Naked-eye detection of inorganic fluoride ion in aqueous media using base labile proton: A different approach

Article abstract of DOI:10.1016/j.jfluchem.2014.01.009

Two new receptors R1 and R2 were designed and synthesized based on 1-naphthohydrazide Schiff's bases for the colorimetric detection of fluoride ion. The receptor R1 was selective toward fluoride ion over other anions in organic media. The presence of carbonyl group in 1-naphthohydrazide makes -NH proton acidic and therefore it could deprotonate with addition of basic anion such as fluoride. However, the acidic -NH proton easily gets solvated even with the trace amount of water. Alternatively, the receptor R2, encompasses highly base labile, hydroxyl (-OH) functionality which detects basic fluoride ions via deprotonation mechanism not only in organic solutions but also in aqueous media. The mechanism involved in the color change of receptor R1 is deprotonation of acidic -NH followed by stabilization of complex through intramolecular charge transfer (ICT) transition which was evidenced by the formation of HF₂ peak in ¹H NMR titration. The color change of receptor R2 involves initial hydrogen bond formation of F^- ion with -NH group and deprotonation at higher concentration of F^- ions which leads to intramolecular charge transfer transition in organic media. On the other hand, in aqueous solution of receptor R2, addition of the basic fluoride ion leads to the deprotonation of base labile hydroxyl (-OH) proton, which is responsible for colorimetric detection.

Full text of DOI:10.1016/j.jfluchem.2014.01.009

Journal of Fluorine Chemistry 160 (2014) 1–7
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
‘‘Naked-eye’’ detection of inorganic fluoride ion in aqueous media
using base labile proton: A different approach
*
Madhuprasad Kigga, Darshak R. Trivedi
Supramolecular Chemistry Laboratory, Department of Chemistry, National Institute of Technology Karnataka (NITK), Surathkal, Mangalore 575025,
Karnataka, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Two new receptors R1 and R2 were designed and synthesized based on 1-naphthohydrazide Schiff’s
bases for the colorimetric detection of fluoride ion. The receptor R1 was selective toward fluoride ion
over other anions in organic media. The presence of carbonyl group in 1-naphthohydrazide makes –NH
proton acidic and therefore it could deprotonate with addition of basic anion such as fluoride. However,
the acidic –NH proton easily gets solvated even with the trace amount of water. Alternatively, the
receptor R2, encompasses highly base labile, hydroxyl (–OH) functionality which detects basic fluoride
ions via deprotonation mechanism not only in organic solutions but also in aqueous media. The
mechanism involved in the color change of receptor R1 is deprotonation of acidic –NH followed by
stabilization of complex through intramolecular charge transfer (ICT) transition which was evidenced by
the formation of HF2 peak in 1H NMR titration. The color change of receptor R2 involves initial hydrogen
bond formation of Fꢀ ion with –NH group and deprotonation at higher concentration of Fꢀ ions which
leads to intramolecular charge transfer transition in organic media. On the other hand, in aqueous
solution of receptor R2, addition of the basic fluoride ion leads to the deprotonation of base labile
hydroxyl (–OH) proton, which is responsible for colorimetric detection.
Received 8 October 2013
Received in revised form 6 January 2014
Accepted 12 January 2014
Available online 23 January 2014
Keywords:
Anion receptor
Aqueous detection
Acidic proton
Fluoride
Hydrogen bonding
Deprotonation
ß 2014 Elsevier B.V. All rights reserved.
1. Introduction
well-known functional groups such as urea/thiourea, amides,
pyrrole and imidazolium are proved themselves as effective binding
The anion sensing via synthetic organic receptor is a field of
interest in supramolecular chemistry because of their importance
in chemical and biological systems [1–8]. Among the range of
anions, fluoride ion has attained significance because of its role in
preventing dental decay [9,10] and in the treatment of osteoporo-
sis [11,12]. However, in excess, fluoride can lead to fluorosis [13–
17] and other bone related diseases [18,19]. This dual functionality
of fluoride, with respect to beneficial as well as non-beneficial has
acquired interest of the supramolecular chemists.
