Communications
Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201609459
German Edition:
DOI: 10.1002/ange.201609459
Organic Electronics Hot Paper
Controlling Singlet–Triplet Energy Splitting for Deep-Blue Thermally
Activated Delayed Fluorescence Emitters
Lin-Song Cui, Hiroko Nomura, Yan Geng, Jong Uk Kim, Hajime Nakanotani, and
Chihaya Adachi*
Abstract: The development of efficient metal-free organic
emitters with thermally activated delayed fluorescence (TADF)
properties for deep-blue emission is still challenging. A new
family of deep-blue TADF emitters based on a donor–acceptor
architecture has been developed. The electronic interaction
between donor and acceptor plays a key role in the TADF
mechanism. Deep-blue OLEDs fabricated with these TADF
emitters achieved high external quantum efficiencies over
19.2 % with CIE coordinates of (0.148, 0.098).
Nowadays, light-emitting diodes based on organic materi-
als, so-called organic light-emitting diodes (OLEDs), have
been widely touted for next-generation displays and solidstate lighting due to their desirable features such as low-cost
production, flexibility, and light weight. According to spin
statistics, electrically injected holes and electrons recombine
to form singlet and triplet excitons in a ratio of 1:3.[1] To
achieve an ideal 100 % internal quantum efficiency (IQE),
many efforts have been devoted to the use of non-emissive
and longer-lived triplet excitons. In comparison to fluorescent
OLEDs, organic phosphorescent devices containing preciousmetal complexes offer a four-fold increase in IQE.[2] However, the high costs and unclear toxicities of precious-metal
materials limit their cost effectiveness and long-term mass
production.[3] Moreover, due to the increasing probability for
thermal population of non-emissive metal-centered states as
the optical gap increases, the development of blue and deepblue precious-metal emitters for OLEDs is significantly
limited and challenging.[4]
[*] L.-S. Cui, Y. Geng, J. U. Kim, H. Nakanotani, Prof. C. Adachi
Department of Applied Chemistry
Center for Organic Photonics and Electronics Research (OPERA)
Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395 (Japan)
E-mail: [email protected]
Homepage: http://www.cstf.kyushu-u.ac.jp/ ~ adachilab/
Prof. C. Adachi
Japan Science and Technology Agency, ERATO
Adachi Molecular Exciton Engineering Project
744 Motooka, Nishi, Fukuoka 819-0395 (Japan)
and
International Institute for Carbon Neutral Energy Research
(WPI-I2CNER), Kyushu University
744 Motooka, Nishi, Fukuoka 819-0395 (Japan)
H. Nomura
Kyulux Inc., Suite 227
Fukuoka Industry-Academia Symphonicity (FiaS), Bldg.2
4-1 Kyudai-Shinmachi, Nishi-ku, Fukuoka 819-0388 (Japan)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201609459.
Angew. Chem. Int. Ed. 2017, 56, 1571 –1575
To address these issues, several alternative strategies have
been employed to exploit the non-emissive triplet energies of
the cheaper fluorescent materials for efficient deep-blue
OLEDs, such as triplet–triplet annihilation (TTA), hybridized
local and charge transfer (HLCT) and thermally activated
delayed fluorescence (TADF).[5] Among them, TADF has
emerged as the most promising alternative because it can
realize ultimate IQE through efficient reverse intersystem
crossing (RISC) from the lowest excited triplet state (T1) to
the lowest excited singlet state (S1).[6] According to Boltzmann statistics, the smaller the energy splitting (DEST)
between T1 and S1 at a given temperature, the easier it is to
achieve RISC.[7] Therefore, a minimal DEST is the most
demanding parameter in the development of TADF emitters.
