Synthesis, fluorescence properties and F? detection performance of Eu(III) complexes based on the novel coumarin Schiff base derivatives

Article abstract of DOI:10.1016/j.molliq.2020.114439

Three novel coumarin Schiff base derivatives and corresponding Eu(III) complexes were designed and synthesized. The aggregation-induced emission (AIE) property of ligands was studied. The fluorescence properties and electrochemical properties of Eu(III) complexes were investigated by comparing different substituent groups. The ion selectivity and ion detection limits of the [EuL²(NO₃)₃] ?H₂O were also studied. The result indicates that all ligands exhibit the AIE property in CH₃CN/H₂O solution and emit green fluorescence with the maximum emission wavelength of 552 nm. All complexes show characteristic red fluorescence of Eu(III) and the influence of different substituents on the fluorescence intensity of Eu(III) complexes is ordered: -OCH₃ > -H > -NO_2, and [EuL²(NO₃)₃] ?H₂O exhibits the strongest fluorescence intensity. The density functional theory (DFT) calculations of the ligands indicate that the introduction of electron-donating groups increases the HOMO energy level and the electron cloud density, which enhance the ability to coordinate with Eu(III) and the fluorescence intensity of the corresponding complex. Besides, the electrochemical properties of Eu(III) complexes indicate that the introduction of electron-donating groups increases the energy gaps (Eg) and the conjugation of the complex, which enhances the fluorescence intensity of the complexes. Ion selectivity experiments show that [EuL²(NO₃)₃] ?H₂O has a specific recognition function for F^?, and its response limit is between 1.0 × 10^?5 mol·L^?1 and 2.2 × 10^?5 mol·L^?1. Based on the above results, all the ligands and Eu(III) complexes have a promising application in optical materials due to the excellent fluorescence properties. In addition, All Eu(III) complexes also have a potential application prospects in the field of fluoride ion detection.

Full text of DOI:10.1016/j.molliq.2020.114439

Journal of Molecular Liquids 320 (2020) 114439
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis, fluorescence properties and F− detection performance of Eu
(III) complexes based on the novel coumarin Schiff base derivatives
⁎
Fangfei Luan, Gangxiang Xiao, Yuxi Zhang, Shiquan Li, Zhongqian Hu, Hongli Du, Dongcai Guo
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 August 2020
Received in revised form 12 September 2020
Accepted 23 September 2020
Available online 30 September 2020
Three novel coumarin Schiff base derivatives and corresponding Eu(III) complexes were designed and synthe-
sized. The aggregation-induced emission (AIE) property of ligands was studied. The fluorescence properties
and electrochemical properties of Eu(III) complexes were investigated by comparing different substituent
groups. The ion selectivity and ion detection limits of the [EuL2(NO3)3] •H2O were also studied. The result indi-
cates that all ligands exhibit the AIE property in CH3CN/H2O solution and emit green fluorescence with the max-
imum emission wavelength of 552 nm. All complexes show characteristic red fluorescence of Eu(III) and the
influence of different substituents on the fluorescence intensity of Eu(III) complexes is ordered: -OCH3 > -
H > -NO2, and [EuL2(NO3)3] •H2O exhibits the strongest fluorescence intensity. The density functional theory
(DFT) calculations of the ligands indicate that the introduction of electron-donating groups increases the
HOMO energy level and the electron cloud density, which enhance the ability to coordinate with Eu(III) and
the fluorescence intensity of the corresponding complex. Besides, the electrochemical properties of Eu(III) com-
plexes indicate that the introduction of electron-donating groups increases the energy gaps (Eg) and the conju-
gation of the complex, which enhances the fluorescence intensity of the complexes. Ion selectivity experiments
show that [EuL2(NO3)3] •H2O has a specific recognition function for F−, and its response limit is between
1.0 × 10−5 mol·L−1 and 2.2 × 10−5 mol·L−1. Based on the above results, all the ligands and Eu(III) complexes
have a promising application in optical materials due to the excellent fluorescence properties. In addition, All
Eu(III) complexes also have a potential application prospects in the field of fluoride ion detection.
© 2020 Elsevier B.V. All rights reserved.
Keywords:
AIE property
Eu(III) complex
Fluorescence property
Electro-chemical property
Ion detection
1. Introduction
the mechanism of restricted intramolecular rotations (RIR) [7,8]. Then,
a lot of works have been done around the AIE effects, such as ion probes,
In recent years, lanthanide complexes have been widely used in ca-
talysis, sensors, magnetic recording materials and optical materials
fields based on their special porous cavities and notable electronic fea-
tures [1–5]. Among many rare earth complexes, Eu(III) complexes
have pure red fluorescence, long lifetime and good monochromaticity.
These features enable a wide range of applications as optical probes
for bioimaging and sensors. However, there are still some shortcomings
that need to be overcome. For example, poor thermal stability, low lu-
minous intensity and low electron transfer efficiency. The fluorescence
intensity of lanthanide complexes mainly depends on two key factors:
(1) Transfer efficiency of triplet energy level and luminescence level of
ligands (2) The extent of non-radiative decay of excited state of lantha-
nide complexes [6]. Therefore, designing and synthesizing excellent
fluorescent ligands is the key of this work.
biotherapeutics, optical materials, OLEDS and cancer detection, and so
on [9–11]. Most of AIE materials have a large conjugated structure,
which is a good choice as a luminescent ligand. Among many AIE mate-
rials, some Schiff-base materials with intramolecular proton transfer
(ESIPT) properties have also been reported recently [12–14]. For the
Schiff-base materials, most of them are used in organic catalysis synthe-
sis, complexes and liquid crystal materials fields. Especially, in com-
plexes fields, the Schiff-base derivatives often are selected as the
ligand due to their ease of preparation and good coordination ability
[15,16]. Therefore, designing and synthesizing Schiff-base derivatives
with AIE properties is a good choice.
