Strategy to improve the efficiency of solution-processed phosphorescent organic light-emitting devices by modified TADF host with tert-butyl carbazole

Article abstract of DOI:10.1016/j.tet.2020.131869

A new bipolar host material t3Cz-SO was designed and synthesized by introducing tert-butyl carbazole unit to the peripheral of parent TADF units. The calculated triplet energy and ΔE_ST of t3Cz-SO are 2.85 and 0.11 eV, respectively. Moreover, t3

Full text of DOI:10.1016/j.tet.2020.131869

Tetrahedron 81 (2021) 131869
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Strategy to improve the efficiency of solution-processed
phosphorescent organic light-emitting devices by modified TADF host
with tert-butyl carbazole
, b, c,
Yan Liu a, Suyu Qiu a, Jianmin Yu a, Xinxin Ban a
*, Jie Pan b, Kun Gao c,
Aiyun Zhu a
**, Dongen Zhang a, Tianlin Zhang a, Zhiwei Tong a
,
b,
c
,
, b, c
a School of Environmental and Chemical Engineering, Jiangsu Ocean University, Lianyungang, Jiangsu, 222005, China
b Jiangsu Key Laboratory of Marine Bioresources and Environment, Jiangsu Ocean University, Lianyungang, Jiangsu, 222005, China
c Jiangsu Key Laboratory of Function Control Technology for Advanced Materials, Jiangsu Ocean University, Lianyungang, Jiangsu, 222005, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new bipolar host material t3Cz-SO was designed and synthesized by introducing tert-butyl carbazole
Received 8 September 2020
Received in revised form
12 November 2020
Accepted 7 December 2020
Available online 13 January 2021
unit to the peripheral of parent TADF units. The calculated triplet energy and DEST of t3Cz-SO are 2.85 and
0.11 eV, respectively. Moreover, t3Cz-SO exhibited excellent film-forming ability and morphological
stability, which are suitable for the fabrication of solution-processed OLEDs. The FIrpic-based device host
with t3Cz-SO achieved low turn-on voltage of 3.5 V and high power efficiency of 16.5 lm Wꢀ1, which is
almost ten times higher than the unmodified parent TADF molecule. This study shows that the intro-
duction of tert-butyl carbazole unit to the periphery of small molecular TADF emitter is an effective
method for the development of efficient electroluminescent materials.
Keywords:
TADF
Bipolar host
© 2020 Elsevier Ltd. All rights reserved.
Solution-process
Organic light-emitting diodes
1. Introduction
and electroluminescence properties of the host material directly
affect the performance of the devices [10,11], such as turn-on
Organic light emitting diodes (OLEDs) have already become the
most promising technology in the display field thanks to its ad-
vantages such as low energy consumption, self-luminescence, high
flexibility and high resolution [1e3]. So far, thermally activated
delayed fluorescent (TADF) OLEDs have become a promising
alternative for high-efficiency organic light emitting diodes
(OLEDs) [4], in which TADF emitters emerge remarkable perfor-
mance because it has the potential to improve efficiency by fully
harvesting singlet and triplet excitons from the triplet (T1) state to
the singlet (S1) state through reverse intersystem crossing (RISC)
process to achieve 100% internal quantum efficiency and avoid the
use of expensive heavy-metal [5e7]. Generally, the emission layer
(EML) of the solution-processed devices mainly comprise the
dopant and host materials [8,9]. Since the optical, electrochemical
voltage, current efficiency, power efficiency and external quan-
tum efficiency (EQE). The host materials occupy an important po-
sition in EML and choosing the proper host materials is the key to
controlling the efficiency and brightness of the OLED devices
[12e14].
Normally, the host materials for phosphorescent emitters are
conventional unipolar electron-transport or hole-transport mate-
rials [15], the narrow recombination zone exists near the EML
interface, which may cause rapid roll-off of efficiency, which will
affects the stability of devices [16,17]. Therefore, the bipolar host
molecules, which contain two parts of electron donating (D) and
electron accepting (A), come to be more functional. Bipolar host
molecules have been shown to facilitate the injection and transport
of holes and electrons, widen the recombination zone, and thus
mitigate the efficiency roll-off [18e20]. For example, Lee [21] and
his group synthesized three triphenylene derivatives for the use as
hosts for green PhOLEDs and achieved a high EQE up to 20.3%. Kang
[22] and co-workers reported three different host molecules with
carbazole, carboline, and DBT moieties in one molecular structure.
They have similar T1 levels, lower driving voltages (4.7e5.0 V),
* Corresponding author. School of Environmental and Chemical Engineering,
Jiangsu Ocean University, Lianyungang, Jiangsu, 222005, China.
