Benzimidazole-based fluorescent sensors for Cr3+ and their resultant complexes for sensing HSO4- and F-

Article abstract of DOI:10.1016/j.tet.2012.08.022

A benzimidazole-based receptor that selectively recognized Cr³⁺ without interference from other metal ions was synthesized in this study. The resultant Cr-complex recognized HSO4- and F^- via two approaches. HSO4- ions bound to the metal center, causing changes in the fluorescent spectra at different wavelengths, whereas F^- ions displaced Cr³⁺, thereby switching ON the fluorescent spectra at the same wavelength.

Full text of DOI:10.1016/j.tet.2012.08.022

Tetrahedron 68 (2012) 8551e8556
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Benzimidazole-based fluorescent sensors for Cr3þ and their resultant complexes
ꢀ
for sensing HSO4 and Fꢀ
,
c,
*
Preeti Saluja a, Navneet Kaur b, Narinder Singh a , Doo Ok Jang
*
a Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India
b Centre for Nanoscience & Nanotechnology, Panjab University, Chandigarh, Punjab 160014, India
c Department of Chemistry, Yonsei University, Wonju 220-710, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
A benzimidazole-based receptor that selectively recognized Cr3þ without interference from other metal
Article history:
Received 9 July 2012
Received in revised form 7 August 2012
Accepted 7 August 2012
Available online 14 August 2012
ꢀ
ions was synthꢀesized in this study. The resultant Cr-complex recognized HSO4 and Fꢀ via two ap-
proaches. HSO4 ions bound to the metal center, causing changes in the fluorescent spectra at different
wavelengths, whereas Fꢀ ions displaced Cr3þ, thereby switching ON the fluorescent spectra at the same
wavelength.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Benzimidazole
Sensor
Cr(III)
Fluorescence
Anion
1. Introduction
assay,8 in which an anionic guest displaces the metal ion causing
the revival of the original UVevis or fluorescence spectrum, or via
Anions play an important role in biochemistry, medicine, and in
clinical and environmental analysis.1 Fluoride has a role in pre-
venting dental caries, in the treatment for osteoporosis, and in the
refinement of uranium used in the manufacture of nuclear
weapons.2 Hydrogen sulfate is usually required to maintain the
level of dissolved calcium in boiling water as it helps to prevent the
loss of calcium due to precipitation.3 However, excess of these
anions can have adverse effects. (a) Fluoride leads to fluorosis,
which is a type of fluoride toxicity,2 and (b) hydrogen sulfate re-
leased in industrial processes has deleterious effects on the envi-
ronment.4 Thus, monitoring these anions in biological or
environmental samples is of great importance because of their di-
verse effects, both beneficial and otherwise.
simple electrostatic interaction of an anion with a metal center,
causing a change in the UVevis or fluorescence spectrum.9 Thus,
a single metal sensor complex can recognize two anions via two
different approaches, i.e., the cation displacement approach, which
causes an ONeOFF switch of fluorescence intensity at the same
wavelength, and electrostatic interaction of anions with a metal
center, which may cause a shift in wavelength.
In this context, we have designed a set of sensors in such a way
that receptors have both hydrogen bond donor and hydrogen bond
acceptor sites (Scheme 1). The receptors bind with metal ions. The
resulting complexes recognize oxyanions such as hydrogen sulfate
and undergo decomplexation with anions such as halides.
Few reports of a single sensor behaving differently for these two
anions exist,6 although several selective and sensitive sensors are
available for individual recognition of these anions.2,5 The recog-
nition of any anion in water is difficult because of the high hydra-
tion energy of anions. Anions usually experience competition from
the solvent for sensor binding sites.7 Nevertheless, a metal complex
may resolve this problem through either a cation displacement
2. Results and discussion
Receptors 1e3 were synthesized by a condensation reaction of 2-
(2-aminophenyl)-1H-benzimidazole with pyridine-2-carbaldehyde,
pyridine-3-carbaldehyde or pyridine-4-carbaldehyde, respectively.
The photophysical properties of receptors 1e3 were examined using
UVevis absorption and fluorescence spectroscopy. UVevis absorp-
tion spectroscopic studies of receptors 1e3 were performed with
20 mM solutions of receptors 1e3 in a HEPES-buffered CH3CN/H2O (8/
2, v/v) solvent system to understand their ground state behaviors.
