Calix[4]arene based molecular probe for sensing copper ions

Article abstract of DOI:10.1016/j.tetlet.2014.02.021

A novel calix[4]arene based molecular probe for metal ions has been designed, synthesized and evaluated. Studies on its binding with different metal ions reveal a noticeable naked eye color change, bathochromic shift in absorption spectrum and remarkable enhancement in fluorescence emission in the presence of Cu²⁺ only. The role of calix[4]arene scaffold for selective recognition of Cu²⁺ has been demonstrated by repeat evaluation and analysis of an appropriate reference molecule. A rational explanation for fluorescence enhancement in 3 on interaction with copper has been suggested.

Full text of DOI:10.1016/j.tetlet.2014.02.021

Tetrahedron Letters 55 (2014) 2173–2176
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Calix[4]arene based molecular probe for sensing copper ions
⇑
Har Mohindra Chawla , Preeti Goel, Richa Shukla
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel calix[4]arene based molecular probe for metal ions has been designed, synthesized and
evaluated. Studies on its binding with different metal ions reveal a noticeable naked eye color change,
bathochromic shift in absorption spectrum and remarkable enhancement in fluorescence emission in
the presence of Cu2+ only. The role of calix[4]arene scaffold for selective recognition of Cu2+ has been
demonstrated by repeat evaluation and analysis of an appropriate reference molecule. A rational
explanation for fluorescence enhancement in 3 on interaction with copper has been suggested.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 26 August 2013
Revised 8 February 2014
Accepted 10 February 2014
Available online 21 February 2014
Keywords:
Calix[4]arenes
Coumarinylhydrazone
Molecular probe
Turn on sensor molecules
The development of chemosensors for recognition of copper
ions has attracted immense interest because of its critical role in
chemical and biological systems.1,2 Disproportionate amounts of
copper ions may lead to neurodegenerative diseases and amyotro-
phic lateral sclerosis.2c A large volume of research investigations
have been documented which describe different types of sensors
for copper ions which utilize UV, near infra red and emission spec-
troscopies and sensing mechanisms that include photoinduced
electron transfer, fluorescence resonance energy transfer, internal
charge transfer, nanotechnology and other photo-physical proper-
ties compatible with analysis of Cu2+ in biological systems.3
Diverse nature of functions of copper ions, both beneficial and
otherwise, and its analysis requirements justify strong academic
interest in Cu2+ selective molecular probes especially because it
is a typical ion that leads to fluorescence quenching inherent to
its paramagnetic nature.4 There is a significant design challenge
for obtaining molecular receptors for copper that lead to enhance-
ment of intensity of the emission spectrum.
and its derivatives have been extensively employed as fluorophores
because of their tunable photophysical properties in the visible
region.8 Schiff bases (imines) are also known to act as good chelat-
ing sites for metal ions.9 Thus a molecular probe possessing a calix-
arene scaffold, fluorescence moiety and a Schiff base substructure
should be worth examining for recognition of metal ions.10 Herein
we report the synthesis of a new calix[4]arene derivative (3) with
different functional units cited above for recognition of copper ions.
The distinct advantage expected from the designed molecular
probe is that it emits in the visible region to promote least interfer-
ence from background emissions.11
The designed molecular receptor 3 was synthesized from p-tert-
butyl calix[4]arene via the known intermediates10 while 2 was
synthesized by following the synthetic protocol depicted in
Scheme S1 (Supplementary data). Reaction of calix[4]arene deriva-
tive (1) with (2) in the presence of glacial acetic acid in ethanol
yielded the product which when washed with methanol gave 3
in satisfactory yield (71%) (Scheme 1).12–14
Fluorescence5 and colorimetric6 sensing modes have several
advantages over other method of evaluating ionic recognition due
to their ease of operation, high sensitivity and straight forward
application modes. Reported chemosensors for copper ions mostly
involve the attachment of fluorophore units to the macrocyclic or
chelating scaffolds. Among well known macrocyclic frameworks,
calix[4]arenes are considered to be extremely useful building
blocks for achieving ionic and molecular recognition7 plausibly
due to their preorganized cavity and facile modifiability. Coumarin
The synthesized compounds were characterized by 1H NMR, 13
C
NMR, IR, and HRMS (ESI, Fig. S2). For example, 3 showed a >C@N–
signal at 1605 cmÀ1 in IR and a sharp pair of doublets at d 3.32 and
d 4.23 for axial and equatorial protons respectively in the 1H NMR
spectrum and a distinct signal at d 31.18 for the methylene carbons
in its 13C NMR spectrum indicated its symmetric cone conforma-
tion for the calix[4]arene scaffold. It was further confirmed by
observing a D2O exchangeable singlet at d 7.76 due to –OH protons
in its 1H NMR spectrum. Non deuterable singlets at d 8.60 and 8.55
for the azo-methine proton (–N@CH) confirmed the structure for 3
as depicted in Scheme 1.
⇑
Corresponding author. Tel.: +91 11 26591517; fax: +91 11 26581 102.