Conventional fluoride ion detection using electrode is costly,
time consuming and requires complicated instrumentation with
confusing handling procedures [20]. On the other hand, colorimetric
receptors appear to be attractive because of their simplicity, high
sensitivity, high selectivity and real-time ‘naked eye’ detection. This
colorimetric detection of fluoride could be achieved by designing a
host molecule where the binding of anion can change the color of
host molecule. Based on this strategy, a number of receptors have
been reported for the selective binding of fluoride [21–25]. Many
sites for the fluoride ion through the formation of a hydrogen bond
with N–H unit of these functional groups [26–38]. Silyl group
deprotection based colorimetric receptors were also verified in
various literatures [39–42]. The affinity of a boron atom toward
fluorideionswerewellutilized for the detectionof fluorideions [43–
50]. Unfortunately, majority of them are capable of detecting
fluoride ions only in absolute non-aqueous conditions and for the
detection of organic fluoride sources such as tetrabutylammonium
fluoride (TBAF) and therefore cannot be used for the real-life
applications. Alternatively, if the receptor could detect inorganic
fluorides such as sodium fluoride in aqueous media then it could be
useful for the real-life application.
The detection ability, selectivity and sensitivity of a receptor
depends on the acidity of protons where the Fꢀ ions bind [51]. If
the acidity of these protons is lesser than that of water then the
Fꢀ ions get solvated. Hence, contamination of receptor even with
trace amount of water could result in the failure of colorimetric
detection process. However, this drawback can be resolved to
some extent by designing a receptor where the Fꢀ ion binding
protons are more acidic than water or by incorporating a base
labile group such as hydroxyl (–OH) functionality, which can
readily deprotonate with the basic ions such as Fꢀ ion [52,53].
*
Corresponding author. Tel.: +91 824 2474000x3205; fax: +91 824 2474033.
E-mail address: darshak_rtrivedi@yahoo.co.in (D.R. Trivedi).
0022-1139/$ – see front matter ß 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2014.01.009

2
M. Kigga, D.R. Trivedi / Journal of Fluorine Chemistry 160 (2014) 1–7
Table 1
H
N
X
O
R1
Crystallographic data of receptor R2.
X=
R2
N
X=
NO2
X=
HO
Parameters
Receptor R2
O
CCDC No.
938645
C19H16N2O3
320.34
Chemical formula
Formula weight
Crystal system
Space group
R3
Orthorhombic
Pbca
Scheme 1. Molecular structures of R1, R2 and R3.
˚
a (A)
13.8637(14)
8.5605(9)
27.679(3)
90.00
˚
b (A)
˚
Few receptors which are capable of detecting fluoride ions in
c (A)
a
b
g
(8)
(8)
(8)
organo–aqueous solvent mixture were reported recently. Unfor-
tunately, majority of them are limited to organic fluoride source
such as TBAF [54–56] or to make test papers, which is not
instantaneous toward detection process [57,58]. On the other
hand, there are few reports which showed admirable results such
as detection of inorganic fluoride ions in aqueous media [59] and
removal of inorganic fluoride ions by extracting them to organic
media [60]. Nevertheless, the synthesis of receptors for the
practical or real-life applications still persist as the detection of
fluoride ions in ‘water-only’ conditions are hard to achieve.
Herein, we report the design and synthesis of new colorimetric
receptors R1, R2 and R3 (Scheme 1), based on 1-naphthohydrazide
Schiff’s base for Fꢀ ion detection. The receptor R1 was synthesized
for the detection of Fꢀ ion in organic media. The receptor R2
comprises hydroxyl (–OH) functional group and therefore it was
able to detect the Fꢀ ion in the form of NaF in organo–aqueous
mixture (9:1 ratio) with this receptor. The receptor R3 was
synthesized to depict the role of aromatic substitution to 1-
naphthohydrazide backbone.
90.00
90.00
3
˚
V (A)
3284.9(6)
8
Z
Crystal size
F (0 0 0)
0.49 ꢂ 0.45 ꢂ 0.39
1344
R-Factor (%)
4.3
Table 2
H-bonding parameters for receptor R2.
