Fluorescent sensors for Ca2+ and Pb2+ based on binaphthyl derivatives

Article abstract of DOI:10.1016/j.saa.2008.09.013

Two novel binaphthyl compounds have been synthesized for the selective fluorescent recognition of Ca²⁺ or Pb²⁺. By introducing different terminal groups to the receptor unit, the fluorescence signals of the receptors are significantl

Full text of DOI:10.1016/j.saa.2008.09.013

Spectrochimica Acta Part A 72 (2009) 322–326
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Fluorescent sensors for Ca2+ and Pb2+ based on binaphthyl derivatives
Ya-Wen Wang∗, Yong-Tao Shi, Yu Peng∗, Ai-Jiang Zhang,
Tian-Hua Ma, Wei Dou, Jiang-Rong Zheng
College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Two novel binaphthyl compounds have been synthesized for the selective fluorescent recognition of Ca2+
or Pb2+. By introducing different terminal groups to the receptor unit, the fluorescence signals of the
receptors are significantly changed: 1 is fluorescence enhancement for Ca2+, 2 is fluorescence quenching
for Pb2+. The binding properties for metal ions were examined by the absorption and fluorescence spectra.
The fluorescence intensity enhancement was ascribed to the complex formation between Ca2+ and 1 which
blocked the photo-induced electron transfer process.
Article history:
Received 20 June 2008
Received in revised form
21 September 2008
Accepted 27 September 2008
Keywords:
© 2008 Elsevier B.V. All rights reserved.
Binaphthyl compounds
Fluorescent sensor
Metal ion
PET process
1. Introduction
into an optical signal owing to the change of its photophysical
characteristics due to the perturbation by the bound cation of var-
Calcium ion play an important role as a messenger in biological
systems. Many physiological processes are triggered, regulated, or
influenced by calcium ion [1,2]. Lead ion is the most toxic heavy
metal ion causing adverse environmental and health problems. A
wide variety of symptoms (which include memory loss, irritabil-
ity, anemia, muscle paralysis, and mental retardation) have been
attributed to lead poisoning, suggesting that Pb2+ affects multi-
ple targets in vivo [3]. Therefore, it is important to explore new
methods for analyzing Ca2+ and Pb2+ in vitro and in vivo. Because
of the importance of ionic calcium and lead in biological processes,
chemists have been interested in the design of chemosensors for
the specific detection of them.
Design of chemosensors for the selective detection of a spe-
cific analyte is a topic of considerable interest, due to their wide
ranging application in the broad areas of chemistry, biology and
optics [4–11]. In optical sensors, the use of fluorescence as detecting
method offers distinct advantages in terms of sensitivity, selectiv-
ity and response time. Fluorescent molecular sensors have attracted
considerable interest because of their intrinsic sensitivity and selec-
tivity [4,6,7]. They are also called fluoroionophores because they
consist in a recognition moiety (ionophore) linked to a trans-
ducting moiety (fluorophore). The choice of the fluorophore is
of major importance because it governs the recognition event
ious photoinduced processes (electron transfer, energy transfer,
charge transfer). The recognition moiety is responsible for the effi-
ciency and selectivity of binding. Among ionophores for metal ions,
binaphthyl-based ligands offer numerous advantages because of
the rigidity of the complexing unit and tuneable strategies to obtain
molecules with appropriate substituents. Although the beginning
of the development can be traced back to the enantioselective
quenching of binaphthyl-based fluorescence first reported about
30 years ago [12]. Only recently binaphthyl-based fluorescent sen-
sors have been used for the monitoring of many chemical species
[13–24].
In the early reports some fluorescent molecular sensors have
been designed for Ca2+ or Pb2+, which include derivatives of crown
ether [25–28], calixarene [29–32], anthracene [33–35], squaraine
[36–39] and so on. Despite having many sensors, chemists continue
endeavoring to design new ones and to improve their sensitiv-
ity, selectivity and reliability in order to satisfy various needs.
In connection with our developments of fluorescent sensors for
metal ions [40,41], and studies about binaphthyl-based ligands for
luminescent lanthanide complexes [42–44], we designed and syn-
thesized two binaphthyl-based compounds in a novel but simple
approach as fluorescent sensors for Ca2+ and Pb2+, which show dis-
tinctly different optical properties. To best of our knowledge, much
less attention has been paid to binaphthyl-based compounds as
fluorescent sensors for meal ions. And to our surprise, only having
different terminal groups, sensor 1 show selectivity and response
toward Ca2+ with fluorescence enhancement, but sensor 2 is fluo-
∗
Corresponding authors. Tel.: +86 931 8912552; fax: +86 931 8912582.
E-mail addresses: ywwang@lzu.edu.cn (Y.-W. Wang), pengyu@lzu.edu.cn
(Y. Peng).
rescence quenching for Pb2+
.
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.09.013

