Donor–σ–Acceptor Motifs: Thermally Activated Delayed Fluorescence Emitters with Dual Upconversion

Article abstract of DOI:10.1002/anie.201708876

A family of organic emitters with a donor–σ–acceptor (D-σ-A) motif is presented. Owing to the weakly coupled D-σ-A intramolecular charge-transfer state, a transition from the localized excited triplet state (³LE) and charge-transfer triplet state (³CT) to the charge-transfer singlet state (¹CT) occurred with a small activation energy and high photoluminescence quantum efficiency. Two thermally activated delayed fluorescence (TADF) components were identified, one of which has a very short lifetime of 200–400 ns and the other a longer TADF lifetime of the order of microseconds. In particular, the two D-σ-A materials presented strong blue emission with TADF properties in toluene. These results will shed light on the molecular design of new TADF emitters with short delayed lifetimes.

Full text of DOI:10.1002/anie.201708876

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Accepted Article
Title: Donor-σ-Acceptor Motif Thermally Activated Delayed
Fluorescence Emitters with Dual Upconversion
Authors: Yan Geng, Anthony D’Aleo, Ko Inada, Lin-Song Cui, Jong Uk
Kim, Hajime Nakanotani, and Chihaya Adachi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708876
Angew. Chem. 10.1002/ange.201708876
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10.1002/anie.201708876
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Donor-σ-Acceptor Motif Thermally Activated Delayed
Fluorescence Emitters with Dual Upconversion
Yan Geng[a,b], Anthony D’Aleo[c], Ko Inada[a], Lin-Song Cui[a], Jong Uk Kim[a], Hajime Nakanotani[a,d,e]
and Chihaya Adachi*[a,d,e]
Abstract: A family of organic emitters having a donor-σ-acceptor
(D-σ-A) motif is demonstrated. Based on the weakly coupled D-σ-A
intramolecular charge transfer state, a transition from the localized
excited triplet states (3LE) and charge transfer triplet states (3CT) to
the charge transfer singlet states (1CT) with a small activation
energy and high photoluminescence quantum efficiency is
accomplished. Two thermally activated delayed fluorescence
(TADF) components are identified, one of which has a very short
lifetime of 200-400 ns and another a longer TADF lifetime of the µs
order. In particular, two D-σ-A materials present strong blue
emission with TADF properties in toluene. This result will shed light
on the molecular design of new TADF emitters having short delayed
lifetimes.
metals, which hinder their long-term mass production and
utilization. Recently, however, the application of metal-free
thermally activated delayed fluorescence (TADF) emitters has
led to the development of TADF-OLEDs. These are among the
most promising classes of practical next-generation OLEDs,
since high-performance EL characteristics can be obtained
through the molecular design of conventional aromatics, which
is in effect unlimited.[4]
In TADF-OLEDs, electrically generated triplet excitons can
be efficiently upconverted from the lowest triplet excited state
(T1) to the lowest singlet excited state (S1) via reverse
intersystem crossing (RISC) when the energy difference
between the two excited states (∆EST
) is small enough,
generally less than a few hundreds meV.[5] Recently, TADF-
OLEDs have made substantial progress in achieving an external
EL quantum efficiency of over 30%, which is comparable with
the best PHOLEDs.[6] The TADF process is central to the
formation of a charge-transfer (CT)excited state from donor-
acceptor (D-A) molecules, where the highest occupied
molecular orbital (HOMO) is located on the donor moiety, and
the lowest unoccupied molecular orbital (LUMO) is offered by
the acceptor moiety. For the conventional D-A type TADF
emitters, the overlap between HOMO and LUMO must be
minimized to decrease ∆EST, which is crucial for efficient thermal
upconversion via RISC.[4]
Nowadays, organic light-emitting diodes (OLEDs) are widely
utilized in flat-panel displays and solid-state lighting, and attract
considerable interest both in academia and industry.[1] The
internal electroluminescence (EL) quantum efficiency (IQE) of
OLEDs with conventional organic fluorophores is in practice
limited to 25% because 75% of the electrically generated
excitons are statistically triplets that decay nonradiatively.[2] In
