Binaphthalene bridged bipolar transporting materials for blue electroluminescence: Toward high EL efficiency via molecular tuning

Article abstract of DOI:10.1016/j.tet.2014.03.017

Two isomeric bipolar transporting molecules containing arylamine and benzimidazole moieties linked by 1,1-binaphthalene bridge have been synthesized and used for blue light-emitting diodes. The highly twisted binaphthalene bridge is beneficial for amorphous morphology, good solubility, high thermal stability and high photoluminescence quantum efficiency (Φ_PL). The charge transfer bands of these compounds exhibit interesting solvent-polarity dependent fluorescence properties. The physical properties of the compounds were tunable upon binding of the benzimidazole with binaphthalene group via C- (BINAPC) or N- (BINAPN) linkage. Three-layered blue-emitting OLEDs using BINAPC or BINAPN as the emitting layer, NPB as the hole transporting layer, and TPBI as the electron transporting layer as well as hole-blocking layer exhibit good performance. External quantum efficiencies of 2.49% with color coordinates of (0.15, 0.11) and 2.67% with color coordinates of (0.16, 0.16) were achieved for BINAPC and BINAPN, respectively.

Full text of DOI:10.1016/j.tet.2014.03.017

Tetrahedron 70 (2014) 2992e2998
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Binaphthalene bridged bipolar transporting materials for blue
electroluminescence: toward high EL efficiency via molecular tuning
,
b
,
,
d
d,
Rossatorn Muangpaisal a c, Wei-I. Hung b, Jiann T. Lin b , San-Yu Ting , Li-Yin Chen
*
*
a Taiwan International Graduate Program, Taiwan, ROC
b Institute of Chemistry, Academia Sinica, 128 Section 2 Academia Road, Nankang, Taipei 11529, Taiwan, ROC
c Department of Chemistry, National Tsing Hua University, 101 Section 2 Kuang-Fu Road, Hsinchu 30031, Taiwan, ROC
d Department of Photonics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Two isomeric bipolar transporting molecules containing arylamine and benzimidazole moieties linked
by 1,10-binaphthalene bridge have been synthesized and used for blue light-emitting diodes. The highly
twisted binaphthalene bridge is beneficial for amorphous morphology, good solubility, high thermal
stability and high photoluminescence quantum efficiency (FPL). The charge transfer bands of these
compounds exhibit interesting solvent-polarity dependent fluorescence properties. The physical prop-
erties of the compounds were tunable upon binding of the benzimidazole with binaphthalene group
via C- (BINAPC) or N- (BINAPN) linkage. Three-layered blue-emitting OLEDs using BINAPC or BINAPN as
the emitting layer, NPB as the hole transporting layer, and TPBI as the electron transporting layer as well
as hole-blocking layer exhibit good performance. External quantum efficiencies of 2.49% with color
coordinates of (0.15, 0.11) and 2.67% with color coordinates of (0.16, 0.16) were achieved for BINAPC and
BINAPN, respectively.
