A novel “turn-off” fluorescent sensor for Al3+ detection based on quinolinecarboxamide-coumarin

Article abstract of DOI:10.1016/j.inoche.2020.108168

Quinolinecarboxamides based on coumarin derivatives were efficiently synthesized and the optical properties were investigated. Compound ZXQ substituted by the diethylamino group on 7-position of coumarin exhibited blue fluorescence (λ_em ≈ 470 nm) with a high fluorescence quantum yield of 0.92 in Toluene and showed “turn-off” fluorescence property for Al³⁺. Probe ZXQ formed the 2:1 complex with Al³⁺ and performed fluorescence quenching in MeCN/H₂O (95:5), which exhibited good selectivity and sensitivity. In addition, the mechanism of binding and fluorescence quenching was illustrated by titration experiment, Job’ plot and DFT calculations.

Full text of DOI:10.1016/j.inoche.2020.108168

Journal Pre-proofs
3
+
A novel “turn-off” fluorescent sensor for Al detection based on quinoline-
carboxamide-coumarin
Zichun Zhou, Wenjie Niu, Zhangqi Lin, Yanhong Cui, Xue Tang, Yujin Li
PII:
S1387-7003(20)30758-9
DOI:
https://doi.org/10.1016/j.inoche.2020.108168
INOCHE 108168
Reference:
To appear in:
Inorganic Chemistry Communications
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Revised Date:
Accepted Date:
17 March 2020
1 August 2020
5 August 2020
Please cite this article as: Z. Zhou, W. Niu, Z. Lin, Y. Cui, X. Tang, Y. Li, A novel “turn-off” fluorescent sensor
3
+
for Al detection based on quinolinecarboxamide-coumarin, Inorganic Chemistry Communications (2020), doi:
https://doi.org/10.1016/j.inoche.2020.108168
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3
+
A novel “turn-off” fluorescent sensor for Al detection
based on quinolinecarboxamide-coumarin
Zichun Zhou, Wenjie Niu, Zhangqi Lin, Yanhong Cui, Xue Tang, Yujin Li*
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014,
People's Republic of China.
Abstract:
Quinolinecarboxamides based on coumarin derivatives were efficiently
synthesized and the optical properties were investigated. Compound ZXQ substituted
by the diethylamino group on 7-position of coumarin exhibited blue fluorescence (λem
≈
470 nm) with a high fluorescence quantum yield of 0.92 in Toluene and showed "turn-
3
+
3+
off" fluorescence property for Al . Probe ZXQ formed the 2:1 complex with Al and
performed fluorescence quenching in MeCN/H O (95:5), which exhibited good
2
selectivity and sensitivity. In addition, the mechanism of binding and fluorescence
quenching was illustrated by titration experiment, Job’ plot and DFT calculations.
Keywords: quinolinecarboxamide; Al3+ fluorescent sensor; coumarin; quenching
mechanism
1
.Introduction
Aluminum plays a significant role in our lives, such as pharmaceuticals, light
alloys and water purification [1,2]. Emerging studies revealed that excessive intake of
3
+
Al ion in human body can cause Alzheimer's disease, amyotrophic lateral sclerosis,
hemochromatosis, osteoporosis [3-5] etc., as well as environmental acidification
increases the amount of free Al3+ ions [6]. Therefore, the development of specific
3
+
fluorescent sensor for Al detection is of great crucial in biological and environment
research and a growing number of scientists are interested in the investigation of
3
+
fluorescent sensors for Al [7].

