A cholesteryl thiazolothiazole derivative for colorimetric sensing of Cu2+ and its sol–gel transition

Dyes and Pigments 122 (2015) 109e115

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A cholesteryl thiazolothiazole derivative for colorimetric sensing of Cu2þ and its solegel transition Xin Zhang a, b, 1, Hyun-A Choi a, 1, Songyi Lee a, 1, Jun Yin c, Sun Hee Kim d, Chongmok Lee a, **, Juyoung Yoon a, * a

Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea College of Chemistry and Materials Science, Hebei Normal University, Yuhua District, Shi Jiazhuang 050-024, China Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, China d Western Seoul Center, Korea Basic Science Institute, Seoul 120-140, Republic of Korea b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 March 2015 Received in revised form 10 June 2015 Accepted 13 June 2015 Available online 23 June 2015

In the current work, a new thiazolothiazole derivative Chol-TZTZ (1) was synthesized, in which a thiazolothiazole bearing aryl amine groups was linked with four cholesteryl units. Thiazolothiazole derivative 1 displayed a unique colorimetric change from green to blue upon the addition of Cu2þ via the formation of organic radicals. UV absorption maximum was changed from 395 nm to 700 nm when Cu2þ was added. Among the various solvents, cyclohexanone could induce gelation of compound 1. pep interaction between the thiazolothiazole core and van der Waals forces between the cholesteryl group should be involved in the molecules self-assembly of the gelator chol-TZTZ into fibrous superstructures. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Copper sensing Cu(II) sensor Colorimetric sensor Thiazolothiazole Solegel transition Organo radical

1. Introduction Fluorescent and colorimetric chemosensors have drawn a lot of attentions for various analytes [1]. Especially, transition metal ions are of great interest due to their biological and environmental importance [2]. Traditional molecular recognition and reactionbased approach have been adopted to design new chemosensors for metal ions. Copper ion selective chemosensors have been developed using similar approaches [3]. It is known that, under moderate oxidizing potentials, the lone pair of electrons on nitrogen of arylamines can be converted to cation radicals [4]. Recently, organic radical formation of arylmines by chemical oxidants such as Cu2þ has been explored as a new tool to detect Cu2þ since unique colorimetric change can be observed when arylamines are connected to chromophores [5].

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Lee), [email protected] (J. Yoon). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.dyepig.2015.06.016 0143-7208/© 2015 Elsevier Ltd. All rights reserved.

Costa group reported a squaramide derivative bearing arylamine unit as chemodosimeter, which can form a colored zwitterionic radical upon the addition of Cu2þ [5a]. Tris(4-anisyl)amine (TAA) radical formation was utilized to sense Cu2þ by Gopidas's group [5b]. Chen group reported that a series of arylaminofluorene derivatives can produce their corresponding arylaminium cation radicals upon the addition of Cu(ClO4)2 in CH3CN [5c]. Our group also utilized this concept to sense Cu2þ using a new thiazolothiazole derivative [5d]. On the other hand, low-molecular-weight gelators (LMWGs) have been actively studied for various applications, such as templates, sensors, devices and drug delivery systems [6,7]. In the current study, a new thiazolothiazole derivative bearing four cholesteryl groups, Chol-TZTZ (1), has been synthesized. Adopting the concept that organic radical can be formed by the chemical oxidants, compound 1 was applied to sense Cu2þ among the various metal ions. In addition, among the various solvents, cyclohexanone could induce gelation of compound 1. The unique color change in gel state via the radical formation could be also induced by electrolysis, which suggested the potential application of Chol-TZTZ as magnetic gel system.

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(75 MHz, CDCl3) d 168.4, 154.4, 149.5, 148.8, 139.2, 127.9, 123.1, 111.8, 78.3, 64.1, 56.7, 56.1, 50.0, 42.3, 39.7, 39.5, 38.0, 36.8, 36.5, 36.2, 35.8, 31.8, 28.2, 28.0, 27.7, 24.3, 23.9, 22.8, 22.6, 21.0, 19.3, 18.8, 11.9. FABHRMS C136H204N4O12S2 ([MþH]þ) calcd 2150.4950, found 2150.4966.

2. Experimental 2.1. Synthesis 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 (230e400 mesh). 1H NMR and 13C NMR spectra were recorded using 300 MHz and 75 MHz, respectively. Chemical shifts were expressed in ppm and coupling constants (J) in Hz.