Received 20 December 2013
Received in revised form 26 February 2014
Accepted 6 March 2014
Available online 16 March 2014
Keywords:
Amorphous materials
Blue light-emitting
Bipolar transport
Structureeproperties correlations
Organic light emitting diode
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
of small molecules for forming amorphous morphology is desired
for OLEDs application. Moreover, amorphous materials with a high
Organic light-emitting diodes (OLEDs) have potential applica-
tions in display and solid-state lighting.1 In recent years, significant
progress has been made on light-emitting materials for full color
display and light source, including luminous efficiency, color purity,
and lifetime.2 Blue-, green-, and red-emitting materials provide
three prime colors needed for OLEDs application. Though the de-
velopment of red- and green-emitters for organic light emitting
diodes (OLEDs) has been very satisfactory so far, progress on blue
emitters with good stability and high efficiency still lag behind and
there is much room for improvement in order to speed up light-
emitting device technology.3 Critical requirements for emissive
materials in OLEDs include high luminescence quantum yield and
color purity in the solid state, bipolar transport, good film mor-
phology, and good thermal and redox stability.2
glass transition temperature have advantages over crystalline ma-
terials due to their good solubility and easy processability.5 The
binaphthalene entity is widely incorporated in molecules for sen-
sor,6 asymmetric catalysis,7 non-linear optics,8 and light-emitting
diodes.9 It was demonstrated that the binaphthalene moiety in
a molecule could resist molecular crystallization and facilitate for-
mation of glassy morphology.10
Our group has been interested in bipolar transporting materials
because these molecules can more effectively confine excitons and
improve the device performance and stability.11 A straightforward
design strategy for bipolar transporting molecules is to incorporate
the hole- and electron-transporting moieties into a single molecule,
i.e., the donor-acceptor type approach.12 For blue emitters, the
choice of suitable interconnection between the hole-transporting,
Both small molecules and polymers are commonly used as
emitters of OLEDs. Compared to polymers, small molecules are
easier to purify and have well-defined molecular weight. In con-
trast, small molecules have higher tendency to form crystals,4
which frequently result in device failure. Therefore, proper design
electron-transporting, and
prominent bathochromic shift due to extended
p
-conjugated spacer is crucial to avoid
p
-conjugation.13 In
2011, Huang and co-workers reported bipolar blue-light emitting
materials containing a hole-transporting triphenylamine moiety
and an electron-transporting benzimidazole moiety at the C-9 and
C-10 position of the anthracene.14 By modified the number and
position of phenyl bridge between functionalized moieties and
anthracene core, the color and the energy levels of these materials
were adjustable.
* Corresponding authors. Tel.: þ886 2 27898522; fax: þ886 2 27831237; e-mail
addresses: jtlin@gate.sinica.edu.tw, jtlin@chem.sinica.edu.tw (J.T. Lin).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.03.017

R. Muangpaisal et al. / Tetrahedron 70 (2014) 2992e2998
2993
In this work, we report the synthesis and investigation of two
isomeric bipolar blue light-emitting molecules consisting of an
electron-donating arylamine, an electron-accepting N-phenyl-
benzimidazole, and an in-between binaphthalene bridge. Zhao and
hole- and electron-transport units, respectively. Extended conju-
gation of the arylamine entity may decrease its color purity.17
Therefore, a bulky binaphthalene bridge was used to avoid strong
charge transfer between the arylamine and the N-phenyl-
benzimidazole. The representative absorption and emission spectra
of the compounds in dichloromethane and in the film cast on a glass
substrate are shown in Fig. 1, and the photophysical data are sum-
marized in Table 1.
co-workers reported that compounds with different p-conjugated
isomeric structures exhibiteddifferentcarrier transport behaviors in
OTFTs.15 The two isomers in this study differ in the interconnection
mode of the electron-accepting N-phenylbenzimidazole entity to
the binaphthalene bridge, i.e., via C- or N-linkage, are therefore ex-
pected to have different carrier transport properties. Blue OLEDs
based on these materials will be also discussed. Some carbazole/
benzimidazole hybrid molecules were studied recently by both
experimental and computational approaches, and they appeared
promising as the host materials for phosphorescent emitters.11b,16
Similar to the previously reported
benzimidazole compounds, the absorption peak around 300 nm
can be attributed to amine-centered
* transition.18 The band of
p-conjugated arylamine/
pep
longer wavelength is due to the charge transfer absorption from the
diphenylamine to the benzimidazole group, which is in agreement
with the theoretical computation (vide infra). The absorption in-
tensity of the charge transfer band (w350e450 nm) is much lower
2. Results and discussion
2.1. Synthesis
than those of the congeners with different
This outcome can be rationalized by the partial disruption of the
p
-conjugated spacers.11a
p
-
conjugation caused by the binaphthalene group. In contrast, a sat-
urated spacer can more effectively decouple the interaction of do-
nor and acceptor and suppress the intramolecular charge transfer
(ICT).19
The synthesis of the target molecules is outlined in Scheme 1.
Compounds 1a and 1b underwent Miyaura borylation reaction to
afford 2a and 2b, respectively. Subsequent palladium-catalyzed
Suzuki cross coupling reaction of 2a and 2b with 40-bromo-N,N-
diphenyl-[1,10-binaphthalen]-4-amine provided the desired prod-
ucts. The isolated yields are 68% and 43% for BINAPC and BINAPN,
respectively. The new compounds were fully characterized by 1H
and 13C NMR spectra, mass spectra, and elemental analysis.
