Selective sensing of Al³⁺ by naphthyridine coupled rhodamine chemosensors

Abstract

Napthyridine-based rhodamine chemosensors 1 and 2 are designed and synthesized. Both the chemosensors selectively recognize Al³⁺ ion over a series of other cations in CH₃CN–H₂O (4 : 1, v/v, 10 μM Tris–HCl buffer, pH = 7.0) by exhibiting change in color (colorless to pink) and emission at 588 nm in the turned on mode. Complexation is reversible and the ensembles of 1·Al³⁺ and 2·Al³⁺ selectively sense F⁻ ion by discharging the pink color of the solutions and bringing a reverse change in both absorption and emission spectra.

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Introduction

Aluminium ion is the third most abundant metal ion found in nature and exists in its ionic form Al³⁺ ion.¹ It is extensively used in modern life such as food packaging, cookware, drinking water supplies, antiperspirants, deodorants, antacids and manufacturing of cars and computers. Aluminium ion enters the human body through food and water.² The average daily intake of Al³⁺ ions for human body is about 3–10 mg per day.³ Excess amount of Al³⁺ is neurotoxic to humans and can cause a wide range of diseases such as Alzheimer's disease, Parkinson's disease, osteoporesis, etc.⁴ In addition, an incremental increase of Al³⁺ in the environment is detrimental to growing plants. Due to poor coordinating ability of Al³⁺ ion, its detection by highly selective and sensitive chemosensors is of great interest for human health and the environment.

In continuation of our work on the sensing of biologically and environmentally important cations,⁵ we report two naphthyridine based receptor modules 1 ⁶ and 2 in this work. Both the receptors, 1 and 2, show high selectivity towards Al³⁺ ion in CH₃CN–H₂O (4 : 1, v/v, 10 μM Tris–HCl buffer, pH = 7.0) by exhibiting color and emission changes.

Rhodamine B and its derivatives (RhB) show good photo stability, high extinction coefficients (>75 000 cm⁻¹ M⁻¹), and high fluorescence quantum yields.⁷ The performance of this moiety in sensing metal ion is linked with the switching of the spirocyclic form (which is colorless and non-fluorescent) to the opened ring amide form which is pink in color and strongly fluorescent. Metal ion recognition using rhodamine labeled chemosensors is recently thoroughly reviewed.⁸ A small number of rhodamine labeled chemosensors, which show color change and selective “turn-on” emission to Al³⁺ ions is reported in the literature.⁹ In the design, 1,8-naphthyridine has been exploited because of its intriguing structure and bonding properties. It is a well established motif and is applied in coordination chemistry,¹⁰ pharmaceutical¹¹ and molecular recognition fields.¹²

Results and discussion

The syntheses of chemosensors 1 and 2 were accomplished according to the Scheme 1. During the course of our study on 1 in cation recognition, Kong et al., recently reported the synthesis of 1 and its anion sensing behavior.⁶ However, in our method, initially rhodamine 3 was converted to the acid chloride 4, which was then reacted with 2-amino-7-methyl-1,8-naphthyridine¹³ in the presence of Et₃N in dry CH₂Cl₂ to afford the desired compound 1 in appreciable yields. Following the same strategy, compound 2 was prepared by coupling the acid chloride 4 with compound 5 in dry CH₂Cl₂. The intermediate compound 5 ¹⁴ was obtained according to our reported procedure. All the compounds were fully characterized by ¹H, ¹³C, FTIR, and mass analyses.

Scheme 1 (i) POCl₃, ClCH₂CH₂Cl, reflux, 2 h; (ii) 2-amino-7-methyl-1,8-naphthyridine, Et₃N, dry CH₂Cl₂, 10 h; (iii) 5, Et₃N, dry CH₂Cl₂, 12 h.

