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Cite this: Chem. Commun., 2014,
50, 6020
A fluorescent molecular probe for the
identification of zinc and cadmium salts by
excited state charge transfer modulation†
Received 16th January 2014,
Accepted 10th April 2014
Kizhmuri P. Divya,a Sivaraman Savithrib and Ayyappanpillai Ajayaghosh*a
DOI: 10.1039/c4cc00379a
www.rsc.org/chemcomm
A fluorescent probe for the identification of a given metal salt is not
known. Herein we present a new fluorescent probe 1 for the
identification of different zinc and cadmium salts by exploiting
the effect of the charge density of counteranions to perturb the
excited state solvatochromic behavior of the probe.
Design of molecular probes for the selective detection of cations and
anions has been a topic of considerable importance.1 In this
context, a large number of fluorophores have been reported for
the sensing of cations, anions and neutral molecules.2 However,
no fluorophores are known to identify different salts of a specific
metal. For example, a small molecule based fluorescent probe that
identifies the different anions present in various salts of Zn2+ or
Cd2+ has not been reported. Fluorophores with strong intramolecular charge transfer (ICT) show substantial changes in fluorescence with respect to the surrounding environment (solvatochromic
probes).3 Therefore, solvatochromic probes have been widely used
for a variety of applications such as polarity sensitive live cell
imaging, cation sensing, and for biosensing.4 Herein we report a
fluorescent molecular probe 1 that is capable of detecting the
counteranions such as ClO4, Cl, NO3 and OAc in zinc and
cadmium salts. We have exploited the excited state solvatochromic
fluorescence property of fluorophore 1 for identifying the counteranions involved in different salts of a specific cation.
We have synthesized a D–p–A–p–D type fluorophore 1 with a
bipyridine moiety as the receptor site since 2,20 -bipyridine is a
versatile ligand in coordination chemistry.5 Particularly, bipyridine
derivatives bind to different transition metal ions and show changes
in optical properties.6 Bipyridine conjugated to heterocyclic moieties
shows specific fluorescence response to zinc and cadmium ions
a
Photosciences and Photonics Group, Chemical Sciences and Technology Division,
National Institute for Interdisciplinary Science and Technology (NIIST), CSIR,
Trivandrum, India. E-mail: [email protected]; Fax: +91-471-249-0186;
Tel: +04712515306
b
Computation and Modelling Section, Process Engineering and Environmental
Technology Division, National Institute for Interdisciplinary Science and
Technology (NIIST), CSIR, Trivandrum, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc00379a
6020 | Chem. Commun., 2014, 50, 6020--6022
whereas other transition metal ions quench the emission. This
property has been successfully utilized for the ratiometric sensing
of zinc ions under various conditions.7 It is known that bipyridine
based fluorophores show intramolecular charge transfer properties
which upon binding of cations will further enhance, allowing
considerable modification of the emission properties.8
Fluorophore 1 is synthesized through the Wittig–Horner reaction
of the carbazole carbaldehyde (3) and the 2,20 -bipyridyldiphosponate
(2) in a 45% yield. The product is characterized by 1H NMR, 13C NMR
and mass spectral analyses (see ESI†). Molecule 1 showed an
absorption maximum at 406 nm in chloroform (6 106 M) with
a slight shift of 5 nm in other solvents such as acetonitrile and
DMSO (Fig. S1, ESI†). However, the emission properties of 1 showed
large dependency on solvent polarity. For example, the emission
spectrum of 1 (C 6 106 M, lex = 400 nm) in hexane showed two
maxima at 433 nm and 462 nm with a shoulder band at 493 nm. In
chloroform, the emission spectrum showed a maximum at 476 nm
and became broader with a shift of the emission maximum to the
longer wavelength region in acetonitrile (lem = 505 nm) and in
DMSO (lem = 515 nm) with an emission color change from blue to
green (Fig. S2a, ESI†). This large solvatochromic shift of 1 could be
due to the stabilization of the excited state charge transfer state in
polar solvents relative to the ground state. This was further proved by
the time resolved fluorescence lifetime studies (Fig. S2b, ESI†). The
fluorescence lifetime of 1 in hexane was 0.81 ns, which gradually
increased to 1.59 ns in DMSO with a mono-exponential decay. The
molecule was highly emissive in chloroform which is a less polar
solvent and showed a quantum yield of 87% (quinine sulphate in
0.1 N H2SO4 as a standard) while the emission intensity was found to
decrease in acetonitrile (ff = 69%).
