Thermally activated delayed fluorescent emitters based on 3-(phenylsulfonyl)pyridine

Article abstract of DOI:10.1016/j.cplett.2021.138474

3-(Phenylsulfonyl)pyridine (PSP), an unreported acceptor, is used to construct two thermally activated delayed fluorescent materials (TADF), PSPP and PSPBP, with rigid PXZ as donor in 3-position or 3, 5-position of PSP. Both PSPP and PSPBP have large twis

Full text of DOI:10.1016/j.cplett.2021.138474

Chemical Physics Letters 771 (2021) 138474
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Chemical Physics Letters
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Research paper
Thermally activated delayed fluorescent emitters based on
3-(phenylsulfonyl)pyridine
,
,
,*
Ping Wua 1, Feng-ming Xieb 1, Huai-xin Weia, Yan-Qing Lib, Guo-liang Daia, Yan Wanga,
Jian-Xin Tangb *, Xin Zhaoa
,
a School of Chemistry and Life Sciences, Suzhou University of Science and Technology, Suzhou, Jiangsu 215009, China
b Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou,
Jiangsu 215123, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
3-(Phenylsulfonyl)pyridine (PSP), an unreported acceptor, is used to construct two thermally activated delayed
fluorescent materials (TADF), PSPP and PSPBP, with rigid PXZ as donor in 3-position or 3, 5-position of PSP. Both
PSPP and PSPBP have large twisted structure due to highly steric hindrance, resulting in small ΔEST. Simulta-
neously, the frontier molecular orbitals of PSPP and PSPBP show limited spatial overlap, which improve pho-
toluminescence quantum yields. Both emitters have distinct TADF performance. The OLEDs based on these
emitters exhibit good photoelectric properties. This work enriches the acceptor selection scope and possesses
crucial significance to developing new TADF molecules.
3-(phenylsulfonyl)pyridine
Phenothiazine
Thermally activated delayed fluorescent
materials
Meta-position
OLEDs
1. Introduction
structure molecule with intramolecular charge transfer (ICT) charac-
teristic by connecting donor (D) and acceptor (A) moieties, which can
Organic light-emitting diodes (OLEDs) have been arousing tremen-
dous attention in the field of display and lighting for the excellent fea-
tures such as flexible display, fast response, low power consumption,
light-weight, and other advantage [1–5]. It is well known that during
the recombination of holes and electrons, 25% excitons spin-statistically
split into the singlet excited state (S1), and 75% excitons split into triplet
excited state (T1) [6]. Traditional fluorescent materials theoretically
achieves 25% internal quantum efficiency (IQE) due to only singlet
excitons are utilized to radiate from S1 excited state to ground state S0
[1,3,7,8]. Although phosphorescent materials can theoretically achieve
100% IQE, the drawbacks of containing precious metals and poor sta-
bility limit the widespread application of phosphorescent OLEDs devices
[3,9]. As the third-generation luminescent materials for OLEDs, ther-
mally activated delayed fluorescent materials (TADF) were developed
rapidly due to the low cost and theoretically 100% IQE by harvesting
singlet and triplet excitons through reverse intersystem crossing (RISC)
process[6,10–14]. A very small singlet–triplet splitting (ΔEST) between
its S 1 and T 1 is beneficial to improving the rate of RISC of TADF ma-
terials and the external quantum efficiency (EQE) of OLEDs [1,15–18].
The main strategy to reduce the ΔEST is adopting a highly pre-twisted
spatially separate the overlap of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
[15,19,20]. But small overlap of the HOMOs and LUMOs can reduce the
radiation decay rate according to Franck-Condon principle [21],
resulting the photoluminescence quantum yields (PLQYs) of TADF ma-
terials reduced. Rigid structures were introduced into TADF emitter can
suppress non-radiation decay and improve radiative decay rate, which
can maximize the PLQYs [22,23]. So the higher PLQYs and smaller ΔEST
are vital to obtain efficient TADF materials.
