Comparison of tetraphenylmethane and tetraphenylsilane as core structures of high-triplet-energy hole- and electron-transport materials

Article abstract of DOI:10.1002/chem.201103640

Tetraphenylmethane and tetraphenylsilane were compared as building units for high-triplet-energy, charge-transport materials. Tetraphenylsilane is effective as the building unit in electron-transport materials, whereas tetraphenylmethane is suitable in hole-transport materials (see figure; TSPO1=diphenylphosphine oxide-4-(triphenylsilyl), TSPA=phenyl-N,N-diphenyl-4- (triphenylsilyl)aniline, and TCPA=N,N-diphenyl-4-tritylaniline). Copyright

Full text of DOI:10.1002/chem.201103640

COMMUNICATION
DOI: 10.1002/chem.201103640
Comparison of Tetraphenylmethane and Tetraphenylsilane as Core
Structures of High-Triplet-Energy Hole and Electron-Transport Materials
Chil Won Lee and Jun Yeob Lee*[a]
It is important to improve the quantum efficiency of or-
ganic light-emitting diodes (OLEDs) to reduce the power
consumption of OLED displays. One effective approach to
enhance the quantum efficiency of OLEDs is to adopt phos-
phorescent-emitting materials instead of the typical fluores-
cent-emitting materials.[1] The use of a phosphorescent emit-
ter could increase the external quantum efficiency of red,
green, and blue OLEDs with more than 20%.[2–5]
To obtain high quantum efficiency in phosphorescent
OLEDs (PHOLEDs), it is important to confine triplet exci-
tons and charges inside the emitting layer.[3–9] For this pur-
pose, hole- and electron-transport layers play an impor-
tant role. Therefore, the hole- and electron-transport materi-
als should have higher triplet energy than the phosphores-
cent emitters and should have the energy levels for hole or
electron blocking. In particular, the triplet energy of the
hole- and electron-transport materials should be above
2.80 eV for applications in all red, green, and deep-blue
PHOLEDs.[5,6]
The most popular approach to get high triplet energy in
charge-transport materials is to separate the conjugated
building units to reduce the conjugation length of the mole-
cule.[5,7–9,10–16] Typically, tetraphenylsilane with silicon as the
linking unit has been widely used as core structures for
high-triplet-energy, charge-transport materials, because sili-
con disconnects the conjugation between different building
units.[5,10–13] Several materials with tetraphenylsilane as core
structures have been developed for applications as high-trip-
let-energy, hole- and electron-transport materials. For exam-
In addition to the tetraphenylsilane, tetraphenylmethane,
with sp3-type carbons as the linking unit to separate the aro-
matic building units, has also been used as high-triplet-
energy core, because sp3 carbons have a tetragonal geomet-
rical structure. However, there have been few reports on tet-
raphenylmethane as core structures of high-triplet-energy,
hole- and electron-transport materials, although several
compounds with sp3 carbons in the fused ring structure are
known.[17,18]
Herein, two high-triplet-energy, hole-transport materials,
with tetraphenylmethane and tetraphenylsilane core struc-
tures, and one high-triplet-energy, electron-transport materi-
al, with a tetraphenylmethane core, are described and the
roles of tetraphenylmethane and tetraphenylsilane as build-
ing units are compared. Tetraphenylmethane show good per-
formance as the core structure of hole-transport materials,
whereas tetraphenylsilane is effective as the core of elec-
tron-transport materials.
Four hole- and electron-transport materials were prepared
to compare tetraphenylsilane and tetraphenylmethane as
core structures of charge-transport materials. Two hole-
transport materials, N,N-diphenyl-4-(triphenylsilyl)aniline
(TSPA) and N,N-diphenyl-4-tritylaniline (TCPA), with tetra-
phenylsilane and tetraphenylmethane core structures, re-
spectively, were modified with a diphenylamine functional
group to obtain good hole-transport properties. Two elec-
tron-transport materials, TSPO1 and diphenyl(4-tritylphe-
nyl)phosphine (TCPO1), have the same backbone structure
and were modified with a diphenylphosphine oxide func-
tional group to improve the electron-transport properties.
The synthesis of the TSPO1 has been reported in our previ-
ous work.[5] The other compounds were newly synthesized in
this work.