Addition of transition metal salts to a solution of 1 (6 106 M)
in chloroform showed a significant change in the absorption and
emission spectra. For example, the intensity of the absorption band
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at 405 nm decreased with the concomitant formation of a new redshifted band at around 460 nm. The emission of 1 at 476 nm was
significantly quenched by different transition metal ions except for
zinc(II) and cadmium(II). In the case of Zn2+ and Cd2+ the emission
band at 476 nm was quenched with a concomitant formation of a
new red-shifted band. For example, the fluorescence response of 1 in
chloroform against Zn(NO3)2 is shown in Fig. S3a (ESI†). Upon
titration of Zn(NO3)2 the emission maximum gradually decreased
with concomitant formation of a new band at 563 nm through an
isoemissive point at 547 nm. The individual emission response of 1
against different transition metal ions is shown in Fig. S3b (ESI†).
Surprisingly, the red-shifted emissions of 1 + Zn2+ and 1 + Cd2+
were strongly influenced by the counteranion present in the respective metal salts (Fig. S4–S6 and Table S1, ESI†). For example, the
individual emission response of 1 with various zinc salts is shown in
Fig. 1a. This observation encouraged us to study the ability of 1 to
discriminate different salts of Zn2+ having various counteranions.
The emission peak of 1 in chloroform (lem = 476 nm) was red-shifted
to 597 nm upon binding with Zn(ClO4)2. In the case of ZnCl2 and
Zn(OAc)2 the emission maxima occurred at 554 nm and 548 nm,
respectively. The Job plot revealed a 1 : 1 complexation between the
fluorophore and the Zn2+ (Fig. S7, ESI†). The binding constants
calculated from Benesi–Hildebrand plots showed the highest
binding constant for Zn(ClO4)2 (3.33 105 M1) and decreased
from Zn(NO3)2 (1.47 105 M1) to ZnCl2 (7.8 104 M1) in
chloroform. Zn(OAc)2 showed the lowest value of 1.77 104 M1.
In the case of cadmium salts, the emission maximum of 1 in
chloroform was shifted to 574 nm with the addition of Cd(ClO4)2,
557 nm with Cd(NO3)2, 550 nm with CdCl2 and 541 nm with
Cd(OAc)2 indicating a gradual decrease in the emission maximum
in the order ClO4 - NO3 - Cl - OAc. The selectivity of the
probe was checked by recording the fluorescence of 1 in the presence
of 5 times excess of different cations. Plots of the intensity of the
emission at 563 nm in the presence of excess of different cations
before and after the addition of Zn2+ are shown in Fig. S8 (ESI†).
Interestingly, perchlorate, nitrate, chloride and acetate salts of alkali
and alkaline earth metals did not show any considerable variation in
the fluorescence properties of 1.
The large shift in the emission maximum with respect to the
couteranion is due to the difference in the coordination ability
between the counteranion and Zn2+. The net charge quantity of
Zn2+ depends on the average distance between zinc cations and
Fig. 1 (a) Emission spectral changes of 1 (
) (6 106 M, CHCl3), with
Zn(OAc)2 (TT), ZnCl2 (
), Zn(NO3)2, (
), Zn(ClO4)2 (
) excited at
430 nm. The inset figure shows the corresponding emission color changes
under 365 nm UV light. (b) Plots of the emission maximum of 1 (6 106
M) and the corresponding quantum yields in chloroform (rhodamine B in
ethanol as a standard) with various zinc salts.
