A new on-fluorescent sensor for Ag+ based on benzimidazole bearing bis(ethoxycarbonylmethyl)amino groups

Article abstract of DOI:10.1515/hc-2015-0002

A new fluorescent sensor based on a benzimidazole unit bearing bis(ethoxycarbonylmethyl)amino groups was designed and synthesized. The ligand exhibits strong sensitivity and selectivity for Ag⁺ by enhanced fluorescent intensity in the presence

Full text of DOI:10.1515/hc-2015-0002

Heterocycl. Commun. 2015; aop
Bing Zhao, Yue Fang, Ming-jie Ma, Qi-Gang Deng*, Yu Xu and Li-yan Wang
A new on-fluorescent sensor for Ag+ based on
benzimidazole bearing bis(ethoxycarbonylmethyl)
amino groups
DOI 10.1515/hc-2015-0002
Received January 19, 2015; accepted March 20, 2015
positive response rather than fluorescent quenching upon
the ion binding are preferred to promote the sensitivity
+
factor [7–10]. The design of such turn-on silver ion (Ag )
sensors is an intriguing challenge because many transi-
tion elements often cause fluorescent quenching [11].
Most of the reported signaling units are mainly based
Abstract:
A new fluorescent sensor based on a
benzimidazole unit bearing bis(ethoxycarbonylmethyl)
amino groups was designed and synthesized. The ligand on rhodamine [12] and coumarin fragments [13]. The imi-
+
exhibits strong sensitivity and selectivity for Ag by dazole unit [14, 15] is a newer fluorescent group that is
enhanced fluorescent intensity in the presence of a wide becoming important in the field of chemosensors given its
range of other tested metal ions in methanol. The colori- fluorescence off-on behavior that results from its particu-
+
metric and fluorescent response to Ag can be conveni- lar structural properties [16]. Park and coworkers [17] have
ently detected even by the naked eye, which offers a facile shown that a proper substitution of a tetraphenylimida-
+
method for visual detection of Ag .
zole scaffold can lead to white light-emitting compounds
by the combination of excited-state intramolecular proton
+
Keywords: Ag ion; benzimidazole; fluorescent sensor; transfer and restricted energy transfer. In addition, a
naked eye.
polymer containing thienoimidazole systems can be
used for both colorimetric and ratiometric detections of
Hg2+ as well as fluorometric detection of Zn2+ via fluores-
cence turn-on response with augmented lifetime via the
chelation of metal ions to both S and N heteroatoms [18].
Unfortunately, in the existing reports, the fluorescence
enhancement in most cases is small and usually suffers
from a high background interference. To our knowledge,
few reports have been devoted to fluorescence enhance-
Introduction
Some metal ions, such as Ca2+, Fe3+, Zn2+, K , Mg2+, Ag , and
Na , are playing a key biological role in human body [1],
+
+
+
whereas other heavy metal ions, including Al3+, Cd2+, Cr3+,
Hg2+, and Pb2+, can be harmful to organisms [2]. Accord-
ingly, the development of fluorescent probes for the
detection of various metal ions is becoming an important
research area [3–6]. Over the last decades, the rational
design and synthesis of efficient sensors to selectively
recognize targets has been a hot topic in molecular recog-
nition studies. Generally, a typical sensor is devised by a
covalent linkage of three parts, namely, a receptor unit, a
spacer unit, and a signaling unit, although there are some
examples of spacer-free probes also. Usually, highly selec-
tive probes for transition or heavy elements that give a
+
ment of probes for Ag .
Results and discussion
In continuation of our work on imidazole derivatives [19],
a new silver ion probe L was synthesized and studied.
The synthetic route to the ligand compound-bearing
bis(ethoxycarbonylmethyl)amino and benzimidazole func-
tionalities is outlined in Scheme 1.
The fluorescence properties of ligand L were inves-
tigated by UV-vis absorption and fluorescence emission
spectra in methanol. The UV absorption spectrum of L in
the presence of various cations is presented in Figure 1.
