The effect of the substitution position of dibenzofuran on the photophysical and charge-transport properties of host materials for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/chem.201203778

A twist in the tail: The effect of varying the substitution position of dibenzofuran on its photophysical properties and organic light-emitting diode device performance was systematically investigated by synthesizing three compounds with 2-, 3- and 4-substitution patterns (see scheme). The proper selection of substitution position can control the photophysical properties and performance of the materials. Copyright

Full text of DOI:10.1002/chem.201203778

DOI: 10.1002/chem.201203778
The Effect of the Substitution Position of Dibenzofuran on the Photophysical
and Charge-Transport Properties of Host Materials for Phosphorescent
Organic Light-Emitting Diodes
Chil Won Lee,[a] Jeong-A Seo,[b] Myoung-Seon Gong,[b] and Jun Yeob Lee*[a]
Dibenzofuran has been used as a high-triplet-energy core
structure of organic materials for phosphorescent organic
light-emitting diodes (PHOLEDs) because of its high triplet
energy of 3.14 eV.[1–4] As it has strong electron-withdrawing
oxygen linkage in the molecular structure, and has been
used as a core structure of electron-transport materials or
electron-transport-type host materials.
performances of green and blue PHOLEDs doped with
phosphorescent dopants were also compared. It was shown
that substitution at the 3-position of dibenzofuran is useful
to lower the driving voltage, while substitution at the 2-posi-
tion is effective in achieving a high triplet energy and high
quantum efficiency.
Three host materials, 9-(3-(dibenzo
yl)-9H-carbazole (2-DFPCz), 9-(3-(dibenzo
phenyl)-9H-carbazole (3-DFPCz) and 9-(3-(dibenzo-
AHCTUNGTREG[NNUN b,d]furan-4-yl)phenyl)-9H-carbazole (4-DFPCz) were syn-
ACHTUNGTRENNUNG
The dibenzofuran core has been modified at the 2- or 4-
position to synthesize various derivatives for use in PHO-
LEDs. The 2-position of dibenzofuran was modified simply
by bromination using N-butylsuccinimide or bromine,[4]
while the 4-position of dibenzofuran was substituted with
functional groups by direct lithiation owing to the electron-
withdrawing character of oxygen in dibenzofuran.[3] Several
high-triplet-energy host materials have been developed by
using this method. However, the 3-position of dibenzofuran
could not be directly functionalized, and no organic material
with substituent at the 3-position of dibenzofuran has been
reported for use in PHOLEDs. Therefore, no systematic in-
vestigation about the relationship between the substitution
position of dibenzofuran and device performances of PHO-
LEDs has been carried out. The synthesis of a dibenzofuran
derivative with a substituent at the 3-position may thus
enable the systematic study of the effect of substitution posi-
tion of dibenzofuran on material characteristics and device
performance.
ACHTUNGTRENNUNG
thesized by a Suzuki coupling reaction of brominated diben-
zofuran with 3-(9H-carbazol-9-yl)phenylboronic acid. Bro-
minated benzofuran and carbazole intermediates were
prepared according to a previously reported procedure.[5–8]
2-DFPCz was also synthesized by a previously reported pro-
cedure.[9] The synthesis of the three host materials is shown
in Scheme 1 (a detailed synthetic method is described in the
Supporting Information).
Herein, we synthesized high-triplet-energy host materials
based on dibenzofuran and phenylcarbazole derived from a
dibenzofuran intermediate with a substituent at 2-, 3- and 4-
positions, and the relationship between the substitution posi-
tion of dibenzofuran and material properties of host materi-
als was systematically investigated. Furthermore, the device
[a] Prof. C. W. Lee, Prof. J. Y. Lee
Department of Polymer Science and Engineering
Dankook Univeristy, 126, Jukjeon-dong, Suji-gu
Yongin-si, Gyeonggi-do, 448-701 (Korea)
Fax: (+82)31-8005-3585
Scheme 1. Overview of the synthesis of 2-DFPCz, 3-DFPCz, and 4-
DFPCz.
E-mail: leej17@dankook.ac.kr
Molecular simulation of the host materials was carried
out to study the effect of substitution position on the highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO). A suite of the Gaussian03 pro-
gram and the nonlocal density functional of Beckeꢀs three
parameters employing the Lee–Yang–Parr functional
(B3LYP) with 6-31G* basis sets were used for the simula-
[b] J.-A. Seo, Prof. M.-S. Gong
Department of Nanobiomedical Science and
WCU Research Center of Nanobiomedical Science
Dankook University, Cheonan
Chungnam 330-714 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203778.
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COMMUNICATION
3-DFPCz and 4-DFPCz were À2.45 eV, À2.48 eV and
À2.40 eV, respectively. Energy levels of host materials are
summarized in Table 1.
