Deep blue organic light-emitting diode using non anthracene-type fused-ring spiro[benzotetraphene-fluorene] with aromatic wings

Article abstract of DOI:10.1016/j.orgel.2014.08.030

We prepared three spirobenzotetraphene-based fused-ring spiro[benzo[ij]tetraphene-7,9′-fluorene] (SBTF) derivatives for use in non anthracene-type deep-blue organic light-emitting diode (OLED) hosts. 3-(2-Naphthyl)-10-naphthylspiro[benzo[ij]tetraphene-7,9′-fluorene] (N-NSBTF), 3-[4-(2-naphthyl)phenyl]-10-naphthylspiro[benzo[ij]tetraphene-7,9′-fluorene] (NP-NSBTF), and 3-(phenyl)-10-naphthylspiro[benzo[ij]tetraphene-7,9′-fluorene] (PNSBTF) were synthesized via multi-step Suzuki coupling reactions. The optimized device structure-ITO/N,N′-bis-[4-(di-m-tolylamino)phenyl]-N,N′-diphenylbiphenyl-4,4′-diamine (DNTPD, 60 nm)/bis[N-(1-naphthyl)-N-phenyl]benzidine (NPB, 30 nm)/NSBTF hosts: LBD (5%) (20 nm)/aluminum tris(8-hydroxyquinoline) (Alq3, 20 nm)/LiF/Al-was characterized by its blue electroluminescence to have a current efficiency of 6.25 cd/A, a power efficiency of 5.07 lm/W, and an external quantum efficiency of 5.24% at 18.7 mA/cm2 at CIE coordinates of 0.130, 0.149.

Full text of DOI:10.1016/j.orgel.2014.08.030

Organic Electronics 15 (2014) 2922–2931
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Deep blue organic light-emitting diode using non
anthracene-type fused-ring spiro[benzotetraphene-fluorene]
with aromatic wings
Min-Ji Kim a, Chil-Won Lee b, Myoung-Seon Gong a,
⇑
a Department of Nanobiomedical Science and BK21 PLUS NBM Global Research Center, 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
We prepared three spirobenzotetraphene-based fused-ring spiro[benzo[ij]tetraphene-7,90-
fluorene] (SBTF) derivatives for use in non anthracene-type deep-blue organic light-emitting
diode (OLED) hosts. 3-(2-Naphthyl)-10-naphthylspiro[benzo[ij]tetraphene-7,90-fluorene]
(N-NSBTF), 3-[4-(2-naphthyl)phenyl]-10-naphthylspiro[benzo[ij]tetraphene-7,90-fluorene]
(NP-NSBTF), and 3-(phenyl)-10-naphthylspiro[benzo[ij]tetraphene-7,90-fluorene] (P-
NSBTF) were synthesized via multi-step Suzuki coupling reactions. The optimized device
structure – ITO/N,N0-bis-[4-(di-m-tolylamino)phenyl]-N,N0-diphenylbiphenyl-4,40-diamine
(DNTPD, 60 nm)/bis[N-(1-naphthyl)-N-phenyl]benzidine (NPB, 30 nm)/NSBTF hosts: LBD
(5%) (20 nm)/aluminum tris(8-hydroxyquinoline) (Alq3, 20 nm)/LiF/Al – was characterized
by its blue electroluminescence to have a current efficiency of 6.25 cd/A, a power efficiency
of 5.07 lm/W, and an external quantum efficiency of 5.24% at 18.7 mA/cm2 at CIE coordinates
of 0.130, 0.149.
Article history:
Received 18 April 2014
Received in revised form 16 August 2014
Accepted 16 August 2014
Available online 28 August 2014
Keywords:
Fluorescence
Deep blue host
Spiro[benzotetraphene-fluorene]
Color purity
Fused-ring
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
derivatives have been previously reviewed and discussed
in terms of their thermal stability and EL performance
Many light-emitting dyes have been extensively stud-
ied for their ability to produce blue light emissions when
used with adequate dopant and host materials in the emis-
sive layer of OLEDs. However, the requirement of a large
energy band-gap makes it difficult to fabricate stable, blue
OLEDs with high electroluminescence (EL) efficiency and
high color purity [1–3].
Anthracene derivatives have been used as blue host
materials in OLEDs because they emit blue light and have
excellent photoluminescence (PL) and EL properties
[4–7]. In addition, anthracene fluorescence can easily
change from blue to green by introducing various aromatic
substituents at the 9- and 10-positions [8]. Anthracene
[9–12], but the color purity is far from adequate for use
in full-color organic light emitting diode (OLED) displays.
