Phenyl-substituted fluorene-dimer cored anthracene derivatives: Highly fluorescent and stable materials for high performance organic blue- and white-light-emitting diodes

Article abstract of DOI:10.1039/b925418h

A new series of highly fluorescent blue-emitting materials based on fluorene and anthracene hybrids are designed and synthesized for organic light-emitting diodes (OLEDs). These materials feature a phenyl-substituted fluorene dimer as a bulky and rigid core and anthracene as a functional active group. The novel use of a phenyl-substituted fluorene dimer as building skeleton to design functional molecules is reported for the first time. The thermal, photophysical, electrochemical, and electroluminescent (EL) properties are presented, as well as combined density functional study of their geometry and electronic structure. These compounds show excellent thermal resistance with high glass transition temperature (T_g) in the range 159-257°C, thermal decomposition temperature (T_d) 441-495°C, and high fluorescent quantum yield (Φ_F = 0.61-0.96, relative to 9,10-diphenylanthracene) as well as good film-forming and morphological stability. Remarkably, high-performance blue OLEDs are also fabricated in a simple three-layer device architecture using these compounds as emissive layer with luminance efficiency of 2.2-5.1 cd A^-1 as a non-doped blue emitter and even higher efficiency of up to 13.6 cd A^-1 and maximum external quantum efficiency 4.8% is obtained when doped a blue fluorescent dye, 4,4′-(1E,1′E)-2,2′(biphenyl-4,4′diyl)bis(ethane-2, 1-diyl)bis(N,N-dip-tolyaniline) (DPAVBi). Furthermore, we fabricate highly efficient fluorescent white OLEDs employing an interesting emission in the longer wavelength of one of our compound combined with DPAVBi emission to achieve stable white light emission in a binary blend single emissive layer with high efficiency of 14.8 cd A^-1 (5.3 lm W^-1) and maximum brightness of 50248 cd m^-2.

Full text of DOI:10.1039/b925418h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Phenyl-substituted fluorene-dimer cored anthracene derivatives: highly
fluorescent and stable materials for high performance organic
blue- and white-light-emitting diodes†
a
a
Shanghui Ye,ab Jianming Chen,ab Chong-an Di, Yunqi Liu, Kun Lu,ab Weiping Wu,ab Chunyan Du,ab
*
Ying Liu,ab Zhigang Shuaic and Gui Yua
Received 3rd December 2009, Accepted 29th January 2010
First published as an Advance Article on the web 1st March 2010
DOI: 10.1039/b925418h
A new series of highly fluorescent blue-emitting materials based on fluorene and anthracene hybrids are
designed and synthesized for organic light-emitting diodes (OLEDs). These materials feature a phenyl-
substituted fluorene dimer as a bulky and rigid core and anthracene as a functional active group. The
novel use of a phenyl-substituted fluorene dimer as building skeleton to design functional molecules is
reported for the first time. The thermal, photophysical, electrochemical, and electroluminescent (EL)
properties are presented, as well as combined density functional study of their geometry and electronic
structure. These compounds show excellent thermal resistance with high glass transition temperature
ꢀ
ꢀ
(Tg) in the range 159–257 C, thermal decomposition temperature (Td) 441–495 C, and high
fluorescent quantum yield (FF ¼ 0.61–0.96, relative to 9,10-diphenylanthracene) as well as good film-
forming and morphological stability. Remarkably, high-performance blue OLEDs are also fabricated
in a simple three-layer device architecture using these compounds as emissive layer with luminance
efficiency of 2.2–5.1 cd Aꢁ1 as a non-doped blue emitter and even higher efficiency of up to 13.6 cd Aꢁ1
and maximum external quantum efficiency 4.8% is obtained when doped a blue fluorescent dye, 4,40-
(1E,10E)-2,20(biphenyl-4,40diyl)bis(ethane-2,1-diyl)bis(N,N-dip-tolyaniline) (DPAVBi). Furthermore,
we fabricate highly efficient fluorescent white OLEDs employing an interesting emission in the longer
wavelength of one of our compound combined with DPAVBi emission to achieve stable white light
emission in a binary blend single emissive layer with high efficiency of 14.8 cd Aꢁ1 (5.3 lm Wꢁ1) and
maximum brightness of 50248 cd mꢁ2
.
as distyrylarylene derivatives (DSA),7–9 hydroxyphenyl-pyridine
beryllium complex,10 bistriphenylenyl,11 fluorenes–especially
spirofluorenes-cored conjugated compounds,12–16 anthracene
derivatives,17–23 and fluorene/anthracene hybrids.24–27 Very
recently, Lyu et al.28 developed a series of new silicon-cored
anthracene derivatives by non-conjugated connecting two
anthracene units through a silicon-core. These compounds
exhibit high glass transition temperatures (Tg) range from 102 to
177 ꢀC and bright blue emission with high color purity and a high
efficiency up to 7.5 cd Aꢁ1 with an emission peak at 460 nm in
a doped device configuration. Although many blue-light emitters
have been reported, highly efficient ones exhibiting high thermal
stability and robust emission are still rare.
