Linear-shaped thermally activated delayed fluorescence emitter using 1,5-naphthyridine as an electron acceptor for efficient light extraction

Article abstract of DOI:10.1016/j.orgel.2019.105600

In this study, we developed a donor-π-acceptor-π-donor-type thermally activated delayed fluorescence (TADF) emitter (NyDPAc) using 1,5-naphthyridine as a core moiety. NyDPAc exhibited a high horizontal dipole ratio and thus a high external quantum efficiency (20.9%) even at a relatively low photoluminescence quantum yield (57%). 1,5-naphthyridine as an electron acceptor represents a promising core moiety for efficient TADF materials.

Full text of DOI:10.1016/j.orgel.2019.105600

Organic Electronics xxx (xxxx) xxx
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: http://www.elsevier.com/locate/orgel
Linear-shaped thermally activated delayed fluorescence emitter using
1,5-naphthyridine as an electron acceptor for efficient light extraction
,
,*
Youngnam Leea, Seung-Je Woob, Jang-Joo Kimb **, Jong-In Honga
a Department of Chemistry, Seoul National University, Seoul, 08826, Republic of Korea
b Department of Materials Science and Engineering, Seoul National University, Seoul, 08826, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this study, we developed a donor-π-acceptor-π-donor-type thermally activated delayed fluorescence (TADF)
Organic light emitting diode
Thermally activated delayed fluorescence
Light out coupling efficiency
Emitting dipole orientation
Naphthyridine
emitter (NyDPAc) using 1,5-naphthyridine as a core moiety. NyDPAc exhibited a high horizontal dipole ratio
and thus a high external quantum efficiency (20.9%) even at a relatively low photoluminescence quantum yield
(57%). 1,5-naphthyridine as an electron acceptor represents a promising core moiety for efficient TADF
materials.
1. Introduction
efficient TADF materials based on combinations of electron donors and
acceptors, OLED device efficiencies containing these emitters with near-
Organic light-emitting diodes (OLEDs) have attracted considerable
attention as a future display technology. Phosphorescence emitters can
attain 100% of the internal quantum efficiency (IQE) due to the strong
spin-orbit coupling of heavy metals [1,2], as compared to conventional
fluorescence emitters that yield only 25% of the IQE [3,4]. However,
because iridium and platinum used in phosphorescence emitters are
rare-earth elements and expensive, considerable effort has been devoted
to developing pure organic emitters which can reach 100% of the IQE.
Recently, thermally activated delayed fluorescence (TADF) materials
have received a great deal of attention as alternative OLED emitters
unity PLQYs are insufficient due to low light out-coupling efficiencies
[15–17]. Efficiency of TADF materials can be enhanced by employing
linear-shaped fluorescent molecules with a horizontal orientation. There
have been some reports about TADF emitters exhibiting a high hori-
zontal dipole ratio using triscarbazole, tri-spiral electron donor,
spiro-linked double electron donors and acceptors, and others [18–22].
However, to the best of our knowledge, 1,5-naphthyridine has not been
used as a core moiety for linear-shaped TADF molecules showing
excellent high horizontal dipole ratios.
1,5-naphthyridine has been known for its applications in pharma-
ceutical communities [23–25]. However, applications to OLEDs as an
electron acceptor remain largely unexplored. In this study, we designed
a linear-shaped TADF emitter (NyDPAc) by connecting 1,5-naphthyli-
dine as an electron acceptor and dimethylacridine as an electron
donor through a phenylene bridge (Fig. 1a). NyDPAc showed TADF
emission due to the orbital separation between the electron donor and
acceptor. Furthermore, the rod-like structure of NyDPAc induced hori-
zontal deposition during thermal evaporation, resulting in maximized
light out-coupling efficiency. Thus, the NyDPAc-based OLED device
showed a high EQE of 20.9% despite a relatively low PLQY (57%). This
is the first case of utilizing 1,5-naphthyridine as a linear acceptor for
achieving high TADF OLED efficiency.
[5–10]. TADF emitters require a small singlet-triplet energy gap (ΔEST
)
and have shown a maximum photoluminescence quantum yield (PLQY)
of 100% when triplet (T1) excitons are fully converted to singlet (S1)
excitons through reverse intersystem crossing (RISC). For high efficiency
OLEDs, both the IQE and external quantum efficiency (EQE) should be
considered, where the EQE is related to light out-coupling [11]. Even
when the IQE of TADF materials reaches 100%, the theoretical
maximum EQE is only 20% for isotropic materials due to limited light
out-coupling efficiencies [12,13]. Therefore, designing emitters which
can fully extract electroluminescence through the glass substrate is
necessary. These materials tend to be highly linear or planar, the dipoles
of which are aligned horizontally when thermally deposited [14].
In general, TADF materials can be designed by reducing the ΔEST
through distortion of electron donors and acceptors. Despite many
* Corresponding author.
** Corresponding author.
E-mail addresses: jjkim@snu.ac.kr (J.-J. Kim), jihong@snu.ac.kr (J.-I. Hong).
https://doi.org/10.1016/j.orgel.2019.105600
Received 10 September 2019; Received in revised form 23 November 2019; Accepted 17 December 2019
Available online 18 December 2019
1566-1199/© 2019 Published by Elsevier B.V.
Please cite this article as: Youngnam Lee, Organic Electronics, https://doi.org/10.1016/j.orgel.2019.105600

