Spirobenzofluorene linked anthracene derivatives: Synthesis and application in blue fluorescent host materials

Article abstract of DOI:10.1016/j.dyepig.2012.08.011

Blue light-emitting anthracene-based host materials with a spiro[benzo[c]fluorene-7,9′-fluorene] core, 5-(10-(naphthalen-2-yl) anthracen-9-yl)spiro[benzo[c]fluorene-7,9′-fluorene] (NA-SBFF) and 5-(10-(4-(naphthalen-1-yl)phenyl)anthracen-9-yl)spiro[benzo[c]fluorene-7, 9′-fluorene] (NPA-SBFF), were designed and synthesized via two-step Suzuki coupling reactions. Introduction of a spiro group into the structure of the anthracene moieties lead to a reduction in crystallization tendency, and a high glass transition temperature was observed. Typical blue fluorescent organic light emitting diodes with the configuration of ITO/N,N′-diphenyl-N, N′-bis[4-(phenyl-m-tolyl-amino)phenyl]biphenyl-4,4′-diamine (DNTPD)/N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (NPB)/HOST: Dopant/tris(8-hydroxyquinoline)aluminum (Alq₃)/LiF were developed using SBFF-type anthracene derivatives as a host material and N,N,N′,N′-tetraphenylspiro[benzo[c]fluorene-7,9′-fluorene]-5, 9-diamine (TPA-SBFF) as a dopant material. A device obtained from NA-SBFF doped with TPA-SBFF showed blue color purity of 0.146 and 0.167, a luminance efficiency of 8.65 cd/A, and an external quantum efficiency >6.01% at 8.0 V.

Full text of DOI:10.1016/j.dyepig.2012.08.011

Dyes and Pigments 96 (2013) 211e219
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Spirobenzofluorene linked anthracene derivatives: Synthesis and application
in blue fluorescent host materials
,
Jeang-A. Seo a, Chil-Won Lee b, Myoung-Seon Gong a
*
a Department of Nanobiomedical Science and WCU Research Center of Nanobiomedical Science, Dankook University, Chungnam 330-714, Republic of Korea
b Department of Polymer Science and Engineering, Dankook University, 126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Blue light-emitting anthracene-based host materials with a spiro[benzo[c]fluorene-7,90-fluorene] core, 5-
(10-(naphthalen-2-yl)anthracen-9-yl)spiro[benzo[c]fluorene-7,90-fluorene] (NA-SBFF) and 5-(10-(4-
(naphthalen-1-yl)phenyl)anthracen-9-yl)spiro[benzo[c]fluorene-7,90-fluorene] (NPA-SBFF), were designed
and synthesized via two-step Suzuki coupling reactions. Introduction of a spiro group into the structure of
the anthracene moieties lead to a reduction in crystallization tendency, and a high glass transition
temperature was observed. Typical blue fluorescent organic light emitting diodes with the configuration of
ITO/N,N0-diphenyl-N,N0-bis[4-(phenyl-m-tolyl-amino)phenyl]biphenyl-4,40-diamine (DNTPD)/N,N0-di(1-
naphthyl)-N,N0-diphenylbenzidine (NPB)/HOST: Dopant/tris(8-hydroxyquinoline)aluminum (Alq3)/LiF
were developed using SBFF-type anthracene derivatives as a host material and N,N,N0,N0-tetraphenylspiro
[benzo[c]fluorene-7,90-fluorene]-5,9-diamine (TPA-SBFF) as a dopant material. A device obtained from NA-
SBFF doped with TPA-SBFF showed blue color purity of 0.146 and 0.167, a luminance efficiency of 8.65 cd/A,
and an external quantum efficiency >6.01% at 8.0 V.
