New limb structured blue light emitting materials for OLEDs

Article abstract of DOI:10.1016/j.dyepig.2012.05.002

We designed new three limb structured anthracene derivatives, PNA, NNA and FNA, which are composed of an anthracene core and naphthalene units at the 9,10-positions of anthracene and phenyl, naphthalene and fluorene units at the 2,3-positons of anthracene. The effect of the introduced limbs on the 2,3,9,10-positions of anthracene was studied. The doped EL devices using ADN as a host and (3% PNA or NNA or FNA) as a dopant showed similar maximum quantum efficiency of 3.9%-4.3% with high color purity of (0.15, 0.13).

Full text of DOI:10.1016/j.dyepig.2012.05.002

Dyes and Pigments 95 (2012) 384e391
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New limb structured blue light emitting materials for OLEDs
a
a
c
c
a
a
Ran Kim , Seung-Jin Yoo , Eun-Kyung Kim , Han Sung Yu , Sung Chul Shin , Sang-Kyung Lee ,
b,
a,
*
**
Soon-Ki Kwon
, Yun-Hi Kim
a
Department of Chemistry and RINS, Gyeongsang National University, Jinju 660-701, Republic of Korea
School of Materials Science & Engineering and ERI, Gyeongsang National University, Gajwa-dong 900, Jinju 660-701, Republic of Korea
EL Materials Development Division, Duksan Hi-Metal Co. Ltd., Seongnam 463-402, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 October 2011
Received in revised form
We designed new three limb structured anthracene derivatives, PNA, NNA and FNA, which are composed
of an anthracene core and naphthalene units at the 9,10-positions of anthracene and phenyl, naphthalene
and fluorene units at the 2,3-positons of anthracene. The effect of the introduced limbs on the 2,3,9,10-
positions of anthracene was studied. The doped EL devices using ADN as a host and (3% PNA or NNA or
FNA) as a dopant showed similar maximum quantum efficiency of 3.9%e4.3% with high color purity of
20 March 2012
Accepted 8 May 2012
Available online 23 May 2012
(
0.15, 0.13).
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Blue OLED
Limb structure
Anthracene derivatives
Quantum efficiency
Color purity
Non-coplanar structure
1
. Introduction
properties. Moreover, it has been also reported that the anthracene
derivatives have ambipolar transporting property, leading high
efficient devices [19e23].
Organic light emitting devices (OLEDs) have been attracting
considerable attention on the basis of their potential application in
flat-panel displays [1e3]. During the past several years, much
progress has been made in this field [4e10]. Great efforts have been
made to develop high performance materials with desirable
materials for developing marketable OLEDs. Many new materials
with RGB (red, green, blue) emission have been developed to meet
the requirements of full-color displays.
Efficient blue-light OLEDs are of particular interest, because they
are desired for use as blue light sources in full-color display
applications; furthermore, they can also serve as hosts for
exothermic energy transfer to lower-energy fluorophores to realize
white-light OLEDs [11e18].
Recently, we reported that the whole molecule becomes highly
twisted by introducing bulky substituents at the 9- and 10-
positions of anthracene. Thus, the fluorescence e quenching
interactions caused by aggregates can be suppressed to some
extent and non-radiative energy decay can be reduced [8,9,22e26].
Here, we designed new limb-structured blue light emitting
materials, which are composed of an anthracene main core,
naphthalene units at the 9,10-positions of anthracene and phenyl
or naphthalene or fluorene units at the 2,3-positions of anthracene.
We studied the effect of the introduced limbs on the 2,3,9,10-
positions of anthracene. In particular, it is expected that the
incorporated limbs on the 2,3-positions are highly twisted to the
anthracene main unit and result in amorphousness of the molecule
and reduced the intermoleular interaction. Moreover, despite that
the new designed molecular structure is composed of an aromatic
unit, the unique twisted limb structure of the molecule leads to
increased solubility. Thus, we expect that the new limb structured
blue light emitting materials will be high efficient and stable blue
light emitting materials.
Anthracene derivatives have been extensively studied and
developed as light emitting materials in OLEDs because of their
interesting photoluminescence (PL) and electroluminescence (EL)
*
Corresponding author. Tel.: þ82 55 772 1491; fax: þ82 55 772 1489.
*
* Corresponding author. Tel.: þ82 55 772 1652.
