Synthesis and crystal structure of a novel pyridine acylhydrazone derivative as a “turn on” fluorescent probe for Al3+

Article abstract of DOI:10.1016/j.molstruc.2020.128138

A novel acylhydrazone-based fluorescent probe HL was synthesized and characterized by ¹H NMR, ¹³C NMR, mass spectrum (MS) and x-ray diffraction analysis. HL showed high selectivity and sensitivity towards Al³⁺ over other metal ions based on excited-state intramolecular proton transfer (ESIPT) and photo-induced electron transfer (PET) processes with a low detection limit (2.14 × 10^?8 M) and reasonable pH application range (4.0–7.5). HL and Al³⁺ formed a 1:1 complex, leading to a remarkable fluorescence enhancement, which was confirmed by Job's plot experiments, ¹H NMR and MS. Fluorescence titration experiments showed the fluorescence intensity of HL had good linearity with Al³⁺ concentration and can be used to detect Al³⁺ in real water samples.

Full text of DOI:10.1016/j.molstruc.2020.128138

Journal of Molecular Structure 1212 (2020) 128138
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis and crystal structure of a novel pyridine acylhydrazone
derivative as a “turn on” fluorescent probe for Al3þ
,
Huanan Peng a *, Xiaoming Peng b, Jiaqi Huang a, Ai Huang a, Shujuan Xu a,
Jingjing Zhou a, Shoushun Huang a, Xinping Cai a
a College of Chemistry and Environmental Sciences, Shangrao Normal University, Shangrao 334001, China
b Shangrao County Environmental Protection Bureau, Shangrao 334100, China
a r t i c l e i n f o
a b s t r a c t
A novel acylhydrazone-based fluorescent probe HL was synthesized and characterized by 1H NMR, 13C
NMR, mass spectrum (MS) and x-ray diffraction analysis. HL showed high selectivity and sensitivity
towards Al3þ over other metal ions based on excited-state intramolecular proton transfer (ESIPT) and
photo-induced electron transfer (PET) processes with a low detection limit (2.14 ꢀ 10ꢁ8 M) and
reasonable pH application range (4.0e7.5). HL and Al3þ formed a 1:1 complex, leading to a remarkable
fluorescence enhancement, which was confirmed by Job’s plot experiments, 1H NMR and MS. Fluores-
cence titration experiments showed the fluorescence intensity of HL had good linearity with Al3þ con-
centration and can be used to detect Al3þ in real water samples.
Article history:
Received 8 February 2020
Received in revised form
24 March 2020
Accepted 25 March 2020
Available online 1 April 2020
Keywords:
Crystal structure
Fluorescent probe
Aluminium detection
Acylhydrazone
ESIPT
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
selective detection of Al3þ, including rhodamine [19], coumarin
[20], pyrene [21], fluorescein [22], acylhydrazone [23e31] and so
In recent years, fluorescent probes have played a significant role
in the broad areas of chemistry, biology, food and the environment
[1e5]. Compared to other analytical approaches such as atomic
absorption spectroscopy (AAS), fluorescent probes for the detection
of metal ions have attracted considerable attention due to their
high selectivity and sensitivity, convenient operation and fast
response time [6e14]. Among these metal ions, aluminum ions
(Al3þ) are widely distributed in natural waters and soils, and
inevitably accumulate in the human body. However, as a nones-
sential element of living systems, excess Al3þ in the human body
can also lead to various diseases, including Alzheimer’s disease,
Parkinson’s disease and cardiovascular disease [15e17]. Further-
more, the maximum concentration of Al3þ in drinking water should
on [32e35]. Many acylhydrazone derivatives are reported as fluo-
rescent probes for Al3þ owing to their simple synthetic methods
and excellent coordination environments [26,36,37]. In general,
many types of sensing mechanism are utilized in the design of
fluorescent probes for Al3þ detection, including photo-induced
electron transfer (PET), intramolecular charge transfer (ICT),
imine isomerization, resonance energy transfer (RET) and excited-
state intramolecular proton transfer (ESIPT) [38e41]. In the struc-
ture of salicylidene hydrazone, the hydroxyl group can transfer a
proton to the neighboring imine to form a six-membered ring
through an intramolecular hydrogen bond, leading to the ESIPT
process [42e44]. In the presence of metal ions, ESIPT is inhibited
and imine isomerization is blocked, which leads to a significant
fluorescence emission enhancement [43e45].
not exceed 7.41
mM according to the regulations of the World
Health Organization (WHO) [18]. Therefore, it is necessary to
rapidly detect Al3þ in environmental and biological systems.
According to previous literature, numerous novel fluorescent
probes have been developed in recent years for the sensitive and
Based on these ideas, we designed and synthesized a novel
acylhydrazone-based fluorescent probe (HL) through condensation
of pyridine-2-carbohydrazide and 2,4-dihydroxybenzaldehyde
(Scheme 1). As expected, HL showed high selectivity towards
Al3þ over other examined metal ions based on ESIPT and PET pro-
cesses. Meanwhile, HL displayed
a remarkable fluorescence
enhancement towards Al3þ with a color change from colorless to
blue.
* Corresponding author.
E-mail address: penghuanan@163.com (H. Peng).
https://doi.org/10.1016/j.molstruc.2020.128138
0022-2860/© 2020 Elsevier B.V. All rights reserved.