** Corresponding author. School of Environmental and Chemical Engineering,
Jiangsu Ocean University, Lianyungang, Jiangsu, 222005, China.
E-mail addresses: banxx@jou.edu.cn (X. Ban), zhuay@jou.edu.cn (A. Zhu).
https://doi.org/10.1016/j.tet.2020.131869
0040-4020/© 2020 Elsevier Ltd. All rights reserved.

Y. Liu, S. Qiu, J. Yu et al.
Tetrahedron 81 (2021) 131869
significantly higher EQEs than the devices containing an mCP host
and an unprecedentedly high EQE of around 25.7%. However,
traditional TADF molecules are not conducive to solution-processed
devices due to their small molecular structure, easy crystallization,
and poor film-forming properties [23]. TADF host molecules used in
the preparation of solution-processed OLED devices should meet
the following conditions: (a) has good solubility in commonly used
organic solvents; (b) has good film-forming properties so that de-
vices can be prepared by spin coating and inkjet [16]; (c) possess
higher triplet energies than those of the dopant emitters to prevent
reverse energy transfer from the guest back to the host, as well as
confine triplet excitons in the emissive layer [22,24].
In this work, we designed and synthesized a new molecule t3Cz-
SO, which fully demonstrated that the introduction of tert-
butylcarbazole in the periphery of the small molecular TADF
molecule can improve the film-forming performance and solubility
of the material, while effectively maintaining the TADF properties
of the material. Electrochemical studies showed that the intro-
duction of the tert-butylcarbazole group effectively inhibited the
self-polymerization of molecules under external stimulus, and after
the film was thermally annealed at 150 ꢁC, it still maintained the
good spectral performance and morphological stability. Compared
with the device made of unmodified molecules, the performance of
the device prepared by the host material of blue phosphorescent
molecules was significantly improved.
are shown in Scheme 1.
2.2. General method
1H and 13C NMR spectra are recorded on a BRUKER AMX in-
strument. Elemental analysis is performed using an Elementar
Vario EL CHN elemental analyzer. Molecular masses are measured
by a BRUKER DALTONICS matrix-assisted laser desorption ioniza-
tion time-of-fight mass spectrometer (MALDI-TOF-MS). The
UVevis absorption spectra of the compounds are measured by
SHIMADZU UV-2450. The photoluminescence emission spectra are
recorded on HORIBA FLUOROMAX-4. Thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) curves are
recorded with a Netzsch simultaneous thermal analyzer (STA)
system (STA 409 PC). Cyclic voltammetry (CV) is performed on a
CHI750C voltammetric analyzer in a typical three-electrode cell
with a platinum plate working electrode, a platinum wire counter
electrode and a silver wire reference electrode. The optimized
structure is calculated by Gaussian09 at the B3LYP functional with
6e31G(d) basis sets. The excitation energies in the singlet and
triplet states are obtained using the TD-DFT method based on an
optimized molecular structure for the ground state.
3. Synthesis
3.1. 9- (4-(phenylsulfonyl)phenyl)-9H-carbazole (Cz-SO)
2. Experimental section
Bromo-4-(phenylsulfonyl)benzene (0.5 g, 1 mmol), 9H-carba-
zole (0.31 g, 1.1 mmol), K2CO3 (0.7 g, 3 mmol), CuI (0.064 g,
0.2 mmol), 1,10- Phenanthroline hydrate (0.062 g, 0.2 mmol) are
added to a 100 mL round-bottom flask, and then N, N- dime-
thylformamide (DMF, 20 mL) is added under a nitrogen atmo-
sphere. After stirring for 24 h at 110 ꢁC, the reaction mixture is
cooled to room temperature. The solvent is removed under reduced
pressure and the mixture is extracted with CH2Cl2 and ultrapure
water. The organic extracts are combined, and dried over NaSO4.
Upon evaporating off the solvent, the crude product is purified by
column chromatography on silica gel with petroleum ether/CH2Cl2
to yield M1 (0.83 g, 61%) as a white powder. 1H NMR (500 MHz,
2.1. Chemicals
The synthesis of the compounds 3,6-di-tert-butyl-9H-carbazole
(1), 3,6-diiodo-9H-carbazole (2), 3,6-diiodo-9-tosyl-9H-carbazole
(3), 3,6-di- tert-butyl-9-(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9-
tosyl-9H-carbazol -6-yl)-9H-carbazole (4) and 3,6-di-tert-butyl-9-
(3-(3,6-di-tert-butyl-9H
-carbazol-9-yl)-9H-carbazol-6-yl)-9H-
carbazole (5) were prepared according to published procedures
[25]. All chemicals were purchased from Energy Chemistry com-
panies and used without further purification unless otherwise
noted. N, N-dimethylformamide (DMF) and methylene chloride
(DCM) solvents were dewatered according to standard procedures.
