Effect of a Pendant Acceptor on Thermally Activated Delayed Fluorescence Properties of Conjugated Polymers with Backbone-Donor/Pendant-Acceptor Architecture

Article abstract of DOI:10.1002/asia.201801813

Three sets of conjugated polymers with backbone-donor/pendant-acceptor architectures, named PCzA3PyB, PCzAB2Py, and PCzAB3Py, are designed and synthesized. The three isomeric benzoylpyridine-based pendant acceptor groups are 6-benzoylpyridin-3-yl (3PyB), 4-((pyridin-2-yl)carbonyl)phenyl (B2Py) and 4-((pyridin-3-yl)carbonyl)phenyl (B3Py), whereas the identical backbone consists of 3,6-carbazolyl and 2,7-acridinyl rings. One acridine ring and each acceptor group constitute a definite thermally activated delayed fluorescence (TADF) unit, incorporated into the main chain of the polymers through the 2,7-position of the acridine ring with the varied content. All of the polymers display legible TADF features with a short microsecond-scale delayed lifetime (0.56–1.62 μs) and a small singlet/triplet energy gap (0.10–0.19 eV). Progressively redshifted emissions are observed in the order PCzAB3Py, PCzA3PyB, and PCzAB2Py owing to the different substitution patterns of the pyridyl group. Photoluminescence quantum yields can be improved by regulating the molar content of the TADF unit in the range 0.5–50 %. The non-doped organic light-emitting devices (OLEDs) fabricated by solution-processing technology emit yellow-green to orange light. The polymers with 5 mol % of the TADF unit exhibit excellent comprehensive electroluminescence performance, in which PCzAB2Py5 achieves a maximum external quantum efficiency (EQE) of 11.9 %, low turn-on voltage of 3.0 V, yellow emission with a wavelength of 573 nm and slow roll-off with EQE of 11.6 % at a luminance of 1000 cd m^?2 and driving voltage of 5.5 V.

Full text of DOI:10.1002/asia.201801813

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Effect of Pendant Acceptor on Thermally Activated Delayed
Fluorescence Properties of Conjugated Polymers with Backbone-
donor/Pendant-acceptor Architecture
Authors: Yanxiang Cheng, Yike Yang, Kuofei Li, Chenxu Wang, and
Hongmei Zhan
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To be cited as: Chem. Asian J. 10.1002/asia.201801813
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10.1002/asia.201801813
Chemistry - An Asian Journal
Effect of Pendant Acceptor on Thermally Activated Delayed Fluorescence
Properties of Conjugated Polymers with Backbone-donor/Pendant-acceptor
Architecture
Yike Yang, Kuofei Li, Chenxu Wang,* Hongmei Zhan, and Yanxiang Cheng*
Mr. Y. Yang, Mr. K. Li, Dr. H. Zhan, Prof. Y. Cheng
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences
Changchun 130022, P. R. China
E-mail: yanxiang@ciac.ac.cn (Y. C.)
Dr. Chenxu Wang
Public Technical Service Center
Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences
Changchun 130102, P. R. China
E-mail: wangchenxu@iga.ac.cn (C.W.)
