Controlled emission colors and singlet-triplet energy gaps of dihydrophenazine-based thermally activated delayed fluorescence emitters

Article abstract of DOI:10.1039/c4tc02530j

We have developed thermally activated delayed fluorescence (TADF) emitters containing 5,10-dihydrophenazine as an electron donor and various electron-acceptor units. The TADF emitters exhibit wide ranges of emission colors from green to orange, singlet-triplet energy gaps ΔE_ST of ～0-0.19 eV, and delayed fluorescence lifetimes τ_d of 0.1-50 μs. An organic light-emitting diode containing one of the TADF emitters exhibits a maximum external quantum efficiency (EQE) of 12%, which is higher than those obtained with conventional fluorescent emitters. Time-resolved photoluminescence measurements of the compounds in a host matrix reveal that TADF makes a large contribution to the EQE of the devices. Our findings provide guidelines for modulating ΔE_ST and τ_d of TADF emitters.

Full text of DOI:10.1039/c4tc02530j

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ARTICLE
Controlled emission color and singlet-triplet energy
gap of dihydrophenazine-based thermally activated
delayed fluorescence emitters†
Cite this: DOI: 10.1039/x0xx00000x
a
b,c
b
b,d
Jiyoung Lee, Katsuyuki Shizu, Hiroyuki Tanaka, Hajime Nakanotani,
a,b,e
c
b,e
Received 00th January 2012,
Accepted 00th January 2012
Takuma Yasuda,
Hironori Kaji and Chihaya Adachi*
DOI: 10.1039/x0xx00000x
We develop thermally activated delayed fluorescence (TADF) emitters containing 5,10ꢀ
dihydrophenazine as an electron donor and various electronꢀacceptor units. The TADF
emitters exhibit wide ranges of emission color from green to orange, singletꢀtriplet energy
gap ꢁEST of ~0–0.19 eV, and delayed fluorescence lifetime τd of 0.1–50 ꢂs. An organic
lightꢀemitting diode containing one of the TADF emitters exhibits a maximum external
quantum efficiency (EQE) of 12%, which is higher than those obtained with conventional
fluorescent emitters. Timeꢀresolved photoluminescence measurements of the compounds in
a host matrix reveal that TADF makes a large contribution to the EQE of the devices. Our
findings provide guidelines for modulating ꢁEST and τd of TADF emitters.
www.rsc.org/
1
1
yl)phenyl)ꢀ1,3,5ꢀtriazine (triꢀPXZꢀTRZ), have ꢁEST values of
0 eV.
By modulating
Introduction
~
ꢁ
EST
,
we can control the delayed
) of TADF compounds. When ꢁEST is
small, the RISC in a TADF compound is expected to be fast,
During the last two decades, organic lightꢀemitting diodes
τ
fluorescence lifetime ( d
(
OLEDs) have attracted considerable attention because of their
1
,2
potential application for flatꢀpanel displays. Small OLED
displays have been used in mobile phones, and are already
commercially available. Furthermore, OLEDs have recently
drawn particular interest because they make it possible to
realize flexible flatꢀpanel displays, leading to nextꢀgeneration
solidꢀstate lighting sources. Materials that exhibit thermally
activated delayed fluorescence (TADF) can be used to fabricate
highly efficient OLEDs without using rare metals. A TADF
emitter containing only carbon, hydrogen, and nitrogen atoms6
achieved an external quantum efficiency (EQE) of nearly 20%,
suggesting that TADFꢀbased OLEDs are viable alternatives to
conventional phosphorescent OLEDs. TADF compounds
possessing diverse molecular structures with various electron
leading to a short T exciton lifetime (τT). When τT is short,
1
exciton annihilation processes such as tripletꢀtriplet (TꢀT) and
singletꢀtriplet (SꢀT) annihilations are expected to be suppressed.
Conversely, when ꢁEST is large, these exciton annihilations
have time to actively occur because of the long τT, resulting in a
large rollꢀoff of EQE. For phosphorescent OLEDs, it is well
known that TꢀT annihilation is a major factor of their rollꢀoff
behavior. By using a TADF compound with a small ꢁEST as
an emitter, we can develop highly efficient OLEDs with small
rollꢀoff. In fact, OLEDs containing PXZꢀTRZ, bisꢀPXZꢀTRZ,
or triꢀPXZꢀTRZ as an emitter exhibit small rollꢀoff, indicating a
3
ꢀ5
2
7
2
8
1
0,11
close relationship between ꢁEST and EQE rollꢀoff.
