A Lewis Acidic, π‑Conjugated Stibaindole with a Colorimetric Response to Anion Binding at Sb(III)

We describe 1-chloro-2,3-diphenylstibaindole (1-Cl), a Sb­(III) heterocycle which exhibits visible-wavelength absorption due to conjugation of the σ*­(Sb–Cl) orbital with the π* system of the conjugated backbone. Because the σ*­(Sb–Cl) orbital is accessible, stibaindole 1-Cl is a Lewis acid that readily binds halide anions at the site opposite the Sb–Cl bond. This binding induces a “turn-off” colorimetric response due to the suppression of the σ*−π* conjugation in the anion-bound form.