4-(2-(4′-(2,2-diphenylvinyl)biphenyl-4-yl)vinyl)-N,N-diphenylaniline derivatives for blue organic light-emitting diodes

Article abstract of DOI:10.1080/15421406.2013.849458

This paper reports the synthesis and electroluminescent properties of the new blue fluorescent diphenylaniline derivatives end-caped with various aryl groups. Multilayered OLEDs were fabricated using these materials as dopants in ADN host, with the configuration of ITO/DNTPD (40 nm)/NPB (20 nm)/ADN: Blue dopants 1-4 (20 nm)/Alq₃ (40 nm)/Liq (1.0 nm)/Al. All devices showed the efficient blue emissions. Among these devices, the devices C as a dopant exhibited blue emission with the luminance of 18020 cd/m² at 11v, the luminous efficiency of 9.6 cd/A at 20 mA/cm², the power efficiency of 3.91 lm/W at 20 mA/cm². In addition, the device D exhibited deep blue emisson with CIE coordinates of (0.151, 0.134) at 8 V showed a luminous efficiency of 5.3 cd/A and power efficiency of 2.3 lm/W at 20 mA/cm².

Full text of DOI:10.1080/15421406.2013.849458

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4-(2-(4′-(2,2-diphenylvinyl)biphenyl-4-
yl)vinyl)-N,N-diphenylaniline Derivatives
for Blue Organic Light-Emitting Diodes
Sung Min Kwon a , Kum Hee Lee a , Seok Jae Lee b , Young Kwan Kim
b & Seung Soo Yoon a
a Department of Chemistry , Sungkyunkwan University , Suwon ,
440-746 , Korea
b Department of Information Display , Hongik University , Seoul ,
121-791 , Korea
Published online: 16 Dec 2013.
To cite this article: Sung Min Kwon , Kum Hee Lee , Seok Jae Lee , Young Kwan Kim & Seung Soo
Yoon (2013) 4-(2-(4′-(2,2-diphenylvinyl)biphenyl-4-yl)vinyl)-N,N-diphenylaniline Derivatives for
Blue Organic Light-Emitting Diodes, Molecular Crystals and Liquid Crystals, 584:1, 103-112, DOI:
10.1080/15421406.2013.849458
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Mol. Cryst. Liq. Cryst., Vol. 584: pp. 103–112, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.849458
4-(2-(4ꢀ-(2,2-diphenylvinyl)biphenyl-4-yl)vinyl)-N,N-
diphenylaniline Derivatives for Blue Organic
Light-Emitting Diodes
SUNG MIN KWON,1 KUM HEE LEE,1 SEOK JAE LEE,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
This paper reports the synthesis and electroluminescent properties of the new blue flu-
orescent diphenylaniline derivatives end-caped with various aryl groups. Multilayered
OLEDs were fabricated using these materials as dopants in ADN host, with the con-
figuration of ITO/DNTPD (40 nm)/NPB (20 nm)/ADN: Blue dopants 1–4 (20 nm)/Alq3
(40 nm)/Liq (1.0 nm)/Al. All devices showed the efficient blue emissions. Among these
devices, the devices C as a dopant exhibited blue emission with the luminance of 18020
cd/m2 at 11v, the luminous efficiency of 9.6 cd/A at 20 mA/cm2, the power efficiency of
3.91 lm/W at 20 mA/cm2. In addition, the device D exhibited deep blue emisson with
CIE coordinates of (0.151, 0.134) at 8 V showed a luminous efficiency of 5.3 cd/A and
power efficiency of 2.3 lm/W at 20 mA/cm2.
Keywords Blue OLEDs; fluorescent dopant; styrylarylene derivatives; unsymmetrical
structure
Introduction
Since the pioneering work of multilayered organic light-emitting devices (OLEDs) by Tang,
this research field has attracted extensive attention because of its huge market potential in
the next-generation flat-panel display technology [1]. For full color OLED display appli-
cations, a set of primary blue, green, and red emitters is required. Red and green OLEDs
with high efficiency and stability have been achieved [2,3]. However, the development of
efficient blue-emitting materials remains an important issue for full color display. There-
fore, there 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 [4–6]. For
this reason, several efficient blue emitters have been developed, including metal-complexes,
distyrylarylenes, oligofluorenes, spirobifluorenes, carbazoles, and others [7–13]. Among
the blue emitters developed thus far, an unsymmetrical mono(styryl)amine-based blue
dopant known as BD-1 has high efficiency and a pure blue color. A current efficiency of
5.4 cd/A with CIE coordinates of (0.14, 0.13) and external quantum efficiency of 5.1%
∗Address correspondence to Prof. Seung Soo Yoon, Department of Chemistry, Sungkyunkwan
University, Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do 440-746, Korea (ROK). Tel.:
(+82)31-290-7071; Fax: (+82)31-290-5971. E-mail: ssyoon@skku.edu
[291]/103

