Making Waxy Salts in Water: Synthetic Control of Hydrophobicity for Anion‐Induced and Aggregation‐Enhanced Light Emission

We show that multipodal polycationic receptors function as anion‐responsive light‐emitters in water. Prevailing paradigms utilize rigid holes and cavities for ion recognition. We instead built open amphiphilic scaffolds that trigger polar‐to‐nonpolar environment transitions around cationic fluorophores upon anion complexation. This ion‐pairing and aggregation event produces a dramatic enhancement in the emission intensity, as demonstrated by perchlorate as a non‐spherical hydrophobic anion model. A synergetic interplay of C−H⋅⋅⋅anion hydrogen bonding and tight anion–π+ contacts underpins this supramolecular phenomenon. By changing the aliphatic chain length, we demonstrate that the response profile and threshold of this signaling event can be controlled at the molecular level. With appropriate molecular design, inherently weak, ill‐defined, and non‐directional van der Waals interaction enables selective, sensitive, and tunable recognition in water. Molecular recognition in water is an unperfected art in synthetic supramolecular chemistry. We demonstrate that the inherent hydrophobicity of tripodal fluorogenic cations can be synthetically modulated to control the sensitivity toward large, non‐spherical, and charge‐diffuse anions in water.