Calix[4]amido crown functionalized visible sensors for cyanide and iodide anions

Article abstract of DOI:10.1039/d1ra03608d

This study comprises the design and development of calix[4] arene-amido-based ionophoresby varying structural stringency and steric hindrance at the lower rim to probe the anion sensing properties. The ionophores are prepared, purified, and characterized using various analytical techniques. The molecular structure of the most active ionophoreIis established by single-crystal X-ray characterisation. Out of various anions investigated, iodide and cyanide show the highest sensitivity towards the ionophores investigated. Both anions are sensitive enough to give a visibly distinct color change. The binding properties of the ionophores are established with¹H &¹²⁷I NMR, fluorescence, and UV-vis spectroscopy, revealing that three ionophores strongly interact with CN^?and I^?. The binding constants are calculatedviaBenesi-Hildebrand plots using absorption data. The time-dependent¹H NMR revealed strong hydrogen bonding between the OH and NH groups of the ionophore and cyanide anion. The¹²⁷I NMR shows the highest 27.6 ppm shift after 6 h for ionophoreI. The crystal structure revealed hydrogen bonding of N-H protons of the amide pendulum and phenolic oxygen of the calix rim. The Job's plot depicted the possibility of a 1?:?1 complex of ionophores with both anions.

Full text of DOI:10.1039/d1ra03608d

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Calix[4]amido crown functionalized visible sensors
for cyanide and iodide anions†
Cite this: RSC Adv., 2021, 11, 26644
*
Pragati R. Sharma,
Shubham Pandey, Apoorva Malik, Ganpat Choudhary,
*
Vineet K. Soni and Rakesh K. Sharma
This study comprises the design and development of calix[4] arene-amido-based ionophores by varying
structural stringency and steric hindrance at the lower rim to probe the anion sensing properties. The
ionophores are prepared, purified, and characterized using various analytical techniques. The molecular
structure of the most active ionophore I is established by single-crystal X-ray characterisation. Out of
various anions investigated, iodide and cyanide show the highest sensitivity towards the ionophores
investigated. Both anions are sensitive enough to give a visibly distinct color change. The binding
1
properties of the ionophores are established with H & 127I NMR, fluorescence, and UV-vis spectroscopy,
revealing that three ionophores strongly interact with CNꢀ and Iꢀ. The binding constants are calculated
via Benesi–Hildebrand plots using absorption data. The time-dependent 1H NMR revealed strong
hydrogen bonding between the OH and NH groups of the ionophore and cyanide anion. The 127I NMR
shows the highest 27.6 ppm shift after 6 h for ionophore I. The crystal structure revealed hydrogen
bonding of N–H protons of the amide pendulum and phenolic oxygen of the calix rim. The Job's plot
depicted the possibility of a 1 : 1 complex of ionophores with both anions.
Received 8th May 2021
Accepted 29th July 2021
DOI: 10.1039/d1ra03608d
rsc.li/rsc-advances
fact that several instrumental methods have been developed for
the detection of cyanide and iodide ions, they are time-
Introduction
consuming, expensive, and cumbersome due to portability,
calibration, and sample preparation. Hence there is a great
need to develop colorimetric chemosensors for anions like
cyanide and iodide, which are low cost and nontoxic and can be
used with ease. Recently impressive advances have been made
towards the development of visible cyanide sensors introducing
molecules and materials such as pillararene,8 naphthofuran
carbohydrazide,9 supramolecular gel,10 naphthalene,11 cyclo-
dextrin,12 curcumin13 chromone,14 pyrene-thiacalix[4]arenes,15
calix[4]arene–naphthalimide,16 calix[3]pyrrole,17 Schiff bases,18
nitrobenzoxadiazole,19 and organogelators.20 For iodide detec-
tion, gold nanoparticles,21 poly(vinylpyrrolidone)-supported
copper nanoclusters,22 bidentate ureido-dihomooxacalix[4]
arene,23 hexahomotrioxacalix[3]arene,24 and porous ionic poly-
mer7 based colorimetric sensors are noteworthy. Recently, we
have reported dipicryl hydrazine based colorimetric sensors for
selective determination of anions like uoride, acetate,
hydroxide, cyanide, and hydrogen sulfate. The anion binding
ability of dipicrylhydrazine with various anions like uoride,
acetate, hydroxide, cyanide, and hydrogen sulfate is due to polar
non-polar interactions and hydrogen bonding.25 Continuing
our efforts, hereby we are reporting furfuryl and benzyl func-
tionalized calix[4]amido crown-based visual molecular sensors
for cyanide and iodide ions with ppm level detection limit.
