Highly efficient chrysene emitters based on optimized side groups for deep blue emission

Article abstract of DOI:10.1016/j.dyepig.2018.04.016

Diphenylamine substituted with methyl groups was used as a side group to realize high efficiency chrysene deep blue emitters. Three chrysene derivatives substituted with side groups were successfully synthesized: tetra-o-tolylchrysene-6,12-diamine (o-DPAC

Full text of DOI:10.1016/j.dyepig.2018.04.016

Accepted Manuscript
Highly efficient chrysene emitters based on optimized side groups for deep blue
emission
Seokwoo Kang, Hayoon Lee, Hyocheol Jung, Minjin Jo, Mina Jung, Jongwook Park
PII:
S0143-7208(18)30366-8
10.1016/j.dyepig.2018.04.016
DYPI 6677
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 14 February 2018
Revised Date: 10 April 2018
Accepted Date: 10 April 2018
Please cite this article as: Kang S, Lee H, Jung H, Jo M, Jung M, Park J, Highly efficient chrysene
emitters based on optimized side groups for deep blue emission, Dyes and Pigments (2018), doi:
10.1016/j.dyepig.2018.04.016.
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ACCEPTED MANUSCRIPT
Highly efficient chrysene emitters based on optimized side groups for
deep blue emission
Seokwoo Kanga, Hayoon Leea, Hyocheol Junga, Minjin Job, Mina Jungb, Jongwook Parka*
a
Department of Chemical Engineering, Kyung Hee University, Gyeonggi, 17104, Korea
b
Department of Chemistry/Display Research Center, The Catholic University of Korea
*E-mail: jongpark@khu.ac.kr
Abstract
Diphenylamine substituted with methyl groups was used as a side group to realize high efficiency
chrysene deep blue emitters. Three chrysene derivatives substituted with side groups were successfully
synthesized: tetra-o-tolylchrysene-6,12-diamine (o-DPAC), tetra-m-tolylchrysene-6,12-diamine (m-DPAC),
and tetra-p-tolyl-chrysene-6,12-diamine (p-DPAC). The maximum PL (Photoluminescence) emission
wavelengths of the three materials in solution and in a film were shortest for o-DPAC and longest for p-
DPAC. The highly twisted structure of o-DPAC showed the narrowest FWHM (47 nm) in the deep blue
region with a PLmax of 449 nm in the film state. The three synthesized materials showed excellent thermal
stability with a high Td over 370 °C. EML was applied to a non-doped OLED device considering the band
gap of synthesized materials. Among the synthesized materials, the m-DPAC device achieved excellent EL
performance of CIE x, y (0.14, 0.09), 4.89 cd/A, 3.60 lm/W, and an EQE of 6.18 %.
1. Introduction
Organic light emitting diodes (OLEDs) have been studied in various fields such as displays and
biosensors due to their tunable optical and electronic properties. Organic light-emitting diodes (OLEDs)
for displays were reported by the Tang group in 1987 and have been the subject of much research due
to the applicability of full-color flat panel displays, next generation lighting, and flexible displays [1-6]. In
OLEDs, electrons and holes are injected into the emitting layer, and emission is generated by the exciton
formed inside. Thus, the characteristics of the emitting material are very important for determining the
efficiency, wavelength, and lifetime of the OLED device. Therefore, it is essential to develop red, green,
and blue emitting materials with pure color emission, high efficiency, and good thermal properties to
realize full color OLED displays with high efficiency [7-12].
In particular, the Commission International de L'Eclairage (CIE) coordinates (x, y) = (0.14, 0.08)

