A highly sensitive and selective ratiometric fluorescent probe based on conjugated donor-acceptor-donor constitution of 1,8-naphthyridine for Hg 2+

Article abstract of DOI:10.1016/j.jphotochem.2011.04.031

A new 1,8-naphthyridine based di-olefinic chemosensor was designed, synthesized and its sensing behavior towards metal ions was investigated by UV-vis, fluorescence and ¹H NMR spectroscopic methods. A highly Hg²⁺-selective fluorescen

Full text of DOI:10.1016/j.jphotochem.2011.04.031

Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 47–51
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A highly sensitive and selective ratiometric fluorescent probe based on
conjugated donor–acceptor–donor constitution of 1,8-naphthyridine for Hg2+
Ajit K. Mahapatraa,∗, Giridhari Hazraa, Nirmal K. Dasa, Prithidipa Sahooa,1
,
Shyamaprosad Goswamia, Hoong-Kun Funb
a Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103, India
b X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
A new 1,8-naphthyridine based di-olefinic chemosensor was designed, synthesized and its sensing behav-
ior towards metal ions was investigated by UV–vis, fluorescence and 1H NMR spectroscopic methods. A
highly Hg2+-selective fluorescence enhancing property (>1.5-fold) in conjunction with a visible colorimet-
ric change from yellow to purple has been observed, leading to a potential fabrication of both “naked-eye”
and fluorescence detection of Hg2+. Our designed sensor of the donor–acceptor–donor system shows high
selectivity towards mercury(II) ion over other competing metal ions.
Received 21 January 2011
Received in revised form 6 April 2011
Accepted 22 April 2011
Available online 30 April 2011
Keywords:
Naphthyridine
© 2011 Elsevier B.V. All rights reserved.
Donor–acceptor–donor
Bathochromic shift
Hypsochromic shift
Hg-sensing
1. Introduction
sors for many heavy transition metal [10–12] (HTM) ions is still a
challenge [13–17]. Hg2+ belongs to the so-called “silent ions” since,
The design and synthesis of fluorescent probes with desirable
properties are of considerable importance in supramolecular chem-
istry due to their fundamental role in medical, environmental,
and biological applications [1–3]. The highly toxic metal ion Hg2+
causes serious health and environmental problems [4]. The soluble
inorganic Hg2+ ion, which provides a pathway for contaminat-
ing large amounts of water, is a caustic and carcinogenic material
with a high cellular toxicity [5]. Methyl mercury is usually gen-
erated through microbial methylation in aquatic sediments from
Hg2+ ions, and the subsequent accumulation of methyl mercury by
humans through the food chain may lead to serious and perma-
nent damage to the brain. Therefore, it is very important for the
routine detection of Hg2+ ion in the environmental monitoring of
rivers and larger bodies of water, and in the evaluation of the safety
of aquatically derived food supplies. Many kinds of chemical and
unlike some biological metal ions (such as Fe2+, Mn2+ or Cu2+), it
does not have an intrinsic spectroscopic or magnetic signal because
of its 5d10 6s0 electronic configuration. The fluorometric detec-
tion of such ions is based in the use of small-molecule fluorescent
sensors that consist of an ionophore–chromophore system. There
are some examples of fluorescence quenching chemosensors for
Hg2+ in organic or aqueous solutions [18–21]. However, there are
few chemosensors with fluorescence enhancement (FE) for Hg2+ in
organic solutions [22–27], and few in aqueous solutions. Most of the
fluoroionophores for Hg2+ consist of fluorophores and macrocycle
receptors (e.g., aza-crown ether) or the receptors containing sul-
fur atoms [28–33], and there are some fluorescent probes for Hg2+
designed on the basis of chemical reaction, redox, and the photody-
namic principle [34–37]. Therefore, novel selective chemosensors
with FE for Hg2+ in aqueous–organic system become our target.
This work is aimed at the design and construction of a new
class of colorimetric and fluorometric dual-channel assay to specif-
ically detect the presence of Hg2+ over a wide range of other
cations. The 1,8-naphthyridine (napy) moiety is one of the most
useful fluorophores for the construction of fluorogenic chemosen-
sors for a variety of important chemical species. Particularly, the
1,8-naphthyridine (napy) and its derivatives have been widely
used as a guanine recognition reagents and bidentate ligands.
However, there are very few reports about the derivatives of 1,8-
naphthyridine as fluorescent sensors for transition metals [38–41].
physical sensors have been developed for the detection of Hg2+
,
among which the fluorescence based sensors represent a simple,
but sensitive technique providing detection limits of as low as ppb
[6–9]. So far, the development of practical fluorescent chemosen-
∗
Corresponding author. Tel.: +91 33 2668 4561; fax: +91 33 2668 4564.
E-mail address: mahapatra574@gmail.com (A.K. Mahapatra).
Present address: Chemistry and Chemical Biology Department, Rutgers Univer-
1
sity, 610 Taylor Road, Piscataway, NJ 08854, USA.
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.04.031

48
A.K. Mahapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 47–51
In order to shift the emission bands to a blue or red region, a con-
jugated donor (D)–acceptor (A)–donor (D) molecule incorporating
a central moiety of naphthyridine and two terminal moieties of
dimethylaniline connected by ethenyl bridges (R1) was selected. In
this report, we describe the synthesis and designed the molecule
R1 of D–A–D assembly for UV–vis and fluorescence spectral studies
of Hg2+ ion.
The naphthyridine moiety acts as the acceptor site, whereas
the dimethylaniline moiety functions as the donor site (Scheme 1).
The acceptor and donor moieties are connected by ethenyl bridges
to form a conjugated scaffold. In general, several binding states
may exist in equilibrium. At the early stage, the binding of the
D–A–D molecule with a metal ion at the acceptor site, forming
D–AM–D (1:1), would enhance the acceptor strength to facilitate
ICT (Intramolecular Charge Transfer). When a metal ion binds at
one of the donor sites, the resulting D–A–DM (1:1) may also facil-
itate ICT due to the resulting A–DM unit function as an enhanced
electron-withdrawing group. When the D–A–D molecule is satu-
rated with metal ions at the late stage, all the acceptor and donor
sites are bound to metal ions to form a 1:3 complex (MD–AM–DM)
(Scheme 1). A hypsochromic spectral shift thus occurs to account
for the great decrease of dipole at this stage [42].
Fig. 1. Ortep diagram of receptor R1.
128.2, 124.8, 117.1, 115.9, 38.9, 36.5, 29.6, 27.3; IR: ꢀ(cm−1): 3440,
2915, 2848, 1616, 1578, 1521, 1502, 1368, 1301, 1197, 1145, 977,
814, 748. TOF MS ES(+); m/z calcd for C28H28N4(M)+: 420.1314.
Found (M+H)+: 421.1245.
The synthetic route is shown in Scheme 2.
2.3. X-ray crystallography
The structure of R1 is further confirmed by single crystal X-ray
diffraction and is shown in Fig. 1.
2. Experimental
3. Results and discussion
2.1. General
3.1. UV–vis titration
2-Methyl-6-pycolin and N,N-dimethyl aniline were purchased
and used without further purification.1H NMR spectra were
obtained using 300 MHz NMR spectrometer and 13C NMR spec-
tra were obtained using 75 MHz or 125 MHz NMR spectrometers.
Column chromatography was performed with silica gel (100–200
mesh). Fluorescence spectra were measured using spectroscopic
grade solvents. The fluorescence measurements were performed at
room temperature (temperature controlled at 18 2 ◦C) and within
1 h after the sample preparation.
The absorption spectra of sensor R1 in the presence of different
concentrations of Hg2+ ions are displayed in Fig. 2. When increasing
concentrations of Hg2+ ions are introduced in CH3CN/H2O (1:1), the
absorbance peak is centered at 441 nm and gradually decreased
with concomitant appearance of a red shifted peak around 579 nm.
This red shift is assigned at the early stage (D–AM–D species in
Scheme 1) to an efficient ICT process from the dual-armed electron-
donor dimethylaniline groups. A double bond is inserted between
the napy ring and the dimethylaniline moiety to elongates the ␲
conjugation, so that the absorption could further extend into the
visible region.
2.2. Synthetic procedures of R1
Thereby, upon binding to a metal ion at the acceptor naph-
thyridine site, the electron-donating capability of the dimethy-
laniline moiety should increase, which leads to stronger ICT. A
bathochromic shift in the absorption spectra upon addition of metal
ions is thus expected. The titrations were carried out by using var-
2,7-Bis
[2-(N,N-dimethyl-4-aminophenyl)-vinyl]-
[1,8]naphthyridine (R1): Synthesis of R1 was accomplished by
modification of an established procedure. N,N-dimethyl-4-
aminobenzaldehyde was synthesized according to reported
methods [43–45]. Equivalent moles of 2,7-dimethyl naphthyri-
dine (100 mg, 0.238 mmol) and p-dimethylaminobenzaldehyde
(71 mg, 0.4765 mmol) were dissolved in 5 ml of fresh and dry DMF.
The solution was stirred in a round bottom flask attached to a
drying tube. Sodium hydride (0.330 g, 12.5 equiv.) and potassium
t-butoxide (0.886 g, 6.25 equiv.) were added to the flask one after
another, making sure that the drying tube remains attached after
each addition. This reaction mixture was stirred for 24 h. About
15 g of ice was taken in a large beaker to which 10 ml of saturated
ammonium chloride (NH4Cl) was added. The stirred reaction mix-
ture was then poured into the beaker of ice, and 5 ml of saturated
ammonium chloride was added to it to dissolve any remaining
product in the flask. The mixture in the beaker was stirred for about
15 min, and the resulting solid was collected using a vacuum filter.
The solid product was washed with cold water while the vacuum
was on, and left in the vacuum to air dry the product overnight.
The yield of this product was 78%. The product was characterized
by 1H NMR, 13C NMR, X-ray crystallography, mass spectroscopy.
Mp 290–292 ◦C; 1H NMR (CDCl3, 300 MHz): ı7.90–8.02 (m, 2H),
7.59 (S, 2H), 7.52–7.56 (m, 2H), 7.2812–7.2838 (d, J = 0.78 Hz, 4H),
7.21 (S, 2H), 6.74–6.77 (d, J = 8.46 Hz, 4H), 3.04 (S, J = 7.5 Hz, 12H);
13C NMR (75 MHz, CDCl3): ı164.5, 155.8, 141.8, 140.9, 133.6, 128.9,
Fig. 2. Absorbance spectra of compound R1 in the presence of increasing Hg2+ con-
centrations (0, 2, 4, 6, 8, 10, 14, 20, 24, 30, 35, 40, 50, 60, 70, and 80 ␮M) in a
CH3CN–water solution (50:50, v/v). The concentration of the R1 was c = 1.0 × 10−5 M.
Inset: (a) only receptor R1, (b) R1 + 10 equiv. of Hg2+, and (c) R1 + more than 10 equiv.
of Hg2+
.

