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Solvent-dependent selective fluorescence sensor for Zn2+ and Al3+ based on a new diarylethene with a salicylal schiff base group
Accepted Manuscript Solvent-dependent selective fluorescence sensor for Zn diarylethene with a salicylal schiff base group
2+
and Al
3+
based on a new
Lele Ma, Gang Liu, Shouzhi Pu, Chunhong Zheng, Congbin Fan PII:
S0040-4020(17)30134-5
DOI:
10.1016/j.tet.2017.02.009
Reference:
TET 28450
To appear in:
Tetrahedron
Received Date: 25 October 2016 Revised Date:
17 January 2017
Accepted Date: 6 February 2017
Please cite this article as: Ma L, Liu G, Pu S, Zheng C, Fan C, Solvent-dependent selective fluorescence 2+ 3+ sensor for Zn and Al based on a new diarylethene with a salicylal schiff base group, Tetrahedron (2017), doi: 10.1016/j.tet.2017.02.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Solvent-dependent selective fluorescence sensor for Zn2+ and Al3+ based on a new diarylethene with a salicylal schiff base group
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Lele Ma, Gang Liu, Shouzhi Pu*, Chunhong Zheng, Congbin Fan Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China
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Tetrahedron journal homepage: www.elsevier.com
Lele Ma, Gang Liu, Shouzhi Pu*, Chunhong Zheng, Congbin Fan
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Solvent-dependent selective fluorescence sensor for Zn2+ and Al3+ based on a new diarylethene with a salicylal schiff base group
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Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China Corresponding author: E-mail address: [email protected] (S. Pu); Tel. & Fax: +86-791-83831996.
ABSTRACT
Article history: Received Received in revised form Accepted Available online
A novel photochromic diarylethene with a salicylidene Schiff base group was synthesized and its multi-controllable fluorescence switching properties induced by light, base, and metal ions were investigated. The diarylethene displayed sequence-dependent responses through efficient interaction of the salicylidene Schiff base group with base and metal ions, and it could serve as a naked-eye chemosensor for highly selective detection of different metal ions in different solvents. When triggered by Zn2+ in acetonitrile, its fluorescence was enhanced by 400 fold with a concomitant color change from dark to bright yellow. In contrast, when it bound Al3+ in methanol, its fluorescence was enhanced by 110 fold accompanied with a color change from dark to bright cyan. On the basis of its light-, base-, and metal-responsive fluorescence behaviors, two complicated logic gates were constructed by using the fluorescence intensity as output and combinational stimuli of light and chemical species as inputs.
1. Introduction
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Keywords: Photochromism Diarylethene Fluorescence chemosensor Logic gate Ion recognition
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ARTICLE INFO
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In recent years, the development of fluorescence sensors for selective detection of various relevant metal ions has gained increasing attention due to their negative biological and environmental effects.1 Among the various metal ions, Zn2+ and Al3+ play important roles in a wide variety of physiological and pathological processes.2 Zn2+, the second most abundant transition metal ion in the human body, plays an important role as an anti-oxidant.3 Besides, Zn2+ is actively involved in diverse fundamental biological processes, such as gene transcription, regulation of metalloenzymes, neural signal transmission, and apoptosis.4 While a small quantity of Zn2+ is necessary for the living organism, high-level uptakes has been implicated in neurodegenerative disorders.5 As the most prevalent metal element in the environment, Al3+ is considered toxic for biological systems. Al3+ may cause induce the accumulation of oxidative damage, resulting in misfunction of the central nervous system in Alzheimer’s disease.6 Among the available detection methods, fluorescence sensors are predominantly attractive in terms of sensitivity, selectivity, response time and live cell applications. Recently, several molecular turn-on fluorescence sensors were reported for the detection of Al3+ and Zn2+ based on photoinduced electron transfer (PET), internal charge transfer (ICT), chelation enhanced fluorescence (CHEF) and
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deprotonation mechanisms.7 However, most of these fluorescence sensors cannot selectively distinguish Al3+ and Zn2+ from each other.8 Thus, it is desirable to develop fluorescence sensors with high selectivity and sensitivity for both ions. Photochromic materials have attracted much attention for their potential applications in optical memories, photoswitches, logic circuits, and chemosensors.9 As one of the most promising photoresponsive compounds, diarylethenes have attracted remarkable research interest because of their excellent thermal stability, notable fatigue resistance, high sensitivity, and rapid response.10 In particular, they can be functioned as fluorescence sensors because their fluorescence can be reversibly modulated by alternating irradiation with UV and visible light. Recently, many fluorescence sensors based on diarylethene derivatives have been reported.11 For instance, Yi and co-workers developed a new multi-responsive fluorescence sensor based on diarylethene with one terpyridine unit and demonstrated its effective switchable fluorescence controlled by UV/Vis or Zn2+/EDTA in solution.12 In a previous work, we reported a novel diarylethene with a triazole-bridged methylquinoline group and its ability for recognizing specific metal ions in different solvent systems.13 The diarylethene was selective towards Zn2+ and Cd2+ in acetonitrile. In contrast, it could selectively recognize Cr3+, Al3+, and Fe3+ in the binary solvent of acetonitrile and water (v/v = 7/3). The
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to room temperature. After filtrated and condensed by column results have contributed to a broad understanding of the MANUSCRIPT ACCEPTED chromatography on silica gel using petroleum ether/ethyl acetate fluorescence sensor based on diarylethenes with various (v/v = 1/1), the sample was recrystallized from dichloromethane functional groups. and finally to get the colorless powder diarylethene 1O (0.25 g, Schiff base derivatives have been extensively used as model yield: 83%). M.p. 507–508 K; 1H NMR (DMSO-d6, 400 MHz), δ systems to study fundamental photophysical processes, such as (ppm): 2.00 (s, 3H, -CH3), 2.06 (s, 3H, -CH3), 2.28 (s, 3H, -CH3), excited-state intramolecular proton transfer (ESIPT), 7.01(d, 1H, J = 4.0 Hz, phenyl-H), 7.41 (s, 1H, thienyl-H), 7.54– 14 photochromism, solvatochromism, and thermochromism. 7.58(m,3H, phenyl-H),7.60(d, 1H, phenyl-H), 7.63(s, 1H, Among the various Schiff base compounds, salicylidene Schiff phenyl-H), 7.84(d, 1H, J = 4.0 Hz, phenyl-H), 7.95 (d, 1H, J = bases are important and versatile class of ligands due to their 4.0 Hz, phenyl-H),8.70 (s, 1H, -CH=N), 11.48(s, 1H, -OH), facile preparation via salicylaldehyde-amine condensation, easy 12.22(s, 1H, -NH); 13C NMR (DMSO-d6, 100 MHz), δ modification of their steric and electronic properties, and high (ppm):10.19, 11.73, 14.02, 103.93, 117.84, 119.28, 121.69, binding affinity toward various metal ions.15 Additionally, the 121.95, 122.31, 124.22, 124.34, 124.98, 125.10, 126.43, 129.46, chemical properties of salicylidene Schiff bases could be induced 134.85, 140.62, 141.60, 151.38, 156.18, 158.06, 163.59, 170.21; 16 by pH changes. In this work, we designed and synthesized a IR (KBr, υ, cm-1): 1437 (−C−N), 1538 (−C=C), 1583 (−N−H), 2+ novel solvent-dependent selective fluorescence sensor for Zn 1653 (−C=N), 3205 (−OH); MS (m/z): 604 [M-H]−. Calcd for and Al3+ based on a diarylethene salicylal Schiff base derivative. C29H21F6N3O3S (%): Calcd C, 57.52; H, 3.50; N, 6.94; Found C, Besides its fluorescence could be dually controlled by both light 57.32; H, 3.58; N, 6.86. and acid/base stimuli, it could serve as a highly selective fluorescence sensor for Zn2+ and Al3+ in specific solvent systems 3. Results and discussion with different fluorescence signals and binding modes. The 3.1. Photochromism of 1O schematic illustration of photochromism of the diarylethene is shown in Scheme 1. The photochromic property of 1O induced by photoirradiation
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was studied in methanol (2.0 × 10-5 mol L-1) at room temperature. The changes in its absorption spectrum and color upon photoirradiation are shown in Fig. 1. The maximum absorption peak of the open-ring isomer 1O was observed at 295 nm (ε = 4.34 × 104 mol−1 L cm−1) due to a π → π* transition.18 Upon irradiation with 297 nm light, the absorption at 295 nm decreased with the increase of a new absorption band at 560 nm (ε = 5.50 × 103 mol−1 L cm−1) due to the formation of the closed-ring isomer 1C. The new band increased upon irradiation for 5 min till the photostationary state (PPS) was reached and a clear isosbestic point was observed at 317 nm. This process was accompanied by a color change from colorless to pale purple. When the solution at the PSS was irradiated with appropriate visible light for 4 min (λ > 450 nm), the purple solution was bleached completely and the absorption spectrum was recovered to that of the open-ring isomer 1O. The quantum yields of the cyclization and cycloreversion are 0.39 and 0.043, respectively.
