Tri-color emission and colorimetric recognition of acetate using semicarbazide and thio-semicarbazide derivatives: Experimental and computational studies

Article abstract of DOI:10.1016/j.saa.2015.10.018

Two new fluorescence probes having semicarbazide (DSC) and thio-semicarbazide (DTSC) units have been derived upon reaction with 2-hydroxy-5-methylbenzene-1,3-dialdehyde. Both the probes show excellent selectivity for acetate ion in DMSO medium whereby DTS

Full text of DOI:10.1016/j.saa.2015.10.018

Tri-color emission and colorimetric recognition of acetate using semicarbazide
and thio-semicarbazide derivatives: Experimental and computational studies
Sisir Lohar, Sougata Sinha, Subrata Ghosh, Debasis Das
PII:
DOI:
Reference:
S1386-1425(15)30268-7
doi: 10.1016/j.saa.2015.10.018
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Accepted date:
7 June 2015
19 September 2015
22 October 2015
Please cite this article as: Sisir Lohar, Sougata Sinha, Subrata Ghosh, Debasis Das,
Tri-color emission and colorimetric recognition of acetate using semicarbazide and thio-
semicarbazide derivatives: Experimental and computational studies, (2015), doi:
1
0.1016/j.saa.2015.10.018
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Tri-color emission and colorimetric recognition of acetate using
semicarbazide and thio-semicarbazide derivatives: Experimental
and computational studies
a
b
b*
a*
Sisir Lohar , Sougata Sinha , Subrata Ghosh and Debasis Das
a
Department of Chemistry, The University of Burdwan, Burdwan, West Bengal, India.
Fax: +91 342 2530452; Tel: +91 342 2533913; E-mail: ddas100in@yahoo.com
b
Department of Chemistry, Indian Institute Technology, Mandi, Himachal Pradesh, India
Abstract
Two new fluorescence probes having semicarbazide (DSC) and thio-semicarbazide (DTSC)
units have been derived upon reaction with 2-hydroxy-5-methylbenzene-1, 3-dialdehyde. Both
the probes show excellent selectivity for acetate ion in DMSO medium whereby DTSC
generates tricolor emission. The association constants of DSC and DTSC for acetate are 6.6 ×
4
-1
3
-1
-7
1
1
0 M and 2 × 10 M respectively with corresponding detection limits,1.06 × 10 M and 2.5×
-6
0 M. Density functional theoretical (DFT) studies nicely demonstrate the interaction between
the DTSC and acetate ion.
Introduction
The design of optical sensor for selective detection of anion has received considerable attention
over the past few years. Anions are ubiquitous in biology, actively participate in enzyme activity,
hormone transport, protein synthesis, and DNA regulation [1–4]. For example, the antibiotic
ristocetin is known to efficiently and selectively bind amino acid carboxylates [5-6]. Acetate
-
(
AcO ) is an important player in biochemistry, environmental, and pharmaceutical science [7–9].
It is considered as an indicator of organic decomposition in marine sediments [10]. Fluorescence,
1

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already established as an efficient and sensitive technique [11] for anion recognition via
significant change of emission color and/or intensity. Varieties of anion sensing mechanisms, viz.
competitive binding, [12] photo-induced electron transfer (PET), [13] metal-to-ligand charge
transfer (MLCT), [14] excimer/exciplex formation, [15] intra-molecular charge transfer (ICT),
[
16] excited-state intra-/ inter-molecular proton transfer (ESIPT) [17] and H-bonding induced
-
loop formation [18] etc. have been reported. Most of the reported AcO / carboxylate receptors
-
-
[
19–27] face interference from other basic anions like H PO and F . Recognition of anions in
2 4
biological systems is very often achieved via hydrogen bonding with highly pre-organized
proteins having sterically well-defined complex sites in the interior of proteins [28-29].
Examples are also available where recognition and discrimination of different anions have been
achieved via suitable receptor coordination sites [30-33]. Herein we describe synthesis,
characterizations and photo-physical interaction of a probe (DTSC) derived from 2, 6-diformyl-
-
4
-methylphenol (DFP) and thiosemicarbazide with AcO . DTSC has a unique tri-color emission
-
in presence of AcO in DMSO medium. A model semicarbazide derivative (DSC) of DFP fails to
-
generate tri-color emission in presence of AcO in DMSO medium.
Results and discussion
DTSC and DSC have been synthesized by a single step condensation reaction between DFP
with thiosemicarbazide and semicarbazide in ethanol as a yellow precipitate with good yield
(Scheme 1).The yellow solids are recrystallized from absolute ethanol to obtain pure DTSC and
1
DSC as established from H NMR, MS and FTIR spectra (Figs. S1–S6, ESI). The UV–vis.
spectrum (Fig. 1) of the DMSO solution of DTSC shows two characteristic bands at 315 and 370
-
nm along with a broad band centered at 450 nm. Upon gradual addition (0.01 to 90 µM) of AcO
,
the bands at 315 and 370 nm decrease with the increase of the 450 nm band. A clear isobestic
2