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corresponding to deep blue were recently requested by the National Television System Committee (NTSC)
standard. So, it is necessary to develop a stable deep blue emitting material with high efficiency. However,
due to the wide band gap, which is the intrinsic property of blue light emitters, a large energy level
difference exists with adjacent hole / electron transporting layers. Therefore, a high driving voltage is
required for operation. During device operation, a higher voltage causes more joule heating, resulting in
degradation of blue emitting materials, which in turn has a detrimental effect on lifetime and efficiency.
Therefore, there is a continuing need for deep blue lighting materials with pure emission and high
efficiency [13-17]. Although many studies on deep blue emitting materials have been reported, there are
few papers on compounds that provide high efficiency [18-21]. Chrysene, anthracene, and pyrene show
maximum absorbance wavelengths of 325 nm, 379 nm, and 338 nm, respectively. Chrysene especially
absorbs light at shorter wavelength regions than other blue emitters. This implies that chrysene has an
intrinsic short conjugation length, and its wide band gap is suitable for deep blue emission based on
molecular design.
In our previous research, various deep blue emitter studies were carried out by introducing electron-
donating groups and bulky side groups in chrysene. Among the developed compounds, DPA-C-DPA
material (in which a diphenylamine group was introduced) showed an excimer peak in the film state,
which resulted in poor efficiency and undesirable CIE (x, y) coordinates [22]. To solve this problem, we
studied the effect of methyl group substituted diphenylamine groups at the ortho, meta, and para
positions, effectively preventing excimer and deep blue emission. As previously reported, introducing a
bulky group as a side group effectively prevents stacking between molecules, increasing color purity,
efficiency, and device lifetime [16,17]. Therefore, we selected diphenylamine as the basic form of side
groups and introduced methyl groups at the ortho, meta, and para sites to prevent self-quenching and
reduce intermolecular interactions and aggregation through steric hindrance.
Thus, three new chrysene derivatives were synthesized: tetra-o-tolylchrysene-6,12-diamine (o-DPAC), tetra-
m-tolylchrysene-6,12-diamine (m-DPAC), and tetra-p-tolylchrysene-6,12-diamine (p-DPAC). We used these
materials as emitting layers (EMLs) of a non-doped device to determine the EL performance for these
three derivatives.

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2. Experimental
2.1. Synthesis
Scheme 1. Synthetic routes of the synthesized compounds.
Compound (1) Di-o-tolyl-amine. 1-Bromo-2-methylbenzene 1mL (8.39 mmol), o-toluidine 1.15mL,
Pd2(dba)3 0.31g (0.336 mmol), sodium tert-butoxide 2.4g (25.2 mmol), and tri-tert-butylphosphine 0.6mL
(0.503 mmol) were added to 120 mL of anhydrous toluene in a 3-neck round bottom flask under a N2
atmosphere. After the reaction was finished, the mixture was extracted with ethyl acetate and DI water.
The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The crude
product was purified by column chromatography on silica gel using THF: hexane = 1 : 20. (2.0g, Yield 27%)
1H NMR (300 MHz, DMSO): δ(ppm) 8.25-8.22(m, 3H), 7.98-7.90(m, 8H), 7.55-7.41(m, 10H), 7.33-7.24(m,
8H), 7.19-7.11(m, 8H), 6.99(t, 2H).
Tetra-o-tolylchrysene-6,12-diamine
(o-DPAC).
Di-o-tolyl-amine
1.33g
(6.77
mmol),
6,12-
dibromochrysene 1.0g (2.60 mmol), Pd(OAc)2 0.0468g (0.208 mmol), sodium tert-butoxide 1.5g (15.6
mmol), and tri-tert-butylphosphine 0.37mL (0.313 mmol) were added to 100 mL of anhydrous toluene in a
3-neck round bottom flask under a N2 atmosphere. The mixture was refluxed at 110 ℃ for 10 h. After the
reaction was finished, the mixture was extracted with chloroform and DI water. The organic layer was
dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The crude product was purified by
column chromatography on silica gel using toluene: hexane = 1 : 10. (0.3g, Yield 19%) 1H NMR (300 MHz,

