A highly selective fluorescent chemosensor for Fe³⁺ based on a new diarylethene with a rhodamine 6G unit

Abstract

Herein, a new diarylethene derivative bearing rhodamine 6G (1O) was synthesized via Schiff base condensation. It exhibited a sensitive response to Fe³⁺ with the solution changing from colorless (no fluorescence) to distinct pink (strong fluorescence) due to the transformation of the spirolactam ring to the open ring of rhodamine 6G. The diarylethene derivative bound to Fe³⁺ in the binding stoichiometry of 1 : 1, and the limit of detection (LOD) of the 1O chemosensor for Fe³⁺ detection was found to be 65 nM. In addition, 1O exhibited an obvious acidichromism to TFA and could function as a reversible fluorescence photoswitch in response to UV/vis light irradiation. Based on these observations, a logic circuit was constructed with the combinational stimuli of UV/vis light and Fe³⁺/EDTA as inputs and the fluorescent emission intensity at 585 nm as the output.

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Introduction

Heavy metal cations, such as Hg²⁺, Cu²⁺, and Fe³⁺, naturally exist and have been widely used in many industrial fields. They are toxic at high concentrations and are responsible for many incidents of industrial and agricultural pollution.^1–3 Compared to the time-consuming and expensive instrumental techniques reported for heavy metal-ion detection, a chemosensor is a faster, cheaper, more powerful, and highly sensitive molecular tool for the detection of many biological and environmental heavy metal pollutants.^4–6 Among all chemosensors, fluorescent chemosensors have attracted significant attention due to their simple operation, high sensitivity, and real-time detection. Fe³⁺ plays a key role in many biochemical processes at the cellular level and is indispensable for most of the organisms.⁷ It provides oxygen-carrying capacity to heme and acts as a cofactor in many enzymatic reactions involved in the mitochondrial respiratory chain. However, high levels of Fe³⁺ within the body can be associated with increased incidence of certain cancers and dysfunction of organs such as heart, pancreas, and liver.^8–11 Therefore, new simple and efficient chemosensors for Fe³⁺ detection are highly desired.¹² Several fluorescent sensors have been developed for Fe³⁺ detection. However, most of them detect Fe³⁺ based on fluorescence quenching by the paramagnetic property of Fe³⁺.¹³ For example, in an early study, Wolf et al. selectively detected Fe³⁺ in aqueous solutions with a 1,8-diacridylnaphthalene-derived fluorosensor based on fluorescence quenching. Therefore, the development of highly selective turn-on response chemosensors for Fe³⁺ is of significance and challenging.¹⁴ Photochromic materials can be potentially applied to optical memories, photoswitches, logic circuits, and chemosensors.^15,16 Diarylethenes, one of the most promising photoresponsive compounds, have attracted significant research interests due to their excellent thermal stability, notable fatigue resistance, high sensitivity, and rapid response.^17,18

In particular, they can function as a fluorescence sensor based on their reversible fluorescence response to UV/vis lights irradiation. Recently, many diarylethene-based sensors have been reported. Zheng et al. prepared a dual-control molecular switch based on photochromic hexahydrogen cyclopentene bearing two rhodamine units and demonstrated it as an integrated logic circuit at the molecular level. Pu et al. exploited diarylethene-based fluorescence sensors to detect Fe³⁺, Hg²⁺, and Cu²⁺.^19,20 Their study has contributed towards the comprehensive understanding of the sensors based on diarylethene-bearing functional groups.

The rhodamine 6G-based fluorescent chemosensor is an excellent fluorophore for metal ion detection and has attracted extensive interest in recent years by virtue of its excellent photophysical properties such as long excitation and emission wavelength, high fluorescence quantum efficiency, and excellent detection sensitivity.^21,22 The off–on fluorescence switching of the chemosensor is based on the transformation between the spirocyclic and open ring forms of rhodamine 6G. The chemosensor is in the spirocyclic form in the absence of metal ions and exhibits no color or fluorescent emission. The addition of suitable metal ions can open the spirocycle via coordination, leading to chromogenic and fluorogenic responses.

