A new fluorescent probe of hydrazine: 2-[4-(imidazo[1,2-a]pyridin-3-ylethynyl)benzylidene]malononitrile

Article abstract of DOI:10.1246/cl.170169

Hydrazine is widely used in a plenty of fields as a raw material and is closely related to our healthy. A new fluorescent probe of hydrazine was designed and synthesized, choosing imidazo[1,2-α]pyridine as the electron acceptor, based on the intramolecular charge-transfer (ICT) effect. The probe, 2-[4-(imidazo[1,2-α]pyridin-3-ylethynyl)benzylidene]malononitrile (5), responds rapidly to hydrazine and exhibits obvious color changes from yellow to colorless, indicating its use as a color indicator for hydrazine. Sensing mechanism of probe 5 toward hydrazine was deduced through HOMOLUMO energy levels and the interfacial plots of the molecular orbitals. Its physical and chemical properties were demonstrated by UV, fluorescence, and singlecrystal X-ray diffraction via computation and experiment. The calculated data are consistent with experimental results.

Full text of DOI:10.1246/cl.170169

Advance Publication Cover Page
A New Fluorescent Probe of Hydrazine:
-(4-(Imidazo[1,2-a]pyridin-3-ylethynyl)benzylidene)malononitrile
2
Chunshen Zhao,* Chao Huang, Shuo Yang, Huifang Chai, Yi Le, and Li Liu*
Advance Publication on the web March 18, 2017
doi:10.1246/cl.170169
©
2017 The Chemical Society of Japan
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1
A New Fluorescent Probe of Hydrazine: 2-(4-(Imidazo[1,2-a]pyridin-3-
ylethynyl)benzylidene)malononitrile
1
,2,3,4
1,2
1,2
5
1,2
1,2,6
Chunshen Zhao, *
Chao Huang, Shuo Yang, Huifang Chai, Yi Le and Li Liu *
1
School of Pharmaceutical Sciences, Guizhou University, Guiyang, 550025, P. R. China
Guizhou Engineering Laboratory for Synthetic Drugs, Guiyang, 550025, P. R. China
Key Laboratory of Guizhou for Fermentation Engineering and Biomedicine, Guiyang, 550025, P. R. China
2
3
4
Key Laboratory for Green Chemical and Clean Energy Technology of Guizhou Province, Guiyang, 550025, P. R. China
5
Guiyang College of Traditional Chinese Medicine, Department of Pharmacy, Guiyang, 550002, P. R. China
6
School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, 550025, P. R. China
(
E-mail: chunshenzhao@163.com)
1
Hydrazine is widely used in plently of fields as a raw hydrazine. Its molecular structure was determined by
H
1
3
material and closely related to our healthy. A new fluorescent
probe of hydrazine was designed and synthesized, choosing
imidazo[1,2-a]pyridine as the electron acceptor, based on the
intrarmolecular charge transfer (ICT) effect. The probe, 2-(4-
NMR, C NMR, MS and IR. We also determined its crystal
structure by single-crystal X-ray diffraction analysis and
calculated its geometric data using Gaussian 09 program
1
6
package
based on density functional theory (DFT)
(
(
imidazo[1,2-a]pyridin-3-ylethynyl)benzylidene)malononitrile
5), responds rapidly toward hydrazine and exhibits obvious
calculations. The crystal structure was extremely similar to
calculated geometries.
color changes from yellow to colorless, indicating its use as a
color indicator for hydrazine. Sensing mechanism of probe 5
toward hydrazine was deduced through HOMO-LUMO
energy levels and the interfacial plots of the molecular orbitals.
And its physical and chemical properties were demonstrated
by UV, fluorescence and single-crystal X-ray diffraction via
computation and experiment. The calculated data are
consistent with experimental results.
Hydrazine, a colorless and transparent oily liquid with
strong alkaline and hygroscopicity, can corrode glass, rubber,
leather, cork and so on. It is widely used in the industry of
aerospace, agriculture, pharmaceutical, chemical, photography
as missile systems, weapons for mass destruction, pesticides,
plant growth regulators, antimicrobial drugs, fuel cells,
catalysts, metal corrosion inhibitor, emulsifiers, dyes,
Scheme 1. Synthesis and proposed sensing mechanism of probe 5.
The synthesis of imidazo[1,2-a]pyridine (2): The
solution of bromacetal (50 g, 0.254 mol) in 1 N hydrochloric
o
acid (200 mL) was refluxed for 30 minutes at 80 C. The pH
of the solution was adjusted to 7 by sodium bicarbonate and to
the mixture was added 2-aminopyridine(15.90 g, 0.169 mol)
followed by stirring for 2 h at room temperature. Adjust the
pH to 10 with sodium hydroxide and the product was
extracted by ethyl acetate. The organic phase was collected
and the solvent was evaporated to get the brown oil (12.5 g,
55.9%).
The synthesis of 3-iodoimidazo[1,2-a]pyridine (3): To
the solution of imidazo[1,2-a] pyridine (6.50 g, 0.055 mol) in
0 mL THF was added 1-iodopyrrolidine-2,5-dione (14.85 g,
0.066 mol) slowly. The reaction mixture was stirred for 1 h at
room temperature and extracted by ethyl acetate. The organic
phase was collected and the solvent was evaporated to afford
the yellow solid (6.10 g, 45.5%).
The synthesis of 4-(imidazo[1,2-a]pyridin-3-ylethynyl)
benzaldehyde (4): To a 100 mL round bottom flask, 3-
iodoimidazo[1,2-a]pyridine (6.10 g, 24.4 mmol) was
dissolved in 100 mL THF followed by adding 4-
ethynylbenzaldehyde (3.28 g, 0.244 mmol), triethylamine
1-5
photography chemicals and so on. Nevertheless, since it’s
toxic effects, hydrazine will potentially leads to serious
environmental pollution and health hazards during its
6
manufacture, use, transport and disposal procedures. The
research indicates that long-term exposure to the atmosphere
1
of hydrazine can induce steatosis, damage of the liver and
7
central nervous system, mutation and cancer. Thus, the
concentration of hydrazine in the environment needs to be
strictly monitored and sophisticated methods are always
8
8
required. The generally described efforts devoted to
determine
hydrazine
include
chemiluminescence,
electrochemistry, flow detectionand spectrofluorimetry
9-14
technique.
In recent years, the fluorescent has been widely
used as a simple but reliable detection method for the rapid
and sensitive detection of hydrazine both qualitatively and
1
5
quantitatively. So we tried to design a new fluorescent probe
for detecting trace amounts of hydrazine.
In our work, the photophysical properties, synthesis,
sensitivity, crystal structure and calculation of 5 are reported
(
3.81 g, 36.6 mmol), triphenylphosphane (0.066 g, 0.244
mmol), copper iodide (0.047 g, 0.244 mmol) and
bis(triphenylphosphine) palladium(II) chloride (0.175 g, 0.244
mmol) in order under nitrogen atmosphere. The reaction was
(
Scheme 1). Compound 5 responded rapidly toward hydrazine
and exhibited distinct color changes from bright yellow to
colorless, indicating a good sensitivity as a color indicator for

2
o
heated to 65 C to stir for 1 h and the mixture was poured into
form with hydrazine 5-N
2
H
4
are consistent with experimental
water after the completion of the reaction that no starting
material could be detected via TLC. THF was removed by a
rotary evaporator and solid precipitation was filtered, washed
with dichloromethane. The aqueous phase was extracted with
dichloromethane and the organic phase was collected and
value (Figure 3).
1
evaporated to afford the yellow solid (3.05 g, 49.6%). H
NMR (400 MHz, CDCl
3
) δ 10.04 (s, 1H), 8.39 (d, J = 6.8 Hz,
1
3
H), 7.99 (s, 1H), 7.91 (d, J = 8.4 Hz, 2H), 7.77 – 7.68 (m,
H), 7.39 – 7.31 (m, 1H), 7.03 (t, J = 6.4 Hz, 1H).
The synthesis of 2-(4-(imidazo[1,2-a]pyridin-3-
Figure 2. Experimental UV absorption spectra (a) and fluorescence
titration spectra (b) of probe 5 and its reacted form with hydrazine 5-N
ylethynyl)benzylidene)malononitrile (5): To the solution of
compound 4 (3.00 g, 11.44 mmol) in 50 mL THF was added
malononitrile (0.79 g, 17.16 mmol) and five drops of
2
4
H .
o
tetrahydropyrrole. The mixture was heated to 65 C and
stirred for 1 h. The yellow solid (2.1 g, 62.4%) was got by
filtration after the reaction was cooled to room temperature.
o
m.p.: 204.6 - 205.8 C. IR (KBr pellets): ν 3452.3, 3037.1,
-
1 1
2
226.7, 2196.4, 1582.8, 1256.8, 1154.2, 835.5, 756.1 cm . H
NMR (400 MHz, DMSO-d ) δ 8.74 (s, 1H), 8.54 (s, 1H), 8.33
7.69 (m, 6H), 7.49 (s, 1H), 7.19 (s, 1H). C NMR (100
MHz, DMSO-d ) δ 160.04, 145.73, 139.45, 139.16, 131.36,
6
1
3
–
6
Figure 3. Calculated UV absorption spectra (a) and fluorescence titration
spectra (b) of probe 5 and its reacted form with hydrazine 5-N
1
1
30.87, 130.75, 129.67, 127.62, 127.16, 126.14, 117.52,
2
4
H .
14.24, 114.12, 113.25, 107.14, 99.11, 81.80, 81.62. MS
+
(
ESI+): m/z = 295.0 [M + H] .
We have performed the computations on the probe with
and without hydrazine based on density functional theory
The crystal of probe 5 was grown by slow evaporation of
chloroform under ambient conditions, and suitable crystal was
obtained for crystallographic analysis. The measured values
reveals that 5 possesses triclinic crystal system having P-1
space group [unit cell: a = 4.9408(16) Å, b = 12.394(4) Å, c =
(
DFT) and had gotten insight into the sensing mechanism of
compound 5 toward hydrazine. As shown in Figure 4,
comparing the level changes of the HOMO and LUMO in
compound 5 and compound 5-N H , the ICT effect in 5-N H
2
4
2
4
1
5.052(5) Å]. ORTEP diagrams of the molecular structure and
is stronger than in compound 5. Thus we can see an obvious
blue-shift in the absorption spectra and an intense
fluorescence enhancement with the adding of N H . In these
the atomic numbering schemes of 5 is shown in Figure 1. The
hydrogen atoms were omitted for clarity. The crystallographic
and refinement data are shown in Supplementary Material
Table S1. The crystal structure of 5 was compared with the
DFT-optimized structure. Some selected experimental and
calculated geometry parameters for 5 are listed in Table S2.
As expected, most of the calculated geometry parameters for
the probe are close to the X-ray data.
2
4
process, the imidazo[1,2-a]pyridine group plays a role of
donor and its electron transfers to the malononitrile accepter
group after excitation. And after reacted with hydrazine, the
malononitrile group is substituted by electron-abundant
2
hydrazine to form 5-N H4, which shows a strong ICT effect.
Figure 1. DFT-optimized and crystal structure of probe 5.
We have investigated the changes of fluorescence
spectra and the UV-vis spectra for the probe 5 in the absence
and presence of hydrazine. The color change following by the
addition of hydrazine are detectable easily by the naked eye of
sensor. As shown in Figure 2(a)，the green line showed a
weak absorption at 419 nm, which was attributed to the
intramolecular charge transfer (ICT) transition in the molecule.
Upon the addition of hydrazine, the absorption intensity at
Figure 4. HOMO-LUMO energy levels and the interfacial plots of
the molecular orbitals for probe 5 and its reacted form with
2 4
hydrazine 5-N H .
4
19 nm moved left immediately and became stronger. And as
In summary, we have designed and synthesized a
fluorescence probe of hydrazine. The probe showed a high
sensitivity character during the experimental verification. The
crystal of probe 5 was grown and its crystal structure was
determined by single-crystal X-ray diffraction analysis, which
showed a extremely similar data to calculated geometries
we can see, Figure 2(b) indicated that the absorption peak
moved to 416 nm from 596 nm when adding hydrazine in the
fluorescence spectra. The calculated UV absorption spectra
and fluorescence titration spectra of probe 5 and its reacted

3
using Gaussian 09 program package based on DFT
calculations. We have also studied its character via UV and
fluorescence. Comparing the experimental UV absorption
spectra and fluorescence titration spectra of probe 5 and its
reacted form with hydrazine with calculated value, the results
are in agreement.
This work was supported by the Science and
Technology Cooperation Program of Guizhou Province
(
LH20147681 and LH20157647). The Major Social
Development Project (SY20143054) and the Modernization
of Traditional Chinese Medicine Support Project in Guizhou
province (20105022). The theoretical calculations were
conducted on the ScGrid and Deepcomp7000 the
Supercomputing Center, Computer Network Information
Center of Chinese Academy of Sciences.
Supporting Information is available electronically on J-
STAGE.
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