Limited studies pertain to the application of calix[4]amido
crowns have been explored for the detection of transition metal
Supramolecular based anion-selective visual sensors have
drawn signicant attention in the past two decades1 due to their
vital role in the environment and biological systems. The
detection of anions is more challenging than cations due to
their bigger size, structural specicity, basicity, and nucleo-
philicity2. Among various water-soluble anions, biologically
important cyanide (CNꢀ) and iodide (Iꢀ) are of great interest.
Cyanide is highly toxic because of its deleterious effects on
human health in minute quantities. Cyanide is also involved in
chemicals, polymers, metal mining industries, and chemical
warfare agents.3 As per the WHO guidelines, the maximum
allowed cyanide concentration in drinking water is two ppb per
liter.4,5
In contrast, iodine is an essential micronutrient for the
human body, responsible for producing thyroid hormones.
Iodine deciency can also cause enlarged thyroid glands,
thyroid cancer, and pregnancy complications for babies.
Excessive ingestion can lead to hyperthyroidism diseases like
Graves' disease.6 Thus, there is a dire need to develop easy,
quick, and selective sensors in diverse samples.7 Despite the
Department of Chemistry, Sustainable Materials and Catalysis Research Laboratory
(SMCRL), Indian Institute of Technology, Jodhpur NH 65, Karwar, Jodhpur 342037,
India. E-mail: rks@iitj.ac.in; pragati@iitj.ac.in
† Electronic supplementary information (ESI) available. CCDC 1506051. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1ra03608d
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cations26 and limited anions.27 The amide moiety of calix[4]
amido crowns have a unique role in sensory behavior since N–H
can interact with anions via selective hydrogen bonding
donor.28–31 The furfuryl and benzyl substituent attach with
amide moiety provide secondary interactions with chromo-
phoric assistance, essential for the colorimetric sensor. These
studies are unexplored in the context of cyanide and iodide. The
current study also throws the light on detailed synthesis of
furfuryl and benzyl functionalized calix[4]amido crowns, their
stereochemistry, rigidity, steric hindrance, and interaction
behavior using diverse analytical techniques such as NMR,
XRD, and UV-vis.
Synthesis of ionophore I
To a solution of calix[4]amidocrown-5 (0.91 g, 1.5 mmol) in dry
CH3CN (70 mL) were added N-benzyl chloroacetamide (0.83 g, 4.5
mmol) and K2CO3 (1.04 g, 7.5 mmol). The reaction mixture was
reuxed for 48 h. The reaction mixture was quenched by addition
of 5% HCl (10 mL) and CH2Cl2 (40 mL). The organic phase was
separated and washed with water (3 ꢁ 40 mL), dried over anhy-
drous sodium sulphate, and evaporated in vacuo. The crude
product was recrystallized with CHCl3 and methanol to yield
a white crystalline compound (0.89 g, 78.95%) as shown in
Scheme 1. mp 229 ꢂC, 1H-NMR: (500 MHz, CDCl3 see ESI Fig. S1†)
d 8.82 (t, 2H, CONH), 7.80 (s, 2H, OH), 7.64 (t, 1H, NH), 7.37–7.31
(m, 5H, benzyl), 7.13 (d, 4H, ArH), 6.95 (d, 4H, ArH), 6.85 (t, 2H,
ArH), 6.78 (t, 2H, ArH), 4.48 (d, 2H, benzyl), 4.40 (s, 4H, OCH2),
4.08 (d, 4H, ArCH2Ar AB system), 3.53 (br s, 4H, CONHCH2), 3.48
(d, ArCH2Ar, AB system), 3.26 (s, 2H, benzyl), 2.93 (br, s, 4H,
CONHCH2CH2). 13CNMR: (see ESI Fig. S2†); d 71.1, 168.7, 151.9,
150.9, 132.5, 129.7, 129.2, 129.1, 128.8, 128.6, 128.5, 127.9, 127.7,
Experimental section
Experimental materials
All reactions and manipulations were routinely performed
under an inert atmosphere. Solvents such as toluene, ethanol,
tetrahydrofuran, acetonitrile were purchased from Merck and 127.6, 127.2, 126.9, 126.7, 120.9 (ArCH, ArC, CONH), 74.8 (OCH2),
puried by standard procedures, and freshly distilled before 59.9, 54.9 (benzylC), 44.19, 43.2, 39.9, 37.1, 31.9, 31.5
use. Reagents such as p-tert-butyl phenol and diphenyl ether, (CONHCH2CH2, ArCH2Ar), 30.3, 30.1, 29.7, 29.4, 27.1, 22.7, 19.8,
ethyl bromoacetate, chloroacetylchloride, and diethylenetri- 14.1 (benzylC). Maldi TOF MS ES+ (m/z) calculated for
amine were purchased from Sigma-Aldrich and used as C45H46O7N4, 754.34, found 755.34 [M + H+]+, 756.34 [M + 2H+]+
(see ESI Fig. S14†) CHN Anal. calcd for C45H46O7N4: C 71.60, H
6.14, N 7.42, O 14.84; found: C 71.59, H 6.12, N 7.41, O 14.82.
