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A fluorescent molecular probe for the identification of zinc and cadmium salts by excited state charge transfer modulation

Authors:
Divya K. P. at National Institute for Interdisciplinary Science and Technology
Divya K. P.
Sivaraman Savithri at National Institute for Interdisciplinary Science and Technology
Sivaraman Savithri
Ayyappanpillai Ajayaghosh at National Institute for Interdisciplinary Science and Technology
Ayyappanpillai Ajayaghosh
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A fluorescent probe for the identification of a given metal salt is not known. Herein we present a new fluorescent probe 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.
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This journal is ©The Royal Society of Chemistry 201 4 Chem. Commun.
Cite this: DOI: 10.1039/c4cc00379a
A fluorescent molecular probe for the
identification of zinc and cadmium salts by
excited state charge transfer modulation†
Kizhmuri P. Divya,
a
Sivaraman Savithri
b
and Ayyappanpillai Ajayaghosh*
a
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 Zn
2+
or
Cd
2+
has not been reported. Fluorophores with strong intra-
molecular charge transfer (ICT) show substantial changes in fluores-
cence with respect to the surrounding environment (solvatochromic
probes).
3
Therefore, solvatochromic probes have been widely used
foravarietyofapplicationssuchaspolaritysensitivelivecell
imaging, cation sensing, and for biosensing.
4
Herein we report a
fluorescent molecular probe 1that is capable of detecting the
counteranionssuchasClO
4
,Cl
,NO
3
and OAc
in zinc and
cadmium salts. We have exploited the excited state solvatochromic
fluorescence property of fluorophore 1for identifying the counter-
anions involved in different salts of a specific cation.
We have synthesized a D–p–A–p–D type fluorophore 1with 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
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 1is synthesized through the Wittig–Horner reaction
of the carbazole carbaldehyde (3)andthe2,2
0-bipyridyldiphosponate
(2) in a 45% yield. The product is characterized by
1
H NMR,
13
CNMR
and mass spectral analyses (see ESI†). Molecule 1showed an
absorption maximum at 406 nm in chloroform (6 10
6
M) with
a slight shift of 5 nm in other solvents such as acetonitrile and
DMSO (Fig. S1, ESI†). However, the emission properties of 1showed
large dependency on solvent polarity. For example, the emission
spectrum of 1(C 6 10
6
M, l
ex
= 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 (l
em
= 505 nm) and in
DMSO (l
em
= 515 nm) with an emission color change from blue to
green (Fig. S2a, ESI†). This large solvatochromic shift of 1could be
duetothestabilizationoftheexcitedstatechargetransferstatein
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 1in 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 H
2
SO
4
as a standard) while the emission intensity was found to
decrease in acetonitrile (f
f
= 69%).
Addition of transition metal salts to a solution of 1(6 10
6
M)
in chloroform showed a significant change in the absorption and
emission spectra. For example, the intensity of the absorption band
a
Photosciences and Photonics Group, Chemical Sciences and Technology Division,
National Institute for Interdisciplinary Science and Technology (NIIST), CSIR,
Trivandrum, India. E-mail: ajayaghosh62@gmail.com; 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
Received 16th January 2014,
Accepted 10th April 2014
DOI: 10.1039/c4cc00379a
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at 405 nm decreased with the concomitant formation of a new red-
shifted band at around 460 nm. The emission of 1at 476 nm was
significantly quenched by different transition metal ions except for
zinc(II)andcadmium(II). In the case of Zn
2+
and Cd
2+
the emission
band at 476 nm was quenched with aconcomitantformationofa
new red-shifted band. For example, the fluorescence response of 1in
chloroform against Zn(NO
3
)
2
isshowninFig.S3a(ESI†). Upon
titration of Zn(NO
3
)
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+Zn
2+
and 1+Cd
2+
were strongly influenced by the counteranion present in the respec-
tive metal salts (Fig. S4–S6 and Table S1, ESI†). For example, the
individual emission response of 1with various zinc salts is shown in
Fig. 1a. This observation encouraged us to study the ability of 1to
discriminate different salts of Zn
2+
having various counteranions.
The emission peak of 1in chloroform (l
em
= 476 nm) was red-shifted
to 597 nm upon binding with Zn(ClO
4
)
2
.InthecaseofZnCl
2
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 Zn
2+
(Fig. S7, ESI†). The binding constants
calculated from Benesi–Hildebrand plots showed the highest
binding constant for Zn(ClO
4
)
2
(3.33 10
5
M
1
)anddecreased
from Zn(NO
3
)
2
(1.47 10
5
M
1
)toZnCl
2
(7.8 10
4
M
1
)in
chloroform. Zn(OAc)
2
showed the lowest value of 1.77 10
4
M
1
.
In the case of cadmium salts, the emission maximum of 1in
chloroform was shifted to 574 nm with the addition of Cd(ClO
4
)
2
,
557 nm with Cd(NO
3
)
2
, 550 nm with CdCl
2
and 541 nm with
Cd(OAc)
2
indicating a gradual decrease in the emission maximum
in the order ClO
4
-NO
3
-Cl
-OAc
. The selectivity of the
probe was checked by recording the fluorescence of 1in 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 Zn
2+
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
thefluorescencepropertiesof1.
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 Zn
2+
. The net charge quantity of
Zn
2+
depends on the average distance between zinc cations and
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 ClO
4
as the counteranion due to the larger ionization
equilibrium coefficient of Zn(ClO
4
)
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 coordina-
tion of Zn(ClO
4
)
2
with the fluorophore facilitates the interaction of
the strongly ionized ClO
4
(weakly basic when compared to NO
3
,
Cl
,andOAc
) 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 1in the presence of different zinc
salts are shown in Fig. 1b. The maximum red shift in the emission
was observed for ClO
4
and decreased in the order of NO
3
,Cl
,
and OAc
. The fluorescence quantum yields showed an increase
from ClO
4
to NO
3
and Cl
whereas OAc
showed more or less the
same value as Cl
.
The detailed photophysical properties of 1upon 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(ClO
4
)
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 1Zn
2+
complexes. The effective discrimination of various zinc
and cadmium salts is possible since the excited state of 1is 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 1in 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 1towards 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. 1 (a) Emission spectral changes of 1()(610
6
M, CHCl
3
), with
Zn(OAc)
2
(TT), ZnCl
2
(), Zn(NO
3
)
2
,( ), Zn(ClO
4
)
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 10
6
M) and the corresponding quantum yields in chloroform (rhodamine B in
ethanol as a standard) with various zinc salts.
Fig. 2 The visual color codes of 1(6 10
6
M) upon addition of metal
salts with different counteranions in chloroform (fluorescence output from
a BioTek cell reader, l
ex
@ 435 nm).
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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 1has
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.
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(b) A two-dimensional principal component analysis (PCA) plot of zinc and
cadmium salts having different counteranions.
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Charge transfer of 1,3,4‐oxadiazole derivative, its functional polymers and study of their different aggregation‐induced emission enhancement behaviors
Article
Full-text available
In this study, a symmetrical D‐π‐A‐π‐D initiator ViOXD, based on substituted amino donor (D) and 1,3,4‐oxadiazole acceptor (A), with rigid stilbene π‐bridge was synthesized. The photophysical investigation of ViOXD in solution and aggregate state showed its solvatochromic and aggregation‐caused quenching characteristics, because of its strong charge transfer and the formation of H‐aggregates, respectively. The introduction of peripheral polystyrene or carbazole‐containing PVPCz chains via atom transfer radical polymerization not only recovered the fluorescence of the ViOXD core, but also tuned its maximum emission wavelength in aggregate state, due to the stronger electron‐donating ability and better conjugation of PVPCz chains than polystyrene chains. On the other hand, the large conjugation length and symmetrical charge transfer from the ends to the cores of ViOXD facilitated the electronic delocalization and bestowed the end‐functionalized polymers with good fluorescence in aggregate state, which we have applied in cellular imaging by preparation of their fluorescent nanoparticles.
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Synthesis of tetrahydrochromenes and dihydronaphthofurans via a cascade process of [3 + 3] and [3 + 2] annulation reactions: mechanistic insight for 6- endo-trig and 5- exo-trig cyclisation
Article
Full-text available
  • Feb 2023
A [3 + 3] and [3 + 2] annulation strategy using nitrostyrene derived MBH primary and secondary nitro allylic acetate for the construction of tetrahydrochromenes and dihydronaphthofurans at room temperature.
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A new lipophilic cationic rhodamine-based chemosensor for detection of Al(III)/Cu(II) and intracellular pH change and its application as a smartphone-assisted sensor in water sample analysis
Article
  • Nov 2022
  • J PHOTOCH PHOTOBIO A
A new probe, RTPP was prepared by condensation of rhodamine B hydrazide with (3-formyl-4-hydroxybenzyl)triphenylphosphonium chloride. The structure of sensor RTPP was elucidated using spectroscopic methods and single crystal X-ray diffraction. Colourless and non-fluorescence sensor RTPP solution turns pink in the presence of Cu²⁺ and showed orange fluorescence enhancement in the presence of Al³⁺. Cation recognition performance and binding mechanisms of the probe were studied through UV, fluorescence, IR, MS, and NMR spectroscopic studies; where the results revealed that sensor RTPP has a high affinity towards Al³⁺/Cu²⁺ with the limit of detection down to at least 10⁻⁷ M. The developed system was applicable for tracing Cu²⁺ through colorimetric method in two selected water samples with satisfactory results. More significantly, “naked-eye” detection of Cu²⁺ using RTPP has been effectively incorporated with a smartphone RGBColorDetector application to make it suitable for on-site qualitative and quantitative colorimetric investigation. In addition, RTPP can act as a sensor for detecting acidic pH change in HCT 116 human colorectal carcinoma cell line through fluorescence response.