E-mail addresses: hmchawla@chemistry.iitd.ac.in, hmchawla@chemistry.iitd.
ernet.in (H.M. Chawla).
http://dx.doi.org/10.1016/j.tetlet.2014.02.021
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2174
H. M. Chawla et al. / Tetrahedron Letters 55 (2014) 2173–2176
Fluorescence emission spectra of 3 in the presence of Cu2+ at
various concentrations are shown in Figure 2. It was determined
that addition of Cu2+ ions (0–130
M) resulted in enhancement
l
O
O
O
O
H
(H2C)3
H
MeO
(CH2)3
O
O
O
of the fluorescence signal with a 90 nm red shift in the position
O
EtOH/AcOH
Reflux
O
O
O
O
H
(H2C)3
H
(CH2)3
O
of fluorescence peak indicating that it can be used as turn-on fluo-
+
O
H
NNH2
rescence sensor for Cu2+
.
HC
N
CH
N
N
C
H
N
H
C
O
The enhancement in intensity of fluorescence of 3 may be ratio-
nalized by considering photoinduced electron transfer (PET) in the
designed molecular probe. Such an electron transfer is facile in the
Schiff base moiety. The presence of a lone pair of electrons on
nitrogen results in an effective quenching of the fluorescence
intensity of 3 in its off state. When Schiff base substructure inter-
acts with Cu2+ ions, this photoinduced electron transfer seems to
get blocked which results in enhancement of fluorescence inten-
sity. That this indeed is the case was substantiated by the observa-
tion that fluorescence intensity also got markedly enhanced when
measurements were made at low pHs plausibly due to protonation
of the nitrogen lone pair of electrons. These findings are in conso-
nance with recent literature precedents.15 Enhancement in fluores-
cence intensity can also be attributed to the suppression of C=N
isomerization15a,16 in the excited state when complexation occurs
with Cu2+ (as shown in Figure 3).
O
(2)
(1)
O
O
O
O
O
OH
MeO
OMe
(3)
OH
OMe
O
N
EtOH/AcOH
reflux
N
H
NNH2
CHO
O
(2)
O
(4)
MeO
Reference
Scheme 1. Synthesis of novel calixarene derivative.
A reference compound (4) with similar binding sites and the
signalling unit was also synthesized (Scheme 1) to evaluate the
role of calix[4]arene platform in the ion recognition process.
The sensing ability of 3 was investigated by monitoring the
absorption spectra on gradual addition of several metal cations
Compound 3 showed a remarkably high selectivity for Cu2+
which was confirmed by observing an insignificant increase in
the fluorescence intensity upon addition of other metal ions (ESI,
Fig. S5). Fluorescence spectra of solution of 3 on addition of differ-
ent metal ions are given in Figure 4.
such as Na+, Li+, K+, Cs+, Ag+, Ca2+, Mn2+, Zn2+, Co2+, Cd2+, Pb2+
,
Hg2+, Fe2+, Ni2+ and Cu2+ as their perchlorate salts in acetonitrile.
The absorption spectrum of receptor 3 in the absence of any metal
ion showed a band centered around 339 nm which is characteristic
of the coumarin moiety. This absorption band remained un-
changed (only slight decrease in the absorbance value due to dilu-
tion was observed) on addition of 5.5 equiv of Na+, Li+, K+, Cs+, Ag+,
Ca2+, Mn2+, Zn2+, Co2+, Cd2+, Pb2+, Hg2+, Fe2+, Ni2+ with no color
change in the solution (ESI, Fig. S3). The 339 nm band red shifted
to 346 nm with decreased intensity on addition of increasing con-
centration of Cu2+ ion which was also accompanied by a color
change of the solution from colorless to light yellow (Fig. 1). Thus
3 showed a specific selectivity for Cu2+ ion to allow its facile naked
eye detection. Job’s plot was deduced to determine the stoichiom-
etry of the interaction between the Cu2+ ion and receptor 3. Anal-
ysis of the data revealed that the stoichiometry of molecular
complex was 1:1 (Fig. 1 inset) which was further confirmed by
straight line obtained from Benesi–Hildebrand plot (ESI, Fig. S4).
Fluorescence spectra of 3 were also recorded after addition of
Cu2+ ions to its acetonitrile solution. When excited at 340 nm,
the emission spectrum of 3 showed a weak fluorescence band at
420 nm.
Selectivity coefficients were calculated for all the studied metal
n+
2+
2+
ions using the relationship KM
=
D
FM
/ . It was
n+ DFCu
,
Cu
determined that ions other than copper have a very small value
n+
2+
for KM
/
Cu
indicating an insignificant change in the selectivity
of 3 for copper.
The practical application of synthesized novel receptor 3 as
fluorescence turn on probe for Cu2+ was also examined by record-
ing its fluorescence response to Cu2+ in the presence of other com-
peting ions. As shown in Figure 5, most of the competing ions such
as Co2+, Hg2+, Zn2+, Cd2+, Mn2+, Fe2+ and Ag+ exhibited no conflict in
the detection of Cu2+ in the presence of other metal ions. A little
interference from nickel ions was however noticed. Thus 3 can
be used for selective detection of Cu2+ ions even in the presence
of mentioned competing ions.
Fluorescence properties of the reference compound 4 in the
presence of Cu2+ were investigated with the aim to assess the
1.19
1.1
1.0
0.9
0.8
0.7
0.6
A
0.5
0.4
0.3
0.2
0.1
0.0
230 240 260 280 300 320 340
380 400 420 440 460 480 500 520
360 nm
Figure 1. Change in the UV–vis spectra of Ligand (15
(0–70
M) in ACN. Inset: Job’s plot of Cu2+ complex formation. {[3]/[3]+[Cu2+] is the
mole fraction of ligand 3}and color change after addition of Cu2+
l
M) upon addition of Cu2+
Figure 2. Enhancement in the fluorescence intensity of 3 (15
presence of Cu2+ (0–130
M); Inset shows change in the fluorescence intensity of
the ligand with varying concentration of Cu2+ (kex = 340 nm).
lM) in ACN in
l
l
.