2
H. Peng et al. / Journal of Molecular Structure 1212 (2020) 128138
Scheme 1. Synthetic routes of HL.
Table 1
Crystal data for HL.
2. Experimental
2.1. Materials and instruments
Compound
HL
Empirical formula
C13H11N3O3
257.25
296(2) K
0.71073 Å
Monoclinic
P21/c
13.8885(18)
7.8686(10)
11.9535(16)
90
113.625(2)
90
1196.8(3)
4
1.428
0.104
All reagents and solvents (analytical grade) for all experiments
were obtained from commercial sources and used without further
purification. Pyridine-2-carbohydrazide was prepared according to
our previous study [44]. 1H NMR and 13C NMR spectra were
recorded on a Bruker AVANCE 400 MHz spectrometer with
DMSO‑d6 as the solvent and tetramethylsilane (TMS) as the internal
standard. Mass spectra were obtained using a Thermo Scientific
LCQ Fleet liquid chromatograph/mass spectrometer (LC/MS).
Fluorescence spectra were measured using a Hitachi F-7000 fluo-
rescence spectrophotometer. All pH values were measured using a
Leici PHS-2 digital pH meter.
Formula weight
Temperature
Wavelength
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
Volume (Å3)
Z
Calculated density (g cmꢁ3
)
m
(mmꢁ1
)
2.2. Synthesis
Crystal size (mm3)
0.120 ꢀ 0.100 x 0.080
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta ¼ 25.242ꢂ
Data/restraints/parameters
Goodness-of-fit on F2
3.044e27.505ꢂ.
Pyridine-2-carbohydrazide (0.137 g, 1 mmol) was dissolved in
ethanol (10 mL), and 2,4-dihydroxybenzaldehyde (0.138 g, 1 mmol)
was added to the solution. The mixture was refluxed for 6 h and
cooled to room temperature. Then the precipitate was filtered and
crystallized from the ethanol to obtain HL as brownish-yellow flake
ꢁ18 ꢃ h ꢃ 18, ꢁ10ꢃ k ꢃ 9, ꢁ14 ꢃ l ꢃ 15
10,326
2740 [R(int) ¼ 0.0300]
100.0%
2740/2/178
1.026
R1 ¼ 0.0459, wR2 ¼ 0.1111
R1 ¼ 0.0761, wR2 ¼ 0.1247
0.172 and ꢁ0.192
crystals 0.175 g, yield: 68%. 1H NMR (400 MHz, DMSO‑d6)
d (ppm):
Final R indices [I > 2sigma(I)]
R indices (all data)
12.34 (s, 1H, NH), 11.63 (s, 1H, OH), 10.03 (s, 1H, OH), 8.72 (d,
J ¼ 5.6 Hz, 2H, ¼CH, Py-H), 8.14e8.04 (m, 2H, Py-H), 7.69e8.66 (m,
1H, Py-H), 7.25 (d, J ¼ 8.4 Hz, 1H, Ph-H), 6.38 (dd, J ¼ 8.4 Hz,
J ¼ 2.0 Hz, 1H, Ph-H), 6.33 (d, 1H, J ¼ 2.0 Hz, Ph-H); 13C NMR
Largest diff. peak and hole (e Åꢁ3
)
the Cambridge Crystallographic Data Centre (CCDC number
1982764).
(100 MHz, DMSO‑d6) d (ppm): 161.3, 160.5, 160.2, 151.3, 149.8, 149.0,
138.5, 132.2, 127.5, 123.2, 110.9, 108.2, 103.2; MS (ESI): calcd for
C
13H11N3O3 [M þ Na]þ 280.24, found 280.28.
3. Results and discussion
2.3. Fluorescence spectra measurements
3.1. Synthesis and characterization
Solutions of metal ions (10 mM) were prepared from nitrate or
chloride salts in double distilled water and diluted to the desired
concentrations with double distilled water. The probe HL was dis-
solved in N, N-dimethylformamide (DMF) to afford a stock solution
(1 mM). Fluorescence emission spectra were determined after
equilibrating the mixtures for 5 min at room temperature. For all
fluorescence measurements, the excitation wavelength, excitation
slit width and emission slit width were set to 370, 5 and 5 nm,
respectively.
The synthetic route of HL is shown in Scheme 1. First, pyridine-
2-carbohydrazide was synthesized by reaction of methyl pyridine-
2-carboxylate with hydrazine hydrate in refluxing ethanol. Then,
pyridine-2-carbohydrazide was allowed to react with 2,4-
dihydroxybenzaldehyde in refluxing ethanol to afford desired
product HL in 68% yield. The structure of HL was confirmed by 1H
NMR (Fig. S1), 13C NMR (Fig. S2), MS (Fig. S3) and single crystal x-
ray diffraction. The 1H NMR spectrum of HL exhibited characteristic
peaks at 12.34 (s, 1H, NH), 11.63 (s, 1H, OH), 10.03 (s, 1H, OH) and
8.72 (d, J ¼ 5.6 Hz, 2H, ¼CH, Py-H), which are in accordance with its
structure. In the mass spectrum, a peak appeared at m/z ¼ 280.28,
which corresponds to the formation of [M þ Na]þ.
2.4. Single crystal x-ray structure determination
A
brownish yellow flake crystal of HL with dimensions
0.12 ꢀ 0.10 ꢀ 0.08 mm3 was selected to determine its single crystal
structure by x-ray diffraction. Diffraction data were collected on a
Bruker APEX II CCD diffractometer equipped with a graphite-
The crystal structure of HL is shown in Fig. 1. The structure
shows that HL crystallizes in a monoclinic system with space group
P21/c (Table 1). The imine (C7-N3), carbonyl group (C6-O1) and
hydrazine (N2-N3) bond lengths are 1.281(2), 1.226(2) and
1.3796(16) Å, respectively, which are supported by previously re-
ported data [42]. The imine(C]N) exists in trans conformation,
which is in agreement with reported literature [47,48]. The two
aromatic rings are planar and comparable to related structures [44].
The picolinyl hydrazide is connected to the substituted benzene
monochromated MoK
a
radiation (
l
¼ 0.71073 Å) at 296(2) K. The
structure was solved using direct methods with SHELXS-2014/7
software [46]. All non-hydrogen atoms were refined anisotropi-
cally by full-matrix least-squares techniques on F2. Hydrogen atoms
were generated geometrically at calculated positions. The crystal
data for HL are summarized in Table 1 and have been deposited at

