Blue-light-emitting bis(diarylamino)stilbene derivatives: Synthesis, photophysical properties, and highly efficient OLEDs

Article abstract of DOI:10.1080/15421406.2010.495886

New electroluminescent materials, 4,4′-bis(2-naphthyl-4-tert- butylphenylamino)stilbene (1), 4,4′-bis(4-tert-butylphenyl-3,5-di-tert- butylphenylamino)stilbene (2), and 4,4′-bis(di-4-tert-butylphenylamino) stilbene (3) were synthesized and characterized. To evaluate the electroluminescent properties of these compounds, multilayered OLED devices 1-3 were designed. By employing blue dopant 1 into a multilayer device, it was possible to achieve a maximum luminance of 8861cdm-²at 12.0V, a luminous efficiency of 6.82cdA^-1 at 20mAcm^-2, a power efficiency of 2.72lmW^-1 at 20mAcm^-2, and CIEx,y coordinates of (x=0.158,y=0.187) at 10.0V. These results demonstrated the superiority of these bis(diarylamino)stilbene derivatives in organic light-emitting devices. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2010.495886

Mol. Cryst. Liq. Cryst., Vol. 530: pp. 40=[196]–47=[203], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.495886
Blue-Light-Emitting Bis(diarylamino)stilbene
Derivatives: Synthesis, Photophysical Properties,
and Highly Efficient OLEDs
SEUL ONG KIM,1 KUM HEE LEE,1
HYUCK JOO KWON,2 YOUNG KWAN KIM,2
AND SEUNG SOO YOON1
1Department of Chemistry, Sungkyunkwan University, Suwon,
Gyeonggi-do, Korea
2Department of Information Display, Hongik University, Seoul, Korea
New electroluminescent materials, 4,40-bis(2-naphthyl-4-tert-butylphenylamino)-
stilbene (1), 4,40-bis(4-tert-butylphenyl-3,5-di-tert-butylphenylamino)stilbene (2),
and 4,40-bis(di-4-tert-butylphenylamino)stilbene (3) were synthesized and charac-
terized. To evaluate the electroluminescent properties of these compounds, multi-
layered OLED devices 1-3 were designed. By employing blue dopant 1 into a
multilayer device, it was possible to achieve a maximum luminance of 8861 cd mꢀ2
at 12.0 V, a luminous efficiency of 6.82 cd Aꢀ1 at 20 mA cmꢀ2, a power efficiency
of 2.72 lm Wꢀ1 at 20 mA cmꢀ2, and CIEx,y coordinates of (x ¼ 0.158,y ¼ 0.187)
at 10.0 V. These results demonstrated the superiority of these bis(diarylamino)-
stilbene derivatives in organic light-emitting devices.
Keywords Bis(diarylamino)stilbene; blue fluorescent materials; Buchwald-
Hartwig cross-coupling reaction; organic light-emitting diode
Introduction
Organic light-emitting diode (OLED) displays are considered 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 [1,2].
Research on OLEDs has focused on the improvement of device performance by
introducing highly efficient materials. In particular, there has been considerable
interest in developing the performance of blue OLEDs, which is relatively poor com-
pared with that of red and green OLEDs [3]. It is therefore important to develop
high-performance blue-light-emitting materials with good color purity and intense
luminescent efficiency. Out of small molecules, several classes of materials based
on anthracenes [4,5], distyrylarylenes [6], oligofluorenes [7], spirobifluorenes [8,9],
Address correspondence to Prof. Seung Soo Yoon, Department of Chemistry, Sung-
kyunkwan University, Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do 440-746, Korea
(ROK). Tel.: (þ82)31-290-7071; Fax: (þ82)31-290-5971; E-mail: ssyoon@skku.edu
40=[196]