In the present study, a diarylethene derivative bearing a rhodamine 6G moiety was synthesized to construct a fluorescent chemosensor for Fe³⁺ detection. The chemosensor exhibited excellent selectivity and sensitivity for Fe³⁺ detection via the turn on fluorescence signal in an aqueous acetonitrile solution (v/v = 1 : 1). Scheme 1 is the schematic of the photochromism of the diarylethene derivative.

Scheme 1 Photochromism of diarylethene 1O.

Experimental

General methods

Chemical reagents were obtained from J & K Scientific Ltd and were used as received. All the reagents and solvents were of analytical grade and diluted as required. All cations were added in the form of metal nitrates except for K⁺, Ba²⁺, Mn²⁺, and Hg²⁺ that were added as metal chlorides. The stock solutions of the metal ions were prepared in distilled water at the concentration of 0.1 mol L⁻¹.

NMR spectra were obtained using a Bruker AV400 (400 MHz) spectrometer with CDCl₃ and DMSO-d₆ as the solvents and tetramethylsilane (TMS) as the internal standard. Fluorescence quantum yield was measured using an Absolute PL Quantum Yield Spectrometer QY C11347-11. UV/vis spectra were obtained using an Agilent 8453 UV/vis spectrophotometer. The fluorescence spectra were obtained via a Hitachi F-4600 fluorescence spectrophotometer. Infrared spectra (IR) were obtained using a Bruker Vertex-70 FT-IR spectrometer.

Synthesis of 1O

Scheme 2 shows the synthesis route of the target compound, [1-(2-methyl-5-phenyl-3-thienyl)-2-{2-methyl-5-[4-N(rhodamine-6G)-hydrazide]phenyl-3-thienyl}perfluorocyclopentene] (1O). 3-Bromo-2-methyl-5-(1,3-dioxolane)thiophene (3), (2-methyl-5-phenyl-3-thieryl)perfluorocyclopentene (4), and 8 were synthesized as described in the literature.^23–27 Compound 3 was coupled with 4 to afford compound 5, which was hydrolyzed to produce compound 6. Compound 8 reacted with 6 in anhydrous ethanol via Schiff base condensation to afford the target compound 1O. The detailed procedure is described as follows.

Scheme 2 Synthesis route for diarylethene 1O.

A solution of n-BuLi/hexane (2.5 mol L⁻¹, 4.90 mL) was added dropwise to 60 mL of compound 3 (3.24 g, 10.00 mmol) solution in anhydrous THF at 195 K under stirring and an argon atmosphere and stirred for 30 min. Compound 4 (3.67 g, 10.00 mmol) was slowly added to the reaction mixture at 195 K, further stirred for 3 h at the same temperature, and quenched with water. The product was extracted with dichloromethane, dried over Na₂SO₄, filtered, and evaporated. The crude product was purified by column chromatography using the petroleum ether–ethyl acetate (v/v = 10 : 1) as the eluent to afford the compound 5 (2.60 g, 4.40 mmol), which was refluxed in acetone–water (v/v = 4 : 1) for 2 h, extracted with dichloromethane, dried over Na₂SO₄, filtered, evaporated, and purified by column chromatography to afford compound 6 as a blue solid with a yield of 84%. Mp: 363–365 K. ¹H NMR (CDCl₃, 400 MHz), δ (ppm): 1.90 (t, 3H, J = 8.0 Hz), 1.93 (s, 3H), 7.21 (s, 1H), 7.25 (d, 1H, J = 8.0 Hz), 7.32 (t, 2H, J = 8.0 Hz), 7.37 (s, 1H), 7.47 (d, 2H, J = 8.0 Hz), 7.63 (d, 2H, J = 8.0 Hz), 7.83 (d, 2H, J = 8.0 Hz), 9.94 (s, 1H).

