Orange phosphorescent organic light-emitting diodes using new spiro[benzoanthracene-fluorene]-type host materials

Article abstract of DOI:10.1016/j.dyepig.2012.01.009

New spirobenzoanthrcene-type orange phosphorescent host materials, 9-(spiro[benzoanthracene-7,9′-fluorene]-2′-yl)-9H-carbazole and 9-(spiro[benzoanthracene-7,9′-fluorene]-3-yl)-9H-carbazole, with stable efficiency roll-off, were prepared by an Ullmann cou

Full text of DOI:10.1016/j.dyepig.2012.01.009

Dyes and Pigments 94 (2012) 304e313
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Dyes and Pigments
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Orange phosphorescent organic light-emitting diodes using new spiro
[benzoanthracene-fluorene]-type host materials
,
Ji-Young Kim a, Chil-Won Lee c, Jee Geun Jang 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 Electronics Engineering, Dankook University, Cheonan 330-714, Republic of Korea
c 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
New spirobenzoanthrcene-type orange phosphorescent host materials, 9-(spiro[benzoanthracene-7,90-flu-
orene]-20-yl)-9H-carbazole and 9-(spiro[benzoanthracene-7,90-fluorene]-3-yl)-9H-carbazole, with stable
efficiency roll-off, were prepared by an Ullmann coupling reaction of 20-bromo-spiro[benzoanthracene-7,90-
fluorene] and 3-bromo-spiro[benzoanthracene-7,90-fluorene] respectively, with carbazole. Orange phos-
phorescent organic light-emitting diodes with the configuration of indium tin oxide/N,N’-diphenyl-N,N’-
bis-[4-(phenyl-m-tolylamino)-phenyl]-biphenyl-4,40-diamine/N,N’-di(1-naphthyl)-N,N’-diphenylbenzidine/
Host:Ir(pq)2acac/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline/tris(8-hydroxyquinoline)aluminum/LiF/Al
were developed using spiro[benzoanthracene-fluorene]-type carbazole derivatives as a host material and
Ir(pq)2acac as a dopant. A device obtained from 9-(spiro[benzoanthracene-7,90-fluorene]-20-yl)-9H-carba-
zole doped with 3% Ir(pq)2acac showed an orange color purity of (0.599, 0.373) and an efficiency of 14.74 cd/A
at 6.0 V. The overall result is a devicewith an external quantum efficiency (EQE) >8.93% at high brightness but
an operating voltage of <6.0 V.
Article history:
Received 1 December 2011
Received in revised form
11 January 2012
Accepted 15 January 2012
Available online 27 January 2012
Keywords:
PHOLED
Spiro[benzoanthracene-fluorene]
Carbazole
Orange
Host
Phosphorescence
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
device due to the broad recombination zone, resulting in reduced
probability of exciton quenching. However, the mixed host device is
Phosphorescent organic light-emitting diodes (PHOLEDs) have
become increasingly important in recent years due to their higher
triplet emission efficiency compared with fluorescent OLEDs. An
increase in internal efficiency to as high as 100% can be theoreti-
cally achieved due to triplet emission, which is particularly
important for practical applications in which the device is expected
to operate at a low voltage and consume less power [1e5].
However, the quantum efficiency of a PHOLED is considerably
reduced at high current density or luminance due to triplet-triplet
annihilation [6], triplet-exciton quenching [7], and triplet-polaron
quenching [7]. Triplet exciton quenching is caused by the long
excited state lifetime of the triplet exciton. Several studies have
attempted to minimize the decrease in efficiency at high current
density [8e12]. Triplet exciton quenching in the emitting layer is
reduced by broadening the recombination zone in the emitting
layer [8]. The recombination zone can be controlled using a mixed
host system with a broad recombination zone and shows little
efficiency roll-off. The exciton density decreases in a mixed host
complicated. Therefore, development of a host material that can
minimize the efficiency of roll-off is needed.
Carbazole derivatives are photoconductors with good hole
transport properties [13e16]. The effectiveness of carbazole-based
host materials for use in deep blue PHOLEDs due to the high triplet
energy of the carbazole unit has recently been demonstrated
[17,18]. Spirobifluorene and spirosilabifluorene derivatives have
been used widely as new amorphous host and dopant materials,
respectively, possessing high morphological stability [19e25].
Spirobifluorene, with a high glass transition temperature, shows
excellent nondispersive hole transporting and ambipolar carrier
transporting properties [26,27]. Asymmetrical spiro compounds
possessing naphthalene groups not only preserve their inherent
characteristics, such as morphological stability, high glass transi-
tion temperature, and amorphous properties, but also provide
a variety of substituents on the aromatic ring, resulting in the
formation of conjugation-controlled OLED host and dopant mate-
rials; therefore, spirobenzofluorenes have recently received
a great deal of attention as fluorescent materials for OLEDs [28e39].
Spirobenzofluorene/carbazole hybrid material contains a carbazole
unit with hole transport properties and a spirobenzofluorene unit
with electron transport properties [36]. Therefore, the combination
* 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.
doi:10.1016/j.dyepig.2012.01.009

J.-Y. Kim et al. / Dyes and Pigments 94 (2012) 304e313
305
of carbazole and spirobenzoanthracene is effective to develop
2.3. Preparation of spiro[benzo[de]anthracene-7,90-fluorene]
a host material with balanced hole and electron injection.