Y. Lee et al.
Organic Electronics xxx (xxxx) xxx
Table 1
Calculated data of NyDPAc.
HOMO
[eV]
LUMO
Egap
S1 [eV]
T1 [eV]
ΔEST
[eV]
[eV]
[eV]
NyDPAc
4.90 eV
2.22 eV
2.68 eV
2.286
eV
2.282
eV
0.004
eV
Fig. 1. (a) Design strategy (b) optimized geometry: HOMO (blue), LUMO
(Red). (For interpretation of the references to color in this figure legend, the
reader is referred to the Web version of this article.)
2. Experimental
Scheme 1. Synthetic routes for NyDPAc. i) m-NO2PhSO3Na, glycerol, H2SO4,
H2O, 150 �C; ii) mCPBA, CH2Cl2/MeOH, rt; iii) POCl3, 110 �C, 20 min; iv) CuI,
trans-1,2-cyclohexanediamine, sodium tert-butoxide, 1,4-dioxane, 100 �C; v) n-
BuLi, trimethyl borate, THF, À 78 �C → rt; vi) Pd(PPh3)4, K2CO3, 1,4-dioxane/
H2O, 100 �C.
2.1. Quantum chemical calculations
All quantum chemical calculations were performed using the
Gaussian ’09 program package. Gas-phase geometry optimizations for
the lowest excited singlet and triplet states were carried out using den-
sity functional theory (DFT) and time-dependent density functional
theory (TD-DFT) calculations at the B3LYP/6-31G(d) level.
2.4. Device fabrication and measurements
The patterned indium-tin-oxide (ITO, 150 nm) substrates were
washed with water and isopropyl alcohol, followed by 10 min UV-ozone
treatment. Organic layers, LiF, and Al were thermally evaporated at a
deposition rate of 1–2 Å sÀ 1 for organic layers, 0.1 Å sÀ 1 for LiF, and 3–5
Å sÀ 1 for the Al electrode. OLED properties were measured using a
Keithley source meter 2400 and a PR-650 spectrascan colorimeter.
2.2. Photophysical property analysis
UV–visible spectra were recorded on a Jasco V-730 spectrophotom-
eter. Fluorescence and phosphorescence spectra were recorded on a
Jasco FP-8300 spectrophotometer. Absolute quantum efficiency was
obtained with a PTI QuantaMaster 40 spectrofluorometer using a 3.2 in.
integrating sphere at room temperature. Transient photoluminescence
(PL) was measured with a streak camera (Hamamatsu Photonics, Japan)
using a nitrogen laser (337 nm; Usho Optical Systems, Japan) as the
excitation source. For angle-dependent PL (ADPL) measurements, p-
polarized light emitted from PL samples was measured by attaching the
film substrate to a half-cylinder lens with index-matching oil and
changing the angle between the sample and the detector from À 90� to
90� using a motorized rotational stage.
2.5. Synthesis and characterization
Commercially available reagents and solvents were used without
further purification unless otherwise noted. 1H and 13C NMR spectra
were recorded using an Agilent 400-MR DD2 400 MHz or Varian/Oxford
As-500 500 MHz in CDCl3. 1H NMR chemical shifts in CDCl3 were
referenced to CHCl3 (7.27 ppm). 13C NMR chemical shifts in CDCl3 were
reported relative to CHCl3 (77.23 ppm). Mass spectra were recorded on a
matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF)
Microflex instrument from Bruker. High-resolution mass spectrometric
(HRMS) data (JEOL, JMS-700) with fast atom bombardment (FAB)
positive mode were received directly from the National Center for Inter-
University Research Facilities (NCIRF). Elemental analyses were carried
out using Thermo Scientific FlashEA 1112.
2.3. Electrochemical and thermal analysis
Cyclic voltammetry (CV) experiments were conducted in DMF so-
lution (1.00 mM) with 0.1 M tetra-n-butylammonium perchlorate
(TBAP) as the supporting electrolyte. A glassy carbon electrode was
employed as the working electrode and referenced to an Ag reference
electrode. All potential values were calibrated against the ferrocene/
ferrocenium (Fc/Fcþ) redox couple. The onset potential was determined
from the intersection of two tangents drawn at the rising and back-
ground current of the cyclic voltammogram. Differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA) were per-
formed with a TA instrument DSC Q10 and Q50 thermogravimetric
3. Results and discussion
3.1. Theoretical calculations
DFT and TD-DFT calculations of NyDPAc were performed to show an
optimized molecular structure and the highest occupied molecular
orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) of
ground states and energies of excited states at the B3LYP/6-31G(d) level.
The calculated data are summarized in Table 1. DFT calculations of
NyDPAc revealed a linear shape as the optimized geometry (Fig. 1b),
where dimethylacridine and a phenylene bridge were distorted with a
analyzer in a nitrogen atmosphere at a heating rate of 10 �C minÀ 1
.
2

