Synthesis and properties of 9,9′-diaryl-4,5-diazafluorenes. A new type of electron-transporting and hole-blocking material in EL device

Article abstract of DOI:10.1246/cl.2004.276

The title compounds were prepared and investigated for electron- transporting and hole-blocking materials in organic electroluminescent (EL) devices. EL efficiencies of the phosphorescent devices indicated high performance of hole-blocking ability in the compounds. A maximum external efficiency of 18% was achieved and a maximum power luminous efficiency was over 45 Lm W^-1.

Full text of DOI:10.1246/cl.2004.276

276
Chemistry Letters Vol.33, No.3 (2004)
Synthesis and Properties of 9,90-Diaryl-4,5-diazafluorenes.
A New Type of Electron-Transporting and Hole-Blocking Material in EL Device
Katsuhiko Ono,ꢀ Tomoki Yanase, Masakazu Ohkita, Katsuhiro Saito,
Yosuke Matsushita,y Shigeki Naka,y Hiroyuki Okada,y and Hiroyoshi Onnagaway
Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555
yDepartment of Electric and Electronic Engineering, Faculty of Engineering, Toyama University, Gofuku, Toyama 930-8555
(Received November 6, 2003; CL-031063)
The title compounds were prepared and investigated for
electron-transporting and hole-blocking materials in organic
electroluminescent (EL) devices. EL efficiencies of the phos-
phorescent devices indicated high performance of hole-blocking
ability in the compounds. A maximum external efficiency of
18% was achieved and a maximum power luminous efficiency
ta and elemental analyses. Compounds 1 were obtained as color-
less crystals (1a: mp 275–276 ꢂC, 1b: mp 208–209 ꢂC, 1c: mp
211–212 ꢂC). The melting point of 1a is similar to that of BCP
(mp 279–283 ꢂC) and those of 1b and 1c are slightly lower.
The absorption maxima of 1 were observed around 323 and
316 nm in dichloromethane (Table 1) and those were shifted to
longer wavelength than those of BCP. Influence of introduction
of methoxy groups was not observed in the UV spectra of 1. Ac-
cording to the absorption edges, the HOMO–LUMO energies of
1 were evaluated as 3.5–3.6 eV. These values are comparable to
that of BCP. The cyclic voltammograms (CV) of 1 in DMF
showed reversible one-electron redox waves. The half-wave re-
duction potentials are listed in Table 1. Although the reduction
potentials of 1 are slightly lower than that of BCP, it was found
that the energy levels of LUMOs are very close to that of BCP.
The decrease of electron affinity in compound 1c is due to sub-
stitution of methoxy groups. The MNDO-PM3 calculations indi-
cated that compound 1a has a lower HOMO energy and a slight-
ly higher LUMO energy than BCP (HOMO/eV: 1a, ꢁ9:37;
BCP, ꢁ8:80, LUMO/eV: 1a, ꢁ0:77; BCP, ꢁ0:81).16 The results
of the UV and CV analyses are consistent with the calculations.
was over 45 Lm Wꢁ1
.
Studies on carrier-transporting materials have been research
subjects of great interest from organic EL devices since effective
recombination balance between electrons and holes at emitting
layers afford high efficiency and long lifetime of luminescence.1
A variety of materials with high electron affinity have been syn-
thesized and investigated for electron-transporting layers.2–7 Re-
cently, electron-transporting materials have been also applied for
hole-blocking layers in EL devices using phosphorescent mate-
rials such as iridium and platinum complexes to afford high
quantum efficiencies.8–13 2,9-Dimethyl-4,7-diphenyl-1,10-phen-
anthroline (BCP) is one of the most powerful candidates as an
electron-transporting and hole-blocking material and that afford-
ed external quantum efficiencies of 8.0 and 13.7% in combina-
tion with fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] in 4,40-
N,N0-dicarbazolylbiphenyl (CBP) host.8,9 In this context, we
have now prepared 9,90-diaryl-4,5-diazafluorenes (1a–c) as a
new type of the materials with high electron affinity due to the
bipyridyl moiety and the spiro structure. EL devices fabricated
