Excimer-emitting single molecules with stacked π-conjugated groups covalently linked at the 1,8-positions of naphthalene for highly efficient blue and green OLEDs

Article abstract of DOI:10.1039/c3tc30319e

Two efficient blue-light-emitting compounds, 1,8-bis(4-(N-carbazolyl) phenyl)naphthalene (BCzPN) and 1,8-bis(4-(10-phenylanthracen-9-yl)-phenyl) naphthalene (BPAPN), are designed and synthesized, in which two phenylcarbazole or diphenylanthracene units are closely stacked through bonding to the 1- and 8-positions of the naphthalene ring, resulting in strong intramolecular excimer emissions in solution or as a film. By utilizing BPAPN as an emitter, high efficiencies of 6 cd A^-1 and 5.8% external quantum efficiency (EQE) at 100 cd m^-2, and 8 cd A^-1 and 5.8% EQE at 1000 cd m ^-2 are achieved in a non-doped blue device. By using BCzPN or BPAPN as a host, a DPAVBi-doped BCzPN based blue device gave high efficiencies of 15 cd A^-1 and 6.5% EQE at 100 cd m^-2, and 12 cd A ^-1 and 5.5% EQE at 1000 cd m^-1, and a C545T-doped BPAPN based green device gave high efficiencies of 23 cd A^-1 and 6.7% EQE at 100 cd m^-2, and 22 cd A^-1 and 6.7% EQE at 1000 cd m^-2, respectively. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3tc30319e

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Materials Chemistry C
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Excimer-emitting single molecules with stacked
p-conjugated groups covalently linked at the 1,8-
positions of naphthalene for highly efficient blue
and green OLEDs†
Cite this: DOI: 10.1039/c3tc30319e
*
Jian-Yong Hu, Yong-Jin Pu, Yusuke Yamashita, Fumiya Satoh, So Kawata,
*
Hiroshi Katagiri, Hisahiro Sasabe and Junji Kido
Two efficient blue-light-emitting compounds, 1,8-bis(4-(N-carbazolyl)phenyl)naphthalene (BCzPN) and
1,8-bis(4-(10-phenylanthracen-9-yl)-phenyl)naphthalene (BPAPN), are designed and synthesized, in
which two phenylcarbazole or diphenylanthracene units are closely stacked through bonding to the 1-
and 8-positions of the naphthalene ring, resulting in strong intramolecular excimer emissions in solution
or as a film. By utilizing BPAPN as an emitter, high efficiencies of 6 cd Aꢀ1 and 5.8% external quantum
efficiency (EQE) at 100 cd mꢀ2, and 8 cd Aꢀ1 and 5.8% EQE at 1000 cd mꢀ2 are achieved in a non-
doped blue device. By using BCzPN or BPAPN as a host, a DPAVBi-doped BCzPN based blue device gave
high efficiencies of 15 cd Aꢀ1 and 6.5% EQE at 100 cd mꢀ2, and 12 cd Aꢀ1 and 5.5% EQE at 1000 cd
Received 19th February 2013
Accepted 19th April 2013
DOI: 10.1039/c3tc30319e
m
ꢀ1, and a C545T-doped BPAPN based green device gave high efficiencies of 23 cd Aꢀ1 and 6.7% EQE
at 100 cd mꢀ2, and 22 cd Aꢀ1 and 6.7% EQE at 1000 cd mꢀ2, respectively.
www.rsc.org/MaterialsC
emitting molecule containing two teruorenyl groups linked
Introduction
with indenouorene, with blue electroluminescent properties.4a
We recently reported OLED properties of an excimer-emitting
molecule with two pyrene groups attached to the 1- and
8-positions of naphthalene, the pyrenes being stacked face-to-
face.4b This stacked pyrene molecule showed highly efficient
sky-blue emission in OLEDs, and high efficiency as a host for a
green uorescent emitter. For the development of uorescence-
emitting compounds for OLEDs, blue emitting compounds are
particularly worth studying because long practical lifetimes are
more difficult to achieve for blue phosphorescent devices than
for other emitting devices.5 In this paper, we present our efforts
to design and synthesize two efficient blue-light emitting
Organic light-emitting devices (OLEDs) have attracted a great
deal of attention because of their promise for use as solid-state
lighting.1 The color rendering index (CRI) is an important
parameter in white lighting applications to evaluate the quality
of the lighting. To achieve a high CRI, broad blue, green, and
red emission spectra are required. Excimer emission could be
an approach for achieving a high CRI because excimers produce
broad spectra with large full width at half maxima (FWHM).2
Excimer emission is also largely bathochromically shied
compared to standard uorescence, so there is almost no
overlap between emission and absorption. Less self-quenching
of the excimer emission also leads to a high uorescence
quantum yield. Excimer emission through intermolecular p–p
interactions has been widely reported for OLED applications.3
However, there have been recent reports on excimer emission
from intramolecular p–p interactions in single molecules and
their application in OLEDs.4 Poriel et al. reported an excimer-
compounds,
1,8-bis(4-(N-carbazolyl)phenyl)naphthalene
(BCzPN) and 1,8-bis(4-(10-phenylanthracen-9-yl)-phenyl)naph-
thalene (BPAPN), in which two identical luminophore units,
phenylcarbazole or diphenylanthracene, are closely stacked
through bonding to the 1- and 8-positions of the naphthalene
ring, resulting in strong single-molecule excimer emissions in
solution or as a lm (Fig. 1). Carbazole and anthracene were
chosen as stacked p-units because they have strong excimer-
emitting properties6 and excellent charge transporting and
electroluminescent properties.7–9 We investigated the potential
application of these excimer-emitting single molecules as OLED
materials and found that OLEDs that used BCzPN or BPAPN as
an emitter (non-doped OLEDs) or as a host material (doped
OLEDs) were highly efficient. A non-doped blue device using
BPAPN as an emitter gave high efficiencies of 6 cd Aꢀ1 and 5.8%
Department of Organic Device Engineering, Research Center for Organic Electronics,
Yamagata University, 4-3-16 Johnan, Yonezawa, Yamagata, 992-8510, Japan.
E-mail: pu@yz.yamagata-u.ac.jp; kid@yz.yamagata-u.ac.jp; Fax: +81-238-263595
† Electronic supplementary information (ESI) available: 1H-NMR spectra of
BCzPN and BPAPN, TGA curves of BCzPN and BPAPN, DSC thermograms of
BCzPN and BPAPN, summary of the BCzPN crystal data, chemical structures
of the compounds used in the devices, the energy diagram, etc. CCDC 909549.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3tc30319e
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dioxaborolane. A Suzuki cross-coupling reaction of 9-bro-
moanthracene with phenylboronic acid, in the presence of
Pd(PPh3)4 and 2 M K2CO3, yielded 9-phenylanthracene 2a.
Brominated product 2b was produced by stirring compound 2a
with N-bromosuccinimide in chloroform for 3 h at room
temperature. Compound 2 was obtained from the Suzuki cross-
coupling reaction of bromide 2b with 4-chlorophenylboronic
acid followed by borylation, following a published method.11
Finally, modied Pd-catalyzed Suzuki cross-coupling reactions
of 1,8-dibromonaphthlene4b with boronate compounds 1 and 2
were used to synthesize BCzPN (total yield: 4.6%) and BPAPN
(total yield: 22%), respectively. The chemical structures of
BCzPN and BPAPN were identied using 1H-NMR spectroscopy,
mass spectrometry (Fig. S3†), and elemental analysis. Four
singlet peaks in the 1H-NMR spectrum of BCzPN at d 6.79, 7.00,
7.14, and 7.33 ppm were broader than the other peaks (Fig. S1†).
These can be ascribed to the four protons on the carbazole rings,
which cannot freely rotate because of steric hindrance. A similar
result was observed in the 1H-NMR spectrum of BPAPN (Fig. S2†).
BCzPN and BPAPN showed high thermal stabilities in ther-
mogravimetric analysis (TGA) measurements, with only 5%
weight loss up to 417 ꢁC and 477 ꢁC (Fig. S4†), respectively. The
melting points (Tm) of BCzPN and BPAPN were 307 ꢁC and 343 ꢁC,
respectively, and no glass-transition temperatures (Tg) were
found from differential scanning calorimetry (DSC) measure-
ments (Fig. S5†). All thermal data are summarized in Table 1.
Fig. 1 Chemical structures of BCzPN and BPAPN, and a schematic diagram of
the p-stacked structure.
external quantum efficiency (EQE) at 100 cd mꢀ2, and 8 cd Aꢀ1
and 5.8% EQE at 1000 cd mꢀ2. A DPAVBi-doped blue device
using BCzPN as a host gave high efficiencies of 15 cd Aꢀ1 and
6.5% EQE at 100 cd mꢀ2, and 12 cd Aꢀ1 and 5.5% EQE at 1000 cd
mꢀ1. A C545T-doped green device using BPAPN as a host gave
high efficiencies of 23 cd Aꢀ1 and 6.7% EQE at 100 cd mꢀ2, and
22 cd Aꢀ1 and 6.7% EQE at 1000 cd mꢀ2
.
Results and discussion
Synthesis
The synthetic routes for BCzPN and BPAPN are described in
Scheme 1. An Ullmann condensation between carbazole and
1,4-dibromobenzene gave 4-bromo-1-carbazolylbenzene 1a in
high yield.10 Compound 1 was synthesized by reacting 1a with
an excess of n-BuLi at ꢀ78 ꢁC, followed by quenching the
lithiated complex with 2-isopropoxy-4,4,5,5-tetra-methyl-1,3,2-
X-ray molecular structure of BCzPN
To conrm the “p-stacked” structure of the molecules, we
attempted to prepare single crystals of each for X-ray measure-
ment. However, we could successfully obtain only a BCzPN
crystal by slowly evaporating a dichloromethane solution at
room temperature.12 Key crystallographic data are summarized
in Table S1,† and the crystal structure is shown in Fig. 2. Fig. 2b
shows that the two carbazole rings, carbazole 1 and carbazole 2,
were stacked face-to-face but slightly out of alignment with each
other to the degree that the dihedral angle of C11–C1–C9–C29
was 18.2ꢁ. The C10–C1–C11 and C10–C9–C29 bond angles were
124.0ꢁ and 124.5ꢁ, respectively, and the two attached groups
were slightly angled to each other because of repulsion. The
Table 1 Physical data of the compounds
BCzPN
BPAPN
a
Tm/Td /ꢁC
307/417
328 (333)
398 (404)
ꢀ5.81 (ꢀ5.42)
ꢀ2.38 (ꢀ1.78)
3.43
343/477
400 (406)
429 (446)
ꢀ5.94 (ꢀ5.33)
ꢀ3.05 (ꢀ2.01)
2.89
labs,maxb/nm
lem,maxb/nm
HOMOc
LUMOc
d
Eg
S1
e
3.18
2.48
2.93
1.71
e
T1
a
b
Melting point and decomposition temperature, 5% weight loss. In
toluene solution at room temperature, the values in parentheses are
c
measured on a thin lm. The values in parentheses are calculated
from DFT calculations. Eg is the band-gap energy estimated from the
intersection of the absorption in the thin lms. Singlet and triplet
d
e
energy levels (S1 and T1) calculated from TD-DFT.
Scheme 1 Synthetic routes for compounds BCzPN and BPAPN.
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Fig. 3 Spatial distributions of the frontier orbitals of BCzPN and BPAPN.
Fig. 2 Molecular structure of BCzPN viewed toward the naphthalene plane (a)
and viewed parallel to the naphthalene plane (b and c).
distances between atoms were (in increasing order): C1–C9,
˚
˚
˚
˚
2.6 A; C11–C29, 3.0 A; C14–C32, 3.9A; N1–N2, 4.1 A. The average
distance between the carbazole 1 and carbazole 2 planes in a
˚
perpendicular direction was 3.47 A (Table S2†), short enough to
create an excimer state.13,14
Theoretical calculations
Density functional theory (DFT) calculations (B3LYP) for BCzPN
and BPAPN were performed with the basis set of 6-311+G(d,p)//
6-31G(d) on the Gaussian 09 program.15 The structure of BCzPN
obtained from the X-ray measurements was used for its energy
Fig. 4 (a) UV/vis and PL spectra of BCzPN in toluene (solid line) and as a film
calculation, and the optimized structure of BPAPN from calcu- (dotted line), compared with the PL spectrum of phenylcarbazole (PCz) in toluene
(solid line labeled PCz). (b) Fluorescence intensity as a function of time for BCzPN
lations was used for its energy calculation. The calculated
density surfaces of the highest occupied molecular orbital
in toluene without O2 (circles) and with O2 (triangles).
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
are shown in Fig. 3. The HOMO of BCzPN is located on the
electron-rich carbazole units and the LUMO is located on the
naphthalene ring. The HOMO and LUMO of BPAPN are both
mainly located on the diphenylanthracene units. The singlet
compared with the PL spectrum of N-phenylcarbazole (PCz) in
toluene. The UV-vis absorption spectrum of BCzPN exhibited
the characteristic vibrational pattern of an isolated PCz at 280–
and triplet energy levels (S1 and T1) of BCzPN and BPAPN were
350 nm (lmax ¼ 328 nm).16 The PL spectra of BCzPN had
calculated using time-dependent DFT (TD-DFT) calculations,
completely lost the PCz monomer emission vibronic structural
and the S1 and T1 energy differences were estimated to be
feature.16 The PL peak maximum of BCzPN in dilute solution
0.70 eV for BCzPN and 1.22 eV for BPAPN (Table 1).
was red-shied from 345 nm (for PCz) to 398 nm. This result
demonstrates that signicant intramolecular excimer emission
occurs between the two closely stacked phenylcarbazole units in
Photophysical properties
The UV-vis absorption and photoluminescence (PL) spectra of BCzPN. The UV-vis absorption and emission spectra of the
BCzPN in toluene and as a lm are shown in Fig. 4a, and are BCzPN lm were almost identical to the solution spectra, and
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exhibited small red shis, of only 5 and 6 nm, to lmaxs of 333
and 404 nm, respectively. The red shi of PL spectra is due to
different dielectric constants of each surrounding medium, and
the PL spectrum in dichloroethane showed further red shi to a
lmax of 417 nm. The UV-vis absorption and photoluminescence
(PL) spectra of BPAPN in toluene and as a lm are shown in
Fig. S6† and are compared with the PL spectrum of 9,10-
diphenylanthracene (DPA) in toluene.17 Intramolecular excimer
emissions were conrmed at 429 and 446 nm for BPAPN in
toluene and as a lm, respectively. Photoexcitation spectra of
BCzPN and BPAPN in lms were almost identical to the UV
spectra respectively (Fig. S7†). The PL quantum efficiencies
(PLQEs) of BCzPN and BPAPN in a neat lm were 44% and 56%,
respectively, measured using an integrating sphere system. The
uorescence lifetimes of BCzPN and BPAPN in toluene were
measured using a streak camera (Fig. 4b and S6c†). The inten-
sity decayed mono-exponentially with lifetimes (se) of 4.5 ns and
7.2 ns for BCzPN and BPAPN, respectively. The excimer emis-
sions were slightly quenched under an oxygen atmosphere (se ¼
2.2 ns for BCzPN and 3.5 ns for BPAPN). The HOMO energy
levels were found to be 5.81 eV and 5.94 eV for BCzPN and
BPAPN, respectively, by photoelectron yield spectroscopy. The
LUMO energy levels were estimated to be 2.38 eV for BCzPN and
3.05 eV for BPAPN from the difference between the HOMO
levels and the optical energy gaps of 3.43 eV for BCzPN and
2.89 eV for BPAPN, which were determined from the UV
absorption edges. The physical properties of BCzPN and BPAPN
are summarized in Table 1.
