Spectral and DFT studies on simple and selective colorimetric sensing of fluoride ions via enhanced charge transfer using a novel signaling unit

Article abstract of DOI:10.1016/j.dyepig.2012.08.014

Two new colorimetric sensors for fluoride ions [N,N′-(anthracene-9, 10-dione-1,2-diyldicarbamothioyl)dibenzamide] and [N,N′-(naphthalene-1,4- dione-2,3-diyldicarbamothioyl) dibenzamide] have been prepared and characterized using various spectral technique

Full text of DOI:10.1016/j.dyepig.2012.08.014

Dyes and Pigments 96 (2013) 364e371
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Spectral and DFT studies on simple and selective colorimetric sensing of fluoride
ions via enhanced charge transfer using a novel signaling unit
*
Angupillai Satheshkumar, Kuppanagounder P. Elango
Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624 302, India
a r t i c l e i n f o
a b s t r a c t
Two new colorimetric sensors for fluoride ions [N,N0-(anthracene-9,10-dione-1,2-diyldicarbamothioyl)
dibenzamide] and [N,N0-(naphthalene-1,4-dione-2,3-diyldicarbamothioyl) dibenzamide] have been
prepared and characterized using various spectral techniques. The quinonoid receptors exhibited high
selectivity for fluoride ion detection over other anions. The association constant of the receptor-F
complexes were found to be 6.3 ꢀ 108 and 7.7 ꢀ 1015 Mꢁ1 for the anthroquinone and naphthoquionone
receptors, respectively. 1H NMR studies indicated that the fluoride ion sensing by the receptors is due to
the formation of H-bonds between F-ions and the eNH moiety of the receptors which has been
enhanced by directly attaching signaling unit to the receptor unit in the naphthoquinone derivative. The
naphthoquinone receptor also exhibited colorimetric sensing of fluoride ions present in commercially
available toothpaste samples in aqueous medium. The structural and electronics properties of the sensors
and their fluoride complexes were also investigated using ab initio DFT calculations.
Article history:
Received 19 May 2012
Received in revised form
11 August 2012
Accepted 13 August 2012
Available online 31 August 2012
Keywords:
Thiourea
Naphthoquinone
Anthroquinone
Fluoride
H-bonding
Charge transfer
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
sensor. Various authors have developed chromogenic sensors
containing urea/thioureas [14e18], calyx e [4] e pyrrole [19e22],
Recently considerable effort has been made to design chemo-
sensors for anions. The reason for this intensive interest is the
importance of the detection of anions in disciplines such as biology,
and environmental science [1]. Among the interest in biologically
functional anions, fluoride is one of particular importance owing to
its established role in dental care [2] and treatment of osteoporosis
[3]. Fluoride is also present in toothpaste in the form of NaF or
sodium monoflurophosphate which are used to prevent cavities in
teeth. However, excessive use may create fluorosis resulting in the
discoloration of teeth [4,5]. As the smallest and the most electro-
negative atom, fluorine has unique chemical properties and can
form the strongest hydrogen bond interaction with hydrogen bond
donors. Consequently, selective detection of this anion becomes
essential either visually or spectroscopically. The visual detection of
analytes has shown huge advantage over sensing methods due to
its quick response and simplicity as it does not require any equip-
ment for analyte detection [6e11].
