Triplet-energy control of polycyclic aromatic hydrocarbons by BN replacement: Development of ambipolar host materials for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1021/cm503102d

In this work, we achieved the triplet-energy control of polycyclic aromatic hydrocarbons (PAHs) by replacing the Carbon-Carbon (CC) unit with a Boron-Nitrogen (BN) unit. Time-dependent density functional theory calculations suggested that the insertion of the BN unit may cause localization of the singly occupied molecular orbitals 1 and 2 (SOMO1/SOMO2) in the triplet state, which in turn can reduce the exchange interaction and dramatically increase the high singlet-triplet excitation energy (E_T). The PAH containing the BN unit, 4b-aza-12b-boradibenzo[g,p]chrysene, showed a large E_T value and ambipolar carrier-transport abilities. The introduction of a phenyl substituent on 4b-aza-12b-boradibenzo[g,p]chrysene slightly reduced the E_T values and the carrier-transport abilities, but increased the glass-transition temperatures. On the basis of these findings, we successfully built phosphorescent organic light-emitting diodes using the BN compounds as host materials, which exhibit a superior performance over the device using a representative host material, 4,4'-bis(N-carbazolyl)-1,1'-biphenyl, not only in terms of efficiency but also in terms of device lifetime. This study demonstrated the potential of BN-embedded polycyclic aromatics in organic electronics and showed a novel strategy to achieve triplet-energy control of aromatic compounds.

Full text of DOI:10.1021/cm503102d

Article
pubs.acs.org/cm
Triplet-Energy Control of Polycyclic Aromatic Hydrocarbons by BN
Replacement: Development of Ambipolar Host Materials for
Phosphorescent Organic Light-Emitting Diodes
Sigma Hashimoto,† Toshiaki Ikuta,# Kazushi Shiren,# Soichiro Nakatsuka,‡ Jingping Ni,#
Masaharu Nakamura,*,† and Takuji Hatakeyama*,‡,§,∥
†International Research Center for Elements Science (IRCELS), Institute for Chemical Research (ICR), Kyoto University, Uji, Kyoto
611-0011, Japan
#JNC Petrochemical Corporation, 5-1 Goi Kaigan, Ichihara, Chiba, 290-8551, Japan
‡Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337,
Japan
§PRESTO, Japan Science and Technology Agency, 5, Sanbancho, Chiyoda, Tokyo, 102-0075, Japan
∥Element Strategy Initiative for Catalyst and Battery, Kyoto University, Nishikyo, Kyoto, 615-8510, Japan
S
* Supporting Information
ABSTRACT: In this work, we achieved the triplet-energy control
of polycyclic aromatic hydrocarbons (PAHs) by replacing the
Carbon−Carbon (CC) unit with a Boron−Nitrogen (BN) unit.
Time-dependent density functional theory calculations suggested
that the insertion of the BN unit may cause localization of the
singly occupied molecular orbitals 1 and 2 (SOMO1/SOMO2) in
the triplet state, which in turn can reduce the exchange interaction
and dramatically increase the high singlet−triplet excitation energy
(ET). The PAH containing the BN unit, 4b-aza-12b-boradibenzo-
[g,p]chrysene, showed a large ET value and ambipolar carrier-
transport abilities. The introduction of a phenyl substituent on 4b-
aza-12b-boradibenzo[g,p]chrysene slightly reduced the ET values and the carrier-transport abilities, but increased the glass-
transition temperatures. On the basis of these findings, we successfully built phosphorescent organic light-emitting diodes using
the BN compounds as host materials, which exhibit a superior performance over the device using a representative host material,
4,4′-bis(N-carbazolyl)-1,1′-biphenyl, not only in terms of efficiency but also in terms of device lifetime. This study demonstrated
the potential of BN-embedded polycyclic aromatics in organic electronics and showed a novel strategy to achieve triplet-energy
control of aromatic compounds.
than the phosphorescent emitting materials are required for the
emitting layer.9c,d In this work, we explored the hypothesis that
replacement of the CC unit of PAHs by an isoelectronic BN
unit10 would localize the singly occupied molecular orbitals
(SOMO1 and SOMO2) in the triplet state (T1).11 This in turn
could suppress the exchange interaction between SOMOs in
the T1 state and increase ET. As shown in Figure 1,
dibenzo[g,p]chrysene 1 was chosen as a PAH with a wide
energy gap.12 Time-dependent density functional theory (TD-
DFT) calculations showed that the isoelectronic 4b-aza-12b-
boradibenzo[g,p]chrysene 2 is characterized by a significant
larger ET (3.03 eV) than that of 1 (2.25 eV); in contrast, the
singlet−singlet excitation energy (Es) was found to be the same
for both (ES = 3.49 eV).13 This finding can be explained if one
INTRODUCTION
■
Polycyclic aromatic hydrocarbons (PAHs) are an important
class of materials that are used in organic electronics,1 dyes,2
batteries,3 liquid-crystal displays,4 and photoresponsive materi-
als.5 In particular, substantial efforts have been devoted to
building organic field-effect transistors based on PAHs6 because
of their excellent charge-transport properties and redox
stability. On the other hand, organic light-emitting diodes
(OLEDs)7 using PAHs as charge-transport materials have not
received much attention.8 This may be due to their small band
gap (Eg) and/or to their poor morphological stability, which
can reduce the efficiency and lifetime of devices.
Recently, a lot of studies focused on OLEDs using
phosphorescent emitting materials (PHOLEDs), which can
show much higher internal quantum efficiency than those using
fluorescent ones because of capability of emission from the
triplet exciton.9 To achieve an optimum efficiency, ambipolar
host materials with higher singlet−triplet excitation energy (ET)
Received: August 23, 2014
Revised: September 26, 2014
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of 2, 2-Ph, and 2-Ph2
Figure 1. Design of ambipolar host materials for phosphorescent
organic light-emitting diodes. Singlet−singlet and singlet−triplet
excitation energies (ES, ET) at the ground-state (S0) structure were
calculated with TD-DFT at the B3LYP/6-31G(d) level of theory.
compares the localized SOMO1/SOMO2 of 2 in the T1 state
with those of 1 (Figure 2). In particular, SOMO1 of 2 is
Spectroscopic Measurements. To estimate ES and ET of
1 and 2, a set of spectroscopic measurements was carried out as
shown in Figure 3. The UV−visible absorption spectrum of 2
Figure 2. Singly occupied Kohn−Sham orbitals of 1 (a) and 2 (b) in
the T1 state calculated at the UB3LYP/6-31G(d) level of theory.
Orbital energies are shown in parentheses.
localized on the boron atom, ortho and para positions to the
boron atom, and meta position to the nitrogen atom; SOMO2
is localized on the nitrogen atom, meta position to the boron
atom, and ortho and para positions to the nitrogen atom.13 To
achieve high glass-transition temperature (Tg), we have
synthesized phenylated derivatives 2-Ph and 2-Ph2. This
allowed us to produce highly efficient PHOLEDs with a
sufficiently long lifetime.
RESULTS AND DISCUSSION
■
Synthesis of 4b-Aza-12b-boradibenzo[g,p]chrysene
Derivatives. The synthesis of the derivatives is illustrated in
Scheme 1. 4b-Aza-12b-boradibenzo[g,p]chrysene 2 and mono-
phenylated derivative 2-Ph were synthesized under optimized
conditions14 from the corresponding diarylamines 3 and 4 via a
tandem electrophilic arene borylation in 71% and 74% yield,
respectively. The electrophilic bromination of 2 was accom-
plished by treatment with 2 equiv of N-bromosuccinimide
(NBS) to give dibrominated derivative 5 in 94% yield. In the
presence of 2 mol % dichlorobis(p-dimethylaminophenyldi-tert-
butylphosphine)palladium(II) (Pd-132) and 5 equiv of K3PO4,
the succeeding Suzuki−Miyaura coupling with phenylboronic
acid smoothly took place to give coupling product 2-Ph2 in
74% yield.
Figure 3. Standardized absorption spectra (blue line, 0.02 mM in
CH2Cl2), fluorescence spectra (red line, 0.02 mM in CH2Cl2), and
phosphorescence spectra (green line, saturated EtOH solution at 77
K) of (a) 1 and (b) 2.
