Blue organic light-emitting diodes based on diarylamino-substituted stilbene derivatives

Article abstract of DOI:10.1080/15421406.2012.689717

A series of diarylamino-substituted stilbene derivatives with various aromatic system were synthesized. multilayered OLEDs with the configuration of ITO/NPB (700 )/ADN: blue dopants 1-5 (300: 2 %)/Alq3 (300 )/Liq (10 )/Al were fabricated. All devices show

Full text of DOI:10.1080/15421406.2012.689717

Mol. Cryst. Liq. Cryst., Vol. 563: pp. 195–205, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.689717
Blue Organic Light-Emitting Diodes Based on
Diarylamino-Substituted Stilbene Derivatives
SUNG MIN KWON,1 KUM HEE LEE,1 HYUCK JOO KWON,2
∗
YOUNG KWAN KIM,2,∗ AND SEUNG SOO YOON1,
1Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea
2Department of Information Display, Hongik University, Seoul 121-791, Korea
A series of diarylamino-substituted stilbene derivatives with various aromatic system
were synthesized. multilayered OLEDs with the configuration of ITO/NPB (700 Å)/ADN:
blue dopants 1–5 (300 Å: 2 %)/Alq3 (300 Å)/Liq (10 Å)/Al were fabricated. All devices
showed the efficient blue emissions. Among these devices, the devices using 4,4ꢀ-bis(9,9-
diethylfluoren-2-yl-3,5-di-tert-butylphenylamino)stilbene 4 as a dopant exhibited blue
emission with the luminance of 12590 cd/m2, the luminous efficiency of 9.10 cd/A at
20 mA/cm2, the power efficiency of 3.73 lm/W at 20 mA/cm2, and the CIE coordinates
of (0.155, 0.195) at 10.0 V.
Keywords Blue fluorescent dopant; buchwald-hartwig cross-coupling; electrolumi-
nescence; OLEDs; stilbene derivatives
Introduction
Organic light-emitting diodes (OLEDs) have received considerable the next generation flat-
panel display because OLEDs have the advantages of low-driving-voltage, high efficiency,
and possibility for flexible and large-area display applications [1]. Red, green, and blue
are needed to obtain a full-color display. While a lager number of exceptional red and
green emitters have been developed that satisfy the requirements for OLEDs, efficient
and stable organic blue emitters are still rare. During the past decades, great efforts were
focused on the development of high-performance blue electroluminescent materials with
desirable properties, because the improved efficiency, raised color purity, and extended
operation lifetime are difficult due to their wide energy band gap. Thus, several efficient
blue emitters have been developed, including metal-complexes [2], distyrylarylenes [3,4],
oligofluorenes [5], spirobifluorenes [6,7], carbazoles [8], and others [9–13]. However, the
electroluminescent properties of blue emitters are necessary to improve full color display
performance.
∗Address correspondence to S.S. Yoon, Department of Chemistry, Sungkyunkwan University,
Cheoncheon-dong, Jangan-gu, Suwon, 440-746, Korea (ROK). Tel: (+82) 31-290-5971; Fax: (+82)
31-290-7075. E-mail: ssyoon@skku.edu; or Y. K. Kim, Department of Information Display, Hongik
University, Mapo-gu, Sangsu-dong, Seoul 121–791, Korea (ROK). Tel: (+82)2-320-1646. Fax:
(+82)2-3141-8928. E-mail: kimyk@hongik.ac.kr
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S. M. Kwon et al.
Stilbene derivatives have attracted great interests in the area of OLED because these
materials are well known to exhibit blue photoluminescence by themselves and have promis-
ing photoluminescent and electroluminescent properties [16]. Particularly, diarylamino-
substituted stilbene derivatives possess the great promise for the efficient blue OLED,
because the diaryamine moieties improve the hole transport properties [17,18] through the
effective conjugation with the stilbene core [14] as well as the fluorescent quantum yield
through a significant decrease of the quantum yields for cis-trans photoisomerization [15].
In this paper, we have designed and synthesized a new class of blue emitter 1–5 based
on bis(diarylamino)stilbene derivatives [19]. In these molecules, various arylamine groups
are introduced at the 4- and 4ꢀ-positions of the central stilbene core to test substituent effect
on the electroluminescent performances. In addition, we expected that the introduction of
a sterically bulky tert-butyl unit might reduce degree of intermolecular π-π stacking and
concentration quenching, leading to the improved EL performances. Herein, synthesis and
electroluminescent properties of these new materials when doped in the suitably matched
host in the emitting layer of an OLED are described.
Experimental
Synthesis
4,4ꢀ-bis(3,5-di-tert-butylphenylamino)stilbene (6): 4,4ꢀ-Dibromostilbene (2.10 g, 6.21
mmol), 3,5-di-tert-butylaniline (2.68 g, 13.0 mmol), Pd2dba3 (0.285 g, 0.31 mmol),
(2-biphenyl)di-tert-butylphosphine (0.185 g, 0.62 mmol), sodium tert-butoxide (2.38 g,
24.8 mmol), and toluene (30 mL) were charged in a two-necked flask kept under nitrogen.
The mixture was heated at reflux for 18 h. After cooling, the solvent was removed under
vacuum and the residue was extracted with dichloromethane/water. The organic layer was
dried over MgSO4 and filtered. The residue obtained after evaporation of the solvent was
recrystallized from dichloromethane/hexane. Compound 6 was obtained as a bright green
1
solid in 52.6% yield. H-NMR (300 MHz, CDCl3) [δ ppm]: 7.39 (d, J = 8.5 Hz, 4H),
7.04–6.93 (m, 12H), 5.81 (s, 2H), 1.32 (s, 36H). 13C-NMR (75 MHz, CDCl3) [δ ppm]:
152.2, 143.2, 141.9, 127.5, 117.1, 116.1, 113.5, 35.2, 31.7. FT-IR [ATR]: ν = 3420, 3029,
2965, 1592, 1517, 1449, 1335, 968, 849, 708 cm−1.EI-MS (m/z): 587 [M+].
General procedure for the Synthesis of Compound (1–5): To a toluene (15 mL)
solution of 4, 4ꢀ-bis(3,5-di-tert-butylphenylamino)stilbene (500 mg, 0.85 mmol) were
added corresponding aryl bromide (2.13 mmol), sodium tert-butoxide (327 mg,
3.41 mmol), Pd2dba3 (39 mg, 0.043 mmol), and (2-biphenyl)di-tert-butylphosphine (25 mg,
0.085 mmol) under nitrogen. The resulting solution was heated to reflux for 18 h. The so-
lution mixture was extracted with toluene and washed twice with water. The combined
organic layers were dried over MgSO4 and the solvent removed under reduced pressure to
afford a crude product that was purified by column chromatography on silica gel using a
mixture dichloromethane/hexane (1:4) as the eluent.
4,4ꢀ-bis(4-trimethylsilylphenyl-3,5-di-tert-butylphenylamino)stilbene (1): Bright
green solid. Yield: 39.9 %. 1H-NMR (300 MHz, CDCl3) [δ ppm]: 7.37-7.34 (m, 8H), 7.13
(s, 2H), 7.08–7.03 (m, 8H), 6.97–6.96 (m, 6H), 1.25 (s, 36H), 0.25 (s, 18H). 13C-NMR
(75 MHz, CDCl3) [δ ppm]: 152.0, 148.5, 147.2, 146.6, 134.3, 132.9, 131.9, 127.2, 126.8,
124.1, 122.2, 120.3, 117.9, 35.2, 31.7, 0.7. FT-IR [ATR]: ν = 2956, 1511, 1321, 1216,
1109, 829, 751,713, 683 cm−1. FAB-MS (m/z): 882 [M+]. HRMS-TOF (M++H) Anal.
calcd for C60H79N2Si2 883.5782: found: 883.5767.