A solution of compound 8 (0.98 g, 0.20 mmol) in 15 mL anhydrous ethanol was added to a solution of compound 6 (0.11 g, 0.20 mmol) under constant stirring, refluxed for 48 h, cooled down to room temperature, and quenched with water. The solvent was removed under reduced pressure. The residue was extracted with dichloromethane, and the organic phase was dried over Na₂SO₄, filtered, and evaporated. The crude product was purified by column chromatography on a silica gel column using petroleum ether–ethyl acetate (v/v = 4 : 1) as the eluent to afford 1O (0.10 g, 0.10 mmol) as a light purple solid in 51% yield. Mp: 430–432 K. ¹H NMR (DMSO-d₆, 400 MHz), δ (ppm) 1.19 (t, 6H, J = 8.0 Hz), 1.82 (s, 6H), 1.96 (s, 6H), 3.10–3.13 (m, 4H), 5.05 (s, 2H), 6.16 (s, 2H), 6.32 (s, 2H), 7.03 (d, 1H, J = 8.0 Hz), 7.33 (d, 1H, J = 8.0 Hz), 7.41 (d, 3H, J = 4.0 Hz), 7.47 (s, 1H), 7.51 (s, 1H), 7.55–7.60 (m, 6H), 7.73 (s, 1H) 7.90 (d, 1H, J = 8.0 Hz), 8.64 (s, 1H) (Fig. S1†). ¹³C NMR (DMSO-d₆, 100 MHz), δ (ppm): 13.97, 14.08, 16.86, 37.40, 62.73, 65.56, 95.67, 104.95, 118.15, 122.48, 122.92, 123.19, 123.72, 125.21, 125.48, 126.78, 127.37, 128.05, 128.10, 128.63, 128.80, 129.13, 132.42, 133.78, 134.08, 140.85, 141.13, 141.65, 141.83, 145.75, 147.74, 150.94, 151.29, 163.69 (Fig. S2†). MS (m/z): 959.28 [M + H]⁺ (Fig. S3†); IR (KBr, ν, cm⁻¹): 1199, 1216, 1267, 1315, 1515, 1608, 1620, 1636, 1684, 1713.

Results and discussion

Photochromism of 1O

The photochromic properties of 1O were determined in acetonitrile (C = 2 × 10⁻⁵ mol L⁻¹) at room temperature. The open ring isomer 1O exhibited an absorption peak at 353 nm (Fig. 1A) due to the π*–π transition.²⁸ A new absorption band centered at 602 nm appeared, and the solution of 1O turned from colorless to blue under the irradiation at 297 nm, indicating that the closed-ring form 1C was formed. The blue solution of 1C could be bleached to colorless under the visible light irradiation at λ > 500 nm, and its absorption peak shifted back to 353 nm. An isosbestic point appeared at 370 nm, indicating that a two-component photochromic reaction occured.^29–31 The cyclization and cycloreversion quantum yields of 1O were determined to be 0.232 and 0.003, respectively, with 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene as a ref. 32 The fatigue resistance of 1O revealed that 1O degraded only 8.3% after 10 coloration–discoloration cycles in response to the alternate irradiations of UV/vis lights at room temperature (Fig. 1B).

Fig. 1 (A) Changes in the absorption spectra of diarylethene 1O in acetonitrile (C = 2.0 × 10⁻⁵ mol L⁻¹) at room temperature. (B) Fatigue resistance of 1O in acetonitrile (C = 2.0 × 10⁻⁵ mol L⁻¹) at room temperature.

Spectral response of 1O to Fe³⁺

It has been reported that rhodamine 6G derivatives are subject to significant changes in absorption or fluorescence spectra upon coordination with metal ions and thus can be used as a sensitive probe for metal ion detection.³³ In the present study, the fluorescence of 1O (C = 2.0 × 10⁻⁵ mol L⁻¹) induced by metal ions including Fe³⁺, Cr³⁺, Al³⁺, Hg²⁺, Cu²⁺, Mg²⁺, Ba²⁺, Zn²⁺, Mn²⁺, Co²⁺, Ni²⁺, Ca²⁺, Cd²⁺, Pb²⁺, Sr²⁺, and K⁺ was determined in an aqueous acetonitrile solution (v/v = 1 : 1) at room temperature. As shown in Fig. 2A, for the absorption spectra of 1O in the presence of 10.0 equiv. of metal ions, only Fe³⁺, Cr³⁺, and Al³⁺ were able to induce strong absorptions at 531 nm, and Fe³⁺ caused a much stronger absorption peak than Cr³⁺ and Al³⁺.

Fig. 2 Changes in fluorescence of 1O in response to various metal ions (10.0 equiv.) in aqueous acetonitrile solution (C = 2.0 × 10⁻⁵ mol L⁻¹, v/v = 1 : 1). (A) Absorption spectral changes. (B) Changes in fluorescence. (C) Color and fluorescence changes of 1O after the addition of 10.0 equiv. of different metal ions.

