1,8-naphthyridine modified naphthalimide derivative: Ratiometric and selective sensor for hg2+ in organic aqueous solution

Article abstract of DOI:10.5012/bkcs.2013.34.1.63

A bottom-modified (4-position) naphthalimide derivative 1 with 1,8-naphthyridine as binding site has been designed and synthesized. Compound 1 is the first 1,8-naphthyridine-modified naphthalimide-based sensor that can detect Hghg²⁺ selectively with respect to ratiometric fluorescent change and blue shift in organic aqueous solution. The Job's plot and FAB mass indicate that 1 formed a 1:1 complex with Hg2+. A top-modified naphthalimide derivative 2 with 1,8-naphthyridin as binding site has also been synthesized for comparison.

Full text of DOI:10.5012/bkcs.2013.34.1.63

1,8-Naphthyridine Modified Naphthalimide Derivative
Bull. Korean Chem. Soc. 2013, Vol. 34, No. 1 63
http://dx.doi.org/10.5012/bkcs.2013.34.1.63
1,8-Naphthyridine Modified Naphthalimide Derivative: Ratiometric and
2+
Selective Sensor for Hg in Organic Aqueous Solution
‡
Yong Gang Shi, Yu Lian Duan, Jian Hua Chen, Xiang Hua Wu, Ying Zhou, and Jun Feng Zhang
†,*
*
Key Laboratory of Molecular Engineering and Photo-functional Materials, College of Chemistry and Chemical Engineering,
*
Yunnan Normal University, Kunming 650500, P.R. China. E-mail: junfengzhang78@yahoo.cn
†
*
School of Chemical Science and Technology, Yunnan University, Kunming 650091, P.R. China. E-mail: yingzhou@ynu.edu.cn
‡
Yunnan Academy of Tobacco Science, Kunming 650106, P.R. China
Received September 19, 2012, Accepted October 4, 2012
A bottom-modified (4-position) naphthalimide derivative 1 with 1,8-naphthyridine as binding site has been
designed and synthesized. Compound 1 is the first 1,8-naphthyridine-modified naphthalimide-based sensor
2+
that can detect Hg selectively with respect to ratiometric fluorescent change and blue shift in organic aqueous
2+
solution. The Job’s plot and FAB mass indicate that 1 formed a 1:1 complex with Hg . A top-modified
naphthalimide derivative 2 with 1,8-naphthyridin as binding site has also been synthesized for comparison.
Key Words : Fluorescent sensor, Naphthalimide, 1,8-Naphthyridine mercury, Ratiometric
Introduction
ing feature of this ligand working as a sensor is the sensitive
response toward D-glucoside or D-glucopyranoside with a
significant blue shift in emission,20 which provides the poten-
tial to form a ratiometric fluorescent sensor that exhibits a
spectral shift upon binding to the analytes of interest. There-
fore, 1,8-naphthyridine, as a typical N-containing receptor,
was introduced to form the ratiometric sensing system in 1.
This paper reports the design and synthesis of a new
naphthalimide derivative bearing a 1,8-naphthyridine bind-
ing group (1), which displays a ratiometric fluorescent change
toward Hg2+ among the other examined metal ions in organic
aqueous solution. After addition Hg2+ to 1 in CH3CN-
HEPES buffer (0.02 M, pH 7.4) (1:1, v/v), 1 showed obvious
blue shift from 547 nm to 534 nm in emission spectra, which
allowed 1 to selectively detect Hg2+ ion by the naked eye
over a great number of environmental ions including Ag+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Fe3+, Cr3+ and other alkali
metal and alkaline earth metal cations. For comparison, a
top-modified naphthalimide derivative 2 with 1,8-naphthyri-
din as binding site has also been designed and synthesized.
To the best of our knowledge, 1 is the first example of 1,8-
naphthyridine-modified naphthalimide-based compounds
used as Hg2+ fluorescent chemosensor with ratiometric fluore-
scent changes and blue shift.