The synthesis of the three charge-transport materials is
shown in Scheme 1. TSPA was easily prepared by the amina-
tion of brominated tetraphenylsilane by using palladiu-
m(II)acetate as the catalyst. TCPA was synthesized by the
reaction of tritylaniline with bromobenzene with palladiu-
m(II)acetate as the catalyst. TCPO1 was synthesized by the
phosphorylation of 1-bromo-4-tritylbenzene prepared from
4-tritylaniline. All compounds were purified by column
chromatography to obtain a purity level above 99% from
high-performance chromatography analysis. The detailed
synthesis of the compounds is described in the Supporting
Information.
ple,
diphenylphosphine
oxide-4-(triphenylsilyl)phenyl
(TSPO1) is effective as a high-triplet-energy, electron-trans-
port material for all red, green, and deep-blue PHOLEDs[5]
and 2,5-bis(4-triphenylsilanyl-phenyl)[1,3,4] oxadiazole can be
used as electron-transport material for green PHOLEDs.[11]
For hole-transport materials, bis[4-(p,p’-ditolylamino)phe-
nyl]diphenylsilane improves the external quantum efficiency
of sky-blue PHOLEDs.[10]
[a] Prof. C. W. Lee, Prof. J. Y. Lee
Department of polymer science and engineering
Dankook university, 126, Jukjeon-dong, Suji-gu
Yongin-si, Gyeonggi-do, 448-701 (Korea)
Fax : (+82)31-8005-3585
E-mail: leej17@dankook.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103640.
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Scheme 1. Synthesis TCPA, TSPA, and TCPO1. The chemical structure of TSPO1 is also shown.
Photophysical properties of the four charge-transport ma-
terials were measured with UV/Vis and photoluminescence
(PL) spectrometers. Table 1 summarizes the UV/Vis and PL
data of the four compounds and the spectra are presented in
the Supporting Information (Figure S1). TSPA and TCPA
showed strong UV/Vis absorption signals at 309 and 304 nm,
respectively, due to the strong absorption of the diphenyla-
mine unit. The UV/Vis absorption was redshifted by 5 nm
for TSPA, with the tetraphenylsilane core. The PL emission
of TSPA was similarly redshifted by 5 nm compared with
that of TCPA, due to the redshifted UV/Vis absorption. The
bandgaps could be calculated from the absorption edges of
the UV/Vis spectra and they were 3.56 and 3.67 eV for
TSPA and TCPA, respectively. The redshifted absorption/
emission spectra and the reduced bandgap of TSPA can be
explained by the LUMO stabilization effect of the silicon
unit in the tetraphenylsilane core.[19–21] Although both
carbon and silicon are known to separate the conjugation of
the aromatic group, silicon tends to extend the LUMO of
the conjugated molecule because the s* orbital of silicon in-
teracts with the p* orbital of the conjugated unit. As the
HOMO is not greatly affected by the silicon linking group,
the LUMO of the silicon-based molecule is stabilized, lead-
ing to a reduced bandgap and redshifted emission. In the
case of carbon, it does not affect the HOMO and LUMO of
the molecules because the conjugation is completely isolated
by the sp3 carbons. This is confirmed by the HOMO/LUMO
levels of TCPA and TSPA. There is little difference between
the HOMO levels of TCPA and TSPA, whereas the LUMO
is stabilized by 0.15 eV in TSPA. Similar results were ob-
tained for the TCPO1 and TSPO1. Redshifts of the UV/Vis
absorption and PL emission signals were observed and the
Table 1. Physical properties of charge-transport materials.
Material UV/Vis PL
HOMO LUMO Bandgap Triplet
[nm]a
[nm][a] [eV][b]
[eV][c]
[eV][d]
Energy
[eV][e]
TCPA
TSPA
TCPO1
TSPO1
304
309
272
278
366
371
324
322
À5.78
À5.82
À6.75
À6.79
À2.11
À2.26
À2.29
À2.52
3.67
3.56
4.46
4.27
2.92
2.90
3.38
3.36
[a] Measured in tetrahydrofuran. [b] HOMOs were measured by CV.
[c] LUMOs were calculated from the difference between HOMO and
bandgap. [d] Bandgaps were calculated from the absorption edges of the
UV/Vis spectra. [e] Triplet energies were calculated from the first emis-
sion peaks of low-temperature PL spectra.
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Core Structures of Electron-Transport Materials
COMMUNICATION
bandgap is decreased in TSPO1, containing the silicon unit.
The LUMO is most stabilized in TSPO1 due to the silicon
unit, leading to a reduced bandgap.
The triplet energy of the four compounds was calculated
from the first emission peak of low-temperature (77 K) PL
spectra. The triplet energies of TSPA and TCPA are 2.90
and 2.92 eV, whereas those of TSPO1 and TCPO1 are 3.36
and 3.38 eV, respectively. Tetraphenylsilane-based, charge-
transport materials exhibit lower triplet energy than the tet-
raphenylmethane-based material due to the extended conju-
gation. All four charge-transport materials show triplet
energy high enough for exciton blocking in deep-blue
PHOLEDs.
