3-Benzoyl-4H-chromen-4-one: A novel twisted acceptor for highly efficient thermally activated delayed fluorescence emitters

Article abstract of DOI:10.1016/j.dyepig.2020.108744

In this work, a novel twisted 3-benzoyl-4H-chromen-4-one (BZC) acceptor is reported and a BZC-based thermally activated delayed fluorescence (TADF) emitter, 3-(4-(10H-phenoxazin-10-yl)benzoyl)-4H-chromen-4-one (BZC-PXZ), is developed. The unique structure of BZC endows BZC-PXZ with favorable molecular and stacking structures. First, the small and twisted structure of BZC group not only brings about large dihedral angle (88.78°) between donor and acceptor groups, but also efficiently prevents π???π interaction. Second, the three oxygen atoms of BZC group bring about abundant weak intermolecular interactions which can enhance the charge-transporting ability of BZC-PXZ and facilitate the exciton recombination on it. As a result, BZC-PXZ exhibits small singlet-triplet energy splitting (0.02 eV) and high photoluminescence quantum yield (93percent) simultaneously. Meanwhile, the BZC-PXZ-based device displays high efficiency (64.6 cd/A, 69.0 lm/W, 21.3percent), high exciton utilization efficiency (77.2percent) and low roll-off of efficiencies even at ultralow doped concentration of 1percent. These results demonstrate that BZC group is a good acceptor for developing high-performance TADF emitters.

Full text of DOI:10.1016/j.dyepig.2020.108744

Journal Pre-proof
3-Benzoyl-4H-chromen-4-one: A Novel Twisted Acceptor for Highly Efficient
Thermally Activated Delayed Fluorescence Emitters
Peng Wang, Junsheng Yu, Shanyong Chen, Hong Yu, Xingwu Yan, Youwei Guan,
Jinlei Chen, Lu Li
PII:
S0143-7208(20)31441-8
DOI:
https://doi.org/10.1016/j.dyepig.2020.108744
DYPI 108744
Reference:
To appear in: Dyes and Pigments
Received Date: 10 March 2020
Revised Date: 9 July 2020
Accepted Date: 26 July 2020
Please cite this article as: Wang P, Yu J, Chen S, Yu H, Yan X, Guan Y, Chen J, Li L, 3-Benzoyl-4H-
chromen-4-one: A Novel Twisted Acceptor for Highly Efficient Thermally Activated Delayed
Fluorescence Emitters, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.108744.
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Author Statement
In this work, the detailed contributions of each author are as following:
Peng Wang: Investigation, Methodology, Software
Junsheng Yu: Validation, Writing-Review & Editing
Shanyong Chen: Conceptualization, Formal analysis, Writing-Review & Editing
Hong Yu: Investigation
Xingwu Yan: Data Curation
Youwei Guan: Project administration, Writing-Original Draft
Jinlei Chen: Visualization, Supervision
Lu Li: Resources, Writing-Review & Editing, Funding acquisition

3-Benzoyl-4H-chromen-4-one: A Novel Twisted Acceptor for Highly
Efficient Thermally Activated Delayed Fluorescence
Emitters
Peng Wangab, Junsheng Yua, Shanyong Chen*b, Hong Yub, Xingwu Yanb, Youwei Guanb, Jinlei
Chenb, Lu Li*b
aState Key Laboratory of Electronic Thin Films and Integrated Devices, School of
Optoelectronic Science and Engineering, University of Electronic Science and Technology of
China (UESTC), Chengdu 610054, P. R. China
bMicro/Nano Optoelectronic Materials and Devices International Science and Technology
Cooperation Base of China, Chongqing University of Arts and Sciences, Chongqing 402160, P.
R. China
*Corresponding Author. E-mail: chensy@cqwu.edu.cn (S. Chen) and lli@cqwu.edu.cn (L. Li).

Abstract
In this work, a novel twisted 3-benzoyl-4H-chromen-4-one (BZC) acceptor is reported and a
BZC-based
thermally
activated
delayed
fluorescence
(TADF)
emitter,
3-(4-(10H-phenoxazin-10-yl)benzoyl)-4H-chromen-4-one (BZC-PXZ), is developed. The
unique structure of BZC endows BZC-PXZ with favorable molecular and stacking structures.
First, the small and twisted structure of BZC group not only brings about large dihedral angle
(88.78°) between donor and acceptor groups, but also efficiently prevents π∙∙∙π interaction.
Second, the three oxygen atoms of BZC group bring about abundant weak intermolecular
interactions which can enhance the charge-transporting ability of BZC-PXZ and facilitate the
exciton recombination on it. As a result, BZC-PXZ exhibits small singlet-triplet energy splitting
(0.02 eV) and high photoluminescence quantum yield (93%) simultaneously. Meanwhile, the
BZC-PXZ-based device displays high efficiency (64.6 cd/A, 69.0 lm/W, 21.3%), high exciton
utilization efficiency (77.2%) and low roll-off of efficiencies even at ultralow doped
concentration of 1%. These results demonstrate that BZC group is a good acceptor for
developing high-performance TADF emitters.
Keywords: Thermally activated delayed fluorescence; 3-Benzoyl-4H-chromen-4-one, Small
singlet-triplet splitting; High photoluminescence quantum yield; Efficient exciton utilization