˚
S. no.
Type of interactions
Distance (A)
Angle (8)
1
2
N–Hꢁ ꢁ ꢁO
O–Hꢁ ꢁ ꢁN
2.810 (2)
2.681 (2)
166 (2)
145.66
hydrogensulphate and dihydrogenphosphate in the form of
tetrabutylammonium (TBA) salts. A color change from colorless
to red and colorless to bright yellow was observed instantaneously
upon adding Fꢀ ions and AcOꢀ ions to the receptors R1 (Fig. 2) and
R2 (Fig. S7, see Supporting information) respectively. The color
intensity was more in case of Fꢀ ion for both the receptors which
indicates strong binding of Fꢀ ion to the receptors whereas, a much
weaker interaction between receptors and AcOꢀ ion was resulted
in decreased intensity in colorimetric detection.
The colorimetric detecting ability of receptors R1 and R2 were
studied in ACN solvent. Unfortunately, the receptor R1 was
partially soluble in ACN and therefore, it was not able to study for
colorimetric applications. The receptor R2 displayed similar color
change from colorless to bright yellow on addition of Fꢀ ions and
AcOꢀ ions (Fig. 3).
However, the color intensity for AcOꢀ ion was less when
compared to Fꢀ ion because of the weaker interaction of AcOꢀ ion
with receptor.
The selectivity of receptor R1 was further confirmed with UV–
vis spectroscopy where it showed significant shift in the
absorption band upon addition of Fꢀ ions and AcOꢀ ions. The
intensity of this newly generated absorption band after addition of
AcOꢀ ions was much less than that of Fꢀ ions (Fig. 4). However, all
other anions did not show any color change as well as did not show
2. Results and discussion
The single crystal of the receptors R2 suitable for X-ray
diffraction analysis was obtained by slow evaporation of metha-
nol–chloroform (1:1) solution at room temperature. The ORTEP
diagrams (50% probability) of the receptor R2 is given in Fig. 1.
Detailed crystallographic data of receptor R2 is given in Table 1.
The receptor R2 was crystallized in orthorhombic lattice with
space group Pbca. The naphthalene ring of the molecule is
distorted from the plane by an angle of 44.4 (2)8. The carbonyl
(–C55O) group of the receptor is linked by intermolecular H-bond
via N–Hꢁ ꢁ ꢁO55C interactions. The hydroxyl group of the receptor is
connected by intramolecular H-bond to the imine N via O–
Hꢁ ꢁ ꢁN55C interactions. The details of H-bonding parameters (bond
lengths and angles) have been mentioned in Table 2.
The receptors R1 and R2 were initially investigated for selective
colorimetric detection of Fꢀ ion over other anions in DMSO solvent.
The receptors (2.5 ꢂ 10ꢀ5 M) were treated with 1 equiv. of various
anions such as fluoride, chloride, bromide, iodide, acetate,
Fig. 1. The ORTEP diagrams (50% probability) of receptor R2.

M. Kigga, D.R. Trivedi / Journal of Fluorine Chemistry 160 (2014) 1–7
3
Fig. 2. Change in color of R1 (2.5 ꢂ 10ꢀ5 M) in DMSO solution on adding 1 equiv. of tetrabutylammonium anions: (a) free receptor R1, (b) Fꢀ, (c) Clꢀ, (d) Brꢀ, (e) Iꢀ, (f) AcOꢀ,
ꢀ
ꢀ
(g) HSO4 and (h) H2PO4 ions.
Fig. 3. Change in color of R2 (2.5 ꢂ 10ꢀ5 M) in ACN solution on adding 1 equiv. of tetrabutylammonium anions: (a) free receptor R2, (b) Fꢀ, (c) Clꢀ, (d) Brꢀ, (e) Iꢀ, (f) AcOꢀ,
ꢀ
ꢀ
(g) HSO4 and (h) H2PO4 ions.
any change in UV–vis absorption band, which indicated either
these anions are not interacted with receptor R1 or the interaction
was much weaker to create any changes in colorimetric detection
and in UV–vis absorption. The same selectivity confirmation was
extended to the receptor R2 in DMSO as well as in ACN solvents
(Fig. S12 and Fig. S13, see Supporting information). In both the
solvents the receptor R2 showed substantial shift in the absorption
band only upon adding Fꢀ ions and AcOꢀ ions. However, the
intensity of absorption band was much lesser upon adding AcOꢀ
ions, which clearly indicates that the interaction between receptor
R2 and AcOꢀ ion is much weaker.
