Two trans-1-(9-anthryl)-2-phenylethene derivatives as blue-green emitting materials for highly bright organic light-emitting diodes application

Article abstract of DOI:10.1016/j.orgel.2017.07.054

1-(9-Anthryl)-2-phenylethene (t-APE) is a blue-green material with high fluorescence quantum yield (Ф_f 0.44). However, it is easily crystallized. Herein, Two asymmetric blue-green emitting materials based on t-APE, (E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-1-yl)anthracene (6) and (E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-2-yl)anthracene (7) were firstly designed and synthesized. The two compounds possess high thermal stability, morphological durability, and bipolar characteristics. The non-doped blue-green organic light-emitting diodes (OLEDs) using 6 and 7 as emitting layers showed emission at 495 nm, full width at half maximum of 80 nm, maximum brightness of 13,814, 10,579 cd m^?2, maximum current efficiency of 3.62, 7.16 cd A^?1, and Commission Internationale de L'Eclairage (CIE) coordinate of (0.20, 0.43), respectively. Furthermore, when employing 6 and 7 as blue-green emitting layers and rubrene doped in tris-(8-hydroxyquinolinato)aluminum (Alq3) as the orange emitting layers to fabricate white OLEDs (WOLEDs), the WOLEDs exhibit a maximum brightness of 10,984, 14,652 cd m^?2, maximum current efficiency of 2.04, 2.70 cd A^?1, and CIE coordinate of (0.30, 0.40), (0.37, 0.47), Color Rendering Index (CRI) of 65, 60, stable EL spectra, respectively. This study demonstrates that the t-APE-type derivatives have the excellent properties for the emitting materials of OLEDs.

Full text of DOI:10.1016/j.orgel.2017.07.054

Accepted Manuscript
Two trans-1-(9-anthryl)-2-phenylethene derivatives as blue-green emitting materials
for highly bright organic light-emitting diodes application
Nonglin Zhou, Xiaona Shao, Shirong Wang, Yin Xiao, Xianggao Li
PII:
S1566-1199(17)30386-5
10.1016/j.orgel.2017.07.054
ORGELE 4246
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 13 June 2017
Revised Date: 24 July 2017
Accepted Date: 31 July 2017
Please cite this article as: N. Zhou, X. Shao, S. Wang, Y. Xiao, X. Li, Two trans-1-(9-anthryl)-2-
phenylethene derivatives as blue-green emitting materials for highly bright organic light-emitting diodes
application, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.07.054.
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ACCEPTED MANUSCRIPT
Graphical Abstract
The new two t-APE-type derivatives are applied to OLEDs devices showing that t-APE-type
derivatives have the excellent properties for the emitting materials of OLEDs.

ACCEPTED MANUSCRIPT
Two trans-1-(9-anthryl)-2-phenylethene derivatives as blue-green emitting materials for
highly bright organic light-emitting diodes application
Nonglin Zhoua,b, Xiaona Shaoa,b,c, Shirong Wanga,b, Yin Xiaoa,b, Xianggao Lia,b*
aSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300354, China
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,
China
cHebei Radio & TV University, Shijiazhuang, Hebei 050080, China.
*Corresponding author. E-mail address: lixianggao@tju.edu.cn (Xianggao Li)
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ABSTRACT: 1-(9-Anthryl)-2-phenylethene (t-APE) is a blue-green material with high
fluorescence quantum yield (Фf 0.44). However, it is easily crystallized. Herein, Two asymmetric
blue-green
emitting
materials
based
on
t-APE,
(E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-1-yl)anthracene
(6)
and
(E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-2-yl)anthracene (7) were firstly
designed and synthesized. The two compounds possess high thermal stability, morphological
durability, and bipolar characteristics. The non-doped blue-green organic light-emitting diodes
(OLEDs) using 6 and 7 as emitting layers showed emission at 495 nm, full width at half maximum
of 80 nm, maximum brightness of 13814, 10579 cd m-2, maximum current efficiency of 3.62, 7.16
cd A-1, and Commission Internationale de L'Eclairage (CIE) coordinate of (0.20, 0.43),
respectively. Furthermore, when employing 6 and 7 as blue-green emitting layers and rubrene
doped in tris-(8-hydroxyquinolinato)aluminum (Alq3) as the orange emitting layers to fabricate
white OLEDs (WOLEDs), the WOLEDs exhibit a maximum brightness of 10984, 14652 cd m-2,
maximum current efficiency of 2.04, 2.70 cd A-1, and CIE coordinate of (0.30, 0.40), (0.37, 0.47),
Color Rendering Index (CRI) of 65, 60, stable EL spectra, respectively. This study demonstrates
that the t-APE-type derivatives have the excellent properties for the emitting materials of OLEDs.
KEWORDS: blue-green, asymmetric, bipolar, organic light-emitting diodes, white
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1. Introduction
Blue-green emitter instead of standard blue emitter is an effective method to improve the color
rendering index (CRI) of white organic light-emitting diodes (WOLEDs) with two-element colors
because its wavelength can complement the gap in wavelength between blue and orange emitters.
