Colorimetric and fluorescence sensing of fluoride anions with potential salicylaldimine based schiff base receptors

Article abstract of DOI:10.1016/j.saa.2009.12.077

Salicylaldimine based schiff base receptors with different substituents showing fluorescent enhancement in the presence of fluoride anion was visualized through naked eye as well as by change in spectral properties (UV-vis and fluorescent techniques). The reason for such fluorescence enhancement may be due to hydrogen bond interaction between receptor recognition site and fluoride anion. Such a hydrogen bond interaction creates a six-membered transition state, which avoids quenching processes. To support this, fluorescence enhancement factor (FEF) was calculated and it was found to be more (FEF = 652) for -NO₂ substituted receptor compared to other receptors.

Full text of DOI:10.1016/j.saa.2009.12.077

Spectrochimica Acta Part A 75 (2010) 1146–1151
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Colorimetric and fluorescence sensing of fluoride anions with potential
salicylaldimine based schiff base receptors
Radhakrishnan Sivakumar, Vijayaragavan Reena, Nallamuthu Ananthi, Muthiya Babu,
Sambandam Anandan∗, Sivan Velmathi∗∗
Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Salicylaldimine based schiff base receptors with different substituents showing fluorescent enhancement
in the presence of fluoride anion was visualized through naked eye as well as by change in spectral
properties (UV–vis and fluorescent techniques). The reason for such fluorescence enhancement may be
due to hydrogen bond interaction between receptor recognition site and fluoride anion. Such a hydrogen
bond interaction creates a six-membered transition state, which avoids quenching processes. To support
this, fluorescence enhancement factor (FEF) was calculated and it was found to be more (FEF = 652) for
–NO2 substituted receptor compared to other receptors.
Received 16 July 2009
Received in revised form
15 December 2009
Accepted 31 December 2009
Keywords:
Schiff base
Anions
© 2010 Elsevier B.V. All rights reserved.
Fluoride sensors
Fluorescence enhancement
1. Introduction
interaction with anions which influence the photo-physical and
electronic transitions within the receptors. It has thus become a
Designing ion-selective molecular sensors for sensing of anions
and cations is a challenging object in supramolecular chemistry,
in view of the fact that ions play a major role in many chemical
and biological processes. An advance in the field of sensor com-
prises investigation of molecular recognition at model interfaces
which provides great insights into biological phenomena and mod-
ern applications such as high selectivity sensors [1]. Among the
biologically important ions, fluoride anion is a common ingredient
in hypnotics, anesthetics, psychiatric drugs and cockroach poisons
and is a contaminant in drinking water. Excess fluoride ion expo-
sure causes fluorosis, thyroid activity depression, bone disorders
and immune system disruption [2]. Besides the above signifi-
cance, detection of fluoride ion with the use of simple preparation
and minimal instrumental assistance is desirable toward practical
applications. Colorimetric chemosensors based on optical and spec-
tral changes upon recognition with ions are particularly attractive
because of their simplicity and high detection limit of fluores-
cence detection methods [3–7]. Nevertheless in most of the cases
[8–11] fluorescence quenching rather than fluorescent enhance-
ment was also observed. The reason for fluorescence quenching
was mainly attributed to the nature of the adduct formed upon
demand for the development of fluorescent “turn on” sensors for
anions of biological importance. Kim and Ahn [12] observed the fact
that, the fluorescence enhancement of the receptors in the presence
of anions was likely owing to the elimination of n–␲* transitions
within the receptor which seems to intervene the ␲–␲* transition
levels which are responsible for emission. Hence, recognition of
receptors towards anions not only involves color changes but also
affects the electronic transitions within the molecule. For this rea-
son, the receptors have to be suitably designed in such a way that
it contains both anion recognition site and fluorophore moiety.
In biological systems, anion recognition is carried out through
hydrogen bonding by proteins with sterically well-defined com-
plex cites in the interior of proteins [13,14]. The neutral type anion
sensors generally consist of heterocyclic units such as pyrrole, pyri-
dine and indole and other units like amide, amine and phenol in
the recognition sites for hydrogen bonding [15,16]. Many reports
have appeared describing the synthesis and study of very sophis-
ticated anion sensors [17–21]. This is because, anions seems to
interact with a hydrogen bond donors of a receptor (NH2, –OH,
–C(O)NHR) more strongly. Among the available anions F− anion
shows better interaction than any other anions. To the best of our
knowledge, only few research groups were working in this field
with receptors having –OH group as a binding site [22–30]. On the
basis of above mentioned background, here we report the fluores-
cence sensing of fluoride anions with receptors having –OH group
as binding site (salicylaldimine based schiff base receptors with
different substituents).
∗
Corresponding author. Tel.: +91 431 2503639; fax: +91 431 2500133.
Corresponding author. Tel.: +91 431 2503640; fax: +91 431 2500133.
E-mail addresses: sanand@nitt.edu (S. Anandan), velmathis@nitt.edu
∗∗
(S. Velmathi).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.12.077

