Blue light emission of new anthracene derivatives produced using optimized side group link positions

Article abstract of DOI:10.1016/j.dyepig.2018.04.029

Using an anthracene chromophore as a core group and a phenyl carbazole chromophore as a side group, three new emitters of blue light, 2-DCPA, 3-DCPA and 4-DCPA, were synthesized. The three compounds differed with regard to the position of the carbazole linked to the core, with 2-DCPA and 4-DCPA using carbazole nodes and 3-DCPA using the lobe position. Density functional theory calculations were performed to determine which positions of the carbazole moiety had node characteristics and which had lobe characteristics. The PL_max values of 2-DCPA, 3-DCPA and 4-DCPA in the film state were in the blue region, at 453, 457, and 452 nm, respectively. Of these materials, 3-DCPA, i.e., that with the linkage to the lobe position, showed the highest efficiency, with a value of 2.91 cd/A, and EQE, with a value of 2.65%. In a doped device using CBP as a host material and 3-DCPA as a dopant, the EL_max emission was observed to be in the deep blue region, at 433 nm, and with a CIE value of (0.150, 0.068).

Full text of DOI:10.1016/j.dyepig.2018.04.029

DyesandPigments156(2018)369–378
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Blue light emission of new anthracene derivatives produced using optimized
side group link positions
Seokwoo Kanga, Hyocheol Junga, Hayoon Leea, Suji Leeb, Mina Jungb, Jaehyun Leeb,
Young Chul Kima, Jongwook Parka,∗
a
Department of Chemical Engineering, Kyunghee University, Gyeonggi, 17104, South Korea
Department of Chemistry, The Catholic University, South Korea
b
A B S T R A C T
Using an anthracene chromophore as a core group and a phenyl carbazole chromophore as a side group, three
new emitters of blue light, 2-DCPA, 3-DCPA and 4-DCPA, were synthesized. The three compounds differed with
regard to the position of the carbazole linked to the core, with 2-DCPA and 4-DCPA using carbazole nodes and 3-
DCPA using the lobe position. Density functional theory calculations were performed to determine which po-
sitions of the carbazole moiety had node characteristics and which had lobe characteristics. The PLmax values of
2-DCPA, 3-DCPA and 4-DCPA in the film state were in the blue region, at 453, 457, and 452 nm, respectively. Of
these materials, 3-DCPA, i.e., that with the linkage to the lobe position, showed the highest efficiency, with a
value of 2.91 cd/A, and EQE, with a value of 2.65%. In a doped device using CBP as a host material and 3-DCPA
as a dopant, the ELmax emission was observed to be in the deep blue region, at 433 nm, and with a CIE value of
(0.150, 0.068).
1. Introduction
various side groups that effectively prevent intermolecular packing and
increase the electroluminescence (EL) efficiency of blue light-emitting
core chromophores having high photoluminescence (PL) efficiency such
as anthracene, pyrene and chrysene [13–15].
Organic light-emitting diodes (OLEDs) were first reported by Tang
and Van Slyke, and are now attracting attention in the market, for
applications such as mobile phones, television displays and solid-state
lighting, because of their potential as next-generation devices [1].
OLED device emits light which comes from exciton in the emission
material layer (EML), and the exciton is produced from electrons and
holes which are injected at the cathode and anode. In here, emitting
material plays an important role in determining the wavelength and
efficiency of the OLEDs.
Therefore, to achieve high performance of full-color OLED display,
it is necessary to develop red, green, and blue emitting materials having
high efficiency and long life-time [2–4]. In particular, in case of a blue
emitting material for a television display, the Commission Inter-
nationale de L'Eclairage coordinates CIE (x, y) of (0.14, 0.08) is re-
quired by the National Television System Committee (NTSC), and CIX
(x, y) of (0.15, 0.06) is required for a deep blue material of high-defi-
nition television (HDTV) [5]. However, blue light-emitting materials
require a wide energy band gap, which causes high charge injection
barriers in the device, and thus blue OLED devices are inferior to green
and red ones in terms of electroluminescence (EL) efficiency and device
life-time [6–12]. Many studies have been carried out to introduce
In this study, we used density functional theory (DFT) to determine
the location of orbital nodes and lobes within a core-side framework.
The selected side group was designed to contain the carbazole moiety,
since this moiety has been shown to contribute to good charge transport
and high PL efficiency [16]. In addition, a phenyl group was introduced
into the side group in order to block the carbazole NH group quenching
site and to add effects resulting from having a bulky chemical structure
[17]. DFT calculations showed the 2 and 4 positions of the carbazole to
have node characteristics, and the 3 position to have lobe character-
istics. Using these three different positions, we synthesized three new
compounds, namely 9,10-bis(9-phenyl-9H-carbazol-2-yl)anthracene (2-
DCPA), 9,10-bis(9-phenyl-9H-carbazol-3-yl)anthracene (3-DCPA), and
9,10-bis(9-phenyl-9H-carbazol-4-yl)anthracene (4-DCPA) as shown in
Scheme 1.
∗
Corresponding author.
E-mail address: jongpark@khu.ac.kr (J. Park).
https://doi.org/10.1016/j.dyepig.2018.04.029
Received 19 March 2018; Received in revised form 15 April 2018; Accepted 16 April 2018
Availableonline18April2018
0143-7208/©2018ElsevierLtd.Allrightsreserved.

S. Kang et al.
DyesandPigments156(2018)369–378
Scheme 1. Synthetic routes to 2-DCPA, 3-DCPA, and 4-DCPA.
