Distinctions of positional isomers of N-(methylthiazol-2-yl)nitrobenzamide by copper and iron ions

Article abstract of DOI:10.1016/j.ica.2015.07.007

Abstract The fluorescence emissions of six positional isomers of N-(methylthiazol-2-yl)-nitrobenzamide are selectively reduced upon interactions with Cu²⁺, Fe²⁺ or Fe³⁺ ions among various metal ions that were studied. One

Full text of DOI:10.1016/j.ica.2015.07.007

Accepted Manuscript
Distinctions of positional isomers of N-(methylthiazol-2-yl)nitrobenzamide by
copper and iron ions
Nithi Phukan, Abir Goswami, Jubaraj B. Baruah
PII:
S0020-1693(15)00341-2
http://dx.doi.org/10.1016/j.ica.2015.07.007
ICA 16600
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
19 June 2015
12 July 2015
13 July 2015
Please cite this article as: N. Phukan, A. Goswami, J.B. Baruah, Distinctions of positional isomers of N-
(methylthiazol-2-yl)nitrobenzamide by copper and iron ions, Inorganica Chimica Acta(2015), doi: http://dx.doi.org/
10.1016/j.ica.2015.07.007
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Distinctions of positional isomers of N-(methylthiazol-2-yl)nitrobenzamide by copper and
iron ions
Nithi Phukan, Abir Goswami, Jubaraj B. Baruah*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039,
Assam, India. Fax: +91-361-2690762; Ph. +91-361-2582311; email: juba@iitg.ernet.in;
url: http://www.iitg.ernet.in/juba.
Abstract: The fluorescence emissions of six positional isomers of N-(methylthiazol-2-yl)-
nitrobenzamide are selectively reduced upon interactions with Cu2+, Fe2+ or Fe3+ ions among
various metal ions that were studied. One isomer, namely N-(4-methylthiazol-2-yl)-3-
nitrobenzamide shows reduction in the fluorescence emission intensity only upon interactions
with Fe2+ or Fe3+ ions, enabling detection of the iron ions. Specific fluorescence emissions
changes observed were used to distinguish N-(4-methylthiazol-2-yl)-3-nitrobenzamide from
the other structural isomers of this compound. The positional isomers of the N-(4-
methylthiazol-2-yl)-3-nitrobenzamide are difficult to distinguish at low concentrations by
using other conventional techniques. On the basis of the evidences obtained from the lifetime
measurements and the ESR studies, selective discrimination of Cu2+ ions by this particular
isomer is explained on the basis of the metal complex formation by the other positional
isomers with Cu2+ ions other than this isomer, such metal complexation influence the internal
charge transfer processes.
Key words: Thiazoles; Positional isomers; Fluorescence; Copper and Iron ions.
1

Introduction:
Sensing of metal ions by various polydenate ligands [1-2] and the analytical chemistry of
metal ions with small molecules as analytes have relevance in present-day research [3-5].
Such research works stem from selective interactions of organic substrates with one or more
metal ions which are generally influenced by position as well as the nature of the functional
group/s on the organic moiety. Hence, there is an avenue to screen positional isomers of
organic molecules of similar backbones. Nuclear magnetic resonance [6] or mass
spectroscopy [7-8] generally distinguishes positional isomers; but such methods
require high-end equipment. From a supramolecular chemistry point of view, proton-
transfer between constituents of a host-guest complex [9-11] can be used to visually
identify positional isomers. Due to high sensitivity with fast and thorough output of
fluorescence emission of fluorescent substrates upon interaction with metal ions,
fluorescence turning on and off is a conventional signal transduction technique to
differentiate analytes [12-16]. The intensity and position of fluorescence emission are
guided by various mechanisms, hence it also provides avenue to design receptors to operate
through a definite path [15-16]. The role of an organogel matrix in the capabilities of
colorimetric sensing of positional isomers of hydroxy-aromatics was demonstrated by
Shinkai and his coworkers [17]. On the other hand, capabilities of different anionic templates
of positional isomers of polyaromatic sulphonates to recognize nucleobases were revealed by
Ward and his group [18]. Suslick and his coworkers elucidated utilities colorimetric and
fluorometric sensor arrays to discriminate similar analytes with high sensitivity [19]. In spite
Figure 1 here
of emerging literature and methodologies in the analytical chemistry of metal ions there is no
concerted literature [20] in which selective interactions of multiple metal ions toward organic
positional isomers have been studied. Such an approach may be considered as bio-inspired, as
many enzymes selectively bind certain metal ions, in which the binding modes are also very
specific [21-23]. We set our study to distinguish a set of isomers that have donor and acceptor
units tethered through an amide linkage which would show fluorescence emission via an
internal charge transfer mechanism as illustrated in figure 1. Additionally the excited state
would be highly sensitive to interacting metal ions where a subtle difference would cause a
2

