Management of Locally Excited States for Purine-based TADF Emitters: A Method to Reduce Device Efficiency Roll-Off

Article abstract of DOI:10.1021/acs.orglett.1c00918

The programmed arylation of purine has been developed to construct a series of efficient thermally activated delayed fluorescent (TADF) materials. The corresponding organic light-emitting diodes (OLEDs) exhibit external quantum efficiency as high as 16.0% alongside small efficiency roll-off. Intriguingly, this work proves that the good management of localized states is an efficient way to reduce device efficiency roll-off and is crucial for the future design of high-performance OLEDs.

Full text of DOI:10.1021/acs.orglett.1c00918

pubs.acs.org/OrgLett
Letter
Management of Locally Excited States for Purine-based TADF
Emitters: A Method to Reduce Device Efficiency Roll-Off
Yangbo Wu, Yuming Zhang, Chunhao Ran, Jingbo Lan, Zhengyang Bin,* and Jingsong You
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ABSTRACT: The programmed arylation of purine has been developed to
construct a series of efficient thermally activated delayed fluorescent (TADF)
materials. The corresponding organic light-emitting diodes (OLEDs) exhibit
external quantum efficiency as high as 16.0% alongside small efficiency roll-off.
Intriguingly, this work proves that the good management of localized states is an
efficient way to reduce device efficiency roll-off and is crucial for the future design
of high-performance OLEDs.
realize state-of-the-art-performance yellow fluorescent
hermally activated delayed fluorescent (TADF) materials
Tthat possesses small single−triplet energy gaps (ΔE ) so OLEDs.20
ST
As a naturally occurring moiety, the purine unit also
as to efficiently upconvert “dark“ triplet excitons to emissive
singlet excitons have received tremendous research interests in
recent years and have been considered as the next generation
possesses a unique structure with a fused hexagonal pyrimidine
and pentagonal imidizole, which provides us with an
opportunity to further investigate the performance of a
nitrogen-containing heterocyclic acceptor in assembling
efficient TADF materials.21−23 By introducing a phenoxaine
(PXZ) donor at a different location of the purine acceptor
through the programmed arylation reaction, we herein wish to
design and synthesize a series of purine-based TADF materials
(2-PXZ-PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-
PXZ-PR, Figure 1) and then systematically investigate their
photoluminescence (PL) and electroluminescence (EL)
performance.
of emitters in organic light-emitting diodes (OLEDs).1−5
A
typical molecular design concept for efficient TADF materials
is to construct compounds by bridging electron-rich donors
(D) and electron-deficient acceptors (A) with highly twisted
phenyl linkages.6−8 The twisted D−π−A conformation can
ensure well-separated highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) distributions, thus leading to a small ΔEST value
alongside a fast reverse intersystem crossing rate (krisc) for an
efficient triplet exciton utilization process.9,10
To understand the structure−property relationship of these
compounds at the molecular level, density functional theory
(DFT) and time-dependent DFT (TD-DFT) calculations were
performed to study the electronic states and the distributions
of their frontier molecular orbitals (FMOs). As shown in
Figure 1, because of the large dihedral angles of almost 90°
between the PXZ donor and the phenyl bridge, all molecules
have well-separated FMO distributions, where the HOMOs
and the LUMOs are mainly localized on the electron-rich PXZ
As reported, electron-deficient nitrogen-containing hetero-
cycles are well-documented as acceptors for developing
efficient TADF materials.11−13 On one hand, the high electron
deficiency of these heterocyclic structures can lead to small
ΔEST values when TADF materials are formed; meanwhile,
heterocyclic structures are also favorable for forming
intermolecular hydrogen bonding between the nitrogen
atoms and the phenyl linkage, so as to inhibit structure
relaxation and increase the photoluminescence quantum yield
(PLQY).14−16 For example, triazine (TRZ) that features a
hexagonal heterocycle is one of the most widely used electron
acceptors and has shown great success in assembling a large
amount of TADF materials.17−19 Recently, by fusing a
pentagonal triazole (TAZ) unit on TRZ acceptor, we have
developed a new type of triazolotriazine (TAZTRZ) acceptor
and constructed a series of highly efficient TADF materials to
Received: March 17, 2021
Published: May 7, 2021
https://doi.org/10.1021/acs.orglett.1c00918
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3839−3843
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Letter
Scheme 1. Programmed Arylation of Purine for the
Synthesis of 2-PXZ-PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-PXZ-
PR, and 6-PXZ-PR
eV for 6-PXZ-PRB, −2.40 eV for 8-PXZ-PRB, −2.30 eV for 2-
PXZ-PR, and −2.45 eV for 6-PXZ-PR, respectively.
