A turn-on AIE active fluorescent sensor for Hg²⁺ by combination of 1,1-bis(2-pyridyl)ethylene and thiophene/bithiophene fragments

Abstract

Fluorescent sensors for Hg²⁺ that combine aggregation-induced emission (AIE) activity of tetraarylethylenes with metal chelating 1,1-bis(2-pyridylethylene) fragments and thiophene/bithiophene substituents have been prepared and characterized. The sensors exhibit red-shifted and enhanced emission in the presence of Hg²⁺ in aqueous solution while exhibiting little to no change in fluorescence in the presence of other metal ions. Job plot analyses indicate 2 : 1 sensor : Hg²⁺ binding stoichiometries in solution. ¹H-NMR spectroscopy was also employed to investigate solution phase binding interactions between the sensors and Hg(ClO₄)₂, and a chelated HgI₂–bis(pyridyl) complex has been characterized by X-ray crystallography. The limit of detection for Hg²⁺ was determined to be 48 nM. In contrast, the fluorescence of structurally analogous materials possessing quinoline rings in place of pyridine groups is completely quenched in the presence of Hg(ClO₄)₂. The high sensitivity and selectivity displayed by these sensors for Hg²⁺ over other metal ions may enable monitoring of mercury in aqueous environments.

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Introduction

Fluorescence-based probes capable of sensing and/or imaging metal ions are important analytical tools in diverse areas of research, including molecular biology, clinical diagnostics, biotechnology, and environmental science.^1–4 Among the metal ions targeted for selective sensor development, Hg²⁺ continues to receive considerable attention because of its non-biodegradable nature and its extreme toxicity toward human health and harmful effects on the environment.⁵ Inorganic Hg²⁺ released into the environment can be converted to methyl mercury by multiple species of bacteria. Methyl mercury readily bio-accumulates in tissues of living animals and plants, and enters the human body through the food chain (e.g. edible fish). Elevated Hg²⁺ levels in humans produce acute neurological effects, kidney failure, and brain damage.⁶ Therefore, it is important to develop sensitive and selective methods for detection of Hg²⁺ in environmental and biological samples.⁷

Traditional methods of Hg²⁺ detection, such as atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS) are limited by the need for expensive and complex instrumentation. In contrast, fluorescence-based methods hold promise as simple, sensitive, non-invasive, portable, and relatively inexpensive alternatives for detection and sensing of Hg²⁺ in biological and environmental samples.⁸ Since Hg²⁺ is a heavy metal, its interaction with organic fluorophores often results in fluorescence quenching as a consequence of mercury's large spin–orbit coupling constant.⁹ Consequently, many fluorescent sensors developed for Hg²⁺ exhibit negative or “turn-off” chelation-enhanced quenching (CHEQ) effects.^10,11 Turn-off fluorescence sensors, however, can suffer from high rates of false negative results. Accordingly, fluorescence “turn-on” sensors are more desirable because of their high sensitivity and elevated signal-to-noise ratio (S/N).

There are, however, relatively few examples of “turn-on” fluorescent receptors suitable for detection of Hg²⁺, especially in aqueous systems. Lippard and Nolan have characterized several Hg²⁺ sensors based on fluorescein derivatives with attached Hg-chelating groups featuring coordinating thioether groups.¹² Similarly, a fluorescein-based sensor functionalized with an azathia-crown ether group has been developed for specific sensing of Hg²⁺ in aqueous solution at ppm concentrations.¹³ Additionally, fluorescent Hg²⁺ sensors based on modulating the dye-quenching ability of gold nanoparticles (AuNP) have been reported with sensitivities as low as 2 ppb in aqueous solution.¹⁴

Increasingly, compounds that display aggregation-induced emission (AIE) are being used as components of turn-on fluorescent sensors. Unlike conventional fluorophores that exhibit aggregation-caused quenching (ACQ), compounds with aggregation-induced emission (AIE) characteristics typically have weak fluorescence in dilute solution but become highly emissive upon aggregation in the presence of poor solvents or in the solid state.¹⁵ This intrinsic behaviour of AIE active compounds is largely attributed to restriction of intramolecular motions that occur upon aggregation.^15g Several turn-on fluorescence sensors featuring AIE characteristics have been developed for monitoring Hg²⁺ in aqueous solution.^8a,16 For example, tetraphenylethylene (TPE) based compound 1 (Fig. 1) was designed as a turn-on AIE active sensor that utilizes thymine for selective coordination of Hg²⁺.^16c Additionally, the intense red emission of aggregated TPE-functionalized quinolinium salt 2 that is quenched in the presence of iodide anion can be fully recovered upon addition of Hg²⁺, allowing fluorescence off–on detection of HgI₂.^8a More recently, rhodamine-TPE derivative 3 has been reported as a ratiometric Hg²⁺ sensor.^16b

Fig. 1 Examples of TPE derivatives as fluorescent metal sensors (1–4).