Mr. Y. Yang, Mr. K. Li
University of Chinese Academy of Sciences
Beijing 100049, P. R. China
Keywords: conjugated polymer; thermally activated delayed fluorescence; pendant; acceptor;
organic light-emitting diode
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Abstract:
Three sets of conjugated polymers with backbone-donor/pendant-acceptor
architecture, named PCzA3PyB, PCzAB2Py and PCzAB3Py, are designed and
synthesized. Three isomeric benzoylpyridine-based pendant acceptor groups are
6-benzoylpyridin-3-yl (3PyB), 4-((pyridin-2-yl)carbonyl)phenyl (B2Py) and
4-((pyridin-3-yl)carbonyl)phenyl (B3Py), respectively, whereas an identical backbone
consists of 3,6-carbazolyl and 2,7-acridinyl rings. One acridine ring and each acceptor
group constitute a definite thermally activated delayed fluorescence (TADF) unit,
incorporated into the main chain of the polymers through 2,7-position of acridine ring
with the varied content. All of the polymers display the legible TADF features with a
short microsecond-scale delayed lifetime (0.56-1.62 μs) and a small singlet/triplet
energy gap (0.10-0.19 eV). The progressively red-shifted emissions are observed in
the order PCzAB3Py, PCzA3PyB and PCzAB2Py due to the different substitution
patterns of pyridyl group. Photoluminescence quantum yields can be improved by
regulating the molar content of the TADF unit in the range of 0.5-50%. The
non-doped organic light-emitting devices fabricated by solution-processing
technology emit yellow-green to orange light. The polymers with the 5% mole content
of the TADF unit exhibit more excellent comprehensive electroluminescence
performance, in which PCzAB2Py5 achieves a maximum external quantum efficiency
(EQE) of 11.9%, low turn-on voltage of 3.0 V, yellow emission with wavelength of
573 nm and slow roll-off with EQE of 11.6% at luminance of 1000 cd m−2 and driving
voltage of 5.5 V.
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Introduction:
Purely organic thermally activated delayed fluorescence (TADF) materials have
attracted significant attention owing to their practical application potential in the field
of panel display and solid state lighting.[1-6] Unlike the noble metal phosphorescent
complexes, TADF emitters enable efficient thermal up-conversion of the triplet (T1)
excitons to the singlet (S1) excitons via a rapid reverse intersystem crossing (RISC)
procedure, facilitating the utilization of all electro-generated excitons for
electroluminescent (EL) emission.[1] Therefore, the organic compounds can be the
most suitable low-cost alternative to the precious complexes. In practice, to ensure an
effective RISC, the energy difference (ΔEST) between the singlet and triplet excited
states has to be minimized, which is proportional to the electron-exchange energy that
relies on the overlap integral between the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO).[7-10] Accordingly, the most
common design strategy for TADF materials is based on highly twisted (or nearly
orthogonal) donor-acceptor structures with strong intramolecular charge-transfer (CT)
feature to sufficiently separate the spatial distribution of these frontier orbitals.[2-6]
Following this fundamental guideline, significant progress has been made in the
development of various efficient TADF materials by appropriately selecting
donor/acceptor moieties, carefully managing their linking topologies and systemically
controlling the torsion angle between donor and acceptor.[11-40] For example, Adachi
and co-workers developed
a
TADF emitter, HAP-3TPA, containing
a
heptaazaphenalene acceptor bridged three diphenylamine donors through phenylene
group, which produced orange-red EL emission in addition to high EL efficiency.[38]
In contrast, two emitters, HAP-3MF with same acceptor but three
4-fluoro-3-methylphenyl donors and 3DPA3CN with similar donor but
tricyanobenzene acceptor, only produced green emissions, although they also
exhibited efficient TADF performance in devices.[39,40] The results clearly declare that
alteration of the donor and/or acceptor moieties can influence the emissive
characteristics of TADF emitters.
Similarly, the above approach is also valid for TADF polymers, which excels in
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Chemistry - An Asian Journal
low-cost and large-area display application through convenient solution processing
technology.[41-63] For example, two TADF polymers pCzBP and pAcBP with the same
electron-accepting benzophenone (BP) group connected to the different strength
electron-donating carbazole or acridine groups, respectively, showed green and yellow
emissions with maximum external quantum efficiency (EQE) of 8.1% and 9.3% in the
corresponding doped devices.[43] In our recent works, the differences in TADF
behaviour were also observed due to using different pendant acceptor in the polymers
with the backbone-donor/pendant-acceptor (BDPA) architecture.[55-61] The
solution-processed device based on the polymers with BP as the pendant acceptor
achieved the higher EQE than those with formylphenyl pendant acceptor, while the
devices had similar emissive wavelength with the low turn-on voltages and slow
efficiency roll-offs.[59,60] The results provide an incentive to further develop the TADF
polymer by means of replacing the pendant acceptor so as to furnish more excellent
EL performance, especially the other emission color in comparison with numerous
green TADF materials. On the basis of the consideration, three isomeric
benzoylpyridine derivatives are selected as pendant acceptors to create three sets of
conjugated polymers PCzA3PyB, PCzAB2Py and PCzAB3Py with identical
backbone. As expected, the pendant acceptors definitively influence the TADF
features of these polymers, especially giving rise to the red-shifted emissions
compared to the reported polymers with benzophenone or formylphenyl as pendant
acceptor. The photophysical properties and EL performances of these polymers are
fully characterized, indicative of their TADF behavior. Combining with the crystal
structures of three precursors, effect of the pendant acceptors on the TADF properties
is discussed in detail.
