Synthesis and electroluminescent properties of red fluorescent 2-(6,8-di-tert-butyl-2-(4-((3,5-di-tert-butylphenyl)(4-(trimethylsilyl)phenyl) amino)styryl)-4H-chromen-4-ylidene)malononitrile (DCCTBPA) for organic light-emitting diodes (OLEDs)

Article abstract of DOI:10.1080/15421406.2012.706556

We synthesize a red fluorescent material, 2-(6,8-di-tert-butyl-2-(4-((3,5- di-tert-buty- lphenyl)(4-(trimethylsilyl)phenyl)amino)styryl)-4H-chromen-4- ylidene)malononitrile (DCCTBPA) using Knoevenagel condensation. To explore the electroluminescent proper

Full text of DOI:10.1080/15421406.2012.706556

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Synthesis and Electroluminescent
Properties of Red Fluorescent
2-(6,8-di-tert-butyl-2-(4-
((3,5-di-tert-butylphenyl)(4-
(trimethylsilyl)phenyl)amino)styryl)-4H-
chromen-4-ylidene)malononitrile
(DCCTBPA) for Organic Light-Emitting
Diodes (OLEDs)
Eun Jae Na a , Kum Hee Lee a , 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: 27 Sep 2012.
To cite this article: Eun Jae Na , Kum Hee Lee , Young Kwan Kim & Seung Soo Yoon (2012) Synthesis
and Electroluminescent Properties of Red Fluorescent 2-(6,8-di-tert-butyl-2-(4-((3,5-di-tert-
butylphenyl)(4-(trimethylsilyl)phenyl)amino)styryl)-4H-chromen-4-ylidene)malononitrile (DCCTBPA)
for Organic Light-Emitting Diodes (OLEDs), Molecular Crystals and Liquid Crystals, 568:1, 8-14, DOI:
10.1080/15421406.2012.706556
To link to this article: http://dx.doi.org/10.1080/15421406.2012.706556
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Mol. Cryst. Liq. Cryst., Vol. 568: pp. 8–14, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.706556
Synthesis and Electroluminescent Properties of Red
Fluorescent 2-(6,8-di-tert-butyl-2-(4-((3,5-di-tert-
butylphenyl)(4-(trimethylsilyl)phenyl)amino)styryl)-
4H-chromen-4-ylidene)malononitrile (DCCTBPA)
for Organic Light-Emitting Diodes (OLEDs)
EUN JAE NA,1 KUM HEE LEE,1 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
We synthesize a red fluorescent material, 2-(6,8-di-tert-butyl-2-(4-((3,5-di-tert-buty-
lphenyl)(4-(trimethylsilyl)phenyl)amino)styryl)-4H-chromen-4-ylidene)malononitrile
(DCCTBPA) using Knoevenagel condensation. To explore the electroluminescent
properties of materials multilayered OLEDs were fabricated by employing DCCTBPA
as a dopant material. This device showed the luminous and power efficiencies of 0.73
cd/A and 0.31 lm/W at 20 mA/cm2, respectively, with the CIE coordinates of (0.66, 0.34)
at 7.0 V. Compared to a device using DCCPA, this device using DCCTBPA showed
the improved EL performances due to reducing the molecular aggregation and thus
preventing concentration quenching.
Keywords Chromene Derivatives; OLED; Red Fluorescence; t-Butyl group
Introduction
Many research efforts have been focused on the development of the emitting materials
for organic light-emitting diodes (OLEDs) due to their potential application in flat panel
displays and solid-state lighting [1,2]. Among the blue-, green-, and red-light-emitting
materials required for full-color displays, red-light-emitting mater-ials remain one of the
greatest challenges because of the lack of high-performance [3] Presently, most high-
performance red OLEDs are made by doping a red dye into a suitable host [4]. The most
widely studied dopants are pyran-containing compounds such as DCM and DCJTB have
been widely studied [5,6], but their EL performances are not pleased with a prerequisite
for the practical applications. Recently, Lee et al. group reported a new red fluorescent ma-
terial based on chromene moiety such as 2-(2-(4-(diphenyl-amino)styryl)-4H-chromen-4-
ylidene) malono-nitrile (DCCPA), and showed the red electro-luminescence with the power
efficiency of 0.139 lm/W and the CIE coordinate of (0.60, 0.39) in an OLED device using
this materials as a dopant. Furthermore, a derivative of DCCPA such as 2-(2-(4-(diphenyl-
amino)styryl)-4H-6,7-dimethyl-chromen-4-ylidene)malononitrile (DCCMPA) has showed
∗Address corresponding to Seung Soo Yoon, Department of Chemistry, Sungkyunkwan Univer-
sity, Suwon 440-746 Korea. Tel: 82 31 290 7071; Fax:+82 31 290 7075. E-mail: ssyoon@skku.edu
[424]/8

