'Naked-eye' detection of F- ions by two novel colorimetric receptors

Article abstract of DOI:10.1016/j.tetlet.2012.11.138

Two novel colorimetric receptors, S1 and S2, linked to one or two nitrophenylurea groups were synthesized in good yields, characterized and their chromogenic properties investigated towards various anions (F^-, Cl^-, Br^-, I<

Full text of DOI:10.1016/j.tetlet.2012.11.138

Tetrahedron Letters 54 (2013) 613–617
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
‘Naked-eye’ detection of FÀ ions by two novel colorimetric receptors
⇑
Serkan Erdemir , Ozcan Kocyigit, Onder Alici, Sait Malkondu
Selcuk University, Science Faculty, Department of Chemistry, 42031 Konya, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel colorimetric receptors, S1 and S2, linked to one or two nitrophenylurea groups were synthe-
sized in good yields, characterized and their chromogenic properties investigated towards various anions
(FÀ, ClÀ, BrÀ, IÀ, NO3À, AcOÀ, ClO4À, HSO4À, and H2PO4À) by UV–vis and 1H NMR techniques. The receptors,
effectively and selectively, recognized and distinguished the biologically important FÀ from other anions
such as ClÀ, BrÀ, IÀ etc. in CH3CN. This selectivity could be easily observed by the naked eye, indicating
that receptors S1 and S2 are potential colorimetric sensors for fluoride ion.
Received 21 September 2012
Revised 15 November 2012
Accepted 28 November 2012
Available online 5 December 2012
Keywords:
Schiff base
Ó 2012 Elsevier Ltd. All rights reserved.
Receptor
Colorimetric
Anion
UV–visible spectroscopy
Sensing of anions in both organic and aqueous solutions has be-
come an active field of research within supramolecular chemistry.
Anions are mainly recognized through hydrogen-bonding interac-
tions,1 electrostatic interactions2 and coordination with metal
ions.3 Among various non-covalent interactions, hydrogen-bond-
ing is particularly useful and effective in this respect. Receptors
bearing functional groups such as amides,4 ureas,5 thioureas,6
and imidazolium7 have been widely used to recognize anions via
hydrogen-bonding interactions.
The development of simple receptors capable of recognizing
biologically relevant anions such as fluoride, chloride, phosphate,
and carboxylate has attracted considerable interest.8 The design
of these receptors has focused on the ability to recognize and sense
selectively biologically important anions through naked eye,
electrochemical and optical responses.9 While the incorporation
of fluorescent chromophores into receptors has gained consider-
able attention owing to their high sensitivities and easy detec-
tion,10 the investigation of anion-selective receptors based on
colored chromophores is less studied.11 In particular, the develop-
ment of colorimetric anion sensing is important and useful since it
allows so-called ‘naked-eye’ detection of anions without the use of
any spectroscopic instrumentation. Such receptors would be more
valuable if they could be obtained by a simple synthetic method.12
Herein, we report the synthesis of two new receptors, S1 and S2
containing one or two nitrophenyl groups as the chromogenic sig-
nalling subunits and urea/phenolic-OH groups as binding sites.
Their anion binding properties were investigated by means of
UV–vis and 1H NMR spectroscopy, as well as by the ‘naked-eye’.
The synhthesis of receptors S1 and S2 is depicted in Scheme 1.
The Schiff bases 1 and 2 were synthesized in one step by conden-
sation of 2-hydroxy-1-naphthaldehyde and diethylenetriamine or
triethylenetetramine in ethanol.
Receptors S1 and S2 were obtained in 71% and 75% yields,
respectively by reacting 4-nitrophenylisocyanate with 1 or 2. Their
molecular structures and purities were established from spectro-
scopic studies including 1H NMR, 13C NMR, HRMS, and FT-IR
analysis (Supplementary data). For example, the 1H NMR
(400 MHz, DMSO-d6) spectra of S1 and S2 showed two singlets at
d 9.09 and d 9.20 characteristic of urea protons, whilst the 13C
NMR (100 MHz, DMSO-d6) spectra showed the expected number
of resonances.
The binding behaviour of receptors S1 and S2 with different an-
ions was studied in acetonitrile. The titrations were carried out in
CH3CN at a 1.0 Â 10À5 M concentration of receptors S1 and S2, by
the addition of incremental amounts from (0–500 lL) of tetrabu-
tylammonium fluoride (1.0 Â 10À3 M); the spectra of the receptors
are shown in Figure 1.
The electronic spectra of receptors S1 and S2 showed four
transitions. As shown in Figure 1, upon a gradual increase of the
fluoride ion concentration, the bands at 300–350 nm gradually
decreased and the bands at 400–440 nm increased, and new bands
appeared at 470 and 455 nm with isosbestic points at 376 and
346 nm, indicating the formation of new complexes between the
receptors S1 and S2 and fluoride anions (Fig. 1). The receptors S1
and S2 (1.0 Â 10À5 M) in CH3CN showed dramatic colour changes
from colourless to yellow in the presence of TBAF, respectively.
The colour changes are most probably due to the formation of
hydrogen bonds or deprotonation of receptors S1 and S2 on addi-
tion of the fluoride ions. These H-bonds or deprotonations affect
⇑
Corresponding author. Tel.: +9 332 223 3858; fax: +9 332 241 2499.
E-mail address: serdemir82@gmail.com (S. Erdemir).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.138

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S. Erdemir et al. / Tetrahedron Letters 54 (2013) 613–617
CHO
OH
(i)
(ii)
H
N
H
N
N
H
N
N
N
N
OH
HO
OH
HO
2
1
(iii)
(iii)
O2N
NO2
NO2
NH
HN
O
NH
O
O
N
N
N
N
OH
N
N
N
HO
OH
HO
S1 (71%)
S2 (75%)
Scheme 1. Synthesis of the receptors S1 and S2: (i) diethylenetriamine, EtOH, rt; (ii) triethylenetetramine, EtOH, rt; (iii) 4-nitrophenylisocyanate, dry CH2Cl2, reflux.
Figure 1. UV–vis titration spectra for receptors S1 (a) and S2 (b) (1.0 Â 10À5) at room temperature on increasing the concentration of TBAF in CH3CN.
the electronic properties of the chromophore, resulting in a colour
change along with a new charge-transfer interaction between the
fluoride-bound OH and NH and the electron-deficient nitro
group.13,14
Titrations were also carried out with various anions such as
AcOÀ, ClÀ, BrÀ, IÀ, NO3À, ClO4À, HSO4À, and H2PO4À as their tetrabu-
tyl ammonium salts. Interestingly, there was no obvious change
observed in the UV spectra, except with acetate, which gave slight
interference (Supplementary data). However new, albeit minor
peaks appeared at 480 and 465 nm which indicated that the recep-
tors S1 and S2 showed a slight response to acetate ions.
The deprotonation occurred at a higher concentration of acetate
than fluoride; this is due to the higher electronegativity, smaller
size and higher basicity of FÀ ions, which makes them bind more
strongly with the receptors S1 and S2. That is why FÀ could be
recognized selectively from the other anions (Figs. 2a and 3a).
Figures 2b and 3b reflect the selectivity for FÀ over the other anions
which are shown by the blue bars for S1 and S2. The slight interfer-
ence of acetate is shown by the pink bars, but it cannot be detected
by the naked eye as shown in Figure 4. This Figure shows the color
change of the solutions of the receptors S1 and S2 in the presence
of various anions in CH3CN. It can be seen with the naked eye that

S. Erdemir et al. / Tetrahedron Letters 54 (2013) 613–617
615
Figure 4. The visible color changes observed for S1 and S2 in CH3CN solution
(1.0 Â 10À5 M) upon addition of 10 equiv of anions as TBAÀsalts: from left to right:
free S1 and S2, FÀ, ClÀ, BrÀ, IÀ, NO3À, HSO4À, ClO4À, H2PO4 and AcOÀ.
the colors of the S1 and S2 solutions change from colourless to
yellow after the addition of fluoride. Other anions did not induce
any color changes even in excess amounts. From these experimen-
tal data, it can be concluded that the receptors S1 and S2 have high
selectivity and sensitivity toward FÀ ions in acetonitrile. The other
anions examined, except acetate, had no significant influence.
Judging from the titrations, the strong binding of fluoride
allowed a Job’s plot15 to be used in the determination of the
binding stoichiometry. The stoichiometries between the receptors
S1 or S2 and fluoride were determined from the Job plots using the
UV-vis spectra, which showed evident 1:2 and 1:3 stoichiometries,
respectively (Fig. 5a and b).
Figure 2. (a) UV–vis absorption spectra and (b) the absorption ratios at 470 nm of
S1 (1.0 Â 10À5 M) in CH3CN after the addition of 10 equiv of each of the different
guest ions.
Figure 3. (a) UV–vis absorption spectra and (b) the absorption ratios at 455 nm of
S2 (1.0 Â 10À5 M) in CH3CN after the addition of 10 equiv of each of the different
guest ions.
Figure 5. The Job plots of S1 (a) and S2 (b) with tetrabutylammonium fluoride
using UV–vis.

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S. Erdemir et al. / Tetrahedron Letters 54 (2013) 613–617
nitrogens and the naphthol –OH groups.17 However, the titration
experiments on the receptors S1 and S2 and tetrabutylammonium
fluoride in DMSO-d6 showed that the proton signals of the naph-
thol hydroxyl disappeared after the addition of 1.0 equiv of fluoride
and there was no appearance of proton signals due to this group
even after the addition of 2, 3, 4 and 5 equiv of fluoride. This dem-
onstrates formation of a new complex between the naphthol
hydroxyl and fluoride.
In summary, we have developed two novel colorimetric anion
sensors which selectively recognize fluoride over other anions
(AcOÀ, ClÀ, BrÀ, IÀ, NO3À, ClO4À, HSO4À, and H2PO4À) in acetonitrile.
More importantly, these chemosensors display naked-eye detec-
tion at room temperature. The nature of the anion receptor inter-
action has been defined by absorption and NMR spectroscopy,
and we have demonstrated that in the case of S1 and S2, deproto-
nation interactions are most likely involved in the recognition
process.
Figure 6. Partial 1H NMR spectra of S1 (7.24 Â 10À2 M) in DMSO-d6 at 25 °C and the
corresponding changes after the addition of various equivalents of tetrabutylam-
monium fluoride (2.0 Â 10À2 M).
Acknowledgements
We thank the Research Foundation of Selcuk University (BAP)
for financial support of this work.
Benesi–Hildebrand plots16 derived from the changes at 470 and
455 nm in the UV–vis spectra of S1 and S2 gave the association
constants. The association constants of FÀ with S1 and S2 were
calculated as 6.38 Â 103 and 1.11 Â 103 MÀ1 from the UV–vis
titrations (Supplementary data).
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
11.138.
In order to understand the effect of fluoride anions on the NH
and OH protons of the receptors S1 and S2, the 1H NMR spectra
were recorded in DMSO-d6. The urea NH and OH signals in receptor
S1 appeared at d 9.09 and d 13.95 (Fig. 6) and at d 9.20 and d 13.98
in receptor S2 (Fig. 7). The urea NH protons in S1 and S2 disap-
peared after the addition of 1.0 equiv of tetrabutylammonium
fluoride to the receptor solutions. The deprotonation of the urea
subunits in receptors S1 and S2 can induce two distinct effects
on the aromatic substituents: (i) it increases the electron density
on the phenyl rings with through bond propagation which gener-
ates a shielding effect, and should produce an upfield shift of the
C–H protons; (ii) it induces polarisation of the C–H bonds via a
through-space effect, where the partial positive charge causes a
deshielding effect and produces a downfield shift.
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