Results and Discussion:
The polymer structures and synthetic routes are depicted in Scheme 1. Three
brominated
benzoylpyridine
derivatives
2-benzoyl-5-bromopyridine
(1),
2-(4-bromobenzoyl)pyridine (2) and 3-(4-bromobenzoyl)pyridine (3) as acceptor
building-blocks were obtained by the optimized synthetic method.[64-66] The
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10.1002/asia.201801813
Chemistry - An Asian Journal
corresponding
precursors
(A3PyB),
9,9-dihexal-10-(6-benzoylpyridin-3-yl)-9,10-dihydroacridine
9,9-dihexal-10-(4-((pyridin-2-yl)carbonyl)phenyl)-9,10-dihydroacridine (AB2Py) and
9,9-dihexal-10-(4-((pyridin-3-yl)carbonyl)phenyl)-9,10-dihydroacridine (AB3Py)
were prepared from the Pd-catalyzed C-N coupling reaction of brominated
benzoylpyridine and 9,9-dihexal-9,10-dihydroacridine with t-BuONa as base in
excellent yield. The further bromination in the dark smoothly afforded almost
quantitative bromoacridine monomers MA3PyB, MAB2Py and MAB3Py, of which the
fluorescence dramatically weakened relative to their precursors. Finally, three sets of
conjugated polymers were synthesized by modified Suzuki coupling copolymerization
of the corresponding bromoacridine and 3,6-substituted carbazole (M2 and M3)
monomers with the feed molar ratios of 50:0:50, 25:25:50, 10:40:50, 5:45:50, 1:49:50,
and 0.5:49.5:50, respectively.[67] In terms of the feed molar ratio of bromoacridine
monomer in polymerization, the obtained polymers are denoted as
PCzA3PyB50-PCzA3PyB0.5,
PCzAB2Py50-PCzAB2Py0.5
and
PCzAB3Py50-PCzAB3Py0.5, respectively. The actual contents of the acridine unit are
in good agreement with the feed ratios (Supporting Information). For example, the
1
contents of the AB2Py unit in PCzAB2Py50-PCzAB2Py0.5 are estimated from H
NMR spectra to be 48.3%, 24.2%, 10.1%, 4.8%, 1.6% and 0.87%, respectively.
Similar to the polymers previously reported, the 3,6-position connections of carbazole
in polymer backbone are advantageous for maintaining a high triplet energy level and
the long alkyl branched chains can improve solubility of the polymers.[68,69] The
number average molecular weights (Mn) and polydispersity indexes (PDIs) of the
polymers are in the 5.5-16.7 kDa and 1.56-2.61 ranges, respectively, determined by
gel permeation chromatography (GPC) using THF as an eluent and calibrated against
polystyrene standards (Figure S1 and Table S1). Furthermore, these polymers show
excellent thermal stability with high decomposition temperatures (Td, with 5% weight
loss) of 385-436 °C, which gradually increase with the increasing ratio of carbazole
ring, ascribed to the better thermal stability of carbazole fragment.
