1,3-Di-amidoquinoline conjugate of calix[4]arene (L) as a ratiometric and colorimetric sensor for Zn2+: Spectroscopy, microscopy and computational studies

Article abstract of DOI:10.1039/c2dt11900e

Carboxamidoquinoline appended calix[4]arene-1,3-di-conjugate (L) has been synthesized and characterized and its single crystal XRD structure has been established. L has been shown to act as selective ratiometric turn-on fluorescence sensor for Zn²⁺ up to a lowest concentration of 183 ± 18 ppb (2.82 μM) with a nine-fold enhancement by exhibiting blue-green emission. The coordination features of the species of recognition have been computationally evaluated by DFT methods and found to have distorted tetrahedral Zn²⁺ center in an N₄ core. The spherical nano-structural features observed for L in TEM are being transformed into the Koosh nano-flower like structure when complexed with Zn²⁺ and hence these two can be easily differentiated. Even the features observed in AFM can distinctly differentiate L from its Zn²⁺ complex.

Full text of DOI:10.1039/c2dt11900e

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1,3-Di-amidoquinoline conjugate of calix[4]arene (L) as a ratiometric and
colorimetric sensor for Zn2+: Spectroscopy, microscopy and computational
studies†
V. V. Sreenivasu Mummidivarapu, Khatija Tabbasum, Jugun Prakash Chinta and Chebrolu P. Rao*
Received 10th October 2011, Accepted 1st December 2011
DOI: 10.1039/c2dt11900e
Carboxamidoquinoline
appended
calix[4]arene-1,3-di-
structural changes present between L and its zinc complex using
microscopy.
conjugate (L) has been synthesized and characterized and its
single crystal XRD structure has been established. L has been
shown to act as selective ratiometric turn-on fluorescence sen-
sor for Zn2+ up to a lowest concentration of 183 18 ppb (2.82
lM) with a nine-fold enhancement by exhibiting blue-green
emission. The coordination features of the species of recogni-
tion have been computationally evaluated by DFT methods
and found to have distorted tetrahedral Zn2+ center in an N4
core. The spherical nano-structural features observed for L
in TEM are being transformed into the Koosh nano-flower
like structure when complexed with Zn2+ and hence these two
can be easily differentiated. Even the features observed in
AFM can distinctly differentiate L from its Zn2+ complex.
The receptor molecule, L has been synthesized in a single
step on calixarene platform by reacting p-tert-butyl calix[4]arene
with 2-chloro-N-(quinolin-8-yl) acetamide in 55% yield and was
characterized (Scheme 1, S01 in ESI†).7 The 2-chloro-N-(quinolin-
8-yl) acetamide was synthesized from 8-aminoquinoline.8 The L
1
complex was characterized by H and 13C NMR, FTIR, HRMS
and elemental analysis (S02 in ESI†).
In biological systems, Zn2+ exhibits structural, catalytic, and
regulatory roles and its cellular presence is crucial for genetic
expression.1–3 This element is present even in the brain tissue in the
concentrations of 0.1–0.5 mM. Therefore, zinc might be involved
in several neurological dysfunctions.4 Thus the development of
synthetic receptors for the selective recognition of Zn2+ continues
to intrigue the researchers. A majority of the calix[4]arene based
zinc sensors rely on the metal induced fluorescence changes,
also influenced by several other factors.5 Recently our group
demonstrated the Zn2+ sensing by a variety of conjugates of
calix[4]areneinorganic solvents as wellas inbio-relevant medium.6
To our knowledge, there are only two reports on the calix[4]arene
derivatives that sense Zn2+ ratiometrically,5c,5i however, reports
on the changes observed in the nano-structural features upon
complexation have not been common in the literature. Therefore,
in the present communication selective Zn2+ sensing by a carbox-
amidoquinoline appended 1,3-di-conjugate of calix[4]arene (L) by
ratio-metric fluorescence method has been demonstrated besides
providing coordination features of the species of recognition by
computational methods and also deriving their interesting nano-
Scheme 1 Scheme for the synthesis of L: (a) chloroacetyl chloride, Et3N,
dry CH2Cl2, 6 h; (b) K2CO3, NaI, dry acetone, 24 h.
