Study on changes in optical properties of phenylbenzothiazole derivatives on metal ion binding

Article abstract of DOI:10.1016/j.saa.2010.04.038

The UV-visible and fluorescence properties of hydroxy substituted benzothiazole derivatives change on interactions with zinc(II), cadmium(II) and mercury(II) ions. The binding of 2-(2′-hydroxyphenyl)benzothiazole is specific to zinc(II) over cadmium(II) and mercury(II). Similar type of interactions of zinc(II), cadmium(II) and mercury(II) with 2-(2′,3′, 4′-trihydroxyphenyl)benzothiazole is observed in their UV-visible absorptions, whereas 2-(4′-hydroxyphenyl)benzothiazole shows interactions with mercury(II) over the other two ions.

Full text of DOI:10.1016/j.saa.2010.04.038

Spectrochimica Acta Part A 77 (2010) 126–129
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Study on changes in optical properties of phenylbenzothiazole derivatives on
metal ion binding
Rupam Sarma, Bhaskar Nath, Anvita Ghritlahre, Jubaraj B. Baruah∗
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 March 2010
Accepted 28 April 2010
The UV–visible and fluorescence properties of hydroxy substituted benzothiazole derivatives
change on interactions with zinc(II), cadmium(II) and mercury(II) ions. The binding of 2-(2ꢀ-
hydroxyphenyl)benzothiazole is specific to zinc(II) over cadmium(II) and mercury(II). Similar type of
interactions of zinc(II), cadmium(II) and mercury(II) with 2-(2ꢀ,3ꢀ,4ꢀ-trihydroxyphenyl)benzothiazole is
observed in their UV–visible absorptions, whereas 2-(4ꢀ-hydroxyphenyl)benzothiazole shows interac-
tions with mercury(II) over the other two ions.
Keywords:
Hydroxybenzothiazoles
Fluorescence emission
UV–visible absorption
Metal ion binding
© 2010 Elsevier B.V. All rights reserved.
Switching of coordination
1. Introduction
2. Materials and methods
Benzimidazole occupies a major status as fluorescence probes
[1–7]. The optical properties of analogous derivatives of benzoth-
iazole are relatively less explored, which leaves scope for further
study [8–13]. Among them, the hydroxybenzothiazoles are of spe-
cial interest as presence of OH-group along with a ring containing
nitrogen and sulphur atoms at two side of the heterocyclic ring can
control coordination modes in a tuneable manner. Moreover, it
would be possible to switch fluorescence or UV–visible properties
of a hydroxyphenylbenzothiazole derivative by the affinity of a
metal ion to either form chelate through lone pair of electrons
