The role of ancillary ligand on regulating photoluminescence properties of Eu(III) helicates

Article abstract of DOI:10.1016/j.ica.2021.120495

Three binuclear Eu³⁺ helicates, [Eu₂(OBTA)₃(H₂O)₃(CH₃COCH₃)]·1.5CH₃COCH₃·H₂O (1), [Eu₂(OBTA)₃(Bpy)₂]·2CH₃

Full text of DOI:10.1016/j.ica.2021.120495

Inorganica Chimica Acta 525 (2021) 120495
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
The role of ancillary ligand on regulating photoluminescence properties of
Eu(III) helicates
Hongjian Ma, Yanyan Zhou*, Ting Gao, Hongfeng Li, Pengfei Yan*
Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University) Ministry of Education, School of Chemistry and Materials Science, Heilongjiang
University, Harbin 150080, PR China
A R T I C L E I N F O
A B S T R A C T
Three binuclear Eu3+ helicates, [Eu2(OBTA)3(H2O)3(CH3COCH3)]⋅1.5CH3COCH3⋅H2O (1), [Eu2(OBTA)3(Bpy)2]⋅
2CH3COCH3⋅CH2Cl2 (2), [Eu2(OBTA)3(Phen)2]⋅2CH3COCH3 (3) based a new bis-β-diketone, 4,4′-bis(4,4,4-tri-
fluoro-1,3-butanedione) diphenyl ether (OBTA) was prepared in order to insight into the effects of complexes
structure on Ln3+ ion emission properties. X-ray single crystal diffraction analyses confirmed the formation of
binuclear triple-stranded helicates. The introduction of different ancillaries (Solvent molecular H2O and
CH3COCH3; 2,2′-bipyridine, Bpy and 1,10-phenanthroline, Phen) to helicates regulate the coordination envi-
ronments around Eu3+ ions, and lead to the obvious variations of photophysical properties. Combination the
structural analyses and photophysical experiment results, the significance of spatial tension of helicate is
emphasized.
Keywords:
Lanthanides
Complex
Luminescence
Ancillary ligands
Helicates
1. Introduction
the coordination structure and luminescence properties.
Recently, bis-β-diketones have been exploited as the excellent
Since the term “helicate” was firstly introduced by Lehn [1,2], this
kind of helical metallosupramolecular architecture has received great
attentions not only due to their fascinating structures, but also the
accompanied optical [3–8], magnetic [9,10] and biomedical properties
[11]. The generation of helicates need the ligand having a modest rigid,
meanwhile the metal ions owning a predictable coordination direction.
In this situation, the transition metals have markedly superiority to
construct helicates due to their well coordination direction [12–14]. In
contrast, the construction of lanthanide helicates is challenging in view
of the large radii and undefined coordination geometries of Ln3+ ions
[15,16].
candidate to prepare lanthanide helicates [26,27]. The doubly negative
charge of bis-β-diketonate anion could strongly bonded to Ln3+ ion, by
electrostatic and coordination interaction, and improve the thermody-
namic stability of helicate. Additionally, the highly effective sensitizing
capable of β-diketone on Ln3+ ion luminescence has also been well
documented [28–30].
Herein, we designed a new bis-β-diketone ligand, 4,4′-bis(4,4,4-tri-
fluoro-1,3-butanedione) diphenyl ether, OBTA. The O-bridge between
two β-diketonate units offers a modest rigid for ligand to wrap around
Ln3+ ion to form helicate. Upon self-assembly of OBTA and Eu3+ in a 3:2
stoichiometric ratio, a triple-stranded binuclear Eu3+ helicate was iso-
The motivation of preparing lanthanide helicates originated from
their excellent luminescence properties [17,18]. In this realm, much
efforts have been devoted by Piguet and Bünzli [19–22]. The stability
and rigid of helicates generally endowed the complexes the higher
luminescent quantum yields [23,24], while the supramolecular chirality
arising from the helical twisting of ligand strands rendered the chirop-
tical properties of Ln3+ ion [25]. To insight into the effects of coordi-
nation structure on Ln3+ ion emission properties is the constant subject
for lanthanide luminescence materials. The excellent structural stability
of helicate provide a perfect platform to study the relationship between
lated
with
the
formula,
[Eu2(OBTA)3(H2O)3(CH3COCH3)]⋅
1.5CH3COCH3⋅H2O (1). In order to investigate the effects of coordina-
tion environment on photophysical properties, two nitrogen-containing
heterocycles, 2,2′-bipyridine (Bpy) and 1,10-phenanthroline (Phen)
were selected as ancillary ligands to replace solvent molecules H2O and
CH3COCH3. Single-crystal X-ray diffraction analyses confirmed the for-
mation
of
[Eu2(OBTA)3(Bpy)2]⋅2CH3COCH3⋅CH2Cl2
(2)
and
[Eu2(OBTA)3(Phen)2]⋅2CH3COCH3 (3). On the basis of the structure
analyses and comprehensive photophysical experiments, it was
concluded that the different spatial tension of helicates probably was the
* Corresponding authors.
