Efficient blue organic light-emitting diodes using N2,N2,N11,N11,5,6,7,8-octaphenyltriphenylene-2,11-diamine derivatives

Article abstract of DOI:10.1002/bio.3070

In this study, we have synthesized phenyl-substituted triphenylene derivatives, using the Diels-Alder reaction and the Buchwald-Hartwig reaction. To investigate electroluminescence properties of these materials, multilayer organic light-emitting diode (OL

Full text of DOI:10.1002/bio.3070

Research article
Received: 26 May 2015,
Revised: 5 November 2015,
Accepted: 5 November 2015
Published online in Wiley Online Library: 22 December 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.3070
Efficient blue organic light-emitting
2 2 11 11
diodes using N ,N ,N ,N ,5,6,7,8-
octaphenyltriphenylene-2,11-diamine
derivatives
Young Seok Kim,a Su Jin Jeong,a Hyun Woo Lee,a Jwajin Kim,a Song Eun Lee,b
Young Kwan Kimb* and Seung Soo Yoona*
Abstract: In this study, we have synthesized phenyl-substituted triphenylene derivatives, using the Diels–Alder reaction and the
Buchwald–Hartwig reaction. To investigate electroluminescence properties of these materials, multilayer organic light-emitting
diode (OLED) devices were fabricated with a structure of indium–tin–oxide (ITO) (180 nm)/4,4′-bis(N-(1-naphthyl)-N-phenylamino)
biphenyl (NPB) (50 nm)/blue-emitting materials (1–3) (30 nm)/bathophenanthroline (Bphen) (35 nm)/lithium quinolate (Liq) (2
nm)/Al (100 nm). A device using N2,N2,N11,N11,5,6,7-heptaphenyltriphenylene-2,11-diamine (2) exhibited efficient blue emission
with luminous, power, and external quantum efficiencies of 0.92 cd/A, 0.67 lm/W, and 1.17% at 20 mA/cm2, respectively. The
Commission International de L’Éclairage coordinates of this device were (x = 0.15, y = 0.09) at 6.0 V. Copyright © 2015 John Wiley
& Sons, Ltd.
Keywords: organic light-emitting diode; blue fluorescence; triphenylene; Diels–Alder reaction; Buchwald–Hartwig reaction 1
formationof excimers and exciplexes, which leads to concentration
Introduction
quenching(14–17).Moreover,byintroducingdiphenylaminemoie-
Organic light-emitting devices (OLEDs) have attracted a lot of atten-
ties, the enhancement of device performance due to effective
tion as a next-generation display technology due to their potential
electron-donatingandthushole-transportingabilitieswasexpected
applications in full-color displays, flat-panel displays, and solid-state
(18). Therefore, OLED devices usingthese materials as blue emitters
lightning. However, OLEDs using blue emitter still suffer from low
wouldbeexpectedtohaveimprovedELpropertiesandcolorpurity.
levels of efficiency and a short lifetime as compared with those using
green and red emitters. This situation is because blue-emitting mate-
rials possess a large optical band gap, which results in inferior device
performances due to limited charge carrier injection. Consequently,
Experimental
color purity and device lifetime are seriously reduced (1–4).
Synthesis and characterization
Many researchers have reported on studies on blue light-emitting
materials that have aimed to achieve highly efficient and stable blue
emissions (5,6). For examples, various blue-emitting materials based
on pyrene (7), anthracene (8), fluorine (9), fluoranthene (10) or
triphenylene (11–13) derivatives have been reported. Among these
examples, triphenylene derivatives are good candidates as blue emit-
ters for OLEDs due to their suitable wide band gap (Eg = 3.3 eV). Also,
they are highly efficient, and have good color purity, thermal
stability and electrochemical properties due to their rigid
structure (14). However, only a few systematic studies have inves-
tigated the electroluminescence (EL) properties of blue-emitting
materials based on triphenylene derivatives (15).