The instrument to attain this is choosing appropriate donor and
acceptor units and putting the donor in an appropriate position of the
acceptor [22,24]. Diphenylsulfone (DPS) was proved to be one of the
most used widely acceptors in TADF molecules [25,26]. In addition, the
sulfonyl group in DPS has a tetrahedral geometry, which limits the
conjugation length of the compound, which is beneficial to reducing
ΔEST and enhance the reverse intersystem crossing rate constant (KRISC
)
[27]. Young Sik Kim’s group found that meta position substituted
2mDTC-DPS has larger dihedral angle (51◦) than para-position
substituted 2DTC-DPS (48.4◦), meaning that the molecular structure of
2mDTC-DPS is more distorted, which achieves more effectively
* Corresponding authors.
E-mail addresses: jxtang@suda.edu.cn (J.-X. Tang), zhaoxinsz@usts.edu.cn (X. Zhao).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.cplett.2021.138474
Received 27 December 2020; Received in revised form 4 February 2021; Accepted 22 February 2021
Available online 8 March 2021
0009-2614/© 2021 Elsevier B.V. All rights reserved.

P. Wu et al.
Chemical Physics Letters 771 (2021) 138474
separated of the frontier molecular orbitals (FMOs) of 2mDTC-DPS and
results in a smaller ΔEST [28]. Kunpeng Guo group synthesized two
TADF material mSOAD and DMAC-DPS by connecting donor 9,9-
dimethyl-9,10-dihydroacridine (DMAC) to the meta-, or para-position
of DPS. The meta position substituted mSOAD obtains very smaller
ΔEST (0.02 eV) than that (0.18 eV) of DMAC-DPS, and result in the
mSOAD-based devices exhibits superior maximum EQE, maximum
to improving the oscillator strength and their PLQYs. PSPP with one
donor has suitable ICT and higher PLQYs, while PSPBP with two donor
PXZ has stronger ICT and lower PLQYs. Theoretical calculation and
photophysical properties demonstrate both emitters have typical TADF
characteristics and good photoelectric properties. Compared with the
OLED based on PSPBP, the PSPP-based OLED has better device perfor-
mance with EQEmax of 4.46% and turn-on voltage of 3.2 V. This work
enriches the acceptor selection scope and possesses crucial significance
to developing new TADF molecules.
ˇ˙
current efficiency and power efficiency [29]. Saulius Jursenas team
found that the ΔEST of the TmCZ (meta-) is smaller than that of the TpCZ
(para-), as compare with TpCZ (para-), the delayed emission intensity of
TmCZ (meta-) is 3 orders of magnitude stronger [30]. So TADF mole-
cules that the donor is located in the meta-position of the acceptor,
usually have large dihedral angle, smaller ΔEST and superior photo-
physical properties. However, it remains a great challenge to obtain
efficient TADF materials by designing and synthesizing proper molec-
ular structure.
2. Experimental section
2.1. General information
Synthetic routes and chemical structures of two TADF emitters can
be seen in the Scheme 1. All solvents used are analytical grade. Part of
the reaction was carried out under nitrogen atmosphere. 1H NMR and
13C NMR spectra were recorded with Germany Bruker AVANCE III type
NMR Spectrometer (400 MHz) at room temperature. The mass spectra
were recorded by GCT-Premier TOF mass spectrometer.
