The role of electron-transporting Benzo[f]quinoline unit as an electron acceptor of new bipolar hosts for green PHOLEDs

Article abstract of DOI:10.1016/j.dyepig.2018.11.024

We prepared three new compounds [3,6-di(9H-carbazol-9-yl)phenanthrene (3,6-DCP), 2,9-di(9H-carbazol-9-yl)benzo[f]quinoline (2,9-DCBQ), and 3,9-di(9H-carbazol-9-yl)benzo[f]-quinoline (3,9-DCBQ)] containing phenanthrene or benzo[f]quinoline as an electron-withdrawing moiety and a carbazole as electron-donating moiety, respectively, as bipolar hosts for green phosphorescent organic light emitting diodes (PHOLEDs). We intentionally substituted nitrogen atom to the C-3 position of phenanthrene moiety to prepare benzo[f]quinolinegroup. And, we found that it allowed better electron transporting behavior than the phenanthrene moiety. Meanwhile, the benzo[f]quinoline/phenanthrene core moieties significantly improved the thermal stability of those host materials, which exhibited glass transition and decomposition temperatures of 132–139 and 395–427 °C, respectively. The green PHOLEDs which were fabricated with those host materials showed the lowest operating voltage of 4.7 V at 1000 cd/m² when we used 3,9-DCBQ. Very interestingly, it has an asymmetric structure with completely separated HOMO and LUMO in space. In contrast, 3,6-DCP having phenanthrene and carbazole moieties showed much higher operating voltage of 6.1 V which imply that replacing nitrogen at the C-3 position of phenanthrene improves carrier transport, that is, electron transporting behavior. As a result, the 3,9-DCBQ-based PHOLED showed the best overall performance, exhibiting current and power efficiencies of 48.5 cd/A and 20.6 lm/W, respectively.

Full text of DOI:10.1016/j.dyepig.2018.11.024

Accepted Manuscript
The role of electron-transporting Benzo[f]quinoline unit as an electron acceptor of new
bipolar hosts for green PHOLEDs
Junseok Seo, So-Ra Park, Mina Kim, Min Chul Suh, Jihoon Lee
PII:
S0143-7208(18)31967-3
https://doi.org/10.1016/j.dyepig.2018.11.024
DYPI 7168
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 5 September 2018
Revised Date: 24 October 2018
Accepted Date: 11 November 2018
Please cite this article as: Seo J, Park S-R, Kim M, Suh MC, Lee J, The role of electron-transporting
Benzo[f]quinoline unit as an electron acceptor of new bipolar hosts for green PHOLEDs, Dyes and
Pigments (2018), doi: https://doi.org/10.1016/j.dyepig.2018.11.024.
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ACCEPTED MANUSCRIPT

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The Role of Electron-Transporting Benzo[f]quinoline Unit
as an Electron Acceptor of New Bipolar Hosts for Green
PHOLEDs
Junseok Seoa,1, So-Ra Parkb,1, Mina Kima, Min Chul Suhb,*, and Jihoon Leea,**
a Department of Polymer Science and Engineering & Department of IT Convergence (BK21 PLUS),
Korea National University of Transportation, 50 Daehak-ro, Chungju, 27469, Republic of Korea
b Department of Information Display, Kyung Hee University, Dongdaemoon-Gu, Seoul 02447,
Republic of Korea
*Corresponding authors. Tel : +82-2-961-0694; Fax: +82-2-968-6924
E-mail: mcsuh@khu.ac.kr (Prof. M. C. Suh)
**Corresponding authors. Tel : +82-43-841-5427; Fax: +82-43-841-5420
E-mail: jihoonli@ut.ac.kr (Prof. J. H. Lee)
