3-Hydroxyflavone derivatives synthesized by a new simple method as chemosensors for cyanide anions

Abstract

Two novel 3-hydroxyflavone derivatives were synthesized by a one-step simple condensation, cyclization and subsequent oxidation reaction catalyzed by pyrrolidine, which shows great convenience compared with the traditional method. The compounds can recognize cyanide anions at very low concentration with remarkable spectral shift and an obvious color change from yellow to colorless. The bonding mechanism analysis via NMR experiments and mass spectra indicates that cyanide anions induce the deprotonation of the compounds.

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Introduction

Flavone derivatives are widely available in nature and have many biological activities like anti-inflammatory, anti-cancer, anti-estrogenic, anti-microbial, anti-allergic, anti-oxidant, anti-viral, and cytotoxic activities,¹ as well as modulate various enzyme systems.² 3-Hydroxyflavones (3-HFs), such as quercetin³ and kaempferol,⁴ are well-known natural flavonoids as representative fluorescent molecular chemosensors possessing favorable photo-physical properties, such as good photostabilities, reasonable fluorescence quantum yields^5,6 and typical excited-state intramolecular photon transfer (ESIPT).^7–9 Modified 3-HF derivatives have been extensively developed as chemosensors for biological molecules^10–15 such as protein, amino acids, DNA, and some specific ions, alkaline earth metals and alkali metals.^16,17 Due to the unique fluorescence properties of 3-HF derivatives, some of them also have been used as luminescent materials.¹⁸

Cyanides are extremely toxic to living organisms, which can cause the body's immune system to collapse, and even lead to death.¹⁹ But cyanides are still widely used in many industrial fields as un-replaceable starting materials. This enables the detection of trace cyanide to become an important part of human's health and social environment security.^20–24 At present, there are a lot of detection methods for cyanide, such as titrimetric analysis, voltammetry and potentiometry.²⁰ Recently fluorescence method has received more and more attention because of its high sensitivity and convenience.^25–29

We have been devoted to the research on chemosensors for cyanide for several years.^30–32 Through our unremitting efforts, two novel 3-hydroxyflavone derivatives were synthesized via optimized one-pot reaction, which include condensation, cyclization and oxidation reactions. Interestingly, the compounds can recognize cyanide with obvious absorption, fluorescence and color change. The synthetic method presented here is simpler than the common methods.^33–35

Experimental

Chemicals and instruments

4-(N,N-Diethylamino)benzaldehyde, 2-hydroxy-5-nitrophenylacetophenone, 2-hydroxy-5-bromoacetophenone and acryloyl chloride were purchased from Aladdin Reagents. Tetra(n-butyl)ammonium cyanide was used as received from Shanghai Macklin Biochemicol Co. Ltd. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III 400 MHz spectrometer at room temperature. Mass spectra were recorded on an Agilent Q-TOF6510 spectrometer. Elemental analyses were carried out on a PE 2400 autoanalyzer.

Synthesis and characterization

The synthetic routes of compounds 1 and 2 are shown in Scheme 1. The compounds were synthesized by a simple reaction between 2-hydroxy-5-nitrophenylacetophenone or 2-hydroxy-5-bromoacetophenone and 4-(N,N-diethylamino)benzaldehyde. 3-Acrylate group substituted flavone 3 similar to compound 2 was also synthesized.

Scheme 1 Synthetic routes to compounds 1–3.