To enrich the acceptor selection scope for developing new TADF
molecules, it’s very significant to evaluate the possibility of using novel
acceptor. Therefore, we designed a new acceptor 3-(phenylsulfonyl)
pyridine (PSP), and synthesized two novel TADF molecules 10-(3-(pyr-
idin-3-yl-sulfonyl)phenyl)-10H-phenoxazine (PSPP) and 10,10′-(5-
(pyridin-3-yl-sulfonyl)-1,3-phenylene)bis(10H-phenoxazine) (PSPBP),
introducing one or two donor PXZ at the meta-position of the benzene
ring in the PSP (Scheme 1). Of which the pyridine moiety has a rigid
plane, and the presence of N atoms can increase electron-withdrawing
properties of PSP, which can effectively adjust the electron transport
characteristics in the molecule. Meanwhile, rigid PXZ as a donor is
beneficial to suppressing non-radiation decay process and increasing
radiation decay rate and PLQYs [31–33], for expecting to obtain effi-
cient TADF materials. Experiments shown that both PSPP and PSPBP
have highly distorted molecular structures with large dihedral angle
(close to 90◦) between the donor and the acceptor, and the ΔESTs of them
are very small (0.01 and 0.02 eV, respectively). Meanwhile, the com-
bination of the donor PXZ and the acceptor PSP enables the FMOs of
PSPP and PSPBP to achieve a limited spatial overlap, which is conducive
2.2. Synthesis
2.2.1. Synthesis and characterization of 10-(3-(pyridin-3-ylsulfonyl)
phenyl)-10H-phenoxazine (PSPP)
3-bromothiophenol (2.10 g, 11.11 mmol), 3-iodopyridine (2.74 g,
13.37 mmol) and K2CO3 (3.95 g, 28.6 mmol) were dissolved in a 150 mL
three-necked flask with 15 mL DMF. And CuI (0.11 g, 0.56 mmol) was
added into the mixture under nitrogen atmosphere. After heating and
stirring at 110 ◦C for 24 h, the reaction was stopped and filtrated. The
filtrate was extracted by CH2Cl2 and washed with NaOH solution. The
crude intermediate product was used for the next reaction without
further separation and purification because of 3-iodopyridine no effect
on the oxidation reaction [34].
The crude intermediate product was dissolved and stirred in HOAc
(20 mL) and CH2Cl2 (20 mL). After the temperature rose to 70 ◦C, H2O2
(20 mL) was added sequentially in three batches. Then the reaction
temperature was controlled at 120 ◦C for 16 h. The solids that precipi-
tated in the reaction flask were filtered off. The intermediate product 1 is
light yellow powder (1.81 g) which recrystallized from ethanol. Inter-
mediate product 1 (0.50 g, 1.68 mmol), PXZ (0.53 g, 2.89 mmol) and
sodium tert-butyl alkoxide(t-BuONa) (1.20 g, 12.48 mmol) were dis-
solved in 50 mL of toluene into a 150 mL three-necked bottle under
nitrogen atmosphere. Tri-tert-butylphosphonium tetrafluoroborate ([(t-
Bu)3PH]BF4) (0.069 g, 0.084 mmol) and tris(dibenzylideneacetone)
dipalladium(pd2(dba)3) (0.065 g, 0.0084 mmol) were added into the
solution, and the reaction was performed at 110 ◦C for 24 h, then cooled
to room temperature. The reaction solution was poured into 200 mL of
NaOH solution, extracted with DCM (30 mL × 3), and the organic phase
was concentrated. The product was further purified by silica gel column
chromatography (pure DCM) to obtain 0.55 g (yield: 80%) of yellow
solid. 1H NMR (400 MHz, d6-DMSO) δ 9.22 (d, J = 2.1 Hz, 1H), 8.90 (dd,
J = 4.8, 1.5 Hz, 1H), 8.49–8.43 (m, 1H), 8.19 (dd, J = 9.9, 4.8 Hz, 2H),
7.95 (t, J = 7.9 Hz, 1H), 7.85 (d, J = 8.6 Hz, 1H), 7.69 (dd, J = 7.7, 4.8
Hz, 1H), 6.78 (dd, J = 7.8, 1.5 Hz, 2H), 6.71 (td, J = 7.6, 1.4 Hz, 2H),
6.64 (td, J = 7.7, 1.6 Hz, 2H), 5.77 (dd, J = 7.9, 1.3 Hz, 2H). 13C NMR
(101 MHz, d6-DMSO) δ 154.93, 148.66, 143.92, 143.65, 140.17,
137.69, 137.55, 136.27, 133.74, 133.67, 130.40, 128.32, 125.19,
124.22, 122.44, 115.98, 113.61. TOF MS (EI, m/z) Calcd for
O
Br
S
HS
Br
O
N
1
(1)
(2)
CuI
Cl
N
I
H O ,HOAC
2
2
O
S
Cl
HS
Cl
O
N
Cl
2
H
N
O
O
S
N
O
O
N
PSPP
t-BuONa,N2,Toluene
pd2(dba)3,t-Bu3PHBF4
O
O
S
H
N
N
O
C
23H16N2O3S [M+]: 400.0882, Found: 400.0880. (Fig. S1-3, Supporting
N
O
Information)
N
O
2.2.2. Synthesis and characterization of 10,10′-(5-(pyridin-3-ylsulfonyl)-
1,3-ethylene)bis(10H- phenoxazine)(PSPBP)
PSPBP
3,5-dichlorothiophenol (2.32 g, 13.04 mmol), 3-iodopyridine (2.81
g, 13.71 mmol) and K2CO3 (4.56 g, 33.02 mmol) were dissolved in a 150
Scheme 1. Synthetic routes and chemical structure of PSPP and PSPBP.
2

P. Wu et al.
Chemical Physics Letters 771 (2021) 138474
mL three-necked flask with 15 mL DMF. And CuI (0.15 g, 0.76 mmol)
was added into the mixture under nitrogen atmosphere. After heating
and stirring at 110 ◦C for 24 h, the reaction was filtrated. The filtrate was
extracted by CH2Cl2 and washed with NaOH solution. The crude inter-
mediate product was used for the oxidation reaction. The oxidation re-
action was same with intermediate product 1 and the intermediate
product 2 is light yellow powder (1.95 g).
3. Results and discussion
3.1. Theoretical calculations
In order to explore the molecular structures, electron cloud distri-
bution and energy gap in theory, the B3LYP/6-31G(d) level in density
functional theory was performed to calculate and optimize the geo-
metric configuration and frontier molecular orbitals (FMOs) of the
target molecules in gas phase using Gaussian 09 program [35]. The
Optimized structure and distribution of the molecular orbitals are shown
in Fig. 1.
Intermediate product 2 (0.56 g, 1.95 mmol), PXZ (0.78 g, 4.26
mmol) and t-BuONa (1.43 g, 14.87 mmol) were dissolved in toluene (50
mL) under nitrogen atmosphere. [(t-Bu)3PH]BF4 (0.081 g, 0.099 mmol)
and pd2(dba)3 (0.055 g, 0.0071 mmol) were added into the solution, and
the reaction was performed at 110 ◦C for 24 h, then cooled to room
temperature. The reaction solution was poured into 200 mL of NaOH
solution, extracted with DCM (30 mL × 3). The product was further
purified by silica gel column chromatography (pure DCM) to obtain
0.80 g (yield: 71%) of yellow solid. 1H NMR (400 MHz, d6-DMSO) δ 9.28
(d, J = 2.0 Hz, 1H), 8.92 (dt, J = 4.9, 2.5 Hz, 1H), 8.54 (ddd, J = 8.1, 2.3,
1.6 Hz, 1H), 8.34 (d, J = 1.9 Hz, 2H), 8.08 (t, J = 1.8 Hz, 1H), 7.71 (ddd,
J = 8.1, 4.9, 0.6 Hz, 1H), 6.73 (dddd, J = 16.6 , 14.4, 7.5, 2.0 Hz, 12H),
6.01 (dd, J = 7.6, 1.8 Hz, 4H). 13C NMR (101 MHz, d6-DMSO) δ 155.11,
149.06, 146.94, 143.69, 143.16, 140.54, 137.39, 136.58, 133.63,
131.03, 125.19, 124.31, 122.59, 116.01, 113.96. MALDI TOF MS (ESI,
The HOMOs of PSPP and PSPBP are mainly distributed in the PXZ
moieties, and the LUMOs of the two molecules are dispersed on the PSP.
Compared with the energy gap of PSPP (3.02 eV) between HOMO and
LUMO, the energy gap of PSPBP (2.87 eV) is reduced, which is because
that increasing of donor number reduces the LUMO energy level, while
the change of the HOMO energy level is negligible. The similar HOMO
energy levels of the emitters are ascribe to their identical donor moieties
[36], while lower LUMO energy level of PSPBP than that of PSPP should
be attributed to the altering of conjugation length of the donor unit and
more stronger ICT effect due to the introduction of one more donor PXZ
[37]. The dihedral angles between PXZ and PSP in PSPP and PSPBP are
84.19◦, 86.26◦ and 83.09◦, respectively, indicating the both emitters
have highly twisted structures. So the FMOs of the two molecules are
m/z) Calcd for
C
35H23N3O4S [M+]: 581.140, Found: 581.150.
(Figure S4-6, Supporting Information)
almost separated, which results the two molecules have a small ΔEST
.
Fig. 1. (a) Optimized structure, (b) HOMO and LUMO distributions, calculated HOMO-LUMO energies of PSPP and PSPBP, respectively.
3

P. Wu et al.
Chemical Physics Letters 771 (2021) 138474
Simultaneously, the FMOs of PSPP and PSPBP possess limited spatial
overlap, which can improve their oscillator strength and PLQYs [23].
ascribed to triplet excitons convert into singlet excited state through
RISC process (T1 → S1), and then radiate to the ground state. Therefore,
these two materials have obvious delayed fluorescence properties, and
delayed fluorescence lifetimes are very short, which is beneficial to
reducing triplet–triplet-annihilation, singlet–triplet-annihilation and
triplet-polaron-Annihilation [39].
3.2. Photophysical properties
The UV–Vis absorption (UV) spectrum, fluorescence (FL) and low-
temperature 77 K phosphorescence (Phos) spectrum of PSPP and
PSPBP were measured in toluene (10ꢀ 5 mol/L). As shown in Fig. 2 and
Table 1, the UV–Vis absorption bands of the two molecules are similar.
The maximum absorption peak of PSPP and PSPBP are at 321 and 317
To further demonstrate the TADF characteristics of two emitters, the
temperature-dependent transient PL decays in non-doped films under
varied temperature were measured (Fig. S8, Supporting Information).
The intensity of the delayed component was intensified with the in-
crease of temperature, suggesting typical TADF property [2].The PLQYs
of them are 46.1% for PSPP and 32.4% for PSPBP (Table S1, Supporting
Information), the lower PLQY for PSPBP may attributed to the stronger
ICT due to one more donor PXZ [37]. The PLQY values of PSPP and
PSPBP are higher than that of some TADF materials with DPS as acceptor
[27]. To further understand the excited state characteristics of PSPP and
PSPBP, the kinetics parameters were investigated [40]. As shown in
Table S1, both PSPP and PSPBP have larger kRISC and radiative rate
constant (kr), however, compared with PSPBP, PSPP possesses smaller
non-radiative rate constant (knr) and intersystem crossing rate constant
(kisc), therefore PSPP obtains larger PLQY than PSPBP.
