A novel quinoline derivative containing a phenanthroimidazole moiety: Synthesis, physical properties and light-emitting diodes application

Article abstract of DOI:10.1016/j.dyepig.2021.109198

The deep-blue emitting material 2-(8-(benzyloxy)quinolin-2-yl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (QL-PPI) which contains 8-(Benzyloxy)quinoline core and phenanthroimidazole moiety, has been designed and synthesized. The non-doped OLED using QL-PPI a

Full text of DOI:10.1016/j.dyepig.2021.109198

Dyes and Pigments 188 (2021) 109198
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
A novel quinoline derivative containing a phenanthroimidazole moiety:
Synthesis, physical properties and light-emitting diodes application
,b,*
Xiaona Shaoa, Wenzhu Liua, Ruike Guoa, Junfeng Chena, Nonglin Zhoua
a College of Chemistry and Materials Engineering, Huaihua University, Huaihua, 418000, China
b Huaihua University Hunan Engineering Laboratory for Preparation Technology of Polyvinyl Alcohol (PVA) Fiber Material, Hunan, Huaihua, 418000, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The deep-blue emitting material 2-(8-(benzyloxy)quinolin-2-yl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (QL-
PPI) which contains 8-(Benzyloxy)quinoline core and phenanthroimidazole moiety, has been designed and
synthesized. The non-doped OLED using QL-PPI as the emitting layer shows emission at 455 nm, full width at
half maximum of 100 nm, maximum brightness of 250 cd mꢀ 2, maximum current efficiency of 0.47 cd Aꢀ 1, and
Commission Internationale de L’Eclairage (CIE) coordinate of (0.20, 0.22). Furthermore, the QL-PPI with a slight
mass concentration in the silica gel is coated on the near ultraviolet (NUV) chip (380 nm) to prepare the hybrid
deep-blue LED device HB1 with a y coordinate = 0.0541. The hybrid WLED device HW1 based on QL-PPI mixed
with green phosphor [(Ba, Sr)2SiO4: Eu2+] and red phosphor [(Sr, Ca)2AlSiN3: Eu2+] is prepared. The hybrid
WLED exhibits luminance efficiency of 18.05 lm Wꢀ 1 (at 19.90 mA), Correlated Color Temperature (CCT) of
4580 K, Color Rendering Index (CRI) of 80, and CIE chromaticity coordinate of (0.3571, 0.3562). These results
reveal that the QL-PPI can apply both in OLED and the hybrid LED simultaneously. The QL-PPI which used in the
hybrid WLED has the potential to reduce reliance on rare-earth metals and improve the CRI of the hybrid WLED.
Phenanthroimidazole
Quinoline
Deep-blue emitting material
OLED
Hybrid WLED
1. Introduction
to realize white light emission [16]. This structure of the hybrid device
can avoid of the reliance on rare earth metal materials because the
Since the multilayered organic light emitting diode (OLED) was
firstly reported by Tang and VanSlyke in 1987, the OLED has received
extensive attention due to its features of solid-state lighting and flexible
and transparent displays [1]. In recent years, aryl substituted benz-
imidazole (PI) has attracted great attention in OLED field because of its
simple synthesis [2], excellent thermal properties [3], high photo-
luminescence quantum yields (PLQYs) [4], and bipolar properties [5].
There are many high performance of OLED applications based on PI
derivatives [6–9], and some of them can be used in preparing deep-blue
OLED with a y coordinate ≤0.064 [10]. Simultaneously, the quinoline
derivatives and its complexes have been extensively exploited. Some of
them are suitable to be used in different opto-electronic devices or po-
tential application in OLED device due to their photoluminescent
properties [1,11–14]. Inspired by these, it was deemed to be a poten-
tially viable strategy to connect the PI unit to the quinoline skeleton with
a view to obtaining a deep-blue/blue emitting material [15].