Fluorescent sensors for the detection and measurement of
environment important ions are actively investigated because
these ions play indispensable roles in vital processes.1 In
particular, the development of a high selective fluorescent
probe for mercury ion in the presence of a variety of other
metal ions has received great attention.2 Mercury is a highly
toxic and widely spread heavy metal pollutant, which damages
DNA, impairs mitosis, and disrupts the central nervous and
endocrine systems.3 In this regard, many fluorescent probes
for mercury ions have been extensively explored.4 For
instance, coordination of Hg2+ to S atom-based receptors and
mercury-mediated desulfurization reactions have been wide-
ly used in development of reversible and irreversible Hg2+
fluorescent probes.5-7 Compared to traditional S-containing
receptors, N-containing receptor for effectively binding Hg2+
in a fluorescent probe is rare.8
1,8-Naphthalimide (Naph) is well known for typical intra-
molecular charge transfer (ICT) fluorophore, strong absorp-
tion and emission in the visible region, high photostability,
large Stokes’ shift and insensitivity to pH.9 Modifications of
naphthalimide have given rise to a great number of deriva-
tives with tunable binding properties and a variety of fluore-
scent properties.10 Some of them have been reported in our
previous works and proved to be effective fluorescent sensors
Results and Discussion
− 11
for F , Cu2+,12 Zn2+,13 and some other ions.14 In the present
work, the 1,8-naphthalimide fluorophore in conjunction with
N-containing binding groups would facilitate the recognition
of metal ions.
Coumpound 1 and 2 were synthesized as shown in
Scheme 1. N-(2-aminoethyl)-5,7-dimethyl-1,8-naphthyridin-
2-amine (5) was first synthesized by displacement reaction
between ethanediamine and 7-chloro-2,4-dimethyl-1,8-
naphthyridine (6) with a moderate yield of 74%. Compound
5 was then reacted with N-propyl-4-bromo-1,8-naphthali-
mides (3) in 2-methoxyethanol to give 1 in 46% yield. When
compound 5 was reacted with 4-(N-piperidine)-1,8-naph-
thalic anhydride in ethanol, 2 was obtained in 70% yield.
1,8-Naphthyridine and its derivatives have been applied in
the coordination chemistry,15 pharmaceutical16 and molecular
recognition fields,17 because of their intriguing structures
and bonding properties, as well as the biocompatibility and
spectroscopic properties.18 It exhibit various coordination
modes and interesting spectroscopic properties.19 A distinguish-

64 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 1
Yong Gang Shi et al.
of diethyl ether into a CH2Cl2 solution of 1 and 2, respec-
tively. In crystal 1, the distances between N2 and N1, N2 and
N3, N2 and N4 are 3.039, 2.995 and 4.417 Å, respectively.
The naphthalimide plane defined by C24-C25-N5 atoms has
a dihedral angle of 46.8º with the naphthyridine C2-C11-N4
mean plane. While in crystal 2, the distances between N1
and N2, N1 and N3, N1 and N4 are 3.758, 5.709 and 7.927
Å, respectively. The naphthalimide plane defined by C1-N1-
C2 atoms has a dihedral angle of 155.3º with the naphthy-
ridine C15-C16-C17 mean plane. These data indicate that
the uncoordinated N atoms in crystal 1 are more appropriate
to bind a metal ion than that in crystal 2.
The photophysical properties of 1 and 2 were evaluated in
CH3CN-HEPES buffer (0.02 M, pH = 7.4) (1:1, v/v) solution.
As shown in Figure S1, S2, both of compound 1 and 2
showed strong absorption at around 445 nm with the
presence of the ICT band of Naph. Addition of Li+, Na+, K+,
Ag+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+ and Cr3+ did not
change the absorption spectra, while only a slight blue shift
was observed upon the addition of Hg2+, Cu2+ and Fe3+ in 1.
Studies on the UV-vis absorption revealed that 1 and 2
showed no obvious selectivity toward a great number of
environmental ions including Ag+, Co2+, Ni2+, Cu2+, Zn2+,
Cd2+, Pb2+, Fe3+, Cr3+ and other alkali metal and alkaline
earth metal cations.
