2-aryloxybenzo[d]oxazoles as deep blue solid-state emitters: Synthesis, aggregation-induced emission properties and crystal structure

Article abstract of DOI:10.1016/j.dyepig.2020.109127

The design and exploration of efficient organic fluorophores have attracted significant scientific interest for their various potential applications. As blue fluorescent materials are essential for the achievement of a full-color display and white emissio

Full text of DOI:10.1016/j.dyepig.2020.109127

Dyes and Pigments 187 (2021) 109127
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Dyes and Pigments
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2-aryloxybenzo[d]oxazoles as deep blue solid-state emitters: Synthesis,
aggregation-induced emission properties and crystal structure
Yuanyuan Menga, Ying Fanga, Chunming Yuanb, Chunhui Dua, Kun-Peng Wanga,
,
,**
Shaojin Chena *, Zhi-Qiang Hua
a State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao,
266042, PR China
b College of Chemistry and Environmental Science, YiLi Normal University, Yining, 835000, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The design and exploration of efficient organic fluorophores have attracted significant scientific interest for their
various potential applications. As blue fluorescent materials are essential for the achievement of a full-color
display and white emission, the development of new efficient organic blue-emitting materials is of great
importance. In this work, we found that simple organic molecules 2-aryloxybenzo[d]oxazoles could act as novel
organic fluorophores with special AIE properties and efficient blue fluorescent emission in the solid states.
Compounds 3 and 4 possess moderately high quantum yields at 10.3% and 11.4%, respectively. The CIE co-
ordinates indicate that most of these compounds are deep-blue emitters. The single crystal structure of compound
3 shows that it has a twisted V-shape molecular structure with J-type aggregation. Noncovalent interactions
Benzoxazoles
Organic fluorophores
Deep blue emitters
Aggregation-induced emission
Solid-state fluorescence
widely exist in the crystal structures, while π-π stacking interactions are not observed between adjacent aromatic
rings. The special fluorescent properties of these new emitters could ascribe to the unique molecular packing
mode in the crystal structure. This work provides a new strategy to access organic solid-state fluorophores with
deep-blue emission.
1. Introduction
attracted much interest. Until now, many AIE fluorophores have been
developed and their applications have been explored in various areas,
The design and exploration of efficient organic luminescent mate-
rials have attracted significant scientific interest for their potential
application in biology and materials science, such as optical sensors and
organic light emitting devices (OLEDs) [1]. Most conjugated organic
fluorophores are weakly emissive or nonemissive in the solid state
because of the aggregation-caused quenching (ACQ) process, which was
arisen by energy transfer and the formation of excimers in the condensed
phase. As fluorescent materials in optical devices are generally utilized
in their film or solid state, the organic emitters with ACQ effects are not
suitable for practical applications. Development of organic lumino-
phores with efficient emission in solid-state is highly required. Tang and
co-workers have reported several fluorescent materials that are strongly
emissive upon aggregation but nonemissive in dilute solutions, exhib-
iting special aggregation-induced emission properties (AIE) [2]. These
fluorophores provide an efficient solution to the ACQ problem, and have
such as optoelectronic devices and biological sensors [3].
As the three primary colors, blue, green and red are essential for the
achievement of a full-color display. The concurrent emission of the three
primary colors is necessary for the realization of white emission, which
is indispensable for lighting devices. [4] Besides acting as blue emitters,
blue fluorescent materials may also act as host materials for the green
and red dopants. However, simple organic AIE fluorophores with effi-
cient deep-blue emission in the solid states have rarely been developed
[5]. As a continous effort for development of organic luminescent ma-
terials, herein, we provide a strategy to develope new organic solid-state
fluorophores with efficient deep blue emissions [6]. These compounds
do not show any fluorescent emission in dilute solution, but show
stronger emission in highly concentrated solutions or solid states.
Further investigation reveals that they have typical AIE properties.
These fluorophores have twisted V-shape molecular structures, which
* Corresponding author.
** Corresponding author.
E-mail addresses: chensj@qust.edu.cn (S. Chen), huzhiqiang@qust.edu.cn (Z.-Q. Hu).
https://doi.org/10.1016/j.dyepig.2020.109127
Received 7 October 2020; Received in revised form 21 December 2020; Accepted 28 December 2020
Available online 31 December 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