Only a limited number of materials have been able to
provide the appropriate brightness and stability necessary
[13–17], and small fluorene-based molecules and polymers
have gained much attention for the potential as efficient
blue emitters [18,19]. We believe that non-anthracene spi-
rofluorene molecules with high conjugation are effectively
avoid molecular aggregation and enhance the efficiency of
electron and hole transport.
Spiro[fluorene-7,90-fluorene] (SBF) or spiro[benzo[c]flu-
orene-7,90-fluorene] (SBFF) are interesting fluorescent
OLED materials because that can provide for a variety of
derivatives. In particular, SBFF has an asymmetric spiro
core structure with naphthalene and phenyl rings in the
spiro molecules. Thus, the substitution of aromatic
⇑
Corresponding author. Tel.: +82 41 5501476; fax: +82 41 550 3431.
E-mail address: msgong@dankook.ac.kr (M.-S. Gong).
http://dx.doi.org/10.1016/j.orgel.2014.08.030
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

M.-J. Kim et al. / Organic Electronics 15 (2014) 2922–2931
2923
moieties to the SBFF core can allow for conjugation
controlled OLED host materials, as shown in Scheme 1
[20–23]. More recently, spiro[benzo[de]anthracene-7,90-
fluorene] (SBAF) derivatives have been prepared and
implemented as OLED host and dopant materials. The
highly conjugated spiro molecules allow for the optical
and electrochemical properties to be delicately tuned over
a wide range by following the appropriate chemical modi-
fication [24,25]. However, more fused and conjugated
spiro-compounds have been studied less, particularly
spiro[benzo[ij]tetraphene-7,90-fluorene] (SBTF) and its
derivatives. Only a few examples of applications for SBTF
derivatives in organic electronics have been published
[26,27].
sodium metal and calcium hydride. Photoluminescence
(PL) spectra were recorded on a fluorescence spectropho-
tometer (Jasco FP-6500; Tokyo, Japan) and UV–vis spectra
were obtained by a UV–vis 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). FT-IR spectra were
obtained with a Thermo Fisher Nicolet 850 spectrophotom-
eter (Waltham, MA, USA), and elemental analyses were per-
formed using CE Instrument EA1110 (Hindley Green, Wigan,
UK). Differential scanning calorimeter (DSC) measurements
were performed on a Shimadzu DSC-60 DSC under N2. The
thermogravimetric analysis (TGA) measurements were per-
formed on a Shimadzu TGA-50 thermogravimetric analyzer.
High resolution mass spectra were recorded using an HP
6890 (Brea, CA, USA) and Agilent Technologies 5975C MSD
in FAB mode (Palo Alto, CA, USA).
In this study, we developed and characterized a novel 10-
naphthylspiro[benzo[ij]tetraphene-7,90-fluorene]
(NSBTF)
core structure for use as a deep-blue, high-fluorescence host
material with good thermal and morphological stability. We
prepared three new blue host materials that consisted of
3-(2-naphthyl)-10-naphthylspiro[benzo[ij]tetraphene-7,90-
fluorene] (N-NSBTF), 3-[4-(2-naphthyl)phenyl]-10-naph-
thylspiro[benzo[ij]tetraphene-7,90-fluorene] (NP-NSBTF),
and 3-phenyl-10-naphthylspiro[benzo[ij]tetraphene-7,90-
fluorene] (P-NSBTF), and these were characterized using
1H nuclear magnetic resonance (NMR), 13C NMR, Fourier
transform-infrared (FT-IR) and mass spectroscopy (MS),
thermal analysis, and UV–vis and PL spectroscopy. Also,
the EL properties of the fabricated multilayered OLEDs were
evaluated.
2.2. Preparation of 10-naphthylspiro[benzo[ij]tetraphene-
7,90-fluorene] (NSBTF)
A solution of 8-[2-[6-(2-naphthyl)]]naphthyl-1-bromo-
naphthalene (14.06 g, 30.6 mmol) in THF (100 mL) was
added to a 250 mL two-necked flask. The reaction flask
was cooled to ꢀ78 °C, and n-BuLi (2.5 M in n-hexane,
14.68 mL) was added slowly in a dropwise fashion. The
solution was stirred at this temperature for 1 h, followed
by adding a solution of 9-fluorenone (5.51 g, 30.6 mmol)
in THF (30 mL) under an argon atmosphere. The resulting
mixture was gradually warmed to ambient temperature
and quenched by adding saturated aqueous NaHCO3
(90 mL). The mixture was extracted with dichloromethane.
