Efficient deep-blue electroluminescence based on phenanthroimidazole-dibenzothiophene derivatives with different oxidation states of the sulfur atom

Article abstract of DOI:10.1002/asia.201601626

Developing efficient deep-blue materials is a longterm research focus in the field of organic light-emitting diodes (OLEDs). In this paper, we report two deep-blue molecules, PITO and PISF, which share similar chemical structures but exhibit different pho

Full text of DOI:10.1002/asia.201601626

CHEMISTRY
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Accepted Article
Title: Efficient Deep Blue Electroluminescence Based on
Phenanthroimidazole-Dibenzothiophene Derivatives with
Different Oxidation States of Sulfur Atom
Authors: Xiangyang Tang, Tong Shan, Qing Bai, Hongwei Ma, Xin He,
and Ping Lu
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Efficient
Deep
Blue
Electroluminescence
Based
on
with
Phenanthroimidazole-Dibenzothiophene
Derivatives
Different Oxidation States of Sulfur Atom
Xiangyang Tang,†[a] Tong Shan,†[a] Qing Bai,[a] Hongwei Ma,[a] Xin He,[a] and Ping Lu*[a]
†: X. Tang and T. Shan contributed equially to this work.
Abstract: Developing efficient deep blue materials is a long term
research focus in the field of organic light emitting diodes (OLEDs).
In this paper, we report two deep blue molecules, PITO and PISF,
which share similar chemical structures but exhibit different
photophysical and device properties. These two molecules consist of
phenanthroimidazole and dibenzothiophene analogues. The
distinction of their chemical structures lies in the different oxidation
states of the S atom. For PITO, the S atom is oxidized and the
resulting structure dibenzothiophene S,S-dioxide becomes electron
deficient. Therefore, the PITO displays remarkable solvatochromism,
implying a charge transfer (CT) excited state formed between the
donor (D) phenanthroimidazole and acceptor (A) dibenzothiophene
S,S-dioxide. For PISF, it is constituted by phenanthroimidazole and
dibenzothiophene of which the S atom is not oxidized. The PISF
displays locally excited (LE) emission with little solvatochromism.
Compared to PISF, the D-A molecule PITO with electron deficient
group shows much lowered LUMO energy level which is in favor of
electron injection in device. In addition, the PITO exhibits more
balanced carrier transport. However, the PISF is capable of emitting
in the shorter wavelength region which is beneficial to obtain better
color purity. The doped electroluminescence (EL) device of the D-A
molecule PITO manifests deep blue emission with a CIE coordinates
of (0.15, 0.08) and maximum external quantum efficiency (EQE) of
4.67%. The doped EL device of the LE molecule PISF, however,
reveals an even bluer emission with CIE coordinates of (0.15, 0.06)
and a maximum EQE of 4.08%.
According to the National Television System Committee (NTSC),
deep blue emission should have a Commission International de
L’Eclairage (CIE) coordinates (x, y) of (0.14, 0.08). The
European Broadcasting Union (EBU) puts forward even more
rigorous requirement on deep blue emission with CIE
coordinates of (0.15, 0.06). To achieve efficient deep blue
emission, fluorescent materials may be superior to
phosphorescent materials. Though heavy metal (commonly Ir
or Pt) based phosphorescent materials can harvest 100%
electrical injected excitons,[5] it still remains a bottleneck to
develop efficient phosphorescent materials with high triplet
energy levels to meet the NTSC deep blue CIE coordinates,[6]
probably due to the increasing nonradiative process via metal d-
orbitals when elevating the metal-ligand charge transfer (MLCT)
state into the deep blue region,[7] and the most classic blue
phosphorescent material FIrpic is in fact a sky blue emitter.[8] In
contrast, there are a wide variety of fluorescent materials that
can emit in the deep blue region with high photoluminescence
quantum efficiency (PLQY).[3] Several short conjugated aromatic
rigid rings such as anthracene,[9] phenanthrene,[10] pyrene[11] and
fluorene[12] are good building blocks for efficient deep blue
materials and some of them exhibit fairly good device
performance. For example, Kim and co-workers recently
reported an anthracene derivative 2-(2,5-dimethyl-4-(10-
(naphthalen-1-yl)anthracen-9-yl)phenyl)-4,6-diphenyl-1,3,5-
triazine (NAXPT) with horizontal emitting dipoles.[13] The
nondoped device based on NAXPT exhibits deep blue emission
with CIE coordinates of (0.145, 0.068) and a high external
quantum efficiency (EQE) of 6.6%. Ma et al designed and
synthesized a triphenylamine-phenanthrene molecule. It shows
efficient deep blue EL with a CIE coordinates of (0.157, 0.073)
and a high EQE of 7.23% in the nondoped device.[10a] In addition,
donor-acceptor (D-A) molecules with limited charge transfer (CT)
strength are also attracting emitters for high performance deep
blue OLEDs.[14] The proper energy alignments of D-A molecules
can effectively reduce charge injection barrier in device.
