An intramolecular charge transfer fluorescent probe: Synthesis and selective fluorescent sensing of Ag+

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Spectrochimica Acta Part A 70 (2008) 923–928

An intramolecular charge transfer fluorescent probe: Synthesis and selective fluorescent sensing of Ag+ Honglei Mu, Rui Gong, Lin Ren, Cheng Zhong, Yimin Sun, Enqin Fu ∗ Hubei Key Laboratory on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, PR China Received 29 July 2007; received in revised form 26 September 2007; accepted 4 October 2007

Abstract An intramolecular charge transfer (ICT) fluorescent probe, in which the thiourea derivative moiety is linked to the fluorescent 4-(dimethylamino) benzamide, has been designed and synthesized. The ions-selective signaling behaviors of the probe were investigated. Upon the addition of Ag+ , an overall emission enhancement of 14-fold was observed. Compound 1 displayed highly selective chelation enhanced fluorescence (CHEF) effect with Ag+ over alkali, alkali earth metal ions and some transition metal ions in aqueous methanol solutions. The prominent selective and efficient fluorescent enhancing behavior could be utilized as a new chemosensing probe for the analysis of Ag+ ion in aqueous environment. © 2007 Elsevier B.V. All rights reserved. Keywords: Molecular discrimination; Fluorescent probe; Fluorescent enhancing; Ag+

1. Introduction Fluorescent probes capable of selectively recognizing guest species are of particular interest in supramolecular chemistry because of their high selectivity, sensitivity, and simplicity [1]. Especially, fluorescent sensing of heavy and transition metal (HTM) ions have received increasing attention due to their importance in many biological processes and environmental relevancy [2]. In the previous literatures, a number of fluorescent probes for HTM ions have been reported [3]. Most of them display fluorescence intensity changes, but relatively few of them result in fluorescence enhancing with HTM analytes, such as Hg (II), Pb (II), Ag (I), and Cu (II), since these ions generally act as quenchers via the electron transfer or facilitated intersystem crossing (isc) processes. It is generally believed that sensors with a fluorescence enhancement signal when interacting with analytes are much more efficient. Whereas, the examples of Ag+ -selective fluorescence enhancement probes have been still scarce [4]. Therefore the development of new fluorescent Ag+ sensors, especially those that exhibit selective Ag+ -amplified emission, is still a challenge.

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Corresponding author. Tel.: +86 27 87219044; fax: +86 27 68756757. E-mail address: [email protected] (E. Fu).

1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.10.006

The intramolecular charge transfer (ICT) mechanism has been widely exploited for metal ions sensing [2a,b,5]. In most cases, the ionophore is the electron-donor or electrondeficient substituent of the fluorophore and coordination of a metal ion leads to a pronounced blue or red shift of the intramolecular charge transfer absorption band but often to change in fluorescence intensity [6]. It is known that the excited-state intramolecular charge transfer and the accompanied dual fluorescence of the electron donor/acceptor substituted benzenes such as 4-(dimethylamino)benzonitrile (DMABN) and 4-(dimethylamino)benzamide (DMABA). The generally favored concept to explain the anomalous, long wavelength fluorescence is a TICT state [7]. The dual fluorescent behavior depends sensitively on the nature of the electron donor/acceptor [8], therefore it is possible to design fluorescent sensors for metal ions sensing. In this paper we report an intramolecular charge transfer (ICT) fluorescent probe, in which the thiourea derivative moiety is linked to the fluorescent 4-(dimethylamino)benzamide, and its Ag+ ion-selective signaling behaviors. The fluorophore in this fluorescent probe is a strong “push–pull” ␲-electron system, with an aniline nitrogen atom as an electron donor and a carbonyl as an electron acceptor. Based on the principle of hard and soft acids and bases, the S atom in the thiourea moiety tends to coordinate the Ag+ [9]. Therefore, we can expect that the complexation of Ag+ ion will have a dramatic influence