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1147
application. In the FT-IR spectroscopy we get information about
the presence of functional groups in the compound such as C C,
CH N, etc. by the position of absorption peaks which arise due to
stretching vibration of the bonds in the groups. The presence of C
N
in the molecule is confirmed by the vibrations between 1690 and
1640 cm−1 and aromatic C C between 1600 and 1473 cm−1. These
vibrations are seen in the IR spectra of all the compounds synthe-
sized and the data obtained are given in the experimental section.
NMR spectroscopy is one of the principal techniques which give
us the structural information about molecules. NMR spectra were
obtained using CDCl3 as the solvent. All the three compounds gave a
singlet at around ı 8.6–9.0 ppm corresponding to the CH N proton
indicating the formation of imine and the aromatic protons res-
onate in the ı 6.9–8.2 region In addition the OH proton of aromatic
ring is shown as a singlet at around ı 12.3–12.6 ppm.
Fig. 1. Structure of the receptors 1–3.
3.1. 1H NMR spectroscopic studies
2. Experimental
To understand the molecular interactions between the receptor
recognition site and fluoride anion, 1H NMR titration experiments
were done with receptor 3 in the presence and in the absence
of TBAF in DMSO-d6. Fig. 2 shows the 1H NMR spectrum of the
receptor 3 in the absence and in the presence of 1 and 3 equiv.
TBAF. In the case of neat receptor 3 i.e. in the absence of TBAF,
–OH proton appears as a singlet at 12.3 ppm and the imine pro-
ton (CH N) comes at 9.0 ppm as a sharp singlet, whereas in the
presence of 1 equiv. of TBAF, the singlet at 12.3 ppm disappeared
immediately indicating the hydrogen bond interaction between
fluoride anion and –OH group, consequently an up field shift
from 9.0 to 8.9 ppm in the imine proton was also observed. The
disappearance of the phenolic OH upon the addition of 1 equiv.
of fluoride anion may be due to the simultaneous formation of
the alkoxide and its hydrogen bonding with the F− ion. In the
presence of 3 equiv. of fluoride ions an additional sharp peak at
16 ppm appeared which is due to the deprotonation of the chro-
mogenic receptor and the subsequent formation of [HF2−] species.
From the NMR spectra, we inferred that complete disappearance
of the –OH singlet upon addition of 1 equiv. of TBAF indicates
strong H-bond interaction between the receptor recognition site
and fluoride anion. This may be due to high sensing ability of the
receptors.
2.1. Materials and methods
All reagents were purchased from Sigma Aldrich and used as
received. Analytical Grade solvents were used as such. 1H NMR
spectra were recorded on a JEOL LA600 (600 MHz) in CDCl3 at 298 K
with TMS as internal standard. FT-IR spectra were measured on a
Perkin–Elmer FT-IR spectrometer using KBr plates. UV–visible and
Fluorescence spectra were recorded in 1 cm path length quartz
cell on a Perkin Elmer EZ301 spectrophotometer and Shimadzu
RF-5301 PC spectrofluorophotometer, respectively.
2.2. General experimental procedure for the synthesis of the
receptors
Typically to 4-nitro aniline (0.2762 g, 2 mmol) dissolved in
10 ml methanol, salicylaldehyde (0.2426 g, 2 mmol) dissolved in
5 ml was slowly added and stirred for 3 h at room temperature.
Completion of the reaction was monitored through TLC for the
disappearance of the starting compounds. Then the solvent was
removed through rotovac yielding the reddish yellow crystals of N-
(4-nitrophenyl) salicylaldimine (receptor 3) yield: 98.3%. The solid
thus obtained was dried in vacuum oven. M. Pt 130–132 ◦C. Sim-
ilarly following the above procedure the receptors 1 and 2 were
synthesized.
3.2. Colorimetric analysis
In order to deduce the anion sensing ability of the receptors 1–3
with halide anions (F−, Cl−, Br− and I−) titrations were carried out
in different solvents namely CHCl3, CH3CN and DMSO. The change
in optical and optoelectronic properties was monitored by visual
(naked-eye), absorption and fluorescence techniques.
2.2.1. Receptor 1
1H NMR (CDCl3, ıppm) 7.0–7.2 m 2H, 7.3–7.5 dd 4H, 8.2–8.4 d
2H (aromatic), 8.6 s, 1H (CH N), 12.5 s, 1H (OH).
IR (KBr plates, cm−1): 1267, 1345, 1463, 1630, 3070, 3427.
First, the halide anions (F−, Cl−, Br− and I−) were added as
tetrabutylammonium salts to 5 × 10−5 M solutions of the receptors
in acetonitrile. In the naked eye experiments, the receptors 1, 2
and 3 (5 × 10−5 M) showed dramatic color change from colorless
to pale yellow, fluorescent yellow and orange color, respectively
in the presence of tetrabutyl ammonium fluoride (0.2 equiv.) in
acetonitrile. The reason for this color change was probably due
to the formation of hydrogen bond interactions between the –OH
groups of the phenyl ring of salicylaldimine and fluoride ions. This
is because, fluoride ions have higher electro negativity and small
size [31,32] interacts with hydroxyl group through intermolecu-
lar hydrogen bond (O–H–F) which will affect the optical properties
of the receptors. However the receptors 2 and 3 showed remark-
able color change compared to the receptor 1 as seen in Fig. 3a–c.
These observations are due to the presence of chromogenic signal-
ing units (−Cl and −NO2) in the aniline ring of the receptors 2 and
3 while compared to the receptor 1. All the receptors were found
2.2.2. Receptor 2
1H NMR (CDCl3, ıppm) 6.9–7.1 m 2H, 7.2 m 2H, 7.4 m 4H (aro-
matic), 8.6 s 1H, 12.6 s, 1H (OH).
IR (KBr plates, cm−1): 1272, 1374, 1484, 1600, 3071, 3481.
2.2.3. Receptor 3
1H NMR (CDCl3, ıppm) (aromatic) 6.9 m 2H, 7.2 m 1H, 7.4 m 5H,
7.6 d 1H (aromatic), 9.0 s 1H (CH N), 12.3 s, 1H (OH).
IR (KBr plates, cm−1): 1272, 1395, 1485, 1611, 3056, 3450.
3. Results and discussion
The chromogenic receptors 1–3 (Fig. 1) were synthesized by
schiff’s base condensation of salicylaldehyde and aniline (1) or 4-
chloroaniline (2) or 4-nitroaniline (3). They were well characterized
by FT-IR, 1H NMR spectroscopic methods before using them in

