3,3'-Bicarbazole-based host materials for high-efficiency blue phosphorescent OLEDs with extremely low driving voltage

Article abstract of DOI:10.1002/adma.201200848

Four types of functionalized 3,3'-bicarbazole derivatives are prepared and investigated as a host material for blue phosphorescent OLEDs. By using a 3,3'-bicarbazole derivative/FIrpic film as an emissive layer, high power efficiency of 46 lm W^-1 (45 cd A^-1, EQE 20%) with an extremely low driving voltage at 3.1 V is obtained at 100 cd m^-2. Copyright

Full text of DOI:10.1002/adma.201200848

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3,3′-Bicarbazole-Based Host Materials for High-Efficiency
Blue Phosphorescent OLEDs with Extremely Low
Driving Voltage
Hisahiro Sasabe,* Naoki Toyota, Hiromi Nakanishi, Tasuku Ishizaka, Yong-Jin Pu,
a n d Junji Kido*
High-performance organic light-emitting device (OLED) incor-
porating a phosphorescent emitter is one of the most promising
candidates for energy-saving solid-state lighting and flat-display
panel.[1,2] Generally, a phosphorescent emitter is dispersed
in a suitable host material to obtain a high photoluminescent
quantum yield (ηPL) suppressing concentration quenching.[3]
Additionally, to realize a high ηPL for a blue phosphorescent
emitter, a host material needs to have a triplet energy (ET) over
2.75 eV. On the other hand, carrier balance is one of the most
important factors to obtain a high efficiency OLED.[4] For a
better carrier balance between electrons and holes in an emis-
sive layer (EML), it is considered to be important to adjust fron-
tier molecular orbitals (FMO) of a host material with FMOs of a
hole-transport layer (HTL) and an electron-transport layer (ETL).
Therefore, OLED performance can be dramatically improved by
using a suitable host material.
0.46 eV.[5] Very recently, Kim and co-workers also have pre-
dicted by DFT calculation that carbazole-based oligomer
linked with 3,3’-position can possess a higher ET than that of
a common blue phosphorescent emitter, iridium(III) bis[(4,6-
difluorophenyl)-pyridinate-N,C2’]picolinate (FIrpic).[7] Due to the
sterically twisted structure, BCz derivatives can greatly reduce
the host-host aggregation causing excimer formation, and a
dimerization of carbazole can improve thermal and morpho-
logical stability of thin film. Additionally, the electronic proper-
ties of BCz derivatives can be tuned by chemical modification
on the 9,9′-positions. These attractive features should promise
tremendous opportunities as a host material in blue phospho-
rescent OLEDs. However, BCz derivative has not been used in
blue phosphorescent OLED so far despite use in green phos-
phorescent OLEDs.[8,9] In this study, we prepare and investigate
a series of functionalized BCz derivatives as a host material for
blue OLEDs. We can successfully develop a FIrpic-based OLED
with a power efficiency (ηp,100) over 45 lm W−1 at 100 cd m−2
with an extremely low driving voltage at 3.1 V.