imidazole [23], and calixarenes [16,24,25] as the binding func-
tionalities due to their hydrogen bond donor properties. Moieties
such as nitrophenyl [26e28], anthroquinone [10,23], and azo-dye
[11,13,29] were used as signaling units in these sensors. A survey
of literature revealed that, many attempts have been made to
increase the H-bond donor property of the receptor moiety in the
urea/thiourea based sensors. Amilan Jose et al. [10] have studied the
fluoride ion sensing property of anthroquinone based urea/thio-
urea systems. The results indicated that the thiourea system was
found to posses relatively higher binding ability with fluoride ion in
DMSO (the formation constant in the order of 105 Mꢁ1). Mei-Zhen
Sun et al. [30] have reported that Hg (II)-thiourea based complexes
promoted intramolecular charge transfer (ICT) between receptor
and signaling units which selectively sense fluoride ion in aceto-
nitrile. Cho and co-workers [31] investigated the fluoride ion
sensing property of anthroquinone based urea system. Vinod
kumar et al. [27] have reported the fluoride ion and CNꢁ sensing
behavior of urea/thiourea based sensor with p-nitrophenyl as the
signaling unit. Likewise, Duke and Gunnlaugsson [32] have inves-
tigated the fluoride ion sensing property of a urea based sensor
with p-trifluoromethylphenyl as the signaling unit. The literature
suggests that the fluoride ion sensing ability of a sensor is based on
the ease with which the occurrence of the ICT between receptor
and signaling units in the sensor occurs. The main objective,
A chemical sensor is generally composed of two units, a receptor
and a signaling unit [12,13]. When signaling unit contains chro-
mophores, the receptor is known as chromogenic (or) colorimetric
* Corresponding author. Tel.: þ91 451 245 2371; fax: þ91 451 2454466.
E-mail address: drkpelango@rediffmail.com (K.P. Elango).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.08.014

A. Satheshkumar, K.P. Elango / Dyes and Pigments 96 (2013) 364e371
365
therefore, of the present endeavor is to tune the electron accepting
2.2. Synthesis of thiourea compounds (1-2)
property of the signaling unit of thiourea based receptors to
selectively sense fluoride ion in an aqueouseorganic medium.
Hence two new thiourea based receptors were prepared, charac-
terized and employed as colorimetric fluoride ion sensors. Various
spectral techniques such as UVeVis, spectrofluorometer, and 1H
NMR have been used to investigate the interaction between the
receptor and Fꢁ ions.
2.2.1. N,N0-(antharacene-9,10-dione-1,2-diyldicarbamothioyl)
dibenzamide (1)
A solution of benzoyl chloride (1.4058 g, 0.01 mol) in acetone
(50 mL) was added drop wise to a suspension of potassium thiocy-
anate (0.9718 g, 0.01 mol) in anhydrous acetone (50 mL). The reaction
mixture was heated under reflux for 45 min and then cooled to room
temperature.
A solution of 1,2-diaminoanthracene-9,10-dione
(1.0826 g, 0.005 mol) in acetone was added and the resulting
mixture was stirred for 2 h. 0.1 N HCl (300 mL) was added to the
reaction mixture and resulting solid was filtered, washed with water
and dried in vacuum. The crude material was purified by 100e200
mesh flash column by using 30% of ethylacetate in petroleum ether
to get the pure product as a dark orange solid (yield 72%). The sche-
maticequationwasmentioned inScheme 1. UVeVis(lmax ¼ 428 nm),
2. Experimental
2.1. Chemical and apparatus
All reagents for synthesis of the receptors were obtained
commercially and were used without further purification. Spectro-
scopic grade solvents wereused as received. UVeVis spectral studies
were carried out in ACN (or) ACN/water (9:1 v/v) on a (JASCO V-630-
double beam spectrophotometer). Steady state fluorescence spectra
were obtained on a spectrofluorometer (JASCO-FP-6200). The exci-
1H NMR (DMSO-D6, 300 MHz):
d (ppm) 7.49e7.59 (m, 3H), 7.66e7.75
(m, 3H), 7.84e7.95 (m, 3H), 8.04 (d, J ¼ 6 Hz, 4H), 8.17 (d, J ¼ 6 Hz,1H),
8.24 (d, J ¼ 6 Hz, 2H), 11.79 (s, 2H), 12.06 (s, 2H). FT-IR (KBr) (cmꢁ1
)
3398 (NHeC¼S), 3244 (NHeC¼O), 1708, 1601 (C]O), 1345 (C¼Se).
