Rationally designed tri-armed imidazole–indole hybrids as naked eye receptors for fluoride ion sensing

Article abstract of DOI:10.1080/00397911.2017.1324625

The present communication describes the design, synthesis, and characterization of unique tri-armed imidazole–indole hybrids RA-RE for naked eye detection of fluoride ion in 9:1 DMSO–water at concentration level of 1.5 ppm, which is recommended permissible limit of fluoride ion. Molecular structures of the receptors are so fabricated that both heterocyclic units, namely, indole and imidazole have been rationally used as chromophore and binding unit, respectively. Strategical introduction of nitro group(s) at the fifth position of indole ring remarkably enhances the binding ability of receptor RD making it highly selective toward fluoride ion (tetrabutylammonium salt) over other typical anionic species. The sensing event can be visualized by naked eyes swiftly with color change. This observation is well corroborated by a redshift of 80 nm in UV–visible spectroscopic studies. A peak at 16.1 ppm due to HF₂^- in ¹H NMR titration validates the deprotonation of imidazolium N?H by fluoride ion.

Full text of DOI:10.1080/00397911.2017.1324625

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Rationally designed tri-armed imidazole-indole
hybrids as naked eye receptors for fluoride ion
sensing
Anshu Jain, Ragini Gupta & Madhu Agarwal
To cite this article: Anshu Jain, Ragini Gupta & Madhu Agarwal (2017): Rationally designed
tri-armed imidazole-indole hybrids as naked eye receptors for fluoride ion sensing, Synthetic
Communications, DOI: 10.1080/00397911.2017.1324625
To link to this article: http://dx.doi.org/10.1080/00397911.2017.1324625
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Date: 02 May 2017, At: 21:59

Rationally designed tri-armed imidazole-indole hybrids as naked eye receptors for
fluoride ion sensing
Anshu Jain1, Ragini Gupta1,2, Madhu Agarwal3
1Department of Chemistry, Malaviya National Institute of Technology, Jaipur, India,
2Materials Research Centre, Malaviya National Institute of Technology, Jaipur, India,
3Department of Chemical Engineering, Malaviya National Institute of Technology,
Jaipur, India
Received 1 February 2017
Corresponding author Ragini Gupta. E-mail: guptaragini@yahoo.com,
rgupta.chy@mnit.ac.in
Abstract
The present communication describes the design, synthesis and characterization of
unique tri-armed imidazole-indole hybrids RA-RE for naked eye detection of fluoride
ion in 9:1 DMSO-water at concentration level of 1.5 ppm, which is recommended
permissible limit of fluoride ion. Molecular structures of the receptors are so fabricated
that both heterocyclic units, viz., indole and imidazole have been rationally employed as
chromophore and binding unit, respectively. Strategical introduction of nitro group(s) at
the 5th position of indole ring remarkably enhances the binding ability of receptor RD
making it highly selective towards fluoride ion (tetrabutylammonium salt) over other
typical anionic species. The sensing event can be visualized by naked eyes swiftly with
colour change. This observation is well corroborated by a red shift of 80 nm in UV-
-
visible spectroscopic studies. A peak at 16.1 ppm due to HF2 in 1H NMR titration
validates the deprotonation of imidazolium N-H by fluoride ion.
GRAPHICAL ABSTRACT
1

KEYWORDS: colorimetric sensing, fluoride receptor, imidazole, indole, naked eye
sensing
INTRODUCTION
Anion complexation chemistry has evolved as a stimulating multidisciplinary research
area across chemistry, environment and biology, considering the importance of anion-
molecule interactions in the areas of medicine, materials development, physiology,
synthetic chemistry and analyte sensing [1]. From spherical fluoride ion to tetrahedral
sulphate and phosphate ions, anions find their importance everywhere in nature, with
involvement in vital functions and thus demand continuous research [2]. Drawing
inspiration from nature, together with acquired knowledge of chemistry, researchers are
artistically manipulating the chemical moieties to craft the diverse array of receptors for
the selective recognition of desired anion [3]. Till date, multitude of approaches have been
attempted with various functionalities to achieve selective binding with anions, most
2