Among host materials, carbazole derivatives show superior
performances in phosphorescent OLEDs. Because carbazole
has a high ET of over 3.0 eV and chemical compatibility of a
phosphorescent emitter, a carbazole derivative can realize a high
We investigated four types of functionalized 3,3’-bicar-
bazole derivatives, which are modified with triphenylamine
(BCzTPA), triphenylphosphine oxide (BCzPO) and tetraphe-
nylmethane (BCzTPM) and phenyl (BCzPh)[10] moieties on the
9,9′-positions, respectively (Figure 1). For better hole-injection,
triphenylamine (TPA) moieties were introduced. On the other
hand, for better electron-injection, tirphenylphosphine-oxide
(PO) moieties were introduced. Prior to material preparation,
we conducted the density functional theory (DFT) calcula-
tions. We used a common host material, N,N′-dicarbazolyl-3,5-
benzene (mCP) as a reference. The calculated highest occupied
molecular orbital (HOMO) energies of BCz derivatives were
estimated to be between–5.15 and–5.44 eV, those are much
shallower than that of mCP, depending on substituents on
9,9′-position. Among BCz derivatives, an electron-donating
effect of TPA makes a HOMO energy shallower. While an
electron-withdrawing inductive effect of PO deepens a HOMO
energy. These results suggest that an enhanced hole-injection
can be created by using BCz derivatives. On the other hand, the
calculated lowest unoccupied molecular orbital (LUMO) ener-
gies were evaluated to be between–1.11 and–1.48 eV, those are
much shallower than that of mCP except BCzPO, suggesting
a poor electron-injection property. Whereas BCzPO is expected
to have a much better electron-injection property than that of
mCP. We also calculated ET and ES from TD-DFT calculation
at the RB3LYP 6-31G(d) level. Although the conjugation length
in a BCz derivative is longer than that of mCP, ET levels were
η
PL. Further, carbazole have a small singlet–triplet energy dif-
ference (ΔEST) of 0.48 eV, due to the reduced exchange energies
involving n-π∗ transition.[5] Thus, a carbazole-based host mate-
rial potentially can reduce a driving voltage of OLED. Although
a carbazole derivative has above mentioned advantages, a car-
bazole derivative without chemical modification shows strong
excimer formation behavior leading to a significant loss of a
triplet energy.[6]
In this regard, 3,3’-bicarbazole (BCz), which is a dimer
of carbazole, has not only a high ET but also a small ΔEST of
Dr. H. Sasabe, Prof. J. Kido
Department of Organic Device Engineering
and Research Center for Organic Electronics (ROEL)
Yamagata university
4-3-16 Johnan, Yonezawa, Yamagata 992-8510, Japan
E-mail: h-sasabe@yz.yamagata-u.ac.jp;
kid@yz.yamagata-u.ac.jp
N. Toyota, H. Nakanishi, T. Ishizaka, Dr. Y.-J. Pu
Department of Organic Device Engineering
Yamagata University
4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
N. Toyota, H. Nakanishi, T. Ishizaka, Dr. Y.-J. Pu
Research Center for Organic Electronics (ROEL)
Yamagata University
4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
DOI: 10.1002/adma.201200848
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Figure 1. Chemical structure of BCz derivatives.
evaluated to be as high as around 3.00 eV, which is applicable
in a blue phosphorescent OLED. ES levels were calculated to be
3.57–3.96 eV, those are smaller than that of mCP. Compared
with mCP and BCzPh, in the cases of BCzTPA and BCzPO, ES
levels were very small maintaining a high ET over 3.00 eV, in
other words, these BCz derivatives can possess very small ΔEST
to reduce an operating voltage of a blue phosphorescent OLED.
Considering HOMO/LUMO locations, this is because intramo-
lecular charge-transfer (ICT) occurs in BCzTPA and BCzPO
(see Supporting Information in detail). All the calculated data
are summarized in Table 1.
The synthetic route of Bz derivatives was shown in Scheme 1.
The precursor 3,3′-bicarbazole was prepared via an oxidation
reaction of carbazole according to the literature.[11] Then, a CuI/
L-proline-catalyzed Ullmann coupling reaction of 3,3′-bicar-
bazole with a corresponding aryl halide gave the target BCz
derivatives in 74–95% yield.[12] Note that we also tried to pre-
pare these materials via palladium-catalyzed Buchwald-Hartwig
reaction, however we could not obtain sufficient amount of BCz
derivatives probably due to the low solubility of 3,3′-bicarbazole
in toluene or xylene.[13] The characterization was established on
the basis of NMR, mass spectrometry, and elemental analyses.