O
COOH
O
N
O
O
O
O
OH
TEP-COOH
PQTEG
N
N
S
N
N
N
N
X
HO
OH HO
receptor R1 X=S
receptor R2 X=O
receptor 1
3
+
Scheme 1 Representative examples of Al fluorescent sensor.
In past decades, a great number of fluorescent sensors have been designed in the
detection of heavy metal ions for medical, biological and environmental science, which
received great attention because of its advantages on simple, convenience and low cost
3
+
[
8-11]. The Al chemosensors based on “turn on” type have been widely studied such
as quinolone [12], hydrazinopyrimidine [13], Schiff base [14-16], etc. For example, as
seen in Scheme 1, Jisha synthesized the quinoline derivative PQTEG as fluorescent
probe acted as a visible colorimetric sensor for Al3+ [17]. Gui reported a novel
3
+
tetraphenylethylene core TEP-COOH as “turn-on” chemosensor for Al by using AIE
3
+
effect and performed high selectivity toward Al [18]. Schiff base is a familiar structure
3
+
of Al chemosensor, which can provide a hard-base environment to coordinate with
the hard-acid Al3+ [19]. Lu designed a “turn-on” type Schiff-base receptor 1 as
3
+
fluorescent sensor for Al in EtOH/H O (95:5 v/v) solution [20]. However, compared
2
3
+
with the “turn-on” type, the “turn-off” type fluorescent sensor for Al detection was
rarely reported in literature, which usually has a lower detection limit [21]. Rangasamy
designed thiophene and furan appended pyrazoline as fluorescent sensor receptors R1
3
+
and R2 for detection of Al by using quenching mechanism [22]. Recently, coumarin
derivatives are considered of the most attractive fluorophore groups with extensive
research and application because of their high molar absorption coefficient, large
Stokes shift, high fluorescence quantum yield, good physiological activity [23,24],
which was suggested that coumarin could be used as desired fluorophore for the

designed of fluorescent chemosensor.
Herein, an effective “turn-off” type fluorescent sensor for Al3+ based on
quinolinecarboxamide-coumarin molecule was designed and synthesized in this paper.
The sensor ZXQ exhibited the blue fluorescence with high fluorescence quantum yield,
3
+
which performed an excellent sensitive and selective response to Al . Moreover, the
DFT calculations were used to confirm the quenching mechanism.
2
. Experimental
2
.1. Materials and equipment
All of the chemicals used in the current study were purchased from commercial
vendors and used as received without further purification, unless otherwise noted. All
solvents were purified and dried using standard methods prior to use. The salts used in
stock solutions of metal ions were AgNO , Al(NO ) , Co(NO ) , CuCl, Cu(NO ) ,
3
3 3
3 2
3 2
FeSO , Fe(NO ) , HgCl , LiNO , MgSO , MnCl , NiCl , Pb(NO ) , ZnCl . Nuclear
4
3 3
2
3
4
2
2
3 2
2
1
13
magnetic resonance ( H, C) spectra were recorded on a Bruker AM 500 spectrometer
with chemical shifts reported as ppm at 500, 125 MHz, respectively, (in DMSO, CDCl3
and TMS as the internal standard). The high-resolution mass spectra were recorded on
a Thermo Scientific LTQ Orbitrap XL (ESI) and melting points (Mp.) were recorded
on a X-4 electro-thermal digital melting point apparatus. All of the chemicals used in
the current study were purchased from commercial vendors and used as received
without further purification, unless otherwise noted. All solvents were purified and
dried using standard methods prior to use.
2
.2. Absorbance, fluorescence and quantum yield
UV-Vis absorption spectra were measured on a UV-2550. Fluorescence spectra
were obtained with an Edinburgh FS5 spectrofluorometer. The solvents used in the
photochemical measurements were spectroscopic grade. All the experiments were

performed repeatedly, and reproducible results were obtained. The Φ values in solution
F
were measured using an integrating sphere method and dilute solutions of the
-5
compounds in organic solvent were used (1×10 mol/L). All the fluorescence spectra
data were recorded 3 times.
2
2
.3 General procedure for the synthesis of compounds
.3.1 General preparation of compounds 3-carboxycoumarin derivatives 2
A mixture of salicylaldehyde 1a (10.0 mmol, 1.210 g), meldrum's acid (10.0 mmol,
.441 g) in ethanol (10 mL) was stirred to reflux for 12 h. The reaction was complete
1
detected by TLC and cooled to room temperature, and then the reaction precipitate was
filtered to give crude product. The crude product was purified by re-crystallized from
anhydrous ethanol solvent (5 mL) to afford target compound 2a (1.616 g, 85%) as white
solid without further purification.
2
.3.2 General preparation of compounds 3-acyl chloride coumarin derivatives 3
A mixture of 3-carboxycoumarin 2a (10.0 mmol, 1.901 g), anhydrous thionyl
chloride (4.0 mL) and anhydrous DMF (0.1 mL) was stirred at room temperature for 1
h. The reaction was complete detected by TLC and the solvent was removed under
reduced pressure to afford target compound 3a (1.447 g, 83%) as white solid.
2
.3.3 General preparation of compounds 3-carboxamide coumarin derivatives HXQ
and ZXQ
-Acyl chloride coumarin 3a (0.4160 g, 2.0 mmol), Et N (2.021 g, 20.0 mmol), 2-
3
3
quinolylamine (0.2884 g, 2.0 mmol) and CHCl (10 mL) were added in turn to a 50 mL
3
round-bottomed flask. The reaction was stirred for 12 h at room temperature and