1. Introduction
Since the first report by Tang et al., organic light-emitting diodes
(OLEDs), whether based on small molecules or polymers, have
been made dramatic progress because of their potential
applications in full-color flat-panel display and solid-state illu-
mination sources.1,2 Among the three primary colors, although
green- and red-emitting diodes have been approached the prac-
tical requirement,3–6 blue-emitting diodes remain relatively poor
due to their intrinsic characteristics such as wide band-gap, low
lying the highest occupied molecular orbit (HOMO) energy
levels, and thus high carrier injection barrier into the emitter. It
remains a great challenge to develop highly efficient blue-emit-
ting diodes with stable and good color purity. Many materials
have been synthesized in the development of blue OLEDs, such
Anthracene derivatives have been extensively explored in
fabrication of highly efficient and stable OLEDs, mainly due to
their high photoluminescence (PL) quantum yield, electrolumi-
nescence (EL) properties, and good electrochemical properties.29
Among these anthracene derivatives, 9,10-di(naphthalene-
2-yl)anthracene (ADN) is one of the most representative blue
fluorescent materials for its fluorescence properties and thermal
stability.30 However, they fail to provide dense films or the vapor-
deposited thin films tend to crystallize, resulting in rough surface,
grain boundaries or pin holes that lead to current leakage or
catastrophic device failure.24,31 In general, amorphous thin films
having high Tg are less vulnerable to heat and thence their devices
aKey Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, P. R. China. E-mail: liuyq@iccas.
ac.cn
bGraduate School of Chinese Academy of Sciences, Beijing 100084, P. R.
China
cDepartment of Chemistry, Tsinghua University, Beijing 100084, P. R.
China
† Electronic supplementary information (ESI) available: UV–vis
absorption spectra of DAPFB, DPAPFB, and DNAPFB thin films on
quartz slice. Cyclic voltammogram of DAPFB, DPAPFB, and
DNAPFB. See DOI: 10.1039/b925418h
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benzoylation of isophthaloyl dichloride and bromobenzene gives
the 1,3-bis(4-bromobenzoyl)benzene, which was treated with
biphenyl-2-yllithium followed by cyclization to produce
1,3-bis(9-(4-bromophenyl)-9H-fluoren-9-yl)benzene (DBPFB).
Suzuki coupling reactions of DBPFB with the corresponding
anthryl boronic ester in the presence of a catalytic amount of
Pd(PPh3)4 and 2 M K2CO3 yields the target products. We chose
Friedel–Crafts benzoylation and cyclization routes to produce
defect-free 9,9-diarylfluorene, inspired by the recent studies
demonstrated by Holmes et al. that the structural perfection of
fluorene-based light-emitting materials depends on the synthetic
route of the monomers.37 We use isophthaloyl dichloride, bro-
mobenzene, and biphenyl-2-yllithium commercially available
materials as starting chemicals to prepare fluorene instead of
fluorenone as the starting materials. This synthetic route should,
in principle, yield structurally perfect fluorene without fluo-
renone, so the problematic fluorenone emission at green region
accompanied fluorene emission was well circumvented. Two
substituent fluorene rings were connected through a phenyl ring
followed by grafting the active anthracene group, thus improved
molecular weight and thermal stability as well as elimination of
intermolecular interactions to ensure high photoluminescent
quantum efficiency of the anthracene moieties due to the huge
hindrance of the tetrahedron fluorene dimer core. Moreover, this
route avoids high-temperature and the reaction proceeds
smoothly at room temperature with high yield (>95%) and purity.
The detailed synthesis and characterization were described in the
experimental section. The molecular structures of the target
Scheme 1 Synthetic route of the three target products and the corre-
sponding intermediates.
perform more stably.32 Consequently, light-emitting materials
possessing high values of Tg are desired to retain the film
morphology during the operation of the device.
In the previously study, we and others have demonstrated that
incorporating a rigid and bulky tetrahedral molecular skeleton,
such as fluorene and carbazole, into a molecule can improve its
film-forming and morphologically stability through the non-
conjugated linking method to pursue high-triplet-energy blue
phosphorescent host materials.33–36 Although fluorene is a well-
known building block, the novel use of phenyl-substituted fluo-
rene dimer as a building skeleton to design functional molecules
is never reported. In this contribution, we firstly report the use
molecules were confirmed by H and 13C nuclear magnetic reso-
1
of
a phenyl-substituted fluorene dimer as a bulky and
nance (NMR), mass spectrometry, and elemental analysis.
rigid core to design functional materials. A new series of blue
light-emitting materials, 1,3-bis(9-(4-(anthracen-9-yl)phenyl)-
9H-fluoren-9-yl)benzene (DAPFB, Scheme 1), 1,3-bis(9-(4-(10-
phenylanthracen-9-yl)phenyl)-9H-fluoren-9-yl)benzene (DPAPFB),
and 1,3-bis(9-(4-(10-(naphthalen-1-yl)anthracen-9-yl)phenyl)-
9H-fluoren-9-yl)benzene (DNAPFB), which features anthracene
as the functional active group and the phenyl-substituted fluo-
rene dimer as the bulky and rigid core. These compounds con-
nected through the sp3-hybridized carbon atom at C–9 position
of the fluorene, which serves as a spacer to effectively block
conjugation between them, thus individual unit’s photophysical
and electrochemical properties are remained unperturbed.