E-mail addresses: zhouyanyan_1216@163.com (Y. Zhou), Yanpf@vip.sina.com (P. Yan).
https://doi.org/10.1016/j.ica.2021.120495
Received 7 March 2021; Received in revised form 20 May 2021; Accepted 4 June 2021
Available online 7 June 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

H. Ma et al.
Inorganica Chimica Acta 525 (2021) 120495
Scheme 1. Synthetic routes of the L and the corresponding lanthanide complexes 1–3.
crucial factor to affect non-radiative decay rate, intrinsic quantum effi-
ciency, even the total luminescence efficiency.
2.3. Synthesis of 4,4′-Bis(4,4,4-trifluoro-1,3-butanedione) diphenyl ether
(OBTA)
2. Experimental section
A mixture of sodium methoxide (1.44 g, 26.67 mmol) and ethyl
trifluoroacetate (3.84 g, 27.00 mmol) in dry DME (70 mL) was stirred for
10 min, followed by the addition of 4,4′-Diacetyl diphenyl ether (2.54 g,
10.00 mmol), which was further stirred at room temperature overnight.
The resulting solution was quenched with water and was acidified to pH
2–3 using hydrochloric acid (1 M solution). The resulting yellow pre-
cipitate was filtered and dried at room temperature. Recrystallization
from cyclohexane gave yellow needle crystals (3.38 g, 72%). Anal. Calc.
for C20H12F6O5 (446.06): C, 53.82; H, 2.71. Found: C, 53.71; H, 2.65. IR
(KBr, cmꢀ 1): 3075, 1588, 1501, 1239, 1201, 1168, 1108, 799, 576. 1H
NMR (CDCl3, 400 MHz): 15.19 (s, 2H), 7.99 (d, 4H), 7.17 (d, 4H), 6.55
(s, 2H). ESI-TOF-MS m/z 501.0481 [M + CH3OH + Na]+.
2.1. Materials and instruments
All commercially available chemicals and the solvents were analyt-
ical reagent grade and used without further purification.
The IR spectra were recorded in the 4000–400 cmꢀ 1 region using KBr
disks and a PerkinElmer Spectrum One spectrophotometer. UV/vis ab-
sorption spectra were measured with a PerkinElmer (Lambda 25)
spectrometer. 1H NMR spectra were measured by using a Bruker Avance
400 (400 MHz) spectrometer in CDCl3. Electrospray mass spectrometry
(ESI-MS) mass spectra were taken on Bruker maXis mass spectrometers.
X-ray crystal data for the complexes were collected on a Xcalibur, Eos,
Gemini diffractometer with Mo Ka radiation. The structures were solved
by direct methods and refined on F2 by full-matrix least-squares using
the SHELXTL-2014 program [31]. The level B alerts in checkCIF file are
caused by some reflections which are omitted due to beam stop or
overflow. The luminescence spectra and luminescence lifetimes were
done on an Edinburgh FSL-920 fluorescence spectrometer. The absolute
quantum yields of Eu(III) ions emissions were measured by an absolute
method using an Edinburgh Instruments integrating sphere equipped
with the Edinburgh FLS-920 fluorescence spectrophotometer. The
values reported are the average of three isolated determinations. The
absolute quantum yield were measured by the following expression:
2.4. Synthesis of [Eu2(OBTA)3(H2O)3(CH3COCH3)]⋅
1.5CH3COCH3⋅H2O
OBTA (0.20 g, 0.45 mmol) and NEt3 (0.10 g, 0.99 mmol) were dis-
solved in methanol (20 mL), and the mixture was allowed to stir for 20
min. EuCl3⋅6H2O (0.11 g, 0.30 mmol) in methanol (5 mL) was added
dropwise and stirred at room temperature for 24 h. The precipitate
formed after the addition of water was filtered and washed with H2O (3
× 10 mL) and CH3OH (3 × 10 mL) and dried in vacuum. Yield: 85%.
