A highly selective fluorescent sensor for Al³⁺ based on a new diarylethene with a 3-hydroxy-2-naphthohydrazide unit

Abstract

A novel diarylethene derivative with a 3-hydroxy-2-naphthohydrazide unit was successfully synthesized. It displayed favorable photochromism upon irradiation with UV/vis lights. The deprotonated derivative of the diarylethene, due to the addition of triethylamine into its solution, also showed excellent photochromism with a distinguishable color change. The deprotonated example could recover its original state using trifluoroacetic acid. Furthermore, the diarylethene was highly selective toward Al³⁺ with an obvious fluorescent color change from dark to green in acetonitrile. Finally, two logic circuits were constructed on the basis of the unimolecular platform with multi-responsive photochromism and fluorescent switching properties.

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Introduction

Aluminum, as the earth's crust third most abundant element, is widely dispersed and used in the environment around us in modern society. When aluminum accumulates in the human body, it can lead to a wide range of diseases, such as bone and joint diseases, neuronal disorders leading to dementia, myopathy and Alzheimer's disease.^1,2 Thus, the detection of Al³⁺ in environmental and biological systems with high selectivity and sensitivity is very important. So far, a lot of methods have been designed for the detection of Al³⁺. Many traditional methods, including atomic absorption spectrometry,³ inductively coupled plasma-mass spectroscopy,⁴ and inductively coupled plasma-atomic emission spectrometry,⁵ required complicated procedures and the involvement of expensive instruments, whereas fluorescent chemosensors provide another privileged approach which do offer the advantages of operation simplicity, high selectivity, sensitivity, and real-time response.^6–8 Therefore, developing highly selective and sensitive fluorescent probes for Al³⁺ have recently attracted much attention.

As one of the most promising photoresponsive materials for applications in photonic devices,⁹ diarylethenes have been well concerned for their fast response, excellent thermal stability, and outstanding fatigue resistance.^10–12 Recently, diarylethenes with versatile functional groups have been widely applied as fluorescent probes due to their special photoswitching properties.^13–15 For example, Tian et al. reported a new multi-state 1,8-naphthalimide-piperazine-tethered dithienylethene, which responded to H⁺ and Cu²⁺.¹⁶ Zeng et al. reported a phenanthrene-bridged photochromic diarylethene with double crown ethers to selectively recognize Cu²⁺ and Hg²⁺ in both solution and solid.¹⁷ In our previous work, we have successfully introduced various functional units (such as terpyridine, quinoline) into diarylethenes and most of them could serve as chemosensors for various metal ions (Hg²⁺, Zn²⁺, etc.).^18,19 However, only few reports on diarylethene-based fluorescent chemosensors for Al³⁺ have been published, because Al³⁺ has always been a problematic analyte due to the lack of spectroscopic characteristics and poor coordination ability compared to other transition metals.²⁰ Although the reported fluorescent chemosensors based on diarylethene with a rhodamine unit were responsive toward Al³⁺ with remarkable color and fluorescence changes, most of them still suffered from the interference of Cr³⁺ and Fe³⁺.^21,22 3-Hydroxy-2-naphthohydrazide derivatives were used for the histochemical demonstration of DNA in the past.²³ Although they have been widely used as selective chemosensors for metal ions,^24,25 their diarylethene derivatives as fluorescent sensors have not been reported.

In this paper, 3-hydroxy-2-naphthohydrazide unit was firstly introduced to diarylethene to construt a fluorescent probe for Al³⁺ with high selectivity and sensitivity. Its multifunctional switching characteristics induced by base/acid, lights, as well as metal ions were systematically investigated. The photochromism of the diarylethene was shown in Scheme 1.

Scheme 1 Photochromism of 1o.

