Tetraphenylethylene imidazolium macrocycle: synthesis and selective fluorescence turn-on sensing of pyrophosphate anions

Abstract

A new imidazolium macrocycle based on tetraphenylethylene (TPE) has been synthesized. This positively charged macrocycle showed a typical aggregation-induced emission (AIE) effect but fluorescence emission could not be induced by common inorganic anions in aqueous solution, including pyrophosphate bearing four negative charges. However, in the presence of a half equivalent of zinc(II) ions, the addition of pyrophosphate anions could arouse a strong fluorescence while other common inorganic anions gave almost no response. The macrocycle has an inherent cavity with the correct size for binding a pyrophosphate anion, so that the pyrophosphate anion can be easily aggregated together with the macrocycle and a zinc cation, which selectively triggers a turn-on fluorescence.

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Introduction

Anions are important chemical species that have a great impact on life and the environment. Among a large variety of anions, phosphate anions especially attract interest due to their important role in life. For example, Arthritis and Mönckeberg’s arteriosclerosis are related to abnormal levels of phosphate and pyrophosphate anions in blood serum.¹ In addition, signalling and energy transduction also require the participation of phosphate anions.² Various techniques, such as colorimetric³ and electro-chemiluminence,⁴ have been employed to detect phosphate anions. A fluorometric method, due to its high sensitivity, low cost and easy operation, has a greater potential in the analysis of compounds, including anions. During the past decades, many fluorescence receptors for the sensing of phosphate anions have been developed.⁵ However, a fluorescence probe with a high selectivity for pyrophosphate anions is still scarce.

In 2001, Tang’s group reported a new class of organic compounds with an amazing fluorescence property, that is, they had no emission in solution but emitted strong fluorescence in an aggregation state or as a solid.⁶ This novel fluorescence phenomenon was coined as aggregation-induced emission (AIE) and a probable mechanism of the restriction of intramolecular rotation (RIR) for the fluorescence was suggested. Since then, AIE fluorophores have attracted a great deal of attention due to their potential application in bio/chemosensors and solid emitters.⁷ Especially, due to the significant fluorescence change between aggregation and de-aggregation, AIE compounds have received more attention in research as bio/chemosensors.^7,8 However, research work for the sensing of phosphate anions by the AIE effect,⁹ especially for the selective sensing of anions in aqueous solution,¹⁰ is very rare. The reason behind this might be because of the solvent competing effect in highly polar solvent.¹¹ Herein, we report the synthesis of an imidazolium macrocycle based on tetraphenylethylene (TPE). The macrocycle could selectively detect pyrophosphate anions in aqueous solution in the presence of zinc(II) ions.

Result and discussion

As shown in Scheme 1, the TPE derivative 1, a known AIE compound,¹² was used as a starting material and was easily transformed into dialdehyde 2 using a Duff reaction. With dialdehyde 2 in hand, dialcohol 3 and dichloride 4 were obtained by reduction with NaBH₄ followed by a chlorinating reaction with thionyl chloride. Upon a nucleophilic substitution with imidazole, dichloride 4 was converted into diimidazole 5. Finally, the target product imidazolium macrocycle 6 containing TPE units was obtained in 24% yield by the reaction of dichloride 4 and diimidazole 5 in the presence of a tetrabutylammonium chloride template.¹³

Scheme 1 The synthesis of tetraphenylethylene macrocycle 6.

Fortunately, the single crystal of compound 6 was obtained by slow evaporation of its solution in THF/H₂O. The crystal structure is monoclinic with C2 space group symmetry and cell parameters of a = 33.698, b = 8.9588, c = 10.793 Å and α = γ = 90°, β = 107.81°.¹⁴ The distance between the two opposite nitrogen atoms of 6 is 5.836 Å as shown in Fig. 1. The macrocycle forms a rectangle cavity approximately 8.294 × 5.836 Ų in size (Fig. 1A). As an interesting packing mode, the cavities can stack upon each other to form a channel which is filled with solvents and counter ions. The channels arrange in parallel to give a 3-dimensional network (Fig. 1B).

Fig. 1 (A) The crystal structure of 6. (B) The crystal packing pattern of 6, hydrogen atoms are hidden for clarity.

