A calixdiquinone bis-indoaniline derivative with two kinds of potential binding sites, discriminates cations according to their charge/size ratio. Harder and highly oxophilic Eu (III) preferentially interacts with the lower rim quinone oxygens; whereas Na (I) binds to the azacrown moieties. As a consequence of the chromogenic sensor's design, these two binding events result in two different responses, Eu (III) causes a remarkable auxochromic ...