Moreover, their bipolar attribute is beneficial for a balanced
carrier transport.[15] Especially, some D-A molecules possess
sufficient small singlet-triplet energy splitting which is in favor of
triplet harvesting via reverse intersystem crossing (RISC).[16] For
example, Adachi et al developed a blue emitting molecule
consists of 3,6-di-tertbutylcarbazole and sulfone with thermally
activated delayed fluorescence (TADF) character.[17] The doped
device displays deep blue EL with CIE coordinates of (0.15,
0.07). Though the efficiency is as high as 9.9%, it decreases
rapidly at high luminescence. Zhang and co-workers also
reported a carbazole-sulfone analogue. The nondoped device
achieves a CIE coordinates of (0.157, 0.055) and a maximum
EQE of 4.21%.[18] Among so many deep blue materials,
Introduction
Organic light emitting diodes (OLEDs) is expected to be the
next-generation of lighting and display technique because of its
unique advantages such as light weight, flexibility, energy
conservation and so on.[1] Though great progress has been
made since the pioneering work of C. W. Tang and co-workers,[2]
some issues are still not well resolved, and one of which is the
realization of efficient deep blue electroluminescence (EL).[3]
Deep blue emission can not only provide one of the RGB
primary colors and enhance the color gamut for full color display,
but also lower power consumption for white lightings.[4]
[a]
X. Tang, T. Shan, Q. Bai, H. Ma, X. He, Prof. P. Lu
Department of chemistry, Jilin University.
State Key Laboratory of Supramolecular Structure and Materials.
2699 Qianjin Avenue, Changchun 130012, P. R. China.
E-mail: lup@jlu.edu.cn.
Supporting information for this article is given via a link at the end of
the document.
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phenanthroimidazole (PI) derivatives have attracted great
attention recently.[19] The PI mainly consists of phenanthrene
and imidazole rings. The relatively short conjugation of PI gives
rise to a near ultraviolet emission.[20] Besides, the rigid PI plane
can reduce nonradiative vibrational and rotational processes,
leading to a high PLQY. What’s more, the different bonding
mode of the two nitrogen atoms in the imidazole ring endows PI
with bipolar character.[21] Various strategies have been applied
to design efficient blue-emitting PI derivatives.[22] By linking two
PI in different modes, efficient blue emission can be expected.[23]
For instance, Cheng et al connected two PI via a stilbene
linker.[23a] The resultant molecule is doped into DMPPP host and
a blue emission with CIE coordinates of (0.14, 0.14) is obtained
with a maximum EQE of 8.1%. PI can also serve as acceptor to
construct blue emitting D-A molecules.[24] Ma and co-workers
employed triphenylamine as donor and PI as acceptor to
construct a blue-emitting molecule TPA-PPI.[24a] The nondoped
device exhibits a maximum EQE of 5.02% and CIE coordinates
of (0.15, 011). Besides, PI is also used as electron donor due to
its bipolar character.[25] Tong et al adopted polycyclic aromatic
hydrocarbon units as electron acceptor and PI as donor and
achieved a maximum EQE of 5.25% with a CIE coordinates of
(0.16, 0.07).[25a] We recently employed PI as a weak donor and
sulfone as acceptor to construct a D-A molecule PMSO with
deep blue emission.[25b] The doped device gives a maximum
EQE of 6.8% and a CIE coordinates of (0.152, 0.077). This
positive result inspires us to further develop other analogues in
order to obtain efficient deep blue OLEDs. We note that
dibenzothiophene S,S-dioxide (DBTSO) as an acceptor
constitutes many efficient blue-emitting D-A polymers and small
molecules.[26] For example, Perepichka et al introduced DBTSO
into oligofluorene and attained stable and efficient blue emission
with improved electron affinity.[26a] However, the color purity of
many DBTSO based D-A materials cannot satisfy the NTSC
deep blue standard.[27] This is due to a pronounced CT character
between donor and acceptor DBTSO according to previous
researches.[28] Considering the weak donating ability and high
PLQY of PI, efficient deep blue emission would be expected
from a D-A molecule in which PI acts as the donor and DBTSO
as acceptor. In general, there are two kinds of linking modes for
DBTSO, i.e., at the 3,7-positions (para sites) or 2,8-positions
(meta sites).[28d] Previous study has shown that if the para sites
of DBTSO are substituted by donors, the whole molecule will
present a linear configuration and the donor/acceptor share
good conjugation which will lead to a bathochromic emission.[28d]
By contrast, if the DBTSO is substituted by donors at the meta-
sites, the donors are twisted against DBTSO, therefore the
conjugation between them is partially interrupted, making it
possible to obtain blue emission. Herein, the PI is connected to
the meta-positions of DBTSO as a weak donor to construct a D-
A-D molecule termed as PITO. The PITO behaves relatively
strong CT strength as evidenced by the remarkable
solvatochromism. The nondoped device exhibits sky blue
emission with a CIE coordinates of (0.17, 0.31) and a maximum
EQE of 3.88%. To tune the emission into deep blue region,
PITO is doped into host and the doped device shows a CIE
coordinates of (0.15, 0.08) which is very close to the NTSC
requirement for deep blue emission. The maximum EQE is
4.67%. To realize emission at even shorter wavelength and
smaller CIEy value, another molecule analogue to PITO is also
synthesized. As shown in Scheme 1, the molecule is termed as
PISF in which the S atom of dibenzothiophene (DBT) is not
oxidized to weaken the CT strength of the whole molecule. As a
result, the PISF exhibits much bluer emission compared to PITO.
