Novel Diarylethene-Based Fluorescent Switching for the Detection of Al3+ and Construction of Logic Circuit.

A novel photochromic diarylethene was synthesized successfully containing a phthalazine unit. Its multistate fluorescence switching properties were investigated by stimulating with UV/vis lights and Al3+/EDTA. The synthesized diarylethene displayed excellent selectivity to Al3+ with a distinct fluorescence change, revealing that it could be used as a sensor for fluorescence identification of Al3+, and a logic circuit was constructed by utilizing this diarylethene molecular platform. Moreover, it also exhibited a high accuracy for the determination of Al3+ in practical water samples.

Introduction

Photochromic compounds can show two isomers when irradiated with ultraviolet and visible lights. During the process of reversible photoisomerization, the physical properties, including refractive index, absorption spectra, fluorescence, and conjugation, could be adjusted.[1,2] They were widely used in optical storages,[3] optical switches,[4−7] optical conversion devices,[8,9] solar cells,[10,11] sensors,[12−14] logic circuits,[15−17] and other fields. Among the reported photochromic compounds, diarylethenes exhibited excellent advantages including good thermal stability and excellent fatigue resistance,[18−21] and are deemed to be the most potential nominees for practical applications. Over the past decade, numerous scientific researchers have put in the efforts to develop diarylethene derivatives for constructing fluorescent switches and sensors.[22−25] A lot of functional units, such as pyridine,[26,27] quinoline,[28] rhodamine,[29] fluorescein,[30] and so on, were introduced to the diarylethene unit to construct multistate systems, the design and construction of efficient multi-addressable fluorescent switching systems with diarylethenes has attracted much attention. Phthalazine is a heterocyclic organic compound with two nitrogen atoms. It is considered as one of the crucial sensors and chelating agents for many metal ions, because it could act as the metal-binding group and fluorophore.[31−34] Until now, there was no report on the fluorescent switching of diarylethene containing the phthalazine unit.

On the other hand, diarylethenes were also widely researched due to the application in numerous optoelectronic devices.[35−37] Many researchers have applied diarylethenes to construct significant electronic logic devices of molecular scale, because the electronic devices are becoming more and more important in our life.[38−42] For instance, Tian et al. structured a complex molecular switching, and its four optical outputs respond to four inputs.[43] A multi-addressable terarylene containing benzo[b]thiophene-1,1-dioxide (BTO) was synthesized by Zhu and colleagues; at the same time, a sequence of molecular logic gates including 1:2 demultiplexer, half-adder, and so on were built on the unimolecular platform.[44] However, considering the potential applications of logic gates, there is still a larger space for development to construct the single-molecule fluorescence switching based on diarylethenes.[45−47]

Aluminum as the third most widespread (8% by weight) element in the biosphere has been widely distributed in natural water and most biological tissues in the form of Al3+.[48,49] Moreover, aluminum is diffusely applied in our daily life such as light alloys, water purification instruments, household utensils, and so on.[50,51] These factors increase the concentration of Al3+ in environment and biosome. Nevertheless, high concentrations of Al3+ are toxic to the human body, flora, and fauna. Excessive aluminum not only causes severe human health issues, such as Alzheimer’s disease, Parkinson’s disease, and anemia, but also affects the growth of plant roots and aquatic wildlife.[52−55] Consequently, it is particularly important to develop effective methods to monitor Al3+ in environmental and biological samples.

To date, multifarious methods for detection of aluminum ion have been developed including inductively coupled plasma mass spectrometry, atomic absorption spectrometry,[56,57] and so on. Except for these analysis techniques, fluorescent sensors are considered to be the most suitable method for detecting Al3+, because of their advantages of high selectively, simplicity, and real-time analysis.[58−60] To date, there have been reported many fluorescent sensors to detect Al3+.[61−65] However, these sensors are either interfered with Fe3+, Cd2+, Cr3+, and Zn2+, or the sensitivity needs to be further improved. Hence, designing a high selective and sensitive fluorescent sensor for the determination of Al3+ is vitally important.

In the work, we triumphantly designed and synthesized a new fluorescent switching of diarylethene containing the phthalazine unit. The diarylethene derivative could be served as a multicontrolled fluorescent switching under the stimulation of UV/vis lights and Al3+/EDTA. Simultaneously, it also could be used as a highly selective sensor for fluorescence identification Al3+. Moreover, it was also suitable for constructing logic gate. Scheme displays the photochromism of the diarylethene derivative, and the structural characterization data of 1o including 1H NMR, 13C NMR, IR spectrum, and electrospray ionization-mass spectrometry (ESI-MS) are exhibited in Supporting Information (Figures S1–S4).

