The Importance of Excited-State Energy Alignment for Efficient Exciplex Systems Based on a Study of Phenylpyridinato Boron Derivatives

Article abstract of DOI:10.1002/anie.201804218

Understanding excited-state dynamics is critical for improving the photoluminescence (PL) efficiency of exciplexes. A series of exciplexes based on conventional hole-transporting materials as donor and newly developed phenylpyridinato boron derivatives as acceptor were investigated. High PL efficiencies were achieved in only some combinations, and a large difference in performance among combinations provided insight into nonradiative processes in exciplex systems. Furthermore, the triplet local excited states (³LE) of each donor and acceptor were found play an important role in triplet exciplex harvesting. Significant contributions from triplets were clearly observed when the charge-transfer excited states (¹CT and ³CT) and ³LE were ideally aligned. We also demonstrated fine control of relative energy alignment via the concentration to improve the PL efficiency.

Full text of DOI:10.1002/anie.201804218

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Accepted Article
Title: The Importance of Excited-State Energy Alignment for Efficient
Exciplex Systems Based on a Study of Phenylpyridinato Boron
Derivatives
Authors: Masashi Mamada, Guojian Tian, Hajime Nakanotani, Jianhua
Su, and Chihaya Adachi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201804218
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10.1002/anie.201804218
Angewandte Chemie International Edition
COMMUNICATION
The Importance of Excited-State Energy Alignment for Efficient
Exciplex Systems Based on a Study of Phenylpyridinato Boron
Derivatives
Masashi Mamada,‡*[a] Guojian Tian,‡[b] Hajime Nakanotani,[a,c] Jianhua Su,[b] and Chihaya Adachi*[a,c]
Abstract: Understanding excited-state dynamics is critical for
improving the photoluminescence (PL) efficiency of exciplexes. Here,
we investigated a series of exciplexes based on conventional hole-
transporting materials as donor and newly developed
phenylpyridinato boron derivatives as acceptor. High PL efficiencies
were achieved in only some combinations, and a large difference in
performance among combinations provided insight into nonradiative
processes in exciplex systems. In addition, the triplet local excited
states (3LE) of each donor and acceptor were found play an important
role in triplet exciplex harvesting. Significant contributions from triplets
were clearly observed when the charge-transfer excited states (1CT
and 3CT) and 3LE were ideally aligned. We also demonstrated fine
control of relative energy alignment via the concentration to improve
the PL efficiency.
designing TADF materials. Therefore, most of the recently
reported TADF molecules are composed of electron donor and
acceptor (D-A) units exhibiting an intramolecular charge-transfer
(CT) transition,[5] with few exceptions.[6] On the other hand,
intermolecular D-A systems can form a charge transfer complex
known as an exciplex.[7]
Exciplex systems exhibit a small ΔEST because of separation
of the hole and electron over two different molecules.[8] So far,
several exciplex systems exhibiting efficient TADF have been
reported, resulting in OLEDs with external EL quantum
efficiencies (ηext) above the theoretical limit for a device using a
traditional fluorescent emitter.[9] However, the mechanisms for
triplet harvesting by exciplexes have not been as thoroughly
investigated as those in intramolecular D-A systems. Exciplex
systems generally suffer from relatively low photoluminescence
(PL) quantum yields (ΦPL),[10] which drastically limits ηext in
devices. Despite recent reports describing vibronic coupling
between a triplet CT state (3CT) and a triplet local excited state
(3LE) as a route for efficient interconversion from a triplet state to
a singlet CT state (1CT),[11] the effect of 3LE of each molecule upon
Spin statistics dictates that 25% of the excitons generated when
holes and electrons recombine in organic light-emitting diodes
(OLEDs) are singlets while 75% are triplets.[1] Therefore, utilizing
triplets is key for realizing internal electroluminescence (EL)
efficiencies of 100%. One approach is to enhance the radiative
transition from triplets through strong spin-orbit coupling at room
temperature in phosphorescent materials, which are mainly
based on heavy-metal complexes.[2] Alternatively, triplets can be
upconverted into emissive singlets through a process called
thermally activated delayed fluorescence (TADF).[3] Efficient
reverse intersystem crossing (RISC) from the lowest-energy
triplet (T1) to the lowest-energy singlet (S1) can be achieved when
a molecule has a small singlet-triplet splitting energy (ΔEST). Since
ΔEST is proportional to the exchange integrals between the wave
functions of the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO),[4] spatial
separation of these orbitals is a generally used strategy for
ΦPL in exciplex systems has not yet been well studied.
