Selective colorimetric sensing of fluoride ion via H-bonding in 80% aqueous solution by transition metal chelates

Accepted Manuscript Title: Selective colorimetric sensing of fluoride ion via H-bonding in 80% aqueous solution by transition metal chelates Authors: C. Parthiban, Kuppanagounder P. Elango PII: DOI: Reference:

S0925-4005(17)30160-0 http://dx.doi.org/doi:10.1016/j.snb.2017.01.153 SNB 21678

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

16-11-2016 16-1-2017 24-1-2017

Please cite this article as: C.Parthiban, Kuppanagounder P.Elango, Selective colorimetric sensing of fluoride ion via H-bonding in 80% aqueous solution by transition metal chelates, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.01.153 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Selective colorimetric sensing of fluoride ion via H-bonding in 80% aqueous solution by transition metal chelates C. Parthiban and Kuppanagounder P. Elango* Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624 302, India

* Corresponding author. Tel.: +91 451 245 2371; Fax: +91 451 2454466 E-mail address: [email protected] (Dr. K.P. Elango)

Graphical abstract

 

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Highlights

 Quinone-imidazole ensemble and its M(II) chelates sense F- selectively.  The mechanism of sensing involves formation of H-bond.  Metal chelates sense fluoride in 80% aqueous solution via H-bonding.

ABSTRACT A series of four coordinated transition metal chelates of composition [M(R1)2], where M = Cu(II), Co(II), Ni(II) and Zn(II) and R1 = quinone-imidazole ensemble, have been prepared and characterized by using standard analytical and spectral techniques. These metal chelates selectively (over many anions) and sensitively (detection limit down to nM) sense fluoride ion colorimetrically with an instantaneous striking colour change from yellow to red. They do so in H2O:DMF (80:20% v/v) medium, while free R1 imparts similar colour change only in DMF. The mechanism of sensing involves formation of H-bond between imidazole N-H group and fluoride ion. The chelates binds strongly (Ka 107 M-1) with fluoride ion to form a 1:2 (receptor:fluoride) complex. The coordination of transition metals to R1 not only enhances the H-bond donor ability of the N-H group but also accommodates 80% of water in the sensing medium, which is a novel finding. Electrochemical and theoretical calculations have also been used to substantiate the results obtained in the spectral studies. The spectral data were applied to construct logic gate at molecular level. Keywords: Complex; fluoride; sensing; colorimetry; chelates

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1. Introduction The development of synthetic receptors for selective and sensitive sensing of anions has been augmented, during the last decade, due to the impact of anions in biological, chemical and environmental processes [1,2]. The development of colorimetric chemosensors has fascinated a large number of focuses because they not only can recognize the target species but also provide qualitative and quantitative analysis without the support of any spectroscopic instrumentation [3]. Among various anions, the colorimetric detection of fluoride ion in aqueous solution is most essential because the beneficially and detrimental role is highly concentration dependent in human beings [4-6]. According to the World Health Organization (WHO) the maximum permissible limit of fluoride ion in drinking water is 1.5 mg/L [7]. Hence, it is important to develop highly selective, sensitive and rapid colorimetric sensor for the detection of fluoride ion in aqueous solutions. Review of literature revealed that the main mechanism of fluoride ion sensing is through either one of the followings viz. H-bonding interaction, Lewis acid-base interaction and fluoride ion induced chemical reaction [8-12]. Of which H-bonding interaction really dominates over the others [13,14]. However, fluoride ion sensing via H-bonding in aqueous solution suffered a lot due to its high hydration energy (-505 kJ mol-1) [15]. To overcome such a setback, recently, we are in the process of developing metal complex based receptors for fluoride ion sensing with a presumption that coordination of metal ions with organic receptors can bring the entire molecule into the aqueous phase and we too have achieved some success [16-18]. Not only that, Chang et al. have highlighted the advantages of metal complexes over pure organic counterparts in their recent review [19]. The advantages include i) the positive charge leads to stronger electrostatic interaction with anionic species, ii) the defined geometries of metal complexes enhance the selectivity towards anions and iii) the most appealing reason is that the metal complexes possess

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a varying unique functionalities that improves the sensor technology. With such a large reported advantage, will it not possible to develop metal complex based receptors for selective sensing of fluoride ion in aqueous solution? During recent past quite a few metal complex based receptors have been reported in literature but majority of them work in organic medium [19-21]. Wang et al. have reported that the Gd(III) based receptor (I) senses fluoride in 50% aqueous THF solution but through replacement of coordinated water by fluoride ion [22]. Amilan Jose et al. have demonstrated that fluoride ion can be sensed in 20% aqueous ACN solution by Ru(II) complex (II) in the form of test papers to avoid hydration of fluoride ion [23]. Lin et al. have shown that Ru(II) complex (III) can colorimetrically senses fluoride ion in aqueous solution as low as 10 mg/L concentration, but also only in the form of test papers [24].

As mentioned above, recently we have also reported few metal complex based receptors (IV and V) for selective sensing of fluoride ion aqueous medium. These receptors have been designed with the assumptions that coordination of metal ions with the receptor unit (N-H) would enhance the H-bond donor property of the N-H moiety towards anion and also could bring

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the molecule into the aqueous solution. As seen from the reports that the results are encouraging that the complexes sense fluoride ion selectively and could accommodate 50% water or more (up to 70%) in the sensing medium [16,18].