Moreover, the resulting molecules have a sterically bulky 3D
cardo structure, which not only hinders their close packing and
crystallization but also increases their molecular rigidity, leading
to amorphous materials having pronounced morphological
stability. In a simple three-layer device architecture these
compounds show respectable high luminance efficiency of about
2.2–5.1 cd Aꢁ1 as a blue emitter and an even higher efficiency of
up to 13.6 cd Aꢁ1 of DPAPFB based blue OLEDs and 14.8 cd
Aꢁ1 of DNAPFB based white OLEDs when doped with a blue
fluorescent dopant, 4,40-(1E,10E)-2,20(biphenyl-4,40diyl)bis(ethane-
2,1-diyl)bis(N,N-dip-tolyaniline) (DPAVBi).
2.2. Density functional theory calculation
Density functional theory (DFT) calculations (B3LYP; 6-31G*)
were carried out to obtain information about the three-dimen-
sional geometries and the HOMO and the lowest unoccupied
molecular orbital (LUMO) energy levels distributions of the
compounds. The anthracene units are significantly twisted against
each other because of the tetrahedral environment of the central
phenyl-substituted fluorene dimer skeleton, resulting in a non-
coplanar structure in each molecule, as shown in Fig. S1.† More-
over, the bigger the substituent on the anthracene, the more
pronounced twisted angles. These geometrical characteristics can
effectively prevent intermolecular interactions between the
p-systems and thus suppress molecular crystallize and improve
morphological stability of thin film. The electron densities of the
HOMOs andLUMOsare mainlylocalizedontheanthraceneunits
with a little on the aromatic rings but not on the fluorenyl skeleton,
indicating no effect on the electronic structures of the anthracene
units imposed by the fluorenyl skeleton through non-conjugated
combination. As a result, the molecular orbital analysis clearly
indicates that the substituent in 9- and/or 10-positions of anthra-
cenedoesnot significantlyinfluence the blue emission nature of the
anthracene unit, which proved our molecular design rational.
2. Result and discussion
2.1. Material synthesis
2.3. Thermochemical properties
The synthetic routes and chemical structures of DAPFB,
DPAPFB, and DNAPFB are shown in Scheme 1. Friedel–Crafts
The thermochemical properties of the three materials were
characterized by differential scanning calorimetry (DSC) and
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thermal gravimetric analysis (TGA), and all of them exhibit
intrinsically amorphous characteristic. In the first heating scan,
a distinct glass transition at temperature of 159 ꢀC for DAPFB,
240.7 ꢀC for DPAPFB, and 256.7 ꢀC for DNAPFB, respectively,
was observed, and this endothermic phase-transition still
remained in the subsequently cooling and heating cycles, as
shown in Fig. S2.† The bigger the substituent on the anthracene,
the higher the Tg value. It not only hinders close packing and
intermolecular interactions in the solid state, but also increases
the molecular rigidity. Consequently, the introduction of 9-aryl
fluorenyl into anthracene leads to reduction of the crystallization
tendency and an increase in Tg. Such high Tgs make them among
the highest values ever reported in the literature.15,38 The thermal
decomposition temperatures (Td) corresponding to 5%-weight-
their surface morphologies employing atomic force microscopy
(AFM) before and after annealing. For comparison, we also
fabricated ADN thin film using the same method. The topo-
graphical images revealed that our anthracene derivatives
provided smooth and uniform surfaces with room-mean-square
(RMS) roughness of 2.0, 1.7, and 2.3 nm for DAPFB, DPAPFB,
and DNAPFB, respectively. In contrast, ADN displayed a very
rougher surface with RMS of 22.9 nm, as shown in Fig. 1.
Moreover, our compounds undꢀerwent negligible morphological
changes after annealing at 110 C under nitrogen for 12 h with
RMS of 1.9, 2.2, and 2.2 nm for DAPFB, DPAPFB, and
DNAPFB, respectively, while ADN film suffered crystallizing
under thermal treating and its surface was broken. This suggested
that our compounds possessing high thermal and morphological
stabilities as well as good miscibility to the dopant, which can be
attributed to the introduction of huge phenyl-substituted fluorene
dimer as a bulky and rigid core as well as high molecular weight.
ꢀ
loss reached as high as 441.4, 488.5, and 495.9 C for DAPFB,
DPAPFB, and DNAPFB, respectively. Such high Tg and Td
indicate high thermal stability of this material, which is very
desirable for high performance OLEDs application. The key
data of the three compounds were listed in Table 1.