In this study, new carbazole-based spirobenzoanthracene
host materials, 9-(spiro[benzoanthracene-7,90-fluorene]-20-yl)-
9H-carbazole (SBAF-2C) and 9-(spiro[benzoanthracene-7,90-fluo-
rene]-3-yl)-9H-carbazole (SBAF-3C), were prepared by intro-
ducing the carbazole moiety at the 20- and 3-positions on SBAF,
and their use as orange phosphorescent host materials was
evaluated. The device performances of the orange PHOLEDs with
the SBAF-2C and SBAF-3C hosts were investigated based on the
red dopant concentration.
(SBAF)
A
solution of 1-bromo-8-phenylnaphthalene (8.67 g,
30.6 mmol) in THF (50 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 pres-
sure. A yellow powdery product was obtained. The crude residue
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
purified as a white powder by silica gel chromatography using
dichloromethane/n-hexane (2/1).
2. Experimental
2.1. Chemicals and instruments
Tetrakis(triphenylphosphine)palladium(0), phenylboronic acid,
9-fluorenone, n-butyllithium, carbazole (95%), potassium
carbonate (99%), copper powder (99%), dichloromethane, 1,8-
diboromonaphthalene, and nitrobenzene (Aldrich Chem. Co.,
St. Louis, MO, USA) were used without further purification.
Ammonia water (Duksan Chem. Co., Seoul, South Korea) was used
as received. Tetrahydrofuran was distilled over sodium and calcium
hydride.
Photoluminescence (PL) spectra were recorded on a fluores-
cence spectrophotometer (Jasco FP-6500) and UV-vis spectra
were obtained by a UV-vis spectrophotometer (Shimadzu, UV-
1601PC). A low-energy photo electron spectrometer (Hitachi
High Tech, AC-2) was used to measure energy levels. Fourier
transform-infrared (FT-IR) spectra were obtained with a Varian
model 640-IR spectrophotometer, and elemental analyses were
performed using a Thermo Fisher Flash 2000 elemental analyzer.
Differential scanning calorimeter (DSC) measurements were
performed on a Shimadzu DSC-60 instrument under nitrogen
at a heating rate of 10ꢀC/min. Thermo gravimetric measure-
ments were performed on a Shimadzu TGA-50 analyzer at
a heating rate of 10ꢀC/min. Low and high resolution mass spectra
were recorded using an HP 6890 and Agilent 5975C MSD in FAB
mode.
Yield 87%. Mp 209ꢀC. FT-IR (KBr, cmꢁ1) 3056, 3033 (aromatic C-
H). 1H-NMR (500 MHz, CDCl3)
d 8.27e8.26(d, 1H, Ar-CH-
naphthalene), 8.22e8.20(d, 1H, Ar-CH-naphthalene), 7.84e7.82(m,
3H, Ar-CH-benzene), 7.68e7.67(d, 1H, Ar-CH-benzene), 7.64e7.61(t,
1H, Ar-CH-naphthalene), 7.37e7.33(t, 2H, Ar-CH-benzene),
7.30e7.26(t, 1H , Ar-CH-naphthalene), 7.20e7.17(t, 1H, Ar-CH-
benzene), 7.14e7.11(t, 2H, Ar-CH-fluorene), 6.99e6.96(t, 3H, Ar-CH-
benzene), 6.62e6.60(d, 1H, Ar-CH-fluorene), 6.53e6.51(d, 1H, Ar-
CH-fluorene). 13C NMR (CDCl3)
d 156.9, 140.0, 139.0, 137.5, 134.0,
131.9, 130.1, 128.6, 128.6, 128.3, 127.7, 127.7, 127.7, 126.7, 126.6, 126.3,
125.6, 125.2, 123.5, 120.2, 119.5, 77.4, 77.2, 76.9, 60.0. Anal. Calcd for
C19H18 (Mw, 366.14): C, 95.05;H, 4.95. Found: C, 95.01; H, 4.92. MS
(FAB) m/z 367.14[(M þ 1)þ].
2.4. Preparation of 3-bromo spiro[benzo[de]anthracene-7,90-
fluorene]
SBAF (3.66 g, 10 mmol) was dissolved in carbon tetrachloride in
a two-necked flask; bromine (2.36 g, 15 mmol) was then added
slowly in a dropwise fashion over a period of 20 min. The mixture
was stirred at room temperature for 3 days. The precipitated solid
was filtered and dried in vacuo to give the crude product, which was
purified by recrystallization from ethyl acetate/n-hexane (1/1) to
give a white powder.
2.2. Preparation of 20-bromospiro[benzo[de]anthracene-7,90-
fluorene] (20-bromo-SBAF)
1,8-Diboromonaphthalene (10.00 g, 34.95 mmol), phenyl-
boronic acid (4.26 g, 34.95 mmol), tetrakis(triphenylphosphine)
palladium(0) (1.7 g, 1.46 mmol), and THF (100 mL) were stirred
in a two-necked flask for 30 min. A degassed H2O solution of
potassium carbonate (3.26 g, 29.1 mmol) was added dropwise to
the above solution over a period of 20 min. The resulting solu-
tion was heated under reflux overnight at 80ꢀC. The reaction
mixture was extracted with dichloromethane and water.