In continuation of our earlier work, here in the present endeavor, we have screened few more transition metal chelates for their ability to sense fluoride ion selectively in aqueous solution. The strategy behind the design in the present attempt is that well known ligating groups such as imidazole and phenolic hydroxyl groups were selected to form chelates with metal ions [17,25,26]. Also, imidazole-quinone ensembles were reported to exhibit good anion sensing properties [17,27]. With these design strategies in mind we have reported the synthesis, characterization and anion sensing behavior of the following receptors. The ligand (R1) and its Co(II), Ni(II), Cu(II) and Zn(II) complexes (R2-R5) were synthesized using conventional chemical methods and their anion sensing properties were screened using several spectral (UVVis, fluorescence and 1H NMR), electrochemical and computational studies. The observed results are encouraging that these transition metal complexes sense fluoride ion selectively colorimetrically in 80% aqueous DMF solution via H-bonding interaction between the receptor and fluoride ion. To the best of our knowledge this the first such result reported in literature. An attempt was also made to utilize YES-OR(3)-INHIBIT logic gate function for the sensing of fluoride ions.

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2. Experimental Section 2.1. Chemicals All the chemicals used in the present study were of high purity analytical grade (Aldrich, India) and were used as received. Commercially available spectroscopic grade solvents (Merck, India) were used as received. The solutions of anions were prepared from their analytical grade tetrabutylammonium salts. Double distilled water was used throughout the work and the second distillation was carried out using alkaline permanganate. 2.2. Instrumentation UV-Vis spectral studies were carried out on a double beam spectrophotometer (JASCO V630, Japan). Steady state fluorescence spectra were obtained on a spectrofluorimeter (Agilent, Carry Eclipse). The excitation and emission slit widths (10 nm) and the scan rate (250 mVs-1) were kept constant for all of the experiments. Nuclear magnetic resonance spectra were recorded in DMSO-d6 (Brucker, Switzerland; 1H NMR 300 MHz,

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C NMR 75 MHz). The 1H NMR

spectral data is expressed in the form: Chemical shift in units of ppm (normalized integration, multiplicity, and the value of J in Hz). Elemental analysis for CHN was performed at CSIR-

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Central Drug Research Institute, Lucknow (EuroVector EA 3000). EPR spectra were recorded at Madurai Kamaraj University, Madurai on JEOL FA3000, X-Band Microwave spectrometer using Mn marker as the standard. FT-IR spectra were recorded in a JASCO (FT-IR 460 Plus, Japan) spectrometer. The differential pulse voltammetric (DPV) experiments, of 1 mmol solutions of the compounds, were carried out using GC as working, Pt wire as reference and Ag wire as auxiliary electrodes in DMF containing 0.1 M tetrabutylammonium perchlorate as supporting electrolyte. The geometrical optimization of the complexes was performed using Density Functional Theory with the B3LYP hybrid functional, by using a basis set of 6-31G. Computations have been performed using the Gaussian 03 Revision D.01 program package. 2.3. Synthesis of 2,3-diamino-1,4-naphthoquinone (1) 2,3-Diamino-1,4-naphthoquinone was prepared as reported earlier (Scheme 1) [27]. To a stirred solution of 2,3-dichloro-1,4-naphthoquinone (10 g, 0.045 mol) in aceotonitrile (200 mL), potassium phthalimide (16.3 g, 0.9 mol) was added. The reaction mixture was refluxed for 12 h under nitrogen atm. Afterwards the reaction mixture was cooled to RT and filtered through a filter paper and the residue was washed with water (200 mL). The yellow solid obtained was dried under vacuum. The fine yellow solid was transferred to a 500 mL round bottom flask containing 250 mL of distilled water and then 50 mL of hydrazine hydrate (90%) was added to it. The reaction mixture was heated to 60ºC for 12 h. Finally the reaction mixture was cooled to RT, filtered through a filter paper and washed with water to get the pure product as a dark blue coloured powder (6 g, Yield= 72%). 1H NMR ( 300 MHz, DMSO-d6) δppm : 5.46 (s, 4H), 7.577.61 (m, 2H), 7.74-7.77 (m, 2H) (Fig. S1). 2.4. Synthesis of receptor (R1)