2.4. Photophysical characterization
To further investigate thermal and morphological stabilities of
these anthracene derivatives as well as their miscibility to the
dopant as possible host materials, we prepared thin films using
vacuum vapor deposition method deposited on indium tin oxide
(ITO) coated glass substrate of these materials and then explored
The UV–vis absorbance and PL spectra of the three anthracene
derivatives, as well as 9,10-diphenylanthracene (DPA) for
comparison were measured in dilute dichloromethane solution.
All of them exhibited characteristic vibronic patterns of the
Table 1 Physical properties of the anthracene derivatives
LUMO/HOMO d/eV
a
b
lmax (em)/nm
Compound
Tg/Tm/Td /ꢀC
lmaxb (abs)/nm
Eg (opt.)/eV
FFc (%)
DPA
ꢁ/248/259
356, 375, 395
349, 367, 387
358, 377, 397
358, 377, 397
411, 430
402, 423
416, 431
414, 433
3.03
3.03
3.02
3.02
0.90
0.61
0.67
0.96
2.51/5.54
2.59/5.59
2.54/5.55
2.55/5.55
DAPFB
DPAPFB
DNAPFB
159/417/441
241/438/488
257/475/495
a
Obtained from DSC and TGA measurement. b Measured in dilute dichloromethane solution. c
FF values measured following a handbook using 9,10-
diphenylanthracene standard (FF ¼ 0.90 in cyclohexane). d Obtained according to the cyclic voltammetry measurement.
Fig. 1 Tapping mode atomic force microscopy imagine of the deposited DAPFB, DPAPFB, DNAPFB, and AND thin films doped with 10% by weight
of DPAVBi surfaces; top, before heated; down, after heated at 110 ꢀC for 12 h under nitrogen atmosphere.
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isolated anthracene units, with three bands located at 355 nm,
375 nm, and 395 nm for DPA, a little of hypochromic shift of 1
nm compared with DPAPFB and DNAPFB, and 6–7 nm bath-
ochromic shift relative to DAPFB, as shown in Fig. 2. The
optical band gaps were calculated to be 3.03 eV, 3.02 eV, and 3,02
eV for DAPFB, DPAPFB, and DNAPFB, respectively,
according to the maximum absorption border. The bigger the
substituent aromatic ring is on the anthracene, the larger red-
shift in the UV–vis spectrum. The same trend was observed in the
PL spectra, since the conjugation length increases with the
increasing size of the substituent aromatic rings on the anthra-
cene from DAPFB, DPAPFB, to DNAPFB. When transferred
from solution to the solid state, the UV–vis spectra only
exhibited 2ꢂ3 nm red-shifts in the absorption peaks with similar
patterns of the solution’s (for detail, see Fig. S3).† Under UV
light excitation, all of the anthracene derivatives emit blue light
whether in solution or in the solid state.
or conformational isomers.11 The observed difference between
the present PL spectra was suggested to explain by the ratio of
conformational isomers. It is conceivable that different confor-
mational isomers existed in the thermal deposited film and the
solid powder. As the substituent group increases form hydrogen
atom, phenyl group, to naphthalene group of DAPFB,
DPAPFB, and DNAPFB molecules, respectively, more atoms
are combined together and thus more energetically conforma-
tional isomer, high or small, were expected to form. Compared
with DAPFB molecules, DPAPFB and DNAPFB molecules,
with two bigger substituent groups on the anthracene, compose
more atoms and thus more energetically conformational isomer.
DFT calculation results also proved a variety of conformational
isomers exist. In general, the ratio of vibronic features is inde-
pendent of the sample preparation. Another possible explanation
for the observed difference (particularly, the evolution of emis-
sion shoulders peaking at 500ꢂ550 nm) in the PL spectra is
intramolecular interactions. Although aggregate of a kind of
molecules in the solid state may also induce new emission peak
characterized by broaden and weak in the PL spectrum, yet the
similar UV–vis absorption spectra of the thin films and the dilute
solutions exclude this possible deduction (for detail, see
Fig. S3).† Another thing worth noting is the relatively high
photoluminescent quantum yield (PLQY) in the range of 0.61 to
0.96 relative to DPA in dilute solution. High PLQY indicate
eliminated interactions between molecular chromophores arising
from the bulky and rigid skeleton of phenyl-substituted fluorene
dimer skeleton incorporated into the anthracene at the C–9
position. Further proved our molecular design strategy rational,
Further study on the PL spectra of the three anthracene
derivatives in the solid state revealed that the preparation
methods of films have profound effects on the photophysical
properties. As for thermal deposited films, they displayed broad
and featureless peaks with a maximum at 441 nm, while
DPAPFB and DNAPFB showed two shoulders at 420 and 508
nm as well as very weak band at 577 nm, as shown in Fig. 2. But
in the powder, they displayed characteristic emission with peak
at 422 nm and shoulder at 444 nm for DAPFB, 435, 460, and
500 nm for DPAPFB, 440, 462, and 535 nm for DNAPFB,
respectively. Observation of multiple bands in the PL spectra of
an emitter is known and generally attributed to vibronic features
Fig. 2 UV–vis absorption (a) and PL (b) spectra of DPA, DAPFB, DPAPFB, and DNAPFB in dilute dichloromethane solution, as well as optical
absorption spectrum of DPAVBi (the dash line). PL spectra of DAPFB, DPAPFB, and DNAPFB powder (c); vacuum deposited film (d).