7
ꢀ25
In this study, we develop TADF compounds with wide
ranges of emission color (green to orange), ꢁEST (~0 to 0.19
eV), and τd (0.14 to 53 ꢂs). The TADF compounds are
acceptorꢀdonorꢀacceptorꢀtype
structures using 5,10ꢀdihydrophenazine as the electron donor
and various electron acceptor units (Fig. 1): 5,10ꢀbis(4ꢀ(1ꢀ
phenylꢀ1 imidazolꢀ2ꢀyl)phenyl)ꢀ5,10ꢀ
dihydrophenazine (DHPZꢀ2BI); 4,4'ꢀ(phenazineꢀ5,10ꢀ
diyl)dibenzonitrile (DHPZꢀ2BN); 5,10ꢀbis (4ꢀ(benzo[ ]thiazolꢀ
ꢀyl)phenyl)ꢀ5,10ꢀdihydrophenazine (DHPZꢀ2BTZ); 5,10ꢀ
donors and acceptors have been developed.
TADF involves reverse intersystem crossing (RISC) from
the lowest triplet excited state (T ) to the lowest singlet excited
1
composed
of
symmetric
state (S ). The rate of RISC is inversely proportional to the
1
2
6
energy gap between S1 and T , ꢁEST
photophysical properties of TADF compounds depend on ꢁEST
For TADF compounds, ꢁEST is related to the degree of spatial
overlap between the highest occupied molecular orbital
.
Therefore, the
1
.
H
ꢀbenzo[
d
]
d
(
HOMO) and lowest unoccupied molecular orbital (LUMO).
2
Thus, by spatially separating HOMO and LUMO, ꢁEST can be
decreased. TADF compounds with wellꢀseparated HOMO and
LUMO, for example, 5,12ꢀdihydroꢀ12ꢀ(4,6ꢀdiphenylꢀ1,3,5ꢀ
bis(4ꢀ(4,6ꢀdiphenylꢀ1,3,5ꢀtriazinꢀ2ꢀyl)
phenyl)ꢀ5,10ꢀ
dihydrophenazine (DHPZꢀ2TRZ). The LUMO energy level and
distribution of the TADF compounds depend largely on the
fragment LUMO of their acceptor moieties. Emission color is
closely related to the LUMO energy level, while ꢁEST and τd
are related to the LUMO distribution. By using
9
triazinꢀ2ꢀyl)ꢀ5ꢀphenylindolo[3,2a]carbazole (PICꢀTRZꢀ2), 2ꢀ
1
0
phenoxazineꢀ4,6ꢀdiphenylꢀ1,3,5ꢀtriazine (PXZꢀTRZ),
bis(4ꢀ(10 ꢀphenoxazinꢀ10 ꢀyl)phenyl)ꢀ6ꢀphenylꢀ1,3,5ꢀtriazine
bisꢀPXZꢀTRZ), and 2,4,6ꢀtri(4ꢀ(10 ꢀphenoxazinꢀ10Hꢀ
2,4ꢀ
H
H
(
H
This journal is © The Royal Society of Chemistry 2013
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Fig. 1 Chemical structures of TADF emitters DHPZꢀ2BI, DHPZꢀ2BN,
DHPZꢀ2BTZ, and DHPZꢀ2TRZ.
dihydrophenazine as an electron donor, we can obtain TADF Fig. 2 Photoluminescence spectra measured for DHPZꢀ2BI, DHPZꢀ2BN,
DHPZꢀ2BTZ, and DHPZꢀ2TRZ in toluene solution.
compounds with various photophysical properties simply by
changing the acceptor moieties.
Benzimidazole, benzonitrile, benzothiazole, and triazine are
wellꢀknown electronꢀaccepting units. Benzimidazole is one of
Results and discussion
the most widely used acceptors in OLEDs because of its good
By introducing four different acceptors onto the N atoms of a
dihydrophenazine donor unit, we obtained TADF compounds
exhibiting green to orange emission (Fig. 1). Synthesis and
characterization data for these compounds are available in ESI.
The photophysical properties of DHPZꢀ2BI, DHPZꢀ2BN,
DHPZꢀ2BTZ, and DHPZꢀ2TRZ in toluene solution suggest that
they exhibit TADF. Figure 2 shows PL spectra for the four
compounds in toluene solution measured at room temperature
using a spectrofluorometer (Fluoromaxꢀ4, Horiba Jobin Yvon,
USA). DHPZꢀ2BI and DHPZꢀ2BN showed green emission,
while DHPZꢀ2BTZ and DHPZꢀ2TRZ exhibited yellow and
orange emission, respectively. The broad emission bands
2
9ꢀ31
electronꢀaccepting ability.
For example, 1,3,5ꢀtris(Nꢀ
phenylbenzimidazoleꢀ2ꢀyl)benzene (TPBi) containing three
benzimidazole moieties has been used as an effective electronꢀ
2
9
transporting and host material in OLEDs.
Compounds
containing benzonitrile acceptor groups can form twisted
intramolecular chargeꢀtransfer (CT) excited states. Derivatives
of 4ꢀdimethylaminobenzonitrile exhibit unique photophysical
properties such as dual fluorescence, and have attracted a great
3
2
deal of interest in photochemistry. Benzothiazole also has a
good electronꢀaccepting ability and has been used to develop
3
3ꢀ
electroluminescent (EL) and electronꢀtransporting materials.
