Ind. Eng. Chem. Res. 2005, 44, 847-851
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Fluorometric Detection of pH and Metal Cations by 1,4,7,10-Tetraazacyclododecane (Cyclen) Bearing Two Anthrylmethyl Groups Yasuhiro Shiraishi,* Yoshiko Kohno, and Takayuki Hirai Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan
1,4,7,10-Tetraazacyclododecane (cyclen) bearing two anthrylmethyl groups (1,7-bis(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane, L2) was synthesized and used as a fluorescent chemosensor for detection of pH and transition metal cations in aqueous solution. The fluorescent response and complexation properties of L2 were studied in comparison with those of a cyclen with a single anthrylmethyl group that was previously reported (1-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane, L1). Although L2 showed a less sharp response than L1, L2 demonstrated fluorescence behavior similar to that of L1, showing strong fluorescence at a low-pH region (pKa ) 8.4) and in the presence of Zn2+ and Cd2+ but showing weak fluorescence at a high-pH region and in the presence of Cu2+. Chemical stability test by 1H NMR revealed that L2 is very stable even if left to stand under a humid atmosphere for 30 days, suggesting that L2 may be a potential effective fluorescent chemosensor for detection of pH and transition metal cations in aqueous solution. Introduction Fluorometric analysis is a simple and useful method for prompt detection of pH and metal cations in aqueous solution. Considerable effort has therefore been made to develop selective and sensitive fluorescent chemosensors.1 Most of the sensors consist of a fluorophore (e.g., anthracene) covalently linked to an ionophore (e.g., amino group), which can bind H+ and metal cations, and are thus called a fluoroionophore.2-4 The fluorescence of the fluorophore is “on” when the ionophore binds H+ or metal cations. In the absence of H+ and metal cations, the fluorescence is, on the contrary, quenched by an electron transfer from the nitrogen lone pair to the photoexcited fluorophore. Among the sensors based on the above photoinduced electron-transfer mechanisms, 1,4,7,10-tetraazacyclododecane (cyclen) bearing a single anthrylmethyl group (1-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane, L1) (Figure 1) is known to be a principle and effective chemosensor for detection of pH and transition metal cations in aqueous solution.5 The L1 is, however, decomposed easily, when left to stand under humid atmosphere, such that the use of L1 for practical fluorometric sensing is difficult.5 Development of a more stable fluorescent chemosensor is therefore necessary. In the present work, a cyclen bearing two anthrylmethyl groups at the opposite side of the nitrogen atom on the cyclen ring, (1,7-bis(9-anthrylmethyl)1,4,7,10-tetraazacyclododecane, L2) (Figure 1), was newly synthesized and used as a fluorescent chemosensor for detection of pH and metal cations in aqueous solution. The fluorescent response of L2 was studied in comparison with that of L1, in regard to the protonation and complexation behaviors of L1 and L2. The chemical stability of L2 was studied by 1H NMR, and the * To whom correspondence should be addressed. Tel.: +816-6850-6271. Fax: +81-6-6850-6273. E-mail: shiraish@cheng. es.osaka-u.ac.jp.
Figure 1. Structure of fluorescent chemosensors.
applicability of L2 for practical fluorometric sensing was also examined. Experimental Section Materials. All of the reagents were of the highest commercial quality, which were supplied by Wako Pure Chemical Industries, Ltd. and Tokyo Kasei Co., Ltd. and used without further purification. Cyclen was synthesized as described.6 L1 was synthesized by the reaction of cyclen with 9-chloromethylanthracene in toluene, as described.5 L2 was synthesized as follows: NaH (0.11 g, 2.3 mmol) was added slowly to DMF (40 mL) containing cyclen (0.12 g, 0.7 mmol) under dry N2 at 273 K and the resultant mixture was stirred for 2 h, to form a 1,7dianion of cyclen.7 Excess quantity of NaH was then filtered off. To the resulting solution, 9-chloromethylanthracene (0.35 g, 1.5 mmol) dissolved in DMF (20 mL) was added slowly at 383 K under dry N2 and stirred for 12 h. The DMF was then removed completely by evaporation from the resulting mixture. The residue was dissolved in ethanol (2 mL), to which concentrated aqueous HCl solution (1.2 mL) was added slowly and stirred for 2 h at room temperature. The formed yellowish precipitate was recovered by filtration and dried in vacuo at 353 K for 12 h, affording a yellowbrown powder of L2 (0.102 g, yield 24%): mp 428 K; 1H NMR (CD3OD, 400 MHz, TMS) δ 2.92-3.26 (m, 16H, CH2 of cyclen), 4.92-5.00 (m, 4H, ArCH2), 7.37-8.68
10.1021/ie049459i CCC: $30.25 © 2005 American Chemical Society Published on Web 01/12/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005
(m, 18H, ArH); 13C NMR (CD3OD, 400 MHz) δ 18.48, 43.69, 44.91, 45.61, 50.90, 51.31, 52.44, 58.62; FAB mass spectrum, m/e 553 (M+), Anal. Calcd for C38H40N4: C 82.57, H 7.29; 10.14. Found: C 82.4, H 7.3, N 10.1. Fluorescence Measurements. The measurements were carried out on a Hitachi F-4500 fluorescence spectrophotometer. Excitation wavelength of 335 nm was employed, where both excitation and emission slit widths were 3 nm. The measurements were done at 298 ( 0.1 K using a 10-mm-path length quartz cell. For pH detection, 3 mL of buffered aqueous solution containing L1 or L2 (3 µM) was used, where the buffered aqueous solutions used for respective pH adjustment were 0.033 M CH3COOH and 0.007 M CH3COONa (pH 4.0), 0.05 KH2PO4 and 0.005 NaOH (pH 6.2), 0.05 KH2PO4 and 0.03 NaOH (pH 7.6), 0.05 KCl, 0.05 H3BO3, and 0.022 NaOH (pH 9.5), 0.05 NaHCO3 and 0.01 NaOH (pH 10.4), and 0.05 KCl and 0.013 NaOH (pH 12.3). For metal cation detection, an aqueous solution (60 µL) containing metal chloride (0.3 mM) was added to 3 mL of the above buffered solution of pH 10.4 containing L1 or L2 (3 µM) and used for measurement. Fluorescence quantum yields (Φ) of L1 and L2 were determined by comparison of the integrated corrected emission spectrum of standard quinine, which was excited at 366 nm in 0.10 M H2SO4 (Φ ) 0.55). Excitation spectra were recorded at an emission wavelength of 417 nm. Fluorescence lifetime was measured on a PTI-3000 apparatus (Photon Technology International) using a Xe nanoflash lamp filled with N2 as an excitation source. The measurements were done at 298 ( 0.1 K with excitation and emission wavelengths of 358 and 417 nm, respectively. Other Analyses. Potentiometric pH titrations were carried out on a COMTITE-550 potentiometric automatic titrator (Hiranuma Co., Ltd.) with a glass electrode GE-101. Aqueous test solutions (70 mL) containing cyclen, L1, or L2 (0.1 mM) in the absence and presence of Zn2+ (0.1 mM) were kept under dry argon with an ionic strength of I ) 0.10 (KCl). The titrations were done at 298 ( 0.1 K using an aqueous KOH (2.5 mM) solution as a base, and at least two independent titrations were performed. Deprotonation constants and intrinsic complexation constants of cyclen, L1, and L2 were determined by means of the nonlinear least-squares program HYPERQUAD,8 where KW () [H+][OH-]) value used was 10-13.78 (at 298 K).9 Absorption spectra were recorded on an UV-visible photodiode-array spectrophotometer (Shimadzu; Multispec-1500) at 298 ( 0.1 K. 1H NMR (in CD3OD) spectra were obtained by JEOL JNM-GSX270 Excalibur using TMS as standard. Electron densities of nitrogen atoms on L1, L2, and their tetraprotonated species (H4L14+ and H4L24+) were calculated using the MNDO-PM3 method within the WinMOPAC ver.3.0 software (Fujitsu Ltd.), in accordance with the previously described procedure.10-12 Results and Discussion Detection of pH. As shown in Figure 2a, L2 dissolved in aqueous solution of pH 4 shows almost the same absorption spectrum as that of L1, where the molar absorption coefficient of L2 is higher than that of L1, owing to the presence of two anthrylmethyl groups. Change in the absorption spectrum of L2 with pH was scarcely observed, as was also the case for L1.5,13 Fluorescence spectra of L1 and L2 in aqueous solution of different pH are summarized in Figure 3. L2 shows strong fluorescence at the low-pH region (