Synthesis of compound 1. Compound 1 was synthesized by the reaction between N,N-diethylaminobenzaldehyde and 2-hydroxy-5-nitrophenylacetophenone. 0.44 g (2.5 mmol) of N,N-diethylaminobenzaldehyde, 0.45 g (2.5 mmol) of 2-hydroxy-5-nitrophenylacetophenone, 2 mL (25 mmol) of pyrrolidine as catalyst and 2.5 mL of ethanol as solvent were put in a 50 mL round bottom flask. The mixture was stirred at room temperature for 12 h and then red precipitate formed. The precipitate was filtered and repeatedly washed with ethanol, then dried. Finally, 0.41 g red solid of compound 1 were obtained in yield 46.3%. ¹H NMR (400 MHz, DMSO) δ: 1.14 (t, J = 7.2 Hz, 6H), 3.44 (q, J = 7.2 Hz, 4H), 6.82 (d, J = 9.2 Hz, 2H), 7.98 (d, J = 9.2 Hz, 1H), 8.14 (d, J = 9.2 Hz, 2H), 8.52 (dd, J = 9.2, 2.8 Hz, 1H), 8.78 (d, J = 2.8 Hz, 1H), 9.66 (s, 1H). ¹³C NMR (100 MHz, DMSO), δ: 12.42, 43.72, 110.81, 116.18, 120.24, 120.84, 121.47, 126.91, 129.54, 137.38, 143.52, 148.07, 148.80, 157.04, 170.80. Calcd exact mass: 355.1294, found 355.1273. Anal. calcd for C₁₉H₁₈N₂O₅: C 64.40, H 5.12, N 7.91; found C 64.18, H 5.11, N 7.90.

Synthesis of compound 2. Compound 2 was synthesized similar to compound 1 with N,N-diethylaminobenzaldehyde and 2-hydroxy-5-bromoacetophenone as the starting materials and 41.4% yield. ¹H NMR (400 MHz, DMSO), δ: 1.14 (t, J = 7.0 Hz, 6H), 3.43 (q, J = 7.0 Hz, 4H), 6.81 (d, J = 9.3 Hz, 2H), 7.73 (d, J = 9.0 Hz, 1H), 7.90 (dd, J = 9.0 Hz, 2.5 Hz, 1H), 8.10 (d, J = 9.2 Hz, 2H), 8.14 (d, J = 2.5 Hz, 1H), 9.36 (s, 1H). ¹³C NMR (100 MHz, DMSO), δ: 12.46, 43.74, 110.78, 116.56, 116.62, 120.84, 123.15, 126.55, 129.45, 135.48, 137.25, 147.64, 148.61, 153.08, 170.50. Calcd exact mass: 388.0548, found 388.0586. Anal. calcd for C₁₉H₁₈BrNO₃: C 58.78, H 4.67, N 3.61; found C 58.59, H 4.68, N 3.60.

Synthesis of compound 3. Compound 3 was synthesized by the reaction between compound 2 and acryloyl chloride in dry dichloromethane according to ref. 8 with 97.5% yield. ¹HNMR (400 MHz, DMSO), δ: 1.22 (t, J = 7.1 Hz, 6H), 3.43 (q, J = 7.1 Hz, 4H), 6.08 (dd, J = 10.4 Hz, 1.1 Hz, 1H), 6.68 (dd, J = 17.3 Hz, 1.2 Hz, 1H), 7.42 (d, J = 8.9 Hz, 1H), 7.74 (dd, J = 8.9 Hz, 2.5 Hz, 1H), 7.83 (d, J = 8.9 Hz, 2H), 8.35 (d, J = 1.2 Hz, 1H). ¹³C NMR (100 MHz, DMSO), δ: 12.67, 44.69, 111.07, 118.26, 119.88, 125.26, 127.35, 128.71, 130.19, 133.59, 136.40, 154.33, 157.27, 163.15, 170.50. Calcd exact mass: 442.0654, found 442.0649. Anal. calcd for C₂₂H₂₀BrNO₄: C 59.74, H 4.56, N 3.17; found C 59.52, H 4.57, N 3.18.

Photophysical properties and response to cyanide anions

Linear absorption and steady state fluorescence spectra of the compounds in acetonitrile with C = 1.0 × 10⁻⁵ mol L⁻¹ at room temperature were recorded on a Shimadzu UV-2550 spectrophotometer and a Horiba Fluoromax-4 fluorescence spectrometer, respectively. The spectral titration was carried out with a solution of tetra(n-butyl)ammonium cyanide (TBACN) in acetonitrile as CN⁻ source.

