Selective recognition of fluoride ions through fluorimetric and colorimetric response of a first mesitylene based dipodal sensor 15employing thiosemicarbazones

Article abstract of DOI:10.1016/j.tetlet.2013.03.003

A new dipodal chromo-fluorogenic anion sensor (2Z,2′Z)-2,2′- (((((2,4,6-trimethyl-1,3-phenylene)bis-(methyl-ene))bis-(oxy))bis(2,1-phenylene) )bis(methanylylidene))bis(N-methylhydrazinecarbothio-amide) has been synthesized and characterized for the detection of F^- ion in DMSO. The sensor has proven to be highly selective and sensitive to F^- ion, moreover it can act as naked eye chromogenic sensor. The mechanism involved in the detection of fluoride ion is the deprotonation of the -NH groups, which was confirmed by using TBAOH. The pre-organization of the dipodal receptor provides chelating H-bonds with the spherical F^- ion. These strong H-bonding interactions result in fast deprotonation of the sensor, producing an optical response. Employing only a weak fluorescence capability of the imine groups, 1 behaves as a fluorogenic sensor by exhibiting fluorescence enhancement at λ_ex 438 nm upon addition of F^- ions.

Full text of DOI:10.1016/j.tetlet.2013.03.003

Tetrahedron Letters 54 (2013) 2423–2427
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selective recognition of fluoride ions through fluorimetric and colorimetric
response of a first mesitylene based dipodal sensor employing
thiosemicarbazones
⇑
⇑
Sanyog Sharma, Maninder Singh Hundal , Geeta Hundal
Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India
a r t i c l e i n f o
a b s t r a c t
A new dipodal chromo-fluorogenic anion sensor (2Z,20Z)-2,20-(((((2,4,6-trimethyl-1,3-phenylene)bis-
(methyl-ene))bis-(oxy))bis(2,1-phenylene))bis(methanylylidene))bis(N-methylhydrazinecarbothio-amide)
has been synthesized and characterized for the detection of Fꢀ ion in DMSO. The sensor has proven to be
highly selective and sensitive to Fꢀ ion, moreover it can act as naked eye chromogenic sensor. The
mechanism involved in the detection of fluoride ion is the deprotonation of the –NH groups, which
was confirmed by using TBAOH. The pre-organization of the dipodal receptor provides chelating H-bonds
with the spherical Fꢀ ion. These strong H-bonding interactions result in fast deprotonation of the sensor,
producing an optical response. Employing only a weak fluorescence capability of the imine groups, 1
behaves as a fluorogenic sensor by exhibiting fluorescence enhancement at kex 438 nm upon addition
of Fꢀ ions.
Article history:
Received 29 December 2012
Revised 27 February 2013
Accepted 2 March 2013
Available online 13 March 2013
Keywords:
Thiosemicarbazones
Anion sensing
H-bonding
Deprotonation
NMR spectroscopy
Ó 2013 Elsevier Ltd. All rights reserved.
The design and synthesis of anion sensing receptors have drawn
considerable attention because anions play an important role in
biological, medical, environmental, and chemical sciences.1 For
example, Chloride ion measurements can aid in monitoring of
blood brain disorders and cystic fibrosis.2 The monitoring of nitrate
levels is used to trace pollution from agriculture, for checking fish
farms, and for surveying nutrient levels in natural water bodies.
Fluoride ion is used in dental care and treatment of osteoporosis
and due to its release during hydrolysis of nerve gas Sarin.3 How-
ever, its excessive use may cause fluorosis.4 Among various chemo-
sensors, those which can detect the presence of anions through
both chromogenic and fluorogenic response are exceptionally
interesting as their sensing process is marked by simple, naked
eye detection and also at very low concentration. Most of the neu-
tral anion sensors employ polarized N–H fragments for anion rec-
ognition and are based upon ureas, thioureas, amides, amidoureas,
pyrroles, and amines.5 The recognition phenomenon in these
examples involves either H-bonding (incipient proton transfer) or
complete deprotonation of N–H protons (frozen proton transfer).6
Previously, our group has reported sensors, based on aromatic
platform which had imine and hydroxyl groups7 or urea/thiourea8
as end groups in the receptors. As an extension of our work, we
have now employed thiosemicarbazide moiety as end groups on
supramolecular platform, in order to get an insight into the com-
bined effect of imine and thiourea groups and study the sensing
properties of the sensor. There are a few reports of sensors contain-
ing thiosemicarbazide moieties with9 or without10 appended fluo-
rophore. Here, we present to the best of our knowledge, the first
example of mesitylene based Fꢀ ion sensor which is a dipodal
probe containing thiosemicarbazones, without any additional fluo-
rophore. Both, binding and signaling for the sensing are done by
the thiosemicarbazone moiety itself. The dipodal probe contains
two thiosemicarbazones lying in close proximity to each other
and thus represents a simple, pre-organized, highly selective and
sensitive colorimetric and fluorogenic sensor based on a supramo-
lecular platform.
The synthesis of sensor 1 involves a three-step reaction accord-
ing to Scheme 1.11 Compound 2 was prepared by previously re-
ported method.12 Dipodal aldehyde (3) was prepared by the
reaction of 2-hydroxybenzaldehyde with dibromide 2 in the
presence of K2CO3 in dry acetonitrile. Then, a simple Schiff’s base
condensation reaction of compound 3 and 4-methyl-3-thiosemi-
carbazide in acetonitrile gave the desired product 1. The products
were characterized by various spectroscopic techniques
(Fig. S1–ꢀS17). A pair of strong stretching bands at 3274 cmꢀ1 and
3106 cm shows the presence of –N–H groups. The IR band at
1527 cmꢀ1 confirms the presence of C@N group. In 1H NMR, the
signals at d 8.4 and 11.4 correspond to NH(f) and NH(e) groups,
respectively while the signal at d 8.3 indicates the formation of
imine (CH@N) group. In 13C NMR spectrum, signals for CH@N,
⇑
Corresponding authors. Tel.: +91 9501114469.
E-mail addresses: hundal_chem@yahoo.com (M.S. Hundal), geetahundal@yahoo.
com (G. Hundal).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.003

