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A molecular chameleon: Fluorometric to Pb2+, fluorescent ratiometric to Hg2+ and colorimetric to Ag+ ions
Journal Pre-proof A Molecular chameleon: Fluorometric to Pb2+ , Fluorescent Ratiometric to Hg2+ and Colorimetric to Ag+ ions Parvathy O. Chandrasekaran, Ajayakumar Aswathy, Kiran James, Kannankutty Kala, Mohanan T. Ragi, Narayanapillai Manoj
PII:
S1010-6030(20)30847-9
DOI:
https://doi.org/10.1016/j.jphotochem.2020.113050
Reference:
JPC 113050
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
21 September 2020
Revised Date:
16 November 2020
Accepted Date:
18 November 2020
Please cite this article as: Chandrasekaran PO, Aswathy A, James K, Kala K, Ragi MT, Manoj N, A Molecular chameleon: Fluorometric to Pb2+ , Fluorescent Ratiometric to Hg2+ and Colorimetric to Ag+ ions, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.113050
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A Molecular chameleon: Fluorometric to Pb2+, Fluorescent Ratiometric to Hg2+ and Colorimetric to Ag+ ions Parvathy O. Chandrasekarana, Ajayakumar Aswathya, Kiran Jamesa, Kannankutty Kalab, Mohanan T. Ragic, Narayanapillai Manoja, * a Department
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of Applied Chemistry and Interuniversity Center for Nanomaterials and Devices, Cochin University of Science and Technology, Kochi-22, India. b Department of Chemical Engineering, National Tsing Hua University, Taiwan. c Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram 695019, India. * Corresponding author. E-mail address: [email protected]
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Graphical Abstract
Research Highlight
Developed a donor-π-acceptor based Chameleonic fluorescent chemosensor for toxic metal ions Inherent ICT character is exploited for tuning selectivity of metal ion binding The solvent polarity dependence of charge on the chelating atoms established through theoretical modelling and validated by experiment Established the applicability in aqueous medium
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Abstract A molecular chameleon, having D-π-A architecture with phenothiazine donor and rhodanine-3-
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acetic acid acceptor (PRAA) was synthesized and demonstrated its use as a fluorescent sensor to toxic metals ions. PRAA is highly selective and sensitive to Ag+, Hg2+ and Pb2+ ions and its selectivity can be tuned by solvent polarity. The chameleonic behavior is attributed to the
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intramolecular charge transfer and variation in the electron density at the ligand atom as a function of solvent polarity. The mechanism of binding and the stoichiometry of interaction is established
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by experimental and theoretical methods. How PRAA can be used as a probe in aqueous solutions
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for the detection of Ag+ and Hg2+ ions by using an anionic surfactant medium is convincingly Keywords: Sensors, Fluoroionophores, Donor-π-Acceptor, Fluorometric sensor, Ratiometric sensor, Colorimetric sensor
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1. Introduction
In recent years, development of new probes for the selective and sensitive detection of transition and heavy metals (THM) and different anions of biological importance has attracted considerable attention because of their quite important role in many environmental and biological
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processes [1-2]. Among the THM ions Pb2+ and Hg2+ ions are highly toxic while Ag+ ions is less toxic [3]. Lead is relatively more abundant and low cost metal, widely used in construction industries, batteries, bullets, solders, weights, paints etc. and is more easily found in the environment [4, 5]. Even very low level of lead exposure can cause neurological, cardiovascular, reproductive and developmental disorders in children [6]. Mercury is considered more dangerous, because both elemental and ionic mercury by the bacterial action gets converted into methyl mercury and subsequently bioaccumulates in animal tissues through the food chain [7-10]. High
dose exposure of mercury leads to the malfunctioning of central nervous system, blood and kidneys, results in diseases like Hunter-Russell syndrome, Minamata disease and acrodynia [11]. According to EPA (Environmental Protection Agency), the maximum permissible level of inorganic Hg2+ ions in drinking water is 2 ppb [12, 13]. Silver is used in photography, electrical, pharmaceutical and cosmetic industry [14]. There are many reports on silver toxicity and bioaccumulation which leads to adverse biological effects [15, 16]. Silver inactivates sulfhydryl enzyme by combining with amine, imidazole and carboxyl group of various metabolites [17]. Thus
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detection of these toxic ions is important. Quicker and easy to use sensor systems based on chelation by molecules with associated changes in their electrochemical and photophysical properties are widely used in this field. Most popular and sensitive method of detection exploits
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the changes in the fluorescence properties of molecules. Such sensors exhibit high sensitivity and low detection limit etc. [18-20]. Even though many such molecules are known in the literature,
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they are either selective to a class or a group of metal ions or specific to one particular metal ion. No report exists so far where a single molecular system shows tunable selectivity by changing the
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medium or mode of use.
Nowadays a new concept of “single sensor for multiple targets” has got more attention of
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researchers [21]. The ability to detect more than one target simultaneously can shorten the overall analytical processing time and potentially reduce the cost. Inspired by this concept, herein we have
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successfully demonstrated the use of a donor-π-acceptor (D-π-A) system based on phenothiazine as a fluorescent sensor PRAA (Scheme 1) to detect and quantify three different metal ions by choosing appropriate solvent media. Organic push-pull molecules commonly known as donor-πacceptor (D-π-A) systems have attracted great attention because facile tuning of their electronic states can be achieved by choosing appropriate D/A component to make them suitable for a variety
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of applications [22-24]. Phenothiazine and its derivatives were used to construct organic luminescent materials and as dyes in organic photovoltaic applications [25-33]. By virtue of the presence of N and S atoms, it has a nonplanar butterfly conformation that inhibit strong intermolecular π-π interactions and also make them good electron donors [34-36]. It exhibits intense fluorescence, good hole transporting capacity and structural regulation. Rhodanine-3acetic acid act as an electron acceptor, possessing S and O, as part of a carbonyl or a thiocarbonyl functionality along with N and S as ring atoms with a pendant carboxylic acid receptor offering a variable platform for metal ion coordination. Moreover, earlier reports showed that rhodanine-3-
acetic acid has been used as spectrophotometric agent for sensing heavy metal ions [37-39]. Thus, our sensor system uses the D-π-A molecule as a tunable fluorescent reporter having a chelating heterocyclic acceptor for analyte binding. 2. Experimental 2.1 Materials All reagents and solvents of analytical grades were obtained commercially and used
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without further purification. Spectroscopic grade solvents from Merck were used for photophysical studies.
(400 MHz) and
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Melting point is uncorrected and was determined on a JSGW melting point apparatus. 1H 13
C NMR (100 MHz) spectra were recorded on a Bruker Avance III FT-NMR
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spectrometer with tetramethylsilane (TMS) as internal standard and DMSO-d6 as the solvent. MALDI-TOF mass spectrum was obtained using Bruker Autoflex max LRF. Absorption spectra
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were measured by using an Evolution 201 UV-visible spectrophotometer. Fluorescence measurements were recorded on a Horiba Fluorolog 3 spectrofluorimeter with excitation and
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emission slit widths of 2 nm. The fluorescence spectra were corrected for the monochromator and detector effects by using the correction method provided with the FluorEscenceTM Software by Horiba. Fluorescence lifetime was measured with Horiba Fluorolog 3 time-correlated single
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photon counting system using Horiba NanoLED-510L pulsed laser diode having 510 ± 10 nm wavelength output and a pulse width of <200ps. The lifetime values were obtained by the deconvolution of the decay from the instrument response function recorded using a scattering solution. The analysis was performed using DAS6 decay analysis software. The average lifetime is the amplitude weighted average lifetime reported by the decay analysis software from the fitting
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results. All measurements were performed at room temperature. Geometry optimization of the compound was carried out by DFT computation using Gaussian 09 software with B3LYP or M06L exchange correlation energy functional and 6-311G (d, p) basis set [40]. The major electronic transitions and their oscillator strengths were calculated by TD-DFT method using the same functional and basis set. The optimized geometry and their respective electronic transitions in different solvents were obtained by polarizable continuum model as implemented in Gaussian 09.