received. The furfuryl amine, benzylamine, and all tetrabuty-
lammonium salts oꢀf anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, CNꢀ, OHꢀ, CH3-
COOꢀ, NO3ꢀ, ClO4 and HSO4ꢀ) were purchased from Alfa-
Aesar. Analytical thin-layer chromatography was carried out
on silica plates (SiO2,Merck 60 F254) Obtained from E. Merck
Chemical Co.
Synthesis of ionophore II
To a solution of calix[4]amidocrown-5 (0.91 g, 1.5 mmol) in dry
CH3CN (70 mL) were added N-furfurylchloroacetamide (0.78 g,
Physical measurement
UV-vis spectra were recorded using a Varian model Cary Win
400 UV/Vis spectrophotometer. The Perkin Elmer LS55 uo-
rescence spectrophotometer was used for recording uores-
cence spectra. Nuclear magnetic resonance spectra (1H NMR &
13C NMR) were recorded on a Bruker 500 MHz WB FT-NMR
spectrometer having proton noise decoupling mode with
a standard 5 mm probe. Chemical shis for 1H NMR spectra are
reported as d in parts per million (ppm) downeld from SiMe4 (d
0.0) and relative to the signal of chloroform-d (d 7.26, singlet).
Multiplicities were given as: s (singlet); d (doublet); t (triplet); q
(quartet) or m (multiplets). For proving strong binding between
ionophore hosts and anionic guests, 127I NMR was recorded
separately at 100 MHz under same solvent mixture ratio. The
melting point was checked by Buchi melting point apparatus M-
565. Mass spectra were recorded by Bruker microTOF-Q II using
ESI source, and elemental analysis was done on FLASH EA 1112
series from Thermo nnigan, Italy.
Synthesis and characterization of ionophores
The p-tert-butylcalix[4]aene, calix[4]arene, 1,3 diester calix[4]
arene and calix[4]amido crown-5 were prepared as per the re-
ported literature procedure.26a,32–34 N-Furfurylchloroacetamide
and N-benzylchloroacetamide reagents were synthesized
according to reported methods.35,36
Scheme 1 Showing synthetic procedure for compounds: (a) K2CO3,
BrCH2COOC2H5, acetone; (b) toluene/ethanol, diethylenetriamine; (c
and d) K2CO3, acetonitrile, 48 h, reflux (e) NaH, THF: DMF: 2.5 : 1, 72 h
reflux.
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mp 244 ꢂC; 1H-NMR: (500 MHz, CDCl3 see ESI Fig. S5†); d 8.82 (t,
2H, CONH), 7.66 (t, 1H, CONH), 7.41 (s, 2H, ArH), 7.28 (m, 6H,
benzyl), 7.20 (m, 4H, ArH), 7.11 (s, 4H, ArH), 6.85 (s, 4H, ArH),
4.47 (d, 2H, benzyl), 4.38 (s, 4H, OCH2), 4.04 (d, 4H, ArCH2Ar AB
system), 3.51 (br s, 4H, CONHCH2), 3.45 (s, 4H, CONHCH2),
3.40–3.37 (d, ArCH2Ar, AB system), 3.26 (s, 2H, benzyl), 2.91
(br, s, 4H, CONHCH2CH2), 1.63 (1H, NH), 1.30 (s, 18H, t-butyl),
0.99 (s, 18H, t-butyl). 13CNMR: (see ESI Fig. S6†); d 170.1, 167.9,
148.4, 147.6, 147.5, 142.53, 137.86, 130.9, 127.5, 126.5, 126.4,
126.1, 125.1, 124.6 (ArCH, ArC, CONH), 73.6 (OCH2), 58.7, 53.9
(benzylC), 42.16, 38.5, 37.1, 33.1, 32.9, (CONHCH2CH2, ArCH2-
Ar), 30.7, 30.5, 29.8, 29.7, 28.6, 21.8, 13.1 (benzyl C). Maldi TOF
MS ES+ (m/z) calculated for C70H87O8N5, 1125.66, found 1126.66
[M + H+]+, 1127.66 [M + 2H+]+ (see ESI Fig. S16†), CHN Anal.