1. Introduction
As the third-generation emitter of organic light emitting diodes (OLEDs), thermally activated
delayed fluorescence (TADF) emitter gains much attention in the past decade [1-11]. For TADF
emitter, two parameters are most important: singlet-triplet splitting (∆EST) and
photoluminescence quantum yield (PLQY). Generally, small ∆EST and high PLQY are essential
for efficient exciton utilization and high-performance TADF emitter. Previous studies indicate
that the donor (D) and acceptor (A) composition groups play a vital role in determining ∆EST
and PLQY[12-15]. For example, the combination of strong donor and strong acceptor can
separate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) to realize small ∆EST[16]. To meet the demand of TADF emitters, researchers
have developed many donor and acceptor groups[17-22]. For donors, their choices are
relatively fixed, including carbazole, aromatic amine, phenoxazine and their derivatives[23-24].
Hence, TADF studies mainly focus on developing new acceptors, molecular structures and
exciplex-TADF[25-31]. In the past decade, many acceptors have been introduced, including
cyanobenzene, pyrimidine, triazine, arylborane, etc[32-34]. Based on these acceptors, many
excellent TADF emitters have been reported[35-39]. However, until now, TADF emitters have
not been adopted in actual products because their performance need to be further improved. To
realize this goal, new composition groups and molecular structures are needed. Therefore,
developing novel acceptor for high-performance TADF emitters is still highly desired.
Herein, we report a novel 3-benzoyl-4H-chromen-4-one (BZC) acceptor and a BZC-based
TADF emitter, 3-(4-(10H-phenoxazin-10-yl)benzoyl)-4H-chromen-4-one (BZC-PXZ) (Scheme
1). From the crystal structure of BZC-PXZ, it could be seen that BZC group was a small and
twisted acceptor and the unique structure of BZC group could endow the material with
favorable molecular and stacking structures. Photophysical measurements indicated that
BZC-PXZ possessed small ∆EST (0.02 eV), high PLQY (93%) (measured in doped film, mCP:
5% BZC-PXZ) and short delayed fluorescence exciton lifetime (4.47 μs). BZC-PXZ-based
device displayed high performance. At the doped concentrations of 1%, 5% and 10%, the
maximum external quantum efficiency (EQE) of devices were 21.3%, 22.0%, 20.9%, while the
maximum exciton utilization efficiency (EUE) were 77.2%, 78.9%, 75.7%, respectively. These

results demonstrated that BZC group was a good acceptor for developing high-performance
TADF emitters.
2. Experimental Section
2.1. General Information
All the starting chemicals for the synthesis were purchased from commercial sources and used
as received without purification unless otherwise noted. All the solvents used for the synthesis
1
were dried by sodium or calcium hydride and distilled under nitrogen atmosphere. H NMR
spectra were measured using an AVANCE 600 MHz spectrometer (Bruker). Mass spectrometry
were measured by Shimazu LCMS2020 instrument. Ultraviolet-visible (UV-Vis) absorption
spectra were tested using a Hitachi U-3900 spectrophotometer. Fluorescence spectra were
recorded using a Horiba FluoroMax-4 spectrophotometer spectrometer. The transient PL decay
characteristic of the material was investigated using an Edinburgh Instruments FLS980
spectrometer. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC)
characteristics were determined by a TA Q600 thermogravimeter and NETZSCH DSC204
instrument using the heating rate of 10 °C min−1 under nitrogen atmosphere to obtain the
compound weight loss. Cyclic voltammetry used an electrochemical workstation, the working
electrode was a platinum electrode, the reference electrode was an Ag electrode, and the counter
electrode was a platinum wire. The redox potential was measured in a 0.1 M solution at a scan
rate of 100 mV s-1 with tetrabutylammonium hexafluorophosphate (TBAPF6) as the electrolyte.
The photoluminescence quantum yields (PLQYs) were measured with the Yokohama
C9920-02G assay system. The transient photoluminescence decay characteristics of the doped
film (mCP: 5% BZC-PXZ) was measured under nitrogen atmosphere at room temperature.
According to the material properties, the excitation wavelength was 325 nm and the emission
wavelength was 430 nm. Single-crystal diffraction data were collected on a Rigaku
R-AXIS-RAPID diffractometer using the ω-scan mode with graphite-monochromator Mo·Kα
radiation. The structure was solved with direct methods using the SHELXTL programs and
refined with full-matrix least squares on F2. The crystal data can be obtained free of charge
from the Cambridge Crystallographic Data Centre (CCDC: 1962908).
The vacuum sublimation was carried out at the sublimation machine from Shenyang Keyou