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Table 2
Physical and photophysical data of NyDPAc.
λPLa/b [nm]
ESa/b [eV]
ETa/b [eV]
ΔESTa/b [eV]
PLQYb [%]
57
τFb [ns]
τTADF [μs]
HOMOc [eV]
5.30
HOMOc [eV]
2.87
θlld [%]
Tge/Tdf [oC]
144/414
b
NyDPAc
486/510
2.81/2.80
2.51/2.51
0.30/0.29
12
451
92
a
Measured using toluene solution (10À 5 M).
b
c
d
e
f
Measured using 10 wt% NyDPAc doped in DPEPO film.
Estimated from the onset potentials (oxEonset and redEonset [eV] against Fc/Fcþ redox couple) in CV experiments.
The horizontal emitting dipole ratio.
The glass transition temperature.
The temperature at 5% weight loss.
Fig. 2. (a) Absorption and PL spectra of NyDPAc in solution (concentration: 10À 5 M). (b) Fluorescence and phosphorescence spectra of NyDPAc in toluene and 10 wt
% NyDPAc doped in DPEPO film. (c) Transient PL decay curve using 10-wt% NyDPAc doped in DPEPO film. (d) Angle-dependent PL intensity.
dihedral angle of 89� because of steric hindrance. The HOMO was
localized on the dimethylacridine moiety, whereas the LUMO was
localized on the linear core of 1,5-naphthyridine and two phenylene
linkers. The energies of singlet (2.286 eV) and triplet (2.282 eV) excited
states were estimated using TD-DFT calculations, resulting in a small
ΔEST of 0.004 eV.
POCl3 to furnish 2,6-dichloro-1,5-naphthyridine (2). 2,6-Bis(4-(9,9-
dimethylacridin-yl)phenyl)-1,5-naphthyridine (NyDPAc) was obtained
through Suzuki-Miyaura coupling between 2 and (4-(9,9-dimethylacri-
din-10(9H)-yl)phenyl) boronic acid (3). NyDPAc was further purified by
train sublimation under reduced pressure (<10À 4 Torr). Details of their
syntheses and purification are provided in the Supporting Information.
3.2. Synthesis
3.3. Photophysical properties
The synthetic procedure is depicted in Scheme 1. First, 3-aminopyr-
idine was heated with sulfuric acid, glycerol, and 3-nitrobenzene sul-
fonate to produce 1,5-naphthyridine through a Skraup reaction. The
resulting 1,5-naphthyridine was oxidized with mCPBA to yield 1,5-
naphthyridine-1,5-dioxide (1), which was then chlorinated using
We investigated the photophysical properties using ultra-
violet–visible (UV–vis), photoluminescence (PL), and transient PL
spectroscopy. The photophysical properties of NyDPAc are summarized
in Table 2. In the UV–vis spectra, charge transfer (CT) absorption
occurred at 350 nm, which represents the charge transfer from
3

