Synthesis and electroluminescence property of green fluorescent dopant including anthracene and diphenylamine moiety

Article abstract of DOI:10.1166/jnn.2019.16691

We have synthesized green fluorescent emitters of N⁹,N⁹,N¹⁰,N¹⁰-tetraphenylanthracene-9, 10-diamine (TAD) and N⁹,N⁹,N¹⁰,N¹⁰-tetra-o-tolylanthracene-9, 10-diamine (oMe-

Full text of DOI:10.1166/jnn.2019.16691

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Nanoscience and Nanotechnology
Vol. 19, 4799–4802, 2019
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Synthesis and Electroluminescence Property of Green
Fluorescent Dopant Including Anthracene and
Diphenylamine Moiety
Hyocheol Jung, Jaemin Ryu, Seokwoo Kang, Hayoon Lee, and Jongwook Park∗
Department of Chemical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do, 17104, Republic of Korea
We have synthesized green fluorescent emitters of N9,N9,N10,N10-tetraphenylanthracene-
9,10-diamine (TAD) and N9,N9,N10,N10-tetra-o-tolylanthracene-9,10-diamine (oMe-TAD) including
anthracene and diphenylamine moiety and evaluated properties of compounds. The methyl groups
were introduced to the diphenylamine to prevent molecular aggregation of the dopant and reduce
self-quenching through steric hindrance of molecular structure. In solution state, photoluminescence
(PL) maximum wavelength is 508 nm for TAD and 519 nm for oMe-TAD. In film state, PL maximum
wavelength is 518 nm for TAD and 527 nm for oMe-TAD. In electroluminescence (EL) spectra, EL
maximum wavelength of TAD is 508 nm and EL maximum wavelength of oMe-TAD is 522 nm. The
doped device using TAD as green fluorescent dopant exhibited CE of 17.7 cd/A, PE of 7.74 lm/W,
and EQE of 6.11%. The doped device using oMe-TAD as dopant exhibited CE of 19.8 cd/A, PE of
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7.23 lm/W, and EQE of 5.97%.
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Keywords: Organic Light-Emitting Diode, Green Fluorescent Dopant, Anthracene,
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Diphenylamine.
1. INTRODUCTION
Recently, many studies have been reported on the intro-
duction of methyl groups into chromophores such as
anthracene and chrysene to form highly twisted molecu-
lar structures.8–9 Highly twisted molecular structure can
effectively prevent intermolecular packing, thus reducing
the concentration quenching effect of emitters. In addition,
substituting functional group at ortho position can help
control not only the absorption and emission wavelength
of material, but also energy levels such as highest occupied
molecular orbital (HOMO) level and lowest unoccupied
molecular orbital (LUMO) level of the material. Because
it can affect the conjugation and electronic effects of the
molecule.8–9
Organic light-emitting diodes (OLEDs), first reported by
Tang and VanSlyke, have received much attention in aca-
demic and industrial fields until recently.1 In particular,
OLEDs have received attention to full-color display and
lighting applications. For application in full-color display
and lightings fields, development of red, green, and blue
emitters with high photoluminescence (PL) and electrolu-
minescence (EL) efficiency is essential.
To design emitters with high PL and EL efficiency,
a core-side concept has been used. Chromophores with
high photoluminescence quantum yield (PLQY) such as
anthracene, pyrene, and chrysene are selected as core
groups. In the case of side groups, bulky side groups
such as m-terphenyl (TP) or triphenylbenzene (TPB) can
effectively prevent intermolecular packing. Also, electron-
donating group such as diphenylamine (DPA) or tripheny-
lamine (TPA) as side groups can improve the EL efficiency
of emitters.2–7
In this study, N 9,N 9,N 10,N 10-tetraphenylanthracene-
9,10-diamine (TAD) and N9,N 9,N 10,N 10-tetra-o-tolyl-
anthracene-9,10-diamine (oMe-TAD) based on anthracene
core and diphenylamine side groups were designed and
synthesized (Scheme 1). In particular, in the molecular
structure of oMe-TAD, the reason for introducing methyl
groups at the ortho position is that it can make a more
highly twisted molecular structure relative to the meta and
para positions.9
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 8
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doi:10.1166/jnn.2019.16691
4799