Y. Meng et al.
Dyes and Pigments 187 (2021) 109127
Scheme 1. Synthesis and chemical structures of 1–8.
lead to their distinct fluorescent properties in solid and aggregated states
[7].
2. Experimental
2.1. Materials and instruments
All the reagents and metal salts were purchased from (Energy
Chemical, Adamas-beta and Heowns). Reagents and solvents are
analytical grade. All chemicals were used as received without further
1
purification, unless otherwise claimed. H NMR and 13C NMR spectra
were recorded on a Bruker Advance 500 spectrometer in appropriated
deuterated solution at room temperature with the solvent residual
proton signal as a standard. High resolution mass spectra (HRMS) were
measured on a GCT premier CAB048 mass spectrometer operating in
MALDI-TOF mode. Fluorescence spectra were obtained by PerkinElmer
LS-55 spectrofluorometer. UV–vis absorption spectra were recorded on a
PerkinElmer Lambda 950 spectrophotometer. Absolute fluorescence
quantum yields of dyes 1–8 in the solid state are measured on an
Edinburgh instrument FLS 1000 by comparing the areas under the cor-
rected emission spectrum of the test sample using integrating sphere
according to the definition of fluorescence efficiency.
2.2. General synthesis procedure
To a solution of phenols (3.59 mmol) in acetonitrile (20 ml) was
added anhydrous potassium carbonate (3.58 mmol), and the reaction
mixture was stirred at 80 ◦C for 2 h. Then, 2-chlorobenzoxazole (0.5 g,
3.26 mmol) was added and the mixture was further stirred at reflux for 3
h. After the reaction finished as detected by TLC, the mixture was cooled
to room temperature. The solid was filtered out and washed twice with
ethanol (30 ml). The solution was concentrated under reduced pressure.
The obtained solid residue was then recrystallized in ethanol (30 ml) to
give the pure product as a white solid.
2.2.1. 2-phenoxybenzo[d]oxazole (1)
Yield: 70%. TLC (SiO2, petroleum ether/ethyl acetate 15:1); Rf =
0.30.1H NMR (500 MHz, CDCl3) δ 7.52 (d, J = 7.4 Hz, 1H), 7.50–7.45
(m, 2H), 7.45–7.40 (m, 3H), 7.31 (d, J = 7.3 Hz, 1H), 7.29–7.21 (m,
Fig. 1. (A) UV–vis absorption of 1–8 in DMSO solution (10ꢀ 4 M) (B) Normal-
ized FL emission of 1–8 in DMSO solution (10ꢀ 2 M).
2

