A turn-on green channel Zn²⁺ sensor and the resulting zinc(II) complex as a red channel HPO₄²⁻ ion sensor: a new approach

Abstract

A newly synthesized and spectroscopically characterized non-fluorescent organic moiety (L′H) selectively sensed Zn²⁺ ions based on a chelation-enhanced fluorescence (CHEF) process at the λ_em of 520 nm through the formation of a new dinuclear zinc(II) complex (1). Upon the addition of Zn²⁺ ions to the solution of L′H in dimethyl sulphoxide at 25 °C, a systematic enhancement of fluorescence was observed, which was not affected by the presence of competitive ions. The reaction of L′H with Zn²⁺ ions led to the formation of a dinuclear zinc(II) complex, featuring a new in situ formed macrocyclic ligand (L), which was isolated in pure form and then characterized. The formulation of 1 was established by spectroscopic tools along with a detailed structural analysis carried out using single crystal X-ray crystallography. In addition, the complex 1 also behaved as a red-shifted, HPO₄²⁻ ion-selective chemosensor at the λ_em of 595 nm based on a displacement approach in dimethyl sulphoxide. Interestingly, 1 showed remarkable sensitivity towards HPO₄²⁻ ions via fluorescence modulation of the dinuclear zinc(II) complex (1) compared with the other anions examined herein.

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Introduction

Designing highly sensitive and specific sensors for ionic species is of great interest because of biological and environmental applications of these sensors.^1–3 These ions are vital because of their sustainable role in the biological processes of living beings. Zinc ion (Zn²⁺) is the second most abundant metal ion in the human body, and the cellular biochemistry of Zn²⁺ is diverse and far ranging.⁴ Moreover, misregulation of Zn²⁺ is also implicated in human health disorders. It is believed that lack of zinc ions can result in an increased risk of several disorders, such as stunted growth, mental retardation, and digestive dysfunction, because majority of biological zinc ions are tightly sequestered by proteins.⁵ Additionally, the presence of excess free zinc in certain cells may be related to severe neurological disorders such as Alzheimer's and Parkinson's disease.^6a,b Therefore, it is necessary to obtain insight into the vital roles of Zn²⁺ ions in biological processes, resulting in great demand for the design and development of efficient systems that can selectively and sensitively detect Zn²⁺ in living systems. Several analytical methods including UV-vis-spectroscopy,^6c potentiometry,^6d and flame atomic absorption spectrometry^6e have played a vital role in the detection of Zn²⁺. A variety of fluorescent sensors for Zn²⁺ have been reported based on various fluorophores,^7–9 and continuous efforts have been dedicated to improve the effectiveness of Zn²⁺ sensors.

The development of selective receptors for phosphate anions and derivatives, such as HPO₄²⁻, H₂PO₄⁻, pyrophosphate (PPi, P₂O₇⁴⁻), adenosine triphosphate (ATP), adenosine diphosphate (ADP), CTP₃, IP₃, and phosphoproteins,¹⁰ has been of particular interest because of the vital roles that these chemical species play in a range of life processes. These processes involve essential events such as energy storage, signal transduction, and gene construction. Especially, PPi is a biologically important target because it is the product of ATP hydrolysis under cellular conditions.¹¹ Similarly, ATP is also a very important sensing target because it is known as a universal energy source and is also involved in many enzymatic reactions inside living organisms.¹¹ Therefore, the development of a quick and sensitive method for phosphate anion detection is of great importance. Recently, metal-based receptors, particularly Zn(II) complexes, for fluorescence sensing of H₂PO₄⁻ ions, pyrophosphate, and phosphate¹² have been reported; however, a dinuclear zinc(II) complex as a HPO₄²⁻ ion sensor has not been reported to date.