Blue Organic Light-Emitting Diodes
197
4,4ꢀ-bis(4-methoxylphenyl-3,5-di-tert-butylphenylamino)stilbene (2): Bright green
1
solid. Yield: 43.5%. H-NMR (300 MHz, CDCl3) [δ ppm]: 7.31 (d, J = 8.4 Hz, 4H),
7.11–7.05 (m, 6H), 6.98–6.92 (m, 10H), 6.83 (d, J = 8.8 Hz, 4H), 3.81 (s, 6H), 1.24 (s,
36H). 13C-NMR (75 MHz, CDCl3) [δ ppm]: 156.2, 151.8, 147.9, 147.2, 141.0, 130.7, 127.4,
127.0, 126.2, 121.9, 118.8, 116.8, 114.8, 55.7, 35.1, 31.7. FT-IR [ATR]: ν = 2967, 1589,
1505, 1301, 1239, 1035, 825, 711 cm−1. FAB-MS (m/z): 798 [M+]. HRMS-TOF (M++H)
Anal. calcd for C56H67N2O2 799.5203: found: 799.5189.
4,4ꢀ-bis(2-naphthyl-3,5-di-tert-butylphenylamino)stilbene (3): Bright green solid.
1
Yield: 42.3 %. H-NMR (500 MHz, CDCl3) [δ ppm]: 7.75 (d, J = 7.5 Hz, 2H), 7.70
(d, J = 8.5 Hz, 2H), 7.59 (d, J = 8.5 Hz, 2H), 7.46 (s, 2H), 7.40–7.32 (m, 8H), 7.30 (dd,
J = 2.0, 9.0 Hz, 2H), 7.14 (s, 2H), 7.09 (s, 4H), 6.99 (s, 2H), 1.25 (s, 18H). 13C-NMR
(75 MHz, CDCl3) [δ ppm]: 152.1, 147.5, 146.9, 145.7, 134.7, 131.9, 130.2, 128.9, 127.8,
127.3, 127.2, 126.8, 126.4, 124.7, 124.5, 123.8, 120.1, 119.9, 117.7, 35.2, 31.7. FT-IR
[ATR]: ν = 3028, 2965, 1589, 1509, 1306, 961, 839, 750, 714, 659 cm−1. FAB-MS (m/z):
838 [M+]. HRMS-TOF (M+ + H) Anal. calcd for C62H67N2 839.5304: found: 839.5266.
4,4ꢀ-bis(9,9-diethylfluoren-2-yl-3,5-di-tert-butylphenylamino)stilbene (4): Bright
1
green solid. Yield: 46.5 %. H-NMR (500 MHz, CDCl3) [δ ppm]: 7.62 (d, J = 7.5 Hz,
2H), 7.57 (d, J = 8.0 Hz, 2H), 7.36 (d, J = 9.0 Hz, 4H), 7.32-7.27 (m, 4H), 7.24-7.23
(m, 2H), 7.11–7.06 (m, 10H), 6.98 (s, 6H), 1.99-1.87 (m, 8H), 1.24 (s, 36H), 0.34 (t, J =
7.0 Hz, 12H). 13C-NMR (75 MHz, CDCl3) [δ ppm]: 151.8, 151.3, 149.9, 147.7, 147.4,
147.1, 141.7, 136.8, 131.5, 127.2, 127.1, 126.6, 126.5, 123.9, 123.4, 123.0, 120.4, 119.5,
119.3, 119.0, 117.0, 56.4, 35.1, 33.0 31.6. FT-IR [ATR]: ν = 2970, 1502, 1460, 1366,
1217, 1033, 738, 661 cm−1. FAB-MS (m/z): 1026 [M+]. HRMS-TOF (M++H) Anal.
calcd for C76H87N2 1027.6869: found: 1027.6866.
4,4ꢀ-bis[(3,5-di-tert-butylphenyl)-4-biphenylamino]stilbene (5): Bright green solid.
1
Yield: 34.3 %. H-NMR (300 MHz, CDCl3) [δ ppm]: 7.60 (d, J = 7.2 Hz, 4H), 7.48
(d, J = 8.7 Hz, 4H), 7.44-7.37 (m, 8H), 7.31 (d, J = 7.3 Hz, 2H), 7.18–7.08 (m, 10H),
7.01–6.98 (m, 6H), 1.26 (s, 36H). 13C-NMR (75 MHz, CDCl3) [δ ppm]: 152.1, 147.3,
146.8, 140.9, 124.9, 131.9, 129.0, 127.8, 127.3, 127.0, 126.9, 126.7, 123.9, 123.8, 120.0,
117.7, 35.2, 31.7. FAB-MS (m/z): FT-IR [ATR]: ν = 3031, 2969, 1598, 1487, 1364, 1321,
966, 827, 758, 693 cm−1. FAB-MS (m/z): 890 [M+]. HRMS-TOF (M++H) Anal. calcd for
C66H71N2 891.5617: found: 891.5583.
Fabrication of OLED
For fabricating OLEDs, indium-tin-oxide (ITO) thin films coated on glass substrates were
used, which were 30 Ω/square of the sheet resistivity. The ITO-coated glass was cleaned
in an ultrasonic bath by the following sequences: acetone, methyl alcohol, distilled water,
storage in isopropyl alcohol for 48 h, drying by an N2 gas gun. The substrates were treated by
O2 plasma treatment at 2 × 10−2 Torr at 125 W for 2 min. All organic materials and metals
were deposited under high vacuum (5 × 10−7 Torr). The OLEDs fabricated in this study had
aconfigurationof indiumtinoxide(ITO)/4,4ꢀ-bis[N-(1-naphthyl)-N-phenylamino]biphenyl
(NPB) (700 Å)/9,10-di(2-naphthyl)anthracene (ADN): blue dopants 1–3 (300 Å: 2%)/tris(8-
hydroxyquinolinato)aluminum (Alq3) (300 Å)/lithium quinolate (Liq) (10 Å)/Al. All of the
optical and electrical properties of OLEDs such as the current density, luminance, luminous
efficiency and CIE coordinate characteristics were measured with Keithly 236, LS-50B,
and MINOLTA CS-100A, respectively.