E-mail addresses: skwon@gnu.ac.kr (S.-K. Kwon), ykim@gnu.ac.kr (Y.-H. Kim).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.05.002

R. Kim et al. / Dyes and Pigments 95 (2012) 384e391
385
2
. Experimental
solution of tetrabutyl ammonium perchlorate in acetonitrile. The
organic electroluminescence (EL) devices were fabricated using
0
0
2.1. Materials
successive vacuum deposition of ITO/N,N -diphenyl-N,N -bis-[4-
0
(
phenyl-m-thonyl-amino)-phenyl]-biphenyl-4,4 -diamine (DNTPD,
All starting materials were purchased from Aldrich and Strem
70 nm)/1,4-bis[(1-naphthylphenyl)-amino]biphenyl (a-NPD, 30 nm)/
and were used without further purification.
(PNA or NNA or FNA, 20 nm) or (PNA or NNA or FNA doped ADN, 3%,
2
0 00
3
0 nm)/2,2 ,2 -tris(8-hydroxyquinoline) aluminium (Alq , 40 nm)/
2
.2. Instrument
LiF/Al. In these devices, ITO (indium tin oxide) and Al were the
anode and the cathode. The ITO glass with a sheet resistance of
1H-NMR spectra were recorded using a Bruker Avance-300 MHz
about 10
U was etched for the anode electrode pattern and cleaned
FT-NMR spectrometer, and chemical shifts were reported in ppm
units with tetramethylsilane as internal standard. FT-IR spectra
were recorded using a Bruker IFS66 spectrometer. Thermogravi-
metric analysis (TGA) was performed under nitrogen using a TA
instruments 2050 thermogravimetric analyzer. Differential scan-
ning calorimeter (DSC) was conducted under nitrogen using a TA
instrument DSC Q10. The both samples were heated at a rate of
in ultrasonic baths of isopropyl alcohol and acetone. The overlap
area of Al and ITO electrodes is about 4 mm . A UV zone cleaner
2
(Jeilight Company) was used for further cleaning before vacuum
deposition of the organic materials. Vacuum deposition of the
ꢂ7
organic materials was carried out under a pressure of 2 ꢁ 10 torr.
The deposition rate for organic materials was about 0.1 nm/s. The
evaporation rate and the thickness of the film were measured with
a quartz oscillator. OLED performance was studied by measuring
the current-voltage-luminescence (I-V-L) characteristics, EL, and PL
spectra at room temperature. I-V-L characteristics and CIE color
coordinates were measured with a Keithley SMU238 and Spec-
trascan PR650. EL spectra of the devices were measured utilizing
ꢀ
1
0 C/min. UVevisible spectra and photoluminescence (PL) spectra
were measured by Shimadsu UV-1065PC UVevisible spectropho-
tometer and Perkin Elmer LS50B fluorescence spectrophotometer,
respectively. The electrochemical properties of the materials were
measured by cyclic voltammetry using an Epsilon C3 in a 0.1 M
Scheme 1. Synthetic route to PNA, NNA and FNA.

386
R. Kim et al. / Dyes and Pigments 95 (2012) 384e391
Fig. 1. Calculated stereostructures and frontier orbitals of PNA, NNA, and FNA.
a
diode array rapid analyzer system (Professional Scientific
350 mL of acetic acid. The reaction mixture was refluxed with for
Instrument Corp.) Fluorescence spectra of the solutions in chloro-
form were measured using a spectrofluorimeter (Shimadzu Corp.).
48 h. The reaction was worked up by adding Na
tion at 0 C. The organic layer was separated by methylene
dichloride, and the solvent was evaporated. The crude product was
2
CO
3
aqueous solu-
ꢀ
2.3. 3,4-Dibromothiophene-1,1-dioxide (1)
purified by column chromatography with n-hexane/CH
v/v) as eluent. Yield: (3.2 g, 40.65%). Mp 278e279 C. IR (KBr):
2
Cl
2
(1:1,
ꢀ
Trifluoroacetic acid (1101 mmol, 153 mL) was slowly added into
ꢀ
the 30% H
2
O
2
(60 mL) at 0 C. After addition of trifluoroacetic acid,
the solution of 3,4-dibromothiophene (70.40 mmol, 8.14 mL) in
CH Cl (90 mL) was added in the mixture. The mixture was stirred
Table 1
Physical data of compounds.
2
2
for 3 h at room temperature. The reaction was worked up by adding
Comp
l
abs/(nm)a
l
em/(nm)a
Solution Film
HOMO LUMO
E
g
(eV)
ꢀ
Na
2
CO
3
aqueous solution at 0 C. The organic layer was separated by
(eV)
(eV)
Solution
Film
methylene dichloride, and the solvent was evaporated. The crude
PNA
NNA
291, 351,
369, 389, 411
260, 275,
3
4
372, 394, 438
416
373, 392, 432
452
5.65
5.60
2.74
2.91
product was recrystallized in co-solventof CH
2
Cl
2
and ethanol Yield:
ꢀ
ꢂ1
(
12.4 g, 49%). Mp 103e104 C. IR (KBr): 1309, 1146 cm (S]O),
457
2.78
2.82
ꢂ1
2
1
3
3
015 cm (sp CeH). H NMR (300 MHz, CDCl , ppm): d 6.85 (1).
18, 371, 392, 415
13
2
.4. 2,3-Dibromoanthraquinone (2)
FNA
260, 287, 321, 375, 394, 448
72, 394, 416 414
260, 360, 377, 374, 392, 442
98 414
468
461
5.63
5.66
2.85
2.65
2.78
3.01
3
ADN
3
,4-Dibromothiophene-1.1-dioxide (5.65 g, 20.62 mmol) and
3
1,4-naphthoquinone (16.31 g, 103.13 mmol) were dissolved in

R. Kim et al. / Dyes and Pigments 95 (2012) 384e391
387
ꢂ1
ꢂ1
2
ꢂ1
1
1
(
681 cm (C]O), 3075 cm (sp CeH),1656 cm (C]C). H NMR
with n-hexane/CH
2
Cl
2
(1:1, v/v) as eluent. Yield: (1.99 g, 68%). Mp
ꢀ
ꢂ1
ꢂ1
2
300 MHz, CDCl , ppm): 8.49 (2), 8.38e8.40 (2), 7.86e7.89 (2).
3
d
226 C. IR (KBr): 3521 cm (eOH), 3017e3029 cm (sp CeH),
ꢂ1
1
1582 cm
3
(C]C). H NMR(300 MHz, CDCl , ppm): d8.52(2),
2.5. 2,3-Dibromo-9,10-dihydroxy-9,10-dinaphthylanthracene (3)
8.38e8.40 (4), 7.86e7.89 (5), 6.94 (2).
2
,3-Dibromoanthraquinone (2.10 g, 5.74 mmol) were dissolved
2.6. 2,3-Dibromo-9,10-dinaphthylanthracene (4)
in 70 mL of THF. Naphthylmagnesium bromide (10.2 mL) was added
ꢀ
to the reaction mixture at 0e15 C. The mixture was stirred for 3 h
2,3-Dibromo-9,10-dihydroxy-9,10-dinaphtylanthracene (2.4 g,
3.86 mmol), NaH PO ‧H O (4.09 g, 38.56 mmol) and KI (1.92 g,
at room temperature. The organic layer was extracted with diethyl
2
4
2
ether. The crude product was purified by column chromatography
11.58 mmol) were dissolved in 50 ml of acetic acid. After the
ꢀ
Fig. 2. TGA thermogram and DSC thermogram of (a) PNA, (b) NNA, and (c) FNA in nitrogen atmosphere at a scan rate 10 C/min.

388
R. Kim et al. / Dyes and Pigments 95 (2012) 384e391
reaction mixture was refluxed for 4 h, the reaction was worked up
by water. The crude solid product was several time filtered by
2,3-Dibromoanthraquinone was obtained by oxidation and the
DielseAlder reaction following degradation of SO . 2,3-Dibromo-
2
ꢀ
ꢂ1
water. Yield: (2.02 g, 83%), Mp 298 C. IR (KBr): 3011e3043 cm
9,10-dihydroxy-9,10-dinaphthylanthracene was synthesized by
reaction of 2-naphthyl magnesium bromide and 2,3-
dibromoanthraquinone. 2,3-Dibromo-9,10-dinaphthylanthracene
obtained through reduction of 2,3-dibromo-9,10-dihydroxy-9,10-
dinaphthylanthracene was reacted with phenyl boronic acid, 2-
naphthyl boronic acid, and 9,9-diethylfluoren-2-ylboronic acid to
afford 2,3-diphenyl-9,10-dinaphthylanthracene (PNA), 2,3,9,10-
2
ꢂ1
1
(
sp CeH), 1752e1734 cm
(C]C). H NMR (300 MHz, CDCl ,
3
ppm): 8.07e7.96 (10), 7.72e7.67 (8), 7.64e7.59 (2).
d
2.7. 2,3-Diphenyl-9,10-dinaphtylanthracene (PNA) (5a)
2
,3-Dibromo-9,10-dinaphthalene anthracene (1 g, 1.7 mmol)
and phenyl boronic acid (0.93 g, 7.64 mmol) and 2M K CO (1.94 g)
2
3
tetranaphthylanthracene
(NNA),
and
2,3-difluorenyl-9,10-
was added to the 50 mL of THF. After Pd(PPh
3
)
4
(0.0038 g, 3 mol/%)
naphthylanthracene (FNA), respectively.
ꢀ
was added to the mixture, the mixture was stirred at 90 C for 48 h.
After the reaction was worked up by adding water, the organic layer
was extracted with chloroform. After the solvent was evaporated,
the crude product was purified by column chromatography using
n-hexane/ethyl acetate (5:1, v/v) as eluent. Yield: (0.86 g, 81%). Mp
The structures of new limb structured materials were confirmed
by various spectroscopic methods such as FT-NMR, IR and mass
spectroscopies. Theoretical calculations using Spartan08 software
in order to fully optimize the molecular structure, were carried out
for the characterization of 3-dimensional structures and the energy
densities of the HOMO and LUMO states of each materials. Fig. 1
shows the stereostructures and the energy densities of the HOMO
and LUMO states of materials derived from the calculations.
The limbs attached at the 2,3,9,10-positions of anthracene are
highly twisted toward the anthracene backbone at angles of
ꢀ
ꢂ1
2
ꢂ1
3
42 C. IR (KBr): 3073e3023 cm (sp CeH), 1732e1715 cm (C]
1
C). H NMR (300 MHz, CDCl
.58e7.43 (4), 7.40e7.32 (2), 7.23e7.02 (10). C NMR (300 MHz,
CDCl , ppm): 141.3, 138.7, 137.1, 136.4, 133.4, 132.8, 130.6, 130.2,
29.9, 129.5, 129.4, 128.3, 128.2, 127.9, 127.7, 127.1, 126.4, 126.2,
25.2. EI-MS: m/z 582.
3
, ppm): d 8.19e7.81 (10), 7.68e7.60 (4),
1
3
7
3
d
1
1
ꢀ
ꢀ
88.73e80.40 for the 9,10-positions and 122.82e123.54 for the 2,3-
positions. From this calculation, it is expected that the newly
obtained limb-structured materials have non-coplanar structures
that bear bulky substituents. These substituents disrupt the inter-
molecular interaction and suppress the problematic recrystallization,
2.8. 2,3,9,10-Tetranaphtylanthracene (NNA) (5b)
2
,3-Dibromo-9,10-dinaphthalene anthracene (1 g, 1.7 mmol)
and 2-naphthalene boronic acid (1.3 g, 7.64 mmol) and 2M K CO
2
3
(
3
1.94 g) was added to the 50 mL of THF. After Pd(PPh
mol/%) was added to the mixture, the mixture was stirred at 90 C
3
)
4
(0.0038 g,
ꢀ
for 48 h. After the reaction was worked up by adding water, the
organic layer was extracted with chloroform. After the solvent was
evaporated, the crude product was purified by column chroma-
tography using n-hexane/ethyl acetate (5:1, v/v) as eluent. Yield:
ꢀ
ꢂ1
2
(
1
(
7
0.82 g, 84%). Mp 328 C. IR (KBr): 3072e3020 cm (sp CeH),
ꢂ1
1
712e1701 cm (C]C). H NMR (300 MHz, CDCl
4), 7.98e7.82 (6), 7.76e7.74 (6), 7.69e7.43 (8), 7.4 (2), 7.26e7.23 (6),
.03 (2). 13C NMR (300 MHz, CDCl
, ppm): 139.1, 133.5, 129.5,
28.9, 128.3, 128.2 (2), 128.1 (2), 127.9, 127.5, 127.1, 127.0, 126.4,
3
, ppm): d 8.09
3
d
1
126.2, 125.8, 125.7, 125.3. EI-MS: m/z 682.
