Thiacalix[4]arene-cinnamaldehyde derivative: ICT-induced preferential nanomolar detection of Ag+ among different transition metal ions

Article abstract of DOI:10.1039/c1ob06294h

A new thiacalix[4]arene-cinnamaldehyde derivative 3, which undergoes red shift in the fluorescence spectrum in the presence of Ag⁺ ions, has been synthesized. This emission shift is attributed to the intramolecular charge transfer process in the presence of Ag⁺ ions with a detection limit in the nanomolar range.

Full text of DOI:10.1039/c1ob06294h

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PAPER
Thiacalix[4]arene-cinnamaldehyde derivative: ICT-induced preferential
nanomolar detection of Ag+ among different transition metal ions†
Manoj Kumar,* Naresh Kumar and Vandana Bhalla
Received 29th July 2011, Accepted 15th November 2011
DOI: 10.1039/c1ob06294h
A new thiacalix[4]arene-cinnamaldehyde derivative 3, which undergoes red shift in the fluorescence
spectrum in the presence of Ag+ ions, has been synthesized. This emission shift is attributed to the
intramolecular charge transfer process in the presence of Ag+ ions with a detection limit in the nanomolar
range.
enhancement with spectral shift for detecting silver ions10 has
been limited till now because silver ions usually quench fluor-
Introduction
Silver, because of its toxic effects, is an important metal ion
among various heavy transition-metal ions.1 The use of silver in
the electrical, photographic imaging, and pharmaceutical indus-
tries results in contamination of the environment.2 Silver ions
also have a negative effect in biological systems, such as interact-
ing with vital enzymes which results in inactivation of these
enzymes,3 interacting with cell membranes,4 and interfering with
electron transport.5 Irreversible darkening of the skin and
mucous membrane is a side effect originating from prolonged
use of silver .6 Thus, keeping in view the role played by silver in
day to day life, simple and rapid sensing of silver in biological
and environmental systems is very important. Many approaches
such as atomic absorption, inductively coupled plasma-mass
atomic emission, fluorescence, and UV-vis absorption spec-
troscopy have been widely employed to quantify trace amounts
of silver ions.7 Among these methodologies, fluorescence signal-
ling is one of the first choices as it is highly sensitive and simple
which translates binding events into tangible fluorescence
signals.8 More importantly, most fluorescent sensors can be used
as cellular imaging reagents which make the fluorescence
approach superior to other analytical methods. Therefore, much
effort has been devoted to design fluorescent sensors for versatile
objectives. A number of photophysical processes can be
exploited to yield visible parameters which can be correlated to
analyte concentration.9 For high sensitivity and simplicity, a sig-
nificant spectral shift in either the absorption or emission spectra
becomes very important. However, the success of fluorescence
escence emission via electron transfer and intersystem crossing
processes.11 Thus, it is extremely advantageous to propose a
novel sensor with spectral shifts for silver ions. In this regard,
sensors based on metal-induced changes in fluorescence are par-
ticularly attractive as they offer significant spectral shifts. The
most fundamental way to assure such a spectral change is to
design a molecule exhibiting intramolecular charge transfer
(ICT) mechanism which involves the observation of changes in
the ratio of the intensities of the absorption or the emission at
two wavelengths.12
Our research involves the design, synthesis and evaluation of
(thia)calix[4]arene based receptors for selective sensing of soft
metal ions, anions and evaluation of their switching behaviour.13
A number of molecular switches based on thiacalix[4]arenes
have been reported in the past by our research group.14 Recently,
we have reported a thiacalix[4]arene based chemosensor bearing
two pyrene groups which demonstrates ratiometric sensing with
Ag+ and fluorescence quenching with Fe3+ ions in mixed
aqueous media.15 Now, we have designed and synthesized a
receptor, 3, based on 1,3-alternate conformation of thiacalix[4]
arene bearing dimethyliminocinnamaldehyde moieties. The che-
mosensor 3 shows a strong fluorescence attributed to the excited
state redistribution of charge which makes a twisted intramolecu-
lar charge transfer (TICT) state.16 Out of the various transition
and alkali metal ions, only in the presence of Ag+ ions, the che-
mosensor 3 displays a selective spectral shift in fluorescence
emission as a consequence of intramolecular charge transfer
(ICT), which involves electron transfer from the nitrogen atom
of the dimethylamino moiety to the imino nitrogen, of which the
electron density is diminished on interaction with Ag+ ions. The
addition of other transition metal ions like Hg2+, Fe2+, Fe3+,
Zn2+, Cu2+, Cd2+ and Pb2+ shows linear decrease of the emis-
sion, while the addition of Ba2+, Mg2+, K+, Li+, Na+, Ni2+, Co2+
did not alter the fluorescence emission of receptor 3.