Vacuum Technology Co., Ltd. This machine had two heating sections to set up different
temperatures. For BZC-PXZ, the two section temperatures were set to 120 and 220
,
respectively. The vacuum pressure was 2.0×10-1 Pa. After 12 h, the sublimation finished and
pure BZC-PXZ was obtained.
The DFT calculations were performed using Gaussian 09 program package. The geometrical
structure was fully optimized with C1 symmetry constraints by using the restricted closed-shell
B3LYP method with 6-31G* basis set. The lowest singlet (S1) and lowest triplet (T1) energies
were evaluated based on a time-dependent DFT (TD-DFT) approach at B3LYP/6-31G* level.
2.2. OLED fabrication and characterization
All the devices were fabricated on indium tin oxide (ITO) substrate with a sheet resistance of 15
Ω/sq and a thickness of 135 nm. The ITO substrate was cleaned with acetone, detergent,
distilled water in sequence, and then in an ultrasonic solvent bath. After dried at 80 oC for 1 h,
the ITO substrates were treated with UV-ozone for 15 min. The substrate was transferred to a
vacuum deposition chamber. HAT-CN, TAPC, TCTA, host-guest doped luminescent layer (mCP:
BZC-PXZ), Bphen, LiF and aluminum were evaporated in sequence through a shadow mask
with an array of 2.25 mm2 under the pressure of 5.0×10-4 Pa. The current-voltage-luminance
(J-V-L) characteristics were tested using a Keithley 2400 Semiconductor Characterization
System and a calibrated silicon photodiode. The electroluminescent (EL) spectra and brightness
were tested by a spectrograph PR-670. All the measurements were carried out under argon
conditions at room temperature.
2.3. Synthesis
Methyl 4-(10H-phenoxazin-10-yl) benzoate (M1): To a two-necks round-bottom flask,
10H-phenoxazine (3.66 g, 20 mmol), methyl iodobenzoate (5.50 g, 21 mmol), cuprous oxide
(6.00 g, 42 mmol) and dimethylacetamide (30 mL) were added. The mixture was stirred at 160 ℃
for 24 hours under nitrogen atmosphere. After cooling to room temperature, the mixture was
filtrated and the filtrate was poured into 500 mL water and stirred for 10 min. The crude product
was collected by filtration and purified by column chromatography on silica gel (eluent:
1
petroleum/dichloromethane = 1:1) to provide a pale green solid (5.78 g, 90% yield). H NMR
(600 MHz, CDCl3, δ): 8.26 (d, J = 8.2 Hz, 2H), 7.44 (d, J = 8.2 Hz, 2H), 6.70 (t, J = 5.8 Hz,
2H), 6.67 (t, J = 7.8 Hz, 2H), 6.59 (t, J = 7.3 Hz, 2H), 5.92 (d, J = 7.9 Hz, 2H), 3.97 (s, 3H).

HRMS: 317.1056 [M]+ (calcd: 317.11). Anal. Calcd for C20H15NO3: C 75.70, H 4.76, N 4.41, O
15.12; found: C 75.58, H 4.77, N 4.43, O 15.20.
4-(10H-phenoxazin-10-yl) benzoic acid (M2): A mixture of M1 (4.82 g, 15 mmol),
methanol (60 mL) and 1 M aqueous NaOH (60 mL) was stirred and refluxed for 6 hours under
nitrogen atmosphere. After cooling to room temperature, the mixture was poured into 250 mL
water and acidified by adding 1 M aqueous HCl until the pH of the mixture was adjusted to 5.0.
The crude product was collected by filtration and purified by column chromatography on silica
gel (eluent: petroleum/dichloromethane = 1:4) to provide a pale green solid (3.91 g, 86% yield).
1H NMR (600 MHz, DMSO-d6, δ): 13.18 (s, 1H), 8.20 (d, J = 7.5 Hz, 2H), 7.56 (d, J = 7.5 Hz,
2H), 6.77 (d, J = 7.3 Hz, 2H), 6.72-6.67 (m, 4H), 5.90 (d, J = 7.5 Hz, 2H). HRMS: 303.0884
[M]+ (calcd: 303.09). Anal. Calcd for C19H13NO3: C 75.24, H 4.32, N 4.62, O 15.82; found: C
75.26, H 4.33, N 4.65, O 15.77.
2-Acetylphenyl 4-(9H-carbazol-9-yl) benzoate (M3): A mixture of M2 (3.64 g, 12 mmol),
3-acetyl-4-hydroxybenzene-1-ylium (1.5 mL, 13 mmol), 4-pyrrolidinopyridine (3.50 g, 5 mmol)
and N, N-dicyclohexylcarbodiimide (7.00 g, 26 mmol) were added into 150 mL dry
dichloromethane under nitrogen atmosphere. The mixture was stirred at room temperature for
12 hours. Then the mixture was filtrated and washed with 200 mL dichloromethane. The solvent
was removed by rotary evaporation. The product was purified by column chromatography on
silica gel (eluent: petroleum/dichloromethane = 1:2) to provide a yellow solid (4.20 g, 83%
1
yield). H NMR (600 MHz, CDCl3, δ): 8.43 (d, J = 8.2 Hz, 1H), 7.90 (d, J = 6.3 Hz, 1H),
7.40-7.62 (m, 4H), 6.63-6.73 (m, 8H), 6.00 (d, J = 7.0 Hz, 2H), 2.60 (s, 3H). HRMS: 421.1278
[M]+ (calcd: 421.13). Anal. Calcd for C27H19NO4: C 76.95, H 4.54, N 3.32, O 15.18; found: C
76.88, H 4.53, N 3.35, O 15.22.
1-(4-(10H-phenoxazin-10-yl) phenyl)-3-(2-hydroxyphenyl) propane-1, 3-dione (M4): The
M3 (4.21 g, 10 mmol), KOH (672 mg, 12 mmol) were added into 150 mL dry pyridine and
heated at 60 for 6 hours. After cooling to room temperature, water was added and the pH of
the mixture was adjusted to 5.0 by adding 10% HCl. The product was collected by filtration and
purified by column chromatography on silica gel (eluent: petroleum/dichloromethane = 1:2) to
1
provide an orange solid (3.62 g, 86% yield). H NMR (600 MHz, CDCl3, δ): 15.53 (s, 1H),
12.06 (s, 1H), 8.17 (d, J = 8.5 Hz, 2H), 7.81 (d, J = 8.5 Hz, 1H), 7.50 (d, J = 6.8 Hz, 2H),