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a non-coplanar conformation not only prevents intermolecular
interactions but also enables to improve the thermal and amor-
phous stability as well as high photoluminescent efficiency.
using thermal deposition in the following configuration: ITO/a-
NPB (40nm)/EML (30nm)/BCP (35nm)/Ca : Ag (100 nm), where
4,40-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (a-NPB) is the
hole-transporting layer (HTL); EML is DAPFB, DPAPFB, or
DNAPFB; 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
(BCP) used as hole-blocking and electron-transporting layer;
2.5. Electrochemical properties
In addition to the photophysical properties the electrochemical
properties is also very important to determine a material char-
acteristics and its potential application. Thus the electrochem-
istry of the three newly synthesized compounds was investigated
by cyclic voltammetry, which was performed in a conventional
three-electrode compartment cell. During the anodic oxidization
voltage sweep measured in 1,2-dichloromethane, reversible
oxidation peak at 1.32, 1.37, 1.32 V with onset potential at 1.19,
1.15, and 1.15 V for DAPFB, DPAPFB, and DNAPFB,
respectively, were observed. Upon the cathodic voltage sweep in
THF, reversible reduction peak at ꢁ2.26, ꢁ2.25, and ꢁ2.24 V
with onset potentials at ꢁ1.81, ꢁ1.86, and ꢁ1.85 V for DAPFB,
DPAPFB, and DNAPFB, respectively, were recorded (for detail,
see Fig. S4).† All of them showed reversible anodic oxidation or
cathodic reduction behavior during the repeat potential sweeps,
indicating the potential utilization for stable hole- and electron-
transporting materials.
ꢀ
a thin (7 A) Ca serves as electron-injecting layer at the Ag elec-
trode interface. The device energy level diagram was present in
Fig. 3 (right). The EL spectra of DAPFB, DPAPFB, and
DNAPFB show blue emissions with peaks at 460, 444, and 444
nm, respectively (see Fig. 3 left). It is interesting to note that the
emission maximum of each device corresponds well to one of the
maxima of the PL spectra. Similar EL spectra and PL spectra
indicates the same light-emitting process derive from the decay of
the singlet excitons. However, DNAPFB device, compared with
DAPFB and DPAPFB devices, showed a little difference in the
EL spectra, which displayed a distinct peak at around 540 nm
and a shoulder at 577 nm while the other two showed negligible
emission in this longer wavelength region. Although the PL
spectrum of DNAPFB also was similar to its EL spectrum, yet
the emission got stronger at the lower energy region, which can
be attributed to the directed charge trapping effect39 in the EL
process other than the PL process. The difference between
DAPFB, DPAPFB and DNAPFB in the EL spectra might be
ascribed to the different low energically conformational isomers
ratios changes in the electrically stimulation. Although exciplex
may also generate new emission in the longer wavelength region,
yet the PL spectrum of the blend films of DNAPFB with NPB or
BCP showed no new emission, thus exclude this deduction (see
Fig. S5).† All the devices showed reasonable high efficiency
(Table 2); the maximum luminance efficiency reached 2.2 cd Aꢁ1
for DAPFB emitter, 3.2 cd Aꢁ1 for DPAPFB emitter, and 5.1 cd
Aꢁ1 for DNAPFB emitter and remained very stable even at high
luminance of 10000 cd mꢁ2 (3.9 cd Aꢁ1), which is comparable to
or higher than other reported fluorescent blue-light-emitting
hydrocarbons, such as anthracene derivatives17,19,23,29,40–42 or
fluoranthene compounds,16,43 though its color purity are not idea
and still need to be improved.
The HOMO energy levels of DAPFB, DPAPFB, and
DNAPFB were estimated from the p-doping onset potential to
be ꢁ5.59, ꢁ5.55, and ꢁ5.55 eV, respectively, which is very close
to that of DPA (ꢁ5.54 eV), using ferrocene as internal standard
which has a value of ꢁ4.8 eV under the vacuum energy level.
Accordingly, the LUMO energy levels of DAPFB, DPAPFB,
and DNAPFB were calculated from the n-doping onset potential
to be ꢁ2.59, ꢁ2.54, and ꢁ2.55 eV for DAPFB, DPAPFB, and
DNAPFB, respectively. The electrochemical band gap (3.00,
3.01, and 3.00 eV) is, in general, well consistent with the optical
band gap (3.03, 3.02, and 3.02 eV). It is also interesting to
mention that the two anthracene moieties connected through the
non-conjugated sp3 hybridized carbon atom of fluorene dimer
core were electrochemically oxidize or reduce simultaneously at
the same potential, which further proved our molecular design
concept rational. Generally speaking, the electrochemical prop-
erties and photophysical properties of DAPFB, DPAPFB, and
DNAPFB in dilute solution are very similar, mirroring the
anthracene unit’s characteristics.