1 Both authors have equally contributed to this work.
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ABSTRACT
We prepared three new compounds [3,6-di(9H-carbazol-9-yl)phenanthrene (3,6-DCP), 2,9-
di(9H-carbazol-9-yl)benzo[f]quinoline (2,9-DCBQ), and 3,9-di(9H-carbazol-9-yl)benzo[f]-
quinoline (3,9-DCBQ)] containing phenanthrene or benzo[f]quinoline as an electron-
withdrawing moiety and a carbazole as electron-donating moiety, respectively, as bipolar
hosts for green phosphorescent organic light emitting diodes (PHOLEDs). We intentionally
substituted nitrogen atom to the C-3 position of phenanthrene moiety to prepare
benzo[f]quinolinegroup. And, we found that it allowed better electron transporting behavior
than the phenanthrene moiety. Meanwhile, the benzo[f]quinoline/phenanthrene core moieties
significantly improved the thermal stability of those host materials, which exhibited glass
transition and decomposition temperatures of 132–139 and 395–427 °C, respectively. The
green PHOLEDs which were fabricated with those host materials showed the lowest
operating voltage of 4.7 V at 1000 cd/m2 when we used 3,9-DCBQ. Very interestingly, it has
an asymmetric structure with completely separated HOMO and LUMO in space. In contrast,
3,6-DCP having phenanthrene and carbazole moieties showed much higher operating voltage
of 6.1 V which imply that replacing nitrogen at the C-3 position of phenanthrene improves
carrier transport, that is, electron transporting behavior. As a result, the 3,9-DCBQ-based
PHOLED showed the best overall performance, exhibiting current and power efficiencies of
48.5 cd/A and 20.6 lm/W, respectively.
Keywords : green host materials, benzo[f]quinoline-based, photodehydrogenation, low operating voltage
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1. Introduction
Organic light-emitting diodes (OLEDs) have been in the limelight of both academic and
industrial research due to their highly efficient light emission and realization of outstanding
colors. However, OLEDs suffer from reduced efficiency at high brightness, spurring
extensive investigations aimed at mitigating this problem.[1] In particular, the low
operating voltage and high thermal stability of OLEDs make them well-suited for display
and illumination applications,[2, 3] with most of the corresponding phosphorescent hosts
being of the bipolar type due to featuring an enhanced balance of electron and hole
densities, wider recombination zone, lower operating voltage, and more simple device
architecture.[4, 5] Generally, bipolar hosts comprise both electron donors (allowing hole
transportation) and electron acceptors (allowing electron transportation), with carbazole
being an example of the former class, and pyridine, triazine, imidazole, and cyano-
substituted aromatics representing the latter one.[6-9] Herein, we describe the rational
design of benzo[f]quinoline-based molecules to improve the thermal stability of hosts and
replace previous electron-transporting materials. Among the numerous candidate
substances, phenanthrene-based molecules enjoy the largest popularity, since their U-
shaped structure suppresses close molecular packing and thus prevents crystallization
.
Moreover, the phenanthrene core can be regarded as a biphenyl derivative stitched in two
ortho-positions by a short spacer and is thus expected to result in increased glass transition
temperatures (Tg) and better thermal stability.[10, 11] Finally, the C-3 carbon of
phenanthrene can be replaced by nitrogen, resulting in improved electron transportation.
Fig. 1 shows the energy levels of the highest occupied molecular orbitals (HOMOs) and the
lowest unoccupied molecular orbitals (LUMOs) of phenanthrene and its nitrogen-
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containing analogues calculated by density functional theory (DFT) at the B3LYP level,
revealing that the introduction of nitrogen decreased both HOMO and LUMO energies,
with the exact magnitude of this decrease depending on the location of the N (nitrogen)
atom. In fact, the exact position of the N atom required for improved performance is
difficult to determine. However, the reduction of the LUMO energy without a significant
change of molecular structure is certainly useful for improving the electron transport
properties of hosts. In view of the above, we have synthesized three novel green PHOLED
hosts, namely 3,6-DCP, 2,9-DCBQ, and 3,9-DCBQ, as described in Scheme 1.
2. EXPERIMENTAL
2.1. General information.
All reagents and solvent were purchased from Sigma Aldrich and Alfa Aesar, TCI,
Oakwood chem. All reactions were carried out under a N2 atmosphere. anhydrous toluene
drying used the Molecular sieve (0.4 nm). The 1H and 13C nuclear magnetic resonance
(NMR) was recorded on a Bruker 400MHz (Bruker DPX) NMR spectrometer. Ultraviolet-
visible (UV-vis) spectrophotometer (HP8453) and fluorescence spectrophotometer (Perkin
Elmer LS50B) were used to measure UV-vis spectra and photoluminescence (PL) spectra.