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A mixture of 1 (0.5 g, 2.66 mmol) and 5-bromosalicylaldehyde (0.535 g, 2.66 mmol) in DMSO (5 mL) was heated at 90ºC with stirring for 6 h. After cooling to RT, the precipitate obtained was filtered using a filter paper and was washed with cold ethanol to obtain the pure product (0.63 g, Yield = 64%) (Scheme 2). The receptor R1 was characterized using various spectral techniques and the results are: 1H NMR (DMSO-d6, 300 MHz) (ppm) δ 13.05 (s, 1H), 8.37 (s, 1H), 8.07 (s, 2H), 7.85 (s, 2H), 7.49-7.46 (d, 1H, J=8.7 Hz), 6.97-6.95 (d, 1H, J=8.4 Hz); (Fig. S2), 13C NMR (DMSO-d6, 75 MHz) δ 179.77, 163.01, 156.24, 140.96, 135.53, 134.53, 132.21, 131.59, 127.56, 127.07, 123.33; (Fig. S3), LCMS (ESI-APCI) m/z: [M+H]+calcd for C17H9BrN2O3, 369.2, found, 370.0 (Fig. S4). 2.5. General procedures for the synthesis of metal complexes Receptor (R1) (1.35 mmol) was dissolved in 20 mL of methanol, then 1.35 mmol of chloride salt of metals dissolved in 20 mL of methanol was added drop-wise and the reaction mixture was stirred and heated at 60°C for 5 h. After completion of the reaction, the solvent was allowed to evaporate slowly to produce a solid. The resultant product was collected by filtration and washed with dichloromethane and methanol to get the pure product (Scheme 3).

3. Results and discussion The five new receptors viz. the ligand (R1), [Cu(R1)2] (R2), [Co(R1)2] (R3), [Ni(R1)2] (R4) and [Zn(R1)2] (R5) were synthesized, characterized and screened for their ability to sense anions. The ligand and its complexes were prepared chemically and characterized using conventional analytical and spectral techniques. The anion sensing behaviors of these receptors were investigated using spectral techniques such as UV-Vis, fluorescence and 1H NMR. Electrochemical and theoretical studies were also carried out to substantiate the results obtained in spectral studies.

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3.1. Characterization of metal complexes The reaction of R1 with chlorides of Cu(II), Co(II), Ni(II) and Zn(II) in methanol afforded the metal complexes as depicted in Scheme 2. The complexes were characterized using analytical and spectral techniques. The complexes were stable in air and soluble in DMF and DMSO. The analytical data of the receptor R1 and its complexes are summarized in Table S1. The elemental analysis results suggested that the general formula of the complexes is [M(R1)2] [where M = Cu(II), Co(II), Ni(II) and Zn(II)]. The molar conductivity data of 1 mM DMF solutions of the complexes suggested that they are non-electrolyte in nature. The FT-IR spectra of the complexes were compared with that of the free R1 in order to ascertain the mode of binding. The major bands observed in the FT-IR spectra of R1 and its metal complexes are listed in Table S2. The FT-IR spectrum (Fig. S5) of the free R1 exhibited a band at 3439 cm-1 assigned to ν(O-H). This band has disappeared, in all the complexes, suggesting coordination of the phenolic O-H group after deprotonation [28]. The FT-IR spectrum of R1 showed a band at 1573 cm-1, which can be attributed to ν(C=N) of the imidazole ring. The upward shift observed in this band, in the complexes, to the tune of 18-20 cm-1 indicated that R1 coordinates to the metal ions through the imidazole N-atom [17]. This is further confirmed by the downward shift occurred in the N-H stretching and the appearance of new band, in the complexes, in the region of 452-463 cm-1 due to ν(M-N) modes [29]. The bands appeared at 1606 and 1676 cm-1 correspond to ν(C=O) of the quinone ring. The appearance of two bands for the carbonyl stretching is due to the attachment two different substituents in 2- and 3-positions of the quinone ring. In the complexes, these bands remained almost unchanged revealing noninvolvement of the carboxyl oxygen atom in complexation with the metal ions [16-18].

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The electronic spectrum of the free R1 exhibited a band at 403 nm corresponds to the intramolecular charge transfer (ICT) transition from imidazole N-atoms (donor) to the electron deficient quinone (acceptor). Parallel observations were made by us in many quinone-imidazole ensembles [17,27]. In all the complexes the red-shift observed (9-40 nm) in this band suggested that the R1 might have coordinated to the metal ions through the imidazole N-atom. The UV-Vis spectrum of the copper complex (R2) showed a broad band centered around 648 nm, which can be assigned to 2B1g→2B2g transition corresponding to a square planar environment around the Cu(II) ion. The observed magnetic moment is 1.76 BM supported the electronic spectral observation [30]. The EPR spectrum of the Cu(II) complex in solid state at 298 K exhibited axial signals with two g-values, g║= 2.2251 and g= 2.0571 (Fig. S6). In square planar complexes the unpaired electron lies in dx2-y2 orbital giving 2B1g as the ground state with g║ > g. In the present case, the observed trend g║ > g> ge (2.0023) confirmed that the structure of [Cu(R1)2] is square planar [16-18]. The electronic spectrum of the Co(II) complex (R3) exhibited two bands at 603 and 672 nm assignable to 4A2g → 4 T1g(P) and 4A2g → 4T1g(F) transitions, respectively suggesting a square planar geometry for the complex. The observed magnetic moment of 2.64 BM further corroborated the electronic spectral finding [16,31]. The UV-Vis spectrum of R4, [Ni(R1)2], exhibited an absorption at 608 nm due to the 3T1(F) →3T1(P) transition for tetrahedral geometry. The observed magnetic moment of 3.98 BM is assignable to the tetrahedral geometry around Ni(II) ion [30]. The Zn(II) complex (R5) is found to be diamagnetic as expected for a d10 configuration. The shifts observed in the FT-IR bands when compared to that of the free R1 indicated the coordination of the imidazole N-atom and phenolic O-H group after deprotonation

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making up a four coordination sphere for the tetrahedral coordinated geometry around the Zn(II) ion [17,29]. The foregoing results of analytical and spectral studies indicated that the R1 coordinates to Cu(II), Co(II), Ni(II) and Zn(II) ions via imidazole N-atom and phenolic hydroxyl group after deprotonation to form four coordinated chelates with a stoichiometry of 1:2 (M:R1). The HRMS spectra of the receptors R2-R5 have confirmed the proposed formulae viz. [M(R1)2] for the metal chelates (Fig. S7).