The molecular structures of three precursors were determined by X-ray
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Chemistry - An Asian Journal
crystallographic analyses and presented in Figure 1. A3PyB, AB2Py and AB3Py
indeed adopt a highly distorted molecular geometry with the dihedral angles of 79.72°,
86.31° and 86.11° between the acridine and adjacent pyridine or benzene rings,
respectively. The large enough dihedral angles ensure the sufficiently spatial
separation of HOMO and LUMO, and are favorable to lower the electronic coupling
of the orbitals for obtaining ΔEST small enough.[10] Another concerned dihedral angles
between the pyridine and benzene rings in the pendant acceptors are 51.28°, 57.81°
and 59.21°, while the torsion angles (N-C-C-O) of the middle carbonyl and pyridine
ring are 33.35°, 45.34° and 30.93° for the precursors A3PyB, AB2Py and AB3Py,
respectively. The above relatively small angles in A3PyB indicate that the donor
acridine more strongly bonds with pyridine ring, also reflected by the shorter bond
length (1.429 Å) of C-N (between the acridine and adjacent pyridine or benzene rings)
than those (1.437 and 1.434 Å) in AB2Py and AB3Py. Unlike the precursor AB3Py, a
hydrogen-bond (Ph-H…N < 2.6 Å) interactions from pyridine and benzene rings
around the carbonyl group are observed in A3PyB and AB2Py, which could increase
the conjugation of the acceptor moiety and the conjugative delocalization of the
π-electrons, and then cause the red-shifted emission. Thus, a further consequence
should be that the electron-withdrawing ability of the acceptor would intensify when
the pyridyl groups were introduced as a linker and/or an ortho-position group bonded
to carbonyl group. In addition, by comparison with the similar precursor with BP as
acceptor group (Table S2), several marked differences in the structural parameters
imply that the replacement of benzene by pyridine is anticipated to modify the
photophysical properties in the corresponding polymers.[59]
As expected, three sets of polymers exhibit the clear red-shifted emissions,
compared to the polymers PABPC with BP as the pendant acceptor (Figure 2(a) and
Table 1), whereas all polymers show the very similar UV-vis absorption and
photoluminescence (PL) patterns whether in toluene or neat film (Figure 3, S2 and
S3).[59] With the polymer PCzAB2Py as an example, a strong absorption around 320
nm primarily corresponds to the π-π* transition of the carbazole fragment, while a
long wavelength shoulder at 345 nm appears and gradually enhances and becomes a
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Chemistry - An Asian Journal
dominant peak from PCzAB2Py10 to PCzAB2Py50 with the increasing content of the
TADF unit (Figure 3 and Table 1), indicating that the absorption band is mainly
contributed to the π-π* transition of acridine group. Moreover, a weak absorption
at >400 nm from the intramolecular CT transition is also observed in the polymers
with the relatively high content of the TADF unit.[10] In dilute solution, only the
polymer PCzAB2Py50 shows a single emission at 587 nm, while others with the
25-0.5% molar content of the TADF unit exhibit a dual emission profile peaked
around 590 and 420 nm, of which the latter is assigned to the carbazole fragment.[57-60]
In contrast, the neat films of PCzAB2Py50-PCzAB2Py5 merely display a single peak
at 579-555 nm, whereas the dual emissions are observed only in PCzAB2Py1 and
PCzAB2Py0.5, implying that the stronger inter-chain interaction occurs in film. These
photophysical behaviors are similar to other two sets of polymers PCzA3PyB and
PCzAB3Py as well as PABPC. In comparison with PCzAB2Py, the polymer
PCzA3PyB with intramolecular hydrogen bond and pyridyl group as a linker between
donor and acceptor possesses almost same emission, while PCzAB3Py without
intramolecular hydrogen bond shows an obvious blue-shifted emission with about 10
nm. Especially, the polymers with the 5% molar content of the TADF unit blue-shift
above 20 nm compared to the reported PABPC5 (Table 1), which is clearly attributed
to effect of the introduction of pyridyl group in the acceptor moiety on emissive
wavelength. Note that the influence of electron-withdrawing pyridyl group is more
apparently reflected in all polymers with the 5% molar content of the TADF unit,
which have more balanced photophysical properties than others (Table 1).