Structure determination

Single crystals of compounds 1 and 2 were obtained by slow evaporation of the compounds solution in a mixed solution of dichloromethane and ethanol. X-ray diffraction data of a dark red single crystal (1, 0.42 mm × 0.18 mm × 0.046 mm) and red single crystal (2, 0.42 mm × 0.088 mm × 0.036 mm) were collected on a Bruker Smart APEX-II CCD X-ray single crystal diffractometer. Crystal data, data collections and structure refinements of compounds 1 and 2 are list in Table S1.† Selected geometric parameters (Å, °) of the crystal structures are shown in Table S2.†

Results and discussion

Synthesis and molecular structures

Both the 3-hydroxyflavone derivatives 1 and 2 were synthesized by a new type of reaction between 5-substituted 2-hydroxy-phenylacetophenone and amino substituted benzaldehyde. The traditional method of synthesizing 3-hydroxyflavone derivatives often requires a lot of steps, and takes a long time and complex process.^33–35 However, after a continuous attempt, we have found a new, very simple synthetic method. The compounds can be obtained only by one step reaction at room temperature and simply treatment on the reaction products. Compared with the traditional method, this method shows great convenience. These compounds were fully characterized by NMR, mass spectra, crystal structure and elemental analyses (Fig. S1 and S2†). The compounds can be obtained successfully with a large excess of pyrrolidine as catalytic agent. The investigation indicates that less pyrrolidine will lead to more by-product and increases difficulty of purification. Further investigation indicates other 3-hydroxyflavone derivatives also can be obtained with hydroxyl-chalcone catalyzed by pyrrolidine. The possible reaction mechanisms maybe include three steps. Firstly, hydroxyl-chalcone was produced via typical condensation reaction between benzaldehyde and acetophenone. Then Michael addition cyclization occurs and at last auto-oxidation catalyzed by alkali leads to the product.

The crystal structures of compounds 1 and 2 (Fig. 1) were obtained, which verify further the structure of the compounds. The dihedral angels between phenyl ring (C10–C15) and 3-hydroxyflavone (C1–C9, O3) are 11.52° (1) and 9.74° (2), respectively (Table S2†), which indicates that there is a certain degree of twist. Packing diagrams of the compounds are shown in Fig. 2. As shown in Fig. 2a and b, molecules of 1 are consisted of (001) layers and the molecules in two adjacent layers are arranged in a head-to-tail mode. All the molecules are parallel and adjacent two molecules in same layer are also arranged in a head-to-tail mode. The distance between adjacent two molecules in same layer is 3.4 Å. Molecules of 2 are consisted of (100) layers (Fig. 2c and d) and the molecules in two adjacent layers are arranged in a head-to-head mode. The molecules are adjacent in head-to-tail mode in same layer and the angel between these two molecules is 12.73°.

Fig. 1 The molecular structures of compounds 1 (a) and 2 (b).

Fig. 2 Packing diagrams of compounds 1 (a, b) and 2 (c, d) along a (a), b (b, d) and c (c) axis.

Spectral response to cyanide anion

UV-vis absorption and fluorescence emission properties of the compounds for cyanide anions detection were investigated (Fig. 3 and S3†). As shown in Fig. 3, both compounds 1 and 2 exhibit strong absorption in blue region with the main absorption peak at 426 nm (1, ε = 3.56 × 10⁴ M⁻¹ cm⁻¹) and 415 nm (2, ε = 4.46 × 10⁴ M⁻¹ cm⁻¹), respectively. Compared with that of 2, the absorption peak of 1 has 11 nm red shift, which may be derived from the stronger electron accepting ability of nitro group. Upon the addition of cyanide anions, the original absorption peak of these compounds decreases gradually while a new absorption peak appears in short wavelength region (1: 318 nm, 2: 317 nm).

Fig. 3 Changes in UV-vis absorption spectra of compounds 1 (a) and 2 (b), fluorescence spectra of 2 (c) in acetonitrile upon the addition of different concentration of TBACN. Insert: absorbance at 318 nm and 426 nm (1), absorbance at 317 nm and 425 nm (2), fluorescence intensity at 538 nm as a function of concentration of TBACN.

As shown in Fig. 3c, compound 2 exhibits strong yellow-green fluorescence emission with fluorescence peak at 538 nm and quantum yield of 0.1. However, compound 1 emits very weak fluorescence emission with low quantum yield of 0.0006. The weaker fluorescence emission of compound 1 may be derived from the nitro group, which is a well-known fluorescence quenching group.

Upon the addition of CN⁻, fluorescence of both the compounds gradually decreases. Especially, strong yellow-green fluorescence of compound 2 at 538 nm is almost quenched.

