Molecular engineering of “A-D-D-A” dual-emitting-cores emitters with thermally activated delayed fluorescence and aggregation-induced emission characteristics

Article abstract of DOI:10.1016/j.orgel.2021.106199

Endowing thermally activated delayed fluorescence (TADF) emitter with aggregation-induced emission (AIE) peculiarity is of great significance for realizing more promising commercial applications. Herein, two new dual-emitting-cores emitters with a structu

Full text of DOI:10.1016/j.orgel.2021.106199

Organic Electronics 96 (2021) 106199
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Molecular engineering of “A-D-D-A” dual-emitting-cores emitters with
thermally activated delayed fluorescence and aggregation-induced
emission characteristics
,
Jinan Tanga *, Yibai Suna, Qifeng Tangb, Yueming Sunb
a Department of Chemical and Pharmaceutical Engineering of Southeast University Chengxian College, Nanjing, Jiangsu, 210088, PR China
b School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu, 211189, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Endowing thermally activated delayed fluorescence (TADF) emitter with aggregation-induced emission (AIE)
peculiarity is of great significance for realizing more promising commercial applications. Herein, two new dual-
emitting-cores emitters with a structure of acceptor-donor-donor-acceptor (A-D-D-A), namely 2DBT-BZ-2Cz and
2DFT-BZ-2Cz, were designed and synthesized to explore their luminescence trait. The emitters, adopting dual
carbazole as donor segments and dual phenyl ketone in peripheral skeleton as electron acceptor units, were
featured with small singlet (S1)–triplet (T1) splitting energy (ΔEST) of 0.02 eV and 0.01 eV. The efficient ther-
mally activated delayed fluorescence (TADF) characteristics and aggregation-induced emission property make
them suitable for nondoped OLED devices. The solution-processed green OLEDs based on 2DBT-BZ-2Cz
demonstrated greater device performance with current efficiency of 20.7 cd Aꢀ 1 and maximum luminescence
of 10,000 cd mꢀ 2. This work thus provides the direction to explore luminogens of dual-emitting-cores with TADF
and AIE features as promising candidates in solid state lighting.
Dual-emitting-cores
Singlet (S1)–Triplet (T1) splitting energy
Thermally activated delayed fluorescence
Aggregation-induced emission
1. Introduction
twisted molecular configurations [31–35]. AIE materials show negli-
gible emission in dilute solutions but enhanced fluorescence in the
Thermally activated delayed fluorescent (TADF) materials have ac-
quired tremendous success in organic light-emitting diodes (OLEDs)
because triplet (T1) excited state excitons can upconvert to singlet (S1)
excited state excitons to realize 100% exciton utilization via a reverse
intersystem crossing (RISC) process and finally emit light, induced by a
small singlet–triplet energy difference (ΔEST) [1–9]. The small ΔEST
values of the TADF emitters can be generally achieved by effectively
separating the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) [10–18]. However, owing
to the long triplet lifetimes, most TADF compounds need to be doped in
suitable host materials to restrain concentration quenching processes,
resulting in the complex device fabrication process [19–22]. Meanwhile,
it requires precise control of the host-guest ratio of the mixture to ach-
ieve the desired effect [23–30]. Hence, building the single-component
emitting layers is urgently needed to develop the non-doped OLEDs.
Given these circumstances, combining TADF materials with aggregation
induced emission (AIE) peculiarity is a promising method to fabricate
host-free OLEDs due to their analogous design tactics of adopting highly
aggregated state, in which the concentration quenching and exciton
annihilation in aggregated state can be restrained [36–42].
It was well-known that the increased molecular rigidity can suppress
non-radiative decay process effectively, leading to a high PLQY. In
addition, the highly twisted donor-acceptor (D-A) structure can make
the HOMO and LUMO energy levels effectively locate on the donor and
acceptor moieties, respectively, resulting in small ΔEST. Therefore, the
dual-emitting-cores design is an attractive method to enhance the ri-
gidity and twist of molecular structure, achieving a small ΔEST as well as
introducing multiple TADF emitting segments for strong absorption and
high PLQY by extending the conjugation [43–49]. In a consequence, Lee
et al. [43] first reported the dual emitting core TADF emitter (DDCzIPN)
to realize a high external quantum efficiency (EQE) of 15.8%; Yang et al.
[49] reported two dual emitting cores (2,20-DPXZ-PN and 3,
30-DPXZ-PN) by connecting two identical TADF emitting units, con-
firming obvious TADF features and obtaining the EQEs of 13.4 and
14.9%, respectively. Although these materials exhibited TADF emission
and high EQE in the range of 10–20%, the molecular structure lacks
* Corresponding author.
E-mail address: 100001427@cxxy.seu.edu.cn (J. Tang).
https://doi.org/10.1016/j.orgel.2021.106199
Received 25 March 2021; Received in revised form 21 April 2021; Accepted 5 May 2021
Available online 4 June 2021
1566-1199/© 2021 Published by Elsevier B.V.