As we all know, coumarin derivatives are often used in the field of
drug therapy and bioprobes, especially in the field of fluorescent mate-
rials [17–19]. Whether natural or artificial, the preparation of coumarin
derivatives is relatively easy to obtain and low in cost. Besides, most
hydroxy-coumarin compounds have a certain fluorescent property,
which is also widely used in the field of fluorescent materials. In our pre-
vious works, some excellent ligands and complexes are synthesized,
such as Schiff bases, pyrazolones, quinolines, and so on [20–23]. Most
In 2001, the phenomenon of aggregation-induced emission (AIE)
was first reported by Tang and co-workers, which was explained by
⁎
Corresponding author.
E-mail address: dcguo2001@hnu.edu.cn (D. Guo).
https://doi.org/10.1016/j.molliq.2020.114439
0167-7322/© 2020 Elsevier B.V. All rights reserved.

F. Luan, G. Xiao, Y. Zhang et al.
Journal of Molecular Liquids 320 (2020) 114439
researchers focus on the function of complexes, and the complexes of
AIE coumarin derivatives ligands has rarely been reported. Therefore,
coumarin derivatives are also a good choice as ligands.
using a CHI 660d electrochemical workstation (electrolyte: 0.1 M
tetrabutylammonium exafluorophosphate, solvent: DMF, sensitiv-
ity: 0.1 mA/V, scan speed: 50 mV s−1).
In this work, three novel coumarin Schiff base derivatives and corre-
sponding Eu(III) complexes are synthesized and characterized by 1H
NMR, 13C NMR, ESI-MS, UV–Vis, Molar conductivity, Elemental analysis,
Thermogravimetric analysis and FT-IR analysis. The AIE property of the
ligands is explored, and the influence of different substituents on the
fluorescence and electrochemical properties of Eu(III) complexes is
discussed. Besides, this work has further studied the ion detection per-
formance of the complex with the best fluorescence performance. Cor-
respondingly, the synthetic route of ligands (L1–3) is presented in
Scheme 1.
2.2. Synthesis methods
2.2.1. Synthesis of intermediates
2.2.1.1. 7-Hydroxy-4-methyl-2H-chromen-2-one (A). Resorcinol (2.20 g,
20 mmol), ethyl3-oxobutanoate (1.30 g, 10 mmol) and concentrated
HCl (5 mmol) were added to three-necked flask. The reaction was
carried out at 100 °C for 5 min in a microwave irradiation reaction,
and TLC was monitored until the consumption of the reactants was
completed. The reaction was then cooled to room temperature and
an appropriate amount of cold water was added and a large white
solid precipitated and then filtered. The precipitate was washed
three times with cold water and dried in a vacuum oven. Off-white,
yield: 87%, 1H NMR (400 MHz, DMSO) δ ppm = 10.52 (s, 1H, OH),
7.61 (d, J = 8.7 Hz, 1H, ArH), 6.82 (dd, J = 8.7, 2.3 Hz, 1H, ArH),
6.72 (d, J = 2.2 Hz, 1H, ArH), 6.15 (s, 1H, ArH), 2.39 (s, 3H, CH3).
13C NMR(126 MHz, CDCl3) δ161.60 (s), 160.74 (s), 155.28 (s),
153.92 (s), 126.98 (s), 113.28 (s), 112.45 (s), 110.69 (s), 102.62 (s),
18.53(s). ESI-MS (CH2Cl2) m/z: 175.12 (M − 1).
2. Experimental
2.1. Materials and physical measurements
The purity of Eu2O3 exceeded 99.99%, all reagents used were of AR
grade without further purification.
1H NMR and 13C NMR were measured with a Bruker-400 MHz nu-
clear magnetic resonance spectrometer with deuterated chloroform
(CDCl3) or dimethyl sulfoxide (DMSO)-d6 as a solvent and TMS as in-
ternal reference. The electron impact mass spectra (ESI-MS) were
measured using a MAT95XP analyzer. The UV–Vis spectra were re-
corded on a LabTech UV-2100 UV Spectrophotometer and dimethyl
sulfoxide (DMSO) was used as a reference and a solvent. The FT-IR
(KBr pellets) were recorded in the region 4000–400 cm−1 on a
Shimadzu IRAffinity-1 FT-IR spectrophotometer. Elemental analysis
was determined by Flash EA 1112 Elemental Analyzer. Thermal
gravimetric (TG) were carried out up to 800 °C with a heating
speed of 10 °C/min in the air atmosphere on an a SHIM ADZU DTG-
60 thermogravimetric analyzer. Fluorescence spectra were moni-
tored by HIACHI F-2700 spectrophotometer at room temperature,
the widths of both the excitation and emission slit were 5.0 nm and
the voltage of photomultiplier tube was 700 V. Density functional
theory (DFT) calculations were performed with Gaussian 09 pro-
gram. Cyclic voltammetry (CV) measurements were performed
2.2.1.2. 7-Hydroxy-4-methyl-2-oxo-2H-chromene-8-carbaldehyde (B). 7-
hydroxy-4-methyl-2H-chromen-2-one (1.76 g, 10 mmol), and hexa-
methylenetetramine (1.40 g, 10 mmol) were added to the round bot-
tom flask with 15 mL acetic acid. Refluxing at 70 °C for 6 h, 5 mL of
water and 2 mL of concentrated hydrochloric acid were added after
the reaction was cooled. The product was extracted with diethyl ether
and recrystallized three times with ethanol to dryness. Light yellow
solid, yield: 57%. 1H NMR (400 MHz, CDCl3) δ ppm = 12.27 (s, 1H,
OH), 10.68 (s, 1H, C\\H), 7.79 (d, J = 9.0 Hz, 1H, ArH), 6.97 (d, J =
9.0 Hz, 1H, ArH), 6.27 (s, 1H, ArH), 2.49 (s, 3H, CH3). 13C NMR
(126 MHz, CDCl3) δ191.77(s), 164.15(s), 159.29(s), 155.58(s), 154.58
(s), 133.78(s), 114.03(s), 112.21(s), 111.57(s), 109.45(s), 18.82(s).
