New hole transporting materials based on tetraphenylbenzene and aromatic amine derivatives for OLEDs

Article abstract of DOI:10.1080/15421406.2013.849447

Three different new hole transport compounds, namely,2-[1-{4-(N,N- diphenylamino) phenyl}-3,4,5,6-tetraphenylbenzene-2-yl]5-N,N- diphenylaminopyridine(DPAP-TB), 2-[1-{4-(N-phenylnaphthalene-1-amino)phenyl}-3, 4,5,6-tetraphenylbenzene-2-yl]5-(N-phenylnaphthalen-1-amino)pyridine(1-PNAP-TB) and 2-[1{4-(N-phenylnaphthal-en-1-amino)phenyl}-3,4-,5,6-tetraphenylbenzene-2- yl]-5-(N-phenylnaphthalen-2-amino)pyridine(2-PNAP-TB) were synthesized. Optical and electronic properties were examined by UV-Visible absorption spectrum and cyclic voltammetry. Luminance efficiencies and external quantum efficiencies of DPAP-TB, 1-PNAP-TB and 2-PNAP-TB were 3.72, 2.82, 3.30 cd/A, and 1.29, 0.97, 1.12% at 10 mA/cm², respectively. Synthesized materials exhibited high T_g in the range of 118 to 133°C. These values are higher than that of NPB which is commonly used as a hole transporting material.

Full text of DOI:10.1080/15421406.2013.849447

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New Hole Transporting Materials Based
on Tetraphenylbenzene and Aromatic
Amine Derivatives for OLEDs
Seungho Kim a , Sang-Ho Lee b , Byeong-Kwan An a , Se-Young Oh c
Kwang-Yol Kay b & Jongwook Park a
,
a Department of Chemistry , The Catholic University of Korea ,
Bucheon , 420-743 , Korea
b Department of Molecular Science and Technology , Ajou
University , Suwon , 443-749 , Korea
c Department of Chemical and Biomolecular Engineering , Sogang
University , Seoul , 121-742 , Korea
Published online: 16 Dec 2013.
To cite this article: Seungho Kim , Sang-Ho Lee , Byeong-Kwan An , Se-Young Oh , Kwang-Yol
Kay & Jongwook Park (2013) New Hole Transporting Materials Based on Tetraphenylbenzene and
Aromatic Amine Derivatives for OLEDs, Molecular Crystals and Liquid Crystals, 584:1, 69-77, DOI:
10.1080/15421406.2013.849447
To link to this article: http://dx.doi.org/10.1080/15421406.2013.849447
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Mol. Cryst. Liq. Cryst., Vol. 584: pp. 69–77, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.849447
New Hole Transporting Materials Based
on Tetraphenylbenzene and Aromatic Amine
Derivatives for OLEDs
SEUNGHO KIM,1 SANG-HO LEE,2 BYEONG-KWAN AN,1