B
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showed a relatively strong absorption band, corresponding to a
π−π* transition, with a maximum at 340 nm (log ε = 4.06), to
compare with that of 1 at 351 nm (log ε = 4.21). When the CC
unit was replaced with the BN unit, a small blue shift in the
UV−visible absorbance was observed, probably due to an
insufficient double-bond character of the BN bond. The
photoluminescence emissions of 1 and 2 showed vibronic
fine structures at 395 and 410 nm and a maximum at 409 nm at
room temperature, respectively. As predicted by the TD-DFT
calculations, the BN replacement does not significantly change
the fluorescence spectrum, but causes a remarkable blue shift in
the phosphorescence spectrum. In particular, at 77 K the
photoluminescence emissions of 1 and 2 showed maxima at
600 and 633 nm and at 436 and 462 nm, respectively. Thus, the
combination of our experimental and computational data
suggested that the replacement of CC unit with the BN unit
hardly changes ES, but significantly affects ET through
suppression of the exchange interaction between SOMOs.
Since 2 showed a high ET value (2.84 eV), we compared its
phosphorescence spectra with those of 4,4′-bis(N-carbazolyl)-
1,1′-biphenyl (CBP) and Ir(ppy)3, which are considered in this
work as a reference host material and a reference green dopant
for PHOLEDs, respectively. As shown in Figure 4, 2 has a
measurements, we prepared films (thickness: 6.0−9.9 μm) of
compounds 2, 2-Ph, and 2-Ph2 and of the carbon analog 1 by
vacuum deposition under 5.0 × 10−3 Pa. The carrier-transport
properties of the films were then evaluated at room
temperature and with electric fields of 5.0−5.8 × 105 V cm−1.
It was found that all the compounds containing the BN unit
have ambipolar carrier-transport abilities (Table 1). In
particular, the parent compound 2 showed the highest hole
(4.1 × 10−4 cm2 V−1 s−1) and electron (2.3 × 10−3 cm2 V−1 s−1)
mobility,15 for example, 3 times larger than those of 1 (1.2 and
7.5 × 10−4 cm2 V−1 s−1, respectively). The ionization potentials
(Ip) and optical band gaps (Eg) of the vacuum-deposited films
were also determined. Ip of the parent compound 2 was
estimated to be 6.01 eV, which is comparable to that of CBP.
The introduction of one or two phenyl groups resulted in a
lower Ip, 5.81 and 5.64 eV, respectively, possibly due to the
expanded π-conjugation. For the same reason, 2 showed the
highest Eg (3.29 eV), in agreement with the trend observed for
ET. Owing to their high Tg, all the derivatives form stable
amorphous films, which were determined by X-ray diffraction
analysis..
Phosphorescent Organic Light-Emitting Diodes. Since
the compounds containing the BN unit showed promising
ambipolar carrier-transport abilities and large ET values, they
were used as host materials, together with Ir(ppy)3 as a green
dopant, to build heterojunction PHOLEDs (Figures 5 and 6,
Figure 4. Standardized phosphorescence spectra of 2 (green line), 2-
Ph (red line), 2-Ph2 (blue line), CBP (black line), and Ir(ppy)3 (black
dotted line) in EtOH at 77 K.
Figure 5. Energy diagram of the materials of PHOLEDs (in eV). The
emissive layer consists of 10 wt % of Ir(ppy)3 and 90 wt % of one of
the five host materials. ITO = indium tin oxide. HAT-CN =
dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonit rile.
TBBD = N4,N4,N4′,N4′-tetra([1,1′-biphenyl]-4-yl)-[1,1′-biphenyl]-
4,4′-diamine. TCTA = tris(4-(9H-carbazoyl-9-yl)phenyl)amine. TPBi
= 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene.
larger ET than those of CBP and Ir(ppy)3 (2.64 and 2.50 eV),
confirming its potential as a host material for a PHOLED.
Similarly, phenylated derivatives, 2-Ph and 2-Ph2, also showed
sufficiently large ET values (2.70 and 2.69 eV).
Charge Mobility and Photoelectron Spectroscopy
Measurements. To perform the time-of-flight (TOF)
Table 1. Physical Properties of 2, 2-Ph, 2-Ph2, 1, and CBP
a
a
b
c
d
e
f
μh
μe
Ip
Eg
Ea
ET
Tg
1
1.2 × 10−4
4.1 × 10−4
7.9 × 10−5
1.7 × 10−4
1 × 10−3
7.5 × 10−4
2.3 × 10−3
3.2 × 10−3
3.2 × 1010−4
1 × 10−3
5.84
6.01
5.81
5.64
6.02
3.13
3.29
3.21
3.15
3.45
2.71
2.72
2.60
2.49
2.57
2.05
2.84
2.70
2.69
2.64
74
71
2
2-Ph
2-Ph2
CBP
99
124
85
a
Hole and electron mobility (cm2 V−1s−1) of the vacuum-deposited thin films were determined by the time-of-flight (TOF) method. See ref 15b for
b
c
CBP. Ionization potentials (eV) were determined by photoelectron spectroscopy in air (PESA). Optical band gaps (eV) were estimated from onset
wavelengths of the UV/vis absorption spectra of the vacuum-deposited thin films. Electron affinities (eV) were calculated from Ip and Eg. Triplet
energies (eV) were estimated from the emission maxima of the phosphorescence spectra of saturated EtOH solutions at 77 K. Glass-transition
d
e
f
temperatures (°C) were determined by differential scanning calorimetry (DSC).