Single crystals of L were obtained from a slow evaporation
of a solution of L dissolved in a minimum quantity of CHCl3
followed by diluting this with CH3CN and were found to be
12,13
¯
triclinic with P1.‡
The XRD structure of L exhibits a cone
conformation for the calixarene by retaining two intra-rim O–
˚
H ◊ ◊ ◊ O hydrogen bonds with O ◊ ◊ ◊ O distance of 2.75 A at the lower
rim (Fig. 1a). The distance between the two phenolic oxygens at
˚
the lower rim is 4.76 A and that between the two centroids of
˚
the quinoline moieties is 3.87 A. The cone-conformation is found
1
to be retained even in the solution as evident from its H NMR.
The crystal packing diagram shows a well ordered arrangement of
calix[4]arene conjugates in a head-to-head fashion reminiscent of
a lipid membrane structure (Fig. 1b).
The interaction of L has been studied with several metal
ions using emission as well as absorption spectroscopy. The ion
recognition of L has been studied by exciting the solutions at
320 nm and measuring its emission spectra from 330 to 600 nm.
Among Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+,
Cd2+, Ag+, and Hg2+ ions studied, only Zn2+ exhibited fluorescence
enhancement in L (Fig. 2a, S04 & S06 in ESI†). Titration of L with
Bioinorganic Laboratory, Department of Chemistry, Indian Institute of
Technology Bombay, Mumbai, 400 076, India. E-mail: cprao@iitb.ac.in
† Electronic supplementary information (ESI) available: Synthesis, charac-
terization, computational and nano-structural features. CCDC reference
number 836001. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2dt11900e
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Fig. 1 (a) Stereo view of the molecular structure of L as obtained from
the single crystal XRD. (b) Lattice diagram of L.
Fig. 3 (a) Absorption spectral traces for the titration of L (10 mM) with
Zn2+, (b) absorbance vs. mole ratio plot for 252 (green triangle), 288 (black
square), 315 (red square) and 356 (blue triangle) bands.
change visually in the presence of different metal ions under an
incident light of 365 nm, and found bluish green fluorescence only
in case of Zn2+, while the other ions do not show such emission
(Fig. 4). Further, this has been carried out in the presence of other
ions added to an initial solution possessing a 1 : 2 L to Zn2+ ratio
and found no changes in the bluish green fluorescence in case of
all the ions except Fe2+, Cu2+ and Hg2+, suggesting that the Zn2+
sensing by L can be monitored even in presence of all the other
ions (S08 in ESI†).
Fig. 2 (a) Fluorescence spectral traces for the titration of L with Zn2+
in methanol (lex = 320 nm). (b) Histogram showing the number of
folds of enhancement during different metal ion titration. Inset: Relative
fluorescence intensity vs. mole ratio plot for 410 nm (ꢀ) and 490 nm (
)
᭺
bands.
Zn2+ showed a red shift of 80 nm in the lem maximum of L. Thus,
while the intensity of the band at 410 nm decreases, the new band
observed at 490 nm increases as a function of the added Zn2+ in a
ratiometric manner with an overall fluorescence enhancement of
Fig. 4 Colorimetric titration of L (10 mM) with different metal ions
(20 mM) in methanol as observed under UV (365 nm) incident light.
9
1 fold at saturation (Fig. 2a & 2b). The binding constant of L
1H NMR studies carried out with Zn2+ showed a considerable
change in the splitting pattern of the amide proton signals observed
at 8.7 ppm (S12 in ESI†). The formation of 1 : 1 complex was
confirmed by ESI MS, where the spectrum shows a molecular ion
peak at m/z of 1081.4 and the observed isotopic peak pattern
supports the presence of Zn2+ (Fig. 5a). Powder XRD studies
showed that L is highly crystalline and its zinc complex is indeed
less crystalline in nature (Fig. 5b).
with Zn2+ has been derived using the Benesi–Hildebrand equation
and the corresponding association constant, Ka was found to be
37150 720 M-1. The minimum concentration of Zn2+ that can
be detected by L was obtained from a dilution experiment carried
out by keeping the mole ratio of L to Zn2+ as 1 : 1 and measuring
the fluorescence intensity and found to be 183 18 ppb (2.82 mM)
(S11 in ESI†).
Competitive metal ion titrations have also been carried out to
study the sensing of Zn2+ by L even in the presence of the other
ions. During the titration, the L:Mn+ ratio was kept at 1 : 5 and the
resultant solution was titrated against varying concentrations of
Zn2+ for each Mn+ (S07 in ESI†). The fluorescence intensity was
found to be that of the original Zn2+ titration with all the ions
studied except Fe2+, Cu2+ and Hg2+, and hence a non-interfering
sensing of Zn2+ is restricted to the presence of the other Mn+. Even
the competitive titrations carried out by keeping a L:Zn2+ ratio
of 1 : 2 and varying the concentration of the other Mn+ showed
similar results (S07 in ESI†).