change in emission features. In such cases interference of a metal ion with the excited or
ground state population would help to modulate the intensity of such fluorescence processes.
Figure 2 here
We have chosen six new positional isomers of N-(methylthiazol-2-yl)nitrobenzamide shown
in Figure 2 for such a study. The choice of these isomers is based on the fact that thiazole
unit is an important constituent of medicinal compounds such as vitamin B1 [24],
cystothiazole A [25] and sulfathiazole [26]. Thiazole derivatives are constituents of
polymeric materials [27], liquid crystals [28], photonucleases [29], fluorescent dyes
[30-31], insecticides [32] and biologically active molecules [33-34].
Experimental:
General:
1H-NMR spectral data were recorded on Varian-AS400 spectrometers. UV-vis spectral data
were recorded using a Perkin-Elmer Lambda 750 UV-vis spectrophotometer. Fluorescence
measurements were performed on Fluoromax-4 spectrofluorometer using 10 mm path length
quartz cuvettes with a slit width of 10 nm at room temperature. Time-resolved fluorescence
decays were measured on a picosecond time-resolved cum steady state luminescence
spectrometer, Eddinburg Instrument model: FSP920. Infrared spectra were obtained by using
a Perkin-Elmer FT-IR spectrophotometer (4000-400 cm-1). ESI-mass spectra were recorded
on a micro mass Q-TOF (Waters) mass spectrometer using an acetonitrile / formic acid
matrix. ESR spectra were run on a JEOL, Model: JES-FA200 ESR spectrometer. Energy
level calculations were carried out in Gaussian03W using density functional theory with a
hybrid B3LYP functional and the 6-31++g (d,p) basis set.
Synthesis and characterisation:
The compounds L1 and
corresponding isomer of 2-amino-methyl thiazole. Similarly for the L3 and L4 2-
nitrobenzoyl chloride and for the
5 and 6 4-nitrobenzoyl chloride were used in place
L
2 were prepared by reacting 3-nitrobenzoyl chloride with the
L
L
of 3-nitrobenzoyl chloride. In a typical experiment 3-nitrobenzoyl chloride (0.185 g, 1
mmol) was dissolved in dichloromethane (20 mL) under nitrogen atmosphere.
3