completed the reaction was detected by TLC analysis. The mixture was washed with
water (20 mL) and extracted with dichloromethane (3×25 mL). The organic layer was
combinated and dried with Na SO , and then removed the solvent under reduced
2
4
pressure. The residue was purified by silica gel chromatography eluting (silica gel,
petroleum ether/ethyl acetate=4:1) to afford complex HXQ (0.08218 g, 26%) as an
white solid.
2
.3.4 2-oxo-N-(quinolin-2-yl)-2H-chromene-3-carboxamide (compound HXQ)
1
White solid. Yield 26%. Mp. 221-223 °C H NMR (500 MHz, DMSO-d ) δ(ppm):
6
1
1.41 (s, 1H), 9.10 (s, 1H), 8.47 (d, J = 10.6 Hz, 2H), 8.08 (d, J = 7.2 Hz, 1H), 7.96 (d,
J = 8.0 Hz, 1H), 7.89 (d, J = 7.8 Hz, 1H), 7.85-7.81 (m, 1H), 7.74 (d, J = 7.8 Hz, 1H),
1
3
7
1
.60 (t, J = 7.4 Hz, 1H), 7.55-7.47 (m, 2H). C NMR (125 MHz, DMSO) δ(ppm):
65.93 (1C), 163.67 (1C), 154.18 (1C), 150.54 (1C), 146.47 (1C), 139.05 (1C), 134.88
(
1C), 130.72 (1C), 130.40 (1C), 127.89 (1C), 127.39 (1C), 126.13 (1C), 125.52 (2C),
25.50 (1C), 124.33 (1C), 118.63 (1C), 116.39 (1C), 114.20 (1C). HRMS (ESI): m/z
1
+
+
calcd for C H N O (M+Na) 339.0740, found 339.0745.
1
9
12
2
3
2
.3.5
7-(diethylamino)-2-oxo-N-(quinolin-2-yl)-2H-chromene-3-carboxamide
(compound ZXQ)
1
Yellow solid. Yield 46%. Mp. 317-318 °C. H NMR (500 MHz, CDCl ) δ(ppm):
3
1
1.52 (s, 1H), 8.81 (s, 1H), 8.56 (d, J = 8.9 Hz, 1H), 8.18 (d, J = 9.0 Hz, 1H), 7.95 (d, J
=
8.4 Hz, 1H), 7.78 (d, J = 8.0 Hz, 1H), 7.66 (t, J = 7.7 Hz, 1H), 7.55-7.38 (m, 2H),
6
.67 (dd, J = 8.9, 2.3 Hz, 1H), 6.55 (d, J = 2.2 Hz, 1H), 3.47 (q, J = 7.1 Hz, 4H), 1.26
1
3
(
s, 6H). C NMR (125 MHz, CDCl ) δ(ppm): 162.52 (1C), 162.01 (1C), 158.09 (1C),
3
1
53.08 (1C), 151.29 (1C), 148.87 (1C), 139.94 (1C), 138.13 (1C), 131.49 (1C), 129.70
1C), 128.06 (1C), 127.37 (1C), 126.39 (1C), 125.03 (1C), 115.09 (1C), 110.19 (1C),
09.83 (1C), 108.63 (1C), 96.77 (1C), 45.22 (2C), 12.46 (2C). HRMS (ESI): m/z calcd
(
1
+
+
for C H N O (M+H) 388.16557, found 388.16525.
2
3
22
3
3
3
. Results and discussion

