Selective fluorescence sensing of Mg2+ions by Schiff base chemosensor: Effect of diamine structural rigidity and solvent

Article abstract of DOI:10.1039/c4ra05827e

Highly selective strong turn-on fluorescence for Mg²⁺(Φ = 0.03 to 0.57) was realized with a simple Salen based Schiff base chemosensor (1a) using dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) as solvent. Importantly, Ca²⁺that

Full text of DOI:10.1039/c4ra05827e

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PAPER
2
+
Selective fluorescence sensing of Mg ions by
Schiff base chemosensor: effect of diamine
structural rigidity and solvent†
Cite this: RSC Adv., 2014, 4, 41565
P. S. Hariharan and Savarimuthu Philip Anthony*
2+
Highly selective strong turn-on fluorescence for Mg (F ¼ 0.03 to 0.57) was realized with a simple Salen
based Schiff base chemosensor (1a) using dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) as
2+
2+
solvent. Importantly, Ca that often interferes in the Mg fluorescence sensing did not show any
2+
significant influence on the selectivity. The fluorescence sensing of Mg is highly solvent as well as
2+
amine structure dependent. Fluorescence sensing of Mg only by 1,2-phenylenediamine condensed
Schiff bases in DMF or DMSO was observed. Different substituents (1b–e) on the salicylaldehyde unit
2+
were synthesized and we explored the effect of substitution on Mg sensing and selectivity. Except 1c,
2+
other chemosensors showed selective fluorescence sensing of Mg in DMF/DMSO. Interestingly, 1a–e
3
+
(
except 1c) exhibited selective strong turn-on fluorescence for Fe (lmax ¼ 462 nm, F ¼ 0.421) in
Received 17th June 2014
different solvents (DMSO, DMF, THF, CH CN) after 1 h. The concentration dependent studies showed
3
Accepted 26th August 2014
2
+
ꢀ7
linear enhancement of fluorescence intensity for Mg with the detection limit of 10 M. The practical
2+
DOI: 10.1039/c4ra05827e
applications of the chemosensor for selective sensing of Mg in real samples such as pond, tap, river
www.rsc.org/advances
and ground water have also been demonstrated.
5
electrodes (ISEs), and NMR in the past few years. However,
Introduction
optical methods that track changes of uorescence or absorp-
2
+
2+
Magnesium ions (Mg ) are among the most abundant divalent tion arisen from the Mg induced perturbation of the chro-
2
+
cations in cells and play pivotal roles in many cellular processes, mophore, is best suitable for Mg detection in biological
such as enzyme-driven biochemical reactions, proliferation of systems.
1
cells, and stabilization of DNA conformation. The estimated
Particularly, molecular chemosensors have drawn signi-
2
+
total concentration of Mg in mammalian cells varies between cant attention in recent years due to its convenience, real-time
4 and 20 mM, the majority of it bound to ATP and a smaller response, high sensitivity, selectivity, versatility and relatively
amount to proteins, phospholipids, and various phosphome- simple handling. Coumarin based diketone or b-keto acid
tabolites. Mg is also involved in many pathological processes, derivatives are the highly explored uorescent sensors for
such as congestive heart failure, lung cancer, and muscle selective detection of Mg ions as well as molecular imaging of
dysfunction. In contrast, high levels of Mg are contribute to a local changes in intracellular Mg concentration. Dong et al.
number of age-related and neuronal diseases ranging from showed Na triggered coumarin based Salen uorescence
hypertension to Alzheimer's disease. The mechanisms by sensor for Mg . Crown ether derivatives, polymer based
which Mg concentration is regulated at the cellular level and ligands and nanoparticles, have also been successfully
the effects in human health has also been poorly understood employed for selective detection of Mg ions. However, most of
due to the scarcity of efficient chemical tools for the study of these chemosensor molecules require intricate synthetic
this ion. As a result, the sensing of Mg has generated methodologies and hence are relatively difficult to prepare. The
increasing interest in the areas of chemical and biological major issues with many of the reported chemosensors for Mg
sciences. Many analytical methods have been developed for the is their low selectivity against Zn and Ca . The similar
1
6
2
2+
2
+
2+
2
+
7
3
+
2
+
8
9
4
2
+
10
11
2
+
2
+
2
+
2
+
2+
2
+
2+
2+
detection of Mg , including atomic absorption, ion-selective chemical properties of Mg and Ca make it difficult for che-
12
mosensor to distinguish them. Hence it is still interesting and
of importance to design a highly selective and sensitive uo-
2
+
School of Chemical & Biotechnology, SASTRA University, Thanjavur-613401, Tamil rescent sensor that can recognize Mg without the interference
Nadu, India. E-mail: philip@biotech.sastra.edu; Fax: +914362264120; Tel:
from other metal ions.