104/[292]
S. M. Kwon et al.
Figure 1. Synthetic routes of blue fluorescent materials 1–4. Conditions: (a) KOtBu, THF, 0◦C to
room temperature, 30 min. (b) Pd(PPh3)4, K2CO3 (2M), Aliquat 336, Toluene, reflux, 3h.
were demonstrated by doping BD-1 in 2-methyl-9,10-di(2-naphthyl)anthracene (MADN).
Unfortunately, blue dopant materials based on aromatic amine units suffer color-shifting
problems at high doping concentrations due to intermolecular interactions between the
dopant materials.
In this paper, we report the synthesis, characterization, and EL properties of the blue
fluorescent materials based on unsymmetrical diphenylaminostyrylarylene derivatives for
OLEDs. The blue fluorescent materials (1–4) were based on a 4,4ꢀ-distyrylbiphenyl with
a diphenylamine unit and various end-capped groups (Fig. 1). In these materials, the 4,4ꢀ-
distyrylbiphenyl cores were combined with diphenylamine units, which have the enhanced
hole-transporting properties [14–17]. Furthermore, the bulky diaryl end-capping groups
could prevent the close-packing of these materials in solid state and improve the EL effi-
ciencies by the suppression of concentration quenching. Also, the unsymmetrical structures
of these materials would have less tendency to pack into crystal lattice and hence favor
amorphous morphology [18–20]. As described herein, blue OLEDs using these novel blue
materials showed the high efficiencies and deep blue color chromaticities.
Experimental
General Procedure for the Synthesis of Compounds 1–4
General procedure for the Horner-Wadsworth-Emmons reaction (1-3): To a mixture of
phosphonate intermediate (0.11 mmol) and the corresponding aldehyde (0.13 mmol) in
10 mL of anhydrous THF at 0◦C was added dropwise 0.13 mL of 1.0 M KOtBu (0.13 mmol)
in THF under nitrogen. The reaction mixture was stirred for 10 min at 0◦C, followed by

N,N-diphenylaniline Derivatives for Blue OLEDs
[293]/105
Figure 2. UV absorption spectra (a) and PL spectra of materials (1–4).
1 hr at room temperature and quenched with water. The solution mixture was filtered with
water. The filtered solid was dissolved in CH2Cl2, dried over Na2SO4, filtered with silica gel,
and the solvent removed under reduced pressure. The crude mixture was purified through
column chromatography using silica-gel and crystallization to give the desired compound.
Table 1. Physical properties of materials 1–4
Compound
UVa
PLa
FWHM
HOMO
LUMO
Eg
Q.Y.b
1
2
3
4
388
390
389
385
488
495
495
479
79
83
82
75
5.58
5.59
5.58
5.60
2.78
2.76
2.75
2.72
2.80
2.83
2.83
2.88
0.21
0.60
0.72
0.89
aCH2Cl2 solution (10−5 M). bUsing BDAVBi as a standard; λex = 360nm (Ф = 0.86 in CH2Cl2).