H. Peng et al. / Journal of Molecular Structure 1212 (2020) 128138
3
Fig. 1. Crystal structure of HL with 30% probability of displacement ellipsoids.
ring through an imine bond, and the dihedral angle between the
pyridine and benzene rings is 5.63(8)ꢂ. Furthermore, the molecule
contains two intramolecular hydrogen bonds: N(1)-H(2A), 2.686(2)
Å and N(3)-H(2O), 2.619(2) Å (Fig. 1). The molecules are linked
through intermolecular hydrogen bond O(3)eH(3O),,,O(1) to
form a chain structure (Fig. 2).
3.2. Fluorescence responses of HL towards metal ions
Fig. 3. Fluorescence emission spectra of HL (10
m
M) with various metal ions (10
mM) in
DMF-H2O (v/v ¼ 1:9) (lex ¼ 370 nm). Inset: color change of HL in the absence and
The fluorescence response of HL (10
mM) towards various metal
presence of Al3þ ion under UV-irradiation.
ions (10
m
M) including Cu2þ, Hg2þ, Co2þ, Fe3þ, Mg2þ, Mn2þ, Pb2þ
,
Kþ, Naþ, Ba2þ, Ca2þ, Cd2þ, Sr2þ, Zn2þ, Al3þ and Cr3þ were first
measured in DMF-H2O (v/v ¼ 1:9) solution at room temperature. As
can be seen in Fig. 3, HL exhibited a strong fluorescence emission
peak at 456 nm in the presence of Al3þ at an excitation wavelength
of 370 nm compared to free HL. However, no obvious fluorescence
intensity changes were observed after addition of other metal ions.
In addition, under UV-irradiation, HL also displayed a color change
from colorless to blue in the presence of Al3þ compared to the other
metal ions tested. These results indicate that HL can be used as a
“turn on” fluorescent probe with high selectivity for Al3þ
Furthermore, time-dependent fluorescence spectra of HL (10 M)
with Al3þ (10
M) were also performed to study the dynamics of
the reaction between HL and Al3þ (Fig. S4). The fluorescence in-
tensity of HL increased remarkably after the addition of Al3þ
.
m
m
,
reaching a steady state within 10 min, which indicates that HL
could provide a rapid analytical method for Al3þ detection.
In order to further investigate the selectivity of HL towards Al3þ
,
competition experiments were performed by adding Al3þ (10
to HL (10
M) in the presence of Co2þ, Fe3þ, Mg2þ, Mn2þ, Pb2þ, Kþ,
Naþ, Ba2þ, Ca2þ, Cd2þ, Sr2þ, Zn2þ, or Cr3þ (100 M), or Cu2þ or Hg2þ
(10 M). As shown in Fig. 4, upon addition of coexistent metal ions,
mM)
m
m
m
Fig. 4. Fluorescence spectra of HL (10
Fe3þ, Mg2þ, Mn2þ, Pb2þ, Kþ, Naþ, Ba2þ, Ca2þ, Cd2þ, Sr2þ, Zn2þ, or Cr3þ (100
or Hg2þ (10
M) in DMF-H2O (v/v ¼ 1:9) (lex ¼ 370 nm).
m
M) with Al3þ (10
m
M) in the presence of Co2þ
M), or Cu2þ
,
negligibly enhanced fluorescence emission at 456 nm was observed
compared to that of HL with Al3þ alone, which is comparable to
reported fluorescent probes [24,49e51]. Although the fluorescence
m
m
Fig. 2. Hydrogen bonds (dashed lines) of between HL molecules viewed along the a-axis.