Due to high sensitivity, rapid response rate, and the relatively low-cost of fluorescence systems, significant attention has also been paid to developing fluorescent chemosensors capable of recognizing and sensing both cations and anions.¹³ In most cases, enhancement or the turn-on of fluorescence towards one species and the resulting complex acting as a quencher or turn-off for another species at the same emission wavelength, or vice versa, have been reported.¹⁴ In some cases, a change in solvent has played a key role in evaluating a sensor for two ions at different emission wavelengths.¹⁵ Herein, an organic moiety selectively sensed Zn²⁺ ions with green fluorescence (λ_em: 520 nm) at a concentration as low as 27.80 nM, and the resulting zinc(II) complex (1) effectively detected HPO₄²⁻ ions with red fluorescence (λ_em: 595 nm) in the same solvent, DMSO, on excitation at the same wavelength (λ_ex: 440 nm). To the best of our knowledge, to date, this type of an organic moiety and the resulting zinc(II) complex have not been explored as sensors based on the enhancement of fluorescence at different emission wavelengths through a significant amount of a red-shift of ca. 75 nm.

Herein, we present a new fluorescent and colorimetric chemosensor, 4-methyl-2,6-bis-[(5-phenyl-1H-pyrazol-3-ylimino)-methyl]-phenol (L′H), for the selective sensing of Zn²⁺ ions and the dinuclear zinc(II) complex of a new in situ formed macrocyclic type ligand (L); this complex resulted from the reaction of L′H with Zn²⁺ ions, which formulated Zn₂L (1) that is selective for HPO₄²⁻ anions in a DMSO solution. The L′H selectively senses Zn²⁺ ions based on a chelation-enhanced fluorescence (CHEF) process, whereas the structurally characterised zinc(II) complex (1) performs as a red-shifted, HPO₄²⁻ ion selective chemosensor based on a displacement approach in dimethyl sulphoxide. The complex 1 showed selective UV-vis and fluorescence responses for the biologically relevant HPO₄²⁻ anion in DMSO with a low detection limit even in the presence of several other anions.

Experimental

Synthesis of 4-methyl-2,6-bis-[(5-phenyl-1H-pyrazol-3-ylimino)-methyl]-phenol (L′H)

A solution of 3-amino-5-phenylpyrazole (1.59 g, 10.0 mmol) in absolute methanol (10 mL) was added dropwise to a stirred solution of 2,6-diformyl-4-methyl-phenol (0.82 g, 5.0 mmol) in the same solvent (20 mL). The resulting solution was gently refluxed under magnetic stirring for ca. 6 h. The yellow powder precipitate that formed was filtered off, washed several times with absolute methanol, dried under vacuum, and characterized as compound L′H.

L′H. C₂₇H₂₂N₆O. Calcd C, 72.63; H, 4.97; N, 18.82; O, 3.58; anal. found: C 72.34, H 4.39, N 18.95. Mp 292 ± 2 °C. ESI-MS: [L + H⁺], m/z, 447.1939 (obs. with 100% abundance) [(calcd: m/z, 447.198; where L′H = molecular weight of organic moiety (L′H))]; ¹H NMR (DMSO-d₆, 400 MHz, δ ppm): 2.32 (s, 3H), 6.72–6.77 (s, 2H), 7.23–7.43 (m, 8H), 7.73–7.96 (m, 4H), 9.18 (s, 2H), 10.40 (s, 1H) and 13.42 (broad, 2H). ¹³C NMR (DMSO-d₆): 162.87, 159.86, 150.05, 144.39, 140.03, 137.83, 129.85, 129.48, 125.98, 124.18, 121.27, 94.91 and 20.87. IR (KBr, cm⁻¹): ν_OH: 3411.46, ν_CH=N: 1561.08, ν_NH: 1455.53, 1426.74. Yield, 97%.