2.2. UV and fluorescence study Stock solutions of Chol-TZTZ (0.01 mM) and various perchlorate salts of metal ions (0.05 mM), such as Al3þ, Agþ, Ca2þ, Co2þ, Cr3þ, Csþ, Cu2þ, Fe2þ, Fe3þ, Hg2þ, Kþ, Liþ, Mg2þ, Mn2þ, Naþ, Ni2þ, Pb2þ, Sr2þ and Zn2þ ions, were prepared in CH3CN. Test solutions were prepared by placing 30 mL 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 CH3CN.

2.1.1. Compound 4 200 mg of 3 [5d] (0.3 mmol) was added into 5 ml of ethanol and 4.5 ml of NaOH (10 wt%) aqueous solution. The mixture was stirred for overnight at room temperature. Neutralization was carried out with diluted HCl solution until pH 7. The resulting precipitate was filtered and the filter cake was washed with water for several times. The solid was dried under vacuum for 1 day. 120 mg of 4 as a yellowish powder was obtained, yield: 80%. M.p. ¼ 277.3e278.5  C. 1 H NMR (300 MHz, (CD3)2SO) d 7.73 (d, 4H, J ¼ 8.1 Hz), 6.79 (d, 4H, J ¼ 8.1 Hz), 4.82 (t, 4H, J ¼ 5.7 Hz), 3.48e3.59 (m, 16H). 13C NMR (75 MHz, (CD3)2SO) d 168.4, 150.5, 148.6, 127.8, 120.7, 111.9, 58.5, 53.6. FAB-HRMS C24H28N4O4S2 ([MþH]þ) calcd 501.1630, found 501.1631.

2.3. Electrochemical study Electrochemical experiments were performed with a BAS 100B electrochemical analyzer at room temperature in a glove box. Cyclic voltammograms (CVs) were measured in a CH2Cl2 solution containing 0.5 mM Chol-TZTZ and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) with a Pt disk working and Pt counter electrode as well as AgjAgNO3 (0.1 M) reference electrode. Then, the potential values were calibrated against the saturated calomel electrode (SCE) [5d]. Spectroelectrochemical data were measured with an ITO-coated glass electrode (Delta Technologies) as the working electrode using a UV cuvette, a diode array spectrophotometer (HewlettePackard 8452A), and an AFRDE5 biopotentiostat (Pine Instruments) under atmospheric conditions [8].

2.1.2. Chol-TZTZ (1) 50 mg of 4 (0.1 mmol) and 270 mg of cholesteryl chloroformate (0.6 mmol) were added into 5 ml of anhydrous dichloromethane. Then 72 mL of pyridine (0.89 mmol) was added at 0  C and the reaction mixture was stirred at room temperature for 48 h. The mixture was filtered and the solvent of filtrate was removed under reduced pressure. The residue was purified by column chromatography (silica gel, dichloromethane/methanol ¼ 200:1, v/v). 75 mg of yellowish powder was obtained, yield: 34.8%. M.p. ¼ 248.3e250.1  C. 1H NMR (300 MHz, CDCl3) d 7.85 (d, 4H, J ¼ 9.0 Hz), 6.81 (d, 4H, J ¼ 9.3 Hz), 5.40 (s, 4H) 4.42e4.52 (m, 4H), 4.33 (t, 8H, J ¼ 6.0 Hz), 3.76 (t, 8H, J ¼ 9.0 Hz), 2.35e2.39 (m, 8H), 1.85e2.00 (m, 20H), 0.87e1.65 (m, 132H), 0.68 (s, 12H). 13C NMR

3. Results and discussion 3.1. Synthesis The synthesis of thiazolothiazole derivative, Chol-TZTZ (1), was carried out in three steps which is outlined in Scheme 1. In the

dithiooxamide

N 2

O

O

O H

O

O

O O

N

in DMF, 160 o C, N2, 39.9%

O O

N

S

S

N

N

O

3

O

O O

OH

HO NaOH in ethanol

N

N

r. t., 80%

S

cholesteryl chloroformate

N S

HO

OH

4

O

O

O

O

O

O N

N

S

S

N

N O

O O O

C5H5N in CH 2Cl2 0 o C to r. t. 34.8%

N

1 Chol-TZTZ

O O

Scheme 1. Synthesis of compound 1 (Chol-TZTZ).