commonly being preorganization of N-H and O-H moieties for effective hydrogen
bonding [4], tailoring the acidity of N-H group for deprotonation to occur [5], anion-π
interactions [6] and hydrophobic effects [7], etc.
Amongst the ubiquitous anions, fluoride anion recognition has captured a substantial
attention of scientific community because it is one of the trace element required in the
formation of teeth and bones in humans [8]. However, it is well documented that excessive
intake of fluoride ion through drinking water causes dental fluorosis, bone diseases and
lesions of thyroid and liver [9]. This proves that beneficial and injurious effects of fluoride
ion intake is concentration dependant, which has been set as 1.5 ppm in drinking water by
World Health Organization (WHO) [10]. Available conventional method to quantitatively
ascertain the concentration of fluoride in drinking water by lanthanum fluoride based
membrane electrode is time consuming and requires skilful handling, since the electrode
is quite fragile and expensive [11]. To overcome these obstacles, development of a simple
and robust procedure for the fabrication of anion receptor is essential, which can detect
analyte via naked eye and give semi quantitative information about safe/ unsafe fluoride
levels. In view of this requirement, colorimetric receptors provide easy detection of
analyte via naked eye under visible light [12]
.
A review of literature reveals the employment of molecules containing functional groups,
urea/thiourea [13], secondary amide [14] and thiosemicarbazide [15] to effectively and
selectively bind fluoride ion through the formation of hydrogen bond with N-H units of
these groups. Covalent attachment of these molecules with appropriate signalling unit,
3

such as, nitrophenyl, azo dye etc. yields the complete receptor, which can convert anion-
receptor binding event into either colour or fluorescence change as an output [16]
.
Heterocycles, which have recently been introduced as anion receptors, can act as both
chromophore and binding units [17]. Interesting photophysical properties of heterocycles
make them strong contenders for chromophoric unit and further, inbuilt acidic hydrogen
bond donor groups (N-H) in several heterocyclic moieties can serve as binding sites for
anions [18]. Amongst these heterocycles, imidazole N-H is acidic enough to act as an
excellent hydrogen bond donor [19]. Moreover, acidity can be tailored by proper insertion
of electron withdrawing substituents in the ring at critical positions and it can be
annulated with another imidazole, benzene, anthraquinone, naphthalene and
naphthalimide units for enhancing its NH acidity, rendering it fit to effectively bind
fluoride ion by several research groups [20]. Recently, quinone-ferrocene systems bridged
by imidazole moiety and quinone-imidazole system have been reported by Elango et. al.,
which exhibited naked eye response towards fluoride and cyanide ions in aqueous media
at neutral pH [21]. Although an appreciable number of receptors are known for fluoride
ion detection, their applicability to real life applications is restricted to very few since
most of them are urea/thiourea based, which exhibit multi-anion sensitivity and lack
selectivity [22]. Further, there exists a challenge to develop receptor for detecting fluoride
ion in competitive media, owing to its high hydration enthalpy (-505 KJ/mol) [23]
.
Taking into perpective all this scenario [24], it was envisaged that the introduction of
electron withdrawing nitro group at 5th position of indole ring could fine-tune the binding
4