To achieve a narrow DEST, molecules should possess excited
states of a charge-transfer (CT) nature to obtain a small
spatial overlap between the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) on the donor (D) and acceptor moieties (A),
respectively. Consequently, the selection of D and A units
with appropriate ionization potential (IP) and electron
affinity (EA) is important for realizing small DEST in TADF
molecules.[8]
So far, most reported TADF molecules tend to be
designed with D-D-A or D-A-D type structures, since the
DEST is smaller for the D-D-A or D-A-D type structure than
for the D-A type structure.[6c, 9] However, molecular design
based on this strategy sometimes results in a decrease of the
optical band gap and thus a redshift of the light emission to
the sky-blue or even green regions, which is unfavorable for
the design of deep-blue TADF emitters. Additionally, less
overlap of the HOMO and LUMO generally gives rise to
a forbidden S1!S0 transition, leading to a decrease of the
radiative efficiency as has been widely observed in many
TADF molecules.[6f, 9c] This trade-off among DEST, optical
band gap and radiative efficiency has to be balanced for deepblue TADF emitters. To overcome these dilemmas, an ideal
design strategy that can individually modulate each property
without influencing the others is intrinsically difficult but
much in demand to realize highly efficient deep-blue TADF
OLEDs.
Here we report a simple but effective for design of organic
molecules that exhibit deep-blue fluorescence based on
TADF by introducing methyl groups in the proper position.
Interestingly, the DEST of the investigated D-A molecules are
well modulated without significantly changing their optical
band gap or radiative efficiency. The twisting D-A type
molecules (Cz-TRZ1-4) shown in Figure 1 a are composed of
cyaphenine as an acceptor and carbazole-based moieties (3,6-
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Figure 1. a) Chemical structures, b) optimized geometries, and c) calculated FMOs for molecules Cz-TRZ1-4.
dimethylcarbazole and 1,3,6,8-tetramethylcarbazole) as
donors. They were easily prepared by an aromatic nucleophilic substitution reaction between the fluoroarene and the
carbazole derivatives with good yields of over 80 %
(Scheme 1).
Angewandte
Chemie
sured in dichloromethane/dimethylformamide solution. As
shown in Figure S3, all compounds possess the same reduction potential at @2.16 : 0.05 V, which can be assigned to the
reduction of the cyaphenine segment. Similarly, Cz-TRZ1,
Cz-TRZ3 and Cz-TRZ4 possess the equally oxidation potential around + 0.97 : 0.05 V due to their common donor units
of 3, 6-dimethylcarbazole. However, the oxidation potential
of Cz-TRZ2 is appreciably lower than those of other three
compounds (+ 0.84 V). Thus, the two methyl substitutions in
the 1, 8-position of 3, 6-dimethylcarbazole fragment could
dramatically reduce its ionization potential (IP). The relationship between the IP of the donor and the excited triplet state
nature (3CT or 3LE) will be discussed later. The HOMO and
LUMO energy levels of Cz-TRZ1-4 were estimated as @5.63,
@5.51, @5.63, @5.65 eV and @2.69, @2.73, @2.71, @2.71 eV,
respectively, in good agreement with the corresponding
individual D and A unit energies, showing that the HOMO–
LUMO gap in Cz-TRZ1-4 is determined by the HOMO and
LUMO levels of the donor and acceptor, respectively.
Photophysical properties of Cz-TRZ1-4 were analyzed
using ultraviolet-visible (UV/Vis) and photoluminescence
(PL) spectroscopies. As shown in Figure 2, all compounds
Scheme 1. Synthetic route to Cz-TRZ series compounds.