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Anion Recognition and Sensing: The State of the Art and Future Perspectives
Article
Anion recognition chemistry has grown from its beginnings in the late 1960s with positively charged ammonium cryptand receptors for halide binding to, at the end of the millennium, a plethora of charged and neutral, cyclic and acyclic, inorganic and organic supramolecular host systems for the selective complexation, detection, and separation of anionic guest species. Solvation effects and pH values have been shown to play crucial roles in the overall anion recognition process. More recent developments include exciting advances in anion‐templated syntheses and directed self‐assembly, ion‐pair recognition, and the function of anions in supramolecular catalysis.
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ChemInform Abstract: Design and Synthesis of 4,4′-π-Conjugated[2,2′]-bipyridines: A Versatile Class of Tunable Chromophores and Fluorophores
Article
  • Jul 2010
  • ChemInform
A series of 4,4′-disubstituted[2,2′]-bipyridines, featuring π-conjugated substituents such as donor-(acceptor-) substituted styryl, thienylvinyl, phenylimino and phenylazo groups, have been synthesized. X-Ray structures are provided for three ligands containing –CC–, –CN– and –NN– linkers, respectively. These chromophores display good to excellent thermal stabilities with decomposition temperatures of up to 350°C. The strong influence of the nature of the endgroups and π-linkers on the optical properties is discussed. The stepwise protonation of 4,4′-dibutylaminostyryl-[2,2′]-bipyridine and its coordination behavior to different metallic moieties [Zn(II), Hg(II), Pd(II), Re(I), Re(VII)] have also been investigated. It is found that the absorption and emission maxima can be easily tuned by these exogenous additives over a wide range of wavelengths (360<λabs<560 nm ; 482<λem<646 nm).
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Neutral Anion Receptors: Design and Application
Article
  • Feb 1998
  • CHEM COMMUN
After the development of synthetic cation receptors in the late 1960s, only in the past decade has work started on the development of synthetic neutral anion receptors. Combination and preorganization of different anion binding groups, like amides, urea moieties, or Lewis acidic metal centers lead to receptor molecules that strongly bind inorganic anions with high selectivity. Combined with neutral cation ligands, ditopic receptors were obtained for the complexation of inorganic salts. The molecular complexation properties of anion receptors has been transduced into macroscopic properties in membrane separation processes and in sensors for selective anion detection.
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Zinc(II)Induced Color-Tunable Fluorescence Emission in the ??-Conjugated Polymers Composed of the Bipyridine Unit: A Way to Get White-Light Emission
Article
  • Apr 2005
A series of fluorene and bipyridine (bpy) copolymers were synthesized, and their optical properties were investigated in the presence of zinc(II). By adjusting the bpy content in the polymer backbone, the emitting color of the polymer solution could be tuned from initial blue to green to white and violet when it interacted with zinc(II) ions. The counteranions of zinc(II) ions were also found to influence the optical properties of copolymers greatly.
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Ionochromic Effects and Structures of Metalated Poly( p -phenylenevinylene) Polymers Incorporating 2,2‘-Bipyridines
Article
  • Mar 2000
The effects of metal ion chelation to the 2,2â²-bipyridine (bpy) groups on the photophysics and exciton dynamics of two conjugated polymers 1 and 2 in solution are investigated. The structures of polymers 1 and 2 have 2,2â²-bipyridyl-5-vinylene units that alternate with one and three 2,5-bis(n-cecyloxy)-1,4-phenylenevinylene monomer units, respectively. The photophysics and exciton dynamics of metalated polymers 1 and 2 are compared to those of the metal-free polymers (Chen et al.J.Phys.Chem.A 1999, 103, 4341--4351). The origins of ionochromic effects due the metal ion chelation were studied using both steady-state and transient optical spectroscopy, and the results indicate that both conformational flattening and participation of Ï electrons from the metal in the Ï-conjugation of the polymer backbone play important roles in metal ion binding induced red shifts in absorption and photoluminescence spectra. The photoluminescence properties of the metalated polymers are determined by the metal ion electronic structures, where the closed shell Zn{sup 2+}-bound polymer 2 has an increased photoluminescence quantum yield and the corresponding open shell Ni{sup 2+}- or Fe{sup 3+}-bound polymers have quenched photoluminescence due to spin-orbit coupling. The dual character of metalated polymer 2 as a conjugated polymer and as a metal-bpy complex is discussed. In addition, the structures of metal ion binding sites are studied via X-ray absorption fine structure (XAFS) and are related to the photophysical properties of the metalated polymers.