calcd for C70H87O8N5: C 74.6, H 7.78, N 6.22, O 11.36; found: C
74.9, H 7.12, N 6.41, O 11.82.
Fig. 1 Colorimetric analysis for (a) ionophore I, (b) ionophore II, and (c)
ionophore III upon addition of different anions.
Visual detection experiment
Visual detection experiments were carried out for recognizing
the host–guest interactions of ionophores I–III with various
anions. In a typical experiment, a series of stock solutions of
anions (5 ꢁ 10ꢀ3 M) and ionophores (5 ꢁ 10ꢀ5 M) were
prepared in freshly distilled acetonitrile. Further, 2 mL aliquots
of stock solution were taken and mixed with 2 mL of the anion
solution. Typical photographs were taken with a digital camera.
All ionophores gave instant color change for iodide and cyanide
ions. Iodide and cyanide ion gave pink and wine color for
ionophore I, respectively, consistent with the color variation
displayed in Fig. 1. The ionophore II showed light pink color
with iodide while cyanide turned yellow. In contrast to iono-
phore I and II, ionophore III gives light yellow color with
cyanide, but there is no visible change in the case of iodide.
4.5 mmol) and K2CO3 (1.04 g, 7.5 mmol). The reaction mixture
was reuxed for 48 h. The solvent was removed in vacuo and the
residue was quenched by addition of 5% HCl (10 mL) and
CH2Cl2 (40 mL). The organic phase was separated and washed
with water (3 ꢁ 40 mL), dried over anhydrous sodium sulphate,
and evaporated in vacuo. The crude product recrystallized with
CHCl3 and methanol to yield a white crystalline compound
(0.80 g, 72.8%) as shown in Scheme 1. mp 258 ꢂC; 1H NMR: (500
MHz, CDCl3 see ESI Fig. S3†); d 8.76 (t, 2H, CONH), 7.79 (s, 2H,
OH), 7.54 (t, 1H, NH), 7.12 (m, 1H, furfuryl) 7.07–7.05 (d, 4H,
ArH), 6.90–6.88 (d, 4H, ArH), 6.78 (t, 2H, ArH), 6.71 (t, 2H, ArH),
6.18 (s, 2H, furfuryl), 4.45 (s, 4H, OCH2), 4.39 (d, 2H, CH2 fur-
furyl), 4.09 (d, 4H, ArCH2Ar, AB system), 3.47 (d, 4H, ArCH2Ar,
AB system), 3.42 (d, 4H, CH2), 3.18 (br s, 2H, CONHCH2), 2.82
(brs, 4H, CONHCH2CH2). 13CNMR: (see ESI Fig. S4†): d 171.1,
168.7, 152.2, 150.9, 141.9, 132.5, 129.7, 129.2, 127.6, 126.7, 120.9
(ArCH, ArC, CONH), 110.5, 106.8 (furfuryl C), 74.9 (OCH2),
59.61, 54.92 (furfuryl C), 39.9, 36.3, 31.9, 31.5 (CONHCH2CH2,
ArCH2Ar), 29.7, 22.70, 14.1 (furfuryl C). Maldi TOF MS ES+ (m/z)
calculated for C43H44O8N4, 744.32, found 745.32 [M + H+]+,
746.32 [M + 2H+]+, 747.32 [M + 3H+]+ (see ESI Fig. S15†), CHN
Anal. calcd for C43H44O8N4: C 69.34, H 5.95, N 7.52, O 17.18;
found: C 69.30, H 5.91 N 7.41, O 17.10.
