Energy transfer efficiency control from the blue emitting aromatics to the yellow fluorescent intramolecular charge transfer type dye for Electroluminescence

Article abstract of DOI:10.1080/15421406.2013.808149

An intramolecular charge transfer type yellow fluorescent material, (Z)-4-(2-[4,4″-bis(dimethylamino)-1,1′:4′,1″-terphenyl- 2′-yl]-2′-1-cyanovinyl)benzonitrile (BDAT-CP) was synthesized for the implimentation to organic light emitting diodes (OLEDs). The molecules of the this catagory often have electro-conductivity and show high luminescence at longer wavelength in visible region. The BDAT-CP exhibited a large Stokes shift from 329 nm (UV absorption maximum) to 628 nm of photoluminescence(PL) in chloroform solvent. Solid powder PL showed yellow emission peaked at 582 nm. The device structures were indium tin oxide (ITO)/N,N′-bis-(1-naphyl)-N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB, 50 nm)/2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN, 30 nm): BDAT-CP (x %)/4,7-diphenyl-1,10-phenanthroline (BPhen, 30 nm)/Liq(2 nm)/Al. The maximum luminance measured were 14700 cd/m2 (x = 5%), 9825 cd/m2 (x = 10%) and 7901 cd/m2 (x = 15%) for each BDAT-CP concentration. The luminance efficiency and quantum efficiency observed were 7.14 (x = 5%), 4.72 (x = 10%) and 3.19 (x = 15%), and 2.77 (x = 5%), 2.09 (x = 10%) and 1.59 (x = 15%), respectively. The device emitted yellow light corresponding to Commission Internationale de IEclairage (CIExy) coordinates of (0.492, 0.494) (x = 5%) and (0.523, 0.470) (x = 10%) and (0.545, 0.451) (x = 15%) at 8 V. This study showed that the energy transfer from MADN to BDAT-CP was dependent on the concentration of yellow dye, which affected the hole transporting properties of the OLED.

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Energy Transfer Efficiency Control
from the Blue Emitting Aromatics to
the Yellow Fluorescent Intramolecular
Charge Transfer Type Dye for
Electroluminescence
Sung Nam Lee a , Seok Jae Lee b , Young Kwan Kim b & Dong Myung
Shin a
a Department of Chemical Engineering , Hong-ik University , Seoul ,
Korea
b Department of Information Display , Hong-ik University , Seoul ,
Korea
Published online: 13 Sep 2013.
To cite this article: Sung Nam Lee , Seok Jae Lee , Young Kwan Kim & Dong Myung Shin (2013) Energy
Transfer Efficiency Control from the Blue Emitting Aromatics to the Yellow Fluorescent Intramolecular
Charge Transfer Type Dye for Electroluminescence, Molecular Crystals and Liquid Crystals, 581:1,
59-69
To link to this article: http://dx.doi.org/10.1080/15421406.2013.808149
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Mol. Cryst. Liq. Cryst., Vol. 581: pp. 59–69, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.808149
Energy Transfer Efficiency Control from the Blue
Emitting Aromatics to the Yellow Fluorescent
Intramolecular Charge Transfer Type Dye for
Electroluminescence
SUNG NAM LEE,1 SEOK JAE LEE,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
An intramolecular charge transfer type yellow fluorescent material, (Z)-4-(2-[4,4ꢀꢀ-
bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ-yl]-2ꢀ-1-cyanovinyl)benzonitrile
(BDAT-
CP) was synthesized for the implimentation to organic light emitting diodes (OLEDs).
The molecules of the this catagory often have electro-conductivity and show high lumi-
nescence at longer wavelength in visible region. The BDAT-CP exhibited a large Stokes
shift from 329 nm (UV absorption maximum) to 628 nm of photoluminescence(PL) in
chloroform solvent. Solid powder PL showed yellow emission peaked at 582 nm. The
device structures were indium tin oxide (ITO) / N,Nꢀ-bis-(1-naphyl)-N,Nꢀ-diphenyl-1,1ꢀ-
biphenyl-4,4ꢀ-diamine (NPB, 50 nm) / 2-methyl-9,10-bis(naphthalen-2-yl)anthracene
(MADN, 30 nm) : BDAT-CP (x %) / 4,7-diphenyl-1,10-phenanthroline (BPhen, 30 nm)
/ Liq(2 nm) / Al. The maximum luminance measured were 14700 cd/m2 (x = 5%),
9825 cd/m2 (x = 10%) and 7901 cd/m2 (x = 15%) for each BDAT-CP concentration.
The luminance efficiency and quantum efficiency observed were 7.14 (x = 5%), 4.72
(x = 10%) and 3.19 (x = 15%), and 2.77 (x = 5%), 2.09 (x = 10%) and 1.59 (x
= 15%), respectively. The device emitted yellow light corresponding to Commission
Internationale de I’Eclairage (CIExy) coordinates of (0.492, 0.494) (x = 5%) and
(0.523, 0.470) (x = 10%) and (0.545, 0.451) (x = 15%) at 8 V. This study showed that
the energy transfer from MADN to BDAT-CP was dependent on the concentration of
yellow dye, which affected the hole transporting properties of the OLED.
Keywords BDAT-CP; intramolecular charge transfer; yellow light-emitting diode
1. Introduction
Organic light-emitting devices (OLEDs) have attracted significant attention in recent years
and are considered with self-emission, low-voltage operation, fast response time, wide-
angle of field, high contrast and mechanical flexibility to be realized in flat-panel display
industry [1–3]. White organic light-emitting diodes (WOLEDs) have application to back-
lights of liquid-crystal display (LCD), large area solid-state lighting and full color flat-panel
display [4–7]. In order to achieve white color, triple emission layers from red, green and
∗Address correspondence to Prof. Dong Myung Shin, Department of Chemical Engineering,
Hong-ik University, Sangsu-dong, Mapo-gu, Seoul 121-791, Korea (ROK). Tel.: (+82)2-320-1652;
Fax: (+82)2-320-1191. E-mail: shindm@hongik.ac.kr
[437]/59

