9,10-Diphenylanthracene derivative substituted with indole moiety for blue organic light-emitting diodes

Article abstract of DOI:10.1080/15421406.2016.1277486

In order to develop blue emitters in organic light-emitting diodes (OLEDs), we synthesized two materials, 9-(9,10-diphenylanthracen-6-yl)-9H-carbazole and 1-(9,10-diphenylanthracen-6-yl)-1H-indole by Buchwald–Hartwig reactions. To investigate their electroluminescent performances, multilayer devices were fabricated by following structure: indium-tin-oxide (ITO) (180?nm) / N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) (50?nm) / blue emitters (30?nm) / 4,7-diphenyl-1,10-phenanthroline (Bphen) (30?nm) / lithium quinolate (Liq):Al (100?nm). A device using 1-(9,10-diphenylanthracen-6-yl)-1H-indole exhibited an efficient blue emission with luminous, power, and external quantum efficiency of 3.11?cd/A, 1.60?lm/W, and 2.82% at 20?mA/cm², respectively and CIE (x, y) coordinates of (0.15, 0.13) at 6.0?V.

Full text of DOI:10.1080/15421406.2016.1277486

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
9,10-Diphenylanthracene derivative substituted
with indole moiety for blue organic light-emitting
diodes
Dong Young Kim, Young Seok Kim, Song Eun Lee, Young Kwan Kim & Seung
Soo Yoon
To cite this article: Dong Young Kim, Young Seok Kim, Song Eun Lee, Young Kwan Kim &
Seung Soo Yoon (2017) 9,10-Diphenylanthracene derivative substituted with indole moiety for
blue organic light-emitting diodes, Molecular Crystals and Liquid Crystals, 644:1, 197-204, DOI:
10.1080/15421406.2016.1277486
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1277486
Published online: 27 Apr 2017.
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Date: 04 May 2017, At: 06:54

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
,-Diphenylanthracene derivative substituted with indole
moiety for blue organic light-emitting diodes
Dong Young Kima, Young Seok Kima, Song Eun Leeb, Young Kwan Kimb,
and Seung Soo Yoona
aDepartment of Chemistry, Sungkyunkwan University, Suwon, Korea; bDepartment of Information Display,
Hongik University, Seoul, Korea
ABSTRACT
KEYWORDS
Blue OLED; anthracene;
carbazole; indole;
Buchwald–Hartwig reactions
In order to develop blue emitters in organic light-emitting diodes
(OLEDs), we synthesized two materials, 9-(9,10-diphenylanthracen-
6-yl)-9H-carbazole and 1-(9,10-diphenylanthracen-6-yl)-1H-indole by
Buchwald–Hartwig reactions. To investigate their electrolumines-
cent performances, multilayer devices were fabricated by following
structure: indium-tin-oxide (ITO) (180 nm) / N,N’-di(1-naphthyl)-N,N’-
diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB) (50 nm) / blue emitters
(30 nm) / 4,7-diphenyl-1,10-phenanthroline (Bphen) (30 nm) / lithium
quinolate (Liq):Al (100 nm). A device using 1-(9,10-diphenylanthracen-6-
yl)-1H-indole exhibited an efficient blue emission with luminous, power,
and external quantum efficiency of 3.11 cd/A, 1.60 lm/W, and 2.82% at
20 mA/cm2, respectively and CIE (x, y) coordinates of (0.15, 0.13) at 6.0 V.
1. Introduction
Organic light-emitting diodes (OLEDs) have many attractive advantages such as low-driving
voltage, solid-state lighting, and wide view angle [1]. For these characteristics, OLEDs
have attracted the public eyes in scientific and commercial fields, and been regarded as
next-generation displays due to the promise of applications for the full-color and flat-panel
displays [2]. In order to achieve them, highly efficient red, green, and blue emitters with
tremendous thermal stabilities, long life-time, and good color-purity are indispensible. Com-
pared to the advances of red and green emitters, the development of blue emitters with high
electroluminescent (EL) efficiency needs to be further progressed because of their intrinsic
weakness; they have wide energy band gap and thus charge-injection from the near layer is
difficult comparing to red and green emitters [3–4].
Most recent researches about blue emitters have been used fluorescent organic materials
based on anthracene [5–7, 15–16]. Especially, 9,10-diphenylanthracene (DPA) is considered
as a representative anthracene derivative because of its high quantum yield in solution state
(Ф = 0.90) and worthy EL properties in OLED device. However, this material has also dis-
advantages of characteristics as a blue emitter and its EL performances should be improved
[8].
CONTACT Seung Soo Yoon
ssyoon@skku.edu; Young Kwan Kim
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
kimyk@hongik.ac.kr
©  Taylor & Francis Group, LLC

