3018
L. Chen et al. / Tetrahedron Letters 55 (2014) 3017–3023
(specification: 20 ꢁ 20 cm) was purchased from Whatman Interna-
tional Ltd., England.
3.0 mL cell, a series of anions in solution was then added. Ethanol
was added to this mixture to a total volume of 3.0 mL. UV–vis spec-
tra were measured after shaking for about 1 min.
1H NMR and 13C NMR spectra were measured by a Bruker
300 MHz superconducting magnet high field NMR spectrometer
using tetramethylsilane as an internal standard. Ultraviolet–visible
(UV–vis) spectra were obtained on an UV-4802 spectrophotometer
(UNICO(Shanghai) Instruments Co. Ltd., China). Mass spectra were
recorded on a PE SCIEX API365 LC/MS/MS system. Elemental anal-
Preparation of colorimetric test paper for cyanide ion
Colorimetric test paper for cyanide was prepared by immersing
Whatman 1 chromatography paper and commercial filter paper in
a methanolic solution of HPEAPB (3.0 ꢁ 10ꢀ2 mol Lꢀ1) for 30 min
and then dried in air.
ysis was performed using
analyzer.
a LECO CHN-900 CHN elemental
Synthesis of HPEAPB
Results and discussion
It was reported by Zimmermann-Dimer and Machado25 that a
change in color could observed upon the addition of CNꢀ, Fꢀ, and
H2POꢀ4 to an acetonitrile solution of protonated merocyanine, in
other words, protonated merocyanine was not a good colorimetric
anionic chemosensor in acetonitrile. The response of HPEAPB to
anions in different solvents has been investigated and found that
in ethanol HPEAPB showed good selectivity and fast response to
CNꢀ over other interfering anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2POꢀ4 ,
and HSOꢀ4 ). Figure 2a illustrates the changes in UV–vis spectra,
the absorbance at 414 nm decreased and a new absorbance at
531 nm appeared upon the addition of CNꢀ. Other interfering an-
ions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ, and HSOꢀ4 ) produced minimal
change in the UV–vis spectra. At the same time, a color change
from yellow to red was immediately observed (less than 2 s) upon
the addition of CNꢀ to the ethanolic solution of HPEAPB. Similarly,
other interfering anions could not produce noticeable change in
color (Fig. 2b). These results demonstrate that HPEAPB can distin-
guish CNꢀ from halogen anions, AcOꢀ, H2PO4ꢀ, and HSO4ꢀ with fast
response and good selectivity, and can be detected by the naked
eye.
HPEAPB was synthesized according to Ref. 24 with some mod-
ification (Fig. 1). Under N2 atmosphere, 4-methylpyridine
a
(12.1 g, 0.13 mol) was added dropwise to a solution of 4-hydroxy-
benzaldehyde (16.0 g, 0.13 mol) and 30 mL of acetic anhydride,
and then refluxed for 24 h under electromagnetic stirring. After
cooling to room temperature, the mixture was poured into
300 mL of ice water and then stirred for 1.5 h to hydrolyze the ex-
cess acetic anhydride. The resultant mixture was filtered, and the
cake obtained was washed with ice water and recrystallized from
ethanol. The obtained solid was introduced to an ethanolic solution
(150 mL) of potassium hydroxide (7.0 g), and the mixture was re-
fluxed for 90 min, yielding a dark solution. The pH of the solution
was adjusted to 5–6 with acetic acid, and a pale yellow precipitate
was formed. 4-[(E)-2-(4-Pyridinyl)ethenyl] phenol (13.1 g) was ob-
tained as a pale yellow solid by filtration and dried in a freeze-
drier. 4-[(E)-2-(4-Pyridinyl)ethenyl]phenol (3.0 g, 15 mmol) and
25 mL of dry acetonitrile were introduced to a 100 mL flask. After
heating to 60 °C under stirring, allyl bromide (2.8 g, 23 mmol)
was added dropwise to the mixture within 10 min, and the resul-
tant mixture was refluxed for 4 h, during which the color of the
mixture changed from blood red to orange. The mixture was cooled
to room temperature and filtered, and the solid was washed with
acetonitrile and methanol. 4.0 g of HPEAPB as a yellow powder
was obtained. Yield: 84.1%. MS-Q1:(m/z) 238.1. 1H NMR
(300 MHz, DMSO-d6) d (ppm): 10.22 (s, broad 1H, OH), 8.83 (d,
2H, Py-H, J = 6.0), 8.18 (d, 2H, Py-H, J = 6.0), 7.98 (d, 1H, @CH-Py,
J = 18.0), 7.63 (d, 2H, Ar-H, J = 9.0), 7.31 (d, 1H, @CH-Ar, J = 15.0),
6.88 (d, 2H, Ar-H, J = 9.0), 6.11–6.23 (m, 1H, @CH, J = 6.0–33.0),
5.42–5.45 (m, 2H, @CH2, J = 6.0–18.0), 5.13 (d, 2H, ACH2A,
J = 6.0). 13C NMR (75 MHz, DMSO-d6) d (ppm): 160.06, 153.73,
143.86, 141.57, 131.87, 130.25, 126.15, 123.14, 121.19, 119.53,
115.99, 60.10. Elemental Anal. Calcd: C, 60.39; H, 5.07; N, 4.40;
found: C, 60.47; H, 5.06; N, 4.48.
UV–vis titration was conducted with CNꢀ (Fig. 3a). It was found
that the intensity of the absorbance at 414 nm decreased gradually
and the absorbance at 531 nm increased gradually as CNꢀ concen-
tration increased, accompanied with the formation of an isosbestic
point at 454 nm and a significant change in color from light yellow
to dark red (Fig. 3c). The isosbestic point at 454 nm indicated the
formation of a new complex between HPEAPB and CNꢀ. The Job
curve indicated that HPEAPB and CNꢀ formed a 1:1 complex
(Fig. 3b) (See ESI). According to Benesi–Hildebrand relation
A0þA1K½Xꢀꢂ
A¼
26, the association constant between HPEAPB and CNꢀ
1þK½Xꢀꢂ
was calculated to be 3.2 ꢁ 104. The limit of detection (LOD) for cya-
nide ion by UV–vis was determined to be 3.4 ꢁ 10ꢀ6 mol Lꢀ1
according to DL = 3r
/K.27 The LOD for cyanide ion sensed by the
naked eye was found to be 8.0 ꢁ 10ꢀ6 mol Lꢀ1 (Fig. 4a). A linear
relationship between the absorbance at 531 nm and CNꢀ concen-
tration in the range of 2.0 ꢁ 10ꢀ6 to 2 ꢁ 10ꢀ4 mol Lꢀ1 (Fig. 4b)
was obtained, illustrating that HPEAPB could be used to roughly
measure CNꢀ concentration in this concentration range.
UV–vis detection
UV–vis spectroscopic characterization of HPEAPB was per-
formed in ethanol at 298 K. Air-tight screw-capped quartz cells
of 1.0 cm optical path length was used in all experiments.
In a 50 mL volumetric flask, 59.5 mg HPEAPB was dissolved in
Figure 5 shows the 1H NMR spectra of HPEAPB in DMSO-d6
(6.0 ꢁ 10ꢀ2 mol Lꢀ1) upon the addition of CNꢀ. The –OH signal that
appeared at 10.20 in HPEAPB disappeared after the addition of
ethanol to obtain HPEAPB solution with
a
concentration of
1.0 ꢁ 10ꢀ3 mol Lꢀ1. 60
lL of HPEAPB solution was placed in a
Figure 1. Synthetic route for HPEAPB.