3.2. Anion sensing 3.2.1. Visual detection First, the colorimetric recognition of various anions such as F-, Cl-, Br-, I-, AcO-, H2PO4-, S2-, CN- and NO3- as their tetrabutylammoniam salts by the receptors were investigated using visual detection experiment. The receptor R1 showed a colour change from yellow to red with fluoride ions in DMF (Fig. 1). All the other receptors R2-R5 also exhibited similar colour change upon addition of fluoride ion in DMF medium (Fig. not shown). To our delight, the receptors R2-R5 showed similar colour change selectively with fluoride ion in Water-DMF (80:20% v/v) solution (Fig. 1 and S8). These observations in the visual detection experiments indicted that these receptors are highly selective towards fluoride ion. The colour changes observed upon addition of one equivalent of fluoride ion to R1 and R5 (as representative cases) at different pH values is depicted in Figure S9. As seen from the figure that in acidic medium these receptors have failed to exhibit any colour change with fluoride ion, which may be due to the protonation of the imidazole N-atom. In basic medium the free receptors themselves showed

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colour change from yellow to red. These observations suggested that the receptors might sense fluoride ions via H-bond formation. 3.2.2. UV-Vis spectral studies To understand the mode of interaction between the receptors and fluoride ion, UV-Vis titration experiments were carried out. The electronic spectrum of R1 in DMF showed a strong absorption peak at 403 nm corresponding to the ICT transition in the molecule as spelt above. The existence of such an ICT transition would impart colour to the receptor molecule itself and any electronic perturbation in the molecule as a result of anion binding would impart striking colour change that can easily be seen visually, we suppose. In line with the presumption, the absorption at 403 nm imparts yellow colour to R1 and upon addition of fluoride ion the ICT transition peak was bathochromically shifted to 467 nm with a colour change to red (Fig. 2). Parallel electronic spectral behaviors were observed in the case of R2-R5 with fluoride ion in 80% aq. DMF solution (Table 1; Fig. 2 and S10). Such a spectral observation suggested the formation of receptor-fluoride complex, R-F- [32]. The stoichiometry of the receptor-fluoride complexes was determined using the Job’s continuous variation method and the results are shown in figure 3. It is evident from the figure that, in the case of R1 curve with a maximum at 0.5 mole fraction indicated the formation of 1:1 (R:F-) complex [33] and in the cases of R2-R5 curves with a maximum at 0.3 mole fraction suggested that the stoichiometry is 1:2 (R:F-) [34]. As seen from the figures 2 and S9, with the addition of incremental amounts of fluoride ion to R1 (in DMF) and R2-R5 (in 80% aq. DMF), the absorption maximum of the receptors (λICT) decreased gradually with a concomitant increase in the absorbance of a new peak. The large red-shift observed in λICT (50-64 nm) upon addition of fluoride ion indicated that the ICT transition is relatively easier in the receptor-fluoride complex than that in the corresponding free

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receptor. This is due to the fact that the binding of fluoride ion to the imidazole N-H moiety through H-bond would increase the electron density on the N-atom and consequently makes the ICT transition relatively easier in R-F- complex when compared to free R.17 The greatest advantages of designing molecules in which receptor unit (N-H) is directly attached to signaling unit (quinone), is that even weak H-bonding interaction would brought out large shift in the absorption maximum, as observed here [16,18,32]. The appearance of a single isosbestic point in the UV-Vis titration experiments suggested that there exists an equilibrium between only two species in the solution viz. R and R-F- [35]. 3.2.3. Fluorescence spectral studies Electronic spectral studies suggested the formation of receptor-fluoride complex via Hbonding. The binding constants of these R-F- complexes were determined using fluorescence titration experiments. The receptor R1 on excitation at 403 nm (λICT) showed an emission band at 548 nm in DMF. Upon addition of incremental amounts of fluoride ion to the solution of R1, the fluorescence of the receptor was found to be quenched progressively. All the other receptors (R2-R5) exhibited similar fluorescence quenching behavior upon addition of fluoride ion in 80% aq. DMF solution (Fig. 4). The binding constants of the receptor-fluoride complexes were computed from the fluorescence titration data using the flowing equation [36]. log (F0 - F)/F = log KA + n log[Q] Where F0 is the emission intensity in the absence of quencher (Q), F is the emission intensity at the quencher concentration [Q] and KA is the binding constant for the receptor–fluoride ion complex. In all the cases, a plot of log (F0 - F) versus log [Q] was found to be linear (r > 0.98) and the binding constants thus determined are also collected in Table 1.

