Full-Color Delayed Fluorescence Materials Based on Wedge-Shaped Phthalonitriles and Dicyanopyrazines: Systematic Design, Tunable Photophysical Properties, and OLED Performance

Article abstract of DOI:10.1002/adfm.201505106

Purely organic light-emitting materials, which can harvest both singlet and triplet excited states to offer high electron-to-photon conversion efficiencies, are essential for the realization of high-performance organic light-emitting diodes (OLEDs) without using precious metal elements. Donor-acceptor architectures with an intramolecular charge-transfer excited state have been proved to be a promising system for achieving these requirements through a mechanism of thermally activated delayed fluorescence (TADF). Here, luminescent wedge-shaped molecules, which comprise a central phthalonitrile or 2,3-dicyanopyrazine acceptor core coupled with various donor units, are reported as TADF emitters. This set of materials allows systematic fine-tuning of the band gap and exhibits TADF emissions that cover the entire visible range from blue to red. Full-color TADF-OLEDs with high maximum external electroluminescence quantum efficiencies of up to 18.9% have been demonstrated by using these phthalonitrile and 2,3-dicyanopyrazine-based TADF emitters.

Full text of DOI:10.1002/adfm.201505106

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Full-Color Delayed Fluorescence Materials Based on
Wedge-Shaped Phthalonitriles and Dicyanopyrazines:
Systematic Design, Tunable Photophysical Properties,
and OLED Performance
In Seob Park, Sae Youn Lee, Chihaya Adachi, and Takuma Yasuda*
electrons recombine to form singlet
Purely organic light-emitting materials, which can harvest both singlet and
triplet excited states to offer high electron-to-photon conversion efficiencies,
are essential for the realization of high-performance organic light-emitting
diodes (OLEDs) without using precious metal elements. Donor–acceptor
architectures with an intramolecular charge-transfer excited state have been
proved to be a promising system for achieving these requirements through
a mechanism of thermally activated delayed fluorescence (TADF). Here,
luminescent wedge-shaped molecules, which comprise a central phthalonitrile
or 2,3-dicyanopyrazine acceptor core coupled with various donor units, are
reported as TADF emitters. This set of materials allows systematic fine-
tuning of the band gap and exhibits TADF emissions that cover the entire
visible range from blue to red. Full-color TADF-OLEDs with high maximum
external electroluminescence quantum efficiencies of up to 18.9% have been
demonstrated by using these phthalonitrile and 2,3-dicyanopyrazine-based
TADF emitters.
and triplet excitons with a ratio of 1:3,
according to spin statistics.[2] Therefore,
internal EL quantum efficiencies (ηint
)
of OLEDs based on conventional fluores-
cent materials are limited to 25% as only
the lowest singlet (S1) excitons can be uti-
lized for light emission under electrical
excitations. In this case, the lowest triplet
(T1) excitons undergo a spin-forbidden
transition, resulting in radiation-less
deactivation. Meanwhile, precious metal-
containing organometallic phosphorescent
materials are capable of harvesting both S1
and T1 excitons for light emission.[2] This
is because of the enhanced intersystem
crossing (ISC) mediated by a strong spin–
orbit coupling of the heavy metals such as
iridium, platinum, and osmium, which
results in ηint of up to 100%.[3] Despite
their desirable EL characteristics, the
1. Introduction
rarity, high cost, and toxicity of these precious metals, as well
as the poor stability of blue phosphors would hamper the wide-
spread applications of these OLEDs in the future. As an alterna-
tive, thermally activated delayed fluorescence (TADF, Figure 1),
which occurs by converting the excited T1 states to emissive S1
states via an efficient reverse intersystem crossing (RISC) using
purely organic emitters, has attracted great attention in recent
years.[4–9] Highly efficient TADF-OLEDs with ηint of nearly
100% have been successfully realized, and hence, TADF is
expected to be a key technology for the next-generation OLEDs.
The major design principle for TADF molecules is to mini-
mize the overlap between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) by localizing them on different moieties in a molecule,
leading to a small energy gap between the excited S1 and T1
states (ΔEST), as a consequence of reduced electron exchange
interactions.[10] Under this condition, the RISC process can be
strongly accelerated. However, a weak frontier orbital overlap
is a prerequisite for an efficient light emission, because a
negligible spatial overlap of the wavefunctions results in the
loss of transition probability according to Fermi’s golden rule.