60/[438]
S. N. Lee et al.
blue emitter were developed [8,9]. Another method to achieve WOLEDs is the two comple-
mentary colors way like as blue mixed with yellow or orange color [10–12]. This method
has advantages such as low material cost and simple fabrication process [10,13]. For this
reasons, there exists a continuing need and innovative opportunity for the development of
highly efficient yellow dopant materials for various OLED display applications. Notwith-
standing many previous researches, yellow emitting materials still showed low luminous
and quantum efficiency and luminance maximum [14–16].
In this study, we designed and synthesized a new yellow fluorescent material,
(2Z)-3-[4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ-yl]-2-(4-cyanophenyl)acetonitrile
(BDAT-CP), 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-CP as a dopant was fabricated and its device performance was measured.
2. Experimental
2.1. Synthesis
Synthesis of BDAT-CP was shown in Scheme 1. All solvents involved in the exper-
iments 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-CP was confirmed by 1H and 13C-NMR spectroscopy, MS (FAB) spectrum and
elemental analysis.
4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ-carbaldehyde (II) was obtained from
the reaction of 2,5-dibromobenzalaldehyde (I) with 4-(dimethylamino)phenylboronic acid
by Suzuki coupling. The mixture of 2,5-dibromobenzalaldehyde (I) (1.0 g, 3.79 mmol),
4-(dimethylamino)phenylboronic acid (1.375 g, 8.33 mmol), Pd(PPh3)4 (175 mg), Na2CO3
(1.20 g) and Aliquat336 (0.693 ml) in mixed solvent of toluene (80 ml)-H2O (50 ml)-THF
(20 ml) was mildly refluxed under N2 atmosphere 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 drying under high vacuum afforded 835 mg (2.42 mmol)
N
N
N
CN
CN
Br
Br
NC
CHO
CHO
B(OH)2
CN
N
N
III
I
II
Scheme 1. Synthesis of BDAT-CP.

