Halide Recognition through Diagnostic “Anion–π” Interactions: Molecular Complexes of Cl⁻, Br⁻, and I⁻ with Olefinic and Aromatic π Receptors

Intense colorations and new charge-transfer absorption bands are observed upon addition of a halide (Cl−, Br−, I−) to neutral organic π acceptors with electron-deficient olefinic and aromatic centers. These phenomena results from noncovalent anion–π interactions (shown schematically), which were confirmed by X-ray crystallography.