Therefore, both these conditions should be met for designing
novel efficient TADF molecules. Based on this design prin-
ciple, various purely organic TADF emitters have been devel-
oped to date;[4–9] most of them adopt a twisted intramolecular
Organic light-emitting diodes (OLEDs) are now used in main-
stream display technologies and are also being explored inten-
sively for future lighting applications owing to their unique
advantages such as the high electroluminescence (EL) effi-
ciency, high contrast, light weight, flexibility, and low manu-
facturing costs.[1] In OLEDs, electrically injected holes and
I. S. Park, Prof. T. Yasuda
INAMORI Frontier Research Center (IFRC)
Kyushu University
744 Motooka, Nishi-ku
Fukuoka 819-0395, Japan
E-mail: yasuda@ifrc.kyushu-u.ac.jp
I. S. Park, Dr. S. Y. Lee, Prof. C. Adachi, Prof. T. Yasuda
Department of Applied Chemistry
Graduate School of Engineering
Kyushu University
744 Motooka, Nishi-ku
Fukuoka 819-0395, Japan
Prof. C. Adachi
Center for Organic Photonics and Electronics Research (OPERA)
Kyushu University
744 Motooka, Nishi-ku
Fukuoka 819-0395, Japan
DOI: 10.1002/adfm.201505106
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EL quantum efficiency (ηext) of almost 20%[4a] or even higher.[11]
It has also been reported that by changing the position of cyano
groups and the number of carbazole units, the emission spectra
of the TADF materials can be tuned. ICT of these molecules
has crucial effects on their band gap and luminescence. For
the realization of full-color TADF-OLEDs, it is vital to modulate
the TADF characteristics and the emission colors systematically
using a versatile donor–acceptor (D–A) system.
In this work, a series of wedge-shaped donor–acceptor–
donor (D–A–D) molecules (Figure 2), comprising a central
phthalonitrile (VPN) or 2,3-dicyanopyrazine (CNP) acceptor
core coupled with various donor units (i.e., 1-methylcarba-
zole (Cz), 9,9-dimethylacridan (Ac), and phenoxazine (Px)),
are reported. π-Conjugated phenylene linkers are introduced
between the donor and acceptor units to enhance the radiative
decay constant of the S1 state to some degree, meanwhile main-
taining a small ΔEST. These new systems allow a systematic
fine-tuning of their band gap and TADF emissions to cover
the entire visible range. Full-color EL with a high ηext of up to
18.9% can be achieved using wedge-shaped phthalonitrile and
dicyanopyrazine derivatives as TADF emitters in OLEDs.
Figure 1. Schematic representation for TADF mechanism in OLEDs.
Upon hole and electron recombination, 25% of the corresponding excited
states are singlets, while 75% are triplets. The triplet excitons are har-
vested through thermally promoted T1→S1 RISC to emit TADF.
charge-transfer (ICT) design, in which the electron donor and
acceptor units are incorporated to be orthogonal to each other.
Among the reported TADF emitters, 1,2,3,5-tetrakis(carbazol-
9-yl)-4,6-dicyanobenzene (4Cz-IPN)[4a] combined with an
electron-withdrawing isophthalonitrile (i.e., 1,3-dicyanobenzene)
with fourfold electron-donating carbazole units is one of the
most efficient TADF emitters, exhibiting a maximum external
2. Results and Discussion
2.1. Molecular Geometric and Electronic Structures
To understand the effect of variation of the D–A–D struc-
tures on the geometric and optoelectronic properties,
Figure 2. Molecular structures (upper), energy levels and Kohn–Sham orbitals of the HOMO and LUMO (lower) of wedge-shaped phthalonitrile and
dicyanopyrazine-based TADF molecules characterized by DFT and TD-DFT calculations at the B3LYP/6-31G(d,p) level.
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quantum-chemical calculations were performed on the
designed phthalonitrile and dicyanopyrazine-based mole-
cules. The calculated energy levels of the HOMO and LUMO
and the respective frontier orbital distributions are presented
in Figure 2. The energy levels and molecular geometries in
the ground state were optimized by density functional theory
(DFT) calculations. The excited S1 and T1 states were com-
puted using the optimized structures with time-dependent
DFT (TD-DFT) calculations. It is found that the calculated
HOMO–LUMO energy gaps (Eg) can be modulated systemat-
ically from 3.2 eV (Cz-VPN) to 1.9 eV (Px-CNP) by a rational
combination of donor and acceptor units. The decrease in the
Eg from Cz-VPN to Px-VPN follows the increasing order of the
electron-donating ability of their donor units (Cz < Ac < Px)
which consequently affects their HOMO energy levels.