phosphors were substituted by organic emitting materials [17]. Some
organic/inorganic hybrid WLED devices can obtain high luminance ef-
ficiency. And the CIE chromaticity coordinate of the organic/inorganic
hybrid WLED can approach to (0.33, 0.33) [18,19]. For instance, Cali
et al., constructed the organic/inorganic hybrid WLED device by
employing GaN/InGaN as the blue LED and using the organic yellow
emitting material as the under-conversion material. The device exhibits
luminance efficiency of 118 lm Wꢀ 1 [20]. Sivakumar Vaidyanathan’s
team synthesized six yellow-orange trianiline derivatives of D-A struc-
ture. The CIE chromaticity coordinate of the organic/inorganic hybrid
white light LED device is (0.32, 0.33). The CIE chromaticity coordinate
value is very close to the ideal white light (0.33, 0.33) [21]. Although
achievements have been made, the color rendering index (CRI) values of
these devices are not mentioned. This may be due to the lack of
blue-green/green light components in the emission spectra of WLED
devices using the complementary color mode (BO mode) resulting in a
low CRI which only higher values are acceptable for illumination ap-
plications. It is worth mentioning that, K.W. Cheah prepared RGB (three
primary colors) organic/inorganic hybrid WLED devices with CIE
An organic emitting material can be coated on the blue InGaN light-
emitting diode (LED) to construct organic/inorganic hybrid white LED
(WLED) which can convert the part of blue light into yellow or red light
* Corresponding author. College of Chemistry and Materials Engineering, Huaihua University, Huaihua, 418000, China.
E-mail address: znl4615@163.com (N. Zhou).
https://doi.org/10.1016/j.dyepig.2021.109198
Received 11 December 2020; Received in revised form 24 January 2021; Accepted 26 January 2021
Available online 30 January 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

X. Shao et al.
Dyes and Pigments 188 (2021) 109198
chromaticity coordinate of (0.321, 0.365) by using SrAl2O4:Eu2+ as
green phosphor, Sr4Al14O25:Eu2+ as blue phosphor, and Eu(BTFA)3Phen
as organic red emitting material however, the CRI value of the device
was not reported [22].
structure. The commercial (Ba, Sr)2SiO4: Eu2+and (Sr, Ca)2AlSiN3: Eu2+
were used as the green and red phosphors, respectively. The green/red
phosphors and organic blue emitting materials were weighted in a
certain proportion (For example, the emitting material mass concen-
tration of 0.4% in this work means that the compound weight is 0.4% of
the total mass of silica gel, and the total weight of silica gel in each
device involved in this work is 1.5 g), and then mixed with organic silica
gel thoroughly. In this step, the organic silica gel was composed of A glue
and B glue. A/B silica gel was mixed according to the weight ratio of 1
(0.3 g):4 (1.2 g). After stirring evenly, vacuum defoaming was con-
ducted for 30 min. The NUV LED chips (380 nm) were coated by the
mixture of organic silica gel to produce hybrid LED devices. After the
However, there are few reports concerning the deep-blue/blue
emitting materials applied both in OLED and the organic/inorganic
hybrid LED simultaneously. In this work, in order to achieve this target,
the new organic deep-blue emitting material 2-(8-(benzyloxy)quinolin-
2-yl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (QL-PPI) was designed
and synthesized. The thermal, photophysical properties and the light-
emitting diodes application of the QL-PPI were investigated.
◦
2. Experiment section
mixture of organic silica gel being solidified at 120 C for 1.0 h, the
obtained hybrid LED devices were used for the subsequent test. All
properties of the hybrid LED devices such as spectra, luminance effi-
ciency, CCT, CRI and CIE chromaticity coordinate of devices were
measured with an Everfine WY power source and SC-CK-09 LED test
system. All measurements were carried out at room temperature under
ambient conditions.
2.1. Materials
The main reagents are described here. Petroleum ether (PE), Ethyl
acetate (EtOAc), 1,4-Dioxane, Selenium dioxide (SeO2), Ammonium
acetate, Tetrahydrofuran (THF), Acetonitrile (MeCN) and Dichloro-
methane (DME) were received from Shanghai Titan Scientific Co., Ltd.
Benzyl chloride and 2-Methyl-8-quinolinol were acquired from
Shanghai Energy Chemical. All materials were used without further
purification unless otherwise stated. The green (BaSi2N2O2: Eu2+) and
red (Sr, Ca)2AlSiN3: Eu2+ phosphors were purchased from Shenzhen
looking long technology co., Ltd. The near ultraviolet (NUV) LED chips
(380 nm) and organic silica gel were purchased from the same company.
2.4. Synthesis
The intermediate products and designed compound were synthe-
sized as outlined in Scheme 1.
