Fluorescence Switching of Intramolecular Lewis Acid–Base Pairs on a Flexible Backbone

A class of intramolecular Lewis acid–base pairs containing Lewis acidic dimesitylboranes paired with phosphine oxide, sulfoxide, and sulfone Lewis basic groups are explored. The absorption and emission properties of the compounds are investigated in different solvent environments, and switching of the photophysical behavior between the Lewis acid–base adducts and free acid–base pairs is examined. We find that phosphine oxide Lewis base groups are effective partners in flexible Lewis pairs; however, the analogous sulfoxide and sulfone groups are not. Additionally, the absorption and emission wavelengths can be systematically tuned by varying the conjugation length and electron-donating and -accepting substituents on the backbone.