on S and O atoms or N and O atoms as illustrated in Scheme 1.
From the hard acid–soft base principle it is obvious that soft metal
ions will preferentially bind to S-coordinating site, while hard ions
will prefer the N-coordinating site. The emission and absorption
properties of dimeric N-coordinating complex of zinc(II) derived
from 2-(2ꢀ-hydroxyphenyl)benzothiazole are reported, however,
the selectivity in binding to other metal ions is not studied [8]. With
an intention of looking at the selectivity in metal binding with
tuneable property of hydroxyphenylbenzothiazole derivatives
(Fig. 1) we have prepared 2-(2ꢀ-hydroxyphenyl)benzothiazole
(1), 2-(4ꢀ-hydroxyphenyl)benzothiazole (2), and 2-(2ꢀ,3ꢀ,4ꢀ-
trihydroxyphenyl)benzothiazole (3) and studied their interactions
with zinc(II), cadmium(II) and mercury(II) ions using UV–visible
absorptions and fluorescence emission properties.
2.1. General
All reagents and solvents were purchased commercially and
were used without further purification, unless otherwise stated.
1H NMR data were recorded with a Varian 400 MHz FTNMR spec-
trometer. The FT-IR spectra were recorded using a Perkin Elmer
spectrometer in the KBr pellets in the range 4000–400 cm−1
.
The UV/visible spectra were recorded using Perkin Elmer Lambda
750 spectrometer. The fluorescence spectra were recorded with a
Perkin Elmer LS 55 fluorescence spectrophotometer. Mass spectra
were recorded in a Water LC–MS in negative mode.
2.2. Synthesis of the compounds 1, 2 and 3
2.2.1. 2-(2ꢀ-hydroxyphenyl)benzothiazole (1)
To a well stirred solution of 2-aminothiophenol (534 ␮l, 5 mmol)
in methanol (20 ml) salicylaldehyde (532 ␮l, 5 mmol) was added.
The reaction mixture was then allowed to stir for 12 h. The reac-
tion progress was monitored at regular intervals using TLC. After
completion of the reaction the solvent was removed under reduced
pressure that gave a yellow solid. The product was purified by re-
crystallization from dichloromethane. Yield: 92%. IR (KBr, cm−1):
3443 (b), 3058 (w), 1622 (m), 1589 (m), 1487 (s), 1458 (m), 1315
(m), 1271 (m), 1219 (s), 1033 (m), 756 (m), 742 (s). 1H NMR (CDCl3,
400 MHz, ppm): 12.5 (1H, b), 8.0 (1H, d, J = 8.0 Hz), 7.9 (1H, d,
J = 8.0 Hz), 7.7 (1H, d, J = 8.0 Hz), 7.5 (1H, m), 7.4 (2H, dd, J = 8.4 Hz,
8.8 Hz), 7.1 (1H, d, J = 8.4 Hz), 6.9 (1H, t, J = 7.6 Hz). LC–MS [M−1]:
226.94.
∗
Corresponding author. Tel.: +91 361 2582311; fax: +91 361 2890762.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.04.038