C
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lifetime of OLEDs was also measured at the initial brightness of
2000 cd/m2. The devices obtained from 2 showed longer
lifetimes than CBP (148 and 80 h, respectively), probably
owing to its redox stability. The introduction of one or two
phenyl groups significantly improved the lifetime (226 and 206
h, respectively). This is possibly caused by their large Tg, 99 and
124 °C, respectively. Thus, these measurements suggested that
the compounds containing BN may be suitable as a new class of
ambipolar host materials for PHOLEDs.
CONCLUSIONS
■
In this work, we have successfully synthesized 4b-aza-12b-
boradibenzo[g,p]chrysene derivatives; by means of time-of-
flight and phosphorescent measurements, we confirmed their
ambipolar carrier-transport abilities and their high singlet−
triplet excitation energy. Notably, phosphorescent organic light-
emitting diodes that employed the synthesized 4b-aza-12b-
boradibenzo[g,p]chrysene derivatives as host materials showed
higher efficiencies and longer device lifetimes than those with
CBP, a representative host material. Density functional theory
calculations suggested that the replacement of the central CC
unit of boradibenzo[g,p]chrysene with a BN unit causes the
localization of singly occupied molecular orbitals in the triplet
state and reduces the exchange interaction between SOMOs,
thereby increasing the singlet−triplet excitation energy
dramatically. We believe that our results clearly indicated a
new strategy to achieve the triplet-energy control of aromatic
compounds, a crucial outcome that can provide not only a new
class of ambipolar host materials but also thermally activated
delayed fluorescence materials.
EXPERIMENTAL SECTION
Figure 6. Characteristics of PHOLED using CBP (black line), 2
(green line), 2-Ph (red line), and 2-Ph2 (blue line) as an ambipolar
host material.
■
General Procedure. All the reactions dealing with air- or
moisture-sensitive compounds were carried out in a dry
reaction vessel under a positive pressure of nitrogen. Air- and
moisture-sensitive liquids and solutions were transferred via a
syringe or a stainless steel cannula. Organic solutions were
concentrated by rotary evaporation at ca. 30−400 mm Hg. Gel
permeation chromatography was performed on a JAIGEL-1H
and 2H (20 mm i.d.) with an LC-9204 (Japan Analytical
Industry Co., Ltd.). Proton nuclear magnetic resonance (1H
NMR) and carbon nuclear magnetic resonance (13C NMR)
spectra were recorded on JEOL ECS400 (400 MHz) NMR
spectrometers. Proton chemical shift values are reported in
parts per million (ppm, δ scale) downfield from tetramethylsi-
lane and are referenced to the tetramethylsilane (δ 0) or o-
dichlorobenzene-d4 (δ 7.20). 13C NMR spectra were recorded
at 101 MHz. Carbon chemical shift values are reported in parts
per million (ppm, δ scale) downfield from tetramethylsilane
and are referenced to the carbon resonance of CDCl3 (δ 77.0).