Ac-CNP and Px-CNP containing a strong electron-with-
drawing dicyanopyrazine core tend to have a lower LUMO
energy and a narrower Eg than the phthalonitrile-based coun-
terparts do. This trend indicates that the donor and acceptor
strengths have a great influence on the resulting optoelec-
tronic functionality since the HOMO–LUMO electronic tran-
sition dominates the S1 excitation with ICT characteristics.
The HOMOs of these molecules are predominantly located
on the peripheral donor units, whereas the LUMOs are distrib-
uted over the phenylene linkers as well as the central acceptor
core (Figure 2). For Cz-VPN, the HOMO is extended slightly
to the neighboring phenylene linkers, owing to the less steric
hindrance of the Cz donor units, compared with the Ac and
Px units. The clear spatial separation of the frontier orbitals
of all these molecules resulted in small calculated ΔEST values
of less than 0.04 eV (Table 1), suggesting the high potential
as TADF emitters. It is also evident that, in the optimized
ground-state structures, the dihedral angles between the
peripheral donor units and the nearby phenylene linkers
increase in the order of Cz (53°–54°) < Px (74°–77°) <
Ac (87°–90°). Such highly distorted geometries of these
molecules should arise from the steric effects of the hydrogen
atoms and/or methyl groups at the peri-positions in the donor
units, contributing to the decrease of the electron exchange
energy for the ICT transitions. In contrast, the dihedral angles
between the central phthalonitrile or dicyanopyrazine acceptor
unit and the phenylene linkers are relatively small (51°–54°
for Ac-VPN and Px-VPN; 34°–35° for Ac-CNP and Px-CNP).
These results suggest that the phenylene linker has a stronger
conjugation with the central acceptor core than the outermost
donor units.
2.2. Synthesis
The synthetic scheme is outlined in Scheme 1, and the detailed
synthetic procedures are described in the Experimental Sec-
tion and the Supporting Information. The key intermediate 2
was synthesized from tetraphenylcyclopentadienone 1 and
fumaronitrile.[12] Then, the Buchwald–Hartwig amination of 2
with 1-methylcarbazole in the presence of Pd(t-Bu3P)2 as a
catalyst afforded Cz-VPN with a high yield. The syntheses of
Ac-VPN and Px-VPN were achieved by Suzuki–Miyaura cross-
coupling reactions between 4,5-dichlorophthalnitrile and the
corresponding boronic esters (5 and 7, respectively). For the
synthesis of Ac-CNP and Px-CNP, the dibromoprecursor 8 was
initially prepared, and the Buchwald–Hartwig aminations with
9,9-dimethylacridan or phenoxazine were carried out. All final
products were purified by temperature-gradient sublimation
under vacuum after column chromatography to obtain highly
pure materials, which can be used for the fabrication of OLED
devices by vacuum deposition. Chemical structures of the final
products were confirmed by 1H and 13C nuclear magnetic reso-
nance (NMR) spectroscopy, matrix-assisted laser desorption
ionization time-of-flight (MALDI-TOF) mass spectrometry, and
elemental analysis (see the Experimental Section for details).
2.3. Photophysical Properties
The molecular orbital distributions are reflected in the photo-
physical properties of the phthalonitrile and dicyanopyrazine-
based materials. Basic photophysical parameters have been
collected from steady-state UV–vis absorption and photolumi-
nescence (PL) spectra, and time-resolved transient PL analyses
for toluene solutions and as doped thin films, and the results
are summarized in Table 1. As shown in Figure 3, these com-
pounds exhibit a broad and weak absorption band at longer
wavelengths in the absorption spectra, which can be assigned
to the ICT transitions from the peripheral donor units to the
acceptor core. The ICT absorption band shifts to lower energies
Table 1. Photophysical data of phthalonitrile and dicyanopyrazine-based TADF materials.