Experimental

General methods

Chemical reagents were purchased from Aldrich and used without further purification. All solvents were spectrometric grade and purified prior to use. NMR spectra were collected on a Bruker AV400 (400 MHz) spectrometer with DMSO-d₆ as the solvent and tetramethylsilane (TMS) as an internal standard. UV-vis spectra were measured on an Agilent 8453 UV/Vis spectrophotometer. Fluorescence spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer. Infrared spectra (IR) were collected on a Bruker Vertex-70 spectrometer. Elemental analysis was carried out with a PE CHN 2400 analyzer. Melting points were measured on a WRS-1B melting point apparatus. Photo-irradiation experiments were performed with a SHG-200 UV lamp, and a BMH-250 visible lamp. The fluorescence quantum yields in methanol were measured with an Absolute PL Quantumn Yield Spectrometer QY C11347^_11.

Synthesis of 1o

The synthetic route of the diarylethene (1o) was shown in Scheme 2. Compound 2 was prepared by the method reported in previous literature.²⁶ The experimental procedures details and characterization data of 1o were showed as follows.

Scheme 2 Synthetic route of the diarylethene 1o.

A mixture of 2 (0.05 g, 0.10 mmol) and 3-hydroxy-2-naphthohydrazide (0.02 g, 0.10 mmol) in anhydrous ethanol (50 mL) was refluxed for 5 h. The mixture was cooled to room temperature and white precipitate was obtained. The crude product was filtered off, washed with ethanol and dried under vacuum. The target product was recrystallized in ethanol to give 0.05 g in 80% yield. M.p. 492–494 K; ¹H NMR (DMSO-d₆, 400 MHz, TMS), δ (ppm): 1.89 (s, 3H), 2.17 (s, 3H), 7.26–7.39 (m, 5H), 7.48 (t, 1H, J = 6.0 Hz), 7.60 (s, 2H), 7.73 (d, 1H, J = 8.0 Hz), 7.88 (d, 1H, J = 8.0 Hz), 8.37 (s, 1H), 8.59 (s, 1H), 11.16 (s, 1H), 11.96 (s, 1H); ¹³C NMR (DMSO-d₆, 100 MHz, TMS), δ (ppm): 13.5, 15.0, 104.7, 111.0, 111.9, 119.8, 121.1, 124.3, 124.5, 124.8, 125.5, 125.9, 126.3, 127.3, 128.7, 129.1, 130.5, 130.7, 136.3, 138.2, 142.9, 145.4, 154.0, 154.3, 157.2, 164.0; IR (ν, KBr, cm⁻¹): 628, 757, 802, 847, 872, 927, 988, 1062, 1104, 1122, 1192, 1225, 1269, 1453, 1556, 1638, 2921; MS(ESI, m/z): calcd for C₈₆H₈₅F₆N₇O₄S₂: 614.6, found: 615.1 [M + H]⁺.

Source of ions

The store solutions of Al³⁺, Co²⁺, Pb²⁺, Cr³⁺, Zn²⁺, Ca²⁺, Mg²⁺, Cu²⁺, Ni²⁺, Cd²⁺, and Fe³⁺ were prepared by dissolving their metal nitrate salts (0.10 mmol) in distilled water (10 mL), respectively. The store solutions of Hg²⁺, Mn²⁺, K⁺ and Ba²⁺ were obtained by dissolving their chloride salts (0.10 mmol) in distilled water (10 mL), respectively.

UV-vis experiments

A diarylethene solution (2.0 × 10⁻⁵ mol L⁻¹) was prepared in acetonitrile. The volume of the diarylethene solution in the UV-vis measurements was 3.0 mL. The effects of adding various metal ions to the solution could be clearly observed by naked eyes.

Fluorescence experiments

A diarylethene solution (2.0 × 10⁻⁵ mol L⁻¹) was prepared in methanol. The fluorescence properties of the diarylethene were investigated by the stimulation of light and Al³⁺. The excitation wavelength of the diarylethene and the complexed with Al³⁺ were determined at 351 nm. The excitation and emission slit widths were 5.0 nm.