Following evaporation of the organic layer with a rotary evap-
orator, the resulting powdery product was purified by column
chromatography using n-hexane to give a white crystalline solid.
Yield 73%. Mp 153ꢀC. FT-IR (KBr, cmꢁ1) 3054 (aromatic C-H),
Yield 79%. Mp 256ꢀC. FT-IR (KBr, cmꢁ1) 3053, 3037(aromatic C-
H), 748(aromatic C-Br). 1H-NMR (500 MHz, CDCl3)
d 8.17e8.16 (d,
1H, Ar-CH-naphthalene), 8.11e8.09 (d, 2H, Ar-CH-benzene),
7.94e7.92 (d, 1H, Ar-CH-fluorene), 7.84e7.82 (d, 2H, Ar-CH-
benzene), 7.37e7.34 (t, 2H, Ar-CH-benzene), 7.31e7.26 (m, 2H, Ar-
CH-benzene), 7.14e7.11 (t, 2H, Ar-CH-benzene) 7.01e6.98 (t, 1H,
Ar-CH-fluorene) , 6.94e6.93 (d, 2H, Ar-CH-benzene), 6.69e6.67 (d,
1H, Ar-CH-fluorene), 6.53e6.51 (d, 1H, Ar-CH-fluorene). 13C NMR
(CDCl3)
d 156.5, 140.0, 138.9, 138.1, 132.4, 131.3, 130.5, 130.2, 128.9,
128.7, 128.6, 128.6, 127.9, 127.5, 126.2, 126.1, 125.5, 123.4, 122.8,
120.3, 120.0, 77.4, 77.2, 76.9, 66.5, 59.9. Anal. Calcd for C29H17Br
(Mw, 445.35): C, 78.21;H, 3.85;Br, 17.94. Found: C, 78.16; H, 3.81. MS
(FAB) m/z 445.05 [(M þ 1)þ].
652 (aromatic C-Br). 1H-NMR (500 MHz, CDCl3)
d 7.85e7.83(d,
2H, Ar-CH-naphthalene), 7.76e7.75(d, 1H, Ar-CH-naphthalene),
7.48e7.45(t, 1H, Ar-CH-naphthalene), 7.42e7.40(d, 1H, Ar-CH-
naphthalene), 7.38e7.35(m, 3H, Ar-CH-benzene), 7.33e7.31(m,
2H, Ar-CH-benzene), 7.26e7.23(t, 1H, Ar-CH-benzene). 13C NMR
2.5. Synthesis of 9-(spiro[benzo[de]anthracene-7,90-fluorene]-20-
yl)-9H-carbazole (SBAF-2C)
(CDCl3)
d 143.0, 140.5, 136.2, 133.9, 131.3, 130.4, 129.8, 129.0,
129.0, 128.4, 127.5, 127.1, 126.2, 125.4, 120.3, 77.4, 77.2, 76.9, 59.9.
Anal. Calcd for C29H17Br (Mw, 445.35): C, 78.21;H, 3.85;Br, 17.94.
Found: C, 78.18; H, 3.82. MS (FAB) m/z 445.05 [(M þ 1)þ].
A mixture of 20-bromo-SBAF (5.00 g, 11.23 mmol), carbazole
(2.00 g, 12.35 mmol), potassium carbonate (3.1 g, 22.45 mmol), and
copper bronze (0.71 g, 11.23 mmol) in nitrobenzene (80 mL) was

306
J.-Y. Kim et al. / Dyes and Pigments 94 (2012) 304e313
Fig. 1. The device configuration and the chemical structure of the materials used in the devices.
heated under reflux with vigorous stirring under a nitrogen
atmosphere for 18 h. After removing the solvent in vacuo,
ammonia solution (80 mL) was added, and the mixture was left to
stand for 2 h. Dichloromethane (150 mL) and distilled water
(100 mL) were then added, and the organic layer was separated.
After evaporation of the organic layer with a rotary evaporator,
the resulting powdery product was purified by column chroma-
tography using a mixture of dichloromethane/n-hexane (v/v, 1/3)
as an eluent to give a white crystalline SBAF-2C.
Ar-CH-carbazole). 13C NMR (CDCl3) 152.0, 148.2, 147.5, 142.3, 142.1,
140.8,136.4,135.9,134.3,129.6,129.4,128.2,128.1,127.3,126.4,126.0,
125.9,124.1,124.0,123.8,123.5,122.7,122.2,120.4,120.4,120.0,109.9,
77.4, 77.2, 76.9, 60.1. Anal. Calcd for C41H25N (Mw, 531.64): C,
92.63; H, 4.74; N, 2.63. Found: C, 92.60; H, 4.69; N, 2.68. MS (FAB) m/z
532.6 [(Mþ1)þ]. UVevis (THF): lmax (Absorption) ¼ 345 nm, lmax
(Emission) ¼ 395 nm.