2424
S. Sharma et al. / Tetrahedron Letters 54 (2013) 2423–2427
The anion recognition ability of sensor 1 was determined by the
H
OH
H
UV–vis and fluorescence studies upon addition of various tetrabu-
tylammonium anions (TBAX, where X = Fꢀ, Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, CNꢀ,
AcOꢀ, HSOꢀ4 , ClO4ꢀ and H2POꢀ4 ) in DMSO (2 ꢁ 10ꢀ4 M). The reaction
mechanism proposed was confirmed by NMR spectroscopy.
The sensor 1 (2 ꢁ 10ꢀ5 M) exhibits a strong absorption band at
CHO
CH3CN, K2CO3
Br
Br
O
O
2
CHO OHC
3
kmax 337 nm (ꢀ
max 65,600 Mꢀ1 cmꢀ1), which has been designated as
an intraligand charge transfer (ICT) band involving imine (CH@N)
groups. As 10 equiv of tetrabutylammonium fluoride was added
to the solution of sensor 1, the absorption band at 337 nm
S
NH
C2H5OH
H2N NH
disappears and
a new band appears at kmax 402 nm (ꢀmax
50,200 Mꢀ1 cmꢀ1) with a bathochromic shift of 65 nm (Fig. 1).
This red shift in absorption band for TBAF was accompanied by
a color change from colorless to yellowish green (Fig. 2). No such
spectral or visual change was observed for any other tetrabutyl
ammonium anions (TBAX) under study (Fig. S8–S9).
c
H
a
a
b
j
O
O
a'
The ratiometric response of the sensor 1 with increase in equiv-
alents of TBAF is shown in Figure 3. The stoichiometry of the reac-
tion of sensor 1 and TBAF is 1:1 according to Job’s plot13 studies
(Fig. S10). The equilibrium constant, K of the sensor 1 for the
fluoride ion has been calculated as 2.66 ꢁ 103 Mꢀ1 from the
Benesi–Hildebrand plot14 (Fig. S11). The stability of the complex
i
h
d
g
N
N
HN
NH
e
f
S
NH
HN
S
was confirmed by the decrease in free energy of ꢀ19.54 kJ molꢀ1
,
k
1
calculated from the equation
D
G = ꢀ2.303 RT logK.
Scheme 1. Synthesis of (2Z,20Z)-2,20-(((((2,4,6-trimethyl-1,3-phenylene)bis(meth-
ylene))bis(oxy))bis(2,1-phenylene))bis-(methanylylidene))bis(N-methylhydrazine
carbothioamide).
Fluorescence studies of sensor 1 were also performed with the
solutions of the same concentration that is, sensor 1 (2 ꢁ 10ꢀ5 M)
and TBAX (2 ꢁ 10ꢀ4 M). Sensor 1 contains no additional fluoro-
phore, hence upon excitation at kex 267 nm, it shows a very weak
broad fluorescence emission band in the range of 350–500 nm.
This band has low intensity because of efficient photoinduced elec-
tron transfer (PET) quenching,15 which comes into play due to
availability of the lone pair of adjacent NH group.
Upon addition of TBAF, the fluorescence spectrum exhibits an
emission band at kmax 438 nm with shoulder at 375 nm and at
480 nm (kex 267 nm) but no such spectral change is observed with
other TBAX ions (Fig. 4, Fig. S12). Upon complexation of the Fꢀ ion,
the PET phenomenon may be reduced when –NH groups are in-
volved in H-bonding with the TBAF anions resulting in the
enhancement of the signal in fluorescence spectrum (Fig. 5a). The
detection limit16 for Fꢀ ion was estimated to be 2 ꢁ 10ꢀ5 mol/L at
I/I0 = 3 (Fig. 5b).
The sensing selectivity of sensor 1 toward Fꢀ ions was evaluated
in the presence of 10 equiv of other tetrabutyl ammꢀonium anions
including Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, CNꢀ, AcOꢀ, HSOꢀ4 , ClO4 and H2POꢀ.
4
The excellent selectivity of sensor 1 for fluoride over other anions
is evident from the pronounced differences in absorbance (Fig. S9),
in color changes (Fig. 2), and in fluorescence (Fig. S13). The selec-
tivity for Fꢀ over other anions, especially CNꢀ and CH3COOꢀ, is
important because many reported fluoride sensors suffer from
the deleterious interference of these anions.
Figure 1. Changes in the UV–vis spectrum of (1) in DMSO (2 ꢁ 10ꢀ5 M) on the
addition of various TBAX ions (2 ꢁ 10ꢀ4 M) in DMSO.
C–O, and C@S appear at d 138.5, 157.3, and 177.5, respectively. The
signals corresponding to quaternary carbons are absent in DEPT
135 spectrum. The formation of sensor 1 was also confirmed by
the peak in mass spectrum at 563.22 for [M+H]+. The CHN data
are also in accordance with the molecular formula.
In order to investigate the binding sites of sensor 1 for the Fꢀ
ions, NMR titrations were carried out in DMSO-d6, with increasing
amounts of TBAF as shown in Figure 6. NMR spectra of sensor 1
Figure 2. Visual color changes in (1) in DMSO (2 ꢁ 10ꢀ5 M) with various TBAX ions (2 ꢁ 10ꢀ4 M).