However, there are still rare of reports on the high efficiency blue-green emitting materials [1]. In
2008, Moorthy et al. employed a blue emitting material of anthracene-anchored-bimesityls and
fabricated non-doped blue-green OLEDs with CIE coordinate of (0.19, 0.38), maximum
brightness of 3199 cd m-2 and maximum current efficiency (CE) of 0.64 cd A-1 [2]. In 2010, Qiu's
group recorded the CE of 10.6 cd A-1 and CIE coordinate of (0.18, 0.45) by doping the TBCPF:
OXD-7 host with a blue-green phosphor [3]. In 2013, Turksoy's group reported a blue-green
OLEDs with CIE coordinate of (0.23, 0.42), maximum brightness of 335 cd m-2 and maximum CE
of 1.1 cd A-1 by employing anthracene-pyridine derivatives [4]. Dumur's group proposed TCTA
host materials and fabricated phosphorescent blue-green OLEDs with the power efficiency (PE) of
0.98 lm W-1 and CIE coordinate of (0.21, 0.41) [5]. In 2014, Chen and colleagues synthesized a
new DPA derivative and fabricated the non-doped blue-green OLEDs with CIE coordinate of
(0.22, 0.37), maximum brightness of 7891 cd m-2 and maximum CE of 2.5 cd A-1 [6]. In 2015,
Huang and his coworkers reported non-doped blue-green OLEDs and obtained CIE coordinate of
(0.17, 0.39), maximum brightness of 4490 cd m-2 and maximum CE of 1.43 cd A-1 by employing a
newly synthesized ADN derivative [7]. Park's group synthesized a new hexaphenyl benzene
derivative and fabricated the non-doped blue-green OLEDs with CIE coordinate of (0.23, 0.45),
and maximum CE of 2.48 cd A-1 [8]. Zhang and colleagues reported extremely high-performance
blue-green OLEDs with CIE coordinate of (0.19, 0.49), establishing a new world record PE of
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11.8 lm W-1 and 5.3% by doping the CBP host with a blue-green phosphor [9]. In 2016, Kim's
group utilizing DPA derivative fabricated the non-doped blue-green OLEDs with CIE coordinate
of (0.19, 0.45), maximum brightness of 1377 cd m-2 and maximum CE of 1.75 cd A-1 [10].
Although achievements have been made, there are still several problems in developing blue-green
OLEDs. Firstly, in some blue-green devices using anthrance derivatives as emitting materials, the
blue-green emission spectra are originated from the excimer of materials which found at the
molecular interactions [2, 4, 6, 7, 10]. Secondly, the turn-on voltage (defined as the operation
voltage at brightness of 1 cd m-2) of some blue-green OLEDs is higher than 7 V without
explanation [4, 8]. Especially, there are a few reports on utilizing blue-green emitting materials to
fabricate WOLEDs [3]. Thus, synthesis of blue-green emission (originating from excited
monomer rather than excimer) and suitable energy level materials applying to WOLEDs is
becoming increasingly important. In general, the HOMO levels of anthracene derivatives are near
5.5-5.6 eV [11-14]. And the excimer emission can be restrained effectively by the bulky or
asymmetric substituents which inhibit intermolecular interactions [15, 16]. In line with the
above-mentioned consideration and facts, the issue can be solved by constructing blue-green
anthracene derivatives with bulky or asymmetric substituents.
As we know, 1-(9-anthryl)-2-phenylethene (t-APE, Scheme 1 ) is a blue-green material with
high fluorescence quantum yield (Фf 0.44) [17]. There is no report about using t-APE to construct
OLEDs because it is easily crystallized (see ESI Fig. S2b). Herein, we firstly introduced the
naphthanthracene group into the t-APE molecule to improve the thermal properties while keeping
their advantageous electronic and optical characteristics. The asymmetric molecule structure
would suppress the aggregation of t-APE segments in the solid state which results in stable
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blue-green EL emission. For this purpose, two asymmetric blue-green material of
(E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-1-yl)anthracene
(6)
and
(E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-2-yl)anthracene (7) were firstly
designed and synthesized. The thermal and electrochemical properties, the bipolar characteristics,
and the device performance of the two materials were investigated. The non-doped OLEDs using
classical and simple structure based on 6 and 7 show blue-green emission (The EL spectra of these
device emissions centered at 495 nm with full width at half maximum of 80 nm.), which originates
from excited monomer. Since the HOMO levels of 6 and 7 are near 5.5 eV, using these two
materials as blue-green emitter and rubrene doped in Alq3 as the orange emitter to fabricate
WOLEDs exhibit a maximum brightness of 10984, 14652 cd m-2, CRI of 65, 60, respectively. The
EL spectra of these WOLEDs are stable under high voltage without any interlayer. We believe the
performance of WOLEDs could be improved by optimizing the structure of device and employing
other orange materials.
2. Experiment section
2.1. Materials
All materials were received from Tianjin Heowns Biochemical Technology Co., Ltd. and used
without further purification unless otherwise stated. All solvents were purchased from Tianjin
Rianlon Corporation. 1,4-Dioxane was carefully dried and distilled from a sodium benzophenone
mixture under argon (Ar) atmosphere before use.
2.2 General procedures
1H and 13C NMR spectra were obtained on a Bruker ACF400 (400 MHz) spectrometer in
chloroform-d (CDCl3) with tetramethylsilane as reference. Mass spectra were carried out on an
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Agilent MALDI-TOF. UV-Vis absorption and photoluminescence spectra of THF solution and
film were collected with Thermo Evolution 300 UV-Vis spectrophotometer and Hitachi F-4500
fluorescence spectrophotometer, respectively. Fluorescence lifetime measurements were recorded
on the HORIBA Jobin-Yvon, NanoLog using time-correlated single photon counting (TCSPC).
