1λ⁵‑Stibaindoles as Lewis Acidic, π‑Conjugated, Fluoride Anion Responsive Platforms

Our interest in anion-responsive materials has led us to investigate the synthesis and properties of derivatives featuring a Lewis acidic antimony­(V) moiety integrated within the π-conjugated structure of a 1λ5-stibaindole (or 1λ5-benzostibole). Starting from an organozirconium reagent, 1,2,3-tris­(phenyl)-1-stibaindole (1) was first synthesized and subsequently allowed to react with o-chloranil and 3,5-di-tert-butyl-o-benzoquinone to afford the corresponding catecholato-1λ5-stibaindole derivatives 2 and 3, respectively, as a result of oxidative addition of the o-benzoquinone reagent to the antimony center. Conversion of 1 into 2 and 3 lowers the energy of the antimony-based σ* orbital, thus promoting its conjugation with the π* orbital of the heterocyclic chromophore. As a result of this σ*−π* conjugation, 2 and 3 experience a narrowing of the π–π* energy gap leading to a red shift of the corresponding UV–vis absorption band. This σ*−π* conjugation can be turned off by addition of a fluoride anion, which interacts with the antimony-based σ* orbital. This turn-off response manifests itself in a color change that can be exploited for the detection of ppm levels of fluoride in drinking water samples.