Bis(alizarinato)silane: In Silico Design and Synthesis of a Powerful Chromogenic Lewis Acid as a Dual-Gated Fluoride Ion Probe

Previously reported Lewis superacids are colorless and do not exhibit visual changes upon adduct formation. Herein, we combine the features of near Lewis superacidity with light absorption and emission in the visible range. Detailed (TD-)DFT considerations lead to the design of bis(alizarinato)silane 1 as a powerful chromogenic Lewis acid, that responds to one or two fluoride ions with different optical outputs. Lewis adducts of 1 are synthesized, and the coordination-state-dependent absorption and emission maxima verified experimentally. Reaction with, for example, sodium fluoride, reveals the simultaneous participation of the Lewis acidic silicon center and the Lewis basic alizarin carbonyl functions, promising its additional use as a salt-selective molecular receptor. Although the poor solubility and moisture intolerance of 1 for the moment hinder real practical applications, this study paves the way for dual-gated fluoride ion sensing.