Blue-Light-Emitting Bis(diarylamino)stilbenes
41=[197]
metal chelates [10], and others [11–13] have been explored as possible blue OLEDs.
However, blue emitters are necessary to improve full color display performance.
In this study, the synthesis and electroluminescent properties of three stilbene
derivatives, 4,40-bis(2-naphthyl-4-tert-butylphenylamino)stilbene (1), 4,40-bis(4-tert-
butylphenyl-3,5-di-tert-butylphenylamino)stilbene (2), and 4,40-bis(di-4-tert-butyl-
phenylamino)stilbene (3) are reported. Since trans-4,40-bis(diarylamino)stilbene deri-
vatives have promising photoluminescent and electroluminescent properties [14,15],
they have served in various applications, including organic light-emitting diodes. In
the three different blue dopant materials studied here, sterically bulky tert-butyl
groups were introduced into the diarylamine moiety to prevent intermolecular p-p
stacking interactions and thus reduce concentration quenching. We herein demon-
strate highly efficient blue OLEDs using these new blue emitters.
Experimental
Materials and Measurements
All reactions were performed under a nitrogen atmosphere. Commercially available
1
reagents were used without further purification unless otherwise stated. H- and
13C-NMR were recorded on a Varian Unity Inova 300Nb spectrometer. FT-IR spec-
tra were recorded using a Bruker VERTEX70 FT-IR spectrometer. Low- and high-
resolution mass spectra were measured using a Jeol JMS-600 spectrometer in the
FAB mode and a JMS-T100TD (AccuTOF-TLC) in the positive ion mode.
Synthesis
Synthesis of 4,40-bis(2-naphthyl-4-tert-butylphenylamino)stilbene (1). To a toluene
(10 mL) solution of 4,40-bis(4-tert-butylphenylamino)stilbene (0.60 g, 1.26 mmol)
was added 2-bromonaphthalene (0.58 g, 2.78 mmol), sodium tert-butoxide (0.36 g,
3.78 mmol), Pd2dba3 (0.058 g, 0.063 mmol), and (2-biphenyl)di-tert-butylphosphine
(0.038 g, 0.13 mmol) under N2. The resulting solution was heated to reflux for
18 h. The reaction mixture was extracted with toluene, washed with aqueous NaCl
solution, and dried with MgSO4. After filtration and evaporation of the solvent,
the crude product was purified by column chromatography (silica gel,
hexanes:EtOAc ¼ 15:1) and recrystallization from THF:MeOH to give the desired
1
compound as a bright green solid (0.81 g, 88.3%). H-NMR (CDCl3, 300 MHz) [d
ppm]: 7.74 (d, J ¼ 7.6 Hz, 2H), 7.69 (d, J ¼ 8.8 Hz, 2H), 7.58 (d, J ¼ 7.6 Hz, 2H),
7.45 (s, 2H), 7.39–7.32 (m, 8H), 7.30–7.26 (m, 6H), 7.07 (dd, J ¼ 2.2, 8.7 Hz, 8H),
6.96 (s, 2H), 1.32 (s, 18H). 13C-NMR (CDCl3, 75 MHz) [d ppm]: 147.5, 146.5,
145.6, 145.0, 134.7, 132.1, 130.3, 129.1, 127.8, 127.4, 127.2, 126.8, 126.5, 126.4,
124.7, 124.6, 124.5, 124.0, 120.4, 34.6, 31.7. FT-IR [ATR]: n ¼ 3028, 2962, 1629,
1506, 1363, 1294, 965, 851, 747, 652 cmꢀ1
.
FAB-MS (m=z): 726 [Mþ].
HRMS-TOF (Mþ þ H) Anal. calcd for C54H51N2 727.40522: found: 727.40056.
Synthesis of 4,40-bis(4-tert-butylphenyl-3,5-di-tert-butylphenylamino)stilbene (2).
The synthetic procedure used was similar to that described for 1 and the desired
1
compound was obtained as a bright green solid (0.41 g, 56.0%). H-NMR (CDCl3,
300 MHz) [d ppm]: 7.33 (d, J ¼ 8.5 HZ, 4H), 7.26–7.23 (m, 4H), 7.09 (s, 2H),
7.05–7.01 (m, 8H), 6.96–6.94 (m, 6H), 1.32 (s, 18H), 1.25 (s, 36H). 13C-NMR