Low-temperature PL measurement of the synthesized materials was carried out at 77 K using
a dilute solution of the materials. Cyclic voltammetry (CV) were measured by a
Autolab/PGSTAT 2 model, which was equipped with a platinum 3 array type as the working
electrode, platinum wire as the counter electrode, and Ag/0.1M AgNO3 as the reference
electrode, at room temperature in a solution of tetrabuthylammonium hexafluorophospate (n-
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Bu4NPF6)(0.1 N) in dimethyl formamide under nitrogen gas protection, at a scan rate of 100
or 50 mV/s. The differential scanning calorimeter (DSC) measurements was performed with
a DSC2010 unit under a nitrogen flow at a heating rate of 10°C min-1. Thermogravimetric
analysis (TGA) was conducted with a Thermogravimetric Analyzer S-1000 instrument under
nitrogen with a heating rate of 10°C min-1 30 to 700°C. High resolution mass spectra were
recorded using a HR-ESI MS SYNAPT G2 (Waters, U.K.) by the Ochang branch of the
Korean Basic Science Institute.
2.2. Synthetic details
2.2.1. Synthesis of (4-bromobenzyl)triphenylphosphonium bromide (1). In a two-necked
round bottom flask a mixture of 1-bromo-4-(bromomethyl)benzene (10 g, 40 mmol),
triphenylphosphine (11.5 g, 44 mmol, 1.1 eq) in acetone (42 ml) was stirred at room
temperature under N2 atmosphere for 12 h. The reaction mixture was then filtered and the
white solids were collected washed with acetone before drying in vacuo (20.3 g, 99 %).
2.2.2. Synthesis of 1,2-bis(4-bromophenyl)ethene (2).
A
mixture of (4-
bromobenzyl)triphenylphosphonium bromide (15 g, 29 mmol) and potassium tert-butoxide
(3.45 g, 31 mmol, 1.05 eq) in THF (156 mL) was stirred in an ice-water bath for several
minutes and then 4-bromobenzaldehyde (5.14 g, 28 mmol, 0.95 eq) was added. The solution
was maintained at 0 ˚C and stirred continuously for a further 12 h. The mixture was then
poured in water (100 mL) and extracted with dichloromethane (3 × 50mL). The organic
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layer was washed with water (2 × 20 mL). The solution was concentrated under reduced
pressure to obtain a viscous liquid, which was purified by column chromatography on silica
1
gel and petroleum ether as eluent to give 1,2-bis(4-bromophenyl)ethene (6.7 g, 71 %). H
NMR (400 MHz, CDCl3) δ (ppm): 7.48 (d, J = 8.5 Hz, 2H), 7.36 (dd, J = 8.4, 3.7 Hz, 4H),
7.14 – 7.00 (m, 3H), 6.54 (s, 1H).
2.2.3. Synthesis of 5-bromo-2-(4-bromostyryl)pyridine (3). Compound 3 was synthesized by
1
the same procedure as described above for (1) using compound. Yield: 77 %; H-NMR (400
MHz, CDCl3) δ (ppm): 8.64 (d, J = 2.3 Hz, 1H), 7.79 (dd, J = 8.3, 2.4 Hz, 1H), 7.56 (d, J =
16.1 Hz, 1H), 7.50 (d, J = 8.5 Hz, 2H), 7.43 (d, J = 8.5 Hz, 2H), 7.28 (s, 1H), 7.09 (d, J = 16.0
Hz, 1H).
2.2.4 Synthesis of 2-bromo-6-(4-bromostyryl)pyridine (4). Compound 4 was synthesized by
1
the same procedure as described above for (1) using compound. Yield: 76 %; H NMR (400
MHz, CDCl3) δ (ppm): 7.43 – 7.37 (m, 2H), 7.32 (d, J = 7.4 Hz, 2H), 7.17 (d, J = 8.4 Hz, 2H),
7.07 (d, J = 6.7 Hz, 1H), 6.77 (d, J = 12.5 Hz, 1H), 6.63 (d, J = 12.4 Hz, 1H).
2.2.5. Synthesis of 3,6-dibromophenanthrene (5). 1,2-bis(4-bromophenyl)ethene (4 g, 11.833
mmol), iodine (3.6 g, 14.2 mmol, 1.2 eq), THF (19.12 mL) and toluene (1 L) was irradiated
using a 320 W HMPV lamp (300 nm) for 48 h/monitored by TLC. After the reaction was over,
the excess of iodine was removed by washing the solution with aqueous Na2S2O3, followed
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by distilled water. The organic layer was concentrated under the reduced pressure. The pure
product was isolated as a yellow solid by column chromatography over silica gel further
1
crystallized from light petroleum ether (3.3 g, 83%). H NMR (400 MHz, CDCl3) δ (ppm):
8.72 (d, J = 1.7 Hz, 2H), 7.76 (d, J = 8.5 Hz, 2H), 7.74 – 7.68 (m, 4H).