Synthesis of dinuclear zinc(II) complex (1)

To the dimethylformamide (DMF) solution of L′H (446.0 mg, 1.0 mmol), a water solution of zinc nitrate hexahydrate (149.00 mg, 0.50 mmol) was very slowly added under vigorous stirring. Then, the reaction mixture was stirred at ambient temperature for another 6.0 h. The resulting solution was kept aside for slow evaporation at room temperature. After a few days, a red-coloured crystalline solid was obtained. Single crystals suitable for X-ray analysis were obtained from the solution of the crude product in DMF–H₂O on slow evaporation.

[Zn₂L]·3H₂O. C₉₀H₇₂N₁₈O₇Zn₂. Anal. found: C, 66.15; H, 4.53; N, 15.76; calc.: C, 66.30; H, 4.33; N, 15.46. IR (cm⁻¹): ν_NH = 3427.45; ν_CH=N = 1577.07. ¹H NMR (500 MHz, DMSO-d₆): 10.18 (s, 1H); 8.85 (d, 1H; j = 50.5 Hz); 8.70 (s, 3H); 7.48 (m, 9H); 7.40 (m, 11H); 7.20 (m, 24H); 6.88 (b, 3H); 6.55 (s, 2H); 5.76 (s, 1H); 5.56 (b, 3H); 2.27 (s, 8H). ESI-MS (in methanol): [M + 2H]²⁺, m/z, 798.12 (100%) [(calcd: m/z, 798.21; where M = molecular weight of C₉₀H₆₆N₁₈O₄Zn₂)] yield, 63%.

X-ray crystallography

Data collection of compound 1 was performed at the X-ray diffraction beamline (XRD1) of the Elettra Sincrotrone Trieste (Italy), with a Pilatus 2M image plate detector. The complete dataset was obtained at 100 K (nitrogen stream supplied by an Oxford Cryostream 700) with a monochromatic wavelength of 0.700 Å through the rotating crystal method. The crystals were dipped in N-paratone and mounted on the goniometer head with a nylon loop. The diffraction data were indexed, integrated, and scaled using XDS.¹⁶ The structure was solved by direct methods using SIR2014 ¹⁷ and subsequent Fourier analysis, and refinement with the full-matrix least-squares method based on F² was performed with SHELXL.¹⁸ Anisotropic thermal motion was allowed for all the non-H atoms. Hydrogen atoms were placed at the calculated positions, except those of the coordinated water molecules. The void in the unit cell (18.5%) was taken into account using the Squeeze program (Platon) since the solvent molecules could not be suitably modelled and refined. Graphics were drawn using the Cameron program.¹⁹ The dinuclear zinc(II) complex (1) crystallized in the monoclinic space group Pc. Crystal data and details are tabulated in Table S1,† and some selected bond distances and angles are listed in Table S2.†

General method of UV-vis and fluorescence titration

For UV-vis and fluorescence titrations, stock solutions of L′H and dinuclear zinc(II) complex (1) were prepared in DMSO and a stock solution of Zn²⁺ and HPO₄²⁻ was prepared in HPLC water at 25 °C. Fluorescence measurements were carried out using a 2.5 nm x 2.5 nm slit width. To the 100 μL (5 μM) stock solution of L′H, 0–300 μL (0–15 μM) Zn²⁺ solution was gradually added, and the total volume was adjusted to 2.0 mL by adding the required amount of DMSO to obtain the absorbance and fluorescence spectra. Similarly, a HPO₄²⁻ solution (0–150 μL) (0–75 μM) was gradually added to the 50 μL (25 μM) stock solution of the dinuclear zinc(II) complex (1), and the total volume was adjusted to 2 mL by the addition of DMSO. All the spectral data were acquired after 10 minutes of mixing to obtain the optimized spectra.