Here, we designed and synthesized new spiro-type four-
coordinate boron derivatives for use as acceptors in exciplex
systems, and we studied the derivatives with various
combinations of donor molecules to get insight into the factors
leading to high ΦPL. Organoboron compounds are promising as
n-type semiconductors and electron-accepting building blocks,
and π-conjugated frameworks with an intramolecular boron-
nitrogen coordinate such as four-coordinate organoboron often
show intense luminescence properties and better chemical
stability
compared
to
three-coordinate
organoboron
derivatives.[12] We focused on 2-phenylpyridinato boron (BPPy)
derivatives based on N,C chelate, which are isoelectronic with
fluorene, possess high electron affinity and good luminescence
properties,[13] and have been used as acceptor in TADF
materials.[14] Although some BPPy derivatives with phenyl
substituents have been synthesized, concerns remain about the
stability of boron-carbon single bonds in the excited state.[15] We
functionalized the BPPy units by bridging with diphenylether
(DPE), a weak donor unit, to obtain molecules (depicted in Figure
1a inset) that are rigid, which should simplify analysis of molecular
geometric arrangement in the excited state, for use as acceptors
in exciplex systems.
[a]
Dr. M. Mamada, Prof. H. Nakanotani, Prof. C. Adachi
Center for Organic Photonics and Electronics Research (OPERA)
Kyushu University
744 Motooka, Nishi, Fukuoka 819-0395 (Japan)
and
JST, ERATO, Adachi Molecular Exciton Engineering Project
744 Motooka, Nishi, Fukuoka 819-0395 (Japan)
E-mail: mamada@opera.kyushu-u.ac.jp
E-mail: adachi@cstf.kyushu-u.ac.jp
[b]
Dr. G. Tian, Prof. J. Su
Key Laboratory for Advanced Materials and Institute of Fine
Chemicals
BPPy-DPE and the fluorinated variant BFPPy-DPE with
higher electron affinity for better exciplex formation (see Methods
S1 for synthesis details), hereafter referred to as BPD and BFPD,
were stable, soluble, and sublimable (Figure S1); therefore, thin
films for optical measurements were prepared by spin-coating,
and devices were fabricated using vacuum evaporation (Methods
S2). We confirmed, based on a subset of the samples, that both
spin-coated and vacuum-deposited films produce the same
photophysical results (Figure S2). The emitters were investigated
East China University of Science and Technology
130 Meilong Road, Shanghai, 200237 (China)
Prof. C. Adachi, Prof. H. Nakanotani
International Institute for Carbon Neutral Energy Research (WPI-
I2CNER), Kyushu University
[c]
[‡]
744 Motooka, Nishi, Fukuoka 819-0395 (Japan)
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201804218
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COMMUNICATION
TAPC
BPD
BFPD
BPD:TAPC
BFPD:TAPC
a)
1
Table 1. Photophysical properties of exciplex films.
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
Conc.
λem
S1
ΦPL
Guest
Host (HTM)
[wt%][a]
[nm][b]
[eV][c]
[%][d]
– (neat)
m-MTDATA
TPD
100
10
10
10
15
20
50
10
50
10
10
100
10
10
20
50
428
565
494
488
490
492
511
475
485
471
407
436
548
472
480
493
3.32
2.53
2.84
2.88
2.85
2.83
2.76
3.00
2.91
3.02
3.44
3.30
2.63
2.98
2.94
2.86
1.0
10.4
13.9
55.1
60.0
56.6
50.2
19.1
25.4
21.6
2.0
b)
BFPD:HTM
m-CBP
Spiro-BPA
TCTA
TAPC
TPD
TAPC
BFPD
m-MTDATA
TCTA
400
500
600
700
800
Spiro-BPA
m-CBP
Wavelength (nm)
c)
d)
– (neat)
1.2
m-MTDATA
9.0
BPD
26.6
33.7
35.4
TAPC
[a] Guest concentration. [b] Measured in ambient conditions. [c] Calculated
from λonset. [d] Absolute PL quantum yield (ΦPL) measured under argon flow
in an integrating sphere at room temperature.