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S. M. Kwon et al.
Measurements
All reactions were performed under a nitrogen atmosphere. Commercially available
reagents and solvents were used without further purification unless otherwise noted. 1H- and
13C- NMR spectra were recorded on a Varian (Unity Inova 300Nb or Unity Inova 500Nb)
spectrometer. FT-IR spectra were recorded using a Bruker VERTEX70 FT-IR spectrome-
ter. Low- and high- resolution mass spectra were measured using a Jeol JMS-AX505WA
spectrometer in FAB mode or Jeol JMS-600 spectrometer in EI mode and a JMS-T100TD
(AccuTOF-TLC) in the positive ion mode.
The UV-Vis absorption measurements of these blue compounds in dichloromethane
(10−5 M) were acquired with a Sinco S-3100 in a quartz cuvette (1.0 cm path). Pho-
toluminescence spectra were measured on an Amincobrowman series 2 luminescence
spectrometer. The fluorescence quantum yields of the blue materials were determined in
dichloromethane at 293 K against BDAVBi as a reference (Ф = 0.86) [20].
HOMO (highest occupied molecular orbital) energy levels were determined with a
low energy photoelectron spectrometer (Riken-Keiki, AC-2). The energy band gaps were
determined from the intersection of the absorption and photoluminescence spectra. LUMO
(lowest unoccupied molecular orbital) energy levels were calculated by subtracting the
corresponding optical band gap energies from the HOMO energy values.
Scheme 1. Molecular structures and synthetic route for bis(diarylamino)stilbene derivatives.