Results and discussion

Photochromism and fluorescence properties of 1o

The absorption spectral change of 1o induced by UV/vis lights was investigated in acetonitrile (2.0 × 10⁻⁵ mol L⁻¹) at room temperature. As shown in Fig. 1a, the absorption maximum of 1o was observed at 332 nm (ε = 3.55 × 10⁴ mol⁻¹ L cm⁻¹) due to π → π* transition.²⁷ Upon irradiation with 297 nm light, the colorless solution of 1o turned purple and a new visible absorption band centered at 547 nm (ε = 1.52 × 10⁴ mol⁻¹ L cm⁻¹) emerged due to the formation of closed-ring isomer 1c. Reversely, the purple color was bleached to colorless upon irradiation with visible light (λ > 500 nm), and the absorption spectrum returned to the initial state of 1o. An isosbestic point at 352 nm was observed, indicating that there was a two component photochromic reaction.^28–30 The quantum yields of cyclization and cycloreversion were determined to be 0.27 and 0.02, respectively, with 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene as a reference.³¹ The fatigue resistance of 1o was tested through alternating irradiation with UV and visible lights at room temperature. The result showed that the coloration and decoloration cycles of 1o could be repeated for 10 times with 7.5% degradation (Fig. 1c).

Fig. 1 (a) Absorption spectral and color changes of 1o by photoirradiation in acetonitrile (2.0 × 10⁻⁵ mol L⁻¹) at room temperature, (b) emission spectral change of 1o by photoirradiation in methanol (2.0 × 10⁻⁵ mol L⁻¹) at room temperature, (c) fatigue resistance of 1o in acetonitrile at room temperature.

Like other reported diarylethenes,^32–34 1o exhibited an evident fluorescent switching between the open-ring isomer and the closed-ring isomer upon irradiation with UV/vis lights. As shown in Fig. 1b, the emission peak of 1o in methanol was observed at 454 nm, and the absolute fluorescence quantum yield of 1o was determined to be 0.003. Upon irradiation with 297 nm UV light, the emission intensity at 454 nm gradually decreased due to the formation of the non-fluorescence closed-ring isomer 1c. When arrived at the photostationary state (PSS), the emission intensity of 1o was quenched ca. 64%, indicating that the diarylethene exhibited relatively strong fluorescence modulation efficiency in methanol solution. The residual fluorescence may be attributed to the incomplete cyclization and the existence of isomers with parallel conformation.³⁵ Reversely, the fluorescence of 1o could be restored upon irradiation with appropriate visible light (λ > 500 nm).

Acidichromism

The dual-controllable photochromic behaviors of 1o were investigated with the stimulation of light and base/acid. The absorption spectra and color changes of 1o by the stimulation of TEA/TFA and UV/vis lights in acetonitrile (2.0 × 10⁻⁵ mol L⁻¹) at room temperature were shown in Fig. 2. Upon addition of 50 equiv. TEA to the solution of 1o produced the deprotonated diarylethene 1o′, accompanied by the solution color changed from colorless to light yellow. Similarly, upon addition of 50 equiv. TEA to the solution of 1c in the PSS generated the deprotonated 1c′, accompanied by the solution color changed from purple to blue. Compared to 1c, the absorption maximum of 1c′ was remarkably increased and redshifted from 547 nm to 575 nm. The absorption spectrums of 1o and 1c could be restored by adding 50 equiv. TFA to the solutions of 1o′ and 1c′, respectively (Fig. 2a). Moreover, 1o′ could also undergo photoisomerization upon alternating the irradiation with UV and visible lights (Fig. 2b). Upon irradiation with 297 nm light, a new visible absorption band centered at 575 nm appeared and increased, due to the formation of the closed-ring isomer 1c′, accompanied by the solution color changed from light yellow to blue. The blue solution could be changed to light yellow upon irradiated with visible light (λ > 500 nm), indicating that 1c′ could return to the initial form 1o′. The solution color changes of 1o and 1c induced by light and TEA/TFA were shown in Fig. 2c. All the results indicated that an efficient dual-controllable colorimetric switch could be constructed based on 1o with TEA/TFA and UV/vis lights as stimuli.

Fig. 2 Absorption spectral and color changes of 1o induced by TEA/TFA and UV/vis light in acetonitrile (2.0 × 10⁻⁵ mol L⁻¹): (a) 1o and 1c induced by TEA/TFA, (b) 1o′ induced by UV/vis lights, (c) photos demonstrating changes in its absorption.