3
5
Green fluorescent compounds containing benzothiazole and
coumarin moieties have been used as EL materials for
Triazineꢀbased compounds have been used as
electronꢀtransporting materials and electron transportꢀtype
host materials because of their good electronꢀaccepting
ability. We have developed various triazineꢀbased TADF
compounds, and shown that triazine is an effective electron
originate from the CT character of S of the four compounds, as
1
discussed below. We previously reported that PXZꢀTRZ
containing a phenoxazine donor and triazine acceptor shows
33,34
OLEDs.
3
6
1
0
green emission. Comparing DHPZꢀ2TRZ with PXZꢀTRZ
suggests that replacing the phenoxazine moiety in a TADF
compound with dihydrophenazine causes upward shift of
HOMO level and red shift of emission color. Interestingly, in
addition to the CT emission peak at 650 nm, DHPZꢀ2TRZ
shows a weak emission peak at around 450 nm. Such dual
emission has been observed for molecules with excited states of
3
7
8
ꢀ12
acceptor in TADF compounds.
In this study, we synthesize novel TADF materials
combining a dihydrophenazine donor with various electronꢀ
acceptor moieties. Dihydrophenazine has good electronꢀ
donating ability, and has been used as an electron donor in
3
2
twisted intramolecular charge transfer character. We assigned
this additional peak to a locally excited state of the
3
8
organic CT crystals. Dihydrophenazine has two NH groups,
and one or two electron acceptors can be introduced into it, so
dihydrophenazine
moiety,
because
dihydrophenazine
3
9
derivatives show emission peaks at 430ꢀ460 nm. For the
compounds other than DHPZꢀ2TRZ, since the intensity of the
emission from the CT excited state is strong compared with that
from the locally excited state, the emission peak at around 450
nm does not clearly appear in the PL spectra. The low PLQY of
DHPZꢀ2TRZ can be explained by the energyꢀgap law, which
states that nonꢀradiative decay rate increases with decreasing
the energy difference between S and S states.
donorꢀacceptorꢀtype
and
acceptorꢀdonorꢀacceptorꢀtype
structures are possible. In a previous study, we reported that
TADF compounds with donorꢀacceptorꢀdonorꢀtype structure
showed larger photoluminescence (PL) efficiency than those
1
3
with just donorꢀacceptor, indicating that a acceptorꢀdonorꢀ
acceptorꢀtype structure is a promising candidate for TADF
emitters. Therefore, we prepare emitters with acceptorꢀdonorꢀ
acceptorꢀtype structure rather than acceptorꢀdonor in this work
1
0
DHPZꢀ2BI, DHPZꢀ2BN, and DHPZꢀ2BTZ were confirmed
to emit delayed fluorescence in toluene solution after excluding
oxygen (which quenches triplet states) by bubbling nitrogen
through the solution for 10 min. For the above three
compounds, after deoxygenation, delayed components with
long lifetimes were observed in their timeꢀresolved PL decays
measured using a fluorescence lifetime spectrometer (C11367
Quantaurusꢀtau, Hamamatsu Photonics, Japan). Although a
delayed component for DHPZꢀ2TRZ was not observed clearly
in toluene solution, as stated below, DHPZꢀ2TRZ was
(
2
Fig. 1). We evaluate the performance of DHPZꢀ2BI, DHPZꢀ
BN, DHPZꢀ2BTZ, and DHPZꢀ2TRZ as emitter dopants for
OLEDs. DHPZꢀ2BI and DHPZꢀ2BNꢀbased OLEDs show green
emission, while DHPZꢀ2BTZ and DHPZꢀ2TRZꢀbased ones
show yellow and orange emission, respectively. We also
confirm TADF makes an important contribution to the EL
efficiency of these OLEDs.
2
| J. Name., 2012, 00, 1-3
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Fig. 4 Experimental HOMO and LUMO energy levels of neat DHPZꢀ2BI,
DHPZꢀ2BN, DHPZꢀ2BTZ, and DHPZꢀ2TRZ films.
Fig. 3 Highest occupied and lowest unoccupied natural transition orbitals for
of DHPZꢀ2BI, DHPZꢀ2BN, DHPZꢀ2BTZ, and DHPZꢀ2TRZ calculated at
S
1
the TDꢀCAMꢀB3LYP/ccꢀpVDZ//CAMꢀB3LYP/ccꢀpVDZ level of theory.
confirmed to emit TADF in a solidꢀstate host matrix. Prompt
fluorescence lifetime (τp), τd, and PL quantum yields (PLQYs)
for the compounds in toluene solution are listed in Table S1 in
ESI. PLQYs were measured using an absolute PLQY
Fig. 5 Photoluminescence spectra of 6 wt% DHPZꢀ2BI:mꢀCBP, DHPZꢀ
2
BN:mꢀCBP, PDHZꢀ2BTZ:mꢀCBP, and DHPZꢀ2TRZ:mꢀCBP films.
spectrometer (C11347 QuantaurusꢀQY, Hamamatsu Photonics, excitation, an electron transfers from donor to acceptor
Japan). PLQYs were higher in the absence of oxygen than in moieties. The S states thus have CT character.