2.4.1. 8-(Benzyloxy)-2-methylquinoline (2)
8-(Benzyloxy)-2-methylquinoline was prepared according to litera-
ture [23]. White solid.
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. Elemental analysis was performed on a VarioMICROCHNOS
elemental analyzer. UV–Vis absorption and photoluminescence spectra
of THF solution and film were collected with Thermo Evolution 300
UV–Vis spectrophotometer and Hitachi F-7000 fluorescence spectro-
photometer, respectively. Differential scanning calorimeter (DSC) was
undertaken using a TA Q20 instrument under nitrogen atmosphere at a
2.4.2. 8-(Benzyloxy)quinoline-2-carbaldehyde (3)
8-Benzyloxy-2-methylquinoline (5.0 g, 20.06 mmol) and selenium
oxide (2.67 g, 24.07 mmol) were added in 1,4-dioxane (400 mL) and the
mixture was stirred under nitrogen at 105 ◦C for 3 h. The 1,4-dioxane
solvent was removed under reduced pressure, the crude product was
purified by chromatography on a silica gel column (PE/EtOAc = 10/1 as
eluent) to give the title compound 3.7 g (70.7%). 1H NMR (400 MHz,
CDCl3) δ 10.34 (s, 1H), 8.28 (d, J = 8.4 Hz, 1H), 8.08 (d, J = 8.4 Hz, 1H),
7.56 (dd, J = 12.8, 7.6 Hz, 4H), 7.41 (ddd, J = 20.4, 14.8, 7.7 Hz, 7H),
7.17 (d, J = 7.7 Hz, 1H), 5.50 (s, 2H). 13C NMR (101 MHz, CDCl3) δ
193.98, 155.14, 151.55, 140.27, 137.29, 136.51, 131.43, 129.67,
128.72, 128.55, 128.04, 127.62, 127.10, 126.98, 119.94, 117.87,
110.97, 71.07. Found C, 77.70; H, 4.88; N, 5.23%; molecular formula
heating rate of 10 C minꢀ 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 minꢀ 1. Cyclic voltammetry (CV)
was recorded on a CHI-600C electrochemical analyzer. The measure-
ments were determined using a conventional three-electrode configu-
ration consisting of a glassy carbon working electrode, a platinum-disk
auxiliary electrode and an Ag/AgCl reference electrode. And the scan
rate was 10 mV sꢀ 1. DFT calculations were performed to characterize the
3D geometries and the frontier molecular orbital energy levels of QL-PPI
at the B3LYP/6-31G(d) level by using the Gaussian 03 program. All
measurements were conducted at room temperature.
C17H13NO2; requires C, 77.55; H, 4.98; N, 5.32%.
2.4.3. 2-(8-(Benzyloxy)quinolin-2-yl)-1-phenyl-1H-phenanthro[9,10-d]
imidazole (QL-PPI)
Compound QL-PPI was prepared according to the method reported
in the literature [7,10]. 9,10-Phenanthrenedione (0.42 g, 2.02 mmol),
2.3. Device fabrication and measurement
The OLED device was fabricated by vacuum thermal evaporation
technology. Before the deposition of an organic layer, the indium tin
oxide (ITO) substrate was rinsed in an ultrasonic bath by the following
sequences: in acetone, methyl alcohol, distilled water and kept in iso-
propyl alcohol for 48 h and dried by N2 gas gun. The clean substrate was
treated with oxygen plasma under the conditions of 10ꢀ 2 Pa at 80 W for
3 min. The deposition rate of organic compounds was 0.7–1.4 Å Sꢀ 1
.
Finally, a cathode composed of LiF (1 nm) and aluminum (100 nm) were
sequentially deposited onto the substrate in the vacuum of 5 × 10ꢀ 4 Pa.
All of properties of the OLED device such as J-V-B curves, current effi-
ciency (CE), and CIE chromaticity coordinates of devices were measured
with a Keithley 2400 Source meter and Chroma meter CS-2000. The
hybrid organic/inorganic blue LED device was prepared with a structure
of “near ultraviolet LED chips + organic blue emitting materials”. The
organic/inorganic hybrid WLED device was fabricated with the similar
Scheme 1. Synthetic routes and molecular structure of QL-PPI.