ESI-MS (CH2Cl2) m/z: 203.00 (M − 1).
Scheme 1. Synthetic route of L1–3
.
2

F. Luan, G. Xiao, Y. Zhang et al.
Journal of Molecular Liquids 320 (2020) 114439
2.2.2. Synthesis of L1–3
The fluorescence quantum yields of L1–3 in acetonitrile solvent were
calculated based on the following formula by quinine sulfate as standard
[24].
2.2.2.1. (E)-7-hydroxy-4-methyl-8-((pyridin-2-ylimino)methyl)-2H-
chromen-2-one L1. 7-HYDROXY-4-methyl-2-oxo-2H-chromene-8-
carbaldehyde (0.20 g, 1 mmol) and pyridin-2-amine (0.01 g, 1 mmol)
were added to the round bottom flask with 10 mL methanol. Refluxing
at 60 °C for 4 h, the reaction was cooled to room temperature and a large
orange precipitate was precipitated. Precipitate was washed three times
with ethanol to give the final pure product. Light yellow solid, yield:
76%. 1H NMR (400 MHz, CDCl3) δ ppm = 15.33 (s, 1H, OH), 10.16 (s,
1H, C\\H), 8.60 (d, J = 4.5 Hz, 1H, ArH), 7.85 (t, J = 7.7 Hz 1H, ArH),
7.65 (d, J = 9.0 Hz, 1H, ArH), 7.37 (d, J = 7.9 Hz 1H, ArH), 7.34–7.29
(m, 1H, ArH), 6.95 (d, J = 9.0 Hz, 1H, ArH), 6.19 (s, 1H, ArH), 2.47 (s,
3H, CH3). 13C NMR (126 MHz, CDCl3) δ168.63(s), 160.17(s), 158.35(s),
155.39(s),153.11(s), 149.31(s), 138.55(s), 130.22(s), 122.98(s), 119.09
(s), 115.58(s), 110.95(s), 110.73(s), 106.82(s), 18.94(s). ESI-MS
(CH2Cl2) m/z: 281.31 (M + 1).
n2x Fx
A
Φfx
¼
∙
∙
std ∙Φfstd
n2std
Fstd Ax
where std. and x represent the standard and the analyte, respectively,
and n represents the refractive index of the solution (nx(C2H3N) = 1.344,
nstd = 1.337). F represents the fluorescence integrated area, and A rep-
resents the ultraviolet absorbance. Фfstd represents the fluorescence
quantum yield of the standard substance solution (Фfstd = 0.55).
According to many related works reports, similar structures show
excellent AIE properties. As expected, from the Fig. 1 and Table 1, the
fluorescence intensity and fluorescence quantum yields of L1 are signif-
icantly enhanced through the fw increases, and the green emission is
exhibited. For the ligand, a typical intramolecular proton transfer phe-
nomenon has been found in which a phenolic hydroxyl group interacts
with C_N to fluoresce by interconversion between an enol isomer and
a keto isomer [25–27]. When the ligand is dissolved in the solution,
since the C_N pole is easily rotated, the conjugation of the molecule
is affected, and the intramolecular proton transfer efficiency is lowered.
Unlike the ACQ phenomenon (aggregation-caused quenching), the ro-
tation of C_N is limited with the increase of fw, and the conjugation
of molecules increases. The fluorescence intensity at fw = 0 is the
weakest, and the fluorescence intensity is continuously enhanced by
the increase of fw. Besides, the corresponding fluorescence quantum
yields (Фfx) increase from 0.081 to 0.255, which also indicates that the
ligand possessesAIE properties.
2.2.2.2. (E)-7-hydroxy-8-(((5-methoxypyridin-2-yl)imino)methyl)-4-
methyl-2H-chromen-2-one
L2.
7-Hydroxy-4-methyl-2-oxo-2H-
chromene-8-carbaldehyde (0.20 g, 1 mmol) and 5-methoxypyridin-2-
amine (0.12 g, 1 mmol) were added to the round bottom flask with
10 mL methanol. Refluxing at 60 °C for 4 h, the reaction was cooled to
room temperature and a large orange precipitate was precipitated. Pre-
cipitate was washed three times with ethanol to give the final pure
product. Light yellow solid, yield: 81%. 1H NMR (400 MHz, DMSO) δ
ppm = 15.39 (s, 1H, OH), 10.04 (s, 1H, C\\H), 8.24 (s, 1H, ArH), 7.60
(d, J = 9.0 Hz, 1H, ArH),7.31 (t, J = 2.9 Hz, 2H, ArH), 6.92 (d, J =
9.0 Hz, 1H, ArH), 6.16 (s, 1H, ArH), 3.95 (s, 3H, CH3), 2.44(s, 3H,CH3).
13C NMR (126 MHz, CDCl3) δ191.85(s), 164.18(s), 163.72(s), 155.66
(s), 154.15(s), 147.48(s), 134.84(s),133.83(s), 133.09(s), 111.60(s),
109.56(s), 107.64(s), 56.49(s), 19.03(s). ESI-MS (CH2Cl2) m/z: 311.09
(M + 1).
3.2. Characterization of the Eu(III) complexes
3.2.1. FT-IR spectral analysis and molar conductivity
In order to prove the structural information of the complexes, FT-IR
and the molar conductivity were measured. All the complexes were sol-
uble in DMSO, DMF, slightly soluble in ethanol, methanol, ethyl acetate
and dichloromethane, insoluble in water, petroleum ether. Molar con-
ductivity was measured in DMF (10−3 mol·L−1, pH = 7, temperature =
25 °C), and the data was presented in Table S1 and Fig. 2 (Table S1 and
FT-IR of L2, 3 and [EuL2, 3(NO3)3] •H2O are provided in the support infor-
mation). Since the structure is similar, we only choose L1 and [EuL1
(NO3)3] •H2O as an explanation [28,29].