Scheme 1

Photochromism of 1o

Results and Discussion

The photochromic properties of 1o were performed by irradiation of UV/vis lights in methanol. According to Figure A, two absorption bands centered at 293 nm (ε = 5.88 × 104 L mol–1 cm–1) and 389 nm (ε = 2.38 × 104 L mol–1 cm–1) were observed, which were ascribed to 1o. When the solution of 1o was irradiated with 297 nm ultraviolet light, the absorbance at 293 nm decreased gradually, while two new absorption bands centered at 363 nm (ε = 3.89 × 104 L mol–1 cm–1) and 594 nm (ε = 2.08 × 104 L mol–1 cm–1) appeared simultaneously and increased gradually, along with which the colorless solution turn blue, which may be caused by the C=N isomerization[67] and the formation of the closed-ring isomer 1c with a larger π-electron delocalization in the molecule.[68] At the photostationary states (PSS), the conversion from 1o to 1c was 100%, according to the absorption coefficients in methanol.[69] Two clear isosbestic points were respectively found at 318 and 409 nm, manifesting a unimolecular process.[70] Contrarily, the absorption spectrum could be restored from 1c to 1o by irradiating visible light (λ > 500 nm), accompanied by which the blue solution turn colorless. The cycloreversion and cyclization quantum yields were calculated as 0.012 and 0.21, and 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene was used as standard.[71]

Figure 1

Changes of the absorption (A) and fluorescence spectrum (B) of 1o by photoirradiation in methanol (λex = 410 nm).

Then, we investigated systematically the fluorescence changes of 1o that were induced by UV/vis lights. As exhibited in Figure B, 1o shows a weak emission peak at 613 nm using 410 nm light as the excitation, and the absolute fluorescence quantum yield was measured as 0.009. When the solution of 1o was irradiated with a 297 nm ultraviolet light, the emission intensity of 1o gradually decreased and was quenched by ca. 86% at the PSS, because the nonfluorescence isomer 1c was formed, revealing that the diarylethene has a high fluorescence modulation efficiency. Similarly, the fluorescent spectrum also could be restored from 1c to 1o by irradiation with a proper wavelength of visible light (λ > 500 nm).

The interactions between 1o and various metal ions (5.0 equiv of 1o) were measured by fluorescence spectroscopy. As described in Figure , the emission intensity at 560 nm enhanced significantly after addition of Al3+, and an prominent variation in fluorescent color was observed from black to green, because a new complex 1o-Al3+ (1o′) was generated. After coordinating with Al3+, the stable complexation was formed, which not only prohibited the photoinduced electron transfer process from the hydroxyl group to the diarylethene unit but also inhibited isomerization of the C=N and increased rigidification of fluorophore structure, resulting in the chelation-enhanced fluorescence effect.[72,73] Moreover, the intramolecular charge transfer effect also exists in the process, and a 53 nm blue shift was observed from 613 to 560 nm with the addition of Al3+. While the remaining test ions were added, respectively, the fluorescent intensity of 1o remained unchanged, with the exception of Zn2+. The fluorescent intensity enhanced slightly after adding Zn2+, which was negligible compared with Al3+. The results revealed that 1o could be used as a highly effective fluorescence sensor to detect Al3+.

Figure 2

Fluorescence responses of 1o to various ions (5.0 equiv) in methanol (λex = 410 nm): (A) evolution of emission spectra; (B) changes of emission intensity; and (C) changes of fluorescence color (under 365 nm UV-lamp).

The fluorometric titration experiments of 1o and 1c with Al3+ were performed in methanol and are described in Figure . With the increase of Al3+ from 0 to 5.0 equiv, the fluorescence intensity of 1o at 560 nm gradually enhanced (Figure A) and reached saturation with further titration (Figure S5, Supporting Information). At that time, the emission intensity is 126-fold higher than that of 1o, and the absolute fluorescence quantum yield was measured as 0.053. The color of fluorescent turned green from black, because of the formation of 1o′. The emission intensity of 1c was enhanced 88-fold when 0 to 5.0 equiv of Al3+ was added, indicating the formation of 1c′, and the emission intensity remained unchanged with further addition of Al3+ (Figure B). The fluorescence intensity of 1o and 1c could be recovered from 1o′ and 1c′ by adding 10.0 equiv of EDTA, demonstrating that the complexation–decomplexation reactions of 1o and 1c with Al3+ were reversible, respectively.