1
air, suggesting that in air, triplet states were quenched by
The difference in the emission wavelengths of our
oxygen, and RISC from triplet to singlet states was suppressed. compounds originates from the variation of their LUMO energy
Of the four compounds, DHPZꢀ2BI showed the highest PLQY levels. Figure 4 shows the energy levels of the HOMO and
of 24.1%.
LUMO of neat films of DHPZꢀ2BI, DHPZꢀ2BTZ, DHPZꢀ
Quantum chemical calculations explain the reason why S1 2TRZ, and DHPZꢀ2BN fabricated by vacuum deposition. The
of DHPZꢀ2BI, DHPZꢀ2BN, DHPZꢀ2BTZ, and DHPZꢀ2TRZ HOMO energy levels were measured using a photoelectron
has CT character. Figure 3 shows the highest occupied and yield spectrometer (ACꢀ3, Riken Keiki, Japan). The HOMOꢀ
4
0
lowest unoccupied natural transition orbitals (NTOs) for S of LUMO energy gaps were determined from UVꢀVis absorption
1
the compounds. S geometries were optimized at the CAMꢀ spectra of the neat films measured using
B3LYP/ccꢀpVDZ level of theory.
a
UVꢀVis
S1 and NTOs were spectrophotometer (LAMBDA950, PerkinElmer, USA). The
calculated with a timeꢀdependent density functional theory LUMO energy levels were estimated from the experimental
0
4
1,42
4
3,44
(
(
TDꢀDFT) method
using the optimized S0 geometries HOMO energy levels and HOMOꢀLUMO energy gaps. The
denoted as TDꢀCAMꢀB3LYP/ccꢀpVDZ//CAMꢀB3LYP/ccꢀ HOMO energy levels are almost the same, while the LUMO
pVDZ). All calculations were performed using Gaussian 09 energy levels vary depending on the acceptor moieties (Fig. 4).
4
5
program package. By taking solvent effects into account The HOMO energy levels do not depend on the acceptor
4
6ꢀ48
within the polarizable continuum model,
this method moieties, suggesting that the HOMOs originate predominantly
reasonably reproduces absorption and emission wavelengths of from the dihydrophenazine moiety. The LUMO energy level of
the compounds in toluene solution (Table S2 in ESI). The the DHPZꢀ2BN neat film was lower than that expected from its
highest occupied NTOs are largely localized on the emission spectrum in toluene solution. In toluene solution,
dihydrophenazine moieties, while the lowest unoccupied NTOs because DHPZꢀ2BN and DHPZꢀ2BI exhibit similar emission
are localized on the acceptor moieties, resulting in good spatial wavelengths, their neat films are expected to have comparable
separation of the two NTOs. This separation results from the LUMO energy levels. However, the LUMO energy level of the
strong electronꢀdonating and ꢀaccepting abilities of the donor neat DHPZꢀ2BN film is 0.17 eV lower than that of the neat
and acceptor moieties, respectively. In addition, large twist DHPZꢀ2BI film. This indicates that interaction between the
angles of nearly 90° between the units also induce the LUMOs of neighboring molecules is stronger for DHPZꢀ2BN
separation of the NTOs. Because the HOMOꢀLUMO transition than for DHPZꢀ2BI in their neat films.
dominates S0ꢀS1 excitation for these compounds, the
To investigate the PL characteristics of DHPZꢀ2BI, DHPZꢀ
distribution patterns of the highest occupied and lowest 2BN, DHPZꢀ2BTZ, and DHPZꢀ2TRZ in a host layer, 6 wt%
unoccupied NTOs reflect those of the HOMO and LUMO, DHPZꢀ2BIꢀ, DHPZꢀ2BNꢀ, DHPZꢀ2BTZꢀ, and DHPZꢀ2TRZꢀ
respectively. The theoretical calculations suggest that in S ꢀS
doped 9,9′ꢀbiphenylꢀ3,3′ꢀdiylꢀbisꢀ9Hꢀcarbazole (mꢀCBP) films
0
1
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Fig. 7 External quantum efficiency and current density characteristics for
OLEDs containing DHPZꢀ2BI, DHPZꢀ2BN, DHPZꢀ2BTZ, and DHPZꢀ2TRZ
as an emitter. The insets show EL emission from the green, yellow, and
orange OLEDs.
RISC. τp and τd at 300 K were approximately 4 ns and 50 ꢂs,
respectively. Figure 6(b) depicts fluorescence and
phosphorescence spectra of the doped film. ꢁEST was
determined to be 0.19 eV from the onset of the spectra.
Transient PL decays for DHPZꢀ2BN, DHPZꢀ2BTZ and DHPZꢀ
2
TRZ are shown in Fig. S1ꢀ3 in ESI. These TADF compounds
showed PL decays similar to that observed for DHPZꢀ2BI.