Solution was cooled to 0 °C by placing on an ice-bath and 2-amino-5-methylthiazole
(0.114 g, 1 mmol) was added to the reaction mixture. It was stirred overnight. A white
precipitate of N-(5-methylthiazol-2-yl)-3-nitrobenzamide (
L
1) was obtained, which
1
was filtered, washed with water and dried in vacuum. Yield, 0.240g (92%). H-NMR
(400MHz, DMSO-d6): 8.65 (s, 1H), 8.48 (d, 9.2 Hz, 1H), 8.36 (d, 7.6 Hz, 1H), 7.91(t,
8.0Hz, 1H), 7.20 (s, 1H), 6.60 (s, 1H), 2.09 (s, 3H). Mass (ESI): Cacld m/z for
C11H9N3O3S, 263.0365; found (m+1), 264.0445. IR (KBr, cm-1): 3458 (w), 3092 (m),
1669 (m), 1614 (m), 1533 (s), 1436 (m), 1343 (s), 1152 (m), 1070 (m), 917 (s), 714
(s), 514 (m). Crystallographic parameters: Crystal system, Monoclinic; Space group,
P21/n; a(Å), 3.9164(5); b(Å), 13.1518(12); c(Å), 22.183(3); α = β(°), 90.00; γ(°),
94.628(11); V(Å3), 1138.9(2); Z, 4; Density (g.cm-3), 1.535; Abs. Coeff. (mm-1),
0.288; Abs. Correction, multi-scan; F (000), 544; Total reflections, 2008; Reflections,
I > 2σ(I), 1164; Max. θ/°, 25.00; Ranges (h, k, l), -4 ≤ h ≤ 4, -9 ≤ k ≤ 15, -26 ≤ l ≤ 11;
Completeness to 2θ (%), 99.9; Data/ restrain/ parameter; 2008/0/164; Goof, 1.005; R
indices [I>2σ(I)], 0.0594; R indices (all data), 0.1144.
Yields and spectroscopic details of compounds L2 6 are listed below:
-L
N-(4-methylthiazol-2-yl)-3-nitrobenzamide (L2): Pale yellow solid; Yield 0.246g
(94%); 1H-NMR (400MHz, DMSO-d6): 8.62 (s, 1H), 8.48 (d, 9.2 Hz, 1H), 8.36 (d, 7.6
Hz, 1H), 8.31 (s, 1H), 6.26 (s, 1H), 2.09 (s, 3H). ESI mass (m/z) 264.0459 (m+1). IR
(KBr, cm-1): 3445 (w), 3109 (s), 1712 (s), 1645 (m), 1522 (s), 1430 (s), 1370 (s), 1343
(s), 1224 (s), 1152 (m), 1072 (s), 948 (m), 823 (m), 788 (m), 720 (s), 583 (s).
N-(5-methylthiazol-2-yl)-2-nitrobenzamide (L3) Yield 0.247g (94%); 1H-NMR
(400MHz, DMSO-d6): 8.44 (d, 9.6 Hz, 1H), 8.07 (d, 7.2 Hz, 1H), 7.97 (t, 8.4 Hz, 1H),
7.86 (t, 6.8 Hz, 1H), 7.05 (s, 1H), 2.21 (s, 3H). ESI-mass (m/z) 264.0445 (m+1). IR
(KBr, cm-1): 3387 (w), 3268 (w), 3093 (w), 1630 (s), 1586 (s), 1361 (m), 1148 (w),
910 (m), 796 (s), 704 (s), 593 (w).
N-(4-methylthiazol-2-yl)-2-nitrobenzamide (L
4): Yield, 0.240g (90%); 1H-NMR
(400MHz, DMSO-d6): 8.44 (d, 9.6 Hz, 1H), 8.07 (d, 7.2 Hz, 1H), 7.97 (t, 8.4 Hz, 1H),
7.86 (t, 6.8 Hz, 1H), 7.05 (s, 1H), 2.21 (s, 3H). ESI-mass (m/z): 264.0445 (m+1). IR
(KBr, cm-1): 3443 (w), 3317 (m), 3095 (w), 1659 (s), 1620 (s), 1582 (s), 1431 (m),
1388 (m), 782 (s), 722 (s), 633 (m), 536 (s).
1
N-(5-methylthiazol-2-yl)-4-nitrobenzamide (L5): Yield, 0.223g (85%); H-NMR (400MHz,
DMSO-d6): 8.28 (d, 8.0 Hz, 2H), 8.23 (d, 8.4 Hz, 2H), 7.20 (s, 1H), 2.27 (s, 3H). ESI-mass
4