nm, respectively, which is attributed to the π-π* transition. Using the
absorption threshold of them, the optical band gap (Eg) were calculated
to be 2.96 eV for PSPP and 2.63 eV for PSPBP, respectively. At room
temperature, the fluorescence emission peaks of PSPP and PSPBP are
located at 524 and 549 nm, respectively, both of which are green fluo-
rescence emission. Fluorescence emission peaks of PSPBP is obvious red-
shifted than that of PSPP due to one more donor PXZ and resulting in
stronger ICT effect [3]. In addition, the small overlaps between the ab-
sorption and the emission spectra for PSPP and PSPBP indicate that
there are ICT effects for both PSPP and PSP [38]. To further investigate
the ICT properties, the photoluminescence (PL) spectra were measured
for two emitters in different polar solvent, such as hexane, toluene,
dichloromethane (CH2Cl2), tetrahydrofuran (THF). As shown in Fig. S7
(Supporting Information), the fluorescence peaks were red-shift up to
90 nm for PSPP and 93 nm for PSPBP, respectively, with the solvent
polarity increased, indicating pronounced ICT effect [1]. However,
compared with PSPBP, PSPP with one donor has suitable ICT perfor-
mance. Usually, stronger ICT characteristic reduces PLQYs of TADF
[37]. So PSPP may achieve higher PLQYs. The phosphorescence of PSPP
shows a characteristic vibronic structure, implying that its lowest triplet
state is local emission, while the phosphorescence spectrum of PSPBP
shows a broad and featureless emission band with a peak at 507 nm. The
S1/T1 levels of PSPP and PSPBP were calculated from the emission peak
of the fluorescence and phosphorescence emission spectra at 77 K to be
2.62/2.61 eV and 2.46/2.44 eV, respectively, and the ΔESTs of them
were calculated to be 0.01 for PSPP and 0.02 eV for PSPBP. It can be seen
that PSPP and PSPBP have small ΔEST, which is very conducive to the
RISC process from triplet state to singlet state.
3.3. Electrochemical properties
To investigate the values of the FMOs energy levels of the emitters,
the cyclic voltammograms (CV) of the two compounds were performed.
As shown in Fig. 4 and Table 1, the left oxidation peak is the oxidation
peak of ferrocene, which is the internal standard. From the initial
oxidation peak of the CV curve, the HOMO levels were ꢀ 5.12 eV for
PSPP and ꢀ 5.22 eV for PSPBP. Compared to those compounds with DPS
as the acceptor [29,41,42], the HOMO levels of the emitters are closed to
the work function of the ITO because the introduction of pyridine
moieties increases the HOMO levels of the emitters, which is benefit to
reducing the hole carrier injection barrier. According to Eg measured by
UV–Vis absorption, the LUMO levels were ꢀ 2.16 and ꢀ 2.59 eV for PSPP
and PSPBP, respectively. The values of FMOs of PSPP and PSPBP
measured by CV are consistent with the DFT calculation values.
In order to confirm the delayed fluorescence properties of the emit-
ters, transient PL curves were measured (Fig. 3 and Table S1). It can be
seen from Fig. 3 that the two compounds show a clear double expo-
nential curve. The fluorescence lifetimes of two compounds are
3.4. Thermal properties
Thermal gravity analysis (TGA) and differential scanning calorim-
etry (DSC) were carried out to explore the thermal properties of PSPP
and PSPBP. As shown in Fig. 5 and Table 1, the thermal decomposition
temperatures (Td) of PSPP and PSPBP are 295 and 413 ◦C, respectively
(Fig. 5a). Due to the introduction of two donor units, the mass of PSPBP
is increased, so it has more excellent thermal stability. It can be clearly
composed of two parts of 44.89 ns/1.83 μs for PSPP and 16.86 ns/0.87
μ
s for PSPBP, respectively. The nanosecond part belongs to prompt
fluorescence radiating from the lowest excited state (S1) to the ground
state (S0). The microsecond part belongs to delayed fluorescence,
Fig. 2. UV–Vis absorption spectra, fluorescence spectra (room temperature and 77 K) and phosphorescence spectra (77 K) and molecular structures of PSPP and
PSPBP in toluene solution.
4

P. Wu et al.
Chemical Physics Letters 771 (2021) 138474
Table 1
Summary of photophysical, electrochemical and thermal properties of PSPP and PSPBP.