Absolute photoluminescence quantum efficiency at room temperature was measured using an
absolute photoluminescence quantum yield measurement system (Edinburg FLS-980 fluorescence
spectrometer) with a calibrated integrating sphere. Differential scanning calorimeter (DSC) was
undertaken using a TA Q20 instrument under nitrogen atmosphere at a heating rate of 10 °C min-1.
Thermo gravimetric analysis (TGA) was carried out on a TA Q500 thermo gravimetric analyzer
under nitrogen atmosphere at a heating rate of 10 °C min-1. Atomic force microscopes (AFM)
were measured using NTEGRA Spectra. Cyclic voltammetry (CV) was taken on a CHI-600C
electrochemical analyzer. The measurements were determined using
a
conventional
three-electrode configuration consisting of a glassy carbon working electrode, a platinum-disk
auxiliary electrode and an Hg/KCl reference electrode. And the scan rate was 10 mV s-1. DFT
calculations were performed to characterize the 3D geometries and the frontier molecular orbital
energy levels of 6 and 7 at the B3LYP/6-31G(d) level by using the Gaussian 03 program. All
measurements were conducted at room temperature.
2.3. Device fabrication and measurement
The EL devices were fabricated by vacuum thermal evaporation technology. Before the
deposition of an organic layer, the indium tin oxide (ITO) substrates were rinsed in an ultrasonic
bath by the following sequences: in acetone, methyl alcohol, distilled water and kept in isopropyl
alcohol for 48 h and dried by N2 gas gun. The clear substrates treated by oxygen plasma with the
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conditions of 10-2 Pa at 80 W for 3 min. The deposition rate of organic compounds was 0.7-1.4 Å
S-1. Finally, a cathode composed of LiF (1 nm) and aluminum (100 nm) were sequentially
deposited onto the substrate in the vacuum of 5×10-4 Pa. All of properties of the EL devices such
as J-V-B curves, current efficiency (CE), and CIE chromaticity coordinates of devices were
measured with a Keithley 2400 Source meter and Chroma meter CS-2000. All measurements were
carried out at room temperature under ambient conditions.
2.4. Synthesis
The designed compounds were synthesized as outlined in Scheme 1.
Scheme 1. Synthetic routes and molecular structures of 6 and 7
2.4.1. Diethyl benzylphosphonate (1)
Diethyl benzylphosphonate as the basis compound for the synthesis of the t-APE isomers was
prepared according to literature [18, 19]. Benzyl bromide (1.50 g, 8.8 mmol) was suspended in 5
mL of toluene, containing triethylphosphite (6.00 g, 35 mmol). The mixture was refluxed at
140 °C for 2 h. Excess triethylphosphite and toluene were removed by distillation to give the
required product as a yellow oil (1.80 g, 89.9%). 1H NMR (400 MHz, DMSO-d6) δ 7.53 – 7.25 (m,
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5H),3.96 – 3.91 (m, 4H), 3.15 (d, J = 21.6 Hz, 2H), 1.21 (t, J = 7.1 Hz, 6H).
2.4.2 Diethyl-4-Bromobenzylphosphonate (2)
Compound 2 was synthesized according to the method described in synthesis of compound 1
with yield of 92.5%. 1H NMR (400 MHz, DMSO-d6) δ 7.51 (dd, J = 8.4, 0.9 Hz, 2H), 7.25 (dd, J
= 8.4, 2.5 Hz, 2H),3.96 – 3.91 (m, 4H), 3.25 (d, J = 21.6 Hz, 2H), 1.19 (t, J = 7.1 Hz, 6H).
2.4.3 (E)-9-styrylanthracene (3, t-APE)
Diethyl benzylphosphonate (1.10 g, 4.8 mmol) and 9-anthraldehyde (1.00 g, 4.8 mmol) were
added into a 100 mL round-bottom flask with the protection of N2. N,N-Dimethylformamide (30
mL) was added to above flask, then cooled to 0 °C. The N,N-Dimethylformamide solution of
t-BuOK (0.54 g, 4.8 mmol) was added dropwise to above flask, stirred for 30 min at 0 °C,
followed with stirring at room temperature until the 9-anthraldehyde was consumed completely
(monitored by thin-layer chromatography). The reaction was terminated with ice water, and the
product was extracted with ethyl acetate 100 mL. After the solvent was evaporated, the crude
product was purified by chromatography on a silica gel column (petroleum ether: ethyl acetate =
4:1 as eluent) to give the title compound as a pure (E)-9-styrylanthracene (1.3 g, 86.9%):
1
Mp:132 °C. H NMR (400 MHz, CDCl3) δ 8.36 (dd, J = 12.3, 6.3 Hz, 3H), 8.06 – 7.96 (m, 2H),
7.90 (d, J = 16.5 Hz, 1H), 7.67 (d, J = 7.6 Hz, 2H), 7.59 – 7.40 (m, 6H), 7.35 (t, J = 6.9 Hz, 1H),
6.94 (d, J = 16.5 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 137.35, 132.81, 131.55, 129.78, 128.87,
128.73, 128.05, 126.64, 126.50, 126.07, 125.50, 125.21, 124.93.