In order to examine the selectivity toward Fꢀ ion over other
anions, receptor R1 was treated with 1 equiv. of Fꢀ ion in presence
of Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ and H2PO4ꢀ ions (1 equiv.). As shown in Fig.
S10 (see Supporting Information), presence of other ions virtually
made no difference on the colorimetric detection of Fꢀ ion. Thus
receptor R1 could selectively detect the fluoride ion even in
presence of other competing anions.
absorption band at 476 nm with the formation of a clear isosbestic
point at 362 nm (Fig. 5) appeared. This bathochromic shift of
148 nm was attributed to the intramolecular charge transfer
transition in the receptor R1.
The UV–vis titration of receptor R2 was carried out in both
DMSO as well as ACN solutions. The DMSO solution showed
constant decrease in the absorption band at 301 nm with gradual
increasing addition of Fꢀ ions. Meanwhile a new band centered at
413 nm appeared and intensity of this absorption band was
increased continuously with increasing concentration of Fꢀ ions
(Fig. S15, see Supporting information). A gradual addition of Fꢀ ions
to receptor R2 in ACN, a new absorption peak at 440 nm
corresponding to intramolecular charge transfer, was established.
Concurrently, the absorption maxima corresponding to –OH
functional group at 296 nm gradually shifted to a new absorption
maxima at 344 nm creating an isosbestic point at 315 nm (Fig. 6).
This 48 nm bathochromic shift in the absorption maxima was due
to the stabilization of keto tautomer of hydroxyl functional group.
However, this enol to keto tautomeric shift was not observed in
case of DMSO solution as the keto tautomer itself is more stable in
more polar solvents like DMSO [61,62] The DMSO solvent forms
hydrogen bond with the –C55O of keto tautomer and therefore, keto
tautomer becomes stable in receptor R2. As a result, keto tautomer
To understand the nature of the receptor–anion interactions, a
UV–vis titration was performed by increasing addition of TBAF to
R1 (2.5 ꢂ 10ꢀ5 M) in absolute dry DMSO. The constant increase in
the concentration of TBAF resulted in steady decrease in the
intensity of absorbance band at 328 nm. Meanwhile, a new
Fig. 4. UV–vis spectral changes of R1 (2.5 ꢂ 10ꢀ5 M) in DMSO after addition of
10 equiv. of (a) Fꢀ, (b) AcOꢀ and (c) Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ and H2PO4ꢀ ions in the form
of TBA salts.
Fig. 5. UV–vis titration of R1 (2.5 ꢂ 10ꢀ5 M) with TBAF (0–15 equiv.) in DMSO.
Inset: Benesi–Hildebrand plot of receptor R1 binding with Fꢀ ion associated with
absorbance change at 476 nm in DMSO.

4
M. Kigga, D.R. Trivedi / Journal of Fluorine Chemistry 160 (2014) 1–7
O
O
O
O
N-
N
N
N
NH
N
N
N
H
F
F
F
-HF2
-O N+
-O N+
O-
-O N+
HO N+
O
O
O
Scheme 2. Proposed mechanism for binding of fluoride ions with receptor R1.
1H NMR titration experiments of R1 and R2 with Fꢀ ion (as TBA
salt) were carried out in DMSO-d6 to understand the mechanism of
receptor–anion interactions.