Both compounds emit in the blue region upon excitation at
290 nm. The emission intensity of both compounds are linearly
proportional to concentration in the range of 10ꢁ5 to 10ꢁ4 mol dmꢁ3
,
suggesting negligible aggregation of the dye molecules.20 There is
red shift of the fluorescence spectra upon increasing the solvent
polarity (Fig. 2). The spectral shift can be correlated with the solvent
Scheme 1. Synthetic pathways of BINAPC and BINAPN.
2.2. Thermal properties
polarity (Df) and the difference of dipole moment between the
ground and excited states according to the LipperteMataga equa-
tion (Eq. S1; Supplementary data).21 The spectral shift is pro-
portional to the solvent polarity, indicating the bipolar nature of the
transition.16a The triplet energies (ET) of both compounds, estimated
from the highest energy peak of the phosphorescence taken in tol-
uene at 77 K, were found to be w2.2 eV. This value is high enough for
the compounds to be used as the host for red phosphors in future
studies.
The thermal properties of the new materials were investigated
by TGA and DSC analyses in a nitrogen atmosphere. BINAPC and
BINAPN possess very high thermal decomposition temperatures
(5% weight loss) at 440 and 404 ꢀC, respectively (Fig. S1; Supple-
mentary data). No apparent exothermic crystallization and endo-
thermic melting peaks were observed for both BINAPC and BINAPN
from the DSC thermograms during heating and cooling cycles, in-
dicating the amorphous nature of these binaphthalene bridge
molecular solid.9a High thermal stability and amorphous charac-
teristics of the compounds should be beneficial to their application
in OLEDs.
2.4. Electrochemical properties
The electrochemical properties of BINAPC and BINAPN were
investigated by cyclic voltammetry (CV). Both compounds exhibited
a quasi-reversible wave due to the oxidation of the arylamine unit.22
No reduction wave was detected. Representative cyclic voltammo-
grams are shown in Fig. S3 (Supplementary data). The two com-
pounds exhibited nearly identical oxidation potentials. The HOMO
energy levels of the compounds were calculated to be ꢁ5.49 eV from
cyclic voltammetry and by comparison with ferrocene (ꢁ4.8 eV,
2.3. Photophysical properties
The main purpose of this study is to construct molecules pos-
sessing bipolar carrier-transport properties and emitting in the blue
region or at even shorter wavelengths. As aforementioned, the
arylamine and N-phenylbenzimidazole entities were chosen for

2994
R. Muangpaisal et al. / Tetrahedron 70 (2014) 2992e2998
(a)
(a)
(b)
(b)
Fig. 1. Normalized UVevisible absorption and PL spectra of (a) BINAPC and (b)
BINAPN in dichloromethane and in the film cast on the glass substrate.
Table 1
Physical data of BINAPC and BINAPN
Fig. 2. PL spectra of (a) BINAPC and (b) BINAPN in various solvents.
c
e
Compound Tg/Td
labs (nm)
lem (nm)
FPL
HOMO/LUMO Eg
(ꢀC)a
[solb/film]
[solb/film]
(eV)d
double-logarithmic representation of the TOF transient, the carrier-
transit time (tT) needed to determine the carrier mobilities can
be evaluated. The two compounds exhibit both electron- and
hole-transport behavior, with dispersive transport characteristics.
The field dependence of the carrier mobilities follows the nearly
universal PooleeFrenkel relationship (Fig. S5, Supplementary
data).23 Relevant data for the electron and hole mobilities are
shown in Table 2. In comparison, the hole mobilities of BINAPC
and BINAPN are nearly three orders lower than that
(w10ꢁ3 cm2 Vꢁ1 sꢁ1) of commonly used diphenylamino-based hole-
transport materials, 1,4-bis(1-naphthylphenylamino)-biphenyl
(NPB).24 In comparison, the electron mobilities of the two com-
pounds are comparable to those of the typical electron-transport
material, 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene) (TPBI)
BINAPC
BINAPN
e/440 304, 364/290 465/449
e/404 296, 352/290 457/446
0.33 ꢁ5.84/ꢁ2.83
0.45 ꢁ5.86/ꢁ2.80
3.01
3.06
a
Tg: glass-transition temperature. Td: thermal decomposition temperature. The
heating and cooling rates were 10 and 30 ꢀC minꢁ1, respectively.
b
Absorption maximum and emission maximum measured in CH2Cl2.