Fig. 5 (a) Current density–luminance–voltage plots and (b) current efficiency–
power efficiency–current density plots for device 2, with BPAPN as an emitter.
Inset (b): EL spectrum of device 2.
blocks electrons from the EML inefficiently (Fig. S8†). In
contrast, non-doped BPAPN-based pure-blue device 2 was much
more efficient, with high efficiency values of 8 cd Aꢀ1 and 5.8%
EQE at 100 cd mꢀ2. Even at a relatively high current density of
20 mA cmꢀ2, device 2 displayed high efficiency levels of 7 cd Aꢀ1
and 5.6% EQE and a high EL of 1410 cd mꢀ2 at 5.2 V. EL data for
devices 1 and 2 are summarized in Table 2. The high perfor-
mance of device 2 can be attributed to: (i) the HOMO and LUMO
energy levels of BPAPN are matched well with the neighboring
Electroluminescence properties
To investigate the electroluminescence (EL) properties of
BCzPN and BPAPN as emitters, two blue uorescent OLEDs with
typical three-layer structures, [ITO/TAPC (45 nm)/BCzPN
(10 nm)/B3PyPB (45 nm)/LiF (1 nm)/Al] (device 1) and [ITO/
TAPC (40 nm)/BPAPN (20 nm)/B3PyPB (40 nm)/LiF (1 nm)/Al]
(device 2), were fabricated. We used 1,10-bis(di-4-tolyl-amino-
phenyl)-cyclohexane (TAPC) as a hole-transporting layer (HTL)
and 3,5,300,500-tetra-3-pyridyl-1,10;30,100-terphenyl (B3PyPB)18 as
an electron-transporting layer (ETL) (Fig. S8†). The EL spectrum
Table 2 Electroluminescent data of devices 1–4d
of BPAPN-based device 2 was almost identical to the PL spec-
trum of the BPAPN lm (Fig. 5b), and emission (l
was solely from the excimer emission of the BPAPN lm, with
EL
¼ 438 nm)
max
Device 1
Device 2
Device 3
Device 4
PL
max
no emission component from TAPC (l
¼ 372 nm)19 or
lema/nm
403
51
0.21, 0.08
5.2
0.8
1.3
3.5
8.5
0.2
0.5
438
60
0.18, 0.17
3.9
6.4
7.9
5.8
4.9
4.8
7.5
478
68
0.19, 0.38
4.9
9.2
14.5
6.5
5.9
6.6
12.3
5.5
505 (534)
72
0.28, 0.62
3.5
20.5
22.5
6.7
PL
max
B3PyPB (l
¼ 365 nm).18 However, although the EL spectrum FWHM/nm
EL
CIE (x, y)
of BCzPN-based device 1 (l
¼ 403 nm) was identical to the PL
max
Vb/V
spectrum of the BCzPN lm, an electromer emission peak at
592 nm (ref. 19) was observed, indicating the poor injection of
holes and electrons into the emitting layer (EML) (Fig. S9†).
PEb/lm Wꢀ1
CEb/cd Aꢀ1
EQEb/%
Non-doped BCzPN-based device 1 showed deep blue emission Vc/V
4.0
PEc/lm Wꢀ1
17.4
22.4
6.7
with a current efficiency of 1.3 cd Aꢀ1 and 3.5% EQE at 100 cd
mꢀ2. These low efficiencies are probably caused by: (i) the low
PLQE of the BCzPN lm; (ii) the LUMO energy level of BCzPN is
not effectively matched with the neighboring B3PyPB electron-
transporting layer (ETL), resulting in an insufficient electron
injection into the EML; and (iii) the difference between the
LUMO energy levels of BCzPN and TAPC is small (0.36 eV) and
CEc/cd Aꢀ1
EQEc/%
1.3
5.8
a
The value in parentheses is the shoulder. b The values are measured at
100 cd mꢀ2
. . Abbreviations:
The values are measured at 1000 cd mꢀ2
c
d
FWHM: full width at half maximum; CIE [x, y]: Commission
International de I'Eclairage coordinates; V: voltage; PE: power
efficiency; CE: current efficiency; EQE: external quantum efficiency.