Department of Chemistry, UGC-Center for Advance Studies-1, Guru
Nanak Dev University, Amritsar, Punjab, India. E-mail: mkshar-
maa@yahoo.co.in; Fax: +91 (0)183 2258820; Tel: +91 (0)183 2258802
9 ext. 3205
†Electronic supplementary information (ESI) available: 1H NMR and
mass spectra. See DOI: 10.1039/c1ob06294h
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Scheme 1 Synthesis of 3.
Results and discussion
Fig. 1 UV-vis spectra of 3 (10.0 μM) with Ag+ ions in THF; (A) in the
presence of 0–1 equiv of Ag+ ions; (B) in the presence of 1–100 equiv
of Ag+ ions; inset showing the colour change: (a) before; (b) after the
addition of Ag+ ions and normalized absorbance spectra; blue, free 3;
red, 3 + 1 equiv Ag+ ions and green, 3 + 100 equiv Ag+ ions.
The condensation of diamine 117 with N,N-dimethylaminocinna-
maldehyde 2 furnished compound 3 in 78% yield (Scheme 1).
The structure of compound 3 was confirmed from its spectro-
scopic and analytical data. The IR spectrum of compound 3
showed a CvN stretching band at 1602 cm−1 (see ESI S12†).
There is no absorption band corresponding to free aldehyde and
amino groups, which indicates that the condensation has taken
1
place. The H NMR spectrum (see ESI S7 and S8†) of com-
pound 3 showed four triplets (6, 4, 4 and 4H) at 0.62, 3.05, 3.82
and 4.14 ppm corresponding to the CH3, NCH2, OCH2 and
OCH2 protons, three multiplets (4, 8 and 8H) at 1.02–1.09,
6.63–6.74 and 7.31–7.34 ppm corresponding to the methylene,
aromatic and CH protons, four singlets (18, 18, 12 and 4H) at
1.26, 1.28, 2.99 and 7.41 ppm corresponding to the tert-butyl,
NCH3 and aromatic protons, and a doublet (2H) at 7.87 ppm
corresponding to the imino protons. The mass spectrum showed
a parent ion peak at m/z 1205.6 corresponding to the compound
3 (see ESI S10†). These spectroscopic data corroborate the struc-
ture 3 for this compound.
The binding behaviour of compound 3 towards different
cations (Ag+, Hg2+, Fe2+, Fe3+, Zn2+, Cu2+, Ni2+, Co2+, Cd2+,
Pb2+, Ba2+, Mg2+, K+, Li+ and Na+) as their perchlorate salts was
studied by UV-vis and fluorescence spectroscopy. The titration
experiments were carried out in dry THF by adding aliquots of
different metal ions. The absorption spectrum of 3 (10.0 μM) is
characterized by typical absorption bands of dimethyliminocin-
namaldehyde moiety at 265 and 352 nm (Fig. 1A) assigned to
the transition from S0 to S2 and S1 states respectively.16 A low
energy absorption at 352 nm indicates that this is a π–π* type of
transition. Addition of Ag+ ions up to one equivalent resulted in
a red shift of the band centered at 352 nm to 396 nm with an iso-
sbestic point at 370 nm (Fig. 1A). The formation of a new band
at 396 nm is due to the interaction of Ag+ ions with imino nitro-
gen atoms leading to weak intramolecular charge transfer (ICT)
from the nitrogen atom of the dimethylamino moiety to the
imino nitrogen atom. On further addition of Ag+ ions (1–100
equiv) the absorption band at 396 nm decreases with simul-
taneous appearance of a new absorption band at 466 nm
(Fig. 1B). The variations in the absorption bands at 396 and
466 nm on addition of Ag+ ions led to a clear isosbestic point at
432 nm. We believe that as the amount of Ag+ ions increases,
the interaction between the Ag+ ions and the imino nitrogen
atoms induces an enhanced ICT from the nitrogen atom of the
dimethylamino moiety to the imino nitrogen atom, of which the
electron density is diminished on interaction with Ag+ ions
Fig. 2 UV-vis spectra of 3 (10.0 μM) with various metal ions
(100 equiv each) in THF.