h)
i)
ΦPLc) [%]
τpd) [ns]/
τdd) [µs]
ΔEST
Calc. ΔEST
Compound
HOMOe)
[eV]
LUMOf)
[eV]
ES/ETg)
[eV]
λ
abs [nm]
λ
PL [nm]
sola)
sola)/filmb)
440 / 450
532 / 499
577 / 540
620 / 570
sola)/filmb)
[eV]
0.36
0.20
0.08
0.09
0.04
[eV]
Cz-VPN
Ac-VPN
Px-VPN
Ac-CNP
Px-CNP
292,325,339j)
282,384
78 / 63
59 / 86
42 / 77
20 / 67
<1 / 15
41 / 173
35 / 5.1
27 / 2.3
30 / 3.3
21 / 1.5
–6.1
–5.8
–5.7
–5.4
–5.2
–2.9
–3.1
–3.3
–3.1
–3.2
3.08 / 2.72
2.90 / 2.70
2.64 / 2.56
2.33 / 2.24
2.45 / 2.41
0.039
0.002
0.015
0.003
0.040
304,406
282,342j),436
321,473
–
k) / 610
a)Measured in oxygen-free toluene solution at room temperature; b)6 wt%-doped thin film in a host matrix (host: PPF for Cz-VPN and mCBP for others); c)Absolute PL
quantum yield evaluated using an integrating sphere under a nitrogen atmosphere; d)PL lifetimes of prompt (τp) and delayed (τd) decay components for the 6 wt%-doped
film measured using a streak camera at 300 K; e)Determined by photoelectron yield spectroscopy in pure neat films; f)Deduced from the HOMO and optical energy gap
(Eg); g)Singlet (ES) and triplet (ET) energies estimated from onset wavelengths of the emission spectra at 300 and 50 K in the doped films, respectively; h)ΔEST = ES – ET;
i)Calculated by TD-DFT at B3LYP/6-31G(d,p); j)Shoulder peak; k)Not determined.
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Scheme 1. Synthetic routes for phthalonitrile and dicyanopyrazine-based TADF molecules.
with increasing donor and acceptor strengths, which is well in
accordance with the calculation results.
light. In contrast, Px-CNP exhibited no PL emission in dissolved
states. The absolute PL quantum yields (ΦPL) of these emitters
in the doped films lie in the range of 63%–86%, with the excep-
tion for the red-emitting Px-CNP film (ΦPL = 15%). The reason
for such a low ΦPL of Px-CNP may be ascribed to the energy-gap
law, which suggests that the ΦPL decreases as the energy gap is
reduced, owing to the vibrational overlap of the ground and the
excited states.[15] Thus, the experimental PL observations con-
firm the expected trend that the multicolor tuning of TADF can
be accomplished by systematic structural variations.
We also examined the transient PL characteristics of the emit-
ters to reveal the TADF behavior. As exemplified in Figure 5,
the transient PL curve of the 6 wt%-Ac-VPN:mCBP doped film
obviously indicated a nanosecond-order prompt decay compo-
nent and a microsecond-order delayed decay component in the
time range of 10 µs, which can be fitted with a biexponential
For further investigation of the photophysical and TADF
properties, doped thin films of the phthalonitrile and dicyano-
pyrazine-based emitters in a host matrix were prepared in order
to avoid concentration quenching. For the blue emitter Cz-VPN,
2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF)[13] pos-
sessing a high T1 energy (ET) value of 3.1 eV was selected as a
host to prevent the backward energy transfer from the excited
Cz-VPN to the host material and to confine the T1 excitons within
the Cz-VPN molecules. For other emitters, 3,3′-bis(carbazol-
9-yl)-1,1′-biphenyl (mCBP)[14] with an ET of 2.9 eV
was adopted as a suitable host material. As can be seen from
Figure 4, the emission colors of the doped films range from
blue (Cz-VPN: λPL = 450 nm) to green (Px-VPN: λPL = 540 nm),
and even red (Px-CNP: λPL = 610 nm), covering the entire visible
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Figure 3. UV–vis absorption spectra of phthalonitrile and dicyanopyra-
zine-based D–A–D type molecules in toluene. The inset shows a magni-
fied view of lower-energy ICT absorptions.
model. While the prompt component had a lifetime (τp) of
35 ns with a quantum efficiency (Φp) of 8%, the delayed com-
ponent had a lifetime (τd) of 5.1 µs with an efficiency (Φd) of
78% at 300 K. Moreover, the delayed component is appeared
with a similar spectral distribution as the prompt one; hence,
the delayed emission can be ascribed to the singlet emission
through RISC processes. Temperature dependence of the tran-
sient PL was also studied for the 6 wt%-Ac-VPN:mCBP doped
film in the temperature range of 50–300 K (Figure 5a). Appar-
ently, the delayed emission was intensified when temperature
was increased from 50 to 300 K, demonstrating that the T1 → S1
RISC process was enhanced by the thermal energy, which is a
direct evidence of TADF. Similar temperature-dependent tran-
sient PL behavior was observed for all the phthalonitrile and
dicyanopyrazine-based emitters (Supporting Information). The
delayed emission lifetime of the green-emitting Px-VPN film
(τd = 2.3 µs) is found to be much shorter
Figure 4. a) Photographs showing PL emission under UV irradiation and
b) steady-state PL spectra of 6 wt%-doped thin films of the phthalonitrile
and dicyanopyrazine-based emitters in a PPF or mCBP host matrix.