2.9. 2,3-bis(9,9-dimethyl-9H-fluoren-2-yl)-9,10-dinaphthylanthracene
(
FNA) (5c)
2
,3-Dibromo-9,10-dinaphthalene anthracene (1 g, 1.7 mmol)
and 2-(9,9-dimethyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (1.96 g, 7.64 mmol) and 2M K CO (1.94 g) was
2
3
added to the 50 mL of THF. After Pd(PPh
3
)
4
(0.0038 g, 3 mol/%) was
ꢀ
added to the mixture, the mixture was stirred at 90 C for 48 h.
After the reaction was worked up by adding water, the organic layer
was extracted with chloroform. After the solvent was evaporated,
the crude product was purified by column chromatography using
n-hexane/ethyl acetate (5:1, v/v) as eluent. Yield (0.95 g, 88%). Mp
ꢀ
ꢂ1
2
ꢂ1
3
(
31 C. IR (KBr): 3068e3022 cm (sp CeH), 2786e2698 cm
3
ꢂ1
ꢂ1
). 1H
sp CeH) 1788e1719 cm (C]C), 1375e1373 cm (eCH
3
NMR (300 MHz, CDCl , ppm): d 7.98 (2), 7.82 (2), 7.67e7.45 (16),
3
13
7
.41e7.38 (2), 7.36e7.21 (10), 7.01 (2), 1.08 (12). C NMR (300 MHz,
3
CDCl , ppm): 153.6, 152.7, 140.5, 139.2, 136.5, 137.4, 137.1, 136.4,
1
1
33.6, 132.8, 130.5, 129.4, 128.5, 128.2, 128.0, 127.6, 127.1, 126.8,
26.6, 126.0, 125.1, 125.0, 122.1, 119.8, 119.4, 46.5, 26.6. EI-MS:
m/z 814.
3
. Results and discussion
The synthetic scheme of the three limb structured anthracene
Fig. 3. Energy level diagrams of devices using PNA as emitting material and ADN-3%
derivatives is shown in Scheme 1.
PNA as emitting material.

R. Kim et al. / Dyes and Pigments 95 (2012) 384e391
389
which would eventually reduce self-aggregation, and they thus
which in turn, improve the morphological stability of the thin film of
OLED device.
A slight emission red-shift is observed for the film states, which
may be accounted for by the different dielectric constants of the
media[ . The PL emission of PNA, NNA, and FNA display blue
emission and exhibit high PL quantum yield in solution; 68 ꢃ 10%,
65 ꢃ 10%, and 59 ꢃ 10%, respectively.
20]
The absorption spectra of the compounds in chloroform solution
reveal the characteristic vibronic patterns from the
tions of the isolated anthracene group ( max are 360, 378, and
97 nm). The absorption spectra of PNA, NNA and FNA in film as
pep* transi-
l
The relevant energy levels for these materials are listed in
Table 1. The band gap energies were estimated from the onset of the
absorption spectra in dilute solution. The results indicate that PNA,
NNA, and FNA have low band-gap energies compared with that of
ADN. Although the similar energy band gap, band gap energy order
is PNA > NNA > FNA. From the results, it is suggested that the
introduction of aromatic substituents at the 2,3-positions leads to
3
well as in solution are almost similar to those of 9,10-
dinaphthylanthracene (ADN) while the absorption edges are
slightly red-shift compared with ADN.
The absorption and emission spectra of PNA, NNA, and FNA in
solution and solid films are summarized in Table 1.