2.2.6. Synthesis of 2,9-dibromobenzo[f]quinoline (6). Compound 6 was synthesized by the
1
same procedure as described above for (5) using compound (3). Yield: 88 %; H NMR (400
MHz, CDCl3) δ (ppm): 9.00 (s, 2H), 8.68 (s, 1H), 7.96 (s, 2H), 7.86 – 7.74 (m, 2H).
2.2.7. Synthesis of 3,9-dibromobenzo[f]quinoline (7). Compound 7 was synthesized by the
1
same procedure as described above for (5) using compound (4). Yield: 83 %; H NMR (400
MHz, CDCl3) δ (ppm): 8.74 – 8.69 (m, 2H), 7.96 (d, J = 3.6 Hz, 2H), 7.83 (d, J = 8.5 Hz, 1H),
7.77 (d, J = 8.5 Hz, 1H), 7.72 (d, J = 8.7 Hz, 1H).
2.2.8.
Synthesis
of
3,6-di(9H-carbazol-9-yl)phenanthrene
(3,6-DCP).
3,6-
dibromophenanthrene (1.6 g, 4.7 mmol), carbazole (1.65 g, 9.85 mmol, 2.07 eq), Pd2(dba)3
(0.28 g, 0.3 mmol), sodium tert-butoxide (0.91g, 9.5 mmol) and tri-tert-butylphosphine (50
wt% in toluene) (1.68 ml, 7.1 mmol, 1.5 eq) were charged to a flask. Then toluene (26 mL)
was added to the flask, and the resulting mixture was stirred under N2 at 90 ˚C overnight. The
organic layer was extracted with toluene (2 × 30 mL) and dried over magnesium sulfate. The
solvent was removed under reduced pressure, and the residue was purified by silica gel
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column chromatography using CH2Cl2 / n-hexane (1:3) as the eluent. The resulting compound
was obtained as a white powder (1.82 g, 75.6 %) and further purified by train sublimation in a
81.97 % yield. 1H-NMR (400 MHz, CD2Cl2) δ (ppm) : 8.82 (d, J = 1.9 Hz, 2H), 8.23 (d, J =
8.4 Hz, 2H), 8.15 (dd, J = 7.7, 0.9 Hz, 4H), 8.01 (s, 2H), 7.88 (dd, J = 8.4, 2.0 Hz, 2H), 7.50
(d, J = 8.2 Hz, 4H), 7.39 (ddd, J = 8.3, 7.2, 1.2 Hz, 4H), 7.31 – 7.24 (m, 4H). 13C-NMR (400
(
)
MHz, CD2Cl2) δ ppm : 141.37 136.71 131.68 131.45 130.95 127.45 126.60 126.52 123.79
121.36 120.69 120.53 110.04. HRMS (ESI) [M+H]+ : m/z calcd for C38H25N2, 509.2018;
found 509.1978.
2.2.9. Synthesis of 2,9-di(9H-carbazol-9-yl)benzo[f]quinoline (2,9-DCBQ). Compound 2,9-
DCBQ was synthesized by the same procedure as described above for 3,6-DCP using
1
(
)
compound (6). Yield: 74.2 %; H-NMR (400 MHz, CD2Cl2) δ ppm : 9.25 (d, J = 2.3 Hz,
1H), 9.10 (d, J = 2.3 Hz, 1H), 8.80 (d, J = 1.9 Hz, 1H), 8.30 (d, J = 8.5 Hz, 1H), 8.27 – 8.19
(m, 2H), 8.19 – 8.12 (m, 4H), 7.96 (dd, J = 8.4, 2.0 Hz, 1H), 7.54 – 7.45 (m, 4H), 7.45 – 7.36
(m, 4H), 7.35 – 7.25 (m, 4H). 13C-NMR (400 MHz, CD2Cl2) δ (ppm): 149.65 147.88 141.29
137.29 132.66 131.18 128.86 127.33 126.78 126.60 126.14 124.05 123.87 121.04 120.86
120.75 120.69 109.97 109.73 HRMS (ESI) [M+H]+ : m/z calcd for C37H24N3, 509.1892;
found 510.1961.