Scheme 1. Synthetic method and photochromism of diarylethene 1O.
2. Experimental 2.1. General methods
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All of the starting materials for synthesis were commercially available and were used as received. NMR spectra were collected on a Bruker AV400 (400 MHz) spectrometer with CDCl3 or dimethyl sulfoxide (DMSO-d6) as the solvent and tetramethylsilane (TMS) as an internal standard. IR spectra were recorded on a Bruker Vertex-70 spectrometer. Mass spectra were obtained on an Agilent 1100 ion trap MSD spectrometer. Melting point was measured using a WRS-1B melting point apparatus. Elemental analysis was carried out with a PE CHN 2400 analyzer. UV/Vis spectra were measured on an Agilent 8453 UV/Vis spectrophotometer. Fluorescence spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer. Fluorescence quantum yield was measured with an Absolute PL Quantum Yield Spectrometer QY C11347-11. Photo-irradiation was carried out using an SHG-200 UV lamp and a BMH-250 visible lamp. Photoirradiation was carried out using an MUΛ-165 UV lamp and a MVL-210 visible lamp. Lights of appropriate wavelengths were isolated using different light filters. All solvents used were of spectrograde and were purified by distillation prior to use. Metal ions were obtained by the dissolution of their respective metal nitrates (0.1 mol L−1) in distilled water expect for Mn2+, K+, and Ba2+ (their counter ions were chloride ions). 2.2 Synthesis of the target compound The synthetic route of the target diarylethene, 1-(3,5-dimethyl4-isoxazolyl)-2-{2-methyl-5-[4-hydroxyl-3-(methylenebenzohydrazide)-phenyl]-3-thienyl}perfluorocyclopentene (1O) is also shown in Scheme 1. Precursor 2 was synthesized by the reported methods.17 The raw material benzohydrazide 3 (0.14 g, 1.0 mmol) and compound 2(0.5 g, 1.0 mmol) were dissolved in a minimum amount of ethanol (10 ml), and then mixed together. The reaction mixture was stirred at 80 °C for 4 h, and then cooled
Fig. 1. Absorption spectral and color changes of 1O by photoirradiation in methanol (2.0 × 10-5 mol L-1).
3.2. Changes in Absorption and Fluorescence by Base/Acid Stimuli The dual-controllable photoswitching behaviors of 1O were investigated by the stimulation of acid/base and light are shown in Scheme 2. The fluorescence spectral changes of 1O (2.0 × 10-5 mol L-1) induced by tetrabutylammonium hydroxide (TBAH) and trifluoroacetic acid (TFA) in acetonitrile at room temperature are shown in Fig. 2A. 1O showed weak fluorescence in the original state. However, when 2.0 equiv of TBAH (1.2 µL, 0.1mol L−1) was added to the colorless solution of 1O, the fluorescence at 553 nm was significantly enhanced due to the formation of
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deprotonated 1O'. This result was in agreement with a previous MANUSCRIPT The dual-controllable photoswitching behaviors of 1O were ACCEPTED report.19 The fluorescence intensity reached a plateau upon in investigated by the stimulation of acid/base and light are shown further titration when the amount of TBAH was more than 2 in Scheme 2. The fluorescence spectral changes of 1O (2.0 × 10-5 mol L-1) induced by tetrabutylammonium hydroxide (TBAH) and equiv (Fig. 2B). Compared to that of 1O, the emission intensity of 1O' was enhanced by 40 fold at 553 nm, accompanied by a trifluoroacetic acid (TFA) in acetonitrile at room temperature are significant fluorescence color change from dark to bright yellow. shown in Fig. 2A. 1O showed weak fluorescence in the original After addition of 2.0 equiv of TFA (1.2 µL, 0.1 mol L−1), 1O' state. However, when 2.0 equiv of TBAH (1.2 µL, 0.1mol L−1) returned to its protonated form of 1O and its fluorescence was was added to the colorless solution of 1O, the fluorescence at 553 quenched. The formation of 1O' was confirmed by 1H NMR nm was significantly enhanced due to the formation of analysis in CD3CN (Fig.S1). After the addition of TBAH, the deprotonated 1O'. This result was in agreement with a previous proton of hydroxyl group at 11.83 ppm and that of imidogen report.19 The fluorescence intensity reached a plateau upon in group at 10.67 ppm disappeared. 1O' exhibited a notable further titration when the amount of TBAH was more than 2 fluorescence switch by photoirradiation in acetonitrile at room equiv (Fig. 2B). Compared to that of 1O, the emission intensity temperature (Fig. 2C). Upon irradiation with 297 nm light, the of 1O' was enhanced by 40 fold at 553 nm, accompanied by a emission intensity at 553 nm decreased with fluorescent color significant fluorescence color change from dark to bright yellow. change from bright yellow to dark. At the photostationary state, After addition of 2.0 equiv of TFA (1.2 µL, 0.1 mol L−1), 1O' the emission intensity was quenched to only 50% of its initial returned to its protonated form of 1O and its fluorescence was value due to the formation of the closed-ring isomer 1C'. quenched. The formation of 1O' was confirmed by 1H NMR Subsequently, irradiation with visible light (λ > 450 nm) analysis in CD3CN (Fig.S1). After the addition of TBAH, the regenerated the open-ring isomer and restored the emission proton of hydroxyl group at 11.83 ppm and that of imidogen spectrum back to its original form 1O'. Consequently, the group at 10.67 ppm disappeared. 1O' exhibited a notable fluorescence of the title diarylethene could be modulated by both fluorescence switch by photoirradiation in acetonitrile at room TBAH/TFA or UV/Vis stimuli, and serve as a dual-controllable temperature (Fig. 2C). Upon irradiation with 297 nm light, the fluorescent molecular switch. emission intensity at 553 nm decreased with fluorescent color change from bright yellow to dark. At the photostationary state, the emission intensity was quenched to only 50% of its initial value due to the formation of the closed-ring isomer 1C'. Subsequently, irradiation with visible light (λ > 450 nm) regenerated the open-ring isomer and restored the emission spectrum back to its original form 1O'. Consequently, the fluorescence of the title diarylethene could be modulated by both TBAH/TFA or UV/Vis stimuli, and serve as a dual-controllable fluorescent molecular switch.
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Scheme 2. Photochromism, color, and fluorescence changes of 1 induced by TBAH/TFA and UV/Vis.
Fig. 2. Changes in fluorescence of 1O in acetonitrile (2.0 × 10−5 mol L−1) induced by TBAH/TFA and light stimuli at room temperature, excited at 438 nm: (A) emission intensity and fluorescence color changes of 1O induced by the addition of TBAH; (B) The curve of fluorescent intensity ratio at 550 nm with the addition of different equiv. TBAH concentration; (C) emission intensity and fluorescence color changes of 1Oʹ by photoirradiation.
