Synthesis and electroluminescent properties of blue fluorescent triphenylamine substituted anthracene derivatives for OLEDs

Article abstract of DOI:10.1080/15421406.2010.495882

We describe the electroluminescent properties of blue fluorescent materials based on mono(triphenylamino)anthracene derivatives synthesized via Suzuki cross coupling reactions. To explore the electroluminescent properties of materials 1-3, multilayered OL

Full text of DOI:10.1080/15421406.2010.495882

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Synthesis and Electroluminescent
Properties of Blue Fluorescent
Triphenylamine Substituted Anthracene
Derivatives for OLEDs
Kum Hee Lee a , Jae Nam You a , Hyuck Joo Kwon b , Young Kwan Kim
b & Seung Soo Yoon a
a Department of Chemistry , Sungkyunkwan University , Suwon,
Kyeonggi-do, Korea
b Department of Information Display , Hongik University , Seoul,
Korea
Published online: 14 Nov 2010.
To cite this article: Kum Hee Lee , Jae Nam You , Hyuck Joo Kwon , Young Kwan Kim & Seung
Soo Yoon (2010) Synthesis and Electroluminescent Properties of Blue Fluorescent Triphenylamine
Substituted Anthracene Derivatives for OLEDs, Molecular Crystals and Liquid Crystals, 530:1, 48/
[204]-55/[211]
To link to this article: http://dx.doi.org/10.1080/15421406.2010.495882
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Mol. Cryst. Liq. Cryst., Vol. 530: pp. 48=[204]–55=[211], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.495882
Synthesis and Electroluminescent Properties of
Blue Fluorescent Triphenylamine Substituted
Anthracene Derivatives for OLEDs
KUM HEE LEE,1 JAE NAM YOU,1
HYUCK JOO KWON,2 YOUNG KWAN KIM,2 AND
SEUNG SOO YOON1
1Department of Chemistry, Sungkyunkwan University, Suwon,
Kyeonggi-do, Korea
2Department of Information Display, Hongik University, Seoul, Korea
We describe the electroluminescent properties of blue fluorescent materials based on
mono(triphenylamino)anthracene derivatives synthesized via Suzuki cross coupling
reactions. To explore the electroluminescent properties of materials 1–3, multi-
layered OLEDs were fabricated and all devices exhibited efficient blue emissions.
Among those, device 1, with CIE x,y coordinates of (0.159, 0.166) at 12.0 V,
exhibited the best performance, with a maximum luminance of 7306 cd=m2 at
12.0 V, and luminous and power efficiencies of 4.95 cd=A and 1.83 lm=W at
20 mA=cm2, respectively.
Keywords Blue fluorescent materials; mono(triphenylamino)anthracene deriva-
tives; multilayered OLEDs; Suzuki cross-coupling reaction
Introduction
The development of highly efficient organic light-emitting diodes (OLEDs) has
attracted significant attention recently due to its potential application in flat panel
displays [1–4]. In this regards a variety of new emitting materials have been
developed to fill the requirements of three primary color (red, green, blue) emitters
for full color OLED displays [5]. However, blue-emitting materials still receive
considerable attention, because its wide band gap is such that highly efficient deep
blue emitting materials are difficult to produce. Recently, a variety of blue-emitting
materials based on anthracene have been reported [6–16]. Among these are
mono(triarylamino)-substituted anthracene derivatives. For example, Tao et al. have
reported that 9-mono(triphenylamino)-10-pyrenyl anthracene derivatives exhibited
efficient blue emission with a maximum efficiency of up to 7.9 cd=A in a nondoped
Address correspondence to Prof. Seung Soo Yoon, Department of Chemistry, Sung-
kyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do 440-746,
Korea (ROK). Tel.: (þ82)31-290-7071; Fax: (þ82)31-290-7075; E-mail: ssyoon@skku.edu
48=[204]

