Stereoelectronics of the Hydrogen-Bond-Induced Fluorescence Quenching of 3-Aminofluorenones with Alcohols.

Two derivatives of 3-amino-9-fluorenone (1) bearing one (2) and two methyl (3) groups flanking the carbonyl group are prepared. Comparison of their photophysical properties show that all suffer efficient radiationless deactivation in the presence of alcohols. Preferential solvation studies with mono alcohols reveal that a single H-bonding interaction quenches the excited states of 1 and 2, but not that of 3. In contrast, a single molecule of ethylene glycol quenches all three. These results are interpreted in a quenching mechanism similar to one proposed by Inoue and co-workers, but where an out-of-plane H-bond with the carbonyl group gives rise to an emissive species, while an in-plane H-bond results in quenching.