Figure 3

Changes in the emission intensity and fluorescence color of 1o (A) and 1c (B) with addition of Al3+ (0–5.0 equiv) in methanol (λex = 410 nm).

The UV/vis titration experiments were also performed to investigate the interaction between 1o and Al3+. When 0–5.0 equiv of Al3+ was added, the intensity of absorption bands at 334 and 434 nm enhanced, while the peak at 389 nm declined gradually, the absorbance at 434 nm remained unchanged with further addition of Al3+, and two clear isosbestic points at 356 and 414 nm were found, respectively (Figure A), suggesting that only one stable complex 1o′ was generated in the process. Simultaneously, the colorless solution turned yellow green. Whereafter, the interaction of 1c with Al3+ was also investigated. The intensity of absorption bands at 460 and 594 nm enhanced, and the peak at 363 nm weakened with increasing amount of Al3+ from 0 to 5.0 equiv, along with which the solution color turned navy blue from blue (Figure B), as a result of the generation of complex 1c′. The absorption spectra of 1o and 1c were restored instantly with addition of 10.0 equiv of EDTA.

Figure 4

Variations of absorption spectra and solution color of 1o (A) and 1c (B) caused by Al3+/EDTA in methanol.

The photoisomerization reactions of 1o′ were also investigated. First, the variation of absorption spectrum of 1o was investigated under the irradiation of ultraviolet light with 297 nm, the intensity of absorption band centered at 293 nm weakened gradually, and two new bands centered at 371 and 598 nm were emerged and increased. It can be found that the yellow green solution turned to navy blue, which was consistent with the variation of absorption spectrum, indicating that closed-ring isomer 1c′ was generated (Figure A). When the PSS was arrived, three isosbestic points were formed at 338, 414, and 438 nm, demonstrating that the process is a unimolecular process. Contrarily, the absorption spectrum and solution color of 1c′ could come back to the original state (1o′) by irradiating visible light (λ > 500 nm). Then, the fluorescence property of 1o′ was also changed under the irradiation of ultraviolet light with 297 nm, the fluorescence emission intensity at 560 nm weakened gradually, and the emission intensity was quenched by 92.7% at the PSS; the decrease in emission intensity was ascribed to the photoisomerization from 1o′ to 1c′. The fluorescence resonance energy transfer (FRET) was taken place from the phthalazine unit to the diarylethene unit, resulting in the emission quenched, accompanied by the green fluorescence turn black (Figure B). The above results revealed that 1o′ displayed excellent fluorescence modulation efficiency and could be used as a fluorescence switching.

Figure 5

Variations of absorption (A) and fluorescence spectrum (B) of 1o′ in methanol by irradiating with ultraviolet and visible lights (λex = 410 nm).

The Job’s plot experiment was implemented in the light of the previous report.[72] The result manifested a 1:1 stoichiometric ratio between Al3+ and 1o (Figure A). Furthermore, the signal peak of ESI-MS at m/z = 794.83 corresponding to [M + Al3+ + NO3– – H+]+ provided a direct evidence to confirm the 1:1 binding stoichiometry of Al3+ with 1o (Figure S4, Supporting Information). In the light of fluorescence titration curve, the relationship between F0/(F0 – F) and 1/[Al3+] exhibited an excellent linear (R = 0.993) (Figure B); the association constant (Ka) was determined as 2.07 × 104 L mol–1 through the Benesi–Hildebrand equation.[74] The limit of detection (LOD) of 1o to Al3+ was determined as 5.5 × 10–8 mol L–1 on the basis of 3δ/s (Figure S6, Supporting Information).

Figure 6

Job’s plot for binding mode of 1o with Al3+ (A), and (B) Hildebrand–Benesi plot based on 1:1 mode of 1o and Al3+ with Ka = 2.07 × 104 L mol–1.