The PL quantum efficiency (FPL) was measured in toluene solution using an-
c
thracene (FPL¼0.28 in ethanol) as the standard.27
d
The HOMO energies were determined through AC II measurements;
LUMO¼HOMOþEg.
e
The value of Eg was calculated based on the absorption onset of the solid film.
below the vacuum level). As the HOMO energy calculated by CV
method normally depends on the solvent used for the measure-
ment, the atmospheric photoelectron spectroscopy was also used to
estimate the HOMO energy of BINAPC and BINAPN. The HOMO
energy of BINAPC estimated from the AC II measurements is slightly
lower than that of BINAPN by w0.02 eV.
(w10ꢁ5 cm2 Vꢁ1 ꢁ1) and N-arylbenzimidazole-containing ana-
s
logues (w10ꢁ5e10ꢁ6 cm2 Vꢁ1 sꢁ1).25 The hole mobility of BINAPN is
about twice of thatof BINAPC, while the electron mobility of BINAPC
is more than four times higher than that of BINAPN. The two isomers
in this study have the same trend in carrier mobility: they all have
higher electron mobility than hole mobility. In BINAPC, the electron
mobility is higher than hole mobility by w2 order. The electron
mobility and hole mobility of BINAPN are more balanced compared
to those of BINAPC. The different behaviors for the two isomers are
not unexpected, as the carrier mobilities of glassy materials are af-
fected by both carrier-transport units and molecular geometries.26
2.5. Charge-transport properties
The carrier-transport properties of BINAPC and BINAPN were
investigated by the time-of-flight (TOF) transient-photocurrent
technique at room temperature. The TOF transient for electrons
and holes of the compounds are shown in Fig. S4 (Supplementary
data). From the intersection point of two asymptotes in the

R. Muangpaisal et al. / Tetrahedron 70 (2014) 2992e2998
2995
Table 2
Electron and hole mobilities of compounds measured from the TOF method
me (cm2 Vꢁ1 sꢁ1
a
)
bb (cm Vꢁ1
)
mh (cm2 Vꢁ1 sꢁ1
a
)
bc (cm Vꢁ1
)
Electric field (V cmꢁ1
)
1/2
1/2
BINAPC
BINAPN
5.4ꢂ10ꢁ6 (d)
7.85ꢂ10ꢁ3
2.3ꢂ10ꢁ4 (d)
2.09ꢂ10ꢁ3
6ꢂ105
1.2ꢂ10ꢁ5 (d)
1.54ꢂ10ꢁ3
5.4ꢂ10ꢁ5 (d)
3.33ꢂ10ꢁ3
5ꢂ105
a
me: electron mobility, mh: hole mobility; d: dispersive.
PooleeFrenkel factor for the electron transport.
PooleeFrenkel factor for the hole transport.
b
c
2.6. Theoretical calculation
characteristics of the compounds shorten the carrier lifetime in
thicker films. Consequently, three-layer device II with a structure of
ITO/NPB (30 nm)/BINAPC or BINAPN (20 nm)/TPBI (30 nm)/LiF
(1 nm)/Al (100 nm) was fabricated. NPB and TPBI were used as the
hole- and electron-transporting materials, respectively, while LiF
and Al were used as the buffer layer and the cathode, respectively.
NPB and TPBI were chosen for our purpose as the HOMO and LUMO
energy levels of the compounds matched reasonably well with the
HOMO and LUMOlevels of BINAPC and BINAPN, respectively (Fig. 4).
The performanceparametersof these devicesare collected inTable 3.
Representative EL spectra, currentevoltageeluminance (IeVeL)
characteristics, and currenteefficiencyevoltage plots of these de-
vices are shown in Fig. 5. When the thinner film of BINAPC or
BINAPN used as the emitting layer, electrons, and holes are effec-
Calculated density surfaces of the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) are shown in Fig. 3. The HOMOs of both compounds are
mainly located on the donor. Conversely, the structure factor ac-
counts for the LUMOs located in different regions for the two
compounds. The LUMO of BINAPC is distributed across the bridge
and acceptor, whereas the LUMO of BINAPN is mainly distributed
on the bridge. This is probably due to the difference in the dihedral
angle between the phenyl and benzimidazole (ca. 30ꢀ for BINAPC;
ca. 55ꢀ for BINAPN). Similar observation was reported on the car-
bazole/phenylbenzimidazole derivatives developed by Wong and
co-workers.16b
Fig. 3. Calculated density surfaces of the molecular orbitals of BINAPC (left) and BINAPN (right) at
uPBE/6-31G* level.
The simulated absorption spectra based on time-dependent
calculations for BINAPC and BINAPN follow the same trend as
that in experimental observation: BINAPC has more bathochromic
charge transfer excitation (longer wavelength bands) and more
hypsochromic localized excitation (shorter wavelength bands). The
simulated absorption bands are shown in Fig. S7 (Supplementary
data). The excitation energies of the S0/S1 transition after opti-
mization of the excited state structure are similar for BINAPC and
BINAPN, which is consistent with the similar emission wavelengths
of the two isomers.
2.7. Electroluminescent properties and OLED characteristic
Based on the thermal, photophysical and carrier mobility prop-
erties, the new compounds appeared to be promising as non-doped
blue-emitting materials for OLED application. Non-doped single-
layerELdeviceIwithstructureofITO/BINAPCorBINAPN(80 nm)/LiF
(1 nm)/Al (100 nm)was firstfabricated. However, theperformance of
the device I was far from satisfactory, despite of the bipolar transport
behavior of the two compounds. Possibly dispersive transport
Fig. 4. Energy levels diagram of relevant materials used for device fabrication.
tively confined in the emitting layers to form excitons. This is also
supported by the nearly superimposable EL and PL spectra. The
slightly better device performance of BINAPN than BINAPC may be
due to the higher quantumyield and better balanced carrier mobility

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R. Muangpaisal et al. / Tetrahedron 70 (2014) 2992e2998
Table 3
Electroluminescent data of devices I and II
(a)
BINAPC
BINAPN
Von (V)a
9.5; 3.5
57 (14.0); 12,448 (14.0)
8.5; 4.0
32 (14.0); 15,250 (14.0)
Lmax (cd mꢁ2
)
(V at Lmax, V)b
lem (nm)c
464; 450
0.16, 0.21; 0.15, 0.11
88; 70
0.08; 2.49
0.03; 1.98
458; 458
0.16, 0.17; 0.16, 0.16
74; 72
0.13; 2.67
0.04; 1.77
CIE (x, y)d
FWHM (nm)e
hext,max (%)f
hp,max (lm Wꢁ1
g
)
hc,max (cd Aꢁ1
)
0.13; 2.43
0.17; 3.32
h
a
Von (turn-on voltage) was obtained from the-intercept of the plot of log (lumi-
nance) against applied voltage.
b
L: luminance; max: maximum.
lem, l maximum at emission wavelength.
CIE (x, y): Commission Internationale de l’Eclairage coordinates.
FWHM: full width at half maximum.
hext: external quantum efficiency; max: maximum.
hp: power efficiency; max: maximum.
c
d
e
f
g
h
hc: current efficiency; max: maximum.
(b)
of the former. At a current density of 100 mA cmꢁ2, they still retain
promising performance: luminance (L)¼2375 cd mꢁ2; external
quantum efficiency (hext)¼2.4%; power efficiency (hp)¼1.0 lm Wꢁ1
and current efficiency (hc)¼2.4 cd Aꢁ1 for BINAPC, and luminance
(L)¼2835 cd mꢁ2
,
hext¼2.3%; hp¼0.9 lm Wꢁ1 and hc¼2.8 cd Aꢁ1 for
BINAPN. Use of the compounds as the host for phosphors should be
also possible in view of the successful fabrication of OLEDs in this
study.