The combined organic layers were dried over magnesium
sulfate, filtered, and evaporated under reduced pressure.
A yellow powdery product was obtained. The crude resi-
due was placed in another two-necked flask (100 mL)
and dissolved in acetic acid (50 mL). A catalytic amount
of aqueous HCl (5 mol%, 12 N) was then added, and the
whole solution was heated under reflux for 12 h. After
cooling to ambient temperature, the compound was puri-
fied as a white powder by silica gel chromatography using
dichloromethane/n-hexane (2/1).
2. Experimental
2.1. Materials and measurements
Tetrakis(triphenylphosphine)palladium(0),
(Aldrich
Chem. Co., St. Louis, MO, USA), 4-(2-naphthyl)phenyl boro-
nic acid, naphthalene-2-boronic acid, and phenylboronic
acid (Frontier Scientific Co., West Logan, UT, USA) were used
as received. 3-Bromo-10-naphthylspiro[benzo[ij]tetraph-
ene-7,90-fluorene] was prepared by a successive Suzuki
reaction using corresponding organic boronic acids and
halides.
1,6-Bis[(p-trimethylsilylphenyl)amino]pyrene
NSBTF: yield 86%. Mp 333 °C. UV–vis (THF): kmax
(LBD; band gap, 2.73 eV; HOMO, 5.47 eV; kmax (absorp-
tion) = 426 nm; kmax (emission) = 460 nm) was adopted as
the deep-blue dopant material. n-Butyllithium (2.5 M solu-
tion in hexane), K2CO3, NaOH and HCl (Duksan Chem. Co.,
Seoul, South Korea) were used without further purification.
Tetrahydrofuran (THF) was purified by distillation over
(absorption) = 362,
379 nm,
kmax
(emission) = 396,
415 nm. 1H NMR (500 MHz, CDCl3, ppm) 8.46 (d, 1H),
8.32 (d, 1H), 8.00 (m, 5H), 7.81 (m, 4H), 7.72 (d, 1H), 7.60
(m, 1H), 7.45 (m, 2H), 7.40 (t, 2H), 7.25 (m, 2H), 7.23 (d,
1H), 7.13 (t, 1H), 7.06 (t, 2H), 6.99 (d, 1H), 6.57 (d, 1H).
13C NMR (CDCl3, ppm) 150.4, 148.1, 147.4, 142.8, 142.2,
DBAF
S
BAF
S
BF
S
BFF
S
Scheme 1. Various spiro molecules: Substituent positions on spiro[benzo[c]fluorene-7,90-fluorene].

2924
M.-J. Kim et al. / Organic Electronics 15 (2014) 2922–2931
140.0, 139.2, 138.4 137.8, 137.2, 136.8, 136.2, 135.2, 134.4,
133.8, 133.2, 130.7, 129.6, 129.2, 128.2, 128.0, 127.8, 127.4,
126.2, 125.8, 125.2, 124.4, 124.0, 123.3, 122.8, 122.4, 120.4,
120.1, 66.5. FT-IR (KBr, cmꢀ1) 3060, 3040, 3018 (aromatic
C–H). Anal. Calcd. for C43H26 (542.67): C, 95.17; H, 4.83.
Found: C, 95.20; H, 4.85. MS (FAB) m/z 543.20 [(M + 1)+].
sequentially onto the substrate at a rate of 1.0 Å/s by ther-
mal evaporation from heated alumina crucibles. The con-
centration of the dopant materials was 5%. The devices
were encapsulated with a glass lid and a CaO getter after
cathode deposition. Current density–voltage 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.
2.3. Representative preparation of 3-(2-naphthyl)-10-
naphthylspiro[benzo[i,j]tetraphene-7,90-fluorene] (N-NSBTF)
3. Results and discussion
A solution of 3-bromo-10-naphthylspiro[benzo[ij]tet-
raphene-7,90-fluorene] (6.22 g, 10 mmol), tetrakis(triphen-
ylphosphine)palladium(0) (0.59 g, 0.51 mmol), and
naphthalene-2-boronic acid (1.72 g, 10 mmol) dissolved
in THF (150 mL) was stirred in a double-necked flask for
30 min. K2CO3 (2 M, 150 mL) was added dropwise over
20 min. The resulting reaction mixture was refluxed over-
night 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 CH2Cl2/n-hexane (1/1) to
give the yellow crystalline product N-NSBTF. NP-NSBTF
and P-NSBTF were prepared using similar procedures
described above.
