Pyridylphenyl appended imidazoquinazoline based ratiometric fluorescence “turn on” chemosensor for Hg²⁺ and Al³⁺ in aqueous media

Abstract

A highly fluorescent and efficient ratiometric ‘turn on’ probe for Hg²⁺ and Al³⁺ based on quinazoline (1) has been designed and synthesized. Limit of detection (LOD) and association constants revealed its superior binding affinity and sensitivity toward Hg²⁺ over Al³⁺. Interference studies signified displacement of the complexed Al³⁺ by Hg²⁺ which has been supported by various studies.

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The design and development of sensitive and selective fluorescent probes for detection of heavy and transition metal ions (HTMs) in aqueous media has emerged as an exciting area of contemporary chemical research.¹ Mercury (Hg) is one of the most toxic and hazardous metals that causes diverse callous problems related to skin, respiratory, gastro-intestinal tissues, central nervous system (CNS) and DNA.^2,3 Various products containing mercury like paints, electronic equipment, batteries etc. are essential in our daily life.⁴ It is distributed in the environment in various forms viz., elemental (Hg), inorganic (Hg²⁺) and organic (CH₃Hg⁺) mercury. The elemental and inorganic mercury are easily converted into extremely toxic organic mercury by several bacterial and chemical processes.⁵ Conversely, aluminium (Al) is the third most abundant metal of the earth's crust and is widely used in diverse areas.⁶ Increasing concentration of the Al causes serious health problems like microcytic hypo-chromic anaemia, Al-related bone disease (ARBD), neuronal disorder, encephalopathy, myopathy and Alzheimer's disease etc.⁷ Due to the excessive usage of Hg and Al based products, people are encircled by these metals and influenced by their harsh effects. Therefore, development of sensitive and selective fluorescent probes for their detection is highly sought-after.

Moreover, quinazolines are essential for many biological processes.⁸ Recently quinazolines derivatives have been used as fluorescent chemosensor for the detection of HTMs (Hg²⁺ and Pb²⁺), anions, and also as efficient pH indicators.^5a,9,10 In this context, many chemosensors for Hg²⁺ exhibiting complexation induced fluorescence quenching have been reported.¹¹ Through an earlier report we too, have reported quinazoline derivatives showing quenching behaviour towards Hg²⁺.^5,9 In addition, chemosensors for detection of the Hg²⁺ via fluorescence enhancement are rather scarce.¹² Considering these points we set out to design and develop new quinazolines exhibiting fluorescence ‘turn on’ in presence of Hg²⁺. Through the present work we describe a new imidazo-quinazoline 6-(4-(pyridine-4-yl)-phenyl)-5,6-dihydrobenzo-[4,5] imidazo-[1,2-c]quinazoline (1) having 4-phenylpyridine together with its applicability as a ratiometric fluorescence ‘turn on’ probe for Hg²⁺ and Al³⁺ in aqueous media. Also, we describe herein our observations made on replacement of the complexed Al³⁺ (1·Al³⁺) by Hg²⁺ and superior selectivity and sensitivity of 1 for Hg²⁺ in the background presence of other metal ions including Al³⁺.

The probe 1 has been synthesized in good yield (81%) by condensation of 2-(2-aminophenyl)-1-benzimidazole with 4-(pyridine-4-yl)-benzaldehyde in ethanol under refluxing conditions (Scheme 1).⁹ It has been thoroughly characterized by satisfactory elemental analyses and spectral (FT-IR, ¹H and ¹³C NMR, and ESI-MS) studies (Fig. S1–S3, ESI†). ¹H NMR spectrum (dmso-d₆) of 1 displays two doublets in the down field region at δ 8.57 (2H, Ha) and 7.71 (2H, Hb) ppm assignable to the characteristic resonances due to pyridine ring protons (Fig. S1, ESI†). Two distinct singlets at δ 7.74 and 7.19 ppm corresponding to –NH (Hf) and –CH (He) protons clearly indicated the formation of 1. It has further been evidenced by presence of the molecular ion peak [M + H]⁺ at m/z 375.1603 (calcd 375.1610) in the ESI-MS spectrum of 1 (Fig. S3, ESI†).

Scheme 1 Synthesis of 1 and its interaction with Hg²⁺ and Al³⁺.

