S. Nishizawa, R. Kato, T. Hayashita
Jun 1, 1998
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Analytical Sciences
Abstract
cally important substrates is one of the current research topics in analytical chemistry and biochemistry. A wide variety of chromoionophores for cations such as alkali and alkaline-earth metal ions1 as well as amines2,3 have been reported. In contrast, only a few chromoionophores have been reported for the colorimetric determination of anions in solution.4–7 In design and fabrication of new anion receptors for applications in aprotic media8–11, the (thio)urea group as hydrogen bond donor has recently drawn much interest as a functional group for neutral receptors to recognize monoand dicarboxylate anions, halide anions, sulfate, as well as dihydrogenphosphate etc.6c,9–17 Wilcox et al.17 have recently shown substituent effects in arylthiourea upon binding with a zwitterionic sulfonic acid in chloroform. In their study, some of the aryl(thio)ureas with a nitro group were found to show changes in their UV-Vis spectra upon complexation. However, these spectral changes were only utilized to assess association strengths, and no further studies of these materials as chromogenic anion receptors have been reported. We now report herein a very simple anion receptor 1 in which a p-nitrophenyl group is linked to a methylthiourea moiety. In this type of donor–acceptor chromophore, the electronic excitation is mostly accompanied by a charge transfer from donor nitrogen of the thiourea to acceptor substituent (–NO2) of the chromophore.18 Thus, the excited state would be more strongly stabilized by anion binding, resulting in a bathochromic shift in the absorption maxima (lmax). Introducing the electron withdrawing substituent enhances the acidity of thiourea moiety, receptor 1 strongly binds with anions to produce an effective color change in aprotic media.