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TAPC HTL and B3PyPB ETL (Fig. S6†); (ii) the high hole mobility congurations of the devices were: [ITO/TAPC (40 nm)/BCzPN:
of TAPC;20 (iii) the low TAPC HOMO level, which results in a 6 wt% DPAVBi (20 nm)/B3PyPB (40 nm)/LiF (1 nm)/Al] (device 3)
reduced barrier to hole injection to the EML; and (iv) TAPC also and [ITO/TAPC (40 nm)/BPAPN: 3 wt% C545T (20 nm)/B3PyPB
has a high LUMO level, which blocks electrons from the EML (40 nm)/LiF (1 nm)/Al] (device 4). In these devices, TAPC and
efficiently, and it has high energy levels in their S1 and T1 states, B3PyPB were still used as the hole-transporting layer (HTL) and
which can efficiently conne the singlet and triplet excitons in the electron-transporting material (ETL).
the EML. The EQE for device 2 was 5.8% at both 100 cd mꢀ2 and
DPAVBi-doped BCzPN-based device 3 showed bright blue
1000 cd mꢀ2, which is much higher than the theoretical upper emission only from DPAVBi with CIE coordinates of (0.19, 0.38),
limit of 2.8% EQE when the outcoupling efficiency is assumed implying that efficient energy transfer from the BCzPN host to
to be 20% and 4.2% EQE with 30% outcoupling efficiency. The DPAVBi can occur. This was conrmed by the large spectral
much higher EQEs than the theoretical values could be caused overlap between DPAVBi absorption and the BCzPN PL spec-
by the increased singlet excitons from triplet–triplet annihila- trum, as shown in Fig. 6a. C545T-doped BPAPN-based device 4
tion or in the BPAPN molecules. The reverse intersystem showed green emission from the dopant C545T, as shown in
crossing from triplet to singlet could also be one of the possible Fig. 6b. This device exhibited a bright green light with CIE
mechanisms, however the S1–T1 energy gap estimated from the coordinates of (0.28, 0.62) at 5.0 V. The voltage–current density–
calculation should be sufficiently large to exclude such possi- luminance characteristics and the efficiency curves of blue
bilities (Table 1), and the single exponential decay of the PL also device 3 and green device 4 are illustrated in Fig. 7 and the data
supported the absence of the delayed uorescence from triplet. are listed in Table 2. Blue device 3 exhibited maximum lumi-
It should be noted that the performance of the BPAPN-based nance efficiency up to 15 cd Aꢀ1, while green device 4 exhibited
device was better than the high efficiencies of non-doped pure- maximum luminance efficiency up to 23 cd Aꢀ1. These effi-
blue OLEDs that have recently been described in the literature,21 ciencies are comparable to the previously reported DPAVBi-
demonstrating the superiority of the single-molecule excimer- based blue OLEDs22 and C545T based-green OLEDs,23 respec-
emitting compound BPAPN for OLEDs.
tively. There were some other aspects worth noting. First, in
To investigate the function of BCzPN and BPAPN as host devices 3 and 4, high efficiencies were achieved at practical
materials, a blue device with DPAVBi as an emitter and a green luminance between 100 and 1000 cd mꢀ2, which indicates
device with C545T as an emitter were fabricated (Fig. S8†). The that these devices have great potential for practical
applications.24 For example, BPAPN-based green device 4 has a
luminance efficiency of 23 cd Aꢀ1 at 100 cd mꢀ2, and 22 cd Aꢀ1
Fig. 6 (a) Comparison of the EL spectrum of DPAVBi-doped BCzPN-based device
3 (solid line and triangles) with the optical absorption spectrum of DPAVBi (dotted
line) and the PL spectrum of BCzPN (solid line). (b) Comparison of the EL spectrum
of C545T-doped BPAPN-based device 4 (solid line and triangles) with the PL
spectrum of BPAPN (dotted line) and the EL spectrum of non-doped device
2 (solid line).
Fig. 7 (a) Current density–luminance–voltage plots and (b) current efficiency–
power efficiency–current density plots for device 3 (circles) and 4 (triangles) using
BCzPN or BPAPN as the host, respectively.
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ꢀ1
ꢁ
under a nitrogen atmosphere at a heating rate of 10 C min
.
Absorption and uorescence spectra were recorded respectively
with a Shimadzu UV-3150 UV-vis-NIR spectrophotometer and a
FluroMax-4 (Jobin-Yvon-Spex) luminescence spectrometer. The
highest occupied molecular orbital (HOMO) value was
measured directly by atmospheric ultraviolet photoelectron
spectroscopy (Rikken Keiki AC-3), whereas the lowest unoccu-
pied molecular orbital (LUMO) value was determined from the
HOMO and position of the lowest energy absorption edge of the
UV absorption spectra.
Synthesis of 1-bromo-4-(carbazole-9-yl)benzene (1a). 1,3-
Dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone
(DMPU)
(0.5 mL) was added to a mixture of carbazole (1) (2.57 g,
15.00 mmol), 1,4-dibromobenzene (3.55 g, 15.00 mmol), CuI
(0.295 g, 1.50 mmol), 18-crown-16-ether (0.133 g, 0.50 mmol),
and K2CO3 (4.15 g, 30.00 mmol). The reaction mixture was
heated to 170 ꢁC for 24 h under nitrogen. Aer it was cooled to
room temperature, the reaction mixture was quenched with HCl
(120 mL, 1.0 N). The combined organic extracts were washed
with NH3 aq. and water, dried with anhydrous Na2CO3 and
evaporated. The crude product was puried by column chro-
matography using hexane as an eluent to yield the target
compound 1a (1.92 g) as a white solid. Yield: 40%, 1H-NMR
(400 MHz, CDCl3): d (ppm) 7.29–7.47 (m, 8H), 7.73 (d, J ¼
8.72 Hz, 2H), 8.15 (d, J ¼ 7.76 Hz, 2H); MS (EI, m/z): 323 [M+].
Synthesis of 4,4,5,5-tetramethyl-2-(4-(carbazole-9-yl))-phenyl-
1,3,2-dioxaborolane (1). n-BuLi (2.04 mL) was added to a solu-
tion of 9-(4-bromophenyl)carbazole (2c) (1.0 g, 3.10 mmol) in
dehydrated THF (30.00 mL) at ꢀ78 ꢁC under nitrogen. The
reaction mixture was stirred for 15 min and then a solution of
2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxa-borolane (0.63 g,
3.41 mmol) in THF (5 mL) was slowly added into this mixture.
The reaction mixture was stirred for 2 h at ꢀ78 ꢁC, then warmed
to room temperature and stirred overnight. The reaction
mixture was quenched with dichloromethane (200 mL), washed
with brine and extracted with dichloromethane (100 mL). The
combined organic extracts were dried with anhydrous MgSO4
and evaporated. The crude products were successfully puried
by column chromatography using hexane : dichloromethane ¼
10 : 1 as eluents, and recrystallized from hexane to yield 0.21 g
Fig. 8 External quantum efficiency–current density plots for devices 1–4.
at 1000 cd mꢀ2. Second, the efficiency roll-off at high current
density in devices 3 and 4 was low, for instance, the best device
performance came from the BPAPN-based device 4, which
showed the highest maximum EQE, 6.8%, at 0.84 mA cmꢀ1
(192 cd mꢀ2) and still maintained 6.1% EQE at 41 mA cmꢀ2
(8400 cdmꢀ2)(Fig. 8). These results can be attributedtotwo facts:
(i) energy transfer from the hosts (BCzPN and BPAPN) to the
dopants (DPAVBi and C545T) was highly efficient because of the
good energy level match; and (ii) the HOMO and LUMO energy
levels of the hosts (BCzPN and BPAPN) are well matched with the
neighboring TAPC hole-transporting layer and the B3PyPB elec-
tron-transporting layer (Fig. S8†). The charge trapping property
of the emitting dopants should also be an advantage to control
the charge balance and suppress the efficiency roll-off.
Conclusions
In summary, we have designed and synthesized two efficient
blue-light excimer-emitting compounds, BCzPN and BPAPN, in
which signicant single-molecule excimer emissions occur,
both in solution and as a lm. With BPAPN as an emitter, the
non-doped device produced bright pure-blue emission around
438 nm with a high efficiency of 6 lm Wꢀ1 (8 cd Aꢀ1) and 5.8%
EQE at 100 cd mꢀ2, corresponding to the excimer emission of
the BPAPN lm. With a 6 wt% DPAVBi-doped BCzPN emissive
layer or a 3 wt% C545T-doped BPAPN emissive layer, the devices
exhibited high efficiencies of 9 lm Wꢀ1 (15 cd Aꢀ1) with 6.5%
EQE and 21 lm Wꢀ1 (23 cd Aꢀ1) with 6.7% EQE, respectively, at
100 cd mꢀ2. These results strongly demonstrate that these
single-molecule excimer-emitting compounds are very prom-
ising candidates as emitters and host materials for highly effi-
cient OLEDs.
1
of 1 as a white solid. Yield: 18%, H-NMR (400 MHz, CDCl3): d
(ppm) 1.40 (s, 12H), 7.29 (t, J ¼ 7.34 Hz, 2H), 7.38–7.46 (m, 4H),
7.59 (d, J ¼ 8.24 Hz, 2H), 8.05 (d, J ¼ 8.72 Hz, 2H), 8.14 (d, J ¼
7.76 Hz, 2H); MS (EI, m/z): 369 [M+].
Synthesis of 9-phenylanthracene (2a). 9-Bromoanthracene
(5.00 g, 19.45 mmol), phenyl-boronic acid (3.56 g, 29.18 mmol),
Pd(PPh3)4 (1.12 g, 0.97 mmol), K2CO3 (40 mL, 26.00 g,
194.50 mmol), and ethanol (40.00 mL) were mixed in a ask
containing nitrogen saturated toluene (180.00 mL). The reac-
tion mixture was reuxed for 24 h under nitrogen. Aer it was
Experimental
General information
1H NMR spectra were recorded on JEOL 400 (400 MHz) spec- cooled to room temperature, the reaction mixture was
trometers. Mass spectra were obtained using a JEOL JMS-K9 or quenched with brine and extracted with toluene (500.00 mL).