11B NMR spectra were recorded at 128 MHz. Boron chemical
shift values are reported in parts per million (ppm, δ scale) and
are referenced to the external standard boron signal of BF3·
Et2O (δ 0). Data are presented as chemical shift, multiplicity (s
= singlet, d = doublet, t = triplet, q = quartet, quint = quintet,
sext = sextet, sept = septet, m = multiplet and/or multiplet
resonances, and br = broad), coupling constant in hertz (Hz),
signal area integration in natural numbers, and assignment
(italic). IR spectra were recorded on an ATR-FTIR
spectrometer (FT/IR-Spectrum One, Perkin-Elmer). Charac-
teristic IR absorptions are reported in cm−1. Melting points
were recorded on a Yanaco MP-500 V. High-resolution mass
Table 2). While the device containing the carbon analog 1
exhibited a very poor performance, those containing 2, 2-Ph,
Table 2. Properties of the 10 wt % Ir(ppy)3-Doped
PHOLEDs
a
a
a
a
b
host
V1000
ηc,1000
ηp,1000
EQE1000
LT80
c
c
c
c
1
(7.5)
4.8
(0.014)
70.2
(0.006)
46.1
(0.008)
19.5
2
148
226
206
80
2-Ph
2-Ph2
CBP
5.3
69.8
41.3
19.3
5.0
63.3
39.6
17.4
5.4
57.7
33.9
16.0
a
Driving voltage (V), current efficiency (cd A−1), power efficiency (lm
b
W−1), and external quantum efficiency (%) at 1000 cd m−2. Time (h)
when brightness (L0 = 2000 cd m−2) decreases to 80% (1600 cd−2).
c
Data at 10 cd m−2.
and 2-Ph2 exhibited a superior performance over the CBP
device in terms of the driving voltage (V1000), current efficiency
(ηc,1000), power efficiency (ηp,1000), and the external quantum
efficiency (EQE1000). Such a small driving voltage can be
attributed to the lower Ip or to the higher Ea of the BN
compounds, which can reduce the contact resistance between
the carrier transport and the emissive layers. The better
performance of the BN compounds in terms of EQE may also
be explained considering their large ET values, which can
prevent back-energy transfer from the dopant. In addition, the
D
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spectra (HRMS) were obtained using the electron impact (EI)
method with a JEOL JMS-700, JMS-SX102A. UV−visible
absorption spectra were measured with a JASCO V-560 and V-
570. Fluorescence spectra were collected with a HORIBA
Scientific FluoroMax-4 and a Hitachi High-Tech F-7000. The
purity of isolated compounds was determined by GC analysis
on a Shimadzu GC-17A instrument equipped with an FID
detector and a capillary column, InertCap 1MS (GL Sciences
yield). IR (neat): cm−1 3055 (Ar−H), 1599, 1483, 1442, 1320,
1256, 1125, 1049, 941, 881, 832, 781, 747, 724, 692, 649, 620.
mp: 214.8−216.4 °C. 1H NMR (CDCl3, 400 MHz): δ 7.34 (td,
J = 1.2 Hz, 7.6 Hz, 1H, NCCCHCH), 7.37−7.41 (m, 2H,
NCCHCHCH, (CHCH)2CH), 7.50 (t, J = 8.0 Hz, 2H,
(CHCH)2CH), 7.59 (dd, J = 2.0 Hz, 8.8 Hz, 1H,
NCCHCHCPh), 7.60−7.63 (m, 2H, BCCHCH), 7.73 (dd, J
= 1.2 Hz, 8.0 Hz, 2H, (CHCH)2CH), 7.78 (td, J = 1.6 Hz, 8.0
Hz, 1H, BCCCHCH), 7.79 (td, J = 1.2 Hz, 8.0 Hz, 1H,
BCCCHCH(Ph-side)), 8.13 (dd, J = 1.2 Hz, 7.6 Hz, 1H,
NCCHCHCH), 8.16 (d, J = 8.8 Hz, 1H, NCCHCHCPh), 8.37
(dd, J = 1.6 Hz, 7.6 Hz, 1H, NCCCHCH), 8.42 (d, J = 8.0 Hz,
1H, BCCCH), 8.48 (d, J = 8.0 Hz, 1H, BCCCH(Ph-side)),
8.56 (d, J = 2.0 Hz, 1H, NCCCHCPh), 8.69 (br, d, J = 7.2 Hz,
2H, BCCH). 13C NMR (CDCl3, 101 MHz): δ 121.4, 121.8,
123.0 (2C), 123.1, 124.0, 125.5, 125.7, 126.8 (2C), 126.9, 127.1
(2C), 127.2, 127.5, 127.7, 128.9 (2C), 131.1 (2C), 132.5 (br,
2C, CBC), 135.6, 135.6, 135.8, 136.4, 137.0, 138.8 (2C), 140.9.
11B NMR (CDCl3, 128 MHz): δ 35.2. Anal. Calcd for
C30H20NB: C, 88.90; H, 4.97; N, 3.46. Found: C, 89.20; H,
5.12; N, 3.48.