As shown in Figure 2(b), the phosphorescent spectra of three polymers with the
5% molar content of the TADF unit reveal the sequentially red-shifted emissive peaks
with respect to that of PABPC5 at 77 K, indicating that the different substitution
patterns of pyridyl group primarily give rise to decrease in the T1 energy level in the
order PCzAB3Py, PCzAB2Py and PCzA3PyB. According to the maximum peak
wavelengths of their phosphorescent spectra, the T1 energy can be estimated to be
2.36, 2.29 and 2.23 eV for PCzAB3Py5, PCzAB2Py5 and PCzA3PyB5, respectively.
Thus, the experimental ΔEST values for three polymers are estimated to be 0.16, 0.10
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and 0.10 eV, respectively, from T1 and S1 calculated from the PL spectra in
oxygen-free toluene solution at 300 K. The values are close to the minima in the same
set of polymers, demonstrating that the polymers with the 5% molar content of the
TADF unit have more efficient RISC (Table 1, S3 and S4). The HOMO energy levels
of the thin films of the polymers measured by cyclic voltammetry in degassed
anhydrous acetonitrile solution are similar under the same molar content of the TADF
unit, which is consistent with the polymer structure composed of identical donor
backbone. However, the HOMO and LUMO energy levels are almost located in the
deeper than PABPC, indicating that the introduction of pyridyl group simultaneously
changes the strength of donor and acceptor moieties and then impacts on the TADF
properties of the polymers.
To fully characterize the TADF characteristics of these polymers, the transient PL
decay spectra were measured in neat film. As shown in Figure 4, S3(d) and S4(d), and
Table1, S3 and S4, the fluorescence decays of the polymers are comprised of a similar
nanosecond-scale component and a varied microsecond-scale component at 300 K,
which belong to the prompt fluorescence (PF) and delayed fluorescence (DF),
respectively. The delayed lifetimes (τDF) of the polymers with the same molar content
of the TADF unit gradually decrease in the order PCzA3PyB, PCzAB2Py and
PCzAB3Py, while the values of the polymers with the 5% molar content of the TADF
unit are near to the minima within the same set of polymers. Compared to the
polymers PCzA3PyB with the pyridyl group as the linker, the τDFs of other two
polymers are significantly shorter, implying that TADF process can be promoted more
effectively when the substituted electron-withdrawing group is sited on the far-end
position of the acceptor moiety. Accordingly, the overall PLQYs of PCzAB2Py and
PCzAB3Py are distinctly higher than those of PCzA3PyB in neat film. Meanwhile,
the percentages of the delayed component gradually increase with the decreasing
content of the TADF unit in each group of polymers, indicating that the triplet and
fluorescent singlet compete with each other through RISC processes. Furthermore, the
proportion of the delayed component apparently increases with the rising temperature
from 50 to 300 K for PCzA3PyB5 in neat film (Figure 4b), which is ascribed to the
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acceleration of up-conversion from T1 to S1 states at the higher temperature. The
corresponding RISC rate constant is as high as 2.15 × 106 s-1 at 300 K.[2,70] Another
noticeable characteristic is that the polymers own short τDFs, which are of benefit to
restraining the efficiency roll-off at high current density or luminance in EL device.