Figure described the changes of the partial 1H NMR spectra. Upon addition of Al3+, the signal (Ha) at 14.56 ppm assigned to −OH was disappeared, indicating that the oxygen atom on −OH participated in the coordination. The signal (Hb) at 10.91 ppm assigned to −NH– exhibited a 0.05 ppm downfield shift. The signal (Hc) at 9.32 ppm high field shift to 9.20 ppm, and the signal (Hd, He) at 9.10 ppm split to 9.09 and 8.90 ppm. Therefore, according to the 1:1 stoichiometry, the possible complexation mode between 1o and Al3+ was exhibited in Figure .

Figure 7

Partial 1H NMR spectra of 1o and 1o′ in DMSO-d6 (inset displays the possible complexation mode of 1o with Al3+).

Based on the above discussion, the multistate fluorescence switching behaviors of 1o could be regulated independently by stimulating with Al3+/EDTA and UV/vis lights. Scheme exhibited the multicontrolled photoswitching properties of 1o. According to the experimental results, we designed a combinational logic circuit, which was composed of four input signals and one output signal. The input signals included UV light of 297 nm, λ > 500 nm visible light, Al3+, and EDTA, which were named as In1, In2, In3, and In4, respectively, while the emission intensity at 560 nm was considered as an output signal (Figure ). The signal of outputs contained “off” or “on” two states, corresponding to the Boolean value of “0” or “1”, respectively. The fluorescence intensity of 1o at 613 nm was deemed as an original value, if the fluorescence intensity at 560 nm in excess of 126 times of the original value, the output signal was considered as “on” corresponding to the Boolean value “1”; otherwise, was considered to be “off” corresponding to the Boolean value “0”. Therefore, 1o displayed fluorescence switching behavior of on or off with different inputs. Such as the strings “0, 0, 1, and 0” corresponding input signals In1, In2, In3, and In4 represented “off, off, on, and off”, respectively. In this case, 1o transformed to 1o′ when 1o was stimulated by Al3+, resulting in a prominent enhancement in emission intensity; the output signal was “on” and the output digit was “1”. Table summarizes the total probable logic strings in this logic circuit.

Scheme 2

Variations of Structures and Colors of 1o in the Presence of UV/Vis Lights and Al3+/EDTA

Figure 8

Integrated logic circuit equal to the truth table shown in Table . In1 (UV light of 297 nm), In2 (λ > 500 nm light), In3 (Al3+), In4 (EDTA), and output (λem = 560 nm).

Table 1

Truth Table of the Total Probable Strings of Four Binary-Input Data and the Corresponding Output Digit

inputs
In1 (UV)	In2 (vis)	In3 (Al3+)	In4 (EDTA)	outputa (λem = 560 nm)
0	0	0	0	0
1	0	0	0	0
0	1	0	0	0
0	0	1	0	1
0	0	0	1	0
1	1	0	0	0
1	0	1	0	0
1	0	0	1	0
0	1	1	0	1
0	1	0	1	0
0	0	1	1	0
1	1	1	0	1
1	1	0	1	0
1	0	1	1	0
0	1	1	1	0
1	1	1	1	0

At 560 nm, the emission intensity in excess of 126 times of the initial value is deemed as 1, otherwise is considered to be 0.

In further to explore the potential application of the sensor in actual water samples, competitive tests were carried out. The results indicated that the equal equiv of coexistence ions hardly interfered fluorescence selective of 1o to Al3+, with the exception of Cu2+, Fe3+, Co2+, and Ni2+ that induced fluorescence quenching (Figure S7, Supporting Information). Whereafter, the practical application of 1o in water samples were measured; three practical water samples including Qingshan Lake, Ganjiang River, and Tap water were collected from Nanchang City. On the basis of the methods reported previously,[75,76] the obtained water samples had undergone pretreatment with 0.2 μm membrane to remove suspended substance, and then different concentrations of  Al3+ were added separately to the real water samples to evaluate the recoveries. The recoveries of Al3+ added in the three water samples were calculated and are summed up in Table . The recoveries for Al3+ were from 99.4 to 102.8%, revealing that the sensor exhibited a high accuracy in the determination of Al3+ in the practical water samples.