1
Inc., 30 m × 0.25 mm i.d., 0.25 μm film thickness) and/or H
NMR analyses. High glass-transition temperatures were
measured with a Perkin-Elmer Diamond DSC.
4b-Aza-12b-boradibenzo[g,p]chrysene (2). A solution
of butyllithium in hexane (29.3 mL, 1.60 M, 46.9 mmol) was
added slowly to a solution of 3 (15.0 g, 46.9 mmol) in toluene
(250 mL) at −75 °C under nitrogen. After the reaction mixture
was stirred at 0 °C for 1 h, a solution of boron trichloride in
heptane (46.9 mL, 1.0 M, 46.9 mmol) was added at −75 °C.
After being stirred at room temperature for 8 h, the solvent was
removed in vacuo, and the reaction mixture was added to a
solution of aluminum trichloride (25.0 g, 188 mmol) and
2,2,6,6-tetramethylpiperidine (13.8 g, 98.5 mmol) in o-
dichlorobenzene (300 mL) at room temperature. After stirring
at 160 °C for 12 h, 1,4-diazabicyclo[2.2.2]octane (21.0 g, 188
mmol) was added. The reaction mixture was filtered with a pad
of Celite. After the solvent was removed in vacuo, the crude
product was purified by alumina column chromatography
[eluent: toluene/AcOEt/Et3N = 90/10/1 (volume ratio)] and
reprecipitation from mixture of ethyl acetate and heptane to
obtain the title compound (10.9 g, 71% yield) as a white-yellow
powder. IR (neat): cm−1 3056 (Ar−H), 1598, 1575, 1485,
1445, 1430, 1319, 1300, 1286, 1259, 1238, 1166, 1134, 1049,
2,7-Dibromo-4b-aza-12b-boradibenzo[g,p]chrysene
(5). N-Bromosuccinimide (0.178 g, 1.00 mmol) was added to a
solution of 2 (0.165 g, 0.50 mmol) in dichloromethane (6.0
mL) and acetonitrile (2.0 mL). After the mixture was stirred at
room temperature for 1 h, the solvent was removed in vacuo.
The residue was triturated with acetonitrile to afford the title
compound (0.228 g, 94% yield) as a colorless powder. IR
(neat): cm−1 3046 (Ar−H), 1598, 1570, 1554, 1542, 1482,
1436, 1392, 1325, 1297, 1281, 1260, 1236, 1194, 1162, 1097,
1047, 1013, 937, 886, 876, 862, 833, 811, 781, 756, 744, 720,
1
1
935, 744, 723, 624, 587, 558. mp 226.5−227.3 °C. H NMR
669, 643, 630, 620, 605. mp: 244−245 °C. H NMR (CDCl3,
(CDCl3, 400 MHz): δ 7.32 (td, J = 1.2, 7.6 Hz, 2H,
NCCCHCH), 7.36 (td, J = 1.6, 7.6 Hz, 2H, NCCHCH), 7.60
(td, J = 0.8, 7.6 Hz, 2H, BCCHCH), 7.77 (td, J = 1.6, 7.6 Hz,
2H, BCCCHCH), 8.10 (dd, J = 1.2, 7.6 Hz, 2H, NCCH), 8.36
(dd, J = 1.6, 7.6 Hz, 2H, NCCCH), 8.41 (d, J = 7.6 Hz, 2H,
BCCCH), 8.68 (d, J = 7.6 Hz, 2H, BCCH). 13C NMR (CDCl3,
101 MHz): δ 121.4 (2C), 123.0 (4C), 125.5 (2C), 126.8 (2C),
127.5 (4C), 131.1 (2C), 132.5 (br, 2C, CBC), 135.6 (2C),
137.1 (2C), 138.8 (2C). 11B NMR (CDCl3, 128 MHz): δ 35.6.