Y. Meng et al.
Dyes and Pigments 187 (2021) 109127
Table 1
The photophysical data of 1–8.
compound
λabs(nm)a
λem(nm)b
Stokes shift (nm)b
λem(nm) solid
ΦFc solid
τ
(ns)d solid
1
2
3
4
5
6
7
8
271,277
272,278
271,277
271,277
271,277
271,277
258,271,277
271,277
373
432
355
346
376
334
361
354
96
154
78
69
99
57
84
77
401
425
423
358
350
350
427
406
1.9%
5.3%
10.3%
11.4%
0.5%
3.5%
6.8%
4.8%
1.30
1.59
5.03
3.33
0.86
9.39
1.40
5.37
a
Measured in 0.1 mM DMSO solution.
Measured in 10 mM DMSO solution.
b
c
Absolute fluorescence quantum yield measured by using the calibrated integrating sphere system.
d
τ
= average fluorescence or phosphorescence lifetime calculated by
τ
= ΣAiτ2i /ΣAiτi, where Ai is the pre-exponential factor for lifetime τi
.
2H)⋅13C NMR (126 MHz, CDCl3) δ 161.20, 151.73, 147.39, 139.72,
139.97, 134.55, 133.68, 124.81, 123.92, 121.63, 118.49, 117.74,
110.30, 109.31.
128.93, 125.40, 123.48, 122.39, 119.12, 117.72, 108.85.
2.2.2. 2-(4-methoxyphenoxy)benzo[d]oxazole (2)
2.2.8. 2-(4-fluorophenoxy)benzo[d]oxazole (8)
Yield: 67%. TLC (SiO2, petroleum ether/ethyl acetate 10:1); Rf =
0.40.1H NMR (500 MHz,CDCl3) δ 7.50 (d, J = 7.4 Hz, 1H), 7.41 (d, J =
9.1 Hz, 1H), 7.32 (d, J = 9.0 Hz, 2H), 7.28–7.20 (m, 2H), 6.97 (d, J = 9.0
Hz, 2H)⋅13C NMR (126 MHz, CDCl3) δ 162.82, 157.65, 148.44, 146.28,
140.77, 124.41, 123.24, 121.22, 118.64, 114.85, 109.80, 55.63.
Yield: 62%. TLC (SiO2, petroleum ether/ethyl acetate 8:1); Rf =
0.40.1H NMR (500 MHz, CDCl3) δ 7.50 (d, J = 7.6 Hz, 1H), 7.44–7.37
(m, 3H), 7.30–7.20 (m, 2H), 7.14 (t, J = 8.4 Hz, 2H)⋅13C NMR (126 MHz,
CDCl3) δ 162.22, 161.39, 159.44, 148.49, 148.41, 140.57, 124.55,
123.50, 121.81, 121.75, 118.72, 116.69, 116.50, 109.88.
2.2.3. 2-(4-chlorophenoxy)benzo[d]oxazole (3)
3. Results and discussion
Yield: 65%. TLC (SiO2, petroleum ether/ethyl acetate 8:1); Rf
=
0.40.1H NMR (500 MHz, CDCl3) δ 7.52 (d, J = 8.9 Hz, 1H), 7.45–7.41
(m, 3H), 7.39 (d, J = 9.0 Hz, 2H), 7.30–7.23 (m, 2H)⋅13C NMR (126
MHz, CDCl3) δ 160.85, 150.10, 147.38, 139.49, 130.82, 128.97, 123.62,
122.61, 120.55, 117.76, 108.93.
3.1. Spectroscopic study in solution
The general scheme for the synthesis of benzoxazole derivatives 1–8
and their molecular structures are shown in Scheme 1. These compounds
are readily prepared in good yields from 2-chlorobenzoxazole by a base-
assisted nucleophilic phenoxide substitution reaction [8]. The structures
of all compounds were characterized by 1H NMR and 13C NMR. These
benzoxazole derivatives could be dissolved in various solvents, such as
toluene, THF, DCM and DMSO.
2.2.4. 2-(4-(trifluoromethyl)phenoxy)benzo[d]oxazole (4)
Yield: 60%. TLC (SiO2, petroleum ether/ethyl acetate 10:1); Rf =
0.30.1H NMR (500 MHz, CDCl3) δ 7.75 (d, J = 8.5 Hz, 2H), 7.60 (d, J =
8.4 Hz, 2H), 7.54 (dd, J = 7.0, 2.2 Hz, 1H), 7.46 (dd, J = 7.2, 2.1 Hz,