2.2 Synthesis Phenothiazine rhodanine-3-acetic acid conjugate (PRAA) was synthesized according to the reported procedure [41, 42] by the Knoevenagel condensation between 10-octyl-10Hphenothiazine-3-carbaldehyde (1) and rhodanine-3-acetic acid (2) in glacial acetic acid medium in the presence of ammonium acetate (Scheme 1). Brick red crystals of the product was filtered and washed with hot water and dried in vacuum. Yield 80%. mp 244 °C. 1H NMR (400 MHz, DMSOd6) ẟ (ppm): 0.76-0.79 (t, 3H), 1.15-1.18 (m, 8H), 1.32-1.33 (m, 2H), 1.63-1.66 (m, 2H), 3.85-
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3.88 (t, 2H), 4.70 (s, 2H), 5.69 (s, 1H), 6.95-7.00 (m, 2H), 7.02-7.12 (m, 2H), 7.17-7.21 (t, 1H), 7.39 (s, 1H), 7.41 (d, 1H), 7.67 (s, 1H); 13C NMR (100 MHz, DMSO-d6) ẟ (ppm) 13.7, 21.8, 25.7,
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25.8, 28.2, 28.3, 30.8, 44.7, 46.6, 115.8, 116.1, 118.3, 122.0, 123.3, 123.7, 126.6, 127.0, 127.7, 129.0, 130.7, 132.8, 142.6, 147.0, 166.1, 167.1, 192.4; MALDI-TOF MS (ESI MS) m/z: [M+H]+
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calcd for C26H29N2O3S3 513.1335, found 513.1139. (See Figures S1 – S3 in supplementary
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information for spectral data)
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Scheme 1 2.3 Metal ion binding studies
A stock solution of PRAA with the concentration of 3.1 x 10-3 M was prepared in THF (tetrahydrofuran) at room temperature. This stock solution was used for all the UV-vis and
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fluorescence study after appropriate dilution. Metal ion binding studies were carried in different solvents and solvent mixtures with a series of transition metal salts in particularly of heavy metal salts. Acetate salts of metals like mercury, lead, cadmium, nickel, cobalt, copper, manganese, zinc, magnesium and silver were prepared by dissolving in milli Q water, while for the study in CHCl 3 methanol is used. Stock solutions of metal salts in water were added systematically to the dye solution, changes in UV-Vis absorption and fluorescence spectra were carefully monitored. 3. Results and Discussion
3.1 Electronic States and Photophysical Properties The optimized geometry and the molecular orbitals of PRAA were obtained by DFT calculations using B3LYP or M06-L functional with 6-311G (d, p) basis set using Gaussian 09 for gas phase, CHCl3, THF and MeCN solution. The optimized geometry and the MO’s obtained in MeCN are given in Figure 1. The results show that the molecule assumes a butterfly like bend geometry for the phenothiazine moiety. Intramolecular charge transfer (ICT) character inherent to the D-π-A systems such as PRAA is evident from the localization of HOMO and HOMO-1 orbitals
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on the phenothiazine moiety and the LUMO and LUMO+1 orbitals on the rhodanine-3-acetic acid, the acceptor moiety. The major electronic transitions computed by TD-DFT using B3LYP, M06-
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L and CAM-B3LYP are given in Table 1. Calculations using CAM-B3LYP functional gave better correlation with the experimental data [43]. The theoretically calculated band at 330 nm has its
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major contribution from HOMO-1 to LUMO transition and the long wavelength band at 429 nm has a major contribution from HOMO to LUMO transition. Based on the nature of the orbitals
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involved, these transitions can be safely assigned as ICT excitation with shift of charge from phenothiazine to the rhodanine-3-acetic acid. The spectra were unusually red shifted in non-polar solvents such as dichloroethane and chloroform which could be due to a specific solvent
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interaction with halogenated solvents. The absorption and emission spectrum of PRAA recorded in different solvents are given in Figure 2(a) and (b) respectively. In agreement with the inherent
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intramolecular charge transfer nature, PRAA showed significant bathochromic shift in the emission spectra with increasing polarity of the solvent medium. However, this bathochromic shift is less significant in the absorption spectra recorded in different solvents. By using the LippertMataga formalism, the solvent polarity effect on this D-π-A system can be better explained (Table S1 and Figure S4) [44, 45]. The linear relationship between Stokes shift (Δνst) and orientational
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polarizability (Δf) well explains the ICT nature of the excited state. The compound showed moderate fluorescence quantum yield of 0.20 in THF and very low values in other solvents. The fluorescence lifetime was also measured in THF, CHCl3 and MeCN (Figure S5). The decay profiles showed biexponential dynamics in these solvents with average lifetime ranging from 0.27 to 2.05 ns. Decay with a very short lifetime (τav) of 0.27 ns was obtained in MeCN. This could be due to the highly stabilized ICT excited state having enhanced non-radiative deactivation rates. The compilation of important photophysical parameters in the three solvents are presented in Table 2.
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Figure 1 Optimized ground state geometry, distribution of molecular orbital coefficient density in HOMO, HOMO-1, HOMO-2, LUMO, LUMO+1 of PRAA in MeCN.
THF
(εmax, x 104 cm-1M-1)
B3LYP
M06-L
CAM-B3LYP
362 (2.14±0.04), 475 (2.27±0.04)
391 (0.67), 549 (0.52)
431 (0.60), 646 (0.37)
330 (0.4502), 427 (0.9301)
359 (2.41±0.05), 465 (2.49±0.05)
392 (0.66) 552 (0.51)
432 (0.57), 650 (0.36)
330 (0.4481), 428 (0.9198)
357 (2.42±0.05), 466 (2.50±0.05)
393 (0.64), 556 (0.50)
435 (0.52), 656 (0.35)
330 (0.4449), 429 (0.9029)
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MeCN a
λmaxb, nm, (f)
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CHCl3
λmaxa, nm,
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Solvent
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Table 1 Experimental and calculated absorption wavelengths λmax in nm and oscillator strengths.
Experimental data, bTheoretical data
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Figure 2 (a) Normalized absorption and (b) normalized emission spectrum (λex = 440 nm) of
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PRAA in different solvents.
THF
MeCN a
(nm)
(cm-1M-1)
(nm)
362,
2.14±0.04,
699
475
2.27±0.04
359,
2.41±0.05,
465
2.49±0.05
357,
2.42±0.05,
466
2.50±0.05
Fluorescence
fa
quantum
yields
τavb (ns)
kr x 108 knr x 108 (s-1)
(s-1)
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εmax x 104 λem
0.070±0.006
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CHCl3
λmax
1.77±0.01 0.39±0.02 5.24±0.02
681
0.200±0.005
2.05±0.01 0.97±0.02 3.89±0.02
741
0.010±0.005
0.27±0.01 0.37±0.02 36.6±0.02
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Solvent
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Table 2 Photophysical properties of PRAA in CHCl3, THF and MeCN
obtained
upon
excitation
at
490
nm
using
Tris(2,2'-
bipyridyl)dichlororuthenium(II) as the standard (f = 0.042 in water) [46]. b𝜏𝑎𝑣 = ∑𝑛𝑖=1 𝛼𝑖 𝜏𝑖 , where α is the
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normalized pre-exponential values.
3.2 Selectivity of PRAA The acceptor, rhodanine-3-acetic acid unit in PRAA is a chelating ligand for metal ions. A number of metal ions readily coordinates with the thiocarbonyl (C=S), carbonyl (C=O) and carboxyl groups in rhodanine-3-acetic acid [37-39]. An initial screening of the metal ion sensing ability of PRAA was carried out using acetate salts of metals like Hg2+, Pb2+, Ni2+, Cu2+, Co2+,
Mn2+, Zn2+, Cd2+, Mg2+ and Ag+ in MeCN. Stock solutions of metal salts in water were added systematically to the dye solution changes in UV-Vis absorption were carefully monitored. Figure 3(a) shows the UV-Vis absorption spectra of PRAA (2.5 x 10-5 M) on addition of 3.0 x 10-5 M solution of metal ions Ag+, Hg2+, Pb2+, Ni2+, Cu2+, Co2+, Mn2+, Zn2+, Cd2+ and Mg2+ in MeCN. A bar diagram showing selectivity of metal ion binding in different solvents is given in Figure S6. The spectral response shows significant changes in the absorption spectrum of the dye on addition of these metal ions, which involves a broadening of the spectrum, reduction in molar absorption
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and a bathochromic shift. This result contradicts with the selectivity claims reported for similar systems in aqueous or polar medium. It seems that the chelation properties of D-A systems with this ligand weigh much on the type of the donor moiety. In this case it is phenothiazine, which is
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a better donor compared to carbazole or triphenylamine reported earlier [37, 38]. Therefore, there exists an influence of the extent of ICT character in these molecules which controls the electron
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density on chelating atoms or groups.