2.4.4. 6 (E)-9-(4-bromostyryl)anthracene (4)
Compound 4 was synthesized according to the method described in synthesis of compound 3
with yield of 59.7%: Mp:167 °C. 1H NMR (400 MHz, CDCl3) δ 8.44 (s, 1H), 8.37 – 8.28 (m, 2H),
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8.05 (dd, J = 5.7, 3.9 Hz, 2H), 7.92 (d, J = 16.5 Hz, 1H), 7.65 – 7.46 (m, 8H), 6.91 (d, J = 16.5 Hz,
1H). 13C NMR (101 MHz, CDCl3) δ 136.24, 136.08, 132.29, 131.96, 131.51, 129.69, 128.78,
128.10, 126.73, 125.85, 125.72, 125.64, 125.25.
2.4.5. (E)-2-(4-(2-(anthracen-9-yl)vinyl)phenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (5)
A mixture of (E)-9-(4-bromostyryl)anthracene (0.72 g, 2 mmol), bis(pinacolato)-diboron (0.65 g,
2.5 mmol), [1,1'-bis(diphenylphophino)ferrocene]-dichloro-palladium(II) (0.05 g, 0.06 mmol),
potassium acetate (0.65 g, 6 mmol), and anhydrous 1,4-dioxane (20 mL) was added into a 50 mL
round-bottom flask with the protection of Ar. The reaction system was refluxed for 24 h under
stirring. After cooling, the mixture was evaporated under reduced pressure, treated with saturated
sodium chloride solution, extracted with ethyl acetate, and the organic layer was dried over
anhydrous MgSO4. After removal of the solvent, the residue was purified by column
chromatography on silica gel using ethyl acetate/petroleum at 1:10 by volume as the eluent to give
a yellow powder (0.70 g, 86.0%): 1H NMR (400 MHz, CDCl3) δ 8.44 (s, 1H), 8.40 – 8.33 (m, 2H),
8.03 (dd, J = 13.4, 9.0 Hz, 3H), 7.93 (d, J = 7.9 Hz, 2H), 7.71 (d, J = 8.0 Hz, 2H), 7.62 – 7.39 (m,
4H), 7.00 (d, J = 16.6 Hz, 1H), 1.42 (s, 12H). 13C NMR (101 MHz, CDCl3) δ 139.97, 137.33,
135.33, 132.62, 131.51, 129.73, 128.70, 126.59, 125.99, 125.88, 125.55, 125.21, 83.87, 24.91.
2.4.6. (E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-1-yl)anthracene (6)
A
mixture of 9-bromo-10-(naphthalen-1-yl)anthracene (0.42 g, 1.1 mmol),
(E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-1-yl)anthracene (0.49 g, 1.2 mmol),
tetrakis(triphenylphosphine)palladium(0) (0.03 g, 0.03 mmol), and 1 M sodium carbonate (2.7
mL) in 40 mL of 1,4-dioxane was stirred at 100 °C for 48 h under nitrogen atmosphere, cooled
down to room temperature. The title compound was obtained by simple filtration from the
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reaction mixture. Yellow solid (0.40 g, 66.4%). MALDITOF-TOF-MS (m/z): [M + H]+ calcd for
1
C46H30, 582.23; found, 582.3. H NMR (400 MHz, CDCl3) δ 8.49 (dd, J = 13.6, 6.5 Hz, 3H),
8.23 – 7.81 (m, 9H), 7.77 – 7.33 (m, 13H), 7.33 – 7.20 (m, 4H), 7.16 (d, J = 16.7 Hz, 1H). 13C
NMR (101 MHz, CDCl3) δ 137.09, 136.78, 136.59, 135.21, 133.70, 133.59, 131.92, 131.60,
130.71, 129.97, 129.85, 129.24, 128.78, 128.26, 128.16, 127.16, 127.01, 126.67, 126.57, 126.31,
126.12, 126.01, 125.64, 125.60, 125.27, 125.25, 125.23.
2.4.7. (E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-2-yl)anthracene (7)
Compound 7 was synthesized according to the method described in synthesis of compound 6
with yellow solid (0.45 g, 74.7%). MALDITOF-TOF-MS (m/z): [M + H]+ calcd for C46H30, 582.23;
found, 582.4. 1H NMR (400 MHz, CDCl3) δ 8.49 (dd, J = 13.6, 6.5 Hz, 3H), 8.23 – 7.81 (m, 9H),
7.77 – 7.33 (m, 13H), 7.33 – 7.20 (m, 4H), 7.16 (d, J = 16.7 Hz, 1H). 13C NMR (101 MHz, CDCl3)
δ 137.09, 136.78, 136.59, 135.21, 133.70, 133.59, 131.92, 131.60, 130.71, 129.97, 129.85, 129.24,
128.78, 128.26, 128.16, 127.16, 127.01, 126.67, 126.57, 126.31, 126.12, 126.01, 125.64, 125.60,
125.27, 125.25, 125.23.