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THF): δ(ppm) 8.21-8.18(d, 1H), 8.08(s, 1H), 7.92-7.89(d, 1H), 7.47-7.41(t, 1H), 7.37-7.32(t, 1H), 7.23-7.19(t,
3H), 7.13-7.08(m, 1H), 6.99-6.88(m, 3H), 6.76-6.73(d, 1H), 2.48(s, 6H). 13C-NMR (75 MHz, CDCl3): δ= 148.91,
148.13, 143.63, 134.17, 132.99, 132.21, 132.15, 131.98, 129.58, 127.19, 127.02, 126.73, 126.55, 126.00,
125.74, 125.22, 124.53, 124.28, 123.78, 117.72, 19.14; LRMS(EI, m/z): [M+] calc’d. for C46H38N2, 618.30;
found, 618; Elemental analysis calc’d. (%) for C46H38N2: C 89.28, H 6.19, N 4.53; found: C 87.28, H 6.26, N
4.17%.
Tetra-m-tolylchrysene-6,12-diamine
(m-DPAC).
Di-o-tolyl-amine
1.33g
(6.77
mmol),
6,12-
dibromochrysene 1.0g (2.60 mmol), Pd(OAc)2 0.07g (0.312 mmol), sodium tert-butoxide 1.5g (15.6 mmol),
and tri-tert-butylphosphine 0.37mL (0.313 mmol) were added to 100 mL of anhydrous toluene in a 3-neck
round bottom flask under a N2 atmosphere. The mixture was refluxed at 110 ℃ for 10 h. After the
reaction was finished, the mixture was extracted with chloroform and DI water. The organic layer was
dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The crude product was purified by
column chromatography on silica gel using toluene: hexane = 1 : 3. (0.8g, Yield 50%) 1H NMR (300 MHz,
THF): δ(ppm) 8.63-8.60(m, 2H), 8.14-8.11(d, 1H), 7.58-7.52(t, 1H), 7.47-7.42(t, 1H), 7.09-7.04(t, 2H), 6.97(s,
2H), 6.92-6.89(d, 2H), 6.76-6.73(d, 2H), 2.18(s, 6H). 13C-NMR (75 MHz, CDCl3): δ= 148.88, 143.07, 139.43,
132.27, 131.15, 129.35, 128.21, 127.35, 127.19, 125.53, 124.08, 122.99, 119.61, 21.80; LRMS(EI, m/z): [M+]
calc’d for C46H38N2, 618.30; found, 618; Elemental analysis calc’d (%) for C46H38N2: C 89.28, H 6.19, N 4.53;
found: C 88.41, H 6.27, N 4.21%.
Tetra-p-tolylchrysene-6,12-diamine
(p-DPAC).
Di-o-tolyl-amine
1.33g
(6.77
mmol),
6,12-
dibromochrysene 1.0g (2.60 mmol), Pd(OAc)2 0.07g (0.312 mmol), sodium tert-butoxide 1.5g (15.6 mmol)
and tri-tert-butylphosphine 0.37mL (0.313 mmol) were added to 100 mL of anhydrous toluene in a 3-neck
round bottom flask under a N2 atmosphere. The mixture was refluxed at 110 ℃ for 10 h. After the
reaction was finished, the mixture was extracted with chloroform and DI water. The organic layer was
dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The crude product was purified by
column chromatography on silica gel using toluene: hexane = 1 : 5. (0.44g, Yield 25%) 1H NMR (300 MHz,
THF): δ(ppm) 8.58-8.55(m, 2H), 8.11-8.08(d, 1H), 7.56-7.51(t, 1H), 7.44-7.39(t, 1H), 6.99(s, 8H), 2.25(s, 6H).
13C-NMR (75 MHz, CDCl3): δ= 146.68, 143.21, 132.23, 131.47, 131.01, 130.16, 128.02, 127.21, 127.11,
125.51, 124.04, 122.50, 122.29, 20.97; LRMS(EI, m/z): [M+] calc’d for C46H38N2, 618.30; found, 618; Elemental
analysis calc’d (%) for C46H38N2: C 89.28, H 6.19, N 4.53; found: C 88.49, H 5.96, N 4.07%.
2.2. Materials, Measurements, and OLED Fabrication
Reactant di-p-tolylamine and 3,3'-dimethyldiphenylamine were purchased from Sigma-Aldrich and Alfa -
Aesar. Reagents and solvents were purchased as reagent grade and were used without further purification.