S. Sharma et al. / Tetrahedron Letters 54 (2013) 2423–2427
2425
Figure 3. (a) Changes in UV–vis spectra of sensor (1) (2 ꢁ 10ꢀ5 M) upon gradual addition of TBAF; (b) Fꢀ titration profile at 337 nm and 402 nm.
tonation also causes significant upfield shifts (Table S1) in the NMR
signals of aromatic protons (Fig. S14).
In the present case, the H-bonding pocket defined by four –NH
protons of dipodal probe 1 which has the potential to bind anions
in a chelate mode through four H-bond donation sites, tends to
bring the fluoride ion to their vicinity (Scheme 2). Fluoride ion
though is a weaker base (pKa = 15 in DMSO), but here it behaves
as a very strong base second to OHꢀ (pKa = 32 in DMSO) only17
,
due to extreme stability of [HF2]ꢀ. It is known to induce
deprotonation of even less acidic moieties like urea protons.18
Pre-organization of the four –NH groups to provide a cavity and
chelation of Fꢀ ions, coupled with the inherent basicity of Fꢀ ions
in DMSO probably, help in making the deprotonation so fast that
the prior H-bonding step is not registered in the 1H NMR. The exis-
tence of [FHF]ꢀ however, is unequivocally proved by the presence
of a tell-tale,19 well formed triplet at d 16.15 ppm in the 1H NMR of
sensor 1 in the presence of 1.0 equiv and above of Fꢀ ion (Fig. 6).
The signal due to –NH(f) which seems to disappear, in fact first gets
broadened due to chelating H-bonding and then appears again
once the deprotonation of the –NH(e) protons occurs. The signifi-
cant upfield shifts in the aromatic protons and not the methyl
group (attached to –NH group) further ratify that it is the –NH
group adjacent to the imine which is deprotonated and not the
other one. The deprotonation of the sensor 1 was also confirmed
Figure 4. Fluorescence ratio [I/Io] of sensor (1) (2 ꢁ 10ꢀ5 M) at kmax 438 nm upon
addition of various anions (2 ꢁ 10ꢀ4 M) with excitation at 267 nm in DMSO. Io and I
are the intensities in the absence and presence of anions, respectively.
show that the signals for the –NH(e) groups, which were observed
at 11.4 ppm disappear completely much before the complete addi-
tion of 1 equiv of TBAF, indicating that the deprotonation is too fast
to register the effect of H-bonding on the –NH signals in 1H NMR.
But the signals for the –NH(f) protons first get so much broadened
that they almost disappear and then are restored again. This depro-
Figure 5. (a) Changes in fluorescence spectra of sensor (1) (2 ꢁ 10ꢀ5 M) upon gradual addition of TBAF (2 ꢁ 10ꢀ4 M); (b) increase in fluorescence intensity of sensor 1 at
438 nm upon Fꢀ ion for the calculation of detection limit.