In this study, we have designed and synthesized three blue-
emitting materials 1–3 based on phenyl-substituted triphenylene
derivatives using the Diels–Alder reaction and the Buchwald–
Hartwig reaction; 1,2,4-triphenyltriphenylene (1), N2,N2,N11,
N11,5,6,8-heptaphenyltriphenylene-2,11-diamine (2), and N2,N2,
N11,N11,5,6,7,8-octaphenyltriphenylene-2,11-diamine (3). In these
blue emitters, non-planar structure due to twisted-phenyl groups
reduces self-aggregation through steric hindrance and disrupts the
All reactions were performed under nitrogen. Solvents were dried
carefully and distilled from appropriate drying agents prior to use.
* Correspondence to: S. S. Yoon, Department of Chemistry, Sungkyunkwan
University, Suwon, 440–746, Korea. E-mail: ssyoon@skku.edu
* Y. K. Kim, Department of Information Display, Hongik University, Seoul, 121–791,
Korea. E-mail: kimyk@hongik.ac.kr
a
Department of Chemistry, Sungkyunkwan University, Suwon440-746, Korea
b
Department of Information Display, Hongik University, Seoul121-791, Korea
Abbreviations: DFT, density functional theory; EA, Elemental analyses; EQE,
external quantum efficiency; ETL, electron-transporting layer; HBL, hole-
blocking layer; HOMO, highest occupied molecular orbital; HTL, hole-
transporting layer; ITO, indium–tin–oxide; LE, luminous efficiency; LUMO,
lowest unoccupied molecular orbital; NMR, nuclear magnetic resonance;
OLED, organic light-emitting diode; PL, photoluminescence.; APCI, Atmo-
spheric pressure chemical ionization; PE, power efficiency; EML, emitting
layer; NRF, National Research Foundation of Korea.
Luminescence 2016; 31: 1031–1036
Copyright © 2015 John Wiley & Sons, Ltd.

Y. S. Kim et al.
Commercially available reagents were used without further purifi-
cation unless otherwise stated. H nuclear magnetic resonance
Physical measurements
1
The UV–vis absorption and photoluminescence (PL) spectra of
the newly designed materials were measured in dichloromethane
(10–5 M) using Shimadzu UV-1650PC and Aminco-Bowman series
2 luminescence spectrometers. The fluorescence quantum yields
of the emitting materials were determined in dichloromethane
at 293 K against diphenylanthracene (DPA) as a reference
(ФDPA = 0.90). The highest occupied molecular orbital (HOMO)
energy levels were measured with a low-energy photoelectron
spectrometer (Riken-Keiki, AC-2). The energy band gaps were
determined from the intersection of the absorption and PL spectra.
The lowest unoccupied molecular orbital (LUMO) energy levels
were calculated by subtracting the corresponding optical band
gap energies from the HOMO energy values.
(NMR) and 13C NMR were recorded on a Varian Unity Inova
300Nb spectrometer or a Bruker Avance ΙΙΙ 500 MHz NMR
spectrometer. Fourier transform infrared (FT-IR) spectra were
recorded using a Bruker VERTEX70 FT-IR spectrometer. Low- and
high-resolution mass spectra were measured using a Jeol
JMS-AX505WA spectrometer in atmospheric pressure chemical
ionization (APCI) mode. Elemental analyses (EA) were determined
by a Flash 2000 autoanalyzer.