In addition to AIE-based systems, chelation-induced fluorescence enhancement is also a useful strategy to develop emissive sensors for Hg²⁺.¹⁷ In this context, we recently reported 1,1-bis(2-pyridyl)-2,2-diphenylethylene (4) as a selective fluorescent sensor for Zn²⁺ ions based on the combination of AIE properties and metal chelating ability.¹⁸ In continuing research aimed at further modifying AIE active sensor 4 for sensing of other biologically and environmentally relevant metal ions, we sought to target Hg²⁺ using an AIE active tetraarylethylene that combines a metal-chelating 1,1-bis(2-pyridyl)ethylene fragment with thioarenes. Specifically, we envisioned that derivatives bearing thiophene or bithiophene moieties might display selectivity for Hg²⁺ over other metals because of the thiophilic nature of Hg²⁺.^19,20 We report here the synthesis and characterization of two new 1,1-bis(2-pyridyl) tetraarylethylenes (6 and 7) and two new 1,1-bis(2-quinolinyl) tetraarylethylenes (10 and 11) that incorporate thiophene or bithiophene units as part of the tetraarylethylene framework. The AIE activity of these compounds and their ability to function as selective fluorescent sensors for detection of Hg²⁺ ions in aqueous solution has been evaluated.

Results and discussion

As illustrated in Scheme 1, the synthesis of the putative fluorescent sensors 6 and 7 was easily accomplished using procedures analogous to those we recently reported for preparation of 4.¹⁸ Double Suzuki coupling of 5 proceeded smoothly with either thiophene-2-boronic acid or 2,2′-bithiophene-5-boronic acid pinacol ester as cross-coupling partners to furnish 6 and 7 in reasonable yields.

Scheme 1 Synthesis of thiophene and bithiophene 1,1-bis(2-pyridyl)tetraarylethylenes 6 and 7.

Both compounds 6 and 7 exhibited similar UV-visible absorption spectra in CH₃CN solution, with longest wavelength absorption appearing at ∼340 nm (see ESI†). However, their fluorescence properties under conditions leading to aggregation induced emission displayed significant differences. The fluorescence spectrum of 6 in pure CH₃CN revealed a weak emission at ∼512 nm (Fig. 2A). Incremental addition of H₂O (a poor solvent for 6) initially produced a slight red-shift in the emission wavelength followed by a decrease in emission intensity at ∼520 nm and the appearance of a new blue-shifted emission peak at ∼404 nm that increases in intensity as a function of added H₂O (Φ₄₀₄ = 6.3%, 9 : 1 H₂O : CH₃CN). The emission profile of 6 as a function of solvent polarity and aggregation state may reflect transition from a locally excited state fluorophore to emission from a charge-transfer state.²¹ In contrast, 7 exhibits a more or less conventional AIE effect that features a marked increase in fluorescence emission (λ_em ∼ 503 nm) upon incremental addition of H₂O (Fig. 2B, Φ₅₀₃ = 7.5%, 9 : 1 H₂O : CH₃CN).

Fig. 2 (A) AIE profile of 6 in CH₃CN/H₂O mixtures (λ_ex = 338 nm, [6] = 10 μM). (B) AIE profile of 7 in CH₃CN/H₂O mixtures (λ_ex = 351 nm, [7] = 10 μM).

The fluorescence response of 6 in 9 : 1 H₂O : CH₃CN was then measured in the presence of 2 equivalents of different metal ions (Hg²⁺, Mg²⁺, Ca²⁺, Ba²⁺, Co²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Ni²⁺, Zn²⁺, Fe³⁺, Fe²⁺, Mn²⁺, Na⁺, and K⁺), and the results are shown in Fig. 3A (perchlorate counterion(s) in each case). Most metal ions produced little to no change in the fluorescence of the solution. Addition of either Cu²⁺ or Fe³⁺ resulted in fluorescence quenching, whereas Cd²⁺ and Zn²⁺ elicited a slight increase in fluorescence in the 400–450 nm range. The effect of added Hg²⁺ ions, however, was much more dramatic as an intense red-shifted emission centered at 600 nm was observed. The fluorescence response of 6 at 600 nm as a function of increasing amounts of Hg(ClO₄)₂ was measured in 9 : 1 H₂O : CH₃CN (Fig. 3B). Emission intensity was observed to increase rapidly up to 2 equivalents Hg²⁺ (Φ = 15.7%), and then gradually plateau at ∼4 equivalents.