6.91-7.05 (m, 3H), 6.62-6.72 (m, 6H), 5.98 (d, J = 7.9 Hz, 2H), 4.70 (s, 1H). HRMS: 421.1277
[M]+ (calcd: 421.13). Anal. Calcd for C27H19NO4: C 76.95, H 4.54, N 3.32, O 15.18; found: C
76.93, H 4.54, N 3.33, O 15.19.
3-(4-(10H-phenoxazin-10-yl) benzoyl)-4H-chromen-4-one (BZC-PXZ): The mixture of
M4 (2.11 g, 5 mmol), DMF-DMA (17 mL, 75 mmol) and toluene (100 mL) was heated at 80 ℃
for 1 hour. After cooling to room temperature, the product was collected by filtration and
purified by column chromatography on silica gel (eluent: dichloromethane) to provide a yellow
1
solid (1.98 g, 92% yield). H NMR (600 MHz, CDCl3, δ): 8.39 (s, 1H), 8.29 (d, J = 7.9 Hz,
1H), 8.09 (d, J = 7.9 Hz, 2H), 7.78 (t, J = 7.9 Hz, 1H), 7.58 (d, J = 8.5 Hz, 1H), 7.52 (t, J = 7.6
Hz, 1H), 7.46 (d, J = 7.9 Hz, 2H), 6.60-6.72 (m, 6H), 6.02 (d, J = 7.9 Hz, 2H). 13C NMR (600
MHz, CDCl3, δ): 113.57, 115.64, 115.67, 118.42, 121.86, 123.33, 124.96, 125.07, 126.34,
126.47, 130.70, 130.72, 132.42, 134.60, 136.83, 144.05, 156.13, 159.08, 174.77, 190.96.
HRMS: 432.1182 [M+H]+ (calcd: 431.12). Anal. Calcd for C28H17NO4: C 77.95, H 3.97, N 3.25,
O 14.83; found: C 77.99, H 3.95, N 3.26, O 14.80.
3. Results and Discussion
3.1. Synthesis and Thermal Properties
BZC-PXZ was synthesized from phenoxazine and obtained as a yellow powder (Scheme 1).
Then, it was further purified by column chromatography and vacuum sublimation. Finally, it
was fully characterized by NMR, mass spectra and element analysis (Fig. S1-S11).
Thermal properties of BZC-PXZ were investigated using differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA) (Fig. S12 and Table 1). From the DSC curve, no
glass transition was observed. From the TGA curve, its decomposition temperature (Td)
(corresponding to 5% weight loss) was determined to be 303 °C. Good thermal stability is
favorable for the formation of uniform thin films during thermal evaporation and prolongs the
device operational lifetime.
3.2. Crystal Structure
The yellowish crystal of BZC-PXZ was obtained by slow vacuum sublimation and its single
crystal structure was examined (Fig. 1 and Table S2). BZC-PXZ possesses a twisted molecular
structure and the dihedral angles between different groups are shown in Fig. 1b. From left to

right (chromanone, carbonyl, benzene and phenoxazine), the dihedral angles are 42.34°, 25.23°
and 88.78°, respectively. Obviously, the acceptor group, 3-benzoyl-4H-chromen-4-one,
possesses twisted structure. BZC-PXZ molecules adopt a parallel arrangement model with four
kinds of molecular rows. Within one molecular row, molecules connect with each other by
C-H∙∙∙O and C-H∙∙∙π interactions with the contact distances of 2.685 Å and 3.123 Å respectively
(yellow lines in Fig. 1c). Between different kinds of molecular rows, these rows connect with
each other by C-H∙∙∙O、C-H∙∙∙π and π∙∙∙π interactions (green lines in Fig. 1c). The contact
distances of C-H∙∙∙O interactions are 2.552 Å、2.559 Å and 2.705 Å respectively. The contact
distance of π∙∙∙π interaction is 3.727 Å with an overlap of half of a benzene ring. For C-H∙∙∙π
interaction, its contact distances are 2.479 Å and 2.763 Å respectively. It can be seen that the
small and twisted BZC group can not only endow BZC-PXZ with large dihedral angle (88.78°)
between donor and acceptor groups, but also efficiently prevent the strong π∙∙∙π interaction.
Meanwhile, the oxygen atoms in BZC group bring about abundant weak intermolecular
interactions.
The nearly vertical structure (dihedral angle: 88.78°) means efficient separation of donor and
acceptor groups which is favorable for obtaining small ΔEST and efficient reverse intersystem
crossing (RISC) process. The lack of π∙∙∙π interaction can avoid fluorescence quenching and
improve the fluorescent quantum efficiency. Finally, the intermolecular interactions can
enhance the charge-transporting ability of BZC-PXZ and this is favorable for the exciton
recombination on it.
3.3. Theoretical Calculations and Electrochemical Properties
To gain insight into the spatial distributions of the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO), density functional theory (DFT)
calculations were carried out and results were shown in Fig. 2a. BZC-PXZ exhibited a
prominent character of a typical TADF molecule: its HOMO was mainly located on
phenoxazine group while its LUMO was mainly distributed on 3-benzoyl-4H-chromen-4-one
group; there was only a little overlap between the HOMO and LUMO. The spatially separated
frontier orbital distribution could reduce the ΔEST value and facilitate the reverse intersystem
crossing from the lowest triplet excited state (T1) to the lowest singlet excited state (S1). The