In order to further improve the efficiency and color purity, we
adopted a host-dopant system using these materials as host and
a highly fluorescent blue dye, DPAVBi, as dopant at a concen-
tration range of 1–6% in the same device structure. The key
performance characteristics of the devices are listed in Table 2.
The doped DPAPFB devices exhibited bright blue emission with
a peak at 460 nm and a shoulder at 492 nm, which is the intrinsic
emission pattern of the dopant; no short wave emission was
2.6. Electroluminescent properties
Encouraged by the high PLQY, high thermal stabilities, and
suitable HOMO and LUMO energy levels, we fabricated devices
Table 2 EL performance of blue OLEDs based on DAPFB, DPAPFB and DNAPFB
CIEb (x, y)
LEc/cd Aꢁ1
PEc (lm Wꢁ1
)
a
Light-emitting layer
EQEmax (%)
LEmax/cd Aꢁ1
PEmax (lm Wꢁ1
)
Lmax /cd mꢁ2
DAPFB
DPAPFB
DNAPFB
DPAPFB:1.1%DPAVBi
DPAPFB:1.8%DPAVBi
DPAPFB:4.0%DPAVBi
DPAPFB:6.3%DPAVBi
0.8
1.5
2.3
4.8
4.1
4.5
4.2
2.2
3.2
5.1
13.6
11.5
12.5
11.7
0.6
1.1
1.8
2.7
2.6
2.7
3.6
6981 (368)
12508 (713)
15559 (514)
29739 (308)
37120 (381)
26878 (364)
34795 (466)
0.18, 0.21
0.18, 0.21
0.24, 0.28
0.16, 0.23
0.16, 0.26
0.16, 0.26
0.16, 0.26
2.1
2.7
4.2
4.7
11.4
12.3
11.2
0.5
0.7
0.8
2.5
2.3
2.3
2.3
a
The data in the parentheses are the corresponding current density. b Recorded at 12 V. c Values obtained at a luminance of 5000 cd mꢁ2
.
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Fig. 3 EL spectra of DAPFB, DPAPFB, DNAPFB, and DPAVBi doped DPAPFB devices (left), and relative HOMO/LUMO energy levels of the
materials used in the device (right).
observed indicating completely energy transfer from the host to
the dopant. Furthermore, we found the emission spectra is
independent of the driving voltage in the range 6–17 V, with the
Commission Internationale de L’Eclairage (CIE) chromaticity
coordinate values x, y ¼ (0.16, 0.23). The maximum brightness
reached 37120 cd mꢁ2 at a current density of 381 mA cmꢁ2 cor-
responding to 1.8% doping concentration. Obviously, the lumi-
nance efficiencies of all the doped devices got pronounced
improvement compared to the undoped ones (Fig. 4). It is
worthy mentioning that the maximum efficiencies was obtained
at high luminance, indicating more practical application.44 For
example, the 1.1% doped device showed a maximum luminance
efficiency of 13.6 cd Aꢁ1 (corresponding to an external quantum
efficiency of 4.8%) at brightness over 2000 cd mꢁ2, more than
two-fold enhancement over spirobifluorene-linked bisanthracene
derivatives as host materials (4.9 cd Aꢁ1).24 As for DNAPFB
host, it exhibited even higher efficiencies than DPAPFB host (for
details, see Table 3). The maximum luminance efficiency reached
14.8 cd Aꢁ1 (corresponding external quantum efficiency of 3.5%)
and the maximum brightness is 53504 cd mꢁ2 (Fig. 5). In addi-
tion, the current efficiencies show only a mild decrease as the lumi-
nance increase. For example, the device based on DANPFB host
with 10% doping concentration shows a slightly decrease in efficiency
to 13.3 cd Aꢁ1 when the luminance increased to 5000 cd mꢁ2.
Fig. 4 Current density–voltage–luminance characteristics (a) and luminance efficiency as a function of luminance (b) for the DAPFB, DPAPFB and
DNAPFB based devices.
Table 3 EL properties characteristics of DPAVBi doped DNAPFB emitter
CIEa (x, y)
Lmax /cd mꢁ2
LEc/cd Aꢁ1
PEc (lm Wꢁ1
)
b
Light-emitting layer
EQEmax (%)
LEmax/cd Aꢁ1
PEmax (lm Wꢁ1
)
DNAPFB:6%DPAVBi
DNAPFB:8%DPAVBi
DNAPFB:10%DPAVBi
DNAPFB:20%DPAVBi
3.0
3.3
3.5
2.7
10.4
13.7
14.8
11.4
4.6
4.8
5.3
6.9
0.32, 0.40
0.32, 0.41
0.32, 0.41
0.33, 0.42
16474 (179)
53504 (607)
50248 (623)
33026 (385)
10.0
13.2
13.3
10.7
2.0
2.9
2.5
1.9
a
b
Recorded at 12 V. The data in the parentheses are the corresponding current density. c Recorded at a luminance of 5000 cd mꢁ2
.