Synthesis and EL Property of Green Fluorescent Dopant Including Anthracene and Diphenylamine Moiety
Jung et al.
PL spectroscopy. The EL performances of the fabricated
doped devices were obtained by Keithley 2400 electrome-
ter, and light intensity was obtained by Minolta CS-1000A.
3. RESULTS AND DISCUSSION
Figure 1(a) shows normalized UV-Visible absorption and
PL spectra in solution state. TAD and oMe-TAD show UV-
Visible absorption in the 330 nm to 500 nm range. In par-
ticular, the UV-Visible maximum peak (458 nm) of TAD
and the UV-Visible maximum peak (470 nm) of oMe-
TAD are intramolecular charge transfer (ICT) peaks that
occur from the diphenylamine moiety to the anthracene
moiety.2ꢂ10ꢂ11 As can be seen in Figure 1(b), the ICT peaks
were also found in the UV-Visible absorption spectra of
the two materials in the film state.
In solution state, the UV maximum wavelength (UVmaxꢀ
of oMe-TAD was 12 nm red-shifted compared to UVmax of
TAD and 9 nm red-shifted in film state. In oMe-TAD, the
methyl group was substituted for the ortho position of the
diphenylamine group, and the ortho position of the phenyl
ring could affect the conjugation. The methyl groups were
substituted at the ortho position to increase the conjugation
Scheme 1. Synthetic routes of synthesized compounds.
2. EXPERIMENTAL DETAILS
2.1. Synthesis
2.1.1. N9,N9,N10,N10-Tetraphenylanthracene-9,10-
Diamine (TAD)
9,10-dibromoanthracene (1.31 g, 3.91 mmol), dipheny-
lamine (1.68 g, 9.77 mmol), Pd2(dba)3 (0.11 g,
0.12 mmol), and sodium tert-butoxide (2.72 g, 19.5 mmol)
were dissolved in toluene (100 mL) under N2. Then, tri-
tert-butylphosphine (0.15 mL, 0.35 mmol) was added to
ꢀ
the mixture. The mixture was stirred at 110 C for 2 h.
After evaporation of toluene in the crude material, the
crude material was extracted with CHCl3 and water. The
organic layer was dried over anhydrous magnesium sul-
fate (MgSO4ꢀ and filtered. After evaporation of CHCl3,
the crude material was purified by silica gel column chro-
matography with CHCl3: n-hexane (1:3) as the eluent
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to obtain a yellow solid. (0.4 g, yield 21%); 1HNMR
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(300 MHz, CDCl , ꢁ): 8.20–8.17 (q, 4H), 7.37–7.33
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3
(q, 4H), 7.23–7.17 (t, 8H), 7.13–7.10 (d, 8H), 6.93–6.88
(t, 4H).
2.1.2. N9,N9,N10,N10-Tetra-o-Tolylanthracene-9,10-
Diamine (oMe-TAD)
9,10-dibromoanthracene (2.00 g, 5.86 mmol), di-o-tolyl-
amine (2.60 mL, 14.2 mmol), Pd2(dba)3 (0.21 g,
0.34 mmol), and sodium tert-butoxide (2.82 g, 56.9 mmol)
were dissolved in toluene (100 mL) under N2. Then, tri-
tert-butylphosphine (0.20 mL, 1.02 mmol) was added to
ꢀ
the mixture. The mixture was stirred at 110 C for 2 h.
After evaporation of toluene in the crude material, the
crude material was extracted with CHCl3 and water. The
organic layer was dried over anhydrous MgSO4 and fil-
tered. After evaporation of CHCl3, the crude material was
purified by silica gel column chromatography with CHCl3:
n-hexane (1:3) as the eluent to obtain a yellow solid.
(0.50 g, yield 15%); 1H NMR (300 MHz, CDCl3, ꢁꢀ:
8.13–8.10 (q, 4H), 7.22–7.17 (q, 4H), 7.09–7.00 (m, 8H),
6.95–6.90 (t, 8H), 1.85-1.83 (s, 12H).
2.2. Measurements
1H-NMR spectra were recorded on Brucker, Advance
300. The optical absorption spectra were obtained by HP
8453 UV-VIS-NIR spectrometer. Perkin Elmer lumines-
cence spectrometer LS55 (Xenon flash tube) was used for
Figure 1. Normalized UV-visible absorption and PL spectra of synthe-
sized compounds (a) solution state, toluene, 1 × 10−5 M (b) film state,
thickness: 50 nm.
4800
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Jung et al.
Synthesis and EL Property of Green Fluorescent Dopant Including Anthracene and Diphenylamine Moiety
Table II. EL performances of doped OLED devices at 10 mA/cm2.
Table I. Optical properties of synthesized compounds.
Solution statea
Film stateb
Volt
(V)
CEa
PEb
EQEc
(%)
CIEd
(x, y)
ELmax
(nm)
Dopant
TAD
(cd/A) (lm/W)
Compounds UVmax (nm) PLmax (nm) UVmax (nm) PLmax (nm)
7.98
17.71
19.78
7.74
7.23
6.11
5.97
(0.23, 0.60)
(0.29, 0.63)
508
522
TAD
oMe-TAD
458
470
508
518
468
477
518
528
oMe-TAD 9.52
Notes: a: Current efficiency, b: Power efficiency, c: External quantum efficiency,
d: Commission Internationale de l’Eclairage.
Notes: a: 1×10−5 M in toluene, b: Film on glass, thermal evaporated film (thick-