Fig. S2 shows the absorption spectral and color changes of 1O (2.0 × 10-5 mol L-1) induced by TBAH and TFA in acetonitrile at room temperature. Addition of 2 equiv. of TBAH (1.2 µL, 0.1 mol L−1) to the colorless solution of 1O, a new visible band centered at 465 nm (ε = 1.24 × 104 mol−1 L cm−1) was observed (Fig. S2A), and the maximum absorption peak of the open-ring isomer 1O was red-shifted from 293 nm to 323 nm. Meanwhile, the color of solution changed from colorless to yellow due to the formation of deprotonated diarylethene 1O'. The deprotonated 1O' could return to 1O by neutralizing with 2.0 equiv of TFA (1.2 µL, 0.1 mol L−1). Moreover, upon irradiation with 297 nm light, the pale yellow solution of 1O' turned dark blue with concomitant appearance of a new visible absorption band centered at 658 nm (ε = 1.80 × 103 mol−1 L cm−1) due to the formation of the closed-ring isomer 1C'. Upon irradiation with visible light (λ > 450 nm), the dark blue solution changed to pale yellow and the absorption spectrum reverted to the original openring isomer 1O'. Therefore, 1O' could also undergo a reversible photoisomerization (Fig. S2B). 3.3. Fluorogenic sensing for Zn2+ in MeCN The selectivity of 1O toward a variety of metal ions through fluorescence emission was investigated in acetonitrile at room temperature. Fig. 3 shows the emission spectra and fluorescence color change of 1O with an excitation of 438 nm when 3 equiv of various metal ions such as K+, Ag+, Cu2+, Ca2+, Mg2+, Fe3+, Ba2+, Sr2+, Pb2+, Ni2+, Hg2+, Mn2+, Zn2+, Cd2+, Co2+, Al3+, and Cr3+ were added to the solution of 1O. It was found that the fluorescence of the solution exhibited no or insignificant increase except for Zn2+. The addition of Zn2+ resulted in drastic enhancement of the emission intensity at 560 nm with concomitant fluorescence color change from dark to bright yellow. It is noteworthy that the
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3+ peak at 604.1 (m/z) for [1O-H]–. When excess amount of Zn2+ individual addition of Ni2+, Cu2+, Co2+, FeACCEPTED , and Sr2+ also MANUSCRIPT resulted in fluorescence intensity changes, but the changes were was added to 1O, the peak at 604.1 (m/z) decreased and a new very small compared to that of Zn2+. Therefore, the diarylethene peak at 667.0 (m/z) appeared due to the formation of [(1O-Zn2+)2+ could serve as a fluorescence chemosensor for detection of Zn H]–. The positive ion mass spectrum indicated the 1:1 binding with high selectivity in acetonitrile. More importantly, the mode between 1O and Zn2+. Upon complexation, the proton of 2+ 2+ the hydroxyl group at 13.2 ppm disappeared, and the proton of diarylethene can clearly distinguish Zn from Cd , whereas the discrimination of Zn2+ from Cd2+ is well known as a major the imine (−CH=N) was shifted from 8.35 ppm to 8.37 ppm (Fig. obstacle. The selective fluorescence enhancement by Zn2+ might S4). be ascribed to the effective coordination of Zn2+ with 1O over other metal ions. It causes the 1O-Zn2+ complex to be more coplanar than 1O itself, resulting in the chelation-enhanced fluorescence (CHEF) effect.20 Also, 1O is poorly fluorescent due to the isomerization of the C=N double bond in the excited state and the excited-state intramolecular proton transfer (ESIPT) involving the phenolic protons.21 Upon stable chelation with Zn2+, the C=N isomerization and ESIPT might be inhibited, leading to fluorescence enhancement.
Fig. 4. Changes in fluorescence of 1O in acetonitrile (2.0 × 10−5 mol L−1) induced by Zn2+/EDTA and light stimuli at room temperature, excited at 438 nm: (A) emission intensity and fluorescence color changes of 1O induced by the addition of Zn2+; (B) The curve of fluorescent intensity ratio at 560 nm with the addition of different equiv. Zn2+ concentration; (C) emission intensity and fluorescence color changes of 1O-Zn2+ by photoirradiation; (D) Job’s plot showing the 1:1 complex between Zn2+ and 1O.
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Fig. 3. Changes in the fluorescence of 1O induced by the addition of various metal ions (3 equiv) in acetonitrile (2.0 × 10−5 mol L−1) excited at 438 nm: (A) emission spectral changes; (B) emission intensity changes; (C) photos demonstrating changes in its fluorescence.
Scheme 3. Photochromism, color, and fluorescence changes of 1O induced by Zn2+/EDTA and UV/Vis.
Upon irradiation at 297 nm light, the fluorescence intensity of 1O-Zn2+ was quenched to ca. 38% due to the formation of the closed-ring isomer 1C-Zn2+ at the photostationary state (Fig. 4C). However, when EDTA was added to the solution of 1O-Zn2+, the fluorescence intensity could not be restored due to the complexation between Zn2+ and 1O is much more tightly than that between EDTA and Zn2+. The binding mode between 1O and Zn2+ was determined by the Job’s plot analysis. The fluorescence intensity of 1O-Zn2+ at 560 nm was plotted against the molar fraction of 1O under the condition of an invariant total concentration. As shown in Fig. 4D, the Job’s plot for 1O-Zn2+ complex exhibited 1:1 stoichiometry. The formation of 1O-Zn2+ complex was further confirmed by ESI-MS method and 1H NMR analysis in CD3CN. As shown in Fig. S3, the free 1O had a main
The absorption spectrum of 1O stimulated by Zn2+ was shown in Fig. S5. Upon addition of 3 equiv of Zn2+ to the solution of 1O, the absorption band of the open-ring isomer at 292 nm decreased and a new absorption band centered at 425 nm (ε = 1.04 × 104 mol−1 L cm−1) emerged. Meanwhie, the solution changed from colorless to light yellow, indicating a clean conversion of 1O into 1O-Zn2+ complex (Fig. S5A). It was noticed that the absorption properties of 1O-Zn2+ could not be restored by adding excess EDTA to the solution. Upon irradiation with 297 nm light, the light yellow solution of 1O-Zn2+ turned gray with concomitant appearance of a new visible absorption band centered at 560 nm (ε = 6.30 × 103 mol−1 L cm−1) due to the formation of the closed-ring isomer 1C-Zn2+ (Fig. S5B). Upon irradiation with visible light (λ > 450 nm), the gray solution
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To further investigate the chemosensing properties of 1O toward Zn2+, the fluorescence titration tests were performed in acetonitrile (2.0 × 10−5 mol L−1) at room temperature as shown in Fig. 4. When Zn2+ was gradually added to the solution of 1O from 0 to 3 equiv, an increase in emission intensity was red-shift from 553 to 560 nm due to the formation of 1O-Zn2+, followed by a plateau with further titration. As compared with 1O, the emission peak of 1O-Zn2+ at 560 nm was enhanced by 400 fold at the plateau (Fig. 4A). Fig. 4B depicts the effect of Zn2+ concentration on the emission intensity at 560 nm. The results indicated that there was a linear relationship between the fluorescence intensity and the Zn2+ concentration in the range of 0−24 µM, suggesting that 1O could be potentially used as a colorimetric fluorescent probe for Zn2+.
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expected because of their exchange with the solvent protons. turned to light yellow and the absorption spectrum reverted to the MANUSCRIPT ACCEPTED The protons of 1O displayed signals at 7.48, 7.55, 7.72, and 7.89 original open-ring isomer 1O-Zn2+. Therefore, 1O-Zn2+ could ppm for the phenyl group. Upon complexation with Al3+, also undergo a reversible photoisomerization. The cyclic graph of 2+ protons-(h,l) shifted from 7.89 to 8.07 ppm, proton-f shifted from 1O induced by Zn /EDTA and UV/Vis was shown in Scheme 3. 7.72 to 7.58 ppm, and protons-(d,f,k,i,j) were broadened. The 3.4. Fluorogenic sensing for Al3+ in MeOH results were attributed to the isomerization of the C=O double bond to C=N double bond after 1O was complexed with Al3+ To gain an insight into the fluorescent properties of 1O toward (Fig. S7). Based on the Job’s plot, ESI-MS, and 1H NMR, we various metal ions in methanol, the emission changes were proposed the structure of the complex between 1O and measured with them at room temperature. When excited at 402 Al3+.(Scheme 4). nm, 1O exhibited a weak fluorescence emission compared to that 3+ 3+ in the presence of Al (Fig. 5). The addition of Al from 0 to 3 equiv resulted in drastic enhancement of the emission intensity at 510 nm with concomitant fluorescent color change from dark to bright cyan. The dramatic fluorescence enhancement responses to Al3+ many be attributed to the chelation of 1O with Al3+ and the C=O isomerization of salicylidene Schiff base unit. As compared with 1O, the emission peak of 1O-Al3+ at 510 nm was enhanced by 110 fold at the plateau. In contrast, upon addition of other metal ions such as K+, Ag+, Cu2+, Ca2+, Mg2+, Fe3+, Ba2+, Sr2+, Pb2+, Ni2+, Hg2+, Mn2+, Zn2+, Cd2+, Co2+, and Cr3+ ，only Zn2+ resulted in tiny fluorescence increase, either no or slight increase in intensity were observed with the rest metal ions. It is noteworthy that 1O could serve as an excellent fluorescent probe for Al3+ by switching the solvent from acetonitrile to methanol. The results indicated that the fluorescence behaviors of 1O-Zn2+ and 1O-Al3+ complexes are solvent-dependent.