3. Conclusions
In summary, we have developed two isomeric blue-emitting
compounds consisting of arylamine and benzimidazole with
a binaphthalene bridge. The new compounds exhibit bipolar
transporting characteristics and can be used for efficient blue-
emitting OLEDs. The use of highly twisted binaphthalene bridge
(c)
results in partial disruption of
p-conjugation. The difference in
linking modes of the benzimidazole entity to the bridge, i.e., C-
linkage (BINAPC) or N-linkage (BINAPN), leads to different photo-
physical and carrier-transport properties of the molecules. The N-
linkage isomer has more balanced carrier mobilities.
4. Experimental section
4.1. General information
The 1H NMR spectra were recorded on a Bruker AV III-400 MHz
spectrometer, 13C were recorded on a Bruker AV 500 MHz spec-
trometer and FAB-mass spectra were collected on a JMS-700 double
focusing mass spectrometer (JEOL, Tokyo, Japan). Elemental anal-
yses were performed on a PerkineElmer 2400 CHN analyzer. TGA
and DSC were recorded on Perkin Elmer Pyris 1 TGA and DSC 7,
respectively. Electronic absorption spectra were obtained on
a Dynamica DB-20 UVevisible spectrometer. Emission spectra were
recorded at room temperature by a Hitachi F-4500 FL spectro-
photometer. Luminescence quantum yields (FPL) were calculated
relative to anthracene (FPL¼0.28 in ethanol).27 Phosphorescence
measurement was performed by Jobin Yvon FL3-21. Cyclic vol-
tammetry experiments were performed with a CHI-621B electro-
chemical analyzer. All measurements were carried out at room
temperature with a conventional three-electrode configuration
consisting of a platinum working electrode, an auxiliary electrode,
and a non-aqueous Ag/AgNO3 reference electrode. The E1/2 values
were determined as 1/2(EaþEc), where Ea and Ec are the anodic and
cathodic peak potentials, respectively. The solvent used was THF
and the supporting electrolyte was 0.1 M tetrabutylammonium
Fig. 5. EL performance of the device II for BINAPC and BINAPN (a) EL spectra. (b)
Current density and luminance versus applied electric field c) Current efficiency versus
voltage.
hexafluorophosphate. HOMOs were obtained from the measure-
ment of ionization potential using low energy photoelectron
spectrometer (RIKEN KEIKI AC II).
4.2. Material synthesis
Chemicals and solvents were reagent grades and purchased
from Aldrich and Acros, respectively. Solvents were dried by stan-
dard procedures. All reactions and manipulations were carried out
under N2 with the use of standard inert atmosphere and Schlenk

R. Muangpaisal et al. / Tetrahedron 70 (2014) 2992e2998
2997
techniques. All column chromatography was performed by using
silica gel (230e400 mesh, MachereyeNagel GmbH & Co.) as the
stationary phase in a column, which is 25e35 cm in length and
2.5 cm in diameter. 2-(4-Bromophenyl)-1-phenyl-1H-benzo[d]im-
idazole (1a)28 and 40-bromo-N,N-diphenyl-[1,10-binaphthalen]-4-
amine29 were synthesized according to literature methods. 1-(4-
Bromophenyl)-2-phenyl-1H-benzo[d]imidazole (1b) was prepared
as described in Scheme S1 (Supplementary data).
(m, 3H), 7.46e7.42 (m, 7H), 7.38e7.34 (m, 3H), 7.32e7.28 (m, 4H),
7.14 (d, J¼8.0 Hz, 4H) and 7.03 (t, J¼7.6 Hz, 2H); 13C NMR (125 MHz,
acetone-d6):
d (ppm) 152.7, 149.7, 144.7, 144.5, 142.1, 140.0, 139.3,
138.6, 137.8, 137.6, 135.6, 134.3, 132.6, 132.3, 131.6, 130.5, 130.4,
130.3, 129.5, 129.3, 128.7, 128.6, 127.9, 127.8, 127.8, 127.5, 127.4,
127.4, 127.3, 126.9, 125.5, 124.3, 123.7, 123.0, 120.7, 111.5; Mass
(FAB): m/z: 690.2 [MþH]þ; Anal. Calcd for C51H35N3: C, 88.79; H,
5.11; N, 6.09. Found: C, 88.74; H, 5.31; N, 6.05.