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X. Shao et al.
Dyes and Pigments 188 (2021) 109198
aniline (0.28 g, 3.03 mmol), 8-(benzyloxy)quinoline-2-carbaldehyde
(0.53 g, 2.02 mmol), ammonium acetate (NH4OAc, 1.09 g, 14.12 mmol),
and acetic acid (HOAc, 10 mL) were added into flask, followed with
refluxed for 12 h under argon (Ar) environment. The reaction solution
was cooled down to room temperature, then deionized water was
poured into flask, then filtered. The residue was purified by column
chromatography on silica gel using ethyl PE/EtOAc at 10:1 by volume as
the eluent to give a white powder with yield of 75.0% (0.80 g): 1H NMR
(400 MHz, CDCl3) δ 8.94 (dd, J = 8.4, 3.6 Hz, 1H), 8.72 (dd, J = 20.4,
8.3 Hz, 2H), 8.54 (d, J = 8.6 Hz, 1H), 8.16 (d, J = 8.6 Hz, 1H), 7.82–7.72
(m, 1H), 7.70–7.58 (m, 3H), 7.53–7.44 (m, 1H), 7.45–7.15 (m, 11H),
7.07 (dd, J = 8.3, 1.3 Hz, 1H), 6.85 (dd, J = 7.3, 1.6 Hz, 1H), 5.21 (s,
2H). 13C NMR (101 MHz, CDCl3) δ 154.63, 149.48, 148.37, 139.75,
139.56, 137.56, 137.28, 136.20, 129.76, 129.54, 129.33, 129.23,
128.82, 128.74, 128.58, 128.52, 127.88, 127.36, 127.26, 127.05,
126.35, 125.78, 125.23, 124.05, 123.26, 123.20, 122.83, 122.52,
121.54, 119.57, 110.34, 70.28. Found C, 84.09; H, 4.56; N, 8.63%;
molecular formula C37H25N3O; requires C, 84.23; H, 4.78; N, 7.96%.
intramolecular charge transfer (ICT) band from the phenanthro[9,10-d]
imidazole moiety to the 8-(benzyloxy)quinolin group [24]. The ab-
sorption bands are observed in the range of 300–325 nm approximately
match to the quinoline group π-π* transition [25]. Compare the UV–Vis
absorption spectra of THF solution with that of thin films, the shape and
peak position of QL-PPI are similar. No obvious red-shift of absorption
peaks manifests that no crystallization occurs in the films [26,27].
Notably, as shown in Fig. 2, three peaks located at around 330–406 nm
in absorption spectra are not obvious than that in thin films. The peak at
about 380 nm was redshift to 390 nm and enhanced greatly. This is
owing to the dispersed state of the QL-PPI in solution and the aggrega-
tion state of the QL-PPI in solid film [28]. In the aggregated state, there is
a large molecular interaction between skeleton of the 8-(benzyloxy)
quinolin group or the phenanthro[9,10-d]imidazole moiety, which
changes the absorption peak intensity.
In the THF solution, the QL-PPI exhibits maximum PL (PLmax) value
at deep-blue wavelengths of 421 nm. As shown in Fig. S3, its emission
wavelength red-shifted (429 nm in MeCN solution) with increasing
solvent polarity, implying the emission originating from the ICT state
[29]. The photoluminescence quantum yields (PLQYs) of compound
QL-PPI was tested by using trans-1-(9-anthryl)-2-phenylethene (t-APE,
PLQYs 0.46 in MeCN) as the reference substance, the PLQYs of com-
pound QL-PPI reached 0.77. The full width at half maximum (FWHM) of
the deep-blue spectrum is 58 nm. Nevertheless, the maximum emission
peak in PL spectrum of the QL-PPI in thin film state showed large
red-shift about 34 nm with FWHM of 82 nm, comparing with its PL
spectrum in THF solution. The red-shift of the emission observed in the
film state is probably due to the presence of strong intermolecular in-
teractions or aggregation in film states.
