A novel electron-acceptor moiety as a building block for efficient donor-acceptor based fluorescent organic lighting-emitting diodes

Article abstract of DOI:10.1039/C6CC09486D

A new electron-withdrawing moiety (BFPz) has been used for the first time as an acceptor in OLEDs and its corresponding core unit (2-Br-BFPz) was synthesized. Combined with an electron-donating moiety triphenylamine, a novel fluorescent material with a D-A structure named TPA-BFPz was synthesized. Encouragingly, the EQE values of non-doped and doped blue OLEDs reach 3.68% and 4.42%, respectively.

Full text of DOI:10.1039/C6CC09486D

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A novel electron-acceptor moiety as a building block for
efficient donor-acceptor based fluorescent organic
lighting-emitting diodes
Received 00th January
0xx,Accepted 00th January 20xx
2
*
DOI: 10.1039/x0xx00000x
MiaoꢀMiao Xue, ChenꢀChao Huang, Yi Yuan, YeꢀXin Zhang, ManꢀKeung Fung and LiangꢀSheng
*
Liao
www.rsc.org/
A new electron-withdrawing moiety (BFPz) is first used as an tricyclic system, as seen in Scheme 1. By combining this new
acceptor in OLED and its corresponding core unit (2-Br-BFPz) electronꢀaccepting moiety with a triphenylamine (TPA) group which
was synthesized. Combined with an electron-donating moiety possesses outstanding holeꢀtransporting ability, a novel fluorescent
triphenylamine,
structure named TPA-BFPz was synthesized. Encouragingly, the
EQEs of non-doped and doped blue OLEDs reach 3.68% and bromobenzofuro[2,3ꢀb]pyrazine (2ꢀBrꢀBFPz) was first synthesized
a
novel fluorescent material with
a
D-A molecule with a DꢀA structure is formed.
To obtain this novel DꢀA structure, a new core unit named 2ꢀ
as a precursor. The 2ꢀBrꢀBFPz was synthesized by the ring closing
4.42%, respectively.
reaction of 5ꢀbromoꢀ3ꢀ(2ꢀmethoxyphenyl)pyrazinꢀ2ꢀamine prepared
Fluorescent materials have been playing a critical role in organic by the Suzuki coupling reaction of (2ꢀmethoxyphenyl)boronic acid
lightꢀemitting diodes (OLEDs) since Tang’s report on tris(8ꢀ and 3,5ꢀdibromopyrazinꢀ2ꢀamine. The DꢀA fluorescent molecule,
[1ꢀ2]
hydroxyquinoline)ꢀaluminum (Alq
3
)ꢀbased multilayer devices.
In known as TPAꢀBFPz, was then prepared by the Suzuki coupling
particular with the introduction of the concepts of hybridized local reaction of 2ꢀBrꢀBFPz with N, Nꢀdiphenylꢀ4ꢀ(4,4,5,5ꢀtetramethylꢀ
and charge transfer (HLCT) and thermally activated delayed 1,3,2ꢀdioxaborolanꢀ2ꢀyl)aniline. The synthetic yield of TPAꢀBFPz
fluorescence (TADF) in recent years, the research based on was ~60%. Its detailed synthetic route is shown in Scheme S1 and
fluorescent molecules with donorꢀacceptor (DꢀA) structure has the synthetic procedure is described in the electronic supplementary
[3ꢀ6]
†
become a hot topic.
A compound with DꢀA structure means the information (ESI ). The thermal properties of TPAꢀBFPz were
material is composed of both an electron donor and acceptor studied using thermal gravimetric analysis (TGA) and differential
moieties. However, electronꢀacceptor moieties with
electronꢀwithdrawing capability are far from adequate.
Therefore, it is crucial to design desirable electronꢀaccepting loss temperature (T ) of up to 350 C and its glassꢀtransition
a
strong scanning calorimetry (DSC), as shown in Fig. S1 and Table S1. It
can be seen that the TPAꢀBFPz is thermally stable with a 5% weightꢀ
o
d
o
moieties. As we know dibenzofuran, a classical tricyclic unit, has temperature (T ) is 63 C.
g
confined conjugation structures which allow delocalization of both
the injected electrons/holes and hence possess outstanding carrier
[
7ꢀ9]
transporting ability.
Nevertheless, its electronꢀwithdrawing
ability is hardly comparable with other prototypical moieties such as
[10]
[11]
[12]
benzimidazole , triazine
and borane derivatives
which have
been widely used as electron acceptors in DꢀA based πꢀconjugated
fluorescent molecules. On the other hand, pyrazine is also known to
[
13]
be an excellent electronꢀwithdrawing molecule.
Therefore, it
would be an innovative strategy if a new electronꢀaccepting moiety
is synthesized based on a hybrid structure of dibenzofuran and
pyrazine. In this way, two nitrogen atoms are embedded into the
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute
of Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center
of Suzhou Nano Science and Technology, Soochow University
Suzhou 215123, P.R. China
E-mail: lsliao@suda.edu.cn; mkfung@suda.edu.cn.