SE-YOUNG OH,3 KWANG-YOL KAY,2,∗
AND JONGWOOK PARK1,∗
1Department of Chemistry, The Catholic University of Korea, Bucheon 420-743,
Korea
2Department of Molecular Science and Technology, Ajou University, Suwon
443-749, Korea
3Department of Chemical and Biomolecular Engineering, Sogang University,
Seoul 121-742, Korea
Three different new hole transport compounds, namely,2-[1-{4-(N,N-diphenylamino)
phenyl}-3,4,5,6-tetraphenylbenzene-2-yl]5-N,N-diphenylaminopyridine(DPAP-TB),
2-[1-{4-(N-phenylnaphthalene-1-amino)phenyl}-3,4,5,6-tetraphenylbenzene-2-yl]5-
(N-phenylnaphthalen-1-amino)pyridine(1-PNAP-TB) and 2-[1{4-(N-phenylnaphthal-
en-1-amino)phenyl}-3,4-,5,6-tetraphenylbenzene-2-yl]-5-(N-phenylnaphthalen-2-
amino)pyridine(2-PNAP-TB) were synthesized. Optical and electronic properties were
examined by UV-Visible absorption spectrum and cyclic voltammetry. Luminance
efficiencies and external quantum efficiencies of DPAP-TB, 1-PNAP-TB and 2-PNAP-
TB were 3.72, 2.82, 3.30 cd/A, and 1.29, 0.97, 1.12% at 10 mA/cm2, respectively.
Synthesized materials exhibited high Tg in the range of 118 to 133◦C. These values are
higher than that of NPB which is commonly used as a hole transporting material.
Keywords Hole transporting materials; organic light-emitting diode
Introduction
Organic light-emitting diodes (OLEDs) have been the subject of intensive investigation in
the past few years because of their potential application as emissive elements for flat panel
displays [1,2]. Generally, multi-layered devices comprised of charge transport and emitting
layers exhibit higher electroluminescence (EL) efficiency than single-layer devices. This is
because multi-layered devices balance hole and electron carriers, and then the exciton
is formed in the middle of emitting layer [3]. Among multi-layers of OLED, charge
transporting materials have played important roles of balancing hole and electron carriers,
and hole transporting layer (HTL) and electron transporting layer (ETL) are used [4].
∗Address correspondence to Prof. Jongwook Park, Department of Chemistry, The Catholic Uni-
versity of Korea, Bucheon 420-743, Korea, Tel.: (+82)2-2164-4331, Fax: (+82)2-2164-4764. E-mail:
hahapark@catholic.ac.kr and Kwang-Yol Kay, Department of Molecular Science and Technology,
Ajou University, Suwon 10 443-749, Korea, E-mail: kykay@ajou.ac.kr.
[257]/69