Emitter
λabs [nm]a
λFl. [nm]b
λPh. [nm]c
Td [◦C]d
Tg [◦C]e
ES1/T1 [eV]f
ΔEST [eV]g
Eg [eV]h
HOMO/LUMO [eV]i
PSPP
321
317
524
549
474
507
295
413
182
220
2.62/2.61
2.46/2.44
0.01
0.02
2.96
2.63
ꢀ 5.12/ꢀ 2.16
ꢀ 5.22/ꢀ 2.59
PSPBP
a
UV–Vis absorption maximum at room temperature.
Fluorescence maximum at room temperature.
Phosphorescence maximum at 77 K.
b
c
d
e
f
g
h
i
Decomposition temperature with 5% weight loss.
Glass transition temperature.
Singlet (ES) and triplet (ET) energies estimated from emission peaks of the fluorescence and phosphorescence spectra in toluene at 77 k, respectively.
ΔEST = ES ꢀ ET.
Eg = 1240/λoneset
.
The HOMO energy levels were determined from the onset oxidation potential of cyclic voltammetry, ELUMO = Eg + EHOMO
.
3.5. Electroluminescent properties
In order to explore the electroluminescence (EL) properties of PSPP
and PSPBP, multilayer OLEDs were fabricated by doping two TADF
emitters into the mCBP host as a light-emitting layer (EML). The devices
structure are: ITO/MoO3 (5 nm)/NPB (40 nm)/TCTA (10 nm)/mCBP:
TADF emitter (20 wt%) (20 nm)/TmPYPB (40 nm)/LiF (1 nm)/Al (100
nm). In the devices, TCTA is selected as the electron blocking layer
(EBL), MoO3 and LiF are the hole injection layer and the electron in-
jection layer, respectively. NPB and TmPYPB are the hole transport layer
and the electron transport layer, respectively (Fig. 6a, 6b). The EL
emission spectra, external quantum efficiency–voltage (EQE-V) curves,
current density–voltage-luminance (J-V-L) curves and chromaticity co-
ordinates of these devices were shown in Fig. 6c–f. The devices perfor-
mance parameters of two TADF-OLEDs are summarized in Table 2.
As shown in Fig. 6c, the devices based on PSPP and PSPBP EL emit
bright green light of 534 nm and green light of 554 nm, respectively. The
maximum EQE of PSPP and PSPBP are 4.46%, and 2.02%, respectively
(Fig. 6d). The Commission Internationale de I’Eclairage (CIE) co-
ordinates of PSPP and PSPBP are (0.34, 0.52) and (0.41, 0.54), respec-
tively, which can be seen in Fig. 6f.
Fig. 3. Transient PL decay of PSPP and PSPBP in non-doped film.
Both emitters have lower turn-on voltages of 3.2 V than the TADF
emitters with DPS as acceptor [29,41,46], indicating that the charge
carrier injection barrier is lower. For the PSPBP-based device, the
maximum luminance (Lmax) values is 4812 cd mꢀ 2, while the device
based on PSPP exhibits Lmax of 1727 cd mꢀ 2. The full width half
maximum of the emitters are closed. The maximum power efficiency
PEmax (12.01 lm Wꢀ 1) and the maximum current efficiency CEmax
(13.38 cd Aꢀ 1) of PSPP-OLED are higher than PEmax (4.23 lm Wꢀ 1) and
CEmax (6.12 cd Aꢀ 1) of PSPBP-OLED.
4. Conclusions
In summary, using a novel acceptor PSP, we designed and synthe-
sized two TADF materials, PSPP and PSPBP, with rigid PXZ as donor in
3-position or 3, 5-position of PSP. Both PSPP and PSPBP have large
twisted structure due to highly steric hindrance, resulting in small ΔESTs
(<0.2 eV). Simultaneously, the FMOs of PSPP and PSPBP possess limited
spatial overlap, which improve their oscillator strength and PLQYs.