HRMS(EI) m/z [M]+ calcd for C24H16NB, 329.1376; observed,
329.1380. Anal. Calcd for C24H16NB: C, 87.56; H, 4.90; N,
4.25. Found: C, 87.79; H, 5.14; N, 4.31.
2-Phenyl-4b-aza-12b-boradibenzo[g,p]chrysene (2-
Ph). A solution of butyllithium in hexane (29.3 mL, 1.60 M,
46.9 mmol) was added slowly to a solution of N-([1,1′-
biphenyl]-2-yl)-[1,1′:3′,1″-terphenyl]-4-amine 4 (18.6 g, 46.8
mmol) in toluene (250 mL) at −70 °C under argon. After 1 h,
the reaction mixture was stirred at 0 °C for 1 h. A solution of
boron trichloride in heptane (46.9 mL, 1.0 M, 46.9 mmol) was
added at −60 °C. After being stirred at room temperature, the
solvent was removed in vacuo, and the reaction mixture was
added to a solution of aluminum trichloride (25.0 g, 188 mmol)
and 2,2,6,6-tetramethylpiperidine (13.9 g, 98.4 mmol) in o-
dichlorobenzene (300 mL). After the solution was stirred at
170 °C for 20 h, a suspension of sodium carbonate (10.0 g) and
sodium acetate (31.0 g) in ice water was poured. The extracted
organic layer was filtered with a pad of Celite. After the solvent
was removed in vacuo, the crude product was purified by
alumina column chromatography (eluent: toluene/Et3N = 100/
1) and trituration with reprecipitation from a mixture of ethyl
acetate and heptane to obtain the title compound (14.0 g, 74%
400 MHz): δ 7.41 (dd, J = 2.3, 9.0 Hz, 2H, NCCHCH), 7.62
(td, J = 0.8, 7.4 Hz, 2H, BCCHCH), 7.78 (td, J = 1.2, 7.6 Hz,
2H, BCCHCHCH), 7.82 (d, J = 9.0 Hz, 2H, NCCH), 8.30 (d, J
= 7.4 Hz, 2H, BCCCH), 8.42 (d, J = 2.3 Hz, 2H, NCCCH),
8.63 (dd, J = 1.2, 7.4 Hz, 2H, BCCH). 13C NMR (CDCl3, 101
MHz): δ 116.2 (2C), 122.8 (2C), 123.1 (2C), 127.5 (2C),
128.3 (2C), 129.5 (2C), 129.6 (2C), 131.4 (2C), 132.4 (br, 2C,
CBC), 135.6 (2C), 135.7 (2C), 137.5 (2C). 11B NMR (CDCl3,
128 MHz): δ 35.5. Anal. Calcd for C24H14NBBr2: C, 59.19; H,
2.90; N, 2.88. Found: C, 59.13; H, 3.08; N, 2.93.
2,7-Diphenyl-4b-aza-12b-boradibenzo[g,p]chrysene
(2-Ph2). Pd-132 (20.0 mg, 0.028 mmol) was added to a
solution of 5 (0.700 g, 1.4 mmol), phenylboronic acid (0.430 g,
3.5 mmol), and tripotassium phosphate (1.49 g, 7.0 mmol) in
toluene (15 mL). After the mixture was stirred at 70 °C for 1 h,
the resulting solution was cooled to room temperature and
water added to quench the reaction. The organic materials were
extracted with toluene, and the solvent was removed in vacuo.
The crude product was purified by Al2O3 column chromatog-
raphy (eluent: toluene/Et3N = 99/1) and washed by heptane
and ethyl acetate. The residue was purified by recrystallization
(heptane/chlorobenzene) to obtain the title compound (0.510
g, 74%). IR (neat): cm−1 3053 (Ar−H), 3026 (Ar−H), 1599,
1566, 1543, 1504, 1482, 1440, 1396, 1335, 1321, 1283, 1254,
1238, 1200, 1178, 1151, 1125, 1103, 1074, 1052, 1028, 1008,
978, 941, 910, 885, 837, 826, 753, 730, 707, 669, 637, 628, 617,
1
609. mp: 269−271 °C. H NMR (CDCl3, 400 MHz): δ 7.39
(tt, J = 1.6, 7.4 Hz, 2H, CHCHCHCHCH), 7.50 (t, J = 7.4 Hz,
4H, CHCHCHCHCH), 7.59−7.64 (m, 4H), 7.73 (d, J = 7.4
Hz, 4H, CHCHCHCHCH), 7.79 (td, J = 0.8, 7.8 Hz, 2H,
BCCHCHCH), 8.18 (d, J = 8.2 Hz, 2H, NCCH), 8.49 (d, J =
E
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Chemistry of Materials
Article
7.8 Hz, 2H, BCCCH), 8.57 (d, J = 2.0 Hz, 2H, NCCCH), 8.70
(d, J = 7.4 Hz, 2H, BCCH). 13C NMR (CDCl3, 101 MHz): δ
121.8 (2C), 123.1 (2C), 124.0 (2C), 125.7 (2C), 127.0 (2C),
127.1 (4C), 127.2 (2C), 127.8 (2C), 128.9 (4C), 131.1 (2C),
132.6 (br, 2C, CBC), 135.7 (2C), 135.9 (2C), 136.4 (2C),
138.8 (2C), 140.9 (2C). 11B NMR (CDCl3, 128 MHz): δ 36.0.