The product was purified by train sublimation method before
device fabrication. The purity of BCz derivatives thus obtained
was confirmed to be >99.0% by HPLC analysis.
stability of thin solid film using BCz derivatives. The ionization
potential (Ip) was measured by a photoelectron yield spectros-
copy (PYS) under the vacuum (∼10−3 Pa). As predicted from
DFT calculation, BCz derivatives show a 0.4–0.5 eV shallower
ionization potential around–5.65 eV by a photoelectron yield
spectrometry (PYS) compared with mCP (6.1 eV) in the solid
state. Considering that Ip of BCzPh was observed at –5.67 eV,
this is characteristic of 3,3’-bicarbazole skeleton. Among BCz
derivatives, an electron-donating effect of TPA makes an Ip 0.06
eV shallower. While an electron-withdrawing inductive effect of
PO slightly deepens the Ip of BCzPO (∼0.02 eV), but the induc-
tive effect is very small in the solid state. The electron affinity (Ea)
was calculated to be around 2.30 eV by subtraction of the optical
energy gap (Eg) from the Ip. Compared with mCP (–2.60 eV),
these values are shallower indicating that electron-injection
to EML should be improved in OLED. The phosphorescent
spectrum of 3 wt%-BCz derivative doped PMMA film was
measured by a streak camera at 5 K. We estimated ET from the
onset of phosphorescence in the solid state. The onset phospho-
rescence was observed at around 2.90 eV, which is comparable
with that of mCP (3.00 eV), with almost no effect of substitu-
ents. From the perspective of ΔEST, all the BCz derivatives has
very small ΔEST of 0.37–0.45 eV, which is even smaller than
that of mCP (0.49 eV).[14] Therefore, BCz derivatives are con-
sidered to be applicable for a blue phosphorescent OLED with a
reduced operating voltage. All the physical properties are sum-
marized in Table 2.
The thermal properties were measured by differential scan-
ning calorimetry (DSC). The glass transition temperature (Tg) of
functionalized BCz derivatrives, such as BCzTPA and BCzPO
were observed at over 136 °C, which is 30 °C higher than that
of phenyl substituted BCzPh and more than double than that
of mCP (60 °C). These results clearly promise excellent thermal
Prior to the device fabrication, photophysical properties of a
11wt% FIrpic-doped host film was evaluated. An ηPL was meas-
ured under N2 fl ow using a integrating sphere excited 331 nm
with a multichannel spectrometer as the optical detector. ηPLs
T a b le 1 . Calculated HOMO, LUMO, ΔEH–L, ES, ET and ΔEST values (eV).
HOMOa)
–5.31
LUMOa)
–1.16
–1.11
–1.14
–1.48
–1.26
ES
ETc)
3.02
3.00
3.02
3.01
3.18
c)
b)
d)
Compound
BCzPh
ΔEH–L
ΔEST
–4.15
–4.04
–4.12
–3.96
–4.54
3.96
3.67
3.73
3.57
4.01
0.97
0.67
0.71
0.56
0.83
BCzTPA
BCzTPM
BCzPO
mCP
–5.15
–5.26
–5.44
–5.80
a)Calculated at RB3LYP 6-311+G(d,p)//RB3LYP 6-31G(d); b)ΔEH–L = HOMO–LUMO; c)Lowest singlet (ES) and triplet (ET) energy from TD-DFT at RB3LYP 6-31G(d)//RB3LYP
6-31G(d); d)Energy difference between lowest singlet (ES) and triplet (ET) energy (ΔEST = ES–ET).
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BCz derivative, sterically bulky substituents, such as TPA, TPM
and PO, can keep a FIrpic molecule away from a direct con-
tact with 3,3′-bicarbazole skeleton, so the back energy transfer
from FIrpic to 3,3′-bicarbazole skeleton is consider to be neg-
ligibly-small. Thus, a chemical modification of 9,9’-position
of 3,3’-bicarbazole by sterically bulky functionalities can be an
effective way to confine the triplet exciton of FIrpic. It should be
noted that an aggregation induced concentration quenching of
FIrpic is thought to be very small in these hosts compared with
other symmetric tris-cyclometallated iridium complexes.[17]
To investigate the function of BCz derivatives as a host mate-
rial, a blue phosphorescent OLEDs using FIrpic as an emitter
were fabricated. We used 1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]
cyclohexane (TAPC) as a hole-transporting layer (HTL) and
3,3″,5,5″-tetra(3-pyridyl)-1,1′;3′,1″-terphenyl (B3PyPB)[18] as an
electron-transporting layer (ETL), these materials have higher
ET than that of FIrpic. Therefore, the triplet exciton quenching
of FIrpic at the HTL/EML and/or EML/ETL interface(s) can be
minimized. Then, we fabricated a blue phosphorescent OLED
with a structure of [ITO (130 nm)/ TAPC (40 nm)/ FIrpic
11 wt% doped BCz derivative (10 nm)/ B3PyPB (50 nm)/ LiF
(1 nm)/ Al (100 nm)]. EL spectra showed an emission only from
FIrpic with no emission from neighboring materials (inset in
Figure 2(b)). The current density–voltage–luminance (J–V–L)
and the power efficiency–luminance (PE–L) characteristics
are shown in Figure 2(a) and Figure 2(b), respectively. All the
OLED performances are summarized in Table 3 (see also Sup-
porting information).