J. Tang et al.
Organic Electronics 96 (2021) 106199
diversification because the combination of the donor and acceptor units
is limited. Hence, developing novel organic luminescent materials with
dual emitting cores and simultaneously exhibiting a TADF characteristic
and AIE feature are impending.
chloride (1.32 g, 10 mmol) in 50 mL dry dichloromethane at 0 ◦C, 4-bro-
mobenzoyl chloride (2.18 g, 10 mmol) was slowly added dropwise. After
the addition was completed, the mixture was stirred at 40 ◦C for 3 h. The
reaction was quenched by adding 40 mL ice-HCl solution, and the
aqueous layer was separated from the organic layer. Then the aquatic
phase was further extracted with dichloromethane for several times. The
combined organic solution was evaporated under vacuum to afford in-
termediate products. The solvent was removed under reduced pressure
and the residue was purified on a silica gel column using petroleum
ether/dichloromethane mixture (3:1, v/v) as eluent to afford the prod-
uct as white powder in 72% yield (2.21 g, 6.04 mmol). 1H NMR (600
MHz, CDCl3, δ): 8.58 (d, J = 1.3 Hz, 1H), 8.21–8.18 (m, 1H), 7.97–7.86
(m, 5H), 7.54–7.48 (m, 2H), 7.23–7.18 (m, 2H). 13C NMR (150 MHz,
CDCl3, δ): 195.12, 166.35, 164.33, 144.16, 139.76, 135.52, 135.19,
134.21, 133.84, 132.69, 127.94, 127.50, 124.93, 122.97, 122.63,
121.97, 115.47. MS (MALDI-TOF) [m/z]: calcd for C19H11BrOS, 365.97;
found, 367.26. Anal. Calcd. for C19H11BrOS: C, 62.14; H, 3.02. Found: C,
62.12; H, 3.00.
In this study, two novel AIE-active TADF materials with a dual
emitting core and similar A-D-D-A molecular configuration, namely
2DBT-BZ-2Cz and 2DFT-BZ-2Cz, were designed and synthesized. Two
materials share the same carbazole as donor groups, dibenzo [b, d]
thiophen-2-yl (phenyl) ketone and dibenzo [b, d] furan-2-yl (phenyl)
ketone, as two different acceptor groups, respectively. The comparative
study of TADF and AIE characteristics is established through physical
properties, thermal stability, electrochemical property and density
functional theory (DFT). The efficient separation of frontier molecular
orbital (FMO) induced similarly narrow ΔEST and overt TADF charac-
teristic with evident delayed fluorescence lifetimes. In addition, highly
twisted molecular structure of the dual emitting cores restricted the π-π
packing and abduct AIE feature. Owing to extended conjugate connec-
tions, these dual emitters 2DBT-BZ-2Cz and 2DFT-BZ-2Cz show red-
shifted spectra of 539 nm and 535 nm green emission, respectively.
Then the solution-processed nondoped OLEDs based on them exhibited
excellent device performance with maximum external quantum effi-
ciency (EQEmax) of 6.8 cd Aꢀ 1 and 4.5 cd Aꢀ 1-1, maximum power effi-
ciencies (PEmax) of 12.4 lm Wꢀ 1 and 7.6 lm Wꢀ 1. This strategy offered a
unique molecular design strategy for realizing luminogens with TADF
and AIE characteristic.
Synthesis of (4-bromophenyl) (dibenzo [b,d]furan-2-yl)methanone
(DFT-BZ-Br).
The synthetic process of DFT-BZ-Br was similar to that of DBT-BZ-Br
as yellow powder with a yield of 70% (2.03 g, 5.8 mmol). 1H NMR (600
MHz, CDCl3, δ): 8.67 (d, J = 1.3 Hz, 1H), 8.24–8.21 (m, 1H), 7.94–7.83
(m, 5H), 7.50–7.42 (m, 2H), 7.18–7.13 (m, 2H). 13C NMR (150 MHz,
CDCl3, δ): 194.22, 165.45, 164.23, 145.16, 139.74, 135.51, 135.42,
134.28, 132.64, 132.69, 127.84, 127.53, 124.95, 122.91, 122.65,
121.94, 115.43. MS (MALDI-TOF) [m/z]: calcd for C19H11BrO2, 349.99;
found, 351.20. Anal. Calcd. for C19H11BrO2: C, 64.98; H, 3.16. Found: C,
64.96; H, 3.13.
2. Experimental section
2.1. Materials
Synthesis of 9H, 9′H-3,3′-bicarbazole (BCz).
All corresponding solvents and raw materials were purchased from
commercial sources and used without any further purification besides
otherwise indicated.
