A versatile carbazole donor design strategy for blue emission switching from normal fluorescence to thermally activated delayed fluorescence

Article abstract of DOI:10.1016/j.dyepig.2021.109581

The weak electron donating ability of carbazole is most suitable for constructing the donor-acceptor type blue emitters, but its low spatial requirements caused by the five-membered ring bridged structure is not favorable for spatial separation of frontier molecular orbitals. So many carbazole based compounds do not exhibit thermally activated delayed fluorescence (TADF). Herein, a more sterically demanding group was introduced at the 1-site of carbazole to form 1-methylcarbazole (1-MeCz), in order to construct a versatile donor for blue TADF materials. 1-MeCz was used as donor in combination with triazine and pyrimidine as acceptors to design novel compounds 1-MeCz-TRZ and 1-MeCz-Pm. It was observed that the presence of the methyl group at the 1-site of carbazole enhanced the twisted angles and reduced frontier molecular orbital overlapping, successfully switching the blue emission from the normal fluorescence of the methyl-free reference compounds (Cz-TRZ and Cz-Pm) to TADF of 1-MeCz-TRZ and 1-MeCz-Pm. The organic light-emitting diodes of 1-MeCz-TRZ and 1-MeCz-Pm exhibited blue emission at 450 and 458 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.11) and (0.17, 0.18), and external quantum efficiencies of 13.07% and 7.53%. This study provides a versatile, simple and practical design strategy with 1-MeCz to construct pure blue TADF emitters in combination with various acceptors.

Full text of DOI:10.1016/j.dyepig.2021.109581

Dyes and Pigments 194 (2021) 109581
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A versatile carbazole donor design strategy for blue emission switching
from normal fluorescence to thermally activated delayed fluorescence
*
**
Rui Niu, Jiuyan Li , Di Liu , Ruizhi Dong, Wenkui Wei, Houru Tian, Chunlong Shi
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian, 116024, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The weak electron donating ability of carbazole is most suitable for constructing the donor-acceptor type blue
emitters, but its low spatial requirements caused by the five-membered ring bridged structure is not favorable for
spatial separation of frontier molecular orbitals. So many carbazole based compounds do not exhibit thermally
activated delayed fluorescence (TADF). Herein, a more sterically demanding group was introduced at the 1-site
of carbazole to form 1-methylcarbazole (1-MeCz), in order to construct a versatile donor for blue TADF materials.
Blue thermally activated delayed fluorescence
(
TADF)
Methyl-substituted carbazole
Steric hindrance
Organic light-emitting diodes (OLEDs)
1
1
-MeCz was used as donor in combination with triazine and pyrimidine as acceptors to design novel compounds
-MeCz-TRZ and 1-MeCz-Pm. It was observed that the presence of the methyl group at the 1-site of carbazole
enhanced the twisted angles and reduced frontier molecular orbital overlapping, successfully switching the blue
emission from the normal fluorescence of the methyl-free reference compounds (Cz-TRZ and Cz-Pm) to TADF of
1
-MeCz-TRZ and 1-MeCz-Pm. The organic light-emitting diodes of 1-MeCz-TRZ and 1-MeCz-Pm exhibited blue
emission at 450 and 458 nm with Commission Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.11) and
0.17, 0.18), and external quantum efficiencies of 13.07% and 7.53%. This study provides a versatile, simple and
(
practical design strategy with 1-MeCz to construct pure blue TADF emitters in combination with various
acceptors.
1
. Introduction
Nowadays, organic light-emitting diodes (OLEDs) have been highly
group [6], as a novel pathway to achieve 100% IQE by using noble-metal
free pure organic materials, only by means of the upconversion of triplet
excitons to singlet ones through reverse intersystem crossing (RISC)
1
process. A small energy gap between the lowest triplet (T ) and (S )
attractive for solid state lighting and new-generation displays due to
their wonderful performance such as flexibility, lightness and low-cost
production, which can be dated back to the report by Tang and Van
Slyke [1]. The singlet and triplet excitons, originating from electrically
injected holes and electrons, combine in a ratio of 1:3 according to spin
statistics [2,3]. For the OLEDs based on the normal fluorescent emitters,
the internal quantum efficiencies (IQEs) are limited to 25% by dark
triplet states. Most efforts have been devoted to achieving the ideal
1
singlet excited states (ΔEST) is basically required for this mechanism,
which can be achieved by appropriate molecular design. Since then, the
OLEDs based on the pure organic TADF emitters manifest bright appli-
cation prospects because they overcome the intrinsic shortcomings
correlated with the traditional fluorescent and noble metal containing
phosphorescent emitters [7–9].
Generally, TADF features can be obtained by the electron-donor-
1
00% IQEs and improving the external quantum efficiencies (EQEs) by
π
-electron-acceptor (D- -A) frameworks with intramolecular charge
π
activating the non-emissive triplet excitons. Hereafter, the emergence of
transition-metal complex phosphors has extremely enhanced the device
performance by harvesting the triplet excitons for light emission because
of the spin-orbit coupling (SOC) effect [4,5]. However, the commercial
applications of phosphorescent OLEDs (PhOLEDs) are difficult to move
forward due to the utilization of noble metals. Fortunately, thermally
activated delayed fluorescence (TADF) was reported first by Adachi’s
transfer (ICT) effect, with the prerequisite of narrow singlet-triplet en-
ergy splitting (ΔEST) [10]. In recent years, great progresses have been
achieved in green and red TADF OLEDs with EQEs around 30% [11–14].
Pure blue TADF emitters for OLEDs, usually defined as y-axis value of
Commission Internationale de l’Eclairage (CIE) coordinates (CIE
y
) is
above 0.10 with CIEx+y below 0.30, are still the grand challenges at
present, because a broad energy gap between the highest occupied
*
Corresponding author.
** Corresponding author.
E-mail addresses: jiuyanli@dlut.edu.cn (J. Li), liudi@dlut.edu.cn (D. Liu).
https://doi.org/10.1016/j.dyepig.2021.109581
Received 18 April 2021; Received in revised form 18 June 2021; Accepted 18 June 2021
Available online 23 June 2021
0
143-7208/© 2021 Elsevier Ltd. All rights reserved.

R. Niu et al.
Dyes and Pigments 194 (2021) 109581
molecular orbital (HOMO) and the lowest unoccupied molecular orbital
2. Experimental section
(
LUMO) is essential for blue emission, composed by a donor with a deep
HOMO and acceptor with a shallow LUMO. The previous reports
demonstrated that the small ΔEST of TADF emitters could be realized by
the rational spatial separation of HOMO and LUMO, which could rein-
2.1. General information
The H NMR and 13C NMR spectra were measured in CDCl
1
using a
3
force the excitons upconversion process from T
1
to S
1
state via thermal
400 MHz and 126 MHz Bruker Avance II 400 and Bruker AVANCE III
500, respectively. The mass spectra were obtained on a HP1100LC/MSD
MS spectrometer and matrix-assisted laser desorption/ionization
(MALDI) micro mass spectrometry (MS) spectrometer. The fluorescence
at room temperature and transient photoluminescence (PL) spectra were
measured with an Edinburgh FLS1000 fluorescence spectrometer, while
the low temperature fluorescence and phosphorescence spectra were
measured on a Hitachi F-7000 fluorescence spectrometer at 77 K in 2-
methyltetrahydrofuran. The ultraviolet–visible (UV–vis) absorption
spectra measurements were performed on a Perkin-Elmer Lambda 650
spectrometer at room temperature. Thermogravimetric analyses (TGA)
were carried out on a Perkin-Elmer thermogravimeter (Model TGA7)
activation. The electron-rich donor (e.g., bicarbazole derivatives) and
the electron-withdrawing group (e.g., triazine) were directly linked to
reduce the overlap between HOMO and LUMO [15]. Another design
strategy is to increase torsion angle between donor and acceptor to
achieve the effective separation of HOMO and LUMO. Recently, Yang
reported CN-pyridine-based TADF emitters, with double-twist structure,
which exhibited high EQEs of 25.8% in green region and 21.1% in
yellow region [16]. A red TADF emitter with highly twisted D- -A ar-
π
chitecture has been developed by Chi’s group with EQE of 7.13% at 635
nm with high photoluminescence quantum yield (PLQY) of 55% [17]. A
sky blue TADF device was demonstrated a high EQE of 27.8% by Lee’s
group, where the selective twisting of the donor structure design strat-
egy was applied [18].
◦
ꢀ 1
under a nitrogen (N
ferential scanning calorimetry (DSC) studies were performed using a
2
) flow at a heating rate of 10 C min . The dif-
◦
ꢀ 1
Carbazole is weakly electron donating in comparison with other
widely used donors including acridine, phenoxazine, and phenothiazine,
and highly suitable for constructing blue emitters, since the charge
transfer between donor and acceptor should be relatively weak if high
excited state energy and thus blue emission is desired. At the same time,
the planar structure of carbazole is beneficial to suppress the energy loss
caused by the conformation change in the blue OLEDs [19–22]. How-
ever, the five-membered ring bridged structure and the resultant less
steric hindrance of carbazole ring are not favorable for spatial separation
of HOMO and LUMO. As a result, most of the carbazole based com-
pounds do not exhibit TADF due to relatively large ΔEST values. To solve
this problem, there are two common ways to design blue TADF mole-
cules with the modified-carbazole: (a) different electron-donating
groups are introduced to the carbazole ring in order to make the
HOMO dispersed; (b) the bulky units, such as methyl or phenyl, are
grafted on the carbazole at 1-site and/or 8-site, likewise on the phe-
DSC 1/500 at a heating rate of 20 C min under a nitrogen atmo-
sphere. Cyclic voltammetry (CV) measurements were performed using
ꢀ 1
an electrochemical workstation (CHI610E) at a scan rate of 100 mV s
and a conventional three electrode configuration, which contains a
glassy carbon working electrode, a Pt-wire counter electrode, and a
saturated calomel electrode (SCE) reference electrode. All of the CV
measurements were demonstrated under nitrogen-purged in dichloro-
methane (DCM) for the anodic scan and dimethylformamide (DMF) for
the cathodic scan at room temperature, with 0.1 M tetrabutylammonium
hexafluorophosphate as the supporting electrolyte. Absolute photo-
luminescence quantum yields (PLQYs) of the compounds were measured
in doped films on a HAMAMATSU absolute PL quantum yield spec-
trometer (C11347).
2.2. OLED fabrication and measurements
nylene bridge for D-
π
-A type molecules. It is an efficient strategy to tune
The pre-cleaned ITO glass substrates with a sheet resistance of 15 Ω
2
ꢀ
were first treated by UV-ozone for 30 min. Then a 40 nm thick
the light-emitting mechanism with the small ΔEST by the delocalization
of the HOMO and LUMO [23–26].
m
PEDOT:PSS film was first deposited on the ITO glass substrate and baked
◦
Herein, we investigate a general chemical modification of carbazole
to switch the emission mechanism from normal fluorescence to TADF. A
methyl (Me) group was introduced at the 1-site of carbazole (Cz) to
generate the 1-methylcarbazole (1-MeCz), with the expectation that the
strong steric hindrance of the relatively bulky methyl group will facili-
tate the rotation of carbazole ring to reduce the spatial repulsion be-
tween this methyl and the neighbouring group. 1-MeCz was used as
versatile donor to design TADF emitters in combination with triazine
and pyrimidine as acceptors. It is expected that the twist angle between
carbazole donor and the bridge or acceptor will be increased and pure
blue TADF can be achieved. Two novel compounds, namely 1-MeCz-TRZ
and 1-MeCz-Pm, were designed and prepared in this way. The methyl-
free compound, i.e. Cz-TRZ, was prepared as a reference for compari-
son. It was demonstrated that the normal fluorescence of methyl-free
compounds (like Cz-TRZ) was successfully switched to blue TADF of
at 120 C for 30 min in air. Subsequently, the substrates were transferred
into a vacuum chamber to deposit the organic layers with a base pres-
ꢀ 6
sure of less than 10 Torr (1 Torr = 133.32 Pa). A thin layer of LiF (1
nm) and subsequently a thin layer of Al (200 nm) were vacuum
ꢀ 1
deposited as the cathode, with the deposition rates of 0.1 Å s for LiF
ꢀ 1
and 3–5 Å s for Al. The emitting area of each pixel was determined by
2
the overlapping of the two electrodes as approximate 9 mm . The cur-
rent density-voltage-brightness (J-V-B) curves of the devices were
measured with Konica Minolta CS200 and a source-measure-unit (SMU)
Keithley 2400 under ambient conditions at room temperature. The EL
spectra and Commission Internationale de l’Eclairage (CIE) coordinates
of the devices were measured with a PR705 photometer and a SMU
Keithley 236 under ambient conditions at room temperature. The for-
ward viewing external quantum efficiency was calculated using the
current efficiency, EL spectra and human photopic sensitivity.
1
-MeCz-TRZ and 1-MeCz-Pm. These compounds were used as doped
emitters to fabricate blue OLEDs and exhibited acceptable performance.
The 1-MeCz-TRZ device exhibited a maximum current efficiency of
2.3. Compound synthesis
ꢀ 1
1
2.16 cd A and an EQE of 13.07%, almost being twice of the Cz-TRZ
The intermediate 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine
was synthesized according to the literature [27]. 1-Methylcarbazole
(1-MeCz) was synthesized following the literature method [27], and
the synthetic procedure is shown in Scheme 1.
based OLED, at the wavelength of 450 nm owing to the extra contri-
bution of the delayed fluorescence proportion. The 1-MeCz-Pm based
ꢀ 1
TADF-OLED also exhibited a current efficiency of 10.56 cd A and EQE
of 7.53%. Obviously, this is a reasonable and effective strategy to rein-
force the electroluminescence efficiency by methyl group introduction
on the carbazole ring at a sterically demanding site, altering the emitting
mechanism from the normal fluorescence to TADF.
2.3.1. Synthesis of 2-(4-chlorophenyl)-4,6-diphenyl-1,3-pyrimidine
A mixture of 4,6-diphenyl-1,3-pyrimidine (2.00 g, 7.50 mmol), 4-
chlorophenylboronic acid (1.17 g, 7.50 mmol), anhydrous sodium car-
bonate (2.38 g, 22.50 mmol), palladium(II) acetate (17.00 mg, 0.07
mmol) and triphenylphosphine (59.00 mg, 0.22 mmol) were dissolved
2

R. Niu et al.
Dyes and Pigments 194 (2021) 109581
Scheme 1. Chemical structures and synthetic routes of Cz-TRZ, 1-MeCz-TRZ, and 1-MeCz-Pm.
in ethylene glycol dimethyl ether (60 mL). Under nitrogen atmosphere,
3H); 13C NMR (126 MHZ, CDCl
3
) δ 171.88 (s), 170.95 (s), 143.61 (s),
◦
the mixture was stirred at 70 C for 24 h. Upon cooling to room tem-
142.57 (s), 139.59 (s), 136.11 (s), 136.05 (s), 132.72 (s), 129.88 (s),
129.56 (s), 129.06 (s), 128.94 (s), 128.74 (s), 126.00 (s), 124.36 (s),
123.50 (s), 121.42 (s), 120.24 (s), 120.16 (s), 120.10 (s), 118.24 (s),
perature, and reaction solvent was removed under reduced pressure.
Then the crude product was purified by column chromatography on
silica gel using dichloromethane/petroleum (1:20) as eluent to obtain a
110.01 (s), 20.06 (s). TOF-EI-MS (m/z): calcd. for C34
H
24
N
4
488.2001,
+
+
white powder. Yield: 78%. MALDI-TOF-MS (m/z): 343.0997 [M ]. The
found 488.2003 [M ]. Anal. Calcd (%) for C34
N, 11.47; Found: C, 83.54; H, 4.98; N, 11.45.
24 4
H N , C, 83.58; H, 4.95;
comprehensive characterization data are identical to those in the liter-
ature report [28].
◦
1
1-MeCz-Pm: Yield: 72%, M
.96 (d, J = 8.1 Hz, 2H), 8.37 (dd, J = 7.5, 2.1 Hz, 4H), 8.18 (d, J = 7.7
Hz, 1H), 8.10 (d, J = 17.3 Hz, 2H), 7.74–7.56 (m, 8H), 7.46–7.37 (m,
p
202 C. H NMR (400 MHZ, CDCl
3
) δ
8
2
.3.2. General procedure for syntheses of Cz-TRZ, 1-MeCz-TRZ, 1-MeCz-
1
3
Pm
1H), 7.32 (t, J = 7.6 Hz, 1H), 7.27–7.18 (m, 3H), 2.18 (s, 3H). C NMR
(126 MHZ, CDCl
2
-
(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine or 2-(4-chlor-
3
) δ 163.99 (s), 162.76 (s), 141.69 (s), 140.84 (s),
ophenyl)-4,6-diphenyl-1,3-pyrimidine (5.00 mmol), Cz or 1-MeCz (5.50
mmol), tris(dibenzylideneacetone)dipalladium (0.25 mmol), tri-tert-
butylphosphine tetrafluoroborate (0.75 mmol) and sodium tert-butoxide
137.02 (s), 136.34 (s), 129.95 (s), 128.44 (s), 128.31 (s), 127.99 (s),
127.75 (s), 126.32 (s), 124.85 (s), 123.12 (s), 122.29 (s), 120.44 (s),
118.96 (s), 118.94 (s), 118.87 (s), 117.13 (s), 109.56 (s), 109.06 (s),
(
10.00 mmol) were dissolved in dry toluene (150 mL) under N
2
atmo-
18.91 (s). MALDI-TOF-MS (m/z): calcd. for C35
H
25
N
3
487.2048, found
◦
+
sphere. After stirring at 120 C for 24 h under N
2
atmosphere, the re-
487.2065 [M ]. Anal. Calcd (%) for C35
8.62. Found: C, 86.20; H, 5.19; N, 8.59.
25 3
H N , C, 86.21; H, 5.17; N,
action solvent was removed by vacuum distillation, and the residue was
purified by column chromatography on silica gel to afford the crude
product. Repeated recrystallization from methanol/chloroform gave the
3. Results and discussion
pure products as white powders.
1
Cz-TRZ: Yield: 82%. H NMR (400 MHZ, CDCl
3
) δ: 9.06 (d, J = 8.3
3.1. Synthesis and thermal properties
Hz, 1H), 8.89–8.82 (m, 2H), 8.21 (d, J = 7.8 Hz, 1H), 7.85 (d, J = 8.2 Hz,
H), 7.64 (ddt, J = 20.1, 13.6, 6.6 Hz, 4H), 7.49 (t, J = 7.6 Hz, 1H), 7.37
1
The chemical structures and synthetic procedures of these emitters
are shown in Scheme 1. The intermediate 2-(4-bromophenyl)-4,6-
diphenyl-1,3,5-triazine was synthesized according to previously re-
ported methods [27]. 2-(4-chlorophenyl)-4,6-diphenyl-1,3-pyrimidine
was synthesized via Suzuki coupling reaction. These three compounds
were successfully prepared through Buchwald-Hartwig C–N coupling
reaction of the corresponding acceptor intermediate with donor Cz or
1-MeCz in high yields. To meet the requirements for OLED application,
all these emitters were thoroughly purified by silica gel column chro-
matography and repeated recrystallization in methanol/chloroform in
1
3
(
t, J = 7.4 Hz, 1H). C NMR (126 MHZ, CDCl
3
) δ 170.83 (s), 140.63 (s),
39.46 (s), 135.15 (s), 133.98 (s), 131.65 (s), 129.64 (s), 128.02 (s),
27.70 (s), 125.74 (s), 125.15 (s), 122.77 (s), 119.42 (s), 119.40 (s),
1
1
1
4
1
08.92 (s). TOF-EI-MS (m/z): calcd. for C33
H
22
N
4
474.1844, found
+
74.1847 [M ]. Anal. calcd (%) for C33
H
22
N
4
, C, 83.52; H, 4.67; N,
1.81; Found: C, 83.51; H, 4.70; N, 11.83.
◦
1
-MeCz-TRZ: Yield: 75%, M
p
231 C. 1H NMR (400 MHz, CDCl
3
) δ
9
1
.54–8.51 (m, 6H), 8.18 (d, J = 7.7 Hz, 1H), 8.09 (dd, J = 7.4, 1.7 Hz,
H), 7.73–7.55 (m, 7H), 7.46–7.38 (m, 1H), 7.37–7.18 (m, 5H), 2.17 (s,
3

R. Niu et al.
Dyes and Pigments 194 (2021) 109581
view of their good solubility in common organic solvents. 1H NMR
spectroscopy, mass spectrometry and elemental analysis were used to
characterize the molecular structures.
the high T
and moderate T
values indicate good thermal stability of
d
g
these compounds and stable OLEDs can be expected if they are used as
emitters.
The thermal properties of the emitters were investigated by ther-
mogravimetric analysis (TGA) and differential scanning calorimetry
3
.2. Theoretical calculation
(
DSC). As shown by the TGA curves in Fig. S1(a), their thermal
decomposition temperatures (T
d
, corresponding to 5% weight loss) are
◦
The Gaussian 09 program suite was used to simulate the geometry
detected at 386.53 and 334.13 C for 1-MeCz-TRZ and 1-MeCz-Pm,
respectively. During the second heating cycle in the DSC measure-
configuration and frontier molecular orbitals of these molecules, based
on density functional theory (DFT) calculations. The ground states (S
geometries and the lowest excited singlet and triplet states geometries
and T ) were optimized at the B3LYP/6-31G(d) level and time-
0
)
ments (Fig. S1(b)), moderate glass transition temperatures (T
g
) of 99 and
◦
7
1 C were observed for 1-MeCz-TRZ and 1-MeCz-Pm. At the same time,
(
S
1
1
the melting points (T
m
, i.e. M
p
) of these two compounds were found at
◦
dependent DFT (TDDFT) at the same level, respectively. As depicted
in Fig. 1, the HOMOs of all molecules locate mainly on the carbazole
2
31 and 202 C for 1-MeCz-TRZ and 1-MeCz-Pm, respectively. As above,
Fig. 1. Optimized geometries, HOMO and LUMO distributions in the optimized ground states, and the calculated energy levels for Cz-TRZ, 1-MeCz-TRZ, and 1-
MeCz-Pm.
4

R. Niu et al.
Dyes and Pigments 194 (2021) 109581
ring, with a little spreading onto the adjacent phenylene ring, and the
LUMOs are distributed over the central electron withdrawing triazine or
pyrimidine ring and the neighbouring phenyl or phenylene rings. The
overlap degree of the HOMO and LUMO correlates with the steric mo-
lecular conformation and the torsion angles. As shown by the optimized
geometries in Fig. 1 and 1-MeCz-TRZ and 1-MeCz-Pm have larger
dihedral angles among three compounds because of the introduction of
methyl group with tetrahedral structures at the 1-site of carbazole,
which is in accordance with the molecular design principle of TADF
g
respectively. The electrochemical band gaps (E s) are determined as the
potential difference between E or en ds et and Eox multiplied by the electron
charge (e) to be 2.69, 2.65, 2.72 eV for Cz-TRZ, 1-MeCz-TRZ, and 1-
MeCz-Pm, respectively. Evidently, the electrochemical results are
consistent with the theoretical simulated ones.
onset
3.4. Photophysical properties
◦
types. For Cz-TRZ, the donor group has a dihedral angle of 50.77 with
Photophysical properties of three emitters were analyzed using
UV–vis and photoluminescence (PL) spectroscopies in dilute toluene
solutions. As illustrated in Fig. 3, all compounds have a common strong
the phenylene ring. When methyl is introduced at the 1-site of carbazole,
◦
the dihedral angle in 1-MeCz-TRZ increased to 68.60 due to steric
hindrance. As above, 1-MeCz-Pm has a similar dihedral angle between
the carbazole ring and the phenylene bridge. As expected, with the
introduction of the methyl at 1-site of carbazole, the degree of molecular
distortion increased due to the steric hindrance effect, resulting in the
effective separation of HOMO and LUMO. It is predicted that the large
torsion will lead to small ΔEST and efficient RISC, ensuring the realiza-
tion of TADF properties.
absorption peaks at around 285 nm which belong to the
of the aromatic rings including the acceptor and donor units, and the
weak absorption peaks at 320–340 nm could be assigned to the n–
transitions of Cz or 1-MeCz. All compounds exhibited a broad and
structureless absorption band in the wavelength range of 350–400 nm,
which can be attributed to the intramolecular charge transfer (ICT)
transition from the carbazole donor to the acceptor unit. All relevant
data are summarized in Table 1. These compounds showed broad and
featureless fluorescence spectra in dilute toluene solution upon optical
π
–
π
* transitions
π*
3
.3. Electrochemical properties
1
excitation at the absorption maxima (Fig. 3), indicating the S states of
The electrochemical properties of these molecules were investigated
these compounds can be identified as 1CT states [27]. As well, the
emission maximum of 1-MeCz-TRZ is redshifted by approximately 13
nm compared with Cz-TRZ (422 nm) because the electron donating
ability is slightly enhanced by methyl group at 1-site of carbazole. The
fluorescence peaks of 1-MeCz-Pm is centered at 420 nm in toluene so-
lution. Furthermore, the ICT characteristic also can be confirmed by the
gradual bathochromic shift of the fluorescence spectra with increasing
the solvent polarity for each compound, as depicted in Fig. S2. Fig. 3 also
illustrates the low-temperature (LT) fluorescence and phosphorescence
spectra of Cz-TRZ, 1-MeCz-TRZ and 1-MeCz-Pm in a frozen 2-methylte-
trahydrofuran (2-MeTHF) matrix at 77 K. Clearly, the phosphorescence
spectra of Cz-TRZ, 1-MeCz-TRZ and 1-MeCz-Pm are well resolved and
by means of cyclic voltammetry (CV) in oxygen-free dichloromethane
anodic scan) or dimethylformamide (cathodic scan) solutions at a scan
(
ꢀ 1
rate of 100 mV s . The cyclic voltammograms are shown in Fig. 2 and
the related data are summarized in Table 1. On the basis of the oxidation
curves, it is not difficult to figure out that all these compounds reveal an
irreversible oxidation waves that should occur on the same carbazole
moiety of each molecule. It can be discerned that the oxidation of two 1-
onset
MeCz containing compounds occurred at less positive potentials (Eox ),
i.e. Cz-TRZ (1.21 V), 1-MeCz-TRZ (1.15 V), and 1-MeCz-Pm (1.18 V).
The HOMO energies were estimated according to the equation of
onset
EHOMO = ꢀ (E
+ 4.4), to be ꢀ 5.61, ꢀ 5.55, ꢀ 5.58 eV for Cz-TRZ, 1-
ox
MeCz-TRZ, and 1-MeCz-Pm, respectively. It confirms that 1-methylcar-
bazole has greater electron-donating ability than the unsubstituted
carbazole ring. The LUMO energies of these molecules were measured
from the cathodic scan in a similar way. The onset potentials of the
reduction waves (E or en ds et) for the three molecules Cz-TRZ, 1-MeCz-TRZ,
show characteristic vibrational structures, indicating that their T
1
states
3
are LE states. The T energies (E ) of these compounds were determined
1 T
from the short-wavelength onset of LT phosphorescence spectra to be
2.84 eV (Cz-TRZ), 2.76 eV (1-MeCz-TRZ), 2.95 eV (1-MeCz-Pm),
respectively. Similarly, their S
.99 and 3.16 eV for Cz-TRZ, 1-MeCz-TRZ and 1-MeCz-Pm, respectively.
Then ΔESTs were estimated to be 0.36, 0.23 and 0.21 eV for Cz-TRZ,
-MeCz-TRZ and 1-MeCz-Pm, respectively. Delayed fluorescence may
1 S
energies (E ) were calculated to be 3.20,
2
and 1-MeCz-Pm are close to each other (ꢀ 1.48, ꢀ 1.50 and ꢀ 1.54 V).
onset
According to the equation ELUMO = ꢀ (E
+ 4.4), the LUMO levels of
red
1
these compounds were estimated to be ꢀ 2.92, ꢀ 2.90, and ꢀ 2.86 eV,
be possible for 1-MeCz-TRZ and 1-MeCz-Pm since the relatively small
ΔEST is the prerequisite to realize efficient RISC process.