HOMO and LUMO energy levels of BZC-PXZ were calculated to be -4.65 and -2.03 eV,
respectively. The S1, T1 and ΔEST were calculated to be 2.2431 eV, 2.1748 eV, 0.0683 eV,
respectively. From the spectra below, the experimental S1, T1 and ΔEST were 2.46 eV, 2.44 eV,
0.02 eV, respectively. Compared with experimental values, the calculated values exhibited
similar variation tendency.
The electrochemical properties of BZC-PXZ were investigated by the cyclic voltammetry
(CV) measurement in dichloromethane (Fig. 2b). BZC-PXZ showed reversible oxidation
behavior. From the oxidation wave, its HOMO energy level was determined to be -5.05 eV
+
according to the equation of EHOMO = − (E[onset, ox vs Fc /Fc] + 4.8) eV. No reduction wave was
observed. Its LUMO energy level was determined to be -2.42 eV from its HOMO and Eg.
3.4. Photophysical Properties
The UV-vis absorption spectra of BZC-PXZ in toluene was shown in Fig. 3a. The absorption
below 350 nm could be ascribed to the local excitation transition absorption of
3-benzoyl-4H-chromen-4-one (BZC) and phenoxazine (PXZ). While the weak absorption
around 408 nm was attributed to the charge-transfer transition from PXZ moiety to BZC moiety.
From the absorption spectra, the energy gap between S1,0 and S0,0 (Eg) was estimated as 2.63 eV
for BZC-PXZ. The absorption spectra of BZC-PXZ in different solvents were exhibited in Fig.
S15. With the increase of solvent polarity, the charge-transfer transition absorption peaks were
almost the same. This indicated that the ground-state dipole moment of BZC-PXZ was small.
To verify the lowest triplet excited state (T1) of BZC-PXZ, its fluorescence and
phosphorescence spectra in toluene at 77 K were measured (Fig. 3a). At room temperature,
BZC-PXZ exhibited a broad emission with an unstructured peak of 561 nm. At 77 K, its
emission showed a blue-shift with a single peak of 505 nm. From the crystal structure of
BZC-PXZ, it could be seen that BZC-PXZ consisted of small composition groups and
possessed twisted molecular structure. Hence, at room temperature, many molecular rotation
and vibration happened. These unfavorable molecular motions consumed much energy. At 77 K,
these motions stopped and those wasted energy could be used for light emission. Hence,
compared with the emission peak at 300 K, the emission peak at 77 K showed an obvious
blue-shift. For its phosphorescence spectra, no fine structure was observed, indicating that its T1

state was not locally-excited (3LE) state but was charge transfer (3CT) state. From the peaks of
fluorescence (505 nm) and phosphorescence (508 nm) spectra at 77 K, the S1 and T1 energy
levels of BZC-PXZ were calculated to be 2.46 eV and 2.44 eV, respectively. Hence, its ΔEST
was 0.02 eV. Small ΔEST facilitates the efficient RISC. The photoluminescence (PL) spectra of
BZC-PXZ in crystalline state, pure-film state and doped-film state (mCP: 5% BZC-PXZ) were
also measured (Fig. S13). Its fluorescence peaks at crystalline state, pure-film state and
doped-film state were 596 nm, 589 nm, 533 nm, respectively. Compared with the solution and
doped-film spectra, the pure-film and crystal spectra had a redshift which should be ascribed to
the intermolecular interactions.
In order to investigate the TADF feature of BZC-PXZ, the transient photoluminescence (PL)
decay characteristic of the doped film (mCP: 5% BZC-PXZ) was measured (Fig. 3b). The
transient PL of BZC-PXZ showed clear secondary exponential decays with a nanosecond-scale
prompt component and a microsecond-scale delayed component. The lifetimes of the prompt
fluorescence (τp) and the delayed fluorescence (τd) were 21.59 ns and 4.47 μs, respectively.
Meanwhile, to further confirm the TADF feature of BZC-PXZ, its temperature-dependent
transient PL decays from 150 to 300 K were investigated (Fig. S14). With the increase of
temperature, the delayed fluorescence intensity increased, indicating that the RISC process
from the triplet to singlet excited states was enhanced by thermal activation. These results
verified the TADF characteristic of BZC-PXZ. The PLQY of its doped film (mCP: 5%
BZC-PXZ) was 93%. From the transient PL, the ratios of prompt component and delayed
component were 52% and 48%, respectively, suggesting that the delayed fluorescence had a
great contribution on the total fluorescence. In the film with mCP host, the PLQY of the prompt
fluorescence (ΦPF) and delayed fluorescence (ΦDF) were 48% and 45%, respectively. The
PLQYs of BZC-PXZ in dichloromethane (10-5 mol/L) and pure film were 0.86 and 0.21,
respectively.
To investigate the charge transfer (CT) characteristic and the lowest singlet excited state (S1)
of BZC-PXZ, its photoluminescence (PL) spectra in different solvents (hexane, triethylamine,
butylether, isopropylether, diethyl ether, ethyl acetate, dichloromethane, dimethylformamide,
acetone, acetonitrile) were measured (Fig. S15). In hexane, the PL spectra of BCZ-PXZ
exhibited a certain feature of vibrational structure, indicating that the fluorescence in hexane