2.6. Synthesis of 9-(spiro[benzo[de]anthracene-7,90-fluorene]-3-
yl)-9H-carbazole (SBAF-3C)
Yield 72%. Mp 261ꢀC. FT-IR (KBr, cmꢁ1) 3052, 3037 (aromatic
C-H),1268 (aromatic C-N).1H-NMR (500 MHz,CDCl3)
d 8.88e8.87 (d,
1H, Ar-CH-naphthalene), 8.62e8.61 (d, 1H, Ar-CH-naphthalene),
8.05e8.04 (d, 2H, Ar-CH-naphthalene), 7.94e7.92 (d, 1H, Ar-CH-
carbazole), 7.82e7.81 (d, 2H, Ar-CH-carbazole), 7.75e7.74 (t, 1H,
Ar-CH-carbazole), 7.67e7.66 (d, 2H, Ar-CH-benzene), 7.60e7.58
(t,1H, Ar-CH-fluorene), 7.37e7.34 (t, 2H, Ar-CH-fluorene), 7.34e7.33
(t, 1H, Ar-CH-fluorene), 7.33e7.32 (d, 1H, Ar-CH-carbazole),
7.28e7.22 (m, 1H, Ar-CH-naphthalene), 7.20e7.18 (m, 3H, Ar-CH-
benzene), 7.18e7.14 (t, 2H, Ar-CH-benzene), 7.11e6.99 (s, 1H, Ar-
CH-benzene), 6.87e6.84 (d, 2H, Ar-CH-benzene), 6.84e6.82 (d, 1H,
A mixture of 3-bromo-SBAF (6.00 g, 13.47 mmol), carbazole
(2.47 g, 14.82 mmol), potassium carbonate (3.72 g, 26.95 mmol),
and copper bronze (0.85 g, 13.47 mmol) in nitrobenzene (70 mL)
was stirred and heated under reflux in a nitrogen atmosphere for
18 h. Following removal of the solvent in vacuo, ammonia solution
(70 mL) was added, and the mixture was left to stand for 2 h.
Dichloromethane (150 mL) and water (100 mL) were added, and
the organic phase was separated. The organic solution was washed
twice with water (100 mL). Following evaporation of the resulting

J.-Y. Kim et al. / Dyes and Pigments 94 (2012) 304e313
307
solution with a rotary evaporator, the resulting powdery product
was purified by column chromatography using a mixture of
dichloromethane/hexane (v/v, 1/3) as an eluent to give SBAF-3C as
a white solid.
nyl]-biphenyl-4,40-diamine (DNTPD, 60 nm)/N,N’-di(1-naphthyl)-
N,N’-diphenylbenzidine (NPB, 30 nm)/SBAF-2C or SBAF-3C: iri-
dium(III) bis(2-phenylquinoline)acetylacetonate (Ir(pq)2acac)
(30 nm, x %)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP,
Yield 62%. Mp 371ꢀC. FT-IR (KBr, cmꢁ1) 3054, 3039 (aromatic C-
5
nm)/tris(8-hydroxyquinoline)aluminum (Alq3, 20 nm)/LiF
H), 1268 (aromatic C-N). 1H-NMR (500 MHz, CDCl3)
d
9.02e9.00 (d,
(1 nm)/Al (200 nm) (Fig. 1), in which SBAF-2C or SBAF-3C acted as
the phosphorescent host emitter and Ir(pq)2acac acted as the
dopant material, Alq3 as the electron-transporting layer, and BCP
as the hole/exciton blocking layer. All organic materials, except the
dopants, were deposited at a rate of 1 Å/s. Various doping
concentrations of Ir(pq)2acac were used such as. 1, 3, 5, 7, and 10%.
The devices were encapsulated with a glass lid and a CaO getter
after cathode deposition. A Keithley 2400 source measurement
unit and a CS 1000 spectroradiometer were used to measure
current density-voltage-luminance and electroluminescence
characteristics of the blue fluorescent OLEDs, respectively.
1H, Ar-CH-benzene), 8.54e8.52 (d, 1H, Ar-CH-carbazole), 8.11e8.10
(d, 1H, Ar-CH-naphthalene), 8.10e8.09 (d, 1H, Ar-CH-carbazole),
7.78e7.77 (d, 1H, Ar-CH-benzene), 7.76e7.75 (d, 2H, Ar-CH-
carbazole), 7.52e7.51 (t, 1H, Ar-CH-benzene), 7.37e7.36 (t, 1H, Ar-
CH-carbazole), 7.35e7.34 (d, 3H, Ar-CH-benzene), 7.24e7.23 (t, 1H,
Ar-CH-carbazole), 7.23e7.22 (t, 1H, Ar-CH-carbazole), 7.20e7.19 (m,
3H, Ar-CH-fluorene), 7.13e7.12 (d, 2H, Ar-CH-benzene), 7.12e7.11 (s,
1H, Ar-CH-fluorene), 6.96e6.87 (d, 1H, Ar-CH-benzene), 6.83e6.81
(m, 3H, Ar-CH-benzene), 6.84e6.82 (d, 1H, Ar-CH-carbazole), 13C
NMR (CDCl3)
d 150.2, 148.1, 147.5, 142.4, 142.3, 142.1, 137.4, 134.2,
131.3, 130.8, 128.2, 128.1, 127.7, 126.6, 126.0, 125.1, 124.6, 124.4, 124.0,
123.4,123.3,123.2,120.4,120.4,119.9,110.3, 77.4, 77.2, 76.9, 60.1. Anal.
Calcd for C41H25N (Mw, 531.64): C, 92.63; H, 4.74; N, 2.63. Found: C,
92.62; H, 4.70; N, 2.70. MS (FAB) m/z 532.6 [(Mþ1)þ]. UVevis (THF):
lmax (Absorption)¼ 337 nm, lmax (Emission)¼ 411 nm.
