Anion induced azo-hydrazone tautomerism for the selective colorimetric sensing of fluoride ion

Article abstract of DOI:10.1016/j.saa.2014.02.200

The design, synthesis, characterization and their anion sensing properties of two receptors capable of exhibiting azo-hydrazone tautomerism are reported. The anion sensing properties have been investigated using electronic, fluorescence and nuclear magnet

Full text of DOI:10.1016/j.saa.2014.02.200

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 798–805
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Anion induced azo-hydrazone tautomerism for the selective colorimetric
sensing of fluoride ion
A. Satheshkumar a, E.H. El-Mossalamy b,c, R. Manivannan a, C. Parthiban a, L.M. Al-Harbi b, S. Kosa b,
Kuppanagounder P. Elango a,
⇑
a Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624302, India
b Chemistry Department, Faculty of Science, King Abdul-Aziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
c Chemistry Department, Faculty of Science, Benha University, Benha 13518, Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
Anion sensing through azo-hydrazone
type tautomerism.
Mechanism of sensing involves anion
induced change of chromophore from
C@N to N@N.
ꢀ
The receptors are highly selective
towards fluoride ion over other
anions.
a r t i c l e i n f o
a b s t r a c t
Article history:
The design, synthesis, characterization and their anion sensing properties of two receptors capable of
exhibiting azo-hydrazone tautomerism are reported. The anion sensing properties have been investigated
using electronic, fluorescence and nuclear magnetic spectral studies in addition to electrochemical and
visual detection experiments. Both the receptors selectively bind fluoride ion with >100 nm red-shift
in the electronic spectrum and the color changes from yellow to red. The results of the spectral studies
revealed that the sensing mechanism involves fluoride ion induced change of chromophore from C@N
(hydrazone form) to N@N (azo form) in these receptors leading to the visible color change. Density
Functional Theory calculations were conducted to rationalize the optical response of the receptors.
Ó 2014 Elsevier B.V. All rights reserved.
Received 30 November 2013
Received in revised form 21 February 2014
Accepted 23 February 2014
Available online 13 March 2014
Keywords:
Fluoride ion
Sensors
Tautomerism
Chromophore
Acenaphthenequinone
DFT calculation
Introduction
intake (>1.5 mg/L) [1]. Review of literature revealed that the
fluoride ion sensors fall on either one of the following types namely
Recently the studies on chemosensors for anions have received
continuous attention as anions play important role in many fields.
Fluoride ion is of particular interest due to its importance in dental
care, treatment of Osteoporosis and its toxicity due to excess
NAH based H-bonding, Lewis acid, metal ion template, metal
complexes, guest displacement and chemoreactants [2]. However,
only very few anion sensors based on tautomerism has been
reported so far [3].
Azo dyes are the largest group of colorants with respect to num-
ber and production volume, with applications in many fields for
coloring variety of substrates [4]. Azo-hydrazone tautomerism is
⇑
Corresponding author. Tel.: +91 451 245 2371; fax: +91 451 245 4466.
E-mail address: drkpelango@rediffmail.com (K.P. Elango).
http://dx.doi.org/10.1016/j.saa.2014.02.200
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

A. Satheshkumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 798–805
799
a phenomenon that occurs in azo dyes possessing substituent con-
jugated to the azo linkage which has labile proton that can be ex-
changed intramolecularly. A typical example of such tautomerism
exist in 2-phenylazo-1-naphthol is shown below [5].
techniques in addition to electrochemical and visual detection
experiments. Theoretical computations, based on the Density
Functional Theory, have also been carried out to get a better insight
into the mechanism (tautomerism) of sensing. The survey of liter-
ature revealed that only very few works has been published with
similar type of compounds. But either they are less studied or
not selective towards fluoride ions [13–16].