2.2.10. Synthesis of 3,9-di(9H-carbazol-9-yl)benzo[f]quinoline (3,9-DCBQ). This compound
was also synthesized by the same procedure as described above for 3,6-DCP using compound
(7). Yield: 75.3%; 1H-NMR (400 MHz, CD2Cl2) δ (ppm): 9.06 (d, J = 8.7 Hz, 1H), 8.86 (d, J
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= 1.8 Hz, 1H), 8.24 (d, J = 8.5 Hz, 1H), 8.22 – 8.18 (m, 3H), 8.17 – 8.11 (m, 3H), 8.11 – 8.07
(m, 2H), 7.94 (d, J = 8.8 Hz, 1H), 7.89 (dd, J = 8.4, 2.0 Hz, 1H), 7.57 – 7.52 (m, 2H), 7.51 –
7.47 (m, 2H), 7.46 – 7.42 (m, 2H), 7.36 (dd, J = 7.2, 0.9 Hz, 2H), 7.32 (dd, J = 7.0, 0.9 Hz,
13
2H). C-NMR (400 MHz, CD2Cl2) δ (ppm): 151.77 148.56 141.45 139.94 137.25 134.12
131.64 131.26 131.08 126.72 126.60 126.56 124.9 123.90 121.7 121.20 120.79 120.67
120.56 117.79 112.16 110.12 HRMS (ESI) [M+H]+ : m/z calcd for C37H24N3, 509.1892;
found 510.1959.
2.3. Device fabrication and measurement
2.3. 1. Materials
We purchased or synthesized the following materials. Plexcore® OC AQ1200
(AQ1200) was purchased from Sigma-Aldrich as hole injection layer (HIL). N,N′-
bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and tris(4-carbazoyl-9-ylphenyl)
amine (TCTA) were purchased from Lumtec Corp. and used as a hole transport layer (HTL)
as well as exciton blocking layer (EBL), respectively. 3,6-DCP, 2,9-DCBQ and 3,9-DCBQ
were synthesized and utilized as host materials for emitting layer (EML). Bis(2-
phenylpyridine)(acetylacetonato) iridium (III) [Ir(ppy)2(acac)] was purchased from Lumtec
Corp. and used as a phosphorescent green dopant for the EML. 4,7-diphenyl-1,10-
phenanthroline (BPhen) as an electron transport layer (ETL), lithium fluoride (LiF) as the
materials for an electron injection layer (EIL), and aluminum (Al) as a cathode were also
purchased from the commercial suppliers and were used without purification.
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2.3. 2. Decvie fabrication
To fabricate OLED devices, a 150 nm thick patterned indium-tin oxide (ITO) glasses
with an open emission area of 4 mm2 were used. The ITO glasses were cleaned in acetone
and isopropyl alcohol with sonication process and rinsed in deionized water. Then, ITO glass
substrates were treated in UV-ozone system to eliminate all the organic impurity remained
during previous fabrication processes. AQ1200 was spin-coated on ITO glass in ambient
condition and annealed at 120 oC for 15min in the nitrogen atmosphere. Subsequently, all the
organic materials were deposited on AQ1200 by vacuum evaporation technique under a
pressure of ~1x10-6 Torr. The deposition rate of organic layers was about 0.5 Å/s. LiF and Al
were deposited with rates of 0.1 Å/s and 3 Å/s, respectively. The current density-voltage (J-V)
and luminance-voltage (L-V) data of OLEDs were collected by Keithley SMU 2635A and
Minolta CS-100A, respectively. Electroluminescence (EL) spectra and the Commission
Internationale De’Eclairage (CIE) coordinates were obtained using a Minolta CS-2000A
spectroradiometer.
3. Results and discussion
3.1. Synthesis
The synthesis and structures of PHOLED hosts are shown in Scheme 1. Compound 1 was
prepared via the phosphine alkylation,[12, 13] and 2–4 were synthesized via Wittig reactions,
which, however, always produce E/Z isomers in a 1:1 ratio.[14, 15] Intermediates 5, 6, and 7
were prepared via photodehydrogenation using a 320-W high-pressure mercury vapor lamp
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(3000 Å). During the above photoreaction, the E/Z-isomers were interconverted, with the
subsequent cyclization shifting the equilibrium to the desired side.[16-18] Finally, Buchwald
Hartwig coupling[19] afforded the desired hosts, with their overall yields over four steps
1
equaling ~48 %. The identities of all compounds were confirmed by H and 13C nuclear
magnetic resonance spectroscopy and electrospray ionization mass spectrometry.