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the counteranion, which is determined by the ionization coefficient
of the zinc salt in solution. Zinc(II) may have more net charge in the
case of ClO4 as the counteranion due to the larger ionization
equilibrium coefficient of Zn(ClO4)2 in solution. In the case of
Zn(OAc)2 which is more covalent in nature has low net effective
charge on zinc(II) due to weak ionization. Thus, the strong coordination of Zn(ClO4)2 with the fluorophore facilitates the interaction of
the strongly ionized ClO4 (weakly basic when compared to NO3,
Cl, and OAc) with the excited fluorophore, thus stabilizing the
excited state which is responsible for the large shift in the emission
maximum. Plots of the emission maximum and the corresponding
fluorescence quantum yields of 1 in the presence of different zinc
salts are shown in Fig. 1b. The maximum red shift in the emission
was observed for ClO4 and decreased in the order of NO3, Cl,
and OAc. The fluorescence quantum yields showed an increase
from ClO4 to NO3 and Cl whereas OAc showed more or less the
same value as Cl.
The detailed photophysical properties of 1 upon complexing
with various zinc and cadmium salts are summarized in Table S1
and S2 (ESI†), respectively. The stabilization of the excited state was
maximum in the case of the Zn(ClO4)2 complex and gradually
decreased with a decrease in the ionization ability of the zinc
complex. This is clear from the quantum yield and lifetime values
of 1Zn2+ complexes. The effective discrimination of various zinc
and cadmium salts is possible since the excited state of 1 is a CT
state, the fluorescence of which can be easily perturbed by the
stabilization of the excited state. Thus, a fluorescence color pattern
of different zinc salts was created by recording the emission color
of 1 in the presence of different metal salts using a BioTeck cell
reader upon excitation at 435 nm (Fig. 2). A control experiment in
the absence of any metal salt showed the original blue fluorescence
of probe 1. The plot of fluorescence intensity versus wavelength can
be applied for the identification of metal salts which are not easy to
find out by visual color changes (Fig. 3a).
The response of probe 1 towards various zinc and cadmium
salts with closely related fluorescence variations were identified
using the principal component analysis (PCA) method.9 PCA is a
linear transformation that can be used to reduce, compress or
simplify a data set and is a valuable tool which has considerable
significance in the discrimination of multiple analyte responses.
It does this by transforming the data to a coordinate system so
that one can choose not to use all the components and still
capture the most important part of the data. A scatter diagram of
the first two principal components is shown in Fig. 3b, which
Fig. 2 The visual color codes of 1 (6 106 M) upon addition of metal
salts with different counteranions in chloroform (fluorescence output from
a BioTek cell reader, lex@ 435 nm).
Chem. Commun., 2014, 50, 6020--6022 | 6021
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Notes and references
Fig. 3 (a) Discrimination of different zinc and cadmium salts by plotting
the relative fluorescence intensity against the wavelength of emission.
(b) A two-dimensional principal component analysis (PCA) plot of zinc and
cadmium salts having different counteranions.
represents a tight clustering of repetitive data with relatively good
spatial separation and demonstrates the clear discrimination of
zinc and cadmium salts with different counteranions.
In conclusion, a new solvatochromic fluorescent probe 1 has
been designed for the detection of various zinc and cadmium
salts having different counteranions, which is otherwise difficult
to achieve by using other fluorescent probes. The excited state
charge transfer property and the associated solvatochromic
emission of the probe are dependent upon the counteranion
size and the charge density which are the key to the anion
differentiation. A solvent which shows minimum initial shift in
the emission and has good compatibility with metal salts is
crucial for the observed effect, for which chloroform is found to
be ideal. In the present case, the probe need not be water
compatible since the use of the present probe is for the identification
of a given salt and not for the sensing of a specific cation or anion in
biological or any other analytical sample. Finally, the methodology
described here can be developed as a simple laboratory test for the
identification of given zinc or cadmium salts.
A.A. is grateful to the Department of Atomic Energy, Government
of India for financial support under a DAE-SRC Outstanding
Researcher Award. K.P.D. thanks the Council of Scientific and
Industrial Research (CSIR), Government of India for research
fellowships.
6022 | Chem. Commun., 2014, 50, 6020--6022
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