which results in a large red shift with appearance of a new band
at 466 nm. In the presence of 100 equivalents of other metal
ions like Hg2+, Fe2+, Fe3+, Zn2+, Cu2+, Ni2+, Co2+, Cd2+, Pb2+,
Ba2+ and Mg2+ the absorption band at 352 nm decreases
accompanied by a red shift to 466 nm (Fig. 2). While the
addition of K+, Li+ and Na+ ions resulting in the small decrease
in the absorbance at 352 nm with slight red shift (Fig. 2). The
addition of just one equivalent of divalent metal ions like Hg2+,
Fe2+, Fe3+, Zn2+, Cu2+, Ni2+, Co2+, Cd2+, Pb2+, Ba2+ and Mg2+
shifted the absorption band of receptor 3 at 352 nm to 466 nm
(see ESI S4†) as divalent metal ions have more affinity towards
electrons, immediately inducing a strong intramolecular charge
transfer in comparison to Ag+ ions, which are of a monovalent
nature. From these UV-vis studies, it is clear that compound 3 is
interacting with different metal ions in the ground state. Fitting
the changes in the absorbance spectra of compound 3 with
different metal ions (Ag+, Hg2+, Fe2+, Fe3+, Zn2+, Cu2+, Ni2+,
Co2+, Cd2+, Pb2+, Ba2+ and Mg2+) that change the absorption
spectrum of 3, the nonlinear regression analysis program
SPECFIT18 demonstrated that 1 : 1 stoichiometry (host : guest)
was the most stable species in solution and gave different
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Table 1 Binding constants of different metal ions with compound 3
Metal ions
log β
Metal ions
log β
Ag+
Fe2+
Zn2+
Ni2+
Cd2+
Ba2+
6.25 0.23
4.83 0.07
4.19 0.32
4.37 0.06
4.52 0.05
3.34 0.05
Hg2+
Fe3+
5.02 0.44
5.26 0.08
5.90 0.23
5.47 0.11
5.33 0.03
4.80 0.08
Cu2+
Co2+
Pb2+
Mg2+
Fig. 4 Fluorescence spectra of 3 (1.0 μM) with Ag+ ions in THF; λex
=
360 nm; (A) in the presence of 0–2 equiv of Ag+ ions; (B) in the pres-
ence of 0–30 equiv of Ag+ ions; inset (1) the fluorescence change: (a)
before; (b) after the addition of Ag+ ions and (2) the fluorescence inten-
sity I/Io (Io = initial fluorescence intensity at 488 nm; I = fluorescence
intensity after the addition of Ag+ ions at 488 nm) as a function of Ag+
ions concentration.
Fig. 3 Fluorescence spectra of 3 (1.0 μM) in the various polar and
non-polar solvents; λex = 360 nm.
binding constants (Table 1). From these results it is clear that the
binding of silver with compound 3 is relatively strong in com-
parison to the binding of metal ions like Hg2+, Fe2+, Fe3+, Zn2+,
Cu2+, Ni2+, Co2+, Cd2+, Pb2+, Ba2+ and Mg2+ with compound 3.