3. Result and discussion
3.1. Synthesis, thermal and Morphology properties of 6 and 7
The synthesis of 6 and 7 is outlined in Scheme 1, involving a series of Suzuki reactions (see
Experiment section). The thermal properties of 6 and 7 were investigated by TGA and DSC under
nitrogen atmosphere. And the related data are listed in Table 1. As displayed in Fig. 1, the
decomposition temperature (Td, 5% weight loss) values of the compound 6 and 7 were determined
to be 427 °C and 425 °C, respectively, which are higher than the 223 °C determined for t-APE
(See ESI Fig. S1). Such high Td values suggest that these compounds are stable and have the
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potential to be fabricated into device by vacuum thermal evaporation technique, which is a
prerequisite for high performance OLED applications [20]. As shown in Fig. 1 (insert), DSC
analysis reveals that the glass transition temperatures (Tg) of both compounds amount to 105 °C.
The similarly thermal data may be attributed to their similar structures. Moreover, no distinct
crystallization was observed in this series upon heating at a temperature beyond Tg. The high Tg
values implies that it is possible to get stable glasses from these materials and means good stability
of the amorphous film during fabrication and operation of OLEDs [21, 22].
Fig. 1. TGA and DSC thermograms (insert) of 6 (a) and 7 (b).
To confirm the thermal stability and morphological durability against thermal stress, the surface
morphologies of vacuum-deposited thin films was studied by atomic force microscopy (AFM).
The vacuum-deposited of the two new compounds on a silicon wafer substrate were about 40 nm
thickness to match that used in the actual electroluminescent device. As illustrated in Fig. 2, the
thin films of 6 and 7 exhibited a uniform surface morphology having a root-mean-square (RMS)
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surface roughness of 0.236 nm and 0.239 nm, respectively. These films did not undergo any
significant morphological change after annealing at 60 °C and 90 °C for 24 h suggesting their
excellent thermal and amorphous stability, which indicates that the asymmetric molecules with
number of conformers increased the energy needed for crystallization and favors the stability of
amorphous films [23-25].
Fig. 2. Atomic force microscope (AFM) topographic images of 6 and 7 with thermal treated
samples (a) 25 °C 24 h, (b) 60 °C 24 h, and (c) 90 °C 24 h).
3.2. Optical properties and energy levels
The normalized UV-Vis absorption and photoluminescence (PL) spectra of 6 and 7 in THF
solution (1×10-5 M) andvacuum-deposited thin films (quartz plate) are shown in Fig. 3, relevant
data are summarized in Table 1. For a direct comparison, the UV-Vis absorption and PL spectra of
t-APE in THF solution and in films was collected (See ESI Fig. S2). In the THF solution, there are
three peaks located at around 350-406 nm in the UV-Vis absorption spectra of t-APE, 6 and 7
indicating that the characteristic absorption peaks are caused by π-π* transition of the t-APE
moiety [26]. Compare the UV-Vis absorption spectra of THF solution with that of thin films, the
shape and peak position of 6 and 7 are similar. No obvious red-shift of absorption peaks manifests
that no significant aggregation or crystallization occurs in the films [27]. However, the UV-Vis
absorption spectral shape and peak position of t-APE show some difference in THF solution and
thin films due to crystallization in the films. As shown in Fig. S2b, the crystallization of t-APE in
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the films probably attributed to the thermal delay [28].
Fig. 3. UV-Vis absorption and PL spectra of 6 (a) and 7 (b) dilute solution (1×10-5 M) in THF and
neat thin films.
In the THF solution, t-APE, 6 and 7 exhibit similar trends with similar maximum PL (PLmax
values at blue-green wavelengths of 470 nm, 475 nm and 475 nm and full width at half maximum
(FWHM) values 76 nm, 80 nm and 80 nm respectively. Compared to t-APE, the PLmax values of 6
and 7 are red-shifted by 5 nm and the FWHM values of 6 and 7 are increased by 4 nm. This
implies that the conjugation lengths of these anthracene derivatives are enhanced by introducing a
naphthanthracene to t-APE group. There were no differences in PL spectra of t-APE between
solution and thin film which might be originated from crystallization-induced emission (See ESI
Fig. S2a) [29-31]. Nevertheless, the maximum emission peak in PL spectra of 6 and 7 in thin film
state showed slight red-shift about 20 nm without change of FWHM, comparing with their PL
spectra in THF solution. The red-shift of the emission observed in the film state is probably due to
)
the difference in relative permittivity or dielectric constant of the environment [32]. And this
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red-shift emission also can suggest that the presence of weak intermolecular interactions or
aggregation in film states due to the twisted asymmetrically bound aryl groups. Nonetheless, the
presence of molecular aggregates will also support the enhancement of charge transport properties
due to specific restrictions on intramolecular rotations [33]. In degassed THF solutions, the
photoluminescence quantum yields (PLQYs, 0.47 for 6 and 0.46 for 7) are very similar with that
of t-APE. The excited-state lifetime (τ) is 2.0 ns for both 6 and 7, which is equal to those of
previously reported anthracene derivatives [34]. However, the lifetime of 6 and 7 is much short
than t-APE (3.3 ns, see Fig. S3) indicating that the kf values of 6 and 7 increased [35].
Fig. 4. Molecular structures (a) and electron density distributions of the HOMO (b)/LUMO (c)
calculated with B3LYB/6-31G(d).