3. Result and discussion
3.1. Synthesis and thermal properties of QL-PPI
The synthesis of QL-PPI is outlined in Scheme 1, involving a series of
hydroxyl protection reaction (benzylation), oxidation reaction and
Debus-Radziszewski reaction (see Experiment section). As described in
Fig. 1, the decomposition temperature (Td, 5% weight loss) value of the
compound QL-PPI was evaluated to be 407 ◦C. The DSC (Fig. 1 insert)
analysis uncovered that the glass transition temperature (Tg) of QL-PPI
was 92 ◦C. Moreover, no distinct crystalline peak was observed at a
temperature beyond Tg. The small molecular compound QL-PPI has
terrific thermodynamic stability due to molecular rigidity. The related
data are listed in Table 1.
To obtain a good understanding on the optical properties in the
molecular level, density functional theory (DFT) calculations (B3LYP/6-
31G(d)) were carried out by using Gaussian 03 software [30]. The
optimized geometry and the electron distribution of the compound
QL-PPI are shown in Fig. 3. The calculation results indicate that the
QL-PPI is not a planar molecule structure. There exists a dihedral angle
of the QL-PPI between benzene ring and imidazole ring and dihedral
angle between benzyloxy and quinoline ring. However, quinoline ring is
coplanar with phenomidazole ring. The HOMO and LUMO orbitals of
the QL-PPI are located on quinoline ring and phenomidazole ring. Thus,
it is suggesting that the QL-PPI shows enhanced absorption peaks
locating at 330–406 nm and more red-shifted emission in the film
comparison to solution, which is consistent with our experimental
results.
3.2. Optical properties and energy levels
The normalized UV–Vis absorption and photoluminescence (PL)
spectra of QL-PPI in THF solution (1 × 10ꢀ 5 M) and thin films (quartz
plate) are shown in Fig. 2, relevant data are summarized in Table 1. In
the THF solution, there are three peaks located at around 330–406 nm in
the UV–Vis absorption spectra of QL-PPI. The absorption peak in the
range of 345 nm can be assigned to the
of 2-imidazole to the imidazole unit. A lower-energy band with peaks at
around 365 and 380 nm can be assigned as the * transition and an
π-π* transition of the substituent
In order to measure the HOMO values of the compound QL-PPI,
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 the QL-PPI. The band gaps and the
HOMO and LUMO levels are summarized in Table 1. The HOMO and
LUMO level of the QL-PPI is ꢀ 5.9 eV and ꢀ 2.9 eV, respectively. This
result implies that the QL-PPI is easy to inject an electron in the OLED.
π-π
3.3. Non-doped blue OLED
To investigate the potential application of the compound QL-PPI,
non-doped OLED with the configuration of ITO/NPB (25 nm)/QL-PPI
(40 nm)/TPBi (15 nm)/LiF (1 nm)/Al (100 nm) was fabricated. In this
device, N,N′-Di-1-naphthyl-N,N′-diphenylbenzidine (NPB), QL-PPI, and
1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) were acted as
the hole-transport layer (HTL), emitting layer (EML) electron-transport
layer (ETL), respectively [31]. LiF was employed as the
electron-injecting layer (EIL) and Al metal served as the cathode [32].
The device configuration and the energy level diagrams of used mate-
rials are shown in Fig. 4.
The electroluminescence (EL) spectrum, current density-voltage-
brightness (J-V-B) characteristics, CE, and PE of the non-doped OLED
device using compound QL-PPI as the emitting materials are shown in
Fig. 1. TGA and DSC thermogram (insert) of QL-PPI.
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X. Shao et al.
Dyes and Pigments 188 (2021) 109198
Table 1
The optical, thermal properties and energy levels of the as-synthesized compound.
PL[d] (nm)
E[ge] (eV)
HOMO[f] (eV)
LUMO[g] (eV)
Фf [l]/Δλ[k] (nm)
[c]
Compound
T[da] (◦C)
T[gb] (◦C)
λ
abs
(nm)
QL-PPI
407
92
300,345,365,380
421/455[h]
3.0
ꢀ 5.9
ꢀ 2.9
0.77/56
/300,350,375,390[h]
[a] Measured by TGA at a heating rate of 10 ◦C minꢀ 1 under nitrogen atmosphere. [b] Measured by DSC according to the heat-cool-heat procedure. [c] Absorption
spectra were recorded in the 1.0 x 10ꢀ 5 mol Lꢀ 1 THF solution. [d] PL spectra were recorded in the (1.0 x 10ꢀ 5 mol Lꢀ 1) THF solution. [e] Optical energy gaps calculated
from the absorption threshold from UV–Vis absorption spectrum of THF solution. [f] Measured with CV. [g]|LUMO| = |HOMO|-Eg. [h] Absorption or PL peaks of thin
films. [l] PLQYs of compound QL-PPI (relative value). [k] Stokes shift of compound QL-PPI (in THF solution).