†
M. M. Xue, C. C. Huang, Y. Yuan, Y. X. Zhang, Prof. M. K. Fung, Prof. L. S. Liao.
Electronic Supplementary Information (ESI) available: Experimental detail, physical
measurements and other electronic format. See DOI: 10.1039/x0xx00000x
Scheme1. Synthetic strategy of TPA-BFPz.
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the materials exhibit strong but diverse fluorescent emissions. The
color change from deep blue in hexane to orange yellow in DMF
indicates that the TPAꢀBFPz is a favorable fluorescent material with
a strong CTꢀstate characteristic. Meanwhile, it also suggests that
TPAꢀBFPz holds high sensitivity of its fluorescent emission to
different types of solvents. Particularly, such solvent effect almost
makes it capable of emitting in the entire visible spectrum. With the
aim to further investigate its fluorescent properties, the fluorescence
quantum yields (Φ
TPAꢀBFPz exhibited strong fluorescence with Φ
8% in DCM solution and in film, respectively. In addition, the
F
) in solution and in film were tested. As a result,
F
up to 92% and
7
transient PL of a neat film of TPAꢀBFPz was also tested, as depicted
in Fig. S3. It shows that the TPAꢀBFPz has a short fluorescent
lifetime of 6.9 ns without “delayed fluorescence” phenomenon.
To gain a deeper insight into the relationship between the D (TPA)
and A (BFPz) at the molecular level, DFT calculation (B3LYP/6ꢀ31
g*) on TPAꢀBFPz molecule was carried out. As illustrated in Fig. S4,
due to the electronꢀdonating ability of the TPA unit and electronꢀ
accepting ability of BFPz moiety, the highest occupied molecular
orbital (HOMO) of TPAꢀBFPz is mainly distributed in the TPA
group and the lowest unoccupied molecular orbital (LUMO) is
Fig. 1. a) UV-Vis and PL spectra of TPA-BFPz; b) PL spectra of TPA-BFPz in mainly located in the BFPz moiety, but their molecular orbitals are
different solvents; c) Color of TPA-BFPz in different solvents under UV partly overlapped in the pyrazine part of the BFPz group and
exposure with a wavelength of 365 nm.
benzene ring of the TPA group. Furthermore, the TPA and BFPz
groups are twistꢀlinked with a slightly distorted angle (θ D–A) of 21 .
o
In order to probe the interaction between D (TPA) and A (BFPz) The distribution of molecular orbitals and small θ D–A further
in the TPAꢀBFPz molecule, its photophysical properties were enhances the radiativeꢀtransition rate and fluorescence efficiency.
As TPAꢀBFPz possesses outstanding fluorescent properties as
characterized. The UVꢀVis spectrum of TPAꢀBFPz diluted in
ꢀ5
mentioned, we further explored it as an emitter for OLED
applications. The HOMO energy level of TPAꢀBFPz was measured
by ultraviolet photoelectron spectroscopy (UPS), which is estimated
to be 5.62 eV, as shown in Fig. S5. Knowing the band gap of TPAꢀ
BFPz as determined from the UV–Vis spectrum in Fig. 1a, the
LUMO energy level is calculated to be 2.79 eV. A nonꢀdoped OLED
toluene solution (1×10 mol/L) and photoluminescence (PL)
spectrum of TPAꢀBFPz prepared as a film are shown in Fig. 1a.
There are two pronounced absorption peaks. The shortꢀwavelength
absorption peak at 308 nm can be ascribed to the nꢀπ* transition of
[14]
triphenylamineꢀcentered,
and the longer wavelength at 394 nm
can be attributed to the charge transfer (CT) transition from D (TPA)
to A (BFPz). It can also be seen that the TPAꢀBFPz (in film) shows a
strong emission at 482 nm. Furthermore, the CT characteristic of
TPAꢀBFPz would be quite obvious if different polar solvents were
used in the fluorescence measurements. As depicted in Fig. 1b and
Table S2, the solvatochromic effect of the PL spectra of TPAꢀBFPz
was observed in different polar solvents. When the solvent polarity
was increased gradually from a lowꢀpolarity (e.g. hexane) to highꢀ
polarity such as N, Nꢀdimethylformamide (DMF), the PL spectra of
TPAꢀBFPz got wider and exhibited a total redꢀshift as large as 135
nm, i.e., 428 nm in hexane (Hex.), 466 nm in toluene (Tol.) and 563
nm in DMF. But there was an exception that the PL spectra of TPAꢀ
BFPz exhibited a blueꢀshift phenomenon from dichloromethane
with
a configuration of ITO/1,4,5,8,9,11ꢀhexaazatriphenyleneꢀ
hexacarbonitrile (HATꢀCN, 10 nm)/diꢀ[4ꢀ(N,Nꢀditolylꢀamino)ꢀ