3. Results and discussion
3.1. Synthesis and characterization
Synthetic routes to SBAF-2C and SBAF-3C are shown in
Scheme 1. As the benzoanthrcene was composed of phenyl and
naphthalene moieties, it was easy to prepare 3-bromo-SBAF by
direct bromination reaction of SBAF with bromine. But it is
impossible to brominate at 20-position, because the rate of
2.7. PHOLED fabrication
PHOLEDs were fabricated with
a configuration of (ITO,
150 nm)/N,N’-diphenyl-N,N’-bis-[4-(phenyl-m-tolyl-amino)-phe-
Scheme 1. Synthesis of spirobenzoanthracene precursors for orange phosphorescent host materials.

308
J.-Y. Kim et al. / Dyes and Pigments 94 (2012) 304e313
substitution reaction of bromine on the naphthalene ring was faster
than other phenyl ring [39]. Thus, the multi-step synthesis was
indispensable to prepare 20-bromo-SBAF. Two bromo compounds
were characterized and confirmed by distinctive isotope peak of
mass spectrometry. SBAF-2C was prepared through the 1) the
Suzuki reaction of 1,8-dibromonaphthalene with phenylboronic
acid, 2) the coupling reaction of 8-phenyl-1-lithio-naphthalene and
2-bromofluorenone, 3) the cyclization reaction followed by an
amination reaction of naphthalene] with carbazole in the presence
of a copper bronze catalyst. SBAF-3C was synthesized by proce-
dures similar to those used to synthesize SBAF-2C, as shown in
Schemes 1 and 2. The synthetic yields of SBAF-2C and SBAF-3C
were 62% and 72%, respectively. Purity of SBAF-2C and SBAF-3C
was >99.5% after purification by column chromatography and train
sublimation. The two host materials were completely soluble in
common organic solvents, which makes them eligible for solution
phase characterization. Chemical structures and compositions of
the resulting precursors and SBAFs were characterized by 1HNMR,
13C-NMR, FT-IR spectriscopy, elemental analysis and gas
chromatography-mass spectrometry. The chiral carbon peak was
observed at 60.0 ppm in the 13C NMR spectra. Results of elemental
analysis and mass spectroscopy also supported formation of SBAF-
2C and SBAF-3C and matched well with the calculated data.
3.2. Optical properties and energy levels
UV-vis absorption and PL spectra of the SBAF carbazole deriv-
atives were measured in THF solution (10ꢁ5 mol/L) and in the solid
film state (Fig. 2 and Table 1). SBAF-2C and SBAF-3C showed spiro
core absorption peaks at 328 nm and 340 nm, respectively, and the
carbazole unit at 291 nm. Solid-state absorption spectra of SBAF-2C
and SBAF-3C differed from those of their solutions. Absorption
maxima of SBAF-2C and SBAF-3C films appeared at 335 and
343 nm and were red-shifted by 7 and 3 nm, respectively. PL
maxima of SBAF-2C and SBAF-3C were 391 and 416 nm in solution,
respectively, whereas solid state PL maxima were observed at
399 nm for SBAF-2C and 419 nm for SBAF-3C. These PL spectra
were slightly red-shifted compared with those of spiro[fluorene-
7,90-benzofluorene] analogs (from 366 to 411 nm) [37e39].
Fig. 2. Normalized absorption, photoluminescence spectra and energy diagram of (a)
SBAF-2C and (b) SBAF-3C.
N
Br
H
N
SBAF-2C
2'-bromo-SBAF
N
Br
SBAF-3C
3-bromo-SBAF
Scheme 2. Chemical structures of spirobenzoanthrcene-type orange phosphorescent host materials.

J.-Y. Kim et al. / Dyes and Pigments 94 (2012) 304e313
309
Table 1
of the devices. As shown, HOMO and LUMO of SBAF-2C were 5.92
and 2.42 eV, and those of SBAF-3C were 5.95 and 2.75 eV, respec-
tively (Table 1). SBAF-carbazole (SBAF-Cz) host materials had lower
LUMO and HOMO values than those of spirobenzofluorene-type
phosphorescent host materials [40,41]; therefore, the effect of the
carbazole substituent on the parent spirobenzoantnracene-
fluorene was weak. Thus, transfer of holes from the hole-transfer
layer to the host layer was difficult.
Phosphorescence spectra were obtained at 77K to measure
SBAF-2C and SBAF-3C triplet energy (Fig. 2). SBAF-2C and SBAF-3C
exhibited phosphorescence emission peaks at 518 nm and 539 nm,
respectively, and triplet energy was identified as the first energy
peak in the phosphorescence spectrum. The calculated triplet
energy levels of SBAF-2C and SBAF-3C were 2.39 eV and 2.30 eV,
respectively. The relatively low triplet energy of the two host
materials was due to the spirobenzoanthracene group in SBAF.
SBAF triplet energy was slightly higher than that of 9-carbazole-
spiro[benzo[c]fluorene-7,90-fluorene] (543 nm) and 5-carbazole-
spiro[benzo[c]fluorene-7,90-fluorene] (541 nm) reported earlier.
Therefore, SBAF may be better than 9-carbazole-spiro[benzo[c]
fluorene-7,90-fluorene] as a host for orange PHOLED [41].