The changes in the fluorescence emission of 1 and 2 were
next investigated. For 1, excitation of the ICT absorption
bands gave rise to long wavelength emission, centered at
547 nm. Addition of Li+, Na+, K+, Mg2+, Ag+, Co2+, Ni2+,
Cu2+, Zn2+, Cd2+, Pb2+, Fe3+ and Cr3+ showed little or no
effect on the emission of 1. However in the case of Hg2+, a
new emission peak centered at 534 nm appeared (Figure 2).
A 13 nm blue-shift of the emission spectra from 547 nm to
534 nm was probably due to the weakened ICT effect caused
by the coordination of the nitrogen atom of the naphth-
alimide with Hg2+. In contrast, 2 showed a very weak emi-
ssion peak centered at 550 nm. A slightly fluorescence quen-
ching was observed, with no changes in max, upon the addi-
tion of Hg2+ in 2 solutions (Figure S3). These results demon-
Scheme 1. Synthesis route of compound 1 and 2.
The detailed experimental procedures and the characteri-
zation of the new compounds are described in Supporting
Information.
The structures of 1 and 2 were further confirmed by X-ray
analysis (Figure 1). The single crystal of 1 and 2 suitable for
X-ray diffraction studies were grown by the vapor diffusion
Figure 2. Fluorescence spectra of 1 (10 µM) in CH3CN-HEPES
buffer (0.02 M, pH = 7.4) (1:1, v/v) in the presence of different
metal perchlorates (5 equiv.). Excitation wavelength was 360 nm.
Figure 1. Crystal structures of compound 1 and 2.

1,8-Naphthyridine Modified Naphthalimide Derivative
Bull. Korean Chem. Soc. 2013, Vol. 34, No. 1 65
Figure 3. Fluorescence spectra of 1 (10 µM) in CH3CN-HEPES
buffer (0.02 M, pH 7.4, v/v, 1:1) in the presence of different
2+
amounts of Hg . Inset: the ratio of emission intensity at 547 nm
2+
and 534 nm as a function of Hg concentration.
strated that the 1,8-naphthyridin in the bottom part (4-
position) in 1 could efficiently be involved in the formation
of Hg2+ complex in the recognition of metal ions; however,
the 1,8-naphthyridin in the top part cannot work like that in 2.
To get further insight into the binding of Hg2+ with 1, the
fluorescence spectra of 1 upon titration with Hg2+ were
recorded (Figure 3). Upon addition of increasing amounts of
Hg2+, the emission intensity at 547 nm decreased slowly
with a 13 nm blue-shift. With 20 equivalent of Hg2+, the
maximum of emission band shifted to 534 nm. The linear
dependence of the intensity ratio within the equivalent range
of Hg2+ ion testified that 1 forms a 1:1 complex with Hg2+,
whose association constant (Ka) was determined to be about
2.3 × 103 from the titration experiments. Moreover, the Job’s
plot (Figures 4) and FAB mass (Figures S4) confirmed the
1:1 stoichiometry for the 1-Hg2+ complex, which also strong-
ly supports the above conclusions.
Figure 5. (a) Emission spectra of 1 (10 µM) mixed with 20 equiv.
2+
of Hg and 40 equiv. of various metal ions in CH3CN-HEPES
buffer (0.02 M, pH 7.4) (1:1, v/v). (b) Histogram representing
emission intensity of 1 to various metal ions (purple bars) and
2+
fluorescence intensity change of the mixture of 1 and Hg after
addition of an excess of the appropriate other metal ions (green
bars), its wavelength is I534nm and I547nm of the fluorescence
intensity ratios. Excitation at 360 nm (slit = 5/5).