The proton Ha at
was initially disappeared on adding TBAF (1 equiv.) as a result of
fast proton exchange. The signals at 8.15 (Hc) and 8.05 (Hd)
corresponding to nitro phenyl group were merged together till
2 equiv. of Fꢀ ions due to the fast proton exchange through the
receptor R1. At 5 equiv. of Fꢀ ions the –NH of receptor deprotonates
and a conjugated quinonoid form of receptor R1 was stabilized. As
a result, splitting of the signals corresponding to nitro phenyl
group reappeared (Fig. 7). As the concentration of Fꢀ ion increased
d 12.34 corresponding to –NH of receptor R1
Fig. 6. UV–vis titration of R2 (2.5 ꢂ 10ꢀ5 M) with TBAF (0–15 equiv.) in ACN. Inset:
Benesi–Hildebrand plot of receptor R2 binding with Fꢀ ion associated with
absorbance change at 440 nm in ACN.
d
d
predominates over enol tautomer in DMSO solvent and the –C55O
functional group of keto tautomer has no binding site to participate
in the detection process.
from 1 equiv. to 5 equiv., signal at
d 8.45 corresponding to imine
The stoichiometry of the anion complexation for both receptors
R1 and R2 was determined by Benesi–Hildebrand method [63]
using UV spectrometric titration data at 476 nm and 440 nm
respectively (Fig. 5, inset and Fig. 6, inset). The linearity in graph
confirms formation of a stable 1:2 stoichiometric complex for both
receptors with Fꢀ ions. The binding constant was calculated using
Benesi–Hildebrand equation (Eq. (1)) and was found to be
2.36 ꢃ 0.33 ꢂ 106 Mꢀ2 for receptor R1 and 0.973 ꢃ 0.09 ꢂ 106 Mꢀ2
for Receptor R2.
proton (Hb) was slowly upfielded owing to the formation of
exocyclic double bond during detection process. The deprotona-
tion lead to increase in electron density over receptor R1 which
resulted in upfielded aromatic protons.
The most important result observed in 1H NMR titration was
appearance of a new peak at
d 16.1 (Fig. 7, inset) at higher
concentration (5 equiv.) of Fꢀ ion. This corresponds to HF2
(hydrogen difluoride) [64] which further confirm deprotonation
of –NH proton in receptor R1.
The binding mechanism of receptor R1 to Fꢀ ions was proposed
by evaluating the results obtained from UV–vis titration and 1H
NMR titration (Scheme 2). The Fꢀ ion detection using receptor R1 is
two-step process. At first, a Fꢀ ion binds through hydrogen bonding
to the –NH proton of the receptor R1 which results in 1:1 adduct
and a R1ꢁ ꢁ ꢁFꢀ complex is formed [65]. The second Fꢀ ion induces
deprotonation of –NH proton in the receptor R1 and as a result, the
electron density increases over the deprotonated receptor R1.
Thus, the charge separation in the receptor is introduced which
1
1
1
¼
þ
(1)
n
K½Fꢀꢄ ðAmax ꢀ A0Þ
ðA ꢀ A0Þ ðAmax ꢀ A0Þ
where A0, A, Amax are the absorption considered in the absence of
Fꢀ, at an intermediate, and at a concentration of saturation. K is
binding constant, [Fꢀ] is concentration of Fꢀ ion and n is the
stoichiometric ratio.
Fig. 7. Partial 1H NMR spectra of receptor R1 in DMSO-d6 after the addition of
(a) 0 equiv., (b) 1 equiv., (c) 2 equiv. and (d) 5 equiv. of Fꢀ ions. Inset: (a) no peak at
16.1 at 0 equiv. Fꢀ ions and (b) peak corresponding to HF2 on adding 5 equiv. of
Fꢀ ions.
Fig. 8. Partial 1H NMR spectra of receptor R2 in DMSO-d6 after the addition of
(a) 0 equiv., (b) 1 equiv., (c) 2 equiv. and (d) 5 equiv. of Fꢀ ions.

M. Kigga, D.R. Trivedi / Journal of Fluorine Chemistry 160 (2014) 1–7
5
-O
HN
O
O
F
O
O
F
DMSO
F
N-
HN
NHa
HbN
N H
N
NH
N
HN
-HF2
O
O
O
O
HO
O
O
O
O
O
Scheme 3. Proposed mechanism for binding of fluoride ions with receptor R2.
results in ICT transition between the electron deficient –NO2 group
at p-position and electron rich –Nꢀ which lead to intense
colorimetric change [66].