4.2.1. 1-Phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) phe-
nyl)-1H-benzo[d]imidazole (2a). A mixture of 1a (0.70 g, 2.0 mmol),
bis(pinacolate)diborane (0.53 g, 2.1 mmol), l,l0-bis(diphenylphosphino)
ferrocene]dichloropalladium (Pd(dppf)Cl2) (0.060 g, 0.08 mmol), and
anhydrous potassium acetate (0.393 g, 4.0 mmol) in 1,4-dioxane
(20 mL) was heated at 80 ꢀC under nitrogen for 12 h. After cooling
to room temperature, the mixture was extracted with ethyl acetate
(30 mLꢂ3). The organic extracts were washed with water, brine, and
then dried with anhydrous MgSO4. After filtration, the filtrate was
pumped dry in vacuo. The crude residue was subjected to column
chromatography by eluting with hexanes/EA (8:1) as the eluent to give
4.3. Light-emitting devices fabrication
Pre-patterned ITO substrates were cleaned by standard pro-
cedure before use. Single-layered EL devices were prepared by
vacuum deposition of 80 nm of BINAPC or BINAPN as emitting
layer. Three-layered EL devices were fabricated by vacuum de-
position of 30 nm of NPB, followed by 20 nm of BINAPC (or
BINAPN) and 30 nm of TPBI as hole-transporting, light-emitting
and electron-transporting layers, respectively. LiF (1 nm thick) was
then deposited as the buffer layer. Aluminum was finally deposited
as the cathode (100 nm). Electroluminescence (EL) spectra were
recorded on a Hitachi fluorescence spectrophotometer F-4500. IeV
curves were measured by using photodiode detector (GW INSTEK,
GPC-3030DQ) under ambient environment.
a white solid (yield: 81%).1H NMR (400 MHz, acetone-d6):
d (ppm) 7.79
(d, J¼7.6 Hz, 1H), 7.71 (d, J¼7.6 Hz, 2H), 7.62e7.56 (m, 5H), 7.45 (d,
J¼7.6 Hz, 2H), 7.37e7.23 (m, 3H) and 1.34 (s, 12H); 13C NMR (125 MHz,
acetone-d6): d (ppm) 152.9, 144.3, 138.6, 138.2, 135.2,134.0, 131.0, 129.7,
129.6, 128.6, 124.3, 123.7, 120.7, 111.4, 84.9, 25.3; Mass (FAB): m/z: 397.1
4.4. Time-of-flight (TOF) mobility measurements
[MþH]þ.
The mobilities of compounds were characterized by the tran-
sient photocurrent technique, i.e., the time-of-flight (TOF) tech-
nique.30 The TOF measurement were performed using the sample
4.2.2. 2-Phenyl-1-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) phe-
nyl)-1H-benzo[d]imidazole (2b). Compound 2b was synthesized by
similar procedures as described for 2a. The product was obtained as
structure of: glass/Ag (30 nm)/BINAPC or BINAPN (1e2
mm)/Al
a white solid (yield: 43%). 1H NMR (400 MHz, acetone-d6):
d
(ppm)
(150 nm) with an active area of 2ꢂ2 mm2 as described in a previous
report. The organic layer was deposited by vacuum deposition. In
the TOF measurement, a frequency-tripled Nd:YAG laser (355 nm)
with w10 ns pulse duration was used for pulsed illumination
through the semi-transparent Ag. Under an applied DC bias, the
transient photocurrent as a function of time was recorded with
a digital storage oscilloscope. The TOF measurements were typi-
cally performed in a 10ꢁ5 Torr vacuum chamber. Depending on the
polarity of the applied bias V, selected photogenerated carriers
(holes or electrons) are swept across the sample thickness D with
a transit time tT, the applied electric field E is then V/D, and the
carrier mobility is given by¼D/(tT$E)¼D2/(V$tT).
7.94 (d, J¼6.4 Hz, 2H), 7.79 (d, J¼7.6 Hz, 1H), 7.62e7.60 (m, 2H), 7.45
(d, J¼8.0 Hz, 2H), 7.37e7.26 (m, 6H) and 1.34 (s, 12H); 13C NMR
(125 MHz, acetone-d6):
d (ppm) 153.1, 144.3, 140.7, 138.3, 137.1,
131.4, 130.9, 130.4, 129.2, 128.6, 127.8, 124.2, 123.7, 120.6, 111.4,
85.09, 25.31; Mass (FAB): m/z: 397.2 [MþH]þ.