H. M. Chawla et al. / Tetrahedron Letters 55 (2014) 2173–2176
2175
O
O
O
O
O
O
O
O
H
H
(H2C)3
H
(H2C)3
H
(CH2)3
O
(CH2)3
O
O
O
Cu2+
HC
HC
N
CH
CH
.
.
N
N
N
N
N
N
Cu2+
N
PET
PET
O
O
O
O
O
O
O
O
MeO
OMe
MeO
OMe
High Fluorescence
Low Fluorescence
Figure 3. Proposed model for rationalizing the enhancement in fluorescence
intensity upon addition of Cu2+ to the solution of receptor 3 (15
M) in ACN.
l
Figure 6. Comparative study of molecular receptor 3 in ACN with its reference
compound 4 by exciting at 340 nm.
3 could be calculated by using calibration sensitivity (m) of relative
fluorescence intensity versus [Cu2+] in the aforementioned range.17
Minimum change in the fluorescence intensity due to the presence
of Cu2+ was taken to be 3 Â so where so represents the standard
deviation of Fo for ten replicate measurements.18 These measure-
ments gave the limit of detection (LOD) of copper as 0.6
l
l
M which
M.
is fair within the recommended US EPA19 limits of ꢀ20
The 1H NMR titrations revealed the presence of broadened
signals for 3-Cu2+ complex as expected due to paramagnetic nature
of Cu2+ 20
.
In conclusion, we have reported the synthesis and evaluation of
a novel calix[4]arene derivative with appended coumarin moiety
for selective recognition of Cu2+ ions. 3 can function as a ‘turn-
on’ fluorescence and a chromogenic molecular probe.
Figure 4. Change in the fluorescence intensity of the ligand 3 upon addition of
10 equiv of various metal ions to ligand in ACN (kex = 340 nm). Inset shows
n+
2+
n+
2+
selectivity coefficients, KM
/
= DFM /DFCu .
Acknowledgements
Cu
Preeti Goel and Richa Shukla thank UGC and CSIR, India for a re-
search fellowship. Financial assistance from DST, MoRD and FIST
grant for HRMS to Dept. of Chemistry, IITD is gratefully
acknowledged.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.02.
021.
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