In 250 mL three-necked flask, carbazole (5 g, 30 mmol) and anhy-
drous FeCl3 (20 g, 120 mmol) were successively added, and then 100 mL
chloroform was added. The mixture was stirred at room temperature
and reacted for 12 h. When the reaction was stopped, reaction mixture
was poured into a large amount of ethanol solution, stirred and filtered,
and the filter cake was dissolved in an appropriate amount of ethyl ac-
etate. Then an appropriate amount of glacial acetic acid and zinc powder
were added, stirred, and the solvent was removed by vacuum evapora-
tion. Finally, the crude product was purified by silica gel column chro-
matography (the stationary phase was silica gel, the eluent was
petroleum ether and ethyl acetate 1:3). The final product was white solid
powder in 60% yield (3.02 g, 9.1 mmol). 1H NMR (600 MHz, CDCl3, δ):
11.27 (s, 2H), 8.52 (s, 2H), 8.25 (d, 2H), 7.82 (d, 2H), 7.65 (d, 2H), 7.55
(d, 2H), 7.40 (t, 2H), 7.18 (t, 2H). 13C NMR (150 MHz, CDCl3, δ): 143.75,
143.68, 140.65, 140.91, 133.91, 133.62, 129.82, 129.24, 125.91,
124.15, 122.75, 121.78, 121.42, 119.45, 116.85, 113.92, 111.25. MS
(MALDI-TOF) [m/z]: calcd for C24H16N2, 332.13; found, 332.41. Anal.
Calcd. for C24H16N2: C, 86.72; H, 4.85; N, 8.43. Found: C, 86.75; H, 4.82;
N, 8.43.
2.2. Instrumentation
1H NMR and 13C NMR spectra were recorded on a BRUKER AMX
600-MHz spectrometer in CDCl3 at 600 MHz and 150 MHz, respectively,
using Si(CH3)4 as internal standard. Molecular masses were recorded on
matrix assisted laser desorption-ionization time-of-flight mass spec-
trometry (MALDI-TOF-MS) using a BRUKER DALTONICS instrument,
with α-cyano-hydroxycinnamic acid as a matrix. UV–Vis absorption and
photoluminescence emission spectra of the targets were tested by a
SHIMADZU UV-2600 spectrophotometer and a HORIBA FLUOROMAX-4
spectrophotometer respectively. Cyclic voltammetry was measured by a
CHI750C voltammetric analyzer in dichloromethane (DCM) solutions
(10ꢀ 3 M) and a scan rate of 50 mV sꢀ 1, with in a typical three-electrode
cell system with a platinum plate as working electrode, a platinum wire
as counter electrode and a silver wire as reference electrode, using the
tetrabutylammonium hexafluorophosphate (0.1 M) and ferrocene as
supporting electrolyte and internal standard, respectively. The HOMO
energy levels were calculated according to the equation: EHOMO = ꢀ
(Eox+4.4) (eV). The LUMO was determined by HOMO and optical band
gap Eg. Thermogravimetric analysis (TGA) was performed with Netzsch
simultaneous thermal analyzer (STA) system (STA409 PC) under a dry
nitrogen gas flow at a heating rate of 10 ◦C minꢀ 1. Differential scanning
calorimetry (DSC) curves were carried out using DSC 2910 modulated
calorimeter. The energy levels of frontier orbital and electronic cloud
distribution were performed with the Gaussian 09 program package and
optimized at the B3LYP/6-31G (d) level in theory.
Synthesis of (9H, 9′H-[3,3′-bicarbazole]-9,9′-diylbis (4,1-phenyl-
ene))bis (dibenzo [b,d]thiophen-2-ylmethanone) (2DBT-BZ-2Cz).
DBT-BZ-Br (2.0 g, 5.45 mmol), 9H, 9′H-3,3′-bicarbazole (BCz, 0.83 g,
2.5 mmol), cuprous iodine (0.1 g, 5 mmol), potassium phosphate (1.38
g, 10 mmol) and 1,10-phenanthroline (0.1 g, 5 mmol) were dissolved in
50 mL solution of N, N-dimethylformamide and refluxed at 140 ◦C
temperature for 24 h under nitrogen. After the reaction finished, reac-
tion mixture was cooled to room temperature and poured into a large
quantity of saturated salt solution. The precipitated organic substance
was pumped out and filtered, and the crude product was purified by
silica gel column chromatography (the stationary phase was silica gel,
the eluent was petroleum ether and ethyl acetate 1:5). The final product
was yellow solid powder in 48% yield (1.08 g, 1.2 mmol). 1H NMR (600
MHz, CDCl3, δ): 8.67 (s, 2H), 8.43 (s, J = 8.5 Hz, 2H), 8.20–8.22 (m, 4H),
8.10–8.11 (d, J = 7.8 Hz, 4H), 7.96 (s, 4H), 7.85–7.86 (m, 2H),
7.77–7.78 (m, 6H), 7.61–7.62 (d, J = 8.4 Hz, 2H), 7.54–7.56 (d, J = 8.4
Hz, 2H), 7.42–7.48 (m, 6H), 7.31–7.33 (m, 2H). 13C NMR (150 MHz,
2.3. Synthesis
Synthesis of (4-bromophenyl) (dibenzo [b,d]thiophen-2-yl)
methanone (DBT-BZ-Br).