Synthesis and Properties of Red Fluorescent OLEDs
[425]/9
the improved EL performance due to the introduction of two methyl substituents on the
chromene moiety by preventing concentration quenching [7,8].
In this paper, we describe the synthesis and electroluminescent properties of a red
fluore-scent material, 2-(6,8-di-tert-butyl-2-(4-((3,5-di-tert-butylphenyl)(4-(trimethylsilyl)
phenyl) amino)styryl)-4H-chromen-4-ylidene)malononitrile (DCCTBPA). In red emitter
DCCTBPA, the bulky t-butyl groups and trimethylsilyl group are introduced into DCCPA
skeleton to increase steric hindrance between red emitters in the emitting layer of devices
and thus improve the EL performances by preventing concentration quenching.
Experimental
Materials and Measurement
All commercially available reagents were purchased from Aldrich and TCI and used with-
out further purification. 1H- and 13C-NMR were recorded on a Varian (Unity Inova 300Nb)
spectrometer. FT-IR spectra were recorded using a Thermo Nicolet Avatar 320 FT-IR
spectrometer. Low- and high-resolution mass spectra were measured using a Jeol JMS-
AX505WA spectrometer in the FAB mode and a Jeol JMS-600W spectrometer in the EI
mode and a JMS-T100TD (AccuTOF-TLC) in the positive ion mode. The UV-Vis absorp-
tion and photoluminescence spectra of these newly designed red dopants were measured in
a 10−5 M solution of 1,2-dichloroethane. Fluorescent quantum yields were determined in
1,2-dichloroethane at 293 K against DCJTB = 0.78 [16]. The HOMO energy levels were
measured with low energy photo-electron spectrometry (Riken-Keiki AC-2). The LUMO
energy levels were calculated by subtracting the corresponding optical band gap energies
from the HOMO energy values.
Synthesis
Synthesis of 6,8-di-tert-butyl-2-methyl-4H-chromen-4-ylidene)malononitrile (1). To a solu-
tion of NaH (5.03 mmol) and 1-(3,5-di-tert-butyl-2-hydroxy phenyl)ethanone (2.01 mmol)
in anhydrous THF, acetyl chloride (2.01 mmol) was added at room temperature. After
stirred for 1 h, NH4Cl was added at 0◦C. The solution was extracted with hexane and brine.
The resulting materials were dissolved in glacial acetic acid (5 ml) and HCl (1 ml) and then
were heated to 100◦C for 1 h. After cooling to room tem-perature, the reaction mixture
was washed to NaHCO3 and extracted with ethyl acetate and brine. After filtration and
evaporation of solvent, the mixture was purified by silica gel column chromatography with
ethyl acetate and hexane (1:4). (Yield : 87%) The resulting compound (0.73 mmol) and
malonitrile (1.50 mmol) were dissolved in acetic anhydride and heated for 24 h. The solu-
tion was washed to NaHCO3 and extracted with CH2Cl2. After filtration and evaporation of
solvent, the mixture was purified by re-crystallized with CH2Cl2 and hexane. (Yield : 79%).
1H-NMR (300 MHz, CDCl3): δ ppm 8.79 (d, J = 2.2 Hz, 1H), 7.75 (d, J = 2.2 Hz, 1H),
6.75 (d, J = 0.4 Hz, 1H), 2.49 (s, 3H), 1.49 (s, 9H), 1.38 (s, 9H); 13C-NMR (125 MHz): δ
ppm 160.5, 155.0, 150.0, 148.7, 138.9, 126.0, 122.8, 120.5, 120.3, 105.5, 61.5, 35.8, 35.7,
31.6, 31.6, 31.5, 30.4, 30.3; Mass (EI) m/z = 320 (M+).
Synthesis of 4-((3,5-di-tert-butylphenyl)-(4-(trimethylsilyl)phenyl)amino)benzaldeh
yde (2). 3,5-di-tert-butylaniline(2.43 mmol) and 2-(4-bromophenyl)-1,3-dioxolane
(2.43 mmol) were dissolved in toluene (7 ml). Pd2dba3 (0.12 mmol), NaOt-Bu (7.29 mmol),
and 2-P(t-bu)2biphenyl (0.24 mmol) were then added. The reaction mixture was heated to
80◦C for 5 h, and then 1-bromo-4-trimethylsilyl benzene (2.43 mmol) was added and heated