(m/z): 264.0445 (m+1). IR (KBr, cm-1): 3448 (w), 2924 (s), 2842 (m), 1687 (m), 1641 (m),
1518 (m), 1345 (s), 1261 (s), 1102 (m), 1015 (s), 803 (s), 717 (s).
1
N-(4-methylthiazol-2-yl)-4-nitrobenzamide (L6): Yield, 0.228g (87%); H-NMR (400MHz,
DMSO-d6): 8.18 (d, 9.2Hz, 2H), 8.13 (d, 9.6Hz, 2H), 6.20 (s, 1H), 2.47 (s, 3H). Mass (ESI)
found m/z: 264.0445 (M+1), IR (KBr, cm-1): 3448 (w), 1698 (w), 1541 (m), 1431 (m) 1348
(m), 1180 (m), 1036 (s), 921 (s), 860 (s), 800 (s), 717 (s).
Results and Discussion:
A series of new positional isomers shown in Figure 2 were synthesized by reacting
two isomers of 2-amino-methylthiazole with corresponding nitrobenzoyl chloride and
were characterised by common spectroscopic techniques.
Study on the fluorescence emission changes of isomers have shown that
-
independent solution of each of L1 L6 in dimethylformamide (DMF) or in aqueous
DMF (30%) shows absorption maxima at ~275 nm and all these compounds show
fluorescence emissions in the range of 400-415 nm on excitation at 280 nm. For
example,
L
1 in DMF shows fluorescence emission at 410 nm when excited at 280 nm,
2. Thus
whereas similar excitation causes broad fluorescence emission at 405 nm for
L
these isomers are difficult to distinguish from their individual emission spectra or from
absorption spectra. Fluorescence emission occurring at longer wavelengths on
excitation at shorter wavelength arises due to internal charge transfer (ICT) and this
phenomenon was observed in thioimidazole tethered to electron accepting units [35].
Figure 3 here
To confirm the feasibility of involvement of charge transfer processes contributing
to the fluorescence changes, we have carried out energy calculations of each isomer
using the density functional theory package in Gaussian03W. We observed that the
energy difference between L1 and L2 are 1.44 kcal/mole, the HOMO and LUMO of
these two compounds visualized with GaussView have shown that the HOMO in these
molecules are localized over the thiazole ring, whereas the LUMO is localized at the
nitrobenzene ring which is relatively less electron-rich than the thiazole ring (Figure
S11). Analysis of the other four isomers L3-L6 have established the in these cases also
HOMO and LUMO are localised in a similar to the L1 or
L
2. Thus, the emissions
5

occurring in these isomers are attributed to the charge transfer occurring upon
excitation from the HOMO to LUMO.
Table 1 here
Since these isomers could not be distinguished easily from their respective
fluorescence emissions, the effects of various chloride salts of metal ions such as Zn2+,
Cd2+, Co2+, Pb2+, Ni2+, Cu2+, Fe3+, Al3+, Cr3+, Hg2+, Ag+, Mg2+, Ba2+, Mn2+, Na+, K+,
Ca2+, and the sulphate salt of Fe2+ with compounds L1-L6 were studied by using
steady-state fluorescence spectroscopy. As an illustrative example, the gradual
decrease observed in the fluorescence emission intensity on addition of Cu2+ ions to a
solution of
L
1, without a shift in the emission maximum, Figure 3a. Bar graphs
showing the relative intensity upon the addition of equimolar amounts of different
metal ions to solutions of L1-L6 are shown in Figures 3b-3f. We also found that
fluorescence emission of L2 was not affected by metal ions that were tested with L1
with the exception of the Fe2+ or Fe3+ ions. Thus, the specific fluorescence change of
compound L2 towards Fe2+ or Fe3+ ions provided an easy way to differentiate two
positional isomers. Two isomers having identical nitrobenzamide motifs, but a methyl
group at different positions of the thiazole ring, for example, positional isomers, L1
and L2 showed similar UV-visible absorption maximum at 270 nm. Addition of
copper(II) chloride or iron(III) chloride to a solution of L L
1 or 2 showed an increase in
intensity of their respective absorption at 270 nm, hence these two isomers could not
be distinguished by UV-visible spectra but this experiment suggested that L1 and L2
bind to these metal ions.
The binding constants of each isomer with the interacting metal ion were
determined by using a Benesi-Hildebrand plot; the data are listed in Table 1. The relative
magnitudes of logarithm of binding constants fall between ~3.4-5.3 suggesting that the logK
values are comparable. Since the structures of positional isomers are based on three closely
related structures, one would expect similar binding constants of the isomers having similar
methylthiazole part, which was not observed. In present examples, a correlation of structures
of the isomers with the binding constants with a metal ion is not possible. On the other hand,
binding constants with iron do not help to distinguish the isomers in a definitive manner.
Thus, the non-observation of changes in intensity of the fluorescence of L2 by Cu2+ ions
becomes the distinguishing factor of L2 from the other positional isomers. The observations
6