Y.-W. Wang et al. / Spectrochimica Acta Part A 72 (2009) 322–326
323
2. Experimental
NMR (300 MHz, CDCl3): ı 7.96 (d, J = 9.0 Hz, 2H), 7.88 (d, J = 7.8 Hz,
2H), 7.39–7.34 (m, 4H), 7.25 (d, J = 8.7 Hz, 2H), 7.20 (d, J = 8.7 Hz, 2H),
4.57 (s, 2H), 4.56 (s, 2H), 3.64 (s, 3H), 3.64 (s, 3H) ppm. IR (KBr film):
ꢀmax 2953, 1755, 1621, 1592, 1508, 1434, 1377, 1333, 1288, 1213,
1149, 1107, 1091, 1001, 819, 749 cm−1. The hydrazine hydrate (1.5 g,
1.4 mL) was added to 4 (5 g, 11.63 mmol) in dry ethanol (10 mL) at
80 ◦C. Then the mixture was allowed to reflux for 4 h to afford 3
as a white solid; Yield: 91%. 1H NMR (300 MHz, d6-DMSO): ı 8.73
(s, 2H, –NH–), 8.05 (d, J = 9.0 Hz, 2H), 7.94 (d, J = 8.1 Hz, 2H), 7.51
(d, J = 8.7 Hz, 2H), 7.34 (t, J = 7.2 Hz, 2H), 7.22 (t, J = 8.4 Hz, 2H), 6.91
(d, J = 8.7 Hz, 2H), 4.58 (d, J = 14.7 Hz, 2H), 4.44 (d, J = 14.7 Hz, 2H),
4.24 (brs, 4H, –NH2) ppm. IR (KBr film): ꢀmax 3428, 3274, 3051,
2921, 1661, 1622, 1594, 1532, 1509, 1264, 1219, 1147, 1086, 985,
2.1. Materials
All commercially available chemicals were of A.R. grade and all
solvents used were purified by standard methods.
2.2. Methods
IR-spectra were measured on Nicolet Nexus 670 FT-IR using KBr
pellets in the range of 400–4000 cm−1. The 1H NMR spectra were
recorded on a Varian Mercury plus 300BB spectrometer in CDCl3
or d6-DMSO solution with TMS as internal standard. Fluorescence
measurements were performed on a Hitachi F-4500 spectropho-
tometer equipped with quartz curettes of 1 cm path length. The
excitation and emission slit widths were 5.0 nm. Absorption spectra
were made on a Shimadzu UV-240 spectrophotometer. HRMS were
determined on a Bruker Daltonics APEXII 47e FT-ICR spectrometer.
802, 737 cm−1
Compounds 1 and 2 were prepared according to the literature
[45].
.
Compound 1: yield: 92%. 1H NMR (300 MHz, d6-DMSO): ı 11.57
(s, 1H, –N CH–), 11.48 (s, 1H, –N CH–), 11.29 (s, 1H, –N C–OH),
10.89 (s, 1H, –N C–OH), 10.69 (s, 1H, –NH–), 10.59 (s, 1H, –NH–),
9.36 (s, 2H, –OH), 8.33–8.27 (m, 2H), 8.15–8.01 (m, 2H), 7.99–7.93
(m, 2H), 7.60 (d, J = 8.1 Hz, 1H), 7.54–7.46 (m, 1H), 7.40–7.20 (m, 4H),
7.14–6.95 (m, 4H), 6.85–6.74 (m, 2H), 5.27–5.08 (m, 2H), 4.82–4.62
(m, 2H), 3.80 (s, 6H) ppm. 13C NMR (75 MHz, d6-DMSO): ı 169.4,
164.5, 164.3, 153.9, 153.6, 148.2, 148.0, 147.1 (2C), 146.0, 141.4, 133.5,
133.4, 129.6, 129.4, 128.9, 128.1, 126.6, 125.2, 124.9, 124.7, 124.1,
123.6, 120.7, 120.5, 120.1, 119.2, 119.1, 118.8, 118.7, 118.0, 117.8, 116.4,
115.7, 113.9, 112.9, 68.4, 67.6, 56.2, 55.8 ppm. IR (KBr film): ꢀmax
3319, 3217, 3056, 2932, 2836, 1691, 1612, 1584, 1530, 1508, 1465,
1362, 1327, 1255, 1219, 1149, 1079, 946, 809, 778, 733 cm−1. HRMS
(ESI) m/z obsd 699.2443([M+H]+, calcd 699.2449 for C40H35N4O8).
Compound 2: yield: 90%. 1H NMR (300 MHz, d6-DMSO): ı 11.41
(s, 1H, –N CH–), 11.29 (s, 1H, –N CH–), 10.78 (s, 1H, –NH–), 10.18 (s,
1H, –NH–), 9.54–9.44 (m, 2H, –OH), 8.07–8.06 (m, 1H), 8.01–7.89
(m, 2H), 7.84–7.81 (m, 1H), 7.57–7.49 (m, 2H), 7.44–7.08 (m, 6H),
7.06–6.95 (m, 4H), 6.80–6.73 (m, 2H), 5.27–5.00 (m, 2H), 4.76–4.55
(m, 2H), 3.78 (s, 3H), 3.75 (s, 3H) ppm. 13C NMR (75 MHz, d6-
DMSO): ı 170.0, 169.7, 164.7, 164.5, 154.8, 154.5, 149.8, 149.5, 149.4,
149.0, 145.1 (2C), 134.0 (2C), 130.3, 130.0, 129.9, 129.6, 129.4, 128.7,
127.3, 126.1, 125.8, 125.4, 125.2, 124.6, 124.2, 123.0, 122.2, 120.4,
119.8, 118.5, 116.5, 116.1, 110.0, 109.5, 68.8, 68.2, 56.2, 56.1 ppm. IR
2.3. Synthesis: compounds 1 and 2
Compounds
1 and 2 were readily prepared according to
Scheme 1. We introduced two different terminal groups into the
1, 1ꢀ-binaphthyl framework to construct the fluorescent receptors.
All of the compounds were characterized by 1H NMR, 13C NMR, IR
and HRMS. They were all obtained in high yield and were soluble
in common organic solvents such as CHCl3, MeOH and DMSO.
2.4. Syntheses: compounds 1–4
Anhydrous K2CO3 (9.5 g, 70.98 mmol) was added into the 10 mL
DMF solution of 1, 1ꢀ-bi-2-naphthol (5 g, 17.48 mmol) at 80 ◦C. After
two hours, a solution of methyl chloroacetate (4 g, 36.89 mmol) in
10 mL DMF was added dropwise to the mixture and maintained
at 100 ◦C for 4 h. When cooled, 60 ml distilled water was poured
and the turbid solution was extracted by 40 mL chloroform three
times. Organic phase combined was washed with water and dried
with anhydrous Na2SO4. Solvent removed, the crude residue was
purified by flash column chromatography on silica gel (Petroleum
ether/AcOEt = 5:1 to 3:1) to afford the compound 4; Yield: 65%. 1
H
Scheme 1. The synthesis of receptors 1 and 2.