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The observed magnitude of the binding constants (107 M-1) proposed a strong interaction between the receptors and fluoride ion. The results in Table 1 indicated that the binding constant for the interaction of metal chelates with fluoride ion was found to be relatively larger than that between the free ligand R1 and fluoride ion. So, coordination of transition metal ions not only accommodates 80% water into the sensing medium but also enhanced the intensity of binding between the receptor and fluoride ion. Further, it is interesting to compare the binding constants determined in the present study with those reported in literature for similar transition metal complexes. The KA (M-1) values observed in the present case in 80% aq. DMF are much higher than that reported for I (7.35x103) in 50% aq. THF [22], II (6.6x103) in acetonitrile [23] and III (2.69x106) in acetonitrile [24]. Therefore, the metal chelates under investigation are novel as they showed strong binding with fluoride ion that to in aqueous solution. The detection limits of R1 (in DMF) and R2-R5 (in 80% aq. DMF) for fluoride ions were calculated at S/N=3 as reported elsewhere [37,38]. The results collected in Table 1 indicated that these receptors can be used to detect fluoride ions at nM concentrations, which is much lower than its permissible limit set by the WHO (1.5 mg/L). 3.2.4. 1H NMR titration studies The mechanism of interaction between the receptors and fluoride ion was established by performing 1H NMR titration experiments in DMSO-d6. The 1H NMR spectra of R1 in the absence and presence of varying amounts of fluoride ion are shown in figure 5. As seen from the spectra, in the free R1 the signal corresponds to N-H and O-H protons appeared as a singlet at δ 13.054 ppm. It is evident from the 1H NMR titration upon addition of 0.5 equivalent of fluoride ion, the signal corresponds to the O-H proton disappeared and that of N-H proton experienced a downfield shift with broadening, which is also disappeared after the addition of one equivalent of

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fluoride ion. These observations indicated that the mechanism of sensing of fluoride ion by R1 involves formation of H-bonds between both these protons and fluoride ion simultaneously followed by deprotonation. Parallel observations were reported earlier during fluoride ion sensing by receptors possessing similar structural features viz. having N-H and O-H groups in close proximity [39]. Since the receptor R5 is a diamagnetic Zn(II) complex, 1H NMR titration experiments were also carried out with the addition of incremental amounts of fluoride ion to R5 in DMSO-d6 (Fig. 6). In the free R5 the imidazole N-H proton appeared at 13.508 ppm, which is slightly more acidic than that of free R1 (13.054 ppm) and thus can act as a relatively better H-bond donor towards fluoride ion. This may be due to the fact that coordination of Zn(II) ion to imidazole Natom might have removed the electron density on this N-atom, which decreased the ICT between this N-atom and quinone and consequently should have enhanced the ICT transition from the other N-atom (N-H) to quinone, making the H-atom relatively more acidic. Such a H-atom would bind to fluoride ion relatively stronger and led to larger binding constant in [M(R1)2] when compared to R1, as observed here (Table 1). As seen from the figure 6, upon addition of 0.5 equivalent of fluoride ion the signal corresponds to the N-H proton showed a downfield shift with broadening and which is also disappeared after the addition of two equivalents of fluoride ion. Thus, in the case of the Zn(II) complex also the mechanism of sensing involves formation of H-bond between the imidazole N-H moiety and fluoride ion. The proposed mechanism of sensing of fluoride ion by these receptors is depicted in Scheme 4. Further, the 1H NMR titration experiments also confirmed the stoichiometry of the receptor-fluoride ion complexes determined using Job’s continuous method. The absence of signal corresponds to phenolic O-H proton in R5

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reiterate that R1 coordinated to Zn(II) ion via phenolic O-H group after deprotonation, as discussed above. 3.2.5. Electrochemical studies With an aim to substantiate the results obtained in spectral investigations, electrochemical studies were also carried out. Such a study is feasible, in the present case, as all these receptors possess a highly redox active quinone ring in them. Also, since the receptor unit N-H is directly attached to the quinone ring, any small perturbation in the electron density upon anion binding would significantly alter the redox behavior of the electro-active quinone ring [16,17]. For comparison purpose, we need to study the electrochemical behavior of all these receptors in a common solvent and therefore Differential Pulse Voltammograms (DPV) of R1-R5 were recorded in DMF. The DP voltammograms of these receptors recorded in the absence and presence of increasing fluoride ion concentrations are shown in figure 7. As shown in the figure, the free R1 exhibited two reduction peaks which are characteristic of the electro-reduction of the quinone ring (Q). The first peak (-0.296 V) corresponds to formation of radical anion (Q•-) and the second peak at a relatively more negative potential (-0.792 V) corresponds to formation of dianion (Q2-) [40]. These peaks in [Cu(R1)2], [Co(R1)2], [Ni(R1)2] and [Zn(R1)2] have appeared at (-0.512; -1.008), (-0.537; -1.033), (-0.565; -1.056) and (-0.616; -1.115 V), respectively. It is interesting to note that, in these metal chelates both the reduction peaks appeared at relatively higher negative potentials than that in the free R1, suggesting that the electro-reduction of the quinone ring is relatively difficult in R2-R5 than in R1. This observation corroborated well with the results of electronic and 1H NMR spectral studies. That is, the slightly higher acidity of the N-H proton in the metal chelates is due to enhanced ICT transition from the N-atom to the quinone ring (leading to higher λICT), which made the electro-reduction of the quinone ring

