A new yellow emissive fluorescent material for highly efficient organic light-emitting diodes

Article abstract of DOI:10.1080/15421406.2011.563651

A new yellow fluorescent material, (2Z)-3-[4,4″-bis(dimethylamino)-1, 1':4',1″-terphenyl-2'-yl]-2-phenylacrylonitrile (BDAT-P), was synthesized for organic light-emitting diodes. This compound has an electron donor-acceptor group which leads to intramolec

Full text of DOI:10.1080/15421406.2011.563651

Mol. Cryst. Liq. Cryst., Vol. 538: pp. 75–83, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.563651
A New Yellow Emissive Fluorescent Material for
Highly Efficient Organic Light-Emitting Diodes
NA RAE PARK,1 GWEON YOUNG RYU,1
SE JIN MOON,2 JI HOON SEO,2 YOUNG KWAN KIM,2
AND DONG MYUNG SHIN1
1Department of Chemical Engineering, Hong-ik University, Seoul, Korea
2Department of Information Display, Hong-ik University, Seoul, Korea
A new yellow fluorescent material, (2Z)-3-[4,400-bis(dimethylamino)-1,10:40,100-
terphenyl-20-yl]-2-phenylacrylonitrile (BDAT-P), was synthesized for organic
light-emitting diodes. This compound has an electron donor-acceptor group which
leads to intramolecular charge transfer property.
Optoelectronic properties of a device with the structure of ITO=NPB=
MADN:BDAT-P (x %)=BPhen=Liq=Al were measured and the measurement
demonstrated that the device using BDAT-P as a dopant revealed highly efficient
device performance with greenish yellow electroluminescent emission. Maximum
luminance of the device was measured to be 18300 (x ¼ 8%), 10880 (x ¼ 12%)
and 11810 (x ¼ 16%) cd=m2, respectively. The maximum luminous and quantum
efficiency values were observed to be 12.5 (x ¼ 8%), 9.17 (x ¼ 12%) and 12.79
(x ¼ 16%) cd=A, and 3.83 (x ¼ 8%), 2.8 (x ¼ 12%) and 3.81 (x ¼ 16%) %,
respectively. The device emitted greenish yellow light corresponding to Commission
Internationale de I’Eclairage (CIExy) coordinates of (0.319, 0.569) (x ¼ 8%) and
(0.321, 0.569) (x ¼ 12%) and (0.356, 0.578) (x ¼ 16%) at 10 V.
Keywords BDAT-P; organic light-emitting diode; yellow dopant; yellow emissive
1. Introduction
Organic light-emitting diodes (OLEDs) have attracted broad attention due to their
potential application with self emission, fast response time, low-power consumption,
low operation voltage, high contrast, high brightness, high efficiency ultrathin struc-
ture and light weight for full color display since Kodak reports green device based on
small molecule [1–4]. There have been many studies including guest-host system to
improve OLEDs performance [5]. White organic light-emitting diodes (WOLEDs)
have application to backlights of liquid-crystal display (LCD), large area solid-state
lighting and full color flat-panel display [6–8]. In order to achieve white color, triple
emission layers from red, green and blue emitter were developed. Another method
to achieve WOLEDs is the two complementary colors way like as blue mixed with
Address correspondence to Prof. Dong Myung Shin, Department of Chemical
Engineering, Hong-ik University, Sangsu-dong, Mapo-gu, Seoul 121-791, Korea (ROK).
Tel.: (þ82)2-320-1652; Fax: (þ82)2-320-1191; E-mail: shindm@hongik.ac.kr
75