Anal. Calc. For C60H38Eu2F18O19 (1708.06): C, 42.09; H, 2.55. Found: C,
42.02; H, 2.47. IR (KBr, cmꢀ 1): 1620, 1459, 1311, 1291, 1244, 1169,
793. ESI-TOF-MS m/z 1658.9395 [Eu2(OBTA)3 + Na]+.
∫
Lemisson
Ereference ꢀ Esample
∫
Φ =
2.5. Synthesis of Eu2(OBTA)3L2 [L = 2,2′-bipyridine (2), 1,10-phenan-
throline (3)]
Where Lemission is the emission spectrum of the sample, collected by the
integrating sphere, Esample is the spectrum of the incident light collected
by the sphere, and Ereference is the spectrum of the light used for exci-
tation with only the reference in the sphere.
[Eu2(OBTA)3(H2O)3(CH3COCH3)]⋅1.5CH3COCH3⋅H2O (0.854 g,
0.50 mmol) was dissolved in methanol (25 mL), co-ligand (1.0 mmol) in
methanol (15 mL) was added and refluxed for 7 h. After cooling to room
temperature, the resulting white precipitate was filtered and dried under
vacuum.
2.2. Synthesis of 4,4′-diacetyl diphenyl ether
[Eu2(OBTA)3(Bpy)2]⋅2CH3COCH3⋅CH2Cl2 (2): Yield: 85%. Anal.
Calc. For C80H46Eu2F18N4O15 (1948.16): C, 49.19; H, 2.65; N, 2.83.
Found: C, 49.11; H, 2.59; N, 2.88. IR (KBr, cmꢀ 1): 1619, 1312, 1249,
1168, 793. ESI-TOF-MS m/z 1971.0735 [Eu2(OBTA)3(Bpy)2 + Na]+.
[Eu2(OBTA)3(Phen)2]⋅2CH3COCH3 (3): Yield: 80%. Anal. Calc. For
Acetyl chloride (2.77 g, 35.28 mmol) and anhydrous AlCl3 (4.70 g,
35.28 mmol) dissolved in anhydrous dichloromethane (30 mL) were
added dropwise to a stirred solution of diphenyl ether (2.00 g, 11.76
mmol) in anhydrous dichloromethane (20 mL). The reaction was stirred
at room temperature overnight and the resultant mixture was poured
into 100 mL ice water. The resulting organic layer was dried over
anhydrous Na2SO4. The solvent was removed under reduced pressure to
give a crude product, which was purified by recrystallization from
ethanol and acetone to afford white flake crystals (2.42 g, 81%). Anal.
calc. for C16H14O3 (254.09): C, 75.57; H, 5.55. Found: C, 75.44; H, 5.42.
IR (KBr, cmꢀ 1): 1601, 1228, 1172, 1109, 789. EI-MS m/z 254.09 M+.
C
84H46Eu2F18N4O15 (1996.16): C, 50.41; H, 2.60; N, 2.77. Found: C,
50.34; H, 2.52; N, 2.82. IR (KBr, cmꢀ 1): 1618, 1505, 1312, 1248, 1168,
791.ESI-TOF-MS m/z 2019.0701 [Eu2(OBTA)3(Phen)2 + Na]+.
3. Results and discussion
3.1. Synthesis and characterization of ligands and complexes
The synthesis procedures of the ligand OBTA and their correspond-
ing Ln(III) complexes 1, 2 and 3 are outlined in Scheme 1. The C2-
symmetric
achiral
bis-β-diketone
4,4′-bis(4,4,4-trifluoro-1,3-
2

H. Ma et al.
Inorganica Chimica Acta 525 (2021) 120495
Table 1
Shape analysis of Eu(III) complexes 1, 2 and 3 by SHAPE 2.1 software.
Complexes
Square
Triangular
Biaugmented trigonal
antiprism (D4d
)
dodecahedron (D2d
)
prism (C2v)
1
2
3
2.376
0.722
1.859
0.408
1.578
0.856
2.250
2.178
1.395
Fig. 1. ESI-TOF mass spectra of complexes 1–3 with insets showing the simu-
lated (Sim.) and observed (Obs.) isotopic pattern.