Interaction with anions by UV-vis study
In UV-vis titration experiment, 1 mL (5 ꢁ 10ꢀ3 M) stock solution
of tetrabutylammoniumanions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, CNꢀ, OHꢀ,
CH3COOꢀ, NO3ꢀ, ClO4ꢀ and HSO4ꢀ) was added to a 1 mL (5 ꢁ
10ꢀ5 M) acetonitrile solution of ionophore I–III in ascending
order in ten factions (1 : 100 ratio). The spectral changes were
recorded to ascertain the interaction/selectivity. Based on
spectral changes, titrations were performed with strongly
interacted anions cyanide and iodide to investigate the binding
sites and determine the binding constant. We have performed
competitive binding with otheꢀr anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, CNꢀ,
OHꢀ, CH3COOꢀ, NO3ꢀ, ClO4 and HSO4ꢀ) and found that
ionophores are showing selective binding and color change
towards cyanide.
Synthesis of ionophore III
To a solution of p-tert-butyl calix[4]amido crown-5 (1.24 g, 1.5
mmol) in dry THF (50 mL) and dry DMF (20 mL) under N2 was
added NaH (0.18 g, 7.5 mmol). The mixture was stirred at RT for
30 min followed by addition of N-benzyl chloroacetamide
(0.83 g, 4.5 mmol). The reaction mixture was reuxed for 72 h,
NMR spectral study
1
the solvent was removed in vacuo and quenched by the addition The H and 13C NMR spectra of ionophore I–III were recorded
of 5% HCl (10 mL) and CH2Cl2 (40 mL). The organic phase was before and aer the addition of selected anions (CNꢀ and Iꢀ) in
separated and washed with water (3 ꢁ 40 mL), dried over CDCl3 : CD3CN (1 : 2.5). In this experiment, 0.003 mmol of
anhydrous sodium sulfate and evaporated in vacuo. The crude ionophore I–III were dissolved separately in 0.5 mL mixture
product recrystallized with CHCl3 and methanol to yield a white containing CDCl3 : CD3CN (1 : 2.5), and further, add various
crystalline compound (1.126 g, 67.42%) as shown in Scheme 1. anions (20 molar equivalents) in the form of TBA salts. Proton
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NMR was recorded at different time intervals up to 24 h aer the
addition of anions.
Result and discussion
UV-vis absorption studies
Quantitative investigations were carried out to understand
cyanide and iodide anions' encapsulation behavior, in presence
of different ionophores using a spectrophotometric titration
method in CH3CN. The absorption spectra of ionophores I, II,
and III at variable concentrations of iodide and cyanide are
shown in Fig. 2 and 3. Ionophores I and II show a strong
absorption maximum at 279 nm, while III shows at 285 nm.
Single spectral band for anions and dual spectral bands were
observed for all ionophores from 273 to 290 nm. Upon
increasing the anions' concentration, the absorbance of iono-
phores is increased at both the maxima as per Fig. 2a–c (for
iodide) and Fig. 3a–c (for cyanide). In the absence of ionophore,
iodide and cyanide anions show their respective peaks at 249
and 275 nm, respectively. The absorbance increased with the
concentration of anions due to interaction with ionophores.
Changes in the absorbance of ionophores (measured against
the solvent as reference) upon the addition of anions were
observed at 279 nm for ionophore's I and II, and 285 nm for
ionophore III. Owing to their hydrophobic cavities, host iono-
phores offer the iodide and cyanide a suitable environment for
Structure determination by single-crystal X-ray study
Appropriate crystal was selected for ionophore I, dipped in
paratone oil, and mounted on cryo loop. Crystal data were
collected at 100 K using graphite monochromatic MoKa (l ¼
ꢀ
0.71073 A) radiation on a Bruker SMART APEX diffractometer
equipped CCD area detector. The SAINT soware37 was used for
data integration and reduction. Empirical absorption correction
was applied to the collected reections with SADABS.38
SHELXTL39 was used to solve the structures by direct methods
using and rened on F2 by the full-matrix least-squares tech-
nique using the SHELXL-97 (ref. 40) package. Non-hydrogen
atoms were rened anisotropically till convergence is reached.
The MERCURY 3.8 (ref. 41) is used to generate graphics. The X-
ray structure of ionophore I is shown in Fig. S17,† wherein
chloroform and water molecules have been omitted for clarity.