3
.1 Synthesis
N
NH2
O
O
O
O
O
O
N
O
CHO
OH
O
O
SOCl2
DCM
OH
Cl
N
H
R1
EtOH
R1
O
R1
O
R1
O
DCM/Et N
3
O
1
1
a R =H, R =H
2a R =H, R =H 85%
3a R =H, R =H 83%
3b R =NEt , R =H 87%
1 2 2
HXQ R1=H 26%
ZXQ R1=NEt2 46%
1
2
1
2
1
2
b R =NEt , R =H
2b R =NEt , R =H 76%
1
2
2
1
2
2
Scheme 2 Synthesis of HXQ and ZXQ.
Preparation route of target molecules HXQ and ZXQ were shown in scheme 2.
Precursors 3-carboxycoumarin 2 were prepared by the reaction of salicylaldehyde
derivatives with meldrum's acid in refluxing ethanol solution, which resulted in 85%
(
R = H) and 76% (R = NEt ) yields. Then, with the acyl chloride reaction of thionyl
1 1 2
chloride, the 3-carboxycoumarin 2 converted to 3-acyl chloride coumarin 3 in 83% (R1
H) and 87% (R = NEt ) yields. The target amide coumarin derivatives were produced
=
1
2
by using the amide reaction of 3-acyl chloride coumarin 3 and 2-quinolylamine under
Et N condition, which were yielded in 26% and 46%, respectively. The structures of
3
1
13
HXQ and ZXQ were characterized by H NMR, C NMR and HR-MS analysis.
3
3
.2 Fluorescence detection
.2.1 The solvent selection of ZXQ
The UV-Vis absorption and fluorescence emission spectra of ZXQ in different
solvents were detected and shown in Fig. S1 (see in supporting information) and Table
. ZXQ exhibited similar high sharp absorption peaks in different organic solvents, that
1
its maximum absorption wavelength and the molar absorption coefficient (εmax) were
rarely affected by the solvent polarity. Meanwhile, the maximum absorption
wavelength ranged in 428-440 nm, which displayed blue fluorescence and could be
seen intuitively under 365 nm UV lamp (Fig. 1). The value of εmax was approximately
4
-1
-1
4
-1
-1
7
.0×10 M ·cm , except 5.2×10 M ·cm in Toluene. Compared with the absorption
spectra, the fluorescence emission spectra of ZXQ was much more affected by the
solvent polarity. It is noteworthy that the fluorescence emission intensity of ZXQ was
much stronger in non-polar solvent than in polar solvent. The fluorescence quantum
yield (Φ ) of ZXQ in DCM, CHCl , EA and THF were examined as 0.85, 0.88, 0.76
F
3

and 0.86, respectively, and its emission wavelength at around 460 nm. Moreover, the
Φ of ZXQ was up to 0.92 in non-polar solvent toluene, with the emission wavelength
F
at 449 nm.
Table 1 Optical data of ZXQ in solvents at room temperature.
Dye
Solvent
UV-Vis
Fluorescence
Stokes shift
(nm)
λabs (nm)a
εmax (M ·cm )
-1
-1
λem (nm)
ФFb
ZXQ
toluene
DCM
CHCl3
EA
428
435
434
428
429
432
437
440
434
434
52100
72200
73000
76700
74100
75500
70900
68800
70300
64600
449
466
459
463
464
475
481
485
477
480
0.92
0.85
0.88
0.76
0.86
0.15
0.07
0.05
0.09
0.04
21
31
25
32
35
43
44
45
43
46
THF
MeCN
DMF
DMSO
EtOH
MeOH
aAll of the values correspond to the strongest absorption peaks.
b
Fluorescence quantum yields (Ф ) are measured using an integrating sphere method. Standard
F
errors are less than 5%.
However, the fluorescence emission intensity significantly reduced and the
emission peak underwent a slight red shift (10-30 nm) in polar solvents (MeCN, DMF,
DMSO, EtOH, MeOH) compared with in non-polar solvents. The fluorescence
quantum yield (Φ ) of ZXQ was 0.15 in polar solvent MeCN with the emission
F
wavelength at 475 nm. As well as the Φ of ZXQ reduced to less than 0.1 in strong
F
polar solvents (DMF and DMSO) and proton solvents (EtOH and MeOH) with the
emission wavelength at around 480 nm. Accordingly, the Stokes shift of ZXQ were
ranged between 21 and 35 nm in toluene, DCM, CHCl , EA and THF. Whereas in polar
3
solvents (MeCN, DMF, DMSO, EtOH, MeOH), ZXQ performed a slightly larger
Stokes shift within 43-46 nm.
3
.2.2 Optical properties of HXQ and ZXQ in CHCl3
The UV-Vis absorption and fluorescence emission spectra of HXQ and ZXQ in