X. Zhang et al. / Dyes and Pigments 122 (2015) 109e115

first step, the commercially available aldehyde 2 was reacted with dithiooxamide to obtain compound 3 according to our reported schemes [5d]. Then one molecule of compound 3 underwent sufficient removal of four molecules of the acetyl group upon treatment with NaOH/ethanol aqueous solution to give intermediate 4. Finally, with a simple synthesis approach, compound 4 was reacted with cholesteryl chloroformate in the presence of pyridine at room temperature in dichloromethane to give compound 1 in a total yield of 11.1%. Compound 1 was fully characterized by 1H and 13C NMR as well as high resolution mass spectroscopy (Figs. S1e4). 3.2. Selective optical change for Cu2þ The UVevis absorption changes of Chol-TZTZ (10 mM) towards various metal ions such as, Al3þ, Agþ, Ca2þ, Co2þ, Cr3þ, Csþ, Cu2þ, Fe2þ, Fe3þ, Hg2þ, Kþ, Liþ, Mg2þ, Mn2þ, Naþ, Ni2þ, Pb2þ, Sr2þ and Zn2þ are reported in Fig. 1a. Among the various metal ions, CholTZTZ displayed a selective UVevis absorption change, which can be also easily observed by naked-eye (Fig. 1b). In the absence of Cu2þ, Chol-TZTZ showed a maximum absorption at 395 nm in CH3CN. The addition of Cu2þ induced appearance of a new peak at 700 nm with a decrease of the peak at 395 nm.

111

3.3. Spectral studies of Chol-TZTZ (1) Various metal ions were examined for UV and fluorescence study of Chol-TZTZ (1) in CH3CN. Among these metal ions, probe 1 displayed a selective UVevis absorption change, which were accompanied by fluorescence quenching effect. Probe 1 showed a major absorption band at 395 nm, and new absorption peak at 700 nm appeared after adding Cu2þ (5 equiv.), as shown in Fig. 1a. In addition, Fig. 2a shows the UVevis titration of 1 with Cu2þ in CH3CN. A clear isosbestic point at 465 nm was observed with the increase at 700 nm and the decrease at 395 nm. Upon the excitation at 400 nm, Chol-TZTZ showed an emission with lmax ¼ 500 nm was observed for Cu2þ. Based on the fluorescence changes of Chol-TZTZ with Cu2þ in CH3CN (Fig. 2b), a linear correlation was observed between 0 and 50 mM of the Cu2þ solution with the detection limit of 4.8  104 M. The CV waves of Chol-TZTZ showed two oxidation peaks at 0.89 and 1.01 V vs. SCE implying the mono- and bis-radical formation (Fig. S5). 3.4. Gelation ability and its property The gelation ability of Chol-TZTZ has been estimated by mixing 6 mg of Chol-TZTZ with 0.25 ml of a solvent in a septum-capped

Fig. 1. (a) UV spectra of 1 (1.0  105 M) upon addition of 5 equiv. of metal ions in CH3CN. (b) Colorimetric changes of 1 (1.0  105 M) upon the addition of various metal ions (5 equiv.) in CH3CN.

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Fig. 2. (a) UV titrations of 1 (1.0  105 M) with Cu2þ in CH3CN. (b) Fluorescence titrations of 1 (1.0  105 M) with Cu2þ in CH3CN (lex ¼ 400 nm, slit: 3 nm/3 nm).

Table 1 Gelation abilities of Chol-TZTZ in various solvents. Entry

Solvent

Phasea at 24 mg/ml

CGCb/wt% (mmol dm3)c

Tgd/ C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Cyclohexanone Dioxane Chloroform Dichloromethane (DCM) Dimethyl sulfoxide (DMSO) N,N-Dimethylformamide (DMF) n-Hexane Dodecane Cyclohexane Decane Ethanol n-Butanol n-Propanol Acetone Acetonitrile (ACN) Ethyl acetate (EtOAc)

OG PG S S I P P I P P I I I I I I

2.52 (11.1) / / / / / / / / / / / / / / /

58e66 / / / / / / / / / / / / / / /

a

OG ¼ Opaque gel; S ¼ Solution; I ¼ Insoluble; P ¼ Precipitation; PG ¼ Partial gel. Critical gelation concentration: the minimum concentration for the gelator to entrap solvents. c The CGC values were represented by the mass percent and the related molar concentrations in parentheses are also given. d Gel-to-sol transition temperature was determined by warming the gel at CGC ranging from the point at which the first part of the gel was melted to the point at which the whole sol formed. b

X. Zhang et al. / Dyes and Pigments 122 (2015) 109e115

113

Fig. 3. SEM (a) and TEM (b) images of the cyclohexanone gel of chol-TZTZ (24 mg/ml).