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Analytical TLC was carried out on a Merck 60 F254 silica gel plate, and column chromatography was
performed on Merck 60 silica gel (230-400 mesh). The 1H-NMR spectra were recorded on Bruker Advance
300 spectrometers. The FAB+-mass and EI+-spectra were recorded on a JMS-600W, JMS-700, 6890 Series,
and Flash1112 or 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 temperatures (Tg) and melting
temperatures (Tm) of the compounds were obtained on a Differential Scanning Calorimetry (DSC)
Discovery DSC25 (TA instruments) under a nitrogen atmosphere. Compounds were heated to 350 ℃ at
rate of 5 ℃/min and cooled at 5 ℃/min. Degradation temperatures (Td) values of the compounds were
measured with Thermal Gravimetric Analysis (TGA) using a TGA4000 (Perkin Elmer). Samples were heated
to 700 ℃ at a rate of 10 ℃/min. The HOMO energy levels were determined with ultraviolet photoelectron
spectroscopy (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 a deposition area of 4 mm2. The LiF and aluminum layers were continuously
deposited under the same vacuum condition. The current-voltage-luminance (I-V-L) characteristics of the
fabricated EL devices were obtained with a Keithley 2400 electrometer. Light intensities were obtained
with a Minolta CS-1000A. The device was stored in a glovebox for stability against moisture and air.
3. Results and Discussion
Molecular design, optical properties, and thermal properties

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Fig. 1. Chemical structures of the chrysene derivatives.
Fig. 1 and Scheme 1 show the chemical structures and synthetic method used to make the materials.
Chrysene was selected as the core chromophore. Diphenylamine substituted with methyl groups was used
as a side group, and the substitution position of the methyl group was changed to the ortho, meta, and
para positions based on the amine group. To compare the optical and electrical properties with the three
newly synthesized materials, a previously reported diphenylamine-substituted chrysene derivative (DPA-C-
DPA) without the methyl group was selected as the reference material [22]. Synthesis was performed
using
a conventional Suzuki-Miyaura cross-coupling reaction. All compounds were purified by
recrystallization and column chromatography. The synthesized compounds were characterized using
nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy, and elemental analysis.
Fig. 2 and 3 show UV-visible (UV-VIS) absorption and photoluminescence (PL) spectra of the synthesized
compounds in a toluene solution and as a vacuum deposited film, respectively. Table 1 summarizes the
optical properties, thermal properties, and energy levels of these compounds

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Fig. 2. (a) UV-VIS absorption and (b) PL spectra of 1 ⅹ10-5 M toluene solution for the synthesized materials.
Fig. 3. (a) UV-VIS absorption and (b) PL spectra of the vacuum-deposited film for the synthesized materials.
Table 1. Optical, thermal, and electrical properties of the synthesized compounds.
Solutiona
Film on glassb
UVmax PLmax FWHM
Band
gap
HOMOe LUMO
Tg
Tm
Td
UVmax
(nm)
PLmax
(nm)
FWHMc
(nm)
Φfd
(eV)
(eV)
(℃) (℃) (℃)
(eV)
(nm)
402
393
407
413
(nm)
(nm)
DPA-C-
DPA
464,
501
395
390
401
408
447
440
450
459
50
50
50
51
80
32 198 321 361 -5.49 -2.67
2.82
2.92
2.83
2.78
o-
449
454
462
47
67
92
90
-
-
379 -5.47 -2.55
DPAC
m-
55
248 308 370
-5.4
-2.57
DPAC
p-
53
263 314 415 -5.37 -2.59
DPAC