In order to confirm the binding of Zn2+, absorption spectral
studies were carried out for the titration of Zn2+ with L in
methanol (Fig. 3a). While the absorbance of the ~250 and 356 nm
bands increases that of the 288 and 315 nm bands decreases
(Fig. 3b). Thus the spectral changes and the isosbestic points
observed at 243, 275 and 331 nm clearly suggest the formation
of a complex between L and Zn2+ (S09 in ESI†), while the other
metal ions exhibit no significant change in the absorption spectra.
The stoichiometry of the complex formed (1 : 1) has been derived
based on the Job’s plot (S10 in ESI†). The Zn2+ sensing property
of L has been further supported by observing the fluorescent color
Fig. 5 (a) ESI mass spectrum showing the isotopic peak pattern of
molecular ion peak for the 1 : 1 complex formed between L and Zn2+
(b) Powder X-ray diffractograms of L and [ZnL].
.
Since single crystals of the zinc complex of L could not be
obtained, the structural features of the complex were characterised
by DFT computational calculations.10 The optimized structure
of L at B3LYP/6-31G showed no conformational changes when
compared to its crystal structure (S13 in ESI†). Taking the
optimized structure of L, its Zn2+ complex was modeled by
employing three different strategies: (i) by using L after removing
1672 | Dalton Trans., 2012, 41, 1671–1674
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both the amidic protons, (ii) by using tautomer of L as reported
in the literature,9 and (iii) as neutral receptor. The structures were
optimized in a cascade fashion similar to that carried out with L by
using an additional polarization function. The structure obtained
at B3LYP/6-31G* level of theory showed a distorted tetrahedral
geometry around Zn2+ in the case of amidic deprotonated (Fig.
6a) as well as tautomeric forms wherein each arm of L acts as a
bidentate (Fig. S13 in ESI†). In the case of the neutral form of L,
the structure showed a distorted trigonal bipyramidal geometry
around Zn2+ bonded through two quinoline nitrogens, one amide
nitrogen and two amide oxygen atoms, resulting in N3O2 core
where both the amide oxygens occupy the axial positions. The
stabilization energy obtained in the case of strategy (i) was higher
than the other two strategies, viz., (ii) and (iii), and hence the
structure obtained from (i) has been given in this paper. The
Fig. 7 Non-contact mode AFM image of (a) L and (b) {L+Zn2+}. The
TEM image of (c) L (d) {L+Zn2+}.
distance between the nitrogens present in L has been found to
2+
˚
be in the range of 5.5–6.4 A. Upon complexation with Zn ,
˚
the distances were found to decrease to 3.3–3.8 A due to the
conformational changes brought in the arms of L to accommodate
the metal ion (Fig. 6b and 6c, and S13 in ESI†). Going from L
to its Zn2+ complex the dihedral angles, C2–C5–N1–C13 on one
arm and C52–C54–N3–C58 on the other arm flip from +175.0
0.1◦ to -168.4 0.1◦ in order to accommodate the metal ion in a
tetrahedral environment.
Fig. 8 Histogram showing the particle size distribution of (a) L and (b)
{L+Zn2+} in AFM.
These spherical particles are being transformed into the bundles
of crystalline fibers emanating from a common core like Koosh
nano-flowers,11 in the presence of Zn2+ as noticed from their
TEM micrographs (Fig. 7d), which is indeed an unprecedented
observation since such micro-structures can be used as their
characteristics. Thus both the AFM and TEM showed nano-
structural features of similar nature and these are sufficient enough
to differentiate L (nano-sphere) from its zinc complex (Koosh
nano-flower). The nano-structural features obtained with the
simple zinc perchlorate used as control differs from that observed
for {L+Zn2+} as studied based on AFM.
Fig. 6 (a) Optimized structure of [ZnL] obtained at B3LYP/6-31G* level
of theory. Comparison of the arms of the optimized structures of (b) L
and (c) [ZnL] (calix[4]arene cavity has been removed for clarity). Metric
data for the coordination geometry around the Zn2+: Zn–N1 = 1.975,
˚
Zn–N2 = 2.050, Zn–N3 = 1.975, Zn–N4 = 2.050 (A), N1–Zn–N2 = 84.3,
Carboxamidoquinoline based 1,3-di-conjugate of calix[4]arene
(L) has been synthesized and was well characterized by various
spectral and analytical techniques. The receptor L exists in a
cone conformation in both solution and the solid state as evident
N1–Zn–N3 = 149.9, N1–Zn–N4 = 111.2, N2–Zn–N3 = 111.2, N2–Zn–N4 =
118.9, N3–Zn–N4 = 84.3 (◦).