Figure 1. a) PL spectra of TAPC, BPD, and BFPD neat films and 10-wt%-doped
BPD:TAPC and BFPD:TAPC films. Inset: Chemical structures of BPD and
BFPD. b) PL spectra of 10 wt% BFPD doped in various HTMs. c) PL decay
curves of 10-wt%-BFPD:TAPC film at 100 K, 200 K, and room temperature.
Inset: PL spectra at 100 K and room temperature resolved into prompt and
delayed components. d) PL decay curves of 10-wt%-BFPD:HTM films at room
temperature.
order (Figure 1c), indicating a large contribution of triplet
harvesting to the emission. A slight redshift for delayed emission
at room temperature and a small ΔEST (0.04 eV) estimated from
the onsets of the emission spectra are also consistent with
emission proceeding through RISC.[8] However, the decay curves
of exciplex emitters are generally complicated. For example, the
decay in the ~1 μs regime was reported to be tri-exponential for
some exciplex emitters,[16] presumably because the molecular
environments changes over time, and the delayed emission >1 μs
includes many components such as from TADF with a broad
distribution of energy levels, room-temperature phosphorescence,
and long persistent luminescence from charge separation and
recombination.[17]
Although it is difficult to assign all components, ΦPL for
BFPD:TAPC at room temperature can be divided into 29
percentage points from short-lived components (Φp) with lifetimes
<1 μs and 26 percentage points from delayed components (Φd)
with lifetimes >1 μs (Table S7). On the other hand, the films of
BFPD:Spiro-BPA and BFPD:TPD showed negligibly weak
delayed emission at both room temperature and 100 K (Figure 1d
and Figure S10). Thus, the difference in ΦPL values (22% and
14%) for exciplexes in these two systems are directly related to
Φp. The films of BFPD:TCTA and BFPD:m-MTDATA have Φp of
16% and 7.0% and Φd of 3.5% and 3.4%, respectively.
for TADF (Note S1, Methods S3, Tables S1–S6, Figures S3-S9),
but no conclusive evidence could be found.
Exciplex systems based on BPD or BFPD as the acceptor
were evaluated using various hole transporting materials (HTMs)
as the donor (full chemical names and structures can be found in
Data S1). The selected HTMs are commonly used for the hole
transporting layer (HTL) in OLEDs. Figure 1a shows PL spectra
from neat films of BPD, BFPD, and TAPC and from 10-wt%-
acceptor-doped films of TAPC. The broad, featureless emission
band with a large redshift with respect to the emission of neat films
of the components confirms the formation of exciplex D+A−
complexes in each of the blend films.[16] Blend films using other
HTMs also exhibited redshifted spectra (Figure 1b) with the
exception of m-CBP, for which emission from each component
was observed without exciplex formation. Interestingly, the choice
of HTM greatly influences ΦPL (Table 1), and BFPD:TAPC exhibits
a ΦPL of 60%, one of the highest values for an exciplex system
and tens of times higher than the values for BFPD (ΦPL = 1%) and
TAPC (ΦPL = 5%) in neat films and BFPD in toluene (ΦPL = 3.1%).
