Isobenzofuranone- and Chromone-Based Blue Delayed Fluorescence Emitters with Low Efficiency Roll-Off in Organic Light-Emitting Diodes

Article abstract of DOI:10.1021/acs.chemmater.7b03371

Significant efforts have been devoted to the development of novel efficient blue-emitting molecules for organic light-emitting diode (OLED) applications. Blue organic emitters exhibiting thermally activated delayed fluorescence (TADF) have the potential to achieve ～100% internal electroluminescence quantum efficiency in OLEDs. In this paper, we report a promising molecular design strategy for obtaining appropriate high singlet and triplet excited energies, short exciton lifetimes, and high quantum efficiencies in blue TADF emitters. We introduce isobenzofuranone and chromone containing a cyclic ketone or lactone moiety as effective acceptor building units to construct donor-acceptor TADF emitters. Owing to their small singlet-triplet energy splitting, properly contracted π-conjugation, and weakened intramolecular charge-transfer character, these new emitters display strong blue TADF emissions with high photoluminescence quantum yields (53-92%) and notably short TADF emission lifetimes (2.8-4.3 μs) in thin films. Blue TADF-OLEDs utilizing these emitters exhibit external electroluminescence quantum efficiencies of up to 16.2% and extremely low efficiency roll-off even at practical high luminance. The current findings open new avenues for designing practically usable high-performance blue TADF emitters with simple molecular structures.

Full text of DOI:10.1021/acs.chemmater.7b03371

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Article
Isobenzofuranone- and Chromone-Based Blue Delayed Fluorescence
Emitters with Low Efficiency Roll-Off in Organic Light-Emitting Diodes
Jiyoung Lee, Naoya Aizawa, and Takuma Yasuda
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03371 • Publication Date (Web): 29 Aug 2017
Downloaded from http://pubs.acs.org on August 29, 2017
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Chemistry of Materials
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Isobenzofuranone- and Chromone-Based Blue Delayed
Fluorescence Emitters with Low Efficiency Roll-Off in Organic
Light-Emitting Diodes
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Jiyoung Lee,† Naoya Aizawa,† and Takuma Yasuda*,†,‡
†INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishiꢀku, Fukuoka 819ꢀ0395, Japan
‡Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishiꢀku, Fukuoka
819ꢀ0395, Japan
ABSTRACT: Significant efforts have been devoted to the development of novel efficient blueꢀemitting molecules for organic
lightꢀemitting diode (OLED) applications. Blue organic emitters exhibiting thermally activated delayed fluorescence (TADF) have
the potential to achieve ~100% internal electroluminescence quantum efficiency in OLEDs. In this paper, we report a promising
molecular design strategy for obtaining appropriate high singlet and triplet excited energies, short exciton lifetimes, and high
quantum efficiencies in blue TADF emitters. We introduce isobenzofuranone and chromone containing a cyclic ketone or lactone
moiety as effective acceptor building units to construct donor–
acceptor TADF emitters. Owing to their small singlet–triplet
energy splitting, properly contracted πꢀconjugation, and
weakened intramolecular chargeꢀtransfer character, these new
emitters display strong blue TADF emissions, with high
photoluminescence quantum yields (53–92%) and notably short
TADF emission lifetimes (2.8–4.3 ꢀs) in thin films. Blue
TADFꢀOLEDs utilizing these emitters exhibit external
electroluminescence quantum efficiencies of up to 16.2% and
extremely low efficiency rollꢀoff even at practical high
luminance. The current findings open new avenues for
designing practically usable highꢀperformance blue TADF
emitters with simple molecular structures.
coupling induced by the heavy metal center. In this context,
numerous efforts have been devoted to the development of
highꢀperformance iridiumꢀ or platinumꢀcontaining
INTRODUCTION
Since the seminal invention of efficient organic
electroluminescence (EL) devices by Tang et al. in 1987,1
organic lightꢀemitting diodes (OLEDs) have seen rapid
advances toward practical applications in fullꢀcolor flatꢀpanel
displays, solidꢀstate lighting, and flexible wearable electronic
devices.2–11 Development of highꢀefficiency organic
luminophores displaying the three primary (RGB: red, green,
and blue) emission colors is critical for future OLED
technologies. The main factor determining the internal EL
quantum efficiency (ηint) of OLEDs is the spin statistics upon
recombination of holes and electrons under electrical
excitation. In traditional longꢀlasting fluorescent OLEDs, only
25% singlet (S1) excitons can be exploited to produce EL
emission and the remaining 75% triplet (T1) excitons are
typically lost through a nonꢀradiative decay path, limiting ηint
to 25%.5
phosphorescent emitters, and great success in terms of EL
efficiency, color purity, and operational stability has been
achieved for green and red phosphorescent OLEDs, which
meet the criteria for commercial applications. Nevertheless,
some issues still remain in current phosphorescent OLED
technology. The main hurdle is the lack of efficient and stable
deepꢀblue phosphorescent emitters, whose development lags
behind their green and red counterparts because of the
difficulty in designing stable and wide energyꢀgap phosphors
possessing intrinsically high excited energy for metalꢀtoꢀ
ligand chargeꢀtransfer transition.13–15 Moreover, the use of
expensive precious metal elements is indispensable for high
ηint in the existing phosphorescence systems. Thus, even with
lower EL efficiencies, considerable attention is still directed to
fully organic fluorophores, particularly for blueꢀemitting
devices with high color purity and longꢀterm stability.