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Fig. 6. Changes in fluorescence of 1O in methanol (2.0 × 10−5 mol L−1) induced by Al3+/EDTA and light stimuli at room temperature, excited at 402 nm: (A) emission intensity and fluorescence color changes of 1O induced by the addition of Al3+; (B) The curve of fluorescent intensity ratio at 510 nm with the addition of different equiv. Al3+ concentration; (C) emission intensity and fluorescence color changes of 1O-Al3+ by photoirradiation; (D) Job’s plot showing the 1:1 complex between Al3+ and 1O.
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Fig. 5. Changes in the fluorescence of 1O induced by the addition of various metal ions (3 equiv) in methanol (2.0 × 10−5 mol L−1) excited at 402 nm: (A) emission spectral changes; (B) emission intensity changes; (C) photos demonstrating changes in its fluorescence.
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Fig. 6A depicts the effect of Al3+ concentration on the emission at 510 nm. A good linear relationship was seen between the fluorescence intensity of 1O and Al3+ concentration in the range of 0−24 µΜ (Fig. 6B). This suggested the potential application of 1O as a colorimetric fluorescent probe for Al3+ in methanol. Upon irradiation with 297 nm light, the fluorescence intensity of 1O-Al3+ was quenched dramatically with a concomitant color change from bright cyan to dark due to the formation of the closed-ring isomer 1C-Al3+ (Fig. 6C). The quenched fluorescence intensity reached ca. 34% at the photostationary state. After adding EDTA solution to the solution of 1O-Al3+, the fluorescence intensity was restored due to the complexation between Al3+ and EDTA. The Job’s plot showed a 1:1 stoichiometry between 1O and Al3+ (Fig. 6D). The formation was also further confirmed by ESI-MS method and 1H NMR analysis in CD3OD. The positive ion mass spectrum indicated that peak at 628.1 (m/z) is assignable to [1O+Na]+ and peak at 725.0 (m/z) is assignable to [(1O-Al3+)+NO3–+CH3OH]+ (Fig. S6). The 1H NMR spectrum of 1O shows weak signals of N–H and phenolic O–H protons at 7.53 ppm and 10.06 ppm as
Scheme 4. Photochromism, color, and fluorescence changes of 1O induced by Al3+/EDTA and UV/Vis.
Fig. S8 shows the absorption spectral and color changes of 1O induced by Al3+/EDTA and UV/Vis in methanol at room temperature. Upon addition of 3 equiv Al3+ into the colorless solution of 1O, the absorption band of the open-ring isomer at 295 nm decreased and a new absorption band at 311 nm increased with an isosbestic point at 303 nm. Meanwhile, the solution color changed from colorless to light yellow accompanied with the emergence of another absorption band at 398 nm (ε = 1.25 × 104 mol−1 L cm−1), which indicated a clean conversion of 1O into 1O-Al3+ complex (Fig. S8A). The complex could return to 1O by adding excess EDTA and this was attributed to the complexation between Al3+ and EDTA. 1O-Al3+
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also underwent a reversible photoisomerization upon irradiation MANUSCRIPT output signal O1 is ‘on’ and the output digit is ‘1’ (Fig.7A). All ACCEPTED with UV and visible light. Upon irradiation with 297 nm light, the possible strings of the three inputs are listed in Table 1. Similarly, when the input string is ‘1, 0, 1, and 0’, corresponding the light yellow solution of 1O-Al3+ turned purple, accompanied by the appearance of a new absorption band centered at 560 nm inputs (In1, In2, In4, and In5) are ‘on, off, on, and off’and the output signal O2 is ‘on’ and the output digit is ‘1’ (Fig. 7B). All (ε = 6.30 × 103 mol−1 L cm−1) due to the formation of the closedthe possible strings of the four inputs are listed in Table 2. ring isomer 1C-Al3+ (Fig. S8B). Upon irradiation with visible light (λ > 450 nm), the purple solution and the absorption Table 1. Truth table for all possible strings of three binary-input data and the spectrum reverted to the original open-ring isomer 1O-Al3+. corresponding output digit. 3+ Scheme 4 shows the cyclic graph of 1O induced by Al /EDTA and UV/Vis. Input 3.5. Application in logic circuits
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In general, molecules can undergo changes in the ground or excited states in response to modulators which can be guest molecules, ions, or light of a certain wavelength. In most cases, these changes could be signaled by changes in the emission intensity.22 On the basis of the fact that the fluorescence intensity of the target compound 1O could be effectively modulated by TBAH, Al3+/EDTA, and UV/Vis stimuli (Schemes 2 and Schemes 4), we further illustrated combinational logic circuits by utilizing light irradiation, TBAH, EDTA, and Al3+ as the input signals, and the change of fluorescence intensity as the output signal.
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a At 553 nm, the emission intensity below 50% of the original value is defined as 0, otherwise defined as 1.
Table 2. Truth table for all possible strings of three binary-input data and the corresponding output digit.
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Fig. 7. Two logic circuits based on the diarylethene with various inputs: (A) the combinational logic circuits equivalent to the truth table given in Table 1: In1 (Vis), In2 (UV), In3 (TBAH), and O1; (B) the combinational logic circuits equivalent to the truth table given in Table 2: In1 (Vis), In2 (UV), In4 (Al3+) In5 (EDTA) and O2.
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As shown in Fig. 7, we constructed two combinational logic circuits, one had three inputs (Vis (λ > 450 nm, In1), UV (297 nm, In2), and TBAH (In3)) and an output (fluorescence intensity at 553 nm (O1)) and the other had four inputs (Vis (λ > 450 nm, In1), UV (297 nm, In2), Al3+ (In4), and EDTA (In5)). All the input signals could be either ‘on’ or ‘off’ state with different Boolean values. For instance, when visible light was used input signal In1 was switched to ‘on’ state with a Boolean value ‘1’. Similarly, input signal In2 was ‘1’ when UV irradiation (297 nm) was employed, In3 was ‘1’ when TBAH was added, In4 was ‘1’ when Al3+ was added, and In5 was ‘1’ when EDTA was added. When TBAH or Al3+ added into the solution, 1O exhibited strong fluorescence, and then its intensity was employed as an initial state. The output signal could be regarded as ‘off’ when the emission intensity was below 50% of the original value. Otherwise, the output signal was regarded as ‘on’. The binary digits (‘0’ and ‘1’) can be used instead of the two levels (‘off’ and ‘on’). Concretely, when the string is ‘1, 0, and 1’, the corresponding inputs (In1, In2, and In3) are ‘on, off, and on’, respectively. Under these conditions, 1O was converted to the deprotonated 1O' and its fluorescence was enhanced, then the
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In conclusion, a novel photochromic diarylethene has been synthesized by using benzohydrazide as a functional group and perfluordiarylethene as photoswitching trigger via a salicylidene Schiff base linkage. Its multi-addressable molecular switching behavior and selective interaction with Zn2+ and Al3+ were investigated systematically. The results demonstrated that the diarylethene was highly selective towards Zn2+ and Al3+ in acetonitrile and methanol with distinct fluorescence signals and binding modes. Moreover, the diarylethene exhibited different absorption and fluorescence behaviors in the presence of base. Based on the fact that its fluorescence could be effectively modulated by stimulation with light and chemical species, two logic circuits were constructed by using the fluorescence intensity as the output signals with the inputs of UV/Vis , TBAH, Al3+, and EDTA stimuli. The results of current work are useful for design and synthesis of new photochromic diarylethenes with multi-controllable switching behavior and solvent-dependent selectivity for multiple targets of interest.
References and notes
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This work was supported by the NSFC (51373072, 201362013, and 21262015), Project of Jiangxi Sci-Tech Innovative Team (20142BCB24012), NSF of Jiangxi Province (20142BAB203005).