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N. R. Park et al.
yellow or orange color. This method has advantages such as low material cost and
simple fabrication process [9–12]. There have been some researches using yellow
and sky blue or yellow and blue emitter for fabrication of WOLEDs [9,13]. Accord-
ing to a recent survey, WOLED using the combination with blue emission material
and its complimentary color, yellow is preferred by most of the customers. Further-
more, one of the sensitive color for human eyes is yellow [14,15]. Thus, yellow emiss-
ive material is important for WOLEDs.
In this study, we designed and synthesized a new yellow fluorescent material,
BDAT-P, that has intramolecular charge transfer (ICT) property for use in yellow
OLEDs. In order to achieve yellow light emission with high device performance,
device using BDAT-P as a dopant was fabricated and its device performance was
measured.
2. Experimental
2.1. Synthesis
Synthesis of BDAT-P was shown in Scheme 1. All solvents involved in the experi-
ments were reagent grade and were purified by the usual methods before use.
2,5-Dibromobenzaldehyde (I) was commercially available (Aldrich). The molecular
structure of BDAT-P was confirmed by 1H and 13C-NMR spectroscopy, MS
(FAB) spectrum and elemental analysis.
4,400-bis(dimethylamino)-1,10:40,100-terphenyl-20-carbaldehyde (II) was obtained
from the reaction of 2,5-dibromobenzalaldehyde (I) with 4-(dimethylamino)phenyl-
boronic acid by Suzuki coupling. The mixture of 2,5-dibromobenzalaldehyde (I)
(800 mg, 3.03 mmol), 4-(dimethylamino)phenylboronic acid (1.10 g, 6.66 mmol),
Pd(PPh3)4 (140 mg), Na2CO3 (963 mg) and Aliquat336 (0.433 ml) in mixed solvent
of toluene (64 ml)-H2O (40 ml)-THF (16 ml) was mildly refluxed under N2 atmos-
phere with stirring for 3 hr. The reaction mixture was cooled, filtered and washed
with mixture of ethyl acetate and n-hexane (1 : 5). Removal of the solvents and dry-
ing under high vacuum afforded 785 mg (2.28 mmole) of the product (II) as a green
1
solid and further purification was not required. Yield: 75%; H-NMR (500 MHz,
CDCl3) d (ppm) 10.069 (s, 1H), 8.189 (s, 1H), 7.804 (d, 1H), 7.475(d, 1H), 7.290
(d, 2H), 7.588 (d, 2H), 6.810 (d, 4H), 3.013 (s, 12H); 13C-NMR (125 MHz, CDCl3)
Scheme 1. Synthesis of BDAT-P.

