Optical and electrochemical properties of heteroditopic ion receptors derived from crown ether-based calix[4]arene with amido-anthraquinone pendants

Abstract

Two heteroditopic receptors based on a calix[4]arene crown ether containing amidoanthraquinone pendants in cone and 1,3-alternate conformations (1 and 2, respectively) were synthesized. Photophysical properties of 1 and 2 were studied by UV-vis and fluorescence spectrophotometry in dried CH₃CN. Both 1 and 2 showed the highest sensitivity towards F⁻ through the appearance of a new charge transfer band at 500 nm and the enhancement of the emission spectra at λ_em = 542 nm and 528 nm respectively. Interestingly, in the presence of K⁺, the fluorescence intensity of 1 at 542 nm increased around 2 fold compared to that in the absence of K⁺ upon addition of F⁻, while this phenomenon was not observed in the case of receptor 2. Cyclic voltammograms of receptors 1 and 2 showed two consecutive one-electron reversible waves in 40% v/v CH₃CN in CH₂Cl₂, corresponding to two single-electron reductions to give mono- and dianions species at E_1/2I = −1.21 V and E_1/2II = −1.66 V as well as E_1/2I = −1.25 V and E_1/2II = −1.71 V, respectively. H₂PO₄⁻ gave remarkable potential shifts (ca. 200 mV) of the second reduction waves (E_1/2II) of both free 1 and 2. In the presence of K⁺, only receptor 1 gave remarkable potential shifts in its redox wave II upon adding F⁻ and AcO⁻. Therefore, receptors 1 and 2 exhibited dual sensing modes by fluorescence spectrophotometry and cyclic voltammetry. The topology of ligands also played an important role in cooperative binding properties of heteroditopic receptor 1 possessing a closer distance between a cation and an anion binding. On the other hand, the two ion binding sites of receptor 2 were separated by a longer distance and did not support the cooperative binding. This resulted in the abstraction of K⁺ from receptor 2 upon addition of anions.