10/[426]
E. J. Na et al.
to 80◦C for 24 h. The solution was extracted with ethyl ether and brine. The combined
organic layers were washed with brine and MgSO4. After filtration and evaporation of
solvent, the mixture was purified by silica gel column chromatography with ethyl acetate
and hexane. The obtained compound was yellowish solid (Yield : 29%). The obtained
compound was yellowish solid. (Yield : 23%) 1H-NMR (300 MHz, CDCl3): δ 9.52, (s, 1H),
7.39 (d, J = 8.9 Hz, 2H), 7.18 (d, J = 8.4 Hz, 2H), 6.99 (m, 1H), 6.87 (d, J = 8.4 Hz, 2H),
6.75 (m, 4H), 0.99 (s, 18H) 0.01 (m, 9H); 13C-NMR (500 MHz): δ 152.1, 146.6, 134.8,
131.5, 127.6, 125.0, 123.6, 122.3, 121.6, 120.5, 119.2, 118.0, 65.6, 35.2, 31.7, 0.9; Mass
(EI) m/z = 457 (M+).
Synthesis of DCCTBPA. Compound 1 (1.09 mmol) and Compound 2 (1.09 mmol)
into RBF were added and connected with deanstark trap (put the molecular sieve), reflux
condenser. And then anhydrous ethyl alcohol (30ml), piperidine (4.91 mmol) were added
and heated to 95◦C for 5 h and then cooled to room temperature. The mixture was filtered
and extracted with ethyl acetate and brine. After filtration and evaporation of solvent,
1
the mixture was purified by re-crystallized with ethyl alcohol. (Yield : 78%) H-NMR
(300 MHz, CDCl3): δ 7.43 (d, J = 9.0 Hz, 1H), 7.22 (d, J = 6.6, 1.9 Hz, 1H), 7.10 (d, J
= 8.5 Hz, 4H), 7.02 (t, J = 1.6 Hz, 1H), 6.93 (m, 4H), 6.80 (m, 5H), 1.03 (s, 10H), 1.01
(s, 22H), 0.04 (s, 3H), 0.01 (s, 10H); 13C-NMR (500 MHz): δ 212.6, 203.7, 190.5, 170.4,
167.3, 153.2, 152.3, 149.8, 149.4, 135.0, 131.0, 128.3, 124.6, 124.2, 123.0, 121.2, 121.1,
118.7, 118.4, 106.3, 104.8, 66.3, 36.4, 35.8, 32.3, 32.2, 31.0, 21.4, 0.9; FT-IR (KBr): ν =
2978, 2341, 1590, 1556, 1505, 1492, 1320, 835 cm−1; Mass (EI) m/z = 759 (M+); HRMS
(EI) calcd for C51H61N3OSi, 759.4584; found, 759.4589; mp. 151◦C.
Fabrication of OLED
OLEDs using red-light-emitting molecules were fabricated by vacuum (10−6 torr) thermal
evaporation onto pre-cleaned ITO coated glass substrates. The device structures were
as follows: (1) ITO/N,Nꢀ-diphenyl-N,Nꢀ-(1-napthyl) -(1,1ꢀ-biphenyl)-4,4ꢀ-diamine (NPB)
(50 nm)/ tris(8-quionlinolato)-aluminium (Alq3): Red dopant (1 or 2%) (30 nm)/Alq3
(5 nm)/Liq (2 nm)/Al. (2) ITO/NPB (50 nm)/Alq3: Red dopant (2%) (30 nm)/bathucuproine
(BCP) (10 nm)/ Alq3 (40 nm)/Liq (2 nm)/Al. All of the pro-perties of the OLEDs such as the
current density (J), luminance ꢀ(L), luminance efficiency (LE), power efficiency (PE), and
´
commission inter-national de 1 Eclairage (CIE) coordinate charac-teristics were measured
using a Keithly 2400 source measurement unit and a Chroma meter MINOLTA CS-1000A.
Electro-luminance was measured using a Roper Scientific Pro 300i.
Results and Discussion
Structure and synthetic scheme of DCCTBPA were shown in Scheme 1. The UV-Vis
absorption and PL spectra of DCCTBPA and DCCPA are shown in Fig. 1. In particular,
Fig. 1(a) shows the good overlap between the emission spectra of a common host material
Alq3, and the absorption spectra of DCCTBPA and DCCPA. This observation imply that the
Fo¨rster singlet energy transfer from host Alq3 to red emitter DCCTBPA and DCCPA would
be efficient, and Alq3 served well as a host in the OLEDs by using these compounds as
red dopant materials. Red emitters DCCTBPA and DCCPA exhibit efficient red emissions
with maximum emission peaks of 663 and 650 nm, respectively, as shown in Fig. 1 (b).
Inter-estingly, red emitter DCCTBPA has the longer maximum emission peak than DCCPA.
The electron-donating t-butyl and trimethylsilyl groups in diaryl-aminophenyl moiety of