One of the most viable strategies for maximizing ηint of
OLEDs relies on the use of organometallic phosphorescent
emitters incorporating heavy metals such as iridium or
platinum. Such phosphorescent OLEDs can reach ηint values as
high as ~100% by harvesting both S1 and T1 excitons7,8,12
owing to fast intersystem crossing (ISC) via intense spin–orbit
Recently, thirdꢀgeneration OLEDs based on metalꢀfree
thermally activated delayed fluorescence (TADF) emitters that
can realize ~100% exciton utilization have been recognized as
attractive alternatives to phosphorescent systems.10,16–23 By
virtue of the considerably small S1–T1 energy splitting (ꢁEST),
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TADF emitters enable efficient upconversion of nonꢀradiative
show superior TADF properties in our previous study.25,31–33,37
1
T1 excitons to radiative S1 excitons via reverse intersystem
crossing (RISC) and hence show highly efficient EL emission
without the use of precious metals. Although numerous TADF
emitters have been developed and tested as RGBꢀemitting
OLEDs since 2012, there are only a few reports on highꢀ
performing blue TADF emitters which meet high external EL
quantum efficiency (ηext) over 20% as well as suitable color
purity (with Commission Internationale de l'Éclairage (CIE)
chromaticity coordinates of x < 0.2 and y < 0.2).18,24–35 Hence,
a rational design strategy for efficient blue TADF emitters is
desired. Furthermore, most blue TADFꢀOLEDs reported so far
suffer from severe efficiency rollꢀoff at high current density.
This rollꢀoff stems from exciton annihilation processes,
including triplet–triplet annihilation (TTA) and singlet–triplet
annihilation (STA), which are primarily caused by the
relatively long excitedꢀstate lifetimes of the blue TADF
emitters.36 Accordingly, suppression of efficiency rollꢀoff
remains another important challenge for the production of
highꢀperformance blue TADFꢀOLEDs.
In this paper, we report a novel series of highꢀperformance
blue TADF emitters (Figure 1) combining isobenzofuranone
(BF) or chromone (CM) as new electronꢀacceptor (A) units
with spiro[acridanꢀ9,9'ꢀxanthene] electronꢀdonor (D) units.
These materials exhibit bright blue TADF emissions centered
around 460–485 nm, with high photoluminescence quantum
yields (ΦPL = 53–92%) and very short delayed fluorescence
lifetimes (τd = 2.8–4.3 ꢀs), in doped thin films and nonꢀdoped
neat films. OLEDs fabricated using these blue TADF emitters
retain high EL efficiencies over a broad brightness range,
demonstrating slight efficiency rollꢀoff. This study will shed
light on the design guidelines for highꢀperformance blueꢀ
emitting materials and devices that can promote the practical
applications of TADF technology.
In contrast to the diverse variety of possible D units, A
building units for highly efficient blue TADF emitters are still
limited to benzonitriles,27 triazines,19,24,26,34 pyrimidines,32,33,38
sulfones,18,29,30 and phenylboranes,25,28,31,39 constraining the
diversity of molecular design. Therefore, a more versatile
design of A units becomes imperative for further expansion of
the blue TADF family. Recently, we,17,37,40–42 Cheng et al.,43–45
Su and Tang et al.,46,47 and other groups35,48,49 explored TADF
emitters bearing a carbonyl group such as benzophenone (BP)
and xanthone (XT) derivatives as A units. OLEDs fabricated
with BPꢀ and XTꢀbased TADF emitters showed excellent EL
performance with maximum external quantum efficiencies
(ηext) exceeding 18%. However, the BPꢀ and XTꢀbased
emitters combining with (spiro)acridanꢀbased donors
displayed turquoise to green EL emissions centered around
490–510 nm with CIE(x,y) chromaticity coordinates of (0.18–
0.26, 0.40–0.55),37,40 far from the deepꢀblue region. As can be
seen from Figure 2, owing to the relatively large πꢀconjugation
systems and strong electronꢀaccepting natures (or deepꢀlying
LUMO energy levels) of BP and XT as well as the typical
carbonylꢀappended aromatic dyes (e.g., anthraquinone and
coumarin), it is difficult to attain efficient deepꢀblue TADF
emission when using them as the A building unit in
combination with (spiro)acridanꢀbased D units. Therefore, we
designed BF and CM with contracted πꢀconjugation as A units
for present D–A systems with the intention of widening the
HOMO–LUMO energy gap and raising the intrinsic T1 energy
level for bluer TADF emission.
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Figure 1. Molecular structures of isobenzofuranone (BF)ꢀ and
chromone (CM)ꢀbased blue TADF emitters, MXAcꢀBF, MXAcꢀ
CM, and XAcꢀCM. Their common electronꢀdonating and
withdrawing substructures are displayed in red and blue,
respectively.
RESULTS AND DISCUSSION
Material Design and Synthesis. To minimize ꢁEST and
accelerate RISC, TADF emitters generally comprise preꢀ
twisted donor–acceptor (D–A) πꢀelectronic systems with
strong intramolecular chargeꢀtransfer (ICT) characteristics
giving rise to a small spatial overlap between the highest
occupied molecular orbital (HOMO) and the lowest
Figure 2. Comparison of intrinsic electronic properties of various
carbonylꢀappended aromatics, including anthraquinone (AQ),
coumarin (CO), benzophenone (BP), xanthone (XT), chromone
(CM), and isobenzofuranone (BF), calculated at the PBE0/6ꢀ
31G(d) level.
unoccupied
molecular
orbital
(LUMO).10,20–23
To
simultaneously achieve deepꢀblue chromaticity and small ꢁEST,
the desired TADF emitters should be constructed by
appropriate choice of weak D–A combinations. Among the
widely used arylamineꢀtype D building units, spiroacridan
derivatives are particularly effective and have been reported to
To understand the electronic transition characteristics of
MXAcꢀBF, MXAcꢀCM, and XAcꢀCM at the molecular level,
the highest occupied and the lowest unoccupied natural
transition orbitals (NTOs) for the S1 and T1 excitations were
simulated using timeꢀdependent density functional theory
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Chemistry of Materials
(TDDFT) calculations (Figure 3). All these molecules adopted Photophysical Properties. Figure 4 depicts the UV–vis
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nearly orthogonal D–A conformations in their optimized
structures with dihedral angles of ~88° between the BF (or
CM) and the adjacent acridan moiety. Because of their highly
twisted geometries and obvious ICT characters, the highest
occupied NTOs are predominantly localized on the electronꢀ
donating acridan moiety but hardly on the peripheral spiroꢀ
fused xanthene moiety; the lowest unoccupied NTOs are
thoroughly distributed over the electronꢀaccepting BF or CM
unit. This clear separation of NTOs results in small calculated
ꢁEST values of 0.04–0.10 eV, allowing for efficient RISC.