No obvious solvatochromism is observed for PISF indicating
much reduced CT strength. The nondoped device of PISF gives
a deep blue emission with a CIE coordinates of (0.16, 0.09) and
a maximum EQE of 2.60%. The doped device of PISF emits at
even shorter wavelength with a CIE coordinates of (0.15, 0.06)
and a maximum EQE of 4.08%. The EL of PISF doped device
has reached the EBU deep blue criterion and the efficiency is
high among the reported deep blue OLEDs that meet the EBU
deep blue requirement.[29]
Scheme 1. Synthetic procedures of PITO and PISF. (a) CHCl3, Br2, 0 °C to
room temperature, overnight; (b) H2O2, acetic acid, 120 °C , overnight; (c)
bis(pinacolato)diboron, Pd(dppf)Cl2, KOAc, 1,4-dioxane, 90 °C, N2 protection,
72 h; (d) 4-bromobenzaldehyde, CH3COONH4, aniline, acetic acid, 120 °C , N2
protection, overnight. (e) Pd(PPh3)4, K2CO3 (aq, 2 M), toluene, 90 °C, N2
protection, 72 h.
Results and Discussion
Synthetic procedures
The molecular structures and synthetic routes of PITO and PISF
are shown in Scheme 1. To begin with, PPIBr was prepared via
one-pot reaction mode using our previous procedure.[20] Then
PPIBr was reacted with bis(pinacolato)diboron through
palladium catalyzed Suzuki coupling to get PPIB. DBTBr was
obtained by the bromination of DBT.[30] Afterwards, DBTBr was
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oxidized by H2O2 to get DBTSOBr. The DBTB was produced by
the borylation reaction of DBTBr. In the end, the target
compound PITO was accessible by using PPIB and DBTSOBr
as starting materials via palladium catalyzed Suzuki coupling.
Likewise, PISF was acquired by Suzuki coupling using PPIBr
and DBTB as reaction substrates. The molecular structure and
purity of PITO and PISF were fully characterized by NMR, MS,
and elemental analysis.
morphology changes easily at high temperature. The PISF
exhibits a glass transition temperature (Tg) of 225 °C which is
comparable to that of other organic light-emitting materials with
good thermal stability.[31] No melting point is observed for PISF.
From the thermal stability point of view, PITO and PISF are
suitable for evaporation fabricated OLEDs and have proper
endurance for joule heat.
Figure 1. (a) TGA and (b) DSC curves of PITO and PISF.
Thermal properties
Thermal stability is very important to organic light-emitting
materials because once the device is working, the electric
current can generate large amount of joule heat which may
decompose the emitter or destroy the morphology of the thin film.
As displayed in Figure 1a, the decomposition temperature (Td,
corresponding to 5% weight loss) of PITO and PISF are 554 °C
and 526 °C, respectively. Such high Td means that PITO and
PISF would not decompose during evaporation and have proper
endurance for joule heat. Another issue is the morphology
stability under high temperature. As can be seen in Figure 1b,
PITO does not show any glass transition or melting points during
the temperature rising process, revealing that PITO tends to
form amorphous morphology and would not undergo
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nature of the S1 state. As shown in Figure 2a, the PITO emits at
the sky blue region in neat film with an emission peak of ~482
nm and the PLQY is measured to be 31%. To suppress
intermolecular interactions and gain deep blue emission, PITO is
dispersed into a wide band gap host matrix 1,3-di(9H-carbazol-
9-yl)benzene (mCP) with a doping concentration of 10%. The
emission peak of PITO doped film blue shifts to ~450 nm and
the corresponding PLQY is improved to 82%. The neat film of
PISF shows an emission peak of 447 nm (Figure 2b). The
corresponding PLQY is 26%. To tune the emission of PISF into
even shorter wavelength region, PISF is also doped into mCP at
a doping level of 25%. The emission peak is dragged to ~430
nm and the PLQY is 60%. Considering the deep blue emission
and high PLQY of PITO and PISF in doped films, they may be
good candidates as deep blue emitters for OLEDs application.
Figure 2. (a) PITO: the absorption spectrum in dilute THF (10-5 M), the PL in
dilute THF (10-5 M), neat film and doped film; (b) PISF: the absorption
spectrum in dilute THF (10-5 M), the PL in dilute THF (10-5 M), neat film and
doped film; (c) Emission spectra of PITO in different solvents; (d) Emission
spectra of PISF in different solvents.