3.2. Photophysical properties
The UV-vis absorption and PL spectra of the above hosts in toluene are shown in Fig. 2
and summarized in Table 1. Despite the shapes of UV-vis absorption peaks being slightly
different for each host, they corresponded to traditional carbazole absorption wavelengths,
with the related absorption maxima (assigned to n-π* transitions) located at 340 nm (3,6-
DCP and 2,9-DCBQ) and 374 nm (3,9-DCBQ).[20] The corresponding PL maxima were
located at 388, 401, and 396 nm for 3,6-DCP, 2,9-DCBQ, and 3,9-DCBQ, respectively,
being slightly red-shifted by 6-13 nm in the case of thin films. Thus, the intermolecular
interaction was significantly suppressed when carbazole moieties were attached to the rigid
core that hindered close molecular packing. The optical bandgap energies (Egopt) of 3,6-DCP,
2,9-DCBQ, and 3,9-DCBQ were determined from their absorption edges (λcutoff) as 3.29,
3.04, and 3.20 eV, respectively, with the corresponding triplet energies (ET) determined from
phosphorescence spectra at 77 K as 2.56, 2.60, and 2.62 eV, respectively, exceeding that of
Ir(ppy)2(acac) (2.47 eV) and indicating that the Ir(ppy)2(acac)-to-host triplet energy transfer
was completely suppressed.[21]
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3.3. Thermal properties
The thermal properties of the hosts were investigated by TGA and DSC in an atmosphere
of nitrogen, which revealed that all compounds showed relatively high grass-transition
temperature (Tg) and decomposition temperature (Td) (Fig. 3 and Table 1). Temperatures
corresponding to 5% weight loss were determined by TGA as 402 °C (3,6-DCP), 395 °C
’
(2,9-DCBQ), and 427 °C (3,9-DCBQ), with DSC measurements revealing Tg s of 132 °C for
3,6-DCP and 139 °C for 2,9-DCBQ and 3,9-DCBQ. Both phenanthrene and
benzo[f]quinoline exhibit a rigid flat structure and are thus regarded as compounds with
outstanding thermal stability. DFT-based calculations showed that the dihedral angles
between the phenan-threne/benzo[f]quinoline core and carbazole side groups were in the
range of 42.1–53.6° (Fig. S1). This might be related to how Tm was not observed in the 2nd
scan as molecules were not piled on the solid form. The three synthesized hosts exhibited an
’
almost two-fold increased Tg s compared to that of a typical green host material, 4,4′-bis(N-
carbazolyl)-1,1′-biphenyl (CBP) (62 °C).
3.4. Theoretical calculations
The electronic structure of the hosts was further investigated by quantum chemical
calculations using the Materials Studio 7.0 D-Mol3 software package and the density
functional theory (DFT) calculations at the Lee-Yang-Parr correlation (B3LYP) level of
theory, with the above dihedral angles determined as 53.6° (C11-C12-N28-C29 for 3,6-DCP),
51.0° (C11-C12-N42-C43 for 2,9-DCBQ), and 42.1° (C12-C13-N16-C21 for 3,9-DCBQ)
(Fig. 4 and Fig. S1). The twisted geometry of these hosts effectively prevented close
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molecular packing in the solid state and thus suppressed recrystallization and improved film
stability. The HOMO of 3,9-DCBQ was localized on the carbazole ring located further away
from the nitrogen of the benzo[f]quinoline core, whereas the corresponding LUMO was
localized on the core, achieving complete spatial HOMO-LUMO separation and thus
realizing bipolar charge transportation. In contrast to the above example, the HOMOs of 3,6-
DCP and 2,9-DCBQ featured non-zero contributions of the core, with the corresponding
LUMOs being localized on phenanthrene and benzo[f]quinoline, respectively. In other words,
the spatial HOMO-LUMO separation was determined by the asymmetrical or pseudo-
symmetrical molecular structure. In pseudo-symmetrical hosts (3,6-DCP and 2,9-DCBQ),
the electron density of HOMO is broadly distributed from both carbazole units through core
unit, while electron density of LUMO is being localized only on the core moiety. Conversely,
the HOMO of the asymmetrical 3,9-DCBQ was localized only on the carbazole ring located
further away from the N atom of the benzo[f]quinoline core, resulting in complete spatial
separation. The reason why this difference arises is that the conjugation between the core
moiety and the right carbazole moiety in 3,9-DCBQ is more efficient than that of 3,6-DCP
and 2,9-DCBQ, so that the core moiety and the right carbazole moiety behave like one unit
(electron transport unit) . On the other hand, since the carbazole on the left side is bent more
than 50 degrees, it can be considered that it will not share the electron density with the core
unit. This might help to separate the each carrier pathway for efficient charge transportation
and recombination. Notably, the substitution of phenanthrene for benzo[f]quinoline
effectively lowered the HOMO and LUMO energies of host molecules. From this change, the
electron energy barrier between BPhen and EML is reduced, and current injection could be
enhanced.