Selectivity of 1o for metal ions

The ion-recognition behavior of 1o was investigated with fluorescence spectroscopy in methanol. To the solution of 1o (2.0 × 10⁻⁵ mol L⁻¹), 1.0 equiv. of various metal ions, including Al³⁺, Zn²⁺, Cd²⁺, Fe³⁺, Cr³⁺, Hg²⁺, Cu²⁺, Mg²⁺, Co²⁺, Mn²⁺, Ca²⁺, K⁺, Sr²⁺, Ba²⁺, Pb²⁺ and Ni²⁺, were added individually under the same conditions. Fig. 3 shows the fluorescence and fluorescent color changes of 1o induced by the addition of the above mentioned metal ions. The fluorescence and fluorescent color showed no remarkable changes in the present of various metal ions, except Al³⁺. When Al³⁺ was added to the methanol solution of 1o, the fluorescence intensity was enhanced evidently and the emission peak red shifted 109 nm with a concomitant fluorescent color change from dark to green. The increase in emission intensity may be attributed to the formation of the 1o-Al³⁺ (1o′′) complex. When 1o was coordinated with the Al³⁺, a large CHEF effect was observed because the stable complexation between 1o and Al³⁺ prohibited the PET process from the electron-donating group to the diarylethene. In addition, the stable chelation of 1o with the aluminum ion inhibited the C=N isomerization and rigidification of fluorophore structure.^36,37 The results suggested that 1o can serve as a fluorescence sensor for highly selective recognition of Al³⁺ in acetonitrile.

Fig. 3 Changes in the fluorescence (λ_ex = 351 nm) of 1o induced by the addition of various metal ions (1.0 eq.) in methanol (2.0 × 10⁻⁵ mol L⁻¹): (a) emission spectral changes, (b) emission intensity changes, (c) photos demonstrating changes in its fluorescence.

Changes in fluorescence by Al³⁺ stimuli

To further evaluate the responsive nature of 1o induced by Al³⁺, the fluorescence titration study was carried out in methanol (2.0 × 10⁻⁵ mol L⁻¹) at room temperature, and the results were shown in Fig. 4. Upon addition of Al³⁺, the fluorescence intensity of 1o at 563 nm gradually increased when Al³⁺ increased from 0 to 1.0 equiv. of 1o, followed by a plateau with further titration. Compared with 1o, the emission intensity of 1o′′ was enhanced by 47 fold at the plateau, and the absolute fluorescence quantum yield was determined to be 0.074. An obvious change in fluorescent color from dark to green could be easily observed by naked eyes (Fig. 4c). Upon irradiation at 297 nm light, the fluorescent color of 1o′′ changed from green to dark green due to the formation of the nonfluorescent closed-ring isomer of 1c-Al³⁺ (1c′′) (Fig. S.3a, ESI†). The fluorescence intensity of 1o′′ was quenched to 18% in the PSS, indicating that the diarylethene exhibited relatively strong fluorescence modulation efficiency in methanol. Furthermore, the fluorescence titration of 1c with Al³⁺ was also performed in methanol at room temperature (Fig. 4b). The results showed that the emission intensity of 1c′′ was reached by adding 1.0 equiv. of Al³⁺. Compared with 1c, the emission intensity of 1c′′ was enhanced by 24 fold. The fluorescence intensity of 1c′′ could not return to 1o′′ upon irradiation with visible light (Fig. S.3b, ESI†). Enough long time exposure of 1c′′ under visible light enhanced the fluorescence intensity by 3 fold. This phenomenon could be applied in nondestructive readout.^38,39 The fluorescent color changes among 1o, 1c, 1o′′, and 1c′′ were shown in Fig. 4c. The results indicated that the fluorescent properties of 1o and 1c could not be restored by adding excess EDTA to the solutions of 1o′′ and 1c′′, respectively.

Fig. 4 Changes in the fluorescence (λ_ex = 351 nm) and color of 1o induced by Al³⁺/EDTA and UV/vis light in methanol (2.0 × 10⁻⁵ mol L⁻¹): (a) 1o induced by Al³⁺, (b) 1c induced by Al³⁺, (c) photos demonstrating changes in its fluorescence.