Blue Fluorescent Anthracene Derivatives for OLEDs
49=[205]
Figure 1. Synthetic routes to blue fluorescent materials (1–3). Conditions: a) Pd(PPh3)4,
K2CO3 (2 M in H2O), Aliquat 336, Toluene, reflux.
device [17]. However, for practical applications, the properties of these emitting
materials need to be improved.
In this study, a series of blue fluorescent materials (1–3) based on mono(triphe-
nylamino) anthracene (Figure 1) were synthesized, and their electroluminescent
properties were investigated in doping system using ADN as a host [7].
Experimental
Materials and Characterization
9-(naphthalen-2-yl)-10-(4-triphenylamine)anthracene (1) and 9-phenyl-10-(4-triphe-
1
nylamine) anthracene (2) were prepared according to literature methods [18]. H-
and 13C-NMR were recorded using a Varian (Unity Inova 300NB) or Varian (Unity
Inova 500NB) spectrometer at 300 MHz and 500 MHz, respectively. Low- and
high-resolution mass spectra were recorded using a Jeol JMS-AX505WA spectro-
meter in FAB mode or Jeol JMS-600 spectrometer in EI mode.
9-(naphthalen-1-yl)-10-(4-triphenylamine)anthracene (3). A solution of 4-bromo-
N,N-diphenylaniline (0.64 mmol), 10-phenylanthracen-9-ylboronic acid (0.84 mmol),
tetrakis(triphenyl phosphine)palladium (0.02 mmol), Aliquat 336 (0.05 mmol) and
aqueous 2.0 M K2CO3 (5.20 mmol) in toluene were heated at 120ꢀC under Ar atmo-
sphere for 4 h. After the solution cooled, the product was extracted with CH2Cl2.
The CH2Cl2 solution was washed with brine and H2O, and then dried on MgSO4.
Evaporation of the solvent, followed by recrystallization with CHCl3=hexane, gave
1
3 (92%) as a yellow solid. H-NMR (300 MHz, CDCl3): d ppm 8.06 (d, J ¼ 8.2 Hz,
1H), 8.01 (d, J ¼ 8.2 Hz, 1H), 7.89 (d, J ¼ 8.8 Hz, 2H), 7.70 (t, J ¼ 7.0 Hz, 1H),
7.56 (d, J ¼ 7.0 Hz, 1H), 7.51–7.15 (m, 21H), 7.12–7.06 (m, 2H); 13C-NMR
(500 MHz, CDCl3): ppm 148.0, 147.4, 137.6, 137.1, 135.0, 133.9, 133.8, 132.8,
132.4, 130.9, 130.3, 129.6, 129.4, 128.4, 128.3, 127.3, 127.2, 126.9, 126.5, 126.2,
125.8, 125.4, 125.2, 124.9, 123.4, 123.3, 123.2; IR (KBr): n 3037, 1592, 1509, 1493,
1374, 1326, 1279, 936, 762, 755, 738, 697 cmꢁ1; EI-MS m=z ¼ 547 [Mþ]; HRMS-TOF
[Mþ þ H] Anal. calcd for C50H53N2 547.2300: found 547.2273.
Physical Measurements
The UV-Vis absorption and photoluminescence spectra of these newly designed
dopants were measured in CH2Cl2 solution (10ꢁ5 M) using a Sinco S-3100 and
Amincobrowman series 2 luminescence spectrometer, respectively. The HOMO
energy levels were determined by low energy photoelectron spectrometry

50=[206]
K. H. Lee et al.
(Riken-Keiki AC-2). LUMO energy levels were estimated by subtracting the energy
gap from the HOMO energy levels.
OLED Fabrication and Measurement
For fabricating the OLEDs, indium-tin-oxide (ITO) thin films coated on glass sub-
strates were used, with a sheet resistivity of 30 X=square and a thickness of 100 nm.
The ITO-coated glass was cleaned in an ultrasonic bath by the following sequence of
solvents: acetone, methyl alcohol, and distilled water, and kept in isopropyl alcohol
for 48 h and dried by N2 gas gun. The substrates were treated by O2 plasma treat-
ment under the conditions of 2 ꢂ 10ꢁ2 Torr at 125 W for 2 min [22]. All organic mate-
rials and metal were deposited under high vacuum (5 ꢂ 10ꢁ7 Torr). The OLEDs were
fabricated in the following sequence: ITO=N,N0-diphenyl-N,N0-(1-napthyl)-(1,10-
phenyl)-4,40-diamine (NPB) (50 nm)=9,10-di(2-naphthyl)anthracene (ADN): Blue
dopant materials (30 nm, 2%)=tris(8-hydroxyquinoline) aluminum (Alq3) (20 nm)=
lithium quinolate (Liq) (2 nm)=Al (100 nm). The current density (J), luminance
(L), luminous efficiency (LE), and CIE chromaticity coordinates of the OLEDs were
measured with a Keithley 2400 and Chroma Meter CS-1000A. Electroluminance was
measured using a Roper Scientific Pro 300i.
Results and Discussion
Figure 1 shows the molecular structures of the dopant compounds used in this study.
Compounds 1–3 were synthesized via Suzuki cross-coupling reactions between
4-bromo-triphenylamine and corresponding arylanthracene boronic acid compounds
with moderate yields. After conventional purifications such as column chromato-
graphy and recrystallization, the newly synthesized blue-emitting materials (1–3) were
purified further by train sublimation at reduced pressure, below 10ꢁ3 torr, and fully
1
characterized using H- and 13C-NMR, infrared (IR), and low- and high-resolution
mass spectrometry. High-pressure liquid chromatography (HPLC) analysis revealed
that the purity of the blue-emitting materials (1–3) was at least 99.0%.
Figure 2 shows the absorption (a) and emission (b) spectra of each of
compounds 1–3 in CH2Cl2 solution. The photophysical data are summarized in
Table 1. The absorption spectra have two major bands in the range of
Figure 2. (a) The absorption and (b) emission spectra of compounds 1–3.