1H), 7.30 (qd, J = 7.4, 1.6 Hz, 2H). 13C NMR (126 MHz, DMSO-D6) δ
160.99, 154.96, 147.96, 140.02, 127.43, 124.76, 123.83, 121.38,
118.41, 110.27.
The absorption and emission spectra of compounds 1–8 in solution
are investigated (Fig. 1), and the detailed photophysical data are sum-
marized in Table 1. These compounds show similar absorption peaks in
DMSO solution at around 271 and 277 nm, which indicate that they
have similar electronic band gaps. The different substitution groups on
phenyl ring have little influence on the absorption profiles except
compound 7, which have a stronger absorption peak at 258 nm. This
2.2.5. 2-(4-bromophenoxy)benzo[d]oxazole (5)
Yield: 75%. TLC (SiO2, petroleum ether/ethyl acetate 10:1); Rf =
0.40.1H NMR (500 MHz, CDCl3) δ 7.59 (d, J = 8.9 Hz, 2H), 7.52 (dd, J =
7.2, 1.6 Hz, 1H), 7.43 (dd, J = 7.6, 1.6 Hz, 1H), 7.34 (d, J = 8.8 Hz, 2H),
7.31–7.19 (m, 2H)⋅13C NMR (126 MHz, CDCl3) δ 161.80, 151.70,
148.41, 140.52, 133.00, 124.66, 123.67, 121.98, 119.53, 118.82,
109.97, 99.99.
could be ascribed to the π-π* transition on the para cyano-phenyl ring.
To obtain the emission spectra of compounds 1–8, their dilute solution
in DMSO (10ꢀ 6 M) were illuminated under UV light with 270–320 nm.
Surprisingly, almost no fluorescent intensity was detected under these
conditions. Further experiments reveal that only at much higher con-
centrations can we detect remarkable fluorescent intensity. Thus, the
fluorescence emission spectra of these compounds were obtained in
their DMSO solution with a concentration of 10ꢀ 2 M. Nearly all com-
pounds exhibit fluorescent emissions in UV light range from 346 to 376
nm except compound 2, which shows a quite different fluorescent
spectrum with a larger Stokes shift. It has an obviously red-shifted
emission band at 432 nm with a shoulder at 376 nm. To explain the
special emission behaviors of compound 2, we studied its optical
properties in various solvents with different polarities (Figure S1). Upon
excitation, the solutions of 2 in toluene and chloroform showed fluo-
rescent emissions at around 375 nm, while the emission peaks were red-
shifted to around 420 nm in relatively polar solvents such as MeCN and
2.2.6. 1,4-bis(benzo[d]oxazol-2-yloxy)benzene (6)
Yield: 63%. TLC (SiO2, petroleum ether/ethyl acetate 15:1); Rf =
0.40.1H NMR (500 MHz, CDCl3) δ 7.56 (s, 4H), 7.53 (d, J = 7.3 Hz, 2H),
7.45 (d, J = 9.2 Hz, 2H), 7.31–7.24 (m, 4H). 13C NMR (126 MHz, CDCl3)
δ 160.82, 149.29, 147.39, 139.56, 123.60, 122.60, 120.61, 117.78,
108.94.
2.2.7. 4-(benzo[d]oxazol-2-yloxy)benzonitrile (7)
Yield: 65%. TLC (SiO2, petroleum ether/ethyl acetate 8:1); Rf
=
0.40.1H NMR (500 MHz, CDCl3) δ 7.79 (d, J = 8.8 Hz, 2H), 7.65 (d, J =
8.8 Hz, 2H), 7.55 (d, J = 7.0 Hz, 1H), 7.47 (d, J = 7.3 Hz, 1H), 7.37–7.27
(m, 2H)⋅13C NMR (126 MHz, DMSO) δ 168.28, 160.68, 155.35, 147.95,
3