198
D. Y. KIM ET AL.
In this work, as the continuing efforts for the efficient blue emitters based on anthracene
derivatives, we designed and synthesized two materials, 9-(9,10-diphenylanthracen-6-yl)
-9H-carbazole (1) and 1-(9,10-diphenylanthracen-6-yl)-1H-indole (2) by Buchwald–Hartwig
reactions. For improved carrier balances in OLED devices, the well-known electron-rich and
-donating arylamino groups such as carbazole and indole groups are incorporated at the
C-2 position of 9,10-diphenylanthracene which enhance hole- transporting abilities. Further-
more, the arylamine moieties have noteworthy thermal stabilities with amorphous film form-
ing abilities [9, 17]. Therefore, these materials would have improved EL performances for blue
OLEDs.
2. Experimental details
Synthesis and characterization
2-Bromo-9,10-diphenylanthracene was synthesized according to literature [10]. 1H NMR and
13C NMR were recorded on Bruker Avance Ⅲ 500 MHz NMR spectrometer. FT-IR spec-
tra were recorded using a Bruker VERTEX70 FT-IR spectrometer. Low- and high-resolution
mass spectra were measured using Jeol JMS-AX505WA spectrometer in APCI mode.
-(,-diphenylanthracen--yl)-H-carbazole ()
2-bromo-9,10-diphenylanthracene (1.0 mmol), 9H-carbazole (1.5 mmol), Tris(dibenzylide-
neacetone)dipalladium(0) (0.05 mmol), dicyclohexylphosphino-2ꢀ,4ꢀ,6ꢀ-triisopropylbiphenyl
(0.08 mmol), sodium tert-butoxide (3.0 mmol) and anhydrous toluene (14 ml) were mixed
in a flask. The mixture was refluxed at 120°C for 4 h. After the reaction finished, the reaction
mixture was extracted with toluene and washed with water. The organic layer was dried with
anhydrous MgSO4 and filtered with silica gel. The solution was then evaporated. The crude
1
product was purified recrystallization from CH2Cl2/EtOH. Yield: 60%. H NMR (CDCl3,
500 MHz, δ); 8.11 (d, J = 8.0 Hz, 2 H), 7.91 (d, J = 9.0 Hz, 1 H), 7.88 (s, 1 H), 7.77–7.73
(m, 2 H), 7.67–7.64 (m, 2 H), 7.60 (d, J = 7.0 Hz, 1 H), 7.58–7.55 (m, 4 H), 7.54–7.51 (m,
4 H), 7.46 (d, J = 7.0 Hz, 1 H), 7.43 (d, J = 8.5 Hz, 2 H), 7.40–7.38 (m, 3 H) 7.37 (d, J =
8.5 Hz, 2 H); 13C NMR (CDCl3, 500 MHz, δ); 140.7, 138.7, 138.5, 137.5, 137.2, 134.2, 131.3,
131.2, 131.1, 130.5, 130.2, 130.2, 129.2, 128.7, 128.6, 127.8, 127.8, 127.1, 127.0, 125.9, 125.6,
125.4, 124.4, 123.8, 123.4, 120.3, 120.0, 109.8; FT-IR [ATR]: ν (cm−1) 3026, 2378, 2351, 2312,
1740, 1368, 1270, 1216; APCI-MS (m/z): 495 [M+].
-(,-diphenylanthracen--yl)-H-indole ()
Yield: 72%. 1H NMR (CDCl3, 500 MHz, δ); 7.85 (d, J = 9.5 Hz, 1 H), 7.76 (s, 1 H), 7.74–7.71
(m, 2 H), 7.66–7.62 (m, 3 H), 7.61–7.57 (m, 3 H), 7.54–7.51 (m, 7 H), 7.39–7.33 (m, 3 H),
7.20–7.13 (m, 2 H), 6.65 (s, 1 H); 13C NMR (CDCl3, 500 MHz, δ); 138.7, 138.6, 137.5, 137.0,
136.2, 135.7, 131.2, 131.2, 130.6, 130.1, 130.0, 129.3, 129.0, 128.6, 128.5, 128.2, 127.8, 127.8,
127.7, 127.1, 126.9, 125.6, 125.2, 122.7, 122.4, 121.1 120.5, 120.0, 110.6, 103.8; FT-IR [ATR]:
ν (cm−1) 3055, 1521, 1467, 1211, 1125, 1022, 763, 739, 703; APCI-MS (m/z): 446 [M+].
Physical measurements
The UV–Vis absorption and photoluminescence (PL) spectra of these materials were mea-
sured in dichloromethane (10−5 M) using Shimadzu UV-1650PC and Aminco-Bowman
series 2 luminescence spectrometers. The fluorescent quantum yield were determined in