By using BCzTPM and BCzPh, an OLED showed extremely
reduced operating voltage of 3.1 V at 100 cd m−2 and gave an
ηp,100 of over 45 lm W−1 at 100 cd m−2 presumably due to the
superior carrier balance in EML. These are among the best per-
formances in FIrpic-based OLEDs. Whereas in a OLED using
mCP (–6.09 eV), an operating voltage of 3.7 V at 100 cd m−2
have been observed in the same device configuration.[19] Among
these hosts, BCzTPA with electron-donating TPAs showed a
superior current density at a low driving voltage (<3.0 V). On
the other hand, BCzPO with electron-accepting POs showed a
less current density. In this FIrpic-based OLED, holes are pref-
erably injected to host from TAPC (Ip =–5.60 eV) because of
the much deeper Ip of FIrpic (–6.15 eV) than these hosts (Ip =
–5.67 ∼–5.72 eV) from the viewpoint of a favorable density-of-
state (DOS) overlap.[20] Thus, a better hole-injection to EML is
one of the critical factors in a FIrpic-based device.
Scheme 1. Synthetic route of BCz derivatives.
of BCz/FIrpic films were estimated to be 62–74%. These
values are 20% lower than that of mCP/FIrpic film.[15] Among
BCz derivatives, BCzTPM showed relatively high ηPL of 74%
despite the similar ET to the other BCz derivatives. In these
highly doped films, emissions via the intermolecular energy
transfer (ET) from host to FIrpic and/or via direct excitation of
FIrpic should be involved. In the UV-vis spectrum of BCzTPM,
smaller absorption at 331 nm can be observed than that in
BCzTPA and BCzPO. Note that there are relatively small dif-
ferences in the spectrum overlaps between the emission spec-
trum of BCz derivative and the absorption spectrum of FIrpic.
(see Supporting information in detail). In the cases in BCzTPA
and BCzPO, host material can absorb 331 nm light more effec-
tive than that in BCzTPM due to their strong n-π∗ absorption
derived from the ICT. Thus, the efficient intermolecular ET
from host to FIrpic should be more important to realize higher
ηPL. However, considering the lower ηPL, the ET to FIrpic
might be less efficient in BCzTPA and BCzPO. A transient PL
decay curve of 11% FIrpic-doped film exhibited almost single-
exponential decay at room temperature except BCzPh/FIrpic
film (see Supporting information in detail). While in the case
of BCzPh/FIrpic, a delayed component involving back energy
transfer from FIrpic to BCzPh was observed. Here, in the
molecular structure of BCz derivative, we should consider an
ET of each subunits,[16] such as 3,3′-bicarbazole, phenyl, TPA,
TPM and PO. In BCz derivative, a lowest ET subunit is core
3,3′-bicarbazole skeleton. In BCzPh/FIrpic film, FIrpic can con-
tact directly with 3,3′-bicarbazole skeleton because the phenyl
groups are sterically compact. While in the other functionalized
T a b le 2 . Physical properties of BCz derivatives.
