Responsive Supramolecular Sensors Based on Pillar[5]arene–BTD Complexes for Aqueous Sensing: From Static Quenching to Anion and DNA Recognition

We report the design and investigation of supramolecular guest-host systems based on cationic pillar[5]-arene macrocycles and fluorescent 2,1,3-benzothiadiazole (BTD) derivatives. The BTD guests, obtained through Sonogashira cross-coupling, display intense visible emission, large Stokes shifts, and strong solvatochromism, while the pillar[5]-arene hosts enable selective recognition in aqueous media. Spectrofluorimetric titrations and NMR experiments revealed the formation of 1:1 inclusion complexes between dianionic BTD derivatives and cationic pillar[5]-arenes, with binding constants on the order of 10⁵ M^-1. Static fluorescence quenching was observed upon complexation, in agreement with molecular docking simulations (Δ<i>G</i> ≈ -8.5 kcal mol^-1) that highlighted the role of electrostatic and π-π interactions. Proof-of-concept sensing studies demonstrated that competitive binding of anions or DNA induces disassembly of the nonemissive BTD <b>4</b>⊂P-[5]-A complex, restoring the intrinsic fluorescence of the guest. These results establish pillar[5]-arene-BTD assemblies as promising supramolecular platforms for responsive fluorescence sensing in aqueous environments.