Fluorescent Calix[4]arene-Oxacyclophane Receptors as Sensors of Transition Metal Cations

The sensing behavior of a calix[4]arene-oxacyclophane-carbazole conjugate (1) towards Cu(I) ions was investigated. Contrary to the noteworthy sensitivity and selectivity previously found for Cu(II), the affinity of 1 for Cu(I) cations is much lower. Through fluorescence titration assays, the apparent binding constants (Ka) for the two ions were determined, showing that an affinity of over thirteen fold was displayed for Cu(II). Two Cu(I) counter-ions were used to determine possible effects on the binding event. It was found that acetate and iodide ions behave similarly, yielding Ka of the same magnitude. Formation of a ground-state supramolecular complex between 1 and Cu(I) ions was not observed on UV-Vis titrations, in contrast to what was previously reported for Cu(II) using the same host. The affinity of 1 for Fe(III), a major biological competitor, was also assessed. The fluorescence of host 1 is quenched by Fe(III) ions although to a lesser extent (32% less efficient than Cu(II)). The study demonstrates that calixarene 1 is able to differentiate copper ions in two common oxidation states using fluorescence techniques, thereby suggesting its application for redox-active centers in biomimetic chemistry.