comparison with fluorescent OLEDs, phosphorescent OLEDs
(PHOLEDs) with precious-metal complexes as emitters can
achieve IQEs of 100%, since both singlet and triplet excitons
can be harvested as a result of singlet-triplet state mixing via
efficient spin-orbit coupling.[3] However, PHOLEDs still suffer
from the high cost, scarcity and toxicity of these precious
Besides the well-established single-molecule TADF
emitters, exciplex emitters based on intermolecular CT
transitions were also confirmed to function via TADF by our
group in 2012.[7] Compared with single-molecule TADF
emitters, a wide variety of exciplex TADF emitters have been
developed by the appropriate combination of donor and
acceptor molecules. While some technical issues, such as low
photoluminescence quantum yield (PLQY), complex device
fabrication and delicate requirements for energy level alignment,
represent a barrier to the development of high-performance
exciplex TADF OLEDs, recent studies suggest the crucial
importance of control over the distance between donor and
acceptor moieties.[8] All the studies to date highlight the
necessity of CT state in TADF mechanism.
In this context, we explored an improved design of TADF
molecules having a donor-σ-acceptor (D-σ-A) structure where
the σ-group allows to some extent to control the distance in an
intramolecular way.[9] In principle, hyperconjugation in such D-σ-
A molecules would not only favor the wide separation of the
HOMO and LUMO, resulting in small ∆EST, but also overcome
the disadvantages of exciplex TADF emitters, where the charge
transfer state is reached through intermolecular charge transfer.
The D-A geometries in exciplexes are uncontrollable in solid
films, which increases nonradiative decay.[8] More importantly,
we aim for very fine tuning and alignment of the CT and local
excited (LE) state energy levels, so that the mixing of the
[a]
Dr. Y. Geng, K. Inada, L.-S. Cui, J. U. Kim, Prof. H. Nakanotani and
Prof. C. Adachi
Center for Organic Photonics and Electronics Research (OPERA),
Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395,
Japan.
E-mail: adachi@cstf.kyushu-u.ac.jp
Homepage: http://www.cstf.kyushu-u.ac.jp/~adachilab/
Dr. Y. Geng
College of Chemistry, Chemical Engineering and Materials Science,
Shandong Normal University, Jinan 250014, P. R. China.
Dr. A. D’Aleo,
Aix Marseille Univ., CNRS, CINaM UMR 7325, Campus de Luminy,
Case 913, 13288 Marseille, France.
Prof. H. Nakanotani and Prof. C. Adachi
Japan Science and Technology Agency, ERATO, Adachi Molecular
Exciton Engineering Project, 744 Motooka, Nishi-ku, Fukuoka 819-
0395, Japan
[b]
[c]
[d]
[e]
Prof. H. Nakanotani and Prof. C. Adachi
International Institute for Carbon Neutral Energy Research (WPI-
I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-
0395, Japan
Supporting information for this article is given via a link at the end of
the document. CCDC 1569936 and 1569947 contain the
supplementary crystallographic data for this paper. These data are
provided free of charge by The Cambridge Crystallographic Data
Centre
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10.1002/anie.201708876
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different excited-state species enhances their radiative decay
rate and hence their emission properties.
In 3, the distance between two TRZ units is above 3.6 Å. These
twisted configurations and the lack of molecular contact prevent
both materials from forming excimers and exciplexes in the solid
state.[11]
To determine experimentally the HOMO and LUMO
energies of these molecules, the first oxidation and reduction
potentials were measured in DCM and dimethylformamide
(DMF) solutions, respectively, by cyclic voltammetry (CV). As
shown in Fig.S2, all of the materials present the same reduction
potential of -1.92 V, which can be assigned to the reduction of
the TRZ acceptor moieties. The oxidation potentials are in the
range of 0.47 to 0.59 V, which is in good agreement with the
electron-donating capabilities of those units. Thus, the LUMO
energy levels of 1-3 were estimated to be -2.50 eV for all three
molecules, and the HOMO energy levels as -5.26, -5.42 and -
5.41 eV, respectively.[12]
Figure 1. Molecular structures, calculated HOMO and LUMO energies and
molecular orbital distribution of HOMO and LUMO for 1-3.