The HOMO and LUMO levels of the four materials were
measured by cyclic voltammetry (CV) and are summarized
in Table 1. The HOMO level of TCPA is À5.78 eV, whereas
that of TSPA is À5.82 eV. There is only 0.04 eV difference
between the HOMO levels of the two hole-transport materi-
als. However, the LUMO level is lowered by 0.15 eV in
TSPA due to the LUMO stabilization effect explained
above. The HOMO and LUMO levels of TCPA and TSPA
are suitable for hole injection and electron blocking in deep-
blue PHOLEDs.
TCPO1 and TSPO1 show deep HOMO levels of À6.75
and À6.79 eV, and LUMO levels of À2.29 and À2.52 eV due
to the electron-withdrawing phosphine oxide unit. As the
bandgap of the two electron-transport materials is large, be-
cause the conjugation is isolated by phosphine oxide unit,
the LUMO levels are rather high.
Figure 1. Molecular-simulation results of TCPA, TSPA, TCPO1, and
TSPO1.
To compare tetraphenylmethane and tetraphenylsilane as
core structures of high-triplet-energy, hole-transport materi-
als, hole-only devices of TCPA and TSPA were fabricated.
The structure of the hole-only devices was as follows:
indium tin oxide (ITO, 50 nm)/N,N’-diphenyl-N,N’-bis-[4-
(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,4’-diamine
(DNTPD, 60 nm)/N,N’-di(1-naphthyl)-N,N’-diphenylbenzi-
dine (NPB, 5 nm)/TSPA or TCPA (30 nm)/Au. Figure 2a
shows the current density of hole-only devices of TCPA and
TSPA. The hole current density of TCPA was higher than
that of TSPA, indicating that TCPA is better than TSPA as
a hole-transport material. Because of the small differences
(0.04 eV) between the HOMO levels of TCPA and TSPA,
the high hole current density of TCPA can be explained by
the better hole-transport properties of TCPA. In the molec-
ular-orbital diagram, the HOMO is dispersed over the tri-
phenylamine unit in both materials and is little affected by
the linking unit, indicating that the different hole-transport
properties are not caused by the HOMO distribution. In-
stead, the different hole-transport properties can be ex-
plained by the molecular size of TCPA and TSPA. The
length of the carbon–carbon bond is shorter than that of
carbon–silicon and the atomic size of carbon is smaller than
that of silicon, indicating that the tetraphenylmethane core
is smaller than the tetraphenylsilane core. Therefore, the in-
termolecular packing is improved in TCPA, leading to
better hole-transport properties.[23]
Electron-only devices of TCPO1 and TSPO1 were fabri-
cated to compare tetraphenylmethane and tetraphenylsilane
as core structures of high-triplet-energy, electron-transport
materials. The device structure was as follows: ITO (50 nm)/
LiF (1 nm)/TSPO1 or TCPO1 (50 nm)/LiF (1 nm)/Al
(200 nm). Data for the electron-only devices of TCPO1 and
TSPO1 are shown in Figure 2b. The electron current density
was much higher in TSPO1 than in TCPO1. In contrast to
the data of the hole-only device, tetraphenylsilane was
better than tetraphenylmethane regarding electron trans-
The thermal stability of TCPA, TSPA, and TCPO1 was
analyzed by using a thermogravimetric analyzer and all
three compounds were stable up to 3508C (Figure S2 in the
Supporting Information).
Molecular simulation of the four materials was carried
out to correlate the molecular-orbital distribution with pho-
tophysical properties of the materials. A suite of the Gaussi-
an 03 program and the nonlocal density functional of
Beckeꢁs 3-parameters employing the Lee–Yang–Parr func-
tional (B3LYP) with the 6-31G* basis sets were used for the
simulation.[22] Figure 1 shows the HOMO and LUMO distri-
bution of the four materials. The HOMOs of TCPA and
TSPA are localized on the triphenylamine unit, whereas the
LUMOs are dispersed over the whole molecule. There was
little difference of the HOMO and LUMO distribution be-
tween TCPA and TSPA. In the case of TCPO1 and TSPO1,
the LUMOs are distributed over the phenyl unit connected
to the diphenylphosphine oxide, whereas the HOMOs are
dispersed over the phenyl units attached to the carbon or
silicon atoms, due to the strong electron-withdrawing char-
acter of the diphenylphosphine oxide. The HOMO and
LUMO distributions of TCPO1 and TSPO1 are similar. The
carbon and silicon atoms do not induce significant change to
the molecular-orbital distribution. Therefore, the HOMO
and LUMO difference between carbon and silicon com-
pounds originates from the LUMO stabilization effect of sil-
icon.