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Anion recognition using dimetallic coordination complexes
Article
  • Dec 2006
  • COORDIN CHEM REV
This article describes advances made over the past 3 years in anion recognition using coordination complexes, with a specific focus on dimetallic architectures that utilize a bridging mechanism. The formation of coordination complexes is a relatively straightforward method of constructing fluorescent and colorimetric chemosensors and imaging agents, and a particularly effective way to develop indicator displacement assays that operate in water. These assays are likely to find increased application in various aspects of analytical and environmental chemistry, as well as biomedical imaging and drug discovery. Significant progress in phosphoesterase mimics has been made, and concomitant with the increased mechanistic insight, is the discovery of a catalyst that cleaves phosphodiesters with poor O-alkyl leaving groups. Also discussed is a macrocyclic coordination complex whose shape and supramolecular function is pH-dependent.
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A naphthalimide based chemosensor for Zn2+, pyrophosphate and H2O2: Sequential logic operations at the molecular level
Article
  • Dec 2012
  • CHEM COMMUN
A naphthalimide based fluorescent probe has been synthesized which shows ratiometric fluorescence response towards Zn(2+) ions and the in situ prepared -Zn(2+) complex was used for the detection of pyrophosphate and hydrogen peroxide which mimics the functions of sequential logic circuits at the molecular level.
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Organometallic Networks Based on 2,2‘-Bipyridine-Containing Poly(p-phenylene ethynylene)s
Article
  • Mar 2005
Conjugated polymers that comprise 2,2‘-bipyridine moieties as part of the macromolecular backbone represent versatile precursors for the formation of conjugated metallo-supramolecular networks, which are readily accessible via ligand-exchange reactions. Poly{2,2‘-bipyridine-5,5‘-diylethynylene[2,5-bis(2-ethylhexyl)oxy-1,4-phenylene]ethynylene} (BipyPPE1) and a statistical copolymer comprising 5,5‘-diethynyl-2,2‘-bipyridine and 1,4-diethynyl-2,5-bis(alkyloxy)benzene moieties (BipyPPE2) were synthesized via the Pd0-catalyzed cross-coupling reaction of 1,4-diethynyl-2,5-bis(octyloxy)benzene, 1,4-bis[(2-ethylhexyl)oxy]-2,5-diiodobenzene, and 5,5‘-diethynyl-2,2‘-bipyridine. Complexation studies involving these polymers and a variety of transition metals suggest that ligand exchange leads to three-dimensional networks, which feature BipyPPE−metal−BipyPPE cross-links and display interesting optoelectronic properties. It is found that complexes with group 12 d10 ions (Zn2+ and Cd2+) are emissive, while other transition metals such as Cu+, Co2+, and Ni2+ form nonradiative metal-to-ligand charge-transfer complexes with the polymers.
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Photoluminescence and Electroluminescence of d6 Metal−Organic Conjugated Oligomers: Correlation of Photophysics and Device Performance
Article
  • Oct 2003
The photophysical and electroluminescent device properties of a series of d6 transition metal complexes that feature an oligo(aryleneethynylene) (OAE) “ligand” have been investigated. The metals are coordinated η2- to the OAE via a 2,2‘-bipyridine moiety that is in the “core” of the oligomer, and the metals used are η2-Ir(ppy)2+, η2-Ru(bpy)22+, and η2-Os(bpy)22+ (Ir-2, Ru-2, and Os-2, respectively, where ppy = 2-phenylpyridine and bpy = 2,2‘-bipyridine). The photoluminescence spectra and excited-state decay parameters for the metal−OAEs indicate that at ambient temperature Ru-2 and Os-2 feature a lowest 3MLCT excited state; however, in Ir-2 the lowest excited state is 3π,π* localized on the OAE. Electroluminescent (EL) devices that contain the metal−OAEs dispersed at 5 wt % in a poly(vinyl carbazole):2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole matrix exhibit electroluminescence at wavelengths that correlate well with the photoluminescence from the complexes in a rigid glass at low temperature. The EL efficiency of the devices varies in the order Ir-2 > Ru-2 Os-2. Factors that may contribute to the difference in EL efficiencies are discussed.
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