106/[294]
S. M. Kwon et al.
Table 2. Electroluminescent characteristics of devices A-D
Device EL [nm]a J [mA/cm2]b L [cd m−2]d LE [cd A−1 c,d
]
PE [lm W−1 c,d
]
CIE [x,y]e
A
B
C
D
458, 60
460, 56
462, 56
454, 55
128
86
102
128
16800
15970
18020
5257
8.08/ 8.17
3.26/ 3.77
3.59/ 3.85
3.92/ 4.28
2.26/ 2.35
0.159, 0.176
0.157, 0.185
0.157, 0.197
0.151, 0.134
8.81/ 8.82
9.57/ 9.59
5.29/ 5.30
aFWHM. bCurrent density at 10V. cAt 20 mA cm−2. dMaximum value. eValue collected at 8.0 V.
1: 4-(2-(4ꢀ-(2,2-diphenylvinyl)biphenyl-4-yl)vinyl)-N,N-diphenylaniline: Yield 48%.
1H-NMR (300MHz, CDCl3): δ ppm 7.54-7.47 (m, 4H), 7.40-7.30 (m, 11H), 7.29-7.28
(m, 1H), 7.26-7.20 (m, 6H), 7.11-7.09 (d, J = 7,5Hz, 5H), 7.06-6.95 (m, 8H). 13C-NMR
(75 MHz, CDCl3): δ ppm 147.5, 147.4, 143.4, 142.7, 140.5, 139.3, 138.8, 136.7, 136.450,
131.5, 130.4, 130.0, 129.3, 128.8, 128.3, 128.2, 127.7, 127.6, 127.6, 127.5, 127.4, 127.0,
126.7, 126.5, 126.3, 124.5, 123.6, 123.1. APCI-MS (m/z): 602 [M+]. HRMS [EI+] calcd
for C46H35N: 601.2770, found: 601.2765.
2: 4-(2-(4ꢀ-(2-(biphenyl-4-yl)-2-phenylvinyl)biphenyl-4-yl)vinyl)-N,N-diphenylani-
line; Yield 56%. 1H-NMR (300MHz, CDCl3): δ ppm 7.68-7.66 (d, J = 6.9Hz, 1H), 7.62-
7.60 (d, J = 8.1 Hz, 2H), 7.57-7.52 (m, 5H), 7.45-7.28 (m, 15H), 7.23 (s, 2H), 7.17-7.14
(d, J = 8.4Hz, 1H), 7.12-7.10 (d, J = 8.1Hz, 5H),7.06-7.02 (m, 7H). 13C-NMR (75 MHz,
CDCl3): δ ppm 147.5, 147.4, 143.5, 142.3, 140.6, 140.1, 139.4, 139.3, 138.8, 136.7, 136.5,
131.5, 130.9, 130.4, 130.1, 129.3, 128.9, 128.8, 128.3, 128.2, 128.0, 127.8, 127.6, 127.4,
127.3, 127.0, 126.9, 126.7, 126.5, 126.4, 126.3, 124.5, 123.6, 123.1. APCI-MS (m/z): 678
[M+]. HRMS [EI+] calcd for C52H39N: 677.3083, found: 677.3081.
3: 4-(2-(4ꢀ-(2-(naphthalen-2-yl)-2-phenylvinyl)biphenyl-4-yl)vinyl)-N,N-diphenyl-
aniline: Yield 64 %. 1H-NMR (300MHz, CDCl3): δ ppm 7.86-7.71 (m, 4H), 7.55-7.46 (m,
Figure 3. Device structure and energy level diagram used in this study.

N,N-diphenylaniline Derivatives for Blue OLEDs
[295]/107
Figure 4. EL spectra of devices A-D.
5H), 7.44-7.41(m, 2H), 7.39-7.27 (m, 9H), 7.25-7.22 (m, 4H), 7.12-7.01 (m, 12H), 6.99 (s,
1H). 13C-NMR (75 MHz, CDCl3): δ ppm 147.6, 147.4, 138.8, 131.5, 130.5, 130.1, 129.3,
128.8, 128.7, 128.4, 128.3, 128.2, 127.8, 127.8, 127.6, 127.4, 127.0, 127.0, 126.9, 126.8,
126.7, 126.5, 126.3, 126.2, 126.1, 126.1, 124.6, 123.6, 123.1. APCI-MS (m/z): 652 [M+].
HRMS [EI+] calcd for C50H37N: 651.2926, found: 651.2922.
General Procedure for the Suzuki cross-coupling reaction, synthesis of 4: A
(0.48 mmol) the corresponding F (0.72 mmol), Pd(PPh3)4 (0.024 mol), aqueous 2.0 M
K2CO3 (6.0 mmol), Aliquat 336, and toluene were mixed in a flask. The mixture was
refluxed for 2.5 hr. After the reaction had finished, the reaction mixture was extracted with
ethyl acetate and washed with water. The organic layer was dried with anhydrous MgSO4
and filtered with silica gel. The crude mixture was purified through column chromatography
using silica-gel and crystallization to give the desired compound.
4: 4-(2-(4ꢀ-(1,2-diphenylvinyl)biphenyl-4-yl)vinyl)-N,N-diphenylaniline: Yield 58%.
1H-NMR (300MHz, CDCl3): δ ppm 7.65-7.55 (m, 4H), 7.41-7.24 (m, 12H), 7.13-6.98 (m,
17H). 13C-NMR (75 MHz, CDCl3): δ ppm 147.6, 147.3, 139.5, 139.4, 137.4, 136.8, 131.5,
131.0, 130.4, 129.6, 129.3, 128.7, 128.4, 128.2, 128.1, 128.0, 127.8, 127.6, 127.5, 127.4,
127.1, 126.9, 126.8, 126.6, 126.5, 124.5, 123.6, 123.1. APCI-MS (m/z): [M+]. HRMS [EI+]
calcd for C46H35N: 601.2770, found: 601.2767.
Optical Measurements
The UV-vis absorption and photoluminescence spectra of the new dopant materials were
measured in dichloromethane (10−5 M) using a Sinco S-3100 and Aminco-Browman series
2 luminescence spectrometer, respectively. The fluorescence quantum yields of the blue
materials were determined in dichloromethane at 293 K against BDAVBi as a reference
(Ф = 0.86) [21]. 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.