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upon the emission of fluorophore and transmit the signal of recognition. 2. Experimental 2.1. Reagents and apparatus Absorption spectra were determined on a UV-2550 UV–visible spectrophotometer. Fluorescence spectra were determined on a F-4500 fluorescence spectrophotometer. Melting points were determined on a X-6 micro-melting point apparatus and were uncorrected. IR spectra were obtained on a Nicolet 170SX FT-IR or a Shimadzu FT-IR 8000 spectrophotometer. NMR spectra were recorded on Varian Mercury VX300 FT-NMR spectrometer with (CH3 )4 Si as internal reference. Elemental analyses were determined on a Perkin-Elmer204B elemental auto analysis apparatus. Twice-distilled water was used throughout the experiments. All the materials for synthesis and test were purchased from Shanghai Chemicals (Shanghai, China) and used as received. Except specified, other chemicals were analytical reagent grade and used without further purification. The solutions of metal ions were prepared from their acetate salts, except for Pb2+ , Ag + from nitrate and Fe2+ from sulfate, respectively. 1 H NMR studies were recorded after adding two equivalents Ag+ into probe (20 mM) in DMSO-d6 . The effect of the metal ions upon the absorption and fluorescence intensity was examined by adding a few microliters of stock solution of the metal cations to a known volume of the solution (2 mL). The addition was limited to 0.1 mL, so that dilution remained insignificant. Association constants (1:2) of 1 with Ag+ are calculated by followed equation [10] and linearly fitted in origin 7.0:   (X − X0 ) log = n log[G] + log Kd (X∞ − X) X0 : fluorescent intensity of host without guest; X∞ : fluorescent intensity reaching a limitation by adding excessive guest; n: the stoichiometric ratio of host and guest; [G]: concentrations of guest. 2.2. Synthesis of 1 and 2 The synthetic route of 1 and 2 is shown in Scheme 1. 4(dimethylamino)benzoic acid [11](5). 3.2 g(0.019 mol) AgNO3 was dissolved into a 60 mL potassium hydroxide aqueous solution (7%) and 1.5 g(0.01 mol) 4-(dimethylamino)-benzaldehyde was added into this solution. The mixture was stirred at 60 ◦ C for 24 h, then cooled to room temperature, and filtered. After the solution was acidified with concentration hydrochloric acid, the product precipitated gradually. The solid was filtrated and then recrystallized from ethanol to give white needle product 5 in 89% yields: 1 H NMR (300 MHz, CDCl3 ): δ = 7.98(d, J = 8.1 Hz, 2H), 6.68(d, J = 8.1 Hz, 2H), 3.06(s, 6H). 4-(dimethylamino)benzoic acid ethyl ester [12] (4). In a 100 mL round-bottomed flask was placed 3.0 g(0.018 mol) 4(dimethylamino)benzoic acid with 50 mL alcohol. Then 2 mL concentrated sulfuric acid was added dropwise under vigorous

stirring. The solution was allowed to reflux for 6 h and cooled to room temperature. Saturated aqueous sodium carbonate was added with stirring until pH was 7–8. The precipitated product was filtered and recrystallized from ethanol and water (1:1) to give white crystals 4 in 71% yields: 1 H NMR (300 MHz, CDCl3 ): δ = 7.93(d, J = 8.7 Hz, 2H), 6.66(d, J = 8.7 Hz, 2H), 4.33(q, J = 7.2 Hz, 2H), 3.03(s, 6H), 1.36(t, J = 7.2 Hz, 3H). 4-(dimethylamino)benzoic acid hydrazide (3). In a 100 mL round-bottomed flask was placed 1.0 g(0.005 mol) 4-(dimethylamino)benzoic acid ethyl ester with 50 mL alcohol. Then 5 g hydrazine hydrate (85%) was dropped into under stirring. The solution was allowed to reflux for 2 days. The reaction mixture was allowed to cool to room temperature and evaporated under reduced pressure to give a solid. The solid recrystallized from ethanol to give white product 3 in 75% yields: 1 H NMR (300 MHz, CDCl3 ): δ = 7.68(d, J = 8.7 Hz, 2H), 7.29(s, 1H), 6.68(d, J = 8.7 Hz, 2H), 4.06(s, 2H), 3.02(s, 6H). N-4-(dimethylamino)benzamido-N’-1-naphthylthiourea (1).A mixture of 0.72 g (0.004 mol) 3 and 0.74 g(0.004 mol) 1-naphthyl isothiocyanate were stirred to dissolve in 50 mL ethylene glycol monomethyl ether. The mixture was then stirred at room temperature for 8 h and filtered. The solid was washed with ethanol (3 × 50 mL) and further dried under a vacuum pump to afford 0.88 g white powder product 1 in 67% yield: 1 H NMR (300 MHz, DMSO-d ): δ = 10.31(s, 1H), 9.96 (s, 1H), 6 9.65 (s, 1H), 7.97–7.90 (m, 2H), 7.85–7.81(m, 3H), 7.50–7.46 (m, 3H), 7.32 (d, J = 9.0 Hz, 1H), 6.71 (d, J = 9.0 Hz, 2H), 2.95 (s, 6H). 13 C NMR (75 MHz, DMSO-d6 ): δ = 183.4, 166.8, 153.1, 136.6, 134.3, 131.5, 130.1, 128.4, 127.4, 127.1, 126.6, 126.4, 126.0, 124.6, 119.6, 111.2; IR (KBr): ␯/cm−1 3336.6, 3298.0, 3261.0, 1649.9, 1603.3, 1501.0, 1464.1, 1295.1, 1203.0, 769.1; EI MS found: m/z = 364.2 (M+ ); Anal. Calcd for C20 H20 N4 OS: C 65.91, H 5.53, N 15.37; found: C 65.76, H 5.55, N 15.34. N-4-(dimethylamino) benzamido-N -1-naphthylurea (2). A mixture of 0.72 g(0.004 mol) 3 and 0.70 g(0.004 mol) 1-naphthyl isothiocyanate were stirred to dissolve in 50 mL ethylene glycol monomethyl ether. The mixture was then stirred at room temperature for 12 h and filtered. The solid was washed with ethanol (3 × 50 mL) and further dried under a vacuum pump to afford 0.94 g white powder product 2 in 55% yield: 1 H NMR (300 MHz, DMSO-d6 ): δ = 10.07(s, 1H), 8.89 (s, 1H), 8.34 (s, 1H), 8.1(d, J = 9.0 Hz, 1H), 7.92 (d, J = 9.0 Hz, 1H), 7.82 (d, J = 9.0 Hz, 3H), 7.65(d, J = 9.0 Hz, 1H), 7.59–7.43 (m, 3H), 6.73 (d, J = 9.0 Hz, 2H), 2.98 (s, 6H); 13 C NMR (75 MHz, DMSO-d6 ): δ = 167.1, 157.2, 153.1, 135.1, 134.4, 129.6, 128.9, 126.5, 126.4, 126.3, 124.1, 122.6, 119.6, 111.4; IR(KBr): ␯/cm−1 3277.7, 1650.1, 1598.7, 1552.1, 1485.5, 1205.6, 765.8; EI MS found: m/z = 347.9 (M+ ); Anal. Calcd for C20 H20 N4 O2 : C 68.95, H 5.79, N 16.08; found: C 69.03, H 5.80, N 16.03. 3. Results and discussion The metal ion binding properties of compound 1 were investigated by UV–vis absorption and fluorescence spectroscopy. The titration experiments were carried out in H2 O–CH3 OH sys-

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Scheme 1. Synthesis of 1 and 2.

tem (1:3, v/v) by adding of Ag+ . Fig. 1 shows the changes in the absorption spectra of probe 1 in a mixture (1:3) of H2 O–CH3 OH containing HEPES (20 mM) with the increasing of Ag+ . As the concentration of Ag+ increased, we observed a regular decreasing in the absorption at 313 nm and a new absorption band at 331 nm occurred and increased prominently to their limiting values. The changes of the absorption suggest the coordination of Ag+ to the probe and the effect of the ICT state had been disturbed. The interaction of Ag+ with probe 1 might enhance the electron-withdrawing character of the amide group in probe 1 (acceptor group), which causes the clear red shift. The fluorescent spectral properties of probe 1 (c = 1 × 10−5 M) were also determined in aqueous methanol

Fig. 1. Changes in the UV–vis spectra of 1 (10−5 M) on addition of Ag+ (0–5 × 10−5 M) in aqueous methanol solution (H2 O–CH3 OH, 1:3, v/v, 20 mM HEPES, pH = 7.0).

solution (H2 O–CH3 OH, 1:3, v/v, 20 mM HEPES, pH = 7.0) (λex = 353 nm). 1 showed a very weak fluorescence (LE state) at ca. 380 nm and the long wavelength CT emission was not found in this condition. The fluorescence intensity enhancement was observed in the LE emission region in the presence of Ag+ ions. Fig. 2a shows the fluorescence titration of 1 with Ag+ . The fluorescence of the solution increased dramatically with increasing concentrations of Ag+ . Addition of two equiv of Ag+ resulted in a 14-fold enhancement of fluorescence intensity of probe 1, the maximum emissive wavelength red-shifted from 378 to 385 nm slightly. The fluorescence intensity increased linearly on the concentrations of Ag+ between 0 and 2.0 × 10−5 M (inset of Fig. 2b). The regression equation is: IF = 82.02 + 5.36 × 107 CAg+ . Then we carried out Job’s plot experiment by varying the concentration of both 1 and Ag+ (Fig. 3). The maximum point at the mole fraction of 0.33, which indicated that a 1:2 (ligand:metal) complex was formed [13]. The complexation of 1 with Ag+ ion was further evidenced by the 1 H NMR measurement (Fig. 4). The binding of silver ion to 1 was monitored by the changes in the 1 H NMR spectrum of 1 in DMSO-d6 in the presence of Ag+ ion. When two equiv of Ag+ ions were added, the peaks, assigned to the thiourea protons of 1 and the amide N–H proton appeared at 9.66, 9.96 and 10.31 ppm, experience clear downfield shifted to 10.47, 10.69 and 10.82 ppm, respectively. This could be attributed to a deshielding effect, arising from the decrease of the electron density of nitrogen in the fluorophore caused by 1-Ag+ complexation. After addition of Ag+ ions, the aromatic protons display almost no change of chemical shift and only small change in the shape of the peak. This proved that the S atom of the thiourea and the O atom of the amide possibly were the coordinating sites for Ag+ . Based on the above information, we calculated the possible interaction model between Ag+ and

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Fig. 2. (a) Fluorescence titration of 1 (10−5 M) in the presence of different concentration of Ag+ in aqueous methanol solution (H2 O–CH3 OH, 1:3, v/v, 20 mM HEPES, pH = 7.0). (λex = 353 nm). (b) The plot of fluorescence intensity at 385 nm vs. equivalents of Ag+.

1 (Fig. 5). The O, N atoms of amide and the S atom of thiourea coordinated two silver ions in the calculated model. This model could well explain the experimental results. The detection limit of 1 as a fluorescent probe for the analysis of Ag+ was also determined by the equation [14]: cL =

Fig. 3. Job plot for 1 and Ag+ .

3sB b

where sB was the blank signal standard deviation and b was the slope of the calibration graph. The detection limit was approximately 8 × 10−7 M, which was sufficiently low for the detection of Ag+ in many chemical and biological systems. Then the apparent association constant, Kd , was determined. The association constant between the host and metal ion was 1.8 × 108 M−2 . As a result, the probe 1 displayed a highly selective chelation enhanced fluorescence (CHEF) effect [15] with Ag+ . The large CHEF effects with Ag+ could be explained possibly when the probe 1 had caught a silver ion, the electron- withdrawing ability of the amide group in probe 1 would be enhanced, and thus

Fig. 4. Evolution of the 1 H NMR spectra of (a) 1, (b) 1 + 2equiv Ag+ in DMSO-d6 .

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Fig. 5. The calculated result for the interaction between Ag+ and 1 based on PM3 of MOPAC 2007 software.

Fig. 8. Fluorescent ratiometric responses of 1 (1 × 10−5 M) containing Ag+ (2 × 10−5 ) and the background metal ions (10−4 M).