2.2.2.3. (E)-7-hydroxy-4-methy-8-(((5-nitropyridin-2-yl)imino)methyl)-
2H-chromen-2-o ne L3. 7-Hydroxy-4-methyl-2-oxo-2H-chromene-8-
carbaldehyde (0.20 g, 1 mmol) and 5-nitropyridin-2-amine (0.14 g,
1 mmol) were added to the round bottom flask with 10 mL methanol.
Refluxing at 60 °C for 4 h, the reaction was cooled to room temperature
and a large orange precipitate was precipitated. Precipitate was washed
three times with ethanol to give the final pure product. Red solid, yield:
76%. 1H NMR (400 MHz, DMSO) δ ppm = 11.91 (s, 1H, OH), 10.47 (s, 1H,
C_H),8.79–8.64 (m, 1H, ArH),7.96 (d, J = 8.8 Hz, 1H, ArH), 7.88 (d, J =
8.5 Hz, 1H, ArH), 7.63 (d, J = 8.5 Hz 1H, ArH), 6.81 (d, J = 9.2 Hz, 1H,
ArH), 6.33 (s, 1H, ArH), 2.40(s, 3H, CH3). 13C NMR (Poor sample solubil-
ity). ESI-MS (CH2Cl2) m/z: 325.16 (M − 1).
As shown in Fig. 2 and Table S1, compared with the L1, the FT-IR of
[EuL1(NO3)3] •H2O has undergone a very dramatic change, which indi-
cates the Eu(III) complexes have been prepared successfully. For the
L1, a certain intensity band at 3469 cm−1 is found, which is assigned
to the stretching frequency of the OH group. In addition, a high intensity
band is observed at 1723 cm−1, which is attributed to the typical
stretching vibration of the C_O group [30]. Meanwhile, medium inten-
sity bands are detected at 1691 cm−1 and 1652 cm−1 due to the differ-
ent azomethine moieties, which is the C_N group of Schiff-base and
pyridine ring, respectively [31]. For the Eu(III) complex, a broad high in-
tensity band of the OH group is observed at 3451 cm−1 with 18 cm−1
red-shifts, which indicates the oxygen atom of the OH group coordi-
nates to Eu(III) ions without deprotonating [32]. In addition, two νC=N
are red shifted to 1667 cm−1 and 1598 cm−1 with 24 cm−1 and
54 cm−1 shifts, respectively, which are corresponding the characteristic
band of the C_N group in Schiff-base and pyridine ring. The obvious
shifts indicate that two C=N groups in Schiff-base and pyridine ring
have been coordinated to Eu(III) ions through the nitrogen atom. How-
ever, we can notice that there is no significant shifts in the position of
C_O group, which indicates that C_O group does not participate in
the coordination. The characteristic peak of the ligand L1 and the com-
plex [EuL1(NO3)3] •H2O at 2920 cm−1 correspond to the stretching vi-
bration of saturated C\\H [33], and the saturated C\\H has no
possibility of coordination with Eu3+, we not discuss it too much here.
2.2.3. Synthesis of Eu(III) complexes
According to our previous reports [20], 0.20 mmol ligands were dis-
solved in 20 mL ethyl acetate at 60 °C, and 1 mL Eu(NO3)3 ethyl acetate
solution was added. Then, pH was adjusted to 6–7 using 1 mol·L−1 so-
dium ethoxide solution. After refluxing for 8 h, the mixture was cooled
to room temperature and poured into petroleum ether, then washed
with water (30 mL) and filtered to give a white solid. Finally, the Eu
(III) complexes were obtained.
3. Results and discussion
3.1. The properties of ligands (L1–3
)
This paper only studies the AIE properties of the ligands. The follow-
ing is a detailed description of the AIE properties of the ligands. Since all
ligand structures are similar, we choose the L1 as an explanation. The
AIE properties were measured in acetonitrile as a solvent (30 μM,
pH = 7, 25 °C). The water fraction (fw) is the ratio of water to solvent
(acetonitrile and water).
3

F. Luan, G. Xiao, Y. Zhang et al.
Journal of Molecular Liquids 320 (2020) 114439
Fig. 1. (a) Fluorescence spectra under different fw (0%–90%) (b) Fluorescence intensity under different fw (0%–90%) (c) Fluorescence under different fw (0%–90%).
Different functional groups exhibit varying degrees of red shifts due to
the conjugation of the molecules increases after coordination, the vibra-
tion frequency becomes lower, the wave number becomes lower, and
the corresponding wavelength red shifts. Thus, we can infer that the
OH group and two C_N groups are successfully coordinated with Eu
(III) ions [34,35].
Meanwhile, the measured values of the molar conductivity are in the
range of 23 to 27 Scm2.mol−1, which indicates that the above materials
are nonelectrolytes [36]. In other words, there is no free nitrate in the
complex molecule. The characteristic frequencies of the coordinating ni-
trate groups appear at about 1370 cm−1 (νas), 1190 cm−1 (ν2),
1020 cm−1 (ν4) and 829 cm−1 (νs). The difference between the two
highest frequency band (Δν = νas-ν2) gives the information on the co-
ordination of nitrate ions with rare earth ions [37,38]. From FT-IR data,
the Δν values of Eu(III) complexes are about 180 cm−1, indicating
three nitrate ions are coordinated with the Eu(III) ion in a bidentate
fashion.
The data of elemental analysis of [EuL1–3(NO3)3] •H2O are listed in
Table 2. We can see that the experimental data of the content C
(29.96%), H(2.33%), N(11.86%), O(31.02%) and Eu(28.92%) in the [EuL1
(NO3)3] •H2O are well consistent with the theoretical calculation values
of C(30.18%), H(2.20%), N(11.00%), O(30.18%) and Eu(28.88%) in the
[EuL1(NO3)3] •H2O, and the [EuL2,3(NO3)3] •H2O are the same result, so
I won't describe them in detail here.