The metal ion sensing properties of 1 and 2 towards metal ions such as Al³⁺, Cr³⁺, Fe³⁺, Hg²⁺, Co²⁺, Pb²⁺, Cu²⁺, Ni²⁺, Mg²⁺, Zn²⁺, Cd²⁺ and Ag⁺ were evaluated in CH₃CN–H₂O (CH₃CN–H₂O = 4 : 1, v/v,10 μM Tris–HCl buffer, pH = 7.0). Without metal ions, both the chemosensors 1 and 2 were almost non-fluorescent. However, on excitation either at 480 nm (Fig. S1 and S2, ESI†) or at 510 nm, both 1 and 2 showed emission at 588 nm with significant intensity upon interaction with Al³⁺ ions (Fig. 1). Other cations in the study brought insignificant change in emission at this wavelength for sensors 1 and 2 (Fig. S1 and S2, ESI†). Upon interaction with Al³⁺, the colorless solutions of 1 and 2 turned pink in color. In fact, addition of a small amount of Al³⁺ to the solution of 2 caused greater change in emission than with 1. This indicated better sensitivity of 2 than 1 in Al³⁺ detection.

Fig. 1 Fluorescence titration spectra of (a) 1 (c = 3 × 10⁻⁵ M) and (b) 2 (c = 3 × 10⁻⁵ M) in CH₃CN–water (4/1, v/v; 10 μM Tris–HCl, pH = 7.0) upon addition of Al₂(SO₄)₃·18H₂O (c = 1.2 × 10⁻³ M). Insets represent the color changes of the receptor solutions under UV light.

Fig. 2 shows the change in fluorescence ratios [(I − I₀)/I₀] of 1 and 2 at 588 nm in the presence of 16 and 9 equiv. amounts of different metal ions, respectively.

Fig. 2 Fluorescence ratios [(I − I₀)/I₀] at 588 nm of (a) 1 (c = 1.5 × 10⁻⁵ M) and (b) 2 (c = 1.5 × 10⁻⁵ M) in CH₃CN–water (4/1, v/v; 10 μM Tris–HCl buffer, pH 7.0) upon the addition of 16 and 9 equiv. amounts of metal ions, respectively.

UV-vis titration of 1 (c = 3 × 10⁻⁵ M) in CH₃CN–H₂O (4/1, v/v; 10 μM Tris–HCl buffer; pH = 7.0) with Al³⁺ resulted in a sharp ratiometric change in absorbance. The absorption at 274 nm and 322 nm gradually decreased along with the appearance of a new peak at 559 nm (Fig. 3a). Other metal ions considered in the study did not produce such a change in absorbance (Fig. S3, ESI†). A similar type of spectral behaviour was observed in the case of 2 (Fig. 3b). Like the case of 1, other metal ions did not cause much change in the absorbance of 2 (Fig. S4, ESI†).

Fig. 3 (a) UV-vis titration spectra of 1 (c = 3 × 10⁻⁵ M) in CH₃CN–H₂O (4/1, v/v; 10 μM Tris–HCl buffer; pH 7.0) upon the addition of Al₂(SO₄)₃·18H₂O (c = 1.2 × 10⁻³ M). Inset: color change upon the addition of Al³⁺ ions (c = 1.2 × 10⁻³ M) and (b) UV-vis titration spectra of 2 (c = 3 × 10⁻⁵ M) in CH₃CN–H₂O (4/1, v/v; 10 μM Tris–HCl buffer; pH 7.0) upon the addition of Al₂(SO₄)₃·18H₂O (c = 1.2 × 10⁻³ M). Inset: color change upon the addition of Al³⁺ ions (c = 1.2 × 10⁻³ M).