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R. Sivakumar et al. / Spectrochimica Acta Part A 75 (2010) 1146–1151
Fig. 2. 1H NMR (DMSO-d6) spectra of the receptor 3 in the presence (a) 3 equiv., (b) 1 equiv., and (c) absence of tetrabutyl ammonium fluoride.
to be insensitive even on addition of large excess of Cl−, Br− and I−
(up to 100 equiv.).
mum wavelength of ␲–␲* transitions for the compounds 1–3 are all
almost within 207–355 nm, there is no absorption band in the vis-
ible region. The bands in the region 207–230 nm could be assigned
to excitation of the ␲ electrons of the aromatic system. The band
at 300 nm for receptors (1 and 2) and at 320 nm for receptor 3
is due to the transition between the ␲ orbital localized on the
azomethine group (C N). The band in the region around 340 nm is
due to existence of charge transfer transition within the molecule
[33]. However, upon addition of fluoride anion to all the receptors
immediate color change was noticed through naked eye, which on
electronic (UV–vis) spectroscopic analysis we find a new peak in
the visible region with noticeable peak intensity changes in the
UV region. The new peak above 400 nm for the receptors 1–3 were
mainly attributed to the formation of keto form [34] of the receptors
resulting from hydrogen bonding interaction with fluoride anion as
shown in Fig. 5. In case of receptors 2 and 3 the color of the solu-
tion become more intense upon increasing the concentration of
the fluoride anion whereas for receptor 1 the color change was not
appreciable due to the absence of chromogenic signaling unit. How-
ever, for all the receptors the increase in the intensity of the peak
in visible region reaches a limit after the addition of 2 equiv. of F−.
Upon successive addition of fluoride anion (from 0.2 to 2 equiv.) to
receptor 1, the intensity of the peaks at 300, 316 and 336 nm grad-
ually decreases, while the intensity of the peaks at 225 and 269 nm
gradually increases. A new peak (420 nm) in the visible region was
noticed upon addition of 0.6 equiv. of the fluoride anion for receptor
1 before that no peaks in the visible region. Similarly for receptor 2,
with the addition of 0.4 equiv. of fluoride anion causes a new peak
at 420 nm and it increased stepwise up to 2 equiv. Also from the
absorption spectrum, we noticed the intensity of the peaks at 270,
300, 318 and 340 nm were decreased and at 230 nm get increased
Similar observable color changes also took place in CHCl3 and
DMSO solutions of the receptors. The receptors 1 and 2 became
pale and fluorescent yellow in both chloroform and DMSO medium
upon addition of fluoride ions, while for receptor 3 the color
changes to pink color in DMSO and orange color in CHCl3. No
color change (CHCl3 and DMSO medium) was observed in the pres-
ence of chloride, bromide and iodide ions as similar to acetonitrile
medium.
3.3. UV–vis spectroscopic studies
The change in optoelectronic properties of receptors 1–3 in
the presence of fluoride anion was investigated by UV–vis spec-
troscopic methods. The titrations were carried out in acetonitrile
medium at 5.0 × 10−5 M concentrations of receptors 1–3 upon the
addition of incremental amounts of 0.02 ml (1.5 × 10−3 M) of tetra-
butylammonium fluoride, the spectra of the receptors are shown
in Fig. 4a–c. The UV–vis absorption data of the receptors 1–3 were
listed in Table 1. From the electronic absorption spectra of the
receptors 1–3 in the absence of anions, it is found that the maxi-
Table 1
UV–visible spectral data for neat receptors 1–3
in acetonitrile medium (5 × 10−5 M).
Receptor
ꢀmax (nm)
1
2
3
336, 316, 300, 269, 225, 207
340, 318, 270, 230
355, 321, 214