Moreover, the calculated S1 energies are 2.65–2.84 eV, which
are higher than that estimated for the corresponding greenꢀ
emitting XTꢀbased TADF molecule XAcꢀXT (2.58 eV).37 We
can thus anticipate bluer TADF emissions from these BFꢀ and
CMꢀbased molecules.
absorption and photoluminescence (PL) spectra of dilute
solutions of MXAcꢀBF, MXAcꢀCM, and XAcꢀCM in toluene.
All molecules showed relatively weak and broad absorptions
in the range of 350–400 nm, which are assigned to the ICT
transition associated with electron transfer from the acridan
moiety to the BF or CM unit (see Figure 3a). Upon
photoexcitation with UV light, these solutions emitted strong
blue PL with emission peaks (λPL) ranging from 449 to 468
nm. In comparison with that of XAcꢀCM, the PL emission of
MXAcꢀCM in toluene was redꢀshifted by ~20 nm because of
the enhanced electronꢀdonating ability of dimethylꢀsubstituted
MXAc relative to nonꢀmethylated XAc. Additionally,
introducing dimethyl substituents in the acridan moiety also
provided large influence on the absolute PL quantum yield
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(ΦPL) of the resulting D–A molecule; the ΦPL values in
deoxygenated toluene solutions increased in the order XAcꢀ
CM (43%) < MXAcꢀBF (64%) < MXAcꢀCM (74%).50
Figure 4. (a) UV–vis absorption and PL spectra of MXAcꢀBF,
MXAcꢀCM, and XAcꢀCM in toluene (1 × 10−5 M). The inset
represents a magnified view of the lowerꢀenergy ICT absorptions.
(b) Photograph of PL emissions from the toluene solutions under
UV irradiation at 365 nm.
To further investigate the photophysical and TADF
properties in solid guest–host systems, doped thin films of
three emitters (guest dopants) in solid host matrices of
Figure 3. (a) Highest occupied and lowest unoccupied natural
transition orbitals (HONTOs and LUNTOs) for the S1 state and
arrangements of the S1 and T1 energy levels with oscillator
strengths (f) for MXAcꢀBF, MXAcꢀCM, and XAcꢀCM. (b) Spin
density distributions of their T1 states calculated at the PBE0/6ꢀ
31G(d) level.
bis(diphenylphosphoryl)dibenzo[b,d]furan
(PPF)56
were
prepared by varying the dopant concentrations in the range of
25–100 wt%. Due to the sufficiently high T1 energy of PPF
(ET = 3.1 eV), backward excitedꢀenergy transfer from the T1
states of the guest emitters to that of the host material is
effectively suppressed in this system. Intriguingly, as
presented in Figure 5a, thin films containing MXAcꢀBF and
MXAcꢀCM retained considerably high ΦPL values (surpassing
70%) at high dopant concentrations even at 100 wt% (for neat
films), revealing suppressed concentration quenching behavior.
The terminal bulky xanthene moiety on the spiroꢀfused D unit
and the preꢀtwisted overall molecular structure are beneficial
in inhibiting intermolecular electronic interactions, which
usually cause excitedꢀenergy loss in the condensed state. The
calculated spin densities of the T1 states of the BFꢀ and CMꢀ
based molecules did not distribute on the terminal xanthene
moiety (see Figure 3b), which may contribute to a reduction in
the shortꢀrange electronꢀexchange interactions for T1 excitons
between adjacent molecules.37
MXAcꢀBF, MXAcꢀCM, and XAcꢀCM were synthesized by
Buchwald–Hartwig
reactions
between
5ꢀ
bromoisobenzofuranone or 7ꢀbromoꢀ2ꢀmethylchromone and
the corresponding D units, i.e., spiro[2,7ꢀdimethylacridanꢀ9,9'ꢀ
xanthene] (MXAc) or spiro[acridanꢀ9,9'ꢀxanthene] (XAc),
using Pd(OAc)2/P(tꢀBu)3HBF4 as the catalyst. All final
products were purified using temperatureꢀgradient vacuum
sublimation after column chromatography. The chemical
structures of these materials were ascertained using NMR
spectroscopy, mass spectrometry, and elemental analysis (see
the Experimental Section and Supporting Information for
details). As revealed by thermogravimetric analysis, MXAcꢀ
BF, MXAcꢀCM, and XAcꢀCM had high thermal stability with
decomposition temperatures (Td) of 363, 360, and 370 °C at
5% weight loss, which possibly resulted from the
incorporation of robust spiroꢀfused D units.
The detailed photophysical parameters obtained for the
emitter:PPF doped thin films with different dopant
concentrations are collected in Table 1. As exemplified in
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shows the molecular structure of PPF. (b) Steadyꢀstate PL spectra
and (c) transient PL decay profiles of 50 wt%ꢀemitter:PPF doped
thin films measured at 300 K.
Figure 5b, the steadyꢀstate PL spectra of the 50 wt%ꢀ
emitter:PPF doped films exhibited structureless intense blue
emissions solely from the dopant emitters, similar to those in
dilute solutions, even though their λPL positions were slightly
redꢀshifted (within 15 nm) with respect to those in toluene
solutions. From the energy differences between the onsets of
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Figure 5c depicts transient PL decay curves of 50 wt%ꢀ
emitter:PPF doped films at room temperature (300 K), which
can be fitted by two exponential decay components as: IPL(t) =
A1exp(−t/τp) + A2exp(−t/τd), where A1 and A2 are frequency
factors; τp and τd are the lifetimes of the prompt and delayed
decay components, respectively. While the prompt component
with τp of 24–33 ns corresponds to conventional fluorescence
(S1 → ground state, S0), the delayed component with τd of 2.8–
4.3 ꢀs originates from TADF involving ISC and RISC
processes, followed by emission decay (S1 → T1 → S1 → S0).