For example, τDFs of the polymers PCzAB3Py5, PCzAB2Py5 and PCzA3PyB5 are
0.88, 0.95 and 1.13 μs, respectively, even less than 1.29 μs of the TADF polymer
PABPC5 previously reported.[59]
As a result of the TADF characteristics, these polymers exhibit efficient EL
performance in solution-processed OLEDs employing neat polymer film as the
emissive layer (EML) with the identical configuration of
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic
ITO/
acid)
(PEDOT:PSS)/Polymers/1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB)/LiF/Al. In
these devices, PEDOT:PSS serves as the hole injection layer (HIL), and TmPyPB acts
as the electron-transporting (ETL) and exciton-blocking layer. All devices exhibit
intrinsic emission from the TADF polymers with slightly red-shifted EL spectra
relative to PL ones in neat films. The polymers PCzA3PyB50, PCzAB2Py50 and
PCzAB3Py50 with the 50% molar content of the TADF unit possess the longest EL
wavelengths in each group, mainly locating in the yellow to orange region with peaks
at 588, 600, and 585 nm, respectively. As the content of the TADF unit decreases in
each group of polymers, the main peak of EL spectra blue-shit gradually, and finally
moves to the green-yellow to yellow region peaked at 568, 573 and 557 nm for the
polymers PCzA3PyB5, PCzAB2Py5 and PCzAB3Py5 with the 5% molar content of
the TADF unit, respectively. Moreover, EL emission of the carbazole fragment around
430 nm becomes very weak relative to its PL emission, and only appears in the
polymers with the very low molar content of the TADF unit below 1%. All these
observations indicate that electrons tend to be trapped by the TADF unit and
recombine with holes to form excitons directly, because of the much lower LUMO
levels of the TADF unit than the polymer backbone. All devices turn on at an
extremely low voltage in the range of 2.6 to 3.2 V, indicating the efficient charge
injection from HIL/ETL to EML, which comes from the well matched energy levels
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between the HIL/ETL and these polymers. In addition, the turn-on voltages decrease
slowly with the increasing content of the TADF unit because electrons will be more
likely to inject into the TADF unit through barrier-free process at the relatively high
content of the TADF unit. More importantly, the turn-on and driving voltages of
PCzA3PyB are almost constant and slightly lower than other two sets of polymers,
indicating that the directly bonded donor/acceptor structure is in favor of
injection/transport of electrons. Correspondingly, the devices realize high luminance
with the maxima 16913 cd m-2 for PCzA3PyB5, 25942 cd m-2 for PCzAB2Py10 and
37235 cd m-2 for PCzAB3Py25, respectively, benefiting from the effective exciton
capture ability of the polymers.
The highest EL efficiencies are attained in the devices using the polymers
PCzA3PyB5, PCzAB2Py5 and PCzAB3Py5 as EMLs (Figure 6 and Table 4). For
example, the maximum EQEs of each group of the devices reach the highest values of
5.2, 11.9 and 12.2% for the above polymers, respectively, and then decrease whether
the content of the TADF unit decreases or increases in the polymers. The regularities
are consistent with the changes of PLQYs in neat films, demonstrating that the main
contribution of EQE is from PLQY. The maximum EQE of the device based on
PCzA3PyB5 is obviously inferior to those of PCzAB2Py5 and PCzAB3Py5, despite
exceeding the theoretical limit of conventional fluorescent OLEDs. The result is
reasonable due to the lack of phenyl linker between donor and acceptor in PCzA3PyB,
leading to the low PLQYs, although their devices have the relatively low turn-on and
driving voltages.[5] By contrast, the devices containing PCzAB2Py and PCzAB3Py
with a phenyl linker achieve more excellent EL performances, especially the very
slow roll-offs of EQE at the high luminance. For example, at 1000 cd m-2, the EQEs
still retain 11.3% and 11.6% for PCzAB2Py5 and PCzAB3Py5 (Table 4), respectively.
The small roll-off rates are undoubtedly ascribed to the short delayed lifetimes
resulting from the introduction of electron-withdrawing pyridyl group, which enable a
high exciton utilization rate to restrain concentration quenching. Overall, in the view
of the obtained data, the device using neat film of PCzAB2Py5 as EML realizes the
best comprehensive EL performances with the high luminance of over 25000 cd m-2,
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Chemistry - An Asian Journal
low turn-on voltage of 3.0 V, and maximum EQE of 11.9% accompanying by slow
efficiency roll-off, in addition to a longer emissive wavelength of 573 nm in the
yellow region.