The phenolic OH of the alternate phenyl rings of the calix
moiety are substituted by amide-linked chains. The presence of
hydrogen bonding between amide NHs and phenolic oxygen
atoms is also observed. Crystallographic parameters for the
ionophore I are given in Table S1.†
Fig. 2 Absorption spectra of 5 ꢁ 10ꢀ5 M ionophores in the absence and presence of iodide at concentrations 5 ꢁ 10ꢀ5 to 5 ꢁ 10ꢀ6 M (a)
ionophore I (b) ionophore II (c) ionophore III (d) Benesi–Hildebrand absorption plot for 1 : 1 complexation of iodide with ionophores I–III.
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Fig. 3 Absorption spectra of 5 ꢁ 10ꢀ5 M ionophore's in the absence and presence of cyanide at concentrations 5 ꢁ 10ꢀ3 to 5 ꢁ 10ꢀ4 M (a)
ionophore I (b) ionophore II (c). Ionophore III (d) Benesi–Hildebrand absorption plot for 1 : 1 complexation of cyanide with ionophores I–III.
interaction to form inclusion complexes. The binding constants cyanide, the values are lower, i.e., 319 Mꢀ1, 304 Mꢀ1 and 209
for the formation of ionophores: iodide and ionophores: Mꢀ1, respectively, at 298 K. This behavior explains strong
cyanide complexes are determined by analyzing the absorbance complexation towards both anions in the following order: I > II
changes. The binding constant Ka and stoichiometry of the > III. It is observed that ionophore I gave a stronger Ka value
inclusion complexes of ionophores with iodide and cyanide can than II and III, indicating its superior interaction compared to
be determined by the Benesi–Hildebrand equation (Table 1).42 heavily alkylated and arylated II and III. Considering the
For ionophores I, II and III, the calculated binding constant structural features of the host and guest, we presumed that the
from the straight-line slope for iodide is found to be 552 Mꢀ1
,
deprotonation and charge transfer via polarization leads to
509 Mꢀ1, and 350 Mꢀ1, respectively. At the same time, for complexation with iodide. In contrast, hydrogen bonding
Table 1 Binding constants of the complexes via absorption spectral maxima for iodide and cyanide anions with Ionophores I, II and III
Monitored lmax
S. No.
Ionophores
l (nm)
(nm)
Binding constant Ka (Mꢀ1
)
R2
Complex
For iodide anion
1
2
3
I
II
III
273–284
273–284
280–290
279
279
285
552.0
509.0
350.0
0.9988
0.9999
0.9982
1 : 1
1 : 1
1 : 1
For cyanide anion
4
5
6
I
II
III
273–284
273–284
280–290
279
279
285
319.0
304.0
209.0
0.9996
0.9996
0.9993
1 : 1
1 : 1
1 : 1
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Table 2 Binding constants of the complexes via emission spectral maxima for iodide and cyanide anions with ionophores I, II and III
Monitored lmax
S. No.
Ionophores
l (nm)
(nm)
Binding constant Ka (Mꢀ1
)
R2
Complex
For iodide anion
1
2
3
I
II
III
375–385
375–385
380–384
379
379
380
834
729
498
0.9997
0.9995
0.9990
1 : 1
1 : 1
1 : 1
For cyanide anion
4
5
6
I
II
III
375–385
375–385
380–384
379
379
380
956
753
604
0.9994
0.9998
0.9991
1 : 1
1 : 1
1 : 1
between cyanide and amines of ionophore leads to deprotona-
tion and strengthens the host–guest association. All the above
phenomena are common in all ionophores. Since steric
hindrances are in the following order: III > II > I, bind in reverse
order.
The plot's good linearity with better correlation co-efficient
R2 more than 0.998 for all complexes shows the strong forma-
tion of 1 : 1 complexes between anions and ionophores I, II, and
III. Stoichiometric ratios are calculated from Job's Plot, which
indicates towards 1 : 1 complex formation (see ESI Fig. S7†).43
Fluorescence studies
Samples prepared for UV-vis analysis were further used for the
spectrouorometric study. Table 2 and Fig. 4 & 5 show the
supramolecular interaction of ionophores with iodide and
cyanide anions. An enrichment of uorescence intensity of
ionophores was observed upon adding iodide and cyanide
aliquots. Such qualitative assessment of the inclusion
complexation behavior by spectral titrations shows a single
emission maximum at 379 nm for ionophore I and II and 380
for ionophore III, respectively, with very low uorescence
Fig. 4 Fluorescence spectra of 5 ꢁ 10ꢀ5 M ionophores in the absence and presence of iodide at concentrations 5 ꢁ 10ꢀ5 M to 5 ꢁ 10ꢀ6 M. (a)
Ionophore I (b) ionophore II (c) ionophore III (d) Benesi–Hildebrand emission plot for 1 : 1 complexation of iodide with ionophores I–III.