Anal. Calcd for C36H24NB: C, 89.82; H, 5.03; N, 2.91. Found:
C, 90.04; H, 5.14; N, 2.89.
was also supported by NEXT Program from the Japan Society
for the Promotion of Science (JSPS) and Asahi Glass
Foundation. We thank Mr. Yohei Ono and Dr. Takeshi
Matsushita (JNC Petrochemical Corporation) and Dr. Yasushi
Honda (HPC Systems Inc.) for valuable discussions.
ABBREVIATIONS
■
PAHs, polycyclic aromatic hydrocarbons; CC, carbon−carbon;
BN, boron−nitogen; OLEDs, organic light-emitting diodes;
PHOLEDs, phosphorescent organic light-emitting diodes;
SOMO, singly occupied molecular orbitals; TD-DFT, time-
dependent density functional theory; CBP, 4,4′-bis(N-carba-
zolyl)-1,1′-biphenyl; TOF, time of flight; EQE, external
quantum efficiency; ITO, indium tin oxide; HIL, hole injection
layer; HTL, hole transport layer; EBL, electron blocking layer;
EML, emissive layer; ETL, electron transport layer
Fabrication of PHOLEDs and Their Characteristics.
PHOLEDs were fabricated on glass substrates coated with a
patterned transparent indium tin oxide (ITO) conductive layer.
Substrates were cleaned in a detergent solution for 5 min and
then in distilled water for 10 min with an ultrasonic bath. They
were predried with a jet spin washer, then dried for 5 min in an
oven at 150 °C, and finally treated with ozone plasma. The
PHOLED structures of ITO (50 nm)/HIL (10 nm)/HTL (60
nm)/EBL (10 nm)/EML (30 nm)/ETL (50 nm)/LiF (1 nm)/
Al (100 nm) were prepared by vacuum evaporation.
Dipyrazino[2,3-f:2′,3′-h]quinoxaline- 2,3,6,7,10,11-hexacarbo-
nitrile (HAT-CN), N4,N4,N4′,N4′-tetra([1,1′-biphenyl]-4-yl)-
[1,1′-bipheny l]-4,4′-diamine (TBBD), tris(4-carbazoyl-9-yl-
phenyl) amine (TCTA), and 1,3,5-tris(1-phenyl-1H-benzo[d]-
imidazol-2-yl)benzene (TPBi) were used as HIL, HTL, EBL,
and ETL, respectively. The EML consists of 10 wt % of
Ir(ppy)3 and 90 wt % of one of the five host materials. The
pressure of the vacuum evaporation was 5.0 × 10−4 Pa, and the
film thickness was controlled with a calibrated quartz crystal
microbalance during deposition. After the deposition of all
layers, the PHOLED test modules were encapsulated with
capping glass in the evaporation chamber filled with nitrogen.
The OLED characteristics of all the fabricated devices were
evaluated at room temperature in air atmosphere using a
voltage−current−luminance measuring system, which consisted
of a source meter (Keithley 2400) and a spectral-radiance meter
(Topcon SR-3AR). Assuming a perfect diffusion surface light
emitting surface, the external quantum efficiency was calculated
with EL spectrum. The operational stability (or lifetime) was
tested at a constant current corresponding to an initial
luminance of 2000 cd/m−2 for each device at room temper-
ature. The elapsed time, luminance, and voltage during the
lifetime tests were recorded in situ by a computer until the end
of the testing.
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ASSOCIATED CONTENT
* Supporting Information
Computational details, charge mobility measurements, spec-
troscopy, and NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: hatake@kwansei.ac.jp.
*E-mail: masaharu@scl.kyoto-u.ac.jp.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by PRESTO from the Japan
Science and Technology Agency (JST), a Grant-in-Aid for
Young Scientists (23685020), and a Grant-in-Aid for Scientific
Research (26288095) from the Ministry of Education, Culture,
Sports, Science & Technology in Japan (MEXT). The study
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dx.doi.org/10.1021/cm503102d | Chem. Mater. XXXX, XXX, XXX−XXX