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a Toluene solution (1.00 ⅹ 10-5 M). b Film thickness is 50 nm on the glass. C Full width at half maximum of PL. d Absolute PLQY of 1
wt% dispersed film in PMMA. e Ultraviolet photoelectron spectroscopy (Riken-Keiki, AC-2).
The maximum UV-VIS absorption wavelength of the three synthesized materials was a 5 nm blue shift for
o-DPAC and a 6 nm red for m-DPAC compared to the reference material DPA-C-DPA in the solution state.
In the case of p-DPAC, the maximum absorption wavelength was 408 nm, or a 13 nm red shift. The
maximum PL emission wavelength of the solution state was 447 nm for the reference DPA-C-DPA, and
the maximum PL emission of o-DPAC showed a blue shifted wavelength of 440 nm, which is the shortest
among the three synthesized materials. This is because the methyl group in the ortho position has a
highly twisted structure due to repulsion between the core and the side group, which resulted in a larger
band gap. In m-DPAC, the electron-donating effect and the twisted angle were relatively weak due to the
methyl group at the meta position, and the maximum PL wavelength was 450 nm. In p-DPAC, the methyl
group of the para position does not cause steric hindrance with the core group, and it even provides an
electron-donating effect for the core moiety. So, the twisted structure is reduced, and the maximum PL
wavelength value is shown at 459 nm, which is the longest.
In the solution state, FWHM values showed similar values (50 nm) for all four materials. In the film state
of DPA-C-DPA, an excimer peak was formed at 501 nm in addition to 464 nm, while excimer peaks were
not observed in all three new diphenylamine derivatives substituted with methyl groups. This means that
the introduction of methyl groups effectively prevented intermolecular packing. In the o-DPAC, methyl
substitution at the ortho position effectively prevented packing in the film state and showed a maximum
emission wavelength at 449 nm. In the m-DPAC, the electron-donating effect was weak due to the
introduction of a methyl group at the meta position, and the steric hindrance effect partially acts to
prevent the excimer, resulting in a maximum emission wavelength of 454 nm. In p-DPAC, the
intermolecular packing was effectively prevented due to introduction of the methyl group, and the
maximum emission wavelength of 462 nm was observed due to the electron-donating effect at the para
position.
In the solution state, the DPA-C-DPA material had a FWHM of 50 nm; that in the film state was large at

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80 nm. On the other hand, the three synthesized materials showed an FWHM value of 47-55 nm, which is
relatively narrow in both in the solution and film state. Especially, in the case of o-DPAC compound, film
and solution states showed similar FWHM values of 47 nm and 50 nm. Film state has larger value of
FWHM compared to solution state in general, but o-DPAC did not have this result tendency. It may be
due to the prevention of molecular packing which comes from the large dihedral angle of o-DPAC
compound in film state. The maximum PL wavelength value of the three compounds in the film state
showed a tendency to red shift 3-9 nm relative to that in the solution.
Optimized structures and dihedral angles of the three materials were calculated using Gaussian density
functional theory (DFT) as shown in Fig. 4. The dihedral angle (a-b-c-d, α position) of the core and side of
DPA-C-DPA was 47.72 °. o-DPAC had the largest angle of 65.83 °, and the steric hindrance between the
core and the side group greatly contributed to the degree of twisting. As a result, the maximum PL
emission wavelength was 449 nm, which is the blue-shifted region in the film state. In addition,
intermolecular packing can be effectively prevented in the film state, and the FWHM value remained
narrow [23,24]. Dihedral angles of m-DPAC and p-DPAC were α = 53.84 and 53.08 °, respectively. All of
synthesized materials including methyl group had increased dihedral angle compared to DPA-C-DPA. It
caused to the twisted chemical structure and preventing intermolecular packing.
Fig. 4. Optimized structures and dihedral angles of chrysene derivatives: DPA-C-DPA, o-DPAC, m-DPAC, and p-DPAC
(calculated at the B3LYP/6-31G(d)).
Density functional theory (DFT) calculations were carried out to confirm the property change when
diphenylamine groups substituted with methyl groups were introduced into the chrysene core. The