Fig. 5. All the device data are listed in Table 2. As can be seen in Fig. 5a,
the device shows a main peak centered at the 455 nm with FWHM 100
nm. The corresponding CIE coordinate of the OLED device is (0.20, 0.22)
which located at sky blue region. The main peak of the EL spectrum of
the OLED device is coincident with the film PL spectrum of compound
QL-PPI. However, there is shoulder peak generated at the 600 nm
making the FWHM of spectrum expanded to 100 nm. As can be seen
from Fig. 4a, the HOMO and LUMO level of QL-PPI is ꢀ 5.9 eV and ꢀ 2.9
eV, respectively. This implies that the QL-PPI is easy to inject an electron
and hard to inject hole from NPB to QL-PPI in the OLED, due to the large
energy barrier between NPB and QL-PPI and narrow energy barrier
between TPBi and QL-PPI. Then we suggest the shoulder peak at 600 nm
in spectrum of OLED device comes from electroplex that originates from
the direct optical transition across QL-PPI and NPB interface, which
result in the FWHM of spectrum expanded to 100 nm [33]. The
non-doped OLED device exhibits a maximum brightness of 250 cd mꢀ 2
,
turn-on voltage (defined as the operation voltage at brightness of 1 cd
Fig. 2. UV–Vis absorption and PL spectra of QL-PPI (a) dilute solution (1 ×
10ꢀ 5 M) in THF and (b) neat thin films.
Fig. 4. The device energy-level diagram (a), configuration (b) and molecular
structures of the materials used in this study (c).
Fig. 3. Molecular structures (a) and electron density distributions of the HOMO (b)/LUMO (c) calculated with B3LYB/6-31G(d).
4

X. Shao et al.
Dyes and Pigments 188 (2021) 109198
Fig. 5. (a) Electroluminescence spectra of devices. (b) current density-voltage-brightness (J-V-B) characteristics. (c) current efficiencies and power efficiencies versus
current density.
Table 2
Key performance parameters of the Non-doped blue OLED.
Coumpound
Turn-on
Voltagea/V
8
Brightness (cd mꢀ 2
)
CEb (cd Aꢀ 1
0.47
)
PEc (lm Wꢀ 1
)
ELdλmax/nm
FWHM/nm
100
CIEd 1931 (x,y)
QL-PPI
a
250
0.14
455
0.20,0.22
Recorded at 1 cd mꢀ 2
.
b
c
Max Current efficiency.
Max Power efficiency.
Recorded at 10V.
d
mꢀ 2) of 8 V, maximum CE of 0.47 cd Aꢀ 1, maximum PE of 0.14 lm Wꢀ 1
.
Table 3
Key performance parameters of the hybrid deep-blue LED and WLED.
Though the device manifests not overly impressive performance, we
believe the device performance can be further improved by changing
different holes transport functional layer which can match the energy
level with the QL-PPI.
Device
Current
(mA)
CCT (K)
CRI
CIE 1931 (x, y)
Luminance
Efficiency(lm
Wꢀ 1
)
HB1
19.90
19.90
–
–
0.1620,
0.0541
0.3571,
0.3562
2.94
3.4. Hybrid deep-blue LED and WLED
HW1
4580
80
18.05
As the QL-PPI shows deep-blue emission in the THF solution, the QL-
PPI with a mass concentration of 0.4% in the silica gel is coated on the
NUV chip (380 nm) to prepare the hybrid deep-blue LED device (HB1).
The device data are listed in Table 3. The EL spectrum and CIE chro-
maticity coordinate of device HB1 are shown in Fig. 6ab. The peak value
of the emission spectrum of device HB1 is located at 421 nm which is
consistent with the spectrum in THF solution. The device HB1 exhibits
deep-blue emission with CIE chromaticity coordinate of (0.1620,
0.0541). Moreover, device HB1 demonstrates performance with lumi-
nance efficiency of 2.94 lm Wꢀ 1 at 19.90 mA which is much better than
that of OLED device. It can be inferred that the concentration of QL-PPI
in silica gel is low, which can effectively inhibit its fluorescence
quenching in device.