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Fig. 3. (a) Energy level diagram for the OLED device, (b) current density-voltage-luminance (J-V-L) curves, (c) EL spectra at 5 V, (d) external quantum efficiency-
current density (ηEQE-J) curve.
dimethylacridine to naphthyridine. As
a result, solvatochromism
Table 3
appeared as the polarity of the solvent increased from toluene to tetra-
hydrofuran (THF) and methylene chloride (MC). The CT emissions were
observed at 486 nm in toluene, 548 nm in THF, and 558 nm in MC
(Fig. 2a). Thin-film PL measurements were conducted using bis[2-
(diphenylphosphino)phenyl] ether oxide (DPEPO) as a host with a high
T1 energy (T1 ¼ 3.0 eV) [26]. The 10 wt% doped film exhibited CT
emission at 510 nm and an absolute PLQY of 57%.
Device data of NyDPAc in a DPEPO host matrix.
a
e
Doping
Turn on
voltage
[V]
EQEmax
[%]
CEb
[cd
AÀ 1
PEc
CIE
λmax
concentration
[lm
Coordinates
[nm]
]
WÀ 1
]
d(x, y)
10%
20%
30%
3.6
3.6
3.6
20.9
18.7
12.6
47.9
46.1
29.8
41.8
40.2
26.0
0.28, 0.53
0.30, 0.55
0.32, 0.55
516
524
528
The time-gated PL spectra were acquired at a low temperature (77 K)
to investigate the characteristics of triplet excited states (Fig. 2b). Both
solution and film phosphorescence spectra were characterized by the
mixing of 3CT and 3LE states [27], compared to the spectrum of the
acceptor moiety (Fig. S1), whereas the fluorescence has a 1CT character.
ΔEST was estimated to be approximately 0.30 eV based on the onset of
spectra. To confirm the TADF characteristics, a transient PL decay pro-
file of NyDPAc in the doped films was obtained (Fig. 2c). Decay curves
showed prompt and delayed components with lifetimes of 12 ns and 451
a
External quantum efficiency.
Current efficiency.
b
c
d
e
Power efficiency.
CIE 1931 Coordinates.
Maximum wavelength at 5V.
3.4. Electrochemical and thermal properties
The HOMO and LUMO levels of NyDPAc measured by cyclic vol-
tammetry were 5.30 and 2.87 eV, respectively (Fig. S2). Therefore,
DPEPO (HOMO ¼ 6.1 eV, LUMO ¼ 2.0 eV) can serve as a suitable host
material for NyDPAc. Differential Scanning calorimetry (DSC) and
thermal gravimetric analysis (TGA) showed extremely high thermal
stability with a glass transition temperature (Tg) of 144 �C and a
decomposition temperature (Td) of 414 �C at 5% weight loss (Fig. S3).
μ
s, respectively. NyDPAc clearly showed TADF emission despite large
ΔEST. This was due to appropriate interactions between 1CT, 3LE, and
3CT for RISC according to the El-Sayed rule [28]. In fact, several studies
on successful TADF emitters have been conducted using large ΔEST
[29–32]. The kinetic constants of NyDPAc were calculated according to
the Adachi’s method (Table S1) [33–35].
4

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3.5. Device properties
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