Article history:
Received 25 June 2012
Received in revised form
5 August 2012
Accepted 12 August 2012
Available online 23 August 2012
Keywords:
Blue OLED
Blue host
Spiro[benzo[c]fluorene-7,90-fluorene]
Color purity
Anthracene
Spirobenzofluorene
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
9,10-Di(2-naphthyl)anthracene (
b-ADN) is one of the best blue
host materials with an efficiency as high as 5.1 cd/A [10].
b
-ADN
Many blue light-emitting dyes have been studied to achieve blue
light emissions with host and dopant materials as the emitter, but
there isstillaclearneedforfurtherimprovements intermsofstability,
efficiency, and color purity [1e3]. Organic electroluminescence (EL)
devices using anthracene derivatives as host blue emitters exhibit
very promising performance. However, because of the considerable
blueshiftabsorption of thesedeepbluedopants, bettermatching host
materials with sufficient spectral overlap for efficient Förster energy
transfer are needed to facilitate the generation of blue dopant emis-
sions with high efficiency as well as a deep blue color [4,5].
The anthracene plays an important role in the development and
application of OLEDs materials such as emitting materials from blue
to red, hole- and electron-transporting materials. Recently, their
derivatives were reviewed and discussed in relation to the thermal
stability and the electroluminescent performance [6e9]. An anthra-
cene based 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) with
a stable thin film morphology and a sufficiently wide band gap has
been reported. Nevertheless, the color purity is far from adequate for
full-color organic light emitting diode (OLED) display applications.
shows excellent stability, the electroluminescent spectrum of the
undoped device is broad and featureless and is greenish blue in
color centering at 460 nm.
Spiro-type host and dopant materials containing fluorene and
benzofluorene as OLED fluorescent materials have received a great
deal of attention, because they preserve the inherent characteristics
of spiro compound such as morphological stability, high glass
transition temperature, and amorphous properties [11e20]. They
can also provide a variety of substituents on the spiro[benzo[c]
fluorene-7,90-fluorene] (SBFF) because of their asymmetrical spiro
core structure with naphthalene, phenyl rings of spiro molecules,
and conjugation controlled OLED host materials, as shown in
Scheme 1 [21e24].
Many studies have considered SBFF as an organic electrolumi-
nescent host material. The performance of OLEDs (the color purity
and luminescence efficiency) based on SBFF is determined by the
position of substitution and a variety of aryl moieties with a variety
of conjugation chain lengths [25e30].
In this study, a novel anthracene derivative with an SBFF core
structure was developed as a high fluorescent deep blue host
material with good thermal/morphological stability, and the
physical properties were investigated. Two new blue host materials
consisting of 5-(10-(naphthalen-2-yl)anthracen-9-yl)spiro[benzo
* Corresponding author. Tel.: þ82 41 5501476; fax: þ82 41 5503431.
E-mail address: msgong@dankook.ac.kr (M.-S. Gong).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.08.011

212
J.-A. Seo et al. / Dyes and Pigments 96 (2013) 211e219
vacuum, and the product was extracted with ethyl acetate and water.
The ethyl acetate solution was washed with water and dried with
MgSO4. Evaporation of the solvent, followed by column chromatog-
raphy using n-hexane on a silica gel, yielded a white yellow product. 9-
Bromo-10-(naphthalene-2-yl)anthracene was prepared using naph-
thalene-1-boronic acid similar to the procedures described above.
2.3. Preparation of 10-(4-(naphthalene-1-yl)phenyl)anthracene-9-
ylboronic acid
9-Bromo-10-(4-(naphthalene-1-yl)phenyl)anthracene (10 g,
21.0 mmol) in dry THF (350 mL) was placed in a double-necked
round-bottomed flask (1000 mL) under an argon gas environ-
ment. After the reaction mixture was maintained at ꢁ78 ꢀC, n-BuLi
(2.5 M in n-hexane, 11.32 mL) was added through a syringe pump
over 90 min with vigorous stirring. Trimethyl borate (3.21 mL,
28.0 mmol) in THF (5 mL) was injected through the syringe pump
for 30 min. The reaction mixture was raised to room temperature
with stirring. After the reaction was completed, 2 N HCl (350 mL)
was introduced and maintained for 5 h. The reaction mixture was
extracted with ethyl acetate/H2O (1:1). The organic layer was
separated and evaporated to yield a light yellow crystal. 10-
(Naphthalene-2-yl)anthracene-9-ylboronic acid was prepared
using 9-bromo-10-(naphthalene-2-yl)anthracene using similar
procedures to those described above.