2. Experimental
to 70 mL of anhydrous 1,2-diclorobenzene solution in 3-neck round
bottom flask under N2 atmosphere. The mixture was refluxed at 180 °C
for 10 h (3.27 g, Yield 56%) 1H NMR (300 MHz, THF): δ(ppm) 10.45 (s,
1H), 8.04–8.01 (d, 1H), 7.96–7.94 (d, 1H), 7.57 (s, 1H), 7.42–7.32 (m,
2H), 7.27–7.24 (d, 1H), 7.17–7.11 (t, 1H).
Compound (3) 2-bromo-9-phenyl-9H-carbazole 2-bromo-9H-
carbazole (2 g, 8 mmol), copper(Ι) iodide (1.24 g, 6.5 mmol), cesium
carbonate (7.98 g, 24 mmol) were added to 25 mL of anhydrous toluene
solution in 3-neck round bottom flask under N2 atmosphere. After the
mixture was heated at 80 °C, iodobenzene (1.82 mL, 16 mmol), ethy-
lenediamine (0.82 mL, 12.2 mmol) was added to mixture. Then the
mixture was refluxed at 110 °C for 15 h 1H NMR (300 MHz, THF):
δ(ppm) 8.14–8.11 (d, 1H), 8.07–8.04 (d, 1H), 7.67–7.62 (m, 2H),
7.59–7.56 (m, 2H), 7.53–7.47 (m, 2H), 7.39–7.32 (m, 3H), 7.27–7.21 (t,
1H).
2.1. Synthesis
Compound (1) 4′-bromo-2-nitro-biphenyl 4-bromophenylboronic
acid (12.9 g, 64.7 mmol), 1-Iodo-2-nitro-benzene (10.0 g, 49.7 mmol),
Pd(pph3)4 (2.9 g, 2.5 mmol) were added to 120 mL of anhydrous THF
solution in 3-neck round bottom flask under N2 atmosphere. Then,
60 mL of 2M K2CO3 was added to the reaction mixture. The mixture was
heated 80 °C for 1 day under N2 atmosphere. (7.5 g, Yield 60%) 1H NMR
(300 MHz, THF): δ(ppm) 7.93–7.90 (d, 1H), 7.70–7.65 (t, 1H),
7.60–7.53 (m, 3H), 7.49–7.47 (d, 1H), 7.27–7.26 (t, 1H), 7.24–7.23 (t,
1H).
Compound (2) 2-bromo-9H-carbazole 4′-bromo-2-nitro-biphenyl
(5.9 g, 21.3 mmol), triphenylphosphine (13.96 g, 53 mmol) were added
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DyesandPigments156(2018)369–378
Compound (4) 2-boro-9-phenyl-9H-carbazole 2-bromo-9-phenyl-
7.81–7.65 (m, 14H), 7.57–7.48 (m, 6H), 7.46–7.40 (t, 2H), 7.31–7.23
(m, 6H). HRMS (FAB-MS, m/z): calcd. for C50H32N2, 660.26; found,
660.2631 [M]+. Anal. calcd for C50H32N2: C 90.88, H 4.88, N 4.24;
found: C 90.66, H 4.90 N 4.22%.
Compound (8) 2′-bromo-2-nitro-biphenyl 2-bromophenylboronic
acid (10 g, 50 mmol), 1-Iodo-2-nitro-benzene (9 g, 45 mmol), Pd(pph3)4
(2.6 g, 4.5 mmol) were added to 120 mL of anhydrous THF solution in
3-neck round bottom flask under N2 atmosphere. Next, 60 mL of 2M
K2CO3 was added to the reaction mixture and then heated 80 °C for 1
day under N2 atmosphere. After vaporizing the solvent, the crude
product was recrystallized using hexane. (8.0 g, Yield 75%) 1H NMR
(300 MHz, THF): δ(ppm) 8.12–8.09 (d, 1H), 7.76–7.70 (t, 1H),
7.66–7.59 (m, 2H), 7.43–7.37 (t, 2H), 7.30–7.25 (t, 2H).
Compound (9) 4-bromo-9H-carbazole 2′-bromo-2-nitro-biphenyl
(7 g, 26 mmol), triphenylphosphine (16.7 g, 65 mmol) were added to
60 mL of anhydrous 1,2-diclorobenzene solution in 3-neck round
bottom flask under N2 atmosphere. The mixture was refluxed at 180 °C
for 10 h (3.0 g, Yield 50%) 1H NMR (300 MHz, THF): δ(ppm) 10.58 (s,
1H), 8.65–8.62 (d, 1H), 7.45–7.36 (m, 3H), 7.31–7.29 (d, 1H),
7.23–7.15 (m, 2H).
Compound (10) 4-bromo-9-phenyl-9H-carbazole 4-bromo-9H-
carbazole (2 g, 8 mmol), copper(Ι) iodide (1.24 g, 6.5 mmol), cesium
carbonate (7.98 g, 24 mmol) were added to 25 mL of anhydrous toluene
solution in 3-neck round bottom flask under N2 atmosphere. After the
mixture was heated at 80 °C, iodobenzene (1.82 mL, 16 mmol), ethy-
lenediamine (0.82 mL, 12.2 mmol) was added to mixture. Then the
mixture was refluxed at 110 °C for 15 h (1.7 g, Yield 68%) 1H NMR
(300 MHz, THF): δ(ppm) 8.81–8.78 (d, 1H), 7.65–7.59 (t, 2H),
7.56–7.47 (m, 3H), 7.45–7.40 (m, 2H), 7.35–7.27 (q, 3H), 7.25–7.20 (t,
1H).