property of imidazole moiety and serve as selective naked eye receptor for fluoride ion
sensing. To the best of our knowledge, this system has not been documented earlier.
RESULTS AND DISCUSSION
Compounds RA-RE have been synthesized by environment friendly protocol (Scheme
1). 1,2-bis(indolyl)-ethane-1,2-dione derivatives (3a-c) were prepared in stepwise
reaction of appropriately substituted indole (1a-c) and oxalyl chloride in dichloroethane
to give compound 2a-c, which on further treatment with indole derivatives in presence of
aluminium chloride afforded compounds 3a-c. Compounds 3a-c are irradiated with
formyl indole derivatives (4a-c) and ammonium acetate in polyethylene glycol (PEG
400) at 300 W and 180o C in microwave to yield desired final product RA-RE in good
yields. Compared with the traditional method, this protocol offers several advantages like
excellent yields, shorter reaction times, minimal environmental effects and clean reaction.
Reaction proceeds without the addition of acid or base catalyst.
All the compounds were characterized by 1H NMR, 13C NMR, FTIR and mass
spectroscopy. FTIR of receptors RA-RD show characteristic broad absorption band due
to N-H stretching of imidazolium N-H and indolic N-H in the region 3417-3313 cm-1
and 3185-3055 cm-1 respectively. Receptor RE shows absorption band at 3375 cm-1 due
to N-H stretching of only imidazole N-H. A broad peak from 2978-2914 cm-1 is assigned
to aromatic C-H vibration. Absorption band from 1644-1618 cm-1 and 1328-1208 cm-1
have been assigned to C=C and C-N streching vibrations. Two bands in the region 1550-
1515 cm-1 and 1390-1350 cm-1 are observed due to N-O stretching in nitro group(s)
5

present in receptors RB-RE. 1H NMR spectra of receptors, RA-RD exhibit characteristic
sharp singlets due to imidazolium N-H and indole N-H in the region δ 12.10-12.55 and δ
9.7-9.81 ppm, respectively. Receptor RE displays a sharp singlet at δ 12.15 ppm due to
imidazolium N-H. A multiplet from δ 7.11-8.28 ppm is observed for aromatic protons.
Peaks at m/z 414, 549, 459, 504 and 849 [M+H]+ in the mass spectra of RA-RE, also
confirm the formation of products.
Colorimetric Sensing, Interference Studies And UV-Visible Titration Experiment
The anion recognition proficiencies of receptors RA-RE (10-4 M in DMSO) were
vigilantly probed by naked eye, UV-visible and NMR spectroscopic techniques using
different anions as their tetrabutylammonium salts (TBA) viz., fluoride, acetate, chloride,
bromide, iodide, dihydrogen phosphate, hydrogen sulphate and nitrate in competitive
media, 9:1 DMSO-water. Receptors RA-RC were found insensitive to the addition of
anions and no change in colour was noticed in their solutions, even at high concentration
(10-3 M) of anions (TBA salts). On the other hand, an instant and distinctive colour
change from yellow to orange was observed on addition of TBA salt of fluoride ion into
receptor RD (10-4 M in DMSO) (Fig 1(a)). It is worthy to mention that detection limit for
fluoride ion detection is found to be 1.5 ppm, which is regarded as the maximum
permissible limit for fluoride in drinking water. Other anions, when were added to
receptor individually, they did not show any colour change (Fig. 1(a)). However, when 1
equivalent of fluoride ion (TBA salt) was added to these solutions, an instantaneous
change in colour was noticed in receptor RD (10-4 M in DMSO) (Fig. 1(b)), similar to
colour change induced by fluoride ion alone. It establishes that receptor RD can
6