Scheme 1. Substituent positions on spiro[benzo[c]fluorene-7,90-fluorene].
[c]fluorene-7,90-fluorene] (NA-SBFF) and 5-(10-(4-(naphthalen-1-
yl)phenyl)anthracen-9-yl)spiro[benzo[c]fluorene-7,90-fluorene]
(NPA-SBFF) were prepared and characterized using 1H nuclear
magnetic resonance (NMR), 13C NMR, Fourier transform-infrared
(FT-IR), mass spectroscopy (MS), thermal analysis, UVevis, and
photoluminescence (PL) spectroscopy. The EL properties of multi-
layered OLEDs fabricated using two host materials and N,N,N0,N0-
tetraphenylspiro[benzo[c]fluorene-7,90-fluorene]-5,9-diamine
(TPA-SBFF) as the dopant were evaluated.
2. Experimental
2.1. Materials and measurements
2.4. Representative preparation of NPA-SBFF
Tetrakis(triphenylphosphine)palladium(0), (Aldrich Chem. Co.,
St. Louis, MO, USA), 9,10-dibromoanthracene (TCI Chem. Co., Tokyo,
Japan), 4-(naphthalene-1-yl)phenylboronic acid, and naphthalene-
2-boronic acid (Frontier Scientific Co., West Logan, UT, USA) were
used as received. n-Butyllithium (2.5 M solution in hexane), bromine,
trimethyl borate (redistilled), potassium carbonate, sodium
hydroxide(DuksanChem. Co., Seoul, SouthKorea)wereused without
further purification. 5-Bromospiro[benzo[c]fluorene-7,90-fluorene]
and TPA-SBFF were prepared using a method previously reported
[01]. Tetrahydrofuran (THF) was purified by distillation over sodium
metal and calcium hydride. PL spectra were recorded on a fluores-
cence spectrophotometer (Jasco FP-6500; Tokyo, Japan) and the UVe
vis spectra were obtained by means of a UVevis spectrophotometer
(Shimadzu, UV-1601PC; Tokyo, Japan). Energy levels were measured
with a low-energy photo-electron spectrometer (AC-2; Riken-Keiki,
Union City, CA, USA). The FT-IR spectra were obtained with
a Thermo Fisher Nicolet 850 spectrophotometer (Waltham, MA,
USA), and the elemental analyses were performed using a CE
Instrument EA1110 (Hindley Green, Wigan, UK). The differential
scanning calorimeter (DSC) measurements were performed on
aShimadzuDSC-60DSCundernitrogenataheatingrateof10ꢀC/min.
The thermogravimetric analysis (TGA) measurements were per-
formed on a Shimadzu TGA-50 thermogravimetric analyzer at
a heating rate of 5 ꢀC/min. The low and high resolution mass spectra
were recorded using a JEOL JMS-AX505WA spectrometer in the fast
atom bombardment mode. High resolution mass spectra were
recorded using an HP 6890 and Agilent 5975C MSD in FAB mode.
A
solution of 5-bromospiro[benzo[c]fluorene-7,90-fluorene]
(4.51 g,10 mmol), tetrakis(triphenylphosphine)palladium(0) (0.59 g,
0.51 mmol), and 10-(4-(naphthalene-1-yl)phenyl)anthracene-9-
ylboronic acid (4.29 g, 0.01 mol) dissolved in THF (150 mL) was stir-
red in a double-necked flask for 30 min. Potassium carbonate (2 M,
150 mL) was added dropwise over 20 min. The resulting reaction
mixture was refluxed overnight at 80 ꢀC and then extracted with
ethyl acetate and water. After the organic layer was evaporated with
a rotary evaporator, the resulting powdery product was purified by
column chromatography from dichloromethane/n-hexane (1/1) to
givetheyellowNPA-SBFFcrystallineproduct. NA-SBFFwasprepared
using similar procedures described above.