ness: 50 nm).
state, and oMe-TAD was 7 nm red-shifted. oMe-TAD
showed a smaller red-shifted value than TAD because
oMe-TAD effectively prevented intermolecular packing
in film state compared to TAD. This may be because
the methyl groups contained in the side groups of oMe-
TAD made intramolecular steric hindrance, and oMe-
TAD had a more twisted molecular structure relative
to TAD.
To investigate the electroluminescence (EL) proper-
ties of the two materials, an EL devices were fabri-
cated. Also, to minimize intermolecular packing, both
materials were applied as dopants. The device structure
is ITO/2TNATA (60 nm)/NPB (15 nm)/ꢃ,ꢄ-ADN + 4%
dopant (35 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (200 nm).
Figure 2(a) shows the overlap graph between the PL of
host and the absorption spectra of dopants. The PL spectra
of host and the absorption spectra of dopants were well-
matched and it could confirm that Foster energy transfer
length of oMe-TAD, and thus the UVmax values of oMe-
TAD were red-shifted compared to UVmax of TAD.
This phenomenon is also seen in PL spectra. As shown
in Table I, in the solution state, the TAD shows a PL max-
imum wavelength (PLmaxꢀ of 508 nm and the oMe-TAD
shows a PLmax of 518 nm. Meanwhile, in the film state,
TAD represents PLmax of 518 nm and oMe-TAD represents
PLmax of 528 nm. In solution and film state, oMe-TAD
showed 10 nm red-shifted PLmax compared to TAD.
Comparing the results between solution state and film
state, in case of TAD, the UVmax of the film state was
10 nm red-shifted compared to UVmax of the solution
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from host to dopant effectively occur. Figure 2(b) shows
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the current efficiency (CE) of the two materials, and the
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EL performances of the fabricated doped devices are sum-
marized in Table II. TAD and oMe-TAD showed an EL
maximum wavelength (ELmaxꢀ of 508 nm and 522 nm,
respectively, which is similar to the PLmax of the solution
state. The CIE (x, y) of TAD and oMe-TAD are (0.23,
0.60) and (0.29, 0.63), respectively, corresponding to the
green emission. Looking at the CE of the two materi-
als, TAD was 17.71 cd/A and oMe-TAD was 19.78 cd/A.
The relatively high CE of oMe-TAD compared to TAD is
due to the effective prevention of intermolecular packing
due to the twisted molecular structure. Also, the methyl
groups in oMe-TAD, which is weak electron donating
group can improve EL efficiency of materials. There-
fore, oMe-TAD showed relatively higher CE value than
TAD not containing methyl group. And two doped OLED
devices showed similar external quantum efficiency (EQE)
values. In power efficiency (PE) values, oMe-TAD showed
slightly lower PE value than the PE value of TAD. This
is because the voltage of oMe-TAD showed higher value
than the voltage of TAD.
4. CONCLUSION
TAD and oMe-TAD were successfully synthesized
through Buchwald-Hartwig amination. UV-Visible absorp-
tion spectra of TAD and oMe-TAD appeared in the range
of about 458 nm to 477 nm. Also, PL spectra of two
Figure 2. (a) PL spectrum of host and absorption spectra of the synthe-
sized dopants and (b) current efficiencies of EL devices using synthesized
materials as dopant.
J. Nanosci. Nanotechnol. 19, 4799–4802, 2019
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Synthesis and EL Property of Green Fluorescent Dopant Including Anthracene and Diphenylamine Moiety
Jung et al.
materials appeared in the range of 508 nm to 528 nm
corresponding to green color. In doped device, TAD and
oMe-TAD applied as dopant of doped device were CE of
17.71 and 19.78 cd/A, respectively. oMe-TAD containing
methyl groups at ortho position showed higher CE than
CE of TAD.
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Chem. 83, 2640 (2018).
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Acknowledgment: This work was supported by a grant
from Kyung Hee University in 2018 (KHU-20180318).
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References and Notes
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(1987).
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Received: 30 May 2018. Accepted: 31 October 2018.
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J. Nanosci. Nanotechnol. 19, 4799–4802, 2019