70/[258]
S. Kim et al.
For high performance HTL material, it needs to get high mobility as well as high glass
transition temperature (Tg) for long life time. In order to achieve two points, tetraphenyl
moiety and aromatic amine moiety were introduced in new HTL compounds. Tetraphenyl
moiety has twisted multi-phenyl groups and it can increase Tg based on multi-aromatic
groups. Aromatic amine can support high mobility because of electron donating effect.
Three materials, 2-[1-{4-(N, N-diphenylamino)phenyl}-3, 4, 5, 6-tetraphenylbenzene-
2-yl]5-N, N-diphenylaminopyridine(DPAP-TB), 2-[1-{4-(N-phenylnaphthalene-1-amino)
phenyl}-3, 4, 5, 6-tetraphenylbenzene-2-yl]5-(N-phenylnaphthalen-1-amino)pyridine(1-
PNAP-TB) and 2-[1{4-(N-phenylnaphthalen-1-amino)phenyl}-3, 4-,5, 6-tetraphenylbenz-
ene-2-yl]-5-(N-phenylnaphthalen-2-amino)pyridine(2-PNAP-TB) include aromatic amine
groups as an electron donor moiety. In particular, we analyzed dependence of the EL device
characteristics on the position at which the naphthylphenyl amine or diphenylamine group
is substituted. Then we compared EL properties with those of NPB, a commercialized
HTL compound. The electronic and optical properties of these materials were character-
ized by cyclic voltammetry (CV), UV-visible and photoluminescence (PL) spectroscopies.
Moreover, multilayer EL devices were fabricated using these materials as hole transporting
materials [5].
Experimental
Syntheses
{(4-Bromophenyl)ethynyl}trimethylsilane (2). To a mixture of 1-bromo-4-iodobenzene
(1) (3.0 g, 10.06 mmol), dichlorobis(triphenylphosphine)palladium (II) (28 mg, 0.04 mmol),
and copper iodide (18 mg, 0.09 mmol) in triethylamine (30 ml) was added trimethylsily-
lacetylene (1.7 ml, 11.94 mmol), and stirred at 48◦C for 2 hours. Solvent was evaporated
under reduced pressure, and the residue was chromatographed on silica gel with hexane
1
to give 2 (2.42 g, 95%) in a white solid. H NMR (CDCl3, 400 MHz): δ = 0.25 (s, 9H),
7.30(d, 2H), 7.40 (d, 2H). Anal. Calcd for C11H13BrSi: C, 52.17%; H, 5.17%. Found: C,
52.29%; H, 5.08%.
1-Bromo-4-ethynylbenzene (3). To a solution of 2 (2.0 g, 7.90 mmol) in methanol
(100 ml) was added potassium fluoride (3.50 g, 37.19 mmol). The mixture was stirred at
room temperature for 24 hours. Solvent was evaporated under reduced pressure, and the
residue was extracted with dichloromethane (3 × 30 ml). The extracts were combined,
1
and the solvent was evaporated to produce 3 (1.19 g, 83%) in a yellow solid. H NMR
(CDCl3, 400 MHz): δ = 3.33 (s, 1H), 7.35 (d, 2H), 7.45 (d, 2H). Anal. Calcd for C8H5Br:
C, 53.07%; H, 2.78%. Found: C, 53.18%; H, 2.90%.
5-Bromo-2-{(4-bromophenyl)ethynyl}pyridine (4). To a mixture of compound 3
(2.0 g, 11.05 mmol), dichlorobis(triphenylphosphine)palladium (II) (50 mg, 0.08 mmol),
and copper iodide (18 mg, 0.09 mmol) in triethylamine (30 ml) was added 2,5-
dibromopyridine (2.7 g, 11.39 mmol), and stirred at 48◦C for 24 hours. Solvent was
evaporated under reduced pressure, and the residue was chromatographed on silica gel with
dichloromethane/hexane (1:1) to produce 4 (3.12 g, 84%) in a brown solid. Mp. 176◦C. 1H
NMR (CDCl3, 400 MHz): δ = 7.38–7.40 (d, 1H), 7.40–7.42 (m, 2H), 7.42–7.49 (m, 2H),
7.78–7.81 (m, 1H), 8.64 (d, 1H). Anal. Calcd for C13H7NBr2: C, 46.32%; H, 2.09%; N,
4.16%. Found: C, 46.21%; H, 2.13%; N, 4.21%.
2-{1-(4-Bromophenyl)-3, 4, 5, 6-tetraphenylbenzene-2-yl}-5-bromopyridine (5).
The mixture of 4 (800 mg, 2.37 mmol) and tetraphenylcyclopentadienone (1.0 g, 2.60 mmol)