Result and discussion

Synthesis and structural characterisation

The organic moiety 4-methyl-2,6-bis-[(5-phenyl-1H-pyrazol-3-ylimino)-methyl]-phenol (L′H) was synthesized by condensing a methanolic solution of 3-amino-5-phenylpyrazole with 2,6-diformyl-4-methyl-phenol in a 2 : 1 ratio (Scheme 1). The formulation of L′H, as shown in Scheme 1, was established by physicochemical and spectroscopic tools. The peaks obtained in the ¹H NMR spectrum of L′H were assigned, and these were in agreement with the structural formula of this ligand in solution (Fig. S1†). The ESI mass spectrum of the compound in methanol showed a peak at m/z 447.1939 with 100% abundance assignable to [L′H + H⁺] (calculated value at m/z 447.198), where L′H = molecular weight of the organic moiety (Fig. S2†). The IR spectrum of L′H showed the characteristic stretching of O–H, N–H, and C=N bonds (Fig. S3†), and the characteristic peaks were observed in the ¹³C NMR spectrum (Fig. S4†).

Scheme 1 Synthetic strategy of L′H and Zn₂L-complex (1).

The dinuclear zinc(II) complex (1) was isolated in the solid state from the reaction mixture of L′H with zinc nitrate hexahydrate in a 2 : 1 molar ratio in a DMF/water medium at room temperature under stirring condition. Upon reaction of L′H with Zn²⁺ ions, a new in situ macrocyclic type ligand (L) was formed and red-colored single crystals of Zn₂L·H₂O were obtained on slow evaporation. The well-resolved ESI mass spectrum (Fig. S5†), IR spectrum (Fig. S6†), and ¹H NMR spectrum (Fig. S7†) of the dinuclear zinc(II) complex (1) support the formulation and the structure established by single crystal X-ray crystallographic analysis. Of the two zinc ions, Zn1 exhibited an octahedral coordination sphere realized through two phenolato oxygen and two imine nitrogen atoms, completing its coordination geometry with two aqua ligands. Zn2 presented a highly distorted octahedral environment by being chelated by two oxygen and two imine nitrogen donors (Fig. 1).

Fig. 1 ORTEP drawing (ellipsoids at 30% probability) of the dinuclear complex Zn₂L (H atoms omitted for clarity).

UV-vis spectroscopy studies of L′H

The UV-vis absorption spectrum of L′H obtained in DMSO solution exhibited intense absorption bands at energy levels below 400 nm, corresponding to π → π* (275 nm, ε = 2.1 × 10⁵ L mol⁻¹ cm⁻¹; 315 nm, ε = 1.61 × 10⁵ L mol⁻¹ cm⁻¹) and n → π* (380 nm, ε = 2.4 × 10⁴ L mol⁻¹ cm⁻¹) transitions. To evaluate the binding affinity of L′H towards Zn²⁺ ions, the UV-vis spectral changes upon the addition of Zn²⁺ ions to the solution of L′H in DMSO were investigated. The incremental addition of Zn²⁺ ions led to a gradual decrease in the absorbance along with the appearance of a lower-energy metal-to-ligand charge-transfer (MLCT) band at around 440 nm. The presence of an isosbestic point at 410 nm indicated the existence of an equilibrium between two species: L′H and the resulting dinuclear zinc(II) complex (1) (Fig. 2).

Fig. 2 UV-vis titration spectra of L′H (5 μM) with Zn²⁺ ions (0–15 μM) in DMSO solution at 25 °C with naked eye visual color change of L′H only and Zn(II) complex (1) [in inset].

To verify the potential of the probe L′H in the intracellular atmosphere, another inspection of the probe was carried out to check the interferences of other competitive ions. Metal ion selectivity assays were performed without changing other experimental conditions. On the addition of an excess of 10 equivalents of relevant metal ions (i.e., Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, and Pb²⁺), no significant change was observed in the UV-vis spectral pattern.