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Highlights A novel diarylethene with a salicylidene Schiff base group was synthesized > It exhibited multi-controllable behaviors induced by light, base, and metal ions > It can serve as a
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naked-eye chemosensor for different metal ions in different solvent
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based on a new
Lele Ma, Gang Liu, Shouzhi Pu, Chunhong Zheng, Congbin Fan PII:
S0040-4020(17)30134-5
DOI:
10.1016/j.tet.2017.02.009
Reference:
TET 28450
To appear in:
Tetrahedron
Received Date: 25 October 2016 Revised Date:
17 January 2017
Accepted Date: 6 February 2017
Please cite this article as: Ma L, Liu G, Pu S, Zheng C, Fan C, Solvent-dependent selective fluorescence 2+ 3+ sensor for Zn and Al based on a new diarylethene with a salicylal schiff base group, Tetrahedron (2017), doi: 10.1016/j.tet.2017.02.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Solvent-dependent selective fluorescence sensor for Zn2+ and Al3+ based on a new diarylethene with a salicylal schiff base group
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Lele Ma, Gang Liu, Shouzhi Pu*, Chunhong Zheng, Congbin Fan Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China
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Tetrahedron journal homepage: www.elsevier.com
Lele Ma, Gang Liu, Shouzhi Pu*, Chunhong Zheng, Congbin Fan
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Solvent-dependent selective fluorescence sensor for Zn2+ and Al3+ based on a new diarylethene with a salicylal schiff base group
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Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China Corresponding author: E-mail address: [email protected] (S. Pu); Tel. & Fax: +86-791-83831996.
ABSTRACT
Article history: Received Received in revised form Accepted Available online
A novel photochromic diarylethene with a salicylidene Schiff base group was synthesized and its multi-controllable fluorescence switching properties induced by light, base, and metal ions were investigated. The diarylethene displayed sequence-dependent responses through efficient interaction of the salicylidene Schiff base group with base and metal ions, and it could serve as a naked-eye chemosensor for highly selective detection of different metal ions in different solvents. When triggered by Zn2+ in acetonitrile, its fluorescence was enhanced by 400 fold with a concomitant color change from dark to bright yellow. In contrast, when it bound Al3+ in methanol, its fluorescence was enhanced by 110 fold accompanied with a color change from dark to bright cyan. On the basis of its light-, base-, and metal-responsive fluorescence behaviors, two complicated logic gates were constructed by using the fluorescence intensity as output and combinational stimuli of light and chemical species as inputs.
1. Introduction
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Keywords: Photochromism Diarylethene Fluorescence chemosensor Logic gate Ion recognition
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ARTICLE INFO
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In recent years, the development of fluorescence sensors for selective detection of various relevant metal ions has gained increasing attention due to their negative biological and environmental effects.1 Among the various metal ions, Zn2+ and Al3+ play important roles in a wide variety of physiological and pathological processes.2 Zn2+, the second most abundant transition metal ion in the human body, plays an important role as an anti-oxidant.3 Besides, Zn2+ is actively involved in diverse fundamental biological processes, such as gene transcription, regulation of metalloenzymes, neural signal transmission, and apoptosis.4 While a small quantity of Zn2+ is necessary for the living organism, high-level uptakes has been implicated in neurodegenerative disorders.5 As the most prevalent metal element in the environment, Al3+ is considered toxic for biological systems. Al3+ may cause induce the accumulation of oxidative damage, resulting in misfunction of the central nervous system in Alzheimer’s disease.6 Among the available detection methods, fluorescence sensors are predominantly attractive in terms of sensitivity, selectivity, response time and live cell applications. Recently, several molecular turn-on fluorescence sensors were reported for the detection of Al3+ and Zn2+ based on photoinduced electron transfer (PET), internal charge transfer (ICT), chelation enhanced fluorescence (CHEF) and
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deprotonation mechanisms.7 However, most of these fluorescence sensors cannot selectively distinguish Al3+ and Zn2+ from each other.8 Thus, it is desirable to develop fluorescence sensors with high selectivity and sensitivity for both ions. Photochromic materials have attracted much attention for their potential applications in optical memories, photoswitches, logic circuits, and chemosensors.9 As one of the most promising photoresponsive compounds, diarylethenes have attracted remarkable research interest because of their excellent thermal stability, notable fatigue resistance, high sensitivity, and rapid response.10 In particular, they can be functioned as fluorescence sensors because their fluorescence can be reversibly modulated by alternating irradiation with UV and visible light. Recently, many fluorescence sensors based on diarylethene derivatives have been reported.11 For instance, Yi and co-workers developed a new multi-responsive fluorescence sensor based on diarylethene with one terpyridine unit and demonstrated its effective switchable fluorescence controlled by UV/Vis or Zn2+/EDTA in solution.12 In a previous work, we reported a novel diarylethene with a triazole-bridged methylquinoline group and its ability for recognizing specific metal ions in different solvent systems.13 The diarylethene was selective towards Zn2+ and Cd2+ in acetonitrile. In contrast, it could selectively recognize Cr3+, Al3+, and Fe3+ in the binary solvent of acetonitrile and water (v/v = 7/3). The
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to room temperature. After filtrated and condensed by column results have contributed to a broad understanding of the MANUSCRIPT ACCEPTED chromatography on silica gel using petroleum ether/ethyl acetate fluorescence sensor based on diarylethenes with various (v/v = 1/1), the sample was recrystallized from dichloromethane functional groups. and finally to get the colorless powder diarylethene 1O (0.25 g, Schiff base derivatives have been extensively used as model yield: 83%). M.p. 507–508 K; 1H NMR (DMSO-d6, 400 MHz), δ systems to study fundamental photophysical processes, such as (ppm): 2.00 (s, 3H, -CH3), 2.06 (s, 3H, -CH3), 2.28 (s, 3H, -CH3), excited-state intramolecular proton transfer (ESIPT), 7.01(d, 1H, J = 4.0 Hz, phenyl-H), 7.41 (s, 1H, thienyl-H), 7.54– 14 photochromism, solvatochromism, and thermochromism. 7.58(m,3H, phenyl-H),7.60(d, 1H, phenyl-H), 7.63(s, 1H, Among the various Schiff base compounds, salicylidene Schiff phenyl-H), 7.84(d, 1H, J = 4.0 Hz, phenyl-H), 7.95 (d, 1H, J = bases are important and versatile class of ligands due to their 4.0 Hz, phenyl-H),8.70 (s, 1H, -CH=N), 11.48(s, 1H, -OH), facile preparation via salicylaldehyde-amine condensation, easy 12.22(s, 1H, -NH); 13C NMR (DMSO-d6, 100 MHz), δ modification of their steric and electronic properties, and high (ppm):10.19, 11.73, 14.02, 103.93, 117.84, 119.28, 121.69, binding affinity toward various metal ions.15 Additionally, the 121.95, 122.31, 124.22, 124.34, 124.98, 125.10, 126.43, 129.46, chemical properties of salicylidene Schiff bases could be induced 134.85, 140.62, 141.60, 151.38, 156.18, 158.06, 163.59, 170.21; 16 by pH changes. In this work, we designed and synthesized a IR (KBr, υ, cm-1): 1437 (−C−N), 1538 (−C=C), 1583 (−N−H), 2+ novel solvent-dependent selective fluorescence sensor for Zn 1653 (−C=N), 3205 (−OH); MS (m/z): 604 [M-H]−. Calcd for and Al3+ based on a diarylethene salicylal Schiff base derivative. C29H21F6N3O3S (%): Calcd C, 57.52; H, 3.50; N, 6.94; Found C, Besides its fluorescence could be dually controlled by both light 57.32; H, 3.58; N, 6.86. and acid/base stimuli, it could serve as a highly selective fluorescence sensor for Zn2+ and Al3+ in specific solvent systems 3. Results and discussion with different fluorescence signals and binding modes. The 3.1. Photochromism of 1O schematic illustration of photochromism of the diarylethene is shown in Scheme 1. The photochromic property of 1O induced by photoirradiation
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was studied in methanol (2.0 × 10-5 mol L-1) at room temperature. The changes in its absorption spectrum and color upon photoirradiation are shown in Fig. 1. The maximum absorption peak of the open-ring isomer 1O was observed at 295 nm (ε = 4.34 × 104 mol−1 L cm−1) due to a π → π* transition.18 Upon irradiation with 297 nm light, the absorption at 295 nm decreased with the increase of a new absorption band at 560 nm (ε = 5.50 × 103 mol−1 L cm−1) due to the formation of the closed-ring isomer 1C. The new band increased upon irradiation for 5 min till the photostationary state (PPS) was reached and a clear isosbestic point was observed at 317 nm. This process was accompanied by a color change from colorless to pale purple. When the solution at the PSS was irradiated with appropriate visible light for 4 min (λ > 450 nm), the purple solution was bleached completely and the absorption spectrum was recovered to that of the open-ring isomer 1O. The quantum yields of the cyclization and cycloreversion are 0.39 and 0.043, respectively.