N
H
N
H
N
N
O
O
Experimental section
Hydrazone form
Azo form
Chemical and apparatus
All reagents for synthesis of the receptors were obtained com-
mercially (Sigma Aldrich, India) and were used without further
purification. All anions, in the form of tetrabutylammonium salts,
and spectroscopic grade solvents (Merck, India) were used as
received. UV–Vis spectral studies were carried out in DMSO on a
(Jasco V-630) double beam spectrophotometer. Steady state
This over 100 year’s old tautomerism has been extensively
studied because the azo and hydrazone tautomers show different
optical and physical properties. The color and properties of one
tautomer may be completely different from the other [6]. Hence,
it is presumed that a chemosensor with similar structural features
would exhibit the anion sensing behavior. This is because when the
added anions abstract the proton, from one tautomeric form, that
will be converted to other tautomer and consequently exhibit
striking color change for visual detection. With this idea in mind
the following sensors have been synthesized and screened for their
anion sensing behavior. The selection of urea moiety as the recep-
tor unit is based on the fact that it out numbered all other types
and in many cases they exhibit high selectivity for fluoride ion over
the other anions [7–12]. As a result of anion sensing we expect the
following azo-hydrazone type tautomerism in these compounds
which would lead to observable color change.
fluorescence spectra were obtained on
a spectrofluorometer
(Jasco-FP-6200). The excitation and emission slit width (5 nm)
and the scan rate (250 nm) was kept constant for all of the exper-
iments. FT-IR spectra were obtained as KBr pellets on FT-IR spec-
trometer (Jasco 460 plus). Nuclear magnetic resonance spectra
were recorded in DMSO-d6 (Bruker, 1H NMR 300 MHz, 13C NMR
75 MHz). The 1H NMR spectral data is expressed in the form:
Chemical shift in units of ppm (normalized integration, multiplic-
ity, the value of J in Hz). Differential pulse voltammetry was per-
formed with a conventional three electrode system consisting of
a glassy-carbon-disk working electrode which was polished using
0.05 lm (alumina/water slurry) and rinsed thoroughly with double
X
distilled water and DMSO. Solutions for DPV were typically 1.0 mM
in the redox-active species and were deoxygenated by purging
with nitrogen.
X
N
N
NH2
N
O
N
NH2
deprotonation
H
A
Synthesis of the receptors
O
Azo-form
The method of synthesis of the receptors is shown in Scheme 1.
hydrazone-form
Where
(Z)-1-(1-oxoacenaphthylen-2(1H)-ylidene)semicarbazide (1)
X=O; Receptor 1
X=S; Receptor 2
A-= Anion
To a stirred solution of acenaphthenequinone (1 g, 0.0054 mol)
in ethanol semicarbazide hydrochloride (0.6122 g, 0.0054 mol)
was added at RT under N2 atmosphere and then few droops of
Con.H2SO4 was added to the reaction mixture. The reaction mix-
ture was refluxed under N2 atmosphere for 5 h. The reaction was
monitored by TLC, after completion of the reaction; the reaction
mixture was cooled to RT and then filtered through the filter paper.
The settled residue was washed with cold ethanol (20 mL) to get
Also, with an aim to shed some light on the mechanism of this
proposed anion sensing the following compound (3) was also pre-
pared and applied as sensor in the present study. The selection of
the compound is based on the fact that it may possess different
H-bonding property and also the azo-hydrazone type tautomerism
may not occur in this compound.
the pure product as a yellow solid. FT-IR (KBr) m
/cmꢁ1: 3407,
3241(ANH2), 3145 (ANHA), 1687 (C@O), 1610 (NHAC@O), 1571
(C@N). 1H NMR (300 MHz, DMSO-d6) d 7.25 (s, 2H), 7.90 (m, 2H),
O
O
O
N
NH2
N
O
NH
O
N
Ethanol
H
ClH.H2N
+
N
H
NH2
5 h, 90 0C
N
O
1
3
HS
O
O
N
NH
S
N
Ethanol
5 h, 90 º C
H
H2N
+
N
H
NH2
The main objective of the present endeavor, therefore, is the
synthesis, characterization and anion sensing properties of these
three compounds (1–3). The anion sensing behavior has been
investigated using UV–Vis, fluorescence and 1H NMR spectral
O
2
Scheme 1. Preparation receptors 1 and 2.

800
A. Satheshkumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 798–805
7.95 (d, 1H, J = 6.9 Hz), 8.11 (t, 2H, J = 7.2 Hz & 6.3 Hz), 8.38 (d, 1H,
J = 8.4 Hz), 11.84 (s, 1H). 13C NMR (75 MHz, DMSO-d6) d 189.02,
155.69, 138.71, 137.06, 133.13, 130.96, 130.33, 129.31, 128.90,
126.91, 122.63, 118.10. LCMS (ESI-APCI) m/z: Cal. for C13H9N3O2,
239.0, Found, 240.0 [M+H]+.
