Novel thiophene based colorimetric and fluorescent receptor for selective recognition of fluoride ions

Article abstract of DOI:10.1016/j.tetlet.2012.06.147

A colorimetric anion sensor 2,2′-(1E,1′E)-(thiophene-2,5- diylbis(methan-1-yl-1-ylidene)) was synthesized and characterized by various spectroscopic techniques. Anion binding studies were carried out using UV-visible spectrophotometric titrations and emis

Full text of DOI:10.1016/j.tetlet.2012.06.147

Tetrahedron Letters 53 (2012) 5068–5070
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Novel thiophene based colorimetric and fluorescent receptor
for selective recognition of fluoride ions
⇑
D. Renuga, D. Udhayakumari, S. Suganya, S. Velmathi
Organic and Polymer Synthesis Laboratory, 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
0
0
Article history:
A colorimetric anion sensor 2,2 -(1E,1 E)-(thiophene-2,5-diylbis(methan-1-yl-1-ylidene)) was synthe-
sized and characterized by various spectroscopic techniques. Anion binding studies were carried out
using UV–visible spectrophotometric titrations and emission spectra studies, revealed that the receptor
Received 28 May 2012
Revised 28 June 2012
Accepted 30 June 2012
Available online 6 July 2012
ꢀ
ꢀ
exhibits selective recognition toward F over other anions. The selectivity for F among the halides is
attributed mainly to the hydrogen-bond interaction of the receptor with F . Receptor 1 showed color
ꢀ
change from fluorescent green to orange in the presence of tetrabutylammonium fluoride with 1:1 stoi-
chiometry. Receptor 1 exhibits remarkably enhanced fluorescence intensity.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Thiophene based receptor
Anion recognition
UV–vis spectroscopy
Fluorescence enhancement
Chemical sensors, especially optical sensors, are elegant alter-
native to the traditional analytical instruments. They have the
advantages of size, cost-effectiveness, simplicity, no necessity of
and fluorescent sensor for fluoride ion. The receptor reported in
this Letter has only one previous report about its conformational
1
8
analysis. Other than that no information is available in the open
literature. To the best of our knowledge this is the first report on
the synthesis, characterization, and utilization of receptor 1 as a
selective fluoride ion sensor. Upon the addition of 2 equivalents
1,2
the reference solution, and fieldwork applicability. Among the
methods available for chemical sensors, a colorimetric technique
has many advantages. It can be easily observed and determined
by the naked eye, rather than by using large, sophisticated, and
expensive analytical instruments (such as mass spectrome-
ꢀ
of F ion, receptor 1 turned from fluorescent green to orange color.
Among the various anions, receptor 1 exhibits high selectivity, sen-
sitivity toward fluoride ion and remarkably enhanced fluorescence
intensity.
3
a–c,4a–e
ters).
Among the anion species, fluoride sensing has at-
tracted considerable attention because of its crucial role in dental
care and the treatment of osteoporosis.5 On the other hand, it is
highly advantageous to develop a high-effective sensor that can
selectively detect fluoride ion with the naked eye ‘no–yes’ re-
Receptor 1 was prepared by a simple condensation method
1
9
ꢀ1
according to Scheme 1. A very strong absorption at 1598 cm
in the FTIR spectrum indicates the presence of C@N group. O-H
6
sponse. The addition of fluoride in drinking water and toothpastes
stretching vibration of this compound was observed around
-1
has become widespread due to the valuable effects in human
health. However, high doses of fluoride are hazardous and can lead
3376 cm . C@C, C–O, and C–N stretching vibrations are seen at
ꢀ
1
ꢀ1
ꢀ1
1452 cm , 1227 cm , and 1368 cm
respectively. Receptor 1
7
to dental or skeletal fluorosis. Charge neutral receptors have been
gave a singlet at d 8.9 ppm corresponding to CH@N proton indicat-
ing the formation of imine and the aromatic protons of receptor
resonate in the d 6.8–7.2 region and the thiophene ring protons
used for anion sensor and most of them contain pyrrole, amide,
indolocarbazole, guanidium, imidazolium, and/or urea/thiourea
moieties, and the anion is recognized via H bonding or deprotona-
8
–16
tion of protons on the receptor NH in organic solvents.
Although thiophene based compounds were reported for several
studies it was not reported anywhere for sensing studies, so our
interest was captured by thiophene based receptors for sensing
studies. In continuation of our ongoing research in the chemo sen-
NH2
S
S
C H OH
2 5
N
N
1
7a–c
OH
O
O
sor field,
herein we report the synthesis, characterization, and
Reflux,3h
OH
HO
application of a simple thiophene based receptor as a colorimetric
Receptor 1
⇑
Corresponding author. Tel.: +91 431 2503640; fax: +91 431 2500133.
E-mail address: velmathis@nitt.edu (S. Velmathi).
0
0
Scheme 1. Synthesis of 2,2 -(1E,1 E)-(thiophene-2,5-diylbis(methan-1-yl-1-yli-
dene))bis(azan-1-yl-1 ylidene)diphenol.
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.147

D. Renuga et al. / Tetrahedron Letters 53 (2012) 5068–5070
5069
1
1
0
0
.2
.0
.8
.6
Rec
-
Rec+2 eq F
-
Rec+2 eq H PO
2
4
-
Rec+2 eq AcO
-
Rec+2 eq OH
-
Rec+2 eq Cl
ꢀ
5
-
Figure 1. Color changes of receptor 1 (2.5 ꢁ 10 M solution in CH
3
CN) before and
Rec+2 eq Br
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
4
after the addition of 100
l
L (2 equiv) of F , Cl , Br , AcO , OH , and H
3
CN) ions, respectively.
2
PO
1.5 ꢁ 10ꢀ3 M solution in CH
0.4
.2
0.0
(
0
gave a singlet at d 7.7. In addition, the –OH protons in the aromatic
rings are shown as a singlet at d 9.2 ppm. In 13C spectrum, the aro-
matic carbons resonate in the d 116–137 region. Thiophene ring
carbons resonate around d 133–146. Imine carbon resonates at d
300
400
500
600
700
Wavenumber (nm)
1
53 ppm. Thus the structure of the receptor was confirmed with-
ꢀ
5
out any ambiguity by various spectroscopic methods.
The recognition properties of the receptor 1 toward different
anions were studied by the naked-eye experiment, UV–visible,
and fluorescence titration.
Figure 2b. UV–vis spectrum of receptor 1 (2.5 ꢁ 10 M) upon titration with anions
ꢀ ꢀ ꢀ ꢀ ꢀ
3 2 4
in CH CN (H PO , AcO , OH , Cl , Br ).
the electronic transition. The second band observed in the wave-
length 320 nm could be due to the transition between the -orbital
localized on the central band of azomethine group (CH@N). The
third band obtained in the wavelength of 420 nm is may be due
to an intra molecular charge transfer (CT) transition within the
In the naked eye experiment, color changes were studied in ace-
p
tonitrile (CH
3
CN). Upon addition of 2 equivalents of fluoride ions in
20
ꢀ
5
3
the form of TBAF salt solution (1.5 ꢁ 10 M in CH
3
CN), fluorescent
3
CN) became or-
ꢀ
green solution of receptor 1 (2.5 ꢁ 10 M in CH
ange. The color changes are shown in Figure 1. No color changes
were observed for the addition of dihydrogen phosphate, chloride,
bromide, and acetate ions even to the addition of large excess of
ꢀ
whole molecule. As a function of F , a new red-shifted absorption
band centered at 520 nm increased with a concomitant decrease of
the band at 420 nm with the isosbestic point. The band at 520 nm
is attributable to the Internal Charge Transfer (ICT). The other an-
ions did not give any response in acetonitrile medium, there was
no extra band observed after the addition of 2 equivalents of an-
ions as their tetrabutylammonium salts (Fig. 2b). Thus receptor 1
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
4
Cl , Br , AcO , OH , and H
2
PO
ions (up to 10 equiv) in the form
of their tetrabutylammonium salts and the receptor was found to
be insensitive. The appearance of the color change signifies the
interaction of F with the OH groups of receptor through hydrogen
bond. These H-bond interactions produced a new interaction be-
tween electron-rich F and receptor, resulting in a visible color
change from fluorescent green to orange.
The sensing behavior of receptor 1 toward fluoride ion was
determined by spectrophotometric methods in CH CN. Absorption
3
ꢀ
-
can sense F ions selectively in the presence of other competing
ꢀ
anions.
The binding constant of the receptor for fluoride ion was calcu-
3
lated as 1.3 ꢁ 10 according to Benesi–Hildebrand (B–H) equation.
Job’s plot studies revealed that the stoichiometry between the
receptor 1 and anion was 1:1.