N-NSBTF: yield 81%. Mp 322 °C. UV–vis (THF): kmax
(absorption) = 370, 387 nm, kmax (emission) = 442 nm. 1H
NMR (500 MHz, CDCl3, ppm) 8.52 (d, 1H), 8.42 (d, 1H),
8.04 (m, 6H), 7.96 (d, 1H), 7.88 (m, 2H), 7.83 (m, 3H),
7.69 (m, 4H), 7.54 (m, 2H), 7.45 (m, 2H), 7.41 (t, 2H),
7.25 (s, 1H), 7.22 (d, 1H), 7.06 (m, 5H), 6.63 (d, 1H). FT-IR
(KBr, cmꢀ1) 3060, 3018 (aromatic C–H). Anal. calcd. for
3.1. Synthesis and characterization
Scheme 2 shows the synthetic route to 3-bromo-
10-naphthylspiro[benzo[ij]tetraphene-7,90-fluorene]. Spe-
cifically, the Pd-catalyzed Suzuki coupling reaction of
3-bromo-10-naphthylspiro[benzo[ij]tetraphene-7,90-fluo-
rene] with naphthalene-2-boronic acid, 4-(2-naph-
thyl)phenyl boronic acid and phenylboronic acid produced
high yields (>87%) of N-NSBTF, NP-NSBTF, and P-NSBTF
(Scheme 3). Bromination of 10-naphthylspiro[benzo[ij]tet-
raphene-7,90-fluorene] with 1.01 equiv. of bromine in car-
bon tetrachloride solution led to the selective, high-yield
formation of 3-bromo-10-naphthylspiro[benzo[ij]tetraph-
ene-7,90-fluorene] (Figs. S1–S6). The chemical structures
and composition of the resulting spiro[benzo[ij]tetraph-
ene-7,90-fluorene] derivatives were characterized via 1H
NMR, FT-IR, gas chromatography–MS, and elemental analy-
sis (Figs. S7–S15).
C
53H32 (668.25): C, 95.18; H, 4.82. Found: C, 95.09; H,
3.2. Thermal properties
4.81. MS (FAB) m/z 669 [(M + 1)+].
NP-NSBTF: yield 78%. Mp 374 °C. UV–vis (THF): kmax
(absorption) = 373 nm, kmax (emission) = 446 nm. 1H NMR
(500 MHz, CDCl3, ppm) 8.52 (d, 1H), 8.42 (d, 1H), 8.15 (s,
1H), 8.03 (m, 5H), 7.95 (m, 2H), 7.78 (m, 7H), 7.78 (d,
1H), 7.74 (d, 1H), 7.57 (m, 3H), 7.51 (m, 2H), 7.46 (m,
2H), 7.42 (t, 2H), 7.21 (d, 1H), 7.13 (m, 3H), 7.07 (m, 2H),
6.63 (d, 1H). FT-IR (KBr, cmꢀ1) 3060, 3018 (aromatic C–
H). anal. calcd. for C59H36 (744.28): C, 95.13; H, 4.87.
Found: C, 95.05; H, 4.85. MS (FAB) m/z 745 [(M + 1)+].
P-NSBTF: yield 84%. Mp 317 °C. UV–vis (THF): kmax
(absorption) = 369, 385 nm, kmax (emission) = 432 nm. 1H
NMR (500 MHz, CDCl3, ppm) 8.50 (d, 1H), 8.39 (d, 1H),
8.04 (m, 5H), 7.82 (m, 3H), 7.74 (m, 1H), 7.67 (m, 1H),
7.59 (d, 1H), 7.53 (d, 2H), 7.51 (m, 2H), 7.44 (m, 2H), 7.41
(m, 3H), 7.21 (d, 1H), 7.10 (m, 3H), 7.04 (d, 2H), 6.61 (d,
1H). FT-IR (KBr, cmꢀ1) 3060, 3038, 3020 (aromatic C–H).
Anal. calcd. for C49H30 (618.23): C, 95.11; H, 4.89. Found:
C, 95.09; H, 4.85. MS (FAB) m/z 619 [(M + 1)+].