on the Fe2+ and Fe3+ ions effectively quenching the fluorescence of all the ligands and Cu2+
ions quenching the fluorescence of all but the L2 ligand suggest that such quenching
processes are not due to the simple effect of the paramagnetic species alone but supports an
quenching process passing through ICT mechanism as elucidated in the Figure 1. Cu2+ ions
failed to quench the fluorescence of the L2, hence we could not calculate a binding constant
in this case. As effective quenching requires metal-ligand interactions, it may be suggested
that there is no effective complex formation between L2 and Cu2+ ions. Paramagnetic ions
such as Mn2+ ions failed to quench fluorescence emissions of these ligands, which suggested
that there must be definite metal-ligand interactions to show quenching of the fluorescence
emissions.
Table 2 here
Fluorescence emission of L1 at 410 nm has lifetime of 9.34 ns, which slightly
changes in the presence of Fe3+ or Cu2+ ions. In the presence of Fe3+ it is 8.57 ns,
whereas in the presence Cu2+ it is biexponential and the major fraction has a lifetime
8.07 ns. On the other hand, L2 shows biexponential lifetime, and the addition of
copper ions lowers the lifetime. It was not possible to determine lifetime from
equimolar concentrations of Fe3+ and ligand
L
2, but it was determined at a thousand-
fold lower ratio of Fe3+ with respect to the ligand. In this case we observe insignificant
change in lifetime from the pure ligand. Thus, the quenching effect of Cu2+ ions in this
case is significantly less than that of the Fe3+ ions. Despite the fact that the copper ions
quench the fluorescence, it does not have a slight effect on the life-time of the process.
The observed changes in fluorescence, which are due to the interaction of the ligand
with a metal ion, not only affect the emission lifetime, but also the probability of ICT
transitions.
The crystal structure of L1 shows (Figure 4) a non-planar structure, in which the
sulphur atom of the thiazole ring and the oxygen atom of amide project toward the
same side of the structure. Although there are examples in the literature of thiazole
copper complexes having distorted octahedral geometry [36], we could not get
crystalline copper or iron complexes from reaction with any of the ligands. From Job
plots determined from absorption changes in the UV region we find that L
1 and L2
7

Figure 4 here
forms a copper complex in 1: 4 metal:ligand ratio in solution (Figure S16). These ligands are
not planar, hence their respective copper complexes could be formed by the coordination of
two bidentate ligands coordinating through nitrogen atom of thiazole and the nitrogen atom
of amide to form four-member chelate, and two mono-dentate ligands (along the z-axis)
binding through their thiazole nitrogen atoms. We tried to get crystals of complexes from
reactions of the ligands with copper and iron salts in acetonitrile and alcohol which was not
successful.
We observed ESR signals from a solution of L1 in dimethylformamide (DMF) with
cupric chloride at g// = 2.117 and g┴ = 2.066 (Figure S17). Whereas, a solution of equimolar
L2 and cupric chloride in DMF has a broad ESR signal at g = 2.170, which is identical to a
solution of cupric chloride in DMF. It is observed that g// > g┴ in the former case, which
suggests that unpaired electron resides in the dx2-y2 -orbital of copper ion whereas in the later
unpaired electron occupies dz2-orbital [37]. It may be mentioned that each HOMO (Figure
S10) of ligands L1-L6 has symmetry appropriate to interact with dx2-y2 orbital. ESR
experiments suggest that among the L1 and L2 only the L1 forms complex with Cu2+ions.
This is consistent with the lack of quenching observed in the Cu2+-L2 mixture. Dihedral
angles of amide bonds may influence internal charge transfer [38-39], but in our case we find
that such differences are not important in this set of compounds because the optimized
geometry of the isomers do not show significant differences.
Conclusions:
Based on a design principle stemming from an internal charge transfer mechanism,
it is shown that interactions of Fe2+ or Fe3+ ions result in drastic decrease in the
fluorescence emission intensity of L1
many other common metal ions. The fluorescence emission changes caused on L2 are
very specific to Fe2+ or Fe3+ ions. Among the L1 L6 isomers L1 L3 L6 also show a
-
L6 which does not happen in the presence of
-
,
-
decrease in fluorescence intensity upon addition of Cu2+ ions, but the fluorescence
emission of L2 is unaffected by Cu2+ ions. Such an exceptional recognition of one
positional isomer among six isomers by ions of two of the most abundant transition
metal ions present in humans and mammals provides a rationale to screen many other
isomeric systems by fluorescence technique which may be difficult to distinguish by
other spectroscopic tools.
8

