Spectroscopic and computational study of a naphthalene derivative as colorimetric and fluorescent sensor for bioactive anions

Article abstract of DOI:10.1007/s10895-013-1178-x

The anion recognition property of a naphthalene based receptor (L) was investigated by naked-eye, UV-Vis, fluorescence, ¹H NMR and computational methods. The receptor L showed fluoride selective naked-eye detectable colorimetric and UV-Vis spectral changes over other tested anions due to the formation of hydrogen bonding complex in 1:1 stoichiometry and/or deprotonation between fluoride and the receptor. Interestingly, the fluorescence of L was quenched by fluoride but enhanced by acetate.

Full text of DOI:10.1007/s10895-013-1178-x

J Fluoresc
DOI 10.1007/s10895-013-1178-x
SHORT COMMUNICATION
Spectroscopic and Computational Study of a Naphthalene
Derivative as Colorimetric and Fluorescent Sensor
for Bioactive Anions
Darshna Sharma & Suban K. Sahoo & Rati Kanta Bera &
Raj Kamal
Received: 26 October 2012 /Accepted: 24 February 2013
#
Springer Science+Business Media New York 2013
Abstract The anion recognition property of a naphthalene
based receptor (L) was investigated by naked-eye, UV-Vis,
fluorescence, H NMR and computational methods. The
receptor L showed fluoride selective naked-eye detectable
colorimetric and UV-Vis spectral changes over other tested
anions due to the formation of hydrogen bonding complex
in 1:1 stoichiometry and/or deprotonation between fluoride
and the receptor. Interestingly, the fluorescence of L was
quenched by fluoride but enhanced by acetate.
essential in the formation of majority of enzyme-
substrate and enzyme-cofactor complexes as well as in
the interactions of proteins with DNA or RNA. Among
the different bio-active anions, fluoride and acetate are
of prime importance. Acetate acts as a critical compo-
nent in numerous metabolic processes [8, 9]. On the
other hand, interest in the detection and recognition of
fluoride ion is because of its significant role in dental
caries, clinical treatment for osteoporosis, toxicity resulting
from it’s over accumulation in the bone, and association with
hydrolysis of the nerve gas sarin [10–12]. Because of
the advancement in the supramolecular concepts on
host-guest chemistry, numbers of suitable receptors have
been developed for the selective encapsulation and sens-
ing the biologically important anions for the qualitative
and quantitative determination [13–16]. Particularly, the sens-
ing of anion through the naked-eye (colorimetric), fluorescent
and/or electrochemistry responses have attracted considerable
attention.
1
.
Keywords Anion recognition Colorimetric and fluorescent
.
.
.
sensor Fluoride Acetate DFT
Introduction
Anion recognition continues to be a major research goal
for many supramolecular chemistry groups around the
world. This is because anions play crucial role in a
wide range of biological phenomena, chemical and en-
vironmental processes [1–6]. Anions are roughly present
in 70 % of all enzymatic sites, which play essentials
roles in many proteins and are critical for the manipu-
lation and storage of genetic information [7]. It is also
In this communication, we have introduced a simple and
easy to prepare anion sensor L by combining 2-hydroxy-1-
naphthaldehyde with 2-aminophenol. Addition of one
−
−
−
equivalent of F , AcO and H PO anions to L solution
2
4
in acetonitrile, naked-eye detectable color changes was ob-
served from yellow to orange. Importantly, spectroscopic
results reveal that the sensor L showed fluoride selective
behavior in the presence of other competitive anions. The
experimental evidences were well supported by theoretically
estimated anion recognition ability of L.
:
D. Sharma S. K. Sahoo (*)
Department of Applied Chemistry, S.V. National Institute
of Technology (SVNIT), Surat, Gujrat, India
e-mail: suban_sahoo@rediffmail.com
R. K. Bera
Department of Chemistry, Sant Longowal Institute of Engineering
&
Technology (SLIET), Longowal, Punjab, India
Materials and Methods
R. Kamal
Unless otherwise specified, all reagents for synthesis
were obtained commercially and used without further
Department of Chemistry, Kurukshetra University, Kurukshetra,
Haryana, India