2426
S. Sharma et al. / Tetrahedron Letters 54 (2013) 2423–2427
Figure 6. Full 1H NMR of (1) in DMSO-d6 (5 mM) showing changes on gradual addition of TBAF.
Supplementary data
Supplementary data associated (general experimental section,
characterization of sensor 1 and compound 3 by spectroscopic
techniques, absorption and fluorescence spectra, Job’s plot, Bene-
si–Hildebrand plot and NMR studies) with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
03.003.
References and notes
Scheme 2. Proposed reaction mechanism for formation of sensor 1.
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In conclusion, a new mesitylene based dipodal sensor contain-
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Acknowledgements
M.S.H. and G.H. are thankful to CSIR (India) for research Grant
No. 01(2406)/10/EMR-II. S. S. is thankful to UGC for senior research
fellowship.

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(161 mg) in 56% yield. IR (KBr, cmꢀ1) 3274, 3106, 2898, 1527, 1224, 1074, 746;
1H NMR (300 MHz, DMSO-d6, d): 2.35 (s, 6H, –CH3), 2.38 (s, 3H, –CH3), 2.97 (d,
6H, –CH3, J = 4.2), 5.15 (s, 4H, –CH2), 6.99 (s, 1H, –Ar), 7.02 (d, 2H, –Ar, J = 7.5),
7.34 (d, 2H, –Ar, J = 8.1), 7.42 (t, 2H, –Ar, J = 6.9), 8.09 (dd, 2H, J = 7.8), 8.30 (s,
2H, –CH@N), 8.40 (q, 2H, –NH, J = 9.0), 11.42 (s, 2H, –NH); 13C NMR (75 MHz,
DMSO-d6, d): 15.1 (–CH3), 19.3 (–CH3), 30.7 (–N–CH3), 65.3 (–CH2), 113.1 (–Ar),
120.7 (–Ar), 122.7 (–Ar), 126.1 (–Ar), 129.9 (–Ar), 130.8 (–Ar), 131.1 (–Ar),
137.3 (–CH@N), 138.4 (–Ar), 138.5 (–Ar), 157.4 (–C–O), 177.6 (–C@S);
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11. Synthesis of sensor 1: To a solution of 199 mg (0.513 mmol) of 2,20-(((2,4,6-
trimethyl-1,3-phenylene)bis(methylene))bis(oxy))dibenzaldehyde (compound
2)
in
acetonitrile
was
added
107 mg
(1.019 mmol)
of
3-
methylthiosemicarbazide and the mixture was refluxed for 5 h. The mixture
was then cooled to room temperature and filtered, and the solid was washed
with cold methanol. The solid was dried and recrystallized in chloroform.
Washing the solid with cold methanol gave a pale yellow solid product 1