The D-σ-A type molecules (1-3) designed in this study are
composed of triazine (TRZ) as acceptor (A), connected to
tetramethyl-substituted carbazole (TMCz), dihydroacridine
(DMAC) or triphenylamine (TPA) moieties as donors (D) via
hexafluoroisopropylidene as a σ-bridge, as illustrated in Fig.1.
They could be easily prepared by a multi-step synthesis,
finishing with a Buchwald-Hartwig amination reaction, in good
yields of ca. 80% (Scheme S1). Details of the synthetic route
are provided in the supporting information. All materials were
fully characterized by 1H-NMR, 13C-NMR, 19F-NMR, mass
spectrometry and elemental analysis.
Figure 2. a) Absorption, fluorescence (298
phosphorescence (77 K in toluene matrix) spectra of molecules 1-3.
K
in toluene) and
The three-dimensional geometries, frontier molecular
orbital energy levels and ∆EST of these molecules were
calculated using density functional theory with the Gaussian 09
package at the B3LYP/6-31G(d) level.[10] Similar to previously
reported TADF emitters, the HOMO and LUMO of the three
molecules are mainly dispersed over the electron-donating
moiety and the acceptor moiety, respectively (Fig.1). The
overlap between the HOMO and LUMO is clearly minimized by
the effective separation of their electron densities caused by the
introduction of the σ-bridge, implying a small energy difference
between the singlet and triplet excited states. Based on the
ground-state geometries, the ∆EST values of compounds 1-3
are calculated to be extremely small, i.e., 0.0006, 0.0009 and
0.0165eV, respectively, as illustrated in detail in the supporting
information (Table S1a). Despite different values using different
calculation methods (Table S1c), the ∆EST values of 1-3 always
showed the same trend of increasing in that order.
To investigate the intramolecular and intermolecular
contacts within these materials, single crystals of 1 and 3,
obtained by the vapor diffusion method with dichloromethane
(DCM) and hexane, were analyzed. The single-crystal X-ray
analysis revealed that both molecules present an asymmetric
molecular geometry. In both molecules, the donor segment
(DMAC or TPA) and acceptor segment (TRZ) adopt a highly
twisted configuration. As evident from the packing pattern,
neither intramolecular nor intermolecular spatial contact
between donor and acceptor moieties is observed in 1 (Fig.S1).
The photophysical properties of the three materials were
investigated using absorption and fluorescence spectroscopy
and compared with those of the control molecules, DMAC-TRZ,
TMCz-TRZ and DPA-TRZ (D-A structure), having no -bridge
(Fig.S3). All the D-A molecules exhibit absorption bands in the
region of 350 to 420 nm, with moderately weak molar
absorption coefficient ε, which can be ascribed to the
intramolecular charge-transfer (ICT) transition from the HOMO
to the LUMO. Unlike their D-A analogs, the absorption spectra of
molecules 1-3 do not present observable ICT states in the
longer-wavelength region, as shown in Fig.2, indicating no CT
interaction in their ground states. Nevertheless, the
photoluminescence (PL) spectra exhibit broad and structureless
emission, indicating that their emissive S1 excited states are
ICT states in nature, which is compatible with conventional
exciplex emission, and with the computational values. Further,
the emission maxima of 2 and 3 are blue-shifted relative to that
of 1, corresponding to their lower electron-donating capabilities.