1,2,4-triphenyltriphenylene (1). (a) General procedure for the
Diels-Alder reaction: o-xylene (20 ml) was added to a mixture of
phen-cyclone (2.6 mmol) and 1-ethynylbenzene (2.6 mmol) in a
flask. The mixture was refluxed at 180°C for 24 h. The reaction mix-
ture was filtered by EtOH. The crude solid dissolved in toluene was
filtered, and evaporated under reduced pressure. The crude prod-
uct was the recrystallized from CH2Cl2/EtOH (yield: 95%). 1H-NMR
(300 MHz, CDCl3) [δ ppm]; 8.43 (d, J = 8.4 Hz, 2H), 7.72 (d, J = 8.4
Hz, 1H), 7.66 (s, 1H), 7.54–7.50 (m, 3H), 7.47–7.38 (m, 5H),
7.25–7.17 (m, 6H), 7.12–6.98 (m, 6H); 13C-NMR (125 MHz, CDCl3)
[δ ppm]; 144.4, 142.0, 141.6, 140.1, 138.1, 136.5, 132.4, 132.0,
131.8, 131.6, 131.3, 130.7, 130.1, 130.0, 129.9, 129.7, 129.6, 129.1,
128.4, 127.6, 127.2, 126.7, 126.6, 126.4, 126.1, 125.5, 125.2, 123.2;
FT-IR [ATR]: ν 3053, 3027, 1596, 1573, 1494, 1478, 1441, 1412,
1369, 1182, 1155, 1072, 1027, 1005, 949, 891, 808, 790, 776, 770,
757, 739, 727, 705, 690, 683, 657, 622, 616, 603, 549, 541, 529,
482, 416 cm–1; APCI-MS (m/z): 456 [M+]; Anal. Calcd: C, 94.70; H,
5.30. Found: C, 94.11; H, 5.22.
OLED fabrication and measurement
For fabricating OLEDs, indium–tin–oxide (ITO) thin films coated on
glass substrates were used, which covered 30 squares sheet resis-
tivity, and 180 nm thickness. The ITO-coated glass was cleaned in
an ultrasonic bath by the following sequence: acetone, methyl al-
cohol, distilled water, and stored in isopropyl alcohol for 48 h
and dried by a N2 gas gun. The substrates were treated by O2
plasma under 2.0 × 10–2 torr at 125 W for 2 min. All organic mate-
rials and metals were deposited under high vacuum (5 × 10–7 torr).
The devices were fabricated in the following sequence: ITO
(180 nm)/4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB)
(50 nm)/blue-emitting materials (1–3) (30 nm)/bathophenan-
throline (Bphen) (35 nm)/lithium quinolate (Liq) (2 nm)/Al
(100 nm), NPB as the hole-transporting layer, Bphen as the
electron-transporting layer, and Liq:Al as the composite cathode.
The current density–voltage–luminance (J–V–L) characteristics of
the devices were measured with a source measure unit
(Keithley 238). The electroluminescence (EL) performance of
the devices including luminous efficiency (LE), external quantum
efficiency (EQE), Commission International de L’Éclairage (CIE)
chromaticity coordinate, and EL spectra were analyzed by
Chroma Meter CS-1000A instruments.
N2,N2,N11,N11,5,6,8-heptaphenyltriphenylene-2,11-diamine (2). (b)
General procedure for the Buchwald–Hartwig cross-coupling reac-
tion: 7,10-dibromo-1,2,4-triphenyltriphenylene (1.0 mmol) and the
corresponding diphenylamine (2.2 mmol), Pd2(dba)3 (0.05 mmol),
2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (Sphos) (0.08 mmol),
NaOt-Bu (5.0 mmol), toluene (30 mL) were mixed in a flask. The
mixture was refluxed at 120°C for 21 h. The reaction mixture
was filtered by CH2Cl2. The crude solution was extracted with
CH2Cl2 and washed with water. The organic layer was dried with
anhydrous MgSO4 and filtered with silica gel. The solution was
then evaporated. The crude product was purified recrystallization
from CH2Cl2/Hex. (yield: 65%). 1H-NMR (300MHz, CDCl3) [δ ppm];
7.60–7.47 (m, 7H), 7.42–7.30 (m, 5H), 7.18–7.15 (m, 14H),
7.08–7.01 (m, 14H), 6.75(dd, J = 7.8 Hz, 1H), 6.67 (dd, J = 7.8 Hz,
1H); 13C-NMR (125 MHz, CDCl3) [δ ppm]; 147.2, 146.1, 145.7,
144.6, 142.3, 141.9, 139.4, 137.6, 136.0, 132.6, 132.3, 132.0,
131.7, 131.1, 130.7, 130.4, 130.0, 129.6, 129.4, 129.3, 129.2,
129.0, 128.3, 127.6, 127.1, 126.5, 126.0, 125.5, 124.9, 124.8,
133.4, 133.3, 120.7, 120.3, 115.9, 115.8; FT-IR [ATR]: ν 3033, 1613,
1593, 1492, 1446, 1407, 1357, 1317, 1277, 1177, 1072, 888, 873,
790, 696 cm–1. APCI-MS (m/z): 791 [M+]; Anal. Calcd: C, 91.11; H,
5.35; N, 3.54. Found: C, 90.54; H, 5.31; N, 3.47.