Elemental analysis : Anal.Calcd. for C30H20N4O4S2: C, 63.81; H, 3.57;
N, 9.92. Found: C, 63.31; H, 3.20; N, 9.78.
tation and emission slit width (5 nm) and the scan rate (250 mV sꢁ1
)
was kept constant for all of the experiments. FT-IR spectra were
obtained as KBr pellets on an FT-IR spectrometer (JASCO 460 Plus,
Japan). Nuclear magnetic resonance spectra were recorded in
DMSO-d6 in Madurai Kamaraj University (Bruker, 1H NMR 300 MHz,
13C NMR 75 MHz). The 1H NMR spectra data is expressed in the form:
chemical shift in units of ppm (normalized integration, multiplicity,
and the value of J in Hz).
2.2.2. N,N0-(naphthalene-1,4-dione-2,3-diyldicarbamothioyl)
dibenzamide (2)
A solution of benzoyl chloride (1.4058 g, 0.01 mol) in acetone
(50 mL) was added drop wise to a suspension of potassium thio-
cyanate (0.9718 g, 0.01 mol) in anhydrous acetone (50 mL). The
NH2
H
N
H
N
O
NH2
O
S
O
O
NH
O
NH
S
O
1
O
O
S
C
Cl
KSCN
N
Acetone,
45 min, 50oC
O
NH2
O
S
S
NH
NH2
O
O
NH
NH
O
NH
O
2
Scheme 1. Preparation of thiourea compounds 1 and 2.

366
A. Satheshkumar, K.P. Elango / Dyes and Pigments 96 (2013) 364e371
reaction mixture was heated under reflux for 45 min and then
cooled to room temperature. A solution of 2,3-diaminonaph-
thoquinone (1.0 g, 0.005 mol) in acetone was added and the
resulting mixture was stirred for 2 h. 0.1 N HCl (300 mL) was added
to the reaction mixture and resulting solid was filtered, washed
with water and dried in vacuum. The crude material was purified by
(100e200) mesh flash column by using 40% of ethylacetate in
petroleum ether to get the pure product as a dark orange solid
(yield 62%). The schematic equation mentioned in Scheme 1. UVe
Vis (lmax ¼ 459 nm), 1H NMR (DMSO-D6, 300 MHz):
d (ppm) 7.08
(t, J ¼ 6 Hz, 2H), 7.31 (t, J ¼ 9 Hz, 4H), 7.45 (t, J ¼ 9 Hz, 4H), 7.76e7.89
(m, 2H), 7.99e8.09 (m, 2H), 12.01 (s, 2H), 13.31 (s, 2H). 13C NMR
(DMSO-D6, 75 MHz):
d (ppm) 121.9, 125.2, 125.8, 128.2, 130.0.130.9,
132.3, 132.5, 132.6, 134.7, 155.3, 168.1, 177.1, 180.4. FT-IR (KBr, cmꢁ1
)
3414 (NHeC¼S), 3218 (NHeC¼O), 1663, 1549 (C]O), 1277 (C]S).
Elemental analysis: Anal.Calcd. for C26H18N4O4S2: C, 60.69; H, 3.53;
N, 10.89. Found: C, 60.50; H, 3.61; N, 10.60.
3. Results and discussion
Two new thiourea based receptors (1-2) with varying signaling
units were efficiently prepared (Scheme 1) and characterized. The
Fꢁ ion sensing behavior of these receptors has been investigated
using various spectral techniques (UVeVis, Fluorescence and 1H
NMR). The colorimetric sensing of Fꢁ ions can effectively been done
either by increasing the H-bond donor ability of the receptor unit or
by increasing the electron accepting ability of the signaling unit of
a sensor. The systems so for reported in the literature contains
a phenyl ring with an electron withdrawing substituent as the
signaling unit [27]. The receptor 1, employed in the present study,
possesses an anthroquinone moiety and the carbonyl group as
signaling units. In receptor 2, the electron acceptor naph-
thoquinone is selected as the signaling unit. Further, in this system,
electron deficient quinone unit is directly attached to the thiourea
nitrogen so as to have easy and maximum ICT transition. To the best
of our knowledge this is the first attempt to prepare such a novel
moiety to sense Fꢁ ions colorimetrically. The results and discussion
of the colorimetric sensing of Fꢁ ions by these receptors are pre-
sented below.
