Anion Binding-Induced White Light Emission using a Water-Tolerant Fluorescent Molecular Tweezer

A novel fluorescent molecular tweezer (FMT), built on the pyridine-2,6-bis-carboxamide framework, has been developed that, in presence of a red emitter, gives rise to white light emission in response to the addition of H₂ PO₄^- anions. The FMT incorporates two pyrene moieties as fluorescent reporter units and a strategically placed amine residue that imparts pH sensitivity to the fluorescence and offers additional electrostatic/hydrogen-bonding interactions to the anions. As a result, this FMT selectively binds monoanionic tetrahedral oxyanions such as H₂ PO₄^- and HSO₄^- that contain hydrogen bond donors and acceptors, and can sense their presence in aqueous acetonitrile through changes in fluorescence. Anion binding results in excimer formation by the pyrenes and a bluish-green emission from the FMT. Both amide and amine residues of the FMT interact with these anions. The binding stoichiometry with H₂ PO₄^- and HSO₄^- was found to be 1:1 and affinity of the FMT for these anions is of the order of 10⁴  m^-1 . The limit of detection for H₂ PO₄^- was found to be 13 nm. Addition of a perylene monoimide-based red emitter gives rise to panchromatic emission perceived as white light.