The fluorescence spectrum of compound 3 (1.0 μM) in THF
exhibits a strong emission at 418 nm when excited at 360 nm
(Fig. 3). The intense fluorescence emission of receptor 3 at
418 nm is due to the extended conjugated system which involves
redistribution of charge in the excited state and makes a twisted
intramolecular charge transfer (TICT) state. To confirm that the
emission observed for receptor 3 in THF is based on this twisted
intramolecular charge transfer, we studied the fluorescence be-
haviour of receptor 3 in various polar and non-polar solvents
(Fig. 3, see ESI S2†). In non-polar solvents (benzene and
toluene) receptor 3 shows unresolved emission corresponding to
both a delocalized excited (DE) state at short wavelength
(402 nm) and a twisted intramolecular charge transfer (TICT)
state at longer wavelength (414 nm). Any increase in polarity of
the solvents did not affect the position of the delocalized excited
band. In comparison to the delocalized excited band, the TICT
band shows a large red shift when moving from non-polar to
polar solvents. For example, in THF a red shift of the TICT band
(4 nm) was observed in comparison to non-polar solvents, while
the presence of the delocalized excited band is hardly observed
as in this case the delocalized excited band goes underneath the
envelope of the TICT emission band. Thus, the appearance of
the red shifted emission band in polar solvents clearly indicates
that the emission of receptor 3 in THF is due to the twisted intra-
molecular charge transfer state.16
Fig. 5 Fluorescence spectra of 3 (1.0 μM) with Hg2+, Fe2+, Fe3+, Zn2+
,
Cu2+, Cd2+ and Pb2+ ions (30 equiv each) showing the linear decrease of
the emission at 418 nm in THF; λex = 360 nm.
418 nm quenches completely, and a new emission band appears
at 488 nm with a bathochromic shift of about 70 nm (Fig. 4).
With further additions of Ag+ ions (2–30 μM) the emission band
at 488 nm shows significant fluorescence enhancement. The flu-
orescence behaviour of compound 3 in the presence of Ag+ ions
is attributed to the alteration in the electronic properties of 3 i.e.
increased intramolecular charge transfer (ICT) on metal ion com-
plexation.19 The nitrogen atoms of the imino moieties get
involved in coordination with the silver ions, which enhances the
electron withdrawing ability of imino nitrogen atoms leading to
an enhanced intramolecular charge transfer process and conse-
quently results in a large red shift. This type of emission shift is
not observed with the addition of any other metal ions (Fig. 6A).
The addition of metal ions like Hg2+, Fe2+, Fe3+, Zn2+, Cu2+,
Cd2+ and Pb2+ shows a linear decrease in the emission at
418 nm (Fig. 5). Fluorescence quenching in the case of Hg2+
and Pb2+ may be attributed to spin–orbit coupling.20 For Cu2+
and Fe3+ it may be due to the paramagnetic nature of these
ions21 and for Fe2+, Zn2+ and Cd2+, it may be due to
Further, upon addition of just two equivalents (0–2 μM) of
Ag+ ions to the receptor 3 in THF, the emission band centered at
This journal is © The Royal Society of Chemistry 2012
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Fig. 8 1H NMR (300 MHz) spectra of 3 in CDCl3/CD3CN (8 : 2); (a)
Free ligand; (b) Free ligand + 1.0 equiv. of silver perchlorate.
Fig. 6 Fluorescence response of 3 (1.0 μM) to various cations (30 μM
each) in THF; λex = 360 nm. Bars represent the emission intensity ratio
(I/Io) (Io = initial fluorescence intensity at 488 nm; I = final fluorescence
intensity at 488 nm after the addition of metal ions). Blue bars represent
selectivity (I/Io) of 3 upon addition of different metal ions; red bars rep-
resent competitive selectivity of receptor 3 towards Ag+ ions (30 μM) in
the presence of other metal ions (30 μM).