Blue Emitting Aromatics Application to the Yellow OLED
[439]/61
of the product (II) as a green solid and further purification was not required. Yield: 62%;
1H-NMR (500 MHz, CDCl3) δ (ppm) 10.093 (s, 1H), 8.210 (s, 1H), 7.831 (d, 1H), 7.586(d,
1H), 7.270 (d, 2H), 7.588 (d, 2H), 6.849 (d, 4H), 3.001 (s, 12H); 13C-NMR (125 MHz,
CDCl3) δ (ppm) 193.57, 150.19, 143.91, 139.43, 133.69, 131.35, 131.11, 128.97, 128.28,
127.46, 127.40, 125.17, 124.63, 112.65, 112.01, 40.45, 40.37.
(Z)-4-(2-[4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ-yl]-2ꢀ-1-cyanovinyl)benzo-
nitrile (BDAT-CP) (III) was prepared from the Knoevenagel condensation reac-
tion of 4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ-carbaldehyde (II) with 4-
cyanophenylacetonitrile. The mixture of 4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ-
carbaldehyde (494 mg, 1.43 mmol), 4-cyanophenylacetonitrile (0.8149 g, 5.73 mmol)
and sodium ethoxide (prepared by the reaction of 79 mg of Na with 15 ml of absolute
EtOH) in 300 ml of EtOH was stirred at room temperature for 7 days. The Orange yellow
solid formed in reaction mixture was filtered and washed with distillated water, EtOH and
MeOH. Removal of the solvents and drying under vacuum afforded 577 mg (1.23 mmol) of
the product (III) as a yellow solid. Mp. 258◦C (DSC); Yield: 86.0%; 1H-NMR (500 MHz,
CDCl3) δ (ppm) 8.405 (d, 1H, aromatic), 8.204 (m, 1H, aromatic), 7.858 (s, 1H, vinyl),
7.718 (m, 1H, aromatic), 7.615 (d, 2H, aromatic), 7.471 (d, 1H, aromatic), 7.441 (t, 2H,
aromatic), 7.332 (t, 1H, aromatic), 7.305 (d, 2H, aromatic), 6.842 (d, 2H, aromatic), 6.899
(m, 2H, aromatic), 3.062 (s, 12H, methyl); 13C-NMR (125 MHz, CDCl3) δ (ppm) 150.319,
146.411, 133.018, 131.123, 130.646, 128.069, 126.674, 113.097, 112.836, 112.276,
77.522, 77.268, 77.014, 40.776, 40.661; Anal. calcd. for C32H28N4: C: 82.02, H: 6.09,
N: 11.96. Found: C: 81.93, H: 5.97, N: 11.82. MS (FAB) calcd for C31H29N3 (M+) m/z
468.23, found 468.23.
Figure 1. The molecular structure of BDAT-CP, MADN, Liq, BPhen, and NPB.

62/[440]
S. N. Lee et al.
2.2. Measurement
1H-NMR and 13C-NMR spectra were recorded on a Varian Unity INOVA 500 spectrom-
eter operating at 499.761 MHz and 125.701 MHz, respectively. Elemental analysis 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)
Figure 2. (a) Normalized UV-Visible absorption and PL spectra of BDAT-CP in chloroform solution
and PL spectrum of MADN in film, (b) ICT property of BDAT-CP in solvents of different polarities
and (c) the HOMO and LUMO energy levels of materials used in device with the structure of
ITO/NPB/MADN:BDAT-CP/BPhen/Liq/Al. (Continued)

Blue Emitting Aromatics Application to the Yellow OLED
[441]/63
Figure 2. (Continued)
using standard conditions. The melting point was measured to be 258◦C by differential scan-
ning calorimetry (DSC) using a Seiko Exstar 7000 (DSC7020) with a scan rate of 10◦C/min
at the temperature range of 40 ∼ 300◦C. UV-visible absorption and photoluminescence (PL)
spectra were measured by HP model 8453 and Perkin Elmer LS55, respectively.
The HOMO energy level of 5.2 eV for BDAT-CP was measured by cyclic voltam-
metric method using a EpslianEC-2000 at a scan rate of 100 mV/s and LUMO energy
level of 3.45 eV was calculated from measured HOMO energy level. The value of optical
band gap was observed as 1.75 eV which was obtained from UV-visible absorption spec-
trum. 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 × 10−7 torr) of organic
materials onto the surface of indium tin oxide (ITO) coated in glass substrate. 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 contamination. The device has the structure of
ITO (100 nm)/NPB (50 nm)/MADN:BDAT-CP (x %) (30 nm)/BPhen (30 nm)/Liq (2 nm)/Al
(100 nm), in which NPB, BDAT-CP, 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. Figure 1 showed molecular structures
of BDAT-CP, MADN, Liq, BPhen, and NPB for fabrication of devices.