Compound 1O in acetonitrile exhibited no obvious emission signal under excitation at 515 nm. The addition of 10.0 equiv. of Fe³⁺ to the 1O solution led to ca. 168-fold increase in the fluorescent emission intensity at 585 nm (Fig. 2B). The 1O solution showed weaker responses to Cr³⁺ and Al³⁺ with 45-fold and 28-fold increases in fluorescent emission peaks at 578 nm and 585 nm, respectively (Fig. S4†). These observations indicate that 1O can be potentially used as a selective fluorescence sensor for Fe³⁺ in an acetonitrile/water binary solvent system (v/v = 1 : 1). In addition, Fe³⁺ turned the 1O solution from colorless to pink and from dark to a bright green yellow color under excitation at 515 nm due to the formation of the open ring amide form of rhodamine 6G (1O′) (Fig. 2C). Other metal ions caused no detectable increase in the emission intensity or solution color of 1O in the aqueous acetonitrile solution except for Cr³⁺, Al³⁺, and Cu²⁺ that turned the 1O solution to pale pink and caused a weak fluorescent emission (Fig. 2C).

The chemosensing properties of 1O towards Fe³⁺ were further investigated via absorption and fluorescence titration. As shown in Fig. 3, the solution of 1O changed from colorless to pink (Fig. 3A), and a new absorption band centered at 530 nm appeared with the increase of Fe³⁺ equiv. from 0 to 10.0 due to the formation of 1O′ (Fig. 3B). Further increase in the Fe³⁺ amount caused no significant change in the absorption of 1O. Moreover, the fluorescent emission of 1O at 585 under excitation at 515 nm increased with the increase of Fe³⁺ equiv and peaked at 10.0 equiv. (Fig. 3C). The absolute fluorescence quantum yield was determined to be 0.43. The fluorescence intensity changes with the addition of Fe³⁺ were also visibly observed with the naked eye, with the solution turning from dark to a bright green-yellow under excitation at 515 nm (Fig. 3D).

Fig. 3 (A) Fluorescence properties of 1O in aqueous acetonitrile (C = 2.0 × 10⁻⁵ mol L⁻¹, v/v = 1 : 1, λ_ex = 515 nm) induced by Fe³⁺/EDTA. (B) Upon addition of different equivalents of Fe³⁺ and the emission intensity curve at 585 nm. (C) The absorption spectra of 1O′ in aqueous acetonitrile (C = 2.0 × 10⁻⁵ mol L⁻¹, v/v = 1 : 1) induced by Fe³⁺/EDTA. (D) The absorption intensity changes of 1O at 531 nm with different equivalents of Fe³⁺. (E) Changes in the fluorescence of 1O′ in aqueous acetonitrile (C = 2.0 × 10⁻⁵ mol L⁻¹, v/v = 1 : 1, λ_ex = 515 nm) upon alternating irradiation with UV/vis light.

The fluorescence was reverted by addition of an excess of EDTA (10.0 equiv.), indicating that the coordination of 1O with Fe³⁺ could be restored. Note that 1O′ also exhibited excellent fluorescent switching properties under alternate irradiations with UV/vis lights in aqueous acetonitrile (v/v = 1 : 1) (Fig. 3E). In the photostationary state, the fluorescence emission intensity was quenched 82% due to the formation of 1C′ with the fluorescence changing from bright green-yellow to dark yellow. These results indicate that the diarylethene derivative 1O is a promising candidate for fluorescence-switching devices due to fluorescence switching behavior induced by both Fe³⁺/EDTA and UV/vis light.

Job's plot analysis was conducted on the fluorescence titration data, as previously reported, to calculate the binding ratio between 1O and Fe³⁺.^34,35 As shown in Fig. 4A, the concentration of 1O′ reached a maximum as the molar fraction of [Fe³⁺]/([Fe³⁺] + [1O]) = 0.5, indicating that the complex ratio between Fe³⁺ and 1O was 1 : 1 in the aqueous acetonitrile solution (v/v = 1 : 1). The association constant (K_a) of 1O with Fe³⁺ was determined to be 5.9793 × 10⁴ mol L⁻¹ by the Benesi–Hildebrand equation (Fig. 4B),³⁶ and the limit of detection of Fe³⁺ by 1O was found to be 65 nM (3δ per slope) (Fig. 4C). These results indicate that 1O is highly selective towards Fe³⁺ and can be used as a fluorescence sensor of Fe³⁺.