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Will changing solvent polarity impose an effect on the metal ions sensing property of PRAA? To verify the effect of solvent polarity on the chelation properties we carried out metal binding studies by UV-Vis titration in solvents or solvent mixtures of varying polarity. We used
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CHCl3 as the non-polar medium, THF as a moderately polar medium and MeCN as the polar medium. In order to obtain a solvent polarity intermediate to THF and MeCN we have also used a
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1:1 mixture of THF and MeCN as the medium (Figure S7). The results obtained for 3.0 x 10-5 M metal salt solution in MeCN, CHCl3, THF and THF:MeCN (1:1) are given in Figure 3. The spectrophotometric titrations for the respective metal ion/solvent combination were performed and the results are presented in Figure 4. Here the concentration of PRAA (2.5 x 10-5 M) was kept constant and metal ion concentration was varied from zero to 3.0 x 10-5 M ie., until no further
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change in absorption spectrum of the respective solution. All spectra evolved with three isosbestic points around 335-350, 375-385 and 410-430 nm depending on the medium and metal ion. The evolution of the spectra with isosbestic points clearly indicates binding interaction between PRAA and the respective metal ion. The limiting concentration of the metal ion close to 1 equivalent is indicative of a complexation equilibrium with 1:1 stoichiometry in all cases except in CHCl3 where no further change was observed after the addition of 0.7 equivalents of Hg2+. Overall spectrophotometric studies show that PRAA is selective to Hg2+ in CHCl3 and Pb2+ in THF:MeCN (1:1).
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Figure 3 Absorption spectra showing the metal ion selectivity of PRAA in (a) MeCN, (b) CHCl3,
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(c) THF and (d) THF:MeCN (1:1).
Figure 4 Spectrophotometric titration curves of PRAA [2.5 x 10-5 M] with Hg2+ in (a) CHCl3 and (b) THF. Spectrophotometric titration curves of PRAA [2.5 x 10-5 M] with Pb2+ in (c) THF and (e) THF:MeCN (1:1) mixture. (d) Spectrophotometric titration curves of PRAA [2.5 x 10-5 M] with Ag+ in THF. 3.3 Spectrofluorometric titration in different solvents Fluorescence spectroscopy is a more sensitive technique than UV-Vis spectrophotometry. We probed the response of fluorescence of PRAA in the presence of Ag+, Hg2+ and Pb2+ where
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binding interaction is observed by spectrophotometry. For the study the respective solvent metal ion combination was used and the isosbestic point was chosen as the excitation wavelength. This
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will ensure constant absorbance at the excitation wavelength at all concentrations of the metal ion enabling proper observation of the excited state that is responsible for the emission. At this
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wavelength both the probe and probe-analyte complex will have equal rate of absorption of photons. In CHCl3, we have observed a quenching of fluorescence intensity at the emission
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maximum (695 nm) with a concomitant appearance of a band at 645 nm until the addition of 1 equivalent of Hg2+ (Figure 5(a)). Thus, in CHCl3 upon Hg2+ complexation PRAA shows a
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ratiometric response in fluorescence. The ratiometric response curve plotted by using ratio of fluorescence intensity at 645 nm and 695 nm against concentration of Hg2+ is given in Figure 5(b). In THF, all three metals ions, Ag+, Hg2+ and Pb2+ show evidence of binding by UV-Vis
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spectrophotometry. In this case, Ag+ and Pb2+ quenched the fluorescence of PRAA and Hg2+ showed ratiometric response. The fluorescence titration curves obtained in THF for these metal ions are given in Figure S8. Interestingly, in THF:MeCN (1:1) mixture Pb2+ ions showed an 2 times enhancement in fluorescence intensity with a 62 nm blue shift in the emission maximum to 655 nm (Figure 5(c)). This response was upto the addition of close to 1 equivalent of Pb2+ ions.
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Here, with the addition of MeCN the fluorescence of PRAA was significantly reduced and in the presence of Pb2+ the fluorescence showed a “turn on” as well as a “fluorometric” response. This fluorometric response makes PRAA a “spectrometric ruler” for the detection Pb2+ ions in 1:1 THF:MeCN. The plot of fluorescence maximum as a function of Pb2+ concentration is given in Figure 5(d) with the array of normalized spectra for different concentrations of Pb2+ as inset. Our repetitive experiments with different initial concentration of PRAA gave identical results with barely minimal error in the spectral shift for the respective Pb2+ concentration. Thus, PRAA
behaves like a molecular chameleon that changes its selectivity to metal ion depending on the polarity of the medium. This makes PRAA a unique molecule that can be used for the detection and quantification of two important toxic metal ions, Hg2+ and Pb2+, just by choosing appropriate
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solvent medium for analysis.
Figure 5 Fluorescence titration spectra of PRAA [1.6 x 10-5M] in (a) CHCl3 (λex = 427 nm) and
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(b) ratiometric response upon titration with Hg2+ ions. Fluorescence titration spectra of PRAA [1.6 x 10-5M] in (c) THF:MeCN (1:1) mixture (λex = 410 nm) and (d) the fluorometric response curve upon titration with Pb2+ ions. Inset: The array of normalized spectra of PRAA for different concentrations of Pb2+ ions in THF:MeCN (1:1) mixture.
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However, for wider applicability as a dye in a kit for the detection and quantification of
toxic metal ions one must explore its utility in the aqueous medium. Since, the response and selectivity depend very much on the polarity of the medium use of water as a solvent may lead to complete loss of selectivity. Moreover, PRAA is sparingly soluble in water and the use of water/MeCN or other combinations may not yield desirable results. We explored the use of anionic surfactant such as SDS (100 mM) as a medium. SDS is ideal as the micellar surface is negatively charge and ensure better interaction of metal ions with the probe. PRAA in SDS gave an absorption
spectrum with maximum 477 nm indicative of a polar binding site in the micelle. The emission maximum was 705 nm which is similar to the maximum observed in 1:1 THF:MeCN mixture. From this we assume that PRAA is located in the micelle with the hydrophobic alkyl chain oriented towards the core of the micelle and the polar rhodanine-3-acetic acid unit on the surface. Such a topology of binding is suitable for interaction with metal ions. We studied the nature of binding with all three metal ions by absorption as well as by fluorescence spectroscopy. Addition of Ag+ and Hg2+ ions affected the absorption and emission spectra, but, Pb2+ ions showed only a slight change in both spectra. The insensitivity towards Pb2+ ions in this medium might be due to the
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strong association of Pb2+ ions with sulfate end groups of the surfactant as evident from the poor solubility of PbSO4 in aqueous medium [47]. Moreover, PRAA gave a colorimetric response to
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Ag+ ions where the solution has turned purple. Hg2+ gave an enhancement in fluorescence with a 55 nm blue shift in the emission maximum. The evolution of absorption and emission spectra of
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PRAA in the presence of Ag+ or Hg2+ are given in Figure 6. A summary of the response of PRAA
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to the metal ions studied and its correlation to medium changes is given in Table 3.
Figure 6 (a) Absorption and (b) emission spectra (λex = 441 nm) of PRAA [3.0 x 10-5 M] in SDS upon titration with Hg2+ ions. (c) Absorption and (d) emission spectra (λex = 490 nm) of PRAA [3.0 x 10-5 M] in SDS upon titration with Ag+ ions. Table 3 Response of PRAA to metal ions in different solvents. Metal ions
MeCN
THF
THF:MeCN
CHCl3
SDS
(1:1)
(100 mM)
Em
UV
Em
UV
Em
UV
Em
UV
Em
Ag+
-
-
+
+
-
-
-
-
+
+
Hg2+
+
+
+
+
-
-
+
+
+
+
Pb2+
+
+
+
+
+
+
Others
+
+
-
-
-
-
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UV
-
-
-
-
-
-
-
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-
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3.4 Stoichiometry of binding
The observation of isosbestic points in the evolution of UV-Vis absorption spectra is
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typically observed when there are overlapping spectra exists for the ligand as well as the complex. A more focused isosbestic point suggests the complexation equilibrium between PRAA and metal ions involve only two absorbing species and the observed spectrum is a simple additive spectrum
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due to PRAA and the comples. Absence of any concentration dependent change in the isosbestic point suggests a 1:1 complexation in most of the cases, except for Hg2+ in CHCl3 where a range is observed. For Hg2+ in CHCl3 the isosbestic region has the nature of an evolving surface suggesting absorption by more than 2 species present in the medium. This indicate formation of complexes with 1:1 stoichiometry followed by formation of complexes with 2:1/1:2 stoichiometry.