4.2.3. N,N-Diphenyl-40-(4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phe-
nyl)-[1,10-binaphthalen]-4-amine (BINAPC). 40-Bromo-N,N-diphenyl-
1,10-binaphthyl-4-amine (1.0 g, 1.99 mmol), 2a (0.79 g, 2.0 mmol),
Pd(dppf)Cl2 (82 mg, 0.1 mmol), and KF (0.35 g, 6.0 mmol) were
dissolved in anhydrous DMF (20 mL) and the mixture was heated
at 120 ꢀC under nitrogen for 12 h. After cooling to room tempera-
ture, the mixture was extracted with ethyl acetate (30 mLꢂ3). The
organic extracts were washed with water, brine, and then dried
with anhydrous MgSO4. After filtration, the filtrate was pumped
dry. The crude residue was subjected to column chromatography by
using a mixture of hexanes/EA (8:1 to 6:1) as the eluent to give
4.5. Quantum chemistry computation
The geometries and electronic structures of BINAPC and
BINAPN were calculated using density functional theory (DFT). All
structures in their ground-state were optimized at B3LYP/6-311G**
level of theory. The results are with a positive definite Hessian.
The singlet excited state and HOMO/LUMO energies were opti-
a yellow solid (yield: 68%). 1H NMR (400 MHz, acetone-d6):
d (ppm)
8.13 (d, J¼8.0 Hz, 1H), 8.01 (d, J¼8.0 Hz, 1H), 7.85e7.82 (m, 3H),
7.70e7.67 (m, 2H), 7.63e7.60 (m, 8H), 7.51 (d, J¼7.6 Hz, 3H),
7.44e7.39 (m, 3H), 7.38e7.35 (m, 2H), 7.34e7.25 (m, 6H), 7.13 (d,
J¼8.4 Hz, 4H) and 7.02 (t, J¼7.2 Hz, 2H); 13C NMR (125 MHz, ace-
mized using
uPBE with 6-311G* basis set to analyze the effect of
the linking topologies in electronic properties. The lowest singlet
excitation (S1) of the compounds was calculated using time-
dependent density functional theory (TDDFT) at
level.
tone-d6):
d
(ppm) 152.8,149.7, 144.6,144.5,142.6,140.3,139.1,138.8,
uPBE/6-311G*
138.4, 137.8, 135.6, 134.2, 132.4, 132.3, 131.1, 130.9, 130.7, 130.4,
130.3,129.8,129.4,128.8,128.5,127.9,127.7,127.5,127.3,127.1,126.9,
125.5, 124.2, 123.7, 122.9, 120.7, 111.4; Mass (FAB): m/z: 689.2 [M]þ;
Anal. Calcd for C51H35N3; C, 88.79; H, 5.11; N, 6.09. Found: C, 88.70;
H, 5.35; N, 6.30.
Acknowledgements
We would like to thank you to Dr. Zhi-Qiang Wesley You and Dr.
Hsien-Hsin Chou for providing the quantum calculation data and
discussion.
4.2.4. N,N-Diphenyl-40-(4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phe-
nyl)-[1,10-binaphthalen]-4-amine (BINAPN). BINAPN was synthe-
sized by similar procedures as described for BINAPC. The product
was obtained as a light green solid (yield: 43%). 1H NMR (400 MHz,
Supplementary data
acetone-d6):
d
(ppm) 8.15 (d, J¼8.4 Hz, 1H), 8.10 (d, J¼8.4 Hz, 1H),
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2014.03.017.
7.87e7.81 (m, 3H), 7.77e7.72 (m, 3H), 7.69e7.63 (m, 4H), 7.59e7.51

2998
R. Muangpaisal et al. / Tetrahedron 70 (2014) 2992e2998
Hung, W. Y.; Chen, C. M.; Liu, Q. Y.; Liu, Y. H.; Wong, K. T. RSC Adv. 2013, 3,
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