Table 1. Energy levels of the host materials.
Material
HOMO
[eV][a]
LUMO
[eV][b]
Band gap
[eV][c]
Triplet energy
[eV][d]
2-DFPCz
3-DFPCz
4-DFPCz
À5.90
À5.98
À5.85
À2.45
À2.48
À2.40
3.45
3.50
3.45
2.89
2.59
2.79
[a] The HOMO was measured by a surface analyzer. [b] The LUMO was
calculated from the difference between the HOMO and the band gap.
[c] The band gap was calculated from the absorption edge of the UV/Vis
spectra. [d] The triplet energy was calculated from the first emission peak
position of the low-temperature PL spectra.
Figure 1. HOMO and LUMO distributions of 3-DFPCz and 4-DFPCz.
tion.[10] Figure 1 shows the HOMO and LUMO distribution
of host materials. Phenylcarbazole plays a role of hole trans-
port unit owing to the electron-donating character of the
carbazole moiety,[11] while dibenzofuran acts as an electron-
transport unit owing to the electron-withdrawing property
of oxygen. Therefore, the HOMO was dispersed over carba-
zole unit, whereas the LUMO was localized on dibenzofu-
The photophysical properties of the three host materials
were analyzed using UV/Vis and photoluminescence (PL)
spectroscopy. UV/Vis spectra were obtained from vacuum-
deposited thin films of the host materials. Figure 2 shows
UV/Vis and PL spectra of the host materials, which exhibit-
ed typical UV/Vis absorption of phenylcarbazole and diben-
zofuran. The optical band gap was calculated from the ab-
sorption edge of UV/Vis spectra. 2-DFPCz, 3-DFPCz and 4-
DFPCz showed optical band gaps of 3.45 eV, 3.50 eV and
3.45 eV, respectively. There was little difference of optical
band gaps between three host materials in solid state. Triplet
energies calculated from peak position of the first emission
peak of low-temperature PL spectra were 2.89 eV, 2.59 eV
and 2.79 eV, for 2-DFPCz, 3-DFPCz and 4-DFPCz, respec-
tively. The triplet energy was low in 3-DFPCz because of ex-
tended conjugation through the para-substituted phenyl
group. In the case of 2-DFPCz and 4-DFPCz, meta substitu-
tion of the phenyl group suppressed the extension of conju-
gation, leading to a high triplet energy. In particular, 2-sub-
stitution was effective in raising the triplet energy of host
materials. The triplet energy of the host materials was rather
low compared with other dibenzofuran derivatives with di-
phenylphosphine oxide owing to the phenyl linkage and car-
bazole unit.
ACHTUNGTRENNUNGran unit. The HOMO distribution was similar, and there was
little difference of simulated HOMO levels between host
materials. However, the simulated LUMO level was differ-
ent depending on the substitution position. The LUMO was
distributed only on dibenzofuran in 2-DFPCz, as reported in
other work[9] (Supporting Information), while the LUMO
was extended to a phenyl unit of phenylcarbazole of 3-
DFPCz and 4-DFPCz. The extension of the LUMO orbital
stabilized the LUMO level of 3-DFPCz and 4-DFPCz. The
simulated LUMO levels of 3-DFPCz and 4-DFPCz were
À1.34 eV and À1.27 eV, respectively, compared with
À1.09 eV of 2-DFPCz.
The HOMO level of the three host materials were mea-
ACHTUNGTRENNUNGsured by a surface analyzer using vacuum-deposited thin
films. The HOMO levels of 2-DFPCz, 3-DFPCz and 4-
DFPCz were À5.90 eV, À5.98 eV and À5.85 eV, respectively.
The LUMO level was calculated from the HOMO level and
the optical band gap from UV/Vis spectra of thermally
evaporated thin filmsACTHNUGRTENUNG(Figure 2). LUMO levels of 2-DFPCz,
Hole-only and electron-only devices of three host materi-
als were fabricated to study the effect of substitution posi-
tion on the hole and electron transport properties of the
host materials. Figure 3 shows current-density–voltage
curves of hole- and electron-only devices. The hole current
density of hole-only devices was similar in all three devices,
while the electron current density of electron-only devices
was high in 3-DFPCz and 4-DFPCz. This can be explained
by the HOMO and LUMO distribution of host materials.
There was little difference of HOMO distribution between
host materials, leading to similar hole transport properties in
all three host materials. However, the LUMO distribution
was different depending on the substitution position of di-
benzofuran. 3-DFPCz and 4-DFPCz with a widely dispersed
LUMO showed high electron current density, but 2-DFPCz
with a localized LUMO distribution exhibited low electron
current density. The dominant electron transport mechanism
&
^ ^
, ) and low-
, ) of 3-DFPCz (filled symbols) and 4-DFPCz
Figure 2. UV/Vis ( ,&), solution photoluminescence (PL;
~ ~
temperature PL spectra (
(open symbols).
