Binding of flavylium ions to sulfonatocalix[4]arene and implication in the photorelease of biologically relevant guests in water

The formation of host-guest complexes between seven flavylium cations and water-soluble p-sulfonatocalix[4]arene (SC4) was investigated by UV/vis absorption, fluorescence, and NMR spectroscopies. The results show the cationic guests form complexes with affinities in the submillimolar range. A representative chalcone/flavylium photoswitch was investigated in more detail regarding its pH- A nd light-triggered interconversion between the two forms. The dramatic affinity differentiation of the SC4 binding of the two switchable species (40 M^-1 for the trans-chalcone versus 3.5 × 10⁴ M^-1 for the flavylium cation) enables the pH-gated photocontrol of the complexation process. These responsive properties were explored to demonstrate the competitive and selective release of biologically relevant guests from their supramolecular complexes with p-sulfonatocalix[4]arene (SC4), following the principle of AND logic. The guest release can be reverted by the thermally activated reaction of the flavylium ion back to the trans-chalcone.