Fig. 6. Changes in the fluorescence spectra of 2 (10−5 M) on addition of Ag+ (0–3 × 10−4 M) in aqueous methanol solution (H2 O–CH3 OH, 1:3, v/v, 20mM HEPES, pH = 7.0).

a red-shift in emission spectra and enhancement of fluorescence intensity should be observed [16]. For comparison, the fluorescent spectral properties of control molecule 2 in the presence of Ag+ were also determined in order to prove the role of S atom in the fluorescent probe. The fluorescence intensity of 2 decreased continually upon the

addition of Ag+ , and even when the total amount of Ag+ ion exceeded 30 equiv of the probe 2, equilibrium had still not achieved (Fig. 6). The 1 H NMR experiment was also carried out in order to investigate the interaction of Ag+ with 2. The addition of 1 equiv of Ag+ did not cause the change of the proton signals of 2. These results indicated the association constant of 2 with Ag+ was small. The result showed the S atom of thiourea in 1 played an important role as the recognition site, which caused the CHEF effect upon the addition of Ag+ . To evaluate the selectivity of probe 1, the effects of other metal ions on the fluorescent spectral properties of probe 1 were also investigated. The results were shown in Fig. 7. Some transition metal ions, for example, Co2+ , Ni2+ , Cu2+ , Zn2+ , Mn2+ , Fe2+ , and the alkali, alkali earth metal ions had only a little effect on the fluorescence intensity (λem = 385 nm) of probe 1 under the same condition, except the Hg2+ caused 6-fold enhancement of fluorescence intensity. These results indicated 1 as a highly selective fluorescent probe for Ag+ ion. To explore practical applicability of 1 as an Ag+ -selective fluorescent probe, competition experiments were also performed. The solution of 1 in the presence of Ag+ (2 × 10−5 M) was mixed

Fig. 7. (a) Fluorescence spectra of 1 (1 × 10−5 M) in the presence of 2 equiv of different metal ions in aqueous methanol solution (H2 O-CH3 OH, 1:3, v/v, 20 mM HEPES, pH = 7.0). (b) fluorescence intensity at 383 nm in the presence of equiv of different metal ions in aqueous methanol solution (H2 O–CH3 OH, 1:3, v/v, 20 mM HEPES, pH = 7.0).

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with background metal ions (10−4 M), including Hg2+ , Co2+ , Ni2+ , Cu2+ , Zn2+ , Mn2+ , Fe2+ the alkali and alkali earth metal ions, respectively. The fluorescence intensity of all above the solutions showed no obvious variation comparing with that only containing Ag+ (Fig. 8). 4. Conclusion In summary, we have synthesized a new fluorescent probe 1 for Ag+ based on the ICT mechanism with high sensitivity and selectivity, using the amide and thiourea group as the recognition moiety and 4-(dimethylamino)benzamide as the signal group. Compound 1 showed chelation enhanced fluorescence (CHEF) effect with Ag+ over other competing metal ions in aqueous methanol solutions. Therefore, probe 1 could be utilized as a new fluorescent probe for the analysis of micromolar concentration range of Ag+ ion in aqueous environment. Acknowledgement We gratefully acknowledge the financial support from the Natural Science Foundation of China (No. 20272045 and No. 20672085). References [1] (a) L. Fabbrizzi, A. Poggi, Chem. Soc. Rev. (1995) 197; (b) D.T. McQuade, A.E. Pullen, T.M. Swager, Chem. Rev. 100 (2000) 2537; (c) G.W. Gokel, W.M. Leevy, M.E. Weber, Chem. Rev. 104 (2004)2723; (d) J.F. Callan, A.P. de Silva, D.C. Magri, Tetrahedron 61 (2005) 8551; (e) K. Rurack, U. Resch-Genger, Chem. Soc. Rev. 31 (2002) 116; (f) F. Davis, S.D. Collyer, S.P.J. Higson, Top. Curr. Chem. 255 (2005) 97; (g) R.J.T. Houk, S.L. Tobey, E.V. Anslyn, Top. Curr. Chem. 255 (2005) 199. [2] (a) A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515; (b) B. Valeur, I. Leray, Coord. Chem. Rev. 205 (2000) 3; (c) A.P. de Silva, D.B. Fox, A.J.M. Huxley, T.S. Moody, Coord. Chem. Rev. 205 (2000) 41; (d) L. Prodi, F. Bolletta, M. Montalti, N. Zaccheroni, Coord. Chem. Rev. 205 (2000) 59; (e) D.W. Boening, Chemosphere 40 (2000) 1335.

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