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Y.-W. Wang et al. / Spectrochimica Acta Part A 72 (2009) 322–326
(KBr film): ꢀmax 3412, 3318, 3054, 2928, 1671, 1593, 1512, 1462,
1430, 1383, 1272, 1210, 1084, 814, 749 cm−1. HRMS (ESI) m/z obsd
699.2441([M+H]+, calcd 699.2449 for C40H35N4O8).
2.5. Binding studies
The studies on the binding properties of 1 and 2 were carried
out in methanol. The perchlorate salts solution (1.0 × 10−2 M) were
prepared by dissolving the desired amount of perchlorate salts in
dry methanol. Association constants were calculated by means of
a linear curve fitting with Origin 7.0 (Origin-Lab Corporation).
3. Results and discussion
3.1. Complexation studies with Ca2+ and Pb2+
Fig. 1 shows the fluorescence titration of 1 with Ca2+ in
methanol. Addition of a 20-fold Ca2+ results in a 3 times enhance-
ment of fluorescence intensity with respect to the metal ion-free
state. The free acceptor 1 in the methanol possessed a fluorescence
peak at 445 nm with over 70 nm red shifts of 1, 1ꢀ-bi-2-naphthol, at
372 nm. This indicates an efficient intramolecular energy transfer
Fig. 3. The fluorescence intensity change profile of 1 (1.0 × 10−5 M in methanol)
in the presence of selected metal ions. Excitation is at 297 nm, and emission is
monitored at 488 nm.
from the phenyl rings to the naphthyl core [15,21]. Addition of cal-
cium ions to a methanol solution of 1 led to a bathochromic shift of
the main fluorescence band to 488 nm, which exhibits 43 nm red-
shift. Further addition of Ca2+ led to an increase in intensity of the
488 nm band and a decrease in the intensity of the 445 nm peak.
The change was marked by the presence of an isosbestic point at
448 nm. The large bathochromic shift for 1-Ca2+ can be attributed
to the strong stabilization of internal charge-transfer (ICT) state by
the complexation of calcium with the hydroxy and the nitrogen
atom of –N CH– groups via chelation.
To obtain an excellent chemosensor, high selectivity is a matter
of necessity. In the present work, studies of selective coordination
of cations of 1 by means of fluorescence spectroscopy were then
extended to related main group, transition and heavy metal ions
(Li+, Na+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+
,
Hg2+ and Ag+). Only the addition of Ca2+ resulted in a prominent
fluorescent enhancement in fluorescence (Fig. 2). The selectivity
of 1 for Ca2+ over other metal ions was investigated by the com-
petition experiments. Fig. 3 shows the fluorescence intensity of 1
in the presence of selected metal ions. It is obvious that 1 has a
Fig. 1. Fluorescence titration of 1 (1.0 × 10−5 M) in the presence of different concen-
trations of Ca2+ in CH3OH, ꢁex = 297 nm. Inset shows Benesi–Hilderbrand plot of 1
with Ca2+ at 488 nm.
Fig. 2. Fluorescence spectrum of 1 (1.0 × 10−5 M) in methanol in the presence of
Fig. 4. Fluorescence titration of 2 (2.0 × 10−5 M) in the presence of different con-
centrations of Pb2+ in CH3OH, ꢁex = 291 nm. Inset shows Benesi–Hilderbrand plot of
2 with Pb2+ at 371 nm.
several metal ions ([M] = 1.0 × 10−5 M (Co, Cu, Ni, Ag, Hg, Pb, Mn) ([M] = 5.0 × 10−5
(Li, Na, Mg, Ca, Sr, Ba, Zn)). ꢁex = 297 nm.
M