Compound (11) 4-boro-9-phenyl-9H-carbazole 4-bromo-9-
phenyl-9H-carbazole (1.5 g, 4.7 mmol) was dissolved in 72 mL of an-
hydrous THF solution and cooled to −78 °C. Next, 4.7 mL of 2 M n-
butyllithium (9.4 mmol) was slowly added dropwise. After the mixture
was stirred at −78 °C for 30 min, 2.77 mL of 2-Isopropoxy-4,4,5,5-tet-
ramethyl-1,3,2-dioxa-borolane (14 mmol) was slowly added. The re-
sulting mixture was stirred for 3 h and gradually allowed warmed to
room temperature. After the reaction was completed, the crude product
was extracted using ethyl acetate and D.I water. Water remaining in the
organic layer is removed using anhydrous magnesium sulfate. The
solvent was vaporized and recrystallized using hexane. (1.0 g, Yield
63%) 1H NMR (300 MHz, DMSO): δ(ppm) 1.44(s, 12H), 7.24–7.31(m,
2H), 7.39–7.46(q, 3H), 7.54–7.58(q, 3H), 7.66–7.72(m, 3H),
8.96–8.99(d, 1H).
9H-carbazole (2.27 g, 7 mmol) was dissolved in 100 mL of anhydrous
THF solution and cooled to −78 °C. Then, 7.07 mL of 2.0 M n-bu-
tyllithium (14 mmol) was slowly added dropwise. After the mixture was
stirred at −78 °C for 30 min, 4 mL of 2-Isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxa-borolane (21 mmol) was slowly added. The resulting mix-
ture was stirred for 3 h and gradually allowed to warm to room tem-
perature. After the reaction was finished, the reaction mixture was
extracted with ethyl acetate and water. The organic layer was dried
with anhydrous MgSO4 and filtered. The solvent was vaporized and
recrystallized using hexane. (1.9 g, Yield 70%) 1H NMR (300 MHz,
THF): δ(ppm) 8.17–7.81 (t, 2H), 7.79 (s, 1H), 7.69–7.62 (t, 2H),
7.60–7.56 (t, 3H), 7.52–7.46 (t, 1H), 7.40–7.33 (m, 2H), 7.25–7.20 (t,
1H), 1.31 (s, 12H).
9,10-bis(9-phenyl-9H-carbazol-2-yl)anthracene (2-DCPA) 9,10-
dibromoanthracene (1.0 g, 2.6 mmol), 2-boro-9-phenyl-9H-carbazole
(2.5 g, 6.5 mmol), Pd(PPh3)4 (0.3 g, 0.2 mmol) were added to 25 mL of
anhydrous toluene under N2 atmosphere and stirred. 12 mL of tetra-
ethylammonium hydroxide (20 wt %) was added to the mixture and
stirred for reflux at 110 °C. After 2 h of reaction, the crude solution was
filtered and the solvent of residue was vaporized. (0.4 g, Yield: 40%) 1H
NMR (300 MHz, THF): δ(ppm) 8.42–8.38 (t, 1H), 8.30–8.27 (d, 1H),
7.70–7.60 (m, 4H), 7.57–7.30 (m, 8H), 7.22–7.26 (q, 2H). HRMS (FAB-
MS, m/z): calcd. for C50H32N2, 660.26; found, 660.2578 [M]+. Anal.
calcd for C50H32N2: C 90.88, H 4.88, N 4.24; found: C 90.58, H 4.67 N
4.19%.
Compound (5) 9-phenyl-9H-carbazole 9H-carbazole (10 g,
60 mmol), copper(Ι) iodide (9.1 g, 48 mmol), cesium carbonate (58.4 g,
180 mmol) were added to 90 mL of anhydrous toluene solution in 3-
neck round bottom flask under N2 atmosphere. After the mixture was
heated at 80 °C, iodobenzene (13.3 mL, 120 mmol), ethylenediamine
(6 mL, 18 mmol) was added to mixture. Then the mixture was refluxed
at 110 °C for 15 h (8.8 g, Yield 60%) 1H NMR (300 MHz, DMSO):
δ(ppm) 8.27–8.24 (d, 2H), 7.72–7.66 (t, 2H), 7.64–7.61 (d, 2H),
7.57–7.52 (t, 1H), 7.47–7.41 (t, 2H), 7.39–7.36 (d, 2H), 7.32–7.30 (t,
2H).
Compound (6) 3-bromo-9-phenyl-9H-carbazole 9-phenyl-9H-
carbazole (6 g, 27 mmol), N-Bromosuccinimide (4.3 g, 27 mmol) were
added to 30 mL of chloroform solution in 3-neck round bottom flask
and stirred for 30 min at room temperature. After vaporizing the sol-
vent, the crude product was recrystallized using hexane. (7.4 g, Yield
85%) 1H NMR (300 MHz, CDCl3): δ(ppm) 8.25–8.24 (d, 1H), 8.10–8.07
(d, 1H), 7.64–7.58 (t, 2H), 7.54–7.45 (m, 4H), 7.44–7.37 (m, 2H),
7.32–7.27 (m, 2H).
Compound (7) 3-boro-9-phenyl-9H-carbazole 3-bromo-9-phenyl-
9H-carbazole (5 g, 15 mmol) was dissolved in 75 mL of anhydrous THF
solution and cooled to −78 °C. Next, 10 mL of 2 M n-butyllithium
(16.5 mmol) was slowly added dropwise. After the mixture was stirred
at −78 °C for 30 min, 2.2 mL of 2-Isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxa-borolane (19 mmol) was slowly added. The resulting mix-
ture was stirred for 3 h and gradually allowed warm to room tem-
perature. After the reaction was completed, the crude product was ex-
tracted using ethyl acetate and D.I water. Water remaining in the
organic layer is removed using anhydrous magnesium sulfate. The
solvent was vaporized and recrystallized using hexane. (3.65 g, Yield
68%) 1H NMR (300 MHz, CDCl3): δ(ppm) 8.64 (s, 1H), 8.19–8.16 (d,
1H), 7.87–7.84 (d, 1H), 7.63–7.54 (m, 4H), 7.54–7.49 (t, 1H),
7.40–7.35 (m, 3H), 7.31–7.28 (d, 1H), 1.40 (s, 12H).