Compared with PSPBP, PSPP with one donor has suitable ICT and higher
PLQYs. Theoretical calculation and photophysical properties demon-
strate both emitters have distinct TADF performance and shorter
Fig. 4. CV curves of PSPP and PSPBP.
found from the DSC curves (Fig. 5b) that the glass transition tempera-
tures (Tg) of PSPP and PSPBP are 182 and 220 ◦C, respectively.
Compared to those compounds with DPS as the acceptor [42–44], PSPP
and PSPBP obtain higher Td and Tg because the introduction of pyridine
moieties enhances the polarity of the emitters, which increases the
intermolecular force. High Td and Tg values demonstrate PSPP and
PSPBP have better thermal properties and superior film-forming prop-
erties, which is vital for improving long-term stability of OLEDs [45].
delayed fluorescence lifetimes (1.83 μs for PSPP, 0.86 μs for PSPBP).
OLEDs based on the both emitters have good photoelectric properties.
The PSPP-based device exhibits bright green emission with peak of 534
nm, while the PSPBP-based device shows green light with peak of 554
nm due to one more donor PXZ. Compared with the PSPBP-based OLED,
the OLED based on PSPP with one donor PXZ achieves higher EQEmax of
4.46% and lower turn-on voltage of 3.2 V. This work enriches the
acceptor selection scope and possesses crucial significance to developing
5

P. Wu et al.
Chemical Physics Letters 771 (2021) 138474
Fig. 5. (a) TGA curves and (b) DSC curves of PSPP and PSPBP.
Fig. 6. (a) The device structure and energy level diagram, (b) the chemical structure of the materials used in the TADF devices, and (c) EL spectra of the TADF
OLEDs. d) EQE–luminance. (e) Current density and luminance versus the voltage (J–V–L) characteristics, and f) Commission Internationale de L’Eclairage (CIE)
coordinates of PSPP and PSPBP.
6

P. Wu et al.
Chemical Physics Letters 771 (2021) 138474
Table 2
Electroluminescence performance data of OLEDs.
b
c
d
Emitters
Von [V]a
L [cd mꢀ 2
]
PE [lm Wꢀ 1
]
CE [cd A-1
]
EQE [%]e
λEL [nm]f
FWHM [nm]g
CIE (x, y)h
PSPP
3.2
3.2
1727
4812
12.01
4.23
13.38
6.12
4.46
2.02
534
554
117
114
(0.34, 0.52)
(0.41, 0.54)
PSPBP
a
The operating voltage at a brightness of 1 cd mꢀ 2
.
b
c
d
e
f
Maximum luminance.
Maximum power efficiency.
Maximum current efficiency.
Maximum external quantum efficiency.
The EL emission peak.
g
h
FWHM of EL.
CIE 1931 coordinates.
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234–238.
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Declaration of Competing Interest
[17] X. Li, Y. Shi, K. Wang, M. Zhang, C. Zheng, D. Sun, G. Dai, X. Fan, D. Wang, W. Liu,
Y. Li, J. Yu, X. Ou, C. Adachi, X. Zhang, ACS Appl. Mater. Inter. 11 (2019)
13472–13480.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
´
[18] R.J. Vazquez, J.H. Yun, A.K. Muthike, M. Howell, H. Kim, I.K. Madu, T. Kim, P.
Zimmerman, J.Y. Lee, T.G. III, J. Am. Chem. Soc. 142 (2020) 8074–8079.
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Inter. 11 (2019) 7192–7198.
Acknowledgements
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This work is financially supported by the National Key Basic
Research and Development Program of China [Nos. 2016YFB0401002,
2016YFB0400700], the National Natural Science Foundation of China
[Nos. 61520106012, 61722404, 51873138, 21203135], the 333 Pro-
gram [No. BRA2019061], Collaborative Innovation Center of Suzhou
Nano Science & Technology.
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Appendix A. Supplementary material
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Supplementary data to this article can be found online at https://doi.
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