Selective Recognition of Fluoride by using a Benzobisimidazolium Derivative through Aggregation-Induced Fluorescence

A new benzobisimidazolium derivative (1) bearing four naphthalene moieties was synthesized and demonstrated as an F^- ion-selective fluorescent chemosensor. The fluorescence of 1 in acetonitrile (CH₃CN) is significantly stronger with F^- and acetate (CH₃CO₂^-), but not with other anions (Cl^-, Br^-, I^-, HSO₄^-, and H₂PO₄^-). The fluorescence of 1 is enhanced selectively with F^- in the presence of a small amount of water. Our DFT calculations indicate that the electrostatic interactions between the positively charged benzobisimidazolium moieties and F^- play an important role in the formation of stable aggregates. The formation of stable aggregates of 1 with F^- in CH₃CN is a key step for the selective sensing of F^-, and the fluorescence of the aggregates is further enhanced in a mixture of 95 % CH₃CN and 5 % water, which can be attributed to the aggregation-induced emission.