Photophysical properties
As shown in Figure 2a and Figure 2b, the absorption spectra of
PITO and PISF are similar in dilute THF. Both PITO and PISF
show an absorption peak around 262 nm which can be
attributed to the π-π* transition of the phenyl ring anchored at N
atom of the imidazole group.[32] The 348 and 366 nm absorption
bands of PITO can be assigned to the π-π* transition of the
conjugation backbone of the molecule. Likewise, the 335 and
365 nm absorption bands of PISF also arise from the π-π*
transition of the conjugation backbone of PISF. The excited state
properties of PITO and PISF can be evaluated by the emission
behavior in different solvents with various polarities (Figure 2c
and 2d). For PITO, it shows remarkable solvatochromism,
suggesting relatively strong CT between PI and DBTSO. The
photoluminescence (PL) of PITO shows vibronic structure in
non-polar solvent n-hexane which implies a locally excited (LE)
state. However, as the solvent alters from ether to THF and
acetone, the emission peaks greatly red shift from ~406 nm in n-
hexane to ~496 nm in acetone. Besides, the PL spectra become
structureless and broadened, indicating the formation of a CT
excited state. The solvatochromism of PITO disclose an
evolution from a LE state to a CT excited state as the solvent
polarity increases. On the contrary, PISF constantly displays LE-
like emission in the four solvents, as evidenced by the PL with
fine vibronic structure. In addition, the PL spectra do not red shift
conspicuously with increasing solvent polarity, further
demonstrating the LE character of the S1 state of PISF.
Specifically, the fine vibronic structure becomes vague in high
polar solvent acetone. We can conclude that the CT emission
between the donor DBT and acceptor PI is initiated for PISF in
high polar solvent. Similar spectroscopic behavior is also
observed in literatures of DBT and DBTSO derivatives which
have demonstrated that an oxidation state of the sulfur atom is
crucial for the CT process and the evolution from a LE to a CT
excited state as the solvent polarity increases.[28] The PLQY of
PISF in dilute THF is close to unity, proving the highly emissive
Figure 3. Cyclic voltammetry of PITO and PISF.
Electrochemical properties
Cyclic voltammetry (CV) is employed to determine the
HOMO/LUMO energy levels which are derived from the onset
oxidation/reduction potentials during positive/negative scans. As
shown in Figure 3, the reduction potential of PITO against Fc+/Fc
is -1.95 V whereas that of PISF is lowered to be -2.54 V.
Comparing the difference of molecular structure between PITO
and PISF, the electron deficient group DBTSO has the strongest
electron affinity compared to PI and DBT which leads to the
higher reduction potential of PITO in comparison with that of
PISF. The LUMO energy levels of PITO and PISF are calculated
to be -2.85 eV and -2.26 eV, respectively. This reveals that by
oxidation of S atom, the resultant electron deficient group
DBTSO can lower LUMO energy and reduce electron injection
barrier in device. The oxidation potential of PISF against Fc+/Fc
is 0.61 V and is a little bit lower than that of PITO (0.72 V). The
oxidation onset takes place on PI for PITO. By contrast, for PISF,
the oxidation preferentially occurs on DBT instead of PI. This
can be rationalized that the DBT is “electron richer” than PI and
therefore is easier to be oxidized. In fact, DBT is commonly used
as hole transporting and hosting functional group in many
OLEDs materials.[33] The HOMO energy levels of PITO and PISF
are estimated to be -5.52 eV and -5.41 eV, respectively. The
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introduction of electron rich group DBT can improve the HOMO
energy level and therefore facilitate hole injection. By oxidation
of DBT, the resultant group DBTSO turns to be electron deficient
and can lower electron injection barrier in device.
Table 1. Photophysical data of PITO and PISF.
Compound
PITO
Td / Tg (°C )[a]
554 / n.o.[b]
526 / 225
λmax, abs (nm)[c]
Eg (eV)[d]
3.02
λmax, PL (nm)[e]
482[g] / 450[h]
447[g] / 430[h]
PLQY (%)[f]
31[g] / 82[h]
26[g] / 60[h]
HOMO / LUMO (eV)[i]
-5.52 / -2.85
262 / 348 / 366
262 / 335 / 365
PISF
3.16
-5.41 / -2.26
[a] Td: decomposition temperature; Tg: glass transition temperature. [b] n.o. = not observed. [c] Absorption peaks in dilute THF
(10-5 M). [d] Optical gap calculated from the absorption onset in dilute THF (10-5 M). [e] Emission peaks. [f] PLQY measured
by integrating sphere. [g] Values of neat film. [h] Values of doped film. [i] HOMO / LUMO energy levels estimated from cyclic
voltammetry measurement.
transporting whereas the hole transporting is mainly undertaken
by PI considering its bipolar property.[21] Since the hole mobility
are much higher than electron mobility for many organic
molecules, the existence of electron withdrawing group DBTSO
can therefore enhance electron mobility and narrow down the
mobility gap between hole and electron, making these opposite
carriers being transported evenly at the price of sacrificing a
certain extent of hole mobility. For PISF, the DBT is an electron
donating group which can further improve the hole mobility of
the molecule while the electron transporting may mainly occur
on PI group. Thanks to the bipolar property of PI, the PISF still
displays balanced carrier transport. But the difference of the
opposite carrier mobility of PISF is much obvious than that of
PITO.
Single Carrier Devices
Single carrier devices were fabricated to investigate whether the
PITO and PISF possess balanced carrier transporting ability.
The hole-only device consists of ITO / HATCN (6 nm) / NPB (10
nm) / PITO or PISF (80 nm) / NPB (20 nm) / Al (100 nm) in
which HATCN is hexaazatriphenylenehexacabonitrile and NPB
is N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine. A thin
layer of HATCN and NPB near anode ITO is used to facilitate
hole injection and NPB near cathode Al is applied to block
electron. The electron-only device constitutes of ITO / TPBi (20
nm) / PITO or PISF (80 nm) / TPBi (10 nm) / LiF (1 nm) / Al (100
nm) in which TPBi is 1,3,5-tris(1-phenyl-1H-benzimidazol-2-
yl)benzene. A thin layer of LiF and TPBi near cathode Al is
designed to enhance electron injection while TPBi near anode
ITO is aimed at preventing hole injection. For both the hole- and
electron-only devices, the PITO or PISF layer is much thicker
than that of NPB / TPBi so that the hole / electron transporting of
the whole device is predominated by PITO or PISF. As shown in
Figure 4, PITO exhibits balanced carrier transporting character
in a wide range of voltages. With increasing voltages, the
electron density increases more rapidly than the hole density,
demonstrating that the electron mobility is better than hole
mobility for PITO. However, even at the voltage as high as 20 V,
the current density of electron-only device is only about as
threefold as that of hole-only device which gives a further proof
that PITO possesses balanced carrier transporting ability with
electron mobility being a little more higher. As for PISF, the
distinction of the opposite carrier mobility is greater than that of
PITO which reveals less balanced carrier transporting property
of PISF. The current density of hole-only device is close to an
order of magnitude higher than that of electron-only device for
PISF at the voltage of 20 V. Considering the fact that the hole
mobility of many organic molecules is often several orders of
magnitude higher than electron mobility,[34] we may conclude
that the carrier transport of PISF is balanced to some extent.
Note that the hole mobility of PISF is higher than that of PITO
while vice versa for the electron mobility. Such distinction
between PITO and PISF is closely in correlation with their
respective chemical structures. For PITO, the DBTSO is a strong
electron withdrawing group which is responsible for the electron
Figure 4. Single carrier devices of PITO and PISF.
Electroluminescence properties
To investigate the application potential of these two molecules
as solid state emitter, we fabricated OLEDs using PITO and
PISF as emitting layer. We firstly fabricated the nondoped
devices of PITO with the configuration of ITO / HATCN (6 nm) /
NPB (40 nm) / TCTA (5 nm) / emitting layer (PITO, 20 nm) /
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TPBi (50 nm) / LiF (0.5 nm) / Al (120 nm) (Device A) where ITO
(indium tin oxide) is the anode, HATCN is hole injecting layer,
NPB is hole transporting layer, TCTA (tris(4-carbazoyl-9-
ylphenyl)amine) is the buffer layer, TPBi is the electron
transporting layer, LiF is the electron injecting layer and Al is the
cathode. The nondoped device shows sky blue emission with a
CIE coordinates of (0.17, 0.31) at 7 V (Table 2). The EL
spectrum corresponds well with the PL counterpart with an
emission peak at ~480 nm (Figure S3b), indicating the emission
comes from the emitting layer. The EL spectra are stable at
various voltages which proves that the carriers are recombined
exactly within the emitting layer (Figure S3a). The turn-on
voltage is 3.2 V and is only a little higher than PITO optical gap
(see in Table 1 and Table 2), disclosing effective charge
injection. The maximum luminescence is ~39000 cd m-2. The
maximum EQE is 3.88% which is achieved at very high
luminescence of ~24000 cd m-2 (Figure 5a). The EQE at 100
and 1000 cd m-2 are 3.22% and 3.57%, respectively. To adjust
the emission of PITO into deep blue region, we also fabricated
doped device. mCP is selected as host considering the good
overlap between the absorption spectrum of PITO and the
emission band of mCP (Figure S1a). The doped device consists
of ITO / HATCN (6 nm) / NPB (40 nm) / TCTA (5 nm) / emitting
layer (PITO: mCP, wt. 10%, 20 nm) / TPBi (50 nm) / LiF (0.5 nm)
/ Al (120 nm) (Device B). The EL is successfully dragged into the
deep blue region with a CIE coordinates of (0.15, 0.08) at 7 V
which is very close to the NTSC requirement of blue emission
(Table 2). The doped EL spectra is in agreement with the PL
counterpart and is very stable at a wide range of voltages
(Figure S3c and S3d), indicating that the carrier is recombined in
the emitting layer and the EL originates from the dopant PITO.
The turn-on voltage of doped device is also 3.2 V. The doped
device can reach a maximum luminescence of ~10000 cd m-2.
maximum EQE of doped device can be rationalized by the
increase in PLQY of the doped film as shown in Table 1. The
EQE at 100 and 1000 cd m-2 are 4.44% and 3.25%, respectively.
The performance of the doped device is good among the
reported deep blue fluorescent OLEDs which can meet the
NTSC deep blue requirement.[35] The PISF exhibits bluer
emission than PITO. Therefore we also fabricated PISF based
nondoped EL device with the structure of ITO / HATCN (6 nm) /
NPB (40 nm) / TCTA (5 nm) / emitting layer (PISF, 20 nm) / TPBi
(20 nm) / LiF (0.5 nm) / Al (120 nm) (Device C). The CIE
coordinates is (0.16, 0.09) under a driving voltage of 7 V. The EL
is stable at various voltages and matches with the PL spectrum
with an emission peak of ~444 nm (Figure S4a and S4b). The
turn-on voltage is 3.2 V, reflecting a low charge injection barrier.
The maximum luminescence is ~15000 cd m-2. The maximum
EQE is 2.60% which is attained at a luminescence of ~100 cd m-
2 (Figure 5c). The EQE at 1000 cd m-2 is 2.27%. To enhance the
device efficiency, PISF is doped into mCP host. The doped
device consists of ITO / HATCN (6 nm) / NPB (40 nm) / TCTA (5
nm) / emitting layer (PISF: mCP, wt. 30%, 20 nm) / TPBi (20 nm)
/ LiF (0.5 nm) / Al (120 nm) (Device D). The device shows
extremely deep blue emission with a CIE coordinates of (0.15,
0.06) which is in accordance with the EBU requirement on deep
blue emission. The EL locates at the deep blue region with an
emission peak of ~427 nm and corresponds with the PL
spectrum of doped film (Figure S4c and S4d). The turn-on
voltage is 3.3 V indicating relatively low charge injection barrier.
The maximum luminescence is ~6000 cd m-2. The maximum
EQE is 4.08% which is achieved at a luminescence of ~20 cd m-
2
(Figure 5c). The EQE at 100 and 1000 cd m-2 are 3.92% and
3.22%, respectively. Though there is a certain degree of
efficiency roll-off, the overall device performance is positive and
is comparable to many other deep blue fluorescent materials
that can reach EBU standard.[29]
The maximum EQE is 4.67% which is obtained at
a
luminescence of ~60 cd m-2 (Figure 5a). The improvement of
Table 2. Electroluminescence data of PITO and PISF based devices.
Device
Von (V)[a]
3.2
Lmax (cd m-2)[b]
39000
CEmax (cd A-1)[c]
EQEmax / 100 / 1000 (%)[d]
3.88 / 3.22 / 3.57
4.67 / 4.44/ 3.25
EL λmax (nm)[e]
CIE (x, y)[f]
0.17, 0.31
0.15, 0.08
0.16, 0.09
0.15, 0.06
A
B
C
D
7.17
3.35
1.96
1.98
480
444
444
427
3.2
10000
3.2
15000
2.60 / 2.60 / 2.27
4.08 / 3.92 / 3.22
3.3
6000
[a] Turn on voltage at the luminescence of 1 cd m-2. [b] Maximum luminescence. [c] Maximum current efficiency. [d] External
quantum efficiency: maximum / at 100 cd m-2 / at 1000 cd m-2. [e] Emission peaks of EL spectra. [f] CIE coordinates of EL devices.
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Figure 5. (a) EQE-Luminescence curves of PITO nondoped and doped devices; (b) EL of PITO nondoped and doped devices; (c) EQE-Luminescence curves of
PISF nondoped and doped devices; (d) EL of PISF nondoped and doped devices.
However, the emission color of PITO is red-shifted compared
with that of PISF. The CIE coordinates of PITO nondoped device
is (0.17, 0.31) and the maximum EQE is 3.88%. By doping PITO
into appropriate host, the EL of doped device can be tuned into
Conclusions
deep blue region with a CIE coordinates of (0.15, 0.08) which is
close to the NTSC requirement on deep blue emission. The
maximum EQE is improved to 4.67%. On the other hand, the
PISF displays bluer EL but a little lower efficiency. The CIE
coordinates of PISF nondoped device is (0.16, 0.09) and the
maximum EQE is 2.60%. The doped device of PISF exhibits
deep blue emission with CIE coordinates of (0.15, 0.06) and
meets with the EBU standard of deep blue emission. The
maximum EQE of PISF doped device is 4.08%. Overall, both
PITO and PISF show deep blue EL in doped device and their
efficiency is comparable to many other deep blue materials
which can satisfy the NTSC and EBU standard.
To conclude, we have successfully designed and synthesized
two blue-emitting molecules PITO and PISF. They share similar
chemical structures with different oxidation states of the S atom.
For PITO, the S atom is oxidized and the resultant structure
DBTSO is electron deficient. Therefore, using PI as donor and
DBTSO as acceptor, the D-A molecule PITO reveals strong
solvatochromism, disclosing a CT excited state. In contrast, the
PISF in which the S atom is not oxidized shows typical LE
emission probably resulting from the π conjugation of the whole
molecule. The D-A molecule PITO shows a much lowered
LUMO energy level than PISF, demonstrating that the S,S-
dioxide is in favor of reducing electron injection barrier. In
addition, the PITO also shows more balanced carrier transport
than PISF at the price of sacrificing a part of the hole mobility.
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crude product was purified by column chromatography using petroleum
ether as eluent to give white solid. Yield: 3.10 g (90%). 1H NMR (500
MHz, DMSO-d6, δ ppm): 8.77 (s, 2H), 8.04 (d, J = 8.5 Hz, 2H), 7.71 (d, J
= 8.7 Hz, 2H). MALDI-TOF (m/z): [M+] Calcd: 339.86; Found: 341.01.
Experimental Section
General information
All the reagents and solvents were used as received. 1H NMR was
recorded on Bruker AVANCE 500 spectrometer with tetramethylsilane
(TMS) as the internal standard and DMSO-d6 as solvent. Elemental
analysis was carried out on a Flash EA 1112, CHNSO elemental analysis
instrument. MALDI-TOF-MS mass spectra were obtained from an
AXIMA-CFRTM plus instrument. Thermal gravimetric analysis (TGA) was
performed on a Perkin-Elmer thermal analysis system from 30 °C to
900 °C . NETZSCH (DSC-204) unit was applied to measure differential
scanning calorimetry (DSC) from 30 °C to 370 °C . Cyclic voltammetry
(CV) were measured by using BAS 100B/W electrochemical analyzer
with standard one-compartment, three-electrode electrochemical cell.
The CV was performed in solutions. The working electrode was a glass-
carbon disk electrode. The counter electrode was a Pt wire. The
reference electrode was Ag/Ag+. The scan rate was 100 mV s-1. The
2,8-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)dibenzo[b,d]thiophene (DBTB)
A mixture of DBTBr (3.4 g, 10 mmol), bis(pinacolato)diboron (10.16 g, 40
mmol), Pd(dppf)Cl2 (467 mg, 0.6 mmol), KOAc (5.9 g, 60 mmol), 60 mL
1,4-dioxane was stirred at 90 °C under N2 protection for 72 h. The
reaction was quenched by water and the mixture was extracted by
dichloromethane. After evaporating the solvent, the crude product was
purified by column chromatography (petroleum ether : dichloromethane =
5 : 1, v/v) to give white solid. Yield: 2.62 g (60%). 1H NMR (500 MHz,
DMSO-d6, δ ppm): 8.64 (s, 2H), 8.02 (d, J = 8.0 Hz, 2H), 7.76 (d, J = 8.2
Hz, 2H), 1.32 (s, 24H). MALDI-TOF (m/z): [M+] Calcd: 436.21; Found:
437.82.
electrolyte
for
negative
or
positive
(TBAPF6)
scan
dissolved
was
in
tetrabutylammoniumhexafluorophosphate
2,8-dibromodibenzo[b,d]thiophene 5,5-dioxide (DBTSOBr)
anhydrous dimethylformamide (DMF) or anhydrous dichloromethane (0.1
M). Following the IUPAC recommendation,[36] ferrocenium/ferrocene
(Fc+/Fc) redox couple was used as the internal standard and the formal
potential of Fc+/Fc is 4.8 eV below vacuum.[37] All potentials relative to
Ag/Ag+ electrode obtained from CV measurement were eventually
referenced against Fc+/Fc to calculate HOMO/LUMO levels. As a result,
the Ag/Ag+ electrode is just a pseudo reference. The HOMO/LUMO
levels are calculated according to the following formalism:
A mixture of DBTBr (2.05 g, 6 mmol), 40 mL acetic acid, 5 mL 30% H2O2
was stirred at 120 °C. After about half an hour, large amount of white
sediment occurred. The reaction mixture was continued stirring overnight.
Upon completion, the mixture was cooled and filtered. The product was
purified by recrystallization to get white solid. Yield: 2.12 g (95%). 1H
NMR (500 MHz, DMSO-d6, δ ppm): 8.65 (s, 2H), 8.00 (d, J = 8.2 Hz, 2H),
7.91 (d, J = 8.5 Hz, 2H). MALDI-TOF (m/z): [M+] Calcd: 371.85; Found:
373.14.
HOMO = - (Eox vs. Fc+/Fc + 4.8) eV
LUMO = - (Ered vs. Fc+/Fc + 4.8) eV.
2-(4-bromophenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (PPIBr)
A
mixture of phenanthrene-9,10-dione (10.5 g, 50 mmol), 4-
where the Eox vs. Fc+/Fc and Ered vs. Fc+/Fc are oxidation and reduction
onset potentials relative to Fc/Fc+ reference, respectively. UV-vis and PL
spectra were recorded on a Shimadzu UV-3100 spectrophotometer. The
PLQY (ΦF) in solutions and solid films were measured by integrating
sphere.
bromobenzaldehyde (9.25 g, 50 mmol), CH3COONH4, (19.25 g, 250
mmol), aniline (18.3 mL, 200 mmol) was dissolved in 200 mL acetic acid
and stirred at 120 °C overnight under N2 protection. The mixture was
poured into 100 mL water and then filtered to get grey solid. The product
was purified by column chromatography (petroleum ether
:
dichloromethane = 2 : 1, v/v) to give white solid. Yield: 19.24 g (85%). 1H
NMR (500 MHz, DMSO-d6, δ ppm): 8.94 (d, J = 8.2 Hz, 1H), 8.89 (d, J =
8.3 Hz, 1H), 8.69 (d, J = 7.9 Hz, 1H), 7.79 (t, J = 7.4 Hz, 1H), 7.72 (m,
6H), 7.58 (t, J = 8.3 Hz, 3H), 7.52 (d, J = 8.5 Hz, 2H), 7.35 (t, J = 7.2 Hz,
1H), 7.09 (d, J = 8.0 Hz, 1H). MALDI-TOF (m/z): [M+] Calcd: 448.06;
Found: 449.84.
Device fabrications and measurements
ITO coated glasses were used as the substrates and the sheet
resistance was 20 Ω square-1. The ITO glasess were cleaned by
deionized water, isopropyl alcohol, acetone, toluene, treated with UV-
zone for 30 min, and then transferred to a vacuum deposition system
with a base pressure lower than 5×10-6 mbar for organic and metal
deposition. The hole injecting layer (HATCN, 6 nm) was deposited at 0.1
Å s-1. The deposition rate of all other organic layers was 1.0 Å s-1. The
electron injecting layer LiF (0.5 nm) was deposited at a rate of 0.1 Å s-1
and then the capping Al metal layer (120 nm) was deposited at a rate of
4.0 Å s-1. The EL characteristics were measured using a Keithley 2400
programmable electrometer and a PR-650 Spectroscan spectrometer
under ambient condition.
1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-
1H-phenanthro[9,10-d]imidazole (PPIB)
A mixture of PPIBr (2.24 g, 5 mmol), bis(pinacolato)diboron (2.54 g, 10
mmol), Pd(dppf)Cl2 (117 mg, 0.15 mmol), KOAc (1.47 g, 15 mmol), 50
mL 1,4-dioxane was stirred at 90 °C under N2 protection for 72 h. The
reaction was quenched by water and the mixture was extracted by
dichloromethane. After evaporating the solvent, the crude product was
purified by column chromatography (petroleum ether : dichloromethane =
1 : 1, v/v) to give white solid. Yield: 1.62 g (65%). 1H NMR (500 MHz,
DMSO-d6, δ ppm): 8.94 (d, J = 8.4 Hz, 1H), 8.89 (d, J = 8.3 Hz, 1H), 8.71
(d, J = 8.0 Hz, 1H), 7.79 (t, J = 7.4 Hz, 1H), 7.71 (m, 6H), 7.60 (dt, J =
19.6, 9.8 Hz, 5H), 7.35 (t, J = 7.5 Hz, 1H), 7.11 (d, J = 8.4 Hz, 1H), 1.30
(s, 12H). MALDI-TOF (m/z): [M+] Calcd: 496.23; Found: 497.73.
Synthesis and characterization
2,8-dibromodibenzo[b,d]thiophene (DBTBr)
Dibenzo[b,d]thiophene (DBT) (1.85 g, 10 mmol) was dissolved in 80 mL
CHCl3 and then 1.1 mL Br2 (22 mmol) was added dropwise at 0 °C. The
reaction mixture was stirred at room temperature overnight. Upon
completion, the reaction mixture was washed by aqueous K2CO3 and
then extracted by dichloromethane. After evaporating the solvent, the
PITO
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A mixture of PPIB (2.24 g, 5 mmol), DBTSOBr (744 mg, 2 mmol),
Pd(PPh3)4 (92 mg, 0.08 mmol), aqueous K2CO3 (15 mL, 2 M) and 30 mL
toluene was stirred at 90 °C under N2 protection for 72 h. Then the
mixture was extracted by dichloromethane. After evaporating the solvent,
the crude product was purified by column chromatography using
dichloromethane as eluent to give light green solid. Yield: 1.06 g (55%).
1H NMR (500 MHz, DMSO-d6, δ ppm): 8.96 (d, J = 8.5 Hz, 2H), 8.91 (d, J
= 8.4 Hz, 2H), 8.76 (m, 4H), 8.06 (dd, J = 24.3, 8.1 Hz, 4H), 7.95 (d, J =
8.1 Hz, 4H), 7.81 (d, J = 8.6 Hz, 16H), 7.73 (d, J = 7.9 Hz, 2H), 7.59 (t, J
= 7.4 Hz, 2H), 7.37 (t, J = 7.6 Hz, 2H), 7.11 (d, J = 8.1 Hz, 2H). MALDI-
TOF (m/z): [M+] Calcd: 952.29; Found: 953.58. Anal. Calcd. for
C66H40N4O2S: C, 83.17; H, 4.23; N, 5.88. Found: C, 83.28; H,
4.29; N, 5.91.
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DMSO-d6, δ ppm): 8.97 (d, J = 8.4 Hz, 2H), 8.92 (d, J = 7.7 Hz, 2H), 8.76
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Acknowledgements
We appreciate the financial support from the Ministry of Science
and Technology of China (2013CB834801, 2016YFB0401001),
National Science Foundation of China (21374038), Jilin
Provincial
Science
and
Technology
Department
(20160101302JC).
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Keywords: deep blue • fluorescence • phenanthroimidazole-
dibenzothiophene • OLEDs • semiconductors
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10.1002/asia.201601626
Chemistry - An Asian Journal
FULL PAPER
FULL PAPER
Two blue-emitting molecules, PITO
and PISF, with different oxidation
states of the S atom are reported.
They share similar molecular structure
but exhibit different photophysical and
device properties. Both the PITO and
PISF doped devices show deep blue
emission with a maximum external
quantum efficiency of 4.67% and
4.08%, respectively.
Xiangyang Tang, Tong Shan, Qing Bai,
Hongwei Ma, Xin He, and Ping Lu*
Page No. – Page No.
Efficient Deep Blue
Electroluminescence Based on
Phenanthroimidazole-
Dibenzothiophene Derivatives with
Different Oxidation States of Sulfur
Atom
For internal use, please do not delete. Submitted_Manuscript
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