The thermal behavior of the NSBTF host materials was
evaluated by means of differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA) under a nitro-
gen atmosphere, and Table 1 summarizes the thermal
analysis data for the three compounds. A 5% weight loss
was observed at 403, 412, and 398 °C, and the melting
points (Tm) of N-NSBTF, NP-NSBTF, and P-NSBTF were
322, 374, and 317 °C, respectively, but no melting points
were observed on the second heating scan, even though
they were given enough time to cool in air. Once the com-
pounds became amorphous solids, they did not revert to
their crystalline state. After the samples had cooled to
room temperature, a second DSC scan showed glass transi-
tion temperatures (Tg) of 179 °C, 186 °C, and 174 °C for N-
NSBTF, NP-NSBTF, and P-NSBTF, respectively, as a result of
the rigid spiro-type backbone. The data indicate that these
three materials are stable enough to endure the high tem-
peratures of vacuum vapor deposition. As a result, the
amorphous glassy state of the transparent films made of
the two host materials indicated that these are good candi-
dates for use as EL materials [28].
2.4. OLED fabrication
A
basic device configuration of indium tin oxide
(150 nm)/N,N0-bis-[4-(di-m-tolylamino)phenyl]-N,N0-diph-
enylbiphenyl-4,40-diamine
(DNTPD, 60 nm)/bis[N-(1-
3.3. Optical properties and energy levels
naphthyl)-N-phenyl]benzidine (NPB, 30 nm)/NSBTF hosts:
LBD (20 nm, 5%)/aluminum tris(8-hydroxyquinoline)(Alq3,
20 nm)/LiF (1 nm)/Al (200 nm) was used for device fabrica-
tion, as shown in Fig. 1. The organic layers were deposited
Fig. 2 shows the UV absorption and PL spectra of N-
NSBTF, NP-NSBTF, and P-NSBTF in solution, as well as in
the form of solid thin films. Specifically, two maxima were

M.-J. Kim et al. / Organic Electronics 15 (2014) 2922–2931
2925
.K(ꢀ#N
#NSꢁ ꢂꢃꢄꢅPOꢆ
05$6( *QUVꢇꢅ.$&
ꢂꢃꢄꢅPOꢈꢅ ꢉꢅꢊꢆ
02$ꢅꢂꢁꢄꢅPOꢆ
&026&ꢅꢂꢋꢄꢅPOꢆ
+61
N
N
N
Al
O
O
N
O
N
N
N
H
3C
H
C
3
H
3C
H
C
3
DNTPD
Alq3
N
N
NPB
Fig. 1. The device configuration and the chemical structure of the materials used in the devices.
observed in the absorption spectra of the material
dissolved in THF. The strong absorption observed at a
wavelength of 370 nm, with a shoulder peak at 387 nm,
was attributed to the tetraphene moiety and the aromatic
substituents in the SBTF core. Upon photoexcitation, the PL
emission of N-NSBTF peaked at approximately 442 nm for
the THF solution and at 449 nm for the solid thin film. NP-
NSBTF had a long conjugation length and showed a red-
shifted absorption and PL spectra. On the other hand, P-
NSBTF had a short conjugation length and showed a blue
shift. The spectral shift in the solid state is a result of the
different dielectric constants of the surrounding medium
[28].
The photophysical properties of these compounds are
summarized in Table 1 where it can be seen that all com-
pounds had high PL QE values (N-NSBTF, 0.74; NP-NSBTF,
0.83; and P-NSBTF, 0.71). A more accurate QE comparison
was possible for the three host materials because they had
similar UV absorption and PL spectra and NP-NSBTF
(naphthylphenyl) was found to have a higher QE than N-
NSBTF (naphthyl) and P-NSBTF (phenyl).
Molecular simulation of N-NSBTF, NP-NSBTF, and P-
NSBTF was carried out to understand their physical prop-
erties at the molecular level. Fig. 3 shows the geometric
structure of the three host materials. Rotation of the aro-
matic substituents in NSBTF was limited due to steric hin-
drance between SBTF and the aromatic moiety. This led to
distortion (angle, 30–35°) of the aromatic substituents, for
which the conjugation can be destroyed by the SBTF group
in N-NSBTF, NP-NSBTF, and P-NSBTF [29,30].