In order to further expand the application of the QL-PPI in organic/
inorganic hybrid WLED devices, the inorganic WLED devices were
fabricated firstly. The inorganic WLED devices were constructed by
using the blue (BaMgAl10O17: Eu2+), green (Ba, Sr)2SiO4: Eu2+ and red
(Sr, Ca)2AlSiN3: Eu2+ phosphors. In order to obtain the equivalent
spectral intensity of blue, green and red light in the EL spectra of
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Dyes and Pigments 188 (2021) 109198
Fig. 6. (a) the EL spectrum and CIE chromaticity coordinate (b) of the device HB1. (c) the EL spectrum and CIE chromaticity coordinate (d) of the device HW1.
inorganic WLED devices (so that the CIE coordinates of the devices are
close to the ideal white light), we first studied the concentration formula
of the tri-phosphor in the devices. Considering the blue light that
emitting from blue phosphors can be absorbed by green and red phos-
phors [34,35], the WLED devices should be appropriate increase in the
concentration of the blue phosphors. In order to further verify the cor-
rectness of this conjecture, the concentration of green phosphor (3.2%)
and red phosphor (2.6%) are kept unchanged except the concentration
of blue phosphor was changed. The WLED devices properties are sum-
marized in Table S1. As shown in Figure. S5, when the concentration of
blue phosphor increases from 1.2% to 4% (devices W1, 2, 3, 4), the
intensity of blue light in the EL spectra of inorganic WLED devices is very
weak. The CIE coordinates of the four devices indicate that the WLED
devices emit warm white light although the CRI of the four WLED de-
vices can reach 83. This phenomenon is terminated in device W5 which
the concentration of blue phosphor is 14%. The emission intensity of
blue light is improved obviously in the device W5. The CIE coordinate
and CRI of the device W5 are (0.3941, 0.3774), 74.4, respectively.
However, the concentration of blue phosphor is high in the device W5
increasing an urgent need to reduce blue phosphor or replace blue
phosphor with organic blue emitting materials to reduce reliance on
rare-earth metals.
deep-blue emitting material to replace the blue phosphor to construct
hybrid WLED device can reduce the reliance on rare earth metals. The
WLED device HW1 based on the QL-PPI mixed with green phosphor
[(Ba, Sr)2SiO4: Eu2+] and red phosphor [(Sr, Ca)2AlSiN3: Eu2+] is pre-
pared. In device HW1, the silica gel total mass concentration of the QL-
PPI, green phosphor and red phosphor are 0.4%, 3.2% and 2.6%,
respectively. The EL spectrum and CIE chromaticity coordinate of device
HW1 are illustrated in Fig. 6cd. The device data are listed in Table 3. The
deep-blue region is very strong in the EL spectrum. The device HW1
exhibits luminance efficiency of 18.05 lm Wꢀ 1 (at 19.90 mA), CCT of
4580 K, CRI of 80, and CIE chromaticity coordinate of (0.3571, 0.3562).
The CIE coordinate of device HW1 is very close to the ideal values of
white light and the CRI value of 80 is much better than that of inorganic
WLED device W5 which is 74.4.
4. Conclusion
In summary, the new deep-blue material, QL-PPI with 8-(benzyloxy)
quinoline containing PI moiety, was designed and synthesized. The QL-
PPI benefit from quinoline and PI skeleton, shows high thermal stability.
The QL-PPI was employed as emitter to fabricate non-doped OLED. The
OLED shows sky-blue emission due to the aggregation in the film and
electroplex that originates from the direct optical transition across QL-
Inspired by these well-founded facts, we believe using the QL-PPI as
6

X. Shao et al.
Dyes and Pigments 188 (2021) 109198
[8] Song Y-L, Jiao B-J, Liu C-M, Peng X-L, Wang M-M, Yang Y, et al. Synthesis,
structures and luminescent properties of red emissive neutral copper(I) complexes
with bisphosphino-substituted benzimidazole. Inorg Chem Commun 2020;112:
107689.