Acknowledgments:
Author N.P. thanks Council of Scientific and Industrial Research, New Delhi, India,
for a Senior Research Fellowship.
Supporting information: Spectroscopic details and fluorescence measurement plots are
available. The CCDC deposition number for the CIF of L
1 is 1058371.
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10

Figures and Tables:
Flow of electron
S1
emission
Acceptor
nitrobenzene
Donor
thiazole
S2
Environment sensitive
amide bond
Figure 1: A design principle to screen positional isomers.
O
HN
O
HN
Me
N
N
S
S
O2N
Me
O2N
N-(4-methylthiazol-2-yl)-3-
nitrobenzamide (L2)
N-(5-methylthiazol-2-yl)-3-
nitrobenzamide (L1)
O
O
Me
N
N
HN
S
HN
S
NO2
Me
NO2
N-(5-methylthiazol-2-yl)-2-
nitrobenzamide (L5)
N-(4-methylthiazol-2-yl)-2-
nitrobenzamide (L6)
O
O
Me
N
N
O2N
O2N
HN
S
HN
S
Me
N-(4-methylthiazol-2-yl)-4-
nitrobenzamide (L6)
N-(5-methylthiazol-2-yl)-4-
nitrobenzamide (L5)
Figure 2: Positional isomers of N-(methylthiazol-2-yl)nitrobenzamide
11

(a)
(b)
(c)
(c)
(d)
(e)
(f)
Figure 3: (a) Changes in fluorescence of L1 (3 mL, 1 × 10-4 M in DMF, λex = 280 nm) on
addition of Cu2+ ions (1 × 10-2 M). Bar diagrams showing relative fluorescence emission
intensity of (b) L1 at λem = 410 nm and (c) L2 at λem = 405 nm and (c-f) are that of L3, L4, L5
and L6 respectively in presence of various metal ions. I400 = normalised emission intensity
with respect to the lowest emission intensity.
12

Table 1: Binding constants with iron and copper ions#
Compound
Metal ion
Binding
constant (M-1)
6.756 × 103
2.024 ×103
7.871 ×103
2.967 ×104
2.087 ×104
9.532 ×103
9.801 ×103
1.3252 ×104
1.064 × 104
1.291 × 104
7.299 × 103
3.4 01× 103
7.518 × 103
3.436 × 103
5.882 × 103
7.042 × 103
2.724 × 103
logK
L1
L1
L1
L2
L2
L3
L3
L3
L4
L4
L4
L5
L5
L5
L6
L6
L6
Fe3+
Fe2+
Cu2+
Fe3+
Fe2+
Fe3+
Fe2+
Cu2+
Fe3+
Fe2+
Cu2+
Fe3+
Fe2+
Cu2+
Fe3+
Fe2+
Cu2+
3.829
5.306
3.895
4.472
4.319
3.979
3.991
4.122
4.026
4.111
3.863
3.532
3.876
3.536
3.769
3.847
3.435
# in all cases chloride salts are used for Cu2+ and Fe3+, whereas
sulphate salt of Fe2+ was used. (to avoid oxidation to Fe3+ the solution
was purged nitrogen gas).
Table 2: Fluorescence lifetime of L L
1 and 2 in the presence of metal ions.
Compound
+ metal ion
L1
Life time in ns (fraction)
9.338 (100 %)
8.857 (100 %)
8.070 (93.3 %)
0.700 (6.7 %)
7.935 (82.14 %)
1.194 (17.86%)
7.935 (82.51 %)
1.044 (17.49 %)
7.305 (83.45 %)
0.945 (16.55 %)
L1 + Fe3+
L1 + Cu2+
L2
L2 + Fe3+*
L2 + Cu2+
# Equimolar concentration of ligand and metal ions.
*at 103-times lower concentration of Fe3+ with respect to L2
.
Figure 4: Structure of L1 (thermal ellipsoids at 50% probability).
13