The energy level of SBAF-2C and SBAF-3C was determined
mainly by the molecular orbital distribution at the molecular level,
and a molecular simulation of the two compounds was performed.
Fig. 4 (a) shows the HOMO and LUMO distributions of SBAF-2C and
SBAF-3C. The HOMO of SBAF-2C was highly delocalized in the
carbazole and spirofluorene units, whereas the LUMO of SBAF-2C
was distributed only over the spirobenzoanthrcene unit. Therefore,
the hole transport property of SBAF-2C was determined by the
carbazole unit, and the electron transport property was dependent
on the SBAF unit. Unexpectedly, the HOMO of SBAF-3C was mostly
localized in the carbazole unit with little orbital distribution in the
spirobenzoanthracene unit, whereas the LUMO of SBAF-3C was
highly delocalized only in the spirobenzoanthracene unit. The
carbazole group had a large effect on the HOMO orbital distribution
of SBAF-3C, indicating that introducing a carbazole group to spi-
robenzoanthracene was efficient for the PHOLED.
Thermal properties and EL properties of the devices obtained from two host and
Ir(pq)2acac dopant materials.
Properties
Host
SBAF-2C
SBAF-3C
Dopant
Ir(pq)2acac
EL at 7V
lmax (nm)
FWHM (nm)
mA/cm2
cd/Aa
601
86
3.98
603
86
4.18
7.28
8.04 (6 V)
3.60
3.81 (6 V)
368.4
0.616
0.374
5.45
4.99 (6 V)
340
343
415.8
419
3.15
5.95
2.80
13.98
14.74 (6 V)
6.26
7.70 (6 V)
521.5
0.599
0.373
8.50
8.93 (6 V)
328
335
391.2
399.3
3.43
5.92
2.49
2.39
3.42
124.5
290.2
368.5
cd/Ab
lm/Wa
lm/Wb
cd/m2
CIE-x
CIE-y
EQE(%)a
EQE(%)b
lmax (nm)c
lmax (nm)d
lmax (nm)c
lmax (nm)d
Bandgap
HOMO
Optical properties
Energy levels
LUMO
e
ET
ES
Tg
2.30
3.11
126.7
286.3
361
f
g
Thermal properties
h
Tm
Td
i
a
Values at 7 V.
b
Values at a peak; Vopt, operating voltage at a specified current.
THF solution.
Solid film.
Triplet energy.
c
d
e
f
Singlet energy.
g
h
i
Glass transition temperature.
Melting temperature.
Decomposition temperature.
The hole and electron injection are closely related to the energy
level of the host material, and the energy levels of the new host
materials were measured by a low energy photo-electron spec-
trometer and estimated from the absorption edge of their UV-
visible spectra. Fig. 3 presents a schematic energy-level alignment
As shown in Fig. 4(b), the carbazole moiety in SBAF-2C can be
freely rotated, so it can be distorted by 53ꢀ. Rotation of the carba-
zole group in SBAF-3C was also limited by steric hindrance of
EML : SBAF-3C
a
b
EML : SBAF-2C
-1.6
-1.8
-1.6
-1.8
HIL
-2.0
HIL
-2.0
2.0
2.0
-2.2
-2.4
-2.6
-2.8
-3.0
-3.2
-3.4
-3.6
-3.8
-4.0
-4.2
-4.4
-4.6
-4.8
-5.0
-2.2
HTL
HTL
2.4
EML
2.45
EML
-2.4
2.75
3.0
-2.6
-2.8
-3.0
-3.2
-3.4
-3.6
-3.8
-4.0
-4.2
-4.4
-4.6
-4.8
-5.0
2.4
HBL
2.8
HBL
2.8
EIL
2.9
EIL
2.9
3.0
3.6
3.6
LiF/Al
LiF/Al
3.8
3.8
ITO
ITO
-5.2 4.8
-5.4
-5.2 4.8
-5.4
5.1
5.1
5.2
5.2
-5.6
-5.6
5.4
5.4
-5.8
-5.8
5.7
5.7
-6.0
-6.0
5.92
5.95
-6.2
-6.2
6.1
6.1
-6.4
-6.4
-6.6
-6.6
-6.8
-6.8
Fig. 3. Energy level diagrams of the devices, (a) SBAF-2C and (b) SBAF-3C doped with Ir(pq)2acac.

310
J.-Y. Kim et al. / Dyes and Pigments 94 (2012) 304e313
Fig. 4. (a) Molecular orbital distribution and (b) optimized geometrical structure of SBAF-2C and SBAF-3C.
spirobenzoanthracene. This lead to distortion of the carbazole core
structure (distorted 65ꢀ), and delocalization with the carbazole
group cannot be guaranteed by either the fluorene or benzoan-
thracene groups in SBAF-2C and SBAF-3C.
revealed relatively high glass transition temperatures (Tg) of 124.5
and 126.7 ꢀC, due the rigid spiro-type backbone. Thermal decom-
position temperatures for SBAF-2C and SBAF-3C, identified as the
peaks of the weight loss derivatives, were 368.5 and 361ꢀC,
respectively. As a result, the amorphous glassy state of the two host
material transparent films makes them good candidates for use as
EL materials.