TADF-OLEDs. To investigate the EL performance, multilayer
OLEDs were fabricated using 6 wt%-emitter:host doped films
as the emitting layer (EML). The structure of the fabricated
devices was as follows: indium–tin-oxide (ITO, 110 nm)/α-NPD
(40 nm)/mCP (10 nm)/EML (20 nm)/PPF (10 nm)/TPBi
(30 nm)/LiF (0.8 nm)/Al (100 nm), where 4,4′-bis[N-(1-
naphthyl)-N-phenylamino]-1,1′-biphenyl
(α-NPD)
and
1,3,5-tris(N-phenylbenzimizazol-2-yl)benzene (TPBi) served as
a hole-transporting layer (HTL) and electron-transporting layer
than those of the blue-emitting Cz-VPN and
light-blue-emitting Ac-VPN films (τd = 173
and 5.1 µs, respectively), suggesting a more
efficient T1→S1 up-conversion arising from
smaller ΔEST (Table 1). From the room-tem-
perature fluorescence and the low-temper-
ature phosphorescence (5 K) spectra of the
doped films, the experimental ΔEST values are
found to be in the order of Cz-VPN (0.36 eV)
> Ac-VPN (0.20 eV) > Px-VPN (0.08 eV)
≈ Ac-CNP (0.09 eV) > Px-CNP (0.04 eV),
which are in good agreement with the trend
of the delayed emission lifetimes.
2.4. Electroluminescence Performance
Color-tunable
with high ΦPL enable these emitters to
be adopted in high-efficiency full-color
TADF
characteristics
Figure 5. a) Temperature dependence of transient PL decay of a 6 wt%-Ac-VPN:mCBP doped
film. b) Streak image of the doped film measured at 300 K under vacuum, in which green dots
represent PL photon counts.
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(ETL), respectively. To prevent the energy transfer and to con-
fine T1 excitons in the EML, thin layers of 1,3-bis(carbazol-9-yl)
benzene (mCP) and PPF with high T1 energies were inserted
at the EML/HTL and EML/ETL interfaces. The current den-
sity–voltage–luminance (J–V–L) and ηext versus luminance
characteristics of the devices are shown in Figure 6, and the key
device parameters are summarized in Table 2.
All the devices employing phthalonitrile and dicyanoben-
zene-based emitters displayed EL spectra similar to the
corresponding PL spectra, confirming that EL emission was
generated solely from the emitters via the same radiative
decay process. These devices indicated similar turn-on volt-
ages (Von) in the range of 4.4–5.5 V. For the Cz-VPN-based
device, a blue EL emission at a maximum wavelength (λEL)
of 465 nm and Commission Internationale de l'Éclairage
(CIE) color coordinates of (0.15, 0.18) were obtained. The EL
efficiency of the blue-emitting Cz-VPN device is drastically
decreased with increasing luminance. This severe efficiency
roll-off of the device is mainly attributed to the relatively longer
T1 excited-state lifetime of the Cz-VPN emitter, which under-
goes exciton quenching such as triplet–triplet annihilation and/
or triplet–polaron annihilation.[16] Another possible reason
can be the poor hole-transport property of the PPF host. PPF
is a unipolar electron-transporting material and the hole/elec-
tron carrier balance is disrupted at high current densities. We
anticipate that the performance and roll-off behavior of the blue
TADF device can further be improved by using suitable bipolar
hosts with high T1 values and blue TADF emitters with shorter
excited-state lifetimes.
High EL efficiencies with ηext of 18.9%, 14.9%, and 13.3%
are attained for the devices containing Ac-VPN, Px-VPN, and
Ac-CNP, respectively (corresponding power efficiencies (ηp) are
34.1, 26.7, and 26.1 lm W–1, respectively), and their EL emis-
sions are located in the light-blue, green, and yellow regions,
respectively. It is noteworthy that the Ac-VPN- and Px-VPN-
based devices showed very small efficiency roll-off; hence, the
ηext of the devices were still high (12.4% and 10.4%, respec-
tively), even at a high luminance of 10 000 cd m−2. For the
Figure 6. a) Energy level diagram and molecular structures of materials used for TADF-OLEDs. b) Current density–voltage–luminance (J–V–L) char-
acteristics and c) external EL quantum efficiency (ηext) versus luminance plots of the TADF-OLEDs. The inset of panel (c) represents normalized EL
spectra of the devices measured at 10 mA cm−2
.