New Hole Transporting Materials
[259]/71
in diphenyl ether (100 ml) was refluxed for 48 hours. Solvent was evaporated under reduced
pressure, and the residue was chromatographed on silica gel with dichloromethane/hexane
to afford 5 (1.16 g, 71%) in a brown solid. Mp. 331◦C. 1H NMR (CDCl3, 400 MHz): δ =
6.70–6.90 (m, 22H), 6.80–7.10 (d, 2H), 7.29–7.30 (m, 2H), 8.20 (d, 1H). Anal. Calcd for
C41H27NBr2: C, 71.01%; H, 3.93%; N, 2.02%. Found: C, 69.94%; H, 4.01%; N, 2.07%.
2-[1-{4-(N, N-diphenylamino)phenyl}-3, 4, 5, 6-tetraphenylbenzene-2-yl]5-N, N-
diphenylaminopyridine (DPAP-TB). To a mixture of 5 (50 mg, 0.07 mmol), palladium
(II) acetate (5 mg, 0.022 mmol), tri(t-butyl)phosphine (0.05 ml), and sodium tert-butoxide
(0.10 g, 1.04 mmol) in p-xylene (10 ml) was added diphenylamine (40 mg, 0.18 mmol).
The mixture was stirred at room temperature for 10 minutes, and then refluxed for 12 hours.
After evaporation of the solvent under reduced pressure, the residue was chromatographed
on silica gel with dichloromethane to give DPAP-TB (15 mg, 24%) in a white solid. Mp.
260◦C. 1H NMR (CDCl3, 400 MHz): δ = 6.62–6.69 (d, 2H), 6.80–6.98 (m, 36H), 7.11–7.19
(m, 8H), 8.00 (d, 1H). Anal. Calcd for C65H47N3: C, 89.73%; H, 5.45%; N, 4.83%. Found:
C, 86.66%; H, 5.49%; N, 4.85%.
2-[1-{4-(N-phenylnaphthalene-1-amino)phenyl}-3, 4, 5, 6-tetraphenylbenzene-2-
yl]5-(N-phenylnaphthalen-1-amino)pyridine (1-PNAP-TB). To a mixture of 5 (100 mg,
0.144 mmol), palladium (II) acetate (5 mg, 0.022 mmol), tri(t-butyl)phosphine (0.05 ml),
and sodium tert-butoxide (30 mg, 0.31 mmol) in p-xylene (20 ml) was added N-phenyl-
1-naphthylamine (100 mg, 0.46 mmol). The mixture was stirred at room temperature for
10 minutes, and then refluxed for 12 hours. After evaporation of the solvent under reduced
pressure, the residue was chromatographed on silica gel with dichloromethane/hexane
1
(4:1) to give 1-PNAP-TB (25 mg, 18%) in a white solid. Mp. 165◦C. H NMR (CDCl3,
400 MHz): δ = 6.20–6.40 (d, 2H), 6.78–6.92(m, 28H), 7.02–7.19 (m, 10H), 7.30–7.40 (m,
4H), 7.60–7.70 (m, 4H), 7.78–7.82 (m, 2H), 7.98 (d, 1H). Anal. Calcd for C73H51N3: C,
90.37%; H, 5.30%; N, 4.33%. Found: C, 90.24%; H, 5.37%; N, 4.39%.
2-[1{4-(N-phenylnaphthalen-1-amino)phenyl}-3, 4-,5, 6-tetraphenylbenzene-2-
yl]-5-(N-phenylnaphthalen-2-amino)pyridine (2-PNAP-TB). To a mixture of 5 (1.00 g,
1.44 mmol), palladium (II) acetate (50 mg, 0.22 mmol), tri(t-butyl)phosphine (0.2 ml),
and sodium tert-butoxide (0.3 g, 3.12 mmol) in p-xylene (20 ml) was added N-phenyl-2-
naphthylamine (631 mg, 2.88 mmol). The mixture was stirred at room temperature for 10
minutes, and then refluxed for 12 hours. After evaporation of the solvent under reduced
pressure, the residue was chromatographed on silica gel with dichloromethane/hexane
(4:1) to give 2-PNAP-TB (343 mg, 24%) in a white solid. Mp. 167◦C. 1H NMR (CDCl3,
400 MHz): δ = 6.70–6.76 (d, 2H), 6.78–7.00 (m, 28H), 7.02–7.22 (m, 10H), 7.24–7.34 (m,
4H), 7.42–7.49 (m, 2H), 7.59–7.68 (m, 4H), 8.09 (d, 1H). Anal. Calcd for C73H51N3: C,
90.37%; H, 5.30%; N, 4.33%. Found: C, 90.29%; H, 5.41%; N, 4.30%.
Measurements
The optical absorption spectra were obtained by HP 8453 UV-VIS-NIR spectrometer.
Perkin Elmer luminescence spectrometer LS50 (Xenon flash tube) was used for photo-
luminescence (PL) spectroscopy. Glass-transition temperature (Tg) and degradation tem-
perature (Td) of the compounds were measured by differential scanning calorimetry (DSC)
under nitrogen atmosphere using a DSC2910 (TA instrument) and thermogravimetric anal-
ysis (TGA) using SDP-TGA2960 (TA instrument). The redox potential of the synthesized
materials was measured at the scanning rate of 50 mV/s by WBCS 3000 system with
cyclic voltammeter (CV), and acetonitrile (AN) containing 0.1 M tetrabutylammonium
perchlorate was used as an electrolyte when measuring. Indium tin oxide (ITO) was used

72/[260]
S. Kim et al.
Scheme 1. Chemical structures and synthetic method of DPAP-TB, 1-PNAP-TB, and 2-PNAP-TB.
as a working electrode and saturated Ag/AgNO3 as a reference electrode. Ferrocene was
a reference compound for potential calibration. The current–voltage-luminance (I-V-L)
characteristics of the fabricated EL devices were obtained by Keithley 2400 electrometer
and light intensity was obtained by Minolta CS-1000 A, respectively.
Preparation of OLED Device
EL devices of ITO/2-TNATA (60 nm)/synthesized materials or NPB (15 nm)/Alq3
(70 nm)/LiF (1 nm) /Al (200 nm) were fabricated using 2-TNATA as a hole injection
layer, NPB or synthesized materials as a hole transporting layer, Alq3 as an electron

New Hole Transporting Materials
[261]/73
Figure 1. UV-Visible absorption spectrum: (a) toluene solution (b) evaporated film.
transporting and emitting layer, LiF as an electron injection layer. The organic layer was
vacuum-deposited using thermal evaporation at a vacuum pressure of 10−6 torr and the rate
of deposition 1 Å/s to give an emitting area of 4 mm2, and the Al layer was continuously
deposited under the same vacuum condition.
Results and Discussion
Scheme 1 shows the chemical structures of the synthesized hole transporting materials.
UV–visible absorption (UV-Vis.) of the synthesized materials are described and sum-
marized in Fig. 1 and Table 1. As shown in Fig. 1 and Table 1, UV–Vis. spectra

74/[262]
S. Kim et al.
Table 1. Optical, electrical, and thermal properties of synthesized materials
UV-Vis.max
(nm)
Compounds Solution Film HOMO (eV) LUMO (eV) Eg (eV) Tg (◦C)a Td (◦C)b
DPAP-TB
1-PNAP-TB
2-PNAP-TB
NPB
309
314
320
339
309
316
322
348
5.27
5.31
5.25
5.34
1.97
2.18
2.16
2.40
3.30
3.13
3.09
2.94
118
133
125
104
394
396
400
378
a,b: measured by DSC and TGA at a heating rate of 10◦C/min.
of 2-[1-{4-(N, N-diphenylamino)phenyl}-3, 4, 5, 6-tetraphenylbenzene-2-yl]5-N, N-
diphenylaminopyridine (DPAP-TB), 2-[1-{4-(N-phenylnaphthalene-1-amino)phenyl}-3,
4, 5, 6-tetraphenylbenzene-2-yl]5-(N-phenylnaphthalen-1-amino)pyridine(1-PNAP-TB),
and 2-[1{4-(N-phenylnaphthalen-1-amino)phenyl}-3, 4-,5, 6-tetraphenylbenzene-2-yl]-5-
(N-phenylnaphthalen-2-amino)pyridine(2-PNAP-TB) in solution showed maximum ab-
sorption wavelengths of 309 nm, 314 nm and 320 nm, respectively. Similarly UV–Vis.
spectra of DPAP-TB, 1-PNAP-TB and 2-PNAP-TB in film showed maximum absorption
wavelengths of 309 nm, 316 nm and 322 nm, respectively. UV–Vis. spectra between so-
lution and film showed a small difference of 2 nm. This indicates that the behavior of
molecules in film state is not very different from that in the solution state and there is not
special packing effect in all three compounds [6]. In both solution state and film state,
UV–Vis. maximum values of 1-PNAP-TB and 2-PNAP-TB have red shift compared to
DPAP-TB. This is because 1-PNAP-TB and 2-PNAP-TB has more conjugation length of
naphthalene moiety than that of DPAP-TB [7]. All three compounds do not show any
Figure 2. Current-Voltage-Luminance (I-V-L) characteristics of the fabricated EL.

New Hole Transporting Materials
[263]/75
Figure 3. Luminance efficiency of devices: ITO/2-TNATA (60 nm)/synthesized materials or
NPB(15 nm)/Alq3 (70 nm)/LiF (1 nm)/Al (200 nm).
Figure 4. External quantum efficiency of devices: ITO/2-TNATA (60 nm)/synthesized materials or
NPB(15 nm)/Alq3 (70 nm)/LiF (1 nm)/Al (200 nm).