Blue Organic Light-Emitting Diodes
199
Figure 1. (a) Absorption and (b) emission spectra of blue-light-emitting materials 1–5 in
dichloromethane solution.
Results and Discussion
The development of palladium-catalyzed aromatic carbon-nitrogen bond formation pro-
vides a feasible method for synthesis of N-diarylamino-substituted stilbenes. As is shown
in Scheme 1, the Pd2dba3/(2-biphenyl)di-tert-butylphosphine/NaOtBu catalyst system was
employed for the coupling of trans-4,4ꢀ-bis(3,5-di-tert-butylphenylamino)stilbene and cor-
responding aryl bromides to afford compounds 1–5. To investigate the influence of the
aromatic end groups, five designed compounds introduced end groups with various aro-
matic units. Compounds 1–5 had 4-trimethylsilylphenyl, 4-methoxyphenyl, 2-naphthyl,
9,9-diethylfluoren-2-yl, and 4-biphenyl end group, respectively. The molecular structures
1
of the compounds were characterized by H- and 13C-NMR, FR-IR, and low- and high-
resolution mass spectroscopy.
The UV-Vis absorption spectra of these blue fluorescent materials in dichloromethane
are shown in Fig. 1(a). Also, the photophysical data are summarized in Table 1. Com-
pounds 1–5 show a main absorption peak at 396–408 nm range and good spectral overlap
was observed between the emission of the host (ADN) and the absorption of the dopant
materials 1–5. These observations indicate that the blue materials 1–5 can effectively accept
energy from the ADN host material by Fo¨rster-type energy transfer. A particularly good
spectral overlap was observed in the dopant 4. The energy transfer efficiency is highly
dependent on the spectral overlap between the emission of the host and the absorption of
the dopant, so this material should be expected to have highly efficient electroluminescent
properties in OLED devices.
The normalized PL emission spectra of the compounds 1–5 are shown in Fig. 1(b). All
materials showed blue fluorescence with maximum emission wavelengths at 451, 465, 454,
464, and 456 nm, respectively. A comparison of the emission wavelengths of compound 4
and 5 clearly indicates that the planar biphenyl unit of the outer fluorene in the former is easy
to the electron delocalization between N-aryl and aminostilbene groups. Compound 5 broke
the conjugation system due to the distorted structure at the biphenyl units. Consequently,
the energy gaps of compound 4 decreased while the absorption/emission peaks were red-
shifted in comparison with compound 5. The FWHM of the spectra is very narrow at
58–65 nm, and these blue-light-emitting bis(diarylamino)stilbene derivatives have high PL