The metal-free ligand shows no absorption higher than
350 nm. The addition of 10 equivalents of metal ion,
*Corresponding author: Qi-Gang Deng, Chemistry and Chemical
Engineering Institute, Qiqihar University, Qiqihar 161006, China,
e-mail: dqigang@163.com
Bing Zhao, Yue Fang, Ming-jie Ma, Yu Xu and Li-yan Wang:
Chemistry and Chemical Engineering Institute, Qiqihar University,
Qiqihar 161006, China
+
+
including K , Na , Ca2+, Mg2+, Ba2++, Zn2+, Fe3+, Pb2+, Cu2+,
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ꢀꢀꢀꢁ
2ꢀ ꢀB. Zhao et al.: A new on-fluorescent sensor for Ag
COOEt
EtOOC
EtOOC
COOEt
EtOOC
COOEt
NH2
NH2
N
NH2
N
N
BrCH2COOEt
K2HPO4, KI, MeCN
DMF
POCl3
MeOH
CHO
1
2
3
HN
N
L
Scheme 1ꢀ
show the selectivity of the interaction of compound L with
silver ion in the presence of other ions.
0.8
0.6
0.4
0.2
0
During the spectrophotometric titration of the solu-
tion of L in methanol by solution of AgNO3 in the same
solvent, the optical density of the ligand absorption band
(λmaxꢀ=ꢀ323 nm) decreases, and the characteristic absorp-
tion band of the complex Ag-L with λmaxꢀ=ꢀ424 nm appears
and grows gradually with the increasing concentration of
Al3+, Cr3+
Ag+
Other cation
+
Ag (not shown). This process parallels the color change
300
400
500
600
700
observed by the naked eye.
Wavelength (nm)
The insert shows color change to the naked eye upon
the addition of Ag+ to the solution of L under otherwise
similar conditions.
Figure 1ꢀAbsorption spectra of L (10 μm) in the presence of 10
equivalents of different metal ions in methanol.
+
To further investigate the interaction of Ag and L,
the fluorescence spectra were analyzed (Figure 2). Com-
Co2+, Ni2+, Cd2+, and Hg2+, results in a little change in pound L shows a very weak fluorescence at 424 nm in
absorption (Figure 1), as analyzed by the UV absorption the absence of metal ions with the fluorescence intensity
spectrum and the visible absorption observed by the of approximately 26 a.u. After the addition of metal ion,
+
naked eye. Upon the addition of 10 equivalents of Cr3+ or including Ba2+, Ca2+, Cd2+, Co2+, Cu2+, Fe3+, Hg2+, K , Mg2+,
Al3+, the UV absorption peak exhibits a slightly red shift Na2+, Ni2+, Pb2+, or Zn2+, changes in the fluorescence emis-
+
from 323 to 345 nm. By contrast, in the presence of Ag
sion are minimal. Even in the presence of Al3+ or Cr3+ when
ion under otherwise similar conditions, the solution of a slight red shift in UV absorption is observed, the fluores-
L undergoes a significant color change from colorless to cence spectra are still very similar to that of the free ligand
+
dark yellow (inset in Figure 1) with an emergence of a new L. By contrast, the addition of Ag (10 equivalents) causes
absorption peak at approximately 424 nm. These results a remarkable enhancement of fluorescence intensity at
B
A
200
180
160
140
120
100
80
1.2
1.0
0.8
0.6
0.4
0.2
0
Ag+
L, Al3+, Ba2+, Ca2+, Cd2+
,
Co2+, Cr3+, Cu2+, Fe3+
Hg2+, K+, Mg2+, Na+,
Ni2+, Pb2+, Zn2+
,
60
40
20
0
380
400
420
440
460
480
500
Wavelength (nm)
Figure 2ꢀSelective fluorescent spectrum of the interaction of Ag ion and L. (A) Fluorescence spectra of L (10.0 μm) in the absence and pre-
sence of various cations (100.0 μm) in methanol solution (λexꢂ=ꢂ323 nm). (B) Fluorescence intensity ratios F/F0 of L (5.0ꢂ×ꢂ10-7 M, constant) at
424 nm (λexꢂ=ꢂ323 nm) in methanol in the presence of Ag+ only (F0) and in the presence of Ag+ and 10 equivalents of additional metal ion (F).
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ꢁꢀꢀꢀ
ꢃ3
B. Zhao et al.: A new on-fluorescent sensor for Ag ꢃ
424 nm by a factor of approximately 5.5 (Figure 2A). To O atoms hinders the PET process leading to fluorescence
+
validate the selectivity of L toward Ag , competition exper- intensity enhancement.