64/[442]
S. N. Lee et al.
Figure 3. (a) Current density vs. voltage and (b) luminance vs. voltage characteristics with variation
of doping concentration.
3. Results and Discussion
Figure 2(a) showed UV absorption and PL spectra of BDAT-CP in chloroform and PL
spectrum of MADN in film. They were used as the dopant and the host materials, respec-
tively. The UV absorption spectrum of BDAT-CP showed a peak at 329 nm and shoulder
at 429 nm. The PL spectrum of BDAT-CP showed yellow emission with a maximum peak
at 620 nm. The intramolecular charge transfer from amino groups to cyano groups made
large Stokes shift and the emission usually came from the twisted excited state [17,18].

Blue Emitting Aromatics Application to the Yellow OLED
[443]/65
The large Stokes shift hardly caused an overlap between absorption and emission bands.
Although the two bands were not fully overlapped, MADN was used as the host material for
OLEDs using BDAT-CP as the dopant. The overlap between UV-visible absorption band
of BDAT-CP and the PL band of MADN was favorable for the Fo¨rster energy transfer from
MADN to BDAT-CP. Figure 2 (b) showed photoluminescence(PL) of BDAT-CP in the Chlo-
roform and n-Hexane mixture. The fluorescence wavelength and intensity were changed
Figure 4. (a) Current density vs. luminous efficiency and (b) quantum efficiency vs. current density
characteristics with variation of doping concentration.

66/[444]
S. N. Lee et al.
by ratio of n-hexane in the solvent. The molecules such as BDAT-CP and (Z)-3-(4,4ꢀꢀ-
bis(dimethylamino)-[1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl]-2ꢀ-yl)-2-phenylacrylonitrile(BDAT-P) [14] have
extended conjugation of electron donating (donor) and accepting (acceptor) moieties within
molecule show a strong intramolecular charge transfer (ICT) character. PL wavelength of
a fluorophore with ICT character can be controlled by modification of donor or acceptor
moiety which changes the degree of ICT. As the electron withdrawing ability in acceptor is
increased, intramolecular charge separation in excited state is enhanced. As concentration
of chloroform increased, PL intensity decreased, which was due to increase in radiation-
less transition caused by extended twisted state and energy orbital overlap. The dramatic
Figure 5. (a) Normalized EL spectra, (b) measured CIExy coordinates and (c) chromaticity diagram
measured with variation of doping concentration. (Continued)

Blue Emitting Aromatics Application to the Yellow OLED
[445]/67
Figure 5. (Continued)
bathochromic shift observed from BDAT-CP compared to BDAT-P can be easily under-
stood by stronger ICT character. Figure 2 (c) represented the HOMO and LUMO energy
levels of the materials used for the device with the structure of ITO/NPB/MADN:BDAT-
CP/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-CP.
Figure 3(a) and (b) showed the current density vs. voltage and current density vs.
luminance of BDAT-CP doped OLEDs at various doping concentration of 5, 10 and 15%.
The luminance maxima were 14700 cd/m2 (x = 5%), 9825 cd/m2 (x = 10%) and 7901 cd/m2
(x = 15%), respectively. As doping concentration increased from 5% to 15%, luminance
was decreased, which was ascribed to the mismatching of holes and electrons to make
excitons. The holes can travel to the Bphen because the BDAT-CP have higher HOMO
level than MADN.
As shown in Fig. 4(a) and (b), device doped with the new yellow fluorescent BDAT-CP
represented suitable luminous efficiency and quantum efficiency for OLEDs. The maximum
luminous and quantum efficiency values were observed to be 7.14 (x = 5%), 4.72 (x = 10%)
and 3.19 (x = 15%), and 2.77 (x = 5%), 2.09 (x = 10%) and 1.59 (x = 15%), respectively.
Based on Figs 3 and 4, EL emission peak was much more shifted to short wavelength
compared to that of PL. The device with doping concentration of 5% was observed to be
the most stable and efficient.
Figure 5(a) showed the EL spectra of BDAT-CP with variation of doping concen-
tration (x = 5, 10 and 15%). EL emission peaks were shifted to longer wavelength as
doping concentration was increased and they were measured to be 576 (x = 5%), 590