2.5. OLED fabrication
A basic device configuration of indium tin oxide (150 nm)/N,N0-
diphenyl-N,N0-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-biphenyl-
4,40-diamine (DNTPD, 60 nm)/N,N0-di(1-naphthyl)-N,N0-diphe-
nylbenzidine (a-NPB, 30 nm)/host: TPA-SBFF (30 nm, x%)/2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP,
hydroxyquinoline)aluminum (Alq3, 20 nm)/LiF (1 nm)/Al (200 nm)
was used for device fabrication, as shown in Fig.1. The organic layers
5
nm)/Tris(8-
ꢀ
were deposited sequentially onto the substrate at a rate of 1.0 A/s by
thermal evaporation from heated alumina crucibles. The doping
concentration of the dopant materials was varied at 5, 10 and 15%.
The devices were encapsulated with a glass lid and a CaO getter after
cathode deposition. Current densityevoltage luminance and EL
characteristics of the blue fluorescent OLEDs were measured with
a Keithley 2400 source measurement unit (Cleveland, OH, USA) and
a CS 1000 spectroradiometer.
2.2. Preparation of 9-bromo-10-(4-(naphthalene-1-yl)phenyl)
anthracene
9,10-Dibromoanthracene (10 g, 0.03 mol), tetrakis(triphenyl
phosphine)palladium(0) (1.72 g,1.49 mmol) and 4-(naphthalene-1-yl)
phenylboronic acid (7.4 g, 0.03 mol) were dissolved in THF (350 mL) in
a double-necked flask and stirred for 30 min. Then, potassium
carbonate (2 M, 200 mL) was added dropwise over 20 min. The
mixturewasdegassedandrefluxedovernight at80ꢀCunderanitrogen
atmosphere. After being cooled, the solvent was evaporated under
3. Results and discussion
3.1. Synthesis and characterization
TPA-SBFF was prepared as a dopant material using an amination
reaction of 5,9-dibromo-SBFF and diphenyl amine. 5-Bromo-SBFF

J.-A. Seo et al. / Dyes and Pigments 96 (2013) 211e219
213
Fig. 1. The device configuration and the chemical structure of the materials used in the devices.
was prepared by selective bromination of SBFF in carbon tetra-
chloride solvent, as shown in Scheme 2. The molecular structures of
the anthracene end-capped SBFF derivatives and their synthetic
routes are shown in Scheme 3. These compounds can be obtained
with high yields using typical two-step Suzuki coupling reactions
according to a procedure reported previously [21,30]. The chemical
structures and composition of the resulting precursors and spiro-
compounds were characterized by 1H NMR, 13C NMR, FT-IR, gas
chromatographyeMS, and elemental analysis.
NA-SBFF and NPA-SBFF, respectively. Table 1 summarizes the DSC
and TGA data for the two host materials. The NPA-SBFF showed
melting points (Tm) of 413 ꢀC, but no melting points were observed
on the second heating scan, even though they were given enough
time to cool in air. Once the compounds became amorphous
solids, they did not revert to the crystalline state. After the
samples had cooled to room temperature, a second DSC scan was
performed at 10 ꢀC/min and revealed high glass transition
temperatures (Tg) of 263 ꢀC and 272 ꢀC for NA-SBFF and NPA-SBFF,
respectively, because of their rigid spiro-type backbone. This result
suggests that their thermal stability improved significantly by
introducing the rigid anthracene unit into the spiro backbone. As
a result, the amorphous glassy state of the transparent films of the
two host materials showed that they are good candidates for use
as EL materials.
3.2. Thermal properties
The thermal properties of the resulting blue host materials
were characterized by DSC and TGA in a nitrogen atmosphere. The
onset decomposition temperatures were 452.0 ꢀC and 433.4 ꢀC for

214
J.-A. Seo et al. / Dyes and Pigments 96 (2013) 211e219
B(OH)2
n-BuLi
Br
B(OH)2
B(OCH3)3
Br
Br
B(OH)2
n-BuLi
Br
B(OH)2
B(OCH3)3
Scheme 2. Preparation of precursors for spiro-type host materials.
Scheme 3. Preparation of NA-SBFF and NPA-SBFF host materials with SBFF and anthracene core.
3.3. Optical properties and energy levels
three major bands with similar absorption peaks of 350e400 nm.
These characteristic absorption patterns were attributed to the
* transitions of the central anthracene core of the compounds.