Tg/Tm/Td5/ °Ca)
105/200/399
157/239/526
n.d./377/529
136/n.d./516
60/187/341
ΔEST/eVd)
0.45
η
PL/%e)
62
Compound
BCzPh
Ip/ Ea/Eg/eVb)
–5.67/–2.35/3.32
–5.61/–2.20/3.31
–5.72/–2.41/3.31
–5.69/–2.41/3.28
–6.09/–2.60/3.49
ET/eVc)
2.87
BCzTPA
BCzTPM
BCzPO
2.91
0.40
63
2.87
0.44
74
2.91
0.37
64
mCPf)
3.00
0.49
80
a)Glass transition temperature (Tg) and melting point (Tm) determined by DSC measurement. Decomposition temperature with 5% weight loss (Td5) obtained from TGA
analysis; b)Ionization potential (Ip) obtained from PYS under the vacuum (∼10−3 Pa). Energy gap (Eg) taken as the point of intersection of the normalized absorption
spectra. Electron affinity (Ea) calculated using Ip and Eg; c)Onset of phosphorescence of 3 wt%-BCz derivative doped PMMA film measured by using a streak camera (C4334
from Hamamatsu Photonics); d) ΔEST = Eg–ET; e) Photoluminescent quantum yield of 11 wt% FIrpic-doped film; f)Data from Ref[13].
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(a)
150
100
50
(a)
50
40
30
20
10
0
104
103
102
101
100
0
0
1
2
3
4
5
6
7
Voltage (V)
0
1
2
3
4
5
6
7
8
Voltage (V)
(b)
60
(b)
60
50
40
30
20
10
0
50
40
30
20
10
0
400
500
600
Wavelength (nm)
1
10
100
1000
10000
Luminance (cd m- 2)
1
10
100
1000
10000
Figure 2. (a) J–V–L characteristics and (b) PE–L characteristics of blue
OLEDs using BCzTPA (circle), BCzTPM (triangle), BCzPO (square) and
BCzPh (diamond), respectively. Inset: EL spectra of the devices.
Luminance (cd m- 2)
Figure 3. (a) J–V characteristics and (b) PE–L characteristics of BCzTPA-
based OLEDs: device A (circle), B (triangle), C (square), respectively.
To get a better understanding into the carrier injection
behaviors in this OLED, we compared three types of devices
with an EML structure: (A) FIrpic 11wt% doped BCzTPA
(10 nm), (B) BCzTPA (5 nm)/FIrpic 11wt% doped BCzTPA (5
nm), (C) 11wt% doped BCzTPA (5 nm)/BCzTPA (5 nm). The
J–V and PE–L characteristics are shown in Figure 3. Compared
with device (A), device (B) with non-doped BCzTPA at HTL
interface showed much higher current density, while device (C)
with non-doped BCzTPA at ETL interface showed much lower
current density. Device (B) gave an ηp,100 of 38 lm W−1 at 100 cd
m−2 , which is comparable with the performance in device (A).
On the other hand, device (C) showed much lower efficiency of
under 10 lm W−1 . From these comparison, it can be considered
that (i) better hole-injection to EML can be realized by using
BCzTPA due to a favorable overlap of DOS distributions with
HTL, (ii) electron-injection to EML can be greatly enhanced
by carrier trapping of FIpic, (iii) FIrpic prevents hole-injection
and transport to EML due to the large difference in Ip levels
between FIrpic and BCzTPA,[21] (iv) electron-injection from
ETL to BCzTPA is difficult without doping of FIrpic due to the
shallow Ea level.
In conclusion, we prepared and investigated a series of
functionalized BCz derivatives as a host material for blue
phosphorescent OLEDs. DFT calculations predicted that a BCz
derivative had (i) a shallower HOMO level than that of mCP
to enhance hole-injection from HTL, (ii) a high ET, which
is applicable for a blue phosphorescent OLED and (iii) small
ΔEST to reduce an operating voltage of an OLED. For the phys-
ical properties, BCz derivatives showed a 0.4–0.5 eV shallower
T a b le 3 . Summary of OLED performances.
