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.