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Stoichiometry of binding was further determined by the method of Job’s continuous variation [8, 48]. The maximum on the plot correspond to the stoichiometry of the two species in the complex. In THF:MeCN (1:1) mixture the titration with Pb2+ ions, Job’s plot gave a maximum at a mole fraction of 0.5, indicating that a 1:1 stoichiometry for the complex between PRAA and Pb2+ ions (Figure S9(a)). Whereas, in the case of Hg2+ ions, the maximum observed was at mole fraction of 0.4, indicating a 2:1 binding stoichiometry, ie., PRAA-Hg2+-PRAA in CHCl3 (Figure S9(b)). While in the case of THF both Pb2+ and Hg2+ ions gave 1:1 binding (Figure S9(c) and (d)
respectively). The binding constant for the complexes were determined by the Benesi-Hildebrand method in cases where there is an enhancement in fluorescence, ie., Pb2+ in THF:MeCN (1:1) and Hg2+ in CHCl3 [8, 49]. In this method we have chosen the intensity at a wavelength where the intensity of emission from the free PRAA is minimal. The equation developed for a 1:1 binding equilibrium is used for fitting the plot of inverse of variation in fluorescence intensity as a function of inverse of metal ion concentration in all medium (see Supplementary information section S 1.5.1 and Figure S10). The binding constant (K) was calculated from the slope and is given in Table S2. For Pb2+ ions in THF:MeCN (1:1) a one order high value of 1.8 x 105 M-1 was obtained
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compared to Hg2+ (2.7 x 104 M-1) in CHCl3. In THF, where all the three metal ions were found to form complexes with PRAA we estimated the binding constant from the Stern–Volmer constants
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determined for the quenching of fluorescence intensity at a wavelength far from the emission of the complex. In this case we assumed a static quenching mechanism where the Stern–Volmer
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constant is also the binding constant of the complex formed (Figure S11). The values are given in
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Table S2 and it shows that Pb2+ ions has a preferential binding compared to other two metal ions. 3.5 Determination of Lowest Detection Limit
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The variation in the absorbance shows a linear dependence in the lower range of metal ion concentration. Each of this response was plotted (Figures S12-15) and detection limit was calculated from the fitted curve (Table S2).
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3.6 Rationalization of binding interaction between PRAA and metal ions To further probe PRAA’s chameleonic behavior in the selective binding of metal ions depending on the solvent medium and the nature of binding interaction we analyzed the complex formed between PRAA with Pb2+ / Hg2+ ions by MALDI-TOF mass spectrometry. PRAA-M2+
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ions solution was prepared by taking a 1:1 stoichiometry using sinapinic acid as the matrix. The HRMS data obtained in the case of PRAA-Pb2+ complex (Figure S16), the peak observed at m/z = 750.1226 correspond to [PRAA+Pb2++MeOH-H+] ion, where the calculated molecular weight is 750.1129. While in the case of PRAA-Hg2+ complex (Figure S17) the peak at m/z = 716.6245 correspond to [PRAA-Hg2++2H+] ion, where calculated mass is 716.1103. Our investigations on 1
H and 13C NMR titrations in DMSO-d6 in the presence of metal ions were inconclusive (Figure
S18). Upon addition of Hg2+ / Pb2+ ions to the solution of PRAA, splitting in the aromatic region was lost maybe due to the paramagnetic nature of metal ions. The differential complexation ability of different chelating atoms in a ligand can be analyzed on the basis of Hard and Soft nature of the chelating atoms vis-à-vis the Hardness of the metal ion. Change in solvent medium generally does not cause vide variations in molecular geometry in the ground state, however solvent stabilization or reorganization may affect geometry especially in the case of molecules having ICT states. Solvents, do affect the extent of
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intramolecular charge transfer in D-π-A systems. Though large changes in ground state properties are not evident in the UV-Vis spectra, subtle changes are accounted for in the results of the TD-
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DFT calculations. Most common method to implicitly account for solvent effects is the use of polarizable continuum model (PCM), in which the solvent is modeled as a continuous static
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medium characterized by a dielectric constant [50-52]. Out of various methods available the conductor-like polarizable continuum model (CPCM), is an implementation of the conductor-like
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screening model (COSMO) in the PCM framework. Optimized structures using CPCM models for CHCl3, THF and MeCN using identical functional and basis sets were further analyzed for the effective charge distribution around various atoms that are potential chelating sites. Mulliken
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population analysis of the electronic structural data thus obtained qualitatively provides an estimate of partial atomic charges on these chelating atoms [53]. Such comparisons are valid only
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when the functional and basis sets used are identical. The comparison of Mulliken charge distribution of different chelating sites on PRAA in different solvents and gas phase is given in Figure 7 (Table S3 for values) along with the electrostatic potential map made using data obtained for gas phase calculations. There is marked changes in the Mulliken charge distribution for the S atom of thiocarbonyl group and the carbonyl O atom of –COOH group of rhodanine-3-acetic acid
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with change in the solvent model. Moving from CHCl3 to MeCN the solvent polarity increases, consequently the S atom on the thiocarbonyl becomes more negative. According to the HSAB principle, charges on atoms control the hard or soft nature of the atoms concerned. In the binding interaction, soft Lewis acids like Ag+, Hg2+ and Cd2+ generally show affinity towards soft atoms such as sulfur [54, 55]. Here, solvent medium changes affect the charge transfer between the phenothiazine, the donor moiety and the rhodanine-3-acetic acid the acceptor moiety. The ring S atom on the rhodanine-3-acetic acid is adjacent to the bridge and the charge on this atom is highly modulated based on the extent of charge transfer. This further affect the charge distribution
on the S atom of thiocarbonyl group. Such variation in charges results in the modulation of hardness on this atom. Thus in a non-polar medium like CHCl3 PRAA binds to Hg2+ only and as the medium changes to polar MeCN, PRAA chelates with most of the metal ions except Ag+. In addition to all these, ability of dissociation of metal salts in the particular solvent medium may also control the nature of binding. Considering all these and the stoichiometry of binding, the
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plausible mode of binding interaction of metal ions is presented in Scheme 2.
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Figure 7 (a) Bar diagram showing Mulliken charge population on the chelating atoms of PRAA
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in different solvents. (b) Electrostatic potential mapped surface of PRAA in the gas phase.
Scheme 2 Schematic representation of the metal ion binding with PRAA 4. Conclusions
Herein, a simple D-π-A system based on phenothiazine donor and rhodanine-3-acetic acid acceptor was used as a fluorescent sensor whose selectivity can be tuned by changing the solvent polarity. PRAA is highly selective and sensitive to Ag+, Hg2+ and Pb2+ ions and its selectivity can be tuned by solvent polarity. This chameleonic behavior is attributed to the intramolecular charge transfer and variation in the electron density at the ligand atom as a function of solvent polarity. The binding stoichiometry was found to be 1:1 for Ag+ and Pb2+ ions in different medium, while 2:1 for Hg2+ ions in CHCl3. It is also demonstrated that how it can be used as a probe in aqueous
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solutions for detection of Ag+ and Hg2+ by using an anionic surfactant medium. Further structural variation and tuning of topology of micellar binding of the probe in the surfactant medium can lead to development of kits for submicromolar Hg2+ detection and quantification in aqueous
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samples.
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2. Outdated reference [46] has been removed.
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Authors statement Authors Statement on corrections made to the manuscript entitled " A Molecular chameleon: Fluorometric to Pb2+, Fluorescent Ratiometric to Hg2+ and Colorimetric to Ag+ ions" As per suggestions of the reviewers we have made the following changes to the manuscript 1. A statement on correction of emission spectra due to the instrument factors has been made in the text.
3. All the figures are modified accordingly.
4. Tables 1 and 2 are modified accordingly.
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5. The formula used for calculating the average lifetime is included in the text. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that
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could have appeared to influence the work reported in this paper.
Acknowledgments
We gratefully acknowledge University Grants Commission (UGC) for research fellowship.