ꢀ5
titrations were carried out with receptor 1 (2.5 ꢁ 10 M) in CH
3
CN
and incremental addition of tetrabutylammonium fluoride
The sensitivity of receptor was also investigated with the help of
emission spectra. Titration experiments were carried out similar to
ꢀ3
(
1.5 ꢁ 10 M) in CH
3
CN up to 2 equivalents to the receptor 1
and the resulting spectra are shown in Fig. 2a. Receptor 1 in the ab-
sence of F ions showed three bands. The first band in the wave-
length 270 nm was assigned to the excitation of the
ꢀ5
UV–vis analysis with the same solutions, receptor 1 (2.5 ꢁ 10 M)
ꢀ
ꢀ3
and anion (1.5 ꢁ 10 M) both in CH
3
CN. Upon excitation at 460 nm
p electrons
due to the strong fluorescent nature of the receptor 1 it gave a
of the aromatic system. This band is sensitive to the substitution
at the aromatic rings and their positions are little influenced by
⁄
changing the solvent polarity confirming the local
p
–p
nature of
7
7
7
7
7
7
8
7
6
5
4
3
8
0
1
1
1
0
0
0
0
0
.4
.2
.0
.8
.6
.4
.2
.0
0.18
0.16
0.14
0.12
0.10
70
60
50
40
72
7
7
1
0
0.08
0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1
-
0
0
.06
.04
Equiv of F
100 L
µ
0.0
0.5
1.0
1.5
2.0
-
Equiv of F
0
1
00 L
µ
3
2
0
0
0
500
520
540
560
580
2
50 300 350 400 450 500 550 600
Wavenumber (nm)
Wavenumber (nm)
ꢀ
5
Figure 3a. Fluorescence titration spectrum of receptor 1 (2.5 ꢁ 10 M in CH
3
CN)
ꢀ
5
ꢀ3
Figure 2a. UV–vis spectral changes of receptor 1 (2.5 ꢁ 10 M in CH
3
CN) upon
upon the gradual addition of (0–100
l
L) TBAF (1.5 ꢁ 10 Min CH
3
CN). (Inset:
ꢀ
ꢀ3
ꢀ
titration with F ion (1.5 ꢁ 10 M in CH
3
CN). (Inset: changes of absorbance upon
changes of fluorescence emission upon addition of F ion at 510 nm.) (Excited at
-
addition of F ion at 520 nm).
460 nm).

5
070
D. Renuga et al. / Tetrahedron Letters 53 (2012) 5068–5070
Acknowledgements
N
C
O
S
C
O
N
S
H
H
H
Authors express their thanks to DRDO (ERIP/ER/1006004/M/01/
1333 dated 23-05-2011) for financial assistance in the form of a
major sponsored project.
N
N
TBAF
F-
OH
HO
H
ꢀ
Scheme 2. Possible structure of complex formed between receptor 1 and F .
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.06.
1
47.
Rec
-
9
7
6
4
3
1
0
5
0
5
0
5
Rec+2 eq F
-
4
References and notes
Rec+2 eq H PO
2
-
Rec+2 eq AcO
-
1. Oehme, O. S.; Wolfbeis, A. Microchim. Acta 1997, 126, 177–192.
2
3
Rec+2 eq OH
.
.
Hisamoto, H.; Suzuki, K. Anal. Chem. 1999, 18, 513–524.
(a) Haddou, H.; Wiskur, S.; Lynch, V.; Anslyn, E. V. J. Am. Chem. Soc. 2001, 123,
11296–11297; (b) Toal, S. J.; Trogler, W. C. J. Mater. Chem. 2006, 16, 2871–2883;
-
Rec+2 eq Cl
-
Rec+2 eq Br
(
c) Gunnlaugsson, T.; Kruger, P.; Jensen, P.; Tierney, J.; Ali, H.; Hussey, G. J. Org.
Chem. 2005, 70, 10875–10878.
4
.
(a) Selective reviews and books for chemosensor; Valeur B; Molecular
Fluorescence; Wiley-VCH: Weinheim, 2002; (b) Lakowicz, J. R.; 4, Probe
Design and Chemical Sensing; 1994.; (c) De Silva, A. P.; Gunaratne, H.;
Gunnlaugsson, T.; Huxley, A.; McCoy, C.; Rademacher, J. T.; Rice, T. E. Chem. Rev.
1997, 97, 1515–1566; (d) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev.
2
2
000, 100, 2537–2574; (e) Que, E. L.; Domaille, D. W.; Chang, C. J. Chem. Rev.
008, 108, 1517–1549.
5. Kleerekoper, M. Endocrinol. Metab. Clin. North Am. 1998, 27, 441–452.
6.
Zhipei, Y.; Kai, Z.; FangbinGong; Shayu, L.; Jun, C.; Jin, S. M.; Lyubov, N. S.;
Albina, I.; Mikhaleva; Boris, T. A.; Guoqiang, Y. Environ. Health Crit. 2002, 8, 227.
Gale, P.A.; Atwood, J.L.; Steed, J.W.; (Eds.), Encyclopedia of Supramolecular
Chemistry; Marcel Dekker, New York, 2004, 31-41.
4
80
500
520
540
560
580
600
Wavenumber (nm)
7.
ꢀ
5
Figure 3b. Fluorescence titration spectrum of receptor 1 (2.5 ꢁ 10 M) upon
8. Gale, P.A.; Sessler, J.L.; Atwood, J.L.; Steed, J.W.; (Eds.), Encyclopedia of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
titration with anions in CH
3
CN (H
2
PO
4
, AcO , OH , Cl , Br ).
Supramolecular Chemistry; Marcel Dekker, New York,2004, 1176-1185.
Pfeffer, F. M.; Buschgens, A. M.; Barnett, N. W.; Gunnlaugsson, T.; Kruger, P. E.
9
.
Tetrahedron Lett. 2005, 46, 6579–6584.
10. Turner, D. R.; Smith, B.; Spencer, E. C.; Goeta, A. E.; Evans, I. R.; Tocher, D. A.;
Howard, J. A. K.; Steed, J. W. New J. Chem. 2005, 29, 90–98.
1. Turner, D. R.; Paterson, M. J.; Steed, J. W. J. Org. Chem. 2006, 71, 1598–1608.
2. Pfeffer, F. M.; Lim, K. F.; Sedgwick, K. J. Org. Biomol. Chem. 2007, 5, 1795–1799.
strong emission band at 510 and 545 nm. Fig. 3a shows the spectral
variation of the receptor upon gradual addition of TBAF.
1
1
Observed enhancement in fluorescent intensity upon the incre-
mental addition (0–2 equiv) of F ions may be due to the efficient
charge transfer between guest–host species. This highly indicates
13. Kang, S. O.; Begum, R. A.; James, K. B. Angew. Chem. Int. Ed. 2006, 45, 7882–
ꢀ
7894.
14. Boiocchi, M.; Boca, L. D.; Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M.;
Monzani, E. J. Am. Chem. Soc. 2004, 126, 16507–16514.
that there is a strong sensing action taking place between receptor
15. Ghosh, K.; Masanta, G.; Chattopadhyay, A. P. Tetrahedron Lett. 2007, 48, 6129–
132.
ꢀ
6
1
and F ion through hydrogen bonding formation as shown in
1
1
6. Veale, E. B.; Gunnlaugsson, T. J. Org. Chem. 2008, 73, 8073–8076.
7. (a) Udhayakumari, D.; Saravanamurthy, S.; Ashok, M.; Velmathi, S. Tetrahedron
Lett. 2011, 52, 4631–4635; (b) Velmathi, S.; Reena, V.; Suganya, S.; Anandan, S.
J. Fluor. 2012, 22, 155–162; (c) Prabhu, S.; Saravanamoorthy, S.; Ashok, M.;
Velmathi, S. J. Lumin. 2012, 132, 979–986.
Scheme 2. Emission spectrum of receptor 1 with other anions is
shown in Figure 3b. Only in the case of acetate ions there was a
marginal increase in the emission intensity and other anions did
not show any change. Thus it can be concluded that the receptor
18. Fridman, N.; Kaftory, M. Pol. J. Chem. 2007, 81, 825–832.
ꢀ
1
can be utilized as a selective chemosensor for F ions in the pres-
1
9. Synthesis of receptor 1: To a hot solution of 2,5-thiophene dicarboxaldehyde
(1 mmol, 0.139 g) in 10 mL of ethanol was added 2 mmol (0.218 g) of o-amino
phenol in 20 mL of ethanol and the mixture was slowly refluxed for 3 h at
ence of other interfering ions.
In conclusion, we have designed and synthesized a highly sen-
70 °C. On cooling, a yellow coloured solid separated out, it was filtered and
ꢀ
sitive thiophene based chemosensor for the detection of F ion
washed with ethanol and then dried in vacuum. Yield was 0.255 g (79%). The
ꢀ
1
selectively. The chemosensor can be utilized for the detection of
structure of the ligand was confirmed by: IR (KBr
(C@N), 1452 (C@C), 1227 (C–O), 1368 (C–N). 1H NMR (d ppm, 400 MHz, DMSO-
): 6.81 (1H, s), 6.83 (1H, dd), 6.95 (1H, s), 7.18 (1H, dd), 7.7 (1H, s), 8.9 (1H, s),
m cm ) 3376 (OH), 1598
ꢀ
ꢀ
ꢀ
ꢀ
F
in the presence of competing anions such as Cl , Br , AcO ,
d
6
ꢀ
ꢀ
OH , and H
2
PO
4
ions. The changes in absorption and fluorescence
13
9
.2 (1H, s); C NMR (d ppm, 100 MHz, DMSO-d
127.68, 137.19, 150.96, 133.11, 146.08, 152.87.
0. Gabr, A. A. Spectrochim. Acta A. 1990, 46, 1751–1757.
6
): 116.22, 119.61, 122.44,
spectra were driven by the hydrogen bonding between receptor 1
and fluoride anions.
2