+
914362264101
The ease of synthesis coupled with tailorability, good bio-
logical activities, strong photophysical properties and coordi-
nation ability with metal ions has made Schiff bases as one of
†
Electronic supplementary information (ESI) available: Colorimetric absorption
spectra for metal ions and turn-on uorescence sensing by 1a–d and 2a–d. See
DOI: 10.1039/c4ra05827e
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the most widely explored molecular chemosensors for selective (aromatic)), 7.40–7.48 (m, 6H (aromatic)), 6.95–7.00 (m, 4H
13
13
sensing of metal ions. Particularly, Salen based p-conjugated (aromatic)). C NMR (200 MHz, d6-DMSO, d ppm) 166.61,
tetradentate [O^N^N^O] chelating ligands have been widely 160.54, 146.41, 133.50, 132.44, 130.24, 126.50, 122.70, 117.64,
employed for selective sensing of various metal ions such as 116.96. C H N O (316.12): calcd C 75.93, H 5.10, N 8.86;
2
0
16 2 2
2
+
2+
3+
3+
2+
Zn , Cu , Al , La , and Pt . The interaction of organic found C 75.57, H 5.31, N 8.67. m/z (LC-MS) 317.00 (M + H).
0
ligands with metal ions and resultant solid state structural
1b (N,N -bis (4-methoxy salicylidine)-O-phenylenediamine).
ꢂ
1
organization are highly inuenced by solvent, temperature, Yield ¼ 85%. m.p. 174–176 C. H NMR (400 MHz, d
6
-DMSO, d
14
anions and pH. A subtle change of organic ligand structure or ppm) 13.525 (s, 2H (OH)), 8.83 (s, 2H (CH]N)), 7.53–7.55 (d, 2H
solvent resulted in completely different molecular organization (aromatic)), 7.41–7.44 (m, 2H (aromatic)), 7.34–7.37 (m, 2H
14b,c
and properties.
Herein, we report the highly selective uo- (aromatic)), 6.53–6.56 (d, 2H (aromatic)), 6.48 (s, 2H (aromatic)),
rescence sensing of Mg ions by simple 1,2-phenylenediamine 3.80 (s, 6H (OCH3)). C NMR (200 MHz, d -DMSO, d ppm)
2
+
13
6
based Salen Schiff base chemosensor (1a–e) in DMF or DMSO. 168.12, 166.76, 162.53, 144.50, 134.74, 133.94, 128.50, 118.80,
2
+
The chemosensor did not show uorescence turn-on for Mg
20 2 4
116.31, 55.84. C22H N O (376.14): calcd C 70.20, H 5.36, N
other than DMF and DMSO solvent. Chemosensors with 7.44; found C 70.11, H 5.23, N 7.52. m/z (LC-MS) 377.00 (M + H).
0
different substituent was prepared and studied their role on
1c (N,N -bis (4-diethylamino salicylidine)-O-phenylenedi-
2
+
2+
ꢂ
1
Mg sensing and selectivity. Importantly, Ca that oen amine). Yield ¼ 80%. m.p. 141–143 C. H NMR (400 MHz, d
6
-
2
+
interferes in the Mg uorescence sensing did not show any DMSO, d ppm) 13.58 (s, 2H (OH)), 8.61 (s, 2H (CH]N)), 7.51–
signicant inuence on the selectivity. The linear enhancement 7.52 (d, 2H (aromatic)), 7.42–7.45 (m, 2H (aromatic)), 7.30–7.33
2
+
of uorescence with concentration of Mg was observed. The (m, 2H (aromatic)), 6.51–6.54 (d, 2H (aromatic)), 6.34 (s, 2H
ꢀ
7
13
chemosensors showed the detection limit of 10
M. The (aromatic)), 3.31–3.37 (q, 8H), 1.05–1.10 (t, 12H). C NMR (200
-DMSO, d ppm) 163.45, 162.19, 155.12, 144.45, 134.22,
Mg in real samples such as pond, tap, and ground water have 133.00, 128.92, 122.78, 117.78, 116.78, 43.73, 12.53. C28
practical application of the chemosensor in selective sensing of MHz, d
6
2
+
34 4 2
H N O
also been demonstrated.