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intensity before the addition of anions at excitation wavelength forces, hydrogen bonding strengths, and steric effects. Not only
of 279 nm for I and II and 285 nm for ionophore III. In the case this, hydrogen-bonding donor binding to an anions involves
of iodide anion, there is no spectral shi observed in ionophore deprotonation of hydrogen binding donor, in which proton
II and III, while ionophore I shows the blue shi to 375 nm (ꢃ5 transferred to basic anions and it varies with size and electro-
nm). Fig. 4 and 5 shows the emission spectra of iodide and negative character of anions CNꢀ and Iꢀ.
cyanide anion with all ionophores.
All the above concepts collaborate and contribute to the
Enhancement of emission maxima shows gradual redshi formation of a stable inclusion. Hence, inclusion formation
for I and II from 379 nm to 375 nm, while no noticeable changes constants or binding constants (Ka) of all the complexes were
were observed for ionophore III. Alteration in the host mole- calculated from uorescence data using the Benesi–Hildebrand
cule's photophysical and photochemical properties establishes equation, and the stoichiometric ratio was determined (Table
transference of the anions from a more protic atmosphere to 2).42,43 A good linearity with a better regression co-efficient, R2 is
a less protic atmosphere, i.e., the cavity of ionophores. The obtained. The binding constant ‘Ka’ is calculated from the
ionophores can form intra and inter-molecular hydrogen bonds straight line's slope, considering both ground and excited state
between the phenolic O–H and the nitrogen of the cyanide. The measurements. It was found to be 834 Mꢀ1, 729 Mꢀ1 & 498 Mꢀ1
supramolecular cavity of ionophores offers a protective envi- in the case of iodide anion, while the derived binding constant
ronment with excitation of singlet species and vibrational from cyanide complexes were 956 Mꢀ1, 753 Mꢀ1 & 604 Mꢀ1 for
restriction to iodide and cyanide molecules during encapsula- ionophore I, II & III, respectively. The above behavior indicates
tion. The study clearly explained that the encapsulation or that all ionophore forms different inclusion complexes with
inclusion phenomenon depends on the tting and selective cyanide and iodide ions. The linearity of the plot shows that the
detection concept between host and guest molecules. Such stoichiometry of the complex between ionophores and both the
encapsulations cannot be clearly explained on single parameter, anions was found to be 1 : 1. The binding constant values for
various spectacles leads to several weak intermolecular forces ionophore I with all anions are higher than other complexes
such as ion–dipole, dipole–dipole, van der Waals, electrostatic (Table 2), indicating that ionophore's capability to form
Fig. 5 Fluorescence spectra of 5 ꢁ 10ꢀ5 M ionophores in the absence and presence of cyanide at concentrations 5 ꢁ 10ꢀ3 M to 5 ꢁ 10ꢀ4 M. (a)
Ionophore I (b) ionophore II (c) ionophore III (d) Benesi–Hildebrand absorption plot for 1 : 1 complexation of cyanide with ionophores.
26650 | RSC Adv., 2021, 11, 26644–26654
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Fig. 8 A part of time-dependent 1H NMR spectra (500 MHz) in CDCl3:
CD3CN at 25 ꢂC for tetrabutylammonium cyanide anion with iono-
phore I.
Fig. 6 A part of time-dependent 1H NMR spectra (500 MHz) in CDCl3:
CD3CN at 25 ꢂC showing protons of ionophore I in the absence
presence of iodide anion.
interaction studies were carried out by adding the equimolar
concentration of ionophore I and anions at 297 K. For iodide
anion there is slight signicant change was seen in time-
dependent 1HNMR (Fig. 6) due to the large size and poor
hydrogen bonding tendency. Peak a get diminished with slight
shi in case of peak c and becomes constant aer 6 h. Inter-
estingly, the 127I showed 27.6 ppm upeld shi due to inclusion
and non-covalent interaction inside the cavity (Fig. 7). No
further changes were observed aer 6 h. The other two iono-
phores II and III, also showed similar trends (see ESI Fig. S8–
S13†).
inclusion complexes with iodide and cyanide anions. The
ionophore I can complex better with the cyanide and iodide
than arylated II and III. For iodide, deprotonation and charge
transfer via polarization are major interacting forces. However,
hydrogen bonding between nitrogen atoms of cyanide and
amines of ionophore leads to complete deprotonation and
strengthening host–guest association. All above phenomena
were common in all ionophores; however, steric hindrances are
highest in III, which have the least binding. Stoichiometric
ratios were calculated from Job's plot that shows 1 : 1 complex
formation (see ESI Fig. S7†).43 Binding constant data's for
emission and absorption data's were different, but dual binding
constants and better R2 values in same sequence clearly explain
that ionophores have a strong binding capability with anions.