As stated previously, the keto tautomer of receptor R2 will be
predominated over the enol tautomer in polar solvents such as
DMSO. This tautomerism is confirmed by 1H NMR spectrum where
O
O
NaF
-HF
NH
NH
N
N
the proton at
10.9 (Hb) corresponds to –NH of keto tautomer. In 1H NMR titration
of receptor R2, both the protons corresponding to –NH at 12.25
and at
10.9, were disappeared upon adding 1 equiv. of Fꢀ ions
d 12.25 (Ha) corresponds to –CONH and proton at d
+Na-O
H O
d
O
O
d
(Fig. 8), which is owing to the formation of hydrogen bond between
Fꢀ ion and –NH proton of –CONH followed by fast proton exchange.
Surprisingly, no other major changes were observed in 1H NMR
signals corresponding to the aromatic protons while adding Fꢀ ions
up to 2 equiv. However, at higher concentration of Fꢀ ions
(5 equiv.) the proton Ha deprotonates which resulted in the
charge transfer transition in receptor R2 arises. Consequently, the
aromatic signals corresponding to naphthyl protons (Hd) were
merged together. This deprotonation, leads to increased electronic
density over the phenyl group and signals corresponding to these
protons (He) were slightly upfielded. The imine –CH in receptor R2
was not participated in the detection process. This was confirmed
Scheme 4. Proposed mechanism for detection of inorganic fluoride ions in aqueous
media.
attached to naphthohydrazide unit is indispensable for the Fꢀ ion
detection process.
To evaluate real-life applicability of any receptor, it should
detect the inorganic fluoride ions such as sodium fluoride
particularly in aqueous media. Therefore, to ensure the real-life
significance, the receptors R1 and R2 were evaluated for the
colorimetric detection of sodium fluoride in 9:1 (1 ꢂ 10ꢀ3 M
solution in DMSO:H2O for R1 and ACN:H2O for R2) organo–
aqueous mixture. Though the receptor R1 showed remarkable
color change in organic solvents for the detection of TBAF, it failed
to detect inorganic fluoride ions in aqueous media. However, the
receptor R2 showed a significant color change from colorless to
pale yellow on adding 5 equiv. of sodium fluoride in aqueous
media (Fig. 9). The receptor R2 showed same color change even
with the addition of 5 equiv. TBAF as Fꢀ ion source in aqueous
media. Unfortunately, UV–vis studies were unable to determine as
the receptor concentration exceeded absorption limit.
The receptor R1 failed to detect inorganic fluoride ions as the
binding site (–NH) readily gets solvated in presence of water even
in trace amounts. On the other hand, receptor R2 has a phenolic –
OH which is highly base labile and immediately binds with basic
inorganic fluoride ions to form respective salt. As a result, upon
adding sodium fluoride to the receptor R2, phenolic –OH readily
gets deprotonated to form corresponding sodium salt (Scheme 4)
and this deprotonation leads to the color change of receptor R2
from colorless to pale yellow.
by 1H NMR titration spectra where the signals at
d 8.6 was not
shifted either upfield or downfield even after adding 5 equiv. of Fꢀ
ions.
Thus, the results of UV–vis titration and 1H NMR titration were
compiled to determine the binding mechanism of receptor R2 with
Fꢀ ions (Scheme 3). Being a keto tautomer in DMSO, the –NH
proton of receptor R2 initially forms hydrogen bond with Fꢀ ion to
stabilize a R2ꢁ ꢁ ꢁFꢀ adduct. Further addition of Fꢀ ion persuades
deprotonation of the –NH proton in receptor R2 and thus
establishes charge on the receptor. This leads charge transfer
transition within the receptor R2 and thus the receptor show a
significant color change.
The receptor R3 was evaluated for colorimetric detection of Fꢀ
ions over other anions in DMSO. The color of receptor was not
changed even after adding 20 equiv. of anions. The absorbance in
UV–vis spectroscopy was also not changed with the addition of any
anions. Therefore, it is clear that the presence of aromatic unit
The receptor R2 was able to colorimetrically detect the Fꢀ ions
with a minimum concentration of 1 ꢂ 10ꢀ6 M in organic media and
1 ꢂ 10ꢀ4 M in organo–aqueous mixture. The amount of Fꢀ ions
required for detection in organo–aqueous mixture is high as the
binding site of receptor gets solvated in the presence of water.