Fluorescent Intensity (a. u.)

vial, heating until solids were dissolved completely, and then cooling to room temperature. As a defining characteristic, the gelation can be observed upon upending the vial in which the gels stay where they are, after several thermoreversible tests. As summarized in Table 1, Chol-TZTZ can only gelate cyclohexanone out of the 16 solvents. The critical gelation concentration, at which the solvent is gelated with the minimum amount of gelator, was found as 24 mg/ml in cyclohexanone. The gel-to-sol transition temperature (Tg) for the cyclohexanone gel (24 mg/ml) was evaluated to be 58e66  C. The resulting gels are robust, opaque and stable for several weeks at room temperature.

400 -3

0.1 mmol dm -3 1.0 mmol dm -3 11.0 mmol dm

300

200

100

0

450

500

550

600

650

700

Wavelength (nm)

Fig. 5. EPR spectra of Chol-TZTZ (1  104 M) in CH3CN solution at 298 K upon the addition of 2 equiv. Cu2þ.

400

3.0

300

2.5

Absorbance

Fluorescent Intensity (a. u.)

The morphology of the xerogel obtained from the organogelator Chol-TZTZ has been examined by scanning electron microscope (SEM) and transmission electron microscope (TEM). The sample for the SEM image was fabricated by drying the gel (24 mg/ml) on a silicon wafer via slow evaporation of the solvent (cyclohexanone) at room temperature under vacuum for 24 h. Then the obtained xerogel was tested with the Pt shield. As shown in Fig. 3a, a floccuslike morphology was observed in the SEM image. For TEM examination, the specimen was prepared by drying the diluted cyclohexanone gel on the surface of carbon-coated copper grid for

200 Gel

100 Sol 0 450

500

550

600

650

700

Wavelength (nm) Fig. 4. Fluorescence emission spectra (top: excitation at 400 nm) of solutions or gel of Chol-TZTZ as the concentration was increased from 0.1 mM to 11.0 mM in cyclohexanone; Bottom: Emission spectra of Chol-TZTZ recorded in cyclohexanone (24 mg/ml) before and after gel formation with excitation at 400 nm (inset: fluorescent photos of gel and sol recorded under a 365 nm UV lamp.

o

25 C o 90 C

2.0 1.5 1.0 0.5 0.0

400

500

600

700

800

Wavelength (nm) Fig. 6. Absorption spectra of Chol-TZTZ with a concentration of 1  104 M in cyclohexanone.

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X. Zhang et al. / Dyes and Pigments 122 (2015) 109e115

Fig. 7. Photographs of Chol-TZTZ gel on ITO glass electrode (a) before electrolysis and (b) after electrolysis at 1.20 V for 9 min. The color change after cease of electrolysis for (c) 10 min, (d) 30 min, (e) 1 h, (f) 2 h, (g) 3 h, (h) 6 h, and (i) 24 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