Fig. 6B no significant variation in emission was observed by
comparison with or without the other metal ions. It was found
that 3 has a detection limit of 70 × 10−9 mol L−1 for Ag+ ions
which is sufficiently low for the detection of a nanomolar con-
centration range of Ag+ ions (see ESI S6†) found in many
chemical systems.24 Fitting the changes in the fluorescence
spectra of compound 3 with Ag+ ions using the nonlinear
regression analysis program SPECFIT18 gave a good fit and
demonstrated that 1 : 1 stoichiometry (host : guest) was the most
stable species in solution with a binding constant (log β) = 6.73
with 0.17 error which is comparable with that determined by
the absorbance method. The method of continuous variation
(Job’s plot) was also used to prove the 1 : 1 stoichiometry
(Fig. 7).25 The binding mode of receptor 3 with Ag+ ions is
1
proved by H and 13C NMR spectroscopy. The imino protons of
receptor 3 undergo a downfield shift of Δδ = 1.69 ppm on
addition of 1.0 equiv of Ag+ ions proving interaction of receptor
3 with Ag+ ions through the nitrogen atoms of the imino moiety
(Fig. 8, see ESI S13†). The participation of sulphur atoms in the
thiacalix[4]arene ring towards Ag+ ions coordination is proved
by 13C NMR studies of compounds 1 and 3 in the presence of
Ag+ ions. In the presence of 1.0 equiv of Ag+ ions compounds 1
and 3 undergo a downfield shift of signals corresponding to the
carbons of the aromatic ring of the thiacalixarene moiety from
126.16–128.68 ppm to 126.23–131.32 ppm (see ESI S16–17†)
and 123.23–130.67 ppm to 126.30–131.40 ppm (see ESI
S14–15†), respectively. The downfield shifts of signals corre-
sponding to the carbons of the aromatic ring of the thiacalixarene
moiety in both cases elucidate the role of the sulphur atoms in
the thiacalix[4]arene ring in the coordination with Ag+ ions. The
binding of Ag+ ions with receptor 3 is also proved by mass spec-
troscopy (see ESI S11†). The mass spectrum showed a peak at
m/z 1412.7 corresponding to the 3-AgClO4 complex, which not
only confirms the binding of Ag+ ions with receptor 3 but also
proves the 1 : 1 stoichiometry of the host and guest species. In
addition to this, we also carried out a reversibility experiment,
which proves that the binding of Ag+ ions to receptor 3 is revers-
ible (Fig. 9). Addition of tetrabutylammonium chloride to the
solution of the 3-Ag+ complex restored the fluorescence signal
of 3 to its original level. Further, addition of Ag+ ions to the
same solution gives a 3-Ag+ complex indicating the reversible
Fig. 7 Job’s plot of 3 with Ag+ ions in THF representing 1 : 1
stoichiometry.
photoinduced electron transfer from the photoexcited dimethyl-
iminocinnamaldehyde moiety to the metal ions.22 However, the
preferred ICT process from the nitrogen atom of the dimethyl-
amino moiety (donor) to the metal bound imino nitrogen atom
(acceptor) may also contribute to the observed fluorescence
quenching. The addition of other metal ions such as Ba2+, Mg2+,
K+, Li+, Na+, Ni2+, Co2+ did not alter the fluorescence emission
of compound 3 (see ESI S3†).
Further, by considering the ratio of the fluorescence intensity
at 488 nm (I488) to that of 418 nm (I418), we observed 13-fold
fluorescence enhancement in the case of 3-Ag+ complex in com-
parison to the free receptor (0.11). The fluorescence quantum
yield23 (see ESI S5†) of 3-Ag+ complex is 0.32 (at λem
=
488 nm) as compared to that of free 3 (0.47, at λem = 418 nm)
which shows good agreement with fluorescence spectra obtained
for receptor 3 in the presence of Ag+ ions. To check the practical
ability of compound 3 as a Ag+ selective fluorescent sensor, we
carried out competitive experiments in the presence of Ag+ at
30 μM mixed with Hg2+, Fe2+, Fe3+, Zn2+, Cu2+, Ni2+, Co2+,
Cd2+, Pb2+, Ba2+, Mg2+, K+, Li+ and Na+ at 30 μM. As shown in
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Li+ and Na+) and standard solutions (10−1 M to 10−3 M) in dry
THF were added to record the UV-vis and fluorescence spectra.
