Bis-Triazole-Appended Azobenzene Chromophore for Selective

May 23, 2011 - Receptor II shows high selectivity toward Cu2ю in acetonitrile/water (80:20, v/v) ... yellow), that can be clearly observed by the nak...
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Bis-Triazole-Appended Azobenzene Chromophore for Selective Sensing of Copper(II) Ion Elango Hrishikesan, Chinnusamy Saravanan, and Palaninathan Kannan* Department of Chemistry, Anna University, Chennai 600-025, India

Ind. Eng. Chem. Res. 2011.50:8225-8229. Downloaded from pubs.acs.org by AUSTRALIAN NATL UNIV on 08/10/18. For personal use only.

bS Supporting Information ABSTRACT: A Cu2þ-specific colorimetric sensor of bis-triazole-appended azobenzene receptors I and II was designed and synthesized. Receptor II shows high selectivity toward Cu2þ in acetonitrile/water (80:20, v/v) solution based on the internal charge transfer (ICT) mechanism. The tridentate coordination behavior was proposed with 1:1 stoichiometry between receptor II and Cu2þ. Herein, the Cu2þ gives rise to a large change in the absorption spectra (from red to pale yellow) that is clearly visible to the naked eye.

1. INTRODUCTION The design and development of highly sensitive and specific probes for various metal ions has been a subject of intense interest because of their potential applications in clinical biochemistry and environmental science.16 Accordingly, it is highly necessary to selectively sense heavy metal ions such as mercury, lead, zinc, and copper ions.711 Copper is the most abundant element, an essential trace mineral of importance for both physical and mental health, and a cofactor that takes an active part in a large variety of enzymes. As a consequence, it plays an important role in fundamental physiological processes in organisms. Thus, the development of new methods for the selective detection and visualization of Cu2þ in chemical and biological systems is significant.1217 Recently, several receptors have been developed by different research groups for sensing Cu2þ in solution based on 4,5-diaminonaphthalimide, dansyl-naphthalimide dyads, coumarin and azobenzene derivatives, and so on.18,19 The click-chemistry-assisted 1,2,3-triazole linkage is a potential strategy for the functionalization of biomolecules because it is efficient, selective, and devoid of the formation of side products during the functionalization.2023 In addition, the triazole ring can be employed not only as a labile ligand for catalysis in organic transformations but also as a stabilizing multidentate ligand motif for co-ordination chemistry, as witnessed by the numerous recent reports involving the coordination chemistry of 1,2,3triazole-containing chelators by a number of research groups.2426 Very recently, Nguyen et al. designed a receptor based on N-alkylated bis-triazole-linked donorπ-electron bridgeacceptor (DπA) fluorene-based chromophoric system in which the bis-triazole ring act as a recognizing center for the metal ions.27 This system particularly senses Zn2þ and Hg2þ ions over other metal ions in aqueous and organic medium. Although triazole-based receptors for various metal ions have been investigated in the past, the reported synthetic methods for these sensors involve tedious reaction routes and poor yields. Azobenzene-based receptor are excellent chromogenic dyes that are widely utilized as receptors in chemosensors.28 Gunnlaugsson et al. reported DπA azobenzene-based chromophoric systems in which the phenyl iminodiacetate acts as both a recognition center and a donor and the nitro group acts as an electron acceptor.29 This r 2011 American Chemical Society

system particularly detects Cu2þ over other metal ions under physiological conditions. Thus, in this study, we describe a concise synthesis of structurally simplified and functionally improved Cu2þ chemosensors through bis-triazole-appended azobenzene-based colorimetric chemosensors I and II. These sensors function based on the well-known intramolecular charge transfer (ICT) mechanism in the azobenzene structure; one of the aromatic rings is integrated with ion receptor (bis-triazole). In this case, N-alkylated bis-triazole receptors were chosen as an electron donor, and the nitro group was chosen as an electron acceptor. Herein, the Cu2þ ion detection gives rise to large changes in the absorption spectra (from red to pale yellow), that can be clearly observed by the naked eye.

2. EXPERIMENTAL SECTION 2.1. Materials. Commercially available chemicals/compounds were used without further purification. Reagents and solvents (AR grade) were purchased from S.R.L Chemicals, Mumbai, India, and Sigma-Aldrich, Bangalore, India, and used as received. Perchlorate salts of all cations (all from Merck) were used without further purification except for vacuum drying. 2.2. Techniques. Infrared spectra were obtained on a Thermo Electron Corporation Nicolet 380 FTIR spectrometer. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a Bruker AM-400 spectrometer with Me4Si as the internal standard. The UVvis measurements were carried out on a Shimadzu UV-1650PC spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Syntheses. The synthetic routes for receptors I and II are depicted in Scheme 1. The route was started from aniline 1. The N-alkylation of aniline with propargyl bromide using K2CO3 in acetone was carried out at 70 °C, yielding compound 2 in 56% yield. Then, it was condensed with the corresponding diazotized Received: March 18, 2011 Accepted: May 23, 2011 Revised: May 23, 2011 Published: May 23, 2011 8225

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Scheme 1. Synthesis of Receptors I and II

Figure 1. UVvis absorption spectral changes of receptor II [2  105 M in CH3CN/H2O (80:20, v/v)] after the addition of 10 equiv of Naþ, Kþ, Ni2þ, Cu2þ, Zn2þ, Agþ, Cd2þ, Hg2þ, and Pb2þ ions.

compounds 3 and 4 in the presence of sodium acetate to give the corresponding compounds 5 and 6. The 6-bromohexanol was reacted with sodium azide in dimethylformamide at 60 °C to obtain the corresponding 6-azidohexanol. Finally, receptors I and II were prepared by the Cu(I)catalyzed 1,3-dipolar cycloaddition (“click-method”) reaction of compound 5 and 6 with 6-azidohexanol in the presence of CuSO4 and sodium ascorbate in a mixture of H2O/t-BuOH (1:4) at room temperature with good yields. Receptors I and II were characterized by 1H NMR and 13C NMR spectroscopy. The synthesized receptors were composed of a bis-triazole-based

ionophore for selective recognition of metal ions and the azobenzene unit, which is responsible for signal transduction during spectroscopic studies. 3.2. Photophysical Studies of Receptors I and II. The photophysical properties of receptor II with several metal cations (Naþ, Kþ, Ni2þ, Cu2þ, Zn2þ, Agþ, Cd2þ, Hg2þ, and Pb2þ) in the form of their perchlorate salts in CH3CN/H2O (80:20, v/v) were investigated by UVvis measurements (Figure 1), and titration studies were conducted at pH 7.2 (50 mM HEPES {2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid} buffer). We selected the carrier solvent based on its miscibility with H2O, which was the target medium for cation detection in the environment. The absorption spectra of receptors I and II (2  105 M) show strong absorption at 400 and 459 nm, respectively, in CH3CN/H2O (80:20, v/v). The maximum at 459 nm for receptor II in CH3CN/H2O (80:20, v/v) was assigned to the internal charge transfer (ICT) of the chromophore because of the pushpull effect of the electrondonating N,N-bis-triazolyl amine and the electron-withdrawing nitro group in stable cis isomer form.30 The existence of cis form of the azobenzene chromophore was confirmed by irradiation with 365-nm UV light reveals that no change in its absorption maximum and intensity (if the receptor can exist in the trans form, it undergo trans f cis photoisomerization under UV light; see the Supporting Information). The energy-minimized diagram of receptor II obtained with the Spartan’08 suite as shown in Figure 2 also supports the existence of a stable scorpion-like cis isomer. Titration experiments were carried out using all of the metal cations (Naþ, Kþ, Ni2þ, Cu2þ, Zn2þ, Agþ, Cd2þ, Hg2þ, and Pb2þ) and demonstrated that Cu2þ alone promotes a marked 8226

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Figure 4. Job plot of a 1:1 complex of receptor II and Cu2þ ion, where the difference in absorbance intensity at 342 nm is plotted against the mole fraction of receptor II at an invariant total concentration of 20 μM in CH3CN/H2O (80:20, v/v). Figure 2. Energy-minimized structure of receptor II.

Figure 3. Changes in absorption spectra of receptor II upon addition of Cu2þ from 0 to 3 equiv.

response (see the Supporting Information) in its UVvis absorption spectrum as well as to the naked eye. Upon addition of 0 3 equiv of Cu2þ to a CH3CN/H2O (80:20, v/v) solution of receptor II (2  105 M), the intensity of the band at 459 nm gradually decreased, and accordingly, one new higher-energy (HE) band emerged around 342 nm with a clear isosbestic point at 383 nm. The absorption usually shifts to short wavelength because of the weakening of pushpull effect of the chromophoric system because the metal ion bound in the donor part can reduces the donating ability of the donor (D) nitrogen atom in the DπA system.29,31,32 The appearance of a clear isosbestic point at 383 nm (Figure 3) indicates that the azobenzene sensor was converted into its Cu2þ complex only. Receptor I did not exhibit any apparent color change in the presence of any of the metal ions including Cu2þ. However, upon addition of Cu2þ to a CH3CN/H2O (80:20, v/v) solution of receptor I, the intensity of the band at 400 nm increased up to the addition of 2 equiv of Cu2þ. (See the Supporting Information.) This is attributed to the absence of a pushpull nature in receptor I. Therefore, receptor II was employed as a good chemosensor for the detection of Cu2þ. To elucidate this complex formation further, the stoichiometry of the receptor II/Cu2þ system was determined by means of continuous-variation plots (Job plots), wherein the variation of

Figure 5. BenesiHildebrand plot for the binding response of Cu2þ on receptor II at 342 nm.

the absorbance at 342 nm was plotted against the molar fraction of the receptor II. The maximum value was clearly observed at 0.5 and indicated that the triazole receptor in the azobenzene chromophore and Cu2þ formed 1:1 complex (Figure 4). This observation obtained by Job’s method supports this conclusion.33 To understand the binding ability of receptor II, an absorption titration experiment was conducted using Cu2þ. By adding Cu2þ, the absorption corresponding to azobenzene receptor II gradually blue-shifted, and the maximum reached 342 nm when 1.0 equiv of Cu2þ was added to the system (Figure 3). Figure 5 shows a plot of the change in absorbance at 342 nm as a function of Cu2þ concentration. The saturation in the absorption change was produced by the addition of an almost equimolar amount of Cu2þ relative to the bistriazole units of azobenzene receptor II, implying that the efficient cation reception event was accomplished. The degree of color changes was no longer affected by the addition of more than 3 equiv of cations. The binding constant for receptor II with Cu2þ was evaluated by the BenesiHildebrand method34 with the association constant (Ka) of 0.7  104 M1. Electronic excitation generally occurs in the azobenzene chromophore through charge transfer from the donor amine nitrogen to the acceptor NO2 group of the chromophore. 8227

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: þ91 44 2235 8666. Fax: þ91 44 2220 0660.

’ ACKNOWLEDGMENT Support for this work from the Department of Science and Technology, New Delhi, India (DST-SR/S1/PC-07/2010), is gratefully acknowledged. ’ REFERENCES

Figure 6. FTIR spectra of receptor II with 1 and 2 equiv of Cu2þ in CH3CN.

When a complex forms between receptor II and a cationic guest, the excited state should be more stabilized by cation binding, resulting in a hypsochromic shift in the absorption maximum (λmax) because of the inhibition of the ICT behavior35 of receptor II, as well as a color change from red to pale yellow. The interaction between receptor II and the metal ions was also investigated by FTIR spectroscopy. Our attempts to obtain the crystal structures to elucidate the tridentate coordination behavior of receptor II with Cu2þ were not successful because of the flexible methylene spacer present in the receptor II. Because of the paramagnetic nature of Cu2þ, the analysis of the nature of the complex between the receptor and the metal ion was not feasible by NMR studies. Instead, we measured the FTIR spectra of receptor II (Figure 6) and its complex with Cu2þ to examine their binding sites. The fact that the characteristic absorption band for the triazole ring decreased from 2362 to 2240 cm1 indicates the involvement of the triazole ring in the formation of a complex with Cu2þ.

4. CONCLUSIONS In summary, we have developed a bis-triazole-containing azobenzene chemosensor for Cu2þ on the basis of the ICT mechanism with high sensitivity and selectivity and simple construction of the receptor. Furthermore, this molecule makes it possible to detect the Cu2þ ratiometrically. The design strategy and remarkable photophysical properties of the sensor would help to extend other supramolecular systems that have selectivity and improve visualization of Cu2þ in aqueous media. ’ ASSOCIATED CONTENT

bS

Details of syntheses, 1H and 13C NMR spectra of synthesized compounds, photograph of solutions of receptor II upon addition of various metal ions in CH3CN/H2O, and UVvis spectra of receptor II with 460-nm visible light irradiation during the time period for the isomerization reaction and of receptor I with addition of Cu2þ in CH3CN/H2O. This material is available free of charge via the Internet at http:// pubs.acs.org. Supporting Information.

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