Fig. 4 (A) The Job's plot showing the 1 : 1 complex of 1O and Fe³⁺. (B) Hildebrand–Benesi plot based on the 1 : 1 ratio between 1O and Fe³⁺, the binding constant of 1O with Fe³⁺ was calculated to be 5.9793 × 10⁴ mol L⁻¹. (C) The limit of detection (LOD) is 65 nM.

The selectivity of a chemosensor towards the analyte over the other competitive species is a very important parameter to evaluate its sensing performance.³⁷ To further confirm the high selectivity of the 1O sensor towards Fe³⁺, competitive tests were conducted in the presence of metal ions with similar properties: Cd²⁺, Ba²⁺, Mg²⁺, Ca²⁺, Pb²⁺, K⁺, Co²⁺, Zn²⁺, Sr²⁺, Mn²⁺, Ni²⁺, Al³⁺, Cr³⁺, Cu²⁺, and Hg²⁺. Fig. 5 shows the fluorescence response of 1O to Fe³⁺ in the aqueous acetonitrile solution as different ions (20.0 equiv.) were added. No obvious interference was observed from other competitive metal ions in aqueous acetonitrile, indicating that 1O was highly selective towards Fe³⁺. The present sensor was compared with other sensors reported for Fe³⁺ detection in recent years,^38–44 as listed in Table 1. These sensors showed some good physical properties such as lower LOD, association constant, high sensitivity and so on. For the 1O–Fe³⁺ complex, a moderate performance in all aspects was observed, suggesting that diarylethene 1O could serve as a fluorescence sensor for Fe³⁺.

Fig. 5 Emission changes at 585 nm of 1O in the presence of different metal ions added in the amount of 20.0 equiv. (λ_ex = 515 nm) in acetonitrile/water binary solvent (C = 2.0 × 10⁻⁵ mol L⁻¹, v/v = 1 : 1), after the initial addition of Fe³⁺ (10.0 equiv.) to the above solution.

Table 1 Comparative study of the analytical performance of 1O–Fe³⁺ with other recently reported sensors

Sensor	Approaches	LOD	Ka	Ref.
	Fluorescence increase	737 nM	—	Sens. Actuators B, 2016, 224, 661–667
	Fluorescence increase	418 nM	1.88 × 10^4	J. Lumin., 2015, 157, 143–148
	Fluorescence increase	31 nM	2.19 × 10^4	Bioorg. Med. Chem., 2014, 22, 4826–4835
	Fluorescence increase	105 nM	1.24 × 10^5	Talanta, 2014, 128, 69–74
	Fluorescence increase	2 × 10^4 nM	—	Sens. Actuators B, 2012, 174, 231–236
	Fluorescence quenching	300 nM	—	Talanta, 2013, 106, 261–265
	Fluorescence increase	1.1 nM	log() = 4.72 ± 0.27	Sens. Actuators B, 2013, 178, 228–232
	Fluorescence increase	65 nM	5.9793 × 10^4	This work

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Acidichromism

It is particularly well-known that the open ring form of the rhodamine moiety in the presence of a proton emits strong fluorescence in the range of 540–700 nm.^45,46 As shown in Fig. 6A and Fig. S5,† the fluorescence intensity of 1O (C = 2.0 × 10⁻⁵ mol L⁻¹) in acetonitrile at 595 nm under excitation at 515 nm was significantly enhanced by TFA due to the formation of an open ring fluorescent rhodamine moiety. The fluorescence intensity reached a maximum in the presence of 30 equiv. TFA and the absolute fluorescent quantum yield of the protonated 1O (1O′′) was determined to be 0.55. The addition of TEA to the 1O′′ solution re-generated the spirolactam form of rhodamine. The fluorescent emission at 583 nm disappeared, and the bright green-yellow fluorescence of the solution diminished (Fig. 6A). Further addition of TFA produced the protonated 1O′′. As shown in Fig. 6B, the spirolactam form of 1O was non-absorbing, whereas the open ring form exhibited a visible absorption band centered at 529 nm and the solution changed from colorless to pink, which could be easily observed with the naked eye. The absorption reached a maximum with 30.0 equiv. TFA and further increasing TFA equiv. caused no significant change in the absorption (Fig. S6†). The reversible modulation of fluorescent emission via alternating additions of TFA and TEA for 10 cycles resulted in no significant decrease in the emission intensity of 1O (Fig. 6D).