Y. Meng et al.
Dyes and Pigments 187 (2021) 109127
Fig. 2. Changes in the fluorescent intensities of compounds 1–8 with different solvent concentrations (10ꢀ 6 M–10ꢀ 2 M) of 1 (A), 2 (B), 3 (C) in MeCN and 4 (D), 5
(E), 6 (F), 7 (G), 8 (H) in DMSO.
4

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Dyes and Pigments 187 (2021) 109127
Fig. 3. Photographs of fluorescent emission of 1–8 in MeCN or DMSO at different concentrations taken under illumination with UV light (λ = 365 nm).
5

Y. Meng et al.
Dyes and Pigments 187 (2021) 109127
Fig. 4. FL spectra of compounds 1 (A) and 2 (C) in MeOH-water mixture with different water fractions; concentration: 0.5 mM. Changes in the fluorescent intensities
of compounds 1 (B) and 2 (D) in MeOH-water mixture with different water fractions.
6

Y. Meng et al.
Dyes and Pigments 187 (2021) 109127
excited luminogen has a planar conformation stabilized by electronic
conjugation and gives a sharp emission spectrum. However, in a polar
solvent, the excited luminogen changes into a TICT state from LE state
due to intramolecular rotation. The twisted molecular conformation
could be stabilized by polar solvent molecules in solution. As the energy
level in ground state was elevated in the twisted conformation, the en-
ergy band gap of excited and ground state became smaller. Thereby, the
emission wavelength showed red-shifting in polar solvents.
As these compounds show quite unusual fluorescent properties in
solution state, the relationship between concentration and emission in-
tensity was further studied, as shown in Figs. 2 and 3. All compounds
were non fluorescent in dilute solution (10ꢀ 6-10ꢀ 4 M), but with strong
emission in higher concentrated solution (10ꢀ 2 M). For example, com-
pound 2 did not exhibit any fluorescence at lower concentration in
MeCN (10ꢀ 6-10ꢀ 3 M). When the concentration of solution was increased
to 10ꢀ 2 M, strong fluorescent emission was detected at 432 nm, and 504-
fold enhancement of emission intensity was achieved compared to that
of 10ꢀ 6 M solution (Fig. 2B). Other compounds also showed similar
fluorescent properties. At high concentration, the molecule could
aggregate through non-covalence interactions and lead to strong fluo-
rescent emission [2a,10].
The AIE properties of these new compounds are studied by investi-
gating their fluorescent behaviors in the aggregated state. Their aggre-
gates are readily prepared by adding a large amount of water to the
MeOH or THF solutions, for water is a poor solvent for these molecules.
As shown in Fig. 4A and B, compound 1 exhibits low emission intensity
in MeOH/water mixture when fw is less than 80 vol%, but the emission
intensity swiftly increased when fw gets higher than 90 vol% and 7-fold
enhancement was achieved when fw was up to 99%. When water frac-
tion is high, the aggregates form in the mixed solution and emission
intensity is enhanced through restricted intramolecular rotation process.
Other compounds also show similar fluorescent properties, exhibiting
weak emission in MeOH or THF solution but strong emission in mixed
solution with fw up to 70%–90% (Fig. 4C and D, Fig. S3 and S4). These
experiment results demonstrate that compounds 1–8 are AIE active
fluorophores.
3.2. Spectroscopic study in the solid state
The fluorescent properties of 1–8 in solid states are also investigated
(Fig. 5 and Table 1). Unlike the emission curve in solution, in the solid
state, there is almost no dependency between the electrical properties of
the substituent and the maximum emission wavelength. Compounds 4, 5
and 6 with CF3, Br and benzoxazoloxy groups emit UV light (350–358
nm) with the excitation wavelength around 290 nm, while compound 1
with no substituent shows the maximum emission peak at 401 nm. When
the substituent groups are changed to OCH3, Cl, CN, and F as in the
compounds 2, 3, 7 and 8, the emission peaks are shifted to blue light
(406–427 nm). The different emission wavelength of these benzoxazole
species might be attributed to the specific electronic effects of various
atoms or groups on aromatic rings. Substituents that form large conju-
gation system with phenyl ring could lead to red-shifted fluorescent
emission (such as OCH3, Cl, CN, and F), while others that do not con-
jugate with phenyl ring effectively could lead to blue-shifted emission
(such as CF3, and Br). In compound 6, two benzoxazoloxy groups are
linked with a phenyl ring symmetrically, and the increasing of electronic
density on the phenyl ring leads to a short emission wavelength.
The CIE coordinates of these fluorophores are as follow: 1 (0.17,
0.11), 2 (0.16, 0.09), 3 (0.16, 0.09), 4 (0.15, 0.14), 5 (0.15, 0.08), and 6
(0.15, 0.14), 7 (0.17, 0.14), and 8 (0.15, 0.11). Each of the CIE x and y
coordinates is smaller than 0.2, which indicate that these compounds are
Fig. 5. (A) Normalized FL emission of 1–8 in solid state. (B) Solid fluorescence
photographs of 1–8 taken under a UV lamp (λ = 365 nm). (C) Emission color
coordinates of 1–8 in the CIE 1931 chromaticity diagram. (For interpretation of
the references to color in this figure legend, the reader is referred to the Web
version of this article.)
DMSO. The solvent effect observed above could be attributed to a
twisted intramolecular charge transfer (TICT) mechanism [9]. Com-
pound 2 is comprised of electron-donating (methoxy group) and
electron-accepting (benzoxazole ring) units, and it takes a planar
conformation in the locally excited (LE) state. In a nonpolar solvent, the
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Y. Meng et al.
Dyes and Pigments 187 (2021) 109127
Fig. 6. Frontier-molecular-orbital distributions and energy levels of 1–8 by DFT calculations.
measured (Figure S2), and the results are summarized in Table 1. The
values of lifetimes ranged from 0.86 to 9.39 ns, which are belonged to
fluorescence.
Table 2
HOMO and LUMO Energies of compound 1–8.a
compound
HOMO (eV)
LUMO (eV)
ΔE (eV)
To better understand the electronic properties of these benzoxazole
derivatives 1–8, the density functional theory (DFT) calculations were
carried out at the B3LYP/6-31G* level (Fig. 6). The energy levels of the
highest occupied molecular orbitals (HOMOs) and the lowest unoccu-
pied molecular orbitals (LUMOs) of these compounds as well as the
energy band gaps are provided in Table 2. The distributions of HOMOs
and LUMOs are depending on the electronic effects of the substituents on
phenyl rings. For example, the HOMO of compound 2 with electron-
donating group (-OMe) is primarily localized over the phenyl ring,
whereas the LUMO is localized over the benzoxazole moiety. For com-
pound 4 and 7 with electron-accepting group (-CF3 and –CN), the
HOMOs are localized over the benzoxazole moiety, while the LUMOs are
localized over the phenyl ring. For other compounds, the HOMOs and
LUMOs are mainly located over the whole molecular frameworks. These
1
2
3
4
5
6
7
8
ꢀ 6.02
ꢀ 5.68
ꢀ 6.14
ꢀ 6.29
ꢀ 6.12
ꢀ 5.91
ꢀ 6.41
ꢀ 6.06
ꢀ 0.61
ꢀ 0.50
ꢀ 0.81
ꢀ 1.03
ꢀ 0.82
ꢀ 0.79
ꢀ 1.54
ꢀ 0.67
5.41
5.18
5.33
5.26
5.30
5.12
4.87
5.39
a
Calculated at the B3LYP/6-31G* level.
deep-blue emitters. To compare the emission efficiency of these com-
pounds, the absolute fluorescence quantum yields in solid state were
measured. It is found that compounds 3 and 4 possess moderately high
quantum yields at 10.3% and 11.4%, respectively. The photo-
luminescence lifetimes (τ) of compound 1–8 in solid state were
theoretical results indicated that compound 2,
4 and 7 have
8