CHCl were shown in Fig. 1. HXQ exhibited a broad absorption peak at about 310 nm
3
4
-1
-1
in CHCl and the value of ε was about 1.4×10 M ·cm . It can be seen that HXQ
3
max
showed colorless in CHCl under the visible light (Fig. 1 left, insert). However, the
3
absorption intensity of ZXQ significantly enhanced and its maximum absorption
wavelength (λ ) underwent an obvious red shift of 121 nm compared with HXQ.
abs
Accordingly, the λabs of the diethylamine substituted ZXQ at about 434 nm and the
molar absorption coefficient (ε) was increased by approximately 5.2 times. Moreover,
the introduction of diethylamino significantly enhanced the fluorescence emission
intensity of ZXQ, while also caused a large red shift on emission wavelength. The
emission wavelength of ZXQ (459 nm) was larger than HXQ (375 nm), suggesting that
the substitution of diethylamine on the coumarin formed a push-pull structure which
caused the molecular charge transfer (ICT). Meanwhile, the fluorescence quantum yield
(
Ф ) was greatly increased from 0.03 (HXQ) to 0.88 (ZXQ). Thus, the Stokes shift of
F
HXQ was 68 nm, yet ZXQ was decreased to 26 nm.
1
2
1
2
Fig. 1. UV-Vis absorption (blue) and fluorescence (red) spectra of HXQ (left) and ZXQ
right) in CHCl Inset shows photographs of HXQ and ZXQ in CHCl taken under (1) the
(
3.
3
visible light and (2) the 365 nm UV lamp.
3
.2.3 Fluorescence spectral properties of detecting Al3+
When studied the fluorescence properties of HXQ and ZXQ, we found that ZXQ
3
+
3+
can be quenched by Al , so it can be used as the fluorescent probe for Al detection.
3
+
In order to determine the possibility of using ZXQ as the fluorescent probe for Al ,
-5
UV-Vis absorption and fluorescence emission spectra of ZXQ solution (1.0×10 mol/L)
+
3+
2+
+
2+
2+
in MeCN/H O (95:5, v/v) before and after addition of Ag , Al , Co , Cu , Cu , Fe ,
2
3
+
2+
+
2+
2+
2+
2+
2+
-5
Fe , Hg , Li , Mg , Mn , Ni , Pb and Zn (1.0×10 mol/L) were measured and

shown in Fig. 2.
-5
Fig. 2. Left: The absorption spectra of ZXQ (1.0×10 mol/L) in the presence of different metal ions
-5
in the MeCN/H O (95:5, v/v). Right: The Fluorescence spectra of ZXQ (1.0×10 mol/L) in the
2
presence of different metal ions in the MeCN/H O (95:5, v/v).
2
The maximum absorption wavelength of ZXQ appeared an obvious red shift from
3
+
4
33 nm to 464 nm in the presence of Al . However, the addition of other metal ions
did not caused any obvious changes on the maximum absorption wavelength, while the
2
+
+
2+
molar absorption coefficient (ε) was increased in the presence of Hg , Ag , Ni and
2
+
2+
2+
2+
2+
Co , and decreased slightly after addition of Cu , Fe , Mg and Pb . Meanwhile,
the color of ZXQ has been significantly deepened under the visible light after addition
of Al3+ as shown in Fig. 3. More interestingly, the fluorescence of ZXQ was
significantly quenched (see Fig. 2) and the emission wavelength underwent a red shift
3
+
of 12 nm due to the presence of Al . Nevertheless, the fluorescence emission spectra
of ZXQ changed slightly with the presence of other metal ions. Accordingly, the
emission wavelength of ZXQ underwent a slight blue shift of 5 nm in the addition of
2
+
Cu and the emission intensity became a little weaker. As well as the fluorescence
intensity of ZXQ reduced a little after addition of other 12 metal ions. Furthermore, as
3
+
shown in Fig. 3, ZXQ can be used to clearly distinguish Al under 365 nm UV lamp
and visible light. These interesting phenomenon indicated that ZXQ could be used as
3
+
the “turn off” type fluorescent probe for detection Al .