Fig. 3 (A) Fluorescence spectra of 6 in the presence of different metal perchlorate salts (2 equivalents). (B) Relative change in fluorescence intensity at 600 nm of 6 as a function of added Hg²⁺ (λ_ex = 338 nm, [6] = 10 μM).

The selective red-shifted and enhanced fluorescence response of 6 in the presence of Hg²⁺ is attributed to the presence of both the chelating 2-pyridyl groups as well as the peripheral thiophene rings. Indeed, we have previously shown that addition of Hg(ClO₄)₂ to 4 (possessing the 1,1-bis(2-pyridyl)ethylene fragment but lacking thiophene substituents) quenches the fluorescence under conditions similar to those outlined above.¹⁸ Likewise, exposure of 1,1-bis(2-thiophenyl)-2,2-diphenylethylene (an analogue of 6 in which the pyridine rings have been replaced with phenyl groups)²² also fails to produce a fluorescence response (see ESI†). In order to gain structural insight into possible adducts between 6 and Hg²⁺, we unsuccessfully attempted to grow crystals from solutions containing 6 and Hg(ClO₄)₂. Efforts to obtain crystals from other Hg(II) salts (Hg(OAc)₂, Hg(NO₃)₂, HgCl₂) were also unsuccessful. X-Ray quality crystals were obtained, however, from slow evaporation of a THF/MeOH solution of 6 and HgI₂. The molecular structure of the resulting complex features chelation of HgI₂ by the bis(2-pyridyl)ethylene moiety (see ESI†). Presumably the strong affinity of Hg²⁺ for iodide anions^8a,16d precludes halide dissociation to allow for potential thiophene ring interactions or formation of 2 : 1 ligand : Hg²⁺ complexes.

To gain insight into the solution phase behaviour of 6 and Hg(ClO₄)₂, a Job plot was constructed by monitoring the change in fluorescence intensity at 600 nm as a function of mole fraction 6 (Fig. 4). The maximum emission response was observed at 0.67 mole fraction of 6, which indicates a 2 : 1 (6 : Hg²⁺) binding stoichiometry. Furthermore, NMR titration experiments between 6 and Hg(ClO₄)₂ in D₂O/CD₃CN revealed a downfield shift of the 2-pyridyl ortho hydrogens, indicating the coordination of Hg²⁺ to the pyridine nitrogen atoms (see ESI†).

Fig. 4 Job plot of 6 and Hg(ClO₄)₂ binding in 9 : 1 H₂O : CH₃CN indicating 2 : 1 (6 : Hg²⁺) binding stoichiometry. The relative concentrations of 6 and Hg(ClO₄)₂ were varied while the total concentration [6 + Hg(ClO₄)₂] remained constant (20 μM).

The effect of counterion on the turn-on fluorescence response of 6 in the presence of Hg²⁺ was also probed (Fig. 5). Addition of Hg(OAc)₂ and HgCl₂ to solutions of 6 produced a fluorescence response comparable to that observed upon addition of Hg(ClO₄)₂. Inclusion of nitrate counterions resulted in diminished fluorescence, and addition of HgI₂ resulted in fluorescence quenching.

Fig. 5 Fluorescence spectra of 6 in the presence of Hg²⁺ with different counterions (λ_ex = 338 nm, [6] = 10 μM).

The fluorescence response of bithiophene derivative 7 in 9 : 1 H₂O : CH₃CN was also examined in the presence of 2 equivalents of various metal ions. Once again, most metal ions elicited little to no change in fluorescence, including Zn²⁺ and Cd²⁺ (which did produce a fluorescent response with 6). Addition of Hg(ClO₄)₂, however, resulted in a significantly red-shifted and enhanced fluorescence signal centered at 633 nm (Φ = 20.8%, Fig. 6A). As illustrated in Fig. 6B, the selective turn-on response of 7 to Hg²⁺ is not affected by the presence of Group I and Group II metal ions (Na⁺, K⁺, Ca²⁺, Mg²⁺ and Ba²⁺). Moreover, the Hg²⁺-selective fluorescence response of 7 is also unaffected by the presence of other transition metal or heavy metal ions, with the exception of Cu²⁺ and, to a lesser extent, Fe³⁺.

Fig. 6 (A) Fluorescence spectra of 7 in the presence of various metal ions (2 equivalents). (B) Emission of 7 at 633 nm in the presence of various metal ions and Hg²⁺–metal ion combinations (normalized to emission in the presence of Hg²⁺). λ_ex = 351 nm, [7] = 10 μM.

A Job plot based on the emission response of 7 in presence of different mole fractions of Hg²⁺ in 9 : 1 H₂O : CH₃CN was almost identical to the plot constructed for 6 (Fig. 4) and indicated a similar 2 : 1 (7 : Hg²⁺) binding stoichiometry (see ESI†). The effect of pH on the turn-on response of 7 to Hg²⁺ was additionally probed over a wide range of pH (Fig. 7). At acidic pH (pH < 5), protonation of the (2-pyridyl) groups will affect the ability of 7 to chelate Hg²⁺ and consequently no emission was observed. At basic pH, the intensity of emission at 633 nm is significantly diminished, which is likely due to formation of Hg(OH)₂. The maximum emission response was observed between pH ∼ 6–9, which indicates that 7 is suitable for sensing Hg²⁺ at physiological pH. Finally, the limit of detection of 7 for Hg²⁺ was determined to be 48 nM (ESI†).

Fig. 7 Fluorescence intensity at 633 nm of 7 in the absence and presence of Hg²⁺ (2 equivalents) as a function of pH (λ_ex = 351 nm, [7] = 10 μM).

Solution phase binding interactions between 7 and Hg(ClO₄)₂ were further investigated by NMR. Fig. 8 shows a series of ¹H-NMR spectra of 7 obtained in the absence and presence of increasing amounts of Hg(ClO₄)₂. These experiments were performed in 1 : 1 D₂O : CD₃CN in order to maintain solubility of 7 at the concentration needed for NMR spectroscopy. The signal corresponding to the ortho-H of the 2-pyridyl group (doublet at 8.42 ppm) is incrementally shifted downfield upon addition of up to 0.5 equivalent of Hg(ClO₄)₂. This shift is attributed to coordination of Hg²⁺ to the pyridine nitrogen atoms and is in agreement with our previous results observed upon coordination of Zn²⁺ to 4.^18,23 Further addition of Hg(ClO₄)₂ (beyond 0.5 equivalents) results in significant shifting and broadening of all peaks in the aromatic region, including signals for the bithiophene hydrogens. The downfield shifts of all aryl hydrogen resonances and peak broadening might reflect formation of ternary ligand:metal complexes, consistent with the results of Job plot analysis which indicates a 2 : 1 ligand : Hg²⁺ stoichiometry. Qualitatively similar peak shifting/broadening as a function of [Hg²⁺] was also observed in NMR titrations of 6 (see Fig. S13, ESI†).

Fig. 8 ¹H NMR spectra of 7 + Hg(ClO₄)₂ at different equivalents of Hg(ClO₄)₂ (x = number of Hg(ClO₄)₂ equivalents) in D₂O : CD₃CN (1 : 1), [7] = 3 mM.

Variation in N-heterocycle was also briefly examined through preparation of tetraarylethylenes possessing 2-quinolinyl substituents in place of 2-pyridyl groups. Dibromoalkene 9 was obtained upon treatment of di(quinolin-2-yl)methanone (8)²⁴ with CBr₄ and PPh₃. Subsequent double Suzuki reaction of 9 with either thiophene-2-boronic acid or 2,2′-bithiophene-5-boronic acid pinacol ester afforded 10 and 11 in serviceable yields (Scheme 2). The AIE properties of 10 and 11 were examined in CH₃CN/H₂O mixtures. The AIE profile of 10 mirrors that of 6 in that incremental addition of H₂O results in enhanced and blue-shifted emission at ∼462 nm compared to emission at ∼515 nm in 100% CH₃CN (Fig. 9A). Compound 11 exhibited a much reduced AIE effect in CH₃CN/water mixture compared to the other compounds examined in this study. A weak emission at ∼413 nm in pure CH₃CN was found to slightly shift to 435 nm upon incremental addition of H₂O, but fluorescence intensity was only modestly enhanced (Fig. 9B).