The imidazolium macrocycle 6 is soluble in highly polar solvents such as acetonitrile and methanol but cannot be dissolved in less polar solvents, such as hexane. Hence, its AIE effect could be observed in the mixed solvent of THF and hexane (Fig. S1†). A solution of 6 in THF (containing 0.5% acetonitrile) was not light emitting, but it appeared fluorescent at 472 nm after 50% hexane (volume percentage, the same below) was added and a clouding effect started to be observed. After that, the fluorescence became stronger with the continued addition of hexane.

Similarly, an aqueous solution of 6 (5.0 × 10⁻⁵ M) in water containing 0.5% DMSO exhibited almost no emission. By addition of various anions, such as pyrophosphate P₂O₇⁴⁻ (PPi), PO₄³⁻ (Pi), HPO₄²⁻ (HPi), H₂PO₄⁻ (H₂Pi), SO₄²⁻, SO₃²⁻, NO₃⁻, NO₂⁻, HCO₃⁻, CO₃²⁻, C₂O₄²⁻, F⁻, Cl⁻, Br⁻ and I⁻ (counter ion, Na⁺), no obvious enhancement of the emission could be observed. However, upon addition of one equivalent of Zn(OAc)₂ before or after the addition of anions, the pyrophosphate anion gave rise to a strong blue emission while other anions resulted in almost no emission under a 365 nm lamp light (Fig. 2). This clearly indicated that 6 could be used as a selective turn-on sensor for pyrophosphate anions in the presence of Zn(II). It was found that other some divalent transition metal ions, such as Cu(II), Ni(II), Pb(II), Co(II), and Cd(II), also changed the emission intensity but the effect was much less than that of Zn(II) (Fig. S2†). The titration of 6 (5 × 10⁻⁵ M) with Zn(II) in the presence of two equivalents of PPi showed that the fluorescence intensity reached a maximum after the addition of 0.5 equivalents of Zn(II) (Fig. S3†), which was 25 times larger than that without Zn(II).

Fig. 2 (A) The fluorescence spectra of 6 in water containing 0.5% DMSO with the addition of different anions in the presence of zinc(II). Insets, photos of the solutions under 365 nm lamp light; λ_ex = 347 nm, ex/em slits = 5/5 nm. (B) Diagram of the emission at 472 nm of 6 in the presence of zinc(II) and different anions. [6] = 2[Zn(OAc)₂] = 1/2[anion] = 5.0 × 10⁻⁵ M.

Moreover, even in the presence of a mixture of other anions with each anion being of one equivalent to 6, the PPi anions aroused a strong fluorescence while the mixture of other anions without PPi only gave rise to a very weak fluorescence (Fig. 3). The intensity difference between the mixtures of anions with and without PPi was more than 10-fold. In addition, the fluorescence aroused by PPi in the presence of other anions was almost the same as that in the absence of the other anions. This result demonstrated that other anions did not interfere with the detection of PPi by this fluorescence receptor.

Fig. 3 The fluorescence spectra of 6 in water containing 0.5% DMSO with the addition of an anion mixture in the presence of zinc(II). λ_ex = 354 nm, ex/em slit width = 5/5 nm, [6] = [each other anion] = 1/2[PPi] = 2[Zn(II)] = 5 × 10⁻⁵ M.

A fluorescence titration of 6 (5 × 10⁻⁵ M) with pyrophosphate anions in the presence of Zn(II) (2.5 × 10⁻⁵ M) in H₂O containing 0.5% DMSO was also performed (Fig. 4). When a small amount of pyrophosphate anions was added, the enhancement of fluorescence was not significant. This might result from the solubility of the complex formed upon the addition of pyrophosphate anions. In the range of 0.6–2.0 equivalents of pyrophosphate anions, the fluorescence intensity rapidly increased with the added anions. After that, the increase in the fluorescence intensity became slow and finally stopped when 2.4 equivalents of the pyrophosphate anions were added.

Fig. 4 The fluorescence change of 6 with PPi in the presence of zinc(II) in water containing 0.5% DMSO. Inset, curve of the intensity at 472 nm vs. the concentration of PPi. λ_ex = 347 nm, ex/em slits = 5/5 nm, [6] = 2[Zn(OAc)₂] = 5.0 × 10⁻⁵ M.