Highly Efficient Organic Light-Emitting Diodes
77
d (ppm) 193.40, 150.16, 143.83, 139.50, 133.71, 131.04, 131.01, 128.99, 128.18,
127.56, 127.39, 125.10, 124.64, 112.67, 112.06, 40.46, 40.39.
(2Z)-3-[4,400-bis(dimethylamino)-1,10:40,100-terphenyl-20-yl]-2-phenylacrylonitrile
(BDAT-P) (III) was prepared from the Knoevenagel reaction of 4,400-bis-
(dimethylamino)-1,10:40,100-terphenyl-20-carbaldehyde (II) with benzyl cyanide. The
mixture of 4,400-bis(dimethylamino)-1,10:40,100-terphenyl-20-carbaldehyde (400 mg,
1.16 mmol), benzyl cyanide (0.536 ml, 4.64 mmol) and sodium ethoxide (prepared
by the reaction of 64 mg of Na with 10 ml of absolute EtOH) in 150 ml of EtOH
was stirred at room temperature for 4 days. The yellow solid formed in reaction mix-
ture was filtered and washed with EtOH and MeOH. Removal of the solvents and
drying under vacuum afforded 343 mg (0.73 mmole) of the product (III) as a yellow
solid. Mp 243.3ꢀC (DSC); Yield: 66.6%; 1H-NMR (500 MHz, CDCl3) d (ppm) 8.338
(d, 1H, aromatic), 7.668 (dd, 1H, aromatic), 7.640 (d, 2H aromatic), 7.624 (s, 1H,
vinyl), 7.615 (d, 2H, aromatic), 7.471 (d, 1H, aromatic), 7.396 (t, 2H, aromatic),
7.344 (t, 1H, aromatic), 7.275 (d, 2H, aromatic), 6.842 (d, 2H, aromatic), 6.776 (d,
2H, aromatic), 3.002 (s, 12H, methyl); 13C-NMR (125 MHz, CDCl3) d (ppm)
150.07, 149.86, 143.37, 140.46, 139.50, 134.48, 131.81, 130.79, 130.21, 128.94,
128.76, 128.19, 127.86, 127.79, 127.33, 126.39, 125.94, 118.41, 112.84, 112.03,
111.75, 40.53, 40.40; Anal. calcd. for C31H29N3: C: 83.92, H: 6.59, N: 9.47. Found:
C: 83.77, H: 6.55, N: 9.25. MS (FAB) calcd for C31H29N3 (Mþ) m=z 443, found 443.
2.2. Measurement
1H-NMR and 13C-NMR spectra were recorded on a Varian Unity INOVA 500 spec-
trometer operating at 499.761 MHz and 125.701 MHz, respectively. Elemental analy-
sis was performed on a CE instrument EA1112 analyzer. Mass spectrum (FAB-MS)
was measured on JEOL, JMS-AX505WA, HP 5890 Series II Hewlett-Packard
5890A (capillary column) using standard conditions. The melting point was
measured to be 366.2ꢀC by differential scanning calorimetry (DSC) using a Seiko
Exstar 7000 (DSC7020) with a scan rate of 10ꢀC=min at the temperature range of
40ꢁ400ꢀC. UV-visible absorption and photoluminescence (PL) spectra were
measured by HP model 8453 and Perkin Elmer LS55, respectively.
The HOMO energy level of 5.23 eV for BDAT-P was measured by cyclic
voltammetric method using a EpslianEC-2000 at a scan rate of 100 mV=s and
LUMO energy level of 2.54 eV was calculated from measured HOMO energy level.
The value of optical band gap was observed as 2.69 eV which was obtained from
UV-visible absorption spectrum. Electroluminescence (EL) spectra and brightness-
current-voltage characteristics of the device were measured by using keithley 2400,
CHROMA METER CS-1000A. All the measurements were carried out at room
temperature.
2.3. Fabrication of OLED
The OLED was fabricated by the high vacuum thermal deposition (8 ꢂ 10ꢃ7 torr)
of organic materials onto the surface of indium tin oxide (ITO) coated in glass sub-
strate. An ITO coated glass was cleaned in an ultrasonic bath by regular sequences:
in acetone, methanol, distilled water and isopropyl alcohol. The cleaned substrate
was immediately loaded into the deposition chamber in order to prevent air

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N. R. Park et al.
contamination. The device has the structure of ITO (100 nm)=NPB (50 nm)=
MADN:BDAT-P (x %) (30 nm)=BPhen (30 nm)=Liq (2 nm)=Al (100 nm), in which
NPB, BDAT-P, BPhen and Liq were used as a hole transporting layer (HTL), an
emitter layer (EML), a hole blocking and electron transporting layer (HBL, ETL)
and an electron injection layer (EIL), respectively.
3. Results and Discussion
Figure 2(a) showed UV absorption and PL spectra of BDAT-P in chloroform sol-
ution and PL spectrum of MADN in film, in which they were used as the dopant
and the host material, respectively. The UV absorption and PL peaks of BDAT-P
measured to be at 325 nm with shoulder at 400 nm and 551.5 nm, respectively. The
PL spectrum of BDAT-P showed yellow light emission and large Stokes shift which
hardly caused an overlap between absorption and emission bands of BDAT-P. This
leads to little self-absorption of the emitted light so that the BDAT-P is very advan-
tageous for OLED application. Although it was not fully overlapped, MADN was
used as the host material for OLEDs using BDAT-P as the dopant because the over-
lap between UV-visible absorption band of BDAT-P and the PL band of MADN
was favorable to Fo¨rster energy transfer from MADN to BDAT-P. Figure 2(b)
represented the HOMO and LUMO energy levels of the materials used in device
with the structure of ITO=NPB=MADN:BDAT-P=BPhen=Liq=Al. It showed that
the correlation of energy band gap of MADN with that of BDAT-P was suitable
to Fo¨rster energy transfer from MADN to BDAT-P.
Figure 3(a) and (b) showed the current density vs. voltage and current density vs.
luminance of BDAT-P doped OLEDs at various doping concentration of 8, 12 and
16%, in which the luminance maxima were 18300 (at 219 mA=cm2), 10880
(at 176 mA=cm2,) and 11810 (at 169 mA=cm2,) cd=m2, respectively. As doping
Figure 1. The molecular structure of BDAT-P., NPB, MADN, BPhen and Liq.