Scheme 1. Synthetic method and photochromism of diarylethene 1O.
2. Experimental 2.1. General methods
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All of the starting materials for synthesis were commercially available and were used as received. NMR spectra were collected on a Bruker AV400 (400 MHz) spectrometer with CDCl3 or dimethyl sulfoxide (DMSO-d6) as the solvent and tetramethylsilane (TMS) as an internal standard. IR spectra were recorded on a Bruker Vertex-70 spectrometer. Mass spectra were obtained on an Agilent 1100 ion trap MSD spectrometer. Melting point was measured using a WRS-1B melting point apparatus. Elemental analysis was carried out with a PE CHN 2400 analyzer. UV/Vis spectra were measured on an Agilent 8453 UV/Vis spectrophotometer. Fluorescence spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer. Fluorescence quantum yield was measured with an Absolute PL Quantum Yield Spectrometer QY C11347-11. Photo-irradiation was carried out using an SHG-200 UV lamp and a BMH-250 visible lamp. Photoirradiation was carried out using an MUΛ-165 UV lamp and a MVL-210 visible lamp. Lights of appropriate wavelengths were isolated using different light filters. All solvents used were of spectrograde and were purified by distillation prior to use. Metal ions were obtained by the dissolution of their respective metal nitrates (0.1 mol L−1) in distilled water expect for Mn2+, K+, and Ba2+ (their counter ions were chloride ions). 2.2 Synthesis of the target compound The synthetic route of the target diarylethene, 1-(3,5-dimethyl4-isoxazolyl)-2-{2-methyl-5-[4-hydroxyl-3-(methylenebenzohydrazide)-phenyl]-3-thienyl}perfluorocyclopentene (1O) is also shown in Scheme 1. Precursor 2 was synthesized by the reported methods.17 The raw material benzohydrazide 3 (0.14 g, 1.0 mmol) and compound 2(0.5 g, 1.0 mmol) were dissolved in a minimum amount of ethanol (10 ml), and then mixed together. The reaction mixture was stirred at 80 °C for 4 h, and then cooled
Fig. 1. Absorption spectral and color changes of 1O by photoirradiation in methanol (2.0 × 10-5 mol L-1).
3.2. Changes in Absorption and Fluorescence by Base/Acid Stimuli The dual-controllable photoswitching behaviors of 1O were investigated by the stimulation of acid/base and light are shown in Scheme 2. The fluorescence spectral changes of 1O (2.0 × 10-5 mol L-1) induced by tetrabutylammonium hydroxide (TBAH) and trifluoroacetic acid (TFA) in acetonitrile at room temperature are shown in Fig. 2A. 1O showed weak fluorescence in the original state. However, when 2.0 equiv of TBAH (1.2 µL, 0.1mol L−1) was added to the colorless solution of 1O, the fluorescence at 553 nm was significantly enhanced due to the formation of
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deprotonated 1O'. This result was in agreement with a previous MANUSCRIPT The dual-controllable photoswitching behaviors of 1O were ACCEPTED report.19 The fluorescence intensity reached a plateau upon in investigated by the stimulation of acid/base and light are shown further titration when the amount of TBAH was more than 2 in Scheme 2. The fluorescence spectral changes of 1O (2.0 × 10-5 mol L-1) induced by tetrabutylammonium hydroxide (TBAH) and equiv (Fig. 2B). Compared to that of 1O, the emission intensity of 1O' was enhanced by 40 fold at 553 nm, accompanied by a trifluoroacetic acid (TFA) in acetonitrile at room temperature are significant fluorescence color change from dark to bright yellow. shown in Fig. 2A. 1O showed weak fluorescence in the original After addition of 2.0 equiv of TFA (1.2 µL, 0.1 mol L−1), 1O' state. However, when 2.0 equiv of TBAH (1.2 µL, 0.1mol L−1) returned to its protonated form of 1O and its fluorescence was was added to the colorless solution of 1O, the fluorescence at 553 quenched. The formation of 1O' was confirmed by 1H NMR nm was significantly enhanced due to the formation of analysis in CD3CN (Fig.S1). After the addition of TBAH, the deprotonated 1O'. This result was in agreement with a previous proton of hydroxyl group at 11.83 ppm and that of imidogen report.19 The fluorescence intensity reached a plateau upon in group at 10.67 ppm disappeared. 1O' exhibited a notable further titration when the amount of TBAH was more than 2 fluorescence switch by photoirradiation in acetonitrile at room equiv (Fig. 2B). Compared to that of 1O, the emission intensity temperature (Fig. 2C). Upon irradiation with 297 nm light, the of 1O' was enhanced by 40 fold at 553 nm, accompanied by a emission intensity at 553 nm decreased with fluorescent color significant fluorescence color change from dark to bright yellow. change from bright yellow to dark. At the photostationary state, After addition of 2.0 equiv of TFA (1.2 µL, 0.1 mol L−1), 1O' the emission intensity was quenched to only 50% of its initial returned to its protonated form of 1O and its fluorescence was value due to the formation of the closed-ring isomer 1C'. quenched. The formation of 1O' was confirmed by 1H NMR Subsequently, irradiation with visible light (λ > 450 nm) analysis in CD3CN (Fig.S1). After the addition of TBAH, the regenerated the open-ring isomer and restored the emission proton of hydroxyl group at 11.83 ppm and that of imidogen spectrum back to its original form 1O'. Consequently, the group at 10.67 ppm disappeared. 1O' exhibited a notable fluorescence of the title diarylethene could be modulated by both fluorescence switch by photoirradiation in acetonitrile at room TBAH/TFA or UV/Vis stimuli, and serve as a dual-controllable temperature (Fig. 2C). Upon irradiation with 297 nm light, the fluorescent molecular switch. emission intensity at 553 nm decreased with fluorescent color change from bright yellow to dark. At the photostationary state, the emission intensity was quenched to only 50% of its initial value due to the formation of the closed-ring isomer 1C'. Subsequently, irradiation with visible light (λ > 450 nm) regenerated the open-ring isomer and restored the emission spectrum back to its original form 1O'. Consequently, the fluorescence of the title diarylethene could be modulated by both TBAH/TFA or UV/Vis stimuli, and serve as a dual-controllable fluorescent molecular switch.
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Scheme 2. Photochromism, color, and fluorescence changes of 1 induced by TBAH/TFA and UV/Vis.
Fig. 2. Changes in fluorescence of 1O in acetonitrile (2.0 × 10−5 mol L−1) induced by TBAH/TFA and light stimuli at room temperature, excited at 438 nm: (A) emission intensity and fluorescence color changes of 1O induced by the addition of TBAH; (B) The curve of fluorescent intensity ratio at 550 nm with the addition of different equiv. TBAH concentration; (C) emission intensity and fluorescence color changes of 1Oʹ by photoirradiation.