1
The ligand adopts spherical shape particles which are well
spread all over themicasurface as shown byAFM studies (Fig. 7a).
However, in the presence of Zn2+, these particles seem to merge and
adopt a distorted shape to result in elongated ones characteristic of
the involvement of Zn2+ which is capable of bringing L’s together
through coordination (Fig. 7b). The majority of these particles
bear sizes in the range 90 to 140 nm, as can be gauged from the
histogram (Fig. 8 and S14 in ESI†). On the other hand, in the
presence of Zn2+ the average size of the particle increases to 185 ¥
110 nm2 owing to their elongated particle shape suggesting that
the particles merge in the presence of Zn2+ which may indicate
that the metal ion induces aggregation. Even the TEM studies
carried out using carbon coated copper grids support the presence
of spherical particles for L (Fig. 7c, S15 in ESI†). Indeed, such
spherical structures have already been observed with some other
calixarene derivatives as reported from our group recently.6f,g
from the H NMR spectrum and single crystal XRD structure
respectively. L showed selectivity towards Zn2+ among all the
metal ions as demonstrated by ratiometric fluorescence sensing.
Metal ion binding of Zn2+ to L has been further supported
by absorption titration and the data could clearly differentiate
the binding ions from those the non-binding ones. Competitive
titration indicates that L could be able to sense Zn2+ even in the
presence of several other metal ions. The Job’s plot suggests the
formation of a 1 : 1 complex between L and Zn2+, which has been
further confirmed by ESI-MS. The species of recognition has been
modelled by computational calculations at DFT level that resulted
in a distorted tetrahedral geometry around Zn2+. These studies
further showed conformational changes in the arms of L upon
complexation with Zn2+. L exhibits colorimetric response towards
Zn2+ among the metal ions studied. Both the AFM and TEM
showed nano-structural features of similar nature and these are
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sufficient enough to differentiate L (nano-sphere) from its zinc
complex (Koosh nano-flower) (Fig. 9).
Souane, J. S. Kim, S. E. Matthews and J. Vicens, J. Org. Chem., 2008,
73, 8212; (d) I. Leray and B. Valeur, Eur. J. Inorg. Chem., 2009, 3525;
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Chem., 2008, 47, 3549; (g) A. Senthilvelan, I.-T. Ho, K.-C. Chang, G.-
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Rao, Tetrahedron Lett, 2009, 50, 2735.
7 Synthesis and characterization of L: To a solution of p-tert-calix[4]arene
(0.6 g, 0.9 mmol), K2CO3 (0.52 g, 3.7 mmol) was added followed by NaI
(0.56 g, 3.7 mmol) and 2-chloro-N-(quinolin-8-yl)acetamide (0.83 g, 3.7
mmol). The reaction mixture was refluxed for 24 h and the temperature
was slowly brought down to RT and the solvent was evaporated to
dryness. The residue was dissolved in CH2Cl2 and finally washed with
NaHCO3 solution and water. The pure compound was obtained by
silica gel column chromatography by using EtOAc/Petrolium ether
as eluents (1 : 9): yield (55%, 0.52 g); FTIR (KBr, cm-1) 1687, 3491,
Anal. Calcd. for C66H72N4O6·2CH3OH:C, 75.53; 7.46; N, 5.18. Found
C, 75.70, H, 7.54; N, 5.01. 1H- NMR: (CDCl3, 400 MHz d (ppm)):0.93
(s, 18H, C(CH3)3), 1.312 (s, 18H, C(CH3)3), 3.4 (d, 4H, Ar-CH2-Ar, J =
13.2), 4.4 (d, 4H, Ar-CH2-Ar J = 12.8), 4.6 (s, 4H, O-CH2), 6.2(s, 4H, Ar-
H), 6.7 (s, 4H, Ar-H), 7.15 (m, 6H, Quinoline -H), 7.6 (d, 2H, Quinoline-
H, J = 8.4), 8.2 (d, 2H, Quinoline -H, J = 7.6), 8.7 (d, 2H, Quinoline
-H, J = 8), 11.1 (s, 2H, NH-CO). 13C- NMR: (CDCl3, 100 MHz d
(ppm)):31.09, 31.53 (C(CH3)3), 31.91,(Ar-CH2-Ar), 34.06 (C(CH3)3)
75.29 (O-CH2), 116.58, 121.68, 122.09 Quinoline-Ar-C, 125.44, 125.96,
126.19, 127.05, 131.64, 133.23, 135.34,Ar-C, 138.15, 142.10, 147.67
Quinoline-Ar-C, 148.11, 149.51, 150.89 Ar-H, 166.48 (NH-CO-Ar);
HRMS m/z 1017.55 (M+ +1).