To obtain a better understanding on the optical and thermal properties in the molecular level,
density functional theory (DFT) calculations (B3LYP/6-31G(d)) were carried out by using
Gaussian 03 software [36]. The optimized geometry and the electron distribution of compounds 6
and 7 are shown in Fig. 4. The calculation results indicate that both 6 and 7 have non-coplanar
molecular structures. The dihedral angles of two compounds between anthrancene and 1-naphthyl,
and 2-naphthyl were calculated 89°, 103°, respectively, and dihedral angles between anthrancene
and phenyl of t-APE-moiety were calculated 103°, 102°, respectively, and dihedral angles between
anthrancene and double bond were calculated 49°, 50°, respectively. The non-coplanar structures
would prevent the intermolecular interaction and self-aggregation from forming molecular
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recrystallization, which improves the morphological stability [37]. The shapes of the HOMO and
LUMO orbitals of 6 and 7 are almost the same. The LUMO is located on the t-APE moiety. The
HOMO is dominated by the orbitals from the t-APE-antrancene core. Thus it is suggest that 6 and
7 show more red-shifted emission in comparison to t-APE.
Table 1. The optical, thermal properties and energy levels of the as-synthesized compound
6
7
Tda (°C)
Tgb (°C)
427
425
105
105
λabsc (nm)
PLd (nm)
362,380,398/364,384,406l
363,381,398/366,386,406l
475/495l
0.47[2.0]
2.85
475/495l
0.46[2.0]
2.85
Фf[τ [ns]]e
Egf (eV)
HOMOg (eV)
LUMOh (eV)
-5.48
-5.51
-2.63
-2.66
a Measured by TGA at a heating rate of 10 °C min-1 under nitrogen atmosphere. b Measured by DSC according to
d
the heat-cool-heat procedure. c Absorption spectra were recorded in the 1.0 x 10-5 mol L-1 THF solution. PL
spectra were recorded in the (1.0 x 10-5 mol L-1) THF solution. e In degassed THF solutions. f Optical energy gaps
g
calculated from the absorption thresholds from UV-Vis absorption spectra of THF solution. Measured with CV.
h｜LUMO｜=｜HOMO｜-Eg. l Absorption or PL peaks of thin films.
In order to measure the HOMO values of the compounds 6 and 7, cyclic voltammetry (CV)
measurements using a three-electrode cell were conducted (See ESI Fig. S4). The edge of the
UV-Vis spectra was used to calculate the band gap (Eg) of 6 and 7. The band gaps and the HOMO
15

ACCEPTED MANUSCRIPT
and LUMO levels are summarized in Table 1. The HOMO levels of 6 and 7 are -5.48 eV and
-5.51 eV which in agreement with the results of previous studies [11-14, 34]. The slight difference
of HOMO levels between 6 and 7 may be caused by the different dihedral angles of two
compounds between anthrancene and naphthyl [38, 39]. This result implies that 6 is easier to
inject holes than 7 in the OLEDs.
3.3. Electroluminescent properties
3.3.1. Non-doped blue-green OLEDs
To check the potential application of the compound 6 and 7 as emitters, non-doped OLEDs with
the optimized structures of ITO/TPD (25 nm)/6 or 7 (40 nm)/Alq3 (15 nm)/LiF (1 nm)/Al (100 nm)
were firstly fabricated. In these devices, N,N'-Diphenyl-N,N'-di(m-tolyl)benzidine (TPD), 6 or 7,
and Alq3 were utilized as the hole-transport layer (HTL), emitting layer (EML) electron-transport
layer (ETL), respectively. LiF was employed as the electron-injecting layer (EIL) and Al metal
served as the cathode. The device structures and the energy level diagrams of used materials are
shown in Fig. 5.
Fig. 5. The device energy-level diagram (a), configuration (b) and molecular structures of the
materials used in this study (c).
The electroluminescence (EL) spectra, current density-voltage-brightness (J-V-B) characteristics,
16

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CE, and PE of the devices 1 and 2 using compounds 6 and 7 as the emitting materials are shown in
Fig. 6, respectively. Devices 1 and 2 show a main peak centered at the 495 nm with FWHM 80 nm.
The corresponding CIE coordinates of the blue-green electroluminescence are (0.20, 0.43). The
tendency of the EL spectra of device 1 and 2 is similar to the film PL spectra of compounds 6 and
7. This implies that the emission of device 1 and 2 originates mainly from the excited monomer of
compounds 6 and 7. In some blue-green devices using anthrance derivatives as emitting materials
the blue-green emission spectra are originated from the excimer of materials which found at
molecular interactions [2, 4, 6, 7, 10]. In considering these results, the absence of emission at
longer wavelength from the excimer in the EL spectra of 1 and 2 should be attributed to the
suitable energy level and the non-coplanar molecular structures of 6 and 7. As can be seen in Fig.
6a and Fig. 6b, The devices 1 and 2 exhibit a maximum brightness of 13814, 10579 cd m-2,
turn-on voltages (defined as the operation voltage at brightness of 1 cd m-2) of 4.2, 4.6V,
maximum current efficiency of 2.34, 2.22 cd A-1, maximum power efficiency of 0.82, 0.53 lm W-1,
respectively. What is more, when their current density is increased to 500 mA cm-2, the CE value
is still remained at high levels (2.18 cd A-1 for 6 and 2.22 cd A-1 for 7), indicating a much slower
CE efficiency roll-off than most previous reports [2, 4, 10]. A comparison between these results
and performance data of recently reported blue-green OLEDs is list in Table 2. The HOMO
energy level of compound 7 (-5.51 eV) in device 2 is lower than that of compound 6 (-5.48 eV) in
device 1 indicating that the hole-injections into the emitting layer of device 1 is more effective
than device 2. The device performance of 1 is superior in turn-on voltage and efficiency
supporting the effective hole-injection properties of compound 6.
17

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Fig. 6. (a) Electroluminescence spectra of devices. (b) current density-voltage-brightness (J-V-B)
characteristics. (c) current efficiencies and power efficiencies versus current density. (d) current
density versus voltage characteristics of hole-only and electron-only devices for 6 and 7.
To best understand the different device performance, the hole and electron injection/transport
properties of the two emitting materials are identified by single-carrier devices with the structures
of ITO/TPD (15 nm)/6 or 7 (30 nm)/TPD (15 nm)/Al (100 nm) for hole only devices and
ITO/TPBI (15 nm)/6 or 7 (30 nm)/TPBI (15 nm)/LiF (1 nm)/Al (100 nm) for electron-only
devices. TPD and 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI) layer are used to
prevent electron and hole injection from the cathode and anode, respectively [9, 20, 40-42]. The
current density versus voltage (J-V) curves of the four single-carrier devices are shown in Fig. 6d.
The J-V characteristics of single-carrier devices indicate that 6 and 7 are capable of transporting
both electron and hole carriers, suggesting their bipolar transporting nature. The current density of
6 in electron-only curve is lower than that in the hole-only curve while it is opposite for 7. This
indicates a better hole transporting characteristic than electron transporting for 6 but better
electron transporting characteristic than hole transporting for 7. Moreover, the current density of 6
is lower than that for 7 at same voltage level suggesting 6 is easier transporting carrier in
18

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comparison with 7. It is noteworthy that 6 and 7 have similar molecular structure, but show
different transporting behaviors. A possible explanation for this behavior is that a twist angle
(dihedral angles between anthrancene and naphthyl) greater than 100 for 7 can suppress the
conjugation and therefore affects the carrier transport [43-45]. As described above, the device
performance differences of 6 and 7 was attributed to the imparity energy level and disparity carrier
balance.
Table 2. Key performance parameters of reported blue-green OLEDs
a
Von
ELmax
Brightness
(cd/m2)
CEb
(cd/A) (lm/W)
2.34 0.82
(3.62) f (1.33) f
2.22 0.53
(7.16) f (2.65) f
PEc
EQEd FWHM
CIEd
Compound
Ref
(V)
(nm)
(%)
(nm)
(x,y)
13814
4.2
4.6
495
495
-
80
0.20,0.43
0.20,0.43
This worke
This worke
6
7
f
(
(
3133)
10579
-
80
f
2868
)
4.1
3.5
486
498
4490
1.43
0.64
0.85
0.31
-
65
82
0.17,0.39
[7]e
[2]e
FA4PADN
2
3199
0.28
0.19ꢀ0.38
7.7
7.37
-
505
496
484
496
335
-
1.1
2.48
1.75
2.5
0.33
1.16
1.27
1.1
0.43
1.06
0.79
0.7
-
-
0.23,0.42
0.23,0.45
0.19,0.45
0.22,0.37
[4]e
[46]e
[10]e
[6]e
5c
5P-2An
2
1377
7891
-
4.0
96
PPODPA
a
"This work" is defined as the devices reported in this article. Maximum electroluminescence
b
c
d
wavelength. Maximum current efficiency. Maximum power efficiency. Maximum external
quantum efficiency. e Non-doped devices. f Optimized device
19

ACCEPTED MANUSCRIPT
In order to further improve the performance of blue-green devices, the HTL (TPD) was replaced
with NPB and ETL (Alq3) was replaced with TPBi using the structures of ITO/NPB (25 nm)/6
(device A) or 7 (device B) (40 nm)/TPBi (15 nm)/LiF (1 nm)/Al (100 nm). As shown in Fig. S5
and Table 2, the devices A and B exhibit maximum CE of 3.62, 7.16 cd A-1, maximum PE of 1.33,
2.65 lm W-1, respectively. The performances of the blue-green devices were significantly
enhanced because of the hole-blocking properties of TPBi [47]. However, the devices A and B
exhibit a maximum brightness of 3133, 2868 cd m-2 due to the low current density comparing with
devices 1 and 2. But overall, the brightness of devices A and B reach 700, 1400 cd m-2 at a current
density of 20 mA cm-2, respectively, indicating the t-APE-type derivatives have the excellent
properties for the emitting materials of OLEDs.
3.3.2. White OLEDs
To realize the application of the blue-green fluorescent emitter in white OLEDs (WOLEDs),
device 4 and 5 with the optimized configuration of ITO/TPD (25 nm)/6 or 7 (20 nm)/[Alq3:4 Wt%
rubrene] (20 nm)/Alq3 (15 nm)/LiF (1 nm)/Al (100 nm) were fabricated. The compound 6 and 7
were used as the blue-green emitter, rubrene doping in Alq3 (ETL) was used as the orange emitter,
respectively. All the devices data are listed in Table 3. The orange device 3 with the configuration
of ITO/TPD (25 nm)/[Alq3:4 Wt% rubrene] (40 nm)/Alq3 (15 nm)/LiF (1 nm)/Al (100 nm) was
constructed for comparison (See ESI Fig. S6, Table S1). Fig. 7 schematically shows an energy
level diagram and the structure of the WOLEDs.
20

ACCEPTED MANUSCRIPT
Fig. 7. (a)The energy-level diagram of the WOLEDs. (b) structure of the WOLEDs. (c) molecular
structures of the materials used in this study.
Fig. 8 reveals the EL spectra of the devices at 7, 9, 11V. As can be seen, the spectra almost
cover the whole visible light region from 400 nm to 750 nm and shows two main peaks centered at
495 nm and 565 nm which match with the emission peaks in monochromatic OLEDs using 6, 7
and rubrene. There is no other emission except for the two characteristic emissions, which
indicates that there is no exciplex formation at the interface between charge transporting layer and
light emitting layer, and the energy transfer from the Alq3 to orange dopant can be efficient. The
CRI and the CIE coordinates of the device 4 and 5 are 65, 60, (0.30, 0.40) and (0.37, 0.47),
respectively. It is well known that the spectral stability of WOLEDs is very significant for the
application in either displays or solid state lighting. In the two WOLEDs, the blue-green and
orange emissions are enhanced simultaneously with the driving voltage increase meaning that CIE
coordinates of the two WOLEDs do not change with the driving voltage increase. In other words,
the EL spectra of device 4 and 5 are stable under high voltage. In the EL process, there exists
charge trapping effect due to the energy level of the rubrene [48]. However, the high concentration
of rubrene (4 Wt%) can accelerate the electron transporting of Alq3 [49]. In addition, both 6 and 7
process bipolar characteristics, which result in charge carrier transmission is balanced in the
21

ACCEPTED MANUSCRIPT
region of EML and finally carrier confined in the EML under the high voltage [50]. This is
possible a reason for the stable EL spectra. And it is noteworthy that the device 4 exhibits cool
white while device 5 is warm white.
Fig. 8. The normalized EL spectra of the device 4 (a) and 5 (b) at various voltages.
As can be seen in Fig. 9, the devices 4 and 5 exhibit a maximum brightness of 10984, 14652 cd
m-2, turn-on voltages of 3.2 V, maximum current efficiency of 2.04, 2.70 cd A-1, maximum power
efficiency of 0.99, 0.82 lm W-1, respectively. Besides, device 5 with compound 7 is more effective
than device 4 with compound 6 which is opposite to their monochromatic OLEDs though
threshold curve is very alike.
Fig. 9. (a) Current density-voltage-brightness (J-V-B) characteristics. (b) current efficiencies and
power efficiencies versus current density.
22

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Fig. 10. Schematic diagrams of the emission mechanisms of device 4 (a) and 5 (b).
To further comprehend the different performance of the two WOLEDs, the emission mechanism
has been investigated. The hole injection barrier between 6, and Alq3 is 0.32 eV while that for 7 is
0.29 eV. For the device 4, due to the Alq3 ETL, the electron transport is relatively quick by it
while holes can easily pass through TPD to reach the blue-green EML and then be blocked by
Alq3. As shown in Fig. 10a, considering the thin blue-green EML, the main exciton generation
zone is located at the blue-green EML and partially at the orange EML, resulting in low
performance and cool white emission in device 4. As can be seen in Fig. 10b, for the device 5, due
to the smaller holes injection barrier between 7 and Alq3 than 6, the holes can easily pass through
TPD to reach the orange EML. As a result, the main exciton generation zone is located at the
orange EML and partially at the blue-green EML. Then, device 5 exhibits high performance with
warm white emission. As described above, the different performance and CIE coordinates of the
two WOLEDs were attributed to the different energy level of 6 and 7.
23

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Table 3. Key performance parameters of the WOLEDs
a
Von
(V)
3.2
3.2
ELmax
Brightness
(cd m-2)
10984
CEb
(cd A-1)
2.04
PEc
(lm W-1)
0.99
CRId
CIEd
(x, y)
Device
(nm)
4
5
495,565
495,565
65
0.30, 0.40
0.37, 0.47
14652
2.70
0.82
60
a
b
c
Maximum electrobrightness wavelength. Maximum current efficiency. Maximum power
efficiency. d Recorded at 7 V.
4. Conclusions
In conclusion, the new two blue-green materials, 6 and 7, were designed and synthesized by
incorporating naphthalene anthracene group into the t-APE molecule. The two compounds possess
high thermal stability and morphological due to the asymmetric molecule structure. These two
materials were employed as emitters to fabricate non-doped blue-green OLEDs that exhibit CE of
3.62, 7.16 cd A-1. When using 6 and 7 to construct the two-element WOLEDs, the WOLEDs
exhibit stable EL spectra at high voltage (7 to 11 V). Further optimizing the structure of WOLEDs
to improve the performance is currently in progress in our laboratory.
Acknowledgements
This work was supported by the National Key R&D Program of China (2016YFB0401303), the
National Nature Science Foundation of China (21676188) and the Key Projects in Natural Science
Foundation of Tianjin (16JCZDJC37100). The calculation in this work was supported by high
performance computing center of Tianjin University, China.
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