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electron density distribution of the highest occupied molecular orbital (HOMO) of the four chrysene
derivatives is
Fig. 5. Pictorial presentation of the frontier orbitals and a plot of HOMO and LUMO energy levels for
DPA-C-DPA, o-DPAC, m-DPAC, and p-DPAC (calculated at the B3LYP/6-31G(d)).
widely distributed throughout the chrysene and side groups. On the other hand, the electron density
distribution of the lowest unoccupied molecular orbital (LUMO) is localized in the chrysene core because
of the electron-donating effect of the side group. The HOMO and LUMO were -5.49 to -5.37 eV and -
2.67 to -2.55 eV, respectively. The calculated band gap (ΔEH-L) follows the order of o-DPAC (3.53 eV)> m-
DPAC
(3.37 eV)> p-DPAC (3.32 eV). The measured band gaps were 2.92 eV, 2.83 eV, and 2.78 eV, which
followed the same trend as that of calculated values (Table 1, Fig. 5).
Absolute photoluminescence quantum yields (PLQYs) were measured by fabricating 1 wt% doped films on

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poly(methyl methacrylate) (PMMA). PLQYs values of DPA-C-DPA, o-DPAC, m-DPAC, and p-DPAC were
32 %, 67 %, 92 %, and 90 %, respectively (Table 1). m-DPAC showed a value of 92 %, the highest among
the four compounds. Since the PLQY value of the emitter is an important factor for determining the EL
efficiency of the OLED device, m-DPAC and p-DPAC are expected to exhibit excellent EL efficiency when
used in OLED devices as a blue light emitting material.
The glass transition temperature (Tg), melting temperature (Tm), and decomposition temperature (Td) of
the synthesized materials were measured by DSC and TGA, respectively as shown in Table 1 and Fig. 6. Tg
value of more than 100 °C is required for commercialization of OLED displays. Two compounds of m-
DPAC, and p-DPAC showed Tg values of 248 °C and 263 °C which is higher than 198 °C of DPA-C-DPA.
Two compounds also had Tm values of 308 °C and 314 °C. o-DPAC did not show Tg and Tm values. The Td
values of o-DPAC, m-DPAC, and p-DPAC are 379, 370, and 415 °C, respectively, while the Td value is
361 °C for DPA-C-DPA. It is because the introduction of the methyl group leads to an increase in
molecular weight, thereby improving the thermal stability. This high thermal property can provide stable
device morphology under the heat generated during OLED device operation [25].
Fig. 6. Thermal properties of the chrysene derivatives: (a) TGA and (b) DSC
Electroluminescence properties
EL performance was observed using OLED devices to confirm the side group effect with methyl group
substitution. The synthesized materials were used as the non-doped emitting layer. Devices were

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fabricated with the configuration of ITO / N,N’-bis(naphthalene-1-yl)-N,N’-bis(phenyl)benzidine (NPB), 40
nm / synthesized emitting materials, 30 nm / 1,3,5-tri(1-phenyl-1H-benzo[d]imidazole-2-yl)phenyl (TPBi),
30 nm / LiF, 1 nm / Al, 200 nm. NPB was used as the hole-injection and transporting layer. TPBi was used
as the electron-transporting and hole-blocking layer.
The performances of the OLED devices are shown in Fig. 7 and Fig. 8, and the results are summarized in
Table 2. Fig. 7-(a) shows current density and luminance as a function of the applied voltage. Normal
diode characteristic curves were observed. Fig. 7-(b), (c), and (d) show three efficiencies based on the
current
Table 2. EL performances of synthesized materials.
a
b
c
d
e
Compounds
DPA-C-DPA
o-DPAC
Von (V)
5.47
LE (cd/A)
2.16
PE (lm/W)
1.38
EQE (%)
1.80
CIE (x, y)
ELmax (nm)
454, 548
446
PLmax (nm)
464, 501
449
0.17, 0.15
0.15, 0.08
0.14, 0.09
3.49
2.67
2.46
4.01
m-DPAC
3.84
4.89
3.60
6.18
450
454
p-DPAC
3.42
4.39
4.07
4.09
0.14, 0.14
460
462
a
b
c
d
e
Operating voltage at 10mA/cm2.
Luminance efficiency.
Power efficiency.
External quantum efficiency.
Commission
International de l’Eclairage.
density in the range of 0 to 50 mA/cm2. Three graphs show stable efficiencies at high current density and
low current density. As shown in Table 2, the ELmax values of the three compounds are similar to the PLmax
values. This means that EL light emitted from the device is consistent with the emitter emission. o-DPAC,
m-DPAC, and p-DPAC have luminance efficiency values of 2.67, 4.89, and 4.39 cd/A, respectively, while
that of the reference material DPA-C-DPA is 2.16 cd/A. All three materials had efficiencies that were 23-
126 % higher than that of DPA-C-DPA without methyl groups. m-DPAC showed the best luminance
efficiency among the four materials. This result is due to the electron-donating effect of the methyl group.
The CIE (x, y) value shows that the three synthesized materials had reduced y-coordinate values compared
to that of DPA-C-DPA and thus had excellent blue luminescence properties. This is consistent with the