UV/vis spectrum of 1 (c, 10 μM, 1 : 99, v/v; EtOH : H₂O) exhibits strong low energy (LE) band at 358 nm (ε, 1.08 × 10⁴, M⁻¹ cm⁻¹) as well as high energy (HE) bands at 262 (ε, 3.16 × 10⁴ M⁻¹ cm⁻¹) and 227 nm (ε, 4.62 × 10⁴ M⁻¹ cm⁻¹) (Fig. S4, ESI†). In its emission spectrum, 1 displayed emission maxima at 425 nm (λ_ex, 350 nm; Φ, 0.23; SS, 75 nm) (Fig. S4, ESI†). Interaction of 1 toward various metal ions has been investigated by absorption and emission spectral studies, which illustrated noticeable alterations in its spectral features only in presence of the Hg²⁺ and Al³⁺. It showed insignificant changes in presence of 10.0 equiv. of the other metal ions viz., Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ag⁺ and Pb²⁺ (Fig. S5, ESI†). Further, to examine the effect of anions on the spectral features of 1, absorption and emission studies has also been performed in presence of various anions [F⁻, Cl⁻, Br⁻, I⁻, SO₄²⁻, S²⁻, HSO₄⁻, SO₃²⁻, S₂O₃²⁻, S₂O₈²⁻, CO₃²⁻, NO₂⁻, NO₃⁻ and PO₄³⁻] which showed almost negligible changes in the spectral pattern (Fig. S6, ESI†).

To gain deep insight into the interaction of 1 with Hg²⁺, absorption titration experiments have been performed at rt. Notably, upon addition of Hg²⁺ (6.0 equiv.) to a solution of 1 the LE band showed hypochromic as well as red shift (Δλ, 8 nm) and appeared at 366 nm (ε, 9.05 × 10³ M⁻¹ cm⁻¹) (Fig. 1a). Further, the band at 291 nm (ε, 4.17 × 10⁴ M⁻¹ cm⁻¹) exhibited a hyperchromic shift and an apparent isosbestic point at 378 nm. In contrary, the LE band of 1 exhibited hypochromism along with an apparent blue shift (351 nm; ε, 8.83 × 10³ M⁻¹ cm⁻¹; Δλ, 7 nm) upon additions of Al³⁺ (0.2–10.0 equiv.) (Fig. 1b). The HE band also exhibited hypochromism with a minute red shift (264 nm; ε, 2.70 × 10⁴ M⁻¹ cm⁻¹; Δλ, 2 nm). Further, the bands at 291 and 301 nm illustrated hyperchromic shift without any change in the position of bands. Emergence of a clear isosbestic point at 276 nm evidently indicated presence of more than two species in the solution. Observed changes in the absorption spectra of 1 in presence of Hg²⁺ and Al³⁺ clearly indicated their disparate interactions (binding modes) with 1.

Fig. 1 UV/vis titration spectra of 1 in presence of various amounts of Hg²⁺ (a) and Al³⁺ (b).

To have better understanding of the sensitivity and binding behavior of 1 toward Hg²⁺ and Al³⁺ fluorescence titration experiments have been performed. It exhibited ratiometric fluorescence ‘turn on’ responses for these cations, but insignificant changes for other tested metal ions (Fig. S5, S7 and S8, ESI†). Addition of Hg²⁺ (0.2 equiv.) to a solution of 1 illustrated significant quenching for the emission band at λ_max 425 nm and appearance of a new band at λ_max 482 nm (Fig. 2a). Further additions of Hg²⁺ (0.4–3.0 equiv.) resulted in an appreciable quenching (∼three fold) for the band at 425 nm and huge fluorescence enhancement (∼six fold) for the one at 482 nm. Likewise, the addition of Al³⁺ (0.2–8.0 equiv.) resulted changes in the spectral features similar to that for Hg²⁺ (∼two fold quenching at 425 nm along with emergence of a new band at 482 nm) (Fig. 2b). Appearance of clear isosemissive points at 441 (Hg²⁺) and 435 nm (Al³⁺) clearly suggested the presence of more than two species in the solution. At the saturation point, emission quantum yield (Φ) enhanced to 0.36 and 0.45, respectively for Hg²⁺ and Al³⁺. Job's plot analysis revealed 1 : 1 stoichiometry for both the Hg²⁺ and Al³⁺ ions (Fig. S9, ESI†). Estimated limit of detection (LOD) came out to be 1.69 × 10⁻⁸ M for Hg²⁺ and 1.44 × 10⁻⁷ M for Al³⁺ (Fig. S10 and S11, ESI†). The spectral features completely restored upon addition of strong chelating agent like EDTA (10.0 equiv.) to a solution of 1 + Hg²⁺ and 1 + Al³⁺, which indicated reversibility and reusability of the probe 1 (Fig. S12 and S13, ESI†).

Fig. 2 Fluorescence titration spectra of 1 in presence of various amounts of Hg²⁺ (a) and Al³⁺ (b).

The absorption and emission spectral studies revealed that 1 can detect both the Hg²⁺ and Al³⁺ separately in aqueous media. However, it shows higher selectivity and sensitivity for Hg²⁺ in background presence of the interfering Al³⁺ and other metal ions (Fig. S14–S16, ESI†). Further, addition of Al³⁺ (20.0 equiv.) to a solution of 1 + Hg²⁺ did not show any spectral changes, whereas addition of Hg²⁺ (only 4.0 equiv.) to a solution of 1 + Al³⁺ resulted in spectral changes which resembled with the spectral features of 1 + Hg²⁺ (Fig. S16, ESI†). Interference studies revealed replacement of the complexed Al³⁺ by Hg²⁺ in absence of any other external chelating agent and showed superiority over previously reported systems.¹³ Overall results indicated high binding affinity of the probe 1 for Hg²⁺ relative to Al³⁺, which has been substantiated by obtained association constants [K_a; 4.12 × 10⁴ M⁻¹, Hg²⁺ and 2.45 × 10³ M⁻¹, Al³⁺] using Benesi–Hildebrand method from the fluorescence titration experiments (Fig. S17 and S18, ESI†).¹⁴ The spectral results revealed that 1 is an efficient probe for both Hg²⁺ and Al³⁺ ions and is superior/comparable to the other reported probes (Table S1 and S2, ESI†) To consider role of the acidic pH [as Hg(NO₃)₂ and Al(NO₃)₃ generate acidic solution upon dissolution in water], absorption and emission titration studies have been performed in the pH range ∼7.19–4.37. Observed results (Fig. S19, ESI†) indicated that changes in the presence of Hg²⁺ and Al³⁺ are not related to alteration in the pH.

Fluorescent probes used for the recognition of various cations and anions are usually based on diverse mechanisms such as metal ligand charge transfer (MLCT), intermolecular charge transfer (ICT), photo-induced electron transfer (PET), chelation-enhanced fluorescence (CHEF), excimer/exciplex formation, imine isomerization, intermolecular hydrogen bonding, excited state intra-molecular proton transfer, displacement approach and fluorescence resonance energy transfer (FRET).¹⁵ Fluorescence enhancement for the probe 1 toward Hg²⁺ and Al³⁺ may be attributed to photo-induced electron transfer. In 1 the lone pair on –NH group is located closer to the quinazoline ring and induces intra-molecular PET. Upon addition of Hg²⁺ and Al³⁺ to a solution of 1 reduces PET owing to the interaction of the lone pair with metal centre leading to the formation of 1·Hg²⁺ and 1·Al³⁺ complexes. Upon complexation, the system becomes more rigid and reduces PET which leads to a significant enhancement in the fluorescence intensity.

To have a clear cut idea about the most probable interaction sites in 1 for Hg²⁺ and Al³⁺, ¹H NMR titration experiments have been performed in dmso-d₆ (Fig. 3 and S20–S22, ESI†). Upon addition of 0.25 equiv. of Hg²⁺ (D₂O) to a solution of 1 the pyridyl (Ha), phenyl (Hd) and –CH (He) protons displayed downfield shift along with decrease in the peak intensity for –NH proton (Hf) (Fig. S21, ESI†). Further, additions of Hg²⁺ (0.5–1.0 equiv.) led to continuous deshielding for the signals due to Ha, Hd and He protons which appeared at δ 8.73 (Δδ, 0.149 ppm), 8.19 (Δδ, 0.571 ppm) and 7.31 ppm (Δδ, 0.125 ppm), respectively while –NH proton completely disappeared. The loss of signal due to –NH and significant downfield shift for pyridyl protons suggested interaction of Hg²⁺ through deprotonated quinazoline (–NH) and pyridyl nitrogen. The positions of both the coordinated nitrogen in the structure of 1 and clear signals due to each proton in the ¹H NMR spectrum suggested the formation of most probably a polymeric system involving Hg²⁺ and/or some other distinct species, existing in equilibrium with the polymeric structure. On the other hand, addition of Al³⁺ (0.5 equiv.) resulted in a downfield shift for the signals due to phenyl (Hd) as well as quinazoline –CH and –NH protons (He and Hf) (Fig. S22, ESI†).