JMS-T100GCV mass spectrometer. Differential scanning calo- The combined organic extracts were dried with anhydrous
rimetry (DSC) was performed using a Perkin-Elmer Diamond MgSO4 and evaporated. The crude products were successfully
DSC Pyris instrument under a nitrogen atmosphere at a heating puried by column chromatography using hexane as an eluent,
rate of 10 ꢁC minꢀ1. Thermogravimetric analysis (TGA) was and recrystallized from the hexane to yield the desired
carried out using a SEIKO EXSTAR 6000 TG/DTA 6200 unit compound 2a (4.74 g) as a light-yellow powder. Yield: 96%,
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1H-NMR (400 MHz, CDCl3): d (ppm) 7.34 (t, J ¼ 7.62 Hz, 1H), mixture was reuxed for 24 h under nitrogen. Aer it was cooled
7.42–7.47 (m, 4H), 7.53–7.61 (m, 4H), 7.66 (d, J ¼ 9.16 Hz, 2H), to room temperature, the reaction mixture was quenched with
8.04 (d, J ¼ 8.68 Hz, 2H), 8.50 (s, 1H); MS (EI, m/z): 254 [M+].
water and extracted with toluene (300 mL). The combined
Synthesis of 9-bromo-10-phenylanthracene (2b). 9-Phenyl- organic extracts were washed with brine and water, dried with
anthracene (2a) (2.5 g, 10.00 mmol) and N-bromosuccinimide anhydrous MgSO4 and evaporated. The crude product was
(NBS) (2.14 g, 12.00 mmol) were added to 200.00 mL of CHCl3 puried by column chromatography using toluene as an eluent
solution. The mixture was heated to 60 ꢁC for 1 h under and recrystallized from toluene to yield the desired compound
nitrogen. Aer it was cooled to room temperature, the reaction BCzPN (0.55 g) as a white crystal. Yield: 64%, 1H-NMR
mixture was evaporated. The residue was re-dissolved in (400 MHz, CDCl3): d (ppm) 6.79 (s, 4H), 7.00 (s, 4H), 7.14 (s, 4H),
acetone and added to methanol. The precipitate was ltered off 7.33 (s, 8H), 7.59 (d, J ¼ 5.92 Hz, 2H), 7.66 (dd, J ¼ 7.32, 7.80 Hz,
and was successfully washed with methanol to yield 2b (3.26 g) 2H), 7.90 (d, J ¼ 7.76 Hz, 4H), 8.05 (d, J ¼ 8.24 Hz, 2H); MS (EI,
as yellow-colored solid. Yield: 98%, 1H-NMR (400 MHz, CDCl3): m/z): 611.00 [M+]; Anal. calcd: C, 90.46; H, 4.95; N, 4.59. Found:
d (ppm) 7.35–7.41 (m, 5H), 7.54–7.60 (m, 4H), 7.64 (d, J ¼ C, 90.44; H, 4.96; N, 4.60%.
9.16 Hz, 2H), 8.60 (d, J ¼ 9.12 Hz, 2H); MS (EI, m/z): 333 [M+].
Synthesis of 1,8-bis(4-(10-phenylanthracen-9-yl)phenyl)-
Synthesis of 10-(4-chlorophenyl)-9-phenylanthracene (2c). naphthalene (BPAPN). 1,8-Dibromonaphthalene (0.50 g,
9-Bromo-10-phenylbenzene (2b) (3.33 g, 10.00 mmol), p-chlor- 1.75 mmol), 4,4,5,5-tetramethyl-2-(4-(10-phenyl-anthracen-9-
ophenylboronic acid (2.35 g, 15.00 mmol), Pd(PPh3)4 (0.580 g, yl))-phenyl-1,3,2-dioxaborolane (2)(2.36 g, 5.25mmol), Pd2(dba)3
0.50 mmol), K2CO3 (27.60 g, 200.00 mmol, 2 M, 80.00 mL), and (0.162 g, 0.175 mmol), PCy3 (0.049 g, 0.175 mmol) and aqueous
ethanol (80.00 mL) were mixed in a ask containing nitrogen 1.35 M K3PO4 (20.0 mL) were mixed ina ask containing nitrogen
saturated toluene (160.00 mL). The reaction mixture was saturated dioxane (50.0 mL). The reaction mixture was reuxed
reuxed overnight under nitrogen. Aer it was cooled to room for 48 h under nitrogen. Aer it was cooled to room temperature,
temperature, the reaction mixture was quenched with brine and the reaction mixture was quenched with water and extracted with
extracted with dichloromethane (500.00 mL). The combined CH2Cl2 (500 mL). The combined organic extracts were washed
organic extracts were dried with anhydrous MgSO4 and evapo- with brine and water, dried with anhydrous MgSO4 and evapo-
rated. The crude products were successfully puried by column rated. The crude product was puried by column chromatog-
chromatography using hexane as an eluent, and recrystallized raphy using a mixture of hexane : CH2Cl2 ¼ 1 : 1 as eluents and
from the hexane to yield the desired compound 2c (3.46 g) as a recrystallized from hexane to yield the desired compound BPAPN
1
1
yellow-white crystal. Yield: 95%, H-NMR (400 MHz, CDCl3): d (0.62 g) as a light-yellow solid. Yield: 45%, H-NMR (400 MHz,
(ppm) 7.33–7.35 (m, 5H), 7.42 (d, J ¼ 8.24 Hz, 2H), 7.48 (d, J ¼ CDCl3): d (ppm) 5.86 (s, 2H), 6.60 (s, 2H), 7.21 (s, 2H), 7.30 (s, 2H),
8.24 Hz, 2H), 7.55–7.63 (m, 4H), 7.64–7.71 (m, 4H); MS (EI, m/z): 7.35–7.39 (m, 10H), 7.46 (d, J ¼ 7.8 Hz, 4H), 7.52 (d, J ¼ 6.88 Hz,
364 [M+].