The photophysical properties of the doped films are listed in
Table 1. ꢁEST decreases in the following order: DHPZꢀ
2
BI>DHPZꢀ2BN>DHPZꢀ2BTZ, which is consistent with the
calculated results (Table S3 in ESI). For DHPZꢀ2BTZ, ꢁEST is
nearly 0 eV. We could not determine ꢁEST for DHPZꢀ2TRZ
because the phosphorescence spectrum of the 6 wt% DHPZꢀ
Fig. 6 (a) Transient photoluminescence decay curves for a 6 wt% DHPZꢀ 2TRZ:mꢀCBP doped film was not clearly observed. τ decreases
d
2
BI:mꢀCBP film measured at temperatures of 50 to 300 K. (b) Fluorescence with ꢁEST, suggesting that small ꢁEST leads to a higher rate of
and phosphorescence spectra of a 6 wt% PZꢀ2BI:mꢀCBP film measured at 4
K. Green and black lines show fluorescence and phosphorescence spectra,
respectively.
T →S RISC and shorter τd. In contrast, τp is unrelated to ꢁEST
.
1
1
DHPZꢀ2BI exhibited the highest PLQY (ΦPL) of 67.6%, and
contributions from the prompt fluorescence (Φp) and delayed
were prepared by vacuum codeposition. Figure 5 shows PL
spectra of the doped films measured at 300 K using the
spectrofluorometer. The DHPZꢀ2BIꢀ and DHPZꢀ2BNꢀdoped
films showed green emission with peak maxima at 537 nm
fluorescence (Φd) to ΦPL are almost equal (Φp≈Φd). Φd
decreases with increasing emission wavelength (decreasing S0ꢀ
S energy gap), suggesting that when the S ꢀS energy is small,
1
0
1
T1→S0 nonradiative decay is fast, leading to poor RISC
efficiency. For DHPZꢀ2TRZ, Φd is quite low even though ꢁEST
(
2
2.31 eV) and 541 nm (2.29 eV), respectively, while the DHPZꢀ
BTZꢀ and DHPZꢀ2TRZꢀdoped films showed yellow and
orange emissions with peak maxima at 577 nm (2.15 eV) and
98 nm (2.07 eV), respectively. Because the T energy level of
is sufficiently small to allow RISC. In addition to the low Φd
,
Φp for DHPZꢀ2TRZ is much lower than those for the other
compounds. These observations indicate that for orange to red
TADF compounds, excessive separation of the spatial overlap
between HOMO and LUMO suppresses S →S radiative decay
5
1
m
ꢀCBP is 2.84 eV, T and S excitons of the TADF compounds
1
1
4
9
are well confined in the
m
ꢀCBP layers.
1
0
Transient PL decays of the doped films show that DHPZꢀ
BI, DHPZꢀ2BN, DHPZꢀ2BTZ, and DHPZꢀ2TRZ emit TADF
(
decreasing Φp), leading to low ΦPL. Increasing the spatial
2
overlap and promoting S →S radiative decay is necessary to
1
0
in the doped films. The transient PL decays were measured
using a streak camera (C4334, Hamamatsu Photonics, Japan).
Because the decay curves show similar temperature
dependence, we present only the results for DHPZꢀ2BI. Figure
realize TADF compounds that exhibit high ΦPL and orange to
red emission.
OLEDs containing DHPZꢀ2BI, DHPZꢀ2BN, DHPZꢀ2BTZ,
or DHPZꢀ2TRZ as an emitter showed green to orange emission.
We fabricated OLEDs with a structure of indium tin oxide
6
2
(a) shows the transient PL decay curve of the 6 wt% DHPZꢀ
BI: ꢀCBP doped film. The intense emission and long tail are
m
(
ITO)/4,4′ꢀbis[
Nꢀ(1ꢀnaphthyl)ꢀ
Nꢀphenyl]biphenyl diamine (αꢀ
attributed to prompt and delayed components, respectively. The
intensity of the delayed component increases with temperature
from 50 to 200 K, suggesting that the delayed component
originates from TADF. In contrast, the intensity of the delayed
component slightly decreases with increasing temperature over
NPD) (35 nm)/6 wt% emitter:
m
ꢀCBP (15 nm)/TPBi (65
nm)/LiF (0.8 nm)/Al (100 nm) by vacuum deposition. The
maximum EQEs of the DHPZꢀ2BIꢀ, DHPZꢀ2BNꢀ, DHPZꢀ
2
BTZꢀ, and DHPZꢀ2TRZꢀbased devices were 12%, 6%, 5%,
and 1%, respectively (Fig. 7). The EQE of DHPZꢀ2BIꢀbased
device is much higher than those of conventional fluorescent
OLEDs. The DHPZꢀ2BIꢀ and DHPZꢀ2BNꢀbased OLEDs
2
00 K, suggesting that over 200 K, a slight increase of T →S0
1
nonradiative decay becomes a competitive process for T →S
1
1
4
| J. Name., 2012, 00, 1-3
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Table 1 Photoluminescence characteristics of 6 wt% DHPZꢀ2BI:mꢀCBP, DHPZꢀ2BN:mꢀCBP, DHPZꢀ2BTZ:mꢀCBP, and DHPZꢀ2TRZ:mꢀCBP doped films.