In order to prove the composition and thermal stability of Eu(III)
complexes further, ESI-MS analysis is measured in DMF with 10−6
M
and the test mode is positive ion mode. Based on the similar structure
of all Eu(III) complexes, only [EuL1(NO3)3] •H2O is selected for detailed
Therefore, we can deduce the ligands are coordinated with Eu(III)
ion successfully. The functional group specifically involved in the coor-
dination was determined, and the three nitrate groups successfully par-
ticipated in the complexation in a bidentate fashion.
3.2.2. Elemental analysis, ESI-MS and TG analysis
To confirm the composition of Eu(III) complexes further, elemental
analysis, ESI-MS and TG analysis of the complex were determined.
Table 1
Fluorescence quantum yields of L1 in different fw (0%–90%).
fw
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
Фfx 0.081 0.102 0.114 0.119 0.141 0.162 0.189 0.192 0.231 0.255
Fig. 2. The FT-IR spectrum of L1 and [EuL1(NO3)3]·H2O.
4

F. Luan, G. Xiao, Y. Zhang et al.
Journal of Molecular Liquids 320 (2020) 114439
Table 2
Elemental analysis of [EuL1–3(NO3)3] •H2O.
Complexes
Found(calculated)%
C
H
N
O
Eu
[EuL1(NO3)3] •H2O
[EuL2(NO3)3] •H2O
[EuL3(NO3)3] •H2O
29.96(30.18)
31.10(30.62)
29.20(28.18)
2.33(2.20)
2.38(2.40)
2.10(1.91)
11.86(11.00)
11.12(10.51)
12.67(12.33)
31.02(30.18)
32.13(31.22)
34.98(35.23)
28.92(28.88)
23.03(22.81)
23.12(22.31)
analysis. ESI-MS data of [EuL1–3(NO3)3] •H2O are shown in Table 3. The
m/z 629.02 [M + H]+ can be obviously observed in Fig. 3, which is con-
sistent with the theoretically calculated value (m/z: 628.04) of [EuL1
(NO3)3] •H2O. Meanwhile the m/z 670.04 [M + H]+ and 682.96
[M + H]+ of [EuL2,3(NO3)3] •H2O are also can be found in support infor-
mation, which matches well with the theoretical value of [EuL2,3(NO3)3]
•H2O (m/z: 669.06 and m/z: 681.20).
As shown in Figs. 6 and 7, the maximum excitation wavelengths of
the three complexes are all located at 393 nm, and the corresponding
excitation intensity order is [EuL2(NO3)3] •H2O > [EuL1(NO3)3]
•H2O > [EuL3(NO3)3] •H2O. All the complexes emit the red characteristic
fluorescence of Eu(III). Three target complexes show obvious emission
peaks at 595 nm and 618 nm, and lower emission peaks appeared at
651 nm and 701 nm, which mainly correspond to the 5D0-7F1, 5D0-7F2,
5D0-7F3 and 5D0-7F4 transitions. This indicates that ligands are success-
Besides, the TG analysis of [EuL1–3(NO3)3] •H2O were investigated in
the air atmosphere. Similarly, only [EuL1(NO3)3] •H2O is explained in
detail here, and the TG experimental data and curve of [EuL1(NO3)3]
•H2O are exhibited in Table 3 and Fig. 4. The TG curve of [EuL2,3(NO3)
3] •H2O can be viewed in the support information. As we can see from
the Table 3 and Fig. 4, the process of the compound losing weight is
mainly divided into three steps: (1) loss of crystal water, (2) loss of ni-
trate (3) the removal of the ligand. Firstly, when the temperature is in
the range of 90–110 °C, the weight loss ratio of the [EuL1(NO3)3] •H2O
is 2.85%, Which is consistent with the theoretical value(2.83%) of crys-
tallization water in [EuL1(NO3)3] •H2O. Secondly, when the temperature
is in the range of 110–349 °C, the weight loss ratio of the complex is
29.05%, which is also very close to the theoretical calculated value
(29.23%) of nitrate in the complex. Finally, when the temperature
rises from 349 °C to 760 °C, the weight loss ratio is 43.05%, which is
also close to the theoretical calculation value (44.05%) of the ligand in
the complex. The weight loss is completed until 760 °C. When the tem-
perature exceeds 760 °C, the curve begins to stagnate, indicating that
the residue corresponds to europium oxide and the residual value
(26.30%) is close to the theoretical analysis value (27.19%) [39]. Corre-
spondingly, the peak of 629.0 in the ESI-MS corresponds to the molecu-
lar ion peak of [EuL1(NO3)3] •H2O (Theoretical relative molecular mass
is 630.2), and that is consistent with the results of FT-IR, TG and molar
conductivity analysis [40]. All the residue values and the peak of ESI-
MS of [EuL1–3(NO3)3] •H2O are consistent with theoretical values.
fully coordinated with Eu(III). Correspondingly, 5D0
→
7F1 and
5D0 → 7F2 transitions are located at 595 nm and 618 nm, which are at-
tributed to magnetic dipole transitions and electric dipole transitions,
respectively [41]. Like many Eu(III) complex materials, the Stokes shift
of above materials is up to 225 nm. Generally, the symmetric positional
information of the Eu(III) in the complexes is mainly judged by the in-
tensity ratio of 5D0 → 7F1 and 5D0 → 7F2 transitions. The fluorescence in-
tensity ratio η (5D0 → 7F2/5D0 → 7F1) of the two orbits is an important
indicator for judging the symmetrical position of Eu(III). When the η
are more than 1, the 5D0
→
7F2 electric dipole transition emits red
light (618 nm). At this time, Eu(III) is in a position away from the inver-
sion center. When the η are less than 1, the 5D0 → 7F1 magnetic dipole
transition emits orange light (596 nm). At this time, Eu(III) is in the lat-
tice position close to the inversion center. The η values of the above
three target complexes are all larger than 1, which indicates that the eu-
ropium ions in these three complexes are all located away from the in-
version center, that is, the asymmetric center [42].
