Published on the web May 18, 2013
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Molecular Design of High-molecular-orientation Electron-transport Materials
and Application to Organic Light-emitting Diodes
Kazunori Togashi,1,4 Yuta Sagara,1,2 Takuma Yasuda,1,2,3 and Chihaya Adachi*1,2,3
1Center for Organic Photonics and Electronics Research (OPERA), Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
2International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
3Department of Applied Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
4Hodogaya Chemical Co., 45 Miyukigaoka, Tsukuba, Ibaraki 305-0841
(Received February 21, 2013; CL-130150; E-mail: adachi@cstf.kyushu-u.ac.jp)
Two novel electron-transport materials (ETMs), 2,3¤-Bpy-
TP and 2,4¤-Bpy-TP, with high horizontal molecular orientation
to substrates, were synthesized. It was shown by measuring the
IR absorption spectra of their deposited thin films that C-H£N
hydrogen bonds are formed between the 2,3¤-Bpy-TP molecules
and between the 2,4¤-Bpy-TP molecules. It was also shown that
there is a closed relationship between their molecular orienta-
tions and the driving voltages of electron-only devices (EODs)
using them as electron-transport layers (ETLs).
Figure 1. Chemical structures of 2,2¤-Bpy-TP, 2,3¤-Bpy-TP, and
2,4¤-Bpy-TP.
Organic light-emitting diodes (OLEDs) are attracting con-
siderable attention because of their great potential for use in
practical applications including flat panel displays and lighting.
In general, suppression of the OLED driving voltage is desirable
to ensure low device power consumption. To suppress the
driving voltage of OLEDs, it has been necessary to improve the
charge injection efficiency from the electrodes and the charge
mobilities of the charge-transport layers. In particular, the
development of electron-transport materials (ETMs) with high
electron mobilities is important, because the hole mobilities are
usually much higher than the electron mobilities in OLEDs.1,2
To improve the electron mobility, strong intermolecular inter-
action is required, because it leads to large overlaps of the
³ orbitals which participate in electron transport.3,4 In particular,
horizontal orientation of the molecules to a substrate is an
efficient way to achieve higher intermolecular interaction.
Previously, the horizontal orientation of molecules with long
rod-like or planar structures to substrates has been reported.5-7
We also reported on the molecular orientation of an ETM (2,7-
bis(2,2¤-bipyridin-5-yl)triphenylene (Bpy-TP2)), where the val-
ue of the orientation parameter, S, was rather low (S = ¹0.1)
despite the ETM’s long rod-like molecular structure.8 Recently,
Kido’s group reported that 4,6-bis[3,5-di(pyridin-3-yl)phenyl]-
2-methylpyrimidine and 4,6-bis[3,5-di(pyridin-4-yl)phenyl]-2-
methylpyrimidine (B3 and B4PyMPM) molecules with nitrogen
atoms in the pyridine rings at the outer sides of the molecules
have larger anisotropies than that of 4,6-bis[3,5-di(pyridin-2-
yl)phenyl]-2-methylpyrimidine (B2PyMPM) in their deposited
thin films, because they are connected by intermolecular C-
H£N hydrogen bonds.9,10 Using this as a guideline for molecu-
lar design, we designed and synthesized two novel ETMs, 2,7-
bis(2,3¤-bipyridin-5-yl)triphenylene (2,3¤-Bpy-TP) and 2,7-
bis(2,4¤-bipyridin-5-yl)triphenylene (2,4¤-Bpy-TP), with nitro-
gen atoms in the pyridine rings at the outer sides of the Bpy-TP2
(2,2¤-Bpy-TP) molecules. Here, we report the molecular ori-
entation of deposited thin films of these ETMs and their
electron-transport properties in electron-only devices (EODs)
and OLEDs.
The chemical structures of 2,3¤-Bpy-TP and 2,4¤-Bpy-TP are
shown in Figure 1. The materials were synthesized by the
Suzuki-Miyaura coupling reaction11,12 and purified by sublima-
tion before characterization (see the Supporting Information for
details).13
The crude ETMs were sublimed, and their thermophysical
properties were then measured by differential scanning calorim-
etry (DSC). Their optical and photophysical properties were also
evaluated by photoelectron spectroscopy (Riken AC-3), ultra-
violet-visible (UV-vis) absorption and photoluminescence (PL)
spectroscopies. The physical properties of 2,2¤-Bpy-TP, 2,3¤-
Bpy-TP, and 2,4¤-Bpy-TP are summarized in Table 1.14
As shown in Table 1, 2,3¤-Bpy-TP and 2,4¤-Bpy-TP have
low-lying highest occupied molecular orbital (HOMO) levels
when compared with 2,2¤-Bpy-TP. The lowest unoccupied
molecular orbital (LUMO) levels of 2,3¤-Bpy-TP and 2,4¤-
Bpy-TP estimated from the difference between the ionization
potential (Ip) and the energy gap (Eg) in each case are also much
deeper than that of 2,2¤-Bpy-TP and are similar to the LUMO
values obtained by density functional theory (DFT) calculations,
indicating that they have better electron injection properties from
cathodes than does 2,2¤-Bpy-TP.
The molecular orientations of the deposited thin films
were investigated by variable angle spectroscopic ellipsometry
(VASE) measurements. We can quantify the molecular orienta-
tion of an amorphous film by using the orientation parameter S.5
As shown in Figure 2, the values of S determined by VASE
analysis are S = ¹0.4 for 2,3¤-Bpy-TP and S = ¹0.3 for 2,4¤-
Bpy-TP. The deposited thin films of 2,3¤-Bpy-TP and 2,4¤-Bpy-
Chem. Lett. 2013, 42, 651-653
© 2013 The Chemical Society of Japan