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 pi-conjugated structure of a 1 lambda(5)-stibaindole (or 1 lambda(5)-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-tent-butyl-o-benzoquinone to afford the corresponding catecholato-1 lambda(5)-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 sigma* orbital, thus promoting its conjugation with the pi* orbital of the heterocyclic chromophore. As a result of this sigma*-pi* conjugation, 2 and 3 experience a narrowing of the pi-pi* energy gap leading to a red shift of the corresponding UV-vis absorption band. This sigma*-pi* conjugation can be turned off by addition of a fluoride anion, which interacts with the antimony-based sigma* 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.