A UV-Vis titration confirmed the interaction between 6 and the pyrophosphate anion (Fig. 5). With the addition of pyrophosphate anions, the absorption peaks both at 253 nm and 313 nm decreased while a new absorption band at about 400 nm increased. Moreover, the absorption at 313 nm had a small bathochromic shift toward 320 nm with the anion. This obvious change demonstrated the formation of a stable complex between 6 and the pyrophosphate anion even without zinc(II). The association constant of the complex, according to a 1 : 1 molar ratio of the complex between 6 and pyrophosphate, was calculated to be (1.41 ± 0.10) × 10⁴ M⁻¹ by non-linear curve fitting of the absorbance at 253 nm vs. the concentration of pyrophosphate (Fig. S4†).¹⁵ In the presence of Zn(II), the UV-Vis titration of 6 with PPi was very similar to that of without Zn(II) (Fig. S5†), indicating that the metal ions did not directly interact with the macrocycle but only with the anion ions.

Fig. 5 Change of the absorption spectra of 6 in water containing 0.5% DMSO with PPi. Inset, curve of the absorbance at 253 nm vs. the concentration of PPi. [6] = 5.0 × 10⁻⁵ M.

From the above absorption spectra, it was found that the mixing of 6 and PPi easily led to the formation of the 6–PPi complex, probably because the two negative charges at the two ends of one pyrophosphate anion (each end is a phosphate unit) are at a correct distance that can just allow the binding of the two positive charges in the molecule of 6 (Fig. 6). As discussed earlier, in the crystal structure of 6 the distance between the nitrogen atoms of each imidazolium unit of 6 is about 5.836 Å while the longest distance between the oxygen atoms of each phosphate unit of PPi is 5.36 Å. Therefore, the cavity of 6 composed of two imidazolium units is suitable for the inclusion of one molecule of PPi to form a 6–PPi complex, driven by an electrostatic attraction (Fig. 6). The new absorption peak at about 400 nm of 6 upon addition of PPi demonstrated that PPi had such a strong interaction with 6 that the acidic proton on the imidazolium unit was probably removed by the pyrophosphate anion. In the presence of a zinc cation, the two-component 6–PPi complex was transformed into a five-component (6–PPi)₄–Zn complex due to one zinc cation being coordinated by two 6–PPi complexes. With continued coordination, an aggregate of (6–PPi)₄–Zn complexes formed, therefore, it gave a strong fluorescence.

Fig. 6 The AIE mechanism of 6 triggered by PPi in the presence of Zn(II).

In addition, both the emission spectrum and the excitation one of the suspension of 6 in THF/hexane 10 : 90 were similar to that of the 6–PPi–Zn mixture in aqueous solution, hinting that the complex of 6–PPi–Zn existed in an aggregation state (Fig. S6†). The dynamic light scattering (DLS) diagram corroborated that the mixture of 6, PPi and Zn(II) in water formed aggregates with an average diameter up to 2800 nm (Fig. S7†). Due to its solubility in water, the 6–PPi complex was not light emitting, but the five component (6–PPi)₄–Zn complex had low solubility, so it was able to emit a strong fluorescence (Fig. 6). The formation of the 6–PPi complex in a 1 : 1 molar ratio was confirmed by the ESI⁺ HRMS spectrum of a mixture of 6 and sodium pyrophosphate (calcd for C₆₆H₅₈N₄Na₃O₁₁P₂ 1213.3270 [6–PPi + 3Na⁺]⁺, found 1213.3265 [6–PPi + 3Na⁺]⁺) (Fig. S8†).

Conclusions

In conclusion, a novel imidazolium macrocycle based on tetraphenylethylene (TPE) was synthesized. It was found that this macrocycle had an inherent cavity with the ideal size for binding a pyrophosphate anion. Without the addition of a zinc cation, the 6–PPi complex was soluble and could not emit light. In the presence of a zinc cation, the 6–PPi complex could form aggregates due to its coordination with zinc, which resulted in strong fluorescence. Therefore, this macrocycle could act as a selective probe for pyrophosphate anions in aqueous solution. Selective inclusion of an analyte into the cavity of a macrocycle bearing TPE units, leading to turn-on fluorescence, provides a new approach for the design of selective fluorescence probes.

Acknowledgements

We thank the National Natural Science Foundation of China (No. 20872040 and 21072067) for funding support and the Analytical and Testing Centre at Huazhong University of Science and Technology for measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis of compounds and additional spectra, ¹H NMR, ¹³C NMR, IR, and HRMS spectra of compounds 2–6. CCDC 1402520. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra09721e

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