contained the emission from locally excited state. In other solvents, the spectra exhibited a
structure-less characteristic and vibration-free fine structure was observed. From hexane to
acetonitrile, its emission peaks were 433 nm, 482 nm, 482 nm, 491 nm, 475 nm, 497 nm, 519
nm, 516 nm, 518 nm, 523 nm, respectively (Table S1). With the increase of solvent polarity, its
emission peak showed a significant red-shift from 433 nm to 523 nm, indicating a remarkable
CT character of excited state. According to Lippert-Mataga model [40], Stokes shift-solvent
polarity curve was drawn to investigate the excited state (Fig. S16). With the increase of solvent
polarity, the Stokes shift exhibited a straight line (R = 0.97015) and no inflection point was
observed, further indicating that BZC-PXZ showed a CT-like character in both low-polarity and
high-polarity solvents and the S1 state of BZC-PXZ was 1CT.
To get insight into the dynamic processes of BZC-PXZ, the prompt-fluorescence (PF) rate
constant (kPF), the delayed-fluorescence (DF) rate constant (kDF), the singlet radiative rate
constant (ꢀꢁꢂ), the singlet nonradiative rate constant (ꢀꢃꢂꢁ), the intersystem crossing rate constant
(kISC), the reverse intersystem crossing rate constant (kRISC), the triplet nonradiative rate constant
(ꢀꢃꢄꢁ), the intersystem crossing efficiency (ΦISC) and the reverse intersystem crossing efficiency
(ΦRISC) were calculated from the following equations [41]:
ꢇ
ꢈꢉ
ꢀꢅꢆ
ꢀꢍꢆ
=
=
(1)
(2)
ꢊ
ꢋꢌ
ꢇ
ꢎꢉ
ꢊ
ꢏꢌ
ꢀꢁꢂ = Φꢐꢑ. ꢀꢅꢆ
(3)
(4)
ꢇ
ꢇ
ꢈꢉ
ꢀꢃꢂꢁ = ꢀꢅꢆ − ꢀꢁꢂ − ꢀꢅꢆ
ꢈꢒ
ꢇ
ꢇ
ꢈꢉ
ꢀꢓꢔꢕ = ꢀꢅꢆ
(5)
(6)
(7)
(8)
(9)
ꢈꢒ
ꢗ
.ꢗ .ꢇ
ꢏꢌ ꢋꢌ ꢎꢉ
ꢀꢖꢓꢔꢕ
=
ꢗ
.ꢇ
ꢘꢙꢚ ꢈꢉ
ꢀꢃꢄꢁ = ꢀꢍꢆ − ꢛ1 − ꢗꢘꢙꢚꢜ . ꢀꢖꢓꢔꢕ
ꢗ
ꢋꢌ
ꢗ
ꢘꢙꢚ
Φꢝꢞꢟ
=
ꢗ
ꢋꢌ
ꢗ
ꢡꢘꢙꢚ
Φꢠꢝꢞꢟ
=
ꢥ
ꢗ
ꢢꢗ
ꢣꢤ
ꢡꢘꢙꢚ
According to these equations, the kPF, kDF, ꢀꢁꢂ, ꢀꢃꢂꢁ, kISC, kRISC, ꢀꢃꢄꢁ, ΦISC and ΦRISC were
2.22×107 s-1, 1.01×105 s-1, 1.07×107 s-1, 8.61×104 s-1, 1.15×107 s-1, 1.83×105 s-1, 1.28×104 s-1,

0.52 and 0.94, respectively (Table 2). The magnitude of kRISC was as high as 105 s-1 which
verified the efficient RISC process.
3.5. Electroluminescent Properties
The crystal structure of BZC-PXZ indicated that BZC-PXZ might possess good
charge-transporting ability. To investigate the charge-transporting ability of BZC-PXZ, the
single-carrier devices with the structures of [ITO/BZC-PXZ (70 nm)/MoO3 (10 nm)/Al] for
hole-only and [ITO/TPBi (20 nm)/BZC-PXZ (60 nm)/LiF (1.5 nm)/Al] for electron-only were
fabricated. Results were shown in Fig. S17. Based on the space charge limited current (SCLC)
method and I–V characteristics, the hole and electron mobility of BZC-PXZ were determined to
be 5.02 × 10–5 cm2 V–1 s–1 and 2.06 × 10–6 cm2 V–1 s–1, respectively. The good
charge-transporting ability should be attributed to the abundant weak intermolecular
interactions between BZC-PXZ molecules. Meanwhile, the electron mobility was close to the
hole mobility, indicating that BZC-PXZ possessed good bipolar transporting ability. As is well
known, balanced charge-transporting nature was favorable for the exciton recombination on it.
Hence, for BZC-PXZ-based device, high performance can be expected.
To investigate the electroluminescent (EL) properties of BZC-PXZ, multilayer OLEDs with
the structures of ITO/HAT-CN (15 nm)/TAPC (50 nm)/TCTA (5 nm)/mCP: xwt% BZC-PXZ
(30 nm)/Bphen (45 nm)/LiF (1 nm)/Al (100 nm) (D1: 1wt%; D2: 5wt%; D3: 10wt%) were
fabricated. The molecular structures in devices and their energy levels were shown in Fig. 4. In
these
quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN), 4,4'-cyclohexylidenebis [N, N-bis
(p-tolyl) aniline] (TAPC), 4,4',4''-tris (carbazol-9-yl)-triphenylamine (TCTA),
devices,
indium
tin
oxide
(ITO),
dipyrazino
[2,3-f:2',3'-h]
1,3-bis-9H-carbazol-9-ylbenzene (mCP), 4,7-diphenyl-1,10-phenanthroline (Bphen), lithium
fluoride (LiF) and aluminium (Al) were utilized as the anode, hole-injection layer,
hole-transporting layer, electron-blocking layer, host, electron-transporting layer,
electron-injection layer and cathode, respectively. BZC-PXZ was doped in mCP as the emitting
layer. The current density-voltage-luminance (J-V-L) characteristics, efficiencies versus
luminance curves, EL spectra as well as external quantum efficiencies (EQE) were shown in
Fig. 5 and Table 3.
As shown in Fig. 5 and Table 3, these devices exhibited green emission and no emission