Fluorescence studies of L′H

The organic probe L′H exhibited a very feeble emission intensity at 520 nm (λ_ex: 440 nm). Upon incremental addition of Zn²⁺ ions (0–15 μM), the fluorescence intensity steadily and significantly increased by about ∼11-fold at 520 nm (Fig. 3). This spectral characteristic for the addition of Zn²⁺ ions was supported by the shift from colourless to fluorescent green. To check the saturation time of the resulting emission, the time-dependent profile for the fluorescent detection of Zn²⁺ ions (Fig. S8†) was also obtained under the same conditions, and it was determined to be 15 min. From the fluorimetric titration curve, a plot of the fluorescence intensity vs. concentration of Zn²⁺ ions is depicted in Fig. S9.†

Fig. 3 Fluorescence titration of L′H (5 μM) with the incremental addition of Zn²⁺ ions (0–15 μM) DMSO solution with naked eye fluorescence color change L′H only and zinc(II) complex (1) [in inset].

The emission intensity increased due to addition of Zn²⁺ ions (0–15 μM) to the solution of L′H (5 μM) with an enhancement of the fluorescence quantum yield²⁰ by ca. three times (Φ = 0.157) in DMSO, which was calculated by integrating the area under the fluorescence curves using the following equation:

where A is the area under the fluorescence spectral curve, OD is the optical density of the compound at the excitation wavelength 440 nm, and η is the refractive index of the solvent used. The standard used for the measurement of fluorescence quantum yield was fluorescein (Φ = 0.79 in a 0.1 M NaOH solution).

Selectivity

The selectivity study of the organic moiety L′H towards various cations was carried out using transition metal ions (Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Hg²⁺), alkali and alkaline earth metal ions (Na⁺, K⁺, Ca²⁺, and Mg²⁺), and some other metal ions (Al³⁺ and Pb²⁺) (Fig. S10†), having a concentration 50 times that of the ligand. The organic moiety (L′H) had a remarkable specificity and selectivity towards Zn²⁺ ions over the other metal ions examined herein. Interestingly, the introduction of other metal ions caused the emission intensity to be either weakened or unchanged. In the presence of a 10 equivalent excess of various tested ions together with L′H and Zn²⁺ ions, no adverse effect on the intensity was observed (Fig. S11†).

Job's plot from fluorescence experiments

A series of solutions containing L′H and Zn²⁺ ions were prepared such that the total concentration of the sample solution remained constant (20 μM) in all the sets. The mole fractions of Zn²⁺ ions were varied from 0.1 to 0.8. The emission intensity (for L′H) at 520 nm was plotted against the mole fraction of [Zn²⁺] ions. From a Job's plot analysis (Fig. S12†), it was revealed that the maximum emission was at 2 : 1 ratio. These data indicate that the complex species in solution should form a 2 : 1 complex with Zn²⁺ ions, in accordance with the single crystal observations.

Effect of pH

A pH study was performed for the probe L′H in the absence and presence of Zn²⁺ ions over the pH range of 4.0–11.0 (Fig. S13†).

Detection limit calculation

The detection limit was calculated from the calibration curve based on the fluorescence enhancement at 520 nm (Fig. 4) focusing on the lower concentration region of Zn²⁺ ions. From the slope of the curve (S) and the standard deviation of seven replicate measurements of the zero level (σ_zero), the detection limit was estimated using the equation 3σ/S.²¹ The data obtained from this graph indicated that this probe can effectively detect Zn²⁺ ion at very low concentrations up to 27.80 nM.

Fig. 4 Calibration curve in the nanomolar range to calculate the LOD of Zn²⁺ ions by L′H in DMSO solution.

Spectroscopic studies of dinuclear zinc(II) complex (1)

Absorption study. The UV-vis absorption spectrum of 1, obtained in the DMSO solution, exhibited intense absorption-bands at the energy levels at 440 nm due to metal-to-ligand charge-transfer (MLCT) transitions. To find the binding affinity of 1 towards HPO₄²⁻, the spectral changes by the addition of HPO₄²⁻ ions to the DMSO solution of 1 were investigated. The incremental addition of HPO₄²⁻ ions to 1 led to a decrease in the intensity of the band at 432 nm with an appearance of new bands at around 315, 365, and 495 nm (Fig. 5).