2H), 7.56–7.59 (m, 4H), 7.66–7.72 (m, 8H), 7.78 (s, 2H), 8.07 (dd,
Synthesis of 4,4,5,5-tetramethyl-2-(4-(10-phenyl-anthracen-9- J ¼ 1.84, 1.84 Hz, 2H); MS (EI, m/z): 785 [M+]; Anal. calcd: C, 94.86;
yl))-phenyl-1,3,2-dioxaborolane
(2).
9-Phenyl-10-(4-chloro- H, 5.14. Found: C, 94.84; H, 5.15%.
phenyl)anthracene (2c) (3.64 g, 10.00 mmol), bis(pinacolato)
diboron (pin2B2) (3.81 g, 15.00 mmol), Pd(OAc)2 (112.26 mg,
0.50 mmol), S-Phos (410.53 mg, 1.00 mmol), and KOAc (2.96 g,
30.00 mmol) were mixed in a ask containing nitrogen dehy-
drated 1,4-dioxane (200.00 mL). The reaction mixture was
reuxed overnight under nitrogen. Aer it was cooled to room
temperature, the reaction mixture was quenched with brine and
extracted with dichloromethane (600 mL). The combined
organic extracts were dried with anhydrous MgSO4 and evapo-
rated. The crude products were successfully puried by column
chromatography using hexane : dichloromethane ¼ 1 : 1 as
eluents, and recrystallized from hexane solution to yield the
Device fabrication and measurement
All of the devices were grown on glass substrates pre-coated with
a 130 nm thick layer of indium tin oxide (ITO) with a sheet
resistance of 15 U squareꢀ1. The substrates were cleaned with
ultra-puried water and organic solvents and then dry-cleaned
for 30 min by exposure to a UV-ozone ambient. The organic
compounds were successively deposited onto the ITO substrate
under vacuum (ꢂ10ꢀ5 Pa). LiF and Al were patterned using a
shadow mask with an array of openings (2 mm ꢃ 2 mm)
without breaking the vacuum (ꢂ10ꢀ5 Pa). All of the devices were
encapsulated under a nitrogen atmosphere immediately aer
preparation using epoxy glue and glass lids. Electrolumines-
cence (EL) spectra and CIE color coordinates were taken by an
optical multichannel analyzer, Hamamatsu PMA 11, and the
current density–luminance–voltage characteristics of the
OLEDs were measured with a Keithley source meter 2400 and
Konica Minolta CS-200, respectively.
1
pure sample 2 (2.55 g) as a white crystal. Yield: 56%, H-NMR
(400 MHz, CDCl3): d (ppm) 1.42 (s, 12H), 7.30 (d, J ¼ 4.56 Hz,
2H), 7.32 (d, J ¼ 4.12 Hz, 2H), 7.62 (d, J ¼ 6.84 Hz, 2H), 7.51 (d,
J ¼ 7.76 Hz, 2H), 7.56 (t, J ¼ 8.12 Hz, 1H), 7.59 (d, J ¼ 7.32 Hz,
2H), 7.66 (d, J ¼ 4.12 Hz, 2H), 7.69 (d, J ¼ 4.12 Hz, 2H), 8.05 (d,
J ¼ 8.24 Hz, 2H); MS (EI, m/z): 456 [M+].
Synthesis of 1,8-bis(4-(carbazole-9-yl)-phenyl)naphthalene
(BCzPN). 1,8-Dibromonaphthalene (0.40 g, 1.40 mmol), 9-[4-
(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-phenyl]-9H carba-
zole (1) (1.55 g, 4.20 mmol), Pd(PPh3)4 (0.324 g, 0.28 mmol),
Acknowledgements
K2CO3 (2 M, 20.0 mL), and ethanol (20 mL) were mixed in a ask This study was supported by an Industrial Technology Research
containing nitrogen saturated toluene (40.0 mL). The reaction Grant for Young Researchers from the New Energy and
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Paper
Industrial Technology Development Organization (NEDO). We 10 (a) H. Zhang, Q. Cai and D. Ma, J. Org. Chem., 2005, 70, 5164;
are grateful for partial support by Japan Regional Innovation (b) F. Ullmann and P. Sponagel, Chem. Ber., 1905, 38, 2211.
Strategy Program by the Excellence (J-RISE) (Creating interna- 11 K. L. Billingsley, T. E. Barder and S. L. Buchwald, Angew.
tional research hub for advanced organic electronics) of Japan
Science and Technology Agency (JST).
Chem., Int. Ed., 2007, 46, 5359.
12 CCDC-909549 (BCzPN) contains the supplementary
crystallographic data for this paper.†
13 K. Tani, Y. Tohda, H. Takemura, H. Ohkita, S. Ito and
M. Yamamoto, Chem. Commun., 2001, 1914.
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