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S. M. Kwon et al.
Table 1. Photophysical data of compounds 1–5
Abs
PL λmax FWHM HOMO LUMO Bandgap
Ф
Compound λmax [nm] [a] [nm] [b]
[nm]
[eV] [c] [eV] [c]
[eV]
[d]
1
2
3
4
5
396
399
403
408
401
451
465
454
464
456
59
65
58
59
58
−5.32
−5.25
−5.17
−5.29
−5.38
−2.44
−2.41
−2.32
−2.49
−2.53
2.88
2.84
2.85
2.80
2.85
0.98
0.82
0.85
0.67
0.75
[a,b] Maximum absorption and emission wavelength, measured in CH2Cl2 solution, [c] Obtained
from AC-2 and the intersection of the absorption and photoluminescence spectra, [d] Using BDAVBi
as a standard; λex = 360 nm (Ф = 0.86 in CH2Cl2).
quantum yields of 0.67–0.98, suggesting that these materials are expected to have highly
efficient electroluminescent properties in OLED devices.
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels of compounds 1–5 are shown in Table 1. The HOMO
energy levels were measured with an AC-2 photoelectron spectrometer and the LUMO
levels calculated by subtracting the corresponding optical band gap energies from the
HOMO values. The HOMO/LUMO energy levels of compounds 1–5 are −5.32/−2.44,
−5.25/−2.41, −5.17/−2.32, 5.29/−2.49, and −5.38/−2.53, respectively. Also the optical
band gap calculated from the threshold of optical absorption are 2.88 eV, 2.84 eV, 2.85 eV,
2.80 eV and 2.85 eV, respectively. The energy levels are sensitive to aryl substituents at the 4-
and 4ꢀ-positions of the stilbene core. It is expected that the introduction of the two electron-
donor methoxy groups can raise the HOMO energy level of the compound. Compared to
compound 1 with same aromatic framework, resonance effects provided directly by lone-
pair electrons in compound 2 will have stronger influence than the electronic inductive
effect of substituents in increase HOMO level. In fact, compared to the PL spectrum and
energy level of 1, those of 2 indicated red shifts of 14 nm and higher HOMO level due to
the introduction of two methoxy groups.
To explore the electroluminescent properties of these molecules, devices were fabri-
cated with a structure of ITO/NPB (700 Å)/ADN: Blue dopants 1-5 (300 Å)/Alq3 (300
Å)/Liq (20 Å)/Al and their electroluminescent performance characteristics of devices are
summarized in Table 2. The doping concentrations of 1–5 in ADN were optimized to 2%,
respectively. As shown in Fig. 2, the energy barrier between the HTL and EML is 0.3 eV,
while that between ETL and EML is 0.7 eV. This suggests that the device should allow more
efficient hole injection than electron injection into the EML. Also, it is well recognized that
the mobility of holes is much higher than that of electrons [21]. Consequently, holes tend to
transport easily to the ETL without recombining efficiently with electrons in the emissive
layer. To confine and enhance electron-hole recombination in the EML and thus to increase
device efficiency, a hole-transport layer thickness (70 nm) fabricated more than two times
that of the electron-transport layer (30 nm).
The EL emission spectra of the devices 1–5 with emitting layers consisting of the ADN
host doped with dopants 1–5 are seen in Fig. 3. All five stilbene-based devices exhibited
blue emissions with maximum emission peaks between 452-482 nm. The corresponding
CIE coordinates are x = 0.158, y = 0.177 for 1, x = 0.174, y = 0.264 for 2, x = 0.154,
y = 0.185 for 3, x = 0.155, y = 0.195 for 4, and x = 0.170, y = 0.224 for 5. Figure 4