Subsequently, thermogravimetric analysis (TGA) and differ-
ential scanning calorimetry (DSC) measurements were carried
out to investigate the thermal properties of these compounds
(Figure S4). These compounds all possess high decomposition
temperatures (Td, which is the temperature for 5% weight loss)
of 432, 425, 428, 416, and 417 °C and high glass-transition
temperatures (Tg) of 143, 139, 133, 120, and 116 °C for 2-
PXZ-PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-PXZ-
PR, respectively. The sufficiently high Td and Tg values of these
compounds reveal their high thermal stability, which makes
them good candidates for vacuum-possessed OLED materials.
To investigate the PL properties of 2-PXZ-PRB, 6-PXZ-
PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-PXZ-PR, UV−vis
absorption spectra (Abs), fluorescence spectra (FL) at room
temperature, and phosphorescence spectra at 77 K with a delay
time of 10 ms were measured in dilute toluene (1 × 10−5 mol
L−1). As depicted in Figure 2a, all of the compounds exhibit
similar broad absorption bands from 380 to 480 nm, which are
attributed to the intramolecular charge-transfer transition from
PXZ donors to purine acceptors. As shown in the FL spectra,
these compounds all show broad and featureless emissions
from 490 to 535 nm, indicating that their singlet excitons
Figure 1. Molecular structures and FMO distributions of 2-PXZ-
PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-PXZ-PR.
and electron-deficient purine units, respectively. Therefore, this
leads to small ΔEST values of 0.0032, 0.0078, 0.0066, 0.0048,
and 0.0070 eV for 2-PXZ-PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-
PXZ-PR, and 6-PXZ-PR, respectively, which indicates their
potential TADF nature. Notably, around the purine acceptor,
there is almost no LUMO distribution on the phenyl ring that
is bonded to the electron-rich nitrogen atom at the nine-
position, indicating the limited influence of this phenyl ring on
the FMO energy level alignment for the targeting TADF
materials.
2-PXZ-PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-
PXZ-PR were synthesized via the programmed arylation
reactions, as shown in Scheme 1. The N9-arylation of purine
was first realized by the Cu-catalyzed N−H arylation reaction
to deliver compound 2 in 70% yield;24 then, Suzuki cross-
coupling reactions were rapidly carried out to afford 2-PXZ-
PR, 6-PXZ-PR, and the intermediate compound 5.25,26
Crucially, the regioselective C2-arylation and C6-arylation
were accomplished at different temperatures, 80 °C for C6-
arylation and 150 °C for C2-arylation. Finally, the palladium-
catalytic C−H arylation reaction was successfully used to
realize C8-arylation in the presence of CuI and Cs2CO3,
furnishing 2-PXZ-PRB, 6-PXZ-PRB, and 8-PXZ-PRB in
yields of 80, 65, and 40%, respectively.27,28
29
1
belong to charge-transfer states (1CT). Therefore, the CT
energies of 2-PXZ-PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-PXZ-
PR, and 6-PXZ-PR were calculated from the onset wave-
lengths of the FL spectra.
As summarized in Table 1, 2-PXZ-PRB, 6-PXZ-PRB, and 8-
PXZ-PRB that have different donor−acceptor linkages possess
1
different CT energies, which are 2.75, 2.58, and 2.66 eV,
Cyclic voltammetry (CV) measurements were used to
investigate the electrochemical properties of these compounds.
As shown in Figure S3, all molecules exhibit clearly reversible
and similar oxidation processes that originated from the same
PXZ donor of these compounds. The HOMO energy levels are
therefore calculated to be the same as −5.05 eV according to
the oxidation peaks in the CV curves; meanwhile, their LUMO
energy levels are calculated from the HOMO energy levels and
the optical energy gaps obtained from the absorption spectra,
which are determined to be −2.28 eV for 2-PXZ-PRB, −2.44
respectively; meanwhile, the phenyl ring on the C8 position of
the purine unit would only slightly change the CT energy,
1
revealing the limited influence of this phenyl ring on charge-
transfer characteristics. As shown in Figure 2b, the
phosphorescence spectra of these compounds all show clearly
vibrational structures, indicating that their triplet excitons
belong to localized excited states (3LE), which can also be
demonstrated by the natural transition orbital calculation
results (Table S5). The 3LE energies can be obtained from the
emission peaks in the short-wavelength region of phosphor-
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Letter
summarized in Table 1, which are 0.34, 0.21, 0.29, 0.25, and
0.15 eV, respectively, indicating that these compounds all have
good TADF nature, as calculated. In addition, the FL spectra
(Figure S5) and transient PL spectra (Figure S6) of these
compounds are also measured in the doped film of the 3,3′-
bis(carbazol-9-yl)biphenyl (mCBP) host. The transient PL
spectra show an obvious delayed component with an average
delayed lifetime (τd) of 58.5, 48.1, 43.4, 32.5, and 24.3 μs,
respectively. Furthermore, the radiative decay rate (kr) and krisc
are calculated and summarized in Table S7. Compared with 6-
PXZ-PRB, the significantly increased krisc and the slightly
decreased kr lead to a large PLQY (ΦPL) for 6-PXZ-PR (Table
1), rendering it a good TADF emitter.