Molecular simulation also showed the electron distri-
bution of N-NSBTF, NP-NSBTF, and P-NSBTF. Fig. 3 shows
the distribution of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO) of N-NSBTF, NP-NSBTF, and P-NSBTF. The HOMO
and LUMO orbitals of the three host materials were con-
centrated in the tetraphene core, and the fluorene group
only played an accessory role. The substituents, such as

2926
M.-J. Kim et al. / Organic Electronics 15 (2014) 2922–2931
a
r
B
b
r
B
r
B
H
O
Tf
O
c
e
d
(
B
)
H
O
2
H
O
r
B
f
r
B
g
c, Triethyl borate; d, 1,8-Dibromnaphthalene;
a, Triflic anhydride; b, Naphthalene 2-boronic acid;
r
B
/
CCl
2
e, Fluorenone
; f Cl
H
,
; ,
g
4
Scheme 2. Synthetic schemes for the preparation of spiro[benzo[ij]tetraphene-7,90-fluorene] core.
naphthyl, naphthylphenyl, and phenyl groups on both
sides of the spiro[benzotetraphene-fluorene], apparently
affected the HOMO and LUMO distribution in the three
host materials.
3.4. EL properties
Fig. 5 shows the EL spectra of the 5% LBD doped Alq3
devices acting as electron-transporting materials. To eluci-
date the effective energy transfer from the NSBTF host
materials to the LBD dopant, we fabricated a device with
the following multilayer structure: ITO/DNTPD (60 nm)/
NPB (20 nm)/NSBTF:LBD 5 wt% (20 nm)/Alq3 (20 nm)/LiF/
Al where we used DNTPD as the hole-injection layer, NPB
as the hole-transport layer, NSBTF derivatives as the fluo-
rescent blue host, LBD as the fluorescent blue dopant,
Alq3 as the electron-transport layers (ETL), and LiF as the
electron-injection layer at the LiF/Al cathode interface.
The peaks in blue-light emission were observed at a wave-
length of 468 nm, which is similar to that of the PL spec-
trum of NSBTFs, except for a blue shift in the maximum
wavelength. The experimental results suggest a similarity
in the maximum EL spectra of the three hosts, but these
were red-shifted according to the conjugation length
(phenyl < naphthyl < naphthylphenyl), as summarized in
The energy levels of the three host materials used in this
study to fabricate the OLEDs are shown in Fig. 4. A low-
energy photoelectron spectrometer was used to obtain
information on the HOMO energies of the host and the dop-
ant materials and to examine the charge injection barriers.
The energy gaps for N-NSBTF, NP-NSBTF, and P-NSBTF
were calculated to be 3.00, 2.98, and 3.03 eV, respectively,
which were associated with the substituents at the wings
[29]. The HOMO energy levels were ꢀ5.82, ꢀ5.84 and
ꢀ5.85 eV for N-NSBTF, NP-NSBTF, and P-NSBTF, respec-
tively, and the HOMO value of dopant LBD (band gap,
2.73 eV; HOMO, ꢀ5.47 eV) was quite high compared to
those of the hosts. The HOMO values of the three hosts were
similar to those of anthracene derivatives (ꢀ5.5 to ꢀ6.0 eV),
such as 2-methyl-9,10-bis(naphthalen-2-yl)anthracene
(MADN) and 9,10-di(naphth-2-yl)anthracene (b-ADN).
r
B
r
A
at
.
P
d
C
r-
+ A B( H)
O
2
Ar
:
-
N N BTF
-
NP N BTF
S
S
-
P N BTF
S
Scheme 3. Preparation of spiro[benzo[ij]tetraphene-7,90-fluorene] host materials. S.K. Kim, B. Yang, Y. Ma, J.H. Lee, J.W. Park, Exceedingly efficient deep-
blue electroluminescence from new anthracenes obtained using rational molecular design. J. Mater. Chem. 18 (2008) 3376–3384.

M.-J. Kim et al. / Organic Electronics 15 (2014) 2922–2931
2927
Table 1
UV absorption, PL, energy levels and thermal properties of NSBTF host materials.
Sample
Properties
NSBTF
N-NSBTF
NP-NSBTF
P-NSBTF
Puritya
HPLC
(%)
99.9
99.9
99.9
99.9
Thermal analysis
DSC
Tg (°C)
Tm (°C)
145
333
362, 379
3.15
179
322
370, 387
3.00
186
374
373
2.98
174
317
369, 385
3.03
UV (THF)
Max (nm)
Bgb (eV)
Optical analysis
PL (THF)
Max (nm)
FWHMc (nm)
Max (nm)
FWHMc (nm)
Uf
396, 415
442
43
449
53
446
61
452
67
432
41
444
49
41
428
48
–
PL (solid)
QEd
0.74
0.83
0.71
Electrical analysis
AC-2
HOMO (eV)
LUMO (eV)
5.88
2.73
5.82
2.82
5.84
2.86
5.85
2.82
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 cyclohexane (Uf = 0.90).
c
d
N-NSBTF
NP-NSBTF
P-NSBTF
1.2
449 nm
444 nm
N-NSBTF
NP-NSBTF
P-NSBTF
452 nm
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 2. UV–vis and PL spectra of spiro[benzo[ij]tetraphene-7,90-fluorene] derivatives; inset figure shows PL spectra in thin film.