Their delayed decay components gradually intensified with
increasing temperature from 10 to 300 K (Supporting
Information), testifying the typical TADF feature. It is worth
noting that the TADF lifetimes for all new BFꢀ and CMꢀbased
emitters were remarkably shorter than those of most existing
blue TADF emitters including 1,2ꢀbis(carbazoleꢀ9ꢀyl)ꢀ4,5ꢀ
dicyanobenzene10 (2CzPN; τd = 280 ꢀs in the PPF host). Their
reduced τd could be attributed to the accelerated RISC (T1 →
S1 upconversion) process. To gain deeper insight into exciton
upconversion through RISC, we evaluated the rate constants
for RISC (kRISC) using eq 1:10
the
respective
timeꢀresolved
fluorescence
and
phosphorescence spectra measured at 10 K, ꢁEST values were
experimentally determined to be 0.08 eV for MXꢀAcꢀBF and
MXAcꢀCM, and 0.11 eV for XAcꢀCM (Table 1 and
Supporting Information), which are in accord with the TDDFT
results (see Figure 3a).
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ꢇ ꢉꢈꢉꢇ
ꢀꢁꢂꢃꢄ ꢅ ꢆ
(1)
ꢆꢈ ꢉꢊꢋꢌ
where
Φ
p and
Φ
d, fractional quantum yields of the prompt and
delayed fluorescence components, which can be derived from
each transient PL decay curve (see Figure 5c); kp and kd, the
rate constants of prompt and delayed fluorescence can be
experimentally determined by equations kp = 1/τp and kd = 1/τd,
respectively; and kISC, the rate constant of ISC (S1 → T1) can
be estimated by kISC = (1 −
Φp) × kp. The calculated kRISC
values for MXAcꢀBF and MXAcꢀCM (7.5 × 105 and 6.6 × 105
s−1, respectively) are slightly larger than that of XAcꢀCM (4.9
Figure 5. (a) Dependence of overall PL quantum yields (
dopant concentration in emitter:PPF doped thin films; the inset
ΦPL) on
Table 1. Photophysical Data for BF- and CM-Based Blue TADF Emitters in Doped Thin Films
Table 1. Photophysical Data for BF- and CM-Based Blue TADF Emitters in Thin Films
e
b
e
f
i
i
j
j
c
d
d
d
e
HOMOg
HOMO
(eV)
(eV)
LUMOh
LUMO
(eV)
(eV)
ES
ES
ET
ET
ꢁEST
ꢁEST
(eV)
(eV)
b
e
f
i
i
conc.a
conc.
(wt%)
(wt%)
λPL
λPL
τp
kRISC
c
d
τd
τd
ΦPL
ΦPL
Φp
Φp
Φd
Φd
τp
kRISC
emitter
emitter
(nm)
(nm)
(ns)
(ns)
(105 s−1)
(10 s )
(eV) (eV)
(eV) (eV)
(%)
(%)
(%)
(%)
(%)
(%)
(ꢀs)
(ꢀs)
25
25
472
92
27
27
65
65
24
24
4.3
4.3
7.7
7.7
472
478
92
88
50
27
61
24
4.1
7.5
7.5
50
478
88
27
61
24
4.1
MXAcꢀBF
MXAcꢀBF
−5.44
−5.44
−2.48
−2.48
2.98 2.90
2.98 2.90
0.08
0.08
75
75
478
478
80
80
27
27
53
53
24
24
3.9
3.9
6.9
6.9
100
478
78
27
51
24
3.8
6.8
6.8
100
478
78
27
51
24
3.8
25
480
79
35
44
33
2.9
6.7
6.7
25
480
79
35
44
33
2.9
50
50
484
484
77
77
35
35
42
42
33
33
2.8
2.8
6.6
6.6
MXAcꢀCM
MXAcꢀCM
−5.56
−5.56
−2.77
−2.77
2.95 2.87
2.95 2.87
0.08
0.08
75
483
74
35
39
33
2.8
6.1
75
483
74
35
39
33
2.8
6.1
100
482
71
35
36
33
2.8
5.7
100
25
482
454
71
76
35
32
36
44
33
25
2.8
4.0
5.7
5.1
25
454
76
32
44
25
4.0
5.1
50
462
74
32
42
25
3.9
4.9
50
462
74
32
42
25
3.9
4.9
XAcꢀCM
XAcꢀCM
−5.73
−5.73
−2.87
−2.87
3.11 3.00
3.11 3.00
0.11
0.11
75
461
68
32
36
25
3.7
4.5
75
100
461
461
68
53
32
32
36
22
25
25
3.7
2.8
4.5
3.6
100
461
53
32
22
25
2.8
3.6
a
b
b
c
c
aDopant concentration in emitter:PPF codeposited thin films. PL emission maximum measured at 300 K. Absolute PL quantum yield
Dopant concentration in emitter:PPF codeposited thin films. PL emission maximum measured at 300 K. Absolute PL quantum yield
d
d
evaluated using an integrating sphere under N2 at 300 K. Fractional quantum yields for prompt fluorescence (Φp) and delayed fluorescence
evaluated using an integrating sphere under N2 at 300 K. Fractional quantum yields for prompt fluorescence (Φp) and delayed fluorescence
e
e
(Φd) components; Φp + Φd = ΦPL. PL lifetimes for prompt fluorescence (τp) and delayed fluorescence (τd) components measured under N2 at
(Φd) components; Φp + Φd = ΦPL. PL lifetimes for prompt fluorescence (τp) and delayed fluorescence (τd) components measured under N2 at
300 K. fRate constant of RISC (T1 → S1). gDetermined using photoelectron yield spectroscopy in a neat film. hLUMO = HOMO + Eg, in which
300 K. Rate constant of RISC (T1 → S1). Determined using photoelectron yield spectroscopy in a neat film. LUMO = HOMO + Eg, in which
i
i
the optical energy gap (Eg) was derived from the absorption onset of the neat film. Lowest excited singlet (ES) and triplet (ET) energies
the optical energy gap (Eg) was derived from the absorption onset of the neat film. Lowest excited singlet (ES) and triplet (ET) energies
estimated from onset wavelengths of the timeꢀresolved fluorescence and phosphorescence spectra, respectively, for the 50 wt%ꢀemitter:PPF
estimated from onset wavelengths of the timeꢀresolved fluorescence and phosphorescence spectra, respectively, for 50 wt%ꢀemitter:PPF doped
doped film atj10 K. jSinglet–triplet energy splitting determined experimentally using ꢁEST = ES − ET.
film at 10 K. Singlet–triplet energy splitting determined experimentally using ꢁEST = ES − ET.
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Chemistry of Materials
× 105 s−1), presumably due to their smaller ꢁEST values. More
biphenyl) act as holeꢀinjection layer and holeꢀtransporting
layer (HTL), respectively, whereas TPBi (1,3,5ꢀtris(Nꢀ
phenylbenzimidazolꢀ2ꢀyl)benzene) and Liq (8ꢀ
1
2
3
4
5
6
7
8
importantly, kRISC values of these three emitters are two orders
of magnitude larger than that of 2CzPN (kRISC ~103 s−1). This
result unambiguously indicates that RISC can be drastically
accelerated in BFꢀ and CMꢀbased blue TADF molecules
featuring a simple cyclic ketone or ester (lactone) constituent.
hydroxyquinoline lithium) serve as electronꢀtransporting layer
(ETL) and electronꢀinjection material, respectively.
Additionally, to suppress unfavorable exciton quenching at the
HTL/EML and EML/ETL interfaces, thin layers of CCP25 (9ꢀ
phenylꢀ3,9'ꢀbicarbazole) and PPF51 with high ET of 3.0 and 3.1
eV, respectively, were incorporated as excitonꢀblocking layers.
The EL spectra of the fabricated devices contained pure
emissions from the incorporated TADF emitters, as exhibited
in Figure 6b; no significant changes in spectra with applied
voltage were observed in the range of 4–8 V. The XAcꢀCMꢀ
based device achieved EL emission with a peak at the shortest
wavelength (λEL = 462 nm) and the CIE coordinates (0.15,
0.19), corresponding to deepꢀblue light. The EL spectrum of
the MXAcꢀBFꢀbased device peaked at 478 nm with the CIE
coordinates (0.17, 0.29) and whose pattern was nearly
identical to that of the corresponding MXAcꢀCMꢀbased device.
Figure 6c,d shows current density–voltage–luminance (J–V–L)
and external EL quantum efficiency–luminance (ηext–L)
characteristics, respectively, of TADFꢀOLEDs fabricated by
Electroluminescence Performance. Blue TADF
emitters that can simultaneously achieve high
ΦPL, large kRISC,
and low concentration quenching are particularly attractive for
developing nonꢀdoped, hostꢀfree blue TADFꢀOLEDs in
addition to heavilyꢀdoped devices. To evaluate the EL
performances of BFꢀ and CMꢀbased blue TADF emitters,
multilayer OLEDs were fabricated using the emitter:PPF
doped films with various dopant concentrations as well as
nonꢀdoped neat films as emission layers (EMLs). As
illustrated in Figure 6a, the device configuration was indiumꢀ
tinꢀoxide (ITO, 100 nm)/HATꢀCN (10 nm)/αꢀNPD (40
nm)/CCP (5 nm)/EML (20 nm)/PPF (10 nm)/TPBi (30
nm)/Liq (1 nm)/Al (80 nm). In this architecture, HATꢀCN
(2,3,6,7,10,11ꢀhexacyanoꢀ1,4,5,8,9,12ꢀhexaazatriphenylene)
and αꢀNPD (4,4'ꢀbisꢀ[Nꢀ(1ꢀnaphthyl)ꢀNꢀphenylamino]ꢀ1,1'ꢀ
9
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Figure 6. (a) Energy level diagram for the blue TADFꢀOLEDs employing MXAcꢀBF, MXAcꢀCM, and XAcꢀCM as emitters. (b) EL
spectra measured at 10 mA cm−2 (left) with corresponding emission colors on the CIE 1931 chromaticity diagram and photos of blue
EL emissions from the devices with 50 wt% doped films as the EML. (c) Current density–voltage–luminance (J–V–L) characteristics
and (d) external EL quantum efficiency (ηext) vs. L plots for the blue TADFꢀOLEDs fabricated with different dopant concentrations.
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Page 6 of 10
varying the dopant concentration, and their key EL parameters
T1 state, kTTA and kSTA are the rate constants of TTA and STA,
respectively, is the singlet exciton production ratio via TTA
= 0.25), J is the injected current density, d is the
recombination zone thickness (assumed 10 nm), and e is the
elementary charge. For steadyꢀstate conditions (i.e., dNS/dt = 0
and dNT/dt = 0), ηext can be described as:
1
2
3
4
5
6
7
8
are compiled in Table 2. Intriguingly, the MXAcꢀBFꢀbased
devices turn on at the lowest bias voltage of 3.0 V and perform
well with high maximum ηext of 16–17% and maximum power
efficiencies of 31–33 lm W−1, irrespective of dopant
concentration (even for the nonꢀdoped EML). In addition, the
efficiency rollꢀoffs of the MXAcꢀBFꢀbased devices are
remarkably small, retaining high ηext over 12% even when
driven at a high practical luminance of 1000 cd m−2 (for
display applications). Meanwhile, devices with 50 wt% dopant
(MXAcꢀCM and XAcꢀBF) concentration were found to
outperform those with other concentrations (25 and 100 wt%)
from an overall perspective. With 50 wt% dopant
concentration, the MXAcꢀCMꢀ and XAcꢀCMꢀbased devices
gave ηext values of up to 15.0% and 12.1%, respectively, with
lower efficiency rollꢀoff. The lower EL efficiencies of the
MXAcꢀCMꢀ and XAcꢀCMꢀbased devices can be attributed to
α
(α
ꢑ
ꢔ
ꢎ
ꢏꢧꢨ,ꢚ
ꢋ
ꢑ ꢔ
ꢠꢡꢢꢣ ꢤ ꢅ ꢠꢡꢢꢣ,ꢥꢦꢢ
(4)
ꢎ
ꢋ
ꢩ
9
where ηext,max denotes the maximum external EL quantum
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efficiency and NS represents singlet exciton density without
0
TTA and STA (i.e., kTTA = kSTA = 0). To analyze the difference
in rollꢀoff characteristics, this TTA–STA model was applied to
the present BFꢀ and CMꢀbased devices as well as the 2CzPNꢀ
based reference device with the same configuration. As shown
in Figure 7, the curves fit using eq 4 are in reasonable
agreement with the experimental ηext–J data, demonstrating
that rollꢀoff in these TADFꢀOLEDs was primarily caused by
TTA and STA processes. In spite of their suppressed rollꢀoff
characteristics, both kTTA and kSTA values estimated for the BFꢀ
and CMꢀbased devices were comparable or slightly larger than
those obtained for 2CzPN (kTTA ~1 × 10−14 cm3 s−1 and kSTA ~2
× 10−11 cm3 s−1; Supporting Information). Given similar krS and
kISC values for all these materials (1–3 × 107 s−1 and 2–3 × 107
s−1, respectively; Supporting Information), it is assumed that
kRISC is the most prominent factor affecting the degree of
device efficiency rollꢀoff. As described above, the kRISC values
of MXAcꢀBF, MXAcꢀCM, and XAcꢀCM were two orders of
magnitude larger than that of 2CzPN. Thus, for the current
BFꢀ and CMꢀbased devices, the larger kRISC makes it possible
to decrease NT upon electrical excitation (Figure 7b) and
suppress the competing exciton deactivation processes to
realize extremely low efficiency rollꢀoff in blue TADFꢀ
OLEDs.
their lower ΦPL (or
Φd) values than those of MXAcꢀBF in the
thin films (see Figure 5a and Table 1).
Table 2. EL Performance of Blue TADF-OLEDs
emittera
MXAcꢀBF
MXAcꢀCM
XAcꢀCM
λELb (nm)
Vonc (V)
478
3.0
478
3.3
462
3.6
ηext,maxd (%)
ηext,100e (%)
ηext,1000e (%)
ηpf (lm W−1)
16.2
15.7
12.0
31.5
15.0
14.6
11.1
25.7
12.1
11.6
7.1
14.7
CIEh (x, y)
(0.17, 0.29)
(0.16, 0.29)
(0.15, 0.19)
aDevice configuration: ITO/HATꢀCN (10 nm)/αꢀNPD (40 nm)/CCP
(5 nm)/50 wt%ꢀemitter:PPF (20 nm)/PPF (10 nm)/TPBi (30 nm)/Liq
b
c
(1 nm)/Al (100 nm). EL emission maximum at 10 mA cm−2. Turnꢀ
on voltage at a luminance over 1 cd m−2. dMaximum external EL
e
quantum efficiency. External EL quantum efficiency driven at 100
and 1000 cd m−2. fMaximum power efficiency. hCommission
Internationale de l'Éclairage (CIE) chromaticity coordinates recorded
at 10 mA cm−2.
Analysis of Efficiency Roll-Off. In general, efficiency
rollꢀoff is serious for TADFꢀOLEDs, particularly for blueꢀ
emitting devices. Because of the long lifetime of electroꢀ
generated T1 excitons ranging from several tens of
microseconds to milliseconds, triplet–triplet annihilation
(TTA)36,52 is typically a major factor causing efficiency rollꢀ
off in TADFꢀOLEDs. Another possible mechanism, singlet–
triplet annihilation (STA),36 can also influence rollꢀoff
behavior. Considering TTA and STA, the singlet and triplet
exciton densities (NS and NT, respectively) in TADFꢀOLEDs
upon electrical excitation can be expressed as:53,54
ꢍꢎ
ꢋ
ꢅ ꢐ ꢀꢒ ꢓ ꢀꢂꢃꢄ ꢕꢃ ꢓ ꢀꢁꢂꢃꢄꢕꢖ ꢐ ꢀꢃꢖꢗꢕꢃꢕꢖ ꢓ ꢘꢀꢖꢖꢗꢕꢖꢙ ꢓ
ꢃ
ꢑ
ꢔ
ꢍꢏ
ꢚ
(2)
ꢛꢍꢜ
ꢍꢎ
ꢟꢚ
ꢝ
ꢅ ꢀꢂꢃꢄꢕꢃ ꢐ ꢀꢁꢂꢃꢄ ꢓ ꢀꢞꢒ ꢕꢖ ꢐ 1 ꢓ ꢘ ꢀꢖꢖꢗꢕꢖꢙ ꢓ ꢛꢍꢜ
ꢖ
ꢑ
ꢔ
ꢑ
ꢔ
Figure 7. (a) ηext–J characteristics of TADFꢀOLEDs employing
MXAcꢀBF, MXAcꢀCM, XAcꢀCM, and 2CzPN (for reference) as
emitters. The solid lines are the fitted results based on the TTA–
STA model. (b) Triplet exciton density (NT) as a function of J for
each device, calculated using the eqs 2 and 3.
ꢍꢏ
(3)
where krTS is the rate constant for radiative decay from the S1
state, knr is the rate constant for nonꢀradiative decay from the
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Chemistry of Materials
hydroxyacetophenone (5.1 g, 24 mmol) and ethyl acetate (5.1
1
g, 58 mmol) in dry THF (10 mL) under N2. The mixture was
allowed to react for 10 min at room temperature, and then
quenched by pouring into a large amount of ice water. After
acidification with aqueous HCl to pH ~6, the product was
extracted with ethyl acetate. The combined organic layers
were washed with water, and dried over anhydrous MgSO4.
After filtration and evaporation, a crude product of 1ꢀ(4ꢀ
bromoꢀ2ꢀhydroxyphenyl)butanꢀ1,3ꢀdione was obtained, which
was used in the next step without further purification. This
compound was dissolved in a mixture of methanol (30 mL)
and conc. HCl (36%, 1 mL). After stirring for 14 h at room
temperature, the reaction mixture was concentrated under
reduced pressure. The product was purified by column
chromatography on silica gel (eluent: hexane/CHCl3 = 1:1,
v/v) and dried under vacuum to produce a light yellow solid
CONCLUSIONS
2
3
4
5
6
7
8
9
In this study, isobenzofuranone and chromone bearing an
electronꢀwithdrawing cyclic ketone or lactone moiety were
introduced for the first time as the effective acceptor building
units for producing efficient blue TADF emitters. Owing to
their rather small ꢁEST (0.08–0.11 eV), properly contracted πꢀ
conjugation, and weakened ICT character, these emitters
successfully achieved blue TADF emissions (λPL = 461–484
nm), with simultaneous high PL quantum yields (ΦPL = 53–
92%) and notably short emission lifetimes (τd = 2.8–4.3 ꢀs) in
thin films. Blue TADFꢀOLEDs fabricated using these emitters
exhibited impressive ηext of up to 16.2%. While the maximum
ηext values were still limited by their moderate ΦPL values in
heavilyꢀdoped (or neat) films, remarkably reduced efficiency
rollꢀoff was attained in these devices, retaining ηext as high as
12.0% at practical high luminance (1000 cd m−2). Excellent
device performance with reduced rollꢀoff characteristics
possibly resulted from their high RISC rates (kRISC >105 s−1),
which contributed to suppressing the competing exciton
deactivation processes such as TTA and STA under electrical
excitation. By utilizing the molecular design concept presented
here and further enhancing intrinsic PL properties, outstanding
deepꢀblue TADF emitters can be rationally designed and
synthesized for future OLED applications.
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(yield = 3.8 g, 68%). 1H NMR (400 MHz, CDCl3):
δ 8.04 (d, J
= 8.4 Hz, 1H), 7.62 (d, J = 2.0 Hz, 1H), 7.50 (dd, J = 8.4 Hz,
2.0 Hz, 1H), 6.17 (s, 1H), 2.38 (s, 3H). MS (MALDIꢀTOF):
m/z calcd 237.96 [M]+; found 238.37.
Synthesis of MXAc-BF: To
a
solution of 5ꢀ
bromophthalide (2.70 g, 12.7 mmol) and spiro[2,7ꢀ
dimethylacridanꢀ9,9′ꢀxanthene] (4.76 g, 12.7 mmol) in dry
toluene (150 mL) were added Pd(OAc)2 (0.09 g, 0.40 mmol),
P(tꢀBu)3HBF4 (0.55 g, 1.90 mmol), and K2CO3 (5.26 g, 38.1
mmol). The mixture was stirred for 5 days at 90 °C under N2.
After cooling to room temperature, the reaction mixture was
poured into water and then extracted with CHCl3. The
combined organic layers were washed with water and dried
over anhydrous Na2SO4. After filtration and evaporation, the
product was purified by column chromatography on silica gel
EXPERIMENTAL SECTION
Materials and Methods. Commercially available
reagents and solvents were used as received unless otherwise
noted. 5ꢀBromoisobenzofuranone (or 5ꢀbromophthalide) was
purchased from Tokyo Chemical Industry Co., Ltd. (TCI).
(eluent: hexane/CHCl3/ethyl acetate
=
7:2:1, v/v),
recrystallized from CHCl3/hexane, and dried under vacuum to
afford MXAcꢀBF as a light yellow solid (yield = 3.79 g, 59%).
(MXAc),33
2,8ꢀ
1
Spiro[2,7ꢀdimethylacridanꢀ9,9'ꢀxanthene]
M.p. 343 °C. H NMR (500 MHz, CDCl3):
δ 8.24 (d, J = 7.9
spiro[acridanꢀ9,9'ꢀxanthene]
(XAc),25
Hz, 1H), 7.63 (dd, J = 7.9, 1.5 Hz, 1H), 7.60 (s, 1H), 7.20ꢀ7.16
(m, 4H), 7.11 (dd, J = 7.9 Hz, 1.5 Hz, 2H), 6.96 (td, J = 6.9 Hz,
2.4 Hz, 2H), 6.68 (dd, J = 8.5 Hz, 1.7 Hz, 2H), 6.64 (d, J = 1.7
Hz, 2H), 6.09 (d, J = 8.5 Hz, 2H), 5.46 (s, 2H), 2.02 (s, 6H).
bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF),51 and 9ꢀ
phenylꢀ3,9'ꢀbicarbazole (CCP)25 were prepared according to
the
literature
procedures.
2,3,6,7,10,11ꢀHexacyanoꢀ
1,4,5,8,9,12ꢀhexaazatriphenylene (HATꢀCN) was donated by
the Nippon Soda Co., Ltd. and was purified using vacuum
sublimation before use. Other OLED materials were purchased
from eꢀRay Optoelectronics Technology Co., Ltd., and were
used for the device fabrication without further purification.
NMR spectra were recorded on an Avance III 500 or III 400
13C NMR (125 MHz, CDCl3):
δ 170.05, 149.40, 148.48,
147.55, 136.63, 132.88, 132.79, 131.79, 131.22, 130.48,
129.71, 128.67, 127.89, 127.61, 125.67, 125.61, 123.64,
116.01, 113.73, 69.36, 44.63, 20.47. MS (MALDIꢀTOF): m/z
calcd 507.18 [M]+; found 507.21. Anal. calcd (%) for
C38H25NO2: C 82.82, H 4.96, N 2.76; found: C 82.74, H 4.93,
N 2.82.
spectrometer (Bruker). H and 13C NMR chemical shifts were
1
determined relative to the signals of tetramethylsilane (
0.00) and CDCl3 ( = 77.0), respectively, as internal standards.
δ =
δ
Matrixꢀassisted laser desorption ionization timeꢀofꢀflight
(MALDIꢀTOF) mass spectra were collected on an Autoflex III
spectrometer (Bruker Daltonics) using dithranol as a matrix.
Elemental analyses were carried out using an MTꢀ5 analyzer
(Yanaco). Density functional theory (DFT) calculations were
performed using the Gaussian 09 program package. The
molecular geometries in the ground state were optimized using
the PBE0 functional with the 6ꢀ31G(d) basis set in the gas
phase. The lowest singlet and triplet excited states were
calculated using the optimized structures with timeꢀdependent
DFT (TDDFT) at the same level.
Synthesis of 7-bromo-2-methylchromone (Br-CM): Brꢀ
CM was synthesized according to Scheme 1. To a stirred
suspension of sodium hydride (2.3 g, 95 mmol) in dry THF
(10 mL) was added dropwise a solution of 4ꢀbromoꢀ2ꢀ
Scheme 1. Synthesis of the CMꢀbased blue TADF emitters.
