Highly sensitive and selective Pd2+ sensor of naphthalimide derivative based on complexation with alkynes and thio-heterocycle

Article abstract of DOI:10.1039/b815298e

A new fluorescent Pd²⁺ sensor 1, N-butyl-4-(p-methyloxy)- phenylethynyl-5-thiophenemethylamino-1,8-naphthalimide, was designed and synthesized. It showed highly selective on-off fluorescence changes for Pd ²⁺ among the representative transition and heavy metallic cations, and its fluorescence was efficiently quenched by 5 equivalents of Pd ²⁺ in buffer solution. The sensor also displayed a selective chromogenic behavior toward Pd²⁺ from yellow to black-red, which could be easily observed by the naked eye. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b815298e

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Highly sensitive and selective Pd2+ sensor of naphthalimide derivative
based on complexation with alkynes and thio-heterocyclew
Liping Duan, Yufang Xu and Xuhong Qian*
Received (in Cambridge, UK) 2nd September 2008, Accepted 2nd October 2008
First published as an Advance Article on the web 29th October 2008
DOI: 10.1039/b815298e
A new fluorescent Pd2+ sensor 1, N-butyl-4-(p-methyloxy)-
phenylethynyl-5-thiophenemethylamino-1,8-naphthalimide, was
designed and synthesized. It showed highly selective on–off
fluorescence changes for Pd2+ among the representative transi-
tion and heavy metallic cations, and its fluorescence was effi-
ciently quenched by 5 equivalents of Pd2+ in buffer solution. The
sensor also displayed a selective chromogenic behavior toward
Pd2+ from yellow to black-red, which could be easily observed
by the naked eye.
Recently we did systematic investigations on fluorescent
sensors of naphthalimide derivatives for Cu2+ and Cd2+
based on the internal charge transfer (ICT) mechanism,5 on
which some substituting groups with potential chelating abilities
were introduced into the naphthalimide ring at 4, 5-position.
As we know, the thiophene moiety is often employed in
material fields such as conjugated polymer, optoelectronic
device, conductivity-based sensory device, bio-diagnostics de-
vice, novel drug, and nonlinear optical material.6 Sulfur-
containing chains were ever used for binding with heavy or
transition metallic cations. Pd2+ is a ‘‘soft’’ metal that may be
easy to form stable complexes with sulfur-containing amino
acids, peptides and proteins.7 On the other hand, it is well-
known that alkynes are able to coordinate with p-philic metal
ions, such as Ag+, Ni2+,Co2+, Hg2+, and Pd2+ through the
(p - d) donations.8 Bearing these in mind, we designed and
synthesized a simple and aqueous-soluble sensor 1, with
N-butyl-4-amino-1,8-naphthalimide as the fluorophore, thio-
phenemethylamine and phenylethyne as receptors for the
detection of Pd2+and its derivatives.
Development of highly sensitive and selective fluorescent sen-
sors for heavy and transition metal ions is of current interest
because of their significant importance in chemistry, biology,
and environmental science. Pd2+ plays an important role in the
production of dental and medicinal devices, jewellery, auto-
mobile and catalytic1 converters, so the investigation on the
methodology for highly selective and sensitive detection of
Pd2+ and its derivatives has attracted tremendous attentions.
The majority of the known methods include atomic absorption
spectrometry (AAS), plasma emission spectroscopy (ICP-AES),
solid phase microextraction–high performance liquid chroma-
tography (SPME–HPLC), X-ray fluorescence, etc.2 These
methods have the characterisctics of fast measurement and
good sensitivity, but they need expensive facilities, complicated
sample-pretreatment procedures and well-controlled experi-
mental conditions. Colorimetric technique is frequently applied
to the determination of Pd2+. The mechanism of this detection
method is the complexation of Pd2+ with either N,N-dimethyl-
4-nitrosoaniline or 5, 10, 15, 20-tetrakis(4-sulfophenyl)
porphine (TPPS4). The former results in a yellowish orange
complex, which can be analyzed by UV-vis spectroscopy with
low detection limits.3a As to the latter, other heavy metallic
cations can also form complex with TPPS4 and thus cause
determination interferences.3b There are also few interesting
examples of fluorescent detection for Pd2+ and its derivatives
based on chemical reaction in solution.4 We are interested in
developing highly sensitive and selective fluorescent Pd2+
sensor based on the complexation in aqueous solution, which
may provide a new strategy for design of Pd2+ sensor without
complicated sample-pretreatment procedures, and eliminate
obvious interferences from the other transition metallic cations
during the fluorescence or UV measurement.
The target compound N-butyl-4-(p-methyloxy) phenylethynyl-
5-thiophenemethylamino-1,8-naphthalimide was synthesized
from the starting material N-butyl-4-bromo-5-nitro-1,8-
naphthalimide,5a through the corresponding 5-thiophene-
methylamino intermediate 3 (Scheme 1).
Scheme 1 Synthesis of 1. (a) 2-Thiophenemethylamine, DMF, room
temperature, 65%; (b) 4-methyoxyphenylethyne, Pd(PPh3)4, CuI,
Et3N, N2, reflux, 85%.
The fluorescence of fluorophores is usually disturbed by
proton in the detection of metal ions, so their low sensitivity to
pH is desired. The effect of pH on the fluorescence of 1 was
first determined in ethanol-water (60 : 40, v/v) solution. The
emission spectrum of 1 had no obvious change in the pH range
12.0–2.0, viz. the change in pH value does not disturb the
detection of Pd2+ (ESI, Fig. S1w). On the other hand, when
pH was larger than 9.0, the fluorescence of the Pd2+–1
complex disappeared and the complex decomposed, as Pd2+
is easy to bind with OHÀ anion and form Pd(OH)2
State Key Laboratory of Bioreactor Engineering and Shanghai Key
Laboratory of Chemical Biology, School of Pharmacy, East China
University of Science and Technology, Shanghai, 200237, China.
E-mail: xhqian@ecust.edu.cn; Fax: +86-21-64252603
w Electronic supplementary information (ESI) available: Synthetic
details, and spectroscopic data. See DOI: 10.1039/b815298e
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Chem. Commun., 2008, 6339–6341 | 6339