AA is grateful to Govt. of Kerala (E-grants) for research fellowship. We are also thankful to CUSAT, DST-FIST/PURSE, UGC-SAP, IUCND CUSAT for financial support. We are grateful to DST-SAIF, Cochin, for characterization facilities. The computer center at CUSAT is also acknowledged for providing the computational facility set up using the DST-PURSE grant.
Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at
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PII:
S1010-6030(20)30847-9
DOI:
https://doi.org/10.1016/j.jphotochem.2020.113050
Reference:
JPC 113050
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
21 September 2020
Revised Date:
16 November 2020
Accepted Date:
18 November 2020
Please cite this article as: Chandrasekaran PO, Aswathy A, James K, Kala K, Ragi MT, Manoj N, A Molecular chameleon: Fluorometric to Pb2+ , Fluorescent Ratiometric to Hg2+ and Colorimetric to Ag+ ions, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.113050
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A Molecular chameleon: Fluorometric to Pb2+, Fluorescent Ratiometric to Hg2+ and Colorimetric to Ag+ ions Parvathy O. Chandrasekarana, Ajayakumar Aswathya, Kiran Jamesa, Kannankutty Kalab, Mohanan T. Ragic, Narayanapillai Manoja, * a Department
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of Applied Chemistry and Interuniversity Center for Nanomaterials and Devices, Cochin University of Science and Technology, Kochi-22, India. b Department of Chemical Engineering, National Tsing Hua University, Taiwan. c Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram 695019, India. * Corresponding author. E-mail address: [email protected]
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Graphical Abstract
Research Highlight
Developed a donor-π-acceptor based Chameleonic fluorescent chemosensor for toxic metal ions Inherent ICT character is exploited for tuning selectivity of metal ion binding The solvent polarity dependence of charge on the chelating atoms established through theoretical modelling and validated by experiment Established the applicability in aqueous medium
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Abstract A molecular chameleon, having D-π-A architecture with phenothiazine donor and rhodanine-3-
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acetic acid acceptor (PRAA) was synthesized and demonstrated its use as a fluorescent sensor to toxic metals ions. PRAA is highly selective and sensitive to Ag+, Hg2+ and Pb2+ ions and its selectivity can be tuned by solvent polarity. The chameleonic behavior is attributed to the
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intramolecular charge transfer and variation in the electron density at the ligand atom as a function of solvent polarity. The mechanism of binding and the stoichiometry of interaction is established
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by experimental and theoretical methods. How PRAA can be used as a probe in aqueous solutions
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for the detection of Ag+ and Hg2+ ions by using an anionic surfactant medium is convincingly Keywords: Sensors, Fluoroionophores, Donor-π-Acceptor, Fluorometric sensor, Ratiometric sensor, Colorimetric sensor
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1. Introduction
In recent years, development of new probes for the selective and sensitive detection of transition and heavy metals (THM) and different anions of biological importance has attracted considerable attention because of their quite important role in many environmental and biological
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processes [1-2]. Among the THM ions Pb2+ and Hg2+ ions are highly toxic while Ag+ ions is less toxic [3]. Lead is relatively more abundant and low cost metal, widely used in construction industries, batteries, bullets, solders, weights, paints etc. and is more easily found in the environment [4, 5]. Even very low level of lead exposure can cause neurological, cardiovascular, reproductive and developmental disorders in children [6]. Mercury is considered more dangerous, because both elemental and ionic mercury by the bacterial action gets converted into methyl mercury and subsequently bioaccumulates in animal tissues through the food chain [7-10]. High
dose exposure of mercury leads to the malfunctioning of central nervous system, blood and kidneys, results in diseases like Hunter-Russell syndrome, Minamata disease and acrodynia [11]. According to EPA (Environmental Protection Agency), the maximum permissible level of inorganic Hg2+ ions in drinking water is 2 ppb [12, 13]. Silver is used in photography, electrical, pharmaceutical and cosmetic industry [14]. There are many reports on silver toxicity and bioaccumulation which leads to adverse biological effects [15, 16]. Silver inactivates sulfhydryl enzyme by combining with amine, imidazole and carboxyl group of various metabolites [17]. Thus
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detection of these toxic ions is important. Quicker and easy to use sensor systems based on chelation by molecules with associated changes in their electrochemical and photophysical properties are widely used in this field. Most popular and sensitive method of detection exploits
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the changes in the fluorescence properties of molecules. Such sensors exhibit high sensitivity and low detection limit etc. [18-20]. Even though many such molecules are known in the literature,
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they are either selective to a class or a group of metal ions or specific to one particular metal ion. No report exists so far where a single molecular system shows tunable selectivity by changing the
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medium or mode of use.
Nowadays a new concept of “single sensor for multiple targets” has got more attention of
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researchers [21]. The ability to detect more than one target simultaneously can shorten the overall analytical processing time and potentially reduce the cost. Inspired by this concept, herein we have
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successfully demonstrated the use of a donor-π-acceptor (D-π-A) system based on phenothiazine as a fluorescent sensor PRAA (Scheme 1) to detect and quantify three different metal ions by choosing appropriate solvent media. Organic push-pull molecules commonly known as donor-πacceptor (D-π-A) systems have attracted great attention because facile tuning of their electronic states can be achieved by choosing appropriate D/A component to make them suitable for a variety
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of applications [22-24]. Phenothiazine and its derivatives were used to construct organic luminescent materials and as dyes in organic photovoltaic applications [25-33]. By virtue of the presence of N and S atoms, it has a nonplanar butterfly conformation that inhibit strong intermolecular π-π interactions and also make them good electron donors [34-36]. It exhibits intense fluorescence, good hole transporting capacity and structural regulation. Rhodanine-3acetic acid act as an electron acceptor, possessing S and O, as part of a carbonyl or a thiocarbonyl functionality along with N and S as ring atoms with a pendant carboxylic acid receptor offering a variable platform for metal ion coordination. Moreover, earlier reports showed that rhodanine-3-
acetic acid has been used as spectrophotometric agent for sensing heavy metal ions [37-39]. Thus, our sensor system uses the D-π-A molecule as a tunable fluorescent reporter having a chelating heterocyclic acceptor for analyte binding. 2. Experimental 2.1 Materials All reagents and solvents of analytical grades were obtained commercially and used
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without further purification. Spectroscopic grade solvents from Merck were used for photophysical studies.
(400 MHz) and
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Melting point is uncorrected and was determined on a JSGW melting point apparatus. 1H 13
C NMR (100 MHz) spectra were recorded on a Bruker Avance III FT-NMR
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spectrometer with tetramethylsilane (TMS) as internal standard and DMSO-d6 as the solvent. MALDI-TOF mass spectrum was obtained using Bruker Autoflex max LRF. Absorption spectra
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were measured by using an Evolution 201 UV-visible spectrophotometer. Fluorescence measurements were recorded on a Horiba Fluorolog 3 spectrofluorimeter with excitation and
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emission slit widths of 2 nm. The fluorescence spectra were corrected for the monochromator and detector effects by using the correction method provided with the FluorEscenceTM Software by Horiba. Fluorescence lifetime was measured with Horiba Fluorolog 3 time-correlated single
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photon counting system using Horiba NanoLED-510L pulsed laser diode having 510 ± 10 nm wavelength output and a pulse width of <200ps. The lifetime values were obtained by the deconvolution of the decay from the instrument response function recorded using a scattering solution. The analysis was performed using DAS6 decay analysis software. The average lifetime is the amplitude weighted average lifetime reported by the decay analysis software from the fitting
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results. All measurements were performed at room temperature. Geometry optimization of the compound was carried out by DFT computation using Gaussian 09 software with B3LYP or M06L exchange correlation energy functional and 6-311G (d, p) basis set [40]. The major electronic transitions and their oscillator strengths were calculated by TD-DFT method using the same functional and basis set. The optimized geometry and their respective electronic transitions in different solvents were obtained by polarizable continuum model as implemented in Gaussian 09.
