Fluorescent alizarin-phenylboronic acid ensembles: Design of self-organized molecular sensors for metal ions and anions

Alizarin-phenylboronic acid interactions were used in a new design strategy for self-organized fluorescent sensors of metal ions and anions. First, Alizarin Red S (ARS), which binds effectively to phenylboronic acid (PBA) at neutral conditions in water, proved to be an excellent fluorescent indicator for the selective detection of metal ions; the fluorescent-active ARS-PBA ensemble causes an efficient and selective quenching in the fluorescence by adding metal ions. It was also found that anion-induced phenylboronates can bind effectively to alizarin and greatly enhance the fluorescence in MeOH. The observation makes it possible to design new self-organized anion sensory systems composed of alizarin (as reporter) and phenylboronic acids (PBAs, as receptor), such as PBA or 3-nitrophenylboronic acid (NPBA). By combining spectroscopic studies (UV-vis and fluorescence) and ¹¹B NMR measurements, a ternary complex (F ^--alizarin-PBAs) has been determined. The resulting self-organized sensory system is applicable even to the detection of anions dissolved in water, when liquid [alizarin, PBA, 18-crown-6 in CH₂ClCH ₂Cl]-liquid [KX (X = halogen, OAc, H₂PO₄) in water at pH 5.5] two-phase extraction is employed.