selectively bind fluoride ion irrespective of the presence of other anions. Another
receptor RE was found to give similar qualitative colour change with fluoride ion as
receptor RD. In order to enhance the binding affinity and selectivity, electron
withdrawing nitro group(s) was introduced at 5th position of indole rings in molecular
framework of receptors and it was observed that receptors RD and RE gave promising
results in terms of naked eye change in presence of fluoride ion (TBA salt).
Further investigation on anion affinity studies of receptor RD is performed using UV-
visible spectrophotometer to corroborate the initial qualitative studies. Receptor RD
reveals a maxima at 330 nm in UV-visible spectra. Different anions, acetate, chloride,
bromide, iodide, dihydrogen phosphate, hydrogen sulphate and nitrate (TBA salts), when
added into the solution of receptor RD in DMSO, produces no change in spectrum of
receptor (Fig. 2). On the other hand, when fluoride is added, peak at 330 nm disappears
and a new peak at 410 nm appears. The red shift of 80 nm establishes that the binding
event takes place between fluoride ion and receptor. The results obtained establishes the
selective and sensitive nature of receptor for fluoride spectroscopically. To ascertain the
receptor-fluoride ion interaction, UV-visible titration experiment was carried out with
incremental addition of fluoride to receptor solution. On titration with fluoride from 10-5
M to 10-3 M, peak at 330 nm decreases and absorbance of new peak at 410 nm increases
(Fig. 3). From the results obtained, the formation of hydrogen bonding interaction
between fluoride ion and receptor is expected to be responsible for the large
bathochromic shift in the maxima of receptor [25]. However, more information on the
binding process can be obtained by 1H NMR titration only.
7

Determination of binding constant
Continuous variation method is employed for the determination of stoichiometric ratio of
the complex formed between fluoride ion and receptor RD, where the concentration of
both receptor and fluoride ion salt are kept constant (10-4 M in DMSO). The molar
fraction of fluoride/ (receptor + fluoride) is continuously varied. At the molar fraction of
0.50, the absorbance reaches its maxima, revealing that receptor forms 1:1 complex with
fluoride ion (Fig. 4). 1:1 Stoichiometry was also proved by mass spectra of receptor RD
with fluoride ion (TBA salt), which exhibited a peak at m/z 568.0351 [Receptor RD + F-
+H+] (Fig. 5). Figure 6 depicts the change in absorbance at wavelength 410 nm upon
gradual addition of fluoride ion (10-5 to 10-3 M) into receptor RD solution (10-4 M in
DMSO).
The binding constant of receptor RD with fluoride is evaluated by Benesi Hildbrand
equation [26]
1/A-Amin = 1/( ΔAmax + (1/K [F−])(1/ΔAmax).
Here, ΔAmax = Amax− Amin, where, Amin, A, Amax are the absorptions of receptor
RD considered in the absence of F−, at an intermediate, and at a concentration of
complete concentration. K is binding constant, [F−] is concentration of F−. From the plot
of 1/ (A−Amin) against [F−] for receptor RD, the value of K (+10%) was calculated from
the ratio of intercept/slope. Binding constant K, calculated from the graph (Fig. 7) is
found to be 0.15 x 104 M-1.
1H NMR Titration Experiment
8

The foregoing result of UV-visible spectroscopic titration indicates towards the
formation of binding interaction between receptor RD and fluoride ion. In order to affirm
the mechanism of sensing event, 1H NMR titration is carried out. Receptor RD solution is
prepared in DMSO-d6 (10-2 M) and fluoride ion in the form of its TBA salt of varying
concentrations (2.5, 5, 7.5 and 10 equivalents) has been added sequentially (Fig. 8). It is
observed in the 1H NMR spectra of receptor RD, the sharp singlet at δ 12.2 ppm,
corresponding to imidazole N-H, shows a downfield shift upon addition of fluoride ion
and intensity of peak decreases as the concentration of fluoride ion increases, which
disappears altogether, when fluoride concentration reaches 10 equivalents. Indole N-H at
δ 9.8 ppm remains unchanged during titration. It is evident that interaction between
receptor and fluoride ion initiates by hydrogen bonding, as depicted by downfield shift of
imidazolium N-H proton in initial stage. Later, excess addition of fluoride ion triggers the
deprotonation process, witnessed by the disappearance of N-H peak and appearance of
-
peak corresponding to HF2 ion at 16.1 ppm [27]
Further evidence is provided by the synthesis of compound RE, where three indole NH
are protected by di-tert-butyl dicarbonate (Boc). It exhibits naked eye colour change from
yellow to orange in presence of fluoride ion similar to receptor RD, which proves that
three indole NH are not involved in binding process.
On the basis of above studies, the plausible mechanism of binding between RD and
fluoride ion is depicted in Scheme 2.
9