3.3. Thermal properties
The thermal properties of the SBAF-Cz derivatives were evalu-
ated by thermo gravimetric analysis and DSC. Purified SBAF-2C and
SBAF-3C samples showed melting points (Tm) of 290.2 and 286.3ꢀC,
respectively, (Table 1). No melting points were observed on the
second heating scan, even though the specimens were given time
to cool in air. Once they become an amorphous solid, they do not
revert to the crystalline state. After the samples had cooled to room
temperature, a second DSC scan was performed at 10ꢀC/min, which
3.4. EL properties
Fig. 5 shows normalized EL emission spectra of the devices. EL
spectra were obtained from SBAF-2C and SBAF-3C doped with
Ir(pq)2acac at various concentrations to determine the optimized
dopant concentration. The EL spectra were different from those of
PL spectra, suggesting that the different excited state species are

J.-Y. Kim et al. / Dyes and Pigments 94 (2012) 304e313
311
responsible for both the PL and EL emissions, resulting from the
triplet emission due to the Ir(pq)2acac complex. The EL emission
was dominated by the Ir(pq)2acac complex emission peak at
around 593e603 nm, but a small emission was observed around
460 nm, which originated from the NPB hole transfer layer. The
LUMO value of SBAF-2C was quite similar to that of NPB. Thus,
electrons or excitons were easily transferred to the host.
The small peak in the EL spectra at 420 nm may have originated
from the SBAF-3C host material, indicating that the energy transfer
from the SBAF-3C host to the Ir(pq)2acac complex was not efficient,
even at the optimum dopant concentration used in this experi-
ment. This was caused by a low SBAF-3C triplet energy compared
with that of SBAF-2C.
The full width at half maximum (82e86 nm) was relatively
broad compared with that of other spiro[benzofluorene-fluorene]
phosphorescent host materials (73 nm) [40,41]. The dependence
of the device chromaticity on the current density was measured to
evaluate stability (Fig. 6). Converting the EL spectrum into chro-
maticity coordinates on the CIE 1931 diagram resulted in increased
chromaticity stability with an increase in current density and
applied voltage. Additionally, the stability of the color coordinates
also increased with increasing dopant concentration. In particular,
the stability of the CIEy coordinates was better than that of the CIEx.
Fig. 6. Stability of the chromaticity depicted by CIE coordination.
3.5. OLED properties
Multilayer devices have a structure of ITO (150 nm)/DNTPD
(60 nm)/NPB (30 nm)/SBAF-2C or SBAF-3C:Ir(pq)2acac (30 nm, 1, 3,
5, 7, and 10%)/BCP (5 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (200 nm). Ten
different multilayered devices were prepared by changing the
dopant concentration. A-1, A-3, A-5, A-7, and A-10 were fabricated
with doped SBAF-2C using 1, 3, 5, 7, and 10% Ir(pq)2acac, respec-
tively, whereas devices B-1, B-3, B-5, B-7, and B-10, made of the
SBAF-2C host, were doped with 1e10% Ir(pq)2acac.
Figs. 7 and 8 show the current density and luminance-voltage
curves for the devices. The turn-on voltage was 4.5 V for SBAF-
2C, whereas the turn-on voltage for the devices obtained from
SBAF-3C were 6.0 V. The maximum brightness was around
27,390 cd/m2 (at 552 mA/cm2) for the device A-3, 28,440 cd/m2 (at
653 mA/cm2) for the device A-5, and 12,950 cd/m2 (at 174 mA/cm2)
for the device A-7. The device A-5 showed the highest brightness, at
28,440 cd/m2, which was more than two times that of the device A-
7. Both the current density and luminance of the devices using
SBAF-2C were higher than those of SBAF-3C within the same
device structure. The phosphorescent device had a lower driving
voltage of 4.5 V, required for initiating light emissions, compared
with that of other fluorescent OLED [28,29].
a
SBAF-2C:Ir(pq) acac (%)
2
1.0
1% (FWHM 82)
3% (FWHM 84)
5% (FWHM 84)
0.8
7% (FWHM 86)
10%(FWHM 86)
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
750
800
Wavelength (nm)
b
SBAF-3C:Ir(pq) acac (%)
2
700
1.2
1.0
0.8
0.6
0.4
0.2
0.0
SBAF-2C:Ir(pq) acac (%)
2
1% (FWHM 87)
3% (FWHM 92)
5% (FWHM 89)
7% (FWHM 91)
10%(FWHM 92)
1%
600
3%
5%
7%
500
10%
SBAF-3C:Ir(pq) acac (%)
2
400
1%
3%
300
5%
7%
10%
200
100
0
400
450
500
550
600
650
700
750
800
2
4
6
8
10
12
14
16
18
Wavelength (nm)
Voltage(V)
Fig. 5. Electroluminescence spectra of orange PHOLEDs with (a) SBAF-2C and (b)
SBAF-3C as a host according to doping concentration.
Fig. 7. Dependence of current density on the applied voltage of the devices SBAF-2C
and SBAF-3C doped with various concentrations of Ir(pq)2acac.

312
J.-Y. Kim et al. / Dyes and Pigments 94 (2012) 304e313
unit. The HOMO of SBAF-3C was delocalized in the cabazole unit,
which decreased the hole transporting property.
30000
25000
20000
15000
10000
5000
0
SBAF-2C:Ir(pq) acac (%)
2
1%
3%
As the concentration of dopant increased, the NPB peak
decreased, because the charge balance was adjusted by the dopant,
which considers charge trapping. Energy transfer of the SBAF-2C
host to the dopant went smoothly, so SBAF-2C showed a higher
efficiency than that of SBAF-3C.