42=[198]
S. O. Kim et al.
(CDCl3, 75 MHz) [d ppm]: 151.9, 147.7, 147.0, 145.6, 145.2, 131.3, 127.1, 126.5,
126.1, 123.8, 123.0, 119.8, 117.3, 35.1, 34.5, 31.7, 31.6. FT-IR [ATR]: n ¼ 2962,
1650, 1510, 1329, 1306, 1248, 833, 712, 623 cmꢀ1. FAB-MS (m=z): 850 [Mþ].
HRMS-TOF (Mþ þ H) Anal. calcd for C62H79N2 851.62433: found: 851.62405.
Synthesis of 4,40-bis(di-4-tert-butylphenylamino)stilbene (3). The synthetic
procedure used was similar to that described for 1 and the desired compound was
1
obtained as a bright green solid (0.33 g, 67.3%). H-NMR (CDCl3, 300 MHz) [d
ppm]: 7.32 (d, J ¼ 8.7 Hz, 4H), 7.25 (d, J ¼ 8.7 Hz, 8H), 7.04–7.00 (m, 12H), 6.91
(s, 2H), 1.31 (s, 36H). 13C-NMR (CDCl3, 75 MHz) [d ppm]: 147.5, 145.9, 145.1,
131.5, 127.2, 126.6, 126.3, 124.2, 123.2, 34.5, 31.7. FT-IR [ATR]: n ¼ 3031, 2959,
1603, 1509, 1364, 1323, 1282, 826, 696, 629 cmꢀ1. FAB-MS (m=z): 738 [Mþ].
HRMS-TOF (Mþ þ H) Anal. calcd for C54H63N2 739.49913: found: 739.49416.
Physical Measurements
The UV-Vis absorption and photoluminescence spectra of the new dopant materials
were measured in dichloromethane (10ꢀ5 M) using Sinco S-3100 and Amincobrow-
man series 2 luminescence spectrometers. The fluorescence quantum yields of the
emitting materials were determined in dichloromethane at 293 K against BDAVBi
as a reference (U ¼ 0.86). HOMO (highest occupied molecular orbital) energy levels
were determined with a low-energy photoelectron spectrometer (Riken-Keiki,
AC-2).
OLED Fabrication and Characterization
For fabricating OLEDs, indium-tin-oxide (ITO) thin films coated on glass substrates
˚
were used, which were 30 X=square of the sheet resistivity at a 1000 A thickness. The
ITO-coated glass was cleaned in an ultrasonic bath by the following sequences: acet-
one, 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 ꢁ 10ꢀ2 Torr
at 125 W for 2 min. All organic materials and metals were deposited under high vac-
uum (5 ꢁ 10ꢀ7 Torr). The OLEDs fabricated in this study had a configuration of
indium tin oxide (ITO)=4,40-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB)
˚
˚
(700 A)=9,10-di(2-naphthyl)anthracene (ADN): blue dopants 1–3 (300 A: 2%)=
˚
˚
tris(8-hydroxyquinolinato)aluminum (Alq3) (300A)=lithium quinolate (Liq) (10 A)=
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 a Keithly 236, LS-50B, and MINOLTA CS-100A, respectively.
Results and Discussion
Scheme 1 shows the synthetic routes of the all compounds used in this study. As
shown in Scheme 1, we prepared bis(diarylamino)stilbene derivatives 1–3 via
Buchwald-Hartwig cross-coupling reactions using Pd2dba3, t-BuONa, and the John-
Phos ligand (dba: dibenzylideneacetone, JohnPhos: (2-biphenyl)di-tert-butyl-
phosphine). A Pd-catalyzed coupling reaction of these species delivered the desired
stilbenes 1–3 with yields in the range of 56–88%. All the compounds were character-
1
ized by H- and 13C-NMR, FT-IR, and low- and high-mass spectroscopy.