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Table 2. EL performance of the TADF-OLEDs.
TADF emittera)
Cz-VPN
Host
PPF
V
on [V]
CIE (x, y)
(0.15, 0.18)
(0.23, 0.50)
(0.35, 0.57)
(0.47, 0.51)
(0.53, 0.44)
λ
EL [nm]
465
Lmax [cd m−2
]
η
ext [%]
8.7
ηc [cd A−1
18.0
]
η
p [lm W−1
]
4.4
4.5
4.7
4.7
5.5
13,590
12.3
Ac-VPN
mCBP
mCBP
mCBP
mCBP
504
65,720
18.9
14.9
13.3
3.0
51.7
34.1
Px-VPN
537
44,380
45.4
26.7
Ac-CNP
580
70,630
38.1
26.1
Px-CNP
606
101,860
5.8
3.1
a)λEL: EL emission maximum, Von: turn-on voltage at 1 cd m−2, Lmax: maximum luminance, ηext: maximum external EL quantum efficiency, ηc: maximum current efficiency,
ηp: maximum power efficiency, CIE: Commission Internationale de l’Éclairage color coordinates measured at 10 mA cm−2, PPF: 2,8-bis(diphenylphosphoryl)dibenzo[b,d]
furan, mCBP: 3,3′-bis(carbazol-9-yl)-1,1′-biphenyl.
red-emitting Px-CNP-based device, the ηext was inferior com-
pared to those of the other devices because of the relatively
lower ΦPL of 15% in its doped film. However, the Px-CNP-
based device exhibited only a slight efficiency roll-off owing to
its short T1 exciton lifetime of 1.5 µs, compared to other TADF
emitters (Table 1).
small efficiency roll-off. We believe that these results open
new avenues for designing high-performance full-color
TADF emitters with simple and versatile structures for OLED
applications.
Under electrical excitation in TADF-OLEDs, the T1 excitons
are directly generated by carrier recombination and are subse-
quently converted into the S1 state via an efficient RISC. Accord-
ingly, the theoretical maxima of ηint and ηext can be given by the
following equations[5a]
4. Experimental Section
General Methods: NMR spectra were recorded on an Avance III
500 or Avance III 400 spectrometer (Bruker). Chemical shifts of 1H
and 13C NMR signals were quoted to tetramethylsilane (δ = 0.00),
CDCl3 (δ = 77.0) as internal standards, respectively. Matrix-assisted
laser desorption ionization time-of-flight mass spectra were collected
on an Autoflex III spectrometer (Bruker Daltonics) using dithranol
as the matrix. Elemental analyses were carried out with a Yanaco
MT-5 CHN corder. UV–vis absorption and PL spectra were measured
(1)
(2)
ηint =ηS ×Φp +ηS ×Φd +ηT ×Φd /ΦISC
with
a UV-2550 spectrometer (Shimadzu) and a Fluoromax-4
ηext = ηint × ηout
spectrophotometer (Horiba Scientific), respectively, using degassed
spectral grade solvents. The PL quantum yields were measured
using an integration sphere system C9920-02 coupled with a PMA-11
photonic multichannel analyzer (Hamamatsu Photonics). The transient
PL measurements of doped thin films were performed using a C4334
Streak camera (Hamamatsu Photonics) with a N2 gas laser (λ =
337 nm, pulse width = 500 ps, repetition rate = 20 Hz) under vacuum
(<4 × 10–1 Pa). The HOMO energy levels of thin films were determined
using an AC-3 ultraviolet photoelectron spectrometer (Riken-Keiki).
The LUMO energy levels were estimated by subtracting the optical
energy gap (Eg) from the measured HOMO energies; Eg values were
determined from the onset position of the PL spectra of thin films. All
quantum-chemical calculations were performed using the Gaussian
09 program package. Geometries in the ground state were optimized
using the B3LYP functional with the 6–31G(d,p) basis set. Low-lying
excited singlet and triplet states were computed using the optimized
structures with TD-DFT at the same level.
where ηS and ηT denote the singlet-exciton and triplet-exciton
production rates (25% and 75%, respectively), ΦISC is the
intersystem-crossing efficiency (ΦISC = 1 – Φp), and ηout is the
light out-coupling efficiency. The theoretical maxima of ηext for
Cz-VPN, Ac-VPN, Px-VPN, Ac-CNP, and Px-CNP-based devices
are estimated to be ≈9%, 17%, 14%, 13%, and 3%, respectively,
by assuming that ηout is ≈20%.[17] These theoretical values
are consistent with the experimentally obtained ηext values
(Table 2), implying that the appropriate carrier balance and
effective exciton confinement are indeed achieved in the EML
for the full-color TADF-OLEDs.