Synthesis of MXAc-CM: The syntheses of MXAcꢀCM and
XAcꢀCM are outlined in Scheme 1. To a solution of BrꢀCM
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Page 8 of 10
(1.00 g, 4.18 mmol) and spiro[2,7ꢀdimethylacridanꢀ9,9′ꢀ water, acetone, and isopropanol. The substrates were then
1
2
3
4
5
6
7
8
xanthene] (1.57 g, 4.18 mmol) in dry toluene (50 mL) were
added Pd(OAc)2 (0.03 g, 0.13 mmol), P(tꢀBu)3HBF4 (0.18 g,
0.62 mmol), and K2CO3 (1.73 g, 12.5 mmol). The mixture was
stirred for 5 days at 90 °C under N2. After cooling to room
temperature, the reaction mixture was poured into water and
then extracted with CHCl3. The combined organic layers were
washed with water and dried over anhydrous Na2SO4. After
filtration and evaporation, the product was purified by column
chromatography on silica gel (eluent: hexane/CHCl3/ethyl
acetate = 5:2:1, v/v), recrystallized from CHCl3/hexane, and
dried under vacuum to afford MXAcꢀCM as a light yellow
solid (yield = 1.16 g, 52%). M.p. 306 °C. 1H NMR (500 MHz,
subjected to UV–ozone treatment for 30 min before they were
loaded into an Eꢀ200 vacuum evaporation system (ALS
Technology). The organic layers and a cathode aluminum
layer were thermally evaporated on the substrates under
vacuum (<6 × 10−5 Pa) with an evaporation rate of <0.3 nm s−1
through a shadow mask. The layer thickness and deposition
rate were monitored in situ during deposition by an oscillating
quartz thickness monitor. The currentꢀdensity–voltage–
luminance characteristics of the fabricated OLEDs were
measured using a Keithley 2400 source meter and a CSꢀ2000
spectroradiometer (Konica Minolta).
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ASSOCIATED CONTENT
Supporting Information
CDCl3):
δ 8.48 (d, J = 8.3 Hz, 1H), 7.56 (d, J = 1.8 Hz, 1H),
7.45 (dd, J = 8.3 Hz, 1.8 Hz, 1H), 7.22ꢀ7.16 (m, 4H), 7.13 (dd,
J = 7.9 Hz, 1.5 Hz, 2H), 6.97 (td, J = 7.0 Hz, 2.2 Hz, 2H), 6.68
(dd, J = 8.4 Hz, 1.5 Hz, 2H), 6.63 (d, J = 1.5 Hz, 2H), 6.28 (s,
1H), 6.16 (d, J = 8.4 Hz, 2H), 2.45 (s, 3H), 2.01 (s, 6H). 13C
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
NMR spectra, TGA data, additional photophysical and
device data (PDF)
NMR (125 MHz, CDCl3):
δ 177.55, 166.53, 158.09, 148.47,
146.52, 136.54, 132.69, 131.85, 131.25, 130.36, 129.65,
128.69, 128.27, 127.89, 127.56, 123.66, 123.41, 120.99,
115.95, 113.81, 110.99, 44.63, 20.63, 20.48. MS (MALDIꢀ
TOF): m/z calcd 533.20 [M]+; found 532.90. Anal. calcd (%)
for C38H23NO2: C 83.28, H 5.10, N 2.62; found: C 83.36, H
5.02, N 2.66.
Synthesis of XAc-CM: XAcꢀCM was synthesized
according to the same procedure as described above for the
synthesis of MXAcꢀCM, except that spiro[acridaneꢀ9,9′ꢀ
xanthene] (1.74 g, 5.00 mmol) was used as the reactant instead
of spiro[2,7ꢀdimethylacridanꢀ9,9′ꢀxanthene] (see Scheme 1),
yielding XAcꢀCM as a light yellow solid (yield = 1.05 g, 50%).
M.p. 366 °C. H NMR (500 MHz, CDCl3):
Hz, 1H), 7.59 (d, J = 1.8 Hz, 1H), 7.48 (dd, J = 8.3 Hz, 1.8 Hz,
1H), 7.21ꢀ7.16 (m, 4H), 7.14 (dd, J = 7.6 Hz, 1.3 Hz, 2H),
6.97 (td, J = 7.0 Hz, 2.2 Hz, 2H), 6.91ꢀ6.87 (m, 4H), 6.72 (td,
J = 7.6 Hz, 1.2 Hz, 2H), 6.30ꢀ6.25 (m, 3H), 2.45 (s, 3H). 13C
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: yasuda@ifrc.kyushuꢀu.ac.jp
ORCID
Takuma Yasuda: 0000ꢀ0003ꢀ1586ꢀ4701
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
1
δ 8.51 (d, J = 8.3
This work was supported in part by GrantsꢀinꢀAid from the Hoso
Bunka Foundation, the KDDI Foundation, and the Canon
Foundation. J.L. is grateful for financial support from the JSPS
Research Fellowship for Young Scientists (No. 14J03825). The
authors acknowledges the support of the Cooperative Research
Program “Network Joint Research Center for Materials and
Devices”.
NMR (125 MHz, CDCl3):
δ 177.48, 166.59, 158.13, 148.48,
145.94, 138.44, 132.54, 131.80, 131.19, 129.86, 128.85,
128.21, 127.70, 127.04, 123.68, 123.59, 121.45, 120.98,
116.03, 113.93, 111.03, 44.64, 20.64. MS (MALDIꢀTOF): m/z
calcd 505.17 [M]+; found 504.99. Anal. calcd (%) for
C29H23NO2: C 83.15, H 4.59, N 2.77; found: C 83.11, H 4.56,
N 2.80.
Photophysical Measurements. UV–vis absorption and
PL spectra were measured with a Vꢀ670 spectrometer (Jasco)
and a FPꢀ8600 spectrophotometer (Jasco), respectively, using
degassed spectral grade solvents. The absolute PL quantum
yields were determined using an ILFꢀ835 integrating sphere
system (Jasco). The transient PL decay measurements were
carried out using a C11367 Quantaurusꢀtau fluorescence
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