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relatively difficult in the metal chelates. Further, the extent of shift noticed in the reduction potentials (Epc) from R1 to the metal chelates is a measure of H-bond donor ability of the N-H group towards fluoride ion. It is clear from the results that higher the ΔEpc value, higher is the KA value for the binding of the receptor with fluoride ion (Table 1). As seen from the DP voltammograms, upon addition of incremental amounts of fluoride ions, the current intensities of both the reduction peaks decreased with a concomitant shift of the peaks to relatively more negative potentials. That is the addition of fluoride ion made the electroreduction of the quinone ring relatively more difficult when compared to that in the corresponding free receptor. This is due to the fact that formation of H-bond between the imidazole N-H group and fluoride ion (followed by deprotonation) would increase the electron density on the N-atom, thereby enhanced the intensity of ICT transition and consequently made the electro-reduction of the quinone ring difficult. Similar results were reported by us in many quinone containing fluoride ion sensing receptors wherein the N-H is directly attached to the quinone ring [16-18,27]. 3.2.6. Theoretical studies Theoretical calculations, based on Density Functional Theory, were also made to examine the structural and electronic properties of the receptors (R1-R5) and their fluoride ion complexes with an aim to gain further insight into the mechanism of sensing. The structural optimization was carried out with B3LYP/6-31G basis set implanted in a Gaussian 03 program [41]. The optimized geometries along with electron density disseminations of frontier molecular orbitals are illustrated in figure 8. As seen from the figure that, in R1, LUMO is localized on the quinone moiety while HOMO is concentrated around the imidazole ring, as expected and such a charge separation is a prerequisite for a compound to be an ICT probe [32]. In these receptors the

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HOMO → LUMO transition corresponds to the ICT absorption band observed around 420 nm. The energy gap ∆E (= EHOMO - ELUMO) gives an idea about the ease with which the ICT transition occur in the receptor-fluoride ion complexes when compared to the corresponding free receptors. The results collected in Table S3 indicated that in all the cases the ∆E value of receptor-fluoride ion complex was found to be lesser than that of the corresponding free receptor suggesting relatively easier ICT transition in the R-F- complex when compared to free R [42]. These results are in accordance with those observed in the spectral studies. 3.2.7. Combinational logic circuit Recently, molecular logic gates and its integrated operations have attracted remarkable attention due to its wide applications in areas like molecular switches, keypad, devices, diagnostics, functional materials, biosensing, biochemical systems, information processing and storage. The sensitive absorbance and emission responses of the receptors motivated us to construct Boolean logic gates and arithmetic calculation at the molecular level. A simple receptor with its stimulating absorption and emission results mimics the logic operations (YES-OR(3)INHIBIT). As a representative case combinational logic circuit was constituted with anion as chemical input and UV-Vis absorption or fluorescence emission of R5 as output. Do to so, different modes were dotted when compound (R5) was treated with various anions (F-, AcO-, S2and CN-). These chemical inputs and optical outputs were implied as binary digits (0,1) which denoted as ‘off’ and ‘on’ respectively. The chemical inputs were designated as In F-, In AcO-, In S2- and In CN-. For the compound R5, the absorbance and emission were recorded at 490 and 564 nm, respectively. The absorbance threshold was set as 0.61 and fluorescence threshold was 272. When the optical output was higher than the threshold value, the output was recorded as ‘1’ (ON state) or else it was accounted as ‘0’ (OFF state). As shown in figure 9, the optical outputs

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were obtained for sixteen combinations (24=16) of four inputs (viz. In F-, In AcO-, In S2- and In CN-). In the absence of chemical input both absorbance (Output-1, λabs= 490 nm) and fluorescence (Output-2, λem= 564 nm) were accounted as ‘0’ because both absorbance band and fluorescence peak were recorded below the threshold value. However, the addition of 1 molar fluoride ion to R5 produced the absorbance band at 490 nm (output-1) via the formation of a complex between them. The output 1 provides a structure that mimicking YES logic function. On the other hand, the addition of chemical input In F- to R5 , the fluorescence intensity was quenched below the threshold level and the output -2 (λem= 564 nm) was credited as ‘0’ (i.e. OFF state). Furthermore the addition of S2-, AcO- and CN- were displayed a fluorescence peak above the threshold level and recorded as ‘1’. The fluorescence behavior of R5 mimic the three OR logic gates with INHIBIT logic function [43,44]. We have constructed a combinational logic circuit with four inputs (In F-, In AcO-, In S2and In CN-) and two outputs (Output-1, λabs= 490 nm, Output-2, λem= 564 nm) (Fig. 9). From the truth table, the addition of F- monitors the output-1 (λabs) and leads to a YES logic function. YES logic gate is a single input gate that delivers a same output given by the input. However output-2 realizes the OR logic gate with INHIBIT logic function. The other anions S2-, AcO- and CNwere obtain as input to the OR logic gate. The OR logic function is, either any one of the input is ON the output is also ON. INHIBIT gate is the combination of NOT and AND logic functions. NOT gate is also a single output gate that delivers the reversed output of a given input. AND logic gate is two input logic function, if both input is ON the output is also ON. One of the input (F-) is inverted in AND gate and other input is given from the output of OR gate. Therefore the strength of the INHIBIT gate is recognized by the output of OR gate. The concatenation of YES-

20   

OR(3)-INHIBIT combinational logic circuit has been simulated and its truth table is verified successfully (Fig. S11).