Fig. 5 UV-vis titration spectra of 1 (25 μM) with HPO₄²⁻ ions (0–75 μM) in DMSO solution at 25 °C. Naked eye visual color change of 1 only and 1 in presence of HPO₄²⁻ ions [inset].

Emission study. The displacement of Zn²⁺ ions from the complex (1) by HPO₄²⁻ ions was studied from the emission spectral change of the titration curve, as shown in Fig. 6. For the emission study, DMSO solution was continuously titrated with an increasing concentration of HPO₄²⁻ ions. The spectra show the generation of a red-shifted emission band with a maximum at ∼595 nm. The continuous increase in the HPO₄²⁻ ions concentration (Fig. S14†) led to a concomitant enhancement of the emission band with a maximum at ∼595 nm. The resultant solution was kept undisturbed for 15 min, and then, the emission spectrum was obtained considering the time-dependent profile for the fluorescent detection of HPO₄²⁻ ions, as shown in Fig. S15.† The emission intensity increased due to addition of HPO₄²⁻ ions (0–75 μM) to the solution of 1 (25 μM), with the enhancement of fluorescence quantum yield by ca. two times (Φ = 0.3421) in DMSO, which was calculated by integrating the area under the fluorescence curves.

Fig. 6 Fluorescence titration of 1 (25 μM) with the incremental addition of HPO₄²⁻ ions (0, 2.5, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, and 75 μM) in DMSO solution (λ_ex = 440 nm). Naked eye fluorescence color change 1 only and 1 in presence of HPO₄²⁻ ions [inset].

To examine the efficacy of the resulting dinuclear zinc(II) complex (1), herein, the rupture of the dinuclear zinc(II) complex (1) to restore L′H and the subsequent change in the spectral properties of the dinuclear zinc(II) complex (1) in DMSO were studied towards different anions (SCN⁻, PO₄³⁻, MO₄²⁻, CN⁻, F⁻, Cl⁻, Br⁻, I⁻, HAsO₄²⁻, AcO⁻, HPO₄²⁻, H₂AsO₄⁻, SO₄²⁻, H₂PO₄⁻, HSO₄⁻, NO₃⁻, CO₃²⁻, HCO₃⁻, ClO₄⁻, and P₂O₇⁴⁻ (1 equiv., as sodium/potassium salts)). The binding studies exhibited a fast, sensitive, and distinct response towards HPO₄²⁻ ions in comparison to the other anions (Fig. S16†). In the fluorescence spectrum, upon addition of HPO₄²⁻ ions, the emission band at 520 nm was quenched, and a new red-shifted band at 595 nm appeared. This result indicated the weakening of the existing dinuclear zinc(II) complex (1) bond due to the interaction of HPO₄²⁻ ions with Zn²⁺ ions to form Zn(HPO₄) species, insoluble in this medium. Almost no interference was observed for the detection of HPO₄²⁻ anions even in the presence of other anions, having 50 times higher concentration (Fig. S16†).

The detection limit was calculated from the calibration curve based on the fluorescence enhancement at 595 nm (Fig. S17†) focusing on the lower concentration region of HPO₄²⁻ anions. From the slope of the curve (S) and the standard deviation of seven replicate measurements of the zero level (σ_zero), the detection limit was estimated using the equation 3σ/S.²¹ The data obtained from this graph indicates that this probe can effectively detect HPO₄²⁻ anions at very low concentrations up to 3.12 × 10⁻⁷ M.

The emission spectra of L′H, L′H + Zn²⁺ ion (1), and 1 + HPO₄²⁻ ions were also obtained with the increasing solvent polarity, as shown in Fig. S18.† From this study, it was revealed that the polarity of the solvent had almost no effect on the emission wavelength (λ_em) but had an effect on the emission intensity, which was highest in DMSO in both cases.