Results and discussion
A series of new blue fluorescent materials was designed and syn-
thesized by Diels–Alder and Buchwald–Hartwig cross-coupling re-
actions in moderate yields, as shown in Scheme 1. Compound 1
was synthesized by the Diels–Alder reaction between the phen-
cyclone and 1-ethynylbenzene with moderate yields. In the case
of compounds 2 and 3, 7,10-dibromo-1,2,4-triphenyltriphenylene
and 7,10-dibromo-1,2,3,4-tetraphenyltriphenylene intermediates
were synthesized by Diels–Alder reaction. Subsequently, the
dibromo-intermediates were reacted with diphenylamine by
Buchwald–Hartwig cross-coupling reactions to achieve 2 and 3.
1H-NMR, 13C-NMR, IR and low- and high-resolution mass
spectrometry were used to characterize these compounds.
Furthermore, in order to check the purity of the materials,
high-performance liquid chromatography was carried out.
Figure 1(a) shows the UV–vis absorption and PL spectra of the
blue emitters 1–3 in dichloromethane solution. Figure 1(b) exhibits
the PL spectra of the blue fluorescent materials 1–3 in solid thin
films formed on quartz plates. The photophysical properties of
them are summarized in Table 1. In the UV–vis absorption spectra,
the maximum absorption wavelengths of compounds 1–3
N2,N2,N11,N11,5,6,7,8-octaphenyltriphenylene-2,11-diamine (3). Yield:
1
40%. H-NMR (300MHz, CDCl3) [δ ppm]; 7.85–7.57(d, J = 2.4Hz, 2H),
7.35(s, 1H), 7.32(s, 1H), 7.22–7.20 (m, 2H), 7.18–7.17 (m, 3H),
7.16–7.14 (m, 3H), 7.02–7.00 (m, 14H), 6.99–6.96 (m, 3H), 6.87–6.85
(d, J = 1.8Hz, 3H), 6.86–6.85 (d, J = 1.8Hz, 2H), 6.71–6.70 (d, J = 2.4,
1H), 6.68–6.65 (m, 5H); FT-IR [ATR]: ν 2981, 1593, 1492, 1442, 1379,
1350, 1315, 1283, 1260, 1172, 1071, 1032, 922, 870, 775, 699 cm–1.
APCI-MS (m/z): 867 [M+]; Anal. Calcd: C, 91.42; H, 5.35; N, 3.23. Found:
C, 90.63; H, 5.31; N, 3.11.
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Luminescence 2016; 31: 1031–1036

Efficient triphenylene derivatives for blue OLED
Scheme 1. Structure and synthetic routes of compounds (1–3). (a) Xylene, reflux, 24 h. (b) Diphenylamine, Pd2(dba)3, 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (Sphos),
NaOt-Bu, toluene, reflux, 21 h.
Figure 1. (a) UV–vis absorption and PL spectra in CH2Cl2 (10–5M), and (b) PL spectra in thin film of blue materials (1–3).