9,10-bis(9-phenyl-9H-carbazol-4-yl)anthracene (4-DCPA) 9,10-
dibromoanthracene (0.7 g, 1.8 mmol), 4-boro-9-phenyl-9H-carbazole
(1.8 g, 4.5 mmol), Pd(PPh3)4 (0.21 g, 0.14 mmol) were added to 40 mL
of anhydrous toluene under N2 atmosphere and stirred. 9 mL of tetra-
ethylammonium hydroxide (20 wt%) was added to the mixture and
stirred for reflux at 110 °C. After 2 h of reaction, the crude solution was
filtered and the solvent of residue was vaporized. (0.35 g, Yield: 46%)
1H NMR (300 MHz, DMSO): δ(ppm) 6.30–6.32(d, 1H), 6.72–6.78(t,
1H), 7.27–7.45(m, 5H), 7.60–7.67(m, 4H), 7.73–7.83(m, 5H). HRMS
(FAB-MS, m/z): calcd. for C50H32N2, 660.26; found, 660.2565 [M]+.
Anal. calcd for C50H32N2: C 90.88, H 4.88, N 4.24; found: C 90.71, H
4.82 N 4.09%.
2.2. General procedure of compound purification
9,10-bis(9-phenyl-9H-carbazol-3-yl)anthracene (3-DCPA) 9,10-
dibromoanthracene (1.3 g, 3.9 mmol), 3-boro-9-phenyl-9H-carbazole
(3.0 g, 8.1 mmol), Pd(OAc)2 (0.08 g, 0.4 mmol), tricyclohexylphosphine
(0.2 g, 0.8 mmol) were added to 20 mL of anhydrous toluene solution in
3-neck round bottom flask under N2 atmosphere. After the mixture was
heated at 80 °C, 8 mL of 2 M potassium carbonate was added to mixture.
Then the mixture was refluxed at 110 °C for 2 h (0.65 g, Yield 57%) 1H
NMR (300 MHz, THF): δ(ppm) 8.31–8.29 (d, 2H), 8.18–8.14 (t, 2H),
All intermediate compounds from compound (1) to (11) except
compound (4), (7), and (11) as well as final compounds of 2-DCPA, 3-
DCPA, and 4-DCPA were purified by the following method: After the
reaction was completed, the crude product was extracted by using
methylene chloride (MC) and deionized water. Water remaining in the
organic layer is removed by using anhydrous magnesium sulfate.
Recrystallization was proceeded by using hexane solvent for the
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S. Kang et al.
DyesandPigments156(2018)369–378
compounds of (1), (4), (6), (7), (8), and (11). Compounds of (2), (3),
(5), (9), (10), 2-DCPA, 3-DCPA, and 4-DCPA were purified by silica gel
chromatography having the moving solvent conditions: MC:
hexane = 1: 3 for (2) and (9), MC: hexane = 1: 4 for (3), (5), and (10),
chloroform: cyclohexane = 1: 4 for 2-DCPA and 4-DCPA, chloroform:
hexane = 1: 3 for 3-DCPA.
2.3. Measurements and OLED fabrication
Reagents and solvents were purchased from Sigma Aldrich and Alfa
aesar without further purification. Analytical TLC was performed on
Merck 60 F254 silica gel plate, and column chromatography was per-
formed on Merck 60 silica gel (230–400 mesh). The 1H NMR spectra
were recorded on Bruker Avance 300 spectrometers. The FAB + -mass
and EI + -spectra were recorded on a JMS-600W, JMS-700, 6890 Series
and Flash1112, Flash2000. The optical UV-Vis absorption spectra were
obtained using a Lambda 1050 UV/Vis/NIR spectrometer (Perkin
Elmer). A Perkin-Elmer luminescence spectrometer LS55 (Xenon flash
tube) was used to perform PL spectroscopy. The glass transition tem-
peratures (Tg) of synthesized materials were differential scanning ca-
lorimetry (DSC) under a nitrogen atmosphere using a DSC 4000 (Perkin
Elmer). Td of the compounds were measured with thermal gravimetric
analysis (TGA) using a TGA4000 (Perkin Elmer). The HOMO energy
levels were determined with ultraviolet photoelectron yield spectro-
scopy (Riken Keiki AC-2). The LUMO energy levels were derived from
the HOMO energy levels and the band gaps. For the EL devices, all
organic layers were deposited under 10−6 torr, with a rate of deposition
of 1 Å/s to give an emitting area of 4 mm2. The LiF and aluminum layers
were continuously deposited under the same vacuum conditions. The
current-voltage-luminance (I-V-L) characteristics of the fabricated EL
devices were obtained with a Keithley 2400 electrometer. Light in-
tensities were obtained with a Minolta CS-1000A. The operational
stabilities of the devices were measured under encapsulation in a glo-
vebox.
Fig. 2. Electron distribution density of carbazole.
respectively. As shown in Fig. 2, our DFT calculations produced an
electron density distribution of phenyl carbazole showing the 3-position
of the carbazole to be rich in electrons and the 2- and 4- positions of the
carbazole to be deficient in electron density. When a side group donates
electrons to a core anthracene chromophore, the electroluminescence
(EL) efficiency of the compound is generally increased. Thus, 3-DCPA is
expected to have the highest EL efficiency among the three compounds.