Table 2

Detection of Al3+ in the Practical Water Samples

samples	Al3+ spiked (×10–6 mol L–1)	Al3+ recovered (×10–6 mol L–1)	recovery (%)
Qingshan Lake	5.0	5.14	102.8
	10.0	9.98	99.8
	20.0	20.2	101.0
Ganjiang River	5.0	4.97	99.4
	10.0	10.2	102.0
	20.0	19.9	99.5
Tap water	5.0	5.06	101.2
	10.0	9.98	99.8
	20.0	20.3	101.5

Conclusions

A novel multiple-state diarylethene with a phthalazine unit was synthesized successfully. It showed not only prominent photochromic properties but also high selectivity to Al3+. What’s more, it also exhibited multistate fluorescent switching capabilities accompanied by remarkable fluorescent color changes under the stimulation of chemical reagents and lights. According to these characteristics, we designed and constructed a combinational logic circuit with four input signals (UV/vis lights and Al3+/EDTA) and an output signal (the emission intensity at 560 nm). At the same time, it was used as a fluorescence sensor to detect Al3+ in the practical water sample. The study could provide certain help for the synthesis of novel diarylethene derivatives with multiple-state fluorescence switching properties.

Experimental Section

General Methods

NMR spectra were measured on a Bruker AV400 spectrometer using DMSO-d6 as the solvent. Absorption and fluorescence spectra were recorded by an Agilent 8453 UV/vis spectrophotometer and a Hitachi F-4600 fluorescence spectrophotometer, respectively. Photoirradiation experiments were carried out on an MUA-165 UV lamp and an MVL-210 visible lamp. Absolute fluorescence quantum yields were performed using an Absolute PL Quantum Yield Spectrometer QYC11347-11. Infrared (IR) spectra were obtained by a Bruker VERTEX-70 spectrometer. A PE CHN 2400 analyzer was used for elemental analysis. Melting point was determined by a WRS-1B melting point apparatus. Without special instructions, all tests were performed at room temperature, and all samples are of concentration 2.0 × 10–5 mol L–1.

All the reagents were analytically pure and were not further purified before use in the experiments. All cationic salts were nitrates with the exception of Mn2+, Ba2+, K+, and Hg2+ (all of them were chlorates). The corresponding metal salts were dissolved in distilled water to obtain metal ion solution (0.1 mol L–1). Similarly, ethylenediaminetetraacetic acid disodium salt (Na2EDTA) was dissolved in deionized water to obtain EDTA aqueous solution (0.1 mol L–1).

Scheme displayed the synthetic procedure of diarylethene derivative (1o). Compound 2 was prepared by referring to the method previously reported,[66] and compound 3 was purchased and used without further purification. Compound 3 (0.071 g, 0.44 mmol) was added to 5 mL of methanol solution containing compound 2 (0.25 g, 0.44 mmol), when the reaction system was continuously stirred and refluxed for 2.0 h, then cooled in a refrigerator to obtain a yellow precipitate. The crude product was recrystallized using ethanol to get the target compound 1o (0.23 g, 0.33 mmol) in 68% yield. mp 475–476 K; Anal. Calcd for C36H24F6N4OS2 (%): C, 61.18; H, 3.42; N, 7.93. Found: C, 61.17; H, 3.43; N, 7.92; 1H NMR (DMSO-d6, 400 MHz, TMS): δ (ppm) 2.04 (s, 3H), 2.06 (s, 3H), 7.12 (d, 1H, J = 8.6 Hz), 7.37 (m, 1H), 7.46 (m, 2H), 7.52 (s, 1H), 7.55 (s, 1H), 7.68 (d, 3H, J = 6.8 Hz), 8.26 (m, 3H), 8.59 (s, 1H), 9.10 (s, 2H), 9.32 (s, 1H), 10.91 (s, 1H), 14.56 (s, 1H); 13C NMR (DMSO-d6, 100 MHz, TMS): δ (ppm) 13.5, 13.6, 116.7, 119.3, 121.0, 122.1, 123.4, 123.9, 124.4, 124.5, 124.6, 124.8, 127.3, 127.6, 128.7, 129.7, 132.0, 133.2, 135.2, 139.7, 140.7, 141.1, 141.2, 143.9, 147.3, 148.7, 148.8, 157.2; ATR-IR (ν, cm–1): 3152, 2919, 2871, 1623, 1610, 1594, 1503, 1471, 1337, 1270, 1189, 1137, 1112, 1053, 986, 901, 823, 757, 689; MS (ESI, m/z): [M]+ calcd. for C36H24F6N4OS2, 706.13; found, 707.40 [M + H+]+.

Scheme 3

Synthetic Route of 1o