Fig. 3. The ZXQ under the 365 nm UV lamp (upper row) and the visible light (lower row) in the
presence of different metal ions in the MeCN/H O (95:5, v/v) solution.
2
3
. 3 Binding stoichiometry and sensing mechanism
3
+
To further understand the binding process of the fluorescent probe ZXQ for Al ,
the chemical titration experiment based on fluorescence spectra analysis was performed.
-5
3+
-5
-5
ZXQ (1.0×10 mol/L) with different concentrations of Al (0.1×10 -1.0×10 mol/L)
were prepared and their UV-Vis absorption spectra and fluorescence emission spectra
were measured (see Fig. 4). It is observed that the absorption intensity of ZXQ
3
+
decreased gradually with the concentration of Al increased, whilst a new absorption
3
+
peak appeared at about 464 nm. Additionally, when the concentration of Al increased
-5
more than 0.5×10 mol/L, the absorption peak (433 nm) quickly disappeared with the
intensity of new absorption peak (464 nm) increased (Fig. 4). It can be seen from Fig.
3
+
3
that as the concentrations of Al gradually increased in the solutions of ZXQ, the
fluorescence emission wavelength perforrmed a slight red shift, while the
concentrations of Al3+ was increased to 0.5×10 mol/L, the maximum emission
wavelengths were maintained at about 487 nm. Then, the fluorescence intensity of
-5
3
+
ZXQ rapidly decreased as the concentrations of Al increased. The inset of Fig. 4
3
+
displayed the relationship between the Al concentration and the maximum emission
3
+
intensity of ZXQ+Al solution. It is worth noting that the fluorescence was completely
-5
3+
3+
quenched with 0.5×10 mol/L Al . When more Al was titrated, the fluorescence
intensity of ZXQ performed tiny change. Thus, the chemical titration experiment
3
+
indicated that a 2:1 stoichiometry complex of ZXQ and Al may be formed.

-5
Fig. 4. UV-Vis absorption (left) and fluorescence (right) response of ZXQ (1.0×10 mol/L) exposed
3
+
-5
-5
-5
-5
-5
-5
-5
to Al of various concentrations: 0.1×10 , 0.2×10 , 0.3×10 , 0.4×10 , 0.5×10 , 0.6×10 , 0.7×10 ,
-5
-5
-5
0
.8×10 , 0.9×10 , 1.0×10 mol/L (from top to bottom). Inset shows fluorescence titration curve of
3
+
ZXQ with Al . These spectra were measured in MeCN/H O (95:5, v/v) solution.
2
In addition, Job’s Plot based on fluorescence intensity was carried out (see Fig. 5).
3
+
-5
The total concentration of ZXQ and Al was constant (1.0×10 mol/L) and the molar
3
+
fraction of ZXQ ([ZXQ]/[ZXQ]+[Al ]) changed. As shown in Fig. 5, the result of
Job’s Plot displayed a maximum at 2/3 fraction ZXQ and it indicated that a 2:1 ligand-
3
+
metal complex was formed between ZXQ and Al . Therefore, it can be concluded that
3
+
ZXQ and Al formed a complex with a ratio of 2:1, which resulted in fluorescence
quenched.
Fig. 5. Job’s plot for determining the stoichiometry for ZXQ and Al3+ in MeCN/H O (95:5, v/v).
2
-5
Total concentration=1.0×10 mol/L.

Based on the above experimental analysis, the binding ratio of the probe ZXQ to
3
+
3+
Al was verified to be 2:1. It can be seen from Scheme 3, an Al coordinated with the
oxygen atom of amide carbonyl and the nitrogen atom of quinoline on two ZXQ
molecules.
O
O
N
H
N
N
O
O
O
O
N
Al3+
Al3+
N
N
H
N
O
N
H
N
O
O
ZXQ+Al3+
ZXQ
Scheme 3 The response mechanism of ZXQ to Al3+
.
3
+
To further investigate the sensing mechanism of ZXQ to Al , a comparison was
1
3+
made on the H NMR spectroscopy between the ZXQ and ZXQ+Al (see Fig. S2 in
1
supporting information). It can be seen from Fig. S2 that H NMR spectroscopy of the
3
+
complex ZXQ+Al changed greatly compared to ZXQ. The chemical shift of NH
3
+
single of the complex ZXQ+Al has moved from 11.5 ppm to 12.7 ppm compared with
ZXQ in CDCl . These observations indicated the ionization support our assumptions.
3
Meanwhile, we also investigated the interference of different metal ions on the
3
+
recognition of Al by fluorescent probe ZXQ. The fluorescence intensity of ZXQ at
+
3+
2+
4
87 nm was measured in the presence of different metal ions, including Ag , Al , Co ,
+
2+
2+
3+
2+
+
2+
2+
2+
2+
2+
-5
Cu , Cu , Fe , Fe , Hg , Li , Mg , Mn , Ni , Pb and Zn (0.5×10 mol/L,
3
+
-5
respectively). When Al (0.5×10 mol/L) was added into the solution of ZXQ+M,
the blue bars exhibited the same fluorescence quenching effect (see Fig. 6). These
studies clearly suggested that the probe ZXQ could be utilized for the selective
3
+
detection of Al without the interference from other metal ions.