3.2. Optical, thermal and electrical properties
The UV-visible (UV-Vis) absorption spectra and photoluminescence
(PL) spectra of the synthesized compounds are shown in Fig. 3, and
their optical, thermal and electrical properties are summarized in
Table 1. In the solution state, 2-DCPA, 3-DCPA, and 4-DCPA showed
similar UV-Vis absorption spectra, with typical vibronic absorption of
anthracene and with essentially identical UVmax values of 398, 399, and
398 nm, respectively. The PLmax values of solution-state 2-DCPA, 3-
DCPA, and 4-DCPA were observed to be in the deep-blue region, with
values of 439, 445, and 433 nm, respectively. In the film state, 2-DCPA,
3-DCPA, and 4-DCPA also exhibited essentially identical UVmax values,
of 395, 396, and 395 nm, and showed blue light emission PLmax values
of 453, 457, and 452 nm, respectively. The full width at half maximum
(FWHM) values of the PLmax peaks for the three compounds were be-
tween 54 and 59 nm and showed only small differences of 3–4 nm
compared to solution state. Generally, the film state of a compound
with a planar structure causes overlapping of the chromophore due to
the short distance between molecules. It also generally causes various
transitions and large FWHM values. However, the FWHM values of the
peaks of the film states of 2-DCPA, 3-DCPA, and 4-DCPA were all just a
little bit different from those of their solution states. The small mag-
nitude of this difference was attributed to the central anthracene core
and the carbazole side group being highly twisted, which effectively
prevented intermolecular packing and overlapping of the chromophore
in the film state. The highly twisted natures of 2-DCPA, 3-DCPA, and 4-
3. Results and discussion
3.1. Molecular design and synthesis
Three compounds, namely 2-DCPA, 3-DCPA, and 4-DCPA, were
synthesized by carrying out Suzuki aryl-aryl coupling reactions as
shown in Fig. 1 and Scheme 1. 9,10-dibromoanthracene and three
boronyl compounds, specifically N-phenyl-2-carbazole boronate, N-
phenyl-3-carbazole boronate, and N-phenyl-4-carbazole boronate, were
used for the final syntheses. The expected structures of the synthesized
compounds were confirmed by performing nuclear magnetic resonance
(NMR) spectroscopy, elemental analysis (EA), and fast atom bom-
bardment (FAB) mass analysis.
The position of the phenyl carbazole side group linked to the an-
thracene core differed for the three synthesized compounds, being the
3-position lobe site of the carbazole moiety for 3-DCPA and the 2- and
4-position node sites of the carbazole moiety for 2-DCPA and 4-DCPA,
Fig. 1. Chemical structures of the synthesized emitting materials.
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DyesandPigments156(2018)369–378
Fig. 3. UV absorption spectra and PL spectra of the three compounds in toluene solutions (1.0 ⅹ 10−5 M) and thin-film states: (a) 2-DCPA, (b) 3-DCPA, and (c) 4-
DCPA.
DCPA were confirmed by performing Gaussian DFT calculations to
produce optimized structures for these three compounds, as shown in
Fig. 4. The α and β dihedral angles between the anthracene and car-
bazole moieties of these optimized structures were measured to be
between 84.2° and 95.1°, indicative of their highly twisted molecular
structures.
Table 1 summarizes the thermal properties of the synthesized
compounds as determined using thermal gravimetric analysis (TGA)
and differential scanning calorimetry (DSC). The two synthesized
compounds of 3-DCPA and 4-DCPA showed not only high Td values
equal to or greater than 412 °C but also high Tm values of at least
273 °C. Molecules with high Tm and Td values have the advantage that
the morphology of the material does not change easily as a result of the
heat generated during the operation of the OLED device [18].
Theoretical molecular orbital calculations were carried out using
Gaussian09, at the B3LYP/6-31G(d,p) level, to characterize optimized
geometries and electron density distributions of the HOMO-1, HOMO,
LUMO and LUMO+1 states of each molecule (Fig. 5).
These calculations produced HOMO and LUMO electron density
distributions of 2-DCPA, 3-DCPA, and 4-DCPA showing more electrons
located in the anthracene moiety than in the phenyl carbazole moiety,
but HOMO-1 and LUMO+1 electron density distributions showing
many electrons distributed in the phenyl carbazole moiety. When the
electron density distributions of 2-DCPA, 3-DCPA, and 4-DCPA were
considered, they were almost the same as each other. These calculations
suggested the main electron transfer from the excited state to ground
state after excitation to first occur at the anthracene moiety. Our results
indicated anthracene and phenyl carbazole to function as the primary
and secondary chromophores, respectively, in each of the three com-
pounds.
Table 1
Optical, thermal and electrical properties of the three compounds.
Solutiona
Film on glassb
Td
Tm
HOMO
(eV)
LUMOd
(eV)
Band gap
(eV)
UVmax
(nm)
PLmax
(nm)
FWHMc
(nm)
UVmax
(nm)
PLmax
(nm)
FWHMc
(nm)
2-DCPA
3-DCPA
4-DCPA
344,359,377, 398
347, 359, 378, 399
325, 342, 359, 378
439
445
433
55
54
55
339, 355, 376, 395
342, 379, 396
338, 355, 374, 395
453
457
452
59
57
59
433
447
412
–
273
320
−5.62
−5.57
−5.65
−2.62
−2.59
−2.62
3.00
2.98
3.03
a
Toluene solution (1.00 ⅹ 10−5 M).
Film thickness was 50 nm on the glass.
Full width at half maximum.
b
c
d
HOMO (eV) + Band gap (eV) = LUMO (eV).
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S. Kang et al.
DyesandPigments156(2018)369–378
Fig. 4. Optimized structures the synthesized compounds. Dihedral angles were measured from these structures and are noted.
Fig. 5. Electron density distributions of the HOMO-1, HOMO, LUMO and LUMO+1 states determined from calculations at the B3LYP/6-31G(d,p) level.