Meanwhile, the phosphorescence spectra of the three molecules
captured at 77 K are all well resolved and show characteristic
vibrational structures, indicating that their lowest emissive T1
excited states are 3LE (Fig.2) at this temperature.[6] Based on
the values calculated from their fluorescence and
phosphorescence spectra, the ∆EST of materials 1, 2 and 3 are
estimated to be 0.06, 0.07 and 0.18eV, respectively. Extremely
small ∆EST values are often accompanied by low PLQY, since
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the oscillator strength (f) is proportional to the HOMO–LUMO
PLQY after removing oxygen. To define the nature of those
three component decays, streak camera experiments in toluene
overlap integral. Therefore, it is crucial to take both parameters,
∆EST and f, into account in the design of highly efficient TADF.[13] solution were performed. These revealed that the first, rapid
1
The computational data (Table S1) of oscillator strength with
extremely small values of 0.0008, 0.0012 and 0.0054 for 1-3,
respectively, repeat our concern to some extent. For all three D-
σ-A molecules, the PLQY values in toluene before degassing
are below 10%, with particularly low values of 1.6% for 1 and 2.
Interestingly, the PLQY values are dramatically increased in
oxygen-free toluene, reaching 70%, 36% and 20% for 1-3,
decays of 1 and 2 both originate from the mixture of LE and
1CT emissions (Fig. S5), similar to the behavior in toluene
before degassing as revealed in Fig. S4. Meanwhile, the second
1
decay can be ascribed to the CT emission with a rather longer
lifetime. Further, we also identified the third delayed component,
having a redshifted broad CT-type emission in molecules 1 and
2 (Fig. S5). Based on the PLQY enhancement and different
lifetimes of the 1CT emission, we conclude that the first delayed
1CT emission (380 ns for 1 and 190 ns for 2) and subsequent
delayed emission both originate from TADF processes. To
clarify the detailed mechanism, we further measured the
temperature dependence of the PL decay characteristics. For 1
and 2, the first delayed lifetime increased with decreasing
temperature (Figs.S6 and S7). Meanwhile, material 3 presented
no second TADF component.
respectively, which is also
a
common observation in
conventional TADF molecules (Table S3). Importantly, the PL
spectra of the three materials in toluene before degassing are all
1
1
composed of the superposition of LE and CT emission, which
can be recognized from the characteristic vibrational structure
(around 360 nm) and broad emission peak (around 450 nm) in
the spectra, respectively (Fig. S4). The emission peaks at the
shorter-wavelength region can be attributed to emission from the
1LE state, and agree well with the emission spectra of the donor
moieties themselves.
Figure 4. Jablonski diagram of molecules 1 and 2.
In Fig. 4, we show the most probable dual TADF mechanism.
For these D-σ-A emitters, the excitation energy must be higher
1
than those of their donor and acceptor analogs, to ensure LE
emission of the donors. Then, because of the fast ISC process,
most of the excitons transfer to 3LE(Donor), where they are
effectively quenched in the presence of 3O2, resulting in very
low PLQY for 1 and 2 in toluene before degassing. After
degassing, in contrast, the excitons in 3LE(Donor) are able to
convert to the 1CT state by the RISC process, and from here
they radiatively decay (the first TADF emission) to the ground
state. Here the upconversion process is rather fast, i.e., 200-
400 ns, because the transition is spin-allowed based on El
Sayed’s rule in particular, the emission efficiency in molecule 1
is quite high at 63% (Table S5). The increase of τd with
decreasing temperature, with almost no change of PLQY, in
molecules 1 and 2 indicates that the transition from the 3LE(Donor)
to the 1CT states is an endothermic process, but nonradiative
decay is heavily suppressed even at room temperature.
However, when the triplet exciton cannot overcome the gap, this
results in the characteristic phosphorescence from the 3LE(Donor)
state, as shown in Fig. 2. Here, we also mention the presence
Figure 3. a) Transient PL decay profilesof 1-3 in toluene (black line) and
oxygen-free toluene (red line); b)Transient PL decay profiles of 1-3 in oxygen-
free toluene measured by streak camera experiments on a short time scale.