(458.27): calcd C 73.33, H 7.47, N 12.22; found C 73.42, H 7.30, N
1
2.41. m/z (LC-MS) 459.20 (M + H).
0
1
d (N,N -bis (4-hydroxy salicylidine)-O-phenylenediamine).
Experimental section
ꢂ
1
Yield ¼ 80%. m.p. 266–269 C (decomp.). H NMR (400 MHz, d -
6
DMSO, d ppm) 13.40 (s, 2H (OH)), 10.30 (s, 2H (OH)), 8.75 (s, 2H
1
,2-phenylenediamine, ethylene diamine, (ꢁ)-trans-1,2-dia-
(
(
(
CH]N)), 7.42–7.44 (d, 2H (aromatic)), 7.37–7.39 (m, 2H
minocyclohexane (99%), salicylaldehyde, 2-hydroxy-4-methoxy
benzaldehyde, 2-hydroxy-4-diethylamino benzaldehyde, 2,4-
dihydroxy benzaldehyde and 2-hydroxynaphthaldehyde was
obtained from Sigma-Aldrich. The spectroscopic grade solvents
were obtained from Merck India. All chemicals are used as
received. The metal ion solutions used for the uorescence
sensor experiments were prepared using Mill-Q water. The
chemosensors were dissolved in organic solvents (DMF, DMSO,
acetonitrile, methanol, tetrahydrofuran (THF)). Absorption and
aromatic)), 7.30–7.33 (m, 2H (aromatic)), 6.37–6.40 (d, 2H
1
3
6
aromatic)), 6.28 (s, 2H (aromatic)). C NMR (200 MHz, d -
DMSO, d ppm) 164.88, 163.42, 161.87, 145.85, 133.33, 128.26,
1
4
(
22.32, 116.23, 115.90. C20
16 2 4
H N O (348.11): calcd C 68.96, H
.63, N 8.04; found C 68.80, H 4.45, N 8.20. m/z (LC-MS) 349.00
M + H).
0
1e
(N,N -bis (2-hydroxy naphthalidine)-O-phenylenedi-
ꢂ
1
amine). Yield ¼ 80%. m.p. 284–287 C (decomp.). H NMR (400
MHz, d -DMSO, d ppm) 15.13 (s, 2H (OH)), 9.70 (s, 2H (CH]N)),
.53–8.55 (d, 2H (aromatic)), 7.96–7.98 (d, 2H (aromatic)), 7.82–
6

uorescence spectra were recorded using Perking Elmer
8
7
2
Lambda 1050 and Jasco uorescence spectrometer-FP-8200
instruments.
.84 (d, 4H (aromatic)), 7.53–7.58 (t, 2H (aromatic)), 7.44–7.46 (t,
H (aromatic)), 7.36–7.40 (t, 2H (aromatic)), 7.08 (s, 2H
1
3
(
1
aromatic)). C NMR (200 MHz, d
62.51, 147.90, 135.22, 134.40, 133.74, 133.02, 128.89, 127.78,
(416.15):
6
-DMSO, d ppm) 166.74,
General synthesis of 1a–e
Schiff bases were prepared according to the reported proce- 124.62, 122.86, 122.12, 118.88, 114.90. C28H N O
20 2 2
15
dure. Typically, aldehyde (2.2 mmol, salicylaldehyde or 4- calcd C 80.75, H 4.84, N 6.73; found C 80.55, H 5.11, N 6.80. m/z
methoxysalicylaldehyde, 4-diethylamino-2-hydroxy benzalde- (LC-MS) 417.00 (M + H).
hyde or 2,4-dihydroxy benzaldehyde) was dissolved in ethanol
(30 mL) and stirred at room temperature. To this solution, 1,2-
phenylenediamine (1 mmol) in ethanol (5 mL) was added in
drop-wise under stirring. The immediate appearance of yellow
Result and discussion
colour indicates the formation of Schiff bases. The solution was Schiff base chemosensors, 1a–e, were synthesized by quite
allowed stir for another 6 h at room temperature that produced straightforward condensation reaction between primary amine
yellow to light yellow coloured precipitates. The formed and an aldehyde precursor in ethanol solution at room
precipitate was ltered and washed with ethanol and dried temperature (Scheme 1,S1†). The uorescence responses of
2
+
under vacuum.