To a solution of dibenzothiophene (1.53 g, 8.3 mmol) and aluminium
2

J. Tang et al.
Organic Electronics 96 (2021) 106199
CDCl3, δ): 194.35, 143.33, 140.70, 139.75, 138.78, 138.47, 135.39,
134.64, 134.14, 133.81, 132.80, 130.95, 127.11, 126.54, 125.42,
125.28, 125.04, 123.97, 123.54, 123.05, 122.55, 122.00, 121.71,
121.06, 120.46, 119.76, 119.61, 118.05, 109.15, 108.98, 99.52. MS
(MALDI-TOF) [m/z]: calcd for C62H36N2O2S2, 904.22; found, 905.10.
Anal. Calcd. for C62H36N2O2S2: C, 82.28; H, 4.01; N, 3.10. Found: C,
82.30; H, 3.98; N, 3.10.
bicarbazole]-9,9′-diylbis (4,1-phenylene))bis (dibenzo [b,d]thiophen-2-
ylmethanone) (2DBT-BZ-2Cz) and (9H, 9′H-[3,3′-bicarbazole]-9,9′-
diylbis (4,1-phenylene))bis (dibenzo [b,d]furan-2-ylmethanone) (2DFT-
BZ-2Cz) were shown in Scheme 1. The complete synthesis routine only
requires simple two steps that was obtained through Friedel-Crafts
acylation and copper-catalyzed Buchwald-Hartwig coupling reaction.
The finally chemical structures were fully certified by 1H NMR, 13C
NMR, element analysis and mass spectrometry. Before device fabrica-
tion, the target product was further separated and purified by silica gel
column.
Synthesis of (9H, 9′H-[3,3′-bicarbazole]-9,9′-diylbis (4,1-phenyl-
ene))bis (dibenzo [b,d]furan-2-ylmethanone) (2DFT-BZ-2Cz).
The synthetic process of 2DFT-BZ-2Cz was analogous to that of
2DBT-BZ-2Cz as yellow solid powder with a yield of 46% (0.96 g, 1.1
mmol). 1H NMR (600 MHz, CDCl3, δ): 8.59 (s, J = 7.8 Hz, 2H), 8.49 (s,
2H), 8.27–8.28 (d, J = 7.2 Hz, 2H), 8.15–8.17 (d, J = 8.4 Hz, 4H),
8.06–8.10 (m, 4H), 7.83–7.85 (m, 6H), 7.72–7.73 (d, J = 9.0 Hz,
2H),7.68–7.69 (d, J = 9.0 Hz, 2H), 7.65–7.66 (d, J = 7.8 Hz, 2H),
7.61–7.62 (d, J = 8.4 Hz, 2H), 7.54–7.56 (t, 2H), 7.49–7.52 (t, 2H),
7.42–7.44 (t, 2H), 7.37–7.40 (t, 2H). 13C NMR (150 MHz, CDCl3, δ):
195.36, 144.34, 141.71, 140.76, 139.79, 139.48, 136.40, 135.65,
135.15, 134.82, 133.81, 131.96, 128.12, 127.55, 126.43, 126.29,
126.05, 124.98, 124.55, 124.06, 123.57, 123.01, 122.72, 122.07,
121.47, 120.77, 120.62, 119.06, 110.16, 109.99, 100.53. MS (MALDI-
TOF) [m/z]: calcd for C62H36N2O4, 872.27; found, 872.98. Anal. Calcd.
for C62H36N2O4: C, 85.30; H, 4.16; N, 3.21. Found: C, 85.32; H, 4.13; N,
3.21.
3.2. Thermal stability and electrochemical properties
The thermal stability of 2DBT-BZ-2CZ and 2DFT-BZ-2CZ were
explored using a thermogravimetric analyzer (TGA) and differential
scanning calorimeter (DSC). As shown in Fig. 1a–b, both emitters display
high decomposition temperatures (Td, the temperature corresponding to
5% weight loss of the initial value) of more than 500 ◦C. When compared
with 2DFT-BZ-2CZ (Td, 525 ◦C), 2DBT-BZ-2CZ displayed a higher value
of 536 ◦C. Meanwhile, the glass transition temperature (Tg) of 2DBT-BZ-
2CZ was 136 ◦C, which was also higher than that of 2DFT-BZ-2CZ
(122 ◦C). The high Tg value guarantees that the materials can form a
uniform amorphous film during the fabrication process. The excellent
thermal stability of 2DBT-BZ-2CZ and 2DFT-BZ-2CZ demonstrated that
the dual donor-acceptor units were conducive to increase the molecular
rigidity.