by diazafluorene 1a and Ir(ppy)3 also demonstrated high external
quantum efficiency. We report here the preparation and proper-
ties of 1a–c and the application of 1a for the EL devices.
Table 1. Absorption maximaa and edgesa and half-wave reduc-
tion potentialsb of 1a–c
Compound ꢀmax (log ")/nm
ꢀ
edge/eV E1=2red/V
1a
1b
1c
BCP
324 (4.05), 316 (4.01)
323 (3.96), 316 (3.97)
323 (3.99), 316 (3.98)
3.6
3.5
3.6
ꢁ2:48
ꢁ2:48
ꢁ2:54
ꢁ2:39c
312sh (4.10), 280 (4.63) 3.5
aIn CH2Cl2. b0.1 M n-Bu4NClO4 in DMF, Pt electrode, scan
R1 R1
rate 500 mV sꢁ1, V vs Fc/Fcþ. Epc + 0.03 V.
c
Ph
Ph
MeO
R2
OMe
R2
In order to investigate electron-transporting and hole-
blocking ability of the diazafluorene derivatives, EL devices us-
ing 1a were fabricated as shown in Figure 1a. N,N0-Diphenyl-
N
N
Me
Me
BCP
N,N0-bis(3-methylphenyl)-1,10-biphenyl-4,40-diamine
(TPD),
N
N
Ir(ppy)3, and CBP were used for a hole-transporting layer, an
emitter, and a carrier combination host, respectively. For the
cathode, an Al electrode with a LiF buffer layer was employed.
Four types of electron-transporting layer (ETL) were examined
(devices 1–4). The ETL of device 1 was composed of only com-
pound 1a. The ETLs of devices 2 and 3 were formed in the com-
bination of 1a with tris(8-hydroxyquinoline)aluminum (Alq3)
and 2,5-bis(bipyridyl)-1,10-dimethyl-3,4-diphenylsilole (PyPy-
SPyPy),17 respectively. BCP was employed for the ETL of de-
vice 4 as a reference.
O
1a R1 = R2 = H
1b
1 = OMe, R2 = H
1c R1 = R2 = OMe
R
N
N
2
4,5-Diazafluoren-9-one (2) was obtained by oxidation of
1,10-phenanthroline with KMnO4 in a KOH solution.14 9,90-Di-
aryl-4,5-diazafluorenes 1a–c were prepared in 70–90% yields by
the Friedel–Crafts reaction15 of 2 with three types of anisole
(10 mol equiv.) in the presence of H2SO4 (10 mol equiv.) at
65 ꢂC. The molecular structures were determined by spectral da-
Figure 1b shows current-voltage (J-V) characteristics in the
EL devices. The current density of device 1 is lower than that of
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.3 (2004)
277
device 4 in the range of the applied voltage, indicating that the
electron-transporting ability of 1a is inferior to that of BCP.
The current density of the EL device is slightly improved in de-
vices 2 and 3. All of the devices began to emit at an applied volt-
age of 3 V. The emission spectra from devices 1–3 were coinci-
dent with that from device 4, implying phosphorescence from
the Ir(ppy)3 emitters in the devices. Devices 1–4 had the Com-
mission Internationale de L’Eclairage (CIE) coordinates of
(0.28, 0.64), (0.29, 0.51), (0.27, 0.61), and (0.24, 0.66), respec-
tively.
to 18% in device 3 and this value is close to the theoretical limit
about 20% from simple classical optics.18 In addition, the max-
imum power luminous efficiency of device 3 was over
45 Lm Wꢁ1 although the device was driven at the relatively high
voltage.
In summary, we designed and prepared 9,90-diaryl-4,5-di-
azafluorenes 1 as electron-transporting and hole-blocking mate-
rials. The EL devices using 1a indicated the high performance of
hole-blocking ability in comparison to BCP. Improvement of the
electron-transporting ability is expected to provide higher per-
formance of phosphorescent EL devices. EL devices with com-
pounds 1b, c are currently under study.
a)
ITO/TPD (500 Å)/4.8%-Ir(ppy)3:CBP (200 Å)/ETL (300
This work was supported by a grant from Innovation Plaza
Tokai ‘‘Investigation of organic device integration using self-
alignment technology.’’ M. O. thanks the Shorai Foundation
for Science and Technology for financial support. We thank
M. Uchida at Chisso Corp. for providing the PyPySPyPy sample.
Å)/LiF (10 Å)/Al (700 Å).
device 1 ( ) : ETL = 1a (300 Å)
device 2 ( ) : ETL = 1a (100 Å) + Alq3 (200 Å)
device 3 ( ) : ETL = 1a (100 Å) + PyPySPyPy (200 Å)
device 4 ( ) : ETL = BCP (300 Å)
×
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Published on the web (Advance View) February 9, 2004; DOI 10.1246/cl.2004.276