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J. Y. Lee et al.
showed the lowest driving voltage owing to the high current
density.
Figure 5 shows quantum-efficiency–luminance curves of
green PHOLEDs. The quantum efficiency of green PHO-
LEDs was high in 2-DFPCz, while 3-DFPCz and 4-DFPCz
Figure 3. Current-density–voltage curves of hole-only (open symbols) and
&
^ ^
, ),
electron-only devices (filled symbols): 2-DFPCz ( ,&), 3-DFPCz (
~ ~
, ).
4-DFPCz (
in host materials is electron hopping between host mole-
cules, which is dependent on orbital overlap between mole-
cules. The orbital overlap is efficient in 3-DFPCz and 4-
DFPCz owing to extended dispersion of the LUMO, result-
ing in high electron current density.
Green PHOLEDs were fabricated to evaluate three mate-
rials as host materials of PHOLEDs. A device consisting of
indium tin oxide (ITO, 50 nm)/poly(3,4-ethylenedioxythio-
phene) : polystyrenesulfonate (PEDOT:PSS) (60 nm)/4,4’-
(cyclohexane-1,1-diyl)bis(N-phenyl-N-p-tolylaniline)
(TAPC, 30 nm)/host : fac-tris(2-phenylpyridine) iridium (Ir-
Figure 5. Quantum-efficiency–luminance curves of green PHOLEDs.
showed a lower quantum efficiency than 2-DFPCz. The
maximum quantum efficiency of 2-DFPCz was 21.2% com-
pared with 16.9% and 17.4% of 3-DFPCz and 4-DFPCz.
The high quantum efficiency of 2-DFPCz can be explained
by the hole and electron current density in the emitting
layer. It is well known that charge balance in the emitting
layer is critical to the quantum efficiency of green PHO-
LEDs. In the case of 3-DFPCz and 4-DFPCz, a high elec-
tron current density relative to hole current density has a
negative effect on the charge balance in the emitting layer.
The high hole current density was reduced in 2-DFPCz, im-
proving the charge balance in the emitting layer. Although
the driving voltage was high in 2-DFPCz owing to low elec-
tron current density, the quantum efficiency was enhanced
owing to a better charge balance in the emitting layer.
ACHTUNGTRENNUNG(ppy)3) (30 nm, 3%)/diphenylphosphine oxide-4-(triphenyl-
silyl)phenyl (TSPO1, 25 nm)/LiF (1 nm)/Al (200 nm) was
used. Figure 4 shows current-density–voltage curves of green
As the triplet energies of 2-DFPCz and 4-DFPCz were
high enough for energy transfer to a blue emitting phospho-
ACHUTNGERNrNUG esAHCTUNGTRNENcUGN ent dopant, blue PHOLEDs were fabricated using the
three host materials. The device for the blue PHOLED con-
sisted of ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (30 nm)/
host : iridium (III) bisACTHNUTRGNEUGN(4,6-(difluorophenyl)-pyridinato-N,C’)
picolinate (FIrpic) (30 nm, 3%)/TSPO1 (25 nm)/LiF (1 nm)/
Al (200 nm). Figure 6 shows current-density–voltage-lumi-
Figure 4. Current-density–voltage–luminance curves of green PHOLEDs.
PHOLEDs. The current density of 3-DFPCz and 4-DFPCz
devices was higher than that of 2-DFPCz device, as can be
expected from hole-only and electron-only device data. The
high electron current density of 3-DFPCz and 4-DFPCz in-
creased the whole current density of the devices. The higher
current density of 3-DFPCz than that of 4-DFPCz is due to
the high electron current density of 3-DFPCz. Luminance
followed the same trend as the current density. The driving
voltage at 1,000 cdmÀ2 was 5.9 V, 4.7 V and 5.5 V for 2-
DFPCz, 3-DFPCz and 4-DFPCz, respectively. 3-DFPCz
Figure 6. Current-density–voltage–luminance curves of blue PHOLEDs.
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Organic Light-Emitting Diodes
COMMUNICATION
nance curves of blue PHOLEDs. The current density of
blue PHOLEDs showed a similar tendency to that of green
PHOLEDs. However, the luminance behavior of blue PHO-
LEDs was different from that of green PHOLEDs. The lu-
minance of 3-DFPCz was low despite the high current densi-
ty, which is related with the triplet energy of 3-DFPCz. The
triplet energy of 3-DFPCz was 2.59 eV, which was lower
than that of FIrpic dopant (2.65 eV). Therefore, energy
transfer from host to FIrpic is not efficient, and triplet exci-
tons of FIrpic are quenched by 3-DFPCz, leading to low lu-
minance in the 3-DFPCz device.
ciency in the 3-DFPCz device. The poor energy transfer is
due to reduced overlap of PL emission of 3-DFPCz and
FIrpic dopant in comparison with 2-DFPCz and 4-DFPCz
because of a red-shift of PL emission in 3-DFPCz. Addition-
ally, triplet exciton quenching of FIrpic by 3-DFPCz sup-
pressed FIrpic emission.