2.4. Theoretical calculation
In order to obtain the frontier molecular orbital energy levels, the
electrochemical behaviors of these emitters were measured using cyclic
voltammetry. As depicted in Figure 1c, 2DBT-BZ-2CZ and 2DFT-BZ-2CZ
showed similar oxidation processes, with initial peak potentials of 1.24
V and 1.26 V, respectively. Based on the onset potentials and the
equation of ELUMO = EHOMO + Eg, the HOMO/LUMO levels of 2DBT-BZ-
2CZ and 2DFT-BZ-2CZ were ꢀ 5.68/-2.64 eV and ꢀ 5.70/-2.59 eV,
respectively, where the Eg is the optical band gap calculated from ab-
sorption spectra. The HOMO levels were similar in both emitters, but the
LUMO level of 2DFT-BZ-2CZ was shallower than that of 2DBT-BZ-2CZ
due to decreased electron-withdrawing character by conjugation be-
tween two benzophenone units. This analysis was also in accord with
above theoretical simulation.
The density functional theory (DFT) with B3LYP/6-31G* basis was
employed to examine the geometrical and electronic properties of 2DBT-
BZ-2CZ and 2DFT-BZ-2CZ, which was set by same basis in Gaussian 09
software. The optimized configuration of ground states (S0) from two
molecules were conducted firstly. The singlet and triplet energies of the
compounds were predicted by the time-dependent DFT, and the energy
level distributions can also be visualized then.
2.5. Device fabrication and measurements
The glass substrates were coated with indium tin oxide (ITO) and
then sequentially cleaned in an ultrasonic bath with deionized water,
acetone and ethanol for three times. Before device fabrication, the ITO
substrate was treated in a UVꢀ ozone oven for 30 min. A PEDOT: PSS
layer (50 nm) was spin-coated onto the ITO surface and baked at 150 ◦C
for 10 min to remove the residual water. Then, an EML with a thickness
of about 40 nm was spin-coated from a filtered 10 mg mLꢀ 1 1, 2-dichlo-
roethane solution onto the PEDOT: PSS layer and annealed at 80 ◦C for
30 min to remove the residual solvent under nitrogen atmosphere.
Sequentially, 40 nm 1, 3, 5-tris(N-phenylbenzimidazol-2-yl)-benzene
(TPBI) was vacuum-deposited as an electron-transporting and hole-
blocking layer. Finally, Cs2CO3 (2 nm) and Al (100 nm) were evapo-
rated acting as the cathode for the devices. The current densityꢀ voltage-
luminance characteristics, current efficiency and power efficiency were
tested using a Keithley 2636A Sourcemeter coupled with Si-potodiodes
calibrated with PR655. The EL spectra were collected with a Photo-
Research PR-655 SpectraScan. All the characterizations were per-
formed at room temperature in ambient condition without encapsula-
tion. External quantum efficiencies of the devices were calculated
assuming a Lambertian emission distribution.
3.3. Theoretical calculation
To gain insight into the diversiform molecular geometry configura-
tion and electron cloud distribution of frontier molecular orbitals of the
designed emitters at the molecular level, density functional theory (DFT)
calculations under B3LYP/6-31G (d) level with the Gaussian 09 package
were executed. As shown in Fig. 2, in the optimized molecular struc-
tures, the dihedral angle between the carbazole and benzophenone
groups have large torsion, resulting in nonplanar spatial molecular
configuration. This structure can effectively limit the conjugate length
and avoid the crystallization of molecules in the process of film forma-
tion. Besides, by designing the dual emitting core to impose restrictions
on the middle carbazole unit, the non-radiation process from the vi-
brations and rotations of the molecules would be greatly reduced [50].
The calculation of two materials, as desired, shows that the HOMOs are
broadened to the double carbazole units with part of the extension to
benzene ring of benzophenone, and the LUMOs are mostly situated at
benzophenone segments, respectively. The electron-withdrawing prop-
erty of benzophenone fragments is stronger than benzothiophene and
benzofuran units. However, their electron-donating capability is inferior
to bicarbazole. Consequently, DFT calculations show that the frontier
molecular orbitals did not extend on the benzothiophene and benzo-
furan units. Owing to the same electron donor carbazole, the target
molecules 2DBT-BZ-2CZ and 2DFT-BZ-2CZ have the same HOMO level
of ꢀ 5.17 eV, and the LUMO levels are ꢀ 1.89 eV and ꢀ 1.83 eV,
respectively. The efficient separated HOMOs and LUMOs, ensure small
3. Results and discussion
3.1. Synthesis and characterization
All the detailed synthetic processes and molecular structures of
precursor, (4-bromophenyl) (dibenzo [b,d]thiophen-2-yl)methanone
(DBT-BZ-Br) and (4-bromophenyl) (dibenzo [b,d]furan-2-yl)
methanone (DFT-BZ-Br), and the desired molecules, (9H, 9′H-[3,3′-
3

J. Tang et al.
Organic Electronics 96 (2021) 106199
Scheme 1. Synthetic route and chemical structure of target molecules 2DBT-BZ-2Cz and 2DFT-BZ-2Cz.
Fig. 1. (a) TGA curves, (b) DSC curves and (c) Electrochemical properties of 2DBT-BZ-2Cz and 2DFT-BZ-2Cz.
ΔEST at 0.23 and 0.24 eV of 2DBT-BZ-2Cz and 2DFT-BZ-2Cz, respec-
tively, and provide an opportunity for fast RISC process. Additionally,
broadening the distribution of frontier molecular orbitals is in favor of
expediting radiation transition rate [51–53]. This also exemplifies the
advantage of the “twin emitter’’ molecular design.
carried out using an ultraviolet–visible (UV–vis) spectrometer and a
fluorescence spectrometer in the solvent (1 × 10ꢀ 5 M) and neat film
state at room temperature. The relevant summary of photophysical in-
formation is depicted in Table 1. Apparently, in Fig. 3a and b, the
stronger higher-energy absorption bands below 310 nm for both com-
pounds can be assigned to the n-π* and π–π* transitions of the conju-
gated skeleton, whereas the weakly broader absorption band from the
long wavelength of 350–410 nm can be observed, indicating the stron-
ger intramolecular charge transfer from the donor moiety carbazole to
the acceptor unit benzophenone. The PL spectra of 2DBT-BZ-2CZ and
3.4. Photophysical properties and aggregation-induced emission (AIE)
behavior
The photophysical investigation of the synthesized compounds was
4

J. Tang et al.
Organic Electronics 96 (2021) 106199
Fig. 2. The optimization of ground state geometry and frontier molecular orbital electron cloud distribution.
Table 1
Photophysical properties and electrochemical properties of these compounds.
Compound
c
c
e
λabsa [nm]
λemb [nm]
Tg [◦C]
Td [◦C]
τp
/
τde [ns]
HOMOf/LUMOf [eV]
Egg[eV]
ΔESTh [eV]
2DBT-BZ-2Cz
2DFT-BZ-2Cz
240/297/351
248/288/349
539
535
136
122
536
525
23/380
26/440
ꢀ 5.68/-2.64
ꢀ 5.70/-2.59
3.04
3.11
0.02
0.01
aMeasured in dichloromethane.bMeasured in dichloromethane solution.cMeasured by DSC/TGA at a heating rate of 10 ◦C min-1.eThe prompt and delayed fluorescence
lifetimes of the neat film at room temperature.fObtained from the cyclic voltammograms. The oxidation potential of Fc/Fc+ was measured to be 0.4 eV. HOMO = ‒ (4.8
ꢀ 0.4) eV，LUMO = HOMO + Eg.gObtained from the onsets of absorption spectra in neat film.hSplitting between S1 and T1 energy calculated from the onset of
OX
+ E
onset
the fluorescence spectra in neat film at 300 K and the phosphorescence spectra at 77 K in neat film.
2DFT-BZ-2CZ in different dilute solution from nonpolar hexane to highly
polar dichloromethane exhibited obvious structureless red-shifts from
blue-violet to orange-red emissions, indicating their strong intra-
molecular CT properties, and large stokes shifts of 100 and 130 nm,
respectively. Obviously, the solvation effect of 2DFT-BZ-2Cz is more
distinct than 2DBT-BZ-2Cz. Oxygen with higher electronegativity and
smaller atomic size relative to sulfur render furan less aromaticity and
weak intermolecular interactions. This maybe leads to furan-based
nm, and 508/511 nm, respectively. Hence, S1 and T1 energies of
2DBT-BZ-2CZ and 2DFT-BZ-2CZ calculated from the onset wavelengths
of the PL spectra and PH spectra were 2.45/2.43 eV, and 2.44/2.43 eV,
respectively. Thus, the ΔEST values of both materials were estimated to
be 0.02 and 0.01 eV, resulting in the effective RISC process of
2DBT-BZ-2CZ and 2DFT-BZ-2CZ. This result also coincided well with
that obtained from the DFT simulations.
In order to explore the AIE features of these materials, the photo-
luminescence behavior in THF/water mixture solvent with various THF/
water ratio fractions was measured and presented in Fig. 4. For 2DBT-
BZ-2CZ and 2DFT-BZ-2CZ, the emissive spectrum in the pure THF so-
lution was weak and broad, and the main peak was around 500 nm.