Fig. 9 Transformation of nano-spheres observed with L as in (a) to Koosh
nano-flowers observed with {L+Zn2+} as in (b).
Thus L can be used as selective receptor towards Zn2+ by
ratiometric and colorimetric response even in the presence of
other metal ions and also by its nano-structural variation. Since
the Koosh nano-flower material possessing Zn2+ ions is expected
to elicit conductivity properties while the same is not true with
the precursor L, it may evoke interest for the researchers of
microelectronics involved in building sensors and devices.
CPR acknowledges the financial support from DST, CSIR
and DAE-BRNS. VVSM and JPC acknowledges CSIR and KT
acknowledges UGC for their fellowships. We thank Dr Venu
Srinivas for some help with the crystallography refinement. We also
acknowledge the Department of Physics for AFM and CNTRS
for TEM facilities.
Notes and references
‡ Crystal data of L: Empirical formula - C73.5H75N7.5O6, Formula weight -
¯
˚
1159.41, T/K - 150, Crystal System - Triclinic, Space group - P1, a/◦A -
◦
˚
˚
11.6535(4), b/A - 12.0093(4), c/A - 22.2811(3), a ( ) - 87.280(4), b ( ) -
◦
3
˚
83.524(4), g ( ) - 86.494(3), V/A - 3090, Z - 2, Absorption Coefficient
(mm-1) - 0.080, F(000) - 1233.0, Density - 1.246 Mg m-3, Reflections
8 Y. Zhang, X. Guo, W. Si, L. Jia and X Qian, Org. Lett, 2008, 10, 473.
9 J. Jiang, H. Jiang, X. Tang, L. Yang, W. Dou, W. Liu, R. Fang and W.
Liu, Dalton. Trans., 2011, 40, 6367.
collected - 22396, Independent reflections - 10860, Parameters - 804, Rint
-
0.068, Final R (I > 2s(I)) - 0.0933, wR2 - 0.2309. SQUEEZE was used to
remove the contributions of disordered solvent from the structure factor
file.
10 J. A. Pople and coworkers, Gaussian 03, revision C.02, Gaussian, Inc.,
Wallingford, CT, 2004 (total reference is given in S16 ESI†).
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Commun., 2010, 46, 3037.
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12 Crystal structure determination by single crystal XRD: Single crystal
X-ray diffraction data were collected on an OXFORD DIFFRAC-
TION XCALIBUR-S CCD system with graphite-monochromated
Mo–Ka radiation by w - 2q scan mode and the absorption corrections
were applied by using multi-scan. The structural determinations by
direct methods and the refinement of atomic parameters based on
full-matrix least squares on F2 were performed using the SHELX-97
programs. Disordered solvent molecules have been removed by make
use of SQUEEZE option in PLATON and the corresponding out put
has been appended in the CIF file.
2 W. J. Bettger and B. L. O’Dell, Life Sci., 1981, 28, 1425.
3 R. J. P. Williams in Zinc in Human Biology (Ed.: C. F. Mills), Springer-
Verlag, London, 1989, pp. 15–31.
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and I Koyama, Epilepsia, 1986, 27, 183; (c) A. I. Bush, W. H. Pettingel,
M. D. Paradis and R. E. Tanzi, J. Biol. Chem., 1994, 269, 12152; (d) P. A.
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D. Troost and J. M. B. Vianney de Jong, Biol. Signals, 1994, 3, 193.
5 (a) J. S. Kim and D. T. Quang, Chem. Rev., 2007, 107, 3780; (b) I.
A. Bagatin, E. S. De Souza, A. S. Ito and H. E. Toma, Inorg. Chem.
Commun., 2003, 6, 288; (c) S. Y. Park, J. H. Yoon, C. S. Hong, R.
13 G. M. Sheldrick, Acta Cryst., 2008, A64, 112.
1674 | Dalton Trans., 2012, 41, 1671–1674
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The Royal Society of Chemistry 2012
©