Highly Efficient Organic Light-Emitting Diodes
79
Figure 2. (a) Normalized UV-Visible absorption and PL spectra of BDAT-P and PL spectrum
of MADN and (b) the schematic energy band diagram of the device structure of ITO=NPB=
MADN:BDAT-P=BPhen=Liq=Al.
concentration increased from 8% to 16%, luminance was decreased, which was
ascribed to the increase of concentration quenching of dopant molecules.
As shown in Figure 4(a) and (b), device doped with the new yellow fluorescent
BDAT-P into MADN represented high luminous efficiency and quantum efficiency.
The maximum luminous and quantum efficiency values were observed to be 12.5
(x ¼ 8%), 9.17 (x ¼ 12%) and 12.79 (x ¼ 16%) cd=A, and 3.83 (x ¼ 8%), 2.8
(x ¼ 12%) and 3.81 (x ¼ 16%) %, respectively. Based on Figures (3) and (4), although
EL emission peak was much more shifted to short wavelength compared to that of
PL, device with doping concentration of 8% was observed to be the most stable and
efficient.

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N. R. Park et al.
Figure 3. (a) Current density vs. voltage and (b) current density vs. luminance characteristics
with variation of doping concentration.
Figure 5(a) showed the EL spectra of BDAT-P with variation of doping con-
centration (x ¼ 8, 12 and 16%). EL emission peaks were shifted to longer wave-
length as doping concentration was increased and they were measured to be 526
(x ¼ 8%), 529 (x ¼ 12%) and 534 (x ¼ 16%) nm, respectively. Although yellow light
emission was improved with the increase of doping concentration, except for EL
emission color, most of EL properties of device with doping concentration of
8% were superior to those of device with doping concentration 12 and 16%. Yellow
light emission is not for full color display but for WOLEDs so it is assumed that

Highly Efficient Organic Light-Emitting Diodes
81
Figure 4. (a) current density vs. luminous efficiency and (b) quantum efficiency vs. current
density characteristics with variation of doping concentration.
doping concentration of 8% is more favorable than that of 12 and 16%. As shown
in Figure 5(b), Commission Internationale de I’Eclairage (CIExy) coordinates of
device were measured to be (0.319, 0.569) (x ¼ 8%), (0.321, 0.569) (x ¼ 12%) and
(0.356, 0.578) (x ¼ 16%) at 10 V, respectively. Observed CIExy coordinates repre-
sented that the device with doping concentration from 8 to 16% emitted yellowish
green EL emission, which was shown in Figure 5(c).

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N. R. Park et al.
Figure 5. (a) Normalized EL spectra, (b) measured CIExy coordinates and (c) chromaticity
diagram measured with variation of doping concentration.

Highly Efficient Organic Light-Emitting Diodes
83
4. Conclusions
We have synthesized a new yellow fluorescent material, BDAT-P, in two steps and
demonstrated that device doped with BDAT-P into MADN resulted in highly
efficient greenish yellow OLEDs.
EL peaks of BDAT-P doped device were observed to be 526 (x ¼ 8%), 529
(x ¼ 12%) and 534 (x ¼ 16%) nm. The devices showed maximum luminance of
18300 (x ¼ 8%), 10880 (x ¼ 12%) and 11810 (x ¼ 16%) cd=m2. The maximum lumi-
nous efficiency and quantum efficiency of the device were achieved to be 12.5
(x ¼ 8%), 9.17 (x ¼ 12%) and 12.79 (x ¼ 16%) cd=A and 3.83 (x ¼ 8%), 2.8
(x ¼ 12%) and 3.81 (x ¼ 16%) %, respectively. Therefore, it can be concluded that
a newly synthesized fluorescent BDAT-P may be a good candidate for use in fabri-
cation of WOLEDs with high efficiency.
Acknowledgment
This work was supported by National Research Foundation of Korea Grant funded
by the Korean Government (2010-0023836).
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