When the fraction of water was increased from 10 to 60%, the PL in-
tensities and their spectra exhibited a slight hypochromatic shift with a
similar weak emissive intensity on account of the enhanced ICT features
through increasing the polarity of the mixture solvent and twisted
intramolecular charge transfer (TICT) process, which makes the exciton
quenching in non-radiative form. Besides, when the water fraction was
increased from 70 to 90%, the PL intensity was significantly increased
with a corresponding blue shift than the value of the pure THF solution,
π
-functional materials inferior than their thiophene counterparts. It is
speculated that the characteristic of oxygen atom endows 2DFT-BZ-2CZ
with stronger electron-withdrawing property and improved intra-
molecular charge transfer, which is beneficial to induce the larger
bathochromic-shift than 2DBT-BZ-2CZ from low polar solvents to high
polar solvents [32,54]. Furthermore, the photoluminescence (PL)
spectra at room temperature and phosphorescence spectra at 77 K with
0.1 ms delay of 2DBT-BZ-2CZ and 2DFT-BZ-2CZ in films were further
studied, as shown in Fig. 3c and d, in which both the spectra revealed
structureless broad bands, thus indicating that its lowest triplet origin
from the CT emission in neat films. The fluorescent and phosphorescent
peaks for 2DBT-BZ-2CZ and 2DFT-BZ-2CZ were centered at 507/510
5

J. Tang et al.
Organic Electronics 96 (2021) 106199
Fig. 3. The UV absorption and fluorescence emission spectra of 2DBT-BZ-2Cz (a) and 2DFT-BZ-2Cz (b); fluorescence(c) and phosphorescence (d) spectra in solid
films at 77K of DBT-BZ-Cz and DFT-BZ-Cz.
Fig. 4. Emission spectra of 2DBT-BZ-2Cz (a) and (b) 2DFT-BZ-2Cz in a mixed solution of THF/Water in a molar concentration of 10 μM.
which can be attributed to the gradual formation of molecular aggre-
gation. Before generation of the aggregates, solvatochromism is
responsible for the red-shift of emission peak with increasing solvent
polarity. By contrast, after production of nano-aggregates, the intra-
molecular rotation is under restrictions and the less polar environment
becomes possible, therefore giving rise to a blueshift in the emission
color. Thus, these results of the above analysis provide significant evi-
dence to prove the AIE properties of the compounds. This also indirectly
reflected that this dual emitting core of A-D-D-A structure can facilitate
the increasement of molecular rigidity and induce the generation of AIE
property.
In order to further investigate the TADF features, the room temper-
ature transient PL decay spectra were measured in solid films which
were fabricated by solution process. Bi-exponential decays of these films
in Fig. 5 show shorter-life component and the longer-life component
representing a prompt decay component and a delayed emission decay
profile respectively. The summarized data of the prompt and delayed
lifetime in the neat film are presented in Table 1. The short lifetime
components with 23–26 ns corresponded to the conventional fluores-
cence, which was transited from S1 to S0. The RISC process from
6

J. Tang et al.
Organic Electronics 96 (2021) 106199
and energy diagram of the used materials in the OLED devices are
depicted in Fig. 6. In these devices, PEDOT: PSS and Cs2CO3 are served
as hole-injection layer (HIL) and electron-injection layer (EIL), respec-
tively; TPBI was employed as an electron transporting layer (ETL),
which could suppress the energy transfer from EML to ETL in view of the
sufficient ET of 2.7 eV. The emission layers were prepared with these
AIEꢀ active TADF emitter as spin-coated films. As can be observed from
the current densityꢀ voltageꢀ luminance (Jꢀ Vꢀ L) characteristics and
current efficiencyꢀ luminance (CEꢀ L) curves in Fig. 6, the peak current
efficiency of 2DBT-BZ-2Cz based nondoped device is 20.7 cd Aꢀ 1, and
the corresponding external quantum efficiency (EQE) is 6.8%. As shown
in Table 2, the device performance based on 2DBT-BZ-2CZ was superior
to that of 2DFT-BZ-2CZ. High current density accounts for the better
carrier mobility and charge transfer ability of 2DBT-BZ-2CZ. However,
the larger carrier injection barrier of 2DFT-BZ-2CZ results in the poor
electroluminescence property. The current efficiency of 2DFT-BZ-2CZ
based nondoped device is only 14.5 cd Aꢀ 1, which should be assigned
to the poor carrier transportation. The stronger molecular rigidity can
explain the non-ideal devices performance due to the inferior molecular
solubility in the process of solution processed technology. This also can
give guidance and recommendations to the further molecular design for
solution process OLEDs.
Fig. 5. Transient fluorescence spectra of 2DBT-BZ-2Cz and 2DFT-BZ-2Cz solid
films at 300 K.
nonradiative T1 to radiative S1 state induced delayed components with
0.38–0.44 μs. Unveiling their TADF nature.
4. Conclusion
3.5. Electroluminescence properties
In summary, two novel emitters with a motif of A-D-D-A simulta-
neously exhibiting TADF and AIE features were designed and synthe-
sized, where dual carbazole units and benzophenone moieties were
employed as donor and acceptor groups, respectively. With the assis-
tance of large torsion between carbazole and benzophenone, efficient
separation of FMO made the intramolecular charge transfer coming true.