The synthetic routes and chemical structures of Cz-SO and t3Cz-SO
CDCl3):
d
8.20e8.15 (m, 2H), 8.12 (d, J ¼ 7.7 Hz, 2H), 8.05e8.02 (m,
2H), 7.74 (d, J ¼ 8.7 Hz, 2H), 7.65e7.53 (m, 3H), 7.44e7.35 (m, 4H),
Scheme 1. Synthetic Routes and Chemical Structures of Cz-SO and t3Cz-SO.
2

Y. Liu, S. Qiu, J. Yu et al.
Tetrahedron 81 (2021) 131869
7.35e7.27 (m, 2H). 13C NMR (126 MHz, CDCl3)
139.97, 139.75, 133.48, 133.18, 129.57, 129.50, 129.28, 127.86, 127.68,
127.09, 126.33, 123.96, 120.93, 120.54, 109.61.
d
142.47, 141.38,
(d, J ¼ 8.2 Hz, 2H), 7.72 (d, J ¼ 8.6 Hz, 2H), 7.44 (dd, J ¼ 8.8, 1.6 Hz,
4H), 7.28e7.32 (m, 6H), 2.40 (s, 3H), 1.45 (s, 36H).
3.6. 3,6-di-tert-butyl-9-(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)-
9H-carbazol-6-yl)-9H-carbazole (5)
3.2. 3,6-di-tert-butyl-9H-carbazole (1)
Carbazole (5.016 g, 30 mmol), AlCl3 (4.0 g, 30 mmol) and CH2Cl2
KOH (0.13 g, 2.32 mmol) is added to a 100 mL round-bottom
flask, and a solution of 4 (1.0 g, 1.14 mmol) in DMSO (6 mL)、THF
(12 mL) and water (2 mL) is added, then the mixture is stirred and
refluxed for 1.5 h. After cooling to room temperature, adding 10%
HCl (20 mL), then adding water (10 mL) and methanol (5 mL). After
filtration, the precipitate is collected and rinsed with water. The
crude product is purified by column chromatography on silica gel
with eluent: petroleum ether/CH2Cl2 to yield 5 (0.68 g, 76%) as a
(120 mL) are added to a 100 mL round-bottom flask. After cooling to
0
ꢁC, tert-butyl chloride (7.5 ml 60 mmol) is placed in CH2Cl2
(20 ml) and added dropwise for 20 min to form a dark yellow so-
lution. The mixture is stirred at 0 ꢁC for 1 h and at room temper-
ature for 9 h, then pour into ice water and extracted with CH2Cl2.
The combined organic phase is washed with water, brine, dried
over anhydrous NaSO4, filtered, and concentrated under reduced
pressure. The crude product is purified by column chromatography
on silica gel with petroleum ether/CH2Cl2 to yield 1 (0.87 g, 65%) as
white solid. 1H NMR (500 MHz, CDCl3):
d 8.40 (s, 1H), 8.17 (d,
J ¼ 4.8 Hz, 6H), 7.66 (d, J ¼ 8.7 Hz, 2H), 7.58 (d, J ¼ 8.6 Hz, 2H), 7.43
a white powder. 1H NMR (500 MHz, CDCl3):
d
8.07 (d, J ¼ 1.8 Hz,
(d, J ¼ 8.6 Hz, 4H), 7.11 (d, J ¼ 8.6 Hz, 4H), 1.45 (s, 36H).
2H), 7.84 (s, 1H), 7.45 (d, J ¼ 8.2 Hz, 2H), 7.31 (d, J ¼ 8.6 Hz, 2H), 1.44
(s, 18H).
3.7. 3,6-di-tert-butyl-9-(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9-
(4-(phenylsulfonyl)phenyl)-9H-carbazol-6-yl)-9H-carbazole (t3Cz-
SO)
3.3. 3,6-diiodo-9H-carbazole (2)
Carbazole (1.00 g, 5.98 mmol), acetic acid (16 mL) are added to a
100 mL round-bottom flask and the solution is heated to 50 ꢁC.
Then KI (1.29 g, 7.77 mmol) is added. After stirring for a few mi-
nutes, a solution of KIO3 (1.279 g, 5.98 mmol) in acetic acid (2 mL) is
added dropwise. After the mixture is refluxed for 1 h, a light red
suspension is obtained. After cooling to room temperature, adding
water and extracted with CH2Cl2. The obtained organic phase is
washed with water, brine, and dried over anhydrous NaSO4, then
filtered, and concentrated. The crude product is purified by column
chromatography on silica gel with ethyl acetate/petroleum ether to
yield 2 (0.87 g, 65%) as a red-brown powder. 1H NMR (500 MHz,
1-bromo-4-(phenylsulfonyl)benzene (0.2 g, 1 mmol), 5 (0.53 g,
1.1 mmol), K2CO3 (0.28 g, 3 mmol), CuI (0.025 g, 0.2 mmol), 1,10-
Phenant-hroline hydrate (0.027 g, 0.2 mmol) are added to a
100 mL round-bottom flask, and then DMF (20 mL) is added under a
nitrogen atmosphere. After stirring for 24 h at 110 ꢁC, the reaction
mixture is cooled to room temperature. The solvent is removed
under reduced pressure and the mixture is extracted with CH2Cl2
and water. The organic extracts are combined, and dried over
Na2SO4. The crude product is purified by column chromatography
on silica gel with eluent: petroleum ether/ethyl acetate to yield M2
(0.84 g, 62%) as a white solid. 1H NMR (500 MHz, CDCl3):
d 8.46 (d,
CDCl3):
2H), 7.21 (d, J ¼ 8.4 Hz, 2H).
d
8.32 (d, J ¼ 1.8 Hz, 2H), 8.10 (s, 1H), 7.66 (dd, J ¼ 8.2, 1.6 Hz,
J ¼ 8.6 Hz, 2H), 8.29 (d, J ¼ 1.6 Hz, 2H), 8.20 (d, J ¼ 1.2 Hz, 4H), 8.06
(d, J ¼ 8.6 Hz, 2H), 7.77 (d, J ¼ 8.7 Hz, 2H), 7.68 (dd, J ¼ 8.7, 1.8 Hz,
2H), 7.65e7.55 (m, 5H), 7.49 (dd, J ¼ 8.6, 1.6 Hz, 4H), 7.36 (d,
3.4. 3,6-diiodo-9-tosyl-9H-carbazole (3)
J ¼ 8.6 Hz, 4H), 1.50 (s, 36H). 13C NMR (126 MHz, CDCl3)
d 142.82,
142.40, 140.10, 139.98, 139.43, 131.98, 130.17, 127.48, 126.32, 124.77,
123.67, 123.26, 119.54, 116.34, 111.03, 108.99, 77.31, 77.06, 76.80,
53.46, 34.78, 32.06.
To a solution of 2 (1.5 g, 3.58 mmol) and KOH (0.92 g, 16.4 mmol)
in acetone (20 mL) is slowly added p-toluenesulfonyl chloride
(3.127 g, 16.4 mmol) in acetone (2 mL). After refluxing for 15 min,
the reaction solution is poured into water and extracted with
CH2Cl2. The obtained organic phase is washed with water and
brine, dried over anhydrous NaSO4, and finally filtered and
concentrated. The crude product is purified by column chroma-
tography on silica gel with petroleum ether/CH2Cl2 to yield 3
(1.23 g, 63%) as a red-brown powder. 1H NMR (500 MHz, CDCl3):
4. Result and discussion
4.1. Thermal Properties
The thermal stability of Cz-SO and t3Cz-SO in solid powder state
is tested by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC), as shown in Fig. 1. The TGA curve
shows that the decomposition temperatures (Td, corresponding to
5% weight loss) of Cz-SO and t3Cz-SO are 278 ꢁC and 513 ꢁC,
respectively. The high thermal stability indicates that t3Cz-SO has
higher thermal stability than Cz-SO. The DSC curve exhibits that the
glass transition temperature (Tg) of Cz-SO is 152 ꢁC, while t3Cz-SO
has no obvious glass transition temperature in the range of
25e200 ꢁC. This means that t3Cz-SO will not undergo any
morphological changes during the annealing process, which is
more advantageous to the spin-coating method for preparing OLED
devices.
In order to further verify the film-forming ability of the com-
pounds and the morphologies of the solution-processed films, the
atomic force microscopy (AFM) measurement was tested under
non-contact mode. The Cz-SO and t3Cz-SO are prepared with 1, 2-
dichloroethane solution (10 mg/mL), then spin-coated on the ITO/
PEDOT: PSS substrate at a speed of 2000 rpm. Fig. 2 shows the
measured surface morphology of these two compounds after spin-
d
8.17 (d, J ¼ 1.2 Hz, 2H), 8.07 (d, J ¼ 8.6 Hz, 2H), 7.76 (d, J ¼ 8.6 Hz,
2H), 7.63 (d, J ¼ 8.4 Hz, 2H), 7.11 (d, J ¼ 7.8 Hz, 2H), 2.27 (s, 3H).
3.5. 3,6-di-tert-butyl-9-(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9-
tosyl-9H-carbazol-6-yl)-9H-carbazole (4)
CuI (0.29 g, 1.5 mmol), K3PO4 (1.592 g, 7.5 mmol), and ( )-trans-
1,2- diaminocycl-ohexane (DACH, 0.2 mL, 1.6 mmol) are added to a
100 mL round-bottom flask. Then adding a solution of 3 (1.72 g,
3 mmol) and 1 (1.85 g, 6.6 mmol) in toluene (40 mL). The above
mixture is refluxed and stirred for 24 h. After cooling to room
temperature, water and CH2Cl2 are added for extraction. The ob-
tained organic phase is washed with water and brine successively,
and finally dried with anhydrous Na2SO4, then filtered and
concentrated. The crude product is purified by column chroma-
tography on silica gel with eluent: petroleum ether/CH2Cl2 to yield
4 (0.78 g, 56%) as a white solid. 1H NMR (500 MHz, CDCl3):
J ¼ 8.2 Hz, 2H), 8.14 (d, J ¼ 1.8 Hz, 4H), 8.02 (d, J ¼ 1.6 Hz, 2H), 7.90
d 8.56 (d,
3

Y. Liu, S. Qiu, J. Yu et al.
Tetrahedron 81 (2021) 131869
4.2. Photophysical Properties
The optical properties of the TADF emitters are obtained using
the ultravioletevisible absorption and photoluminescent (PL)
spectra in toluene solution (Fig. 3a). The absorption peaks of both
compounds at around 300 nm can be attributed to the p-p* tran-
sition absorption of carbazole while the peak around 350 nm can be
attributed to the intramolecular charge transfer (ICT) of the D-A
system formed between the electron-donating and electron-
withdrawing group. The initial wavelength of the absorption
spectra of Cz-SO and t3Cz-SO are 367 nm and 406 nm, respectively.
Therefore, the optical energy gaps of the Cz-SO and t3Cz-SO are
estimated to be 2.79 and 2.52 eV according to the absorption edges.
The emission peaks of Cz-SO and t3Cz-SO in toluene solution are
located at 397 nm and 440 nm, respectively. Indicating that the
introduction of the tertbutyl-carbazole group improved the elec-
tron donating properties of the donor group, and making the
spectrum red-shifted. However, the emission spectra in the con-
gested film state are significantly influenced that the emission
peaks of Cz-SO and t3Cz-SO are 424 nm and 446 nm, respectively.
Moreover, Cz-SO and t3Cz-SO in the film state have different de-
grees of red-shift compared to the solution state, which is caused by
Fig. 1. TGA curves of Cz-SO and t3Cz-SO. (inset) Corresponding DSC traces.
coating. It can be seen from the pictures under the microscope that
Cz-SO is not uniformly coated on the surface of the substrate, and
there are numerous cracks and blank areas, indicating that the
solubility and film-forming ability of Cz-SO are poor. In contrast, the
t3Cz-SO film is very smooth and uniform, without any pinholes and
cracks. which is a prerequisite for preparing high efficiency
solution-processed devices [26]. Therefore, the introduction of tert-
butylcarbazole groups on the periphery of carbazole is an effective
way to improve the performance of solution processing of tradi-
tional small molecule TADF materials. In addition, the roughness
the
p-p accumulation in the solid state [27]. Furthermore, from
solution to solid film state, Cz-SO red-shifts 27 nm, while t3Cz-SO
only red-shifts 6 nm, which illustrated that the access of tert-
butylcarbazole inhibits the aggregation of molecules in the solid
state effectively and improves the luminous efficiency of the ma-
terial (see Fig. 4).
The emission spectra in different solvents (petroleum ether,
toluene, THF, acetone, DMSO, 10ꢀ3 mol/L) are determined that the
emission spectra of the compounds show significant solvent po-
larity dependence, and the emission peak has a large red-shift with
the increase of the solvent polarity. This is because both Cz-SO and
t3Cz-SO contain strong electron-withdrawing and electron-
donating group to undergo obvious ICT effect after excitation.
Thus, as the polarity of the solvent increases, the emission peaks
red-shift.
value (RMS) of t3Cz-SO in the range of 5 ꢂ 5
mm measured by AFM
is 0.35 nm, which is lower than that of Cz-SO (0.63 nm). The
excellent film-forming properties and low film roughness indicate
that t3Cz-SO is more conducive to the preparation of solution-
processed OLED devices.
Fig. 2. AFM topographic images of Cz-SO and t3Cz-SO.
4

Y. Liu, S. Qiu, J. Yu et al.
Tetrahedron 81 (2021) 131869
Fig. 3. (a) UVevis absorption and PL emission in toluene solution; (b) PL spectra in film state.
Fig. 4. PL and UV spectra of Cz-SO (a,c) and t3Cz-SO (b,d) in difffferent solvents.
In order to probe the phenomena, a Lippert-Magada diagram is
constructed, as shown in Fig. 5. Which expresses the Stokes shift as
dielectric constant, and a is Onsanger Radius, C is a constant and n
represents the refractive index of the solvent [28]. Related spectral
a function of the solvent polarity parameter
Df (ε, n).
data are listed in Table 1. According to formula (2), the
different solvents can be calculated as
f (petroleum ether) ¼ 0.001,
f (toluene) ¼ 0.013, f (THF) ¼ 0.210, f (acetone) ¼ 0.162,
(DMSO) ¼ 0.286 (see Table 2).
From the data in the table, the plots of f as a function of v with
Df of
D
2ðDmÞ2
D
D
D
Df
Dn
¼
nabs
ꢀ
nf
y
Df ðε; nÞ þ A
(1)
hca3
D
D
the slope values can be calculated to be 21246 and 14457 cmꢀ1 for
Cz-SO and t3Cz-SO (Fig. 5). Accordingly, the calculated Dm values
are 13.64 D and 11.25 D. These results suggest that the ICT effect of
t3Cz-SO is weaker than Cz-SO, which contributes to the spectral
stability in OLED devices. Besides, we test the photoluminescence
spectra of the materials in a certain volume of mixed solvent
(10ꢀ5 mol/L) with different volume ratios of THF and water so as to
explore the luminescence properties of the materials in the
aggregated state. As shown in Fig. 6. Apparently, the luminous
Where
D
f (ε, n) is obtained by the following formula:
ε ꢀ 1
n2 ꢀ 1
D
fðε; nÞ ¼
ꢀ
(2)
2n2 þ 1
2ε þ 1
Here, h is the Planck’s constant, c is the velocity of light, nabs and
nf are the absorption wavelength and emission wavelength,
respectively. Dm means the difference between the dipole moments
of the ground state and the excited state, ε represents the static
5

Y. Liu, S. Qiu, J. Yu et al.
Tetrahedron 81 (2021) 131869
intensity of both compounds increases with the volume fraction
(Fw) of water gradually increasing from 0 to 90%, which means that
Cz-SO and t3Cz-SO have the characteristics of aggregation-induced
luminescence enhancement.
4.3. Electrochemical properties
Using tetrabutylammonium hexafluorophosphate (TBAPF6) as
the supporting electrolyte, the electrochemical characteristics of
the two compounds are studied by CV with CH2Cl2 for anodic
sweeping. Fig. 7 (a) (c) show that both compounds have obvious
oxidation behaviors, which means that they have the potential in
bipolar carrier transporting. Based on the formula (EHOMO ¼ - 4.75 -
Eox), we calculate the highest occupied molecular orbital (HOMO)
energy values of Cz-SO and t3Cz-SO to be ꢀ5.65V and ꢀ5.62V,
respectively. And it is basically consistent with the theoretically
calculated values. Fig. 7b (d) show the cyclic voltammetric curves of
Cz-SO and t3Cz-SO in CH2Cl2 measured at room temperature for
scanning 20 times. It can be seen from the figures that after 20
times of continuous scanning, the curve of Cz-SO appears
Fig. 5. LipperteꢀMataga plot of Cz-SO and t3Cz-SO in various solutions.
Table 1
Spectral properties of Cz-SO and t3Cz-SO in different solvents.
SO-Cz
SOCz-2tBuCz
solvents
Dfa (ε,n)
labsb(nm)
lfc (nm)
D
vd (cmꢀ1
)
labsb(nm)
lfc (nm)
D )
vd (cmꢀ1
n-hexane
toluene
THF
acetone
DMSO
0.001
0.013
0.210
0.162
0.286
249, 295, 335
292, 338
258, 291, 325
292, 324
363
388
458
427
442
2303
3813
8935
7445
7957
296, 348
300, 348
253, 298, 348
293, 348
299, 349
414
440
477
492
518
4581
6008
7771
8410
9348
292, 327
a
D
f is the orientation polarizability parameter of the solvent.
b
C
d
labs is absorption wavelength.
lf is fluorescent wavelength.