+
iments in the presence of Ag only (fluorescence intensity
+
F0) and in the presence of Ag and other ions (fluorescence
intensity F) were conducted. As shown in Figure 2B, the
maximum fluorescence intensity ratio F/F0 is approxi- Experimental
+
mately 1.28 for K and the minimum value of 0.86 is
1H NMR (600 MHz) and 13C NMR (150 MHz) spectra were measured on
the Bruker instrument. UV-vis absorption spectra were recorded on a
TU-1901 ultraviolet and visible spectrophotometer (1 cm quartz cell)
at 25°C. Fluorescence measurements were conducted on a Perkin
Elmer LS55 fluorescence spectrometer using a 1-cm quartz cell at
25°C, with excitation and emission slit widths of 10 and 4 nm, respec-
tively, and excitation wavelength at 323 nm. Starting materials 2 and
3 were synthesized according to the reported procedures [20].
observed in the presence of Pb2+. These results demon-
strate that the enhancement in fluorescence intensity
+
resulting from the addition of Ag is not influenced sig-
nificantly by the addition of the background metal ions.
Additional experiments showed that the fluorescence
quantum yield of the ligand L at 424 nm is increased from
+
0.21 to 0.72 in the presence of Ag in methanol solution.
+
The fluorescence titration experiments of L with Ag
in a methanol solution were also investigated in the range
Synthesis of ethyl N-{[4-(1H-benzimidazol-2-yl)]
phenyl]}-N-[(ethoxycarbonyl)methyl]-aminoacetate (L)
+
of molar ratios [Ag ]/[L]ꢀ=ꢀ0–60. The fluorescence intensity
increases with the increasing concentration of Ag+ and
+
reaches a plateau with a ratio of [Ag ]/[L] ꢀ>ꢀ20 (not shown).
A mixture of o-phenylenediamine (5.30 g, 5 mmol) and compound
3 (1.46 g, 5 mmol) in methanol was heated under reflux at 65°C for
approximately 6 h until the reaction was completed, as judged by
TLC analysis, and then concentrated under reduced pressure. The
To determine the stoichiometry of the Ag-L complex,
Job’s method was applied using fluorescence titration
experiments (Figure 3). As can be seen, the fluorescence
+
+
intensity reaches a maximum for the ratio of [Ag ]/{[Ag ]+ crude product L was subjected to silica gel column chromatography
+
eluting with mixtures of hexanes and ethyl acetate in the ratios from
[L]} of 0.5, which indicates a 1:1 stoichiometry of Ag to L
10:1 to 7:3. The free base L was obtained in 77% yield as a gray solid;
in the complex.
mp 170.4–171.4°C; IR (KBr, cm-1): 3241, 3109, 2947, 2865, 1678, 1603,
1
1559, 1511, 1432, 1285, 1103; H NMR (600 MHz, DMSO-d6): δ 12.55
(s, 1H), 7.96 (d, 2H, J ꢀ=ꢀ 8.4 Hz), 7.57 (d, 1H, J ꢀ=ꢀ 7.2 Hz), 7.45 (d, 1H,
Conclusions
J ꢀ=ꢀ 7.2 Hz), 7.13 (m, 2H), 6.71 (d, 2H, J ꢀ=ꢀ 8.4 Hz), 4.29 (s, 4H), 4.15
(q, 4H, Jꢀ=ꢀ 13.8 Hz), 1.22 (t, 6H, Jꢀ=ꢀ 7.2 Hz); 13C NMR (150 MHz, DMSO-d6):
δ 170.7, 152.3, 149.5, 128.0, 119.4, 112.4, 61.0, 53.0, 14.6. Anal. Calcd for
A new fluorescence probe L for Ag+ was synthesized and
C21H23N3O4: C, 66.13; H, 6.08; N, 11.02. Found: C, 66.29; H, 6.17; N, 10.99.
analyzed. The enhancement in fluorescence intensity
+
of L upon interaction with Ag may be explained on the
basis of the thermodynamically favorable photoinduced Acknowledgments: This work was supported by the
+
electron transfer (PET) mechanism between L and Ag . Program for Education Department of Heilongjiang
+
The binding of Ag to L through the lone pairs of N and Province (no. 12541862) and Qiqihar University Graduate
Innovation Fund Grants (No. YJSCX2014-028X).
70
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50
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