2.2 Synthesis Phenothiazine rhodanine-3-acetic acid conjugate (PRAA) was synthesized according to the reported procedure [41, 42] by the Knoevenagel condensation between 10-octyl-10Hphenothiazine-3-carbaldehyde (1) and rhodanine-3-acetic acid (2) in glacial acetic acid medium in the presence of ammonium acetate (Scheme 1). Brick red crystals of the product was filtered and washed with hot water and dried in vacuum. Yield 80%. mp 244 °C. 1H NMR (400 MHz, DMSOd6) ẟ (ppm): 0.76-0.79 (t, 3H), 1.15-1.18 (m, 8H), 1.32-1.33 (m, 2H), 1.63-1.66 (m, 2H), 3.85-
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3.88 (t, 2H), 4.70 (s, 2H), 5.69 (s, 1H), 6.95-7.00 (m, 2H), 7.02-7.12 (m, 2H), 7.17-7.21 (t, 1H), 7.39 (s, 1H), 7.41 (d, 1H), 7.67 (s, 1H); 13C NMR (100 MHz, DMSO-d6) ẟ (ppm) 13.7, 21.8, 25.7,
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25.8, 28.2, 28.3, 30.8, 44.7, 46.6, 115.8, 116.1, 118.3, 122.0, 123.3, 123.7, 126.6, 127.0, 127.7, 129.0, 130.7, 132.8, 142.6, 147.0, 166.1, 167.1, 192.4; MALDI-TOF MS (ESI MS) m/z: [M+H]+
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calcd for C26H29N2O3S3 513.1335, found 513.1139. (See Figures S1 – S3 in supplementary
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Scheme 1 2.3 Metal ion binding studies
A stock solution of PRAA with the concentration of 3.1 x 10-3 M was prepared in THF (tetrahydrofuran) at room temperature. This stock solution was used for all the UV-vis and
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fluorescence study after appropriate dilution. Metal ion binding studies were carried in different solvents and solvent mixtures with a series of transition metal salts in particularly of heavy metal salts. Acetate salts of metals like mercury, lead, cadmium, nickel, cobalt, copper, manganese, zinc, magnesium and silver were prepared by dissolving in milli Q water, while for the study in CHCl 3 methanol is used. Stock solutions of metal salts in water were added systematically to the dye solution, changes in UV-Vis absorption and fluorescence spectra were carefully monitored. 3. Results and Discussion
3.1 Electronic States and Photophysical Properties The optimized geometry and the molecular orbitals of PRAA were obtained by DFT calculations using B3LYP or M06-L functional with 6-311G (d, p) basis set using Gaussian 09 for gas phase, CHCl3, THF and MeCN solution. The optimized geometry and the MO’s obtained in MeCN are given in Figure 1. The results show that the molecule assumes a butterfly like bend geometry for the phenothiazine moiety. Intramolecular charge transfer (ICT) character inherent to the D-π-A systems such as PRAA is evident from the localization of HOMO and HOMO-1 orbitals
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on the phenothiazine moiety and the LUMO and LUMO+1 orbitals on the rhodanine-3-acetic acid, the acceptor moiety. The major electronic transitions computed by TD-DFT using B3LYP, M06-
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L and CAM-B3LYP are given in Table 1. Calculations using CAM-B3LYP functional gave better correlation with the experimental data [43]. The theoretically calculated band at 330 nm has its
-p
major contribution from HOMO-1 to LUMO transition and the long wavelength band at 429 nm has a major contribution from HOMO to LUMO transition. Based on the nature of the orbitals
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involved, these transitions can be safely assigned as ICT excitation with shift of charge from phenothiazine to the rhodanine-3-acetic acid. The spectra were unusually red shifted in non-polar solvents such as dichloroethane and chloroform which could be due to a specific solvent
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interaction with halogenated solvents. The absorption and emission spectrum of PRAA recorded in different solvents are given in Figure 2(a) and (b) respectively. In agreement with the inherent
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intramolecular charge transfer nature, PRAA showed significant bathochromic shift in the emission spectra with increasing polarity of the solvent medium. However, this bathochromic shift is less significant in the absorption spectra recorded in different solvents. By using the LippertMataga formalism, the solvent polarity effect on this D-π-A system can be better explained (Table S1 and Figure S4) [44, 45]. The linear relationship between Stokes shift (Δνst) and orientational
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polarizability (Δf) well explains the ICT nature of the excited state. The compound showed moderate fluorescence quantum yield of 0.20 in THF and very low values in other solvents. The fluorescence lifetime was also measured in THF, CHCl3 and MeCN (Figure S5). The decay profiles showed biexponential dynamics in these solvents with average lifetime ranging from 0.27 to 2.05 ns. Decay with a very short lifetime (τav) of 0.27 ns was obtained in MeCN. This could be due to the highly stabilized ICT excited state having enhanced non-radiative deactivation rates. The compilation of important photophysical parameters in the three solvents are presented in Table 2.
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Figure 1 Optimized ground state geometry, distribution of molecular orbital coefficient density in HOMO, HOMO-1, HOMO-2, LUMO, LUMO+1 of PRAA in MeCN.
THF
(εmax, x 104 cm-1M-1)
B3LYP
M06-L
CAM-B3LYP
362 (2.14±0.04), 475 (2.27±0.04)
391 (0.67), 549 (0.52)
431 (0.60), 646 (0.37)
330 (0.4502), 427 (0.9301)
359 (2.41±0.05), 465 (2.49±0.05)
392 (0.66) 552 (0.51)
432 (0.57), 650 (0.36)
330 (0.4481), 428 (0.9198)
357 (2.42±0.05), 466 (2.50±0.05)
393 (0.64), 556 (0.50)
435 (0.52), 656 (0.35)
330 (0.4449), 429 (0.9029)
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MeCN a
λmaxb, nm, (f)
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CHCl3
λmaxa, nm,
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Solvent
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Table 1 Experimental and calculated absorption wavelengths λmax in nm and oscillator strengths.
Experimental data, bTheoretical data
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Figure 2 (a) Normalized absorption and (b) normalized emission spectrum (λex = 440 nm) of
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PRAA in different solvents.
THF
MeCN a
(nm)
(cm-1M-1)
(nm)
362,
2.14±0.04,
699
475
2.27±0.04
359,
2.41±0.05,
465
2.49±0.05
357,
2.42±0.05,
466
2.50±0.05
Fluorescence
fa
quantum
yields
τavb (ns)
kr x 108 knr x 108 (s-1)
(s-1)
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εmax x 104 λem
0.070±0.006
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CHCl3
λmax
1.77±0.01 0.39±0.02 5.24±0.02
681
0.200±0.005
2.05±0.01 0.97±0.02 3.89±0.02
741
0.010±0.005
0.27±0.01 0.37±0.02 36.6±0.02
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Solvent
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Table 2 Photophysical properties of PRAA in CHCl3, THF and MeCN
obtained
upon
excitation
at
490
nm
using
Tris(2,2'-
bipyridyl)dichlororuthenium(II) as the standard (f = 0.042 in water) [46]. b𝜏𝑎𝑣 = ∑𝑛𝑖=1 𝛼𝑖 𝜏𝑖 , where α is the
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normalized pre-exponential values.
3.2 Selectivity of PRAA The acceptor, rhodanine-3-acetic acid unit in PRAA is a chelating ligand for metal ions. A number of metal ions readily coordinates with the thiocarbonyl (C=S), carbonyl (C=O) and carboxyl groups in rhodanine-3-acetic acid [37-39]. An initial screening of the metal ion sensing ability of PRAA was carried out using acetate salts of metals like Hg2+, Pb2+, Ni2+, Cu2+, Co2+,
Mn2+, Zn2+, Cd2+, Mg2+ and Ag+ in MeCN. Stock solutions of metal salts in water were added systematically to the dye solution changes in UV-Vis absorption were carefully monitored. Figure 3(a) shows the UV-Vis absorption spectra of PRAA (2.5 x 10-5 M) on addition of 3.0 x 10-5 M solution of metal ions Ag+, Hg2+, Pb2+, Ni2+, Cu2+, Co2+, Mn2+, Zn2+, Cd2+ and Mg2+ in MeCN. A bar diagram showing selectivity of metal ion binding in different solvents is given in Figure S6. The spectral response shows significant changes in the absorption spectrum of the dye on addition of these metal ions, which involves a broadening of the spectrum, reduction in molar absorption
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and a bathochromic shift. This result contradicts with the selectivity claims reported for similar systems in aqueous or polar medium. It seems that the chelation properties of D-A systems with this ligand weigh much on the type of the donor moiety. In this case it is phenothiazine, which is
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a better donor compared to carbazole or triphenylamine reported earlier [37, 38]. Therefore, there exists an influence of the extent of ICT character in these molecules which controls the electron
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density on chelating atoms or groups.