The optimized geometry and the electron density distribution of the frontier molecular orbitals (FMOs) were
investigated using density functional theory (DFT) calculations. As shown in Figure 1 b, the dihedral angles between the
carbazole-based donor planes and the cyaphenine acceptor
plane are about 49.888, 86.788, 71.388 and 82.388, respectively, for
Cz-TRZ1-4 molecules. Importantly, the singlet-triplet energy
splitting is strongly dependent on the D-A orientation, which
will be discussed in detail in the following sections. Figure 1 c
depicts the separated HOMO and LUMO orbitals of these
four molecules on their optimized geometries. Apparently,
the HOMOs of these molecules are predominantly located on
the carbazole-based donor units, whereas the LUMOs are
distributed over the cyaphenine groups. For Cz-TRZ1, the
HOMO is extended slightly to the neighboring phenylene
bridge, owing to less steric hindrance between the carbazole
donor unit and the phenylene bridge. Thus, the DEST of CzTRZ1 is larger than those of Cz-TRZ2-4 due to the molecular
design rules of TADF emitters. This can be further illustrated
by the calculated values in Table S1 in the Supporting
Information.
To study the electrochemical properties of the four
compounds, the oxidation/reduction potentials were mea-
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Figure 2. UV/Vis absorption, fluorescence (298 K), and phosphorescent (77 K) spectra of Cz-TRZ1-4.
reveal broad S0 !1CT absorption bands at 370–380 nm, which
can be attributed to the intramolecular charge transfer (ICT)
from the carbazole-based donors to the cyaphenine acceptor.
The four compounds clearly exhibit broad and featureless
emission spectra, indicating that their S1 states are identified
as 1CT states, which is also confirmed by the spatial separation
of their FMOs. In addition, the emission maximum of CzTRZ2 (465 nm) is redshifted by approximately 33 nm compared with those of Cz-TRZ1, Cz-TRZ3 and Cz-TRZ4 (about
432 nm) because of the lower IP of its donor unit. The 1CT
energies of Cz-TRZ1-4 calculated from the onset of their
prompt emission spectra are 3.10, 2.93, 3.10 and 3.10 eV,
respectively. Figure 2 also depicts the phosphorescence
(Phos) spectra of Cz-TRZ1-4 in a frozen toluene matrix at
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77 K. Interestingly, the Phos spectra of Cz-TRZ1, Cz-TRZ3
and Cz-TRZ4 are well resolved and show characteristic
vibrational structures, indicating that their T1 states are 3LE
states. Conversely, Cz-TRZ2 exhibits a rather broad and less
structured Phos spectra, indicating that its T1 state can be
identified as a 3CT state. This difference in T1 state characters
can be explained well in terms of the molecular structures.
The introduction of two methyl substitutions in the 1, 8position of the 3, 6-dimethylcarbazole unit efficiently reduce
its IP despite the large twisted angle of 86.788, consequently
causing a 3CT energy that is lower than the 3LE energy in CzTRZ2 and resulting in a T1 of Cz-TRZ2 that is dominated by
a charge transfer triplet state. However, the IP of 3, 6dimethylcarbazole is not enough to lower the 3CT energies of
Cz-TRZ1, Cz-TRZ3 and Cz-TRZ4, which implies that the
3
LEs are their lowest triplet states. Here we note that the T1 of
Cz-TRZ1 is 2.67 eV, which is appreciably lower than 2.85, 2.93
and 2.95 eV for Cz-TRZ2, Cz-TRZ3 and Cz-TRZ4, respectively. To better understand these experimental results, the
natural transition orbitals (NTOs) were calculated to investigate their S0 !T1 transition characters of these four molecules (see Figure S1). Analysis of the NTOs obviously
demonstrates that the T1 state of Cz-TRZ2 is dominated by
CT character. On the other hand, Cz-TRZ1, Cz-TRZ3 and
Cz-TRZ4 show a main LE feature in their T1 states. Therefore, the hole-particle NTOs of Cz-TRZ1 expand across the
whole molecule due to its strong electronic coupling between
carbazole and cyaphenine units, while for Cz-TRZ3 and CzTRZ4 they become localized on the cyaphenine segment,
which is thanks to their weak D-A electronic coupling
originating from their perpendicularly oriented structures.
Thus, the T1 energy of Cz-TRZ1 is significantly lower than
those of the other three ones and the DEST of Cz-TRZ1-4 are
calculated to be 0.43, 0.08, 0.17 and 0.15 eV, respectively. In
other words, the electronic coupling between the D and A
units and the oxidation potential of the D unit significantly
affect the singlet–triplet energy splitting and T1 state features
of TADF molecules.