R. Sarma et al. / Spectrochimica Acta Part A 77 (2010) 126–129
127
Scheme 1. Two different types of chelates.
2.2.2. 2-(4ꢀ-hydroxyphenyl)benzothiazole (2)
plex formation is further confirmed by a Job’s plot and it is found
to be a 1:2 complex (Fig. 2b). The UV–visible spectra of 1 in the
presence of cadmium(II) and mercury(II) acetates having similar d-
electron configuration does not show significant change. We have
also checked the UV–visible spectra of 1 in the presence of toxic ele-
ments cadmium and mercury in their +2 oxidation state and found
that zinc(II) detection by 1 is unaffected by these ions. Moreover,
the compound 1 does not show any changes in visible absorbances
with metal ions like Mn, Fe, Co, Ni, Cu at their +2 oxidation state.
Thus, zinc(II) can be selectively screened in the presence of these
metal ions by 1.
To a well stirred solution of 2-aminothiophenol (534 ␮l, 5 mmol)
in methanol (20 ml) 4-hydroxybenzaldehyde (0.61 g, 5 mmol) was
added. The reaction mixture was then allowed to stir for 24 h. The
reaction progress was monitored at regular intervals using TLC.
After completion of the reaction the solvent was removed under
reduced pressure. The crude product was purified by chromatogra-
phy (silica gel; hexane/ethyl acetate 3:1). Yield: 73%. IR (KBr, cm−1):
3434 (b), 2922 (w), 1605 (m), 1523 (m), 1482 (m), 1454 (m), 1431
(s), 1284 (s), 1250 (m), 1225 (s), 1167 (m), 977 (m), 826 (m). 1H
NMR (DMSO-d6, 400 MHz, ppm): 9.7 (1H, b), 7.9 (4H, m), 7.4 (1H,
m), 7.3 (1H, m), 6.9 (2H, d, J = 8.0 Hz). LC–MS [M−2]: 225.58.
3.2. Selective detection of zinc(II) by 1 with fluorescence
spectroscopy
2.2.3. 2-(2ꢀ,3ꢀ,4ꢀ-trihydroxyphenyl)benzothiazole (3)
The compound 3 was synthesized in a similar procedure as 2.
Yield: 67% IR (KBr, cm−1): 3437 (b), 3012 (w), 1612 (m), 1589 (m),
1488 (s), 1455 (m), 1279 (m), 1229 (s), 759 (m), 732(m). 1H NMR
(DMSO-d6, 400 MHz, ppm): 11.5 (1H, s), 9.8 (1H, s), 8.6 (1H, s), 8.1
(1H, d, J = 7.2 Hz), 7.9 (1H, t, J = 8.0 Hz), 7.5 (1H, m), 7.4 (1H, m), 7.3
(1H, d, J = 7.2 Hz), 6.5 (1H, d, J = 8.8 Hz). LC–MS [M−1]: 259.92.
Similar study is carried out by fluorimetric titration of com-
pound 1 with different metal ion solutions. Fluorescence emission
spectra of 1 in methanol (10−4 M) in the presence of zinc(II), cad-
mium(II) and mercury(II) show distinct changes. Upon excitation
at 270 nm compound 1 emits at two wavelengths, one at 375 nm
and other at 449 nm. On addition of a zinc(II) acetate solution in
methanol the fluorescence emission at 375 nm decreases, while
the emission intensity at 449 nm increases. Control addition of
a solution of zinc(II) acetate results in an isoemissive point at
410 nm (Fig. 3a). The fluorescence changes support the observa-
tion of UV–visible study where zinc complex formation is shown.
Titration of the compound 1 with relatively higher concentration
of cadmium(II) acetate solution (10−2 M) results in changes in the
fluorescence emission (Fig. 3b). Since change in this case is not
observed in the UV–visible spectra, it can be attributed to the
difference in sensitivity of the two tools. However, fluorescence
study shows significant differences with mercury(II) ions over the
observations made in the case of zinc(II) and cadmium(II). With
incremental addition of mercury(II) acetate (10−2 M in methanol)
to a solution of 1 the fluorescence emission intensity at 449 nm
decreases (Fig. 3c). Zinc(II) and cadmium(II) being comparatively
harder than mercury(II), they are expected to prefer binding to 1
through the N and O atoms to form chelate while mercury(II) will
prefer binding through the S and O atoms. From these studies it may
be inferred that in the case of zinc and cadmium 1 forms chelate
3. Results and discussion
3.1. Selective detection of zinc(II) by 1 with UV–visible
spectroscopy
Ultraviolet absorption as well as the fluorescence emission of
the compounds 1, 2 and 3 was monitored to study their interactions
with different commonly available metal ions and among them
we could see significant changes with three metal ions viz. Zn2+
,
Cd2+ and Hg2+. It is observed that the methanolic solution of com-
pound 1 (10−4 M) absorbs initially at 335 nm. Incremental addition
(20 ␮l) of a zinc(II)acetate solution (10−2 M) to this parent solution
results in a red shift. A new absorption peak appears at 380 nm
with an obvious isosbestic point at 350 nm. With the gradual addi-
tion of Zn2+ solution the peak at 335 nm undergoes a hypochromic
effect while the peak at 380 nm undergoes a hyperchromic effect
(Fig. 2). The presence of the isosbestic point clearly shows the pres-
ence of a zinc-[2-(2ꢀ-hydroxyphenyl)benzothiazolate] complex in
solution which is already reported in the literature [8]. The com-
Fig. 1. Structure of the hydroxybenzothiazoles.