-5
Fig. 6. Fluorescence intensities of ZXQ (1.0×10 mol/L) in the presence of various metal ions and
3
+
Al mixtures in MeCN/H O (95:5, v/v). The red bars represent the emission intensities of ZXQ in
2
the presence of different metal ions, the blue bars represent the emission intensities of ZXQ in the
3+
presence of different metal ions and Al .
3
.4. Effect of pH on fluorescence properties
To investigate the effect of the fluorescence properties of ZXQ and ZXQ+Al on
the pH value, the UV-Vis absorption and fluorescence spectra of ZXQ and ZXQ+Al3+
3
+
-5
(
1.0×10 mol/L) in MeCN/H O (95:5, v/v) solution were measured at different pH
2
values. The absorption spectra (Fig. 7) shown that the maximum absorption peak
decreased and a new absorption peak appeared at 463 nm when the pH of the buffer
solution decreased below 3. Meanwhile, ZXQ displayed a fluorescence quenching at
approximately 478 nm when pH ≤ 3 and the fluorescence of ZXQ did not vary with pH
3
+
over that range of 4 to 12 (Fig. 7). Moreover, with the addition of Al , pH of buffer
3
+
solution significantly affected the fluorescence intensity of ZXQ+Al complex. When
3
+
pH > 10, the fluorescence intensity of ZXQ+Al complex displayed an enhancement
at 478 nm, which was convenient for practical application of ZXQ to the determination
3
+
of Al at pH ranging from 4 to 10.

-5
Fig. 7. Left: The absorption spectra of ZXQ (1.0×10 mol/L) with different pH in MeCN/H O (95:5,
2
v/v). Right: effect of pH on the fluorescence intensity at 478 nm of ZXQ and ZXQ+Al3+ in
MeCN/H O (95:5, v/v).
2
4
. Theoretical calculation
4
.1 Mechanism of detecting Al3
3
+
In order to further study the binding mechanism of Al and probe ZXQ, density
functional theory (DFT) calculations had been performed by applying the polarizable
continuum model (PCM) at the B3LYP/6-311G(d) level in MeCN to optimize the
structure of the fluorescent probe ZXQ, as implemented in the Gaussian 09 program
package [25,26].
The stable structure and MEP surfaces of ZXQ were displayed in Fig. S3 (see in
supporting information), the negative region represents with red color, while the
positive region represents with blue color. Obviously, the negative region was
concentrated between the oxygen atom of amide carbonyl and the nitrogen atom of
3
+
quinoline, suggesting its potential to combine with Al formed a stable six-membered
ring.
According to the binding ratio of ZXQ: Al3+ acquired from the Job’s plot
3
+
experiment and the MEP simulation results, we proposed that one Al coordinated with
the oxygen atom of amide carbonyl and the nitrogen atom of quinoline on two ZXQ
molecules. Fig. S4 (see in supporting information) displayed the geometry of metal-
organic coordination clathrate ZXQ+Al3+ optimized using DFT calculation at the
B3LYP/6-311G(d) level in MeCN, and the red region was the newly formed six-

