Intramolecular excimer formation and complexing behavior of tridentate pyridine podand having two naphthalene rings as a fluorescent chemosensor for zinc ion.

established.1,2 However, the same and other metal ions can be toxic to life when present at certain concentrations in the environment, water supplies, food chain, and industrial chemicals and products. Consequently, an intensive effort has been devoted to develop various sensory molecular receptors capable of recognizing, sensing, and selectively transporting these positively charged substrates so that the concentrations of these metal ions of commercial value can be recovered from waste solutions, and certain toxic transition metal ions in the environment can be removed.1–8 The goal of this research is the design and construction of ion-selective sensors by assembling a specific metal ion-receptor with a subunit capable of signaling the occurrence of a receptor-substrate interaction.8 Many kinds of crown ether type macrocyclic compounds have been used for analytical applications, such as chemical sensors9 and spectrophotometries.10 In the fluorimetry application fluorescent reagents, which have two aromatic hydrocarbons at both terminals of a linear polyether as an analogue of a crown ether, have also been used.11 We have developed compounds capable of selectively responding, via an increase in excimer fluorescence, to several metal ions.12–14 We now wish to report on preliminary results of our study on zinc ion recognition by podand 1. The structural formula is shown in Fig. 1. Podand 1 was prepared by esterification from the corresponding alcohol with carboxylic acid or acid chloride. Measurements of the fluorescence spectra were carried out in an acetonitrile solution of podand 1 (1.0 × 10–5 mol dm–3) at room temperature, and Mg(ClO4)2, Ca(ClO4)2, Ba(ClO4)2, Ni(ClO4)2, Co(ClO4)2, Cu(ClO4)2, Zn(ClO4)2, Ag(ClO4)2 and Cd(ClO4)2 were added to the solution. To prevent any nonlinearity of the fluorescence intensity, 290 nm was chosen as the excitation wavelength. The absorption and excitation spectra of podand 1 are essentially identical. A ground-state intramolecular interaction, such as charge transfer (CT), was excluded by the absence of a new band at a longer wavelength for podand 1. When Mg2+, Ca2+, Ba2+, Ni2+, Co2+, Cu2+, Ag2+ and Cd2+ were added to an acetonitrile solution of podand 1, the shape and intensity of the fluorescence spectra were not changed. However, the spectra of podand 1 changed greatly with the addition of Zn2+. The fluorescence spectra of podand 1 in the presence of several concentrations of Zn(ClO4)2 are shown in Fig. 2. These new longer wavelength emissions are due to excited-state complex formation, i.e., intramolecular excimer formation. We express the metal salt interactions in terms of the equilibrium: