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C. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 174 (2017) 214–222
previously [49]. A solution of 2-furan formylhydrazine (3) (0.126 g,
1.000 mmol) in absolute ethanol (10 mL) was added dropwise to anoth-
er solution containing 7-diethylamino-3-formylcoumarin (6) (0.245 g,
1.000 mmol) in absolute ethanol (30 mL) under stirring. The reaction
mixture was stirred vigorously at room temperature for 12 h, during
which time the color of the solution turned from orange yellow to or-
ange red. Then half of the ethanol was evaporated under reduced pres-
sure and the rest of solution was cooled to room temperature. After
placing the solution into refrigerator for 4 h, an orange red solid was
separated out from the solution and filtered under reduced pressure,
washing five times with ice-cold ethanol (10 mL). The obtained crude
product was recrystallized from absolute ethanol (30 mL) to furnish
the desired product 1 as an orange yellow powder (Scheme 1). Yield:
0.11 g (31.16%). m.p. 239–242 °C, 1H NMR (400 MHz, CDCl3) (Fig. S1):
8.66 (s, 1H,\\NH\\), 7.80 (s, 1H, H4), 7.69 (s, 1H, H7), 7.02 (s, 1H, H5),
6.91 (d, 1H, J = 7.2 Hz, H3), 6.88 (s, 1H, H6), 6.31 (dd, 1H, J = 7.2 Hz,
J = 1.6 Hz, H2), 6.26 (dd, 1H, J = 2.8 Hz, J = 1.2 Hz, \\CH_ N\\),
6.20 (d, 1H, J = 1.6 Hz, H1), 3.76 (q, 4H, J = 5.6 Hz,\\CH2\\), 1.99 (t,
refluxing in absolute ethanol. Then ethyl 7-diethylaminocoumarin-3-
formate (4) was acidated in absolute ethanol to give (7-
5
diethylaminocoumarin-3-formic acid). Subsequently, 5 was reacted
with phosphorus oxychloride in N,N-dimethyl formamide (DMF) and
7-diethylamino-3-formylcoumarin (6) was obtained. Finally, the con-
densation reaction between 2-furan formylhydrazine (3) and 7-
diethylamino-3-formylcoumarin (6) in absolute ethanol furnished the
desired compound 1 as an orange yellow powder. The structure of com-
pound 1 was characterized by 1H NMR and mass spectrometry (ESI-
MS), and the details of the characterization data of compound 1 were
presented in the Supporting Information (Fig. S1–S2).
3.2. UV–vis Titration of Compound 1 with Increasing Amounts of Zn2+
In order to clarify the interaction of compound 1 with metal ions, the
spectroscopic properties of 1 towards various chemically and biological-
ly important metal ions were investigated by UV–vis and fluorescence
methods in ethanol-water (V : V = 9 : 1) solution. In order to gain in-
sight into the UV–vis response of compound 1 to Zn2+, we firstly con-
ducted the UV–vis absorption titration spectrum of compound 1 in the
presence of increasing amounts of Zn2+ in ethanol-water (V : V = 9 :
1) solution and the results were shown in Fig. 1. There were almost no
bands in the range from 230 nm to 600 nm in UV–vis absorption spec-
trum of free 1, but two new bands centered at 342 nm and 474 nm ap-
peared with increasing absorbance upon addition of various
concentrations of Zn2+ (Fig. 1), which indicated that a new complex
had been formed between compound 1 and Zn2+ in ethanol-water (V
: V = 9 : 1) solution.
6H, J = 5.6 Hz, \\CH3). MS (ESI) (Fig. S2): m/z [M + H+ +
354.3599, found 354.1438; [M + Na+ +
calcd 376.3418, found
376.1244; [M + K
+ + calcd 392.4503, found 392.0950; [2M + Na+]+
]
calcd
]
]
calcd 729.6937, found 729.2406. Anal. Calcd. for C19H19N3O4 (%): C,
64.58; H, 5.42; N, 11.89; O, 18.11. found: C, 65.24; H, 4.84; N, 8.84; O,
21.08.
2.4. General Information
Test solutions were prepared by placing 10 μL of the probe stock so-
lution into cuvettes, adding an appropriate aliquot of each metal ion
stock, and diluting the solution to 2 mL with ethanol-water (V : V = 9
: 1) solution. For all fluorescence measurements of compound 1, the ex-
citation wavelength was set at 322 nm and the fluorescence emission
spectra were recorded over the range of 470–640 nm. The excitation
and emission slit widths were 3 nm and 1.5 nm in fluorescence emission
spectra of 1, respectively.
3.3. Fluorescence Titration of Compound 1 with Increasing Amounts of
Zn2+
The quantitative nature of compound 1 for sensing Zn2+ was then
elucidated by conducting fluorescence emission spectrum of 1 in the
presence of increasing amounts of Zn2+ in ethanol-water (V : V = 9 :
1) solution as described in Fig. 2. Upon excitation at 322 nm, compound
1 alone exhibited an intense emission peak centered at 511 nm. Wher-
evers, with a continuous increase in Zn2+ concentration, a gradual de-
crease in emission intensity at 511 nm was observed. Simultaneously,
a new emission peak centered at 520 nm appeared with increasing in-
tensity and a well-defined isoemission point was obtained at 516 nm.
It was probably because that the proton of hydrazone nitrogen atom
The binding constant value for complex 1-Zn2+ was determined on
the basis of the nonlinear filtting of the fluorescence titration curve as-
suming a 2 : 1 stoichiometry by the Benesi–Hildebrand method (1)
[50–51]:
1
1
1
≡
0:5 þ
i
ð1Þ
h
F−Fmin
ðFmax−FminÞ
KðFmax−FminÞ Zn2þ
where Fmin, F, and Fmax are the emission intensities at 520 nm of the or-
ganic moiety considered in the absence of zinc ion, at an intermediate
zinc concentration, and at a concentration of complete interaction, re-
spectively, and where K is the binding constant.
The limit of detection (LOD) of compound 1 for detecting Zn2+ was
calculated from the fluorescence titration. The ratio of emission intensi-
ties at 520 nm and 511 nm (F520 nm/F511 nm) of compound 1 without any
anion was measured to determine the S/N ratios [52–53], and the stan-
dard deviation of blank measurements was calculated. The LOD value
was calculated based on 3 × σblank/k, where σblank is the standard devi-
ation of the blank solution and k is the slope of the calibration plot.
3. Results and Discussion
3.1. Synthesis and Characterization of Compound 1
Compound 1 was synthesized according to the synthetic route
outlined in Scheme 1. Ethyl 2-furan formate (2) and 2-furan
formylhydrazine (3) were synthesized according to the reported meth-
od [48]. Firstly, ethyl 7-diethylaminocoumarin-3-formate (4) was pre-
pared by reacting 4-diethylaminosalicylaldehyde with diethyl
malonate using piperidine and glacial acetic acid as catalysts under
Fig. 1. Change in UV–vis absorption spectrum of 1 (100 μM) upon addition of increasing
amounts of Zn2+ (0.2, 0.4, 0.6, 0.8, 1.0, 1.4, 1.8, 2.2, 2.6, 3.0, 3.4, 3.8, 4.2, 4.6, 5.0 equiv.,
respectively) in ethanol-water (V : V = 9 : 1) solution.