The absorption bands at 300e400 nm originated from the
combination of the * transition of the SBFF core and the pep*
transitions of the anthracene core. Two compounds exhibited blue
emission in solution with a maximum peak of about 435 nm. PL
spectra of these compounds in films showed slight red shifts with
long tail emission relative to those in solution. For NA-SBFF, there
is a clear emission peak at about 650 nm. It seems that the SBFF
pe
Fig. 2 shows the absorption and fluorescence spectra of the two
host materials in dilute THF solution. All absorption spectra had
p
p
ep
Table 1
UV absorption, PL, energy levels and thermal properties of two hosts.
Properties
Sample
NA-SBFF
NPA-SBFF
Puritya
Thermal analysis
HPLC(%)
DSC
99.9
263
413
433.4
377
2.95
436
52.36
449
54.6
0.72
5.96
3.01
99.8
272
439
452
376
2.96
435
52.34
437
50.59
0.59
5.89
2.93
Tg (ꢀC)
Tm (ꢀC)
moiety does not minimized intermolecular
p-stacking in their
Td (ꢀC)
solid states. The two hosts produced moderate fluorescence
quantum efficiency of 0.72 and 0.59 in dilute cyclohexane solution
using 9,10-diphenylanthracene (Ff ¼ 0.9 as the reference) from
NA-SBFF and NPA-SBFF, respectively. The photophysical and
thermal properties of these compounds are summarized in Table 1.
In a previous report, naphthylanthrcenyl group substituted at the
9-position of SBFF showed a higher Ff value than that of the 5-
position. The torsional angle between the phenyl group at the 5-
position and that of anthracene was 83.72ꢀ [30]. It is well known
that a twist angle >90ꢀ in NA-SBFF and NPA-SBFF diminishes or
even suppresses the conjugation, which limits fluorescence [31].
A molecular simulation of NA-SBFF and NPA-SBFF was carried
out to understand the physical properties at the molecular level.
Optical analysis
UV
Max (nm)
Bgb (eV)
PL
Max (nm)
FWHMc (nm)
Max (nm)
FWHMc (nm)
Solid PL
d
QE
AC-2
Ff
Electrical analysis
HOMO (eV)
LUMO (eV)
a
The purity of the samples were finally determined by high performance liquid
chromatography (HPLC) using the above prepared samples after train sublimation.
b
Bandgap.
Full width at half maximum.
Fluorescence quantum efficiency, relative to 9,10-diphenylanthracene in
c
d
cyclohexane (Ff ¼ 0.90).

J.-A. Seo et al. / Dyes and Pigments 96 (2013) 211e219
215
a
NA-SBFF
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
solution UV
solid UV
solution PL
solid PL
400
500
600
700
800
Wavelength (nm)
b
NPA-SBFF
1.0
0.8
0.6
0.4
0.2
0.0
1.0
solution UV
solid UV
solution PL
solid PL
0.8
0.6
0.4
0.2
0.0
Fig. 4. Energy diagram of the blue fluorescence devices using NA-SBFF and NPA-SBFF.
structure, and conjugation of the anthracene core can be destroyed
by the spirobifluorene group in NA-SBFF and NPA-SBFF [32].
Molecular simulation also showed the electron distribution of NA-
SBFF and NPA-SBFF. The highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
distribution of NA-SBFF and NPA-SBFF are shown in Fig. 3. HOMO
and LUMO orbitals of the two host materials were concentrated in
the anthracene core, and the substitution position of the spirobi-
fluorene did not affect the HOMO and LUMO distribution in the
host materials.
400
500
600
700
800
Wavelength (nm)
Fig. 2. UVevis and PL spectra of (a) NA-SBFF and (b) PNA-SBFF.
The energy levels of the two host and dopant materials used
to fabricate the OLEDs in the present study are shown in Fig. 4. A
low-energy photoelectron spectrometer was used to obtain
information on the HOMO energies of the host and dopant
materials and to examine the charge injection barriers. Energy
gaps for NA-SBFF, NPA-SBFF, and TPA-SBFF were calculated as
2.95, 2.96, and 2.71 eV, respectively. The HOMO energy levels
Fig. 3 shows the geometric structure of NA-SBFF and NPA-SBFF. The
anthracene core structure was affected by SBFF in both NA-SBFF
and NPA-SBFF. Rotation of the spirobifluorene in NPA-SBFF and
NA-SBFF was limited by steric hindrance of the spirobifluorene
with anthracene. This lead to distortion of the anthracene core
Fig. 3. HOMO and LUMO electronic density distributions of (a) NA-SBFF and (b) NPA-SBFF, calculated at the DFT/B3LYP/6-31G* for optimization and Time Dependent DFT (TDDFT)
using Gaussian 03.

