Dibenzoborole-Containing π-Electron Systems:  Remarkable Fluorescence Change Based on the “On/Off” Control of the pπ−π* Conjugation

A series of dibenzoborole derivatives with various groups such as (N,N-diphenylamino)phenyl, thienyl, and bithienyl groups at the 3,7-positions have been synthesized and their photophysical properties studied. These new pi-electron systems show significant solvatochromism in the fluorescence spectra. Thus, about 100-140 nm blue shifts in the emission maxima and 20-30-fold increments in the quantum yields are observed upon changing the solvent from THF to DMF. Similar fluorescence changes are observed upon the addition of n-Bu4NF to their THF solutions, demonstrating their sensing abilities toward a fluoride ion. These fluorescence changes result from the "on/off" control of the ppi-pi* conjugation in their LUMO by the coordination of donor solvents or fluoride ion to the boron atom in the dibenzoborole skeleton.