peak of host was observed even at low doped concentration of 1%, indicating the complete
energy transfer from host to BZC-PXZ (Fig. 5a). With the increase of doped concentration, the
emission peak had a small redshift from 542 nm to 546 nm which should be attributed to the
enhanced intermolecular interaction. With the variation of driving voltage, the EL spectra
exhibited good stabilities that were important for high performance displays and lighting
sources. Because of the direct charge trap of BZC-PXZ, these devices had low turn-on voltage
(about 2.7 V) regardless of the doped concentration. Meanwhile, the increased luminance
features of devices were not affected by the doped concentration yet and their maximum
brightness exceeded 33000 cd/m2 (Fig. 5b).
The maximum brightness (Lmax), maximum external quantum efficiency (ηext.max), current
(ηc.max) and power (ηp.max) efficiencies of these three devices were shown in Table 3. With the
increase of doped concentration from 1% to 10%, the devices displayed similar performance:
64.6 cd/A, 69.0 lm/W and 21.3% for D1; 66.5 cd/A, 70.6 lm/W and 22.0% for D2; 63.5 cd/A,
66.8 lm/W and 20.9% for D3. The high device efficiencies should be ascribed to the unique
structure of BZC acceptor. Because of the small and twisted BZC group, the donor was almost
perpendicular to the acceptor and π∙∙∙π interaction was efficiently prevented. Hence, BZC-PXZ
exhibited small ΔEST (0.02 eV) and high PLQY (0.92-0.93, the emitting layers) simultaneously
which made it possess high utilization efficiencies of singlet and triplet excitons (Table 3).
These devices exhibited low roll-off of efficiencies (Fig. 5 and Table 3). Except for the above
reason, the intermolecular interactions might be another reason for the low roll-off of
efficiencies. BZC group possessed three oxygen atoms which brought about abundant weak
intermolecular interactions (Fig. 1) that could enhance the charge-transporting ability of
BZC-PXZ. From the single-carrier devices, its hole and electron mobility were determined to
be 5.02 × 10–5 cm2 V–1 s–1 and 2.06 × 10–6 cm2 V–1 s–1, respectively. The high and balanced
charge-transporting ability was favorable for the exciton recombination on it to obtain high
efficiency and low roll-off of efficiency.
To evaluate the exciton utilization efficiency (ηEUE) of BZC-PXZ, the ηEUE of these three
devices were calculated according to the following equation [42]:
EQE = ηIQE × ηout = ηhe × ΦPL × ηEUE × ηout
Here, ηIQE was the internal quantum efficiency, ηout was the light out-coupling efficiency

(0.2-0.3), ηhe was the electron-hole recombination efficiency which was usually set as 100%,
ΦPL was the photoluminescence quantum yield of emitting layer. If ηout was assumed to be 0.3,
the maximum ηEUE of D1, D2 and D3 were 77.2%, 78.9% and 75.7%, respectively. The ηEUE
was about 3 times higher than the theoretical limit of 25% for conventional fluorescent emitters,
indicating that most of triplet exciton was transformed into singlet exciton for light harvesting.
4. Conclusions
In summary, we reported a novel 3-benzoyl-4H-chromen-4-one (BZC) acceptor whose unique
structure could endow the material with favorable molecular and stacking structures. For
molecular structure, the twisted BZC group brought about nearly vertical structure between
donor and acceptor groups. For stacking structure, because of the three oxygen atoms and
structure of BZC group, abundant weak intermolecular interactions could be formed but π∙∙∙π
interaction was prevented. Based on BZC group, a TADF emitter, BZC-PXZ, was developed.
BZC-PXZ exhibited small ΔEST (0.02 eV) and high PLQY (93%) simultaneously. Meanwhile,
the BZC-PXZ-based device displayed high efficiency (64.6 cd/A, 69.0 lm/W, 21.3%), high
exciton utilization efficiency (77.2%) and low roll-off of efficiencies even at ultralow doped
concentration of 1%. These results demonstrated that BZC group was a good acceptor for
developing high-performance TADF emitters.
Acknowledgments
This work was supported by National Key Research and Development Project of China
(2018YFB0407100-02), National Natural Science Foundation of China (51903026, 61505018,
U1663229, 21603020, 61705026, 51903027), the China Postdoctoral Science Foundation (2019
M653375), Financial Projects of Sichuan Science and Technology Department
(2018ZYZF0062), Natural Science Foundation of Chongqing Science & Technology
Commission
(cstc2017zdcy-yszxX0004,
cstc2019jcyjjq0092,
cstc2016jcyjys0006,
cstc2016jcyjA0148, cstc2018jszx-cyzdX0137, cstc2016jcyjA0577, cstc2018jcyjAX0212,
cstc2017jcyjAX0097), Science and Technology Innovation Leader Plan of Chongqing
(T04040012), Science and Technology Research Program of Chongqing Municipal Education
Commission (KJQN201901314, KJZD-K201901302, KJ1711269), Natural Science Foundation

