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.