membered ring. Further, we have calculated the Mülliken charge distribution about
ZXQ and its metal complexes, and the results were shown in Table 2 and Fig. S4.
Table 2 Mülliken charge distribution of ZXQ and ZXQ+Al3+
.
C
C
O
NH
O
N
O
N
Al
(9-10) (21-24)
(15)
(12,101)
(7)
(16)
(13)
(25)
(59)
ZXQ
-0.451 -0.371 -0.402
-0.315
-0.322 -0.382 -0.390 -0.376
ZXQ+
Al
-0.433 -0.047 -0.389
-0.281
-0.305 -0.361 -0.542 -0.798 1.423
3
+
It is clearly seen that the charge distribution of the coumarin ring and quinoline
ring changed obviously, with the charge of the carbon atom (9-10) on the coumarin and
the charge of the carbon atom (21-24) on the quinoline became more positive after
3
+
binded with the Al . Moreover, the charge of the oxygen atom (7, 14) and the nitrogen
atom (12, 16) also became more positive in the formation of the complex, while the
charge of the oxygen atom (13) and the nitrogen atom (25) become more negative,
3
+
indicating that charge transfer had taken place. Meanwhile, the charge of Al was 1.423,
which was less than its apparent charge 3, suggesting that electron was transferred from
3
+
3+
ZXQ to Al . As result, the positive charge of Al delocalized to the electron cloud of
the oxygen atom (13, 38) and the nitrogen atom (25, 50) to form a coordinate bond.
Fig. 8. Frontier molecular orbitals of ZXQ and metal-organic coordination clathrate ZXQ+Al3+.
On the other hand, we also calculated the molecular frontier orbitals of probe ZXQ
and the corresponding complex at the B3LYP/6-311G(d) level in MeCN. As shown in

Fig. 8, the HOMO of ZXQ was evenly distributed with a large conjugates and electron
delocalization resulted an absorption wavelength red shift and an increase in
fluorescence intensity. It is also apparent from the Fig. 8 that the HOMO of ZXQ+Al3+
complex was concentrated on the coumarin, while the LUMO was concentrated on the
amide and quinoline. Therefore, it revealed that a strong intramolecular charge transfer
(ICT) occurred, which lead to fluorescence quenching [27]. In addition, the energy gap
3
+
of ZXQ+Al complex was reduced to 2.65 eV lower than ZXQ (3.45 eV) lead to a red
shift of the absorption and emission wavelength, which was consistent with the
experimental results.
5
. Conclusion
In summary, a “tun off” type Al3+ fluorescent probe ZXQ was designed and
synthesized, which displayed blue fluorescence and high fluorescence quantum yield
3
+
as well as favorable sensitivity and selectivity for the detection of Al . The possible
3
+
quenching mechanism of the Al probe ZXQ was confirmed by chemometric methods
and DFT calculations. From this, the probe ZXQ can be widely applied to biological,
pharmaceutical and environmental monitoring.
Acknowledgement
We gratefully acknowledge the financial supported by the Natural Science
Foundation of China (21606201) and the National Natural Science Foundation of
Zhejiang (LY13B020016) and the Undergraduate Science and Technological
Innovation Program in Zhejiang Province (Zhejiang Xinmiao Talents Program)
(2017R403066).
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Zichun Zhou:
Graduated from Zhejiang University of Technology with a master's degree. The main
research direction is the synthesis of organic dye molecules.
Wenjie Niu:
Graduated from Zhejiang University of Technology with a master's degree. The main
research direction is the synthesis of organic dye molecules.
Zhangqi Lin:
Graduated from Zhejiang University of Technology with a master's degree. The main
research direction is the synthesis of organic dye molecules.
Yanhong Cui:
Associate Professor, Zhejiang University of Technology, PhD, research direction is
computational chemistry.
Yujin Li:
Associate Professor, Zhejiang University of Technology, PhD, research direction is
green organic synthesis.

Wenjie Niu
Zhangqi Lin

Xue Tang
Yanhong Cui

Conflict of interest
We declare that we have no financial and personal relationships with other people
or organizations that can inappropriately influence our work, there is no professional or
other personal interest of any nature or kind in any product, service and/or company
that could be construed as influencing the position presented in, or the review of, the
manuscript entitled.

Graphic abstract

Highlights



Quinolinecarboxamides based on coumarin derivatives were efficiently designed
and synthesized.
Probe ZXQ formed the 2:1 complex with Al3+ and performed fluorescence
quenching, which showed good selectivity and sensitivity.
The mechanism of binding and fluorescence quenching was illustrated by titration
experiment, Job’ plot and DFT calculations.