* Corresponding authors. E-mail addresses: nsingh@iitrpr.ac.in (N. Singh),
dojang@yonsei.ac.kr (D.O. Jang).
The solutions of receptors 1e3 exhibited absorption bands at 352,
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.08.022

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P. Saluja et al. / Tetrahedron 68 (2012) 8551e8556
Scheme 1. Structure of receptors 1e3.
340, and 334 nm, respectively. These bands are the result of intra-
molecularcharge transfertransitions dueto iminelinkages.10 UVevis
absorption spectra of receptors1e3 were recorded immediatelyafter
preparing a solution and then after 24 h (Fig. S1). If there would be
any hydrolysis of imine linkages, then the intensity of band around
350 nm is expected to fall. In the present case, no such changes in
absorption spectra were observed. This indicates that receptors 1e3
are stable in a HEPES-buffered CH3CN/H2O solvent system.
fluorescence intensity at 470 nm (Fig. 2). This gave rise to a new
emission band at 470 nm with an isosbestic point. The selective
recognition of Cr3þ with receptors 1 and 2 seems to be due to the
compatible binding sites and the optimum size of the pseudocavity
of receptors. On the basis of the fluorescence titration data, binding
constants for the Cr3þ complexes were calculated using a Ben-
seieHildebrand plot12 and were found to be (7.67ꢁ0.30)ꢂ104
and (7.09ꢁ0.35)ꢂ104 Mꢀ1 for 1 and 2, respectively (Fig. S4).
Job’s plots were constructed for 1$Cr3þ and 2$Cr3þ, showing a 1:1
stoichiometry (Figs. S5 and S6). The mass spectrum showed
The absorption profiles of receptors 1e3 were examined in the
absence or presence of various metal species. The absorption pro-
files on addition of different metal ions to solutions of receptors
m/z¼151.8, which corresponds to [MþH]3þ
,
where
M
is
1e3 showed no selectivity (Fig. S2). A 1.0
mM concentration of re-
[1þCr3þþNO3þCH3CN], indicating 1:1 binding of receptor 1 to Cr3þ
(Fig. S7). Similarly, m/z¼254.9 was detected, which corresponds to
[Mþ1]2þ, where M is [2þCr3þþ2NO3ꢀþCH3OH] (Fig. S8). The de-
tection limits for the estimation of Cr3þ by receptors 1 and 2 turned
ceptors 1e3 in a HEPES-buffered CH3CN/H2O (8/2, v/v) solvent
system was excited at 352, 340, and 334 nm, respectively, to de-
termine the fluorescence behavior of these sensors. Receptors 1 and
2 showed emission at 420 nm (Fig. 1), whereas receptor 3 had dual
emission at 420 and 510 nm (Fig. S3). The addition of some selected
metal ions caused fluorescence quenching of receptors 1 and 2 to
different extents. However, the binding event of Cr3þ led to unique
modulation. Receptor 3 showed no specific binding with any of the
tested metal ions.
out to be 1.25 and 0.63 m
M, respectively (Fig. S9).13 The detection
limits for receptors 1 and 2 are sufficiently low for use in any bi-
ological or industrial samples containing low concentrations of
Cr3þ 11e,14
.
The specific responses of receptors 1 and 2 for Cr3þ were
assessed by performing competitive binding experiments in the
800
600
A
B
Host
Host
Na(I), K(I), Ba(II),
Ca(II), Mg(II), Zn(II), Mn(II),
Sr(II), Co(II), Ni(II), Cu(II),
Ag(I), Pb(II), Cd(II), Al(III)
Na(I), K(I), Mn(II), Ca(II),
Sr(II),Mg(II), Cd(II), Ba(II),
Co(II), Ag(I) , Ni(II), Pb(II)
600
400
200
0
400
200
0
Cr(III)
Zn(II)
Cr(III)
Hg(II), Fe(III)
Fe(III), Hg(II),
Cu(II)
Al(III)
360
410
460
510
560
370
420
470
520
570
Wavelength (nm)
Wavelength (nm)
Fig. 1. (A) Changes in the fluorescence spectra of receptor 1 (1
mM) upon addition of a particular metal nitrate salt (10
mM) in HEPES-buffered CH3CN/H2O (8:2, v/v) with excitation
at 352 nm. (B) Changes in the fluorescence spectra of receptor 2 (1
mM) upon the addition of a particular metal nitrate salt (10 mM) in HEPES-buffered CH3CN/H2O (8:2, v/v) with
excitation at 340 nm.