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J. Y. Lee and C. W. Lee
nyl)pyridine) iridium picolinate (30 nm, 3%)/TSPO1 or
TCPO1 (25 nm)/LiF (1 nm)/Al (200 nm). Figure 3 shows the
current density–voltage–luminance and quantum efficiency–
luminance curves of deep-blue PHOLEDs. The current den-
sity of the deep-blue PHOLEDs is maximized in the device
Figure 2. Current density–voltage curves of a) hole-only devices of TCPA
~
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( ) and TSPA ( ) and b) electron-only devices of TCPO1 ( ) and
Figure 3. a) Current density–voltage–luminance and b) quantum efficien-
cy–luminance data of deep-blue PHOLEDs with different hole- and elec-
~
TSPO1 ( ).
&
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tron-transport materials: TCPA/TSPO1 ( ), TSPA/TSPO1 ( ), TCPA/
~
*
TCPO1 ( ), TSPA/TCPO1 ( ).
port. On the basis of the molecular size, TSPO1 should be
worse than TCPO1, because TCPO1 possesses short
carbon–carbon bonds. However, TSPO1 show high electron
current density. This can be explained by differences in
LUMO levels and molecular-orbital distributions between
TCPO1 and TSPO1. The LUMO level of the electron-trans-
port material is critical for the electron injection. The
LUMO level of TSPO1 (À2.52 eV) is more suitable, than
that of TCPO1 (À2.29 eV), for electron injection from
a LiF/Al electrode to the electron-transport layer, because
of smaller energy barriers for electron injection. The re-
duced energy barrier facilitates electron injection from the
LiF/Al cathode to TSPO1. The extended LUMO dispersion
of TSPO1 also contributes the high electron current density
of TSPO1. The extended LUMO distribution facilitates
electron transport and improves the electron current density
of TSPO1. In the case of TCPO1, all phenyl units in the
molecule are separated; this has a negative effect on the
electron transport.
with TCPA and TSPO1 as hole- and electron-transport
layers; this is in agreement with the hole-only and electron-
only device data. The deep-blue PHOLED with TSPA and
TCPO1 shows the lowest current density. The luminance fol-
lowed the same tendency as the current density in the differ-
ent hole- and electron-transport materials. The TCPA/
TSPO1 deep-blue PHOLED also exhibited higher quantum
efficiency than other devices, because of better charge bal-
ance in the emitting layer. Other devices were not as good
as the TCPA/TSPO1 device, due to unbalanced hole and
electron injection.
In conclusion, tetraphenylmethane and tetraphenylsilane
were compared for the use as building units in hole- and
electron-transport materials. Tetraphenylmethane works ef-
fectively as building units of hole-transport materials, where-
as tetraphenylsilane is a good building unit in electron-trans-
port materials. Therefore, the combination of tetraphenyl-
methane-based, hole-transport materials and tetraphenylsi-
lane-based, electron-transport materials gives the best per-
formances in deep-blue PHOLEDs.
Deep-blue PHOLEDs were fabricated by using tetraphe-
nylmethane- and tetraphenylsilane-based, hole- and elec-
tron-transport materials. The device structure was as fol-
lows: ITO (50 nm)/DNTPD (60 nm)/NPB (5 nm)/TSPA or
TCPA (30 nm)/(9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-
3-yl)diphenylphosphine oxide: bis((3,5-difluoro-4-cyanophe-
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Keywords: deep-blue PHOLEDs · high triplet energy · hole
transport material · tetraphenylmethane · tetraphenylsilane
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Received: November 18, 2011
Revised: February 28, 2012
Published online: && &&, 0000
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J. Y. Lee and C. W. Lee
Tetraphenylmethane and tetraphenyl-
silane were compared as building units
for high-triplet-energy, charge-trans-
port materials. Tetraphenylsilane is
effective as the building unit in elec-
tron-transport materials, whereas tetra-
phenylmethane is suitable in hole-
transport materials (see figure;
Phosphorescence
C. W. Lee, J. Y. Lee* ......... &&&& — &&&&
Comparison of Tetraphenylmethane
and Tetraphenylsilane as Core Struc-
tures of High-Triplet-Energy Hole and
Electron-Transport Materials
TSPO1=diphenylphosphine oxide-4-
(triphenylsilyl), TSPA=phenyl-N,N-
diphenyl-4-(triphenylsilyl)aniline, and
TCPA=N,N-diphenyl-4-tritylaniline).
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