108/[296]
S. M. Kwon et al.
Figure 5. Current density and luminance versus applied electric voltage characteristics of devices
A-D.
Fabrication of OLED
For fabricating OLEDs, indium-tin-oxide (ITO) thin films coated on glass substrates were
used, 30 ꢀ/square of the sheet resistivity with a thickness of 1000 A. The ITO-coated
glass was cleaned in an ultrasonic bath by the following sequence: acetone, methyl alcohol,
distilled water, storage in isopropyl alcohol for 48 h, and dried by N2 gas gun. The substrates
were treated by O2 plasma under conditions of 2 × 10−2 Torr at 125 W for 2 min. All
organic materials and metal were deposited under high vacuum (5 × 10−7 Torr). The
OLEDs were fabricated in the following sequence: ITO/ DNTPD (40 nm)/ NPB (20 nm)/
ADN: Blue dopants 1–4 (20 nm)/ Alq3 (40 nm)/ Liq (1.0 nm)/ Al. The current density (J),
luminance (L), luminous efficiency (LE), power efficiency (PE), and the CIE chromaticity
coordinates of the OLEDs were measured with Keithly 2400, Chroma meter CS-1000A.
Electroluminescence was measured using a Roper Scientific Pro 300i.

N,N-diphenylaniline Derivatives for Blue OLEDs
[297]/109
Figure 6. Luminous and power efficiencies as a function of current density for devices A-D.
Results and Discussion
The blue fluorescent materials (1–4) used for this study were synthesized by the Honor-
Wadsworth-Emmons reaction and Suzuki coupling reaction, and molecular structures are
shown in Fig. 1. The molecular structures of these materials were characterized by 1H- and
13C-NMR and low- and high-resolution mass spectroscopy.
The UV-Vis absorption spectra of these blue fluorescent materials in dichloromethane
are shown in Fig. 2(a). Materials 1–4 showed the maximum absorption peaks in the 385-
390 nm range. The good spectral overlap was observed between the emission of the
commom blue host (ADN) and the absorptions of materials 1–4. These observations indicate
that the blue materials 1–4 can effectively accept energy from the ADN host material by
Fo¨rster-type energy transfer. A particularly good spectral overlap was observed for material
3. The efficiency of Fo¨rster-type energy transfer is highly dependent on the spectral overlap
between the emission of the host and the absorption of the dopant, thus this material 3 as a