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Will changing solvent polarity impose an effect on the metal ions sensing property of PRAA? To verify the effect of solvent polarity on the chelation properties we carried out metal binding studies by UV-Vis titration in solvents or solvent mixtures of varying polarity. We used
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CHCl3 as the non-polar medium, THF as a moderately polar medium and MeCN as the polar medium. In order to obtain a solvent polarity intermediate to THF and MeCN we have also used a
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1:1 mixture of THF and MeCN as the medium (Figure S7). The results obtained for 3.0 x 10-5 M metal salt solution in MeCN, CHCl3, THF and THF:MeCN (1:1) are given in Figure 3. The spectrophotometric titrations for the respective metal ion/solvent combination were performed and the results are presented in Figure 4. Here the concentration of PRAA (2.5 x 10-5 M) was kept constant and metal ion concentration was varied from zero to 3.0 x 10-5 M ie., until no further
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change in absorption spectrum of the respective solution. All spectra evolved with three isosbestic points around 335-350, 375-385 and 410-430 nm depending on the medium and metal ion. The evolution of the spectra with isosbestic points clearly indicates binding interaction between PRAA and the respective metal ion. The limiting concentration of the metal ion close to 1 equivalent is indicative of a complexation equilibrium with 1:1 stoichiometry in all cases except in CHCl3 where no further change was observed after the addition of 0.7 equivalents of Hg2+. Overall spectrophotometric studies show that PRAA is selective to Hg2+ in CHCl3 and Pb2+ in THF:MeCN (1:1).
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Figure 3 Absorption spectra showing the metal ion selectivity of PRAA in (a) MeCN, (b) CHCl3,
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(c) THF and (d) THF:MeCN (1:1).
Figure 4 Spectrophotometric titration curves of PRAA [2.5 x 10-5 M] with Hg2+ in (a) CHCl3 and (b) THF. Spectrophotometric titration curves of PRAA [2.5 x 10-5 M] with Pb2+ in (c) THF and (e) THF:MeCN (1:1) mixture. (d) Spectrophotometric titration curves of PRAA [2.5 x 10-5 M] with Ag+ in THF. 3.3 Spectrofluorometric titration in different solvents Fluorescence spectroscopy is a more sensitive technique than UV-Vis spectrophotometry. We probed the response of fluorescence of PRAA in the presence of Ag+, Hg2+ and Pb2+ where
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binding interaction is observed by spectrophotometry. For the study the respective solvent metal ion combination was used and the isosbestic point was chosen as the excitation wavelength. This
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will ensure constant absorbance at the excitation wavelength at all concentrations of the metal ion enabling proper observation of the excited state that is responsible for the emission. At this
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wavelength both the probe and probe-analyte complex will have equal rate of absorption of photons. In CHCl3, we have observed a quenching of fluorescence intensity at the emission
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maximum (695 nm) with a concomitant appearance of a band at 645 nm until the addition of 1 equivalent of Hg2+ (Figure 5(a)). Thus, in CHCl3 upon Hg2+ complexation PRAA shows a
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ratiometric response in fluorescence. The ratiometric response curve plotted by using ratio of fluorescence intensity at 645 nm and 695 nm against concentration of Hg2+ is given in Figure 5(b). In THF, all three metals ions, Ag+, Hg2+ and Pb2+ show evidence of binding by UV-Vis
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spectrophotometry. In this case, Ag+ and Pb2+ quenched the fluorescence of PRAA and Hg2+ showed ratiometric response. The fluorescence titration curves obtained in THF for these metal ions are given in Figure S8. Interestingly, in THF:MeCN (1:1) mixture Pb2+ ions showed an 2 times enhancement in fluorescence intensity with a 62 nm blue shift in the emission maximum to 655 nm (Figure 5(c)). This response was upto the addition of close to 1 equivalent of Pb2+ ions.
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Here, with the addition of MeCN the fluorescence of PRAA was significantly reduced and in the presence of Pb2+ the fluorescence showed a “turn on” as well as a “fluorometric” response. This fluorometric response makes PRAA a “spectrometric ruler” for the detection Pb2+ ions in 1:1 THF:MeCN. The plot of fluorescence maximum as a function of Pb2+ concentration is given in Figure 5(d) with the array of normalized spectra for different concentrations of Pb2+ as inset. Our repetitive experiments with different initial concentration of PRAA gave identical results with barely minimal error in the spectral shift for the respective Pb2+ concentration. Thus, PRAA
behaves like a molecular chameleon that changes its selectivity to metal ion depending on the polarity of the medium. This makes PRAA a unique molecule that can be used for the detection and quantification of two important toxic metal ions, Hg2+ and Pb2+, just by choosing appropriate
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solvent medium for analysis.
Figure 5 Fluorescence titration spectra of PRAA [1.6 x 10-5M] in (a) CHCl3 (λex = 427 nm) and
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(b) ratiometric response upon titration with Hg2+ ions. Fluorescence titration spectra of PRAA [1.6 x 10-5M] in (c) THF:MeCN (1:1) mixture (λex = 410 nm) and (d) the fluorometric response curve upon titration with Pb2+ ions. Inset: The array of normalized spectra of PRAA for different concentrations of Pb2+ ions in THF:MeCN (1:1) mixture.
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However, for wider applicability as a dye in a kit for the detection and quantification of
toxic metal ions one must explore its utility in the aqueous medium. Since, the response and selectivity depend very much on the polarity of the medium use of water as a solvent may lead to complete loss of selectivity. Moreover, PRAA is sparingly soluble in water and the use of water/MeCN or other combinations may not yield desirable results. We explored the use of anionic surfactant such as SDS (100 mM) as a medium. SDS is ideal as the micellar surface is negatively charge and ensure better interaction of metal ions with the probe. PRAA in SDS gave an absorption
spectrum with maximum 477 nm indicative of a polar binding site in the micelle. The emission maximum was 705 nm which is similar to the maximum observed in 1:1 THF:MeCN mixture. From this we assume that PRAA is located in the micelle with the hydrophobic alkyl chain oriented towards the core of the micelle and the polar rhodanine-3-acetic acid unit on the surface. Such a topology of binding is suitable for interaction with metal ions. We studied the nature of binding with all three metal ions by absorption as well as by fluorescence spectroscopy. Addition of Ag+ and Hg2+ ions affected the absorption and emission spectra, but, Pb2+ ions showed only a slight change in both spectra. The insensitivity towards Pb2+ ions in this medium might be due to the
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strong association of Pb2+ ions with sulfate end groups of the surfactant as evident from the poor solubility of PbSO4 in aqueous medium [47]. Moreover, PRAA gave a colorimetric response to
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Ag+ ions where the solution has turned purple. Hg2+ gave an enhancement in fluorescence with a 55 nm blue shift in the emission maximum. The evolution of absorption and emission spectra of
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PRAA in the presence of Ag+ or Hg2+ are given in Figure 6. A summary of the response of PRAA
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to the metal ions studied and its correlation to medium changes is given in Table 3.
Figure 6 (a) Absorption and (b) emission spectra (λex = 441 nm) of PRAA [3.0 x 10-5 M] in SDS upon titration with Hg2+ ions. (c) Absorption and (d) emission spectra (λex = 490 nm) of PRAA [3.0 x 10-5 M] in SDS upon titration with Ag+ ions. Table 3 Response of PRAA to metal ions in different solvents. Metal ions
MeCN
THF
THF:MeCN
CHCl3
SDS
(1:1)
(100 mM)
Em
UV
Em
UV
Em
UV
Em
UV
Em
Ag+
-
-
+
+
-
-
-
-
+
+
Hg2+
+
+
+
+
-
-
+
+
+
+
Pb2+
+
+
+
+
+
+
Others
+
+
-
-
-
-
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UV
-
-
-
-
-
-
-
-p
-
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3.4 Stoichiometry of binding
The observation of isosbestic points in the evolution of UV-Vis absorption spectra is
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typically observed when there are overlapping spectra exists for the ligand as well as the complex. A more focused isosbestic point suggests the complexation equilibrium between PRAA and metal ions involve only two absorbing species and the observed spectrum is a simple additive spectrum
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due to PRAA and the comples. Absence of any concentration dependent change in the isosbestic point suggests a 1:1 complexation in most of the cases, except for Hg2+ in CHCl3 where a range is observed. For Hg2+ in CHCl3 the isosbestic region has the nature of an evolving surface suggesting absorption by more than 2 species present in the medium. This indicate formation of complexes with 1:1 stoichiometry followed by formation of complexes with 2:1/1:2 stoichiometry.