Table 2. This seemed to indicate that the energy transfer
from the host to the LBD dopant was quite efficient at
the optimum dopant concentration employed in this
experiment. A relatively narrow full width half maximum
of 34 nm was observed for the b-ADN device with 5% dop-
ant. The EL spectra and chromaticity of our devices, as
depicted according to CIE coordination–luminance, were
stable with an increasing luminance from 100 to 2000 cd/
m2. Little or no change was detected for the different
brightnesses in this range.
sity characteristics of the OLEDs with an EML composed
of the four hosts with 5% LBD dopant. N-NSBTF and NP-
NSBAF showed a higher current density and luminance
than P-NSBAF and anthracene-type b-ADN did at the same
driving voltage. The decrease in the current density and
luminance of the P-NSBTF device originated from its wide
band gap. The P-NSBTF band gap was 3.03 eV, compared to
the 3.00 eV and 2.98 eV of N-NSBTF and NP-NSBAF,
respectively. The slightly deep HOMO level of the P-NSBTF
hinders hole injection from the NPB hole transport layer to
the emitting layer due to the large energy barrier for hole
injection, leading to low current density and low lumi-
nance [30].
3.5. OLED device properties
N-NSBTF, NP-NSBTF, P-NSBTF, and commercial b-ADN
were evaluated for use as host materials in blue fluorescent
OLEDs. Fig. 6 shows the luminance–voltage–current den-
As shown in Fig. 7, the device with b-ADN with 5% dopant
showed a low efficiency. In the case of the N-NSBTF device
doped with 5% LBD, a maximum brightness of 2205 cd/m2

2928
M.-J. Kim et al. / Organic Electronics 15 (2014) 2922–2931
Fig. 3. HOMO and LUMO electronic density distributions of N-NSBTF derivatives, calculated at the DFT/B3LYP/6-31G* for optimization and time dependent
DFT (TDDFT) using Gaussian 03.
2.0
2.4
Unit:eV
2.73
2.82
2.82
3.2
2.86
2.95
DNTPD
5.1
NPB
5.4
LBD
Alq3
5.47
NP-NSBTF
N-NSBTF
P-NSBTF
β
-ADN
5.7
5.82
5.84
5.85
5.95
Fig. 4. Energy diagram of the blue fluorescence devices using NSBTF host materials.
and 5.92 cd/A occurred at 6 V, as seen in Table 2, and a quan-
The holes injected from a hole transfer layer, a-NPB,
tum efficiency of 5.19% is maintained at a current density of
were transferred to the light emitting layer and were
trapped at the dopant sites. The LBD dopant had a large
capacity to catch holes, and it minimized the loss of holes,
resulting in good EL efficiency. Thus, the HOMO level of
LBD as a dopant was suitable for hole trapping and hole
transport, and the N-NSBTF and NP-NSBAF host materials
were moderate for balancing holes and electrons in the
emitting layer.
The maximum external quantum efficiency (EQE) of the
device obtained from the N-NSBTF device doped with 5%
LBD was of 5.19% as shown in Fig. 8. The luminance effi-
ciency increased rapidly to a maximum of approximately
5.92 cd/A at a low current density of 43.2 mA/cm2 at 6 V.
up to 43.2 mA/cm2, as shown in Fig. 8.
In the device, the HOMO level (5.70 eV) of Alq3 was
higher than that of b-ADN (5.95 eV). This difference
explains why electron–hole recombination may be unbal-
anced in the emitting zone of the device. The HOMO level
(5.82–5.85 eV) of the three NSBTFs was slightly higher
than that of b-ADN. This recombination with a higher
charge also shifted the blue light-emitting zone to a region
close to the ETL (Alq3) and, therefore, facilitates electron–
hole recombination in the 5% doped emitting zone. Conse-
quently, the EL performance of NSBTFs was superior to
that of b-ADN.

M.-J. Kim et al. / Organic Electronics 15 (2014) 2922–2931
2929
W, and CIE coordinates of 0.134 and 0.172 at 6 V. These
results suggest that an exciton was formed and that light
was emitted at specific thresholds. It should be noted that
the efficiencies of these devices showed a small decrease
when the luminance increased to 2018 cd/m2.