Table 2
Absorption frequency and oscillator strength calculated using TD-DFT at the CAM-B3LYP/6-311G(d,p) level for the synthesized compounds.
Compounds
2-DCPA
Wavelength (nm)
292
Oscillator strength
0.2950
Transition
Contribution (%)a
HOMO→LUMO+2
HOMO→LUMO+1
HOMO-1→LUMO
HOMO→LUMO
HOMO-3→LUMO+2
HOMO-4→LUMO+1
HOMO-3→LUMO+1
HOMO-1→LUMO
HOMO→LUMO
34.0
36.0
4.0
403
255
0.6160
0.2570
98.0
19.0
11.0
14.0
31.0
98.0
33.0
37.0
98.0
3-DCPA
366
264
0.3530
0.0750
4-DCPA
a
HOMO-2→LUMO+2
HOMO-1→LUMO+1
HOMO→LUMO
367
0.2430
Contribution (%) = (coefficient)2 × 2 × 100.
374

S. Kang et al.
DyesandPigments156(2018)369–378
Time dependent (TD)-DFT calculations were carried out to further
3.3. Electroluminescence properties
understand the changes in the characteristics of the compounds due to
the change in the linkage to the side group. The results of the molecular
absorption, oscillator strength and transition level calculations are
shown in Table 2. The contribution of the electronic transition using the
coefficient values was examined. For each of the 2-DCPA, 3-DCPA, and
4-DCPA compounds, the HOMO →LUMO transition was calculated to
be predominant, with a contribution of 98%, and to have oscillator
strengths of 0.6160, 0.3530, and 0.2430, respectively, higher than
those of the other transitions. These calculations indicated the HOMO→
LUMO transition to be the main electronic transition route and the
anthracene with the electron density as main to be the core component
of the electronic transition in the molecule. The second highest oscil-
lator strengths calculated for these compounds were 0.2950, 0.2570,
and 0.0750, respectively, but corresponded to different transitions for
the different compounds, revealing these compounds to have slightly
different electronic properties. The contributions of the HOMO-1→
LUMO transition of the three compounds were calculated to be 4%,
31.0%, and 0%, respectively, at 292 nm, 255 nm, and 264 nm. That is,
changing the position of the carbazole linked to the anthracene from
node to lobe was calculated to result in an increase in the contribution
of this transition, which might be helpful for increasing the EL effi-
ciency [19].
These HOMO, LUMO and band gap values are summarized in Fig. 6
and Table 1. The HOMO levels of 2-DCPA, 3-DCPA, and 4-DCPA were
found to be very similar, with values of −5.62, −5.57 and −5.65 eV,
respectively. The LUMO levels of the synthesized materials were also
found to be similar, with values of −2.62, −2.59 and −2.62 eV, re-
spectively, and the band gaps were 3.00, 2.98, and 3.03 eV, respec-
tively. 3-DCPA, having the carbazole lobe position linked to the an-
thracene, exhibited the smallest band gap of the three compounds, as its
PL wavelength was red-shifted.
The three new synthesized compounds were each used as an emit-
ting layer (EML) to fabricate, respectively, three non-doped OLED de-
vices, and we measured the EL performances of the devices. Device
configuration was as follows: Indium tin oxide (ITO)/4,4′,4″-tris(N-(2-
naphthyl)-N-phenylamino)triphenylamine (2-TNATA) (60 nm)/N,N′-
bis (naphthalene-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (20 nm)/syn-
thesized emitting material (35 nm)/Tris-(8-hydroxyquinoline)alu-
minum (Alq3) (15 nm)/lithium fluoride (LiF) (1 nm)/Al (200 nm). 2-
TNATA was used as a hole injection layer and NPB was used as a hole
transport and exciton blocking layer. Alq3 was used as an electron
transport layer. The EL performances of the devices are summarized in
Fig. 7 and Table 3.
As shown in Fig. 7 (a), I-V-L curves characteristic of normal diodes
were observed. Fig. 7 (c) and (d) show luminance efficiency (LE) and
external quantum efficiency (EQE) values of the devices measured at
various current density levels in the range of 0–50 mA/cm2. For each of
the three devices, neither efficiency measure decreased much as the
current density was increased. 2-DCPA, 3-DCPA, and 4-DCPA yielded
ELmax values of 453, 457, and 455 nm and corresponding FWHM values
of 58, 55, and 59 nm, respectively. The EML in each OLED device ef-
fectively prevented close packing of the molecules, with such preven-
tion of packing also suggested from our PL results. 2-DCPA, 3-DCPA,
and 4-DCPA yielded CIE y values of 0.12–0.15, i.e., real blue color.
These compounds may therefore be applied to mobile phone applica-
tions. For the 2-DCPA and 4-DCPA devices, respectively, the LE values
were determined to be 2.61 and 1.96 cd/A, the power efficiency (PE)
values were 1.32 and 1.00 lm/W, and the EQEs were 2.48 and 2.07%.
In the case of the 3-DCPA device, the operating voltage was slightly
increased, but this device showed LE, PE, and EQE values, at 2.91 cd/A,
1.44 lm/W, and 2.65%, respectively, higher than those of the other two
devices. These high efficiency values for the 3-DCPA device were at-
tributed to 3-DCPA having the lobe position of its carbazole moiety
linked to the anthracene core, and the resulting electron donation effect
Fig. 6. Plots of calculated molecular orbitals at the LUMO (red top lines), HOMO (red bottom lines), and HOMO-1 (black lines) energy levels of each of 2-DCPA, 3-
DCPA, and 4-DCPA. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
375

S. Kang et al.
DyesandPigments156(2018)369–378
Fig. 7. EL performances of fabricated devices using the synthesized materials as EMLs: (a) I-V (solid symbols) and L-V (opened symbols) curves, (b) energy levels of
the organic materials, (c) luminance efficiency versus current density, and (d) external quantum efficiency versus current density.