Obviously, the fluorescence intensity and the fluorescence quantum
yields of [EuL2(NO3)3]·H2O is higher than [EuL1(NO3)3]·H2O, while the
fluorescence intensity and the fluorescence quantum yields of [EuL3
(NO3)3]·H2O is lower than [EuL1(NO3)3]·H2O. These indicate that the
introduction of electron-donating groups can enhance the fluorescence
intensity of Eu complexes, while the introduction of electron-
withdrawing groups can weaken the fluorescence intensity of Eu com-
plexes. This is mainly because the introduction of an electron-
donating group increases the conjugation of molecules and the density
of electron clouds, which promotes the energy transfer efficiency be-
tween the ligand and the characteristic ions. In other words, there will
be more electrons in the ligand entering the empty orbit of the rare
earth ions as the electron donating capacity increases, which further il-
lustrates that the electron-donating group can improve the fluorescence
properties of the complexes.
3.2.3. Presumed structural formula of [EuL1–3(NO3)3] •H2O
Based on the above FT-IR, molar conductivity, elemental analysis,
ESI-MS, and TG analysis, the chemical molecular formula of the Eu(III)
complexes can be successfully determined to be [EuL1–3(NO3)3] •H2O,
and the structural formula of [EuL1–3(NO3)3] •H2O is shown in Fig. 5.
3.2.4. Fluorescence properties
The fluorescence performance of the complex was measured in
DMSO and the corresponding data was placed in the Fig. 6 and
Table 4. The fluorescence quantum yields were calculated according to
the above formula (nx(DMSO) = 1.478). The corresponding fluorescence
spectrum data was listed in the Table 4. The excitation and emission
spectra were shown in Fig. 6. The energy transfer mechanism was
shown in Fig. 7.
To study the effect of different substituents on the fluorescence in-
tensity of the complexes, the density functional theory (DFT) calculation
of L1–3 was performed with Gaussian 09 program [43–45]. The geome-
tries were optimized by the B3LYP hybrid density functional and the
basis set was 6-31G (d). The frontier molecular orbitals and energy
levels of the highest occupied molecular orbital (HOMO) and lowest un-
occupied molecular orbital (LUMO) of L1–3 are shown in Fig. 8, and the
Table 3
TG and ESI-MS data of [EuL1−3(NO3)3] •H2O.
Complexes
Found (calculated) of lose weight (%)
ESI-MS
Crystal water
Nitrate
ligand
Residue
Found(calcd)
[EuL1(NO3)3] •H2O
[EuL2(NO3)3] •H2O
[EuL3(NO3)3] •H2O
2.85(2.83)
3.00(2.70)
2.79(2.64)
29.05(29.23)
27.00(27.92)
27.20(27.30)
43.05(44.05)
44.50(46.57)
43.80(47.75)
26.30(27.19)
25.50(25.96)
25.20(25.39)
629.02(628.04)
670.04(669.06)
682.96(681.20)
5

F. Luan, G. Xiao, Y. Zhang et al.
Journal of Molecular Liquids 320 (2020) 114439
Fig. 3. The ESI-MS spectrum of [EuL1(NO3)3] •H2O.
calculated HOMO and LUMO energy level and corresponding energy
gaps (Eg) are listed.
It can be seen from Fig. 8, the electron density distribution in orbital
of L1–3 suggests that the HOMOs are extended over almost all the
R
O
N
O
O
N
O
O
N
O
O
Eu
H2O
N
O
N
O
O
OH
O
R=H, OCH3, NO2
Fig. 4. The TG curves of [EuL1(NO3)3]•H2O.
Fig. 5. The structural formula of [EuL1–3(NO3)3] •H2O.
6

F. Luan, G. Xiao, Y. Zhang et al.
Journal of Molecular Liquids 320 (2020) 114439
Fig. 6. (a) Excitation spectrum of [EuL1–3(NO3)3]•H2O (b) Emission spectrum of [EuL1–3(NO3)3]•H2O.
molecule excepting the substituent groups (Methyl group, C_O or
C_C). Compared with L1 (LUMO: −1.91 eV, HOMO: −5.96 eV), the
strong electron-donating substituent –OCH3 on pyridine ring (L2) in-
creases the LUMO energy level (−1.82 eV) and the HOMO energy
level (−5.58 eV), However, the strong electron-withdrawing substitu-
ent –NO2 on pyridine ring (L3) decreases the LUMO energy level
(−2.68 eV) and the HOMO energy level (−6.45 eV). Compared with
the HOMO energy level of L1–3, the electron-donating group (-OCH3) in-
creases the HOMO energy level of the ligand(L2), that is, the electron
cloud density is enhanced, and it is concentrated in the pyridine ring
(including C_N participating in the coordination), C_N double bond
and benzene ring (including hydroxyl groups), which makes the ligand
more likely to undergo electronic transition after coordination with eu-
ropium ions, and the conjugation of the corresponding complex is en-
hanced, so the fluorescence intensity is relatively strong. The
introduction of electron-withdrawing group (-NO2) makes the fluores-
cence intensity of the complex have different result, that is, the fluores-
cence intensity is weakened. As can be seen from Fig. 8, this is because
the introduction of electron-withdrawing group (-NO2) reduces its
HOMO energy level, which reduces the density of the electron cloud
around the coordination functional group. Based on the above analysis,
substituents on pyridine ring have a great influence on the electronic
cloud distribution of the molecule, which leads to different fluorescence
intensity emission of [EuL1–3(NO3)3] •H2O. This may be an effective ref-
erence for the design of excellent ligands in the field of optical materials.
Fig. 7. Energy transfer mechanism of [EuL1–3(NO3)3]•H2O.
cyclic voltammetry and λonset is the starting value of the maximum ab-
sorption peak of the ultraviolet absorption spectrum [46,47].