201

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S. M. Kwon et al.
Figure 2. Relative energy-level alignments and layer thickness of OLEDs.
shows the operational lifetime of these blue devices. The measured lifetime of devices 1–5
at L/L0 = 0.7 were 263, 212, 76, 162, and 124 h at an initial luminance of 4000 cd m−2
,
respectively. Renormalizing to a initial luminance of 100 cd m−2, the lifetimes of devices
1–5 were measured to be 10500, 8480, 3040, 6480, and 4960 h, respectively. Interestingly,
device 3 employing 3 as a dopant with the higher LUMO and HOMO level than the others
showed the worst device operational lifetime.
Figure 5(a) and (b) shows the luminance-voltage and current density-voltage character-
istics of the devices containing ADN doped with various dopant materials. The maximum
Figure 3. EL spectra of devices using the five stilbene derivatives as dopant emitters.

Blue Organic Light-Emitting Diodes
203
Figure 4. Operational lifetime tests for the blue devices used in this study.
luminance of devices 1–5 were 7207, 8252 14810, 12590 and 10540 cd/m2, respectively.
Variations in the luminous and power efficiencies of devices 1–5, as a function of the
current density, are shown in Fig. 6(a) and (b). The luminous efficiency were 6.10, 7.35,
7.23, 9.10, and 7.66 cd/A at 20 mA/cm2, respectively. And the power efficiencies of de-
vices 1–5 were 2.39, 2.89, 3.21, 3.74, and 3.15 lm/W at 20 mA/cm2, respectively. Also,
the maximum luminous/ power efficiencies of compounds 1-5 are 6.65/3.38, 8.37/4.48,
7.39/3.98, 9.66/5.35, and 7.79(cd/A)/3.88(lm/W), respectively. The EL performances are
sensitive to the substituents at the 4- and 4ꢀ-positions of the central stilbene core. Notably,
the efficiency of the OLED device with dopant 4 were much higher than those of dopant
1, 2, 3, and 5 due to the superior matched spectral overlap between the emission of the
host and the absorption of the dopant, as shown in Fig. 1(a). The maximum brightness of
this device was 12590 cd m−2, with a luminous efficiency of 9.10 cd A−1 at 20 mA cm−1
and power efficiency of 3.74 lm W−1 at 20 mA cm−1. The EL parameters for blue dopant
Figure 5. (a) Luminance and (b) current density versus applied electric voltage characteristics of
devices 1–5.

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S. M. Kwon et al.
Figure 6. (a) Luminous efficiencies and (b) power efficiencies as a function of current density for
the device.
4 reflect its promising application in OLED devices. This study clearly suggests that not
only the high quantum yield of the dopant, but also the superior matched spectral overlap
between the emission of the host and the absorption of the dopant can improve efficiencies
of blue OLEDs composed of a host and dopant.
Conclusions
A series of blue-emitting materials based on a stilbene core with various arylamine groups
were designed and synthesized for OLEDs. When doped in the ADN host in the OLED
device, these materials showed blue electroluminescent properties. Particularly, the device
4 doped with blue dopant 4 showed the CIE coordinate of (0.155, 0.195) at 10 V, a
maximum luminance of 12590 cd/m2, a luminous efficiency of 9.10 cd/A at 20 mA/cm2,
and a power efficiency 3.74 lm/W at 20 mA/cm2. These results demonstrated that these new
bis(diarylamino)stilbene derivatives are promising blue dopants for OLED applications.
Acknowledgment
This research was supported by Basic Science Research Program through the NRF funded
by the Ministry of Education, Science and Technology (20110004655).
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