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M. Ichikawa et al. / Organic Electronics 11 (2010) 1966–1973
2.2.3. 1,3,5-Tris(pinacorato boryl)benzene ((PinB)3B)
plasma for 5 min and then the substrate was immediately
loaded into a high-vacuum chamber (base pressure below
ꢂ6 ꢁ 10ꢀ4 Pa) where organic layers, 0.5-nm-thick LiF, and
200-nm-thick aluminum cathode layers were deposited
by thermal evaporation. Deposition rates were 0.6 Å/s for
organic materials, 0.1 Å/s for LiF, and 6 Å/s for Al. The cur-
rent density-applied voltage–luminance (J–V–L) character-
istics of the OLEDs were measured with a commercial
OLED-characteristics measurement system (Precise Gauge
EL1003) and a source meter (Keithley 2400). Electrolumi-
nescence (EL) spectra were recorded simultaneously with
the EL1003 during J–V–L measurements.
(PinB)3B was synthesized by Suzuki–Miyaura coupling
reaction using 1,3,5-tribromobenzene (8.6 g, 27.3 mmol),
bis(pinacorato)diboron (25.0 g, 98.4 mmol), potassium
acetate (24.1 g, 245.7 mmol), and [1,10-bis(diphenylphos-
phino)ferrocene] dichloropalladium(II) as catalyst in di-
methyl sulfoxide. The reaction mixture was refluxed for
21 h, and then cooled to room temperature. Pure water
(1000 ml) was added and the mixture was stirred for
30 min and filtered to remove precipitates. Addition of
methanol gave a crude white crystalline powder, which
was dissolved in ethyl acetate, filtered to remove the cata-
lyst, and distilled to yield pure (PinB)3B (7.1 g; 57% yield).
dH (600 MHz, CDCl3): 8.37 (s, 3H), 1.33 (s, 36H).
1H- and 13C NMR spectra were recorded on an FT–NMR
spectrometer (JEOL JNM-ECA600). Thermal analyses were
performed on a differential scanning calorimeter (Seiko
Instruments DSC-6200) at a heating rate of 10 °C/min un-
der N2 gas. Ultraviolet (UV) and visible absorption spectra
were recorded with a spectrophotometer (Shimadzu UV-
3150); photoluminescence (fluorescence and phosphores-
cence) spectra were recorded with a spectrofluorometer
(Horiba FluoroMax4P). HOMO energy was determined
with a photoelectron emission yield spectrometer (Riken
Keiki AC-3), and the HOMO energy was defined as being
equal to the ionization potential measured by photoelec-
tron emission spectroscopy. Optical band gaps were deter-
mined by the spectral onset of each UV–visible absorption
spectrum; the lowest unoccupied molecular orbital
(LUMO) energy was then estimated to be the difference be-
tween the HOMO energy and the optical bandgap. Spectro-
scopic measurements were conducted with thin films
prepared by thermal evaporation on quartz substrates.
Electron mobility was measured by conventional
time-of-flight (TOF) techniques. The excitation light was
a 500-ps-duration optical pulse from an N2 gas laser
(k = 337 nm, Usho Optical Systems KEN-150). Test samples
were prepared by thermal evaporation in vacuum and
encapsulated with fresh desiccant under a highly inert
atmosphere of N2 at the dew point of almost ꢀ60 °C and
O2 concentration of <10 ppm. A 100-nm-thick fullerene
2.2.4. 1,3,5-Tris([20,200]bipyridin-60-yl)benzene (BpyB)
BpyB was synthesized from (PinB)3B (2.5 g, 5.5 mmol)
and Br-Bpy (3.8 g, 15.0 mmol) by Suzuki–Miyaura coupling
reaction using 1 M K2CO3 aqueous solution (32.4 ml,
32.4 mmol) as base, tetrakis(trisphenylphosphine)palla-
dium(0) (0.32 g, 0.28 mmol) as catalyst, and 135 ml of a
4/1 ethanol/toluene mixture as solvent. The reaction mix-
ture was refluxed with stirring for 33 h, and then cooled
to room temperature. Pure water (500 ml) was added
and the mixture was stirred for 30 min. Extraction by chlo-
roform, drying with MgSO4, filtration to remove precipi-
tates, and distillation to remove the chloroform gave a
crude brown powder. Purification by silica gel column
chromatography with a 1/3 chloroform/hexane mixture
as eluent yielded BpyB as a white crystalline powder
(1.1 g; 41% yield). dH (600 MHz, CDCl3): 9.01(s, 3H), 8.76
(d, 3H, J 7.2 Hz), 8.74 (d, 3H, J 4.8 Hz), 8.47 (d, 3H, J
7.2 Hz), 8.01 (dd, 3H, J 8.7 Hz), 7.98 (d, 3H, J 7.8 Hz), 7.88
(dd, 3H, J 7.8 Hz), 7.36 (dd, 3H, J 6.3 Hz). dC (150 MHz,
CDCl3): 156.43, 156.31, 155.90, 149.14, 140.43, 137.82,
136.82, 126.25, 123.78, 121.37, 120.65, 119.65.
2.2.5. 1,3,5-Tris([20,200,600,2000]terpyridin-60-yl)benzene (TpyB)
TpyB was synthesized by Suzuki–Miyaura coupling
reaction using (PinB)3B (2.4 g, 5.26 mmol) and Br-Tpy
(5.0 g, 16.0 mmol) essentially as described for BpyB using
1 M K2CO3 aqueous solution (32.3 g, 32.4 mmol), tetra-
kis(trisphenylphosphine)palladium(0) (0.32 g, 0.28 mmol),
and 150 ml of a 4/1 ethanol/toluene mixture. The reaction
mixture was refluxed for 3 h, cooled, and then filtered.
Recrystallization from o-dichlorobenzene solution yielded
TpyB as a white powder (2.7 g; 67% yield). dH (600 MHz,
CDCl3): 9.06 (s, 3H), 8.81 (d, 3H, J 7.8 Hz), 8.73(d, 3H, J
3 Hz), 8.68 (d, 3H, J 8.4 Hz), 8.67 (d, 3H, 7.8 Hz), 8.50 (d,
3H, J 8.4 Hz), 8.03 (m, 9H), 7.89 (dd, 3H, J 7.8 Hz), 7.35 (d,
C60 layer was used as a charge-generation layer for optical
excitation [27].
Computational chemistry was conducted with commer-
cial software (Wavefunction Spartan 06W) on a personal
computer with a dual-core processer. Density function the-
ory (DFT) was used to determine conformations, with a
B3LYP hybrid functional and a 6-311G* basis set for deter-
mining optimized geometries and estimating activation
energies for conformation change. During the estimation
of activation energies, we constrained the dihedral angle
that was responsible for the conformation change and con-
ducted each geometry optimization with the dihedral con-
straint at several angles.
3H,
J 7.8 Hz). dC (150 MHz, CDCl3): 156.21, 155.85,
155.54, 155.30, 149.15, 140.28, 137.77, 136.87, 130.51,
127.70, 126.17, 123.75, 121.26, 121.21, 121.05, 120.60,
119.69.
3. Results and discussion
2.3. Device fabrication and measurements
3.1. Thermal and electronic properties
All OLEDs were fabricated on 150-nm-thick layers of in-
dium-tin oxide (ITO) that were commercially precoated
Fig. 2 shows differential scanning calorimetry (DSC)
curves for BpyB and TpyB. During the first heating proce-
dure, endothermic peaks due to melting are observed for
onto glass substrates with a sheet resistance of 14
X/sq.
The solvent-cleaned ITO surface was treated with O2