Fig. 3. UV–visible absorption spectra of OBTA, and complexes 1, 2 and 3 in
CH3OH (1.0 × 10–5 M).
CH3OH. While the complexes [Eu2(OBTA)3(Bpy)2]⋅2CH3COCH3⋅CH2Cl2
(2) and [Eu2(OBTA)3(Phen)2]⋅2CH3COCH3 (3) were obtained by adding
2.0 equivalent of 2,2′-bipyridine or 1,10-phenanthroline into the solu-
tion of 1 in refluxing condition. High resolution ESI-TOF-MS analyses
affirmed the successful preparation of triple-stranded helicates. From
Fig. 1, three clusters of peaks with m/z at 1658.9395, 1971.0735 and
2019.0701 corresponding to [M + H]+ molecule ion peaks of three
helicates,
[Eu2(OBTA)3(H2O)3(CH3COCH3)]⋅1.5CH3COCH3⋅H2O,
[Eu2(OBTA)3(Bpy)2]⋅2CH3CO CH3⋅CH2Cl2 and [Eu2(OBTA)3(Phen)2]⋅
2CH3COCH3, respectively, could be clearly observed. The assignments
were further affirmed by comparing the isotopic distributions of the
experimental with simulated results.
3.2. X-ray crystallographic analysis
The dinuclear triple-stranded helical structures of three complexes
were affirmed by single crystal X-ray diffraction analyses (Table S1). The
complex 1 crystallized in the monoclinic space group P21/c, while the
complexes 2 and 3 both crystallized in the monoclinic space group C2/c.
In complex 1, each Eu3+ ion was eight coordinated to six O atoms from
three β-diketone chelate units and two O atoms from solvent molecules
H2O or CH3COCH3 (Fig. 2(a)). The Eu–O (bis-β-diketonate) distances fall
in the range of 2.361–2.411 Å, which are slightly shorter than those of
Eu–O bond lengths of coordination solvent molecules, 2.435 and 2.472
Å. In complexes 2 and 3, the eight coordinated geometry of Eu3+ ion was
saturated by six O atoms of β-diketones and two N atoms from ancillary
ligands Bpy or Phen, as shown in Fig. 2(b) and (c). The Eu–O distances
are in range of 2.342–2.391 Å and 2.324–2.414 Å for complexes 2 and 3,
respectively, while the Eu–N distances obviously lengthened, reaching
to 2.554–2.568 Å in Bpy and 2.546–2.582 Å in Phen complexes. It was
rational to consider that the O atoms had the stronger affinity than N
atoms.
Fig. 2. Ball-and-stick representation of the crystal structures of 1 (a), 2 (b),
3 (c).
dioxobutyl) diphenyl ether (L) was prepared via two steps, the first was
the Friedel ꢀ Crafts acylation of diphenyl ether, then followed with a
Claisen condensation of 4,4′-diacetyl diphenyl ether and ethyl tri-
fluoroacetate. The corresponding intermediate and ligand were char-
acterized by ESI-MS, ESI-TOF-MS and 1H NMR (Figs. S1–S3).
The complex [Eu2(OBTA)3(H2O)3(CH3COCH3)]⋅1.5CH3COCH3⋅H2O
(1), was isolated by stirring a 3:2 mixture of OBTA with the corre-
sponding LnCl3⋅6H2O salts (Ln = Eu, Gd) with a triethylamine as base in
As previously reported by us and others [32–37], the coordination
geometry around Eu3+ ions have been proved to relate to their emission
3

H. Ma et al.
Inorganica Chimica Acta 525 (2021) 120495
emission intensity is far higher than those of 5D0→7F1,3-6 transitions. In
fact, according to the Judd–Ofelt theory [38–40], this transition is
strictly forbidden if a Eu3+ ion locate at a site with an inversion center.
5
Therefore, the larger D0→7F2 emission intensity generally means the
stronger the distortion of the Eu3+ site from a highly symmetric coor-
dination polyhedron. The integrated intensity ratio of I(5D0→7F2)/I
(5D0→7F1) was often employed as a probe to infer the coordination ge-
ometry symmetry of Eu3+ ion [41–47]. Here, the intensity ratios reach to
16.82, 16.66 and 16.96 for complexes 1, 2 and 3, respectively. It indi-
cated the presence of a low symmetry around the metal ions, which were
in line with the coordination geometries calculated from crystal
structures.