The transient PL decay of the BFPD:TAPC film clearly
exhibits delayed emission with a lifetime on the microsecond
To investigate the origin of the differences in PL efficiency, we
first evaluated the properties of the HTMs using the same
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Acceptor
Donor (HTM)
Acceptor Donor (HTM)
−1.97
−2.34
−1.97
−2.02
−2.02
−2.37
−5.50
−2.49
−5.74
−2.57
−6.08
−2.66
−2.67
LUMO
HOMO
−
−
+
+
−5.16
−5.16
−5.57
−5.57
−5.72
−6.24
−6.22
3.51
3.64
3.51
3.36
± 0.04
3.36
± 0.06
3.42
3.37
molecule S1
(1LE)
3.44
3.25
3.12
3.12
2.90
± 0.05
3.02
3.00
2.98
2.90
2.87
2.81
± 0.07
2.88 2.87
2.84
molecule T1
(3LE)
2.84
2.38
2.79
2.78
2.81
(1CT)
2.63
2.60
2.54
2.60
Exciplex S1
Exciplex T1
2.53
2.47
2.46
(3CT)
exciplex
not formed
Type III
Type I
Type II
Type I
Type I
Type II
Type II
Figure 2. HOMO, LUMO, and excited-state energies of BFPD, BPD, the HTMs, and the exciplex systems.
methods (Methods S4) as for BPD and BFPD for reliable
comparison. The results are detailed in Figures S11–S13 and
Tables S8–S9 and summarized in Figure 2. Since 3LE of BPD and
BFPD might shift depending on the polarity of the surrounding
media but cannot be measured in each blend, we show an
average and upper and lower limits based on the levels in doped
polystyrene (less polar conditions) and neat (more polar
conditions) films.
evidenced by phosphorescence from 3LE (Figure S15),
preventing RISC and resulting in a nearly negligible delayed
component. The blend BFPD:TCTA is also categorized as type I,
but the closer proximity of the 3LE levels of both molecules to 1CT
leads to a ΔEST of ca. 0.2 eV and a very small contribution from
delayed emission. On the other hand, the strongest delayed
emission results from the type II alignment of BFPD:TAPC, which
has 1CT and 3CT that are very close in energy to the 3LE of each
3
The energy offsets between the HOMOs and LUMOs of the
donor and acceptor drive the electron transfer that leads to
exciplex formation. Therefore, BFPD:m-CBP exhibits prompt
emission alone with low ΦPL and no clear exciplex formation
because of its small offsets. The energy of exciplex emission in
the other combinations is clearly correlated with the HOMO of the
HTM, that is, the S1 energies of the exciplex (1CT) are in good
agreement with Δ(ALUMO-DHOMO), the difference between the
LUMO of the acceptor and the HOMO of the donor, though the
values may be offset by a small binding energy. Furthermore, we
found that the relative energy ordering of T1 of the donor and
acceptor molecules (3LE) and 1CT plays a critical role in the
delayed emission, which, based on the dependence of the
integrated delayed emission area on the excitation intensity
(Figure S14), was confirmed to not originate from triplet-triplet
annihilation (TTA) in any of the films. The energy ordering can be
classified into three types with distinct emission properties: Type
I, with at least one 3LE (donor and/or acceptor) well below the CT
molecule. In this case, the CT states are in resonance with LE,
which enhances coupling between 1CT and 3CT.
Despite the type III blend BFPD:m-MTDATA having a quite
small ΔEST of 0.06 eV, which is comparable to that of BFPD:TAPC,
the triplet contribution (Φd/ΦPL = 33%) is much less than that of
the TAPC blend (Φd/ΦPL = 47%) (Figure 1d). Though the type III
alignment should be able to confine triplets to the exciplex,[8] these
results indicate that the RISC process is not simply controlled by
1
3
only the CT and CT levels but can also be accelerated by the
3LE of each molecule even in an exciplex system. This model of
non-adiabatic coupling between 3CT and 3LE is also confirmed by
the strong phosphorescence at low temperatures for the m-
MTDATA, TAPC, and TCTA blends and the very weak
3
phosphorescence from LE in the TPD blend (Figure S10 and
S14). Previous reports of high ΦPL from exciplex emitters can also
be attributed to large triplet contributions. For example, a ΦPL of
100% at low temperatures (60% at room temperature) was
reported for exciplex emission from TCTA:B4PYMPM, in which
the 3LE of TCTA and B4PYMPM seem to be located very close to
the CT states.[18,19] Therefore, 3LE needs to be considered along
with Δ(ALUMO-DHOMO) when selecting materials for highly efficient
exciplex emission.