Fig. S2 shows the absorption spectral and color changes of 1O (2.0 × 10-5 mol L-1) induced by TBAH and TFA in acetonitrile at room temperature. Addition of 2 equiv. of TBAH (1.2 µL, 0.1 mol L−1) to the colorless solution of 1O, a new visible band centered at 465 nm (ε = 1.24 × 104 mol−1 L cm−1) was observed (Fig. S2A), and the maximum absorption peak of the open-ring isomer 1O was red-shifted from 293 nm to 323 nm. Meanwhile, the color of solution changed from colorless to yellow due to the formation of deprotonated diarylethene 1O'. The deprotonated 1O' could return to 1O by neutralizing with 2.0 equiv of TFA (1.2 µL, 0.1 mol L−1). Moreover, upon irradiation with 297 nm light, the pale yellow solution of 1O' turned dark blue with concomitant appearance of a new visible absorption band centered at 658 nm (ε = 1.80 × 103 mol−1 L cm−1) due to the formation of the closed-ring isomer 1C'. Upon irradiation with visible light (λ > 450 nm), the dark blue solution changed to pale yellow and the absorption spectrum reverted to the original openring isomer 1O'. Therefore, 1O' could also undergo a reversible photoisomerization (Fig. S2B). 3.3. Fluorogenic sensing for Zn2+ in MeCN The selectivity of 1O toward a variety of metal ions through fluorescence emission was investigated in acetonitrile at room temperature. Fig. 3 shows the emission spectra and fluorescence color change of 1O with an excitation of 438 nm when 3 equiv of various metal ions such as K+, Ag+, Cu2+, Ca2+, Mg2+, Fe3+, Ba2+, Sr2+, Pb2+, Ni2+, Hg2+, Mn2+, Zn2+, Cd2+, Co2+, Al3+, and Cr3+ were added to the solution of 1O. It was found that the fluorescence of the solution exhibited no or insignificant increase except for Zn2+. The addition of Zn2+ resulted in drastic enhancement of the emission intensity at 560 nm with concomitant fluorescence color change from dark to bright yellow. It is noteworthy that the
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3+ peak at 604.1 (m/z) for [1O-H]–. When excess amount of Zn2+ individual addition of Ni2+, Cu2+, Co2+, FeACCEPTED , and Sr2+ also MANUSCRIPT resulted in fluorescence intensity changes, but the changes were was added to 1O, the peak at 604.1 (m/z) decreased and a new very small compared to that of Zn2+. Therefore, the diarylethene peak at 667.0 (m/z) appeared due to the formation of [(1O-Zn2+)2+ could serve as a fluorescence chemosensor for detection of Zn H]–. The positive ion mass spectrum indicated the 1:1 binding with high selectivity in acetonitrile. More importantly, the mode between 1O and Zn2+. Upon complexation, the proton of 2+ 2+ the hydroxyl group at 13.2 ppm disappeared, and the proton of diarylethene can clearly distinguish Zn from Cd , whereas the discrimination of Zn2+ from Cd2+ is well known as a major the imine (−CH=N) was shifted from 8.35 ppm to 8.37 ppm (Fig. obstacle. The selective fluorescence enhancement by Zn2+ might S4). be ascribed to the effective coordination of Zn2+ with 1O over other metal ions. It causes the 1O-Zn2+ complex to be more coplanar than 1O itself, resulting in the chelation-enhanced fluorescence (CHEF) effect.20 Also, 1O is poorly fluorescent due to the isomerization of the C=N double bond in the excited state and the excited-state intramolecular proton transfer (ESIPT) involving the phenolic protons.21 Upon stable chelation with Zn2+, the C=N isomerization and ESIPT might be inhibited, leading to fluorescence enhancement.
Fig. 4. Changes in fluorescence of 1O in acetonitrile (2.0 × 10−5 mol L−1) induced by Zn2+/EDTA and light stimuli at room temperature, excited at 438 nm: (A) emission intensity and fluorescence color changes of 1O induced by the addition of Zn2+; (B) The curve of fluorescent intensity ratio at 560 nm with the addition of different equiv. Zn2+ concentration; (C) emission intensity and fluorescence color changes of 1O-Zn2+ by photoirradiation; (D) Job’s plot showing the 1:1 complex between Zn2+ and 1O.
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Fig. 3. Changes in the fluorescence of 1O induced by the addition of various metal ions (3 equiv) in acetonitrile (2.0 × 10−5 mol L−1) excited at 438 nm: (A) emission spectral changes; (B) emission intensity changes; (C) photos demonstrating changes in its fluorescence.
Scheme 3. Photochromism, color, and fluorescence changes of 1O induced by Zn2+/EDTA and UV/Vis.
Upon irradiation at 297 nm light, the fluorescence intensity of 1O-Zn2+ was quenched to ca. 38% due to the formation of the closed-ring isomer 1C-Zn2+ at the photostationary state (Fig. 4C). However, when EDTA was added to the solution of 1O-Zn2+, the fluorescence intensity could not be restored due to the complexation between Zn2+ and 1O is much more tightly than that between EDTA and Zn2+. The binding mode between 1O and Zn2+ was determined by the Job’s plot analysis. The fluorescence intensity of 1O-Zn2+ at 560 nm was plotted against the molar fraction of 1O under the condition of an invariant total concentration. As shown in Fig. 4D, the Job’s plot for 1O-Zn2+ complex exhibited 1:1 stoichiometry. The formation of 1O-Zn2+ complex was further confirmed by ESI-MS method and 1H NMR analysis in CD3CN. As shown in Fig. S3, the free 1O had a main
The absorption spectrum of 1O stimulated by Zn2+ was shown in Fig. S5. Upon addition of 3 equiv of Zn2+ to the solution of 1O, the absorption band of the open-ring isomer at 292 nm decreased and a new absorption band centered at 425 nm (ε = 1.04 × 104 mol−1 L cm−1) emerged. Meanwhie, the solution changed from colorless to light yellow, indicating a clean conversion of 1O into 1O-Zn2+ complex (Fig. S5A). It was noticed that the absorption properties of 1O-Zn2+ could not be restored by adding excess EDTA to the solution. Upon irradiation with 297 nm light, the light yellow solution of 1O-Zn2+ turned gray with concomitant appearance of a new visible absorption band centered at 560 nm (ε = 6.30 × 103 mol−1 L cm−1) due to the formation of the closed-ring isomer 1C-Zn2+ (Fig. S5B). Upon irradiation with visible light (λ > 450 nm), the gray solution
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To further investigate the chemosensing properties of 1O toward Zn2+, the fluorescence titration tests were performed in acetonitrile (2.0 × 10−5 mol L−1) at room temperature as shown in Fig. 4. When Zn2+ was gradually added to the solution of 1O from 0 to 3 equiv, an increase in emission intensity was red-shift from 553 to 560 nm due to the formation of 1O-Zn2+, followed by a plateau with further titration. As compared with 1O, the emission peak of 1O-Zn2+ at 560 nm was enhanced by 400 fold at the plateau (Fig. 4A). Fig. 4B depicts the effect of Zn2+ concentration on the emission intensity at 560 nm. The results indicated that there was a linear relationship between the fluorescence intensity and the Zn2+ concentration in the range of 0−24 µM, suggesting that 1O could be potentially used as a colorimetric fluorescent probe for Zn2+.
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expected because of their exchange with the solvent protons. turned to light yellow and the absorption spectrum reverted to the MANUSCRIPT ACCEPTED The protons of 1O displayed signals at 7.48, 7.55, 7.72, and 7.89 original open-ring isomer 1O-Zn2+. Therefore, 1O-Zn2+ could ppm for the phenyl group. Upon complexation with Al3+, also undergo a reversible photoisomerization. The cyclic graph of 2+ protons-(h,l) shifted from 7.89 to 8.07 ppm, proton-f shifted from 1O induced by Zn /EDTA and UV/Vis was shown in Scheme 3. 7.72 to 7.58 ppm, and protons-(d,f,k,i,j) were broadened. The 3.4. Fluorogenic sensing for Al3+ in MeOH results were attributed to the isomerization of the C=O double bond to C=N double bond after 1O was complexed with Al3+ To gain an insight into the fluorescent properties of 1O toward (Fig. S7). Based on the Job’s plot, ESI-MS, and 1H NMR, we various metal ions in methanol, the emission changes were proposed the structure of the complex between 1O and measured with them at room temperature. When excited at 402 Al3+.(Scheme 4). nm, 1O exhibited a weak fluorescence emission compared to that 3+ 3+ in the presence of Al (Fig. 5). The addition of Al from 0 to 3 equiv resulted in drastic enhancement of the emission intensity at 510 nm with concomitant fluorescent color change from dark to bright cyan. The dramatic fluorescence enhancement responses to Al3+ many be attributed to the chelation of 1O with Al3+ and the C=O isomerization of salicylidene Schiff base unit. As compared with 1O, the emission peak of 1O-Al3+ at 510 nm was enhanced by 110 fold at the plateau. In contrast, upon addition of other metal ions such as K+, Ag+, Cu2+, Ca2+, Mg2+, Fe3+, Ba2+, Sr2+, Pb2+, Ni2+, Hg2+, Mn2+, Zn2+, Cd2+, Co2+, and Cr3+ ，only Zn2+ resulted in tiny fluorescence increase, either no or slight increase in intensity were observed with the rest metal ions. It is noteworthy that 1O could serve as an excellent fluorescent probe for Al3+ by switching the solvent from acetonitrile to methanol. The results indicated that the fluorescence behaviors of 1O-Zn2+ and 1O-Al3+ complexes are solvent-dependent.