The luminescence quantum yields (QYs) experiments showed that
three helicates all present the excellent luminescence efficiencies, with
the absolute QYs reaching up to 33%, 39% and 47% for
[Eu2(OBTA)3(H2O)3(CH3COCH3)]⋅1.5CH3COCH3⋅H2O,
[Eu2(OBTA)3(Bpy)2]⋅2CH3COCH3⋅CH2Cl2 and [Eu2(OBTA)3(Phen)2]⋅
2CH3COCH3. The lower value of complex 1 compared with 2 and 3 were
easily understood as the consideration of solvent molecules ligated on
one metal center. Generally, the replacement of coordination solvents
with ancillary ligands would markedly enhance the Ln3+ center emis-
sion [48–52]. Herein, the enhanced degree of luminescence for two
substituted complexes were different, 3 display an increase of 14%,
while 2 only give a 6% enhancement. This result demonstrates that the
introduction of ancillary ligand is an effective strategy for lanthanide
complexes, but the enhanced extent was not only determined by the
reduction or disappearance of high energy oscillators, such as N–H and
O–H vibrations. Herein, the bidentate ancillary ligands, Bpy and Phen
have similar electron structures and the same coordination fashion. It is
suggested that the luminescence properties of Ln3+ center should relate
to the coordiantion geometry around Eu3+ ion. The different split pat-
terns of 5D0→7F2 and 5D0→7F1 transitions in complexes 2 and 3 also
reflected their different coordination environment [53–56].
Fig. 4. Solid state emission spectra of 1–3 with excitation at 374 nm.
properties. Therefore, the coordination geometries around Eu3+ ions in
three complexes were calculated based on the crystal data by employing
the SHAPE 2.1 software (Fig. S4). The smaller of calculated value, it was
closer to the perfect coordination polyhedra. From the calculated values
(Table 1), the coordination geometries of complexes 1 and 3 were sug-
gested to fulfill an eight-coordinated triangular dodecahedral structure
with the D2d symmetry. In complex 2, a high symmetry eight-
coordinated square antiprismatic structure (8-SAPR, D4d) should be
more reasonable.
To get insight into the relationship between the coordination ge-
ometry and the luminescent properties, we estimated the related pho-
tophysical parameters, such as the excited state lifetimes, the intrinsic
quantum yields (ΦLn) and sensitizing efficiency (ηsen). For Ln3+ com-
plexes, the absolute luminescence quantum yields are proportional to
the ΦLn and ηsen by eq. (1):
Φoverall
=
ηsenΦLn
(1)
The intrinsic quantum yields (ΦLn) reflect the competition efficiency
of radiative transition and nonradiative transition to deactivate the
excited state (Eq. (2)). The values of radiative rate constant (kr) and non-
radiative constant (knr) are the scale to assess the two transitions.
3.3. Photophysical properties of helicates
The absorption spectra of free ligand OBTA and its corresponding
Eu3+ complexes 1, 2 and 3 are shown in Fig. 3. They all display two clear
bands in ranges of 250–300 nm and 300–380 nm, respectively. The low
energy bands with maxima at about 330 nm could be attributed to the
ILCT transition from benzene ring to β-diketone units. While the high
kr
τobs
ΦLn
=
=
(2)
kr + knr τrad
The radiative rate constant from Eu3+ center can be estimated from
Eq. (3):
energy bands are ascribed to the π–π* transitions localizing at benzene
ring of ligand, 2,2′-bipyridine (Bpy) and 1,10-phenanthroline (Phen). In
comparison with free ligand, the complexes all show a shoulder at about
354 nm, which was an exciton coupling feature usually observed in
β-diketone complexes. It originated from the spatial proximity of three
strand ligands. In addition, we also estimated the possible transition
characters of ligand OBTA by DFT calculation with the Gaussian 09
package (basis set, B3LYP/6–31 + g(d,p)). From the optimized electron
structure (Fig. S5), it can be observed that the electron cloud of HOMO
orbital mainly locates at the diphenyl ether moiety, while in LUMO
orbital the electron cloud spreads to diketone units. This result indicates
that the electron transitions in the ligand probably have ILCT character.