Blue Fluorescent Anthracene Derivatives for OLEDs
Table 1. Optical properties of Compounds (1–3)
51=[207]
UVmax
[nm]
PLmax
[nm]
FWHM
[nm]
HOMO=LUMOc
[eV]
a
b
Compound
Eg
Ud
1
2
3
397
396
397
492
492
494
74
50
75
5.71=2.82
5.75=2.83
5.63=2.68
2.89
2.92
2.95
0.67
0.72
0.72
a,bMaximum absorption or emission wavelength, measured in CH2Cl2 solution.
cObtained from AC-2 and UV-vis absorption measurements.
dUsing BDAVBi as a standard; kex ¼ 360 nm (U ¼ 0.86 in CH2Cl2 solution).
360–400 nm. The peaks at 360–400 nm with a characteristic vibronic pattern are
attributed to the p-pꢃ transitions of the anthracene core [19,20]. The absorption bands
at 280–350 nm can be interpreted as the combination of the n-pꢃ transition of triaryl-
amine moieties and the p-pꢃ transitions of the substituted aryl groups on the anthra-
cene core [21]. These compounds 1–3 all have similar emission properties in CH2Cl2
solution, each with a maximum peak at 492–494 nm, in the blue region. We calculated
the fluorescence quantum yield (U) of 1–3 in the CH2Cl2 solution to be 0.67, 0.72 and
0.72, respectively, using BDAVBi (U ¼ 0.86 in CH2Cl2 solution) as a standard [22],
suggesting that these materials were expected to have highly efficient electrolumines-
cent properties in OLED devices. Their full widths at half maximum (FWHM) were
74, 50 and 75 nm, respectively, in CH2Cl2 solution. The edge of the UV-Vis spectra
was used to calculate the band gap of these blue materials. The calculated
HOMO-LUMO energy gaps (Eg) for 1–3 were 2.89, 2.92, and 2.95 eV, respectively.
The highest occupied molecular orbital (HOMO) levels were determined by using
a photoelectron spectrometer (Riken-Keiki AC-2), and the lowest unoccupied mol-
ecular orbital (LUMO) levels were calculated by subtracting the corresponding
optical band gap energies from the HOMO values. The HOMO and LUMO energy
levels for blue fluorescent materials varied from ꢁ5.63 to ꢁ5.75 eV and ꢁ2.68 to
ꢁ2.83 eV, respectively (Table 1). In particular, the blue material dopant 1, with a sub-
stituted 2-naphthylanthracene unit had lower HOMO (ꢁ0.08 eV) and LUMO energy
levels (ꢁ0.14 eV) in comparison with dopant 3, with a substituted 1-naphthylanth-
racene core. These results suggest that 2-position of naphthylanthracene group
decreases the HOMO and LUMO energy levels through the effective p-conjugation.
The device performance of the blue fluorescent materials was studied by doping
these materials in the ADN host at a 2% concentration. Figure 3 shows the device
structure and relative HOMO=LUMO energy levels of the materials used in this
study. NPB was employed as a hole-transporting layer (HTL) to provide hole trans-
port, and also functioned as host in the emitting layer (EML), while Alq3 was used as
the electron-transporting layer (ETL) [23,24].
Figure 4 shows the EL emission spectra of the OLEDs obtained from devices
1–3, and indicate that doped devices exhibited deeper blue than nondoped devices.
All devices exhibited blue emissions with maximum emission peaks at 460, 456,
and 458 nm for device 1, 2, and 3 (Table 2). Interestingly, there is a blue shift in
EL spectra of the device 3 as compared to device 1, because the steric effect of the
1-naphthalene group of dopant 3 decreases the p-conjugation length. The CIE
(Commission Internationale de l’Eclairage) coordinates of devices 1–3 were (0.16,
0.17), (0.15, 0.14), and (0.15, 0.14), respectively, which were close to the NTSC