110/[298]
S. M. Kwon et al.
dopant would be expected to have highly efficient electroluminescent properties in OLED
devices using AND host.
The normalized PL emission spectra of 1–4 are shown in Fig. 2(b). All materials
showed blue fluorescence with maximum emission wavelengths of 488, 495, 495 and
479 nm, respectively. Compared to 1, the PL spectra of 2 and 3 show the red-shift of 7 nm
owing to the lengthening the π-conjugation length by the additional end-capped groups
such as naphthalene and biphenyl. On the other hand, compared to 1, the PL spectrum of
4 shows the blue shift of 9 nm due to the shortening the π-conjugation length due to the
steric repulsion between two end-capping phenyl groups. These observations imply that
the conjugation length of 1–4 is sensitively dependent on end-capped aryl substituents.
These materials 1–4, with the exception of 1, have high PL quantum yields of 0.60-0.89,
suggesting that these materials were 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 1–4 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 1–4 are 5.58/2.78, 5.59/2.76, 5.58/2.75 and 5.60/2.72,
respectively.
To explore the electroluminescent properties of these materials, devices were fabri-
cated and the electroluminescent performance characteristics of devices are summarized in
Table 2. The doping concentrations of 1–4 in ADN host were optimized to 2%, respectively.
The EL emission spectra of devices A-D with emitting layers consisting of the ADN host
doped with dopants 1–4 are seen in Fig. 4. All four doped devices exhibited blue emissions
with the maximum emission peaks between 454-462 nm. The corresponding CIE coordi-
nates are x = 0.159, y = 0.176 for A, x = 0.157, y = 0.185 for B, x = 0.157, y = 0.197 for
C, and x = 0.151, y = 0.134 for D, and their full width at half maximum (FWHM) values
are 60, 56, 56, and 55nm, respectively. The FWHM of the EL spectra of device D is found
to be quite narrow and therefore the color purity is excellent. Compare to CIE coordinates
of device D, those of A-C show red-shifts from 0.042 to 0.063 in the CIEy coordinate due to
the lengthening π-conjugation length of the dopants, which were in good agreement with
the trends of the PL spectra of materials 1–4.
Figure 5(a) and (b) shows the current density-voltage and luminance-voltage character-
istics of devices A-D containing ADN doped with various dopant materials. The maximum
current densities of these blue OLEDs varied from 86 mA/cm2 in device B to as high as
128 mA/cm2 in device A, D. The current density of devices A-D is determined by several
factors such as the differences in HOMO/ LUMO energy levels between the adjacent or-
ganic layers and the charge carrier mobility of each materials in devices. Since materials
and structure in devices A-D are under the same condition of except dopants, the charge
carrier mobility of dopants 1–4 seems to be profound effects on electronic properties of
devices. The maximum brightness of these blue OLEDs varied from 5257 cd/m2 in device
D to as high as 18020 cd/m2 in device 3. The luminous efficiencies and power efficiencies of
the doped devices A-D, as a function of the current density, are shown in Fig. 6(a) and (b).
Their power efficiency of devices A-D were 8.08, 8.81, 9.57, and 5.29 cd/A at 20 mA/cm2,
respectively. Their power efficiency of devices A-D were 3.26, 3.59, 3.92, and 2.26 lm/W at
20 mA/cm2, respectively. Notably, the EL efficiencies of device C with dopant 3 was much
higher than those of devices A, B and D with dopants 1, 2 and 4. Presumably, compared to
dopants 1, 2 and 4, the superior spectral overlap between the emission of the host (ADN)
and the absorption of dopant 3, as shown in Fig. 2(a), and thus the effective energy transfer

N,N-diphenylaniline Derivatives for Blue OLEDs
[299]/111
from ADN host to dopant 3 in device C would lead to the improved EL efficiencies of de-
vice C in comparison with devices A, B and D. This study clearly suggests that efficiencies
of blue OLEDs composed of a host and dopant can be improved by a superior matched
spectral overlap between the emission of the host and the absorption of the dopant.
Conclusions
We have synthesized blue fluorescent materials (1–4) via Horner-Wadsworth-Emmons
reaction and Suzuki cross-coupling reaction. When doped in the ADN host in the OLED
device, these materials showed the efficient blue electroluminescent properties. Particularly,
the device using dopant 3 exhibited a highly efficient blue emission with the maximum
luminance of 18020 cd/m2 at 11 V, the luminous efficiency of 9.6 cd/A at 20 mA/cm2, power
efficiency of 3.9 lm/W at 20 mA/cm2, and CIE x, y coordinates of (0.157, 0.197) at 8 V. In
addition, an OLED device using dopant 4 exhibited a efficient deep-blue emission with a
maximum luminanace, luminous efficiency and power efficiency of 5257 cd/m2 at 10 V, 5.29
cd/A, and 2.26 lm/W at 20 mA/cm2, respectively. The maximum peak wavelength of the EL
spectrum of this device was 454nm with CIE x, y coordinates of (0.151, 0.134) at 8 V. These
results demonstrated the superiority of these unsymmetrical diphenylamino-styrylarylene
derivatives as the blue emitting materials in OLED devices.
Acknowledgment
This research (S2012-0870-000) was supported by the MKE (The Ministry of Knowledge
Economy), Korea, under the ITRC (Information Technology Research Center)/CITRC
(Convergence Information Technology Research Center) support program (NIPA-2012-
H0301-12-4013) supervised by the NIPA (National IT Industry Promotion Agency).
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