J Fluoresc
Scheme 1 Synthesis of the
receptor L
purification. In the titration experiments all the anions
were added in the form of tetra-n-butyl ammonium
symmetry constraints by applying B3LYP/6-31G(d,p)
method in gas phase. The harmonic vibrational frequency
calculations using the same methods as for the geometry
optimizations were used to ascertain the presence of a local
minimum.
(
TBA) salts, which were purchased from Spectrochem
Pvt. Ltd., India or Acros Organic, and stored in a
vacuum desiccator containing self-indicating silica and
dried fully before using. Analytical grade acetonitrile
1
and absolute ethanol were used after distillation.
NMR and
H
Synthesis of L
1
3
C NMR spectra were determined in
DMSO-d6 on BRUKER AVANCE II 400 MHz NMR
using TMS as an internal standard. Melting point were
measured on digital melting point apparatus VMP-DS
2-hydroxy-1-naphthaldehyde (0.5 g, 0.0029 mol) in 20 ml
absolute ethanol was added drop wise into a solution of 2-
aminophenol (0.31 g, 0.0029 mol) in 30 ml absolute ethanol
with magnetic stirring. After the addition, the mixture was
heated at reflux for 5 h, and then the solvent was concen-
trated under reduced pressure. The solid obtained was
recrystallized from ethanol to give orange-yellow crystals.
“
VEEGO” which was uncorrected. UV-Vis spectra were
recorded on VARIAN CARY 50 Spectrophotometer with
a quartz cuvette (path length=1 cm). The fluorescence spectra
were recorded on a Perkin-Elmer LS55 luminescence
spectrometer.
−1
Yield: 85 %., M.P: 210 °C. IR (KBr pellet, 400–4,000 cm ):
−
3
1
Stock solution of the receptor (1.0×10 M) and an-
3,433, 1,632; H NMR (DMSO-d , δ, ppm): 15.65 (s, 1H),
6
−
3
ions (1.0×10 M) were prepared in acetonitrile. These
solutions were used for all spectroscopic studies after
appropriate dilution. For spectroscopic titrations, re-
10.11 (s, 1H), 9.41 (s, 1H), 8.24 (d, 1H), 7.73 (d, 1H), 7.72
(d, 1H), 7.61 (d, 1H), 7.45 (d, 1H), 7.24 (d, 1H), 7.08-7.00
13
(m, 2H), 6.92 (t, 1H), 6.81 (d, 1H); C NMR (DMSO-d , δ,
6
−
5
quired amount of the receptor (C =1.0×10 M) was
ppm): 177.08, 149.22, 148.53, 137.47, 133.69, 128.75, 128.68,
127.81, 126.41, 125.81, 124.73, 122.75, 119.57, 118.99,
117.23, 115.96, 107.60; LC-MS for C H NO : calculated
L
taken directly into cuvette and spectra were recorded
after successive addition of anion by using micropipette.
17
13
2
1
The sample for H NMR study was prepared by mixing
263.30, found 264.10.
both anion and receptor in an appropriate ratio. Then,
the mixture was made soluble in DMSO-d6 and the
spectrum was recorded.
Results and Discussion
All theoretical calculations were carried out with the
Gaussian 09W computer program [17] using the density
functional theory (DFT) method. Optimizations of the
receptor and `anions have been carried out without
Receptor L was synthesized by simple condensation method
according to Scheme 1 and the molecular structure was
established from different spectroscopic studies. The
Fig. 1 Color change of the
sensor L (5.0×10− M) upon
addition of equivalent amount
of different anions in
5
acetonitrile

J Fluoresc
−
5
Fig. 2 UV-Vis spectra changes of sensor L (5.0×10−5M) upon addition
of one equivalent of different anions in acetonitrile
Fig. 4 UV-Vis absorbance spectra of L (1.0×10 M) upon successive
addition of fluoride (1.0×10 M) in acetonitrile
−
4
recognition property of the receptor L towards different
anions was studied by naked-eye experiment, UV-Vis and
fluorescence titrations.
any structural changes. Spectrophotometrically, the UV-
−
5
Vis absorption spectra of L (5.0×10 M) were recorded
in absence and presence of equivalent amount of differ-
ent anions in acetonitrile. As shown in Fig. 2, the free
receptor showed a broad absorption between 300 nm
and 500 nm with λmax at 321 nm, 441 nm and
463 nm presumably due to π-π* transitions. On addi-
In naked-eye experiment, the yellow of the receptor
L becomes orange upon addition of equivalent amount
−
of F due to the formation of H-bond interactions be-
−
tween electron-rich F and the receptor L (Fig. 1).
−
Similarly, a detectable color change from yellow to light
tion of F to L solution, the receptor peaks at 321 nm
orange was observed with receptor L in presence of
and 441 nm decreases with the appearance of new
broad peak between 500 nm and 600 nm due to the
Internal Charge Transfer (ICT). Similarly, the new broad
−
−
AcO and H PO . However, no obvious color changes
2
4
−
−
−
−
was observed in presence of HSO , Cl , Br and I ,
4
−
−
even the anions were excessive presumably due to
weaker interactions with the receptor that failed to alter
peak was also detected due to AcO and H PO but
2 4
−
the intensity was appreciably lower than the F which
indicate the fluoride selective recognition ability of L.
−
−
−
−
The addition of other anions Cl Br , I and HSO did
,
4
Fig. 3 UV-Vis spectra and color changes of sensor L (1.0×10−4M)
upon addition of one equivalent of different anions in 1%water:aceto-
nitrile solution. Insert showing Color change of the sensor L upon
Fig. 5 Job’s plots for the complexation of L with F− and AcO− in
acetonitrile
−
−
−
addition of equivalent amount of different F , AcO and H
2
PO
4

J Fluoresc
1
Fig. 6 H NMR spectra of L in
absence (a) and presence (b) of
one equivalent of fluoride anion
in DMSO-d
6
not result in obvious spectral responses even in
abundance.
still be observed when the aqueous content was as high as
4 % in acetonitrile, which makes the receptor L suitable for
different biochemical applications. In addition, the fluoride-
induced color change remains the same even in the presence
Further, it was observed that the intensity of color de-
creases reversibly on addition of small amount of water. The
presence of protic solvents such as water can compete with
anions for binding sites and disturb the H-bond interactions
between the host and the anionic guest that lead to a reversal
of the visual color [18]. However, at 1 % water:acetonitrile
solution, the receptor L showed fluoride selective naked-eye
detectable color and spectral changes (Fig. 3). Also, the
color and spectral changes induced by the fluoride ion can
−
−
of other anions including AcO and H PO .
2
4
Spectrophotometric titrations were carried out in aceto-
−5
nitrile at 1.0×10 M concentration of receptor L upon the
−
−
addition of incremental amounts of F or AcO . These
results are shown for the most selective anion, fluoride
(Fig. 4). The spectral changes with the formation of
isosbectic point at 473 nm indicate the formation of single
Fig. 7 UV-Vis absorbance
spectra of L (1.0×10−4M) upon
successive addition of TBAOH
1.0×10− M) in acetonitrile.
3
(
Insert showing the B-H plot

J Fluoresc
Fig. 8 Binding modes of L with F− and the color change observed
with test paper kit
complex species between the receptor L and the anion
added. The calculated binding constant (K) by applying
−
4
−1
Benesi-Hildebrand equation for F (7.76×10 M ) is higher
−
3
−1
than AcO (6.24×10 M ), which supported the fact that
the F binds strongly with the receptor as compared to
−
−
AcO . The reason was probably that the higher electroneg-
−
ativity and smaller size of the F favors stronger binding
Fig. 10 Optimized structure of L and L-F− in gas phase at
B3LYP/6-31G(d,p)
with the receptor L by forming hydrogen bonding interac-
tions and/or deprotonating the OH groups.
The Job’s plot suggested that there was only one type of
1
:1 binding interaction between the receptor L and anions
addition, no peaks around ~16 ppm observed due to the
formation of F-H-F complex [19]. These observations in-
−
−
−
(
F or AcO ) added (Fig. 5). Further, to gain an insight into
1
the host-guest interaction between L and anions, H NMR
of L was measured in absence and presence of one equiv-
alent of the fluoride anion as TBA salt in DMSO-d solution
ferred that the receptor is recognizing fluoride anion through
O-H…F hydrogen bonds without any apparent conforma-
tional changes and deprotonation. Further to confirm the
anion binding modes of the receptor L, the absorption
titration of L with tetrabutylammonium hydroxide
(TBAOH) was performed in acetonitrile. As shown in
Fig. 7, the absorption band induced by TBAOH matched
well with the band formed in the presence of fluoride ion
(Fig. 4). The results clearly demonstrate that both anions are
6
(
1
Fig. 6). The free receptor showed signals at 15.65 ppm and
0.11 ppm due to OH groups attached to the naphthalene
and phenolic units, respectively. Addition of just one equiv-
alent of F resulted complete disappearance of receptor
peaks due to OH protons. No remarkable variations were
observed with other signals due to imine proton at 9.41 ppm
and aromatic protons between 8.25 ppm and 6.80 ppm. In
−
−
5
Fig. 9 Fluorescence titration spectra of receptor L (1.0×10 M,
−
8
−
λ
ex=430 nm) upon the gradual addition of fluoride (8.0×10 M to
Fig. 11 HOMO and LUMO diagrams for L and L(F ) obtained at
B3LYP/6-31G(d,p) method
−5
−8
−5
1
.0×10 M) and acetate (6.0×10 M to 1.0×10 M) in acetonitrile

J Fluoresc
functioning here as a base, giving rise to the deprotonation
upon interaction with the receptor L [20]. Based on the
above experimental evidences, the possible complexation
modes of the receptor L was shown in Fig. 8.
Conclusions
We have introduced a simple chemosensor L for bioactive
anions. Sensor L portrayed a colorimetric change from yellow
to orange on interaction with F , AcO and H PO in 1:1
2 4
−5
−
−
−
The anion binding behavior of the receptor L (1.0×10 M)
was also investigated by fluorescence titrations in acetonitrile
stoichiometry in pure CH CN. In mixed solvent 1%H O:
3 2
(
4
Fig. 9). The sensor L showed an emission band centered at
90 nm (λ =430 nm). Upon addition of fluoride, there was a
CH CN, sensor L exhibited selective recognition towards
3
fluoride anion. In addition, receptor L can be implemented
to develop test paper kit for analytical application (Fig. 8).
More importantly, the fluorescence of L was quenched by
fluoride but enhanced by acetate. The enhancement and
quenching of L fluorescence can be helpful to discriminate
ex
significant quenching in the emission intensity of L. The
fluorescence quenching may be due to the intermolecular
hydrogen bonding interactions between fluoride and L, where
the hydrogen bond acts as an efficient accepting mode for
radiationless deactivation processes from the excited state via
internal conversion (IC) [21]. Interestingly, under similar con-
dition the fluorescence of receptor L was enhanced upon
incremental addition of acetate presumably because of the
formation of rigid acetate-L complex through multiple hydro-
gen bonding that restricts tautomeric transformation at the
excited state and therefore, ceases non-radiative decay and
consequently the emission intensity was increased [22]. Fur-
ther study under competitive environment inferred that the
receptor L with equivalent amount of both acetate and
fluoride ions showed fluorescence enhancement as ob-
served only with acetate.
In order to complement our experimental findings, struc-
tural optimization of receptor L and its host-guest complexes
were performed at B3LYP/6-31G(d,p) level by focusing on
the proposed 1:1 binding modes between L and anions
(
[
for the complexes formed between receptor L and an-
ions to provide a useful scale to assess the relative
strengths of these interactions. The calculated results
−
−
the two bioactive anions AcO and F .
Acknowledgment This work was made possible by a grant from the
DST, New Delhi under Fast Track Young Scientist Scheme.
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2
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3
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−
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charge transfer occurred during the anion recognition process.
The HOMO electron density distributed mainly over naphtha-
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unit on interaction with fluoride ion because of the charge
delocalization occurred by deprotonation process. In addition,
the HOMO-LUMO band gap of L decreases on interaction
with anion, which supports the absorbance at longer wave-
length and orange coloration.
1
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