T.S. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 520–526
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feature of chemosensors is that signal transduction of the analyte
binding offer the possibility to monitor its concentration in real-
time and real-space. Chemosensors are use in many disciplines;
giving attracting attention in the scientific community, especially
among chemists, biologists, physicists and material scientists.
Among the different chemosensors, fluorescence-based ones pres-
ent many advantages as fluorescence measurements are usually
very sensitive, low cost, easily performed and versatile, offering
subnanometer spatial resolution with submicron visualization
and submillisecond temporal resolution [1,2]. Furthermore, many
opportunities exist for modulating the photophysical properties
of a fluorophore, such as the introduction of proton-, energy- and
electron-transfer processes, the presence of heavy-atom effects,
changes of electronic density and the destabilization of a non-
emissive np* excited state [1]. This offers a wide number of possi-
bilities for tailoring efficient luminescent chemosensors. At the
same time, the advancement of photochemistry gave the basis
for designing systems in which changes in absorbance and/or fluo-
rescence bands could signal analyte complexation. Nowadays,
among the different analyte, special interest is devoted to develop
chemosensors for transition metal ions: usually they represent an
environmental concern when present in uncontrolled amounts,
but at the same time some of them such as iron, zinc, copper and
cobalt are present as essential elements in biological systems.
Among these metal ions, Zn2+ plays a myriad of roles in numerous
cellular functions such as regulation of gene expression, apoptosis,
co-factors in metalloenzyme catalysis and neurotransmission in
biological systems [3,4]. Many server neurological diseases, includ-
ing Alzheimer’s disease, cerebral ischemia and epilepsy [5–7] are
associated with the disorder of Zn2+ metabolism. Therefore, mea-
surement of Zn2+ is very important in neurobiology. However,
there is a great need for developing Zn2+ selective sensors that
can distinguish Zn2+ from other transition metal ions especially
Cd2+, because Zn2+ and Cd2+ have very similar chemical properties
often respond together with similar spectral changes.
Schiff bases (imines) are known to be good ligand for metal ions
[8,9]. A number of the Schiff base metal complexes have antitumor
properties [10], antioxidative activities [11], and attractive elec-
tronic and photophysical properties [12]. In addition, Schiff base
derivatives incorporating a fluorescent moiety are appealing tools
for optical sensing of metal ions. Tetradentate ligands such as
salen- or pyridine-type symmetrical Schiff bases are capable of
forming complexes with certain metal ions which can exhibit unu-
sual coordination, high thermodynamic stability, good fluorescent
properties and biological activities [13]. However, controllable syn-
thesis of these compounds is a great challenge because many fac-
tors may affect their self-assembly, in which the coordination
geometry of the complex depends upon the chemical structure of
ligand chosen, the coordination geometry preferred by metal, pH
dependent, metal-to-ligand ratio, reaction temperature, solvent
system [14]. Nowadays, designing and synthesis of fluorescent sen-
sors with high selectivity and sensitivity to metal ions is an impor-
tant and vibrant field. Having much availability of commercial
metal ions sensors, chemists still continue endeavoring to design
new ones to improve their sensitivity, selectivity and reliability
in order to satisfy various needs that are due to the wide existence
of metal ions in organisms and its extensive significance. Many
excellent metal ions sensors has been contributed significantly
based on quinoline, anthracene, BODIPY, benzoxazole and fluores-
cein as fluorophore [15,16], but some of the reported synthesis
methods are always too complicated. In this paper, we have de-
signed and synthesized a Schiff based fluorescent compound – N,
N/-bis(salicylidene) – 1, 2 – phenylenediamine (LH2) by one step
condensation of salicylaldehyde and 1, 2 – phenylenediamine in
absolute ethanol (Scheme 1) and characterized by FT-IR, 1H
and 13C NMR and elemental analysis. Photophysical behaviors of
LH2 were studied in different homogeneous solvents and in pres-
ence of different metal ions, focussing the attention on their
absorption and emission properties using steady state absorption
and fluorescence spectroscopy. Till now, a variety of fluorescent
sensors for Zn2+ have been developed with successful applications
[17–24]. However, many of these sensors suffer from interference
of some heavy metal ions and transition metal ions. In addition,
some available Zn2+ sensors have difficulty in distinguishing Zn2+
and Cd2+, because they have very similar chemical properties often
respond together with similar spectral changes including change in
fluorescence intensity and the wavelength shift. Herein, we have
observed its prominent fluorescence enhancement in the presence
of Zn2+, while there was no enhancement in presence of other me-
tal ions. In particular, it was able to distinguish Zn2+ from Cd2+
.
Materials and experimental method
Materials
Salicylaldehyde and 1, 2 – phenylenediamine were obtained
from Aldrich Chemical Company. All the spectroscopic grade sol-
vents used were obtained from Sisco Research Laboratory (SRL)
Pvt. Ltd. and in some cases, from Aldrich Chemical Company.
Chemical reagents were obtained from Lancaster as well as S.D.
Fine Chemical Ltd. and used without further purification. All exper-
imental solutions of varying pH were made with buffer (Qualigen).
The analytical grade type – II water used in the measurements was
obtained from Elix10 water purification system (Millipore India
Pvt. Ltd.). All experiments were carried out at room temperature
(293 K) unless mentioned otherwise.
Synthesis and characterization of N, N/-bis(salicylidene) - 1,2 –
phenylenediamine (LH2)
A portion of salicylaldehyde (1.22 g, 10 mmol) and 1, 2 – phenyl-
enediamine (0.54 g, 5 mmol) were separately dissolved in absolute
ethanol and combined together to get yellow color solution. The
solution was stirred under reflux conditions for 3 h in presence of
2–3 drops of acetic acid, and precipitate was filtrated, washed with
cold absolute ethanol three times, then recrystallized with ethanol/
chloroform (1/3, v/v) to get yellow microcrystal (LH2) in 72% yield.
m.p. 85 °C; 1H NMR (400 MHz, CDCl3 TMS, d, ppm): 13.4 (br s, H-5,
H-5/), 8.4 (s, H-6, H-6/), 7.1–7.3 (m, H-1, H-1/, H-3, H-3/), 6.8–7.2 (m,
H-2, H-2/, H-4, H-4/), 1.5–2.0 (m, H-8, H-8/, H-9, H-9/), 3.3–3.6 (d H-
7, H-7/); 13C NMR(400 MHz,CDCl3, TMS, d, ppm): 117.12, 120.83,
122.98, 127.92, 131.52, 144.10, 162.06; IR (
m
max, cmꢁ1, KBr):
s(CAH),), 1628( C@N),
m(CAN)), 762(d(CAH)); Anal.
3340(
m
m
OH),
2935(mas(CAH),
2864(
m
m
1580(
C@C), 1280(
m
(CAO)), 1150(
Calcd. for C20H22N2O2 (322.20): C 74.55%; H 6.83%; N 8.69%. Found:
C 74.60%; H 6.9%; N 8.61%.
Physical measurements
The IR spectra were measured on a PerkinElmer L 120-000A
spectrometer with KBr pellets in the range 4000–400 cmꢁ1 1H
.
and 13C Nuclear magnetic resonance spectra were recorded on
Bruker DPX- 400 MHz spectrometer and chemical shifts are
expressed in ppm using tetramethylsilane as internal standard.
Elemental analyses were carried out using PE2400 elemental
analyzer.
Steady-state absorption spectra were recorded on a Shimadzu
UV – 1601PC absorption spectrophotometer. Fluorescence spectra
were obtained in a PerkinElmer LS 45 spectrofluorimeter and all
the spectra were corrected for the instrument response function.
Quartz cuvettes of 10 mm optical path length received from