4. Practical application To explore the potential practical application of these receptors, we have prepared test kits using Whatman 40 filter paper coated with DMF solution of R5 (as a representative case) and then dried in air. As shown in figure 10, when the pale yellow coloured strip was immersed in aqueous solution of fluoride ions, an instantaneous colour change was observed. Thus, the test strips revealed the potential application of the receptor to detect fluoride ion in water. To evaluate the analytical applicability of the proposed method, an attempt was made to determine the amount of fluoride ion present in ground water samples using the receptor R5 as a representative case using fluorescence spectral data (Fig. S12). The results given in Table S4 indicated that this method is suitable for the determination of fluoride ions in the ground water samples.

5. Conclusion A quinone-imidazole ensemble and its metal chelates (Cu(II), Co(II), Ni(II) and Zn(II)) were prepared and characterized. The anion sensing properties of these receptors have been studied using UV-Vis, fluorescence and 1H NMR spectral techniques. The quinone-imidazole ensemble selectively senses fluoride ion in neat DMF, while to our delight its metal chelates do so in 80% aq. DMF solution with an instantaneous colour change from yellow to red. The fluoride ion sensing was through formation of H-bond between imidazole N-H group and fluoride ion leading to strong receptor-fluoride ion complex with binding constants in the order of 107 M-1. Coordination of metal ions with the quione-imidazole ensemble was found to

21   

increase the H-bond donor proprty of the N-H group towards fluoride ion. Electrochemical, theoretical studies and construction of logic gates strongly supported the results of the spectral investigations.

Acknowledgement The authors are highly thankful to the Council of Scientific and Industrial Research, New Delhi for financial support (CSIR No. 02(0118)/13/EMR-II).

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their Use as Selective Colorimetric Fluoride ion Sensor, Synth. React. Inorg. Met-Org and Nano-Met. Chem., 44 (2014) 1104-1119. [29] R. Bhaskar, N. Salunkhe, A. Yaul, A. Aswar,  Bivalent transition metal complexes of ONO donor hydrazone ligand: Synthesis, structural characterization and antimicrobial activity, Spectrochim. Acta A, 151 (2015) 621-627. [30] D. Arish, M. Sivasankaran Nair,  Synthesis, characterization, antimicrobial, and nuclease activity studies of some metal Schiff-base complexes, J. Coord. Chem., 63 (2010) 1619-1628. [31] E. K. Barefield, D. H. Busch, S. M. Nelson, Iron, cobalt, and nickel complexes having anomalous magnetic moments, Q. Rev. Chem. Soc., 22 (1968) 457-498. [32] A. Satheshkumar, K. P. Elango, Spectral and DFT studies on simple and selective colorimetric sensing of fluoride ions via enhanced charge transfer using a novel signaling unit, Dyes Pigments, 96 (2013) 364-371. [33] B. Vidya, G. Sivaraman, R. V. Sumesh, D. Chellappa,  Fluorescein-Based ‘‘Turn On’’ Fluorescence Detection of Zn2+ and Its Applications in Imaging of Zn2+ in Apoptotic Cells, ChemistrySelect, 1 (2016) 4024 -4029. [34] R. Pegu, R. Mandal, A. K. Guha, S. Pratihar, A selective ratiometric fluoride ion sensor with a (2, 4-dinitrophenyl) hydrazine derivative of bis (indolyl) methane and its mode of interaction, New J. Chem., 39 (2015) 5984-5990. [35] B. Vidyaa, M. Iniya, G. Sivaraman, R. V. Sumesh, D. chellappaa, Diverse Benzothiazole based chemodosimeters for the detection of cyanide in aqueous media and in HeLa cells, Sens. Actuators B, 242 (2016) 434-442.

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[36] A. Satheshkumar, E. H. El-Mossalamy, R. Manivannan, C. Parthiban, L. M. Al-Harbi, S. Kosa, K. P. Elango, Anion induced azo-hydrazone tautomerism for the selective colorimetric sensing of fluoride ion, Spectrochim. Acta A, 128 (2014) 798-805. [37] C. Parthiban, R. Manivannan, K. P. Elango, Highly selective colorimetric sensing of Hg (II) ions in aqueous medium and in the solid state via formation of a novel M-C bond, Dalton Trans., 44 (2015) 3259-3264. [38] G. Sivaraman, B. Vidya, D. Chellappa, Rhodamine based selective turn-on sensing of picric acid, RSC Adv., 4 (2014) 30828-30831. [39] P. Jayasudha, R. Manivannan, K. P. Elango, Highly selective colorimetric receptors for detection of fluoride ion in aqueous solution based on quinone-imidazole ensembleInfluence of hydroxyl group, Sens. Actuators B, 237 (2016) 230-238. [40] V. A. Nikitina, R. R. Nazmutdinov, G. A. Tsirlina, Quinones electrochemistry in roomtemperature ionic liquids, J. Phys. Chem., 115 (2011) 668-677. [41] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,M. A. Robb, J. R. Cheeseman, J. A. Jr., Montgomery, T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J.