In the case of cyanide, clear changes were observed in time-
dependent proton NMR with deprotonation of peak 8.45 ppm
(–NH) and 7.75 ppm (–OH) of ionophores within 15 min, as
shown in Fig. 8. For ionophore I, aromatic peaks shied
downeld due to the strong nucleophilicity of cyanide ion
inclusion. It has been observed that ionophores do not behave
chemidosmetrically as the color started fading aer 72 h and
the N–H at 8.45 ppm proton started reappearing (Fig. S18†). The
sterically crowded ionophore III showed the least binding
1H & 127I NMR study
The inclusion of guest ions inside the ionophore's cavity was
1
ascertained by changes observed in H & 127I NMR. The NMR
127
capability as observed in I NMR shi (see ESI Fig. S13†).7b
Conclusions
Three new functionalized furfuryl and benzyl derivated of calix
[4]amido crown based molecular ionophores were synthesized
to investigate their anion sensing properties. These ionophores
were designed with variation in lower rim conformation based
on rigidity and steric hindrance at the binding site. A number of
anions were investigated with three ionophores indicating that
ionophore I bind strongly with both iodide and cyanide ions
with interaction order I > II > III. The ionophore I shows drastic
color change from colorless to pink and wine red for iodide and
cyanide anion. For iodide ion, ionophore II and III do not show
very sharp color change while CNꢀ gave colorless to light yellow.
The strong binding for iodide ion was evident by 127I NMR
Fig. 7 Time-dependent 127I NMR spectra (500 MHz) in CDCl3: CD3CN
at 25 ꢂC of tetrabutylammonium iodide with ionophore I.
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where ionophore I gave 27.6 ppm upeld shi while for iono-
phore II and III, 13.9 and 10 ppm, respectively. The cyanide ion
shows strong interaction with N–H and O–H groups, with all
three ionophores, resulted in a color change. The color change
is attributed to the charge transfer process among the anions
and lower amide pendulum. The ionophore I interact strongly
with iodide and cyanide because of the less hindered binding
site, for effective interaction with anions. The binding
isotherms were tted to a 1 : 1 binding model with better
binding constants, as suggested by the Benesi–Hildebrand plot
for all ionophores.
8 (a) Q. Lin, K. P. Zhong, J. H. Zhu, L. Ding, J. X. Su, H. Yao,
T. B. Wei and Y. M. Zhang, Iodine controlled pillar [5]
arene-based multiresponsive supramolecular polymer for
uorescence detection of cyanide, mercury, and cysteine,
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9 W. J. Qu, W. T. Li, H. L. Zhang, T. B. Wei, Q. Lin, H. Yao and
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A rational designed uorescent and
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Conflicts of interest
10 (a) H. H. Yang, P. P. Liu, J. P. Hu, H. Fang, Q. Lin, Y. Hong,
Y. M. Zhang, W. J. Qu and T. B. Wei, A uorescent
supramolecular gel and its application in the ultrasensitive
detection of CNꢀ by anion–p interactions, So Matter,
2020, 16, 9876–9881; (b) W. J. Qu, H. H. Yang, J. P. Hu,
P. Qin, X. X. Zhao, Q. Lin, H. Yao, Y. M. Zhang and
T. B. Wei, A novel bis-acylhydrazone supramolecular gel
and its application in ultrasensitive detection of CNꢀ, Dyes
Pigm., 2021, 186, 108949.
There are no conicts to declare.
Acknowledgements
The authors like to thank the Indian Institute of Technology
Jodhpur for the infrastructure and CASE facilities. PRS grate-
fully acknowledges DST Women Scientists Scheme-A (WOS-A)
for the research grant SR/WOS-A/CS-117/2018.
11 S. D. Padghan, C. Y. Wang, W. C. Liu, S. S. Sun, K. M. Liu and
K. Y. Chen,
A naphthalene-based colorimetric and
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