Preparation of Materials: All regents and solvents for the synthesis
were purchased from Sigma-Aldrich, Tokyo Chemical Industry, or Wako
Pure Chemical Industries, and were used as received unless otherwise
stated. 9,9-Dimethylacridan (Ac)[18] and PPF[13] were prepared according
to the reported procedures. Other OLED materials were purchased from
Luminescence Technology, Corp. The detailed synthetic procedures and
characterization data of compounds 1–8 are described in the Supporting
Information. All reactions were performed under nitrogen atmospheres
in dry solvents. All final products were purified by temperature-gradient
sublimation under vacuum before the measurements and device fabrication.
Synthesis of Cz-VPN: A mixture of 2 (1.65 g, 2.8 mmol), 3 (1.05 g,
5.8 mmol), bis(tri-tert-butylphosphine)palladium(0) (Pd(t-Bu3P)2, 0.03 g,
0.06 mmol), and potassium carbonate (2.28 g, 16.5 mmol) in dry toluene
(30 mL) was refluxed for 48 h. After cooling to room temperature, the
reaction mixture was filtered through a Celite pad, and the filtrate was
concentrated under reduced pressure. The crude product was purified by
column chromatography on silica gel (hexane/dichloromethane = 4:1, v/v)
3. Conclusion
A new series of full-color TADF emitters was designed and
synthesized by combining a phthalonitrile or dicyanopyrazine
acceptor core with various donor units. The photophysical
properties of these materials could be tuned by a rational
combination of the donor and acceptor constituents. The
TADF emissions of the doped films depend on their HOMO–
LUMO energy gap, and the emission occurred at wavelengths
ranging from the blue (450 nm) to red (610 nm) region, span-
ning the entire visible region. Moreover, their small ΔEST
originating from the angular-linked donor–acceptor struc-
tures induces efficient TADF characteristics. The OLEDs
based on these emitters exhibited a high maximum external
EL quantum efficiencies of up to 18.9% and an extremely
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Adv. Funct. Mater. 2016, 26, 1813–1821
2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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to afford Cz-VPN as a white solid (yield = 1.85 g, 84%). 1H NMR (500 MHz,
CDCl3, δ): 8.06 (t, J = 8.0 Hz, 2H), 7.97 (d, J = 7.4 Hz, 2H), 7.39–7.26
(m, 12H), 7.19–7.01 (m, 12H), 6.89 (t, J = 7.0 Hz, 2H), 6.73–6.61 (m,
2H), 1.70 (s, 6H); 13C NMR (125 MHz, CDCl3, δ): 146.54, 139.44, 138.95,
137.26, 136.02, 131.32, 129.81, 128.90, 128.86, 128.73, 128.56, 126.35,
123.99, 121.09, 120.06, 120.03, 118.16, 115.19, 109.60, 109.11, 19.92;
MS (MALDI-TOF) m/z: [M]+ calcd 790.31; found, 790.56. Anal. calcd for
C58H38N4: C 88.07, H 4.84, N 7.08; found: C 88.16, H 4.74, N 7.07.
Synthesis of Ac-VPN: A mixture of 4,5-dichlorophthalonitrile (0.99 g,
<0.3 nm s–1. A cathode aluminum layer was then deposited through
a shadow mask. The layer thickness and the deposition rate were
monitored in situ during deposition by an oscillating quartz thickness
monitor. The current density–voltage–luminance characteristics of the
devices were measured using an E5273A semiconductor parameter
analyzer (Agilent) and a 1930-C optical power meter (Newport). The EL
spectra were recorded using an SD2000 multi-channel analyzer (Ocean
Optics).