Elemental analysis: Anal. Calcd. for C13H9N3O2: C, 65.27; H,
3.79; N, 17.56. Found: C, 65.39; H, 3.62; N, 17.44. M.P: 209–211 °C
Results and discussion
Facile condensation of acenaphthoquinone with semicarbazide
and thiosemicarbazide in ethanol gave receptors 1 and 2, respec-
tively in good yields. The receptors 1 and 2 were characterized
using UV–Vis, FT-IR, 1H and 13C NMR and LCMS spectral tech-
niques. Based on the results the following structures were
proposed for 1 and 2.
1-(1-Oxoacenaphthylen-2(1H)-ylideneamino)isothiourea (2)
O
SH
NH
To a stirred solution of acenaphthenequinone (1 g, 0.0054 mol)
in ethanol thiosemicarbazide (0.4921 g, 0.0054 mol) was added at
RT under N2 atmosphere and then few drops of Con.H2SO4 was
added to the reaction mixture. The reaction mixture was refluxed
under N2 atmosphere for 5 h. The reaction was monitored by
TLC, after completion of the reaction; the reaction mixture was
cooled to RT and then filter through the filter paper. The settled
residue was washed with cold ethanol (20 mL) to get the pure
NH2
N N
H
N N
H
O
O
2
1
product as a yellow solid. FT-IR (KBr) m
/cmꢁ1: 3477 (ANHA),
3280 (NH@C), 2638 (SAH), 1743 (C@O), 1689 (N@CAS), 1589
(C@N). 1H NMR (300 MHz, DMSO-d6) d 7.90 (m, 2H), 8.01 (d, 1H,
J = 6.6 Hz), 8.15 (m, 2H), 8.39 (d, 1H, J = 8.4 Hz), 8.84 (s, 1H), 9.13
(s, 1H) 12.52 (s, 1H). 13C NMR (75 MHz, DMSO-d6) d 189.00,
179.46, 139.65, 137.83, 133.22, 130.86, 130.46, 130.32, 129.36,
129.00, 127.54, 122.87, 118.83. LCMS (ESI-APCI) m/z: Cal. for C13H9-
N3OS, 255.0, Found, 256.0 [M+H]+.
Elemental analysis: Anal. Calcd. for C13H9N3OS: C, 61.16; H,
3.55; N, 16.46, S, 12.56. Found: C, 61.21; H, 3.58; N, 16.39, S,
12.47. M.P: 220–224° C.
Visual detection
Visual inspection of solutions of the compounds 1 and 2
(1.25 ꢂ 10ꢁ4 M) in DMSO before and after addition of 1 eqv. of tet-
rabutylammonium salt of various anions such as Fꢁ, Clꢁ, Brꢁ, Iꢁ,
NOꢁ3 , H2POꢁ4 , CNꢁ and AcOꢁ was carried out. As depicted in Fig. 1
solution of 1 and 2 turned yellow to1 intense red color after the
addition of fluoride ions. However, the color remained unchanged
after the addition of the other chosen anions. This observation indi-
cated the selectivity of 1 and 2 towards fluoride ion.
UV–Vis spectral studies
Acenaphtho[1,2-e][1,2,4]triazin-9(8H)-one (3)
The compound 1 was dissolved in 10 mL of acetic acid and
refluxed for 12 h, the reaction mixture was monitored by TLC
and then acetic acid was concentrated under reduced pressure.
The crude material was basified with saturated NaHCO3 solution
and extracted with ethyl acetate. The extracted organic layer was
washed with water and brine solution. The separated organic layer
was evaporated under vaccum to get the yellow colored powder.
The crude material was purified by (60–120 mesh) silica gel col-
umn chromatography by using 15% ethyl acetate-pet ether mixture
to get the pure.
The anion sensing ability of 1 and 2 with all the chosen anions
such as Fꢁ, Clꢁ, Brꢁ, Iꢁ, AcOꢁ, NO3ꢁ, H2POꢁ4 and CNꢁ were examined
by using UV–Vis spectral technique. The UV–Vis spectral changes of
1 and 2 upon addition of these anions are shown in Fig. 2. Likewise,
the intensity of absorbance of the 1-Fꢁ and 2-Fꢁ complex showed
no appreciable change in the presence of other anions (Fig. S1).