The plethora of anion binding studies conducted has shown that the introduction of nitro
group at the 5th position of the indole ring enhances the binding ability of receptor RD
(10-4 M in DMSO) towards fluoride ion (1.5 ppm). Further, it gave a visible colour
change from yellow to orange upon addition of fluoride ion into the solution of receptor
RD in DMSO. This phenomenon may be explained by the fact that nitro groups are
electron withdrawing in nature and their insertion onto the molecular framework of
receptor RD increased the acidity of imidazole N-H, which can establish hydrogen
binding interactions with highly basic fluoride ion. This event was followed by
deprotonation of N-H, which was evident in 1H NMR titration, where a new peak at 16.1
-
ppm appeared due to HF2 ion. Deprotonation triggers an extended conjugation or π-
delocalisation and alters the dipole associated to the charge-transfer transition or in other
words, stabilizes the excited state of chromophoric group [28]. This ultimately is observed
as a vivid colour change as output. Similar findings have been documented by other
research groups too [6, 29]
.
Ph Effect On RD-F- Interaction
To investigate the effect of pH on receptor RD-F- binding affinity, changes in the
intensity of the absorbance band at 410 nm were observed over a pH range 2-12 (Fig. 9).
The working pH range was found to be 6.5-8.0, where the intensity of absorbance
remains constant. Below pH 6.5, intensity of absorbance band at 410 nm decreased
rapidly. This is probably due to protonation of fluoride ion, to form weakly ionized
hydrofluoric acid, which decreases the affinity of fluoride ion to bind receptor binding
site, N-H. Above pH 8.0, intensity of this absorption band increased, which can be
10

attributed to increased availability of deprotonated fluoride ion to establish stronger
hydrogen binding interactions with receptor’s binding site, N-H.
CONCLUSIONS
Structurally simple and easy to synthesize imidazole-indole based receptor RD has been
synthesized for easy and robust detection of inorganic fluoride in 9:1 DMSO-water. It
senses fluoride at 1.5 ppm with colour change from yellow to orange, perceivable by
naked eye. Detection of fluoride in aqueous media makes the receptor suitable for
practical applications.
EXPERIMENTAL
All chemicals were purchased from Sigma Aldrich, TCI and Spectrochem Chemicals,
India and were used without further purification. Starting substrates, 3-formylindoles
were prepared by following literature [30]
.
Melting points were determined in open glass capillaries and are reported uncorrected. 1H
NMR and 13C NMR were recorded on a Jeol ECS 400 MHz spectrophotometer using
DMSO-d6 as solvent. TMS was taken as an internal standard and the chemical shifts are
reported in δ ppm. Resonance multiplicities are described as s (singlet), d (doublet), t
(triplet), q (quartet) and m (multiplet). FTIR spectra were recorded on a Perkin Elmer
Spectrum Two spectrophotometer using pressed KBr discs in the region of 400 – 4000
cm-1. Mass spectra were recorded on a Xevo G2-S Q-Tof spectrometer (Waters, USA),
capable of recording high-resolution mass spectrum (HRMS) in the ESI (Electrospray
11