Supplementary Information
Distinctions of positional isomers of N-(methylthiazol-2-yl)nitrobenzamide by copper and
iron ions
Nithi Phukan, Abir Goswami, Jubaraj B. Baruah*
Figure S1: UV–visible spectra of L1 in dimethylformamide (3ml, 1 × 10-4 M, λmax 270 nm)
with Fe3+ ions solution added 10µl aliquots.
14

Figure S2: UV–visible spectra of L1 in dimethylformamide (3 ml, 1 × 10-4 M, λmax 270 nm)
with Cu2+ ions solution added 10µl aliquots.
Figure S3: Fluorescence spectra of L1 in the presence of different metal ions such as Ni2+,
Co2+, Zn2+, Cd2+, Hg2+, Mn2+, Al3+ and Pb2+.
15

Figure S4: A representative spectra of Fluorescence spectra of L2 in presence of Ni2+ ions to
suggest no changes in fluorescence (Similar spectra were observed for all metal ions, except
Fe2+ and Fe3+)
Figure S5: Time-resolved fluorescence decay of L1 (10-5 M in DMF) (ex = 300 nm).
Figure S6: Time-resolved fluorescence decay of L1-Fe3+ system (10-5 M in DMF/water,
3:1,v/v) (ex = 300 nm).
16

Figure S7: Time-resolved fluorescence decay of L1-Cu2+ system (10-5 M in DMF/water,
3:1,v/v) (ex = 300 nm).
Figure S8: Time-resolved fluorescence decay of L2 (10-5 M in DMF) (ex = 300 nm).
Figure S9: Time-resolved fluorescence decay of L2-Fe3+ system (in DMF/water, 3:1,v/v) (ex
= 300 nm).
17

Figure S10: Time-resolved fluorescence decay of L2-Cu2+ system (10-5 M in DMF/water,
3:1,v/v) (ex = 300 nm).
HOMO of the L1
LUMO of the L1
HOMO of the L2
LUMO of the L1
18

HOMO of the L3
LUMO of the L3
HOMO of the L4
LUMO of the L4
HOMO of the L5
LUMO of the L5
HOMO of the L6
LUMO of the L6
Figure S11: View of the HOMO and LUMO of L1-L6 from GaussView.
19

(a)
(b)
(c)
(d)
(e)
(f)
20

(g)
Figure S12: (a) Changes in fluorescence spectra of L3 in DMF/H2O (3:1, 3 mL) upon
addition of (a) Cu2+ ions (1 × 10-2 M) solution (b) Fe3+ ions (1×10-2 M) solution (c) ) Fe2+
ions (1×10-2 M) solution by 10 µl in each aliquot. Binding constant determination of L3
(Benesi-Hildebrand plot) by fluorescence method with (d) Cu2+ (e) Fe3+ (f) Fe2+. (g) The
purple bars represent the intensity plot of L3 at λ= 400 nm with various metal ions.
(a)
(b)
21