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
199
Scheme . Structures and synthetic routes of compounds 1 and 2. (a) Pd(dba), XPhos, NaOt-Bu, toluene,
reflux, °C,  h.
dichloromethane solution at 293 K against DPA as a reference (Ф = 0.90) [11]. The highest
occupied molecular orbital (HOMO) energy levels were measured with a low energy photo-
electron spectrometry (Riken-Keiki AC-2). The energy band gaps were determined from the
intersection of the absorption and photoluminescence spectra. The lowest unoccupied molec-
ular orbital (LUMO) energy levels were estimated by the energy band gap and the HOMO
energy levels.
OLED fabrication and measurements
For fabricating OLEDs, the indium-tin-oxide (ITO) thin films coated on glass substrates were
used, which was 30 ꢀ / square of the sheet resistivity. The ITO coated glass was cleaned in
an ultrasonic bath by the following sequences: in acetone, methyl alcohol, distilled water and
kept in isopropyl alcohol for 48 hours and dried by N2 gas gun. The substrates were treated
by O2 plasma treatment with the conditions of 2 × 10−2 Torr at 125 W for 2 minutes. All
organic materials and metal were deposited under high vacuum (5 × 10−7 Torr). The OLEDs
were fabricated in the following sequence: indium-tim-oxide (ITO) / N,N’-di(1-naphthyl)-
N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB) (50 nm) / blue light-emitting materials
(1 and 2) (30 nm) / 4,7-diphenyl-1,10-phenanthroline (Bphen) (30 nm) / lithium quino-
late (Liq):Aluminium (Al) (100 nm) (Device A and B). All of the properties of the OLEDs
such as current density (J), luminance (L), luminous efficiency (LE), power efficiency (PE),
external quantum efficiency (EQE), and the CIE chromaticity coordinates of OLEDs were
measured with Keithley 2400, Chroma meter CS-1000A. Electroluminescence was measured
using Roper Scientific Pro 300i.
3. Results and discussion
The synthetic scheme and chemical structures of materials (1 and 2) are shown in the
Scheme 1. They were synthesized by Pd-catalyzed Buchwald–Hartwig reactions between
2-bromo-9,10-diphenylanthracene and arylamino groups such as 9H-carbazole or 1H-indole
with good yields. Their chemical structures were characterized by 1H-, 13C-NMR, FT-IR, and
mass spectrometer.
Figure 1 illustrates the UV–Vis absorption and photoluminescence (PL) spectra of mate-
rials 1 and 2 at room temperature in diluted CH2Cl2 solutions and thin films; their physical
data are shown in the Table 1. In UV–Vis spectra in CH2Cl2, the higher-intensity bands in
both compounds 1 and 2 around 310–350 nm may be attributed to the delocalized π–π∗

200
D. Y. KIM ET AL.
Figure . UV–Vis absorption spectra (no symbol), PL spectra in CHCl (closed symbol), and in thin films
(opened symbol) of blue emitters 1 and 2.
Table . Physical properties of compounds 1 and 2.
a
a
b
d
FWHMa [nm]
HOMO / LUMOc [eV]
Eg [eV]
Ф e
/
Compound
UVmax [nm]
PLmax [nm]
1
2