The PL quantum yields (PLQYs) of Cz-TRZ1-4 in
oxygen-free toluene are 72 %, 86 %, 60 % and 35 %, respectively. In contrast, the films of Cz-TRZ1-4 doped into DPEPO
(bis[2-(di-(phenyl)phosphino)-phenyl]ether oxide) exhibited
higher PLQYs of 87 %, 98 %, 92 % and 85 %, respectively,
because intramolecular rotation and vibrational motions are
suppressed in their solid state. To better understand the
fluorescent behavior, the transient PL characteristics were
analyzed using a streak camera at room temperature.
Transient PL decay curves of 6 % Cz-TRZ1-4: DPEPO codeposited films are shown in Figure 3. Notably, the Cz-TRZ1
doped film exhibits strong prompt and negligible delayed
fluorescence components, which can be ascribed to its large
DEST. However, prompt and delayed fluorescence components are clearly identified in Cz-TRZ2-4 doped films. The
delayed fluorescence components are thus unambiguously
assigned to the TADF mechanism. The lifetimes of the
delayed components of Cz-TRZ2-4 doped films were 3.5, 13.0
and 10.3 ms, respectively. In addition, the radiative decay rate
constants of fluorescence (kr), internal crossing (kISC), and
reverse internal crossing (kRISC) are estimated from the
Angew. Chem. Int. Ed. 2017, 56, 1571 –1575
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Chemie
Figure 3. Transient PL decay spectra of Cz-TRZ1-4 doped into DPEPO
films (6 wt %) at room temperature.
PLQYs and the lifetime of the prompt and delayed components. All the physical property data is listed in Table S2.
Unexpectedly, the kRISC of Cz-TRZ2 is the highest among the
four materials. Spin-orbit coupling is virtually not operative
between the 1CT and 3CT states in Cz-TRZ2 because the
orbitals involved in both states are the same and thus the
matrix element vanishes, h 1 CT jH SOC j 3 CTi ¼ 0.[9a, 10]
According to the Laporte rule, RISC processes from 3CT to
1
CT state is forbidden. Instead, another mechanism exists for
explaining the high kRISC of Cz-TRZ2. Hyperfine coupling has
been proposed to facilitate RISC processes from 3CT to 1CT
state[11] and this process can be a possible mechanism.
Conversely, for upconversion from 3LE to 1CT (Cz-TRZ3
and 4), RISC processes are strongly dominated by spin-orbit
coupling. While we cannot provide the conclusive mechanism,
the quite small DEST of Cz-TRZ2 should accelerate kRISC. At
any rate, the introduction of methyl groups into the 1, 8position of 3, 6-dimethylcarbazole (Cz-TRZ2) is rather
harmful for deep-blue emission due to the significant
decrease of the oxidation potential regardless of the accelerated kRISC. In short, substitution on the central phenylene
moieties (Cz-TRZ3 and 4) induced a large twisting, similar to
the 1, 8-substitution, that provided the localization of the T1
state while keeping the large optical band gap.
The thermal properties of the four compounds were
investigated by thermogravimetric analysis (TGA). Their
decomposition temperatures (Td) with 5 % loss are higher
than 405 8C (see Figure S2), suggesting that they could form
morphologically stable and uniform amorphous films by
vacuum deposition for OLED fabrication.