Ionization) modes. CEM discover mono mode microwave reactor with magnetron
frequency of 2455 MHz was used for microwave reaction. Ultrasonication synthesis was
performed using Elma S 70 H Ultrasonicator with 37 KHz output frequency. UV-visible
spectra was recorded on a Perkin Elmer Lambda 750 UV-Vis NIR spectrophotometer in
standard 3.5 mL quartz cells with 10 mm path length. The purity of all compounds was
checked by TLC using silica gel as adsorbent and solvents of increasing polarity as
mobile phase.
Synthesis Of Receptors 3-[2,5-{(Un)Substituted-1H-Indol-3-Yl}-1H-Imidazol-4-Yl]-
(Un)Substituted-1H-Indole (RA-RE)
A mixture of 1,2-bis(substituted-indolyl)-ethane-1,2-dione (3a-c) (1mmol), indole
aldehyde (4a-c) ( 1mmol) and ammonium acetate (4 mmol) in polyethylene glycol (2 ml)
was irradiated at 180 °C and 350 W power for 8-10 minutes. The progress of reaction was
monitored by thin film chromatography 4:1 (ethyl acetate: methanol). The reaction
mixture was cooled to room temperature and poured on 100 ml ice water. The separated
solid was filtered and washed with water to yield brown solid as desired title product.
3-[2,5-(1H-indol-3-yl)-1H-imidazol-4-yl]-1H-indole (RA): Brown solid, yield: 89.10 %,
m.p.: 164 oC. FTIR (KBr, υ cm-1): 3406 (Imidazole N-H), 3170 (Indole N-H), 2924 (=C-
H), 1644 (C=C), 1234 (C-N). 1H NMR (DMSO-d6, 400 MHz, ppm): δ (ppm) 7.38-7.94
(m, 15H, Ar-H), 9.23 (s, 3H, indole N-H), 12.10 (s, 1H, imidazole N-H). 13C NMR
(DMSO-d6, 100 MHz, ppm) 104.48, 105.55, 112.35, 117.811, 129.80, 133.46, 142.06,
147.98. MS (ESI) m/z: 414.1884 [M+H]+ Calculated for C27H19N5 : 413.1840.
12

SUPPLEMENTARY MATERIAL
Experimental details and 1H NMR, 13C NMR and mass spectral data for this article can
be accessed on the publisher’s website.
ACKNOWLEDGEMENTS
Authors are thankful to Materials Research Centre, Malaviya National Institute of
Technology, Jaipur for providing the spectral facilities. Financial support from
Department of Science and Technology-Water Technology Initiative (DST-WTI), New
Delhi is deeply acknowledged. One of the authors, A. Jain is also grateful to Council of
Scientific and Industrial Research (CSIR), New Delhi for awarding senior research
fellowship (SRF).
REFERENCES
[1] (a) Beer, P. D.; Gale, P. A. Angew. Chem. Int. Ed. 2001, 40, 486-516; (b) Sessler, J.
L.; Gale, P. A.; Cho, W. S. Anion Receptor Chemistry, Royal Society of Chemistry,
Cambridge, 2006; (c) Gale, P. A. Acc. Chem. Res. 2006, 36, 465-475.
[2] Martinez-Manez, R.; Sancenon, F. Chem. Rev. 2003, 4419-4476.
[3] Dydio, P.; Lichosyt, D.; Jurczak, J. Chem. Soc. Rev. 2011, 40, 2971-2985.
[4] Juwarker, H.; Suk, J. –M.; Jeong, K. –S. Top Heterocycl. Chem. 2010, 24, 177-204.
[5] Velmathi, S.; Reena, V.; Suganya, S.; Anandan, S. J. Fluoresc 2012, 22, 155-162.
[6] Amenola, V.; Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M. Acc. Chem. Res. 2006,
39, 343-353.
13