Device
η
p,100/ηc,100/V100/EQEa)
η
p,1000/ηp,1000/V1000/EQEb)
[lm W−1 /cd A−1 /V/%]
23.8/29.7/3.9/12.9
21.8/26.4/3.8/16.1
22.6/35.5/4.9/15.6
32.3/36.6/3.6/16.4
21.5/30.1/4.4/13.8
[lm W−1 /cd A−1 /V/%]
39.3/38.2/3.1/16.6
45.5/44.5/3.1/19.6
34.4/39.3/3.6/17.2
45.2/43.9/3.1/19.8
28.7/33.9/3.7/16.6
BCzTPA
BCzTPM
BCzPO
BCzPh
mCPc)
a)Power efficiency (ηp), current efficiency (ηc), voltage (V) and external quantum
efficiency (ηext) at 100 cd m−2
;
b)ηp, ηc, V and ηext at 1000 cd m−2 c)Data from
;
Ref [19].
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The resulting solid was purified by chromatography on silica gel (eluent:
toluene/hexane = 1:4) to afford BCzTPA (1.82 g, 74%) as a pale yellow
solid: 1H-NMR (400 MHz, CDCl3) : δ 8 . 44 (d, 2H, J = 1.8 Hz), 8.23
(d, 2H, J = 7.8 Hz), 7.79 (dd, 2H, J = 1.8, 7.8 Hz), 7.55-7.43 (m, 8H), 7.35-
7.21 (m, 26H), 7.09 (t, 4H, J = 7.3 Hz) ppm; 13C-NMR(100 MHz,CDCl3) δ
147.7, 147.2, 141.6, 140.3, 134.4, 131.5, 129.6, 127.9, 126.1, 125.9, 124.9,
124.1, 123.9, 123.5, 120.5, 119.0, 110.2, 110.1 ppm (several signals were
superimposed); UV-vis (fi lm ): λmax = 310, 341 nm; PL (fi lm ): λmax = 397,
418 nm; MS (EI) : m/z = 820 [M+H]+; Anal. Calcd for C60H42N4: C, 87.99;
H, 5.17; N, 6.84%. Found: C, 87.86; H, 5.03; N, 6.81%.
ionization potential around–5.6 ∼ –5.7 eV by a PYS compared
with a well-known carbazole derivative, mCP (–6.1 eV) in the
solid state. An ηPL of a 11 wt%-doped FIrpic/BCz derivative film
was observed to be moderate value of ∼74% despite a high ET
probably due to the imperfect energy transfer from BCz deriva-
tive to FIrpic. A time-resolved photoluminescence analysis of a
FIrpic-doped film revealed that a modification of 9,9’-position
of BCz by sterically bulky functionalities is an effective way to
confine the triplet exciton of FIrpic. Although a BCzTPM/FIrpic
film showed a moderate ηPL of 74%, we successfully developed
an OLED with high PE of 45.5 lm W−1 (44.5 cd A−1 , EQE 19.6%)
with an extremely low driving voltage at 3.1 V at 100 cd m−2 .
A BCzPh/FIrpic film showed a relatively low ηPL of 62%, a
BCzPh-based blue OLED exhibited high PE of 45.2 lm W−1
(43.9 cd A−1 , EQE 19.7%) with an extremely low driving voltage at
3.1 V at 100 cd m−2 . These performances are the highest levels
in the scientific literature. Investigation of the device perform-
ances showed that the hole-injection was greatly enhanced by
using a BCz derivative and that the electron-injection was gen-
erated by carrier trapping process. Our results shows that a host
material based on BCz is one of the most promising candidates
for a high-performance blue phosphorescent OLED. Further-
more, these findings can also provide a powerful guideline to
design a high-performance host material and a device architec-
ture as well as a better understanding into the carrier injection
process in a blue phosphorescent OLED.