Fig. 4 show the emission curves of three helicates upon excitation at
374 nm, the maxima value of excitation bands (Fig. S6). In the spectra,
five emission bands at 580, 594, 611, 651 and 702 nm were observed,
which corresponds to the 5D0→7FJ (J = 0–4) transitions of Eu3+ ion. In
these bands, the 5D0→7F2 transition is hypersensitive transition, its
(
)
1
kr =
Itot
IMD
= AMD,0n3
(3)
τrad
Itot and IMD represent the total integrated emission of 5D0→7FJ and
5D0→7F1 transitions, respectively. AMD,0 is a constant (14.65 sꢀ 1), rep-
5
resenting the spontaneous emission probability of D0→7F1. With this
equation, the kr are calculated to be 1.0 × 103 sꢀ 1 for three complexes 1,
2 and 3. Calculated radiative lifetimes τrad is the reciprocal of kr, the
values of τrad are 9.6 × 102, 9.8 × 102 and 9.5 × 102
μs, respectively.
These values are almost equivalent, consequently we cannot build an
empirical rule based on this result.
The non-radiative rate constants can be obtained by equation (2)
after intrinsic quantum yields (ΦLn) being estimated. On the basis of the
emission decay curves monitored within the 5D0→7F2 transition (Fig. 4),
the lifetimes of the complexes 1, 2 and 3 were measured to be 3.8 × 102,
4.4 × 102 and 4.7 × 102
μs, respectively (Figs. S7–S9). Based on the
4

H. Ma et al.
Inorganica Chimica Acta 525 (2021) 120495
Table 2
Radiative (kr) and nonradiative (knr) decay rates, observed luminescence lifetime (τobs), calculated radiative lifetimes (τrad), intrinsic quantum yield (ΦLn), sensitization
efficiency (ηsen), overall quantum yield (Φoverall) for complexes 1–3 at 298 K. Error in τobs Eu: ±0.05 ms; 10% relative error in the other values.
Complexes
I7F2/I7F1
kr (sꢀ 1
)
knr (sꢀ 1
)
τobs
(
μs)
τrad
(
μs)
ΦLn (%)
ηsen (%)
Φoverall (%)
1
2
3
16.82
16.66
16.96
1.0 × 103
1.0 × 103
1.0 × 103
1.6 × 103
1.2 × 103
1.0 × 103
3.8 × 102
4.4 × 102
4.7 × 102
9.6 × 102
9.8 × 102
9.5 × 102
39
45
50
84
86
93
33
39
47
calculated radiative lifetimes (τrad) and measured lifetimes, ΦEu were
calculated to reach up to 39%, 45% and 50% for complex 1, 2 and 3.
With Eq. (2), the nonradiative rate constants (knr) were calculated to be
1.6 × 103, 1.2 × 103 and 1.0 × 103 sꢀ 1 for complex 1, 2 and 3. The larger
value for 1 was due to the effectively deactivated capable of O–H
oscillator present in coordination water molecules. While the about 200
sꢀ 1 difference of knr values between 2 and 3 could bring their different
ΦEu, 45% and 50%. Because the excitation wavelength used to emission
measurement was 374 nm, which beyond the absorbance region of
ancillary ligands. Therefore, the participation of electron structure of
Bpy or Phen on Eu3+ ion emission could be excluded. Thus, the different
knr values should originate from the slightly variation of helical struc-
tures caused by ancillary ligands. After focusing on the single crystal
structure once again, we found that the Eu–Eu distances in one helicate
were difference, with 13.469 Å for 2 and 14.108 Å for 3. The shorter
distance implies 2 was slightly “fatter” than 3. In helicates, it indicates
there are a little stronger pulling force between two metal centers. From
the overlapped partial crystal structure (Fig. S10), one benzene ring on
one helical strand of 3 was obviously away from the helical axis. This
result demonstrated there was a larger tension in 2 than that in 3. On the
other word, the smaller tension implies the more stable structure of 3
and the smaller contribution to the non-radiative decay.
Declaration of Competing Interest
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.
Acknowledgments
This work is financially supported by The National Natural Science
Foundation of China (Nos. 51773054, 51872077 and 52073080). We
also thank the Key Laboratory of Functional Inorganic Material Chem-
istry, Ministry of Education, PR China, for supporting this work.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120495.
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