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1149
Fig. 4. Absorption spectra of the receptors 1, 2 and 3 (a, b and c, respectively)
recorded in acetonitrile (5.0 × 10−5 M) after addition of 0–2 equiv. of tetrabutylam-
monium fluoride.
Fig. 3. (a) Color changes of receptor 1 in acetonitrile (5.0 × 10−5 M) before and after
the addition of 2 equiv. of representative anions (from left to the right: R, R + F−
,
3.4. Fluorescence spectroscopic studies
R + Cl−, R + Br−, R + I−). (b) Color changes of receptor 2 in acetonitrile (5.0 × 10−5 M)
before and after the addition of 2 equiv. of representative anions (from left to the
right: R, R + F−, R + Cl−, R + Br−, R + I−). (c) Color changes of receptor 3 in acetonitrile
(5.0 × 10−5 M) before and after the addition of 2 equiv. of representative anions (from
left to the right: R, R + F−, R + Cl−, R + Br−, R + I−).
In order to learn more about the sensing ability of the receptors,
fluorescence measurements were carried out similar to UV–visible
measurements. The emission maximum (ꢀemi) of the receptors
(1 × 10−3 M) was around 490 nm for all the three receptors. It was
reported that the enol form of the schiff base shows only very weak
fluorescence [35–37]. For this reason, the concentration depen-
dence of fluorescence spectra was investigated between 1 × 10−3
and 5 × 10−5 M for all receptors in study. At 5 × 10−5 M, the emis-
sion maximum of the receptors was red shifted and show broad
upon addition of fluoride anion. However, for receptor 3, a new
peak (487 nm) was observed just upon addition of 0.2 equiv. of flu-
oride anion with decrease in the intensity of absorption bands at UV
region. This suggests that, the presence of nitro group in the recep-
tor 3 acts as an excellent signaling unit compared to other receptors.
Exposure of receptors with other anions (chloride, bromide and
iodide) did not result in any spectral changes in the absorption
spectrum.
From the UV–vis absorption measurements the binding con-
stant (Ka) of the fluoride complexes of the receptors 1, 2 and
3 were calculated from the variation in the absorbance at 420,
420 and 48l nm, respectively and were found to be 2.787 × 103,
5.684 × 103 and 2.53 × 104. This suggests that the receptor 3 forms
strong binding with fluoride anion compared to other recep-
tors.
Fig. 5. The possible structure of the complex formed between receptor 3 and fluo-
ride anion.