Fig. 6 (A) Fluorescence (λ_ex = 515 nm) change of 1O in acetonitrile (C = 2.0 × 10⁻⁵ mol L⁻¹) induced by TFA/TEA. (B) Changes in absorption spectra of 1O′′ in acetonitrile (C = 2.0 × 10⁻⁵ mol L⁻¹) induced by TFA/TEA. (C) Fluorescence (λ_ex = 515 nm) change of 1O′′ upon alternating irradiation with UV/vis light. (D) Acid–base cycle of 1O in acetonitrile (C = 2.0 × 10⁻⁵ mol L⁻¹) at room temperature. (E) Principle of reversible molecular switching and the color changes of 1O.

As discussed above, the color of 1O′′ solution could be changed via alternating irradiating with UV/vis lights. Based on this feature, it can be anticipated that the diarylethene derivatives bearing chemo-responsive function groups offer a promising approach to the design of optical molecular switches.^47,48 1O′′ in acetonitrile underwent a photocyclization reaction under irradiation at 297 nm, exhibiting a strong emission peak at 582 nm that significantly decreased with the isomerization from 1O′′ to 1C′′ under visible light irradiation (Fig. 6C). The fluorescence intensity of 1O′′ decreased 85%, accompanied by the fluorescence change from bright green-yellow to dark at the photostationary state, which was a typical behaviour of the FRET mechanism. FRET usually occurs between the open ring rhodamine acylhydrazine (the donor) and the closed-ring diarylethene (the acceptor), resulting in intrinsic photoluminescence properties. The fluorescence color and emission spectrum of 1O′′ can be restored via irradiation with visible light of λ > 500 nm. Fig. 6E shows the switching behavior of 1O induced by the stimulations of proton and UV/vis lights. These results indicate that 1O can be potentially used as a dual-control molecular switch by TFA/TEA and UV/vis light.

Application in a logic circuit

As discussed above, the emission intensity of diarylethene 1O was successfully modulated by either UV/vis light or Fe³⁺/EDTA stimuli in an aqueous acetonitrile solution (Fig. 7A). Based on this observation, a logic circuit was constructed by four inputs including 297 nm UV light, visible light (λ > 500 nm), Fe³⁺, and EDTA. The fluorescent emission intensity at 585 nm was set as the output. The output with the emission intensity at 585 nm higher than ca. 2500 was considered as the on state with a Boolean value of 1 and the emission intensity lower than ca. 2500 was regarded as the off state with a Boolean value of 0. As shown in Fig. 7B, the state can be either on (on = 1) or off (off = 0) with different Boolean values. For example, In 1 can be switched to the ‘on’ state with the Boolean value of 1 in response to irradiation at 297 nm and In 2 can be switched on by the irradiation of appropriate visible light (λ > 500 nm). In 3 was 1 with the addition of Fe³⁺ and In 4 was 1 with the addition of EDTA. The output was on with the Boolean value of 1 under the stimuli of all four inputs. The fluorescent emission intensity at 585 nm below ca. 2500 was set as the initial state where the logic circuit was off. The input strings In 3 = 1, In1 = 0, In 2 = 0, and In 4 = 0 gave an output signal of 1 (on). Table 2 shows all possibilities of the logic strings of the four inputs.

Fig. 7 (A) Dual-controlled fluorescent-switching behavior of 1O induced by Fe³⁺/EDTA and UV/vis light stimuli. (B) The combinational logic circuits equivalent to the truth table given in Table 1: In 1 (297 nm light), In 2 (λ > 500 nm light), In 3 (Fe³⁺), In 4 (EDTA) and output (fluorescence at 585 nm).