chemosensor 1a–e for various metal ions such as such as, Mg ,
0
2+
3+
3+
2+
2+
2+
2+
2+
2+
2+
2+
1
a (N, N -bis-(salicylidine)-O-phenylenediamine). Yield ¼ Ca , Cr , Al , Fe , Cu , Co , Ni , Zn , Cd , Hg and Pb
ꢂ
1
87%. m.p. 160–162 C. H NMR (400 MHz, d
6
-DMSO, d ppm) have been investigated in DMF, DMSO, acetonitrile, methanol
12.95 (s, 2H (OH)), 8.94 (s, 2H (CH]N)), 7.66–7.68 (d, 2H and THF. The chemosensors and metal ions were dissolved in
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Scheme 1 Molecular structures of Schiff base chemosensors.
organic solvents (DMF, DMSO, acetonitrile and ethanol) and
water, respectively.
Fig. 2 Absorption spectra of 1a in DMF.
1
a showed weak uorescence in DMF (l
¼ 499 nm, F ¼
max
2
+
0
.0092). Addition of Mg ions into 1a lead to the strong
enhancement of uorescence without altering the uorescence
max (F ¼ 0.1787, Fig. 1). Importantly, other metal ions with 1a
did not show any signicant uorescence enhancements. It is
noted that Ca addition showed only a small enhancement in
the uorescence intensity. Absorption studies of 1a with Mg in
l
2
+
2
+
DMF revealed distinct changes that suggest the formation of
2
+
coordination complex between Mg and 1a (Fig. 2). 1a in DMF
showed a strong absorption at 270 nm and 330 nm. The former
is assigned to p–p* transition involving the molecular orbitals
of imine. As Scheme 2 describes, 1a exist in equilibrium
between the two congurations (normal and tautomer) due to
16
excited state intramolecular proton transfer (ESIPT). The
absorption at 330 nm could be assigned to the transition of
O]C–C]C–(NH).
2
+
2+
2+
2+
3+
2+
2+
2+
Addition of Mg , Cu , Zn , Cd , Cr , Mn , Co and Ni
reduced the intensity at 330 nm since the metal coordination ESIPT.
Scheme 2 Mg-complex structure with 1a and tautomer formation by
prevents the ESIPT and tautomer formation (Scheme 2). The
metal coordination with imine nitrogen that increase the elec-
tron withdrawing property of imine leads to the strong
17
2+
absorption at longer wavelength (380 nm to 440 nm). Mg
with 1a in DMF showed a strong red shied absorption at 396
nm. 1a exhibited similar strong selective turn-on uorescence
2
+
with higher intensity for Mg in DMSO (Table 1).
2
+
The strong enhancement of uorescence upon Mg coor-
dination is due to the restriction of C]N isomerisation and
18
rigidication of uorophore structure. It is noted that 1a
showed strong turn-on uorescence as well as absorption
2
+
changes for Mg either in DMF or DMSO. Other solvents such
as acetonitrile, methanol and THF did not show any turn-on
2
+

uorescence for Mg . Absorption studies also showed that
2
+
Mg did not show any changes in other solvents with 1a
Fig. S1†). This suggests that both DMF and DMSO facilitate the
formation of stable Mg coordination complex with 1a by
(
2
+
2
+
involving the coordination. The possible structure of the Mg
complex with 1a is shown in Scheme 2. The concentration
2
+
dependent studies of 1a with Mg are shown in Fig. 3a. A
steady uorescence enhancement was observed with 1a in DMF
Fig. 1 Digital and fluorescence spectra of 1a in DMF with different
metal ions.
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2
+
3+
2+
2+
Table 1 Quantum yield (F) of 1a–e and with Mg and Fe in different was observed for Mg in presence of Zn . Transition metal
solvents. Quinine sulfate in 0.5 M H SO was used as reference
3+
3+
2+
2+
2+
2+
2
4
ions (Cr , Fe , Mn , Co , Ni and Cu ) showed strong
2
+
inuence on the Mg selectivity that could be due to the
stronger coordination with 1a (Fig. 3b).