Blue-Light-Emitting Bis(diarylamino)stilbenes
43=[199]
Scheme 1. Synthetic routes of compounds 1–3. Condition (i), (ii): Pd2(dba)3 (5 mol %),
(2-biphenyl)di-tert-butylphosphine (10 mol %), t-BuONa (3.0 eq), toluene, 120ꢃC, 18 hr.
Figure 1a shows the normalized optical absorption spectra of the three dopant
materials and emission of the host material in the dichloromethane solution. A good
spectral overlap was observed between the emission of the host (ADN) and the
absorption of the dopant materials 1–3. These observations indicate that the blue
materials 1–3 can effectively accept energy from the ADN host material by
Fo¨rster-type energy transfer. The energy transfer efficiency is highly dependent on
the spectral overlap between the emission of the host and the absorption of the
dopant, so these materials should be expected to have highly efficient electrolumines-
cent properties in OLED devices. The absorption spectra show maximum peaks in
the 396–400 nm range. These absorption bands are thus associated with n ! pꢂ
and p ! pꢂ transitions of the stilbene derivatives. The normalized PL emission spec-
tra of the compounds 1–3 are shown in Figure 1b. Blue emissions with a PL
maximum of 452–456 nm were observed. Notably, the three blue materials show very
similar UV (396–400 nm) and photoluminescence (452–456 nm) spectra. The simi-
larity of absorption and emission spectra in the blue materials reflect the fact that
Figure 1. (a) UV-vis absorption and (b) photoluminescence spectra of compounds 1–3 in a
10ꢀ5 M dichloromethane solution.

44=[200]
S. O. Kim et al.
Table 1. Photophysical data for the compounds 1–3
UVkmax
[a] [nm]
PLkmax
[b] [nm]
FWHM
[nm]
HOMO
[c] [eV]
LUMO
[c] [eV]
Compound
Eg
U[d]
1
2
3
400
398
396
452
456
456
58
60
60
5.29
5.34
5.27
2.42
2.49
2.39
2.87
2.85
2.88
0.82
0.92
0.99
[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; kex ¼ 360 nm (U ¼ 0.86 in CH2Cl2).
the effective conjugation length in these molecules is the same, since the t-butyl unit
have no influence on conjugation length. The FWHM of the spectra is very narrow
at 58–60 nm, and these blue-light-emitting bis(diarylamino)stilbene derivatives have
very high PL quantum yields of 0.82–0.99, suggesting that these materials are
expected to have highly efficient electroluminescent properties in OLED devices.
The highest occupied molecular orbital (HOMO) and lowest unoccupied mol-
ecular orbital (LUMO) energy levels of compounds 1–3 are shown in Table 1 and
Figure 2. The HOMO energy levels were measured with an AC-2 photoelectron spec-
trometer and the LUMO levels calculated by subtracting the corresponding optical
band gap energies from the HOMO values. The HOMO=LUMO energy levels of
compounds 1–3 are 5.29=2.42, 5.34=2.49, and 5.27=2.39, respectively. Compounds
1–3 show a slight difference between the HOMO and LUMO energy levels. Because
N-substituted phenyl and naphthyl end groups are in a distorted p-orbital plane of
the stilbene core unit, these aryl units contribute little to the molecular conjugation
level. As expected, this result causes similar electron delocalization of blue-light-
emitting materials 1–3, and these compounds have almost the same energy levels.
Their optical energy band gaps (Eg) are 2.87, 2.85, and 2.88 eV, respectively, as
determined from the intersection of the absorption and photoluminescence spectra.
To evaluate the EL properties of using these compounds as dopants in the emit-
ting layer, multilayered devices 1–3 were fabricated with the structure of (ITO)=
Figure 2. Relative energy-level alignments and layer thickness of OLEDs.