4
H. Peng et al. / Journal of Molecular Structure 1212 (2020) 128138
Fig. 5. (a) Fluorescence spectra of HL with different concentrations of Al3þ (0e2.2 equiv) (lex ¼ 370 nm); (b) The linear relationship between the fluorescence intensities of HL at
456 nm and concentrations of Al3þ (0e10
mM).
intensities of HL with Al3þ in the presence of Co2þ, Fe3þ, Cr3þ, Hg2þ
or Cu2þ were relatively low, they were clearly detectable and were
attributed to the ability of Al3þ to eliminate possible interference
from other metal ions. Thus, HL could serve as a highly selective
fluorescent probe for the detection of Al3þ in DMF-H2O (v/v ¼ 1:9)
solution.
determined to be 2.35 ꢀ 104 based on fluorescence emission in-
tensity data using the Benesi-Hildebrand equation(Fig. S5) [39].
These results suggest that HL has the potential to be used as a
fluorescent probe for quantitative determination of Al3þ
.
3.4. pH effect
3.3. Fluorescence titration of HL with Al3þ
It is well known that pH is an extremely important factor for
probe measurement of Al3þ. Therefore, fluorescence intensity
measurements of HL in the absence and presence of Al3þ were
performed at different pH conditions in DMF-H2O (v/v ¼ 1:9). As
shown in Fig. 6, the fluorescence intensity of free HL did not change
in the pH range of 1e10. Conversely, obvious fluorescence
enhancement was observed in the presence of Al3þ in the pH range
of 4.0e7.5. Fluorescence intensity clearly decreased under acidic
conditions (pH < 4.0) due to protonation of HL. Meanwhile, fluo-
rescence intensity also significantly decreased under alkaline con-
dition (pH > 7.5) due to Al3þ being to bond with hydroxyl ions to
form aluminum hydroxide, preventing it from forming the HL-Al3þ
complex. These results demonstrate that HL is suitable for detect-
ing Al3þ in the pH range of 4.0e7.5.
To understand the sensing properties of HL towards Al3þ, fluo-
rescent titration of probe HL with various amounts of Al3þ were
carried out in DMF-H2O (v/v ¼ 1:9). Upon addition of Al3þ to the
solution of HL, the fluorescence emission intensity at 456 nm
gradually increased, showing less enhancement when the con-
centration of Al3þ reached at 2.2equiv (Fig. 5a). As shown in Fig. 5b,
a good linear relationship was observed between the fluorescence
intensity of HL and concentration of Al3þ in the range of 0e10
mM.
The linear equation was y ¼ 2127.6182ꢀ - 118.1782, R2 ¼ 0.9912,
and the detection limit was calculated to be 2.14 ꢀ 10ꢁ8 M ac-
cording to the formula LOD ¼ 3
s/k based on a previously reported
method [45]. The LOD is superior to some previously reported Al3þ
fluorescent probes, which indicates HL has relatively better sensi-
tivity among similar probes types [20,21,23e26,30,39,40].
Furthermore, the binding constant (Ka) of HL with Al3þ was
Fig. 6. Effects of pH on the fluorescent intensity of HL (10
mM) in the absence and
presence of Al3þ (10
mM) in DMF-H2O (v/v ¼ 1:9) (lex ¼ 370 nm).
Fig. 7. Job’s plot of HL with Al3þ. (total concentration ¼ 20
M) (lex ¼ 370 nm).
m