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point is observed at 400 nm. Similarly for DSC, two bands viz. at 287 and 357 nm decrease with
the appearance of a new band at 425 nm which gradually enhances upon gradual addition (0.01
-
to 90 µM) of AcO . Here too, an isobestic point at 382 nm is observed (Fig. S7). The ratiometric
change of absorbance along with the appearance of an isosbestic point indicates the formation of
-
a new intermolecular charge transfer complex between the receptors (DTSC and DSC) and AcO
,
responsible for visible color change from colorless to yellow in case of DSC. Whereas, addition
-
of AcO to DTSC develops an orange color which changes to yellow after 2 h. Time dependent
-
UV-vis. titration (Fig. 2) shows that addition of AcO to DMSO solution of DTSC develops a
band centered at 500 nm, responsible for the orange color. However, with time the absorbance of
5
00 nm band decreases with the enhancement of a new band at 450 nm, responsible for yellow
color, along with an isobestic point at 480 nm. Additionally, the band at 370 nm decreases with
the increase of 315 nm band. Two more isobestic points viz. at 397 and 337 nm has also been
observed. It is not the deprotonation of the phenolic –OH of DSC, rather inter-molecular H-
-
-6
bonding between amide –NH- with AcO is responsible for the sensing. As low as 1.5× 10 M
-
AcO may be detected using DSC following UV–vis. titration. After addition of 400 equiv. of
sodium acetate, it reaches a saturation level. However, for DTSC, a different mode of sensing is
-
proposed where at first inter-molecular H-bonding between –NH unit of DTSC with AcO
2
occurs followed by deprotonation of the phenolic –OH group leading to sequential color change
-7
-
from orange to yellow to green is observed. Here, the lowest LOD is 6.5× 10 M AcO .
Interestingly, other anions do not show any significant change for both of DTSC and DSC (Fig.
-
S8 and S9). High selectivity of AcO over other competing anions may be rationalized in terms of
-
the compatible geometries of AcO with the cavity of DSC as well as acidity of two -NH protons
-
of the semicarbazide moiety. AcO being Y-shaped planar oxyanion, fits and binds well with two
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amide –NH-. In case of DTSC, out of two conformations at equilibrium, one is suitable for
-
binding and hence sensing of AcO .
Addition of NaOAc to DMSO solution of DSC decreases the 415 nm band (λ = 370 nm)
ex
abruptly while a new band at 482 nm appears and gradually increases with increasing amount of
-
AcO (Fig. S10). The fluorescence quantum yield enhances from 0 .009 to 0.015.The association
-
3
-1
constant of DSC for AcO is 2 × 10 M as calculated from Benesi–Hildebrand plot [34] (Fig.
-
-6
S11). The detection limit of DSC for AcO is 2.5 × 10 M, observed from fluorescence
method.Weak emission of DTSC(λem = 515 nm, λex = 370 nm) enhances gradually with
-
increasing AcO concentration, monitored after 2 h of their mixing (Fig. S12). The effect of
different anions on the emission intensity of DTSC and DSC are shown in Fig. 3 and Fig. 4
respectively. Interesting observations have been found during time dependent fluorescence
-
titration (Fig. 5) of DTSC upon addition of fixed amount of AcO (the amount after which no
further emission enhancement is observed). At the beginning, emission at 562 nm (orange)
gradually decreases with blue shift to 548 nm (yellow) up to 1 h, then the emission intensity
increases at the same wavelength (at 548 nm)from 1h to 1.5 h. After1.5htill 2 h, emission
intensity further decreases along with blue shift. Emission at 515 nm (green) continuously
increases from 30 sec to 2 h. During the entire phenomena, two isobestic points, viz. at 540 and
-
5
25 nm are observed. The association constant of DTSC for AcO is determined from Benesi–
4
-1
Hildebrand plot (Fig. S13) and found as 6.6 × 10 M . The lowest detection limit of DTSC for
-
-7
AcO is 1.06 × 10 M. Job’s plot (Fig. S14 and S15, ESI) confirms 1:1 stoichiometry between
-
DTSC/DSC and AcO .Fig. S16 and S17 (ESI) show insignificant interference from other anions.
1
H NMR titration has been performed by stepwise addition of NaOAc to the DMSO-d solution
6
of DTSC and DSC (Fig. 6 and 7). The phenolic-OH and amide –NH- protons of free DSC viz. at
4