The electronic spectrum of the receptor 1 exhibited maximum
absorbance at 408 nm (e
= 3768 Mꢁ1 cmꢁ1) in DMSO corresponds
to the electronic transition in the imine (C@N) chromophore [17].
This observation also indicated that the receptor 1 exist in hydra-
zone form in DMSO solutions [17]. As evidenced from the Fig. 2A,
addition of fluoride ions to 1, bathochromically shifted the kmax to
1H NMR (300 MHz, DMSO-d6) d 8.28 (m, 2H), 8.13 (t, 2H,
J = 6 Hz), 7.90 (m, 2H), 6.16 (s, 1H). LCMS (ESI-APCI) m/z: Cal. for
C
13H7N3O, 221.0, Found, 222.0 [M+H]+.
517 nm with an instantaneous formation of red color with a
Dkmax
Elemental analysis: Anal. Calcd. for C13H7N3O: C, 70.58; H, 3.19;
N, 19.00. Found: C, 70.63; H, 3.17; N, 19.07. M.P: 190–193° C.
of 109 nm. The new peak appeared at 517 nm may corresponds to
the azo (N@N) chromophore [17]. However, addition of other an-
ions produced no significant shifts in kmax. Likewise, the receptor
2 also selectively senses Fꢁ ions with a formation of red color and
a concurrent shift in the kmax from 375 (
500 nm in DMSO with a kmax of 125 nm (Fig. 2B).
e
= 3898 Mꢁ1 cmꢁ1) to
D
With the addition of incremental amounts of fluoride ions to
the solution of both 1 and 2, the absorbance of the higher energy
peak (due to C@N chromophore) diminished gradually and that
of the lower energy peak (due to N@N chromophore) increased
gradually (Fig. 3). This observation suggested that addition of fluo-
ride ion might have converted the hydrazone form (with C@N
chromophore) of the receptors to the azo form (with N@N chromo-
phore). The stoichiometry of the interaction between the receptors
1 and 2 and fluoride ions was determined using the electronic
spectral data. The Job’s plot for 1 and 2 is shown in Fig. 4. In both
the cases curve with a maximum at 0.5 mol fraction indicated the
1
For interpretation of color in Figs. 1 and 2, the reader is referred to the web
Fig. 1. Color change of receptors 1 and 2 with anions.
version of this article.

A. Satheshkumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 798–805
801
0.6
0.9
B
A
0.5
0.4
0.3
0.2
0.1
0.0
2+F-
2+Other anions
0.6
0.3
0.0
1+Other anions
1+F-
200 300 400 500 600 700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. UV–Vis absorption changes of compound 1 and 2 (6.25 ꢂ 10ꢁ5 M) upon addition of [(Bu)4N] salts of Fꢁ, CNꢁ, Clꢁ, Brꢁ, Iꢁ, AcOꢁ, NOꢁ3 , H2POꢁ4 in DMSO.
0.8
0.6
0.4
0.2
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
A
B
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. UV–Vis absorption spectra of 1 and 2 (6.25 ꢂ 10ꢁ5 M) with Fꢁ ion (0–8.50 ꢂ 10ꢁ6 M).
formation of a 1:1 (receptor:Fꢁ) complex [18]. The formation con-
stant for these complexes was determined using the Scott equation
[19]. The values thus obtained for 1.Fꢁ and 2.Fꢁ complexes (Fig. S2)
were found to be 7.7 ꢂ 103 and 24 ꢂ 103 L molꢁ1, respectively. The
relatively higher value obtained for 2.Fꢁ indicated that the interac-
tion between the receptor 2 with Fꢁ ions is stronger than that in
receptor 1. This is well supported by the observation made in
0.25
1
2
0.20
0.15
0.10
0.05
0.00
D
kmax with the addition of fluoride ions to the receptors.
Fluorescence study
The fluoride ion binding ability of the receptors 1 and 2 was also
evaluated using fluorescence titration studies. The fluorescence
spectra of the receptors 1 and 2 (6.25 ꢂ 10ꢁ5 M) in DMSO were
recorded with an excitation at 388 and 398 nm, respectively. The
fluorescence emission spectra of the receptors 1 and 2 showed a
broad peak at 420 nm and 430 nm respectively (Fig. 5). In both
the cases, upon gradual addition of fluoride ions to the receptor
solution, quenching of fluorescence of 1 and 2 was observed indi-
cating the complex formation between the receptors and fluoride
ions [20]. From the decrease in fluorescence intensity the binding
constant of the receptor-Fꢁ complex was calculated using the fol-
lowing equation [21].
0.0
0.2
0.4
0.6
0.8
1.0
[R]+[F]/[R]
Fig. 4. Job’s plot for 1 and 2.
1H NMR titration
The results of electronic and fluorescence spectral studies indi-
cated that both the receptors 1 and 2 sense fluoride ions by inter-
acting with it. To investigate the nature of the interaction between
them, 1H NMR titration experiments were carried out in DMSO-d6.
The results obtained are depicted in Fig. 6. It is evident from the
Fig. 6 that, in the receptor 1, the signal for NAH proton (hydrazone
proton) appeared at d = 11.84 ppm (Fig. 6A) while that in the
receptor 2, it appeared at d = 12.52 ppm (Fig. 6B). This indicated
that the H-atom in 2 is relatively more acidic than that in 1 and
thus can act as a better H-bond donor towards fluoride ion. Upon
addition of 0.25 eqv. of fluoride ions to 1, the signal due to this
proton exhibited a downfield shift. This may be due to the H-bond
logðF0 ꢁ F=FÞ ¼ log KA þ n log½Qꢃ
ð1Þ
where F0 is emission intensity in the absence of quencher (Q), F is
that at quencher concentration [Q] and KA is the binding constant.
In both the cases, a plot of log(F0ꢁF/F) versus log[Q] is linear
(Fig. S3). The KA values computed for the receptors 1 and 2 were
found to be 1.4 ꢂ 103 and 4.5 ꢂ 103 L molꢁ1, respectively. In line
with the observation made in the electronic spectral studies, the
fluorescence spectral studies also indicated that the 2.Fꢁ complex
is relatively stronger than 1.Fꢁ complex.

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A. Satheshkumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 798–805
500
a
750
B
a
a=0 M
a=0 M
A
b
400
300
200
100
0
b
600
450
300
150
0
c
c
d
-6M
-6
g=6.25x10
g=6.2510 M
d
e
e
f
g
f
g
400
410
420
430
440
450
400
425
450
475
Wavelength (nm)
Wavelength (nm)
Fig. 5. Change in fluorescence emission spectra for (A) receptor 1 (6.25 ꢂ 105 M), (B) receptor 2 (6.25 ꢂ 105 M) in DMSO upon the addition of TBAF in DMSO from (0–
6.25 ꢂ 106 M).
Fig. 6. Change in partial 1H NMR (300 MHz) spectra of (A) receptor 1 in DMSO-d6 with various concentration of Fꢁ; (B) receptor 2 with various concentration of Fꢁ.
formation between the NAH proton and fluoride ions. The results
depicted in the Fig. 6A indicated that with the incremental addition
of fluoride ions to 1, the NAH proton signal experienced progres-
sive downfield shift, simultaneously broadness and disappeared
at a fluoride ion concentration of 5 equivalent i.e. deprotonation
has occurred. In the case of 2 also, similar H-bond formation fol-
lowed by deprotonation of the NAH proton has occurred with
the addition of incremental amounts of fluoride ions to the recep-
tor solution. However, we did not observe any signal for HFꢁ2 ion.
This may be due to its instability in highly polar solvent like DMSO
fluoride ions while in 1, it happened at five equivalents of fluoride
ions.
Electrochemical studies
In order to substantiate the nature of the interaction between
the receptors and fluoride ions, the electrochemical properties of
1 and 2 in the absence and presence of variable concentrations of
fluoride ions were investigated using differential pulse voltamme-
try (DPV) technique. The DP voltammograms obtained for the
reduction of the ring carbonyl moiety in 1 and 2 are shown in
Fig. 7. It is evident from the figure that the reduction peak for 1
and 2 appeared at ꢁ1.45 and ꢁ1.25 V, respectively [23]. In both
the cases, on the addition of incremental amounts of fluoride ions,
the current intensity of the peak decreased with a concomitant
shift of the peak to a more negative potential. That is addition of
fluoride ions to these receptors made the electro-reduction of the
[22]. It is interesting to note that the
Dd observed in 1 and 2 on
adding 0.25 equivalents of fluoride ions were found to 0.04 and
0.1 ppm, respectively. This indicated that the receptor 2 binds rel-
atively stronger with fluoride ion than the receptor 1 does. This is
because in the receptor 2, the NAH proton is relatively more acidic
than that in 1. This is well supported by the observation that in 2,
complete deprotonation occurred on adding two equivalents of

A. Satheshkumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 798–805
803
keto group relatively more difficult [24]. This may be due to the
-5.0x10-6
-1.0x10-5
-1.5x10-5
-2.0x10-5
-2.5x10-5
-3.0x10-5
-3.5x10-5
A
fact that deprotonation of the hydrazone NAH proton by fluoride
ions, render the O-atom (C@O) relatively more electron rich by
delocalization of electrons through the conjugated system. Such a
conversion of C@O to CAOꢁ (hydrazone form to azo form) render
the electro-reduction of the keto group progressively more
difficult.
f
e
d
F= 1.25x10-4 M
c
b
a
Theoretical study
F=0
DFT calculations [25] have been performed for the two receptors
and their complexes with fluoride ions. The optimized geometries
of 1 and 2 and their complexes 1.Fꢁ and 2.Fꢁ obtained at the
B3LYP/6-311G level are shown in Fig. S4. The optimized geome-
tries of the hydrazone and azo forms of 1 and 2 are given in
Fig. S5. As spelt in the introduction section, these receptors may
exhibit azo-hydrazone tautomerism. The calculated energies, in
the case of 1, for the hydrazone and azo forms are found to be
ꢁ511701.67 and ꢁ511721.90 hartree, respectively. Their relative
energies indicated that the hydrazone form is more stable than
azo form by 20.23 kcal molꢁ1. In the case of the receptor 2, the
energies of the hydrazone and azo tautomers are ꢁ714342.67
and ꢁ714356.08 hartree, respectively. The calculated results
showed that the hydrazone tautomer of 2 is 13.09 kcal molꢁ1 more
stable in energy than the azo tautomer. These results ruled out the
existence of the following tautomerism in 1 and 2 [26].
-1.4
-1.5
Potential (V)
-1.3
-1.6
-1.7
0.0
B
-5.0x10-6
-1.0x10-5
-1.5x10-5
-2.0x10-5
-2.5x10-5
-3.0x10-5
-3.5x10-5
-4.0x10-5
-4
F= 1.25x10 M
d
c
F= 0
b
a
-1.6
-1.5
-1.4
-1.3
-1.2
-1.1
X
X
Potential (V)
N
N
NH2
NH2
N
N
H
Fig. 7. The electrochemical sensing property of receptor 1 (A) and 2 (B) in the
presence of various concentration of [(Bu)4N]F in DMSO from 0–1.25 ꢂ 10ꢁ4 M.
OH
O
ions to these receptors, the formed N@N chromophore absorbs at
a higher wavelength in the electronic spectrum. The distance be-
tween H and F in the receptor–fluoride complexes is 0.97 Å which
is close to the bond length in hydrogen fluoride molecule (0.92 Å)
[28], suggesting a very strong interaction between the H-atom and
fluoride ion.
Hydrazone form
Azo form
Thus, these two receptors exist exclusively in the hydrazone
form. This is well supported by the results of UV–Vis and 1H
NMR spectral studies. The calculated spatial representations of
the frontier molecular orbitals (HOMO and LUMO) for these recep-
tors and their complexes with fluoride ions are depicted in Fig. 8.
The evenly distribution of HOMO and LUMO in 1 and 2 showed
that the intramolecular charge transfer character is only modest
[27,28]. Therefore, the electronic transition observed (Fig. 2) in
these receptors may be due to the C@N chromophore as discussed
in the UV–Vis spectral studies. Same is the case with the receptor–
fluoride complex also. The energies of the MOs and the energy
Mechanism of fluoride ion sensing
Based on the foregoing results and discussion the following
plausible mechanism has been proposed for the fluoride ion sens-
ing by these two receptors (Scheme 2). The UV–Vis and 1H NMR
spectral studies indicated that the receptors exist exclusively in
the hydrazone form. In the free receptors, the electronic transition
corresponds to the kmax is due to the C@N chromophore. The vari-
ous spectral and electrochemical investigations indicated that the
fluoride ion sensing by 1 and 2 is through the anion induced forma-
tion of azo form. Such an azo form formation involves the following
step: deprotonation of NAH proton by fluoride ion, delocalization
of electrons through the conjugated system and which lead to
the formation of N@N chromophore. This newly formed chromo-
phore absorbs at a different wavelength and thus imparts a notice-
able color change which can be seen visually. The results of the
spectral studies corroborate well with that of the DFT calculations.
With an aim to substantiate the proposed mechanism for the
sensing behavior of these two receptors, the effect of addition of
hydroxide ion has also been carried out as hydroxide ion is well
known for its deprotonation capacity. The electronic spectra of
the receptors 1 and 2 recorded in the presence of hydroxide ion
(as its tetrabutylammonium salt) are shown in Fig. S6. As seen
from the Figures, addition of one equivalent of fluoride and
hydroxide ions exhibited identical shifts in the absorption maxi-
mum of both the receptors. Such a similar electronic spectral
corresponds to the electronic transition
collected in Table 1. The results indicated that, in both the cases,
the magnitude of E in receptor–fluoride complex is less than that
D
E (= EHOMOꢁELUMO) are
D
in free receptor. Thus, in the receptor–fluoride complexes, the elec-
tronic transition occurred at a higher wavelength than in the free
receptor as observed in the UV–Vis spectra. In the complexes, the
resultant N@N chromophore absorbs at a higher wavelength
(Fig. 3.)
The calculated bond lengths of some important bonds are also
given in Table 1. In the free receptors the NAN bond length is close
to the standard single bond length (1.40 Å) [29]. This suggested
that these two receptors exist predominantly in hydrazone form.
One can see that, in the receptor–fluoride complexes, the NAN dis-
tance is shortened significantly. This is due to the fact that depro-
tonation of the NAH proton by fluoride ion converts the hydrazone
form to azo form wherein the NAN is a double bond. This is well
supported by the observation that, after the addition of fluoride

804
A. Satheshkumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 798–805
Fig. 8. Forntier MOs of free receptor and their complexes with fluoride ion.
behavior exhibited by 1 and 2 on the addition of hydroxide and
fluoride anions indicated that fluoride ion also results deprotona-
tion through initial H-bond formation. It is interesting to note that,
the electronic spectrum of 3 in the presence of either fluoride or
hydroxide ions exhibited no change in the absorption maximum
(Fig. S6). This is due to the absence of an anion induced azo-hydra-
zone tautomerism i.e. there is no generation of N@N chromophore.
And thus this compound failed to sense fluoride ions with notice-
able color change.
Table 1
Energies of selected MOs and lengths of selected bonds.
System HOMO
(eV)
LUMO
(eV)
D
(eV)
E
NAN
(Å)
NAH
(Å)
HAF
(Å)
1
1.F
2
ꢁ6.2518
ꢁ6.8377
ꢁ6.5272
ꢁ6.7688
ꢁ2.3453
ꢁ4.8461
ꢁ2.6150
ꢁ4.3671
3.9065
1.9916
3.9122
2.4017
1.3910
1.3083
1.3528
1.3125
1.0087
1.6779
1.0126
1.6456
–
0.9759
–
0.9790
2.F
Conclusion
For the first time, selective fluoride ion sensing via anion
induced change of chromophore from C@N to N@N (azo) in two
receptors has been investigated and reported. The mechanism of
‘azo-hydrazone’ tautomeric signaling has been supported by
UV–Vis, fluorescence and 1H NMR spectral studies in addition to
electrochemical and theoretical investigations.
O
O
NH2
N
O
NH2
N
H
N
N
F
-
O
1
Acknowledgement
HS
This project was funded by the Deanship of Scientific Research
(DRS), King Abdul Aziz University, Jeddah. The authors, therefore,
acknowledge with thanks DRS technical and financial support.
HS
NH
N
O
N
H
N
NH
N
-
F
O
Appendix A. Supplementary material
2
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.02.200.
Scheme 2. Mechanism of fluoride ion sensing. .

A. Satheshkumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 798–805
805
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