3
3
states (>0.2 eV); Type II, with at least one LE near and no LE
3
below the CT states; and Type III, with LE of both molecules
above the CT states.
Clear examples of type
I are BFPD:Spiro-BPA and
BFPD:TPD, which have 3LE that are much lower than their 1CT.
3
In these blends, the triplet exciplexes decay to the LE state, as
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10.1002/anie.201804218
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dissociation processes. Although the
nonradiative processes of the exciplexes
have still not been fully elucidated and
further investigation of the relationship
between nonradiative decay and
molecular structure is still needed, our
results provide additional guidelines for
the improvement of Φp in exciplex
systems.
a) BPD:TAPC
Exciplex
b) BFPD:TAPC
c) BFPD:TCTA
TAPC
T1
BPD
T1
Exciplex
TAPC
BFPD
T1
Exciplex
TCTA
BFPD
T1
S1
S1
T1
S1
T1
3.0
10 wt%
10 wt%
2.9
10 wt%
50 wt%
50 wt%
50 wt%
2.8
Ф = 27% → 35%
(10 wt%) (50 wt%)
Ф = 60% → 50%
Ф = 19% → 25%
(10 wt%) (50 wt%)
2.7
(15 wt%) (50 wt%)
Finally, we evaluated OLEDs using
BFPD:TAPC films as an emitting layer
(Figure 4 and Table S10) sandwiched on
each side by a layer of one of the
components to confine the exciplexes.
The ηext are higher than the theoretical
limit of 5% for conventional fluorescence
emitters (ηext = 7.6, 9.8, and 10.5% for
devices A, B, and C, respectively),
although devices A and B showed ηext
slightly lower than those expected from
their ΦPL. This might be due to charge
carrier imbalance and carrier traps, as
evidenced by a large rolloff arising from
the limited number of electron transport
sites in the emitter layers with a lower
BFPD concentrations. The EL spectra (λELmax = 508, 509, and 518
for devices A, B, and C, respectively) were slightly redshifted
compared to the corresponding PL spectra. This has been
observed for exciplex-based TADF emitters in OLEDs.[8] The
Figure 3. Exciplex emission spectra at different concentrations and the phosphorescence spectra of each
molecule.
Rough matching of 1CT and 3LE may be easily achieved, but
fine-tuning of the energy levels is likely needed to help optimize
performance. An easy approach is to change the concentration.
As shown in Figure 3, the 1CT energies were shifted down by up
to ca. 0.1 eV when the doping concentration was increased from
10 wt% to 50 wt%, while the 3LE of the host may not change so
much from 90 wt% to 50 wt%. As a result, ΦPL of BPD:TAPC and
BFPD:TCTA were higher for 50 wt% films, in which the 1CT levels
were shifted toward 3LE. On the other hand, 10-wt%-BFPD:TAPC
already has well-aligned energy levels and doping at higher
concentrations moves 1CT away from 3LE while still keeping it in
a reasonable range for RISC to occur (though less effectively),[9g]
resulting in a slight decrease of ΦPL. In all cases, the change of
ΦPL can be attributed to changes in the triplet contribution (Figure
S16). Thus, concentration tuning provides a simple method for
optimizing triplet harvesting in exciplex systems.
2.0
a)
b)
2.6
2.7
4.1
LiF/Al
4.7
ITO
5.6
Device A: 10 wt% 6.2
Device B: 15 wt%
Device C: 50 wt%
6.7
F
F
N
B
O
BFPD
Although the differences in Φd have primarily been discussed,
a large difference in Φp also exists. While the strong mixing of 1CT
and LE provides a high RISC yield according to the large spin-
c)
d)
3
101
100
orbit coupling element between them, the ISC yield should also
be efficient. However, Φp of BFPD:TAPC is much higher than
those of the others, indicating the presence of additional
nonradiative decay channels in the latter. The BPD-based
systems also show similar behavior (Φp = 6.1 vs 19% in m-
MTDATA and TAPC blends; Table S7). In general, there is a trend
Device A
Device B
Device C
10-1
10-2 10-1 100
101
102
of decreasing Φp with decreasing Δ(ALUMO-DHOMO). Since Δ(ALUMO
-
Current density (mA cm−2
)
D
HOMO) is inversely related to the CT interaction strength, this
indicates that stronger CT interaction leads to an increase in the
effect of nonradiative quenching, which originates from phonon-
mediated processes and charge dissociation.[18] However, among
the BFPD-based systems, BFPD:TAPC does not follow this trend.