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

Input				Output λem = 585 nm
In 1 (UV light)	In 2 (vis light)	In 3 (Fe^3+)	In 4 (EDTA)
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

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Conclusions

In summary, a new diarylethene derivative bearing a rhodamine 6G unit was designed and synthesized. It exhibited highly selective and sensitive turn-on fluorescence and a color change response visible with the naked eye towards Fe³⁺ in aqueous acetonitrile. The Job's plot of experimental data reveals that a 1 : 1 stoichiometry is the most favorable binding mode between 1O and Fe³⁺. In addition, the fluorescent emission of the complex can be turned on/off by UV/vis lights irradiation. Based on these results, an integrated digital circuit was constructed with the fluorescent emission intensity at 585 nm as the output.

Acknowledgements

The authors are grateful for the financial support received from the National Natural Science Foundation of China (21662015, 21363009, 221362013, 51373072), the Project of Jiangxi Academic and Technological leaders (20142BCB22010), the Project of the Science Funds of the Jiangxi Education Office (KJLD13069), and the innovation fund of graduate students (YC2016-X02).

Notes and references

1. E. Erdema, N. Karapinarb and R. Donata, J. Colloid Interface Sci., 2004, 280, 309–314.
2. D. Wang, X. Y. Xiang, X. L. Yang, X. D. Wang, Y. L. Guo, W. S. Liu and W. W. Qin, Sens. Actuators, B, 2014, 201, 246–254.
3. W. Sun, D. Y. Hu, Z. B. Wu, B. A. Song and S. Yang, Chin. J. Org. Chem., 2011, 31, 997–1010.
4. G. J. Park, I. H. Hwang, E. J. Song, H. Kim and C. Kim, Tetrahedron, 2014, 70, 2822–2828.
5. K. Rurack, Spectrochim. Acta, Part A, 2001, 57, 2161–2195.
6. J. F. Zhang, Y. Zhou, J. Yoon and J. S. Kim, Chem. Soc. Rev., 2011, 40, 3416–3429.
7. K. P. Carter, A. M. Young and A. E. Palmer, Chem. Rev., 2014, 114, 4564–4601.
8. P. S. Hardikar, S. M. Joshi, D. S. Bhat, D. A. Raut, P. A. Katre, H. G. Lubree and C. S. Yajnik, Diabetes Care, 2012, 35, 797–802.
9. J. Comin-Colet, M. Lainscak, K. Dickstein, G. S. Filippatos, P. Johnson, T. F. Lüscher, C. Mori, R. Willenheimenr, P. Poniknowski and S. D. Anker, Eur. Heart J., 2013, 34, 30–38.
10. A. Malek, T. Thomas and E. Prasad, ACS Sustainable Chem. Eng., 2016, 4, 3497–3503.
11. T. Ganz, Blood, 2003, 102, 783–788.
12. A. P. De Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97, 1515–1566.
13. X. Y. Qu, Q. Liu, X. N. Ji, H. C. Chen, Z. K. Zhou and Z. Shen, Chem. Commun., 2012, 48, 4600–4602.
14. C. Wolf, X. Mei and H. K. Rokadia, Tetrahedron Lett., 2004, 45, 7867–7871.
15. J. Zhang, Q. Zou and H. Tian, Adv. Mater., 2013, 25, 378–399.
16. A. Toriumi, S. Kawata and M. Gu, Opt. Lett., 1998, 23, 1924–1926.
17. M. Irie, Chem. Rev., 2000, 100, 1685–1716.
18. K. Y. Liu, Y. Wen, T. Shi, Y. Li, F. Y. Li, Y. L. Zhao, C. H. Huang and T. Yi, Chem. Commun., 2014, 50, 9141–9144.
19. G. M. Liao, C. H. Zheng, D. D. Xue, C. B. Fan, G. Liu and S. Z. Pu, RSC Adv., 2016, 6, 34748–34753.
