A controlled supramolecular approach toward cation-specific chemosensors: alkaline earth metal ion-driven exciton signaling in squaraine tethered podands

Abstract

Three different squaraine tethered bichromophoric podands 3a-c with one, two, and three oxygen atoms in the podand chain and an analogous monochromophore 4a were synthesized and characterized. Among these, the bichromophores 3a-c showed high selectivity toward alkaline earth metal cations, particularly to Mg²⁺ and Ca²⁺ ions, whereas they were optically silent toward alkali metal ions. From the absorption and emission changes as well as from the Job plots, it is established that Mg²⁺ ions form 1:1 folded complexes with 3a and 3b whereas Ca²⁺ ions prefer to form 1:2 sandwich dimers. However, 3c invariably forms weak 1:1 complexes with Mg²⁺, Ca²⁺, and Sr²⁺ ions. The signal output in all of these cases was achieved by the formation of a sharp blue-shifted absorption and strong quenching of the emission of 3a-c. The signal transduction is achieved by the exciton interaction of the face-to-face stacked squaraine chromophores of the cation complex, which is a novel approach of specific cation sensing. The observed cation-induced changes in the optical properties are analogous to those of the "H" aggregates of squaraine dyes. Interestingly, a monochromophore 4a despite its binding, as evident from 1H NMR studies, remained optically silent toward Mg²⁺ and Ca²⁺ ions. While the behavior of 4a toward Mg²⁺ ion is understood, its optical silence toward Ca²⁺ ion is rationalized to the preferential formation of a "Head-Tail-Tail-Head" arrangement in which exciton coupling is not possible. The present study is different from other known reports on chemosensors in the sense that cation-specific supramolecular host-guest complexation has been exploited for controlling chromophore interaction via cation-steered exciton coupling as the mode of signaling.