Zn2+, Cd2+, Pb2+, Fe3+ and Cr3+. Fluorescence emission spec-
troscopy was used to monitor the competition events. As
shown in Figure 5, in the presence of Li+, Na+, K+, Mg2+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Cr3+, the emission spectra
were almost identical to which was obtained in the presence
of Hg2+ alone. In the case of Ag+, Pb2+ and Fe3+, the emission
intensities diminished to different extents, but they still had
sufficient detections of Hg2+. Therefore, 1 was proved to be
To explore the possibility of using 1 as a Hg2+ selective
fluorescent chemosensor, competition experiments were
carried out. 1 (10 μM) was first mixed with 20 equivalent of
Hg2+, followed by adding 40 equivalent of various metal
ions including Li+, Na+, K+, Mg2+, Ag+, Co2+, Ni2+, Cu2+,
2+
a promising selective fluorescent sensor for Hg in the
presence of most competing metal.
Conclusions
In conclusion, we have developed a novel ratiometric
fluorescent sensor 1 for Hg2+. It displays a 13 nm blue-shift
of fluorescence emission, with dramatic fluorescence color
change from yellow to green yellow upon addition of Hg2+
in organic aqueous solution, as a result of the weakened ICT
effect caused by the coordination of the nitrogen atom of the
naphthalimide with Hg2+. The Job’s plot and FAB mass
indicate that 1 formed a 1:1 complex with Hg2+. Moreover,
as far as we are aware, 1 is the first 1,8-naphthyridine-modi-
fied naphthalimide-based chemosensor that can selectively
detect Hg2+ with ratiometric fluorescent change.
2+
Figure 4. The Job’s plot showing the 1:1 binding of 1 to Hg ions.

66 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 1
Yong Gang Shi et al.
Experimental
(CDCl3, 500 MHz) δ 8.59 (d, J = 7.2 Hz, 1H), 8.51 (d, J =
8.1 Hz, 1H), 8.39 (d, J = 8.4 Hz, 1H), 7.90 (d, J = 8.8 Hz,
1H), 7.68 (t, J = 7.8 Hz, 1H), 7.18 (d, J = 8.1 Hz, 1H), 6.83
(d, J = 9.9 Hz, 1H), 6.71 (d, J = 7.9 Hz, 1H), 5.81 (s, 1H),
4.60-4.54 (m, 2H), 4.00 (s, 2H), 3.24 (s, 4H), 2.61 (d, J =
14.5 Hz, 3H), 2.50 (d, J = 10.6 Hz, 3H), 1.90 (s, 6H). 13C
NMR (CDCl3, 150 MHz) δ 165.21, 164.77, 161.01, 158.94,
157.54, 144.80, 133.84-133.23 (m), 133.00, 131.30, 130.87,
130.01, 126.17, 125.34, 122.84, 119.39, 115.51, 114.70,
54.53, 53.44, 41.84, 39.46, 26.21, 25.13, 24.34, 17.90.
1:12 Under argon, a solution of 4 (367.5 mg, 1.16 mmol), 5
(250.4 mg, 1.16 mmol), and potassium iodide (50 mg) in 2-
methoxyethanol (30 mL) were refluxed for 24 h. The solvent
was evaporated under reduced pressure, a yellow solid was
obtained, the crude product was purified by column chromato-
graphy (silica, CH2Cl2/MeOH, 100:1, v/v) to afford 85 mg
of 1 (46% yield). 1H NMR (DMSO-d6, 500 MHz) δ 8.91 (d,
J = 8.4 Hz, 1H), 8.60 (s, 1H), 8.40 (d, J = 7.3 Hz, 1H), 8.31
(d, J = 8.5 Hz, 1H), 8.08 (d, J = 9.0 Hz, 1H), 7.80 (t, J = 5.8
Hz, 1H), 7.59-7.48 (m, 1H), 7.03 (d, J = 4.6 Hz, 1H), 7.01
(d, J = 8.7 Hz, 1H), 6.83 (d, J = 9.0 Hz, 1H), 4.00-3.93 (m,
2H), 3.83 (dd, J = 11.4, 5.8 Hz, 2H), 3.59 (dd, J = 10.3, 5.2
Hz, 2H), 2.65 (s, 3H), 2.52 (s, 3H), 1.68-1.52 (m, 2H), 0.90
(t, J = 7.4 Hz, 3H). 13C NMR (DMSO-d6, 150 MHz) δ 164.19,
163.40, 160.51, 160.26, 156.38, 151.05, 145.47, 134.76,
134.29, 131.02, 129.66, 129.47, 124.50, 122.14, 120.31,
119.53, 114.89, 112.91, 108.14, 104.31, 55.38, 45.81, 41.21,
25.48, 21.45, 17.97, 11.89. FAB mass calcd for C28H27N5O2
[M + H]+: 454.22, found 454.3.