In the B devices group, device B-3 showed the highest lumi-
nance efficiency of around 8.04 cd/A (at 368.4 mA/cm2). Increasing
the doping concentration to 10 wt% resulted in an abrupt increase
at the initial state, followed by sudden decreases in luminance
efficiency. Due to the mismatch of the charge transport properties
and energy level of the host materials, these poor efficiency char-
acteristics of devices using SBAF-3C are not unexpected.
Electron transfer from host to dopant in SBAF-3C was not
favorable because an emission peak from the host was observed.
This can also be explained by the triplet energy value. Triplet
energy was the same or lower than that of the dopant (Table 1). EL
emissions generally occurred by primarily injecting electrons into
the dopant. The EL efficiency increased to 5% and then decreased
due to concentration quenching, as the doping concentration
increased.
5%
7%
10%
SBAF-3C:Ir(pq) acac (%)
2
1%
3%
5%
7%
10%
2
4
6
8
10
12
14
16
18
Voltage (V)
Fig. 8. Dependence of luminance on the applied voltage of the devices SBAF-2C and
SBAF-3C doped with various concentrations of Ir(pq)2acac.
Increasing the doping concentration to 10 wt% resulted in
decreased luminance efficiency due to a concentration quenching
effect. Fig. 9 shows the dependence of luminance efficiency on
current density for the ten PHOLEDs with different doping
concentrations. Maximum luminescence efficiency was obtained
for the red PHOLED with a doping concentration of 3%. As a result,
the SBAF-2C device: 3% Ir(pq)2acac showed a luminance efficiency
of 14.74 cd/A at 6 V and a quantum efficiency of 8.92%, which was
slightly lower than that of the spiro[benzofluorene-fluorene] series
[40,41]. Maximum luminance efficiencies of devices B-3, B-5, and
B-7 were 6.92 cd/A, 8.04 cd/A, and 6.50 cd/A, respectively. B series
devices exhibited lower luminance efficiencies than those of
devices obtained from SBAF-2C. Luminance efficiency showed
a gradual decrease with <10% luminance efficiency. At low doping
concentrations, electron transfer occurred mainly by the host, and
SBAF-2C electron transfer seemed to be faster than that of SBAF-3C.
Actually, the carbazole moiety was usually adopted as the hole
injection and enhancer of hole movement but did not improve the
movement of electrons. SBAF-3C was electron rich, because the
carbazole moiety contributes an electron. Therefore, SBAF-3C was
incapable of accepting an electron around the benzoanthracene
The dopant triplet energy was 2.2 eV, and the difference in
triplet energy between the host and dopant was smaller than that
of SBAF-2C. Thus, energy transition was not efficient.
Fig. 10 shows the dependence of the power efficiency on the
current density for the ten PHOLEDs with different doping
concentrations. The A-3 device exhibited the highest performance
at 6.0 V with a maximum power efficiency of 7.70 lm/W at 4.81 mA/
cm2, which was ascribed to the larger number of excitons generated
in the emitting layer. The high quantum efficiency of the red
PHOLED was kept constant up to 300 cd/m2, irrespective of doping
concentration. SBAF-Cz was synthesized as the host for the red
PHOLED, and little efficiency roll-off was achieved in the SBAF-Cz-
based orange PHOLED. In general, quantum efficiency of the
PHOLED decreased significantly at high luminance due to triplet
exciton quenching [36]. According to one report, efficiency roll-off
can be avoided by balancing holes and electron injections from the
charge transport layer to the emitting layer [8]. SBAF-Cz has both
hole transport and electron transport units; therefore, the hole and
electron injection can be balanced. Carbazole plays the role of
a hole transport unit, whereas SBAF works as an electron transport
SBAF-2C:Ir(pq) acac (%)
2
16
SBAF-2C:Ir(pq) acac (%)
2
8
1%
1%
3%
5%
14
3%
5%
7%
12
7%
6
10%
10%
SBAF-3C:Ir(pq) acac (%)
2
SBAF-3C:Ir(pq) acac (%)
2
10
1%
1%
4
3%
3%
8
5%
7%
5%
7%
6
10%
10%
2
4
2
0
0
0
50
100
150
200
250
300
0
50
100
150
200
250
300
2
Current density (mA/cm )
2
Current density (mA/cm )
Fig. 9. Luminance efficiency-current density characteristics of the device using SBAF-
2C and SBAF-3C.
Fig. 10. Power efficiency-current density characteristics of the device using SBAF-2C
and SBAF-3C.

J.-Y. Kim et al. / Dyes and Pigments 94 (2012) 304e313
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New spirobenzoanthrcene-type orange phosphorescent host
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carbazole. When the device configuration of ITO/DNTPD/NPB/
HOST: Ir(pq)2acac/BCP/Alq3/LiF/Al was constructed, EL emissions of
the devices (SBAF-2C doped with 3% Ir(pq)2acac) were observed at
602 nm. The device obtained using SBAF-2C:3% Ir(pq)2acac had the
highest external quantum efficiency of 8.9% and CIE coordinates of
0.599 and 0.373. Based on these characteristics, these orange host
emitting materials are useful for investigating PHOLED with
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Acknowledgment
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).
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