Table 1. Photophysical properties of compounds 1–3
Compound
UVmax a [nm]
PLmaxa [nm]
PLmaxb [nm]
Фc [%]
HOMO/ LUMO [eV]
Eg [eV]
Tg/Tdd [°C]
1
2
3
287
349
342
406
455
442
400
451
448
0.06
0.19
0.19
À6.01/À2.44
À5.49/À2.49
À5.41/À2.37
3.57
3.00
3.04
67/210
103/324
117/332
aCH2Cl2 solution (10–5 M). bThin film. cUsing DPA (9,10-diphenylanthracene) as a standard; λex = 360 nm (Ф = 0.90 in CH2Cl2). dTg and Td
were measured by Universal V4.5A TA instrument at the scan rates of 10°C/min under N2 atmosphere.
appeared at 287, 349, and 342, respectively. Also, the maximum
emission wavelengths of compounds 1–3 showed at 406, 455,
and 442 nm in solution. All compounds have the maximum emis-
sion wavelengths located in the blue region of the visible light in
the PL spectra. In the solution-state PL spectra, the maximum emis-
sion wavelengths of 2 and 3 showed red shifts (49 and 36 nm)
compared with compound 1 due to the extended π-conjugation
lengths by introduction of diphenylamine moiety. Interestingly,
solid-state PL spectra of compounds 1–3 showed insignificant
red shifts than those in solution. Compounds 1–3 have a
non-planar structure that disrupts intermolecular interaction and
reduces self-aggregation through steric hindrance. Thus, this
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Y. S. Kim et al.
Figure 2. Frontier molecular orbitals of compounds 1–3 by using the DFT calculation.
structure would contribute to the insignificant solid-state effect on
PL spectra of compounds 1–3.
The HOMO energy levels of 1–3 measured by a photoelectron
spectrometer (Riken-Keiki AC-2) are À6.01, À5.49, and À5.41 eV,
respectively. Compounds 2 and 3 show higher HOMO energy
levels than that of compound 1, due to the introduction of an
electron-donating diphenylamine moiety (19). The energy band
gaps (Eg) of the blue fluorescent materials 1–3 are 3.57, 3.00, and
3.04 eV, respectively. The LUMO energy levels calculated from their
Eg and the HOMO energy levels are À2.44, À2.49, and À2.37 eV,
respectively.
To understand the observed photophysical properties of the
emitting materials at the molecular level, density functional theory
(DFT) calculations of compounds 1–3 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 (Fig. 2). The
Figure 3. Energy levels diagram of the materials used in devices A–C.
Figure 4. EL spectra of devices A–C.
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Efficient triphenylene derivatives for blue OLED
non-planar structures of 1–3 were attributed to the twisted linkage
between triphenylene backbone and phenyl and diphenylamine
moieties. These non-planar structures would decrease self-
aggregation by steric hindrance, which affected their electronic
and physical properties.
To investigate the EL properties of these materials, the devices
using compounds 1–3 as emitting materials were made with the
following sequence: ITO (180 nm)/4,4′-bis(N-(1-naphthyl)-
N-phenylamino)biphenyl (NPB) (50 nm)/blue-emitting materials
(1–3) (30 nm)/bathophenanthroline (Bphen) (35 nm)/lithium
quinolate (Liq) (2 nm)/Al (100 nm). NPB was used as a hole-
transporting layer (HTL), Bphen as an electron-transporting layer
(ETL) as well as a hole-blocking layer (HBL), and Liq:Al as a compos-
ite cathode. Figure 3 shows the energy level diagrams of devices
using compounds 1–3.
As shown in Fig. 4, all devices Aev exhibited blue emission at the
maximum wavelength of the EL spectra at 454, 449, and 445 nm,
respectively. The CIE coordinates of devices A T were (0.15, 0.10),
(0.15, 0.09), and (0.15, 0.08), respectively at 6.0 V. Interestingly,
devices B and C showed deep-blue emissions with CIE values of
y < 0.10. As shown in Fig. 5, we made the device B, six times,
repeatedly, and measured their CIE coordinates at different
applied voltages. All B devices showed little change in CIE
coordinates even at the different applied voltages. Therefore, it is
reasonable that these OLED devices exhibited stable blue
emissions with excellent reliability and reproducibility.
Current density–voltage–luminance (J–V–L) characteristics of
the devices A c were illustrated in Fig. 6. Table 2 summarizes
J–V–L characteristics and EL efficiencies of devices Ach.