Synthesis and Properties of Red Fluorescent OLEDs
[427]/11
Scheme 1. Synthesis and structures of the red materials DCCTBPA. a) i) NaH, AcCl, ii) HCl,
AcOH, iii) Malonitrile, Ac2O, b) 1-bromo-4-trimethysilylbenzene, 3,5-di-tertButylaniline, Pd2dba3,
2-(di-tert-butylphosphino)-biphenyl, Na-O-(t-Bu), c) 1, Piperidine, EtOH.
DCCT-BPA increase the electron density around nitro-gen atom of DCCTBPA and thus
improve the donor-acceptor interaction in DCCTBPA. Pre-sumably, this would make the
energy band-gap of DCCTBPA narrower than that of DCCPA[9]. All physical properties
were shown in Table 1.
Figure 1. (a) UV-Vis spectra and (b) PL spectra of red emitters DCCTBPA and DCCPA. (c)
Luminous efficiencies-current density and (d) Power efficiencies-current density characteristics of
devices 1–4.

12/[428]
E. J. Na et al.
Table 1. Physical properties of red emitters DCCTBPA and DCCPA
UVMax
(nm)[a]
PLMax
HOMO
(eV)[b]
LUMO
(eV)[b]
Compound
(nm)[a]
FWHM
Eg
Q.Y[c]
DCCTBPA
DCCPA
513
502
663
650
103
105
5.63
5.60
3.54
3.45
2.09
2.15
0.40
0.54
[a]Maximum absorption or emission wavelength in 1,2-dichloroethane (ca. 1 × 10−5 M). [b] Obtained
from AC-2 and UV-vis absorption measurements. [c] Using DCJTB as a standard; lex = 550 nm (ꢀ =
0.78 in 1,2-dichloroethene).
To explore the electroluminescent properties of DCCTBPA, OLED devices using
DCCTBPA as a dopant in Alq3 host were fabricated. Particularly, to optimize the device
structure, two kind of devices with or without BCP were fabricated. BCP with the high
HOMO energy level was used as hole blocking material to improve the EL efficiencies of
devices by blocking hole-leakage from the emitting layer to the electron transporting layer
and thus the con-finement of excitons in the emitting layer [10]. Also, for the comparison, the
control device using DCCPA as a dopant in Alq3 host was fabricated. All electroluminescent
data on devices using DCCTBPA and DCCPA was summarized in Table 2. The luminous
and power efficiencies of devices are shown in Figure 1 (c) and (d), respectively. Also, the
EL spectra of devices are shown in Figure 2. In devices 1 and 2 using DCCTBPA, with the
increase of doping percentage of dopants, the luminous and power efficiencies decreased
due to the concentration quenching effect. The EL spectra of devices 1 and 2 showed Alq3
emissions around 510 nm due to the exciton leakages from emitting layer to Alq3 in the
electron transporting layer. Also, the EL spectrum of device 2 showed the shoulder peak
around 700 nm, which originated from the excimers of dopant DCCTBPA. Thus the CIE