Conclusion:
Acceptor moiety is an ingredient of the adjustable parts in the donor/acceptor
type of TADF emitters. To acquire long wavelength emission, three isomeric
benzoylpyridine derivatives, with stronger electron-withdrawing ability than BP, are
chosen as the pendant acceptor groups to synthesize three sets of corresponding
polymers PCzA3PyB, PCzAB2Py and PCzAB3Py based on the BDPA strategy. As
expected, all polymers show the red-shifted emission and the clear TADF
characteristic with shorter delayed lifetime, compared to the analogues containing BP,
while the different substitution patterns of pyridyl group in acceptor moiety also
produce obvious changes in the PL properties. The directly bonded pyridyl group to
acridine of the backbone results in a large red shift but low PLQY in the polymers
PCzA3PyB, whereas the far-end pyridyl group gives rise to a small red shift but high
PLQY in the PCzAB3Py. By contrast, the preferred substitution mode of pyridyl is
that the group is fixed to carbonyl group with its 2-position and locates at the far-end
of acceptor moiety, forming the polymers with the intramolecular hydrogen bond and
phenyl linker, which synergistically provide the large red shift and high PLQY. The
effects are also verified through their EL performance in the corresponding devices
employing neat film of the polymers as EML, which produce yellow-green to orange
emissions with peak wavelength of 557-600 nm. The polymers all achieve the
maximum EQEs of above 5%, while PCzAB2Py5 exhibits the better comprehensive
performances including the low turn-on and driving voltages, high luminance and
high EL efficiencies with slow roll-offs. The results demonstrate that finely adjusting
the electron-withdrawing ability of the pendant acceptor of the BDPA polymer is an
effective method for improving the photophysical properties of the TADF polymer
and the EL performance of the TADF device. Further works for realizing the saturated
red emission is in progress.
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Supporting Information:
Supporting Information is available from the Wiley Online Library or from the
author.
Acknowledgements:
The authors gratefully acknowledge the financial support from the National Key
R&D Program of China (No. 2016YFB0401001) and the National Natural Science
Foundation of China (Nos. 51473159, 21674110, and 21404101).
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Scheme 1. Polymer structures and synthetic routes.
Figure 1. Crystal structures of three precursors A3PyB, AB2Py and AB3Py. C(1) and
C(2) represent C6H13 groups.
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Figure 2. PL spectra of the polymers with the 5% molar content of the TADF unit in
oxygen-free toluene: (a) fluorescence spectra at 300 K. (b) phosphorescence spectra
(delayed by 5 ms) at 77 K.
Figure 3. UV-vis absorption and PL spectra of PCzAB2Py in (a) oxygen-free toluene
and (b) neat film at 300 K.
Figure 4. Transient PL decay characteristics in neat film under nitrogen atmosphere:
(a) PCzAB2Py at 300 K; (b) PCzAB3Py5 from 50 to 300 K (inset: the contributive
proportion for PLQYs from prompt and delayed fluorescence).
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Figure 5. Device performance for the polymers: (a) Energy level diagrams and device
configuration. (b) EL spectra for PCzAB2Py. (c) Current density-voltage-luminance
curves for PCzAB2Py. (d) External quantum efficiency versus luminance curves for
PCzAB2Py.