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Fig. 5 Current density–voltage–luminance characteristics (a) and luminance efficiency as a function of luminance (b) for the DPAVBi doped DNAPFB
devices.
Interestingly, we found the DNAPFB host incorporating a
sky-blue dopant DPAVBi emits white light, which is never
reported in the literatures. The maximum brightness and lumi-
nance efficiency are also high, superior to that of other conven-
tional fluorescent white OLEDs based on anthracene blue host
and pyran derivatives orange dye two-spectrum single layer white
light-emitting system.22,45 This kind of white fluorescent OLEDs
may be very promising, considering its over performance and
simple device architecture: a three-layer device structure, which is
consist of hole-transporting layer, emissive layer, and hole-
blocking/electron transporting layer without electrode modifica-
tion or complicate device structural engineering.
concentration, which is not difficult to understand. In a doped
€
host–guest system, Forster energy transfer can take place from
the higher energy emissive host to the lower energy emissive
dopant, while the lower energy section can not transfer its
energy to the dopant exothermically and remained; that is the
emission peak of DNAPFB at 444 nm is energetically trans-
ferred to the dopant while the lower energy emission band at
540 nm and 577 nm did not transfer their energy to the dopant.
And due to the lower energy sites competed with the dopant to
get excitons, energy transfer is more favorable to transfer to the
lower emission site in addition to directly charge trapping by the
lower emission sites, so the emission at longer wavelength
region get stronger in the doped system than in the non-doped
system and became dominant. This demonstrated why we can
not get blue emission by doping DPAVBi into DNAPFB host.
Fortunately, by employing these low energy emissions and
a sky-blue dopant controlled at suitable doping level we got
white emission. The CIE chromaticity coordinates vary from
(0.32, 0.38) to (0.32, 0.41) in the driving voltage range of 9–17 V
(corresponding brightness of 77–13608 cd mꢁ2), very close to the
balanced equienergy white point of (0.33, 0.33) (see the inset of
Fig. 6). The negligible variation in the CIE values and EL
efficiency can be attributed to the balanced recombination and
emission of the excitons well confined in the single emissive
layer of the DNAPFB and DPAVBi binary blend emitting
system.
As for the EL spectra of doped DNAPFB devices, however,
they displayed another peak at 540 nm and a shoulder at
577 nm, in addition to the short waves at 460 and 492 nm which
is the intrinsic emission of the dopant, very different from
DPAPFB host devices, which showed white emission other than
blue light of DPAPFB host devices, as shown in Fig. 6. Like the
non-doped EL spectrum, this low energy emissions at 540 nm
and 577 nm are also occurred in the doped devices, which is well
consistent with the PL spectra of DNAPFB in the solid state,
only the relative intensity getting stronger, and we found this
low energy emission can not be eliminated even at high doping
For comparison, we also fabricated another device based on
ADN host in the same device architecture with the same doping
concentration, and found the maximum efficiency is only 8.5 cd
Aꢁ1 (CIE coordinates ¼ (0.21, 0.37), EQE ¼ 3.2%). A similar
device configuration combining the organic materials of
DPAVBi and ADN showed comparable EL efficiency of 7.96 cd
Aꢁ1 with CIEx,y (0.28, 0.34) reported by Lee et al.46 Further
improving its EL efficiency to 9.7 cd Aꢁ1 with CIEx,y (0.16, 0.32)
by adding CFx as the hole injection layer was achieved by Lee
et al.47 Compared with ADN host material, our host materials
greatly improved efficiency and color purity. In general, OLED
performance is described as a product of four factors: (1) carrier
balance factor, (2) exciton formation ratio, (3) photo-
luminescent quantum yield (PLQY), and (4) light out-coupling
factor. Among them, there seem to be mainly two factors, (1)
and (3), to determine these device performances. In energy level
point of view, both DNAPFB and DPAPFB have suitable
Fig. 6 EL spectra of DPAVBi doped DNAPFB device at a range of
driving voltage (inset is the CIE coordinate).
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HOMO and LUMO energy levels, with value at ꢁ5.51 and
ꢁ2.56 eV, respectively, well encompassed those of the dopant,
thus facilitate charge injecting and transporting into the dopant
and well confined excitons on the dopant. Moreover, there are
good spectral overlaps between the absorption spectrum of the
dopant and the PL spectra of the anthracene derivatives, thus
mass spectrometer. Elemental analyses were carried out on
a Carlo-Erba 1160 elemental analyzer. Thermogravimetric
analyses (TGA) were carried out using a Perkin-Elmer ther-
mogravimeter (Model TGA7) under a dry nitrogen gas flow at
a heating rate of 10 ꢀC minꢁ1. Glass transition temperatures (Tg)
and melting phase-transition temperature (Tm) were determined
by differential scanning calorimetry (DSC) at a heating rate of 10
ꢀC minꢁ1 using a Perkin-Elmer differential Scanning calorimeter
(DSC7). The cyclic voltammetry (CV) characterization was
performed on CHI 600C Electrochemical Workstation in
a conventional three-electrode cell using dichloromethane for
anodic sweep (THF for cathodic sweep) containing 0.1 M
n-Bu4NPF6 as supporting electrolyte versus Ag/AgCl reference
electrode, platinum disk as working electrode, and platinum wire
as counter electrode. UV–vis absorption and fluorescence spectra
were collected with a Hitachi U–3010 and Hitachi F–4500
spectrophotometer, respectively.