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3.5. Device characteristics
To verify the design concept of new synthetic host materials, we prepared green
PHOLEDs as follows (see also Fig. 5):
Device A: ITO / AQ1200 (40 nm) / NPB (30 nm) / TCTA (10 nm) / 3,6-DCP :
Ir(ppy)2(acac) (20 nm, 3 wt.%) / BPhen (50 nm) / LiF (1 nm) / Al (100 nm)
Device B: ITO / AQ1200 (40 nm) / NPB (30 nm) / TCTA (10 nm) / 2,9-DCBQ :
Ir(ppy)2(acac) (20 nm, 3 wt.%) / BPhen (50 nm) / LiF (1 nm) / Al (100 nm)
Device C: ITO / AQ1200 (40 nm) / NPB (30 nm) / TCTA (10 nm) / 3,9-DCBQ :
Ir(ppy)2(acac) (20 nm, 3 wt.%) / BPhen (50 nm) / LiF (1 nm) / Al (100 nm)
Fig. 6(a) shows the J-V-L characteristics of the fabricated green PHOLEDs. The
representative results are summarized in Table 2. At a given constant voltage of 5.0 V, the J
values of 0.17, 0.37, and 2.90 mA/cm2 were observed in the fabricated Devices A, B, and C,
respectively. This is the result which was affected by the different electron injection barrier
from ETL to EML as aforementioned as shown in Fig. 5. Turn-on voltages (Von) of 4.0, 3.5,
and 3.0 V were obtained from the Device A, B, and C, respectively. Meanwhile, the
operating voltages (Vop) required to reach 1,000 cd/m2 were 6.1, 6.6, and 4.7 V for the
Devices A, B, and C, respectively. (see also Table 2). The maximum current efficiencies
were 44.0, 35.5, and 48.5 cd/A for Devices A, B, and C, respectively. The maximum power
efficiencies were 13.1, 12.3, and 20.6 lm/W, for Device A, B, and C, respectively. Those data
were also summarized in Table 2. The maximum external quantum efficiency (EQE) values
also increased in the order of Device B (9.8%) < Device A (12.1%) < Device C (14.3%), as
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shown in Fig. 6(b). From those results, we found that Device C (3,9-DCBQ) showed the
greatest current density and better efficiency than the other Devices, which might be
attributed to its improvement of electron affinity by substituting carbon for nitrogen (i.e.,
phenanthrene → benzoquinoline). However, Device B (2,9-DCBQ) showed lower the
current density than Device C (3,9-DCBQ), despite they also have same benzoquinoline
moiety, which might be due to the inefficient electron transporting characteristics originated
from the difference in their molecular structure (i.g. symmetric and asymmetric).[22-24] In
other words, since electron transport units (e.g. phenanthrene and benzoquinoline) in 3,6-
DCP and 2,9-DCBQ have slightly occupied HOMOs which means that some hole carriers
can also be transported through those moieties while they have completely vacant cabazole
moiety in their LUMOs (e.g. electron can only be transported through those moieties). This
might affect that the hole carriers transmitted through central moieties could impede an
electron transport ability of those groups during operation. In contrast, the electron transport
properties of 3,9-DCBQ could be ideal because this molecule has completely separated
HOMO and LUMO.
In addition, the charge trapping behavior of an electron and/or a hole strongly depends on
the energy gap of HOMOs or LUMOs between the host and the dopant molecules. In our
EML system [3,6-DCP, 2,9-DCBQ, and 3,9-DCBQ doped with Ir(ppy)2(acac)] they show
very deep hole trapping level while they show very shallow and negligible electron trapping
level, as shown in Fig. 5. As a result, an exciton recombination zone (RZ) could be shifted to
the HTL/EML interface because of the limited flow of holes as compared to that of electrons.