of Yongchuan District (Ycstc, 2018nb0601, 2019nc0601), the Chongqing University
Outstanding Achievement Transformation Projects (KJZH17130), Overseas Returnees Support
Program for Innovation and Entrepreneurship of Chongqing (cx2018136). This work is also
sponsored by Sichuan Province Key Laboratory of Display Science and Technology.
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Figure and table captions
Scheme 1. The synthetic procedures of BZC-PXZ.
Fig. 1. a) The molecular structure with 50% thermal ellipsoids. b) The dihedral angles between
different groups in BZC-PXZ. c) The packing structure of BZC-PXZ.
Fig. 2. a) Frontier-molecular-orbital distributions, energy levels, and HOMO-LUMO energy
gap for BZC-PXZ characterized by DFT calculations. b) Cyclic voltammetry (CV) curve of
BZC-PXZ in dichloromethane.
Fig. 3. a) Absorption spectra, fluorescence spectra (at 300 K and 77K) and phosphorescence
spectra (at 77 K) of BZC-PXZ in toluene (1.0 × 10-4 mol/L). b) Transient PL decay profile of
BZC-PXZ-doped mCP film (5wt%); Inset: decay measured over a time range of 200 ns.
Fig. 4. a) Energy level diagram and b) chemical structures of the materials used in OLEDs.
Fig. 5. a) EL spectra of D1, D2 and D3 at the luminance of 1000 cd/m2. b) Current
density-voltage-luminance (J-V-L) curves of D1, D2 and D3. c) Current
efficiency-luminance-power efficiency (CE-L-PE) curves of D1, D2 and D3. d) External
quantum efficiency-luminance curves of D1, D2 and D3.
Table 1. Photophysical and electrochemical properties of BZC-PXZ.
Table 2. Photophysical properties of BZC-PXZ-doped mCP film (doped concentration: 5%).
Table 3. The electroluminescent performance of BZC-PXZ-doped devices.

Scheme 1.
Fig. 1.

Fig. 2.
Fig. 3.
Fig. 4.

Fig. 5.
Table 1. Photophysical and electrochemical properties of BZC-PXZ.
a)
b)
b)
d)
d)
e)
Td
[℃]
λabs
λPL
ΦPL
τp
τd
HOMO/LUMO
[eV]
ES/ET
ΔEST
[eV]
0.02
Compound
[nm] [nm] filmc)
[ns] [μs]
[eV]
BZC-PXZ
303 325/408 561 0.93 21.59 4.47
-5.05/-2.42
2.46/2.44
b)
a) Td: decomposition temperature corresponding to 5% weight loss. Measured in toluene at
c)
d)
room temperature. Measured in doped film (mCP: 5% BZC-PXZ). PL lifetimes of prompt
(τp) and delayed (τd) decay components for doped film (mCP: 5% BZC-PXZ) at room
temperature. e) Singlet (ES) and triplet (ET) energies measured in the diluted toluene solution and
estimated from the peaks of the fluorescence and phosphorescence spectra (at 77 K),
respectively.

Table 2. Photophysical properties of BZC-PXZ-doped mCP film (doped concentration: 5%).
a)
b)
c)
d)
e)
f)
g)
h)
j)
k)
i)
ꢀꢃꢄꢁ
Dopant
ΦPF
ΦDF
ΦISC
ΦRISC
kPF
kDF
ꢀꢁꢂ
ꢀꢃꢂꢁ
kISC
kRISC
[107 s-1] [105 s-1] [107 s-1] [104 s-1] [104 s-1] [107 s-1] [105 s-1]
BZC-PXZ 0.48
0.45
0.52
0.94
2.22
1.01
1.07
8.61
1.28
c)
1.15
1.83
a) Prompt fluorescence efficiency. b) Delayed fluorescence efficiency. Intersystem crossing
d)
e)
f)
efficiency. Reverse intersystem crossing efficiency. Prompt-fluorescence rate constant.
Delayed-fluorescence rate constant. g) Singlet radiative rate constant. h) Singlet nonradiative rate
i)
k)
constant. Triplet nonradiative rate constant. j) Intersystem crossing rate constant. Reverse
intersystem crossing rate constant.
Table 3. The electroluminescent performance of BZC-PXZ-doped devices.
a)
b)
e)
Device
Von Lmax
CE[cd/A]/PE[lm/W]/EQE[%]c)
1000 cd m-2 Roll-off rate (%)d)
ηEUE, max
Φf)
[V] [cd/m2]
Maximum
D1 (1wt%)
D2 (5wt%)
2.6
2.7
33114 64.6/69.0/21.3 57.6/41.1/18.9
37729 66.5/70.6/22.0 61.5/43.9/20.4
35024 63.5/66.8/20.9 56.8/39.2/18.7
10.8/40.4/11.3
7.5/37.8/7.3
77.2% 0.92
78.9% 0.93
75.7% 0.92
D3 (10wt%) 2.7
10.6/41.3/10.5
a)
V : voltage recorded at 1 cd m−2. b) Maximum luminescence. c) CE: current efficiency; PE:
on
d)
power efficiency; EQE: external quantum efficiency.
The efficiency variation rate from
e)
maximum efficiency to efficiency at 1000 cd m-2.
η : maximum exciton utilization
EUE, max
efficiency. f) PLQY of BZC-PXZ-doped mCP film with different concentrations.

1. A novel twisted 3-benzoyl-4H-chromen-4-one (BZC) acceptor is reported.
2. The unique structure of BZC group can endow the material with favorable
molecular and stacking structures.
3. BZC-based thermally activated delayed fluorescence emitter, BZC-PXZ, displays
high performance in devices.

Declaration of interests
☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: