Organic radical-induced Cu2+ selective sensing based on thiazolothiazole derivatives

Sensors and Actuators B 192 (2014) 691–696

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Organic radical-induced Cu2+ selective sensing based on thiazolothiazole derivatives Xin Zhang 1 , Minkyung Kang 1 , Hyun-A Choi, Ji Young Jung, K.M.K. Swamy, Soojin Kim, Dabin Kim, Jinheung Kim, Chongmok Lee ∗ , Juyoung Yoon ∗ Department of Chemistry and Nano Sciences, Global Top 5 Research Program, Ewha Womans University, Seoul 120-750, Republic of Korea

a r t i c l e

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Article history: Received 10 September 2013 Received in revised form 8 November 2013 Accepted 8 November 2013 Available online 17 November 2013 Keywords: Cu2+ sensor Organic radical Colorimetric sensor Fluorescent sensor Thiazolothiazole

a b s t r a c t As a new approach to detect Cu2+ , colorimetric detection of Cu2+ via organo radical formation of thiazolothiazole is reported. Upon the addition of Cu2+ , thiazolothiazole derivatives 1 and 2 form relatively stable organo radicals, resulting in a distinct colorimetric change from greenish yellow to blue. Among the various metal ions, only Cu2+ showed a new peak appearance at 610 nm. In addition, a selective fluorescence quenching was also observed with Cu2+ . To understand the origin of the Cu2+ selectivity of 1 and 2, series of electrochemical data are reported. Finally, EPR (electron paramagnetic resonance) data clearly support the formation of this unique organic radical formation. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Copper is not only the third most abundant essential element in the human body, but also plays a key role in neurological diseases, such as Alzheimer’s and Wilson’s diseases [1]. Copper is known as a significant metal pollutant due to its widespread use. Accordingly, colorimetric or fluorescent probes for copper ion have been extensively explored owing to the biological and environmental significance of this metal ion [2–24]. As a general approach to design a fluorescent or colorimetric chemosensor, typical fluorescent/colorimetric chemosensors for copper ions contain a fluorophore covalently linked to a receptor, which can usually show selective binding affinity for copper ions [2–12]. Another more recent yet popular approach is utilizing reaction-based chemodosimeters, in which a copper selective reaction occurs, resulting in fluorescence or colorimetric change [13–20]. On the other hand, arylamine radical formation in the presence of Cu2+ has been adopted as a new approach to design Cu2+ selective chemosensors by Costa’s group [21], Gopidas’s group [22], and the Chen group [23]. For example, the formation of a colored zwitterionic radical was reported in the presence of Cu2+ [21]; the tris(4-anisyl)amine (TAA) radical and its dication were utilized to sense Cu2+ [22]; and cation radicals of arylaminofluorene

∗ Corresponding authors. Tel.: +82 2 3277 2400; fax: +82 2 3277 2384. E-mail addresses: [email protected] (C. Lee), [email protected] (J. Yoon). 1 Both authors contributed equally to this work. 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.11.027

derivatives were used to recognize Cu(ClO4 )2 in CH3 CN by distinct colorimetric change [23]. We also recently reported a thiazolothiazole derivative bearing a bis(2-(acetyloxy)ethyl)amino ligand as a Cu2+ -selective chemosensor [24]. In the current study, we report that the thiazolothiazole derivatives 1 and 2 can effectively sense Cu2+ . A highly selective colorimetric change from greenish yellow to blue was observed with the distinct appearance of a new absorption peak at 610 nm in the presence of copper. The formation of an organic radical was confirmed by EPR. In addition, an elaborate electrochemical study was also reported to explain the mechanism of this process. 2. Experimental 2.1. Synthesis 2.1.1. Materials and methods General methods were used unless otherwise noted; materials were obtained from commercial suppliers and were used without further purification. Flash chromatography was carried out on silica gel (230–400 mesh). 1 H NMR and 13 C NMR spectra were recorded using 300 MHz and 75 MHz, respectively. Chemical shifts were expressed in ppm and coupling constants (J) in Hz. 2.1.2. Synthesis of 1 and 2 2.1.2.1. Synthesis of (4-dimethylaminophenyl)thiazolothiazole (1). Compound 1 was synthesized by modifying the reported

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Scheme 1. Synthesis of compounds 1 and 2.

procedure [25]. A mixture of 4-(dimethylamino)benzaldehyde (0.62 g, 4.16 mmol) and dithiooxamide (0.21 g, 1.75 mmol) in DMF (5 ml) was heated to 160 ◦ C. The reaction was refluxed under N2 for 60 h, and then the solvent was evaporated. The crude product was purified by column chromatography using hexane as eluent to give compound 1 as a yellow powder (0.1 g, 15%). 1 H NMR (CDCl3 , 300 MHz): ı 7.79 (d, 2H, J = 6.9 Hz), 7.69 (d, 2H, J = 6.9 Hz), 6.71–6.77 (dd, 4H), 3.06 (d, 12H); 13 C NMR (CDCl3 , 75 MHz): 165.6, 151.9, 151.2, 148.9, 129.0, 127.8, 120.4, 118.8, 116.3, 116.2, 112.1, 111.7, 40.2; HRMS (FAB) calculated for C20 H20 N4 S2 380.1129; observed 381.1208 (M+H)+ . 2.1.2.2. Synthesis of 4,4 -(thiazolo[5,4-d]thiazole(2). 1.37 g of 2,5-diyl)bis(N,N-diphenylbenzenamine) 4-(diphenylamino)benzaldehyde (5.0 mmol) and dithiooxamide (169 mg, 1.4 mmol) in DMF (5 ml) were stirred at 160 ◦ C under nitrogen atmosphere for 60 h. After cooling to room temperature, the solvent was evaporated under reduced pressure. The residue was purified by column chromatography (silica gel, dichloromethane/hexane = 1:1, v/v). 430 mg of orange-yellowish solid was obtained, yield: 48.5%. 1 H NMR (300 MHz, CDCl3 ) ı 7.81 (d, 4H, J = 8.7 Hz), 7.33 (t, 8H, J = 6.9 Hz), 7.08–7.19 (m, 16H); 13 C NMR (CDCl3 , 75 MHz): 168.4, 150.2, 150.0, 146.9, 129.5, 127.3, 127.2, 125.4, 124.0, 121.9; HRMS (FAB) calculated for C40 H29 N4 S2 629.1834; observed 629.1833 (M+H)+ .

2.2. UV and fluorescence study Stock solutions (10 mM) of the perchlorate salts of Al3+ , Ag+ , Cd2+ , Co2+ , Cr3+ , Cs+ , Cu+ , Cu2+ , Fe2+ , Fe3+ , Hg2+ , K+ , Li+ , Mg2+ , Mn2+ , Na+ , Ni2+ , Pb2+ , and Zn2+ ions in CH3 CN were prepared. Stock solutions of 1 and 2 (0.1 mM) were also prepared in CH3 CN. Test solutions were prepared by placing 300 ␮L of the probe stock solution into a test tube, adding an appropriate aliquot of each metal stock, and then diluting the solution to 3 mL with CH3 CN or CH3 CN and distilled water. The absorption and fluorescence properties were tested in CH3 CN:H2 O (95:5, v/v) or in CH3 CN. Ca2+ ,

2.3. Electrochemistry Electrochemical data were measured using a BAS 100B electrochemical analyzer at room temperature in a glove box. Cyclic voltammograms were obtained in a CH3 CN:CH2 Cl2 (9:1) solution containing 0.5 mM 1 and 0.1 M tetrabutylammonium perchlorate (TBAP) using a Pt disk (1.6 mm diameter) working electrode, Pt coil auxiliary electrode, and Ag|AgNO3 (0.1 M) reference electrode. Compound 2 was examined in CH2 Cl2 due to the solubility problem. The potential values were compared against the saturated calomel electrode (SCE) via the calibration procedure described elsewhere [24]. Spectroelectrochemical experiments were performed with an indium tin oxide (ITO)-coated glass electrode (Delta Technologies) as the working electrode using a UV cuvette (light path-length: 1cm), a diode array spectrophotometer (Hewlett-Packard 8452A), and an AFRDE5 biopotentiostat (Pine Instruments) under ambient conditions [26].

3. Results and discussion 3.1. Synthesis (4-Dimethylaminophenyl)thiazolothiazole 1 and (4diphenylaminophenyl)thiazolothiazole 2 were prepared from the reactions of 4-(dimethylamino)benzaldehyde and 4(diphenylamino)benzaldehyde, respectively, with dithiooxamide by modifying the previously reported procedure [25]. The detailed synthetic procedure and the characterization data are explained in Section 2, and 1 H and 13 C NMR spectra are reported in the supporting information (Scheme 1). 3.2. UV and fluorescent studies To test the selectivity of 1 and 2 toward various metal ions, Al3+ , Ca2+ , Cd2+ , Co2+ , Cr3+ , Cs+ , Cu+ , Cu2+ , Fe2+ , Fe3+ , Hg2+ , K+ , Li+ , Mg2+ , Mn2+ , Na+ , Ni2+ , Pb2+ , and Zn2+ were examined in CH3 CN:H2 O (95:5, v/v) and CH3 CN, respectively. As shown in Fig. 1, only Cu2+ induced the appearance of new absorption peaks at 553 nm and 610 nm, which can be attributed to the formation of the organic radical. The inset picture explains the distinct colorimetric change from greenish yellow to blue upon the addition of Cu2+ , which can be easily observed by the naked eye. Fig. 2 illustrates the UV absorption changes as the amount of Cu2+ increases. The UV absorption titrations of 2 as well as its colorimetric changes with Cu2+ are explained in Fig. S5. Due to the solubility problem, 100% CH3 CN was used as solvent for compound 2. Fig. S6 explains the UV absorption of Cu2+ only, which confirms that new UV absorption peak was not affected by Cu2+ itself at this low concentrations. In addition, selective fluorescence quenching was observed for Cu2+ among the various metal ions (Fig. 3). When thiazolothiazole derivative 1 was excited at 420 nm, emission with max = 510 nm was observed, as shown in Fig. 3. The fluorescence titration experiments of compound 1 were carried out with Cu(ClO4 )2 in CH3 CN as shown in Fig. S7a (supporting information). Furthermore, this quenching changes afforded a linear correlation between the Cu(ClO4 )2 concentration (0–4 ␮M) and relative fluorescence intensities at 485 nm (I0 /I) (Fig. S7b). Based on the calibration curve obtained from this correlation, the detection limit was calculated as 1.7 × 10−6 M. Detection limit and linear correlation range were also examined in CH3 CN–H2 O (95:5, v/v). As shown in Fig. S9, a linear correlation between the Cu(ClO4 )2 concentration (0–16 ␮M) and the detection limit was calculated as 1.3 × 10−5 M. The similar responding behaviors toward Cu2+ in CH3 CN were also observed with receptor 2. The measurement results were shown in the supporting information (Fig. S8). Interference of different metal ions (Ag+ , Al3+ , Cd2+ , Cr3+ , Cs+ , Fe3+ , Fe2+ , Hg2+ , K+ , Mg2+ , Mn2+ , Ni2+ , Pb2+ , Zn2+ ) was examined via fluorescence changes. As shown in Fig. S10, we could not observe any interference from other metal ions even though Fe3+ induced small fluorescence quenching effect. The colorimetric changes of 1 and 2 were observed for few hours after the addition of Cu2+ (Fig. S11 and S12). The blue color of 1 remains about 3 h in CH3 CN even though, in CH3 CN–H2 O (95:5, v/v), there was a color change only after 10 min (Fig. S11). On the other hand, there was almost no significant colorimetric change Ag+ ,

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Fig. 1. (a) UV spectra of 1 (10 ␮M) in CH3 CN–H2 O (95:5, v/v) upon addition of 5 equiv. of metal ions. (b) The picture of 1 (10 ␮M) in CH3 CN–H2 O (95:5, v/v) (left); the picture of 1 (10 ␮M) in CH3 CN–H2 O (95:5, v/v) upon the addition of 10 equiv. of Cu2+ (right).

of 2 for over 12 h in CH3 CN (Fig. S12), which can be attributed to the possible resonance stabilization of organic radical by adjacent phenyl groups.

The reversible redox wave of Cu(ClO4 )2 in CV appeared at 1.04 V with an anodic peak potential (Epa ) of 1.10 V and a cathodic peak potential (Epc ) of 0.97 V (Fig. 5a). Considering the thermodynamics related to the redox potential shown in the CVs, the amine

3.3. Electrochemistry and EPR data To understand the origin of the Cu2+ selectivity of 1, its electrochemical properties in the absence and presence of Cu(ClO4 )2 were also examined in CH3 CN/CH2 Cl2 solution by cyclic voltammetry (CV), as shown in Figs. 4 and 5. The mixture solution was used to circumvent the low solubility of 1 in CH3 CN/H2 O. The CV result of 1 revealed two oxidation waves in the positive potential range at 0.78 and 0.90 V via one- and two-electron processes, respec• •• tively, to produce a cation radical (1+ ) and diradical (12+ ) (Fig. 4). This result can be interpreted as the oxidation of two dimethyl amine groups in 1 owing to electronic communication between the two amine centers as a result of non-trivial charge delocalization through the extended ␲-system of the thiazolothiazole ring [23,27,28]. Similar shape of oxidation waves was also observed for 2 (Fig. S14). Fig. 3. Fluorescence spectra of 1 (10 ␮M) in CH3 CN–H2 O (95:5, v/v) upon the addition of 5 equiv. of metal ions (excitation at 420 nm).

Current / A

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Potential / V vs SCE Fig. 2. UV titration of 1 (10 ␮M) with Cu2+ in CH3 CN–H2 O (95:5, v/v).

Fig. 4. CVs of 1 (0.5 mM) dissolved in CH3 CN:CH2 Cl2 (9:1) containing 0.1 M TBAP at scan rate of 0.1 V s−1 .

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0.10 Intensity

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Wavelength / nm Fig. 7. Change of spectra of 1 after cease of electrolysis at 1.00 V for 3 min (in 10, 20, 30, 60, 90, 150, 210 s; from black to light gray).

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Potential / V vs SCE Fig. 5. CVs of 0.5 mM (1 equiv.) of Cu(ClO4 )2 (a), 1 (0.5 mM) only (b), 1 (0.5 mM) in the presence of 1 equiv. (c), and 2 equiv. (d) of Cu(ClO4 )2 in CH3 CN:CH2 Cl2 (9:1) solution containing 0.1 M (Bu)4 NClO4 (TBAP) at scan rate of 0.1 V s−1 .

centers in 1 can be oxidized at the expense of reduction of Cu2+ . •• In other words, ‘1 + 2Cu2+ → 12+ + 2Cu+ ’ is a spontaneous reaction and is the origin of the Cu2+ selectivity. The Epa of ‘Cu+ − e− → Cu2+ ’ shifted from 1.10 V to 1.25 V upon addition of 1.0 equiv. of Cu2+ in the presence of 1 (Fig. 5c). With 2.0 equiv. of Cu2+ in the presence of 1, the oxidation peak current of Cu+ /Cu2+ increased and was noted at 1.26 V, whereas the position of the reduction peak was negatively shifted by approximately 0.01 V (Fig. 5d). The movement of Cu2+ /Cu+ redox peak potentials could be ascribed to the relative •• binding affinities of 12+ between Cu+ and Cu2+ , i.e., the stronger •• binding of 12+ with Cu+ positively shifted the Epa by 0.15 V, while •• the weaker binding of 12+ with liberated Cu2+ did not significantly affect the position of Epc of ‘Cu2+ + e− → Cu+ ’. It should be noted that most of 1 remains as the cation diradical form at potentials >0.9 V. To examine the absorption spectra change of 1 depending on its oxidation state, spectroelectrochemical experiments were also carried out by applying a fixed potential to the ITO-coated glass electrode. Fig. 6a represents the absorption spectra of 1 measured

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by electrolysis at 0.80 V. The obtained spectrum at a 6 min electrolysis time showed an absorbance plateau at 550–610 nm, which is similar to that of 1 in a solution containing Cu2+ in CH3 CN–H2 O (Fig. 2) and is believed to be the spectrum of the monocation radical • of 1 (1+ ). The color change of 2 by oxidation was also confirmed via similar experiments. The spectrum of the electrochemically oxi• dized 2 (2+ ) resembled that of 2 in the presence of Cu2+ , which were shown in Figs. S13 and S15, respectively. The obtained spectrum with the same experimental conditions except an electrolysis potential of 0.93 V was recorded as shown in Fig. 6b, where the absorption maximum appeared at 650 nm. Further increase of electrolysis potential to 1.00 V (Fig. 7) showed little spectral change from that obtained at 0.93 V, which could be •• ascribed to the generation of dication radical species (12+ ). To support our interpretation of the absorption spectra of 1 depending on its oxidation state, we also measured the absorbance change of 1 in the presence of Cu2+ using a CH3 CN/CH2 Cl2 solution (Fig. S13). Interestingly, the spectrum of 1 with 1.0 equiv. of Cu2+ was similar to that obtained by electrolysis at 0.80 V. Furthermore, the spectrum of 1 with 2.0 equiv. of Cu2+ was also similar to that obtained by electrolysis at >0.93 V. The spectral change of 1 upon varying the amount of Cu2+ is thus produced mainly from the redox process of 1 itself, which is also evidenced by ESR to clearly show the existence of a cation radical of 1 (Fig. 8). It is notable, however, that a similar spectrum was not observed in CH3 CN–H2 O (Fig. 2), which is likely due to instability of dication radical species in the presence of water. Fig. 7 shows the change in the spectra of 1 after cessation of electrolysis at 1.00 V for 3 min. After electrolysis, the spectral pattern

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Fig. 6. Absorption spectra of oxidized 1 taken using ITO glass electrode at 0.80 V (a) and 0.93 V (b), respectively, in solution of 1 (100 ␮M) and 0.1 M TBAP in CH3 CN:CH2 Cl2 (9:1) (insets: spectral change in the range of 480–800 nm).

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Field strength (G) Fig. 8. EPR data of 1 (100 ␮M) in CH3 CN solution at 298 K, (a) only 1; (b) 1 + 1 equiv. Cu2+ ; (c) 1 + 2 equiv. Cu2+ .

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changes from that shown in Fig. 6b to that noted in Fig. 6a, which is presumably due to the comproportionation reaction between •• the electro-generated 12+ and 1 (in bulk solution) inside the UV • + cuvette to produce 1 . Notably, however, the large decrease of •• absorbance at the initial stage after electrolysis indicating that 12+ • species are more unstable than 1+ in air. The formation of the organic radical induced by Cu2+ was further investigated by EPR (Fig. 8). EPR data confirmed the formation of a radical cation of 1 at room temperature, as shown in Fig. 8 in the presence of 1.0 and 2.0 equiv. Cu(ClO4 )2 . EPR data was also tried at 110 K, too (Fig. S16). Upon treatment with two equiv. Cu(ClO4 )2 , a signal of Cu(II) complex at g⊥ = 2.36 appeared together with the organic radical cation signal at 110 K. 4. Conclusion In conclusion, we report that two simple thiazolothiazole derivative 1 and 2 can act as selective sensors for Cu2+ . Among the various metal ions, a distinct and highly selective colorimetric change from greenish yellow to blue was observed upon the addition of Cu2+ . A fluorescence quenching effect was also observed for Cu2+ . Electrochemical and EPR data clearly support the formation of an organic radical. We believe this unique approach presents a new direction for designing metal ion sensors as well as organic radicals. Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (CRI No. 2012R1A3A2048814 for J.Y.; NRF-2013-020688 for C.L.). Mass spectral data were obtained from the Korea Basic Science Institute (Daegu) on a Jeol JMS 700 high resolution mass spectrometer. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.11.027.

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Biographies Xin Zhang received his Ph.D. in Applied Chemistry from Dalian University of Technology (China) in 2011. In 2012 he joined Professor Yoon’s group at Ewha Womans University (Korea) as a post-doctoral fellow. Minkyung Kang received her B.S. in 2011 and her M.S. in 2013 from Ewha Womans University. She just started her doctoral research at The University of Warwick. Hyun-A Choi received her B.S. from Ewha Womans University in 2013 and is currently working with Prof. Chongmok Lee for her M.S. Ji Young Jung got her B.S. degree of Chemistry at Ewha Womans University in 2010 and her master degree under the guidance of Prof. Juyoung Yoon in 2012. K.M.K. Swamy was born in 1963 in India. He received his B.S (1986), M.S. (1988) and Ph.D. (1996) degrees in Chemistry from the Gulbarga University, India. Presently he is a Professor in the Department of Pharmaceutical Chemistry at V.L. College of Pharmacy Raichur, India. His research interests are design & synthesis of fluorescent chemosensors, synthesis and optimization of pharmacologically active compounds and combinatorial chemistry. Soojin Kim got her B.S. degree of science and her M.S. degree under the guidance of Prof. Jinheung Kim in 2013. Dabin Kim received her B.S. from Ewha Womans University in 2013 and is currently working with Prof. Juyoung Yoon for her M.S. Jinheung Kim is currently a professor of the Department of Chemistry and Nano Science, Ewha Womans University, Seoul, Korea. He received his Ph.D. (1995) from

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Univ. of Minnesota at Minneapolis/St. Paul. His research field is in the area of bioinorganic chemistry and nano biomimetic materials. Chongmok Lee is currently a professor of the Department of Chemistry and Nano Science, Ewha Womans University, Seoul, Korea. He received his Ph.D. (1990) from The University of Texas at Austin. His research field is in the area of electrochemistry.

Juyoung Yoon is currently a professor of the Department of Chemistry-Nano Science, Ewha Womans University, Seoul, Korea. He received his Ph.D. (1994) from The Ohio State University. His research interests include investigations of fluorescent chemosensors, molecular recognition and organo EL materials.