Meanwhile, recombination of free charge carriers in materials with a low mobility is often
described by the Langevin recombination rate.[25] Generally, if electron and holes – being
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potential recombination partners – wish to recombine, the effective recombination rate is
proportional to the “direct” recombination rate or the possibility for finding each other. In
high mobility semiconductors, the former is dominant. However, in disordered solids, and
particularly disordered organic semiconductors, the low mobility limits the effective
recombination rate. The process of finding each other can be described as diffusion limited,
which is proportional to the charge carrier mobility as follow:[26]
ꢄ(ꢅꢆ + ꢅꢇ)
ꢀꢁꢂꢃ
=
ꢈꢉꢈꢊ
where ꢀꢁꢂꢃ is the Langevin recombination rate, ꢄ is the elementary charge, ꢅꢆ is the
electron mobility, ꢅꢇ is the hole mobility, ꢈꢊ is the relative permittivity (≈ 3), and ꢈꢉ is the
vaccum permittivity (≈ 8.85 x 10-14 F/cm).
For this reason, we can conclude that 3,9-DCBQ has the best molecular structure to realize
the most desirable device performance because it shows the best electron transporting
behavior than the 3,6-DCP and 2,9-DCBQ. Indeed, the Device C showed the highest the
current density while Device A and B showed the similar level of the current density as we
commented previously [see also Fig. 6(a)].
Fig. 6(c) shows the normalized EL spectra of Devices A-C at a brightness of 1000 cd/m2.
All devices exhibited similar EL spectra with a peak wavelength (
λ
max
) at 520 nm.
4. Conclusions
Novel bipolar hosts with benzo[f]quinoline as an electron acceptor moiety were successfully
synthesized in high overall yields. They exhibited moderately high glass transition
temperatures (> 132 °C) and good thermal stability (Td, > 395 °C). Replacement of the C-3
carbon of phenanthrene by nitrogen facilitated electron transporting behavior which enhances
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the charge recombination inside EML. Among the prepared hosts, 3,9-DCBQ with an
asymmetric structure which realizes complete HOMO-LUMO spatial separation showed best
EML behavior (i.e., a maximal current efficiency of 48.5 cd/A and a maximal power
efficiency of 20.6 lm/W) compared to those of pseudo-symmetrical 3,6-DCP and 2,9-DCBQ
molecules.
Supplementary data
3D molecular structures and dihedral angles of three hosts. 1H- and 13C-NMR spectra of 3,6-
DCP, 2,9-DCBQ, and 3,9-DCBQ.
Acknowledgments
This work was supported by Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education (NRF-
2015R1D1A3A01020008). This research was also supported by National R&D Program
through the National Research Foundation of Korea(NRF) funded by the Ministry of Science
& ICT (NRF-2017M3A7B4041701).
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FIG. CAPTIONS
Scheme 1. Syntheses of three hosts
Fig. 1. HOMO and LUMO energies of phenantherene and its nitrogen-containing analogues
calculated by DFT/B3LYP.
Fig. 2. UV-visible absorption and PL spectra of (a) 3,6-DCP, (b) 2,9-DCBQ, and (c) 3,9-
DCBQ.
Fig. 3. (a) DSC and (b) TGA curves of 3,6-DCP, 2,9-DCBQ, and 3,9-DCBQ.
Fig. 4. Optimized molecular geometries of (a) 3,6-DCP, (b) 2,9-DCBQ, and (c) 3,9-DCBQ
showing LUMOs (left), HOMOs (middle), and side views (right).
Fig. 5. Energy diagrams of three green PHOLEDs.
Fig. 6. (a) current density-voltage-luminance curves, (b) current and external quantum
efficiencies, (c) normalized electroluminescence spectra of fabricated green PHOLEDs (at a
brightness of 1,000 cd/m2).
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Scheme 1. Syntheses of three hosts
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Fig. 1. HOMO and LUMO energies of phenantherene and its nitrogen-containing analogues
calculated by DFT/B3LYP.
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Fig. 2. UV-visible absorption and PL spectra of (a) 3,6-DCP, (b) 2,9-DCBQ,
and (c) 3,9-DCBQ.
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Fig. 3. (a) DSC and (b) TGA curves of 3,6-DCP, 2,9-DCBQ, and 3,9-DCBQ.
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Fig. 4. Optimized molecular geometries of (a) 3,6-DCP, (b) 2,9-DCBQ, and
(c) 3,9-DCBQ showing LUMOs (left), HOMOs (middle), and side views (right).
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Fig. 5. Energy diagrams of three green PHOLEDs.
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