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

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 lambda(cm) = 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 E1/2I = -1.21 V and E1/2II = -1.66 V as well as E1/2I = -1.25 V and E1/2II = -1.71 V, respectively. H2PO4- gave remarkable potential shifts (ca. 200 mV) of the second reduction waves (E1/2II) 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.