216
J.-A. Seo et al. / Dyes and Pigments 96 (2013) 211e219
Fig. 6. Stability of the chromaticity depicted by CIE coordination.
3.4. EL properties
The EL spectra of NA-SBFF and NPA-SBFF doped by the TPA-
SBFF dopant at different concentrations (5%, 10%, and 15%) were
investigated to find the optimum dopant concentration. The EL
spectra of the blue fluorescent NPA-SBFF device doped with TPA-
SBFF are shown in Fig. 5, which showed a maximum peak at
460 nm. A blue emission was observed from the TPA-SBFF-doped
NPA-SBFF device, and the color coordinates of the blue device were
about 0.146 and 0.167. The EL spectra of other host NA-SBFF were
quite similar to those of NPA-SBFF, as summarized in Table 2. The
peak maximum of EL spectra was about 460 nm, and the spectra
were consistent even at a high doping concentration. The EL
emission was dominated by the emission peak of the dopant. This
seemed to indicate that the energy transfer from the host to TPA-
SBFF dopant was quite efficient at the optimum dopant concen-
tration employed in this experiment. The full width at half
maximum of 60 nm was relatively small, which lead to good color
purity, as illustrated in the PL spectrum (46e52 nm).
Fig. 5. Electroluminescence spectra of NA-SBFF and NPA-SBFF devices according to
the doping concentration of TPA-SBFF.
were ꢁ5.89, ꢁ5.96, and ꢁ5.48 eV for NA-SBFF, NPA-SBFF,
and TPA-SBFF, respectively. The HOMO value of TPA-SBFF was
quite high compared with those of the other two hosts. The
HOMO values of the two hosts were similar to those of other
The dependence of chromaticity on current density was
measured to evaluate the stability of the devices, as shown in Fig. 6.
When the EL spectrum was converted to chromaticity coordinates
on the CIE 1931 diagram, chromaticity stability increased with
a decrease in dopant concentration. The stability of the CIEx
anthracene derivatives (ꢁ5.5 to ꢁ6.0 eV) such as MADN and
b-
ADN [33].
Table 2
Eelectroluminescence properties of the devices obtained from two host and TPA-
SBF dopant materials.
Properties
Host
Dopant
(%)
NA-SBFF
TPA-SBFF
5%
NPA-SBFF
10%
5%
10%
EL at 7 V
lmax (nm)
FWHM (nm)
mA/cm2
cd/Aa
460
58.23
0.66
7.92
8.65(8 V)
3.55
462
57.62
1.37
8.36
8.36
460
58.14
1.99
7.50
7.50
462
57.70
3.95
7.15
7.47(6 V)
3.18
3.90(6 V)
280.4
0.146
0.169
4.88
cd/Ab
lm/Wa
3.75
3.58
lm/Wb
cd/m2
3.55
3.79(6 V)
114.6
0.145
0.172
5.69
3.86(5 V)
149.7
0.146
0.162
5.28
51.98
0.146
0.167
5.47
CIE-x
CIE-y
EQE(%)a
EQE(%)b
6.01(8 V)
5.69
5.28
5.11(6 V)
a
Values at 7 V.
Values at a highest peak.
Fig. 7. Current densityevoltageeluminescence characteristics of the device using NA-
SBFF and NPA-SBFF doped with 5% TPA-SBFF.
b

J.-A. Seo et al. / Dyes and Pigments 96 (2013) 211e219
217
a slightly higher current density and luminance than that of NA-
SBFF at the same driving voltage.
As can be seen in the luminance efficiency curves of the six
devices in Fig. 8, high efficiencies >6 cd/A were obtained. A low
efficiency was observed only in the devices doped with 15% dopant.