5.0 mmol),
5 (4.17 g, 10.1 mmol), tetrakis(triphenylphosphine)
palladium(0) (Pd(PPh3)4, 0.06 g 0.05 mmol), potassium iodide (1.66 g,
10.0 mmol), and potassium phosphate (4.31 g, 20.0 mmol) in dry
1,4-dioxane (10 mL) was refluxed for 48 h. After cooling to room
temperature, the reaction mixture was added into water and was
extracted with dichloromethane. The combined organic layers were
washed with water and dried over anhydrous magnesium sulfate. After
filtration and evaporation, the crude product was purified by column
chromatography on silica gel (hexane/dichloromethane = 4:1, v/v)
to give Ac-VPN as a yellow solid (yield = 1.18 g, 34%). 1H NMR (500
MHz, CDCl3, δ): 7.99 (s, 1H), 7.95 (s, 1H), 7.69 (d, J = 8.5 Hz, 4H), 7.49
(td, J = 7.5 Hz, 1.5 Hz, 8H), 7.03–6.94 (m, 8H), 6.30 (dd, J = 8.0 Hz,
1.5 Hz, 4H), 1.53 (s, 12H); 13C NMR (125 MHz, CDCl3, δ): 145.39,
142.84, 140.58, 138.43, 135.87, 135.25, 135.09, 131.81, 131.55, 130.41,
126.42, 125.39, 121.02, 115.78, 114.54, 114.01, 36.04, 31.15; MS
(MALDI-TOF) m/z: [M+H]+ calcd 695.32; found, 695.71. Anal. calcd for
C50H38N4: C 86.42, H 5.51, N 8.06; found: C 86.32, H 5.48, N 7.99.
Synthesis of Px-VPN: Px-VPN was synthesized according to the same
procedure as described above for the synthesis of Ac-VPN, except that
7 (3.91 g, 10.1 mmol) was used as the reactant instead of 5, yielding an
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was partially supported by the ACCEL project from Japan
Science and Technology Agency (JST), and Grants-in-Aid for Scientific
Research on Innovative Areas “3D Active-Site Science” (Grant No.
15H01049), Young Scientists (A) (Grant No. 25708032), and Challenging
Exploratory Research (Grant No. 26620168) from Japan Society for the
Promotion of Science (JSPS), the Cooperative Research Program of
“Network Joint Research Center for Materials and Devices,” the Casio
Science Promotion Foundation, the Ogasawara Foundation for the
Promotion of Science and Engineering, and the Kurata Memorial Hitachi
Science and Technology Foundation.
1
orange solid (yield = 0.98 g, 31%). H NMR (500 MHz, CDCl3, δ): 8.01
(s, 2H), 7.39 (d, J = 8.0 Hz, 4H), 7.33 (d, J = 8.0 Hz, 4H), 6.69 (d, J = 7.8 Hz,
4H), 6.65–6.54 (m, 4H), 6.48 (t, J = 7.4 Hz, 4H), 5.82 (d, J = 7.4 Hz,
4H); 13C NMR (125 MHz, CDCl3, δ): 145.15, 143.92, 139.87, 137.60, 135.17,
133.77, 132.13, 131.32, 123.41, 121.76, 115.69, 115.24, 115.06, 112.93;
MS (MALDI-TOF) m/z: [M]+ calcd 642.21; found, 642.35. Anal. calcd for
C44H26N4O2: C 82.23, H 4.08, N 8.72; found: C 82.23, H 3.99, N 8.77.
Received: November 28, 2015
Revised: January 6, 2016
Published online: February 12, 2016
Synthesis of Ac-CNP:
A mixture of 8 (1.01 g, 2.3 mmol),
9,9-dimethylacridan (1.05 g, 5.0 mmol), bis(tri-tert-butylphosphine)
palladium(0) (Pd(t-Bu3P)2, 0.12 g, 0.23 mmol), and potassium carbonate
(0.95 g, 6.9 mmol) in dry toluene (30 mL) was refluxed for 72 h. After
cooling to room temperature, the reaction mixture was filtered through
a Celite pad, and the filtrate was concentrated under reduced pressure.
The crude product was purified by column chromatography on silica gel
(hexane/ethyl acetate = 5:1, v/v) to afford Ac-CNP as an orange solid
(yield = 0.64 g, 40%). 1H NMR (500 MHz, CDCl3, δ): 7.86 (d, J = 8.5 Hz,
4H), 7.47–7.43 (m, 8H), 6.97–6.90 (m, 8H), 6.35 (dd, J = 7.8 Hz, 1.5 Hz,
4H), 1.66 (s, 12H); 13C NMR (125 MHz, CDCl3, δ): 154.77, 144.69,
140.35, 134.21, 132.33, 131.42, 130.84, 130.16, 126.47, 125.28, 121.48,
114.64, 113.04, 36.18, 30.66; MS (MALDI-TOF) m/z: [M]+ calcd 696.30;
found, 696.49. Anal. calcd for C48H36N6: C 82.73, H 5.21, N 12.06; found:
C 82.21, H 5.16, N 11.85.
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1
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Adv. Funct. Mater. 2016, 26, 1813–1821
2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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