Fig. 2. Changes of UVeVis spectra of receptor 1 (a) and 2 (b) upon the addition of 1
equiv. of Fꢁ and other anions.
3.1. Visual detection
The colorimetric sensitivity of 1 and 2 toward various anions
such as Fꢁ, Clꢁ, Brꢁ, Iꢁ, NO3ꢁ, H2PO4ꢁ, AcOꢁ, and CNꢁ in their tetra-
butylammonium form were monitored visually. As depicted in
Fig. 1 solutions of compound 1 (in ACN) and 2 (in ACN/water) turn
yellow to intense red color after the addition of fluoride. However,
their color remained unchanged after the addition of other chosen
anions. This indicated the selectivity of 1 and 2 toward fluoride ion.
1 (Fig. 2a) exhibited maximum absorbance at 428 nm (log
3
¼ 4.39)
in ACN. As evidenced from Fig. 3a, addition of anions such as Clꢁ,
Brꢁ, Iꢁ, H2POꢁ4 , AcOꢁ, NO3ꢁ and CNꢁ did not produce any significant
change in lmax. However, addition of Fꢁ ions bathochromically
shifts the lmax to 476 nm with an instantaneous formation of red
color. Likewise receptor 2 also selectively senses Fꢁ ions with
a formation of red color and a concurrent shift it lmax from 468 nm
(log
3
¼ 4.48) to 540 nm in ACN/water (Fig. 2b). With the addition of
3.2. UVeVis titration
incremental amounts of Fꢁ ions to the solution of 1, the absorption
peak at 428 nm diminished gradually accompanying the formation
of a new band at 476 nm (Fig. 3a.). This new band is due to ICT
between the receptor (>NH/Fꢁ) and signaling (quinone) units
The molecular interaction of 1 and 2 with all the anions under
investigation were studied using UVeVis spectral changes and the
results are depicted in (Fig. 2). The electronic spectrum of receptor
Fig. 1. Color changes upon addition of (1.875 ꢀ 10ꢁ3 M) of various anions in (1.25 ꢀ 10ꢁ5 M) ACN solution of receptor 1 (A) and 2 (B).

A. Satheshkumar, K.P. Elango / Dyes and Pigments 96 (2013) 364e371
367
The emission maximum for the receptors 1 and 2 is 560 and
580 nm, respectively (Fig. 4a and b). It is evident from the figures
that, in both the cases, with the addition of incremental amount of
F
ꢁ ions to the receptors, the emission intensity decreased gradually.
From the decrease in the emission intensity the association
constant of the receptor-Fꢁ ion complex was calculated using the
following equation [34].
log10ðFo ꢁ FÞ=F ¼ log KA þ n log10½Qꢂ
where, Fo is emission intensity in the absence of quencher (Fluoride
ion) (Q), F is the emission intensity at quencher concentration [Q],
KA is the binding constant for the receptors (1 and 2) with the Fꢁ ion
complex. In the present study, a plot of log10 (FoꢁF)/F versus log10
[Q] is linear in both cases (Fig. 3S). The association constants thus
computed were 6.3 ꢀ 108 and 7.7 ꢀ 1015 molꢁ1 L for the receptor 1
and 2, respectively. The results of fluorescence study also indicated
that the association of receptor 2 with Fꢁ ion is relatively stronger
than that in the case of receptor 1.
3.4. Job’s plot
The Job’s continuous variation method [27] has been used to
determine the stoichiometry of the interaction between the
receptors (1 and 2) and Fꢁ ions. The results are depicted in (Fig. 5a
500
a
400
300
200
100
0
Fig. 3. Change in UVeVis spectra for (a) receptor 1 (1.25 ꢀ 10ꢁ5 M) in ACN upon the
addition of (0.187
ꢀ
10ꢁ4 Me1.875
ꢀ
10ꢁ4 M) of fluoride ion, (b) receptor
2
(1.25 ꢀ 10ꢁ5) in ACN/water (9:1) upon the addition of (7.5 ꢀ 10ꢁ7 Me2.25 ꢀ 10ꢁ5 M) of
fluoride ion. No further change was observed on addition of even higher concentration
of fluoride ion.
500
550
600
650
[27]. The isobetic point (at 455 nm) indicated that there exists only
one type of receptorefluoride complex formation [27]. Parallel to
this observation, with the addition of incremental amount of Fꢁ
ions to the ACN/water solution of 2, the absorption of the band at
468 nm (lmax) diminished gradually and that of the new band at
540 nm increased gradually (Fig. 3b) with a clear isobetic point at
477 nm. The association constant of receptor-Fꢁ ion complex, in
both the cases, has been estimated using the Scott equation [33]. In
both the cases the Scott plot is linear (Fig. 2S) and the association
constants were found to be 1.14 ꢀ 105 and 4.09 ꢀ 106 Mꢁ1 for the
receptors 1 and 2, respectively. The relatively higher values for the
receptor 2 may be due to the fact that in this case the signaling unit
(electron deficient quinone) is directly attached to the receptor
unit. The observed magnitude of the association constant is higher
than that reported for similar thiourea based receptor [10].
Wavelength (nm)
b
120
100
80
60
40
20
0
3.3. Fluorescence study
500
550
600
650
The association constants for the receptor-Fꢁ ion complex
formation were also determined by a fluorescence study. The
fluorescence responses of the interaction of 1 and 2 with Fꢁ ions
were recorded with an excitation at 428 and 468 nm, respectively.
Wavelength (nm)
Fig. 4. Change in fluorescence emission spectra for (a) receptor 1 (1.25 ꢀ 10ꢁ5 M), (b)
receptor 2 (1.25 ꢀ 10ꢁ5 M) in ACN upon the addition of TBAF in ACN from 0 to
0.875 ꢀ 10ꢁ6 M.

368
A. Satheshkumar, K.P. Elango / Dyes and Pigments 96 (2013) 364e371
a
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.0
0.2
0.4
0.6
0.8
1.0
-
[R]/[R]+ [F ]
b
0.5
0.4
0.3
0.2
0.1
0.0
Fig. 6. Change in partial 1H NMR (300 MHz) spectra of (a) receptor 1 in DMSO-D6; (b)
receptor 1 þ 0.1 equiv. Fꢁ; (c) receptor 1 þ 0.2 equiv. Fꢁ; (d) receptor 1 þ 0.3 equiv. Fꢁ.
0.0
0.2
0.4
0.6
0.8
1.0
-
[R]/[R]+[F ]
Fig. 5. The stoichiometry analysis of 1 (a) and 2 (b) with Fꢁ.
and b). In both the cases, the curve with a maximum at 0.3 mol
fraction, indicated the formation of 1:2 (receptor:Fꢁ) complex.
3.5. 1H NMR titration
Fig. 7. Change in partial 1H NMR (300 MHz) spectra of (a) receptor 2 in DMSO-D6; (b)
receptor 2 þ 0.1 equiv. Fꢁ; (c) receptor 2 þ 0.2 equiv. Fꢁ; (d) receptor 2 þ 0.3 equiv. Fꢁ
;
The mechanism of binding of Fꢁ ions with the receptor 1 and 2
has been studied using the 1H NMR titration experiment in DMSO-
d6. The results of this study are shown in (Figs. 6 and 7), respec-
tively, for the receptors 1 and 2. In the case of pure receptors, 1H-
(e) receptor 2 þ 0.4 equiv. Fꢁ.
NMR chemical shift, for NH protons were observed at
d 11.7 and
O
12.0 ppm (For 1) and at 12.0 and 13.3 ppm (For 2). In both the
d
O
S
O
cases, after the addition of 0.1 equivalents of Fꢁ ions to the receptor,
the characteristic NH signal becomes broad. In the case of receptor
1, fluoride ion causes shielding of NH protons while in the case of 2
deshielding of NH protons occurs. This is due to the fact that
complexation of Fꢁ ions with the receptors through the H-bonding
[27]. It is also interesting to note that, in the case of receptor 2, the
NH protons which are directly attached to the signaling unit
O
O
N
H
N
H
S
S
F-
O
HN HN
O
NH
NH
NH
HN
F-
S
F-
F-
O
(quinone) are relatively more acidic (d 13.3 ppm). When compared
to that in the other receptor. Such an NeH moiety would behave as
a better receptor unit and consequently binds relatively stronger
with Fꢁ ions leading to a higher association constant for the
Fig. 8. The possible structure of the complex formed between sensors 1 and 2 with Fꢁ
ion.

A. Satheshkumar, K.P. Elango / Dyes and Pigments 96 (2013) 364e371
369
receptor-Fꢁ ion complex and corroborates the trend observed from
UVeVis and fluorescence studies.
Based on the foregoing results and discussion it is evident that
the receptors 1 and 2 form 1:2 (receptor:Fꢁ) complex via H-
bonding as shown in (Fig. 8).
4. Practical application
With an aim to investigate the real time application of these
receptors, an attempt was made to qualitatively detect Fꢁ ions in
toothpaste. For this, 200 mg each of two commercially available
toothpastes (sample S1 and S2 belonging to different companies)
were dissolved in water and filtered. Then 0.1 mL of the filtrate was
added separately to ACN solution of both 1 and 2 (1.25 ꢀ 10ꢁ5 M).
The results are shown in (Fig. 9). The results indicated that addition
of the toothpaste samples instantaneously produced a red color
with receptor 2. However, receptor 1 failed to impart any color
change which may be due to the addition of water [35]. Qualitative
analysis of the toothpaste samples was also studied using UVeVis
spectroscopy (Fig. 10). The results are in line with that discussed
earlier (with Fꢁ ion solution).
Fig. 9. From left to right: receptor 2, S1 and S2 are toothpaste solution in water, 2 þ S1,
2 þ S2.
1
2+TBAF
2
2+Sample 1
2+Sample 2
5. Theoretical study
The foregoing discussion indicated that the receptors 1 and 2
selectively sense fluoride ions through H-bond formation between
the thiourea NeH and Fꢁ ion. To elaborate upon the mechanism of
recognition abilities of 1 and 2 toward Fꢁ ions are have investigated
the structural and electronic properties, of the receptors and their
complexes with Fꢁ ions were investigated using ab initio Density
Functional Theory (DFT) calculations as implemented in the
0
400
600
Wavelength (nm)
Fig. 10. UVeVis spectrum of receptor 2 (2.50 ꢀ 10ꢁ5 M), 2 þ TBAF, 2 þ Sample 1 (S1),
2 þ sample 2 (S2).
Fig. 11. Optimized structure of receptor 1, 2 and receptor-Fꢁ complex.

370
A. Satheshkumar, K.P. Elango / Dyes and Pigments 96 (2013) 364e371
Fig. 12. HOMO-LUMO orbitals of the receptor and receptor-Fꢁ complex.
Gaussian 03 package [36]. DFT calculations with Becker’s three
parameterized Lee-Yang-Par (B3LYP) exchange functional with
6311G basis sets were carried out for the geometry optimizations.
All of the DFT optimized geometries are shown in Fig. 11. The
relevant frontier molecular orbitals of the receptor and their
complexes with Fꢁ ions are shown in Fig. 12. The bond lengths and
the Mulliken charges on selected atoms of 1 and 2 and their
complexes with Fꢁ ions are collected in Tables S1 and S2 (Supple-
mentary data).
As shown in Fig. 12, the HOMO distribution of 1 is concentrated
on the NeH moiety of one of the thiourea arms. The LUMO, as
expected, is concentrated on the quinone ring. The HOMO to LUMO
excitation is responsible for ICT observed at 428 nm in the elec-
tronic spectrum of 1. In the case of 1-Fꢁ complex the LUMO is
delocalized over the thiourea moiety but not on the quinone as in
free 1. Hence, after the formation of H-bond with Fꢁ ions, the
HOMO-LUMO energy gap is increased (
when compared to that in the free receptor 2 (
delocalization of both HOMO and LUMO over the NeH moiety in
the 1-Fꢁ complex might have caused the up field shift of NeH
protons in the 1H NMR spectrum. Also, the existence of a strong
D
E ¼ 3.3476 eV, Fig. 13) and
D
E ¼ 2.2316 eV). The
Fig. 13. Energy level diagram of HOMO and LUMO orbitals of receptor 1 and 2 and
receptors-Fꢁ complex.

A. Satheshkumar, K.P. Elango / Dyes and Pigments 96 (2013) 364e371
371
ꢀ
H-bond (1.792 A) between H7 of the thiourea NeH and O1 of the
same time, the practical application of this receptor was demon-
strated by the visual selective detection of fluoride ion in tooth-
paste samples.
quinone carbonyl group (Fig. 11) might also have contributed to the
increase in D
E value after the addition of Fꢁ ions to 1. This may be
due to the fact that Fꢁ ions have to expend energy to overcome the
H7/O1 H-bonding to form H7/Fꢁ H-bond in other words this is
the energy required for the structural reorganization to form the 1-
Appendix A. Supplementary material
ꢁ complex. This is supported by the weakening of H7/O1 H-bond
Supplementary data (characterization data and 1H and 13C NMR, spectra
of all the products) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.dyepig.2012.08.014.
F
(1.810 A) in the 1-Fꢁ complex.
ꢀ
In the case of receptor 2, the HOMO is concentrated on the NeH
moiety and the LUMO is concentrated on the quinone ring. The ICT
transition exhibited on free 2 (at 468 nm) in the electronic spec-
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energy gap (
D
E ¼ 1.803 eV, Fig. 13) (makes NeH/Fꢁ moiety
a better donor) and red shifted the ICT to 540 nm (DlICT ¼ 72 nm)
and consequently makes the colorimetric recognition of Fꢁ ions
energetically easier. Such an observed large red shift in lICT, after
complexation with Fꢁ ions renders the compound 2, a novel
receptor wherein the receptor unit is directly attached to the
signaling unit.
Furthermore, in both the receptor 1 and 2, the Mulliken charge
on the four NHs increased significantly after complexation with
fluoride ions indicating formation of NeH/Fꢁ H-bond. This is
because Fꢁ ions will abstract the H-atom of the NeH moiety as Hþ
to form the H-bond. This fact is well supported by the results of 1H
NMR titration experiments. Interestingly it is observed that the
results of the theoretical study corroborate well with the experi-
mental findings.
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6. Conclusion
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We have designed, synthesized and characterized two new
thiourea based sensors for fluoride ions with different signaling
units. The results of UVeVis, fluorescence and 1H NMR studies
indicated that the receptors 1 and 2 showed high selectivity for
fluoride ion detection over other anions such as Clꢁ, Brꢁ, Iꢁ, OAcꢁ,
NOꢁ3 , H2POꢁ4 , and CNꢁ. As the receptor 2 possesses a highly electron
deficient signaling unit directly attached to the receptor unit with
enhanced Fꢁ ion sensing, it works well for the selective sensing of
Fꢁ ions in an aqueous medium. Hence it is no doubt that the
receptor 2 is novel. The results obtained from the ab initio DFT
calculations agree well with the experimental observations. At the
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