That is to say, the compounds exhibit fluorescence turn-off response to cyanide anions. The compounds also exhibit obvious color and fluorescence change, which can be observed even by naked eyes, as shown in the inserts of Fig. 3. The solution color of compounds 1 and 2 changes from light yellow to colorless and the fluorescence is quenched upon the addition of cyanide anions.

A 3-acrylate group substituted flavone 3 similar to compound 2 was also synthesized in order to analysis the recognition mechanism. As shown in Fig. S4,† compound 3 shows un-detectable response to cyanide anions, which indicates that the hydroxyl group is necessary. The bonding mechanism was investigated using in situ ¹H NMR. As shown in Fig. 4, upon the addition of CN⁻, hydrogen proton signal of hydroxyl at 9.37 ppm (H^a) disappears and other hydrogen proton signals exhibit slight shift. The high resolution mass spectra before and after the addition of cyanide anions were also carried out. As shown in Fig. S2,† the ion peak of compound 2 at 388.0586 is attributed to [2 + H]⁺. After the addition of CN⁻, same ion peak was observed. The possible bonding mechanism of the compounds for cyanide anions is intermolecular hydrogen bonding reaction between CN⁻ and the hydroxyl group of the chemosensors.

Fig. 4 The ¹H NMR spectra of compound 2 in dimethyl sulfoxide before and after the addition of cyanide anions and bonding mechanism.

The detection limits of compounds 1 and 2 to CN⁻ with absorption spectra as detection signal (Fig. S5†) are 0.070 μM (1) and 0.053 μM (2), respectively, which were obtained according to ref. 36 and 37. And the detection limit of compound 2 in acetonitrile with fluorescent spectra as detection signal (Fig. S6†) is 0.000060 μM (2). The compounds exhibit higher sensitivity with fluorescence as detection signal compared to absorption.

It is well known that the high selectivity of a chemosensor is a very important criterion. Therefore, the influence of other anions compared with CN⁻ on the compounds 1 and 2 was also examined. As shown in Fig. S7,† other anions such as F⁻, Cl⁻, Br⁻, I⁻, CO₃²⁻, H₂PO₄⁻, HSO₄⁻ and SCN⁻ induce negligible absorption changes. That is to say, both the two compounds can recognize cyanide anions with high selectivity. To further confirm the specific selective properties of the compounds toward cyanide anion, competitive experiments were also carried out. At the presence of 3 equiv. of various anions (F⁻, Cl⁻, Br⁻, I⁻, CO₃²⁻, H₂PO₄⁻, HSO₄⁻ or SCN⁻) and 3 equiv. of CN⁻, the changes in UV-vis absorption and fluorescence spectra of the compounds are shown in Fig. 5 and S8.† It can be seen that these competing anions can neither lead to obvious response nor interfere the recognition reaction between the compounds and CN⁻.

Fig. 5 Changes in UV-vis absorption (a, 415 nm) and fluorescence (b, 538 nm) spectra of compound 2 in the presence of various anions (3 equiv.) in acetonitrile in response to CN⁻ (3 equiv.).

Conclusion

In summary, two novel 3-hydroxyflavone derivatives were synthesized using simple one step condensation and their effectiveness as chemosensors for CN⁻ was demonstrated. The results indicate that both the compounds exhibit obvious UV-vis absorption and fluorescence response in acetonitrile. Especially, compound 2 exhibits obvious fluorescence change. The bonding mechanism analysis indicates that intermolecular hydrogen bonding reaction induces the obvious spectral change.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51373069) and the Natural Science Foundation of Shandong Province (ZR2013BM005, ZR2015BL011), Scientific Research Foundation for the Returned Overseas Chinese Scholars (20121707), Colleges and Universities Science and Technology Foundation of Shandong Province (J16LA08) and the Fund of Graduate Innovation Foundation of University of Jinan, GIFUJN (S1504).

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Footnote

† Electronic supplementary information (ESI) available: High-resolution mass spectra and crystal data of compounds 1 and 2. Fluorescence titration of 1 and absorption change of 3 for cyanide anions. Selectivity and determination of the detection limit. CCDC 1477460 and 1477461. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra11415f

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