Synthesis of compound 3
A mixture of compound 1 (0.10 g, 0.08 mmol) and N,N-
dimethylaminocinnamaldehyde (0.03 g, 0.17 mmol) in a
1 : 1 mixture of dry dichloromethane and dry methanol was
refluxed for 24 h. After completion of the reaction, the solvent
was evaporated and the residue left was crystallized from CHCl3/
CH3OH to give compound 3 in 78% yield; m.p. 212 °C; IR
(KBr) νmax = 1602 cm−1; 1H NMR (CDCl3, 300 MHz): δ = 0.62
(t, J = 9 Hz, 6 H, CH3), 1.02–1.09 (m, 4 H, CH2), 1.26 (s, 18 H,
C(CH3)3), 1.28 (s, 18 H, C(CH3)3), 2.99 (s, 12 H, CH3), 3.05 (t,
4H, J = 9, NCH2) 3.82 (t, 4 H, J = 9 Hz, OCH2), 4.14 (t, J = 6
Hz, 4 H, OCH2), 6.63–6.74 (m, 8 H, Ar-H, CH), 7.31–7.34 (m,
8 H, Ar-H), 7.41 (s, 4 H, Ar-H), 7.87 (d, J = 9 Hz, 2 H, HCvN)
ppm. 13C NMR (CDCl3, 300 MHz): δ = 10.08, 22.10, 31.20,
31.50, 34.19, 40.02, 59.00, 67.25, 70.07, 112.06, 123.59,
123.80, 127.64, 127.80, 128.01, 128.68, 142.29, 145.50, 151.04,
Fig. 9 Fluorescence spectra showing the reversibility of Ag+ coordi-
nation to receptor 3 by Cl− ions; blue line, free 3 (1 μM), red line, 3 +
30 μM Ag+, green line, 3 + 30 μM Ag+ + 30 μM Cl−, orange line, 3 +
30 μM Ag+ + 30 μM Cl− + 80 μM Ag+ in THF; λex = 360 nm.
behaviour of chemosensor 3. However, no precipitation was
observed during the reversibility experiment.
156.45, 157.18, 164.29. MS ES+, m/z:
=
1205.6;
C72H92N4O4S4: calcd. C 71.72, H 7.69, N 4.65; Found C 71.81,
H 7.42, N 4.29.
Conclusions
In conclusion, we have designed and synthesized a receptor
based on the 1,3-alternate conformation of thiacalix[4]arene pos-
sessing two dimethyliminocinnamaldehyde moieties. The Ag+
ions bind with imino nitrogens of receptor 3 and thus, preferen-
tially enhance the intramolecular charge transfer process from
nitrogen atoms of the dimethylamino moiety to the imino moiety
resulting in the significant spectral shift in fluorescence emission
with a detection limit up to the nanomolar range. This type of
emission shift is not observed in the case of any other transition
or alkali metal ions.
Acknowledgements
We are thankful to CSIR (New Delhi) (Ref. No. 01 (2326)/09/
EMR-II) for financial support, N. K. is thankful to CSIR (New
Delhi) for a junior research fellowship and Guru Nanak Dev
University for laboratory facilities.
Notes and references
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Experimental
2 H. I. Scheinberg, Metals and their Compounds in the Environment:
Occurrence, Analysis and Biological Relevance, ed. E. Merian, VCH
New York, 1991.
General information
All reagents were purchased from Aldrich and were used without
further purification. THF was dried over sodium and benzophe-
none and kept over molecular sieves overnight before use. UV-
vis spectra were recorded on a SHIMADZU UV-2450 spectro-
photometer, with a quartz cuvette (path length 1 cm). The cell
holder was thermostatted at 25 °C. The fluorescence spectra were
recorded with a VARIAN CARY ECLIPSE spectrofluorimeter.
1H spectra were recorded on a JEOL-FT NMR-AL 300 MHz
spectrophotometer using CDCl3/CD3CN as solvent and tetra-
methylsilane as internal standards. Data are reported as follows:
chemical shift in ppm (δ), multiplicity (s = singlet, d = doublet,
t = triplet, m = multiplet, br = broad singlet), coupling constants
J (Hz), integration and interpretation.
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