As shown in Fig. 9 and Table 5, the maximum ultraviolet absorption
wavelengths (λonset) of the three target complexes are 343 nm,
339 nm and 342 nm, respectively. The oxidation potential (Eox) of Eu
(III) complexes is found in the range of 0.6935–0.7762. Compared
with [EuL1(NO3)3] •H2O (LUMO: −1.8265 eV, HOMO: −5.4417 eV, Eg:
3.6152), the electron-donating substituent -OCH3 on [EuL2(NO3)3]
•H2O increases the LUMO energy level (−1.7757 eV), HOMO energy
level (−5.4335 eV) and the Eg value (3.6578 eV). However, the effect
of introducing electron withdrawing groups is opposite to that of
electron-donating groups. The Eg value of the three target Eu(III) com-
plexes are relatively high (all around 3.65), which means that all target
Eu(III) complexes have a good chemical stability. This indicates that
more electrons in [EuL2(NO3)3] •H2O enter the empty orbital of the
Eu3+, a larger conjugated structure is formed, and the chemical stability
and fluorescence intensity are enhanced.
3.2.5. Electrochemical properties
The cyclic voltammetry curves (CV) of the EuL1–3(NO3)3.H2O were
measured at the electrochemical workstation. The lowest unoccupied
orbital energy level (LUMO) of the Eu(III) complexes, the highest occu-
pied orbital energy level (HOMO) and the oxidation potential EOX were
investigated. The HOMO energy level can be calculated by the formula
EHOMO = −(4.74 + EOX) eV, and the corresponding LUMO energy
level can be obtained by the formula ELUMO = EHOMO + Eg, Eg is calcu-
lated by the formula Eg = 1240/λonset (eV). EOX is measured by the
3.3. Ion detection performance
3.3.1. Ion-selective analysis of Eu(III) complexes
By comparing the fluorescence properties of the three complexes,
we selected [EuL2 (NO3)3] •H2O to conduct anion selectivity
Table 4
Fluorescence intensity of [EuL1−3(NO3)3] •H2O.
Compounds
λex
(nm)
5D0—7F1
em/nm
5D0—7F2
em/nm
Фfx
η
(5D0—7F2/5D0—7F1)
λ
I/a.u.
λ
I/a.u.
[EuL1(NO3)3]·H2O
[EuL2(NO3)3]·H2O
[EuL3(NO3)3]·H2O
393
393
393
596
595
595
502
1134
325
618
618
618
1645
4929
896
0.189
0.533
0.105
3.28
4.35
2.76
7

F. Luan, G. Xiao, Y. Zhang et al.
Journal of Molecular Liquids 320 (2020) 114439
Fig. 8. Optimized molecular structure and molecular orbital amplitude plots of HOMO and LUMO energy level of L1–3
.
Table 5
experiments due to its excellent fluorescence performance. In order
to explore the anions specifically recognized by this complex, F−
,
The electrochemical data of complexes.
Cl−, Br−, I−, SO42−,NO3− and blank control samples were selected
for selective experiments. This selective experiment was conducted
under the condition of DMSO/H2O(V/V) = 8:2, and the test concen-
tration was 1.0 × 10−4 mol·L−1. The selective fluorescence curve is
shown in Fig. 8.
Compounds
λonset (nm) Eox (V) EHOMO (eV) Eg (eV) ELUMO (eV)
EuL1(NO3)3·2H2O 343
EuL2(NO3)3·2H2O 339
EuL3(NO3)3·2H2O 342
0.7017
0.6935
0.7762
−5.4417
−5.4335
−5.5162
3.6152
3.6578
3.6257
−1.8265
−1.7757
−2.1163
It can be seen from Fig. 10, when Cl−, Br−,I−, SO42−, and NO3− are
added, the fluorescence intensity of the Eu(III) characteristic fluores-
cence of [EuL2(NO3)3] •H2O all changes significantly. The fluorescence
intensity of the 5D0-7F1 and 5D0-7F2 transitions is continuously de-
creased, and there is no obvious change in the fluorescence peak around
455 nm. And the fluorescence intensity of the complex solution at
595 nm and 620 nm is always higher than the broad fluorescence
peak around 455 nm, which shows that the fluorescence of the complex
solution is still dominated by the red characteristic fluorescence of euro-
pium ion at this time. In other words, there is no obvious fluorescence
color change, that is, the complex has no specific recognition function
for these anions. When F− is added, the apparent color of the solution
becomes turbid from the colorless transparent solution. The fluores-
cence intensity of the complex at 455 nm increased rapidly, the fluores-
cence intensity at 595 nm and 620 nm decreased sharply, and the
solution showed a blue fluorescent around 455 nm, which shows that
the complex has a specific recognition function for F−. Since the ligand
is coordinated by two -C=N and -OH with Eu3+ to form a metal chem-
ical bond, in general, the five-membered ring and the six-membered
ring are more stable, while the two C_N and Eu3+ are formed the
Fig. 9. (a) The cyclic voltammetry curves (CV) of Eu complexes (b) The UV–Vis spectrum of Eu complexes.
8

F. Luan, G. Xiao, Y. Zhang et al.
Journal of Molecular Liquids 320 (2020) 114439
characteristic red fluorescence in this concentration range of F−. At
this time, the detection sensitivity of the complex to F− is weak. With
the increasing concentration of F−, when the concentration of F−
reaches 1.0 × 10−5 mol·L−1, at this time, I455nm/I620nm > 1, the fluores-
cence at 455 nm wavelength becomes dominant, and the complex
mainly emits blue fluorescence, that is, the detection of the complex
to F− reached a critical value. When the concentration increased from
1.2 × 10−5 mol·L−1 to 1.6 × 10−5 mol·L−1, the fluorescence intensity
of the solution showed a sharp increase, and the fluorescence intensity
reached 3761. Then as the fluoride ion continue increases, the ratio of
I
455nm/I620nm increases linearly. When the concentration reaches
2.2 × 10−5 mol·L−1, the ratio of I455nm/I620nm no longer increases,
which indicates that the maximum response range of the complex has
been reached, and the response sensitivity of the complex is between
1.0 × 10−5 mol·L−1 and 2.2 × 10−5 mol·L−1. This is because with the
continuous increase of F−, the new bond formed is broken, that is, the
structure of the complex is destroyed, and the fluorescence intensity
of the complex in the characteristic wavelength region of europium
ions decreases. When the concentration of F− increases to a certain
value, the complex no longer responds, this indicates that the response
limit has been reached at this concentration. The above ion selectivity
experiments and F− quantitative exploration indicate that the target
Eu(III) complex has certain application potential in the detection of F−.