5
4
3
2
1
0
000
000
000
000
000
a
d
6
000
250
200
150
100
50
4
.5
4.0
.5
3.0
.5
2.0
.5
2
50
00
50
3
.0
3
5000
4000
3000
2
2.5
PNA
2
1
2.0
1
1.5
1.0
100
0
50
100
150
200
250
0
50
100
150
200
250
2000
Current density (mA/cm )
Current density (mA/cm )
5
0
1000
0
0
0
3
4
5
6
7
8
4
5
6
7
8
Voltage (V)
Voltage (V)
b
5000
e
3.5
8000
7000
2
50
250
200
150
100
50
5.0
4
.5
.0
3.5
.0
2.5
.0
.5
3
.0
4
3
2
1
0
000
000
000
000
4
200
6000
2.5
NNA
3
5000
150
2.0
2
4000
3000
1.5
1
100
0
50
100
150
200
250
0
50
100
150
200
250
Current density (mA/cm )
Current density (mA/cm )
2000
5
0
1000
0
0
0
3
4
5
6
7
8
4
5
6
7
8
Voltage (V)
Voltage (V)
c
5
.5
.0
.5
8000
000
000
5
4
4
3
3
2
2
.0
.5
.0
.5
.0
.5
.0
7000
000
5000
000
3000
f
2
50
250
200
150
100
50
5
7
4
6
4.0
200
6
3.5
3.0
2.5
FNA
5000
4000
3000
150
4
2.0
1.5
100
0
50
100
150
200
250
0
50
100
150
200
250
Current density (mA/cm )
Current density (mA/cm )
2
1
0
000
000
2000
5
0
1000
0
0
0
3
.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
4
5
6
7
8
Voltage (V)
Voltage (V)
Fig. 4. Current density- voltage-luminescence and current density efficiency characteristics of the devices (a) using PNA, (b) using NNA, (c) using FNA, (d) using ADN-3% PNA, (e)
using ADN-3% NNA (f) using ADN-3% FNA.

390
R. Kim et al. / Dyes and Pigments 95 (2012) 384e391
Table 2
Key device performance parameters and EL emission characteristics.
Host
Dopant
wt%
V
A
J (mA/cm2)
LE (Cd/A)
PE (lm/W)
EQE
L (Cd/m2)
CIE
x
y
PNA
NNA
FNA
ADN
ADN
ADN
X
X
X
PNA
NNA
FNA
5.17
4.92
4.99
5.59
5.60
5.78
0.0004
0.0004
0.0004
0.0004
0.0004
0.0004
10
10
10
10
10
10
2.96
2.98
4.33
3.72
4.44
4.64
1.80
1.90
2.73
2.09
2.49
2.52
2.74
2.68
2.81
3.62
4.16
4.32
295.7
297.6
433.1
372.4
444.2
463.7
0.15
0.15
0.16
0.15
0.15
0.15
0.14
0.14
0.22
0.13
0.14
0.14
3
3
3
increased conjugation length and decreased band gap, although
they render the molecules non-coplanar amorphous due to steric
hindrance. The experimental results are consistent with frontier
orbitals of the materials. (Fig. 1) In particular, FNA has expanded
conjugation through fluorene.
transporting layer (ETL) and the newly prepared limb-type anthra-
cene derivatives were used as the emitting layer (EML) (Fig. 3).
Current density-voltage-luminescence, current density-effi-
ciency characteristics of the devices are shown in Fig. 4.
Key device performance parameters and EL emission charac-
teristics are summarized in Table 2 and Fig. 5.
The electrochemical behaviors of PNA, NNA, and FNA were
investigated by cyclic voltammetry (CV). The HOMO, LUMO, and
The non-doped device using PNA exhibits the maximum
external quantum efficiency of 2.74% (2.95 cd/A) with color purity
of (0.15, 0.13) and the non-doped device using NNA exhibits
maximum quantum efficiency of 2.67% (2.97 cd/A) with color
purity of (0.15, 0.14) while the non-doped device using FNA shows
maximum quantum efficiency of 2.8% (4.33 cd/A) with color purity
of (0.16, 0.22). The non-doped devices using FNA with naphthalene
and fluorene limbs showed slightly lower color purity than those of
PNA with naphthalene and phenyl limbs or NNA with naphthalene
limbs while PNA and NNA showed similar quantum efficiency and
color purity. These results can be explained by the extended
conjugation of FNA. However, the newly designed limb structured
anthracene derivatives have better color purity than those of
anthracene derivatives with 2,6 substituents [27]. The results
indirectly show that the limb structured anthracene derivatives
have a twisted structure because of effective steric hindrance of the
limbs at the 2,3-positions[ . Furthermore, the non-doped devices
using PNA and NNA showed higher color purity than that of
reported ADN without the substituents at the 2,3-positions. It
appears that the 3-dimensionally highly twisted limb structured
PNA and NNA can not be easily stacked, leading to inhibited
intermolecular interaction.
band gap (E
g
) are summarized in Table 1. The oxidation peak
potentials are high, Eox ¼ 1.2 for PNA, Eox ¼ 1.25 for NNA, and
E
5
ox ¼ 1.23 for FNA. The HOMO levels of PNA, NNA, and FNA are
.65 eV, 5.6 eV, and 5.63 eV, respectively.