It is noteworthy that three decay components were
revealed in the transient PL decay profiles of materials 1and 2
in oxygen-free toluene, as shown in Fig.3. The first decay
components clearly appeared as a small bump in 1 and 2 but
could not be discerned in 3. These small, fast amplitudes
indicate that the ISC process is much faster than the
14]
fluorescent radiative decay.[5c,
In comparison, there is only
one component decay for molecule 3 in the transient PL decay
profile, and an appreciable enhancement of the lifetime and
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A. Baldo, R. J. Holmes, S. R. Forrest, Z. G. Soos, Phys. Rev.B:
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of the second delayed component, which can be ascribed to the
upconversion from the 3CT state to the 1CT state after the
forward ISC decay. Note that the efficiency of this process is
only 6% and 3% in molecules 1 and 2, respectively, indicating
that the first delayed channel is the major TADF delay process.
In the case of 3, the PL behavior is appreciably different from the
other two. The PLQY is 10% and 20% with and without O2,
respectively, indicating the presence of highly efficient
nonradiative decay processes, which can be ascribed to the
donor unit of TPA. Since the TPA unit is rather rotationally
flexible compared with the other donors, the excitons in 3LE(Donor)
undergo efficient nonradiative decay, and the decrease of
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temperature contributes to
a slight enhancement of the
upconversion. Also, the lack of a clearly observed second
TADF component indicates that nonradiative decay is also
dominant from the 3CT state. Overall, we have confirmed that
molecules 1 and 2 have very fine-tuned energy alignments,
demonstrating two sources of upconversion.
Finally, we note further aspects of this molecular design
concept with respect to OLED application. In view of crucial
issues such as device lifetime and efficiency roll-off in OLEDs,
the decrease of delayed lifetime in TADF emitters will
undoubtedly enhance device performance. However, at the
current stage of design, the presence of the second delayed
component will prevent their effective application because of the
direct exciton formation at this energy level. In a follow-up study,
we intend to finely tune the σ length to enable the complete
mixing of 3CT and 1CT, allowing the disappearance of the
second slow TADF component.
In summary, we have demonstrated a novel molecular
design strategy for TADF emitters based on donor-σ-acceptor
molecules. By separating the HOMO and LUMO through the
introduction of hyperconjugation, a small ∆EST with dual TADF
was obtained, arising from fine-tuned energy alignment of the
1CT, 3CT and 3LE states. In spite of the weakly coupled
intramolecular CT state, considerably high PLQY values can be
achieved using these materials because of their rigid molecular
structures. More interestingly, relatively short TADF decayed
lifetimes of less than 400 ns were achieved with 1 and 2. We
believe our present study provides a new design strategy using
hyperconjugation for advanced TADF materials.
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Acknowledgements
This work was supported by JST ERATO Grant Number
JPMJER1305, Japan.
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Conflicts of interest
[10] 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.
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The authors declare no conflicts of interest.
Keywords: Delayed fluorescence･donor-σ-acceptor･
hyperconjugation･lifetime･organic light-emitting diodes
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10.1002/anie.201708876
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Yan Geng, Anthony D’Aleo, Ko Inada,
Lin-Song Cui, Jong Uk Kim, Hajime
Nakanotani and Chihaya Adachi*
Page No. – Page No.
Donor-σ-Acceptor Motif Thermally
Activated Delayed Fluorescence
Emitters with Dual Upconversion
We explored a novel molecular design strategy for TADF emitters based on a
donor-σ-acceptor motif. Two D-σ-A emittersdisplayed short TADF decayed lifetimes
of less than 400 ns. Since the decrease of delayed lifetime in TADF emitters is
highly beneficial fordevice performance, we believe this work provides an effective
approach for advanced TADF materials.
This article is protected by copyright. All rights reserved.