Fig. 10. Fluorescence spectra of the complex solution under the same amount of different
anions.
four-membered ring is not very stable. When F− is added, F− is more
likely to combine with Eu3+ to generate europium fluoride. The new
bond of the complex is broken, that is, the existing structure of the com-
plex is destroyed. Therefore, the characteristic fluorescence intensity of
Eu(III) complex is significantly reduced. This selective experiment
shows that [EuL2(NO3)3] •H2O possesses a specific recognition function
for F−, which also shows that the complex has a potential application
prospects in the field of ion detection.
4. Conclusions
In summary, three novel coumarin derivatives and corresponding Eu
(III) complexes were synthesized and characterized by 1H NMR, 13C
NMR, UV–Vis, FT-IR, ESI-MS, TG, elemental analysis and molar conduc-
tivity. All the ligands exhibit the AIE property in CH3CN/H2O solution
and emit green fluorescence with the maximum emission wavelength
of 552 nm. All the Eu(III) complexes show characteristic red fluores-
cence of Eu(III) and the influence of different substituents on the fluo-
rescence intensity of Eu(III) complexes is ordered: -OCH3 > -H > -
NO2, and [EuL2(NO3)3] •H2O shows the strongest fluorescence intensity.
The density functional theory (DFT) calculations of the ligands indicate
that the introduction of electron-donating groups increases the HOMO
energy level and the electron cloud density, which enhance the ability
of ligand to coordinate with Eu(III) and the fluorescence intensity of
the corresponding Eu(III) complex. Besides, the electrochemical proper-
ties of Eu(III) complexes indicate that the introduction of electron-
donating groups increases the energy gaps (Eg) and the conjugation of
the complex, which enhances the fluorescence intensity of the complex.
Ion selectivity experiments show that [EuL2(NO3)3] •H2O has a specific
recognition function for F−, and its response limit is between
1.0 × 10−5 mol·L−1 and 2.2 × 10−5 mol·L−1. Compared with other re-
ported coumarin Schiff base derivatives [48–51], the ligands L1–3 syn-
thesized in this work possess outstanding AIE properties and emit
strong green fluorescence, which have great application prospects in
the field of optical materials. Besides, the [EuL1–3(NO3)3] •H2O synthe-
sized in this work show excellent fluorescence property and [EuL2
(NO3)3] •H2O has the strongest fluorescence intensity. What's more in-
teresting is that [EuL2(NO3)3] •H2O is selective for F−, and its response
limit is between 1.0 × 10−5 mol·L−1 and 2.2 × 10−5 mol·L−1, which in-
dicates the Eu(III) complex has potential application prospects in the
field of fluoride ion detection. This provides a new idea for the applica-
tion of rare earth complexes based on coumarin Schiff base derivatives
for our group, and also provides a direction for subsequent research
work.
3.3.2. Response range of Eu(III) complex to F− detection
Through the anion selectivity experiment, we found that the com-
plex has a specific recognition function for F−, and further explore the
detection range of the complex for F−, the experimental condition is
DMSO/H2O(V/V)
= 8:2. The concentration of the complex is
1.0 × 10−4 mol·L−1, and the ion concentration is controlled at
0.0–3.0 × 10−5 mol·L−1. The fluorescence intensity data of the complex
solution at different F− concentrations are shown in Table 6. The corre-
sponding fluorescence spectrum is shown in Fig. 10.
It can be seen from the above Fig. 11 and Table 6, through the contin-
uous increase of the concentration of F−, the fluorescence intensity of
the complex solution at 455 nm continues to increase, and the fluores-
cence intensity of the complex at 620 nm continues to decrease,
which shows that the complex has a good recognition function and
high sensitivity of response to F−. When the concentration of F− is in
the range of 2.0 × 10−6 mol·L−1 to 8.0 × 10−6 mol·L−1, the fluores-
cence intensity of the complex at 620 nm is significantly higher than
that at 455 nm, namely I455nm/I620nm < 1. and the complex exhibits
Table 6
Fluorescence intensity data of different F− concentrations in complex solution.
Concentration (F−)/M
Wavelength/nm
I/a.u.
Wavelength/nm
I/a.u.
2.0 × 10−6
4.0 × 10−6
6.0 × 10−6
8.0 × 10−6
1.0 × 10−5
1.2 × 10−5
1.4 × 10−5
1.6 × 10−5
1.8 × 10−5
2.0 × 10−5
2.2 × 10−5
2.4 × 10−5
2.6 × 10−5
2.8 × 10−5
455
455
455
455
455
455
455
455
455
455
455
455
455
455
26
42
89
188
712
1035
3761
4629
5134
6892
7719
7732
7739
7742
620
620
620
620
620
620
620
620
620
620
620
620
620
620
631
530
482
439
421
392
384
361
258
247
129
121
120
58
CRediT authorship contribution statement
Fangfei Luan: Conceptualization, Formal analysis, Investigation,
Methodology, Project administration, Software, Visualization, Writing
- Original Draft, Writing - review & editing. Gangxiang Xiao: Formal
analysis, Investigation, Methodology, Validation, Writing - review &
9

Fig. 11. (a) Fluorescence spectra of complex solution at different concentrations of F− (b) The curve of the ratio of I455nm/I620nm with the concentration of F−
.

F. Luan, G. Xiao, Y. Zhang et al.
Journal of Molecular Liquids 320 (2020) 114439
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The authors are grateful for the financial support of the National Nat-
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