1
BCzTPM: H-NMR (400 MHz, CDCl3) : δ 8 . 42 (s, 2H), 8.22 (d, 2H,
J = 7.8 Hz), 7.76 (dd, 2H, J = 1.8, 8.7 Hz), 7.57-7.40 (m, 16H), 7.34-
7.25 (m, 30H) ppm; 13C-NMR (100 MHz, CDCl3) δ 146.7, 146.1, 141.3,
140.0, 135.6, 134.5, 132.7, 131.3, 127.8, 126.3, 126.1, 125.9, 125.8,
124.1, 123.7, 120.6, 120.1, 119.0, 110.3, 110.1 ppm (several signals were
superimposed); UV-vis (fi lm ): λmax = 358 nm; PL (fi lm ): λmax = 392,
412 nm; MS (EI) : m/z = 969 [M] +; Anal. Calcd for C74H52N2: C, 91.70;
H, 5.41; N, 2.89%. Found: C, 91.67; H, 5.51; N, 2.80%.
BCzPO: 1H-NMR (400 MHz, CDCl3) : δ 8 . 43 (s, 2H), 8.23 (d, 2H, J =
7.3 Hz), 7.95-7.90 (m, 4H), 7.82-7.76 (m, 14H), 7.63-7.51 (m, 16H), 7.45
(t, 2H, J = 7.3 Hz), 7.43 (t, 2H, J = 7.3 Hz) ppm; 13C-NMR (100 MHz,
CDCl3) δ 141.4, 140.7, 139.4, 134.8, 134.0, 133.9, 132.7, 132.3, 132.3,
132.2, 131.9, 131.7, 130.8, 128.9, 128.8, 126.6, 126.5, 126.1, 124.5, 124.0,
120.8, 120.7, 119.1, 110.1, 110.0 (several signals were superimposed);
UV-vis (fi lm ): λmax = 308, 340 nm; PL (fi lm ): λmax = 412 nm; MS: m/z =
885 [M] +; Anal. Calcd for C60H42N2O2; C, 81.43; H, 4.78; N, 3.17%.
Found: C, 81.24; H, 4.82; N, 3.10%.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Experimental Section
General Procedures: The optimized structures and single-point
energies were calculated by Gaussian09 at the B3LYP 6-31G(d) and
6-311+G(d,p) levels, respectively. TD-DFT calculations for singlet and
triplet energies were performed at the corresponding RB3LYP 6-31G(d)
levels. NMR spectra were recorded on a JEOL 400 spectrometer
(400 MHz for 1H-NMR and 100 MHz for 13C-NMR). Mass spectra were
obtained using a JEOL JMS-K9 mass spectrometer. Differential scanning
calorimetry (DSC) was performed using a Perkin-Elmer Diamond
DSC Pyris instrument under nitrogen atmosphere at a heating rate of
10 °C min−1 . Thermogravimetric analysis (TGA) was undertaken using
a SEIKO EXSTAR 6000 TG/DTA 6200 unit under nitrogen atmosphere
at a heating rate of 10 °C min−1 . UV-Vis spectra were measured using a
Shimadzu UV-3150 UV-vis-NIR spectrophotometer. Photoluminescence
spectra were measured using a FluroMax-4 (HORIBA Jobin-Yvon)
luminescence spectrometer. The ionization potentials (Ip) were
determined by an photoelectron yield spectroscopy (PYS) under
the vacuum (∼10−3 Pa). The phosphorescent spectra of 3 wt%-BCz
derivative doped PMMA films were measured by using a streak camera
(C4334 from Hamamatsu Photonics) at 5 K. All organic materials
were purified by temperature-gradient sublimation in vacuum. The EL
spectra were taken using an optical multichannel analyzer Hamamatsu
Photonics PMA-11. The current density–voltage and luminance–voltage
characteristics were measured using a Keithley source measure unit
2400 and a Minolta CS200 luminance-meter, respectively. External
quantum efficiencies were calculated from the front luminance, current
density and EL spectrum.
Acknowledgements
We greatly acknowledge the financial support in part by Japan Regional
Innovation Strategy Program by the Excellence (J-RISE) (Creating
international research hub for advanced organic electronics) of Japan
Science and Technology Agency (JST) and by KAKENHI (23750204).
Received: February 9, 2012
Revised: April 9, 2012
Published online: May 29, 2012
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