20. S. H. Jing, C. H. Zheng, S. Z. Pu, C. B. Fan and G. Liu, Dyes Pigm., 2014, 107, 38–44.
21. S. Pal, N. Chatterjee and P. K. Bharadwaj, RSC Adv., 2014, 4, 26585–26620.
22. X. Zeng, L. Dong, C. Wu, L. Mu, S. F. Xue and Z. Tao, Sens. Actuators, B, 2009, 141, 506–510.
23. M. Morimoto and M. Irie, Chem. Commun., 2005, 31, 3895–3905.
24. S. Yamamoto, K. Matsuda and M. Irie, Org. Lett., 2003, 5, 1769–1772.
25. R. J. Wang, S. Z. Pu, G. Liu and S. Q. Cui, Beilstein J. Org. Chem., 2012, 8, 1018–1026.
26. Z. Wang, D. Wu, G. Wu, N. Yang and A. Wu, J. Hazard. Mater., 2013, 244, 621–627.
27. D. Wu, W. Huang, C. Duan, Z. Lin and Q. Meng, Inorg. Chem., 2007, 46, 1538–1540.
28. Z. X. Li, L. Y. Liao, W. Sun, C. H. Xu, C. Zhang, C. J. Fang and C. H. Yan, J. Phys. Chem. C, 2008, 112, 5190–5196.
29. S. Kawai, T. Nakashima, K. Atsumi, T. Sakai, M. Harigai, Y. Imamoto and T. Kawai, Chem. Mater., 2007, 19, 3479–3483.
30. M. Morimoto and M. Irie, Chem.–Eur. J., 2006, 12, 4275–4282.
31. T. Nakashima, K. Miyamura, T. Sakai and T. Kawai, Chem.–Eur. J., 2009, 15, 1977–1984.
32. M. Irie, T. Lifka, S. Kobatake and N. Kato, J. Am. Chem. Soc., 2000, 122, 4871–4876.
33. M. J. Yuan, Y. L. Li, J. B. Li, C. H. Li, X. F. Liu, J. Lv, J. L. Xu, H. B. Liu, S. Wang and D. B. Zhu, Org. Lett., 2007, 9, 2313–2316.
34. X. B. Yang, B. X. Yang, J. F. Ge, Y. J. Xu, Q. F. Xu, J. Liang and J. M. Lu, Org. Lett., 2011, 13, 2710–2713.
35. R. Kato, S. Nishizawa, T. Hayashita and N. Teramae, Tetrahedron Lett., 2001, 42, 5053–5056.
36. H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949, 71, 2703–2707.
37. M. Saleem and K. H. Lee, RSC Adv., 2015, 5, 72150–72287.
38. S. Li, D. Zhang, X. Y. Xie, S. G. Ma, Y. Liu, Z. H. Xu, Y. F. Gao and Y. Ye, Sens. Actuators, B, 2016, 224, 661–667.
39. C. C. Wang, Y. Q. Liu, J. Y. Cheng, J. H. Song, Y. F. Zhao and Y. Ye, J. Lumin., 2015, 157, 143–148.
40. X. F. Bao, J. X. Shi, X. M. Nie, B. J. Zhou, X. L. Wang, L. Y. Zhang, H. Liao and T. Pang, Bioorg. Med. Chem., 2014, 22, 4826–4835.
41. C. C. Wang, D. Zhang, X. Y. Huang, P. G. Ding, Z. J. Wang, Y. F. Zhao and Y. Ye, Talanta, 2014, 128, 69–74.
42. M. M. Chai, D. Zhang, M. Wang, H. J. Hong, Y. Ye and Y. F. Zhao, Sens. Actuators, B, 2012, 174, 231–236.
43. Y. Y. Du, M. Chen, Y. X. Zhang, F. Luo, C. Y. He, M. J. Li and X. Chen, Talanta, 2013, 106, 261–265.
44. V. Bhalla, N. Sharma, N. Kumar and M. Kumar, Sens. Actuators, B, 2013, 178, 228–232.
45. K. Ghosh, T. Sarkar, A. Majumdar, S. K. Mandal and A. R. Khuda-Bukhsh, Anal. Methods, 2014, 6, 2648–2654.
46. S. J. Lim, J. Seo and S. Y. Park, J. Am. Chem. Soc., 2006, 128, 14542–14547.
47. L. N. Lucas, J. J. D. de Jong, J. H. Van Esch, R. M. Kellogg and B. L. Feringa, Eur. J. Org. Chem., 2003, 2003, 155–166.
48. W. R. Algar, M. Massey and U. J. Krull, TrAC, Trends Anal. Chem., 2009, 28, 292–306.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra04728b

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