Complexation and fluorescence behavior of a copolymer bearing azacrown ether and anthracene moieties
Introduction
Many fluorescent ionophores based on crown ethers have been reported to date [1], [2], [3], [4], [5], most of which are small molecules consisting of crown and fluorophore moieties. These ionophores were investigated as ON/OFF switching devices and fluorescent reagents for the determination of metal ions in the field of analytical chemistry. In particular, fused aromatic moieties, such as anthryl and pyrenyl groups, have been widely used in many fluoroionophores because the fluorescence spectra of these derivatives are highly responsive to their surroundings, including the electronic conditions of the neighboring heteroatoms and media [6], [7]. For example, the fluorescence of an azacrown ether derivative bearing the anthracene moiety is quenched by photo-induced electron transfer (PET) from the nitrogen atom of the azacrown ether to the anthracene moiety. However, an azacrown ether cannot form a stable complex with an alkali metal ion in aqueous solution due to competition with hydration of the metal ion. Thus, fluorescent ionophores bearing crown ethers are generally not useful in aqueous solution for detecting metal ions.
The complexation behaviors of crown ether-based polymers with metal ions in aqueous solution have been investigated by examining their fluorescence spectral changes. There are three categories of fluorescence that involve complexation between a polymer and metal ions: (1) The metal ion, such as a lanthanide ion, bound to the crown ether polymer is photoluminescent [8]. (2) The fluorescent dye competes with the metal ion for complexation with the crown ether polymer [9]. (3) The crown ether polymer has fluorophores or fluorescent parts whose fluorescence spectra change upon complexation of the crown ether moiety and the metal ion [10], [11], [12]. These studies have shown that crown ether polymers can form relatively stable complexes with metal ions in aqueous solution. The crown ether polymer, which forms micelles, can form a stable complex with K+ even in aqueous solution [12].
In this study, water soluble polymers bearing crown ether and anthracene moieties were synthesized, and their fluorescence spectral changes were examined in aqueous solution in the absence and presence of alkali metal ions. The time-dependent changes in the fluorescence intensity of the polymers were investigated under photo-irradiation conditions.
Section snippets
Materials and equipment
All reactions were carried out under an atmosphere of argon unless otherwise noted. Anhydrous THF was purchased from Kanto Chemicals and used without further purification. Perchlorates of lithium, sodium, potassium and cesium were of analytical grade. Infrared spectra were recorded on a FT-SP FT/IR spectrometer (JASCO). UV–vis spectra were measured with a UV–VIS V-530 spectrophotometer (JASCO). Fluorescence spectra were recorded on a FP-6200 (JASCO). Elemental analyses were performed using a
Synthesis
N-(2-Hydroxyethyl)azacrown ethers, e.g., aza-12-crown-4, 15-crown-5 and 18-crown-6 derivatives, were synthesized via the reaction of 2-aminoethanol and the corresponding oligoethyleneglycol diiodides in the presence of Na2CO3 as a base in CH3CN [13]. The reaction of N-(2-hydroxyethyl)azacrown ether and methacryloyl chloride using triethylamine as a base in CH2Cl2 yielded the methacrylate of the N-2-ethylazacrown ether, which is referred to as the azacrown-methacrylate. 9-Anthrylmethyl
Conclusion
Copolymers bearing anthracene and aza-12-crown-4, 15-crown-5 and 18-crown-6 moieties complex most favorably with Na+, K+ and K+ ions, respectively, in THF:H2O (10:1) solution. In particular, the aza-18-crown-6 copolymer 3 recognized K+ selectively, exhibiting a large change in fluorescence in the presence of this metal ion. These results show that crown ether polymers are candidates for fluorescent dyes to detect alkali metal ions in aqueous solution.
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