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1
0.675 and 10.273 ppm disappear upon addition of 1.0 equiv. NaOAc, indicating deprotonation
-
1
of phenolic –OH and formation of H-bond between amide –NH- and AcO . On the other hand, H
NMR of DTSC in DMSO-d clearly indicates the existence of two conformers at equilibrium.
6
The phenolic –OH appears as doublet at 11.528 ppm while two imine protons have been found at
-
1
0.044 and 9.562 ppm. Addition of 1.0 equiv. AcO to DTSC causes deprotonation of the
phenolic –OH and imine proton (at 9.562 ppm) of one conformer. Among four –NH protons
2
(7.644 ppm) of free DTSC, two are downfield shifted to 7.884 ppm while the other two are up-
-
field shifted to 7.632 ppm. Up-field shifted protons are H-bonded with AcO .The deprotonation
of the phenolic -OH (both of DTSC and DSC) may cause two distinct effects on the aromatic
substituents: (i) increases the electron density on the phenyl rings through bond propagation that
generates a shielding effect to cause up-field shift of the C-H protons; (ii) induces polarization of
the C-H bonds via through-space effect, where the partial positive charge causes a de-shielding
to produce downfield shift. We have observed the first case for both DTSC and DSC as aromatic
C-H of DTSC and DSC have shifted up-field from 8.185 ppm to 8.138 ppm and 7.449 ppm to
-
7
.425 ppm respectively. The probable AcO sensing mechanism of DTSC and DSC are portrayed
in Fig. 8 and 9 respectively.
-
To corroborate the experimental findings, molecular level interactions between DTSC and AcO
have been examined by DFT studies using DFT/B3LYP/6-31++G (d, p) level of theory [35]. As
free DTSC exists in two conformers, designated as A and B, their HOMO-LUMO energy gaps
-
are 3.1908 and 3.3511 eV (Fig. 10) whereas it is lowest for [B-AcO ] adduct, viz. 2.184 eV
-
compared to others. Fig. 11 shows the energy optimized structure of [B-AcO ] adduct showing
-
several H-bonds between AcO and DTSC (form B). The H-bonds between O(41)-H(19), O(41)-
H(24), O(40)-H(34), O(40)-H(29), O(40)-H(20), O(41)-H(20), N(17)-H(24), O(23)-H(20) and
5

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O(40)-H(24) have distances viz. 1.66177, 3.30389, 1.78863, 1.78848, 2.84439, 3.73922,
1
.71472, 2.01123 and 3.41005 Å respectively. The Cartesian coordinates of the optimized
geometry for each of the structure may be found in the supporting information (Table S1 to S3)
Acknowledgement
We are grateful to State DST, Government of West Bengal for financial assistance. We are
thankful to Professor Asok K Mukherjee for valuable advice. S. L. is grateful to CSIR, India for
fellowship. The instrument facility at the Advanced Materials Research Center (AMRC), IIT
Mandi is thankfully acknowledged.
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Figures
Scheme1
-
Fig. 1 Changes of absorbance of DTSC (10 µM) upon addition of AcO (0.01, 0.05,0.1,0.5,1, 5,
1
0, 20, 30, 50, 60, 70 and 90 µM) in DMSO.
1
0

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-
-
Fig.2. Time dependent UV-vis. titration of DTSC with AcO , [DTSC] = 10µM, [AcO ] = 90 µM
1
1

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Fig. 3 Relative emission intensity of DTSC (10 μM) in presence of different anions (100 μM) in
DMSO (λ = 515 nm)
ex
1
2

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Fig. 4 Relative emission intensity of DSC (20 μM) in presence of different anions (1000 μM) in
DMSO (λ = 482 nm)
ex
1
3

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-
-
Fig.5. Time dependent fluorescence titration of DTSC with AcO , [DTSC] = 10 µM, [AcO ] =
9
0 µM.
1
4

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1
-
Fig. 6 H NMR spectral changes of DTSC upon addition of AcO
1
5

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1
-
Fig. 7 H NMR spectral changes of DSC upon addition of AcO
1
6

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-
Fig. 8. Proposed mechanism of AcO sensing by DTSC
1
7

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-
Fig. 9. Proposed mechanism of AcO sensing by DSC
-
Fig. 10. HOMO-LUMO energy gap of DTSC (form A and B) and adduct between B and AcO
1
8

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-
Fig. 11. Optimized geometry of the adduct between DTSC (B form) and AcO
1
9

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Graphical Abstract
A thio-semicarbazide (DTSC) derivative shows excellent selectivity for acetate ion in DMSO
medium through tricolor emission. It also allows selective naked eye detection of acetate ion in
DMSO. Density functional theoretical (DFT) studies nicely demonstrate the interaction between
DTSC and acetate ion.
2
0

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Highlights



Thio-semicarbazide (DTSC) derivative shows excellent selectivity for acetate ion.
Naked eye and tricolor emission based detection.
Density functional theory (DFT) based studies of interaction between DTSC and acetate ion.
2
1