Fig. 3 ¹H NMR spectra of 1 (a), 1 + Hg²⁺ (1.0 equiv.) (b), 1 + Al³⁺ (3.0 equiv.) (c) and 1 + Hg²⁺ + Al³⁺ (d), [red and blue star showing shifting in Ha and Hd protons]. Images of the solid samples of 1, 1 + Hg²⁺ and 1 + Al³⁺ (above) and fluorescence image in solution below (e).

Further, addition of Al³⁺ (1.0–3.0 equiv.) led the Hd and He protons to resonate at δ 7.79 ppm (Δδ, 0.177 ppm) and 7.29 ppm (Δδ, 0.107 ppm). The Hf (–NH) proton showed continual downfield shift and finally merged with the signal at δ 7.92 ppm (Δδ, 0.183 ppm). Herein, the resonance due to pyridyl ring proton (Ha) remains unaffected which strongly indicated that it is not involved in interaction with the metal centre Al³⁺. Thus, it is assumed that Al³⁺ interacts through only –NH of the quinazoline in protonated form. High downfield shift for ¹H NMR signals in presence of Hg²⁺ indicated its stronger binding with deprotonated quinazoline nitrogen. Addition of Hg²⁺ to a solution of 1 + Al³⁺ led to a spectrum that resembled well to that for 1 + Hg²⁺ and supported replacement of the Al³⁺ by Hg²⁺ (Fig. 3 and S20, ESI†).

Plausible structures of 1·Hg²⁺ and 1·Al³⁺ have been proposed on the basis of absorption, emission, ¹H NMR titrations, and Job's plots analyses. It has further been evidenced by ESI-MS studies (Fig. S23 and S24, ESI†). In its mass spectrum 1·Hg²⁺ displayed prominent peaks at m/z 1030.4714 and 967.4067 corresponding to [(1)₂ + Hg²⁺ + NO₃ + H₂O + H]⁺ and [(1)₂ + Hg²⁺ + H₂O]⁺ which supported binding of the Hg²⁺ with two nitrogen (pyridyl and deprotonated quinazoline nitrogen) of 1 and one each of the nitrate and water (Scheme 1). The presented structure is a monomeric unit of a linear polymeric structure. On the other hand, appearance of a peak at m/z 543.4426 in ESI-MS of 1·Al³⁺ has been assigned to [1 + Al³⁺ + (NO₃)₂ + H₂O]⁺ moiety.

To support expected structure of 1 and its mode of interaction in the ensuing Hg²⁺/Al³⁺ complexes, quantum chemical calculations have been performed using DFT (Fig. S25, ESI†). In the optimized structure of [1·Hg²⁺] metal centre is coordinated with two nitrogen (one each from pyridyl and deprotonated quinazoline) one nitrate and water molecule arranged in tetrahedral manner. In the [1·Al³⁺] it involves one quinazoline nitrogen, two nitrates and one water molecule arranged in distorted octahedral manner. Total energy of the optimized structure for [1·Hg²⁺] adduct (−2764 a.u.) is lower than that for [1·Al³⁺] (−1821 a.u.) and stabilised by −943 a.u.

Conclusions

Through this contribution we have presented synthesis and characterization of the novel fluorescent imidazo-quinazoline (1) and demonstrated that it acts as a proficient probe for Hg²⁺ and Al³⁺ over other cations and anions in aqueous media. The sensitivity (nano molar) and selectivity of 1 has been found to be higher for Hg²⁺ relative to Al³⁺. Selective detection of the Hg²⁺ over interfering Al³⁺ and other metal ions in absence of any external chelator has also been thoroughly established. The stronger binding behaviour of 1 toward Hg²⁺ over Al³⁺ may be attributed to binding of the deprotonated –NH with Hg²⁺ and in protonated form with Al³⁺. Overall, this report deals with straightforward recognition of the HTMs (Hg²⁺ over Al³⁺) with red shifted fluorescence ‘turn on’ signalling.

Acknowledgements

We are thankful to the Department of Science and Technology (DST) New Delhi, India for financial assistance through the scheme [SR/S1/IC-25/2011] and one of the authors A.K. acknowledges Council of Scientific and Industrial Research (CSIR), New Delhi, India for the award of Junior and Senior Research Fellowship (no. 09/013(0330)/2009-EMR-I).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental sections, NMR, ESI-MS, UV/vis, fluorescence titration spectra, and figures. See DOI: 10.1039/c4ra09804h

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