Scheme 2 Synthesis of 2,2′-bis(quinolinyl) tetraarylethylenes 10 and 11.

Fig. 9 (A) AIE profile of 10 in CH₃CN/H₂O mixtures (λ_ex = 353 nm, [10] = 10 μM). (B) AIE profile of 11 in CH₃CN/H₂O mixtures (λ_ex = 364 nm, [11] = 10 μM).

In contrast to the selective turn-on fluorescence response observed for 6 and 7 upon exposure to Hg²⁺, addition of Hg(ClO₄)₂ (2 equivalents) to 10 in 90% aqueous CH₃CN resulted in complete fluorescence quenching (Fig. 10). A similar result was obtained with compound 11 as well (ESI†). The behaviour of 10 and 11 is attributed to chelation-enhanced quenching (CHEQ) effects, which are often observed upon coordination of Hg²⁺ to fluorophores.^9,11 Interestingly, only the most downfield hydrogens in the ¹H-NMR spectra of 10 and 11 (corresponding to the H4 and H5 signals of the quinoline rings) are shifted upon addition of up to 1 equivalent Hg(ClO₄)₂, and the remaining quinoline and thiophene/bithiophene signals are unaffected by the presence of mercuric ion (Fig. S14 and S15, ESI†). This behaviour contrasts with the spectra of 6 or 7 + Hg²⁺ in which all the ¹H-NMR signals exhibited appreciable downfield shifting and broadening (see Fig. 8 and Fig. S13, ESI†). While uncovering the exact mechanisms underlying the divergent fluorescence and NMR responses exhibited by 6/7 and 10/11 toward Hg²⁺ ions requires additional research, steric hindrance and/or extended π systems introduced through incorporation of quinoline rings into the tetraarylethylene framework has clearly altered the photophysical response of 10 and 11 toward Hg²⁺, and the bis(2-pyridyl)ethylene fragment in 6 and 7 appears crucial for selective turn-on fluorescence detection of Hg²⁺.

Fig. 10 Fluorescence spectra of 10 in 9 : 1 H₂O : CH₃CN in the absence and presence of Hg(ClO₄)₂ (2 equivalents). λ_ex = 353 nm, [10] = 10 μM.

Conclusions

In summary, two new fluorescent turn-on probes (6 and 7) have been developed for sensing of Hg²⁺ in aqueous solution. These probes were designed by combining metal chelating 1,1-bis(2-pyridylethylene) fragments and thiophene/bithiophene substituents in a single AIE-active platform. Job plots and ¹H-NMR titrations were used to investigate solution phase binding of the new sensors and Hg²⁺. An X-ray crystal structure of the complex between 6 and HgI₂ confirmed the Hg²⁺-chelating ability of the 1,1-bis(2-pyridyl)ethylene moiety. Replacement of the 2-pyridyl groups with 2-quinoline rings (as in 10 and 11), however, completely abolished the selective fluorescence turn-on response toward Hg²⁺. Examples of turn-on fluorescent sensors for Hg²⁺ are rare, and the utilization of an AIE-active scaffold in 6 and 7 eliminates the need for incorporation of additional fluorophores.^8a,12–16 Further modification of the relatively simple tetraarylethylene framework in 6 and 7 to provide even greater selectivity and sensitivity toward Hg²⁺ is being examined, as is the design and construction of analogues that are expected to exhibit altered metal ion selectivities. Indeed, combining the AIE activity of tetraarylethylenes with metal ligating/chelating groups offers a wealth of opportunity for development of selective metal ion sensors with numerous potential biomedical and environmental applications.

Experimental

General

All commercially available starting materials, reagents, and solvents were used as supplied unless otherwise stated. Reported yields are isolated yields. Proton (¹H) and carbon (¹³C) NMR were collected on Bruker NMR spectrometers at 300 MHz or 400 MHz for ¹H and 100 MHz or 75 MHz for ¹³C. Chemical shifts (δ) are reported in parts-per million (ppm) relative to residual undeuterated solvent. Melting points were recorded using a capillary melting point apparatus and are uncorrected. High resolution mass spectra were obtained in positive ion mode using electrospray ionization (ESI) on a double-focusing magnetic sector mass spectrometer. UV-visible spectra were obtained using quartz cuvettes on a Varian Cary 100-Scan dual-beam spectrophotometer. Each measurement was done in duplicate and compared to solvent blank. Blank samples were prepared using HPLC grade acetonitrile. Fluorescence spectra were obtained in air at room temperature using a Horiba Jobin Yvon Fluoromax-4 spectrofluorimeter using 3 mL quartz cuvettes. Quantum yields (Φ) were determined relative to quinine sulfate. Samples were prepared using HPLC grade solvents. X-Ray diffraction data were collected on a Nonius Kappa CCD diffractometer equipped with Mo Kα radiation with λ = 0.71073 Å. Structures were solved by direct methods and data was refined by full-matrix least squares refinement on F² against all reflections.

Synthetic procedures

2,2′-(2,2-Di(thiophen-2-yl)ethene-1,1-diyl)dipyridine (6). Compound 5¹⁸ (692 mg, 2.00 mmol) was dissolved in 100 mL of dioxane : water (4 : 1). The flask was charged with Na₂CO₃ (1.38 g, 10.00 mmol), Pd(OAc)₂ (100 mg, 0.43 mmol), PPh₃ (470 mg, 1.80 mmol) and thiophene-2-boronic acid (1.28 g, 10.00 mmol). The reaction was heated to reflux under argon overnight. After cooling, the reaction mixture was diluted with water and extracted with ethyl acetate (2 × 50 mL) and the combined organic fractions were dried over anhydrous Na₂SO₄, filtered, and concentrated under reduced pressure. The crude product was purified by flash column chromatography using ethyl acetate/hexane 1 : 2 and 2 : 1 as eluent to yield 6 (506 mg, 73%) as a yellow solid. Mp 191–193 °C. ¹H NMR (400 MHz, CDCl₃) δ 6.81–6.84 (m, 4H), 7.06–7.10 (m, 2H), 7.22–7.24 (m, 2H), 7.26–7.29 (m, 2H), 7.65–7.70 (m, 2H), 8.54 (d, J = 5.4 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃) δ 122.7, 127.2, 127.5, 129.5, 129.6, 131.3, 133.1, 137.1, 145.3, 150.5, 161.6. HRMS (ESI): calcd for C₂₀H₁₅N₂S₂ [M + H]⁺, 347.0677; found, 347.0672.

6·HgI₂. To a solution of 6 (20 mg, 0.058 mmol) in THF/methanol (1 : 4, 10 ml), HgI₂ (26 mg, 0.058 mmol) was added. The solution was allowed to slowly evaporate over 2 days to give 6·HgI₂ (25.1 mg, 54%) as yellowish brown crystals.²⁵ Mp >200 °C.

2,2′-(2,2-Di([2,2′-bithiophen]-5-yl)ethene-1,1-diyl)dipyridine (7). Using the procedure given for the preparation of 6, coupling of 5 (340 mg, 1.00 mmol) and 2,2′-bithiophene-5-boronic acid pinacol ester (1.46 g, 5.00 mmol) gave 7 (296 mg, 58%) as an orange-yellow solid. Mp 174–176 °C. ¹H NMR (400 MHz, CDCl₃) δ 6.76 (d, 2H, J = 6.5 Hz), 6.90 (d, 2H, J = 6.5 Hz), 6.92–6.95 (m, 2H), 7.04–7.09 (m, 4H), 7.15 (d, J = 5.8 Hz, 2H), 7.32–7.35 (m, 2H), 7.48–7.53 (m, 2H), 8.55 (d, J = 5.8 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃) δ 122.9, 124.2, 124.9, 125.7, 127.2, 128.9, 130.9, 132.5, 137.3, 138.2, 140.4, 142.0, 143.8, 150.6, 161.5. HRMS (ESI): calcd for C₂₈H₁₉N₂S₄ [M + H]⁺, 511.0431; found, 511.0428.

2,2′-(2,2-Dibromoethene-1,1-diyl)diquinoline (9). Di(quinolin-2-yl)methanone (8)²⁴ (1.70 g, 6.00 mmol) was dissolved in toluene (150 mL). Carbon tetrabromide (3.99 g, 12.00 mmol) and PPh₃ (6.3 g, 24.00 mmol) were added and the reaction mixture was heated to reflux and maintained for 2 days. After this time the reaction mixture was allowed to cool to rt and insoluble material was removed by filtration. The filtrate was treated with 100 mL of 1 M aq. HCl. The aqueous layer was separated and made basic with 1 M aq. NaOH until pH 12. The aqueous phase was extracted with CH₂Cl₂ (3 × 50 mL), and the combined organic fractions were dried over anhydrous Na₂SO₄, filtered, and concentrated under reduced pressure. The crude product was purified by flash column chromatography using ethyl acetate/hexane 1 : 1 as eluent to yield 9 (1.47 g, 56%) as dark yellow solid. Mp 183–185 °C. ¹H NMR (300 MHz, CDCl₃) δ 7.52–7.58 (m, 2H), 7.72–7.86 (m, 6H), 8.14–8.26 (m, 4H). ¹³C NMR (75 MHz, CDCl₃) δ 99.4, 124.7, 129.7, 129.9, 130.3, 132.3, 132.5, 139.3, 149.5, 150.5, 160.4. HRMS (ESI): calcd for C₂₀H₁₃N₂Br₂ [M + H]⁺, 438.9445; found, 438.9452.

2,2′-(2,2-Di(thiophen-2-yl)ethene-1,1-diyl)diquinoline (10). Using the procedure given for the preparation of 6, coupling of 9 (440 mg, 1.00 mmol) and thiophene-2-boronic acid (640 mg, 5.00 mmol) gave 10 (362 mg, 81%) as an orange-yellow solid. Mp 162–163 °C. ¹H NMR (400 MHz, CDCl₃) δ 6.78–6.82 (m, 2H), 6.87–6.91 (m, 2H), 7.21 (d, J = 6.1 Hz, 2H), 7.43–7.50 (m, 4H), 7.57–7.62 (m, 2H), 7.73 (d, J = 6.1 Hz, 2H), 7.90 (d, J = 6.1 Hz, 2H), 7.98 (d, J = 6.1 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃) δ 125.1, 127.4, 127.6, 127.9, 128.5, 128.8, 130.1, 130.6, 131.9, 132.6, 136.6, 137.6, 145.0, 149.1, 162.3. HRMS (ESI): calcd for C₂₈H₁₉N₂S₂ [M + H]⁺, 447.0990; found, 447.0995.

2,2′-(2,2-Di([2,2′-bithiophen]-5-yl)ethene-1,1-diyl)diquinoline (11). Using the procedure given for the preparation of 6, coupling of 9 (440 mg, 1.00 mmol) and 2,2′-bithiophene-5-boronic acid pinacol ester (1.46 g, 5.00 mmol) gave 11 (409 mg, 67%) as a brown-yellow solid. Mp 179–180 °C. ¹H NMR (400 MHz, CDCl₃) δ 6.78–6.80 (m, 2H), 6.86–6.88 (m, 2H), 6.93–6.96 (m, 2H), 7.02–7.04 (m, 2H), 7.16–7.18 (m, 2H), 7.45–7.52 (m, 4H), 7.58–7.62 (m, 2H), 7.75 (d, J = 6.3 Hz, 2H), 7.89 (d, J = 6.3 Hz, 2H) 8.00 (d, J = 6.3 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃) δ 124.3, 125.0, 125.1, 125.6, 125.8, 127.5, 127.9, 128.6, 128.9, 130.1, 130.7, 133.0, 136.7, 136.8, 138.1, 140.7, 143.5, 149.2, 162.2. HRMS (ESI): calcd for C₃₆H₂₃N₂S₄ [M + H]⁺, 611.0744; found, 611.0749.

Job plots

Job plots were constructed to determine the binding stoichiometry between sensors and Hg(ClO₄)₂ by monitoring the change in fluorescence emission intensity as a function of sensor/Hg²⁺ mole fraction. Stock solutions (20 μM each) of sensor and Hg(ClO₄)₂ were prepared in H₂O : CH₃CN (9 : 1). Samples were prepared with different mole fractions of sensor and Hg(ClO₄)₂ while maintaining the total concentration of ([sensor] + [Hg(ClO₄)₂]) for each sample at 20 μM.

Competition experiments

Hg(ClO₄)₂ (2 equivalents) was added to solutions of sensor (10 μM) and the tested metal ion (2 equivalents) in H₂O : CH₃CN (9 : 1). Test solutions were stirred for 5 min and then allowed to stand at room temperature for 30 min. Fluorescence measurements were done at λ_ex = 338 nm for 6, and λ_ex = 351 nm for 7.

Acknowledgements

We thank the Department of Chemistry and the Graduate College of the University of Iowa for support. We thank Dr Dale Swenson for assistance with X-ray crystallography.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, compound characterization data, crystallographic data. CCDC 1533224. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7qm00085e

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