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tendency of the ELmax values. EQE values can be determined based on the luminance efficiency and CIE
value.
The EQE value of the DPA-C-DPA was 1.8 %, but those of the three synthesized materials were 4.01-
6.18 %, which is an increase of 122-243%. Especially, m-DPAC exhibited EQE value of 6.18 % which is over
5 %. EQE value is determined by four factors which are the light out-coupling factor, the ratio of singlet
or triplet exciton, the photoluminescence quantum yield, and the carrier balance. Based on our previous
study, it may be explained by the increased light out-coupling factor and the increased ratio of singlet
exciton. Light out-coupling factor can be enhanced by horizontal orientation of emitter film during
thermal deposition and the ratio of singlet exciton can be increased by triplet-triplet annihilation during
device operation. Further studies are underway [26,27]. The operating voltage at a current density of 10
mA/cm2 was also found to be 3.42-3.84 V lower than that of the reference material, which resulted in
better power efficiency than for DPA-C-DPA.
Fig. 8 shows (a) EL spectra and (b) CIE (x, y) values for the synthesized materials, and (c) band diagram of
materials using OLED device. Smaller x and y coordinate values indicate better performance for blue
colors. In particular, the y coordinate value must be 0.15 or less for a mobile phone and 0.08 or less for a
TV. For the reference material, x and y coordinate values are shown at the highest position, while all the
synthesized materials show better x and y coordinate values. In particular, o-DPAC showed an NTSC y
coordinate value of 0.08, and m-DPAC, which had the highest luminance efficiency, showed 0.09, which is
very close to the NTSC y coordinate value.

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Fig. 7. EL characteristics of devices using o-DPAC, m-DPAC, and p-DPAC as EMLs: (a) l-V-L curve, (b) luminance
efficiency versus current density, (c) power efficiency versus current density, and (d) external quantum efficiency
versus current density.

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Fig. 8. (a) EL spectra of devices and (b) CIE x, y values of the synthesized compounds, and
(c) band diagram of materials using OLED device.
4. Conclusion
o-DPAC, m-DPAC, and p-DPAC were successfully synthesized by replacing the methyl group substituted
in the ortho, meta, and para positions of diphenylamine with a chrysene core. The optical properties of
the synthesized materials show the same tendency to red shift according to the ortho, meta, and para
substitution positions of the methyl group in both solution and film states. o-DPAC showed the highest
dihedral angle (65.83 °) due to the introduction of the methyl group at the ortho position, indicating a
highly twisted structure. Among the synthesized materials, the maximum emission wavelength was
observed for o-DPAC at 449 nm, which is the most blue-shifted region in the film state. The synthesized
materials were applied to a non-doped OLED device as an EML. o-DPAC, which had the widest band gap,
showed deep blue emission of CIE x, y (0.14, 0.08) and an EQE of 4.01 %. m-DPAC exhibited the highest
luminance value of 4.89 cd/A among the four materials and an EQE of 6.18 %, which was about 54 %
higher than those of the other two materials. Optimization of side groups would be very helpful for the
development of high EL efficiency and high color purity deep blue emitting material.

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Acknowledgements
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)
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- Diphenylamine substituted with methyl groups was used as a side group to realize high
efficiency chrysene deep blue emitters.
- The synthesized emitters showed excellent thermal stability with a high Td over 370 °C.
- m-DPAC compound among the synthesized materials achieved excellent blue EL performance of
CIE x, y (0.14, 0.09), 4.89 cd/A, and an EQE of 6.18 %.