In the case of the NA-SBFF device doped with 10% TPA-SBFF, the
maximum brightness of 51.98 cd/m2 and 8.65 cd/A occurred at 8 V,
as summarized in Table 2. The maximum quantum efficiency and
luminance efficiency of the NA-SBFF and NPA-SBFF devices were
similar. The quantum efficiency >4% maintained at high current
density up to 400 mA/cm2 in the two devices as shown in inset of
Fig. 8.
The maximum quantum efficiency of the device obtained from
the NA-SBFF device doped with 10% TPA-SBFF was an external
quantum efficiency (EQE) of 5.69%, as shown in Fig. 9. The device
showed a maximum power efficiency of 3.75 lm/W, and efficiency
increased rapidly to a maximum of approximately 8.36 cd/A at
a low current density of 1.37 mA/cm2 at 7 V. The holes injected from
a hole transfer layer,
a-NPB, were transferred to the lighting
Fig. 8. Current efficiency-current density characteristics of the device using NA-SBFF
and NPA-SBFF doped with 5, 10 and 15% TPA-SBFF; the inset is a figure of external
quantum efficiency vs current density.
emitting layer and were trapped at dopant sites. The same spiro-
type dopant TPA-SBFF had a large capacity to catch the holes,
and minimized the loss of holes resulting in good EL efficiency.
Thus, the HOMO level of TPA-SBFF was suitable for hole trapping
and hole transporting as a dopant, and the NA-SBFF host was
moderate for balancing holes and electrons in the emitting layer.
NPA-SBFF doped with 5% TPA-SBFF showed high efficiency and
a small decrease in efficiency as the current density was increased
from 0 to 1.99 mA/cm2, i.e., a weak-current-induced fluorescent
quenching. It reached a current efficiency of 7.50 cd/A, EQE of 5.28%,
power efficiency of 3.58 lm/W, and CIE coordinates of 0.146 and
0.162 at 7 V. These results suggest that an exciton was formed and
light was emitted at specific thresholds. It should be noted that the
efficiencies of these devices remained stable when the luminance
was increased to 149.7 cd/m2.
coordinates was better than that of the CIEy coordinates. The NA-
SBFF-based device emitted a blue light with CIE coordinates of
0.146 and 0.167.
3.5. OLED device properties
NA-SBFF and NPA-SBFF were evaluated as host materials for
blue fluorescent OLEDs. TPA-SBFF was doped as a blue dopant in
the NA-SBFF and NPA-SBFF host materials, and the device perfor-
mance of the blue OLEDs was studied. Fig. 7 shows the luminancee
voltageecurrent density characteristics of the OLEDs with two
hosts doped with 5% TPA-SBFF dopant as the EML. Although the
two host materials had similar bandgaps, NPA-SBFF showed
The PL spectrum of NA-SBFF and NPA-SBFF overlapped with
a major portion of the absorption spectrum of TPA-SBFF, as shown
Fig. 9. External quantum efficiencyevoltage curves of the NA-SBFF and NPA-SBFF devices according to the doping concentration of TPA-SBFF.

218
J.-A. Seo et al. / Dyes and Pigments 96 (2013) 211e219
improved glass transition temperatures (Tg) for good thermal
Solid UV of TPA-SBFF
Solid PL of NA-SBFF
Solid PL of NPA-SBFF
1.0
0.5
0.0
1.0
0.5
0.0
stability. The EL emission spectra of the devices was about 462 nm.
The typical OLED devices showed excellent performance; the NA-
SBFF-based device exhibited highly efficient blue-light emission
with a maximum efficiency of 8.65 cd/A (EQE, 6.01%). According to
these characteristics, these blue light emitting materials have
sufficient potential for fluorescent OLED applications.
Acknowledgments
This research was supported by WCU (World Class University)
program through the National Research Foundation of Korea funded
by the Ministry of Education, Science and Technology (R31-10069).
350
400
450
500
550
600
650
700
750
Wavelength (nm)
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Fig. 10. Overlapping of UVevis absorption spectrum of TPA-SBFF and PL spectra of NA-
SBFF and NPA-SBFF.
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