A 1.0
(0e10
m
M solution of receptors 1 and 2 was titrated against Cr3þ
presence of all tested metal ions, i.e., Naþ, Kþ, Mn2þ, Mg2þ, Ca2þ
Ba2þ, Sr2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Pb2þ, Agþ, Hg2þ, and Cd2þ
,
.
m
M) with a fluorospectrometer. A gradual increase in the
concentration of Cr3þ quenched the intensity of the emission band
The emission profile of receptor 1 with Cr3þ was unperturbed in the
presence of these metal ions (Fig. 3), confirming its selective
of receptors 1 and 2 at 420 nm, and there was a small increase in the

P. Saluja et al. / Tetrahedron 68 (2012) 8551e8556
8553
500
600
450
300
150
0
B
A
400
300
200
100
0
360
410
460
510
560
370
420
470
520
570
Wavelength (nm)
Wavelength (nm)
Fig. 2. (A) Fluorescence emission spectra of receptor 1 (1
at 352 nm. (B) Fluorescence emission spectra of receptor 2 (1
m
M) showing changes upon successive addition of Cr3þ (0e10
m
M) in HEPES-buffered CH3CN/H2O (8:2, v/v) with excitation
m
M), showing changes upon successive addition of Cr3þ (0e10
mM) in HEPES-buffered CH3CN/H2O (8:2, v/v) with
excitation at 340 nm.
250
200
150
100
50
300
200
100
0
0
Fig. 3. Fluorescent response of receptor 1 (1
metal ions (10
m
M) to Cr3þ (10
m
M) over other selected
Fig. 4. Fluorescent response of receptor 2 (1
metal ions (10 M). The intensity at 470 nm was used for the bar diagram.
m mM) over other selected
M) to Cr3þ (10
m
M). The intensity at 470 nm was used for the bar diagram.
m
binding with Cr3þ. In contrast, the binding of receptor 2 with Cr3þ
was interfered with by many of the tested metal ions, as indicated
by the changes in its emission profile (Fig. 4).
solvent system along with 20 mM of tetrabutylammonium salt of
anions such as Fꢀ, Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, CNꢀ, HSO4ꢀ, PO43ꢀ, ClO4ꢀ, and
AcOꢀ (Fig. 5). Complex 1$Cr3þ showed two different profiles in the
presence of Fꢀ and HSO4ꢀ. In the presence of Fꢀ, the fluorescencꢀe
intensity at 420 nm was enhanced, while in the presence of HSO4
there was a decrease in fluorescence intensity at 420 nm and an
increase in fluorescence intensity at 475 nm. These phenomena
suggest that complex 1$Cr3þ behaves differently with these two
anions and can recognize both. However, complex 2$Cr3þ was not
selective for any of the anions (Fig. S13).
The ground state behavior of receptors 1 and 2 was examined by
titrating 20
m
M solutions of receptors 1 and 2 with Cr3þ using
UVevis absorption spectroscopy. When the concentration of Cr3þ
was increased steadily, the absorption band shifted to 380 nm with
an isosbestic point at 370 nm (Fig. S10). The shifts in absorption and
emission spectra imply that the addition of Cr3þ modulates internal
charge transfer.11 UVevis absorption spectra and fluorescence
spectra were recorded while varying the pH of the solution of re-
ceptor 1 in order to examine the effect of pH on the photophysical
properties of the receptor (Figs. S11 and S12). Changes in pH values
resulted in changes in both the UVevis absorption and fluorescence
spectra of receptor 1. Thus, we conducted all studies in HEPES-
buffered solutions. Our findings indicate that the photophysical
properties are not prone to change with pH under buffered
conditions.
The fluorescence behavior of 1$Cr3þ in the presence of different
anions shows that fluoride causes decomplexation of the 1$Cr3þ
complex, thereby restoring the original emission band of receptor 1.
Interestingly, hydrogen sulfate coordinated with the metal complex
and further modulated the internal charge transfer. The effect of pH
on the 1$Cr3þ was examined by recording fluorescence spectra of
1$Cr3þ (5
mM) in CH3CN/H2O (8/2, v/v) under variable pH (Fig. S14).
To examine the applicability of complexes 1$Cr3þ and 2$Cr3þ as
anion sensors,8,9 we have examined the fluorescence behavior of
complexes 1$Cr3þ and 2$Cr3þ in the presence of different anions.