To investigate the electroluminescence properties of all
four D-A molecules, we constructed the devices with
commonly used multi-layered structure: ITO/HAT-CN
(5 nm)/a-NPD (20 nm)/TCTA (20 nm)/mCP (10 nm)/
DPEPO: 6 wt % Cz-TRZ1-4 (20 nm)/ DPEPO (10 nm)/
TPBi (30 nm)/LiF (0.8 nm)/Al (120 nm) (dopant = Cz-TRZ1
for device 1, Cz-TRZ2 for device 2, Cz-TRZ3 for device 3 and
Cz-TRZ4 for device 4), in which 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) and LiF served as hole-
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and electron-injection materials, respectively; N,N’-diphenylN,N’-bis(1-naphthyl)-1,10-biphenyl-4,4’-diamine
(a-NPD)
and 4,4’,4’’-tris(N-carbazolyl)triphenylamine were utilized as
a hole transport layer; N,N’-dicarbazolyl-3,5-benzene (mCP)
as an exciton blocking layer; 6 wt % Cz-TRZ1-4 doped in
DPEPO host was used as the emitting layer (EML); 1,3,5tris(1-phenyl-benzimidazol-2-yl)benzene (TPBi) acted as an
electron-transporting material. The high T1 of mCP and
DPEPO favorably confine triplet excitons in the EMLs.
Figure 4 shows the electroluminescence (EL) spectra and
external quantum efficiency (EQE) versus current density (J)
characteristics of the OLEDs. The turn-on voltage for all four
OLEDs was 3.5 V. The current density-voltage (J–V) curves
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Chemie
and acceptor units. On the basis of this principle, a series of
deep-blue TADF materials were designed. TADF OLEDs
based on Cz-TRZ3 and Cz-TRZ4 exhibited high maximum
EQEs of 19.2 and 18.3 % and CIE coordinates of (0.148,
0.098) and (0.150, 0.097), respectively. The CIE coordinates
using Cz-TRZ3 and Cz-TRZ4 represent some of the deepest
blue emissions achieved from TADF OLEDs with considerably high EQEs. We believe our results can be used to guide
the further molecular design of deep-blue TADF emitter
materials.
Acknowledgements
This work was supported financially by the Exploratory
Research for Advanced Technology (ERATO) of Japan. The
authors also acknowledge Ms. Nozomi Nakamura and Dr.
Chen Zhong for their technical assistance with this research.
Conflict of interest
The authors declare no conflict of interest.
Keywords: carbazole · fluorescence · organic electronics ·
organic light-emitting diodes · singlet–triplet splitting
How to cite: Angew. Chem. Int. Ed. 2017, 56, 1571 – 1575
Angew. Chem. 2017, 129, 1593 – 1597
Figure 4. a) Energy level diagrams of OLED devices. b) EQE–current
density characteristics of devices 1–4. c) EL spectra of devices 1–4.
d) CIE diagram of Cz-TRZ3-based device.
of all four devices were similar because they shared the same
device structures (see Figure S4). The maximum EQEs were
22.0, 19.2 and 18.3 % for the devices with Cz-TRZ2-4,
respectively, which are much higher than that of Cz-TRZ1based device (7.2 %). This was because the DEST of Cz-TRZ1
is too large to efficiently upconvert its triplet excitons to
singlet ones. More importantly, the CIE coordinates of the
devices with Cz-TRZ3 and Cz-TRZ4 were (0.148, 0.098) and
(0.150, 0.097), respectively, which are closer to the standard
deep blue emission. To our knowledge, Cz-TRZ3- and CzTRZ4-based devices exhibit the highest efficiency among the
deep-blue TADF OLEDs.
In summary, we demonstrated a rational molecular design
strategy for deep blue TADF emitters that can realize the
diminution of DESTs in D-A type molecules without lowering
their optical band gaps and radiative efficiencies. In order to
fulfill this requirement, both the electronic interaction and
the oxidation/ reduction potential of the donor and acceptor
fragments should be taken into account. Moreover, the
photophysical spectra and DFT calculations showed that the
feature of the lowest triplet excited states (3CT or 3LE) of D-A
type molecules were well controlled by introduction of methyl
groups that can finely tune the electronic interaction of donor
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Manuscript received: September 27, 2016
Revised: November 28, 2016
Final Article published: December 30, 2016
T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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