Subscripts p and d denote prompt and delayed components, respectively. τ, Φ, and η are PL lifetime, PLQY, and theoretical maximum EQE, respectively. ΦPL
and ηPL are total PLQY and EQE, respectively: ΦPL=Φ
p
d p d
+Φ ; ηEQE=η +η . γηout is assumed to be 0.2−0.3.
η
Calc.
11ꢀ17
5ꢀ8
EQE (%)
Compound
ꢁEST (eV)
τ
p
(ns)
τ
d
(ꢀs)
Φ
PL (%)
Φ
p
(%)
Φ
d
(%)
η
p
(%)
d
η (%)
Exp.
DHPZꢀ2BI
0.19
0.10
~0
4
7
50
7
67.6
35.2
33.0
6.6
34.0
16.8
20.6
5.7
33.6
18.4
12.4
0.9
1.86
0.99
1.25
0.43
10.20
4.98
3.60
0.27
12
6
DHPZꢀ2BN
DHPZꢀ2BTZ
DHPZꢀ2TRZ
12
6
1
4ꢀ6
5
−
0.1
0.5ꢀ0.7
1
showed green emission with EL peak maxima at 542 and 546 the Academic Center for Computing and Media Studies
nm, respectively, while the DHPZꢀ2BTZꢀ and DHPZꢀ2TRZꢀ (ACCMS), Kyoto University.
based ones exhibited yellow and orange emission with peak
maxima at 601 and 617 nm, respectively.
TADF makes a substantial contribution to the EL efficiency
of the OLEDs. The theoretical maximum EQE (ηEQE) was
Notes and references
Department of Automotive Science, Graduate School of Integrated
a
Frontier Sciences, Kyushu University, 744 Motooka, Nishi, Fukuoka 819ꢀ
calculated using the following equation:
0
b
395, Japan.


Φd
ηEQE = 0.25Φ +
{
0.75+ 0.25
(
1−Φp )}
γηout
,
(1)
Center for Organic Photonics and Electronics Research (OPERA),
Kyushu University, 744 Motooka, Nishi, Fukuoka 819ꢀ0395, Japan. Eꢀ
mail: adachi@opera.kyushuꢀu.ac.jp

p

1
− Φp


where
γ is the charge recombination factor and ηout is the outꢀ
8
coupling constant. Assuming that
DHPZꢀ2BIꢀ, DHPZꢀ2BNꢀ, and DHPZꢀ2BTZꢀbased OLEDs, the
γ
ηout is 0.2–0.3, for the
c
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611ꢀ0011,
Japan
experimental ηEQE values are within the range predicted by Eq.
d
1
3
Innovative Organic Device Laboratory, Institute of Systems, Information
(
1). Prompt and delayed components of ηEQE are defined as:
Technologies and NanoꢀTechnologies (ISIT), 744, Motooka, Nishi,
Fukuoka 819ꢀ0395, Japan.
ηp = 0.25Φpγηout
,
(2)
(3)
Φd
− Φp
ηd
=
{
0.75 + 0.25
(
1− Φp )}
γηout
.
e
International Institute for Carbon Neutral Energy Research (WPIꢀ
1
I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819ꢀ0395,
Japan.
For a conventional fluorescent molecule with Φ =0, ηEQE is
equal to ηp because ηd is zero. For the DHPZꢀ2BIꢀ, DHPZꢀ
d
†
Electronic Supplementary Information (ESI) available: synthesis,
2
BNꢀ, and DHPZꢀ2TRZꢀbased OLEDs, ηd is larger than ηp,
indicating that TADF contributes considerably to ηEQE
.
ηd is characterization data, and quantum chemical calculations of DHPZꢀ2BI,
especially large for the DHPZꢀ2BIꢀbased device: it is higher DHPZꢀ2BN, DHPZꢀ2BTZ, and DHPZꢀ2TRZ. Timeꢀresolved
than 10%, suggesting that the contribution from TADF alone
exceeds the EQEs of fluorescent OLEDs. For the DHPZꢀ2BNꢀ
and DHPZꢀ2TRZꢀbased OLEDs, although ΦPL is small,
because ηd is large, ηEQE is comparable to those obtained with
conventional fluorescent emitters with ΦPL≈100%. For the
DHPZꢀ2TRZꢀbased OLED, because T →S nonradiative decay
photoluminescence measurements of 6 wt% DHPZꢀ2BN:
BTZ: ꢀCBP, and DHPZꢀ2TRZ: ꢀCBP films. Fluorescence and
phosphorescence spectra of the 6 wt% DHPZꢀ2BN: ꢀCBP and DHPZꢀ
BTZ: ꢀCBP films. See DOI: 10.1039/b000000x/
mꢀCBP, DHPZꢀ
2
m
m
m
2
m
1
0
is large and ηd is small, ηEQE is less than 1%. However, ηd still 1. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913ꢀ915.
makes a nonꢀnegligible contribution to ηEQE
.
2. C. W. Tang, S. A. VanSlyke and C. H. Chen, J. Appl. Phys., 1989, 65
610ꢀ3616.
3. S. R. Forrest, Nature, 2004, 428, 911ꢀ918.
. S. Reineke, M. Thomschke, B. Lüssem and K. Leo, Rev. Mod. Phys.
013, 85, 1245ꢀ1293.
the acceptor, we controlled the emission color from green to 5. H. Sasabe and J. Kido, J. Mater. Chem. C, 2013,
orange, singletꢀtriplet energy gap, and lifetime of TADF from 6. H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature
,
,
,
3
Conclusions
4
We
developed
TADF
compounds
containing
a
2
dihydrophenazine donor and various acceptors. By changing
1
, 1699ꢀ1707.
the compounds. An OLED using DHPZꢀ2BI as a green dopant
exhibited a maximum EQE of 12%, which is higher than those
obtained with conventional fluorescent emitters.
2
012, 492, 234ꢀ238.
7
8
9
. A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y. Kato and C.
Adachi, Adv. Mater., 2009, 21, 4802ꢀ4806.
. A. Endo, K. Sato, K. Yoshimura, T. Kai, A. Kawada, H. Miyazaki and
C. Adachi, Appl. Phys. Lett., 2011, 98, 083302.
Acknowledgements
This research was funded by the Japan Society for the
Promotion of Science (JSPS) through the “Funding Program for
WorldꢀLeading Innovative R&D on Science and Technology
. K. Sato, K. Shizu, K. Yoshimura, A. Kawada, H. Miyazaki and C.
Adachi, Phys. Rev. Lett., 2013, 110, 247401.
(
FIRST Program)” and JST, ERATO, Adachi Molecular 10.H. Tanaka, K. Shizu, H. Miyazaki and C. Adachi, Chem. Commun.
,
Exciton Engineering Project. Part of the computation was
carried out using the computer facilities at the Research
Institute for Information Technology, Kyushu University, and
2
012, 48, 11392ꢀ11394.
1.H. Tanaka, K. Shizu, H. Nakanotani and C. Adachi, Chem. Mater.
013, 25, 3766ꢀ3771.
1
,
2
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Journal of Materials Chemistry C
Page 6 of 7
View Article Online
ARTICLE
DOI: 10.1039/C4TC02530J
Journal Name
1
1
1
2.S. Y. Lee, T. Yasuda, H. Nomura and C. Adachi, Appl. Phys. Lett.
,
39.J. V. Morris, U. Brühlmann, O. Serafimov and J. R. Huber, Ber. Buns.
Gesell., 1974, 78, 1348ꢀ1353.
2
012, 101, 093306ꢀ093304.
3.J. Lee, K. Shizu, H. Tanaka, H. Nomura, T. Yasuda and C. Adachi, J. 40.R. Martin, J. Chem. Phys., 2003, 118, 4775ꢀ4777.
Mater. Chem. C, 2013,
1
, 4599ꢀ4604.
41.T. Yanai, D. P. Tew and N. C. Handy, Chem. Phys. Lett., 2004, 393,
4.T. Serevicius, T. Nakagawa, M.ꢀC. Kuo, S.ꢀH. Cheng, K.ꢀT. Wong,
51ꢀ57.
C.ꢀH. Chang, R. C. Kwong, S. Xia and C. Adachi, Phys. Chem. Chem. 42.J. T. H. Dunning, J. Chem. Phys., 1989, 90, 1007ꢀ1023.
Phys., 2013, 15, 15850ꢀ15855.
43.R. Bauernschmitt and R. Ahlrichs, Chem. Phys. Lett., 1996, 256, 454ꢀ
464.
1
1
1
1
1
2
2
2
2
5.Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki and C.
Adachi, J. Am. Chem. Soc., 2012, 134, 14706ꢀ14709.
44.M. E. Casida, C. Jamorski, K. C. Casida and D. R. Salahub, J. Chem.
Phys., 1998, 108, 4439ꢀ4449.
6.T. Nakagawa, S.ꢀY. Ku, K.ꢀT. Wong and C. Adachi, Chem. Commun.
012, 48, 9580ꢀ9582.
,
2
45.M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A.
Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F.
Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, J. E.
Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A.
Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.
M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma,
V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J.
Cioslowski and D. J. Fox, GAUSSIAN 09, Revision C.01, Gaussian,
Inc., Wallingford, CT, 2009.
7.G. Méhes, H. Nomura, Q. Zhang, T. Nakagawa and C. Adachi,
Angew. Chem. Int. Ed., 2012, 51, 11311ꢀ11315.