For investigating the influence of these TADF emitters on electrolu-
minescence (EL) performances, non-doped TADF devices were fabri-
cated with the configuration of ITO/PEDOT: PSS (50 nm)/emitter
(device A: 2DBT-BZ-2CZ; device B: 2DFT-BZ-2CZ) (40 nm)/TPBI (40
nm)/Cs2CO3 (2 nm)/Al (100 nm) via the solution process. The structures
Reduced ΔEST, efficient reverse intersystem crossing, suppressed
Fig. 6. (a) Energy level and molecular structure of materials used in OLED device; (b) current Density –voltage-luminance curves; and (c) Current density -current
efficiency of 2DBT-BZ-2Cz and 2DFT-BZ-2Cz, inset: the electroluminescence spectra in 10 V; (d) the curves of power efficiency versus current density.
7

J. Tang et al.
Organic Electronics 96 (2021) 106199
Table 2
Device performances of the solution-processed nondoped OLEDs.
CIE (x,y)f
a
b
c
d
e
Device
emitter
Von
EL (nm)
CEmax
PEmax
EQEmax
Lmax
A
B
2DBT-BZ-2Cz
2DFT-BZ-2Cz
3.6
3.9
536
540
20.7
14.5
12.4
7.6
6.8
4.5
10,000
6700
(0.37,0.58)
(0.38,0.59)
a
Von = turn-on voltage at 1 cd mꢀ 2
.
b
c
d
e
f
CEmax = maximum current efficiency.
PEmax = maximum power efficiency.
EQEmax = maximum external quantum efficiency.
Lmax = maximum luminance.
CIE = the Commission Internationale de L’Eclairage coordinates at 10 V.
emitting diodes employing a sensitizer with fast triplet upconversion, Adv. Mater.
32 (2020) 1908355–1908364.
nonradiative decay and obvious thermally activated delayed fluores-
cence feature can be achieved in the solid state of the emitters concur-
rently. Utilizing the AIE-TADF materials as emitters, non-doped device
was successfully prepared. Remarkably, employing 2DBT-BZ-2Cz as the
dual-emitting-core emitter, an optimized solution-processed green de-
vice achieved a high CEmax of 20.7 cd Aꢀ 1 and EQEmax of 6.8%, This
work demonstrates the molecular design strategy of introducing dual
emitter core into the emitters possessing TADF and AIE features is
promising to develop efficient solution-processed non-doped OLEDs.
[9] T. Huang, W. Jiang, L. Duan, Recent progress in solution processable TADF
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towards realization of high efficiency through strategic small molecular design,
Inside Chem. 25 (2019) 5623–5642.
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applications of organic fluorescence materials with reduced singlet–triplet energy
gaps, Coord. Chem. Rev. 425 (2020) 213545.
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[13] Q. Wei, Z. Ge, B. Voit, Thermally activated delayed fluorescent polymers:
structures, properties, and applications in OLED devices, Macromol. Rapid
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Declaration of competing interest
[14] Z. Yang, Z. Mao, Z. Xie, Y. Zhang, S. Liu, J. Zhao, J. Xu, Z. Chi, M.P. Aldred, Recent
advances in organic thermally activated delayed fluorescence materials, Chem.
Soc. Rev. 46 (2017) 915–1016.
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.
[15] X. Zheng, R. Huang, C. Zhong, G. Xie, W. Ning, M. Huang, F. Ni, F.B. Dias, C. Yang,
Achieving 21% external quantum efficiency for nondoped solution-processed sky-
blue thermally activated delayed fluorescence OLEDs by means of multi-(donor/
acceptor) emitter with through-space/-bond charge transfer, Adv. Sci. 7 (2020)
1902087–1902094.
Acknowledgements
[16] Y.P. Zhang, X. Liang, X.F. Luo, S.Q. Song, S. Li, Y. Wang, Z.P. Mao, W.Y. Xu, Y.
X. Zheng, J.L. Zuo, Y. Pan, Chiral spiro-Axis induced blue thermally activated
delayed fluorescence material for efficient circularly polarized OLEDs with low
efficiency roll-off, Angew. Chem. Int. Ed. 60 (2021) 1–7.
We are grateful for the grants from the National Natural Science
Foundation of China (21875036), and we are also thankful for the
support of the Doctoral Research Foundation of Southeast University
Chengxian College.
[17] X. Ban, W. Jiang, T. Lu, X. Jing, Q. Tang, S. Huang, K. Sun, B. Huang, B. Lin, Y. Sun,
Self-host thermally activated delayed fluorescent dendrimers with flexible chains:
an effective strategy for non-doped electroluminescent devices based on solution
processing, J. Mater. Chem. C. 4 (2016) 8810–8816.
Appendix A. Supplementary data
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thermally activated delayed fluorescence blue host for high performance solution-
processed hybrid white organic light emitting diodes, Chem. Sci. 10 (2019)
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Supplementary data to this article can be found online at https://doi.
org/10.1016/j.orgel.2021.106199.
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9