Compound
F
(DMF)
F
DMSO
F
CH3CN
F
THF
1
1
1
1
1
1
1
1
1
1
1
1
a
b
d
e
a-Mg
b-Mg
0.0092
0.0083
0.015
0.003 a
0.021
0.019
0.034
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
In order to study the effect of substitutional change on the
2
+
selectivity and sensitivity of Mg , chemosensor 1b–e was
synthesized (Scheme 1). Except 1c, other three chemosensors
0.007 a
2+
2
+
exhibited highly selective strong turn-on uorescence for Mg
0.1787
0.3603
2
+
+
+
a
a
in DMF and DMSO (Fig. 4, S2†). Surprisingly 1c did not show
turn-on uorescence for any metal ions in explored solvents. 1b
0.1811a
0.4194a
2
d-Mg2
0.1861
0.5767
a
a
e-Mg
0.025
0.088b
showed a weak broad uorescence with two peaks (480 nm and
3+
b
b
b
a-Fe
b-Fe
0.1318b
0.421
0.353b
0.431b
2+
5
04 nm). Addition of Mg showed 10 fold uorescence
3+
b
0.0942
0.406b
0.092
0.316
3+
b
b
b
enhancements with a small red shi lmax (480 nm to 484 nm,
Fig. 4a). 1d chemosensor exhibited weak and two clear uo-
d-Fe3
+
0.021b
0.193
0.073
b
0.073b
0.174b
e-Fe
0.041
0.035
0.122
2
+
rescence peaks at 520 nm and 547 nm. Addition of Mg
a
b
Measured immediately. Measured aer 1 h.
increases the uorescence by 4 fold without affecting the uo-
rescence lmax (Fig. 4b). 1e chemosensor showed 30 fold uo-
2
+
rescence enhancements selectively for Mg ions in DMF
(

Fig. S2†). The quantum yield measurements revealed strong
2
+
uorescence for chemosensors with Mg in DMSO compared
2
+
to DMF (Table 1). Among the chemosensors, 1e with Mg
exhibited weak uorescence both in DMF and DMSO and 1d
2+
Fig. 3 (a) 1a (10–6 M in DMF) fluorescence vs. concentration of Mg
10–6 M in water) and (b) interference studies of other metal ions (mM)
(
2+
on the selectivity of Mg (mM).
ꢀ
7
2+
ꢀ7
(
10 M) up to the addition of 1 equivalent of Mg (10 M).
These results suggest the formation of 1 : 1 coordination
2
+
complex between 1a and Mg . It is noted that most of the
2
+
ꢀ6
2+
reported Mg chemosensor showed only micromolar (10 M)
8ꢀ11
detection limit.
The selectivity studies of 1a for Mg in
2
+
3+
2+
presence of other metal ions revealed that Ca , Al , Cd and
Fig. 4 (a) 1b and (b) 1d fluorescence spectra with different metal ions
Hg had negligible interference. But no turn-on uorescence in DMF.
2
+
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2
+
with Mg
exhibited strong uorescence in DMSO. The
concentration dependent studies of 1b, 1d, 1e also showed
steady enhancement of uorescence up to the addition of 1
2
+
equivalent of Mg (Fig. S3†). However, the detection limit of
2
+
Mg was differed for 1e. 1a, 1b and 1d showed selective
2
+
ꢀ7
detection of Mg up to the level of 10 M. But 1e showed
ꢀ4
selective turn-on uorescence only for the addition of 10
Mg . The selectivity studies of 1b, 1d and 1e for Mg showed
M
2
+
2+
2
+
2+
that Ca , a strong competitor with Mg , had little interference
on the selectivity with all chemosensor in DMF (Fig. 5, S4†). The
transition metal ions had strong interference (no uorescence
turn-on) on the selectivity of Mg that is due to the strong
coordination character of these metal ions.
2
+
2
+
The selective uorescence sensing of Mg by 1a–e is highly
solvent dependent. Similarly, Mg uorescence sensing also
2
+
depends on the structure of amine moiety in the chemosensor
(Scheme S2†). For example, use of ethylene diamine or cyclo-
hexane diamine instead of ortho-phenylenediamine did not
2
+
show any uorescence sensing for Mg in any solvents
including DMF or DMSO. Instead, the ethylene or cyclohexane
diamine based Schiff base chemosensor showed highly selective
2
+
Fig. 6 Selective sensing of Mg by 1a in different water. 1a was dis-
2
+
ꢀ6
2
+
solved in DMF (10–6 M) and Mg (10 M) was dissolved in different
water. 100 mL of Mg was added into 1a-DMF.
turn-on uorescence for Zn ions (Fig. S5–8†). These results
2+
indicate that along with solvent the structural rigidity amine
2
+
also plays a role in the formation of Mg coordination complex.