3. Conclusion
To summarize, the receptors R1 and R2 were designed and
synthesized based on 1-naphthohydrazide Schiff’s base for the
selective colorimetric detection of Fꢀ ions over other anions. The
detection process involves initial 1:1 receptor–Fꢀ ion adduct
formation followed by deprotonation at higher concentration of
Fig. 9. Color change of the receptor R2 after adding the Fꢀ ions in 9:1 ACN:H2O
solvent mixture: (a) R2 (1 ꢂ 10ꢀ3 M), (b) R2 + 3 equiv. NaF and (c) R2 + 3 equiv.
TBAF.

6
M. Kigga, D.R. Trivedi / Journal of Fluorine Chemistry 160 (2014) 1–7
Fꢀ ions. This resulted in charge transfer transition through the
receptors and thus remarkable color change from colorless to red
in case of receptor R1 and colorless to bright yellow for receptor R2
was observed. Though the receptor R1 displayed prominent color
change in organic media, it failed to show the same result for the
detection of inorganic fluoride ions in aqueous media as the
binding site easily gets solvated with trace amount of water.
Alternatively, the receptorR2wasabletodetect inorganicfluoridein
aqueous media. This observation was owing to the presence of base
labile –OH functionality in receptor R2 which will be deprotonated
with basic Fꢀ ions, even in aqueous media. Thus, present study could
be further explored for the development of new colorimetric anion
receptors in aqueous media with a new approach.
4.3. (E)-N0-(4-nitrobenzylidene)-1-naphthohydrazide (R1)
Yield: 92%; m.p. 313–314 8C.
Elemental analysis; Calculated for C18H13N3O3: C 67.71, H 4.10,
N 13.16, Found: C 67.53, H 4.23, N 13.24.
1H NMR (DMSO-d6)
8.34 (d, 3H, ArH, J = 8.5 Hz),
1H, ArH, J = 8.0 Hz), 8.04 (d, 2H, ArH, J = 8.0 Hz),
J = 7.0 Hz), 7.63 (d, 3H, ArH, J = 6.0 Hz).
d
12.34 (s, 1H, NH),
8.24 (d, 1H, ArH, J = 7.5 Hz),
7.80 (d, 1H, ArH,
d
8.45 (s, 1H, –CH),
d
d
d
8.14 (d,
d
d
d
FT-IR in cmꢀ1: 3163.5 (br.w), 3005.7 (br.m), 2842.0 (br.w),
1640.7 (s), 1556.6 (m), 1509.9 (s), 1334.6 (s), 1293.0 (s), 1254.1
(m), 1201.1 (m), 1143.8 (w), 1075.1 (w), 1027.5 (w), 779.9 (m).
MS (ESI): m/z: Calculated: 318.3062 [MꢀH]+, Experimental:
318.0919 [MꢀH]+.
4. Experimental
4.4. (E)-N0-(2-hydroxy-3-methoxybenzylidene)-1-naphthohydrazide
(R2)
4.1. General information
Yield: 90%; m.p. 169–170 8C.
Elemental analysis; Calculated for C19H16N2O3: C 71.24, H 5.03,
N 8.74, Experimental: C 71.28, H 5.09, N 8.76.
All chemicals were purchased from Sigma–Aldrich, Alfa Aesar
or from Spectrochem and used without further purification. All the
solvents were procured from SD Fine, India of HPLC grade and used
without further distillation.
1H NMR (DMSO-d6)
8.56 (s, 1H, –CH),
J = 8.5 Hz), 8.04–8.03 (m, 1H, ArH),
7.64–7.60 (m, 3H, ArH), 7.19 (d, 1H, ArH, J = 8.5 Hz),
d
12.22 (s, 1H, –NH),
8.26 (d, 1H, ArH, J = 9.0 Hz),
7.81 (d, 1H, ArH, J = 8.5 Hz),
7.07 (d, 1,
d 6.87 (t, 1H, ArH J = 8.25 Hz), d 3.83 (s, 3H, –CH3).
d
10.89 (s, 1H, –OH),
d
The 1H NMR spectra were recorded on a Bruker, Avance II
(500 MHz)instrumentusingTMSasinternalreferenceandDMSO-d6
as solvent. Resonance multiplicities are described as s (singlet), br s
(broad singlet), d (doublet), t (triplet), q (quartet) and m (multiplet).