The pH values influenced the profile of the fluorescence spectra of
1$Cr3þ. Thus, a buffered solvent system was used. The effect of Fꢀ
ꢀ
and HSO4 on 1$Cr3þ in its ground state was investigated by re-
Fluorescence spectra were recorded by mixing a 5.0
mM solution of
cording UVevis spectra of 20
CH3CN/H2O (8/2, v/v) solvent system with 100
m
M 1$Cr3þ in a HEPES-buffered
1$Cr3þ and 2$Cr3þ in a HEPES-buffered CH3CN/H2O (8/2, v/v)
mM

8554
P. Saluja et al. / Tetrahedron 68 (2012) 8551e8556
Brꢀ and Iꢀ were also recorded. No interference from any of the
competitive anions was detected (Fig. 7). From the titration plot,
ꢀ
the detection limits for HSO4 and Fꢀ were calculated to be 1.58
and 0.125 ꢀ
m
M, respectively (Fig. S18). The differential sensing of Fꢀ
and HSO4 with 1$Cr3þ is explained by a hardehard interaction
between hard Lewis acid (Cr3þ) and hard Lewis base (Fꢀ). Sec-
ondly, the high charge density of Fꢀ leads to equip Fꢀ as proficient
anion to displace Cr3þ from the coordination sphere of 1$Cr3þ 15
.
On the other hand, the charge is not localized in HSO4ꢀ. Hence,
ꢀ
relatively lower charge density of HSO4 restricts the cation dis-
ꢀ
placement, although HSO4 has a good a binding affinity for Cr3þ
through hydroxo groups.16
3. Conclusion
In summary, we synthesized receptor 1, which recognizes Cr3þ
Fig. 5. Changes in the fluorescence spectra of 1$Cr3þ (5
mM) upon addition of a
selectively without interference from other metal ions. The re-
particular tetrabutylammonium anion salt (20
(8:2, v/v) with excitation at 352 nm.
mM) in HEPES-buffered CH3CN/H2O
ꢀ
sultant 1$Cr3þ complex behaves differently with HSO4 and Fꢀ.
Specifically, 1$Cr3þ in the presence of HSO4ꢀ results in a decrease in
fluorescence intensity at 420 nm and enhanced fluorescence in-
tensity at 475 nm. In the presence of Fꢀ, cation displacement occurs,
i.e., Cr3þ is displaced and the fluorescence spectrum is switched ON
at the same wavelength (420 nm). None of the competitive anions
tested interfered with either interaction.
tetrabutylammonium salt of fluoride and hydrogen sulfate
ꢀ
(Fig. S15). The interactions of Fꢀ and HSO4 with 1$Cr3þ were
further confirmed by stepwise addition of these anions to 5 mM
1$Cr3þ in a HEPES-buffered CH3CN/H2O (8/2, v/v) solvent system
(Fig. 6).
1000
600
A
B
800
600
400
200
0
450
300
150
0
370
420
470
520
570
620
370
420
470
520
570
620
Wavelength (nm)
Wavelength (nm)
Fig. 6. (A) Fluorescence emission spectra of 1$Cr3þ (5
CH3CN/H2O (8:2, v/v) with excitation at 352 nm. (B) Fluorescence emission spectra of 1$Cr3þ (5
ride (0e20 M) in HEPES-buffered CH3CN/H2O (8:2, v/v) with excitation at 352 nm.
m
M) showing changes upon successive addition of tetrabutylammonium hydrogen sulfate (0e20
mM) in HEPES-buffered
mM) showing changes upon successive addition of tetrabutylammonium fluo-
m
To exclude the possibility of receptor 1 binding to other anions,
we have assessed the effect of additional anions on the fluores-
cence profile of receptor 1. The fluorescence signal of receptor 1
4. Experimental section
4.1. Materials and methods
(1 mM) was unaffected in the presence of all tested anions (10 mM)
in HEPES-buffered CH3CN/H2O (8:2, v/v) with excitation at 352 nm
(Fig. S16). Binding constants of 1$Cr3þ for hydrogen sulfate and
fluoride ions were calculated using a BenseieHildebrand plot
(Fig. S17) and were found to be (2.89ꢁ0.25)ꢂ105 and
Chemicals were purchased from Sigma Aldrich and used with-
out further purification. The NMR spectra were recorded on an
Avance-II (Bruker) instrument, which was operated at 400 MHz for
1H NMR and 100 MHz for 13C NMR. IR spectra were recorded on
a Perkin Elmer Spectrum One for the compounds in the solid state,
which were prepared as KBr discs. The absorption spectra were
recorded on a Perkin Elmer Lambda 25. Fluorescence measure-
ments were performed on a Perkin Elmer LS55 fluorescence
spectrophotometer.
