers among each other allow for excellent comparison.
Currently, much effort is directed toward finding alternative
carbazole derivatives as host materials for small-molecule
OLEDs that address the above-mentioned issues. These efforts
have resulted for example in the use of 4,4′,4′′-tris(9-carbazolyl)-
triphenylamine (TCTA),24 4,4′-bis(9-carbazolyl)-2,2′-dimethyl-
biphenyl (CDBP),25 and 3,5-bis(9-carbazolyl)benzene (mCP)10
as host materials.
Common to the molecular structure of all the carbazole
derivatives mentioned thus far is the derivatization via the
nitrogen of the carbazole unit. In this report, we introduce
alternative carbazole derivatives for small-molecule OLEDs
based on derivatization at the phenyl rings of the carbazole
unit.26-28 The carbazole is either connected to other carbazoles
to form carbazole dimers and trimers or to fluorene and
oxadiazole to form mixed compounds. We have investigated
the influence of substitution of carbazoles at the phenyl rings
and at the nitrogen on spectroscopical and electrochemical
properties relevant for OLEDs. It will be shown that the position
Results and Discussion
(17) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750.
(18) D′Andrade, B. W.; Baldo, M. A.; Adachi, C.; Brooks, J.; Thompson, M.
E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 1045.
1. Carbazole Oligomers. A series of carbazole compounds
was synthesized and characterized spectroscopically, electro-
chemically, and electrically (as a layer in an OLED). Carbazole
monomers are linked via the 3 (6, 3′, 6′, etc.) positions to form
dimers and trimers and substituted with either aryl- or alkyl-
groups at the 9 positions (see Figure 1).
REDOX Stability of Carbazole Oligomers. Cyclic volta-
mmetry measurements showed one or more oxidation waves
and no reduction wave in the range of -1.4 V to +1.8 V vs
SCE with dichloromethane as solvent. The first oxidation wave
of the carbazole oligomers is reversible as opposed to the
(19) Watanabe, T.; Nakamura, K.; Kawami, S.; Fukuda, Y.; Tsuji, T.; Wakimoto,
T.; Miyaguchi, S.; Yahiro, M.; Yang, M. J.; Tsutsui, T. Synth. Metals 2001,
122, 203.
(20) As estimate for the exchange energy here the energy difference between
the Sν1)0 and Tν1)0 levels is taken.
(21) Ba¨ssler, H.; Arkhipov, V. I.; Emelianova, E. V.; Gerhard, A.; Hayer, A.;
Im, C.; Rissler, J. Synth. Metals 2003, 135-136, 377.
(22) Pai, D. M.; Yanus, J. F.; Stolka, M. J. Phys. Chem. 1984, 88, 4714.
(23) Kanai, H.; Ichinosawa, S.; Sato, Y. Synth. Metals 1997, 91, 195.
(24) Ikai, M.; Tokito, S.; Sakamoto, Y.; Suzuki, T.; Taga, Y. Appl. Phys. Lett.
2001, 79, 156.
(25) Tokito, S.; Iijima, T.; Suzuri, Y.; Kita, H.; Tsuzuki, T.; Sato, F. Appl. Phys.
Lett. 2003, 83, 569.
(26) Justin Thomas, J. R.; Lin, J. T.; Tao, Y. T.; Ko, C. W. J. Am. Chem. Soc.
2001, 123, 9404.
(27) Grazulevicius, J. V.; Strohriegl, P.; Pielichowski, J.; Pielichowski, K. Prog.
Polym. Sci. 2003, 28, 1297.
(29) Bockris, J. O. M.; Khan, S. U. M. Surface Electrochemistry. A Molecular
LeVel Approach; Kluwer Academic/Plenum Publishers: New York, 1993.
(28) Bonesi, S. M.; Erra-Balsells, R. J. Lumin. 2001, 93, 51.
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6036 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004