phenyl]cyclohexan (TAPC, 40 nm)/TPAꢀBFPz (20 nm)/1,3,5ꢀtri[(3ꢀ
pyridyl)ꢀphenꢀ3ꢀyl]benzene
hydroxyquinolinolatolithium (Liq, 2 nm)/Al was fabricated. In
addition, the energy level diagram for each material used in the
devices is summarized in Fig. S6. To our delight, the nonꢀdoped
device has a low driving voltage of 3.4 V (at a current density of 0.2
(TmPyPB,
45
nm)/8ꢀ
2
mA/cm ) and a maximum current efficiency (CE), power efficiency
(
PE) and external quantum efficiency (EQE) of 8.0 cd/A, 7.4 lm/W
and 3.68%, respectively, with an electroluminescent (EL) peak at
84 nm (Fig. 2, Fig. S7 and Fig. S8). In order to further improve the
4
device performance, we optimized the device structure with a
configuration of ITO/HATꢀCN (10 nm)/TAPC (40 nm)/bis[2ꢀ
(
DCM) to tetrahydrofuran (THF) while the polarity of THF is larger
than that of DCM. That may be because the large electronegativity
of oxygen (O) atom in THF molecule may have an shortꢀrange effect
with αꢀH in pyrazine part of TPAꢀBFPz molecule, which may reduce
the energy of the ground state and then lead to the blueꢀshift
(
diphenylphosphino)phenyl]ether oxide (DPEPO):TPAꢀBFPz (X
wt%, 20 nm)/TmPyPB (45 nm)/Liq (2 nm)/Al, in which DPEPO
serves as a host material and X represents the doping concentration
of TPAꢀBFPz in DPEPO. As shown in Fig. 2 and Fig. S7, all
devices have the same EL peak at 492 nm and driving voltages of
[
15]
phenomenon.
For better understanding of the solvent effects on
PL behaviors, the UVꢀVis absorption spectra of TPAꢀBFPz
measured in different solvents were also shown on Fig. S2. In order
to unveil the solvatochromic effect chromatically, the fluorescence
of TPAꢀBFPz dissolved by different polar solvents was observed
under a UV lamp (365 nm), as shown in Fig. 1c. It is obvious that all
2
less than 3.7 V when driven at 0.2 mA/cm . All the doped devices
show a better device performance than the nonꢀdoped OLED. But
the EL spectra of the doped devices have a redshift about 8 nm
compared to that of the nonꢀdoped one because DPEPO is a polar
2
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molecule. The corresponding device performance is summarized in and doped devices reach 3.68% and 4.42%, respectively, which
2
Table 1. As shown in the Table 1, the voltages tested at 0.2 mA/cm
illustrates that the BFPz unit is a promising electronꢀ
of devices decreased when the doping concentration of TPAꢀBFPz in withdrawing moiety. It is foreseen that the electronꢀ
DPEPO increased, which indicated TPAꢀBFPz might have good
electron and hole transporting abilities. When the doping
concentration of TPAꢀBFPz is 30%, the DꢀA based OLED exhibits
maximum efficiencies of 11.5 cd/A, 10.3 lm/W and 4.42%. The
superior device performance implies that TPAꢀBFPz is a favorable
DꢀA molecule and a promising blue fluorescent material in OLED.
withdrawing unit can be used for designing novel TADF or
HLCT materials in the future.
We thank the financial support from the Natural Science
Foundation of China (Nos. 61575136 and 61475106) and the
Natural
Science
Foundation
of
Jiangsu
Province
(
BK20151264). This project is also financially supported by the
Collaborative Innovation Center (CIC) of Suzhou Nano Science
and Technology and the Priority Academic Program
Development of the Jiangsu Higher Education Institutions
Table 1. Summary of electroluminescence data for OLEDs.
a
b
c
c
c
d
Devices
Voltage
CEmax
PEmax
EQEmax
CIE
(
PAPD).
(V)
cd/A
lm/W
%
1
2
2
3
5
5%
0%
5%
0%
0%
3.7
3.6
3.5
3.5
3.4
3.4
11
11
11
9.2
9.4
9.7
10.3
8.9
7.4
4.4
0.19, 0.42
0.19, 0.43
0.19, 0.44
0.19, 0.44
0.18, 0.42
0.16, 0.35
4.3
Notes and references
4.27
4.42
3.85
1
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Fig. 2. a) J-V-L curves of devices; b) CE-L and EQE-L curves of devices
In summary, an innovative electronꢀwithdrawing moiety, 2ꢀ
BrꢀBFPz, which possesses a highly confined conjugated
structure, has been designed and synthesized. We have
combined 2ꢀBrꢀBFPz with the TPA group which is a classical
electronꢀdonating unit to form a novel fluorescent material with
a DꢀA structure named TPAꢀBFPz. Interestingly, the TPAꢀ
BFPz exhibits strong fluorescence and has been utilized as a
blue fluorescent emitter for OLEDs. The EQEs for nonꢀdoped
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