D
V is the Stokes shift.
Table 2
displacement, while the curve of t3Cz-SO is relatively stable.
Therefore, t3Cz-SO has stronger stability under continuous voltage
and is more suitable for device preparation.
Physical properties of Cz-SO and t3Cz-SO.
HOMO/LUMOd lab
(eV) (nm)
lfb
lfc
D
EST S1/T1
a
b
e
Td/Tg
(ꢁC)
(nm) (nm) (eV) (eV)
Cz-SO
t3Cz-SO 513/–
278/152 ꢀ5.62/-1.55
292, 338 388
300, 348 440
424
446
0.41 3.54/3.13
0.11 3.02/2.91
4.4. Density functional theory simulation
ꢀ5.08/-1.72
a
Corresponding to 5% weight loss.
Measured in CH2Cl2 solutions.
Measured in neat film.
Determined using cyclic voltammetry.
Obtained from Gauss simulation.
To systematically evaluate the influence of molecular structure
on charge properties, the molecular conformation of Cz-SO and
t3Cz-SO are optimized at the B3LYP/6-31G level by using Gaussian
09 program, and density functional theory (DFT) calculations are
b
c
d
e
Fig. 6. PL spectra of Cz-SO and t3Cz-SO in mixed solvents with different volume ratios of THF and water.
6

Y. Liu, S. Qiu, J. Yu et al.
Tetrahedron 81 (2021) 131869
Fig. 7. (a) (c) Cyclic voltammograms of Cz-SO and t3Cz-SO measured at room temperature in CH2Cl2; (b) (d) Cyclic voltammograms of Cz-SO and t3Cz-SO measured at room
temperature in CH2Cl2 with scanned 20 times.
Fig. 8. DFT calculations of the molecular configurations in the ground states.
carried out on their molecular energy levels. As shown in Fig. 8, the
lowest unoccupied orbital (LUMOs) of the two compounds are
mainly centered on the acceptor group of diphenyl sulfone, the
highest occupied orbital (HOMO) of Cz-SO is distributed on the
carbazole group, while t3Cz-SO is located on the tricarbazole group.
Simultaneously, HOMO and LUMO have a small amount of overlap,
which would lead to an effective reversed inter-system crossing
(RISC) process and achieve a relatively small DEST and higher PLQY.
Moreover, the theoretically calculated HOMO/LUMO energy levels
of Cz-SO and t3Cz-SO are ꢀ5.62 eV/-1.55 eV and ꢀ5.08 eV/-1.72 eV,
respectively. Clearly, the HOMO energy level of t3Cz-SO is 0.54 eV
higher than Cz-SO, while the LUMO energy is slightly shifted down
(0.17 eV). Therefore, the electron donating ability of t3Cz-SO is
stronger than Cz-SO. In addition, time-dependent density
7

Y. Liu, S. Qiu, J. Yu et al.
Tetrahedron 81 (2021) 131869
functional theory (TD-DFT) is performed to calculate the excited
state energy that the S1 levels of Cz-SO and t3Cz-SO are 3.54 eV and
3.02 eV, respectively, and the T1 levels are 3.13 eV and 2.91 eV. Thus,
luminous efficiency of 19.94 cd Aꢀ1, and the maximum power ef-
ficiency of 13.03 lm Wꢀ1 due to the
DEST is smaller. These results
consist with the thermal and optical results. Since t3Cz-SO in-
troduces the tert-butylcarbazole group on Cz-SO, the molecular
weight increases, and performing more excellent film-forming
ability and smaller roughness. Currently, in the process of fabri-
cating solution-processed devices, the flatness of the film has a
decisive influence on the stability and efficiency of the device, and
different annealing temperatures are required according to the
diverse boiling point and volatility of different solvents. Notably,
the annealing temperature during the device fabrication is gener-
ally high, which means that the better thermal and morphological
stability of t3Cz-SO ensure the high device property of the solution-
processed devices. In addition, the bulky spatial structure of t-butyl
effectively inhibits the aggregation induced exciton quenching of
t3Cz-SO, which also play a key role of the high device performance.
the
DEST of 0.41 eV and 0.11 eV was achieved for the two molecules.
Obviously, comparing with Cz-SO, t3Cz-SO has a smaller
D
EST value,
which would lead to an effective reversed intersystem crossing
(RISC) process to achieve delayed fluorescence characteristics.
4.5. Electroluminescent Properties
To estimate the applicability of the two materials as solution-
processed bipolar hosts, we examine the performance of FIrpic-
based single-layer blue electro-phosphorescence devices with the
simple configuration of ITO/PEDOT: PSS (40 nm)/host: FIrpic (1 wt
%, 60 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). Where ITO serves
as the anode, PEDOT and TPBI are used as the hole injection layer
and electron injection layer, respectively. LiF is used as the electron
transport layer, and Al serves as the cathode. A represents the de-
vice with Cz-SO as the host material, while B is the device with
t3Cz-SO as the host, and the structure and relative energy level are
shown in Fig. 9.
The electroluminescence spectrum, current density-voltage-
luminance (J-V-L) curve, the curves of luminous and power effi-
ciency versus brightness are shown in Fig. 10, and the detailed data
of the devices are summarized in Table 3. Both devices show typical
blue emission at around 475 nm that originates from the guest, and
the emission spectra of the devices are all typical FIrpic emission
peaks, indicating a complete energy transfer from the bipolar host
to the gust. Among them, the device A displays a turn-on voltage of
4.0 V, a maximum luminous efficiency of 3.82 cd Aꢀ1, and a
maximum power efficiency of 1.75 lm Wꢀ1. In contrast, device B
with t3Cz-SO as host reveals a turn-on voltage of 3.5V, a maximum
5. Conclusion
In summary, a simple peripheral modification by introducing
tri-tert-butylcarbazole group to the small molecule TADF material
is denmostrated to be an effective strategy to obtain efficient TADF
host material. the t3Cz-SO has higher morphological stability and
enhance film forming ability to enable the fabrication of solution-
processed OLEDs. Calculated triplet energy of the two compounds
are 397 nm and 440 nm, which are higher than the blue emitter
FIrpic to suppress the back energy transfer. Moreover, the
DEST of
t3Cz-SO is 0.11 eV, which is much smaller than Cz-SO. Both com-
pounds show aggregation-induced luminescence in a mixed sol-
vent of THF and water. Thanks to the excellent photophysical
property, the device based on t3Cz-SO achieves a turn-on voltage of
3.5 V, the maximum luminous efficiency of 19.94 cd Aꢀ1, the
Fig. 9. The energy diagram of the devices and the molecule structures.
8

Y. Liu, S. Qiu, J. Yu et al.
Tetrahedron 81 (2021) 131869
Fig. 10. (a) Current density (hollow)ꢀvoltage (J-V)curves; (b) voltageꢀluminance (solid) (VeL) curves; (c) Current efficiency (hollow) as a function of current density; (d) Power
efficiency (solid) as a function of current density; (e) EL spectra of the devices.
Table 3
Device performances of the devices.
CEmaxb (cd Aꢀ1
)
PEmax (lm Wꢀ1
)
Lmaxd (cd mꢀ2
)
a
c
Device
Host
Von (V)
A
B
Cz-SO
t3Cz-SO
4.0
3.5
3.82
19.94
1.75
13.03
3540
15320
a
Turn-on voltage at 1 cd mꢀ2
Maximum current efficiency.
Maximum power efficiency.
Maximum luminance.
.
b
c
d
maximum power efficiency of 13.03 lm Wꢀ1, and the maximum
luminance of 15320 cd mꢀ2, which is higher than the small mo-
lecular Cz-SO. This research demonstrates that the introduction of
tertiary carbazole units on the basis of the Cz-SO is an effective
method for developing high-efficiency electroluminescent
materials.
Notes
The authors declare no competing financial interest.
Declaration of competing interest
The authors declare that they have no known competing
9

Y. Liu, S. Qiu, J. Yu et al.
Tetrahedron 81 (2021) 131869
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financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
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Acknowledgements
We are grateful for the Grants from the National Natural Science
Foundation of China (21805106), the Natural Science Foundation of
Jiangsu Province (BK20181073), the Natural Science Fund for Col-
leges and Universities in Jiangsu Province (17KJB150007), the
Postdoctoral Science Foundation of China (1107040175), and the
Science Foundation of Huaihai Institute of Technology (KQ16025,
Z2016010). The First-class Undergraduate Majors Construction
Program of Jiangsu Province, the Key Discipline Construction Pro-
gram of Jiangsu Province, a Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD). The Postgraduate Research & Practice Innovation Program
of Jiangsu Province (KYCX19_2257, KYCX19_2262). We also thank
for a project funded by the priority academic program development
of Jiangsu Higher Education Institutions (PAPD) and the project
supported by Scientific and Technological Program of Lianyungang
(JC1603, CG1602).
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