Indeed, the appearance of the peak at 559 nm in UV-vis and 588 nm is due to the opening of spirolactam rings in 1 and 2 to form metal chelated equilibrium structures 1A and 2A, respectively (Fig. 4). The amide carbonyl stretching at 1705 cm⁻¹ in 1 was observed at 1658 cm⁻¹ upon the complexation of Al³⁺. Similarly, the carbonyl stretching at 1696 cm⁻¹ in 2 moved to 1648 cm⁻¹ upon complexation. This significant reduction in carbonyl stretching substantiated the lactam ring opening during the interaction. Thus, napthyridine and lactam anion in 1 participate in the chelation of Al³⁺ ion. In 2, the contribution of the extra pyridine ring, attached to naphthyridine, is noteworthy for its strong complexation and greater sensitivity towards Al³⁺ ion. DFT¹⁵ optimized geometry in Fig. 5 clearly depicts the participation of naphthyridine, pyridine and the lactam moieties in the complexation of Al³⁺ ion.

Fig. 4 Suggested metal chelated structures.

Fig. 5 DFT optimized geometries (using b3lyp functional and 6-31G basis set) of (a) 2 and (b) its complex with Al³⁺.

To further substantiate the binding, ¹H NMR spectra of both 1 and 2 in the absence and presence of Al³⁺ ion were recorded. As can be seen in Fig. 6, the naphthyridine ring protons in both cases exhibited a downfield chemical shift on the complexation of Al³⁺ ion. In addition, the signal for the –CH₂– group, adjacent to the pyridine ring, moved downfield and supported the additional involvement of the pyridine ring in complexation. The change in chemical shift values has been shown in the caption of Fig. 6.

Fig. 6 (A) ¹H NMR (d₆-DMSO, 400 MHz) of (i) 1 (c = 8.6 × 10⁻³ M), (ii) 1 with 1 equiv. amount of Al₂(SO₄)₃·18H₂O (Δδ in ppm: H_a = 0.30, H_b = 0.26, H_c = 0.02, H_d = 0.05) and (B) ¹H NMR (d₆-DMSO, 400 MHz) of (i) 2 (c = 8.9 × 10⁻³ M), (ii) 2 with 1 equiv. amount of Al₂(SO₄)₃·18H₂O (Δδ in ppm: H_a = 0.05, H_b = 0.05, H_c = 0.06, H_d = 0.10, H_e = 0.05, H_f = 0.06) [for the identification of protons see the labeled structures in Fig. 4].

In the interactions, the stoichiometries¹⁶ of 1 and 2 were evaluated to be 1 : 1 (Fig. S5, ESI†), and the binding constant¹⁷ values were determined to be (5.39 ± 0.01) × 10³ M⁻¹ and (6.5 ± 0.009) × 10³ M⁻¹ for 1 and 2, respectively (Fig. S6, ESI†). Due to poor change in emission spectra, we were unable to determine the binding constant values for other metal ions. However, the binding constant values for Al³⁺ ion indicate that the contribution of pyridine in 2 to the stability of the metal complex is not significantly high. But in practice, the sensitivity level of 2 is greater than 1. The analysis of the fluorescence titration data provided the detection limits¹⁸ of 2.94 × 10⁻⁵ M and 1.38 × 10⁻⁵ M for 1 and 2, respectively (Fig. S7, ESI†).

To understand the selectivity in the sensing process, the change in the emission of 1 was observed in the presence and absence of other metal ions. Any metal ion considered in the study did not interfere in the binding of Al³⁺ (Fig. 7a). Similar results were observed for 2 (Fig. 7b). It is important to mention that chemosensors 1 and 2 gave the same results with AlCl₃ and K₂SO₄·Al₂(SO₄)₃·24H₂O under similar conditions (Fig. S8, ESI†), and thereby ruled out the possibility of any role of the anionic part of the aluminium salts in the sensing process.

Fig. 7 Fluorescence responses of (a) 1 (c = 3 × 10⁻⁵ M) and (b) 2 (c = 3 × 10⁻⁵ M) towards Al³⁺ (c = 1.2 × 10⁻³ M) over the selected metal ions (c = 1.2 × 10⁻³ M).