2
+
The practical applicability of 1a for selective sensing of Mg ion
in different samples such as river, ground, pond and tap water
have also been demonstrated. Mg was dissolved in different
2
+
ꢀ6
ꢀ6
water (10 M) and addition of 100 ml into 1a in DMF (10 m)
clearly showed the strong bluish green uorescence (Fig. 6).
3
+
Interestingly, 1a with Fe also exhibited selectively strong
blue uorescence (lmax ¼ 462 nm) in DMF/DMSO but aer 1 h
(
Fig. 7). However, it takes 1 h to produce strong blue uores-
3
+
3+
cence with Fe . 1a-Fe uorescence is completely different
2
+
from 1a-Mg
uorescence. The absorption studies also
revealed a clear change in the absorption aer 1 h. A new peak
appeared at 317 nm along with 330 nm peak. It is noted that the
absorption peak of 1a has also been changed from broad to
sharp (Fig. S9†). It is noted that absorption spectra of 1a and 1a
2
+
ꢀ6
3+
Fig. 5 Mg interference studies of 1b and (b) 1d in presence of other Fig. 7 The change of 1a (10 M in DMF) fluorescence with Fe
ꢀ6
metal cations in DMF.
(10 M in water) with time.
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2
+
with Mg did not show signicant variation aer one hour.
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K. Kikuchi, H. Kojima, Y. Urano, H. Komatsu, K. Suzuki
and T. Nagano, Analyst, 2003, 128, 719; H. Komatsu,
N. Iwasawa, D. Citterio, Y. Suzuki, T. Kubota, K. Tokuno,
Y. Kitamura, K. Oka and K. Suzuki, J. Am. Chem. Soc., 2004,
126, 16353; H. M. Kim, P. R. Yang, M. S. Seo, J.-S. Yi,
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Org. Chem., 2007, 72, 2088.
2
+
Unlike Mg that showed turn-on uorescence only in DMSO
3
+
and DMF, Fe addition into 1a as well as 1b, 1d,e exhibited
strong blue uorescence in DMF, DMSO, acetonitrile and THF
aer 1 h (Fig. S10,† Table 1).
Conclusion
We have demonstrated a highly selective strong turn-on uo-
2
+
rescence for Mg (F ¼ 0.03 to 0.57) with simple Salen based
Schiff base chemosensors using DMF or DMSO as solvent. The
solvent and rigidity of the amine structure was found to be
2
+
critical for uorescence sensing of Mg . The concentration
dependent studies showed linear enhancement of uorescence
2
+
ꢀ7
for Mg with the detection limit of 10 M. Importantly, the
2
+
chemosensors displayed good Mg selectivity in presence of
Ca that oen interferes in the Mg uorescence sensing.
Salen chemosensors with different substitution in the salycy-
laldehyde unit (1b, 1d–e) also exhibited similar Mg uores-
cence sensing except small variation in the sensitivity. 1b and
d showed strong turn-on uorescence for Mg with similar
sensitivity whereas 1e exhibited reduced sensitivity (10 M).
Thus simple Salen chemosensor have been effectively used to
detect biologically important Mg ions by changing the solvent
medium. The practical application of the chemosensor in
selective sensing of Mg in real samples such as pond, tap, and
ground water have also been demonstrated.
2
+
2+
2
+
2
+
1
ꢀ
5
2
+
2
+
Acknowledgements
8
Y. Dong, J. Li, X. Jiang, F. Song, Y. Cheng and C. Zhu, Org.
Lett., 2011, 13, 2252.
Financial supports from DST, New Delhi, India DST Fast Track
Scheme no. SR/FT/CS-03/2011 (G), SR/FST/ETI-284/2011(c),
SASTRA University (TRR Scheme) and instrumentation facility
under CRF facility, SASTRA University are acknowledged with
gratitude.
9 G. Farruggia, S. Iotti, L. Prodi, M. Montalti, N. Zaccheroni,
P. B. Savage, V. Trapani, P. Sale and F. I. Wolf, J. Am. Chem.
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2
873; X. Xiang, D. Wang, Y. Guo, W. Liu and W. Qin,
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