€
ensured efficient Forster energy transfer from the host to the
dopant (Fig. 2). Furthermore, the high PLQY of the host
materials partly accounts for the high efficiency of the doped or
undoped devices. At last, this host-dopant system possesses
exceptional miscibility, film-forming ability, and morphology
stability as revealed by the AFM surface morphology
measurement.
3. Conclusions
In conclusion, we have successfully designed and synthesized
a new series of anthracene derivatives via simple Suzuki coupling
reaction by incorporating a rigid and bulky tetrahedral molec-
ular skeleton, featuring a phenyl-substituted fluorene dimer as
bulky and rigid core and anthracene as a functional active group.
The non-coplanar molecular structure resulting from the tetra-
hedral environment of the phenyl-substituted fluorene dimer and
aryl-substitution group of the anthracene units provide steric
hindrance that prevents close packing of the molecules in the
solid state and also enables to form high thermal and morpho-
logical stable film, high PLQY, and well miscibility with the
dopant. Devices based on these anthracene derivatives show
bright blue emission with color purity and high efficiency up to
5.1 cd Aꢁ1 in an undoped device structure. Moreover, the doped
DPAPFB-based devices exhibit outstanding efficiencies, with
maximum luminance efficiency of 13.6 cd Aꢁ1 (EQE 4.8%) at 43.1
4.3. Fluorescence quantum yield measurements
The FF values in solution were measured following a general
method using 9,10-diphenylanthracene, which has a absolute FF
values of 0.9 in cyclohexane, as the standard.48 Dilute solutions
of DAPFB, DPAPFB, and DNAPFB in cyclohexane solvent
were used for recording the fluorescence spectra. Sample solu-
tions were degassed for 15 min and taken in quartz cuvettes to
record the absorption spectra, which the absorption intensity of
the excitation wavelength was adjusted at around 0.05. The
fluorescence spectra of each were excited at the same wavelength
and recorded 5 times, and an average value of integrated areas of
fluorescence was used for the calculation of FF values. The
refractive indices of solvents were used for the correlation of the
FF values.
mA cmꢁ2, corresponding to a brightness of 5820.9 cd mꢁ2
.
Furthermore, we have fabricated highly efficient white fluores-
cent OLEDs based on incomplete energy transfer between
DNAPFB and DPAVBi binary host-gust emitter system in
a doped device configuration with maximum EL efficiency of
14.8 cd Aꢁ1 and maximum brightness of 53504 cd mꢁ2 as well as
stable CIEx,y value of (0.33, 0.38). The comprehensive physical
and chemical characteristics examination results indicate that
our molecular design is successful and our anthracene derivatives
are promising candidates for photoelectronics application.
4.4. Device fabrication and measurements
In a general procedure, indium-tin oxide (ITO)-coated glass
substrates were etched, patterned, and washed with detergent,
deionized water, acetone, and ethanol in turn. All the organic
layers were successively deposited by vacuum deposition onto the
ITO coated glass substrates at a pressure of 8 ꢃ 10ꢁ5 Pa. In all
ꢀ
devices, a thin (7 A) Ca layer serves as electron-injecting layer at
the Ag electrode interface and a 100-nm-thick Ag capping layer
were deposited through a shadow mask. The active area of device
was 5 mm2. EL spectra and chromaticity coordinates were
measured with a SpectraScan PR650 photometer. Current
density–voltage–luminance (J–V–L) measurements were recor-
4. Experimental
4.1. Synthesis
All starting materials were obtained from commercial suppliers
and used without further purification. 9-Bromoanthracene, iso-
phthaloyl dichloride, and AlCl3 were purchased from Alfa Aesar.
n-Butyllithium was purchased from Merck. 2-Isopropoxy-
4,4,5,5-tetramethyl-[1,3,2]dioxaborolane was purchased from
Aldrich. All the solvents were distilled before use according to
literature procedure (fee detailed synthesis process, see ESI†)
ded simultaneously using
a Keithley 4200 semiconductor
parameter analyzer and a Newport multifunction 2835–C optical
meter, with luminance was measured in the forward direction.
All device characterizations were carried out under ambient
laboratory air at room temperature.
Acknowledgements
We acknowledge financial support from the National Natural
Science Foundation of China (20825208, 60736004, 20721061),
the National Major State Basic Research Development Program
(2006CB806203, 2006CB932103, 2009CB623603), and the
Chinese Academy of Sciences.
4.2. Characteristics
1H NMR and 13C NMR measurements were carried out on
a Bruker DMX 400 NMR spectrometer. MS spectrometry
(MALDI-TOF-MS) was determined on a Bruker BIFLEX III
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