The LEs, PEs, and EQEs of devices Aof as a function of the current
efficiency are shown in Fig. 7. The devices A–C exhibited the max-
imum LEs of 0.80, 0.97, and 1.07 cd/A, maximum PEs of 0.50, 0.83,
and 0.63 lm/W, and maximum EQEs of 1.06, 1.23, and 1.24%, re-
spectively. Compared with device A, devices B and C showed
higher EL efficiencies because the HOMO energy levels of the
emitters were closer to that of NPB, which was used as the HTL.
Thus, devices B and C had a lower hole-injection energy barrier
from HTL to the emitting layer than that of device A. Additionally,
the HOMO energy differences between Bphen and the emitters
(2 and 3) in devices B and C were larger than that with emitter 1
in device A. The resulting effective hole-blocking properties of
devices B and C would lead to improved EL efficiencies of devices
B and C in comparison with device A. Furthermore, the effective
hole-transporting properties of the emitters 2 and 3 due to their
diphenylamine moieties would contribute partially to better EL ef-
ficiencies for devices B and C than those of device A. Interestingly,
the emitters 2 and 3 had higher fluorescent quantum yields than
emitter 1. This effective intrinsic luminescence ability of emitters
2 and 3 played a partial role in the improvement of EL efficiencies
of devices B and C in comparison with device A.
Figure 5. CIE coordinates of the device B at different applied voltages repeated six
times.
Intriguingly, the roll-off in EL efficiency of device C is much
smaller than that of devices A and B. Consequently, in the high
current-density region, device C showed greatly improved EL effi-
ciencies in comparison with devices A and B. Emitter 3 in device C
has four twisted-phenyl groups around the triphenylene emitting
core, while emitters 1 and 2 in devices A and B had three phenyl
groups. This finding suggests that the tendency for self-
aggregation and the resulting quenching effects in emitters 1
and 2 are stronger than that in emitter 3, due to the increased
non-planar structure of emitter 3 compared with emitters 1 and
2. Presumably, in high current-density region with the highly
populated excitons in emitting layer (EML), the excitons in EML
of devices A and B would interact with each other more so than
that of device C. The resulting self-quenching in the EML of devices
A and B would contribute to reduced EL efficiency, and thus
high-efficiency roll-off of devices A and B in the high current-
density region, compared with device C (20).
Figure 6. Current density–voltage–luminance (J–V–L) curves of the devices A–C.
Table 2. EL properties of devices A–C
Device
λ
,max [nm]
Ja [mA/cm2]
L
a [cd/m2]
LE b/c [cd/A]
PE b/c [lm/W]
EQE b/c [%]
CIEd (x, y)
A
B
C
454
449
445
19.4
151.7
16.4
146.8
688.3
163.4
0.80/0.75
0.97/0.92
1.07/1.01
0.50/0.36
0.83/0.67
0.63/0.48
1.06/1.02
1.23/1.17
1.24/1.10
(0.15, 0.10)
(0.15, 0.09)
(0.15, 0.08)
aCurrent density and luminance at 6.5 V. bMaximum values. cAt 20 mA/cm2. dCommission Internationale d’Énclairage (CIE) at 6.0 V.
Luminescence 2016; 31: 1031–1036
Copyright © 2015 John Wiley & Sons, Ltd.
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Y. S. Kim et al.
Figure 7. (a) Luminous efficiencies and power efficiencies; and (b) external quantum efficiencies as a function of current density for the devices Ate.
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Conclusion
In summary, we have synthesized three blue emitters 1–3 based
on triphenylene derivatives using the Diels–Alder reaction and
the Buchwald–Hartwig reaction. Their EL properties were investi-
gated by construction of multi-layered OLED devices. In particular,
devices using N2,N2,N11,N11,5,6,7-heptaphenyltriphenylene-2,11-
diamine (2) and N2,N2,N11,N11,5,6,7,8-octaphenyltriphenylene-
2,11-diamine (3) as emitting materials exhibited outstanding
device performances due to effective hole-injection and
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Acknowledgements
This research was supported by the Basic Science Research Pro-
gram through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology,
South Korea (NRF-2013R1A1A2A10008105).
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