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Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian, 03, Revision D.01, Gaussian, Inc., Wallingford CT, (2004). [42] A. Satheshkumar, K. P. Elango, Intermolecular charge transfer facilitated synthesis and spectral characterization of Schiff bases of a weak nucleophile 2, 3-diamino-1, 4naphthoquinone, RSC Adv., 3 (2013) 1502-1508. [43] C. Parthiban, K. P. Elango, Selective and sensitive colorimetric detection of Hg (II) in aqueous solution by quinone-diimidazole ensemble with mimicking YES-OR-INHIBIT logic gate operation, Sens. Actuators B, 237 (2016) 284-290. [44] C. Parthiban, K. P. Elango, Design, synthesis, characterization and cation sensing behavior of amino-naphthoquinone receptor: Selective colorimetric sensing of Cu(II) ion in nearly aqueous solution with mimicking logic gate operation, Spectrochim. Acta Part A, 174 (2017) 147-153.

Author Biographies    K.  P.  Elango  received  his  PhD  degree  in  the  area  of  Coordination  Complexes.  He  is  now  working  as  a  Professor  of  Chemistry  in  Gandhigram  Rural  Institute‐Deemed  University.  His  area  of  research  interest  includes  development  of  chemosensors  and  coordination  complexes.    C. Parthiban has obtained his Master Degree in Chemistry in 2010 and now is pursuing for  his Doctoral degree.    

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29   

Figure captions

Fig. 1. Color changes of R1 (in DMF) and R5 (in 80% aq. DMF) upon addition of various anions (5 eqv.). Fig. 2. UV-Vis absorbance changes of R1 (in DMF) and R5 (in 80% aq. DMF) upon addition of fluoride ion. [Receptor] = 6.25x10-6 M; [F-] = 0 to 6.25x10-5 M Fig. 3. Job’s plot for the receptors with F- ion. Fig. 4. Fluorescence titration curves of R1 (in DMF) and R2-R5 (in 80% aq. DMF) upon addition of fluoride ion. [Receptor] = 6.25x10-6 M; [F-] = 0 to 3.19x10-6 M Fig. 5. 1H NMR spectra of R1 with (a) 0, (b) 0.5, (c) 0.75, (d) 1 eqv. of F- in DMSO-d6. Fig. 6. 1H NMR spectra of R5 with (a) 0, (b) 0.5, (c) 1, (d) 2 eqv. of F- in DMSO-d6. Fig. 7. Change in redox properties of the receptors (1 mM) in DMF upon addition of F- ions. Fig. 8. Electron density distribution of HOMO and LUMO of the free receptors and their fluoride ion complexes. Fig. 9. (A) Truth table (B) Combinational Logic Circuit (YES-OR (3)-INHIBIT). Fig. 10. Color changes of test papers prepared using R5 in aqueous solution of F- ion. Scheme 1. Synthesis of 1 Scheme 2. Synthesis of R1 Scheme 3. Synthesis of M(II)-Complexes Scheme 4. Mechanism of sensing of fluoride ion by the receptors R1-R5.

30   

Fig. 1

31   

Fig. 2

32   

Fig. 3

33   

Fig. 4

34   

Fig. 5

35   

Fig. 6

36   

Fig. 7

37   

Fig. 8

38   

(A) Inputs In F-

In CN-

In AcO-

In S2-

0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1

0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1

0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

(B)

Fig. 9

Outputs Output-1 Output-2 (λabs= 490 nm) (λem= 564 nm) 0 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0

39   

Fig. 10

O NH

O Cl Cl O

O O

O N

O ACN, reflux, 80ºC, 12 h

O O N O O

N 2H 4 Hydrate

NH2

60ºC, 12 h

NH2 O

1

Scheme 1.

40   

Scheme 2.

O

HO N

O

MCl2

N H

Br

R1

[M(R1)2]

Methanol, 60ºC, 5 h

Where M = Co(II), Ni(II), Cu(II) and Zn(II)

Scheme 3.

Scheme 4.

41   

Table 1. Detection limit and binding constants (Ka) of the receptors and their complexes with Fions. λICT (nm) Receptor

Without With FF-

ΔλICT (nm)

Isosbestic point (nm)

λex λem (nm) (nm)

Binding Constant (K)/mol-1L

Detection Limit (nM)

R1

403

467

64

427

403

548

3.42x107

9.01

R2

412

470

58

437

412

554

4.74x107

4.43

R3

418

468

50

439

418

557

5.86x107

2.95

R4

429

484

55

451

429

560

6.49x107

1.84

R5

435

490

55

460

435

564

8.13x107

0.96