coordinates of devices 1 and 2 are in the yellow-orange not the red region. Interestingly,
compared to device 2 without BCP layer, the EL spectrum of device 3 with BCP layer
showed no Alq3 emission due to the blocking hole leakages from emitting layer to Alq3 in
the electron transporting layer. Thus, compared to device 2, device 3 had the improved CIE
coordinates of (0.66, 0.34) at 7.0 V, approaching saturated red emission. The luminous and
power efficiencies of the device 3 were 0.73 cd/A and 0.31 lm/W at 20 mA/cm2, respectively.
Notably, compared to device 4 using DCCPA as a dopant, the power efficiency of device 3
using DCCTBPA as a dopant increased by 48% at 20 mA/cm2. The bulky t-butyl groups
and trimethylsilylmethyl group of dopant DCCTBPA could prevent molecular aggregation
and thus reduce concentration quenching. This would contribute to the improved luminous
Table 2. EL performance characteristic of devices DCCTBPA and DCCPA
L[a]
LE-J[a]/[b]
(cd/A)
PE-J[a]/[b]
(lm/W)
CIE[c]
(x,y)
Device
Dopant (doping%)
(cd/m2)
1
2
3
4
DCCTBPA(1%)
DCCTBPA(2%)
DCCTBPA(2%—BCP)
DCCPA(2%—BCP)
2928
1096
1465
203.4
1.71/1.71
1.10/1.13
0.73/0.80
0.76/0.81
0.72/0.85
0.38/0.49
0.31/0.42
0.21/0.30
(0.50,0.44)
(0.51,0.44)
(0.66,0.34)
(0.64,0.36)
[a]Maximum values. [b] Values at 20 mA/cm2. [c] Values at 7.0 V.

Synthesis and Properties of Red Fluorescent OLEDs
[429]/13
Figure 2. EL spectra the devices 1 – 4
efficiency of device 3 [11,12]. Furthermore, compared to device 4 with the CIE coordinate
of (0.64, 0.36), device 3 had the improved CIE coordinate of (0.66, 0.34) at 7.0 V, which
might be originated from the electron-donating effects of t-butyl and trimethylsilyl groups
of DCCTBPA dopant in device 3.
Conclusions
We designed and synthesized new red emitter DCCTBPA based on 2-(2-(4-(diarylamino)
styryl)-4H-chromen-4-ylidene)malononitrile (DCCPA) skeleton. A device with the struc-
ture of ITO/NPB (50 nm)/Alq3 : DCCTBPA (2%) (30 nm)/ BCP (10 nm)/Alq3 (40 nm)/Liq
(2 nm)/Al showed the luminous and power efficiencies of 0.73 cd/A and 0.31 lm/W at
20 mA/cm2, respectively. The CIE coordinates of this device was (0.66, 0.34) at 7.0 V,
approaching saturated red emission with the CIE coordinate of (0.67, 0.32). Compared to a
device using DCCPA, this device using DCCTBPA showed the improved EL performances
due to reducing the molecular aggregation and thus preventing concentration quenching.
This study demonstrates that a DCCPA derivative, DCCTBPA possesses excellent proper-
ties for efficient red fluorescent OLEDs.
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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