To further explore the EL performance of these compounds,
we have designed and assembled multilayer OLED devices
with the optimized device structure of ITO/HAT-CN (5 nm)/
NPB (30 nm)/TCTA (5 nm)/20% emitter:mCBP (30 nm)/
TmPyPb (40 nm)/LiF (0.8 nm)/Al (100 nm), in which 2-
PXZ-PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-PXZ-
PR are separately used as the emitters in the mCBP host. The
molecular structures, energy diagrams, and EL characteristics
of the OLED devices are shown in Figure 3. The device
Figure 2. (a) Absorption (Ab) and fluorescence (FL) spectra
measured at room temperature and (b) phosphorescence spectra
measured at 77 K with a delay time of 10 ms of 2-PXZ-PRB, 6-PXZ-
PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-PXZ-PR in dilute toluene (1 ×
1
3
10−5 mol L−1). (c) CT and LE energy level alignments of 2-PXZ-
PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-PXZ-PR. (d)
Crystal structures of 6-PXZ-PRB and 6-PXZ-PR.
escence spectra.30,31 As summarized in Table 1 and depicted in
Figure 2c, although the 1CT energies of 6-PXZ-PRB (2.58 eV)
3
and 6-PXZ-PR (2.59 eV) are similar, the LE energy of 6-
PXZ-PR (2.44 eV) is significantly higher than that of 6-PXZ-
PRB (2.37 eV). This leads to a smaller ΔE(1CT‑3LE) of 6-PXZ-
PR (0.15 eV) than that of 6-PXZ-PRB (0.21 eV). A similar
phenomenon is seen for 2-PXZ-PRB and 2-PXZ-PR. The
crystal structures of 6-PXZ-PRB and 6-PXZ-PR are obtained
by the temperature gradient vacuum sublimation method
(Figures S1 and S2). As shown in Figure 2d, a small dihedral
angle of 23.7° of the purine−phenyl (ring 8) linkage is
observed in the crystal structure of 6-PXZ-PRB, which is
indicative of its extended π-conjugation length at the C8
position compared with that of 6-PXZ-PR, which increases the
3LE energy.32,33 The ΔE(1CT‑3LE) energies of 2-PXZ-PRB, 6-
PXZ-PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-PXZ-PR are
Figure 3. Device performance. (a) Device structures. (b) Molecular
structures used in OLED devices. (c) EL spectra at 5000 cd m−2. (d)
EQE−luminance and power efficiency−luminance characteristics of
2-PXZ-PRB-, 6-PXZ-PRB-, 8-PXZ-PRB-, 2-PXZ-PR-, and 6-PXZ-
PR-based OLEDs.
performance is summarized in Table 2. OLEDs devices based
on 2-PXZ-PRB, 6-PXZ-PRB, and 8-PXZ-PRB that feature a
PXZ donor at a different position of the purine acceptor
exhibit green to yellow emission with emission peaks of 518,
550, and 530 nm, respectively. Moreover, 6-PXZ-PRB that
Table 1. Summary of Thermal and Photophysical Properties of 2-PXZ-PRB, 6-PXZ-PRB, 8-PXZ-PRB, 2-PXZ-PR, and 6-PXZ-
PR
a
b
c
d
e
compound
Td/Tg (°C)
λabs/λem (nm)
1CT (eV)
3LE (eV)
ΔE(1CT‑3LE) (eV)
ΦPL (%)
2-PXZ-PRB
6-PXZ-PRB
8-PXZ-PRB
2-PXZ-PR
6-PXZ-PR
432/143
425/139
428/133
416/120
417/116
389/504
415/535
400/522
392/494
413/531
2.75
2.58
2.66
2.80
2.59
2.41
2.37
2.37
2.55
2.44
0.34
0.21
0.29
0.25
0.15
24.8
37.6
27.2
20.2
60.8
a
b
Measured in dry toluene solution (1 × 10−5 mol L−1). Estimated from the onset of the fluorescence spectra measured at room temperature in
c
toluene solution (1 × 10−5 mol L−1). Estimated from the phosphorescence spectra measured at 77 K in toluene solution (1 × 10−5 mol L−1).
d
e
1
3
Calculated from CT and LE. PLQY (ΦPL) measured in mCBP films.