For the fluorescent host-dopant system, the probability
for energy transfer from the excited energy donor to the
energy acceptor depends on the overlap of the emission
spectrum of the donor with the absorption spectrum of
the acceptor. This is evident from the spectral overlap
between the PL of NSBTF and the UV absorption of LBD
[32–36], as shown in Fig. 9.
A combination of good efficiency and color purity
enabled the non anthracene-type fused ring spirotetraph-
ene derivatives to be good candidates for use a blue emit-
ters in OLEDs.
The life-time tests were performed at a initial lumi-
nance of 5000 cd/m2. The life-times of the three new host
materials were 139.2 h for N-NSBTF, 130.6 h for NP-
NSBTF, and 119.3 h for P-NSBTF. The life-times of N-NSBTF
(139 h) and NP-NSBTF (130 h) were more than those of b-
ADN:5% LBD (130–135 h), which can be explained to be a
result of a more stable morphology of the NSBTF host
materials.
ββ-ADN(0.142, 0.160)
N-NSBTF(0.130, 0.155)
NP-NSBTF(0.134, 0.172)
1.0
0.8
0.6
0.4
0.2
0.0
(0.130, 0.149)
P-NSBTF
400
450
500
550
600
Wavelength (nm)
Fig. 5. Electroluminescence spectra of the devices prepared from NSBTF
host materials.
NP-NSBTF doped with 5% LBD showed a small decrease
in efficiency as the current density increased from 0 to
a weak-current-induced fluorescent
quenching [31]. The sample reached a current efficiency
of 5.84 cd/A, EQE of 5.16%, power efficiency of 4.98 lum/
44.6 mA/cm2, i.e.,
Table 2
Electroluminescence properties of the devices obtained from NSBTF host materials.
Properties
Host
N-NSBTF
NP-NSBTF
P-NSBTF
b-ADN
Dopant
(%)
LBD
5%
EL at 6 V
kmax (nm)
FWHM (nm)
mA/cm2
cd/Aa
468
40
43.2
5.92
6.24 (5.5 V)
2205
0.130
0.155
5.19
467
42
44.6
5.84
6.12 (5.0 V)
2018
0.134
0.172
5.16
469
38
18.7
6.25
6.32 (5.5 V)
1152
0.130
0.149
5.24
466
34
41.8
3.36
3.63 (7.5 V)
1928
0.142
0.160
2.87
cd/Ab
cd/m2
CIE-x
CIE-y
EQE (%)a
lum/W
5.02
4.98
5.07
2.14
a
Values at 6 V.
Values at a highest peak.
b
1000
10000
β
-ADN
900
800
700
600
500
400
300
200
100
0
N-NSBTF
NP-NSBTF
P-NSBTF
β-ADN
N-NSBTF
NP-NSBTF
P-NSBTF
1000
100
0
2
4
6
8
10
12
Voltage (V)
Fig. 6. Current density–voltage–luminescence characteristics of the devices using NSBTF host materials doped with 5% LBD.

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M.-J. Kim et al. / Organic Electronics 15 (2014) 2922–2931
8
7
6
5
4
3
2
1
0
4. Conclusion
β
-ADN (0.142, 0.160)
N-NSBTF (0.130, 0.155)
NP-NSBTF (0.134, 0.172)
P-NSBTF (0.130, 0.149)
In summary, we synthesized new spiro[benzo[ij]tet-
raphene-7,90-fluorene]-based host materials and used
them to fabricate efficient deep-blue OLEDs. The NSBTF
derivatives consisted of a non anthracene-type fused-ring
SBTF core as well as aromatic substituents at both sides
of the SBTF. The device with the optimum structure –
ITO/DNTPD (60 nm)/NPB (20 nm)/NSBTF:LBD 5 wt%
(20 nm)/Alq3 (20 nm)/LiF (0.5 nm)/Al (100 nm) – was char-
acterized by a high blue EL performance with a current
efficiency of 6.25 cd/A, a power efficiency of 5.17 lm/W,
and an external quantum efficiency of 5.54% at 18.7 mA/
cm2 with CIE coordinates of 0.130, 0.149. These results
0
1
2
3
4
5
6
7
8
9
show that non anthracene-type
aromatic SBTF with aromatic wings can be used as efficient
p-conjugated fused-ring
Voltage (V)
hosts for deep-blue OLEDs.
Fig. 7. Current efficiency–voltage of the devices using NSBTF host
materials doped with 5% LBD.
Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2014.08.030.
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