Figure 7.

The benzothiazole-substituted calix[4]arene probe 14 presents the 1,3-alternate conformation. This probe functions as a turn-off sensor for Cu²⁺, binding two equivalents of this metal ion which quench its fluorescence output. The resulting copper complex is an effective turn-on sensor for S²⁻, which—alone among anions tested—rescues the fluorescence of 14. Alternatively, 14 also functions as a turn-on probe for HSO₄⁻ which hydrolyses the imines and thereby enhances the fluorescence output [87].