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Table 2. Summary of Device Performances
a
b
c
emitter
λEL (nm)
Von (V)
EQEmax (%)
PEmax (lm W1−
)
EQE/PE (at 1000 cd m−2
)
EQE/PE (at 5000 cd m−2
)
2-PXZ-PRB
6-PXZ-PRB
8-PXZ-PRB
2-PXZ-PR
6-PXZ-PR
518
550
530
510
541
2.7
2.8
3.0
3.5
3.1
13.6
15.5
11.1
14.9
16.0
45.8
52.6
35.1
38.1
2.4/3.6
6.9/13.9
3.7/6.7
7.8/11.9
13.1/26.5
1.5/1.6
3.2/4.5
2.2/2.8
3.3/3.6
9.2/14.4
47.5
c
a
b
Turn-on voltage measured at 1 cd m−2. External quantum efficiency. Power efficiency.
features a PXZ donor at the C6-position of the purine acceptor
displays a relatively better EL performance with a higher
EQEmax of 15.5% compared with those of 2-PXZ-PRB (13.6%)
and 8-PXZ-PRB (11.1%). However, devices based on 2-PXZ-
PRB, 6-PXZ-PRB, and 8-PXZ-PRB emitters all suffer from
serious efficiency roll-off. For example, at a luminance of 1000
cd m−2, the EQEs of 2-PXZ-PRB, 6-PXZ-PRB, and 8-PXZ-
PRB rapidly dropped to 2.4, 6.9, and 3.7%, respectively. The
severe efficiency roll-off is mainly due to the accumulation of
triplet excitons in the emitting layer.34 As discussed, moving
out a benzene ring at the C8-position can significantly increase
the energy level of 3LE, and thus it enhances the up-conversion
process of triplet excitons and inhibits the triplet exciton
quenching, which is beneficial for reducing the efficiency roll-
off. Therefore, compared with 2-PXZ-PRB- and 6-PXZ-PRB-
based devices, devices using 2-PXZ-PR and 6-PXZ-PR as the
emitter exhibit remarkably reduced efficiency roll-off. For
instance, 2-PXZ-PR- and 6-PXZ-PR-based devices display
EQEmax values of 14.9 and 16.0%, respectively, which are
comparable with those of 2-PXZ-PRB-based device (13.6%)
and 6-PXZ-PRB-based device (15.5%). However, at a
luminance of 1000 cd m−2, the EQE1000 values of 2-PXZ-
PR- and 6-PXZ-PR-based devices can be maintained at 7.8
and 13.1%, respectively, which are significantly higher than
those of the 2-PXZ-PRB-based device (2.4%) and the 6-PXZ-
PRB-based device (6.9%). Crucially, even at a luminance of
5000 cd m−2, the EQE5000 of the 6-PXZ-PR-based device can
still be maintained at 9.2%, indicating a significantly reduced
efficiency roll-off.
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Zhengyang Bin − Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, P. R. China;
orcid.org/0000-0001-6216-7234;
Email: binzhengyang@scu.edu.cn
Authors
Yangbo Wu − Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, P. R. China
Yuming Zhang − Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, P. R. China
Chunhao Ran − Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, P. R. China
Jingbo Lan − Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, P. R. China;
orcid.org/0000-0001-5937-0987
Jingsong You − Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, P. R. China;
orcid.org/0000-0002-0493-2388
In conclusion, we designed and synthesized a series of
purine-based TADF materials (2-PXZ-PRB, 6-PXZ-PRB, 8-
PXZ-PRB, 2-PXZ-PR, and 6-PXZ-PR) that exhibit good PL
performance with emission peaks from 494 to 535 nm and
ΔE(1CT‑3LE) values as small as 0.15 eV. Moreover, these
compounds also achieve good EL performance with turn-on
voltage as low as 2.7 V and EQEmax as high as 16.0%. Crucially,
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00918
Notes
The authors declare no competing financial interest.
3
ACKNOWLEDGMENTS
this work demonstrates that the good management of LE
■
states in TADF emitters is an efficient way to reduce device
efficiency roll-off and is important for the future design of high-
performance OLED devices associated with TADF materials.
We acknowledge financial support from the National NSF of
China (nos. 22031007 and 22005204) and the Sichuan
Science and Technology Program (nos. 2020YJ0245 and
2020YJ0302). We also thank the Comprehensive Training
Platform Specialized Laboratory, College of Chemistry,
Sichuan University.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00918.
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