76/[264]
S. Kim et al.
Table 2. EL performance of the multilayered devices with the synthesized compounds at
10 mA/cm2
hole transporting materials ELmax (nm) L. E. (cd/A)a E.Q.E (%)b
CIE (x,y)c
DPAP-TB
1-PNAP-TB
2-PNAP-TB
NPB
520
520
526
531
3.72
2.82
3.30
3.95
1.29
0.97
1.12
1.35
0.327, 0.544
0.336. 0.547
0.350, 0.555
0.345, 0.537
b
c
aL. E.: luminance efficiency, E.Q.E: external quantum efficiency, C.I.E.: Commission Interna-
tionale de l’Eclairage.
absorption in the visible light area, and thus are suitable as hole transporting layer (HTL)
or hole injection layer (HIL).
Based on UV-Vis. and cyclic voltammetry (CV) results, highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were calculated
as shown in Table 1 [8]. LUMO values of DPAP-TB, 1-PNAP-TB and 2-PNAP-TB were
−1.97 eV, −2.18 eV and −2.16 eV. All three compounds showed higher LUMO levels
than the LUMO value of NPB which is the commercialized as HTL. Through a higher
LUMO level, electron blocking to hole transporting layer is available when electron is
injected to emitting layer. Based on electron blocking effect, luminescence in HTL layer
can be prevented and EML layer would be only allowed to be emitting. HOMO values of
DPAP-TB, 1-PNAP-TB and 2-PNAP-TB were −5.27 eV, −5.31 eV and −5.25 eV.
Non-doped OLED devices were fabricated by using the synthesized materials
or NPB(15 nm) as an hole transport layer. Device configuration is ITO/2-TNATA
(60 nm)/synthesized materials or NPB(15 nm)/Alq3 (70 nm)/LiF (1 nm)/Al (200 nm).
In Fig. 2, current-voltage-luminance (I-V-L) curve according to the applied voltage was
described. All three compounds showed the typical rectifying characteristics and turn-on
voltage was around 5V. The EL performances of devices were described and summarized
in Fig. 3, Fig. 4, and Table 2. Luminance efficiency and external quantum efficiency of
the devices were 3.72, 2.82, 3.30 cd/A, and 1.29, 0.97, 1.12%, respectively when DPAP-
TB, 1-PNAP-TB and 2-PNAP-TB were used as a HTL in devices. These are comparable
efficiencies compared to 3.95 cd/A and 1.35% of NPB, a commercialized HTL material.
The reason for slightly decreased efficiency seems to be caused by the fact that the energy
gap between the HOMO value of the three compounds and the HOMO value of Alq3
emitting layer was greater than 5.34 eV of NPB. In three compounds, DPAP-TB showed
the highest efficiency value and it can be explained by the high LUMO level effect. Among
three synthesized new compounds, DPAP-TB showed the highest efficiency. It can’t be
explained by clear reason, but it may be due to the improved charge balance in this device
configuration. TGA and DSC were measured to identify thermal properties of synthesized
materials. Glass transition temperature (Tg) and initial thermal degradation temperature
(Td) of the materials are summarized in Table 1. Synthesized materials exhibited high Tg
in the range of 118 to 133◦C. These values are higher than that of NPB which is commonly
used as a hole transporting material [9].
Conclusions
UV–Vis. spectra in film state of the three compounds of DPAP-TB, 1-PNAP-TB and 2-
PNAP-TB showed maximum values of 309 nm, 316 nm, and 322 nm. The HOMO and

New Hole Transporting Materials
[265]/77
LUMO values of three synthesized compounds are -5.27, -1.97 eV (DPAP-TB), −5.31,
−2.18 eV (1-PNAP-TB), and −5.25, −2.16 eV (2-PNAP-TB). Luminance and external
quantum efficiencies of nondoped EL devices of DPAP-TB, 1-PNAP-TB and 2-PNAP-TB
were 3.72, 2.82, 3.30 cd/A, and 1.29, 0.97, 1.12%, respectively. The Tg of the synthesized
materials are more than 10◦C higher than that of NPB.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant
funded by the Korea government (MEST) (No. 2012001846).
This research was supported by Basic Science Research Program through the NRF
funded by the Ministry of Education, Science and Technology (No. 2012-047833).
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