The turn on green fluorescence (λ_em = 520 nm) was due to the formation of a new dinuclear zinc(II) complex (1), which was based on chelation-enhanced fluorescence (CHEF) process, and the fluorescence arose due to the formation of a dinuclear zinc(II) complex of a newly in situ formed macrocyclic ligand (L) during the reaction of L′H with Zn²⁺ ions.

The displacement mechanistic pathway in the complex (1) by hydrogen phosphate anion was established by UV-vis and fluorescence spectra (Fig. S19†), ESI mass spectra (Fig. S20†), and ¹H NMR spectra (Fig. S21†). We obtained a new absorption peak at ca. 495 nm due to the addition of HPO₄²⁻ anions to the zinc(II) complex (1); moreover, a similar type of peak was obtained at ca. 495 nm in the case of L′H in the presence of a base, a signature of the anionic species L′ (viz. Fig. S13†) formed with a high fluorescence quantum yield (Φ = 0.300). The findings from these studies indicate that the appearance of the red fluorescence (λ_em = 595 nm) of 1 upon the gradual addition of HPO₄²⁻ anions was due to the formation of a deprotonated form of L′H. On the basis of these spectroscopic observations, a plausible sensing pathway is shown in Scheme 2.

Scheme 2 Probable mechanism for Zn²⁺ sensing by L′H and HPO₄²⁻ ions sensing by the Zn(II) complex (1).

TCSPC experiment

To strengthen the proposed mechanism, the fluorescence average life time measurement of 1 was performed, and it was found to be 0.56 ns at λ_em = 595 nm. The average lifetime of the formed species, due to the addition of HPO₄²⁻ ions to the solution of 1, increased to 2.25 ns at λ_em = 595 nm (viz. 1 : HPO₄²⁻; 1 : 1 in Fig. 7). According to the equations:²² τ⁻¹ = k_r + k_nr and k_r = Φ_f/τ, the radiative rate constant k_r and total nonradiative rate constant k_nr of dinuclear zinc(II) complex (1) in the absence and presence of HPO₄²⁻ ions are listed in Table S3.† The data suggests that the k_r/k_nr ratio was reasonably enhanced due to the reasonable decrease of k_nr in support of the increased in fluorescence in the presence of HPO₄²⁻ ions at λ_em = 595 nm.

Fig. 7 Time-resolved fluorescence decay of dinuclear zinc(II) complex (1) and 1 in the presence of HPO₄²⁻ ions (at λ_em = 595 nm) in DMSO using a nano-LED at 470 nm as the light source.

Conclusions

A new non-fluorescent Schiff base-type organic probe (L′H) was synthesized and spectroscopically characterised. L′H selectively sensed Zn²⁺ ions based on a chelation-enhanced fluorescence (CHEF) process at a λ_em of 520 nm (green fluorescence) through the formation of a new dinuclear zinc(II) complex (1) with almost no interference of competitive ions. A red-colored solid dinuclear zinc(II) complex of a new in situ formed macrocyclic-type ligand (L), formulated as Zn₂L, (1) was isolated from the reaction mixture of L′H with Zn²⁺ ions and was characterized by spectroscopic tools along with detailed structural analyses using single crystal X-ray crystallography. Interestingly, complex (1) also selectively sensed HPO₄²⁻ anions with a red fluorescence (λ_em: 595 nm) in the same solvent, DMSO, on excitation at the same wavelength (λ_ex: 440 nm).

Acknowledgements

The financial assistance received from Department of Science and Technology, Govt. of West Bengal (DST, GoWB, vide project no. 698 (Sanc.)/ST/P/S & T/15-G/2015) is gratefully acknowledged. S. Lohar is thankful to UGC, New Delhi, India for the fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials and physical methods, tables, figures, characterization data, and some spectra. CCDC 1528839. CCDC no. for dinuclear zinc(II) complex (1). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7ra02175e

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