A bipolar triphenylamine-dibenzothiophene S, S-dioxide hybrid compound for solution-processable single-layer green OLEDs and as a host for red emitters

Article abstract of DOI:10.1039/c9nj00537d

A novel bipolar compound triphenylamine-dibenzothiophene S,S-dioxide (TPAFSO) based on a dibenzothiophene-S,S-dioxide (DBTO) moiety and a triphenylamine (TPA) unit, in which the TPA unit is linked at its 4-position to the DBTO group at its 3,7-position, i

Full text of DOI:10.1039/c9nj00537d

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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A Bipolar Triphenylamine-Dibenzothiophene S,S-Dioxide Hybrid
Compound for Solution-Processable Single-Layer Green OLEDs
and as a Host for Red Emitters
Received 00th January 20xx,
Accepted 00th January 20xx
Qinglin Jiang,a Yuwei Xu,a Tiancheng Yu,a Xu Qiu,a Ruiyang Zhao,b Duokai Zhao,a Nan Zheng,a Dehua
Hu,*a Zengqi Xiea and Yuguang Ma*a
DOI: 10.1039/x0xx00000x
A novel bipolar compound triphenylamine-dibenzothiophene S,S-dioxide (TPAFSO) based on a dibenzothiophene-S,S-
dioxide (DBTO) moiety and a triphenylamine (TPA) unit, in which the TPA unit is linked at its 4-position to the DBTO group
at its 3,7-position, is designed and synthesized. The experimental results clearly show that TPAFSO has a high
photoluminescence efficiency of 0.52 in film form and good/balanced carrier injecting properties, as well as an adequate
triplet energy (ET) of 2.1 eV to sensitize red phosphor. Hence, solution-processable single-layer green OLEDs using TPAFSO
as emitters give a maximum luminous efficiency (LE) of 4.7 cd A-1 and a maximum brightness of 5952 cd m-2 with a low turn-
on voltage of 2.5 V, which is superior to the performance of double-layer devices. Furthermore, taking the TPAFSO as a
solution-processed host for red emitters, 5% bis(2-(2’-benzothienyl)-pyridinato-N,C3’)iridium(acetylacetonate)
(Ir(btp)2(acac))-doped single-layer devices give a maximum quantum yield of 3.9% and a saturated red emission with CIE
coordinates of (0.68, 0.32), which are much better than that of Ir(btp)2(acac)-doped poly(N-vinylcarbazole)- 2-tert-
butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK-PBD) single-layer devices. Additionally, the red OLEDs using 4-
(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran (DCJTB)-doped TPAFSO as the emitting layer
obtain a maximum LE of 1.8 cd A-1, and can be comparable to the performances of DCJTB/Alq3-based multilayer devices via
an evaporation process.
Furthermore, it is more challenging for solution-processing
methods to construct multilayer devices because the deposition
of one layer frequently dissolves the layer beneath it.14-16 To
simplify the fabrication process and cut down the cost, a
solution-processable simple device structure, for instance,
double-layer devices, especially single-layer devices, has been
desired and actively pursued, without sacrificing device
efficiency.17
Introduction
Organic light-emitting diodes (OLEDs) have attracted both
scientific and industrial interest for their applications to the next
generation of flat-panel displays and solid-state lighting
sources.1-6 To balance the holes and electrons injected into the
devices, state-of-the-art OLEDs are generally based on
sandwich-like multilayer configurations, in which the involved
materials play a respective role,7-9 covering the active light-
emitting layer and charge carrier injecting/blocking and
transporting layers. However, despite OLEDs with multilayer
architectures usually possess highly efficiency, their
complicated device structures inevitably cause a longer
production cycle and increase the fabrication cost, which are
stumbling block for the wide commercialization of OLEDs.10,11 In
addition, the presence of heterojunction interfaces in a stack of
multiple functional layers is detrimental to the long-term
stability of devices, because of the accumulation of charges and
the generation of higher electric fields at the interfaces.12,13
To this end, the design and synthesis of new multifunctional
luminescent materials are necessary. The device can be
simplified to a two-layered structure if one of the carrier-
transporting
materials
also
possesses
emission
characteristics.18,19 Zhao et al. reported an n-type light emitter
featuring aggregation-induced emission characteristics.19
Double-layer OLEDs, in which the materials work as both light
emitters and electron transporters, exhibit better performances
than triple-layer devices.20 In particular, highly efficient single-
layer devices are likely to be achieved if bipolar luminous
materials, which have a unique capacity for accepting and
transporting both holes and electrons, are developed. Zhang et
al. reported a highly efficient solution-processed single-layer
green OLED based on the bipolar emitter TCBzC, which is
composed of four alkyl-linked peripheral carbazole groups and
an emissive rigid core of 2,1,3-benzothiadiazole.21 The single-
layer devices exhibited a high luminous efficiency of 5.7 cd A-1,
but a high turn-on voltage of 3.5 V resulting from the high
ionization potential of the carbazoles. These ambipolar
a. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory
of Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510640, P. R. China E-mail: msdhhu@scut.edu.cn; ygma@scut.edu.cn
b. College of Chemical Engineering, Qingdao University of Science and Technology,
Qingdao 266042, P. R. China.
Qinglin Jiang and Yuwei Xu contributed equally.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI:10.1039/x0xx00000x

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materials can increase the charge recombination rate and broad In general, bipolar materials are constructed by D and A units
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DOI: 10.1039/C9NJ00537D
charge recombination zones, offering high efficiencies, as well with a certain covalent linkage. The D and A units can endow
as low efficiency roll-off at high current densities.22-26 the materials with carrier injecting and/or transporting
Furthermore, they can be employed not only as the emitters but properties; however, this tailoring also leads to low PL efficiency
also the host materials for the dopants.
due to the formed pure CT characteristic. Thus, to develop
An effective strategy for designing bipolar materials is to bipolar multifunctional materials, not only to be used as
appropriately integrate both electron-donating (D) and emitters but also as universal hosts for phosphors and
electron-accepting (A) units into one molecule.27 In these fluorescence dopants, a tactic should be followed where the
bipolar compounds, the highest occupied molecular orbital HOMO and LUMO can be allowed distribution mainly on the D
(HOMO) and lowest unoccupied molecular orbital (LUMO), and A moieties, respectively. However, a moderate orbital
responsible for transporting holes and electrons, are usually overlap is also necessary to ensure a high value of oscillator
separated, and an intramolecular charge transfer (CT) strength. The molecular structure and electronic density
interaction occurs between the donor and acceptor moieties. distribution of TPAFSO are shown in Fig. 1. The electronic
To reduce both hole and electron injection barriers from the structure of the frontier molecular orbitals in the ground state
anode and cathode, strong D and A units have been commonly simulated by the density functional theory (DFT) using Gaussian
used to construct bipolar host materials because of their 09 serious programs with B3LYP/6-31G(d) basis set. Two
intrinsic low ionization potentials and high electron affinities for triphenylamine units are linked at their 4-position to the DBTO
easily accepting charges.28-32 However, suitable D-A pairs should core at its 3,7-position. As expected, theoretical calculations
be selected carefully while the mode to link should also be show that the HOMO is distributed on the entire molecule and
optimized to avoid the strong CT interaction between them, localized primarily on the peripheral TPA units, and the LUMO
guaranteeing the reasonably high triplet energies for phosphors predominantly lies on the central DBTO core. Due to the small
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and high photoluminescence (PL) efficiency for emitters.33-35
In this study, a novel bipolar compound triphenylamine- DBTO and the DBTO core, a slight overlap occurs between the
twist angle between the phenyl rings of TPA adjacent to the
dibenzothiophene S,S-dioxide (TPAFSO) based on
dibenzothiophene-S,S-dioxide (DBTO) moiety and
a
a
HOMO and LUMO. This endows TPAFSO with high PL
efficiencies, as discussed later, of 0.96 in dichloromethane and
triphenylamine (TPA) unit is designed and synthesized. DBTO 0.52 in film form, which enables it to be a good emitter and a
has been demonstrated to be a strongly electron-withdrawing host for fluorescence dopants.
group with a high electron affinity,36-38 while the TPA unit is well
known as a hole-transporting functional group and matches
well with the work function of the anode (i.e. indium tin oxide
(ITO)). Although analogous molecules have been reported, the
TPA unit is always connected at its 4-position with the DBTO
group at its 2,8-position.39 In this case, the HOMO and LUMO
are completely separated, giving rise to a low PL efficiency.28,39
Fig. 1 Molecular structure of TPAFSO and electronic density
In this work, we optimize the style of connection between the
distribution of its HOMO and LUMO.
TPA unit and the DBTO group, namely, the TPA unit is linked at
Photophysical properties
its 4-position to the DBTO group at its 3,7-position. Besides,
theoretical calculations show an incomplete separation
between the HOMO and LUMO, which endows the TPAFSO with
a high PL efficiency and an adequate triplet energy, ET. Solution-
processable single-layer green OLEDs using TPAFSO as an
emitter give a maximum CE of 4.7 cd A-1 and a maximum
brightness of 5952 cd m-2 with a low turn-on voltage of 2.5 V,
which is superior to the performance of double-layer devices.
Fig. 2 (a) Normalized absorption (abs) and PL spectra of TPAFSO
Taking the TPAFSO as a host for red emitters, 1 wt.% 4-
(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-
in toluene solution (10-5 M, red line) and in film form (spin-
coated from the chloroform solution with a concentration of 10
enyl)-4Hpyran (DCJTB)-doped red OLEDs achieve a maximum
mg mL-1, blue line) at room temperature, and the low-
external quantum efficiency (EQE), brightness and LE of 1.2%,
temperature (77 K) phosphorescence spectra of TPAFSO in
1966 cd m-2 and 1.8 cd A-1, respectively. Deep red devices doped
tetrahydrofuran solution (10-5 M, dark line). (b) PL spectra of
TPAFSO measured in different solvents with increasing polarity.
with
5
wt.%
bis(2-(2’-benzothienyl)-pyridinato-
N,C3’)iridium(acetylacetonate) (Ir(btp)2(acac)) display
a
maximum EQE of 3.9% and a maximum LE of 2.6 cd A-1 with CIE
coordinates of (0.68, 0.32).
The absorption and PL spectrum of TPAFSO in toluene solution
and in film form at room temperature are shown in Fig. 2(a),
and the data are listed in Table 1. The high-energy absorption
band at ~298 nm is assigned to the π-π* and n-π* transitions of
the conjugated aromatic segments, and the 403 nm absorption
peak at a longer wavelength can be ascribed to CT transitions
Results and discussion
Molecular design
2 |New J. Chem., 2019, 00, 1-3
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Table 1 Photophysical and electrochemical properties of TPAFSO.
c
d
f
λabsa [nm]
λemb [nm] (Φf [%])
Tg /Tc /Tme/Td
HOMO/LUMOg
toluene
n-hexanete
438(81)
THF
ACN
553(55)
film
[°C]
[eV]
298, 403
498(87)
524(52) 142/215/254/481
5.24/2.61
a
b
c
d
e
f
Absorption peak. Emission peak. Glass transition temperature. Crystallization temperature. Melting temperature.
Decomposition temperature (5% weight loss). g Measured by CV.
from the peripheral TPA unit to the DBTO core.19 The PL of the 2,8-position of DBTO group (0.08 in acetonitrile solution),
TPAFSO in toluene solution exhibits a maximum emission peak which may be attributed to the large overlap of HOMO and
at 459 nm with an inconspicuous shoulder at ~ 483 nm. The LUMO in TPAFSO.18
absorption of TPAFSO in film form is similar to that in toluene
solution, only with a slight broadening of the absorption band,
Thermal properties
DSC and TGA were carried out to examine the thermal
properties of TPAFSO, as shown in Fig. 3. The DSC plot shows
that TPAFSO exhibits a glass transition temperature (Tg) of
142 °C, a crystallization temperature (Tc) of 215 °C and a melting
temperature (Tm) of 254 °C. In addition, the TGA plot gives a
decomposition temperature (Td, corresponding to a 5% weight
loss) of 481 °C. The excellent thermal properties with high Tg
and Td enable the film to keep a stable morphology under the
operation of devices, which is absolutely vital for device
stability.42
while its PL spectra red shift to longer wavelengths with a
maximum emission peak at 524 nm, which may be partially
attributed to aggregation in the solid state. In fact, the PL
feature of TPAFSO in the film is similar to that in the
dichloromethane solution, indicating that the film state has a
similar polar environment to the dichloromethane solution. The
normalized phosphorescence spectrum of TPAFSO in
tetrahydrofuran solution at 77 K is also shown in Fig. 2(a). The
phosphorescence spectrum exhibits two peaks at 597 and 637
nm, which correspond to the vibronic 0-0 and 0-1 transitions
between these two electronic states, respectively. By taking the
highest-energy peak as the transition of T1
→ S0, the ET of
TPAFSO is estimated to be 2.1 eV, which is high enough to
sensitize red phosphor, such as Ir(btp)2(acac) (ET = 2.0 eV).40
We measured its PL spectra in different solvents with
different polarities to verify the CT characteristic of TPAFSO. As
shown in Fig. 2(b), the PL spectra of TPAFSO exhibit a significant
solvatochromism effect, namely, the emission maximum (λem
)
shifts towards longer wavelengths and the full width at half
Maximum (FWHM) becomes larger as the solvent polarity
increases. Compared to the emission spectrum in n-hexane
solution, the λem shows a large red shift of 60 nm with a FWHM
of 92 nm in tetrahydrofuran solution, while a larger red shift of
115 nm is found with a FWHM of 122 nm in acetonitrile solution.
This large solvatochromic shift, which is consistent with the
large dipole moment of the CT state, indicates that the low-lying
excited state (S1) of TPAFSO possesses a strong CT state
characteristic.33,41 With increased solvent polarity, a strong
interaction occurs between the solvent field and the CT excited
state, increasing the proportion of CT excited states in the S1
excited state and reducing the PL efficiency (i.e. 0.52 in
acetonitrile solution). However, this value is also much larger
than that of the analogous material with TPA unit connected at
Fig. 3 DSC and TGA trace of TPAFSO measured at a heating rate
of 10 °C min-1 under N2.
Electrochemical properties
To probe the electrochemical properties of the synthesized
TPAFSO, CV has been performed in a three-electrode cell setup
with 0.1 M n-Bu4NPF6 as the supporting electrolyte and
ferrocene as the internal standard (Fig. 4). The HOMO and
LUMO energy levels were calculated from the onset potentials

(0.891훾
)
(3)
휇 = 휇0exp
(훾 퐸)
√
(2)
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for oxidation
ferrocene/ferrocenium reference) to be 5.24 and 2.61 eV,
respectively, which match well with the work functions of
commonly used electrodes, for example, 5.2 eV for PEDOT:PSS-
coated ITO anodes and CsF/Al composite cathodes. Therefore,
it is expected that the TPAFSO will behave well in a single-layer
device as an emitter or host as a result of the small injection
barrier from the electrodes.
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Fig. 5 (a) Current density-voltage (J-V) curves for the hole- and
electron-only devices. (b) Electric field-dependent hole and
electron mobilities (μ).
In order to verify the bipolar property of TPAFSO, we
prepared single-layer devices with
a
structure of
ITO/PEDOT:PSS (40 nm)/TPAFSO (80 nm)/CsF (1.5 nm)/Al (120
nm) and bilayer devices based on the structure of
ITO/PEDOT:PSS (40 nm)/TPAFSO (80 nm)/(1,3,5-tris(2-N-
phenylbenzimidazolyl) benzene) (TPBi) (40 nm)/CsF (1.5 nm)/Al
(120 nm), where PEDOT:PSS is typical hole injection material
while TPBi functions as the electron-transporting and hole-
blocking layer. The J-V-B and LE-EQE-J characteristics are
depicted in Figs. 6(a) and (b). It is worth noting that the
performance of single-layer devices are superior to that of
bilayer devices, namely, the former displays a lower turn-on
voltage of 2.5 V, a higher luminance of 5952 cd m-2 and an equal
luminous efficiency of 4.7 cd A-1. In particular, the single-layer
devices show a smaller efficiency roll-off when the current
density increases, which is of significance because the
multilayer device architectures are necessary to avoid
catastrophic efficiency roll-off in high luminance. Furthermore,
the EL maximum peak of the single-layer device is located at 520
nm (Fig. 6(c)), which is blue-shifted by just 4 nm in comparison
with its PL in film form (Fig. 2(b)). However, a large red shift from
the PL to EL spectrum commonly occurs in most organic light-
emitting molecules, which is ascribed to space-charge
accumulation induced by the imbalance in charge transport.20,43
It should be noted that the EL of TPAFSO is red-shift as
compared with the analogous material with TPA unit connected
at the 2,8-position of DBTO group, resulting in a pure green
emission with CIE coordinates of (0.28, 0.55), while the reported
Fig. 4 CV of TPAFSO with oxidation tested in CH2Cl2 and
reduction tested in DMF.
Electroluminescence properties
To evaluate the carrier injection and transport properties of
TPAFSO, hole-only devices with a structure of ITO/poly(3,4-
ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)
(40 nm)/TPAFSO (120 nm)/MoO3 (10 nm)/Al (100 nm) and
electron-only devices with
a structure of ITO/ZnO (20
nm)/TPAFSO (120 nm)/CsF (1.5 nm)/Al (100nm) were fabricated.
Molybdenum trioxide (MoO3) and zinc oxide (ZnO) were utilized
to block electron and hole injection from the cathode and
anode, respectively. Fig. 5 shows the current density versus
voltage characteristics in the hole- and electron-only devices.
The SCLC characteristics can be described by the following
Mott-Gurney equation:
2
퐽 = 89 휀0휀푟휇 퐸
(1)
퐿
where 휀0 is the free-space permittivity (8.55 * 10-14 C V-1 cm-1),
휀푟 is the relative dielectric constant (assumed to be 3.0 for
organic semiconductors), E is the electric field, and L is the
thickness. In general, the carrier mobility (μ) of organic
semiconductors depends on the electric fields (E), which can be
expressed via the Poole-Frenkel formula:
analogous material is
a blue-green emission with CIE
coordinates of (0.16, 0.40).18 The slighter efficiency roll-off and
smaller shift of the EL spectrum in the single-layer device offer
further proof that TPAFSO is an ambipolar transport material. In
addition, a low turn-on voltage of 2.5 V indicates the HOMO and
LUMO levels of TPAFSO are in good alignment with the work
functions of the electrodes, which makes TPAFSO adequate for
single-layer devices.
where
γ is the Poole-Frenkel factor and 휇0 is the zero-field
mobility. According to Equations (1) and (2), the current density
(J) of SCLC region can be further expressed as:
푉2
J = 89 휀0휀푟 휇0exp
퐿3
푉
√
퐿
By fitting the J-V curves in SCLC region to Equation (3), the 휇0
(4.73 × 10-7 and 2.46 × 10-7 cm2 V-1 s-1 for hole mobility and
In view of the bipolar carrier transport properties, high
fluorescence quantum yields, adequate triplet energy and good
energy level matching with both electrodes, TPAFSO was
utilized as host materials for red dyes, including phosphor
(Ir(btp)2(acac)) and fluorophor (DCJTB), to fabricate single-layer
red OLEDs devices based on the structure of ITO/PEDOT:PSS (40
nm)/TPAFSO: dopant (80 nm)/CsF (1.5 nm)/Al (120 nm). The EL
spectra of red dopant-doped devices are shown in Fig. 6(f).
Ir(btp)2(acac) is a commonly used dark red iridium complex, and
electron mobility, respectively) and
훾 values are obtained, thus
generating the field-dependent electron mobility according to
Equation (2). At an electric field of 4 × 104 V cm-1, the hole
mobility is 1.83× 10-6 cm2 V-1 s-1 and the electron mobility of
TPAFSO is 4.20 × 10-7 cm2 V-1 s-1 under the same condition,
indicating that the TPAFSO is a bipolar transporting material.
4 |New J. Chem., 2019, 00, 1-3
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the multilayer devices with 4% Ir(btp)2(acac)-doped CBP Ir(btp)2(acac)-doped
poly(N-vinylcarbazole)-View Artic2le-Otenlrinte-
exhibited a peak quantum efficiency of 6.6%.39 As described in butylphenyl-5-biphenyl-1,3,4-oxadiazoleDOI(:P1V0.K10-P39B/DC9)NJ0s0i5n3g7lDe
Figs. 6(d) and (e), when the Ir(btp)2(acac) was doped into devices.44 Meanwhile, the DCJTB, an archetypical red dopant,
TPAFSO at a concentration of 5%, the solution-processable was doped into TPAFSO by 1% to construct single-layer red
single-layer devices gave a maximum quantum yield of 3.9% and OLED devices with a maximum LE of 1.8 cd A-1, close to 1.95 cd
a saturated red emission with CIE coordinates of (0.68, 0.32) A-1 for 2% DCJTB-doped Alq3 devices.45
(Table 2). These performances are better than that of
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Fig. 6 (a) Current density-brightness-voltage (J-B-V) and (b) luminous efficiency-external quantum efficiency-current density (LE-
EQE-J) characteristics of the single-layer and double-layer green devices based on TPAFSO. (c) Normalized EL spectrum of single-
layer and bilayer green devices based on TPAFSO. (d) J-B-V and (e) LE-EQE-J characteristics of single-layer doped red devices. (f)
Normalized EL spectrum of single-layer doped red devices.
Table 2 Performance of green and red OLEDs.
b
Lmax
EQEc
(%)
LEd
CIEe
(x,y)
Device
Single layer
Double layer
5% Ir(btp)2(acac)
1% DCJTB
Vona (V)
2.5
(cd m-2)
(cd A-1)
5952
3117
1971
1.9
2.0
3.9
1.2
4.7
4.7
2.6
1.8
(0.28,0.55)
(0.31,0.55)
(0.68,0.32)
(0.59,0.40)
2.9
2.9
3.3
1966
a
b
c
d
Turn-on voltage recorded at the luminance of 1 cd m-2. Maximum luminance. Maximum external quantum efficiency.
Maximum current efficiency. e Measured at 5 V.
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Caesium fluoride (CsF) was purchased commercially and was
used without purification. OLED devices were fabricated using
the procedures as follows: ITO-coated glass substrates were
cleaned prior to device fabrication by sonication in acetone,
detergent, distilled water and isopropyl alcohol in sequence,
and finally dried in an oven. After treatment with an oxygen
Conclusion
In summary, TPAFSO as a new bipolar material, by connecting
two triphenylamine units to dibenzothiophene-S,S-dioxide core
at its 3,7-position was synthesized and characterized. The
theoretical calculation (DFT) reveals that the HOMO and LUMO
are mainly distributed on the TPA and DBTO units, respectively,
but not separated absolutely, which endows the TPAFSO with
high PLQYs of 52% in the solid state and an appropriate ET of 2.1
eV. The well-balanced carrier transport capacities are
demonstrated by the single-carrier devices. Furthermore, the
applications of TPAFSO in the solution-processed single-layer
OLEDs devices are studied comprehensively. The TPAFSO-based
green devices are turned on at a low voltage of 2.5 V and offer
a maximum LE of 4.7 cd A-1. Additionally, the performance of
TPAFSO-hosted devices with 5% Ir(btp)2(acac) are efficient with
a peak quantum efficiency of 3.9%, far beyond that of polymer-
based single-layer devices. The red OLEDs, using DCJTB-doped
TPAFSO as the emitting layer, obtain a maximum LE of 1.8 cd A-
1, which is comparable to the performances of DCJTB/Alq3-
based multilayer devices via an evaporation process. Our
strategy to design molecules may provide an alternative
approach to develop multifunctional luminescent materials.
plasma
for
4
min,
a
40
nm-thick
polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS)
(Bayer Baytron 4083) layer was spin-coated on the ITO-coated
glass substrates at 2500 rpm for 30 s. The substrates were
subsequently dried at 120 °C for 20 min in air and then
transferred into a N2-glove box. For the fabrication of the single-
layer green OLEDs, a solution of TPAFSO in chloroform at a
concentration of 12 mg mL-1 was spin-cast onto PEDOT:PSS and
then annealed at 90 °C for 20 min to yield the green emitting
layers, with a thickness of 80 nm. For the fabrication of the
single-layer red OLEDs,
corresponding emitters in
a
blend of TPAFSO and the
chloroform solution at
a
a
concentration of 13 mg mL-1 was spin-cast onto PEDOT:PSS and
then annealed at 90 °C for 20 min to yield the red emitting layers,
with a thickness of 80 nm. Then, the CsF and Al layers were
deposited by the vacuum evaporation method. The background
pressure of the chamber was under 2 × 10-4 Pa during the
deposition process. The layer thickness of the deposited
material was monitored in situ using an oscillating quartz
thickness monitor. The current density-brightness-voltage (J-B-
V) characteristics were measured using a Keithley 236 source
measurement unit, with a calibrated silicon photodiode in the
N2 atmosphere dry-box (Vacuum Atmosphere Co.). The
electroluminescent (EL) spectra were recorded using a PR-705
Spectroscan spectrometer. The device fabrication, except
PEDOT:PSS coating, was carried out in a nitrogen atmosphere
dry-box containing less than 10 ppm oxygen and moisture. The
film thickness was measured by Dektak 150.
Experimental section
General information
1H- and 13C-NMR spectra were measured on a Bruker AVANCE
500 spectrometer at 298 K, using CDCl3 as the solvent and
tetramethylsilane as the internal standard. The elemental
analysis was carried out by using a Flash EA 1112 CHNS-O
elemental analysis instrument. An AXIMA-CFRTM plus
instrument was used to record the MALDI-TOF-MS mass spectra.
UV- vis and PL spectra were recorded on a UV-3100
spectrophotometer and a RF-5301PC fluorometer, respectively.
Thermal gravimetric analysis (TGA) was undertaken on a Perkin
Elmer thermal analysis system at a heating rate of 10 °C/min
and a nitrogen flow rate of 80 mL min-1. Differential scanning
calorimetry (DSC) analysis was carried out using a NETZSCH
(DSC-204) instrument at 10 °C min-1 while flushing with nitrogen.
The electrochemical properties of the compound were
examined by cyclic voltammetry (CV) using a standard one-
compartment, three-electrode electrochemical cell given by a
BAS 100B/electrochemical analyzer. The solvent was anhydrous
acetonitrile and 0.1 M n-Bu4NPF6 was the supporting electrolyte.
The potentials were measured against an Ag/Ag+ (0.1 M AgNO3)
reference electrode with ferrocene as the internal standard.
Device fabrication and characterization
Synthesis
In a 100-mL round flask, 3,7-dibromo-dibenzothiophene S,S-
dioxide (0.56 g, 1.5 mmol), N,N-diphenyl-4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-aniline (1.14 g, 3.06
mmol), [(PPh3)4]Pd(0) (5mol%, 86 mg), 15 mL of toluene and 10
mL of K2CO3 (2 M) were mixed firstly. All the starting materials
and solvents were purchased from Tokyo Chemical Industry Co.,
Ltd. (TCI). Then, the solution was degassed and stirred at 85 °C
for 2 d under a nitrogen atmosphere. After cooling down to
room temperature, the mixture was washed by water and
extracted with chloroform consecutively. The product was then
further dried with MgSO4 and purified by column
chromatography on silica gel to obtain 0.79 g pure product
(yield 75%). TPAFSO: 1H NMR (CDCl3, 500 MHz, ppm): δ 8.01(2H,
s, Ar-H), 7.81 (4H, m, Ar-H), 7.50 (4H, d, J = 8.55 Hz, Ar-H), 7.30

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19 W. C. Chen, Y, Yuan, S. F. Ni, Z. L. Zhu, J. F. Zhang, Z. Q. Jiang,
(8H, t, J = 7.94 Hz, Ar-H), 7.15 (12H, d, J = 7.94 Hz, Ar-H), 7.08
(4H, t, J = 7.32 Hz, Ar-H). 13C NMR (CDCl3, 125 MHz, ppm):148.59,
147.46, 143.13, 138.82, 132.13, 131.89, 129.76, 129.58, 127.79,
125.07, 123.67, 123.31, 121.94, 120.07. MALDI-TOF-MS (M):
m/z 702.8 (100%). Anal. calcd. for C43H34N2O2S: C 82.02, H 4.88,
N 3.99, S 4.56; found: C 82.21, H 4.76, N 4.10, S 4.48.
L. S. Liao, F. L. Wong and C. S. Lee, ACS Appl. MatVeire.wInArtteicrlefaOcnelinse,
DOI: 10.1039/C9NJ00537D
2017, 9, 7331–7338.
20 L. Chen, Y. B. Jiang, H. Nie, P. Lu, H. H. Y. Sung, I. D. Williams,
H. S. Kwok, F. Huang, A. Qin, Z. J. Zhao and B. Z. Tang, Adv.
Funct. Mater., 2014, 24, 3621–3630.
21 M. Zhang, S. F. Xue, W. Y. Dong, Q. Wang, T. Fei, C. Gu and Y.
G. Ma, Chem. Commun., 2010, 46, 3923–3925.
22 A. Chaskar, H. F. Chen and K. T. Wong, Adv. Mater., 2011, 23
,
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3876–3895.
Conflicts of interest
There are no conflicts to declare.
23 L. Duan, J. Qiao, Y. D. Sun and Y. Qiu, Adv. Mater., 2011, 23
1137–1144.
,
24 S. S. Reddy, V. G. Sree, K. Gunasekar, W. Cho, Y. S. Gal, M. Song,
J. W. Kang and S. H. Jin, Adv. Optical Mater., 2016, 4, 1236–
1246.
Acknowledgements
The authors express thanks to the Natural Science Foundation
of China (21334002, 51521002, 51403063), the Major Science
25 B. Liu, Z. W. Yu, D, He, Z. L. Zhu, J. Zheng, Y. D. Yu, W. F. Xie, Q.
X. Tong and C. S. Lee, J. Mater. Chem. C, 2017, 5, 5402–5410.
26 J. Liang, C. L. Li, X. M. Zhuang, K. Q. Ye, Y. Liu and Y. Wang. Adv.
Funct. Mater., 2018, 28, 1707002.
27 X. Han, Q. Bai, L. Yao, H. C. Liu, Y. Gao, J. Y. Li, L. Q. Liu, Y. L.
and
Technology
Project
of
Guangdong
Province
(2015B090913002), the Foundation of Guangzhou Science and
Technology Project (201504010012), and Open Fund of the
State Key Laboratory of Luminescent Materials and Devices
(2017-skllmd-10, 2019-skllmd-01, South China University of
Technology) for their support.
Liu, X. X. Li, P. Lu and B. Yang, Adv. Funct. Mater., 2015, 25
7521–7529.
,
28 J. Ye, Z. Chen, K. Wang, F. F. An, Y. Yuan, W. C. Chen, Q. D.
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29 Y. T. Tao, Q. Wang, C. L. Yang, Q. Wang, Z. Q. Zhang, T. T. Zou,
J. G. Qin and D. G. Ma, Angew. Chem. Int. Ed., 2008, 47, 8104–
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30 K. K. Liu, X. L. Li, M. Liu, D. C. Chen, X. Y. Cai, Y. C. Wu, C. C. Lo,
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A bipolar triphenylamine-dibenzothiophene S,S-dioxide hybrid compound
for solution-processable single-layer green OLEDs and as a host for red
emitters.