Y.-W. Wang et al. / Spectrochimica Acta Part A 72 (2009) 322–326
325
Fig. 6. UV–vis titration of 2 (1.0 × 10−5 M) in the presence of different concentrations
Fig. 5. Fluorescence spectrum of 2 (2.0 × 10−5 M) in methanol in the presence of
several metal ions ([M] = 4.0 × 10−5 M). ꢁex = 291 nm.
of Pb2+ in CH3OH.
highly selective response to Ca2+. On the basis of the photospectro-
scopic data, it seems that Li+, Na+, Ag+, Sr2+, Ba2+, Ni2+ and Zn2+
behave like Mg2+, whereas Mn2+, Co2+, Cu2+, Hg2+ and Pb2+ act
intensity was quenched more than 75% while the concentration
of Pb2+ reached that 10 equivalents of 2. The emission wavelength
upon Pb2+ binding did not exhibit any detectable shift, which means
that binding site between 2 and lead ion is different from 1 with
differently. The fluorescence spectra of 1, with Li+, Na+, Ag+, Sr2+
,
Ba2+, Ni2+, Zn2+ and Mg2+ in methanol, cause emission increases
along with red-shift. But no red-shift phenomenon are observed in
the one of Mn2+, Co2+, Cu2+, Hg2+ and Pb2+, which probably shows
that these metal ions bound in the same binding site with different
affinities.
Ca2+
.
We also study of selective coordination of cations of 2. Addition
of Li+, Na+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Hg2+
and Ag+ led to slightly changes in the fluorescence of 2. Only the
addition of Pb2+ resulted in a prominent fluorescent quenching in
fluorescence (Fig. 5).
The UV–vis spectrum of 2 show that the signals at the short
wavelengths (<300 nm) increased significantly with Pb2+, but much
smaller changes were observed at the long wavelengths (Fig. 6).
The absorption maximum wavelengths for 2-Pb2+ are 234 nm. So
the interaction between host and guest was only evaluated by flu-
orescent spectra.
There was almost no change observed on the UV–vis spectra
of receptor 1 upon addition of metal ions, the interaction between
host and guest was only evaluated by fluorescent spectra.
The fluorescence titration of 2 with Pb2+ was carried out in
methanol, which is shown in Fig. 4. Gradually increasing the con-
centration of Pb2+ caused the fluorescence emission intensities of 2
(2.0 × 10−5 M) at 371 nm to decrease remarkably. The fluorescence
Scheme 2. Proposed binding model of 1 with Ca2+ and 2 with Pb2+
.

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The Job plot is extensively used to find the complexation mode
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Acknowledgments
We are grateful for the generous financial support from a new
faculty grant, Interdisciplinary Innovation Research Fund For Young
Scholars (LZUJC2007003) from Lanzhou University.
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