To better understand the light emitting mechanism of these com-
pounds, the transient PL characteristics were analyzed for the doped
films of these emitters in bis[2-(diphenylphosphino)phenyl]ether oxide
(
DPEPO) host at a doping concentration of 10 wt%. Transient PL decay
curves of the doped films are shown in Fig. 4 and Fig. S3. Notably,
prompt and delayed fluorescence components are clearly identified in
doped films of 1-MeCz-TRZ and 1-MeCz-Pm. The lifetimes of the delayed
components of 1-MeCz-TRZ and 1-MeCz-Pm doped films were deter-
mined as 24.81 and 159.91 s, respectively. Conversely, the Cz-TRZ film
μ
did not exhibit any delayed fluorescence component, which should be
because of its relatively large ΔEST. The reference compound of 1-MeCz-
Pm, i.e. Cz-Pm, was ever reported in literature that it is only the tradi-
tional fluorescence compound [29]. Moreover, the temperature depen-
dent transient PL decay was monitored from 100 to 300 K on the doped
films to confirm the TADF nature for the two emitters. As depicted in
Fig. S3, the intensity of the delayed fluorescence increased with
increasing temperature gradually, especially in the short time range (e.g.
in the first 100
μ
s range), indicating the long lifetime fluorescence
components can be explicitly attributed to the TADF mechanism. Un-
desirably, 1-MeCz-TRZ and 1-MeCz-Pm both have a small proportion of
delayed components, and their ratio of the prompt fluorescence (PF) to
the delayed fluorescence (DF) is to be 5:1 and 20:1, respectively. The PL
Fig. 2. Cyclic voltammograms measured in dilute DCM (anodic) and DMF
cathodic) solutions.
(
5

R. Niu et al.
Dyes and Pigments 194 (2021) 109581
Table 1
Basic parameters of the compounds.
a
b
c
g
d
d
e
f
Compounds
λ
abs/λem [nm]
HOMO/LUMO [eV]
E
[eV]
E
T
[eV]
ΔEST [eV]
τ
PF
/τ
DF [ns/
μs]
ΦPL [%]
Δλ [nm]
Cz-TRZ
280,325, 340,365/422
285,326, 340,361/435
289,330, 366/420
ꢀ 5.61/-2.94
ꢀ 5.54/-2.97
ꢀ 5.53/-2.81
2.67
2.57
2.72
2.84
2.76
2.95
0.36
0.23
0.21
7.42/-
38.70
51.30
42.10
57
74
54
1
1
-MeCz-TRZ
-MeCz-Pm
8.86/24.81
16.74/159.91
a
b
c
d
e
f
Absorption and fluorescence peak wavelengths in toluene solutions.
Determined from cyclic voltammetry measurements.
E
g
= ELUMO - EHOMO
.
Estimated from the short-wavelength onset of the fluorescence/phosphorescence spectra at 77 K in frozen 2-MeTHF, ΔEST = E
S T
- E .
Average lifetimes of prompt and delayed fluorescence in doped films.
Δλ: Stokes shift, calculated as the wavelength difference of the charge transfer absorption and fluorescence bands in absorption and fluorescence spectra in toluene
solution at room temperature.
Fig. 3. UV–vis absorption and PL spectra of Cz-TRZ, 1-MeCz-TRZ and 1-MeCz-Pm in dilute toluene solutions at room temperature, and their LT fluorescence and
phosphorscence spectra in frozen 2-MeTHF at 77 K.
Fig. 4. Transient PL decay curves for the films of 1-MeCz-TRZ (a) and 1-MeCz-Pm (b) doped in DPEPO host (10 wt%) at 298 K.
spectra of 1-MeCz-TRZ and 1-MeCz-Pm in doped films (Fig. S4, Fig. S5)
are slightly red-shifted to those in dilute toluene solutions. The fluo-
rescence peaks of these films are centered at 449 and 456 nm, probably
due to the high polarity of DPEPO [30]. In comparison with the normal
fluorescence of the two reference compounds Cz-TRZ and Cz-Pm,
1-MeCz-TRZ and 1-MeCz-Pm were proved to have TADF character. It
is evident that with the incorporation of methyl at the 1-site of carba-
zole, the blue emission was successfully switched from the normal
fluorescence to TADF.
The absolute PL quantum yields (ΦPL) of these emitters in doped
6

R. Niu et al.
Dyes and Pigments 194 (2021) 109581
films (10 wt% in DPEPO) were measured as 38.70%, 51.30%, and
1
emitters, even resulting in the nonradiative decay of T state exacerba-
4
2.10% for Cz-TRZ, 1-MeCz-TRZ and 1-MeCz-Pm, respectively. Ac-
tion and the poor device performance. Apparently, 1-MeCz-Pm has
much lower kISC and kRISC values than 1-MeCz-TRZ, which should be
responsible for its longer delayed fluorescence lifetime in doped film.
cording to the equations 1-8 listed in the supporting information, the
exciton dynamics of the key transitions during the TADF process for
TADF emitters were further analyzed and summarized in Table S1. As
r
depicted, the radiative rate constants (k ) of 1-MeCz-TRZ and 1-MeCz-
3
.5. Electroluminescent devices
Pm in doped films are higher than their intersystem crossing rate con-
stants (kISC), indicating the singlet excitons that are thermally up-
To investigate the electroluminescence (EL) performance of the three
converted from T
remaining excitons go back to T
constants (kRISC) of the TADF emitters were determined as 4.85 × 10
1
state can generate delayed fluorescence before the
emitters, the multilayered devices were fabricated with the structure of
ITO/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:
PSS, 40 nm)/1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane
1
. The reverse intersystem crossing rate
4
ꢀ 1
4
ꢀ 1
S
and 0.65 × 10 S , respectively, which are far below their inter-
system crossing rates (kISC). The difficult RISC process from T to S lead
to the relatively long delayed fluorescence lifetimes for these two TADF
(
TAPC, 20 nm)/1,3-di(9H-carbazol-9-yl)benzene (mCP, 10 nm)/emitter:
1
1
DPEPO (10 wt%, 20 nm)/DPEPO (10 nm)/1,3,5-tri[(3-pyridyl)-phen-3-
yl]benzene (TmPyPB, 40 nm)/LiF(1 nm)/Al (200 nm) (devices D1 - D3
Fig. 5. (a) Device architecture and energy diagram for the vacuum deposited OLEDs and chemical structures of the relevant materials used for device fabrication, (b)
J-V-B curves, (c, d) efficiency curves, and (e) EL spectra of blue OLEDs D1-D3.
7

R. Niu et al.
Dyes and Pigments 194 (2021) 109581
for Cz-TRZ, 1-MeCz-TRZ and 1-MeCz-Pm, respectively). Therein,
PEDOT:PSS played the role of hole injection layer due to its shallow
HOMO level (ꢀ 5.2 eV). TAPC and TmPyPB were used as the hole
transporting layer (HTL) and electron transporting layer (ETL). mCP was
selected as the electron blocking layer (EBL), and DPEPO was utilized as
the host matrix and hole blocking layer (HBL) on the basis of their high
Table 2
EL performances of devices D1-3.
a
a
b
Devices
V
on
η
CE
η
PE
B
max
η
ext
CIE (x, y)
λ
EL
(nm)
(V)
(cd
(lm
ꢀ 1
(cd
(%)
ꢀ
1
ꢀ 2
A
)
W
)
m )
D1
D2
D3
4.0
3.9
4.4
4.86
3.46
9.11
7.07
3246
596
7.61,
6.63
(0.15,0.07)
(0.15,0.11)
(0.17,0.18)
442
450
458
E
T
of mCP and DPEPO (mCP, E
T
= 2.9 eV; DPEPO, E = 3.1 eV) [30,31].
T
12.16
10.56
13.07,
The chemical structures and energy levels of these materials are shown
in Fig. 5a, and all the key device performance parameters are summa-
rized in Table 2. Three devices turned on (Von, to deliver a luminance of
4
.88
7.53,
.69
817
5
ꢀ 2
1
cd m ) at moderate voltages of 3.9–4.4 V. The J-V-B characteristics
a
The efficiencies are maximum values.
and efficiency curves of these devices are depicted in Fig. 5. As shown by
the EL spectra, the TADF-OLEDs based on 1-MeCz-TRZ and 1-MeCz-Pm
exhibited blue emission with peak wavelengths at 450 and 458 nm and
CIE coordinates of (0.15, 0.11) and (0.17, 0.18) respectively.
b
ꢀ 2
Order of measured values: maximum, then at 100 cd m
.
Declaration of competing interest
As compared with Cz-TRZ based device D1, 1-MeCz-TRZ based de-
vice D2 revealed the superior performance with a maximum current
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.
ꢀ 1
ꢀ 1
efficiency (
η
CE) of 12.16 cd A , a power efficiency (
η
PE) of 9.11 lm W
,
and a maximum external quantum efficiency (
η
ext, EQE) of 13.07%,
which are all two times as high as reference device D1. This is reasonable
because the TADF character of 1-MeCz-TRZ has additional contribution
to the superior efficiency by means of harvesting the electrogenerated
triplet excitons for light emission in OLEDs. For another TADF emitter 1-
Acknowledgments
We thank the National Natural Science Foundation of China
(22078051 and U1801258), the Fundamental Research Funds for the
Central Universities (DUT19ZD217, DUT19TD28), the Open Funds of
State Key Laboratory of Fine Chemicals (KF1901), and the Open Fund of
the State Key Laboratory of Luminescent Materials and Devices (South
China University of Technology, 2021-skllmd-03) for financial support
of this work.
ꢀ 1
MeCz-Pm, device D3 showed the maximum
η
CE of 10.56 cd A and
η
PE
ꢀ
1
of 7.07 lm W , corresponding to a maximum ext of 7.53%. It should be
η
noted that the device efficiencies of both 1-MeCz-TRZ and 1-MeCz-Pm
still have opportunity to be optimized, the relatively low proportion of
DF in the overall emission for both two TADF emitters should be one of
the main reasons responsible for their moderate performance. In addi-
tion, the slightly inferior efficiency of 1-MeCz-Pm device in comparison
with 1-MeCz-TRZ may be because of the rather long DF lifetime of 1-
Appendix A. Supplementary data
MeCz-Pm (159.91 s), which is definitely not favorable for an efficient
μ
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109581.
radiation process. Therefore, it is still challenging to explore as many
acceptors to construct blue TADF emitters to realized pure blue color
and high efficiencies using 1-methylcarbazole as donor.
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