 / 
 / 
. / .
. / .
− . / −.
− . / −.
.
.
.
.
a
In CHCl (− M). b Solid thin film on quartz plates. c The HOMO energy level was determined by low-energy
photoelectron spectrometer (Riken-Keiki, AC-) and the LUMO = HOMO + ꢁE. d Using DPA as a standard; λex =  nm
(Ф = . in CHCl).
transition of corresponding arylamino groups and the lower-intensity bands in them around
350–400 nm are derived from the characteristic π–π∗ transition of anthracene moiety. The
maximum wavelength (λmax) in PL spectra of materials 1 and 2 in CH2Cl2 were 451 nm and
439 nm, respectively. Compared to λmax of compound 2 with indole moiety, that of compound
1 shows red-shifted emissions due to the extended π-conjugation including carbazole moiety.
The full widths at half maximum (FWHM) values in solution state are 42.1 nm and 30.2 nm,
respectively. Compared to PL spectra of compounds 1 and 2 in CH2Cl2, each value of λmax of
compounds 1 and 2 in thin film state shows red-shifts due to the effect of molecular packing
in the solid states.
Figure . Frontier molecular orbitals of compounds 1 and 2.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
201
Figure . Energy-level diagrams of the materials used in devices A and B.
The HOMO energy levels of compounds 1 and 2 measured by a photoelectron spectrome-
ter (Riken-Keiki AC-2) are −5.81 and −5.70 eV, respectively. Compound 2 exhibits shallower
HOMO level than that of compound 1 due to the increased electron-donating ability of lone-
pair electrons at nitrogen atom in indole moiety compared to in carbazole moiety [12]. The
energy band gaps (Eg) of the materials 1 and 2 are 2.95 and 2.97 eV, respectively. The LUMO
energy levels which were calculated by the HOMO energy levels with their corresponding Eg
values are −2.86 and −2.73 eV, respectively.
To understand the photophysical properties of the compounds 1 and 2 at the molecular
level, density functional theory (DFT) calculations of compounds 1 and 2 were carried out
using the Becke’s three parameterized Lee–Yang–Parr (B3LYP) functional with 6–31G∗basis
sets using a suite of Gaussian programs, as shown in the Figure 2. In both compounds 1 and 2,
the electron clouds in HOMOs spread anthracene over carbazole or indole units; they indicate
that chromophores are a combination anthracene and arylamino groups. On the other hands,
those in LUMOs are distributed only anthracene unit; it means that the only lumophores are
anthracene units.
To investigate the EL properties of these materials 1 and 2, devices A and B using materials
1 and 2 as emitters were fabricated with a following structure: ITO (180 nm) / NPB (50 nm) /
blue emitters 1 or 2 (30 nm) / Bphen (30 nm) / Liq:Al (100 nm) (Devices A or B); ITO, NPB,
Bphen, and Liq:Al are used as an anode, hole-transporting layer (HTL), electron-transporting
layer (ETL), and a cathode, respectively. The HOMO and LUMO energy-level diagrams of all
the materials used in the EL devices are illustrated in Figure 3 and their EL properties are
summarized in Table 2.
Figure 4 shows the EL spectra of devices A and B which exhibit blue fluorescence with
λmax values of 456 and 447 nm, respectively. The EL emission spectra of devices A and B
show similar trend with the PL emission spectra of materials 1 and 2. This means that the
Table . EL properties of devices A and B.
a
b
b
b
c
d
LEa [cd/A]
PEa [lm/W]
EQEa [%]
λ,max [nm]
CIEc (x, y)
/
/
/
/
Device
Lmax [cd/m]
A
B


. / .
. / .
. / .
. / .
. / .
. / .


(., .)
(., .)
a
b
c
Maximum value.
Value at  mA/cm.
At . V.
d
Commission Internationale dꢀÉclairage (CIE).

202
D. Y. KIM ET AL.
Figure . EL spectra of blue OLEDs A and B at . V.
Figure . (a) J-V-L characteristics, (b) luminous efficiencies and power efficiencies, and (c) external quantum
efficiencies of the devices A and B.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
203
emissions from devices A and B are originated from the singlet excitons in the corresponding
emitters 1 and 2. The CIE coordinates of devices A and B are (0.16, 0.16) and (0.15, 0.13) at
6.0 V, respectively. Particularly, device B showed efficient deep-blue emission.
Figure 5 shows J-V-L characteristics, luminous efficiencies (LE), power efficiencies (PE),
and external quantum efficiencies (EQE) of the devices A and B, respectively. Particularly,
device B has better EL efficiencies than device A. This phenomenon may have been attributed
by effective charge-carrier injection, especially holes-injection. In device B, hole could be eas-
ily injected to the EML from the HTL through the shallower HOMO energy level of 2 than that
of compound 1 in device A. Furthermore, the hole-blocking capability in device B is larger
than that in device A, because the energy barrier between the EML and the ETL in device B is
higher than that in the device A. Therefore, holes are effectively injected in the emitting layer
and prevented from leakages to the ETL in device B in comparison with device A [13]. These
would leads to more efficient EL performances in device B than device A. Interestingly, device
A shows the higher values of LE than device B, even though EQE of device B is higher than
that of device A. This is the result from the differences in luminance by what color they are.
The luminance is under control with the visual recognition from the emission or reflection
from the light sources and their values are increased in order of blue < red < green. Thus,
device B whose CIE coordinates are located in deeper blue region shows lower LE value than
that of device A, although it has the higher EQE values [14, 18].
4. Conclusion
In summary, a series of highly efficient blue light-emitting materials was synthesized by
Buchwald–Hartwig reactions. Particularly, the device B using compound 2, which has shal-
low HOMO energy level (−5.70 eV), exhibiting the efficient blue emission with high LE,
PE, and EQE values of 3.11 cd/A, 1.60 lm/W, 2.82% at 20 mA/cm2, respectively, with CIE
coordinates of (0.15, 0.13) at 6.0 V. This study demonstrates that arylamine substituted 9,10-
diphenylanthracene derivatives would have the excellent properties as emitters for efficient
blue OLEDs.
Funding
This research was supported by Basic Science Research Program through the NRF funded by the Min-
istry of Education, Science and Technology, South Korea (NRF-2013R1A1A2A10008105) and (NRF-
2015R1A6A1A03031833).
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