(20 nm)/CBP: 4wt% synthesized materials (35 nm)/TPBi (15 nm)/LiF
(1 nm)/Al (200 nm).
Table 3
EL performances at 10 mA/cm2 of non-doped devices made using synthesized
materials as emitting layers.
The EL performances of the doped devices using 2-DCPA, 3-DCPA,
and 4-DCPA are summarized in Fig. 9 and Table 4. The I-V and L-V
curves of the doped devices also showed typical rectifying curve char-
acteristics, and the luminance efficiency at high current density was in
each case similar to that at low current density, as shown in Fig. 9 (a)
and (c). For the 2-DCPA and 4-DCPA dopant devices, respectively, the
LE values were determined to be 1.23 and 0.87 cd/A, the PE values
were 0.53 and 0.33 lm/W, and EQE values were 2.21 and 1.74%. The
doped device containing 3-DCPA showed an LE of 1.61 cd/A, PE of 0.69
lm/W, and EQE of 1.74%. That is, the doped device using 3-DCPA
showed an overall efficiency better than those of the other doped de-
vices, as was found for the non-doped devices. This result was attrib-
uted to the structure of the molecule with the linkage made to the lobe
position being the most effective for Forster energy transfer from CBP.
Electronic states of 2-DCPA and 4-DCPA are basically same, but lumi-
nance efficiency was different as 1.23 cd/A and 0.87 cd/A, respectively.
It can be explained by the different transition contribution of HOMO to
LUMO+1. As shown in transition case of Table 2, 2-DCPA only had
transition from HOMO to LUMO+1 unlike 4-DCPA, which is related
with carbazole moiety. This difference may cause the increment of EL
efficiency between 2-DCPA and 4-DCPA. For each of the three mate-
rials, the ELmax wavelength nearly coincided with the solution PLmax
wavelength of the dopant material, and a deep-blue color with a wa-
velength of about 440 nm could be realized. FWHM values of 2-DCPA,
3-DCPA, and 4-DCPA were 55, 54, and 55 nm in solution PL spectra, but
when three compounds were used in doped EL device, it was decreased
2–4 nm compared to solution PL. This is not big difference of FWHM as
less than 5 nm between spectra of doped EL device and solution PL. It
Synthesized
compounds
Volta LEb
(V) (cd/A) (lm/W) (%)
PEc
EQEd CIEe
(x, y)
ELmax FWHM
(nm)
(nm)
2-DCPA
3-DCPA
4-DCPA
6.90 2.61
7.05 2.91
6.81 1.96
1.32
1.44
1.00
2.48
2.65
2.07
0.16, 0.14 453
0.16, 0.15 457
0.16, 0.12 455
58
55
59
a
Operating voltage at 10 mA/cm2.
Luminance efficiency.
Power efficiency.
External quantum efficiency.
b
c
d
e
Commission International de l’Eclairage.
of the phenyl carbazole side group. Donation of electrons to a core
chromophore generally increases the EL efficiency of the chromophore
compound. Y. Park et al. also reported the relationship between lobe
position and EL efficiency [19].
In addition, although the PL of the film state well matched the EL
data, doped OLED devices were fabricated because the synthesized
molecules showed deep-blue PLmax wavelengths of 433–445 nm in the
solution state. In order to evaluate the performances of the devices, the
synthesized compounds were used as dopants and 4,4′-bis(N-carba-
zolyl)-1,1′-biphenyl (CBP) was used as the host material. As shown in
Fig. 8, the PL spectrum of the CBP host material and the UV spectra of
the three synthesized dopant compounds overlapped well, suggesting
there would be no considerable barrier to the transfer of energy from
the CBP host material to any of the three dopant compounds. The
structures of the devices were ITO/2-TNATA (60 nm-thick)/NPB
376

S. Kang et al.
DyesandPigments156(2018)369–378
Fig. 8. UV-visible absorption spectra of the synthesized materials (dopant) and PL spectrum of CBP (host).
Fig. 9. EL characteristics of doped devices using the synthesized materials as dopants: (a) I-V (solid symbols) and L-V (opened symbols) curves, (b) energy levels of
the organic materials, (c) luminance efficiency versus current density, and (d) power efficiency versus current density.
377

S. Kang et al.
DyesandPigments156(2018)369–378
Table 4
References
EL performances of the doped devices using the synthesized materials as the
dopants.
[1] Tang CW, Van slyke SA. Organic electroluminescent diodes. Appl Phys Lett
1987;51:913.
[2] Raffaella C, Stefano T, Gianluca G, Hakan U, Antonio F, Michele M. Organic light-
emitting transistors with an efficiency that outperforms the equivalent light-emit-
ting diodes. Nat Mater 2010;9:496–503.
[3] Yang L, Shuming C, Jacky WY, Ping L, Ryan TK, Faisal M, Hoi SK, Ben ZT. Tuning
the electronic nature of aggregation-induced emission luminogens with enhanced
hole-transporting property. Chem Mater 2011;23:2536–44.
[4] Cocchi C, Virgili D, Fattori V, Rochester DL, Williams J. N∧C∧N-Coordinated pla-
tinum(II) complexes as phosphorescent emitters in high-performance organic light-
emitting devices. Adv Funct Mater 2007;17:285–9.