One possibility is that, as a result of strong localization of the
HOMO on oxidation sites in TAPC, BFPD:TAPC forms more
tightly bound radical pairs, leading to the suppression of charge
Figure 4. a) Energy level diagram of OLEDs using BFPD:TAPC as the emitting
layer. Energy levels are in units of eV, and values in parentheses indicate layer
thicknesses. b) Current-density−voltage (J−V) characteristics of the OLED
devices. c) PL spectra for 10 wt% and 50 wt% BFPD:TAPC films and EL spectra
for devices A and C. (d) External electroluminescence quantum efficiency of the
OLEDs as a function of current density.
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COMMUNICATION
K. Nakajima, S. Nomura, S. Nakatsuka, K. Kinoshita, J. Ni, Y. Ono, T.
Ikuta, Adv. Mater. 2016, 28, 2777-2781; c) T. Hofbeck, U. Monkowius, H.
Yersin, J. Am. Chem. Soc. 2015, 137, 399-404; d) M. Mamada, K. Inada,
T. Komino, W. J. Potscavage, Jr., H. Nakanotani, C. Adachi, ACS Cent.
Sci. 2017, 3, 769-777.
enhancement of delayed fluorescence under electrical excitation
also proves the efficient triplet harvesting of this exciplex system.
Since the BFPD:TAPC films have relatively high excited-state
energies, this system is potentially promising for application as an
exciplex host for phosphorescent or fluorescent emitters.[20]
[7]
L. Rothberg in Semiconducting Polymers, Vol. 1, (Eds.: G. Hadziioannou,
G. G. Malliaras) Wiley-VCH, Weinheim, 2007, pp.179-204.
In summary, we demonstrated highly efficient exciplex
systems based on phenylpyridinato boron derivatives and TAPC.
The high ΦPL from exciplexes is attributed to the appropriate
combination of D-A molecules providing tight binding of radical
pairs. In addition, we clearly showed that the triplet contribution is
strongly related to the relative energy ordering of 1CT and 3CT of
[8]
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3
the exciplex and LE of the donor and acceptor. Efficient TADF
was achieved when 3CT and 3LE are aligned as to allow 3LE to be
involved in the RISC process. Furthermore, different
concentrations of blend films could provide a shift of the 1CT level,
allowing the energy alignment to be tuned. These findings open
up the possibility to achieve very high photoluminescence
quantum yield from exciplex emitters.
[10] H. Nakanotani, K. Masui, J. Nishide, T. Shibata, C. Adachi, Sci. Rep.
2013, 3, 2127.
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Acknowledgements
We thank Dr. W. J. Potscavage, Jr., for thorough and critical
editing of the manuscript. This work was financially supported by
JST ERATO Grant Number JPMJER1305 and JSPS KAKENHI
Grant Number JP17K14531. G. Tian thanks the China
Scholarship Council for a scholarship.
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Keywords: exciplex • phenylpyridinato boron • donor-acceptor •
thermally activated delayed fluorescence (TADF) • organic light
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This article is protected by copyright. All rights reserved.

10.1002/anie.201804218
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Highly efficient exciplex:
M. Mamada,* G. Tian, H. Nakanotani,
J. Su, C. Adachi*
Contribution of thermally activated
delayed fluorescence to emission
improves photoluminescence
Page No. – Page No.
quantum yield of exciplexes and is
found to be strongly affected by the
location of the triplet local excited
states (3LE). The ideal alignment of
the charge-transfer excited states
and the 3LE has been realized in an
exciplex based on a phenylpyridinato
boron derivative and TAPC.
The Importance of Excited-State
Energy Alignment for Efficient
Exciplex Systems Based on a Study
of Phenylpyridinato Boron
Derivatives
This article is protected by copyright. All rights reserved.