Materials and Measurement.
General Methods − Unless otherwise noted, materials
were obtained from commercial suppliers and were used
without further purification. Flash chromatography was
carried out on silica gel (100-200 mesh). 1H NMR spectra
were recorded using 500 MHz and 13C NMR was recorded
using 150 MHz. Chemical shifts were expressed in ppm and
coupling constants (J) in Hz.
UV-Vis and Fluorescence Titration of 1 with Metal Ions.
The UV-Vis spectra were obtained using U-3010 spectrophoto
meter. The fluorescence spectra were obtained with F-4500
FL spectrometer with a 1 cm standard quartz cell. Stock
solutions (0.01 M) of the perchlorate salts of Li+, Na+, K+,
Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Mg2+, Hg2+, Pb2+, Fe3+,
Cr3+ in CH3CN were prepared. Stock solutions of host (10
μM) were prepared in CH3CN/HEPES (0.02 M, pH 7.4)
(1:1, v/v). Test solutions were prepared by placing 50 μL of
each ions stock, and diluting the solution to 3 mL with
CH3CN/HEPES (0.02 M, pH 7.4) (1:1, v/v). All of the titra-
tion experiments were recorded at room temperature.
Structural Determination. Crystals suitable for X-ray
diffraction were obtained by diffusion of diethyl ether into a
CH2Cl2 solution. All complexes reported here were struc-
turally characterized by X-ray crystal analysis. Diffraction
date were collected on a Rigaku RAXIS-RAPID X-Ray
diffractometer using a graphite monochromator with Mo Kα
radiation (=0.71073 nm) at 298(2) k. Structures were solved
by direct methods (SHELXS-97) and refined (SHELXS-97)
by full-matrix least-squares methods on all F2 date.21 The
unweighted and weighted agreement factors (Rf, Rw) and the
goodness of fit were calculated. Crystal data and details on
data collection and refinement are summarized in Table S1.
Synthesis. N-propyl-4-bromo-1,8-naphthalimides (4) and
7-chloro-2,4-dimethyl-1,8-naphthyridine (6) were synthe-
sized by an improved method according to the literature.22-24
Synthesis of other compounds is described below.
Acknowledgments. This work was supported by the
Foundation of the Department of Science and Technology of
Yunnan Province of China (2011FB013, 2011FB047), the
Natural Science Foundation of China (NSFC Grant Nos.
21002086, 21102127, 21262045, 21262050).
Notes and References
†Electronic Supplementary Information (ESI) available: CCDC 878377
(1) & 878378 (2) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/
5: Under argon, 7-choro-2,4-dimethy-1,8-naphthyridine
(6) (0.2 g, 1.04 mmol) was dissolved in excess ethane diamine
(10 mL), the reaction mixture were refluxed for 48 h, after
which the solution was clear and the solvent was evaporated
under reduced pressure, the crude product was purified by
silica gel column chromatography using CH2Cl2/MeOH
(12:1, v/v) as eluent to afford a white solid product 5 (0.1661
g, 7% yield). 1H NMR (DMSO-d6, 500 MHz) δ 7.98 (d, 1H,
J = 7.2 HZ), 7.37 (s, 1H), 6.90 (s, 1H), 6.76 (d, J = 8.8, 1H),
2.85 (t, J = 6.15, 2H), 2.49 (S, 6H). 13C NMR (DMSO-d6,
150 MHz) δ 160.06, 159.43, 156.58, 145.03, 133.53, 118.97,
114.44, 112.68, 42.86, 40.93, 17.81. FAB mass calcd for
C12H16N4 [M + H]+: 217.14, found 217.13.
1
13
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