Power-efficiency–luminance curves of three host materials
are shown in Figure 9. Despite a high driving voltage of 2-
DFPCz, the high quantum efficiency of 2-DFPCz resulted in
Quantum-efficiency–luminance curves of blue PHOLEDs
are shown in Figure 7. A high quantum efficiency was ob-
tained in the 2-DFPCz device because of balanced hole and
Figure 9. Power-efficiency–luminance curves of blue PHOLEDs.
a high power efficiency compared with the other host mate-
rials.
Figure 7. Quantum-efficiency–luminance curves of blue PHOLEDs.
In conclusion, the effect of substitution position of diben-
zofuran on photophysical properties and device performan-
ces of host materials was systematically investigated by syn-
thesizing three host materials with different substitution po-
sitions. Substitution of dibenzofuran from 3-position reduced
triplet energy, but improved charge transport properties of
the host materials. Substitution at the 4-position maintained
the high triplet energy of dibenzofuran and also enhanced
the charge-transport properties. Substitution at the 2-posi-
tion had a negative effect on the charge-transport proper-
ties, but it retained the high triplet energy of the core.
Therefore, this work has revealed that proper selection of
substitution position can control the photophysical proper-
ties and device performances of host materials.
electron density in the emitting layer. The maximum quan-
tum efficiency of the 2-DFPCz blue device was 19.6% and
the quantum efficiency at 1,000 cdmÀ2 was 17.2%. The
quantum efficiency of 3-DFPCz and 4-DFPCz devices was
lower than that of the 2-DFPCz device. In particular, the
quantum efficiency of the 3-DFPCz device was very low
owing to triplet exciton quenching by the low triplet energy
of 3-DFPCz and poor energy transfer from host to dopant
material. The poor energy transfer can be confirmed by PL
spectra of the vacuum-deposited host:FIrpic film (Figure 8).
The 2-DFPCz and 4-DFPCz films exhibited a strong FIrpic
emission peak, but the 3-DFPCz film showed strong host
emission peak at 354 nm owing to incomplete energy trans-
fer, which lead to a substantial decrease of quantum effi-
Keywords: charge transport
· dibenzofurans · quantum
efficiency · substituent effects · triplet energy
[1] P. A. Vecchi, A. B. Padmaperuma, H. Qiao, L. S. Sapochak, P. E.
Burrows, Org. Lett. 2006, 8, 4211.
[2] C. Han, G. Xie, J. Li, Z. Zhang, H. Xu, Z. Deng, Y. Zhao, P. Yan, S.
Liu, Chem. Eur. J. 2011, 17, 8947.
[3] C. Han, G. Xiw, H. Xu, Z. Zhang, L. Xie, Y. Zhao, S. Liu, W.
Huang, Adv. Mater. 2011, 23, 2491.
[4] S. H. Jeong, C. W. Seo, J. Y. Lee, N. S. Cho, J. K. Kim, J. H. Yang,
Chem. Asian J. 2011, 6, 2895.
[5] E. S. Hand, S. C. Johnson, D. C. Baker, J. Org. Chem. 1997, 62, 1348.
[6] Y. Wei, N. Yoshikai, Org. Lett. 2011, 13, 5504.
[7] D. J. Cram, B. Dicker, M. Lauer, C. B. Knobler, K. N. Trueblood, J.
Am. Chem. Soc. 1984, 106, 7150.
Figure 8. Photoluminescence spectra of FIrpic-doped host films.
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www.chemeurj.org
1197

J. Y. Lee et al.
[8] S. J. Su, C. Cai, J. Kido, Chem. Mater. 2011, 23, 274.
[9] C. W. Lee, K. S. Yook, J. Y. Lee, unpublished results.
[10] Gaussian 03, Revision B05, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgom-
Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Fores-
man, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Na-
nayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wallingford
CT 2004.
ACHTUNGTRENNUNGery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y.
Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O.
[11] S. U. Pandya, H. A. Al Attar, V. Jankus, Y. Zheng, M. R. Bryce,
A. P. Monkman, J. Mater. Chem. 2011, 21, 18439.
Received: October 23, 2012
Revised: November 16, 2012
Published online: December 18, 2012
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Chem. Eur. J. 2013, 19, 1194 – 1198