(c)
(d)
(e)
(f)
(g)
Figure S13: (a) Changes in fluorescence spectra of the L4 in DMF/H2O (3:1, 3mL) upon
addition of (a) Cu2+ ions (1 × 10-2 M) solution (b) Fe3+ ions (1 × 10-2 M) solution (c) ) Fe2+
ions (1 × 10-2 M) solution by 10 µl in each aliquot. Binding constant determination of L4
(Benesi-Hildebrand plot) by fluorescence method with (d) Cu2+, (e) Fe3+, (f) Fe2+. (g) The
red bars represent the intensity plot of the L4 at λ= 400 nm with various metal ions.
22

(a)
(c)
(e)
(b)
(d)
(f)
23

(g)
Figure S14: (a) Changes in fluorescence spectra of the L5 in DMF/H2O (3:1, 3mL) upon
addition of (a) Cu2+ ions (1 × 10-2 M) solution (b) Fe3+ ions (1×10-2 M) solution (c) ) Fe2+
ions (1x10-2 M) solution by 10 µl in each aliquot. Binding constant determination of L5
(Benesi-Hildebrand plot) by fluorescence method with (d) Cu2+, (e) Fe3+, (f) Fe2+. (g) The red
bars represent the intensity plot of the L5 at λ= 400 nm with various metal ions.
(a)
(b)
(c)
(d)
24

(e)
(f)
(g)
Figure S15: (a) Changes in fluorescence spectra of the L6 in DMF/H2O (3:1, 3mL) upon
addition of (a) Cu2+ ions (1×10-2 M) solution (b) Fe3+ ions (1×10-2 M) solution (c) ) Fe2+ ions
(1×10-2 M) solution by 10 µl in each aliquot. Binding constant determination of L6 (Benesi-
Hildebrand plot) by fluorescence method with (d) Cu2+, (e) Fe3+, (f) Fe2+. (g) The blue bars
represents the intensity plot of the L6 at λ= 400 nm with various metal ions.
25

Figure S16: Job plot for binding of L1 with Cu2+.
(a)
26

(b)
Figure S 17: (a) ESR spectra of a solution of equimolar solution of L1 cupric chloride in
dimethylformamide solution. (b) ESR spectra of a solution of equimolar solution of L2 cupric
chloride in dimethylformamide solution.
Table S1: Crystallographic parameters of the ligand L1
Compound No.
Formulae
Mol. wt.
Crystal system
Space group
UTemperature (K)
Wavelength (Å)
a (Å)
L1
C11 H9 N3 O3 S
263.27
Monoclinic
P 21/n
296(2)
0.71073
3.9164(5)
13.1518(12)
22.183(3)
90.00
b (Å)
c (Å)
α (°)
β (°)
94.628(11)
90.00
1138.9(2)
4
1.535
0.288
γ (°)
V (Å3)
Z
-3
Density/g.cm
Abs. Coeff. /mm-1
Abs. Correction
F(000)
multi-scan
544
Total reflections
Reflections, I > 2σ(I)
Max. θ/°
2008
1164
25.00
-4 ≤ h ≤ 4
-9 ≤ k ≤ 15
-26 ≤ l ≤ 11
Ranges (h, k, l)
Complete to 2θ (%)
Data/ restrain/
parameter
99.9
2008/ 0/ 164
Goof(F2)
1.005
R indices [I > 2σ(I)]
0.0594
0.1144
R indices (all data)
27

Distinctions of positional isomers of N-(methylthiazol-2-yl)nitrobenzamide by copper and
iron ions
Fluorescence emission intensities of positional isomers of N-(methylthiazol-2-yl)
nitrobenzamide were reduced by Cu2+, Fe2+ and Fe3+ ions among a series of commonly
abundant metal ions. One of the isomer showed an exceptional fluorescence decrease only in
the presence of Fe2+ or Fe3+ ions making it distinguishable from other isomers in the series.
N. Phukan, A. Goswami, J. B. Baruah
28

Highlight:
Positional isomers of N-(methylthiazol-2-yl)nitrobenzamide show selective fluorescence
responses to copper and iron ions.
One such positional isomer could be distinguished from the difference in emission changes
with copper ions.
Electronic factors play an important role in the differentiation of fluorescence emission.
29