24 h in a vacuum oven at 25  C. The TEM image in Fig. 3b showed the formation of short entangled fibers with 0.5e3 mm in length and approximately 10e80 nm in width. This indicated that the floccuslike structure in the SEM image of the gel was the result of the aggregation of these fibers. It is well known that several weak noncovalent interactions, such as hydrogen bonding, pep stacking, electrostatic interactions, and van der Waals forces can be generally the driving forces for a gelator to self-assemble into gel phase. In case of gelator chol-TZTZ, pep interaction between the thiazolothiazole core and van der Waals forces between the cholesteryl groups should be involved in the molecules self-assembly of the gelator into fibrous superstructures. To gain understanding of the pep interaction style, concentration-dependent fluorescent spectral experiments have been carried out. As shown in Fig. 4 (top), a significant red-shift in the fluorescent emission has been observed with the increase of the gelator concentration in cyclohexanone at room temperature. The emission maximum of the gel phase (11.1 mmol dm3) is redshifted by 42 nm as compared to the emission maximum in the diluted solution (0.1 mmol dm3) of Chol-TZTZ. The fluorescence of Chol-TZTZ (24 mg/ml in cyclohexanone) in the gel phase at 509 nm was ca. 19-fold higher than that in the sol state and the gel formation lead to a red-shift of 11 nm as shown in Fig. 4 (bottom). 3.5. Electrochemical experiments The organic radical formation of Chol-TZTZ upon the addition of Cu2þ was also confirmed by EPR (Fig. 5). As shown in Fig. 5, the EPR signal centered at 3440 G (g~2.00) was observed in the presence of 2.0 equiv. Cu (ClO4)2, which was attributed to the formation of a radical cation of Chol-TZTZ. In addition, temperature-dependent absorption spectral test has been also implemented in a cyclohexanone solution of chol-TZTZ (1  104 M). A slight hypsochromic shift of 6 nm for the absorption bands was detected with the temperature increase from 25 to 90  C as shown in Fig. 6. The above results suggested that J-type aggregation existed during the gel formation by the pep stacking [9]. 3.6. Electrochemical experiments To examine the spectral change of Chol-TZTZ in a gel state, ITO glass was dipped into the heated sol (6 mg/0.25 ml) for 4 s and pulled out with a rate of 5 mm s1 to form a gel state in 1 h. Since the addition of Cu2þ solution to the Chol-TZTZ gel was practically difficult due to the very slow diffusion of Cu2þ solution. As alternative option to make organic radical of Chol-TZTZ in its gel state, dip-coated Chol-TZTZ gel on ITO glass showed distinct colorimetric change from light green to blue after electrolysis, which confirms the formation of organic radical of Chol-TZTZ. The blue color was maintained over 6 h as shown in Fig. 7. 3.7. Time-dependent density functional theory (TD-DFT) calculations To further understand the optical behavior of Chol-TZTZ, a more detailed analysis of the structure and electron density of a simplified version of Chol-TZTZ, dimethyl derivative 5, was carried out in

an attempt to gain insight into its UVeVis absorption and electron distribution of radical. Accordingly, the time-dependent density functional theory (TD-DFT) calculations were used to optimize the structures of compound 5 and its radical at the B3LYP/6-31G* level using a suite of Gaussian 09 programs (Figs. S6eS8, Tables S1eS3). As shown in Fig. S6, the HOMO orbital energy of compound 5 was almost distributed over the whole molecule while the orbital energy of LUMO is largely distributed the dithiazole moiety due to the strong influence of strong electron-deficient N-heterocycle. It is worth mentioning that the calculated electronic absorption spectrum discloses a similar spectrum to the experimental data. For the radical of compound 5, the result of electronic absorption spectrum implied that compound 5 was more inclined to form the diradical, as presented in Fig. S8. More, it displays a similar electron distribution not only in HOMO but also in LUMO with compound 5. The result suggested that the central dithiazole unit had a larger effect towards the electron distribution of compound 5 and its radical. 4. Conclusion In this paper, we report a new thiazolothiazole derivative CholTZTZ (1) as selective chemosensor system for Cu2þ via unique organic radical formation as well as a new supramolecular gel system. Chol-TZTZ consists of a thiazolothiazole bearing aryl amine groups linked four cholesteryl units. The UV absorption maximum was changed from 403 nm to 650 nm when Cu2þ was added, which was also confirmed by distinct colorimetric change from green to blue. Chol-TZTZ showed a unique colorimetric change from green to blue upon the addition of Cu2þ, which can be attributed to the formation of organo radicals. Among the various solvents, cyclohexanone could induce gelation of compound 1. The unique color change in gel state via the radical formation could be also induced by electrolysis, which suggested the potential application of CholTZTZ as magnetic gel system. Acknowledgments J. Y. acknowledges a grant from the National Creative Research Initiative programs of the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2012R1A3A2048814). C. L. acknowledges a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) No. 2014R1A2A2A01005479). X. Z. acknowledges the Youth Top-notch Talent Foundation funded by the Education Department of Hebei Province of P. R. China (No. BJ2014039). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.06.016. References [1] (a) Zhang X, Yin J, Yoon J. Recent advances in development of chiral fluorescent and colorimetric sensors. Chem Rev 2014;114:4918. (b) Lee S, Lee J, Lee M, Cho YK, Baek J, Kim J, et al. Construction and molecular understanding of an unprecedented, reversibly thermochromic bis-polydiacetylene. Adv Funct Mater 2014;24:3699.

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[6]

[7]

[8]

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