[7] (a) Schottel, B. L.; Chifotides, H. T.; Dunbar, K. R. Chem. Soc. Rev. 2008, 37, 68-
83; (b) Hay, B. P.; Custelcean, R. Cryst. Growth Des. 2009, 9, 2539-2545; (c) Inoue, Y.;
Hakushi, T.; Liu, Y.; Tong, L.; Shen, B.; Jin, D. J. Am. Chem. Soc. 1993, 115, 475-481.
[8] (a) Lu, W.; Jiang, H.; Hu, L.; Shen, Z. Tetrahedron 2011, 67, 7909-7912; (b) Hu, S.;
Guo, Y.; Xu, J.; Shao, S. Org. Biomol. Chem. 2008, 6, 2071-2075; (c) Sun, Y.; Liu, Y.;
Guo, W. Sens Actuators B 2009, 143, 171-176; (d) Lee, G. W.; Kim, N. –K.; Jeong, K. -
S. Org. Lett. 2010, 12, 2634-2637.
[9] Shang, X.; Yuang, J.; Wang, Y.; Zhang, J.; Xu, X. J. Mol. Struct. 2012, 1010, 52-58.
[10] Li, Q.; Guo, Y.; Xu, J.; Shao, S. Sens. Actuators B 2011, 158, 427-431.
[11] Frant, M. S.; Ross, J. W. Science 1966, 54, 1553-1555.
[12] Gunnlaugsson, T.; Kruger, P. E.; Jensen, P.; Pfeffer, F. M.; Hussy, G. M.
Tetrahedron Lett. 2003, 44, 8909-8913.
[13] (a) Formica, M.; Fusi, V.; Macedi, E.; Paoli, P.; Piersanti, G.; Rossi, P.; Zappia, G.;
Orlando, P. New J. Chem. 2008, 32, 1204-1214; (b) Brooks, S. J.; Edwards, P. R.; Gale,
P. A.; Light. M .E. New J. Chem. 2006, 30, 65-70.
[14] (a) Velmathi, S.; Reena, V.; Suganya, S.; Anandan, S. J. Fluoresc 2012, 22, 155-
162; (b) Lin, Z.; Ma, Y.; Zheng, X.; Huang, L.; Yang, E.; Wu, C. –Y., Chow, T. J.; Ling,
Q. Dyes Pigments 2015, 113, 129-137.
[15] (a) Chakraborty, S.; Arunachalam, M.; Dutta, R.; Ghosh, P. RSC Adv. 2015, 5,
48060-48070; (b) Kumar, S. L. A.; Kumar, M. S.; Sreeja, P. B.; Sreekanth, A.
Spectrochimica Acta A 2013, 113, 123-129.
[16] (a) Li, Q.; Guo, Y.; Xu, J.; Shao, S.; J. Photochem. Photobiol. B 2011, 103, 140-
144; (b) Hong, K. H.; Kim, H. J. Supramol. Chem. 2013, 25, 24-27.
14

[17] (a) Figueroa, L. E. S.; Moragues, M. E.; Raposo, M. M. M.; Batista, R. M. F.;
Ferreira, R. C. M.; Costa, S. P. G.; Sancenon, F.; Manez, R. M.; Soto, J.; Lis, J. V. R.
Tetrahedron Lett. 2012, 68, 7179-7186; (b) Su H., Li J., Lin H. and Lin H. J. Braz. Chem.
Soc. 2010, 21 (3), 541-545.
[18] Shiraishi, Y.; Sumiya, S.; Hirai, T.; Org. Biomol. Chem. 2010, 8, 1310-1314.
[19] Bao, X. –P,; Zheng, P. –C.; Liu, Y.; Tan, Z.; Zhou, Y. –H.; Song, B. –A.
Supramolecular Chem., 2013, 25, 4, 246-253.
[20] (a) Tan, C.; Wang, Q. Synthetic Metals 2012, 162, 1416-1420; (b) Shao, J.; Quiao,
Y.; Lin, H.; Lin, H. K. J Fluoresc 2009, 19, 183-188; (c) Zheng, P.; Shi, B. -B.; Wu, T. –
B.; Yu, Y. –M. Dyes Pigments 2013, 99, 857-862; (d) Chawla, H. M.; Gupta, T.
Tetrahedron Lett. 2013, 54, 1794-1797.
[21] (a) Satheshkumar, A.; Manivannam, R.; Elango, K. P.; J. Organ. Chem. 2014, 750,
98-106. (b) Jayasudha, P.; Manivannan, R.; Elango, K. P. Sens Accutator B, 2016, 237,
230-238.
[22] (a) Molina, P.; Tarraga, A.; Oton, F. Org. Biomol. Chem, 2012, 10, 1711-1724; (b)
Batista, R. M. F.; Costa, S. P. G.; Raposo, M. M. M. J. Photo. Chem. 2013, 259, 33-40.
[23] Kumari, N.; Jha, S.; Bhattacharya, S. J. Org. Chem. 2011, 76, 8215-8222.
[24] (a) Wang, J.; Yang, L.; Hou, C.; Cao, H. Org. Biomol. Chem., 2012, 10, 6271-6274.
(b) Wang, Y.; Lin, H.; Shao, J.; Cai, Z. S.; Lin; H. K. Talanta 2008, 74, 1122-1125.
[25] (a) Shao, J.; Wang, Y.; Lin, H.; Li, J.; Lin, H. Sens. Actuator B 2008, 134, 849-853;
(b) Jeyanthi, D.; Iniya, M.; Krishnaveni, K.; Chellappa, D. Spectrochimica Acta A, 2015,
136, 1269-1274.
[26] Benesi, H. A.; Hildebrand, J. H. J Am Chem Soc 1949, 71, 2703-2707.
15