Synthesized
compounds
Volta LEb
(V) (cd/A) (lm/W) (%)
PEc
EQEd CIEe
ELmax FWHM
(x, y)
(nm)
440
443
439
(nm)
2-DCPA
3-DCPA
4-DCPA
8.01 1.23
8.17 1.61
9.22 0.87
0.53
0.69
0.33
2.21
2.74
1.74
0.153,
0.066
0.150,
0.068
0.153,
0.060
52
50
53
[5] Jian-Yong H, Yong-Jin P, Fumiya S, So K, Hiroshi K, Hisahiro S, Junji K.
Bisanthracene-Based donor–acceptor-type light-emitting dopants: highly efficient
deep-blue emission in organic light-emitting devices. Adv Funct Mater
2014;24:2064–71.
[6] Zhang D, Cai M, Bin Z, Zhang Y, Zhang D, Duan L. Highly efficient blue thermally
activated delayed fluorescent OLEDs with record-low driving voltages utilizing high
triplet energy hosts with small singlet-triplet splittings. Chem Sci 2016;7:3355–63.
[7] Komatsu R, Sasabe H, Seino Y, Nakao K, Kido J. Light-blue thermally activated
delayed fluorescent emitters realizing a high external quantum efficiency of 25%
and unprecedented low drive voltages in OLEDs. J Mater Chem C 2016;4:2274–8.
[8] Fleetham T, Ecton J, Li G, Li J. Improved out-coupling efficiency from a green
microcavity OLED with a narrow band emission source. Org Electron
2016;37:141–7.
[9] Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. Highly efficient organic light-
emitting diodes from delayed fluorescence. Nature 2012;492:234–8.
[10] Giridhar T, Han TH, Cho W, Saravanan C, Lee TW, Jin SH. An easy route to red
emitting homoleptic IrIII complex for highly efficient solution-processed phos-
phorescent organic light-emitting diodes. Chem Eur J 2014;20:8260–4.
[11] Kuei CY, Tsai WL, Tong B, Jiao M, Lee WK, Chi Y, et al. Bis-tridentate Ir(III)
complexes with nearly unitary RGB phosphorescence and organic light-emitting
diodes with external quantum efficiency exceeding 31 %. Adv Mater
2016;28:2795–800.
[12] Lee S, Kim KH, Limbach D, Park YS, Kim JJ. Low roll-off and high efficiency orange
organic light emitting diodes with controlled co-doping of green and red phos-
phorescent dopants in an exciplex forming co-host. Adv Funct Mater
2013;23:4105–10.
[13] Lee H, Kim B, Kim S, Kim J, Lee J, Shin H, Lee JH, Park J. Synthesis and electro-
luminescence properties of highly efficient dual core chromophores with side
groups for blue emission. J Mater Chem C 2014;2:4737–47.
[14] Shin H, Jung H, Kim B, Lee J, Moon J, Kim J, Park J. Highly efficient emitters of
ultra-deep-blue light made from chrysene chromophores. J Mater Chem C
2016;4:3833–42.
[15] Lee J, Jung H, Shin H, Kim J, Yokohama D, Nishimura H, Wakamiya A, Park J.
Excimer emission based on the control of molecular structure and intermolecular
interactions. J Mater Chem C 2016;4:2784–92.
[16] Wu CL, Chen CT, Chen CT. Synthesis and characterization of heteroatom-bridged
bis-spirobifluorenes for the application of organic light-emitting diodes. Org Lett
2014;16:2114–7.
a
Operating voltage at 10 mA/cm2.
Luminance efficiency.
Power efficiency.
External quantum efficiency.
Commission International de l’Eclairage.
b
c
d
e
could be happened by the effect of solvent polarity. Smaller x and y
coordinate values indicate better performance for blue colors. In par-
ticular, the y coordinate value must be 0.15 or less for a mobile phone
and 0.08 or less for an HD-TV. All of the synthesized materials showed y
coordinate values of 0.060–0.068, and can hence be used for HD-TV
applications.
4. Conclusions
A new blue light-emitting material was successfully synthesized by
linking the node 2- and 4-positions and the lobe 3-position of a car-
bazole to the anthracene 9- and 10-positions. Non-doped devices made
using the synthesized 2-DCPA, 3-DCPA and 4-DCPA showed emission of
real blue light. In the case of 3-DCPA, i.e., that with the linkage at the
lobe position, the efficiency was 2.91 cd/A and EQE was 2.65%, which
showed that the efficiency can be increased by moving the linkage
position of the side group to the lobe position. The synthesized material
was also used as a dopant to increase color purity of the device; and of
the three synthesized materials, 3-DCPA showed the highest efficiency,
with a value of 1.61 cd/A, and an EQE of 2.74%, and emitted light in
the deep blue region with a CIE (x, y) value of (0.150, 0.068).
Acknowledgements
[17] Kim SK, Yang B, Ma Y, Lee JH, Park J. Exceedingly efficient deep-blue electro-
luminescence from new anthracenes obtained using rational molecular design. J
Mater Chem 2008;18:3376–84.
[18] Park YI, Kim B, Lee C, Hyun A, Jang S, Lee JH, Gal YS, Kim TH, Kim KS, Park J.
Highly efficient new hole injection materials for OLEDs based on dimeric phe-
nothiazine and phenoxazine derivatives. J Phys Chem C 2011;115:4843–50.
[19] Park YI, Kim S, Lee JH, Jung DH, Wu CC, Park J. New blue-violet emitters based on
an indenopyrazine core for OLEDs: effects of the position of m-terphenyl side group
substitution on optical and electroluminescence properties. Org Electron
2010;11:864–71.
This research was supported by National R&D Program through the
National Research Foundation of Korea(NRF) funded by the Ministry of
Science & ICT (No. 2017M3A7B4041699). This research was supported
by Basic Science Research Program through the National Research
Foundation of Korea(NRF) funded by the Ministry of Education (No.
2017R1D1A1A09082138).
378