Figure 6. Device performance for the polymers with the 5% molar content of the
TADF unit: (a) EL spectra. (b) External quantum efficiency versus luminance curves.
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Table 1. Photophysical Properties of the Polymers
d)
d)
g)
h)
λabs [nm]
λPL[nm]
sola)/filmb)
ΦPL[%]c)
τPF (ΦPF
)
τDF (ΦDF
)
HOMOe)/LUMOf)
[eV]
ES/ET
ΔEST
Polymer
sola)/filmb)
filmb)
23
[ns (%)]
25(78.1)
30(67.3)
31(58.5)
32(54.3)
30(45.1)
29(43.5)
19(61.9)
32(51.4)
22 (35.1)
[ns (%)]
594(21.9)
744(32.7)
870(41.5)
951(45.7)
1140(53.9)
1203(56.5)
1134(38.1)
876(48.6)
1286 (41.9)
[eV]
[eV]
0.16
0.15
0.12
0.10
0.15
0.16
0.16
0.10
0.12
345, 438/344, 441
319, 438/319, 441
320, 440/319, 441
320, 440/319, 441
320, 440/318, 441
318, 440/320, 414
319, 440/319, 442
320, 430/320, 441
320, 425/327, 426
587/579
-5.32/-2.87
-5.34/-2.89
-5.38/-2.93
-5.42/-2.97
-5.45/-3.00
-5.45/-3.00
-5.44/-3.24
-5.42/-3.15
-5.38/-3.00
2.41/2.25
2.38/2.23
2.37/2.25
2.39/2.29
2.45/2.30
2.50/2.34
2.39/2.23
2.46/2.36
2.60/2.48
PCzAB2Py50
PCzAB2Py25
PCzAB2Py10
PCzAB2Py5
PCzAB2Py1
PCzAB2Py0.5
PCzA3PyB5
PCzAB3Py5
PABPC5
419, 592/572
421, 591/560
420, 593/555
420, 589/431, 540
421, 571/430, 541
421, 587/555
421, 559/544
420, 538/525
37
53
62
68
59
35
65
77
a Measured in oxygen-free toluene solution at room temperature; b Measured in neat film at room temperature; c Absolute PL
quantum yield evaluated using an integrating sphere under nitrogen atmosphere; d The fractional contributions of the fluorescence
(ΦPF) and delayed fluorescence (ΦDF) to the total ΦPL and lifetimes of the prompt (τPF) and delayed (τDF) decay components for
the thin film measured under nitrogen atmosphere at 300 K; e Determined by cyclic voltammetry for the thin film in degassed
f
anhydrous acetonitrile solution at room temperature; Deduced from the HOMO and the optical energy gap (Eg) values; g Singlet
(ES) and triplet (ET) excited energies estimated from the onset and maximum peak wavelengths of emission spectra in
oxygen-free toluene solution at 300 and 77 K, respectively; h ΔEST = ES - ET.
Table 2. Device Performance of the Polymers
Max performance
Device performance at1000/5000/10000 cd m-2
Von
(V)
Device
LE
(cd A-1)
7.6
PE
(lm W-1)
6.0
EQE
Vd
LE
PE
EQE
(%)
λEL [nm]
at 1000 cd m-2
600
CIE(x,y)
at 1000 cd m-2
(0.56, 0.44)
(0.54, 0.46)
(0.50, 0.49)
(0.48, 0.51)
(0.45, 0.52)
(0.43, 0.51)
(0.47, 0.51)
(0.44, 0.54)
(0.40, 0.56)
(%)
3.8
(V)
(cd A-1)
(lm W-1)
PCzAB2Py50
PCzAB2Py25
PCzAB2Py10
PCzAB2Py5
PCzAB2Py1
PCzAB2Py0.5
PCzA3PyB5
PCzAB3Py5
PABPC5
2.6
2.7
2.9
3.0
3.1
3.2
2.6
2.9
2.6
4.6/6.0/-
7.6/5.1/-
5.2/2.7/-
3.8/2.5/-
15.5
27.7
34.2
34.2
32.8
15.0
39.2
60.5
11.8
6.7
4.7/5.7/6.6
5.8/7.2/8.0
5.5/6.8/7.7
6.1/7.2/7.8
5.5/6.2/7.2
4.2/5.3/6.4
4.9/6.0/6.7
3.7/4.5/5.1
15.4/13.3/10.6
27.2/22.9/18.8
32.4/25.4/20.0
28.1/20.6/15.4
26.5/19.4/9.9
14.1/10.2/7.4
37.2/31.2/25.7
59.4/53.2/46.0
10.3/7.3/5.0
14.7/10.0/7.4
18.7/11.7/8.2
14.4/9.0/6.2
15.3/7.3/4.3
10.5/6.0/3.6
23.6/16.4/12.0
50.4/37.2/29.0
6.7/5.7/4.6
10.3/8.6/7.1
11.3/8.9/7.0
10.3/6.9/5.2
8.7/6.4/3.3
4.9/3.6/2.6
11.6/9.7/8.0
17.8/15.9/13.8
592
20.5
10.4
11.9
11.5
10.8
5.2
581
26.3
573
24.6
430/564
429/563
568
25.7
14.4
33.1
12.2
18.1
557
61.2
548
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Table of Content
Substitution pattern of pyridyl group in pendant acceptor distinctly modifies thermally
activated delayed fluorescence properties of the corresponding polymers, which
achieves efficient yellow-green to orange emissions in the non-doped OLEDs.
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