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Fig. 6. Changes in fluorescence of 1O in methanol (2.0 × 10−5 mol L−1) induced by Al3+/EDTA and light stimuli at room temperature, excited at 402 nm: (A) emission intensity and fluorescence color changes of 1O induced by the addition of Al3+; (B) The curve of fluorescent intensity ratio at 510 nm with the addition of different equiv. Al3+ concentration; (C) emission intensity and fluorescence color changes of 1O-Al3+ by photoirradiation; (D) Job’s plot showing the 1:1 complex between Al3+ and 1O.
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Fig. 5. Changes in the fluorescence of 1O induced by the addition of various metal ions (3 equiv) in methanol (2.0 × 10−5 mol L−1) excited at 402 nm: (A) emission spectral changes; (B) emission intensity changes; (C) photos demonstrating changes in its fluorescence.
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Fig. 6A depicts the effect of Al3+ concentration on the emission at 510 nm. A good linear relationship was seen between the fluorescence intensity of 1O and Al3+ concentration in the range of 0−24 µΜ (Fig. 6B). This suggested the potential application of 1O as a colorimetric fluorescent probe for Al3+ in methanol. Upon irradiation with 297 nm light, the fluorescence intensity of 1O-Al3+ was quenched dramatically with a concomitant color change from bright cyan to dark due to the formation of the closed-ring isomer 1C-Al3+ (Fig. 6C). The quenched fluorescence intensity reached ca. 34% at the photostationary state. After adding EDTA solution to the solution of 1O-Al3+, the fluorescence intensity was restored due to the complexation between Al3+ and EDTA. The Job’s plot showed a 1:1 stoichiometry between 1O and Al3+ (Fig. 6D). The formation was also further confirmed by ESI-MS method and 1H NMR analysis in CD3OD. The positive ion mass spectrum indicated that peak at 628.1 (m/z) is assignable to [1O+Na]+ and peak at 725.0 (m/z) is assignable to [(1O-Al3+)+NO3–+CH3OH]+ (Fig. S6). The 1H NMR spectrum of 1O shows weak signals of N–H and phenolic O–H protons at 7.53 ppm and 10.06 ppm as
Scheme 4. Photochromism, color, and fluorescence changes of 1O induced by Al3+/EDTA and UV/Vis.
Fig. S8 shows the absorption spectral and color changes of 1O induced by Al3+/EDTA and UV/Vis in methanol at room temperature. Upon addition of 3 equiv Al3+ into the colorless solution of 1O, the absorption band of the open-ring isomer at 295 nm decreased and a new absorption band at 311 nm increased with an isosbestic point at 303 nm. Meanwhile, the solution color changed from colorless to light yellow accompanied with the emergence of another absorption band at 398 nm (ε = 1.25 × 104 mol−1 L cm−1), which indicated a clean conversion of 1O into 1O-Al3+ complex (Fig. S8A). The complex could return to 1O by adding excess EDTA and this was attributed to the complexation between Al3+ and EDTA. 1O-Al3+
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also underwent a reversible photoisomerization upon irradiation MANUSCRIPT output signal O1 is ‘on’ and the output digit is ‘1’ (Fig.7A). All ACCEPTED with UV and visible light. Upon irradiation with 297 nm light, the possible strings of the three inputs are listed in Table 1. Similarly, when the input string is ‘1, 0, 1, and 0’, corresponding the light yellow solution of 1O-Al3+ turned purple, accompanied by the appearance of a new absorption band centered at 560 nm inputs (In1, In2, In4, and In5) are ‘on, off, on, and off’and the output signal O2 is ‘on’ and the output digit is ‘1’ (Fig. 7B). All (ε = 6.30 × 103 mol−1 L cm−1) due to the formation of the closedthe possible strings of the four inputs are listed in Table 2. ring isomer 1C-Al3+ (Fig. S8B). Upon irradiation with visible light (λ > 450 nm), the purple solution and the absorption Table 1. Truth table for all possible strings of three binary-input data and the spectrum reverted to the original open-ring isomer 1O-Al3+. corresponding output digit. 3+ Scheme 4 shows the cyclic graph of 1O induced by Al /EDTA and UV/Vis. Input 3.5. Application in logic circuits
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In general, molecules can undergo changes in the ground or excited states in response to modulators which can be guest molecules, ions, or light of a certain wavelength. In most cases, these changes could be signaled by changes in the emission intensity.22 On the basis of the fact that the fluorescence intensity of the target compound 1O could be effectively modulated by TBAH, Al3+/EDTA, and UV/Vis stimuli (Schemes 2 and Schemes 4), we further illustrated combinational logic circuits by utilizing light irradiation, TBAH, EDTA, and Al3+ as the input signals, and the change of fluorescence intensity as the output signal.
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Table 2. Truth table for all possible strings of three binary-input data and the corresponding output digit.
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Fig. 7. Two logic circuits based on the diarylethene with various inputs: (A) the combinational logic circuits equivalent to the truth table given in Table 1: In1 (Vis), In2 (UV), In3 (TBAH), and O1; (B) the combinational logic circuits equivalent to the truth table given in Table 2: In1 (Vis), In2 (UV), In4 (Al3+) In5 (EDTA) and O2.
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As shown in Fig. 7, we constructed two combinational logic circuits, one had three inputs (Vis (λ > 450 nm, In1), UV (297 nm, In2), and TBAH (In3)) and an output (fluorescence intensity at 553 nm (O1)) and the other had four inputs (Vis (λ > 450 nm, In1), UV (297 nm, In2), Al3+ (In4), and EDTA (In5)). All the input signals could be either ‘on’ or ‘off’ state with different Boolean values. For instance, when visible light was used input signal In1 was switched to ‘on’ state with a Boolean value ‘1’. Similarly, input signal In2 was ‘1’ when UV irradiation (297 nm) was employed, In3 was ‘1’ when TBAH was added, In4 was ‘1’ when Al3+ was added, and In5 was ‘1’ when EDTA was added. When TBAH or Al3+ added into the solution, 1O exhibited strong fluorescence, and then its intensity was employed as an initial state. The output signal could be regarded as ‘off’ when the emission intensity was below 50% of the original value. Otherwise, the output signal was regarded as ‘on’. The binary digits (‘0’ and ‘1’) can be used instead of the two levels (‘off’ and ‘on’). Concretely, when the string is ‘1, 0, and 1’, the corresponding inputs (In1, In2, and In3) are ‘on, off, and on’, respectively. Under these conditions, 1O was converted to the deprotonated 1O' and its fluorescence was enhanced, then the
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S.; Zhao, L. C.; Zeng, X. S. Org. Lett. 2011, 13, 5274–5277; ACCEPTED MANUSCRIPT 8.
In conclusion, a novel photochromic diarylethene has been synthesized by using benzohydrazide as a functional group and perfluordiarylethene as photoswitching trigger via a salicylidene Schiff base linkage. Its multi-addressable molecular switching behavior and selective interaction with Zn2+ and Al3+ were investigated systematically. The results demonstrated that the diarylethene was highly selective towards Zn2+ and Al3+ in acetonitrile and methanol with distinct fluorescence signals and binding modes. Moreover, the diarylethene exhibited different absorption and fluorescence behaviors in the presence of base. Based on the fact that its fluorescence could be effectively modulated by stimulation with light and chemical species, two logic circuits were constructed by using the fluorescence intensity as the output signals with the inputs of UV/Vis , TBAH, Al3+, and EDTA stimuli. The results of current work are useful for design and synthesis of new photochromic diarylethenes with multi-controllable switching behavior and solvent-dependent selectivity for multiple targets of interest.
References and notes
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Highlights A novel diarylethene with a salicylidene Schiff base group was synthesized > It exhibited multi-controllable behaviors induced by light, base, and metal ions > It can serve as a
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naked-eye chemosensor for different metal ions in different solvent