52=[208]
K. H. Lee et al.
Figure 3. Energy-level diagram of the materials in devices 1–3.
standard blue (x ¼ 0.15, y ¼ 0.16). However, the CIE coordinates of the nondoped
devices corresponding to 1 and 2 were (0.14, 0.21), (0.14, 0.20) respectively, which
indicates that the doped devices exhibited a deeper blue color than the nondoped
device because the dopant materials functioned as a carrier trap in the doping system.
All devices 1–3 showed efficient blue emission, with results summarized in
Table 2. The device 3, which uses dopant 3 containing 1-naphtyl unit, exhibited
CIE coordinates of (0.15, 0.14) at 12 V, a luminance of 7225 cd=m2 at 12 V, a
luminous efficiency of 4.62 cd=A, and a power efficiency of 1.81 lm=W at 20mA=cm2.
Intriguing, similar results were obtained in the device 2 and 3. In other words,
Figure 4. Electroluminescence (EL) spectra of the devices 1–3.

Blue Fluorescent Anthracene Derivatives for OLEDs
53=[209]
Figure 5. (a) L-V characteristics and (b) I-V characteristics of the devices 1–3.
Figure 6. (a) Luminous and (b) Power efficiencies as a function of current density for the
devices 1–3.

54=[210]
K. H. Lee et al.
Table 2. EL performance characteristic of devices (1–3)
Lb
[cd=m2] [cd=A]
LEc
Max LEd
[cd=A]
PEe Max PEf
[lm=W] [lm=W]
CIEa
(x, y)
a
ELmax
[nm]
Device
1
460=472g
456=470g
458
7306
6541
7225
4.95
4.61
4.62
5.18=5.5g
4.71=4.0g
4.71
1.83
1.81
1.81
3.72=3.5g (0.16, 0.17)
(0.14, 0.21)g
2
3
2.30=4.0g (0.15, 0.14)
(0.14, 0.20)g
2.29
(0.15, 0.14)
Device (ITO=NPB=ADN: blue dopant materials 1–3 (2%)=Alq3=Liq=Al).
aValue collected at 12.0 V.
bMaximum luminance at 12.0 V.
cLuminous efficiency at 20 mA=cm2.
dMaximum luminous efficiency.
ePower efficiency at 20 mA=cm2.
fMaximum power efficiency.
gNondoped device with a configuration of ITO=NPB (50 nm)=EML=TPBI (30 nm)=LiF=
MgAg [17].
device 2, employing compound 2 with a phenyl substituent as a dopant material,
indicated deep blue emission with CIE coordinates of (0.15, 0.14) at 12.0 V, the
maximum luminance of 6541 cd=m2 at 12.0 V, and the luminous and power efficien-
cies of 4.61 cd=A and 1.81 lm=W at 20 mA=cm2, respectively. The EL performances
of devices 1–3 are sensitive to the structure of blue dopant materials in the emitting
layer. Among those, a device using dopant 1 comprising 2-naphtyl moiety exhibited
the best performance, with the maximum luminance of 7306 cd=m2 at 12.0 V, and the
luminous and power efficiencies of 4.95 cd=A and 1.83 lm=W at 20 mA=cm2, respect-
ively. However, the EL performances of the doped systems were not improved over
those of undoped systems. This indicates that energy transfer between host (AND)
and dopants (1–3) is not efficient because the absorption of dopants (1–3) do not
overlap effectively with the emission of a common host (ADN) as shown in
Figure 1 (a). This study implies that the effective energy transfer between host and
dopant is a key factor in improving the EL performances in a doped system.
Conclusions
In conclusion, highly efficient blue dopant materials based on mono(triphenylamino)
anthracene derivatives were synthesized via Suzuki cross-coupling. Multilayered
OLEDs were fabricated with the device structure of ITO=NPB=2% anthracene-
triphenylamine derivatives 1–3 doped in ADN=Alq3=Liq=Al. All devices exhibited
efficient blue emissions. Among those, device 1, with CIE x, y coordinates of
(0.16, 0.17) at 12.0 V, exhibited the best performance, with maximum luminance
of 7306 cd=m2 at 12.0 V, and luminous and power efficiencies of 4.95 cd=A and
1.83 lm=W at 20 mA=cm2, respectively.
Acknowledgments
This research was supported by Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (20090073679).

Blue Fluorescent Anthracene Derivatives for OLEDs
55=[211]
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