To further check the reversibility in complexation, a halide addition experiment was performed. The addition of F⁻ to the ensembles of 1 and 2 with Al³⁺ brought about a reverse change in the absorption as well as in the emission spectra (Fig. 8). In contrast, other halides considered in the study were unable to decomplex Al³⁺ ions from the complexes 1A and 2A. The stronger affinity of F⁻ towards Al³⁺ ions caused complete decomplexation and retrieved the spiro lactam rings of 1 and 2, and the pink color of the ensembles was completely discharged (Fig. 9). Thus, the ensembles are useful in the selective recognition of F⁻ over other halides. Fluoride recognition is important in supramolecular chemistry research due to its biological significance.¹⁹

Fig. 8 Changes in (a) absorbance and (b) emission of 1-Al³⁺ complex in CH₃CN–H₂O (4/1, v/v; 10 μM Tris–HCl buffer; pH 7.0) upon the addition of different halides (c = 3 × 10⁻³ M); changes in (c) absorbance and (b) emission of 2-Al³⁺ complex in CH₃CN–H₂O (4/1, v/v; 10 μM Tris–HCl buffer; pH 7.0) upon the addition of different halides (c = 3 × 10⁻³ M).

Fig. 9 (A) Color changes of ensemble 1·Al³⁺ in CH₃CN–H₂O (4/1, v/v; 10 μM Tris–HCl buffer; pH 7.0), after the addition of 250 μL (c = 3 × 10⁻³ M) of each halide solution: (a) 1·Al³⁺ ensemble, (b) 1·Al³⁺ + F⁻, (c) 1·Al³⁺ + Cl⁻, (d) 1·Al³⁺ + Br⁻ and (e) 1·Al³⁺ + I⁻, (B) color change of ensemble 2·Al³⁺ in CH₃CN–H₂O (4/1, v/v; 10 μM Tris–HCl buffer; pH 7.0), after the addition of 200 μL (c = 3 × 10⁻³ M) of each halide solution: (a) 2·Al³⁺ ensemble, (b) 2·Al³⁺ + F⁻, (c) 2·Al³⁺ + Cl⁻, (d) 2·Al³⁺ + Br⁻ and (e) 2·Al³⁺ + I⁻.

Conclusion

In conclusion, naphthyridine-based rhodamine sensors 1 and 2 selectively and effectively recognize Al³⁺ ion over a series of other cations in aqueous CH₃CN at pH 7.0 by exhibiting a sharp change in color and emission in the turn on mode. The ensembles of the sensors 1 and 2 with Al³⁺ further recognize F⁻ ions selectively through a change in color (pink to colorless) as well as emission in the turn off mode. The chemosensors reported in this account are the new addendum to the existing few reports on aluminium sensor in the literature.⁹

Experimental

Synthesis

Compound 1. To a stirred solution of rhodamine B (0.15 g, 0.313 mmol) in 1,2-dichloroethane (10 mL), phosphorus oxychloride (300 μL) was added dropwise at room temperature. The resulting solution was refluxed for 2 h. The reaction mixture was cooled to room temperature and the excess solvent was evaporated off in vacuo to give rhodamine B acid chloride 4, which was impure and directly used in the next step. The crude acid chloride 4 was dissolved in dry CH₂Cl₂ (10 mL) and was added dropwise over 10 min to the solution of 2-amino-7-methyl-1,8-napthyridine (0.06 g, 0.408 mmol) in CH₂Cl₂ (10 mL) containing Et₃N (100 μL). The reaction mixture was stirred at room temperature for 10 h. After the completion of the reaction, solvent was removed under reduced pressure. Water was added to the residue, and the product was extracted with chloroform (20 mL × 3), dried over anhydrous Na₂SO₄. Evaporation of the solvent gave the crude product, which was purified by silica gel column chromatography using petroleum ether–ethyl acetate (7 : 3, v/v) as an eluent to give the pure compound 1 (0.06 g, 32%), mp 192 °C, ¹H NMR (400 MHz, CDCl₃): δ 8.15 (d, 1H, J = 8 Hz), 8.03 (d, 1H, J = 8 Hz), 7.90 (d, 1H, J = 8 Hz), 7.81 (d, 1H, J = 8 Hz), 7.51–7.46 (m, 2H), 7.12 (d, 2H, J = 8 Hz), 6.60 (d, 2H, J = 8 Hz), 6.45 (d, 2H, J = 2.4 Hz), 6.16 (dd, 2H, J₁ = 8 Hz, J₂ = 2.4 Hz), 3.30–3.25 (m, 8H), 2.66 (s, 3H), 1.11 (t, 12H, J = 8 Hz); ¹³C NMR (100 MHz, CDCl₃): δ 169.0, 162.3, 154.4, 154.3, 153.0, 152.7, 148.4, 137.3, 136.1, 133.9, 129.1, 128.1, 127.8, 124.1, 123.5, 121.5, 118.3, 116.3, 107.8, 107.5, 97.9, 66.7, 44.2, 29.7, 12.6; FT-IR: ν cm⁻¹ (KBr): 2767, 2918, 1705, 1614, 1513, 1503; HRMS (ESI): calcd C₃₇H₃₉N₅O₂ [M + 2H]²⁺ 292.6552, found 292.6727; calcd [M + H]⁺ 584.3026, found 584.3334.