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precipitation in high pH solution, which would reduce its
complexation with 1. Therefore, all of the detections of metal
cations were operated in the ethanol–water (60 : 40, v/v) solu-
tion at pH 7.2 maintained with HEPES buffer.
benzylic or naphthalene ring with Pd2+, and the binding mode
was suggested in Scheme 2.
1 has two absorption peaks centered at 340 nm and 458 nm in
ethanol–water (60 : 40, v/v) solution at pH 7.2. The addition of
5 equiv of Pd2+ to the solution of 1 within 1 min caused an
enhancement in the absorbance at 340 nm with expanding the
absorbance at 458 nm, meanwhile, new peak was found in
absorbance at 270 nm which may indicate the formation of a
stable complex (ESI, Fig. S2w). The sensor also showed a
selective chromogenic behavior toward Pd2+ ion, that is, a color
change from light-yellow to black-red, which was observed by
naked eye. Fig. 1 displays the changes emission spectra of 1 with
Pd2+ concentrations. It could be seen that with the increase of
Pd2+ concentration, the fluorescence intensity of 1 at 565 nm
decreased dramatically. The fluorescence quantum yield of 1
decreased from 0.24 to 0.03 (Determined by comparison with
rhodamine B in ethanol (F = 0.49)).9 Fig. 1(b) was the plot of
fluorescence intensity vs. Pd2+ concentration, an approximate
plateau was observed at 1 : 1 molar ratio. It suggested that 1
formed a 1 : 1 stoichiometrical complex with Pd2+. The same
result was obtained in DMSO. (ESI, Fig. S3w).
Scheme 2 Proposed Pd2+ sensing process.
High selectivity is important for an excellent sensor. The
addition of other metal ions such as Cu+, Pt2+, Cu2+, Co2+
,
Zn2+, Hg2+, Ag+, and Pb2+ produced minor changes in both
absorption and fluorescent spectra of 1. The above results
implied that its selectivity for Pd2+ was remarkable (Fig. 2).
At the same time, to further confirm the role of coordination
sites during the sensing of 1 to Pd2+, we prepared a pair of
model compounds (4 and 5, Scheme 1). It was found that the
fluorescence was quenched slightly with the addition of heavy
The quenching of fluorescence was apparently caused by the
binding of Pd2+ with 1. An obvious evidence for the forma-
tion of the complex came from the ESI-MS data. In the
presence of Pd2+, two peaks at 685.3 and 1093.2 m/z were
observed, which attributed to the fragments [L1PdCH2Cl2]+
and [2L1+Pd]+, respectively (ESI, Fig. S4w).
and transition metal ions, e.g. Hg2+, Ag+, Pt2+ and Pd2+
,
owing to heavy atom effect, and no selectivity was observed
under the same condition (ESI, Fig. S9w). The high selectivity
and sensitivity of 1 towards Pd2+ implies that it is essential to
combine thiophenemethylamine and phenylethyne groups
within 1. Both the ligating moieties and steric interaction play
1H-NMR experiments were carried out to confirm the co-
ordination sites of 1 during the formation of the complex. When
1.0 equivalent of PdCl2 (in DMSO) was added to the solution of
1, the peak of the thiophenemethylamine C–H and N–H showed
significant differences, which suggested two active coordination
sites (N, S) were involved in the complexation with Pd2+. The
peak of proton in the benzylic-OCH3 unit was shifted to down-
field from 3.80 to 3.90, which attributed to the deshielding effect
arising from the decrease of the electron density in the
fluorophore caused by alkynes–Pd2+ complexation (ESI,
Fig. S5w). In addition, the chemical shifts of the aromatic
protons in the naphthalene moiety as well as CH2 in benzyl
peaks did not show any significant changes in the 1H-NMR
spectrum, therefore, we were able to exclude the interaction of
important roles in the combination of Pd2+
.
Fig. 1 Fluorescence spectra of compound 1. (Conditions: (1 Â
10À5 M) in ethanol–water (60 : 40, v/v) solution at pH 7.2 maintained
with HEPES buffer (50 mM) after the addition of 0, 0.1, 0.15, 0.2,
0.3, 0.4, 0.45, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4 and 5 equivalents of
Pd2+ ion, Excited slit width 5 nm, Emission slit width 5 nm, excitation
at 365 nm.)
Fig. 2 Top: Fluorescence spectra of 1. (Conditions: 1 (10 mM) in
ethanol–water (60 : 40, v/v) solution at pH 7.2 maintained with
HEPES buffer (50 mM) in the presence of different metal ions
(5 Â 10À5 M) at 25 1C.) Bottom: The fluorescence responses of 1
corresponding to the graph above.
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In summary, an aqueous soluble fluorescent Pd2+ sensor 1
was designed and synthesized, and it displayed high selective
and sensitive fluorometric and colorimetric responses towards
Pd2+ in aqueous solution within 1 min, which might be the
better Pd2+ sensor based on complexation. It was selective,
simple and easy for application.
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This work was supported by the National Basic Research
Program of China (2003CB114400) and Program of Shanghai
Subject Chief Scientist and the National Natural Science
Foundation of China (20536010). We also thank Dr J. Qian
for valuable discussions.
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