PPI and NPB interface. When using QL-PPI to construct a hybrid LED, the
LED exhibits deep-blue emission with CIE chromaticity coordinate of
(0.1620, 0.0541). In addition, the hybrid WLED device HW1 based on
QL-PPI slightly mixed with green phosphor [(Ba, Sr)2SiO4: Eu2+] and red
phosphor [(Sr, Ca)2AlSiN3: Eu2+] is prepared. The hybrid WLED shows
better performance than that inorganic WLED (W5). The research work
presented here provides a new emitting material to fabricate OLED and
hybrid LED devices simultaneously. Further optimizing the hybrid
WLED fabrication (improving the performance) and synthesizing quin-
oline complexes (based on QL-PPI) are currently in progress in our
laboratory.
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Author statement
Xiaona Shao: Participation in the whole work; drafting of the article;
data analysis. Wenzhu Liu: Verification. Data curation. Ruike Guo:
Prepare the hybrid LED devices, Investigation. Junfeng Chen: Synthesize
the compounds of 8-(benzyloxy)-2-methylquinoline and 2-(8-(benzy-
[15] Tao YT, Balasubramaniam E, Danel A, Jarosz B, Tomasik P. Organic light-emitting
diodes based on variously substituted Pyrazoloquinolines as emitting material.
Chem Mater 2001;13(4):1207–12.
loxy)quinolin-2-yl)-1-phenyl-1H-phenanthro[9,10-d]imidazole
(QL-
PPI). Nonglin Zhou：perception and design; prepared OLED; final
[16] Smith R, Liu B, Bai J, Wang T. Hybrid III-nitride/organic semiconductor
nanostructure with high efficiency nonradiative energy transfer for white light
emitters. Nano Lett 2013;13(7):3042–7.
approval of the version to be published.
[17] Yue Z, Cheung YF, Choi HW, Zhao Z, Tang BZ, Wong KS. Hybrid GaN/Organic
white light emitters with aggregation induced emission organic molecule. Opt
Mater Express 2013;3(11):1906.
Declaration of competing interest
[18] Zhang L, Li B, Lei B, Hong Z, Li W. A triphenylamine derivative as an efficient
organic light color-conversion material for white LEDs. J Lumin 2008;128(1):
67–73.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[19] Findlay NJ, Bruckbauer J, Inigo AR, Breig B, Arumugam S, Wallis DJ, et al. An
organic down-converting material for white-light emission from hybrid LEDs. Adv
Mater 2014;26(43):7290–4.
Acknowledgements
[20] Caruso F, Mosca M, Macaluso R, Feltin E, Calı̀C. Generation of white LED light by
frequency downconversion using perylene-based dye. Electron Lett 2012;48(22):
1417.
This work was supported by the Natural Science Foundation of
Hunan Province, China (2020JJ5449), Scientific Research Fund of
Hunan Provincial Education Department (20C1474), Scientific Research
Program of Huaihua University (HHUY2018-06) and the Foundation of
[21] Kajjam AB, Kumar PSV, Subramanian V, Vaidyanathan S. Triphenylamine based
yellowish-orange light emitting organic dyes (donor-pi-acceptor) for hybrid WLEDs
and OLEDs: synthesis, characterization and theoretical study. Phys Chem Chem
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[22] Lee KM, Cheah KW, An BL, Gong ML, Liu YL. Emission characteristics of inorganic/
organic hybrid white-light phosphor. Appl Phys A Mater 2005;80(2):337–9.
[23] Maffeo D, Williams JAG. Intramolecular sensitisation of europium(III)
luminescence by 8-benzyloxyquinoline in aqueous solution. Inorg Chim Acta 2003;
355:127–36.
Hunan
Double
First-rate
Discipline
Construction
Projects
(HHUY2019–05). The calculation in this work was supported by high
performance computing center of Tianjin University, China. The OLED
device in this work was constructed in school of chemical engineering
and technology, Tianjin University, China.
[24] Zhang LK, Tong QX, Shi LJ. A highly selective ratiometric fluorescent chemosensor
for Cd2+ ions. Dalton Trans 2013;42(24):8567–70.
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de Laurentiz R. Synthesis and luminescent properties of new naphthoquinoline
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Appendix A. Supplementary data
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containing fluorenyl core and triple-carbazolyl terminals: synthesis and application
to enhancement of electroluminescence. Phys Chem Chem Phys 2013;15(36):
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Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109198.
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