(2.30ꢁ0.20)ꢂ106
M
ꢀ1, respectively. Interference of other oxy-
anions and halides in the binding of hydrogen sulfate and fluoride
ion, respectively, with receptor 1 was examined. Fluorescence
spectra of 5
solvent system with 20
sulfate in the presence of 20
m
M 1$Cr3þ in a HEPES-buffered CH3CN/H2O (8/2, v/v)
mM tetrabutylammonium salt of hydrogen
m
M of a tetꢀrabutylammonium salt of
oxyanions including NO3ꢀ, PO43ꢀ, ClO4 , and AcOꢀ were recor-
4.2. General procedure for the synthesis of receptors 1e3
A solution of 2-(2-aminophenyl)-1H-benzimidazole (209 mg,
ded. Similarly, spectra for a 5.0
a HEPES-buffered CH3CN/H2O (8/2, v/v) solvent system with
20 M tetrabutylammonium salt of fluoride anion in the presence
of 20
M of a tetrabutylammonium salt of halides including Clꢀ,
m
M concentration of 1$Cr3þ in
m
1
mmol) and the corresponding pyridine carboxaldehyde
m
(1.5 mmol) in dry methanol (50 mL) was heated to reflux for 12 h.

P. Saluja et al. / Tetrahedron 68 (2012) 8551e8556
8555
122.3, 123.9, 124.7,131.8, 132.6, 133.3, 135.8, 142.7, 143.8, 146.7, 147.1,
150.1; FTIR (nmax KBr pellet) 1620 cmꢀ1. Anal. Calcd for C19H14N4: C,
76.49; H, 4.73; N, 18.78. Found: C, 76.17; H, 4.90; N, 18.93.
500
400
300
200
100
0
A
4.5. Synthesis of receptor 3
Following the general procedure above, 3 was obtained as
a white solid in 78% yield. Mp 239e240 ꢃC; 1H NMR (400 MHz,
DMSO-d6)
d
8.96 (d, J¼6.0 Hz, 2H), 8.67 (d, J¼9.2 Hz, 1H), 8.00e7.81
(m, 6H), 7.51 (t, J¼6.4 Hz, 1H), 7.22 (t, J¼9.2 Hz, 1H), 6.55 (d,
J¼8.4 Hz, 1H); 13C NMR (100 MHz, DMSO-d6) 113.7, 118.3, 119.7,
122.7, 122.9, 123.8, 125.4, 127.9,128.7,132.0,141.5,141.8, 143.9, 146.1,
147.2, 150.6; FTIR (nmax KBr pellet) 1630 cmꢀ1; Anal. Calcd for
C19H14N4: C, 76.49; H, 4.73; N, 18.78. Found: C, 76.23; H, 4.83; N,
18.94.
4.6. Synthesis of 1$Cr3D complex
500
400
300
200
100
0
Compound 1 (298 mg, 1.0 mmol) was dissolved along with
Cr(NO3)3 (400 mg, 1.0 mmol) in a CH3CN/CH3OH solvent system.
The solution was heated to reflux for 8 h. Upon completion of the
reaction, the product was separated via slow diffusion of diethyl
B
ether into the reaction mixture. The solid metal complex (1$Cr3þ
)
was washed with cold methanol and a dark green solid was ob-
tained in 87% yield (305 mg). Mp >300 ꢃC; IR (KBr pellet)
n
1620 cmꢀ1; Anal. Calcd for C21H17CrN8O9: C, 43.68; H, 2.97; Cr, 9.01;
N, 19.41; O, 24.94. Found: C, 43.93; H, 2.88; N, 19.36.
4.7. Synthesis of 2$Cr3D complex
Following the procedure for the synthesis of the 1$Cr3þ complex
described above, the 2$Cr3þ complex was obtained as a dark green
solid in 86% yield (300 mg). Mp >300 ꢃC; IR (KBr pellet)
n
1620 cmꢀ1; Anal. Calcd for C20H18CrN7O10: C, 42.26; H, 3.19; Cr,
9.15; N, 17.25; O, 28.15. Found: C, 42.33; H, 3.07; N, 17.16.