In order to calculate the binding ratio between 1o and Al³⁺, Job's plots were obtained by fluorescence titration according to the method reported previously.⁴⁰ As shown in Fig. 5, the molar fraction of [1o]/([1o] + [Al³⁺]) was about 0.5 when the maximum value reached, indicating that the complex ratio between Al³⁺ and 1o was 1 : 1. In addition, the mass spectra of 1o and 1o-Al³⁺ were recorded on the ESI mass spectrometry. As shown in Fig. S.2, ESI,† 1o displayed a characteristic peak at 615.1 m/z for [1o + H⁺]⁺ (calcd 615.6). When excess amounts of Al³⁺ were added, a new peak at 763.6 m/z for [1o + Al³⁺ + 2NO₃⁻–2H⁺]⁻ (calcd 763.6) emerged due to the formation of complex 1o′′. The result further confirmed that the binding stoichiometry was 1 : 1 in complex 1o′′. Based on the 1 : 1 stoichiometry and fluorescence titration data, the binding constant Kα was calculated to be 4.88 × 10⁴ L mol⁻¹ (Fig. S.3a, ESI†), using the Benesi–Hildebrand expression.⁴¹ According to the reported method,⁴² the detection limits were calculated to be 2.2 × 10⁻⁹ mol L⁻¹ for Al³⁺ (Fig. S.3b, ESI†). In this complex, Al³⁺ may coordinate with oxygen atoms of hydroxy and the nitrogen atom of the tertiary amine. The proposed binding mode between 1o and Al³⁺ was shown in Fig. 8a.

Fig. 5 Job' plot showing 1 : 1 complex of 1o and Al³⁺.

In order to confirm the selectivity of 1o as a chemosensor for Al³⁺ in methanol, competitive experiments were performed in the presence of other metal ions, such as Zn²⁺, Cd²⁺, Fe³⁺, Cr³⁺, Hg²⁺, Cu²⁺, Mg²⁺, Co²⁺, Mn²⁺, Ca²⁺, K⁺, Sr²⁺, Ba²⁺, Pb²⁺ and Ni²⁺. As shown in Fig. 6, there was no interference when Al³⁺ was added in the presence of Cd²⁺, Hg²⁺, Cu²⁺, Mg²⁺, Co²⁺, Mn²⁺, K⁺, Sr²⁺, Ba²⁺, Pb²⁺ and Ni²⁺. The fluorescence intensity increased slightly with Ca²⁺ and Zn²⁺, and inhibited slightly with Fe³⁺ and Cr³⁺. The results suggested that 1o could be served as a selective sensor for recognition of Al³⁺ even competing with other related species in methanol.

Fig. 6 Competitive tests for the fluorescence responses of 1o to various metal ions in methanol (2.0 × 10⁻⁵ mol L⁻¹). Bars represent the ratio of emission intensity at 563 nm. Red bars represent the addition of 1.0 equiv. various metal ions to the solution of 1o. Gray bars represent the addition of Al³⁺ (1.0 equiv.) to the above solution, respectively.

Application in logic circuit

As described above, the photochromic behaviors of 1o could be effectively modulated by base/acid, and lights. The photochromism, color and absorbent changes of 1o induced by TEA/TFA and UV/vis lights were shown in Fig. 7a. On the basis of these properties, a combinational logic circuit was constructed by the combination of four input signals (In1: 297 nm light, In2: λ > 500 nm light, In3: TEA, and In4: TFA) and one output signal (O1: absorbance at 575 nm) (Fig. 7b). In this logic circuit, four input signals were either ‘on’ or ‘off’ with different Boolean values of ‘1’ or ‘0’. When 297 nm light was used, In1 was switched to “on” state with a Boolean value of ‘1’. Similarly, In2 was ‘1’ corresponding to irradiation with appropriate visible light (λ > 500 nm), In3 was ‘1’ corresponding to the addition of TEA, and In4 was ‘1’ corresponding to the addition of TFA. The output signal could be regarded as ‘on’ when the absorbance at 575 nm was larger than 0.35. Otherwise, it was regarded as ‘off’ state with a Boolean value of ‘0’. Under the stimuli of different inputs, the diarylethene showed an on-off-on colorimetric switching behavior. Consequently, 1o can read a string of four inputs and write one output (Table 1).