Compound 2. The crude acid chloride 4 was dissolved in dry CH₂Cl₂ (10 mL) and was added dropwise over 10 min to the amine 5 (ref. 14a) (0.10 g, 0.396 mmol) taken in CH₂Cl₂ (10 mL) containing Et₃N (200 μL). The reaction mixture was then stirred for 12 h. After the completion of the reaction, solvent was removed under reduced pressure and the remaining residue was dissolved in water, extracted with CHCl₃ and dried over anhydrous Na₂SO₄. The crude mass was purified by silica gel column chromatography using petroleum ether–ethyl acetate (3 : 2, v/v) as an eluent to yield the yellowish powdery compound 2 (0.08 g, 40%), mp 248 °C, ¹H NMR (400 MHz, CDCl₃): δ 8.70 (d, 1H, J = 4 Hz), 8.37 (d, 1H, J = 8 Hz), 8.06 (d, 1H, J = 8 Hz), 7.85 (d, 1H, J = 8 Hz), 7.78 (d, 1H, J = 8 Hz), 7.75 (d, 1H, J = 8 Hz), 7.70 (d, 1H, J = 8 Hz), 7.54 (t, 2H, J = 8 Hz), 7.26 (d, 1H, J = 8 Hz), 7.20 (d, 1H, J = 8 Hz), 6.84 (d, 1H, J = 8 Hz), 6.50 (d, 2H, J = 8 Hz), 6.44 (d, 2H, J = 2.4 Hz), 6.14 (dd, 2H, J₁ = 8 Hz, J₂ = 4 Hz), 5.78 (s, 2H), 3.32–3.20 (m, 8H), 1.10 (t, 12H, J = 7.2 Hz); ¹³C NMR (100 MHz, CDCl₃): δ 168.6, 163.6, 157.4, 154.1, 153.9, 153.4, 152.0, 149.3, 148.4, 138.3, 137.0, 136.6, 133.8, 130.0, 128.1, 127.8, 124.6, 123.2, 123.1, 122.5, 116.6, 113.6, 111.9, 108.4, 106.7, 97.9, 68.2, 66.9, 44.2, 12.6; FT-IR: ν cm⁻¹ (KBr): 2966, 2727, 1696, 1603, 1514, 1495; HRMS (ESI): calcd C₄₂H₄₂N₆O₃ [M + 2H]²⁺ 339.1659, found 339.1875; calcd [M + H]⁺ 677.3240, found 677.3627.

Acknowledgements

AM thanks CSIR, New Delhi, India for a fellowship.

References

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Footnote

† Electronic supplementary information (ESI) available: Figures showing the change in fluorescence and UV-vis titrations of receptors 1 and 2 with various metal ions, Job plot, binding constant curves, detection limit, F⁻ induced spectral changes and other spectral data. See DOI: 10.1039/c4ra01737d

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