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Stoichiometry of binding was further determined by the method of Job’s continuous variation [8, 48]. The maximum on the plot correspond to the stoichiometry of the two species in the complex. In THF:MeCN (1:1) mixture the titration with Pb2+ ions, Job’s plot gave a maximum at a mole fraction of 0.5, indicating that a 1:1 stoichiometry for the complex between PRAA and Pb2+ ions (Figure S9(a)). Whereas, in the case of Hg2+ ions, the maximum observed was at mole fraction of 0.4, indicating a 2:1 binding stoichiometry, ie., PRAA-Hg2+-PRAA in CHCl3 (Figure S9(b)). While in the case of THF both Pb2+ and Hg2+ ions gave 1:1 binding (Figure S9(c) and (d)
respectively). The binding constant for the complexes were determined by the Benesi-Hildebrand method in cases where there is an enhancement in fluorescence, ie., Pb2+ in THF:MeCN (1:1) and Hg2+ in CHCl3 [8, 49]. In this method we have chosen the intensity at a wavelength where the intensity of emission from the free PRAA is minimal. The equation developed for a 1:1 binding equilibrium is used for fitting the plot of inverse of variation in fluorescence intensity as a function of inverse of metal ion concentration in all medium (see Supplementary information section S 1.5.1 and Figure S10). The binding constant (K) was calculated from the slope and is given in Table S2. For Pb2+ ions in THF:MeCN (1:1) a one order high value of 1.8 x 105 M-1 was obtained
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compared to Hg2+ (2.7 x 104 M-1) in CHCl3. In THF, where all the three metal ions were found to form complexes with PRAA we estimated the binding constant from the Stern–Volmer constants
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determined for the quenching of fluorescence intensity at a wavelength far from the emission of the complex. In this case we assumed a static quenching mechanism where the Stern–Volmer
-p
constant is also the binding constant of the complex formed (Figure S11). The values are given in
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Table S2 and it shows that Pb2+ ions has a preferential binding compared to other two metal ions. 3.5 Determination of Lowest Detection Limit
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The variation in the absorbance shows a linear dependence in the lower range of metal ion concentration. Each of this response was plotted (Figures S12-15) and detection limit was calculated from the fitted curve (Table S2).
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3.6 Rationalization of binding interaction between PRAA and metal ions To further probe PRAA’s chameleonic behavior in the selective binding of metal ions depending on the solvent medium and the nature of binding interaction we analyzed the complex formed between PRAA with Pb2+ / Hg2+ ions by MALDI-TOF mass spectrometry. PRAA-M2+
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ions solution was prepared by taking a 1:1 stoichiometry using sinapinic acid as the matrix. The HRMS data obtained in the case of PRAA-Pb2+ complex (Figure S16), the peak observed at m/z = 750.1226 correspond to [PRAA+Pb2++MeOH-H+] ion, where the calculated molecular weight is 750.1129. While in the case of PRAA-Hg2+ complex (Figure S17) the peak at m/z = 716.6245 correspond to [PRAA-Hg2++2H+] ion, where calculated mass is 716.1103. Our investigations on 1
H and 13C NMR titrations in DMSO-d6 in the presence of metal ions were inconclusive (Figure
S18). Upon addition of Hg2+ / Pb2+ ions to the solution of PRAA, splitting in the aromatic region was lost maybe due to the paramagnetic nature of metal ions. The differential complexation ability of different chelating atoms in a ligand can be analyzed on the basis of Hard and Soft nature of the chelating atoms vis-à-vis the Hardness of the metal ion. Change in solvent medium generally does not cause vide variations in molecular geometry in the ground state, however solvent stabilization or reorganization may affect geometry especially in the case of molecules having ICT states. Solvents, do affect the extent of
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intramolecular charge transfer in D-π-A systems. Though large changes in ground state properties are not evident in the UV-Vis spectra, subtle changes are accounted for in the results of the TD-
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DFT calculations. Most common method to implicitly account for solvent effects is the use of polarizable continuum model (PCM), in which the solvent is modeled as a continuous static
-p
medium characterized by a dielectric constant [50-52]. Out of various methods available the conductor-like polarizable continuum model (CPCM), is an implementation of the conductor-like
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screening model (COSMO) in the PCM framework. Optimized structures using CPCM models for CHCl3, THF and MeCN using identical functional and basis sets were further analyzed for the effective charge distribution around various atoms that are potential chelating sites. Mulliken
lP
population analysis of the electronic structural data thus obtained qualitatively provides an estimate of partial atomic charges on these chelating atoms [53]. Such comparisons are valid only
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when the functional and basis sets used are identical. The comparison of Mulliken charge distribution of different chelating sites on PRAA in different solvents and gas phase is given in Figure 7 (Table S3 for values) along with the electrostatic potential map made using data obtained for gas phase calculations. There is marked changes in the Mulliken charge distribution for the S atom of thiocarbonyl group and the carbonyl O atom of –COOH group of rhodanine-3-acetic acid
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with change in the solvent model. Moving from CHCl3 to MeCN the solvent polarity increases, consequently the S atom on the thiocarbonyl becomes more negative. According to the HSAB principle, charges on atoms control the hard or soft nature of the atoms concerned. In the binding interaction, soft Lewis acids like Ag+, Hg2+ and Cd2+ generally show affinity towards soft atoms such as sulfur [54, 55]. Here, solvent medium changes affect the charge transfer between the phenothiazine, the donor moiety and the rhodanine-3-acetic acid the acceptor moiety. The ring S atom on the rhodanine-3-acetic acid is adjacent to the bridge and the charge on this atom is highly modulated based on the extent of charge transfer. This further affect the charge distribution
on the S atom of thiocarbonyl group. Such variation in charges results in the modulation of hardness on this atom. Thus in a non-polar medium like CHCl3 PRAA binds to Hg2+ only and as the medium changes to polar MeCN, PRAA chelates with most of the metal ions except Ag+. In addition to all these, ability of dissociation of metal salts in the particular solvent medium may also control the nature of binding. Considering all these and the stoichiometry of binding, the
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of
plausible mode of binding interaction of metal ions is presented in Scheme 2.
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Figure 7 (a) Bar diagram showing Mulliken charge population on the chelating atoms of PRAA
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in different solvents. (b) Electrostatic potential mapped surface of PRAA in the gas phase.
Scheme 2 Schematic representation of the metal ion binding with PRAA 4. Conclusions
Herein, a simple D-π-A system based on phenothiazine donor and rhodanine-3-acetic acid acceptor was used as a fluorescent sensor whose selectivity can be tuned by changing the solvent polarity. PRAA is highly selective and sensitive to Ag+, Hg2+ and Pb2+ ions and its selectivity can be tuned by solvent polarity. This chameleonic behavior is attributed to the intramolecular charge transfer and variation in the electron density at the ligand atom as a function of solvent polarity. The binding stoichiometry was found to be 1:1 for Ag+ and Pb2+ ions in different medium, while 2:1 for Hg2+ ions in CHCl3. It is also demonstrated that how it can be used as a probe in aqueous
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solutions for detection of Ag+ and Hg2+ by using an anionic surfactant medium. Further structural variation and tuning of topology of micellar binding of the probe in the surfactant medium can lead to development of kits for submicromolar Hg2+ detection and quantification in aqueous
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samples.
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2. Outdated reference [46] has been removed.
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Authors statement Authors Statement on corrections made to the manuscript entitled " A Molecular chameleon: Fluorometric to Pb2+, Fluorescent Ratiometric to Hg2+ and Colorimetric to Ag+ ions" As per suggestions of the reviewers we have made the following changes to the manuscript 1. A statement on correction of emission spectra due to the instrument factors has been made in the text.
3. All the figures are modified accordingly.
4. Tables 1 and 2 are modified accordingly.
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5. The formula used for calculating the average lifetime is included in the text. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that
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could have appeared to influence the work reported in this paper.
Acknowledgments
We gratefully acknowledge University Grants Commission (UGC) for research fellowship.
AA is grateful to Govt. of Kerala (E-grants) for research fellowship. We are also thankful to CUSAT, DST-FIST/PURSE, UGC-SAP, IUCND CUSAT for financial support. We are grateful to DST-SAIF, Cochin, for characterization facilities. The computer center at CUSAT is also acknowledged for providing the computational facility set up using the DST-PURSE grant.
Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at
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