Thermal properties of the obtained materials were evaluated via
a thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) (Fig. 2).
The TGA (10 C/min) showed that the three materials were
highly stable in nitrogen, with a thermal decomposition tempera-
ꢀ
ture (T
tively. In the DSC, T
d
) of 382, 410, and 410 nm for PNA, NNA, and FNA, respec-
was not observed for PNA and NNA, while FNA
g
ꢀ
has a T
g
of 158 C. Additionally, the materials showed melting
ꢀ
temperatures higher than 310 C. These results clearly reveal that
the materials possess good thermal stability, a very desirable
characteristics for OLEDs stability.
Non-doped and doped OLED devices were fabricated using the
synthesized materials as the emitting layer in the following struc-
25]
0
0
ture:
ITO/N,N -diphenyl-N,N -bis-[4-(phenyl-m-thonyl-amino)-
0
phenyl]-biphenyl-4,4 -diamine
(DNTPD,
-NPD, 30 nm)/(PNA or NNA or
FNA, 20 nm) or (PNA or NNA or FNA doped ADN, 3%, 20 nm)/2,2,’2”-
tris(8-hydroxyquinoline) aluminium (Alq , 40 nm)/LiF/Al. In these
devices, ITO (indium tin oxide) and Al were the anode and the
cathode, respectively. DNTPD was the hole injection layer (HIL);
NPD was the hole transporting layer (HTL); Alq was the electron
70
nm)/1,4-bis[(1-
naphthylphenyl)-amino]biphenyl (
a
3
The doped device using ADN as a host and (3% PNA or NNA or
FNA) as a dopant showed similar maximum quantum efficiency of
(3.9%e4.3%) with high color purity of (0.15, 0.13) regardless of the
dopant. In particular, FNA with naphthalene and fluorene limbs
showed maximum quantum efficiency of 4.3% (4.6 cd/A) with
high color purity of (0.15, 0.13).
a-
3
PNA
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
4. Conclusion
NNA
FNA
ADN:PNA(3%)
ADN:NNA(3%)
ADN:FNA(3%)
We designed three new limb structured anthracene derivatives,
PNA, NNA, and FNA, which are composed of an anthracene core and
naphthalene units at the 9,10-positions of anthracene and phenyl,
naphthyl or fluorene units at the 2,3-position of anthracene. The
theoretical calculations supports that the synthesized materials
have 3-dimensionally highly twisted non-coplanar structures due
to steric hindrance of the introduced limbs. The maximum PL
emission intensity was obtained by excitation at corresponding
limbs at the 2,3,9,10-positions of anthracene due to intramolecular
energy transfer from the limbs to the anthracene core. The intro-
duced limbs at the 2,3-positions induces steric hindrance, although
FNA with fluorene substituents has slightly increased conjugation
length and decreased band gap. The non-doped blue EL devices
using PNA, NNA, or FNA as an emitting material showed efficiency
of 2.67%e2.8% and the doped EL devices using ADN as a host and
(3% PNA or NNA or FNA) as a dopant showed similar maximum
400
500
600
700
Wavelength (nm)
Fig. 5. EL spectra of the devices.

R. Kim et al. / Dyes and Pigments 95 (2012) 384e391
391
quantum efficiency of (3.9%e4.3%) with high color purity of (0.15,
.13) regardless of the dopant.
[12] Jou JH, Chiu YS, Wang RY, Hu HC, Wang CP, Lin HW. Efficient, color-stable
fluorescent white organic light-emitting diodes with an effective exciton-
confining device architecture. Org Electron 2006;7:8e15.
0
[
13] Kido J, Shionoya H, Nagai K. White organic light-emitting diodes based on 2,7-
0
Acknowledgments
bis(2,2-diphenylvinyl)-9,9 -spirobifluorene: improvement in operational life-
time. Appl Phys Lett 2004;85:4609e11.
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14] Zheng CJ, Zhao WM, Wang ZQ, Huang A, Ye J, Ou XM, et al. Highly efficient
non-doped deep-blue organic light-emitting diodes based on anthracene
derivatives. J Mater Chem 2010;20:1550e6.
This research was financially supported by Basic Science
Research Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and
Technology (2011-0003336, 2011-0000310 and 2010-0029084).
Ran Kim thanks the support by MKE and KIAT through the Work-
force Development Program in Strategic Technology.
[
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