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R. Sarma et al. / Spectrochimica Acta Part A 77 (2010) 126–129
Fig. 2. (a) UV–visible spectra of 1 in methanol solution (10−4 M) in the presence of different concentrations of zinc(II) acetate (10−2 M in methanol in 20 ␮l aliquot). (b) Job’s
plot of 1 against zinc(II) acetate to show the formation of an 1:2 complex.
through N and O atoms and there is an enhancement in fluorescence
emission at 449 nm and 454 nm respectively (Fig. 3). Whereas in
the case of mercury high thiophilicity of mercury leads to chelates
through S and O atoms as well as N and O atoms (Scheme 1); in
the presence of excess of ligand chelation through S and O atom
is favoured and consequently quenching in fluorescence emission
intensity at 449 nm is observed.
3.3. Selective detection of mercury(II) by 2
The UV–visible spectroscopic study is not a suitable tool to show
binding property of 2 with either of zinc(II), cadmium(II) or mer-
cury(II) as no significant changes in the visible absorption of 2 is
observed upon interaction with these ions. The changes in the flu-
orescence emission of 2 with incremental addition of zinc(II) and
cadmium(II) are also not systematic (please refer to Supplementary
materials). This could be due to flexible ambidentate interaction of
2 with the metal ions due to the absence of chelating effect. How-
ever mercury having higher affinity for sulphur shows decrease in
emission of a methanolic solution of 2 at 375 nm with increase in
concentration of mercury(II) acetate (Fig. 4).
3.4. Detection of zinc(II), cadmium(II) and mercury(II) by 3
Compound 3 shows interesting UV absorption on interaction
with metal ion (Zn2+, Cd2+ and Hg2+) solutions. The UV–visible
absorption of this compound is affected by interaction with zinc(II),
cadmium(II) and mercury(II) acetates. The UV–visible spectrum of
3 with increasing amount of mercury(II) is shown in Fig. 5c. It is
observed that the methanolic solution of compound 3 (10−4 M)
absorbs initially at 335 nm. The spectra show red shift near to
380 nm with incremental addition of mercury(II) acetate solution.
Moreover the band at 335 nm undergoes a hypochromic effect
with the addition of the metal ion. An isosbestic point at 355 nm
appears suggesting the formation of mercury(II) complex with
3. The complex formation in solution is further confirmed by a
Job’s plot and it is found to be a 1:4 complex (please refer to
Supplementary materials). Similar changes in absorbance of 3 on
addition of with zinc(II) and cadmium(II) along with the appearance
of isosbestic points are observed (Fig. 5a and b) and the complexes
in solution are found to be of 1:1 and 1:2 composition from their
Job’s plot (please refer to Supplementary materials). These results
Fig. 3. Fluorescence emission spectra (ꢀex 270 nm) of 1 in methanol (10−4 M, 2 ml)
solution in the presence of different concentrations of (a) zinc(II) (10−3 M), (b) cad-
mium(II) (10−2 M), and (c) mercury(II) (10−2 M) in 20 ␮l each aliquot.
Fig. 4. Changes in the fluorescence emission of 2 (ꢀex 270 nm, 10−6 M in methanol
solution) on addition of mercury(II) (10−2 M in methanol).

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129
4. Conclusions
In conclusion, we have synthesized
a
few benzothiazole
derivatives and studied their optical properties towards
selective binding of metals. The optical properties of 2-(2ꢀ-
hydroxyphenyl)benzothiazole(1) change on binding to zinc(II),
cadmium(II) and mercury(II) as evidenced from their UV–visible
spectra. This compound is found to be selective towards binding
to zinc(II) over cadmium(II) or mercury(II). The changes in the
fluorescence emission of 1 on addition of zinc(II) is opposite to that
of mercury(II) which is attributed to change in coordination mode
from N–O type to S–O type of chelate. The compound 2 is useful
in studying the role of the 2ꢀ-hydroxy group of the benzothiazole
derivatives. Compound 2 has a 4ꢀ-hydroxy binding site and is
found to be less active towards coordination. The interactions
of compound 3 with zinc(II), cadmium(II) and mercury(II) are
identical, this is in contrast to the selective metal interactions of 1
and 2.
Acknowledgements
The authors thank Department of Science and Technology, New
Delhi, India for financial support and one of the author RS thanks
Council of Scientific and Industrial Research, New Delhi, India for
Junior Research Fellowship.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2010.04.038.
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