A.K. Mahapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 47–51
49
Scheme 1. Design of metal ion sensor with conjugated D–A–D assemblies. A: acceptor; D: donor. Metal ion (M): ᭹; dipolar vector:
.
ious amounts of Hg2+ ion as the aqueous solution of perchlorate
salt. Furthermore, a well-defined isosbestic point was observed at
486 nm, which suggests a new complex formation (R1 with Hg2+
produces a single component). The 138 nm red shift should result in
a significant change in the absorption ratios. Indeed, sensor R1 dis-
plays a remarkable enhancement in absorption ratios (A579/A441
)
from 0.021 to 6.18 upon Hg2+ treatment. More importantly, the
absorption changes are clearly visible to the naked eye. The pale
yellow solution of R1 immediately changed to purple in accor-
dance with this large red-shift (ꢁꢂ = 138 nm). The purple color of
the mixture solution faded when more than 10 equiv. of Hg2+ ions
were added (Fig. 2, inset). Accordingly, an extraordinarily large
hypsochromic shift (ꢁꢂ = 220 and 286 nm) from 579 nm to 359
and 311 nm respectively were observed. By addition of more than
10 equiv. of Hg2+ ions, all the acceptor and donor sites in molecule
R1 were likely saturated to reach a late stage of complexation
(DM–AM–DM species in Scheme 1), in which the saturated complex
would only possess the least ICT property. The prohibition of ␲ con-
jugation thus results in a great hypsochromic shift of the absorption
band.
Fig. 3. Emission spectra of R1 in the presence of increasing Hg2+ concentration (0,
2, 4, 6, 8, 10, 14, 20, 24, 30, 35, 40, 50, 60, 70, and 80 ␮M) in a CH3CN–H2O solu-
tion (50:50, v/v). Excitation wavelength was 441 nm with 5 nm slit widths. The
concentration of the chemosensor was c = 1.0 × 10−6 M.
(i)
N
N
N
NH2
(ii)
N
N
N
N
R1
Scheme 2. Synthesis of R1. Reagents and conditions: (i) crotonaldehyde, H3PO4, 120 ◦C, 12 h and (ii) N,N-dimethyl p-amino benzaldehyde, NaH, dry DMF, N2, rt, 24 h.

50
A.K. Mahapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 47–51
Fig. 4. (a) Fluorescence response of R1 (1 × 10−6 M) with various metal ions (100 equiv.) except Hg2+ in a CH3CN–water solution (50:50, v/v) at 298 K. ꢂex = 441 nm. (b) The
relative fluorescence intensities of R1 (1 × 10−6 M) at 514 nm with various metal ions (100 equiv.) except Hg2+ in CH3CN–water solution (50:50, v/v) at 298 K. ꢂex = 441 nm.
3.2. Fluorescence titration
The fluorescence properties of R1-Hg2+ are surveyed in typical
aqueous–organic solvent systems. R1 exhibits relative strong fluo-
rescence emission at 608 nm (˚f = 0.07) [46,47] in CH3CN/H2O (1:1)
upon excitation at 441 nm.
During titration of Hg2+, a new fluorescence emission peak
appears at about 514 nm and the intensity is dramatically enhanced
with initial emission intensity at 608 nm which is decreased indi-
cating an Hg2+-selective dual-emissive-type (on–off–on) signaling
behavior (Fig. 3). Upon gradual addition of Hg2+ ion, the fluores-
cence intensity of R1 increases by over 1.5-fold accompanied by a
large blue shift (ꢁꢂ = 94 nm) in the emission spectrum. The dra-
matic change in visual fluorescence color from orange to green of
Fig. 5. 1H NMR spectra of R1 (bottom) and in the presence of Hg2+ (top) in CDCl3.
R1 in the absence and presence of Hg2+ further supported the blue
shift emission response (inset in Fig. 3).
However, when other cations are added, the fluorescence of
is due to the decrease in electron density of the NMe2 moieties by
Hg2+ co-ordination at the donor sites. The complexation of R1-Hg2+
results in shifts of an aromatic and ethylenic protons to downfield,
which indicates that D–A–D molecule is saturated with metal ions,
all the acceptor and donor sites are bound to metal ions to form a
1:3 complex (DM–AM–DM).
R1 is significantly quenched (Fig. 4a and b), indicating an efficient
Hg2+-selective (on–off–on) behavior. How could we explain these
phenomenons? On the basis of the above fact, the hypsochromic
shift or blue shift in the emission spectra upon addition of Hg2+
ion can be rationalized by ICT in the excited state of the napy
moiety by the lone pair of electrons on the nitrogen atoms in the
electron-donor dimethylaniline groups [48,49]. We speculate that
the fluorescence change could contribute to two points. First, in the
early stage the system is expected to follow the PET (Photoinduced
Electron Transfer) mechanism where, upon excitation of the free
probe, an electron would be transferred from the dimethylaniline
groups to the fluorophore and hence diminishing fluorescence. Sec-
ond, coordination of the Hg2+ to all the acceptor and donor sites in
molecule R1 were likely saturated to reach a late stage of complex-
ation (DM–AM–DM species in Scheme 1) and hence suppressed the
ICT process and this process also contributes to the turn-on fluo-
rescence enhancement [50]. ICT, instead of PET, is responsible for
the large wavelength shift. Upon titration of Hg(II), more and more
molecules undergo complexation and hence suppressed the ICT,
leading the stronger fluorescence at the ICT emission wavelength
(514 nm).
4. Conclusion
In conclusion, the 1,8-naphthyridine based dual-armed D–A–D
molecule R1 exhibits a new class of colorimetric and fluorimetric
probe for Hg2+ ion detection with high selectivity and sensitivity
on the basis of the PET and ICT. A highly Hg2+-selective fluores-
cence enhancing property (>1.5-fold) in conjunction with a more
convenient “naked-eye” colorimetric detection of the Hg2+ion. The
two stage colorimetric property of R1 was unique in the detection of
Hg2+ ion in aqueous media, e.g., CH3CN/H2O (1:1). It represents one
of the few acyclic fluorescent probes that allow a selective detec-
tion of Hg2+ in aqueous–organic solvent instead of the commonly
used macrocycles for metal ion detection. The D–A–D constitution
demonstrated in this study can serve as a protocol for the future
design of metal ions and other possible analytes.
3.3. 1H NMR titration
To look further into the nature of the interaction between R1
and the Hg2+ ion, 1H NMR experiments are carried out in CDCl3
(Fig. 5). Thus, the aliphatic protons of the NMe2 units experience
more downfield shifts of 0.25 ppm upon addition of Hg2+, whereas
an aromatic and ethylenic protons show relatively small downfield
shifts (0.056–0.15 ppm). The downfield shift of the NMe2 protons
Acknowledgements
We thank the DST [W.B. Government, project no. 694
(sanc.)/ST/P/S&T/6G-6/2007] for financial support. GH thanks the
CSIR, New Delhi for a fellowship.

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