[27] Pandian, T. S.; Choi, Y.; Srinivasadesikan, V.; Lin, M. –C.; Kang, J. New J. Chem.,
2015, 39, 650-658.
[28] (a) Lee, G. W.; Kim, N. –K; Jeong, K. –S. Org. Lett. 2010, 12(11), 2634-2637; (b)
Hu, S.; Guo, Y.; Xu, J; Shao, S. Spectrochimica Acta A 2009, 72, 1043-1046.
[29] Shang, X. F.; Xu, X. F. BioSystems 2009, 96, 165-171.
[30] Joshi, K. C.; Pathak, V. N., Chand, P. Indian J. Chem 1978, 16, 933-936.
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Scheme 1.
17

Scheme 2.
18

Figure 1. (a) Colour changes in the receptor RD (10-4 M in DMSO), in presence of
1equivalent of different anions (TBA salts) in 9:1 DMSO-water, where A, B, C, D, E, F
and G represent F-, Cl-, Br-, I-, CH3COO-, H2PO4-, HSO4- and NO3- ions, respectively.
(b) Colour changes in the receptor RD (10-4 M in DMSO), in presence of 1equivalent
each of fluoride and different anions (TBA salts) in 9:1 DMSO-water, where A, B, C, D,
E, F, G and H are receptor and, F- + Cl-, F- + Br-, F- + I-, F- + CH3COO-, F- + H2PO4-,
F- + HSO4- and F- + NO3-, respectively
19

Figure 2. UV-visible spectra of receptor RD (10-4 M in DMSO) upon addition of
different anions (TBA salts, 10-4 M) in 9:1 DMSO-water
20

Figure 3. UV-visible spectra of receptor RD (10-4 M in DMSO) upon addition from 0.1
to 10 equivalents of fluoride ion (TBA salt) in 9:1 DMSO-water
21

Figure 4. Jobs Plot with receptor RD (10-4 M in DMSO) and fluoride ion (TBA salt) 10-
4 in 9:1 DMSO-water
22

Figure 5. ESI-mass spectrum of complex of fluoride ion and receptor RD
23

Figure 6. Changes in absorbance at 410 nm of receptor RD (10-4 M in DMSO) with
increase in fluoride ion concentration (10-5 to 10-3 M in 9:1 DMSO-water)
24

Figure 7. Fitting curve of Benesi Hildbrand equation
25

Figure 8. Partial 1H NMR (400 MHz) spectra of receptor RD in DMSO-d6 (10-2 M) in
the presence of (a) 2.5, (b) 5, (c) 7.5 and (d) 10 equivalents of TBAF in DMSO-d6
26

Figure 9. Changes in absorbance of receptor RD (10-4 M in DMSO) and fluoride ion
(10-4 M in 9:1 DMSO-water) with varying pH (2-12)
27