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R. Sivakumar et al. / Spectrochimica Acta Part A 75 (2010) 1146–1151
Fig. 7. Dependence of fluorescence intensity (I/Io) with respect to [F−]o/[R]o. Inset
shows the fluorescence response of the receptors 1–3 with 2 equiv. of TBAF.
the fluorescence intensity of the receptors in the presence and
in the absence of the anion and [F−]o and [R]o are the concen-
tration of the anion and receptors, respectively. From this plot,
the fluorescent enhancement factor was calculated. Particularly,
the –NO2 substituted receptor show large enhancement factor of
652, when compared to receptors 1 [FEF = 79] and 2 [FEF = 16].
This suggests that, among the three receptors, Receptor 3 shows
maximum sensitivity to the anion which may due to the presence
of strong electron withdrawing substituent (−NO2). The reason
for fluorescence enhancement in the presence of nitro group may
be explained on the basis of electron transfer mechanism as fol-
lows: it has generally been recognized that emission was usually
accompanied by the delocalization of the electrons within the
molecules. In the case of salicylaldimine based schiff base recep-
tors the excited state of the fluorophore was primarily quenched by
reduced electron transfer from receptor (–OH) to the fluorophore
(–C N) unit. However, upon interaction with anions, the electron
transfer from the electron rich –OH moiety bonded with anion to
the electron deficient –NO2 became more feasible and hence flu-
orescence enhancement was observed. Upon further addition of
anion it appeared that the deprotonated species being more elec-
tron rich compared to the hydrogen bonded complex with fluoride,
undergo very fast electron transfer showed up greater fluorescence
enhancement. These findings open up the way to design and syn-
thesis a new efficient ion-selective molecular sensor for biological
molecules containing fluoride ions.
Fig. 6. Emission spectra of the receptors 1, 2 and 3 (a, b and c, respectively) recorded
in acetonitrile (5.0 × 10−5 M) after addition of 0–2 equiv. of tetrabutylammonium
fluoride. Excitation wavelength (ꢀexi) for the receptors 1, 2 and 3 was 358, 380 and
360 nm, respectively.
4. Conclusion
In conclusion, we have systematically prepared and studied
the fluorescence sensing properties of salicylaldimine based schiff
base receptors via optical and spectroscopic techniques. Among the
three receptors –NO2 substituted receptor shows high fluorescence
enhancement. Fluorescence enhancement factor (FEF) calculations
lend further support for the strong affinity and high selectivity
towards fluoride ion for salicylaldimine based schiff base recep-
tors. The molecular interactions between schiff base receptors and
anions are really important because the knowledge base in the field
of chemosensors and fluorescence “turn on” sensors still needs to
be improved before going for applications.
emission above 500 nm for all the three receptors with dimin-
ished fluorescence intensity. However, the addition of fluoride
anion to the receptors markedly enhances the emission intensity
as shown in Fig. 6a–c (upon adding 0.02 ml of fluoride anion to
the receptors 1, 2 and 3 showed emission maxima at 475, 475 and
472 nm in acetonitrile solution, respectively) whereas no emission
was observed with other anions. Further, the emission intensity
increases on increasing the concentration of the fluoride anions
with marginal changes in their emission maxima. This observed
enhancement in the fluorescent intensity upon addition of flu-
oride anion may be due to the formation of new geometrically
restricted six-membered transition state as shown in Fig. 5. That
is upon addition of fluoride anion to the receptors, an increase in
the conformational restriction occurs due to delocalization of the
charge created with in the molecule. Thus, the increase in rigid-
ity of the system makes the non-radiative decay from the excited
state less probable, consequently, the emission intensity increases.
Fig. 7 shows the dependence of fluorescence intensity (I/Io) with
respect to [F−]o/[R]o for the three receptors, where I and Io are
Acknowledgements
Authors thank Dr. M. Chidambaram, Director, NITT for his con-
stant support and encouragement for research. Authors also, thank
DST, New Delhi (SR/FST/CSI-066/2008 dated 12.01.2009) for the
sanction of major research fund for improvement of Science and
Technology (FIST) for the development of the department.

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1151
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