Fig. 7 (a) Photochromism, color and absorbent changes of 1o induced by TEA/TFA and UV/vis. (b) The combinational logic circuits equivalent to the truth table given in Table 1: In1 (297 nm light), In2 (λ > 500 nm light), In3 (TEA), In4 (TFA) and Output1 (absorption at 575 nm).

Table 1 Truth table for all possible strings of four binary-input data and the corresponding output digit^a

Inputs				Output λab = 575 nm
In1 (UV)	In2 (vis)	In3 (TEA)	In4 (TFA)
a At 575 nm, the absorption intensity more than 0.35 is defined as 1, otherwise defined as 0.
0	0	0	0	0
1	0	0	0	0
0	1	0	0	0
0	0	1	0	0
0	0	0	1	0
1	1	0	0	0
1	0	1	0	1
1	0	0	1	0
0	1	1	0	0
0	1	0	1	0
0	0	1	1	0
1	1	1	0	0
1	1	0	1	0
1	0	1	1	0
0	1	1	1	0
1	1	1	1	0

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In addition, the fluorescence of 1o could be independently modulated by Al³⁺ and UV/vis light. The photochromism, color and fluorescence changes of 1o induced by Al³⁺ and UV/vis were shown in Fig. 8a. Consequently, another logic circuit could be constructed by three input signals (In1: 297 nm light, In2: >500 nm light, and In5: Al³⁺) and one output signal (O2: fluorescence intensity at 563 nm) (Fig. 8b). 1o exhibited strong emission by the addition of Al³⁺, and its fluorescence intensity at 454 nm was regarded as the original value. The output signal could serve as ‘on’ when the fluorescence intensity was 30-fold larger than the original value, otherwise defined as ‘off’. For example, when the input string is ‘0, 0, and 1’, corresponding to the In1, In2, and In5 were ‘off, off, and on’. Under these conditions, 1o was by the stimulation of Al³⁺ and its emission intensity at 563 nm increased significantly. As a result, the output signal O2 was ‘on’, and the output digit was ‘1’. All of the possible logic strings of the three inputs and their corresponding outputs were listed in Table 2.

Fig. 8 (a) Photochromism, color and fluorescence changes of 1o induced by Al³⁺/EDTA and UV/vis. (b) The combinational logic circuits equivalent to the truth table given in Table 2: In1 (297 nm light), In2 (>500 nm light), In5 (Al³⁺) and Output2 (fluorescence at 563 nm).

Table 2 Truth table for all possible strings of four binary-input data and the corresponding output digit^a

Input			Output λem = 563 nm
In1 (UV)	In2 (vis)	In5 (Al^3+)
a At 563 nm, the emission intensity more than 30-fold of the original value is defined as 1, otherwise defined as 0.
0	0	0	0
1	0	0	0
0	1	0	0
0	0	1	1
1	1	0	0
1	0	1	0
0	1	1	1
1	1	1	1

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Conclusions

In conclusion, a novel fluorescent sensor based on a photochromic diarylethene with a 3-hydroxy-2-naphthohydrazide unit was developed for the first time. It exhibited multi-responsive behaviors when stimulated by lights, base/acid, and Al³⁺. In addition, the diarylethene could be utilized as a fluorescent sensor for the recognition of Al³⁺ with high selectivity. Moreover, two logic circuits were constructed on the basis of the unimolecular platform. This work provides a demonstration for the construction of diarylethene-based fluorescent sensors for the recognition of specific metal ions of interest by rational structural design.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (21362013, 51373072), the Natural Science Foundation of Jiangxi Province (20151BAB203019), and the Young Talents Project of Jiangxi Science and Technology Normal University (2015QNBJRC004).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18068j

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