ꢀ
mM) to HSO4
Fig. 7. (A) Bar diagram showing the fluorescence response of 1$Cr3þ (5
(20 M) in the presence of other oxyanions (20 M). (B) Bar diagram showing the
fluorescence response of 1$Cr3þ (5 M) to Fꢀ (20
M) in the presence of other halides
(20 M).
m
m
m
4.8. Metal recognition studies of receptors 1e3
m
m
All of the recognition studies were performed at 25ꢁ1 ꢃC, and,
before recording any spectrum, sufficient time was allowed for
shaking to ensure the uniformity of the solution. The effect of pH on
the UVevis absorption and fluorescence spectrum of receptor 1
was investigated with a solution of receptor 1 prepared in a CH3CN/
H2O (8:2, v/v) solvent system. The cation binding ability of re-
ceptors 1e3 in a HEPES-buffered CH3CN/H2O (8:2, v/v) solvent
system was determined by preparing standard solutions of receptor
1 along with fixed amounts of particular metal nitrate salts in
HEPES-buffered CH3CN/H2O (8:2, v/v). The cation recognition be-
havior of receptors 1e3 was evaluated according to changes in the
fluorescence spectrum of the receptor upon addition of the metal
salt. The fluorescence spectra of receptors 1e3 were recorded with
the excitation wavelengths shown in Fig. 1 and Fig. S3. Volumetric
flasks containing standard solutions of receptors 1 and 2 and varied
amounts of a particular metal nitrate salt in HEPES-buffered
CH3CN/H2O (8:2, v/v) were used for titrations. To evaluate any
possible interference in the estimation of Cr3þ due to the presence
After the reaction was completed, the solvent was evaporated to
25 mL and was kept at 5 ꢃC to facilitate slow evaporation. A white
solid material separated out. This solid was filtered and washed
with cold methanol three times.
4.3. Synthesis of receptor 1
Following the general procedure described above, 1 was ob-
tained as a white solid in 80% yield. Mp 228e230 ꢃC; 1H NMR
(400 MHz, DMSO-d6)
d
8.45 (d, J¼4.4 Hz, 1H), 7.93 (d, J¼7.6 Hz, 1H),
7.75 (t, J¼7.6 Hz, 1H), 7.70 (s, 1H), 7.64 (d, J¼8.0 Hz, 1H), 7.34 (d,
J¼8.0 Hz, 1H), 7.31e7.10 (m, 6H), 6.85 (d, J¼8.0 Hz, 1H), 6.80 (t,
J¼7.2 Hz, 1H); 13C NMR (100 MHz, DMSO-d6)
d 110.3, 112.0, 115.7,
118.0, 118.6, 120.2, 122.0, 122.2, 123.9, 124.6, 131.5, 133.0, 137.4,
142.9, 143.8, 146.8, 149.5, 158.4; IR (KBr pellet)
n
1645 cmꢀ1; Anal.
Calcd for C19H14N4: C, 76.49; H, 4.73; N, 18.78. Found: C, 76.03; H,
4.94; N, 19.03.
of other metal ions, solutions containing receptor 1 (1 mM) along
with a fixed concentration of Cr3þ both with and without other
background cations were prepared in HEPES-buffered CH3CN/H2O
(8:2, v/v). The fluorescence intensity of each solution was recorded.
4.4. Synthesis of receptor 2
Following the general procedure above, 2 was obtained as
a white solid in 81% yield. Mp 178e180 ꢃC; 1H NMR (400 MHz,
4.9. Anion recognition properties of 1$Cr3D and 2$Cr3D
DMSO-d6)
2H), 7.52e7.49 (m, 1H), 7.35e7.13 (m, 6H), 6.88e6.84 (m, 2H); 13C
NMR (100 MHz, DMSO-d6) 110.3, 112.0, 115.0, 118.6, 118.8, 119.6,
d 8.53e8.51 (m, 2H), 7.98e7.96 (m, 1H), 7.70e7.65 (m,
The studies were performed in a fashion similar to the cation
d
recognition
experiments,
except
for
the
fact
that

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Acknowledgements
This work was supported by the India-Korea Joint Program of
Cooperation in Science & Technology (2011-0027710) and by an
CSIR project grant (01(2417)/10 EMR-II). P.S. is thankful to UGC
India for a research fellowship.
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