Melting points were measured on a Stuart-SMP3 melting-point
apparatus in open capillaries. Infrared spectra were recorded on a
ThermoNicoletAvatar-330FT-IRspectrometer;signaldesignations:
s (strong), m (medium), w (weak), br.m (broad medium) and br.w
(broad weak). UV–vis spectroscopy was carried out with Ocean
Optics SD2000-Fibre Optics Spectrometer and Analytikjena Specord
S600 Spectrometer in standard 3.5 mL quartz cells (2 optical
windows) with 10 mm path length. Elemental analyses were done
using Flash EA1112 CHNS analyzer (Thermo Electron Corporation).
d
d
8.13 (d, 1H, ArH,
d
d
d
ArH, J = 8.0 Hz),
FT-IR in cmꢀ1: 3170.3 (w), 3041.1 (w), 2856.2 (w), 1640.0 (s),
1564.5 (m), 1468.5 (m), 1245.8 (s), 722.8 (w), 659.5 (w).
4.5. (E)-N0-propylidene-1-naphthohydrazide (R3)
Yield: 88%; m.p. 191–192 8C.
Elemental analysis: Calculated for C14H14N2O: C 74.31, H 6.24,
N 12.32, Experimental: C 74.23, H 6.21, N 12.41.
1H NMR (DMSO-d6)
ArH), 8.07 (d, 1H, –CH, J = 8.5 Hz),
7.67–7.64 (m, 2H, ArH), 7.60–7.57 (m, 3H, ArH),
2H, –CH2), 1.08 (t, 3H, –CH3, J = 7.5 Hz).
d
11.58 (s, 1H, NH),
8.01–8.7.98 (m, 1H, ArH),
2.33–2.28 (m,
d 8.16–8.12 (m, 1H,
d
d
d
d
d
4.2. Synthesis of target receptors R1, R2 and R3 and characterization
d
FT-IR in cmꢀ1: 3181.1 (br.m), 3034.6 (m), 2068.0 (m), 1870.3
(br.w), 1633.6 (s), 1549.1 (s), 1352.8 (m), 1295.6 (m), 1252.7 (m),
1147.4 (w), 777.6 (s).
The synthesis of receptors R1, R2 and R3 were achieved by a
straightforward process (Scheme 5). The structures were charac-
terized by standard spectroscopic methods.
Intermediate 1 was synthesized according to the reported
procedure in literature [67].
MS (ESI): m/z: Calculated: 249.2635 [M+Na]+, Experimental:
249.1578 [M+Na]+.
A mixture of aldehyde (2.69 mmol) and 1-naphthohydrazide 1
(2.69 mmol) reacted in ethanol under reflux for 5 h. The reaction
was catalyzed by a drop of acetic acid. After cooling, the formed
solid was filtered and washed with ethanol to obtain the target
compounds (R1, R2 and R3). All the synthesized compounds were
solid. Their structures were confirmed by 1H NMR, FT-IR, elemental
analysis and mass spectra.
Acknowledgements
The authors acknowledge the Director and the HOD (Depart-
ment of Chemistry), NITK, for providing the research infrastruc-
ture. MP is thankful to NITK for the research fellowship. DRT
acknowledges Department of Science and Technology (DST,
Government of India, New Delhi) for the financial support. MP
and DRT thank Optoelectronics Laboratory, Department of Physics,
NITK and CSMCRI, Bhavnagar for the spectral analysis.
H
H
O
N
O
N
NH2
N
X
EtOH, H+
80 oC
O
+
X
Appendix A. Supplementary data
1
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jfluchem.2014.
01.009.
R3
R2
R1
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X=
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Scheme 5. Synthesis of the receptors R1, R2 and R3.

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