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Dyes and Pigments 187 (2021) 109127
Fig. 7. (A) The molecular conformation of compound 3. (B) The dihedral angles between the central benzoxazole plane and the phenol ring. (C) The noncovalent
interactions between adjacent molecules. (D) The molecular stacking structures of compound 3 in the crystal structures.
intramolecular charge transfer (ICT) character.
crystal structures, such as C–H⋯Cl, C–H⋯N and C–H⋯O with distances
of 3.170 Å, 2.806 Å and 2.536 Å, respectively. The stacking inter-
To further understand the fluorescent properties of these emitters in
solid state, the single-crystal of compound 3 was prepared and measured
by X-ray structural analysis [11]. As shown in Fig. 7, compound 3 dis-
played a V-shape conformation and the dihedral angle between the
central benzoxazole plane and the phenol ring was 68.97◦. The non-
covalent interactions between adjacent molecules were observed in
π-π
action is not observed between the adjacent molecules. In the crystal
structures, the distances between the phenol rings of two adjacent
molecules are greater than 5.50 Å, and the distances between the phenol
rings and benzoxazole rings of two adjacent molecules are greater than
3.90 Å (Fig. 7D). More importantly, compound 3 adopts coplanar
9

Y. Meng et al.
Dyes and Pigments 187 (2021) 109127
inclined arrangements with slip angle of 35.9◦ for its transition dipoles
(Fig. 7D), which is characteristic arrangement of J-type aggregation (θ
< 54.7◦) [12]. These special arrangements and the strong fluorescence
emission of compound 3 in solid state are in good agreement with Ka-
sha’s molecular exciton model.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.109127.
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Author contribution statement
Yuanyuan Meng: Investigation, Writing-Original draft preparation.
Ying Fang: Investigation. Chunming Yuan: Investigation. Chunhui
Du: Investigation. Kun-Peng Wang: Investigation. Shaojin Chen:
Conceptualization, Methodology, Resources, Supervision. Zhi-Qiang
Hu: Resources, Supervision.
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Declaration of competing interest
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Zhong Y, Wong KS, Pena-Cabrera E, Tang BZ. J Phys Chem C 2009;113:15845–53.
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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.
[11] CCDC-2034494 (3) contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via. www.ccdc.cam.ac.uk/data.request/cif.
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Acknowledgements
We thank the National Natural Science Foundation of China (No.
21702114and 21977060), and Doctoral Fund of Qingdao University of
Science and Technology (010022761) for financial support.
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