H. Peng et al. / Journal of Molecular Structure 1212 (2020) 128138
5
Fig. 8. 1H NMR spectrum of free HL and HL þ Al3þ in DMSO‑d6.
3.5. Proposed sensing mechanism
water samples according to the previously published protocol
[44,52]. Tap water was collected from our laboratory and river
water was obtained from the Xinjiang river (Shangrao, China). The
water samples were first filtered. Then, various concentrations of
The mode of interaction between HL and Al3þ was investigated
by Job’s plot experiments, 1H NMR and MS. Job’s plot experiments
showed that the maximum fluorescence intensity was achieved
when the molar fraction [Al3þ]/([HL] þ [Al3þ]) was 0.5, which
suggests that a 1:1 complex of HL þ Al3þ was formed (Fig. 7). These
results were further confirmed by the presence of a peak at m/
z ¼ 340.70 [HL þ Al3þ þ 2H2O e 3Hþ þ Naþ]þ in the mass spectrum
(Fig. S6). Fig. 8 shows the 1H NMR spectrum of free HL and HL with
excess Al3þ in DMSO‑d6. In the absence of Al3þ, HL exhibited three
single peaks at 12.34, 11.63 and 10.03 ppm, which were assigned to
the protons of its -NH, -OH and -OH groups, respectively. After
addition of excessive Al3þ, the peaks at 11.63 and 10.03 ppm nearly
disappeared. In contrast, the -NH peak at 12.34 ppm remained.
Additionally, the protons in the pyridine ring shifted downfield. The
1H NMR results suggest that the O atom of the hydroxyl group, the
N atom of the imine group and the N atom of the pyridine unit
might take part in coordination of Al3þ. Based on these in-
vestigations, a proposed sensing mechanism is shown in Scheme
S1.
Al3þ (0e10
solution containing HL (10
m
M) were added to the water samples/DMF (v/v ¼ 9:1)
m
M), and the fluorescence intensities of
the samples were collected at room temperature. A good linear
relationship between the fluorescence intensity of HL and Al3þ
concentration was observed for the two real water samples (Fig. 9),
which indicates that HL can be used to detect Al3þ in natural water
sources.
4. Conclusions
In summary, a novel acylhydrazone-based fluorescent probe HL
was synthesized and its structure was confirmed by x-ray diffrac-
tion analysis. HL showed high selectivity and sensitivity towards
Al3þ compared to other metal ions via a 1:1 binding mode in DMF-
H2O (v/v
¼
1:9). HL exhibited a remarkable fluorescence
enhancement with a color change from colorless to blue based on
ESIPT and PET processes. Furthermore, HL was successfully applied
to the quantitative detection of Al3þ in real water samples.
3.6. Application to real water samples
Declaration of competing interest
In order to determine the potential application of the probe HL
in real water samples, HL was applied to detect Al3þ in tap and river
The authors declare that there are no conflict of interest to
report in this research work.
CRediT authorship contribution statement
Huanan Peng: Conceptualization, Methodology, Formal anal-
ysis, Investigation, Data curation, Software, Writing - original draft,
Writing
- review & editing, Resources, Supervision, Project
administration. Xiaoming Peng: Investigation, Data curation,
Software, Methodology. Jiaqi Huang: Investigation, Data curation,
Software, Formal analysis. Ai Huang: Investigation, Data curation.
Shujuan Xu: Investigation, Data curation. Jingjing Zhou: Investi-
gation, Data curation. Shoushun Huang: Investigation, Data cura-
tion. Xinping Cai: Investigation, Data curation.
Acknowledgements
We would like to thank the Science and Technology Research
Program of the Educational Department of Jiangxi Province, China
(No.: GJJ170928) for financial support.
Appendix A. Supplementary data
Fig. 9. Linear relationships between fluorescence intensities of HL and concentrations
of Al3þ in tap and river water samples (lex ¼ 370 nm).
Supplementary data to this article can be found online at

6
H. Peng et al. / Journal of Molecular Structure 1212 (2020) 128138
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https://doi.org/10.1016/j.molstruc.2020.128138.
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