8.K. Nasu, T. Nakagawa, H. Nomura, C.ꢀJ. Lin, C.ꢀH. Cheng, M.ꢀR.
Tseng, T. Yasuda and C. Adachi, Chem. Commun., 2013.
9.J. Li, T. Nakagawa, J. MacDonald, Q. Zhang, H. Nomura, H.
Miyazaki and C. Adachi, Adv. Mater., 2013, 25, 3319ꢀ3323.
0.S. Y. Lee, T. Yasuda, Y. S. Yang, Q. Zhang and C. Adachi, Angew.
Chem. Int. Ed., 2014, 126, 6520ꢀ6524.
1.H. Ohkuma, T. Nakagawa, K. Shizu, T. Yasuda and C. Adachi, Chem.
Lett., 2014, 43, 1017ꢀ1019.
2.T. Takahashi, K. Shizu, T. Yasuda, K. Togashi and C. Adachi, Sci.
Tech. Adv. Mater., 2014, 15, 034202.
3.J. C. Deaton, S. C. Switalski, D. Y. Kondakov, R. H. Young, T. D.
Pawlik, D. J. Giesen, S. B. Harkins, A. J. M. Miller, S. F. Mickenberg
and J. C. Peters, J. Am. Chem. Soc., 2010, 132, 9499ꢀ9508.
2
2
2
2
2
2
3
3
4.R. Czerwieniec, J. Yu and H. Yersin, Inorg. Chem., 2011, 50, 8293ꢀ 46.J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005, 105, 2999ꢀ
8
301.
5.R. Czerwieniec, K. Kowalski and H. Yersin, Dalton Trans., 2013, 42
826ꢀ9830.
3094.
,
47.R. Improta, V. Barone, G. Scalmani and M. J. Frisch, J. Chem. Phys.
,
,
9
2006, 125, 054103.
6.M. N. BerberanꢀSantos and J. M. M. Garcia, J. Am. Chem. Soc., 1996, 48.R. Improta, G. Scalmani, M. J. Frisch and V. Barone, J. Chem. Phys.
18, 9391ꢀ9394. 2007, 127, 074504.
7.K. Masui, H. Nakanotani and C. Adachi, Org. Electron., 2013, 14
721ꢀ2726.
8.M. A. Baldo, C. Adachi and S. R. Forrest, Phys. Rev. B, 2000, 62
0967ꢀ10977.
1
,
,
49.S. Gong, X. He, Y. Chen, Z. Jiang, C. Zhong, D. Ma, J. Qin and C.
2
Yang, J. Mater. Chem., 2012, 22, 2894ꢀ2899.
1
9.Z. Gao, C. S. Lee, I. Bello, S. T. Lee, R.ꢀM. Chen, T.ꢀY. Luh, J. Shi
and C. W. Tang, Appl. Phys. Lett., 1999, 74, 865ꢀ867.
0.M.ꢀY. Lai, C.ꢀH. Chen, W.ꢀS. Huang, J. T. Lin, T.ꢀH. Ke, L.ꢀY. Chen,
M.ꢀH. Tsai and C.ꢀC. Wu, Angew. Chem. Int. Ed., 2008, 47, 581ꢀ585.
1.C.ꢀH. Chen, W.ꢀS. Huang, M.ꢀY. Lai, W.ꢀC. Tsao, J. T. Lin, Y.ꢀH.
Wu, T.ꢀH. Ke, L.ꢀY. Chen and C.ꢀC. Wu, Adv. Func. Mater., 2009, 19
661ꢀ2670.
2.Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev., 2003,
03, 3899ꢀ4032.
,
2
3
1
3
3
3.C. H. Chen and C. W. Tang, Appl. Phys. Lett., 2001, 79, 3711ꢀ3713.
4.K. Okumoto, H. Kanno, Y. Hamaa, H. Takahashi and K. Shibata,
Appl. Phys. Lett., 2006, 89, 063504.
3
3
3
3
5.K. Okumoto, H. Kanno, Y. Hamada, H. Takahashi and K. Shibata,
Appl. Phys. Lett., 2006, 89, 013502.
6.H. Inomata, K. Goushi, T. Masuko, T. Konno, T. Imai, H. Sasabe, J. J.
Brown and C. Adachi, Chem. Mater., 2004, 16, 1285ꢀ1291.
7.H.ꢀF. Chen, S.ꢀJ. Yang, Z.ꢀH. Tsai, W.ꢀY. Hung, T.ꢀC. Wang and K.ꢀ
T. Wong, J. Mater. Chem., 2009, 19, 8112ꢀ8118.
8.Y. Iwasa, T. Koda, S. Koshihara, Y. Tokura, N. Iwasawa and G. Saito,
Phys. Rev. B, 1989, 39, 10441ꢀ10444.
6
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Table of contents entry
We develop thermally activated delayed fluorescence emitters
with wide ranges of emission color and singletꢀtriplet energy gap.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7