Blue-Light-Emitting Bis(diarylamino)stilbenes
45=[201]
Figure 3. Electroluminescence spectra of devices 1–3.
4,40-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) (70 nm)=9,10-di(2-naphthyl)
anthracene (ADN): blue dopants 1–3 (30 nm: 2%)=tris(8-hydroxyquinolinato)alumi-
num (Alq3) (30 nm)=lithium quinolate (Liq) (1 nm)=Al, in which ITO served as the
substrate, ADN acted as a host material, NPB as a hole transport material, and
Alq3 as an electron transport material.
The EL emission spectra of the devices 1–3 with emitting layers consisting of the
ADN host doped with dopants 1–3 are shown in Figure 3. All three stilbene-based
devices exhibited blue emissions in the region of 454–456 nm, with a vibrational
shoulder at 478–480 nm. The corresponding CIE coordinates are x ¼ 0.158,
y ¼ 0.187 for 1, x ¼ 0.165, y ¼ 0.208 for 2, and x ¼ 0.170, y ¼ 0.230 for 3. The device
employing dopant 1, with a narrower FWHM and shorter emission wavelength,
shows CIEy ¼ 0.187 in the deeper blue region.
The current density-voltage (J-V) and luminance-voltage (L-V) characteristics of
the three stilbene OLEDs are shown in Figure 4a and b, respectively. The key perform-
ance parameters of the devices and EL emission characteristics are summarized in
Figure 4. (a) Current density versus applied electric voltage and (b) luminance versus applied
electric voltage characteristics of devices 1–3.

46=[202]
S. O. Kim et al.
Table 2. Electroluminescence data for the devices 1–3
L. E.
[cd=A]
[c]=[d]
P. E.
[lm=W]
[c]=[d]
ELmax J [mA=cm2] L [cd=m2]
CIE
(x,y) [e]
Device
[nm]
[a]
[b]
1
2
3
456=480
454=478
456=478
225.0
186.7
208.3
8861
6296
6934
7.01=6.82 3.27=2.72 (0.158, 0.187)
7.39=6.49 3.92=2.49 (0.165, 0.208)
7.56=6.73 3.85=2.64 (0.170, 0.230)
[a, b] The maximum current density and luminance, [c] Maximum values, [d] At 20 mA=cm2,
´
[e] Commission Internationale d’Enclairage (CIE) coordinates at 10.0 V.
Figure 5. (a) Luminous efficiencies and (b) power efficiencies as a function of current density
for the devices 1–3.
Table 2. The maximum brightness of the stilbene-based blue OLEDs varied from
6296 cd mꢀ2 in device 2 to as high as 8861 cd mꢀ2 in device 1. Variations in the luminous
and power efficiencies of devices 1–3, as a function of the current density, are shown
in Figure 5a and b. These devices show high luminous efficiencies, between
7.01–7.56 cd Aꢀ1, as blue emitters because of the superior matched spectral overlap
between the emission of the host and the absorption of the dopant, as shown in
Figure 1. In particular, compound 1 with 2-naphthyl end groups gave the best device
performance among the three stilbene derivatives 1–3. The maximum brightness of this
device was 8861 cd mꢀ2, with a luminous efficiency of 6.82 cd Aꢀ1 at 20 mA cmꢀ2 and
power efficiency of 2.72 lm Wꢀ1 at 20 mA cmꢀ2. The EL parameters for blue dopant 1
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 over-
lap between the emission of the host and the absorption of the dopant can improve effi-
ciencies of blue OLEDs composed of a host and dopant.
Conclusions
In summary, new blue-light-emitting bis(diarylamino)stilbene derivatives 1–3 con-
taining bulky tert-butyl groups were designed and synthesized. The stilbenes each

Blue-Light-Emitting Bis(diarylamino)stilbenes
47=[203]
exhibited very high photoluminescence quantum yield. By employing blue dopant 1
into a multilayer device structure, it was possible to achieve a maximum luminance
of 8861 cd mꢀ2 at 12.0 V, a luminous efficiency of 6.82 cd Aꢀ1 at 20 mA cmꢀ2, a
power efficiency of 2.72 lm Wꢀ1 at 20 mA cmꢀ2, and CIEx,y coordinates of (x ¼ 0.158,
y ¼ 0.187) at 10.0 V. These results demonstrate the superiority of these bis(diarylami-
no)stilbene derivatives in OLED devices.
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).
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