68/[446]
S. N. Lee et al.
Table 1. EL characteristics of yellow OLEDs, including the reported maximum luminance
`
(Lmax), luminous efficiency (e), and CIE
2
`
Material/method
Lmax (cd/m ) e (cd/A) CIE (x, y)
ref
BDAT-CP
14,700
19,383
7.13a (0.49, 0.49) This work
4-(dicyanomethylene)-2-tert-butyl-6-
(p-diphenylaminostyryl)-4Hpyran
(DCTP) in Alq3 (2 wt%)
rubrene in TPDS (5 mol%)
rubrene in o-TPD:m-OXD (0.4 wt%)
rubrene in NPB (5 wt%)
5.3b
(0.47, 0.51)
19
1.87c
2.30d
4.20e
1.58e
20
21
22
23
27,700
4,200
rubrene in Alq3 (10 mol%)
b
c
aMaximum efficiency. At current density J = 20 mA/cm2. At current density J = 10 mA/cm2.
dAt current density J = 100 mA/cm2. eMaximum efficiency.
(x = 10%) and 598(x = 15%) nm, respectively. Yellow light was more pronounced with
the increase of doping concentration. Many other of EL properties observed from the de-
vice with doping concentration of 5% were superior to those observed from the device
with doping concentration of 10% and 15%. Yellow light emission is not for full color
display but for WOLEDs so it is assumed that doping concentration of 5% is more fa-
vorable than that of 10 and 15%. As shown in Fig. 5(b), Commission Internationale de
I’Eclairage (CIExy) coordinates of device were measured to be (0.492, 0.494) (x = 5%)
and (0.523, 0.470) (x = 10%) and (0.545, 0.451) (x = 15%) at 8 V, respectively. Observed
CIExy coordinates represented that the device with doping concentration from 5 to 15%
emitted yellowish green EL emission, which was shown in Fig. 5(c). This yellow emitting
OLED made with BDAT-CP are concentration dye concentration dependent, which is influ-
enced by hole-transporting properties. The energy transfer of MADN to BDAT-CP is very
efficient.
As mentioned above, BDAT-CP has better properties than other existing yellow
fluorescent materials. Table 1 showed Characteristics of BDAT-CP and several established
yellow OLEDs, Including the reported maximum luminance, luminous efficiency, and CIE
coordinates. It showed that BDAT-CP has higher luminous efficiency. But luminance maxi-
mum is normal level as compared with other materials. However, researches like alterations
of device structures and improvement of external quantum efficiency will be able to easily
solve this problem.
4. Conclusions
A yellow fluorescent material, (2Z)-3-[4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ-yl]-
2-(4-cyanophenyl)acetonitrile (BDAT-CP), was synthesized for use in OLED application.
This fluorescent material has two acceptors that made intramolecular charge transfer (ICT)
characters. As the electron withdrawing ability in acceptor is increased, intermolecular
charge separation in excited state is enhanced, and thus, dramatic shift in PL emission
of BDAT-CP to much longer wavelength compared to BDAT-P can be achieved. The
maximum luminous and quantum efficiency of device that the devices fabricated by BDAT-
CP as emitting layer material was measured to be 7.14 cd/A and quantum efficiency of
2.77%, respectively. Based on both of device efficiency and stability of the efficiency, device

Blue Emitting Aromatics Application to the Yellow OLED
[447]/69
A which was using BDAT-CP resulted in the better efficient device than device B which
was using BDAT-P. Therefore, it can be concluded that a newly synthesized fluorescent
BDAT-CP may be a good candidate for use in fabrication of WOLEDs.
Acknowledgment
This work was supported by the Technology Innovation Program (201210940001, Materials
Development for 50-inches UD OLED TV Using Super Hybrid Process) funded by the
Ministry of Knowledge Economy (MKE, Korea).
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